Role of pyroptosis in the pathogenesis of various neurological diseases

Abstract

Pyroptosis, an inflammatory programmed cell death process, has recently garnered significant attention due to its pivotal role in various neurological diseases. This review delves into the intricate molecular signaling pathways governing pyroptosis, encompassing both caspase-1 dependent and caspase-1 independent routes, while emphasizing the critical role played by the inflammasome machinery in initiating cell death. Notably, we explore the Nucleotide-binding domain leucine-rich repeat (NLR) containing protein family, the Absent in melanoma 2-like receptor family, and the Pyrin receptor family as essential activators of pyroptosis. Additionally, we comprehensively examine the Gasdermin family, renowned for their role as executioner proteins in pyroptosis. Central to our review is the interplay between pyroptosis and various central nervous system (CNS) cell types, including astrocytes, microglia, neurons, and the blood-brain barrier (BBB). Pyroptosis emerges as a significant factor in the pathophysiology of each cell type, highlighting its far-reaching impact on neurological diseases. This review also thoroughly addresses the involvement of pyroptosis in specific neurological conditions, such as HIV infection, drug abuse-mediated pathologies, Alzheimer’s disease, and Parkinson’s disease. These discussions illuminate the intricate connections between pyroptosis, chronic inflammation, and cell death in the development of these disorders. We also conducted a comparative analysis, contrasting pyroptosis with other cell death mechanisms, thereby shedding light on their unique aspects. This approach helps clarify the distinct contributions of pyroptosis to neuroinflammatory processes. In conclusion, this review offers a comprehensive exploration of the role of pyroptosis in various neurological diseases, emphasizing its multifaceted molecular mechanisms within various CNS cell types. By elucidating the link between pyroptosis and chronic inflammation in the context of neurodegenerative disorders and infections, it provides valuable insights into potential therapeutic targets for mitigating these conditions.

Introduction:

In the complex network of cellular responses to disease pathogenesis, an intriguing phenomenon has emerged as a focal point - referred to as regulated cell death. Cells, in their quest to maintain homeostasis, actively undergo programmed cell death as a strategic response to various threats, whether they are cancerous cells or insidious infectious agents (Vande Walle and Lamkanfi, 2016; Vanden Berghe et al., 2014). Among these diverse mechanisms of cell death, pyroptosis stands out as a unique and interesting process, providing insights into the complex interplay between life and death at the cellular level.

Pyroptosis, an unusual and inflammatory form of programmed cell death, first made its mark in the scientific world when Zychlinsky and colleagues observed its emergence in response to invasive pathogenic bacteria in 1992 (Zychlinsky et al., 1992). This discovery marked a turning point, unveiling the existence of a cellular self-destruct mechanism that was hitherto unknown. Since its inception, pyroptosis has been intimately associated with the progression of various human diseases, ranging from neurodegenerative disorders to HIV-associated neurocognitive disorders, as well as cancer and cardiovascular diseases (He et al., 2020; Ma et al., 2018). However, it is within the domain of neurodegenerative disorders and neurocognitive impairments that pyroptosis has notably attracted significant attention in the fields of neuroscience and experimental pharmacology.

Pyroptosis is a process distinguished by a distinct set of morphological and pathophysiological features. These include chromatin condensation, DNA fragmentation, the activation of inflammatory caspases, cell membrane rupture, cell swelling, and the release of proinflammatory cytokines and other cytoplasmic contents (Ketelut-Carneiro and Fitzgerald, 2022; Shi et al., 2015; Vanden Berghe et al., 2014; Zychlinsky et al., 1992).

The pivotal role of inflammatory caspases, originating from both human and mouse sources, is traditionally considered a defining characteristic that distinguishes pyroptosis from other cellular death pathways like apoptosis, necroptosis, and ferroptosis. (Jorgensen and Miao, 2015; Man et al., 2017). However, it is important to note that pathways such as caspase-3/8- and granzyme-mediated pathways, which do not primarily involve inflammatory caspases, also contribute to orchestrating pyroptosis, adding complexity to its mechanisms. Also, the differentiation between pyroptosis and apoptosis became apparent when the former was found to rely on inflammatory caspases, resulting in substantial morphological transformations (Brennan and Cookson, 2000; Chen et al., 1996; Hersh et al., 1999; Hilbi et al., 1998; Watson et al., 2000).

Furthermore, pyroptosis distinguishes itself from apoptosis by the selective release of specific cytoplasmic contents, including proinflammatory cytokines (such as IL-1α, IL-1β, and IL-18) and damage-associated molecular patterns (DAMPs) like double-stranded DNA, adenosine triphosphate (ATP), and high mobility group box 1 (HMGB1) (Frank and Vince, 2019; Rao et al., 2022). The cells undergoing pyroptosis undergo a transformative process, staining positively for propidium iodide, ethidium bromide, and 7-AAD due to the permeability and low molecular weight of these dyes, setting them apart from the characteristic features of apoptosis (Fink and Cookson, 2006). Additionally, a distinctive form of DNA damage is observed in the early stages of pyroptosis. This damage is of lower intensity and can be detected through terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining. It is characterized by fragmented DNA and an intact nucleus, a feature shared with apoptosis (Fink and Cookson, 2006; Tajima et al., 2019; Xu et al., 2018; Zhao et al., 2019a).

The activation of pyroptosis is a finely tuned orchestration, carefully balanced to mount a precise defense against pathogenic invaders, maintain homeostasis, and prevent excessive harm to the host. Many molecular and cellular signals are involved in orchestrating this intricate tango between the release of proinflammatory cytokines/chemokines and the resolution of the response. In this context, we explore the molecular and cellular events leading to the activation and execution of pyroptosis within the complex realms of neurodegeneration and neurocognitive impairment. As we delve deeper into this journey, we will uncover the intriguing connections and implications of pyroptosis in the context of neurological diseases, providing valuable insights into potential therapeutic avenues for mitigating these complex conditions.

Pyroptosis - molecular signaling:

The molecular signaling pathways governing pyroptosis are a captivating labyrinth of intricate mechanisms orchestrating this unique form of programmed cell death. Central to this process is a group of inflammatory caspases, including caspase 1, which exists as a human and mouse homolog, caspase 11 (mouse homolog), and caspase 4 and 5 (human homologs). These caspases function as crucial effectors, orchestrating the initiation of pyroptosis through the executioner protein known as Gasdermin (GSDM), ultimately leading to membrane rupture (Jorgensen and Miao, 2015; Kayagaki et al., 2015; Man et al., 2017; Shi et al., 2015). The pyroptosis signaling pathway can be conveniently categorized into four distinct pathways: the caspase-1-dependent pathway, the caspase-1-independent pathway, the caspase 3/8-mediated pathway, and the granzyme-mediated pathway. The molecular mechanism of pyroptosis is depicted in Figure 1.

Figure 1:

The molecular mechanism of pyroptosis. The caspase-1-dependent inflammasome involves the assembly of intracellular inflammasome machinery sensor proteins in response to PAMPs and DAMPs (viruses, toxins, double-stranded DNA, and bacteria), which activate multiple upstream signaling events such as K⁺ efflux, Ca²⁺ flux, Cl efflux, and ROS production. The sensors recruit ASC (adaptor) and procaspase-1 (effector) to form the inflammasome complex, which undergoes autocatalytic cleavage to generate cleaved caspase-1. Cleaved caspase-1 cleaves pro-IL-1β and pro-IL-18 into their mature forms, subsequently released through the N-GSDMD pore. Additionally, cleaved caspase-1 cleaves GSDMD, releasing N-GSDMD to create a nonselective pore on the plasma membrane, allowing the release of mature IL-1β and IL-18. In the caspase-1 independent pathway, LPS from gram-negative bacteria directly binds to pro-caspase-4/5/11, leading to caspase-4/5/11 activation, which in turn cleaves GSDMD, triggering pyroptosis. In caspase-3/8-mediated pyroptosis, DAMPs induce TNFR/IFNR signaling to activate caspase-3 via caspase-8 or caspase-8 directly, which cleaves GSDMC and GSDME to mediate pyroptosis. Furthermore, chemotherapeutic drugs induce pyroptosis through mechanisms involving ROS via the caspase-3/GSDME, caspase-1/GSDMD, or caspase-8/GSDMC cascades. The Granzyme-mediated pyroptosis pathway involves the release of Granzyme A and Granzyme B from Natural Killer cells, B cells, and T cells, which enter the target cells via perforin to recognize GSDMB and GSDME, respectively.

Caspase-1-dependent pathway

The pathway, predominantly associated with caspase-1 activation, serves as the backdrop for processing both the executioner protein of the GSDM family and the immature form of the proinflammatory cytokines, and this intricate process is pivotal to orchestrating pyroptosis (Cookson and Brennan, 2001; Li et al., 1995; Xia et al., 2019). Recent research has revealed that inhibiting caspase-1 enhances the integrity of the blood-brain barrier (BBB) in ischemia-induced injuries resulting from middle cerebral artery occlusion. This inhibition reduces the leakage of both Evans Blue and matrix metalloproteinase (MMP) proteins while also increasing the levels of tight junctions and tissue inhibitors of metalloproteinases (Liang et al., 2020). Neutrophil caspase-1 activation has also been shown to induce the production of proinflammatory cytokine interleukin-1β, promoting Pseudomonas aeruginosa-mediated pyroptosis in vivo (Santoni et al., 2022). VX765, a well-known caspase-1 inhibitor, robustly inhibits caspase-1-mediated interleukin-1β production and pyroptosis (Jin et al., 2022). In a remarkable stride toward therapeutics, a combination therapy consisting of mycophenolate mofetil, tacrolimus, and steroids has demonstrated the inhibition of caspase-1-induced pyroptosis in kidney tissue specimens from patients and mice with lupus nephritis (Man et al., 2017; Martinon et al., 2002). Unlike other signaling cascades, the caspase-1-dependent pathway operates through a multifaceted cytosolic protein complex known as an inflammasome (Man et al., 2017; Martinon et al., 2002). Typically, the induction of an inflammasome triggers the activation of pro-caspase 1 via caspase activation and recruitment domain (CARD)-CARD interaction from an upstream trigger. This activation leads to the cleavage of GSDM and pro-IL-1β/IL-18 into their active forms, initiating pyroptosis (Burdette et al., 2021). These complex molecular interactions highlight the complexity and precision in regulating pyroptosis, emphasizing its significance in both health and disease. Further exploration of the molecular signaling pathways of pyroptosis provides deeper insights into how these pathways orchestrate cellular responses, paving the way for potential therapeutic interventions in various pathophysiological contexts.

Caspase-1-independent pathway

The caspase-1-independent pyroptosis pathway primarily involves inflammatory caspases, with murine homologs (caspase 11) and human homologs (caspase 4 and caspase 5) taking center stage. While mouse and human caspase-1 are likely orthologues, extensive evolutionary analysis and sequence comparisons of the caspase domain and prodomain reveal that human caspase-4 and −5 may have originated from a caspase-11 ancestral gene duplication event (Martinon and Tschopp, 2004, 2007). Recent research has also unveiled that caspase 11, caspase 4, and caspase 5 can directly bind to cytosolic lipopolysaccharide (LPS) through their CARD. This interaction leads to the cleavage of GSDMD, ultimately triggering pyroptosis (Shi et al., 2014). In 2020, a groundbreaking study demonstrated that LPS-induced TLR4 signaling increases caspase-11/GSDMD activation and cleavage, resulting in the cytosolic accumulation of HMGB1 in hepatocytes (Li et al., 2020b). Furthermore, the activation of caspase-4/11 by LPS faces competition from a TLR4 agonist, oxidized phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine, in macrophages (Chu et al., 2018). Other intriguing findings reveal that l-adrenaline can act on the ADRA2B receptor (Alpha-2B adrenergic receptor) to inhibit caspase-11 activation induced by cytosolic LPS or Escherichia coli infection in macrophages (Chen et al., 2019b). Additionally, the silencing of caspase 4 has been shown to repress TNFα-induction and the executioner proteins of pyroptosis, the GSDM protein family, in human pulmonary arterial endothelial cells (Wu et al., 2022). A study focusing on corneal injury demonstrated that wedelolactone, an active component of Eclipta prostrata, targets caspase-4/5/11 activation, inhibiting the development of Pseudomonas aeruginosa keratitis and the release of proinflammatory cytokines (Xu et al., 2021).

Inflammasome machinery: a critical activator of pyroptosis

In the intricate realm of cellular responses, inflammasomes emerge as pivotal multimeric cytosolic protein complexes that assemble in response to the presence of pathogen-associated molecular patterns (PAMPs) and DAMPs (Sharma and Kanneganti, 2016). While the activation of inflammasome machinery is essential for recognizing and responding to damaged cells and pathogen-invaded environments, it is important to note that overt activation of these complexes has been implicated in driving metabolic and autoimmune disorders. The inflammasome multimeric proteins can be broadly categorized into three distinct families based on their structural features: the Nucleotide-binding domain Leucine-rich Repeat (NLR) containing proteins family (NLRP1, NLRP3, NLRC4, NLRP6, NLRP12), the absent in melanoma 2–like receptors family (AIM2), and the pyrin family (Burckstummer et al., 2009; Chae et al., 2011; Zhou et al., 2022). Within these inflammasome families, a crucial mechanism emerges as they are capable of recruiting two key components: apoptosis-associated speck-like protein containing a caspase activation and recruitment domain (ASC) and the cysteine protease caspase-1, the latter serving as the effector molecule (Sharma and Kanneganti, 2016). This recruitment leads to the formation of an inflammasome complex, a dynamic assembly with profound implications.

The inflammasome complex subsequently undergoes an induced autocatalytic cleavage process, culminating in the production of two active cleaved caspase 1 subunits, denoted as p10 and p20 (Jiang et al., 2021a; Ran et al., 2021; Sharma and Kanneganti, 2016; Sun et al., 2019). These activated caspase 1 subunits wield immense power as they venture into the cellular milieu, poised to initiate a cascade of events. One of the most striking roles of these active cleaved caspase 1 subunits is their ability to process the inactive forms of proinflammatory cytokines, namely IL-1β and IL-18, into their bioactive counterparts through precise proteolytic cleavage. These transformed cytokines are then liberated into the extracellular space through pyroptosis (Jiang et al., 2021a; Ran et al., 2021; Sharma and Kanneganti, 2016; Sun et al., 2019). This dual action, involving the activation of caspase 1 and the subsequent release of proinflammatory cytokines via pore-forming molecules called GSDMs (Evavold et al., 2018) via pyroptosis, amplifies the impact of inflammasome machinery in shaping the cellular response to threats. Thus, inflammasome machinery is a sentinel, ready to mobilize and respond to danger signals derived from pathogens or cellular damage. Its ability to orchestrate the activation of caspase 1 and the release of proinflammatory cytokines through pyroptosis highlights its critical role in the intricate web of cellular defense mechanisms (Figure 2). Understanding the fine balance between appropriate inflammasome activation and its potential for dysregulation is key to deciphering its contributions to health and disease. As we delve deeper into the intricacies of this molecular machinery, we gain valuable insights into the complexities of cellular immunity and the potential for targeted therapeutic interventions in various pathological contexts.

Figure 2:

The different types of inflammasome signaling. The inflammasome machinery, encompassing NLRP1, NLRP3, NLRC4, NLRP6, and AIM2, accumulates in the cytosol upon detection of various PAMPs or DAMPs (such as Anthrax lethal toxin MDP, Reactive Oxygen Species (ROS), cytoplasmic dsDNA, lysosomal damage, flagellin components, and intestinal bacteria). Anthrax lethal toxin MDP induces the cytosolic expression of the NLRP1 inflammasome. NLRP3 inflammasome activation occurs in response to various DAMPs and PAMPs, including ROS generation, K+ efflux, or cathepsin B release resulting from lysosomal damage, triggering inflammasome complex formation. NLRC4 is activated by binding to NLR family apoptosis inhibitory proteins (NAIPs; NAIP5, NAIP6, NAIP2) to sense bacterial flagellin from type III secretion system rod proteins. Intracellular microbe components like LTA lead to the assembly of the NLRP6 inflammasome complex, while AIM2 assembles through binding to pathogen-derived dsDNA. All inflammasome machinery recruit ASC and caspase 1, forming the inflammasome complex, which activates caspase 1, subsequently cleaving proIL-1b and proIL-18 (derived from the nucleus), and ultimately leading to the secretion of mature cytokines from the activated gasdermin pore.

NLR-containing proteins family: Critical activator of pyroptosis

Within the complex landscape of pyroptosis, the NLR-containing protein family plays a pivotal role, serving as critical activators of this intriguing form of programmed cell death. This family typically exhibits a characteristic structural composition comprising a central nucleotide-binding domain (NBD), a C-terminal leucine-rich repeat (LRR) domain, and an N-terminal domain. This intricate architecture allows them to function as key players in the orchestration of pyroptosis (Sharma and Kanneganti, 2016). The NLR-containing protein family can be further classified based on the presence of two distinctive domains in their N-terminal: pyrin or CARD. Accordingly, they are divided into NLRP (P for pyrin) or NLRC (C for CARD) subfamilies. While several members of this family, such as NLRP1, NLRP3, NLRC4, and NLRP6, are well-established inflammasome sensors, others, like NLRP12, are still considered putative inflammasome sensors, warranting further investigation (Sharma and Kanneganti, 2016).

NLRP1, the first inflammasome protein recognized for its caspase-1 activation mechanism, exhibits noteworthy differences between mice and humans. While the mouse NLRP1 genome encodes three paralogs (a, b, and c), none possess the pyrin domain. In contrast, human NLRP1 contains NBD, LRR domains, and a pyrin domain (Sharma and Kanneganti, 2016). The expression of NLRP1 is widespread in various brain cell types, including astrocytes, microglia, neurons, and vascular endothelial cells, underscoring its relevance in neurological contexts (Brickler et al., 2016; de Rivero Vaccari et al., 2009). NLRP1 inflammasome signaling has been implicated in neuronal pyroptosis and neurological deficits following experimental intracerebral hemorrhage, an association mediated through C-C chemokine receptor 5 (Yan et al., 2021). Recent studies have demonstrated that inhibiting or silencing Nlrp1 can prevent inflammasome-driven responses, thereby promoting neuroprotection and cognitive improvement in conditions such as Alzheimer’s disease and stress-induced depression behaviors (Cassel et al., 2008; Chiu et al., 2021; Lee et al., 2012; Song et al., 2020; Zhou et al., 2011).

Among the NLR family, NLRP3 is one of the most extensively studied inflammasome members. It exhibits a remarkable capacity to respond to various exogenous and endogenous ligands, implicating its involvement in various diseases that show accompanying inflammation, including Alzheimer’s disease, arthritis, diabetes, and obesity (Guo et al., 2015). What sets NLRP3 apart is its ability to recognize an assortment of inflammasome activators, spanning microbial cell wall components, environmental crystalline pollutants, endogenous danger signals, reactive oxygen species, calcium signaling, potassium efflux, and lysosomal disruption (Cruz et al., 2007; Dostert et al., 2008; Man and Kanneganti, 2015; Petrilli et al., 2007). The activation of NLRP3 inflammasome typically involves two distinct steps: a priming signal that activates the transcription factor NF-κB and a subsequent activation step driven by a wide range of stimulators (Bauernfeind et al., 2009; Cruz et al., 2007; Dostert et al., 2008; Man and Kanneganti, 2015; Petrilli et al., 2007). In the brain, NLRP3 inflammasome finds expression in multiple cell types, including microglia, astrocytes, and neurons (Fann et al., 2013; Jian et al., 2016; Kannan et al., 2022). Recent data has illuminated its role in HIV gp120 protein-induced NLRP3-dependent pyroptosis in microglia, with inhibiting microglial NLRP3 inflammasome offering alleviation from gp120-mediated neuroinflammatory factor release and neuronal injury (He et al., 2020). Furthermore, the silencing of NLRP3 through siRNA transfection has been shown to significantly enhance the inhibitory effects of curcumin on microglial pyroptosis and the release of proinflammatory responses, both in vitro and in vivo, emphasizing the therapeutic potential of targeting NLRP3 (Ran et al., 2021).

In contrast to the well-characterized NLRP1 and NLRP3, NLRP6 remains less explored due to its unique nature concerning ligand recognition and downstream signaling cascade (Sharma and Kanneganti, 2016). Early studies revealed that in pathogenic bacterial infection, Nlrp6-deficient cells exhibited increased expression of intestinal NFκB and MAPK-dependent inflammation in macrophages (Anand et al., 2012). Within the brain, knocking down NLRP6 was observed to alleviate inflammation, suppress autophagy, and reduce brain injury following intracerebral hemorrhage, illustrating the multifaceted roles of the NLRP6 inflammasome that can vary based on cell or organ specificity (Xiao et al., 2020). Further insights emerged when researchers identified that the NLRP6 inflammasome primarily colocalizes in astrocytes instead of neurons, microglia, or endothelial cells in intracerebral hemorrhage-induced mice (Wang et al., 2017a). Mechanistically, the NLRP6 inflammasome induces caspase 1-regulated pyroptosis, thereby exerting control over neuronal survival in ischemic stroke (Zhang et al., 2020a). Additionally, it has been revealed that the NLRP6 inflammasome triggers pyroptosis in human gingival fibroblasts through caspase 1 activation, leading to the extracellular release of proinflammatory cytokines (Liu et al., 2018).

The specific triggers inducing NLRP6 inflammasome signaling in neurological diseases remain elusive. However, recent findings revealed that lipoteichoic acid (LTA), a molecule produced by Gram-positive bacteria, activates NLRP6, leading to the recruitment and processing of caspase-11 via the adaptor ASC. This activation occurs during Listeria monocytogenes infection or cytosolic presence of LTA. Notably, NLRP6 forms a protein complex with caspase-11 and caspase-1, regulating IL-18 secretion in macrophages. These results offer novel insights into the activation mechanism of the NLRP6 inflammasome and its recruitment of proinflammatory caspases in macrophages following microbial infection (Hara et al., 2018). Studies also unveiled that the NLRP6 promoter region harbors binding sites for peroxisome proliferator-activated receptor-γ, chicken ovalbumin upstream promoter transcription factor 1, and retinoid X receptor α. Exposure to rosiglitazone, a PPAR-γ agonist, also increased NLRP6 mRNA expression in human intestinal epithelial cells (Kempster et al., 2011; Li and Zhu, 2020). Furthermore, another study reported an increase in NLRP6 mRNA expression when exposed to encephalomyocarditis virus, polyinosinic:polycytidylic acid, or interferon (IFN) α in mouse embryonic fibroblasts (Wang et al., 2015a).

A recent report on the NLRP12 inflammasome also reveals its role in PANoptosome activation. This process integrates components from multiple cell death pathways and drives a form of lytic, innate immune inflammatory cell death called PANoptosis (Sundaram et al., 2023). The study found that a combination of heme and PAMPs induced a regulatory NLRP12-PANoptosome complex, thereby driving inflammatory cell death through caspase-8/RIPK3 signaling. Consequently, this led to a complex tripartite PANoptosis cell death involving pyroptosis, necroptosis, and apoptosis. Notably, the deletion of NLRP12 in mice exhibited protection against acute kidney injury and lethality in a hemolytic model (Sundaram et al., 2023). Moreover, another study highlighted an intriguing synergistic relationship among NLRP12, NLRP3, and NLRC4, inducing pyroptosis through CASP1-dependent GSDMD cleavage during ischemic injury (Chen et al., 2020). Additionally, studies using BV2 microglia exposed to oxygen and glucose deprivation/reoxygenation unveiled that silencing NLRP3, NLRP12, or NLRC4 suppressed neuroinflammation and pyroptosis (Chen et al., 2020).

Overall, the NLR-containing protein family, comprising members like NLRP1, NLRP3, and NLRP6, emerges as pivotal regulators of pyroptosis, with each member contributing to distinct cellular responses and pathophysiological processes. Their presence in various brain cell types highlights their significance in neurological contexts, with implications for health and disease. As we unravel the roles of these NLR family members, we gain a deeper understanding of their multifaceted functions and potential as targets for therapeutic interventions in a range of neurological conditions.

Absent in melanoma 2 (AIM2)-like receptor family: Pyroptosis involvement.

In pyroptosis, the AIM2 receptor family is a well-characterized group, with AIM2 being its most prominent member. AIM2 has a structural composition that includes an N-terminal pyrin domain (PYD) and a 200-amino acid repeat C-terminal interferon-inducible nuclear protein domain (HIN200 domain) (Cridland et al., 2012; Sharma and Kanneganti, 2016). Within AIM2, the HIN200 domain plays a crucial role in ligand binding, specifically through the recognition of dsDNA, and exhibits structural autoinhibition in the absence of ligands (Jin et al., 2012; Jin et al., 2013). The recognition of dsDNA by the HIN200 sensor domain effectively relieves its autoinhibitory property, thereby enabling the PYD domain to engage in homotypic interactions with ASC and caspase 1. This interaction gives rise to a star-shaped structure characterized by multiple filaments radiating from a central hub (Jin et al., 2013; Lu et al., 2014).

AIM2 is functionally conserved across various mammalian species, demonstrating a consistent cytosolic localization and featuring a structural N-terminal PYD domain that facilitates ASC recruitment (Cridland et al., 2012; Hornung et al., 2009; Wang et al., 2019a). This receptor family is prevalent and plays a critical role in neurodevelopment, with AIM2 Inflammasome exhibiting high expression in microglia, astrocytes, and neurons (Li et al., 2021b). Recent investigations have shed light on the significance of AIM2 in neurological conditions. In a study conducted in 2021, AIM2 knockout mice displayed attenuated inflammasome-mediated secretion of proinflammatory cytokines, reduced levels of apoptosis, and diminished pyroptosis following bilateral common carotid artery stenosis (BCAS) (Poh et al., 2021). This same study unveiled that AIM2 deletion conferred resistance to chronic microglial activation, neuronal loss, myelin breakdown, and neurocognitive deficits post-BCAS (Poh et al., 2021). Another study delving into early brain injury following subarachnoid hemorrhage found that GSDMD-induced pyroptosis, mediated through the AIM2 inflammasome, could be alleviated by AIM2 inhibition, emphasizing the receptor family’s role in modulating neuroinflammatory responses (Yuan et al., 2020).

Overall, the AIM2 receptor family, with AIM2 as its prime representative, emerges as a critical player in pyroptosis within the neurological context. Its ability to recognize dsDNA and initiate inflammasome assembly underscores its importance in neuroinflammatory processes and highlights its potential as a target for therapeutic interventions in various neurological disorders.

Pyrin-receptor family

A recent discovery has revealed that a receptor family that has emerged as a key player in inflammasome formation is the Pyrin-receptor family. This revelation came to light in a mouse model expressing pyrin-containing mutations associated with familial Mediterranean fever (Chae et al., 2011). The critical component of this receptor family, pyrin, is encoded by the Mediterranean virus gene, which is frequently mutated in individuals afflicted with familial Mediterranean fever (Schnappauf et al., 2019).

Pyrin, the central figure in this context, serves as an innate immune sensor designed to detect the presence of bacterial toxins that induce the activation of Rho guanosine triphosphatase (Rho GTPase). These Rho GTPases are molecular switches that regulate various signal transduction pathways, including those governing cytoskeletal organization (Schnappauf et al., 2019; Xu et al., 2014). The expression of the Pyrin inflammasome is primarily observed within the innate immune system, encompassing cell types such as granulocytes, eosinophils, monocytes, and dendritic cells (Schnappauf et al., 2019). Recent research has revealed that the Pyrin inflammasome engages in molecular interactions with AIM2 and Z-DNA binding protein 1 (ZBP1). These interactions drive the assembly of a multiprotein complex, ultimately culminating in pyroptosis, apoptosis, and necroptosis, collectively called PANoptosis (Lee et al., 2021). This newfound understanding of the role of the Pyrin-receptor family in inflammasome activation adds another layer of complexity to the intricate web of molecular interactions orchestrating cell death pathways. It underscores the significance of this receptor family in regulating immune responses and cellular integrity, shedding light on potential therapeutic avenues for addressing various pathological conditions.

Caspase-3/8-mediated pyroptosis pathway

This pathway offers a new perspective on the interplay between pyroptosis and other cell death mechanisms. Recent studies have shattered the conventional belief that caspase 3/8, primarily associated with apoptosis, cannot trigger GSDM-mediated pyroptosis (Wang et al., 2017b). In a groundbreaking revelation from 2017, researchers discovered that GSDME, a prominent member of the GSDM protein family, possesses the remarkable ability to switch apoptosis induced by various factors, including TNF, DNA-binding/modifying compounds (such as doxorubicin, cisplatin, and actinomycin-D), and topoisomerase inhibitors (including topotecan, CPT-11, etoposide, and mitoxantrone), into pyroptosis (Wang et al., 2017b). This paramount study explained the process by showcasing that upon TNF stimulation, GSDME undergoes cleavage while caspases 3 and 7 become activated. Cells undergoing this transformation exhibit noticeable swelling and the formation of characteristic large bubbles on the plasma membrane (Wang et al., 2017b). The research also unveiled that pyroptosis mediated by GSDME is highly sensitive to pharmacological inhibition or genetic knockout of caspase 3, while caspase 7 remains non-essential (Wang et al., 2017b). A recent study has also revealed that mitochondrial neurotoxins induce caspase 3-dependent GSDME cleavage, resulting in the formation of numerous intracellular puncta in the distal and proximal neurites, as well as cell bodies at early timepoints (Neel et al., 2023). Also, GSDME puncta is primarily dispersed in the cytosol, contrasting with the plasma membrane and cytosolic enrichment observed in the human neuroblastoma cell line. This phenomenon promotes mitochondrial depolarization, trafficking defects, and neurite retraction, suggesting an earlier occurrence of neuronal intracellular GSDME localization compared to plasma membrane involvement in pyroptosis. However, this observation does not negate the potential involvement of pore formation in neurons, as multiple pore-forming molecules may regulate mitochondrial membrane integrity alongside GSDME.

Further enriching our understanding of this complex network, another study highlighted the role of TGF-β activated kinase-1 (TAK1) and IκB kinase (IKK) inhibition, orchestrated by the Yersinia pestis effector protein YopJ. This inhibition leads to RIPK1-caspase-8-mediated cleavage of GSDMD, ultimately inducing pyroptosis (Orning et al., 2018). Additionally, in cancer biology, macrophage-derived TNF-α has been found to induce pyroptosis in breast cancer cells. This occurs through a unique sequence of events involving the nuclear localization of the immune checkpoint programmed death ligand 1 (PD-L1), enhanced gene sequencing of GSDMC, and specific cleavage by caspase 8, leading to the formation of membrane pores (Hou et al., 2020b). The discovery of this apoptosis-to-pyroptosis molecular switch presents a tantalizing opportunity for deeper exploration, particularly in neurodegenerative disorders. Unlocking the precise mechanisms behind this switch holds immense potential for future therapeutic interventions and a more comprehensive comprehension of cell death pathways in various disease states.

Granzyme-mediated pyroptosis pathway

The granzyme-mediated form of pyroptosis emerges as a remarkable feature, enabling immune cells to precisely identify and eliminate pathogenic or malignant intruders (Nussing et al., 2022). Granzymes, small serine proteases with a diameter of approximately 32 kDa, hold a diverse array of substrate specificities. These proteases are stored within the cytoplasmic secretory vesicles of cytotoxic lymphocytes and natural killer cells, poised for release when needed (Nussing et al., 2022). Unlike digestive proteases, granzymes are finely tuned processing enzymes that selectively target specific substrates to initiate various signaling pathways or incapacitate their intended targets (Nussing et al., 2022). The granzyme family encompasses several genes, with granzyme B (GzmB) standing out as the most potent member due to its exceptional substrate specificity. In contrast, granzyme A (GzmA) exhibits lower cytotoxicity (Kaiserman et al., 2006; Susanto et al., 2013). Recent research unveiled an intriguing revelation in the context of chimeric antigen receptor T (CAR-T) cells. These engineered immune cells rapidly activate caspase-3 in primary B leukemic cells by releasing GzmB. Subsequently, this cascade of events induces pyroptosis through the caspase-3/GSDME-mediated pathway (Liu et al., 2020b). Another study shed light on the role of natural killer cells and cytotoxic T lymphocytes, which employ GzmA to cleave GSDMB at a specific site (Lys229/Lys244), triggering pyroptosis (Zhou et al., 2020). Remarkably, this hydrolysis of GSDMB at a non-aspartic acid site leads to oligomerization and pore formation, challenging the conventional notion of a pyroptosis mechanism solely mediated by caspases (Yu et al., 2021; Zhou et al., 2020). Despite these fascinating discoveries, the full scope of this mechanism remains uncharted, particularly concerning its significance to the development of neurodegenerative disorders and neurocognitive impairment. Exploring the limitations of granzyme-mediated pyroptosis promises to uncover novel insights into the role of immune cells in these conditions and may lead to innovative therapeutic strategies.

GSDM: Pyroptosis executioner protein

In 2015, a groundbreaking discovery unveiled the central role of the GSDM superfamily of proteins in mediating pyroptosis, primarily characterized by GSDM-induced pore formation (Rao et al., 2022; Shi et al., 2015). This superfamily includes GSDMA, B, C, D, E (also known as DFNA5), and DFNB59 in humans, while in mice, it comprises GSDMA (with 1–3 isoforms), C (with 1–4 isoforms), D, E, and DFNB59 (Bergsbaken et al., 2011; Ding et al., 2016; He et al., 2015). Among these evolutionarily conserved GSDM proteins, GSDMD and GSDME have emerged as the most extensively studied executioner proteins, elucidating the core mechanisms of pyroptosis. Notably, DFNB59 stands apart due to its distinct pore-forming mechanism that is not yet fully understood (Ding et al., 2016; Liu et al., 2019; Rogers et al., 2019).

Gasdermins generally maintain their inactivity through close interactions between the N-terminal pore-forming and C-terminal inhibitory domains (Kuang et al., 2017; Liu et al., 2019). Activation of gasdermins is initiated through proteolytic cleavage, a consequence of host stimulation by either exogenous or endogenous factors involving lytic or non-lytic caspases or granzymes (Yu et al., 2021). This proteolytic cleavage uniquely divides GSDM into an N-terminal pore-forming fragment and an autoinhibitory C-terminal fragment (Aglietti and Dueber, 2017; Ding et al., 2016; Liu et al., 2016). Subsequently, the cleaved N-terminal pore-forming fragment binds to phospholipids and cardiolipin within the plasma membrane, leading to oligomerization and the formation of large death-inducing pores with a diameter ranging from 10 to 20 nm (Frank and Vince, 2019; Galluzzi et al., 2018; Shi et al., 2017; Vande Walle and Lamkanfi, 2020). Despite extensive research on various pyroptotic executioner proteins in the context of numerous human disorders, the functional correlations and the underlying mechanisms of these findings remain elusive. Unraveling these intricacies promises to shed light on the intricate interplay between GSDM proteins and the pathophysiology of various diseases, paving the way for potential therapeutic breakthroughs.

GSDMA

The expression of GSDMA exhibits intriguing disparities between humans and mice. While human GSDMA is predominantly found in the gastrointestinal tract and skin, murine GSDMA is further divided into GSDMA1–2 in the stomach and GSDMA3 in the skin (Runkel et al., 2004; Saeki et al., 2000). Genomic mapping reveals human GSDMA on chromosome 17q21 and murine GSDMA on chromosome 11 (Zahid et al., 2021). In structural insights, Cryo-Electron microscopy has depicted GSDMA pores as composed of 27–28 protomers, featuring an internal diameter of approximately 180 Å. These pores are formed by collaborating two β-hairpins from each N-terminal protomer, which assemble into an antiparallel β-barrel that inserts into the cellular membrane (Ruan et al., 2018). The importance of GSDMA extends to its association with severe skin inflammation, skin stem cell exhaustion, mammary gland defects, asthma, and systemic sclerosis in humans and murine models (Guo et al., 2017b; Madore et al., 2020; Moreno-Moral et al., 2018; Zhou et al., 2012). Intriguingly, GSDMA3 gene mutations are linked to severe hair loss, with eight distinct alopecia-causing mutations mapped to this gene (Tanaka et al., 2013). However, the role of GSDMA in neurological diseases remains elusive.

GSDMB

GSDMB gene resides on chromosome 17q21 and is composed of 411 amino acids, displaying a unique expression pattern in humans, encompassing the gastrointestinal tract, thyroid, skin, lung, and kidney (Saeki et al., 2000). Cryo-electron microscopy captures the distinct structural features of GSDMB pores, characterized by a GSDMB β-barrel with 24–26-fold symmetry, estimated inner diameter of 150 Å, outer diameter of 250 Å, and height of 60 Å (Wang et al., 2023). The implications of GSDMB in disease are profound, as it is linked to tumorigenesis through its expression in various human tumor tissues and its increased presence in conditions like Crohn’s disease, ulcerative colitis, and asthma (Das et al., 2016; Li et al., 2020a; Soderman et al., 2015). GSDMB stands apart from other GSDM family proteins due to its two different promoters - an Alu-derived promoter expressed solely in normal stomach tissues and a long-terminal-repeat derived promoter directing GSDMB expression in cancer and normal tissues (Li et al., 2020a). Recent studies have also revealed its susceptibility to activation through caspase 1 dependent and independent signaling and caspase 4 cleavage (Chen et al., 2019a; Panganiban et al., 2018). Notably, natural killer cells and cytotoxic T lymphocytes, through GzmA, cleave GSDMB at the interlinker (Lys229/Lys244 site) to induce pyroptosis, challenging the notion of a pyroptosis caspase-only mediated mechanism (Yu et al., 2021; Zhou et al., 2020). However, GSDMB’s role in the central nervous system, particularly concerning neurological disorders, remains insufficiently elucidated.

GSDMC

Human GSDMC boasts a singular homolog, while mice harbor four murine GSDMC variants, such as GSDMC 1, 2, 3, and 4 (Tamura et al., 2007; Zou et al., 2021). In mice, these GSDMC genes are located on chromosome 15 (GSDMC1, GSDMC2, GSDMC3, and GSDMC4) (Zahid et al., 2021). Human GSDMC is expressed in diverse tissues, including the cerebral cortex, endocrine tissues, skin, trachea, spleen, and vagina, while murine mucin orthologous genes find expression in the stomach, large and small intestines, bladder, and prostate (Fagerberg et al., 2014; Katoh and Katoh, 2004; Tamura et al., 2007; Zou et al., 2021). Emerging research has unveiled the involvement of GSDMC involvement in skin injury, mediated by GSDMC activation in human skin keratinocytes through the induction of MMP-1/ERK/JNK signaling pathways (Kusumaningrum et al., 2018). Furthermore, GSDMC contributes to a molecular switch between apoptosis and pyroptosis in Macrophage-derived TNF-α-induced breast cancer tumors. This switch is facilitated by nuclear localization of the immune checkpoint programmed death ligand 1 (PD-L1) and caspase 8 cleavage (Hou et al., 2020b). However, the role of GSDMC in the central nervous system, especially with neurological disorders, remains an intriguing enigma.

GSDMD

Among the GSDM family, GSDMD takes center stage as the most extensively studied protein, positioned as a key executor of inflammasome-induced pyroptosis, driving disease-associated inflammation (Liu et al., 2021a). Its genomic location is on chromosome 8 (8q24.3), with a molecular weight of approximately 53 kDa (Burdette et al., 2021). Notably, Cryo-electron microscopy reveals a hydrophobic core composed of structural components from the C-terminal (L292, E295, Y376, A380, S470, and A474), along with two aromatic residues from the N-terminal (F50 and W51 in mice and F49 and W50 in humans) (Liu et al., 2019). The expression pattern of human GSDMD spans various tissues, including the spleen, liver, lung, gastrointestinal tract, lymph nodes, and thyroid gland. In contrast, murine GSDMD is found in the intestine, lymph nodes, bone marrow, stomach, spleen, urinary bladder, and blood (Shi et al., 2017). Within the CNS, GSDMD is expressed under physiological conditions, with secondary increases occurring during CNS diseases or injuries due to infiltrating peripheral immune cells (Li et al., 2019). Recent research has unveiled astrocytic pyroptosis as a driving force behind astrocyte loss in depressed mice, mediated by NLRP3/caspase-1/GSDMD-mediated pyroptosis (Li et al., 2021a). Additionally, GSDMD has been implicated in HIV-1 gp120-induced NLRP3/GSDMD-dependent pyroptosis and IL-1β production in microglia, resulting in neuronal injury and behavioral impairment in mice (He et al., 2020). Notably, Paeoniflorin has been observed to alleviate hypoxia-induced pyroptosis in astrocytes by downregulating HIF1α/miR-210/caspase1/GSDMD signaling in rats (Jiang et al., 2021b).

Recent research highlights the significance of GSDMD electrostatic filtering, underscoring the role of charge and size in cargo transportation through pyroptotic channels. Cryo-electron microscopy studies of GSDMD pore and prepore structures revealed distinct conformational states, showing extensive membrane-binding elements like hydrophobic anchors and three positively charged patches. When permeabilized, GSDMD pores, primarily negatively charged, facilitate faster release of neutral and positively charged cargoes compared to negatively charged ones of similar sizes. This mechanism tends to favor the passage of matured IL-1β and IL-18 over their precursors due to caspase 1 proteolytic cleavage in the acidic domain (Xia et al., 2021). However, it remains unclear whether IL-1α primarily uses the GSDMD pore as a secretion pathway, given that both pro- and mature IL-1α possess an acidic nature (Xia et al., 2021). Additionally, separate pathways might be involved in IL-1α release, suggested by a study where the membrane-stabilizing agent punicalagin inhibited IL-1β and IL-18 release but had no effect on IL-1α release (Tapia et al., 2019). These findings underscore the complex dynamics of cargo release and the selective permeability of GSDMD, shedding light on the complex mechanisms governing cytokine release in pyroptosis.

GSDME

GSDME, also known as deafness, autosomal dominant 5 (DFNA5), is mapped to chromosome 7p15.3 (Van Laer et al., 1998). Initially, GSDME was identified in patients with a specific form of autosomal dominant, progressive, sensorineural, and nonsyndromic hearing loss (Van Laer et al., 1998). In humans, GSDME exhibits expression in the gastrointestinal tract, thyroid, cochlea, and testis, while murine GSDME is found in the pineal body, cochlea, liver, lung, intestine, dorsal striatum, olfactory bulb, cerebellum, and embryo (Op de Beeck et al., 2011; Wu et al., 2009). The distinctive role of GSDME emerges in its ability to switch apoptosis to pyroptosis through Caspase-3 cleavage of GSDME in the linker region, releasing the N-terminal fragment responsible for triggering pyroptosis (Wang et al., 2017b; Zhang et al., 2020c). Within the central nervous system, GSDME’s involvement assumes significance, as caspase-dependent GSDME cleavage promotes mitochondrial depolarization, trafficking defects, and neurite retraction, hallmarks of neurological diseases (Neel et al., 2023). Moreover, methamphetamine-induced neurotoxicity has been linked to endoplasmic reticulum/GSDME-dependent cell death in hippocampal neuronal cells (Liu et al., 2020c). The myriad functions of GSDM proteins continue to unravel, offering promising avenues for understanding their roles in various diseases, including those affecting the CNS.

Ninjurin 1 (NINJ1)-mediated pyroptosis

An essential hallmark of pyroptosis involves the rupture of the plasma membrane, which is a critical step initiating the release of proinflammatory cytokines. While GSDMs-mediated membrane pore formation through caspase cleavage is a well-recognized mechanism, emerging evidence suggests that cell surface protein NINJ1 is also implicated in the rupture of the plasma membrane (Araki and Milbrandt, 1996; Araki et al., 1997). Structurally, NINJ1, a 16 kDa cell-surface protein, is characterized by two transmembrane regions and extracellular N and C termini, facilitating hydrophilic adhesion and oligomerization to initiate the rupture of the plasma membrane (Araki and Milbrandt, 1996; Araki et al., 1997). Intriguingly, NINJ1 has associations with various inflammatory diseases, including multiple sclerosis, diabetic erectile dysfunction, rheumatoid arthritis, and asthma, underscoring its relevance in these disorders and pyroptosis (Ahn et al., 2014; Ifergan et al., 2011; Yin et al., 2013).

A recent investigation demonstrated that the exposure of cultured human neurons to the HIV-encoded viral protein R (Vpr) upregulates the expression of GSDME and leads to NINJ1 colocalization, contributing to cell lysis. Inhibition of either GSDME or NINJ1 through a gene silencing approach was shown to suppress the rupture of the plasma membrane, highlighting a cooperative interaction between both proteins in an oligomerized state for the non-selective release of proinflammatory cytokines (Fernandes et al., 2023). Additionally, NINJ1 was found to be upregulated in brain endothelial cells and infiltrating antigen-presenting cells during lesions associated with multiple sclerosis and experimental allergic encephalomyelitis (Ifergan et al., 2011). NINJ1 inhibition was also shown to decrease clinical disease outcomes by restricting CD14+ monocyte adhesion and migration across human brain endothelial cells (Ifergan et al., 2011).

Furthermore, NINJ1-mediated plasma membrane rupture was observed to release a broader array of proteins than just lactate dehydrogenase (Kayagaki et al., 2021). Secretomic analysis of wildtype bone marrow-derived macrophages (BMDMs) detected the release of approximately 780 molecules, including plectin, in a NINJ1-dependent manner in response to LPS stimuli, which was diminished in NINJ1 knockout BMDMs (Kayagaki et al., 2021). Interestingly, the release of HMGB1 was also diminished in NINJ1 knockout BMDMs exposed to LPS despite exhibiting normal GSDMD-dependent release of IL-1α (Kayagaki et al., 2021). HMGB1 is known to be a relatively small nuclear protein of approximately 28-kDa size, but it forms large complexes with nucleosomes and transcription factors, coupled with GSDMD pore of about 18 nm, could have hindered its release (Kayagaki et al., 2021; Stros, 2010). However, NINJ1 was found to be dispensable in the release of IL-1β and IL-18-dependent pyroptosis, as wild-type and NINJ1 knockout BMDMs exhibited comparable expression of IL-1β, IL-6, and IL-18 in response to either nigericin or LPS (Kayagaki et al., 2021). Further studies are warranted to comprehensively explore this exciting phenomenon and its implications in various physiological and pathological conditions.

Pyroptosis, CNS cells, and BBB

Several studies have revealed that pyroptosis-associated mediators, such as the inflammasome machinery, caspase cleavage, and gasdermin, are differentially expressed in various CNS cell types. The unique role of pyroptosis in alerting and attracting various immune cells, including glial cells in the CNS, through GSDM-pore formation and proinflammatory cytokine leakage is currently an exciting area of exploration in several disease conditions. This process has been shown to complicate, rather than resolve, immune responses in the CNS, as depicted in Figure 3.

Figure 3:

Pyroptosis, CNS cells, and BBB. Upon recognition of DAMPs and PAMPs, cells internalize these foreign intruders and initiate a unique self-destruction form of cell death termed pyroptosis. This is accompanied by Gasdermin pore formation and the leakage of inflammatory cytokines, which alert and attract immune cells to the CNS. This cell death pathway and inflammation complicate the immune response in the CNS.

Astrocytes

Astrocytes constitute a substantial portion (20–40%) of the total cells in the mammalian brain, underscoring their significance in the CNS (Khakh and Sofroniew, 2015; Liddelow and Barres, 2017). These versatile cells play critical roles in maintaining the brain microenvironment, safeguarding the integrity of the blood-brain barrier, regulating synaptic function, supplying nutrients to neurons, and even contributing to neurogenesis (Farhy-Tselnicker and Allen, 2018; Qian et al., 2018; Stadelmann et al., 2019; Verkhratsky and Nedergaard, 2018; Walker et al., 2020). However, in neurodegenerative disorders, these functions are disrupted due to changes in astrocyte morphology, molecular expression, and reactivity (Escartin et al., 2019). Concomitant with these molecular and cellular changes in astrocytes is the release of sustained proinflammatory cytokines and chemokines through pyroptotic cell death, exacerbating brain damage (Mulica et al., 2021). Furthermore, astrocytic pyroptosis intensifies the inflammatory response within the brain by compromising the supportive function of the BBB (Xie et al., 2019).

Astrocytic pyroptosis often leads to cell swelling, a phenomenon observed in several neurodegenerative disorders (McKenzie et al., 2018; Wang et al., 2019b; Zhao et al., 2019b). This swelling is accompanied by the formation of membrane pores - a characteristic feature of astrocytic pyroptosis. These pores, alongside cell swelling, lead to the release of neurotoxins and the loss of physiological functions, driven by intracellular inflammasome assembly (Clarke et al., 2018; Stogsdill et al., 2017). These pores serve as both small molecule channels and non-selective ion channels, causing calcium influx, endoplasmic reticulum stress, and mitochondrial dysfunction (Baev et al., 2022; Clarke et al., 2018). Calcium overload-induced mitochondrial dysfunction further amplifies inflammasome assembly, exacerbating calcium influx and enhancing protein nanoparticle-induced osmotic pressure on astrocyte membranes, initiating the first phase of pyroptosis (Baev et al., 2022; Long et al., 2023). Recent research emphasizes the critical role of inflammasome-mediated nanoparticle-induced osmotic pressure in astrocyte plasma membrane swelling (Zheng et al., 2021). The second phase of astrocyte pyroptosis ultimately leads to a loss of physiological function, reducing these cells to agents that amplify inflammation (Long et al., 2023). The excessive influx of ions into astrocytes induces hyperosmosis, resulting in the opening of ion channels, irreversible swelling, cell rupture, extracellular release of content, and cell death (Wang et al., 2020). Studies on subarachnoid hemorrhage-induced rats have revealed a significant correlation between caspase-1-mediated inflammasome activation and the extrinsic coagulation initiator tissue factor in astrocytes (Fang et al., 2022). Additionally, these studies have shown that inhibiting pyroptosis-induced neuroinflammation and tissue factor using VX-765 improved learning and memory capacity in these rats (Fang et al., 2022). Furthermore, astrocyte NLRP6 upregulates inflammation factors through inflammasome formation after oxygen-glucose deprivation, resulting in decreased neuron viability and apoptosis in a primary neuron-astrocyte co-culture model (Zhang et al., 2020a).

Microglia

As resident immune macrophages in the brain, microglia are essential for maintaining CNS homeostasis, defending against pathogens, and addressing CNS disorders (El Khoury, 2010; Ransohoff and El Khoury, 2015). These principal resident cells constitute 5–12% of CNS cells that can be region-specific (Lawson et al., 1990). Microglia ontological studies have confirmed their mesenchymal myeloid origin in the yolk sac and their capacity for self-renewal, independent of hematopoietic stem cells (Kierdorf et al., 2013; Tay et al., 2017). Comprehensive gene profiling and functional assays have identified several microglia-specific markers in the healthy brain (such as Iba1, HexB, CD11b, P2ry12, Cx3CR1, S100A8, S100A9, F4/80, Tmem119, Gpr34, SiglecH, TREM2, and Olfml3) (Hickman et al., 2013; Ransohoff and El Khoury, 2015). Microglia serve three critical functions in the CNS: sensing their environment, conducting physiological housekeeping, and protecting against DAMPs and PAMPs (Hickman et al., 2018; Hickman et al., 2013; Ransohoff and El Khoury, 2015; Vasek et al., 2016).

In normal conditions, microglial responses maintain tissue homeostasis through a finely regulated process, but in pathological conditions, these responses become dysregulated, leading to extremes in immune balance and cellular loss or dysfunction (Tang and Le, 2016). Initially, the M1 and M2 paradigm was proposed to characterize microglia phenotypes in relation to inflammatory responses, heavily studied in various human diseases (Block et al., 2007; Le et al., 2001; Tang and Le, 2016). Microglia exhibit states of ‘classical activation,’ ‘alternative activation,’ and ‘acquired deactivation,’ largely dependent on the activating stimulus (Colton and Wilcock, 2010; Colton, 2009). Classical activation (M1) involves proinflammatory cytokine production like IL-1β, IL18, TNF-α, and nitric oxide, while the M2 phenotype, including alternative activation, promotes anti-inflammatory responses, tissue repair, and extracellular matrix reconstruction upon treatment with IL13 or IL4 (Block et al., 2007; Tang and Le, 2016). Acquired deactivation, also part of the M2 phenotype, mitigates acute inflammation and is induced by apoptotic cell uptake or anti-inflammatory cytokines like IL10 and transforming growth factor-β (Colton, 2009; Ponomarev et al., 2007). However, diverse microglia lineages have been observed in murine Alzheimer’s disease models, notably disease-associated microglia expressing a neuroprotective signature and TREM2-expressing disease inflammatory macrophages accumulating in aging brains (Silvin et al., 2022). Disease-associated microglia exhibits specific gene expression profiles like Cd11c, Csf1, and Cd9, whereas disease-inflammatory macrophages show higher expression of Cd11c and TREM2 in Alzheimer’s disease models (Keren-Shaul et al., 2017; Silvin et al., 2022). While studies associate bone marrow mesenchymal stem cell-derived exosomes with attenuating NLPR3-mediated neuronal pyroptosis by modulating microglial polarization from M1 to M2 phase in a cerebral ischemia-reperfusion injury model (Liu et al., 2021b), it remains unclear whether pyroptosis, IL-1β, or IL18 directly impact microglial transcriptional responses or function. This aspect remains important for future investigations.

In neurodegenerative disorders, microglia undergo polarization from dormant to pro-inflammatory and anti-inflammatory phenotypes, marked by inflammation and phagocytosis, respectively (Hu et al., 2015; Long et al., 2023; Periyasamy et al., 2019). The dynamic interactions among various microglia subpopulations, including pro-inflammatory and anti-inflammatory phenotypes, dictate the course of microglia-mediated inflammation and hemostasis (Long et al., 2023). Early in many neurodegenerative diseases, the microglia’s anti-inflammatory phenotype predominates, followed by a rapid switch to a pro-inflammatory phenotype, often driven by pyroptosis (Long et al., 2023). Toll-like receptors like TLR4 on microglial membranes recognize PAMPs and DAMPs, activating NLRP3 inflammasome signaling through the NFκB/MyD88 pathway (Long et al., 2023). NLRP3 inflammasome activation leads to the assembly of ASC and procaspase 1 into an NLRP3 inflammasome complex, resulting in caspase 1-dependent pyroptosis and pore formation (Long et al., 2023).

Kaempferol (KAE), a natural flavonoid, has been shown to exert neuroprotection in 6-hydroxydopamine (6-OHDA)-induced Parkinson’s disease rats by inhibiting microglia pyroptosis (Cai et al., 2022). This study also demonstrated that KAE inhibits pyroptosis mediated by lipopolysaccharides (LPS) through the p38MAPK/NF-κB signaling pathway in microglia BV2 cell lines, attenuating inflammation (Cai et al., 2022). Resveratrol, known for its antioxidant and anti-aging properties, inhibits LPS- and ATP-activated NLRP3 inflammasome signaling, thereby protecting microglial cells from oxidative stress and mitigating proinflammatory cytokine release and pyroptotic cell death (Tufekci et al., 2021). As the first line of defense against pathogen invasion and stress, microglia mediate the early inflammatory response that induces pyroptosis, a process essential for the death of other CNS cells (Ma et al., 2017).

Neurons

Neurons, the nervous system’s fundamental structural and functional units, play a crucial role in sustaining normal life activities. Damage to neurons can have debilitating consequences (Long et al., 2023). In neurodegenerative disorders, neuronal death often precedes clinical features such as limited motor function, memory impairment, and death (Long et al., 2023). The severity and progression of most neurodegenerative disorders are closely correlated with the degree of neuronal death (Long et al., 2023; Wang et al., 2015c). Neuronal cell death is widespread during pathological conditions, significantly limiting the capacity of adult neurons to proliferate or be replaced. Pyroptosis has been proposed to play a critical role in this process (Fricker et al., 2018; Tuo et al., 2022). Notably, neuronal pyroptosis is primarily initiated by inflammatory factors released from glial cells, which induce the assembly of NLRP3 inflammasomes in neurons, leading to programmed cell death (Li et al., 2020c; Long et al., 2023). Brain injuries result in the death of many cells, leading to the abundant presence of abnormal dsDNA in the blood. This dsDNA is sensed by AIM2, triggering AIM2 inflammasome assembly, caspase 1 activation, and subsequent proteolytic cleavage of IL-1β, IL-18, and GSDMD (Lammert et al., 2020; Long et al., 2023). NLRP1 inflammasomes, primarily expressed in neurons, can directly activate caspase 1 and downstream cleavage of GSDMD and release proinflammatory cytokines (Yap et al., 2019). Additionally, chemokine receptors on neurons like CCR5 and CXCR4 can initiate NLRP1 inflammasome assembly after a stroke (Gu et al., 2021; Yan et al., 2021). Studies also indicate that the loss of parkin, an inhibitor of inflammasome priming, in both mouse and human dopamine neurons triggers spontaneous neuronal NLRP3 inflammasome assembly and subsequent cell death (Panicker et al., 2022). This investigation reveals that the loss of parkin activity contributes to NLRP3 inflammasome complex assembly via the generation of mitochondrial-derived reactive oxygen species, facilitated by the accumulation of another parkin ubiquitination substrate, Zinc finger protein 746/Parkin-interacting substrate (Panicker et al., 2022). Notably, inhibiting neuronal NLRP3 inflammasome assembly was observed to prevent the degeneration of dopamine neurons in both familial and sporadic Parkinson’s disease models (Panicker et al., 2022).

BBB

The BBB is a tightly regulated component of the CNS, consisting of closely bound cells such as astrocytes, pericytes, and brain microvascular endothelial cells (BMECs) (Adriani et al., 2017; Sweeney et al., 2019). Through gap junctions, tight junctions, and adhesion junction proteins, these cells form a barrier that strictly controls the transportation of substances in and out of the CNS, including neurotoxic plasma components, blood cells, and pathogens (Adriani et al., 2017; Sweeney et al., 2019). BBB dysfunction and damage have been linked to neurological deficits and various pathologies in neurodegenerative disorders and acute CNS disorders (Sweeney et al., 2019). The extent of BBB damage correlates with the severity of deterioration observed in these disorders, as disruption in BBB integrity leads to the leakage of harmful blood components into the CNS, cellular infiltration, aberrant transport, and molecule clearance, all contributing to neurological deficits (Montagne et al., 2017; Sweeney et al., 2019; Zlokovic, 2011). To compound the severity of BBB dysfunction, the cellular components can undergo pyroptosis, a phenomenon observed in astrocytes, pericytes, and BMECs (An et al., 2023; Fang et al., 2022; Zhang et al., 2021). Structural and molecular changes in these cells have been closely associated with early onset in many neurodegenerative disorders, leading to increased BBB permeability facilitating neutrophil passage to inflammation sites in the brain (Long et al., 2023). Consequently, glial cells like astrocytes and microglia release MMP-2 and MMP-9, which hydrolyze tight junctions, weakening BBB integrity (Long et al., 2023; Shi et al., 2016; Zhang et al., 2021).

During pyroptosis, releasing pro-inflammatory cytokines and reactive oxygen species (ROS) by glial cells promotes BMEC pyroptosis, causing ligand protein function loss and carbon skeleton rearrangement. This, in turn, leads to BBB damage (Long et al., 2023). BMECs contain numerous mitochondria that, when dysfunctional due to pyroptosis, release ROS, exacerbating damage to the cellular components of the BBB (Chen et al., 2017; Forrester et al., 2018). Additionally, BMECs can amplify inflammation by releasing inflammatory factors such as intercellular cell adhesion molecule-1, which attracts neutrophil infiltration during pyroptosis (Bui et al., 2020). While the direct damage to the brain remains elusive, pyroptosis in the BBB is considered an early event that initiates an irreversible cycle of inflammation (Long et al., 2023). Therefore, targeting pyroptosis in the BBB during the early stages may be key to preventing the progression of neurodegenerative disorders.

Role of pyroptosis in HIV, drug abuse, and neurodegenerative disorders

The role of pyroptosis and its relationships with neuroinflammatory mediators have been suggested to be involved in the progression and pathophysiology of HIV, drug abuse, and several neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease. Thus, the interrelationships and contributions of pyroptosis to neurological disorders via immune cell activation, cell death, and neuroinflammation are shown in Figure 4.

Figure 4:

Role of Pyroptosis in HIV, drug abuse, and neurodegenerative disorders. Pyroptosis may contribute to the progression of neurodegenerative diseases through immune cell activation, cell death, and neuroinflammation. Infectious pathogens can activate resident CNS immune cells, including microglia and astrocytes, resulting in pro-inflammatory release and neuronal death. Additionally, this programmed proinflammatory cell death, caused by infectious pathogens and the release of DAMPs, can exacerbate the inflammatory state in the CNS, leading to further activation of CNS immune cells and perpetuating a vicious cycle of inflammation. In Alzheimer’s disease, there is inflammasome machinery activation, high levels of pro-inflammatory cytokines IL-18 and IL-1β release, reduced clearance of infected cells, and accumulation of neurotoxic Aβ plaques and Tau tangles. Pro-inflammatory cytokine release is also well-documented in Parkinson’s disease, where increased neurotoxic α-synuclein accumulation is known to be accompanied by high levels of IL-18, TNF-α, IFN-α, and IFN-β, produced by activated microglia and astrocytes. Drug abuse triggers a self-perpetuating cycle of proinflammatory cytokine release, accompanied by apoptosis, immune cell activation, BBB damage, and recruitment of the immune response of the peripheral nervous system. HIV-1 damages the BBB, depleting immune resistance in the CNS, and results in the release of large doses of chemoattractant from activated CNS immune cells. Additionally, pyroptosis-induced neuroinflammation plays a crucial role in other neurological disorders such as multiple sclerosis, amyotrophic lateral sclerosis, and Huntington’s disease. Some pathogens can also directly damage neurons, leading to alterations in metabolism, enhanced excitotoxicity, and apoptosis, as observed in drug abuse. The overall contribution of pyroptosis can be associated with long-term changes, such as cognitive and behavioral impairments.

Role of pyroptosis in HIV

Despite the success of combination antiretroviral therapy (cART), HIV-1 infections still impair immune function and contribute to neuronal death, ultimately leading to the development of HIV-associated neurocognitive disorders (HAND) (Clifford and Ances, 2013; Eggers et al., 2017; Saylor et al., 2016). HAND is increasingly recognized as a complex neurodegenerative disease characterized by widespread, progressive, accelerated neuronal death and dementia (He et al., 2020). HIV-1-related mechanisms have been shown to induce CNS cells, such as microglia and astrocytes, through direct cell-to-cell contact with productively infected cells (Cevallos et al., 2022; He et al., 2020). A recent study observed that HIV-1 exploits pyroptosis to deplete 95% of CD4+ T cells in the lymph nodes, promoting its vicious infectious cycle and disease progression (Doitsh et al., 2014; Wang et al., 2015b). This pyroptosis-dependent cell death is accompanied by the release of inflammatory mediators that attract more CD4+ T cells from other regions to become infected by HIV-1 (Doitsh et al., 2014; Wang et al., 2015b). Specifically, a study utilized a mathematical model to demonstrate that the decline in CD4+ T cells during HIV-1 infection consists of two major phases: the first rapid phase caused by enormous HIV-1 virus infection and a second phase resulting from chronic inflammatory cytokines released during pyroptosis, which can attract CD4+ T cells from the circulatory system to inflamed lymphoid tissues (Wang et al., 2015b). The same study, which showed that the majority of quiescent lymphoid CD4+ T cells die through caspase-1-mediated pyroptosis, revealed that only 3% undergo caspase-3-mediated apoptosis in both activated and productively infected cells (Doitsh et al., 2014). This result highlights that pyroptosis-mediated chronic inflammation during chronic HIV infection acts as an erosive force that causes a gradual decline in the CD4+ T cell population in plasma (Wang et al., 2015b). Also, HIV-1-associated proteins, including Tat, gp120, and Nef, complicate HAND by persisting in microglia, brain microvascular endothelial cells, neurons, and astrocytes, even in the presence of cART (He et al., 2020; Marino et al., 2020). These HIV-1-associated proteins have been linked to persistent low-level inflammation, inflammatory cytokine expression, ceramide accumulation, brain volume reduction, and synaptic and axonal damage (Dickens et al., 2017; Kannan et al., 2022).

Pyroptosis has recently been implicated in the progression of HAND, as gp120-induced microglial NLRP3 inflammasome signaling mediates pyroptosis and IL-1β release. This initiates neuroinflammation, ultimately leading to neuronal death and behavioral impairment (He et al., 2020). Another study reported that HIV-1 Tat induces the expression of the microglial NLRP3 inflammasome and IL-1β, which are then packaged into microglial exosomes and taken up by neurons, resulting in synaptodendritic injury (Kannan et al., 2022). The cytokines released during HIV-1-associated pyroptosis play an essential role in initiating and amplifying neuroinflammation and neuropathology as neurons express IL-1 receptors (Allan et al., 2005; He et al., 2020; Song et al., 2017). The presence of IL-1 receptors across CNS cells increases the sensitivity of the CNS to proinflammatory cytokines (Allan et al., 2005; He et al., 2020; Song et al., 2017). Thus, pyroptosis has emerged as a critical player in the progression of HAND, exacerbating the neuronal damage and cognitive deficits observed in affected individuals.

Role of pyroptosis in drug abuse

Drug abuse is a pressing issue in the United States, disproportionately affecting marginalized groups such as Native Americans, LGBTQ+ individuals, and those incarcerated (Schulden et al., 2012). This presents a significant social and public health problem (Schulden et al., 2012). Among the most commonly abused substances are alcohol, cannabis, methamphetamines, heroin, cocaine, and ecstasy (Lo et al., 2020). Quantitative studies have shown a permanent deletion of neurons in the developing brain, accompanied by a reduction in brain volume, following exposure to drugs of abuse (Dikranian et al., 2005; Ikonomidou et al., 2000; Olney et al., 2002; Sanders et al., 2008). Studies have also investigated the effects of various drugs on the brain and their relationship to pro-inflammatory cell death mechanisms through inflammasome machinery proteins (Wang et al., 2021). In a study involving rats that received daily intraperitoneal injections of methamphetamine, upregulation of NLRP1, cleaved Caspase-1, IL-1β, and TNF-α was observed (Fan et al., 2022). Additionally, there was a significant increase in GSDMD expression in the hippocampus of rats treated with methamphetamine compared to those receiving saline (Fan et al., 2022). These findings suggest the involvement of pyroptotic cell death, as GSDMD is responsible for pore formation and is a key element in this pathway.

Another study examined the effects of repeated injections of morphine and fentanyl on NLRP3 expression in male rats. When repeatedly injected, both drugs resulted in increased NLRP3 expression compared to a control group receiving saline injections. The researchers also noted increased activation markers of astrocytes (GFAP) and microglia (CD11b), indicating drug-induced neuroinflammation. Morphine and fentanyl also increased the expression of Caspase-1 and GSDMD-N, further suggesting active pyroptosis (Carranza-Aguilar et al., 2022). In an experiment investigating chronic alcohol consumption in mice, ethanol treatment led to the activation of IL-1β and IL-18. However, when a group of mice was treated with both ethanol and a Caspase-1 inhibitor, the activation levels of these pro-inflammatory cytokines were significantly reduced. This suggests caspase-1-dependent pyroptosis in response to ethanol in the gastric epithelial cells of these mice (Li et al., 2018). Overall, there is increasing evidence linking pyroptosis to the effects of drug abuse on the brain. Different drugs have been shown to induce pyroptotic cell death pathways, leading to neuroinflammation and potential neurological consequences.

Role of pyroptosis in Alzheimer’s disease (AD)

AD is characterized by neuropathological hallmarks, including extracellular amyloid-β plaques, intracellular neurofibrillary tangles, neuroinflammation, and neuronal loss (Moonen et al., 2023; Trejo-Lopez et al., 2022). Inflammasome activation has been primarily observed in microglia and astrocytes in AD (Hu et al., 2022). The neuropathogenic proteins associated with AD have been reported to induce NLRP3 inflammasome signaling and caspase-1 activation, leading to microglial polarization toward a proinflammatory phenotype (Liu et al., 2020a; Luciunaite et al., 2020; Stancu et al., 2019). Inflammasome proteins and mediators have also been found to facilitate the formation, spread, and aggregation of toxic Aβ plaques (Friker et al., 2020; Venegas et al., 2017). In APPSwePSEN1dE9 mice, NLRP3 inflammasome-dependent pyroptosis in microglia was associated with forming ASC specks, which promoted the deposition and spread of Aβ. This effect was attenuated by ASC deficiency or anti-ASC antibodies (Friker et al., 2020; Venegas et al., 2017). Another study reported the inhibition of inflammasome activation in microglia and improved cognitive function in APP/PS1 mice by MCC950, an NLRP3 inhibitor (Dempsey et al., 2017).

Additionally, NLRP3 inflammasome activation, cleaved GSDMD, IL-18, and nucleus shrinkage were reported in sparse neurons in the cornu ammonis of the hippocampus during AD pathology (Moonen et al., 2023). Furthermore, NLRP3 inflammasome and ASC activation have been observed in astrocytes following exposure to Aβ plaques, in vitro and in vivo (Couturier et al., 2016; Ebrahimi et al., 2018). Although some studies have reported the absence of NLRP3 inflammasome in AD patients, they exhibited cleaved GSDMD and caspase-8 activation in astrocytes (Moonen et al., 2023). Clinical correlation in the brain and cerebrospinal fluid (CSF) of AD patients revealed increased proteolytic cleavage of caspase-1 and elevated levels of the inflammatory cytokine IL-1β (Cacabelos et al., 1991; Heneka et al., 2013). Recent findings have shown a positive correlation between GSDMD and AD biomarkers Aβ1–42 and Tau181p in the CSF of AD patients (Shen et al., 2021). Overall, AD-associated neuropathogenic proteins stimulate inflammasome-dependent pyroptosis, facilitating the formation of ASC specks, proinflammatory cytokine release, and the chronic inflammatory microenvironment, all of which exacerbate AD pathology (Hu et al., 2022).

Role of pyroptosis in Parkinson’s disease (PD)

PD is a prominent neurodegenerative disorder, ranking second in prevalence following AD. The number of individuals affected by PD is expected to increase as the global population ages (De Virgilio et al., 2016). The characteristic pathology of PD begins with the misfolding of α-synuclein proteins, leading to the formation of Lewy Bodies in the somata of affected neurons (Braak et al., 2004). This pathology is accompanied by the loss of dopaminergic neurons in the substantia nigra pars compacta, resulting in motor symptoms such as bradykinesia and tremors (Marmion and Kordower, 2018). Emerging evidence suggests that pyroptosis may induce PD, a proinflammatory cell death pathway. One study used 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (Salsolinol) to induce pyroptosis both in vitro and in vivo. SH-SY5Y cells, a neuroblastoma neural cell line treated with Salsolinol, showed upregulation of pyroptosis-associated proteins, including NLRP3, ASC, Caspase-1, GSDMD, as well as pro-inflammatory cytokines IL-18 and IL-1β. In vivo, mice receiving intraperitoneal administration of Salsolinol exhibited significant upregulation of pyroptosis-associated proteins in the substantia nigra pars compacta (Wang et al., 2022). These results strongly indicate an association between PD and pyroptosis. Another study induced PD using 6-hydroxydopamine injection into the corpus striatum of male rats and observed an upregulation of pyroptosis markers in addition to Parkinson’s-like symptoms (Cai et al., 2022). In a PD model utilizing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice, increased levels of cleaved GSDMD, IL-18, and IL-1β were detected in the brains of these mice. The study also found an increase in ASC, NLRP3, and cleaved Caspase-1 in the substantia nigra of the mice with PD. In PC-12 cells, the upregulation of GSDMD, IL-18, and IL-1β was accompanied by an increase in α-synuclein and a decrease in TH (Zhang et al., 2020b). Also, an investigation of substantia nigra samples from PD patients and control samples revealed increased levels of cleaved Caspase-1 and ASC in patients with PD compared to the control group (Gordon et al., 2018). In summary, there is accumulating evidence linking pyroptosis to the pathogenesis of PD. Different models of PD induction, both in vivo and in vitro, have shown upregulation of pyroptosis-related markers and proinflammatory cytokines, highlighting the potential role of pyroptosis in the development and progression of PD.

Other neurological disorders

Multiple sclerosis (MS), a chronic inflammatory demyelination disorder of the CNS, shares pathological features with experimental autoimmune encephalomyelitis (EAE), an established animal model for MS (Coll et al., 2022; Lunemann et al., 2021). Studies indicate caspase 1-mediated GSDMD pyroptosis and inflammasome activation in microglia and myelin-forming oligodendrocytes in MS patients and EAE models (McKenzie et al., 2018). VX-765 treatment in EAE animals suppressed pyroptosis-related proteins, prevented axonal injury, and improved neurobehavioral performance (McKenzie et al., 2018). Further evidence revealed pyroptotic microglia/macrophages within demyelinating lesions in progressive MS patients, with in vitro studies indicating caspase-3/7 activation and attenuated pyroptosis features post-caspase 1 inhibition (McKenzie et al., 2020). NLRP3 inhibitors, MCC950, OLT1177, and JC-171, mitigated EAE severity by improving pathology and reducing neuronal demyelination (Coll et al., 2015; Guo et al., 2017a; Hou et al., 2020a; Malhotra et al., 2020; Sanchez-Fernandez et al., 2019).

Amyotrophic lateral sclerosis (ALS), characterized by motor neuron loss, exhibited increased canonical inflammasome-triggered pyroptosis in microglia, contributing to neuronal loss (Van Schoor et al., 2022). Mutant and aggregated forms of TDP-43 triggered NLRP3 inflammasome-dependent proinflammatory cytokines secretion (IL-1β and IL-18), thereby killing the motor neurons (Van Schoor et al., 2022). Studies also revealed mSOD1-induced NLRP3 inflammasome activation, leading to chronic neuroinflammation and ALS progression (Zhang et al., 2022). Consistently, NLRP3, NLRC4, AIM2, and caspase-1 activation has also been observed in neural tissue of mutant SOD1 transgenic animals (Gugliandolo et al., 2018; Johann et al., 2015; Pasinelli et al., 1998). However, debates persist on the significance of pyroptosis in ALS, as IL-1 receptor antagonist Anakinra did not significantly impact disease progression in ALS patients (Maier et al., 2015). These studies highlight the need to explore distinct glial subtypes in future research. Huntington’s disease (HD), another neurodegenerative disorder, recently showed increased NLRP3, Caspase 8, caspase 1, IL1β, and IL18 expression in the striatum of transgenic R6/2 mouse models (Paldino et al., 2020). While the involvement of GSDM-dependent pyroptosis in HD remains unclear, further investigation is needed to elucidate its role. Currently, there is limited information available regarding the involvement of pyroptosis in these neurological disorders.

Similarities and differences between pyroptosis and other cell deaths

Numerous similarities and distinctions exist among four distinct forms of programmed cell death, each initiated by Toll-like receptors (TLR3/TLR4) and death receptors (TNFR) but characterized by markedly different downstream pathways (Bertheloot et al., 2021). Apoptosis stands out as the most divergent among these cell death mechanisms due to its immunologically inert nature, maintenance of membrane integrity, and cellular contraction, accompanied by the release of apoptotic extracellular vesicles (Bertheloot et al., 2021). In sharp contrast, ferroptosis, pyroptosis, and necroptosis exhibit pro-inflammatory features involving the release of cellular contents due to compromised cellular membranes (Yu et al., 2021). Cells undergoing these death pathways typically swell before cellular lysis while their nuclei remain intact. Notably, ferroptosis and necroptosis operate independently of caspases, setting them apart from pyroptosis and apoptosis, both of which rely on caspase activation (Yu et al., 2021). The characteristics of ferroptosis, necroptosis, pyroptosis, and apoptosis, indicating the differences and similarities among these forms of cell death, are shown in Table 1.

Table 1:

Characteristics of ferroptosis, Necroptosis, pyroptosis and Apoptosis

Characteristic feature	Ferroptosis	Necroptosis	Pyroptosis	Apoptosis
Inflammation	yes	yes	yes	no
Apoptotic bodies	no	no	no	yes
Pyroptotic bodies	no	no	Yes	no
Intact nucleus	yes	yes	Yes	no
Pore formation	no	yes	Yes	no
Cell swelling	yes	yes	Yes	no
Cell shrinks	no	no	no	yes
Osmotic lysis	no	yes	yes	no
Membrane integrity	yes	no	no	yes
7-AAD staining	yes	yes	yes	no
PI staining	yes	yes	yes	no
EtBr staining	yes	yes	yes	no
Caspase-1 activation	no	no	yes	no
Caspase-4 activation	no	no	yes	no
Caspase-5 activation	no	no	yes	no
Caspase-11 activation	no	no	yes	no
Caspase-2 activation	no	no	no	yes
Caspase-7 activation	no	no	no	yes
Caspase-10 activation	no	yes	no	yes
PARP cleavage	no	no	no	yes
Gasdermin cleavage	no	no	yes	no
Programmed cell death	yes	yes	yes	yes
PS exposure	yes	yes	yes	yes
Annexin V staining	yes	yes	yes	yes
TUNEL staining	yes	yes	yes	yes
DNA damage	yes	yes	yes	yes
Chromatin condensation	no	no	yes	yes
Membrane blebbing	yes	yes	yes	yes
Diameters of pyroptotic or apoptotic bodies (1–5 μm)	no	no	yes	yes
Caspase-3 activation	no	no	yes	yes
Caspase-6 activation	no	no	yes	yes
Caspase-8 activation	no	yes	yes	yes
Caspase-9 activation	no	no	yes	yes
Death receptors (TNFR)	yes	yes	yes	yes
Toll-like receptors (TLR4/TLR3)	yes	yes	yes	yes
RIPK1/RIPK3	no	yes	no	no
NF-kB	yes	yes	yes	yes
MLKL phosphorylation	no	yes	no	no
GPX4 activity decrease and GSH depletion	yes	no	no	no
Caspase independent	yes	no	no	no

To explore deeper into these distinctions, let us explore the characteristics of each form of programmed cell death:

Apoptosis:

Immunological Inertness:

Apoptosis is immunologically silent, avoiding the activation of inflammatory responses within the surrounding tissue.

Membrane Integrity:

In apoptosis, the cell maintains its membrane integrity until the late stages of cell death, preventing the release of cellular components.

Cellular contraction and apoptotic extracellular vesicles release:

Apoptotic cells undergo a series of coordinated changes, leading to cell shrinkage and the formation of apoptotic extracellular vesicles containing cellular materials for phagocytic clearance.

Executioner caspase:

Apart from the common caspases (caspase-3, −6, −8, and −9) shared with pyroptosis, apoptosis intrinsic and extrinsic pathways involve the essential functions of caspase-2, −7, and −10.

Ferroptosis:

Inflammatory Response:

Ferroptosis is characterized by an inflammatory response involving the release of cellular contents due to membrane damage.

Caspase Independence:

Unlike apoptosis, ferroptosis does not involve caspase activation but lipid peroxidation and iron-dependent mechanisms (Kapralov et al., 2020).

Executioner protein:

Sensitivity to ferroptosis is determined by many essential molecules, including upregulated expression of Acyl-CoA synthetase long-chain family member 4 (ACSL4), which enriches cellular membranes with long polyunsaturated ω6 fatty acids, increasing cells vulnerability to ferroptosis execution (Fernandez and Ellis, 2020). Also associated with ferroptosis are decreased cellular levels of Glutathione peroxidase 4 (GPX4) and cystine-glutamate antiporter, system Xc- (Zeng et al., 2020).

Pyroptosis:

Inflammatory Nature:

Pyroptosis is highly inflammatory, releasing pro-inflammatory cytokines and cellular contents upon membrane rupture.

Inflammasome Activation:

Pyroptosis often involves inflammasome activation, especially the NLRP3/NLRP6 inflammasome, leading to caspase-1 activation and subsequent cytokine release.

Executioner protein:

The GSDM family protein (GSDMA, GSDMB, GSDMC, GSDMD, and GSDME) are responsible for pyroptotic cell death following proteolytic cleavage by cleaved caspase −1, −4, −5, and −11.

Necroptosis:

Inflammatory Characteristics:

Similar to pyroptosis, necroptosis is pro-inflammatory and results in the release of cellular contents.

Receptor-Interacting Protein Kinase (RIPK) Activation:

Necroptosis relies on the activation of receptor-interacting protein kinases (RIPKs), particularly RIPK1 and RIPK3, ultimately leading to necrotic cell death.

Executioner protein:

Mixed Lineage Kinase Domain-like (MLKL), a pseudokinase, is the necroptosis executioner protein, following phosphorylation by the RIPK. This phosphorylation event of MLKL will cause a shift in equilibrium from a monomeric inert cytoplasmic MLKL to an oligomer membrane-associated MLKL. This indicates that a phosphorylation event is critical during necroptosis as compared to proteolytic cleavage observed in pyroptosis. Also, MLKL-mediated membrane pore size has been determined to be ~4 nm (Ros et al., 2017).

In summary, programmed cell death mechanisms, including apoptosis, ferroptosis, pyroptosis, and necroptosis, share common initiation points involving receptors but exhibit distinct characteristics in terms of immunological response, membrane integrity, cellular morphology, and the role of caspases or specific kinases. Understanding these differences is crucial for elucidating their roles in various physiological and pathological contexts.

Conclusion

In conclusion, this comprehensive review has provided a detailed overview of pyroptosis, including its defining characteristics, intricate cellular and molecular mechanisms, regulatory mediators, and potential therapeutic targets. We have also delved into its significant implications and contributions to the progression of neurodegenerative disorders. While substantial progress has been made in unraveling the functional roles and underlying mechanisms of pyroptosis in these disorders, several critical questions remain unanswered, highlighting areas for future exploration and research. One of the pressing challenges in the field is the limited occurrence of pyroptosis in certain neurodegenerative disorders, such as autism, Huntington’s disease, and epilepsy, and identifying the specific DAMPs responsible for activating pyroptosis in others. Understanding the dominant stimuli for pyroptosis in different neurodegenerative diseases is essential for tailoring effective therapeutic strategies. Moreover, pinpointing the precise stage of neurodegenerative disorders at which DAMPs and PAMPs influence is crucial for devising targeted therapeutic interventions.

The regulatory crosstalk between pyroptosis and other programmed cell death pathways, such as apoptosis, ferroptosis, and necroptosis, warrant further investigation. Although recent findings have begun to shed light on the interconnectedness of these pathways, a comprehensive understanding of their interplay remains elusive. In pursuing safe and effective therapeutics, it is noteworthy that certain FDA-approved drugs, like disulfiram, though capable of modifying critical residues in GSDMD to block pore formation, may still permit the processing of IL-1β and GSDMD. Consequently, exploring interventions that target upstream components of the pyroptotic pathway holds promise as a more effective approach. Small molecule inhibitors and natural compounds that specifically target pyroptosis regulators, such as NLRP3 inhibitors (e.g., MCC950 and JC-171) or caspase inhibitors (e.g., VX765, Ac-FLTD-CMK, Ac-YVAD-CMK), have shown encouraging results in preclinical studies, offering potential avenues for therapeutic development.

Furthermore, clinical studies have unveiled a substantial increase in inflammasome-associated proteins in peripheral blood and the CNS of patients with neurodegenerative disorders, suggesting that inflammasome-dependent pyroptosis mediators could serve as valuable biomarkers for clinical diagnosis. As we move forward, future perspectives in pyroptosis research should focus on addressing these pressing questions and exploring novel avenues of investigation. Enhanced understanding of the roles, regulation, and crosstalk of pyroptosis with other cell death pathways in the context of neurodegenerative disorders will pave the way for innovative therapeutic strategies and potential biomarkers, ultimately contributing to improved patient care and the development of more effective treatments for these debilitating conditions.

Within the complex network of cellular survival mechanisms, the endosomal sorting complex required for transport (ESCRT) system emerges as a key player, offering a promising avenue for investigating its interplay with pyroptosis in neurological disorders. Understanding the involvement of ESCRT machinery and its impact on calcium accumulation in relation to pyroptosis and membrane repair stands as a crucial area for investigation. This intricate relationship presents an intriguing avenue for research, exploring how ESCRT-mediated membrane repair intersects with pyroptosis, potentially unveiling innovative therapeutic approaches to safeguard cellular integrity and mitigate the progression of neurodegenerative diseases. Delving into these connections holds the potential to reveal vital insights into cellular survival mechanisms and novel therapeutic targets for neurological disorders.