Mitochondrial unfolded protein response (UPR^mt) as novel therapeutic targets for neurological disorders

Abstract

Neurological disorders, including brain cancer, neurodegenerative diseases and ischemic/reperfusion injury, pose a significant threat to global human health. Due to the high metabolic demands of nerve cells, mitochondrial dysfunction is a critical feature of these disorders. The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved mitochondrial response, which is critical for maintaining mitochondrial and energetic homeostasis under stress. Previous studies have found that UPR^mt participates in diverse physiological processes especially metabolism and immunity. Currently, increasing evidence suggest that targeted regulation of UPR^mt can also effectively delay the progression of neurological diseases and improve patients’ prognosis. This review provides a comprehensive overview of UPR^mt in the context of neurological diseases, with a particular emphasis on its regulatory functions. Additionally, we summarize the mechanistic insights into UPR^mt in neurological disorders as investigated in preclinical studies, as well as its potential as a therapeutic target in the clinical management of neurological tumors. By highlighting the importance of UPR^mt in the complex processes underlying neurological disorders, this review aims to bridge current knowledge gaps and inspire novel therapeutic strategies for these conditions.

Introduction

Cellular homeostasis is crucial for regulating various intracellular processes, such as protein folding, ion balance, and the removal of damaged organelles and proteins. ¹ Disruptions in homeostasis can result in protein aggregation, calcium dysregulation, and impaired autophagy, all of which are implicated in the development of neurological disorders that severely impact human health.^2,3 Neurons, with their exceptionally high metabolic demands, ⁴ depend on mitochondria to generate adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS), which is vital for their structural and functional integrity. ⁵ Because of these energy requirements, neurons are particularly susceptible to mitochondrial dysfunction, which can lead to cognitive and motor deficits characteristic of numerous neurological conditions.

Recent studies underscore the critical role of the mitochondrial unfolded protein response (UPR^mt) in preserving mitochondrial proteostasis and cellular homeostasis. Under mitochondrial stress, UPR^mt activation induces the expression of molecular chaperones and proteases that facilitate the recovery of mitochondrial function. This mechanism has garnered substantial interest, particularly in cancer research, where UPR^mt contributes to tumor progression and resistance to therapy.^6,7 Additionally, UPR^mt has been extensively investigated in neurodegenerative diseases, offering insights into its fundamental role in sustaining cellular health. However, its involvement in the context of ischemia/reperfusion (I/R) injury, ⁸ especially in heart and brain, remains underexplored and merits further attention, as it may provide innovative perspectives for understanding cerebrovascular diseases and developing therapeutic strategies.

This review highlights the evolving understanding of UPR^mt and its relevance across a range of neurological disorders, including brain cancer, neurodegenerative diseases and I/R injury. Furthermore, we discuss the role of UPR^mt in shaping the pathophysiological mechanisms underlying these conditions. Advancing our knowledge in this area could help redefine therapeutic targets and the optimal timing for interventions, as well as to identify novel targeted drugs and therapeutic strategies for neurological diseases within the context of clinical translation.

Physiological and pathophysiological mechanisms of UPR^mt

Mitochondrial stress and protein misfolding

Mitochondria are essential to cellular energy metabolism, primarily generating ATP and participating in various biosynthetic pathways such as fatty acid synthesis, heme biogenesis, and calcium homeostasis. ⁹ The proteins required for these processes are primarily synthesized in the cytosol and imported into mitochondria, where they are essential for maintaining mitochondrial integrity.^10–12 Upon entry into the mitochondria, mitochondrial chaperones, such as mitochondrial heat shock protein 70 (mtHsp70), facilitate the proper folding and assembly of these proteins into functional complexes.^13,14

The proper function of mitochondria is critical for cellular health. Disruptions in mitochondrial function can lead to the accumulation of misfolded proteins within the mitochondrial matrix, triggering UPR^mt, a sophisticated regulatory mechanism that restores mitochondrial function. This is achieved by enhancing the transcription of genes encoding mitochondrial chaperones and proteases. ¹⁵ Moreover, UPR^mt adjusts cellular metabolism toward glycolysis and enhances reactive oxygen species (ROS) scavenging, reducing cells’ dependence on mitochondria function during cellular stress. ¹⁶

Key components of UPR^mt

Central to UPR^mt are mitochondrial chaperones and proteases. Mitochondrial proteases, such as caseinolytic protease P (CLPP), work in concert with chaperones to manage and degrade misfolded proteins that accumulate under stress.^17,18 This coordination action between chaperones and proteases is crucial for maintaining proteostasis within mitochondria.

The regulation of UPR^mt is governed by several key proteins, including the transcription factor activating transcription factor associated with stress 1 (ATFS-1), which plays a crucial role in UPR^mt regulation in C.elegans. When mitochondrial protein misfolding exceeds the capacity of mitochondrial chaperones, ATFS-1 is prevented from degradation in the mitochondria and instead translocates to the nucleus.^18–20 In the nucleus, ATFS-1 promotes the expression of genes encoding mitochondrial chaperones and proteases, facilitating an adaptive response that is crucial for cellular resilience and recovery from mitochondrial stress.^19,21,22

In mammalian systems, UPR^mt regulation is more complex, involving three transcription factors: activating transcription factor 5 (ATF5), activating transcription factor 4 (ATF4), and C/EBP homologous protein (CHOP), all members of the basic leucine zipper (bZip) family.^23–25

Crosstalk between UPR^mt and UPR^ER

While ATF5 is mitochondria-specific, both CHOP and ATF4, are also involved in the regulating the endoplasmic reticulum (ER) unfolded protein response (UPR^ER), highlighting the complex interaction between mitochondrial and ER stress responses.^7,26 Members of the bZIP family of transcription factors play crucial roles in the integrated stress response (ISR) under stress conditions. ⁹ The expression of these three bZIP proteins in mammalian cells requires activation of the ISR, which is primarily mediated by the phosphorylation of the eukaryotic initiation factor-2 alpha subunit (p-eIF2α). ²⁷ The ISR-induced reduction in tRNA ternary complex formation suppresses global protein synthesis, alleviating the burden on mitochondrial protein folding. ²⁸ However, it selectively triggers the translation of specific mRNAs with upstream open reading frames, such as ATF5, ATF4, and CHOP.^29,30 The phosphorylation of eIF2α subsequently enhances the translation of ATF4. It has been reported that ATF5 expression is decreased in Chop-/- mouse embryonic fibroblasts under both basal and mitochondrial stress conditions, while ATF4 expression is upregulated in CHOP-deficient cells under stress. ²⁵ These findings suggest that ATF4 may function upstream of CHOP, while ATF5 seems to be a downstream target of CHOP. Both ATF4 and CHOP are involved in activating UPR^ER, and although both transcription factors play roles in UPR^mt, the activation of UPR^ER typically exceeds that of UPR^mt.

A recent study also highlights the role of VMP1 in regulating ER-mitochondria interactions. Deletion of VMP1 disrupts the UPR balance by selectively activating PERK while suppressing IRE1α and ATF6, underscoring its key role in coordinating stress responses. This modulation ensures that the cell can manage ER stress without triggering harmful UPR overactivation. ³¹

This interdependence between UPR^mt and UPR^ER not only ensures cellular survival under stress but also emphasizes the importance of maintaining the balance between these two systems.^32,33 When this balance is disrupted, it may lead to pathological conditions such as neurodegenerative diseases, where both ER and mitochondrial dysfunction contribute to cellular damage and disease progression. ³⁴ Therefore, understanding the molecular mechanisms through which UPR^mt and UPR^ER communicate and regulate each other is essential for developing therapeutic strategies to address diseases caused by cellular stress.

UPR^mt in cellular survival and stress response

The UPR^mt plays a critical role in cellular survival under stress conditions, particularly when mitochondria are overwhelmed by unfolded proteins. ⁷ (Figure 1) Under such stress, UPR^mt is activated to restore degrade misfolded proteins. This adaptive response is essential for maintaining mitochondrial health and cellular survival, as it helps mitigate mitochondrial dysfunction and protein aggregation, which could otherwise lead to cellular damage. ³⁵ However, if UPR^mt activation becomes unregulated or prolonged, it can overwhelm cellular homeostasis, leading to mitochondrial dysfunction and cell death. For instance, during sustained hypoxia, while UPR^mt activation helps reduce hypoxia-induced mitochondrial protein aggregation (HIMPA), it is insufficient to completely protect the cell from death. ³⁶ This suggests that excessive UPR^mt activity can cause an imbalance in mitochondrial dynamics, possible leading to mitochondrial fragmentation or impaired bioenergetic function. Cell survival is ultimately enhanced when UPR^mt works in concert with other mitochondrial quality control mechanisms, such as mitophagy and mitochondrial fusion, which help to clear damaged mitochondria and maintain mitochondrial integrity. This underscores the importance of understanding how UPR^mt is coordinated with other stress responses to regulate cellular fate and prevent pathological outcomes.

Figure 1.

Summary of the molecular mechanisms of UPRmt. ATF5 can translocate to the mitochondria under conditions of mitochondrial stress. Once stress is detected, ATF5 moves to the nucleus where it promotes the transcription of mitochondrial chaperones, such as HSP10 and HSP60, as well as mitochondrial proteases. Both ATF4 and CHOP act upstream of ATF5, enhancing the expression of UPRmt-related genes and contributing to the activation of UPRER. In response to mitochondrial stress, the transcription of HSP10 and HSP60 is upregulated. Additionally, the activation of SIRT3 induces the deacetylation of FOXO3α, facilitating its relocalization to the nucleus and boosting the cell’s antioxidant capacity.

Emerging technologies in UPR^mt research

Recent advances in omics technologies, including transcriptomics, proteomics, metabolomics, and single-cell approaches, have substantially expanded our understanding of UPR^mt. These high-throughput methods provide comprehensive and quantitative insights into how UPR^mt is regulated across different cellular contexts and stress conditions.

Transcriptomic and proteomic analyses have enabled the identification of UPR^mt-associated gene and protein expression profiles under physiological and pathological stress.^24,37–39 These approaches have revealed that the activation of UPR^mt involves coordinated changes in mitochondrial chaperones, proteases, and metabolic regulators. In particular, single-cell RNA sequencing (scRNA-seq), as an extension of transcriptomic analysis, allows for the resolution of cell-type-specific UPR^mt responses, uncovering heterogeneous activation patterns that would be masked in bulk analyses. For example, scRNA-seq has been widely applied in cancer research to characterize UPR^mt-related transcriptional alterations within the tumor microenvironment,^40,41 demonstrating its utility in dissecting complex tissue heterogeneity.

In addition, by analyzing the transcriptional landscape of individual cells, scRNA-seq enables the identification of distinct UPR^mt activation patterns, helping to determine how different cellular populations respond to mitochondrial stress. Furthermore, time-course transcriptomic studies suggest that UPR^mt activation may transition from an adaptive to a maladaptive response, underscoring the importance of precise temporal control when considering UPR^mt-targeted interventions. ⁴² Notably, single-cell techniques can also be integrated with spatial transcriptomics to spatially map regions of high UPR^mt activity within tissue sections. This approach is particularly valuable for investigating UPR^mt responses in pathologically affected regions of the nervous system, where it may help elucidate the influence of local microenvironmental factors on UPR^mt regulation.^43,44

In parallel, advanced imaging techniques such as super-resolution microscopy and live-cell imaging have provided spatiotemporal insights into UPR^mt dynamics. These tools have been used to visualize the localization of key UPR^mt components and track mitochondrial remodeling in real time under stress conditions. For instance, the redistribution of mitochondrial chaperones and the morphological changes of mitochondria can be directly observed in response to UPR^mt activation.^45,46

Collectively, advancements in omics and imaging technologies have significantly enhanced our understanding of UPR^mt regulation and function. These technologies provide high-resolution insights into UPR^mt activation across different cell types and stress conditions, offering valuable tools for elucidating its physiological and pathological roles and accelerating the development of targeted therapeutic strategies.

UPR^mt in aging

Aging is a crucial physiological process in the lifecycle, influencing various cellular mechanisms, including mitochondrial function. UPR^mt was initially identified as a pivotal mechanism in aging in C. elegans. The knockdown of various UPR^mt components, including ubiquitin-like protein 5 (UBL-5), defective proVentriE 1 (DVE-1), HSP6, HSP60, and CLPP-1, has been shown to suppress lifespan extension. ⁴⁷ Furthermore, knockout of cytochrome c oxidase subunit 1(cco-1), a key component of respiratory chain complex IV, activates the UPR^mt and consequently extends lifespan. ⁴⁷ In a similar vein, depletion of the mitochondrial complex IV assembly protein Surf1 results in UPR^mt activation, enhancing resistance to oxidative stress-induced cell death. ⁴⁸ These findings demonstrate that silencing genes that regulate UPR^mt can reduce lifespan, while targeted knockout of electron transport chain (ETC) components can promote lifespan extension through UPR^mt activation.

Several epigenetic regulators have been implicated in sustaining UPR^mt activation and promoting lifespan extension across species, from worms to humans. ⁴⁹ A recent study revealed that the heat shock factor binding protein 1 (HSB-1)/HSF1 pathway enhances longevity by upregulating histone H4 expression. This increase in histone H4 protein levels results in the compaction of both nuclear and mitochondrial chromatin, subsequently reducing the transcription of mitochondrial genes and activating the UPR^mt, thereby promoting lifespan extension.^50,51 The sirtuin family is closely linked with UPR^mt and longevity. Specifically, silent information regulator 2.1 (Sir-2.1), a homolog of SIRT1 in C. elegans, plays a pivotal role in this process. Deletion of Sir-2.1 inhibits the induction of HSP-6, thereby suppressing UPR^mt and impairing lifespan extension.^52,53 Moreover, supplementation with nicotinamide adenine dinucleotide (NAD+) activates Sir-2.1-mediated UPR^mt, delaying the aging process. ⁵² SIRT1 directly deacetylates mitochondrial ribosomal protein S5 (MRPS5), thereby initiating UPR^mt. The knockout of MRPS5 enhances the expression of ATF4 and ATF5, triggering UPR^mt activation and promoting extending lifespan. ⁵⁴ Additionally, SIRT3 has been shown to regulate antioxidant activity and extend longevity through UPR^mt activation.^55,56

In mammals, distinct regulatory mechanisms of UPR^mt contribute to longevity. Increasing evidence underscores the importance of UPR^mt in activating stem cells, thereby offering protection against senescence. Supplementation with nicotinamide riboside in mouse chow has been shown to induce UPR^mt, which helps prevent senescence in muscle stem cells (MuSC), neural stem cells (NSC), and melanocyte stem cells (McSC). ⁵⁷ In hematopoietic stem cells (HSCs), SIRT7 has been identified as a key regulator of UPR^mt that is essential for maintaining HSC function during aging. ⁵⁸ Moreover, UPR^mt is elevated in NSCs with aging, where SIRT7 plays a protective role by mitigating mitochondrial protein folding stress. Overexpression of SIRT7 has been shown to enhance neurogenesis and improves cognitive function in aged mice. ⁵⁹ However, constitutive activation of UPR^mt, particularly with tissue-specific loss of the mitochondrial chaperone protein HSP60, can have detrimental effects, leading to the loss of stemness in intestinal stem cells (ISCs). ⁶⁰ Further research is needed to clarify how the relative level of UPR^mt signaling influences stem cells function and the extent to which these effects are context-dependent.

The regulatory role of UPR^mt in tissues and organs is crucial for modulation aging. It has been reported that UPR^mt activity is increased in the hearts of aged mice. ⁶¹ In heart tissues from 13 mammalian and avian species, the expression of HSP60 has been found to correlate positively with maximum lifespan. However, research exploring the role of UPR^mt in cardiac aging remains limited. Further studies are needed to determine whether UPRmt is a fundamental mechanism for promoting longevity in both basic research and clinical settings, especially with regard to cardiac aging. Moreover, a study examining serum transcriptomics after exercise in young and older adults highlighted an age-related decline in the adaptive transcriptional response of UPR following a single bout of exercise. This decline may contribute to reduced exercise responsiveness with advancing age. ⁶² Given the critical role of UPR^mt in the aging, particularly in motor function, further investigation is necessary to elucidate its effects and underlying mechanisms.

UPR^mt in cancer

As aging contributes to mitochondrial dysfunction, this process is also implicated in the altered cellular homeostasis observed in cancer, where UPR^mt plays a crucial role in maintaining mitochondrial integrity and promoting tumor progression. The initiation and progression of cancer are complex processes during which cancer cells activate UPR^mt to maintain proteostasis and regulate metabolic reactions. ⁶³ Additionally, UPR^mt activation plays a critical role in tumor metastasis. Numerous studies have identified individual components of UPR^mt as promoters of tumor growth. However, recent research highlights the broader role of UPR^mt in alleviating mitochondrial stress and promoting tumor proliferation. ⁶⁴ Given its tumor-supportive function, targeting specific UPR^mt proteins to inhibit tumor growth has emerged as a novel and intensely researched therapeutic strategy. Both upstream signaling molecules and downstream effector proteins of UPR^mt have shown efficacy in experimental and clinical studies. Encouragingly, several inhibitors targeting these proteins have advanced to clinical trials, demonstrating promising preclinical results. ⁶⁵ (Table 1).

Table 1.

Key cancer therapeutics that target the mitochondrial unfolded protein response (UPR^mt).

Target Protein	Drug/compound	Clinical/research stage	Refs
ATF5	Dominant-Negative ATF5 (DN-ATF5)	Preclinical	71
HSP60/HSP10 Complex	Mizoribine	Phase I	150
	Epolactaene	Preclinical	81
	gold (III) porphyrin complexes	Preclinical	82
	5-sulfonamido-2-phenylbenzoxazole	Preclinical	83
	salicylanilide hybrid analogues	Preclinical	83
mtHSP70	MKT 077	Phase I	84
	JG-231	Preclinical	85
LONP1	CDDO-Me	Phase I	91
CLPP	ONC201	Phase I/II	NCT02250781

An upstream signaling molecules in the UPR^mt - ATF5

ATF5, a member of the bZip family of transcription factors, ⁶⁶ plays a crucial role in upregulating the transcription of UPR^mt components during mitochondrial stress. ²³ ATF5 facilitates tumor growth and progression by maintaining mitochondrial proteostasis, resulting in unfavorable outcomes for cancer patients. ATF5 expression is notably elevated in patients with glioblastoma and its high expression levels are associated with poorer survival rates in glioma cancer patients.^67,68 Additionally, ATF5 modulates the expression of various anti-apoptotic genes in cancer cells. For example, it regulates the expression of early growth response 1 (Egr-1) in glioma cells, promoting cell proliferation. ⁶⁹ ATF5 also transactivates B-cell lymphoma 2 (BCL-2) in glioma cells, enhancing their survival. ⁷⁰ The detrimental effects of ATF5 have been corroborated by an increasing body of experimental evidence. Consequently, researchers are focusing on modulating its expression as a strategy to impede tumor progression.

Researchers have engineered a modified version of the ATF5 protein, in which the DNA-binding domain was altered to abrogate its transcriptional activity. A dominant-negative ATF5 mutant (DN-ATF5), which lacks of DNA-binding capability, has been developed. This mutant effectively blocks certain signaling pathways within cells and has been synthesized to selectively induce apoptosis in a broad spectrum of treatment-resistant tumors in murine models. ⁷¹ The DN-ATF5 peptides had a pronounced and tumor-selective efficacy, promoting them as promising candidates for clinical trials. Furthermore, other upstream signaling molecules, such as ERα,^72,73 single-stranded DNA binding protein 1 (SSBP1) ⁷⁴ and HSF1 ⁷⁵ , have also been identified as potential cancer therapy targets. Further preclinical trials are required to access their targeting potential and therapeutic effectiveness.

Downstream effector proteins in the UPR^mt -Heat shock protein (HSP)

UPR^mt-specific chaperones, particularly the HSP60/HSP10 complex, are among the most extensively studied targets in cancer therapy. Knockdown of HSP60 inhibits cell proliferation in glioblastoma and suppresses tumor growth.^76,77 Similarly, HSP10 is often overexpressed in astrocytoma and is correlated with higher recurrence rates in these patients. Elevated HSP10 levels are also associated with reduced overall survival in astrocytoma cases. Like HSP60, mtHSP70 performs essential housekeeping functions and mediates the refolding of misfolded proteins. Knockdown of mtHSP70 has been shown to inhibit proliferation, migration, and invasion in various cancer cell lines.^78–80

It has been established that numerous inhibitors interact effectively with the HSP60/HSP10 complex. Notably, compounds derived from Eupenicillium brefeldianum have demonstrated the ability to bind to this chaperonin complex. The safety profiles of these drugs have been confirmed through randomized, double-blind, placebo-controlled Phase I clinical trials. However, mizoribine, an active compound extracted from Eupenicillium brefeldianum, exhibited limited bioavailability of approximately 30 μM in humans, which is insufficient to inhibit the HSP60/HSP10 complex at this concentration. Furthermore, additional molecules, such as Epolactaene, gold (III) porphyrin complexes, 5-sulfonamido-2-phenylbenzoxazole, and salicylanilide hybrid analogues targeting HSP60, have been identified or synthesized.^81–83 However, the specificity and efficacy of these compounds require further investigation before clinical advancement.

MKT 077 (3-Methylindolium-2-ylmethylene-3-methyl-1-propylindolium-2,3-dithioate chloride), a rhodacyanine dye, has been shown to reduce the chaperone activity of mtHSP70. A phase I clinical trial demonstrated that MKT 077 can inhibit mitochondrial functions. ⁸⁴ Structure-based design has led to the development of novel rhodacyanine analogues, such as JG-231, to enhance selectivity and bioavailability. However, challenges such as light sensitivity and difficulties in synthesis and solubilization must be addressed before these compounds can proceed to clinical trials. ⁸⁵

Targeting UPR^mt-specific proteases in cancer

Lon peptidase 1 (LONP1)

Mitochondrial proteins undergoing folding are susceptible to aggregation. LONP1 plays a pivotal role in cleaving these aggregates into short peptides for clearance, thereby maintaining mitochondrial proteostasis. ⁸⁶ Elevated LONP1 expression has been observed in various tumors, including melanoma, prostate cancer, pancreatic cancer, and colorectal cancer.^87–89 Moreover, increased LONP1 levels correlate with reduced overall survival in neuroblastoma patients. LONP1 regulates the expression of Bcl-2, cyclin D1, and β-catenin in several tumor types.^87,89,90 Knockdown of LONP1 results in decreased expression of MMP2, MMP9, and phosphorylated JNK, ⁸⁹ suggesting that LONP1 may be a potential target for inhibiting migratory and invasive capabilities of cancer cells.

CDDO-Me, a triterpenoid compound derived from both natural and synthetic methods, inhibits LONP1-mediated proteolysis in cancer cells, inducing apoptosis. ⁹¹ A nonrandomized Phase I clinical trial of CDDO-Me (NCT00508807) was conducted to evaluate dose-limiting toxicities and establish the maximum tolerated dose, providing important guidance for Phase II studies. Furthermore, CDDO-Me impedes multiple tumor-supportive pathways, underscoring its therapeutic potential. Future research should continue to explore this compound, as well as other specific inhibitors.

Clpp

Like LONP1, CLPP accepts protein substrates for degradation. ⁹² CLPP interacts with several respiratory chain proteins and metabolic enzymes in mitochondria, playing a essential role in metabolic regulation within cancer cells. ⁹³

The reduction in CLPP is associated with decreases levels of cyclin A, B1, D1, and MMP7, and inhibition of PI3K and AKT activation, collectively leading to reduced proliferation, migration, and invasion of various cancer cells.^93–95 Interestingly, elevated CLPP levels in cervical cancer cells promote cisplatin resistance by inhibiting cisplatin accumulation, reducing the chemotherapy efficacy. ⁹⁶ Thus, the role of UPR^mt in chemotherapy resistance warrants further investigation to improve patient outcomes.

The imipridone ONC201 binds to and hyperactivates CLPP, inducing excessive proteolysis, mitochondrial dysfunction, and cell death across multiple cancer cell lines. ⁹⁷ A Phase I dose-escalation study has demonstrated a favorable safety profile for ONC201 at the recommended Phase II dose of 625 mg per week. However, initially designed to inactivate AKT/ERK signaling and inhibit the dopamine receptor D2, the potential of ONC201 to target CLPP warrants further investigation. ⁹³

UPR^mt in neurological disorders

Neurodegenerative diseases

Mitochondrial dysfunction is a well-known pathological mechanism underlying age-associated neurodegenerative diseases. A growing body of evidence suggests that the accumulation of unfolded, misfolded, or aberrant proteins is a hallmark of neurodegenerative disorders.^98,99

Alzheimer’s disease (AD)

Alzheimer’s disease (AD) is the most prevalent neurodegenerative disorder worldwide, characterized by the intracellular accumulation of phosphorylated Tau (p-Tau) in neurofibrillary tangles and extracellular aggregation of amyloid-β (Aβ) in senile plaques.^100,101 Emerging evidence suggests that in the early stages of AD pathogenesis, UPR^ER plays a protective role by limiting the formation of neurofibrillary tangles and shielding cells from tau-mediated neurotoxicity.^102–104 However, as AD progresses, the continued accumulation of Aβ or p-Tau leads to sustained endoplasmic reticulum stress (ERS), which contributes to synaptic dysfunction and neurodegeneration. ¹⁰⁵ Notably, there is growing evidence that upregulation of X-box binding protein 1 (XBP1s) can mitigate Aβ neurotoxicity and promote clearance of pathological tau in C. elegans via the activation of UPR^ER. Moreover, forced expression of XBP1s in the hippocampus of transgenic AD mice has been shown to reduce Aβ burden, enhance synaptic plasticity, and improve cognitive performance. ¹⁰⁶ Consistently, as revealed by proteomic profiling, hippocampal overexpression of XBP1 restored the expression of multiple synaptic proteins, as well as key regulators of actin cytoskeleton dynamics and axonal growth. ¹⁰⁷ Together, these findings underscore the critical role of ER stress dysregulation and aberrant activation of UPR sensors and downstream effectors in the pathological progression of AD.

In parallel with studies of UPR^ER, UPR^mt has emerged as a new focal point in AD research. In human studies, Beck et al. reported the activation of UPR^mt in frontal cortex samples from both sporadic and familial AD cases, with significantly elevated levels of UPR^mt-related genes such as HSP60, HSP10, CLPP, and YME1L1.^108,109 Experimental models have further corroborated these findings. Sorrentino et al. demonstrated that Aβ-induced UPR^mt activation in both C. elegans and murine models, helped reduce harmful protein accumulation and deposition by restoring mitochondrial function, thereby mitigating Aβ toxicity and delaying disease progression. ¹¹⁰ Furthermore, robust activation of UPR^mt was shown to protect against Aβ-induced toxicity in mouse brains, human SHSY5Y cells, and neurons exhibiting AD-like pathological features, including Aβ aggregation. In contrast, pharmacological inhibition of UPR^mt exacerbated Aβ toxicity, highlighting its protective role.^111,112

The therapeutic potential of targeting UPR^mt in AD has gained further support with the development of specific drugs. For instance, Honokiol (HKL, C18H18O2), a natural derived from the bark of Magnolia officinalis, has been shown to activate mitophagy, induce UPR^mt, reduce oxidative stress, and restore mitochondrial dynamics in the hippocampus. ¹¹³ These findings suggest that activating UPR^mt can mitigate symptoms during the pathological development of AD, and that enhancing UPR^mt activation may present a promising therapeutic strategy for AD treatment.

Parkinson’s disease (PD)

Parkinson’s disease (PD) is characterized by the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta of the brain. The proteins Parkin and PTEN-induced putative kinase 1 (PINK1) play critical roles in this process by rescuing the phenotypes effects associated with the loss of mortalin. They achieve this by enhancing lysosomal-mediated mitochondrial clearance, which requires intact autophagic machinery.^114–117

Recent studies have revealed that UPR is also integral to the progression of PD. ScRNA-seq has revealed activation of the UPR^ER pathway in the midbrain tissue of individuals with idiopathic PD, suggesting an ongoing ER stress response in affected neuronal populations. ¹¹⁸ In parallel, studies using C. elegans models of PD have shown that loss-of-function mutations in PINK1, Parkin, and parkinson disease protein 7 (PARK7) can induce UPR^mt, which appears to confer neuroprotection by attenuating dopaminergic neuronal loss. ¹¹⁹

Various interventions targeting UPR^mt have been developed across cell lines and animal models to delay PD progression. Studies using 1-methyl-4-phenylpyridinium (MPP+)-treated SHSY5Y cells further elucidated the relationship between UPR^mt and mitochondrial dysfunction, with UPR^mt activation evidenced by increased expression of mitochondrial chaperones, proteases, and transcription mediators. These findings underscore the therapeutic potential of activating UPR^mt to mitigate mitochondrial dysfunction, a hallmark of PD pathogenesis. ¹²⁰

In Drosophila melanogaster models, ginseng protein has been shown to delay the onset of Parkinson-like phenotypes in PINK1-deficient flies, as evidenced by prolonged lifespan and improved motor function. ¹²¹ Additionally, S-(+)-linalool, a major component in essential oils, has demonstrated significant neuroprotective effects against 6-hydroxydopamine (6-OHDA)-induced damage in dopaminergic neurons and has been shown to enhance UPR^mt in C. elegans, positioning it as a promising candidate for PD therapy. ¹²²

These emerging therapeutic strategies emphasize the complexity of PD and highlight the need for further validation in mammalian models to confirm their efficacy and the translation of these findings into clinical practice.

Huntington’s disease (HD)

Huntington’s disease (HD) is a fatal, inherited neurodegenerative disorder characterized by progressive motor, cognitive, and psychiatric decline over a span of 15–20 years following symptom onset. ¹²³ The disorder is caused by an abnormal expansion of Cytosine-Adenine-Guanine (CAG) repeats within the huntingtin gene, resulting in the production of a mutant huntingtin protein (mtHtt) with an extended polyglutamine (polyQ) tract in its N-terminus.^124,125 In HD, chronic activation of UPR^mt due to the accumulation of mut-Htt causes mitochondrial dysfunction, elevated oxidative stress, and the buildup of damaged mitochondria. This is accompanied by reduced levels of critical chaperones like Grp75 and Hsp60, which further contribute to neuronal damage and cell death. ¹²⁶ The ongoing stress overwhelms UPR^mt’s protective capacity, leading to mitochondrial failure and aggravating neurodegeneration.

Mutant huntingtin impairs UPR^mt by destabilizing ATP binding cassette subfamily B member 10(ABCB10) mRNA, which highlights a promising therapeutic target for reducing neuronal loss in the striatum and improving motor performance in HD. ¹²⁷ To better understand the pathophysiological role of UPR^mt in HD, Almeida et al. developed a pheochromocytoma 12 (PC12) HD cell model that key features of the disease, including activation of the integrated stress response and disruptions in UPR^mt and heat shock response pathways. ¹²⁶ This N-terminal HD model is instrumental for elucidating the roles of stress response pathways in HD pathogenesis and for conducting preliminary assessments of therapeutic interventions targeting these pathways. By providing a robust experimental platform, this model significantly expands the prospects for developing novel UPR^mt-targeted therapies in HD.

Other neurodegenerative diseases

TAR DNA-binding protein 43(TDP-43) proteinopathy is defined by the accumulation of TDP-43-immunoreactive inclusion bodies in affected tissues. Clinically, it presents across a spectrum of neurodegenerative diseases, including dementia (notably frontotemporal lobar degeneration, FTLD) and motor neuron disease (MND).^128–131 Elevated experssion of TDP-43 has been shown to induce mitochondrial dysfunction, characterized by a reduction in mitochondrial membrane potential and an increase in ROS production. Moreover, TDP-43 has been demonstrated to activate UPR^mt in both cellular and animal models. Downregulation of the mitochondrial protease LONP1 exacerbates TDP-43-induced mitochondrial dysfunction by promoting the accumulation of mitochondrial TDP-43, thereby accelerating neurodegeneration. ¹³² These findings highlight UPR^mt as a promising therapeutic target for the treatment of TDP-43 proteinopathies.

Mutations in the mitochondrial protease pitrilysin metallopeptidase 1 (PITRM1), which is essential for processing and degrading of mitochondrial precursor proteins, have also been identified in neurodegenerative diseases. PITRM1 deficiency robustly induces UPR^mt activation and enhances mitochondrial clearance in neurons derived from induced pluripotent stem cells(iPSCs). Furthermore, PITRM1-knockout neurons exhibit elevated levels of amyloid precursor protein and Aβ. This PITRM1-related syndrome, characterized by impaired mitochondrial presequence processing, triggers early UPR^mt activation, which subsequently impacts cytosolic quality control pathways. Clinically, this condition manifests as a slowly progressive syndrome with features such as cerebellar ataxia, psychotic episodes, obsessive behavior, and cognitive decline. ¹¹² These findings establish a mechanistic link between mitochondrial function and neurodegenerative proteinopathies, reaffirming UPR^mt as a promising therapeutic target for these debilitating disorders.

Ischemia/reperfusion injury

I/R injury occurs when the blood supply to previously ischemic tissues is restored, often resulting in more extensive functional injury. The pathological mechanisms underlying I/R injury are multifactorial and complex. However, mitochondrial dysfunction serves as a central contributor to the progression of I/R injury.^133–135 Upon reperfusion, mitochondrial respiratory chain dysfunction leads to ATP depletion, ROS accumulation, and oxidative damage to mitochondrial DNA and proteins, ultimately promoting cellular apoptosis and necrosis.^134,136 Emerging data indicate that the degree of mitochondrial disruption during I/R is a critical determinant of the severity of tissue or organ injury.^137–139

Given that UPR^mt has been shown to reduce ROS production and alleviates oxidative stress, there has been growing interest in its potential role of UPR^mt in I/R injury. For instance, Wang et al. demonstrated that pharmacologic activation of UPR^mt using oligomycin or doxycycline could protect the heart from I/R injury in an ATF5-dependent manner. ⁸ In this study, oligomycin and doxycycline were administered 6 h prior to ex-vivo heart I/R injury, leading to obviously upregulated of several known UPR^mt-related genes and a reduction in infarct sizes. However, silencing ATF5 abolished the protective effects of these compounds, underscoring the importance of ATF5 in UPR^mt-mediated protection. Ji et al. also reported that UPR^mt is slightly activated in murine hearts subjected to I/R injury, and that enhancing UPR^mt activation could alleviate heart I/R injury, whereas inhibition of UPR^mt exacerbated the damage. ¹⁴⁰

In a related study, our team recently found that inducing ATF5-mediated UPR^mt could ameliorate mitochondrial dysfunction in neurons following oxygen-glucose deprivation or cerebral ischemia, providing further evidence for the neuroprotective role of UPR^mt in ischemic stroke. ¹⁴¹ Additionally, our studies demonstrated that the Food and Drug Administration (FDA)-approved drug meclizine can activate UPR^mt, suggesting that this drug may serve as a novel translational neuroprotective strategy for mitigating mitochondrial dysfunction in stroke. This approach holds promise for harnessing UPR^mt to develop effective therapies aimed at addressing the complex pathology of I/R injury.

Furthermore, research into UPR^mt modulation in cardiac disease, has extended the cerebral I/R injury. In cardiac tissue, Sirtuin-3(Sirt3) has been shown to activate the mRNA expression of UPR^mt-related genes via the AMPK pathway, thereby improving metabolic remodeling and reducing cardiac hypertrophy. ¹⁴² Building on this work, recent studies have demonstrated that overexpression of Sirt3 protects neuronal mitochondria from post-ischemic dysfunction by activating UPR^mt through the forkhead box O3 (Foxo3)/sphingosine kinase 1 (Sphk1) pathway. ¹⁴³ Inhibition of either UPR^mt or the Foxo3/Sphk1 pathway diminishes the beneficial effects of Sirt3 on neural function and mitochondrial integrity. Conversely, overexpression of Sphk1 alone has been shown to reduce infarction size, attenuate neuroinflammation, preserve neuronal viability, and prevent mitochondrial dysfunction during post-ischemic brain injury. Therefore, UPR^mt plays a critical role in protecting neural viability and maintaining mitochondrial homeostasis, and the Sirt3/Foxo3/Sphk1 pathway emerges as a promising therapeutic target for ischemic stroke. (Figure 2)

Figure 2.

I/R injury causes mitochondrial dysfunction, leading to ATP depletion, ROS accumulation, and oxidative damage, which ultimately contributes to cell apoptosis and necrosis. UPRmt, by reducing ROS production and alleviating oxidative stress, plays a protective role in I/R injury. Pharmacologic activation of UPRmt using agents like oligomycin and doxycycline has shown promise in mitigating heart I/R injury in an ATF5-dependent manner. Additionally, UPRmt activation in cerebral ischemia and stroke models has demonstrated neuroprotective effects by ameliorating mitochondrial dysfunction. The Sirt3/Foxo3/Sphk1 pathway, which modulates UPRmt, emerges as a potential therapeutic target for ischemic stroke, emphasizing the importance of UPRmt in protecting mitochondria and neural viability during I/R injury.

Perspective

UPR^mt plays a critical role in maintaining mitochondrial proteostasis and overall cellular function. While significant strides have been made in understanding the physiological and pathological mechanisms underlying UPR^mt, many questions remain unanswered, and there are numerous opportunities for further investigation. Future research should focus on several key areas to expand our understanding of UPR^mt’s complexity and therapeutic potential.

Expanding the understanding of UPR^mt in mammalian systems

Current knowledge of UPR^mt activation and regulation is primarily derived from model organisms such as C. elegans, and while valuable, this research does not fully capture the complexity seen in mammalian systems. Recent work has suggested that mammalian systems involve multiple transcription factors and signaling networks, which probably involve multiple transcription factors and signaling networks, coordinating UPR^mt activation under both physiological and stress-induced conditions. As such, future studies should explore the tissue-specific regulation of UPR^mt in response to various forms of cellular stress, including oxidative stress, nutrient deprivation, and protein misfolding. Detailed understanding of how different tissues and cell types regulate UPR^mt could provide insights into how to fine-tune UPR^mt responses to treat diseases associated with mitochondrial dysfunction.

Interplay between UPR^mt and other cellular stress pathways

Moreover, UPR^mt does not operate in isolation but likely interacts with other cellular stress responses, such as UPR^ER. Recent research has highlighted interactions between UPR^mt and UPR^ER in cancer cells, which influence both cell survival and apoptosis. ²⁶ Investigating these interactions will enhance our understanding of how cells manage multiple stress pathways to maintain homeostasis, particularly in the context of chronic diseases that are often multifactorial. Understanding these complex cross-talks may offer new insights into disease mechanisms and provide opportunities for therapeutic intervention.

Therapeutic potential and strategies for targeting UPR^mt

The therapeutic potential of modulating UPR^mt is an exciting area of investigation, particularly for diseases associated with mitochondrial dysfunction, such as neurodegenerative disorders. ³⁴

Recent studies have highlighted promising pharmacological agents, such as meclizine, which aim to enhance or restore UPR^mt activity to maintain mitochondrial health. Several strategies can be considered for targeting UPR^mt, including the overexpression of mitochondrial chaperones, the use of antioxidants, and precision modulation of UPR^mt. ¹⁴¹

Enhancing UPR^mt by overexpressing mitochondrial chaperones, such as mtHSP70 and HSP60, is one potential approach. This can improve mitochondrial protein-folding capacity, restore mitochondrial homeostasis, and protect cells from stress. However, careful regulation is necessary, as overexpression could result in unwanted cellular stress or an imbalance in mitochondrial function.

UPR^mt is activated under oxidative stress and plays a role in mitigating oxidative damage by promoting ROS scavenging to protect mitochondrial function. However, its antioxidant capacity is limited, and thus, combining UPR^mt modulation with antioxidants may enhance its ability to maintain cellular homeostasis. This synergistic approach could strengthen the overall stress response, improving the balance between mitochondrial function and oxidative stress. Such a combination may offer more effective neuroprotection, particularly in the context of neurodegenerative diseases, by supporting both mitochondrial function and oxidative defense. Additionally, the combined use of antioxidants and UPR^mt overexpression may achieve better therapeutic efficacy, providing a more comprehensive strategy for maintaining mitochondrial health and cellular function.

Given UPR^mt’s dual roles in cellular survival and death, future therapies should focus on selectively modulating its activation based on the disease context. The therapeutic potential of targeting UPR^mt is particularly promising for diseases associated with mitochondrial dysfunction, such as neurodegenerative disorders like AD and PD. In these diseases, UPR^mt activation is often constitutive, driven by ongoing mitochondrial dysfunction, in contrast to its transient activation during acute stress. While in acute conditions, UPR^mt plays a protective role by aiding mitochondrial protein folding through the activation of chaperones and proteases, its chronic activation in neurodegenerative diseases may exacerbate cellular dysfunction, contributing to disease progression. Therefore, therapeutic strategies should aim to precisely modulate UPR^mt activation—enhancing it during acute stress while suppressing excessive activation in chronic conditions to protect cells from further damage.

Biomarkers for UPR^mt activity and disease monitoring

Additionally, reliable biomarkers that reflect UPR^mt activity are essential for advancing both research and clinical applications. Recent efforts have focused on identifying UPR^mt-related biomarkers using multi-omics approaches, which may be useful for early diagnosis and monitoring of disease progression.^144–146 The identification of biomarkers specific to UPR^mt is essential for the advancement of precision medicine and the development of personalized therapeutic strategies.

Leveraging emerging technologies to advance UPR^mt research

To further accelerate progress in the UPR^mt field, integrating advanced omics and imaging technologies will be essential. Multi-omics platforms—including transcriptomics, proteomics, and metabolomics—offer comprehensive insights into UPR^mt regulatory networks under diverse cellular conditions. In particular, single-cell and spatial transcriptomic approaches enable the characterization of cell-type-specific and region-specific UPR^mt activity, which is especially valuable in complex tissues such as the brain. ¹⁴⁷ Moreover, advanced live-cell and super-resolution imaging techniques allow for real-time monitoring of UPR^mt dynamics and mitochondrial remodeling in response to stress.^148,149 The application of these technologies will not only facilitate the discovery of precise biomarkers but also aid in defining therapeutic windows, refining intervention strategies, and enabling the development of targeted, context-specific therapies. Continued advancement and integration of these tools will be pivotal in translating UPR^mt biology into effective clinical applications.

Conclusion

In summary, the loss of mitochondrial proteostasis leads to mitochondrial dysfunction, which is a central factor implicated in a wide range of pathologies, especially for brain disorders. (Figure 3) This dysfunction results in impaired energy production, increased oxidative stress, and the accumulation of damaged proteins, all of which contribute to disease progression. The activation of UPR^mt has demonstrated beneficial effects in numerous models of neurological disorders, including I/R injury and neurodegenerative diseases, as well as in the physiological aging process. These findings highlight the critical role of UPR^mt in maintaining mitochondrial function and protecting cells from stress-induced damage.

Figure 3.

The relationship between UPRmt -associated regulators and various human disorders is complex and multifaceted. Upstream regulators and downstream effectors of the UPRmt influence the pathological processes of numerous human disorders, including ischemic/reperfusion injury, cancer, aging, and neurodegenerative diseases. These influences are mediated through different signaling pathways and molecular mechanisms.

Notably, in the context of cancer, the situation is contrary. Cancer cells often exploit mitochondrial pathways to support their rapid growth and survival, and the activation of UPR^mt can sometimes enhance these processes. This paradox underscores the complexity of mitochondrial mechanisms across different pathological conditions and suggests that a nuanced approach will be necessary when considering UPR^mt as a therapeutic target in cancer therapy.

Given its crucial role in mitochondrial health, modulating UPR^mt has emerged as a potential therapeutic target for complex diseases that severely impact quality of life and for which no curative treatments currently exist. Precise modulation of UPR^mt pathways holds the potential to restore mitochondrial function, alleviate cellular stress, and mitigate disease progression. Future research should aim to elucidate the intricate mechanisms governing UPR^mt activation and regulation, while also developing targeted therapeutic strategies capable of selectively modulating these pathways in various disease states.

In conclusion, UPR^mt represents a promising frontier in the development of novel therapies for a wide range of neurological diseases. Its critical role in maintaining mitochondrial integrity underscores its significance in cellular health and longevity. As our understanding of UPR^mt continues to advance, so does the potential for developing innovative therapeutic strategies to mitigate mitochondrial dysfunction, a key driver of numerous debilitating diseases.