Neuroinflammation and Disease: Pathways and Opportunities

Abstract

We explore three emerging molecular pathways driving neuroinflammation in chronic and acute brain diseases: the EP2 receptor for prostaglandin E2, the CCR2 receptor for chemokine CCL2, and JAK/STAT signaling. Inflammation is now recognized as a causative factor in neurodegenerative disorders, with neuroinflammation preceding symptom onset in Alzheimer’s disease and likely heralding the onset of epilepsy and Parkinson’s disease. The EP2 receptor modulates immune cell activation and exacerbates inflammatory responses, while CCR2 regulates peripheral immune cell recruitment to sites of brain inflammation. JAK/STAT pathways regulate neuronal and glial function across brain regions and can both amplify and resolve neuroinflammatory processes. These three signaling pathways converge at multiple nodes—immune cell recruitment, cytokine amplification, and transcriptional regulation—establishing feedforward loops that sustain pathology in chronic diseases. Understanding these mechanisms and their complex interactions provides opportunities for novel therapeutic interventions in neurological conditions characterized by inflammation, potentially leading to disease-modifying treatments.

INTRODUCTION

Inflammation is an acknowledged feature and probable driver of chronic diseases and many acute injury sequelae. In the brain, the neuroinflammatory response is primarily orchestrated by astrocytes and microglia, with significant contributions from invading monocytes (1), cerebrovascular smooth muscle cells, perivascular macrophages, and pericytes. Recent research has highlighted the causative role of neuroinflammation in various neurodegenerative disorders. Neuroinflammation is now considered a causative factor in Alzheimer’s disease (AD) pathogenesis, preceding symptom onset (2), and neuroinflammation likely also heralds the onset of epilepsy (3) and Parkinson’s disease (4). A homeostatic, beneficial role for inflammation in response to infection has been known for many decades, whereas the purpose of sterile inflammation in response to tissue damage is less clear. Intriguingly, select pathways and mediators of purposeful inflammation are co-opted to fuel chronic pathological inflammation, suggesting that understanding these processes could reveal novel anti-inflammatory therapeutic strategies.

This review explores three emerging players in neuroinflammation: the prostaglandin E2 (PGE2) receptor subtype 2 (EP2), the C-C chemokine receptor type 2 (CCR2) for chemokine ligand 2 (CCL2), and novel insights into Janus kinase/signal transducer and activator of transcription ( JAK/STAT) pathways. Each of these pathways contributes to neuroinflammatory cascades in distinct but interrelated ways, shaping disease progression in neurodegenerative and neurological disorders. Understanding their mechanisms may provide new strategies for therapeutic intervention in chronic brain diseases. Figure 1 summarizes the main pathways by which EP2 and CCR2 signal to produce inflammation.

Figure 1.

Pathways to inflammation mediated by EP2 and CCR2. Activation of CCR2 and EP2 receptors by their respective ligands triggers multiple intersecting biochemical pathways that converge in both the cytoplasm and nucleus to drive inflammatory gene transcription. Abbreviations: Akt, protein kinase B; AP1, activator protein 1; CCL2, chemokine ligand 2; CCR2, C-C chemokine receptor type 2; COX-2, cyclooxygenase-2; CREB, cAMP response element–binding protein; EP2, prostaglandin E2 receptor; EPAC, exchange protein directly activated by cAMP; ERK, extracellular signal–regulated kinase; IκBα, nuclear factor of kappa light polypeptide gene enhancer in B cells inhibitor alpha; IKKβ, IκB kinase; JAK, Janus kinase; MAPK, mitogen-activated protein kinase; MEK, mitogen-activated protein kinase kinase; NF-κB, nuclear factor κB; PGE2, prostaglandin E2; PI3K, phosphoinositide 3-kinase; PKA, protein kinase A; RAF, rapidly accelerated fibrosarcoma; RAS, rat sarcoma virus; STAT, signal transducer and activator of transcription. Figure created in BioRender; Varvel N. 2025. https://BioRender.com/bzee9od.

EP2: A MAJOR DRIVER OF NEUROINFLAMMATION ASSOCIATED WITH COX-2 INDUCTION

EP2 is a versatile G protein–coupled receptor (GPCR) that plays key roles in regulating diverse physiological processes. Its functions span multiple systems, including bronchodilation via inhibition of mast cell activation (5), vasodilation (6), synaptic plasticity (7, 8), implantation of fertilized ova (9), aging processes (10), and T cell maturation (11, 12). Beyond its physiological roles, EP2 has long been implicated in a wide range of pathological conditions. These conditions include cancer progression (13-15), endometriosis (16), aortic aneurysms (17), cerebral ischemia (18), arthritis (19), epilepsy (20-22), sepsis (23), and AD (24-26). The common thread linking these diverse pathologies is inflammation, which is directly modulated by EP2 activation.

EP2 function is tightly regulated through several intricate mechanisms. EP2 signals through the Gα_s protein to activate adenylyl cyclase, leading to an increase in cyclic AMP and subsequent activation of protein kinase A (PKA) and exchange protein directly activated by cAMP (EPAC) enzymes. However, EP2’s signaling repertoire extends beyond this primary pathway. Notably, it also activates G protein–independent pathways via β-arrestin complexes, initiating signaling cascades involving phosphoinositide 3-kinase (PI3K), protein kinase B (Akt), extracellular signal–regulated kinase (ERK), and epidermal growth factor receptor (EGFR). A unique feature of EP2, setting it apart from other prostaglandin receptors, is its resistance to homologous desensitization upon activation. This characteristic allows EP2 to sustain prolonged signaling during chronic inflammatory conditions, potentially contributing to its significant impact on various pathologies. Furthermore, EP2 signaling can create a positive feedback loop by upregulating cyclooxygenase-2 (COX-2) expression through mechanisms involving β-catenin and cAMP response element–binding protein (CREB). This upregulation amplifies the production of PGE2, further enhancing EP2-mediated effects and potentially exacerbating inflammatory responses (27, 28).

Competitive and Noncompetitive EP2 Receptor Antagonists for Therapeutic Development

By 2010, selective agonists and antagonists had been developed for all prostanoid receptors save for EP2 antagonists. This changed in 2011 when Pfizer published the first selective EP2 antagonist, PF-04418948, an azetidine-3-carboxylic acid (6). The following year, a suite of selective EP2 antagonists based on a 3-aryl-acrylamide scaffold was reported, exemplified by TG4-155 and TG6-10-6 (22, 29). Other scaffolds soon followed, such as Amgen’s benzoxazepine compound 52 (30). Today, approximately ten well-characterized compounds are available as competitive antagonists for human and rodent EP2 receptors (31-33).

Interestingly, a negative allosteric modulator of EP2 was identified that docks into a pocket on the intracellular surface near the G protein binding site, contrasting with agonists that recognize a pocket closer to the extracellular surface (34, 35). This cytoplasm-facing binding site is similarly located to allosteric modulator binding sites in other GPCRs, suggesting a potentially conserved intracellular-facing pocket for GPCR allosteric modulators.

Despite initial promise, Pfizer’s PF-04418948 was withdrawn from further development due to mild hyperbilirubinemia in a Phase I clinical trial caused by potent inhibition of the organic anion transporting polypeptide 1B1 (OATP1B1) bilirubin uptake transporter in hepatocytes (36). Subsequent EP2 antagonists developed at Emory University have addressed this issue by lacking activity at OATP1B1 and other liver transporters. Currently, the only selective EP2 antagonist to have entered clinical trials is the Pfizer compound. However, a dual EP2-EP4 antagonist, TPST-1495 (37), is expected to enter Phase II clinical testing for familial adenomatous polyposis in mid-2025.

Early concerns about the adverse effects of EP2 antagonism, based on global EP2 knockout mouse phenotypes (38), initially hindered development. However, subsequent studies with EP2 antagonists TG11-77.HCl and TG6-10-1 in adult mice and rats showed no adverse effects on cardiovascular and respiratory systems, blood counts, or bone structure, regardless of diet or administration protocol (39). These findings suggest that the adverse effects initially identified in genetic ablation models are likely developmental issues, potentially opening the door for therapeutic applications in diseases involving EP2 dysfunction.

Regulation of Myeloid Bioenergetics by EP2 and Its Role in Aging and Tumor Progression

The role of EP2 activation in aging and cancer-related inflammation reveals a complex interplay between cellular metabolism and immune function. In myeloid cells from aged mice and humans over 65 years old, EP2 activation by PGE2 triggers a cascade of metabolic changes. It promotes glucose sequestration into glycogen via the glycogen synthase kinase 3 beta (GSK3β)-Akt pathway while simultaneously increasing cellular dependence on glucose for energy. These changes result in decreased glycolysis and reduced mitochondrial oxygen consumption, leading to a bioenergetically deficient state (10). Remarkably, EP2 antagonists can restore cell metabolism in aged myeloid cells to a more youthful state. This effect has been observed in both mouse peritoneal macrophages and human monocyte-derived macrophages. The impaired bioenergetics of aged macrophages correlates with a shift toward a proinflammatory state, characterized by increased production of inflammatory cytokines and reduced phagocytic ability. EP2 antagonists effectively counteract this inflammatory polarization.

In the context of cancer, EP2 activation plays a similar role in metabolic reprogramming and immune suppression. In tumors with elevated COX-2 expression, EP2 activation downregulates both oxidative phosphorylation and glycolysis in tumor-invading myeloid cells and CD8⁺ T cells. This metabolic shift contributes to immunosuppression within the tumor microenvironment, ultimately promoting tumor progression (40). Studies using mouse models of inflammation-rich colon cancer have further elucidated the mechanisms by which EP2 mediates these effects. In EP2-deficient mice, neutrophil infiltration is severely reduced compared to wild-type mice. Wildtype infiltrating neutrophils express EP2, tumor necrosis factor (TNF), COX-2, and IL-6, all of which are suppressed in EP2-deficient mice (41). Moreover, EP2 activation enhances the ability of TNF to induce IL-6 and COX-2 expression, suggesting a positive feedback loop that amplifies inflammatory signaling.

These findings highlight the central role of EP2 in modulating cellular metabolism and inflammatory responses in both aging and cancer contexts. The potential for EP2 antagonists to reverse these effects presents a promising therapeutic avenue for addressing age-related immune dysfunction and cancer-associated inflammation.

Roles of EP2 in Neurologic Disorders

COX-2 (encoded by PTGS2), the inducible isoform of cyclooxygenase, is an immediate early gene that rapidly responds to tissue injury or intense neuronal firing. During neuroinflammation, PGE2 becomes a prevalent prostanoid produced by COX-2 in the brain, due to the tight functional coupling between COX-2 and microsomal prostaglandin E synthase-1. Among the four PGE2 receptors, EP2 appears to mediate much of the sequelae of COX-2 induction. Following prolonged seizures, COX-2 is strongly induced in hippocampal pyramidal neurons, dentate granule cells and selected neurons in the amygdala and neocortex. Selective ablation of COX-2 in principal forebrain neurons substantially reduces several detrimental effects of seizures, including delayed mortality, proinflammatory cytokine production, neurodegeneration of pyramidal neurons, blood-brain barrier (BBB) breakdown, and cognitive deficits (42, 43). These findings are largely replicated by administering selective EP2 antagonists to mice or rats during the period of elevated COX-2 expression (22, 29, 44, 45).

The EP2 receptor plays an important role in modulating microglial activation and neuroinflammation. EP2 receptor levels are elevated in primary microglia exposed to lipopolysaccharide (LPS) compared to resting microglia (46). In classically activated rat microglia, EP2 receptor activation exacerbates the production of proinflammatory cytokines and enzymes while reducing expression of other inflammatory regulators such as TNF (47). Importantly, EP2 activation appears to have a modulatory rather than initiating role in inflammation, primarily serving to potentiate existing microglial activation and enhance the induction of specific proinflammatory mediators. The implications of EP2-mediated neuroinflammation have been extensively studied in animal models of AD and epilepsy. These studies have provided valuable insights into the complex role of EP2 signaling in neurological disorders associated with inflammation.

In AD, EP2 activation exacerbates the impairment of beneficial microglial functions. EP2 activation prompts myeloid cells to release proinflammatory factors, thereby reducing the expression of chemokines and proteases involved in attracting microglia to amyloid plaques and amyloid clearance (48). This amplifies the immune response and contributes to neurotoxicity, creating a vicious cycle of inflammation and neurodegeneration characteristic of AD progression. Studies in mouse models of AD have shown that global deletion or myeloid-restricted ablation of EP2 reduces oxidative damage, lipid peroxidation, and amyloid-beta oligomer burden while improving microglial function and suppressing neuroinflammation (24). Moreover, myeloid-restricted ablation of EP2 improves chemotaxis to nascent amyloid plaques and Aβ clearance by microglia and suppresses neuroinflammation (48). Finally, chronic treatment with an EP2 antagonist reduces neuroinflammation in female 5XFAD mice chronically exposed to a low dose of LPS to mimic the inflammatory conditions of a Western diet (26). Taken together, these studies point to an important role for EP2 in suppressing the beneficial effects of microglia and in exacerbating neuroinflammation in AD. We described above how EP2 activation in myeloid cells from aged individuals impairs energy metabolism. A recent study showed that inhibition of indoleamine-2, 3-dioxygenase 1 (IDO1), which metabolizes tryptophan to kynurenine (KYN), rescues hippocampal memory function in mouse preclinical models of AD by restoring astrocyte glucose metabolism (49). It would be useful to test EP2 inhibitors in these AD models.

Status epilepticus (SE) is a severe neurological condition that can trigger a cascade of events leading to cognitive deficits and the development of epilepsy. This process involves a complex, multidimensional inflammatory response in the brain, engaging neurons, activated microglia, infiltrating monocytes, and reactive astrocytes. Recent research has shed light on the critical role of COX-2 and its downstream signaling pathways, mainly through EP2, in mediating these detrimental effects. Conditional ablation of COX-2 from principal neurons yields a range of beneficial outcomes following SE. These include dampening of inflammation, preservation of BBB integrity, reduction in hippocampal neurodegeneration, and normalization of cognitive deficits (42, 43). These findings strongly suggest that neuronal COX-2 signaling initiates the breakdown of the BBB after SE, although the precise mechanism—whether direct action on the neurovascular unit or indirect effects mediated by neuronal injury or inflammation—remains to be elucidated.

Importantly, EP2 antagonism or myeloid-specific conditional ablation of EP2 has demonstrated similar protective effects as neuron-conditional ablation of COX-2 (20, 22, 44, 45, 50-52). Moreover, systemic EP2 blockade has been found to prevent the brain infiltration of inflammatory monocytes (20), a key component of the neuroinflammatory response following SE.

The neuroprotective effects of EP2 antagonism extend beyond the acute phase of SE. Recent data suggest that EP2 blockade may also delay the onset of spontaneous seizures (R Rojas, W Wang, H Xing, T Ganesh & R Dingledine, unpublished manuscript), potentially interfering with the epileptogenic process that follows SE. This finding, if confirmed, could have significant implications for preventing the development of chronic epilepsy in patients who experience SE.

The accumulating evidence highlights the crucial role of EP2 in mediating neuroinflammation and its potential as a therapeutic target in neurological disorders, including epilepsy and potentially AD. EP2 antagonists have demonstrated the ability to mitigate detrimental effects associated with these conditions, including reducing delayed mortality, accelerating recovery from weight loss, reducing brain inflammation, preventing BBB opening, and providing neuroprotection in the hippocampus.

These findings collectively point to EP2 receptor antagonism as a promising avenue for disease-modifying treatments in epilepsy, AD, and potentially other neuroinflammatory conditions. By targeting the EP2 receptor, it may be possible to interrupt the self-reinforcing cycle of inflammation and neuronal injury that contributes to the progression of these disorders.

CCR2: A CHEMOKINE RECEPTOR THAT REGULATES PERIPHERAL IMMUNE CELL RECRUITMENT

The chemokine receptor CCR2 is a Gα_i-linked GPCR that plays a critical role in the immune system by controlling the chemotaxis of circulating monocytes and other immune cells to sites of inflammation or injury. In addition to chemotaxis, CCR2 facilitates monocyte egress from the bone marrow into circulation, promotes an inflammatory phenotype in immune cells, and serves as a chemokine scavenger receptor. In addition to its physiological roles, CCR2 has long been implicated in several pathological conditions, including cancer progression (53), fibrotic diseases (54), cardiovascular diseases, AD (55, 56), multiple sclerosis (MS) (57, 58), traumatic brain injury (59), and epilepsy (1, 60). The common theme in these diseases is immune cell recruitment to inflamed tissues.

CCR2 is constitutively expressed in monocytes and is downregulated after differentiation into macrophages (61, 62). In mice, CCR2 expression is highest on a subset of Ly6C⁺ inflammatory monocytes, which are recruited to inflamed tissues in a CCR2-dependent manner (63). Ccr2 messenger RNA (mRNA) has been detected in subsets of T cells, immature B cells, and dendritic cells under basal and inflammatory conditions (64).

Data describing CCR2 expression in the brain are limited and inconsistent. Early studies reported CCR2 immunostaining in various cell types in human, mouse, and rat brains (65-67), which now appears to be due to nonspecific immunostaining. These findings were not confirmed in a CCR2–red fluorescent protein (RFP) knock-in mouse, a CCR2 transcriptional reporter. CCR2-RFP⁺ cells were not detected in healthy mouse brains. When experimental autoimmune encephalopathy (EAE) was induced in the CCR2-RFP knock-in mouse, CCR2-RFP⁺ monocytes invaded the brain, and no CCR2-RFP expression was observed in neurons, microglia, or epithelial cells (68). These discordant findings stress the importance of rigorous reagent verification approaches.

CCR2 is activated by several chemokines of the monocyte chemotactic protein (MCP) family, including C-C motif chemokine ligand 2 (CCL2) (69), CCL7, CCL8, CCL12 (mouse only), and CCL13 (human only) (70). In vitro studies have revealed that CCL2 does not bind exclusively to CCR2 (71). In contrast, in vivo studies using knockout models suggest that CCL2 is the primary CCR2 ligand and functions predominately as a monocyte chemoattractant in mice with EAE (72).

CCR2 also acts as a scavenger receptor by internalizing chemokines to regulate extracellular chemokine levels. CCR2 scavenges through constitutive internalization, removing CCL2 from the extracellular space and then recycling back to the cell surface for further ligand sequestration. This internalization occurs independently of G proteins, GPCR kinases, β-arrestins, and clathrin (73).

Proinflammatory Role for CCR2 Beyond Immune Cell Trafficking

As judged by cytokine expression, monocytes isolated from the brain and blood after SE have a more proinflammatory profile than activated brain-resident microglia (1). CCL2-stimulated monocytes trigger activation of the JAK2/STAT3 pathway and CCR2 tyrosine phosphorylation. Cell migration and calcium mobilization are blocked by a JAK2 kinase inhibitor (74). Gα_i activation by CCL2 triggers mitogen-activated protein kinase (MAPK) pathways (e.g., ERK, p38) and PI3K-Akt signaling (Figure 1). In monocytes, p38 MAPK is phosphorylated, enhancing activator protein 1 (AP1) and nuclear factor κB (NF-κB) activity to upregulate proinflammatory cytokines such as tumor necrosis factor (TNF), IL-6, and CCL2 itself (75). In a second pathway, CCR2 recruits TNF receptor-associated factor (TRAF) proteins, activating TGF-β-activated kinase 1 (TAK1) (53, 76). TAK1 phosphorylates IκB kinase (IKKβ), which subsequently phosphorylates IκBα, leading to the nuclear translocation of NF-κB, which in turn binds to the promoters of proinflammatory genes such as IL-6, TNF, and CCL2 itself.

CCR2 Signaling Engagement in Epilepsy

Several chemokines, including CCL2, are elevated in the brain tissues of people with epilepsy (77, 78). Brain CCL2 and CCR2 levels are increased in rodent models of SE (79-81). Within 30 min of SE onset in mice, hippocampal Ccl2 mRNA levels increase more than tenfold and reach almost one-hundredfold higher 4 days post-SE (82), indicating a rapid and robust involvement of CCL2 (Figure 2). One hour after SE onset induced by pilocarpine, smooth muscle cells closely juxtaposed to cortical and hippocampal blood vessels express Ccl2 mRNA (R Dingledine, W Wang & N Varvel, unpublished manuscript). The cellular sources of CCL2 at later times are perivascular macrophages and brain-resident microglia 24 h post-SE onset in the systemic kainic acid mouse model (1). Increased CCL2 brain levels are followed by circulating monocyte recruitment into brain tissue. CCR2⁺ monocyte infiltration has been reported in several models of SE produced by intracerebroventricular (60, 83) or intrahippocampal kainic acid administration (84), systemic kainic acid and pilocarpine injection (1, 20), and in the Theiler’s virus model of encephalitis (85). These data reveal that monocyte brain recruitment after SE is model independent.

Figure 2.

Immediate and delayed inflammatory events after seizure activity. (①) Within 30 min of seizure initiation, neuronal COX-2 is induced, producing PGE2. (②) A defective BBB allows serum proteins, including albumin, to enter the brain. BBB leakage occurs as early as 5 h postseizure (174). (③) Vascular smooth muscle cells, perivascular macrophages, and microglia express CCL2. (④) Monocytes enter the brain in an EP2- and CCR2-dependent manner. (⑤) Astrocytes, microglia, and monocytes contribute to an inflammatory response typified by morphological changes and secretion of immune cytokines and chemokines. (⑥) Mural pericytes detach from the BBB endothelium, further eroding the integrity of the BBB (175). Abbreviations: BBB, blood-brain barrier; CCL2, chemokine ligand 2; CCR2, C-C chemokine receptor type 2; COX-2, cyclooxygenase-2; EP2, prostaglandin E2 receptor subtype 2; PGE2, prostaglandin E2; RBC, red blood cell. Figure created in BioRender; Varvel N. 2025. https://BioRender.com/h30r747.

In systemic kainic acid and pilocarpine models, monocyte brain invasion is delayed and transient, beginning 1 day after SE onset, and ending by 14 days after SE onset. Global ablation of Ccr2 prevents brain infiltration of circulating monocytes, blunts inflammatory gene induction, inhibits microgliosis, accelerates weight regain, is neuroprotective, prevents erosion of the BBB, and alleviates memory deficits (1, 60). The neurotoxic mechanism underlying CCR2 activation involves STAT3 signaling and IL-1β. CCR2 antagonism with CCX598 in the rat pilocarpine model showed modest neuroprotection but no change in the rate of behavioral recovery or the number of behaviorally observed spontaneous seizures between 11 and 30 days after SE (86). Microgliosis was not dampened, and monocyte brain invasion was not assessed, so target engagement was unclear. In another report, systemic CCR2 antagonism with INCB3344 (87), beginning 24 h after kainic acid–induced SE, blocked monocyte brain recruitment and largely recapitulated the beneficial consequences observed in the Ccr2 knockout mouse (88). These findings provide strong support for a deleterious role of brain-invading monocytes in the acute phase after SE.

Similar to our findings with EP2 antagonists, the protective effects of CCR2 blockade extend beyond the acute phase of SE. Recent data suggest that transient CCR2 antagonism may also delay or prevent seizure-associated cognitive decline. Blocking monocyte entry with the CCR2 antagonist improves working memory in the Y-maze and memory retention in novel object recognition test (176). Given the profound degradation in life quality and cognitive impairments that can accompany SE and epilepsy (89, 90), these findings, if confirmed, could have significant implications for preventing the debilitating cognitive comorbidities in patients who experience SE.

Roles of CCR2 and CCL2 in Cerebrovascular Disease

Following middle cerebral artery occlusion lasting 90 min in P7 rats, CCR2 and CCL2 expression in the brain rises within 24 h (91). Ccr2 knockout mice subject to ischemia-reperfusion have smaller infarct sizes, less BBB erosion, and reduced edema formation at 1 and 5 days postinjury. These benefits are accompanied by reduced monocyte and neutrophil infiltration into the affected hemisphere (92). CCR2 antagonism with RS504393 after subarachnoid hemorrhage was beneficial as it reduced brain edema and neuroinflammation, maintained BBB integrity, and provided neuroprotection (93). Monocyte infiltration was not assessed. Notably, whereas CCR2 deficiency in monocytes reduced the acute inflammatory response and limited monocyte brain entry after ischemia, angiogenesis and motor function were impaired 8 days postischemia (94). Thus, monocyte recruitment into the brain might have opposing outcomes after stroke, as brain-invading monocytes might facilitate long-term functional recovery at the expense of early neuroinflammation and neuronal damage.

CCR2/CCL2 Signaling Axis in Multiple Sclerosis

In EAE, a widely used animal model of MS, CCR2 expression is elevated in the priming and peak phases of the disease. Levels of spinal cord–derived CCL2 correlate with disease severity during relapses (95), and CCL2 is expressed by astrocytes (96) and microglia (97) in spinal cord tissue. Global knockouts of CCR2 (58) or CCL2 (72) resist EAE induction. Moreover, CCR2 deficiency in three different mouse strains resulted in delayed disease onset and reduced disease severity (98), further supporting the idea that CCR2 antagonism may have therapeutic potential for the treatment of MS. Several drugs targeting CCR2 have progressed into clinical trials, including one potent CCR2 antagonist, MK-0812, which showed favorable pharmacokinetic profiles and demonstrated efficacy in animal models. MK-0812 entered a Phase II clinical trial (NCT00239655) for relapsing-remitting MS but was terminated for lack of efficacy (99). A humanized anti-CCR2 monoclonal antibody (ML1202) reduced the number of lesions in relapsing-remitting MS patients (100), but there was no further development of this therapy (NCT01199640). One reason for these adverse outcomes may include corresponding increases in mRNA levels of chemokines and compensatory neutrophil infiltration in the absence of CCR2, which results in demyelination (98). Also, CCR2⁺ monocytes are potent suppressors of active T cells after EAE (101); thus, CCR2 antagonism could negate anti-inflammatory monocyte function and exacerbate the disease. The benefits of the new therapies have not yet supplanted available therapies.

CCR2 and CCL2 in Alzheimer’s Disease

CCL2 levels are elevated in the brain of AD patients and found in senile plaques, microglia (102), and microvessels (103). AD patients have higher serum (104) and plasma CCL2 levels compared to patients with mild cognitive impairment (MCI) and cognitively intact controls (105). Higher CCL2 levels in cerebrospinal fluid are associated with faster cognitive decline in the prodromal AD period (106). The relationship between genetic variants in CCL2 and AD and MCI risk has been inconclusive. One study found that a single polynucleotide polymorphism modestly influenced the conversion of MCI to AD (107), but another did not find an association between the same polymorphism and AD risk or clinical outcomes (108).

The impact of Ccr2 genetic deletion has been investigated in preclinical models of cerebral amyloidosis. Both the Tg2576 (55) and APP/PS1 (56) models bred onto a Ccr2-deficient mouse showed reduced monocyte recruitment into the brain, elevated amyloid-beta pathology, and more severe cognitive deficits compared to CCR2-sufficient mice. Both studies support a neuroprotective and beneficial role for brain-invading monocytes in AD pathogenesis. Two independent groups promoted circulating monocyte entry into the brain, expecting that the invading monocytes would clear the beta-amyloid deposits. However, beta-amyloid deposition was not alleviated, challenging the idea that monocyte brain entry is beneficial (109, 110). Further complicating the picture, overexpression of CCL2 accelerates tau pathology (111) and increases diffuse amyloid deposition (112). Thus, it remains unclear whether promoting peripheral monocyte recruitment to the brain would exert a net negative, net positive, or no effect on AD progression.

JAK/STAT SIGNALING DRIVES ACUTE AND CHRONIC NEUROINFLAMMATION ACROSS CELL TYPES OF THE CENTRAL NERVOUS SYSTEM

JAK/STAT Signaling in the Central Nervous System

JAK/STAT pathways regulate neuronal and glial function, homeostasis, and development across brain regions. JAK/STAT pathways involve three major structural components: transmembrane receptors, intracellular receptor-associated tyrosine kinases ( JAK1, JAK2, JAK3, TYK2), and signal transducers and activators of transcription (STAT1–6) (113, 114) (Figure 3). Receptor activation by one of over 50 different interleukins, growth factors, or interferons (IFNs) triggers JAK autophosphorylation and subsequent STAT phosphorylation. This leads to dimerization, nuclear translocation, and binding to gamma-activated sequences (GAS) to regulate gene transcription. Additionally, the IFN-stimulated gene factor 3 (ISGF3) complex (STAT1, STAT2, and IRF9) binds interferon-stimulated response elements (ISREs) to regulate genes required for innate immunity (115).

Figure 3.

The JAK/STAT signaling pathway. (a) Ligand-receptor pairs and the JAK/STAT isoforms classically associated with each. Following receptor activation, autophosphorylation of JAKs ( JAK1, JAK2, JAK3, or TYK2), and subsequent phosphorylation of STATs (STAT1–6), STATs regulate gene transcription in several ways. STAT-STAT dimers as well as unphosphorylated STAT3 recognize GAS, the ISGF3 complex (STAT1, STAT2, and IRF9) recognizes ISREs, and unphosphorylated STAT1 interacts with ICS-2 to prolong expression of interferon-stimulated genes. (b) Heat maps showing the levels of JAK and STAT isoforms across brain regions as well as across cell populations in the central nervous system. Abbreviations: EPO, erythropoietin; G-CSF, granulocyte colony stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; GAS, gamma-activated sequences; ICS-2, interferon consensus sequence 2; ISGF3, IFN-stimulated gene factor 3; IRF9, interferon regulatory factor 9; ISRE, interferon-stimulated response element; JAK, Janus kinase; LIF, leukemia inhibitory factor; nTPM, normalized transcripts per million; OSM, oncostatin M; STAT, signal transducer and activator of transcription; TPO, thrombopoietin; Treg, regulatory T cell; TYK2, tyrosine kinase 2; uSTAT, unphosphorylated STAT. Panel a created in BioRender; Hoffman O. 2025. https://BioRender.com/n84z537.

Classical JAK/STAT activation occurs in neurons (116-118), where this pathway regulates synaptic plasticity. JAK activation appears to be necessary for N-methyl-d-aspartate receptor–dependent long-term depression in CA1 pyramidal cells but not long-term potentiation, suggesting a selective role in synaptic weakening (119). In glial cells, STAT3 regulates astrogliosis in diseases such as stroke (120), AD (121), spinal cord injury (122, 123), MS (124), and epilepsy (125). Microglial JAK/STAT activation drives context-dependent pro- or anti-inflammatory responses (126-129). JAK/STAT signaling helps regulate the balance between hippocampal neuronal and glial cell populations during development (130). The distinct regional and cellular distribution of individual isoforms further refine these roles (Figure 3), as JAK1 and JAK2 are broadly expressed in neurons and glia (118, 131), while STAT3 is ubiquitously expressed across cell types and brain regions.

JAK/STAT signaling also exhibits noncanonical mechanisms. Unphosphorylated STATs (uSTATs) regulate gene expression independently of phosphorylation (132-134). Unlike phosphorylated STATs, which transiently accumulate in the nucleus, uSTATs exhibit constitutive nuclear localization, influencing chromatin organization and transcriptional regulation. Nuclear uSTAT1 and uSTAT3 have been shown, respectively, to prolong transcription of IFN-induced genes (133) and activate transcription of NF-κB via IL-6 signaling (132). JAKs can also localize to the nucleus; JAK1 and JAK2 phosphorylate histone H3 at tyrosine 41 (H3Y41) (135, 136). This modification disrupts heterochromatin protein 1α (HP1α) binding, leading to chromatin decompaction and increased gene expression, influencing long-term transcriptional programs in basal and pathological contexts.

JAK/STAT Signaling in Neuroinflammation: Friend or Foe?

JAK/STAT signaling generally amplifies and prolongs neuroinflammation but can also trigger anti-inflammatory immune responses in the central nervous system (CNS). Cytokine-mediated activation of STATs upregulates inflammatory mediators but also promotes negative feedback mechanisms, such as the induction of suppressor of cytokine signaling (SOCS) proteins, which dampen inflammation (124, 129). JAK/STAT activation contributes to neuroinflammatory processes required for immune defense and repair but can become detrimental if chronically sustained, possibly due to reductions in SOCS-mediated negative feedback (137) or ongoing traumatic insults [e.g., spontaneous seizures in epilepsy (118)].

STAT3 plays a key role in promoting reactive astrogliosis, a process characterized by upregulation of glial fibrillary acidic protein (GFAP) and the formation of a glial scar following injury. While this response helps contain damage and protect neurons, persistent astrogliosis can create a physical and biochemical barrier that limits neuronal regeneration (120-125). Microglia also engage JAK/STAT signaling in response to injury and disease. In some contexts, STAT3 activation triggers a proinflammatory microglial phenotype that exacerbates neurodegeneration (128), while promoting reparative states that facilitate recovery in others (129, 138). STAT3 activity in neurons is less understood but appears to be linked to neuronal survival under stress conditions (139). While STAT3 can support cell viability and regeneration, its dysregulation may contribute to maladaptive plasticity or neurotoxicity (60).

JAK/STAT signaling is closely interconnected with other inflammatory pathways. IL-6, a cytokine regulated by NF-κB, is a strong activator of STAT3 via JAK1 or JAK2, establishing a positive feedback loop that amplifies inflammation. Interplay between STAT3 and NF-κB helps propagate chronic neuroinflammation in conditions such as epilepsy (140, 141), AD (142), and MS (143). The cyclic GMP/AMP synthase–stimulator of interferon genes (cGAS-STING) pathway, a crucial component of the innate immune system, can activate JAK/STAT signaling. cGAS-STING is primarily activated by cytosolic DNA, which is indicative of infection and/or cellular damage, triggering a signaling cascade leading to the production of type I IFNs, proinflammatory cytokines, and subsequent activation of the JAK/STAT pathway (144-146).

Understanding these complex regulatory mechanisms is crucial for designing therapeutic interventions that selectively modulate JAK/STAT activity to mitigate pathological inflammation while preserving its essential roles in immune defense and tissue repair.

Therapeutic Repurposing of JAK/STAT Antagonists for Neurologic Disease

Several small-molecule inhibitors targeting the JAK/STAT pathway have been developed, primarily for autoimmune disorders and hematological malignancies. Existing evidence points to an association between peripheral inflammatory diseases and neurological disorders. For example, when adjusted hazard ratios for developing epilepsy are compared between cohorts of rheumatoid arthritis (RA) patients and sex- and age-matched controls, RA patients show an elevated risk (147). Similarly, there is an increased incidence of seizures requiring hospitalization or emergency room visits during an 8-year period following sepsis that was severe enough to have required hospitalization (148). WP1066 is a JAK2/STAT3 inhibitor (149), ruxolitinib a JAK1/2 inhibitor (150), and tofacitinib a JAK1/3 inhibitor (151). JAK inhibitors that are already US Food and Drug Administration (FDA) approved for systemic inflammatory diseases offer an attractive avenue for therapeutic repurposing in CNS disorders.

Epilepsy.

The JAK/STAT pathway has been increasingly implicated in epilepsy, particularly in regulating neuroinflammation, glial activation, and neuronal survival. JAK/STAT signaling has been linked to the upregulation of the long noncoding RNA H19, which promotes hippocampal glial activation in temporal lobe epilepsy (152). Noncanonical mechanisms of STAT3-mediated gene regulation, including chromatin remodeling and transcriptional repression (117, 153, 154), suggest that STAT3 has functions beyond its classical role as a gene activator.

Studies using hippocampal organotypic slice cultures have demonstrated that WP1066 prevents the loss of GABAergic neurons and the establishment of the epileptic state (155), suggesting a role for STAT3 in seizure-induced plasticity. In vivo transgenic approaches further support this, as neuronal STAT3 knockout reduces epilepsy progression (116). Inhibition of STAT3 during SE with WP1066 (156) or ruxolitinib (157) has been explored to determine whether acute intervention can reduce seizure burden. While both inhibitors reduce seizure frequency, the extent of seizure suppression varies, and long-term effects remain uncertain. JAK/STAT pathways are rapidly induced then quenched within days of an epileptogenic insult, followed by a resurgent activation weeks to months later with the onset of spontaneous seizures. Targeting the first wave of activation after epileptic insult does not prevent disease (118, 156, 157). However, brief inhibition of the second wave with tofacitinib enduringly suppresses seizures, rescues deficits in spatial memory, and alleviates epilepsy-associated histopathological alterations. These results suggest tofacitinib as a promising candidate for disease modification and further reinforce the role of JAK/STAT signaling in epilepsies with a robust neuroinflammatory component. Together, these findings suggest that the timing of delivery may be as critical for seizure suppression as JAK isoform selectivity.

Multiple sclerosis.

MS is characterized by immune-mediated demyelination and neuroinflammation, and JAK/STAT signaling plays a central role in modulating these processes. Aberrant STAT phosphorylation has been detected in peripheral blood mononuclear cells of MS patients, suggesting systemic immune dysregulation (158). Additionally, an interaction between JAK/STAT and peroxisome proliferator-activated receptor gamma (PPARγ) signaling has been implicated in T cell modulation and immune response progression in MS (159).

A key therapeutic avenue in MS involves IFN-β treatment, which relies on STAT4 activation. However, studies indicate that decreased IFN-β-induced STAT4 activation correlates with worse clinical outcomes (160). This suggests that JAK/STAT signaling may exert both proinflammatory and neuroprotective effects depending on context. The pathway also influences the transcriptional landscape in MS, as shown by IFN target–gene expression analyses (161). Targeting the blood–spinal cord barrier (BSCB) has emerged as another potential strategy, as modulation of STAT3/SOCS3 signaling can protect BSCB integrity and prevent immune cell infiltration (143).

Parkinson’s disease.

PD is driven by progressive dopaminergic neuronal degeneration, and JAK/STAT signaling has been identified as a major contributor to neuroinflammation and neuronal survival. The pathway is involved in mitophagy and stress responses in motor neurons, where niclosamide has been shown to alter intracellular TAR DNA binding protein 43 (TDP-43) distribution and promote mitochondrial quality control (162). Direct evidence for JAK/STAT involvement in PD pathology has been demonstrated in animal models, with studies indicating that inhibition of this pathway attenuates neuroinflammation and improves motor outcomes (163).

Brain-derived neurotrophic factor (BDNF) has been suggested to exert neuroprotective effects by promoting STAT3 phosphorylation and regulating neuronal autophagy, suggesting a potential mechanism for slowing dopaminergic neurodegeneration (164). Similarly, compounds such as echinacoside protect dopaminergic neurons through IL-6/JAK2/STAT3 signaling, reinforcing the idea that modulating this pathway could be a therapeutic target (165). Animal models further support STAT3’s role in dopaminergic neuron survival, with STAT3 activation conferring protection against neurodegeneration (139). Interestingly, blocking IL-6 signaling has been shown to prevent astrocyte-induced neurodegeneration in induced pluripotent stem cell–based PD models, suggesting that targeting astrocyte-mediated inflammation via the JAK/STAT pathway may be beneficial (166). Finally, inhibition of the JAK/STAT pathway has been found to protect against α-synuclein-induced dopaminergic neurodegeneration (167).

Amyotrophic lateral sclerosis.

In amyotrophic lateral sclerosis (ALS), JAK/STAT signaling contributes to neuroinflammation and glial dysfunction. Tofacitinib, a JAK inhibitor, has been found to suppress natural killer cells in vitro and in vivo, highlighting its potential immunomodulatory role in ALS (168). Activation of STAT3 is observed in ALS models, and treatment with pioglitazone inhibits STAT3 activity, suggesting that JAK/STAT modulation may help mitigate neuroinflammation (169).

The role of astrocytes in ALS progression is particularly notable. In healthy conditions, neuronal signals such as EphB1 induce a neuroprotective astrocyte phenotype, but this process fails in ALS models (60). This disruption suggests that restoring normal JAK/STAT signaling in astrocytes could be a potential therapeutic approach.

Alzheimer’s disease.

The JAK/STAT pathway has been increasingly recognized as a contributor to AD pathology, primarily through its role in neuroinflammation, apoptosis, and β-amyloid accumulation. Apoptotic mechanisms in AD involve JAK/STAT-mediated regulation of neuronal survival, with studies identifying STAT3 as a critical regulator of cell death pathways (170).

JAK inhibitors such as baricitinib and tofacitinib have been explored for their off-target effects in AD. Despite potential modulation of relevant neuroinflammatory pathways in AD, off-target effects, especially with baricitinib, remain a challenge (171). Additionally, Reelin signaling, which has been implicated in both brain development and inflammation, interacts with JAK/STAT to regulate synaptic function and neuronal survival (172). Recent evidence also suggests that apolipoprotein E aggregation in microglia initiates β-amyloidosis through JAK/STAT-dependent mechanisms, further linking this pathway to AD pathology (173).

CHALLENGES AND FUTURE DIRECTIONS

This review explores three emerging molecular pathways that drive neuroinflammation in neurological disorders: the EP2 receptor for PGE2, the CCR2 receptor for chemokine CCL2, and JAK/STAT signaling. Inflammation is now recognized as a causative factor in neurodegenerative disorders, with neuroinflammation preceding symptom onset in several conditions. These pathways converge at multiple nodes—immune cell recruitment, cytokine amplification, and transcriptional regulation—establishing feedforward loops that sustain pathology in chronic diseases. A major challenge is to move beyond isolated pathway studies toward understanding the convergence of multiple inflammatory processes. Future studies of mechanisms underlying the convergence of these signaling pathways should provide opportunities to develop novel disease-modifying treatments for neurological conditions characterized by inflammation. This integrated approach would better reflect the complex reality of neuroinflammatory conditions in which multiple pathways interact simultaneously.

The EP2 receptor modulates immune cell activation and exacerbates inflammatory responses across various pathologies, including epilepsy and AD. The development of selective EP2 antagonists represents a promising therapeutic strategy that could bypass adverse cardiovascular events associated with COX-2 inhibitors. Future work could focus on uncovering the detailed signaling mechanisms involved in EP2-mediated inflammation, optimizing delivery methods for EP2 antagonists, and determining ideal treatment windows following neurological insults.

CCR2 is a central regulator of immune cell trafficking into the CNS. Global ablation of Ccr2 prevents monocyte brain infiltration, reducing inflammation and providing neuroprotection in several disease models. Recent data suggest that transient CCR2 antagonism may delay or prevent seizure-associated cognitive decline, offering potential benefits for patients experiencing SE.

JAK/STAT signaling plays a central role in acute and chronic neuroinflammation, shaping cellular responses across the CNS. Its activation can both amplify and resolve neuroinflammatory processes. Future work could include repurposing existing JAK inhibitors ( Jakinibs) that are already FDA approved for systemic inflammatory diseases, developing selective JAK inhibitors with improved CNS penetration, identifying biomarkers for patient stratification, and identifying critical timing windows for intervention. A refined understanding of cell type–specific JAK/STAT dynamics will help in developing precise, disease-modifying interventions.