GPR3 in neuro-metabolic-immune-reproductive nexus – a potential therapeutic target for Multi-System diseases
B.-D. Feng et al.
Department of Neurology, Affiliated Hospital of Zunyi Medical University, Zunyi Guizhou, China
CONTACT Zui-Cai Xu docxzc@126.comHai-Qing Zhang cqmuhaiqing@126.com Department of Neurology, Affiliated Hospital of Zunyi Medical University, 149 Dalian road, Zunyi, Guizhou 563003, P.R. China.Abstract
Background
GPR3(G-protein-coupled receptor 3), an orphan G-protein-coupled receptor (GPCR) with constitutive Gs activity, is expressed in the brain, liver, ovary, and other tissues, regulating cell proliferation, differentiation, and apoptosis across the nervous, reproductive, immune, and metabolic systems. This review synthesizes evidence on its integrated signaling and physiological functions to address the lack of a comprehensive multisystem pathophysiology overview.
Methods
A systematic literature search was conducted on PubMed and Web of Science, using keywords such as “GPR3”, “GPCR”, “neurodegeneration”, “metabolism”, “immune”, “reproduction”, “agonist”, “inhibitor”, and “therapeutic target”. This search identified GPR3′s roles in neurodegenerative diseases, immune inflammation, reproduction, and energy metabolism. The analysis focused on signaling pathways, ligand regulation, and therapeutic potential.
Results
The research indicates that GPR3 is involved in neuronal survival, synaptic plasticity, and microglial activity via the cAMP/PKA, PI3K/Akt, and β - arrestin pathways. It promotes amyloid - β formation in Alzheimer’s disease (AD), yet provides neuroprotection in Parkinson’s disease (PD) models. It may contribute to anxiety/depression - like states, maintain oocyte meiotic arrest in the ovary, and activate thermogenic genes in adipose tissue. GPR3 modulates immune responses. Using oleic acid (OA) and diphenyleneiodonium (DPI) as activators, and AF64394 and cannabidiol (CBD) as antagonists, it shows potential in disease models.
Conclusion
GPR3 acts as a central molecular hub integrating neural, metabolic, immune, and reproductive signaling, highlighting its potential as a therapeutic target for chronic multisystem disorders. However, its dual roles in certain pathologies and translation challenges necessitate further research.
1.Introduction
GPR3 is a member of the GPCR (G protein-coupled receptor), or G protein-coupled receptor, family with constitutive Gs signaling activity. It is a membrane protein with extensive biological functions that can stimulate cAMP-dependent signaling in the absence of exogenous ligands [1,2]. GPR3 transmits extracellular signals by interacting with G proteins and other signaling molecules [3], thereby exerting physiological and pathological effects. Cryo-electron microscopy observations show that GPR3 exists in both monomeric and dimeric forms, and its oligomerization is related to an intracellular auto-inhibitory mechanism that may regulate its signaling [4] (Figure 1). GPR3 plays a crucial role in regulating physiological functions such as synaptic growth, neuropathic pain, and neuronal survival. It also accelerates neurite growth and promotes neuronal polarity formation [5,6], demonstrating neuroprotective potential. Additionally, GPR3 influences axon regeneration in retinal ganglion cells [7] and modulates the activity of gamma-secretase, making it a potential target for AD treatment [8]. Furthermore, GPR3 is involved in the regulation of reproductive and metabolic processes; its expression positively correlates with thermogenesis in adipose tissue [9]. It also serves as a target for the endogenous ligand oleate, playing an important role in lipid metabolism [10]. Given its expression and functional roles across multiple organ systems, GPR3 represents a unique nexus point for neuro-metabolic-immune-reproductive crosstalk.This review explores the basic characteristics of GPR3, its roles in various biological processes, and its prospects as a potential drug target, this article synthesizes recent evidence to highlight the integrated pathophysiology of the condition and its clinical relevance across multiple organ systems.
2.Methods: Literature search strategy
A systematic literature search was conducted in PubMed and Web of Science databases for articles published from 1994 to 2025. The primary search keywords included: “GPR3”, “G-protein-coupled receptor 3”, “orphan GPCR”, combined with “nervous system”, “neurodegeneration”, “Alzheimer’s”, “Parkinson’s”, “metabolism”, “obesity”, “adipose”, “immune”, “inflammation”, “reproduction”, “oocyte”, “meiosis”, “signal transduction”, “cAMP”, “agonist”, “antagonist”, “inverse agonist”, and “therapeutic target”. Boolean operators (AND, OR) were used to combine search terms. The inclusion criteria encompassed original research articles and reviews focusing on GPR3′s biology, signaling, and pathophysiological roles. Exclusion criteria included studies not published in English. Additionally, studies not directly relevant to GPR3′s core functions, as defined within the scope of this review, were excluded. While a quantitative meta-analysis was not feasible due to the heterogeneity of the studies, this qualitative synthesis aims to comprehensively integrate findings across the neural, metabolic, immune, and reproductive systems. The inherent limitations of the cited works, such as the use of diverse model systems and sometimes conflicting results, are acknowledged and discussed critically throughout the text.
3.Basic introduction to GPR3
3.1.Structure of GPR3
GPR3 is an orphan rhodopsin (class A) GPCR composed of 126 amino acids with a molecular weight of 18.9 kDa [11]. It consists of a single polypeptide chain and exhibits constitutive Gs signaling activity, stimulating cAMP-dependent signaling [1,2,12]. GPR3 is primarily located in the Golgi apparatus, endosomes [13], and in the plasma membrane and subcellular membranes [14]. Its gene is located on human autosomal chromosome 1p34.3 and contains two exons [15]. Structurally, GPR3 features seven transmembrane helices that form a typical GPCR fold. It has three intracellular loops and three extracellular loops, with serine and/or threonine residues in the third intracellular loop involved in regulating GPR3 activity [16]. The N-terminus is extracellular, while the C-terminus is intracellular [8], enabling interaction with G proteins to regulate multiple cell signaling pathways [1,3,8]. The receptor interacts with G proteins through its intracellular domains to mediate signaling; however, the GTP-binding activity is characteristic of the associated G proteins rather than GPR3 itself. Additionally, GPR3 influences intracellular calcium (Ca2+) signaling, which in turn regulates the transmission of intracellular signals [4]. The functional implications of the protein’s dimerization and auto-inhibition, which potentially regulate signal amplitude and specificity, warrant further investigation. (Figure 1 and Table 2).
3.2.Distribution and biological function of GPR3
GPR3 regulates intracellular signal transduction through specific structural features located in its seventh transmembrane helix, modulating physiological processes such as cell proliferation, differentiation, and apoptosis [8]. Moreover, GPR3 shows variable expression in different cell and tissue types, especially in the cerebral cortex and hippocampus [11,17,18], suggesting its importance in neuronal function and cognitive function [12,19]. GPR3 regulates intracellular signal transduction through its structural characteristics, particularly the seventh transmembrane helix, and participates in physiological processes such as cell proliferation, differentiation, and apoptosis [8]. GPR3 is essential for neuronal development and function; it induces the growth of neural synapses and neuronal survival and has neuroprotective and anti-apoptotic effects [5,20], which may influence higher neural functions such as learning and memory.
In AD, GPR3 contributes to the production of Aβ by regulating γ-secretase activity and hydrolysis of amyloid precursor protein [8,21], a process that depends on β-arrestin signaling and C-terminal phosphorylation. Additionally, GRK2 can increase Aβ production by altering the intracellular trafficking of GPR3[21]. In retinal ganglion cells, overexpression of GPR3 promotes regeneration after optic nerve injury through the CaMKII-CREB pathway [7], which can reduce glaucoma-induced axonal degeneration (Figure 2). Moreover, GPR3 is enriched in neurons associated with schizophrenia symptoms and is considered an important target for the disease [22].
During oocyte maturation, GPR3 maintains meiotic arrest through a Gs-dependent mechanism [17,23]. Its PKA and the resulting increase in cAMP levels promote meiotic arrest in oocytes [24–26]. Mice lacking GPR3 exhibit premature ovarian failure and loss of fertility [27–29]. In adipocytes, the high constitutive activity of GPR3 is associated with adipogenesis, and its expression is induced by cold exposure and high-fat diets [9]. Furthermore, GPR3 is regulated through interaction with endogenous ligands such as oleoylethanolamide (OEA), and this interaction exerts signaling functions [8].
These studies demonstrate that GPR3 plays a significant role in the nervous system and is involved in developmental, metabolic, and neurodegenerative diseases. Overall, the biological functions of GPR3 are complex and diverse, and further research is needed to deepen our understanding of its role in cell signal transduction and disease mechanisms (Table 2).
4.The role of GPR3 in the nervous system and its underlying mechanisms linking to the immune response
4.1.Role of GPR3 in the nervous system
GPR3 plays an important role in the nervous system by coupling to the Gαs protein to transmit signals that continuously activate adenylyl cyclase (AC), thereby increasing intracellular cyclic adenosine monophosphate (cAMP) levels [5,14,29,30,31] resulting in a significant increase in intracellular cAMP levels [30] (Table 1). cAMP plays a key role in neuronal survival and differentiation, neurite growth, axon regeneration, neuroprotection, and axonal guidance [5,20,29]. GPR3 is predominantly expressed in the central nervous system, especially in regions such as cerebellar granule neurons, hippocampus, cortex, striatum, and olfactory bulb (Figure 2). It colocalizes with calcium-binding proteins such as calbindin and parvalbumin, which suggests its involvement in the regulation of specific neural circuits [12,13,42]. Furthermore, the expression of GPR3 gradually increases during development and it is localized in the plasma membrane, Golgi apparatus, and endosomes of neurons. GPR3 is dynamically transported to the tips of neurites [13]. The continuous activation of GPR3 may play an important role in neuromodulation and neuro-humoral regulation and is associated with various physiological and pathological processes, including neuronal function and synaptic plasticity [5].
| Site/Cell Type | cAMP signal changes | Neuroprotective/Injury Mitigation Manifestations | Main References |
|---|---|---|---|
| Neurons in cerebral cortex and hippocampus | GPR3 constitutive activation/GPR3 biased G protein signaling → sustained cAMP elevation | Promotes neuronal protrusion growth, anti-apoptosis, improves synaptic plasticity, reduces soluble Aβ, decreases plaque density, and improves cognitive scores | [5,13,20] /[8,20,32,33] |
| Cerebellar granule cells (CGN) | GPR3-Gs-AC → cAMP↑ | Promotes axonal extension; Lack of GPR3 leads to increased apoptosis | [5,34,35] |
| Retinal ganglion cells (RGCs) | GPR3 overexpression → cAMP↑ | Axonal regeneration rate and neuronal survival rate increase after optic nerve entrapment | [7] |
| Dopaminergic neurons (PD model) | Activation of GPR3 → cAMP↑ | Inhibition of microglial hyperactivation leads to the reduction of inflammatory dopaminergic neuron death. | [36,37] |
| Ischemia-reperfusion whole brain model | Activation of GPR3 → cAMP↑ | Activation of PKA/ERK/Akt leads to decreased caspase − 3, reduced neuronal apoptosis, and a decreased infarct volume. Deficiency of GPR3 results in an increased infarct area. | [13,38] |
| The emotion-related circuit (hippocampus-frontal lobe-amygdala) | GPR3 deficiency → cAMP↓ | Decreased monoamine neurotransmitters lead to increased anxiety/depressive-like behaviors; activation of GPR3 can reverse this. | [22,39] |
| Dorsal horn of spinal cord (pain model) | GPR3-AC → cAMP↑ | Inhibition of TRPV1 hyperexcitability → Decreased thermal pain sensitivity; Deficiency of GPR3 leads to a lowered thermal pain threshold | [40,41] |
GPR3 promotes neurite growth through the cAMP-PKA signaling pathway, which regulates PKA activity. GPR3 also affects the polarization and maturation of neurons. The regulation of this local cAMP-PKA signal may play an important role in synaptic formation and plasticity. In cerebellar granule neurons, overexpression of GPR3 significantly enhances neurite elongation and overcomes the inhibitory effect of myelin-related glycoproteins. Conversely, knockdown of GPR3 inhibits neurite growth [5]. In addition, GPR3 exerts anti-apoptotic effects by activating the PKA, ERK, and Akt pathways [13] (Figure 3). For example, the absence of GPR3 under oxidative stress or hypoxia conditions can lead to increased caspase-3 activity and reduced neuronal survival. Activation of the PKA, ERK, and Akt pathways can reverse this effect [13]. Activation of GPR3 can also promote neuronal survival, especially in cases of nerve injury or ischemia. The absence of GPR3 can aggravate the infarct area after cerebral ischemia. In contrast, activation of its downstream ERK and Akt pathways can mitigate neuronal damage and protect neurons from apoptosis induced by various stimuli [34]. Furthermore, GPR3 regulates the release of neurotrophic factors by astrocytes, thereby promoting neuronal survival and regeneration and participating in neuroprotection [7,11,13,40]. During postnatal cerebellar development, GPR3 inhibits the proliferation of granule precursor cells by upregulating p27/kip protein and promotes cell cycle exit. GPR3 knockout mice show thickening of the granular layer and an increase in the number of BrdU-positive proliferative cells [35,43].
4.2.Functional role of GPR3 in neuroinflammation
GPR3, a member of the G protein-coupled receptor (GPCR) family, also plays an important role in mediating the relationship between neuroinflammation and neurological function. Studies have shown that GPR3 controls inflammation-related gene expression by regulating the cAMP/PKA signaling pathway. Additionally, GPR3 plays a key role in the axonal polarization and dendritic protrusion growth of cerebellar granule neurons, which further affects neuronal differentiation and maturation [13] (Figure 3). In addition, GPR3 is implicated in inflammatory signaling pathways such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and signal transducer and activator of transcription 3 (STAT3) [35]. In inflammatory responses, transcription factors of the NF-κB family initiate the expression of inflammatory genes by entering the nucleus. Their activation is usually triggered by extracellular signals such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β). Activation of GPR3 can alter intracellular signaling pathways, thereby affecting the activity of NF-κB [38,43]. Specifically, NF-κB dephosphorylation and transcriptional activity are indirectly regulated through modulation of cAMP levels (Figure 3).
GPR3 affects neuronal growth and polarity formation by regulating the PI3K/Akt signaling pathway [6,7], and indirectly regulates glial cell function and the inflammatory response. Studies have shown that GPR3 is expressed in both microglia and astrocytes, and may affect the neuroinflammatory response by regulating the activation state of these cells [35], and the absence of GPR3 leads to more significant astrocyte activation after seizures [35], This activation may play an inhibitory role in epileptic pathology by regulating microglial activation [34]. Glial overactivation is associated with a variety of neurological diseases, and activation of the GPR3 receptor can affect astrocyte response to inflammatory stimuli. One study investigated the effects of CBD on the inflammatory response of primary astrocytes and microglia in mice and its relationship with GPR3 [36]. The results showed that CBD can inhibit the inflammatory response triggered by lipopolysaccharide (LPS) stimulation; however, its inhibitory effect on pro-inflammatory cytokine release does not depend on the expression of GPR3 [44]. These findings suggest that GPR3 may play a regulatory role in astrocyte activation, although its specific mechanism requires further investigation. Moreover, activation of GPR3 may promote neuronal survival and function by affecting intracellular signaling pathways, such as the PI3K and ERK pathways, thereby combating the adverse effects of neuroinflammatio [36,41].
Studies have shown that GPR3-mediated modulation of β-amyloid pathology can improve amyloid clearance and is accompanied by hypertrophy of astrocytes and microglia, suggesting a protective role in the glial response [45]. However, the specific molecular mechanisms by which G protein-biased GPR3 regulates glial cells to produce protective responses remain unclear. For instance, how it triggers the hypertrophy of microglia and astrocytes, as well as the molecular pathways through which these cells subsequently participate in Aβ clearance, still require in-depth investigation. Furthermore, it will be necessary to explore whether G protein-biased activation of GPR3 can be achieved through approaches such as small-molecule ligands. This exploration will lay the foundation for translating these basic research findings into clinically applicable therapeutic strategies.The expression of GPR3 in microglia is influenced by various stimuli; notably, LPS stimulation significantly upregulates its expression, which may contribute to regulating the inflammatory response of microglia. In addition, GPR3 affects microglial activation and the release of inflammatory factors by regulating the cAMP signaling pathway [46]. Notably, the absence of GPR3 leads to delayed axonal polarization of mouse hippocampal neurons, a process associated with altered phosphorylation of downstream effectors in the PI3K signaling pathway [6]. Furthermore, sustained expression of GPR3 in T cells can upregulate NR4A2 expression, thereby enhancing neuronal resistance to injury through neuroimmune interactions [46] (Figure 3). It is still necessary to further clarify the downstream molecular mechanisms by which GPR3 inhibits effector T cell activation. In addition to NR4A2, whether GPR3 crosstalks with the T cell receptor signaling pathway and the inflammatory cytokine secretion pathway remains to be determined. Neuroinflammation plays a central role in various neurological diseases, with astrocytes and microglia acting as key mediators. Therefore, in-depth research on the GPCR family, specifically GPR3, combined with emerging therapies such as CBD, which has anti-inflammatory properties, is expected to provide new strategies for the treatment of neurological diseases.
4.3.Pathological mechanisms of GPR3 in Alzheimer’s disease (AD)
GPR3 plays a complex role in neurodegenerative diseases, particularly AD. It affects the progression of diseases by regulating neuronal survival and apoptosis [7,12], and is closely related to the formation of amyloid plaques, which is involved in promoting the cleavage of amyloid precursor proteins, thereby increasing the production of Aβ [47,48]. Studies have shown that knockout of GPR3 can reduce amyloid plaque load in mouse models of AD and improve their cognitive function [32,49]. In addition, GPR3 expression levels are elevated in the brain tissue of AD patients and positively correlated with disease severity. This makes GPR3 a potential biomarker [32]. This presents a paradox: GPR3 appears to promote Aβ production, yet its activation can also be neuroprotective. This duality may depend on factors such as cell types (neurons vs. glia), subcellular localization, signaling bias (G protein vs. β-arrestin pathways), or disease stage, which highlights a critical area for future research to reconcile these conflicting observations.Moreover, studies have shown that β-amyloid production in turn interacts with other GPCRs (such as M3 muscarinic receptors) and may exacerbate neuroinflammation by affecting glial cell function [50,51]. Activation of GPR3 leads to polarization and activation of glial cells, which in turn promotes the release of inflammatory factors. Although this inflammatory response has some protective effects, if it becomes uncontrolled, it may exacerbate the progression of neurodegenerative lesions [4,45] and accelerate neuronal damage by promoting the expression of pro-inflammatory cytokines. In addition, the conformational changes and dimerization status of GPR3 have important influences on its function, and its dimerization properties are related to self-inhibition mechanisms, which may play a key role in the pathological mechanism of AD [4]. Therefore, in-depth study of the structural properties and functions of GPR3 can provide a new perspective for understanding the pathogenesis of AD and provide potential targets for treatment strategies, which is expected to delay the progression of AD by intervening in GPR3-mediated signaling pathways.
4.3.1.Role of GPR3 in AD
GPR3 has been found to be strongly associated with the development of amyloid plaques in the brain, neuroinflammation, and neuronal survival in the study of AD [7,12]. Studies of GPR3 mutants show that its binding to β-arrestin 2, endocytic vesicle localization, and binding affinity to amyloid precursor protein (APP) are all related to Aβ production. Among the proteins studied, only GPR3 and PTGER2 interact with APP and stimulate Aβ production. Additionally, GPR3 can bind to APP and promote Aβ production in the absence of exogenous agonists, whereas PTGER2 requires long-term agonist stimulation [48]. In future studies, it is necessary to further investigate the impact of modulating the interactions among GPR3, APP, and β-arrestin2 with respect to in vivo Aβ deposition and disease progression. Furthermore, future studies should explore whether other molecules contribute to the formation or stabilization of this complex, or investigate strategies to inhibit Aβ production by blocking these three interactions, thereby providing experimental insights for developing targeted therapies for AD.GPR3 regulates γ-secretase activity and mediates amyloid hydrolysis of APP [8,32]. Regarding neuronal survival and function, activation of GPR3 may have a protective effect, helping to protect neurons, which is associated with improved cognitive function [34].
GPR3 is involved in Aβ pathology through intracellular signaling mediated by β-arrestin-2. GPR3-biased G protein signaling reduces soluble Aβ levels. In the AppNL-G-FAD mouse model, GPR3 not only increased the compactness of Aβ plaques but also reduced the area of the plaques. It was found that deletion of the GPR3 gene reduced the burden of Aβ plaques and cognitive decline in AD mice [52]. Selective inhibition of GPR3-mediated β-arrestin-2 signaling can maintain the beneficial physiological function of GPR3-mediated G protein signaling while reducing the Aβ plaque burden in AppNL-G-FAD knock-in mice. In addition, GPR3 is expressed at higher levels in astrocytes than in neurons and microglia, and the increased proliferation of astrocytes along with elevated GPR3 expression in these cells may suggest a protective glial response to limit the formation of Aβ plaques [52,53]. Biased GPR3 signaling reduces Aβ production in HEK-APP cells, while Grk2 deficiency reduces GPR3 recruitment, which in turn leads to reduced Aβ production. GRK2 also regulates the localization of GPR3 and APP in the Golgi apparatus, early endosomes, and endoplasmic reticulum. Inhibition of GRK2 leads to increased phosphorylation levels of APP at Thr668. Inhibition of endocytosis increases the levels of GPR3 and cAMP on the cell membrane; however, overexpression of GRK2 and β-arrestin-2 decreases the levels of both GPR3 and cAMP. GRK2 also reduces cAMP production and, together with β-arrestin-2, reduces the expression of GPR3 on the cell membrane. Serine and threonine residues in the third intracellular loop of GPR3 are involved in the regulation of its activity, suggesting that GPR3 signaling on the cell membrane may be silenced by overexpression of GRK2 and β-arrestin-2[16]. In summary, GRK2 plays an important role in regulating the intracellular trafficking and localization of GPR3 and APP associated with biased G proteins, which may directly affect the production of Aβ [21]. In the G protein-biased GPR3 mouse model, GPR3 reduces Aβ accumulation, activates microglia and astrocytes, and limits amyloid plaque development.
4.4.Role of GPR3 in Parkinson’s disease (PD)
GPR3 has shown potential neuroprotective effects in PD, and its activation can promote neuronal polarization and neurite growth, both of which are important for nerve regeneration. In addition, GPR3 may influence the pathological process of PD by modulating neuroinflammatory responses and oxidative stress [35]. Studies have shown that the inhibition of GPR3 may lead to increased neuronal apoptosis, thereby accelerating the progression of PD [33]. In PD models, GPR3 expression plays a significant role. Its activation may protect dopaminergic neurons by inhibiting microglial activation, and the release of inflammatory factors. Microglia are immune cells of the central nervous system, and their overactivation is closely related to the pathological development of PD. Moreover, CBD has been shown to activate multiple receptors, including GPR3, which contributes to its neuroprotective effects [35]. In animal models of PD, CBD reduces neuroinflammation by activating GPR3, thereby protecting dopaminergic neurons from damage [33,35], However, the mechanism of its therapeutic effect is still unclear.These findings support the potential of GPR3 as a therapeutic target, particularly in the development of novel neuroprotective strategies. Research on GPR3 may open new avenues for creating neuroprotective drugs that could potentially improve the quality of life for people with PD.
4.5.Potential role of GPR3 in emotion regulation and psychiatric disorders
GPR3 is widely expressed in the emotional and cognitive centers of the brain (such as the hippocampus, amygdala, and prefrontal cortex) [12,22], and GPR3 is involved in regulating neurotransmitter balance, promoting neurite growth, maintaining neuronal survival, regulating neural network plasticity, and plays a regulatory role in stress response [5]. This multiple regulatory effect suggests a close correlation between the expression level of GPR3 and the risk of mood disorders such as anxiety and depression [20,54,55]. Abnormalities in GPR3 function can lead to imbalances in the regulation of neurotransmitters such as serotonin and norepinephrine, which can affect emotional responses and the ability to cope with stress. Animal experiments have shown that the deletion of GPR3 leads to significant alterations in the levels of monoamine neurotransmitters in the hippocampus, hypothalamus, and prefrontal cortex [23,54]. GPR3 regulates the transmission of monoamine neurotransmitters through the cAMP signaling pathway. Mice lacking GPR3 exhibit anxiety- and depression-like behaviors, which may be related to decreased levels of monoamine neurotransmitters [20,23]. In addition, GPR3 also affects emotional states in stress-related behavioral responses, and GPR3 receptors in the hippocampus play a key role in learning and memory [23], especially in emotion-related memory. GPR3 may also influence behavioral patterns by altering responsiveness to new environmental stimuli [29]. This, in turn, affects the desire to explore and adaptability, and may influence social interaction by modulating the function of neural circuits [23]. Therefore, in-depth study of the role of GPR3 provides a new perspective for the prevention and treatment of related diseases.
4.6.Role of GPR3 in immune response and pathological mechanisms
GPR3 plays a key role in the immune response, and provides a new target for the treatment of autoimmune diseases by modulating the function of macrophages and T cells [46]. Its activation can affect the metabolic reprogramming of macrophages, thereby reducing inflammation in the liver. For example, in a non-alcoholic fatty liver disease (NAFLD) model, the GPR3 agonist DPI inhibits the expression of pro-inflammatory factors through the Gs/cAMP/PKA signaling cascade and activates the β-arrestin2-PKM2 complex. DPI also promotes the expression of glycolytic genes, protects mice from liver lesions and obesity-associated inflammation induced by high-fat diets, and inhibits the release of inflammatory factors. In addition, the interaction of GPR3 with NLRP3 inflammasomes can modulate the immune response. Specifically, GPR3 and other GPCRs inhibit the activation of NLRP3 by recognizing metabolites, thereby reducing IL-1β secretion and inflammation [16,56]. In T cell-mediated autoimmune diseases, GPR3 exhibits a protective effect; it is expressed in both macrophages and T cells and plays an important role in regulating inflammatory factors. GPR3-mediated NR4A2 induction is partially regulated by PKA and MAPK signaling pathways, which together inhibit the spontaneous activation of T cells [46]. In experimental autoimmune uveitis models, mice with GPR3 deletion were more sensitive to pathological conditions and exhibited increased T cell proliferation and production of inflammatory cytokines, suggesting that GPR3 can inhibit T cell overactivation, thereby reducing inflammation [57] (Figures 1 and 2). Furthermore, dynamic changes in GPR3 expression in T lymphocytes are influenced by NR4A2 transcriptional activation [46], which may regulate T cell migration and activation. In the context of chronic inflammation, the regulatory role of GPR3 is particularly important; its absence may lead to increased production of pro-inflammatory cytokines in macrophages [6,48,49], which is closely related to the pathogenesis of various chronic inflammation-related diseases.
5.Role of GPR3 in the reproductive system
5.1.Interaction between GPR3 and reproductive hormones: Regulatory roles of GPR3 in estrogen and progesterone signaling
GPR3, which interacts with reproductive hormones, is an important receptor that regulates ovarian function by integrating FSH, LH, and estrogen signals through the cAMP signaling mechanism to promote follicular development and oocyte maturation [37]. FSH regulates GPR3 and Protein Kinase A I (PKAI) activity through gap junction communication (GJC), inhibiting meiotic resumption in the early stage and promoting maturation in the later stage [58]. In this process, the upregulation of GPR3 depends on cAMP levels, rather than PKA activation. GPR3 contributes to FSH in promoting granulosa cell proliferation and estrogen secretion by increasing cAMP levels [59]. Before ovulation, the expression level of GPR3 changes with the activation of luteinizing hormone (LH). LH activates the NPR2 receptor in granulosa cells to produce cGMP, which is transferred through gap junctions into oocytes, where it inhibits PDE3A activity. This process maintains cAMP levels and participates in the regulation of follicle maturation and ovulation [29,60]. Estrogen may further affect follicle development and corpus luteum function by regulating GPR3 expression and its downstream signaling pathways [61]. In-depth study of the interaction between GPR3 and various reproductive hormones provides new perspectives and strategies for reproductive health.
5.2.Role of GPR3 in mammalian oocyte meiotic arrest and follicle development
GPR3 plays a crucial regulatory role in the arrest of oocyte meiosis [62], and its high expression in multiple tissues suggests that it is key to the arrest and maturation of oocyte meiosis in the early stages [11,15]. When oocytes synthesize CDK1 and become meiotically competent, GPR3, as a continuously active Gs-coupled receptor, controls meiotic arrest by maintaining high levels of cAMP within the oocyte [24]. GPR3 maintains oocyte arrest at prophase I by activating the cAMP/PKA signaling pathway, a process crucial for the regulation of meiosis (Figure 3). If GPR3 is missing or its activity is inhibited, it can lead to early oocyte senescence, which in turn affects reproductive function and egg quality [28,39,42]. The G protein-coupled receptor GPR3 in oocytes can affect the function of surrounding granulosa cells through sustained activation of Gs to produce cAMP required for meiotic arrest [15,39]. It may also influence growth factors and cytokines related to follicle development, thereby regulating the growth and differentiation of these cells and affecting the overall development of follicles [17,63]. GPR3 affects meiosis together with other members of the GPCR family. The downregulation of both GPR3 and GPR12 restores meiosis in mouse and rat oocytes, while GPR12 gene ablation alone does not cause meiotic recovery. This suggests that GPR3 is the main receptor, or that GPR3 has a compensatory role in the absence of GPR12 [30,64]. The maintenance of prophase arrest depends on the Gs activity of GPR3 in oocytes, while follicular cell signaling acts through mechanisms other than increasing GPR3 activity [39]. Studies have demonstrated possible molecular mechanisms by which overexpression of GPR3 promotes apoptosis in porcine follicular granulosa cells [65], and silencing the GPR3 gene inhibits apoptosis in these cells [66], both of which may be related to the regulation of Bcl-2 and Bax expression. However, the study did not demonstrate whether the regulatory effect of GPR3 on porcine granulosa cells affects the quality of oocyte maturation, ovulation rate, or other indicators closely related to the reproductive performance of pigs.These results suggest that abnormal expression of GPR3 may affect female fertility. Studying the function of GPR3 modulation provides new ideas for the treatment of infertility and may offer novel therapeutic options for reproductive problems such as infertility.
7.The role of GPR3 in other disease models is through G protein-coupled receptors
7.1.GPR3 and pain
In some inflammatory models, GPR3 enhances pain sensation by modulating microglial activation and release of inflammatory mediators [35]. After nerve injury, GPR3 knockout (GPR3-/-) mice exhibited higher sensitivity to pain stimuli. For example, in key areas of pain conduction, such as the dorsal horns of the spinal cord, high expression of GPR3 may regulate neuronal excitability through the cAMP-PKA signaling pathway, which in turn affects pain transmission and synergizes with opioid receptors (such as μ-opioid receptors), possibly involving downstream signals cross-regulation [29]. GPR3 knockout (GPR3-/-) mice exhibit significant hypersensitivity to thermal stimuli, both noxious and non-noxious, while there is no significant change in mechanical allodynia [29], suggesting that GPR3 is specifically involved in thermal pain sensation regulation by regulating the activity of cAMP-dependent ion channels such as TRPV1[34]. In addition, the infarct area significantly increased in ischemic brain injury models of GPR3 knockout mice, indicating that GPR3′s neuroprotective function may indirectly affect the pathological process of chronic pain [13,34].
7.2.GPR3 and cardiovascular disease
GPR3 is also expressed in the cardiovascular system, and its functions may include roles in cardiac remodeling, angiogenesis, and inflammation during cardiac pathological conditions. Modulation of GPR3 may influence the adaptive response of the heart [32]. Bresnick et al. demonstrated that GPR3 is able to activate the Gs pathway, increasing the activity of cAMP response elements (CRE) [40]. In addition, Uhlenbrock et al. reported that GPR3 can also activate Gi proteins, which typically inhibit adenylate cyclase activity [8,10,78]. Activation of these signaling pathways may contribute to the development of heart failure. GPR3 and other members of the GPCR family are involved in ischemic events, mainly in the brain and heart, where they may play roles in the pathological or protective signaling responses [5]. These receptors represent potential therapeutic targets for the treatment of ischemia/reperfusion (I/R) injury in vivo.
7.3.GPR3 and tumors
GPR3 has high constitutive activity, leading to increased intracellular cAMP levels by coupling with Gs alpha subunits, which in turn activates downstream effector molecules such as PKA [10]. This pathway is involved in malignant progression in various tumors by promoting cell proliferation, inhibiting apoptosis, and enhancing invasive capacities. Studies have found that GPR3 is highly expressed in tumor tissues, such as cutaneous melanoma, paraganglioma, sarcoma, esophageal cancer, adrenal cortical carcinoma, uterine carcinosarcoma, glioblastoma multiforme, ovarian cancer, and other tumors (Figure 2). High expression of GPR3 is associated with poor prognosis in melanoma. GPR3 deletion inhibits the malignant phenotype of melanoma cells. Moreover, GPR3 regulates NER damage repair in melanoma through XPC and promotes tumor behaviors such as growth, migration, and invasion [79,80]. In oral squamous cell carcinoma (OSCC) and pancreatic ductal adenocarcinoma (PDAC), GPR3 is aberrantly expressed and binds to the chemokine CCL18 to form the CCL18-NIR1/GPR3 axis. This axis activates the JAK2/STAT3 signaling pathway, promotes proliferation, migration, and epithelial-mesenchymal transition (EMT) of OSCC cells, and correlates with TNM stage of patients [81,82]. Phosphorylation of STAT3 further upregulates oncogenes (e.g.Bcl-2, Cyclin D1) and inhibits the expression of pro-apoptotic factors.
8.Research progress on the signaling pathways involving GPR3, its ligands, and their potential applications in diseases
8.1.Functional characteristics and disease applications of GPR3 receptors and agonists
8.1.1.Mechanism of action of major agonists
Agonists of GPR3 include endogenous ligands and synthetic small molecules (Table 2). OA, one of its endogenous ligands, activates the Gs signaling pathway by binding to GPR3; this triggers the cAMP/PKA signaling pathway and promotes thermogenesis and metabolic regulation in adipocytes [8,10]. GPR3 also exhibits high basal activity through a self-activation mechanism, maintaining high activity even in the absence of ligands [4]. Cryo-EM technology revealed the complex structure of GPR3 and OA, and verified the high constitutive activity of GPR3. DPI (Specific pharmacokinetic data are detailed in Supplement 1.)was the first GPR3 agonist to be discovered. It promotes GPR3 activation through multiple signaling pathways, providing a tool for studying its downstream signal transduction and pharmacological functions. DPI-activated GPR3 affects macrophage metabolic reprogramming, underlying molecular mechanisms, and physiological roles in foodborne fat-induced obesity and liver morbidity. DPI induces the formation of a GPR3-β-arrestin2-GAPDH-PKM2 supercomplex that stimulates glycolysis. The rapid increase in supercomplex formation significantly enhances enzyme activity and induces macrophages to shift from oxidative phosphorylation (OxPhos) to glycolytic metabolism. GPR3 activation also stimulates the translocation of PKM2 from the cytoplasm to the nucleus, leading to reactivation of c-Myc and transcription of glycolytic genes, which induces a long-term increase in glycolytic activity (Figure 2). DPI inhibits foodborne fat-induced obesity and liver disease in mice by stimulating the expression of PKM2 in Kupffer cells (KCs) to promote glycolysis and inhibit inflammation. DPI can also stimulate glycolysis and inhibit inflammation in KCs in patients with NAFLD [56]. Sphingosine 1-phosphate (S1P) (Specific pharmacokinetic data are detailed in Supplement 1.), a lipid signaling molecule, activates downstream signaling pathways by binding to GPR3, elevating intracellular cAMP and exerting biological effects [8,14,83]. Additionally, S1P can activate multiple signaling pathways through different GPCRs [84], participate in the regulation of various biological and pathological processes [85,86], and act as a second messenger to mobilize intracellular Ca2+, further activating related signaling pathways [87]. S1P stimulates glycolysis in macrophages and is involved in metabolic reprogramming [56]. The structure of the allosteric sodium-binding cavity of the GPR3 receptor plays a role in agonist binding [3]. Upon ligand activation, GPR3 couples to G proteins (especially Gs proteins), increasing intracellular cAMP levels and affecting cell function, which in turn participates in neuroprotection and regeneration processes [20]. Activation of GPR3 leads to activation of PKA, affecting downstream signaling [10]. GPR3 can also participate in neuronal axon growth and polarity formation through PI3K-mediated signaling pathways, activate downstream CRMP2 dephosphorylation [6,34], and activate ERK signaling pathways (Figure 3).
| Type | Name | Key Features/Mechanism | Associated Pathways/Effects | Disease Models/Notes | Pharmacokinetics |
|---|---|---|---|---|---|
| Endogenous Ligand/Agonist | Oleic Acid (OA) | Binds ligand pocket, stabilizes Gs coupling | cAMP/PKA ↑, Thermogenesis ↑ | AD, Metabolism (Obesity) | t1/2:2–4 min in blood; a few weeks or a few months in organized triglycerides and membrane phospholipids; F: 95% in take orally;CL:he clearance rate of free oleic acid in plasma is extremely high, primarily determined by the uptake and oxidation rates in tissues (especially muscles, heart, and liver). |
| Synthetic Agonist | Diphenyleneiodonium (DPI) | First discovered synthetic agonist | Promotes GPR3-β-arrestin2-PKM2 complex, Glycolysis ↑ | NAFLD, Obesity, Immune metabolism | t1/2: Due to its rapid tissue binding, initial plasma clearance is expected to be rapid;CL: Cannot be calculated using conventional drug models; F:Administered non-orally, with unknown oral bioavailability |
| Lipid Agonist | Sphingosine-1-phosphate (S1P) | Binds GPR3, elevates cAMP | cAMP ↑, Ca²⁺ signaling | Neuroprotection, Inflammation | t1/2: elatively stable in blood (due to carrier protection), with a half-life of several hours; in tissue cells, the half-life is only a few minutes due to the high activity of lytic enzymes;CL:The clearance of S1P is extremely rapid, primarily through irreversible enzymatic degradation within cells rather than organ excretion. F:The concentration of S1P in human plasma is approximately 0.2–0.9 μM, with a rapidly renewing systemic S1P pool. Approximately 60–70% of plasma S1P is bound to ApoM on HDL. |
| Inverse Agonist/Inhibitor | AF64394 | Binds TM3-TM5-TM6 interface, allosteric inhibitor | cAMP accumulation ↓, Gs coupling ↓ | AD models, reduces Aβ | No literature has explicitly reported the specific half-life, oral bioavailability, or systemic clearance of CBD in humans. |
| Inverse Agonist/Inhibitor | Cannabidiol (CBD) | Reduces basal cAMP, β-arrestin2 recruitment ↓ | Anti-neuroinflammation, Aβ production ↓ | AD, PD, Anxiety models | No literature has explicitly reported the specific half-life, oral bioavailability, or systemic clearance of CBD in humans. |
8.1.2.Role of GPR3 agonists in disease
In a mouse model of AD, GPR3 agonists may be regulated by γ-secretase activity to reduce the accumulation of Aβ. For example, a GPR3 agonist may activate the G protein signaling pathway, thereby inhibiting the production of Aβ [32]. In addition, GPR3 agonists may improve the pathological features of AD by modulating the activity of astrocytes and microglia and reducing the inflammatory response [20]. Beyond neurodegenerative diseases, GPR3 agonists also show potential in metabolic disorders. For example, activation of GPR3 can promote thermogenesis in brown adipocytes, thereby reducing the occurrence of obesity and fatty liver. In a mouse model of obesity induced by a high-fat diet, GPR3 overexpression significantly reduced body weight and adipose tissue weight [88]. DPI upregulates the expression of genes involved in glycolysis and lipid metabolism in macrophages, thereby improving metabolic diseases [56,89]. In the immune system, GPR3 agonists have also been found to enhance T cell activity, suggesting an important role in the immune response [46]. In addition, GPR3 agonists may affect nervous system function by regulating the release of neurotransmitters [35], providing a new direction for the treatment of neurodegenerative diseases.
8.2.Functional characteristics of GPR3 receptor inhibitors and their application in diseases
8.2.1.Mechanism of action of major inhibitors
AF64394 (Specific pharmacokinetic data are detailed in Supplement 1.)is the first small molecule inhibitor of GPR3 receptor function and acts as a selective inverse agonist (Table 2). AF64394 and its derivatives bind to the GPR3 dimerization interface (the hydrophobic groove formed by TM3-TM5-TM6) and inhibit receptor activity through allosteric effects [90]. They reduce cAMP accumulation, inhibit coupling of the Gs protein, and modulate downstream signaling pathways. Studies have shown that AF64394 can effectively block GPR3 signal transduction, thereby influencing related disease models [90,91]. Furthermore, inhibitors targeting the cAMP signaling pathway may be effective in GPR3-targeted drug interventions [1,75,92]. Inhibition of GPR3 activity is thought to improve amyloid pathology in mouse models of AD, indicating the potential therapeutic value of GPR3 in neurodegenerative diseases [20]. This suggests that the therapeutic effects in treating AD and Parkinson’s disease may involve modulation of GPR3 and GPR6 receptors [91]. G-protein-coupled receptors, including GPR3, are promising new targets for the treatment of major central nervous system (CNS) diseases [53]. In addition, CBD (Specific pharmacokinetic data are detailed in Supplement 1.) has been identified as an inverse agonist of GPR3; it reduces basal cAMP levels and GPR3-mediated β-arrestin2 recruitment, thereby decreasing Aβ production. CBD also inhibits GPR3 and GPR6[34], exhibiting potential neuroprotective effects such as anti-neuroinflammation. These studies provide an important chemical framework for the development of drugs targeting GPR3.
8.2.2.The application of inhibitors in disease models is a crucial area of study
GPR3 inhibitors play an important role in a variety of diseases. For example, in AD models, GPR3 promotes γ-secretase activity and Aβ production. GPR3 inhibitors reduce the formation of β-arrestin2/γ-secretase complexes through allosteric regulation, significantly reducing amyloid plaque burden and improving cognitive function [20,40]. These findings provide strong evidence for GPR3 as a therapeutic target for AD. In metabolic regulation, late-onset obesity occurs in GPR3-knockout mice. GPR3 inhibitors may reduce thermogenesis by blocking the cAMP-PKA pathway. Additionally, GPR3 expression is upregulated during cold stimulation or high-fat diets, and GPR3 inhibitors can intervene in lipolysis-driven thermogenesis [9,18]. In pain management, GPR3 promotes neuropathic pain, and GPR3 inhibitors may alleviate pain through the cAMP/PKA-RhoA pathway [5]. Furthermore, the use of GPR3 inhibitors in tumor models has shown potential to inhibit tumor cell proliferation, indicating their potential value in tumor treatment [42]. Further studies on GPR3 agonists and inhibitors to reveal their mechanisms of action in different physiological and pathological states will help develop new therapeutic strategies.
8.3.Research progress on GPR3 as a therapeutic target
Specific types of GPCRs, such as serotonin (5-HT) receptors, norepinephrine receptors, and dopamine receptors, play an important role in regulating mood, behavior, and cognitive function [34,93–96]. GPR3 affects the release of dopamine, norepinephrine, and serotonin by modulating signaling pathways such as cAMP, which may influence the pathogenesis of mental illness [16,97]. Studies have shown that GPR3 interacts with multiple neural circuits, including associations with endogenous endorphins, providing new insights into mood regulation [29]. In models of depression, GPR3 expression is elevated, and downregulation of GPR3 function can improve depression-like behavior [54]. Additionally, GPR3 activity is abnormal in patients with anxiety disorders, and inhibition of GPR3 expression can effectively reduce anxiety-like behavior [20]. These findings suggest that GPR3 may serve as a therapeutic target for specific psychiatric disorders such as depression and anxiety disorders. GPCR activity in the central nervous system (CNS) is regulated by major neuropharmacological drugs [29]. Molecular dynamics (MD) simulations of 45 ns duration have been used to evaluate the compound properties of potential agonists, and the optimized structures are of great significance for the virtual screening of potential ligands [8,34,98]. However, significant drug development challenges remain. These include achieving tissue specificity to avoid side effects resulting from target engagement and managing potential receptor desensitization upon prolonged agonist exposure. Additionally, minimizing off-target effects of compounds is particularly challenging due to the structural similarities among Class A GPCRs.The expression of GPR3 in brain regions such as the hippocampus may improve cognitive function in patients with cognitive impairment disorders related to learning and memory by affecting GPR3 activity and increasing cAMP levels [20]. Furthermore, reducing GPR3 activity may provide a theoretical basis for decreasing amyloid pathology in AD, representing a potential target for AD drug development [32]. Translating these findings into clinical practice faces several barriers. These include the need for detailed pharmacokinetic (Table 1) and safety profiles of GPR3-targeting compounds, the development of biomarkers to stratify patient populations, and a deeper understanding of GPR3′s pathophysiological roles in humans versus animal models.Current GPR3 drug development remains at the ‘target validation’ stage and has not yet advanced to the ‘drug optimization’ phase. Although tissue specificity, desensitization, and off - target effects have been mentioned, they have not been systematically assessed, and key data are either contradictory or missing. Future research needs to transition from ‘functional genomics’ to ‘chemical genomics’ and from ‘animal behavior’ to ‘human pharmacology’ to enable the transformation of GPR3 from ‘promising’ to ‘druggable’.
9.Conclusions and prospects
GPR3 is an orphan GPCR that plays an important role in a variety of physiological and pathological processes, especially in the nervous system, and is closely related to neurotransmitter regulation, learning and memory, and neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease. Its activation may influence disease progression by regulating neuronal survival and apoptosis, and its inhibition may improve cognitive function and reduce amyloid accumulation. In the reproductive system, GPR3 helps maintain the meiotic arrest of oocytes, which is critical for follicle development; abnormal expression may lead to premature ovarian failure and loss of fertility. In terms of metabolism, the activation of GPR3 promotes adipocyte thermogenesis, affects body weight and metabolic health, and is expected to become a new target for the treatment of obesity and diabetes. In addition, GPR3 has been associated with psychiatric disorders, immune responses, and energy metabolism, demonstrating its diverse roles.Notably, various agonists, such as oleic acid and DPI, as well as inverse agonists like AF64394 and cannabidiol CBD, have demonstrated the potential to ameliorate pathological conditions in specific disease models.
Although progress has been made in the study of GPR3, the results are not entirely consistent, suggesting that its diverse functions in different physiological and pathological states need to be comprehensively considered.Key limitations of current research include conflicting data on its role in AD the lack of highly specific ligands, and a limited understanding of its regulation in vivo. To address these limitations, future research should prioritize: 1) resolving the context-dependent mechanisms and signaling bias (G protein vs. β-arrestin) of GPR3; 2) developing more selective modulators and characterizing their pharmacokinetics and pharmacodynamics; 3) elucidating GPR3′s function in human tissues and patient populations; 4) exploring its role in the tumor microenvironment and its interactions with other GPCRs. GPR3 gene polymorphisms are associated with individual treatment responses, and gene knock-in models can help study the manifestation of gene variants in individuals, providing a basis for personalized medicine. Through multidisciplinary cooperation, it is expected to reach a consensus on the function of GPR3 and open up new directions for drug development. GPR3 has great potential in clinical applications and deserves continued attention and in-depth research, though its path to the clinic will require carefully addressing the identified challenges.
Supplementary Material
Acknowledgements
We would like to thank all for providing comments on this article. Bi-Dan Feng, Ji-Yao Qin, Qian-qiong Qin designed and written this article, Hai-Qing Zhang and Zu-Cai Xu helped with proofreading and revision. All authors read, revised, and approved the final manuscript.
Consent to publish
All authors agreed to the published version of the manuscript.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Transparency statement
The authors affirm that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant,registered) have been explained.
Sources of the images
Figure 1, 3 are all from FigDraw(https://www.figdraw.com/static/index.html#/).
Figure 2 from CorelDRAW.
Data availability statement
No new data has been generated during the study; The quantitative synthesis and critical evaluation of specific literature are detailed in Supplementary File 2.