Recombinant Human Probable G-protein coupled receptor 37 (GPR37)

What is GPR37 and where is it primarily expressed?

GPR37 is an orphan G protein-coupled receptor (oGPCR) with significant expression in the central nervous system (CNS), particularly in the spinal cord and oligodendrocytes. It belongs to the larger family of GPCRs, which constitute approximately 35% of FDA-approved medication targets . While its cellular signaling mechanisms remain incompletely understood, GPR37 has garnered interest due to its potential role in several neurological conditions, including Parkinson's disease, inflammatory processes, pain perception, autism spectrum disorders, and various brain tumors .

The receptor's amino acid sequence shows significant homology to peptide-specific GPCRs, suggesting potential peptidergic ligands, though complete validation of endogenous ligand-receptor pairings remains challenging . Understanding the distribution and expression patterns of GPR37 provides fundamental insights into its physiological roles and potential as a therapeutic target.

How is GPR37 implicated in Parkinson's disease pathophysiology?

GPR37 has a well-documented association with Parkinson's disease (PD) through several mechanisms:

• Presence in Lewy bodies: Misfolded GPR37 is prominently found within Lewy bodies, making it a biomarker for PD pathology .

• Mutations and ER stress: Mutations in GPR37 are implicated in endoplasmic reticulum (ER) stress, leading to loss-of-function effects that exacerbate dopaminergic neuron death by promoting the accumulation and aggregation of misfolded proteins .

• Juvenile Parkinson's disease: Loss-of-function mutations in GPR37 are associated with autosomal recessive juvenile PD, an early-onset form of the disease .

• Biomarker potential: Peptides from the N-terminus-cleaved domain of GPR37 (ecto-GPR37) show increased levels in the cerebrospinal fluid of PD patients but not in Alzheimer's disease patients, suggesting potential utility as a specific biomarker .

• Therapeutic implications: In cell culture models of PD with GPR37 overexpression, indole-3-propionic acid has been shown to prevent β-amyloid aggregation and ER stress, resulting in reduced neuronal cell death .

These findings collectively highlight the multifaceted roles of GPR37 in PD pathogenesis and its potential as both a biomarker and therapeutic target.

What are the current approaches to studying GPR37 structure in the absence of crystal structures?

Due to the lack of high-resolution structures for orphan GPCRs including GPR37, researchers rely on several computational and experimental approaches:

• Homology modeling: This approach leverages the increasing availability of high-quality protein 3D structures, particularly those obtained through cryo-electron microscopy (Cryo-EM), to predict the 3D structure of GPR37 based on sequence similarity with other GPCRs .

• Ligand-sensitive modeling methods: These address the limitation of traditional homology modeling that often neglects ligand information present in experimental structures, though they require time and expertise for manual interventions .

• Multiple template modeling: In cases of low sequence similarity, using multiple templates can enhance accuracy, although careful template selection is necessary to avoid alignment aberrations .

• Model refinement techniques: Methods for model refinement and coordinate adjustments parallel to the native state contribute to better coverage and accuracy. Molecular dynamics simulations play a crucial role in refining generated models .

• Model assessment parameters: Various parameters like DOPE score, TM score, and RMSD value are used for model comparison, with the choice of determinant parameter based on the modeling purpose .

The development of fully automated homology modeling tools capable of handling ligand-related challenges remains a significant goal in the field. These computational approaches provide valuable starting points for understanding receptor-ligand interactions and designing selective modulators for GPR37 .

What are the known or proposed endogenous ligands for GPR37 and how were they identified?

Research into GPR37 ligands has yielded several candidates:

• Head activator (HA): This invertebrate-derived neuropeptide has been shown to activate Ca²⁺ signaling via GPR37 using the Gα16/aequorin assay with an EC₅₀ value of 3.3 nM. Additionally, HA has been reported to modulate NFAT signaling and inhibit Forskolin-mediated cAMP production through GPR37 .

• Osteocalcin (OCN): This bone-derived protein crucial for brain development and neural cognitive function interacts with GPR37 to regulate oligodendrocyte differentiation, myelination, myelin production, and remyelination after demyelinating injury. Dose-response studies revealed that OCN activates GPR37 with an apparent EC₅₀ of 10.2 nM .

The identification of these ligands has been challenging due to difficulties in validating ligand-GPR37 pairings using recombinant GPR37 expressed in HEK293 cells. Recent successful approaches have turned to primary cell cultures for ligand identification . The OCN/GPR37 regulatory axis, in particular, shows promise for treating inflammatory diseases, as OCN treatment has demonstrated protective effects against LPS-induced inflammation (effects absent in GPR37-deficient mice) .

How does GPR37 expression correlate with prognosis in gliomas and other cancers?

GPR37 expression shows significant correlations with prognosis across various cancer types:

Table 1: GPR37 expression correlation with clinicopathological characteristics in glioma patients .

What molecular mechanisms underlie GPR37's role in cancer cell proliferation and invasion?

GPR37 influences cancer cell behavior through several molecular pathways:

• TGF-β1/Smad signaling: Gain- or loss-of-function assays have demonstrated that increased GPR37 expression enhances the activation of TGF-β1, Smad2, and Smad3 phosphorylation, leading to improved proliferation, migration, and invasion of carcinoma cells in vitro .

• PI3K-Akt signaling pathway: Transient knockdown of GPR37 using siRNA in HuH7 cells has shown a significant decrease in hepatoma cell apoptosis by activating the PI3K-Akt signaling pathway. AKT plays a crucial role in promoting cell survival and growth .

• Cell cycle regulation: The upregulation of GPR37 in human glioma U251 cells leads to increased proliferation, a decrease in G1/G0 phase cells, an increase in S and G2 phase cells, and enhanced phosphorylation of p-AKT (Ser473), suggesting activation of signaling pathways associated with cell survival and proliferation .

• REG4-GPR37 complex: GPR37 is found in the same complex as REG4, which mediates signal transduction and promotes peritoneal metastasis of gastric cancer cells. High expression of REG4 is associated with advanced stage and poor survival prognosis in gastric cancer patients .

• Cell adhesion modulation: In multiple myeloma cells, GPR37 is implicated in regulating cell proliferation through the modulation of cell adhesion ability and AKT and ERK activity .

These diverse mechanisms highlight GPR37's multifaceted role in cancer biology and suggest that targeting this receptor could be a promising approach for developing novel treatments for various types of cancers and their metastases .

What techniques are recommended for analyzing GPR37 expression in patient samples?

Researchers employ multiple complementary techniques to analyze GPR37 expression:

These methodologies provide complementary information about GPR37 expression and its clinical relevance, enabling researchers to comprehensively assess this receptor's role in various pathological contexts.

What are the challenges in establishing reliable in vitro models for studying GPR37 function?

Several challenges exist in developing reliable in vitro models for GPR37 research:

• Receptor misfolding: GPR37 has a tendency to misfold when overexpressed in heterologous cell systems, which can lead to aggregation and ER stress, potentially masking its natural signaling properties .

• Ligand validation difficulties: There are challenges in validating ligand-GPR37 pairings using recombinant GPR37 expressed in common cell lines like HEK293 cells, necessitating the use of primary cell cultures for successful ligand identification .

• Lack of structural information: The absence of high-resolution structures for GPR37 hinders structure-based drug design and understanding of ligand-receptor interactions .

• Appropriate cell models: Selecting cellular models that maintain proper receptor folding, trafficking, and signaling is crucial. Primary neural cultures may better recapitulate the native environment of GPR37 compared to heterologous expression systems .

• Signaling pathway complexity: The exact signaling pathways activated by GPR37 remain incompletely characterized, making it difficult to establish appropriate readouts for receptor activity .

To overcome these challenges, researchers have turned to:

• Using primary cell cultures instead of recombinant systems for ligand identification

• Developing improved homology models to guide experimental design

• Employing multiple complementary assay systems to validate receptor function

• Investigating cell-specific expression and localization using advanced techniques like single-cell RNA sequencing and immunohistochemistry

These approaches help provide more physiologically relevant insights into GPR37 function while minimizing artifacts associated with heterologous expression systems.

What are the most promising therapeutic approaches targeting GPR37?

Based on current research, several therapeutic approaches targeting GPR37 show promise:

• Ligand-based approaches:

  • Developing small molecule agonists, antagonists, and allosteric modulators that selectively target GPR37 .

  • Utilizing the osteocalcin (OCN)/GPR37 regulatory axis for treating inflammatory diseases, based on OCN's protective effects against LPS-induced inflammation .

  • Exploring indole-3-propionic acid (IPA) as a therapeutic agent, as it has shown effectiveness in preventing β-amyloid aggregation and ER stress in PD models with GPR37 overexpression .

• Prevention of protein aggregation:

  • Developing compounds that inhibit GPR37 aggregation, similar to how dextromethorphan mitigates buprenorphine-induced GPR37 accumulation and associated proapoptotic ER stress responses .

• Cancer therapeutics:

  • Targeting GPR37 in various cancers where it promotes proliferation, migration, and invasion, particularly through modulation of TGF-β1/Smad and PI3K-Akt signaling pathways .

  • Disrupting the REG4-GPR37 complex that promotes peritoneal metastasis in gastric cancer .

• Remyelination strategies:

  • Leveraging GPR37's role in oligodendrocyte differentiation, myelination, and remyelination to develop treatments for demyelinating conditions .

Future therapeutic development will benefit from high-throughput screening and structure-based drug design approaches to identify novel compounds that selectively modulate GPR37 signaling. Such ligands could serve as chemical leads for further optimization and valuable pharmacological tools for elucidating receptor functions, potentially leading to novel therapeutics for neurological disorders and cancer .

What are the critical knowledge gaps that need to be addressed to advance GPR37 research?

Several critical knowledge gaps must be addressed to advance GPR37 research:

• Physiological functions and endogenous ligands: The physiological functions of GPR37 and a complete understanding of its endogenous ligands remain largely unknown. Validating and characterizing proposed ligand-receptor interactions and elucidating downstream signaling pathways is crucial .

• Cell-specific expression and localization: Further investigation into the cell-specific expression and localization of GPR37 in different CNS cell types is essential for understanding its functional implications. Advanced techniques such as single-cell RNA sequencing and immunohistochemistry can provide valuable insights into expression patterns and subcellular localization .

• Structural characterization: Solving crystal structures and elucidating the molecular interactions of GPR37 would significantly advance our understanding. This includes characterizing consequences of interactions with PDZ domain-containing proteins and other binding partners to reveal downstream signaling cascades .

• Appropriate preclinical models: Development of preclinical models that accurately recapitulate the pathophysiological aspects of human diseases associated with GPR37 dysfunction is crucial. Genetically modified animal models, such as conditional knockout or transgenic mice targeting GPR37, can serve as valuable tools for in vivo studies .

• Pharmacological tools: There is a need for potent and specific ligands, including small molecule agonists, antagonists, and allosteric modulators. These would serve as valuable tools for elucidating receptor functions and could potentially lead to therapeutic development .

Addressing these knowledge gaps will require multidisciplinary approaches combining structural biology, molecular pharmacology, cell biology, and translational research to fully unravel the therapeutic potential of GPR37.