GPR37 modulates progenitor cell dynamics in a mouse model of ischemic stroke

Orphan G Protein-Coupled Receptor GPR37 as an Emerging Therapeutic Target

G protein-coupled receptors (GPCRs) are successful druggable targets, making up around 35% of all FDA-approved medications. However, a large number of receptors remain orphaned, with no known endogenous ligand, representing a challenging but untapped area to discover new therapeutic targets. Among orphan GPCRs (oGPCRs) of interest, G protein-coupled receptor 37 (GPR37) is highly expressed in the central nervous system (CNS), particularly in the spinal cord and oligodendrocytes. While its cellular signaling mechanisms and endogenous receptor ligands remain elusive, GPR37 has been implicated in several important neurological conditions, including Parkinson's disease (PD), inflammation, pain, autism, and brain tumors. GPR37 structure, signaling, emerging physiology, and pharmacology are reviewed while integrating a discussion on potential therapeutic indications and opportunities.

Chlorogenic acid enhances alveolar macrophages phagocytosis in acute respiratory distress syndrome by activating G protein-coupled receptor 37 (GPR 37)

BACKGROUND

Impaired alveolar macrophages phagocytosis can contribute to pathogenesis of acute respiratory distress syndrome (ARDS) and negatively impacts clinical outcomes. Chlorogenic acid (CGA) is a naturally occurring polyphenolic compound with potential anti-inflammatory and antioxidant bioactivities. Studies have shown that CGA plays a protective role in ARDS, however, the precise protective mechanism of CGA against ARDS, is still unclear.

PURPOSE

The aim of this study was to investigate whether CGA enhances alveolar macrophages phagocytosis to attenuate lung injury during ARDS.

METHODS

RAW264.7 cells were stimulated with lipopolysaccharides (100 μg/ml for 24 h) and treated with CGA (100, 200, and 400 μM CGA for 1 h) to measure pro-inflammatory cytokine levels, GPR37 expression and macrophages phagocytosis. Mouse models of ARDS induced by cecal ligation and perforation (CLP) surgery were treated with CGA (100 or 200 mg/kg) to investigate lung inflammatory injury and alveolar macrophages phagocytosis. Computational modeling was performed to examine potential binding sites of G protein-coupled receptor 37 (GPR37) with CGA, and the results were validated by interfering with the binding sites.

RESULT

In vitro, CGA notably ameliorated inflammatory response and increased phagocytosis in lipopolysaccharides-induced RAW264.7 cells. In vivo, CGA administration significantly alleviated lung inflammatory injury, decreased the bacteria load in the lung, promoted alveolar macrophages phagocytosis and improved the survival rate in mice with CLP-induced ARDS. Moreover, CGA markedly upregulated the expression of GPR37 in vivo and in vitro. However, the protective effect of CGA against ARDS were reversed after silencing the expression of GPR37.

CONCLUSION

CGA has a protective effect against ARDS and may enhance alveolar macrophages phagocytosis and attenuate lung inflammatory injury by upregulating GPR37 expression.

Inflammation and Infection in Pain and the Role of GPR37

Inflammation is known to cause pain, and pain is of one of the cardinal signs of inflammation. Mounting evidence suggests that acute inflammation also resolves pain through specialized pro-resolving mediators (SPMs) and macrophage signaling. GPR37 is expressed by neurons and oligodendrocytes in the brain and has been implicated in multiple disorders, such as demyelination, Parkinson's disease, stroke, and cancer. Recent studies have demonstrated that GPR37 is expressed by macrophages and confers protection against infection by bacteria and parasites. Furthermore, GPR37 promotes the resolution of inflammatory pain and infection-induced pain, as the duration of pain after tissue injury and infection is prolonged in mice lacking Gpr37. Mechanistically, activation of GPR37 enhances macrophage phagocytosis, and Gpr37-deficient macrophages exhibit dysregulations of pro-inflammatory and anti-inflammatory cytokines, switching from M2- to M1-like phenotypes. We also discuss novel ligands of GPR37, including neuroprotectin D1 (NPD1), a SPM derived from docosahexaenoic acid (DHA), and bone-derived hormone osteocalcin (OCN), which can suppress oligodendrocyte differentiation and myelination. NPD1 stimulates macrophage phagocytosis via GPR37 and exhibits potent analgesic actions in various animal models of inflammatory and neuropathic pain. Targeting GPR37 may lead to novel therapeutics for treating inflammation, infection, pain, and neurological diseases.

Mouse Mutants of Gpr37 and Gpr37l1 Receptor Genes: Disease Modeling Applications

The vertebrate G protein–coupled receptor 37 and G protein–coupled receptor 37-like 1 (GPR37 and GPR37L1) proteins have amino acid sequence homology to endothelin and bombesin-specific receptors. The prosaposin glycoprotein, its derived peptides, and analogues have been reported to interact with and activate both putative receptors. The GPR37 and GPR37L1 genes are highly expressed in human and rodent brains. GPR37 transcripts are most abundant in oligodendrocytes and in the neurons of the substantia nigra and hippocampus, while the GPR37L1 gene is markedly expressed in cerebellar Bergmann glia astrocytes. The human GPR37 protein is a substrate of parkin, and its insoluble form accumulates in brain samples from patients of inherited juvenile Parkinson’s disease. Several Gpr37 and Gpr37l1 mouse mutant strains have been produced and applied to extensive in vivo and ex vivo analyses of respective receptor functions and involvement in brain and other organ pathologies. The genotypic and phenotypic characteristics of the different mouse strains so far published are reported and discussed, and their current and proposed applications to human disease modeling are highlighted.

Emerging Roles for the Orphan GPCRs, GPR37 and GPR37 L1, in Stroke Pathophysiology

Recent studies have shed light on the diverse and complex roles of G-protein coupled receptors (GPCRs) in the pathophysiology of stroke. These receptors constitute a large family of seven transmembrane-spanning proteins that play an intricate role in cellular communication mechanisms which drive both tissue injury and repair following ischemic stroke. Orphan GPCRs represent a unique sub-class of GPCRs for which no natural ligands have been found. Interestingly, the majority of these receptors are expressed within the central nervous system where they represent a largely untapped resource for the treatment of neurological diseases. The focus of this review will thus be on the emerging roles of two brain-expressed orphan GPCRs, GPR37 and GPR37 L1, in regulating various cellular and molecular processes underlying ischemic stroke.