GPR37 protein trafficking to the plasma membrane regulated by prosaposin and GM1 gangliosides promotes cell viability

GPR37 and its neuroprotective mechanisms: bridging osteocalcin signaling and brain function

Osteocalcin (OCN) is a hormone secreted by osteoblasts and has attracted widespread attention for its role in regulating brain function. Clinical studies indicate a positive correlation between levels of circulating OCN and cognitive performance. Indeed, lower circulating OCN has been detected in various neurodegenerative diseases (NDs), while OCN supplementation under certain conditions may improve cognitive function. GPR37, a G protein-coupled receptor, has recently been identified as a receptor for OCN. It exhibits distinct expression patterns across various brain regions and cell types, potentially influencing its functional roles within the brain. Research indicates that GPR37 regulates neuronal migration, cell proliferation, differentiation, and myelination. Furthermore, GPR37 has been shown to mitigate inflammation and apoptosis through various mechanisms, exerting neuroprotective effects. However, its regulatory influence on brain function exhibits inconsistency, highlighting a duality in its actions. Therefore, this review thoroughly summarizes the roles and mechanisms of GPR37 in modulating cellular physiological activities and its involvement in immune responses, stress reactions, and neuroprotection. It aims to enhance the understanding of how GPR37 modulates brain function and facilitate the identification of novel therapeutic targets or strategies for related diseases.

Emerging roles of the G-protein-coupled receptor 37 in neurological diseases and pain

Neurological disorders and pain are prevalent clinical issues that severely impact patients' quality of life and daily functioning. With the advancing exploration of these disease mechanisms, G protein-coupled receptor 37 (GPR37) has emerged as a critical protein, garnering widespread attention in the scientific community. As a member of the G protein-coupled receptor family, GPR37 features a seven-transmembrane helix structure and is widely expressed in various brain regions, including the substantia nigra and striatum. In addition to neurons, GPR37 is also detected in immune cells within the nervous system, indicating its potential role in neuron-immune cell interactions. Research has shown that the expression level of GPR37 in neurological disorders can affect neuron survival, cellular signaling, and overall neurological health. Abnormal expression of GPR37 is often associated with disease progression and symptom exacerbation in neurological disorders such as Parkinson's disease and stroke. In the context of pain, GPR37 alleviates pain and inflammatory responses by regulating the phagocytic activity and polarization state of macrophages. This article aims to delve into the mechanistic roles of GPR37 in neurological disorders and pain. Through a comprehensive literature review, we summarize the latest research on GPR37's involvement in neurological diseases and pain, highlighting its critical roles in neural signaling, inflammatory responses, and neuroprotection. This understanding expands the comprehension of GPR37's biological functions and provides new perspectives for improving the clinical outcomes of patients with neurological disorders and pain.

Dissecting the roles of prosaposin as an emerging therapeutic target for tumors and its underlying mechanisms

As a dual-function protein, prosaposin (PSAP) is a lysosome-associated protein that participates in a variety of cellular processes. In the lysosome, PSAP is processed to activate enzymes that degrade lipids. In addition, PSAP proteins located extracellularly are involved in cancer progression, such as proliferation and tumor death suppression signaling. Moreover, under different situations, PSAP exhibits distinct metastasis potentials in tumors. However, comprehensive insight into PSAP in cancer progression has been lacking. Here, we provide a framework of the role of PSAP in cancer and its clinical application in cancer patients, providing a novel perspective on the clinical translation of PSAP.

Role of orphan G-protein coupled receptors in tissue ischemia: A comprehensive review

Ischemic events lead to many diseases and deaths worldwide. Ischemia/reperfusion (I/R) occurs due to reduced blood circulation in tissues followed by blood reflow. Reoxygenation of ischemic tissues is characterized by oxidative stress, inflammation, energy distress, and endoplasmic reticulum stress. There are still no adequate clinical protocols or pharmacological approaches to address the consequences of I/R damage. G protein-coupled receptors (GPCRs) are important therapeutic targets. They compose a large family of seven transmembrane-spanning proteins that are involved in many biological functions. Orphan GPCRs are a large subgroup of these receptors expressed in different organs. In the present review, we summarized the literature regarding the role of orphan GPCRs in I/R in different organs. We focused on the effect of these receptors on modulating cellular and molecular processes underlying ischemia including apoptosis, inflammation, and autophagy. The study showed that GPR3, GPR4, GPR17, GPR30, GPR31, GPR35, GPR37, GPR39, GPR55, GPR65, GPR68, GPR75, GPR81, and GPR91 are involved in ischemic events, mainly in the brain and heart. These receptors offer new possibilities for treating I/R injuries in the body.

Intranasal administration of recombinant prosaposin attenuates neuronal apoptosis through GPR37/PI3K/Akt/ASK1 pathway in MCAO rats

Studies have reported that Prosaposin (PSAP) is neuroprotective in cerebrovascular diseases. We hypothesized that PSAP would reduce infarct volume by attenuating neuronal apoptosis and promoting cell survival through G protein-coupled receptor 37(GPR37)/PI3K/Akt/ASK1 pathway in middle cerebral artery occlusion (MCAO) rats. Two hundred and thirty-five male and eighteen female Sprague-Dawley rats were used. Recombinant human PSAP (rPSAP) was administered intranasally 1 h (h) after reperfusion. PSAP small interfering ribonucleic acid (siRNA), GPR37 siRNA, and PI3K specific inhibitor LY294002 were administered intracerebroventricularly 48 h before MCAO. Infarct volume, neurological score, immunofluorescence staining, Western blot, Fluoro-Jade C (FJC) and TUNEL staining were examined. The expression of endogenous PSAP and GPR37 were increased after MCAO. Intranasal administration of rPSAP reduced brain infarction, neuronal apoptosis, and improved both short- and long-term neurological function. Knockdown of endogenous PSAP aggravated neurological deficits. Treatment with exogenous rPSAP increased PI3K expression, Akt and ASK1 phosphorylation, and Bcl-2 expression; phosphorylated-JNK and Bax levels were reduced along with the number of FJC and TUNEL positive neurons. GPR37 siRNA and LY294002 abolished the anti-apoptotic effect of rPSAP at 24 h after MCAO. In conclusion, rPSAP attenuated neuronal apoptosis and improved neurological function through GPR37/PI3K/Akt/ASK1 pathway after MCAO in rats. Therefore, further exploration of PSAP as a potential treatment option in ischemic stroke is warranted.

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.

GPR37L1 controls maturation and organization of cortical astrocytes during development

Astrocyte maturation is crucial to proper brain development and function. This maturation process includes the ramification of astrocytic morphology and the establishment of astrocytic domains. While this process has been well-studied, the mechanisms by which astrocyte maturation is initiated are not well understood. GPR37L1 is an astrocyte-specific G protein-coupled receptor (GPCR) that is predominantly expressed in mature astrocytes and has been linked to the modulation of seizure susceptibility in both humans and mice. To investigate the role of GPR37L1 in astrocyte biology, RNA-seq analyses were performed on astrocytes immunopanned from P7 Gpr37L1-/- knockout (L1KO) mouse cortex and compared to those from wild-type (WT) mouse cortex. These RNA-seq studies revealed that pathways involved in central nervous system development were altered and that L1KO cortical astrocytes express lower amounts of mature astrocytic genes compared to WT astrocytes. Immunohistochemical studies of astrocytes from L1KO mouse brain revealed that these astrocytes exhibit overall shorter total process length, and are also less complex and spaced further apart from each other in the mouse cortex. This work sheds light on how GPR37L1 regulates cellular processes involved in the control of astrocyte biology and maturation.

Multi-omics analysis unravels dysregulated lysosomal function and lipid metabolism involved in sub-chronic particulate matter-induced pulmonary injury

Particulate matter (PM) is a huge environmental threat and is of major public concern. Oxidative stress and systemic inflammation are known factors that contribute to PM- related damage; however, a systematic understanding of the deleterious pulmonary effects of PM using multi-omics analysis is lacking. In this study, we performed transcriptomic, proteomic, and metabolomic analyses in a mouse model exposed to PM for three months to identify molecular changes in lung tissues. We identified 1690 genes, 326 proteins, and 67 metabolites exhibiting significant differences between PM-challenged and control mice (p < 0.05). Differentially expressed genes and proteins regulated in PM-challenged mice were involved in lipid metabolism and in the immune and inflammatory response processes. Moreover, a comprehensive analysis of transcript, protein, and metabolite datasets revealed that the genes, proteins, and metabolites in the PM-treated group were involved in lysosomal function and lipid metabolism. Specifically, Cathepsin D (Ctsd), Ferritin light chain (Ftl), Lactotransferrin (Ltf), Lipocalin 2 (Lcn2), and Prosaposin (Psap) were major proteins/genes associated with PM-induced pulmonary damage, while two lipid molecules PC (18:1(11Z)/16:0) and PA (16:0/18:1(11Z)) were major metabolites related to PM-induced pulmonary injury. In summary, lipid metabolism might be used as successful precautions and therapeutic targets in PM-induced pulmonary injury to maintain the stability of cellular lysosomal function.

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.

The Effect of Early Life Stress on Emotional Behaviors in GPR37KO Mice

GPR37 is an orphan G-protein-coupled receptor, a substrate of parkin which is linked to Parkinson's disease (PD) and affective disorders. In this study, we sought to address the effects of early life stress (ELS) by employing the paradigm of limited nesting material on emotional behaviors in adult GPR37 knockout (KO) mice. Our results showed that, while there was an adverse effect of ELS on various domains of emotional behaviors in wild type (WT) mice in a sex specific manner (anxiety in females, depression and context-dependent fear memory in males), GPR37KO mice subjected to ELS exhibited less deteriorated emotional behaviors. GPR37KO female mice under ELS conditions displayed reduced anxiety compared to WT mice. This was paralleled by lower plasma corticosterone in GPR37KO females and a lower increase in P-T286-CaMKII by ELS in the amygdala. GPR37KO male mice, under ELS conditions, showed better retention of hippocampal-dependent emotional processing in the passive avoidance behavioral task. GPR37KO male mice showed increased immobility in the forced swim task and increased P-T286-CaMKII in the ventral hippocampus under baseline conditions. Taken together, our data showed overall long-term effects of ELS-deleterious or beneficial depending on the genotype, sex of the mice and the emotional context.

GM1 Is Cytoprotective in GPR37-Expressing Cells and Downregulates Signaling

G-protein-coupled receptors (GPCRs) are commonly pharmacologically modulated due to their ability to translate extracellular events to intracellular changes. Previously, studies have mostly focused on protein–protein interactions, but the focus has now expanded also to protein–lipid connections. GM1, a brain-expressed ganglioside known for neuroprotective effects, and GPR37, an orphan GPCR often reported as a potential drug target for diseases in the central nervous system, have been shown to form a complex. In this study, we looked into the functional effects. Endogenous GM1 was downregulated when stably overexpressing GPR37 in N2a cells (N2a^GPR37-eGFP). However, exogenous GM1 specifically rescued N2a^GPR37-eGFP from toxicity induced by the neurotoxin MPP+. The treatment did not alter transcription levels of GPR37 or the enzyme responsible for GM1 production, both potential mechanisms for the effect. However, GM1 treatment inhibited cAMP-dependent signaling from GPR37, here reported as potentially consecutively active, possibly contributing to the protective effects. We propose an interplay between GPR37 and GM1 as one of the many cytoprotective effects reported for GM1.

Cytosolic GPR37, but not GPR37L1, multimerization and its reversal by Parkin: A live cell imaging study

Biochemical data have shown aggregated G protein-coupled receptor 37 (GPR37) in the cytoplasm and Lewy bodies in Parkinson's disease (PD). Properly folded GPR37 at the plasma membrane appears to be neuroprotective. GPR37, and its homologue GPR37L1, are orphan G protein-coupled receptors and their homo- and hetero-dimers have not been established. We therefore examined GPR37 and GPR37L1 dimerization and extended studies of multimerization of GPR37 to live cells. In this study, we investigated GPR37 and GPR37L1 dimerization and multimerization in live cells using three quantitative imaging methods: Fluorescence Cross-Correlation Spectroscopy, Förster Resonance Energy Transfer, and Fluorescence Lifetime Imaging Microscopy. Our data show that GPR37 and GPR37L1 form homo- and heterodimers in live N2a cells. Importantly, aggregation of GPR37, but not GPR37L1, was identified in the cytoplasm, which could be counteracted by Parkin overexpression. These data provide further evidence that GPR37 participate in cytosolic aggregation processes implicated in PD pathology.

Identification of the GlialCAM interactome: the G protein-coupled receptors GPRC5B and GPR37L1 modulate megalencephalic leukoencephalopathy proteins

Megalencephalic Leukoencephalopathy with subcortical Cysts (MLC) is a type of vacuolating leukodystrophy, which is mainly caused by mutations in MLC1 or GLIALCAM. The two MLC-causing genes encode for membrane proteins of yet unknown function that have been linked to the regulation of different chloride channels such as the ClC-2 and VRAC. To gain insight into the role of MLC proteins, we have determined the brain GlialCAM interacting proteome. The proteome includes different transporters and ion channels known to be involved in the regulation of brain homeostasis, proteins related to adhesion or signaling as several G protein-coupled receptors (GPCRs), including the orphan GPRC5B and the proposed prosaposin receptor GPR37L1. Focusing on these two GPCRs, we could validate that they interact directly with MLC proteins. The inactivation of Gpr37l1 in mice upregulated MLC proteins without altering their localization. Conversely, a reduction of GPRC5B levels in primary astrocytes downregulated MLC proteins, leading to an impaired activation of ClC-2 and VRAC. The interaction between the GPCRs and MLC1 was dynamically regulated upon changes in the osmolarity or potassium concentration. We propose that GlialCAM and MLC1 associate with different integral membrane proteins modulating their functions and acting as a recruitment site for various signaling components as the GPCRs identified here. We hypothesized that the GlialCAM/MLC1 complex is working as an adhesion molecule coupled to a tetraspanin-like molecule performing regulatory effects through direct binding or influencing signal transduction events.

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

The generation of neural stem and progenitor cells following injury is critical for the function of the central nervous system, but the molecular mechanisms modulating this response remain largely unknown. We have previously identified the G protein-coupled receptor 37 (GPR37) as a modulator of ischemic damage in a mouse model of stroke. Here we demonstrate that GPR37 functions as a critical negative regulator of progenitor cell dynamics and gliosis following ischemic injury. In the central nervous system, GPR37 is enriched in mature oligodendrocytes, but following injury we have found that its expression is dramatically increased within a population of Sox2-positive progenitor cells. Moreover, the genetic deletion of GPR37 did not alter the number of mature oligodendrocytes following injury but did markedly increase the number of both progenitor cells and injury-induced Olig2-expressing glia. Alterations in the glial environment were further evidenced by the decreased activation of oligodendrocyte precursor cells. These data reveal that GPR37 regulates the response of progenitor cells to ischemic injury and provides new perspectives into the potential for manipulating endogenous progenitor cells following stroke.

Transcriptomics-Based Phenotypic Screening Supports Drug Discovery in Human Glioblastoma Cells

Simple Summary

Glioblastoma (GBM) remains a particularly challenging cancer, with an aggressive phenotype and few promising treatment options. Future therapy will rely heavily on diagnosing and targeting aggressive GBM cellular phenotypes, both before and after drug treatment, as part of personalized therapy programs. Here, we use a genome-wide drug-induced gene expression (DIGEX) approach to define the cellular drug response phenotypes associated with two clinical drug candidates, the phosphodiesterase 10A inhibitor Mardepodect and the multi-kinase inhibitor Regorafenib. We identify genes encoding specific drug targets, some of which we validate as effective antiproliferative agents and combination therapies in human GBM cell models, including HMGCoA reductase (HMGCR), salt-inducible kinase 1 (SIK1), bradykinin receptor subtype B2 (BDKRB2), and Janus kinase isoform 2 (JAK2). Individual, personalized treatments will be essential if we are to address and overcome the pharmacological plasticity that GBM exhibits, and DIGEX will play a central role in validating future drugs, diagnostics, and possibly vaccine candidates for this challenging cancer.

Abstract

We have used three established human glioblastoma (GBM) cell lines—U87MG, A172, and T98G—as cellular systems to examine the plasticity of the drug-induced GBM cell phenotype, focusing on two clinical drugs, the phosphodiesterase PDE10A inhibitor Mardepodect and the multi-kinase inhibitor Regorafenib, using genome-wide drug-induced gene expression (DIGEX) to examine the drug response. Both drugs upregulate genes encoding specific growth factors, transcription factors, cellular signaling molecules, and cell surface proteins, while downregulating a broad range of targetable cell cycle and apoptosis-associated genes. A few upregulated genes encode therapeutic targets already addressed by FDA approved drugs, but the majority encode targets for which there are no approved drugs. Amongst the latter, we identify many novel druggable targets that could qualify for chemistry-led drug discovery campaigns. We also observe several highly upregulated transmembrane proteins suitable for combined drug, immunotherapy, and RNA vaccine approaches. DIGEX is a powerful way of visualizing the complex drug response networks emerging during GBM drug treatment, defining a phenotypic landscape which offers many new diagnostic and therapeutic opportunities. Nevertheless, the extreme heterogeneity we observe within drug-treated cells using this technique suggests that effective pan-GBM drug treatment will remain a significant challenge for many years to come.

Quantitative Proteomics Reveal an Altered Pattern of Protein Expression in Brain Tissue from Mice Lacking GPR37 and GPR37L1

GPR37 and GPR37L1 are glia-enriched G protein-coupled receptors that have been implicated in several neurological and neurodegenerative diseases. To gain insight into the potential molecular mechanisms by which GPR37 and GPR37L1 regulate cellular physiology, proteomic analyses of whole mouse brain tissue from wild-type (WT) versus GPR37/GPR37L1 double knockout (DKO) mice were performed in order to identify proteins regulated by the absence versus presence of these receptors (data are available via ProteomeXchange with identifier PXD015202). These analyses revealed a number of proteins that were significantly increased or decreased by the absence of GPR37 and GPR37L1. One of the most decreased proteins in the DKO versus WT brain tissue was S100A5, a calcium-binding protein, and the reduction of S100A5 expression in KO brain tissue was validated via Western blot. Coexpression of S100A5 with either GPR37 or GPR37L1 in HEK293T cells did not result in any change in S100A5 expression but did robustly increase secretion of S100A5. To dissect the mechanism by which S100A5 secretion was enhanced, cells coexpressing S100A5 with the receptors were treated with different pharmacological reagents. These studies revealed that calcium is essential for the secretion of S100A5 downstream of GPR37 and GPR37L1 signaling, as treatment with BAPTA-AM, an intracellular Ca²⁺ chelator, reduced S100A5 secretion from transfected HEK293T cells. Collectively, these findings provide a panoramic view of proteomic changes resulting from loss of GPR37 and GPR37L1 and also impart mechanistic insight into the regulation of S100A5 by these receptors, thereby shedding light on the functions of GPR37 and GPR37L1 in brain tissue.

Gpr37l1/prosaposin receptor regulates Ptch1 trafficking, Shh production, and cell proliferation in cerebellar primary astrocytes

Mammalian cerebellar astrocytes critically regulate the differentiation and maturation of neuronal Purkinje cells and granule precursors. The G protein-coupled receptor 37-like 1 (Gpr37l1) is expressed by Bergmann astrocytes and interacts with patched 1 (Ptch1) at peri-ciliary membranes. Cerebellar primary astrocyte cultures from wild-type and Gpr37l1 null mutant mouse pups were established and studied. Primary cilia were produced by cultures of both genotypes, as well as Ptch1 and smoothened (Smo) components of the sonic hedgehog (Shh) mitogenic pathway. Compared to wild-type cells, Gpr37l1-/- astrocytes displayed striking increases in proliferative activity, Ptch1 protein expression and internalization, intracellular cholesterol content, ciliary localization of Smo, as well as a marked production of active Shh. Similar effects were reproduced by treating wild-type astrocytes with a putative prosaptide ligand of Gpr37l1. These findings indicate that Gpr37l1-Ptch1 interactions specifically regulate Ptch1 internalization and trafficking, with consequent stimulation of Shh production and activation of proliferative signaling.

Pro-cathepsin D, Prosaposin, and Progranulin: Lysosomal Networks in Parkinsonism

Mutations in GBA1, the gene encoding the lysosomal hydrolase glucocerebrosidase (GCase), are a risk factor for parkinsonism. Pursuing the potential mechanisms underlying this risk in aging neurons, we propose a new network uniting three major lysosomal proteins: (i) cathepsin D (CTSD), which plays a major role in α-synuclein (SNCA) degradation and prosaposin (PSAP) cleavage; (ii) PSAP, essential for GCase activation and progranulin (PGRN) transport; and (iii) PGRN, impacting lysosomal biogenesis, PSAP trafficking, and CTSD maturation. We hypothesize that alterations to this network and associated receptors modify lysosomal function and subsequently impact both SNCA degradation and GCase activity. By exploring the interactions between this protein trio and each of their respective transporters and receptors, we may identify secondary risk factors that provide insight into the relationship between these lysosomal proteins, GCase, and SNCA, and reveal novel therapeutic targets.

Deficits in Motor Performance, Neurotransmitters and Synaptic Plasticity in Elderly and Experimental Parkinsonian Mice Lacking GPR37

Parkinson’s disease (PD) etiology is attributed to aging and the progressive neurodegeneration of dopamine (DA) neurons of substantia nigra pars compacta (SNc). GPR37 is an orphan G-protein Coupled Receptor (GPCR) that is linked to the juvenile form of PD. In addition, misfolded GPR37 has been found in Lewy bodies. However, properly folded GPR37 found at the cell membrane appears to exert neuroprotection. In the present study we investigated the role of GPR37 in motor deficits due to aging or toxin-induced experimental parkinsonism. Elderly GPR37 knock out (KO) mice displayed hypolocomotion and worse fine movement performance compared to their WT counterparts. Striatal slice electrophysiology reveiled that GPR37 KO mice show profound decrease in long term potentiation (LTP) formation which is accompanied by an alteration in glutamate receptor subunit content. GPR37 KO animals exposed to intrastriatal 6-hydroxydopamine (6-OHDA) show poorer score in the behavioral cylinder test and more loss of the DA transporter (DAT) in striatum. The GPR37 KO striata exhibit a significant increase in GABA which is aggravated after DA depletion. Our data indicate that GPR37 KO mice have DA neuron deficit, enhanced striatal GABA levels and deficient corticostriatal LTP. They also respond stronger to 6-OHDA-induced neurotoxicity. Taken together, the data indicate that properly functional GPR37 may counteract aging processes and parkinsonism.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

Protective effects of GPR37 via regulation of inflammation and multiple cell death pathways after ischemic stroke in mice

GPCR 37 (GPR37) is a GPCR expressed in the CNS; its physiological and pathophysiological functions are largely unknown. We tested the role of GPR37 in the ischemic brain of GPR37 knockout (KO) mice, exploring the idea that GPR37 might be protective against ischemic damage. In an ischemic stroke model, GPR37 KO mice exhibited increased infarction and cell death compared with wild-type (WT) mice, measured by 2,3,5-triphenyl-2H-tetrazolium chloride and TUNEL staining 24 h after stroke. Moreover, more severe functional deficits were detected in GPR37 KO mice in the adhesive-removal and corner tests. In the peri-infarct region of GPR37 KO mice, there was significantly more apoptotic and autophagic cell death accompanied by caspase-3 activation and attenuated mechanistic target of rapamycin signaling. GPR37 deletion attenuated astrocyte activation and astrogliosis compared with WT stroke controls 24-72 h after stroke. Immunohistochemical staining showed more ionized calcium-binding adapter molecule 1-positive cells in the ischemic cortex of GPR37 KO mice, and RT-PCR identified an enrichment of M1-type microglia or macrophage markers in the GPR37 KO ischemic cortex. Western blotting demonstrated higher levels of inflammatory factors IL-1β, IL-6, monocyte chemoattractant protein, and macrophage inflammatory protein-1α in GPR37-KO mice after ischemia. Thus, GPR37 plays a multifaceted role after stroke, suggesting a novel target for stroke therapy.-McCrary, M. R., Jiang, M. Q., Giddens, M. M., Zhang, J. Y., Owino, S., Wei, Z. Z., Zhong, W., Gu, X., Xin, H., Hall, R. A., Wei, L., Yu, S. P. Protective effects of GPR37 via regulation of inflammation and multiple cell death pathways after ischemic stroke in mice.

The Role of Lipids in Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by a progressive loss of dopaminergic neurons from the nigrostriatal pathway, formation of Lewy bodies, and microgliosis. During the past decades multiple cellular pathways have been associated with PD pathology (i.e., oxidative stress, endosomal-lysosomal dysfunction, endoplasmic reticulum stress, and immune response), yet disease-modifying treatments are not available. We have recently used genetic data from familial and sporadic cases in an unbiased approach to build a molecular landscape for PD, revealing lipids as central players in this disease. Here we extensively review the current knowledge concerning the involvement of various subclasses of fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, sterols, and lipoproteins in PD pathogenesis. Our review corroborates a central role for most lipid classes, but the available information is fragmented, not always reproducible, and sometimes differs by sex, age or PD etiology of the patients. This hinders drawing firm conclusions about causal or associative effects of dietary lipids or defects in specific steps of lipid metabolism in PD. Future technological advances in lipidomics and additional systematic studies on lipid species from PD patient material may improve this situation and lead to a better appreciation of the significance of lipids for this devastating disease.

GPR37 and GPR37L1 differently interact with dopamine 2 receptors in live cells

Receptor-receptor interactions are essential to fine tune receptor responses and new techniques enable closer characterization of the interactions between involved proteins directly in the plasma membrane. Fluorescence cross-correlation spectroscopy (FCCS), which analyses concurrent movement of bound molecules with single-molecule detection limit, was here used to, in live N2a cells, study interactions between the Parkinson's disease (PD) associated orphan receptor GPR37, its homologue GPR37L1, and the two splice variants of the dopamine 2 receptor (D2R). An interaction between GPR37 and both splice forms of D2R was detected. 4-phenylbutyrate (4-PBA), a neuroprotective chemical chaperone known to increase GPR37 expression at the cell surface, increased the fraction of interacting molecules. The interaction was also increased by pramipexole, a D2R agonist commonly used in the treatment of PD, indicating a possible clinically relevance. Cross-correlation, indicating interaction between GPR37L1 and the short isoform of D2R, was also detected. However, this interaction was not changed with 4-PBA or pramipexole treatment. Overall, these data provide further evidence that heteromeric GPR37-D2R exist and can be pharmacologically modulated, which is relevant for the treatment of PD.

This article is part of the Special Issue entitled ‘Receptor heteromers and their allosteric receptor-receptor interactions’.

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GPCR interaction is studied with fluorescence cross-correlation spectroscopy.

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Interaction between GPR37 and both isoforms of D2R is detected in live cells.

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GPR37's homologue GPR37L1 is detected to interact with D2RS in live cells.

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GPR37-D2R interaction is increased by D2-like agonist and 4-PBA treatment.

Spatiotemporal Control of GPR37 Signaling and Its Behavioral Effects by Optogenetics

Despite the progress in deorphanization of G Protein-Coupled Receptors (GPCRs), ≈100 GPCRs are still classified as orphan receptors without identified endogenous ligands and with unknown physiological functions. The lack of endogenous ligands triggering GPCR signaling has hampered the study of orphan GPCR functions. Using GPR37 as an example, we provide here the first demonstration of the channelrhodopsin 2 (ChR2)-GPCR approach to bypass the endogenous ligand and selectively activate the orphan GPCR signal by optogenetics. Inspired by the opto-XR approach, we designed the ChR2-GPR37 chimera, in which the corresponding parts of GPR37 replaced the intracellular portions of ChR2. We showed that optogenetic activation of ChR2/opto-GPR37 elicited specific GPR37 signaling, as evidenced by reduced cAMP level, enhanced ERK phosphorylation and increased motor activity, confirming the specificity of opto-GPR37 signaling. Besides, optogenetic activation of opto-GPR37 uncovered novel aspects of GPR37 signaling (such as IP-3 signaling) and anxiety-related behavior. Optogenetic activation of opto-GPR37 permits the causal analysis of GPR37 activity in the defined cells and behavioral responses of freely moving animals. Importantly, given the evolutionarily conserved seven-helix transmembrane structures of ChR2 and orphan GPCRs, we propose that opto-GPR37 approach can be readily applied to other orphan GPCRs for their deorphanization in freely moving animals.

Gangliosides of the Nervous System

This review begins by attempting to recount some of the pioneering discoveries that first identified the presence of gangliosides in the nervous system, their structures and topography. This is presented as prelude to the current emphasis on physiological function, about which much has been learned but still remains to be elucidated. These areas include ganglioside roles in nervous system development including stem cell biology, membranes and organelles within neurons and glia, ion transport mechanisms, receptor modulation including neurotrophic factor receptors, and importantly the pathophysiological role of ganglioside aberrations in neurodegenerative disorders. This relates to their potential as therapeutic agents, especially in those conditions characterized by deficiency of one or more specific gangliosides. Finally we attempt to speculate on future directions ganglioside research is likely to take so as to capitalize on the impressive progress to date.

GPR37L1 modulates seizure susceptibility: Evidence from mouse studies and analyses of a human GPR37L1 variant

Progressive myoclonus epilepsies (PMEs) are disorders characterized by myoclonic and generalized seizures with progressive neurological deterioration. While several genetic causes for PMEs have been identified, the underlying causes remain unknown for a substantial portion of cases. Here we describe several affected individuals from a large, consanguineous family presenting with a novel PME in which symptoms begin in adolescence and result in death by early adulthood. Whole exome analyses revealed that affected individuals have a homozygous variant in GPR37L1 (c.1047G>T [Lys349Asn]), an orphan G protein-coupled receptor (GPCR) expressed predominantly in the brain. In vitro studies demonstrated that the K349N substitution in Gpr37L1 did not grossly alter receptor expression, surface trafficking or constitutive signaling in transfected cells. However, in vivo studies revealed that a complete loss of Gpr37L1 function in mice results in increased seizure susceptibility. Mice lacking the related receptor Gpr37 also exhibited an increase in seizure susceptibility, while genetic deletion of both receptors resulted in an even more dramatic increase in vulnerability to seizures. These findings provide evidence linking GPR37L1 and GPR37 to seizure etiology and demonstrate an association between a GPR37L1 variant and a novel progressive myoclonus epilepsy.

Dopamine Receptor Signaling in MIN6 β-Cells Revealed by Fluorescence Fluctuation Spectroscopy

Insulin secretion defects are central to the development of type II diabetes mellitus. Glucose stimulation of insulin secretion has been extensively studied, but its regulation by other stimuli such as incretins and neurotransmitters is not as well understood. We investigated the mechanisms underlying the inhibition of insulin secretion by dopamine, which is synthesized in pancreatic β-cells from circulating L-dopa. Previous research has shown that this inhibition is mediated primarily by activation of the dopamine receptor D3 subtype (DRD3), even though both DRD2 and DRD3 are expressed in β-cells. To understand this dichotomy, we investigated the dynamic interactions between the dopamine receptor subtypes and their G-proteins using two-color fluorescence fluctuation spectroscopy (FFS) of mouse MIN6 β-cells. We show that proper membrane localization of exogenous G-proteins depends on both the Gβ and Gγ subunits being overexpressed in the cell. Triple transfections of the dopamine receptor subtype and Gβ and Gγ subunits, each labeled with a different-colored fluorescent protein (FP), yielded plasma membrane expression of all three FPs and permitted an FFS evaluation of interactions between the dopamine receptors and the Gβγ complex. Upon dopamine stimulation, we measured a significant decrease in interactions between DRD3 and the Gβγ complex, which is consistent with receptor activation. In contrast, dopamine stimulation did not cause significant changes in the interactions between DRD2 and the Gβγ complex. These results demonstrate that two-color FFS is a powerful tool for measuring dynamic protein interactions in living cells, and show that preferential DRD3 signaling in β-cells occurs at the level of G-protein release.

Mice lacking Gpr37 exhibit decreased expression of the myelin-associated glycoprotein MAG and increased susceptibility to demyelination

GPR37 is an orphan G protein-coupled receptor that is predominantly expressed in the brain and found at particularly high levels in oligodendrocytes. GPR37 has been shown to exert effects on oligodendrocyte differentiation and myelination during development, but the molecular basis of these actions is incompletely understood and moreover nothing is known about the potential role(s) of this receptor under demyelinating conditions. To shed light on the fundamental biology of GPR37, we performed proteomic studies comparing protein expression levels in the brains of mice lacking GPR37 and its close relative GPR37-like 1 (GPR37L1). These studies revealed that one of the proteins most sharply decreased in the brains of Gpr37/Gpr37L1 double knockout mice is the myelin-associated glycoprotein MAG. Follow-up Western blot studies confirmed this finding and demonstrated that genetic deletion of Gpr37, but not Gpr37L1, results in strikingly decreased brain expression of MAG. Further in vitro studies demonstrated that GPR37 and MAG form a complex when expressed together in cells. As loss of MAG has previously been shown to result in increased susceptibility to brain insults, we additionally assessed Gpr37-knockout (Gpr37^-/-) vs. wild-type mice in the cuprizone model of demyelination. These studies revealed that Gpr37^-/- mice exhibit dramatically increased loss of myelin in response to cuprizone, yet do not show any increased loss of oligodendrocyte precursor cells or mature oligodendrocytes. These findings reveal that loss of GPR37 alters oligodendrocyte physiology and increases susceptibility to demyelination, indicating that GPR37 could be a potential drug target for the treatment of demyelinating diseases such as multiple sclerosis.

Folding Underlies Bidirectional Role of GPR37/Pael-R in Parkinson Disease

Since conformational flexibility, which is required for the function of a protein, comes at the expense of structural stability, many proteins, including G-protein-coupled receptors (GPCRs), are under constant risk of misfolding and aggregation. In this regard GPR37 (also named PAEL-R and ETBR-LP-1) takes a prominent role, particularly in relation to Parkinson disease (PD). GPR37 is a substrate for parkin and accumulates abnormally in autosomal recessive juvenile parkinsonism, contributing to endoplasmic reticulum stress and death of dopaminergic neurons. GPR37 also constitutes a core structure of Lewy bodies, demonstrating a more general involvement in PD pathology. However, if folded and matured properly, GPR37 seems to be neuroprotective. Moreover, GPR37 modulates functionality of the dopamine transporter and the dopamine D2 receptor and stimulates dopamine neurotransmission. Here we review the multiple roles of GPR37 with relevance to potential disease modification and symptomatic therapies of PD and highlight unsolved issues in this field.

Parkinson's disease-associated GPR37 receptor regulates cocaine-mediated synaptic depression in corticostriatal synapses

GPR37 is an orphan G protein-coupled receptor highly expressed in the brain. The precise function of GPR37 is still unknown, but a number of evidences indicate it modulates the dopaminergic system. Here, we aimed to determine the role of GPR37 on the control of cocaine-mediated electrophysiological effects (synaptic transmission and short-term plasticity) in corticostriatal synapses. Accordingly, we evaluated basal synaptic transmission and paired-pulse stimulation (PPS) in wild-type and GPR37KO mice slices. Regardless of the genotype, a low concentration of cocaine (2μM) did not modify basal synaptic transmission. Conversely, a higher dose of cocaine (30μM) decreased synaptic transmission in both genotypes, although with different intensities: approximately 30% in slices from wild-type mice and 45% in slices from GPR37-KO mice. On the other hand, no differences in PPS ratio were observed between wild-type and GPR37-KO cocaine-treated mice. Overall, our data suggest that GPR37 is involved in cocaine-induced modification of basal synaptic transmission without modifying cocaine effects in short-term plasticity.

Metalloprotease cleavage of the N terminus of the orphan G protein-coupled receptor GPR37L1 reduces its constitutive activity

Little is known about the pharmacology or physiology of GPR37L1, a G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor that is abundant in the cerebellum. Mice deficient in this receptor exhibit precocious cerebellar development and hypertension. We showed that GPR37L1 coupled to the G protein Gα(s) when heterologously expressed in cultured cells in the absence of any added ligand, whereas a mutant receptor that lacked the amino terminus was inactive. Conversely, inhibition of ADAMs (a disintegrin and metalloproteases) enhanced receptor activity, indicating that the presence of the amino terminus is necessary for GPR37L1 signaling. Metalloprotease-dependent processing of GPR37L1 was evident in rodent cerebellum, where we detected predominantly the cleaved, inactive form. However, comparison of the accumulation of cAMP (adenosine 3',5'-monophosphate) in response to phosphodiesterase inhibition in cerebellar slice preparations from wild-type and GPR37L1-null mice showed that some constitutive signaling remained in the wild-type mice. In reporter assays of Gα(s) or Gα(i) signaling, the synthetic, prosaposin-derived peptide prosaptide (TX14A) did not increase GPR37L1 activity. Our data indicate that GPR37L1 may be a constitutively active receptor, or perhaps its ligand is present under the conditions that we used for analysis, and that the activity of this receptor is instead controlled by signals that regulate metalloprotease activity in the tissue.

The Parkinson's-disease-associated receptor GPR37 undergoes metalloproteinase-mediated N-terminal cleavage and ectodomain shedding

The G-protein-coupled receptor 37 ( GPR37) has been implicated in the juvenile form of Parkinson's disease, in dopamine signalling and in the survival of dopaminergic cells in animal models. The structure and function of the receptor, however, have remained enigmatic. Here, we demonstrate that although GPR37 matures and is exported from the endoplasmic reticulum in a normal manner upon heterologous expression in HEK293 and SH-SY5Y cells, its long extracellular N-terminus is subject to metalloproteinase-mediated limited proteolysis between E167 and Q168. The proteolytic processing is a rapid and efficient process that occurs constitutively. Moreover, the GPR37 ectodomain is released from cells by shedding, a phenomenon rarely described for GPCRs. Immunofluorescence microscopy further established that although full-length receptors are present in the secretory pathway until the trans-Golgi network, GPR37 is expressed at the cell surface predominantly in the N-terminally truncated form. This notion was verified by flow cytometry and cell surface biotinylation assays. These new findings on the GPR37 N-terminal limited proteolysis may help us to understand the role of this GPCR in the pathophysiology of Parkinson's disease and in neuronal function in general.

Drug Discovery Opportunities at the Endothelin B Receptor-Related Orphan G Protein-Coupled Receptors, GPR37 and GPR37L1

Orphan G protein-coupled receptors (GPCRs) represent a largely untapped resource for the treatment of a variety of diseases, despite sophisticated advances in drug discovery. Two promising orphan GPCRs are the endothelin B receptor-like proteins, GPR37 [ET(B)R-LP, Pael-R] and GPR37L1 [ET(B)R-LP-2]. Originally identified through searches for homologs of endothelin and bombesin receptors, neither GPR37 nor GPR37L1 were found to bind endothelins or related peptides. Instead, GPR37 was proposed to be activated by head activator (HA) and both GPR37 and GPR37L1 have been linked to the neuropeptides prosaposin and prosaptide, although these pairings are yet to be universally acknowledged. Both orphan GPCRs are widely expressed in the brain, where GPR37 has received the most attention for its link to Parkinson’s disease and parkinsonism, while GPR37L1 deletion leads to precocious cerebellar development and hypertension. In this review, the existing pharmacology and physiology of GPR37 and GPR37L1 is discussed and the potential therapeutic benefits of targeting these receptors are explored.

The multi-tasked life of GM1 ganglioside, a true factotum of nature

GM1 ganglioside occurs widely in vertebrate tissues, where it exhibits many essential functions, both in the plasma membrane and intracellular loci. Its essentiality is revealed in the dire consequences resulting from genetic deletion. This derives from its key roles in several signalosome systems, characteristically located in membrane rafts, where it associates with specific proteins that have glycolipid-binding domains. Thus, GM1 interacts with proteins that modulate mechanisms such as ion transport, neuronal differentiation, G protein-coupled receptors (GPCRs), immune system reactivities, and neuroprotective signaling. The latter occurs through intimate association with neurotrophin receptors, which has relevance to the etiopathogenesis of neurodegenerative diseases and potential therapies. Here, we review the current state of knowledge of these GM1-associated mechanisms.

The role of parkinson's disease-associated receptor GPR37 in the hippocampus: functional interplay with the adenosinergic system

GPR37 is an orphan G protein-coupled receptor mostly enriched in brain areas such as the cerebellum, striatum, and hippocampus. Identified as a substrate of parkin, GPR37 has been suggested to play a role in Parkinson's disease. Distributed throughout the brain, the function of GPR37, however, remains unknown. We now provide the first mapping of GPR37 within the hippocampus, where GPR37 is widely expressed and localized at the level of the extrasynaptic plasma membrane of dendritic spines, dendritic shafts, and axon terminals. GPR37 per se does not appear to play a role in learning and memory, since knocking out GPR37 (GPR37-KO) did not alter the performance in different hippocampal-related memory tasks. This is in agreement with slice electrophysiology experiments showing no differences both in short-term plasticity paired-pulse facilitation and long-term potentiation between WT and GPR37-KO mice. However, we report a potential functional interaction between GPR37 and adenosine A2A receptors (A2 A R) in the hippocampus, with A2 A R modulating the GPR37-associated phenotype. Thus, the absence of GPR37 appeared to sensitize mice to hippocampal A2 A R-mediated signaling, as observed by the effect of the A2 A R antagonist SCH58261 increasing synaptic depotentiation, reducing novel object recognition memory and reverting the anxiolytic effect of GPR37 deletion. Collectively, these findings afford insight into the localization and role of the orphan GPR37 within the hippocampus with potential involvement in A2 A R function (i.e., A2 A R sensitization). GPR37 is an orphan G protein-coupled receptor widely expressed in the hippocampus and localized at the level of the extrasynaptic plasma membrane of dendritic spines, dendritic shafts and axon terminals. This orphan receptor per se does not appear to directly control the learning and memory processes; however knocking-out GPR37 triggers anxiolytic-like effects and sensitizes mice to hippocampal A2A R-mediated signalling.

The protective role of prosaposin and its receptors in the nervous system

Prosaposin (also known as SGP-1) is an intriguing multifunctional protein that plays roles both intracellularly, as a regulator of lysosomal enzyme function, and extracellularly, as a secreted factor with neuroprotective and glioprotective effects. Following secretion, prosaposin can undergo endocytosis via an interaction with the low-density lipoprotein-related receptor 1 (LRP1). The ability of secreted prosaposin to promote protective effects in the nervous system is known to involve activation of G proteins, and the orphan G protein-coupled receptors GPR37 and GPR37L1 have recently been shown to mediate signaling induced by both prosaposin and a fragment of prosaposin known as prosaptide. In this review, we describe recent advances in our understanding of prosaposin, its receptors and their importance in the nervous system.