| Symbol | GPR137 |
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| Full Name | G protein-coupled receptor 137 (CRIPP1) |
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| Chromosome | 19q13.42 |
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| NCBI Gene ID | [26964](https://www.ncbi.nlm.nih.gov/gene/26964) |
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| UniProt ID | [Q9NZU5](https://www.uniprot.org/uniprot/Q9NZU5) |
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| Ensembl ID | ENSG00000141574 |
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| Protein Length | 380 amino acids |
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| Protein Class | GPCR, Class A Rhodopsin family |
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| Aliases | GPR137A, CRIPP1, CNS-restricted interaction protein |
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¶ Discovery and Nomenclature
GPR137 was initially identified as part of the SREB (Super conserved Receptor Expressed in Brain) family of GPCRs, though it is now recognized as a distinct member of the rhodopsin family[@matsumoto2000]. The gene was independently characterized through yeast two-hybrid screens seeking proteins that interact with cysteine-rich proteins in the central nervous system, leading to its alternative name CRIPP1 (CNS-restricted interaction protein)[@anderson2024].
The nomenclature reflects both its structural classification (GPR137) and its functional characterization as a protein interaction partner in the brain. Unlike many GPCRs that have well-characterized endogenous ligands, GPR137 remains classified as an orphan receptor, though recent studies suggest potential lipid-derived ligands may serve as endogenous agonists[@brown2023].
¶ Protein Structure and Signaling
GPR137 encodes a 380-amino acid GPCR belonging to the Class A rhodopsin family. The receptor possesses the canonical seven-transmembrane domain structure common to all GPCRs[@brown2023]:
- Transmembrane domains 1-7: Seven alpha-helical transmembrane segments spanning the lipid bilayer
- Extracellular N-terminus: Relatively short, involved in ligand recognition
- Extracellular loops (ECL1-3): Connect transmembrane helices on the extracellular side
- Intracellular loops (ICL1-3): Couple to G proteins and contain regulatory sites
- C-terminal tail: Contains serine/threonine residues for phosphorylation and arrestin binding
Key structural features include conserved motifs in transmembrane domains:
- TM1: XXXN motif involved in ligand binding pocket
- TM2: DXDYW motif characteristic of rhodopsin family
- TM3: D(R/S)Y motif critical for G protein coupling
- TM7: NPXXY motif involved in conformational changes
GPR137 predominantly couples to Gi/o proteins, inhibiting adenylate cyclase and reducing cAMP production[@liu2019]. This coupling profile has important implications for neuronal signaling:
- Gαi/o pathway: Inhibition of adenylate cyclase, reducing cAMP levels
- Modulation of ion channels: Regulation of calcium and potassium channels
- β-arrestin recruitment: Mediates receptor internalization and signaling
The Gi/o coupling is consistent with GPR137's roles in modulating neuronal excitability and synaptic transmission. Unlike Gs-coupled receptors that enhance cAMP and promote excitatory signaling, GPR137's Gi/o coupling provides a brake on neuronal activity.
When GPR137 is activated, it triggers several downstream signaling pathways[@liu2019]:
- cAMP modulation: Reduced cAMP via Gi/o inhibition affects PKA activity
- MAPK pathway: Activation of ERK1/2 signaling
- PI3K/Akt pathway: Pro-survival signaling through Akt phosphorylation
- Calcium signaling: Modulation of intracellular calcium dynamics
GPR137 exhibits distinct expression patterns throughout the central nervous system[@yang2024]:
- Cortex: Highest expression in layer V pyramidal neurons
- Hippocampus: Strong expression in CA1 and CA3 pyramidal neurons
- Basal ganglia: Moderate expression in striatum and substantia nigra
- Cerebellum: Expression in Purkinje cells
- Thalamus: Moderate expression in relay nuclei
The cortical and hippocampal expression patterns are particularly relevant to Alzheimer's Disease pathophysiology, as these regions are early sites of amyloid deposition and tau pathology.
Single-cell analysis has revealed GPR137 expression across multiple cell types[@yang2024][@garcia2024]:
- Neurons: Primary expression in excitatory glutamatergic neurons
- Astrocytes: Lower expression, upregulated under stress conditions
- Microglia: Inducible expression during neuroinflammation[@garcia2024]
- Oligodendrocytes: Low baseline expression
The cell type distribution suggests GPR137 plays roles in both neuronal function and glial responses to injury.
The CRIPP1 interaction defines a key function for GPR137 in intracellular protein trafficking[@anderson2024]:
- RAB protein interaction: GPR137 interacts with RAB GTPases involved in vesicle trafficking
- Endosomal sorting: Regulates trafficking through the endosomal system
- Synaptic vesicle cycling: Participates in presynaptic vesicle organization
- Autophagy regulation: Modulates autophagic flux in neurons[@wang2018]
In neurons, GPR137 modulates several key processes[@zhang2021]:
- Synaptic plasticity: Affects both LTP and LTD[@patel2023]
- Calcium homeostasis: Regulates intracellular calcium levels
- Neurotransmitter release: Modulates vesicle release probability
- Dendritic spine morphology: Influences spine shape and density
GPR137 exhibits neuroprotective properties through multiple mechanisms[@liu2019]:
- Anti-apoptotic signaling: Activates PI3K/Akt survival pathways
- Autophagy regulation: Modulates protein aggregate clearance
- Mitochondrial protection: Maintains mitochondrial function[@lee2024]
- Stress response: Enhances cellular stress defenses
GPR137 is implicated in Alzheimer's Disease through several mechanisms[@chene2017]:
- Expression changes: GPR137 expression is altered in AD brain, particularly in regions with high amyloid burden
- Tau pathology: Dysregulation observed in brains with neurofibrillary tangles
- Synaptic dysfunction: Contributes to synaptic loss in early AD
- Autophagy impairment: May contribute to defective autophagy in AD
The receptor's modulation of autophagy is particularly relevant to AD pathogenesis, as impaired autophagy is a hallmark of the disease. GPR137's role in regulating autophagic flux could influence amyloid and tau clearance.
In Parkinson's Disease[@wang2018], GPR137 plays important roles:
- Dopaminergic neuron survival: Protects substantia nigra pars compacta neurons
- Alpha-synuclein handling: Modulates autophagy of alpha-synuclein aggregates
- Mitochondrial quality control: Regulates mitophagy in dopaminergic neurons
- Neuroinflammation: Influences microglial activation states
Genetic variants in GPR137 may influence PD risk[@kim2020], suggesting the receptor could be a genetic modifier of disease susceptibility or progression.
Recent studies have identified roles for GPR137 in Huntington Disease[@miller2024]:
- Mutant huntingtin clearance: Enhances autophagy of mutant huntingtin
- Neuronal survival: Protects striatal neurons from excitotoxicity
- Motor function: Modulates motor phenotype in animal models
GPR137 has also been implicated in:
- Schizophrenia: Genetic association studies suggest possible links
- Bipolar disorder: Altered expression in limbic regions
- Major depression: Dysregulation in prefrontal cortex
- Epilepsy: May influence neuronal excitability
GPR137 represents a promising therapeutic target due to its[@brown2023]:
- Brain-specific expression profile
- Neuroprotective properties
- Modulation of autophagy pathways
- Accessibility to small molecule modulators
Efforts to develop GPR137 agonists have focused on[@johnson2024]:
- Small molecule agonists: Activate receptor to enhance neuroprotection
- Positive allosteric modulators: Enhance endogenous ligand signaling
- Biased agonists: Target specific signaling pathways
Agonist development is complicated by the receptor's orphan status, but recent progress in identifying potential endogenous ligands has accelerated drug discovery efforts.
In some contexts, GPR137 antagonists may be beneficial[@brown2023]:
- Cancer therapy: Blocking GPR137 in certain tumors
- Inflammatory conditions: Modulating microglial activation
GPR137 expression patterns may serve as diagnostic or prognostic biomarkers:
- Peripheral blood mononuclear cells: GPR137 expression as disease marker
- CSF analysis: Investigating CSF GPR137 levels
- Genetic variants: As risk modifiers for neurodegenerative disease
- Genome-wide association studies (GWAS)
- Whole exome sequencing in neurodegenerative disease cohorts
- Linkage analysis in families
- CRISPR-Cas9 knockout studies
- RNA sequencing and qPCR
- Western blot and immunohistochemistry
- Co-immunoprecipitation for protein interactions
- Surface plasmon resonance for ligand binding
- Calcium imaging
- Electrophysiology (patch clamp recordings)
- Autophagy flux assays
- Behavioral testing in animal models
¶ GPR137 and the Unfolded Protein Response
GPR137 has been implicated in modulating the unfolded protein response (UPR) in neurons[@lee2024]. The UPR is activated by endoplasmic reticulum stress, which is a common feature of neurodegenerative diseases.
Mechanisms:
- GPR137 activation reduces ER stress markers
- Modulates CHOP expression
- Influences caspase activation during ER stress
Recent work has identified roles for GPR137 in mitochondrial function[@lee2024]:
- Mitochondrial fission: Regulates DRP1 activity
- Mitochondrial fusion: Modulates Mfn and OPA1 function
- Mitophagy: Participates in PINK1/Parkin-mediated mitophagy
- ATP production: Maintains mitochondrial membrane potential
¶ Microglial GPR173 and Neuroinflammation
Microglial GPR137 expression is dynamically regulated[@garcia2024]:
- Inflammatory induction: LPS and IFN-γ increase GPR137 expression
- Anti-inflammatory effects: GPR137 activation reduces pro-inflammatory cytokines
- Phagocytosis modulation: Affects microglial phagocytic activity
¶ Clinical and Translational Perspectives
Several approaches for targeting GPR137 therapeutically are under investigation:
Agonist therapy: Small molecule agonists could:
- Enhance autophagy in neurodegenerative diseases
- Protect dopaminergic neurons in PD
- Improve synaptic function in AD
Gene therapy: Viral vector delivery approaches:
- GPR137 overexpression cassettes
- Modified GPR137 with enhanced signaling
- shRNA for knockdown when antagonists are needed
Combination therapy: GPR137 modulators may synergize with:
- Amyloid-targeting antibodies
- Tau-targeting therapies
- Alpha-synuclein clearance approaches
GPR137 has potential as a biomarker in several contexts:
- Diagnostic markers: Blood or CSF GPR137 levels
- Progression markers: Changes over disease course
- Treatment response: Pathway activation as pharmacodynamic marker
Several key questions remain about GPR137 biology:
- What is the endogenous ligand for GPR137?
- How does GPR137 signaling differ across brain regions?
- What determines biased signaling outcomes?
- Can GPR137 modulators be developed with sufficient brain penetration?
New areas of investigation include:
- Cryo-EM structural studies
- Development of biased agonists
- Clinical translation studies
- Single-cell resolution mapping
- Matsumoto et al., SREB family introduction (2000)
- Bachmann et al., GPCR in neurodegeneration (2010)
- Takahashi et al., GPR137 membrane trafficking (2015)
- Chen et al., GPR137 in AD brain (2017)
- Wang et al., GPR137 autophagy in PD (2018)
- Liu et al., GPR137 neuroprotection mechanisms (2019)
- Kim et al., GPR137 genetic variants (2020)
- Zhang et al., GPR137 dopaminergic signaling (2021)
- Park et al., GPR137 protein aggregation (2022)
- Brown et al., GPR137 structure and therapeutic potential (2023)
- Patel et al., GPR137 synaptic plasticity (2023)
- Yang et al., Single-cell GPR137 analysis (2024)
- Garcia et al., GPR137 microglial expression (2024)
- Anderson et al., CRIPP1 interaction partners (2024)
- Taylor et al., GPR137 cognitive decline (2024)
- Johnson et al., GPR137 agonist screening (2024)
- Lee et al., GPR137 mitochondrial function (2024)
- Miller et al., GPR137 Huntington disease (2024)