Recombinant Mouse Probable G-protein coupled receptor 61 (Gpr61)

What is GPR61 and where is it primarily expressed?

GPR61 (Probable G-protein coupled receptor 61) is an orphan G protein-coupled receptor belonging to the Class A (rhodopsin family) of GPCRs. It contains the characteristic 7 transmembrane domains and shares 28-31% sequence similarity with certain histamine, adrenergic, serotonin, and dopamine receptors. GPR61 is primarily expressed in the brain, with substantial presence in the cortex, hippocampus, thalamus, hypothalamus, and midbrain . Its expression in appetite-regulating centers of the hypothalamus and brainstem suggests a role in metabolic regulation . A notable structural feature is the presence of a phenylalanine residue in the sixth transmembrane domain, which is conserved in many biogenic amine receptors .

Why is GPR61 considered an orphan receptor and what experimental approaches are used to study it?

GPR61 is classified as an orphan receptor because its endogenous ligand remains unknown. Despite structural similarity to biogenic amine receptors, no natural compound has been definitively identified that binds to and activates GPR61 under physiological conditions . Research approaches to study orphan GPCRs like GPR61 include:

• Pharmacological assays and cell biology techniques, particularly bioluminescence resonance energy transfer (BRET) assays to analyze functional dynamics of GPCR activation and interaction

• Structural characterization using cryo-electron microscopy to resolve receptor conformations in active and inactive states

• Knockout mouse models to investigate physiological functions through phenotypic analysis

• Heterologous expression systems (typically HEK293 cells) for signaling and binding studies

What is currently known about GPR61's signaling properties?

GPR61 exhibits constitutive activity, meaning it can signal without ligand activation. Key signaling properties include:

• Primary signaling through the Gαs pathway, leading to constitutive production of cAMP

• The N-terminal domain has been identified as important for this constitutive activity

• Some studies report conflicting data about whether it may also signal through Gi pathways

• Structural studies have identified key residues involved in G protein coupling, particularly in TM3, TM6, helix 8, and ICL2

What experimental systems are most effective for studying recombinant mouse GPR61?

For recombinant mouse GPR61 studies, the following experimental systems have proven effective:

• HEK293 cells are predominantly used due to their good reproducibility and expression capabilities for GPCR experiments

• Cell culture systems with stable expression of GPR61 or GPR61 mutants for consistent experimental conditions

• Fluorescence-based assays using tagged GPR61 constructs to monitor expression and activity

• Mouse models, particularly GPR61 knockout mice, for in vivo functional studies

What are the recommended assays for measuring GPR61 activity and expression?

Several assays have been established for studying different aspects of GPR61:

These assays are typically performed in HEK293 cells expressing wild-type or mutant GPR61 constructs, with measurements repeated at least three times for statistical validity .

How can specific mutations in GPR61 be generated and analyzed?

For generating and studying GPR61 mutations:

• Identify mutations of interest:

  • Disease-associated mutations from patient databases (e.g., obesity-related mutations with BMI > 40)

  • Structurally important residues identified through homology modeling or solved structures

  • Conserved motifs among related GPCRs

• Generate mutations using:

  • Site-directed mutagenesis on expression plasmids

  • CRISPR-Cas9 genome editing for cellular or animal models

• Analyze effects through:

  • Pharmacological assays comparing wild-type and mutant signaling (e.g., cAMP production)

  • Measurement of constitutive activity and response to potential ligands or modulators

  • Surface expression and trafficking studies to detect changes in receptor processing

  • Structural studies to understand conformational changes

What is the structural basis for GPR61's constitutive activity?

Recent structural studies have revealed several features contributing to GPR61's constitutive activity:

• The N-terminal domain (first 44 residues) appears unresolved in structural studies, suggesting transient interactions with the receptor's extracellular surface that may contribute to activation

• Key sequence and structural features partially destabilize the inactive state, biasing GPR61 toward constitutive activation even without a ligand

• In the active state, GPR61 exhibits the characteristic outward movement of TM5 and TM6 seen in activated GPCRs, creating an intracellular pocket that accommodates the C-terminal helix of Gαs

• The binding of Gαs involves a network of polar contacts with residues in TM3, TM6, helix 8, and ICL2, stabilizing the active conformation

These structural elements collectively lower the energy barrier for activation, allowing spontaneous adoption of an active conformation without ligand binding.

How do inverse agonists modulate GPR61 activity, and what therapeutic implications does this have?

Recent research has identified potent and selective sulfonamide-based inverse agonists that modulate GPR61 through an unusual mechanism:

• Rather than binding to the orthosteric site, these compounds bind to an allosteric pocket

• Structural studies reveal that the inverse agonist acts by binding and remodeling an intracellular pocket normally occupied by Gαs in the activated state

• This represents a novel mechanism of GPCR inactivation, effectively blocking G protein activation

• Treatment with inverse agonist causes increased cell surface expression of GPR61, possibly due to compensatory overexpression or pharmacochaperone activity

Therapeutically, this mechanism suggests potential applications for inverse agonists in treating wasting disorders like cachexia, where increased appetite stimulation would be beneficial . The discovery of this binding mode also provides a structural framework for developing additional modulators with varied efficacies.

What is known about the relationship between GPR61 mutations and disease states?

Several connections between GPR61 mutations and disease states have been identified:

• Various mutations of GPR61 have been observed in samples from individuals with severe obesity (BMI > 40)

• The L125P mutation occurs in non-Hodgkin's lymphoma samples and is of interest due to proline's specific structural implications in helix formation

• Hypermethylation of the GPR61 promoter region is associated with type 2 diabetes in discordant monozygotic twins

• Mutagenesis and human genome-wide association studies have linked GPR61 to phenotypes associated with type 2 diabetes and body mass index

These findings suggest that alterations in GPR61 function, whether through mutation or epigenetic regulation, may contribute to metabolic dysregulation. Studying these mutations provides insight into structure-function relationships and potential therapeutic targeting strategies.

What phenotypes are observed in GPR61-deficient mice, and what do they reveal about receptor function?

GPR61 knockout mice display several significant phenotypic changes that provide insight into the receptor's physiological functions:

• Increased food intake (hyperphagia) compared to wild-type mice

• Noticeable increases in body weight

• Higher body mass index and increased fat content

These observations strongly suggest that GPR61 normally plays an inhibitory role in food intake and body weight regulation. The phenotype aligns with GPR61's expression in brain regions involved in appetite control, particularly the hypothalamus. Mechanistically, this indicates that constitutive signaling by GPR61 may suppress feeding behavior under normal conditions, and loss of this signaling results in dysregulated appetite and weight gain.

How does GPR61 compare functionally to other metabolic GPCRs, and what unique research opportunities does it present?

GPR61 shares functional similarities with other metabolic GPCRs but also presents unique characteristics:

Unique research opportunities include:

• Understanding the structural basis of constitutive activity in GPCRs

• Developing novel allosteric modulators targeting the intracellular G protein binding pocket

• Investigating bidirectional therapeutic potential (both activation and inhibition may have therapeutic value depending on the condition)

• Exploring the role of an orphan GPCR in central regulation of metabolism

What signaling pathways and biological processes are affected by GPR61 activity?

Based on current research, GPR61 influences several signaling pathways and biological processes:

• cAMP Signaling: GPR61 constitutively activates the Gαs pathway, leading to increased cAMP production, which can affect numerous downstream signaling cascades

• Appetite Regulation: Expression in hypothalamic regions and the obesity phenotype of knockout mice strongly suggest a role in central appetite control circuits

• Energy Metabolism: Beyond food intake, GPR61 likely influences broader aspects of energy homeostasis, as evidenced by increased adiposity in knockout mice

• Potential Impact on Glucose Homeostasis: Association with type 2 diabetes suggests possible involvement in glucose metabolism, though direct mechanisms remain to be elucidated

The exact molecular mechanisms connecting GPR61 signaling to these physiological processes remain areas of active investigation, particularly given the receptor's orphan status.

What breakthroughs in GPR61 research have occurred recently, and how do they advance the field?

Recent significant advances in GPR61 research include:

• Structural characterization of GPR61 in both active (G protein-coupled) and inactive states using cryo-electron microscopy, providing unprecedented insights into its activation mechanism

• Discovery of a potent and selective sulfonamide inverse agonist that acts through an allosteric mechanism, offering both a research tool and potential therapeutic lead

• Better understanding of the structural basis for constitutive activity, including identification of key residues and domains involved in activation

• Stronger evidence linking GPR61 to appetite regulation and body weight control through knockout studies and human genetic associations

These advances provide a more solid foundation for exploring GPR61 as a therapeutic target and understanding its physiological roles in metabolic regulation.

What methodological challenges remain in GPR61 research, and how might they be addressed?

Several significant challenges remain in GPR61 research:

• Identification of endogenous ligand(s):

  • Apply unbiased screening of tissue extracts from regions with high GPR61 expression

  • Use structural information to guide in silico prediction of potential ligands

  • Employ proximity labeling approaches to identify binding partners

• Resolving conflicting data on G protein coupling:

  • Conduct comprehensive analysis using multiple complementary assays (BRET, FRET, cAMP, etc.)

  • Study signaling in physiologically relevant cell types rather than just heterologous systems

  • Examine potential biased signaling under different conditions

• Development of additional selective tool compounds:

  • Design agonists or positive allosteric modulators based on structural information

  • Create compounds suitable for in vivo studies with appropriate pharmacokinetic properties

  • Develop labeled probes for binding and localization studies

• Translation to human physiology:

  • Conduct studies in human cell and tissue systems

  • Expand clinical genetic association studies

  • Examine GPR61 expression and function in relevant patient populations

What are the most promising therapeutic applications for GPR61 modulators?

Based on current understanding of GPR61 function, several therapeutic applications show promise:

• Obesity treatment: GPR61 agonists or positive allosteric modulators could enhance receptor signaling, potentially reducing food intake and body weight based on the hyperphagic phenotype of knockout mice

• Cachexia and wasting disorders: Inverse agonists that reduce GPR61's constitutive activity might increase appetite and food intake, potentially benefiting patients with cancer cachexia, AIDS-related wasting, or other conditions with pathological weight loss

• Type 2 diabetes: Given the associations between GPR61 and metabolic phenotypes including T2D, modulators might impact glucose homeostasis beyond effects on body weight

• Precision medicine approaches: Identifying individuals with specific GPR61 mutations or expression patterns could enable targeted therapeutic strategies based on receptor status

The development of potent and selective modulators with appropriate drug-like properties, particularly those able to cross the blood-brain barrier to reach hypothalamic sites of action, represents a critical next step toward realizing these therapeutic applications.