Recombinant Macaca mulatta Melanin-concentrating hormone receptor 2 (MCHR2)

What is Melanin-concentrating hormone receptor 2 (MCHR2) and why is it significant in research?

MCHR2 is a G-protein coupled receptor that functions as a receptor for melanin-concentrating hormone (MCH), a 19-amino-acid neuropeptide critical for maintaining energy balance in mammals. MCHR2 is encoded by the MCHR2 gene, also known as GPR145 (G-protein coupled receptor 145) . Its significance in research stems from its restricted evolutionary expression pattern, being present in primates, cats, and dogs, but notably absent in rodents such as mice and rats . This absence in commonly used laboratory animals has delayed pharmaceutical research targeting this receptor, making studies in non-human primates like Macaca mulatta particularly valuable for translational research to humans .

How does MCHR2 differ from MCHR1 in terms of signaling pathways?

MCHR1 and MCHR2 utilize distinct signaling mechanisms despite responding to the same ligand (MCH):

• MCHR1 couples primarily to Gi/o proteins, which typically inhibit adenylyl cyclase and are associated with the inhibition of neuronal activity .

• MCHR2 couples to Gq proteins, which trigger intracellular signaling pathways associated with stimulation of neuronal activity .

This fundamental difference in signaling mechanisms may explain why these receptors can potentially have opposing physiological effects, as demonstrated in transgenic models . This divergence is particularly important when designing experiments to study MCH receptor function, as findings from MCHR1-only models may not translate directly to systems expressing both receptors.

Why are rhesus macaque models valuable for studying MCHR2 compared to rodent models?

Rodents (mice and rats) naturally lack the MCHR2 gene, making them unsuitable for studying this receptor's native function . In contrast, rhesus macaques express both MCHR1 and MCHR2 similar to humans, providing a more translational model for understanding how these receptors function in human physiology and pathology . This is particularly important since:

What expression systems are available for producing recombinant Macaca mulatta MCHR2?

Multiple expression systems are available for producing recombinant Macaca mulatta MCHR2, each with distinct advantages depending on the research application:

The choice of expression system should be guided by the specific requirements of the experimental setup, including the need for post-translational modifications, protein folding, and functional activity .

How can researchers validate the specificity of antibodies against Macaca mulatta MCHR2?

Validating antibody specificity for Macaca mulatta MCHR2 requires a multi-method approach:

• Western Blotting (WB): Confirm antibody detects a band at the expected molecular weight (approximately 40-45 kDa) in macaque tissues expressing MCHR2. Compare with positive controls (human MCHR2) and negative controls (tissues from species lacking MCHR2) .

• Immunofluorescence (IF): Verify cellular localization patterns in tissues known to express MCHR2, such as hypothalamus and striatum. The antibody should show membrane localization consistent with a G-protein coupled receptor .

• Cross-reactivity testing: Test the antibody against closely related GPCRs, particularly MCHR1, to ensure specificity. Commercial antibodies like CSB-PA070027 and CSB-PA013585LA01HU have been validated for human MCHR2 with potential cross-reactivity to macaque MCHR2 .

• Knockout/knockdown controls: If available, use tissues or cells with MCHR2 knocked down as negative controls to confirm specificity.

• Epitope mapping: Determine which region of MCHR2 the antibody recognizes and assess conservation of this epitope between human and macaque MCHR2.

Several validated antibodies are available commercially with applications including WB, IF, IHC, and ELISA .

What methodological approaches have been used to study MCHR2's role in energy homeostasis?

Research into MCHR2's role in energy homeostasis has utilized innovative approaches to overcome the absence of MCHR2 in rodent models:

• Transgenic MCHR1R2 mouse model: Researchers generated mice expressing human MCHR2 in MCHR1-expressing neurons using homologous recombination, allowing the study of MCHR2 in an otherwise MCHR2-deficient species .

• BAC recombination technique: The human MCHR2 coding sequence (1023 bp) was inserted into exon 1 of the mouse MCHR1 gene present in a bacterial artificial chromosome, enabling coexpression of MCHR2 in MCHR1-expressing neurons .

• Comparative metabolic phenotyping: Researchers compared body weight, food intake, energy expenditure, and metabolic profiles between transgenic MCHR1R2 mice and wild-type littermates using:

  • High-fat diet challenge

  • Glucose and insulin tolerance tests

  • Serum metabolic parameter measurements

  • Quantitative PCR analysis for gene expression

• Neuronal activation studies: The effects of MCH treatment on immediate-early gene c-fos activation were examined to assess how MCHR2 expression modifies MCH signaling in the central nervous system .

These approaches revealed that MCHR1R2 mice gained less weight on a high-fat diet and showed improved metabolic parameters compared to wild-type mice, suggesting MCHR2 may attenuate some obesogenic effects of MCHR1 activation .

What challenges exist in creating experimental models for studying MCHR2 function?

Creating experimental models to study MCHR2 function presents several technical and biological challenges:

• Species expression limitations: The absence of MCHR2 in rodents means that conventional mouse and rat models cannot be used to study native MCHR2 function .

• BAC recombination complexity: When creating transgenic models like MCHR1R2 mice, ensuring proper expression patterns and levels requires careful design of the BAC construct and validation of expression .

• Receptor colocalization issues: In transgenic models, ensuring that MCHR2 is expressed in the appropriate cells and at physiological levels comparable to natural expression in primates is challenging .

• Signaling cross-talk considerations: Since MCHR1 and MCHR2 couple to different G-proteins with potentially opposing effects, determining the net outcome of MCH signaling in cells expressing both receptors requires sophisticated signaling assays .

• Tissue-specific expression patterns: Expression patterns of MCHR2 may vary across brain regions compared to MCHR1, as observed in the MCHR1R2 model where "differences were noted in MCHR1R2 cerebellum where MCHR2 expression was greater than MCHR1" and "MCHR2 expression in the striatum was less than MCHR1" .

• Evolutionary differences: Subtle species differences in MCHR2 structure may impact ligand binding and signaling properties, potentially limiting the translational value of certain models.

How do experimental findings from transgenic models inform our understanding of MCHR2 function?

Transgenic models have provided critical insights into MCHR2 function despite the inherent limitations:

• Opposing actions to MCHR1: The MCHR1R2 transgenic mouse model revealed that MCHR2 may counteract some effects of MCHR1 activation. While wild-type mice developed diet-induced obesity on a high-fat diet, MCHR1R2 mice had lower food intake and were resistant to obesity .

• Metabolic improvements: MCHR1R2 mice showed improved metabolic parameters including lower serum glucose levels in both fed and fasted states (see table below) and reduced insulin levels compared to wild-type mice .

• Neuronal activation differences: MCH treatment inhibited the activation of the immediate-early gene c-fos in wild-type mice, but this inhibition was attenuated in MCHR1R2 mice, suggesting that MCHR2 modifies the cellular response to MCH .

• Coexpression effects: The study demonstrated that when both receptors are present (as in primates naturally), the net effect of MCH signaling differs from when only MCHR1 is present, which has significant implications for translational research .

These findings suggest that MCHR2 may serve as a natural "brake" on some MCHR1-mediated effects related to energy balance, potentially explaining some species differences in susceptibility to obesity.

What quantitative PCR methodologies are recommended for studying MCHR2 expression in Macaca mulatta tissues?

For accurate quantification of MCHR2 expression in Macaca mulatta tissues, the following qPCR methodology is recommended based on published research protocols:

• Tissue preparation:

  • Collect fresh tissue samples from various regions (brain regions, colon, pancreas, liver, skeletal muscle, adipose tissue)

  • Microdissect brain regions for more precise localization of expression

  • Extract RNA using QIAzol extraction and purify using QIAGEN RNeasy minicolumns

• cDNA synthesis:

  • Use 0.5 μg of total RNA from each region

  • Employ high-capacity cDNA reverse transcription kit (Applied Biosystems)

• qPCR protocol:

  • Perform using a thermal cycler such as 7900HT (Applied Biosystems)

  • Use SYBR Green master mix (Applied Biosystems)

  • Quantify relative mRNA levels by transformation against a standard curve

  • Normalize to housekeeping gene expression (e.g., cyclophilin)

• Primer design for Macaca mulatta MCHR2:

  • Forward primer: 5′-CTGCAATCCCAGTGTACCAA-3′

  • Reverse primer: 5′-ACTATGTGGACCAAATCAGCC-3′

  • These primers can be obtained from commercial suppliers like Invitrogen

• Controls:

  • Include cyclophilin as a housekeeping gene control

  • Cyclophilin primers: sense, 5′-GGTGGAGAGCACCAAGACAGA-3′; antisense, 5′-GCCGGAGTCGACAATGATG-3′

  • Include MCHR1 expression analysis for comparison: sense, 5′-CAATGCCAGCAACATCTCC-3′; antisense, 5′-ACCAAACACTGAAGGCATGA-3′

  • Include positive controls (tissues known to express MCHR2) and negative controls (no template)

This methodology allows for reliable comparison of MCHR2 expression across different tissues and brain regions in rhesus macaques, enabling correlation with functional outcomes.

How do expression patterns of MCHR2 differ between Macaca mulatta and humans?

While both species express MCHR2, there are subtle differences in expression patterns:

Understanding these species differences is crucial when using macaque models to study human MCHR2 function and when developing therapeutics targeting this receptor.

What is the significance of using recombinant Macaca mulatta MCHR2 for drug discovery compared to human MCHR2?

Using recombinant Macaca mulatta MCHR2 for drug discovery offers several advantages and considerations:

• Translational relevance: Macaques provide a more tractable experimental model than humans while maintaining high evolutionary proximity. Their MCHR2 shares high sequence homology with human MCHR2, making them valuable for predicting human responses to MCHR2-targeting compounds .

• In vivo validation pathway: Compounds that show promise against recombinant Macaca mulatta MCHR2 in vitro can be tested in vivo in the same species, providing a consistent translational pathway that's not possible with rodent models lacking MCHR2 .

• Species-specific responses: Despite high homology, subtle differences in receptor structure between macaque and human MCHR2 may lead to species-specific responses to certain compounds. Therefore, testing against both macaque and human MCHR2 provides a more comprehensive assessment .

• Dual receptor system: Like humans, macaques naturally express both MCHR1 and MCHR2, allowing assessment of compounds in the context of natural receptor balance—a critical factor since MCHR1 and MCHR2 may have opposing effects .

• Metabolic relevance: Given MCHR2's proposed role in energy homeostasis, testing in macaques provides insight into potential metabolic effects in a physiological system more similar to humans than transgenic rodent models .

The pharmaceutical significance of this approach is underscored by findings from the MCHR1R2 transgenic mouse model, which showed that MCHR2 expression protected against diet-induced obesity, suggesting MCHR2 agonism might have therapeutic potential .

What are emerging techniques for studying the interaction between MCHR1 and MCHR2 signaling pathways?

Emerging techniques for investigating MCHR1-MCHR2 signaling interactions include:

• CRISPR-engineered cell lines: Creating cell lines with precise control over expression levels of both receptors enables detailed study of signaling cross-talk in controlled environments.

• Optogenetic approaches: Using light-activated MCH receptor variants allows temporal control over receptor activation, helping delineate immediate versus delayed signaling events when both receptors are present.

• Transcriptomics and proteomics: RNA-seq and mass spectrometry analyses of tissues from models expressing both receptors (like macaques) versus MCHR1-only models (like rodents) can reveal downstream genetic and protein expression differences.

• Live-cell signaling sensors: FRET-based or bioluminescence-based reporters for second messengers downstream of Gi/o (MCHR1) and Gq (MCHR2) pathways allow real-time monitoring of signaling dynamics in the same cell.

• Proximity labeling techniques: BioID or APEX2 fusion proteins can identify proteins that interact with each receptor type in shared subcellular environments, revealing potential competition or cooperation at the level of protein complexes.

• Single-cell approaches: Analyzing single-cell transcriptomes from hypothalamic and striatal neurons can identify cell populations where both receptors are expressed versus those expressing only one receptor type, helping map the anatomical substrate for interaction.

These techniques will help elucidate how MCHR2 modifies MCHR1 signaling to produce the protective effect against diet-induced obesity observed in the MCHR1R2 mouse model .

How might sex differences impact research on MCHR2 function in Macaca mulatta?

Sex differences should be considered when studying MCHR2 function in Macaca mulatta:

• Sexual dimorphism in brain structure: Rhesus macaques exhibit greater variability in endocranial volume among males than females , which may impact the neuroanatomical distribution and density of MCHR2 expression. This could lead to sex-specific differences in MCH signaling patterns.

• Metabolic differences: Given MCHR2's role in energy homeostasis , sex differences in metabolism and susceptibility to obesity in macaques may interact with MCHR2 function. Female and male macaques may respond differently to MCHR2 modulation.

• Hormonal influences: Sex hormones may regulate MCHR2 expression or signaling sensitivity, potentially creating cyclical variations in female macaques that should be accounted for in experimental design.

• Statistical considerations: The greater male variability seen in brain structure suggests studies should be powered appropriately to detect effects in both sexes, potentially requiring larger sample sizes for males if their MCHR2-related phenotypes are more variable.

• Translational implications: Human neurological and psychiatric conditions that might involve MCH signaling often show sex biases in prevalence or presentation. Understanding sex differences in macaque MCHR2 function may illuminate the neurobiological basis of these human sex differences.

Future studies should systematically compare MCHR2 expression, signaling, and functional outcomes between male and female macaques to build a comprehensive understanding of this receptor's role in both sexes.