Recombinant Macaca fascicularis Mas-related G-protein coupled receptor member E (MRGPRE)

What is MRGPRE and how does it differ from other MRG family members?

MRGPRE (Mas-related G-protein coupled receptor member E) belongs to the MRG family of G-protein coupled receptors that are predominantly expressed in sensory neurons. Unlike other MRG family members that are mainly restricted to dorsal root ganglia (DRG), MRGPRE exhibits a broader expression pattern, including numerous brain regions in macaques, mice, and humans . Structurally, MRGPRE contains an alanine residue at position 34.51, which distinguishes it from other MRGPRs and may significantly affect its signaling properties, potentially limiting its reliance on Gq protein signaling pathways . This receptor represents one of the several MRG orthologs identified in Macaca fascicularis and is part of a receptor family that has evolved to detect noxious stimuli.

How conserved is MRGPRE across species compared to other MRG family receptors?

MRGPRE shows considerable interspecies variation, though less dramatic than some other MRG family members. While human MRGPRs generally display 50-100 times higher allele frequencies of missense mutations compared to other GPCRs like 5-HT1AR, indicating rapid evolutionary adaptations . The MRG family demonstrates remarkable species diversity - mice possess 27 MRGPRs with intact open reading frames (including members from MRGPRA to MRGPRG), while humans have only eight MRGPRs, lacking members in the MRGPRA, B, and C subfamilies . Macaca fascicularis MRGPRE represents an important evolutionary comparison point, as non-human primates generally have MRGPR repertoires more closely resembling humans than rodents do, making them valuable research models for human MRGPR function .

What are the key structural features that differentiate MRGPRs including MRGPRE from canonical GPCRs?

MRGPRs, including MRGPRE, possess several unique structural characteristics that distinguish them from canonical GPCRs:

• MRGPRs lack most canonical activation motifs found in family A GPCRs, including the CWxP motif, PIF motif, and the semi-conserved DRY motif .

• They feature a distinctive TM4-TM5 disulfide bond rather than the TM3-ECL2 disulfide bond observed in most family A GPCRs (Figure 4B in original research) .

• The absence of the TM3-ECL2 disulfide bond in MRGPRs is significant as this bond typically stabilizes the ECL2 conformation above the ligand binding pocket in other GPCRs, affecting ligand retention and dissociation rates .

• MRGPRs demonstrate considerable conformational plasticity, with evidence from related receptors (MRGPRX2, MRGPRD) showing inward movements of TM6 and large conformational changes in extracellular loops upon agonist binding .

What is the expression profile of MRGPRE in Macaca fascicularis tissues?

MRGPRE in Macaca fascicularis shows a distinctive expression pattern compared to other MRG family members. While most MRG receptors are predominantly expressed in small-diameter sensory neurons of the dorsal root ganglia (DRG), MRGPRE exhibits a broader distribution . In macaques, MRGPE is expressed in DRG neurons but also found in numerous brain regions, similar to its expression pattern in mice and humans . This broader neuronal distribution suggests MRGPRE may serve functions beyond peripheral nociception, potentially influencing central nervous system processes. The specific brain regions expressing MRGPRE include areas associated with sensory processing and integration, though detailed mapping studies of MRGPRE expression across the macaque brain would provide more comprehensive information.

How can researchers effectively detect MRGPRE expression in tissue samples?

For detecting MRGPRE expression in Macaca fascicularis tissue samples, researchers should employ a multi-modal approach combining:

• RNA detection methods:

  • RT-PCR using primers specifically designed for Macaca fascicularis MRGPRE

  • In situ hybridization with species-specific probes

  • RNAscope® for high-sensitivity single-molecule detection

• Protein detection methods:

  • Immunohistochemistry using validated antibodies against macaque MRGPRE

  • Western blotting for protein quantification

  • Proximity ligation assays to detect protein-protein interactions

• Co-localization studies:

  • Double-labeling with established neuronal markers (IB4, VR1, SP) as performed for other MRG receptors

  • Comparison with known expression patterns of related receptors (MRGD, MRGX)

When working with macaque samples, it's essential to use species-specific tools as the high sequence diversity of MRGPRs across species means that tools designed for human or rodent MRGPRs may not cross-react effectively with macaque receptors .

What downstream signaling pathways are activated by MRGPRE in Macaca fascicularis?

The signaling profile of Macaca fascicularis MRGPRE remains to be fully characterized, but structural features provide important insights. MRGPRE contains an alanine residue at position 34.51, which has been shown to significantly reduce agonist-stimulated Gq signaling in other MRGPRs and several other GPCRs . This suggests that MRGPRE might not primarily rely on Gq protein for signaling, unlike some other MRGPRs.

For experimental investigations of MRGPRE signaling, researchers should consider:

• Using G15, a permissive G protein that can couple with many GPCRs, to mediate Ca²⁺ flux in functional assays

• Investigating alternate G protein coupling, particularly examining Gi/o pathways

• Exploring Gβγ-dependent modulation of ion channels, as observed with related receptors MRGPRA3, MRGPRC11, and MRGPRX1

• Examining potential interactions with TRP channels and N-type HVA calcium channels

The signaling complexity of MRGPRs suggests that MRGPRE likely exhibits a diverse signaling profile that may differ substantially from related receptors despite sequence similarities.

How does MRGPRE respond to potential endogenous and synthetic ligands?

While specific ligands for Macaca fascicularis MRGPRE have not been definitively identified in the provided research, the MRGPRs family is known to respond to diverse ligands ranging from small molecules to large peptides . For experimental identification of MRGPRE ligands, researchers should:

• Conduct comprehensive screening with:

  • Neuropeptides (examining related peptides that activate other MRGPRs)

  • Small molecule libraries

  • Known ligands of human MRGPRE

• Use functional assays measuring:

  • G protein activation (focusing on multiple G protein subtypes)

  • β-arrestin recruitment

  • Calcium mobilization

  • Membrane potential changes

• Validate hits with:

  • Dose-response analyses

  • Selectivity profiling against other MRGPRs

  • Structure-activity relationship studies

Research should account for potential species differences in ligand responsiveness, as MRGPRs show high sequence diversity across species that affects ligand binding pockets . For example, even closely related species like humans and rhesus macaques can show substantial differences in ligand responses due to variations in the extracellular binding pocket .

What expression systems are most effective for producing functional recombinant Macaca fascicularis MRGPRE?

For successful expression of functional recombinant Macaca fascicularis MRGPRE, researchers should consider the following expression systems with their respective advantages:

For structural studies of MRGPRs, recent advances have employed cryo-electron microscopy (cryoEM) techniques that successfully determined structures of related receptors (MRGPRX2, MRGPRX4, MRGPRD, and MRGPRX1) . These techniques required optimized expression systems, typically insect cells, to overcome challenges associated with non-canonical structural motifs and intrinsic high basal activity of MRGPRs .

What are the optimal purification strategies for obtaining high-quality recombinant MRGPRE protein?

Purification of recombinant Macaca fascicularis MRGPRE requires specialized approaches due to its membrane protein nature and structural characteristics. Based on successful strategies for related MRGPRs, the following protocol is recommended:

• Solubilization and Extraction:

  • Use mild detergents (DDM, LMNG, or GDN) for initial solubilization

  • Consider lipid nanodisc or SMALP technology for maintaining native-like environment

  • Optimize detergent:protein ratios to maximize extraction while preserving structure

• Affinity Purification:

  • Include affinity tags (His6, FLAG, or SUMO) at N-terminus with cleavable linkers

  • Consider using fusion partners (T4 lysozyme or thermostabilized apocytochrome b562) for stability enhancement

  • Employ two-step affinity chromatography for higher purity

• Size Exclusion Chromatography:

  • Critical final step to ensure homogeneity and remove aggregates

  • Buffer optimization to maintain stability (consider including cholesterol hemisuccinate)

  • Quality assessment via SDS-PAGE (>90% purity standard)

• Storage Conditions:

  • Store in stabilizing buffer containing appropriate detergent/lipid mixture

  • Consider flash-freezing aliquots in liquid nitrogen

  • Avoid repeated freeze-thaw cycles

  • For short-term storage, maintain at 4°C; for long-term, store at -80°C

This approach should yield protein with >90% purity as assessed by SDS-PAGE, similar to the quality standards for other recombinant Macaca fascicularis proteins .

How do polymorphisms in MRGPRE affect receptor function and potential research applications?

MRGPRs display remarkably high polymorphism rates compared to other GPCRs, with missense mutation frequencies 50-100 times higher than well-characterized receptors like 5-HT1AR . While specific polymorphism data for Macaca fascicularis MRGPRE is not detailed in the provided research, this high mutation rate across the MRGPR family suggests several important considerations for researchers:

• Functional Consequences:

  • Polymorphisms can significantly affect ligand binding and signaling efficacy, as demonstrated with the L83S variant of MRGPRX4 showing reduced sensitivity to nateglinide and synthetic agonists

  • Some variants may affect protein conformation and stability rather than directly impacting the binding pocket

  • Expression levels and trafficking efficiency may be altered by certain polymorphisms

• Research Strategy Implications:

  • Source animals for receptor isolation should be genotyped when possible

  • Experiments should account for potential allelic variations

  • Comparative studies between variants can provide valuable insights into structure-function relationships

• Translational Relevance:

  • High polymorphism rates suggest MRGPRs may be continuously evolving to respond to diverse noxious stimuli

  • Population-specific variations (as seen with the N245S mutation in MRGPRX4 affecting mentholated cigarette preference in African populations) highlight the importance of considering genetic diversity in translational research

How does Macaca fascicularis MRGPRE compare structurally and functionally to human and rodent orthologs?

The structural and functional comparison between Macaca fascicularis MRGPRE and its human and rodent counterparts reveals important evolutionary adaptations and species differences:

• Sequence Conservation and Divergence:

  • MRGPRs show high sequence diversity across species, with even closely related species showing substantial differences

  • While specific sequence identity percentages for MRGPRE aren't provided, the example of rhesus macaque MRGPRX4 sharing only 81% sequence identity with human MRGPRX4 suggests potentially significant divergence in MRGPRE as well

  • These sequence variations often concentrate around the extracellular ligand binding pocket, affecting pharmacological responses

• Structural Implications:

  • Predicted structural models would likely show divergent extracellular pockets between species, similar to the comparison between rhesus macaque and human MRGPRX4

  • All MRGPRs share unique structural features including the TM4-TM5 disulfide bond and absence of canonical GPCR motifs

• Functional Considerations:

  • The lack of conservation between MRGPR orthologs complicates cross-species extrapolation of functional data

  • Macaca fascicularis models provide a more translatable system for human MRGPR research than rodent models

  • Humanized mice are often employed for studying human MRGPRs function due to these significant interspecies differences

The evolutionary diversity of MRGPRs underscores the importance of using appropriate species-specific models when studying these receptors in the context of nociception and other physiological roles.

What roles might MRGPRE play in nociception and how can this be experimentally evaluated?

While the specific role of MRGPRE in nociception hasn't been fully characterized in the provided research, its expression pattern and relationship to other MRG family members suggest potential involvement in pain processing. MRG family receptors are predominantly expressed in small diameter sensory neurons of the dorsal root ganglia (DRG), suggesting roles in nociception .

To experimentally evaluate MRGPRE's role in nociception, researchers should consider:

• Cellular Localization Studies:

  • Determine if Macaca fascicularis MRGPRE co-localizes with established nociceptive neuronal markers (IB4, VR1, SP) as observed with related receptors MRGX2 and MRGD

  • Map expression across different neuronal populations using single-cell RNA sequencing

• Functional Assays:

  • Calcium imaging in MRGPRE-expressing DRG neurons to assess responses to noxious stimuli

  • Electrophysiological recordings to characterize how MRGPRE activation alters neuronal excitability

  • Investigation of downstream signaling through G proteins and potential modulation of ion channels including TRP channels and N-type HVA calcium channels as observed with related MRGPRs

• In Vivo Models:

  • Generation of MRGPRE knockout or conditional knockout models

  • Behavioral testing using established pain paradigms

  • Pharmacological manipulation using identified agonists/antagonists

The broader expression of MRGPRE in brain regions suggests it may have functions beyond peripheral nociception, potentially in central processing of sensory information.

How might recombinant MRGPRE be used in drug discovery for novel pain or itch therapeutics?

Recombinant Macaca fascicularis MRGPRE represents a valuable tool for drug discovery efforts targeting pain and itch pathways. MRGPRs have emerged as novel drug targets for treating itch, allergy, neuroinflammation, and pain . To leverage recombinant MRGPRE for therapeutic development:

• High-Throughput Screening Approaches:

  • Develop cell-based assays using recombinant MRGPRE for compound library screening

  • Implement BRET-based G protein dissociation assays to assess promiscuous signaling activation as shown for related receptors

  • Design functional readouts appropriate to MRGPRE's signaling profile

• Structure-Based Drug Design:

  • Apply recent structural insights from related MRGPRs to guide rational drug design

  • Account for MRGPRE's unique structural features, particularly the distinctive disulfide bonding pattern and extracellular pocket architecture

  • Develop homology models based on recently solved MRGPR structures to predict ligand binding

• Translational Considerations:

  • Compare pharmacological responses between macaque and human MRGPRE to assess translational potential

  • Address species differences in MRGPR sequence and function when extrapolating to human applications

  • Consider using humanized animal models for in vivo validation of promising compounds

The non-opioid nature of MRGPR-targeted therapies makes them particularly attractive given the urgent need for non-opioid medications to treat both acute and chronic pain conditions . Additionally, the involvement of MRGPRs in itch suggests potential applications for treating drug-induced pruritus, liver dysfunction-associated itch, and chronic skin diseases .