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

What expression systems are commonly used for recombinant Macaca fascicularis MCHR2 production?

Recombinant Macaca fascicularis MCHR2 is commonly produced in several expression systems, with each offering specific advantages:

• E. coli expression system: Most frequently used for full-length MCHR2 production, particularly with N-terminal His-tags for purification purposes. This system offers high protein yields but may require optimization for proper folding of membrane proteins .

• Mammalian cell expression: Used when post-translational modifications are critical for receptor functionality. HEK293 cells are commonly employed for expressing functional MCHR2 .

• Baculovirus expression: Offers a compromise between bacterial and mammalian systems, providing some post-translational modifications with higher yields than mammalian cells .

The choice of expression system should align with experimental requirements, considering factors like protein folding, post-translational modifications, and downstream applications .

What are the key differences between MCHR2 in Macaca fascicularis and other species?

MCHR2 shows important species-specific differences that researchers must consider:

• Absence in rodents: MCHR2 is notably absent in common laboratory rodents (mice, rats), which has significantly delayed research into this receptor as a therapeutic target. This absence makes macaque models particularly valuable for MCHR2 research .

• Human vs. Macaca fascicularis homology: The Macaca fascicularis MCHR2 shares high sequence similarity with human MCHR2, making it an excellent translational model. The conserved functional domains suggest similar ligand binding properties .

• Other macaque species: Studies comparing MCHR2 across macaque species (Macaca mulatta, Macaca fascicularis, Macaca nemestrina) show high conservation, with some specific polymorphisms that may affect receptor function or ligand binding .

This species variation must be considered when designing experiments and interpreting results, particularly for translational studies .

What purification strategies yield the highest purity for recombinant Macaca fascicularis MCHR2?

Optimal purification of recombinant Macaca fascicularis MCHR2 typically follows a multi-step approach:

• Affinity chromatography: For His-tagged MCHR2, immobilized metal affinity chromatography (IMAC) using Ni-NTA resins achieves >90% purity in a single step. Multiple short imidazole gradient elutions rather than a single step elution improve separation from contaminating proteins .

• Size exclusion chromatography (SEC): Critical for removing protein aggregates and achieving higher purity. Use of Tris/PBS-based buffers with 6% trehalose at pH 8.0 helps maintain protein stability during this step .

• Detergent considerations: For functional studies requiring properly folded transmembrane domains, mild detergents like DDM (n-Dodecyl β-D-maltoside) or LMNG (lauryl maltose neopentyl glycol) should be incorporated in all purification buffers .

Final purification typically achieves >90% purity as determined by SDS-PAGE, with proper storage in Tris/PBS-based buffer containing 6% trehalose at pH 8.0 .

How should recombinant Macaca fascicularis MCHR2 be reconstituted and stored for optimal stability?

Proper reconstitution and storage of recombinant Macaca fascicularis MCHR2 is critical for maintaining functionality:

• Reconstitution protocol:

  • Briefly centrifuge the vial before opening to bring contents to the bottom

  • Reconstitute in deionized sterile water to a concentration of 0.1-1.0 mg/mL

  • Add glycerol to a final concentration of 50% for long-term storage

  • Prepare working aliquots to avoid repeated freeze-thaw cycles

• Storage conditions:

  • Long-term storage: -20°C/-80°C in Tris/PBS-based buffer with 50% glycerol

  • Working aliquots: Store at 4°C for up to one week

  • Avoid repeated freeze-thaw cycles as this significantly reduces protein stability and functional activity

• Stability considerations:

  • Monitor purity regularly using SDS-PAGE

  • For functional studies, validate receptor activity after storage using binding assays or G protein-dissociation assays

Properly stored protein maintains functionality for approximately 6 months at -20°C/-80°C in liquid form and 12 months when lyophilized .

What methods are used to assess the functional activity of recombinant Macaca fascicularis MCHR2?

Several complementary methods can evaluate functional activity of recombinant Macaca fascicularis MCHR2:

• G protein-dissociation assays: These monitor the ability of activated MCHR2 to promote dissociation of G proteins (typically Gαi1 from Gβγ) using BRET (Bioluminescence Resonance Energy Transfer) technology. This approach requires:

  • Co-transfection of MCHR2 with Gαi1-RLuc8, Gβ3, and GFP2-Gγ9 in HEK293T cells

  • Stimulation with MCH at varying concentrations

  • Measurement of BRET signal ratio (515 nm/410 nm)

• Calcium mobilization assays: Using FLIPR (Fluorescent Imaging Plate Reader) technology to detect intracellular calcium changes upon receptor activation. This was successfully employed for screening MCHR2 antagonists .

• Radioligand binding assays: For determining binding affinity constants (Ki) of ligands to MCHR2. These typically employ membrane preparations from cells expressing recombinant MCHR2 .

• Real-time kinetic measurements: Using techniques like LigandTracer® technology for monitoring binding kinetics, which more closely mimics in vivo conditions than equilibrium binding assays .

Functional activity should be validated before using the recombinant protein in downstream applications .

How do antagonists bind to Macaca fascicularis MCHR2 and how can they be characterized?

Characterization of antagonist binding to Macaca fascicularis MCHR2 involves several complementary approaches:

• Structural basis of antagonist binding:

  • Cryo-EM structures reveal that antagonists like SNAP-94847 bind within the transmembrane helices of MCH receptors

  • Key binding involves the inward movement of the intracellular end of TM6, which prevents G protein coupling

  • This causes receptor inactivation by triggering closure of the cytoplasmic pocket

• Antagonist screening methodologies:

  • FLIPR (Fluorescent Imaging Plate Reader) assays using cells expressing recombinant MCHR2

  • Spiropiperidine carbazoles have been identified as potent and specific MCHR2 antagonists

  • Compound 38 (a specific antagonist) showed favorable pharmacokinetic properties across species including rhesus monkey

• Selectivity profiling:

  • Cross-reactivity testing against related receptors (such as MCHR1)

  • Assessment of binding to off-target receptors like ADRB3 (β3-adrenergic receptor)

  • Determination of Ki values through competitive binding assays with reference compounds

The antagonist testing provides appropriate tool compounds for studying MCHR2 function in vivo and potential therapeutic applications .

How does the genetic diversity of MCHR2 across macaque species impact experimental design?

Genetic diversity of MCHR2 across macaque species has significant implications for experimental design:

• Known polymorphisms: Research has identified single nucleotide polymorphisms (SNPs) in macaque receptors that may impact ligand binding and receptor signaling. While comprehensive MCHR2-specific polymorphism data is limited, studies on related receptors suggest considerable variation exists .

• Cross-species considerations:

  • Cynomolgus macaques (Macaca fascicularis), rhesus macaques (Macaca mulatta), and pig-tailed macaques (Macaca nemestrina) show some receptor polymorphisms that can influence experimental outcomes

  • These polymorphisms may affect binding affinity, signaling efficacy, and antagonist potency

• Experimental design implications:

  • Genotyping of MCHR2 should be performed in macaques used for pharmacological studies

  • Colony-specific variations may exist and should be characterized before initiating large studies

  • Comparison of results across different macaque colonies requires consideration of genetic background

For translational research, incorporating MCHR2 sequencing in humoral immunity-based studies is critical to account for these variations and prevent confounding study outcomes .

What structural insights have been gained about MCH receptors that might apply to Macaca fascicularis MCHR2?

Recent structural studies provide valuable insights potentially applicable to Macaca fascicularis MCHR2:

• Cryo-EM structures of MCHR1:

  • The active-state MCHR1 structure complexed with MCH and Gi1 reveals the mechanism of receptor activation

  • Multiple conformational states have been identified, distinguished by the relative orientation between MCHR1 and the G protein heterotrimer

  • These conformers show differences in ICL1 (first intracellular loop) conformation and G protein coupling modes

• Antagonist binding mechanisms:

  • Structures of antagonist-bound MCHR1 (with SNAP-94847) reveal that antagonists bind within the transmembrane helices

  • The inward movement of TM6's intracellular end prevents receptor coupling with downstream effectors

  • This structural feature is likely conserved in MCHR2

• Ligand recognition mechanisms:

  • Key residues for MCH peptide recognition have been identified including D192^3.32, Q196^3.36, Y341^6.51, I366^7.39, and Y370^7.43

  • Mutation of these residues dramatically impairs MCH-induced G protein signaling

  • The binding pocket for MCH resembles that of somatostatin in SSTR2, suggesting evolutionary relationships

• MCHR1 vs MCHR2 structural comparison:

  • Alignment of 33 residues responsible for MCH recognition by MCHR1 shows that 22 are similar in MCHR2

  • Non-conserved residues are predominantly in peripheral areas with fewer MCH contacts

  • This suggests relatively conserved MCH-binding mechanisms between the two receptors

These structural insights provide a framework for understanding MCHR2 function and designing selective ligands .

What are the methodological considerations for studying MCHR2 in the context of energy homeostasis?

Investigating MCHR2's role in energy homeostasis requires specialized methodological approaches:

• Tissue expression profiling:

  • Western blot analysis has been used to detect MCHR1 expression in brown adipose tissue (BAT)

  • Similar techniques can be applied to study MCHR2 expression in metabolically active tissues of Macaca fascicularis

  • The detected expression patterns should be validated using multiple antibodies due to potential cross-reactivity

• PET-tracer development and imaging:

  • [¹⁸F]FE@SNAP and [¹¹C]SNAP-7941 have been developed as PET-tracers for MCH receptors

  • These tracers show high selectivity for MCH receptors over ADRB3 (β3-adrenergic receptor)

  • In vivo imaging allows non-invasive assessment of receptor occupancy and distribution

• Transgenic approaches:

  • The MCHR1R2 mouse model with human MCHR2 inserted into the mouse Mchr1 gene locus provides a system to study MCHR2 function in vivo

  • This model expresses hMCHR2 selectively in cells normally expressing mouse MCHR1

  • Similar approaches could be developed for studying specific MCHR2 functions in non-human primates

• Biological sample collection considerations:

  • For in vivo studies, humane blood collection methods have been developed for macaques

  • Training macaques for voluntary in-homecage venipuncture reduces stress and improves data validity

  • This approach allows blood collection without disrupting normal hormone levels that could confound metabolic studies

These methodological considerations are essential for designing robust experiments to understand MCHR2's role in energy homeostasis .

What are the major challenges in producing functional recombinant Macaca fascicularis MCHR2?

Production of functional recombinant Macaca fascicularis MCHR2 faces several technical challenges:

• Membrane protein expression issues:

  • As a seven-transmembrane G protein-coupled receptor, MCHR2 is challenging to express in correctly folded form

  • E. coli expression systems often result in inclusion bodies requiring refolding

  • Solution: Use of specialized E. coli strains (like C41/C43) with optimized membrane protein expression conditions or switching to eukaryotic expression systems

• Protein stability concerns:

  • The transmembrane domains of MCHR2 require proper detergent selection for stability

  • Repeated freeze-thaw cycles significantly reduce activity

  • Solution: Addition of trehalose (6%) and glycerol (50%) to storage buffers and preparation of single-use aliquots

• Functional validation challenges:

  • Confirming proper folding and function of recombinant MCHR2 requires specialized assays

  • Solution: Implementation of G protein-dissociation assays using BRET technology in transfected cells to verify signaling capability

• Tag interference with function:

  • N-terminal tags may interfere with ligand binding or signaling

  • Solution: Use of cleavable tags or careful positioning of tags based on structural information; validate function with both tagged and untagged versions

Each challenge requires specific technical solutions to ensure production of functional receptor protein for research applications .

How can species-specific differences in MCHR2 be addressed when translating findings from macaque models to humans?

Addressing species-specific differences when translating MCHR2 findings requires systematic approaches:

• Comparative receptor pharmacology:

  • Direct comparison of human and Macaca fascicularis MCHR2 responses to the same ligands

  • Determination of binding affinities (Kd) and potencies (EC50) for both receptors

  • Analysis of signaling pathway differences between species

• Structural biology approaches:

  • Homology modeling based on available MCH receptor structures

  • Identification of species-specific amino acid differences in binding pockets

  • Molecular dynamics simulations to predict functional consequences of sequence variations

• Creation of humanized models:

  • Development of cell lines expressing human MCHR2 alongside macaque MCHR2

  • Generation of transgenic models where appropriate (as demonstrated with the MCHR1R2 mouse)

• Consideration of genetic diversity:

  • Sequencing MCHR2 from multiple individuals of both species

  • Identifying polymorphisms that might affect drug responses

  • Creating databases of genetic variants and their functional consequences

These approaches help establish reliable translational bridges between macaque studies and human applications, essential for drug development targeting MCHR2 .

What emerging technologies might advance Macaca fascicularis MCHR2 research?

Several cutting-edge technologies hold promise for advancing MCHR2 research:

• Cryo-EM advancements:

  • Higher resolution structures of MCHR2 in various conformational states

  • Visualization of ligand-receptor interactions at atomic resolution

  • Application of methods like single-particle cryo-EM that have already yielded insights into related receptors

• AI-assisted structural prediction:

  • Tools like AlphaFold2 have been successfully applied to predict structures of MCH receptors and their isoforms

  • These predictions can map locations of genetic variability onto structural models

  • Such approaches can provide insight into functional significance of polymorphisms

• Long-read RNA sequencing:

  • Applied to characterize genetic diversity in macaque receptors

  • Can identify novel isoforms and coding variants

  • Pacific Biosciences (PacBio) technology has already cataloged diversity in related receptors

• GPCR-specific nanobody development:

  • Nanobodies that stabilize specific MCHR2 conformations for structural studies

  • Application of techniques like NanoBiT tethering strategies to promote complex formation and increase stability for structural analysis

• NMR studies of conformational dynamics:

  • Investigation of MCHR2 dynamics in different ligand-bound states

  • Complementary to static structural approaches for understanding receptor activation and signaling

These technologies will provide deeper insights into MCHR2 structure, function, and potential therapeutic applications .

What are the most promising research applications for recombinant Macaca fascicularis MCHR2?

Recombinant Macaca fascicularis MCHR2 offers several promising research applications:

• Drug discovery platforms:

  • Development of selective MCHR2 antagonists as tool compounds

  • Screening libraries for novel ligands with therapeutic potential

  • Structure-based drug design informed by recent structural insights

• Metabolic disease research:

  • Investigation of MCHR2's role in energy homeostasis and obesity

  • Exploration of the receptor's function in brown adipose tissue (BAT)

  • Development of potential therapeutics for metabolic disorders

• Comparative receptor pharmacology:

  • Direct comparison with human MCHR2 to establish translational validity

  • Investigation of species-specific differences in ligand binding and signaling

  • Development of predictive models for drug efficacy across species

• Neuropsychiatric research:

  • Exploration of MCHR2's role in sleep regulation and anxiety

  • Development of PET tracers for in vivo imaging of receptor distribution

  • Investigation of the receptor as a target for mental health conditions

• Model development:

  • Creation of cell-based and transgenic models expressing Macaca fascicularis MCHR2

  • Development of bioassays for screening therapeutic candidates

  • Establishment of systems for studying receptor signaling pathways

These research directions leverage the evolutionary closeness of Macaca fascicularis to humans, offering valuable insights into MCHR2 biology with translational relevance .