MRAP2 Interaction with Melanocortin-4 Receptor in SnakeHead (Channa argus)

Research on the mechanism of buyang huanwu decoction in the amelioration of age-associated memory impairment based on the "co-occurrence network regulation of intestinal microecology-host metabolism-immune function"

ETHNOPHARMACOLOGICAL RELEVANCE

Brain aging can promote neuronal damage, contributing to aging-related diseases like memory dysfunction. Buyang Huanwu Decoction (BYHWD), a traditional Chinese medicine formula known for tonifying qi and activating blood circulation, shows neuroprotective properties. Despite this, the specific mechanism by which BYHWD improves age-associated memory impairment (AAMI) has not been explored in existing literature.

AIM OF THE STUDY

This study aimed to investigate the mechanism of BYHWD in the improvement of AAMI based on the "co-occurrence network regulation of intestinal microecology-host metabolism-immune function".

MATERIALS AND METHODS

Firstly, D-galactose was performed to induce a rat model of AAMI. Learning and memory deficits was assessed by the Morris water maze test. H&E and Nissl staining were used to observe the pathological changes in neurons in the hippocampus of rats. Meanwhile, the levels of pro-inflammatory cytokines and the activation of antioxidant enzymes in rat serum were measured using ELISA. Finally, an integrated pharmacological approach was applied to explore the potential mechanism of BYHWD in improving AAMI.

RESULTS

Our results indicated that BYHWD significantly mitigated the pathological structure of the hippocampus, reversed the levels of IL-6, TNF-α, GSH, and CAT in the serum, and improved learning and memory in aging rats. Transcriptomics combined with network pharmacology showed that energy metabolism and the inflammatory response were the key biological pathways for BYHWD to ameliorate AAMI. Integrative analysis of the microbiome and metabolomics revealed that BYHWD has the potential to restore the balance of abundance between probiotics and harmful bacteria, and ameliorate the reprogramming of energy metabolism caused by aging in the brain. The co-occurrence network analysis demonstrated that a strong correlation between the treatment of AAMI and the stability of intestinal microecology, host metabolism, and immune network.

CONCLUSION

The findings of this study collectively support the notion that BYHWD has a superior therapeutic effect in an AAMI rat model. The mechanism involves regulating the "intestinal microecology-metabolism-immune function co-occurrence network" system to restore the composition of gut microbiota and metabolites. This further improves the metabolic phenotype of brain tissue and maintains the homeostasis of central nervous system's immunity, leading to an improvement in AAMI. Consequently, this study offers a unique perspective on the prevention and treatment of AAMI. And, BYHWD is also considered to be a promising preclinical treatment for improving AAMI.

Molecular characterization, spatio-temporal expression patterns of crtc2 gene and its immune roles in yellow catfish (Pelteobagrus fulvidraco)

cAMP response element binding (CREB) protein 2 (CRTC2) is a transcriptional coactivator of CREB and plays an important role in the immune system. Thus far, the physiological roles of Crtc2 in teleost are still poorly understood. In this study, the crtc2 gene was identified and characterized from yellow catfish (Pelteobagrus fulvidraco; therefore, the gene is termed as pfcrtc2), and its evolutionary and molecular characteristics as well as potential immunity-related roles were investigated. Our results showed that the open reading frame of pfcrtc2 was 2346 bp in length, encoding a protein with 781 amino acids. Gene structure analysis revealed its existence of 14 exons and 13 introns. A phylogenetic analysis proved that the tree of crtc2 was clustered into five groups, exhibiting a similar evolutionary topology with species evolution. Multiple protein sequences alignment demonstrated high conservation of the crtc2 in various vertebrates with similar structure. Syntenic and gene structural comparisons further established that crtc2 was highly conserved, implying its similar roles in diverse vertebrates. Tissue distribution pattern detected by quantitative real-time PCR showed that the pfcrtc2 gene was almost expressed in all detected tissues except for eyes, with the highest expression levels in the gonad, indicating that Crtc2 may play important roles in various tissues. In addition, pfcrtc2 was transcribed at all developmental stages in yellow catfish, showing the highest expression levels at 12 h after fertilization. Finally, the transcriptional profiles of crtc2 were significantly increased in yellow catfishes injected with Aeromonas hydrophila or Poly I:C, which shared a consistent change pattern with four immune-related genes including IL-17A, IL-10, MAPKp38, and NF-κBp65, suggesting pfCrtc2 may play critical roles in preventing both exogenous bacteria and virus invasion. In summary, our findings lay a solid foundation for further studies on the functions of pfcrtc2, and provide novel genetic loci for developing new strategies to control disease outbreak in teleost.

MRAP2a Binds and Modulates Activity and Localisation of Prokineticin Receptor 1 in Zebrafish

The prokineticin system plays a role in hypothalamic neurons in the control of energy homeostasis. Prokineticin receptors (PKR1 and PKR2), like other G-protein-coupled receptors (GPCRs) are involved in the regulation of energy intake and expenditure and are modulated by the accessory membrane protein 2 of the melanocortin receptor (MRAP2). The aim of this work is to characterise the interaction and regulation of the non-melanocortin receptor PKR1 by MRAP2a in zebrafish (zMRAP2a) in order to use zebrafish as a model for the development of drugs targeting accessory proteins that can alter the localisation and activity of GPCRs. To this end, we first showed that zebrafish PKR1 (zPKR1) is able to interact with both zMRAP2a and human MRAP2 (hMRAP2). This interaction occurs between the N-terminal region of zPKR1 and the C-terminal domain of zMRAP2a, which shows high sequence identity with hMRAP2 and a similar propensity for dimer formation. Moreover, we demonstrated that in Chinese hamster ovary (CHO) cells, zMRAP2a or hMRAP2 are able to modulate zPKR1 activation induced by zebrafish PK2 (zPK2) resulting in an impaired ERK and STAT3 activation.

Structural and pharmacological characterization of a medium-chain fatty acid receptor GPR84 in common carp (Cyprinus carpio)

The medium-chain fatty acid receptor GPR84, a member of the G protein-coupled receptor family, is mainly expressed in macrophages and microglia, and is involved in the regulation of inflammatory responses and retinal development in mammals and amphibians. However, structure, tissue distribution, and pharmacology of this receptor have rarely been reported in fish. In this study, we cloned the coding sequence (CDS) of common carp GPR84 (ccGPR84), examined its tissue distribution, and explored its cellular signaling function. The results showed that the CDS of ccGPR84 is 1191 bp and encodes a putative protein with 396 amino acids. Phylogenetic and chromosomal synteny analyses revealed that ccGPR84 was evolutionarily conserved with Cyprinids. Real-time quantitative PCR (qPCR) indicated that ccGPR84 was predominantly expressed in the intestine and spleen. Luciferase reporter assay demonstrated that nonanoic acid, capric acid (decanoic acid), undecanoic acid and lauric acid could inhibit cAMP signaling pathway and activate MAPK/ERK signaling pathway, while the potencies of these four fatty acids on the two signaling pathways were different. Lauric acid has the highest inhibitory potency on cAMP signaling pathway, followed by undecanoic acid, nonanoic acid, and capric acid. While for MAPK/ERK signaling pathway, nonanoic acid has the highest activation potency, followed by undecanoic acid, capric acid, and lauric acid. These findings lay the foundation for revealing the roles of different medium-chain fatty acids in the inflammatory response of common carp.

Genome-wide detection of positive and balancing signatures of selection shared by four domesticated rainbow trout populations (Oncorhynchus mykiss)

BACKGROUND

Evolutionary processes leave footprints along the genome over time. Highly homozygous regions may correspond to positive selection of favorable alleles, while maintenance of heterozygous regions may be due to balancing selection phenomena. We analyzed data from 176 fish from four disconnected domestic rainbow trout populations that were genotyped using a high-density Axiom Trout genotyping 665K single nucleotide polymorphism array, including 20 from the US and 156 from three French lines. Using methods based on runs of homozygosity and extended haplotype homozygosity, we detected signatures of selection in these four populations.

RESULTS

Nine genomic regions that included 253 genes were identified as being under positive selection in all four populations Most were located on chromosome 2 but also on chromosomes 12, 15, 16, and 20. In addition, four heterozygous regions that contain 29 genes that are putatively under balancing selection were also shared by the four populations. These were located on chromosomes 10, 13, and 19. Regardless of the homozygous or heterozygous nature of the regions, in each region, we detected several genes that are highly conserved among vertebrates due to their critical roles in cellular and nuclear organization, embryonic development, or immunity. We identified new candidate genes involved in rainbow trout fitness, as well as 17 genes that were previously identified to be under positive selection, 10 of which in other fishes (auts2, atp1b3, zp4, znf135, igf-1α, brd2, col9a2, mrap2, pbx1, and emilin-3).

CONCLUSIONS

Using material from disconnected populations of different origins allowed us to draw a genome-wide map of signatures of positive selection that are shared between these rainbow trout populations, and to identify several regions that are putatively under balancing selection. These results provide a valuable resource for future investigations of the dynamics of genetic diversity and genome evolution during domestication.

Molecular Cloning, Tissue Distribution, and Pharmacological Characterization of GPR84 in Grass Carp (Ctenopharyngodon Idella)

The G-protein-coupled receptor GPR84, activated by medium-chain fatty acids, primarily expressed in macrophages and microglia, is involved in inflammatory responses and retinal development in mammals and amphibians. However, our understanding of its structure, function, tissue expression, and signaling pathways in fish is limited. In this study, we cloned and characterized the coding sequence of GPR84 (ciGPR84) in grass carp. A phylogenetic analysis revealed its close relationship with bony fishes. High expression levels of GPR84 were observed in the liver and spleen. The transfection of HEK293T cells with ciGPR84 demonstrated its responsiveness to medium-chain fatty acids and diindolylmethane (DIM). Capric acid, undecanoic acid, and lauric acid activated ERK and inhibited cAMP signaling. Lauric acid showed the highest efficiency in activating the ERK pathway, while capric acid was the most effective in inhibiting cAMP signaling. Notably, DIM did not activate GPR84 in grass carp, unlike in mammals. These findings provide valuable insights for mitigating chronic inflammation in grass carp farming and warrant further exploration of the role of medium-chain fatty acids in inflammation regulation in this species.

Divergent Pharmacology and Biased Signaling of the Four Melanocortin-4 Receptor Isoforms in Rainbow Trout (Oncorhynchus mykiss)

The melanocortin-4 receptor (MC4R) is essential for the modulation of energy balance and reproduction in both fish and mammals. Rainbow trout (Oncorhynchus mykiss) has been extensively studied in various fields and provides a unique opportunity to investigate divergent physiological roles of paralogues. Herein we identified four trout mc4r (mc4ra1, mc4ra2, mc4rb1, and mc4rb2) genes. Four trout Mc4rs (omMc4rs) were homologous to those of teleost and mammalian MC4Rs. Multiple sequence alignments, a phylogenetic tree, chromosomal synteny analyses, and pharmacological studies showed that trout mc4r genes may have undergone different evolutionary processes. All four trout Mc4rs bound to two peptide agonists and elevated intracellular cAMP levels dose-dependently. High basal cAMP levels were observed at two omMc4rs, which were decreased by Agouti-related peptide. Only omMc4rb2 was constitutively active in the ERK1/2 signaling pathway. Ipsen 5i, ML00253764, and MCL0020 were biased allosteric modulators of omMc4rb1 with selective activation upon ERK1/2 signaling. ML00253764 behaved as an allosteric agonist in Gs-cAMP signaling of omMc4rb2. This study will lay the foundation for future physiological studies of various mc4r paralogs and reveal the evolution of MC4R in vertebrates.

Molecular characterization of a teleost-specific toll-like receptor 22 (tlr22) gene from yellow catfish (Pelteobagrus fulvidraco) and its transcriptional change in response to poly I:C and Aeromonas hydrophila stimuli

Toll-like receptors (TLRs) are a class of pattern recognition receptors (PRRs) that can recognize pathogen-associated molecular patterns (PMPs) and play important roles in the innate immune system in vertebrates. In this study, we identified a teleost-specific tlr22 gene from yellow catfish (Pelteobagrus fulvidraco) and its immune roles in response to different pathogens were also determined. The open reading frame (ORF) of the tlr22 was 2892 bp in length, encoding a protein of 963 amino acids. Multiple protein sequences alignment, secondary and three-dimensional structure analyses revealed that TLR22 is highly conserved among different fish species. Phylogenetic analysis showed that the phylogenetic topology was divided into six families of TLR1, TLR3, TLR4, TLR5, TLR7 and TLR11, and TLR22 subfamily was clustered into TLR11 family. Meanwhile, synteny and gene structure comparisons revealed functional and evolutionary conservation of the tlr22 gene in teleosts. Furthermore, tlr22 gene was shown to be widely expressed in detected tissues except barbel and eye, with highest expression level in liver. The transcription of tlr22 was significantly increased in spleen, kidney, liver and gill tissues at different timepoints after Poly I:C infection, suggesting TLR22 plays critical roles in defensing virus invasion. Similarly, the transcription of tlr22 was also dramatically up-regulated in spleen, kidney and gill tissues with different patterns after Aeromonas hydrophila infection, indicating that TLR22 is also involved in resisting bacteria invasion. Our findings will provide a solid basis for the investigation the immune functions of tlr22 gene in teleosts, as well as provide useful information for disease control and treatment for yellow catfish.

Pharmacology of orange-spotted grouper (Epinephelus coioides) melanocortin-5 receptor and its modulation by Mrap2

The mammalian melanocortin-5 receptors (MC5Rs) are involved in various functions, including exocrine gland secretion, glucose uptake, adipocyte lipolysis, and immunity. However, the physiological role of fish Mc5r is rarely studied. Melanocortin-2 receptor accessory protein 2 (MRAP2) modulates pharmacological properties of melanocortin receptors. Herein, to lay the foundation for future physiological studies, we cloned the orange-spotted grouper (Epinephelus coioides) mc5r, with a 1008 bp open reading frame and a predicted protein of 334 amino acids. Grouper mc5r had abundant expression in the brain, skin, and kidney. Four ligands could bind to grouper Mc5r and dose-dependently increase intracellular cAMP levels. Grouper Mrap2 did not affect binding affinity or potency of Mc5r; however, grouper Mrap2 decreased cell surface expression and maximal binding of Mc5r. Mrap2 also significantly decreased the maximal response to a superpotent agonist but not the endogenous agonist. This study provided new data on fish Mc5r pharmacology and its regulation by Mrap2.

The interaction of MC3R and MC4R with MRAP2a in rainbow trout (Oncorhynchus mykiss)

Melanocortin 3 and 4 receptors are two important neural G protein-coupled receptors that regulate energy homeostasis in vertebrates. Melanocortin receptor accessory protein 2 (MRAP2) is also involved in the regulation of food intake and body weight as a variable regulator of melanocortin receptors. Rainbow trout (Oncorhynchus mykiss) is a valuable cold-water fish cultured worldwide. In the rainbow trout model, we cloned and identified mrap2a, a paralog of mrap2. Rainbow trout mrap2a consisted of a 690 bp ORF and was expected to encode a putative protein of 229 amino acids. The qPCR results showed that rainbow trout mrap2a was expressed at high levels in brain tissue similar to mc3r and mc4r. In addition, co-immunoprecipitation verified that MRAP2a interacts with MC3R and MC4R in vitro and that MRAP2a is involved in and regulates the constitutive activity and signaling of MC3R and MC4R. MRAP2a reduced constitutive and agonist-stimulated cAMP levels of MC3R; furthermore, MRAP2a increased constitutive ERK1/2 activation but reduced ligand-induced stimulation at high levels of expression. For MC4R, MRAP2a showed decreased cAMP basal activity but increased agonist-stimulated cAMP signaling and increased ACTH ligand sensitivity. However, MRAP2a failed to affect MC4R constitutive activity and agonist-induced ERK1/2 signaling. Undoubtedly, our study will have great significance for revealing the conserved role of MC4R and MC3R signaling in teleost fish, especially in cold-water fish growth and energy homeostasis.

Toll-like Receptor 3 in the Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × P. vachelli ♂): Protein Structure, Evolution and Immune Response to Exogenous Aeromonas hydrophila and Poly (I:C) Stimuli

As a major mediator of cellular response to viral infection in mammals, Toll-like receptor 3 (TLR3) was proved to respond to double-stranded RNA (dsRNA). However, the molecular mechanism by which TLR3 functions in the viral infection response in teleosts remains to be investigated. In this study, the Toll-like receptor 3 gene of the hybrid yellow catfish was identified and characterized by comparative genomics. Furthermore, multiple sequence alignment, genomic synteny and phylogenetic analysis suggested that the homologous TLR3 genes were unique to teleosts. Gene structure analysis showed that five exons and four introns were common components of TLR3s in the 12 examined species, and interestingly the third exon in teleosts was the same length of 194 bp. Genomic synteny analysis indicated that TLR3s were highly conserved in various teleosts, with similar organizations of gene arrangement. De novo predictions showed that TLR3s were horseshoe-shaped in multiple taxa except for avian (with a round-shaped structure). Phylogenetic topology showed that the evolution of TLR3 was consistent with the evolution of the studied species. Selection analysis showed that the evolution rates of TLR3 proteins were usually higher than those of TLR3-TIR domains, indicating that the latter were more conserved. Tissue distribution analysis showed that TLR3s were widely distributed in the 12 tested tissues, with the highest transcriptions in liver and intestine. In addition, the transcription levels of TLR3 were significantly increased in immune-related tissues after infection of exogenous Aeromonas hydrophila and poly (I:C). Molecular docking showed that TLR3 in teleosts forms a complex with poly (I:C). In summary, our present results suggest that TLR3 is a pattern recognition receptor (PRR) gene in the immune response to pathogen infections in hybrid yellow catfish.

Modulation of Canine Melanocortin-3 and -4 Receptors by Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), have crucial roles in regulating energy homeostasis. The melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) have been shown to regulate neural MCRs in a species-specific manner. The potential effects of MRAP1 and MRAP2 on canine neural MCRs have not been investigated before. Herein, we cloned canine (c) MC3R and identified one canine MRAP2 splice variant, MRAP2b, with N-terminal extension of cMRAP2a. Canine MC3R showed higher maximal responses to five agonists than those of human MC3R. We further investigated the modulation of cMRAP1, cMRAP2a, and cMRAP2b, on cMC3R and cMC4R pharmacology. For the cMC3R, all MRAPs had no effect on trafficking; cMRAP1 significantly decreased B_max whereas cMRAP2a and cMRAP2b significantly increased B_max. Both MRAP1 and MRAP2a decreased R_maxs in response to α-MSH and ACTH; MRAP2b only decreased α-MSH-stimulated cAMP generation. For the MC4R, MRAP1 and MRAP2a increased cell surface expression, and MRAP1 and MRAP2a increased B_maxs. All MRAPs had increased affinities to α-MSH and ACTH. MRAP2a increased ACTH-induced cAMP levels, whereas MRAP2b decreased α-MSH- and ACTH-stimulated cAMP production. These findings may lead to a better understanding of the regulation of neural MCRs by MRAP1 and MRAP2s.

Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Ligands for Melanocortin Receptors: Beyond Melanocyte-Stimulating Hormones and Adrenocorticotropin

The discovery of melanocortins in 1916 has resulted in more than 100 years of research focused on these peptides. Extensive studies have elucidated well-established functions of melanocortins mediated by cell surface receptors, including MSHR (melanocyte-stimulating hormone receptor) and ACTHR (adrenocorticotropin receptor). Subsequently, three additional melanocortin receptors (MCRs) were identified. Among these five MCRs, MC3R and MC4R are expressed primarily in the central nervous system, and are therefore referred to as the neural MCRs. Since the central melanocortin system plays important roles in regulating energy homeostasis, targeting neural MCRs is emerging as a therapeutic approach for treating metabolic conditions such as obesity and cachexia. Early efforts modifying endogenous ligands resulted in the development of many potent and selective ligands. This review focuses on the ligands for neural MCRs, including classical ligands (MSH and agouti-related peptide), nonclassical ligands (lipocalin 2, β-defensin, small molecules, and pharmacoperones), and clinically approved ligands (ACTH, setmelanotide, bremelanotide, and several repurposed drugs).

Reversion of MRAP2 Protein Sequence Generates a Functional Novel Pharmacological Modulator for MC4R Signaling

Simple Summary

Reversion of the wild-type protein sequences of single transmembrane melanocortin accessory protein families (MRAP2) in mice and zebrafish created novel functional pharmacological modulators for regulating melanocortin 4 receptor (MC4R) signaling. All of the brand new reversed MRAP2 (rMRAP2) proteins could form proper dimeric topology on the plasma membrane and interact with and affect the ligand-stimulated pharmacological profiles of zebrafish and mouse MC4R signaling in vitro.

Abstract

As a member of the melanocortin receptor family, melanocortin 4 receptor (MC4R) plays a critical role in regulating energy homeostasis and feeding behavior, and has been proven as a promising therapeutic target for treating severe obesity syndrome. Numerous studies have demonstrated that central MC4R signaling is significantly affected by melanocortin receptor accessory protein 2 (MRAP2) in humans, mice and zebrafish. MRAP2 proteins exist as parallel or antiparallel dimers on the plasma membrane, but the structural insight of dual orientations with the pharmacological profiles has not yet been fully studied. Investigation and optimization of the conformational topology of MRAP2 are critical for the development of transmembrane allosteric modulators to treat MC4R-associated disorders. In this study, we synthesized a brand new single transmembrane protein by reversing wild-type mouse and zebrafish MRAP2 sequences and examined their dimerization, interaction and pharmacological activities on mouse and zebrafish MC4R signaling. We showed that the reversed zebrafish MRAPa exhibited an opposite function on modulating zMC4R signaling and the reversed mouse MRAP2 lost the capability for regulating MC4R trafficking but exhibited a novel function for cAMP cascades, despite proper expression and folding. Taken together, our results provided new biochemical insights on the oligomeric states and membrane orientations of MRAP2 proteins, as well as its pharmacological assistance for modulating MC4R signaling.

Mutations in melanocortin-4 receptor: From fish to men

Melanocortin-4 receptor (MC4R), expressed abundantly in the hypothalamus, is a critical regulator of energy homeostasis, including both food intake and energy expenditure. Shortly after the publication in 1997 of the Mc4r knockout phenotypes in mice, including increased food intake and severe obesity, the first mutations in MC4R were reported in humans in 1998. Studies in the subsequent two decades have established MC4R mutation as the most common monogenic form of obesity, especially in early-onset severe obesity. Studies in animals, from fish to mammals, have established the conserved physiological roles of MC4R in all vertebrates in regulating energy balance. Drug targeting MC4R has been recently approved for treating morbid genetic obesity. How the MC4R can be exploited for animal production is highly worthy of active investigation.

Pharmacological Evaluation of Melanocortin 2 Receptor Accessory Protein 2 on Axolotl Neural Melanocortin Signaling

The Melanocortin-3 receptor (MC3R) and Melanocortin-4 receptor (MC4R), two members of the key hypothalamic neuropeptide signaling, function as complex mediators to control the central appetitive and energy homeostasis. The melanocortin 2 receptor accessory protein 2 (MRAP2) is well-known for its modulation on the trafficking and signaling of MC3R and MC4R in mammals. In this study, we cloned and elucidated the pharmacological profiles of MRAP2 on the regulation of central melanocortin signaling in a relatively primitive poikilotherm amphibian species, the Mexican axolotl (Ambystoma mexicanum). Our results showed the higher conservation of axolotl mc3r and mc4r across species than mrap2, especially the transmembrane regions in these proteins. Phylogenetic analysis indicated that the axolotl MC3R/MC4R clustered closer to their counterparts in the clawed frog, whereas MRAP2 fell in between the reptile and amphibian clade. We also identified a clear co-expression of mc3r, mc4r, and mrap2 along with pomc and agrp in the axolotl brain tissue. In the presence of MRAP2, the pharmacological stimulation of MC3R by α-MSH or ACTH significantly decreased. MRAP2 significantly decreased the cell surface expression of MC4R in a dose dependent manner. The co-localization and formation of the functional complex of axolotl MC3R/MC4R and MRAP2 on the plasma membrane were further confirmed in vitro. Dramatic changes of the expression levels of mc3r, mrap2, pomc, and agrp in the fasting axolotl hypothalamus indicated their critical roles in the metabolic regulation of feeding behavior and energy homeostasis in the poikilotherm aquatic amphibian.

Regulation of Melanocortin-3 and -4 Receptors by Isoforms of Melanocortin-2 Receptor Accessory Protein 1 and 2

The neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play essential non-redundant roles in the regulation of energy homeostasis. Interaction of neural MCRs and melanocortin-2 receptor accessory proteins (MRAPs, MRAP1 and MRAP2) is suggested to play pivotal roles in MC3R and MC4R signaling. In the present study, we identified two new human (h) MRAP2 splice variants, MRAP2b (465 bp open reading frame) and MRAP2c (381 bp open reading frame). Human MRAP2s are different in C-termini. We investigated the effects of five isoforms of MRAPs, hMRAP1a, hMRAP1b, hMRAP2a, hMRAP2b, and hMRAP2c, on MC3R and MC4R pharmacology. At the hMC3R, hMRAP1a and hMRAP2c increased and hMRAP1b decreased the cell surface expression. hMRAP1a increased affinity to ACTH. Four MRAPs (hMRAP1a, hMRAP1b, hMRAP2a, and hMRAP2c) decreased the maximal responses in response to α-MSH and ACTH. For hMC4R, hMRAP1a, hMRAP2a, and hMRAP2c increased the cell surface expression of hMC4R. Human MRAP1b significantly increased affinity to ACTH while MRAP2a decreased affinity to ACTH. Human MRAP1a increased ACTH potency. MRAPs also affected hMC4R basal activities, with hMRAP1s increasing and hMRAP2s decreasing the basal activities. In summary, the newly identified splicing variants, hMRAP2b and hMRAP2c, could regulate MC3R and MC4R pharmacology. The two MRAP1s and three MRAP2s had differential effects on MC3R and MC4R trafficking, binding, and signaling. These findings led to a better understanding of the regulation of neural MCRs by MRAP1s and MRAP2s.

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

Topmouth culter melanocortin-3 receptor: regulation by two isoforms of melanocortin-2 receptor accessory protein 2

Melanocortin-3 receptor (MC3R) is a regulator of energy homeostasis, and interaction of MC3R and melanocortin-2 receptor accessory protein 2 (MRAP2) plays a critical role in MC3R signaling of mammals. However, the physiological roles of MC3R in teleosts are not well understood. In this study, qRT-PCR was used to measure gene expression. Radioligand binding assay was used to study the binding properties of topmouth culter MC3R (caMC3R). Intracellular cAMP generation was determined by RIA, and caMC3R expression was quantified with flow cytometry. We showed that culter mc3r had higher expression in the CNS. All agonists could bind and stimulate caMC3R to increase dose dependently intracellular cAMP accumulation. Compared to human MC3R, culter MC3R showed higher constitutive activity, higher efficacies, and Rmax to alpha-melanocyte-stimulating hormone (α-MSH), des-α-MSH, and adrenocorticotrophic hormone. Both caMRAP2a and caMRAP2b markedly decreased caMC3R basal cAMP production. However, only caMRAP2a significantly decreased cell surface expression, Bmax, and Rmax of caMC3R. Expression analysis suggested that MRAP2a and MRAP2b might be more important in regulating MC3R/MC4R signaling during larval period, and reduced mc3r, mc4r, and pomc expression might be primarily involved in modulation of MC3R/MC4R in adults. These data indicated that the cloned caMC3R was a functional receptor. MRAP2a and MRAP2b had different effects on expression and signaling of caMC3R. In addition, expression analysis suggested that MRAP2s, receptors, and hormones might play different roles in regulating culter development and growth.

Identification of novel GPCR partners of the central melanocortin signaling

OBJECTIVE

Homo- or heterodimerization of G protein-coupled receptors (GPCRs) generally affects the normal functioning of these receptors and mediates the responses to a variety of physiological stimuli in vivo. It is well known that melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) are key regulators of appetite and energy homeostasis in the central nervous system. However, the GPCR partners of MC3R and MC4R are not well understood. Our objective is to analyze single cell RNA-seq datasets of the hypothalamus to explore and identify novel GPCR partners of MC3R and MC4R and examine the pharmacological effect on the downstream signal transduction and membrane translocation of melanocortin receptors.

METHODS

We conducted an integrative analysis of multiple single cell RNA-seq datasets to reveal the expression pattern and correlation of GPCR families in the mouse hypothalamus. The emerging GPCRs with important metabolic functions were selected for cloning and co-immunoprecipitation validation. The positive GPCR partners were then tested for the pharmacological activation, competitive binding assay and surface translocation ELISA experiments.

RESULTS

Based on the expression pattern of GPCRs and their function enrichment results, we narrowed down the range of potential GPCR interaction with MC3R and MC4R for further confirmation. Co-immunoprecipitation assay verified 23 and 32 novel GPCR partners that interacted with MC3R and MC4R in vitro. The presence of these GPCR partners exhibited different effects in the physiological regulation and signal transduction of MC3R and MC4R.

CONCLUSIONS

This work represented the first large-scale screen for the functional GPCR complex of central melanocortin receptors and defined a composite metabolic regulatory GPCR network of the hypothalamic nucleuses.