Melanocortin-4 Receptor in Fish: A Review

The melanocortin-4 receptor (MC4R) belongs to the melanocortin system, has been well investigated in mammals. Which plays important roles in several physiological processes, such as regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain infl ammation, and anxiety. Although there are some studies have performed, its role still remain mystery in fi sh. This review will summarize the studies on the fi sh MC4R from its cloning and tissue distribution to its physiological roles. Which should be useful for further studies in fi sh and might provide a new insight in aquaculture industry. Review Article Melanocortin-4 Receptor in Fish: A Review Zheng-Yong Wen*, Yuan-Chao Zou, Chuan-Jie Qin*, Deng-Yue Yuan and Rui Li College of Life Sciences, Conservation and Utilization of Fishes resources in the Upper Reaches of the Yangtze River Key Laboratory of Sichuan Province, Neijiang Normal University, Neijiang, Sichuan 641100, China Dates: Received: 23 December, 2016; Accepted: 16 January, 2017; Published: 20 January, 2017 *Corresponding author: Zheng-Yong Wen, College of Life Sciences, Neijiang Normal University, Neijiang, Sichuan 641100, China, Tel: + 86 18582681220; Fax: + 86 18582681220; E-mail: Chuan-Jie Qin, College of Life Sciences, Neijiang Normal University, Neijiang, Sichuan 641100, China, Tel: + 86 18582681220; Fax: + 86 18582681220; E-mail: https://www.peertechz.com


Introduction
The melanocortin system is an ancient system conserved from teleosts to mammals. It consists of four agonists, two antagonists, and fi ve receptors. The four agonists including , , and  melanocyte-stimulating hormone (MSH), and adrenocorticotropin (ACTH), are derived from tissuespecifi c posttranslational processing of the prohormone proopiomelanocortin (POMC) [1][2][3]. The two endogenous antagonists are consist of agouti and agouti related peptide (A g RP), Agouti is an antagonist for MC1R whereas AgRP for MC3R and MC4R [2,4]. The fi ve melanocortin receptors are called melanocortin receptors 1-5 (MC1R-MC5R) based on the sequence of their cloning. They are all members of Family A G protein coupled receptors (GPCRs) consisting of seven transmembrane domains (TMs) connected by alternating intracellular and extracellular loops, with the N-terminus extracellular, and the C-terminus intracellular [5][6][7].
MC4R is a member of family A GPCRs with seven TMs connected by alternating extracellular loops (ELs) and intracellular loops. The molecular processes involved in the binding of the MSH peptides to the MC4R were extensively studied. Most notably, the acidic residues Asp 122/126 (TM3) and the basic residue His 264 (TM4) are essential for ligand binding [8].
The human MC4R is an intron less gene with an open reading frame of 999 bp that encodes a protein of 332 amino acids. Alignment of MC4R with other MCRs showed that it has the highest homology with the MC3R, with 58% identity and 76% similarity. By fl uorescent in situ hybridization, the MC4R gene was localized to chromosome 18q21.3 [9,10].
Since the cloning of hMC4R, the MC4R has been cloned from mouse, rat, hamster, guinea pig, dog, cat, fox, pig, sheep, cow, and several primates including marmoset, cynomolgus macaque, vervet monkey, and orangutan. It has also been cloned from several nonmammalian species including fi sh, chicken, and pigeon. The amino acid sequences between the different species are highly conserved [11,12].
It has been p r oved that MC4R is involved in regulating energy homeostasis in mammals [7]. Knockout of the MC4R resulted in increased food intake and decreased energy expenditure therefore obesity in mouse [13]. A similar phenotype is also observed in mice ubiquitously overexpressing Agouti or AgRP genes [14,15]. The frameshift mutations found in MC4R lead to a dominant form of obesity in human [16,17]. Furthermore, central administration of the MC4R agonist -MSH has been shown to inhibit appetite and increase basal metabolic rate [18].

Molecular cloning of t h e fi sh MC4R
The MC4R was fi rst cloned from zebrafi sh in 2002 [19], since then, more and more MC4R were cloned from different fi shes. To date, there are more than twenty fi sh MC4R can be found in the National Center for Biotechnology Information database (NCBI: https://www.ncbi.nlm.nih.gov/), including Osteichyes fi shes, Chondrichthyes fi shes and Cyclostomata fi shes (

Tissue distribution of the fi sh MC4R
In mammals, over the past two decades, since the initial cloning [9,36], signifi cant attention has been paid to the melanocortin-4 receptor (MC4R), due to its central expression and roles in regulation of energy homeostasis and body weight. Simultaneously, it also expressed in some peripheral tissues such as heart, lung, kidney medulla, renal nerve, ureter, intercostal muscle and skull bone [37]. The extensive expression of mammalian MC4R suggested it could be involved in many physiological processes.
Of the fi ve MCRs, only MC3R and MC4R are signifi cantly expressed within the central nervous system, hence these two MCRs are also called neural MCRs [38,39]. MC4R is also expressed in some peripheral tissues such as enteroendocrine L cells [40].
In fi sh, the MC4R is extensively expressed in a variety of reported that the spotted scats MC4R was found to be expressed in the brain, pituitary, and gonads in both male and female [21].
Further research showed that common carp MC4R was highly expressed in brain, testis, and eye, followed by expression in the pituitary and heart [26]. Recent study on grass carp showed that MC4R was found to be highly expressed in the brain and eye, but expressed at low levels in the muscle, heart, intestine, liver, gill, spleen, and kidney [28]. The extensive expression pattern of MC4R in fi sh suggested it might act as different physiological functions in different tissues.

Regulating f o od intake and energy homeostasis:
The roles of mammalian MC4R in regulating energy h o meostasis were well studied [7]. MC4R knockout mice exhibit maturity-onset obesity, hyperphagia, increased linear growth, hyperinsulinemia, hyperglycemia, and delayed meal termination and reduced sensitivity to cholecystokinin [13].
Conversely, MC4R over expressed mice found to be obese, similar to mice over-expressing AgRP [41]. In addition, Human genetic studies demonstrated that mutations in MC4R are the most common form of monogenic obesity, characterized by its early-onset and severity [2,16,17,39]. These studies suggest that MC4R plays a key role in regulating energy homeostasis and body weight in mammals including human.
Recent studies showed that the mechanism of regulation of energy h o meostasis by the MC4R is also operational in lower vertebrates including fi sh. In goldfi sh, ICV injection of NDP-MSH or MTII inhibits food intake, whereas the MC4Rspecifi c antagonist HS024 increases food intake [27,42]. These experiments suggested that the MC4R is exerting a tonic inhibitory effect on food intake. Similar results were obtained in rainbow trout, ICV injection of MTII decreases food intake, whereas ICV injection of HS024 and the MC3/4R antagonist SHU9119 increases food intake in rainbow trout [43]. Moreover, nonsynonymous mutations in cavefi sh MC4R are identifi ed to contribute to enhanced appetite, growth, and starvation resistance [44]. Taking together, these studies indicating the piscine also involved in regulating food intake and energy homeostasis.

Dasyatis akajei
Takahashi et al. [34] Cyclostomata Petromyzoniformes Lam p etra fluviatilis Haitina et al. [35] Citation: Wen  Modulating reproductive founction: Several recent studies suggested that the MC4R is also involved in modulating reproductive function through affecting the secretion of reproductive hormones, and consequent sexual maturation in fi sh [7]. Moreover, the MC4R was found to be highly expressed in gonads of several fi sh, such as goldfi sh [42], Ya-fi sh [30], and snakeskin gourami [45]. In addition, some previous studies revealed that MC4R copies located on the sex chromosomes participate in the functional modulation of onset of sexual maturity in both male and female swordtails and platyfi sh, suggesting the potential participation of MC4R in the regulation of reproduction [31,46]. Hence, understanding how MC4R mediates the reproductive function in economically important species is important for artifi cial breeding. However, these hypothesizes have not yet established, and it need to be further studied.

Conclusions and Future Directions
Tremendous progress has been made on the MC4R since 1993 when it was cloned. As an important transmembrane protein, it's involved in many physiological processes not only in higher vertebrate but also in lower vertebrate. Although it has well studied in mammals, there are still exsit many mysteries need to be resolved especially in fi sh. In future, the MC4R should be well known by people following extensive research carrying out, and it might be as a useful tool for improving aquaculture industry.