Structural and functional diversity of proopiomelanocortin in fish with special reference to barfin flounder

Dietary tryptophan intervention counteracts stress-induced transcriptional changes in a teleost fish HPI axis during inflammation

Immune nutrition is currently used to enhance fish health by incorporating functional ingredients into aquafeeds. This study aimed to investigate the connections between tryptophan nutrition and the network that regulates the communication pathways between neuroendocrine and immune systems in European seabass (Dicentrarchus labrax). When tryptophan was supplemented in the diet of unstressed fish, it induced changes in the hypothalamic-pituitary-interrenal axis response to stress. Tryptophan-mediated effects were observed in the expression of anti-inflammatory cytokines and glucocorticoid receptors. Tryptophan supplementation decreased pro-opiomelanocortin b-like levels, that are related with adrenocorticotropic hormone and cortisol secretion. When stressed fish fed a tryptophan-supplemented diet were subjected to an inflammatory stimulus, plasma cortisol levels decreased and the expression of genes involved in the neuroendocrine response was altered. Modulatory effects of tryptophan dietary intervention on molecular patterns seem to be mediated by altered patterns in serotonergic activity.

LC-MS/MS based characterisation and differential expression of proteins in Himalayan snow trout, Schizothorax labiatus using LFQ technique

Molecular characterization of fish muscle proteins are nowadays considered as a key component to understand the role of specific proteins involved in various physiological and metabolic processes including their up and down regulation in the organisms. Coldwater fish specimens including snow trouts hold different types of proteins which help them to survive in highly diversified temperatures fluctuating from 0 to 20 °C. So, in current study, the liquid chromatography mass spectrometry using label free quantification technique has been used to investigate the muscle proteome profile of Schizothorax labiatus. For proteomic study, two weight groups of S. labiatus were taken from river Sindh. The proteomic analysis of group 1 revealed that a total of 235 proteins in male and 238 in female fish were recorded. However, when male and female S. labiatus were compared with each other on the basis of spectral count and abundance of peptides by ProteinLynx Global Server software, a total of 14 down-regulated and 22 up-regulated proteins were noted in this group. The highly down-regulated ones included homeodomain protein HoxA2b, retinol-binding protein 4, MHC class II beta chain and proopiomelanocortin while as the highly expressed up-regulated proteins comprised of gonadotropin I beta subunit, NADH dehydrogenase subunit 4, manganese superoxide dismutase, recombinase-activating protein 2, glycosyltransferase, chymotrypsin and cytochrome b. On the other hand, the proteomic characterisation of group 2 of S. labiatus revealed that a total of 227 proteins in male and 194 in female fish were recorded. When male and female S. labiatus were compared with each other by label free quantification, a total of 20 down-regulated and 18 up-regulated proteins were recorded. The down-regulated protein expression of group 2 comprised hepatic lipase, allograft inflammatory factor-1, NADH dehydrogenase subunit 4 and myostatin 1 while the highly expressed up-regulated proteins included glycogen synthase kinase-3 beta variant 2, glycogen synthase kinase-3 beta variant 5, cholecystokinin, glycogen synthase kinase-3 beta variant 3 and cytochrome b. Significant (P < 0.05) difference in the expression of down-regulated and up-regulated proteins was also noted between the two sexes of S. labiatus in each group. According to MS analysis, the proteins primarily concerned with the growth, skeletal muscle development and metabolism were down-regulated in river Sindh, which indicates that growth of fish during the season of collection i.e., winter was slow owing to less food availability, gonad development and low metabolic activity. While, the proteins related to immune response of fish were also noted to be down-regulated thereby signifying that the ecosystem has less pollution loads, microbial, pathogenic and anthropogenic activities. It was also found that the proteins involved in glycogen metabolism, reproductive and metabolic processes, particularly lipid metabolism were up-regulated in S. labiatus. The significant expression of these proteins may be connected to pre-spawning, gonad development and use of stored food as source of energy. The information generated in this study can be applied to future research aimed at enhancing food traceability, food safety, risk management and authenticity analysis.

Transcriptome Sequencing Analysis Reveals Dynamic Changes in Major Biological Functions during the Early Development of Clearhead Icefish, Protosalanx chinensis

Early development, when many important developmental events occur, is a critical period for fish. However, research on the early development of clearhead icefish is very limited, especially in molecular research. In this study, we aimed to explore the dynamic changes in the biological functions of five key periods in clearhead icefish early development, namely the YL (embryonic), PM (first day after hatching), KK (fourth day after hatching), LC (seventh day after hatching), and SL (tenth day after hatching) stages, through transcriptome sequencing and different analysis strategies. A trend expression analysis and an enrichment analysis revealed that the expression ofgenes encoding G protein-coupled receptors and their ligands, i.e., prss1_2_3, pomc, npy, npb, sst, rln3, crh, gh, and prl that are associated with digestion and feeding regulation gradually increased during early development. In addition, a weighted gene co-expression network analysis (WGCNA) showed that eleven modules were significantly associated with early development, among which nine modules were significantly positively correlated. Through the enrichment analysis and hub gene identification results of these nine modules, it was found that the pathways related to eye, bone, and heart development were significantly enriched in the YL stage, and the ccnd2, seh1l, kdm6a, arf4, and ankrd28 genes that are associated with cell proliferation and differentiation played important roles in these developmental processes; the pak3, dlx3, dgat2, and tas1r1 genes that are associated with jaw and tooth development, TG (triacylglycerol) synthesis, and umami amino acid receptors were identified as hub genes for the PM stage; the pathways associated with aerobic metabolism and unsaturated fatty acid synthesis were significantly enriched in the KK stage, with the foxk, slc13a2_3_5, ndufa5, and lsc2 genes playing important roles; the pathways related to visual perception were significantly enriched in the LC stage; and the bile acid biosynthetic and serine-type peptidase activity pathways were significantly enriched in the SL stage. These results provide a more detailed understanding of the processes of early development of clearhead icefish.

Two ACTH analogs exert differential effects on monocytes/macrophages function regulation in ayu (Plecoglossus altivelis)

Adrenocorticotropic hormone (ACTH), a bioactive peptide of the family of melanocortins, is generated from pro-opiomelanocortin (POMC). So far, the research on the specific functions of ACTH in the immune system of teleosts is limited. We determined two complementary DNA (cDNA) sequences of POMC in ayu (Plecoglossus altivelis), termed PaPOMC-A and PaPOMC-B. PaPOMCs transcripts occurred in all examined tissues, and their expression in immune tissues changed following experimental infection with Vibrio anguillarum. PaACTH-B, but not PaACTH-A, suppressed the phagocytosis of monocytes/macrophages (MO/MФ). Two isoforms of PaACTH increased the bactericidal capacity of MO/MФ. PaACTH-A increased anti-inflammatory cytokine expression, while PaACTH-B decreased pro-inflammatory cytokine expression in MO/MФ. Compared with PaACTH-B treatment, the PaACTH-A treatment improved survival rate and reduced the bacterial load in V. anguillarum-infected ayu through interleukin (IL)-10. Our results indicate that the two PaACTH isoforms exert different effects in the host defense against bacterial infection.

Estrogen mediates sex differences in preoptic neuropeptide and pituitary hormone production in medaka

The preoptic area (POA) is one of the most evolutionarily conserved regions of the vertebrate brain and contains subsets of neuropeptide-expressing neurons. Here we found in the teleost medaka that two neuropeptides belonging to the secretin family, pituitary adenylate cyclase-activating polypeptide (Pacap) and vasoactive intestinal peptide (Vip), exhibit opposite patterns of sexually dimorphic expression in the same population of POA neurons that project to the anterior pituitary: Pacap is male-biased, whereas Vip is female-biased. Estrogen secreted by the ovary in adulthood was found to attenuate Pacap expression and, conversely, stimulate Vip expression in the female POA, thereby establishing and maintaining their opposite sexual dimorphism. Pituitary organ culture experiments demonstrated that both Pacap and Vip can markedly alter the expression of various anterior pituitary hormones. Collectively, these findings show that males and females use alternative preoptic neuropeptides to regulate anterior pituitary hormones as a result of their different estrogen milieu.

Melanocortin Receptor 4 (MC4R) Signaling System in Nile Tilapia

The melanocortin receptor 4 (MC4R) signaling system consists of MC4R, MC4R ligands [melanocyte-stimulating hormone (MSH), adrenocorticotropin (ACTH), agouti-related protein (AgRP)], and melanocortin-2 receptor accessory protein 2 (MRAP2), and it has been proposed to play important roles in feeding and growth in vertebrates. However, the expression and functionality of this system have not been fully characterized in teleosts. Here, we cloned tilapia MC4R, MRAP2b, AgRPs (AgRP, AgRP2), and POMCs (POMCa1, POMCb) genes and characterized the interaction of tilapia MC4R with MRAP2b, AgRP, α-MSH, and ACTH in vitro. The results indicate the following. (1) Tilapia MC4R, MRAP2b, AgRPs, and POMCs share high amino acid identity with their mammalian counterparts. (2) Tilapia MRAP2b could interact with MC4R expressed in CHO cells, as demonstrated by Co-IP assay, and thus decrease MC4R constitutive activity and enhance its sensitivity to ACTH_1-40. (3) As in mammals, AgRP can function as an inverse agonist and antagonist of MC4R, either in the presence or absence of MRAP2b. These data, together with the co-expression of MC4R, MRAP2b, AgRPs, and POMCs in tilapia hypothalamus, suggest that as in mammals, ACTH/α-MSH, AgRP, and MRAP2 can interact with MC4R to control energy balance and thus play conserved roles in the feeding and growth of teleosts.

Appetite-Controlling Endocrine Systems in Teleosts

Mammalian studies have shaped our understanding of the endocrine control of appetite and body weight in vertebrates and provided the basic vertebrate model that involves central (brain) and peripheral signaling pathways as well as environmental cues. The hypothalamus has a crucial function in the control of food intake, but other parts of the brain are also involved. The description of a range of key neuropeptides and hormones as well as more details of their specific roles in appetite control continues to be in progress. Endocrine signals are based on hormones that can be divided into two groups: those that induce (orexigenic), and those that inhibit (anorexigenic) appetite and food consumption. Peripheral signals originate in the gastrointestinal tract, liver, adipose tissue, and other tissues and reach the hypothalamus through both endocrine and neuroendocrine actions. While many mammalian-like endocrine appetite-controlling networks and mechanisms have been described for some key model teleosts, mainly zebrafish and goldfish, very little knowledge exists on these systems in fishes as a group. Fishes represent over 30,000 species, and there is a large variability in their ecological niches and habitats as well as life history adaptations, transitions between life stages and feeding behaviors. In the context of food intake and appetite control, common adaptations to extended periods of starvation or periods of abundant food availability are of particular interest. This review summarizes the recent findings on endocrine appetite-controlling systems in fish, highlights their impact on growth and survival, and discusses the perspectives in this research field to shed light on the intriguing adaptations that exist in fish and their underlying mechanisms.

Dimerization of melanocortin receptor 1 (MC1R) and MC5R creates a ligand-dependent signal modulation: Potential participation in physiological color change in the flounder

Vertebrates produce α-melanocyte-stimulating hormone (α-MSH), which contains an N-terminal acetyl group, and desacetyl-α-MSH, which does not contain an N-terminal acetyl group. In teleosts and amphibians, α-MSH-related peptides stimulate pigment dispersion via melanocortin receptors 1-5 (MC1R-MC5R), which are members of the G-protein-coupled receptor (GPCR) family. We previously reported an interesting phenomenon associated with physiological color changes in the skin of a flatfish, barfin flounder (bf). Specifically, pigments in xanthophores expressing only the bfMC5R gene were dispersed by both α-MSH and desacetyl-α-MSH, whereas those in melanophores expressing both the bfMC1R and bfMC5R genes were dispersed by desacetyl-α-MSH, but not by α-MSH. In this study, we examined whether heterodimers of bfMC1R and bfMC5R can act as significant inhibitory receptors for the N-terminal acetylation of α-MSH in mammalian Chinese hamster ovary cells. Immunofluorescence analyses showed that bfMC1R and bfMC5R were localized together at the plasma membrane when expressed in the same cells. Indeed, after coexpression of Flag-bfMC1R and HA-bfMC5R, immunoprecipitation with anti-Flag antibodies resulted in the presence of anti-HA immunoreactivity in the precipitate, and vice versa. Importantly, cyclic AMP assays showed that cotransfection of bfMC1R with bfMC5R inhibited the cyclic AMP accumulation induced by α-MSH to a greater extent than that observed after transfection of bfMC1R alone. Of note, this inhibitory response was not caused by desacetyl-α-MSH. Thus, we show a ligand-dependent signaling through functional heterodimerization of MC1R and MC5R in mammalian cells. The ligand-selective receptor complex also provide the first mechanistic implication that may play a role in the control of color change in teleosts.

Interrelation between melanocyte-stimulating hormone and melanin-concentrating hormone in physiological body color change: roles emerging from barfin flounder Verasper moseri

In teleosts, as their names suggest, the main target cells of melanocyte-stimulating hormone (MSH) and melanin-concentrating hormone (MCH) are the chromatophores in the skin, where these peptide hormones play opposing roles in regulating pigment migration. These effects are obvious especially when their activities are examined in vitro. On the contrary, while MCH also exhibits activity in vivo, MSH does not always stimulate pigment dispersion in vivo because of predominant sympathetic nervous system. A series of our investigations indicates that this is also the case in barfin flounder, Verasper moseri. Interestingly, we observed that mch expression and the tissue contents of MCH can be easily influenced by changes in environmental color conditions, while gene expression and tissue contents related to MSH scarcely respond to color changes. Transcripts of MSH and MCH receptor genes have been identified in a variety of tissues of this fish species, suggesting that these are multifunctional peptide hormones. Nevertheless, chromatophores in the skin still offer important clues in the efforts to elucidate the functions of melanotropic peptides. Herein, we review the most recent advancements of our studies on MSH and MCH and their receptors in the barfin flounder and discuss the interrelations between these peptides, focusing on their roles in influencing pigment migration in the skin.

Further evidence on acetylation-induced inhibition of the pigment-dispersing activity of α-melanocyte-stimulating hormone

Our previous studies showed that in barfin flounder, α-melanocyte-stimulating hormone (α-MSH) stimulates pigment dispersion in xanthophores, while it shows negligible effects in melanophores. The present study was undertaken to evaluate whether these results are limited to barfin flounder by using Japanese flounder. Three subtypes of proopiomelanocortin gene encoding melanocortins (MCs) were expressed in the Japanese flounder pituitary, one of which was also expressed in the skin. Expression of melanocortin 5 receptor gene (Mc5r) was observed in isolated xanthophores, while that of Mc1r and Mc5r was found in melanophores. In the xanthophores of Japanese flounder skin, α-MSH as well as desacetyl (Des-Ac)-α-MSH and diacetyl (Di-Ac)-α-MSH exhibited dose-dependent pigment-dispersing activities, indicating that the signals of α-MSH-related peptides were mediated by MC5R. On the other hand, α-MSH did not stimulate pigment dispersion in melanophores, while Des-Ac-α-MSH and Di-Ac-α-MSH did, thus indicating that the expression of two different types of Mcr is related to the decrease in α-MSH activity. Thus, the molecular repertoire in MC system observed in Japanese flounder is similar to that in barfin flounder. Moreover, the relationship between the pigment-dispersing activities of α-MSH-related peptides and the expression of Mcr subtypes in xanthophores and melanophores were also similar between Japanese flounder and barfin flounder. Consequently, we hypothesize that inhibition of α-MSH activity could be due to the formation of heterodimers comprising MC1R and MC5R, often observed in G-protein-coupled receptors.

Divergence of duplicate POMC genes in gilthead sea bream Sparus auratus

Proopiomelanocorticotrophin (POMC) in vertebrates is produced in the pituitary gland and undergoes post-translational processing to give rise to a range of biologically active peptides. Teleosts possess 2-3 different POMC transcripts which have been proposed to have originated from a whole or partial genome duplication. In the present study 2 transcripts of gilthead sea bream POMC (sbPOMC-α1 and α2) were cloned and characterised. sbPOMC-α1 is expressed principally in the melanotroph cells of the pars intermedia (PI) and sbPOMC-α2 is expressed in the corticotroph cells of the rostral pars distalis and probably also in the PI. The 2 sbPOMC transcripts have a differential tissue distribution in extra-pituitary sites. An appraisal of POMC evolution indicates sbPOMCs belong to one of the two main clades that exist in teleosts and that overall a non conservative process of gene loss occurred in this infraclass.

Pigment-dispersing activities and cortisol-releasing activities of melanocortins and their receptors in xanthophores and head kidneys of the goldfish Carassius auratus

The five subtypes of melanocortin receptors (MCRs) mediate the functions of α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotropic hormone (ACTH). In fish, these hormones are involved in pigment dispersion and cortisol release, respectively. α-MSH-related peptides exhibit ACTH-like activity in certain fishes. We recently found that multiple Mcr transcripts are expressed in some cell types in the barfin flounder, which is related to regulation of α-MSH activities. Similar results were also observed for the cortisol-releasing activity of α-MSH-related peptides in the head kidney. The present study was undertaken to assess relationship between the expression of multiply expressed Mcrs and α-MSH activities using goldfish. We also determined if α-MSH-related peptides exhibit ACTH-like activity in goldfish. The transcripts of Mc1r, but not those of other subtypes, were observed in xanthophores. α-MSH, which has an acetyl group at the N-terminus, was found to disperse pigment in a dose-dependent manner in xanthophores. This potency was found to be slightly greater than that of desacetyl-α-MSH. These results support our findings that MCR has a higher affinity for α-MSH when single Mcr subtype is expressed. On the other hand, transcripts of Mc2r, but not those of other subtypes, were observed in the head kidney. ACTH(1-24)-stimulated cortisol release was observed in a dose-dependent manner, while α-MSH-related peptides showed no activity. It therefore appears that MC2R also acts as an ACTH-specific receptor in goldfish and that association of α-MSH-related peptides upon release of cortisol is uncommon in fishes.

Inhibiting roles of melanin-concentrating hormone for skin pigment dispersion in barfin flounder, Verasper moseri

Barfin flounders change their surface color pattern to match their background. We have reported evidence of the association between hormones and body color changes in this fish. First, bolus intraperitoneal injection with melanin-concentrating hormone (MCH) immediately turned the skin color pale, while injection with melanocyte-stimulating hormone (MSH) did not change the skin color. Second, gene expression levels of MCH change in response to background color, while those of MSH do not. We also reported the expression of an MCH receptor gene (Mch-r2) in the skin of this fish. In this study, we aimed to further evaluate the roles of MCH in skin color change. First, long-term adaptation of adult barfin flounder to black or white background colors induced significantly different pigment migration patterns in both melanophores and xanthophores (P<0.05). However, continuous intraperitoneal injection with MCH did not influence chromatophore proliferation. Then, using in vitro experiments, we found that MCH aggregates both melanophores and xanthophores, and inhibits the pigment-dispersing activity of MSH in a similar manner. Finally, we identified transcripts of Mch-r2 in cells isolated from both melanophores and xanthophores. Taken together, the evidence suggests that MCH aggregates pigments via MCH-R2 in concert with the nervous system by overcoming the melanin-dispersing activities of MSH in barfin flounder.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Differential expressions of melanocortin receptor subtypes in melanophores and xanthophores of barfin flounder

alpha-Melanocyte-stimulating hormone (alpha-MSH) is a member of the melanocortin (MC) family, and the MC receptor (MCR) is a member of the G protein-coupled receptor (GPCR) superfamily. We previously found that in barfin flounder, a flatfish, alpha-MSH with an acetyl group at the N-terminus stimulated pigment dispersion in xanthophores; however, this effect was not observed in melanophores. Therefore, the present study was undertaken to find which MCR subtypes are expressed in these pigment cells in order to elucidate how acetylation regulates activities of alpha-MSH-related peptides. Here, we also report the cloning of Mc1r and Mc5r from barfin flounder. Three types of cells-melanophores, xanthophores, and nonchromatophoric dermal cells-were isolated from the skin samples collected from the dorsal fin. These cells were then tested for the expression of Mc1r and Mc5r as well as Mc2r and Mc4r that we had previously cloned. Mc1r and Mc5r transcripts were detected in melanophores, and a sole Mc5r transcript was detected in xanthophores. We had previously found that the efficiency of alpha-MSH was higher than that of desacetyl-alpha-MSH for pigment dispersion in xanthophores. Acetylated MSH peptide may have increased binding affinity to MC5R, whereas alpha-MSH lacks melanin-dispersing activity. Increasing evidences indicate that many GPCRs form heterodimers, and this may affect the affinity of the ligand for the corresponding GPCR. Taken together, the expression of two different Mcr subtypes in melanophores may suggest that a heterodimer consisting of MC1R and MC5R may have a low binding affinity toward alpha-MSH. The present results clarify the types of MCRs that are expressed in melanophores and xanthophores of barfin flounder; furthermore, the results provide important clues about the functional regulation of alpha-MSH-related peptides.

Concomitant duplications of opioid peptide and receptor genes before the origin of jawed vertebrates

Background

The opioid system is involved in reward and pain mechanisms and consists in mammals of four receptors and several peptides. The peptides are derived from four prepropeptide genes, PENK, PDYN, PNOC and POMC, encoding enkephalins, dynorphins, orphanin/nociceptin and beta-endorphin, respectively. Previously we have described how two rounds of genome doubling (2R) before the origin of jawed vertebrates formed the receptor family.

Methodology/Principal Findings

Opioid peptide gene family members were investigated using a combination of sequence-based phylogeny and chromosomal locations of the peptide genes in various vertebrates. Several adjacent gene families were investigated similarly. The results show that the ancestral peptide gene gave rise to two additional copies in the genome doublings. The fourth member was generated by a local gene duplication, as the genes encoding POMC and PNOC are located on the same chromosome in the chicken genome and all three teleost genomes that we have studied. A translocation has disrupted this synteny in mammals. The PDYN gene seems to have been lost in chicken, but not in zebra finch. Duplicates of some peptide genes have arisen in the teleost fishes. Within the prepropeptide precursors, peptides have been lost or gained in different lineages.

Conclusions/Significance

The ancestral peptide and receptor genes were located on the same chromosome and were thus duplicated concomitantly. However, subsequently genetic linkage has been lost. In conclusion, the system of opioid peptides and receptors was largely formed by the genome doublings that took place early in vertebrate evolution.