Identification and Genomic Sequence of a Ghrelin Receptor (GHS-R)-like Receptor in the Mozambique Tilapia,Oreochromis mossambicus

The ghrelin receptor GHSR has two efficient agonists in the lobe-finned fish Latimeria chalumnae

The peptide hormone ghrelin (an agonist) and LEAP2 (an antagonist) play important functions in energy metabolism via their receptor GHSR, an A-class G protein-coupled receptor. Ghrelin, LEAP2, and GHSR are widely present from fishes to mammals. However, our recent study suggested that fish GHSRs have different binding properties to ghrelin: a GHSR from the lobe-finned fish Latimeria chalumnae (coelacanth) is efficiently activated by ghrelin, but GHSRs from the ray-finned fish Danio rerio (zebrafish) and Larimichthys crocea (large yellow croaker) have lost binding to ghrelin. Do fish GHSRs use another peptide as their agonist? In the present study we tested to two fish motilins from D. rerio and L. chalumnae because motilin is distantly related to ghrelin. In ligand binding and activation assays, the fish GHSRs from D. rerio and L. crocea displayed no detectable or very low binding to all tested motilins; however, the fish GHSR from L. chalumnae bound to its motilin with high affinity and was efficiently activated by it. Therefore, it seemed that motilin is not a ligand for GHSR in the ray-finned fish D. rerio and L. crocea, but is an efficient agonist for GHSR in the lobe-finned fish L. chalumnae, one of the closest fish relatives of tetrapods. The results of present study suggested that GHSR might have two efficient agonists, ghrelin and motilin, in ancient fishes; however, this feature might be only preserved in some extant fishes with ancient evolutionary origins.

LEAP2 is a more conserved ligand than ghrelin for fish GHSRs

Recently, liver-expressed antimicrobial peptide 2 (LEAP2) was identified as an endogenous antagonist and an inverse agonist of the ghrelin receptor GHSR. However, its functions in lower vertebrates are not well understood. Our recent study demonstrated that both LEAP2 and ghrelin are functional towards a fish GHSR from Latimeria chalumnae, an extant coelacanth believed to be one of the closest ancestors of tetrapods. However, amino acid sequence alignment identified that the 6.58 position (Ballesteros-Weinstein numbering system) of most fish GHSRs are not occupied by an aromatic Phe residue, which is absolutely conserved in all known GHSRs from amphibians to mammals, and is responsible for human GHSR binding to its agonist, ghrelin. To test whether these unusual fish receptors are functional, we studied the ligand binding properties of three representative fish GHSRs, two from Danio rerio (zebrafish) and one from Larimichthys crocea (large yellow croaker). After overexpression in human embryonic kidney 293T cells, the three fish GHSRs retained normal binding to all tested LEAP2s, except for a second LEAP2 from L. crocea. However, they displayed almost no binding to all chemically synthesized n-octanoylated ghrelins, despite these ghrelins all retaining normal function towards human and coelacanth GHSRs. Thus, it seems that LEAP2 is a more conserved ligand than ghrelin towards fish GHSRs. Our results not only provided new insights into the interaction mechanism of GHSRs with LEAP2s and ghrelins, but also shed new light on the functions of LEAP2 and ghrelin in different fish species.

In silico strategy for detailing the binding modes of a novel family of peptides proven as ghrelin receptor agonists

Ghrelin is a peptide hormone involved in multiple functions, including growth hormone release stimulation, food intake regulation, and metabolic and cytoprotective effect. A novel family of peptides with internal cycles was designed as ghrelin analogs and the biological activity of two of them (A228 and A233) was experimentally studied in-depth. In this work, an in silico strategy was developed for describing and assessing the binding modes of A228 and A233 to GHS-R1a (ghrelin receptor) comparing it with ghrelin and GHRP-6 peptides. Several reported structures of different G protein coupled receptors were used as templates, to obtain a good quality model of GHS-R1a. The best model was selected by preliminary molecular docking with ghrelin and GHRP-6. Docking was used to estimate peptide orientations in the binding site of the best model, observing a superposition of its N-terminal and its first aromatic residue. To test the complex stability in time, the C-terminal fragments of each peptide were added and the complexes were inserted a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane, performing a molecular dynamic simulation for 100 ns using the CHARMM36 force field. Despite of the structural differences, the studied peptides share a common binding mode; the N-terminal interacts with E124 and the aromatic residue close to it, with the aromatic cluster (F279, F309, and F312). A preliminary pharmacophore model, consisting in a positive charged amine and an aromatic ring at an approximate distance of 0.79 nm, can be proposed. The results here described could represent a step forward in the efficient search of new ghrelin analogs.

Expression of neuropeptide Y and gonadotropin-releasing hormone gene types in the brain of female Nile tilapia (Oreochromis niloticus) during mouthbrooding and food restriction

A cichlid fish, the Nile tilapia (Oreochromis niloticus), is a maternal mouthbrooder, which exhibits minimum energy expenditure and slower ovarian cycles during mouthbrooding. The objective of this study was to observe changes in the gene expression of key neuropeptides involved in the control of appetite and reproduction, including neuropeptide Y a (NPYa), reproductive neuropeptides: gonadotropin-releasing hormone (GnRH1, GnRH2 and GnRH3) and kisspeptin (Kiss2) during mouthbrooding (4- and 12-days), 12-days of food restriction and 12-days of food restriction followed by refeeding. The food restriction regime showed a significant increase in npya mRNA levels in the telencephalon. However, there were no significant alterations in npya mRNA levels during mouthbrooding. gnrh1 mRNA levels were significantly lower in mouthbrooding female as compared with females with food restriction. gnrh3 mRNA levels were also significantly lower in female with 12-days of mouthbrooding, 12-days of food restriction followed by 12-days of refeeding when compared with controls. There were no significant differences in gnrh2 and kiss2 mRNA levels between groups under different feeding regimes. No significant changes were observed in mRNA levels of receptors for peripheral metabolic signaling molecules: ghrelin (GHS-R1a and GHS-R1b) and leptin (Lep-R). These results suggested that unaffected npya mRNA levels in the telencephalon might contribute to suppression of appetite in mouthbrooding female tilapia. Furthermore, lower gnrh1 and gnrh3 mRNA levels may influence the suppression of reproductive functions such as progression of ovarian cycle and reproductive behaviours, while GnRH2 and Kiss2 may not play a significant roles in reproduction under food restriction condition.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

Evidence that ghrelin may be associated with the food intake of gibel carp (Carassius auratus gibelio)

Ghrelin, a non-amidated peptide hormone, is a potent anorectic neuropeptide implicated in feeding regulation in mammals and non-mammalian vertebrates. However, the involvement of ghrelin in the feeding behavior of teleosts has not been well understood. To better understand the role of ghrelin in the regulation of appetite in fish, in this study, we cloned the cDNAs encoding ghrelin and investigated their mRNA distributions in gibel carp tissues. We also assessed the effects of different nutritional status on ghrelin mRNA abundance. Ghrelin mRNAs were ubiquitously expressed in ten tissues (intestine, liver, brain, mesonephron, head kidney, spleen, skin, heart, muscle, gill and pituitary gland), and relatively high expression levels were detected in the gut. Postprandial studies analysis revealed a significant postprandial decrease in ghrelin mRNA expression in the gut (1 and 3 h after the regular feeding time). In addition, ghrelin mRNA expression in the gut significantly increased at day 7 after fasting and declined sharply after refeeding, which suggested that ghrelin might be involved in the regulation of appetite in gibel carp. Overall, our result provides basis for further investigation into the regulation of feeding in gibel carp.

Roles of leptin and ghrelin in adipogenesis and lipid metabolism of rainbow trout adipocytes in vitro

Leptin and ghrelin are important regulators of energy homeostasis in mammals, whereas their physiological roles in fish have not been fully elucidated. In the present study, the effects of leptin and ghrelin on adipogenesis, lipolysis and on expression of lipid metabolism-related genes were examined in rainbow trout adipocytes in vitro. Leptin expression and release increased from preadipocytes to mature adipocytes in culture, but did not affect the process of adipogenesis. While ghrelin and its receptor were identified in cultured differentiated adipocytes, ghrelin did not influence either preadipocyte proliferation or differentiation, indicating that it may have other adipose-related roles. Leptin and ghrelin increased lipolysis in mature freshly isolated adipocytes, but mRNA expression of lipolysis markers was not significantly modified. Leptin significantly suppressed the fatty acid transporter-1 expression, suggesting a decrease in fatty acid uptake and storage, but did not affect expression of any of the lipogenesis or β-oxidation genes studied. Ghrelin significantly increased the mRNA levels of lipoprotein lipase, fatty acid synthase and peroxisome proliferator-activated receptor-β, and thus appears to stimulate synthesis of triglycerides as well as their mobilization. Overall, the study indicates that ghrelin, but not leptin seems to be an enhancer of lipid turn-over in adipose tissue of rainbow trout, and this regulation may at least partly be mediated through autocrine/paracrine mechanisms. The mode of action of both hormones needs to be further explored to better understand their roles in regulating adiposity in fish.

Identification, tissue distribution and functional characterization of the ghrelin receptor in West African lungfish, Protopterus annectens

We identified two ghrelin receptor isoforms, the ghrelin receptor type-1a (GHS-R1a) and its alternative splice form (GHS-R1b) for West African lungfish, Protopterus annectens. Lungfish GHS-R1a and 1b comprised 361 and 281 amino acids, respectively. Lungfish GHS-R1a showed the highest identity to coelacanth GHS-R1a (80.4%). The highest expression of GHS-R1a mRNAs was seen in the brain, liver, ovary, heart, intestine, and gills. GHS-R1b mRNAs were also detected in the same tissues with GHS-R1a, but their expression level was 1/20 that of GHS-R1a. In human embryonic kidney 293 cells transiently expressing lungfish GHS-R1a, rat and bullfrog ghrelin, and two GHS-R1a agonists, GHRP-6 and hexarelin, increased intracellular Ca(2+) concentrations. The intensity of the Ca(2+) increases induced by GHS-R1a agonists was twice when compared to that induced by ghrelin, although the median effective doses (ED50) were similar, suggesting a long-lasting effect of GHS-R1a agonists with similar affinity. We also examined changes in the GHS-R gene expression during an eight-week estivation. Body weight was slightly lowered, but plasma sodium and glucose concentrations decreased; plasma urea concentration increased significantly 4weeks after the start of estivation. Overall, expression of GHS-R1a mRNA decreased, but changes in GHS-R1b mRNA expression were inconsistent with those of GHS-R1a during estivation, suggesting an involvement of GHS-R in energy homeostasis, as seen in mammals. Our results suggest that the ghrelin-GHS-R1a system is present in this lungfish although ghrelin has not yet been found. The structure of GHS-R1a is closer to that of tetrapods than Actinopterygian fish, indicating a process of evolution that follows the Crossopterygii such as coelacanth.

Molecular evolution of GPCRs: Ghrelin/ghrelin receptors

After the discovery in 1996 of the GH secretagogue-receptor type-1a (GHS-R1a) as an orphan G-protein coupled receptor, many research groups attempted to identify the endogenous ligand. Finally, Kojima and colleagues successfully isolated the peptide ligand from rat stomach extracts, determined its structure, and named it ghrelin. The GHS-R1a is now accepted to be the ghrelin receptor. The existence of the ghrelin system has been demonstrated in many animal classes through biochemical and molecular biological strategies as well as through genome projects. Our work, focused on identifying the ghrelin receptor and its ligand ghrelin in laboratory animals, particularly nonmammalian vertebrates, has provided new insights into the molecular evolution of the ghrelin receptor. In mammals, it is assumed that the ghrelin receptor evolution is in line with the plate tectonics theory. In contrast, the evolution of the ghrelin receptor in nonmammalian vertebrates differs from that of mammals: multiplicity of the ghrelin receptor isoforms is observed in nonmammalian vertebrates only. This multiplicity is due to genome duplication and polyploidization events that particularly occurred in Teleostei. Furthermore, it is likely that the evolution of the ghrelin receptor is distinct from that of its ligand, ghrelin, because only one ghrelin isoform has been detected in all species examined so far. In this review, we summarize current knowledge related to the molecular evolution of the ghrelin receptor in mammalian and nonmammalian vertebrates.

Schizothorax davidi ghrelin: cDNA cloning, tissue distribution and indication for its stimulatory character in food intake

Ghrelin is a gut/brain hormone with a unique acyl modification and various biological functions in fish and mammals. The objectives of this project were to identify ghrelin gene organization, study tissue specific ghrelin mRNA expression and investigate the short- (0, 0.5, 1.5, 3, 6, 9, 12h post-fasting) and long- (1, 3, 5, 7 days) term fasting as well as refeeding after a 7 day fasting induced changes in the expression of ghrelin mRNA in Schizothorax davidi. Our reverse transcription polymerase chain reaction analysis confirmed the predicted ghrelin sequence available in the GenBank and identified ghrelin mRNA expression in several tissues including the gut, liver, brain, heart, spleen, head kidney, gill and muscle. Quantitative PCR studies indicated that the expression level of ghrelin mRNA presented ascendant trend in short-term fasting group compared to the fed group, but it did not reach the significant level on statistics, while there is a significant increase in ghrelin mRNA expression in the gut of Schizothorax davidi fasted for 3, 5 and 7 days when compared to the expression in ad libitum fed fish. Refeeding after a 7 day fasting caused a significant and dramatic decrease in ghrelin mRNA expression in the gut of Schizothorax davidi. An increase in the expression of ghrelin mRNA during fasting, and its decrease following refeeding suggests an orexigenic role for ghrelin in Schizothorax davidi. Overall, our results provide evidence for a highly conserved structure and biological actions of ghrelin during evolution.

Different forms of ghrelin exhibit distinct biological roles in tilapia

Ghrelin has been identified in all vertebrate classes, including sharks. Each species possesses multiple forms of ghrelin that vary in peptide length and acyl modifications (e.g., n-hexanoic, n-non-anoic, n-octanoic, and n-decanoic acids) including des-acyl ghrelin. Octanoylated ghrelin has been shown to be a potent GH secretagogue, orexigenic factor, and plays a role in overall metabolism in vertebrates. In the tilapia model, octanoylated ghrelin (ghrelin-C8) and decanoylated ghrelin (ghrelin-C10) exhibit different biological actions. This mini review highlights the current knowledge of the differential actions of ghrelin-C8 and ghrelin-C10 from studies in the tilapia model. These findings suggest that the multiple forms of ghrelin may exhibit distinct yet complimentary actions directed toward maintaining overall energy balance in other vertebrates.

The role of ghrelin in energy balance regulation in fish

Knowledge about the endocrine regulation of energy balance in fish is of interest for basic as well as aquaculture research. Ghrelin is a peptide hormone that was first identified in fish 10 years ago and has important roles in the control of food intake and metabolism. Both ghrelin and its receptor, the growth hormone secretagogue receptor (GHS-R), have been found in numerous fish species. Their tissue distributions support the idea that ghrelin has an integrative role in the regulation of energy balance at both the central nervous system level and systemic level. In tilapia and goldfish, ghrelin treatment appears to increase food intake and to stimulate lipogenesis and tissue fat deposition to promote a more positive energy status. In rainbow trout, on the other hand, ghrelin decreases food intake. Goldfish and rainbow trout are the fish species in which the mode of action of ghrelin on food intake has been most thoroughly investigated. The results from these studies indicate that ghrelin alters food intake by acting on well-known appetite signals, such as CRH, NPY and orexin, in the hypothalamus in a species-specific manner. In goldfish, sensory fibres of the vagus nerve convey the signal from gut-derived ghrelin to modulate appetite. The data also indicate that ghrelin may modulate foraging/swimming activity and the perception of food in fish. Results related to the effects of energy status, temperature, and stressors on plasma ghrelin/tissue ghrelin mRNA levels are occasionally inconsistent between short- and long-term studies, between the protein and mRNA, and between species. Recent data also imply a role of ghrelin in carbohydrate metabolism. More functional studies are required to understand the role of ghrelin and its mechanisms of action in the regulation of energy balance among fish.

Acute stress inhibits food intake and alters ghrelin signaling in the brain of tilapia (Oreochromis mossambicus)

This study investigated the effect of an acute stress on food intake and on the expression of neuropeptide Y (NPY), corticotropin-releasing hormone (CRH), and ghrelin and its receptors, growth hormone secretagogue receptors (GHSRs) in the tilapia (Oreochromis mossambicus). Food intake was significantly (P < 0.01) reduced in fish after a 30-min crowding and handling stress. In a second group of animals exposed to the same 30-min stressor, tissue samples were collected immediately after the stressor to determine changes in the neuroendocrine regulators of food intake. Although CRH and NPY are considered the major mediators of appetite during stress, both mRNA levels were unaltered in the telencephalon/pre-optic area and in the hypothalamic/optic tectum. Interestingly, there was an elevation in the ghrelin transcript (P < 0.05) in the telencephalon/pre-optic area and elevation of its functional receptor (GHSR1a-LR) (P < 0.001) in the hypothalamic/optic tectum. Elevation of GHSR-LR heteronuclear RNA (P < 0.01) in the telencephalon/pre-optic area and suppression in the hypothalamic/optic tectum (P < 0.001) suggest rapid control of the ghrelin regulatory system in response to acute stress. These results suggest that ghrelin signaling is altered during acute stress. It is not clear if these changes result in altered feeding behavior because no changes in CRH or NPY mRNA expression were observed or if ghrelin is playing a role in regulating overall metabolic changes after acute stress.

Ghrelin receptors in non-Mammalian vertebrates

The growth hormone secretagogue-receptor (GHS-R) was discovered in humans and pigs in 1996. The endogenous ligand, ghrelin, was discovered 3 years later, in 1999, and our understanding of the physiological significance of the ghrelin system in vertebrates has grown steadily since then. Although the ghrelin system in non-mammalian vertebrates is a subject of great interest, protein sequence data for the receptor in non-mammalian vertebrates has been limited until recently, and related biological information has not been well organized. In this review, we summarize current information related to the ghrelin receptor in non-mammalian vertebrates.

Cortisol treatment reduces ghrelin signaling and food intake in tilapia, Oreochromis mossambicus

It is well known that after a stressor, levels of plasma cortisol rise, inducing physiological changes within the animal that are directed toward maintaining homeostasis. Less well understood is the role of cortisol in regulating food intake in teleosts. This study investigated the effect of cortisol on food intake and regulation of the neuroendocrine appetite-stimulating hormones, neuropeptide Y (NPY) and ghrelin, in tilapia (Oreochromis mossambicus). Male and female tilapia were randomly assigned to one of the following treatments: unhandled control, vehicle-injected control, or cortisol (2 μg/g BW). Food intake was determined 24 h after injection during a 1-h feeding trial. Cortisol reduced food intake (P<0.001). An identical study was conducted to measure the effects of 24-h cortisol treatment on the endocrine regulators of food intake. Cortisol reduced stomach expression of ghrelin mRNA (P<0.05) and plasma concentrations of ghrelin (P<0.05). In the hypothalamus/optic tectum cortisol reduced levels of GHSR1a-LR (biologically active ghrelin receptor) mRNA. In the telencephalon/preoptic area cortisol significantly reduced levels of NPY and GHSR1b-LR (biologically inactive ghrelin receptor) mRNA. These findings suggest that anorexigenic actions of cortisol may be mediated via two separate pathways: (1) reducing circulating ghrelin levels as well as GHSR1a-LR expression in the hypothalamus/optic tectum and/or (2) suppressing NPY expression in the telencephalon/preoptic area.

Ghrelin effect on nutritional indices, midgut and fat body of Lymantria dispar L. (Lymantriidae)

Ghrelin is a 28-amino acid peptide that has significant effects on appetite and growth in humans and animals. The aim of this study was to examine 4th instar larvae of the pest insect Lymantria dispar L. after ghrelin treatment. Parameters included changes in nutritional indices (efficiency of conversion of ingested food, efficiency of conversion of digested food, approximate digestibility); midgut and fat body mass; total proteases, trypsin and leucine aminopeptidase activities in the midgut; number, height and width of columnar and goblet cells and their nuclei in the midgut epithelium and detection of ghrelin-like immunoreactivity in the midgut tissue. Four subpicomolar injections of ghrelin (0.3pmol) or physiological saline (control) were applied every 24h. The nutritional indices were higher in the ghrelin treated than in the control group. Ghrelin treatment was also associated with elevation of midgut mass, induced digestive enzyme activities, increased fat body mass and morphometric changes in columnar and goblet cells. This is the first report of the presence of ghrelin-like hormone in endocrine cells of an insect midgut. Such information provides additional evidence for application of this relatively simple model system in the future studies of the mechanisms underlying of digestion and energy balance in more complex organisms.

A novel GH secretagogue, A233, exhibits enhanced growth activity and innate immune system stimulation in teleosts fish

In teleosts fish, secretion of GH is regulated by several hypothalamic factors that are influenced by the physiological state of the animal. There is an interaction between immune and endocrine systems through hormones and cytokines. GH in fish is involved in many physiological processes that are not overtly growth related, such as saltwater osmoregulation, antifreeze synthesis, and the regulation of sexual maturation and immune functions. This study was conducted to characterize a decapeptide compound A233 (GKFDLSPEHQ) designed by molecular modeling to evaluate its function as a GH secretagogue (GHS). In pituitary cell culture, the peptide A233 induces GH secretion and it is also able to increase superoxide production in tilapia head-kidney leukocyte cultures. This effect is blocked by preincubation with the GHS receptor antagonist [d-Lys(3)]-GHRP6. Immunoneutralization of GH by addition of anti-tilapia GH monoclonal antibody blocked the stimulatory effect of A233 on superoxide production. These experiments propose a GH-mediated mechanism for the action of A233. The in vivo biological action of the decapeptide was also demonstrated for growth stimulation in goldfish and tilapia larvae (P<0.001). Superoxide dismutase levels, antiprotease activity, and lectin titer were enhanced in tilapia larvae treated with this novel molecule. The decapeptide A233 designed by molecular modeling is able to function as a GHS in teleosts and enhance parameters of the innate immune system in the fish larvae.

Recent advances in the phylogenetic study of ghrelin

To understand fully the biology of ghrelin, it is important to know the evolutionary history of ghrelin and its receptor. Phylogenetic and comparative genomic studies of mammalian and non-mammalian vertebrates are a useful approach to that end. Ghrelin is a hormone that has apparently evaded natural selection during a long evolutionary history. Surely ghrelin plays crucial physiological roles in living animals. Phylogenetic studies reveal the nature and evolutionary history of this important signaling system.

Effects of short-term starvation on ghrelin, GH-IGF system, and IGF-binding proteins in Atlantic salmon

The effects of short-time fasting on appetite, growth, and nutrient were studied in Atlantic salmon (Salmo salar) smolts. Feed deprivation did change the energy metabolism with reduced plasma protein and muscle indispensible amino acid levels. Plasma levels of ghrelin were significantly higher in starved salmon compared with fed fish after 2 days, but no differences in circulating ghrelin were found between treatments after 14 days. Two mRNA sequences for ghrelin-1 and ghrelin-2, 430 and 533 bp long, respectively, were detected. In addition, the growth hormone secretagogues-receptor like receptor (GHSR-LR) 1a and 1b were identified. Ghrelin-1 but not ghrelin-2 mRNA levels were affected by starvation in the stomach. Lower ghrelin-1 mRNA levels were detected at day 2 in starved fish compared with fed fish. The mRNA levels of GHSR-LR1a were not affected by starvation. Fasting reduced the phenotypic growth and the transcription of insulin-like growth factor (IGF)-II together with IGF-IIR, but IGF-I mRNA were not regulated in fasted salmon after 14 days. Three IGF-binding proteins (IGFBP) at 23, 32, and 43 kDa were found in salmon, and circulating 23 kDa was significantly increased after 14 days of starvation compared with fed fish, indicating increased catabolism. The levels of IGFBP-1 mRNA were significantly higher in fed and starved fish after 14 days compared to those at the start of the experiment, but no significant difference was observed between the treatments. In conclusion, we have shown that circulating ghrelin and ghrelin-1 mRNA is related to changes in energy metabolism in Atlantic salmon.

Ghrelin Receptor in Two Species of Anuran Amphibian, Bullfrog (Rana catesbeiana), and Japanese Tree Frog (Hyla japonica)

We have identified cDNA encoding a functional growth hormone secretagogue-receptor 1a (GHS-R1a, ghrelin receptor) in two species of anuran amphibian, the bullfrog (Rana catesbeiana), and the Japanese tree frog (Hyla japonica). Deduced receptor protein for bullfrog and Japanese tree frog (tree frog) was comprised of 374- and 371-amino acids, respectively. The two receptors shared 86% identity, and are grouped to the clade of the tetrapod homologs by phylogenetic analysis. In functional analyses, ghrelin and GHS-R1a agonists increased intracellular Ca(2+) concentration in GHS-R1a-transfected-HEK293 cell, but ligand selectivity of ghrelin with Ser(3) and Thr(3) was not observed between the two receptors. Bullfrog GHS-R1a mRNA was mainly expressed in the brain, stomach, and testis. In the brain, the gene expression was detected in the diencephalon and mesencephalon, but not in the pituitary. Tree frog GHS-R1a mRNA was predominantly expressed in the gastrointestinal tract and ovary, but not detected in the pituitary. In bullfrog stomach but not the brain, GHS-R1a mRNA expression increased after 10 days of fasting. For tree frog, GHS-R1a mRNA expression was increased in the brain, stomach and ventral skin by 10 days of fasting, and in the stomach and ventral skin by a dehydration treatment. Intracerebroventricular injection of ghrelin in dehydrated tree frog did not affect water absorption from the ventral skin. These results suggest that ghrelin is involved in energy homeostasis and possibly in osmoregulation in frogs.

Ghrelin affects carbohydrate-glycogen metabolism via insulin inhibition and glucagon stimulation in the zebrafish (Danio rerio) brain

Carbohydrate-glycogen metabolism (CGM) is critical for emergency energy supplies in the central nervous system (CNS). Ghrelin (GHRL) in pancreas is known to significantly regulate a dominant player in CGM, insulin (INS). However, its regulatory effect on extrapancreatic INS synthesis is yet unknown. In this study, we used adult zebrafish to elucidate the expression and role of zebrafish GHRL (zGHRL) in genes primarily involved in the brain's CGM. Results showed that zebrafish brain expressed zghrl and its receptor, growth hormone secretagogue-receptor (GHS-R: zghs-r1a and zghs-r2a), according to RT-PCR and in situ hybridization. Protein localization coupled with mRNA spatial expression further verified zGHRL's presence in the brain. For the in vivo study, significant increases in zghs-r1a and zghs-r2a synthesis were observed after injection of synthetic peptide goldfish GHRL-12 (gGHRL) using brain templates analyzed by quantitative real-time PCR (qPCR). Ligand-receptor synthesis of INS (zinsa; zins-r1 and zins-r2) significantly decreased, while glucagon (GCG) (zgcgb1 and zgcgb2; zgcg-r1 and zgcg-r2) exhibited a significant transient increase. In CGM, subsequent processes indicate urgent glucose-sensing response that will balance glycogen degradation and energy storage. Taken together, these findings suggest that GHRL regulates INS synthesis by mediating its action on GHS-R in the CNS and partly involved in CGM.

Sequence, genomic organization and expression of two channel catfish, Ictalurus punctatus, ghrelin receptors

Two ghrelin receptor (GHS-R) genes were isolated from channel catfish tissue and a bacterial artificial chromosome (BAC) library. The two receptors were characterized by determining tissue distribution, ontogeny of receptor mRNA expression, and effects of exogenous homologous ghrelin administration on target tissue mRNA expression. Analysis of sequence similarities indicated two genes putatively encoding GHS-R1 and GHS-R2, respectively, which have been known to be present in zebrafish. Organization and tissue expression of the GHS-R1 gene was similar to that reported for other species, and likewise yielded two detectable mRNA products as a result of alternative splicing. Expression of both full-length, GHS-R1a, and splice variant, GHS-R1b, mRNA was highest in the pituitary. Gene organization of GHS-R2 was similar to GHS-R1, but no splice variant was identified. Expression of GHS-R2a mRNA was highest in the Brockmann bodies. GHS-R1a mRNA was detected in unfertilized eggs and throughout embryogenesis, whereas GHR-R2a mRNA was not expressed in unfertilized eggs or early developing embryos and was the highest at the time of hatching. Catfish intraperitoneally injected with catfish ghrelin-Gly had greater mRNA expression of GHS-R1a in pituitaries at 2 h and Brockmann bodies at 4 h, and of GHS-R2a in Brockmann bodies at 6 h post injection. Amidated catfish ghrelin (ghrelin-amide) had no observable effect on expression of either pituitary receptor; however, GHS-R1a and GHS-R2a mRNA expression levels were increased 4 h post injection of ghrelin-amide in Brockmann bodies. This is the first characterization of GHS-R2a and suggests regulatory and functional differences between the two catfish receptors.

Pre- and postprandial effects on ghrelin signaling in the brain and on the GH/IGF-I axis in the Mozambique tilapia (Oreochromis mossambicus)

The discovery of ghrelin (GRLN) has broadened our understanding of the regulation of energy homeostasis in vertebrates. In addition to stimulating growth hormone release from the pituitary, GRLN has been implicated as a hunger signal stimulating food intake in mammals and goldfish. Indeed, GRLN levels rise preprandial and fall following a meal. The current study investigated pre- and postprandial changes (3 h before and after a meal) in GRLN signaling in the tilapia (Oreochromis mossambicus). Significant elevations in preprandial brain mRNA levels of the GRLN receptor (GHS-R1a) and GRLN were observed; though not significant brain neuropeptide Y (NPY) mRNA levels did increase preprandially. GHS-R1b, and NPY mRNA levels were reduced significantly 3 h after a meal; whereas GHS-R1a levels were unaltered postprandially. Brain ghrelin mRNA levels exhibited a transient significant increase 1 h postprandially. Tilapia that missed the scheduled feeding exhibited no changes in brain GHS-R1a, GRLN and NPY postprandial mRNA levels; whereas GHS-R1b mRNA levels were significantly reduced 1 and 3 h postprandially. Brain GHSR preprocessed RNA (heteronuclear mRNA) levels were significantly elevated 3 h preprandially. GHS-R hnRNA levels were significantly elevated 1h postprandial in fed and fasted tilapia. No preprandial rise in plasma GRLN was observed. Following a meal, plasma GRLN levels were significantly elevated; whereas there was no change in tilapia missing the scheduled feeding. Stomach mRNA levels of GRLN rose preprandially and remained unchanged following a meal. In animals that missed the scheduled feeding stomach GRLN levels dropped significantly 1 h following a meal. There was no change in plasma growth hormone levels in the fed fish, although there was a significant rise in the fasted fish 1h after the scheduled feeding. Postprandial levels of plasma IGF-I were elevated in both fed and fasted tilapia. These results suggest that brain derived GRLN is likely driving day-to-day appetite through GHS-R1a and NPY; while systemic GRLN may play a role in postprandial metabolism.