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

The unexpected loss of the 'hunger hormone' ghrelin in true passerines: a game changer in migration physiology

Migratory birds must accumulate large amounts of fat prior to migration to sustain long flights. In passerines, the small body size limits the amount of energy stores that can be transported, and therefore birds undergo cycles of extreme fattening and rapid exhaustion of reserves. Research on these physiological adaptations was rattled by the discovery that birds have lost the main vertebrate regulator of fat deposition, leptin. Recent studies have thus focused on ghrelin, known as 'hunger hormone', a peptide secreted by the gastrointestinal tract to regulate, e.g. food intake and body mass in vertebrates. Studies on domestic species showed that, in birds, ghrelin has effects opposite to those described in mammals such as inhibiting instead of promoting food intake. Furthermore, recent studies have shown that ghrelin administration influences migratory behaviour in passerine birds. Using comparative genomics and immunoaffinity chromatography, we show that ghrelin has been lost in Eupasseres after the basic split from Acanthisitti about 50 Ma. We found that the ghrelin receptor is still conserved in passerines. The maintenance of a functional receptor system suggests that in Eupasserines, another ligand has replaced ghrelin, perhaps to bypass the feedback system that would hinder the large pre-migratory accumulation of subcutaneous fat.

Ultrastructural study of neuronal cells and localization of ghrelin-like peptide and its receptor in the ganglia of the golden apple snail (Pomacea canaliculata)

Pomacea canaliculata is an invasive snail species causing major problems in agriculture. The snail biology was then investigated. The main objective of the present study was to investigate the nervous system of the snail. The nervous system comprises pairs of cerebral, buccal, pedal, pleural, parietal ganglia and an unpaired visceral ganglion. Most neurons were concentrated at the periphery of the ganglia. The neurons were classified into four types: NR1, NR2, NR3, and NR4. The percentages of the NR3 and NR4 in the pleural and pedal ganglia were significantly higher than those of other ganglia. Ultrastructural study revealed that nuclei of all neuronal types exhibited mostly euchromatins. Many organelles including ribosomes and endoplasmic reticulum were found in their cytoplasm. However, various mitochondria were found in the NR2 and NR3. The immunohistochemistry revealed immunoreactivity of ghrelin-like peptide in the neurons of the cerebral, pleural and pedal ganglia. However, immunoreactivity of GHS-R1a-like peptide existed only in the neurons of the pleural and pedal ganglia. The present study is the first to demonstrate the existence of ghrelin-like peptide and its receptor in P. canaliculata nervous system.

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.

Ghrelin and ghrelin receptor (GHSR) in Chinese alligator, alligator sinensis: Molecular characterization, tissue distribution and mRNA expression changes during the active and hibernating periods

The Chinese alligator (Alligator sinensis) is a freshwater crocodilian endemic to China. So far, the endocrine regulation of feeding and growth in Chinese alligator is poorly understood. In this study, the molecular structure and tissue expression profiles of ghrelin and its receptor GHSR in the Chinese alligator were characterized for the first time. The full-length cDNA of ghrelin was 1770 bp, including a 37 bp 5 '-UTR (untranslated region), a 435 bp ORF (open reading frame) and a 1298 bp 3 '-UTR. The ORF encodes a ghrelin precursor, which consists of 145 amino acid residues, including a signal peptide with 52 amino acid residues at the N-terminus, a mature peptide with 28 amino acid residues, and a possibly obestain at the C-terminus. The full-length cDNA of GHSR was 3961 bp, including a 5'-UTR of 375-bp, an ORF of 1059-bp and a 3' -UTR of 2527-bp. The ORF encodes a protein of 352 amino acid residues containing seven transmembrane domains, with multiple N glycosylation modification sites and conserved cysteine residue sites. The active core "GSSF" of Chinese alligator ghrelin was identical to that of mammals and birds, and the ghrelin binding site of GHSR was similar to that of mammals. The amino acid sequences of both ghrelin and GHSR share high identity with American alligator (Alligator mississippiensis) and birds. Ghrelin was highly expressed in cerebrum, mesencephalon, hypothalamus and multiple peripheral tissues, including lung, stomach and intestine, suggesting that it could play functions in paracrine and/or autocrine manners in addition to endocrine manner. GHSR expression level was higher in hypothalamus, epencephalon and medulla oblongata, and moderate in multiple peripheral tissues including lung, kindey, stomach and oviduct, implicating that ghrelin/GHSR system may participate in the regulation of energy balance, food intake, water and mineral balance, gastrointestinal motility, gastric acid secretion and reproduction. During hibernation, the expression of ghrelin and GHSR in the brain was significantly increased, while ghrelin was significantly decreased in heart, liver, lung, stomach, pancreas and ovary, and GHSR was significantly decreased in heart, liver, spleen, lung, kindey, stomach, ovary and oviduct. These temporal changes in ghrelin and GHSR expression could facilitate the physiological adaption to the hibernation of Chinese alligator. Our study could provide basic data for further studies on the regulation of feeding, physiological metabolism and reproduction of Chinese alligator, which could also be useful for the improvement of artificial breeding of this endangered species.

Expression of ghrelin and leptin in the chemosensory system of adult zebrafish

Numerous data show that the chemosensory system seems to be modulated by changes in the circulating levels of different molecules such as ghrelin, orexin, leptin, NPY, CCK. The chemosensory system of the zebrafish is represented by the taste buds (skin, oral and oropharyngeal), the olfactory rosette and the solitary chemosensorial cells (SCCs). The purpose of our study was to analyze the distribution of two peripheral hormones such as ghrelin and leptin in the chemosensory organs of the zebrafish. Our results demonstrated the presence of immunoreaction for all antibodies used in the zebrafish chemosensory organs even if with different distribution. In particular, IR was observed for ghrelin in the olfactory rosette while IR for leptin was found in the olfactory rosette, in the skin and oropharyngeal taste buds and in the gills. Both these hormones were detected in the intestine, used as a control.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.

Diversification and coevolution of the ghrelin/growth hormone secretagogue receptor system in vertebrates

The gut hormone ghrelin is involved in numerous metabolic functions, such as the stimulation of growth hormone secretion, gastric motility, and food intake. Ghrelin is modified by ghrelin O-acyltransferase (GOAT) or membrane-bound O-acyltransferase domain-containing 4 (MBOAT4) enabling action through the growth hormone secretagogue receptors (GHS-R). During the course of evolution, initially strong ligand/receptor specificities can be disrupted by genomic changes, potentially modifying physiological roles of the ligand/receptor system. Here, we investigated the coevolution of ghrelin, GOAT, and GHS-R in vertebrates. We combined similarity search, conserved synteny analyses, phylogenetic reconstructions, and protein structure comparisons to reconstruct the evolutionary history of the ghrelin system. Ghrelin remained a single-gene locus in all vertebrate species, and accordingly, a single GHS-R isoform was identified in all tetrapods. Similar patterns of the nonsynonymous (dN) and synonymous (dS) ratio (dN/dS) in the vertebrate lineage strongly suggest coevolution of the ghrelin and GHS-R genes, supporting specific functional interactions and common physiological pathways. The selection profiles do not allow confirmation as to whether ghrelin binds specifically to GOAT, but the ghrelin dN/dS patterns are more similar to those of GOAT compared to MBOAT1 and MBOAT2 isoforms. Four GHS-R isoforms were identified in teleost genomes. This diversification of GHS-R resulted from successive rounds of duplications, some of which remained specific to the teleost lineage. Coevolution signals are lost in teleosts, presumably due to the diversification of GHS-R but not the ghrelin gene. The identification of the GHS-R diversity in teleosts provides a molecular basis for comparative studies on ghrelin's physiological roles and regulation, while the comparative sequence and structure analyses will assist translational medicine to determine structure-function relationships of the ghrelin/GHS-R system.