Stimulation of gastric enzyme secretion by porcine cholecystokinin in the ascidian Styela clava

Ancient role of sulfakinin/cholecystokinin-type signalling in inhibitory regulation of feeding processes revealed in an echinoderm

Sulfakinin (SK)/cholecystokinin (CCK)-type neuropeptides regulate feeding and digestion in protostomes (e.g. insects) and chordates. Here, we characterised SK/CCK-type signalling for the first time in a non-chordate deuterostome - the starfish Asterias rubens (phylum Echinodermata). In this species, two neuropeptides (ArSK/CCK1, ArSK/CCK2) derived from the precursor protein ArSK/CCKP act as ligands for an SK/CCK-type receptor (ArSK/CCKR) and these peptides/proteins are expressed in the nervous system, digestive system, tube feet, and body wall. Furthermore, ArSK/CCK1 and ArSK/CCK2 cause dose-dependent contraction of cardiac stomach, tube foot, and apical muscle preparations in vitro, and injection of these neuropeptides in vivo triggers cardiac stomach retraction and inhibition of the onset of feeding in A. rubens. Thus, an evolutionarily ancient role of SK/CCK-type neuropeptides as inhibitory regulators of feeding-related processes in the Bilateria has been conserved in the unusual and unique context of the extra-oral feeding behaviour and pentaradial body plan of an echinoderm.

Molecular and functional characterization of cionin receptors in the ascidian, Ciona intestinalis: the evolutionary origin of the vertebrate cholecystokinin/gastrin family

Cholecystokinin (CCK) and gastrin are vertebrate brain-gut peptides featured by a sulfated tyrosine residue and a C-terminally amidated tetrapeptide consensus sequence. Cionin, identified in the ascidian, Ciona intestinalis, the closest species to vertebrates, harbors two sulfated tyrosines and the CCK/gastrin consensus tetrapeptide sequence. While a putative cionin receptor, cior, was cloned, the ligand-receptor relationship between cionin and CioR remains unidentified. Here, we identify two cionin receptors, CioR1 and CioR2, which are the aforementioned putative cionin receptor and its novel paralog respectively. Phylogenetic analysis revealed that CioRs are homologous to vertebrate CCK receptors (CCKRs) and diverged from a common ancestor in the Ciona-specific lineage. Cionin activates intracellular calcium mobilization in cultured cells expressing CioR1 or CioR2. Monosulfated and nonsulfated cionin exhibited less potent or no activity, indicating that CioRs possess pharmacological features similar to the vertebrate CCK-specific receptor CCK1R, rather than its subtype CCK2R, given that a sulfated tyrosine in CCK is required for binding to CCK1R, but not to CCK2R. Collectively, the present data reveal that CioRs share a common ancestor with vertebrate CCKRs and indicate that CCK and CCK1R form the ancestral ligand-receptor pair in the vertebrate CCK/gastrin system. Cionin is expressed in the neural complex, digestive organs, oral siphon and atrial siphons, whereas the expression of ciors was detected mainly in these tissues and the ovary. Furthermore, cioninergic neurons innervate both of the siphons. These results suggest that cionin is involved in the regulation of siphonal functions.

Neuropeptides, hormone peptides, and their receptors in Ciona intestinalis: an update

The critical phylogenetic position of ascidians leads to the presumption that neuropeptides and hormones in vertebrates are highly likely to be evolutionarily conserved in ascidians, and the cosmopolitan species Ciona intestinalis is expected to be an excellent deuterostome Invertebrate model for studies on neuropeptides and hormones. Nevertheless, molecular and functional characterization of Ciona neuropeptides and hormone peptides was restricted to a few peptides such as a cholecystokinin (CCK)/gastrin peptide, cionin, and gonadotropin-releasing hormones (GnRHs). In the past few years, mass spectrometric analyses and database searches have detected Ciona orthologs or prototypes of vertebrate peptides and their receptors, including tachykinin, insulin/relaxin, calcitonin, and vasopressin. Furthermore, studies have shown that several Ciona peptides, including vasopressin and a novel GnRH-related peptide, have acquired ascidian-specific molecular forms and/or biological functions. These findings provided indisputable evidence that ascidians, unlike other invertebrates (including the traditional protostome model animals), possess neuropeptides and hormone peptides structurally and functionally related to vertebrate counterparts, and that several peptides have uniquely diverged in ascidian evolutionary lineages. Moreover, recent functional analyses of Ciona tachykinin in the ovary substantiated the novel tachykininergic protease-assoclated oocyte growth pathway, which could not have been revealed in studies on vertebrates. These findings confirm the outstanding advantages of ascidians in understanding the neuroscience, endocrinology, and evolution of vertebrate neuropeptides and hormone peptides. This article provides an overview of basic findings and reviews new knowledge on ascidian neuropeptides and hormone peptides.

Endocrinology of protochordates

Large-scale gene duplications occurred early in the vertebrate lineage after the split with protochordates. Thus, protochordate hormones and their receptors, transcription factors, and signaling pathways may be the foundation for the endocrine system in vertebrates. A number of hormones have been identified including cionin, a likely ancestor of cholecytokinin (CCK) and gastrin. Both insulin and insulin-like growth hormone (IGF) have been identified in separate cDNAs in a tunicate, whereas only a single insulin-like peptide was found in amphioxus. In tunicates, nine distinct forms of gonadotropin-releasing hormone (GnRH) are shown to induce gamete release, even though a pituitary gland and sex steroids are lacking. In both tunicates and amphioxus, there is evidence of some components of a thyroid system, but the lack of a sequenced genome for amphioxus has slowed progress in the structural identification of its hormones. Immunocytochemistry has been used to tentatively identify a number of hormones in protochordates, but structural and functional studies are needed. For receptors, protochordates have many vertebrate homologs of nuclear receptors, such as the thyroid, retinoic acid, and retinoid X receptors. Also, tunicates have cell surface receptors including the G-protein-coupled type, such as β-adrenergic, putative endocannabinoid, cionin (CCK-like), and two GnRH receptors. Several tyrosine kinase receptors include two epidermal growth factor (EGF) receptors (tunicates) and an insulin/IGF receptor (amphioxus). Interestingly, neither steroid receptors nor a full complement of enzymes for synthesis of sex steroids are encoded in the Ciona genome. Tunicates appear to have some but not all of the necessary molecules to develop a vertebrate-like pituitary or complete thyroid system.

Molecular cloning of a putative Ciona intestinalis cionin receptor, a new member of the CCK/gastrin receptor family

Cionin, a peptide showing similarities with cholecystokinin and gastrin has been shown to be expressed in the gut and neural ganglion of the protochordate Ciona intestinalis. The present report describes the cloning of a putative cionin receptor (CioR), a new member of the CCK/gastrin family from the gastrointestinal tract of C. intestinalis. mRNA from the stomach of C. intestinalis was isolated using a modified RNA extraction procedure and, subsequently, reverse-transcribed into single-stranded cDNA by means of rapid amplification of 5'- and 3'-cDNA ends (RACE-PCR), followed by full-length PCR amplification. The cloned full-length PCR amplicons contained a short upstream open-reading frame (uORF) coding for a putative 16 amino acid long peptide, followed by a long open reading frame encoding a 526 amino acid putative CioR protein. At the amino acid level, the putative CioR protein shared 35-40% homology with cloned mammalian, chicken, and Xenopus laevis CCK receptors. Phylogenetic analysis revealed that the chicken and X. laevis CCK receptors are orthologues of the mammalian CCK2 receptors whereas CioR protein forms a clade with vertebrate cholecystokinin receptors. Moreover, we found that the CioR cDNA and deduced amino acid sequences were found to correspond to the annotated CCK/gastrin-like receptor gene on Scaffold 117 (C. intestinalis draft genome project, Joint Genome Institute database; http://www.jgi.doe.gov).

Gastrin/CCK-like immunoreactivity in Hatschek's groove of Branchiostoma lanceolatum (Cephalochordata)

Gastrin/CCK-like immunoreactivity was demonstrated in a specific group of epithelial cells in Hatschek's groove. Positive reactions were obtained with antisera directed against the C-terminal amino acid sequence common to mammalian CCK and gastrin, but not with antisera against the midportions of these hormones. No immunoreactivity was obtained with antisera against peptides from hypophysis, neuroendocrine peptides in the gastrointestinal tract, islet hormones, calcitonin, NSE, and S-100. The function of the gastrin/CCK-like peptide found in Hatschek's groove is discussed.

Cholecystokinin and its receptors in vertebrates and invertebrates

Recent studies have indicated that cholecystokinin (CCK) peptides have a long evolutionary history. However, whereas all vertebrates examined have been shown to contain CCK-like peptides, this has not been possible to demonstrate for all invertebrate groups. Immunostaining studies indicate that CCK peptides originate only in neurons in groups below the level of the protochordates. It seems likely that CCK gastrointestinal endocrine cells evolved first at the level of the protochordates, possibly from sensory gut neurons similar to those seen in the invertebrates. Immunochemical and biological studies of a few invertebrate CCK-like peptides suggest that those molecules are substantially different in structure from vertebrate CCKs. Gastrin appears to have evolved from CCK at the level of the appearance of amniotes in vertebrate phylogeny. In mammals, central and peripheral CCK receptors differ in specificity for CCK- and gastrin-like peptides. Comparative studies reveal that this is true for birds as well, but reptiles, amphibia, and fish brain and pancreas CCK receptors exhibit nearly identical specificity patterns. This suggests that the lower vertebrate CCK receptor is ancestral to the distinct brain and pancreas CCK receptors seen in birds and mammals.

CCK-like peptides in the neural complex of a protochordate

The neural complex of the ascidian Styela plicata has been investigated by means of cytochemical and immunocytochemical methods. In the cerebral ganglion, using a mammalian antibody to synthetic CCK-8, immunoreactive neurons and nerve fibers have been localized; at the same time immunofluorescent cells are scattered in some glandular lobules of the neural gland. The possible functions of a CCK-8-like peptide in ascidians is suggested and discussed.

Comparative studies on the effects of cholecystokinins, caerulein, bombesin 6-14 nonapeptide, and physalaemin on gastric secretion in the ascidian Styela clava

The effects of cholecystokinins (CCK), caerulein, bombesin 6-14 nonapeptide (bombesin), and physalaemin on gastric secretion in Styela clava were measured using a perfusion technique. Varying concentrations of both CCK33 and CCK8 produced a significant dose-dependent response. Dose for dose, CCK33 was more potent than CCK8, while the assay was unable to show any discrimination between sulphated and nonsulphated forms of CCK8. The specific CCK inhibitor Bt2cGMP significantly reduced the response to both CCK33 and CCK8. Both caerulein and physalaemin were effective although with a considerably reduced response compared with CCK. Bombesin was the most potent of all secretagogues tested while glucagon was without effect on gastric secretion. It is suggested that the primitive prepancreatic zymogen cells in Styela possess a receptor or receptors with an ability to recognize those peptides which are also active on vertebrate pancreatic acinar cells. It is further suggested that while the results indicate a receptor system less sophisticated than that found in vertebrates, they also imply the presence of an endogenous polypeptide hormone or hormones with a sequence which might be expected to show similarities to more than one vertebrate gastrointestinal peptide.

Gastrin/cholecystokinin-like immunoreactive peptides in the Dungeness crab, Cancer magister (Dana): immunochemical and biological characterization

The purpose of this investigation was to characterize a gastrin/cholecystokinin-like immunoreactant (G/CCK-LI) extractable from the crab, Cancer magister. G/CCK-LI was extracted best in boiling water and was found mainly in the stomach, hemolymph and carapace. A relatively large immunoreactive peptide in the stomach and apparently smaller forms in the hemolymph and carapace were separated by Sephadex G-50 fractionation. Anion-exchange chromatography further fractionated the stomach form into three major peaks. The crab material cross-reacted with three antisera specific for the common C-terminus of gastrin/CCK, but cross-reacted much less with three antisera directed against other portions of the gastrin molecule. Partially purified crab stomach G/CCK-LI inhibited the binding of labeled CCK to mouse brain G/CCK receptors but not to rat pancreatic CCK receptors. The crab peptide did not stimulate rat gastric acid or rat pancreatic amylase secretion. These results indicate that the crab peptides are structurally similar to, but distinguishable from, the bioactive C-terminal amino acid sequence common to gastrins and CCKs.

Occurrence of different secretin-like cells in the digestive tract of the ascidian Styela plicata (Urochordata, Ascidiacea)

Secretin-like cells have been detected in the digestive tract of the ascidian Styela plicata by means of immunofluorescent and immunocytochemical methods. Especially, in the esophageal epithelium there are immunoreactive cells (S2) in which a biogenic amine (5-HT) and a regulatory peptide (secretin) occur together. In the gastric epithelium only secretin-like cells (S1) are present. Tests of cross-reactivity performed with glucagon, GIP and VIP, have confirmed the presence of a secretin-like molecule only in the S1 and S2 cells.