Identification and localization of material with gastrin-like immunoreactivity in the neural ganglion of a protochordate, Ciona intestinalis

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.

The Phylogeny and Biological Function of Gastric Juice-Microbiological Consequences of Removing Gastric Acid

Gastric juice is a unique combination of hydrochloric acid (HCl), lipase, and pepsin. Acidic gastric juice is found in all vertebrates, and its main function is to inactivate microorganisms. The phylogenetic preservation of this energy-consuming and, at times, hazardous function (acid-related diseases) reflects its biological importance. Proton pump inhibitors (PPIs) are one of the most widely used drugs in the world. Due to the reduced prevalence of Helicobacter pylori infection as well as the increased use of inhibitors of gastric acid secretion, the latter has become the most important cause of gastric hypoacidity. In the present manuscript, we review the microbiological consequences of removing gastric acidity. The resulting susceptibility to infections has not been studied extensively, and focus has mainly been restricted to bacterial and parasitic agents only. The strongest evidence concerning the relationship between hypochlorhydria and predisposition to infections relates to bacterial infections affecting the gastrointestinal tract. However, several other clinical settings with increased susceptibility to infections due to inhibited gastric acidity are discussed. We also discuss the impact of hypochlorhydria on the gut microbiome.

CCK(-like) and receptors: structure and phylogeny in a comparative perspective

Cholecystokinin (CCK) and gastrin are regulatory peptides in vertebrates. Their homologues are widely present in metazoan animals, in form of cionin in tunicates, neuropeptide-like protein 12 in nematodes and sulfakinin (SK) in arthropods. CCK(-like) peptides exert diverse physiological effects through binding their corresponding receptors, which are important members of the hormone-binding G-protein-coupled receptors. In this paper, CCK(-like) peptides and receptors are reviewed in a comparative way at levels of molecular structure, physiological functions and phylogeny. CCK signalling system is widely involved in the regulation of satiety, gastric acid secretion, pancreatic secretion, anxiety and memory processes in vertebrates. Its counterpart SK in arthropods is also found with similar functions on regulation of satiety and gastrointestinal motility. Co-evolution of peptide and receptor has been recognized through metazoans. The CCK(-like) receptors seem to be evolved from a common ancestor based on the phylogenetic analysis, with species-specific events in arthropods. In addition, tetraploidization has been brought up to study the evolution of receptors. There are 2 receptors in chordates and nematodes, whereas, the number of sulfakinin receptor varies in arthropods from 0 to 2. We discussed here that the presence or absence of the SK signalling system is likely to be related to feeding behaviour.

Evolution of gastrointestinal hormones: the cholecystokinin/gastrin family

PURPOSE OF REVIEW

To describe recent advances in our understanding of the evolution of gastrointestinal hormones, with the gastrin/cholecystokinin (CCK) family as a model.

RECENT FINDINGS

The release of 11 genomic sequences in the last year has provided a wealth of additional information on peptide hormone sequences. The alternative approach of reverse genetics has identified a separate class of CCK receptor ligands in the nematode Caenorhabditis elegans.

SUMMARY

Three classes of ligands, insect sulfakinins, nematode neuropeptide-like proteins and vertebrate gastrins/cholecystokinins, have now been described for the family of CCK receptors. Although all terminate in an amidated phenylalanine, similarity between the three classes is minimal elsewhere in the sequences. The occurrence of separate gastrin and CCK genes in the dogfish Squalus acanthias dates the divergence of gastrin and CCK to at least 528 +/- 56 Myr ago. The presence of a polyglutamate sequence in marsupial gastrins suggests that the ability to bind ferric ions, which is a critical determinant of biological activity for nonamidated gastrins, was acquired at least 173 +/- 12 Myr ago. Comparison of gastrin or CCK sequences between species suggests that, apart from the C-terminal tetrapeptide amide that is required for receptor binding, conservation is largely restricted to the dibasic processing sites and to the C-terminal flanking peptides of gastrin and CCK. The independent conservation of the latter peptide may be either a consequence of a requirement for precise processing, or may indicate a separate function.

Neuroendocrinology of protochordates: Insights from Ciona genomics

The genome for two species of Ciona is available making these tunicates excellent models for studies on the evolution of the chordates. In this review most of the data is from Ciona intestinalis, as the annotation of the C. savignyi genome is not yet available. The phylogenetic position of tunicates at the origin of the chordates and the nature of the genome before expansion in vertebrates allows tunicates to be used as a touchstone for understanding genes that either preceded or arose in vertebrates. A comparison of Ciona, a sea squirt, to other model organisms such as a nematode, fruit fly, zebrafish, frog, chicken and mouse shows that Ciona has many useful traits including accessibility for embryological, lineage tracing, forward genetics, and loss- or gain-of-function experiments. For neuroendocrine studies, these traits are important for determining gene function, whereas the availability of the genome is critical for identification of ligands, receptors, transcription factors and signaling pathways. Four major neurohormones and their receptors have been identified by cloning and to some extent by function in Ciona: gonadotropin-releasing hormone, insulin, insulin-like growth factor, and cionin, a member of the CCK/gastrin family. The simplicity of tunicates should be an advantage in searching for novel functions for these hormones. Other neuroendocrine components that have been annotated in the genome are a multitude of receptors, which are available for cloning, expression and functional studies.

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).

Secretagogue activities in cod (Gadus morhua) and shrimp (Penaeus aztecus) extracts and alcalase hydrolysates determined in AR4-2J pancreatic tumour cells

Peptides with gastrin immunoreactivity were measured in cod muscle (Gadus morhua) and shrimp heads (Penaeus aztecus) extracts and alcalase hydrolysates and separated by two chromatographic steps. Secretagogue activities present in crude extracts fractions were examined with or without specific antagonists of CCK receptors in AR4-2J cells. Several sub-fractions significantly stimulate amylase release, up to 110%. These stimulatory effects could be completely inhibited by the presence of L 365, 260 specific antagonist of CCKB receptors. After hydrolysis of the raw material, the samples were partially fractionated by two chromatographic steps and potential active fractions detected by a gastrin-CCK radioimmunoassay. The molecular masses of the active fractions were lower than for the extracts. Stimulation of amylase release was higher than with extracts, and the inhibition by L 365, 260, less pronounced. These results show that some peptides remaining after hydrolysis or extraction still exert biological activities and have to be tested in nutritional studies.

Phylogeny of the cholecystokinin/gastrin family

The neuroendocrine peptides cholecystokinin (CCK) and gastrin, originally identified in mammals, are characterized by a common amidated C-terminal tetrapeptide sequence, Trp-Met-Asp-Phe.NH2, which also constitutes the minimal structure necessary for biological activity of both. Hence, it has been proposed that CCK and gastrin have evolved from a common ancestor. Although the occurrence of CCK/gastrin-related peptides has been suggested in representatives of several invertebrate phyla, the evidence, mostly based on immunoreactivity, has not been substantiated by peptide identification. Instead, CCK/gastrin-specific antibodies might be cross-reacting with Asp-Phe-amides, like the lymnaDFamides, isolated from the freshwater snail Lymnaea stagnalis. Cionin, isolated from Ciona intestinalis, a representative of the protochordates that occupy a key position at the transition to vertebrates, so far represents the oldest genuine member of the CCK/gastrin family, dating the emergence of these peptides back to at least 500 million years ago. The CCK/gastrin family appears to be represented in the whole chordate phylum, and in addition to mammals, CCK and gastrin have recently been identified in a number of nonmammalian species representing the major vertebrate classes, including fishes, amphibians, reptiles, and birds. This now makes it possible to consider the CCK/gastrin phylogeny based on structural information. A duplication of the ancestral gene appears to have already occurred before or during the appearance of cartilaginous fish, giving rise to two peptides most likely homologous to mammalian CCK and gastrin. Indicative of a function of gastrin, the acid secretory system appears to have developed concomitantly in sharks. The segregation of CCK and gastrin early in vertebrate evolution resembles the situation in other peptide families, in accordance with a suggested widespread pattern of multiplication within vertebrate peptide and protein families around 400 million years ago. At the amphibian level, two separate peptide systems, resembling mammalian CCK and gastrin, have been characterized by identification of the mature bioactive peptides, cDNAs, gene structures, primary and secondary sites of gene expression, and their physiological actions. The overall gene structure, including exon/intron organization, is similar in all mammalian and nonmammalian CCK/gastrin genes. CCK is well conserved in all vertebrate species investigated, while the mammalian gastrins at first sight appear as a distinct group with little similarity to the nonmammalian gastrins outside the invariant C-terminal tetrapeptide and the C-terminal flanking peptide of the prohormone. However, evidence indicates that the transition from nonmammalian to mammalian gastrin may not be as dramatic as first anticipated. In conclusion, the CCK/gastrin family appears to be represented in most, if not all, chordates, to which group it may also be limited. The two major classes, CCK and gastrin, probably arose as distinct peptide systems early in vertebrate history. While CCK is well conserved in all vertebrates, a major structural change of gastrin accompanied the transition to mammals.

Heterogeneity of cholecystokinin/gastrin-like immunoreactivity in the nervous system of the nematode Ascaris suum

A wholemount immunocytochemical method was used for the localization of cholecystokinin (CCK8)-like and gastrin-like immunoreactivity in Ascaris. The patterns of specific neuronal staining given by two antisera and four monoclonal antibodies made against CCK8, and one antiserum made against gastrin were investigated. Preabsorption of these antibodies with CCK8 or gastrin 17 resulted in complete loss of immunoreactivity in almost all of the neurons (two antisera also contained nonspecific antibodies), suggesting that all of the antibodies recognize epitopes, in Ascaris neurons, that include some or all of the C-terminal five amino acids that are identical in CCK8 and gastrin 17. However, the seven different antibodies showed immunoreactivity in different subpopulations of neurons, implying that there are at least seven different species of CCK-like molecules in Ascaris. Fractionation of Ascaris peptide extracts by high performance liquid chromatography (HPLC), monitoring fractions with a CCK8 radioimmunoassay (RIA), also shows heterogeneity of molecules immunologically related to CCK8.

Evolution of the gastrointestinal endocrine system (with special reference to gastrin and CCK)

The evolution of gut endocrine cells can be seen to have depended in the first instance on the expression of genes encoding regulatory peptides in cells that had evolved the regulated pathway of secretion. It seems probable that the endocrine cells made use of molecules and mechanisms that first emerged in early nervous systems. However, by the start of the vertebrate line of evolution, most of the major families of gut hormones were already found in association with endocrine cells. A single common class of receptor with seven transmembrane domains and acting via association with G-proteins transduces many (perhaps all) gut peptide actions. The duplication and divergence of receptors and peptides can now be traced, in outline at least, for gastrin and CCK in vertebrates. Even in phylogenetically similar groups such as birds and mammals, quite different molecular approaches have been applied to solving the same physiological problem. Evolution of the modern gastrointestinal control system evidently depended in this case both on molecular evolution of peptides and receptors and on cells expressing the genes encoding them.

cDNA deduced procionin. Structure and expression in protochordates resemble that of procholecystokinin in mammals

Using an improved 3' RACE (PCR) amplification system containing oligonucleotide primer with an inosine at ambiguous codon positions and inverse PCR to amplify the 5' ends, we have isolated and characterized cDNA clones which encode cionin, a protochordean homologue of the mammalian hormones, cholecystokinin (CCK) and gastrin. The full-length cloned cDNA of 510 bp encoded a 128 amino acid preprocionin. Reverse transcription-PCR and subsequent cDNA cloning revealed that cionin mRNA is expressed in both the neuronal ganglion and the gut of the protochordate Ciona intestinalis. The primary structure of procionin resembles that of proCCK more than that of progastrin. Sequence-specific immunochemical analysis showed that the cionin gene is expressed also at peptide level in both the gut and the neural ganglion. The neuronal processing of procionin is, however, more complete both with respect to carboxyamidation and tyrosine O-sulfation. Hence, the tissue-specific expression of the cionin gene in Ciona intestinalis resembles that of the CCK gene in mammals.

Occurrence of members of the insulin superfamily in central nervous system and digestive tract of protochordates

Antisera specific for mammalian insulin-like growth factor 1 (IGF-1) and mammalian insulin and the double immunofluorescence technique were used for this study. IGF-1-like-immunoreactivity was localized in entero-endocrine cells in the gastro-intestinal tract of the protochordates Ciona intestinalis and Branchiostoma lanceolatum. Some of the specimens also showed IGF-1-like-immunoreactive (-IR) perikarya and fibers in the central nervous system. Whilst in rat endocrine pancreas, IGF-1-IR and insulin-IR occurred in different cell populations, in Ciona and Branchiostoma the vast majority of entero-endocrine cells and central neurons were IGF-1-like- +insulin-IR. A minor portion exhibited IGF-1-like-IR alone. For further characterization of the IGF-1-like-IR material, in Ciona intestinalis, peptides related to IGF-1 were identified by radioimmunoassay and gel chromatography. In accordance with the immunohistochemical results, IGF-I-like-IR was detected both in cerebral ganglion and in gastro-intestinal tract. Using acid gel chromatography, in Ciona gastro-intestinal tract the IGF-1-like-IR was found to occur in two peaks, with apparent molecular weights of approximately 16 kDa and 3 kDa. Absorption studies with insulin- and IGF-related peptides, with crude extracts and the peak material obtained after gel chromatography, indicated that the IGF-1-like peptides in Ciona are different from mammalian insulin and IGF-1. The findings are in accordance with the presence of a common insulin/IGF precursor molecule in protochordates.

Pattern of substance P- and cholecystokinin-like immunoreactivity during regeneration of the neural complex in the ascidian Ciona intestinalis

The neural ganglion of ascidians exhibits a novel and rapid pattern of regeneration whereby within approximately 28-35 days of total ablation an entirely new neural complex is formed. In normal adults, neuronal cell bodies expressing substance P- (SP-Li), neurokinin A-(NKA-Li), CCK/gastrin- (CCK-Li), and insulin-like immunoreactivity exhibit a clearly defined pattern of localization in the cortical rind of the ganglion with characteristic long processes arising from the perikarya running throughout the neuropile. CCK-Li cell bodies are particularly concentrated close to the points of exit of the main nerve trunks. We have used antisera raised against these peptides to monitor the process of regeneration up to postoperative (pa) day 35. Only SP and CCK antisera produced positive staining in the regenerating tissue. Immunoreactive cell bodies first appear following 14 days pa. At this time CCK-Li neurons are more abundant than SP-Li neurons and in contrast to the pattern found in the normal adult ganglion, immunoreactive cell bodies are located both peripherally and centrally in the core of the ganglion and processes were rarely seen. Later stages exhibited an increasing number of SP-Li neurons and at 35 days pa SP-Li cell bodies clearly predominate. CCK-Li neurons typically become clustered close to the points of emergence of the anterior nerve roots. The early expression of CCK-Li and SP-Li molecules during regeneration is considered in terms of their potential role in development and cell proliferation in the newly forming ganglion.

Structure and biological activity of crustacean gastrointestinal peptides identified with antibodies to gastrin/cholecystokinin

Four gastrin/cholecystokinin-like peptides (G/CCK) which cross-react with a specific C-terminal gastrin/CCK antiserum have been isolated from the stomach of the marine crustacean Nephrops norvegicus. The molecular weight of the four peptides was estimated between 1000 and 2000 Da by molecular sieving. By radioimmunoassay, the cross-reactivity of these peptides with human gastrin 17-I was found to be around 0.03%. Pure peptidic fractions were recovered after four successive steps of HPLC. Amino-acid analysis suggested a similarity between the four peptides identified which may belong to a new family. A limited homology between the C-terminus of one Nephrops peptide and vertebrate G/CCK was found after sequencing. Two of the peptides exhibited secretagogue effects on crustacean isolated midgut glands. The Nephrops peptides, although structurally distinct from the vertebrate G/CCKs, appear to serve similar biological functions in crustaceans.

Immunocytochemical Localization of Glucagon-Related Peptides in the Cerebral Ganglion of the Protochordate Ascidian, Styela plicata

Abstract The central nervous system of the protochordate ascidian Styela plicata has been investigated by means of immunocytochemical techniques in order to localize peptides of the glucagon/secretin family. In particular, glucagon- and peptide histidine isoleucine-containing neurons were localized in the cortex of the cerebral ganglion, and numerous nerve cell bodies containing vasoactive intestinal polypeptide occurred mainly in the medullary zone. Moreover, coexistence of glucagon/peptide histidine isoleucine and glucagon/vasoactive intestinal polypeptide was detected in a few cortical neurons. The localization of peptide histidine isoleucine-like peptides in the central nervous system and alimentary tract supports the occurrence of a 'brain-gut axis' in protochordates as well as in vertebrates and some invertebrates, while glucagon- and vasoactive intestinal polypeptide-like substances can be considered exclusively as neuropeptides.

Gastrin/CCK-like peptides in the spiny dogfish, Squalus acanthias; concentrations and actions in the gut

1. The presence of gastrin/CCK-like immunoreactive material in both muscle and mucosal layers of the whole gut, except the cardiac stomach, in Squalus acanthias has been confirmed by radioimmunoassay. The highest levels were measured in rectum and spiral intestine respectively. 2. Fractionation of the spiral intestine mucosal extract on DEAE 52 columns indicated the presence of multiple forms of gastrin/CCK in the elasmobranch gut. 3. Synthetic mammalian gastrin- and CCK-like peptides, when effective, increased the rhythmic activity and sometimes the basal tonus in smooth muscle preparations from the intestine or rectum. 4. The irregular effects of mammalian peptides on the motility indicated an essential difference from the mammalian counterpart in the structure of the elasmobranch peptide present in the gastrointestinal nerves.

Sulphated CCK-8-like peptides in the neural ganglion of the protochordate Ciona intestinalis

Immunochemical studies were carried out on extracts of the neural ganglion from the ascidian Ciona intestinalis in order to the characterize the peptide(s), which react with antibodies against the C-terminal sequence common for the mammalian hormones, cholecystokinin (CCK) and gastrin. Radioimmunoassays specific for the sulphotyrosyl-containing N-terminus of CCK-8, for the common alpha-carboxyamidated C-terminus and for gastrin were used to monitor gel chromatography and reverse-phase HPLC of the extracts. Only neutral extracts contained immunoreactive material (634 (524-785) pmol eqv.CCK-8/g) (mean and range, n = 4)). HPLC revealed a small peak eluting almost like CCK-8 and a larger peak eluting earlier. By subsequent gel chromatography the larger peak eluted in the same position as sulphated CCK-8. The material was recognized almost equally by the N- and C-terminal CCK radioimmunoassays, whereas the specific C-terminal gastrin radioimmunoassay did not measure the peptides. Treatment with arylsulphatase removed the binding to the antiserum specific for the sulphotyrosyl-containing sequence of CCK. The results indicate that the ganglion of Ciona intestinalis contains a tyrosyl-sulphated peptide resembling mammalian CCK-8.

Localization of immunoreactive gastrin-like cells in the alimentary tract of the ascidian Styela plicata

The occurrence of a gastrin-like immunoreactivity in the alimentary tract of the ascidian Styela plicata has been investigated using immunocytochemical methods. Gastrin-like cells are present only in the gastric epithelium among the cell types responsible for digestion and absorption of food. The physiological role played by the ascidian gastrin-like peptides is discussed together with the evolutionary history of peptides of the gastrin/cholecystokinin family.