Gastro-intestinal and neurohormonal peptides in the alimentary tract and cerebral complex of Ciona intestinalis (Ascidiaceae)

The evolution of neuropeptide signalling: insights from echinoderms

Neuropeptides are evolutionarily ancient mediators of neuronal signalling that regulate a wide range of physiological processes and behaviours in animals. Neuropeptide signalling has been investigated extensively in vertebrates and protostomian invertebrates, which include the ecdysozoans Drosophila melanogaster (Phylum Arthropoda) and Caenorhabditis elegans (Phylum Nematoda). However, until recently, an understanding of evolutionary relationships between neuropeptide signalling systems in vertebrates and protostomes has been impaired by a lack of genome/transcriptome sequence data from non-ecdysozoan invertebrates. The echinoderms—a deuterostomian phylum that includes sea urchins, sea cucumbers and starfish—have been particularly important in providing new insights into neuropeptide evolution. Sequencing of the genome of the sea urchin Strongylocentrotus purpuratus (Class Echinoidea) enabled discovery of (i) the first invertebrate thyrotropin-releasing hormone-type precursor, (ii) the first deuterostomian pedal peptide/orcokinin-type precursors and (iii) NG peptides—the ‘missing link’ between neuropeptide S in tetrapod vertebrates and crustacean cardioactive peptide in protostomes. More recently, sequencing of the neural transcriptome of the starfish Asterias rubens (Class Asteroidea) enabled identification of 40 neuropeptide precursors, including the first kisspeptin and melanin-concentrating hormone-type precursors to be identified outside of the chordates. Furthermore, the characterization of a corazonin-type neuropeptide signalling system in A. rubens has provided important new insights into the evolution of gonadotropin-releasing hormone-related neuropeptides. Looking forward, the discovery of multiple neuropeptide signalling systems in echinoderms provides opportunities to investigate how these systems are used to regulate physiological and behavioural processes in the unique context of a decentralized, pentaradial bauplan.

Comparative localization of serotonin-like immunoreactive cells in Thaliacea informs tunicate phylogeny

BACKGROUND

Thaliaceans is one of the understudied classes of the phylum Tunicata. In particular, their phylogenetic relationships remain an issue of debate. The overall pattern of serotonin (5-HT) distribution is an excellent biochemical trait to interpret internal relationships at order level. In the experiments reported here we compared serotonin-like immunoreactivity at different life cycle stages of two salpid, one doliolid, and one pyrosomatid species. This multi-species comparison provides new neuroanatomical data for better resolving the phylogeny of the class Thaliacea.

RESULTS

Adults of all four examined thaliacean species exhibited serotonin-like immunoreactivity in neuronal and non-neuronal cell types, whose anatomical position with respect to the nervous system is consistently identifiable due to α-tubulin immunoreactivity. The results indicate an extensive pattern that is consistent with the presence of serotonin in cell bodies of variable morphology and position, with some variation within and among orders. Serotonin-like immunoreactivity was not found in immature forms such as blastozooids (Salpida), tadpole larvae (Doliolida) and young zooids (Pyrosomatida).

CONCLUSIONS

Comparative anatomy of serotonin-like immunoreactivity in all three thaliacean clades has not been reported previously. These results are discussed with regard to studies of serotonin-like immunoreactivity in adult ascidians. Lack of serotonin-like immunoreactivity in the endostyle of Salpida and Doliolida compared to Pyrosomella verticillata might be the result of secondary loss of serotonin control over ciliary beating and mucus secretion. These data, when combined with other plesiomorphic characters, support the hypothesis that Pyrosomatida is basal to these clades within Phlebobranchiata and that Salpida and Doliolida constitute sister-groups.

Insulin-like genes in ascidians: findings in Ciona and hypotheses on the evolutionary origins of the pancreas

Insulin plays an extensively characterized role in the control of sugar metabolism, growth and homeostasis in a wide range of organisms. In vertebrate chordates, insulin is mainly produced by the beta cells of the endocrine pancreas, while in non-chordate animals insulin-producing cells are mainly found in the nervous system and/or scattered along the digestive tract. However, recent studies have indicated the notochord, the defining feature of the chordate phylum, as an additional site of expression of insulin-like peptides. Here we show that two of the three insulin-like genes identified in Ciona intestinalis, an invertebrate chordate with a dual life cycle, are first expressed in the developing notochord during embryogenesis and transition to distinct areas of the adult digestive tract after metamorphosis. In addition, we present data suggesting that the transcription factor Ciona Brachyury is involved in the control of notochord expression of at least one of these genes, Ciona insulin-like 2. Finally, we review the information currently available on insulin-producing cells in ascidians and on pancreas-related transcription factors that might control their expression.

Evolution and phylogeny of the corticotropin-releasing factor (CRF) family of peptides: expansion and specialization in the vertebrates

New sequence data on CRF family members from a number of genomes has led to the modification of our understanding of CRF evolution in the Metazoa. The corticotropin-releasing factor (CRF) family of peptides include four paralogous lineages in jawed vertebrates; CRF, urotensin-I/urocortin/sauvagine, urocortin 2 (Ucn2) and urocortin 3 (Ucn3). CRF and the urotensin-I/urocortin/sauvagine group represent a gene duplication from one lineage, whereas Ucns 2 and 3 are the result of a gene duplication in the other paralogous lineage. Both paralogous lineages are the result of a gene duplication from a single ancestral peptide that occurred after the divergence of the tunicates from the ancestor that led to the evolution of chordates and vertebrates. The presence of a single CRF-like peptide in tunicates and insects suggests that a single CRF-like ancestor was present before the separation of deuterostomes and protostomes. Currently there is no strong evidence that indicates that CRF-like peptides were present in metazoan taxa that evolved before this time although the structural similarity between some CRF peptides in insects, tunicates and vertebrates with the calcitonin family of peptides hints that prior to the formation of deuterostomes and protostomes the ancestral peptide possessed both CRF and calcitonin-like structural attributes. Here, we show evidences of conservation of CRF-like function dating back to early prokaryotes. This ancestral CRF-calcitonin-like peptide may have initially resulted from a horizontal gene transfer event from prokaryotes to a protistan species that later gave rise to the metazoans.

Molecular evolution of peptidergic signaling systems in bilaterians

Peptide hormones and their receptors are widespread in metazoans, but the knowledge we have of their evolutionary relationships remains unclear. Recently, accumulating genome sequences from many different species have offered the opportunity to reassess the relationships between protostomian and deuterostomian peptidergic systems (PSs). Here we used sequences of all human rhodopsin and secretin-type G protein-coupled receptors as bait to retrieve potential homologs in the genomes of 15 bilaterian species, including nonchordate deuterostomian and lophotrochozoan species. Our phylogenetic analysis of these receptors revealed 29 well-supported subtrees containing mixed sets of protostomian and deuterostomian sequences. This indicated that many vertebrate and arthropod PSs that were previously thought to be phyla specific are in fact of bilaterian origin. By screening sequence databases for potential peptides, we then reconstructed entire bilaterian peptide families and showed that protostomian and deuterostomian peptides that are ligands of orthologous receptors displayed some similarity at the level of their primary sequence, suggesting an ancient coevolution between peptide and receptor genes. In addition to shedding light on the function of human G protein-coupled receptor PSs, this work presents orthology markers to study ancestral neuron types that were probably present in the last common bilaterian ancestor.

Systems approaches to genomic and epigenetic inter-regulation of peptide hormones in stress and reproduction

The evolution of the organismal stress response and fertility are two of the most important aspects that drive the fitness of a species. However, the integrated regulation of the hypothalamic pituitary adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes has been traditionally thwarted by the complexity of these systems. Pepidergic signalling systems have emerged as critical integrating systems for stress and reproduction. Current high throughput systems approaches are now providing a detailed understanding of peptide signalling in stress and reproduction. These approaches were dependent upon a long history of discovery aimed at the structural characterization of the associated molecular components. The combination of comparative genomics, microarray and epigenetic studies has led not only to a much greater understanding of the integration of stress and reproduction but also to the discovery of novel physiological systems. Recent epigenomic approaches have similarly yielded a new level of complexity in the interaction of these physiological systems. Together, such studies have provided a greater understanding of the effects of stress and reproduction.

The postbranchial digestive tract of the ascidian, Polyandrocarpa misakiensis (Tunicata: Ascidiacea). 2. Stomach

The organization of the stomach in the compound styelid ascidian, Polyandrocarpa misakiensis, is described, and the morphology and cell types of the stomach is discussed from the phylogenetic viewpoint. The stomach is a sac-like organ whose wall is formed into longitudinal folds. The stomach consists of external and internal epithelium. The internal epithelium is simple columnar, except for the bottom of the folds. There are five cell types: absorptive cells, zymogenic cells, endocrine cells, ciliated mucous cells, and undifferentiated cells. The absorptive cells have numerous microvilli. The apical region of these cells is occupied by coated vesicles. The zymogenic cells have a conical outline and a few microvilli on their apical surfaces. There are secretory granules in the apical region of zymogenic cells. The endocrine cells have low cell height and electron-dense granules around the nucleus. Endocrine cells have one or two cilia and a few microvilli on the apical surfaces. The basolateral part of these cells often bulges into the adjoining cells. Immunoelectron microscopy revealed that some endocrine cells have serotonin-like immunoreactivity. The ciliated mucous cells are restricted to a single ventral groove. They have numerous microvilli and a few cilia on their apical surfaces. Moderately electron-dense granules are accumulated in the apical part of the ciliated mucous cells. Undifferentiated cells, filled with free ribosomes, form a pseudostratified epithelium in the base of each fold. The nucleus of undifferentiated cells has a prominent nucleolus. The pseudostratified epithelium of the pyloric caecum consists of electron-dense and electron-light cells.

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.

Nanobiology and physiology of growth hormone secretion

Growth hormone (GH) secretion is controlled by hypothalamic releasing hormones from the median eminence together with hormones and neuropeptides produced by peripheral organs. Secretion of GH involves movement of secretory vesicles along microtubules, transient ‘docking’ with the porosome in the cell membrane and subsequent release of GH. Release of GH is stimulated by GH releasing hormone (GHRH) and inhibited by somatostatin (SRIF). Ghrelin may be functioning to stimulate GH release from somatotropes acting via the GH secretagogue (GHS) receptor (GHSR). However, recent physiological studies militate against this. In addition, ghrelin does influence GH release acting within the hypothalamus. Release of GH from the somatotropes involves the GH-containing secretory granules moving close to the cell surface followed by transitory fusion of the secretory granules with the porosomes located in multiple secretory pits in the cell membrane. Other peptides/proteins can influence GH secretion, particularly in species of non-mammalian vertebrates.

The evolution of pro-opiomelanocortin: looking for the invertebrate fingerprints

The presence and role of the pro-opiomelanocortin (POMC) gene and encoded peptides in invertebrates are here summarized and discussed. Some of the POMC-derived peptides show a significant similarity regarding their functions, suggesting their appearance before the split of protostomian-deuterostomian lineages and their maintenance during evolution. The basic mechanisms that govern the exchange of information between cells are usually well conserved, and this could have also been for POMC-derived peptides, that are mainly involved in fundamental functions such as immune and neuroendocrine responses. However, the presence and functions that POMC-derived peptides exhibit in taxonomically distant models, are not always reflected by the expected gene homology, leaving the problem of POMC evolution in invertebrates in need of additional study.

The postbranchial digestive tract of the ascidian, Polyandrocarpa misakiensis (Tunicata: Ascidiacea). 1. Oesophagus

The organization of the oesophagus in the budding styelid ascidian, Polyandrocarpa misakiensis, is described. The oesophagus consists of external and internal epithelium, and there are loose connective tissue, blood sinuses, and a muscular layer between them. The internal epithelium is simple columnar, except for the bottom of three folds. The external epithelium is simple squamous. The internal epithelium contains four cell types, i.e., ciliated mucous cells, band cells, endocrine cells, and undifferentiated cells. The ciliated mucous cells have apical cilia and microvilli, and two types of mucous vesicle. The band cells also have apical cilia and electron-dense granules in the apical cytoplasm. The endocrine cells are bottle-shaped, and have electron-dense granules both above and below the nucleus. The undifferentiated cells form pseudostratified epithelium at the bottom of each fold, and they have nuclei with prominent nucleoli. One type of coelomic cell, which has retractile cytoplasm, often migrates in the internal epithelium. Near the stomach, there are many darkly stained round cells clustered around the posterior end of the oesophagus. These two types of coelomic cells may be involved in the defense mechanism against the invasion of foreign organisms. The basic organization of the oesophagus of P. misakiensis is similar to those of other ascidians. However, the presence of three folds is a characteristic of a solitary species, rather than of a colonial species. Although ascidians are chordate invertebrates, the organization of their oesophagus is not very complex, which might reflect their life style.

Generation of prolactin-like neurons in the dorsal strand of ascidians

The adult ascidian neural complex forms from a thin tube called the neurohypophyseal duct and from the primordium of the cerebral ganglion from the sensory vesicle in metamorphosing larvae. Neurohypophyseal duct cells, located in the anterior left side of the sensory vesicle of swimming larvae, are derived from the anterior embryonic neural plate, which expresses common transcription factors in vertebrates and urochordates. The cerebral ganglion primordium is probably derived from the posterior sensory vesicle during metamorphosis. After metamorphosis begins, the duct elongates anteriorly and fuses with the stomodeal ectoderm, where the dorsal tubercle, a large ciliated structure that opens into the upper part of the pharynx, later develops. The rudiment of the cerebral ganglion and the duct elongate posteriorly. The duct also differentiates into the neural gland. The dorsal wall of the neural gland in adult ascidians has a thick epithelium (placode), the central part of which forms the dorsal strand by repeated invaginations along the visceral nerve. Both gonadotropin-releasing hormone (GnRH) neurons and prolactin-like (non-GnRH) neurons are generated in the dorsal strand and migrate to the cerebral ganglion along the visceral nerve throughout adulthood. Thus, the epithelium derived from the neurohypophyseal duct possesses neurogenic potential to generate neural stem cells of the central (cerebral ganglion) and peripheral (dorsal strand) nervous systems. The generation of prolactin-like neurons and their migration into the brain with GnRH neurons suggest that the ascidian dorsal strand is homologous to the craniate olfactory placode, and provide unequivocal support for the existence of the clade Olfactores.

The serendipitous origin of chordate secretin peptide family members

Background

The secretin family is a pleotropic group of brain-gut peptides with affinity for class 2 G-protein coupled receptors (secretin family GPCRs) proposed to have emerged early in the metazoan radiation via gene or genome duplications. In human, 10 members exist and sequence and functional homologues and ligand-receptor pairs have been characterised in representatives of most vertebrate classes. Secretin-like family GPCR homologues have also been isolated in non-vertebrate genomes however their corresponding ligands have not been convincingly identified and their evolution remains enigmatic.

Results

In silico sequence comparisons failed to retrieve a non-vertebrate (porifera, cnidaria, protostome and early deuterostome) secretin family homologue. In contrast, secretin family members were identified in lamprey, several teleosts and tetrapods and comparative studies revealed that sequence and structure is in general maintained. Sequence comparisons and phylogenetic analysis revealed that PACAP, VIP and GCG are the most highly conserved members and two major peptide subfamilies exist; i) PACAP-like which includes PACAP, PRP, VIP, PH, GHRH, SCT and ii) GCG-like which includes GCG, GLP1, GLP2 and GIP. Conserved regions flanking secretin family members were established by comparative analysis of the Takifugu, Xenopus, chicken and human genomes and gene homologues were identified in nematode, Drosophila and Ciona genomes but no gene linkage occurred. However, in Drosophila and nematode genes which flank vertebrate secretin family members were identified in the same chromosome.

Conclusions

Receptors of the secretin-like family GPCRs are present in protostomes but no sequence homologues of the vertebrate cognate ligands have been identified. It has not been possible to determine when the ligands evolved but it seems likely that it was after the protostome-deuterostome divergence from an exon that was part of an existing gene or gene fragment by rounds of gene/genome duplication. The duplicate exon under different evolutionary pressures originated the chordate PACAP-like and GCG-like subfamily groups. This event occurred after the emergence of the metazoan secretin GPCRs and led to the establishment of novel peptide-receptor interactions that contributed to the generation of novel physiological functions in the chordate lineage.

Characterization of a corticotropin-releasing factor (CRF)/diuretic hormone-like peptide from tunicates: insight into the origins of the vertebrate CRF family

The corticotropin-releasing factor (CRF) superfamily of peptides includes the four paralogous vertebrate peptide lineages of CRF, urotensin-1/urocortin/sauvagine, urocortin 2 and urocortin 3, as well as the diuretic hormones (DH) and peptides of the arthropods. However, there are considerable sequence differences between the group of vertebrate peptides and those of the arthropods, notably insects. Because of the likely incidence of the formation of paralogous forms within the insects and the great variation in primary structures among these peptides, establishing the structure of the ancestral vertebrate version has not been possible. We screened the genomes of the tunicates, Ciona intestinalis and Ciona savignii, in silico, using the various conserved motifs of both the vertebrate CRF paralogues and the insect diuretic hormone sequences to identify the structure of the Ciona CRF/DH-like peptide genes. A single peptide gene was found in both genomes that possessed motifs reflective of both groups of peptides. These structures suggest a single CRF-like peptide was inherited by vertebrates and possibly chordates. Moreover, the conserved structure of the CRF peptide may have become constrained once it became associated with the regulation of the hypothalamus-pituitary-adrenal/interrenal axis.

Structural evolution of urotensin-I: reflections of life before corticotropin releasing factor

Peptides have a long evolutionary history that predates the appearance of metazoans. The corticotropin releasing factor (CRF) family of peptides is among the most ancient peptide lineages. The identification and characterization of urotensin-I and related orthologues led the way for the elucidation of the entire CRF peptide family. A comparative analysis of the CRF paralogue sequences suggest that CRF is the most derived of these peptides and has lost many of its ancestral characteristics after it became associated with the hypothalamic-pituitary-adrenal/interrenal (HPA/I axis). In vertebrates, the urotensin-I group of orthologues, which includes sauvagine and urocortin, possess a number of shared characteristics that may be indicative of the ancestral peptide. Given the early origin of the CRF family peptides, it is likely that other peptide lineages are distantly related to the CRF family. In silico or cDNA library screening using probes based on urotensin-I/urocortin characteristics have been used to identify novel CRF family and related sequences that provide clues the evolutionary origin of the CRF family.

Sensory cells associated with the tentacular tunic of the ascidian Polyandrocarpa misakiensis (Tunicata: Ascidiacea)

New probable mechanosensory cell groups were found in Polyandrocarpa misakiensis. In this species, the tunic with epithelium penetrates into the oral and atrial tentacles (oral and atrial tentacular tunic), which is continuous with a tunic layer intervening between the descending and ascending epithelium of the siphons. In the oral tentacles, the tunic only extends into the basal part, but in the atrial siphon the tunic extends the full length of the tentacle. Intraepithelial sensory cells were found in the basal part of the oral and atrial tentacular tunic. These sensory cells are usually solitary or paired in the atrial tentacles, and a few cells are grouped together in the oral tentacles. In the oral tentacles, the apicolateral parts of the sensory cells are joined together by two types of junctions, i.e., adherent junctions and modified tight junctions. The supporting cells and sensory cells are connected by adherent junctions. Certain sensory cells are coupled to what seem to be neurosecretory cells. The sensory cells have one apical cilium surrounded by microvilli and a basal axonal process. The apical cytoplasm of the sensory cells has a few organelles. The basal cytoplasm of the sensory cells is dense with rough endoplasmic reticulum, mitochondria, lipid droplets, and dense bodies. The morphology of these sensory cells is comparable with that of the ciliated intraepithelial sensory neurons found in many tunicates, such as those of cupular organ and capusular organ.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

The nervous system in adult tunicates: current research directions

This review covers 25 years of progress on structural, functional, and developmental neurobiology of adult tunicates. The focus is on ascidians rather than pelagic species. The ascidian brain and peripheral nervous system are considered from the point of view of ultrastructure, neurotransmitters, regulatory peptides, and electrical activity. Sensory reception and effector control are stressed. Discussion of the dorsal strand plexus centres on its relationship with photoreceptors, the presence in it of gonadotropin-releasing hormone and its role in reproductive control. In addition to hydrodynamic sense organs based on primary sensory neurons (cupular organs), ascidians are now known to have coronal sense organs based on axonless hair cells resembling those of the vertebrate acustico-lateralis system. The peripheral nervous system is remarkable in that the motor neuron terminals are apparently interconnected synaptically, providing the equivalent of a nerve net. Development of the neural complex in ascidians is reviewed, highlighting recent embryological and molecular evidence for stomodeal, neurohypophyseal, and atrial placodes. The nervous system forms similarly during embryogenesis in the oozooid and blastogenesis in colonial forms. The regeneration of the brain in Ciona intestinalis (L., 1767) is discussed in relation to normal neurogenesis. Finally, the viviparous development of salps is considered, where recent work traces the early development of the brain, outgrowth of nerve roots, and the targetting of motor nerves to the appropriate muscles.

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.

Tachykinin and Tachykinin Receptor of an Ascidian, Ciona intestinalis

Tachykinins (TKs) are the most prevalent vertebrate brain/gut peptides. In this study, we originally identified authentic TKs and their receptor from a protochordate, Ciona intestinalis. The Ciona TK (Ci-TK) precursor, like mammalian gamma-preprotachykinin A (gamma-PPTA), encodes two TKs, Ci-TK-I and -II, including the -FXGLM-NH(2) vertebrate TK consensus. Mass spectrometry of the neural extract revealed the production of both Ci-TKs. Ci-TK-I contains several Substance P (SP)-typical amino acids, whereas a Thr is exceptionally located at position 4 from the C terminus of Ci-TK-II. The Ci-TK gene encodes both Ci-TKs in the same exon, indicating no alternative generation of Ci-TKs, unlike the PPTA gene. These results suggested that the alternative splicing of the PPTA gene was established during evolution of vertebrates. The only Ci-TK receptor, Ci-TK-R, was equivalently activated by Ci-TK-I, SP, and neurokinin A at physiological concentrations, whereas Ci-TK-II showed 100-fold less potent activity, indicating that the ligand selectivity of Ci-TK-R is distinct from those of vertebrate TK receptors. Ci-TK-I, like SP, also elicited the typical contraction on the guinea pig ileum. The Ci-TK gene was expressed in neurons of the brain ganglion, small cells in the intestine, and the zone 7 in the endostyle, which corresponds to the vertebrate thyroid gland. Furthermore, the Ci-TK-R mRNA was distributed in these three tissues plus the gonad. These results showed that Ci-TKs play major roles in sexual behavior and feeding in protochordates as brain/gut peptides and endocrine/paracrine molecules. Taken together, our data revealed the biochemical and structural origins of vertebrate TKs and their receptors.

Occurrence of prohormone convertase-like substances in the neural complex cells of the ascidian Halocynthia roretzi

Our previous study on the distribution of adrenocorticotropin (ACTH)-like substances in the neural complex (cerebral ganglion, dorsal strand, and neural gland) of an ascidian Halocynthia roretzi revealed that some of the cells in the cerebral ganglion and the cells scattered along the dorsal strand were immunopositive with antiserum against ACTH. In order to ascertain whether these cells are equipped with prohormone convertases, we performed immunohistochemical studies on the neural complex by using antisera against PC1 and PC2. A considerable number of cells around the dorsal strand and a few cells in the neural ganglion were immunopositive with PC1 and/or PC2 antibodies. Immunoelectron microscopic study demonstrated that some granulated cells situated in the cerebral ganglion and along the dorsal strand contained PC1- or PC2-like substances within their secretory granules. Western blot analysis revealed the presence of 66-kDa PC1-like and 70-kDa PC2-like substances in the neural complex. Moreover, immunostaining of consecutive sections showed that the majority of the cells containing PC1- and/or PC2-like substances corresponded to the cells immunoreactive with antisera against ACTH and CLIP but not to those immunoreactive with an antiserum against PRL. Cells belonging to the neural gland neither contained electron-dense granules nor showed immunoreactivity with any antisera employed in this experiment. The possibility that some of the cells situated in the cerebral ganglion and along the dorsal strand are progenitors of vertebrate adenohypophyseal cells is discussed.

Hypophyseal development in vertebrates from amphibians to mammals

Classically, it has been widely accepted that epithelial hypophysis develops from the stomodeal ectoderm. This notion was based on purely morphological observation of normal specimens. Recently, evidence is being accumulated that indicates another lineage for the developmental origin of adenohypophysis. This minireview focuses on several series of experimental works that have demonstrated a neuroectodermal origin of the hypophysis in vertebrates from amphibians to mammals. Phylogenetic consideration of the pituitary-like structures of prechordal animals is also presented.

Serotonin localization in Phallusia mammillata larvae and effects of 5-HT antagonists during larval development

The neurotransmitter 5-hydroxytryptamine (5-HT, serotonin) plays an important role in a wide range of non-neural processes. Using immunofluorescence with an antiserotonin antibody, 5-HT was localized in the brain and in some neurons of the larval tail of Phallusia mammillata. To test the effect of 5-HT on development, we treated embryos with two different 5-HT receptor subtype antagonists. Treatment at the gastrula stage with 10 microM ondansetron, an antagonist of the 5-HT(3) receptor, induced anterior truncation and a short tail. At 10 microM, ritanserin, a 5-HT(2B) receptor antagonist, induced larval phenotypes characterized by a roundish trunk region with flat papillae. The juveniles developed from these larvae had an abnormal cardiocirculatory system: their heart contractions were ineffective and their blood cells accumulated in the heart cavity. We conclude that an appropriate level of 5-HT is necessary for correct development and morphogenesis. Moreover, a different key role for multiple receptors in modulating the morphogenetic effects of 5-HT is suggested.

Immunocytochemical localization of serotonin in embryos, larvae and adults of the lancelet, Branchiostoma floridae

Serotonin (5-hydroxytryptamine) is a biogenic amine distributed throughout the metazoans and has an old evolutionary history. It is involved as a developmental signal in the early morphogenesis of both invertebrates and vertebrates, whereas in adults it acts mainly as a neurotransmitter and gastrointestinal hormone. In vertebrates, serotonin regulates the morphogenesis of the central nervous system and the specification of serotonergic as well as dopaminergic neurons. The present study uses, as an experimental model, an invertebrate chordate, the lancelet Branchiostomafloridae, characterized by its remarkable homologies with vertebrates that allows the 'bauplan' of the probable ancestor of vertebrates to be outlined. In particular, the involvement of serotonin as a developmental signal in embryos and larvae, as well as a neurotransmitter and gastrointestinal hormone in adult specimens of Branchiostoma floridae, gives further support to a common origin of cephalocordates and vertebrates.

Mechanism of neurogenesis during the embryonic development of a tunicate

Ascidian and vertebrate nervous systems share basic characteristics, such as their origin from a neural plate, a tripartite regionalization of the brain, and the expression of similar genes during development. In ascidians, the larval chordate-like nervous system regresses during metamorphosis, and the adult's neural complex, composed of the cerebral ganglion and the associated neural gland is formed. Classically, the homology of the neural gland with the vertebrate hypophysis has long been debated. We show that in the colonial ascidian Botryllus schlosseri, the primordium of the neural complex consists of the ectodermal neurohypophysial duct, which forms from the left side of the anterior end of the embryonal neural tube. The duct contacts and fuses with the ciliated duct rudiment, a pharyngeal dorsal evagination whose cells exhibit ectodermic markers being covered by a tunic. The neurohypophysial duct then differentiates into the neural gland rudiment whereas its ventral wall begins to proliferate pioneer nerve cells which migrate and converge to make up the cerebral ganglion. The most posterior part of the neural gland differentiates into the dorsal organ, homologous to the dorsal strand. Neurogenetic mechanisms in embryogenesis and vegetative reproduction of B. schlosseri are compared, and the possible homology of the neurohypophysial duct with the olfactory/adenohypophysial/hypothalamic placodes of vertebrates is discussed. In particular, the evidence that neurohypophysial duct cells are able to delaminate and migrate as neuronal cells suggests that the common ancestor of all chordates possessed the precursor of vertebrate neural crest/placode cells.

Expression of pro-opiomelanocortin (POMC) in the cerebral ganglion and ovary of a protochordate

The distribution of neurones expressing POMC mRNA in the cerebral ganglion of the protochordate ascidian, Styela plicata, was investigated using a non-radioactive in situ hybridization technique. Nerve cell bodies of mono and bipolar types expressing POMC mRNA, were observed mainly in the outer layer of the ganglion. Discrete groups of neurones containing POMC mRNA were also localized in the inner portion of the ganglion, and few small monopolar perykaria expressing POMC mRNA were visible at the emergence of the main nerve trunks. POMC mRNA labeling was also found at level of the cytoplasm of previtellogenic and vitellogenic oocytes, and of follicular cells. Our results demonstrate the expression of one or more genes in the cerebral ganglion and ovary, that may be similar to one or more regions of the mammalian POMC gene. Therefore POMC-related molecules seem to be involved in neuromodulatory pathways and regulatory mechanisms of the oogenesis of ascidians.

This histamine-like effects of Phallusia nigra extract: evidences for direct activity at H1 receptors

The methanol extract (mol. wt lower than 3,000 Da) of the sea squirt Phallusia nigra has stimulatory activity on guinea-pig ileum preparations. This effect was inhibited by cyproheptadine and mepyramine, but not be atropine. Mepyramine antagonized competitively the extract activity with a pA2 of 10.09 +/- 1.12, suggesting a direct activity on H1 histamine receptors. The extract was also assayed on guinea-pig right atria, however, only a mild increase in spontaneous contractions was observed compared to histamine, showing that the extract was a rather poor activator of cardiac H2 receptors. Histamine was not detected upon TLC analysis of the extract by comparison with an authentic standard.

Pro-opiomelanocortin-derived peptides, cytokines, and nitric oxide in immune responses and stress: an evolutionary approach

In vertebrates, including man, the study of stress has contributed substantially to unravelling the complex relationship between immune-neuroendocrine interactions and the systems involved. On the basis of data on the presence and distribution of the main actors (POMC products, cytokines, biogenic amines, and steroid hormones) in different species and taxa from invertebrates to vertebrates, we argue that these responses have been deeply connected and interrelated since the beginning of life. Moreover, the study of nitric oxide suggests that the inflammatory reaction is located precisely between the immune and stress responses, sharing the same fundamental evolutionary roots. The major argument in favor of this hypothesis is that the immune, stress, and inflammation responses appear to be mediated by a common pool of molecules that have been conserved throughout evolution and that from a network of adaptive mechanisms. One cell type, the macrophage, appears to emerge as that most capable of supporting this network critical for survival; it was probably a major target of selective pressure. All these data fit the unitarian hypothesis we propose, by which evolution favors what has been conserved, rather than what has changed, as far as both molecules and functions are concerned.

Distribution and seasonal variation of vasoactive intestinal (VIP)-like peptides in the nervous system of Helix pomatia

The distribution of neuropeptides immunologically related to vasoactive intestinal peptide (VIP) and its precursor peptide preproVIP(111-122), as well as to other peptides of the VIP-family, was studied in the central and peripheral nervous and sensory system of the snail, Helix pomatia, by use of immunocytochemical methods. VIP and preproVIP immunoreactivity was present in somata and nerve fibres of all central ganglia. Hibernating snails contained on average a total of 670 VIP- and 763 preproVIP-immunoreactive neurons. The number of immunoreactive cells was substantially reduced by more than 50% in active snails during summer with an average of 289 VIP- and 356 preproVIP-immunoreactive neurons. Antiserum against VIP labelled nerve fibres next to blood vessels and smooth muscle cells, whereas preproVIP-like material was localized in nerve fibres and endocrine-like cells among dorsal body cells and in the connective tissue along fiber tracts. VIP-immunoreactive material was also found in accessory ganglia of small and large tentacles, ganglia of the lips, the sensory epithelium of the tentacles, free nerve endings between skin epithelial cells, neuronal cells in the retina and in the sensory epithelium of statocysts. The cell-specific distribution and the seasonal variation of VIP- and preproVIP-like peptides suggest that they may act as transmitters or modulators in the nervous and sensory system and may be involved in the physiological adaptation of central neurons during long-term resting periods of snails.

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.

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.

Phylogenetic aspects of pancreastatin- and chromogranin-like immunoreactive cells in the gastro-entero-pancreatic neuroendocrine system of vertebrates

Using a battery of region-specific antisera raised against different amino acid sequences of pancreastatin (Pst) (Pst-1-6, Pst-1-17, Pst-14-49, Pst-33-49) as well as two antisera raised against chromogranin (Cg) A and CgA/B and the biotin-avidin technique, the phylogenetic distribution of Pst-immunoreactive (-IR) and Cg-IR cells was studied in the gastroentero-pancreatic (GEP) neuroendocrine system. The investigation was carried out with representatives of all vertebrate classes as well as with the protochordates Branchiostoma lanceolatum and Ciona intestinalis. The study revealed the presence of Pst-IR and Cg-IR cells in the gastro-intestinal mucosal epithelium as well as in the islet parenchyma of all vertebrates studied with the only exception found in rat. In the rat GEP system unequivocal immunoreactions were obtained only by the use of antiserum CgA/B. In the gastro-intestinal tract of the deuterostomian invertebrates no Pst-IR or Cg-IR cells could be observed with any of our antisera. Whether this might indicate that Pst-like or Cg-like peptides are characteristic for vertebrates or, more likely, whether similar proteins/peptides might be present in the alimentary tract of protochordates which do not react with the antisera at hand, remains to be clarified. Thouh pronounced interspecies and some intraspecies differences were found, several general conclusions can be drawn. In all vertebrate species, the Pst-IR and Cg-IR cells observed in the mucosal epithelium of the gastro-intestinal tract showed an endocrine structure and were of the so-called open type. The Pst and the Cg antisera which gave immunoreactions with parenchymal cells in the islets of Langerhans also reacted with cells in the epithelium of the pancreatic ducts. Comparative analysis of the reaction properties of the region-specific antisera used indicated that the Pst-like material in the islet cells of the cartilaginous fish species studied seems to be "mammalian-like," whereas it appears to be different in the other (phylogenetically younger) submammalian vertebrates. In addition, the Pst-like peptides in the gastro-intestinal mucosal epithelium and in those in the islets seem to differ in most submammalians. Finally, in the pyloric-duodenal junction of the quail (Coturnix c. japonica) the presence of a so far unknown peptide of the Cg family is presumed. In general, our results seem to indicate that the phylogeny of Pst-like and Cg-like peptides is not as "straight" as those which have been demonstrated for several other neurohormonal peptides.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Relaxin-like peptide in ascidians. I. Identification of the peptide and its mRNA in ovary of Herdmania momus

The ovaries of the ascidian Herdmania momus were extracted for relaxin. Relaxin immunoactivity was eluted from Sephadex G-50 in the position of authentic porcine ovarian relaxin and these fractions were parallel to porcine relaxin in an homologous porcine relaxin radioimmunoassay. Poly(A)+ RNA from ascidian ovaries hybridized to a 32P-labeled porcine relaxin B-chain 48 mer oligonucleotide probe. There was no hybridization with 32P-labeled probes of equal length but was rather directed to the B-chain of human relaxin or to two different regions of the C-peptide of porcine relaxin. We conclude that the ascidian ovary produces a relaxin-like molecule, possibly with greater homology in the B-chain to mammalian relaxins than in the C-peptide region.

Phylogeny and ontogeny of gonadotropin-releasing hormone: comparison of guinea pig, rat, and a protochordate

Immunoreactive gonadotropin-releasing hormone (ir-GnRH) was detected in brain extracts of newborn and 10-day-old rats and in adult guinea pigs; it was also present in extracts of the neural ganglion and gland of a protochordate. Radioimmunoassay (RIA) using different GnRH antisera after high-performance liquid chromatography (HPLC) revealed that the dominant form of GnRH is the mammalian form (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) both during ontogenesis in the rat and in the adult guinea pig known to have variant forms of other peptide hormones. None of the other forms of GnRH identified in nonmammalian species to date appear to be present in the rat or guinea pig. A small amount of an unidentified HPLC early eluting form of GnRH is present, but detection by antiserum B-6 implies that it is also mammalian GnRH, with the possibility of changes in positions 2-4. The molecular form of GnRH in a protochordate, the sea squirt Chelyosoma productum, is distinct from salmon and mammalian GnRHs. Cross-reactivity with the sea squirt GnRH-like molecule was highest with an antiserum made against lamprey GnRH; the same antiserum was used to stain nerve fibers in the neural ganglion and some of its roots. This is the first report using RIA, HPLC, and immunocytochemistry to show that protochordates have GnRH-like material. The results suggest that GnRH may have been present at the transition between the invertebrates and vertebrates.

The serotoninergic, cholinergic and peptidergic components of the nervous system in the monogenean parasite, Diclidophora merlangi: a cytochemical study

Confocal scanning laser microscopy has been employed with immunocytochemical techniques to map the distribution of serotoninergic and peptidergic components in the nervous system of the monogenean gill-parasite, Diclidophora merlangi; results are compared with the distribution of cholinergic components, following histochemical staining for cholinesterase activity. While all three neurochemical elements are present in the central and peripheral nervous systems, the cholinergic and peptidergic systems dominate the CNS, whereas the PNS has a majority of serotoninergic nerve fibres. The cholinergic and peptidergic neuronal pathways overlap extensively in staining patterns, suggesting possible co-localization of acetylcholine and neuropeptides. Within the peptidergic nervous system, immunoreactivity to the pancreatic polypeptide family of peptides and FMRFamide were the most prevalent. Gastrin/cholecystokinin (CCK)-, neuropeptide Y-, substance P-, neurokinin A- and eledoisin-like immunoreactivities have been demonstrated for the first time in a monogenean parasite. The gastrin/CCK- and tachykinin-like immunoreactivities had an apparently restricted distribution in the worm.

Localization, quantification, and characterization of pancreatic polypeptide immunoreactivity in the parasitic flatworm Diclidophora merlangi and its fish host (Merlangius merlangus)

Pancreatic polypeptide (PP) immunoreactivity (IR) has been identified, quantified, and subsequently chemically characterised in the parasitic platyhelminth, Diclidophora merlangi, and its specific teleostean host the whiting, Merlangius merlangus. Immunocytochemistry demonstrated PP-IR throughout the central and peripheral nervous systems of the parasite and in open-type endocrine cells of the gastric mucosa of its host. Radioimmunoassay detected PP-IR in alcoholic extracts of whole parasites (39.2 ng/g) and in extracts of gastrointestinal tract (2.1 ng/g), brain (4.6 ng/g), and pancreas (12 ng/g) of the host. Chromatographic analysis of parasite extracts revealed a single immunoreactive species of PP in both high-performance gel permeation and reverse-phase systems. The molecular size of this peptide was similar to bovine PP standard. In contrast, whiting tissues contained two immunoreactive species of PP in both gel permeation and reverse-phase systems. The major species was similar in size to bovine PP standard and the minor species was smaller, with a molecular size comparable to bovine neurotensin. Reverse-phase HPLC revealed that parasite and host peptides were not identical.