Cytochemical and immunofluorescence investigations of insulin-like producing cells in the intestine of Mytilus edulis L. (Bivalvia)

Molecular evolution and functional characterisation of insulin related peptides in molluscs: Contributions of Crassostrea gigas genomic and transcriptomic-wide screening

Insulin Related Peptides (IRPs) belong to the insulin superfamily and possess a typical structure with two chains, B and A, linked by disulphide bonds. As the sequence conservation is usually low between members, IRPs are classified according to the number and position of their disulphide bonds. In molluscan species, the first IRPs identified, named Molluscan Insulin-related Peptides (MIPs), exhibit four disulphide bonds. The genomic and transcriptomic data screening in the Pacific oyster Crassostrea gigas (Mollusc, Bivalvia) allowed us to identify six IRP sequences belonging to three structural groups. Cg-MIP1 to 4 have the typical structure of MIPs with four disulphide bonds. Cg-ILP has three disulphide bonds like vertebrate Insulin-Like Peptides (ILPs). The last one, Cg-MILP7 has a significant homology with Drosophila ILP7 (DILP7) associated with two additional cysteines allowing the formation of a fourth disulphide bond. The phylogenetic analysis points out that ILPs may be the most ancestral form. Moreover, it appears that ILP7 orthologs are probably anterior to lophotrochozoa and ecdysozoa segregation. In order to investigate the diversity of physiological functions of the oyster IRPs, we combine in silico expression data, qPCR measurements and in situ hybridization. The Cg-ilp transcript, mainly detected in the digestive gland and in the gonadal area, is potentially involved in the control of digestion and gametogenesis. The expression of Cg-mip4 is mainly associated with the larval development. The Cg-mip transcript shared by the Cg-MIP1, 2 and 3, is mainly expressed in visceral ganglia but its expression was also observed in the gonads of mature males. This pattern suggested the key roles of IRPs in the control of sexual reproduction in molluscan species.

A Microarray Study of Carpet-Shell Clam (Ruditapes decussatus) Shows Common and Organ-Specific Growth-Related Gene Expression Differences in Gills and Digestive Gland

Growth rate is one of the most important traits from the point of view of individual fitness and commercial production in mollusks, but its molecular and physiological basis is poorly known. We have studied differential gene expression related to differences in growth rate in adult individuals of the commercial marine clam Ruditapes decussatus. Gene expression in the gills and the digestive gland was analyzed in 5 fast-growing and five slow-growing animals by means of an oligonucleotide microarray containing 14,003 probes. A total of 356 differentially expressed genes (DEG) were found. We tested the hypothesis that differential expression might be concentrated at the growth control gene core (GCGC), i.e., the set of genes that underlie the molecular mechanisms of genetic control of tissue and organ growth and body size, as demonstrated in model organisms. The GCGC includes the genes coding for enzymes of the insulin/insulin-like growth factor signaling pathway (IIS), enzymes of four additional signaling pathways (Raf/Ras/Mapk, Jnk, TOR, and Hippo), and transcription factors acting at the end of those pathways. Only two out of 97 GCGC genes present in the microarray showed differential expression, indicating a very little contribution of GCGC genes to growth-related differential gene expression. Forty eight DEGs were shared by both organs, with gene ontology (GO) annotations corresponding to transcription regulation, RNA splicing, sugar metabolism, protein catabolism, immunity, defense against pathogens, and fatty acid biosynthesis. GO term enrichment tests indicated that genes related to growth regulation, development and morphogenesis, extracellular matrix proteins, and proteolysis were overrepresented in the gills. In the digestive gland overrepresented GO terms referred to gene expression control through chromatin rearrangement, RAS-related small GTPases, glucolysis, and energy metabolism. These analyses suggest a relevant role of, among others, some genes related to the IIS, such as the ParaHox gene Xlox, CCAR and the CCN family of secreted proteins, in the regulation of growth in bivalves.

Internal chemical communication within flatworms

An understanding of reproductive function is important for control of parasitic helminths. In cestodes and trematodes virtually nothing is known about regulatory and coordinating mechanisms that control maturation, gamete formation, egg production, and related processes. Neurosecretory neurons have been reported in various species but specific modes of action of neurohormones have yet to be demonstrated. The role of ecdysone is being investigated.

Identification of Ras, Pten and p70S6K homologs in the Pacific oyster Crassostrea gigas and diet control of insulin pathway

Insulin pathways were demonstrated from invertebrates to vertebrates to be involved in the regulation of numerous processes including storage metabolism and reproduction. In addition, insulin system may integrate variations of environmental conditions like dietary restrictions. In the Pacific oyster Crassostrea gigas, reproductive and storage compartments are closely intricated in the gonadal area and their respective development was found to be dependant of trophic conditions. For these reasons, C. gigas is an original and interesting model for investigating the role of insulin control in the balance between storage and reproduction and the integration of environmental parameters. On the basis of sequence conservation, we identified three potential elements of the oyster insulin pathway, Ras, Pten and p70S6K and we investigated their expression levels in various tissues. In the gonadal area, we used laser microdissection in order to precise the targeted contribution of insulin signaling to the restoration of storage tissue and to the control of vitellogenesis. Food deprivation during gametogenesis reinitiation stage led to reduced proliferations of gonia and also to modulate insulin signal by transcriptional activation of insulin pathway elements.

Atrial granular cells of the snailAchatina fulicarelease proteins into hemolymph after stimulation of the heart nerve

The atrium of the gastropod mollusc Achatina fulica receives rich innervation and contains numerous granular cells (GCs). We studied the atrial innervation and discovered that axon profiles typical in appearance of peptidergic neurons form close unspecialized membrane contacts with GCs. Then,we investigated, at both morphological and biochemical levels, the effect of electrical stimulation of the heart nerve on GCs of Achatina heart perfused in situ. The ultrastructural study demonstrated changes in granule morphology consistent with secretion. These events included alteration of granule content, intracellular granule fusion and formation of complex degranulation channels, within which the granule matrix solubilized. It was shown that electrical stimulation resulted in a significant increase of the total protein concentration in the perfusate. Furthermore, SDS-PAGE analysis of the perfusate revealed three new proteins with molecular masses of 16, 22,and 57 kDa. Affinity-purified polyclonal antibodies against the 16 kDa protein were obtained; the whole-mount immunofluorescence technique revealed the presence of this protein in the granules of atrial GCs. In GCs of the stimulated atrium, a progressive loss of their granular content was observed. The results suggest that the central nervous system can modulate the secretory activity of the atrial GCs through non-synaptic pathways.

Mining incretin hormone pathways for novel therapies

The incretin hormones, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are produced predominantly by enteroendocrine cells and have multiple blood glucose-lowering effects. Recent years have seen a surge of interest in understanding the basic physiology and pathophysiology of incretins and in applying this knowledge to the treatment of diabetes and obesity. Considerable gains have been made in elucidating the mechanisms controlling incretin secretion, and there is growing evidence to suggest that incretins might be involved in the rapid reversal of diabetes observed in gastric bypass patients. Here, we review these recent advances and outline the multiple strategies being pursued to exploit the potential therapeutic benefits of GIP and GLP-1.

Insulin-like system and growth regulation in the Pacific oyster Crassostrea gigas: hrIGF-1 effect on protein synthesis of mantle edge cells and expression of an homologous insulin receptor-related receptor

The involvement of molecules belonging to the insulin/IGF family in regulation of growth has been investigated in the Pacific oyster Crassostrea gigas. In vitro biological effects of human recombinant IGF-1 (hrIGF-1) on mantle edge cells, involved in oyster shell and soft body growth, were studied over an annual cycle. In mantle edge cells hrIGF-1 stimulates protein synthesis of 56+/-5.1% over basal for 10(-10) M in September with in addition a clear dose-effect corresponding to the highest shell growth period, and 57.5+/-3.45% over basal for 10(-11) M in March and 51+/-5.4% over basal for 10(-10) M in April corresponding to the period of mantle growth. These insulin-like effects were associated with the expression of a recently identified C. gigas insulin receptor-related receptor (CIR) in mantle edge cells as demonstrated by RT-PCR. Moreover, in situ hybridisation (ISH) confirmed this expression at the level of the inner and outer epithelia involved in mantle growth and shell formation.

Localization and immunological characterization of insulin-like peptide(s) in the land snail Otala lactea (Mollusca: Pulmonata)

Insulin-like peptides were detected by means of immunological techniques in tissues of the land snail Otala lactea. Insulin-positive cells were detected in all the ganglia except the right parietal ganglion and visceral ganglion. In the digestive tract, insulin-positive cells were found in the muscle and connective tissue layer of the intestine. The amount of insulin-like peptide detected in acid-ethanol extract of brains and digestive tracts from active snails did not differ significantly from that in the corresponding tissues from estivating (dormant) ones. More insulin-like peptide was detected in hemolymph from active snails than in hemolymph from estivating ones. Brains from active snails released insulin-like peptide in vitro. Analysis of the cerebral ganglia or digestive tract extracts by size-exclusion chromatography and insulin RIA revealed more than one fraction with insulin immunoreactivity. Some of these fractions contained material with molecular masses close to those of mammalian insulin or its subunits. Further analysis of the extracts by reverse-phase chromatography also revealed more than one fraction with immunoreactivity. The immunoreactive material from the digestive tract was found to be less hydrophobic than insulin. Western blot analysis of the cerebral ganglia extract revealed more than one band with insulin immunoreactivity. Three of these bands had molecular masses very similar to those of vertebrate insulin, its subunits, and its precursor.

Identification of insulin-like peptides in cerebral ganglia neurosecretory cells of the mussel Mytilus edulis

The immunostaining patterns of cerebral ganglia sections from the mussel Mytilus edulis with monoclonal antibodies raised against cerebral ganglia (CG) extracts were compared to those obtained with various polyclonal anti-insulin-like antibodies. One of the monoclonal antibodies (MAB 46) revealed clusters of positive cells in localization comparable to those revealed by the polyclonal antibodies. The nature of the antigen recognized by MAB 46 and the polyclonal antibodies was compared by gel filtration-HPLC of a cerebral ganglia extract. Similar peaks were revealed by the monoclonal and polyclonal antibodies. MAB 46 significantly inhibited the cerebral ganglia induced stimulation of amino-acid incorporation by mantle edge cell suspensions, suggesting that the antigen recognized by MAB 46 is involved in the control of growth.

Insulin-like peptide(s) in the central nervous system of the snail Helisoma duryi

The central nervous system of the snail Helisoma duryi contains porcine insulin-immunoreactive cells. Most of these cells are known as neurosecretory mediodorsal cells (MDC), and are involved in regulation of growth. There are about 25-40 large and small MDC in each cerebral ganglion near the commissure. Besides the MDC, 6-8 insulin-immunoreactive cells are also seen in the left parietal ganglion and the visceral ganglion. Insulin-immunoreactive material can be traced from the MDC perikarya into their axons in the median lip nerve. The elementary granules of the MDC are 150-250 nm in diameter, and are insulin-immunoreactive. Insulin-like material is released by the CNS in vitro, and can be measured by radioimmunoassay using an antiserum against human insulin. High potassium and 4-aminopyridine increase such release in vitro. It is likely that insulin-like peptide is a growth hormone in H. duryi.

Immunocytochemical evidence of vertebrate bioactive peptide-like molecules in the immuno cell types of the freshwater snail Planorbarius corneus (L.) (Gastropoda, Pulmonata)

An immunocytochemical investigation was carried out on round and spreading hemocytes of Planorbarius corneus by using 20 antisera to vertebrate bioactive peptides. The immunotests showed the presence of alpha 1-antichymotrypsin-bombesin-, calcitonin-, CCK-8 (INC)-, CCK-39-, gastrin-, glucagon-, Met-enkephalin-, neurotensin-, oxytocin-, somatostatin-, substance P-, VIP-, and vasopressin-immunoreactive molecules in the spreading hemocytes. The round hemocytes were only positive to anti-bombesin, anticalcitonin, anti-CCK-8 (INC), anti-CCK-39, anti-neurotensin, anti-oxytocin, anti-substance P and anti-vasopressin antibodies. No immunostaining was observed with anti-CCK-8 (Peninsula), anti-insulin, anti-prolactin, anti-thyroglobulin and anti-thyroxin (T4) antibodies. As probably in vertebrates, these bioactive peptides may modulate immuno cell function.

A putative neuroendocrine factor that stimulates glycogen mobilization in isolated glycogen cells from the marine mussel Mytilus edulis

Glycogen synthesized by purified glycogen cells, from the labial palps of Mytilus edulis, was labeled by preincubation in culture medium containing D-[U-14C]glucose. It was stable for at least 4 hr of postincubation in the absence of 14C. Glycogen mobilization was provoked by an acid extract of the cerebral ganglia. The active factor was also found in the hemolymph. The glycogen cells exhibited a dose-dependent response to the cerebral extract. The active cerebral factor is methanol soluble and possesses hydrophobic properties. It is postulated that this factor is a neurohormone.

Development of the dorsal pancreatic primordium transplanted into the third ventricle of rats

The dorsal pancreatic primordia of 12.5-day-old rat embryos transplanted into the third ventricle of adult female rats were immunohistochemically examined 10, 20 and 40 days after transplantation. On day 10, the grafts grew into an epithelial sacculus (S) with a thick subepithelial tissue (ST). Tubular and vesicular structures with a single cuboidal epithelium were found within the wall of the S, but they underwent thereafter a regression without allowing the primordia to differentiate into the exocrine acinar tissues. In contrast with this, pancreatic hormone-containing cells existed in the ST, and were arranged like the islands of a mature animal. The tissue also has smooth muscle fibers and neurons. When the primordium was grafted along with its root connected to the duodenum, gut-like tubular structures differentiated, showing mucosa with villi and crypts, submucous mesenchymal tissue and muscle layers. The mucosa possesses epithelial cells immunoreactive for the pancreatic hormones, and the muscle layers have the myenteric plexuses. These findings seem to provide further evidence that in the rat pancreas, pancreatic-hormone-containing cells differ from the acinar cells in origin.

Identification of neurohypophysial peptides in the ovaries of several mammalian and nonmammalian species

Ovarian tissue from a variety of mammalian and nonmammalian species were extracted in acid. All extracts contained both oxytocin- and vasopressin-like immunoreactivites as determined by radioimmunoassay. Analysis by high performance liquid chromatography revealed the presence of oxytocin in all ovarian extracts examined. This was in contrast to the corresponding posterior pituitary gland which other workers have shown do not necessarily contain the oxytocin peptide. It is suggested that oxytocin may play an important role in ovarian function in species of differing phylogeny.

Gut endocrine cells in the snail Helix aspersa

A microscopic study of the endocrine cells present in the gut of the snail Helix aspersa is made. Electron microscopy is necessary in most cases to identify the enteroendocrine cells, since neither silver impregnations nor immunocytochemical staining have rendered positive results. Endocrine cells are scarce and rest on the basement membrane. They display a clear cytoplasm and variable amounts of small (143 nm) secretory granules of diverse electron-density. They are ovoid or rounded and possess apical processes which extend into the lumen of the gut. The nucleus, located in the basal region of the cell, presents characteristic cytoplasmic indentations. Intraepithelial nerve bundles in contact with endocrine cells are present.

Insulin-like material from the digestive tract of the tunicate Pyura pachydermatina (sea tulip)

Gut tissue from the tunicate Pyura pachydermatina (sea tulip) was found to contain a compound or compounds which react with anti-porcine insulin antibodies, but not anti-hag-fish insulin antibodies, and which also stimulate lipogenesis in isolated rat fat cells. The insulin-like material is present in two immunologically active forms, a species of Mr 6000 apparently similar to mammalian insulin, and a high Mr form which expresses biological activity only after further purification by reverse-phase HPLC. The bioactivity of both species is suppressed in the presence of anti-porcine insulin antibodies.

Immunohistological detection of methionine-enkephalin-like and endorphin-like material in the digestive tract and in the nervous system of the mussel: Mytilus edulis L

Using the indirect immunofluorescence technique, methionine-enkephaline-like, alpha- and beta-endorphin-like peptides were detected on whole body sections of Mytilus edulis L. Met-enkephalin-like immunoreactivity was localized in the epithelium of the digestive tract, in the hepatopancreas, and in the nervous system. The immunoreactive cell bodies were very abundant in the anterior gastric epithelium, but sparse in the terminal portion of the digestive tract. By their basal processes the immunoreactive cells were in contact with a plexus of immunoreactive cells and fibers located in the connective tissue underlying the digestive epithelium. In the principal hepatopancreatic ducts, isolated cells showing met-enkephalin-like immunoreactivity were detected between the epithelial cells and the basal lamina. A few immunoreactive cells and fibers were observed between the hepatopancreatic tubules. The three pairs of nervous ganglia contained in their cortical layer numerous met-enkephalin-like immunoreactive perikarya. Their central area possessed fluorescent immunoreactive bundles of fibers extending to the commissures, the connectives, and the nerves. Met-enkephalin-like immunoreactive fibers were detected between the smooth muscle cells. At the surface of these smooth muscle cells, immunopositive met-enkephalin-like tapered nervous endings were observed. The alpha- and beta-endorphin antisera produced a positive immunoreaction in some gastric epithelial cells, in some perikarya of the pedal ganglia, and in some nervous fibers. The endorphin-like structures were far less abundant than the met-enkephalin-like structures, but very close to them.

Species and tissue distribution of cholecystokinin/gastrin-like substances in some invertebrates

Twenty-six species of invertebrates representing eight phyla were surveyed for the presence of cholecystokinin/gastrin-like (CCK/gastrin-like) peptides by radioimmunoassay of various tissue extracts. This is the first report of the presence of CCK/gastrin-like peptides in representatives of the phylum Ectoprocta, the arthropodan classes Crustacea and Merostomata, and in the nervous systems of the gastropod mollusc Aplysia californica and the oligochaete annelid Lumbricus terrestris. It has been proposed that CCK/gastrin evolved in the invertebrates as a neural peptide and was subsequently exploited by the vertebrates as a regulatory peptide in both the nervous system and the gastrointestinal endocrine system. The present results indicate that some gastropod molluscs, a merostomatan arthropod, and an annelid have detectable CCK/gastrin in both nervous and gut tissue. However, extractable CCK/gastrin was found only in gut tissue and not in the central nervous system of a crustacean arthropod. The tissue origin of the extracted CCK/gastrin in Bugula (phylum Ectoprocta) was not determined. Final resolution of the question of the nervous versus gut endocrine cellular origin of CCK/gastrin in invertebrates awaits further investigation. CCK/gastrin-like peptides are widely distributed among the invertebrates, which thus provide a rich source of comparative material for study of these regulatory substances.

Insulin-like peptides in the lobster Homarus americanus. I. Insulin immunoreactivity

Experiments were conducted to determine if insulin-like peptides are present in the lobster Homarus americanus. Peptides were found that bind specifically to bovine insulin antibodies in a modified vertebrate radioimmunoassay. Extracts of whole hepatopancreas, gut, and hemolymph contained insulin immunoreactivity (IRI) concentrations of 67.5, 14.0, and 11.0 ng, respectively, per 700-g lobster. No insulin immunoreactivity was detected in neurosecretory cells of the eyestalk. The highest immunoreactivity was measured in the hepatopancreas, in the same fractions of eluate which contained the highest immunoreactivity when a bovine insulin standard was passed through the same chromatographic column.

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

Polypeptide-hormone producing cells were localized in the alimentary tract and cerebral ganglion of Ciona intestinalis using cytochemical, immunocytochemical and electron-microscopical methods. Antisera to the following peptides of vertebrate type were employed: bombesin, human prolactin (hPRL), bovine pancreatic polypeptide (PP), porcine secretin, motilin, vasoactive intestinal polypeptide (VIP), beta-endorphin, leu-enkephalin, met-enkephalin, neurotensin, 5-hydroxytryptamin (5-HT), cholecystokinin (CCK), human growth (GH), ACTH, corticotropin-like intermediate lobe peptide (CLIP) and gastric inhibitory peptide (GIP). Immunoreactive cells were found both in the alimentary tract epithelium and in the cerebral ganglion for bombesin, PP, substance P, somatostatin, secretin and neurotensin. Additionally, in the cerebral ganglion only, there were cells immunoreactive for beta-endorphin, VIP, motilin and human prolactin. 5-HT positive cells, however, were restricted to the alimentary tract. No immunoreactivity was obtained either in the cerebral ganglion or in the alimentary tract with antibodies to leu-enkephalin, met-enkephalin, CCK, growth hormone, ACTH, CLIP and GIP. Prolactin-immunoreactive and pancreatic polypeptide-immunoreactive cells were argyrophilic with the Grimelius' stain and were found in neighbouring positions in the cerebral ganglion. At the ultrastructural level five differently granulated cell types were distinguished in the cerebral ganglion. Granules were present in the perikarya as well as in axons. The possible functions of the peptides as neurohormones, neuroregulators and neuromodulators are discussed.

Evolutionary aspects of "brain-gut peptides": an immunohistochemical study

Phylogeny of biogenic peptides and their source cells was studied by immunohistochemistry and electron microscopy. The distribution of the peptide containing neurons and paraneurons in the brain and in the gastroenteropancreatic endocrine system was depicted, especially in the bullfrog as the representative of deuterostomia and in the cockroach and some other insects as the representatives of protostomia. Stress was given to: (1) calcitonin-immunoreactive neurons in bullfrog hypothalamus and PP-reactive neurons in the cockroach protocerebrum as instances of transmissional-hormonal partition of a neuropeptide, (2) open-type endocrine cells in the gut structurally and functionally common to the protostomia and deuterostomia, and (3) phylogeny of the prohormones with special reference to big gastrin and proglucagon (glicentin).

Evolutionary origins of vertebrate hormones: substances similar to mammalian insulins are native to unicellular eukaryotes

Tetrahymena pyriformis, Neurospora crassa, and Aspergillus fumigatus that had been grown in simple defined media were extracted with acid ethanol by a classic method for recovering insulin from pancreas. After filtration of the extracts on Sephadex G-50, distinct peaks of insulin immunoreactivity were recovered in the region typical of insulin. The gel-filtered material from the Tetrahymena had reactivity in the pork insulin radioimmunoassay about equal to its reactivity in the insulin bioassay (stimulation of lipogenesis in isolated rat adipocytes), and the gel-filtered material from neurospora had an immunoreactivity-to-bioactivity ratio of about 1:3. The material that stimulated lipogenesis could be neturalized by anti-insulin sera (i.e., 75-95% of the Tetrahymena material and 60% of the Neurospora material). Bioactive and immunoactive insulin was found in the conditioned medium equal in amount to that in the cells. The findings suggest that insulin did not arise evolutionarily in the intestinal or neural tissues of primitive vertebrates or complex invertebrates but rather has its molecular origins at least as far back as the simplest unicellular eukaryotes.

Substance P-, neurotensin- and bombesin-like immunoreactivities in the gill epithelium of Ciona intestinalis L

Substance P-, neurotensin- and bombesin-like immunoreactivities were localised in some gill epithelial cells in the pharynx of Ciona intestinalis L. No immunoreactivity was obtained with antisera to gastrin, glucagon, insulin, pancreatic polypeptide or calcitonin. Some of the epithelial cells of the gills were shown to be argyrophilic with the Grimelius technique.

Localization of somatostatin-, substance P- and calcitonin-like immunoreactivity in the neural ganglion of Ciona intestinalis L. (Ascidiaceae)

Indirect immunofluorescence studies using antisera to synthetic somatostatin, human calcitonin and substance P indicate, in the neural complex of the sea-squirt, Ciona intestinalis L., that these polypeptides are present in large perikarya situated at the periphery of the cerebral ganglion as well as in some smaller perikarya in the medulla. In the medullary and transitional zone, there are nerve fibres that cross-react positively with anti-calcitonin and anti-substance P.

Localisation of somatostatin- and gastrin-like immunoreactivity in the gastrointestinal tract of Ciona intestinalis L

Somatostatin- and gastrin-like immunoreactivity has been found by immunofluorescence in cells of the stomach and intestinal epithelia of Ciona intestinalis L. The cells containing the peptide immunoreactive to mammalian anti-gastrin can be restained with Grimelius' technique for argyrophilia.

Phylogeny of insulin. Some evolutionary aspects of insulin production with particular regard to the biosynthesis of insulin in Myxine glutinosa

Preceding phylogenetic studies on the occurrence of insulin have shown--e.g. by bioassays and immunocytochemical procedures--that insulin producing B-cells are present in all vertebrates and even in several invertebrates, both protostomian and deuterostomian. The most original B-cells are obviously endocrine cells of open type, situated in the mucosa of the alimentary tract. Moreover, the results of these studies show that insulin is not only a polypeptide hormone of considerable age but also that the insulin molecule seems to have been kept surprisingly stable during evolution. Best known of all non-mammalian insulins is that from the hagfish, Myxine glutinosa. It is probably the most original insulin of all in the vertebrate series. Both the amino-acid sequence and the three-dimensional structure of the dimer of hagfish insulin differ only little from those of pig insulin. The biosynthesis occurs via proinsulin and is also in most respects similar to mammalian insulin biosynthesis. There are, however, some differences. Although it readily crystallizes as tetragonal bipyramids, hagfish insulin does not form hexamers. In a test system, with isolated rat fat cells, its binding affinity is 23% and its potency 5% of that of pig insulin, a discrepancy indicating a "partial antagonism" on the receptors. Although the conversion of proinsulin to insulin seems to occur in the secretion granules, they contain no crystalline cores. Since a strictly tryptic-like enzyme was found to destroy hagfish insulin rapidly, the enzyme converting proinsulin to insulin must--in addition to a carboxy-peptidase-B-like activity--have a different specificity in Myxine.

The occurrence of argyrophilic and argentaffin cells in the gut of Ciona intestinalis L

Argyrophilic and argentaffin cells occur in the stomach and intestinal epithelium of the sea-squirt, Ciona intestinalis L. These cells are characterized by their basal swelling which contains the nucleus surrounded by small secretory granules and by a filamentous cell-apex which reaches the gut lumen. The cells are scattered unevenly within the epithelium. Their number decreases rapidly towards the lower part of the intestine. The localization, size of granules and their shape are features which differentiate these cells from other secretory cells in the gut epithelium such as mucous cells. These cells are thought to possess an endocrine function.