Somatostatin-related and glucagon-related peptides with unusual structural features from the European eel (Anguilla anguilla)

Recent advances in neuropeptide-related omics and gene editing: Spotlight on NPY and somatostatin and their roles in growth and food intake of fish

Feeding and growth are two closely related and important physiological processes in living organisms. Studies in mammals have provided us with a series of characterizations of neuropeptides and their receptors as well as their roles in appetite control and growth. The central nervous system, especially the hypothalamus, plays an important role in the regulation of appetite. Based on their role in the regulation of feeding, neuropeptides can be classified as orexigenic peptide and anorexigenic peptide. To date, the regulation mechanism of neuropeptide on feeding and growth has been explored mainly from mammalian models, however, as a lower and diverse vertebrate, little is known in fish regarding the knowledge of regulatory roles of neuropeptides and their receptors. In recent years, the development of omics and gene editing technology has accelerated the speed and depth of research on neuropeptides and their receptors. These powerful techniques and tools allow a more precise and comprehensive perspective to explore the functional mechanisms of neuropeptides. This paper reviews the recent advance of omics and gene editing technologies in neuropeptides and receptors and their progresses in the regulation of feeding and growth of fish. The purpose of this review is to contribute to a comparative understanding of the functional mechanisms of neuropeptides in non-mammalians, especially fish.

Evolution of the glucagon-like system across fish

In fishes, including the jawless lampreys, the most ancient lineage of extant vertebrates, plasma glucose levels are highly variable and regulation is more relaxed than in mammals. The regulation of glucose and lipid in fishes in common with mammals involves members of the glucagon (GCG)-like family of gastrointestinal peptides. In mammals, four peptides GCG, glucagon-like peptide 1 and 2 (GLP1 and GLP2) and glucose-dependent insulinotropic peptide (GIP) that activate four specific receptors exist. However, in lamprey and other fishes the glucagon-like family evolved differently and they retained additional gene family members (glucagon-related peptide, gcrp and its receptor, gcrpr) that are absent from mammals. In the present study, we analysed the evolution of the glucagon-like system in fish and characterized gene expression of the family members in the European sea bass (Dicentrarchus labrax) a teleost fish. Phylogenetic analysis revealed that multiple receptors and peptides of the glucagon-like family emerged early during the vertebrate radiation and evolved via lineage specific events. Synteny analysis suggested that family member gene loss is likely to be the result of a single gene deletion event. Lamprey was the only fish where a putative glp1r persisted and the presence of the receptor gene in the genomes of the elephant shark and coelacanth remains unresolved. In the coelacanth and elephant shark, unique proglucagon genes were acquired which in the former only encoded Gcg and Glp2 and in the latter, shared a similar structure to the teleost proglucagon gene but possessed an extra exon coding for Glp-like peptide that was most similar to Glp2. The variable tissue distribution of the gene transcripts encoding the ligands and receptors of the glucagon-like system in an advanced teleost, the European sea bass, suggested that, as occurs in mammals, they have acquired distinct functions. Statistically significant (p < .05) down-regulation of teleost proglucagon a in sea bass with modified plasma glucose levels confirmed the link between these peptides and metabolism. The tissue distribution of members of the glucagon-like system in sea bass and human suggests that evolution of the brain-gut-peptide regulatory loop diverged between teleosts and mammals despite the overall conservation and similarity of glucagon-like family members.

Molecular characterization and expression of three preprosomatostatin genes and their association with growth in common carp (Cyprinus carpio)

Somatostatins (SSs) are a structurally diverse family of peptides that play important roles in the regulation of growth, development and metabolism in vertebrates. In this study, three preprosomatostatin genes (PSSs) in the common carp, Cyprinus carpio (Cc) were identified and characterized. Based on cloned sequences and genome BLAST, six isoforms of the PSS gene in C. carpio (CcPSS) were identified and included CcPSS1a and CcPSS1b, CcPSS2a and CcPSS2b, and finally, CcPSS3a and CcPSS3b. The open reading frames (ORF) of CcPSS1a, CcPSS2a and CcPSS3a consist of 345, 336 and 363 nucleotides. During embryonic development, the expressions of CcPSS2 and CcPSS3 were first observed at the stage of optic vesicle, and CcPSS1 mRNA was initially detected at the stage of muscular effect. The highest mRNA levels of CcPSS1, CcPSS2 and CcPSS3 were observed at 1-day post-hatch (dph), 2-dph and the stage of heart beating, respectively. In the adult brain, the distributions of three CcPSS mRNAs were differential but overlapping in the hypothalamus, telencephalon and medulla oblongata. For peripheral tissues, all three CcPSS mRNAs were detected in the mid-intestine, and CcPSS1 and CcPSS3 mRNAs were also expressed in the liver. Owing to the importance of somatostatins on regulating growth, functional mutations of CcPSSs were identified in a C. carpio population. A total of 23 polymorphic sites were detected in CcPSS1a and CcPSS3a. Of them, two SNPs (CcPSS1a-g.922C>T, and CcPSS3a-g.1125C>A) were significantly associated with growth traits, indicating their potential applications in gene (marker)-assisted selective breeding in C. carpio.

Molecular evolution of GPCRs: Somatostatin/urotensin II receptors

Somatostatin (SS) and urotensin II (UII) are members of two families of structurally related neuropeptides present in all vertebrates. They exert a large array of biological activities that are mediated by two families of G-protein-coupled receptors called SSTR and UTS2R respectively. It is proposed that the two families of peptides as well as those of their receptors probably derive from a single ancestral ligand-receptor pair. This pair had already been duplicated before the emergence of vertebrates to generate one SS peptide with two receptors and one UII peptide with one receptor. Thereafter, each family expanded in the three whole-genome duplications (1R, 2R, and 3R) that occurred during the evolution of vertebrates, whereupon some local duplications and gene losses occurred. Following the 2R event, the vertebrate ancestor is deduced to have possessed three SS (SS1, SS2, and SS5) and six SSTR (SSTR1-6) genes, on the one hand, and four UII (UII, URP, URP1, and URP2) and five UTS2R (UTS2R1-5) genes, on the other hand. In the teleost lineage, all these have been preserved with the exception of SSTR4. Moreover, several additional genes have been gained through the 3R event, such as SS4 and a second copy of the UII, SSTR2, SSTR3, and SSTR5 genes, and through local duplications, such as SS3. In mammals, all the genes of the SSTR family have been preserved, with the exception of SSTR6. In contrast, for the other families, extensive gene losses occurred, as only the SS1, SS2, UII, and URP genes and one UTS2R gene are still present.

Insights into the evolution of proglucagon-derived peptides and receptors in fish and amphibians

Glucagon and the glucagon-like peptides (GLP-1 and GLP-2) share a common evolutionary origin and are triplication products of an ancestral glucagon exon. In mammals, a standard scenario is found where only a single proglucagon-derived peptide set exists. However, fish and amphibians have either multiple proglucagon genes or exons that are likely resultant of duplication events. Through phylogenetic analysis and examination of their respective functions, the proglucagon ligand-receptor pairs are believed to have evolved independently before acquiring specificity for one another. This review will provide a comprehensive overview of current knowledge of proglucagon-derived peptides and receptors, with particular focus on fish and amphibian species.

New insight into the molecular evolution of the somatostatin family

The present review describes the molecular evolution of the somatostatin family and its relationships with that of the urotensin II family. Most of the somatostatin sequences collected from different vertebrate species can be grouped as the products of at least four loci. The somatostatin 1 (SS1) gene is present in all vertebrate classes from agnathans to mammals. The SS1 gene has given rise to the somatostatin 2 (SS2) gene by a segment/chromosome duplication that is probably the result of a tetraploidization event according to the 2R hypothesis. The somatostatin-related peptide cortistatin, first identified in rodents and human, is the counterpart of SS2 in placental mammals. In fish, the existence of two additional somatostatin genes has been reported. The first gene, which encodes a peptide usually named somatostatin II (SSII), exists in almost all teleost species investigated so far and is thought to have arisen through local duplication of the SS1 gene. The second gene, which has been characterized in only a few teleost species, encodes a peptide also named SSII that exhibits a totally atypical structure. The origin of this gene is currently unknown. Nevertheless, because the two latter genes are clearly paralogous genes, we propose to rename them SS3 and SS4, respectively, in order to clarify the current confusing nomenclature. The urotensin II family consists of two genes, namely the urotensin II (UII) gene and the UII-related peptide (URP) gene. Both UII and URP exhibit limited structural identity to somatostatin so that UII was originally described as a "somatostatin-like peptide". Recent comparative genomics studies have revealed that the SS1 and URP genes, on the one hand, and the SS2 and UII genes, on the other hand, are closely linked on the same chromosomes, thus confirming that the SS1/SS2 and the UII/URP genes belong to the same superfamily. According to these data, it appears that an ancestral somatostatin/urotensin II gene gave rise by local duplication to a somatostatin ancestor and a urotensin II ancestor, whereupon this pair was duplicated (presumably by a segment/chromosome duplication) to give rise to the SS1-UII pair and the SS2-URP pair.

Regulation of somatostatins and their receptors in fish

The multifunctional nature of the somatostatin (SS) family of peptides results from a multifaceted signaling system consisting of many forms of SS peptides that bind to a variety of receptor (SSTR) subtypes. Research in fish has contributed important information about the components, function, evolution, and regulation of this system. Somatostatins or mRNAs encoding SSs have been isolated from over 20 species of fish. Peptides and deduced peptides differ in their amino acid chain length and/or composition, and most species of fish possess more than one form of SS. The structural heterogeneity of SSs results from differential processing of the hormone precursor, preprosomatostatin (PPSS), and from the existence of multiple genes that give rise to multiple PPSSs. The PPSS genes appear to have arisen through a series of gene duplication events over the course of vertebrate evolution. The numerous PPSSs of fish are differentially expressed, both in terms of the distribution among tissues and in terms of the relative abundance within a tissue. Accumulated evidence suggests that nutritional state, season/stage of sexual maturation, and many hormones [insulin (INS), glucagon, growth hormone (GH), insulin-like growth factor-I (IGF-I), and 17beta-estradiol (E2)] regulate the synthesis and release of particular SSs. Fish and mammals possess multiple SSTRs; four different SSTRs have been described in fish and several of these occur as isoforms. SSTRs are also wide spread and are differentially expressed, both in terms of distribution of tissues as well as in terms of relative abundance within tissues. The pattern of distribution of SSTRs may underlie tissue-specific responses of SSs. The synthesis of SSTR mRNA and SS-binding capacity are regulated by nutritional state and numerous hormones (INS, GH, IGF-I, and E2). Accumulated evidence suggests the possibility of both tissue- and subtype-specific mechanisms of regulation. In many instances, there appears to be coordinate regulation of PPSS and of SSTR; such regulation may prove important for many processes, including nutrient homeostasis and growth control.

Glucagon-like peptide isolated from the eel intestine: effects on atrial beating

A new glucagon-like peptide was isolated from the intestine of the eel Anguilla japonica. The primary structure was determined by sequence analysis after cleavage with lysyl endopeptidase, quantitative amino acid analysis and fast atom bombardment mass spectrometry as HSQGTFTNDY10SKYLETRRAQ20DFVQWLMNSK30RSGGPT. Since its structure is similar to that of oxyntomodulins (OXMs) reported in various vertebrates, we named this peptide eel oxyntomodulin (eOXM). We found that eOXM enhanced the contractile force and the beating rate of the eel atrium in a dose-dependent manner. These effects of eOXM were not inhibited by betaxolol, a β1-adrenoceptor antagonist, indicating that the actions of eOXM were independent of those of adrenaline. eOXM enhanced the intracellular Ca2+ concentration of the myocardium. The contractility of the eel atrium was greatly reduced after omitting Ca2+ from the bathing medium or after treatment with verapamil, a Ca2+ channel blocker. After inhibiting Ca2+ entry under these conditions, the inotropic effect of eOXM was markedly reduced, but the chronotropic effect was not altered significantly. These results indicate that the inotropic effect of eOXM is via a stimulation of Ca2+ influx but that the chronotropic effect may be independent of extracellular Ca2+.

Somatostatins and their receptors in fish

Somatostatin (SRIF) is a multigene family of peptides. SRIF-14 is conserved with identical primary structure in species across the vertebrates. The presence of multiple SRIF genes has been demonstrated in a number of fish species. Notably, three distinct SRIF genes have been identified in goldfish. One of these genes, which encodes [Pro(2)]SRIF-14, has also been identified in sturgeon and African lungfish, and is closely associated with the amphibian [Pro(2),Met(13)]SRIF-14 gene and mammalian cortistatin gene. The main neuroendocrine role of SRIF-14 peptide that has been determined in fish is the inhibition of pituitary growth hormone secretion. The functions of SRIF-14 variant or larger forms of SRIF peptide and the regulation of SRIF gene expression remain to be explored. Type one and two SRIF receptors have been identified from goldfish and type three SRIF receptor from an electric fish. Fish SRIF receptors display considerable homology to mammalian counterparts in terms of primary structure and negative coupling to adenylate cyclase. The identification of the multiple gene family of SRIF peptides and multiple types of SRIF receptors in fish opens a new avenue for the study of physiological roles of SRIF, and the molecular and cellular mechanisms of SRIF actions in fish.

Multiple molecular forms of glucagon and insulin in the kaluga sturgeon, Huso dauricus

Five molecular forms of glucagon and two molecular forms of insulin were characterized from the kaluga sturgeon. Substitutions occurred at two to thirteen internal amino acid residues among the five molecular forms of glucagons, indicating that these glucagons were encoded by five distinct genes. The amino acid sequences of two insulins from the kaluga sturgeon were identical to those of paddlefish insulin-II and Russian sturgeon insulin except that kaluga sturgeon insulin-I had an extension of five residues at the B-chain N-terminus. This is the first demonstration that more than two molecular forms of glucagon have been characterized from a single animal species.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Somatostatin family of peptides and its receptors in fish

Somatostatin (SRIF or SS) is a phylogenetically ancient, multigene family of peptides. SRIF-14 is conserved with identical primary structure in species of all classes of vertebrates. The presence of multiple SRIF genes has been demonstrated in a number of fish species and could extend to tetrapods. Three distinct SRIF genes have been identified in goldfish. One of these genes, which encodes [Pro2]SRIF-14, is also present in sturgeon and African lungfish, and is closely associated with amphibian [Pro2,Met13]SRIF-14 gene and mammalian cortistatin gene. The post-translational processing of SRIF precursors could result in multiple forms of mature SRIF peptides, with differential abundance and tissue- or cell type-specific patterns. The main neuroendocrine role of SRIF-14 peptide that has been determined in fish is the inhibition of pituitary growth hormone secretion. The functions of SRIF-14 variant or larger forms of SRIF peptide and the regulation of SRIF gene expression remain to be explored. Type 1 and type 2 SRIF receptors have been identified from goldfish and a type 3 SRIF receptor has been identified from an electric fish. Fish SRIF receptors display considerable homology with mammalian counterparts in terms of primary structure and negative coupling to adenylate cyclase. Although additional types of receptors remain to be determined, identification of the multiple gene family of SRIF peptides and multiple types of SRIF receptors opens a new avenue for the study of physiological roles of SRIF, and the molecular and cellular mechanisms of SRIF action in fish.Key words: somatostatin, somatostatin receptor, growth hormone, fish.

Ultrastructure and immunocytochemistry of the islet organ of osteoglossomorpha (Teleostei)

Both routine electron microscopy and immunocytochemistry with protein A-gold were used to identify the cell types within the islet organs of four species of teleosts (Osteoglossum bicirrhosum, Pantodon buchholzi, Notopterus chitala, and Gnathonemus petersii) within Osteoglossomorpha, a subdivision with an ancient lineage. Four primary endocrine cell types, A, B, D, and F, were identified within the islets of the four species examined. The B- and D-cells were located mainly in the central core of the islet in the four species. In general, the A-cells were located at the islet periphery in all of the four species but in P. buchholzi and N. chitala they were also differently distributed toward the islet core. F-cells were present only at the islet periphery. Granules of B-cells in three species had a relatively homogeneous shape of the matrix core, but in O. bicirrhosum, the shape varied greatly. Variation in matrix shape of B-cell granules may indicate a different conformation of insulin molecules among at least some species of osteoglossomorphs, and this observation may have some taxonomic significance. Two somatostatin-containing (SST) D-cell types (D1 and DX) with granules of different shape were observed in all four species of osteoglossomorphs. The granules of the two D-cells immunostained either with anti-SST-25 and anti-SST-14 (D1-cells) or with anti-SST-34 (DX-cells). Immunocytochemistry confirmed that A-cells, containing glucagon-family peptides, and F-cells, containing peptides of the pancreatic polypeptide family, had granules of different shape. The cells of the islet organs of these osteoglossomorphs are more similar to those in more derived teleosts than they are to those of nonteleost actinopterygians.

Rainbow trout (Oncorhynchus mykiss) possess two somatostatin mRNAs that are differentially expressed

Previously, we isolated a 624-bp cDNA encoding for a 115-amino acid preprosomatostatin containing [Tyr7,Gly10]-somatostatin (SS)-14 (now designated PPSS-II') obtained from the endocrine pancreas (Brockmann bodies) of rainbow trout. In this study we have characterized a second cDNA obtained from trout pancreas that is 600-bp in length and encodes for a 111-amino acid precursor containing [Tyr7,Gly10]-SS-14 (PPSS-II''). The nucleotide and amino acid identity between the two cDNAs is 82.3 and 80.5%, respectively. Both PPSS-II' and PPSS-II'' mRNA were present in esophagus, pyloric ceca, stomach, upper and lower intestine, and pancreas, whereas only SS-II" mRNA was present in brain. PPSS-II'' mRNA was more abundant than PPSS-II' mRNA in pancreas, whereas PPSS-II' mRNA was more abundant than PPSS-II" mRNA in stomach. Fasting increased pancreatic PPSS-II'' mRNA levels but had no effect on the levels of PPSS-II' mRNA. These results indicate the existence of two nonallelic pancreatic SS-II genes that are differentially expressed, both in terms of distribution among tissues and in terms of relative abundance within the tissues.

Molecular cloning and pharmacological characterization of a somatostatin receptor subtype in the gymnotiform fish Apteronotus albifrons

The actions of the various forms of somatostatin (SRIF), including those of the tetradecapeptide SRIF(14), are mediated by specific receptors. In mammals, five subtypes of SRIF receptors, termed sst(1-5), have been cloned. Using a combination of reverse transcriptase-polymerase chain reaction and genomic library screening in the gymnotiform fish Apteronotus albifrons, a gene encoding the first-known nonmammalian SRIF receptor has been isolated. The deduced amino acid sequence displays 59% identity with the human sst(3) receptor protein; hence, the gene is termed "Apteronotus sst(3)." The predicted protein consists of 494 amino acid residues exhibiting a putative seven-transmembrane domain topology typical of G protein-coupled receptors. A signal corresponding to the Apteronotus sst(3) receptor was detected in brain after amplification of poly(A)(+)-RNA by reverse transcriptase-polymerase chain reaction, but not by Northern blot analysis or in situ hybridization, suggesting a low level of expression. Membranes prepared from CCL39 cells stably expressing the Apteronotus sst(3) receptor gene bound [(125)I][Leu(8),d-Trp(22), (125) I-Tyr(25)]SRIF(28) with high affinity and in a saturable manner (B(max) = 4470 fmol/mg protein; pK(D) = 10.5). SRIF(14) and various synthetic SRIF receptor agonists produced a dose-dependent inhibition of radioligand binding, with the following rank order of potency: SRIF(14) approximately SRIF(28) > BIM 23052 > octreotide > BIM 23056. Under low stringency conditions, an Apteronotus sst(3) probe hybridized to multiple DNA fragments in HindIII or EcoRI digests of A. albifrons DNA, indicating that the Apteronotus sst(3) receptor is a member of a larger family of Apteronotus SRIF receptors.

Molecular cloning of the cDNAs and distribution of the mRNAs encoding two somatostatin precursors in the African lungfish Protopterus annectens

The occurrence of two somatostatin precursors, PSS1 and PSS2, yielding S-14 (SS1) and the variant [Pro2, Met13]S-14 (SS2), has been recently reported in the frog Rana ridibunda. The evolutionary significance of frog PSS2 is unclear because its sequence exhibits very little similarity with other known vertebrate somatostatin precursors. In the present study, we report on the characterization of two somatostatin precursor cDNAs from the brain of the African lungfish Protopterus annectens. One of the cDNAs encodes a 115-amino-acid protein that contains the SS1 sequence at its C-terminal extremity and thus is clearly homologous to PSS1. Comparison with other vertebrate PSS1 showed that lungfish PSS1 is more closely related to PSS1 from tetrapods than to PSS1 from fish. The other cDNA encodes a 109-amino-acid protein that contains a somatostatin variant [Pro2]S-14 at its C-terminal extremity. Sequence analysis of this second precursor indicated that it is the lungfish counterpart of frog PSS2. Northern blot analysis showed that lungfish PSS1 mRNA is widely distributed in the central nervous system and in peripheral organs, including the pancreas and gastrointestinal tract. In contrast, PSS2 mRNA was primarily found in the central nervous system but not in the pancreas or gut. In situ hybridization studies showed that the two genes are differentially expressed in various regions of the lungfish brain. The present data indicate that the PSS2 gene, initially discovered in frog, appeared early in vertebrate evolution, before the emergence of the tetrapod lineage. The recent isolation of a [Pro2]S-14 variant in the sturgeon, whose sequence is identical to that of lungfish SS2, suggests that the PSS2 gene may actually be present in the genome of all Osteichthyii.

Expression of three distinct somatostatin messenger ribonucleic acids (mRNAs) in goldfish brain: characterization of the complementary deoxyribonucleic acids, distribution and seasonal variation of the mRNAs, and action of a somatostatin-14 variant

In this study, three somatostatin (SRIF) complementary DNAs (cDNAs) were characterized from goldfish brain. The cDNAs encode three distinct preprosomatostatins (PSS), designated as PSS-I, PSS-II, and PSS-III. The goldfish PSS-I, PSS-II, and PSS-III contain enzymatic cleavage recognition sites, potentially yielding SRIF-14 with sequence identical to mammalian SRIF-14, SRIF-28 with [Glu1, Tyr7, Gly10]SRIF-14 at its C-terminus, and [Pro2]SRIF-14, respectively. The brain distribution of the three SRIF messenger RNAs (mRNAs) were differential but overlapping in the telencephalon, hypothalamus and optic tectum-thalamus regions. Seasonal variations in the levels of the three mRNAs were observed, with differential patterns between the three mRNAs and differences between the sexes. However, only the seasonal alteration in the levels of the mRNA encoding PSS-I showed close association with the seasonal variation in brain contents of immunoreactive SRIF-14 and inversely correlated with the seasonal variation in serum GH levels described in the previous studies, suggesting that SRIF-14 is involved in the control of the seasonal variation in serum GH levels. The putative SRIF-14 variant, [Pro2]SRIF-14, inhibited basal GH secretion from in vitro perifused goldfish pituitary fragments, with similar potency to SRIF-14; [Pro2]SRIF-14 also inhibited stimulated GH release from the pituitary fragments, supporting that [Pro2] SRIF-14 is a biologically active form of SRIF in goldfish.

Purification and characterization of insulin and peptides derived from proglucagon and prosomatostatin from the fruit-eating fish, the pacu Piaractus mesopotamicus

The fruit-eating teleost fish, the pacu Piaractus mesopotamicus (Characiformes, Characidae) is classified along with the carp and the catfish in the superorder Ostariophysi. The pacu is able to survive and grow in captive conditions feeding exclusively on carbohydrates. Hormonal polypeptides in an extract of pacu Brockmann bodies were purified to homogeneity by reversed phase HPLC and their primary structures determined by automated Edman degradation. Pacu insulin contains only two substitutions, Glu-->Asp at A15 and Thr-->Ser at B24 (corresponding to B22 in mammalian insulins) compared with carp insulin. The B-chains of both insulins contain a dipeptide extension to the N-terminus and a deletion of the C-terminal residue compared with human insulin. Pacu glucagon differs from catfish glucagon by a single substitution at position 17 (Arg-->Gln. The primary structure of the 34 amino acid residue glucagon-like peptide (GLP) differs from catfish GLP only at positions 12 (Ser-->Ala) and 33 (Pro-->Gln). In common with other teleost species, the pacu expresses two somatostatin genes. Somatostatin-14, derived from preprosomatostatin-I (PSS-I), is identical to mammalian/catfish somatostatin-14. Although pacu somatostatin-II was not identified in this study, a peptide was purified that shows 67% sequence identity with residues (1-58) of catfish preprosomatostatin-II (PSS-II). This relatively high degree of sequence similarity contrasts with the fact that catfish PSS-II shows virtually no sequence identity with the corresponding PSS-II from anglerfish (Acanthopterygii) and trout (Protoacanthopterygii). A comparison of the primary structures of the islet hormones suggest that amino acid sequences may have been better conserved within the Ostariophysi than in other groups of the taxon Euteleostei that have been studied.

Morphological basis of the interactions between endocrine cell types in the pancreatic islets of the teleost, Blennius gattoruggine

The endocrine pancreas of the teleost fish Blennius gattoruggine was studied by immunochemistry using both light and electron microscopy. Generally, one large Brockmann body, along with intermediate and small islets, was found. Cells immunoreactive (IR) to anti-insulin (B), anti-glucagon (A) anti-somatostatin (D) anti-pancreatic polypeptide and anti-PYY sera were detected with B cells located at the center of the islet and the other cell types forming a peripheral mantle. The B-cell cytoplasm showed rows of microtubules close to the secretory granules and perpendicular to the plasmalemma. The ultrathin section images revealed exocytotic and endocytotic features, and the presence of intercellular gap junctions between the plasmalemma of contiguous cells, suggesting intercellular routes of communication, e.g. via autocrine and/or paracrine mechanism. These features were observed in all of the cell types, and were abundant in D cells. D cells were particularly numerous in the islets and were disposed close to A and B cells, as observed in other teleost species. The most peripheral B cells, in closer contact with D cells than the central ones, appeared strongly immunolabeled, perhaps owing to the inhibitory action of somatostatin. Some D cells exhibited a long protrusion directed towards the center of the islet. In view of their cytological characteristics and their secretion, D cells might have an important role in the modulation of A and B-cell secretion in an endocrine and/or paracrine fashion.

Effects of somatostatin on lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus

This study was designed to examine the role of somatostatin in regulating changes in lipid metabolism of larvae and metamorphosing landlocked sea lamprey, Petromyzon marinus. Larvae and animals in late metamorphosis (stage 6 on a 7-stage scale) were injected intraperitoneally once per day for 2 days with either saline (0.6%) or somatostatin-14 (SS-14; 500 ng/g body wt). Injection of SS-14 into larval and stage 6 metamorphosing animals resulted in elevated plasma fatty acids levels. In larvae, SS-14-induced hyperlipidemia was supported by enhanced lipolysis, as indicated by increased triacylglycerol lipase (TGL) activity in the liver and kidney. Mobilization of larval renal lipid was accompanied by reduced TG synthesis, as indicated by decreased diacylglycerol acyltransferase (DGAT) activity. In stage 6 metamorphosing lamprey, SS-14 did not significantly affect TGL activity; however, SS-14 significantly reduced fatty acid synthesis, as measured by acetyl-CoA carboxylase activity, in kidney, liver, and muscle, as well as muscular TG synthesis. SS-14-stimulated lipid depletion is reminiscent of the pattern of lipid metabolism displayed by P. marinus during their spontaneous metamorphosis-an observation which suggests that somatostatin may play a role in metamorphosis-associated changes in lipid metabolism in this species.

Evolution of neuroendocrine peptide systems: gonadotropin-releasing hormone and somatostatin

Nine vertebrate and two protochordate gonadotropin-releasing hormone (GnRH) decapeptides have been identified and sequenced. Multiple molecular forms of GnRH peptide were present in the brain of most species examined, and cGnRH-II generally coexists with one or more GnRH forms in all the major vertebrate groups. The presence of multiple GnRH forms has been further confirmed by the deduced GnRH peptide structure from cDNA and/or gene sequences in several teleost species and tree shrew. High conservation of the primary structure of GnRH decapeptides and the overall structure of GnRH genes and precursors suggests that they are derived from a common ancestor. Somatostatin (SRIF) is a phylogenetically ancient, multigene family of peptides. A tetradecapeptide, SRIF (SRIF14) has been conserved, with the same amino acid sequence, in representative species of all classes of vertebrate. Four molecular variants of SRIF14 have been identified. SRIF14 is processed from preprosomatostatin-I, which contains SRIF14 at its C-terminus; preprosomatostatin-I is also processed to SRIF28 in mammals and SRIF26 in bowfin. Teleost fish possess a second somatostatin precursor, preprosomatostatin-II, containing [Tyr7, Gly10]-SRIF14 at the C-terminus, that is mainly processed into large forms of SRIF.

Neuropeptide families: evolutionary perspectives

Examination of neuropeptide families can provide information about phyletic relationships and evolutionary processes. In this article the oxytocin/vasopressin family, growth hormone releasing factor (GRF) superfamily and the substance P/tachykinin family have been considered in detail because they have been isolated from an extraordinarily diverse array of species from several vertebrate classes and invertebrate phyla. More important is that the nucleotide sequence of mRNA or cDNA encoding many of these peptides has been determined, which has allowed evolutionary distances to be estimated based on the DNA mutation rate. The origin of a given family lies in a primordial gene that arose many millions of years ago, and through time, exon duplication and insertion, gene duplication, point mutation and exon loss, the family developed into the forms that are now recognised. For example, in birds, GRF and pituitary adenylate cyclase activating peptide (PACAP) are encoded by the same gene, which probably arose as a result of exon duplication and tandem insertion of the ancestral GRF gene. In mammals GRF is the sole product on one gene, and PACAP is the product of a gene that also produces PACAP-related peptide (PRP), which is homologous to GRF. Thus it appears that between birds and mammals the GRF/PACAP gene duplicated: exon loss gave rise to the mammalian GRF gene, while mutation led to the formation of the mammalian PRP/PACAP gene. The neuropeptide Y superfamily is considered briefly, as is cionin, which is an invertebrate peptide that is closely related to the mammalian gastrin/cholecystokinin family.

Somatostatin in lungfish kidney: an immunohistochemical, autoradiographical and in situ hybridisation study

The localisation of somatostatin-14 (SST-14) was examined immunohistochemically using the antibody Ab-SST-14 in the kidney of the African lungfish Protopterus annectens. Immunoreactive cells were present in the proximal tubules. In situ hybridisation, using an oligonucleotide probe complementary to mRNA for SST-14 and labeled at the 3'-end with alpha-35S, showed SST-14 mRNA distributed in cells with the same localisation as seen for SST-14 immunoreactive cells. Binding sites for SST-14 were identified with autoradiography using 125I SST-14. Binding sites were concentrated on cells of the proximal tubules. It is suggested that SST-14 may be synthesised in the lungfish mesonephros.

Immunocytochemical localization of somatostatin and autoradiographic distribution of somatostatin binding sites in the brain of the African lungfish, Protopterus annectens

The anatomical distribution of somatostatin-immunoreactive structures and the autoradiographic localization of somatostatin binding sites were investigated in the brain of the African lungfish, Protopterus annectens. In general, there was a good correlation between the distribution of somatostatin-immunoreactive elements and the location of somatostatin binding sites in several areas of the brain, particularly in the anterior olfactory nucleus, the rostral part of the dorsal pallium, the medial subpallium, the anterior preoptic area, the tectum, and the tegmentum of the mesencephalon. However, mismatching was found in the mid-caudal dorsal pallium, the reticular formation, and the cerebellum, which contained moderate to high concentrations of binding sites and very low densities of immunoreactive fibers. In contrast, the caudal hypothalamus and the neural lobe of the pituitary exhibited low concentrations of binding sites and a high to moderate density of somatostatin-immunoreactive fibers. The present results provide the first localization of somatostatin in the brain of a dipnoan and the first anatomical distribution of somatostatin binding sites in the brain of a fish. The location of somatostatin-immunoreactive elements in the brain of P. annectens is consistent with that reported in anuran amphibians, suggesting that the general organization of the somatostatin peptidergic systems occurred in a common ancestor of dipnoans and tetrapods. The anatomical distribution of somatostatin-immunoreactive elements and somatostatin binding sites suggests that somatostatin acts as a hypophysiotropic neurohormone as well as a neurotransmitter and/or neuromodulator in the lungfish brain.

Occurrence of two somatostatin variants in the frog brain: characterization of the cDNAs, distribution of the mRNAs, and receptor-binding affinities of the peptides

In tetrapods, only one gene encoding a somatostatin precursor has been identified so far. The present study reports the characterization of the cDNA clones that encode two distinct somatostatin precursors in the brain of the frog Rana ridibunda. The cDNAs were isolated by using degenerate oligonucleotides based on the sequence of the central region of somatostatin to screen a frog brain cDNA library. One of the cDNAs encodes a 115-amino acid protein (prepro-somatostatin-14; PSS1) that exhibits a high degree of structural similarity with the mammalian somatostatin precursor. The other cDNA encodes a 103-amino acid protein (prepro-[Pro2, Met13]somatostatin-14; PSS2) that contains the sequence of the somatostatin analog (peptide SS2) at its C terminus, but does not exhibit appreciable sequence similarity with PSS1 in the remaining region. In situ hybridization studies indicate differential expression of the PSS1 and PSS2 genes in the septum, the lateral part of the pallium, the amygdaloid complex, the posterior nuclei of the thalamus, the ventral hypothalamic nucleus, the torus semicircularis and the optic tectum. The somatostatin variant SS2 was significantly more potent (4-6 fold) than somatostatin itself in displacing [125I-Tyr0, D-Trp8] somatostatin-14 from its specific binding sites. The present study indicates that the two somatostatin variants could exert different functions in the frog brain and pituitary. These data also suggest that distinct genes encoding somatostatin variants may be expressed in the brain of other tetrapods.

Glucagon and glucagon-like peptides in fishes

Glucagon and glucagon-like peptides (GLPs) are coencoded in the vertebrate proglucagon gene. Large differences exist between fishes and other vertebrates in gene structure, peptide expression, peptide chemistry, and function of the hormones produced. Here we review selected aspects of glucagon and glucagon-like peptides in vertebrates with special focus on the contributions made by analysis of piscine systems. Our topics range from the history of discovery to gene structure and expression, through primary structures and regulation of plasma concentrations to physiological effects and message transduction. In fishes, the pancreas synthesizes glucagon and GLP-1, while the intestine may contribute oxyntomodulin, glucagon, GLP-1, and GLP-2. The pancreatic gene is short and lacks the sequence for GLP-2. GLP-1, which is produced exclusively in its biologically active form, is a potent metabolic hormone involved in regulation of liver glycogenolysis and gluconeogenesis. The responsiveness of isolated hepatocytes to glucagon is limited to high concentrations, while physiological concentrations of GLP-1 effectively regulate hepatic metabolism. Plasma concentrations of GLP-1 are higher than those of glucagon, and liver is identified as the major site of removal of both hormones from fish plasma. Ultimately, GLP-1 and glucagon exert effects on glucose metabolism that directly and indirectly oppose several key actions of insulin. Both glucagon and GLP-1 show very weak insulinotropic activity, if any, when tested on fish pancreas. Intracellular message transduction for glucagon, especially at slightly supraphysiological concentrations, involves cAMP and protein kinase A, while pathways for GLP are largely unknown and may involve a multitude of messengers, including cAMP. In spite of fundamental differences in GLP-1 function between fishes and mammals, fish GLP-1 is as powerful an insulinotropin for mammalian B-cells as mammalian GLP-1 is a metabolic hormone if tested on piscine liver.

Polygenic expression of somatostatin in lamprey

Previous studies have led to the identification of three biosynthetically related molecular forms of somatostatin (somatostatin-14, -34 and -37) from the pancreas of the sea lamprey (Petromyzon marinus). We have now isolated from the brain of the same species a second form of somatostatin-14 that is identical to mammalian somatostatin-14 and differs from lamprey pancreatic somatostatin-14 by the substitution Ser12 to Thr. Larger forms of somatostatin were not identified in lamprey brain in this study. These data suggest that the two molecular forms of lamprey somatostatin-14 are the products of different genes that are expressed in a tissue-specific manner.

Prosomatostatin-I is processed to somatostatin-26 and somatostatin-14 in the pancreas of the bowfin, Amia calva

With the exception of the Agnatha (lampreys and hagfishes), somatostatin-14 is the predominant molecular form of somatostatin in the pancreas of species from all classes of vertebrates yet studied. The pancreas of the holostean fish, Amia calva (bowfin; order Amiiformes) contained somatostatin-like immunoreactivity that was resolved by reversed phase HPLC in two components. The primary structure of the more abundant peptide (somatostatin-26) was established as: Ser-Ala-Asn-Pro-Ala5-Leu-Ala-Pro-Arg-Glu10-Arg-Lys-Ala-Gly-+ ++Cys15-Lys-Asn-Phe- Phe-Trp20-Lys-Thr-Phe-Thr-Ser25-Cys. This amino acid sequence shows one substitution (Leu for Met at position 6) and two deletions compared with mammalian somatostatin-28. The minor component was identical to somatostatin-14. The data show that the pathway of post-translational processing of prosomatostatin-I in the bowfin pancreas is appreciably different from the corresponding pathway in teleost fish and higher vertebrates.

Isolation of [Pro2,Met13]Somatostatin-14 and somatostatin-14 from the frog brain reveals the existence of a somatostatin gene family in a tetrapod

Two somatostatin-related peptides were isolated in pure form from an extract of the brain of the European green frog, Rana ridibunda. The primary structure of the most abundant component was identical to that of mammalian somatostatin-14. The primary structure of the second component, present in approximately 5% of the abundance of somatostatin-14, was established as Ala-Pro-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Met-Cys. This sequence shows two substitutions (Pro for Gly2 and Met for Ser13) compared with mammalian somatostatin-14. The data provide evidence for a somatostatin gene family in tetrapods as well as in teleost fish.

Somatostatin 14- and somatostatin 25-like peptides in pancreatic endocrine cells of Sparus aurata (teleost): a light and electron microscopic immunocytochemical study

An immunocytochemical investigation demonstrates the presence of somatostatin (SST) 14- and salmon somatostatin (sSST) 25-like peptides in two populations of somatostatin (D) cells in the islets of gilthead sea bream (Sparus aurata). Both cell types were identified by their differing immunoreactivities to the somatostatin antisera used. D1 cells in the islet periphery between glucagon cells showed sSST 25-like immunoreactivity and contained large moderate to low electron-dense granules. D2 cells, present only in the central region of the islets between insulin cells, were immunoreactive to the SST 14 antisera and had smaller electron-dense granules. In S. aurata, as in other teleosts, preprosomatostatin I and II are probably synthesized and processed to SST 14- and sSST 25-like peptides, respectively, in different D cell types.

Light and electron microscopic immunocytochemical demonstration of the coexistence of somatostatin 14- and somatostatin 25-like peptides in endocrine cells of the stomach of Sparus aurata (Teleost)

An immunofluorescence double-staining method colocalized somatostatin 14 (SST 14)- and somatostatin 25 (SST 25)-like immunoreactivities in endocrine cells located in the depth of gastric folds and upper part of the stomach glands of Sparus aurata (gilthead sea bream). An immunogold method identified somatostatin-like peptides in the secretory granules of the previously described Type IV endocrine cells. Appropriate preabsorption controls demonstrated two different granule populations with somatostatin-like immunoreactivity. SST 14-like peptides seemed to be located in the most commonly found granules, which showed a fibrillar content, whereas SST 25-like peptides were identified in more scarce and denser granules.

The primary structure of glucagon-like peptide but not insulin has been conserved between the American eel, Anguilla rostrata and the European eel, Anguilla anguilla

Insulin was isolated from the pancreas of the American eel, Anguilla rostrata, and its primary structure was established as (Formula: see text). Eel insulin contains unusual substitutions at B-21, B-22, and B-26 in the putative receptor-binding region of the molecule compared with other mammalian and fish insulins. The A-chain of insulin from the European eel contains an asparagine rather than a serine residue at position A-12. Similarly, amino acid composition data indicate the B-chain of insulin from the European eel is appreciably different from that from the American eel. The primary structure of glucagon-like peptide (GLP) from the American eel is identical to that from the European eel, Anguilla anguilla. The primary structure of the peptide was established as (Formula: see text). Fast-atom bombardment mass spectrometry demonstrated that the COOH-terminal arginyl residue is alpha-amidated. The strong evolutionary pressure to conserve the structure of GLP provides further support for the assertion that the peptide plays an important regulatory role in teleost fish.

[Ser5]-somatostatin-14: isolation from the pancreas of a holocephalan fish, the Pacific ratfish (Hydrolagus colliei)

The holocephalan fishes were the first class of vertebrate in evolution to develop a pancreatic gland with both endocrine and exocrine parenchyma. An extract of the pancreas of one such fish, the Pacific ratfish (Hydrolagus colliei) contained somatostatin-like immunoreactivity (141 pmol/g wet wt), measured with an antiserum raised against mammalian somatostatin-14. Automated Edman degradation and fast atom bombardment-mass spectrometry established the primary structure of the major molecular form as Ala-Gly-Cys-Lys-Ser-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys. A minor component of somatostatin-like immunoreactivity, constituting 8% of the total, was of approximate molecular weight 6000. Thus, in the ratfish pancreas prosomatostatin-I is processed predominantly to somatostatin-14, as in the mammalian pancreas, but the resulting tetradecapeptide contains the substitution Ser for Asn at position 5.

Distribution of two forms of somatostatin in the brain, anterior intestine, and pancreas of adult lampreys (Petromyzon marinus)

The distribution of two major immunoreactive forms of somatostatin, somatostatin-14 and somatostatin-34, within the brain, pancreas and intestine of adult lampreys, Petromyzon marinus, was identified using antisera raised against these peptides. Immunostaining of the brain is similar in juveniles and upstream migrants, and somatostatin-14 is the major somatostatin form demonstrated. A few somatostatin-34-containing cells are localized within the olfactory bulbs, thalamus and hypothalamus, but cells immunoreactive to anti-somatostatin-34 in the hypothalamus and thalamus do not co-localize somatostatin-14. Immunostaining of pinealocytes within the pineal pellucida with anti-somatostatin-14 may infer a novel function for this structure. Somatostatin-14 and somatostatin-34 are co-localized within D-cells of the cranial pancreas and caudal pancreas of juveniles and upstream migrants. Numerous somatostatin-34-immunoreactive cells are distributed within the epithelial mucosa of the anterior intestine but not all of these cells cross-react with anti-somatostatin-14. It appears that somatostatin-34 is the major somatostatin in the pancreo-gastrointestinal system of adult lampreys.

The role of glycogen phosphorylase in the regulation of glycogenolysis by insulin and glucagon in isolated eel (Anguilla rostrata) hepatocytes

The effects of porcine, scombroid, and salmon insulins, and bovine and anglerfish glucagons on glycogen depletion and glycogen phosphorylase (GPase) activities were examined in freshly isolated American eel (Anguilla rostrata) hepatocytes. Eel liver GPase in crude homogenates was activated (increase in % GPase a) by phosphorylating conditions and was rapidly inactivated (less than 1 h) when a phosphatase inhibitor (fluoride) was absent. Caffeine inhibits, and AMP activates, the b form of GPase consistent with their effects on rat liver GPase. Both mammalian and fish glucagons increased glucose production in eel hepatocytes, but had more ambiguous effects on glycogen levels and GPase activities. The magnitude of bovine glucagon effects were dependent on the initial glycogen content of the cells; only at glycogen concentrations less than approximately 70 μmoles.g(-1) did glucagon significantly increase % GPase a. Anglerfish glucagon significantly increased cyclic AMP (cAMP) concentrations by 90% at 10(-7) M, but had no effects at 10(-9) M and 10(-8) M. Scombroid and salmon insulins maintained hepatocyte glycogen concentrations and decreased glucose production, with these effects more pronounced at low (10(-9) to 10(-8) M) rather than high (10(-7) M) hormone concentrations. Porcine and salmon insulins decreased total GPase and % GPase a activities, and salmon insulin decreased CAMP levels, but only at 10(-8) M (by 44%).Glycogen is, therefore, depleted by glucagon and maintained by insulin in freshly isolated American eel hepatocytes, and these changes are accomplished, at least in part, by changes in the activities of GPase. Changes in cAMP do not explain all of the observed hormone effects.

Neuropeptide families: an evolutionary perspective

Changes in the structure and function of five neuropeptide families during evolution are considered. The families of gonadotropin-releasing hormone (GnRH), corticotropin-releasing factor (CRF), growth hormone-releasing hormone (GH-RH), somatostatin (SS), and vasopressin/oxytocin (VP/Oxy) are used as models to illustrate the importance of a phylogenetic approach in understanding neuropeptide structure/activity relationships, precursors, processing, gene duplication, novel locations and functions, and gene-associated peptides.

Isolation and primary structure of the C-peptide of proinsulin from the European eel (Anguilla anguilla)

1. The primary structure of the C-peptide of proinsulin from the European eel has been established as: DVEPLLGFLSPKSGQENEVDDFPYKGQGEL. The peptide was isolated from the extract of eel pancreas in a yield that was approximately equimolar with insulin. A comparison with the predicted structures of C-peptides from other teleost fishes has identified a domain in the central region of the peptide that has been more highly conserved than the rest of the molecule.