Polygenic expression of somatostatin in the sturgeon Acipenser transmontanus: molecular cloning and distribution of the mRNAs encoding two somatostatin precursors

Neuropeptides and hormones in hypothalamus-pituitary axis of Chinese sturgeon (Acipenser sinensis)

The hypothalamus and pituitary serve as important neuroendocrine center, which is able to secrete a variety of neuropeptides and hormones to participate in the regulation of reproduction, growth, stress and feeding in fish. Chinese sturgeon is a basal vertebrate lineage fish with a special evolutionary status, but the information on its neuroendocrine system is relatively scarce. Using the transcriptome data on the hypothalamus-pituitary axis of Chinese sturgeon as reference, we found out 46 hypothalamus neuropeptide genes, which were involved in regulation of reproduction, growth, stress and feeding. The results of sequence alignment showed that the neuroendocrine system of Chinese sturgeon evolves slowly, which confirms that Chinese sturgeon is a species with a slow phenotypic evolution rate. In addition, we also isolated six pituitary hormones genes from Chinese sturgeon, including reproductive hormones: follicle-stimulating homone (FSH) and luteinizing hormone (LH), growth-related hormones: growth hormone (GH)/prolactin (PRL)/somatolactin (SL), and stress-related hormone gene: proopiomelanocortin (POMC). Similar to teleost, immunostaining localization analysis in Chinese sturgeon pituitary showed that LH and FSH were located in the pituitary proximal pars distalis, SL was located in the pituitary rostral pars distalis, and POMC was located in the pituitary pars intermedia and pituitary rostral pars distalis. This study will give a contribution to enrich our information on the neuroendocrine system in Chinese sturgeon.

Differential expression of somatostatin genes in the central nervous system of the sea lamprey

The identification of three somatostatin (SST) genes (SSTa, SSTb, and SSTc) in lampreys (Tostivint et al. Gen Comp Endocrinol 237:89-97 https://doi.org/10.1016/j.ygcen.2016.08.006 , 2016) prompted us to study their expression in the brain and spinal cord of the sea lamprey by in situ hybridization. These three genes were only expressed in equivalent neuronal populations in the hypothalamus. In other regions, SST transcripts showed clear differential expression. In the telencephalon, SSTc-positive cells were observed in the medial pallium, ventral part of the lateral pallium, striatum, subhippocampal lobe, and preoptic region. In the diencephalon, SSTa-positive cells were observed in the thalamus and SSTc-positive cells in the prethalamus, posterior tubercle, pretectal area, and nucleus of the medial longitudinal fascicle. In the midbrain, SSTc-positive cells were observed in the torus semicircularis, lateral reticular area, and perioculomotor tegmentum. Different SSTa- and SSTc-positive populations were observed in the isthmus. SSTc neurons were also observed in the rostral octavolateralis area and caudal rhombencephalon. In the spinal cord, SSTa was expressed in cerebrospinal-fluid-contacting (CSF-c) neurons and SSTc in non-CSF-c interneurons. Comparison with previous immunohistochemical studies using anti-SST-14 antibodies strongly suggests that SST-14-like neurons correspond with the SSTa populations. Thus, the SSTc populations were not reported previously in immunohistochemical studies. Cluster-based analyses and alignments of mature peptides suggested that SSTa is an ortholog of SST1 and that SSTb is closely related to SST2 and SST6. These results provide important new insights into the evolution of the somatostatinergic system in vertebrates.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization. Additionally, we combined in situ hybridization with tyrosine hydroxylase (TH) immunochemistry for better characterization of PSST1 and PSST6 expressing populations. We observed differential expression of PSST1 and PSST6, which are the most widely expressed PSST transcripts, in cell populations of many CNS regions, including the pallium, subpallium, hypothalamus, diencephalon, optic tectum, midbrain tegmentum, and rhombencephalon. Interestingly, numerous small pallial neurons express PSST1 and PSST6, although in different populations judging from the colocalization of TH immunoreactivity and PSST6 expression but not with PSST1. We observed expression of PSST1 in cerebrospinal fluid-contacting (CSF-c) neurons of the hypothalamic paraventricular organ and the central canal of the spinal cord. Unlike PSST1 and PSST6, PSST2, and PSST3 are only expressed in cells of the hypothalamus and in some hindbrain lateral reticular neurons, and PSST5 in cells of the region of the entopeduncular nucleus. Comparative data of brain expression of PSST genes indicate that the somatostatinergic system of sharks is the most complex reported in any fish.

Identification, Expression, and Functions of the Somatostatin Gene Family in Spotted Scat (Scatophagus argus)

Somatostatins (SSTs) are a family of proteins consisting of structurally diverse polypeptides that play important roles in the growth regulation in vertebrates. In the present study, four somatostatin genes (SST1, SST3, SST5, and SST6) were identified and characterized in the spotted scat (Scatophagus argus). The open reading frames (ORFs) of SST1, SST3, SST5, and SST6 cDNA consist of 372, 384, 321, and 333 bp, respectively, and encode proteins of 123, 127, 106, and 110 amino acids, respectively. Amino acid sequence alignments indicated that all SST genes contained conserved somatostatin signature motifs. Real-time PCR analysis showed that the SST genes were expressed in a tissue specific manner. When liver fragments were cultured in vitro with synthetic peptides (SST1, SST2, or SST6 at 1 μM or 10 μM) for 3 h or 6 h, the expression of insulin-like growth factor 1 and 2 (Igf-1 and Igf-2) in the liver decreased significantly. Treatment with SST5 had no significant effect on Igf-1 and Igf-2 gene expression. This study provides an enhanced understanding of the gene structure and expression patterns of the SST gene family in S. argus. Furthermore, this study provides a foundation for future exploration into the role of SST genes in growth and development.

Revisiting the evolution of the somatostatin family: Already five genes in the gnathostome ancestor

The somatostatin (SST) family members are a group of neuropeptides that are best known for their role in the regulation of growth, development and metabolism. The occurrence of six paralogous SST genes named SST1, SST2, SST3, SST4, SST5 and SST6 has been reported in vertebrates. It has been proposed that SST1, SST2 and SST5 arose in 2R from a common ancestral gene. SST3 and SST6 would have been subsequently generated by tandem duplications of the SST1 and SST2 genes respectively, at the base of the actinopterygian lineage. SST4 is thought to have appeared more recently from SST1, in teleost-specific 3R. In order to gain more insights into the SST gene family in vertebrates, we sought to identify which paralogs of this family are present in cartilaginous fish. For this purpose, we first searched the recently available genome and transcriptome databases from the catshark Scyliorhinus canicula. In a previous study, three S. canicula SST genes, called at that time SSTa, SSTb and SSTc, were identified and proposed to correspond to SST1, SST5 and SST2 respectively. In the present work, two additional SST genes, called SSTd and SSTe, were found in S. canicula plus two other chondrichtyan species, elephant shark (Callorhinchus milii) and whale shark (Rhincodon typus). Phylogeny and synteny analyses were then carried out in order to reveal the evolutionary relationships of SSTd and SSTe with other vertbrates SSTs. We showed that SSTd and SSTe correspond to SST2 and SST3 respectively, while SSTc corresponds to SST6 and not to SST2 as initially proposed. Our investigations in other vertebrate species also led us to find that the so-called SST2 gene in chicken, lungfish, sturgeons and teleosts actually corresponds to SST6. Conversely, the so-called SST6 gene in actinopterygians corresponds to SST2. Taken together, our results suggest that: i) SST3 and SST6 were already present in the gnathostome ancestor, much earlier than previously thought; ii) SST6 was also present in the tetrapod ancestor and still occurs in living birds; with this respect, it is likely that SST6 was independently lost several times during evolution: in amphibians, squamates and mammals; iii) SST2, SST3 and SST5 were probably lost in euteleosts, sarcopterygians and tetrapods, respectively.

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.

Impact of gene/genome duplications on the evolution of the urotensin II and somatostatin families

The present review describes the molecular evolution of two phylogenetically related families of neuropeptides, the urotensin II (UII) and somatatostatin (SS) families. The UII family consists of four paralogous genes called UII, URP, URP1 and URP2 and the SS family is composed of six paralogous genes named SS1, SS2, SS3, SS4, SS5 and SS6. All these paralogs are present in teleosts, while only four of them, UII, URP, SS1 and SS2 are detected in tetrapods. Comparative genomics showed that most of these genes, namely UII, URP, URP1 and URP2 on the one hand and SS1, SS2 and SS5 on the other hand arose through the 2R. In contrast, the teleost-specific 3R had a much more moderate impact since it only concerned the UII and SS1 genes, which once duplicated, generated a second UII copy and SS4, respectively. The two remaining genes, SS3 and SS6, arose through tandem duplications of the SS1 and SS2 genes respectively, probably in the stem lineage of actinopterygians, before the emergence of teleosts. The history of the UII and SS families has also been marked by massive gene lost, both in tetrapods and in teleosts, but only after the 3R in this latter lineage. Finally, ancestral UII and SS genes are thought to have arisen through tandem duplication of a single ancestral gene, largely before the 1R. An important challenge for the future will be to understand the physiological significance of the molecular diversity of these two families.

Molecular cloning of the cDNAs encoding three somatostatin variants in the dogfish (Scylorhinus canicula)

It has been recently shown that the somatostatin gene family was likely composed of at least three paralogous genes in the common ancestor of all extant jawed vertebrates. These three genes, namely SS1, SS2 and SS5, are thought to have been generated through the two rounds of whole-genome duplications (2R) that took place early during the vertebrate evolution. In the present study, we report the cloning of three distinct somatostatin cDNAs from the dogfish Scylorhinus canicula, a member of the group of cartilaginous fish. We decided to call these cDNAs, at least provisionally, SSa, SSb and SSc, respectively. Two of them, SSa and SSb, encode proteins that both contain the same tetradecapeptide sequence at their C-terminal extremity (AGCKNFFWKTFTSC). This putative peptide is identical to that generated by the SS1 gene in other vertebrate species. The last cDNA, SSc, encodes a protein that contains at its C-terminal extremity the same peptide sequence as that generated by the SS2 gene in teleosts (APCKNFFWKTFTSC). Phylogenetic analysis showed that the SSa and SSc genes likely correspond to the dogfish counterparts of the SS1 and SS2 genes, respectively. In contrast, the phylogenetic status of the SSb gene is less clear. Several lines of evidence suggest that it could correspond to the SS5 gene, but this view will need to be confirmed, for example by synteny analysis. Finally, RT-PCR analysis revealed that SSa, SSb and SSc genes are differentially expressed in dogfish tissues, suggesting that the corresponding peptides may exert distinct functions.

[Molecular biological study on hormones in Acipenseriformes]

Sturgeons belong to the Acipenseriformes, which provide an ideal model for evolutionary studies due to their unique characters. At present, many species of Acipenseriformes are very rare and near extinct. It is urgent to protect these species. However, data on molecular mechanisms of their growth and reproduction regulation are still limited. Hormones are important factors involved in these processes. In this paper, we summarized recent research progresses in the hormones in sturgeon, which will provide valuable information for further studies on molecular mechanisms of growth, sexual development, and reproduction regulation in Acipenseriformes.

Somatostatin and its receptors from fish to mammals

Somatostatin (SST) and its receptors (sst) make up a molecular family with unique functional complexity and versatility. Widespread distribution and frequent coexpression of sst subtypes underlies the multiplicity of (patho)physiological processes controlled by SST (central nervous system functions, endocrine and exocrine secretion, cell proliferation). This complexity is clearly reflected in the intricate evolutionary development of this molecular family. Recent studies postulate the existence of an ancestral somatostatin/urotensin II (SST/UII) gene, which originated two ancestral, SST and UII, genes by local duplication. Subsequently, segment duplication would have originated two diverging SST genes in both fish (SS1/SS2) and tetrapods [(SST/cortistatin(CST))]. SST/CST actions are mediated by a family of GPCRs (sst1-5) encoded by five different genes. sst1-4 sequences are highly conserved compared with sst5, suggesting unique evolutionary and functional relevance for the latter. Indeed, we recently identified novel truncated but functional sst5 variants in several species, which may help to explain part of the complexity of the SST/CST/sst family. Comparative and phylogenetic analysis of this molecular family would enhance our understanding of its paradigmatic evolutionary complexity and functional versatility.

Evolutionary history of the somatostatin and somatostatin receptors

Somatostatin and its receptors have a critical role in mammalian growth through their control pattern of secretion of growth hormone, but the evolutionary history of somatostatin and somatostatin receptors are ill defined. We used comparative whole genome analysis of Danio rerio, Carassius auratus, Xenopus tropicalis, Gallus gallus, Monodelphis domestica, Homo sapiens, Sus scrofa, Bos taurus, Mus musculus, Rattus norvegicus, Canis lupus familiaris, Ovis aries, Equus caballus, Pan troglodytes and Macaca mulatto to identify somatostatin and somatostatin receptors in each species. To date, we have identified a minimum of two genes of somatostatin and five somatostatin receptor genes in mammalian species with variable forms. We established a clear evolutionary history of the somatostatin system and traced the origin of the somatostatin system to 395 million years ago (MYA), identifying critical steps in their evolution.

Understanding the multifactorial control of growth hormone release by somatotropes: lessons from comparative endocrinology

Control of postnatal growth is the main, but not the only, role for growth hormone (GH) as this hormone also contributes to regulating metabolism, reproduction, immunity, development, and osmoregulation in different species. Likely owing to this variety of group-specific functions, GH production is differentially regulated across vertebrates, with an apparent evolutionary trend to simplification, especially in the number of stimulatory factors governing substantially GH release. Thus, teleosts exhibit a multifactorial regulation of GH secretion, with a number of factors, from the newly discovered fish GH-releasing hormone (GHRH) to pituitary adenylate cyclase-activating peptide (PACAP) but also gonadotropin-releasing hormone, dopamine, corticotropin-releasing hormone, and somatostatin(s) directly controlling somatotropes. In amphibians and reptiles, GH secretion is primarily stimulated by the major hypothalamic peptides GHRH and PACAP and inhibited by somatostatin(s), while other factors (ghrelin, thyrotropin-releasing hormone) also influence GH release. Finally, in birds and mammals, primary control of GH secretion is exerted by a dual interplay between GHRH and somatostatin. In addition, somatotrope function is modulated by additional hypothalamic and peripheral factors (e.g., ghrelin, leptin, insulin-like growth factor-I), which together enable a balanced integration of feedback signals related to processes in which GH plays a relevant regulatory role, such as metabolic and energy status, reproductive, and immune function. Interestingly, in contrast to the high number of stimulatory factors impinging upon somatotropes, somatostatin(s) stand(s) as the main primary inhibitory regulator(s) for this cell type.

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.

Distribution of somatostatin immunoreactive neurons and fibres in the central nervous system of a chondrostean, the Siberian sturgeon (Acipenser baeri)

Somatostatin (SOM) is a neuropeptide that is widely distributed in the central nervous system of vertebrates. Two isoforms of somatostatin (SS1 and SS2) have been characterized in sturgeon and in situ hybridisation studies in the sturgeon brain have demonstrated that mRNAs of the two somatostatin precursors (PSS1 and PSS2) are differentially expressed in neurons [Trabucchi, M., Tostivint, H., Lihrmann, I., Sollars, C., Vallarino, M., Dores, R.M., Vaudry, H., 2002. Polygenic expression of somatostatin in the sturgeon Acipenser transmontanus: molecular cloning and distribution of the mRNAs encoding two somatostatin precursors. J. Comp. Neurol. 443, 332-345.]. However, neither the morphology of somatostatinergic neurons nor the patterns of innervation have yet been characterized. To gain further insight into the evolution of this system in primitive bony fishes, we studied the distribution of somatostatin-immunoreactive (SOM-ir) cells and fibres in the brain of the Siberian sturgeon (Acipenser baeri). Most SOM-ir cells were found in the preoptic area and hypothalamus and abundant SOM-ir fibres coursed along the hypothalamic floor towards the median eminence, suggesting a hypophysiotrophic role for SOM in sturgeon. In addition, SOM-ir cells and fibres were observed in extrahypothalamic regions such as the telencephalon thalamus, rhombencephalon and spinal cord, which also suggests neuromodulatory and/or neurotransmitter functions for this peptide. Overall there was a good correlation between the distribution of SOM-ir neurons throughout the brain of A. baeri and that of PSS1 mRNA in Acipenser transmontanus. Comparative analysis of the results with those obtained in other groups of fishes and tetrapods indicates that widespread distribution of this peptide in the brain is shared by early vertebrate lines and that the general organization of the somatostatinergic systems has been well-conserved during evolution.

Distribution of somatostatin-like immunoreactivity in the brain of the caecilian Dermophis mexicanus (Amphibia: Gymnophiona): comparative aspects in amphibians

The organization of the somatostatin-like-immunoreactive (SOM-ir) structures in the brain of anuran and urodele amphibians has been well documented, and significant differences were noted between the two amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study, we analyzed the anatomical distribution of SOM-ir cells and fibers in the brain of the gymnophionan Dermophis mexicanus. In addition, because of its known relationship with catecholamines in other vertebrates, double immunostaining for SOM and tyrosine hydroxylase was used to investigate this situation in the gymnophionan. Abundant SOM-ir cell bodies and fibers were widely distributed throughout the brain. In the telencephalon, pallial and subpallial cells were labeled, being most numerous in the medial pallium and amygdaloid region. Most of the SOM-ir neurons were found in the preoptic area and hypothalamus and showed a clear projection to the median eminence. Less conspicuously, SOM-ir structures were found in the thalamus, tectum, tegmentum, and reticular formation. Both SOM-ir cells and fibers were demonstrated in the spinal cord. The double-immunohistofluorescence technique revealed that catecholaminergic neurons and SOM-ir cells are largely intermingled in many brain regions but form totally separated populations. Many differences were found between the distribution of SOM-ir structures in Dermophis and that in anurans or urodeles. Some features were shared only with anurans, such as the abundant pallial SOM-ir cells, whereas others were common only to urodeles, such as the organization of the hypothalamohypophysial SOM-ir system. In addition, some characteristics were found only in Dermophis, such as the localization of the SOM-ir spinal cells and the lack of colocalization of catecholamines and SOM throughout the brain. Therefore, any conclusions concerning the SOM system in amphibians are incomplete without considering evidence for gymnophionans.

Neuroendocrine control of growth hormone in fish

The biological actions of growth hormone (GH) are pleiotropic, including growth promotion, energy mobilization, gonadal development, appetite, and social behavior. Accordingly, the regulatory network for GH is complex and includes many endocrine and environmental factors. In fish, the neuroendocrine control of GH is multifactorial with multiple inhibitors and stimulators of pituitary GH secretion. In fish, GH release is under a tonic negative control exerted mainly by somatostatin. Sex steroid hormones and nutritional status influence the level of brain expression and effectiveness of some of these GH neuroendocrine regulatory factors, suggesting that their relative importance differs under different physiological conditions. At the pituitary level, some, if not all, somatotropes can respond to multiple regulators. Therefore, ligand- and function-specificity, as well as the integrative responses to multiple signals must be achieved at the level of signal transduction mechanisms. Results from investigations on a limited number of stimulatory and inhibitory GH-release regulators indicate that activation of different but convergent intracellular pathways and the utilization of specific intracellular Ca(2+) stores are some of the strategies utilized. However, more work remains to be done in order to better understand the integrative mechanisms of signal transduction at the somatotrope level and the relevance of various GH regulators in different physiological circumstances.

Comparative genomics provides evidence for close evolutionary relationships between the urotensin II and somatostatin gene families

Although urotensin II (UII) and somatostatin 1 (SS1) exhibit some structural similarities, their precursors do not show any appreciable sequence identity and, thus, it is widely accepted that the UII and SS1 genes do not derive from a common ancestral gene. The recent characterization of novel isoforms of these two peptides, namely urotensin II-related peptide (URP) and somatostatin 2 (SS2)/cortistatin (CST), provides new opportunity to revisit the phylogenetic relationships of UII and SS1 using a comparative genomics approach. In the present study, by radiation hybrid mapping and in silico sequence analysis, we have determined the chromosomal localization of the genes encoding UII- and somatostatin-related peptides in several vertebrate species, including human, chicken, and zebrafish. In most of the species investigated, the UII and URP genes are closely linked to the SS2/CST and SS1 genes, respectively. We also found that the UII-SS2/CST locus and the URP/SS1 locus are paralogous. Taken together, these data indicate that the UII and URP genes, on the one hand, and the SS1 and SS2/CST genes, on the other hand, arose through a segmental duplication of two ancestral genes that were already physically linked to each other. Our results also suggest that these two genes arose themselves through a tandem duplication of a single ancestral gene. It thus appears that the genes encoding UII- and somatostatin-related peptides belong to the same superfamily.

Ontogeny of the somatostatin variant [Pro2,Met13]somatostatin-14 in the brain, pituitary, and sensory organs of the frogRana esculenta

Two forms of somatostatin are expressed in the frog brain, i.e., somatostatin-14 (SS1) and the [Pro(2), Met(13)]somatostatin-14 variant (SS2). We have previously described the ontogeny of SS1-immunoreactive cells in the brain of Rana esculenta. In the present study, we have investigated the distribution of prepro-SS2 (PSS2)-expressing neurons in the brain of the same species during development by using antibodies directed against the N-flanking region of SS2 (PSS2(54-66)). Immunoreactive perikarya first appeared in the ventral hypothalamus at stages IV-VII. Subsequently, positive neurons were seen in the nucleus of the diagonal band of Broca, the anterior preoptic area, the posterior tuberculum (stages VIII-XII), as well as the dorsal (stages XIII-XV) and medial (stages XIX-XX) periventricular preoptic nucleus. At metamorphic climax and in newly metamorphosed frogs, positive perikarya were found in the striatum and in the interpeduncular nucleus. PSS2(54-66)-immunoreactive fibers were already widely distributed during the first stages of development, indicating that SS2 may act as a neuromodulator and/or neurotransmitter during ontogeny. The presence of PSS2(54-66)-positive nerve fibers in olfactory structures suggests that, in tadpoles, SS2 may be involved in the processing of olfactory information. The occurrence of PSS2(54-66)-like immunoreactivity in taste buds, and in the olfactory and vomeronasal organs indicates that SS2 may mediate the unconditioned and reinforcing properties of natural chemicals. Finally, the intenseexpression of PSS2(54-66)-like immunoreactivity in melanotrope cells of the pituitary suggests that SS2 may diffuse toward the pars distalis to regulate the activity of adenohypophysial cells during tadpole development.

Polygenic expression of somatostatin in orange-spotted grouper (Epinephelus coioides): molecular cloning and distribution of the mRNAs encoding three somatostatin precursors

In the present study, three preprosomatostatin (PSS) cDNAs were characterized from hypothalamus of orange-spotted grouper Epinephelus coioides. The first cDNA encodes a 123-amino acid protein (PSSI) that contains the SS14 sequence at its C-terminal extremity and that is identical to that of PSSI of human and other vertebrates. The second cDNA encodes a 127-amino acid protein (PSSII) that contains the SS28 sequence with [Tyr7, Gly10]-SS14 at its C-terminus. The third cDNA encodes a 110-amino acid protein (PSSIII) that contains the somatostatin variant [Pro2]-SS14 at its C-terminal extremity. All these three PSS mRNAs were expressed in brain and pituitary with different mRNA levels. In peripheral tissues, PSSII was more widely distributed than PSSI and PSSIII. High mRNA levels of PSS were found in stomach, intestine and ovary. PSS mRNAs were detected throughout embryogeny and early larval development. Its levels increased with the embryonic development and maintained a higher level during larva developing. The mRNA distribution suggests that the three grouper PSS products play important physiological functions in adult fish as well as in cell growth and organ differentiation in embryo and larva development.

Brain mapping of three somatostatin encoding genes in the goldfish

In the present study the brain distribution of three somatostatin (SRIF)-encoding genes, PSS-I, PSS-II, and PSS-III, was analyzed by in situ hybridization (ISH) in the goldfish. The PSS-I mRNA showed the widest distribution throughout the brain, whereas PSS-II transcripts were restricted to some hypothalamic nuclei. On the other hand, PSS-III presents an intermediate distribution pattern. All SRIF encoding genes are expressed in hypophysiotropic nuclei supporting the idea that, in addition to SRIF-14, [Pro(2)] SRIF-14, and gSRIF-28 have pituitary-controlling functions. Moreover, each of the genes is expressed in nuclei directly associated with feeding behavior, suggesting a role for SRIF peptides in the central control of food intake and energy balance. Alternatively, they might have a role in processing sensory information related with feeding behavior, since PSS genes are expressed in the main gustatory, olfactory, and visual centers, which project to the hypothalamic feeding center in teleost fish.

Somatostatin in the brain of the cave salamander,Hydromantes genei (Amphibia, Plethodontidae): Immunohistochemical localization and biochemical characterization

The distribution of somatostatin-like immunoreactivity in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using two distinct antisera raised against somatostatin-14. Most somatostatin-positive cells were detected in the ependymal cell layer surrounding the ventricles. These cells possessed the typical morphological characteristics of tanycytes or radial glial cells. Double-labeling with an antiserum against somatostatin and a monoclonal antibody against glial fibrillary acidic protein showed that somatostatin-immunoreactive cells lining the ventricles also exhibited GFAP-like immunoreactivity. Injection of the neurotracer biocytin into the lateral ventricle revealed that neurons lining the ventricles did not contain somatostatin-like immunoreactivity. In the telencephalon, somatostatin-like immunoreactivity was confined to radial glial cells. In the diencephalon, in addition to somatostatin-immunoreactive cells in the ependyma, positive cell bodies were also found in the periventricular preoptic nucleus, the infundibular nucleus, the epiphysis, and the subcommissural organ. In the metencephalon, positive cell bodies were found in the auricula cerebelli, whereas in the rhombencephalon numerous somatostatin-immunoreactive cells were seen lining the ventricular cavity. Immunoreactive nerve fibers were observed in the hypothalamus-median eminence complex. In the pituitary, a discrete group of somatostatin-positive cells was found in the pars distalis. High-performance liquid chromatography analysis of brain extracts revealed that the immunoreactive material coeluted with somatostatin-14. The present results show that the somatostatin peptidergic system in the brain of the cave salamander has a more simple organization than those described in the brain of frog and other vertebrates. This feature is probably related to the expression of high pedomorphic characters in plethodontids. The distribution of somatostatin-like immunoreactivity suggests that, in the cave salamander, somatostatin may act as a neurotransmitter and/or neuromodulator, a central regulator of fluid homeostasis, and a hypophysiotropic neurohormone.

Endogenous hypothalamic somatostatins differentially regulate growth hormone secretion from goldfish pituitary somatotropes in vitro

Using Southern blot analysis of RT-PCR products, mRNA for three different somatostatin (SS) precursors (PSS-I, -II, and -III), which encode for SS(14), goldfish brain (gb)SS(28), and [Pro(2)]SS(14), respectively, were detected in goldfish hypothalamus. PSS-I and -II mRNA, but not PSS-III mRNA, were also detected in cultured pituitary cells. We subsequently examined the effects of the mature peptides, SS(14), gbSS(28), and [Pro(2)]SS(14), on somatotrope signaling and GH secretion. The gbSS(28) was more potent than either SS(14) or [Pro(2)]SS(14) in reducing basal GH release but was the least effective in reducing basal cellular cAMP. The ability of SS(14), [Pro(2)]SS(14), and gbSS(28) to attenuate GH responses to GnRH were comparable. However, gbSS(28) was less effective than SS(14) and [Pro(2)]SS(14) in diminishing dopamine- and pituitary adenylate cyclase-activating polypeptide-stimulated GH release, as well as GH release resulting from the activation of their underlying signaling cascades. In contrast, the actions of a different 28-amino-acid SS, mammalian SS(28), were more similar to those of SS(14) and [Pro(2)]SS(14). We conclude that, in goldfish, SSs differentially couple to the intracellular cascades regulating GH secretion from pituitary somatotropes. This raises the possibility that such differences may allow for the selective regulation of various aspects of somatotrope function by different SS peptides.

Characterization of the cDNA encoding a somatostatin variant in the chicken brain: comparison of the distribution of the two somatostatin precursor mRNAs

Although the existence of two somatostatin variants (SS1 and SS2) has now been demonstrated in the brain of mammals, amphibians, and fish, only one isoform of somatostatin (SS1) has been characterized to date in the brain of birds. Here we report cloning of the cDNA encoding a 101-amino-acid protein (PSS2) that encompasses the somatostatin variant [Pro(2)]somatostatin-14 (SS2) at its C-terminus. Sequence analysis indicated that chicken PSS2 is more closely related to fish PSS2 than to mammalian cortistatin precursors. Northern blot analysis showed that the chicken PSS1 gene is expressed in the central nervous system (CNS) and in the pancreas, whereas the PSS2 gene is expressed only in the CNS and not in peripheral organs. In situ hybridization histochemistry revealed that, in the chicken brain, PSS1 mRNA is more widely distributed than PSS2 mRNA. In particular, PSS1 mRNA expression was found in the hippocampus, the hyperstriatum, the preoptic area, the ventricular hypothalamic nuclei, the optic tectum, and several nuclei of the mesencephalon and rhombencephalon. In contrast, the distribution of PSS2 mRNA was restricted to a few regions of the brain, including the paraolfactory lobe, the paleostriatum, and some nuclei of the mesencephalon and rhombencephalon. The fact that the PSS1 and PSS2 genes are differently expressed in the brain and in peripheral organs indicates that, in chicken, the two somatostatin variants likely exert distinct functions. In particular, the observation that PSS1 mRNA, but not PSS2 mRNA, occurs in the preoptic area and in the ventral hypothalamic nuclei suggests that, of the two somatostatin isoforms, only SS1 acts as a hypophysiotropic factor.