Comparative analyses of sequence structure, evolution, and expression of four somatostatin receptors in orange-spotted grouper (Epinephelus coioides)

Molecular identification of somatostatin family in tilapia and the effect of food intake on pss2 and sstr3a

Somatostatin (SS) plays an important role in regulating food intake and following digestive functions of vertebrates mediated via somatostatin receptor (SSTR). However, the feedback regulation by which food intake regulate SS and SSTRs expression, and thereby their functions, remains poorly understood. Here, we cloned somatostatin genes (pss1, pss2 and pss3) and somatostatin receptors genes (sstr2a, sstr2b, sstr2c, sstr3a, sstr3b, sstr5a and sstr5b) from tilapia, Oreochromis niloticus and conducted their tissue distribution. Phylogenetic analysis and genomic synteny maps revealed their high homology across the vertebrates and evolution history. Following, synthesized SS-14 and SS-28 were both confirmed to bind to SSTRs and mediated Erk1/2 or Akt phosphorylation in a dose-dependent manner and time-course effect in 293T cell line as well as tilapia primary hepatocytes. It was observed that the expression levels of SS family showed dynamic change during food intake. In the early stages of feeding, pss2 expressed in gastric epithelium was down-regulated by food intake, and the following low pH after feeding reversed to up-regulate the pss2 mRNA level. This down and up dynamic change hinted that the first down pss2 prompted food digesting and the next up pss2 inhibited digestion, but it might need further explore to prove. Compared with it, sstr3a expression levels in submucosa up-regulated after feeding but did not display dynamic change. Our results contribute to the understanding of how somatostatin family respond to food intake during different stages of feeding, which provides basis for subsequent study of their function in gastrointestine of tilapia.

Transcriptomic Responses and Larval-Stage Growth of Protandrous Yellowfin Seabream (Acanthopagrus Latus) to Different Polyethylene Microplastics Exposure

Polyethylene microplastics (PE-MPs) were widespread in the marine environment; thus, their influences on marine hermaphroditic fish cannot be ignored. This study intends to evaluate the adverse biological effects of two different sources of PE, identified by Raman spectroscopy, on protandrous yellowfin seabream (Acanthopagrus latus) larvae. Growth retardation, brain lesions, head/body length ratio increase, and neuroendocrine system disorders were found, and growth and neuroendocrine regulation-related genes such as sstr2, ghrb, irs1, UGT2B15, UGT2C1, drd4a, esr2b, hsd3b7, and hsd17b2 were identified. PE microbeads (100 μm) showed more severe tissue damage on fish, while environmental PE fibers (500-2500 μm) showed more imperceptible adverse effects. There were 218 DEGs up-regulated and 147 DEGs down-regulated in the environmental PE group, while 1284 (up) and 1267 (down) DEGs were identified in the virgin PE group. PE-MP stress influenced physiological processes like growth and neuroendocrine regulation and cholesterol-steroid metabolism, and caused tissue damage in the fish larvae. The study highlights the effects of environmental PE exposure on hermaphroditic protandrous fish.

Loss of function in somatostatin receptor 5 has no impact on the growth of medaka fish due to compensation by the other paralogs

Somatic growth in vertebrates is regulated endocrinologically by the somatotropic axis, headed by the growth hormone (GH) and the insulin growth factor-I (IGF-I). Somatostatin (Sst), a peptide hormone synthesized in the hypothalamus, modulates GH actions through its receptors (Sstr). Four Sstr subtypes (Sstr 1-3 and 5) have been identified in teleosts. However, little is known about whether they have a specific function or tissue expression. The aim of this study was to determine the role of sstr2 and sstr5 in the growth of the medaka (Oryzias latipes). The assessed expression pattern across diverse tissues highlighted greater prevalence of sstr1 and sstr3 in brain, intestine and muscle than in pituitary or liver. The expression of sstr2 was high in all the tissues tested, while sstr5 was predominantly expressed in the pituitary gland. A CRISPR/Cas9 sstr5 mutant with loss of function (sstr5-/-) was produced. Assessment of sstr5-/- indicated no significant difference with the wild type regarding growth parameters such as standard length, body depth, or peduncle depth. Furthermore, the functional loss of sstr5 had no impact on the response to a nutritional challenge. The fact that several sstr subtypes were upregulated in different tissues in sstr5-/- medaka suggests that in the mutant fish, there may be a compensatory effect on the different tissues, predominantly by sstr1 in the liver, brain and pituitary, with sstr2 being upregulated in pituitary and liver, and sstr3 only presenting differential expression in the brain. Analysis of the sstr subtype and the sstr5-/- fish showed that sstr5 was not the only somatostatin receptor responsible for Sst-mediated Gh regulation.

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.

Hypothalamic- and pituitary-derived growth and reproductive hormones and the control of energy balance in fish

Most endocrine systems in the body are influenced by the hypothalamic-pituitary axis. Within this axis, the hypothalamus delivers precise signals to the pituitary gland, which in turn releases hormones that directly affect target tissues including the liver, thyroid gland, adrenal glands and gonads. This action modulates the release of additional hormones from the sites of action, regulating key physiological processes, including growth, metabolism, stress and reproduction. Pituitary hormones are released by five distinct hormone-producing cell types: somatotropes (which produce growth hormone), thyrotropes (thyrotropin), corticotropes (adrenocorticotropin), lactotropes (prolactin) and gonadotropes (follicle stimulating hormone and luteinizing hormone), each modulated by specific hypothalamic signals. This careful and distinct organization of the hypothalamo-pituitary axis has been classically associated with the existence of many lineal axes (e.g., the hypothalamic-pituitary-gonadal axis) in charge of the control of the different physiological processes. While this traditional concept is valid, it is becoming apparent that hormones produced by the hypothalamo-pituitary axis have diverse effects. For instance, gonadotropin-releasing hormone II has been associated with a suppressive effect on food intake in fish. Likewise, growth hormone has been shown to influence appetite, swimming activity and aggressive behavior in fish. This review will focus on the hypothalamic and pituitary hormones classically involved in regulating growth and reproduction, and will attempt to provide a general overview of the current knowledge on their actions on energy balance and appetite in fish. It will also give a brief perspective of the role of some of these peptides in integrating feeding, metabolism, growth and reproduction.

Molecular identification, tissue distribution and functional analysis of somatostatin receptors (SSTRs) in red-spotted grouper (Epinephelus akaara)

In the present study, four full-length cDNAs of somatostatin receptor (sstr) were cloned from the forebrain and pituitary of red-spotted grouper. The four full-length cDNAs were designated 2292, 1522, 1873 and 1789 bp and identified as sstr1, sstr2, sstr3, and sstr5 by BLAST analysis; the corresponding sizes of the open reading frames (ORFs) were 1155, 1113, 1467 and 1503 bp, which encoding 384, 370, 488 and 500 aa, respectively. The four receptors have seven transmembrane structures and contain the YANSCANPI/VLY sequence, which is the conserved amino acid sequence of the SSTR family. A tissue distribution study showed that the four sstrs had different expression patterns, suggesting that they may play different roles in regulating different physiological processes. The four receptors mediate ERK1/2 phosphorylation by SS-14 in HEK293 cells, and SS-14 promotes ATK and ERK1/2 phosphorylation in primary hepatocytes of red-spotted grouper. These results facilitate the study of SSTRs-mediated intracellular signaling pathways.

Structural and functional analysis of two novel somatostatin receptors identified from topmouth culter (Erythroculter ilishaeformis)

In the present study, we cloned and characterized two somatostatin (SS) receptors (SSTRs) from topmouth culter (Erythroculter ilishaeformis) designated as EISSTR6 and EISSTR7. Analysis of EISSTR6 and EISSTR7 signature motifs, 3D structures, and homology with the known members of the SSTR family indicated that the novel receptors had high similarity to the SSTRs of other vertebrates. EISSTR6 and EISSTR7 mRNA expression was detected in 17 topmouth culter tissues, and the highest level was observed in the pituitary. Luciferase reporter assay revealed that SS14 significantly inhibited forskolin-stimulated pCRE-luc promoter activity in HEK293 cells transiently expressing EISSTR6 and EISSTR7, indicating that the receptors can be activated by SS14. We also identified phosphorylation sites important for the functional activity of EISSTR6 and EISSTR7 by mutating Ser^{23, 43, 107, 196, 311} and Ser^{7, 29, 61, 222, 225} residues, respectively, to Ala, which significantly reduced the inhibitory effects of SS14 on the CRE promoter mediated by EISSTR6 and EISSTR7. Furthermore, treatment of juvenile topmouth culters with microcystin-LR or 17β-estradiol significantly affected EISSTR6 and EISSTR7 transcription in the brain, liver and spleen, suggesting that these receptors may be involved in the pathogenic mechanisms induced by endocrine disruptors. Our findings should contribute to the understanding of the structure-function relationship and evolution of the SSTR family.

In vitro effects of somatostatin on the growth hormone-insulin-like growth factor axis in orange-spotted grouper (Epinephelus coioides)

Growth in vertebrates is mainly mediated by the growth hormone (GH)-insulin-like growth factor (IGF) axis, and somatostatin (SRIF) inhibits growth by decreasing GH release at the pituitary level and antagonizing the release and action of GHRH in the hypothalamus. However, the effects of SRIF on the regulation of growth at levels other than GH release from the pituitary gland are less well known. In the present study, we comprehensively examined the pituitary and peripheral actions of SRIF on the GH-IGF axis in grouper using a primary pituitary and hepatocyte cell culture system. Our results showed that SRIF inhibited GH release at the pituitary level, but had no influence on GH mRNA expression. Basal hepatic GH receptor 1 (GHR1), IGF-I and IGF-II mRNA levels declined over time, whereas GHR2 mRNA levels remained stable throughout the culture period. GH stimulated the hepatic expression of GHR and IGF mRNAs in a dose-dependent manner, while SRIF suppressed both basal and GH-stimulated expression of GHR and IGF mRNAs in primary cultured hepatocytes. The inhibition of GHR and IGF mRNA levels by SRIF was not attributed to the rate of mRNA degradation. To the best of our knowledge, we demonstrated the effects of SRIF on basal and GH-stimulated IGF-II mRNA levels in teleosts for the first time. These results indicate that SRIF regulates growth at the level of the pituitary and peripheral liver.

Somatostatin receptor 1-5; expression profiles during rat development

BACKGROUND

Somatostatin acts through five receptor subtypes (SSTRs 1-5). We aimed to investigate SSTRs mRNA expression and protein distribution in whole rat embryos, with special emphasis on the pancreas.

MATERIAL AND METHODS

Rat embryos were collected on embryonal days 10, 11, 12, 14, 15, 17, 19, 21, and at birth. Presence of SSTRs was investigated with RT-PCR techniques and immunohistochemistry.

RESULTS

There was no SSTR5 mRNA expression in the whole rat embryos. All SSTR1-5 proteins were observed at embryonal day 10, but the localization varied between the different subtypes. From day 11 to birth SSTRs protein presence increased with time in major structures such as skin and cartilage. It remained similar over time in the heart and liver. In the fetal pancreas mRNA expression of SSTR2 and 4 was detected at day 14, and there was an increase up to birth. Only SSTR1 protein co-localized to a higher extent with the islet hormones studied. SSTR2 was present in all islet endocrine cells except for β-cells. In contrast, the immunostaining for SSTR3-4 was co-localized with insulin and PP, and, finally, SSTR5 with glucagon and pancreatic polypeptide. In mRNA isolated from whole rat embryos SSTR1-2 and SSTR4 expression showed a peak at day 14, while SSTR3 mRNA was not present until day 15.

CONCLUSION

The present data suggest a role for SSTRs during the development of the rat embryo. Subsequent functional studies may elucidate regulatory roles of specific SSTRs for the growth and differentiation of the pancreas as well as other organs.

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.

Differential involvement of signaling pathways in the regulation of growth hormone release by somatostatin and growth hormone-releasing hormone in orange-spotted grouper (Epinephelus coioides)

Somatostatin is the most effective inhibitor of GH release, and GHRH was recently identified as one of the primary GH-releasing factors in teleosts. In this study, we analyzed the possible intracellular transduction pathways that are involved in the mechanisms induced by SRIF and GHRH to regulate GH release. Using a pharmacological approach, the blockade of the PLC/IP/PKC pathway reversed the SRIF-induced inhibition of GH release but did not affect the GHRH-induced stimulation of GH release. Furthermore, SRIF reduced the GH release induced by two PKC activators. Inhibitors of the AC/cAMP/PKA pathway reversed both the SRIF- and GHRH-induced effects on GH release. Moreover, the GH release evoked by forskolin and 8-Br-cAMP were completely abolished by SRIF. The blockade of the NOS/NO pathway attenuated the GHRH-induced GH release but had minimal effects on the inhibitory actions of SRIF. In addition, inhibitors of the sGC/cGMP pathway did not modify the SRIF- or GHRH-induced regulation of GH release. Taken together, these findings indicate that the SRIF-induced inhibition of GH release is mediated by both the PLC/IP/PKC and the AC/cAMP/PKA pathways and not by the NOS/NO/sGC/cGMP pathway. In contrast, the GHRH-induced stimulation of GH secretion is mediated by both the AC/cAMP/PKA and the NOS/NO pathways and is independent of the sGC/cGMP pathway and the PLC/IP/PKC system.

UII and UT in grouper: cloning and effects on the transcription of hormones related to growth control

Urotensin II (UII) is a cyclic peptide that was originally extracted from the caudal neurosecretory system (CNSS) of fish. UII is well known to exhibit cardiovascular, ventilatory, and motor effects in vertebrates. Studies have reported that UII exerts mitogenic effects and can act as an autocrine/paracrine growth factor in mammals. However, similar information in fish is limited. In this study, the full-length cDNAs of UII and its receptor (UT) were cloned and characterized in the orange-spotted grouper. UII and UT were expressed ubiquitously in various tissues in grouper, and particularly high levels were observed in the CNSS, CNS, and ovary. A functional study showed that UT was coupled with intracellular Ca2+ mobilization in HEK293 cells. Studies carried out using i.p. injections of UII in grouper showed the following: i) in the hypothalamus, UII can significantly stimulate the mRNA expression of ghrh and simultaneously inhibit the mRNA expression of somatostatin 1 (ss1) and ss2 3 h after injection; ii) in the pituitary, UII also significantly induced the mRNA expression of gh 6 and 12  h after injection; and iii) in the liver, the mRNA expression levels of ghr1/ghr2 and igf1/igf2 were markedly increased 12 and 3  h after the i.p. injection of UII respectively. These results collectively indicate that the UII/UT system may play a role in the promotion of the growth of the orange-spotted grouper.

Genes involved in fatty acid metabolism: molecular characterization and hypothalamic mRNA response to energy status and neuropeptide Y treatment in the orange-spotted grouper Epinephelus coioides

As in mammals, fatty acid (FA) metabolism plays diverse and vital roles in regulating food intake in fish. Multiple lines of evidence suggest that the effect of FA metabolism on food intake is linked to changes in the level of neuropeptide Y (NPY) in the hypothalamus of the rainbow trout. In mammals, the evidence suggests that FA metabolism regulates feeding via hypothalamic NPY. NPY is therefore considered an important factor that mediates the modulation of food intake by FA metabolism in vertebrates. The stimulatory effect of NPY on food intake is well known. However, to the best of our knowledge, the effect of NPY on FA metabolism in the hypothalamus has not been examined. In this study, we cloned the cDNA of four key enzymes involved in FA metabolism and assessed the effect of energy status and NPY on their mRNA expression in the hypothalamus of grouper. The full-length cDNAs of UCP2 and CPT1a and the partial coding sequence (CDS) of ACC1 and FAS were isolated from the grouper hypothalamus. These genes are expressed in the hypothalamus and during the organogenetic stage of embryogenesis. A feeding rhythm study showed that the hypothalamic expression level of NPY and CPT1a was highly correlated with feeding rhythm. Long-term fasting was found to significantly induce the hypothalamic mRNA expression of NPY, CPT1a and UCP2. An in vitro study demonstrated that NPY strongly stimulated CPT1a and UCP2 mRNA expression in a time- and dose-dependent manner. Collectively, these results suggest that these four genes related to FA metabolism may play a role in regulating food intake in grouper and, that NPY modulates FA metabolism in the grouper hypothalamus. This study showed, for the first time in vertebrates, the effect of NPY on the gene expression of FA metabolism-related enzymes.

Ontogeny of mRNA expression of somatostatin and its receptors in chicken embryos in association with methylation status of their promoters

The present study was designed to investigate the ontogeny and tissue distribution of somatostatin and its five receptor subtypes (SSTR1-5) mRNA expression in embryonic chicken (Gallus gallus). Brain, gonads (male), intestine, kidney, liver, muscle, stomach and yolk sac membrane (YSM) of chicken embryos on the embryonic (E) ages of 10, 16 and 21days (right before hatch) were investigated. Bisulfite sequencing PCR (BSP) was performed to determine the methylation status of the promoter region of all the six genes in the liver. Somatostatin (SST) was predominately expressed in intestine, brain and gonads (male) with different ontogenic patterns. The highest expression in intestine was detected at E10. There was ontogenic shift from intestine to brain as development progressed. Expression pattern of SSTRs in brain, intestine and kidney was similar to human embryonic expression. In liver, the ontogenic expression pattern of SST and its receptors was associated to methylation status of the respective promoters. Methylation of site Sp1 determines expression level of SST, SSTR1, SSTR2 and SSTR3 while site a is important in governing the expression of SSTR4 and SSTR5. The results show that ontogenic expression profile of chicken SST and SSTRs is time and tissue specific.

The evolution of vertebrate somatostatin receptors and their gene regions involves extensive chromosomal rearrangements

Background

Somatostatin and its related neuroendocrine peptides have a wide variety of physiological functions that are mediated by five somatostatin receptors with gene names SSTR1-5 in mammals. To resolve their evolution in vertebrates we have investigated the SSTR genes and a large number of adjacent gene families by phylogeny and conserved synteny analyses in a broad range of vertebrate species.

Results

We find that the SSTRs form two families that belong to distinct paralogons. We observe not only chromosomal similarities reflecting the paralogy relationships between the SSTR-bearing chromosome regions, but also extensive rearrangements between these regions in teleost fish genomes, including fusions and translocations followed by reshuffling through intrachromosomal rearrangements. These events obscure the paralogy relationships but are still tractable thanks to the many genomes now available. We have identified a previously unrecognized SSTR subtype, SSTR6, previously misidentified as either SSTR1 or SSTR4.

Conclusions

Two ancestral SSTR-bearing chromosome regions were duplicated in the two basal vertebrate tetraploidizations (2R). One of these ancestral SSTR genes generated SSTR2, -3 and -5, the other gave rise to SSTR1, -4 and -6. Subsequently SSTR6 was lost in tetrapods and SSTR4 in teleosts. Our study shows that extensive chromosomal rearrangements have taken place between related chromosome regions in teleosts, but that these events can be resolved by investigating several distantly related species.

Isolation, characterization, and distribution of somatostatin receptor subtype 2 (SSTR 2) mRNA in rainbow trout (Oncorhynchus mykiss), and regulation of its expression by glucose

In this study, cDNA for a somatostatin receptor variant (somatostatin receptor subtype 2, SSTR 2) was isolated, cloned, and sequenced from rainbow trout. A 1821-nt cDNA was isolated and found to contain a single initiation site 387-nt from the most 5' end, an open reading frame of 1116-nt, and a single putative polyadenylation site 189-nt from the most 3' end. The encoded protein contains 372 amino acids and contains seven membrane-spanning domains. Based on structural analysis, the protein was identified as a subtype 2 SSTR. These data support the emergence of a multigenic SSTR family early in the course of vertebrate evolution, concomitant with or perhaps prior to the divergence of boney fish. The distribution of SSTR 2 mRNA in tissues was determined by quantitative real time-PCR (QRT-PCR). SSTR 2 was most abundant in the brain (where it was detected in the telencephalon, optic tectum, and hypothalamus), skeletal muscle, and liver, but it also was present in the endocrine pancreas (Brockmann body) and various regions of the gastrointestinal tract (esophagus, stomach, intestine). SSTR 2 mRNA was most abundant in the brain, muscle, and liver. In vitro the Brockmann body and liver with increasing concentrations of glucose (1, 4, 10mM) resulted in increased expression of SSTR 2 mRNA. These findings contribute to the understanding of the evolution of the SSTR family and provide insight into the roles of SSTR 2 in fish.