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

Molecular and structural analysis of Hdh-MIRP3 and its impact on reproductive regulation in female Pacific abalone, Haliotis discus hannai

Molluscan insulin-related peptides (MIRP) play a crucial role in various biological processes, including reproduction and larval development in mollusk species. To investigate the involvement of MIRP in the ovarian development of Pacific abalone (Haliotis discus hannai), the Hdh-MIRP3 was cloned from cerebral ganglion (CG). Hdh-MIRP3 cDNA was 993 bp long, encoded a 13.22 kDa peptide, comprising 118 amino acids. Fluorescence in situ hybridization confirmed the localization of Hdh-MIRP3 in the CG and ovary. Molecular docking revealed that Hdh-MIRP3 binds to the N-terminal region of Hdh-IRP-R. Tissue expression analysis showed the highest Hdh-MIRP3 expression in the CG, followed by ovarian tissue. Hdh-MIRP3 expression was significantly upregulated in the CG and ovary during the ripe stage of seasonal ovarian development and in effective accumulative temperature conditioned abalone. Furthermore, siRNA silencing of Hdh-MIRP3 significantly downregulated the expression of four reproduction-related genes, including Hdh-GnRH, Hdh-GnRH-R, Hdh-IRP-R, and Hdh-VTG in both the CG and ovary, and Hdh-MIRP3 as well. These results indicate that Hdh-MIRP3 acts as a regulator of ovarian development in Pacific abalone. Additionally, expression analysis indicated that Hdh-MIRP3 plays a role in embryonic and larval development. Overall, the present findings elucidate the role of Hdh-MIRP3 in reproductive development in female Pacific abalone.

Characterization of Insulin-like Peptide (ILP) and Its Potential Role in Ovarian Development of the Cuttlefish Sepiella japonica

The insulin-like peptide (ILP) family is well known for regulating reproduction in invertebrates, while its role in mollusks remains largely unknown. In this study, we first isolated and characterized the ILP gene in the cuttlefish Sepiella japonica. The full-length SjILP cDNA obtained was 926 bp and encoded a precursor protein of 161 amino acids. The precursor protein consisted of a signal peptide, a B chain, a C-peptide, and an A chain. It possessed the typical features of ILP proteins, including two cleavage sites (KR) and eight conserved cysteines. To define the function of SjILP, the expression of SjILP in different tissues and ovarian development stages were analyzed using qRT-PCR. SjILP was mainly expressed in the ovary, and its gene expression correlated with ovarian development. Furthermore, silencing SjILP using RNA interference (RNAi) dramatically decreased the expression levels of four ovarian-development-related genes (vitellogenin1, vitellogenin2, cathepsin L1-like, and follistatin). These data suggest the critical role of SjILP in the regulation of ovarian development in S. japonica.

Crosstalk between dopamine and insulin signaling in growth control of the oyster

Neuroendocrine hormones such as dopamine and insulin/insulin-like peptides play indispensable roles in growth regulation of animals, while the interplay between dopamine and insulin signaling pathways remains largely unknown in invertebrates. In the present study, we showed that tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis, was highly expressed in all tissues of the fast-growing oysters, and gradually increased with the development, which indicated the potential role of dopamine in growth regulation. Incubated with dopamine hydrochloride and insulin-like peptide recombinant proteins in vitro induced the expression of TH, suggesting a mutual regulatory relationship between insulin and dopamine signaling. Fasting and re-feeding experiments confirmed the role of TH in food intake regulation, also provide a clue about the potential regulatory relationship between the FoxO and TH. Further luciferase assay experiment confirmed that FoxO was involved in transcriptional regulation of TH gene through binding to its specific promoter region. This work provided insights into the crosstalk between dopamine and insulin signaling in growth control of mollusks.

Identification, characterization, and expression profiles of insulin-like peptides suggest their critical roles in growth regulation of the Pacific oyster, Crassostrea gigas

The insulin/insulin-like growth factor signaling (IIS) pathway is well-known in regulation of cell growth and proliferation in vertebrates, while its role in invertebrates such as mollusks remains largely unknown. In this study, we performed an extensive multi-omics data mining and identified four insulin-like peptide genes, including ILP, MIRP3, MIRP3-like and ILP7, in the Pacific oyster, Crassostrea gigas. Their potential roles in growth regulation were further investigated using the selectively bred fast-growing C. gigas variety "Haida No.1". Expression profiling and in situ hybridization of these insulin-like peptides suggested their distinct tissue-specific expression pattern, with dominant expression in the neural enrichment tissues such as labial palp, visceral ganglia, adductor muscle, and digestive gland. The expressions of insulin-like peptides were significantly altered by food abundance in a gene-specific fashion. The expression of ILP was reduced during fasting and increased after re-feeding, the expressions of MIRP3 and ILP7 were generally induced during fasting and down-regulated after re-feeding, while the expression of MIRP3-like was firstly up-regulated and then down-regulated during the fasting and re-feeding process. Furthermore, the expressions of all four insulin-like peptide genes were significantly suppressed at low temperature, in accordance with the growth inhibition. These results indicated that all four insulin-like peptides would play critical but different roles in regulation of growth in the oysters. This work provides valuable information for further investigation on growth regulation mechanism in mollusks and molecular assisted breeding of growth with other production traits in the Pacific oyster.

Comparative Transcriptome Analysis Reveals Molecular Basis Underlying Fast Growth of the Selectively Bred Pacific Oyster, Crassostrea gigas

Fast growth is one of the most desired traits for all food animals, which affects the profitability of animal production. The Pacific oyster, Crassostrea gigas, is an important aquaculture shellfish around the world with the largest annual production. Growth of the Pacific oyster has been greatly improved by artificial selection breeding, but molecular mechanisms underlying growth remains poorly understood, which limited the molecular integrative breeding of fast growth with other superior traits. In this study, comparative transcriptome analyses between the fast-growing selectively bred Pacific oyster and unselected wild Pacific oysters were conducted by RNA-Seq. A total of 1,303 protein-coding genes differentially expressed between fast-growing oysters and wild controls were identified, of which 888 genes were expressed at higher levels in the fast-growing oysters. Functional analysis of the differentially expressed genes (DEGs) indicated that genes involved in microtubule motor activity and biosynthesis of nucleotides and proteins are potentially important for growth in the oyster. Positive selection analysis of genes at the transcriptome level showed that a significant number of ribosomal protein genes had undergone positive selection during the artificial selection breeding process. These results also indicated the importance of protein biosynthesis and metabolism for the growth of oysters. The alternative splicing (AS) of genes was also compared between the two groups of oysters. A total of 3,230 differential alternative splicing events (DAS) were identified, involved in 1,818 genes. These DAS genes were associated with specific functional pathways related to growth, such as “long-term potentiation,” “salivary secretion,” and “phosphatidylinositol signaling system.” The findings of this study will be valuable resources for future investigation to unravel molecular mechanisms underlying growth regulation in the oyster and other marine invertebrates and to provide solid support for breeding application to integrate fast growth with other superior traits in the Pacific oyster.

Inhibition of growth in juvenile manila clam Ruditapes philippinarum: Potential adverse outcome pathway of TBBPA

Tetrabromobisphenol A (TBBPA) is ubiquitous and its contents showing an increasing trend in the coastal environment. In order to investigate the effects of TBBPA on marine bivalves, juvenile manila clams Ruditapes phillipinarum were exposed to TBBPA for 28 days. The results showed that shell growth rate of juvenile clams after exposure to 62.5-1000 μg L^-1 TBBPA for 28 d were significantly inhibited (p < 0.05). Then in order to link the changes in filtration rate, mRNA expression of insulin-like growth factor homologue (IGF) and tissue thyroid hormone (TH) contents to growth, juvenile clams were exposed to 62.5 and 500 μg L^-1 TBBPA for 14 days. The transcriptional levels of neuroendocrine signals (NPF and insulin homologue) associated with filter feeding regulation, and genes of TH synthesis-related enzymes were also examined. The results showed that filtration rates was significantly reduced to 44.1% and 14% of controls after 14 d of exposure. In parallel, exposure to TBBPA significantly increased the expression levels of insulin which may elicit the filter feeding inhibition. TBBPA exposure caused alterations in tissue content of THs and mRNA expression of TH synthesis-related enzymes. However, the data showed increased T₃ content, T₃/T₄ ratio and mRNA expression of IGF. These data demonstrated that the most important key event of TBBPA could be linked to growth impairment in juveniles was the reduction of filtration rate. These results provide a robust framework towards revealing the underlying mechanism of the growth inhibition caused by TBBPA on bivalves and understanding the adverse outcome pathway across taxonomic phyla.

The scallop IGF2 mRNA-binding protein gene PyIMP and association of a synonymous mutation with growth traits

Insulin-like growth factor 2 mRNA-binding proteins (IMPs) function in localization, stability and translational control of their target RNAs. In this study, we identified an IMP gene (PyIMP) from Yesso scallop, Patinopecten yessoensis. The complete DNA sequence of PyIMP was 22,875 bp, consisting of seventeen exons and sixteen introns. The full-length cDNA sequence was 3,293 bp, with an open reading frame of 1,776 bp, encoding 592 amino acids. PyIMP exhibited characters typical of IMPs, namely two RNA recognition motifs and four hnRNP K homology domains. Real-time quantitative reverse transcription PCR analysis indicated that PyIMP was universally expressed, with higher expression levels in the gonad of adult scallops, and in gastrulae and trochophore larvae at developmental stages. A synonymous mutation SNP, c.852A>G, which showed significant associations with growth traits of Yesso scallop, was identified in this gene. Scallops with the AA genotype at this locus had significantly higher trait values than those with the GG genotype for shell length, shell height, body weight, soft tissue weight and striated muscle weight (P < 0.05). Meanwhile, the expression of PyIMP in AA type scallops was significantly higher than that in the GG type, implying a positive effect of PyIMP on scallop growth. PyIMP represents the first mRNA-binding protein gene characterized in mollusks, and SNP c.852A>G will be useful for a better understanding of the role of mRNA-binding proteins in bivalves and for scallop breeding.

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

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

A scallop IGF binding protein gene: molecular characterization and association of variants with growth traits

Background

Scallops represent economically important aquaculture shellfish. The identification of genes and genetic variants related to scallop growth could benefit high-yielding scallop breeding. The insulin-like growth factor (IGF) system is essential for growth and development, with IGF binding proteins (IGFBPs) serving as the major regulators of IGF actions. Although an effect of IGF on growth was detected in bivalve, IGFBP has not been reported, and members of the IGF system have not been characterized in scallop.

Results

We cloned and characterized an IGFBP (PyIGFBP) gene from the aquaculture bivalve species, Yesso scallop (Patinopecten yessoensis, Jay, 1857). Its full-length cDNA sequence was 1,445 bp, with an open reading frame of 378 bp, encoding 125 amino acids, and its genomic sequence was 10,193 bp, consisting of three exons and two introns. The amino acid sequence exhibited the characteristics of IGFBPs, including multiple cysteine residues and relatively conserved motifs in the N-terminal and C-terminal domains. Expression analysis indicated that PyIGFBP was expressed in all the tissues and developmental stages examined, with a significantly higher level in the mantle than in other tissues and a significantly higher level in gastrulae and trochophore larvae than in other stages. Furthermore, three single nucleotide polymorphisms (SNPs) were identified in this gene. SNP c.1054A>G was significantly associated with both shell and soft body traits in two populations, with the highest trait values in GG type scallops and lowest in AG type ones.

Conclusion

We cloned and characterized an IGFBP gene in a bivalve, and this report also represents the first characterizing an IGF system gene in scallops. A SNP associated with scallop growth for both the shell and soft body was identified in this gene. In addition to providing a candidate marker for scallop breeding, our results also suggest the role of PyIGFBP in scallop growth.

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

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

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

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

Immunocytochemical demonstration of glucagon-like peptides in Mytilus edulis cerebral ganglia and an in vitro effect of vertebrate glucagon on glycogen metabolism

Immunological detection of glucagon-like peptides was performed in the cerebral ganglia of the mussel Mytilus edulis using an anti-vertebrate glucagon antibody. Two clusters of positive neurosecretory cells were observed, as well as stained nervous fibers. The effect of vertebrate glucagon on glucose incorporation into glycogen of reserve cells was tested using an in vitro microplate bioassay. Optimal incubation conditions were previously defined and an inhibitory effect of porcine glucagon was obtained for concentrations ranging from 10(-6) to 10(-9)M. It is postulated that the glucagon-like peptide may be implicated in the regulation of glucose metabolism in bivalves.

Protein synthesis in vitro by mantle tissue of the land snail Otala lactea: possible insulin-like peptide function

Mantle-collar tissue from adult land snails Otala lactea continuously incorporated labelled amino acids over a 72-h period of incubation in modified culture medium. Acid-saline extract of cerebral ganglia stimulated protein synthesis by the mantle-collar tissue in vitro. This effect was dose-dependent, with the minimum and maximum doses at 0.5 and 2 cerebral ganglion equivalents, respectively. The protein synthesis-stimulating factor(s) from the cerebral ganglia appeared to be proteinaceous and hydrophobic in nature. The cerebral ganglion extract was fractionated by means of a size-exclusion HPLC column. The biological activity was induced by three fractions with estimated molecular masses of 0.82, 1.88, and 4.33 kilodaltons (kDa). Porcine insulin antiserum abolished the activity of the 4.33- and 1.88-kDa fractions but had no significant effect on the activity of the 0.82-kDa fraction. The results suggest the existence in the cerebral ganglia of more than one factor with protein synthesis-stimulating activity. One of these factors could be related to mammalian insulin. Porcine insulin, however, had no significant effect on protein synthesis by the mantle collar in vitro.

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

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

Effects of five natural gonadotropin-releasing hormones on cell suspensions of marine bivalve gonad: stimulation of gonial DNA synthesis

Gonadotropin-releasing hormones (GnRH) constitute a family of neuropeptides which are important regulators of reproduction in vertebrates. The effect of mammalian GnRH (mGnRH), salmon GnRH, chicken GnRH-I, chicken GnRH-II, and lamprey GnRH-I on [3H]thymidine incorporation into DNA of dissociated gonadal cells of marine bivalves has been studied. The incorporation of [3H]thymidine is linear between 1.5 and 8 h of incubation. All five GnRHs significantly increased DNA synthesis in gonial cells of Crassostrea gigas. The maximal activation was about of 135-140% above control. The activation is dose dependent, over the range 10(-11) to 10(-6) M, but is modulated by the physiological condition of the cells and the stage of sexual maturity of the gonad. mGnRH has also a mitogenic effect in dissociated mantle cells of Mytilus edulis. The effect of mGnRH is blocked by a GnRH antagonist ([D-pGlu1,D-Phe2, D-Trp3,6]GnRH, 5 x 10(-6)M) in C. gigas as well as in M. edulis, suggesting that the GnRH action in the gonad is mediated by specific receptors for GnRH or GnRH-like peptides. The existence of GnRH-immunoreactive neurons and fibers in the cerebral and pedal ganglia of M. edulis was demonstrated by immunocytochemistry. They are located principally in the anterior internal area of the cerebral ganglia, close to the cerebral commissure and in the posterior part of the pedal ganglia. The presence of GnRH-responsive cells and GnRH-like immunoreactive material suggests that peptides of the GnRH-like family are present and functional in bivalve molluscs.