In vitro insulin stimulatory effect on glucose uptake and glycogen synthesis in the gills of the estuarine crab Chasmagnathus granulata

An insulin-like peptide serves as a regulator of glucose metabolism in the immune response of Chinese mitten crab Eriocheir sinensis

A robust immune response against invading pathogens greatly depends on the balance of metabolism, which could be vigorously modulated by insulin/IGF signaling (IIS) pathway in vertebrates. However, knowledge on the IIS pathway, especially the function of insulin-like peptides (ILPs) in invertebrates remained largely unknown. In the present study, a novel ILP was identified from Eriocheir sinensisis (designated EsILP). The coding sequence of EsILP was of 216 bp, which encoded a polypeptide of 71 amino acids containing an IlGF-like domain with four conserved cysteine residues. The mRNA transcripts of EsILP were found to be expressed dominantly in eyestalks and hepatopancreas, and EsILP protein was found to be distributed in the anterior median area of thoracic ganglion mass and the edges of hepatic tubules correspondingly. After Aeromonas hydrophila stimulation, EsILP transcripts were significantly increased at 3, 12 and 24 h post-stimulation in eyestalks and 6 and 48 h in hemocytes, respectively. In contrast, the expression level of EsILP decreased significantly in hepatopancreas from 6 h to 12 h after the stimulation. The glucose level in the hemolymph of crabs was significantly decreased from 6 to 12 h after the injection of recombinant EsILP. These results collectively demonstrated that the ancient ILP protein in E. sinensisis could negatively regulate glucose metabolism and participate in the immune response of the crabs against pathogen infection, which provided clues for the further investigation about the evolution and function of the IIS pathway in invertebrates.

Metabolic effects of epinephrine on the crab Neohelice granulata

Although widely known for their involvement in the control of carbohydrate and lipid metabolism of vertebrates, the participation of catecholamines (CAs) in the metabolism of invertebrates is less understood. This study was designed to identify the physiological role of Epinephrine (E) in the intermediary metabolism of the burrowing crab Neohelice granulata and how E regulates the metabolism in crabs fed with a high-carbohydrate (HC) or a high-protein (HP) diet. To answer these questions, we evaluated in vivo the effects of E injections on glucose and triglycerides in the hemolymph and tissue glycogen levels of crabs fed with HC or HP diet. An in vitro investigation was carried out to assess the direct effects of E on glycogenolysis, lipolysis and glycolysis pathways in the hepatopancreas, mandibular muscle and anterior and posterior gills of this crab. E injections increased glucose and did not affect triglycerides levels in the hemolymph of either group of crabs, and E decreased glycogen in the hepatopancreas and mandibular muscle only in HP crabs, suggesting that these effects may be mediated by the crustacean hyperglycemic hormone (CHH). When the tissues were incubated with different concentrations of E, the concentration of glucose released to the medium decreased in the hepatopancreas and posterior gills, while glucose oxidation increased in the posterior gills of HP crabs. Incubation with E did not alter any parameter in tissues of HC crabs. These effects suggest that E may be involved in the metabolic response to osmotic stress.

Insulin-like receptors and carbohydrate metabolism in gills of the euryhaline crab Neohelice granulata: Effects of osmotic stress

The present study determined the effect of osmotic stress on the insulin-like receptor binding characteristics and on glucose metabolism in the anterior (AG) and posterior (PG) gills of the crab Neohelice granulata. Bovine insulin increased the capacity of the PG cell membrane to phosphorylate exogenous substrate poly (Glu:Tyr 4:1) and the glucose uptake in the control crab group. The crabs were submitted to three periods of hyperosmotic (HR) and hyposmotic (HO) stress, for 24, 72 and 144 h, to investigate the insulin-like receptor phosphorylation capacity of gills. Acclimation to HO for 24 h or HR for 144 h of stress inhibited the effects of insulin in the PG, decreasing the capacity of insulin to phosphorylate exogenous substrate poly (Glu:Tyr 4:1) and decreasing the glucose uptake. Hyperosmotic stress for the same period of 144 h significantly affected ¹²⁵I-insulin binding in the AG and PG. However, HO stress for 24 h significantly reduced ¹²⁵I-insulin-specific uptake only in the PG. Therefore, osmotic stress induces alterations in the gill insulin-like receptors that decrease insulin binding in the PG. These findings indicate that osmotic stress induced a pattern of insulin resistance in the PG. The free-glucose concentration in the PG decreased during acclimation to 144 h of HR stress and 24 h of HO stress. This decrease in the cell free-glucose concentration was not accompanied by a significant change in hemolymph glucose levels. In AG from the control group, neither the capacity of bovine insulin to phosphorylate exogenous substrate poly (Glu:Tyr 4:1) nor the glucose uptake changed; however, genistein decreased tyrosine-kinase activity, confirming that this receptor belongs to the tyrosine-kinase family. Acclimation to HO (24 h) or HR (144 h) stress decreased tyrosine-kinase activity in the AG. This study provided new information on the mechanisms involved in the osmoregulation process in crustaceans, demonstrating for the first time in an estuarine crab that osmotic challenge inhibited insulin-like signaling and the effect of insulin on glucose uptake in the PG.

Insights into insulin-like peptide system in invertebrates from studies on IGF binding domain-containing proteins in the female mud crab, Scylla paramamosain

Insulin-like peptides (ILPs) have been proved to exist extensively in invertebrates and play critical roles in regulating growth, metabolism and reproduction. ILP signaling system has been well defined in insects, with all key components homologous with vertebrate IGF signaling; however, counterparts of IGF binding proteins (IGFBPs) in vertebrates are not included in this system because of lacking sufficient researches in the related aspect. The present study firstly reports the identification of three kinds of invertebrate IGF binding (IB) domain-containing protein genes from the mud crab Scylla paramamosain. Gene expression analysis suggested that they might be closely involved in ovarian development, but with separate roles. Subsequent bioinformatics analysis and in vitro experiments indicated that they are likely to serve as endogenous ILP-specific binding proteins in invertebrates. More importantly, based on the current evidence we inferred that in invertebrate, ILP system might take the place of IGF system in vertebrate species.

Mechanistic target of rapamycin (mTOR) signaling genes in decapod crustaceans: cloning and tissue expression of mTOR, Akt, Rheb, and p70 S6 kinase in the green crab, Carcinus maenas, and blackback land crab, Gecarcinus lateralis

Mechanistic target of rapamycin (mTOR) controls global translation of mRNA into protein by phosphorylating p70 S6 kinase (S6K) and eIF4E-binding protein-1. Akt and Rheb, a GTP-binding protein, regulate mTOR protein kinase activity. Molting in crustaceans is regulated by ecdysteroids synthesized by a pair of molting glands, or Y-organs (YOs), located in the cephalothorax. During premolt, the YOs hypertrophy and increase production of ecdysteroids. Rapamycin (1μM) inhibited ecdysteroid secretion in Carcinus maenas and Gecarcinus lateralis YOs in vitro, indicating that ecdysteroidogenesis requires mTOR-dependent protein synthesis. The effects of molting on the expression of four key mTOR signaling genes (mTOR, Akt, Rheb, and S6K) in the YO was investigated. Partial cDNAs encoding green crab (C. maenas) mTOR (4031bp), Akt (855bp), and S6K (918bp) were obtained from expressed sequence tags. Identity/similarity of the deduced amino acid sequence of the C. maenas cDNAs to human orthologs were 72%/81% for Cm-mTOR, 58%/73% for Cm-Akt, and 77%/88% for Cm-S6K. mTOR, Akt, S6K, and elongation factor 2 (EF2) in C. maenas and blackback land crab (G. lateralis) were expressed in all tissues examined. The two species differed in the effects of molting on gene expression in the YO. In G. lateralis, Gl-mTOR, Gl-Akt, and Gl-EF2 mRNA levels were increased during premolt. By contrast, molting had no effect on the expression of Cm-mTOR, Cm-Akt, Cm-S6K, Cm-Rheb, and Cm-EF2. These data suggest that YO activation during premolt involves up regulation of mTOR signaling genes in G. lateralis, but is not required in C. maenas.

Rheb, an activator of target of rapamycin, in the blackback land crab, Gecarcinus lateralis: cloning and effects of molting and unweighting on expression in skeletal muscle

Molt-induced claw muscle atrophy in decapod crustaceans facilitates exuviation and is coordinated by ecdysteroid hormones. There is a 4-fold reduction in mass accompanied by remodeling of the contractile apparatus, which is associated with an 11-fold increase in myofibrillar protein synthesis by the end of the premolt period. Loss of a walking limb or claw causes a loss of mass in the associated thoracic musculature; this unweighting atrophy occurs in intermolt and is ecdysteroid independent. Myostatin (Mstn) is a negative regulator of muscle growth in mammals; it suppresses protein synthesis, in part, by inhibiting the insulin/metazoan target of rapamycin (mTOR) signaling pathway. Signaling via mTOR activates translation by phosphorylating ribosomal S6 kinase (s6k) and 4E-binding protein 1. Rheb (Ras homolog enriched in brain), a GTP-binding protein, is a key activator of mTOR and is inhibited by Rheb-GTPase-activating protein (GAP). Akt protein kinase inactivates Rheb-GAP, thus slowing Rheb-GTPase activity and maintaining mTOR in the active state. We hypothesized that the large increase in global protein synthesis in claw muscle was due to regulation of mTOR activity by ecdysteroids, caused either directly or indirectly via Mstn. In the blackback land crab, Gecarcinus lateralis, a Mstn-like gene (Gl-Mstn) is downregulated as much as 17-fold in claw muscle during premolt and upregulated 3-fold in unweighted thoracic muscle during intermolt. Gl-Mstn expression in claw muscle is negatively correlated with hemolymph ecdysteroid level. Full-length cDNAs encoding Rheb orthologs from three crustacean species (G. lateralis, Carcinus maenas and Homarus americanus), as well as partial cDNAs encoding Akt (Gl-Akt), mTOR (Gl-mTOR) and s6k (Gl-s6k) from G. lateralis, were cloned. The effects of molting on insulin/mTOR signaling components were quantified in claw closer, weighted thoracic and unweighted thoracic muscles using quantitative polymerase chain reaction. Gl-Rheb mRNA levels increased 3.4-fold and 3.9-fold during premolt in claw muscles from animals induced to molt by eyestalk ablation (ESA) and multiple leg autotomy (MLA), respectively, and mRNA levels were positively correlated with hemolymph ecdysteroids. There was little or no effect of molting on Gl-Rheb expression in weighted thoracic muscle and no correlation of Gl-Rheb mRNA with ecdysteroid titer. There were significant changes in Gl-Akt, Gl-mTOR and Gl-s6k expression with molt stage. These changes were transient and were not correlated with hemolymph ecdysteroids. The two muscles differed in terms of the relationship between Gl-Rheb and Gl-Mstn expression. In thoracic muscle, Gl-Rheb mRNA was positively correlated with Gl-Mstn mRNA in both ESA and MLA animals. By contrast, Gl-Rheb mRNA in claw muscle was negatively correlated with Gl-Mstn mRNA in ESA animals, and no correlation was observed in MLA animals. Unweighting increased Gl-Rheb expression in thoracic muscle at all molt stages; the greatest difference (2.2-fold) was observed in intermolt animals. There was also a 1.3-fold increase in Gl-s6k mRNA level in unweighted thoracic muscle. These data indicate that the mTOR pathway is upregulated in atrophic muscles. Gl-Rheb, in particular, appears to play a role in the molt-induced increase in protein synthesis in the claw muscle.

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.

Effect of insulin/IGF-I like peptides on glucose metabolism in the white shrimp Penaeus vannamei

The insulin-like hormone superfamily encompasses insulin, relaxin, and insulin-like growth factors I (IGF1) and II (IGF2). Insulin hormones regulate cell growth, metabolism, and tissue-specific functions. The presence of insulin has been demonstrated in various invertebrates, and their function as growth promoting or controlling factors has been established in molluscs and insects. In crustaceans, the presence of insulin/insulin-like growth factor (IGF)-like peptides has also been suggested and functional studies have been associated with metabolic control. The general aim of the current study was to elucidate the functional significance of insulin-like peptides in the white shrimp Penaeus vannamei. Because the primary structure of Penaeus insulin is yet unknown, we examined the effect of mammalian insulin/IGF-I on glucose metabolism in P. vannamei. Juvenile shrimps were injected with a single dose of recombinant human (rh) IGF-I or bovine insulin in intermolt stage. Glucose/glycogen levels in shrimp hemolymph and tissues (muscle, hepatopancreas and gills) were determined over a 5h period by means of an enzymatic analysis. We showed that an injection of rhIGF-I induced a significant (P<0.01) increase in glucose levels in hemolymph, 1h after injection and followed by a decrease (P<0.05) 5h post-injection. In the hepatopancreas, an increase (P<0.05) in the glycogen content was observed 3h after insulin treatment. Finally, a significant elevation (P<0.01) of glycogen content in the gills throughout the entire sampling period was detected. Our study suggests the presence of endogenous Penaeus insulin(s) that, just like its vertebrate counterparts, is likely to be involved in the regulation of carbohydrate metabolism in crustaceans.

Insulin and gender: an insulin-like gene expressed exclusively in the androgenic gland of the male crayfish

Members of the insulin family of hormones are generally not regarded as gender-specific, although there is sporadic evidence for the possible involvement of insulin pathways in sexual differentiation. In crustaceans, sexual differentiation is controlled by the androgenic gland (AG), an organ unique to males. To date, attempts to identify active AG factors in decapods through either classical purification methods or sequence similarity with isopod AG hormones have proven unsuccessful. In the present study, the first subtractive cDNA library from a decapod AG was constructed from the red-claw crayfish Cherax quadricarinatus. During library screening, an AG-specific gene, expressed exclusively in males even at early stages of maturation and termed Cq-IAG (C. quadricarinatus insulin-like AG factor), was discovered. In situ hybridization of Cq-IAG confirmed the exclusive localization of its expression to the AG. Following cloning and complete sequencing of the gene, its cDNA was found to contain 1445 nucleotides encoding a deduced translation product of 176 amino acids. The proposed protein sequence encompasses Cys residue and putative cleaved peptide patterns whose linear and 3D organization are similar to those of members of the insulin/insulin-like growth factor/relaxin family and their receptor recognition surface. The identification of Cq-IAG is the first report of a pro-insulin-like gene expressed in a decapod crustacean in a gender-specific manner. Its expression in a male-specific endocrine gland controlling sex differentiation supports the notion that insulin may have evolved in the context of regulating sexual differentiation.

Signaling and function of insulin-like peptides in insects

Insulin-like peptides (ILPs) exist in insects and are encoded by multigene families that are expressed in the brain and other tissues. Upon secretion, these peptides likely serve as hormones, neurotransmitters, and growth factors, but to date, few direct functions have been demonstrated. In Drosophila melanogaster, molecular genetic studies have revealed elements of a conserved insulin signaling pathway, and as in other animal models, it appears to play a key role in metabolism, growth, reproduction, and aging. This review offers (a) an integrated summary of the efforts to characterize the distribution of ILPs in insects and to define this pathway and its functions in Drosophila and (b) a few considerations for future studies of ILP endocrinology in insects.

Effect of hyper or hypo-osmotic conditions on neutral amino acid uptake and oxidation in tissues of the crab Chasmagnathus granulata

We investigated the transport of (14)C-methylaminoisobutyric acid ((14)C-MeAIB) and (14)C-alanine oxidation in hepatopancreas and jaw muscle of Chasmagnathus granulata submitted to 24, 72, and 144 h of hypo- or hyperosmotic stress. While (14)C-MeAIB uptake increased in jaw muscle and hepatopancreas from crabs submitted to hyperosmotic stress, it did not change in tissues from animals submitted to hypo-osmotic stress. Incubation of jaw muscle and hepatopancreas from control groups with 1 mM ouabain did not decrease (14)C-MeAIB uptake. However, ouabain prevented (14)C-MeAIB uptake in hepatopancreas at 24 h of hyperosmotic stress. In contrast, in jaw muscle from crabs submitted to the same conditions, (14)C-MeAIB uptake was not prevented by ouabain in the incubation medium. Jaw muscle from the control group produced four times more (14)CO(2) from (14)C-alanine than the hepatopancreas. During hypo-osmotic stress, amino acid oxidation does not seem to be one of the pathways implicated in the decrease of the amino acid pools in hepatopancreas and jaw muscle. In contrast, during hyperosmotic stress the reduction in (14)C-alanine oxidation appears to be one of the mechanisms involved in the increase of the amino acid pool in the hepatopancreas.

Isolation and biological characterization of a 6-kDa protein from hepatopancreas of lobster Panulirus argus with insulin-like effects

A protein with insulin-like effects was isolated from the hepatopancreas of the lobster Panulirus argus following a classic method for mammalian insulin purification from the pancreas. After acid-alcoholic extraction and ethanol-ether precipitation followed by molecular filtration chromatography, a protein with an apparent molecular weight of 6 kDa was isolated. This protein is characterized by its ability to interact with anti-insulin antibodies and by mimicking insulin actions as the stimulation of glucose oxidation to CO(2) and lipogenesis in isolated rat adipocytes. In addition, this insulin immunoreactive protein (IIP) was able to stimulate the autophosphorylation of the insulin receptor present in rat adipocyte plasma membranes, in a dose-dependent manner. The immunological and biochemical results obtained are consistent with the hypothesis that protein(s) with insulin-like effects occur in the digestive gland of the lobster P. argus and may be of significance to control metabolic and growth related processes in crustaceans.