Characterization of insulin-related peptides and their relation to molt and metabolism in the shrimp Litopenaeus vannamei.

Insulin-like peptides are key regulators of metabolism, reproduction, and senescence in higher eukaryotic organisms. They have been described in different invertebrates, including nematodes, molluscs, and insects. Insulin-like peptides have also been detected in some decapod crustaceans, where they may display typical insulin effects. On the basis that common biological function may be related to a similar structure, we searched for an insulin-related substance in the shrimp Litopenaeus vannamei and further investigated its function. An IGF-I-like molecule was detected in neurosecretory cells of the brain and thoracic ganglion in shrimps at different molt stages. Furthermore, we demonstrated that ILP did not differentially stimulate ecdysteroid secretion as it is observed in some insect species. An in vivo assay was performed and the results revealed that injections of heterologous insulin/IGF-I increased the glycogen contents of gills and digestive gland from intermolt animals (fasting state), suggesting that an endogenous ILP may display a conserved function in decapods. Moreover, cloning and sequencing of a cDNA from hemocytes revealed that the sequence code for a precursor similar to the Insulin-like Growth Factor binding proteins. A quantitative real-time RT-PCR study revealed the presence of LivIGFBP-related peptide transcript in a variety of tissues, including nervous tissues, gills, and muscle, brain, thoracic ganglion and nerve cord. In addition, the expression of this transcript differs according to the animal’s size, suggesting a possible role in growth regulation.