The LvCHH-ITP gene of the shrimp ( Litopenaeus vannamei) produces a widely expressed putative ion transport peptide (LvITP) for osmo-regulation

Abstract

The LvCHH-ITP gene of Litopenaeus vannamei consists of 4 exons and 3 introns spanning 3.8 kb in size. The primary gene transcript produced a cDNA encoded for the putative ion transport pxeptide (LvITP) from exons 1, 2, 3, 4 and an alternative spliced transcript encoded for a putative CHH-like peptide from exons 1, 2 and 4. The mature LvITP is identical to the eyestalk CHH-like peptides at the N-terminal end but it only shows low amino acid sequence identity at the C-terminal end. In inter-molt shrimp, LvITP transcripts level is low in the eyestalk but is high in other non-eyestalk nervous tissues (i.e. brain, thoracic ganglion and ventral nerve cord), the epidermis, gill and gut. The transcript levels increase gradually towards the pre-molt and reach the maximum at the post-molt (stages A and B). In the gill and nerve cord, LvITP transcript level is low in the post-molt and early inter-molt but is high at the late inter-molt and pre-molt. Because of its high degree of amino acid sequence homology to the insect ion transport peptide, we studied the expression of LvITP in the gills of shrimp. The results indicated that a much higher level of LvITP transcript in the posterior gills which suggests its potential role in osmo-regulation. LvITP transcript level in the gill decreased when shrimp were exposed to both salinity extremes. Injection of 20-hydroxyecdysone (20-HE) that stimulate the pre-molt condition could cause a significant increase in LvITP transcript level in 48 h. Injection of dsRNA for LvITP, on the contrary, caused a significant reduction in LvITP transcript level and shrimp showed a serious hemorrhage of the posterior gill in a dose-dependent manner. Shrimp injected with high concentration of dsRNA for LvITP died within 24 h. In conclusion, this study provides molecular evidence to support the osmo-regulatory function of LvITP.