Biological functions and Synthesis of ITP

ITP was originally purified and identified based on the stimulatory effect on chloride ion transport measured as changes in short circuit current through the epithelium of the ileum.Its complete primary structure was determined by RT-PCR-based cDNA cloning in the desert locust Schistocerca gregaria.

The cDNA cloning also revealed the existence of a longer isoform of ITP called ITP-like (ITPL). Genes and mRNAs encoding ITP/ITPL and their homologs have been found in diverse arthropod species. 

Structural features 

Both ITP and ITPL share structural characteristics with the crustacean hyperglycemic hormone (CHH). Six Cys residues that form three intramolecular disulfide bonds are located in corresponding positions, and the bonding patterns are inferred to be the same as those of other peptides belonging to the CHH family. 

Synthesis and release 

Gene, mRNA, and precursor The ITP gene of the tobacco hornworm Manduca sexta consists of three exons, from which mRNAs for ITPL (exons 1 and 3) and ITP (exons 1, 2, and 3) are produced by alternative splicing. The MasITP mRNA has 909 bases that encode a putative signal peptide of 24 aa, an ITP precursor-related peptide (IPRP) of 11 aa, a dibasic processing site, a mature hormone of 72 aa, and an amidation/cleavage site of 3 aa. The MasITPL precursor encoded by 1030 bases of mRNA has a similar structure to the MasITP precursor—a signal peptide, an IPRP, and a dibasic processing site—but the amidation/cleavage site is absent. 

In Drosophila melanogaster, the ITP gene generates several mRNA splice forms that correspond to three molecular isoforms: DrmITP from ITP-RE transcript, DrmITPL1 from ITP-RC transcript, and DrmITPL2 from ITP-RD transcript.6 A complex alternative splicing mechanism could be involved in the production of the mRNA forms.

Biological functions

ITP facilitates chloride ion transport from the lumen to the hemolymph and inhibits acid secretion into the lumen in the ileum, leading to the generation of the electrochemical gradient driving water resorption.As ITPL inhibits the stimulatory effect on ion transport exerted by ITP, it is proposed that ITPL released from peripheral neurosecretory cells may act as a competitive inhibitor of ITP. In Drosophila, ITP (ITP-RE) expression is elevated under osmotic and desiccation stresses, and genetic manipulations of ITP lead to changes in body water content, survival in the above stress conditions, palatability to water and food, and water excretion. 

These suggest the importance of this hormone as a central neuroendocrine factor integrating water homeostasis. Moreover, ITP and ITPL are thought to be involved in various biological processes. In T. castaneum, the knockdown of ITP/ITPL- 1/ITPL-2 using double-stranded RNAs in the larval stages causes an increase of mortality in subsequent developmental stages, finally attaining almost 100% around the time of eclosion. Knockdown during the pupal stage results in a significant reduction in the number of eggs produced and in the survival rate of offspring, accompanied by developmental defects of the ovary.ITP and/or ITPL possibly contribute to the control of ecdysis.Additionally, ITP and ITPL are each supposed to function as a neurotransmitter/neuromodulator as well as a hormone circulating in the hemolymph. For instance, it is suggested that ITP is one of the neurotransmitters released from the clock neurons of the Drosophila brain.