Abstract
Calcitonin is a vertebrate hormone peptide that has a critical role in calcium homeostasis and ion excretion, but no orthologous peptide has ever been characterized in any invertebrate species. In this study, we originally identified an ascidian calcitonin peptide, Ci-CT, in Ciona intestinalis, an emerging deuterostome invertebrate model. The putative peptide sequence conserved several amino acids essential for functions of vertebrate calcitonins, and mass spectrometry confirmed the presence of the matured form of Ci-CT in the ascidian neural complex. However, the Ci-CT gene, unlike vertebrate calcitonin genes, failed to generate any splicing variants. We also detected and cloned a Ci-CT receptor (Ci-CT-R) candidate homologous to vertebrate calcitonin receptors. Ci-CT was found to activate cAMP production upon binding to Ci-CT-R expressed in HEK 293 cells. RT-PCR demonstrated that the Ci-CT gene is expressed in the neural complex, heart and endostyle, whereas the expression of Ci-CT-R gene was observed in the neural complex, pharynx, and endostyle. In situ hybridization revealed that Ci-CT mRNA is localized exclusively in the neural gland region. These results suggest that the original calcitonin was present in ancestral species of chordates. Our data will contribute to investigation of the molecular and functional evolution of the calcitonin family.
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