Involvement of calcitonin and its receptor in the control of calcium regulating-genes and calcium homeostasis in zebrafish (Danio rerio)

Calcitonin (CT) is one of the hormones involved in the vertebrate calcium regulation. It was proposed to act as a hypocalcemic factor, but the regulatory pathways remain to be clarified. We investigated the CT/c alcitonin gene-related peptide (CGRP) family in zebrafish and its potential involvement in the calcium homeostasis. We identified the presence of four receptors: CTR, CRLR1, CRLR2 and CRLR3. From the phylogenetic analysis, together with the effect observed after CT and CGRP overexpression, we concluded that CTR appears to be a CT receptor and CRLR1 a CGRP receptor. The distribution of these two receptors shows a major presence in the central nervous system and in tissues involved in ionoregulation. Zebrafish embryos kept in high Ca(2+) concentration medium, presented an up regulation of CT and CTR expression and a down regulation of epithelial calcium channel (ECaC). Embryos injected with CT morpholino (CALC MO) incubated in high Ca(2+) medium, showed down regulation of CTR while an up regulation on ECaC mRNA expression. On the other hand, over expression of CT cRNA induced the down regulation of ECaC mRNA synthesis, concomitant to the down regulation in the calcium content after 30 hpf. At 4 dpf, CT cRNA injection induced an up regulation of hypercalcemic factors, with subsequent increase in the calcium content . These results suggest that CT acts as a hypocalcemic factor in calcium regulation, probably through an inhibition of ECaC synthesis.