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 vertebrate calcium regulation. It has been proposed to act as a hypocalcemic factor, but the regulatory pathways remain to be clarified. We investigated the CT/calcitonin gene-related peptide (CGRP) family in zebrafish and its potential involvement in 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-Ca²⁺-concentration medium showed upregulation of CT and CTR expression and downregulation of the epithelial calcium channel (ECaC). Embryos injected with CT morpholino (CALC MO) incubated in high-Ca²⁺ medium, showed downregulation of CTR together with upregulation on ECaC mRNA expression. In contrast, overexpression of CT cRNA induced the downregulation of ECaC mRNA synthesis, concomitant with the downregulation in the calcium content after 30 hours postfertilization. At 4 days postfertilization, CT cRNA injection induced upregulation 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 inhibition of ECaC synthesis.