A novel function of calcitonin gene-related peptide in body fluid Cl− homeostasis

Vertebrates need to maintain extracellular chloride (Cl−) concentrations to ensure the normal operation of physiological processes; the transition from aquatic to terrestrial environments necessitated the development of sophisticated mechanisms to ensure Cl− homeostasis in the face of fluctuating Cl− levels. Zebrafish calcitonin gene-related peptide (CGRP), unlike its splice variant calcitonin, does not respond to environmental Ca2+ levels. This study aimed to test the hypothesis that CGRP is involved in the control of body fluid Cl− homeostasis. Acclimation to high-Cl− artificial water stimulated the mRNA expression of cgrp and the receptor (crlr1) when compared with low-Cl−. CGRP knockdown induced upregulation of the Na+-Cl− co-transporter (ncc2b), while overexpression of CGRP resulted in the downregulation of ncc2b mRNA synthesis and a simultaneous decrease in Cl− uptake in embryos. Consistent with these findings, knockdown of either cgrp or crlr1 was found to increase the density of NCC2b-expressing cells in embryos. This is the first demonstration that CGRP acts as a hypochloremic hormone through suppressing NCC2b expression and the differentiation of NCC-expressing ionocytes. Elucidation of this novel function of CGRP in fish body fluid Cl− homeostasis promises to enhance our understanding of the related physiology in vertebrates.