Salinity and prolactin regulate anoctamin 1 in the model teleost, Fundulus heteroclitus

To maintain internal ion balance in marine environments, teleost fishes leverage seawater (SW)-type ionocytes to actively secrete Na⁺ and Cl^- into the environment. It is well established that SW-type ionocytes use apically expressed cystic fibrosis transmembrane conductance regulator 1 (Cftr1) as a conduit for Cl^- to exit the gill. Here, we investigated whether the Ca²⁺-activated Cl^- channel, anoctamin 1 (Ano1), provides an additional path for Cl^--secretion in euryhaline mummichogs (Fundulus heteroclitus). Two ano1 gene isoforms, denoted ano1.1a and -1.1b, exhibited higher expression in the gill and opercular epithelium of mummichogs long-term acclimated to SW versus fresh water (FW). Branchial ano1.1b and cftr1 expression was increased in mummichogs sampled 24 h after transfer from FW to SW; ano1.1a and -1.1b were upregulated in the gill and opercular epithelium following transfer from SW to hypersaline SW. Alternatively, the expression of ano1.1a, -1.1b, and cftr1 in the gill and opercular epithelium was markedly decreased after transfer from SW to FW. Given its role in attenuating ion secretion, we probed whether prolactin downregulates ano1 isoforms. In addition to attenuating cftr1 expression, a prolactin injection reduced branchial ano1.1a and -1.1b levels. Given how Ano1 mediates Cl^- secretion by mammalian epithelial cells, the salinity- and prolactin-sensitive nature of ano1 expression reported here indicates that Ano1 may constitute a novel Cl^--secretion pathway in ionocytes. This study encourages a wider evaluation of this putative Cl^--secretion pathway and its regulation by hormones in teleost fishes.NEW & NOTEWORTHY In this study, we provide evidence in a teleost fish that the Ca²⁺-activated Cl^- channel, anoctamin 1 may provide an additional path for Cl^- secretion by seawater-type ionocytes. Not only is this the first report of a Cftr-independent Cl^--secreting pathway conferring survival in seawater but also the first description of its regulation by the pituitary hormone prolactin.