Salinity and prolactin regulate <i>anoctamin 1</i> in the model teleost, <i>Fundulus heteroclitus</i>

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 (<i>Fundulus heteroclitus</i>). Two <i>ano1</i> gene isoforms, denoted <i>ano1.1a</i> and <i>-1.1b</i>, exhibited higher expression in the gill and opercular epithelium of mummichogs long-term acclimated to SW versus fresh water (FW). Branchial <i>ano1.1b</i> and <i>cftr1</i> expression was increased in mummichogs sampled 24 h after transfer from FW to SW; <i>ano1.1a</i> and <i>-1.1b</i> were upregulated in the gill and opercular epithelium following transfer from SW to hypersaline SW. Alternatively, the expression of <i>ano1.1a</i>, <i>-1.1b</i>, and <i>cftr1</i> 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 <i>ano1</i> isoforms. In addition to attenuating <i>cftr1</i> expression, a prolactin injection reduced branchial <i>ano1.1a</i> and <i>-1.1b</i> levels. Given how Ano1 mediates Cl^- secretion by mammalian epithelial cells, the salinity- and prolactin-sensitive nature of <i>ano1</i> 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.<b>NEW & NOTEWORTHY</b> 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.