Salinity-dependent expression of ncc2 in opercular epithelium and gill of mummichog ( Fundulus heteroclitus )

Mummichogs ( Fundulus heteroclitus ) can tolerate abrupt changes in environmental salinity because of their ability to rapidly adjust the activities of ionocytes in branchial and opercular epithelia. In turn, the concerted expression of sub-cellular effectors of ion transport underlies adaptive responses to fluctuating salinities. Exposure to seawater (SW) stimulates the expression of Na + /K + /2Cl − cotransporter 1 ( nkcc1 ) and cystic fibrosis transmembrane regulator ( cftr ) mRNAs in support of ion extrusion by SW-type ionocytes. Given the incomplete understanding of how freshwater (FW)-type ionocytes actually operate in mummichogs, the transcriptional responses essential for ion absorption in FW environments remain unresolved. In a subset of species, a ‘fish-specific’ Na + /Cl − cotransporter denoted Ncc2 (Slc12a10) is responsible for the uptake of Na + and Cl − across the apical surface of FW-type ionocytes. In the current study, we identified an ncc2 transcript that is highly expressed in gill filaments and opercular epithelium of FW-acclimated mummichogs. Within 1 day of transfer from SW to FW, ncc2 levels in both tissues increased in parallel with reductions in nkcc1 and cftr . Conversely, mummichogs transferred from FW to SW exhibited marked reductions in ncc2 concurrent with increases in nkcc1 and cftr . Immunohistochemical analyses employing a homologous antibody revealed apical Ncc2-immunoreactivity in Na + /K + -ATPase-immunoreactive ionocytes of FW-acclimated animals. Our combined observations suggest that Ncc2/ ncc2 -expressing ionocytes support the capacity of mummichogs to inhabit FW environments.