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.
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We are grateful to Kirsten Tomlinson for collecting animals included in this report. We appreciate the excellent fish care provided by Kristin Salisbury, Aaron Cordiale, and Tracy Broderson. Jennifer Bonner and Eleanore Ritter provided valuable assistance with immunohistochemistry and confocal imaging.
This study was supported by the National Science Foundation (IOS-1755131 to J.P.B and IOS-1251616 to C.K.T.), Arkansas Biosciences Institute (C.K.T) and Skidmore College (Start-Up Funds to J.P.B.).
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Communicated by B. Pelster.
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Breves, J.P., Starling, J.A., Popovski, C.M. et al. Salinity-dependent expression of ncc2 in opercular epithelium and gill of mummichog (Fundulus heteroclitus). J Comp Physiol B 190, 219–230 (2020). https://doi.org/10.1007/s00360-020-01260-x
- Na+/Cl− cotransporter
- Atlantic killifish
- Opercular epithelium