isoform expression and cation selectivity change with salinity in salt-secreting epithelia of .

To provide insight into claudin (Cldn) tight junction (TJ) protein contributions to branchial salt secretion in marine teleost fishes, this study examined TJ protein isoforms of a euryhaline teleost (mummichog; ) in association with salinity change and measurements of transepithelial cation selectivity. Mummichogs were transferred from freshwater (FW) to seawater (SW, 35‰) and from SW to hypersaline SW (2SW, 60‰) in a time course with transfer control groups (FW to FW, and SW to SW). FW to SW transfer increased mRNA abundance of and twofold, whilst and transcripts were unchanged. Transfer from SW to 2SW did not alter , and transiently altered abundance, but increased and fourfold. This was coincident with an increased number of single-stranded junctions (observed by transmission electron microscopy). For both salinity transfers, (1) mRNA was acutely responsive (i.e. after 24 h), (2) other responsive isoforms increased later (3-7 days), and (3) cystic fibrosis transmembrane conductance regulator () mRNA was elevated in accordance with established changes in transcellular Cl movement. Changes in mRNA encoding and appeared linked, consistent with the tandem repeat locus in the genome, whereas mRNA for tandem and seemed independent of each other. Cation selectivity sequence measured by voltage and conductance responses to artificial SW revealed Eisenman sequence VII: Na>K>Rb∼Cs>Li Collectively, these data support the idea that Cldn-10 TJ proteins create and maintain cation-selective pore junctions in salt-secreting tissues of teleost fishes.