WNK1 and p38-MAPK distribution in ionocytes and accessory cells of euryhaline teleost fish implies ionoregulatory function

Mummichog gill and operculum exhibit functionally consistent claudin-10 paralog profiles and Claudin-10c hypersaline response

Claudin (Cldn)-10 tight junction (TJ) proteins are hypothesized to form the paracellular Na⁺ secretion pathway of hyposmoregulating mummichog (Fundulus heteroclitus) branchial epithelia. Organ-specific expression profiles showed that only branchial organs [the gill and opercular epithelium (OE)] exhibited abundant cldn-10 paralog transcripts, which typically increased following seawater (SW) to hypersaline (2SW) challenge. Post-translational properties, protein abundance, and ionocyte localization of Cldn-10c, were then examined in gill and OE. Western blot analysis revealed two Cldn-10c immunoreactive bands in the mummichog gill and OE at ∼29 kDa and ∼40 kDa. The heavier protein could be eliminated by glycosidase treatment, demonstrating the novel presence of a glycosylated Cldn-10c. Protein abundance of Cldn-10c increased in gill and OE of 2SW-exposed fish. Cldn-10c localized to the sides of gill and OE ionocyte apical crypts and partially colocalized with cystic fibrosis transmembrane conductance regulator and F-actin, consistent with TJ complex localization. Cldn-10c immunofluorescent intensity increased but localization was unaltered by 2SW conditions. In support of our hypothesis, cldn-10/Cldn-10 TJ protein dynamics in gill and OE of mummichogs and TJ localization are functionally consistent with the creation and maintenance of salinity-responsive, cation-selective pores that facilitate Na⁺ secretion in hyperosmotic environments.

Summary: The role of claudin-10 tight junction proteins in paracellular salt secretion across fish branchial epithelia is indicated by organ-specific responses to hyperosmotic conditions and their association with salt secreting transcellular proteins

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.