Claudin-31 contributes to corticosteroid-induced alterations in the barrier properties of the gill epithelium

•Knockdown of claudin-31 (cldn-31 KD) compromises gill epithelium integrity.•Cldn-31 is a cortisol-responsive barrier protein in the gill epithelium.•cldn-31 KD limits cortisol-induced gill epithelium tightening.•Select cldn isoforms compensate for cldn-31 KD in the presence of cortisol.•cldn-31 KD abolishes a cortisol-stimulated increase in gill Cldn-8d.

The contribution of Claudin-31 (Cldn-31) to corticosteroid-induced tightening of the trout gill epithelium was examined using a primary cultured model preparation. Cldn-31 is a ∼23 kDa protein that localizes to the periphery of gill epithelial cells and diffusely in select gill cells that are Na+-K+-ATPase-immunoreactive. Transcriptional knockdown (KD) of cldn-31 reduced Cldn-31 abundance and increased epithelium permeability. Under simulated in vivo conditions (apical freshwater), cldn-31 KD increased net ion flux rates (≡ efflux). Cortisol treatment increased Cldn-31 abundance and decreased epithelium permeability. This tightening effect was diminished, but not eliminated, by cldn-31 KD, most likely due to other cortisol-sensitive TJ proteins that were transcriptionally unperturbed or enhanced in cortisol-treated cldn-31 KD preparations. However, cldn-31 KD abolished a cortisol-induced increase in Cldn-8d abundance, which may contribute to compromised cldn-31 KD epithelium permeability. Data suggest an important barrier function for Cldn-31 and an integral role for Cldn-31 in corticosteroid-induced gill epithelium tightening.