C-type natriuretic peptide regulates the molecular components of the rainbow trout gill epithelium tight junction complex

Abstract

Freshwater (FW) fish experience passive paracellular loss of ions into the surrounding environment across water-exposed epithelia such as the gill. The mitigation of paracellular ion loss is thought to be regulated by proteins of the tight junction (TJ) complex and in particular, the large superfamily of claudin (cldn) TJ proteins plays an important role. Transcript and protein levels of TJ proteins in teleosts are known to be under endocrine control of several important osmoregulatory hormones and the current study was aimed at determining whether the osmoregulatory hormone, C-type natriuretic peptide (CNP), can alter paracellular permeability and TJ protein abundance in a primary cultured gill epithelium derived from rainbow trout. Natriuretic peptide receptors were detected in the cultured trout gill epithelium. It was found that (i) developing cultured gill epithelia "grown" in the presence of 10 nM CNP, and (ii) mature cultured gill epithelia exposed to 10 nM CNP for 48 h, exhibited augmented barrier properties. This occurred in association with reduced flux rates of a paracellular permeability marker (polyethylene glycol, molecular mass 400; PEG-400) and, reduced ion efflux (i.e. ion loss) when preparations were exposed to apical FW. Exposure to CNP altered mRNA abundance of cldn-3a, -5a, -6, - 8c, -20a, -25b, -28a, -32a and cgn, but differences in the transcriptional response were observed between chronic and acute CNP exposure. In contrast, chronic and acute exposure to CNP resulted in reduced cldn-10e/Cldn-10e abundance. Data suggest that CNP may play a role in regulating the molecular physiology of the TJ complex in the fish gill epithelium and contribute to the regulation of salt and water balance by influencing the paracellular permeability properties of this tissue.