Cloning, characterization, and functional expression of a CNP receptor regulating CFTR in the shark rectal gland

In the shark, C-type natriuretic peptide (CNP) is the onlycardiac natriuretic hormone identified and is a potent activator ofCl secretion in the rectalgland, an epithelial organ of this species that contains cysticfibrosis transmembrane conductance regulator (CFTR) Clchannels. We have cloned an ancestral CNP receptor (NPR-B) from theshark rectal gland that has an overall amino acid identity to the humanhomologue of 67%. The shark sequence maintains six extracellular Cyspresent in other NPR-B but lacks a glycosylation site and a Glu residuepreviously considered important for CNP binding. When shark NPR-B andhuman CFTR were coexpressed in Xenopus oocytes, CNP increased the cGMP content of oocytes(EC₅₀ 12 nM) and activated CFTRCl channels(EC₅₀ 8 nM). Oocyte cGMP increased36-fold (from 0.11 ± 0.03 to 4.03 ± 0.45 pmol/oocyte) andCl current increased37-fold (from 34 ± 14 to 1,226 ± 151 nA) in thepresence of 50 nM CNP. These findings identify the specific natriureticpeptide receptor responsible forCl secretion in the sharkrectal gland and provide the first evidence for activation of CFTRCl channels by a clonedNPR-B receptor.