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

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Abstract
In the shark, C-type natriuretic peptide (CNP) is the only cardiac natriuretic hormone identified and is a potent activator of Cl−secretion in the rectal gland, an epithelial organ of this species that contains cystic fibrosis transmembrane conductance regulator (CFTR) Cl−channels. We have cloned an ancestral CNP receptor (NPR-B) from the shark rectal gland that has an overall amino acid identity to the human homologue of 67%. The shark sequence maintains six extracellular Cys present in other NPR-B but lacks a glycosylation site and a Glu residue previously considered important for CNP binding. When shark NPR-B and human CFTR were coexpressed in Xenopusoocytes, CNP increased the cGMP content of oocytes (EC5012 nM) and activated CFTR Cl−channels (EC508 nM). Oocyte cGMP increased 36-fold (from 0.11 ± 0.03 to 4.03 ± 0.45 pmol/oocyte) and Cl−current increased 37-fold (from −34 ± 14 to −1,226 ± 151 nA) in the presence of 50 nM CNP. These findings identify the specific natriuretic peptide receptor responsible for Cl−secretion in the shark rectal gland and provide the first evidence for activation of CFTR Cl−channels by a cloned NPR-B receptor.

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