Cloning and expression of eel natriuretic‐peptide receptor B and comparison with its mammalian counterparts

A comparative study of the natriuretic‐peptide receptor NPR‐B was performed by cloning and expressing, in COS‐1 cells, the NPR‐B receptor subtype from the eel gill which exhibited a strong C‐type‐natriuretic‐peptide(CNP)‐induced guanylate cyclase activity. Like other mammalian NPR‐B receptors, the eel NPR‐B receptor consisted of a ligand‐binding extracellular domain, a hydrophobic transmembrane domain, a kinase‐like domain and a guanylate cyclase domain. Sequence comparison among the eel and mammalian receptors revealed a relatively low similarity (∼44%) in the extracellular domain compared to a very high similarity (∼ 84 %) in the cytoplasmic regulatory and catalytic domains. This low similarity allowed identification of the amino acid residues or candidate regions important for the ligand‐binding activity. RNase protection analysis of the eel NPR‐B mRNA demonstrated that the message was predominantly expressed in the liver and atrium as well as in the gill with moderate‐to‐small amounts in the brain, ventricle, esophageal sphincter, stomach, posterior intestine and kidney. The high NPR‐B mRNA levels in the liver, atrium and gill were found to decrease markedly when eels were transferred from fresh water to seawater and kept there for 2 weeks. Since similar changes are known to occur in the ligand CNP levels when eels are facing osmotic challenges, the CNP/NPR‐B system appears to play an important role in their successful adaptation to salinity changes.