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 perf ormed by cloning and expressing, in COS-1 cells, the NPR-B receptor su btype from the eel gill which exhibited a strong C-type-natriuretic-pe ptide(CNP)-induced guanylate cyclase activity. Like other mammalian NP R-B receptors, the eel NPR-B receptor consisted of a ligand-binding ex tracellular domain, a hydrophobic transmembrane domain, a kinase-like domain and a guanylate cyclase domain. Sequence comparison among the e el and mammalian receptors revealed a relatively low similarity (appro ximate to 44%) in the extracellular domain compared to a very high sim ilarity (approximate to 84%) in the cytoplasmic regulatory and catalyt ic domains. This low similarity allowed identification of the amino ac id residues or candidate regions important for the ligand-binding acti vity. RNase protection analysis of the eel NPR-B mRNA demonstrated tha t the message was predominantly expressed in the liver and atrium as w ell as in the gill with moderate-to-small amounts in the brain, ventri cle, esophageal sphincter, stomach, posterior intestine and kidney. Th e high NPR-B mRNA levels in the liver, atrium and gill were found to d ecrease markedly when eels were transferred from fresh water to seawat er and kept there for 2 weeks. Since similar changes are known to occu r in the ligand CNP levels when eels are facing osmotic challenges, th e CNP/NPR-B system appears to play an important role in their successf ul adaptation to salinity changes.