Structural and Functional Evolution of Three Cardiac Natriuretic Peptides

Natriuretic peptides (NPs) are a group of hormones playing important roles in cardiovascular and osmoregulatory systemsin vertebrates. Among the NP subtypes, atrial NP (ANP), B-type NP (BNP), and ventricular NP (VNP) are circulatinghormones expressed exclusively in the heart (cardiac NPs). The constitution of cardiac NPs is variable among species ofvertebrates. In order to understand the evolutionary and functional significance of such variation, we performed a systematicsurvey of cardiac NP cDNAs in nine taxonomically diverse teleosts inhabiting environments of varying salinity. Thediscovery of the coexistence of the ANP, BNP, and VNP genes in the eel and rainbow trout suggested that the ancestralteleost had all three cardiac NPs. As the VNP cDNA was undetectable in ayu and six species of Neoteleostei, it is possiblethat VNP was lost before the divergence of Osmeroidei. The ANP gene was also undetectable in the medaka. Thus, only theBNP gene is universal in species examined in the present study. Synthetic medaka BNP preferentially activated two medakaGC-A–type receptors, suggesting that the three cardiac NPs share the same receptor. However, the regulation of BNPexpression may be the most strict because ATTTA repeats in the 3#-untranslated region and the dibasic motif in the ring areconserved among teleosts and tetrapods. Linkage analyses in the rainbow trout located ANP, BNP, and VNP genes on thesame chromosome, which suggested the generation of the VNP gene by tandem duplication as observed with ANP andBNP genes. If the duplication occurred before the divergence of tetrapods and teleosts, VNP may exist in the tetrapodlineage