Gene duplication of C-type natriuretic peptide-4 (CNP4) in teleost lineage elicits subfunctionalization of ancestral CNP

The diversified natriuretic peptide (NP) family, consisting of four CNPs (CNP1-4), ANP, BNP, and VNP, has been identified in the eel. Here, we successfully cloned additional cnp genes from the brain of eel (a basal teleost) and zebrafish (a later branching teleost). The genes were identified as paralogues of cnp4 generated by the third round of whole genome duplication (3R) in the teleost lineage, thereby being named eel cnp4b and zebrafish cnp4-like, respectively. To examine the histological patterns of their expressions, we employed a newly developed in situ hybridization (ISH) chain reaction using short hairpin DNAs, in addition to conventional ISH. Eel cnp4b was expressed in the medulla oblongata, while mRNAs of eel cnp4a (former cnp4) were localized in the preoptic area. In the zebrafish brain, cnp4-like mRNA was undetectable, while the known cnp4 was expressed in both the preoptic area and medulla oblongata. Together with the different mRNA distribution of cnp4a and cnp4b in eel peripheral tissues determined by RT-PCR and ISH, it is suggested that subfunctionalization by duplicated cnp4s in ancestral teleosts has been retained only in basal teleosts. Intriguingly, cnp4b-expressing neurons in the glossopharyngeal-vagal motor complex of the medulla oblongata were co-localized with choline acetyltransferase, suggesting an involvement of Cnp4b in swallowing and respiration functions that are modulated by the vagus. Since teleost Cnp4 is an ortholog of mammalian CNP, the identified localization of teleost Cnp4 will contribute to future studies aimed at deciphering the physiological functions of CNP.