Characterization of zebrafish (Danio rerio) NCX4: a novel NCX with distinct electrophysiological properties

Members of the [Na.sup.2+]/[Ca.sup.2+] exchanger (NCX) family are important regulators of cytosolic [Ca.sup.2+] in myriad tissues and are highly conserved across a wide range of species. Three distinct NCX genes and numerous splice variants exist in mammals, many of which have been characterized in a variety of heterologous expression systems. Recently, however, we discovered a fourth NCX gene (NCX4), which is found exclusively in teleost, amphibian, and reptilian genomes. Zebrafish (Danio rerio) NCX4a encodes for a protein of 939 amino acids and shows a high degree of identity with known NCXs. Although knockdown of NCX4a activity in zebrafish embryos has been shown to alter left-right patterning, it has not been demonstrated that NCX4a functions as a NCX. In this study, we 1) demonstrated, for the first time, that this gene encodes for a novel NCX; 2) characterized the tissue distribution of zebrafish NCX4a; and 3) evaluated its kinetic and transport properties. While ubiquitously expressed, the highest levels of NCX4a expression occurred in the brain and eyes. NCX4a exhibits modest levels of [Na.sup.+]-dependent inactivation and requires much higher levels of regulatory [Ca.sup.2+] to activate outward exchange currents. NCX4a also exhibited extremely fast recovery from [Na.sup.+]-dependent inactivation of outward currents, faster than any previously characterized wild-type exchanger. While this result suggests that the [Na.sup.+]-dependent inactive state of NCX4a is far less stable than in other NCX family members, this exchanger was still strongly inhibited by 2 [micro]M exchanger inhibitory peptide. We demonstrated that a new putative member of the NCX gene family, NCX4a, encodes for a NCX with unique functional properties. These data will be useful in understanding the role that NCX4a plays in embryological development as well as in the adult, where it is expressed ubiquitously. calcium homeostatis; [Na.sup.+]/[Ca.sup.2+] exchanger; development