Cloning of a second form of activin-βA cDNA and regulation of activin-βA subunits and activin type II receptor mRNA expression by gonadotropin in the zebrafish ovary

Activins are dimeric proteins consisting of two inhibin β subunits. Homo- and hetero-dimerizations of two isoforms of β subunits, βA and βB, produce three forms of activins, activin-A, -B, and -AB. Recent studies have suggested that activin-A mediates gonadotropin-induced oocyte maturation in the zebrafish. To further understand the physiological role of activin-A in the zebrafish ovary, we have cloned cDNAs for a second isoform of the activin-βA subunit and the activin type IIA (ActRIIA) receptor and determined their regulation by gonadotropin. Two sequences were obtained during the cloning of activin-βA subunit, both of which showed high identity to βA subunits of other species, and were therefore designated as isoform 1 and 2. Real-time PCR quantification was used to measure mRNA levels of activin-βA1 and -βA2, as well as two type II receptors, ActRIIA and ActRIIB, in the zebrafish ovary. Activin-βA1 mRNA levels in stages III and IV follicles were similar and higher than those in stage II while high activin-βA2 mRNA levels were only found in stage IV follicles. Highest levels of mRNA expression were detected in small and large stage III follicles for ActRIIA and ActRIIB, respectively. Treatment with human chorionic gonadotropin induced dose- and time-dependent increases in mRNA levels of activin-βA1 and -βA, as well as ActRIIA and ActRIIB. These findings further support the involvement of the activin signaling cascade in gonadotropin-regulated gonadal activities.