The α4 isoform of the Na,K-ATPase is expressed in the germ cells of the testes

In addition to the three isoforms of the catalytic subunit of the Na,K-ATPase originally identified (alpha 1, alpha 2, and alpha 3), a fourth a polypeptide (alpha 4) has recently been found in mammalian cells. This novel a-subunit of the Na, K-ATPase is selectively expressed in male gonadal tissues. In the testes, alpha 4 is functionally active and comprises approximately half of the Na,K-ATPase activity of the organ. At present, the pattern of expression of the alpha 4 polypeptide within the cells of the male gonad is unknown. By in situ hybridization, immunocytochemistry, and the ouabain inhibition profile of Na,K ATPase activity, we show that the alpha 4-subunit is expressed in the germ cells of rat testes. The highest amounts of the isoform are found in spermatozoa, where it constitutes two thirds of the Na,K-ATPase activity of the gametes. The other Na pump present in the cells is the ubiquitously expressed alpha 1 polypeptide. The characteristic localization of alpha 4 in the gonad is further supported by the drastic reduction of the polypeptide in mice that are infertile as a consequence of arrest in maturation of the germ cells. In addition, GC-1spg cells, a murine cell line derived from testis spermatogonia, also contain the Na,K-ATPase alpha 4 polypeptide. However, the level of expression of the isoform in these cells is much lower than in the spermatozoa, a fact that may depend on the limited ability of the GC-1spg cells to differentiate in vitro. The particular expression of the Na,K-ATPase alpha 4 isoform we encounter and the specific enzymatic properties of the polypeptide suggests its importance for ionic homeostasis of the germ cells of the testes.