In addition to the three isoforms of the catalytic subunit of the Na,K-ATPase originally identified (α1, α2, and α3), a fourth α polypeptide (α4) has recently been found in mammalian cells. This novel α-subunit of the Na,K-ATPase is selectively expressed in male gonadal tissues. In the testes, α4 is functionally active and comprises approximately half of the Na,K-ATPase activity of the organ. At present, the pattern of expression of the α4 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 α4-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 α1 polypeptide. The characteristic localization of α4 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 α4 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 α4 isoform we encounter and the specific enzymatic properties of the polypeptide suggests its importance for ionic homeostasis of the germ cells of the testes.
- α4 Isoform