The Sperm Acrosomal Reaction Depends on the Function of Both Na,K-ATPase α1 and α4 Isoforms

Na,K-ATPase (NKA) is expressed as two isoforms in mouse sperm: the testis-specific Na,K-ATPase α4 (NKAα4) and the somatic Na,K-ATPase α1 (NKAα1). Currently, the role that NKAα4 and NKAα1 play in the sperm acrosome reaction (AR) is unknown, and is the subject of our investigation. We studied the following: (1) the differential sensitivity of NKAα4 and NKAα1 to inhibition by ouabain; (2) the effects of deleting NKAα4 in mice (using NKAα4-KO mouse). In sperm from wild type (WT) mice, inhibiting NKAα4 with a low concentration of ouabain reduced AR. Inhibiting NKAα1 with a higher ouabain concentration further reduced AR, indicating that both NKA isoforms are necessary for AR. Surprisingly, sperm from NKAα4-KO mice exhibited an abnormally high AR. This was not due to a lack of acrosome development during sperm differentiation, but rather from premature release of the acrosome after they were isolated from the epididymis. When WT and NKAα4-KO sperm were exposed to media with or without Na+, K+, and Ca2+, or with the ionophores nigericin, valinomycin, and A23187, they displayed abnormal AR, indicating that NKAα4-KO sperm are unable to control intracellular Na+, K+, and Ca2+ levels. NKAα4 directly maintains intracellular Na+ and K+ and indirectly influences Ca2+ levels. This provides the necessary ion environment for full development of sperm AR. Along with NKAα4, NKAα1 also contributes to sperm AR.