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

TL;DR

This study shows that two forms of the Na,K-ATPase enzyme are essential for the sperm acrosome reaction, and their absence leads to abnormal ion regulation and premature acrosome release.

Contribution

The study reveals the specific roles of NKAα4 and NKAα1 in regulating the sperm acrosome reaction through ion balance.

Findings

Inhibiting NKAα4 or NKAα1 with ouabain reduces the acrosome reaction in wild-type sperm.

NKAα4-KO sperm show abnormally high and premature acrosome reaction due to ion imbalance.

NKAα4 maintains intracellular Na+ and K+ levels, indirectly affecting Ca2+ and enabling proper acrosome function.

Abstract

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.

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