The Na,K-ATPase α4 isoform from humans has distinct enzymatic properties and is important for sperm motility

Abstract

In the rat, the Na,K-ATPase α4 isoform exhibits unique enzymatic characteristics and is important for sperm motility. In this work, we studied expression, localization and function of α4 in human spermatozoa. We show two catalytically active Na,K-ATPase α polypeptides with different ouabain affinity and identified expression of α1, α4, β1 and β3 isoforms in the gametes. In addition, human sperm presented two Na,K-ATPases composed of α4, α4β1 and α4β3. Kinetic analysis of these isozymes produced in insect cells showed that, compared with human α1β1, α4β1 and α4β3 exhibit higher Na+ and lower K+ affinity and higher sensitivity to ouabain. These particular enzymatic properties suggested a role for α4 in sperm function. Using computer-assisted sperm analysis (CASA), we found that ouabain inhibition of α4 significantly decreased percentage sperm motility. In contrast, ouabain did not affect linearity of forward progression, amplitude of lateral head displacement, beat cross frequency and sperm straight-line, curvilinear or average path velocities. This suggests a primary role of α4 in flagellar motility. Accordingly, we found α4 in the sperm tail, predominating in the mid-piece of the flagellum. Therefore, similar to the rat ortholog, human Na,K-ATPase α4 isoform has a distinct activity that is essential for sperm function. © Copyright 2006 Oxford University Press.