Na+,K+-ATPase α isoforms in sperm show a highly structured and distinct pattern of distribution

This manuscript shows that the Na+ and K+ transporter Na+,K+-ATPase α4, specific to sperm, is expressed on the surface of the sperm head and flagellum in a very structured manner. This is also true for Na+,K+-ATPase α1, the other Na+,K+-ATPase isoform present in sperm and also in all cells. However, Na+,K+-ATPase α4 distribution changes when the cells are capacitated, an event necessary for fertilization. This dynamic remodeling, along with the distinct functional properties of Na+,K+-ATPase α4 and α1, provides evidence for the refined level of specialization that sperm have developed to achieve the amazing goal of fertilizing the oocyte.

Na+,K+-ATPase α4 is a unique Na+ and K+ transporter of the plasma membrane of spermatozoa, which is essential for male fertility. While previous studies have found Na+,K+-ATPase α4 to be mainly expressed in the sperm flagellum, less is known about its localization in the sperm head. Moreover, the spatial arrangement of Na+,K+-ATPase α4 at the subcellular level and its relationship to the functional state of the cells are unclear. We studied this using stimulated emission depletion (STED) super-resolution microscopy. We show that, under non-capacitated conditions, Na+,K+-ATPase α4 is distributed in a trilinear pattern along the midpiece and as a scattered single line along the principal piece of the sperm flagellum. Under capacitated conditions, Na+,K+-ATPase α4 pattern undergoes remodeling and its distribution shifts to a single line along the flagellum. On the other hand, Na+,K+-ATPase α1, the somatic isoform of Na+,K+-ATPase also present in sperm, exhibits a similar trilaminar localization at the flagellar midpiece but a bilinear pattern in the principal piece. This distribution, unlike that of Na+,K+-ATPase α4, does not change during sperm capacitation. We also found Na+,K+-ATPase α1 and α4 in the sperm head, where they present a complex distribution under both non-capacitated and capacitated conditions. These differences in the localization pattern and spatial dynamics of Na+,K+-ATPase isoform expression, along with their different functional properties, highlight the distinct roles that both isoforms play to support sperm function.