Na,K-ATPase Atp1a4 isoform is important for maintaining sperm flagellar shape

The aim of this study is to investigate the mechanisms by which the testis specific Na,K-ATPase ion transport system (Atp1a4) controls sperm morphology and shape.

Sperm from wild-type (WT) and Atp1a4 knockout (Atp1a4 KO) mice were analyzed morphologically, using light, transmission, and scanning electron microscopy; and functionally, applying sperm osmotic challenge and viability tests. In addition, a sperm proteomic study was performed.

Light microscopy confirmed that sperm lacking Atp1a4 present a bend at the junction of the mid- and principal piece of the flagellum. This bend had different degrees of angulation, reaching occasionally a complete flagellar retroflexion. The defect appeared in sperm collected from the cauda epididymis, but not the epididymal caput or the testis. Transmission and scanning electron microscopy revealed a dilation of the cytoplasm at the site of the bend, with fusion of the plasma membrane in overlapping segments of the flagellum. This was accompanied by defects in the axoneme and peri-axonemal structures. Sperm from Atp1a4 KO mice showed an abnormal response to hypoosmotic challenge with decreased viability, suggesting reduced capacity for volume regulation. Exposure to Triton X-100 only partially recovered the flagellar bend of Atp1a4 KO sperm, showing that factors other than osmotic regulation contribute to the flagellar defect. Interestingly, several key sperm structural proteins were expressed in lower amounts in Atp1a4 KO sperm, with no changes in their localization.

Altogether, our results show that Atp1a4 plays an important role in maintaining the proper shape of the sperm flagellum through both osmotic control and structurally related mechanisms.