RAC1 controls progressive movement and competitiveness of mammalian spermatozoa

Author summary Spermatozoa are highly specialized cells designed for one particular purpose: to swim towards the egg cells and fertilize them. Swimming is achieved by propelling rotation of the flagellum, a complex molecular motor. Many proteins building the flagellum have been identified, and mechanisms generating forward movement have been elucidated in some detail. However, how movement is directed towards the egg cells, is not well understood. In this study, we provide evidence that the Rho small GTPase RAC1 plays a pivotal role in sperm progressive movement. We show that inappropriately high or low RAC1 activity impairs progressive movement and the competitiveness of sperm within a sperm population. Sperm with balanced RAC1 activity therefore gain a selective advantage in reaching and fertilizing egg cells, as exemplified by the mouse t-haplotype. Our data highlight the importance of Rho signaling in controlling progressive motility, in sperm competition, non-Mendelian inheritance and male (in)fertility. Mammalian spermatozoa employ calcium (Ca2+) and cyclic adenosine monophosphate (cAMP) signaling in generating flagellar beat. However, how sperm direct their movement towards the egg cells has remained elusive. Here we show that the Rho small G protein RAC1 plays an important role in controlling progressive motility, in particular average path velocity and linearity. Upon RAC1 inhibition of wild type sperm with the drug NSC23766, progressive movement is impaired. Moreover, sperm from mice homozygous for the genetically variant t-haplotype region (t(w5)/t(w32)), which are sterile, show strongly enhanced RAC1 activity in comparison to wild type (+/+) controls, and quickly become immotile in vitro. Sperm from heterozygous (t/+) males, on the other hand, display intermediate RAC1 activity, impaired progressive motility and transmission ratio distortion (TRD) in favor of t-sperm. We show that t/+-derived sperm consist of two subpopulations, highly progressive and less progressive. The majority of highly progressive sperm carry the t-haplotype, while most less progressive sperm contain the wild type (+) chromosome. Dosage-controlled RAC1 inhibition in t/+ sperm by NSC23766 rescues progressive movement of (+)-sperm in vitro, directly demonstrating that impairment of progressive motility in the latter is caused by enhanced RAC1 activity. The combined data show that RAC1 plays a pivotal role in controlling progressive motility in sperm, and that inappropriate, enhanced or reduced RAC1 activity interferes with sperm progressive movement. Differential RAC1 activity within a sperm population impairs the competitiveness of sperm cells expressing suboptimal RAC1 activity and thus their fertilization success, as demonstrated by t/+-derived sperm. In conjunction with t-haplotype triggered TRD, we propose that Rho GTPase signaling is essential for directing sperm towards the egg cells.

Automated summary

The Rho small GTPase RAC1 plays a crucial role in controlling progressive movement of sperm, impacting their speed and linearity. Abnormal RAC1 activity impairs sperm's progressive movement, while balanced activity gives them a reproductive advantage.