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Complexin I deficient sperm are subfertile due to a defect in zona pellucida penetration

L Zhao, Animal Sciences, University of Illinois at Urbana-Champaign, Urbana, United States
K Reim, Molecular Neurobiology, Max-Plank-Institute for Experimental Medicine, Göttingen, Germany
D Miller, Animal Sciences, University of Illinois, Urbana, 61801, United States

Correspondence: David Miller, Email: djmille{at}uiuc.edu

Abstract

Upon adhesion to the zona pellucida, sperm undergo regulated exocytosis of the acrosome. Although it is necessary for sperm to penetrate the zona pellucida and fertilize an egg, the acrosomal membrane fusion process is poorly understood. Complexin I and II are small, cytosolic proteins that bind to a complex of proteins termed the SNARE complex (soluble N-ethyl-maleimide sensitive factor attachment protein receptor) to regulate synaptic vesicle exocytosis. Complexin II deficient mice are fertile but the fertility of sperm from complexin I deficient male mice is unclear because the mice have ataxia and cannot mate. Here we show that the genes encoding complexin I and II are expressed in primary spermatocytes and spermatids. Complexin proteins were found in/near the developing acrosome in spermatids and in or around the acrosome of mature sperm. Cell fractionation demonstrated complexins I and II were predominantly found in the cytosolic fraction. Furthermore, sperm from complexin I deficient mice had normal morphology, number and only small differences in motility, as assessed by computer assisted semen analysis. Complexin I deficient sperm capacitated normally and bound to the zona pellucida. But when sperm from complexin I deficient mice were inseminated into females, a defect in fertility was observed, in concordance with previous data showing that in vitro fertilization rate was also reduced. If the zona pellucida was removed prior to in vitro fertilization, fertility was normal, demonstrating that zona pellucida penetration was defective, a step requiring acrosomal exocytosis. Therefore, complexin I deficient sperm are subfertile due to faulty zona pellucida penetration.