Membrane remodeling and matrix dispersal intermediates during mammalian acrosomal exocytosis

Abstract

To become fertilization-competent, mammalian sperm must undergo a complex series of biochemical and morphological changes in the female reproductive tract. These changes, collectively called capacitation, culminate in the exocytosis of the acrosome, a large vesicle overlying the nucleus. Acrosomal exocytosis is not an all-or-nothing event, but rather a regulated process in which vesicle cargo disperses gradually. However, the structural mechanisms underlying this controlled release remain undefined. In addition, unlike other exocytotic events, fusing membranes are shed as vesicles; the cell thus loses the entire anterior two-thirds of its plasma membrane and yet remains intact while the remaining non-vesiculated plasma membrane becomes fusogenic. Precisely how cell integrity is maintained through-out this drastic vesiculation process is unclear, as is how it ultimately leads to the acquisition of fusion competence. Here, we use cryo-electron tomography to visualize these processes in unfixed, unstained, fully-hydrated sperm. We show that crystalline structures within the acrosome disassemble during capacitation and acrosomal exocytosis, representing a plausible mechanism for gradual dispersal of the acrosomal matrix. We find that the architecture of the sperm head supports an atypical membrane fission-fusion pathway that maintains cell integrity. Finally, we detail how the acrosome reaction transforms both the micron-scale topography and the nano-scale protein landscape of the sperm surface, thus priming the sperm for fertilization. SignificanceMammalian sperm must undergo a complex series of biochemical and morphological changes in the female reproductive tract in order to become fertilization-competent. These changes culminate in acrosomal exocytosis, during which multiple membrane fusions destabilize the acrosomal vesicle and liberate its contents, which include proteins implicated in penetrating and binding to the egg vestments. Here, we use cryo-electron tomography to visualize acrosomal exocytosis intermediates in unfixed, unstained sperm. Our results suggest structural bases for how gradual dispersal of acrosome contents is regulated, as well as for how the cell remains intact after losing much of its plasma membrane, We also show that acrosomal exocytosis transforms both the micron-scale topography and the nano-scale molecular landscape of the sperm surface, thus priming it for interaction and fusion with the egg. These findings yield important insights into sperm physiology and contribute to our understanding of the fundamental yet enigmatic process of mammalian fertilization.