Cell-penetrating alpha-synuclein accelerates the late stages of the acrosome reaction in human sperm

Secretory vesicles discharge their contents into the extracellular space through fusion pores; the behaviour of these structures regulates release kinetics. In sperm, acrosomal contents are released after the expansion of multiple pores formed when the plasma and acrosomal membranes fuse. Alpha-synuclein is required after pore opening during the human sperm acrosome reaction (AR). Here, we investigated its role in AR kinetics, specifically in pore expansion. We introduced cell-penetrating His6-TAT-alpha-synuclein into human sperm and monitored progesterone-induced AR by time-lapse fluorescence microscopy using an anti-CD46 antibody. Fluorescence traces from individual cells revealed that the AR follows a sigmoidal profile with a steeper slope in cells exposed to His6-TAT-alpha-synuclein plus progesterone than in progesterone-only controls. This acceleration was most pronounced in the later phase of the curves, which suggests that alpha-synuclein enhances pore dilation. It is generally assumed that pore expansion proceeds immediately after opening without modulation; our results challenge this view. Thus, the contribution of this study to the sperm biology field is that pore dilation is regulated. Its contribution to the cell biology field is to provide new insight into the physiological role of alpha-synuclein beyond the nervous system and to raise the possibility of its conserved function across different cell types.