Super-resolution imaging of live sperm reveals dynamic changes of the actin cytoskeleton during acrosomal exocytosis

Abstract

Filamentous actin (F-actin) is a key factor in exocytosis in many cell types. In mammalian sperm, acrosomal exocytosis (AR), a special type of controlled secretion, is regulated by multiple signaling pathways including the actin cytoskeleton. However, the dynamic changes of the actin cytoskeleton in live sperm remain largely. Here, we used the powerful properties of SiR-actin, to examine actin dynamics in live mouse sperm at the onset of the AR. By using a combination of super-resolution microscopy techniques to image sperm loaded with SiR-actin or from transgenic mouse containing Lifeact-EGFP, six regions containing F-actin within the sperm head were revealed. The proportion of sperm possessing these structures changed upon capacitation. By using live cell imaging experiments, we report that dynamic changes of F-actin during AR occur in specific regions of the sperm head. While certain F-actin regions undergo depolymerization prior to the initiation of AR, others remain unaltered or were lost after exocytosis occurred. Our work emphasizes the utility of live-cell nanoscopy, which will undoubtedly impact the search for mechanisms that underlie basic sperm functions.