Molecular mechanisms of membrane fusion during acrosomal exocytosis

Sperm are attractive cells. Understanding their physiology has motivated researchers from all over the globe for decades. Initially came the description of sperm's overall shape and properties, together with their genesis and development in the testis. Later, the study of exocytosis took off owing to ultrastructural analysis that achieved exquisite levels of detail. Biochemical analysis ensued, identifying ligands and signalling pathways whose end point was exocytosis. Somehow, the unveiling of the molecular mechanisms involved in membrane fusion itself lagged behind all this progress. The picture changed dramatically in the last few years, due to an explosion in our knowledge of the many proteins required for exocytosis and its regulation, and the discovery that very similar versions of these proteins play the same roles in virtually all membrane fusion models. Luckily, sperm are not the exception to this rule. For instance, fusion of the outer acrosomal to the plasma membrane depends on Rab3 activation, alpha-SNAP/NSF, synaptotagmin, and SNAREs; it also requires an efflux of calcium from the acrosomal lumen. Convergence of Rab- and toxin-sensitive SNARE-dependent pathways is a hallmark of the acrosome reaction that makes it an attractive mammalian model to study the different phases of the membrane fusion cascade. Finally, because nature has endowed sperm with a cellular specialization that gives them a single, irreversible chance to fertilise an egg, the acrosome reaction is more straightforward to dissect than fusion in other cell types, where the same substances are secreted again and again, requiring the membranes and fusion machinery to recycle multiple times.