Claudia Tomes, Ph.D.

Research

Sexual reproduction to perpetuate a given species occurs through fertilization, during which a diploid zygote is formed to produce a genetically distinct individual. To this end, the haploid sperm and haploid egg must collide to allow entry of the sperm head delivering the male chromatin into the egg cytoplasm. Both the male and female gametes undergo regulated exocytosis termed the acrosome reaction (AR) and the cortical reaction respectively at different times during their encounter. The success of fertilization depends on these exocytoses. In all exocytotic cell lines, membrane fusion is governed by an exquisite and highly conserved protein machinery responsible for avoiding premature secretion and guarantee instantaneous release at the same time. Some components of this machinery are integral proteins localized to the vesicle (Rabs, R-SNAREs, synaptotagmins) and plasma membrane (Q-SNAREs), whereas others do not have transmembrane domains or lipid modifications (-SNAP, PTP1B, NSF, Munc18, complexins). In sperm the AR requires calcium, cyclic AMP and all the fusion proteins mentioned above, orchestrated in a highly regulated protein-protein interaction network. The relevant cAMP target is Epac, a guanine nucleotide exchange factor for the small GTPase Rap. A soluble adenylyl cyclase, regulated by calcium and bicarbonate, synthesizes the cAMP required for the AR. Epac sits at a critical point during the exocytotic cascade after which the pathway splits into two limbs, one (Rab3A-PTP1B--SNAP/NSF-SNAREs) that assembles the fusion machinery into place, and another (Rap1-phospholipase C) that mobilizes intracellular calcium through IP3-sensitive channels. My lab is currently concerned with understanding more deeply the nature and regulation of these signaling pathways, determining at what level/s do these pathways cross talk, and where do they converge to achieve exocytosis. The discovery that very similar versions of the proteins involved play the same roles in virtually all membrane fusion models has greatly simplified our thinking and means that these mechanisms need not be studied in detail in all cell types; instead, future work can concentrate on the analysis of a few favorable secretory cell models. The AR is a relatively simple model and because of the universality of mechanisms underlying exocytosis, the knowledge arising from studying sperm exocytosis needs not to be restricted to the Reproductive Biology field but could be extended to more complex membrane fusion scenarios.