I-6: Role of Actin Cytoskeleton during Mouse Sperm Acrosomal Exocytosis

Background: Mammalian sperm must undergo a process termed capacitation to become competent to fertilize an egg. Capacitation renders the sperm competent by priming the cells to undergo a rapid exocytotic event called acrosomal exocytosis that is stimulated by the zona pellucida (ZP) of the egg or progesterone. Over the years, several biochemical events have been associated with the capacitation process; however, the question that has remained unanswered in investigations of capacitation is: What is the underlying reaction or set of reactions that transform the sperm cell from a state unresponsive to ZP or progesterone-stimulated acrosomal exocytosis to the state primed to respond to these stimuli? Our preliminary results demonstrate that the actin cytoskeleton plays a role in this process. Our long-term goal of this research: to elucidate the molecular mechanism whereby the actin cytoskeleton controls acrosomal exocytosis in mammalian sperm. In this proposal, we evaluate the establishment and stabilization of the primed state of acrosomal exocytosis that develops during the course of sperm capacitation. Additionally, we will examine the roles of intracellular calcium and actin in the destabilization of the primed state of acrosomal exocytosis that results in the propagation of the fusion of the outer acrosomal and plasma membranes. There are several human health-related reasons these studies are significant. For example, an understanding this process may lead to a better understanding of certain cases of male infertility and to the development of pharmacological approaches to interfere with this process, leading to new contraceptive agents. Most importantly, since actin has been implicated in exocytosis occurring in many types of somatic cells, information gathered from studying the less complicated sperm system will likely impact our understanding of secretion in other organ systems such as endocrine or digestive tissues. Materials and Methods: This application also proposes several novel methodological approaches. These include: a. the simultaneous imaging of acrosomal GFP and calcium to study exocytosis, a novel approach that has several advantages compared to studies previously performed to evaluate this process; b. we will be analyzing live cells in real time, without fixation or any other procedure that may introduce artifacts; c. we will use flow cytometry to monitor the formation of intermediate stages of exocytosis by analyzing the acrosomal GFP and the particulate acrosomal matrix protein sp56. The use of GFP sperm provides an objective and sensitive method to study exocytosis; d. we will use natural stimulants of acrosomal exocytosis such as zona pellucida and progesterone, not chemical agents such as calcium ionophores. Results: We have investigated the pathways governing the establishment and stabilization of the primed state of acrosomal exocytosis that develops during the course of sperm capacitation through the formation of intermediate stages of exocytosis. Conclusion: We have investigate the connection between capacitation and acrosomal exocytosis, because it is not known why sperm do not undergo acrosomal exocytosis if they are not fully capacitated. Moreover, we propose a testable model of how actin helps to establish a metastable state during capacitation to prepare sperm for acrosomal exocytosis and how calcium may cause the dissociation of the peri-acrosomal cytoskeleton, effecting the complete fusion of the outer acrosomal membrane and the plasma membrane.