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Role of Phosphatidic Acid in the Sperm Acrosome Reaction

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The acrosome reaction (AR) is found in all vertebrate sperm and is required for the success of subsequent fertilization. AR is the exocytosis of the very large acrosome that is located at the forward edge of the head of the sperm. This exocytosis releases crucial enzymes that cut through the coats surround eggs to allow the approach of the sperm. The exocytosis also moves crucial sperm membrane proteins to the sperm surface to allow for sperm and egg (or egg surface) binding. Thus, the AR is a crucial step to fertilization. However, the mechanism of the AR is not well described. The series of experiments outlined here will examine the role of a lipid, phosphatidic acid (PA) in the induction of the AR. As the mass of the lipid PA increased during agonist (both artificial or with the natural inducer found in egg coats) induction of the AR, there is preliminary evidence supporting our hypothesis. These results utilize our new lipid analysis methodology (HPLC with evaporative light scattering mass detection). In further support, the numerous reports that exocytosis involves the lipid PA are summarized. However, the increase in PA associated with the AR may be a result not a causative event. Thus, experiments will test whether lowering of the sperm PA levels with a proven method will inhibit the acrosome reaction. Furthermore, the percent of sperm undergoing the AR will be determined after addition of exogenous PA. Once again, all methods (use of confocal to detect the Xenopus AR, procedures for the addition of appropriate levels of commercially available PA, lipid analysis, etc) have been worked out. Twenty years of experience with Xenopus laevis (a species that has been designated by the NIH as an animal model organism), the fact that Xenopus sperm do not have to be capacitated (a complication and variable to the study of the AR), that all methodology has been worked out, the reduced human ethical issues make our selection of experimental model appropriate. Although future studies will involve human sperm if our model for the AR is supported, the studies outlined here will increase our knowledge of the male gamete and may lead to improvements of in vitro fertilization or human fertility. Artificial induction of the AR may enable some infertile males to conceive.

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