Signalling compartmentalization involved in the boar sperm acrosome reaction

Actin, a major cytoskeletal protein involved in many functions in mammalian cells, supports the process of spermatogenesis by sustaining the morphological changes occurring in the germ cells as well as the acquisition of motility by the spermatozoon (Lecuyer et al., 2000). Moreover, the biological transformations that take place at a molecular level during the process of capacitation and leading to acrosome reaction also seem to be dependent on both actin localization and its polymeric status (F-actin). In the pig, as for other species, cytoskeletal reorganization has been related to either release of the acrosomal content or the fertilization rate (Castellani-Ceresa et al., 1993; Liu et al., 2002). It can be hypothesized that in the sperm the cytoskeleton acts as an organizer of specific domains on the plasma membrane. This arrangement occurs during the process of sperm maturation by interaction of the actin cyto- skeleton with enzymatic or receptor proteins and/or fusogenic lipids in the membrane, allowing the translocation of specific molecules with antigenic properties to the sites where they are more directly involved in functional processes (AR, fusion etc.). On the basis of this knowledge, the authors have studied the process of capacitation in boar spermatozoa by modulating actin polymerization with cytochalasin D, a specific inhibitor of filament formation. In particular the effect of the actin cytoskeleton on a) acquisition of the capacitation pattern by CTC, and b) incidence of acrosomal exo- cytosis elicited in the sperm after exposure to zonae pellucidae proteins has been evaluated. Finally, the study has focussed on the major events taking place in the lag time from zona stimulation to acrosomal exocytosis, in particular i) the Ca waves triggered by the exposure of single sperm cells to solubilized ZP and ii) the localiza- tion of PLCc1, a membrane linked enzyme acting in the Ca-dependent transduction pathways of AR.[...]