Before the mammalian spermatozoon can participate in fertilisation it must undergo a series of changes. First, it must undergo capacitation, a physiological "switching-on", that takes place around the time of ovulation, in response to factors present in the female tract, and that confers spermatozoa the capacity to interact with the ovum. During its interaction with the female gamete, the spermatoozon engages in mechanisms of cell-cell recognition and undergoes a process of exocytosis of the acrosomal contents, the so-called "acrosome reaction". This acrosomal exocytosis seems to occur in response to the combined action of progesterone (a steroid present in the cells surrounding the oocyte) and the zona pellucida (a glycoprotein coat of the oocyte).

We are interested in understanding how this interaction between gametes takes place and how do different agonists associated with the ovum initiate acrosomal exocytosis. We have found cellular and molecular evidence to support the idea that progesterone and zona pellucida have a synergistic effect triggering exocytosis (Roldan et al. 1994; Shi et al. 2005). We have characterised early signal transduction events activated by these agonists uncovering that each differentially activate protein tyrosine kinase or G proteins leading to donswtream events (Murase and Roldan 1996). In particular, progesterone acts on GABA_A-like receptors similar, but not identical, to those in neurons, and on other receptors of unknown identity somehow linked to Ca²⁺ channels (Shi et al. 1995).

Considerable efforts have been devoted to unravelling signalling pathways underlying acrosomal exocytosis. We have characterised phosphoinositide and phosphatidylcholine hydrolysis by various phospholipases C and the generation and roles of diacyl- and alkyl-acyl-glycerols in response to molecular probes and physiological agonists (Roldan and Harrison, 1989; Roldan and Murase 1994; Roldan et al. 1994), demonstrating their essential function during exocytosis. The participation of phospholipase A₂ has also been addressed and several studies have allowed us to characterise its essential role (Roldan and Fragio 1993; Yuan et al. 2003; Shi et al. 2005), mechanisms regulating enzyme activity (Shi et al. 2005; Chen et al. 2005), cross-talks with other signalling pathways, and the participation of metabolites generated by this enzyme in membrane function (Garde and Roldan 1996, 2000). Reviews summarising current information are available (Brewis et al. 2005; Roldan and Shi 2006).

Currently, we are interested in the different types of PLA₂ that may be present in mammalian spermatozoa, how they are regulated, and their role in the molecular sequence leading to membrane fusion.

Collaborators:

• Qi-xian Shi, Department of Reproductive Physiology and Toxicology, Zhejiang Academy of Medical Sciences, Hangzhou, Zhejiang (People's Republic of China).

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