Although high concentrations of reactive oxygen species (ROS) cause sperm pathology (ATP depletion leading to insufficient axonemal phosphorylation, lipid peroxidation and loss of motility and viability), recent evidence demonstrates that low and controlled concentrations of these ROS play an important role in sperm physiology. Reactive oxygen species, such as the superoxide anion, hydrogen peroxide and nitric oxide, induce sperm hyperactivation, capacitation or the acrosome reaction in vitro. The ROS involved in these processes may vary depending on experimental conditions, but all the evidence converges to describe these events as 'oxidative' or 'redox regulated'. Human sperm capacitation and acrosome reaction are associated with extracellular production of a superoxide anion that is thought to originate from a membrane 'oxidase'. The enzymes responsible for tyrosine phosphorylation-dephosphorylation of sperm proteins are possible targets for ROS since mild oxidative conditions cause increases in protein tyrosine phosphorylation and acrosome reaction. The lipid peroxidation resulting from low concentrations of ROS promotes binding to the zona pellucida and may trigger the release of unesterified fatty acids from the sperm plasma membrane. The fine balance between ROS production and scavenging, as well as the right timing and site for ROS production are of paramount importance for acquisition of fertilizing ability.

Capacitation is defined as the series of transformations that spermatozoa normally undergo during their migration through the female genital tract, in order to reach and bind to the zona pellucida, undergo the acrosome reaction, and fertilize the egg. During this process, extensive changes occur in all sperm compartments (head and flagellum; membrane, cytosol, cytoskeleton), factors originating from epididymal fluid and seminal plasma are lost or redistributed and membrane lipids and proteins are reorganized; ion fluxes induce biochemical modifications and controlled amounts of reactive oxygen species are generated; spermatozoa develop hyperactivated motility; and complex signal transduction mechanisms are initiated. The main purpose of capacitation is to ensure that spermatozoa reach the eggs at the appropriate time and in the appropriate state to fertilize these eggs, by finely-controlling the rate of the changes necessary to prime spermatozoa and by activating all the mechanisms needed for the subsequent acrosome reaction. The reversibility of some of the mechanisms leading to sperm capacitation may therefore be a very important aspect of the fine regulation and perfect timing of this process.

The involvement of cAMP in the process of sperm capacitation has been the subject of several studies. In addition, the importance of protein-tyrosine phosphorylation in this process has been investigated, although only a few studies have been reported in the human. Since agents regulating the intracellular concentrations of cAMP affect sperm capacitation rates, the role of cAMP on the expression of phosphotyrosine-containing proteins was investigated during human sperm capacitation. Fetal cord serum ultrafiltrate, a known capacitation inducer in human spermatozoa, caused an increase in the phosphotyrosine content of 105- and 81-kDa proteins (p105 and p81), the two major phosphotyrosine-containing proteins of human spermatozoa. Similar effects were observed when spermatozoa were incubated with phosphodiesterase inhibitors or cell-permeant cAMP analogs, suggesting that cAMP is involved in these two processes. Forskolin, an adenylyl cyclase activator, also caused an increase in both sperm capacitation rates and tyrosine phosphorylation of p105 and p81, while 12-O-tetradecanoyl phorbol 13-acetate stimulated both capacitation and tyrosine phosphorylation of p105 and p81 only when spermatozoa were incubated in the presence of bicarbonate, in agreement with its reported effects on cAMP production and hamster sperm capacitation. The inhibition of these phenomena by cAMP-dependent protein kinase inhibitors, and the stimulation by protein phosphatase inhibitors, suggest that Ser/Thr protein phosphorylation plays an important role in the regulation of both sperm capacitation and protein-tyrosine phosphorylation pathways. However, observations that both calyculin A and okadaic acid stimulated sperm capacitation, whereas only calyculin A increased p105 and p81 phosphotyrosine content and sperm velocity, suggest that protein phosphatase PP1 is involved in the two latter phenomena while PP2A mediates sperm capacitation. These results suggest that divergent pathways might regulate tyrosine phosphorylation of p105 and p81 and sperm capacitation after cAMP-dependent phosphorylation of an intermediate protein.

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