EXTRACELLULAR SIGNAL-REGULATED KINASES MODULATE CAPACITATION OF HUMANSPERMATOZOA

Recent evidence indicates the presence of p21 Ras and of a protein wit h characteristics similar to mitogen-activated protein kinases (MAPKs) , also known as extracellular signal-regulated kinases (ERKs), in mamm alian spermatozoa, suggesting the occurrence of the Ras/ERK cascade in these cells. In the present study we investigated the subcellular loc alization of ERKs and their biological functions in human spermatozoa. Immunohistochemistry, immunofluorescence, confocal microscopy, and im munoelectron microscopy demonstrated localization of ERKs in the posta crosomal region of spermatozoa. After stimulation of acrosome reaction with the calcium ionophore A23187 and progesterone, ERKs were mostly localized at the level of the equatorial region, indicating redistribu tion of these proteins in acrosome-reacted spermatozoa. Two proteins o f 42 and 44 kDa that are tyrosine phosphorylated in a time-dependent m anner during in vitro capacitation were identified as p42 (ERK-2) and p44 (ERK-1) by means of specific antibodies. The increase in tyrosine phosphorylation of these proteins during capacitation was accompanied by increased kinase activity, as determined by the ability of ERK-1 an d ERK-2 to phosphorylate the substrate myelin basic protein. The role of this activity in the occurrence of sperm capacitation was also inve stigated by using PD098059, an inhibitor of the MAPK cascade. The pres ence of this compound during in vitro capacitation inhibits ERK activa tion and significantly reduces the ability of spermatozoa to undergo t he acrosome reaction in response to progesterone. Since only capacitat ed spermatozoa are able to respond to progesterone, these data strongl y indicate that ERKs are involved in the regulation of capacitation. I n summary, our data demonstrate the presence of functional ERKs in hum an spermatozoa and indicate that these enzymes are involved in activat ion of these cells during capacitation, providing new insight in clari fying the molecular mechanisms and the signal transduction pathways of this process.