Interactions between apoptotic signal transduction and capacitation in human spermatozoa

Capacitation of sperm is a prerequisite for successful fertilization, determined by hyperactivated motility, increased tyrosine phosphorylation (TyrP) and membrane changes. However, the exact molecular mechanism is not fully clarified. The calpain-calmodulin-system is essential for membrane fusion during capacitation. Recently, interactions with caspase (CP) activation, a main feature of apoptotic cells, were postulated. The objective of our study was to examine interactions between apoptosis signalling and the calpain-calmodulin-system during capacitation. Semen samples from 20 healthy donors were incubated in human tubal fluid at 37 degrees C, 5% CO2 for 3 h without additives (control), with 3% BSA (capacitation), 10 mu M calpain-inhibitor III, 20 mu M CP-1 inhibitor or 20 mu M calmodulin-antagonist. Capacitation was monitored by computer assisted sperm motion analyzer, chlortetracycline (CTC)-assay and western blot (TyrP). Activation of caspases and integrity of transmembrane mitochondrial potential (TMP) were evaluated by flow cytometry. Capacitation, as measured by CTC assay, increased TyrP levels and hyperactivation, resulted in inactivation of CP-9, CP-3 and improved integrity of the TMP. Inhibition of calpain and CP-1 during capacitation reduced the capacitation-related parameters, but did not lead to apoptosis. Inhibition of calmodulin resulted in blocking of capacitation and stimulation of apoptosis. Interaction of the capacitation and apoptosis signalling systems seems to enable the capacitation process by prevention of apoptosis.