Interactions between apoptotic signal transduction and capacitation in human spermatozoa

Abstract

BACKGROUND: 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. METHODS: Semen samples from 20 healthy donors were incubated in human tubal fluid at 37°C, 5% CO 2 for 3 h without additives (control), with 3% BSA (capacitation), 10 μM calpain-inhibitor III, 20 μM CP-1 inhibitor or 20 μM 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. RESULTS: 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. CONCLUSION: Interaction of the capacitation and apoptosis signalling systems seems to enable the capacitation process by prevention of apoptosis. © The Author 2009. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved.