Epac activation induces an extracellular Ca -independent Ca wave that triggers acrosome reaction in human spermatozoa.

Background: The signaling pathways of the intracellular second messengers cAMP and Ca play a crucial role in numerous physiological processes in human spermatozoa. One such process is the acrosome reaction (AR), which is necessary for spermatozoa to traverse the egg envelope and to expose a fusogenic membrane allowing the egg-sperm fusion. Progesterone and zona pellucida elicit an intracellular Ca increase that is needed for the AR in the mammalian spermatozoa. This increase is mediated by an initial Ca influx but also by a release from intracellular Ca stores. It is known that intracellular Ca stores play a central role in the regulation of [Ca ] and in the generation of complex Ca signals such as oscillations and waves. In the human spermatozoa, it has been proposed that the cAMP analog and specific agonist of Epac 8-(p-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (2'-O-Me-cAMP) elicits an intracellular Ca release involved in the AR.

Objective: To identify the molecular entities involved in the Ca mobilization triggered by 2'-O-Me-cAMP in human spermatozoa.

Materials And Methods: In capacitated human spermatozoa, we monitored Ca dynamics and the occurrence of the AR in real time using Fluo 3-AM and FM4-64 in a Ca -free medium.

Results: Epac activation by 2'-O-Me-cAMP induced a Ca wave that started in the midpiece and propagated to the acrosome region. This Ca response was sensitive to rotenone, CGP, xestospongin, NED-19, and thapsigargin, suggesting the participation of different ion transporters (mitochondrial complex I and Na /Ca exchanger, inositol 3-phosphate receptors, two-pore channels and internal store Ca -ATPases).

Discussion: Our results suggest that Epac activation promotes a dynamic crosstalk between three different intracellular Ca stores: the mitochondria, the redundant nuclear envelope, and the acrosome.

Conclusion: The Ca wave triggered by Epac activation is necessary to induce the AR and to enhance the flagellar beat.