Calcium-induced acrosomal exocytosis requires cAMP acting through a protein kinase A-independent, Epac-mediated pathway

BRANHAM MT; MAYORGA LS; TOMES CN

JOURNAL OF BIOLOGICAL CHEMISTRY (ONLINE)

American Society for Biochemistry and Molecular Biology, Inc.

Lugar: Bethesda, MD, USA; Año: 2006 vol. 281 p. 8656 - 8666

1083-351X

Epac, a guanine nucleotide exchange factor for the small GTPase Rap, binds to and is activated by the second messenger cyclic adenosine 3´, 5´-monophosphate (cAMP).  In sperm there are a number of signaling pathways required to achieve egg-fertilizing ability that depend upon an intracellular rise of cAMP.  Most of these processes were thought to be mediated by cAMP-dependent protein kinases (PKAs). Here we report a new dependence for the cAMP-induced acrosome reaction involving Epac.  The acrosome reaction is a specialized type of regulated exocytosis leading to a massive fusion between the outer acrosomal and the plasma membranes of sperm cells. Ca2+ is the archetypical trigger of regulated exocytosis, and we show here that its effects on acrosomal release are fully mediated by cAMP.  Ca2+ failed to trigger acrosomal exocytosis when intracellular cAMP was depleted by an exogenously added phosphodiesterase or when Epac was sequestered by specific blocking antibodies.  The non-discriminating dibutyryl cAMP and the Epac-selective 8-pCPT-2Me-cAMP analogues triggered the acrosome reaction in the effective absence of extracellular Ca2+.  This indicates that cAMP - via Epac activation - has the ability to drive the whole cascade of events necessary to bring exocytosis to completion, including tethering and docking of the acrosome to the plasma membrane, priming of the fusion machinery, mobilization of intravesicular Ca2+, and ultimately, bilayer mixing and fusion. cAMP-elicited exocytosis was sensitive to anti-a-SNAP, anti-NSF and anti-Rab3A antibodies, to intra-acrosomal Ca2+ chelators, and botulinum toxins, but was resistant to PKA blockers. These experiments thus identify Epac in human sperm and evince its indispensable role downstream of Ca2+, in exocytosis.