Vps4, an ATPase involved in the biogenesis of multivesicular endosomes, is necessary for the deformation of the acrosomal membrane during acrosomal exocytosis in human spermatozoa.

POCOGNONI, CRISTIÁN A.; BELMONTE, SILVIA A; MAYORGA, LUIS S

Sevilla

Congreso; 22º Congreso IUBMB, 37º FEBS y 35º SEBBM; 2012

IUMB, FEBS, SEBBM

P05-47The acrosome reaction of human spermatozoa is a complex, calcium-dependent regulated exocytosis. Fusion at multiple sitesbetween the outer acrosomal membrane and the cell membranecauses the vesiculation of theses membranes and the release ofthe acrosomal contents. We have reported previously that by preventingthe release of calcium from the acrosome, the exocyticprocess can be arrested at a stage where SNARE proteins areassembled in loose trans complexes. Transmission electron micrographsof sperm at this stage showed that the acrosomes wereprofusely swollen, with deep invaginations of the outer acrosomalmembrane. The protruding edges of these invaginations weretightly apposed to the plasma membrane. We have proposed thatthese membrane deformations are part of the mechanism ofvesiculation. Invagination of the acrosomal membrane is topologicallyequivalent to the formation of internal vesicles in endosomes,a process that depends on the assembly of membranebendingESCRT complexes on the endosomal surface. We areexploring the possibility that the same mechanism is involved inacrosomal exocytosis. A dominant-negative mutant of VPS4, theATPase responsible for the disassembly of the membraneattached ESCRT proteins inhibited acrosomal exocytosis of permeabilizedhuman spermatozoa. Moreover, an anti-VPS4 antibodywas also inhibitory. TEM images show abnormal bendingof the acrosomal membrane when sperm were stimulated in thepresence of the dominant negative VPS4. These observations suggestthat the deformation of the acrosomal membrane necessaryfor acrosomal exocytosis are shaped by an ESCRT-dependentmechanism.