MAYORGA Luis Segundo
congresos y reuniones científicas
ESCRT complexes are involved in acrosomal membrane deformations required for regulated exocytosis in human spermatozoa.
POCOGNONI CA; BELMONTE SA; MAYORGA LS
Simposio; Gordon Research Conference. Molecular Membrane Biology.; 2013
Gordon Research Conference
During spermiogenesis, the male gamete loses most of its endomembrane system. Moreover, it becomes transcriptionally and translationally silent. Spermatozoa are one of the few cell types from multicellular organisms that are adapted to survive individually. These highly differentiated cells are capable of performing a limited set of functions with high efficiency. Mammalian spermatozoa carry a single dense-core secretory granule (named the acrosome), large enough to be observed by fluorescense microscopy. Therefore, sperm are excellent model cells to study secretion. The acrosome reaction is a calcium-dependent exocytosis. Fusion at multiple sites between the outer acrosomal membrane and the cell membrane causes the vesiculation of these membranes and the release of the acrosomal contents. We have reported previously that, by preventing the release of calcium from the acrosome, the exocytic process is arrested at a stage where SNARE proteins are assembled in loose trans complexes. Transmission electron micrographs of sperm at this stage show profusely swollen acrosomes, with deep invaginations of the outer acrosomal membrane. The protruding edges of these invaginations are tightly apposed to the plasma membrane. We have proposed that these membrane deformations are part of the mechanism of the formation of hybrid vesicles, having patches of outer acrosomal and plasma membranes. Invaginations of the acrosomal membrane are topologically equivalent to the formation of internal vesicles in endosomes, a process that depends on the assembly of membrane-bending ESCRT complexes on the endosomal surface. We are exploring the possibility that the same mechanism is involved in acrosomal exocytosis. A dominant-negative mutant of VPS4, the ATPase responsible for the disassembly of the membrane attached ESCRT proteins, inhibited acrosomal exocytosis in streptolysin O-permeabilized human spermatozoa. Moreover, an anti-VPS4 antibody was also inhibitory. TEM images showed abnormal bending of the acrosomal membrane when sperm were stimulated in the presence of the dominant negative VPS4. These observations suggest that the deformation of the acrosomal membrane necessary for acrosomal exocytosis are shaped by an ESCRT-dependent mechanism