MAYORGA Luis Segundo
congresos y reuniones científicas
Exocytosis of the Acrosomal Granule in Human Sperm
Conferencia; Zing Conference ?Lysosome Related Organelles?.; 2014
The two main components of human sperm are the head and flagellum, joined by the connecting piece. The head contains the nucleus, the acrosome, and a small amount of cytoplasm. The acrosomal granule is a membrane-enclosed large secretory and flat organelle with an acidic pH that contains several hydrolytic enzymes. It covers the anterior part to the nucleus. The acrosome is originated from the Golgi apparatus during spermiogenesis, although a role for the endocytic pathway in its biogenesis has also been proposed. The acrosomal membrane underlying the plasma membrane is referred to as the ??outer?? acrosomal membrane, and that overlying the nucleus is referred to as the ??inner?? acrosomal membrane. The outer acrosomal membrane is in close proximity with the plasmalema; however, these two membranes do not interact until the sperm is stimulated in the proximity of the mature oocyte to undergo the acrosome reaction, a special type of calcium-regulated secretion. During acrosomal exocytosis the membranes fuse at multiple points and the fusion pores expand in a particular manner leading to the release of hybrid vesicles, formed by patches of plasma and outer acrosomal membrane. Probably the most crucial role for the acrosomal exocytosis is the change in the topology and composition of the limiting membrane of the sperm. These changes are fundamental for the interaction and fusion of the sperm and oocyte. The acrosomal exocytosis differs from other exocytic processes occurring in neuroendocrine cells. For example, in neurons, small secretory vesicles attached to specialized regions of the plasma membrane undergo fast secretion caused by the opening of a single fusion pore milliseconds after an increase of cytosolic calcium. However, results from several laboratories, including ours, indicate that the membrane fusion machineries involved in the opening of fusion pores in neuroendocrine cells and in sperm share several components, including neurotoxin sensitive SNARE proteins, SNARE complex interacting proteins (such as NSF, SNAP, complexin, synaptotagmin, Munc18, Munc13), Rab3 and Rab3 interacting proteins (such as RIM). Acrosomal exocytosis can be arrested at a stage where SNARE proteins are assembled in loose trans complexes by preventing the release of calcium from the acrosome. Transmission electron micrographs at this stage showed that the acrosomes were profusely swollen, with deep invaginations of the outer acrosomal membrane. We have proposed that these membrane deformations are part of the mechanism of hybrid vesicle formation. Invaginations of the outer acrosomal membrane are topologically equivalent to the formation of internal vesicles in endosomes, a process that depends on the assembly of the ESCRT (Endosomal Sorting Complexes Required for Transport) machinery. 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, and the anti-VPS4 antibody inhibited acrosomal exocytosis of permeabilized human spermatozoa. Moreover, the ubiquitin binding domain of TSG101 ? an ESCRT I protein-, and Snf7 -an ESCRT III protein-, were also inhibitory. TEM images showed abnormal bending of the acrosomal membrane when sperm were stimulated in the presence of the dominant negative VPS4 or with anti-VPS4 antibodies. These observations suggest that the deformations of the acrosomal membrane necessary for acrosomal exocytosis are shaped by an ESCRT-dependent mechanism.