Abstract
After sperm-oocyte fusion, cortical granules (CGs) located in oocyte cortex undergo exocytosis and their content is released into the perivitelline space to avoid polyspermy. Thus, cortical granule exocytosis (CGE) is a key process for fertilization success. We have demonstrated that alpha-SNAP -and its functional partner NSF- mediate fusion of CGs with the plasma membrane in mouse oocytes. Here, we examined at cellular and ultrastructural level oocytes from hyh (hydrocephalus with hop gait) mice, which present a missense mutation in the Napa gene that results in the substitution of methionine for isoleucine at position 105 (M105I) of alpha-SNAP. Mutated alpha-SNAP was mislocalized in hyh oocytes while NSF expression increased during oocyte maturation. Staining of CGs showed that 9.8% of hyh oocytes had abnormal localization of CGs and oval shape. Functional tests showed that CGE was impaired in hyh oocytes. Interestingly, in vitro fertilization assays showed a decreased fertilization rate for hyh oocytes. Furthermore, fertilized hyh oocytes presented an increased polyspermy rate compared to wild type ones. At ultrastructural level, hyh oocytes showed small mitochondria and a striking accumulation and secretion of degradative structures. Our findings demonstrate the negative effects of alpha-SNAP M105 mutation on oocyte biology and further confirm the relevance of alpha-SNAP in female fertility.
Highlights
M105I mutation on oocyte biology: ultrastructural and cellular changes that adversely affect female fertility in mice Matilde de Paola[1,2,3], María Paz Miró[4], Marcelo Ratto[5], Luis Federico Bátiz4,6* & Marcela Alejandra Michaut 1,2,7*
The fusion of cortical granules with the oocyte plasma membrane –known as cortical granule exocytosis or cortical reaction- is the most significant event to prevent polyspermy. This exocytosis is different from other regulated secretory vesicles because their kinetic is slower and cortical granules are not renewed after their fusion with the plasma membrane[27]
We have previously documented that alpha-soluble NSF attachment proteins (SNAPs) and N-ethylmaleimide-sensitive factor (NSF) participate in cortical granule exocytosis in mouse oocytes
Summary
M105I mutation on oocyte biology: ultrastructural and cellular changes that adversely affect female fertility in mice Matilde de Paola[1,2,3], María Paz Miró[4], Marcelo Ratto[5], Luis Federico Bátiz4,6* & Marcela Alejandra Michaut 1,2,7*. After sperm-oocyte fusion, cortical granules (CGs) located in oocyte cortex undergo exocytosis and their content is released into the perivitelline space to avoid polyspermy. We have demonstrated that alpha-SNAP -and its functional partner NSF- mediate fusion of CGs with the plasma membrane in mouse oocytes. In order to prevent polyspermy, the fusion of the first sperm (which occurs only in CGs area in mouse oocyte) induces the exocytosis of CGs, known as cortical reaction, and the content of granules is released into the perivitelline space. The signal transduction pathway accountable for cortical reaction is not yet completely understood and is thought to be mediated by SNARE (Soluble N-ethylmaleimide Sensitive Factor [NSF] Attachment Protein [SNAP] Receptor) proteins: SNAP-25, Syntaxin and VAMP, which are key elements of the membrane fusion machinery. We have previoulsy described that in mammalian gametes, alpha-SNAP and NSF participate in a prefusion step[24,25]
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