Dubois, Y. | Search bioRxiv

bioRxiv2024

Key roles of the zona pellucida and perivitelline space in promoting gamete fusion and fast block to polyspermy inferred from the choreography of spermatozoa in mice oocytes

Mammalian fertilization is still lacking a comprehensive understanding of gamete fusion and block to polyspermy mechanisms. One reason is that they are highly dynamic processes involving transient events that can only be revealed and characterized by direct observation. To extend while challenging existing knowledge, this study applies real time brightfield and confocal imaging to inseminated ZP-intact mouse oocytes and statistical analyses to establish an accurate dynamic picture of the cascade of events leading to fusion and prevention of polyspermy in conditions as close to physiology as possible. These observations allow to characterize the roles of the different components of the oocyte (i.e. zona pellucida (ZP), perivitelline space (PVS), oocyte plasma membrane (OPM)) and the spermatozoon (i.e. head, flagellum) in promoting fertilization and preventing polyspermy. The kinetics we have determined challenge dogmas by showing that: (i) the first sperm is not necessarily the one that fertilizes in mice pointing to the existence of other post-penetration fertilization factors, (ii) the ZP block resulting from the cortical reaction is too slow to contribute to the prevention of polyspermy in mice. On the other hand, it evidences that the ZP directly contributes to polyspermy block in two other ways: (a) as a naturally effective entry barrier for the spermatozoon (independent of any block caused by fertilization), (b) as an effective exit barrier for components released by the OPM, which may contribute to a fast PVS block to polyspermy through the neutralizing of unwanted spermatozoa in the PVS. Moreover, our observations reveal that the ZP plays a key role in fertilization itself by channeling the flagellar oscillations of spermatozoa in the PVS to make them conducive to fusion.

cell biology↗

bioRxiv2020

Elevated expression of complement C4 in the mouse prefrontal cortex causes schizophrenia-associated phenotypes

Accumulating evidence supports immune involvement in the pathogenesis of schizophrenia, a severe psychiatric disorder. In particular, high expression variants of C4, a gene of the innate immune complement system, were shown to confer susceptibility to schizophrenia. However, how elevated C4 expression may impact brain circuits remains largely unknown. We used in utero electroporation to overexpress C4 in the mouse prefrontal cortex (PFC). We found reduced glutamatergic input to pyramidal cells of juvenile and adult, but not of newborn C4-overexpressing (C4-OE) mice, together with decreased spine density, which mirrors spine loss observed in the schizophrenic cortex. Using time-lapse two-photon imaging in vivo, we observed that these deficits were associated with decreased dendritic spine gain and elimination in juvenile C4-OE mice, which may reflect poor formation and/or stabilization of immature spines. In juvenile and adult C4-OE mice we found evidence for NMDA receptor hypofunction, another schizophrenia-associated phenotype, and synaptic accumulation of calcium-permeable AMPA receptors. Alterations in cortical GABAergic networks have been repeatedly associated with schizophrenia. We found that functional GABAergic transmission was reduced in C4-OE mice, in line with diminished GABA release probability from parvalbumin interneurons, lower GAD67 expression and decreased intrinsic excitability in parvalbumin interneurons. These cellular abnormalities were associated with working memory impairment. Our results substantiate the causal relationship between an immunogenetic risk factor and several distinct cortical endophenotypes of schizophrenia, and shed light on the underlying cellular mechanisms.

neuroscience↗