Changes in cortical endoplasmic reticulum clusters in the fertilized mouse oocyte†

Area of Science:

• Cell Biology
• Reproductive Biology
• Developmental Biology

Background:

• Oocytes possess specialized endoplasmic reticulum (ER) structures called cortical ER clusters, vital for egg activation.
• The precise mechanism of cortical ER cluster involvement in fertilization remains incompletely understood.

Purpose of the Study:

• To investigate the structural relationship between cortical ER clusters and the cortical actin layer.
• To elucidate the dynamic changes and interactions of cortical ER clusters during sperm incorporation.
• To explore the implications of these interactions for natural fertilization and assisted reproductive technologies.

Main Methods:

• High-resolution imaging of oocytes to examine cortical ER clusters and actin structures.
• Analysis of sperm-oocyte interactions, including binding and fusion events.
• Investigation of ER cluster dynamics in response to fertilization, including comparisons with Izumo1 knockout models.

Main Results:

• Cortical ER clusters are tethered to novel fenestrae in the cortical actin layer.
• Sperm preferentially bind to the plasma membrane overlying these fenestrae, positioning them near ER clusters.
• Following fusion, ER clusters exhibit global shape and volume changes during sperm incorporation, with localized modifications surrounding the sperm head and nucleus.
• These ER dynamics are dependent on sperm-derived factors, as evidenced by Izumo1 knockout studies.

Conclusions:

• Cortical ER clusters interact with fertilizing sperm indirectly via actin fenestrae and directly during incorporation.
• These ER-sperm interactions are a novel aspect of fertilization and may confer a competitive advantage to the oocyte.
• Current assisted reproductive technologies like intracytoplasmic sperm injection might bypass these natural interactions.