Cell News 01/2017
26
Exocytosis of mouse cortical granules is regulated
by the subcortical maternal complex
Edgar John Vogt, Keizo Tokuhiro, Seung Wook Shin, and Jurrien Dean
SESSION 3: NEW INSIGHTS INTO CELL-CELL FUSION:
MECHANISMS OF MEMBRANE FUSION
Presenting author: Edgar John Vogt
National Institutes of Health, Bethesda, USA
Regulated exocytosis is a fundamental process to all cells for
delivering cargo molecules to the cell surface and extracellular
space. Exocytosis of cortical granules (CGs) in mouse eggs is
essential for the post-fertilization block to polyspermy and
successful progression through early mammalian embryo-
genesis. Timely release of the metalloendoprotease ovastacin
ensures cleavage of the zona pellucida protein ZP2 making the
zona pellucida unreceptive to additional sperm. Several forms
of regulated exocytosis have been described based on fusion
between the lipid bilayers of secretory vesicles and plasma
membrane. Notably, in Xenopus eggs a kiss-and-coat mech-
anism of CG exocytosis has been proposed involving de novo
actin polymerization around granular membranes and motor
activity of different myosins. However, little is known about
how mouse CGs are delivered to and fuse with the plasma
membrane.
Using ovastacin-mCherry transgenic mice for high resolution
confocal live imaging at single granule resolution, we show
that polymerizing actin filaments capture CGs upon Ca2+
stimulation. Retraction of actin filaments brings CGs to the
vicinity of microvillar plasma membrane for compartment mix-
ing. Total internal reflection fluorescence microscopy (TIRFM)
revealed that 1) depolymerization of microvillar F-actin occurs
near exocytosing CGs, and 2) exocytosis leads to the disassoci-
ation of non-muscle myosin IIA from CGs.
We also uncover a presently unknown role of the subcortical
maternal complex (SCMC). Mouse eggs lacking the SCMC
component MATER fail to tether CGs, polymerize microvillar
F-actin and express myosin IIA. This leads to delay of exocyto-
sis and cleavage of ZP2 in MATER-deficient eggs. Consequently,
significantly more sperm penetrate the zona matrix surround-
ing knockout eggs in vitro and in vivo. In conclusion, our data
shows that coupling and fusion of mouse CGs with the egg’s
microvillar plasma membrane is regulated by the SCMC.
