Protein Tyrosine Kinase Wee1B
Is Essential for Metaphase II
Exit in Mouse Oocytes
Jeong Su Oh, Andrej Susor, Marco Conti*
Waves of cyclin synthesis and degradation regulate the activity of Cdc2 protein kinase during the
cell cycle. Cdc2 inactivation by Wee1B-mediated phosphorylation is necessary for arrest of the
oocyte at G2-prophase, but it is unclear whether this regulation functions later during the
metaphase-to-anaphase transition. We show that reactivation of a Wee1B pathway triggers the
decrease in Cdc2 activity during egg activation. When Wee1B is down-regulated, oocytes fail to
form a pronucleus in response to Ca2+ signals. Calcium-calmodulin–dependent kinase II (CaMKII)
activates Wee1B, and CaMKII-driven exit from metaphase II is inhibited by Wee1B down-regulation,
demonstrating that exit from metaphase requires not only a proteolytic degradation of cyclin B but
also the inhibitory phosphorylation of Cdc2 by Wee1B.
Mature oocytes are arrested in metaphase of the second meiosis (MII). Fertiliza- tion triggers a series of rapid, transient
increases in the concentration of intracellular Ca2+.
These Ca2+ signals drive resumption of the cell
cycle by inactivating maturation-promoting factor
(MPF), a complex of Cdc2 and cyclin B (1). During the entry into mitosis, the activity of MPF is
regulated by the inhibitory phosphorylation of Cdc2,
which is catalyzed by the Wee kinase family (2).
However, the generally accepted view is that
proteolytic degradation of cyclin B controls MPF
inactivation and M-phase exit (1, 3). In oocytes,
cyclin B degradation occurs through the activation
of the anaphase-promoting complex (APC) in
association with its coactivator Cdc20 (APCcdc20).
However, the activation of APC through the
overexpression of Cdc20 is not sufficient to overcome the MII arrest (4, 5), suggesting that the
inactivation of MPF might be initiated by mechanisms other than cyclin B degradation. We show
that the inhibitory phosphorylation of Cdc2
through the reactivation of Wee1B is necessary
to initiate MPF inactivation during exit from meiosis II in mouse oocytes.
To investigate Wee1B function during meio-
sis, we examined whether Wee1B is expressed
during the various stages of oocyte-to-embryo
transition (6). Unlike many maternally expressed
mRNAs that are degraded during the onset of
oocyte maturation (7), Wee1B mRNA was con-
tinuously expressed during the meiotic cell cycle
and was replaced by somatic Wee1 (hereafter termed
Wee1A) during the two- or four-cell embryo mi-
totic divisions (Fig. 1A). In contrast, expression of
the Myt1 protein kinase remained stable through-
out oocyte and preimplantation development. Con-
sistent with the mRNA data, Wee1B protein
abundance increased during the meiotic cell cycle
(Fig. 1B). These results corroborate that Wee1B
is expressed in oocytes at the germinal vesicle
(GV) and MII stages and remains stable during
the meiotic cell cycle (8).
Fig. 1. Expression of
Wee1B during the oocyte-to-embryo transition. (A)
Reverse transcription polymerase chain reaction of
Wee kinase mRNAs in oocyte and preimplantation
embryos at different stages
of development. G3PDH
was used as an internal
control. (B) GV and MII oo-
cytes along with Wee1B MO–injected MII oocytes were immuno-blotted for Wee1B and a-tubulin. Each lane contains 100 oocytes.
Band intensities for Wee1B were quantified and normalized to
a-tubulin level. a.u., arbitrary units. P = 0.0283.
bla sto c y st
Center for Reproductive Sciences and The Eli and Edythe Broad
Center of Regeneration Medicine and Stem Cell Research, Department of Obstetrics, Gynecology and Reproductive Sciences,
University of California, San Francisco, CA 94143–0556, USA.
*To whom correspondence should be addressed. E-mail:
MII + MO