Sd. Gross et al., The critical role of the MAP kinase pathway in meiosis II in Xenopus oocytes is mediated by p90(Rsk), CURR BIOL, 10(8), 2000, pp. 430-438
Background: During oocyte maturation in Xenopus, progesterone induces entry
into meiosis I, and the M phases of meiosis I and II occur consecutively w
ithout an intervening S phase. The mitogen-activated protein (MAP) kinase i
s activated during meiotic entry, and it has been suggested that the linkag
e of M phases reflects activation of the MAP kinase pathway and the failure
to fully degrade cyclin B during anaphase I. To analyze the function of th
e MAP kinase pathway in oocyte maturation, we used U0126, a potent inhibito
r of MAP kinase kinase, and a constitutively active mutant of the protein k
inase p90(Rsk), a MAP kinase target.
Results: Even with complete inhibition of the MAP kinase pathway by U0126,
up to 90% of oocytes were able to enter meiosis I after progesterone treatm
ent, most likely through activation of the phosphatase Cdc25C by the polo-l
ike kinase Plx1. Subsequently, however, U0126-treated oocytes failed to for
m metaphase I spindles, failed to reaccumulate cyclin B to a high level and
failed to hyperphosphorylate Cdc27, a component of the anaphase-promoting
complex (APC) that controls cyclin B degradation. Such oocytes entered S ph
ase rather than meiosis II. U0126-treated oocytes expressing a constitutive
ly active form of p90(Rsk) were able to reaccumulate cyclin B, hyperphospho
rylate Cdc27 and form metaphase spindles in the absence of detectable MAP k
inase activity.
Conclusions: The MAP kinase pathway is not essential for entry into meiosis
I in Xenopus but is required during the onset of meiosis II to suppress en
try into S phase, to regulate the APC so as to support cyclin B accumulatio
n, and to support spindle formation. Moreover, one substrate of MAP kinase,
p90(Rsk), is sufficient to mediate these effects during oocyte maturation.