Mos is an upstream activator of mitogenactivated protein kinase (MAPK) and, in mouse oocytes, is responsible for metaphase II arrest. This activity has been likened to its function in Xenopus oocytes as a component of cytostatic factor. Thus, Mos-deficient female mice are less fertile and oocytes derived from these animals fail to arrest at metaphase II Here we show that maturing MOS-'-oocytes fail to activate MAPK throughout meiosis, while p34edc2 kinase activity is normal until late in metaphase II when it decreases prematurely. Phenotypically, the first meiotic division of MOS-/-oocytes frequently resembles mitotic cleavage or produces an abnormally large polar body. In these oocytes, the spindle shape is altered and the spindle fails to translocate to the cortex, leading to the establishment of an altered cleavage plane. Moreover, the first polar body persists instead of degrading and sometimes undergoes an additional cleavage, thereby providing conditions for parthenogenesis. These studies identify meiotic spindle formation and programmed degradation of the first polar body as new and important roles for the Mos/MAPK pathway.The mos protooncogene encodes a protein serine/threonine kinase (1) and Mos is expressed at high levels in oocytes undergoing meiotic maturation (2,3). InXenopus oocytes, Mos has been shown to function as a meiotic initiator (4, 5) and an active component of cytostatic factor (CSF) (6), an activity that is responsible for the arrest of an unfertilized egg at metaphase II of meiosis (7). More recently, mitogen-activated protein kinase (MAPK), which is highly activated throughout oocyte maturation (8-12), has been identified as one of the major downstream targets of Mos (13)(14)(15)(16)(17). Mos has been implicated in the activation and stabilization of p34cdc2 kinase as maturation promoting factor (MPF) (4,(18)(19)(20), and evidence for the mutual dependency between MPF and the Mos/MAPK pathway in Xenopus oocytes has been reported (8,21,22).More recently, it has been shown that oocytes from mice homozygously deficient in mos (MOS-/-) fail to arrest at metaphase II and undergo parthenogenetic activation (23,24). These observations clearly demonstrated that Mos is an active component of CSF, but left unclear whether Mos functions prior to metaphase II arrest as it does in Xenopus.We generated MOS-'-mice and confirmed previous reports (23,24) that oocytes from these animals fail to arrest at metaphase II and instead undergo parthenogenetic activation.Also we show that maturing oocytes from MOS-1-mice fail to activate MAPK, while p34cdc2 kinase activation is normal until metaphase, when it decreases prematurely. Moreover, in MOS-'-oocytes, we observe that the first polar bodies can be abnormally large and sometimes undergo an additional cleavage instead of undergoing rapid degeneration. Thus, in addition to CSF activity, Mos/MAPK are required for regulating the size and degradation of the first polar body. MATERIALS AND METHODSGeneration of MOS-1-Knockout Mice. A replacement-type ...

We have used okadaic acid (OA), a cell-permeable inhibitor of serine/threonine protein phosphatase types 1 (PP-1) and 2A (PP-2A), to demonstrate that the subcellular distribution of glucocorticoid receptor (GR) in rat fibroblasts is influenced by its phosphorylation state. Nuclear GRs in OA-treated cells retain transcriptional enhancement activity. Nuclear import or export of hormone agonist-bound GRs is not affected by OA. However, a dose of OA that fully inhibits PP-2A and partially inhibits PP-1, but not a lower dose that only partially inhibits PP-2A, leads to inefficient nuclear retention of agonist-bound GRs, and their redistribution into the cytoplasm. These receptors appear to be trapped in the cytoplasmic compartment and are unable to recycle (i.e. reenter the nucleus). Addition of OA during different steps of GR recycling demonstrates that OA must be present during nuclear export of GRs to block GR recycling. A direct role for PP-1 and/or PP-2A in GR recycling is suggested by site-specific hyperphosphorylation of GRs in vivo during OA inhibition of recycling. These are the same sites that undergo in vitro site-specific dephosphorylation by PP-1 and PP-2A. The block in GR recycling that results from inhibition of PP-1 and/or PP-2A resembles effects previously observed in v-mos-transformed rat fibroblasts. Interestingly, OA inhibition of PP-2A in v-mos-transformed cells leads to the reversal of oncoprotein effects on GR recycling and retention of receptors within the nuclear compartment. We propose that GR recycling is influenced by the activities of distinct protein phosphatases (PP-1 and/or PP-2A), and that the interference of this pathway observed in v-mos-transformed cells may be the result of effects of the oncoprotein on the phosphatases or a specific subset of their targets.