M-phase entry in eukaryotic cells is driven by activation of MPF, a regulatory factor composed of cyclin B and the protein kinase p34 cdc2 . In G 2 -arrested Xenopus oocytes, there is a stock of p34 cdc2 /cyclin B complexes (pre-MPF) which is maintained in an inactive state by p34 cdc2 phosphorylation on Thr14 and Tyr15. This suggests an important role for the p34 cdc2 inhibitory kinase(s) such as Wee1 and Myt1 in regulating the G 2 →M transition during oocyte maturation. MAP kinase (MAPK) activation is required for M-phase entry in Xenopus oocytes, but its precise contribution to the activation of pre-MPF is unknown. Here we show that the C-terminal regulatory domain of Myt1 specifically binds to p90 rsk , a protein kinase that can be phosphorylated and activated by MAPK. p90 rsk in turn phosphorylates the C-terminus of Myt1 and downregulates its inhibitory activity on p34 cdc2 /cyclin B in vitro. Consistent with these results, Myt1 becomes phosphorylated during oocyte maturation, and activation of the MAPK-p90 rsk cascade can trigger some Myt1 phosphorylation prior to pre-MPF activation. We found that Myt1 preferentially associates with hyperphosphorylated p90 rsk , and complexes can be detected in immunoprecipitates from mature oocytes. Our results suggest that during oocyte maturation MAPK activates p90 rsk and that p90 rsk in turn downregulates Myt1, leading to the activation of p34 cdc2 / cyclin B.

Eph receptors and their membrane-associated ephrin ligands mediate cell-cell repulsion to guide migrating cells and axons. Repulsion requires that the ligand-receptor complex be removed from the cell surface, for example by proteolytic processing of the ephrin ectodomain. Here we show that cell contact-induced EphB-ephrinB complexes are rapidly endocytosed during the retraction of cells and neuronal growth cones. Endocytosis occurs in a bi-directional manner that comprises of full-length receptor and ligand complexes. Endocytosis is sufficient to promote cell detachment and seems necessary for axon withdrawal during growth cone collapse. Here, we show a mechanism for the termination of adhesion and the promotion of cell repulsion after intercellular (trans) interaction between two transmembrane proteins.

The activation of maturation-promoting factor (MPF) is required for G 2 /M progression in eukaryotic cells. Xenopus oocytes are arrested in G 2 and are induced to enter M phase of meiosis by progesterone stimulation. This process is known as meiotic maturation and requires the translation of specific maternal mRNAs stored in the oocytes. We have used an expression cloning strategy to functionally identify proteins involved in G 2 /M progression in Xenopus oocytes. Here we report the cloning of two novel cDNAs that when expressed in oocytes induce meiotic maturation efficiently. The two cDNAs encode proteins of 33 kD that are 88% identical and have no significant homologies to other sequences in databases. These proteins, which we refer to as p33 ringo (rapid inducer of G 2 /M progression in oocytes), induce very rapid MPF activation in cycloheximide-treated oocytes. Conversely, ablation of endogenous p33 ringo mRNAs using antisense oligonucleotides inhibits progesterone-induced maturation, suggesting that synthesis of p33 ringo is required for this process. We also show that p33 ringo binds to and activates the kinase activity of p34 cdc2 but does not associate with p34 cdc2 /cyclin B complexes. Our results identify a novel p34 cdc2 binding and activating protein that regulates the G 2 /M transition during oocyte maturation.

Cell-to-cell communication during development and plasticity is controlled to a large extent by signaling events downstream of receptor tyrosine kinases (RTKs). Most RTKs bind soluble ligands, which are often produced at a distance from the RTK-expressing cells, and therefore these interactions typically mediate long-range communication. Eph receptors (or Ephs), instead, bind membrane-bound ephrin ligands expressed by neighboring cells and mediate short-range cell-to-cell communication. The influence of ephrin-Eph interaction on cell behavior depends on the cell type, but can in most cases be interpreted as repulsion of neighboring cells or of cellular processes, such as the neuronal growth cone. However, in some cases ephrin-Eph activation can have the opposite effect, that is, increased adhesion/attraction. One subclass of ephrins, the ephrinB ligands, are transmembrane proteins with intrinsic (so-called reverse) signaling properties (for review, see Kullander and Klein 2002). This complicates the interpretation of functional assays and genetic phenotypes, because manipulations intended to eliminate forward receptor function often have consequences for reverse signaling as well. This review summarizes the diverse biological roles of ephrins and Ephs in embryonic development, including patterning and morphogenetic processes of the nervous and vascular systems, and in the adult, such as synaptic plasticity. We further touch upon more recent observations on ephrin functions in neurogenesis, nervous system regeneration, and tumorigenesis. Our focus is on, but is not restricted to, recent findings using genetically amenable systems. General features of ephrins and EphsDuring embryonic and postnatal development, cells need to respond to a changing environment, for example, the release of growth factors and morphogens, the migration of neighboring cells, and the production of extracellular matrix proteins by differentiating cells. Through binding to their cognate protein ligands, receptor tyrosine kinases are sensors of such environmental changes and transmit information to the inside of the cell. Of all the RTKs in the human genome, Eph receptors constitute the largest subfamily, which probably arose through rather recent gene duplications. Its 13 members (in mammals) are subdivided based on sequence similarity and ligand-binding characteristics into an A-subclass (EphA1-EphA8) and a B-subclass (EphB1-EphB4, EphB6) with partially overlapping functions (for reviews, see Wilkinson 2001;Kullander and Klein 2002). Their ligands, the ephrins, are also subdivided into an A-subclass (ephrinA1-ephrinA5), which are tethered to the exoplasmic leaflet of the cell membrane by a glycosylphosphatidylinositol (GPI) anchor, and the B-subclass (ephrinB1-ephrinB3), which contain transmembrane and cytoplasmic regions. EphrinA ligands typically bind to EphA receptors, and ephrinB ligands bind to EphB receptors. The EphA4 receptor has a broader ligand-binding spectrum, as it can bind most ephrinA, as well as ephrinB2 and ephrinB3, but n...