1. Setting the Clock on OPC Differentiation
Jason C. Dugas, Adiljan Ibrahim, and Ben A. Barres

Oligodendrocyte precursor cells (OPCs) divide a set number of times, about eight, before daughter cells finally heed external cues and differentiate into oligodendrocytes. This week, Dugas et al. tracked down the molecule that makes up this mitogen-dependent intracellular timer. The cyclin-dependent kinase inhibitor p57^Kip2 was transiently upregulated right after oligodendrocytes differentiated. In a population of cells derived from a single proliferating OPC, p57^Kip2 expression rose uniformly in successive generations. Overexpression of p57^Kip2 slowed the proliferation rate of OPCs in vitro to various degrees, depending on the availability of mitogen and of external differentiation cues. Buildup of p57^Kip2 also triggered expression of early gene markers of myelination. When the authors knocked down expression of p57^Kip2 using RNA silencing, OPC proliferation increased markedly, and differentiation ceased. Thus, p57^Kip2 fulfills the criteria for the primary component of this cellular clock, informing the cells when to abandon the cell cycle and become full-fledged oligodendrocytes.

2. B-Type Plexins in Neural Development
Suhua Deng, Alexandra Hirschberg, Thomas Worzfeld, Junia Y. Penachioni, Alexander Korostylev, Jakub M. Swiercz, Peter Vodrazka, Olivier Mauti, Esther T. Stoeckli, Luca Tamagnone, Stefan Offermanns, and Rohini Kuner

Signaling between plexin receptor molecules and their semaphorin ligands guides many developmental processes. This week, Deng et al. looked beyond the well mapped Plexin-A molecules to track the influence of neuronal Plexin-B proteins. Granule cell precursors (GCPs) of the cerebellum, the dentate gyrus, and the olfactory bulb strongly expressed Plexin-B2 mRNA within their migratory streams, but not once the cells
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Contact: Sara Harris
sharris@sfn.org
202-962-4000
Society for Neuroscience
5-Jun-2007