The Rb-CDK4/6 signaling pathway is critical in neural precursor cell cycleregulation

The tumor suppressor, retinoblastoma (Rb), is involved in both terminal mit osis and neuronal differentiation. We hypothesized that activation of the R b pathway would induce cell cycle arrest in primary neural precursor cells, independent of the proposed function of cyclin-dependent kinases 4/6 (CDK4 /6) to sequester the CIP/KIP CDK inhibitors (CKIs) p21 and p27 from CDK2, W e expressed dominant negative adenovirus mutants of CDKs 2, 4, and 6 (dnCDK 2, dnCDK4, and dnCDK6) in neural progenitor cells derived from E12.5 wild t ype and Rb-deficient mouse embryos. In contrast to previous studies, our re sults demonstrate that in addition to dnCDK2, the dnCDK4/6 mutants can indu ce growth arrest. Moreover, the dnCDK4/6-mediated inhibition is Rb-dependen t. The dnCDK2 partially inhibited cell growth in Rb-deficient cells, sugges ting that CDK2 may have additional targets, A previously proposed function of CDK4/6 is CKI sequestration, thereby preventing the resulting inhibition of CDK2, believed to be the key regulator of cell cycle. However, our immu noprecipitations revealed that the dominant negative CDK mutants could arre st cell growth despite their interaction with p21 and p27, Taken together, our results demonstrate that both CDK2 and CDK4/6 are crucial for cell cycl e regulation. Furthermore, our data underscore the importance of the Rb reg ulatory pathway in neuronal development and cell cycle regulation, independ ent of CRI sequestration.