Neural Hu proteins (HuB͞C͞D) are RNA-binding proteins that have been shown to induce neuronal differentiation activity when overexpressed in immature neural progenitor cells or undifferentiated neuronal tumors. Newly generated HuD-deficient mice exhibited a transient impaired-cranial-nerve-development phenotype at an early embryonic stage. Adult HuD-deficient mice exhibited an abnormal hind-limb reflex and poor rotarod performance. Analysis of neurosphere formation revealed that the number and self-renewal capacity of the neural stem͞progenitor cells were increased in HuD-deficient mice. HuD-deficient primary neurospheres also generated a smaller number of neurons. Cohort analysis of the cellular proliferative activity by using BrdUrd and iododeoxuridine labeling revealed that the number of differentiating quiescent cells in the embryonic cerebral wall was decreased. Long-term administration of BrdUrd revealed that the number of slowly dividing stem cells in the adult subventricular zone was increased in the HuD-deficient mice. Taken together, the results suggest that HuD is required at multiple points during neuronal development, including negative regulation of proliferative activity and neuronal cell-fate acquisition of neural stem͞progenitor cells.

We describe a mouse model in which p27 Kip1 transgene expression is spatially restricted to the central nervous system neuroepithelium and temporally controlled with doxycycline. Transgenespecific transcripts are detectable within 6 h of doxycycline administration, and maximum nonlethal expression is approached within 12 h. After 18 -26 h of transgene expression, the G1 phase of the cell cycle is estimated to increase from 9 to 13 h in the neocortical neuroepithelium, the maximum G 1 phase length attainable in this proliferative population in normal mice. Thus our data establish a direct link between p27 Kip1 and control of G1 phase length in the mammalian central nervous system and unveil intrinsic mechanisms that constrain the G 1 phase length to a putative physiological maximum despite ongoing p27 Kip1 transgene expression.