Foxg1 is a transcription factor that is critical for forebrain development. Foxg1(+/Cre) mice were used to test the hypotheses 1) that the subventricular zone (SZ) generates supragranular neurons, 2) that Foxg1-regulated activities define the output from the SZ, and 3) that Foxg1 is involved in the suppression of p21-initiated cell-cycle exit. Foxg1(+/Cre) mice have thinner neocortices than wild-type controls, specifically in the supragranular layers, as detected by Brn2 immunostaining. Cell proliferation in the ventricular zone (VZ) and SZ was examined to investigate the reduction in upper layer neurons. The number of cycling VZ cells was similar in Foxg1(+/+) and Foxg1(+/Cre) brains. Interestingly, cell proliferation in the SZ and intermediate progenitor cell (IPC) production (noted by Tbr2-immunostaining) was reduced in Foxg1(+/Cre) brains. These decreases coincided with increased expression of the cell-cycle inhibitor p21 in the VZ and SZ. Furthermore, colocalization of p21 with markers of cell proliferation and IPCs indicated that p21 was temporally expressed to influence the proliferative fate of IPCs. Thus, the present data are consistent with the above hypotheses, particularly, that during corticogenesis, Foxg1-regulated activities enable the expansion of the IPC population likely through suppression of p21-dependent cell-cycle exit.

CaMKIV is highly enriched in the nucleus and thought to be critical for improved survival. Here, we demonstrate by immunolocalization/ confocal microscopy and subcellular fractionation that the regulatory and catalytic subunits of m-calpain are enriched in GC nuclei, including GCs grown in medium containing 5 mM KCl. Calpain-mediated proteolysis of CaMKIV is selective, as several other nuclear and nonnuclear calpain substrates were not degraded under chronic depolarizing culture conditions. Depolarization and Ca 2؉ -dependent down-regulation of CaMKIV were associated with significant alterations in other components of the Ca 2؉ -CaMKIV signaling cascade: the ratio of phosphorylated to total cAMP response element-binding protein (a downstream CaMKIV substrate) was reduced by ϳ10-fold, and the amount of CaMK kinase (an upstream activator of CaMKIV) protein and mRNA was significantly reduced. We hypothesize that calpain-mediated CaMKIV proteolysis is an autoregulatory feedback response to sustained activation of a Ca 2؉ -CaMKIV signaling pathway, resulting from growth of cultures in medium containing 25 mM KCl. This study establishes nuclear m-calpain as a regulator of CaMKIV and associated signaling molecules under conditions of sustained Ca 2؉ influx.

There is compelling evidence that bioelectric activity promotes neuronal survival in vivo (Ikonomidou et al. 1999) and in primary cultures of sympathetic (Koike et al. 1989), cerebral cortical (Ghosh and Greenberg 1995) and cerebellar granule (Hack et al. 1993) neurons in a Ca 2+ -dependent manner. As a consequence, numerous investigators have used neuronal cultures, chronically exposed to trophic agents that activate voltage-sensitive calcium channels (VSCCs) or NMDA receptors (NRs), as a model to delineate the molecular underpinnings of Ca 2+ -mediated survival and development. In particular, granule neurons have been extensively studied as highly enriched preparations can be obtained using the cerebellar cortex of neonatal rats as a tissue source. The neurons demonstrate enhanced long-term survival when elevated KCl or NMDA is added to the medium. Alternatively, when cells are grown in medium lacking elevated KCl or NMDA, 70% (serum medium) or 50% (defined medium) undergo apoptosis by 7 days in vitro (DIV). This Ôactivity-dependenceÕ is acquired at 2-3DIV and continual exposure of cultures to elevated KCl or NMDA is required for their maintenance (Gallo et al. 1987). Pharmacological agents that interfere with CaM kinases or Ca 2+ influx also interfere with the trophic effects of KCl or NMDA (Gallo et al. 1987;Hack et al. 1993). Specifically, Type IV Ca 2+ /calmodulin-dependent protein kinase (CaMKIV) is a key component of this survival signaling pathway.Localized in CNS nuclei, CaMKIV is enriched in cerebrum, hippocampus and the granular layer of cerebellar cortex (Sakagami et al. 1999