Notch receptors modulate transcriptional targets following the proteolytic release of the Notch intracellular domain (NotchIC). Phosphorylated forms of NotchIC have been identified within the nucleus and have been associated with CSL members, as well as correlated with regions of the receptor that are required for activity. Genetic studies have suggested that the Drosophila homolog of glycogen synthase kinase-3beta (GSK3beta), Shaggy, may act as a positive modulator of the Notch signaling. GSK3beta is a serine/threonine kinase and is a component of the Wnt/wingless signaling cascade. Here, we observed that GSK3beta was able to bind and phosphorylate Notch1IC in vitro, and attenuation of GSK3beta activity reduced phosphorylation of NotchIC in vivo. Functionally, ligand-activated signaling through the endogenous Notch1 receptor was reduced in GSK3beta null fibroblasts, implying a positive role for GSK3beta in mammalian Notch signaling. As a possible mechanistic explanation of the effect of GSK3beta on Notch signaling, we observed that inhibition of GSK3beta shortened the half-life of Notch1IC. Conversely, activated GSK3beta reduced the quantity of Notch1IC that was degraded by the proteasome. These studies reveal that GSK3beta modulates Notch1 signaling, possibly through direct phosphorylation of the intracellular domain of Notch, and that the activity of GSK3beta protects the intracellular domain from proteasome degradation.

Radial glial cells and astrocytes function to support the construction and maintenance, respectively, of the cerebral cortex. However, the mechanisms that determine how radial glial cells are established, maintained, and transformed into astrocytes in the cerebral cortex are not well understood. Here, we show that neuregulin-1 (NRG-1) exerts a critical role in the establishment of radial glial cells. Radial glial cell generation is significantly impaired in NRG mutants, and this defect can be rescued by exogenous NRG-1. Down-regulation of expression and activity of erbB2, a member of the NRG-1 receptor complex, leads to the transformation of radial glial cells into astrocytes. Reintroduction of erbB2 transforms astrocytes into radial glia. The activated form of the Notch1 receptor, which promotes the radial glial phenotype, activates the erbB2 promoter in radial glial cells. These results suggest that developmental changes in NRG-1-erbB2 interactions modulate the establishment of radial glia and contribute to their appropriate transformation into astrocytes.R adial glial cells play a critical role in the construction of the mammalian brain by contributing to the formation of neurons and astrocytes and providing an instructive scaffold for neuronal migration (1-7). The establishment of radial glial cells from an undifferentiated sheet of neuroepithelium precedes the generation and migration of neurons in the cerebral cortex. During early stages of corticogenesis, radial glial cells can give rise to neurons (3-5). Subsequent neuronal cell movement in the developing mammalian cerebral cortex occurs mainly along radial glial fibers, although nonpyramidal neurons initially migrate into the cortex in a radial glial-independent manner (1, 2, 4-7).During late stages of corticogenesis, as neurogenesis and migration dwindles, the radial glial scaffolding in the telencephalon is dismantled and transformed into type 1 astrocytes (8-10). Astrocytes contribute to the emergence and maintenance of mature brain circuitry through their function as modulators of neuronal activity (11), neurogenesis (12), and as potential neuronal precursors (13). Neurodevelopmental disorders affecting the development of radial glial cells lead to faulty neuronal and glial differentiation, thus resulting in gross CNS malformations characterized by neuronal misplacement and connectivity (14). Despite their significance, the molecular signals controlling the establishment, maintenance, and transformation of radial glial cells in the developing cerebral cortex are poorly defined.Neuregulin-1 (NRG-1), a member of the NRG family of proteins, has been implicated in the differentiation of glial cells from both central and peripheral nervous systems (15, 16). In the developing CNS, NRG-1 is expressed in migrating neurons and oligodendrocyte precursors and is thought to positively influence radial glial cell function and oligodendrocyte development (17-21). NRG-1 mediates its effects via the activation of dimers of protein tyrosine kinase receptors, erbB2, ...

The ␤-amyloid precursor protein (APP) and the Notch receptor undergo intramembranous proteolysis by the Presenilin-dependent ␥-secretase.