Glycogen synthase kinase-3β modulates notch signaling and stability

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 [1, 2] and have been associated with CSL members [3, 4], as well as correlated with regions of the receptor that are required for activity [4, 5]. Genetic studies have suggested that the Drosophila homolog of glycogen synthase kinase-3p (GSK3p), Shaggy, may act as a positive modulator of the Notch signaling [6, 7]. GSK3P is a serine/threonine kinase and is a component of the Wnt/wingless signaling cascade [8]. Here, we observed that GSK3P was able to bind and phosphorylate Notch1IC in vitro, and attenuation of GSK3P activity reduced phosphorylation of NotchIC in vivo. Functionally, ligand-activated signaling through the endogenous Notch1 receptor was reduced in GSK3P null fibroblasts, implying a positive role for GSK3P in mammalian Notch signaling. As a possible mechanistic explanation of the effect of GSK3P on Notch signaling, we observed that inhibition of GSK30 shortened the half-life of Notch1 IC. Conversely, activated GSK3P reduced the quantity of Notch1IC that was degraded by the proteasome. These studies reveal that GSK30 modulates Notch1 signaling, possibly through direct phosphorylation of the intracellular domain of Notch, and that the activity of GSK30 protects the intracellular domain from proteasome degradation.