Experimental and theoretical analysis of Notch1 signaling in astrocytic differentiation of adult neural stem cells

The discovery of active neurogenesis in the adult human brain and the isolation and culture of adult neural progenitors has created a host of possibilities for the therapeutic treatment of a whole family of neurodegenerative diseases. However, before this promise can become a clinical reality, significant progress must be made in understanding the basic mechanisms that govern the behavior of these stem cells. In particular, there is a need to understand how these cells respond to various extracellular cues by either proliferating in their undifferentiated state or adopting a particular cell fate. Numerous cell-fate related decisions in these cells are regulated by a relatively small number of evolutionarily conserved signaling pathways. The Notch1 pathway is one such signaling pathway that is known to influence cell fate decisions at various stages of development in different tissue systems, including the nervous system. It has been shown to induce astrocytic differentiation in adult neural progenitor cells upon constitutive activation. We have employed a combined experimental and computational approach to analyze the effect of endogenous Notch activation in these cells and examined the underlying mechanisms that lead to the differentiation of these cells downstream of Notch signaling.; Our experimental results have demonstrated that neural progenitor cells undergo astrocytic differentiation in response to endogenous Notch activation. Hes1, an immediate downstream target of Notch signaling is both necessary and sufficient to cause this astrocytic differentiation upon Notch activation. Furthermore, transcriptional profiling of endogenous Notch activated neural stem cells have elucidated the role of GSK3beta, a component of the Wnt signaling pathway, in orchestrating the astrocytic differentiation downstream of Notch signaling in a STAT3 dependent manner.; We have also developed deterministic and stochastic predictive models to analyze the dynamics of the network of three key genes in the Notch signaling pathway, namely the notch1, hes1 and RBP-Jk genes, The models predict that the Notch-Hes1 system acts as a bistable switch by changing the expression levels of Hes1 within the cell from a low "OFF" state to a high "ON" state above a threshold level of input signal to activate the Notch pathway.