The adult brain displays a low capacity for neuronal regeneration, which makes injuries or diseases that result in a loss of neurons particularly debilitating. Although the adult brain contains cells that display stem cell-like properties, these multipotent, self-replicating cells have a limited fate in vivo. As part of the first phase of this priority program (TA-310-1) we have shown that Notch1 is required by adult-type neural stem cells to remain multipotent (Nyfeler et al., 2004). We also identified Jagged1 as the ligand for Notch1 in the neurogenic regions of the adult mammalian brain that, by lateral signaling maintains Notch1 activity on neural stem cells. We have evidence that Jagged1 plays a dual role in regulating neurogenesis, firstly as a ligand for Notch1 to maintain self renewal on neighboring neural stem cells, and secondly by inducing a cell intrinsic signal to the nucleus of the Jagged1 presenting cell. It is this cell intrinsic signal that we will address in this project. We will analyze Jagged1 knockout mice, in combination with loss-of-function Notch1 alleles to elucidate the role of Jagged1/Notch1 signaling in neural stem cells in vivo. We have also generated a conditional allele for Jagged1, which we will utilize to conditionally ablate the Jagged1 gene from neural stem cells of the embryonic and adult brain. Using a conditional gene ablation approach we will circumvent the embryonic lethality caused by Jagged1 deficiency. Furthermore, we will analyze the function of the Jagged1 intracellular domain, which we propose is released from the transmembrane portion of Jagged1 and translocates to the nucleus. Based on its expression pattern, we believe that the activation of Jagged1 and its subsequent signal may regulate the differentiation of neural stem cells and interact with the Notch signaling pathway. We will express the intracellular domain of Jagged1 in cultured cells, including neural stem cells, and analyze the effects on fate and differentiation. The elucidation of lateral signals during neurogenesis will greatly facilitate our understanding of the mechanisms controlling neural stem cell differentiation and demonstration of Jagged1-mediated cell autonomous signals may define new targets for therapeutic intervention.