The human Cdc42 GTPase has been highly conserved throughout evolution, and together with its different regulatory proteins and signaling partners, helps to ensure proper cell growth and differentiation, as well as the establishment of cell polarity and the stimulation of cell migration. These diverse functions account for the essential roles played by Cdc42 in development, as well as explain how disruptions in its normal regulation give rise to developmental disorders and disease states. An over-riding goal of our research has been to define and characterize the actions of Cdc42 and its signaling partners, as a means for obtaining new insights into organism development, and to better understand their relationship to the onset of cancer. A particular area of interest has been neurogenesis and the unique but critically important roles played by the two splice-variant forms of Cdc42, which differ only in their carboxyl-terminal ten amino acids. The Cdc42 splice variant that is ubiquitously distributed (from here on called Cdc42) stimulates the transition of undifferentiated, multi-potent cells to neuroprogenitors, whereas the other splice variant that is brain-specific, Cdc42b, triggers the differentiation of neuroprogenitors into neurons. We discovered that their unique functions are due to the opposing effects they exert on EGF receptor (EGFR) - and mTORC1- signaling. Cdc42 enhances EGFR-signaling lifetimes and activates mTORC1, while Cdc42b promotes both EGFR and mTOR ubiquitylation and degradation. In this renewal application, we will build on these and other recent discoveries by pursuing the following lines of study. 1) Exploring how the ubiquitous form of Cdc42 regulates mTORC1 during neurogenesis. Based on recent findings, we will test new ideas regarding how Cdc42 contributes to the activation of the Rheb GTPase and consequently mTORC1. 2) Determining how the brain-specific Cdc42b mediates the degradation of EGFRs and mTOR during neural differentiation. We are especially interested in seeing how Cdc42b works together with its signaling partner ACK (for Activated Cdc42-associated kinase) to down-regulate the EGFR and mTOR, and whether this leads to autophagy. 3) Examining how Cdc42 functions in glioma initiating (stem) cells. Here we will investigate the significance of the specific expression of Cdc42 (and not Cdc42b) in an aggressive class of glioma initiating cells (often referred to as glioma stem cells (GSCs)) implicated in the development of glioblastoma, and see whether the Cdc42-signaling functions during neurogenesis carry-over to GSCs. These studies should provide new insights into the actions of Cdc42 and Cdc42b, as well as those of the important signaling proteins that they influence. Moreover, they should yield fundamentally important information regarding neurogenesis and the development of the brain, as well as how the improper regulation of developmental events contributes to the onset of aggressive brain cancers.