Regulation of Neuronal Survival by the Rit GTPase

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Signaling pathways that contribute to cell death/survival influence diseases including cancer and many neurodegenerative conditions. Elaborating signal transduction mechanisms that regulate these processes are thus important for understanding both basic biology and for therapeutic intervention. Neurotrophins potently stimulate neuronal survival in part by activating the small GTP-binding protein Ras, which functions by translating neurotrophin-initiated signals into multiple signaling pathways, including PI-3 kinase/Akt and MEK/ERK, to promote survival. In the initial funding period it was proposed that a novel, and evolutionarily conserved group of Ras-related GTPases, including two mammalian proteins (Rit and Rin) and a single Drosophila ortholog (Ric), play critical roles in regulating apoptotic signaling. From this work, it is now clear that both Rit and Ric promote neuronal survival in a manner distinct from that of Ras. The original hypothesis is now expanded to address how these anti-apoptotic signaling cascades are regulated. The central hypothesis of this proposal is that the Rit GTPase functions as a molecular switch in neurons, responding to both apoptotic stresses and neurotrophin-initiated signals, to activate a distinct pro-survival signaling cascade that relies upon p38 MAP kinase signaling. Three specific aims are proposed: Aim 1 will characterize the ability of activated Rit to promote neuronal survival. In particular, we will assess the ability of Rit signaling to protect neurons from trophic factor-withdrawal mediated apoptosis. Using primary neurons from a transgenic mouse model expressing activated Rit specifically in neurons (developed during the previous period) it is now possible to analyze these critical issues. In addition, microarray analysis will be used to catalog the neuronal transcriptional program regulated by Rit signaling. Aim 2 will determine the regulatory mechanism that couples NGF-stimulated TrkA to Rit activation and the nature of Rit-dependent regulation of the p38 MAP kinase cascade. Aim 3 will explore the critical signaling pathways utilized for Rit anti-apoptotic signaling. In particular, Rit-mediated activation of both the p38-MSK1/2 kinase cascade and CREB transcriptional pathways appear to play central roles and will be tested using a combination of both cell model systems and primary neurons. Importantly, data developed since the previous review suggests that a second novel Rit-p38-HSP27-MK2 pathway may stimulate AKT signaling to afford neuronal protection. In summary, these studies will establish a role for Rit in neuronal survival. Regulation of this novel Ras-related G-protein may have a pronounced impact on neuronal physiology and would make Rit and its effectors, potential targets for the development of new therapeutic strategies.