This project focuses on the role of the scaffolding protein Kinase Suppressor of Ras 1 (KSR1) in the neuronal regulation of the signaling pathway involving Ras and extracellular signal-regulated kinase (ERK), a member of the mitogen activated protein kinases (MARK). The evolutionary conserved Ras-ERK cascade is critical for long-term synaptic plasticity and morphogenesis of dendritic spines. Many diseases that cause cognitive impairment and learning deficits, such as autism, X-linked mental retardation and neurofibromatosis 1, are associated with dysfunctions in Ras-MAPK signaling. Although the importance of the Ras-ERK signaling in neuronal plasticity is widely recognized, it is not yet clear how ERK couples specific upstream regulators to specific cell responses. Our hypothesis is that the Ras-ERK pathway is compartmentalized in neurons by MARK scaffolding proteins such as KSR1. KSR1 is highly expressed in the hippocampus and is the only known MARK scaffolding protein whose localization changes in response to extracellular stimuli. Moreover recent studies suggest that KSR1 is needed for ERK-dependent synaptic plasticity and neuronal survival. Given the clear importance of understanding the roles and regulation of the Ras-ERK pathway in morphogenesis of dendritic spines, I propose to investigate the involvement of KSR1 in regulation of spatiotemporal dynamics of the Ras-ERK pathway in neurons following two different proplasticity stimuli: calcium entry into via NMDA receptors and stimulation with BDNF. Using 2-photon live cell imaging and electrophysiology, in combination with biochemical assays, I will pursue the following three Specific Aims: 1: To elucidate the role of the regulation of KSR1-ERK interaction in ERK-dependent gene transcription. 2: To measure the dynamics of interaction between KSR1 and ERK. 3: To identify the role of KSR1-ERK interaction in dendritic spine morphogenesis. As such, the proposed research is expected to provide insights into the spatiotemporal regulation of the Ras-ERK pathway by a MARK scaffolding protein in neurons. This study will advance our understanding of the fundamental mechanisms of learning and memory and the pathophysiological basis of mental diseases related to the Ras-ERK pathway.