Abstract PTEN is a phosphatidylinositol phosphatase that antagonizes signaling downstream of growth factor receptors. Mutations in PTEN have been identified in patients with autism spectrum disorder (ASD) and macrocephaly. Further, experimental deletion of Pten in the mouse brain causes macrocephaly and deficits in social behavior, suggesting a causative role in the development of ASD. In neurons, Pten knockout results in aberrant growth and increased excitatory synaptic function. Thus, studying Pten fits with our long-term goal of understanding how synaptic connectivity and activity contribute to cognitive and emotional functions. My central hypothesis is that PTEN dysfunction causes aberrant neuronal growth leading to altered synaptic circuit formation during development. Guided by this hypothesis, the specific aims of this proposal will strengthen our understanding of the molecular and neurophysiological basis of ASD. Manipulation of Pten in vivo presents a unique opportunity to examine the neurophysiological basis of ASD in a model organism. Our first aim will test our central hypothesis by determining the downstream signaling intermediates necessary for neuronal growth, synapse formation, and increased excitability after Pten knockout. We have discovered that cytoskeletal polymerization rate is increased in dendritic growth cones of neurons lacking Pten. We hypothesize that the change in polymerization rate is not dependent on mTORC1 or protein synthesis and therefore represents a parallel molecular pathway that could be targeted for treatment. Understanding the molecular mechanisms underlying these cellular phenotypes could lead to new treatments for ASDs. Our second aim will define how PTEN dependent cytoskeletal polymerization contributes to neuronal growth and synapse function and test if this is mTORC2 dependent. A gap in our understanding of neuronal cell biology exists because signaling downstream of Pten has been largely defined in immortalized cell lines. Thus, we do not know the requirements for specific signaling intermediates regulating translation and cytoskeletal remodeling in specialized processes such as dendritic branching, filopodial growth, and synapse formation. I hypothesize that PTEN function must be regulated in distinct subcellular compartments to regulate soma migration and growth versus dendritic growth and synapse formation. For the third aim, we will test subcellular localization of PTEN necessary to regulate neuronal growth, synapse formation, and synapse function. This proposal will use innovative genetic approaches to both knockout endogenous mouse genes and replace them with mutated forms in vivo. The strategy of gene knockout and molecular substitution will allow us to test specific mechanisms regulating neuronal development and function. The broad goal of this research is to understand the molecular basis of neuronal dendritic arborization facilitating synaptic contact and function.