The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during spinogenesis and experience-dependent plasticity. Mutations in pathways that regulate synaptic actin in humans and mice are associated with neurological disorders (humans) and related endophenotypes (mice). My laboratory studies a signaling pathway that drives de novo actin polymerization in spines by the activation of WAVE1 downstream of Rac. The objective of this application is to analyze epistasis between Fmr1, the causative gene in Fragile X Syndrome, and WAVE1. Our central hypothesis is that elevated WAVE1 and dysregulation of actin significantly contributes to the synaptic and behavioral phenotypes of Fragile X Syndrome. This hypothesis is guided by strong preliminary data based on: 1) In Fmr1 null mice, Rac activity is elevated and inhibition of Rac normalizes LTD. 2) Our previous work showing that loss of WAVE1 results in synaptic phenotypes opposite those of Fmr1 loss. 3) WAVE1 mRNA is a direct target of FMRP and our preliminary data shows an increase in WAVE1 protein in Fmr1 null mice. 4) Our data that demonstrates that mGluR activation reorganizes spine actin and that synaptic actin dynamics are significantly altered in a mouse model of FXS. 5) Our preliminary data showing Fmr1 null memory impairments are rescued in mice also heterozygous for Wave1. The specific aims of this grant are: 1) Quantitatively analyze the link between spine actin dynamics and loss of FMRP. 2) Test if genetic reduction of WAVE1 normalizes Fmr1 null deficits. 3) Test for mimicry of Fmr1 null phenotypes by WAVE1 overexpression in vivo. Because this proposal utilizes a multidisciplinary approach to analyze how loss of Fmr1 results in abnormal WAVE1 levels, altered spine actin dynamics, synaptic plasticity, and behavioral deficits, a fundamental advance in understanding the mechanisms linking actin signaling to neuronal dysfunction in a model of Fragile X Syndrome can be anticipated. Thus the proposed research is relevant to that part of NIH's mission that pertains to the investigation of the mechanisms linking genetic mutations to mechanisms of neurological disease.