Use-dependent selection of optimal connections is a key feature of neural circuit development and in the mature brain underlies functional adaptation of sensory maps as well as learning and memory. Patterned activity guides this circuit refinement through selective stabilization or elimination of specific neuronal branches and synapses. The thalamocortical circuit is central to mammalian brain function, yet the molecular signals that mediate activity-dependent synapse and arbor stabilization and maintenance in this circuit are unknown. cpg15, discovered in a screen for activity-regulated genes, is expressed in the visual system in response to light. Spatio-temporal cpg15 expression coincides with activity-dependent wiring of thalamocortical circuits. The CPG15 protein promotes dendritic and axonal arbor growth, and enhances synaptic maturation. Preliminary analysis of general and cortex-specific cpg15 KO mice shows that although cpg15 is present in both thalamus and cortex, it is thalamic CPG15 that may be critical for maturation of thalamocortical synapses and cortical neuronal arbors. These findings suggest a novel mechanism by which thalamocortical inputs can regulate maturation of cortical synapses through signaling by CPG15. We propose to test the hypothesis that CPG15 is required and sufficient for experience-dependent development of the visual system, specifically in thalamocortical circuits. Aim 1: To test whether cpg15 is required for synapse and dendrite stabilization and experience-dependent plasticity during visual cortex development, and whether the source of CPG15 is thalamic or cortical. Synapse, spine, and dendritic arbor development as well as ocular dominance (OD) plasticity will be assessed during the critical period for OD plasticity in mouse visual cortex. To dissect the role o input- versus target-derived CPG15 signaling in the development of thalamocortical circuits, analyses will be done first in cpg15 KO mice and then in mice with cortex or thalamus-specific deletion of cpg15. Functional synapse development will be assayed by whole-cell patch-clamp recording in visual cortex slices, monitoring spontaneous AMPAR- and NMDAR-mediated mEPSCs and evoked NMDAR/AMPAR EPSC ratios. During recording, cells will be filled with biocytin and later traced to assess for delayed or stunted dendritic arbor growth. Synapse and spine dynamics on pyramidal neurons in visual cortex will be visualized in vivo using newly developed tools for dual color two-photon microscopy, and posthoc immunohistochemistry will discriminate subcortical vs. cortical inputs onto in vivo imaged synapses. Optical intrinsic signal imaging will be used to measure eye-specific responses, as well as OD plasticity in response to MD. Aim 2: To determine whether cpg15 expression from a thalamic vs. cortical source is sufficient to rescue defects in synapse and dendritic arbor stabilization and experience-dependent plasticity. Lentivirus-mediated gene transfer will be used to re-introduce CPG15 into visual cortex or thalamus of cpg15 KO mice in vivo and assess if CPG15 rescues defects in synapse and arbor stabilization and functional maturation, as well as OD plasticity.