Synaptic plasticity, the ability of synapses to change in strength, underlies complex brain functions such as learning and memory, yet little is known about the precise molecular mechanisms and downstream signaling pathways involved. The major goal of my doctoral thesis was to understand these molecular mechanisms and cellular processes underlying synaptic plasticity using the Drosophilalarval neuromuscular junction (NMJ) as a model system. My work centered on a signaling pathway, the Wg/Wnt signaling pathway, which was found to be crucial for activity-driven synapse formation. The Wg/Wnt family of secreted proteins, besides its well-characterized roles in embryonic patterning, cell growth and cancer, is beginning to be recognized as a pivotal player during synaptic differentiation and plasticity in the brain. At the DrosophilaNMJ, the Wnt-1 homolog Wingless (Wg) is secreted from presynaptic terminals and binds to Frizzled-2 (DFz2) receptors in the postsynaptic muscle. Perturbations in Wg signaling lead to poorly differentiated NMJs, containing synaptic sites that lack both neurotransmitter release sites and postsynaptic structures. In collaboration with other members of the Budnik lab, I set out to unravel the mechanisms by which Wg regulates synapse differentiation. We identified a novel transduction pathway that provides communication between the postsynaptic membrane and the nucleus, and which is responsible for proper synapse development. In this novel Frizzled Nuclear Import (FNI) pathway, the DFz2 receptor is internalized and transported towards the nucleus. The C-terminus of DFz2 is subsequently cleaved and imported into the postsynaptic nucleus for potential transcriptional regulation of synapse development (Mathews, Ataman, et al. Science (2005) 310:1344). My studies also centered on the genetic analysis of Glutamate Receptor (GluR) Interacting Protein (dGRIP), which in mammals has been suggested to regulate the localization of GluRs and more recently, synapse development. I generated mutations in the gene, transgenic strains carrying a dGRIP-RNAi and fluorescently tagged dGRIP, and antibodies against the protein. Remarkably, I found dgrip mutants had synaptic phenotypes that closely resembled those in mutations altering the FNI pathway. Through the genetic analysis of dgrip and components of the FNI pathway, immunoprecipitation studies, electron microscopy, in vivotrafficking assays, time-lapse imaging, and yeast two-hybrid assays, I demonstrated that dGRIP had a hitherto unknown role as an essential component of the FNI pathway. dGRIP was found in trafficking vesicles that contain internalized DFz2. Further, DFz2 and dGRIP likely interact directly. Through the use of pulse chase experiments I found that dGRIP is required for the transport of DFz2 from the synapse to the nucleus. These studies thus provided a molecular mechanism by which the Wnt receptor, DFz2, is trafficked from the postsynaptic membrane to the nucleus during synapse development and implicated dGRIP as an essential component of the FNI pathway (Ataman et al. PNAS (2006) 103:7841). In the final part of my dissertation, I concentrated on understanding the mechanisms by which neuronal activity regulates synapse formation, and the role of the Wnt pathway in this process. I found that acute changes in patterned activity lead to rapid modifications in synaptic structure and function, resulting in the formation of undifferentiated synaptic sites and to the potentiation of spontaneous neurotransmitter release. I also found that these rapid modifications required a bidirectional Wg transduction pathway. Evoked activity induced Wg release from synaptic sites, which stimulated both the postsynaptic FNI pathway, as well as an alternative presynaptic Wg pathway involving GSK-3ß/Shaggy. I suggest that the concurrent activation of these alternative pathways by the same ligand is employed as a mechanism for the simultaneous and coordinated assembly of the pre- and postsynaptic apparatus during activity-dependent synapse remodeling (Ataman et al. Neuron (2008) in press). In summary, my thesis work identified and characterized a previously unrecognized synaptic Wg/Wnt transduction pathway. Further, it established a mechanistic link between activity-dependent synaptic plasticity and bidirectional Wg/Wnt signaling. These findings provide novel mechanistic insight into synaptic plasticity.