Coordinated movement depends on the creation of active connections, or synapses, between specific neurons in the motor circuit. Developmental defects or environmental factors can disrupt neuronal circuit wiring, leading to impaired movement or disease. The long-term goal of the proposed studies is to understand how gene expression regulates the specificity of synaptic inputs to motor neurons. Fundamental properties of motor neurons are evolutionarily conserved from the simple C. elegans motor circuit to the complex human spinal cord. The Miller Laboratory has shown that the UNC-4 homeodomain transcription factor regulates the specificity of synaptic inputs to VA motor neurons in C. elegans. UNC-4 works with UNC-37/Groucho to repress VB genes, such as ceh-12/HB9, in VA motor neurons. VA motor neurons, which synapse with the backward motor circuit, are miswired in unc-4 mutants and instead receive inputs normally reserved for VB motor neurons, their lineal sister cells, from the forward circuit. This miswiring results in the inability to execute backward locomotion. Preliminary data indicate that opposing Wnt signaling pathways regulate synaptic choice. UNC-4 antagonizes an EGL-20/Wnt-mediated pathway upstream of ceh-12 in posterior VA motor neurons to specify correct synaptic inputs. In addition, an opposing LIN-44/Wnt- mediated pathway functions in parallel to UNC-4 to repress ceh-12 expression in VA motor neurons. I propose to test these hypotheses, and elucidate the mechanism of Wnt-regulated synaptic choice, with the following experiments: (1) Identify components of the EGL-20/Wnt signaling pathway that is antagonized by UNC-4; (2) Establish the mechanism of unc-4 regulation of EGL-20/Wnt signaling; (3) Identify Wnt signaling components that preserve VA inputs. Experiments in this proposal will identify the mechanism of the conserved developmental process of Wnt signaling in specification of synaptic partners. The proposed studies are of central importance to the field of developmental neuroscience, as results from these experiments will provide insight into the basic biology of synaptic partner specification and the effects of the Wnt pathway on this critical decision. Disease-related phenomena, such as schizophrenia and memory retention, have been attributed to disruption of neuronal circuitry. Understanding the mechanisms important for constructing the neuronal circuit will shed light on possible regeneration technology following disease or injury and provide valuable insight into the complexity of neuron specification.