Communication between glial cells and neurons is emerging as a critical parameter of synaptic function. However, the molecular mechanisms that glial cells use to modify synaptic structure and physiology are poorly understood. We have found that the receptor tyrosine kinase EphA4 has remarkably high expression in the adult hippocampus, a major brain center for learning and memory. EphA4 is discretely localized on dendritic spines of hippocampal pyramidal neurons. Dendritic spines are small protrusions on the surface of neurons that mediate contact with presynaptic excitatory terminals and are believed to be important for memory and cognitive processes. Spines can undergo geometrical remodeling, which has been linked to physiological effectiveness. However, the factors that regulate spine motility and organization remain unclear. Interestingly ephrin-A3, a ligand that stimulates the signaling ability of EphA4, is expressed on the surface of glial processes that surround dendritic spines. Furthermore, activation of EphA4 induces spine retraction whereas inhibiting ephrin/EphA4 interaction distorts spine shape and organization in hippocampal slices. Spine irregularities in EphA4 knock-out mice and hippocampal neurons expressing a kinase-inactive form of EphA4 further indicate that EphA4 signaling is critical for spine morphology. Thus, our data support a model where ephrin-A3/EphA4 receptor signaling between glia and neurons regulates the shape and organization of dendritic spines. Our goal is to characterize this novel form of cross-talk between glial cells and neurons. We will use organotypic cultures of hippocampal slices as a model to examine the effects of EphA4 activity on dendritic spine morphology and identify the signaling pathways activated downstream of EphA4 in dendritic spines. We will also examine whether other EphA receptors as well as EphB receptors function in concert with EphA4 to impart changes in dendritic spine morphology and organization. This project will elucidate a new and intriguing mechanism that could be critical for modulating synaptic function and perhaps the processes of learning and memory formation. Furthermore, it may clarify the neurological aspects of diseases characterized by abnormal dendritic spine structure, such as mental retardation, William's syndrome, Down's syndrome, and autism.