The mammalian brain is formed through a series of intricately orchestrated events whereby neurons born in germinal zones migrate great distances to reach their final positions and form specific connections. Abnormalities in neuronal migration and positioning are believed to be responsible in part for disorders such as lissencephaly, pediatric epilepsy, schizophrenia and autism. Recent genetic studies in mice have identified a key signaling pathway that controls cell positioning and formation of laminated structures throughout the mammalian brain. Mice with disruptions in reelin, disabled-1 (Dab1), or both very low-density lipoprotein receptor (VLDLR) and apolipoprotein E receptor 2 (ApoER2) genes exhibit nearly identical histopathological abnormalities. Reelin is an extracellular protein that directly binds to the lipoprotein receptors and induces tyrosine phosphorylation of Dab1. Dab1 is an intracellular adapter protein that is required for Reelin signaling. The long-range goal of this project is to identify molecular components downstream of Dab1 in the Reelin signaling pathway and to understand the mechanism by which Reelin controls neuronal positioning, dendritic maturation, and synaptic function. Using a yeast two-hybrid strategy, it was found that Dab1 interacts with amyloid precursor family proteins, protocadherin-18 and the novel GTPase activating protein Dab2 interacting protein (Dab2IP). The deduced amino acid sequence of Dab2IP encodes a Ras GAP related domain and several protein-protein interaction domains, including an NPxY PTB-interacting motif. It is hypothesized that Dab2IP functions as a regulator of GTPases and, by virtue of its interaction with Dab1 and other intracellular proteins, is the downstream effector in the Reelin signaling pathway. The specific aims of this proposal are to: 1) Characterize the activity, regulation and cellular function of Dab2IP;2) Determine if Dab2IP plays a critical role in Reelin signaling;3) Characterize the Dab2IP-P1-/- mice which have recently been generated in this laboratory. Understanding the biological function of Dab2IP and its role in Reelin signaling will provide valuable insight into the molecular mechanisms of neuronal migration, cell positioning and dendrite maturation during brain development.