Brain function depends on the flow of information through precisely wired connections between axons and dendrites. Neurons within a circuit can vary widely in the number and arrangement of their dendrites, with some neurons extending only one primary dendrite into a well-defined neuropil and others developing multiple dendrites that extend symmetrically about the cell body. However, in contrast to excellent progress in uncovering mechanisms of axon specification and guidance, relatively little is known about the initial specification and outgrowth of dendrites. In vitro studies suggest that neurons initially extend multipotent neurites, one of which becomes an axon, leaving the remainder to differentiate as dendrites. These results suggest that many aspects of dendrite differentiation are intrinsically regulated. However, in vivo, dendrite development must also be coordinated with the surrounding tissue, such that dendrites are properly positioned to form the appropriate synaptic connections. How extracellular signals induce the intracellular rearrangements that drive the initial specification and subsequent morphogenesis of dendrites is unknown. In the past, this issue has been hard to tackle due to the lack of a suitable assay and the absence of any obvious molecular players. We have been addressing these problems by establishing a system for studying dendrite development in the amacrine cells of the retina. Amacrine cells are typically unipolar, extending a single apical dendrite into a discrete synaptic layer called the inner plexiform layer (IPL). However, in mice lacking the atypical cadherin Fat3, amacrine cells develop a second dendritic arbor that points away from the IPL. Since Fat3 is a cell surface receptor, these results suggest that Fat3 acts by inducing migrating precursors to retract their trailing processes in response to a signal encountered in the IPL. How Fat3 signaling ultimately promotes development of the apical dendrite is a mystery, with no known effectors or ligands. To establish a baseline of knowledge for more detailed analysis of dendrite development, two exploratory studies will be performed. First, we will develop a live imaging assay that can be used to describe the dynamic changes in neurite behavior and Golgi localization that occur as the leading process becomes a dendrite and the trailing process is retracted. Second, to work our way inside the dendrite, we will search for downstream effectors for Fat3, both by testing likely candidate proteins and by performing an unbiased screen for proteins that interact with the Fat3 intracellular domain. Together, these studies will define the salient events of dendrite specification and elucidate the signaling events that occur downstream of Fat3.