The long-term objective of this study is to understand the molecular and cellular mechanisms that underlie axon development and the establishment of neuronal polarity. The ability of neuronal cells to polarize is essential for the organization of the nervous system and has profound functional implications. Typically, a neuron polarizes to elaborate one long axon and multiple, shorter dendrites so as to transmit and receive information and establish the circuitry that is critical for normal cognitive functions. Moreover, abundant evidence indicates that loss of neuronal polarity is associated with numerous neurodegenerative diseases. Thus understanding the machineries involved in neuronal polarization is of utmost importance to human health. The initial event in establishing a polarized neuron is the development of a single axon. We have recently identified DOCK7 as a novel activator of Rac GTPases, and demonstrated that the protein plays a crucial role in early steps of axon development. Moreover, our data have unveiled a link between DOCK7 and the microtubule regulatory protein, stathmin, and highlight the contribution of microtubule network regulation to axon development. These findings provide a unique framework for obtaining novel insight into the molecular and cellular underpinnings of axon development. This application aims to delineate further the role and mechanism of DOCK7 function in this important developmental step. Towards these goals, the first specific aim will characterize the signaling pathway that mediates the effect of DOCK7 on stathmin phosphorylation and axon development. Specific aim 2 focuses on the regulation of DOCK7, including the mechanisms that define how DOCK7 becomes activated and selectively localized in the nascent axon. Molecular, biochemical, and cell biological approaches will be used to address these objectives. The third specific aim will identify DOCK7-interacting proteins important for its function in neuronal polarization, by using biochemical purification techniques and mass spectrometry, and the yeast two-hybrid method. Finally, specific aim 4 scrutinizes the role of DOCK7 in the polarization of migrating cortical neurons in brain slices. To this end, in utero electroporation technology and two-photon microscopy will be employed. Information gained from these studies will greatly contribute to understanding the mechanisms that underlie axon development and neuronal polarization. As such, they will have significant implications for understanding biomedically relevant processes, including neuronal development, associated cognitive function, nerve regeneration and neurodegenerative disease. PUBLIC HEALTH RELEVANCE: The ability of neuronal cells to polarize - i.e. to elaborate one axon and multiple, shorter dendrites - is essential for the organization of the nervous system and has profound functional ramifications for normal cognitive functions and the regeneration of nerve cells. The proposed studies are aimed at understanding the molecular and cellular mechanisms that underlie axon development and neuronal polarization. As such, they will have significant implications for numerous biomedically relevant processes, including neuronal development, associated cognitive function, nerve regeneration and neurodegenerative disease, and may provide the basis for developing therapeutic agents for the generation and regeneration of new and damaged axons, respectively.