Our research aim is to understand how neuronal networks are established during development with the long- term goal of determining how this process can be recapitulated to repair circuits damaged after injury or disease. To extend towards its target, an axon must process directional information in the embryonic environment, reach signals at the correct time in development and be competent to interpret the cue in the correct context. However, although the mechanisms that control directionality for axons have been extensively described, it remains unresolved how either the growth rate or competence of growth cones is regulated. We will examine these questions by determining how Bone Morphogenetic Proteins (BMPs) guide commissural (C) axons in the developing spinal cord. In our previous work, we identified a new class of guidance signals: morphogens that also induce specific cell fates. We showed that BMPs, secreted from the roof plate, provide directional signals for C axons by repelling their initial projections away from the dorsal midline. We have now identified two additional critical roles for BMP signaling in C axon guidance: regulating the rate of C axonal outgrowth and directing the responsiveness of C axons to subsequent guidance cues. To investigate these previously unrecognized activities, we will define the mechanism by which BMPs inhibit the rate of C axon outgrowth in Aim 1 and determine the developmental consequences of unregulated axon outgrowth in Aim 2. In Aim 3, we will assess an integrative mechanism regulated by BMP signaling that permits a C growth cone to change its response to signals over time. By advancing our understanding of the basic mechanisms of axon guidance during development, these studies will facilitate the regeneration of neuronal circuits in the CNS. Approach: We will use convergent in vitro and in vivo methods in these studies, combining biochemical and tissue grafting assays with mouse genetics and chick in ovo electroporation, in the following aims: Aim 1: Determine how BMP signaling regulates Limk1/cofilin to control the rate of C axon outgrowth. Hypothesis: BMPs regulate the rate of C axon outgrowth by upregulating Lim kinase 1 (Limk1) in C neurons and thereby inactivating cofilin, a direct regulator of actin polymerization. Aim 2. Determine the mechanism by which axon extension speed regulates the recognition of guidance cues Hypothesis: The timing of axon outgrowth determines the response of an axon to guidance cues. Guidance errors occur if the rate of C axon outgrowth is increased because either the response of accelerated C growth cones is intrinsically altered to guidance cues, or the extrinsic environment is not in place to guide them. Aim 3. Determine how BMP signaling regulates the response of C axons to Netrin1 Hypothesis: The activation of BMP signaling in C growth cones directs C axons to respond to Netrin1 as a repellent rather than an attractant. Thus, axons accumulate a history of signaling events that informs future guidance decisions, thereby permitting a few guidance cues to generate multiple distinct axon trajectories.