Neurotrophic factor trafficking and signaling in development and disease

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A fundamental question in neuronal cell biology is how membrane proteins are transported long-distance to axons after biosynthesis in cell bodies. Axon targeting of membrane proteins is critical for the formation and maintenance of neuronal connections and for a functional nervous system. Yet, how most membrane proteins are delivered to axons remains undefined. A long-held view in neurobiology is that signaling receptors are constitutively delivered to axons via secretory trafficking. In contrast, we found that TrkA neurotrophin receptors that are essential regulators of neuron survival, axon growth, and inflammatory pain are actively recruited to axons via transcytosis, an endocytosis-based mechanism where receptors embedded in soma surfaces are internalized and anterogradely transported to axons. Strikingly, anterograde TrkA transcytosis is triggered by the ligand, Nerve Growth Factor (NGF), acting on axon terminals, suggesting a positive feedback mechanism that serves to dynamically scale up receptor availability in axons during times of need. Furthermore, we identified that TrkA transcytosis is primed by the activity of PTP1B, an ER-resident protein tyrosine phosphatase, in cell bodies. The overall goal of this application is to elucidate the signaling and trafficking mechanisms underlying a poorly characterized mode of ligand-triggered targeting of receptors to axons. In Aim 1, we will define NGF-mediated mechanisms that initiate transcytosis in cell bodies, elucidate the trafficking itinerary and transport kinetics of receptor transcytosis, and investigate TrkA transcytosis in vivo. In Aim 2, we will test the hypothesis that ER- anchored PTP1B phosphatase promotes a gain of TrkA biological function by controlling the long-distance transcytosis of receptors. We will employ live imaging, biochemical, and functional analyses in compartmentalized neuron cultures in combination with in vivo analyses of genetically modified mice to accomplish these goals. These studies will address a fundamental, yet poorly studied, cell biological question of how signaling receptors are directed to axons, and will provide insight into specialized mechanisms that enhance neuronal responsiveness to spatially acting extrinsic cues.