Axons of sympathetic neurons extend millimeters to meters to innervate peripheral tissues and organs, which is essential for tissue homeostasis and fundamental physiological processes including cardiac output, body temperature, regulation of blood glucose levels, and immune functions. Target-derived signals, such as the neurotrophin, Nerve Growth Factor (NGF), recruit growing axons by binding to TrkA receptors located on axon terminals, to initiate local and long-range retrograde signaling events to support neuron development, maturation, and synaptic functions. To sustain functional responses to target derived ligand, TrkA receptors must be continually localized to axon terminals. Yet, how TrkA receptors are delivered to axons remains largely undefined. Previously, we reported that TrkA receptors are delivered to sympathetic axons by transcytosis, where newly synthesized receptors are first inserted on soma surfaces, then internalized and anterogradely transported to axons. 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 my thesis project and this application is to test the hypothesis that PTP1B phosphatase promotes a gain of TrkA biological function by controlling the long- distance transcytosis of receptors. I will employ imaging, biochemical, and functional analyses in compartmentalized neuron cultures in combination with in vivo analyses of genetically modified mice to accomplish these goals. Specifically, I have generated a new mouse model, TrkAR685A knock-in mice, where a point mutation in TrkA prevents PTP1B binding, which will allow me to test the functional relevance of TrkA- PTP1B interactions on TrkA localization, neuron survival, and axon innervation of target tissues. Through this fellowship application, I will develop a basic understanding of the mechanisms of Trk receptor delivery to axons, and how this contributes to neuronal development. I will also develop the technical, communication, and leadership skills necessary to accomplish my goal of becoming an independent investigator. My training will be facilitated by the rigorous research plan, the expertise and guidance of my mentor and thesis committee, and the outstanding training resources and facilities available through Johns Hopkins University.
The development of neuronal connections, neuron function, and long-term viability of neurons relies on the correct targeting of membrane proteins to their functional sites in axons after biosynthesis in cell bodies. In this application, I will investigate a non-canonical pathway for axon targeting of a key family of signaling receptors which mediate neuron survival and axon growth. These studies will significantly contribute to a basic understanding of how proteins are delivered long-distances to axons, and its relevance in neural development, synaptic communication, and nerve regeneration.
