Functional roles of local and retrograde NGF-TrkA signaling in sympathetic neurons

Nerve Growth Factor (NGF), the prototypical target-derived growth factor, is critical for growth and survival of post-ganglionic sympathetic neurons and nociceptive sensory neurons during development. However, the functional roles of local and retrograde NGF-TrkA signaling in survival and axon growth of sympathetic neurons had remained unclear. An unresolved issue in retrograde neurotrophin survival signaling has been the precise mechanism by which signals initiated at distal axons are propagated over long distances to neuronal cell bodies. Although several models have been proposed to explain long-distance retrograde survival signaling, the available evidence has neither been conclusive nor discounted the existence of any of the models. Our results provide strong support for a “Signaling Endosome Model” in which retrogradely transported NGF-TrkA complexes in signaling endosomes support neuronal survival. Among the various downstream signaling pathways of NGF-TrkA, PI3-K signaling within both cell bodies and distal axons plays critical role in NGF-dependent retrograde survival of sympathetic neurons. PI3-K signaling within cell bodies is necessary for survival, probably through activation of Akt and other downstream pro-survival effectors. In contrast, PI3-K signaling in distal axons indirectly contributes to survival, because it is critical for initiation of retrograde transport of NGF in distal axons and retrograde signaling to cell bodies. Thus, a single TrkA effector pathway has multiple roles within spatially distinct cellular locales during NGF-dependent survival of sympathetic neurons. Besides its function in supporting survival of sympathetic and sensory neurons, NGF also promotes axon growth of these neurons both in vitro and in vivo. Our results support a model in which local NGF-TrkA signaling is indispensable for axon outgrowth, and retrograde NGF-TrkA signaling in the cell bodies potentiates the axon-growth-promoting effect of local NGF signaling within distal axons.; Neurotrophins have effects on numerous types of neurons and non-neuronal cells throughout development, adult life and during aging. Neurotrophins have also been implicated in certain neuropathological processes. The lack of effective and specific tools to study the functions of neurotrophin-Trk signaling has prompted us to develop a kinase-sensitization approach to achieve specific inhibition of Trk receptor kinases. This approach is based on the engineering of both the ATP-binding site of Trk kinases and a chemical inhibitor, PP1. Knock-in mice carrying these modified Trk kinases would combine pharmacological specificity and temporal resolution of chemical inhibition with molecular specificity that genetic techniques can offer. The successful accomplishment of this project will provide powerful chemical genetic tools for future studies of neurotrophin-Trk signaling in vivo.