Retrograde signaling mechanisms of nerve growth factor regulating the survival and apoptosis of sympathetic neurons

During development, the survival of neurons is strictly regulated; only 50% of the neurons initially produced during development survive. The survival of several neuron populations during development, including sympathetic neurons, is regulated by neurotrophins such as nerve growth factor (NGF) released from innervation targets. NGF activates its receptor, TrkA, at axon terminals, to generate signals that are transmitted retrogradely to the cell bodies to induce signaling cascades regulating survival. A general view of this process is that NGF generates retrograde survival signals that, when delivered to cell bodies, induce downstream survival signaling that prevents apoptosis. A retrograde survival signal proposed to be necessary for sympathetic neuron survival consists of endosomes containing NGF and phosphorylated TrkA. For this signal, phosphorylated TrkA arriving at the cell bodies is required to initiate survival signaling. Studies have tested the necessity of TrkA phosphorylation in the cell bodies for survival: results from different studies contradict each other. Moreover, the Trk inhibitor, K252a, used in these studies, has reported non-specific effects. Using an alternate Trk inhibitor, Go6976, data presented in this thesis demonstrates that NGF can promote survival by retrograde signaling that does not require TrkA phosphorylation in the cell bodies. These retrograde signals may be composed of signaling molecules activated downstream of TrkA in axons since pro-survival molecules downstream of TrkA, Akt and CREB, were found activated in the cell bodies/proximal axons.;Data presented in this thesis also reveals the existence of a fundamentally different mechanism for how NGF promotes sympathetic neuron survival: a retrograde apoptotic signal that is suppressed by NGF. NGF withdrawal from axons induced the "axon apoptotic signal" that was retrogradely transmitted to the cell bodies to activate a key pro-apoptotic molecule, c-jun. The axon apoptotic signal, which was blocked by the kinase inhibitors rottlerin and chelerythrine, was necessary for apoptosis in response to NGF deprivation. Evidence GSK3 is involved in the generation or transmission of the axon apoptotic signal was provided by experiments with GSK3 inhibitors and siRNA. The discovery of the axon apoptotic signal refutes the previous view that NGF acting on axon terminals supports survival exclusively by generating retrograde survival signals. The existence of the axon apoptotic signal has broad implications for understanding not only nervous system development but also other conditions where neuronal apoptosis occurs, such as those found in neurotrauma and neurodegenerative diseases.