Depolarization and Neurotrophins Converge on the Phosphatidylinositol 3-Kinase–Akt Pathway to Synergistically Regulate Neuronal Survival

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Abstract
In this report, we have examined the mechanisms whereby neurotrophins and neural activity coordinately regulate neuronal survival, focussing on sympathetic neurons, which require target-derived NGF and neural activity for survival during development. When sympathetic neurons were maintained in suboptimal concentrations of NGF, coincident depolarization with concentrations of KCl that on their own had no survival effect, synergistically enhanced survival. Biochemical analysis revealed that depolarization was sufficient to activate a Ras-phosphatidylinositol 3-kinase–Akt pathway (Ras–PI3-kinase–Akt), and function-blocking experiments using recombinant adenovirus indicated that this pathway was essential for ∼50% of depolarization-mediated neuronal survival. At concentrations of NGF and KCl that promoted synergistic survival, these two stimuli converged to promote increased PI3-kinase–dependent Akt phosphorylation. This convergent PI3-kinase–Akt pathway was essential for synergistic survival. In contrast, inhibition of calcium/calmodulin-dependent protein kinase II revealed that, while this molecule was essential for depolarization-induced survival, it had no role in KCl- induced Akt phosphorylation, nor was it important for synergistic survival by NGF and KCl. Thus, NGF and depolarization together mediate survival of sympathetic neurons via intracellular convergence on a Ras–PI3-kinase–Akt pathway. This convergent regulation of Akt may provide a general mechanism for coordinating the effects of growth factors and neural activity on neuronal survival throughout the nervous system.