Abstract
Nerve Growth Factor (NGF) promotes the elaboration of axonal filopodia and branches through PI3K-Akt. NGF activates the TrkA receptor resulting in an initial transient high amplitude burst of PI3K-Akt signaling followed by a maintained lower steady state, hereafter referred to as initiation and steady state phases. Akt initially undergoes phosphorylation at T308 followed by phosphorylation at S473, resulting in maximal kinase activation. We report that during the initiation phase the localization of PI3K signaling, reported by visualizing sites of PIP3 formation, and Akt signaling, reflected by Akt phosphorylation at T308, correlates with the positioning of axonal mitochondria. Mitochondrial oxidative phosphorylation but not glycolysis is required for Akt phosphorylation at T308. In contrast, the phosphorylation of Akt at S473 is not spatially associated with mitochondria and is dependent on both oxidative phosphorylation and glycolysis. Under NGF steady state conditions, maintenance of phosphorylation at T308 shows dual dependence on oxidative phosphorylation and glycolysis. Phosphorylation at S473 is more dependent on glycolysis but also requires oxidative phosphorylation for maintenance over longer time periods. The data indicate that NGF induced PI3K-Akt signaling along axons is preferentially initiated at sites containing mitochondria, in a manner dependent on oxidative phosphorylation. Steady state signaling is discussed in the context of combined contributions by mitochondria and the possibility of glycolysis occurring in association with endocytosed signalosomes.
Highlights
Nerve growth factor (NGF) regulates multiple aspects of the development of sensory neurons including morphology
We have previously detailed that NGF induces the formation of localized actin filament patches that serve as precursors to the emergence of filopodia, and that actin patch formation is dependent on PI3K-Akt signaling and the sites of formation of actin patches colocalize with microdomains of phosphatidylinositol (3,4,5)-trisphosphate (PIP3) formation, the product of PI3K activity (Ketschek and Gallo, 2010)
Axonal segments populated by mitochondria are preferential sites for intra-axonal protein synthesis (Spillane et al, 2013; Sainath et al, 2017a) and for the formation of axonal actin filament patches and the filopodia that emerge from these patches (Ketschek and Gallo, 2010; Sainath et al, 2017a), and mitochondrial Oxidative Phosphorylation (OxP) is required for mitochondria associated protein synthesis, actin patch/filopodia formation and the emergence of axon branches in response to NGF treatment (Ketschek and Gallo, 2010; Spillane et al, 2013; Sainath et al, 2017a)
Summary
Nerve growth factor (NGF) regulates multiple aspects of the development of sensory neurons including morphology. PI3K-Akt signaling is required for the promotion of axonal actin filament dynamics underlying the emergence of axonal filopodia, the activation of intra-axonal protein synthesis of actin regulatory proteins and the fission of axonal mitochondria (Ketschek and Gallo, 2010; Spillane et al, 2012, 2013; Armijo-Weingart et al, 2019). The positioning of axonal mitochondria along axons determines sites where axon branches can form (Courchet et al, 2013; Spillane et al, 2013; Tao et al, 2014; Sainath et al, 2017a; Wong et al, 2017; Lewis et al, 2018; Smith and Gallo, 2018; Rangaraju et al, 2019) and in the context of NGF-induced branching sites of preferential intra-axonal protein synthesis of required actin regulatory proteins (Spillane et al, 2013)
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