Nerve Growth Factor Induces Axonal Filopodia through Localized Microdomains of Phosphoinositide 3-Kinase Activity That Drive the Formation of Cytoskeletal Precursors to Filopodia

Abstract

The initiation of axonal filopodia is the first step in the formation of collateral branches and synaptic structures. In sensory neurons, nerve growth factor (NGF) promotes the formation of axonal filopodia and branches. However, the signaling and cytoskeletal mechanisms of NGF-induced initiation of axonal filopodia are not clear. Axonal filopodia arise from precursor axonal cytoskeletal structures termed filamentous actin (F-actin) patches. Patches form spontaneously and are transient. Although filopodia emerge from patches, only a fraction of patches normally gives rise to filopodia. Using chicken sensory neurons and live imaging of enhanced yellow fluorescent protein (eYFP)-actin dynamics, we report that NGF promotes the formation of axonal filopodia by increasing the rate of F-actin patch formation but not the fraction of patches that give rise to filopodia. We also demonstrate that activation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway is sufficient and required for driving the formation of axonal F-actin patches, filopodia, and axon branches. Using the green fluorescent protein-plekstrin homology domain of Akt, which targets to PI3K-generated phosphatidylinositol-3,4,5-triphosphate (PIP 3 ), we report localized microdomains of PIP 3 accumulation that form in synchrony with F-actin patches and that NGF promotes the formation of microdomains of PIP 3 and patches. Finally, we find that, in NGF, F-actin patches form in association with axonal mitochondria and oxidative phosphorylation is required for patch formation. This investigation demonstrates that surprisingly NGF promotes formation of axonal filopodia by increasing the formation of cytoskeletal filopodial precursors (patches) through localized microdomains of PI3K signaling but not the emergence of filopodia from patches.

Regulation of ECM degradation and axon guidance by growth cone invadosomes

“…2E-H), and we suspect that these two processes could be linked (Chan et al, 2009;Liu et al, 2010). It is noteworthy that the F-actin foci that we observed in growth cones share many similarities to actin patches that have been found to be responsible for collateral branching of chick DRG axons (Ketschek and Gallo, 2010;Spillane et al, 2011).…”

Actin Aggregations Mark the Sites of Neurite Initiation

“…(2) Cell adhesion molecules: N-cadherin has been reported to be asymmetrically distributed during early neuronal development, in one of the earliest events during the establishment of asymmetry [48,72]. (3) Phosphorylated derivatives of the phospholipid phosphatidylinositol or phosphoinositides: as membrane-localized signaling molecules which interact with the actin cytoskeleton [73,74], PIP3, a member of this family, has been shown to promote neuronal polarization [75,76] and the formation of axonal filopodia [77].…”

The Formins FMNL1 and mDia1 Regulate Coiling Phagocytosis of Borrelia burgdorferi by Primary Human Macrophages

“…In particular, (i) phagocytosis of Listeria by murine macrophages has been shown to be regulated by a TLR2-MyD88-phosphatidylinositol 3-kinase (PI3K) signaling axis (68), and (ii) MyD88-PI3K signaling, resulting in Arp2/3-dependent actin nucleation, has been demonstrated for phagocytosis of Borrelia burgdorferi by murine macrophages (13). Considering that PI3K signaling is central to filopodium formation, including such diverse scenarios as axonal filopodium stimulation by nerve growth factor (69) and herpes simplex virus 1 (HSV-1)-induced filopodium formation during virus uptake (70), it is tempting to speculate that Borrelia-stimulated filopodium formation involves a signal cascade involving PI3K-regulated actin nucleation downstream of TLR2-MyD88 signaling. Of note, phagocytic uptake of borreliae is a complex phenomenon that can involve a variety of receptors (10).…”