The Actin-Based Motility Machinery of Neuronal Growth Cone Veils.Public Deposited
The growth cone is a highly specialized motile structure with a distinctive and dynamically variable morphology. Current understanding of actin-based protrusive motility has been formulated in terms of the dendritic nucleation/array treadmilling model for lamellipodial protrusion and the convergent-elongation model for filopodial protrusion, that were based upon results obtained primarily with non-neuronal motile cells. A major question in cellular neurobiology is whether the same basic mechanisms operate in the neuronal growth cone or whether they have undergone significant modification. I hypothesized that a treadmilling dendritic network, similar to that observed in other motile cells, would support normal growth cone motility. Using correlative phase and platinum replica electron microscopy I correlated directional motility in individual chick dorsal root ganglion growth cone veils to the underlying actin ultrastructure. Protruding veils appeared to contain dendritic network similar in appearance to that found in motile fibroblasts, and the network stained positively for the actin nucleator Arp2/3. Retracting veils, by contrast, contained few actin filaments; suggesting a rapid and aggressive loss of actin filaments accompanies retrograde flow of F-actin. The edge of retracting veils is lined with actin bundles oriented parallel to the membrane. The mechanism of bundling is unknown, but preliminary observations suggest a role for myosin II. Analysis of neuronal filopodia revealed that they support complex filopodia-veil dynamics, not previously reported in fibroblasts, indicating that neuronal filopodia may possess unique functional capabilities that should be evaluated further. Studies of targeted Arp2/3 depletion, performed to evaluate its role in growth cone motility, demonstrated an inconsistent knockdown phenotype. From this we concluded that while normal motility appeared supported by a dendritic network, other redundant pathways might also support growth cone motility.