Regulation of growth cone actin filaments by guidance cues

Journal of Neurochemistry

Motile growth cones lead growing axons through developing tissues to synaptic targets. These behaviors depend on the organization and dynamics of actin filaments that fill the growth cone leading margin (peripheral (P-) domain). Actin filament organization in growth cones is regulated by actin-binding proteins that control all aspects of filament assembly, turnover, interactions with other filaments and cytoplasmic components, and participation in producing mechanical forces. Actin filament polymerization drives protrusion of sensory filopodia and lamellipodia, and actin filament connections to the plasma membrane link the filament network to adhesive contacts of filopodia and lamellipodia with other surfaces. These contacts stabilize protrusions and transduce mechanical forces generated by actomyosin activity into traction that pulls an elongating axon along the path towards its target. Adhesive ligands and extrinsic guidance cues bind growth cone receptors and trigger signaling ac...

Journal of visualized experiments : JoVE, 2011

The motile tips of growing axons are called growth cones. Growth cones lead navigating axons through developing tissues by interacting with locally expressed molecular guidance cues that bind growth cone receptors and regulate the dynamics and organization of the growth cone cytoskeleton. The main target of these navigational signals is the actin filament meshwork that fills the growth cone periphery and that drives growth cone motility through continual actin polymerization and dynamic remodeling. Positive or attractive guidance cues induce growth cone turning by stimulating actin filament (F-actin) polymerization in the region of the growth cone periphery that is nearer the source of the attractant cue. This actin polymerization drives local growth cone protrusion, adhesion of the leading margin and axonal elongation toward the attractant. Actin filament polymerization depends on the availability of sufficient actin monomer and on polymerization nuclei or actin filament barbed end...

Cytoskeleton, 2012

Axonal growth cones turn away from repulsive guidance cues. This may start with reduced protrusive motility in the region the growth cone leading margin that is closer to the source of repulsive cue. Using explants of E7 chick temporal retina, we examine the effects of two repulsive guidance cues, ephrin-A2 and slit3, on retinal ganglion cell growth cone protrusive activity, total F-actin, free F-actin barbed ends, and the activities (phosphorylation states) of actin regulatory proteins, ADF/cofilin and ezrin, radixin, moesin (ERM) proteins. Ephrin-A2 rapidly stops protrusive activity simultaneously with reducing F-actin, free barbed ends and the activities of ADF/cofilin and ERM proteins. Slit3 also stops protrusion and reduces the activities of ADF/cofilin and ERM proteins. We interpret these results as indicating that repulsive guidance cues inhibit actin polymerization and actinmembrane linkage to stop protrusive activity. Retrograde F-actin flow withdraws actin to the C-domain, where F-actin bundles interact with myosin II to generate contractile forces that can collapse and retract the growth cone. Our results suggest that common mechanisms are used by repulsive guidance cue to disable growth cone motility and remodel growing axon terminals. V C 2012 Wiley Periodicals, Inc

Seminars in Neuroscience, 1996

During axonal pathfinding, the direction of nerve fiber extension is established by the growth cone, the motile structure at the distal tip of an elongating axon. It is the growth cone that navigates and directs axonal outgrowth by detecting and responding to complex molecular cues in the nervous system environment. Changes in growth cone behavior and morphology that result from contact with these cues depend on the regulated assembly and dynamic reorganization of actin filaments and microtubules. Therefore, an understanding of growth cone guidance requires resolution of the cytoskeletal rearrangements that occur as navigating growth cones respond to stimulatory and inhibitory molecular signals in their milieu. In this review, we discuss the role of the cytoskeleton in growth cone navigation.

Neuron, 2003

grade actin flow ( ). The second structure, actin arcs, appear in the T zone and move into the C domain, where they contribute to central actin bundle structure. Interestingly, MTs associated with actin arcs are less dynamic than those in the P domain, exhibiting prolonged periods of slow growth due to dramatically reduced catastrophe frequencies. Our initial report identi-New Haven, Connecticut 06520 fied arcs as a novel motile actin structure in growth cones and raised questions about their physiological significance.

Journal of Cell Science, 2006

Axon guidance is mediated by the effects of attractant and repellent guidance cues on the cytoskeleton of growth cones and axons. During development, axon retraction is an important aspect of the pruning of inappropriately targeted axons in response to repellent guidance cues. I investigated the roles of RhoA-kinase and myosin II in semaphorin-3A-induced growth cone collapse and axon retraction. I report that semaphorin 3A activates myosin II in growth cones and axons. Myosin II activity is required for axon retraction but not growth cone collapse. Furthermore, semaphorin 3A promotes the formation of intra-axonal F-actin bundles in concert with the loss of F-actin in growth cone lamellipodia and filopodia. Formation of axonal F-actin bundles was independent of myosin II, but partially required RhoA-kinase activity. Conversely, RhoA-kinase activity was required to shut down F-actin polymerization underlying protrusive activity. Collectively, these observations suggest that guidance c...

Journal of Neuroscience, 2012

The development of a functioning neural network relies on responses of axonal growth cones to molecular guidance cues that are encountered en route to their target tissue. Nerve growth factor (NGF) and neurotrophin-3 serve as attractive cues for chick embryo sensory growth cones in vitro and in vivo, but little is known about the actin-binding proteins necessary to mediate this response. The evolutionarily conserved ezrin/radixin/moesin (ERM) family of proteins can tether actin filaments to the cell membrane when phosphorylated at a conserved threonine residue. Here we show that acute neurotrophin stimulation rapidly increases active phospho-ERM levels in chick sensory neuron growth cone filopodia, coincident with an increase in filopodial L1 and ␤-integrin.

Developmental Neurobiology, 2009

Repulsive guidance cues induce growth cone collapse or collapse and retraction. Collapse results from disruption and loss of the actin cytoskeleton. Actin rich regions of growth cones contain binding proteins that influence filament organization, such as Arp2/3, cortactin, and fascin, but little is known about the role that these proteins play in collapse. Here we show that Semaphorin 3A (Sema 3A), which is repulsive to mouse dorsal root ganglion neurons, has unequal effects on actin binding proteins and their associated filaments. The immunofluorescence staining intensity of Arp-2 and cortactin decreases relative to total protein, while in unextracted growth cones fascin increases. Fascin and myosin IIB staining redistribute and show increased overlap. The degree of actin filament loss during collapse correlates with filament superstructures detected by rotary shadow electron microscopy. Collapse results in the loss of branched f-actin meshworks, while actin bundles are partially retained to varying degrees. Taken together with the known affects of Sema 3A on actin, this suggests a model for collapse that follows a sequence; depolymerization of actin meshworks followed by partial depolymerization of fascin associated actin bundles and their movement to the neurite to complete collapse. The relocated fascin associated actin bundles may provide the substrate for actomyosin contractions that produce retraction.

The Journal of Neuroscience : The Official Journal of the Society for Neuroscience

During development extrinsic guidance cues modulate the peripheral actin network in growth cones to direct axons to their targets. We wanted to understand the role of the actin nucleator Arp2/3 in growth cone actin dynamics and guidance. Since growth cones migrate in association with diverse adhesive substrates during development, we probed the hypothesis that the functional significance of Arp2/3 is substrate dependent. We report that Arp2/3 inhibition led to a reduction in the number of filopodia and growth cone F-actin content on laminin and L1. However, we found substrate-dependent differences in growth cone motility, actin retrograde flow, and guidance after Arp2/3 inhibition, suggesting that its role, and perhaps that of other actin binding proteins, in growth cone motility is substrate dependent.

Neuroscience Letters, 2008

Growth cone guidance and axon elongation require the dynamic coordinated regulation of the actin cytoskeleton. As the growth cone moves, actin-dependent forces generate tension that enables protrusive activity in the periphery and drives growth cone translocation. This dynamic remodeling of the actin cytoskeleton in response to membrane tension requires activation of Src kinase. Although it has been proposed that these actin-dependent forces vary with the extent of actin cross-linking, the identity of the cross-linking protein(s) remains unknown. AFAP120 is a nervous system specific actin cross-linking protein that is regulated by Src kinase phosphorylation. Here, we report that AFAP120 is expressed and tyrosine phosphorylated in differentiating cerebellar granule cells, where it is enriched in the axon and growth cone. Over-expression of AFAP120 enhances neurite elongation in a tyrosine phosphorylation-dependent manner. These findings suggest that AFAP120 may coordinate Src signaling with the dynamic changes in the actin cytoskeleton that drive growth cone motility and axon elongation.