Growth cone behavior and production of traction force

Live imaging of regenerating lamprey spinal axons

BACKGROUND

The sea lamprey has been used as a model for the study of axonal regeneration after spinal cord injury. Although the growing tips of developing axons in lamprey have not been described, in all species studied, growth cones are complex in shape, consisting of a lamellipodium and filopodia, rich in F-actin and lacking neurofilaments (NF). By contrast, static immunohistochemical and electron microscopic observations of fixed tissue suggested that the tips of regenerating lamprey spinal axons are simple in shape, densely packed with NF, but contain very little F-actin. Thus, it has been proposed that regeneration of axons in the CNS of mature animals is not based on the canonical pulling mechanism of growth cones but involves an internal protrusive force, perhaps generated by the transport and assembly of NF. To eliminate the possibility that these histological features are due to fixation artifact, fluorescently labeled regenerating axon tips were imaged live.

METHODS

Spinal cords were transected, and after 0 to 10 weeks, the CNS was isolated in lamprey Ringer at 5 degrees C to 12 degrees C and the large reticulospinal axons were microinjected with fluorescent tracers. The proximal axon tips were imaged with a fluorescence dissecting microscope repeatedly over 2 to 5 days and photographed with confocal microscopy. Experiments were also performed through a dorsal incision in the living animal. Axon tips were microinjected as above or retrogradely labeled with tracer applied to the transection site and photographed through the fluorescence dissecting scope or with two-photon microscopy. The spinal cords were then fixed and processed for wholemount NF immunohistochemistry.

RESULTS

The living axon tips were simple in shape, not significantly different from those in fixed spinal cords, and filled with NF. In isolated CNS preparations, very little axon retraction and no regeneration was observed. In the living animal, rapid retraction, up to 3 mm/day, was seen during the 1st few days posttransection. At more than 2 weeks posttransection, some fibers showed regeneration of up to 35 microm/day.

CONCLUSIONS

1) The tips of regenerating lamprey axons are simple in shape and filled with NF. 2) Both axon retraction and axon extension are active processes, requiring factors present in the living animal that are missing in the isolated CNS.

Extreme stretch growth of integrated axons

Large animals can undergo enormous growth during development, suggesting that axons in nerves and white matter tracts rapidly expand as well. Because integrated axons have no growth cones to extend from, it has been postulated that mechanical forces may stimulate axon elongation matching the growth of the animal. However, this distinct form of rapid and sustained growth of integrated axons has never been demonstrated. Here, we used a microstepper motor system to evaluate the effects of escalating rates of stretch on integrated axon tracts over days to weeks in culture. We found that axon tracts could be stretch grown at rates of 8 mm/d and reach lengths of 10 cm without disconnection. Despite dynamic and long-term elongation, stretched axons increased in caliber by 35%, while the morphology and density of cytoskeletal constituents and organelles were maintained. These data provide the first evidence that mechanical stimuli can induce extreme "stretch growth" of integrated axon tracts, far exceeding any previously observed limits of axon growth.

Regulating filopodial dynamics through actin-depolymerizing factor/cofilin

The regulation of filopodial dynamics by neurotrophins and other guidance cues plays an integral role in growth cone pathfinding. Filopodia are F-actin-based structures that explore the local environment, generate forces and play a role in growth cone translocation. Here, we review recent research showing that the actin-depolymerizing factor (ADF)/cofilin family of proteins mediates changes in the length and number of growth cone filopodia in response to brain-derived neurotrophic factor (BDNF). Although inhibition of myosin contractility also causes filopodial elongation, the elongation in response to BDNF does not occur through a myosin-dependent pathway. Active ADF/cofilin increases the rate of cycling between the monomer and polymer pools and is critical for the BDNF-induced changes. Thus, we discuss potential mechanisms by which ADF/cofilin may affect filopodial initiation and length change via its effects on F-actin dynamics in light of past research on actin and myosin function in growth cones.

How do cells move along surfaces?

The movement of cells along surfaces is a complex phenomenon that consists of several interrelated processes, including cell-substratum adhesion, and extension and retraction of the cell edge, in which the actin cytoskeleton plays a crucial role. The past decade has seen increasingly detailed molecular-based investigations into cell motility, but it is still not known how molecular events are integrated to give cell movement. Molecular studies are now beginning to be linked to a more global concept of how whole cells move, and this combined approach promises to yield new insights into cell locomotion.

Role of myosin II in axon outgrowth

The initial stages of nerve outgrowth carried out by growth cones occur in three fundamental cyclic steps. Each of these steps appears to require myosin II activity to variable degrees. The steps include the following: (a) exploration, involving extensions and retractions that are driven and controlled by the interaction of actin retrograde flow and polymerization; (b) adhesion of new extensions to the substrate, which has been shown to be mediated by complex interactions between extracellular matrix proteins, cell adhesion proteins, and the actin cytoskeleton; and (c) traction force generated during forward advance of the growth cone, resulting in the production of tension on the neurite.

Slow axonal transport and the genesis of neuronal morphology

The classic view of slow axonal transport maintains that microtubules, neurofilaments, and actin filaments move down the axon relatively coherently at rates significantly slower than those characteristic of known motor proteins. Recent studies indicate that the movement of these cytoskeletal polymers is actually rapid, asynchronous, intermittent, and most probably fueled by familiar motors such as kinesins, myosins, and cytoplasmic dynein. This new view, which is supported by both live-cell imaging and mechanistic analyses, suggests that slow axonal transport is both rapid and plastic, and hence could underlie transformations in neuronal morphology.

Molecular mechanisms of axon guidance

Axons are guided along specific pathways by attractive and repulsive cues in the extracellular environment. Genetic and biochemical studies have led to the identification of highly conserved families of guidance molecules, including netrins, Slits, semaphorins, and ephrins. Guidance cues steer axons by regulating cytoskeletal dynamics in the growth cone through signaling pathways that are still only poorly understood. Elaborate regulatory mechanisms ensure that a given cue elicits the right response from the right axons at the right time but is otherwise ignored. With such regulatory mechanisms in place, a relatively small number of guidance factors can be used to generate intricate patterns of neuronal wiring.

Building a bridge: engineering spinal cord repair

Injuries to the spinal cord that result in disruption of axonal continuity have devastating consequences for injured patients. Current therapies that use biologically active agents to promote neuronal survival and/or growth have had modest success in allowing injured neurons to regrow through the area of the lesion. Strategies for successful regeneration will require an engineering approach. We propose the design of cell-free grafts of biocompatible materials to build a bridge across the injured area through which axons can regenerate. There are three critical regions of this bridge: the on-ramp, the surface of the bridge itself, and the off-ramp. Each of these regions has specific design requirements, which, if met, can promote regeneration of axons in the injured spinal cord. These requirements, and proposed solutions, are discussed.

Calcium-dependent regulation of interactions of caldesmon with calcium-binding proteins found in growth cones of chick forebrain neurons

1. This study was undertaken to determine if caldesmon, calmodulin, S100beta, and neurocalcin delta were present in chick forebrain neurons, and if so, to investigate the interactions of these proteins in the presence of different concentrations of calcium. 2. Immunocytochemistry was used to determine the presence and localization of these proteins in cultured forebrain neurons. Western blotting, gel electrophoresis in the presence of different concentrations of calcium, chemical cross-linking, and affinity chromatography were used to investigate the interactions of these proteins with each other. 3. Our data show that caldesmon and three calcium-binding proteins (S100beta, calmodulin, and neurocalcin 3) are localized in growth cones and neurites of chick forebrain neurons in culture. In the presence of different concentration of calcium, these calcium-binding proteins have different affinities to caldesmon and to each other. S100beta binds with greater affinity than calmodulin to caldesmon, and its ability to bind to caldesmon is regulated by neurocalcin delta. 4. These findings suggest a specific calcium-dependent regulatory pathway for modulating actomyosin during growth cone motility.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

The clutch hypothesis revisited: ascribing the roles of actin-associated proteins in filopodial protrusion in the nerve growth cone

We seek to understand how the nerve growth cone acts as a sensory motile machine to respond to chemical cues in the developing embryo. This review focuses on filopodial protrusion and F-actin-based motility because there is good evidence that these processes are required for axon guidance. The clutch hypothesis, which states that filopodial protrusion occurs by actin assembly when an actin filament is fixed with respect to the substrate (i.e., a clutch is engaged), was postulated by Mitchison and Kirscher to link protrusion to actin dynamics. Protrusion would require functional modules for movement of material into filopodia, clutching the F-actin, F-actin assembly at the tip, and retrograde flow. In this review, recent studies of actin-associated proteins involved in filopodial protrusion will be summarized, and their roles will be assessed in the context of the clutch hypothesis. The large number of proteins involved in filopodial motility and their complex interactions make it difficult to understand how these proteins act in protrusion. Recently, we have used microscale chromophore-assisted laser inactivation (micro-CALI) for the focal and acute inactivation of specific actin-associated proteins during filopodial protrusion to address their in situ roles. Our findings suggest that myosin V functions in moving membranes or other material forward in extending filopodia, that talin acts in the clutch module, and that zyxin acts in actin assembly at the tip during filopodial protrusion, perhaps by recruiting Ena/VASP family members to promote actin elongation at this site.

Major role for active extension in the formation of processes by ras-transformed fibroblasts

Expression of constitutively active Ras protein in fibroblasts results in enhanced cell motility, invasion competence and morphological changes including the formation of elongate cellular processes. These processes have been shown to resemble retraction tails formed passively behind nontransformed cells by movement relative to sites of cell-substrate attachment. However, analysis presented here reveals that active extension mechanisms also play a role in the formation of these processes. Extension of distal process ends occurs at 0.42+/-0.44 microm/min in ras-transformed fibroblasts and accounts for 63.6+/-27.5% of observed process lengths. Active process extension by ras-transformed fibroblasts also persists in the presence of cell-cell contacts. Studies conducted using actin or microtubule antagonists, and correlation of process behavior followed by fixation and immunostaining reveal that process extension requires intact actin and microtubule networks. Other analyses reveal that active extension plays a significantly smaller role in the formation of processes by non-transformed control fibroblasts. These observations demonstrate that constitutively active Ras enhances process extension in fibroblasts and is a causal factor in process extension by fibroblasts in the presence of cell-cell contacts. Moreover, these studies demonstrate that process extension by ras-transformed fibroblasts is accomplished through mechanisms similar to those thought to drive active extension of processes by other cell types including neurons. These findings suggest that extension of cellular processes could play an important role in the metastatic behavior of ras-transformed fibroblasts as well as the response of untransformed fibroblasts to receptor mediated signal transduction events.

Changes in membrane trafficking and actin dynamics during axon formation in cultured hippocampal neurons

Neurons begin to polarize when one of the neurites becomes the axon. Hippocampal neurons in cell culture have a sharp transition between their unpolarized and polarized stage revealed by the rapid growth of the future axon. Recent progress shows that both a cytoplasmic membrane flow and actin dynamics govern axon formation, and thereby initial neuronal polarization. We here review these mechanisms, evaluate their physiological role, and show similarities to the transient polarization of migrating fibroblasts. Finally, we present a model how actin dynamics and vectorial membrane flow may interact to achieve axon formation.

Attachment of growth cones on substrate observed by multi-mode light microscopy

Evanescent light illumination was introduced into a multi-mode microscope to construct a new type of total internal reflection fluorescence microscope (TIRFM). This microscope, capable of TIRFM, high resolution video-enhanced differential interference contrast (DIC), epifluorescence, interference reflection (IR) imaging, was combined with an image acquisition system for time-lapse microscopy. Neuronal growth cones of a rat hippocampal neuron were stained with membrane labeling fluorescence dyes (DiI or octadecyl rhodamine B). Dynamic changes of the cell substrate contact of the neuronal growth cone were observed using the multi-imaging capacities of this system. When growth cone regions were stimulated by pressure ejection of a high potassium solution, TIRFM intensity at the basal membrane of the growth cone increased, suggesting that basal membrane of growth cone approaches the glass substrate when excited. The approach of the ventral membrane to the substrate during excitatory stimulation was also observed with IR microscope. The functional importance of cell/substrate contact in growth cones is discussed.

Stimulation-induced changes in filopodial dynamics determine the action radius of growth cones in the snail Helisoma trivolvis

Filopodia on neuronal growth cones constantly extend and retract, thereby functioning as both sensory probes and structural devices during neuronal pathfinding. To better understand filopodial dynamics and their regulation by encounters with molecules in the environment, we investigated filopodial dynamics of identified B5 neurons from the buccal ganglion of the snail Helisoma trivolvis before and after treatment with nitric oxide (NO). We have previously demonstrated that treatment with several NO-donors caused a transient, cGMP-mediated elevation in [Ca(2+)](i), which was causally related to an increase in filopodial length and a reduction in the number of filopodia on growth cones. We demonstrate here that these effects were the result of distinct changes in filopodial dynamics. The NO-donor SIN-1 induced a general increase in filopodial motility. Filopodial elongation after treatment with SIN-1 resulted from a significant increase in the rate at which filopodia extended, as well as a significant increase in the time filopodia spent elongating. The reduction in filopodial number was caused by a significant decrease in the frequency with which new filopodia were inserted into the growth cone. With the exception of the back where filopodia appeared less motile, filopodial dynamics appeared to be mostly independent of the location on the growth cone. These results suggest that NO can regulate filopodial dynamics on migrating growth cones and might function as a messenger to adjust the action radius of a growth cone during pathfinding.

Regulated actin cytoskeleton assembly at filopodium tips controls their extension and retraction

The extension and retraction of filopodia in response to extracellular cues is thought to be an important initial step that determines the direction of growth cone advance. We sought to understand how the dynamic behavior of the actin cytoskeleton is regulated to produce extension or retraction. By observing the movement of fiduciary marks on actin filaments in growth cones of a neuroblastoma cell line, we found that filopodium extension and retraction are governed by a balance between the rate of actin cytoskeleton assembly at the tip and retrograde flow. Both assembly and flow rate can vary with time in a single filopodium and between filopodia in a single growth cone. Regulation of assembly rate is the dominant factor in controlling filopodia behavior in our system.

Regulation of chick dorsal root ganglion growth cone filopodia by protein kinase C

Growth cone filopodia function both as structural and sensory devices during neuronal pathfinding and their presence is important for correct growth cone navigation. It is assumed that a growth cone can adjust the area of the environment it can explore by regulating the length and number of its filopodial sensors, and in several cell types, these parameters are controlled by the intracellular calcium concentration ([Ca(2+)](i)). In the present report, we address the question whether [Ca(2+)](i) is a general regulator of growth cone filopodia, or whether different cell types utilize different second-messenger systems for this purpose. We show that increasing [Ca(2+)](i) in growth cones of chick dorsal root ganglion (DRG) neurons does not affect average filopodial length in this cell type, suggesting that this parameter is not controlled by [Ca(2+)](i) in chick DRG neurons. Further, we demonstrate that the second-messenger protein kinase C (PKC) is involved in the regulation of filopodial length in chick DRG neurons. Activation of PKC with the phorbol ester, phorbol myristate-13-acetate (PMA), caused filopodial shortening, whereas inhibition of PKC with either bisindolylmaleimide I or calphostin C caused a significant elongation of filopodia. Although the pathway through which PKC mediates its effect on growth cone filopodia in chick DRG neurons remains to be identified, our results indicate that filopodial regulation by [Ca(2+)](i), though clearly important in several other neuronal cell types in vitro, appears to be less important in chick DRG neurons. Rather, we find that in chick DRG neurons, filopodial parameters are controlled by PKC.

Filopodial adhesion does not predict growth cone steering events in vivo

Migration of growth cones is in part mediated by adhesive interactions between filopodia and the extracellular environment, transmitting forces and signals necessary for pathfinding. To elucidate the role of substrate adhesivity in growth cone pathfinding, we developed an in vivo assay for measuring filopodial-substrate adhesivity using the well-characterized Ti pioneer neuron pathway of the embryonic grasshopper limb. Using time-lapse imaging and a combination of rhodamine-phalloidin injections and DiI labeling, we demonstrate that the filopodial retraction rate after treatment with cytochalasin D or elastase reflects the degree of filopodial-substrate adhesivity. Measurements of filopodial retraction rates along regions of known differing substrate adhesivities confirmed the use of this assay to examine filopodial-substrate adhesion during in vivo pathfinding events. We analyzed 359 filopodia from 22 Ti growth cones and found that there is no difference between the retraction rates of filopodia extending toward the correct target (on-axis) and filopodia extending away from the correct target (off-axis). These results indicate on-axis and off-axis filopodia have similar substrate adherence. Interestingly, we observed a 300% increase in the extension rates of on-axis filopodia during Ti growth cone turning events. Therefore, in addition to providing filopodia with important guidance information, regional cues are capable of modulating the filopodial extension rate. The homogeneity in filopodial retraction rates, even among these turning growth cones in which differential adhesivity might be expected to be greatest, strongly establishes that differential adhesion does not govern Ti pioneer neuron migration rate or pathfinding. We propose that the presence of local differences in receptor-mediated second messenger cascades and the resulting assembly of force-generating machinery may underlie the ability of filopodial contacts to regulate growth cone steering in vivo.

Filopodial adhesion does not predict growth cone steering events in vivo

Migration of growth cones is in part mediated by adhesive interactions between filopodia and the extracellular environment, transmitting forces and signals necessary for pathfinding. To elucidate the role of substrate adhesivity in growth cone pathfinding, we developed an in vivo assay for measuring filopodial-substrate adhesivity using the well-characterized Ti pioneer neuron pathway of the embryonic grasshopper limb. Using time-lapse imaging and a combination of rhodamine-phalloidin injections and DiI labeling, we demonstrate that the filopodial retraction rate after treatment with cytochalasin D or elastase reflects the degree of filopodial-substrate adhesivity. Measurements of filopodial retraction rates along regions of known differing substrate adhesivities confirmed the use of this assay to examine filopodial-substrate adhesion during in vivo pathfinding events. We analyzed 359 filopodia from 22 Ti growth cones and found that there is no difference between the retraction rates of filopodia extending toward the correct target (on-axis) and filopodia extending away from the correct target (off-axis). These results indicate on-axis and off-axis filopodia have similar substrate adherence. Interestingly, we observed a 300% increase in the extension rates of on-axis filopodia during Ti growth cone turning events. Therefore, in addition to providing filopodia with important guidance information, regional cues are capable of modulating the filopodial extension rate. The homogeneity in filopodial retraction rates, even among these turning growth cones in which differential adhesivity might be expected to be greatest, strongly establishes that differential adhesion does not govern Ti pioneer neuron migration rate or pathfinding. We propose that the presence of local differences in receptor-mediated second messenger cascades and the resulting assembly of force-generating machinery may underlie the ability of filopodial contacts to regulate growth cone steering in vivo.

The role of local actin instability in axon formation

The role of localized instability of the actin network in specifying axonal fate was examined with the use of rat hippocampal neurons in culture. During normal neuronal development, actin dynamics and instability polarized to a single growth cone before axon formation. Consistently, global application of actin-depolymerizing drugs and of the Rho-signaling inactivator toxin B to nonpolarized cells produced neurons with multiple axons. Moreover, disruption of the actin network in one individual growth cone induced its neurite to become the axon. Thus, local instability of the actin network restricted to a single growth cone is a physiological signal specifying neuronal polarization.

Disruption of dynamic cell surface architecture of NIH3T3 fibroblasts by the N-terminal domains of moesin and ezrin: in vivo imaging with GFP fusion proteins

Lamellipodia, filopodia, microspikes and retraction fibers are characteristic features of a dynamic and continuously changing cell surface architecture and moesin, ezrin and radixin are thought to function in these microextensions as reversible links between plasma membrane proteins and actin microfilaments. Full-length and truncated domains of the three proteins were fused to green fluorescent protein (GFP), expressed in NIH3T3 cells, and distribution and behaviour of cells were analysed by using digitally enhanced differential interference contrast (DIC) and fluorescence video microscopy. The amino-terminal (N-)domains of all three proteins localize to the plasma membrane and fluorescence recordings parallel the dynamic changes in cell surface morphology observed by DIC microscopy of cultured cells. Expression of this domain, however, significantly affects cell surface architecture by the formation of abnormally long and fragile filopodia that poorly attach and retract abnormally. Even more striking are abundant irregular, branched and motionless membraneous structures that accumulate during retraction of lamellipodia. These are devoid of actin, endogenous moesin, ezrin and radixin, but contain the GFP-labeled domain. While a large proportion of endogenous proteins can be extracted with non-ionic detergents as in untransfected control cells, >90% of N-moesin and >60% of N-ezrin and N-radixin remain insoluble. The minimal size of the domain of moesin required for membrane localization and change in behavior includes residues 1–320. Deletions of amino acid residues from either end result in diffuse intracellular distribution, but also in normal cell behavior. Expression of GFP-fusions of full-length moesin or its carboxy-terminal domain has no effect on cell behavior during the observation period of 6–8 hours. The data suggest that, in the absence of the carboxy-terminal domain, N-moesin, -ezrin and -radixin interact tightly with the plasma membrane and interfere with normal functions of endogeneous proteins mainly during retraction.

Axonal outgrowth of cultured neurons is not limited by growth cone competition

We have examined the question of scarcity-driven competition for outgrowth among growth cones of a single neuron. We measured spontaneous neurite elongation rates from 85 hours of videotape of the arbors of 31 chick sensory neurons in culture. These rate measurements were analyzed in ten minute periods that allowed cell bodies to be classified as to the number of their growth cones and the elongation to be analyzed as a series of discrete events. Comparing periods in which neurons maintained simple bipolar morphology we find no temporal competition between the two growth cones. That is, periods of above-average growth by one growth cone are not compensated by below-average growth during the same period by its sibling growth cone. Analyzing all outgrowth from a neuron based on its number of growth cones shows that net elongation rate from a single cell body is a linear function of the number of growth cones from 1 to 11. These observations suggest that growth cones behave independently and are not limited by availability of structural precursors. A surplus pool of structural precursors available for normal growth is also indicated by the high capacity for growth from single neurites when experimentally stimulated by mechanical tension. In addition, towing one or more neurites at above average rates does not cause any decline in simultaneous growth cone-mediated outgrowth from a single neuron compared to the 2–3 hour period prior to experimentally induced elongation. This high capacity for growth combined with the often observed, intermittant growth behavior of individual growth cones suggests that neurite outgrowth is intrinsically limited primarily by poor growth cone ‘performance,’ not scarcity-driven competition. We postulate that growth cones are poor ‘tractors,’ exerting too little tension to exploit the available capacity for axonal elongation.

Arginine kinase expression and localization in growth cone migration

Migrating neuronal growth cones exert traction forces that are generated by ATP-driven F-actin/myosin interactions. Sustained generation of these forces may require an energy supply mediated by the guanidino kinases, creatine kinase and arginine kinase. We cloned and sequenced grasshopper arginine kinase and examined its expression during embryogenesis and its subcellular localization in vivo and in vitro. During the first half of embryogenesis, arginine kinase is expressed selectively in a small percentage of ectodermal cells (dorsal closure cells), in a small percentage of mesodermal cells (muscle pioneers), and throughout the developing CNS. Most of these cell types are motile, including nascent neurons, muscle pioneers, dorsal closure cells, and many CNS glia. Neuroblasts also strongly express arginine kinase; they are nonmotile but are undergoing repeated rounds of (ATP-dependent) mitosis. Arginine kinase is colocalized with F-actin in a narrow band along the leading edges of lamellipodia of migrating glia. In neurons undergoing axonogenesis, arginine kinase is concentrated in growth cones and extends to the tips of filopodia. The amount of arginine kinase varies widely between growth cones, even between different growth cones of the same neuron. Energy for growth cone migration appears to be mobilized by (1) selective expression of arginine kinase by neurons, (2) localization of arginine kinase within growth cones, and (3) concentration of arginine kinase within specific growth cones, depending on the traction forces being generated. Mobilization of guanidino kinases may participate in the selective growth of specific growth cones.

Autonomous regulation of growth cone filopodia

The fan-shaped array of filopodia is the first site of contact of a neuronal growth cone with molecules encountered during neuronal pathfinding. Filopodia are highly dynamic structures, and the "action radius" of a growth cone is strongly determined by the length and number of its filopodia. Since interactions of filopodia with instructive cues in the vicinity of the growth cone can have effects on growth cone morphology within minutes, it has to be assumed that a large part of the signaling underlying such morphological changes resides locally within the growth cone proper. In this study, we tested the hypothesis that two important growth cone parameters-namely, the length and number of its filopodia-are regulated autonomously in the growth cone. We previously demonstrated in identified neurons from the snail Helisoma trivolvis that filopodial length and number are regulated by intracellular calcium. Here, we investigated filopodial dynamics and their regulation by the second-messenger calcium in growth cones which were physically isolated from their parent neuron by neurite transection. Our results show that isolated growth cones have longer but fewer filopodia than growth cones attached to their parent cell. These isolated growth cones, however, are fully capable of undergoing calcium-induced cytoskeletal changes, suggesting that the machinery necessary to perform changes in filopodial length and number is fully intrinsic to the growth cone proper.

Arginine kinase expression and localization in growth cone migration

Migrating neuronal growth cones exert traction forces that are generated by ATP-driven F-actin/myosin interactions. Sustained generation of these forces may require an energy supply mediated by the guanidino kinases, creatine kinase and arginine kinase. We cloned and sequenced grasshopper arginine kinase and examined its expression during embryogenesis and its subcellular localization in vivo and in vitro. During the first half of embryogenesis, arginine kinase is expressed selectively in a small percentage of ectodermal cells (dorsal closure cells), in a small percentage of mesodermal cells (muscle pioneers), and throughout the developing CNS. Most of these cell types are motile, including nascent neurons, muscle pioneers, dorsal closure cells, and many CNS glia. Neuroblasts also strongly express arginine kinase; they are nonmotile but are undergoing repeated rounds of (ATP-dependent) mitosis. Arginine kinase is colocalized with F-actin in a narrow band along the leading edges of lamellipodia of migrating glia. In neurons undergoing axonogenesis, arginine kinase is concentrated in growth cones and extends to the tips of filopodia. The amount of arginine kinase varies widely between growth cones, even between different growth cones of the same neuron. Energy for growth cone migration appears to be mobilized by (1) selective expression of arginine kinase by neurons, (2) localization of arginine kinase within growth cones, and (3) concentration of arginine kinase within specific growth cones, depending on the traction forces being generated. Mobilization of guanidino kinases may participate in the selective growth of specific growth cones.

PC12 cell aggregation and neurite growth in gels of collagen, laminin and fibronectin

PC12 cells form aggregates when suspended within three-dimensional, self-assembled, type I collagen gels; these aggregates increase in size over time. In addition, when the cells are cultured in the presence of nerve growth factor, they express neurites, which extend through the three-dimensional matrix. In this report, the roles of fibronectin, laminin and nerve growth factor in PC12 cell aggregation and neurite growth following suspension in collagen matrices were evaluated. Single cells and small clusters of cells were suspended in collagen gels; the kinetics of aggregation were determined by measurement of the projected area of each aggregate, and neurite lengths were determined by measurement of end-to-end distance. Fibronectin and laminin inhibited the aggregation of PC12 cells at 50 micrograms/ml, and fibronectin, but not laminin, inhibited the growth of neurites at 100 micrograms/ml. In the absence of serum, the aggregation of cells cultured with nerve growth factor was almost completely inhibited, but the average neurite length was unaffected. In the presence of nerve growth factor, the extent of cell aggregation could not be explained simply by an increase in cell number, suggesting the presence of two separate mechanisms for aggregate growth: one dependent on cell motility and another dependent on cell proliferation.

The cytoskeletal linking proteins, moesin and radixin, are upregulated by platelet-derived growth factor, but not basic fibroblast growth factor in experimental mesangial proliferative glomerulonephritis

The expression of the two cytoskeletal linking proteins, moesin and radixin, was examined in experimental mesangial proliferative nephritis in rats (anti-Thy1 model). Moesin and radixin mRNA and protein are constitutively expressed in all cell types of normal rat glomeruli, except podocytes. In the anti-Thy1 model the expression of moesin and radixin was increased in infiltrating macrophages and in activated, alpha-smooth muscle actin-positive mesangial cells and was concentrated in the cellular extensions of mesangial cells in areas of glomerular remodelling. Studies using neutralizing antibodies demonstrated that the increase in moesin and radixin expression by mesangial cells is mediated by PDGF, but not bFGF. The increase in these cytoskeletal proteins appears to be regulated primarily (radixin) or partially (moesin) posttranscriptionally. The data suggest that PDGF mediated upregulation of the cytoskeletal proteins, moesin and radixin, is important for cell migration and other changes that accompany the coordinated restoration of glomerular architecture after injury.

A Drosophila homolog of the Rac- and Cdc42-activated serine/threonine kinase PAK is a potential focal adhesion and focal complex protein that colocalizes with dynamic actin structures

Changes in cell morphology are essential in the development of a multicellular organism. The regulation of the cytoskeleton by the Rho subfamily of small GTP-binding proteins is an important determinant of cell shape. The Rho subfamily has been shown to participate in a variety of morphogenetic processes during Drosophila melanogaster development. We describe here a Drosophila homolog, DPAK, of the serine/threonine kinase PAK, a protein which is a target of the Rho subfamily proteins Rac and Cdc42. Rac, Cdc42, and PAK have previously been implicated in signaling by c-Jun amino-terminal kinases. DPAK bound to activated (GTP-bound) Drosophila Rac (DRacA) and Drosophila Cdc42. Similarities in the distributions of DPAK, integrin, and phosphotyrosine suggested an association of DPAK with focal adhesions and Cdc42- and Rac-induced focal adhesion-like focal complexes. DPAK was elevated in the leading edge of epidermal cells, whose morphological changes drive dorsal closure of the embryo. We have previously shown that the accumulation of cytoskeletal elements initiating cell shape changes in these cells could be inhibited by expression of a dominant-negative DRacA transgene. We show that leading-edge epidermal cells flanking segment borders, which express particularly large amounts of DPAK, undergo transient losses of cytoskeletal structures during dorsal closure. We propose that DPAK may be regulating the cytoskeleton through its association with focal adhesions and focal complexes and may be participating with DRacA in a c-Jun amino-terminal kinase signaling pathway recently demonstrated to be required for dorsal closure.

The dynamics of growing axons. Membrane biophysics

Experiments with laser optical tweezers throw light on the old question of where membrane is added in a growing nerve axon.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Mammalian myosin I alpha is concentrated near the plasma membrane in nerve growth cones

To determine if unconventional myosins play a role in nerve outgrowth, antibodies specific for rat brain derived mammalian myosin I alpha (MMI alpha) were used to label cultured rat superior cervical ganglion nerve cells. Observations were made at both the light and electron microscopic level of resolution using preparative procedures designed to enhance the ability to precisely determine the relationship between antibody label and cellular structures in order to map the distribution and structural association of this myosin. Immunofluorescence showed that MMI alpha has a punctate distribution throughout the nerve cell body, neurites, and growth cones. In growth cones, MMI alpha staining is sometimes elevated in thin peripheral regions of high actin content at the leading edge. Immunoelectron microscopy using colloidal gold conjugated antibodies showed that in growth cones MMI alpha is absent from membranous organelles and is concentrated primarily in the cell cortex adjacent to the cell membrane. The cortical label is equally distributed between upper and lower membranes. The plasma membrane association of the MMI alpha label persists under conditions in which the actin cytoskeleton is perturbed or removed, suggesting a direct association between a fraction of MMI alpha and the plasma membrane. MMI alpha label is also associated with the non-cortical actin cytoskeleton. Partial disruption of the actin cytoskeleton using cytochalasin B causes redistribution of only a subset of MMI alpha label. These data suggest a complex relationship between MMI alpha, the actin cytoskeleton, and the plasma membrane in the growth cone. In contrast to its localization in the growth cone, in neuronal cell bodies MMI alpha is also associated with tubulovesicular structures. This suggests that at this location MMI alpha may either act as an organelle motor or is passively transported to the plasma membrane on vesicles.

Axon membrane flows from the growth cone to the cell body

During the growth of axons, the surface area of the neuron increases dramatically. Membrane addition as well as exchange could contribute to rapid membrane dynamics or flow. Using diffusing latex beads to monitor membrane flow, we find that axonal membrane flows rapidly (7 microns/min) from growth cone to cell body during axon growth and that flow is inhibited by brefeldin A. To power this flow, there is a membrane lesion gradient from growth cone to cell body that could draw the membrane over the axon at that rate. Further, when an artificial flow is induced to the center of the axon by use of laser tweezers, the primary source of the membrane is from the growth cone. We suggest that during neuron growth, there is excess membrane added at the growth cone in chick dorsal ganglia (DRGs) that undergoes edcocytosis at the cell body, thereby creating a flow that can rapidly alter the content of the axon membrane.

Inhibition of formation of filopodia after axotomy by inhibitors of protein tyrosine kinases

The activity of motile protrusions of the growth cone--filopodia, veils, and lamellipodia--is essential for directed growth of a neuronal process. The regulation of the formation of these protrusions is not well understood. Numerous filopodia and veils or lamellipodia form within minutes of transection of an Aplysia axon in culture, as the initial components of growth cones of regenerating neurites. Axotomy, therefore, provides a robust and reliable protocol for analyzing the formation of these protrusions. We evaluated the involvement of protein phosphorylation in the regulation of protrusive activity. Of the inhibitors of protein kinases assayed, only the inhibitors of protein tyrosine kinases--genistein, lavendustin A, herbimycin A, and erbstatin analogue--suppressed the formation of protrusions, as assessed by high magnification video microscopy. These drugs did not work by preventing resealing of the axon, as evident from visual inspection and by the unimpaired effectiveness of genistein or lavendustin in preventing formation of filopodia when applied after resealing. Inhibition of protein tyrosine kinases not only prevented the formation of actin-based protrusions, but also caused deterioration of the actin network underlying the protrusive area of preexisting growth cones. Consistent with an involvement of protein tyrosine phosphorylation in the generation of protrusive structures, immunocytochemistry revealed that aggregates of phosphotyrosine appeared at the margins of the axon, from which protrusions emerge shortly after axotomy. These results suggest a role for protein tyrosine phosphorylation in the formation and maintenance of actin-based protrusive structures.

Localized collapsing cues can steer growth cones without inducing their full collapse

Collapsing factors are proteins that induce growth cone collapse and paralysis when added in a soluble form to cultured embryonic neurons. Here we examine the responses of growth cones to localized collapsing signals. Temporal retinal ganglion cell growth cones exposed to a localized collapsing stimulus from nasal retinal ganglion cell axons frequently turn smoothly away from the axons without collapsing. Turning is rare on contact with retinal axons that are unable to induce collapse. In a separate series of experiments, dorsal root ganglion growth cones tend to turn away from beads coated with a brain extract enriched for the motility-inhibiting protein collapsin. Many turns are accomplished with filopodial contact alone. Growth cones do not turn away from control beads coated with heat-inactivated collapsin. These results suggest that inhibitory guidance cues can steer growth cones through a localized inhibition of lamellipodial protrusion.

Cytomechanics of axonal development

Mechanical tension is a robust regulator of axonal development of cultured neurons. We review work from our laboratory, using calibrated glass needles to measure or apply tension to chick sensory neurons, chick forebrain neurons, and rat PC12 cells. We survey direct evidence for two different regimes of tension effects on neurons, a fluid-like growth regime, and a nongrowth, elastic regime. Above a minimum tension threshold, we observe growth effects of tension regulating four phases of axonal development: 1. Initiation of process outgrowth from the cell body; 2. Growth cone-mediated elongation of the axon; 3. Elongation of the axon after synaptogenesis, which normally accommodates the skeletal growth of vertebrates; and 4. Axonal elimination by retraction. Significantly, the quantitative relationship between the force and the growth response is surprisingly similar to the simple relationship characteristic of Newtonian fluid mechanical elements: elongation rate is directly proportional to tension (above the threshold), and this robust linear relationship extends from physiological growth rates to far-above-physiological rates. Thus, tension apparently integrates the complex biochemistry of axonal elongation, including cytoskeletal and membrane dynamics, to produce a simple "force input/growth output" relationship. In addition to this fluid-like growth response, peripheral neurons show elastic behaviors at low tensions (below the threshold tension for growth), as do most cell types. Thus, neurites could exert small static forces without diminution for long periods. In addition, axons of peripheral neurons can actively generate modest tensions, presumably similar to muscle contraction, at tensions near zero. The elastic and force-generating capability of neural axons has recently been proposed to play a major role in the morphogenesis of the brain.

Traction forces generated by locomoting keratocytes

Traction forces produced by moving fibroblasts have been observed as distortions in flexible substrata including wrinkling of thin, silicone rubber films. Traction forces generated by fibroblast lamellae were thought to represent the forces required to move the cell forwards. However, traction forces could not be detected with faster moving cell types such as leukocytes and growth cones (Harris, A. K., D. Stopak, and P. Wild. 1981. Nature (Lond.). 290:249-251). We have developed a new assay in which traction forces produced by rapidly locomoting fish keratocytes can be detected by the two-dimensional displacements of small beads embedded in the plane of an elastic substratum. Traction forces were not detected at the rapidly extending front edge of the cell. Instead the largest traction forces were exerted perpendicular to the left and right cell margins. The maximum traction forces exerted by keratocytes were estimated to be approximately 2 x 10(-8) N. The pattern of traction forces can be related to the locomotion of a single keratocyte in terms of lamellar contractility and area of close cell-substratum contact.

Measurements of growth cone adhesion to culture surfaces by micromanipulation

Neurons were grown on plastic surfaces that were untreated, or treated with polylysine, laminin, or L1 and their growth cones were detached from their culture surface by applying known forces with calibrated glass needles. This detachment force was taken as a measure of the force of adhesion of the growth cone. We find that on all surfaces, lamellipodial growth cones require significantly greater detachment force than filopodial growth cones, but this differences is, in general, due to the greater area of lamellipodial growth cones compared to filopodial growth cones. That is, the stress (force/unit area) required for detachment was similar for growth cones of lamellipodial and filopodial morphology on all surfaces, with the exception of lamellipodial growth cones on L1-treated surfaces, which had a significantly lower stress of detachment than on other surfaces. Surprisingly, the forces required for detachment (760-3,340 mudynes) were three to 15 times greater than the typical resting axonal tension, the force exerted by advancing growth cones, or the forces of retraction previously measured by essentially the same method. Nor did we observe significant differences in detachment force among growth cones of similar morphology on different culture surfaces, with the exception of lamellipodial growth cones on L1-treated surfaces. These data argue against the differential adhesion mechanism for growth cone guidance preferences in culture. Our micromanipulations revealed that the most mechanically resistant regions of growth cone attachment were confined to quite small regions typically located at the ends of filopodia and lamellipodia. Detached growth cones remained connected to the substratum at these regions by highly elastic retraction fibers. The closeness of contact of growth cones to the substratum as revealed by interference reflection microscopy (IRM) did not correlate with our mechanical measurements of adhesion, suggesting that IRM cannot be used as a reliable estimator of growth cone adhesion.

Regenerating sciatic nerve axons contain the adult rather than the embryonic pattern of microtubule associated proteins

Microtubule associated proteins play a central role in the control of axon growth. We have used immunohistochemical techniques to establish which microtubule-associated proteins are present in the rat hindlimb spinal cord, dorsal root ganglia and peripheral nerves during axonal growth during embryogenesis, in adulthood, and during regeneration of crushed sciatic nerves. During embryogenesis microtubule-associated protein-1b and tau are present in all neurons and axons, microtubule-associated protein-2 is present in neurons but not in axons, and there is no microtubule-associated protein-1a. In adults, microtubule-associated protein-1a and microtubule-associated protein-1b are present in all sciatic nerve axons and in motor and dorsal root ganglion neurons. Tau, in its adult form, is present in many fine probably sensory axons, but not in most larger axons, and in motor and sensory neurons. Microtubule-associated protein-2 is present only in neurons. During regeneration the pattern of microtubule-associated protein expression retains the adult pattern. All regenerating axons contain microtubule-associated protein-1a and microtubule-associated protein-1b, none contain microtubule-associated protein-2, and a subset of fine axons contain tau. There is no detectable change in microtubule-associated protein expression by motoneurons. While axons are clearly able to regenerate without either microtubule-associated protein-2 or tau, tau containing axons appear to regenerate faster than those which lack it. It is possible that the failure of neurons to recapitulate the embryonic pattern of microtubule-associated protein expression during regeneration could be a reason why regenerative axon growth is slower and less vigorous than axon growth in embryos.

Simulation of integrin-cytoskeletal interactions in migrating fibroblasts

Cell migration is a dynamic phenomenon requiring a physical interaction between the internal cell motile machinery and the external substratum in which adhesion receptors, such as integrins, serve as the transmembrane link. To analyze quantitatively this interaction, we apply a modified Brownian dynamics algorithm to simulate cytoskeleton-mediated transport of integrin on the dorsal surfaces of migrating fibroblasts. Previously, we experimentally demonstrated that integrin is transported in an intermittent fashion, with directed excursions interspersed by diffusive periods, preferentially toward the cell edge where the integrin is likely used in the formation of nascent adhesions. Integrins containing mutations in the cytoskeleton-binding region of the cytoplasmic domain display statistically different degrees of directed transport, indicating that this phenomenon is dependent on cytoskeletal associations. In the present work, we develop a computer algorithm generating simulated integrin transport trajectories, given estimates for the rate constants defining coupling (kc) and uncoupling (ku) of integrin with cytoskeletal components. Other parameters supplied to the program, the diffusion coefficient (D) for integrin in the membrane and the instantaneous velocity (vi) of the integrin/cytoskeleton complex, have been measured independently in our experimental system. By comparing the simulated trajectories with those obtained experimentally, we are able to estimate the coupling and uncoupling rate constants for the interaction of integrin with cytoskeletal elements in vivo. We find that integrin couples with cytoskeletal elements at a rate approximately 10 times slower than its rate of uncoupling (kc = 0.3 s-1, ku = 3 s-1). Comparison of these rate constants with an equivalent rate constant for diffusion, k+ = 0.4 s-1, indicates that the coupling interaction is likely a diffusion-limited process, as is typically expected for membrane processes. We further show by calculation that directed transport is necessary for integrin to traverse the length of an extending lamellipod to its leading edge; diffusion alone is not sufficiently fast to supply adhesion receptors to points of new cell/substratum contact.

Neuritic growth rate described by modeling microtubule dynamics

A model is developed to describe neuronal elongation as a result of the polymerization of microtubules and elastic stretching of the neurites by force produced by the growth cone. The model for a single segment with a single growth cone revealed a constant elongation rate, while the concentration of tubulin in the soma rises, and the concentration of tubulin becomes constant in the growth cone. Extending the model to a neurite with a single branch point and two growth cones revealed the same results. When the assembly or the disassembly rate of microtubules is unequal in both growth cones, transient retraction of one of the terminal segments occurs, which results in complete retraction of the segment when the difference in (dis)assembly rate between the two growth cones is large enough. When the model is applied to large trees, a maximal sustainable number of terminal segments as a function of the production rate of tubulin appears. Mechanisms to stop outgrowth are discussed in relation to the establishment of synaptical contacts between cells.

Cytoskeletal events in growth cone steering

Oriented neural outgrowth is dependent upon the capability of growth cones to reorient the direction of their migration in response to contact with guidance information. Recent observations have directed attention toward the role of actin distribution and concentration in transmitting localized peripheral signals to central elements of the growth cone, particularly microtubules.

The unique and shared properties of neuronal growth cones that enable navigation and specific pathfinding

The neuronal growth cone has as one of its primary functions the role of guiding the elongating neurite to its appropriate target. The events during pathfinding require that the neuronal growth cone responds to particular molecules in the environment by making appropriate changes in the behavior of the growth cone. This paper discusses the role of intracellular calcium as a signalling system mediating these responses. The specific roles of the growth cone filopodia in acting as antennae in advance of the motile growth cone are discussed in the context of calcium mediated pathfinding events.