Rapid growth cone translocation on laminin is supported by lamellipodial not filopodial structures

To determine the relationship between growth cone structure and motility, we compared the neurite extension rate, the form of individual growth cones, and the organization of f-actin in embryonic (E21) and postnatal (P30) sympathetic neurons in culture. Neurites extended faster on laminin than on collagen, but the P30 nerites were less than half as long as E21 neurites on both substrata. Growth cone shape was classified into one of five categories, ranging from fully lamellipodial to blunt endings. The leading margins of lamellipodia advanced smoothly across the substratum ahead of any filopodial activity and contained meshworks of actin filaments with no linear f-actin bundles, indicating that filopodia need not underlie lamellipodia. Rapid translocation (averaging 0.9-1.4 microns/min) was correlated with the presence of lamellipodia; translocation associated with filopodia averaged only 0.3-0.5 microns/min. This relationship extended to growth cones on a branched neurite where the translocation of each growth cone was dependent on its shape. Growth cones with both filopodial and lamellipodial components moved at intermediate rates. The prevalence of lamellipodial growth cones depended on age of the neurites; early in culture, 70% of E21 growth cones were primarily lamellipodial compared to 38% of P30 growth cones. A high percentage of E21 lamellipodial growth cones were associated with rapid neurite elongation (1.2 mm/day), whereas a week later, only 16% were lamellipodial, and neurites extended at 0.5 mm/day. Age-related differences in neurite extension thus reflected the proportion of lamellipodial growth cones present rather than disparities in basic structure or in the rates at which growth cones of a given type moved at different ages. Filopodia and lamellipodia are each sufficient to advance the neurite margin; however, rapid extension of superior cervical ganglion neurites was supported by lamellipodia independent of filopodial activity.     

Growth cone-growth cone interactions in cultures of rat sympathetic neurons

Growth cones of sympathetic neurons from the superior cervical ganglia of neonatal rats were studied using video-microscopy to determine events following contact between growth cones and other cell surfaces, including other growth cones and neurites. A variety of behaviors were observed to occur upon contact between growth cones. Most commonly, one growth cone would collapse and subsequently retract upon establishing filopodial contact with the growth cone of another sympathetic neuron. Contacts resulting in collapse and retraction were often accompanied by a rapid and transient burst of lamellipodial activity along the neurite 30-50 microns proximal to the retracting growth cone. In no instances did interactions between growth cones and either fibroblasts or red blood cells result in the growth cone collapsing, suggesting that a specific recognition event was involved. On several occasions, growth cones were seen to track other growth cones, although fasciculation was rare. In some cases, there was no obvious response between contacting growth cones. Growth cone-growth cone contact was almost four times more likely to result in collapse and retraction than was growth cone-neurite contact (45% vs 12%, respectively). These observations suggest that the superior cervical ganglion may be composed of neurons with different cell surface determinants and that these determinants are more concentrated on the surface of growth cones than on neurites. These results further suggest that contact-mediated inhibition of growth cone locomotion may play an important role in growth cone guidance.     

Modulation of growth cone morphology by substrate-bound adhesion molecules.

The growth cone, a terminal structure on developing and regenerating axons, is specialized for motility and guidance functions. In vivo the growth cone responds to environmental cues to guide the axon to its appropriate target. These cues are thought to be responsible for position-specific morphological changes in the growth cone, but the molecules that control growth cone behavior are poorly characterized. We used scanning electron microscopy to analyze the morphology of retinal ganglion cell growth cones in vitro on different adhesion molecules that axons normally encounter in vivo. L1/8D9, N-cadherin, and laminin each induced distinctive morphological characteristics in growth cones. Growth cones elaborated lamellipodial structures in response to the cell adhesion molecules L1/8D9 and N-cadherin, whereas laminin supported filopodial growth cones with small veils. On L1/8D9, the growth cones were larger and produced more filopodia. Filopodial associations between adjacent growth cones and neurites were frequent on L1/8D9 but were uncommon on laminin or N-cadherin. These results demonstrate that different adhesion molecules have profoundly different effects on growth cone morphology. This is consistent with previous reports suggesting that changes in growth cone morphology in vivo occur in response to changes in substrate composition.     

Modeling the role of myosin 1c in neuronal growth cone turning

We addressed the mechanical basis for how embryonic chick dorsal root ganglion growth cones turn on a uniform substrate of laminin-1. Turning is significantly correlated with lamellipodial area but not with filopodial length. We assessed the lamellipodial contribution to turning by asymmetric micro-CALI of myosin isoforms that causes localized lamellipodial expansion (myosin 1c) or filopodial retraction (myosin V). Episodes of asymmetric micro-CALI of myosin 1c (or myosin 1c and V together) caused significant turning of the growth cone. In contrast, repeated micro-CALI of myosin V or irradiation without added antibody did not turn growth cones. These findings argue that lamellipodia and not filopodia are necessary for growth cone turning. To model the role of myosin 1c on growth cone turning, we fitted the measured trajectories from asymmetric micro-CALI of myosin 1c-treated and untreated growth cones to the persistent random walk model. The first parameter in this equation, root-mean-square speed, is indistinguishable between the two data sets whereas the second parameter, the persistence of motion, is significantly increased (2.5-fold) as a result of asymmetric inactivation of myosin 1c by micro-CALI. This analysis demonstrates that growth cone turning results from an increase in the persistence of directional motion rather than a change in speed. Taken together, our results suggest that myosin 1c is a molecular correlate for directional persistence underlying growth cone motility.     

Adhesion sites of neural tumor cells : Morphogenesis of substratum-attached material

An analysis by scanning electron microscopy (SEM) has been performed of the attachment, neurite outgrowth, EGTA-mediated detachment, and morphological characteristics of substratum-attached material (SAM) for non-neurite- or neurite-containing rat neuroblastoma cells growing on serum-coated plastic coverslips. Attachment is initiated by filopodial contact with the substratum and with subsequent broad spreading of the surface membrane; footpad-type adhesion sites commonly observed in fibroblasts are not apparent at the periphery of these neuronal cells. During serum starvation, neurite extension occurs by elongation into bipolar cells, membrane ruffling and filopodial extension at these polar ends, and growth cone extension over the substratum. With time, some growth cones terminate membrane ruffling and spread extensively into a footpad-like morphology. EGTA-mediated detachment occurs by cell body rounding and pulling away from small focal areas of contact between the surface membrane and the substratum. After complete detachment, two morphologically different classes of SAM are identified. Non-neurite-containing neuroblastoma cells leave large membranous pools of SAM which are rigid and raised off the substratum, revealing small focal contact areas. A second morphological class of SAM is identified in neurite-containing cultures as small pools of membranous material tightly bound to the substratum and reminiscent of the footpad SAM deposited by fibroblasts. Along with the biochemical differences noted previously for the SAMs from non-neurite- or neurite-containing cultures, these studies indicate that the adhesion between the growth cone of neurites and the serum-coated substratum is significantly different from the adhesion processes occurring between the cell body and the substratum.     

Immunocytochemical evidence for colocalization in neurite growth cones of actin and myosin and their relationship to cell-substratum adhesions

Sensory neurons from 8- to 11-day chick embryos were cultured on polyornithine-treated coverslips, fixed with glutaraldehyde, and stained for immunofluorescent localization of actin. Actin was distributed in a fibrous form in the growth cones, extending into filopodia and lamellipodial expansions of the growth cone margin. Often, these actin fibers were located at sites of linear adhesions to the glass substratum, as viewed by interference reflection optics. Our antisera to myosin did not recognize myosin in glutaraldehyde-fixed cells, and paraformaldehyde, which preserves the antigenicity of myosin, did not fix embryonic neurons well. Thus, myosin was localized in NGF-stimulated PC12 cells, whose morphology is better preserved by paraformaldehyde. Within the growth cones of PC12 neurites, actin and myosin are distributed into fibrous arrays which resemble the actin fibers seen in the growth cones of sensory neurons. Thus, actomyosin-like contractile forces may be exerted in neurite growth cones. These forces may act in concert with cell-substratum adhesive bonds to move the growth cone across the substratum or move organelles within the growth cone.     

Growth cone behavior and production of traction force

The growth cone must push its substrate rearward via some traction force in order to propel itself forward. To determine which growth cone behaviors produce traction force, we observed chick sensory growth cones under conditions in which force production was accommodated by movement of obstacles in the environment, namely, neurites of other sensory neurons or glass fibers. The movements of these obstacles occurred via three, different, stereotyped growth cone behaviors: (a) filopodial contractions, (b) smooth rearward movement on the dorsal surface of the growth cone, and (c) interactions with ruffling lamellipodia. More than 70% of the obstacle movements were caused by filopodial contractions in which the obstacle attached at the extreme distal end of a filopodium and moved only as the filopodium changed its extension. Filopodial contractions were characterized by frequent changes of obstacle velocity and direction. Contraction of a single filopodium is estimated to exert 50-90 microdyn of force, which can account for the pull exerted by chick sensory growth cones. Importantly, all five cases of growth cones growing over the top of obstacle neurites (i.e., geometry that mimics the usual growth cone/substrate interaction), were of the filopodial contraction type. Some 25% of obstacle movements occurred by a smooth backward movement along the top surface of growth cones. Both the appearance and rate of movements were similar to that reported for retrograde flow of cortical actin near the dorsal growth cone surface. Although these retrograde flow movements also exerted enough force to account for growth cone pulling, we did not observe such movements on ventral growth cone surfaces. Occasionally obstacles were moved by interaction with ruffling lamellipodia. However, we obtained no evidence for attachment of the obstacles to ruffling lamellipodia or for directed obstacle movements by this mechanism. These data suggest that chick sensory growth cones move forward by contractile activity of filopodia, i.e., isometric contraction on a rigid substrate. Our data argue against retrograde flow of actin producing traction force.     

Suppression of Radixin and Moesin Alters Growth Cone Morphology,Motility, and Process Formation In Primary Cultured Neurons

In this study we have examined the cellular functions of ERM proteins in developing neurons. The results obtained indicate that there is a high degree of spatial and temporal correlation between the expression and subcellular localization of radixin and moesin with the morphological development of neuritic growth cones. More importantly, we show that double suppression of radixin and moesin, but not of ezrin–radixin or ezrin–moesin, results in reduction of growth cone size, disappearance of radial striations, retraction of the growth cone lamellipodial veil, and disorganization of actin filaments that invade the central region of growth cones where they colocalize with microtubules. Neuritic tips from radixin–moesin suppressed neurons displayed high filopodial protrusive activity; however, its rate of advance is 8–10 times slower than the one of growth cones from control neurons. Radixin–moesin suppressed neurons have short neurites and failed to develop an axon-like neurite, a phenomenon that appears to be directly linked with the alterations in growth cone structure and motility. Taken collectively, our data suggest that by regulating key aspects of growth cone development and maintenance, radixin and moesin modulate neurite formation and the development of neuronal polarity.     

Involvement of microtubules in the regulation of neuronal growth cone morphologic remodeling

The guidance of nerve fibers depends on the constant protrusion, movement, and retraction (i.e., remodeling) of growth cone lamellae and filopodia. We used drugs that interfere with the dynamics of microtubules to investigate the role of microtubules in the remodeling of larval amphibian spinal cord neuronal growth cones. Vinblastine (8–100 nM), taxol (10 nM), and nocodazole (330 nM) altered microtubule distributions in growth cones and decreased the percentage of lamellar perimeter undergoing remodeling, while not affecting the rates of lamellar protrusion and retraction. Also, 8–20 nM vinblastine caused temporary losses of the continuity of the originally fan-shaped lamella, resulting in two or more lamellae at the growth cone. At higher concentrations of microtubule drugs, the originally fan-shaped lamella broke up into separate smaller lamellae followed by the centrifugal displacement from the base of the growth cone and eventual collapse of the resultant lamellae. Low doses of cytochalasin B prevented the centrifugal displacement of lamellae in response to microtubule drugs. During microtubule drug-mediated loss of growth cone lamellae, some filopodia were observed to elongate to greater than normal lengths. Similarly, exposure to 20 nM vinblastine resulted in an increase in filopodial length but not filopodial number. As evidenced by DiOC₆(3) staining, 8–20 nM vinblastine altered the distribution of membranous organelles within growth cones, suggesting that the effects of microtubule drugs on growth cones may be mediated in part by alterations in organelle localization. Our data show that microtubules are involved in the maintenance and regulation of lamellar and filopodial structures at the neuronal growth cone. These findings have implications for the mechanisms by which growth cones are guided during development and regeneration. © 1998 John Wiley & Sons, Inc. J Neurobiol 35: 121–140, 1998     

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.     

The timing of initial neuropeptide expression by an identified insect neuron does not depend on interactions with its normal peripheral target

To study the developmental regulation of a neuropeptide phenotype, we have analyzed the biochemical and morphological differentiation of two identifiable neurons in embryos of the moth, Manduca sexta. The central cell, CF, and the peripheral cell, L1, are both neuroendocrine neurons that express neuropeptides related to the molluscan tetrapeptide FMRFamide. Both neurons project axons to the transvers nerve in each thoracic segment. Within the CF and L1 cells, neuropeptide-like immunoreactivity was localized to secretory granules that had cell specific morphologies and sizes. The onset of neuropeptide expression in the two cell types displayed a similar pattern: immunoreactivity was first detected in distal processes and soon after within cells bodies. However, the onsets occurred at different times: for the CF cell, neuropeptides were first seen at 60%-63% of embryonic development, after the neuron had extended a long axon into the periphery, while L1 neuropeptide expression began at ～42%, as it first extended its growth cone. These times were related in that they corresponded to the arrival times of the respective growth cones at a similar position in the developing peripheral nerve. Withinthis region of the nerve, the growth cones of both cell typesexhibited a transient and cell-specific interaction with an identified mesodermal cell, called the Syncytium. Like the L1 and B neurons (Carr and Taghert, 1988b), the CF growth cones typically grew past this cell, yet remained attached to it by lamellipodial and filopodial processes of the axon. Ultrastructurally, the interaction involved filopodial adhesion to and insertion within the Syncytial cell. Two other nonneuroendocrine cell types grew axons past this same region, but showed no such tendencies. To test the hypothesis that the morphological and biochemical differentiation of these cells was somehow linked, central ganglia were isolated (as individuals or connected as ganglionic chains) in tissue culture, prior to the time when CF growth cones entered the periphery and prior to the development of CF neuropeptide expression. In the majority of cases, CF neurons nevertheless displayed their neuropeptide phenotype at a normal and cell-specific stage. We conclude that the initiation of neuropeptide expression is highly correlated with schedules of morphological differentiation in these neurons, but that, in the case of the CF neuron, it is not regulated by interactions of the growth cone with peripheral structures.     

The key feature for early migratory processes

Migration of cells is one of the most essential prerequisites to form higher organisms and depends on a strongly coordinated sequence of processes. Early migratory events include substrate sensing, adhesion formation, actin bundle assembly and force generation. While substrate sensing was ascribed to filopodia, all other processes were believed to depend mainly on lamellipodia of migrating cells. In this work we show for motile keratinocytes that all processes from substrate sensing to force generation strongly depend on filopodial focal complexes as well as on filopodial actin bundles. In a coordinated step by step process filopodial focal complexes have to be tightly adhered to the substrate and to filopodial actin bundles to enlarge upon lamellipodial contact forming classical focal adhesions. Lamellipodial actin filaments attached to those focal adhesions originate from filopodia. Upon cell progression, the incorporation of filopodial actin bundles into the lamellipodium goes along with a complete change in actin cross-linker composition from filopodial fascin to lamellipodial α-actinin. α-Actinin in turn is replaced by myosin II and becomes incorporated directly behind the leading edge. Myosin II activity makes this class of actin bundles with their attached FAs the major source of force generation and transmission at the cell front. Furthermore, connection of FAs to force generating actin bundles leads to their stabilization and further enlargement. Consequently, adhesion sites formed independently of filopodia are not connected to detectable actin bundles, transmit weak forces to the substrate and disassemble within a few minutes without having been increased in size.     

Cdc42 stimulates neurite outgrowth and formation of growth cone filopodia and lamellipodia

To assess the role of cdc42 during neurite development, cmyc-tagged constitutively active (CA) and dominant negative (DN) cdc42 were expressed in dissociated primary chick spinal cord neurons using adenoviral-mediated gene transfer. Three days after infection, >85% of the neurons in infected cultures expressed cdc42 proteins, as detected by indirect immunofluorescence against cmyc. Growth cones of infected neurons displayed 1.83- (CAcdc42) and 1.93-fold (DNcdc42) higher cmyc immunofluorescence per square micrometer than uninfected controls. CAcdc42 expression stimulated growth cones, almost doubling growth cone size and number of filopodia, and increased neurite growth rates by 65-89%. In neurons plated onto fibronectin, the percent of growth cones with both filopodia and lamellipodia increased from 71 to 92%. Total Texas Red-phalloidin staining in these growth cones doubled, and the percent of growth cones with F-actin localized to peripheral regions increased from 52% in controls to 78% after CAcdc42 expression. Expression of DNcdc42 did not significantly alter growth cone morphology or neurite growth rates. Addition of soluble laminin to spinal cord neurons resulted in the identical phenotype as CAcdc42 expression, including changes in growth cone morphology, F-actin localization, and neurite growth rates. Significantly, expression of DNcdc42 blocked the effects of laminin on growth cones. These results show that cdc42 promotes neurite outgrowth and filopodial and lamellipodial formation in growth cones and suggests that cdc42 and laminin share a common signaling pathway during neurite development. Addition of laminin to CAcdc42-expressing neurons is inhibitory to growth cones, indicating that laminin also may activate some other pathways.     

Src and cortactin promote lamellipodia protrusion and filopodia formation and stability in growth cones

Src tyrosine kinases have been implicated in axonal growth and guidance; however, the underlying cellular mechanisms are not well understood. Specifically, it is unclear which aspects of actin organization and dynamics are regulated by Src in neuronal growth cones. Here, we investigated the function of Src2 and one of its substrates, cortactin, in lamellipodia and filopodia of Aplysia growth cones. We found that up-regulation of Src2 activation state or cortactin increased lamellipodial length, protrusion time, and actin network density, whereas down-regulation had opposite effects. Furthermore, Src2 or cortactin up-regulation increased filopodial density, length, and protrusion time, whereas down-regulation promoted lateral movements of filopodia. Fluorescent speckle microscopy revealed that rates of actin assembly and retrograde flow were not affected in either case. In summary, our results support a model in which Src and cortactin regulate growth cone motility by increasing actin network density and protrusion persistence of lamellipodia by controlling the state of actin-driven protrusion versus retraction. In addition, both proteins promote the formation and stability of actin bundles in filopodia.     

The key feature for early migratory processes: Dependence of adhesion,actin bundles,force generation and transmission on filopodia

Migration of cells is one of the most essential prerequisites to form higher organisms and depends on a strongly coordinated sequence of processes. Early migratory events include substrate sensing, adhesion formation, actin bundle assembly and force generation. While substrate sensing was ascribed to filopodia, all other processes were believed to depend mainly on lamellipodia of migrating cells. In this work we show for motile keratinocytes that all processes from substrate sensing to force generation strongly depend on filopodial focal complexes as well as on filopodial actin bundles. In a coordinated step by step process, filopodial focal complexes have to be tightly adhered to the substrate and to filopodial actin bundles to enlarge upon lamellipodial contact forming classical focal adhesions. Lamellipodial actin filaments attached to those focal adhesions originate from filopodia. Upon cell progression, the incorporation of filopodial actin bundles into the lamellipodium goes along with a complete change in actin cross-linker composition from filopodial fascin to lamellipodial α-actinin. α-Actinin in turn is replaced by myosin II and becomes incorporated directly behind the leading edge. Myosin II activity makes this class of actin bundles with their attached FAs the major source of force generation and transmission at the cell front. Furthermore, connection of FAs to force generating actin bundles leads to their stabilization and further enlargement. Consequently, adhesion sites formed independently of filopodia are not connected to detectable actin bundles, transmit weak forces to the substrate and disassemble within a few minutes without having been increased in size.Key words: filopodia, focal complexes, cell migration, focal adhesion, myosin II, force, actin flow, maturation     

Regulation of actomyosin contractility by PI3K in sensory axons

Phosphatidylinositol 3-kinase (PI3K) activity is known to be required for the extension of embryonic sensory axons. Inhibition of PI3K has also been shown to mediate axon retraction and growth cone collapse in response to semaphorin 3A. However, the effects of inhibiting PI3K on the neuronal cytoskeleton are not well characterized. We have previously reported that semaphorin 3A-induced axon retraction involves activation of myosin II, the formation of an intra-axonal F-actin bundle cytoskeleton, and blocks the formation of F-actin patches that serve as precursors to filopodial formation in axons. We now report that inhibition of PI3K results in activation of myosin II in axons. Inhibition of myosin II activity, or its upstream regulatory kinase RhoA-kinase, blocked axon retraction induced by inhibition of PI3K. In addition, inhibition of PI3K also induced intra-axonal F-actin bundles, which likely serve as a substratum for myosin II-based force generation during axon retraction. In axons, filopodia are formed from axonal F-actin patch precursors. Analysis of axonal F-actin patch formation in eYFP-actin expressing neurons revealed that inhibition of PI3K blocked formation of axonal F-actin patches, and thus filopodial formation. These data provide insights into the regulation of the neuronal cytoskeleton by PI3K and are consistent with the notion that decreased levels of PI3K activity mediate axon retraction and growth cone collapse in response to semaphorin 3A.     

Cell-to-substratum adhesion and guidance of axonal elongation

The behavior of axonal growth cones on surfaces with patterned variations in substratum was observed. Cells from sensory ganglia of 8-day-old chicken embryos were cultured on plastic petri dishes, plastic tissue culture dishes, and polyornithine-coated tissue culture dishes, all of which contained gridlike patterns of palladium (Pd) deposition.The results indicated that growth cones elongated on the Pd-shadowed areas vs areas lacking Pd deposits depending on the relative adhesivity of the growth cones to the substrata. In petri dishes, growth cones stay on the Pd; in tissue culture dishes, they cross from one surface to the other; and in polyornithine-coated dishes, they elongate for great distances on the Pd-free areas. Analyses of time-lapse movies showed that, on Pd-shadowed polyornithine dishes, growth cones often approach the Pd-coated areas and microspikes touch the Pd surface. Yet, the axon tip continues to elongate on the Pd-free polyornithine surface.The conclusion is offered that interactions between microspikes and the substratum adjacent to the growth cone are important determinants of the directions and pathways of axonal elongation.     

Hippocampal growth cone responses to focally applied electric fields

A wide variety of cell types respond to electric fields in culture. Despite evidence for electric fields existing in the mammalian embryo, there are few studies testing the effects electric fields exert on neurons from the mammalian central nervous system (CNS). The present study demonstrates orientation responses to focally applied electric fields of embryonic rat hippocampal neurons isolated in culture. The most striking result from this study is that different growth cones of the same neuron can show differential responsiveness to focally applied electric fields: growth cones on the short straight processes that are destined to become dendrites, oriented toward the cathode, whereas growth cones on the longest process, the presumptive axon, did not orient. The present experiments bring a significant increase in resolution to the study of neuronal growth cone orientation by applied electric fields: a novel examination of the early events leading to orientation. Growth cones on dendrites displayed a spectrum of orientation responses: directed lamellipodial extension, directed filopodial extension and/or reorientation, cytoplasmic swelling of existing filopodia, consolidation of filopodia, and rapid elongation of the entire process. Individual growth cones displayed only one or two of these responses. Additionally, not all growth cones on these short processes sustained their initial orientation response: 35% adapted within 6 min. © 1993 John Wiley & Sons, Inc.     

Common mechanisms underlying growth cone guidance and axon branching

During development, growth cones direct growing axons into appropriate targets. However, in some cortical pathways target innervation occurs through the development of collateral branches that extend interstitially from the axon shaft. How do such branches form? Direct observations of living cortical brain slices revealed that growth cones of callosal axons pause for many hours beneath their cortical targets prior to the development of interstitial branches. High resolution imaging of dissociated living cortical neurons for many hours revealed that the growth cone demarcates sites of future axon branching by lengthy pausing behaviors and enlargement of the growth cone. After a new growth cone forms and resumes forward advance, filopodial and lamellipodial remnants of the large paused growth cone are left behind on the axon shaft from which interstitial branches later emerge. To investigate how the cytoskeleton reorganizes at axon branch points, we fluorescently labeled microtubules in living cortical neurons and imaged the behaviors of microtubules during new growth from the axon shaft and the growth cone. In both regions microtubules reorganize into a more plastic form by splaying apart and fragmenting. These shorter microtubules then invade newly developing branches with anterograde and retrograde movements. Although axon branching of dissociated cortical neurons occurs in the absence of targets, application of a target-derived growth factor, FGF-2, greatly enhances branching. Taken together, these results demonstrate that growth cone pausing is closely related to axon branching and suggest that common mechanisms underlie directed axon growth from the terminal growth cone and the axon shaft.     

Geometry of isolated sensory neurons in culture. Effects of embryonic age and culture substratum

Sensory neurons were dissociated from lumbar dorsal root ganglia of embryonic chick and put into culture, either directly or after removing non-neuronal cells by density gradient centrifugation. The cells were grown on culture substrata of various kinds in medium containing nerve growth factor (NGF). After 24 h the cultures were fixed, mounted and analysed. Lengths of neurites were measured, and the numbers of primary processes formed at the cell body and of growth cones were counted. From these values, the rates of growth cone advance and frequency of growth cone branching were calculated. Neuronal outgrowths increased strikingly in length and complexity with embryonic age; there was a 3.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones when neurons from 15-day embryos (E15) were compared with those from 8-day embryos (E8) grown on the same substratum (glass). Growth was markedly greater on surfaces prepared with laminin or conditioned medium compared with plain glass or air-dried collagen. When E15 neurons grown on glass were compared with those grown on laminin, for example, a 2.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones was observed. Calculations showed that a major factor in these changes was an increase in the frequency of growth cone branching. The number of initial processes emanating from the cell body changed with age, but not with the different substrata tested. Non-neuronal cells when present in low numbers and in contact with neurons did not appear to influence neuronal geometry in a systematic way. Our results document the fact that both external factors (in this case, the nature of the culture substratum) and intrinsic factors (stage of development of the neuron) can influence the geometry of neurite outgrowth.