Transport and turnover of microtubules in frog neurons depend on the pattern of axonal growth

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Dynein-mediated microtubule translocation powering neurite outgrowth in chick and Aplysia neurons requires microtubule assembly

Previously, we have shown that bulk microtubule (MT) movement correlates with neurite elongation, and blocking either dynein activity or MT assembly inhibits both processes. However, whether the contributions of MT dynamics and dynein activity to neurite elongation are separate or interdependent is unclear. Here, we investigated the underlying mechanism by testing the roles of dynein and MT assembly in neurite elongation of Aplysia and chick neurites using time-lapse imaging, fluorescent speckle microscopy, super-resolution imaging and biophysical analysis. Pharmacologically inhibiting either dynein activity or MT assembly reduced neurite elongation rates as well as bulk and individual MT anterograde translocation. Simultaneously suppressing both processes did not have additive effects, suggesting a shared mechanism of action. Single-molecule switching nanoscopy revealed that inhibition of MT assembly decreased the association of dynein with MTs. Finally, inhibiting MT assembly prevented the rise in tension induced by dynein inhibition. Taken together, our results suggest that MT assembly is required for dynein-driven MT translocation and neurite outgrowth.

Neurite elongation is highly correlated with bulk forward translocation of microtubules

During the development of the nervous system and regeneration following injury, microtubules (MTs) are required for neurite elongation. Whether this elongation occurs primarily through tubulin assembly at the tip of the axon, the transport of individual MTs, or because MTs translocate forward in bulk is unclear. Using fluorescent speckle microscopy (FSM), differential interference contrast (DIC), and phase contrast microscopy, we tracked the movement of MTs, phase dense material, and docked mitochondria in chick sensory and Aplysia bag cell neurons growing rapidly on physiological substrates. In all cases, we find that MTs and other neuritic components move forward in bulk at a rate that on average matches the velocity of neurite elongation. To better understand whether and why MT assembly is required for bulk translocation, we disrupted it with nocodazole. We found this blocked the forward bulk advance of material along the neurite and was paired with a transient increase in axonal tension. This indicates that disruption of MT dynamics interferes with neurite outgrowth, not by disrupting the net assembly of MTs at the growth cone, but rather because it alters the balance of forces that power the bulk forward translocation of MTs.

A mathematical model explains saturating axon guidance responses to molecular gradients

Correct wiring is crucial for the proper functioning of the nervous system. Molecular gradients provide critical signals to guide growth cones, which are the motile tips of developing axons, to their targets. However, in vitro, growth cones trace highly stochastic trajectories, and exactly how molecular gradients bias their movement is unclear. Here, we introduce a mathematical model based on persistence, bias, and noise to describe this behaviour, constrained directly by measurements of the detailed statistics of growth cone movements in both attractive and repulsive gradients in a microfluidic device. This model provides a mathematical explanation for why average axon turning angles in gradients in vitro saturate very rapidly with time at relatively small values. This work introduces the most accurate predictive model of growth cone trajectories to date, and deepens our understanding of axon guidance events both in vitro and in vivo.

Cytoplasmic dynein pushes the cytoskeletal meshwork forward during axonal elongation

During development, neurons send out axonal processes that can reach lengths hundreds of times longer than the diameter of their cell bodies. Recent studies indicate that en masse microtubule translocation is a significant mechanism underlying axonal elongation, but how cellular forces drive this process is unknown. Cytoplasmic dynein generates forces on microtubules in axons to power their movement through 'stop-and-go' transport, but whether these forces influence the bulk translocation of long microtubules embedded in the cytoskeletal meshwork has not been tested. Here, we use both function-blocking antibodies targeted to the dynein intermediate chain and the pharmacological dynein inhibitor ciliobrevin D to ask whether dynein forces contribute to en bloc cytoskeleton translocation. By tracking docked mitochondria as fiducial markers for bulk cytoskeleton movements, we find that translocation is reduced after dynein disruption. We then directly measure net force generation after dynein disruption and find a dramatic increase in axonal tension. Taken together, these data indicate that dynein generates forces that push the cytoskeletal meshwork forward en masse during axonal elongation.

Drosophila growth cones advance by forward translocation of the neuronal cytoskeletal meshwork in vivo

In vitro studies conducted in Aplysia and chick sensory neurons indicate that in addition to microtubule assembly, long microtubules in the C-domain of the growth cone move forward as a coherent bundle during axonal elongation. Nonetheless, whether this mode of microtubule translocation contributes to growth cone motility in vivo is unknown. To address this question, we turned to the model system Drosophila. Using docked mitochondria as fiduciary markers for the translocation of long microtubules, we first examined motion along the axon to test if the pattern of axonal elongation is conserved between Drosophila and other species in vitro. When Drosophila neurons were cultured on Drosophila extracellular matrix proteins collected from the Drosophila Kc167 cell line, docked mitochondria moved in a pattern indicative of bulk microtubule translocation, similar to that observed in chick sensory neurons grown on laminin. To investigate whether the C-domain is stationary or advances in vivo, we tracked the movement of mitochondria during elongation of the aCC motor neuron in stage 16 Drosophila embryos. We found docked mitochondria moved forward along the axon shaft and in the growth cone C-domain. This work confirms that the physical mechanism of growth cone advance is similar between Drosophila and vertebrate neurons and suggests forward translocation of the microtubule meshwork in the axon underlies the advance of the growth cone C-domain in vivo. These results highlight the need for incorporating en masse microtubule translocation, in addition to assembly, into models of axonal elongation.

Xenopus cytoplasmic linker-associated protein 1 (XCLASP1) promotes axon elongation and advance of pioneer microtubules

Dynamic microtubules (MTs) are required for neuronal guidance, in which axons extend directionally toward their target tissues. We found that depletion of the MT-binding protein Xenopus cytoplasmic linker-associated protein 1 (XCLASP1) or treatment with the MT drug Taxol reduced axon outgrowth in spinal cord neurons. To quantify the dynamic distribution of MTs in axons, we developed an automated algorithm to detect and track MT plus ends that have been fluorescently labeled by end-binding protein 3 (EB3). XCLASP1 depletion reduced MT advance rates in neuronal growth cones, very much like treatment with Taxol, demonstrating a potential link between MT dynamics in the growth cone and axon extension. Automatic tracking of EB3 comets in different compartments revealed that MTs increasingly slowed as they passed from the axon shaft into the growth cone and filopodia. We used speckle microscopy to demonstrate that MTs experience retrograde flow at the leading edge. Microtubule advance in growth cone and filopodia was strongly reduced in XCLASP1-depleted axons as compared with control axons, but actin retrograde flow remained unchanged. Instead, we found that XCLASP1-depleted growth cones lacked lamellipodial actin organization characteristic of protrusion. Lamellipodial architecture depended on XCLASP1 and its capacity to associate with MTs, highlighting the importance of XCLASP1 in actin-microtubule interactions.

Numerical study of axonal outgrowth in grooved nerve conduits

Nerve conduits with grooved inner texture, working as a topographical guidance cue, have been experimentally proved to play a significant role in axonal alignment. How grooved conduits guide axonal outgrowth is of particular interest for studying nerve regeneration. A viscoelastic model of axonal outgrowth in a conduit with a defined grooved geometry characterized by its width in the circumferential direction and its height in the radial direction is developed in this work. In this model, the axon is considered as an elastic beam and the axonal deformation and motion, including stretching, bending and torsion, are described using a Cosserat rod theory. The friction between axon and substrate is also considered in this model as well as the tip outgrowth. It is found that the directional outgrowth of the axon can be significantly improved by the grooved texture: when the groove width decreases or the groove height increases, the axonal elongation in the longitudinal direction of the conduit can be increased, which is in good agreement with experimental observations. This work is the first numerical model to study the effect of the substrate geometry on axonal outgrowth.

Neurogenetics of slow axonal transport: from cells to animals

Slow axonal transport is a multivariate phenomenon implicated in several neurodegenerative disorders. Recent reports have unraveled the molecular basis of the transport of certain slow component proteins, such as the neurofilament subunits, tubulin, and certain soluble enzymes such as Ca(2+)/calmodulin-dependent protein kinase IIa (CaM kinase IIa), etc., in tissue cultured neurons. In addition, genetic analyses also implicate microtubule-dependent motors and other housekeeping proteins in this process. However, the biological relevance of this phenomenon is not so well understood. Here, the authors have discussed the possibility of adopting neurogenetic analyses in multiple model organisms to correlate molecular level measurements of the slow transport phenomenon to animal behavior, thus facilitating the investigation of its biological efficacy.

The glucuronyltransferase GlcAT-P is required for stretch growth of peripheral nerves in Drosophila

During development, the growth of the animal body is accompanied by a concomitant elongation of the peripheral nerves, which requires the elongation of integrated nerve fibers and the axons projecting therein. Although this process is of fundamental importance to almost all organisms of the animal kingdom, very little is known about the mechanisms regulating this process. Here, we describe the identification and characterization of novel mutant alleles of GlcAT-P, the Drosophila ortholog of the mammalian glucuronyltransferase b3gat1. GlcAT-P mutants reveal shorter larval peripheral nerves and an elongated ventral nerve cord (VNC). We show that GlcAT-P is expressed in a subset of neurons in the central brain hemispheres, in some motoneurons of the ventral nerve cord as well as in central and peripheral nerve glia. We demonstrate that in GlcAT-P mutants the VNC is under tension of shorter peripheral nerves suggesting that the VNC elongates as a consequence of tension imparted by retarded peripheral nerve growth during larval development. We also provide evidence that for growth of peripheral nerve fibers GlcAT-P is critically required in hemocytes; however, glial cells are also important in this process. The glial specific repo gene acts as a modifier of GlcAT-P and loss or reduction of repo function in a GlcAT-P mutant background enhances VNC elongation. We propose a model in which hemocytes are required for aspects of glial cell biology which in turn affects the elongation of peripheral nerves during larval development. Our data also identifies GlcAT-P as a first candidate gene involved in growth of integrated peripheral nerves and therefore establishes Drosophila as an amenable in-vivo model system to study this process at the cellular and molecular level in more detail.

The emerging role of forces in axonal elongation

An understanding of how axons elongate is needed to develop rational strategies to treat neurological diseases and nerve injury. Growth cone-mediated neuronal elongation is currently viewed as occurring through cytoskeletal dynamics involving the polymerization of actin and tubulin subunits at the tip of the axon. However, recent work suggests that axons and growth cones also generate forces (through cytoskeletal dynamics, kinesin, dynein, and myosin), forces induce axonal elongation, and axons lengthen by stretching. This review highlights results from various model systems (Drosophila, Aplysia, Xenopus, chicken, mouse, rat, and PC12 cells), supporting a role for forces, bulk microtubule movements, and intercalated mass addition in the process of axonal elongation. We think that a satisfying answer to the question, "How do axons grow?" will come by integrating the best aspects of biophysics, genetics, and cell biology.

The role of stretching in slow axonal transport

Axonal stretching is linked to rapid rates of axonal elongation. Yet the impact of stretching on elongation and slow axonal transport is unclear. Here, we develop a mathematical model of slow axonal transport that incorporates the rate of axonal elongation, protein half-life, protein density, adhesion strength, and axonal viscosity to quantify the effects of axonal stretching. We find that under conditions where the axon (or nerve) is free of a substrate and lengthens at rapid rates (>4 mm day⁻¹), stretching can account for almost 50% of total anterograde axonal transport. These results suggest that it is possible to accelerate elongation and transport simultaneously by increasing either the axon's susceptibility to stretching or the forces that induce stretching. To our knowledge, this work is the first to incorporate the effects of stretching in a model of slow axonal transport. It has relevance to our understanding of neurite outgrowth during development and peripheral nerve regeneration after trauma, and hence to the development of treatments for spinal cord injury.

Slowing of axonal regeneration is correlated with increased axonal viscosity during aging

Background

As we age, the speed of axonal regeneration declines. At the biophysical level, why this occurs is not well understood.

Results

To investigate we first measured the rate of axonal elongation of sensory neurons cultured from neonatal and adult rats. We found that neonatal axons grew 40% faster than adult axons (11.5 µm/hour vs. 8.2 µm/hour). To determine how the mechanical properties of axons change during maturation, we used force calibrated towing needles to measure the viscosity (stiffness) and strength of substrate adhesion of neonatal and adult sensory axons. We found no significant difference in the strength of adhesions, but did find that adult axons were 3 times intrinsically stiffer than neonatal axons.

Conclusions

Taken together, our results suggest decreasing axonal stiffness may be part of an effective strategy to accelerate the regeneration of axons in the adult peripheral nervous system.

Growth and elongation within and along the axon

Mechanical tension is a particularly effective stimulus for axonal elongation, but little is known about how it leads to the formation of new axon. To better understand this process, we examined the movement of axonal branch points, beads bound to the axon, and docked mitochondria while monitoring axonal width. We found these markers moved in a pattern that suggests elongation occurs by viscoelastic stretching and volume addition along the axon. To test the coupling between "lengthening" and "growth," we measured axonal width while forcing axons to grow and then pause by controlling the tension applied to the growth cone or to the cell body. We found axons thinned during high rates of elongation and thickened when the growth cones were stationary. These findings suggest that forces cause lengthening because they stretch the axon and that growth occurs, in a loosely coupled step, by volume addition along the axon.

Analysis of tubulin transport in nerve processes

In neurons, the molecular machinery for axonal growth and navigation is localized to the growth cone region, whereas tubulin is synthesized primarily in the cell body. Because diffusion serves as an efficient transport mechanism only for very short distances, tubulin has to be actively transported from the cell body down the axon. Two mechanistically distinct models for tubulin transport have been proposed. "Polymer model" postulates that tubulin moves in the form of microtubules preassembled in the cell body, whereas "subunit model" assumes that axonal microtubules are stationary, and that tubulin is delivered from the cell body in unassembled form. We used three independent quantitative approaches (photobleaching, fluorescence speckle microscopy, and microtubule plus end tracking) to demonstrate that axonal microtubules are stationary in rapidly growing axons produced by Xenopus spinal cord neurons in culture. These experiments strongly support subunit model for tubulin delivery.

A physical model of axonal elongation: force, viscosity, and adhesions govern the mode of outgrowth

Whether the axonal framework is stationary or moves is a central debate in cell biology. To better understand this problem, we developed a mathematical model that incorporates force generation at the growth cone, the viscoelastic properties of the axon, and adhesions between the axon and substrate. Using force-calibrated needles to apply and measure forces at the growth cone, we used docked mitochondria as markers to monitor movement of the axonal framework. We found coherent axonal transport that decreased away from the growth cone. Based on the velocity profiles of movement and the force applied at the growth cone, and by varying the attachment of the axonal shaft to the coverslip, we estimate values for the axial viscosity of the axon (3 x 10(6) +/- 2.4 x 10(6) Pa.s) and the friction coefficient for laminin/polyornithine-based adhesions along the axon (9.6 x 10(3) +/- 7.5 x 10(3) Pa.s). Our model suggests that whether axons elongate by tip growth or stretching depends on the level of force generation at the growth cone, the viscosity of the axon, and the level of adhesions along the axon.

Direct evidence for coherent low velocity axonal transport of mitochondria

Axonal growth depends on axonal transport. We report the first global analysis of mitochondrial transport during axonal growth and pauses. In the proximal axon, we found that docked mitochondria attached to the cytoskeletal framework that were stationary relative to the substrate and fast axonal transport fully accounted for mitochondrial transport. In the distal axon, we found both fast mitochondrial transport and a coherent slow transport of the mitochondria docked to the axonal framework (low velocity transport [LVT]). LVT was distinct from previously described transport processes; it was coupled with stretching of the axonal framework and, surprisingly, was independent of growth cone advance. Fast mitochondrial transport decreased and LVT increased in a proximodistal gradient along the axon, but together they generated a constant mitochondrial flux. These findings suggest that the viscoelastic stretching/creep of axons caused by tension exerted by the growth cone, with or without advance, is seen as LVT that is followed by compensatory intercalated addition of new mitochondria by fast axonal transport.

Tau interaction with microtubules in vivo

Tau is a major microtubule-associated protein which induces bundling and stabilization of axonal microtubules (MTs). To investigate the interaction of tau with MTs in living cells, we expressed GFP-tau fusion protein in cultured Xenopus embryo neurons and performed time-lapse imaging of tau-labeled MTs. Tau uniformly labeled individual MTs regardless of their assembly/disassembly status and location along the axon. Photobleaching experiments indicated that interaction of tau with MTs is very dynamic, with a half-time of fluorescence recovery of the order of 3 seconds. Treatment of cells with taxol, a drug that suppresses MT dynamics, rapidly induced detachment of tau from MTs. Although binding of tau to straight MTs was uniform, there was a heightened concentration of tau at the sites of high MT curvature. Our results suggest that dynamic interaction of tau with MTs may modify local mechanical properties of individual MTs and play a crucial role in the remodeling of the MT cytoskeleton during neuronal plasticity.

Modulation of actomyosin contractility by myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling specifies axon formation in neurons

Actin depolymerization through Rho GTPases or exogenous mechanical tension has been suggested as a key determinant for the first step of neuronal polarization, the axonogenesis, in which one of the neurites starts to grow becoming the axon. The underlying mechanism and the relationship between two forces in the cells, however, are mostly unknown. Here, we report that the myosin-dependent contractility is a common effector between two forces and a critical determinant in axonogenesis and neuronal polarization. We have found that inhibition of myosin ATPase activity and modulation of myosin light chain phosphorylation/dephosphorylation through Rho GTPases signaling induced multiple axons. Moreover, overexpression of wild-type myosin light chain kinase dramatically increased filopodial structures and produced multi-axonal structures. Our results suggest that MLC phosphorylation/dephosphorylation through Rho GTPases signaling modulates the actomyosin contractility, and then in turn provides a physiological tension in neurons to induce axon.

Where does slow axonal transport go?

Axonal transport is the specialized and well-developed intracellular transport system for regulated and/or long-distance transport based on generalized cellular machineries. Among them, slow axonal transport conveys cytoplasmic proteins. The motor molecule, the nature of transporting complex and the transport regulation mechanism for slow transport are still unclarified. There has been a dispute regarding the nature of transporting complex of cytoskeletal proteins, polymer-sliding hypothesis versus subunit-transport theory. Recent data supporting the hypothesis of polymer sliding in cultured neurons only reconfirm the previously reported structure and this inference suffers from the lack of ultrastructural evidence and the direct relevance to the physiological slow transport phenomenon in vivo. Observation of the moving cytoskeletal proteins in vivo using transgenic mice or squid giant axons revealed that subunits do move in a microtubule-dependent manner, strongly indicating the involvement of microtubule-based motor kinesin. If the slow transport rate reflects the intermittent fast transport dependent on kinesin motor, we have to investigate the molecular constituents of the transporting complex in more detail and evaluate why the motor and cargo interaction is so unstable. This kind of weak and fluctuating interaction between various molecular pairs could not be detected by conventional techniques, thus necessitating the establishment of a new experimental system before approaching the molecular regulation problem.

Block of slow axonal transport and axonal growth by brefeldin A in compartmented cultures of rat sympathetic neurons

Disruption of the Golgi by brefeldin A (BFA) has been reported to block fast axonal transport and axonal growth. We used compartmented cultures of rat sympathetic neurons to investigate its effects on slow axonal transport. BFA (1 micro g/ml) applied to cell bodies/proximal axons for 6-20 h disrupted the Golgi, reversibly blocked axonal growth, and reversibly blocked anterograde transport of all proteins, including tubulin. The retrograde transport of nerve growth factor (NGF) was also blocked. The phosphorylation of Erk1 and Erk2 in response to NGF was unaffected after 6 h of treatment with BFA, suggesting that the block of axonal transport was specific and direct. Consistent with its principal site of action at the Golgi, no effects were observed when BFA was applied only to the distal axons. Block of fast anterograde and retrograde axonal transport is consistent with the role of the Golgi in supplying transport vesicles. Block of slow axonal transport was surprising, and further results indicated that transport of tubulin en route along the axon was arrested by application of BFA to the cell bodies, suggesting that a continuous supply of anterograde transport vesicles from the Golgi is required to maintain slow axonal transport of cytoskeletal proteins.

Live-cell imaging of slow axonal transport in cultured neurons

Cytoskeletal polymers and other cytosolic protein complexes are transported along axons in the slow components of axonal transport. Studies on the movement of neurofilaments and microtubules in the axons of cultured neurons indicate that these polymers actually move at fast rates and that the movements are also infrequent and highly asynchronous. These observations indicate that the slow rate of slow axonal transport is due to rapid movements interrupted by prolonged pauses which presents special challenges for studies on the mechanism of movement. This chapter describes the procedures that the author's laboratory has used to observe and analyze the movement of neurofilaments and microtubules in axons of cultured neurons from the superior cervical ganglia of neonatal rats. In particular, the author describes how to culture these neurons, how to transfect them by nuclear injection, and how to detect the rapid and infrequent movement of cytoskeletal polymers using time-lapse fluorescence imaging.

Growth cone turning induced by direct local modification of microtubule dynamics

Pathfinding by nerve growth cones depends on attractive and repulsive turning in response to a variety of guidance cues. Here we present direct evidence to demonstrate an essential and instructive role for microtubules (MTs) in growth cone steering. First, both growth cone attraction and repulsion induced by diffusible cues in culture can be completely blocked by low concentrations of drugs that specifically inhibit dynamic microtubule ends in the growth cone. Second, direct focal photoactivated release of the microtubule-stabilizing drug taxol on one side of the growth cone consistently induces attraction (turning toward the site of application). Using the focal pipette application method, we also show that local MT stabilization by taxol induces growth cone attraction, whereas local MT destabilization by the microtubule-disrupting drug nocodazole induces repulsion (turning away). Finally, the microtubule-initiated attractive turning requires the participation of the actin cytoskeleton: local microtubule stabilization induces preferential protrusion of lamellipodia before the attractive turning, and the attraction can be abolished by inhibition of either actin polymerization or the Rho family GTPases. Together, these results demonstrate a novel steering mechanism for growth cones in which local and selective modification of dynamic microtubules can initiate and instruct directional steering. With the subsequent concerted activity of the actin cytoskeleton, this microtubule-initiated mechanism provides the growth cone with the additional means to efficiently navigate through its environment.

Rapid movement of microtubules in axons

Cytoskeletal and cytosolic proteins are transported along axons in the slow components of axonal transport at average rates of about 0.002-0.1 microm/s. This movement is essential for axonal growth and survival, yet the mechanism is poorly understood. Many studies on slow axonal transport have focused on tubulin, the subunit protein of microtubules, but attempts to observe the movement of this protein in cultured nerve cells have been largely unsuccessful. Here, we report direct observations of the movement of microtubules in cultured nerve cells using a modified fluorescence photobleaching strategy combined with difference imaging. The movements are rapid, with average rates of 1 microm/s, but they are also infrequent and highly asynchronous. These observations indicate that microtubules are propelled along axons by fast motors. We propose that the overall rate of movement is slow because the microtubules spend only a small proportion of their time moving. The rapid, infrequent, and highly asynchronous nature of the movement may explain why the axonal transport of tubulin has eluded detection in so many other studies.

Microtubule transport in the axon

There has been a great deal of interest in how the microtubule array of the axon is established and maintained. In an early model, it was proposed that microtubules are actively transported from the cell body of the neuron down the length of the axon. This model has been contested over the years in favor of very different models based on stationary microtubules. It appears that a corner has finally been turned in this long-standing controversy. It is now clear that cells contain molecular motor proteins capable of transporting microtubules and that microtubule transport is an essential component in the formation of microtubule arrays across many cells types. A wide variety of cell biological approaches have provided strong indirect evidence that microtubules are indeed transported within axons, and new live-cell imaging approaches are beginning to permit the direct visualization of this transport. The molecules and mechanisms that transport microtubules within axons are also under intense study.

Regulation and subcellular localization of the microtubule-destabilizing stathmin family phosphoproteins in cortical neurons

Stathmin is a ubiquitous cytosolic phosphoprotein, preferentially expressed in the nervous system, and the generic element of a protein family that includes the neural-specific proteins SCG10, SCLIP, and RB3 and its splice variants, RB3' and RB3". All phosphoproteins of the family share with stathmin its tubulin binding and microtubule (MT)-destabilizing activities. To understand better the specific roles of these proteins in neuronal cells, we performed a comparative study of their expression, regulation, and intracellular distribution in embryonic cortical neurons in culture. We found that stathmin is highly expressed ( approximately 0.25% of total proteins) and uniformly present in the various neuronal compartments (cell body, dendrites, axon, growth cones). It appeared mainly unphosphorylated or weakly phosphorylated on one site, and antisera to specific phosphorylated sites (serines 16, 25, or 38) did not reveal a differential regulation of its phosphorylation among neuronal cell compartments. However, they revealed a subpopulation of cells in which stathmin was highly phosphorylated on serine 16, possibly by CaM kinase II also active in a similar subpopulation. The other proteins of the stathmin family are expressed about 100-fold less than stathmin in partially distinct neuronal populations, RB3 being detected in only about 20% of neurons in culture. In contrast to stathmin, they are each mostly concentrated at the Golgi apparatus and are also present along dendrites and axons, including growth cones. Altogether, our results suggest that the different members of the stathmin family have complementary, at least partially distinct functions in neuronal cell regulation, in particular in relation to MT dynamics.

Rapid intermittent movement of axonal neurofilaments observed by fluorescence photobleaching

Observations on naturally occurring gaps in the axonal neurofilament array of cultured neurons have demonstrated that neurofilament polymers move along axons in a rapid, intermittent, and highly asynchronous manner. In contrast, studies on axonal neurofilaments using laser photobleaching have not detected movement. Here, we describe a modified photobleaching strategy that does permit the direct observation of neurofilament movement. Axons of cultured neurons expressing GFP-tagged neurofilament protein were bleached by excitation with the mercury arc lamp of a conventional epifluorescence microscope for 12-60 s. The length of the bleached region ranged from 10 to 60 microm. By bleaching thin axons, which have relatively few neurofilaments, we were able to reduce the fluorescent intensity enough to allow the detection of neurofilaments that moved in from the surrounding unbleached regions. Time-lapse imaging at short intervals revealed rapid, intermittent, and highly asynchronous movement of fluorescent filaments through the bleached regions at peak rates of up to 2.8 microm/s. The kinetics of movement were very similar to our previous observations on neurofilaments moving through naturally occurring gaps, which indicates that the movement was not impaired by the photobleaching process. These results demonstrate that fluorescence photobleaching can be used to study the slow axonal transport of cytoskeletal polymers, but only if the experimental strategy is designed to ensure that rapid asynchronous movements can be detected. This may explain the failure of previous photobleaching studies to reveal the movement of neurofilament proteins and other cytoskeletal proteins in axons.

Slow axonal transport: stop and go traffic in the axon

Efforts to observe the slow axonal transport of cytoskeletal polymers during the past decade have yielded conflicting results, and this has generated considerable controversy. The movement of neurofilaments has now been seen, and it is rapid, infrequent and highly asynchronous. This motile behaviour could explain why slow axonal transport has eluded observation for so long.

Neurofilaments are transported rapidly but intermittently in axons: implications for slow axonal transport

Slow axonal transport conveys cytoskeletal proteins from cell body to axon tip. This transport provides the axon with the architectural elements that are required to generate and maintain its elongate shape and also generates forces within the axon that are necessary for axon growth and navigation. The mechanisms of cytoskeletal transport in axons are unknown. One hypothesis states that cytoskeletal proteins are transported within the axon as polymers. We tested this hypothesis by visualizing individual cytoskeletal polymers in living axons and determining whether they undergo vectorial movement. We focused on neurofilaments in axons of cultured sympathetic neurons because individual neurofilaments in these axons can be visualized by optical microscopy. Cultured sympathetic neurons were infected with recombinant adenovirus containing a construct encoding a fusion protein combining green fluorescent protein (GFP) with the heavy neurofilament protein subunit (NFH). The chimeric GFP-NFH coassembled with endogenous neurofilaments. Time lapse imaging revealed that individual GFP-NFH-labeled neurofilaments undergo vigorous vectorial transport in the axon in both anterograde and retrograde directions but with a strong anterograde bias. NF transport in both directions exhibited a broad spectrum of rates with averages of approximately 0.6-0.7 microm/sec. However, movement was intermittent, with individual neurofilaments pausing during their transit within the axon. Some NFs either moved or paused for the most of the time they were observed, whereas others were intermediate in behavior. On average, neurofilaments spend at most 20% of the time moving and rest of the time paused. These results establish that the slow axonal transport machinery conveys neurofilaments.

Plasma membrane recycling and flow in growing neurites

Axonal growth requires insertion of newly synthesized membrane components into the plasmalemma. Imbalance between exocytotic membrane addition and endocytic retrieval at specific axonal sites may lead to the bulk plasma membrane flow along the axon and, thus, contribute to the renewal of plasma membrane components. By using fluorescent lipid analogs incorporated into the plasma membrane, we determined the sites of membrane internalization in growing Xenopus embryo neurons. Vectorial traffic of endocytic membranes from the distal axon to the cell body was observed, suggesting bulk retrieval of plasma membrane at the growth cone. No long-range axonal transport of membrane material internalized at the cell body or along the axon was detected. In addition, we measured the rate of plasma membrane flow in Xenopus neurites. Axonal plasma membrane was found to flow anterogradely with the rate equal to approximately 30% of the rate of neurite elongation. Our results suggest that the "growth from the tip" pattern of neurite elongation may coexist with transport of new membrane components along plasmalemma by anterograde membrane flow.

Rapid movement of axonal neurofilaments interrupted by prolonged pauses

Axonal cytoskeletal and cytosolic proteins are synthesized in the neuronal cell body and transported along axons by slow axonal transport, but attempts to observe this movement directly in living cells have yielded conflicting results. Here we report the direct observation of the axonal transport of neurofilament protein tagged with green fluorescent protein in cultured nerve cells. Live-cell imaging of naturally occurring gaps in the axonal neurofilament array reveals rapid, intermittent and highly asynchronous movement of fluorescent neurofilaments. The movement is bidirectional, but predominantly anterograde. Our data indicate that the slow rate of slow axonal transport may be the result of rapid movements interrupted by prolonged pauses.

Strategies for studying microtubule transport in the neuron

Microtubules are prominent cytoskeletal elements within the neuron. They are essential for the differentiation, growth, and maintenance of axons and dendrites. The microtubules within each type of process have a distinct pattern of organization, and these distinct patterns result in many of the morphological and structural features that distinguish axons and dendrites from one another. There are a number of challenges that must be met in order for the neuron to establish the microtubule arrays of axons and dendrites. One attractive model invokes the active transport of microtubules from the cell body of the neuron into and down these processes. In support of this model, specific motor proteins have now been identified within neurons that have the necessary properties to transport microtubules into developing axons and dendrites with the appropriate orientation for each type of process. An important goal is to develop microscopic methods that permit the visualization of microtubule transport within different regions of the neuron. To date, achieving this goal has met with mixed success, probably as a result of the geometry of the neuron and the inherent complexity of the neuronal microtubule arrays. While some approaches have failed to reveal microtubule transport, other more recent approaches have proven successful. These approaches provide strong visual support for a model based on microtubule transport, and provide hope that future approaches can provide even clearer demonstrations of this transport.

Speckle microscopic evaluation of microtubule transport in growing nerve processes

Assembly of microtubules is fundamental to neuronal morphogenesis. Microtubules typically form crosslinked bundles in nerve processes, precluding resolution of single microtubules at the light microscopic level. Therefore, previous studies of microtubule transport in neurites have had to rely on indirect approaches. Here we show that individual microtubules can be visualized directly in the axonal shafts of Xenopus embryo neurons by using digital fluorescence microscopy. We find that, although the array of axonal microtubules is dynamic, microtubules are stationary relative to the substrate. These results argue against a model in which newly synthesized tubulin is transported down the axon in the form of microtubules.

Reorganization and movement of microtubules in axonal growth cones and developing interstitial branches

Local changes in microtubule organization and distribution are required for the axon to grow and navigate appropriately; however, little is known about how microtubules (MTs) reorganize during directed axon outgrowth. We have used time-lapse digital imaging of developing cortical neurons microinjected with fluorescently labeled tubulin to follow the movements of individual MTs in two regions of the axon where directed growth occurs: the terminal growth cone and the developing interstitial branch. In both regions, transitions from quiescent to growth states were accompanied by reorganization of MTs from looped or bundled arrays to dispersed arrays and fragmentation of long MTs into short MTs. We also found that long-term redistribution of MTs accompanied the withdrawal of some axonal processes and the growth and stabilization of others. Individual MTs moved independently in both anterograde and retrograde directions to explore developing processes. Their velocities were inversely proportional to their lengths. Our results demonstrate directly that MTs move within axonal growth cones and developing interstitial branches. Our findings also provide the first direct evidence that similar reorganization and movement of individual MTs occur in the two regions of the axon where directed outgrowth occurs. These results suggest a model whereby short exploratory MTs could direct axonal growth cones and interstitial branches toward appropriate locations.

Slow transport of unpolymerized tubulin and polymerized neurofilament in the squid giant axon

A major issue in the slow transport of cytoskeletal proteins is the form in which they are transported. We have investigated the possibility that unpolymerized as well as polymerized cytoskeletal proteins can be actively transported in axons. We report the active transport of highly diffusible tubulin oligomers, as well as transport of the less diffusible neurofilament polymers. After injection into the squid giant axon, tubulin was transported in an anterograde direction at an average rate of 2.3 mm/day, whereas neurofilament was moved at 1.1 mm/day. Addition of the metabolic poisons cyanide or dinitrophenol reduced the active transport of both proteins to less than 10% of control values, whereas disruption of microtubules by treatment of the axon with cold in the presence of nocodazole reduced transport of both proteins to approximately 20% of control levels. Passive diffusion of these proteins occurred in parallel with transport. The diffusion coefficient of the moving tubulin in axoplasm was 8.6 micrometer(2)/s compared with only 0.43 micrometer(2)/s for neurofilament. These results suggest that the tubulin was transported in the unpolymerized state and that the neurofilament was transported in the polymerized state by an energy-dependent nocodazole/cold-sensitive transport mechanism.

Different contributions of microtubule dynamics and transport to the growth of axons and collateral sprouts

Axonal growth is believed to depend on microtubule transport and microtubule dynamic instability. We now report that the growth of axon collateral branches can occur independent of microtubule dynamic instability and can rely mostly on the transport of preassembled polymer. Raising embryonic sensory neurons in concentrations of either taxol or nocodazole (NOC) that largely inhibit microtubule dynamics significantly inhibited growth of main axonal shafts but had only minor effects on collateral branch growth. The collaterals of axons raised in taxol or nocodazole often contained single microtubules with both ends clearly visible within the collateral branch ("floating" microtubules), which we interpret as microtubules undergoing transport. Furthermore, in these collaterals there was a distoproximal gradient in microtubule mass, indicating the distal accumulation of transported polymer. Treatment of cultures with a high dose of nocodazole to deplete microtubules from collaterals, followed by treatment with 4-20 nM vinblastine to inhibit microtubule repolymerization, resulted in the time-dependent reappearance and subsequent distal accumulation of floating microtubules in collaterals, providing further evidence for microtubule transport into collateral branches. Our data show that, surprisingly, the contribution of microtubule dynamics to collateral branch growth is minor compared with the important role of microtubule dynamics in growth cone migration, and they indicate that the transport of microtubules may provide sufficient cytoskeletal material for the initial growth of collateral branches.

Role of moving growth cone-like "wave" structures in the outgrowth of cultured hippocampal axons and dendrites

Hippocampal neurons exhibit periodically recurring growth cone-like structures, referred to as "waves," that emerge at the base of neurites and travel distally to the tip. As a wave nears the tip, the neurite undergoes retraction, and when it reaches the tip, the neurite undergoes a burst of growth. At 1 day in culture, during early axon outgrowth, axons undergo an average 7.5-microm retraction immediately preceding wave arrival at the tip followed by 12-microm growth immediately after arrival (an average net growth of 4.5 microm). In branched axons, waves often selectively travel down one branch or the other. Growth selectively occurs in the branch chosen by the wave. In dendrites, which grow much slower on average, wave-associated retractions are much greater, resulting in less net growth. In the presence of Brefeldin A, which disrupts membrane traffic through the Golgi apparatus and leads to retraction of the axon, axonal waves continue to be associated with both growth spurts and retractions. The magnitude of the growth spurts is not significantly different from untreated axons, but wave-associated retractions are significantly increased. The close association between waves and cyclical elongation suggests that waves may act to bring about this pattern of growth. Our results also show that modulation of regularly occurring retraction phases plays a prominent role in determining average outgrowth rates.

Distribution of neurotransmitter secretion in growing axons

Neurotransmitter secretion from the nerve terminal is mediated by the fusion of synaptic vesicles with the plasma membrane. It is generally believed that neurotransmitter release in mature synapses is localized to the presynaptic nerve terminals. To probe the topology of neurotransmitter secretion along developing axons in culture, we recorded membrane currents from myocytes manipulated into contact with axons. At the early stages of growth, exocytotic events were detected along the axon as well as at the growth cone. At the later stages of growth, neurotransmitter secretion adopted the form of a smooth proximodistal gradient, with the highest level of activity at the growth cone region. Our results reveal the existence of a previously undetected early stage of axonal growth and suggest developmental regulation in the pattern of neurotransmitter secretion along the growing nerve processes.

Slow axonal transport of neurofilament protein in cultured neurons

We have investigated the axonal transport of neurofilament protein in cultured neurons by constricting single axons with fine glass fibers. We observed a rapid accumulation of anterogradely and retrogradely transported membranous organelles on both sides of the constrictions and a more gradual accumulation of neurofilament protein proximal to the constrictions. Neurofilament protein accumulation was dependent on the presence of metabolic substrates and was blocked by iodoacetate, which is an inhibitor of glycolysis. These data indicate that neurofilament protein moves anterogradely in these axons by a mechanism that is directly or indirectly dependent on nucleoside triphosphates. The average transport rate was estimated to be at least 130 micrometer/h (3.1 mm/d), and approximately 90% of the accumulated neurofilament protein remained in the axon after detergent extraction, suggesting that it was present in a polymerized form. Electron microscopy demonstrated that there were an abnormally large number of neurofilament polymers proximal to the constrictions. These data suggest that the neurofilament proteins were transported either as assembled polymers or in a nonpolymeric form that assembled locally at the site of accumulation. This study represents the first demonstration of the axonal transport of neurofilament protein in cultured neurons.

Neurotransmitter secretion along growing nerve processes: comparison with synaptic vesicle exocytosis

In mature neurons, synaptic vesicles continuously recycle within the presynaptic nerve terminal. In developing axons which are free of contact with a postsynaptic target, constitutive membrane recycling is not localized to the nerve terminal; instead, plasma membrane components undergo cycles of exoendocytosis throughout the whole axonal surface (Matteoli et al., 1992; Kraszewski et al., 1995). Moreover, in growing Xenopus spinal cord neurons in culture, acetylcholine (ACh) is spontaneously secreted in the quantal fashion along the axonal shaft (Evers et al., 1989; Antonov et al., 1998). Here we demonstrate that in Xenopus neurons ACh secretion is mediated by vesicles which recycle locally within the axon. Similar to neurotransmitter release at the presynaptic nerve terminal, ACh secretion along the axon could be elicited by the action potential or by hypertonic solutions. We found that the parameters of neurotransmitter secretion at the nerve terminal and at the middle axon were strikingly similar. These results lead us to conclude that, as in the case of the presynaptic nerve terminal, synaptic vesicles involved in neurotransmitter release along the axon contain a complement of proteins for vesicle docking and Ca2+-dependent fusion. Taken together, our results support the idea that, in developing axons, the rudimentary machinery for quantal neurotransmitter secretion is distributed throughout the whole axonal surface. Maturation of this machinery in the process of synaptic development would improve the fidelity of synaptic transmission during high-frequency stimulation of the presynaptic cell.

Dynamics of axonal microtubules regulate the topology of new membrane insertion into the growing neurites

Nerve growth depends on the delivery of cell body-synthesized material to the growing neuronal processes. The cellular mechanisms that determine the topology of new membrane addition to the axon are not known. Here we describe a technique to visualize the transport and sites of exocytosis of cell body- derived vesicles in growing axons. We found that in Xenopus embryo neurons in culture, cell body-derived vesicles were rapidly transported all the way down to the growth cone region, where they fused with the plasma membrane. Suppression of microtubule (MT) dynamic instability did not interfere with the delivery of new membrane material to the growth cone region; however, the insertion of vesicles into the plasma membrane was dramatically inhibited. Local disassembly of MTs by focal application of nocodazole to the middle axonal segment resulted in the addition of new membrane at the site of drug application. Our results suggest that the local destabilization of axonal MTs is necessary and sufficient for the delivery of membrane material to specific neuronal sites.

The direction of neurite growth in a weak DC electric field depends on the substratum: contributions of adhesivity and net surface charge

We investigated the influence of the growth surface on the direction of Xenopus spinal neurite growth in the presence of a dc electric field of physiological magnitude. The direction of galvanotropism was determined by the substratum; neurites grew toward the negative electrode (cathode) on untreated Falcon tissue culture plastic or on laminin substrata, which are negatively charged, but neurites growing on polylysine, which is positively charged, turned toward the positive electrode (anode). Growth was oriented randomly on all substrata without an electric field. We tested the hypothesis that the charge of the growth surface was responsible for reversed galvanotropism on polylysine by growing neurons on tissue culture dishes with different net surface charges. Although neurites grew cathodally on both Plastek substrata, the frequency of anodal turning was greater on dishes with a net positive charge (Plastek C) than on those with a net negative charge (Plastek M). The charge of the growth surface therefore influenced the frequency of anodal galvanotropism but a reversal in surface charge was insufficient to reverse galvanotropism completely, possibly because of differences in the relative magnitude of the substratum charge densities. The influence of substratum adhesion on galvanotropism was considered by growing neurites on a range of polylysine concentrations. Growth cone to substratum adhesivity was measured using a blasting assay. Adhesivity and the frequency of anodal turning were graded over the range of polylysine concentrations (0 = 0.1 < 1 < 10 = 100 microg/ml). The direction of neurite growth in an electric field is therefore influenced by both substratum charge and growth cone-to-substratum adhesivity. These data are consistent with the idea that spatial or temporal variation in the expression of adhesion molecules in embryos may interact with naturally occurring electric fields to enhance growth cone pathfinding.

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.

Selective in vivo fluorescence labelling of cholinergic neurons containing p75(NTR) in the rat basal forebrain

The cholinergic system of the rat basal forebrain is used as a model for the homologous region in humans which is highly susceptible to neuropathological alterations as in Alzheimer's disease. Cholinergic cells in the basal forebrain express the low-affinity neurotrophin receptor p75NTR. This has been utilized for selective immunolesioning of cholinergic neurons after internalization of an immunotoxin composed of anti-p75NTR and the ribosome-inactivating toxin saporin. However, the goal of many studies may be not the lesion, but the identification of cholinergic cells after other experimentally induced alterations in the basal forebrain. Therefore, a novel cholinergic marker was prepared by conjugating the monoclonal antibody 192IgG directed against p75NTR with the bright red fluorochrome carbocyanine 3 (Cy3). Three days after intraventricular injection of Cy3-192IgG the fluorescence microscopic analysis revealed a pattern of Cy3-labelled cells matching the distribution of cholinergic neurons. Apparently the marker was internalized within complexes of p75NTR and Cy3-192IgG which were then retrogradely transported to the cholinergic perikarya of the basal forebrain. In addition to the even labelling of somata, a strong punctate-like Cy3-immunofluorescence was seen in structures resembling lysosomes. The specificity of the in vivo staining was proven by subsequent immunolabelling of choline acetyltransferase (ChAT) with green fluorescent Cy2-tagged secondary antibodies. In the medial septum, the diagonal band and the nucleus basalis only cholinergic neurons were marked by Cy3-192IgG. In parallel experiments, digoxigenylated 192IgG was not detectable within cholinergic basal forebrain neurons after intraventricular injection. Presumably, this modified antibody could not be internalized. On the other hand, digoxigenylated 192IgG was found to be an excellent immunocytochemical marker for p75NTR as shown by double labelling including highly sensitive mouse antibodies directed against ChAT. Based on the present findings, future applications of the apparently non-toxic Cy3-192IgG and other antibodies for fluorescent in vivo and in vitro labelling are discussed.