Rapid movement of axonal neurofilaments interrupted by prolonged pauses

Bidirectional actin transport is influenced by microtubule and actin stability

Local and long-distance transport of cytoskeletal proteins is vital to neuronal maintenance and growth. Though recent progress has provided insight into the movement of microtubules and neurofilaments, mechanisms underlying the movement of actin remain elusive, in large part due to rapid transitions between its filament states and its diverse cellular localization and function. In this work, we integrated live imaging of rat sensory neurons, image processing, multiple regression analysis, and mathematical modeling to perform the first quantitative, high-resolution investigation of GFP-actin identity and movement in individual axons. Our data revealed that filamentous actin densities arise along the length of the axon and move short but significant distances bidirectionally, with a net anterograde bias. We directly tested the role of actin and microtubules in this movement. We also confirmed a role for actin densities in extension of axonal filopodia, and demonstrated intermittent correlation of actin and mitochondrial movement. Our results support a novel mechanism underlying slow component axonal transport, in which the stability of both microtubule and actin cytoskeletal components influence the mobility of filamentous actin.

Live-cell imaging of neurofilament transport in cultured neurons

Neurofilaments, which are the intermediate filaments of nerve cells, are space-filling cytoskeletal polymers that contribute to the growth of axonal caliber. In addition to their structural role, neurofilaments are cargos of axonal transport that move along microtubule tracks in a rapid, intermittent, and bidirectional manner. Though they measure just 10nm in diameter, which is well below the diffraction limit of optical microscopes, these polymers can reach 100 μm or more in length and are often packed densely, just tens of nanometers apart. These properties of neurofilaments present unique challenges for studies on their movement. In this article, we describe several live-cell fluorescence imaging strategies that we have developed to image neurofilament transport in axons of cultured neurons on short and long timescales. Together, these methods form a powerful set of complementary tools with which to study the axonal transport of these unique intracellular cargos.

A Stochastic Multiscale Model That Explains the Segregation of Axonal Microtubules and Neurofilaments in Neurological Diseases

The organization of the axonal cytoskeleton is a key determinant of the normal function of an axon, which is a long thin projection of a neuron. Under normal conditions two axonal cytoskeletal polymers, microtubules and neurofilaments, align longitudinally in axons and are interspersed in axonal cross-sections. However, in many neurotoxic and neurodegenerative disorders, microtubules and neurofilaments segregate apart from each other, with microtubules and membranous organelles clustered centrally and neurofilaments displaced to the periphery. This striking segregation precedes the abnormal and excessive neurofilament accumulation in these diseases, which in turn leads to focal axonal swellings. While neurofilament accumulation suggests an impairment of neurofilament transport along axons, the underlying mechanism of their segregation from microtubules remains poorly understood for over 30 years. To address this question, we developed a stochastic multiscale model for the cross-sectional distribution of microtubules and neurofilaments in axons. The model describes microtubules, neurofilaments and organelles as interacting particles in a 2D cross-section, and is built upon molecular processes that occur on a time scale of seconds or shorter. It incorporates the longitudinal transport of neurofilaments and organelles through this domain by allowing stochastic arrival and departure of these cargoes, and integrates the dynamic interactions of these cargoes with microtubules mediated by molecular motors. Simulations of the model demonstrate that organelles can pull nearby microtubules together, and in the absence of neurofilament transport, this mechanism gradually segregates microtubules from neurofilaments on a time scale of hours, similar to that observed in toxic neuropathies. This suggests that the microtubule-neurofilament segregation can be a consequence of the selective impairment of neurofilament transport. The model generates the experimentally testable prediction that the rate and extent of segregation will be dependent on the sizes of the moving organelles as well as the density of their traffic.

The shape and function of axons is dependent on a dynamic system of microscopic intracellular protein polymers (microtubules, neurofilaments and microfilaments) that comprise the axonal cytoskeleton. Neurofilaments are cargoes of intracellular transport that move along microtubule tracks, and they accumulate abnormally in axons in many neurotoxic and neurodegenerative disorders. Intriguingly, it has been reported that neurofilaments and microtubules, which are normally interspersed in axonal cross-sections, often segregate apart from each other in these disorders, which is something that is never observed in healthy axons. Here we describe a stochastic multiscale computational model that explains the mechanism of this striking segregation and offers insights into the mechanism of neurofilament accumulation in disease.

Microtubule-dependent transport and dynamics of vimentin intermediate filaments

We studied two aspects of vimentin intermediate filament dynamics-transport of filaments and subunit exchange. We observed transport of long filaments in the periphery of cells using live-cell structured illumination microscopy. We studied filament transport elsewhere in cells using a photoconvertible-vimentin probe and total internal reflection microscopy. We found that filaments were rapidly transported along linear tracks in both anterograde and retrograde directions. Filament transport was microtubule dependent but independent of microtubule polymerization and/or an interaction with the plus end-binding protein APC. We also studied subunit exchange in filaments by long-term imaging after photoconversion. We found that converted vimentin remained in small clusters along the length of filaments rather than redistributing uniformly throughout the network, even in cells that divided after photoconversion. These data show that vimentin filaments do not depolymerize into individual subunits; they recompose by severing and reannealing. Together these results show that vimentin filaments are very dynamic and that their transport is required for network maintenance.

Drag of the cytosol as a transport mechanism in neurons

Axonal transport is typically divided into two components, which can be distinguished by their mean velocity. The fast component includes steady trafficking of different organelles and vesicles actively transported by motor proteins. The slow component comprises nonmembranous materials that undergo infrequent bidirectional motion. The underlying mechanism of slow axonal transport has been under debate during the past three decades. We propose a simple displacement mechanism that may be central for the distribution of molecules not carried by vesicles. It relies on the cytoplasmic drag induced by organelle movement and readily accounts for key experimental observations pertaining to slow-component transport. The induced cytoplasmic drag is predicted to depend mainly on the distribution of microtubules in the axon and the organelle transport rate.

Axonal transport: cargo-specific mechanisms of motility and regulation

Axonal transport is essential for neuronal function, and many neurodevelopmental and neurodegenerative diseases result from mutations in the axonal transport machinery. Anterograde transport supplies distal axons with newly synthesized proteins and lipids, including synaptic components required to maintain presynaptic activity. Retrograde transport is required to maintain homeostasis by removing aging proteins and organelles from the distal axon for degradation and recycling of components. Retrograde axonal transport also plays a major role in neurotrophic and injury response signaling. This review provides an overview of axonal transport pathways and discusses their role in neuronal function.

Biophysical challenges to axonal transport: motor-cargo deficiencies and neurodegeneration

Axonal transport is indispensable for the distribution of vesicles, organelles, messenger RNAs (mRNAs), and signaling molecules along the axon. This process is mediated by kinesins and dyneins, molecular motors that bind to cargoes and translocate on microtubule tracks. Tight modulation of motor protein activity is necessary, but little is known about the molecules and mechanisms that regulate transport. Moreover, evidence suggests that transport impairments contribute to the initiation or progression of neurodegenerative diseases, or both, but the mechanisms by which motor activity is affected in disease are unclear. In this review, we discuss some of the physical and biophysical properties that influence motor regulation in healthy neurons. We further discuss the evidence for the role of transport in neurodegeneration, highlighting two pathways that may contribute to transport impairment-dependent disease: genetic mutations or variation, and protein aggregation. Understanding how and when transport parameters change in disease will help delineate molecular mechanisms of neurodegeneration.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

Local regulation of neurofilament transport by myelinating cells

Axons in the vertebrate nervous system only expand beyond ∼ 1 μm in diameter if they become myelinated. This expansion is due in large part to the accumulation of space-filling cytoskeletal polymers called neurofilaments, which are cargoes of axonal transport. One possible mechanism for this accumulation is a decrease in the rate of neurofilament transport. To test this hypothesis, we used a fluorescence photoactivation pulse-escape technique to compare the kinetics of neurofilament transport in contiguous myelinated and unmyelinated segments of axons in long-term myelinating cocultures established from the dorsal root ganglia of embryonic rats. The myelinated segments contained more neurofilaments and had a larger cross-sectional area than the contiguous unmyelinated segments, and this correlated with a local slowing of neurofilament transport. By computational modeling of the pulse-escape kinetics, we found that this slowing of neurofilament transport could be explained by an increase in the proportion of the time that the neurofilaments spent pausing and that this increase in pausing was sufficient to explain the observed neurofilament accumulation. Thus we propose that myelinating cells can regulate the neurofilament content and morphology of axons locally by modulating the kinetics of neurofilament transport.

Deciphering the axonal transport kinetics of neurofilaments using the fluorescence photoactivation pulse-escape method

Neurofilaments are transported along axons stochastically in a stop-and-go manner, cycling between brief bouts of rapid movement and pauses that can vary from seconds to hours in length. Presently the only way to analyze neurofilament pausing experimentally on both long and short time scales is the pulse-escape method. In this method, fluorescence photoactivation is used to mark a population of axonal neurofilaments and then the loss of fluorescence from the activated region due to neurofilament movement is monitored by time-lapse imaging. Here we develop a mathematical description of the pulse-escape kinetics in terms of the rate constants of a tested mathematical model and we show how this model can be used to characterize neurofilament transport kinetics from fluorescence photoactivation pulse-escape experiments. This combined experimental and computational approach is a powerful tool for the analysis of the moving and pausing behavior of neurofilaments in axons.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

Fast vesicle transport is required for the slow axonal transport of synapsin

Although it is known that cytosolic/soluble proteins synthesized in cell bodies are transported at much lower overall velocities than vesicles in fast axonal transport, the fundamental basis for this slow movement is unknown. Recently, we found that cytosolic proteins in axons of mouse cultured neurons are conveyed in a manner that superficially resembles diffusion, but with a slow anterograde bias that is energy- and motor-dependent (Scott et al., 2011). Here we show that slow axonal transport of synapsin, a prototypical member of this rate class, is dependent upon fast vesicle transport. Despite the distinct overall dynamics of slow and fast transport, experimentally induced and intrinsic variations in vesicle transport have analogous effects on slow transport of synapsin as well. Dynamic cotransport of vesicles and synapsin particles is also seen in axons, consistent with a model where higher-order assemblies of synapsin are conveyed by transient and probabilistic associations with vesicles moving in fast axonal transport. We posit that such dynamic associations generate the slow overall anterogradely biased flow of the population ("dynamic-recruitment model"). Our studies uncover the underlying kinetic basis for a classic cytosolic/soluble protein moving in slow axonal transport and reveal previously unknown links between slow and fast transport, offering a clearer conceptual picture of this curious phenomenon.

Cell biology in neuroscience: Cellular and molecular mechanisms underlying axon formation, growth, and branching

Proper brain wiring during development is pivotal for adult brain function. Neurons display a high degree of polarization both morphologically and functionally, and this polarization requires the segregation of mRNA, proteins, and lipids into the axonal or somatodendritic domains. Recent discoveries have provided insight into many aspects of the cell biology of axonal development including axon specification during neuronal polarization, axon growth, and terminal axon branching during synaptogenesis.

Analytical comparison between Nixon-Logvinenko's and Jung-Brown's theories of slow neurofilament transport in axons

This paper develops analytical solutions describing slow neurofilament (NF) transport in axons. The obtained solutions are based on two theories of NF transport: Nixon-Logvinenko's theory that postulates that most NFs are incorporated into a stationary cross-linked network and only a small pool is slowly transported and Jung-Brown's theory that postulates a single dynamic pool of NFs that are transported according to the stop-and-go hypothesis. The simplest two-kinetic state version of the model developed by Jung and Brown was compared with the theory developed by Nixon and Logvinenko. The model for Nixon-Logvinenko's theory included stationary, pausing, and running NF populations while the model used for Jung-Brown's theory only included pausing and running NF populations. Distributions of NF concentrations resulting from Nixon-Logvinenko's and Jung-Brown's theories were compared. In previous publications, Brown and colleagues successfully incorporated slowing of NF transport into their model by assuming that some kinetic constants depend on the distance from the axon hillock. In this paper we defined the average rate of NF transport as the rate of motion of the center of mass of radiolabeled NFs. We have shown that for this definition, if all kinetic rates are assumed constant, Jung-Brown's theory predicts a constant average rate of NF transport. We also demonstrated that Nixon-Logvinenko's theory predicts slowing of NF transport even if all kinetic rates are assumed constant, and the obtained slowing agrees well with published experimental data.

Seeing the unseen: the hidden world of slow axonal transport

Axonal transport is the lifeline of axons and synapses. After synthesis in neuronal cell bodies, proteins are conveyed into axons in two distinct rate classes-fast and slow axonal transport. Whereas fast transport delivers vesicular cargoes, slow transport carries cytoskeletal and cytosolic (or soluble) proteins that have critical roles in neuronal structure and function. Although significant progress has been made in dissecting the molecular mechanisms of fast vesicle transport, mechanisms of slow axonal transport are less clear. Why is this so? Historically, conceptual advances in the axonal transport field have paralleled innovations in imaging the movement, and slow-transport cargoes are not as readily seen as motile vesicles. However, new ways of visualizing slow transport have reenergized the field, leading to fundamental insights that have changed our views on axonal transport, motor regulation, and intracellular trafficking in general. This review first summarizes classic studies that characterized axonal transport, and then discusses recent technical and conceptual advances in slow axonal transport that have provided insights into some long-standing mysteries.

Severing and end-to-end annealing of neurofilaments in neurons

We have shown previously that neurofilaments and vimentin filaments expressed in nonneuronal cell lines can lengthen by joining ends in a process known as "end-to-end annealing." To test if this also occurs for neurofilaments in neurons, we transfected cultured rat cortical neurons with fluorescent neurofilament fusion proteins and then used photoconversion or photoactivation strategies to create distinct populations of red and green fluorescent filaments. Within several hours we observed the appearance of chimeric filaments consisting of alternating red and green segments, which is indicative of end-to-end annealing of red and green filaments. However, the appearance of these chimeric filaments was accompanied by a gradual fragmentation of the red and green filament segments, which is indicative of severing. Over time we observed a progressive increase in the number of red-green junctions along the filaments accompanied by a progressive decrease in the average length of the alternating red and green fluorescent segments that comprised those filaments, suggesting a dynamic cycle of severing and end-to-end-annealing. Time-lapse imaging of the axonal transport of chimeric filaments demonstrated that the red and green segments moved together, confirming that they were indeed part of the same filament. Moreover, in several instances, we also were able to capture annealing and severing events live in time-lapse movies. We propose that the length of intermediate filaments in cells is regulated by the opposing actions of severing and end-to-end annealing, and we speculate that this regulatory mechanism may influence neurofilament transport within axons.

Charcot-Marie-Tooth causing HSPB1 mutations increase Cdk5-mediated phosphorylation of neurofilaments

Mutations in the small heat shock protein HSPB1 (HSP27) are a cause of axonal Charcot-Marie-Tooth neuropathy (CMT2F) and distal hereditary motor neuropathy. To better understand the effect of mutations in HSPB1 on the neuronal cytoskeleton, we stably transduced neuronal cells with wild-type and mutant HSPB1 and investigated axonal transport of neurofilaments (NFs). We observed that mutant HSPB1 affected the binding of NFs to the anterograde motor protein kinesin, reducing anterograde transport of NFs. These deficits were associated with an increased phosphorylation of NFs and cyclin-dependent kinase Cdk5. As Cdk5 mediates NF phosphorylation, inhibition of Cdk5/p35 restored NF phosphorylation level, as well as NF binding to kinesin in mutant HSPB1 neuronal cells. Altogether, we demonstrate that HSPB1 mutations induce hyperphosphorylation of NFs through Cdk5 and reduce anterograde transport of NFs.

Toxic neurofilamentous axonopathies -- accumulation of neurofilaments and axonal degeneration

A number of neurotoxic chemicals induce accumulation of neurofilaments in axonal swellings that appear at varying distances from the cell body. This pathology is associated with axonal degeneration of different degrees. The clinical manifestation is most commonly that of a mixed motor-sensory peripheral axonopathy with a disto-proximal pattern of progression, as in cases of chronic exposure to n-hexane and carbon disulphide. It has been demonstrated that protein adduct formation is a primary molecular mechanism of toxicity in these axonopathies, but how this mechanism leads to neurofilament accumulation and axonal degeneration remains unclear. Furthermore, little is known regarding the mechanisms of neurofilamentous axonopathy caused by 3,3'-iminodipropionitrile, an experimental toxin that induces proximal axon swelling that is strikingly similar to that found in early amyotrophic lateral sclerosis. Here, we review the available data and main hypotheses regarding the toxic axonopathies and compare them with the current knowledge of the biological basis of neurofilament transport. We also review recent studies addressing the question of how these axonopathies may cause axonal degeneration. Understanding the mechanisms underlying the toxic axonopathies may provide insight into the relationship between neurofilament behaviour and axonal degeneration, hopefully enabling the identification of new targets for therapeutic intervention. Because neurofilament abnormalities are a common feature of many neurodegenerative diseases, advances in this area may have a wider impact beyond toxicological significance.

Tau phosphorylation affects its axonal transport and degradation

Phosphorylated forms of microtubule-associated protein tau accumulate in neurofibrillary tangles in Alzheimer's disease. To investigate the effects of specific phosphorylated tau residues on its function, wild type or phosphomutant tau was expressed in cells. Elevated tau phosphorylation decreased its microtubule binding and bundling, and increased the number of motile tau particles, without affecting axonal transport kinetics. In contrast, reducing tau phosphorylation enhanced the amount of tau bound to microtubules and inhibited axonal transport of tau. To determine whether differential tau clearance is responsible for the increase in phosphomimic tau, we inhibited autophagy in neurons which resulted in a 3-fold accumulation of phosphomimic tau compared with wild type tau, and endogenous tau was unaffected. In autophagy-deficient mouse embryonic fibroblasts, but not in neurons, proteasomal degradation of phosphomutant tau was also reduced compared with wild type tau. Therefore, autophagic and proteasomal pathways are involved in tau degradation, with autophagy appearing to be the primary route for clearing phosphorylated tau in neurons. Defective autophagy might contribute to the accumulaton of tau in neurodegenerative diseases.

Imaging axonal transport in the rat visual pathway

A technique was developed for assaying axonal transport in retinal ganglion cells using 2 µl injections of 1% cholera toxin b-subunit conjugated to AlexaFluor488 (CTB). In vivo retinal and post-mortem brain imaging by confocal scanning laser ophthalmoscopy and post-mortem microscopy were performed. The transport of CTB was sensitive to colchicine, which disrupts axonal microtubules. The bulk rates of transport were determined to be approximately 80-90 mm/day (anterograde) and 160 mm/day (retrograde). Results demonstrate that axonal transport of CTB can be monitored in vivo in the rodent anterior visual pathway, is dependent on intact microtubules, and occurs by active transport mechanisms.

The discontinuous nature of neurofilament transport accommodates both establishment and repair of the axonal neurofilament array

Neurofilaments (NFs) provide structural support to axons. Timely and regional deposition of NFs is essential during axonogenesis, since progressive stabilization of proximal axons is essential to support continued pathfinding of distal axonal regions. NFs undergo short bursts of microtubule-mediated axonal transport interspersed by prolonged pauses. We demonstrate herein that it is this unique "on-off" method of axonal transport, coupled with the ability of NFs to form cation-dependent, phosphomediated lateral associations that allow neurons to mediate the orderly transition from exploratory process to stabilized axon following synaptogenesis. We further demonstrate how this transport method provides for NF maintenance following maturation and encompasses the potential for regeneration.

A critical reevaluation of the stationary axonal cytoskeleton hypothesis

Neurofilaments are transported along axons in a rapid intermittent and bidirectional manner but there is a long-standing controversy about whether this applies to all axonal neurofilaments. Some have proposed that only a small proportion of axonal neurofilaments are mobile and that most are deposited into a persistently stationary and extensively cross-linked cytoskeleton that remains fixed in place for many months without movement, turning over very slowly. In contrast, others have proposed that this hypothesis is based on a misinterpretation of the experimental data and that, in fact, all axonal neurofilaments move. These contrary perspectives have distinct implications for our understanding of how neurofilaments are organized and reorganized in axons both in health and disease. Here, we discuss the history and substance of this controversy. We show that the published data on the kinetics and distribution of neurofilaments along axons favor a simple "stop and go" transport model in which axons contain a single population of neurofilaments that all move in a stochastic, bidirectional and intermittent manner. Based on these considerations, we propose a dynamic view of the neuronal cytoskeleton in which all neurofilaments cycle repeatedly between moving and pausing states throughout their journey along the axon. The filaments move infrequently, but the average pause duration is on the order of hours rather than weeks or months. Against this fluid backdrop, the action of molecular motors on neurofilaments can have dramatic effects on neurofilament organization that would not be possible if the neurofilaments were extensively cross-linked into a truly stationary network.

FluoroMyelin™ Red is a bright, photostable and non-toxic fluorescent stain for live imaging of myelin

FluoroMyelin™ Red is a commercially available water-soluble fluorescent dye that has selectivity for myelin. This dye is marketed for the visualization of myelin in brain cryosections, though it is also used widely to stain myelin in chemically fixed tissue. Here we have investigated the suitability of FluoroMyelin™ Red as a vital stain for live imaging of myelin in myelinating co-cultures of Schwann cells and dorsal root ganglion neurons. We show that addition of FluoroMyelin™ Red to the culture medium results in selective staining of myelin sheaths, with an optimal staining time of 2h, and has no apparent adverse effect on the neurons, their axons, or the myelinating cells at the light microscopic level. The fluorescence is bright and photostable, permitting long-term time-lapse imaging. After rinsing the cultures with medium lacking FluoroMyelin™ Red, the dye diffuses out of the myelin with a half life of about 130 min resulting in negligible fluorescence remaining after 18-24h. In addition, the large Stokes shift exhibited by FluoroMyelin™ Red makes it possible to readily distinguish it from popular and widely used green and red fluorescent probes such as GFP and mCherry. Thus FluoroMyelin™ Red is a useful reagent for live fluorescence imaging studies on myelinated axons.

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.

Neurofilaments are flexible polymers that often fold and unfold, but they move in a fully extended configuration

Time-lapse imaging of neurofilaments in axons of cultured nerve cells has demonstrated that these cytoskeletal polymers move along microtubule tracks in both anterograde and retrograde directions, powered by microtubule motors. The filaments exhibit short bouts of rapid intermittent movement interrupted by prolonged pauses, and the average velocity is slow because they spend most of their time pausing. Here, we show that axonal neurofilaments are also very flexible and frequently exhibit complex and dynamic folding and unfolding behaviors while they are pausing. Remarkably, however, when the filaments move in a sustained manner, we find that they always adopt an unfolded, that is, fully extended configuration, and this applies to movement in both anterograde and retrograde directions. Given the flexibility of neurofilament polymers and the apparent ease with which they can fold back on themselves, the fact that they move in a fully extended configuration suggests that moving neurofilaments may be pulled from their leading end. Thus, we speculate that motors may bind to the leading ends of neurofilaments polymers during both anterograde and retrograde motion.

Interaction with a kinesin-2 tail propels choline acetyltransferase flow towards synapse

Bulk flow constitutes a substantial part of the slow transport of soluble proteins in axons. Though the underlying mechanism is unclear, evidences indicate that intermittent, kinesin-based movement of large protein-aggregates aids this process. Choline acetyltransferase (ChAT), a soluble enzyme catalyzing acetylcholine synthesis, propagates toward the synapse at an intermediate, slow rate. The presynaptic enrichment of ChAT requires heterotrimeric kinesin-2, comprising KLP64D, KLP68D and DmKAP, in Drosophila. Here, we show that the bulk flow of a recombinant Green Fluorescent Protein-tagged ChAT (GFP::ChAT), in Drosophila axons, lacks particulate features. It occurs for a brief period during the larval stages. In addition, both the endogenous ChAT and GFP::ChAT directly bind to the KLP64D tail, which is essential for the GFP::ChAT entry and anterograde flow in axon. These evidences suggest that a direct interaction with motor proteins could regulate the bulk flow of soluble proteins, and thus establish their asymmetric distribution.

Interference with kinesin-based anterograde neurofilament axonal transport increases neurofilament-neurofilament bundling

Neurofilaments (NFs) associate with each other and with other cytoskeletal elements to form a lattice that supports the mature axon. Phosphorylation contributes to formation of this stationary population of NFs by fostering cation-dependent interactions among NF sidearms. Association of NFs with the stationary phase indirectly competes with NF axonal transport by withdrawing NFs from kinesin-dependent motility along microtubules. We therefore hypothesized that inhibition of anterograde NF transport may increase incorporation into the stationary phase. To test this hypothesis, we treated differentiated NB2a/d1 cells expressing GFP-tagged NF subunits with monastrol, a specific inhibitor of kinesin-5. Monastrol significantly inhibited anterograde axonal transport of NF-H but not NF-M, and increased the incorporation of newly-transported NF subunits into axonal NF bundles. These findings support the notion that NF transport and bundling exert opposing forces on axonal NF dynamics, and that inhibition of anterograde transport of NFs can increase their incorporation into the stationary phase.

Axonal transport of neurofilaments: a single population of intermittently moving polymers

Studies on mouse optic nerve have led to the controversial proposal that only a small proportion of neurofilaments are transported in axons and that the majority are deposited into a persistently stationary and extensively cross-linked cytoskeletal network that remains fixed in place for months without movement. We have used computational modeling to address this issue, taking advantage of the wealth of published kinetic and morphometric data available for neurofilaments in the mouse visual system. We show that the transport kinetics and distribution of neurofilaments in mouse optic nerve can all be explained fully by a "stop-and-go" model of neurofilament transport, in which axons contain a single population of neurofilaments that all move stochastically in a rapid, intermittent, and bidirectional manner. Importantly, we find that the transport kinetics are not consistent with deposition of neurofilaments into a persistently stationary phase, and that deposition models cannot account for the observed distribution of neurofilaments along mouse optic nerve axons. Finally, we show that the apparent existence of a stationary neurofilament network in mouse optic nerve is most likely an experimental artifact due to contamination of the neurofilament transport kinetics with cytosolic proteins that move at faster rates. Thus, there is no evidence for the deposition of axonally transported neurofilaments into a persistently stationary neurofilament network in optic nerve axons. We conclude that all of the neurofilaments move and that they do so with a single broad and continuous distribution of average rates that is dictated by their intermittent and stochastic motile behavior.

The slow axonal transport of alpha-synuclein--mechanistic commonalities amongst diverse cytosolic cargoes

Slow axonal transport conveys perikaryally-synthesized cytosolic proteins in a rate-class called Slow Component-b (SCb). One such protein--α-synuclein--is largely conveyed in SCb, and is also a key player in a group of neurodegenerative diseases called synucleinopathies. Axonal transport defects of α-synuclein have been hypothesized to play a role in synucleinopathies, but mechanisms moving α-synuclein in slow axonal transport are unclear. Here we use a recently developed model-system in our laboratory to visualize the slow transport of α-synuclein, comparing it to another SCb protein synapsin. Despite differences inbiological properties and overall-solubility in axons, the anterograde transport of both SCb proteins was strikingly similar, suggesting commonalities in slow axonal transport mechanisms of seemingly diverse cytosolic cargoes. The data support a model where SCb proteins dynamically organize into 'transport-competent' complexes that are conveyed via transient associations with other persistently-moving cargoes ("mobile-units"). The identity of the latter is yet unknown. Visualizing normal α-synuclein transport may also open the door to studies of α-synuclein transport in pathologic states.

Traumatic white matter injury and toxic leukoencephalopathies

White matter injury may be secondary to a range of neurodegenerative disorders, such as the common dementing disorders of the elderly, or may be a consequence of specific white matter disorders, such as multiple sclerosis and the rare leukodystrophies. This article will focus on two relatively common primary groups of disorders of the white matter, traumatic white matter injury and toxic leukoencephalopathies. Traumatic axonal injury may be focal or diffuse, and is associated with a clinical spectrum ranging from concussion through to coma and death. The molecular mechanisms underlying axonal degeneration secondary to traumatic axonal degeneration are being elucidated and may give an insight into potential therapeutic targets. Toxic leukoencephalopathy may be secondary to exposure to a wide range of compounds, including chemotherapeutic drugs. These toxins may produce white matter injury through a range of mechanisms, and the potential toxic effects of compounds need to be considered when assessing a patient with a nonspecific leukoencephalopathy.

Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia

Epithelia are exposed to multiple forms of stress. Keratin intermediate filaments are abundant in epithelia and form cytoskeletal networks that contribute to cell type–specific functions, such as adhesion, migration, and metabolism. A perpetual keratin filament turnover cycle supports these functions. This multistep process keeps the cytoskeleton in motion, facilitating rapid and protein biosynthesis–independent network remodeling while maintaining an intact network. The current challenge is to unravel the molecular mechanisms underlying the regulation of the keratin cycle in relation to actin and microtubule networks and in the context of epithelial tissue function.

Object tracking with particle filtering in fluorescence microscopy images: application to the motion of neurofilaments in axons

Neurofilaments are long flexible cytoplasmic protein polymers that are transported rapidly but intermittently along the axonal processes of nerve cells. Current methods for studying this movement involve manual tracking of fluorescently tagged neurofilament polymers in videos acquired by time-lapse fluorescence microscopy. Here, we describe an automated tracking method that uses particle filtering to implement a recursive Bayesian estimation of the filament location in successive frames of video sequences. To increase the efficiency of this approach, we take advantage of the fact that neurofilament movement is confined within the boundaries of the axon. We use piecewise cubic spline interpolation to model the path of the axon and then we use this model to limit both the orientation and location of the neurofilament in the particle tracking algorithm. Based on these two spatial constraints, we develop a prior dynamic state model that generates significantly fewer particles than generic particle filtering, and we select an adequate observation model to produce a robust tracking method. We demonstrate the efficacy and efficiency of our method by performing tracking experiments on real time-lapse image sequences of neurofilament movement, and we show that the method performs well compared to manual tracking by an experienced user. This spatially constrained particle filtering approach should also be applicable to the movement of other axonally transported cargoes.

A simple photoactivation and image analysis module for visualizing and analyzing axonal transport with high temporal resolution

We describe a strategy for analyzing axonal transport of cytosolic proteins (CPs) using photoactivatable GFP-PAGFP-with modifications of standard imaging components that can be retroactively fitted to a conventional epifluorescence microscope. The photoactivation and visualization are nearly simultaneous, allowing studies of proteins with rapidly mobile fractions. Cultured hippocampal neurons are transfected with PAGFP-tagged constructs, a discrete protein population within axons is photoactivated, and then the activated population is tracked by live imaging. We show the utility of this method in analyzing axonal transport of CPs that have inherent diffusible pools and distinguish this transport modality from passive diffusion and vesicle transport. The analytical tools used to quantify the motion are also described. Aside from the time needed for preparation of neuronal cultures/transfection, the experiment takes 2-3 h, during which time several axons can be imaged and analyzed. These methods should be easy to adopt by most laboratories and may also be useful for monitoring CP movement in other cell types.

Molecular motor proteins and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor neurons in the brain, brainstem and spinal cord, which is characterized by motor dysfunction, muscle dystrophy and progressive paralysis. Both inherited and sporadic forms of ALS share common pathological features, however, the initial trigger of neurodegeneration remains unknown. Motor neurons are uniquely targeted by ubiquitously expressed proteins in ALS but the reason for this selectively vulnerability is unclear. However motor neurons have unique characteristics such as very long axons, large cell bodies and high energetic metabolism, therefore placing high demands on cellular transport processes. Defects in cellular trafficking are now widely reported in ALS, including dysfunction to the molecular motors dynein and kinesin. Abnormalities to dynein in particular are linked to ALS, and defects in dynein-mediated axonal transport processes have been reported as one of the earliest pathologies in transgenic SOD1 mice. Furthermore, dynein is very highly expressed in neurons and neurons are particularly sensitive to dynein dysfunction. Hence, unravelling cellular transport processes mediated by molecular motor proteins may help shed light on motor neuron loss in ALS.

Novel diffusion barrier for axonal retention of Tau in neurons and its failure in neurodegeneration

Missorting of Tau from axons to the somatodendritic compartment of neurons is a hallmark of Alzheimer's disease, but the mechanisms underlying normal sorting and pathological failure are poorly understood. Here, we used several Tau constructs labelled with photoconvertible Dendra2 to analyse its mobility in polarized neurons. This revealed a novel mechanism of sorting-a retrograde barrier in the axon initial segment (AIS) operating as cellular rectifier. It allows anterograde flow of axonal Tau but prevents retrograde flow back into soma and dendrites. The barrier requires binding of Tau to microtubules but does not require F-actin and thus is distinct from the sorting of membrane-associated proteins at the AIS. The barrier breaks down when Tau is phosphorylated in its repeat domain and detached from microtubules, for example, by the kinase MARK/Par1. These observations link the pathological hallmarks of Tau missorting and hyperphosphorylation in neurodegenerative diseases.

Mechanistic logic underlying the axonal transport of cytosolic proteins

Proteins vital to presynaptic function are synthesized in the neuronal perikarya and delivered into synapses via two modes of axonal transport. While membrane-anchoring proteins are conveyed in fast axonal transport via motor-driven vesicles, cytosolic proteins travel in slow axonal transport via mechanisms that are poorly understood. We found that in cultured axons, populations of cytosolic proteins tagged to photoactivatable GFP (PAGFP) move with a slow motor-dependent anterograde bias distinct from both vesicular trafficking and diffusion of untagged PAGFP. The overall bias is likely generated by an intricate particle kinetics involving transient assembly and short-range vectorial spurts. In vivo biochemical studies reveal that cytosolic proteins are organized into higher order structures within axon-enriched fractions that are largely segregated from vesicles. Data-driven biophysical modeling best predicts a scenario where soluble molecules dynamically assemble into mobile supramolecular structures. We propose a model where cytosolic proteins are transported by dynamically assembling into multiprotein complexes that are directly/indirectly conveyed by motors.

Differential roles of kinesin and dynein in translocation of neurofilaments into axonal neurites

Neurofilament (NF) subunits translocate within axons as short NFs, non-filamentous punctate structures ('puncta') and diffuse material that might comprise individual subunits and/or oligomers. Transport of NFs into and along axons is mediated by the microtubule (MT) motor proteins kinesin and dynein. Despite being characterized as a retrograde motor, dynein nevertheless participates in anterograde NF transport through associating with long MTs or the actin cortex through its cargo domain; relatively shorter MTs associated with the motor domain are then propelled in an anterograde direction, along with any linked NFs. Here, we show that inhibition of dynein function, through dynamitin overexpression or intracellular delivery of anti-dynein antibody, selectively reduced delivery of GFP-tagged short NFs into the axonal hillock, with a corresponding increase in the delivery of puncta, suggesting that dynein selectively delivered short NFs into axonal neurites. Nocodazole-mediated depletion of short MTs had the same effect. By contrast, intracellular delivery of anti-kinesin antibody inhibited anterograde transport of short NFs and puncta to an equal extent. These findings suggest that anterograde axonal transport of linear NFs is more dependent upon association with translocating MTs (which are themselves translocated by dynein) than is transport of NF puncta or oligomers.

The myosin Va head domain binds to the neurofilament-L rod and modulates endoplasmic reticulum (ER) content and distribution within axons

The neurofilament light subunit (NF-L) binds to myosin Va (Myo Va) in neurons but the sites of interaction and functional significance are not clear. We show by deletion analysis that motor domain of Myo Va binds to the NF-L rod domain that forms the NF backbone. Loss of NF-L and Myo Va binding from axons significantly reduces the axonal content of ER, and redistributes ER to the periphery of axon. Our data are consistent with a novel function for NFs as a scaffold in axons for maintaining the content and proper distribution of vesicular organelles, mediated in part by Myo Va. Based on observations that the Myo Va motor domain binds to intermediate filament (IF) proteins of several classes, Myo Va interactions with IFs may serve similar roles in organizing organelle topography in different cell types.

A hereditary spastic paraplegia mutation in kinesin-1A/KIF5A disrupts neurofilament transport

BACKGROUND

Hereditary spastic paraplegias are a group of neurological disorders characterized by progressive distal degeneration of the longest ascending and descending axons in the spinal cord, leading to lower limb spasticity and weakness. One of the dominantly inherited forms of this disease (spastic gait type 10, or SPG10) is caused by point mutations in kinesin-1A (also known as KIF5A), which is thought to be an anterograde motor for neurofilaments.

RESULTS

We investigated the effect of an SPG10 mutation in kinesin-1A (N256S-kinesin-1A) on neurofilament transport in cultured mouse cortical neurons using live-cell fluorescent imaging. N256S-kinesin-1A decreased both anterograde and retrograde neurofilament transport flux by decreasing the frequency of anterograde and retrograde movements. Anterograde velocity was not affected, whereas retrograde velocity actually increased.

CONCLUSIONS

These data reveal subtle complexities to the functional interdependence of the anterograde and retrograde neurofilament motors and they also raise the possibility that anterograde and retrograde neurofilament transport may be disrupted in patients with SPG10.

Peptidyl-prolyl isomerase 1 regulates protein phosphatase 2A-mediated topographic phosphorylation of neurofilament proteins

In normal neurons, neurofilament (NF) proteins are phosphorylated in the axonal compartment. However, in neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), NF proteins are aberrantly hyperphosphorylated within the cell bodies. The aberrant hyperphosphorylation of NF accumulations found in neurodegeneration could be attributable to either deregulation of proline-directed Ser/Thr kinase(s) activity or downregulation of protein phosphatase(s) activity. In this study, we found that protein phosphatase 2A (PP2A) expression is high in neuronal cell bodies and that inhibition of PP2A activity by okadaic acid (OA), microcystin LR (mLR), or fostriecin (Fos) leads to perikaryal hyperphosphorylation of NF. Peptidyl-prolyl isomerase Pin1 inhibits the dephosphorylation of NF by PP2A in vitro. In cortical neurons, Pin1 modulates the topographic phosphorylation of the proline-directed Ser/Thr residues within the tail domain of NF proteins by inhibiting the dephosphorylation by PP2A. Inhibition of Pin1 inhibits OA-induced aberrant perikaryal phosphorylation of NF. Treatment of cortical neurons with OA or Fos prevents the general anterograde transport of transfected green fluorescent protein-high-molecular-mass (NF-H) into axons caused by hyperphosphorylation of NF-H, and inhibition of Pin1 rescues this effect. Furthermore, inhibition of Pin1 inhibits the OA- or Fos-induced neuronal apoptosis. We show that OA-induced hyperphosphorylation of NF is a consequence of dephosphorylation of NF and is independent of c-Jun N-terminal protein kinase, extracellular signal-regulated kinase, and cyclin-dependent kinase-5 pathways. This study highlights a novel signaling role of PP2A by Pin1 and implicates Pin1 as a therapeutic target to reduce aberrant phosphorylation of NF proteins in neurodegenerative disorders such as AD, PD, and ALS.

Tight functional coupling of kinesin-1A and dynein motors in the bidirectional transport of neurofilaments

We have tested the hypothesis that kinesin-1A (formerly KIF5A) is an anterograde motor for axonal neurofilaments. In cultured sympathetic neurons from kinesin-1A knockout mice, we observed a 75% reduction in the frequency of both anterograde and retrograde neurofilament movement. This transport defect could be rescued by kinesin-1A, and with successively decreasing efficacy by kinesin-1B and kinesin-1C. In wild-type neurons, headless mutants of kinesin-1A and kinesin-1C inhibited both anterograde and retrograde movement in a dominant-negative manner. Because dynein is thought to be the retrograde motor for axonal neurofilaments, we investigated the effect of dynein inhibition on anterograde and retrograde neurofilament transport. Disruption of dynein function by using RNA interference, dominant-negative approaches, or a function-blocking antibody also inhibited both anterograde and retrograde neurofilament movement. These data suggest that kinesin-1A is the principal but not exclusive anterograde motor for neurofilaments in these neurons, that there may be some functional redundancy among the kinesin-1 isoforms with respect to neurofilament transport, and that the activities of the anterograde and retrograde neurofilament motors are tightly coordinated.

Neurofilaments form a highly stable stationary cytoskeleton after reaching a critical level in axons

The ultrastructural view of the axonal cytoskeleton as an extensively cross-linked network of neurofilaments (NFs) and other cytoskeletal polymers contrasts with the dynamic view suggested by axonal transport studies on cytoskeletal elements. Here we reconcile these perspectives by showing that neurons form a large NF network along axons which is unequivocally stationary, metabolically stable, and maintained by NFs and nonfilamentous subunit assemblies undergoing slow transport by intermittent rapid movements and pauses. In mouse primary cortical neurons transfected with EGFP-NFL, formation of this stationary NF network requires a critical level of NFs, which explains its absence in NF-poor developing neurons studied previously. Most NFs at proximal axon regions were in a stationary structure coexisting with a smaller pool of moving EGFP-NFL assemblies that were mainly nonfilamentous. Distally along the same axon, EGFP-labeled NFL was much less abundant, and we detected only short filaments moving bidirectionally by slow transport (rapid movements and pauses) as previously described. In living mice, >25% of radiolabeled newly synthesized NFs remained in optic axons after slowly transported NFs had exited. Retained NF remained fixed over several months in a nonuniform distribution and exhibited exceptionally slow turnover (t(1/2) >2.5 months), implying that, at steady state, >90% of NFs in mature optic axons comprise the stationary cytoskeleton and <10% are undergoing slow transport. These findings reconcile in vitro and in vivo axonal transport observations, showing that slowly transported NFs or subunit oligomers are precursors to a highly stable stationary cytoskeletal network that supports mature axons.