Axonal transport of microtubules: the long and short of it

The various forms of hereditary motor neuron disorders and their historical descriptions

Motor neuron disorders comprise a clinically and pathologically heterogeneous group of neurologic diseases characterized by progressive degeneration of motor neurons (including both sporadic and hereditary diseases), affecting the upper motor neurons, lower motor neurons, or both. Hereditary motor neuron disorders themselves represent a vast and heterogeneous group, with numerous clinical and genetic overlaps that can be a source of error. This narrative review aims at providing an overview of the main types of inherited motor neuron disorders by recounting the stages in their historical descriptions. For practical purposes, this review of the literature sets out their various clinical characteristics and updates the list of all the genes involved in the various forms of inherited motor neuron disorders, including spinal muscular atrophy, familial amyotrophic lateral sclerosis, hereditary spastic paraplegia, distal hereditary motor neuropathies/neuronopathies, Kennedy's disease, riboflavin transporter deficiencies, VCPopathy and the neurogenic scapuloperoneal syndrome.

Quantifying Yeast Microtubules and Spindles Using the Toolkit for Automated Microtubule Tracking (TAMiT)

Fluorescently labeled proteins absorb and emit light, appearing as Gaussian spots in fluorescence imaging. When fluorescent tags are added to cytoskeletal polymers such as microtubules, a line of fluorescence and even non-linear structures results. While much progress has been made in techniques for imaging and microscopy, image analysis is less well-developed. Current analysis of fluorescent microtubules uses either manual tools, such as kymographs, or automated software. As a result, our ability to quantify microtubule dynamics and organization from light microscopy remains limited. Despite the development of automated microtubule analysis tools for in vitro studies, analysis of images from cells often depends heavily on manual analysis. One of the main reasons for this disparity is the low signal-to-noise ratio in cells, where background fluorescence is typically higher than in reconstituted systems. Here, we present the Toolkit for Automated Microtubule Tracking (TAMiT), which automatically detects, optimizes, and tracks fluorescent microtubules in living yeast cells with sub-pixel accuracy. Using basic information about microtubule organization, TAMiT detects linear and curved polymers using a geometrical scanning technique. Images are fit via an optimization problem for the microtubule image parameters that are solved using non-linear least squares in Matlab. We benchmark our software using simulated images and show that it reliably detects microtubules, even at low signal-to-noise ratios. Then, we use TAMiT to measure monopolar spindle microtubule bundle number, length, and lifetime in a large dataset that includes several S. pombe mutants that affect microtubule dynamics and bundling. The results from the automated analysis are consistent with previous work and suggest a direct role for CLASP/Cls1 in bundling spindle microtubules. We also illustrate automated tracking of single curved astral microtubules in S. cerevisiae, with measurement of dynamic instability parameters. The results obtained with our fully-automated software are similar to results using hand-tracked measurements. Therefore, TAMiT can facilitate automated analysis of spindle and microtubule dynamics in yeast cells.

The interconnection of endoplasmic reticulum and microtubule and its implication in Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) and microtubule (MT) network form extensive contact with each other and their interconnection plays a pivotal role in ER maintenance and distribution as well as MT stability. The ER participates in a variety of biological processes including protein folding and processing, lipid biosynthesis, and Ca²⁺ storage. MTs specifically regulate cellular architecture, provide routes for transport of molecules or organelles, and mediate signaling events. The ER morphology and dynamics are regulated by a class of ER shaping proteins, which also provide the physical contact structure for linking of ER and MT. In addition to these ER-localized and MT-binding proteins, specific motor proteins and adaptor-linking proteins also mediate bidirectional communication between the two structures. In this review, we summarize the current understanding of the structure and function of ER-MT interconnection. We further highlight the morphologic factors which coordinate the ER-MT network and maintain the normal physiological function of neurons, with their defect causing neurodegenerative diseases such as Hereditary Spastic Paraplegia (HSP). These findings promote our understanding of the pathogenesis of HSP and provide important therapeutic targets for treatment of these diseases.

Quantifying yeast microtubules and spindles using the Toolkit for Automated Microtubule Tracking (TAMiT)

Fluorescently labeled proteins absorb and emit light, appearing as Gaussian spots in fluorescence imaging. When fluorescent tags are added to cytoskeletal polymers such as microtubules, a line of fluorescence and even non-linear structures results. While much progress has been made in techniques for imaging and microscopy, image analysis is less well developed. Current analysis of fluorescent microtubules uses either manual tools, such as kymographs, or automated software. As a result, our ability to quantify microtubule dynamics and organization from light microscopy remains limited. Despite development of automated microtubule analysis tools for in vitro studies, analysis of images from cells often depends heavily on manual analysis. One of the main reasons for this disparity is the low signal-to-noise ratio in cells, where background fluorescence is typically higher than in reconstituted systems. Here, we present the Toolkit for Automated Microtubule Tracking (TAMiT), which automatically detects, optimizes and tracks fluorescent microtubules in living yeast cells with sub-pixel accuracy. Using basic information about microtubule organization, TAMiT detects linear and curved polymers using a geometrical scanning technique. Images are fit via an optimization problem for the microtubule image parameters that is solved using non-linear least squares in Matlab. We benchmark our software using simulated images and show that it reliably detects microtubules, even at low signal-to-noise ratios. Then, we use TAMiT to measure monopolar spindle microtubule bundle number, length, and lifetime in a large dataset that includes several S. pombe mutants that affect microtubule dynamics and bundling. The results from the automated analysis are consistent with previous work, and suggest a direct role for CLASP/Cls1 in bundling spindle microtubules. We also illustrate automated tracking of single curved astral microtubules in S. cerevisiae , with measurement of dynamic instability parameters. The results obtained with our fully-automated software are similar to results using hand-tracked measurements. Therefore, TAMiT can facilitate automated analysis of spindle and microtubule dynamics in yeast cells.

Rab3A interacts with spastin to regulate neurite outgrowth in hippocampal neurons

Investigating novel mechanisms of neurite outgrowth via cytoskeleton is critical for developing therapeutic strategies against neural disorders. Rab3A is a vesicle-related protein distributed throughout the nervous system, but the detailed mechanism related to cytoskeleton remains largely unknown. Our previous reports show that spastin serves microtubule to regulate neurite outgrowth. Here, we asked whether Rab3A could function via modulating spastin during neuronal development. The results revealed that Rab3A colocalized with spastin in cultured hippocampal neurons. Immunoprecipitation assays showed that Rab3A physically interacted with spastin in rat brain lysates. Rab3A overexpression significantly induced spastin degradation; this effect was reversed by leupeptin- or MG-132- administration, suggesting the lysosomal and ubiquitin-mediated degradation system. Immunofluorescence staining further confirmed that Rab3A and spastin immune-colocalized with the lysosome marker lysotracker. In COS7 cells, Rab3A overexpression significantly downregulated spastin expression and abolished the spastin-mediated microtubule severing. Furthermore, overexpression inhibited while genetic knockdown of Rab3A promoted neurite outgrowth. However, this inhibitory effect on neurite outgrowth in hippocampal neurons could be reversed via co-transfection of spastin, indicating that Rab3A functions via its interaction protein spastin. In general, our data identify an interaction between Rab3A and spastin, and this interaction affects the protein stability of spastin and eliminates its microtubule severing function, thereby modulating neurite outgrowth.

Role of tubulin post-translational modifications in peripheral neuropathy

Peripheral neuropathy is a common disorder that results from nerve damage in the periphery. The degeneration of sensory axon terminals leads to changes or loss of sensory functions, often manifesting as debilitating pain, weakness, numbness, tingling, and disability. The pathogenesis of most peripheral neuropathies remains to be fully elucidated. Cumulative evidence from both early and recent studies indicates that tubulin damage may provide a common underlying mechanism of axonal injury in various peripheral neuropathies. In particular, tubulin post-translational modifications have been recently implicated in both toxic and inherited forms of peripheral neuropathy through regulation of axonal transport and mitochondria dynamics. This knowledge forms a new area of investigation with the potential for developing therapeutic strategies to prevent or delay peripheral neuropathy by restoring tubulin homeostasis.

Phosphorylation mutation impairs the promoting effect of spastin on neurite outgrowth without affecting its microtubule severing ability

Spastin, a microtubule-severing enzyme, is known to be important for neurite outgrowth. However, the role of spastin post-translational modification, particularly its phosphorylation regulation in neuronal outgrowth, remains unclear. This study aimed to investigate the effects of eliminating spastin phosphorylation on the neurite outgrowth of rat hippocampal neurons. To accomplish this, we constructed a spastin mutant with eleven potential phosphorylation sites mutated to alanine. The phosphorylation levels of the wildtype spastin (WT) and the mutant (11A) were then detected using Phos-tag SDS-PAGE. The spastin constructs were transfected into COS7 cells for the observation of microtubule severing, and into rat hippocampal neurons for the detection of neuronal outgrowth. The results showed that compared to the spastin WT, the phosphorylation levels were significantly reduced in the spastin 11A mutant. The spastin mutant 11A impaired its ability to promote neurite length, branching, and complexity in hippocampal neurons, but did not affect its ability to sever microtubules in COS7 cells. In conclusion, the data suggest that mutations at multiple phosphorylation sites of spastin do not impair its microtubule cleavage ability in COS7 cells, but reduce its ability to promote neurite outgrowth in rat hippocampal neurons.

Microtubule retrograde flow retains neuronal polarization in a fluctuating state

In developing vertebrate neurons, a neurite is formed by more than a hundred microtubules. While individual microtubules are dynamic, the microtubule array has been regarded as stationary. Using live-cell imaging of neurons in culture or in brain slices, combined with photoconversion techniques and pharmacological manipulations, we uncovered that the microtubule array flows retrogradely within neurites to the soma. This flow drives cycles of microtubule density, a hallmark of the fluctuating state before axon formation, thereby inhibiting neurite growth. The motor protein dynein fuels this process. Shortly after axon formation, microtubule retrograde flow slows down in the axon, reducing microtubule density cycles and enabling axon extension. Thus, keeping neurites short is an active process. Microtubule retrograde flow is a previously unknown type of cytoskeletal dynamics, which changes the hitherto axon-centric view of neuronal polarization.

PTRN-1 (CAMSAP) and NOCA-2 (NINEIN) are required for microtubule polarity in Caenorhabditis elegans dendrites

The neuronal microtubule cytoskeleton is key to establish axon-dendrite polarity. Dendrites are characterized by the presence of minus-end out microtubules. However, the mechanisms that organize these microtubules with the correct orientation are still poorly understood. Using Caenorhabditis elegans as a model system for microtubule organization, we characterized the role of 2 microtubule minus-end related proteins in this process, the microtubule minus-end stabilizing protein calmodulin-regulated spectrin-associated protein (CAMSAP/PTRN-1), and the NINEIN homologue, NOCA-2 (noncentrosomal microtubule array). We found that CAMSAP and NINEIN function in parallel to mediate microtubule organization in dendrites. During dendrite outgrowth, RAB-11-positive vesicles localized to the dendrite tip to nucleate microtubules and function as a microtubule organizing center (MTOC). In the absence of either CAMSAP or NINEIN, we observed a low penetrance MTOC vesicles mislocalization to the cell body, and a nearly fully penetrant phenotype in double mutant animals. This suggests that both proteins are important for localizing the MTOC vesicles to the growing dendrite tip to organize microtubules minus-end out. Whereas NINEIN localizes to the MTOC vesicles where it is important for the recruitment of the microtubule nucleator γ-tubulin, CAMSAP localizes around the MTOC vesicles and is cotranslocated forward with the MTOC vesicles upon dendritic growth. Together, these results indicate that microtubule nucleation from the MTOC vesicles and microtubule stabilization are both important to localize the MTOC vesicles distally to organize dendritic microtubules minus-end out.

The Role of Spastin in Axon Biology

Neurons are highly polarized cells with elaborate shapes that allow them to perform their function. In neurons, microtubule organization—length, density, and dynamics—are essential for the establishment of polarity, growth, and transport. A mounting body of evidence shows that modulation of the microtubule cytoskeleton by microtubule-associated proteins fine tunes key aspects of neuronal cell biology. In this respect, microtubule severing enzymes—spastin, katanin and fidgetin—a group of microtubule-associated proteins that bind to and generate internal breaks in the microtubule lattice, are emerging as key modulators of the microtubule cytoskeleton in different model systems. In this review, we provide an integrative view on the latest research demonstrating the key role of spastin in neurons, specifically in the context of axonal cell biology. We focus on the function of spastin in the regulation of microtubule organization, and axonal transport, that underlie its importance in the intricate control of axon growth, branching and regeneration.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Simulation of a sudden drop-off in distal dense core vesicle concentration in Drosophila type II motoneuron terminals

Recent experimental observations have shown evidence of an unexpected sudden drop-off in the dense core vesicles (DCVs) content at the ends of certain types of axon endings. This article seeks to determine whether these observations may be explained without modifying the parameters characterizing the ability of distal en passant boutons to capture and accumulate DCVs. We developed a mathematical model that is based on the conservation of captured and transiting DCVs in boutons. The model consists of 77 ordinary differential equations and is solved using a standard Matlab solver. We hypothesize that the drop in DCV content in distal boutons is due to an insufficient supply of anterogradely moving DCVs coming from the soma. As anterogradely moving DCVs are captured (and eventually destroyed) in more proximal boutons on their way to the end of the terminal, the fluxes of anterogradely moving DCVs between the boutons become increasingly smaller, and the most distal boutons are left without DCVs. We tested this hypothesis by modifying the flux of DCVs entering the terminal and found that the number of most distal boutons left unfilled increases if the DCV flux entering the terminal is decreased. The number of anterogradely moving DCVs in the axon can be increased either by the release of a portion of captured DCVs into the anterograde component or by an increase of the anterograde DCV flux into the terminal. This increase could lead to having enough anterogradely moving DCVs such that they could reach the most distal bouton and then turn around by changing molecular motors that propel them. The model suggests that this could result in an increased concentration of resident DCVs in distal boutons beginning with bouton 2 (the most distal is bouton 1). This is because in distal boutons, DCVs have a larger chance to be captured from the transiting state as they pass the boutons moving anterogradely and then again as they pass the same boutons moving retrogradely.

ER Morphology in the Pathogenesis of Hereditary Spastic Paraplegia

The endoplasmic reticulum (ER) is the most abundant and widespread organelle in cells. Its peculiar membrane architecture, formed by an intricate network of tubules and cisternae, is critical to its multifaceted function. Regulation of ER morphology is coordinated by a few ER-specific membrane proteins and is thought to be particularly important in neurons, where organized ER membranes are found even in the most distant neurite terminals. Mutation of ER-shaping proteins has been implicated in the neurodegenerative disease hereditary spastic paraplegia (HSP). In this review we discuss the involvement of these proteins in the pathogenesis of HSP, focusing on the experimental evidence linking their molecular function to disease onset. Although the precise biochemical activity of some ER-related HSP proteins has been elucidated, the pathological mechanism underlying ER-linked HSP is still undetermined and needs to be further investigated.

Tracking Fungal Growth: Establishment of Arp1 as a Marker for Polarity Establishment and Active Hyphal Growth in Filamentous Ascomycetes

Polar growth is a key characteristic of all filamentous fungi. It allows these eukaryotes to not only effectively explore organic matter but also interact within its own colony, mating partners, and hosts. Therefore, a detailed understanding of the dynamics in polar growth establishment and maintenance is crucial for several fields of fungal research. We developed a new marker protein, the actin-related protein 1 (Arp1) fused to red and green fluorescent proteins, which allows for the tracking of polar axis establishment and active hyphal growth in microscopy approaches. To exclude a probable redundancy with known polarity markers, we compared the localizations of the Spitzenkörper (SPK) and Arp1 using an FM4-64 staining approach. As we show in applications with the coprophilous fungus Sordaria macrospora and the hemibiotrophic plant pathogen Colletotrichum graminicola, the monitoring of Arp1 can be used for detailed studies of hyphal growth dynamics and ascospore germination, the interpretation of chemotropic growth processes, and the tracking of elongating penetration pegs into plant material. Since the Arp1 marker showed the same dynamics in both fungi tested, we believe this marker can be broadly applied in fungal research to study the manifold polar growth processes determining fungal life.

Chronic neuronal activation increases dynamic microtubules to enhance functional axon regeneration after dorsal root crush injury

After a dorsal root crush injury, centrally-projecting sensory axons fail to regenerate across the dorsal root entry zone (DREZ) to extend into the spinal cord. We find that chemogenetic activation of adult dorsal root ganglion (DRG) neurons improves axon growth on an in vitro model of the inhibitory environment after injury. Moreover, repeated bouts of daily chemogenetic activation of adult DRG neurons for 12 weeks post-crush in vivo enhances axon regeneration across a chondroitinase-digested DREZ into spinal gray matter, where the regenerating axons form functional synapses and mediate behavioral recovery in a sensorimotor task. Neuronal activation-mediated axon extension is dependent upon changes in the status of tubulin post-translational modifications indicative of highly dynamic microtubules (as opposed to stable microtubules) within the distal axon, illuminating a novel mechanism underlying stimulation-mediated axon growth. We have identified an effective combinatory strategy to promote functionally-relevant axon regeneration of adult neurons into the CNS after injury.

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

p60-katanin: a novel interacting partner for p53

Katanin, one of the best-characterized microtubule (MT) severing proteins, is composed of two subunits: catalytic p60-katanin, and regulatory p80-katanin. p60-katanin triggers MT reorganization by severing them. MT reorganization is essential for both mitotic cells and post-mitotic neurons in numerous vital processes such as intracellular transport, mitosis, cellular differentiation and apoptosis. Due to the deleterious effect of continuous severing for cells, p60-katanin requires a strategic regulation. However, there are only a few known regulators of p60-katanin. p53 functions in similar cellular processes as katanin such as cell cycle, differentiation, and apoptosis depending on its interacting partners. Considering this similarity, in this study we investigated p53 as a potential regulatory candidate of p60-katanin, and examined their interaction. Co-immunoprecipitation analyses revealed that p60-katanin interacts with p53. We were able to locate a potential interaction site for the two proteins by deleting different candidate regions We showed for the first time that p53 and p60-katanin interact. This interaction appears to occur via p53's DNA binding domain and p60-katanin's C-terminal. This study will pave the way for future studies regarding the functional outcomes of this interaction which is vital for understanding the regulation of cellular events such as cell cycle, differentiation, and apoptosis in disease and in health.

Role of dynein-dynactin complex, kinesins, motor adaptors, and their phosphorylation in dendritogenesis

One of the characteristic features of different classes of neurons that is vital for their proper functioning within neuronal networks is the shape of their dendritic arbors. To properly develop dendritic trees, neurons need to accurately control the intracellular transport of various cellular cargo (e.g., mRNA, proteins, and organelles). Microtubules and motor proteins (e.g., dynein and kinesins) that move along microtubule tracks play an essential role in cargo sorting and transport to the most distal ends of neurons. Equally important are motor adaptors, which may affect motor activity and specify cargo that is transported by the motor. Such transport undergoes very dynamic fine-tuning in response to changes in the extracellular environment and synaptic transmission. Such regulation is achieved by the phosphorylation of motors, motor adaptors, and cargo, among other mechanisms. This review focuses on the contribution of the dynein-dynactin complex, kinesins, their adaptors, and the phosphorylation of these proteins in the formation of dendritic trees by maturing neurons. We primarily review the effects of the motor activity of these proteins in dendrites on dendritogenesis. We also discuss less anticipated mechanisms that contribute to dendrite growth, such as dynein-driven axonal transport and non-motor functions of kinesins.

Dynactin1 depletion leads to neuromuscular synapse instability and functional abnormalities

BACKGROUND

Dynactin subunit 1 is the largest subunit of the dynactin complex, an activator of the molecular motor protein complex dynein. Reduced levels of DCTN1 mRNA and protein have been found in sporadic amyotrophic lateral sclerosis (ALS) patients, and mutations have been associated with disease, but the role of this protein in disease pathogenesis is still unknown.

METHODS

We characterized a Dynactin1a depletion model in the zebrafish embryo and combined in vivo molecular analysis of primary motor neuron development with live in vivo axonal transport assays in single cells to investigate ALS-related defects. To probe neuromuscular junction (NMJ) function and organization we performed paired motor neuron-muscle electrophysiological recordings and GCaMP calcium imaging in live, intact larvae, and the synapse structure was investigated by electron microscopy.

RESULTS

Here we show that Dynactin1a depletion is sufficient to induce defects in the development of spinal cord motor neurons and in the function of the NMJ. We observe synapse instability, impaired growth of primary motor neurons, and higher failure rates of action potentials at the NMJ. In addition, the embryos display locomotion defects consistent with NMJ dysfunction. Rescue of the observed phenotype by overexpression of wild-type human DCTN1-GFP indicates a cell-autonomous mechanism. Synaptic accumulation of DCTN1-GFP, as well as ultrastructural analysis of NMJ synapses exhibiting wider synaptic clefts, support a local role for Dynactin1a in synaptic function. Furthermore, live in vivo analysis of axonal transport and cytoskeleton dynamics in primary motor neurons show that the phenotype reported here is independent of modulation of these processes.

CONCLUSIONS

Our study reveals a novel role for Dynactin1 in ALS pathogenesis, where it acts cell-autonomously to promote motor neuron synapse stability independently of dynein-mediated axonal transport.

Hereditary spastic paraplegia: gain-of-function mechanisms revealed by new transgenic mouse

Mutations of the SPAST gene, which encodes the microtubule-severing protein spastin, are the most common cause of hereditary spastic paraplegia (HSP). Haploinsufficiency is the prevalent opinion as to the mechanism of the disease, but gain-of-function toxicity of the mutant proteins is another possibility. Here, we report a new transgenic mouse (termed SPASTC448Y mouse) that is not haploinsufficient but expresses human spastin bearing the HSP pathogenic C448Y mutation. Expression of the mutant spastin was documented from fetus to adult, but gait defects reminiscent of HSP (not observed in spastin knockout mice) were adult onset, as is typical of human patients. Results of histological and tracer studies on the mouse are consistent with progressive dying back of corticospinal axons, which is characteristic of the disease. The C448Y-mutated spastin alters microtubule stability in a manner that is opposite to the expectations of haploinsufficiency. Neurons cultured from the mouse display deficits in organelle transport typical of axonal degenerative diseases, and these deficits were worsened by depletion of endogenous mouse spastin. These results on the SPASTC448Y mouse are consistent with a gain-of-function mechanism underlying HSP, with spastin haploinsufficiency exacerbating the toxicity of the mutant spastin proteins. These findings reveal the need for a different therapeutic approach than indicated by haploinsufficiency alone.

Mitotic Motor KIFC1 Is an Organizer of Microtubules in the Axon

KIFC1 (also called HSET or kinesin-14a) is best known as a multifunctional motor protein essential for mitosis. The present studies are the first to explore KIFC1 in terminally postmitotic neurons. Using RNA interference to partially deplete KIFC1 from rat neurons (from animals of either gender) in culture, pharmacologic agents that inhibit KIFC1, and expression of mutant KIFC1 constructs, we demonstrate critical roles for KIFC1 in regulating axonal growth and retraction as well as growth cone morphology. Experimental manipulations of KIFC1 elicit morphological changes in the axon as well as changes in the organization, distribution, and polarity orientation of its microtubules. Together, the results indicate a mechanism by which KIFC1 binds to microtubules in the axon and slides them into alignment in an ATP-dependent fashion and then cross-links them in an ATP-independent fashion to oppose their subsequent sliding by other motors.SIGNIFICANCE STATEMENT Here, we establish that KIFC1, a molecular motor well characterized in mitosis, is robustly expressed in neurons, where it has profound influence on the organization of microtubules in a number of different functional contexts. KIFC1 may help answer long-standing questions in cellular neuroscience such as, mechanistically, how growth cones stall and how axonal microtubules resist forces that would otherwise cause the axon to retract. Knowledge about KIFC1 may help researchers to devise strategies for treating disorders of the nervous system involving axonal retraction given that KIFC1 is expressed in adult neurons as well as developing neurons.

Microtubules are reorganised and fragmented for uterine receptivity

For the development of uterine receptivity, many morphological and molecular changes occur in the apical surface of luminal uterine epithelial cells (UECs) including an increase in vesicular activity. Vesicular movements for exocytosis and endocytosis are dependent on microtubules; however, changes in microtubules in UECs during early pregnancy have received little attention. β-tubulin, one of the main component of microtubules, is distributed throughout the cytoplasm of UECs on day 1 (non-receptive) of pregnancy in the rat. On day 5.5, β-tubulin is concentrated above the nuclei and by day 6 (receptive), β-tubulin is concentrated in a band-like fashion above the nucleus. Western blot analysis of isolated UECs found two bands (50 and 34 kDa) for β-tubulin in UECs during early pregnancy. The intensity of the 34 kDa band was significantly higher on day 6 compared to day 1. The increase in the 34 kDa band may be due to higher proteolytic activity associated with microtubule polymerisation during the receptive state. Transmission electron microscopy showed fragmented microtubules at the time of receptivity in UECs. This is the first study to show that microtubules are reorganised during uterine receptivity. This re-organisation likely facilitates vesicular movement and promotes the reorganisation of the apical plasma membrane for uterine receptivity.

Altered Regulation of KIAA0566, and Katanin Signaling Expression in the Locus Coeruleus With Neurofibrillary Tangle Pathology

The locus coeruleus (LC), which contains the largest group of noradrenergic neurons in the central nervous system innervating the telencephalon, is an early and constantly vulnerable region to neurofibrillary tangle (NFT) pathology in aging and Alzheimer's disease (AD). The present study using whole genome bisulfite sequencing and Infinium Human Methylation 450 BeadChip was designed to learn about DNA methylation profiles in LC with age and NFT pathology. This method identified decreased DNA methylation of Katanin-Interacting Protein gene (KIAA0566) linked to age and presence of NFT pathology. KIAA0566 mRNA expression demonstrated with RT-qPCR significantly decreased in cases with NFT pathology. Importantly, KIAA0566 immunoreactivity was significantly decreased only in LC neurons with NFTs, but not in neurons without tau pathology when compared with neurons of middle-aged individuals. These changes were accompanied by a similar pattern of selective p80-katanin reduced protein expression in neurons with NFTs. In contrast, p60-katanin subunit expression levels in the neuropil were similar in MA cases and cases with NFT pathology. Since katanin is a major microtubule-severing protein and KIAA0566 binds and interacts with katanin, de-regulation of the katanin-signaling pathway may have implications in the regulation of microtubule homeostasis in LC neurons with NFTs, thereby potentially interfering with maintenance of the cytoskeleton and transport.

Polarity Sorting of Microtubules in the Axon

A longstanding question in cellular neuroscience is how microtubules in the axon become organized with their plus ends out, a pattern starkly different from the mixed orientation of microtubules in vertebrate dendrites. Recent attention has focused on a mechanism called polarity sorting, in which microtubules of opposite orientation are spatially separated by molecular motor proteins. Here we discuss this mechanism, and conclude that microtubules are polarity sorted in the axon by cytoplasmic dynein but that additional factors are also needed. In particular, computational modeling and experimental evidence suggest that static crosslinking proteins are required to appropriately restrict microtubule movements so that polarity sorting by cytoplasmic dynein can occur in a manner unimpeded by other motor proteins.

Truncating mutations of SPAST associated with hereditary spastic paraplegia indicate greater accumulation and toxicity of the M1 isoform of spastin

The SPAST gene, which produces two isoforms of the microtubule-severing protein spastin, is the chief gene mutated in hereditary spastic paraplegia. Truncated M1 spastin proteins are toxic and have the potential to accumulate in these patients.

The SPAST gene, which produces two isoforms (M1 and M87) of the microtubule-severing protein spastin, is the chief gene mutated in hereditary spastic paraplegia. Haploinsufficiency is a popular explanation for the disease, in part because most of the >200 pathogenic mutations of the gene are truncating and expected to produce only vanishingly small amounts of shortened proteins. Here we studied two such mutations, N184X and S245X, and our results suggest another possibility. We found that the truncated M1 proteins can accumulate to notably higher levels than their truncated M87 or wild-type counterparts. Reminiscent of our earlier studies on a pathogenic mutation that generates full-length M1 and M87 proteins, truncated M1 was notably more detrimental to neurite outgrowth than truncated M87, and this was true for both N184X and S245X. The greater toxicity and tendency to accumulate suggest that, over time, truncated M1 could damage the corticospinal tracts of human patients. Curiously, the N184X mutation triggers the reinitiation of translation at a third start codon in SPAST, resulting in synthesis of a novel M187 spastin isoform that is able to sever microtubules. Thus microtubule severing may not be as reduced as previously assumed in the case of that mutation.

Local inhibition of microtubule dynamics by dynein is required for neuronal cargo distribution

Abnormal axonal transport is associated with neuronal disease. We identified a role for DHC-1, the C. elegans dynein heavy chain, in maintaining neuronal cargo distribution. Surprisingly, this does not involve dynein's role as a retrograde motor in cargo transport, hinging instead on its ability to inhibit microtubule (MT) dynamics. Neuronal MTs are highly static, yet the mechanisms and functional significance of this property are not well understood. In disease-mimicking dhc-1 alleles, excessive MT growth and collapse occur at the dendrite tip, resulting in the formation of aberrant MT loops. These unstable MTs act as cargo traps, leading to ectopic accumulations of cargo and reduced availability of cargo at normal locations. Our data suggest that an anchored dynein pool interacts with plus-end-out MTs to stabilize MTs and allow efficient retrograde transport. These results identify functional significance for neuronal MT stability and suggest a mechanism for cellular dysfunction in dynein-linked disease.

Sliding of centrosome-unattached microtubules defines key features of neuronal phenotype

Contemporary models for neuronal migration are grounded in the view that virtually all functionally relevant microtubules (MTs) in migrating neurons are attached to the centrosome, which occupies a position between the nucleus and a short leading process. It is assumed that MTs do not undergo independent movements but rather transduce forces that enable movements of the centrosome and nucleus. The present results demonstrate that although this is mostly true, a small fraction of the MTs are centrosome-unattached, and this permits limited sliding of MTs. When this sliding is pharmacologically inhibited, the leading process becomes shorter, migration of the neuron deviates from its normal path, and the MTs within the leading process become buckled. Partial depletion of ninein, a protein that attaches MTs to the centrosome, leads to greater numbers of centrosome-unattached MTs as well as greater sliding of MTs. Concomitantly, the soma becomes less mobile and the leading process acquires an elongated morphology akin to an axon.

Buckling behavior of individual and bundled microtubules

As the major structural constituent of the cytoskeleton, microtubules (MTs) serve a variety of biological functions that range from facilitating organelle transport to maintaining the mechanical integrity of the cell. Neuronal MTs exhibit a distinct configuration, hexagonally packed bundles of MT filaments, interconnected by MT-associated protein (MAP) tau. Building on our previous work on mechanical response of axonal MT bundles under uniaxial tension, this study is focused on exploring the compression scenarios. Intracellular MTs carry a large fraction of the compressive loads sensed by the cell and therefore, like any other column-like structure, are prone to substantial bending and buckling. Various biological activities, e.g., actomyosin contractility and many pathological conditions are driven or followed by bending, looping, and buckling of MT filaments. The coarse-grained model previously developed in our lab has been used to study the mechanical behavior of individual and bundled in vivo MT filaments under uniaxial compression. Both configurations show tip-localized, decaying, and short-wavelength buckling. This behavior highlights the role of the surrounding cytoplasm and MAP tau on MT buckling behavior, which allows MT filaments to bear much larger compressive forces. It is observed that MAP tau interconnections improve this effect by a factor of two. The enhanced ability of MT bundles to damp buckling waves relative to individual MT filaments, may be interpreted as a self-defense mechanism because it helps axonal MTs to endure harsher environments while maintaining their function. The results indicate that MT filaments in a bundle do not buckle simultaneously implying that the applied stress is not equally shared among the MT filaments, that is a consequence of the nonuniform distribution of MAP tau proteins along the bundle length. Furthermore, from a pathological perspective, it is observed that axonal MT bundles are more vulnerable to failure in compression than tension.

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.

Inhibition of kinesin-5 improves regeneration of injured axons by a novel microtubule-based mechanism

Microtubules have been identified as a powerful target for augmenting regeneration of injured adult axons in the central nervous system. Drugs that stabilize microtubules have shown some promise, but there are concerns that abnormally stabilizing microtubules may have only limited benefits for regeneration, while at the same time may be detrimental to the normal work that microtubules perform for the axon. Kinesin-5 (also called kif11 or Eg5), a molecular motor protein best known for its crucial role in mitosis, acts as a brake on microtubule movements by other motor proteins in the axon. Drugs that inhibit kinesin-5, originally developed to treat cancer, result in greater mobility of microtubules in the axon and an overall shift in the forces on the microtubule array. As a result, the axon grows faster, retracts less, and more readily enters environments that are inhibitory to axonal regeneration. Thus, drugs that inhibit kinesin-5 offer a novel microtubule-based means to boost axonal regeneration without the concerns that accompany abnormal stabilization of the microtubule array. Even so, inhibiting kinesin-5 is not without its own caveats, such as potential problems with navigation of the regenerating axon to its target, as well as morphological effects on dendrites that could affect learning and memory if the drugs reach the brain.

Ciliobrevins as tools for studying dynein motor function

Dyneins are a small class of molecular motors that bind to microtubules and walk toward their minus ends. They are essential for the transport and distribution of organelles, signaling complexes and cytoskeletal elements. In addition dyneins generate forces on microtubule arrays that power the beating of cilia and flagella, cell division, migration and growth cone motility. Classical approaches to the study of dynein function in axons involve the depletion of dynein, expression of mutant/truncated forms of the motor, or interference with accessory subunits. By necessity, these approaches require prolonged time periods for the expression or manipulation of cellular dynein levels. With the discovery of the ciliobrevins, a class of cell permeable small molecule inhibitors of dynein, it is now possible to acutely disrupt dynein both globally and locally. In this review, we briefly summarize recent work using ciliobrevins to inhibit dynein and discuss the insights ciliobrevins have provided about dynein function in various cell types with a focus on neurons. We temper this with a discussion of the need for studies that will elucidate the mechanism of action of ciliobrevin and as well as the need for experiments to further analyze the specificity of ciliobreviens for dynein. Although much remains to be learned about ciliobrevins, these small molecules are proving themselves to be valuable novel tools to assess the cellular functions of dynein.

Hereditary spastic paraplegia SPG4: what is known and not known about the disease

Mutations in more than 70 distinct loci and more than 50 mutated gene products have been identified in patients with hereditary spastic paraplegias, a diverse group of neurological disorders characterized predominantly, but not exclusively, by progressive lower limb spasticity and weakness resulting from distal degeneration of corticospinal tract axons. Mutations in the SPAST (previously known as SPG4) gene that encodes the microtubule-severing protein called spastin, are the most common cause of the disease. The aetiology of the disease is poorly understood, but partial loss of microtubule-severing activity resulting from inactivating mutations in one SPAST allele is the most postulated explanation. Microtubule severing is important for regulating various aspects of the microtubule array, including microtubule number, length, and mobility. In addition, higher numbers of dynamic plus-ends of microtubules, resulting from microtubule-severing events, may play a role in endosomal tubulation and fission. Even so, there is growing evidence that decreased severing of microtubules does not fully explain HSP-SPG4. The presence of two translation initiation codons in SPAST allows synthesis of two spastin isoforms: a full-length isoform called M1 and a slightly shorter isoform called M87. M87 is more abundant in both neuronal and non-neuronal tissues. Studies on rodents suggest that M1 is only readily detected in adult spinal cord, which is where nerve degeneration mainly occurs in humans with HSP-SPG4. M1, due to its hydrophobic N-terminal domain not shared by M87, may insert into endoplasmic reticulum membrane, and together with reticulons, atlastin and REEP1, may play a role in the morphogenesis of this organelle. Some mutated spastins may act in dominant-negative fashion to lower microtubule-severing activity, but others have detrimental effects on neurons without further lowering microtubule severing. The observed adverse effects on microtubule dynamics, axonal transport, endoplasmic reticulum, and endosomal trafficking are likely caused not only by diminished severing of microtubules, but also by neurotoxicity of mutant spastin proteins, chiefly M1. Some large deletions in SPAST might also affect the function of adjacent genes, further complicating the aetiology of the disease.

The dynein inhibitor Ciliobrevin D inhibits the bidirectional transport of organelles along sensory axons and impairs NGF-mediated regulation of growth cones and axon branches

The axonal transport of organelles is critical for the development, maintenance, and survival of neurons, and its dysfunction has been implicated in several neurodegenerative diseases. Retrograde axon transport is mediated by the motor protein dynein. In this study, using embryonic chicken dorsal root ganglion neurons, we investigate the effects of Ciliobrevin D, a pharmacological dynein inhibitor, on the transport of axonal organelles, axon extension, nerve growth factor (NGF)-induced branching and growth cone expansion, and axon thinning in response to actin filament depolymerization. Live imaging of mitochondria, lysosomes, and Golgi-derived vesicles in axons revealed that both the retrograde and anterograde transport of these organelles was inhibited by treatment with Ciliobrevin D. Treatment with Ciliobrevin D reversibly inhibits axon extension and transport, with effects detectable within the first 20 min of treatment. NGF induces growth cone expansion, axonal filopodia formation and branching. Ciliobrevin D prevented NGF-induced formation of axonal filopodia and branching but not growth cone expansion. Finally, we report that the retrograde reorganization of the axonal cytoplasm which occurs on actin filament depolymerization is inhibited by treatment with Ciliobrevin D, indicating a role for microtubule based transport in this process, as well as Ciliobrevin D accelerating Wallerian degeneration. This study identifies Ciliobrevin D as an inhibitor of the bidirectional transport of multiple axonal organelles, indicating this drug may be a valuable tool for both the study of dynein function and a first pass analysis of the role of axonal transport.

Pharmacologically inhibiting kinesin-5 activity with monastrol promotes axonal regeneration following spinal cord injury

While it is well established that the axons of adult neurons have a lower capacity for regrowth, some regeneration of certain CNS populations after spinal cord injury (SCI) is possible if their axons are provided with a permissive substrate, such as an injured peripheral nerve. While some axons readily regenerate into a peripheral nerve graft (PNG), these axons almost always stall at the distal interface and fail to reinnervate spinal cord tissue. Treatment of the glial scar at the distal graft interface with chondroitinase ABC (ChABC) can improve regeneration, but most regenerated axons need further stimulation to extend beyond the interface. Previous studies demonstrate that pharmacologically inhibiting kinesin-5, a motor protein best known for its essential role in mitosis but also expressed in neurons, with the pharmacological agent monastrol increases axon growth on inhibitory substrates in vitro. We sought to determine if monastrol treatment after an SCI improves functional axon regeneration. Animals received complete thoracic level 7 (T7) transections and PNGs and were treated intrathecally with ChABC and either monastrol or DMSO vehicle. We found that combining ChABC with monastrol significantly enhanced axon regeneration. However, there were no further improvements in function or enhanced c-Fos induction upon stimulation of spinal cord rostral to the transection. This indicates that monastrol improves ChABC-mediated axon regeneration but that further treatments are needed to enhance the integration of these regrown axons.

The high molecular weight neurofilament subunit plays an essential role in axonal outgrowth and stabilization

Neurofilaments (NFs) are thought to provide structural support to mature axons via crosslinking of cytoskeletal elements mediated by the C-terminal region of the high molecular weight NF subunit (NF-H). Herein, we inhibited NF-H expression in differentiating mouse NB2a/d1 cells with shRNA directed against murine NF-H without affecting other NF subunits, microtubules or actin. shRNA-mediated NF-H knockdown not only in compromised of late-stage axonal neurite stabilization but also compromised early stages of axonal neurite elongation. Expression of exogenous rat NF-H was able to compensate for knockdown of endogenous NF-H and restored the development and stabilization of axonal neurites. This rescue was prevented by simultaneous treatment with shRNA that inhibited both rat and murine NF-H, or by expression of exogenous rat NF-H lacking the C-terminal sidearm during knockdown of endogenous NF-H. Demonstration of a role for NF-H in the early stages of axonal elaboration suggests that axonal stabilization is not delayed until synaptogenesis, but rather that the developing axon undergoes sequential NF-H-mediated stabilization along its length in a proximal–distal manner, which supports continued pathfinding in distal, unstabilized regions.

Motor protein traffic regulation by supply-demand balance of resources

In cells and in in vitro assays the number of motor proteins involved in biological transport processes is far from being unlimited. The cytoskeletal binding sites are in contact with the same finite reservoir of motors (either the cytosol or the flow chamber) and hence compete for recruiting the available motors, potentially depleting the reservoir and affecting cytoskeletal transport. In this work we provide a theoretical framework in which to study, analytically and numerically, how motor density profiles and crowding along cytoskeletal filaments depend on the competition of motors for their binding sites. We propose two models in which finite processive motor proteins actively advance along cytoskeletal filaments and are continuously exchanged with the motor pool. We first look at homogeneous reservoirs and then examine the effects of free motor diffusion in the surrounding medium. We consider as a reference situation recent in vitro experimental setups of kinesin-8 motors binding and moving along microtubule filaments in a flow chamber. We investigate how the crowding of linear motor proteins moving on a filament can be regulated by the balance between supply (concentration of motor proteins in the flow chamber) and demand (total number of polymerized tubulin heterodimers). We present analytical results for the density profiles of bound motors and the reservoir depletion, and propose novel phase diagrams that present the formation of jams of motor proteins on the filament as a function of two tuneable experimental parameters: the motor protein concentration and the concentration of tubulins polymerized into cytoskeletal filaments. Extensive numerical simulations corroborate the analytical results for parameters in the experimental range and also address the effects of diffusion of motor proteins in the reservoir. We then propose experiments for validating our models and discuss how the 'supply-demand' effects can regulate motor traffic also in in vivo conditions.

Transport and diffusion of Tau protein in neurons

In highly polarized and elongated cells such as neurons, Tau protein must enter and move down the axon to fulfill its biological task of stabilizing axonal microtubules. Therefore, cellular systems for distributing Tau molecules are needed. This review discusses different mechanisms that have been proposed to contribute to the dispersion of Tau molecules in neurons. They include (1) directed transport along microtubules as cargo of tubulin complexes and/or motor proteins, (2) diffusion, either through the cytosolic space or along microtubules, and (3) mRNA-based mechanisms such as transport of Tau mRNA into axons and local translation. Diffusion along the microtubule lattice or through the cytosol appear to be the major mechanisms for axonal distribution of Tau protein in the short-to-intermediate range over distances of up to a millimetre. The high diffusion coefficients ensure that Tau can distribute evenly throughout the axonal volume as well as along microtubules. Motor protein-dependent transport of Tau dominates over longer distances and time scales. At low near-physiological levels, Tau is co-transported along with short microtubules from cell bodies into axons by cytoplasmic dynein and kinesin family members at rates of slow axonal transport.

Pathogenic mutation of spastin has gain-of-function effects on microtubule dynamics

Mutations to the SPG4 gene encoding the microtubule-severing protein spastin are the most common cause of hereditary spastic paraplegia. Haploinsufficiency, the prevalent model for the disease, cannot readily explain many of its key aspects, such as its adult onset or its specificity for the corticospinal tracts. Treatment strategies based solely on haploinsufficiency are therefore likely to fail. Toward developing effective therapies, here we investigated potential gain-of-function effects of mutant spastins. The full-length human spastin isoform called M1 or a slightly shorter isoform called M87, both carrying the same pathogenic mutation C448Y, were expressed in three model systems: primary rat cortical neurons, fibroblasts, and transgenic Drosophila. Although both isoforms had ill effects on motor function in transgenic flies and decreased neurite outgrowth from primary cortical neurons, mutant M1 was notably more toxic than mutant M87. The observed phenotypes did not result from dominant-negative effects of mutated spastins. Studies in cultured cells revealed that microtubules can be heavily decorated by mutant M1 but not mutant M87. Microtubule-bound mutant M1 decreased microtubule dynamics, whereas unbound M1 or M87 mutant spastins increased microtubule dynamics. The alterations in microtubule dynamics observed in the presence of mutated spastins are not consistent with haploinsufficiency and are better explained by a gain-of-function mechanism. Our results fortify a model wherein toxicity of mutant spastin proteins, especially mutant M1, contributes to axonal degeneration in the corticospinal tracts. Furthermore, our results provide details on the mechanism of the toxicity that may chart a course toward more effective treatment regimens.

Cell biology in neuroscience: cellular and molecular mechanisms underlying presynapse formation

Synapse formation is a highly regulated process that requires the coordination of many cell biological events. Decades of research have identified a long list of molecular components involved in assembling a functioning synapse. Yet how the various steps, from transporting synaptic components to adhering synaptic partners and assembling the synaptic structure, are regulated and precisely executed during development and maintenance is still unclear. With the improvement of imaging and molecular tools, recent work in vertebrate and invertebrate systems has provided important insight into various aspects of presynaptic development, maintenance, and trans-synaptic signals, thereby increasing our understanding of how extrinsic organizers and intracellular mechanisms contribute to presynapse formation.

Modeling anterograde and retrograde transport of short mobile microtubules from the site of axonal branch formation

This theoretical research is motivated by a recent model of microtubule (MT) transport put forward by Baas and Mozgova (Cytoskeleton 69:416-425, 2012). According to their model, in an axon all plus-end-distal mobile MTs move anterogradely while all minus-end-distal mobile MTs move retrogradely. Retrograde MT transport thus represents a mechanism by which minus-end-distal MTs are removed from the axon. We suggested equations that implement Baas and Mozgova's model. We employed these equations to simulate transport of short mobile MTs from a region (such as the site of axonal branch formation) where MT severing activity results in generation of a large number of short MTs of both orientations. We obtained the exact and approximate transient solutions of these equations utilizing the Laplace transform technique. We applied the obtained solutions to calculate the average rates of anterograde and retrograde transport of short MTs.

Saccharin enhances neurite extension by regulating organization of the microtubules

AIMS

In the present study, we found that saccharin, an artificial calorie-free sweetener, promotes neurite extension in the cultured neuronal cells. The purposes of this study are to characterize the effect of saccharine on neurite extension and to determine how saccharin enhances neurite extension.

MAIN METHODS

The analyses were performed using mouse neuroblastoma N1E-115 cells and rat pheochromocytoma PC12 cells. Neurite extension was evaluated by counting the cells bearing neurites and measuring the length of neurites. Formation, severing and transportation of the microtubules were evaluated by immunostaining and western blotting analysis.

KEY FINDINGS

Deprivation of glucose increased the number of N1E-115 cells bearing long processes. And the effect was inhibited by addition of glucose. Saccharin increased the number of these cells bearing long processes in a dose-dependent manner and total neurite length and longest neurite length in each cell. Saccharin also had a similar effect on NGF-treated PC12 cells. Saccharin increased the amount of the microtubules reconstructed after treatment with nocodazole, a disruptor of microtubules. The effect of saccharin on microtubule reconstruction was not influenced by dihydrocytochalasin B, an inhibitor of actin polymerization, indicating that saccharin enhances microtubule formation without requiring actin dynamics. In the cells treated with vinblastine, an inhibitor of microtubule polymerization, after microtubule reorganization, filamentous microtubules were observed more distantly from the centrosome in saccharin-treated cells, indicating that saccharin enhances microtubule severing and/or transportation.

SIGNIFICANCE

These results suggest that saccharin enhances neurite extension by promoting microtubule organization.

The intricate relationship between microtubules and their associated motor proteins during axon growth and maintenance

The hallmarks of neurons are their slender axons which represent the longest cellular processes of animals and which act as the cables that electrically wire the brain, and the brain to the body. Axons extend along reproducible paths during development and regeneration, and they have to be maintained for the lifetime of an organism. Both axon extension and maintenance essentially depend on the microtubule (MT) cytoskeleton. For this, MTs organize into parallel bundles that are established through extension at the leading axon tips within growth cones, and these bundles then form the architectural backbones, as well as the highways for axonal transport essential for supply and intracellular communication. Axon transport over these enormous distances takes days or even weeks and is a substantial logistical challenge. It is performed by kinesins and dynein/dynactin, which are molecular motors that form close functional links to the MTs they walk along. The intricate machinery which regulates MT dynamics, axonal transport and the motors is essential for nervous system development and function, and its investigation has huge potential to bring urgently required progress in understanding the causes of many developmental and degenerative brain disorders. During the last years new explanations for the highly specific properties of axonal MTs and for their close functional links to motor proteins have emerged, and it has become increasingly clear that motors play active roles also in regulating axonal MT networks. Here, I will provide an overview of these new developments.

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.

Beyond taxol: microtubule-based treatment of disease and injury of the nervous system

Contemporary research has revealed a great deal of information on the behaviours of microtubules that underlie critical events in the lives of neurons. Microtubules in the neuron undergo dynamic assembly and disassembly, bundling and splaying, severing, and rapid transport as well as integration with other cytoskeletal elements such as actin filaments. These various behaviours are regulated by signalling pathways that affect microtubule-related proteins such as molecular motor proteins and microtubule severing enzymes, as well as a variety of proteins that promote the assembly, stabilization and bundling of microtubules. In recent years, translational neuroscientists have earmarked microtubules as a promising target for therapy of injury and disease of the nervous system. Proof-of-principle has come mainly from studies using taxol and related drugs to pharmacologically stabilize microtubules in animal models of nerve injury and disease. However, concerns persist that the negative consequences of abnormal microtubule stabilization may outweigh the positive effects. Other potential approaches include microtubule-active drugs with somewhat different properties, but also expanding the therapeutic toolkit to include intervention at the level of microtubule regulatory proteins.