Cytoplasmic Dynein Transports Axonal Microtubules in a Polarity-Sorting Manner

'Mitotic' kinesin-5 is a dynamic brake for axonal growth in Drosophila

During neuronal development, microtubule reorganization shapes axons and dendrites, establishing the framework for efficient nervous system wiring. Our previous work has demonstrated the role of kinesin-1 in driving microtubule sliding, which powers early axon outgrowth and regeneration in Drosophila melanogaster. Here, we reveal a crucial new role for kinesin-5, a mitotic motor, in modulating postmitotic neuron development. The Drosophila kinesin-5, Klp61F, is expressed in larval brain neurons, with high levels in ventral nerve cord (VNC) neurons. Knockdown of Klp61F in neurons leads to severe adult locomotion defects and lethality, primarily due to defects in VNC motor neurons. Klp61F depletion results in excessive microtubule penetration into the axon growth cone, causing significant axon growth defects in culture and in vivo. These defects are rescued by a chimeric human-Drosophila kinesin-5 motor, indicating a conserved role for kinesin-5 in neuronal development. Altogether, we propose that kinesin-5 acts as a brake on kinesin-1-driven microtubule sliding, ensuring proper axon pathfinding in growing neurons.

Asymmetric microtubule nucleation from Golgi stacks promotes opposite microtubule polarity in axons and dendrites

The neuronal microtubule cytoskeleton is highly polarized, with most microtubules growing away from the soma in axons (plus-end-out), but many microtubules growing toward the soma in dendrites (minus-end-out). This differential microtubule polarity allows directional trafficking of specific organelles, vesicles, and molecules into either axons or dendrites, but how it is established and maintained remains unclear. We showed previously that microtubules are nucleated asymmetrically from Golgi stacks within the soma of Drosophila neurons, with their plus ends growing preferentially toward and into axons and away from dendrites. Here, we show that this microtubule nucleation asymmetry correlates with a cis-to-trans orientation of specific Golgi stacks toward the axon and depends on microtubule-nucleating γ-tubulin ring complexes (γ-TuRCs) at the cis-Golgi and the plus-end-stabilizing protein CLASP at the trans-Golgi. Depleting CLASP or reducing γ-TuRC localization to the Golgi by depleting the Golgin protein GMAP (Golgi microtubule-associated protein) perturbs asymmetric microtubule nucleation and growth within the soma and results in polarity changes in proximal axons and dendrites. We propose that the plus ends of microtubules nucleated by γ-TuRCs at the cis-Golgi are stabilized by CLASP at the trans-Golgi to promote the growth of microtubules along the cis-to-trans Golgi axis. This, coupled with oriented Golgi stacks, promotes microtubule growth toward and into axons and away from dendrites, helping promote plus-end-out microtubule polarity in axons and maintain minus-end-out microtubule polarity in dendrites.

Tau and MAP6 establish labile and stable domains on microtubules

We previously documented that individual microtubules in the axons of cultured juvenile rodent neurons consist of a labile domain and a stable domain and that experimental depletion of tau results in selective shortening and partial stabilization of the labile domain. After first confirming these findings in adult axons, we sought to understand the mechanism that accounts for the formation and maintenance of these microtubule domains. We found that fluorescent tau and MAP6 ectopically expressed in RFL-6 fibroblasts predominantly segregate on different microtubules or different domains on the same microtubule, with the tau-rich ones becoming more labile than in control cells and the MAP6-rich ones being more stable than in control cells. These and other experimental findings, which we studied further using computational modeling with tunable parameters, indicate that these two MAPs do not merely bind to pre-existing stable and labile domains but actually create stable and labile domains on microtubules.

"Mitotic" kinesin-5 is a dynamic brake for axonal growth

During neuronal development, neurons undergo significant microtubule reorganization to shape axons and dendrites, establishing the framework for efficient wiring of the nervous system. Previous studies from our laboratory demonstrated the key role of kinesin-1 in driving microtubule-microtubule sliding, which provides the mechanical forces necessary for early axon outgrowth and regeneration in Drosophila melanogaster. In this study, we reveal the critical role of kinesin-5, a mitotic motor, in modulating the development of postmitotic neurons. Kinesin-5, a conserved homotetrameric motor, typically functions in mitosis by sliding antiparallel microtubules apart in the spindle. Here, we demonstrate that the Drosophila kinesin-5 homolog, Klp61F, is expressed in larval brain neurons, with high levels in ventral nerve cord (VNC) neurons. Knockdown of Klp61F using a pan-neuronal driver leads to severe locomotion defects and complete lethality in adult flies, mainly due to the absence of kinesin-5 in VNC motor neurons during early larval development. Klp61F depletion results in significant axon growth defects, both in cultured and in vivo neurons. By imaging individual microtubules, we observe a significant increase in microtubule motility, and excessive penetration of microtubules into the axon growth cone in Klp61F-depleted neurons. Adult lethality and axon growth defects are fully rescued by a chimeric human-Drosophila kinesin-5 motor, which accumulates at the axon tips, suggesting a conserved role of kinesin-5 in neuronal development. Altogether, our findings show that at the growth cone, kinesin-5 acts as a brake on kinesin-1-driven microtubule sliding, preventing premature microtubule entry into the growth cone. This regulatory role of kinesin-5 is essential for precise axon pathfinding during nervous system development.

Trim46 knockout impaired neuronal architecture and caused hypoactive behavior in rats

BACKGROUND

Tripartite motif (TRIM46) is a relatively novel protein that belongs to tripartite motif family. TRIM46 organizes parallel microtubule arrays on the axons, which are important for neuronal polarity and axonal function. TRIM46 is highly expressed in the brain, but its biological function in adults has not yet been determined.

RESULTS

Trim46 knockout (KO) rat line was established using CRISPR/cas9. Trim46 KO rats had smaller hippocampus sizes, fewer neuronal dendritic arbors and dendritic spines, and shorter and more distant axon initial segment. Furthermore, the protein interaction between endogenous TRIM46 and FK506 binding protein 5 (FKBP5) in brain tissues was determined; Trim46 KO increased hippocampal FKBP5 protein levels and decreased hippocampal protein kinase B (Akt) phosphorylation, gamma-aminobutyric acid type A receptor subunit alpha1 (GABRA1) and glutamate ionotropic receptor NMDA type subunit 1 (NMDAR1) protein levels. Trim46 KO rats exhibited hypoactive behavioral changes such as reduced spontaneous activity, social interaction, sucrose preference, impaired prepulse inhibition (PPI), and short-term reference memory.

CONCLUSIONS

These results demonstrate the significant impact of Trim46 KO on brain structure and behavioral function. This study revealed a novel potential association of TRIM46 with dendritic development and neuropsychiatric behavior, providing new insights into the role of TRIM46 in the brain.

Tensile Stress on Microtubules Facilitates Dynein-Driven Cargo Transport

Mechanical stress significantly affects the physiological functions of cells, including tissue homeostasis, cytoskeletal alterations, and intracellular transport. As a major cytoskeletal component, microtubules respond to mechanical stimulation by altering their alignment and polymerization dynamics. Previously, we reported that microtubules may modulate cargo transport by one of the microtubule-associated motor proteins, dynein, under compressive mechanical stress. Despite the critical role of tensile stress in many biological functions, how tensile stress on microtubules regulates cargo transport is yet to be unveiled. The present study demonstrates that the low-level tensile stress-induced microtubule deformation facilitates dynein-driven transport. We validate our experimental findings using all-atom molecular dynamics simulation. Our study may provide important implications for developing new therapies for diseases that involve impaired intracellular transport.

AMPK-SP1-Guided Dynein Expression Represents a New Energy-Responsive Mechanism and Therapeutic Target for Diabetic Nephropathy

Key Points

AMP kinase senses diabetic stresses in podocytes, subsequently upregulates specificity protein 1–mediated dynein expression and promotes podocyte injury. Pharmaceutical restoration of dynein expression by targeting specificity protein 1 represents an innovative therapeutic strategy for diabetic nephropathy.

Background

Diabetic nephropathy (DN) is a major complication of diabetes. Injury to podocytes, epithelial cells that form the molecular sieve of a kidney, is a preclinical feature of DN. Protein trafficking mediated by dynein, a motor protein complex, is a newly recognized pathophysiology of diabetic podocytopathy and is believed to be derived from the hyperglycemia-induced expression of subunits crucial for the transportation activity of the dynein complex. However, the mechanism underlying this transcriptional signature remains unknown.

Methods

Through promoter analysis, we identified binding sites for transcription factor specificity protein 1 (SP1) as the most shared motif among hyperglycemia-responsive dynein genes. We demonstrated the essential role of AMP-activated protein kinase (AMPK)–regulated SP1 in the transcription of dynein subunits and dynein-mediated trafficking in diabetic podocytopathy using chromatin immunoprecipitation quantitative PCR and live cell imaging. SP1-dependent dynein-driven pathogenesis of diabetic podocytopathy was demonstrated by pharmaceutical intervention with SP1 in a mouse model of streptozotocin-induced diabetes.

Results

Hyperglycemic conditions enhance SP1 binding to dynein promoters, promoted dynein expression, and enhanced dynein-mediated mistrafficking in cultured podocytes. These changes can be rescued by chemical inhibition or genetic silencing of SP1. The direct repression of AMPK, an energy sensor, replicates hyperglycemia-induced dynein expression by activating SP1. Mithramycin inhibition of SP1-directed dynein expression in streptozotocin-induced diabetic mice protected them from developing podocytopathy and prevented DN progression.

Conclusions

Our work implicates AMPK-SP1–regulated dynein expression as an early mechanism that translates energy disturbances in diabetes into podocyte dysfunction. Pharmaceutical restoration of dynein expression by targeting SP1 offers a new therapeutic strategy to prevent DN.

A kinesin-1 adaptor complex controls bimodal slow axonal transport of spectrin in Caenorhabditis elegans

An actin-spectrin lattice, the membrane periodic skeleton (MPS), protects axons from breakage. MPS integrity relies on spectrin delivery via slow axonal transport, a process that remains poorly understood. We designed a probe to visualize endogenous spectrin dynamics at single-axon resolution in vivo. Surprisingly, spectrin transport is bimodal, comprising fast runs and movements that are 100-fold slower than previously reported. Modeling and genetic analysis suggest that the two rates are independent, yet both require kinesin-1 and the coiled-coil proteins UNC-76/FEZ1 and UNC-69/SCOC, which we identify as spectrin-kinesin adaptors. Knockdown of either protein led to disrupted spectrin motility and reduced distal MPS, and UNC-76 overexpression instructed excessive transport of spectrin. Artificially linking spectrin to kinesin-1 drove robust motility but inefficient MPS assembly, whereas impairing MPS assembly led to excessive spectrin transport, suggesting a balance between transport and assembly. These results provide insight into slow axonal transport and MPS integrity.

The role of mechanics in axonal stability and development

Much of the focus of neuronal cell biology has been devoted to growth cone guidance, synaptogenesis, synaptic activity, plasticity, etc. The axonal shaft too has received much attention, mainly for its astounding ability to transmit action potentials and the transport of material over long distances. For these functions, the axonal cytoskeleton and membrane have been often assumed to play static structural roles. Recent experiments have changed this view by revealing an ultrastructure much richer in features than previously perceived and one that seems to be maintained at a dynamic steady state. The role of mechanics in this is only beginning to be broadly appreciated and appears to involve passive and active modes of coupling different biopolymer filaments, filament turnover dynamics and membrane biophysics. Axons, being unique cellular processes in terms of high aspect ratios and often extreme lengths, also exhibit unique passive mechanical properties that might have evolved to stabilize them under mechanical stress. In this review, we summarize the experiments that have exposed some of these features. It is our view that axonal mechanics deserves much more attention not only due to its significance in the development and maintenance of the nervous system but also due to the susceptibility of axons to injury and neurodegeneration.

Cellular cartography: Towards an atlas of the neuronal microtubule cytoskeleton

Microtubules, one of the major components of the cytoskeleton, play a crucial role during many aspects of neuronal development and function, such as neuronal polarization and axon outgrowth. Consequently, the microtubule cytoskeleton has been implicated in many neurodevelopmental and neurodegenerative disorders. The polar nature of microtubules is quintessential for their function, allowing them to serve as tracks for long-distance, directed intracellular transport by kinesin and dynein motors. Most of these motors move exclusively towards either the plus- or minus-end of a microtubule and some have been shown to have a preference for either dynamic or stable microtubules, those bearing a particular post-translational modification or those decorated by a specific microtubule-associated protein. Thus, it becomes important to consider the interplay of these features and their combinatorial effects on transport, as well as how different types of microtubules are organized in the cell. Here, we discuss microtubule subsets in terms of tubulin isotypes, tubulin post-translational modifications, microtubule-associated proteins, microtubule stability or dynamicity, and microtubule orientation. We highlight techniques used to study these features of the microtubule cytoskeleton and, using the information from these studies, try to define the composition, role, and organization of some of these subsets in neurons.

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

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.

Microtubule polarity flaws as a treatable driver of neurodegeneration

Microtubule disruption is a common downstream mechanism leading to axonal degeneration in a number of neurological diseases. To date, most studies on this topic have focused on the loss of microtubule mass from the axon, as well as changes in the stability properties of the microtubules and/or their tubulin composition. Here we posit corruption of the normal pattern of microtubule polarity orientation as an underappreciated and yet treatable contributor to axonal degeneration. We include computational modeling to fortify the rigor of our considerations. Our simulations demonstrate that even a small deviation from the usual polarity pattern of axonal microtubules is detrimental to motor-based trafficking of organelles and other intracellular cargo. Additional modeling predicts that axons with such deviations will exhibit significantly reduced speed and reliability of organelle transport, and that localized clusters of wrongly oriented microtubules will result in traffic jams of accumulated organelles.

Impact of Copper Nanoparticles and Copper Ions on Transcripts Involved in Neural Repair Mechanisms in Rainbow Trout Olfactory Mucosa

Olfactory mucosa is well known for its lifelong ability for regeneration. Regeneration of neurons and regrowth of severed axons are the most common neural repair mechanisms in olfactory mucosa. Nonetheless, exposure to neurotoxic contaminants, such as copper nanoparticles (CuNPs) and copper ions (Cu²⁺), may alter the reparative capacity of olfactory mucosa. Here, using RNA-sequencing, we investigated the molecular basis of neural repair mechanisms that were affected by CuNPs and Cu²⁺ in rainbow trout olfactory mucosa. The transcript profile of olfactory mucosa suggested that regeneration of neurons was inhibited by CuNPs. Exposure to CuNPs reduced the transcript abundances of pro-inflammatory proteins which are required to initiate neuroregeneration. Moreover, the transcript of genes encoding regeneration promoters, including canonical Wnt/β-catenin signaling proteins and developmental transcription factors, were downregulated in the CuNP-treated fish. The mRNA levels of genes regulating axonal regrowth, including the growth-promoting signals secreted from olfactory ensheathing cells, were mainly increased in the CuNP treatment. However, the reduced transcript abundances of a few cell adhesion molecules and neural polarity genes may restrict axonogenesis in the CuNP-exposed olfactory mucosa. In the Cu²⁺-treated olfactory mucosa, both neural repair strategies were initiated at the transcript level. The stimulation of repair mechanisms can lead to the recovery of Cu²⁺-induced olfactory dysfunction. These results indicated CuNPs and Cu²⁺ differentially affected the neural repair mechanism in olfactory mucosa. Exposure to CuNP had greater effects on the expression of genes involved in olfactory repair mechanisms relative to Cu²⁺ and dysregulated the transcripts associated with stem cell proliferation and neural reconstitution.

Unrestrained growth of correctly oriented microtubules instructs axonal microtubule orientation

In many eukaryotic cells, directed molecular transport occurs along microtubules. Within neuronal axons, transport over vast distances particularly relies on uniformly oriented microtubules, whose plus-ends point towards the distal axon tip (anterogradely polymerizing, or plus-end-out). However, axonal microtubules initially have mixed orientations, and how they orient during development is not yet fully understood. Using live imaging of primary Drosophila melanogaster neurons, we found that, in the distal part of the axon, catastrophe rates of plus-end-out microtubules were significantly reduced compared to those of minus-end-out microtubules. Physical modelling revealed that plus-end-out microtubules should therefore exhibit persistent long-term growth, while growth of minus-end-out microtubules should be limited, leading to a bias in overall axonal microtubule orientation. Using chemical and physical perturbations of microtubule growth and genetic perturbations of the anti -catastrophe factor p150, which was enriched in the distal axon tip, we confirmed that the enhanced growth of plus-end-out microtubules is critical for achieving uniform microtubule orientation. Computer simulations of axon development integrating the enhanced plus-end-out microtubule growth identified here with previously suggested mechanisms, that is, dynein-based microtubule sliding and augmin-mediated templating, correctly predicted the long-term evolution of axonal microtubule orientation as found in our experiments. Our study thus leads to a holistic explanation of how axonal microtubules orient uniformly, a prerequisite for efficient long-range transport essential for neuronal functioning.

Multilayered regulations of alternative splicing, NMD, and protein stability control temporal induction and tissue-specific expression of TRIM46 during axon formation

The gene regulation underlying axon formation and its exclusiveness to neurons remains elusive. TRIM46 is postulated to determine axonal fate. We show Trim46 mRNA is expressed before axonogenesis, but TRIM46 protein level is inhibited by alternative splicing of two cassette exons coupled separately to stability controls of Trim46 mRNA and proteins, effectively inducing functional knockout of TRIM46 proteins. Exon 8 inclusion causes nonsense-mediated mRNA decay of Trim46 transcripts. PTBP2-mediated exon 10 skipping produces transcripts encoding unstable TRIM46 proteins. During axonogenesis, transcriptional activation, decreased exon 8 inclusion, and enhanced exon 10 inclusion converge to increase TRIM46 proteins, leading to its neural-specific expression. Genetic deletion of these exons alters TRIM46 protein levels and shows TRIM46 is instructive though not always required for AnkG localization nor a determinant of AnkG density. Therefore, two concurrently but independently regulated alternative exons orchestrate the temporal induction and tissue-specific expression of TRIM46 proteins to mediate axon formation.

Centrosome-dependent microtubule modifications set the conditions for axon formation

Microtubule (MT) modifications are critical during axon development, with stable MTs populating the axon. How these modifications are spatially coordinated is unclear. Here, via high-resolution microscopy, we show that early developing neurons have fewer somatic acetylated MTs restricted near the centrosome. At later stages, however, acetylated MTs spread out in soma and concentrate in growing axon. Live imaging in early plated neurons of the MT plus-end protein, EB3, show increased displacement and growth rate near the MTOC, suggesting local differences that might support axon selection. Moreover, F-actin disruption in early developing neurons, which show fewer somatic acetylated MTs, does not induce multiple axons, unlike later stages. Overexpression of centrosomal protein 120 (Cep120), which promotes MT acetylation/stabilization, induces multiple axons, while its knockdown downregulates proteins modulating MT dynamics and stability, hampering axon formation. Collectively, we show how centrosome-dependent MT modifications contribute to axon formation.

LIS1 and NDEL1 Regulate Axonal Trafficking of Mitochondria in Mature Neurons

Defective mitochondrial dynamics in axons have been linked to both developmental and late-onset neurological disorders. Axonal trafficking is in large part governed by the microtubule motors kinesin-1 and cytoplasmic dynein 1 (dynein). Dynein is the primary retrograde transport motor in axons, and mutations in dynein and many of its regulators also cause neurological diseases. Depletion of LIS1, famous for linking dynein deregulation to lissencephaly (smooth brain), in adult mice leads to severe neurological phenotypes, demonstrating post-developmental roles. LIS1 stimulates retrograde transport of acidic organelles in cultured adult rat dorsal root ganglion (DRG) axons but findings on its role in mitochondrial trafficking have been inconsistent and have not been reported for adult axons. Here we report that there is an increased number of mitochondria in cross-sections of sciatic nerve axons from adult LIS1^+/– mice. This is probably related to reduced dynein activity as axons from adult rat nerves exposed to the dynein inhibitor, ciliobrevin D also had increased numbers of mitochondria. Moreover, LIS1 overexpression (OE) in cultured adult rat DRG axons stimulated retrograde mitochondrial transport while LIS1 knockdown (KD) or expression of a LIS1 dynein-binding mutant (LIS1-K147A) inhibited retrograde transport, as did KD of dynein heavy chain (DHC). These findings are consistent with our report on acidic organelles. However, KD of NDEL1, a LIS1 and dynein binding protein, or expression of a LIS1 NDEL1-binding mutant (LIS1-R212A) also dramatically impacted retrograde mitochondrial transport, which was not the case for acidic organelles. Manipulations that disrupted retrograde mitochondrial transport also increased the average length of axonal mitochondria, suggesting a role for dynein in fusion or fission events. Our data point to cargo specificity in NDEL1 function and raise the possibility that defects in the LIS1/NDEL1 dynein regulatory pathway could contribute to mitochondrial diseases with axonal pathologies.

Mechanisms of microtubule organization in differentiated animal cells

Microtubules are polarized cytoskeletal filaments that serve as tracks for intracellular transport and form a scaffold that positions organelles and other cellular components and modulates cell shape and mechanics. In animal cells, the geometry, density and directionality of microtubule networks are major determinants of cellular architecture, polarity and proliferation. In dividing cells, microtubules form bipolar spindles that pull chromosomes apart, whereas in interphase cells, microtubules are organized in a cell type-specific fashion, which strongly correlates with cell physiology. In motile cells, such as fibroblasts and immune cells, microtubules are organized as radial asters, whereas in immotile epithelial and neuronal cells and in muscles, microtubules form parallel or antiparallel arrays and cortical meshworks. Here, we review recent work addressing how the formation of such microtubule networks is driven by the plethora of microtubule regulatory proteins. These include proteins that nucleate or anchor microtubule ends at different cellular structures and those that sever or move microtubules, as well as regulators of microtubule elongation, stability, bundling or modifications. The emerging picture, although still very incomplete, shows a remarkable diversity of cell-specific mechanisms that employ conserved building blocks to adjust microtubule organization in order to facilitate different cellular functions.

Imaging Diversity in Slow Axonal Transport

The polarized morphology of neurons necessitates the delivery of proteins synthesized in the soma along the length of the axon to distal synapses; critical for sustaining communication between neurons. This constitutive and dynamic process of protein transport along axons termed "axonal transport" was initially characterized by classic pulse-chase radiolabeling studies which identified two major rate components: a fast component and a slow component. Early radiolabeling studies indicated "cohesive co-transport" of slow transport cargos. However, this approach could not be used to visualize or provide mechanistic insights on this highly dynamic process. The advent of fluorescent and photoactivatable imaging probes have now enabled real-time imaging of axonal transport. Conventional fluorescent probes have helped visualize and characterize the molecular mechanisms of transport of vesicular proteins. These proteins typically move in the fast component of axonal transport and appear as "punctate structures" along axons. However, a large majority of transported proteins that move in the slow component of transport, typically show a "uniform diffusive glow" along axons when tagged to conventional fluorescent probes. This makes it challenging to unequivocally track them in real time. Our lab has used photoactivatable fluorescent probes to tag three individual cytosolic proteins moving in the slow component of axonal transport, and identified three distinct modes of transport along axons. Our data from these experiments argue against the prevailing hypothesis based on classic radiolabeling studies, which suggested that all slow-transport proteins may move along the axon as one large macromolecular protein complex. Although other labs have started using photoactivation to study axonal transport of cytosolic proteins, this technique remains largely under-utilized. Here, we describe the detailed protocols to image and analyze axonal transport of three typical slow-component cargoes along axons of cultured hippocampal neurons.

With the Permission of Microtubules: An Updated Overview on Microtubule Function During Axon Pathfinding

During the establishment of neural circuitry axons often need to cover long distances to reach remote targets. The stereotyped navigation of these axons defines the connectivity between brain regions and cellular subtypes. This chemotrophic guidance process mostly relies on the spatio-temporal expression patterns of extracellular proteins and the selective expression of their receptors in projection neurons. Axon guidance is stimulated by guidance proteins and implemented by neuronal traction forces at the growth cones, which engage local cytoskeleton regulators and cell adhesion proteins. Different layers of guidance signaling regulation, such as the cleavage and processing of receptors, the expression of co-receptors and a wide variety of intracellular cascades downstream of receptors activation, have been progressively unveiled. Also, in the last decades, the regulation of microtubule (MT) assembly, stability and interactions with the submembranous actin network in the growth cone have emerged as crucial effector mechanisms in axon pathfinding. In this review, we will delve into the intracellular signaling cascades downstream of guidance receptors that converge on the MT cytoskeleton of the growing axon. In particular, we will focus on the microtubule-associated proteins (MAPs) network responsible of MT dynamics in the axon and growth cone. Complementarily, we will discuss new evidences that connect defects in MT scaffold proteins, MAPs or MT-based motors and axon misrouting during brain development.

Mechanical Counterbalance of Kinesin and Dynein Motors in a Microtubular Network Regulates Cell Mechanics, 3D Architecture, and Mechanosensing

Microtubules (MTs) and MT motor proteins form active 3D networks made of unstretchable cables with rod-like bending mechanics that provide cells with a dynamically changing structural scaffold. In this study, we report an antagonistic mechanical balance within the dynein-kinesin microtubular motor system. Dynein activity drives the microtubular network inward compaction, while isolated activity of kinesins bundles and expands MTs into giant circular bands that deform the cell cortex into discoids. Furthermore, we show that dyneins recruit MTs to sites of cell adhesion, increasing the topographic contact guidance of cells, while kinesins antagonize it via retraction of MTs from sites of cell adhesion. Actin-to-microtubule translocation of septin-9 enhances kinesin-MT interactions, outbalances the activity of kinesins over that of dyneins, and induces the discoid architecture of cells. These orthogonal mechanisms of MT network reorganization highlight the existence of an intricate mechanical balance between motor activities of kinesins and dyneins that controls cell 3D architecture, mechanics, and cell-microenvironment interactions.

Therapeutic Strategies for Mutant SPAST-Based Hereditary Spastic Paraplegia

Mutations of the SPAST gene that encodes the microtubule-severing enzyme called spastin are the chief cause of Hereditary Spastic Paraplegia. Growing evidence indicates that pathogenic mutations functionally compromise the spastin protein and endow it with toxic gain-of-function properties. With each of these two factors potentially relevant to disease etiology, the present article discusses possible therapeutic strategies that may ameliorate symptoms in patients suffering from SPAST-based Hereditary Spastic Paraplegia, which is usually termed SPG4-HSP.

Modeling a disease-correlated tubulin mutation in budding yeast reveals insight into MAP-mediated dynein function

Mutations in the genes that encode α- and β-tubulin underlie many neurological diseases, most notably malformations in cortical development. In addition to revealing the molecular basis for disease etiology, studying such mutations can provide insight into microtubule function and the role of the large family of microtubule effectors. In this study, we use budding yeast to model one such mutation—Gly436Arg in α-tubulin, which is causative of malformations in cortical development—in order to understand how it impacts microtubule function in a simple eukaryotic system. Using a combination of in vitro and in vivo methodologies, including live cell imaging and electron tomography, we find that the mutant tubulin is incorporated into microtubules, causes a shift in α-tubulin isotype usage, and dramatically enhances dynein activity, which leads to spindle-positioning defects. We find that the basis for the latter phenotype is an impaired interaction between She1—a dynein inhibitor—and the mutant microtubules. In addition to revealing the natural balance of α-tubulin isotype utilization in cells, our results provide evidence of an impaired interaction between microtubules and a dynein regulator as a consequence of a tubulin mutation and sheds light on a mechanism that may be causative of neurodevelopmental diseases.

Engagement of Neurotropic Viruses in Fast Axonal Transport: Mechanisms, Potential Role of Host Kinases and Implications for Neuronal Dysfunction

Much remains unknown about mechanisms sustaining the various stages in the life cycle of neurotropic viruses. An understanding of those mechanisms operating before their replication and propagation could advance the development of effective anti-viral strategies. Here, we review our current knowledge of strategies used by neurotropic viruses to undergo bidirectional movement along axons. We discuss how the invasion strategies used by specific viruses might influence their mode of interaction with selected components of the host’s fast axonal transport (FAT) machinery, including specialized membrane-bounded organelles and microtubule-based motor proteins. As part of this discussion, we provide a critical evaluation of various reported interactions among viral and motor proteins and highlight limitations of some in vitro approaches that led to their identification. Based on a large body of evidence documenting activation of host kinases by neurotropic viruses, and on recent work revealing regulation of FAT through phosphorylation-based mechanisms, we posit a potential role of host kinases on the engagement of viruses in retrograde FAT. Finally, we briefly describe recent evidence linking aberrant activation of kinase pathways to deficits in FAT and neuronal degeneration in the context of human neurodegenerative diseases. Based on these findings, we speculate that neurotoxicity elicited by viral infection may involve deregulation of host kinases involved in the regulation of FAT and other cellular processes sustaining neuronal function and survival.

Mini-review: Microtubule sliding in neurons

Microtubule sliding is an underappreciated mechanism that contributes to the establishment, organization, preservation, and plasticity of neuronal microtubule arrays. Powered by molecular motor proteins and regulated in part by static crosslinker proteins, microtubule sliding is the movement of microtubules relative to other microtubules or to non-microtubule structures such as the actin cytoskeleton. In addition to other important functions, microtubule sliding significantly contributes to the establishment and maintenance of microtubule polarity patterns in different regions of the neuron. The purpose of this article is to review the state of knowledge on microtubule sliding in the neuron, with emphasis on its mechanistic underpinnings as well as its functional significance.

Microtubule Dysfunction: A Common Feature of Neurodegenerative Diseases

Neurons are particularly susceptible to microtubule (MT) defects and deregulation of the MT cytoskeleton is considered to be a common insult during the pathogenesis of neurodegenerative disorders. Evidence that dysfunctions in the MT system have a direct role in neurodegeneration comes from findings that several forms of neurodegenerative diseases are associated with changes in genes encoding tubulins, the structural units of MTs, MT-associated proteins (MAPs), or additional factors such as MT modifying enzymes which modulating tubulin post-translational modifications (PTMs) regulate MT functions and dynamics. Efforts to use MT-targeting therapeutic agents for the treatment of neurodegenerative diseases are underway. Many of these agents have provided several benefits when tested on both in vitro and in vivo neurodegenerative model systems. Currently, the most frequently addressed therapeutic interventions include drugs that modulate MT stability or that target tubulin PTMs, such as tubulin acetylation. The purpose of this review is to provide an update on the relevance of MT dysfunctions to the process of neurodegeneration and briefly discuss advances in the use of MT-targeting drugs for the treatment of neurodegenerative disorders.

The Spatiotemporal Construction of the Axon Initial Segment via KIF3/KAP3/TRIM46 Transport under MARK2 Signaling

The axon initial segment (AIS) is a compartment that serves as a molecular barrier to achieve axon-dendrite differentiation. Distribution of specific proteins during early neuronal development has been proposed to be critical for AIS construction. However, it remains unknown how these proteins are specifically targeted to the proximal axon within this limited time period. Here, we reveal spatiotemporal regulation driven by the microtubule (MT)-based motor KIF3A/B/KAP3 that transports TRIM46, influenced by a specific MARK2 phosphorylation cascade. In the proximal part of the future axon under low MARK2 activity, the KIF3/KAP3 motor recognizes TRIM46 as cargo and transports it to the future AIS. In contrast, in the somatodendritic area under high MARK2 activity, KAP3 phosphorylated at serine 60 by MARK2 cannot bind with TRIM46 and be transported. This spatiotemporal regulation between KIF3/KAP3 and TRIM46 under specific MARK2 activity underlies the specific transport needed for axonal differentiation.

Microtubules originate asymmetrically at the somatic golgi and are guided via Kinesin2 to maintain polarity within neurons

Neurons contain polarised microtubule arrays essential for neuronal function. How microtubule nucleation and polarity are regulated within neurons remains unclear. We show that γ-tubulin localises asymmetrically to the somatic Golgi within Drosophila neurons. Microtubules originate from the Golgi with an initial growth preference towards the axon. Their growing plus ends also turn towards and into the axon, adding to the plus-end-out microtubule pool. Any plus ends that reach a dendrite, however, do not readily enter, maintaining minus-end-out polarity. Both turning towards the axon and exclusion from dendrites depend on Kinesin-2, a plus-end-associated motor that guides growing plus ends along adjacent microtubules. We propose that Kinesin-2 engages with a polarised microtubule network within the soma to guide growing microtubules towards the axon; while at dendrite entry sites engagement with microtubules of opposite polarity generates a backward stalling force that prevents entry into dendrites and thus maintains minus-end-out polarity within proximal dendrites.

Golgi Outposts Locally Regulate Microtubule Orientation in Neurons but Are Not Required for the Overall Polarity of the Dendritic Cytoskeleton

Microtubule-organizing centers often play a central role in organizing the cellular microtubule networks that underlie cell function. In neurons, microtubules in axons and dendrites have distinct polarities. Dendrite-specific Golgi "outposts," in particular multicompartment outposts, have emerged as regulators of acentrosomal microtubule growth, raising the question of whether outposts contribute to establishing or maintaining the overall polarity of the dendritic microtubule cytoskeleton. Using a combination of genetic approaches and live imaging in a Drosophila model, we found that dendritic microtubule polarity is unaffected by eliminating known regulators of Golgi-dependent microtubule organization including the cis-Golgi matrix protein GM130, the fly AKAP450 ortholog pericentrin-like protein, and centrosomin. This indicates that Golgi outposts are not essential for the formation or maintenance of a dendrite-specific cytoskeleton. However, the overexpression of GM130, which promotes the formation of ectopic multicompartment units, is sufficient to alter dendritic microtubule polarity. Axonal microtubule polarity is similarly disrupted by the presence of ectopic multicompartment Golgi outposts. Notably, multicompartment outposts alter microtubule polarity independently of microtubule nucleation mediated by the γ-tubulin ring complex. Thus, although Golgi outposts are not essential to dendritic microtubule polarity, altering their organization correlates with changes to microtubule polarity. Based on these data, we propose that the organization of Golgi outposts is carefully regulated to ensure proper dendritic microtubule polarity.

Extremely Low Forces Induce Extreme Axon Growth

Stretch-growth has been defined as a process that extends axons via the application of mechanical forces. In the present article, we used a protocol based on magnetic nanoparticles (NPs) for labeling the entire axon tract of hippocampal neurons, and an external magnetic field gradient to generate a dragging force. We found that the application of forces below 10 pN induces growth at a rate of 0.66 ± 0.02 µm h^-1 pN^-1 Calcium imaging confirmed the strong increase in elongation rate, in comparison with the condition of tip-growth. Enhanced growth in stretched axons was also accompanied by endoplasmic reticulum (ER) accumulation and, accordingly, it was blocked by an inhibition of translation. Stretch-growth was also found to stimulate axonal branching, glutamatergic synaptic transmission, and neuronal excitability. Moreover, stretched axons showed increased microtubule (MT) density and MT assembly was key to sustaining stretch-growth, suggesting a possible role of tensile forces in MT translocation/assembly. Additionally, our data showed that stretched axons do not respond to BDNF signaling, suggesting interference between the two pathways. As these extremely low mechanical forces are physiologically relevant, stretch-growth could be an important endogenous mechanism of axon growth, with a potential for designing novel strategies for axonal regrowth.SIGNIFICANCE STATEMENT Axon growth involves motion, and motion is driven by forces. The growth cone (GC) itself can generate very low intracellular forces by inducing a drastic cytoskeleton remodeling, in response to signaling molecules. Here, we investigated the key role of intracellular force as an endogenous regulator of axon outgrowth, which it has been neglected for decades because of the lack of methodologies to investigate the topic. Our results indicate a critical role of force in promoting axon growth by facilitating microtubule (MT) polymerization.

Kinetochore protein Spindly controls microtubule polarity in Drosophila axons

Microtubule polarity in axons and dendrites defines the direction of intracellular transport in neurons. Axons contain arrays of uniformly polarized microtubules with plus-ends facing the tips of the processes (plus-end-out), while dendrites contain microtubules with a minus-end-out orientation. It has been shown that cytoplasmic dynein, targeted to cortical actin, removes minus-end-out microtubules from axons. Here we have identified Spindly, a protein known for recruitment of dynein to kinetochores in mitosis, as a key factor required for dynein-dependent microtubule sorting in axons of Drosophila neurons. Depletion of Spindly affects polarity of axonal microtubules in vivo and in primary neuronal cultures. In addition to these defects, depletion of Spindly in neurons causes major collapse of axonal patterning in the third-instar larval brain as well as severe coordination impairment in adult flies. These defects can be fully rescued by full-length Spindly, but not by variants with mutations in its dynein-binding site. Biochemical analysis demonstrated that Spindly binds F-actin, suggesting that Spindly serves as a link between dynein and cortical actin in axons. Therefore, Spindly plays a critical role during neurodevelopment by mediating dynein-driven sorting of axonal microtubules.

Nucleating microtubules in neurons: Challenges and solutions

The highly polarized morphology of neurons is crucial for their function and involves formation of two distinct types of cellular extensions, the axonal and dendritic compartments. An important effector required for the morphogenesis and maintenance and thus the identity of axons and dendrites is the microtubule cytoskeleton. Microtubules in axons and dendrites are arranged with distinct polarities, to allow motor-dependent, compartment-specific sorting of cargo. Despite the importance of the microtubule cytoskeleton in neurons, the molecular mechanisms that generate the intricate compartment-specific microtubule configurations remain largely obscure. Work in other cell types has identified microtubule nucleation, the de novo formation of microtubules, and its spatio-temporal regulation as essential for the proper organization of the microtubule cytoskeleton. Whereas regulation of microtubule nucleation usually involves microtubule organizing centers such as the centrosome, neurons seem to rely largely on decentralized nucleation mechanisms. In this review, I will discuss recent advances in deciphering nucleation mechanisms in neurons, how they contribute to the arrangement of microtubules with specific polarities, and how this affects neuron morphogenesis. While this work has shed some light on these important processes, we are far from a comprehensive understanding. Thus, to provide a coherent model, my discussion will include both well-established mechanisms and mechanisms with more limited supporting data. Finally, I will also highlight important outstanding questions for future investigation.

Finding order in slow axonal transport

Slow axonal transport conveys cytosolic and cytoskeletal proteins into axons and synapses at overall velocities that are several orders of magnitude slower than the fast transport of membranous organelles such as vesicles and mitochondria. The phenomenon of slow transport was characterized by in vivo pulse-chase radiolabeling studies done decades ago, and proposed models emphasized an orderly cargo-movement, with apparent cohesive transport of multiple proteins and subcellular structures along axons over weeks to months. However, visualization of cytosolic and cytoskeletal cargoes in cultured neurons at much higher temporal and spatial resolution has revealed an unexpected diversity in movement - ranging from a diffusion-like biased motion, to intermittent cargo dynamics and unusual polymerization-based transport paradigms. This review provides an updated view of slow axonal transport and explores emergent mechanistic themes in this enigmatic rate-class.

Mechanical Regulation of Neurite Polarization and Growth: A Computational Study

The densely packed microtubule (MT) array found in neuronal cell projections (neurites) serves two fundamental functions simultaneously: it provides a mechanically stable track for molecular motor-based transport and produces forces that drive neurite growth. The local pattern of MT polarity along the neurite shaft has been found to differ between axons and dendrites. In axons, the neurons' dominating long projections, roughly 90% of the MTs orient with their rapidly growing plus end away from the cell body, whereas in vertebrate dendrites, their orientations are locally mixed. Molecular motors are known to be responsible for cytoskeletal ordering and force generation, but their collective function in the dense MT cytoskeleton of neurites remains elusive. We here hypothesized that both the polarity pattern of MTs along the neurite shaft and the shaft's global extension are simultaneously driven by molecular motor forces and should thus be regulated by the mechanical load acting on the MT array as a whole. To investigate this, we simulated cylindrical bundles of MTs that are cross-linked and powered by molecular motors by iteratively solving a set of force-balance equations. The bundles were subjected to a fixed load arising from actively generated tension in the actomyosin cortex enveloping the MTs. The magnitude of the load and the level of motor-induced connectivity between the MTs have been varied systematically. With an increasing load and decreasing motor-induced connectivity between MTs, the bundles became wider in cross section and extended more slowly, and the local MT orientational order was reduced. These results reveal two, to our knowledge, novel mechanical factors that may underlie the distinctive development of the MT cytoskeleton in axons and dendrites: the cross-linking level of MTs by motors and the load acting on this cytoskeleton during growth.

Endosomal Wnt signaling proteins control microtubule nucleation in dendrites

Dendrite microtubules are polarized with minus-end-out orientation in Drosophila neurons. Nucleation sites concentrate at dendrite branch points, but how they localize is not known. Using Drosophila, we found that canonical Wnt signaling proteins regulate localization of the core nucleation protein γTubulin (γTub). Reduction of frizzleds (fz), arrow (low-density lipoprotein receptor-related protein [LRP] 5/6), dishevelled (dsh), casein kinase Iγ, G proteins, and Axin reduced γTub-green fluorescent protein (GFP) at branch points, and two functional readouts of dendritic nucleation confirmed a role for Wnt signaling proteins. Both dsh and Axin localized to branch points, with dsh upstream of Axin. Moreover, tethering Axin to mitochondria was sufficient to recruit ectopic γTub-GFP and increase microtubule dynamics in dendrites. At dendrite branch points, Axin and dsh colocalized with early endosomal marker Rab5, and new microtubule growth initiated at puncta marked with fz, dsh, Axin, and Rab5. We propose that in dendrites, canonical Wnt signaling proteins are housed on early endosomes and recruit nucleation sites to branch points.

Mechanisms of Neuronal Alternative Splicing and Strategies for Therapeutic Interventions

Many cellular and physiological processes are coordinated by regulatory networks that produce a remarkable complexity of transcript isoforms. In the mammalian nervous system, alternative pre-mRNA splicing generates functionally distinct isoforms that play key roles in normal physiology, supporting development, plasticity, complex behaviors, and cognition. Neuronal splicing programs controlled by RNA-binding proteins, are influenced by chromatin modifications and can exhibit neuronal subtype specificity. As highlighted in recent publications, aberrant alternative splicing is a major contributor to disease phenotypes. Therefore, understanding the underlying mechanisms of alternative splicing regulation and identifying functional splicing isoforms with critical phenotypic roles are expected to provide a comprehensive resource for therapeutic development, as illuminated by recent successful interventions of spinal muscular atrophy. Here, we discuss the latest progress in the study of the emerging complexity of alternative splicing mechanisms in neurons, and how these findings inform new therapies to correct and control splicing defects.

Unconventional Roles of Cytoskeletal Mitotic Machinery in Neurodevelopment

At first look, cell division and neurite formation seem to be two different, essential biological processes. However, both processes require extensive reorganization of the cytoskeleton, and especially microtubules. Remarkably, in recent years, independent work from several groups has shown that multiple cytoskeletal components previously considered specific for the mitotic machinery play important roles in neurite initiation and extension. In this review article, we describe how several cytoplasmic and mitotic microtubule motors, components of mitotic kinetochores, and cortical actin participate in reorganization of the microtubule network required to form and maintain axons and dendrites. The emerging similarities between these two biological processes will certainly generate new insights into the mechanisms generating the unique morphology of neurons.

Molecular basis for dyneinopathies reveals insight into dynein regulation and dysfunction

Cytoplasmic dynein plays critical roles within the developing and mature nervous systems, including effecting nuclear migration, and retrograde transport of various cargos. Unsurprisingly, mutations in dynein are causative of various developmental neuropathies and motor neuron diseases. These ‘dyneinopathies’ define a broad spectrum of diseases with no known correlation between mutation identity and disease state. To circumvent complications associated with dynein studies in human cells, we employed budding yeast as a screening platform to characterize the motility properties of seventeen disease-correlated dynein mutants. Using this system, we determined the molecular basis for several classes of etiologically related diseases. Moreover, by engineering compensatory mutations, we alleviated the mutant phenotypes in two of these cases, one of which we confirmed with recombinant human dynein. In addition to revealing molecular insight into dynein regulation, our data provide additional evidence that the type of disease may in fact be dictated by the degree of dynein dysfunction.

Motor proteins maintain order by transporting biomolecules and various structures within living cells. Dynein is one such motor that moves many types of cargoes along tracks called microtubules, which are spread across the cell’s interior. This motor is particularly important in nerve cells, which can be very long and thus depend heavily on motor proteins to ensure cargoes end up where they are needed. This becomes especially apparent in human diseases that arise as a consequence of mutations in the genes that produce components of the dynein motor.

It is assumed that these genetic changes simply prevent dynein from working properly, which ultimately affects the health and survival of cells. However, it is currently unknown what specific effect these mutations have on dynein’s role within the cell, and how these changes lead to particular diseases.

Marzo et al. have now used dynein from a budding yeast to closely examine 17 mutations in the dynein gene that are associated with developmental and/or motor neuron diseases in humans. For each mutation, various aspects of how dynein moves (e.g. average speed, distance travelled) were measured and quantitatively compared. The results show that the severity of the effect of each mutation can be directly correlated with the type of disease caused by the mutation. In particular, mutations that lead to less severe defects are found in patients that suffer from various motor neuron diseases, while more severe dynein mutations are found in patients with developmental brain disorders. Marzo et al. confirmed the likely structural changes that caused the defects in dynein’s activity in two of the 17 cases, by engineering additional, restorative mutations that lessened the effects of the primary mutation.

These findings reveal links between the molecular impact of defects in the dynein gene and human health. They also confirm that budding yeast is a powerful tool for investigating newly discovered dynein mutations that correlate with disease. This study provides a potential system that could be used to screen drugs that might lessen the effects of specific dynein mutations. However, further work is needed to determine how effective this system will be for drug discovery.

TUBA1A mutations identified in lissencephaly patients dominantly disrupt neuronal migration and impair dynein activity

The microtubule cytoskeleton supports diverse cellular morphogenesis and migration processes during brain development. Mutations in tubulin genes are associated with severe human brain malformations known as 'tubulinopathies'; however, it is not understood how molecular-level changes in microtubule subunits lead to brain malformations. In this study, we demonstrate that missense mutations affecting arginine at position 402 (R402) of TUBA1A α-tubulin selectively impair dynein motor activity and severely and dominantly disrupt cortical neuronal migration. TUBA1A is the most commonly affected tubulin gene in tubulinopathy patients, and mutations altering R402 account for 30% of all reported TUBA1A mutations. We show for the first time that ectopic expression of TUBA1A-R402C and TUBA1A-R402H patient alleles is sufficient to dominantly disrupt cortical neuronal migration in the developing mouse brain, strongly supporting a causal role in the pathology of brain malformation. To isolate the precise molecular impact of R402 mutations, we generated analogous R402C and R402H mutations in budding yeast α-tubulin, which exhibit a simplified microtubule cytoskeleton. We find that R402 mutant tubulins assemble into microtubules that support normal kinesin motor activity but fail to support the activity of dynein motors. Importantly, the level of dynein impairment scales with the expression level of the mutant in the cell, suggesting a 'poisoning' mechanism in which R402 mutant α-tubulin acts dominantly by populating microtubules with defective binding sites for dynein. Based on our results, we propose a new model for the molecular pathology of tubulinopathies that may also extend to other tubulin-related neuropathies.

Investigation of multiple-dynein transport of melanosomes by non-invasive force measurement using fluctuation unit χ

Pigment organelles known as melanosomes disperse or aggregate in a melanophore in response to hormones. These movements are mediated by the microtubule motors kinesin-2 and cytoplasmic dynein. However, the force generation mechanism of dynein, unlike that of kinesin, is not well understood. In this study, to address this issue, we investigated the dynein-mediated aggregation of melanosomes in zebrafish melanophores. We applied the fluctuation theorem of non-equilibrium statistical mechanics to estimate forces acting on melanosomes during transport by dynein, given that the energy of a system is related to its fluctuation. Our results demonstrate that multiple force-producing units cooperatively transport a single melanosome. Since the force is generated by dynein, this suggests that multiple dyneins carry a single melanosome. Cooperative transport has been reported for other organelles; thus, multiple-motor transport may be a universal mechanism for moving organelles within the cell.

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.

Dynein promotes sustained axonal growth and Schwann cell remodeling early during peripheral nerve regeneration

Following injury, axons of the peripheral nervous system have retained the capacity for regeneration. While it is well established that injury signals require molecular motors for their transport from the injury site to the nucleus, whether kinesin and dynein motors play additional roles in peripheral nerve regeneration is not well understood. Here we use genetic mutants of motor proteins in a zebrafish peripheral nerve regeneration model to visualize and define in vivo roles for kinesin and dynein. We find that both kinesin-1 and dynein are required for zebrafish peripheral nerve regeneration. While loss of kinesin-1 reduced the overall robustness of axonal regrowth, loss of dynein dramatically impaired axonal regeneration and also reduced injury-induced Schwann cell remodeling. Chimeras between wild type and dynein mutant embryos demonstrate that dynein function in neurons is sufficient to promote axonal regrowth. Finally, by simultaneously monitoring actin and microtubule dynamics in regenerating axons we find that dynein appears dispensable to initiate axonal regrowth, but is critical to stabilize microtubules, thereby sustaining axonal regeneration. These results reveal two previously unappreciated roles for dynein during peripheral nerve regeneration, initiating injury induced Schwann cell remodeling and stabilizing axonal microtubules to sustain axonal regrowth.

Nerve regeneration requires coordinated responses from multiple cell types after injury. Axons must extend from the neuronal cell body back towards their targets, while surrounding Schwann cells enter a repair cell state in which they promote regeneration. While nerves of the peripheral nervous system can regrow, it is estimated that fewer than 10 percent of patients fully recover function after nerve injury. In order to understand the mechanisms by which peripheral nerves regrow, we used live cell imaging in the zebrafish to observe the process of nerve regeneration, monitoring axons and Schwann cells simultaneously during this process. Using genetic mutants, we identified a role for the molecular motors kinesin-1 and dynein in promoting axonal regrowth. Furthermore, we found that dynein plays an additional role in Schwann cell response to injury. Thus, we demonstrate that molecular motors are required in multiple cell types to promote nerve regeneration.

Microtubule control of functional architecture in neurons

Neurons are exquisitely polarized cells whose structure and function relies on microtubules. Microtubules in signal-receiving dendrites and signal-sending axons differ in their organization and microtubule-associated proteins. These differences, coupled with microtubule post-translational modifications, combine to locally regulate intracellular transport, morphology, and function. Recent discoveries provide new insight into the regulation of non-centrosomal microtubule arrays in neurons, the relationship between microtubule acetylation and mechanosensation, and the spatial patterning of microtubules that regulates motor activity and cargo delivery in axons and dendrites. Together, these new studies bring us closer to understanding how microtubule function is locally tuned to match the specialized tasks associated with signal reception and transmission.

Axonogenesis Is Coordinated by Neuron-Specific Alternative Splicing Programming and Splicing Regulator PTBP2

How a neuron acquires an axon is a fundamental question. Piecemeal identification of many axonogenesis-related genes has been done, but coordinated regulation is unknown. Through unbiased transcriptome profiling of immature primary cortical neurons during early axon formation, we discovered an association between axonogenesis and neuron-specific alternative splicing. Known axonogenesis genes exhibit little expression alternation but widespread splicing changes. Axonogenesis-associated splicing is governed by RNA binding protein PTBP2, which is enriched in neurons and peaks around axonogenesis in the brain. Cortical depletion of PTBP2 prematurely induces axonogenesis-associated splicing, causes imbalanced expression of axonogenesis-associated isoforms, and specifically affects axon formation in vitro and in vivo. PTBP2-controlled axonogenesis-associated Shtn1 splicing determines SHTN1's capacity to regulate actin interaction, polymerization, and axon growth. Precocious Shtn1 isoform switch contributes to disorganized axon formation of Ptbp2^-/- neurons. We conclude that PTBP2-orchestrated alternative splicing programming is required for robust generation of a single axon in mammals.

Microtubules: From understanding their dynamics to using them as potential therapeutic targets

Microtubules (MT) and actin microfilaments are dynamic cytoskeleton components involved in a range of intracellular processes. MTs play a role in cell division, beating of cilia and flagella, and intracellular transport. Over the past decades, much knowledge has been gained regarding MT function and structure, and its role in underlying disease progression. This makes MT potential therapeutic targets for various disorders. Disturbances in MT and their associated proteins are the underlying cause of diseases such as Alzheimer's disease, cancer, and several genetic diseases. Some of the advances in the field of MT research, as well as the potenti G beta gamma, is needed al uses of MT-targeting agents in various conditions have been reviewed here.

An Integrated Cytoskeletal Model of Neurite Outgrowth

Neurite outgrowth underlies the wiring of the nervous system during development and regeneration. Despite a significant body of research, the underlying cytoskeletal mechanics of growth and guidance are not fully understood, and the relative contributions of individual cytoskeletal processes to neurite growth are controversial. Here, we review the structural organization and biophysical properties of neurons to make a semi-quantitative comparison of the relative contributions of different processes to neurite growth. From this, we develop the idea that neurons are active fluids, which generate strong contractile forces in the growth cone and weaker contractile forces along the axon. As a result of subcellular gradients in forces and material properties, actin flows rapidly rearward in the growth cone periphery, and microtubules flow forward in bulk along the axon. With this framework, an integrated model of neurite outgrowth is proposed that hopefully will guide new approaches to stimulate neuronal growth.

Local microtubule organization promotes cargo transport in C. elegans dendrites

The specific organization of the neuronal microtubule cytoskeleton in axons and dendrites is an evolutionarily conserved determinant of neuronal polarity that allows for selective cargo sorting. However, how dendritic microtubules are organized and whether local differences influence cargo transport remains largely unknown. Here, we use live-cell imaging to systematically probe the microtubule organization in Caenorhabditis elegans neurons, and demonstrate the contribution of distinct mechanisms in the organization of dendritic microtubules. We found that most non-ciliated neurons depend on unc-116 (kinesin-1), unc-33 (CRMP) and unc-44 (ankyrin) for correct microtubule organization and polarized cargo transport, as previously reported. Ciliated neurons and the URX neuron, however, use an additional pathway to nucleate microtubules at the tip of the dendrite, from the base of the cilium in ciliated neurons. Since inhibition of distal microtubule nucleation affects distal dendritic transport, we propose a model in which the presence of a microtubule-organizing center at the dendrite tip ensures correct dendritic cargo transport.