Dynactin is required for transport initiation from the distal axon

Interfacial Interactions between Neural Tracing Lectin-Gold Nanoparticle Conjugate and Cell Membrane Glycoproteins

The use of neural tracers as targeting molecules for drug delivery has been previously established as a novel and efficient method of neural drug delivery. The wheat germ agglutinin-horseradish peroxidase conjugate (WGAHRP) is a common neural tracer, which has been used extensively to decipher neural pathways in vertebrates. It has been widely reported to bind to cell surfaces and be transported in a retrograde fashion (from synapses toward the cell body) via dynein motors along microtubules within axons and transynaptically between neurons. Here we report on the differential binding between WGAHRP and gold-conjugated WGAHRP (AuNP-WGAHRP) to the glycoprotein profiles extracted from two neuronal cell lines and one skeletal muscle cell line, as well as the binding kinetics to heparin. From proteomic analysis of the extracted glycoproteins, we suggest the identity of cell surface glycoproteins involved in the retrograde transport of WGAHRP. This study illuminates the interfacial and molecular interactions of protein-gold conjugates with native ligands and opens the door for the identification of new targets for neural tracing and nervous system-related drug delivery.

Direct observation of importin α family member KPNA1 in axonal transport with or without a schizophrenia-related mutation

Karyopherin α1 (KPNA1)/(human importin α5; mouse importin α1) facilitates cargo transport into the nucleus by forming a complex with a nuclear localization sequence containing cargo and importin β1 (IPOB1). The elevated KPNA1 expression in neurons and the correlation between mutations and psychiatric disorders suggest its broader significance beyond nucleocytoplasmic transport. Although KPNA1 is localized in the neurites of neurons, its role in axonal transport mechanisms remains unclear, and data on the connection between psychiatric disorders and signaling at the periphery of neurons remain limited. To address this knowledge gap, we investigated the dynamics of KPNA1 and related factors within axons. Our results showed that many of the axonal KPNA1 did not form a complex with IPOB1 in noninjured steady-state neurons. Axonal KPNA1 exhibited relatively stationary mobility and some showed bidirectional motility with fluctuating motion. KPNA1 partly comigrated with endosome/lysosome-associated factors, suggesting the presence of novel mechanisms underlie axonal transport and nucleocytoplasmic shuttling involving KPNA1 and IPOB1. Mutated KPNA1, which has been shown to be associated with psychiatric disorders (KPNA1E448X), was predominantly localized to the nucleus and lost from the axon. Incorporating a nuclear export signal (KPNA1E448X-NES) enhanced its subcellular localization and dynamics in the axon. Our findings demonstrate that KPNA1 functions not only as a shuttle between the cytoplasm and nucleus but also as a transporter in neuronal axons, relying on the endosomes for movement away from the nucleus with relatively slow net motions. Furthermore, a mutation in the Kpna1 gene can affect the dynamics of axonal transport. The insights from these mutations provide valuable knowledge for expanding our understanding of psychiatric disorders and facilitate the development of novel treatment strategies.

Dynein-driven regulation of postsynaptic membrane architecture and synaptic function

Cytoplasmic dynein is essential in motor neurons for retrograde cargo transport that sustains neuronal connectivity. Little, however, is known about dynein function on the postsynaptic side of the circuit. Here, we report distinct postsynaptic roles for dynein at neuromuscular junctions in Drosophila. Intriguingly, we show that dynein puncta accumulate postsynaptically at glutamatergic synaptic terminals. Moreover, Skittles (Sktl), a phosphatidylinositol 4-phosphate 5-kinase that produces phosphatidylinositol 4,5-bisphosphate (PIP2) to organize the spectrin cytoskeleton, also localizes specifically to glutamatergic synaptic terminals. Depletion of postsynaptic dynein disrupted the accumulation of Skittles and the PIP2 phospholipid, and organization of the spectrin cytoskeleton at the postsynaptic membrane. Coincidental with dynein depletion, we observed an increase in the size of ionotropic glutamate receptor (iGluR) fields and an increase in the amplitude and frequency of miniature excitatory junctional potentials. PIP2 levels did not affect iGluR clustering, nor did dynein affect the levels of iGluR subunits at the neuromuscular junction. Our observations suggest a separate, transport-independent function for dynein in iGluR cluster organization. Based on the close apposition of dynein puncta to the iGluR fields, we speculate that dynein at the postsynaptic membrane contributes to the organization of the receptor fields, hence ensuring proper synaptic transmission.

The Power of Three: Dynactin associates with three dyneins under load for greater force production

Cytoplasmic dynein is an essential microtubule motor protein that powers organelle transport and mitotic spindle assembly. Its activity depends on dynein-dynactin-cargo adaptor complexes, such as dynein-dynactin-BicD2 (DDB), which typically function with two dynein motors. We show that mechanical tension recruits a third dynein motor via an auxiliary BicD adaptor binding the light intermediate chain of the third dynein, stabilizing multi-dynein assemblies and enhancing force generation. Lis1 prevents dynein from transitioning into a force-limiting phi-like conformation, allowing single-dynein DDB to sustain forces up to ~4.5 pN, whereas force generation often ends at ~2.5 pN without Lis1. Complexes with two or three dyneins generate ~7 pN and ~9 pN, respectively, consistent with a staggered motor arrangement that enhances collective output. Under load, DDB primarily takes ~8 nm steps, challenging existing dynein coordination models. These findings reveal adaptive mechanisms that enable robust intracellular transport under varying mechanical demands.

VezA/vezatin facilitates proper assembly of the dynactin complex in vivo

Cytoplasmic dynein-mediated intracellular transport needs the multi-component dynactin complex for cargo binding and motor activation. However, the cellular factors involved in dynactin assembly remain unexplored. Here, we found in Aspergillus nidulans that the vezatin homolog VezA is important for dynactin assembly. VezA affects the microtubule plus-end accumulation of dynein before cargo binding and cargo-adapter-mediated dynein activation, two processes that both need dynactin. The dynactin complex contains multiple components, including p150, p50, and an Arp1 (actin-related protein 1) mini-filament associated with a pointed-end sub-complex. VezA physically interacts with the Arp1 mini-filament either directly or indirectly. Loss of VezA significantly decreases the amount of Arp1 pulled down with pointed-end proteins, as well as the protein levels of p50 and p150 in cell extract. Using various dynactin mutants, we further revealed that the dynactin assembly process must be highly coordinated. Together, these results shed light on dynactin assembly in vivo.

Herpes simplex virus type-1 cVAC formation in neuronal cells is mediated by dynein motor function and glycoprotein retrieval from the plasma membrane

Herpesvirus assembly requires the cytoplasmic association of large macromolecular and membrane structures that derive from both the nucleus and cytoplasmic membrane systems. Results from the study of human cytomegalovirus (HCMV) in cells where it organizes a perinuclear cytoplasmic virus assembly compartment (cVAC) show a clear requirement for the minus-end-directed microtubule motor, dynein, for virus assembly. In contrast, the assembly of herpes simplex virus -1 (HSV-1) in epithelial cells where it forms multiple dispersed, peripheral assembly sites is only mildly inhibited by the microtubule-depolymerizing agent, nocodazole. Here, we make use of a neuronal cell line system in which HSV-1 forms a single cVAC and show that dynein and its co-factor dynactin localize to the cVAC, and dynactin is associated with membranes that contain the virion tegument protein pUL11. We also show that the virus membrane-associated structural proteins pUL51 and the viral envelope glycoprotein gE arrive at the cVAC by different routes. Specifically, gE arrives at the cVAC after retrieval from the plasma membrane, suggesting the need for an intact retrograde transport system. Finally, we demonstrate that inhibition of dynactin function profoundly inhibits cVAC formation and virus production during the cytoplasmic assembly phase of infection.IMPORTANCEMany viruses reorganize cytoplasmic membrane systems and macromolecular transport systems to promote the production of progeny virions. Clarifying the mechanisms by which they accomplish this may reveal novel therapeutic strategies and illustrate mechanisms that are critical for normal cellular organization. Here, we explore the mechanism by which HSV-1 moves macromolecular and membrane cargo to generate a virus assembly compartment in the infected cell. We find that the virus makes use of a well-characterized, microtubule-based transport system that is stabilized against drugs that disrupt microtubules.

Gonadal hormone deprivation regulates response to tibolone in neurodegenerative pathways

Gonadal hormone deprivation (GHD) and decline such as menopause and bilateral oophorectomy are associated with an increased risk of neurodegeneration. Yet, hormone therapies (HTs) show varying efficacy, influenced by factors such as sex, drug type, and timing of treatment relative to hormone decline. We hypothesize that the molecular environment of the brain undergoes a transition following GHD, impacting the effectiveness of HTs. Using a GHD model in mice treated with Tibolone, we conducted proteomic analysis and identified a reprogrammed response to Tibolone, a compound that stimulates estrogenic, progestogenic, and androgenic pathways. Through a comprehensive network pharmacological workflow, we identified a reprogrammed response to Tibolone, particularly within "Pathways of Neurodegeneration", as well as interconnected pathways including "cellular respiration", "carbon metabolism", and "cellular homeostasis". Analysis revealed 23 proteins whose Tibolone response depended on GHD and/or sex, implicating critical processes like oxidative phosphorylation and calcium signalling. Our findings suggest the therapeutic efficacy of HTs may depend on these variables, suggesting a need for greater precision medicine considerations whilst highlighting the need to uncover underlying mechanisms.

VezA/vezatin facilitates proper assembly of the dynactin complex in vivo

Cytoplasmic dynein-mediated intracellular transport needs the multi-component dynactin complex for cargo binding and motor activation. However, cellular factors involved in dynactin assembly remain unexplored. Here we found in Aspergillus nidulans that the vezatin homolog VezA is important for dynactin assembly. VezA affects the microtubule plus-end accumulation of dynein before cargo binding and cargo adapter-mediated dynein activation, two processes that both need dynactin. The dynactin complex contains multiple components including an Arp1 (actin-related protein 1) mini-filament associated with a pointed-end sub-complex. VezA physically interacts with dynactin either directly or indirectly via the Arp1 mini-filament and its pointed-end sub-complex. Loss of VezA causes a defect in dynactin integrity, most likely by affecting the connection between the Arp1 mini-filament and its pointed-end sub-complex. Using various dynactin mutants, we further revealed that assembly of the dynactin complex must be highly coordinated. Together, these results shed important new light on dynactin assembly in vivo.

Dysregulation of stress granule dynamics by DCTN1 deficiency exacerbates TDP-43 pathology in Drosophila models of ALS/FTD

The abnormal aggregation of TDP-43 into cytoplasmic inclusions in affected neurons is a major pathological hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although TDP-43 is aberrantly accumulated in the neurons of most patients with sporadic ALS/FTD and other TDP-43 proteinopathies, how TDP-43 forms cytoplasmic aggregates remains unknown. In this study, we show that a deficiency in DCTN1, a subunit of the microtubule-associated motor protein complex dynactin, perturbs the dynamics of stress granules and drives the formation of TDP-43 cytoplasmic aggregation in cultured cells, leading to the exacerbation of TDP-43 pathology and neurodegeneration in vivo. We demonstrated using a Drosophila model of ALS/FTD that genetic knockdown of DCTN1 accelerates the formation of ubiquitin-positive cytoplasmic inclusions of TDP-43. Knockdown of components of other microtubule-associated motor protein complexes, including dynein and kinesin, also increased the formation of TDP-43 inclusions, indicating that intracellular transport along microtubules plays a key role in TDP-43 pathology. Notably, DCTN1 knockdown delayed the disassembly of stress granules in stressed cells, leading to an increase in the formation of pathological cytoplasmic inclusions of TDP-43. Our results indicate that a deficiency in DCTN1, as well as disruption of intracellular transport along microtubules, is a modifier that drives the formation of TDP-43 pathology through the dysregulation of stress granule dynamics.

Perry Disease: Bench to Bedside Circulation and a Team Approach

With technological applications, especially in genetic testing, new diseases have been discovered and new disease concepts have been proposed in recent years; however, the pathogenesis and treatment of these rare diseases are not as well established as those of common diseases. To demonstrate the importance of rare disease research, in this paper we focus on our research topic, Perry disease (Perry syndrome). Perry disease is a rare autosomal dominant neurodegenerative disorder clinically characterized by parkinsonism, depression/apathy, weight loss, and respiratory symptoms including central hypoventilation and central sleep apnea. The pathological classification of Perry disease falls under TAR DNA-binding protein 43 (TDP-43) proteinopathies. Patients with Perry disease exhibit DCTN1 mutations, which is the causative gene for the disease; they also show relatively uniform pathological and clinical features. This review summarizes recent findings regarding Perry disease from both basic and clinical perspectives. In addition, we describe technological innovations and outline future challenges and treatment prospects. We discuss the expansion of research from rare diseases to common diseases and the importance of collaboration between clinicians and researchers. Here, we highlight the importance of researching rare diseases as it contributes to a deeper understanding of more common diseases, thereby opening up new avenues for scientific exploration.

Axonal transport of autophagosomes is regulated by dynein activators JIP3/JIP4 and ARF/RAB GTPases

Neuronal autophagosomes form and engulf cargos at presynaptic sites in the axon and are then transported to the soma to recycle their cargo. Autophagic vacuoles (AVs) mature en route via fusion with lysosomes to become degradatively competent organelles; transport is driven by the microtubule motor protein cytoplasmic dynein, with motor activity regulated by a sequential series of adaptors. Using lysate-based single-molecule motility assays and live-cell imaging in primary neurons, we show that JNK-interacting proteins 3 (JIP3) and 4 (JIP4) are activating adaptors for dynein that are regulated on autophagosomes and lysosomes by the small GTPases ARF6 and RAB10. GTP-bound ARF6 promotes formation of the JIP3/4-dynein-dynactin complex. Either knockdown or overexpression of RAB10 stalls transport, suggesting that this GTPase is also required to coordinate the opposing activities of bound dynein and kinesin motors. These findings highlight the complex coordination of motor regulation during organelle transport in neurons.

Control of motor landing and processivity by the CAP-Gly domain in the KIF13B tail

Microtubules are major components of the eukaryotic cytoskeleton. Posttranslational modifications (PTMs) of tubulin regulates interactions with microtubule-associated proteins (MAPs). One unique PTM is the cyclical removal and re-addition of the C-terminal tyrosine of α-tubulin and MAPs containing CAP-Gly domains specifically recognize tyrosinated microtubules. KIF13B, a long-distance transport kinesin, contains a conserved CAP-Gly domain, but the role of the CAP-Gly domain in KIF13B's motility along microtubules remains unknown. To address this, we investigate the interaction between KIF13B's CAP-Gly domain, and tyrosinated microtubules. We find that KIF13B's CAP-Gly domain influences the initial motor-microtubule interaction, as well as processive motility along microtubules. The effect of the CAP-Gly domain is enhanced when the motor domain is in the ADP state, suggesting an interplay between the N-terminal motor domain and C-terminal CAP-Gly domain. These results reveal that specialized kinesin tail domains play active roles in the initiation and continuation of motor movement.

Speech- and language-linked FOXP2 mutation targets protein motors in striatal neurons

Human speech and language are among the most complex motor and cognitive abilities. The discovery of a mutation in the transcription factor FOXP2 in KE family members with speech disturbances has been a landmark example of the genetic control of vocal communication in humans. Cellular mechanisms underlying this control have remained unclear. By leveraging FOXP2 mutation/deletion mouse models, we found that the KE family FOXP2R553H mutation directly disables intracellular dynein-dynactin 'protein motors' in the striatum by induction of a disruptive high level of dynactin1 that impairs TrkB endosome trafficking, microtubule dynamics, dendritic outgrowth and electrophysiological activity in striatal neurons alongside vocalization deficits. Dynactin1 knockdown in mice carrying FOXP2R553H mutations rescued these cellular abnormalities and improved vocalization. We suggest that FOXP2 controls vocal circuit formation by regulating protein motor homeostasis in striatal neurons, and that its disruption could contribute to the pathophysiology of FOXP2 mutation/deletion-associated speech disorders.

Disruption of axonal transport in neurodegeneration

Neurons are markedly compartmentalized, which makes them reliant on axonal transport to maintain their health. Axonal transport is important for anterograde delivery of newly synthesized macromolecules and organelles from the cell body to the synapse and for the retrograde delivery of signaling endosomes and autophagosomes for degradation. Dysregulation of axonal transport occurs early in neurodegenerative diseases and plays a key role in axonal degeneration. Here, we provide an overview of mechanisms for regulation of axonal transport; discuss how these mechanisms are disrupted in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, hereditary spastic paraplegia, amyotrophic lateral sclerosis, and Charcot-Marie-Tooth disease; and discuss therapeutic approaches targeting axonal transport.

Deficiency of Perry syndrome-associated p150Glued in midbrain dopaminergic neurons leads to progressive neurodegeneration and endoplasmic reticulum abnormalities

Multiple missense mutations in p150Glued are linked to Perry syndrome (PS), a rare neurodegenerative disease pathologically characterized by loss of nigral dopaminergic (DAergic) neurons. Here we generated p150Glued conditional knockout (cKO) mice by deleting p150Glued in midbrain DAergic neurons. The young cKO mice displayed impaired motor coordination, dystrophic DAergic dendrites, swollen axon terminals, reduced striatal dopamine transporter (DAT), and dysregulated dopamine transmission. The aged cKO mice showed loss of DAergic neurons and axons, somatic accumulation of α-synuclein, and astrogliosis. Further mechanistic studies revealed that p150Glued deficiency in DAergic neurons led to the reorganization of endoplasmic reticulum (ER) in dystrophic dendrites, upregulation of ER tubule-shaping protein reticulon 3, accumulation of DAT in reorganized ERs, dysfunction of COPII-mediated ER export, activation of unfolded protein response, and exacerbation of ER stress-induced cell death. Our findings demonstrate the importance of p150Glued in controlling the structure and function of ER, which is critical for the survival and function of midbrain DAergic neurons in PS.

Kinesin-1 autoinhibition facilitates the initiation of dynein cargo transport

The functional significance of Kinesin-1 autoinhibition has been unclear. Kinesin-1 transports multiple cargoes including cytoplasmic dynein to microtubule plus ends. From a genetic screen for Aspergillus mutants defective in dynein-mediated early endosome transport, we identified a kinesin-1 mutation kinAK895* at the C-terminal IAK motif involved in autoinhibition. The kinA∆IAK and kinAK895E mutants exhibited a similar defect in dynein-mediated early endosome transport, verifying the importance of kinesin-1 autoinhibition in dynein-mediated transport. Kinesin-1 autoinhibition is not critical for dynein accumulation at microtubule plus ends or for the secretory vesicle cargoes of kinesin-1 to reach the hyphal tip. However, it facilitates dynein to initiate early endosome transport. This is unrelated to a direct competition between dynein and kinesin-1 on early endosomes because kinesin-3 rather than kinesin-1 drives the plus-end-directed early endosome movement. This effect of kinesin-1 autoinhibition on dynein-mediated early endosome transport is related to cargo adapter-mediated dynein activation but at a step beyond the switching of dynein from its autoinhibited conformation.

Axonal transport of autophagosomes is regulated by dynein activators JIP3/JIP4 and ARF/RAB GTPases

Neuronal autophagosomes, "self-eating" degradative organelles, form at presynaptic sites in the distal axon and are transported to the soma to recycle their cargo. During transit, autophagic vacuoles (AVs) mature through fusion with lysosomes to acquire the enzymes necessary to breakdown their cargo. AV transport is driven primarily by the microtubule motor cytoplasmic dynein in concert with dynactin and a series of activating adaptors that change depending on organelle maturation state. The transport of mature AVs is regulated by the scaffolding proteins JIP3 and JIP4, both of which activate dynein motility in vitro. AV transport is also regulated by ARF6 in a GTP-dependent fashion. While GTP-bound ARF6 promotes the formation of the JIP3/4-dynein-dynactin complex, RAB10 competes with the activity of this complex by increasing kinesin recruitment to axonal AVs and lysosomes. These interactions highlight the complex coordination of motors regulating organelle transport in neurons.

Employing Live-Cell Imaging to Study Motor-Mediated Transport

Microtubule-based transport is a highly regulated process, requiring kinesin and/or dynein motors, a multitude of motor-associated regulatory proteins including activating adaptors and scaffolding proteins, and microtubule tracks that also provide regulatory cues. While in vitro studies are invaluable, fully replicating the physiological conditions under which motility occurs in cells is not yet possible. Here, we describe two methods that can be employed to study motor-based transport and motor regulation in a cellular context. Live-cell imaging of organelle transport in neurons leverages the uniform polarity of microtubules in axons to better understand the factors regulating microtubule-based motility. Peroxisome recruitment assays allow users to examine the net effect of motors and motor-regulatory proteins on organelle distribution. Together, these methods open the door to motility experiments that more fully interrogate the complex cellular environment.

NudC regulated Lis1 stability is essential for the maintenance of dynamic microtubule ends in axon terminals

In the axon terminal, microtubule stability is decreased relative to the axon shaft. The dynamic microtubule plus ends found in the axon terminal have many functions, including serving as a docking site for the Cytoplasmic dynein motor. Here, we report an unexplored function of dynein in microtubule regulation in axon terminals: regulation of microtubule stability. Using a forward genetic screen, we identified a mutant with an abnormal axon terminal structure owing to a loss of function mutation in NudC. We show that, in the axon terminal, NudC is a chaperone for the protein Lis1. Decreased Lis1 in nudc axon terminals causes dynein/dynactin accumulation and increased microtubule stability. Microtubule dynamics can be restored by pharmacologically inhibiting dynein, implicating excess dynein motor function in microtubule stabilization. Together, our data support a model in which local NudC-Lis1 modulation of the dynein motor is critical for the regulation of microtubule stability in the axon terminal.

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NudC, a dynein regulator, is crucial for axon terminal structure

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NudC mutation leads to a near complete loss of Lis1 protein in axon terminals

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Lis1 deficits cause accumulation of dynein and cargo in axon terminals

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Local elevation of dynein increases axon terminal microtubule stability

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.

Defective axonal transport of endo-lysosomes and dense core vesicles in a Drosophila model of C9-ALS/FTD

A GGGGCC (G₄ C₂ ) repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although disruptions in axonal transport are implicated in the pathogenesis of multiple neurodegenerative diseases, the underlying mechanisms causing these defects remain unclear. Here, we performed live imaging of Drosophila motor neurons expressing expanded G₄ C₂ repeats in third-instar larvae and investigated the axonal transport of multiple organelles in vivo. Expression of expanded G₄ C₂ repeats causes an increase in static axonal lysosomes, while it impairs trafficking of late endosomes (LEs) and dense core vesicles (DCVs). Surprisingly, however, axonal transport of mitochondria is unaffected in motor axons expressing expanded G₄ C₂ repeats. Thus, our data indicate that expanded G₄ C₂ repeat expression differentially impacts axonal transport of vesicular organelles and mitochondria in Drosophila models of C9orf72-associated ALS/FTD. This article is protected by copyright. All rights reserved.

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.

On and off controls within dynein-dynactin on native cargoes

The dynein-dynactin nanomachine transports cargoes along microtubules in cells. Why dynactin interacts separately with the dynein motor and also with microtubules is hotly debated. Here we disrupted these interactions in a targeted manner on phagosomes extracted from cells, followed by optical trapping to interrogate native dynein-dynactin teams on single phagosomes. Perturbing the dynactin-dynein interaction reduced dynein's on rate to microtubules. In contrast, perturbing the dynactin-microtubule interaction increased dynein's off rate markedly when dynein was generating force against the optical trap. The dynactin-microtubule link is therefore required for persistence against load, a finding of importance because disease-relevant mutations in dynein-dynactin are known to interfere with "high-load" functions of dynein in cells. Our findings call attention to a less studied property of dynein-dynactin, namely, its detachment against load, in understanding dynein dysfunction.

Increased LRRK2 kinase activity alters neuronal autophagy by disrupting the axonal transport of autophagosomes

Parkinson's disease-causing mutations in the leucine-rich repeat kinase 2 (LRRK2) gene hyperactivate LRRK2 kinase activity and cause increased phosphorylation of Rab GTPases, important regulators of intracellular trafficking. We found that the most common LRRK2 mutation, LRRK2-G2019S, dramatically reduces the processivity of autophagosome transport in neurons in a kinase-dependent manner. This effect was consistent across an overexpression model, neurons from a G2019S knockin mouse, and human induced pluripotent stem cell (iPSC)-derived neurons gene edited to express the G2019S mutation, and the effect was reversed by genetic or pharmacological inhibition of LRRK2. Furthermore, LRRK2 hyperactivation induced by overexpression of Rab29, a known activator of LRRK2 kinase, disrupted autophagosome transport to a similar extent. Mechanistically, we found that hyperactive LRRK2 recruits the motor adaptor JNK-interacting protein 4 (JIP4) to the autophagosomal membrane, inducing abnormal activation of kinesin that we propose leads to an unproductive tug of war between anterograde and retrograde motors. Disruption of autophagosome transport correlated with a significant defect in autophagosome acidification, suggesting that the observed transport deficit impairs effective degradation of autophagosomal cargo in neurons. Our results robustly link increased LRRK2 kinase activity to defects in autophagosome transport and maturation, further implicating defective autophagy in the pathogenesis of Parkinson's disease.

Evidence Linking Protein Misfolding to Quality Control in Progressive Neurodegenerative Diseases

Several proteolytic systems including ubiquitin (Ub)-proteasome system (UPS), chaperonemediated autophagy (CMA), and macroautophagy are used by the mammalian cells to remove misfolded proteins (MPs). UPS mediates degradation of most of the MPs, where Ub-conjugated substrates are deubiquitinated, unfolded, and passed through the proteasome's narrow chamber, and eventually break into smaller peptides. It has been observed that the substrates that show a specific degradation signal, the KFERQ sequence motif, can be delivered to and go through CMA-mediated degradation in lysosomes. Macroautophagy can help in the degradation of substrates that are prone to aggregation and resistant to both the CMA and UPS. In the aforesaid case, cargoes are separated into autophagosomes before lysosomal hydrolase-mediated degradation. Even though the majority of the aggregated and MPs in the human proteome can be removed via cellular protein quality control (PQC), some mutant and native proteins tend to aggregate into β-sheet-rich oligomers that exhibit resistance to all identified proteolytic processes and can, therefore, grow into extracellular plaques or inclusion bodies. Indeed, the buildup of protease-resistant aggregated and MPs is a usual process underlying various protein misfolding disorders, including neurodegenerative diseases (NDs) for example Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and prion diseases. In this article, we have focused on the contribution of PQC in the degradation of pathogenic proteins in NDs.

DCTN1 Binds to TDP-43 and Regulates TDP-43 Aggregation

A common pathological hallmark of several neurodegenerative diseases, including amyotrophic lateral sclerosis, is cytoplasmic mislocalization and aggregation of nuclear RNA-binding protein TDP-43. Perry disease, which displays inherited atypical parkinsonism, is a type of TDP-43 proteinopathy. The causative gene DCTN1 encodes the largest subunit of the dynactin complex. Dynactin associates with the microtubule-based motor cytoplasmic dynein and is required for dynein-mediated long-distance retrograde transport. Perry disease-linked missense mutations (e.g., p.G71A) reside within the CAP-Gly domain and impair the microtubule-binding abilities of DCTN1. However, molecular mechanisms by which such DCTN1 mutations cause TDP-43 proteinopathy remain unclear. We found that DCTN1 bound to TDP-43. Biochemical analysis using a panel of truncated mutants revealed that the DCTN1 CAP-Gly-basic supradomain, dynactin domain, and C-terminal region interacted with TDP-43, preferentially through its C-terminal region. Remarkably, the p.G71A mutation affected the TDP-43-interacting ability of DCTN1. Overexpression of DCTN1^G71A, the dynactin-domain fragment, or C-terminal fragment, but not the CAP-Gly-basic fragment, induced cytoplasmic mislocalization and aggregation of TDP-43, suggesting functional modularity among TDP-43-interacting domains of DCTN1. We thus identified DCTN1 as a new player in TDP-43 cytoplasmic-nuclear transport, and showed that dysregulation of DCTN1-TDP-43 interactions triggers mislocalization and aggregation of TDP-43, thus providing insights into the pathological mechanisms of Perry disease and other TDP-43 proteinopathies.

Long-distance regressive signaling in neural development and disease

Nervous system development proceeds via well-orchestrated processes involving a balance between progressive and regressive events including stabilization or elimination of axons, synapses, and even entire neurons. These progressive and regressive events are driven by functionally antagonistic signaling pathways with the dominant pathway eventually determining whether a neural element is retained or removed. Many of these developmental sculpting events are triggered by final target innervation necessitating a long-distance mode of communication. While long-distance progressive signaling has been well characterized, particularly for neurotrophic factors, there remains relatively little known about how regressive events are triggered from a distance. Here we discuss the emergent phenomenon of long-distance regressive signaling pathways. In particular, we will cover (a) progressive and regressive cues known to be employed after target innervation, (b) the mechanisms of long-distance signaling from an endosomal platform, (c) recent evidence that long-distance regressive cues emanate from platforms like death receptors or repulsive axon guidance receptors, and (d) evidence that these pathways are exploited in pathological scenarios. This article is categorized under: Nervous System Development > Vertebrates: General Principles Signaling Pathways > Global Signaling Mechanisms Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Dynein-dynactin segregate meiotic chromosomes in C. elegans spermatocytes

The microtubule motor cytoplasmic dynein 1 (dynein) and its essential activator dynactin have conserved roles in spindle assembly and positioning during female meiosis and mitosis, but their contribution to male meiosis remains poorly understood. Here, we characterize the G33S mutation in the C. elegans dynactin subunit DNC-1, which corresponds to G59S in human p150^Glued that causes motor neuron disease. In spermatocytes, dnc-1(G33S) delays spindle assembly and penetrantly inhibits anaphase spindle elongation in meiosis I, which prevents the segregation of homologous chromosomes. By contrast, chromosomes segregate without errors in the early dnc-1(G33S) embryo. Deletion of the DNC-1 N-terminus shows that defective meiosis in dnc-1(G33S) spermatocytes is not due to the inability of DNC-1 to interact with microtubules. Instead, our results suggest that the DNC-1(G33S) protein, which is aggregation prone in vitro, is less stable in spermatocytes than the early embryo, resulting in different phenotypic severity in the two dividing tissues. Thus, the dnc-1(G33S) mutant reveals that dynein-dynactin drive meiotic chromosome segregation in spermatocytes and illustrates that the extent to which protein misfolding leads to loss of function can vary significantly between cell types.

The Neglected Genes of ALS: Cytoskeletal Dynamics Impact Synaptic Degeneration in ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that selectively affects motor neurons (MNs) of the cortex, brainstem, and spinal cord. Several genes have been linked to both familial (fALS) and sporadic (sALS) cases of ALS. Among all the ALS-related genes, a group of genes known to directly affect cytoskeletal dynamics (ALS2, DCTN1, PFN1, KIF5A, NF-L, NF-H, PRPH, SPAST, and TUBA4A) is of high importance for MN health and survival, considering that MNs are large polarized cells with axons that can reach up to 1 m in length. In particular, cytoskeletal dynamics facilitate the transport of organelles and molecules across the long axonal distances within the cell, playing a key role in synapse maintenance. The majority of ALS-related genes affecting cytoskeletal dynamics were identified within the past two decades, making it a new area to explore for ALS. The purpose of this review is to provide insights into ALS-associated cytoskeletal genes and outline how recent studies have pointed towards novel pathways that might be impacted in ALS. Further studies making use of extensive analysis models to look for true hits, the newest technologies such as CRIPSR/Cas9, human induced pluripotent stem cells (iPSCs) and axon sequencing, as well as the development of more transgenic animal models could potentially help to: differentiate the variants that truly act as a primary cause of the disease from the ones that act as risk factors or disease modifiers, identify potential interactions between two or more ALS-related genes in disease onset and progression and increase our understanding of the molecular mechanisms leading to cytoskeletal defects. Altogether, this information will give us a hint on the real contribution of the cytoskeletal ALS-related genes during this lethal disease.

Cargo-Mediated Activation of Cytoplasmic Dynein in vivo

Cytoplasmic dynein-1 is a minus-end-directed microtubule motor that transports a variety of cargoes including early endosomes, late endosomes and other organelles. In many cell types, dynein accumulates at the microtubule plus end, where it interacts with its cargo to be moved toward the minus end. Dynein binds to its various cargoes via the dynactin complex and specific cargo adapters. Dynactin and some of the coiled-coil-domain-containing cargo adapters not only link dynein to cargo but also activate dynein motility, which implies that dynein is activated by its cellular cargo. Structural studies indicate that a dynein dimer switches between the autoinhibited phi state and an open state; and the binding of dynactin and a cargo adapter to the dynein tails causes the dynein motor domains to have a parallel configuration, allowing dynein to walk processively along a microtubule. Recently, the dynein regulator LIS1 has been shown to be required for dynein activation in vivo, and its mechanism of action involves preventing dynein from switching back to the autoinhibited state. In this review, we will discuss our current understanding of dynein activation and point out the gaps of knowledge on the spatial regulation of dynein in live cells. In addition, we will emphasize the importance of studying a complete set of dynein regulators for a better understanding of dynein regulation in vivo.

Mechanisms of neuronal survival safeguarded by endocytosis and autophagy

Multiple aspects of neuronal physiology crucially depend on two cellular pathways, autophagy and endocytosis. During endocytosis, extracellular components either unbound or recognized by membrane-localized receptors (termed "cargo") become internalized into plasma membrane-derived vesicles. These can serve to either recycle the material back to the plasma membrane or send it for degradation to lysosomes. Autophagy also uses lysosomes as a terminal degradation point, although instead of degrading the plasma membrane-derived cargo, autophagy eliminates detrimental cytosolic material and intracellular organelles, which are transported to lysosomes by means of double-membrane vesicles, referred to as autophagosomes. Neurons, like all non-neuronal cells, capitalize on autophagy and endocytosis to communicate with the environment and maintain protein and organelle homeostasis. Additionally, the highly polarized, post-mitotic nature of neurons made them adopt these two pathways for cell-specific functions. These include the maintenance of the synaptic vesicle pool in the pre-synaptic terminal and the long-distance transport of signaling molecules. Originally discovered independently from each other, it is now clear that autophagy and endocytosis are closely interconnected and share several common participating molecules. Considering the crucial role of autophagy and endocytosis in cell type-specific functions in neurons, it is not surprising that defects in both pathways have been linked to the pathology of numerous neurodegenerative diseases. In this review, we highlight the recent knowledge of the role of endocytosis and autophagy in neurons with a special focus on synaptic physiology and discuss how impairments in genes coding for autophagy and endocytosis proteins can cause neurodegeneration.

A Conserved Role for Vezatin Proteins in Cargo-Specific Regulation of Retrograde Axonal Transport

Active transport of organelles within axons is critical for neuronal health. Retrograde axonal transport, in particular, relays neurotrophic signals received by axon terminals to the nucleus and circulates new material among enpassant synapses. A single motor protein complex, cytoplasmic dynein, is responsible for nearly all retrograde transport within axons: its linkage to and transport of diverse cargos is achieved by cargo-specific regulators. Here, we identify Vezatin as a conserved regulator of retrograde axonal transport. Vertebrate Vezatin (Vezt) is required for the maturation and maintenance of cell-cell junctions and has not previously been implicated in axonal transport. However, a related fungal protein, VezA, has been shown to regulate retrograde transport of endosomes in hyphae. In a forward genetic screen, we identified a loss-of-function mutation in the Drosophila vezatin-like (vezl) gene. We here show that vezl loss prevents a subset of endosomes, including signaling endosomes containing activated BMP receptors, from initiating transport out of motor neuron terminal boutons. vezl loss also decreases the transport of endosomes and dense core vesicles, but not mitochondria, within axon shafts. We disrupted vezt in zebrafish and found that vezt loss specifically impairs the retrograde axonal transport of late endosomes, causing their accumulation in axon terminals. Our work establishes a conserved, cargo-specific role for Vezatin proteins in retrograde axonal transport.

Spatial control of membrane traffic in neuronal dendrites

Neuronal dendrites are highly branched and specialized compartments with distinct structures and secretory organelles (e.g., spines, Golgi outposts), and a unique cytoskeletal organization that includes microtubules of mixed polarity. Dendritic membranes are enriched with proteins, which specialize in the formation and function of the post-synaptic membrane of the neuronal synapse. How these proteins partition preferentially in dendrites, and how they traffic in a manner that is spatiotemporally accurate and regulated by synaptic activity are long-standing questions of neuronal cell biology. Recent studies have shed new insights into the spatial control of dendritic membrane traffic, revealing new classes of proteins (e.g., septins) and cytoskeleton-based mechanisms with dendrite-specific functions. Here, we review these advances by revisiting the fundamental mechanisms that control membrane traffic at the levels of protein sorting and motor-driven transport on microtubules and actin filaments. Overall, dendrites possess unique mechanisms for the spatial control of membrane traffic, which might have specialized and co-evolved with their highly arborized morphology.

Activation and Regulation of Cytoplasmic Dynein

Cytoplasmic dynein is an AAA+ motor that drives the transport of many intracellular cargoes towards the minus end of microtubules (MTs). Previous in vitro studies characterized isolated dynein as an exceptionally weak motor that moves slowly and diffuses on an MT. Recent studies altered this view by demonstrating that dynein remains in an autoinhibited conformation on its own, and processive motility is activated when it forms a ternary complex with dynactin and a cargo adaptor. This complex assembles more efficiently in the presence of Lis1, providing an explanation for why Lis1 is a required cofactor for most cytoplasmic dynein-driven processes in cells. This review describes how dynein motility is activated and regulated by cargo adaptors and accessory proteins.

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.

Conformational diversity of dynactin sidearm and domain organization of its subunit p150

Dynactin is a principal regulator of the minus-end directed microtubule motor dynein. The sidearm of dynactin is essential for binding to microtubules and regulation of dynein activity. Although our understanding of the structure of the dynactin backbone (Arp1 rod) has greatly improved recently, structural details of the sidearm subcomplex remain elusive. Here, we report the flexible nature and diverse conformations of dynactin sidearm observed by electron microscopy. Using nanogold labeling and deletion mutant analysis, we determined the domain organization of the largest subunit p150 and discovered that its coiled-coil (CC1), dynein-binding domain, adopted either a folded or an extended form. Furthermore, the entire sidearm exhibited several characteristic forms, and the equilibrium among them depended on salt concentrations. These conformational diversities of the dynactin complex provide clues to understanding how it binds to microtubules and regulates dynein.

Dynactin p150 promotes processive motility of DDB complexes by minimizing diffusional behavior of dynein

Cytoplasmic dynein is activated by forming a complex with dynactin and the adaptor protein BicD2. We used interferometric scattering (iSCAT) microscopy to track dynein-dynactin-BicD2 (DDB) complexes in vitro and developed a regression-based algorithm to classify switching between processive, diffusive, and stuck motility states. We find that DDB spends 65% of its time undergoing processive stepping, 4% undergoing 1D diffusion, and the remaining time transiently stuck to the microtubule. Although the p150 subunit was previously shown to enable dynactin diffusion along microtubules, blocking p150 enhanced the proportion of time DDB diffused and reduced the time DDB processively walked. Thus, DDB diffusive behavior most likely results from dynein switching into an inactive (diffusive) state, rather than p150 tethering the complex to the microtubule. DDB-kinesin-1 complexes, formed using a DNA adapter, moved slowly and persistently, and blocking p150 led to a 70 nm/s plus-end shift in the average velocity of the complexes, in quantitative agreement with the shift of isolated DDB into the diffusive state. The data suggest a DDB activation model in which dynactin p150 enhances dynein processivity not solely by acting as a diffusive tether that maintains microtubule association, but rather by acting as an allosteric activator that promotes a conformation of dynein optimal for processive stepping. In bidirectional cargo transport driven by the opposing activities of kinesin and dynein-dynactin-BicD2, the dynactin p150 subunit promotes retrograde transport and could serve as a target for regulators of transport.

Axonal transport: Driving synaptic function

The intracellular transport system in neurons is specialized to an extraordinary degree, enabling the delivery of critical cargo to sites in axons or dendrites that are far removed from the cell center. Vesicles formed in the cell body are actively transported by kinesin motors along axonal microtubules to presynaptic sites that can be located more than a meter away. Both growth factors and degradative vesicles carrying aged organelles or aggregated proteins take the opposite route, driven by dynein motors. Distance is not the only challenge; precise delivery of cargos to sites of need must also be accomplished. For example, localized delivery of presynaptic components to hundreds of thousands of "en passant" synapses distributed along the length of a single axon in some neuronal subtypes provides a layer of complexity that must be successfully navigated to maintain synaptic transmission. We review recent advances in the field of axonal transport, with a focus on conceptual developments, and highlight our growing quantitative understanding of neuronal trafficking and its role in maintaining the synaptic function that underlies higher cognitive processes such as learning and memory.

An alternative mechanism of early nodal clustering and myelination onset in GABAergic neurons of the central nervous system

In vertebrates, fast saltatory conduction along myelinated axons relies on the node of Ranvier. How nodes assemble on CNS neurons is not yet fully understood. We previously described that node-like clusters can form prior to myelin deposition in hippocampal GABAergic neurons and are associated with increased conduction velocity. Here, we used a live imaging approach to characterize the intrinsic mechanisms underlying the assembly of these clusters prior to myelination. We first demonstrated that their components can partially preassemble prior to membrane targeting and determined the molecular motors involved in their trafficking. We then demonstrated the key role of the protein β2Na_v for node-like clustering initiation. We further assessed the fate of these clusters when myelination proceeds. Our results shed light on the intrinsic mechanisms involved in node-like clustering prior to myelination and unravel a potential role of these clusters in node of Ranvier formation and in guiding myelination onset.

The Generation of Dynein Networks by Multi-Layered Regulation and Their Implication in Cell Division

Cytoplasmic dynein-1 (hereafter referred to as dynein) is a major microtubule-based motor critical for cell division. Dynein is essential for the formation and positioning of the mitotic spindle as well as the transport of various cargos in the cell. A striking feature of dynein is that, despite having a wide variety of functions, the catalytic subunit is coded in a single gene. To perform various cellular activities, there seem to be different types of dynein that share a common catalytic subunit. In this review, we will refer to the different kinds of dynein as “dyneins.” This review attempts to classify the mechanisms underlying the emergence of multiple dyneins into four layers. Inside a cell, multiple dyneins generated through the multi-layered regulations interact with each other to form a network of dyneins. These dynein networks may be responsible for the accurate regulation of cellular activities, including cell division. How these networks function inside a cell, with a focus on the early embryogenesis of Caenorhabditis elegans embryos, is discussed, as well as future directions for the integration of our understanding of molecular layering to understand the totality of dynein’s function in living cells.

New phenotype of DCTN1-related spectrum: early-onset dHMN plus congenital foot deformity

OBJECTIVE

To describe the clinical and genetic features of two patients with different phenotypes due to various Dynactin 1 (DCTN1) gene mutations and further explore the phenotype-genotype relationship.

METHODS

Patient 1 is a 23-year-old man with congenital foot deformity and life-long distal muscle weakness and atrophy. Patient 2 is a 48-year-old woman with adult-onset progressive weakness, lower limbs atrophy, and pyramid bundle signs. Electrophysiology test showed normal nerve conduction velocity of both patients and neurogenic changes in needle electromyography. Open sural nerve biopsy for Patient 1 showed slight loss of myelinated nerve fibers. Both patients were performed with whole-exome sequencing followed by functional study of identified variants.

RESULTS

Two mutations in DCTN1 gene were identified in Patient 1 (c.626dupC) and Patient 2 (c.3823C>T), respectively. In vitro, the wild type mostly located in cytoplasm and colocalized with α-tubulin. However, c.626dupC tended to be trapped into nuclear and the c.3823C>T formed cytoplasmic aggregates, both losing colocalization with α-tubulin. Western blotting showed a truncated mutant with less molecular weight of c.626dupC was expressed.

INTERPRETATION

We identify two novel DCTN1 mutations causing different phenotypes: (1) early-onset distal hereditary motor neuropathy plus congenital foot malformation and (2) amyotrophic lateral sclerosis, respectively. We provide the initial evidence that foot developmental deficiency probably arises from subcellular localizing abnormality of Dynactin 1, revealing DCTN1-related spectrum is still expanding.

Orchestrating mitochondria in neurons: Cytoskeleton as the conductor

Mitochondria are crucial to support synaptic activity, particularly through ATP production and Ca²⁺ homeostasis. This implies that mitochondria need to be well distributed throughout the different neuronal sub-compartments. To achieve this, a tight and precise regulation of several neuronal cytoskeleton players is necessary to transport and dock mitochondria. As post-mitotic cells, neurons are highly dependent on mitochondrial quality control mechanisms and several cytoskeleton proteins have been implicated in mitophagy. Therefore, all of these processes are orchestrated by the crosstalk between mitochondria and the neuronal cytoskeleton to form a coordinated and tuned symphony.

Cargo adaptors regulate stepping and force generation of mammalian dynein-dynactin

Cytoplasmic dynein is an ATP-driven motor that transports intracellular cargos along microtubules. Dynein adopts an inactive conformation when not attached to a cargo, and motility is activated when dynein assembles with dynactin and a cargo adaptor. It was unclear how active dynein-dynactin complexes step along microtubules and transport cargos under tension. Using single-molecule imaging, we showed that dynein-dynactin advances by taking 8 to 32-nm steps toward the microtubule minus end with frequent sideways and backward steps. Multiple dyneins collectively bear a large amount of tension because the backward stepping rate of dynein is insensitive to load. Recruitment of two dyneins to dynactin increases the force generation and the likelihood of winning against kinesin in a tug-of-war but does not directly affect velocity. Instead, velocity is determined by cargo adaptors and tail-tail interactions between two closely packed dyneins. Our results show that cargo adaptors modulate dynein motility and force generation for a wide range of cellular functions.

Enhancing the retrograde axonal transport by curcumin promotes autophagic flux in N2a/APP695swe cells

The accumulation of autophagosomes and dysfunction at the axonal terminal of neurons play crucial roles in the genesis and development of Alzheimer’s disease (AD). Abnormalities in neuron axonal transport-related proteins prevent autophagosome maturation in AD. Curcumin, a polyphenol plant compound, has been shown to exert neuroprotective effects by increasing autophagy in AD, but the underlying mechanism of its effect on autophagy axon transport remains elusive. This study investigated the effects of curcumin on autophagosome formation and axonal transport in N2a/APP695swe cells (AD cell model) as well as the mechanism underlying those effects. Curcumin treatment significantly increased the expression of Beclin1, Atg5, and Atg16L1, induced the formation of autophagosomes, and promoted autophagosome–lysosome fusion in N2a/APP695swe cells. At the same time, curcumin promoted the expression of dynein, dynactin, and BICD2 as well as their binding to form the retrograde axonal transport molecular motor complex. Moreover, curcumin also increased the expression of the scaffolding proteins Rab7- interacting lysosomal protein (RILP) and huntingtin in N2a/APP695swe cells. Taken together, our findings indicate that curcumin increases autophagic flux by promoting interactions among autophagic axonal transport-related proteins and inducing lysosome–autophagosome fusion. This study provides evidence suggesting the potential use of curcumin as a novel treatment for AD.

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.

Axonal autophagy: Mini-review for autophagy in the CNS

Neurons are long-lived and highly polarized cells that depend on autophagy to maintain cellular homeostasis. The robust, constitutive biogenesis of autophagosomes in the distal axon occurs via a conserved pathway that is required to maintain functional synapses and prevent axon degeneration. Autophagosomes are formed de novo at the axon terminal in a stepwise assembly process, engulfing mitochondrial fragments, aggregated proteins, and bulk cytosol in what appears to be a nonselective uptake mechanism. Following formation, autophagosomes fuse with late endosomes/lysosomes and then are rapidly and efficiently transported along the axon toward the soma, driven by the microtubule motor cytoplasmic dynein. Motile autophagosomes mature to autolysosomes in transit by fusing with additional late endosomes/lysosomes, arriving at the soma as fully competent degradative organelles. Misregulation of neuronal autophagy leads to axonal degeneration and synaptic destabilization, and has been implicated in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, and ALS.