Competition between motor molecules (kinesin and cytoplasmic dynein) and fibrous microtubule-associated proteins in binding to microtubules

Effects of three microtubule-associated proteins (MAP2, MAP4, and Tau) on microtubules' physical properties and neurite morphology

The physical properties of cytoskeletal microtubules have a multifaceted effect on the expression of their cellular functions. A superfamily of microtubule-associated proteins, MAP2, MAP4, and tau, promote the polymerization of microtubules, stabilize the formed microtubules, and affect the physical properties of microtubules. Here, we show differences in the effects of these three MAPs on the physical properties of microtubules. When microtubule-binding domain fragments of MAP2, tau, and three MAP4 isoforms were added to microtubules in vitro and observed by fluorescence microscopy, tau-bound microtubules showed a straighter morphology than the microtubules bound by MAP2 and the three MAP4 isoforms. Flexural rigidity was evaluated by the shape of the teardrop pattern formed when microtubules were placed in a hydrodynamic flow, revealing that tau-bound microtubules were the least flexible. When full-length MAPs fused with EGFP were expressed in human neuroblastoma (SH-SY5Y) cells, the microtubules in apical regions of protrusions expressing tau were straighter than in cells expressing MAP2 and MAP4. On the other hand, the protrusions of tau-expressing cells had the fewest branches. These results suggest that the properties of microtubules, which are regulated by MAPs, contribute to the morphogenesis of neurites.

Active Transport by Cytoplasmic Dynein Maintains the Localization of MAP2 in Developing Neurons

MAP2 has been widely used as a marker of neuronal dendrites because of its extensive restriction in the somatodendritic region of neurons. Despite that, how the precise localization of such a soluble protein is established and maintained against thermal forces and diffusion has been elusive and long remained a mystery in neuroscience. In this study, we aimed to uncover the mechanism behind how MAP2 is retained in the somatodendritic region. Using GFP-tagged MAP2 expressed in cultured hippocampal neurons, we discovered a crucial protein region responsible for the localization of MAP2, the serine/proline-rich (S/P) region. Our pulse-chase live-cell imaging revealed the slow but steady migration of MAP2 toward distal dendrites, which was not observed in a MAP2 mutant lacking the S/P region, indicating that S/P-dependent transport is vital for the proper localization of MAP2. Furthermore, our experiments using an inhibitor of cytoplasmic Dynein, ciliobrevin D, as well as Dynein knockdown, showed that cytoplasmic Dynein is involved in the transport of MAP2 in dendrites. We also found that Dynein complex binds to MAP2 through the S/P region in heterologous cells. Using mathematical modeling based on experimental data, we confirmed that an intermittent active transport mechanism is essential. Thus, we propose that the cytoplasmic Dynein recruits and transports free MAP2 toward distal dendrites, thereby maintaining the precise dendritic localization of MAP2 in neurons. Our findings shed light on the previously unknown mechanism behind MAP2 localization and provide a new direction for soluble protein trafficking research in the field of cell biology of neurons.

Choreographing the motor-driven endosomal dance

The endosomal system orchestrates the transport of lipids, proteins and nutrients across the entire cell. Along their journey, endosomes mature, change shape via fusion and fission, and communicate with other organelles. This intriguing endosomal choreography, which includes bidirectional and stop-and-go motions, is coordinated by the microtubule-based motor proteins dynein and kinesin. These motors bridge various endosomal subtypes to the microtubule tracks thanks to their cargo-binding domain interacting with endosome-associated proteins, and their motor domain interacting with microtubules and associated proteins. Together, these interactions determine the mobility of different endosomal structures. In this Review, we provide a comprehensive overview of the factors regulating the different interactions to tune the fascinating dance of endosomes along microtubules.

More than a marker: potential pathogenic functions of MAP2

Microtubule-associated protein 2 (MAP2) is the predominant cytoskeletal regulator within neuronal dendrites, abundant and specific enough to serve as a robust somatodendritic marker. It influences microtubule dynamics and microtubule/actin interactions to control neurite outgrowth and synaptic functions, similarly to the closely related MAP Tau. Though pathology of Tau has been well appreciated in the context of neurodegenerative disorders, the consequences of pathologically dysregulated MAP2 have been little explored, despite alterations in its immunoreactivity, expression, splicing and/or stability being observed in a variety of neurodegenerative and neuropsychiatric disorders including Huntington’s disease, prion disease, schizophrenia, autism, major depression and bipolar disorder. Here we review the understood structure and functions of MAP2, including in neurite outgrowth, synaptic plasticity, and regulation of protein folding/transport. We also describe known and potential mechanisms by which MAP2 can be regulated via post-translational modification. Then, we assess existing evidence of its dysregulation in various brain disorders, including from immunohistochemical and (phospho) proteomic data. We propose pathways by which MAP2 pathology could contribute to endophenotypes which characterize these disorders, giving rise to the concept of a “MAP2opathy”—a series of disorders characterized by alterations in MAP2 function.

Caspase-cleaved tau is senescence-associated and induces a toxic gain of function by putting a brake on axonal transport

The microtubule-associated protein tau plays a central role in tauopathies such as Alzheimer's disease (AD). The exact molecular mechanisms underlying tau toxicity are unclear, but aging is irrefutably the biggest risk factor. This raises the question of how cellular senescence affects the function of tau as a microtubule regulator. Here we report that the proportion of tau that is proteolytically cleaved at the caspase-3 site (TauC3) doubles in the hippocampus of senescent mice. TauC3 is also elevated in AD patients. Through quantitative live-cell imaging, we show that TauC3 has a drastically reduced dynamics of its microtubule interaction. Single-molecule tracking of tau confirmed that TauC3 has a longer residence time on axonal microtubules. The reduced dynamics of the TauC3-microtubule interaction correlated with a decreased transport of mitochondria, a reduced processivity of APP-vesicle transport and an induction of region-specific dendritic atrophy in CA1 neurons of the hippocampus. The microtubule-targeting drug Epothilone D normalized the interaction of TauC3 with microtubules and modulated the transport of APP-vesicles dependent on the presence of overexpressed human tau. The results indicate a novel toxic gain of function, in which a post-translational modification of tau changes the dynamics of the tau-microtubule interaction and thus leads to axonal transport defects and neuronal degeneration. The data also introduce microtubule-targeting drugs as pharmacological modifiers of the tau-microtubule interaction with the potential to restore the physiological interaction of pathologically altered tau with microtubules.

Aging, Osteo-Sarcopenia, and Musculoskeletal Mechano-Transduction

The decline in the mass and function of bone and muscle is an inevitable consequence of healthy aging with early onset and accelerated decline in those with chronic disease. Termed osteo-sarcopenia, this condition predisposes the decreased activity, falls, low-energy fractures, and increased risk of co-morbid disease that leads to musculoskeletal frailty. The biology of osteo-sarcopenia is most understood in the context of systemic neuro-endocrine and immune/inflammatory alterations that drive inflammation, oxidative stress, reduced autophagy, and cellular senescence in the bone and muscle. Here we integrate these concepts to our growing understanding of how bone and muscle senses, responds and adapts to mechanical load. We propose that age-related alterations in cytoskeletal mechanics alter load-sensing and mechano-transduction in bone osteocytes and muscle fibers which underscores osteo-sarcopenia. Lastly, we examine the evidence for exercise as an effective countermeasure to osteo-sarcopenia.

Mitophagy Failure in APP and Tau Overexpression Model of Alzheimer's Disease

Mitochondrial alterations and oxidative stress are common features of Alzheimer's disease brain and peripheral tissues. Moreover, mitochondrial recycling process by autophagy has been found altered in the sporadic form of the disease. However, the contribution of the main proteins involved in this pathology such as amyloid-β protein precursor (AβPP) and tau needs to be achieved. With this aim, human unmodified fibroblasts were transduced with lentivectors encoding APP and Tau and treated with CCCP to study the mitophagy process. Both AβPP and tau separately increased autophagy flux mainly by improving degradation phase. However, in the specific case of mitophagy, labeling of mitochondria by PINK1 and PARK2 to be degraded by autophagy seemed reduced, which correlates with the long-term accumulation of mitochondria. Nevertheless, the combination of tau and AβPP was necessary to cause a mitophagy functional impairment reflected in the accumulation of depolarized mitochondria labeled by PINK1. The overexpression of Tau and APP recapitulates the mitophagy failure previously found in sporadic Alzheimer's disease.

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.

Microtubules in Influenza Virus Entry and Egress

Influenza viruses are respiratory pathogens that represent a significant threat to public health, despite the large-scale implementation of vaccination programs. It is necessary to understand the detailed and complex interactions between influenza virus and its host cells in order to identify successful strategies for therapeutic intervention. During viral entry, the cellular microenvironment presents invading pathogens with a series of obstacles that must be overcome to infect permissive cells. Influenza hijacks numerous host cell proteins and associated biological pathways during its journey into the cell, responding to environmental cues in order to successfully replicate. The cellular cytoskeleton and its constituent microtubules represent a heavily exploited network during viral infection. Cytoskeletal filaments provide a dynamic scaffold for subcellular viral trafficking, as well as virus-host interactions with cellular machineries that are essential for efficient uncoating, replication, and egress. In addition, influenza virus infection results in structural changes in the microtubule network, which itself has consequences for viral replication. Microtubules, their functional roles in normal cell biology, and their exploitation by influenza viruses will be the focus of this review.

Crowd Control: Effects of Physical Crowding on Cargo Movement in Healthy and Diseased Neurons

High concentration of cytoskeletal filaments, organelles, and proteins along with the space constraints due to the axon's narrow geometry lead inevitably to intracellular physical crowding along the axon of a neuron. Local cargo movement is essential for maintaining steady cargo transport in the axon, and this may be impeded by physical crowding. Molecular motors that mediate active transport share movement mechanisms that allow them to bypass physical crowding present on microtubule tracks. Many neurodegenerative diseases, irrespective of how they are initiated, show increased physical crowding owing to the greater number of stalled organelles and structural changes associated with the cytoskeleton. Increased physical crowding may be a significant factor in slowing cargo transport to synapses, contributing to disease progression and culminating in the dying back of the neuronal process. This review explores the idea that physical crowding can impede cargo movement along the neuronal process. We examine the sources of physical crowding and strategies used by molecular motors that might enable cargo to circumvent physically crowded locations. Finally, we describe sub-cellular changes in neurodegenerative diseases that may alter physical crowding and discuss the implications of such changes on cargo movement.

Microtubules gate tau condensation to spatially regulate microtubule functions

Tau is an abundant microtubule-associated protein in neurons. Tau aggregation into insoluble fibrils is a hallmark of Alzheimer's disease and other types of dementia1, yet the physiological state of tau molecules within cells remains unclear. Using single-molecule imaging, we directly observe that the microtubule lattice regulates reversible tau self-association, leading to localized, dynamic condensation of tau molecules on the microtubule surface. Tau condensates form selectively permissible barriers, spatially regulating the activity of microtubule-severing enzymes and the movement of molecular motors through their boundaries. We propose that reversible self-association of tau molecules, gated by the microtubule lattice, is an important mechanism of the biological functions of tau, and that oligomerization of tau is a common property shared between the physiological and disease-associated forms of the molecule.

H2O2 induces nuclear transport of the receptor tyrosine kinase c-MET in breast cancer cells via a membrane-bound retrograde trafficking mechanism

Reactive oxygen species (ROS) are cellular by-products produced from metabolism and also anticancer agents, such as ionizing irradiation and chemotherapy drugs. The ROS H₂O₂ has high rates of production in cancer cells because of their rapid proliferation. ROS oxidize DNA, protein, and lipids, causing oxidative stress in cancer cells and making them vulnerable to other stresses. Therefore, cancer cell survival relies on maintaining ROS-induced stress at tolerable levels. Hepatocyte growth factor receptor (c-MET) is a receptor tyrosine kinase overexpressed in malignant cancer types, including breast cancer. Full-length c-MET triggers a signal transduction cascade from the plasma membrane that, through downstream signaling proteins, up-regulates cell proliferation and migration. Recently, c-MET was shown to interact and phosphorylate poly(ADP-ribose) polymerase 1 in the nucleus and to induce poly(ADP-ribose) polymerase inhibitor resistance. However, it remains unclear how c-MET moves from the cell membrane to the nucleus. Here, we demonstrate that H₂O₂ induces retrograde transport of membrane-associated full-length c-MET into the nucleus of human MCF10A and MCF12A or primary breast cancer cells. We further show that knocking down either coatomer protein complex subunit γ1 (COPG1) or Sec61 translocon β subunit (SEC61β) attenuates the accumulation of full-length nuclear c-MET. However, a c-MET kinase inhibitor did not block nuclear c-MET transport. Moreover, nuclear c-MET interacted with KU proteins in breast cancer cells, suggesting a role of full-length nuclear c-MET in ROS-induced DNA damage repair. We conclude that a membrane-bound retrograde vesicle transport mechanism facilitates membrane-to-nucleus transport of c-MET in breast cancer cells.

Structural insight into microtubule stabilization and kinesin inhibition by Tau family MAPs

The Tau family of microtubule-associated proteins promote microtubule stabilization or regulate microtubule-based motility. Shigematsu et al. visualized MAP4 and microtubules complexed with kinesin-1 by cryo-EM, which suggests a structural basis of microtubule stabilization and kinesin inhibition by Tau family MAPs.

The Tau family microtubule-associated proteins (MAPs) promote microtubule stabilization and regulate microtubule-based motility. They share the C-terminal microtubule-binding domain, which includes three to five tubulin-binding repeats. Different numbers of repeats formed by alternative splicing have distinct effects on the activities of these proteins, and the distribution of these variants regulates fundamental physiological phenomena in cells. In this study, using cryo-EM, we visualized the MAP4 microtubule complex with the molecular motor kinesin-1. MAP4 bound to the C-terminal domains of tubulins along the protofilaments stabilizes the longitudinal contacts of the microtubule. The strongest bond of MAP4 was found around the intertubulin–dimer interface such that MAP4 coexists on the microtubule with kinesin-1 bound to the intratubulin–dimer interface as well. MAP4, consisting of five repeats, further folds and accumulates above the intertubulin–dimer interface, interfering with kinesin-1 movement. Therefore, these cryo-EM studies reveal new insight into the structural basis of microtubule stabilization and inhibition of kinesin motility by the Tau family MAPs.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

The calcium-binding protein ALG-2 regulates protein secretion and trafficking via interactions with MISSL and MAP1B proteins

Mobilization of intracellular calcium is essential for a wide range of cellular processes, including signal transduction, apoptosis, and vesicular trafficking. Several lines of evidence have suggested that apoptosis-linked gene 2 (ALG-2, also known as PDCD6), a calcium-binding protein, acts as a calcium sensor linking calcium levels with efficient vesicular trafficking, especially at the endoplasmic reticulum (ER)-to-Golgi transport step. However, how ALG-2 regulates these processes remains largely unclear. Here, we report that MAPK1-interacting and spindle-stabilizing (MISS)-like (MISSL), a previously uncharacterized protein, interacts with ALG-2 in a calcium-dependent manner. Live-cell imaging revealed that upon a rise in intracellular calcium levels, GFP-tagged MISSL (GFP-MISSL) dynamically relocalizes in a punctate pattern and colocalizes with ALG-2. MISSL knockdown caused disorganization of the components of the ER exit site, the ER-Golgi intermediate compartment, and Golgi. Importantly, knockdown of either MISSL or ALG-2 attenuated the secretion of secreted alkaline phosphatase (SEAP), a model secreted cargo protein, with similar reductions in secretion by single- and double-protein knockdowns, suggesting that MISSL and ALG-2 act in the same pathway to regulate the secretion process. Furthermore, ALG-2 or MISSL knockdown delayed ER-to-Golgi transport of procollagen type I. We also found that ALG-2 and MISSL interact with microtubule-associated protein 1B (MAP1B) and that MAP1B knockdown reverts the reduced secretion of SEAP caused by MISSL or ALG-2 depletion. These results suggest that a change in the intracellular calcium level plays a role in regulation of the secretory pathway via interaction of ALG-2 with MISSL and MAP1B.

Neuronal Polarity: MAP2 Shifts Secretory Vesicles into High Gear for Long-Haul Transport down the Axon

Accurate control of polarized cargo trafficking is essential for neuronal function. In this issue of Neuron, Gumy et al. (2017) show that MAP2 defines a pre-axonal filtering zone and controls axonal cargo transport by influencing the activities of distinct kinesin motors.

MAP2 Defines a Pre-axonal Filtering Zone to Regulate KIF1- versus KIF5-Dependent Cargo Transport in Sensory Neurons

Polarized cargo transport is essential for neuronal function. However, the minimal basic components required for selective cargo sorting and distribution in neurons remain elusive. We found that in sensory neurons the axon initial segment is largely absent and that microtubule-associated protein 2 (MAP2) defines the cargo-filtering zone in the proximal axon. Here, MAP2 directs axonal cargo entry by coordinating the activities of molecular motors. We show that distinct kinesins differentially regulate cargo velocity: kinesin-3 drives fast axonal cargo trafficking, while kinesin-1 slows down axonal cargo transport. MAP2 inhibits "slow" kinesin-1 motor activity and allows kinesin-3 to drive robust cargo transport from the soma into the axon. In the distal axon, the inhibitory action of MAP2 decreases, leading to regained kinesin-1 activity and vesicle distribution. We propose that selective axonal cargo trafficking requires the MAP2-defined pre-axonal filtering zone and the ability of cargos to switch between distinct kinesin motor activities.

A biophysical model of how α-tubulin carboxy-terminal tails tune kinesin-1 processivity along microtubule

It appears that so-called post-translational modifications of tubulin heterodimers are mostly focussed at positions of amino acid sequences of carboxy-terminal tails. These changes have very profound effects on microtubule functions especially in connection with cellular traffic in terms of motor proteins. In this study, we elaborated the biophysical model aimed to explain the strategy governing these subtle interplays between structural and functional properties of microtubules. We relied onto Langevin equations including fluctuation-dissipation processes. In that context we found out that small interaction between a charged motor neck domain and oppositely charged carboxy-terminal tail of the α-tubulin plays the decisive role in tuning kinesin-1 motor processivity along microtubules.

Interactions between Tau and Different Conformations of Tubulin: Implications for Tau Function and Mechanism

Tau is a multifaceted neuronal protein that stabilizes microtubules (MTs), but the mechanism of this activity remains poorly understood. Questions include whether Tau binds MTs laterally or longitudinally and whether Tau's binding affinity depends on the nucleotide state of tubulin. We observed that Tau binds tightly to Dolastatin-10 tubulin rings and promotes the formation of Dolastatin-10 ring stacks, implying that Tau can crosslink MT protofilaments laterally. In addition, we found that Tau prefers GDP-like tubulin conformations, which implies that Tau binding to the MT surface is biased away from the dynamic GTP-rich MT tip. To investigate the potential impact of these Tau activities on MT stabilization, we incorporated them into our previously developed dimer-scale computational model of MT dynamics. We found that lateral crosslinking activities have a much greater effect on MT stability than do longitudinal crosslinking activities, and that introducing a bias toward GDP tubulin has little impact on the observed MT stabilization. To address the question of why Tau is GDP-tubulin-biased, we tested whether Tau might affect MT binding of the +TIP EB1. We confirmed recent reports that Tau binds directly to EB1 and that Tau competes with EB1 for MT binding. Our results lead to a conceptual model where Tau stabilizes the MT lattice by strengthening lateral interactions between protofilaments. We propose that Tau's GDP preference allows the cell to independently regulate the dynamics of the MT tip and the stability of the lattice.

Role of non-motile microtubule-associated proteins in virus trafficking

Viruses are entirely dependent on their ability to infect a host cell in order to replicate. To reach their site of replication as rapidly and efficiently as possible following cell entry, many have evolved elaborate mechanisms to hijack the cellular transport machinery to propel themselves across the cytoplasm. Long-range movements have been shown to involve motor proteins along microtubules (MTs) and direct interactions between viral proteins and dynein and/or kinesin motors have been well described. Although less well-characterized, it is also becoming increasingly clear that non-motile microtubule-associated proteins (MAPs), including structural MAPs of the MAP1 and MAP2 families, and microtubule plus-end tracking proteins (+TIPs), can also promote viral trafficking in infected cells, by mediating interaction of viruses with filaments and/or motor proteins, and modulating filament stability. Here we review our current knowledge on non-motile MAPs, their role in the regulation of cytoskeletal dynamics and in viral trafficking during the early steps of infection.

Impaired protein degradation in FTLD and related disorders

Impaired protein degradation has been discussed as a cause or consequence of various neurodegenerative diseases, such as Alzheimer's, Parkinson's and Huntington's disease. More recently, evidence accumulated that dysfunctional protein degradation may play a role in frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). Since in almost all neurodegenerative diseases, protein aggregates are disease-defining hallmarks, it is most likely that impaired protein degradation contributes to disease onset and progression. In the majority of FTD cases, the pathological protein aggregates contain either microtubuleassociated protein tau or TAR DNA-binding protein (TDP)-43. Aggregates are also positive for ubiquitin and p62/sequestosome 1 (SQSTM1) indicating that these aggregates are targeted for degradation. FTD-linked mutations in genes encoding three autophagy adaptor proteins, p62/SQSTM1, ubiquilin 2 and optineurin, indicate that impaired autophagy might cause FTD. Furthermore, the strongest evidence for lysosomal impairment in FTD is provided by the progranulin (GRN) gene, which is linked to FTD and neuronal ceroid lipofuscinosis. In this review, we summarize the observations that have been made during the last years linking the accumulation of disease-associated proteins in FTD to impaired protein degradation pathways. In addition, we take resent findings for nucleocytoplasmic transport defects of TDP-43, as discussed for hexanucleotide repeat expansions in C9orf72 into account and provide a hypothesis how the interplay of altered nuclear transport and protein degradation leads to the accumulation of protein deposits.

MAP4-regulated dynein-dependent trafficking of BTN3A1 controls the TBK1-IRF3 signaling axis

The innate immune system detects viral nucleic acids and induces type I interferon (IFN) responses. The RNA- and DNA-sensing pathways converge on the protein kinase TANK-binding kinase 1 (TBK1) and the transcription factor IFN-regulatory factor 3 (IRF3). Activation of the IFN signaling pathway is known to trigger the redistribution of key signaling molecules to punctate perinuclear structures, but the mediators of this spatiotemporal regulation have yet to be defined. Here we identify butyrophilin 3A1 (BTN3A1) as a positive regulator of nucleic acid-mediated type I IFN signaling. Depletion of BTN3A1 inhibits the cytoplasmic nucleic acid- or virus-triggered activation of IFN-β production. In the resting state, BTN3A1 is constitutively associated with TBK1. Stimulation with nucleic acids induces the redistribution of the BTN3A1-TBK1 complex to the perinuclear region, where BTN3A1 mediates the interaction between TBK1 and IRF3, leading to the phosphorylation of IRF3. Furthermore, we show that microtubule-associated protein 4 (MAP4) controls the dynein-dependent transport of BTN3A1 in response to nucleic acid stimulation, thereby identifying MAP4 as an upstream regulator of BTN3A1. Thus, the depletion of either MAP4 or BTN3A1 impairs cytosolic DNA- or RNA-mediated type I IFN responses. Our findings demonstrate a critical role for MAP4 and BTN3A1 in the spatiotemporal regulation of TBK1, a central player in the intracellular nucleic acid-sensing pathways involved in antiviral signaling.

On-chip microtubule gliding assay for parallel measurement of tau protein species

Tau protein is a well-established biomarker for a group of neurodegenerative diseases collectively called tauopathies. So far, clinically relevant detection of tau species in cerebrospinal fluid (CSF) cannot be achieved without immunological methods. Recently, it was shown that different tau isoforms including the ones carrying various types of mutations affect microtubule (MT)-kinesin binding and velocity in an isoform specific manner. Here, based on these observations, we developed a microfluidic device to analyze tau mutations, isoforms and their ratios. The assay device consists of three regions: a MT reservoir which captures MTs from a solution to a kinesin-coated surface, a microchannel which guides gliding MTs, and an arrowhead-shaped collector which concentrates MTs. Tau-bound fluorescently labeled MTs (tau-MTs) were assayed, and the increase in fluorescence intensity (FI) corresponding to the total number of MTs accumulated was measured at the collector. We show that our device is capable of differentiating 3R and 4R tau isoform ratios and effects of point mutations within 5 minutes. Furthermore, radially oriented collector regions enable simultaneous FI measurements for six independent assays. Performing parallel assays in the proposed device with minimal image processing provides a cost-efficient, easy-to-use and fast tau detection platform.

Navigating the cell: how motors overcome roadblocks and traffic jams to efficiently transport cargo

Intracellular transport plays an essential role in maintaining the organization of cells. The importance of long-range, bi-directional transport is evidenced by the fact that its failure goes hand in hand with several diseases including neurodegenerative diseases such as Alzheimer's and Amyotrophic Lateral Sclerosis. The nanoscale cellular transport machinery consisting of cytoskeletal tracks and motor-proteins is responsible for effectively delivering important materials to specific locations inside the cell. Motor-proteins manage to overcome several challenges in the crowded cellular environment to achieve well-coordinated and effective transport. In recent years, thanks to state-of-the-art single molecule biophysical tools, we have started to gain insights into the cellular traffic rules. This perspective summarizes the challenges that motors face in navigating the complex cytoskeleton and the lessons learned about transport in crowded environments from both bottom-up in vitro studies as well as top-down in vivo studies.

Transport and diffusion of Tau protein in neurons

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

MAPping out distribution routes for kinesin couriers

In the crowded environment of eukaryotic cells, diffusion is an inefficient distribution mechanism for cellular components. Long-distance active transport is required and is performed by molecular motors including kinesins. Furthermore, in highly polarised, compartmentalised and plastic cells such as neurons, regulatory mechanisms are required to ensure appropriate spatio-temporal delivery of neuronal components. The kinesin machinery has diversified into a large number of kinesin motor proteins as well as adaptor proteins that are associated with subsets of cargo. However, many mechanisms contribute to the correct delivery of these cargos to their target domains. One mechanism is through motor recognition of sub-domain-specific microtubule (MT) tracks, sign-posted by different tubulin isoforms, tubulin post-translational modifications, tubulin GTPase activity and MT-associated proteins (MAPs). With neurons as a model system, a critical review of these regulatory mechanisms is presented here, with a particular focus on the emerging contribution of compartmentalised MAPs. Overall, we conclude that - especially for axonal cargo - alterations to the MT track can influence transport, although in vivo, it is likely that multiple track-based effects act synergistically to ensure accurate cargo distribution.

Spatiotemporal resolution of BDNF neuroprotection against glutamate excitotoxicity in cultured hippocampal neurons

Brain-derived neurotrophic factor (BDNF) protects hippocampal neurons from glutamate excitotoxicity as determined by analysis of chromatin condensation, through activation of extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3-K) signaling pathways. However, it is still unknown whether BDNF also prevents the degeneration of axons and dendrites, and the functional demise of synapses, which would be required to preserve neuronal activity. Herein, we have studied the time-dependent changes in several neurobiological markers, and the regulation of proteolytic mechanisms in cultured rat hippocampal neurons, through quantitative western blot and immunocytochemistry. Calpain activation peaked immediately after the neurodegenerative input, followed by a transient increase in ubiquitin-conjugated proteins and increased abundance of cleaved-caspase-3. Proteasome and calpain inhibition did not reproduce the protective effect of BDNF and caspase inhibition in preventing chromatin condensation. However, proteasome and calpain inhibition did protect the neuronal markers for dendrites (MAP-2), axons (Neurofilament-H) and the vesicular glutamate transporters (VGLUT1-2), whereas caspase inhibition was unable to mimic the protective effect of BDNF on neurites and synaptic markers. BDNF partially prevented the downregulation of synaptic activity measured by the KCl-evoked glutamate release using a Förster (Fluorescence) resonance energy transfer (FRET) glutamate nanosensor. These results translate a time-dependent activation of proteases and spatial segregation of these mechanisms, where calpain activation is followed by proteasome deregulation, from neuronal processes to the soma, and finally by caspase activation in the cell body. Moreover, PI3-K and PLCγ small molecule inhibitors significantly blocked the protective action of BDNF, suggesting an activity-dependent mechanism of neuroprotection. Ultimately, we hypothesize that neuronal repair after a degenerative insult is initiated at the synaptic level.

Neurodegeneration as an RNA disorder

Neurodegenerative diseases constitute one of the single most important public health challenges of the coming decades, and yet we presently have only a limited understanding of the underlying genetic, cellular and molecular causes. As a result, no effective disease-modifying therapies are currently available, and no method exists to allow detection at early disease stages, and as a result diagnoses are only made decades after disease pathogenesis, by which time the majority of physical damage has already occurred. Since the sequencing of the human genome, we have come to appreciate that the transcriptional output of the human genome is extremely rich in non-protein coding RNAs (ncRNAs). This heterogeneous class of transcripts is widely expressed in the nervous system, and is likely to play many crucial roles in the development and functioning of this organ. Most exciting, evidence has recently been presented that ncRNAs play central, but hitherto unappreciated roles in neurodegenerative processes. Here, we review the diverse available evidence demonstrating involvement of ncRNAs in neurodegenerative diseases, and discuss their possible implications in the development of therapies and biomarkers for these conditions.

The light chains of microtubule-associated proteins MAP1A and MAP1B interact with α1-syntrophin in the central and peripheral nervous system

Microtubule-associated proteins of the MAP1 family (MAP1A, MAP1B, and MAP1S) share, among other features, a highly conserved COOH-terminal domain approximately 125 amino acids in length. We conducted a yeast 2-hybrid screen to search for proteins interacting with this domain and identified α1-syntrophin, a member of a multigene family of adapter proteins involved in signal transduction. We further demonstrate that the interaction between the conserved COOH-terminal 125-amino acid domain (which is located in the light chains of MAP1A, MAP1B, and MAP1S) and α1-syntrophin is direct and occurs through the pleckstrin homology domain 2 (PH2) and the postsynaptic density protein 95/disk large/zonula occludens-1 protein homology domain (PDZ) of α1-syntrophin. We confirmed the interaction of MAP1B and α1-syntrophin by co-localization of the two proteins in transfected cells and by co-immunoprecipitation experiments from mouse brain. In addition, we show that MAP1B and α1-syntrophin partially co-localize in Schwann cells of the murine sciatic nerve during postnatal development and in the adult. However, intracellular localization of α1-syntrophin and other Schwann cell proteins such as ezrin and dystrophin-related protein 2 (DRP2) and the localization of the axonal node of Ranvier-associated protein Caspr1/paranodin were not affected in MAP1B null mice. Our findings add to a growing body of evidence that classical MAPs are likely to be involved in signal transduction not only by directly modulating microtubule function, but also through their interaction with signal transduction proteins.

Bidirectional intracellular transport: utility and mechanism

Bidirectional transport of intracellular cargo along microtubule tracks is the subject of intense debate in the motility field. In the present review, we provide an overview of the models describing the possible mechanisms driving intracellular saltatory transport, taking into account current experimental results that may at first seem contradictory. We examine the phenomenon of saltatory motion, in an attempt to interpret the mechanistic debate in terms of the utility of saltatory motion.

Facilitated intracellular transport of TrkA by an interaction with nerve growth factor

Intracellular transport of neurotrophin receptors together with neurotrophins is one of the key events of neurotrophin signaling for the growth and the survival of neurons. However, the involvement of neurotrophin signaling in the regulation of intracellular transport of neurotrophin receptors has been remained unclear. We visualized the behavior of TrkA, a receptor of nerve growth factor (NGF), by labeling with GFP in PC12 cells. We found remarkable changes of the behavior of TrkA-GFP upon the application of NGF. Before the application, only ~37% of the fluorescent dots of TrkA showed translocations along neurites of PC12 cells. After the application, number of the dots showing the directional movement increased to ~65%. The averaged velocities of the directional movement of TrkA-GFP dots became higher after the application of NGF. We tested the idea whether NGF binding accelerated the translocations of TrkA by simultaneously observing TrkA-GFP and fluorescently labeled NGF, Cy3.5-NGF. The velocity of TrkA-GFP dots associated with Cy3.5-NGF was remarkably higher than that of TrkA-GFP dots without Cy3.5-NGF. On the basis of these observations, we hypothesize that there is a signaling mechanism within a single vesicle that facilitates the intracellular transport of each vesicle containing the activated TrkA.

Functional genomic screen and network analysis reveal novel modifiers of tauopathy dissociated from tau phosphorylation

A functional genetic screen using loss-of-function and gain-of-function alleles was performed to identify modifiers of tau-induced neurotoxicity using the 2N/4R (full-length) isoform of wild-type human tau expressed in the fly retina. We previously reported eye pigment mutations, which create dysfunctional lysosomes, as potent modifiers; here, we report 37 additional genes identified from ∼1900 genes screened, including the kinases shaggy/GSK-3beta, par-1/MARK, CamKI and Mekk1. Tau acts synergistically with Mekk1 and p38 to down-regulate extracellular regulated kinase activity, with a corresponding decrease in AT8 immunoreactivity (pS202/T205), suggesting that tau can participate in signaling pathways to regulate its own kinases. Modifiers showed poor correlation with tau phosphorylation (using the AT8, 12E8 and AT270 epitopes); moreover, tested suppressors of wild-type tau were equally effective in suppressing toxicity of a phosphorylation-resistant S11A tau construct, demonstrating that changes in tau phosphorylation state are not required to suppress or enhance its toxicity. Genes related to autophagy, the cell cycle, RNA-associated proteins and chromatin-binding proteins constitute a large percentage of identified modifiers. Other functional categories identified include mitochondrial proteins, lipid trafficking, Golgi proteins, kinesins and dynein and the Hsp70/Hsp90-organizing protein (Hop). Network analysis uncovered several other genes highly associated with the functional modifiers, including genes related to the PI3K, Notch, BMP/TGF-β and Hedgehog pathways, and nuclear trafficking. Activity of GSK-3β is strongly upregulated due to TDP-43 expression, and reduced GSK-3β dosage is also a common suppressor of Aβ42 and TDP-43 toxicity. These findings suggest therapeutic targets other than mitigation of tau phosphorylation.

Tau isoform-specific modulation of kinesin-driven microtubule gliding rates and trajectories as determined with tau-stabilized microtubules

We have utilized tau-assembled and tau-stabilized microtubules (MTs), in the absence of taxol, to investigate the effects of tau isoforms with three and four MT binding repeats upon kinesin-driven MT gliding. MTs were assembled in the presence of either 3-repeat tau (3R tau) or 4-repeat tau (4R tau) at tau:tubulin dimer molar ratios that approximate those found in neurons. MTs assembled with 3R tau glided at 31.1 μm/min versus 25.8 μm/min for 4R tau, a statistically significant 17% difference. Importantly, the gliding rates for either isoform did not change over a fourfold range of tau concentrations. Further, tau-assembled MTs underwent minimal dynamic instability behavior while gliding and moved with linear trajectories. In contrast, MTs assembled with taxol in the absence of tau displayed curved gliding trajectories. Interestingly, addition of 4R tau to taxol-stabilized MTs restored linear gliding, while addition of 3R tau did not. The data are consistent with the ideas that (i) 3R and 4R tau-assembled MTs possess at least some isoform-specific features that impact upon kinesin translocation, (ii) tau-assembled MTs possess different structural features than do taxol-assembled MTs, and (iii) some features of tau-assembled MTs can be masked by prior assembly by taxol. The differences in kinesin-driven gliding between 3R and 4R tau suggest important features of tau function related to the normal shift in tau isoform composition that occurs during neural development as well as in neurodegeneration caused by altered expression ratios of otherwise normal tau isoforms.

Prion protein region 23-32 interacts with tubulin and inhibits microtubule assembly

In previous studies we have demonstrated that prion protein (PrP) binds directly to tubulin and this interaction leads to the inhibition of microtubule formation by inducement of tubulin oligomerization. This report is aimed at mapping the regions of PrP and tubulin involved in the interaction and identification of PrP domains responsible for tubulin oligomerization. Preliminary studies focused our attention to the N-terminal flexible part of PrP encompassing residues 23-110. Using a panel of deletion mutants of PrP, we identified two microtubule-binding motifs at both ends of this part of the molecule. We found that residues 23-32 constitute a major site of interaction, whereas residues 101-110 represent a weak binding site. The crucial role of the 23-32 sequence in the interaction with tubulin was confirmed employing chymotryptic fragments of PrP. Surprisingly, the octarepeat region linking the above motifs plays only a supporting role in the interaction. The binding of Cu(2+) to PrP did not affect the interaction. We also demonstrate that PrP deletion mutants lacking residues 23-32 exhibit very low efficiency in the inducement of tubulin oligomerization. Moreover, a synthetic peptide corresponding to this sequence, but not that identical with fragment 101-110, mimics the effects of the full-length protein on tubulin oligomerization and microtubule assembly. At the cellular level, peptide composed of the PrP motive 23-30 and signal sequence (1-22) disrupted the microtubular cytoskeleton. Using tryptic and chymotryptic fragments of alpha- and beta-tubulin, we mapped the docking sites for PrP within the C-terminal domains constituting the outer surface of microtubule.

Roles of beta-tubulin residues Ala428 and Thr429 in microtubule formation in vivo

The C termini of beta-tubulin isotypes are regions of high sequence variability that bind to microtubule-associated proteins and motors and undergo various post-translational modifications such as polyglutamylation and polyglycylation. Crystallographic analyses have been unsuccessful in resolving tubulin C termini. Here, we used a stepwise approach to study the role of this region in microtubule assembly. We generated a series of truncation mutants of human betaI and betaIII tubulin. Transient transfection of HeLa cells with the mutants shows that mutants with deletions of up to 22 residues from betaIII and 16 from betaI can assemble normally. Interestingly, removal of the next residue (Ala(428)) results in a complete loss of microtubule formation without affecting dimer formation. C-terminal tail switching of human betaI and betaIII tubulin suggests that C-terminal tails are functionally equivalent. In short, residues outside of 1-429 of human beta-tubulins make no contribution to microtubule assembly. Ala(428), in the C-terminal sequence motif N-QQYQDA(428), lies at the end of helix H12 of beta-tubulin. We hypothesize that this residue is important for maintaining helix H12 structure. Deletion of Ala(428) may lead to unwinding of helix H12, resulting in tubulin dimers incapable of assembly. Thr(429) plays a more complex role. In the betaI isotype of tubulin, Thr(429) is not at all necessary for assembly; however, in the betaIII isotype, its presence strongly favors assembly. This result is consistent with a likely more complex function of betaIII as well as with the observation that evolutionary conservation is total for Ala(428) and frequent for Thr(429).

Tuning microtubule-based transport through filamentous MAPs: the problem of dynein

We recently proposed that regulating the single-to-multiple motor transition was a likely strategy for regulating kinesin-based transport in vivo. In this study, we use an in vitro bead assay coupled with an optical trap to investigate how this proposed regulatory mechanism affects dynein-based transport. We show that tau's regulation of kinesin function can proceed without interfering with dynein-based transport. Surprisingly, at extremely high tau levels--where kinesin cannot bind microtubules (MTs)--dynein can still contact MTs. The difference between tau's effects on kinesin- and dynein-based motility suggests that tau can be used to tune relative amounts of plus-end and minus-end-directed transport. As in the case of kinesin, we find that the 3RS isoform of tau is a more potent inhibitor of dynein binding to MTs. We show that this isoform-specific effect is not because of steric interference of tau's projection domains but rather because of tau's interactions with the motor at the MT surface. Nonetheless, we do observe a modest steric interference effect of tau away from the MT and discuss the potential implications of this for molecular motor structure.

Impairment of spatial learning and memory in ELKL Motif Kinase1 (EMK1/MARK2) knockout mice

The hyperphosphorylation of tau protein is one of the hallmarks of Alzheimer's disease (AD) and of the associated cognitive decline. EMK1 (MARK2) is a serine/threonine kinase which phosphorylates tau and MAP2. An involvement of this kinase in memory functions is not established. We used a behavioral approach to study the phenotype of EMK1-null mice (EMK1-KO) as a possible model of MAP2/tau altered phophorylation. Compared to wild type mice, EMK1-KO mice did not differ in non-cognitive aspects of behavior, such as locomotion in activity cages, or anxiety in the elevated plus maze. However, they exhibited lower performance in the first stage of acquisition of a hippocampal-dependent spatial learning, as assessed in a radial water maze, although, they acquired the task with repeated training. They were again found to be impaired on re-learning a new platform position. In addition, they exhibited poor long-term retention performance. These data underline the importance on both early memory processes and long-term retrieval, of the dynamic instability of microtubules generated by the phosphorylation of MAPs.

Tobacco mosaic virus movement protein functions as a structural microtubule-associated protein

The cell-to-cell spread of Tobacco mosaic virus infection depends on virus-encoded movement protein (MP), which is believed to form a ribonucleoprotein complex with viral RNA (vRNA) and to participate in the intercellular spread of infectious particles through plasmodesmata. Previous studies in our laboratory have provided evidence that the vRNA movement process is correlated with the ability of the MP to interact with microtubules, although the exact role of this interaction during infection is not known. Here, we have used a variety of in vivo and in vitro assays to determine that the MP functions as a genuine microtubule-associated protein that binds microtubules directly and modulates microtubule stability. We demonstrate that, unlike MP in whole-cell extract, microtubule-associated MP is not ubiquitinated, which strongly argues against the hypothesis that microtubules target the MP for degradation. In addition, we found that MP interferes with kinesin motor activity in vitro, suggesting that microtubule-associated MP may interfere with kinesin-driven transport processes during infection.

Immunohistochemical and microarray analyses of a mouse model for the smith-lemli-opitz syndrome

The Smith-Lemli-Opitz syndrome is a mental retardation/malformation syndrome with behavioral components of autism. It is caused by a deficiency in 3beta-hydroxysteroid-Delta7-reductase (DHCR7), the enzyme required for the terminal enzymatic step of cholesterol biosynthesis. The availability of Smith-Lemli-Opitz syndrome mouse models has made it possible to investigate the genesis of the malformations associated with this syndrome. Dhcr7 gene modification (Dhcr7-/-) results in neonatal lethality and multiple organ system malformations. Pathology includes cleft palate, pulmonary hypoplasia, cyanosis, impaired cortical response to glutamate, and hypermorphic development of hindbrain serotonergic neurons. For the current study, hindbrain regions microdissected from gestational day 14 Dhcr7-/-, Dhcr7+/- and Dhcr7+/+ fetuses were processed for expression profiling analyses using Affymetrix oligonucleotide arrays and filtered using statistical significance (S-score) of change in gene expression. Of the 12,000 genes analyzed, 91 were upregulated and 98 were downregulated in the Dhcr7-/- hindbrains when compared to wild-type animals. Fewer affected genes, representing a reduced affect on these pathways, were identified in heterozygous animals. Hierarchical clustering identified altered expression of genes associated with cholesterol homeostasis, cell cycle control and apoptosis, neurodifferentiation and embryogenesis, transcription and translation, cellular transport, neurodegeneration, and neuronal cytoskeleton. Of particular interest, Dhcr7 gene modification elicited dynamic changes in genes involved in axonal guidance. In support of the microarray findings, immunohistochemical analyses of the netrin/deleted in colorectal cancer axon guidance pathway illustrated midline commissural deficiencies and hippocampal pathfinding errors in Dhcr7-/- mice. The results of these studies aid in providing insight into the genesis of human cholesterol-related birth defects and neurodevelopmental disorders and highlight specific areas for future investigation.

Gigaxonin-controlled degradation of MAP1B light chain is critical to neuronal survival

Giant axonal neuropathy (GAN) is a devastating sensory and motor neuropathy caused by mutations in the GAN gene, which encodes the ubiquitously expressed protein gigaxonin. Cytopathological features of GAN include axonal degeneration, with accumulation and aggregation of cytoskeletal components. Little is currently known about the molecular mechanisms underlying this recessive disorder. Here we show that gigaxonin controls protein degradation, and is essential for neuronal function and survival. We present evidence that gigaxonin binds to the ubiquitin-activating enzyme E1 through its amino-terminal BTB domain, while the carboxy-terminal kelch repeat domain interacts directly with the light chain (LC) of microtubule-associated protein 1B (MAP1B). Overexpression of gigaxonin leads to enhanced degradation of MAP1B-LC, which can be antagonized by proteasome inhibitors. Ablation of gigaxonin causes a substantial accumulation of MAP1B-LC in GAN-null neurons. Moreover, we show that overexpression of MAP1B in wild-type cortical neurons leads to cell death characteristic of GAN-null neurons, whereas reducing MAP1B levels significantly improves the survival rate of null neurons. Our results identify gigaxonin as a ubiquitin scaffolding protein that controls MAP1B-LC degradation, and provide insight into the molecular mechanisms underlying human neurodegenerative disorders.

MARK/PAR1 kinase is a regulator of microtubule-dependent transport in axons

Microtubule-dependent transport of vesicles and organelles appears saltatory because particles switch between periods of rest, random Brownian motion, and active transport. The transport can be regulated through motor proteins, cargo adaptors, or microtubule tracks. We report here a mechanism whereby microtubule associated proteins (MAPs) represent obstacles to motors which can be regulated by microtubule affinity regulating kinase (MARK)/Par-1, a family of kinases that is known for its involvement in establishing cell polarity and in phosphorylating tau protein during Alzheimer neurodegeneration. Expression of MARK causes the phosphorylation of MAPs at their KXGS motifs, thereby detaching MAPs from the microtubules and thus facilitating the transport of particles. This occurs without impairing the intrinsic activity of motors because the velocity during active movement remains unchanged. In primary retinal ganglion cells, transfection with tau leads to the inhibition of axonal transport of mitochondria, APP vesicles, and other cell components which leads to starvation of axons and vulnerability against stress. This transport inhibition can be rescued by phosphorylating tau with MARK.

Dynein and kinesin share an overlapping microtubule-binding site

Dyneins and kinesins move in opposite directions on microtubules. The question of how the same-track microtubules are able to support movement in two directions remains unanswered due to the absence of details on dynein-microtubule interactions. To address this issue, we studied dynein-microtubule interactions using the tip of the microtubule-binding stalk, the dynein stalk head (DSH), which directly interacts with microtubules upon receiving conformational change from the ATPase domain. Biochemical and cryo-electron microscopic studies revealed that DSH bound to tubulin dimers with a periodicity of 80 A, corresponding to the step size of dyneins. The DSH molecule was observed as a globular corn grain-like shape that bound the same region as kinesin. Biochemical crosslinking experiments and image analyses of the DSH-kinesin head-microtubule complex revealed competition between DSH and the kinesin head for microtubule binding. Our results demonstrate that dynein and kinesin share an overlapping microtubule-binding site, and imply that binding at this site has an essential role for these motor proteins.

Microtubule-associated protein 2 (MAP2) expression during synaptic plasticity in the guinea pig cochlea

The expression of different isoforms of microtubule-associated proteins 2 (MAP2), including the low molecular weight form MAP2c present mainly in developing neurons, was investigated in the primary auditory neurons after alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) perfusion in the guinea pig cochlea. MAP2 expression appeared to be tightly regulated in the repairing neurons. Neurite regrowth seems to involve the MAP2c isoform. In cochlear neurons, mechanisms involved in the period of development might be reactivated after excitotoxic injury in the mature cochlea.

Discodermolide interferes with the binding of tau protein to microtubules

We investigated whether discodermolide, a novel antimitotic agent, affects the binding to microtubules of tau protein repeat motifs. Like taxol, the new drug reduces the proportion of tau that pellets with microtubules. Despite their differing structures, discodermolide, taxol and tau repeats all bind to a site on beta-tubulin that lies within the microtubule lumen and is crucial in controlling microtubule assembly. Low concentrations of tau still bind strongly to the outer surfaces of preformed microtubules when the acidic C-terminal regions of at least six tubulin dimers are available for interaction with each tau molecule; otherwise binding is very weak.

Coexpression of GSK-3beta corrects phenotypic aberrations of dorsal root ganglion cells, cultured from adult transgenic mice overexpressing human protein tau

Coexpression of constitutively active GSK-3beta[S9A] rescued the axonal pathology induced by overexpression of human tau in transgenic mice (Spittaels et al., (2000) J. Biol. Chem. 275, 41340-41349). We isolated dorsal root ganglion (DRG) neuronal cultures from adult tau4R- and tau4R x GSK-3beta-transgenic mice to define the mechanisms at the cellular and subcellular level. DRG from tau4R-transgenics showed a reduced sprouting capacity while density and stability of microtubules in the axonal processes were significantly increased. Video-enhanced contrast microscopy demonstrated a dramatic inhibition of fast axonal transport. Coexpression of GSK-3beta increased tau phosphorylation and reversed the effects on microtubule stability and saltatory motion. In DRG from GSK-3beta single transgenics, increased tau phosphorylation was evident without any major effects on microtubule stability or axonal transport. These observations support the hypothesis that excess tau competed with motor-proteins for binding to microtubules and/or that a rigid microtubular system inhibits axonal transport.

The role of microtubules in guard cell function

Guard cells are able to sense a multitude of environmental signals and appropriately adjust the stomatal pore to regulate gas exchange in and out of the leaf. The role of the microtubule cytoskeleton during these stomatal movements has been debated. To help resolve this debate, in vivo stomatal aperture assays with different microtubule inhibitors were performed. We observed that guard cells expressing the microtubule-binding green fluorescent fusion protein (green fluorescent protein::microtubule binding domain) fail to open for all major environmental triggers of stomatal opening. Furthermore, guard cells treated with the anti-microtubule drugs, propyzamide, oryzalin, and trifluralin also failed to open under the same environmental conditions. The inhibitory conditions caused by green fluorescent protein::microtubule binding domain and these anti-microtubule drugs could be reversed using the proton pump activator, fusicoccin. Therefore, we conclude that microtubules are involved in an upstream event prior to the ionic fluxes leading to stomatal opening. In a mechanistic manner, evidence is presented to implicate a microtubule-associated protein in this putative microtubule-based signal transduction event.