Microtubule-associated protein 2c reorganizes both microtubules and microfilaments into distinct cytological structures in an actin-binding protein-280-deficient melanoma cell line

Chemical Genetic Identification of PKC Epsilon Substrates in Mouse Brain

PKC epsilon (PKCε) plays important roles in behavioral responses to alcohol and in anxiety-like behavior in rodents, making it a potential drug target for reducing alcohol consumption and anxiety. Identifying signals downstream of PKCε could reveal additional targets and strategies for interfering with PKCε signaling. We used a chemical genetic screen combined with mass spectrometry to identify direct substrates of PKCε in mouse brain and validated findings for 39 of them using peptide arrays and in vitro kinase assays. Prioritizing substrates with several public databases such as LINCS-L1000, STRING, GeneFriends, and GeneMAINA predicted interactions between these putative substrates and PKCε and identified substrates associated with alcohol-related behaviors, actions of benzodiazepines, and chronic stress. The 39 substrates could be broadly classified in three functional categories: cytoskeletal regulation, morphogenesis, and synaptic function. These results provide a list of brain PKCε substrates, many of which are novel, for future investigation to determine the role of PKCε signaling in alcohol responses, anxiety, responses to stress, and other related behaviors.

Direct and Indirect Effects of Filamin A on Tau Pathology in Neuronal Cells

In Alzheimer disease (AD), Tau, an axonal microtubule-associated protein, becomes hyperphosphorylated, detaches from microtubules, accumulates, and self-aggregates in the somatodendritic (SD) compartment. The accumulation of hyperphosphorylated and aggregated Tau is also seen in other neurodegenerative diseases such as frontotemporal lobar degeneration (FTLD-Tau). Previous studies reported a link between filamin A (FLNA), an actin-binding protein found in the SD compartment, and Tau pathology. In the present study, we further explored this link. We confirmed the interaction of Tau with FLNA in neuroblastoma 2a (N2a) cells. This interaction was mediated by a domain located between the 157 and 383 amino acids (a.a.) of Tau. Our results also revealed that the overexpression of FLNA resulted in an intracellular accumulation of wild-type Tau and Tau mutants (P301L, V337M, and R406W) in N2a cells. Tau phosphorylation and cleavage by caspase-3 but not its aggregation were increased upon FLNA overexpression in N2a cells. In the parietal cortex of AD brain, insoluble FLNA was increased compared to control brain, but it did not correlate with Tau pathology. Interestingly, Tau binding to microtubules and F-actin was preserved upon FLNA overexpression in N2a cells. Lastly, our results revealed that FLNA also induced the accumulation of annexin A2, a Tau interacting partner involved in its axonal localization. Collectively, our data indicated that in Tauopathies, FLNA could contribute to Tau pathology by acting on Tau and annexin A2.

Intrinsically Disordered Proteins: An Overview

Many proteins and protein segments cannot attain a single stable three-dimensional structure under physiological conditions; instead, they adopt multiple interconverting conformational states. Such intrinsically disordered proteins or protein segments are highly abundant across proteomes, and are involved in various effector functions. This review focuses on different aspects of disordered proteins and disordered protein regions, which form the basis of the so-called "Disorder-function paradigm" of proteins. Additionally, various experimental approaches and computational tools used for characterizing disordered regions in proteins are discussed. Finally, the role of disordered proteins in diseases and their utility as potential drug targets are explored.

Super-multiplexing excitation spectral microscopy with multiple fluorescence bands

Fluorescence microscopy, with high molecular specificity and selectivity, is a valuable tool for studying complex biological systems and processes. However, the ability to distinguish a large number of distinct subcellular structures in a single sample is impeded by the broad spectra of molecular fluorescence. We have recently shown that excitation spectral microscopy provides a powerful means to unmix up to six fluorophores in a single fluorescence band. Here, by working with multiple fluorescence bands, we extend this approach to the simultaneous imaging of up to ten targets, with the potential for further expansions. By covering the excitation/emission bandwidth across the full visible range, an ultra-broad 24-wavelength excitation scheme is established through frame-synchronized scanning of the excitation wavelength from a white lamp via an acousto-optic tunable filter (AOTF), so that full-frame excitation-spectral images are obtained every 24 camera frames, offering superior spectral information and multiplexing capability. With numerical simulations, we validate the concurrent imaging of 10 fluorophores spanning the visible range to achieve exceptionally low (∼0.5%) crosstalks. For cell imaging experiments, we demonstrate unambiguous identification of up to eight different intracellular structures labeled by common fluorophores of substantial spectral overlap with minimal color crosstalks. We thus showcase an easy-to-implement, cost-effective microscopy system for visualizing complex cellular components with more colors and lower crosstalks.

Knock-Down of CD24 in Astrocytes Aggravates Oxyhemoglobin-Induced Hippocampal Neuron Impairment

Subarachnoid hemorrhage (SAH), as one of the most severe hemorrhagic strokes, is closely related to neuronal damage. Neurogenesis is a promising therapy, however, reliable targets are currently lacking. Increasing evidence has indicated that CD24 is associated with the growth of hippocampal neurons and the regulation of neural stem/precursor cell proliferation. To investigate the potential effect of CD24 in astrocytes on neuron growth in the hippocampus, we used a Transwell co-culture system of hippocampal astrocytes and neurons, and oxyhemoglobin (OxyHb) was added to the culture medium to mimic SAH in vitro. A specific lentivirus was used to knock down CD24 expression in astrocytes, which was verified by western blot, quantitative real-time polymerase chain reaction, and immunofluorescent staining. Astrocyte activation, neurite elongation, neuronal apoptosis, and cell viability were also assessed. We first determined the augmented expression level of CD24 in hippocampal astrocytes after SAH. A similar result was observed in cultured astrocytes exposed to OxyHb, and a corresponding change in SHP2/ERK was also noticed. CD24 in astrocytes was then downregulated by the lentivirus, which led to the impairment of axons and dendrites on the co-cultured neurons. Aggravated neuronal apoptosis was induced by the CD24 downregulation in astrocytes, which might be a result of a lower level of brain derived neurotrophic factor (BDNF). In conclusion, the knock-down of CD24 in astrocytes suppressed hippocampal neuron growth, in which the SHP2-ERK signaling pathway and BNDF were possibly involved.

An acetylation mimicking mutation, K274Q, in tau imparts neurotoxicity by enhancing tau aggregation and inhibiting tubulin polymerization

In Alzheimer's disease, tau is predominantly acetylated at K174, K274, K280, and K281 residues. The acetylation of K274-tau is linked with memory loss and dementia. In this study, we have examined the molecular mechanism of the toxicity of acetylated K274-tau. We incorporated an acetylation mimicking mutation at K274 (K→Q) residue of tau. The mutation (K274Q) strongly reduced the ability of tau to bind to tubulin and also to polymerize tubulin while K274R mutation did not reduce the ability of tau either to bind or polymerize tubulin. In addition, K274Q-tau displayed a higher aggregation propensity than wild-type tau as evident from thioflavin S fluorescence, tryptophan fluorescence, and electron microscopic images. Furthermore, dynamic light scattering, atomic force microscopy, and dot blot analysis using an oligomer-specific antibody suggested that K274Q mutation enhanced the oligomerization of tau. The K274Q mutation also strongly decreased the critical concentration for the liquid-liquid phase separation of tau. The oligomeric forms of K274Q-tau were found to be more toxic than wild tau to neuroblastoma cells. Using circular dichroism and fluorescence spectroscopy, we provide evidence indicating that the acetylation mimicking mutation (K274Q) induced conformational changes in tau. The results suggested that the acetylation of tau at 274 residues can increase tau aggregation and enhance the cytotoxicity of tau oligomers.

Regulation of Tau Homeostasis and Toxicity by Acetylation

Multiple neurodegenerative conditions including Alzheimer's disease and frontotemporal dementia are characterized by the accumulation of tau in the brain, associated with synapse loss and cognitive decline. Currently, the molecular events that lead to tau aggregation, and the pathological effects of the tau protein, are incompletely understood. Recent work has highlighted aberrant acetylation of tau as a key to understanding the pathophysiological roles of this protein. Specific acetylation sites regulate the formation of tau aggregates, synaptic signaling and long-term potentiation. Unraveling the details of this emerging story may offer novel insights into potential therapeutic approaches for devastating neurodegenerative diseases.

In-Cell NMR Study of Tau and MARK2 Phosphorylated Tau

The intrinsically disordered protein, Tau, is abundant in neurons and contributes to the regulation of the microtubule (MT) and actin network, while its intracellular abnormal aggregation is closely associated with Alzheimer's disease. Here, using in-cell Nuclear Magnetic Resonance (NMR) spectroscopy, we investigated the conformations of two different isoforms of Tau, Tau40 and k19, in mammalian cells. Combined with immunofluorescence imaging and western blot analyses, we found that the isotope-enriched Tau, which was delivered into the cultured mammalian cells by electroporation, is partially colocalized with MT and actin filaments (F-actin). We acquired the NMR spectrum of Tau in human embryonic kidney 293 (HEK-293T) cells, and compared it with the NMR spectra of Tau added with MT, F-actin, and a variety of crowding agents, respectively. We found that the NMR spectrum of Tau in complex with MT best recapitulates the in-cell NMR spectrum of Tau, suggesting that Tau predominantly binds to MT at its MT-binding repeats in HEK-293T cells. Moreover, we found that disease-associated phosphorylation of Tau was immediately eliminated once phosphorylated Tau was delivered into HEK-293T cells, implying a potential cellular protection mechanism under stressful conditions. Collectively, the results of our study reveal that Tau utilizes its MT-binding repeats to bind MT in mammalian cells and highlight the potential of using in-cell NMR to study protein structures at the residue level in mammalian cells.

Multivalent cross-linking of actin filaments and microtubules through the microtubule-associated protein Tau

Microtubule-associated proteins regulate microtubule dynamics, bundle actin filaments, and cross-link actin filaments with microtubules. In addition, aberrant interaction of the microtubule-associated protein Tau with filamentous actin is connected to synaptic impairment in Alzheimer’s disease. Here we provide insight into the nature of interaction between Tau and actin filaments. We show that Tau uses several short helical segments to bind in a dynamic, multivalent process to the hydrophobic pocket between subdomains 1 and 3 of actin. Although a single Tau helix is sufficient to bind to filamentous actin, at least two, flexibly linked helices are required for actin bundling. In agreement with a structural model of Tau repeat sequences in complex with actin filaments, phosphorylation at serine 262 attenuates binding of Tau to filamentous actin. Taken together the data demonstrate that bundling of filamentous actin and cross-linking of the cellular cytoskeleton depend on the metamorphic and multivalent nature of microtubule-associated proteins.

The microtubule associated protein Tau also interacts with filamentous actin. Here the authors combine biophysical experiments and NMR studies to characterize the structural changes that occur in Tau upon binding to filamentous actin and show that phosphorylation of serine 262 attenuates actin binding of Tau.

Haplotype-specific MAPT exon 3 expression regulated by common intronic polymorphisms associated with Parkinsonian disorders

BACKGROUND

Genome wide association studies have identified microtubule associated protein tau (MAPT) H1 haplotype single nucleotide polymorphisms (SNPs) as leading common risk variants for Parkinson's disease, progressive supranuclear palsy and corticobasal degeneration. The MAPT risk variants fall within a large 1.8 Mb region of high linkage disequilibrium, making it difficult to discern the functionally important risk variants. Here, we leverage the strong haplotype-specific expression of MAPT exon 3 to investigate the functionality of SNPs that fall within this H1 haplotype region of linkage disequilibrium.

METHODS

In this study, we dissect the molecular mechanisms by which haplotype-specific SNPs confer allele-specific effects on the alternative splicing of MAPT exon 3. Firstly, we use haplotype-hybrid whole-locus genomic MAPT vectors studies to identify functional SNPs. Next, we characterise the RNA-protein interactions at two loci by mass spectrometry. Lastly, we knockdown candidate splice factors to determine their effect on MAPT exon 3 using a novel allele-specific qPCR assay.

RESULTS

Using whole-locus genomic DNA expression vectors to express MAPT haplotype variants, we demonstrate that rs17651213 regulates exon 3 inclusion in a haplotype-specific manner. We further investigated the functionality of this region using RNA-electrophoretic mobility shift assays to show differential RNA-protein complex formation at the H1 and H2 sequence variants of SNP rs17651213 and rs1800547 and subsequently identified candidate trans-acting splicing factors interacting with these functional SNPs sequences by RNA-protein pull-down experiment and mass spectrometry. Finally, gene knockdown of candidate splice factors identified by mass spectrometry demonstrate a role for hnRNP F and hnRNP Q in the haplotype-specific regulation of exon 3 inclusion.

CONCLUSIONS

We identified common splice factors hnRNP F and hnRNP Q regulating the haplotype-specific splicing of MAPT exon 3 through intronic variants rs1800547 and rs17651213. This work demonstrates an integrated approach to characterise the functionality of risk variants in large regions of linkage disequilibrium.

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.

Acetylated tau in Alzheimer's disease: An instigator of synaptic dysfunction underlying memory loss: Increased levels of acetylated tau blocks the postsynaptic signaling required for plasticity and promotes memory deficits associated with tauopathy

Pathogenesis in tauopathies involves the accumulation of tau in the brain and progressive synapse loss accompanied by cognitive decline. Pathological tau is found at synapses, and it promotes synaptic dysfunction and memory deficits. The specific role of toxic tau in disrupting the molecular networks that regulate synaptic strength has been elusive. A novel mechanistic link between tau toxicity and synaptic plasticity involves the acetylation of two lysines on tau, K274, and K281, which are associated with dementia in Alzheimer's disease (AD). We propose that an increase in tau acetylated on these lysines blocks the expression of long-term potentiation at hippocampal synapses leading to impaired memory in AD. Acetylated tau could inhibit the activity-dependent recruitment of postsynaptic AMPA-type glutamate receptors required for plasticity by interfering with the postsynaptic localization of KIBRA, a memory-associated protein. Strategies that reduce the acetylation of tau may lead to effective treatments for cognitive decline in AD.

Dephosphorylation of MAP2D enhances its binding to vimentin in preovulatory ovarian granulosa cells

Preovulatory granulosa cells express the low-molecular-mass MAP2D variant of microtubule-associated protein 2 (MAP2). Activation of the luteinizing hormone choriogonadotropin receptor by human choriogonadotropin (hCG) promotes dephosphorylation of MAP2D on Thr256 and Thr259. We sought to evaluate the association of MAP2D with the cytoskeleton, and the effect of hCG on this association. MAP2D partially colocalized, as assessed by confocal immunofluorescence microscopy, with the vimentin intermediate filament and microtubule cytoskeletons in naive cells. In vitro binding studies showed that MAP2D bound directly to vimentin and β-tubulin. Phosphorylation of recombinant MAP2D on Thr256 and Thr259, which mimics the phosphorylation status of MAP2D in naive cells, reduces binding of MAP2D to vimentin and tubulin by two- and three-fold, respectively. PKA-dependent phosphorylation of vimentin (Ser32 and Ser38) promoted binding of vimentin to MAP2D and increased contraction of granulosa cells with reorganization of vimentin filaments and MAP2D from the periphery into a thickened layer surrounding the nucleus and into prominent cellular extensions. Chemical disruption of vimentin filament organization increased progesterone production. Taken together, these results suggest that hCG-stimulated dephosphorylation of MAP2D at Thr256 and Thr259, phosphorylation of vimentin at Ser38 and Ser72, and the resulting enhanced binding of MAP2D to vimentin might contribute to the progesterone synthetic response required for ovulation.

Acetylated Tau Obstructs KIBRA-Mediated Signaling in Synaptic Plasticity and Promotes Tauopathy-Related Memory Loss

Tau toxicity has been implicated in the emergence of synaptic dysfunction in Alzheimer's disease (AD), but the mechanism by which tau alters synapse physiology and leads to cognitive decline is unclear. Here we report abnormal acetylation of K274 and K281 on tau, identified in AD brains, promotes memory loss and disrupts synaptic plasticity by reducing postsynaptic KIdney/BRAin (KIBRA) protein, a memory-associated protein. Transgenic mice expressing human tau with lysine-to-glutamine mutations to mimic K274 and K281 acetylation (tauKQ) exhibit AD-related memory deficits and impaired hippocampal long-term potentiation (LTP). TauKQ reduces synaptic KIBRA levels and disrupts activity-induced postsynaptic actin remodeling and AMPA receptor insertion. The LTP deficit was rescued by promoting actin polymerization or by KIBRA expression. In AD patients with dementia, we found enhanced tau acetylation is linked to loss of KIBRA. These findings suggest a novel mechanism by which pathogenic tau causes synaptic dysfunction and cognitive decline in AD pathogenesis.

Microtubule-associated proteins as direct crosslinkers of actin filaments and microtubules

The cytoskeletal polymers--actin, microtubules, and intermediate filaments--are interlinked by coordinated protein interactions to form a complex three-dimensional cytoskeletal network. Association of actin filaments with microtubules is important for various cellular processes such as cell division, migration, vesicle and organelle transport, and axonal growth. Several proteins including signaling molecules, motor proteins, and proteins directly or indirectly associated with microtubules and actin are involved in bridging the cytoskeletal components. Microtubule-associated proteins (MAPs) belonging to the MAP1, 2, 4 family and Tau proteins have been identified as key players that directly crosslink the two cytoskeletons. This review summarizes the current understanding of the interactions of these MAPs with actin filaments and their role in forming the actin-microtubule network and further discusses how the in vitro reconstitution assays can be used to study the dynamics of coordinated networks. Understanding the mechanisms by which actin and microtubules interact is key to decipher cancer, wound healing, and neuronal regeneration.

The cockayne syndrome B protein is essential for neuronal differentiation and neuritogenesis

Cockayne syndrome (CS) is a progressive developmental and neurodegenerative disorder resulting in premature death at childhood and cells derived from CS patients display DNA repair and transcriptional defects. CS is caused by mutations in csa and csb genes, and patients with csb mutation are more prevalent. A hallmark feature of CSB patients is neurodegeneration but the precise molecular cause for this defect remains enigmatic. Further, it is not clear whether the neurodegenerative condition is due to loss of CSB-mediated functions in adult neurogenesis. In this study, we examined the role of CSB in neurogenesis by using the human neural progenitor cells that have self-renewal and differentiation capabilities. In this model system, stable CSB knockdown dramatically reduced the differentiation potential of human neural progenitor cells revealing a key role for CSB in neurogenesis. Neurite outgrowth, a characteristic feature of differentiated neurons, was also greatly abolished in CSB-suppressed cells. In corroboration with this, expression of MAP2 (microtubule-associated protein 2), a crucial player in neuritogenesis, was also impaired in CSB-suppressed cells. Consistent with reduced MAP2 expression in CSB-depleted neural cells, tandem affinity purification and chromatin immunoprecipitation studies revealed a potential role for CSB in the assembly of transcription complex on MAP2 promoter. Altogether, our data led us to conclude that CSB has a crucial role in coordinated regulation of transcription and chromatin remodeling activities that are required during neurogenesis.

Polyelectrolyte properties of filamentous biopolymers and their consequences in biological fluids

Anionic polyelectrolyte filaments are common in biological cells. DNA, RNA, the cytoskeletal filaments F-actin, microtubules, and intermediate filaments, and polysaccharides such as hyaluronan that form the pericellular matrix all have large net negative charge densities distributed over their surfaces. Several filamentous viruses with diameters and stiffnesses similar to those of cytoskeletal polymers also have similar negative charge densities. Extracellular protein filaments such collagen, fibrin and elastin, in contrast, have notably smaller charge densities and do not behave as highly charged polyelectrolytes in solution. This review summarizes data that demonstrate generic counterion-mediated effects on four structurally unrelated biopolymers of similar charge density: F-actin, vimentin, Pf1 virus, and DNA, and explores the possible biological and pathophysiological consequences of the polyelectrolyte properties of biological filaments.

Dia-interacting protein (DIP) imposes migratory plasticity in mDia2-dependent tumor cells in three-dimensional matrices

Tumor cells rely upon membrane pliancy to escape primary lesions and invade secondary metastatic sites. This process relies upon localized assembly and disassembly cycles of F-actin that support and underlie the plasma membrane. Dynamic actin generates both spear-like and bleb structures respectively characterizing mesenchymal and amoeboid motility programs utilized by metastatic cells in three-dimensional matrices. The molecular mechanism and physiological trigger(s) driving membrane plasticity are poorly understood. mDia formins are F-actin assembly factors directing membrane pliancy in motile cells. mDia2 is functionally coupled with its binding partner DIP, regulating cortical actin and inducing membrane blebbing in amoeboid cells. Here we show that mDia2 and DIP co-tether to nascent blebs and this linkage is required for bleb formation. DIP controls mesenchymal/amoeboid cell interconvertability, while CXCL12 induces assembly of mDia2:DIP complexes to bleb cortices in 3D matrices. These results demonstrate how DIP-directed mDia2-dependent F-actin dynamics regulate morphological plasticity in motile cancer cells.

Role of the oligodendroglial cytoskeleton in differentiation and myelination

Oligodendrocytes, the myelin-forming cells of the central nervous system, are in culture characterized by an elaborate process network, terminating in flat membranous sheets that are rich in myelin-specific proteins and lipids, and spirally wrap axons forming a compact insulating layer in vivo. By analogy with other cell types, maintenance and stability of these processes, as well as the formation of the myelin sheath, likely rely on a pronounced cytoskeleton consisting of microtubules and microfilaments. While the specialized process of wrapping and compaction forming the myelin sheath is not well understood, considerably more is known about how cytoskeletal organization is mediated by extracellular and intracellular signals and other interaction partners during oligodendrocyte differentiation and myelination. Here, we review the current state of knowledge on the role of the oligodendrocyte cytoskeleton in differentiation with an emphasis on signal transduction mechanisms and will attempt to draw out implications for its significance in myelination.

MAP2-mediated in vitro interactions of brain microtubules and their modulation by cAMP

Microtubule-associated proteins (MAPs) are involved in microtubule (MT) bundling and in crossbridges between MTs and other organelles. Previous studies have assigned the MT bundling function of MAPs to their MT-binding domain and its modulation by the projection domain. In the present work, we analyse the viscoelastic properties of MT suspensions in the presence or the absence of cAMP. The experimental data reveal the occurrence of interactions between MT polymers involving MAP2 and modulated by cAMP. Two distinct mechanisms of action of cAMP are identified, which involve on one hand the phosphorylation of MT proteins by the cAMP-dependent protein kinase A (PKA) bound to the end of the N-terminal projection of MAP2, and on the other hand the binding of cAMP to the RII subunit of the PKA affecting interactions between MTs in a phosphorylation-independent manner. These findings imply a role for the complex of PKA with the projection domain of MAP2 in MT-MT interactions and suggest that cAMP may influence directly the density and bundling of MT arrays in dendrites of neurons.

Filamin B mediates ICAM-1-driven leukocyte transendothelial migration

During inflammation, the endothelium mediates rolling and firm adhesion of activated leukocytes. Integrin-mediated adhesion to endothelial ligands of the Ig-superfamily induces intracellular signaling in endothelial cells, which promotes leukocyte transendothelial migration. We identified the actin cross-linking molecule filamin B as a novel binding partner for intracellular adhesion molecule-1 (ICAM-1). Immune precipitation as well as laser scanning confocal microscopy confirmed the specific interaction and co-localization of endogenous filamin B with ICAM-1. Importantly, clustering of ICAM-1 promotes the ICAM-1-filamin B interaction. To investigate the functional consequences of filamin B binding to ICAM-1, we used small interfering RNA to reduce filamin B expression in ICAM-1-GFP expressing HeLa cells. We found that filamin B is required for the lateral mobility of ICAM-1 and for ICAM-1-induced transmigration of leukocytes. Reducing filamin B expression in primary human endothelial cells resulted in reduced recruitment of ICAM-1 to endothelial docking structures, reduced firm adhesion of the leukocytes to the endothelium, and inhibition of transendothelial migration. In conclusion, this study identifies filamin B as a molecular linker that mediates ICAM-1-driven transendothelial migration.

Association of hepatitis C virus replication complexes with microtubules and actin filaments is dependent on the interaction of NS3 and NS5A

The hepatitis C virus (HCV) RNA replication complex (RC), which is composed of viral nonstructural (NS) proteins and host cellular proteins, replicates the viral RNA genome in association with intracellular membranes. Two viral NS proteins, NS3 and NS5A, are essential elements of the RC. Here, by using immunoprecipitation and fluorescence resonance energy transfer assays, we demonstrated that NS3 and NS5A interact with tubulin and actin. Furthermore, immunofluorescence microscopy and electron microscopy revealed that HCV RCs were aligned along microtubules and actin filaments in both HCV replicon cells and HCV-infected cells. In addition, the movement of RCs was inhibited when microtubules or actin filaments were depolymerized by colchicine and cytochalasin B, respectively. Based on our observations, we propose that microtubules and actin filaments provide the tracks for the movement of HCV RCs to other regions in the cell, and the molecular interactions between RCs and microtubules, or RCs and actin filaments, are mediated by NS3 and NS5A.

Microtubule-binding proteins CLASP1 and CLASP2 interact with actin filaments

Cell morphogenesis requires dynamic communication between actin filaments and microtubules which is mediated, at least in part, by direct structural links between the two cytoskeletal systems. Here, we examined interaction between the CLIP-associated proteins (CLASP) CLASP1 and CLASP2, and actin filaments. We demonstrate that, in addition to a well-established association with the distal ends of microtubules, CLASP2alpha co-localizes with stress fibers, and that both CLASP1alpha and CLASP2alpha co-immunoprecipitate with actin. GFP-CLASP2alpha exhibits retrograde flow in the lamellipodia of Xenopus primary fibroblasts and in the filopodia of Xenopus spinal cord neurons. A deletion mapping analysis reveals that both the microtubule-binding domain of CLASP2 (which is homologous between all CLASPs) and the N-terminal dis1/TOG domain of CLASP2alpha (which is homologous between alpha isoforms) possess actin-binding activity. Fluorescence resonance energy transfer experiments demonstrate significant energy transfer between YFP-CLASP2alpha and CFP-actin. Our results indicate that CLASPs function as actin/microtubule crosslinkers in interphase cells. We propose that CLASPs facilitate recognition of actin filaments by the plus ends of growing microtubules at the initial stages of actin-microtubule interaction. Cell Motil.

JWA as a functional molecule to regulate cancer cells migration via MAPK cascades and F-actin cytoskeleton

Mitogen activated protein kinase (MAPK) cascades are thought to mediate diverse biological functions such as cell growth, differentiation and migration. Activated MAPK may affect microtubule (MT) which is essential for cellular polarity, differentiation and motility. Data in this study show that JWA, a newly identified novel microtubule-associated protein (MAP) was essential for the rearrangement of F-actin cytoskeleton and activation of MAPK cascades induced by arsenic trioxide (As2O3) and phorbol ester (PMA). Over-expression of JWA alone in HeLa, B16 and HCCLM3 cancer cells effectively inhibited cellular migration; whereas, cellular migration was significantly accelerated when cells were deficient in JWA expression. The mechanism underlying these phenomena might be due to JWA affected F-actin rearrangement. Furthermore, JWA deficiency blocked anti-migratory effect produced by As2O3 but enhanced the migratory effect initiated by PMA in HeLa cells. JWA SDR-SLR motifs are not only critical for the MAPK cascades activation, but also for cell migration. Further studies found that JWA differentially regulated cell migration via ERK downstream effectors focal adhesion kinase (FAK) and cyclooxygenase-2 (COX-2). Therefore, JWA regulated-tumor cellular migration might involve MAPK cascades activation and F-actin cytoskeleton rearrangement mechanisms. Our data provide an unexpected role for JWA in tumor cell migration behaviors.

Abnormal bundling and accumulation of F-actin mediates tau-induced neuronal degeneration in vivo

Hyperphosphorylated forms of the microtubule-associated protein (MAP) tau accumulate in Alzheimer's disease and related tauopathies and are thought to have an important role in neurodegeneration. However, the mechanisms through which phosphorylated tau induces neurodegeneration have remained elusive. Here, we show that tau-induced neurodegeneration is associated with accumulation of filamentous actin (F-actin) and the formation of actin-rich rods in Drosophila and mouse models of tauopathy. Importantly, modulating F-actin levels genetically leads to dramatic modification of tau-induced neurodegeneration. The ability of tau to interact with F-actin in vivo and in vitro provides a molecular mechanism for the observed phenotypes. Finally, we show that the Alzheimer's disease-linked human beta-amyloid protein (Abeta) synergistically enhances the ability of wild-type tau to promote alterations in the actin cytoskeleton and neurodegeneration. These findings raise the possibility that a direct interaction between tau and actin may be a critical mediator of tau-induced neurotoxicity in Alzheimer's disease and related disorders.

Do axonal defects in tau and amyloid precursor protein transgenic animals model axonopathy in Alzheimer's disease?

The subcellular localization of organelles, mRNAs and proteins is particularly challenging in neurons. Owing to their extended morphology, with axons in humans exceeding a meter in length, in addition to which they are not renewed but persist for the entire lifespan, it is no surprise that neurons are highly vulnerable to any perturbation of their sophisticated transport machinery. There is emerging evidence that impaired transport is not only causative for a range of motor disorders, but possibly also for Alzheimer's disease (AD) and related neurodegenerative disorders. Support for this hypothesis comes from transgenic animal models. Overexpression of human tau and amyloid precursor protein (APP) in mice and flies models the key hallmark histopathological characteristics of AD, such as somatodendritic accumulation of phosphorylated forms of tau and beta-amyloid (Abeta) peptide-containing amyloid plaques, as well as axonopathy. The latter has also been demonstrated in mutant mice with altered levels of Alzheimer-associated genes, such as presenilin (PS). In Abeta-producing APP transgenic mice, axonopathy was observed before the onset of plaque formation and tau hyperphosphorylation. In human AD brain, an axonopathy was revealed for early but not late Braak stages. The overall picture is that key players in AD, such as tau, APP and PS, perturb axonal transport early on in AD, causing impaired synaptic plasticity and reducing survival rates. It will be challenging to determine the molecular mechanisms of these different axonopathies, as this might assist in the development of new therapeutic strategies.

Snail induction is an early response to Gli1 that determines the efficiency of epithelial transformation

Gli family members mediate constitutive Hedgehog signaling in the common skin cancer, basal cell carcinoma (BCC). Snail/Snai1 is rapidly induced by Gli1 in vitro, and is coexpressed with Gli1 in human hair follicles and skin tumors. In the current study, we generated a dominant-negative allele of Snail, SnaZFD, composed of the zinc-finger domain and flanking sequence. In promoter-reporter assays, SnaZFD blocked the activity of wild-type Snail on the E-cadherin promoter. Snail loss-of-function mediated by SnaZFD or by one of several short hairpin RNAs inhibited transformation of RK3E epithelial cells by Gli1. Conversely, enforced expression of Snail promoted transformation in vitro by Gli1, but not by other genes that were tested, including Notch1, ErbB2, and N-Ras. As observed for Gli1, wild-type Snail repressed E-cadherin in RK3E cells and induced blebbing of the cytoplasmic membrane. Induction of a conditional Gli1 transgene in the basal keratinocytes of mouse skin led to rapid upregulation of Snail transcripts and to cell proliferation in the interfollicular epidermis. Established Gli1-induced skin lesions exhibited molecular similarities to BCC, including loss of E-cadherin. The results identify Snail as a Gli1-inducible effector of transformation in vitro, and an early Gli1-responsive gene in the skin.

Cytoskeletal mechanisms of neuronal morphogenesis

The cytoskeleton is the major intracellular structure that determines the morphology of a neuron. Thus, mechanisms that ensure a precisely regulated assembly of cytoskeletal elements in time and space have an important role in the development from a morphologically simple neuronal precursor cell to a complex polarized neuron that can establish contacts to several hundreds of other cells. Here, cytoskeletal mechanisms that underlie the formation of neurites, directed elongation and stabilization of neuronal processes are summarized. It has become evident that different cytoskeletal elements are highly crosslinked with each other by several classes of specific linker proteins. Of these, microtubule-associated proteins (MAPs) appear to have an important role in connecting the microtubule skeleton to other cytoskeletal filaments and plasma membrane components during neuronal morphogenesis. Future experiments will have to elucidate the function and the regulation of the neuronal cytoskeleton in an authentic nervous system environment during development. Recent approaches are discussed at the end of this article.

Tau interaction with microtubules in vivo

Tau is a major microtubule-associated protein which induces bundling and stabilization of axonal microtubules (MTs). To investigate the interaction of tau with MTs in living cells, we expressed GFP-tau fusion protein in cultured Xenopus embryo neurons and performed time-lapse imaging of tau-labeled MTs. Tau uniformly labeled individual MTs regardless of their assembly/disassembly status and location along the axon. Photobleaching experiments indicated that interaction of tau with MTs is very dynamic, with a half-time of fluorescence recovery of the order of 3 seconds. Treatment of cells with taxol, a drug that suppresses MT dynamics, rapidly induced detachment of tau from MTs. Although binding of tau to straight MTs was uniform, there was a heightened concentration of tau at the sites of high MT curvature. Our results suggest that dynamic interaction of tau with MTs may modify local mechanical properties of individual MTs and play a crucial role in the remodeling of the MT cytoskeleton during neuronal plasticity.

Interaction of microtubule-associated protein-2 and p63: a new link between microtubules and rough endoplasmic reticulum membranes in neurons

Neurons are polarized cells presenting two distinct compartments, dendrites and an axon. Dendrites can be distinguished from the axon by the presence of rough endoplasmic reticulum (RER). The mechanism by which the structure and distribution of the RER is maintained in these cells is poorly understood. In the present study, we investigated the role of the dendritic microtubule-associated protein-2 (MAP2) in the RER membrane positioning by comparing their distribution in brain subcellular fractions and in primary hippocampal cells and by examining the MAP2-microtubule interaction with RER membranes in vitro. Subcellular fractionation of rat brain revealed a high MAP2 content in a subfraction enriched with the endoplasmic reticulum markers ribophorin and p63. Electron microscope morphometry confirmed the enrichment of this subfraction with RER membranes. In cultured hippocampal neurons, MAP2 and p63 were found to concomitantly compartmentalize to the dendritic processes during neuronal differentiation. Protein blot overlays using purified MAP2c protein revealed its interaction with p63, and immunoprecipitation experiments performed in HeLa cells showed that this interaction involves the projection domain of MAP2. In an in vitro reconstitution assay, MAP2-containing microtubules were observed to bind to RER membranes in contrast to microtubules containing tau, the axonal MAP. This binding of MAP2c microtubules was reduced when an anti-p63 antibody was added to the assay. The present results suggest that MAP2 is involved in the association of RER membranes with microtubules and thereby could participate in the differential distribution of RER membranes within a neuron.

MAP2c, but not tau, binds and bundles F-actin via its microtubule binding domain

BACKGROUND

MAP2 and tau are abundant microtubule-associated proteins (MAPs) in neurons. The development of neuronal dendrites and axons requires a dynamic interaction between microtubules and actin filaments. MAPs represent good candidates to mediate such interactions. Although MAP2c and tau have similar, well-characterized microtubule binding activities, their actin interaction is poorly understood.

RESULTS

Here, we show by using a cosedimentation assay that MAP2c binds F-actin. Upon actin binding, MAP2c organizes F-actin into closely packed actin bundles. Moreover, we show by using a deletion approach that MAP2c's microtubule binding domain (MTBD) is both necessary and sufficient for both F-actin binding and bundling activities. Surprisingly, even though the MAP2 and tau MTBDs share high sequence homology and possess similar microtubule binding activities, tau is unable to bind or bundle F-actin. Furthermore, experiments with chimeric proteins demonstrate that the actin binding activity fully correlates with the ability to promote neurite initiation in neuroblastoma cells.

CONCLUSIONS

These results provide the first demonstration that the MAP2c and tau MTBD domains exhibit distinct properties, diverging in actin binding and neurite initiation activities. These results implicate a novel actin function for MAP2c in neuronal morphogenesis and furthermore suggest that actin interactions could contribute to functional differences between MAP2 and tau in neurons.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

The role of microtubule-associated protein 2c in the reorganization of microtubules and lamellipodia during neurite initiation

During neurite initiation, cells surrounded by a flattened, actin-rich lamellipodium transform to produce thin, microtubule-filled neurite shafts tipped by actin-rich growth cones, but little is known about this transformation. Our detailed time-lapse analyses of cultured hippocampal neurons, a widely used model system for neuronal development, revealed that neurites emerge from segmented lamellipodia, which then gradually extend from the cell body to become nascent growth cones. This suggests that actin- and microtubule-rich structures are reorganized in a coordinated manner. We hypothesized that proteins such as microtubule-associated protein 2 (MAP2), which can interact with both cytoskeletal components, might be critically involved in neurite initiation. Live-cell video and fluorescence microscopy in Neuro-2a cells showed that expression of MAP2c triggers neurite formation via rapid accumulation and bundling of stable, MAP2c-bound microtubules, concurrent with a gradual transformation of lamellipodia into nascent growth cones. The microtubule-stabilizing agent Taxol did not mimic this effect, suggesting that the ability of MAP2c to stabilize microtubules is not sufficient for neurite initiation. However, combination of Taxol treatment with actin disruption induced robust process formation, suggesting that inhibitory effects of F-actin need to be overcome as well. Neurite initiation by MAP2c required its microtubule-binding domain and was enhanced by its binding domain for cAMP-dependent protein kinase (PKA). MAP2c mutants defective in both PKA and microtubule binding acted as dominant negative inhibitors of neurite initiation in neuroblastoma cells and primary hippocampal neurons. Together, these data suggest that MAP2c bears functions that both stabilize microtubules and directly or indirectly alter actin organization during neurite initiation.

The role of microtubule-associated protein 2c in the reorganization of microtubules and lamellipodia during neurite initiation

During neurite initiation, cells surrounded by a flattened, actin-rich lamellipodium transform to produce thin, microtubule-filled neurite shafts tipped by actin-rich growth cones, but little is known about this transformation. Our detailed time-lapse analyses of cultured hippocampal neurons, a widely used model system for neuronal development, revealed that neurites emerge from segmented lamellipodia, which then gradually extend from the cell body to become nascent growth cones. This suggests that actin- and microtubule-rich structures are reorganized in a coordinated manner. We hypothesized that proteins such as microtubule-associated protein 2 (MAP2), which can interact with both cytoskeletal components, might be critically involved in neurite initiation. Live-cell video and fluorescence microscopy in Neuro-2a cells showed that expression of MAP2c triggers neurite formation via rapid accumulation and bundling of stable, MAP2c-bound microtubules, concurrent with a gradual transformation of lamellipodia into nascent growth cones. The microtubule-stabilizing agent Taxol did not mimic this effect, suggesting that the ability of MAP2c to stabilize microtubules is not sufficient for neurite initiation. However, combination of Taxol treatment with actin disruption induced robust process formation, suggesting that inhibitory effects of F-actin need to be overcome as well. Neurite initiation by MAP2c required its microtubule-binding domain and was enhanced by its binding domain for cAMP-dependent protein kinase (PKA). MAP2c mutants defective in both PKA and microtubule binding acted as dominant negative inhibitors of neurite initiation in neuroblastoma cells and primary hippocampal neurons. Together, these data suggest that MAP2c bears functions that both stabilize microtubules and directly or indirectly alter actin organization during neurite initiation.

Conserved microtubule-actin interactions in cell movement and morphogenesis

Interactions between microtubules and actin are a basic phenomenon that underlies many fundamental processes in which dynamic cellular asymmetries need to be established and maintained. These are processes as diverse as cell motility, neuronal pathfinding, cellular wound healing, cell division and cortical flow. Microtubules and actin exhibit two mechanistic classes of interactions--regulatory and structural. These interactions comprise at least three conserved 'mechanochemical activity modules' that perform similar roles in these diverse cell functions.

Cytoskeletal disrupting agents prevent calmodulin kinase, IQ domain and voltage-dependent facilitation of L-type Ca2+ channels

A calmodulin (CaM) binding 'IQ' domain on the L-type Ca(2+) channel (LTCC) C terminus and calmodulin kinase II (CaMK) both signal increases in LTCC opening probability (P(o)) by shifting LTCCs into a gating mode (mode 2) with long openings through a process called facilitation. However, the mechanism whereby CaMK and the IQ domain are targeted to LTCCs is unknown. Endogenous CaMK is targeted to LTCCs in excised cell membrane patches because LTCC P(o) increased significantly in CaM-enriched (20 microM) bath solution and this effect was prevented by a specific CaMK inhibitory peptide, but not by an inactive control peptide. Pre-exposure of myocytes to the cytoskeletal disrupting agents nocodazole (microtubule specific) or cytochalasin D (microfilament specific) prevented the effects of CaM-dependent increases in P(o) of LTCCs in excised membrane patches. Neither cytochalasin D nor nocodazole altered the distribution of LTCC gating modes under basal conditions in on-cell mode or excised cell membrane patches, but each of these agents occluded the response of LTCCs to exogenous, constitutively active CaMK and to an IQ-mimetic peptide (IQmp). Cytochalasin D and nocodazole pretreatment also prevented LTCC facilitation that followed a cell membrane depolarizing prepulse. In contrast, cytochalasin D and nocodazole did not affect the increase in LTCC P(o) or prevent the shift to mode 2 gating in response to protein kinase A, indicating that cytoskeletal disruption specifically prevents prepulse, CaMK and IQ-dependent LTCC facilitation.

Protein kinase MARK/PAR-1 is required for neurite outgrowth and establishment of neuronal polarity

Protein kinases of the microtubule affinity-regulating kinase (MARK) family were originally discovered because of their ability to phosphorylate certain sites in tau protein (KXGS motifs in the repeat domain). This type of phosphorylation is enhanced in abnormal tau from Alzheimer brain tissue and causes the detachment of tau from microtubules. MARK-related kinases (PAR-1 and KIN1) occur in various organisms and are involved in establishing and maintaining cell polarity. Herein, we report the ability of MARK2 to affect the differentiation and outgrowth of cell processes from neuroblastoma and other cell models. MARK2 phosphorylates tau protein at the KXGS motifs; this results in the detachment of tau from microtubules and their destabilization. The formation of neurites in N2a cells is blocked if MARK2 is inactivated, either by transfecting a dominant negative mutant, or by MARK2 inhibitors such as hymenialdisine. Alternatively, neurites are blocked if the target KXGS motifs on tau are rendered nonphosphorylatable by point mutations. The results suggest that MARK2 contributes to the plasticity of microtubules needed for neuronal polarity and the growth of neurites.

Differential localization of high- and low-molecular-weight variants of microtubule-associated protein 2 in the developing rat telencephalon

Microtubule-associated protein 2 (MAP2) occurs in developing mammalian neuronal tissue as both high- and low-molecular-weight forms with temporally regulated expression. We studied the MAP2 expression in the developing rat telencephalon with monoclonal antibodies that recognized both the high- and low-molecular-weight forms of MAP2 variants or that specifically recognized high-molecular-weight forms of MAP2 variants. Differences in the staining patterns of these antibodies reflected differences in the distribution of the high- and low-molecular-weight MAP2s. The immunoreactive sites of high- and low-molecular-weight MAP2 had a more widespread distribution in the embryonic telencephalon than those of high-molecular-weight MAP2. Many bipolar cells in the ganglionic eminence (GE) and in the intermediate zone (IZ) of the neocortex showed low-molecular-weight MAP2 immunoreactivity, but they showed weak or no high-molecular-weight MAP2 immunoreactivity. Expression of mRNA containing exons common to high- and low-molecular-weight MAP2 was detected in the tangentially ellipsoidal cells in the IZ, but expression of mRNA containing an exon specific to high-molecular-weight MAP2 was not detected in these cells by in situ hybridization. We interpreted these observations as indicating that the bipolar cells contained MAP2c preferentially, but contained MAP2a and MAP2b (MAP2a/b) at a very low or negligible level. The cells that expressed MAP2c preferentially among the MAP2 splicing variants composed 50% of the preplate cells, most of the MAP2-positive cells in the hippocampus and the corpus callosum. Double labeling by DiI staining and Dlx2 immunohistochemistry, or by Dlx2 and MAP2 immunohistochemistry, revealed that most of the Dlx2-positive cells in the IZ expressed MAP2c preferentially at embryonic day 16. Another double-labeling study revealed that most GAD-positive cells in the preplate were MAP2a/b positive, whereas most GAD-positive cells in the IZ expressed MAP2c preferentially, with only a negligible level of MAP2a/b immunoreactivity. We conclude that MAP2 immunoreactivity in the IZ was localized in the tangentially migrating neurons. The tangentially migrating neurons seemed to acquire MAP2a/b immunoreactivity as they entered the preplate or cortical plate and developed into mature neurons. Radially migrating neurons in the IZ were MAP2 negative. After entering to the preplate or the cortical plate, they became MAP2a/b positive as they developed into mature neurons.

Tubulin, actin, and tau protein interactions and the study of their macromolecular assemblies

The intracellular polymerization of cytoskeletal proteins into their supramolecular assemblies raises many questions regarding the regulatory patterns that control this process. Binding experiments using the ELISA solid phase system, together with protein assembly assays and electron microscopical studies provided clues on the protein-protein associations in the polymerization of tubulin and actin networks. In vitro reconstitution experiments of these cytoskeletal filaments using purified tau, tubulin, and actin proteins were carried out. Tau protein association with tubulin immobilized in a solid phase support system was inhibited by actin monomer, and a higher inhibition was attained in the presence of preassembled actin filaments. Conversely, tubulin and assembled microtubules strongly inhibited tau interaction with actin in the solid phase system. Actin filaments decreased the extent of in vitro tau-induced tubulin assembly. Studies on the morphological aspects of microtubules and actin filaments coexisting in vitro, revealed the association between both cytoskeletal filaments, and in some cases, the presence of fine filamentous structures bridging these polymers. Immunogold studies showed the association of tau along polymerized microtubules and actin filaments, even though a preferential localization of labeled tau with microtubules was revealed. The studies provide further evidence for the involvement of tau protein in modulating the interactions of microtubules and actin polymers in the organization of the cytsokeletal network.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

The projection domain of MAP2b regulates microtubule protrusion and process formation in Sf9 cells

The expression of microtubule-associated protein 2 (MAP2), developmentally regulated by alternative splicing, coincides with neurite outgrowth. MAP2 proteins contain a microtubule-binding domain (C-terminal) that promotes microtubule assembly and a poorly characterized domain, the projection domain(N-terminal), extending at the surface of microtubules. MAP2b differs from MAP2c by an additional sequence of 1372 amino acids in the projection domain. In this study, we examined the role of the projection domain in the protrusion of microtubules from the cell surface and the subsequent process formation in Sf9 cells. In this system, MAP2b has a lower capacity to induce process formation than MAP2c. To investigate the role of the projection domain in this event, we expressed truncated forms of MAP2b and MAP2c that have partial or complete deletion of their projection domain in Sf9 cells. Our results indicate that process formation is induced by the microtubule-binding domain of these MAP2 proteins and is regulated by their projection domain. Furthermore, the microtubule-binding activity of MAP2b and MAP2c truncated forms as well as the structural properties of the microtubule bundles induced by them do not seem to be the only determinants that control the protrusion of microtubules from the cell surface in Sf9 cells. Rather, our data suggest that microtubule protrusion and process formation are regulated by intramolecular interactions between the projection domain and its microtubule-binding domain in MAP2b.

Synergistic effects of MAP2 and MAP1B knockout in neuronal migration, dendritic outgrowth, and microtubule organization

MAP1B and MAP2 are major members of neuronal microtubule-associated proteins (MAPs). To gain insights into the function of MAP2 in vivo, we generated MAP2-deficient (map2(-/-)) mice. They developed without any apparent abnormalities, which indicates that MAP2 is dispensable in mouse survival. Because previous reports suggest a functional redundancy among MAPs, we next generated mice lacking both MAP2 and MAP1B to test their possible synergistic functions in vivo. Map2(-/-)map1b(-/-) mice died in their perinatal period. They showed not only fiber tract malformations but also disrupted cortical patterning caused by retarded neuronal migration. In spite of this, their cortical layer maintained an "inside-out" pattern. Detailed observation of primary cultures of hippocampal neurons from map2(-/-)map1b(-/-) mice revealed inhibited microtubule bundling and neurite elongation. In these neurons, synergistic effects caused by the loss of MAP2 and MAP1B were more apparent in dendrites than in axons. The spacing of microtubules was reduced significantly in map2(-/-)map1b(-/-) mice in vitro and in vivo. These results suggest that MAP2 and MAP1B have overlapping functions in neuronal migration and neurite outgrowth by organizing microtubules in developing neurons both for axonal and dendritic morphogenesis but more dominantly for dendritic morphogenesis.

Tau and transgenic animal models

Advances in genetics and transgenic approaches have a continuous impact on our understanding of Alzheimer's disease (AD) and related disorders, especially as aspects of the histopathology and neurodegeneration can be reproduced in animal models. AD is characterized by extracellular Abeta peptide-containing plaques and neurofibrillary aggregates of hyperphosphorylated isoforms of microtubule-associated protein tau. A causal link between Abeta production, neurodegeneration and dementia has been established with the identification of familial forms of AD which are linked to mutations in the amyloid precursor protein APP, from which the Abeta peptide is derived by proteolysis. No mutations have been identified in the tau gene in AD until today. Tau filament formation, in the absence of Abeta production, is also a feature of several additional neurodegenerative diseases including progressive supranuclear palsy, corticobasal degeneration, Pick's disease, and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in the tau gene which are linked to FTDP-17 established that dysfunction of tau can, as well as Abeta formation, lead to neurodegeneration and dementia. In this review, newly recognized cellular functions of tau, and the neuropathology and clinical syndrome of FTDP-17 will be presented, as well as recent advances that have been achieved in studies of transgenic mice expressing tau and AD-related kinases and phosphatases. These models link neurofibrillary lesion formation to neuronal loss, provide an in vivo model in which therapies can be assessed, and may contribute to determine the relationship between Abeta production and tau pathology.

Phosphorylation-dependent localization of microtubule-associated protein MAP2c to the actin cytoskeleton

Microtubule-associated protein 2 (MAP2) is a neuronal phosphoprotein that promotes net microtubule growth and actin cross-linking and bundling in vitro. Little is known about MAP2 regulation or its interaction with the cytoskeleton in vivo. Here we investigate the in vivo function of three specific sites of phosphorylation on MAP2. cAMP-dependent protein kinase activity disrupts the MAP2-microtubule interaction in living HeLa cells and promotes MAP2c localization to peripheral membrane ruffles enriched in actin. cAMP-dependent protein kinase phosphorylates serines within three KXGS motifs, one within each tubulin-binding repeat. These highly conserved motifs are also found in homologous proteins tau and MAP4. Phosphorylation at two of these sites was detected in brain tissue. Constitutive phosphorylation at these sites was mimicked by single, double, and triple mutations to glutamic acid. Biochemical and microscopy-based assays indicated that mutation of a single residue was adequate to disrupt the MAP2-microtubule interaction in HeLa cells. Double or triple point mutation promoted MAP2c localization to the actin cytoskeleton. Specific association between MAP2c and the actin cytoskeleton was demonstrated by retention of MAP2c-actin colocalization after detergent extraction. Specific phosphorylation states may enhance the interaction of MAP2 with the actin cytoskeleton, thereby providing a regulated mechanism for MAP2 function within distinct cytoskeletal domains.