Near-atomic model of microtubule-tau interactions

The whole is greater than the sum of its parts: at the intersection of order, disorder, and kinetochore function

The kinetochore (KT) field has matured tremendously since Earnshaw first identified CENP-A, CENP-B, and CENP-C [1,2]. In the past 35 years, the accumulation of knowledge has included: defining the parts list, identifying epistatic networks of interdependence within the parts list, understanding the spatial organization of subcomplexes into a massive structure - hundreds of megadaltons in size, and dissecting the functions of the KT in its entirety as well as of its individual parts. Like nearly all cell and molecular biology fields, the structure-function paradigm has been foundational to advances in the KT field. A point nicely highlighted by the fact that we are at the precipice of the in vitro reconstitution of a functional KT holo complex. Yet conventional notions of structure cannot provide a complete picture of the KT especially since it contains an abundance of unstructured or intrinsically disordered constituents. The combination of structured and disordered proteins within the KT results in an assembled system that is functionally greater than the sum of its parts.

Coevolution of Atomic Resolution and Whole-Brain Imaging for Tau Neurofibrillary Tangles

Neurofibrillary tangle (NFT) imaging methods at the distinct scales of atomic and whole-brain resolutions have coevolved rapidly. Linking these two areas of research provides insight into how and why certain tau radiotracers, using positron emission tomography (PET), bind selectively to certain morphological forms of the NFT fibril. In this Review, a brief history and background for each research area is presented leading to a summary of the current state of knowledge, with a synopsis of PET NFT radiotracers and an outlook for near-term research efforts. The continued integration of information provided at the level of each of these scales of resolution will catalyze the next generation of clinical imaging technique development and enhance our interpretations of them.

Phosphorylated tau interactome in the human Alzheimer's disease brain

Accumulation of phosphorylated tau is a key pathological feature of Alzheimer's disease. Phosphorylated tau accumulation causes synaptic impairment, neuronal dysfunction and formation of neurofibrillary tangles. The pathological actions of phosphorylated tau are mediated by surrounding neuronal proteins; however, a comprehensive understanding of the proteins that phosphorylated tau interacts with in Alzheimer's disease is surprisingly limited. Therefore, the aim of this study was to determine the phosphorylated tau interactome. To this end, we used two complementary proteomics approaches: (i) quantitative proteomics was performed on neurofibrillary tangles microdissected from patients with advanced Alzheimer's disease; and (ii) affinity purification-mass spectrometry was used to identify which of these proteins specifically bound to phosphorylated tau. We identified 542 proteins in neurofibrillary tangles. This included the abundant detection of many proteins known to be present in neurofibrillary tangles such as tau, ubiquitin, neurofilament proteins and apolipoprotein E. Affinity purification-mass spectrometry confirmed that 75 proteins present in neurofibrillary tangles interacted with PHF1-immunoreactive phosphorylated tau. Twenty-nine of these proteins have been previously associated with phosphorylated tau, therefore validating our proteomic approach. More importantly, 34 proteins had previously been associated with total tau, but not yet linked directly to phosphorylated tau (e.g. synaptic protein VAMP2, vacuolar-ATPase subunit ATP6V0D1); therefore, we provide new evidence that they directly interact with phosphorylated tau in Alzheimer's disease. In addition, we also identified 12 novel proteins, not previously known to be physiologically or pathologically associated with tau (e.g. RNA binding protein HNRNPA1). Network analysis showed that the phosphorylated tau interactome was enriched in proteins involved in the protein ubiquitination pathway and phagosome maturation. Importantly, we were able to pinpoint specific proteins that phosphorylated tau interacts with in these pathways for the first time, therefore providing novel potential pathogenic mechanisms that can be explored in future studies. Combined, our results reveal new potential drug targets for the treatment of tauopathies and provide insight into how phosphorylated tau mediates its toxicity in Alzheimer's disease.

In Silico Evaluation of Acetylation Mimics in the 27 Lysine Residues of Human Tau Protein

BACKGROUND

Various neurodegenerative diseases, including Alzheimer's disease (AD), are related to abnormal hyperphosphorylated microtubule-associated protein tau accumulation in brain lesions. Recent studies have focused on toxicity caused by another post-translational modification (PTM), acetylation of the lysine (K) residues of tau protein. Because there are numerous acetylation sites, several studies have introduced mimics of tau acetylation using amino acid substitutions from lysine to glutamine (Q). However, human tau protein contains over 20 acetylation sites; thus, investigation of the effects of an acetylated tau is difficult.

OBJECTIVE

Here, the authors in silico evaluated acetylation effects using SIFT, PolyPhen-2 and PROVEAN which can estimate the effects of amino acid substitutions based on the sequence homology or protein structure in tau isoforms. In addition, they also investigated 27 acetylation effects on the amyloid formation of tau proteins using Waltz.

RESULTS

15 acetylation mimics were estimated to be the most detrimental, which indicates that there may be novel pathogenic acetylation sites in the human tau protein. Interestingly, the deleterious effect of acetylation mimics was different according to the type of isoforms. Furthermore, all acetylation mimics were predicted to be a region of amyloid formation at the codons 274-279 of human tau protein. Notably, acetylation mimic of codon 311 (K311Q) induced the formation of an additional amyloid region located on codons 306-311 of the human tau protein.

CONCLUSION

To the best of our knowledge, this is the first simultaneous in-silico evaluation of the acetylation state of 27 human tau protein residues.

Effects of pharmacological modulators of α-synuclein and tau aggregation and internalization

Parkinson's disease (PD) and Alzheimer's disease (AD) are common neurodegenerative disorders of the elderly and, therefore, affect a growing number of patients worldwide. Both diseases share, as a common hallmark, the accumulation of characteristic protein aggregates, known as Lewy bodies (LB) in PD, and neurofibrillary tangles in AD. LBs are primarily composed of misfolded α-synuclein (aSyn), and neurofibrillary tangles are primarily composed of tau protein. Importantly, upon pathological evaluation, most AD and PD/Lewy body dementia cases exhibit mixed pathology, with the co-occurrence of both LB and neurofibrillary tangles, among other protein inclusions. Recent studies suggest that both aSyn and tau pathology can spread and propagate through neuronal connections. Therefore, it is important to investigate the mechanisms underlying aggregation and propagation of these proteins for the development of novel therapeutic strategies. Here, we assessed the effects of different pharmacological interventions on the aggregation and internalization of tau and aSyn. We found that anle138b and fulvic acid decrease aSyn and tau aggregation, that epigallocatechin gallate decreases aSyn aggregation, and that dynasore reduces tau internalization. Establishing the effects of small molecules with different chemical properties on the aggregation and spreading of aSyn and tau will be important for the development of future therapeutic interventions.

It's complicated: The relationship between sleep and Alzheimer's disease in humans

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by an asymptomatic period of amyloid-β (Aβ) deposition as insoluble extracellular plaque, intracellular tau aggregation, neuronal and synaptic loss, and subsequent cognitive dysfunction and dementia. A growing public health crisis, the worldwide prevalence of AD is expected to rise from 46.8 million individuals affected in 2015 to 131.5 million in 2050. Sleep disturbances have been associated with increased future risk of AD. A bi-directional relationship is hypothesized between sleep and AD with sleep disturbances as either markers for AD pathology and/or a mechanism mediating increased risk of AD. In this review, the evidence in humans supporting this complex relationship between sleep and AD will be discussed as well as the therapeutic potential and challenges of treating sleep disturbances to prevent or delay the onset of AD.

Bibliometric Analysis of Chronic Traumatic Encephalopathy Research from 1999 to 2019

Research on chronic traumatic encephalopathy (CTE) has increased over the past two decades. However, few studies have statistically analyzed these publications. In this work, we conducted a bibliometric analysis of studies on CTE to track research trends and highlight current research hotspots. Relevant original articles were obtained from the Web of Science Core Collection database between 1999 and 2019. CiteSpace and VOSviewer software were used to perform analysis and visualization of scientific productivity and emerging trends. Our results show that the publications related to CTE dramatically increased from four publications in 1999 to 160 publications in 2019. The United States dominated this field with 732 publications (75.934%), followed by Canada with 88 publications (9.129%). Most of related publications were published in the journals with a focus on molecular biology, immunology, neurology, sports and ophthalmology, as represented by the dual-map overlay. A total of 11 major clusters were explored based on the reference co-citation analysis. In addition, three predominant research topics were summarized by clustering high-frequency keywords: epidemiological, clinical and pathological studies. The research frontiers were the diagnosis of diseases using new neuroimaging techniques, and the investigation of the molecular mechanism of tau aggregation. This study provides researchers with valuable guidance in the selection of research topics.

Molecular and cellular mechanisms underlying the pathogenesis of Alzheimer's disease

Alzheimer’s disease (AD) is the most common neurodegenerative disorder seen in age-dependent dementia. There is currently no effective treatment for AD, which may be attributed in part to lack of a clear underlying mechanism. Studies within the last few decades provide growing evidence for a central role of amyloid β (Aβ) and tau, as well as glial contributions to various molecular and cellular pathways in AD pathogenesis. Herein, we review recent progress with respect to Aβ- and tau-associated mechanisms, and discuss glial dysfunction in AD with emphasis on neuronal and glial receptors that mediate Aβ-induced toxicity. We also discuss other critical factors that may affect AD pathogenesis, including genetics, aging, variables related to environment, lifestyle habits, and describe the potential role of apolipoprotein E (APOE), viral and bacterial infection, sleep, and microbiota. Although we have gained much towards understanding various aspects underlying this devastating neurodegenerative disorder, greater commitment towards research in molecular mechanism, diagnostics and treatment will be needed in future AD research.

Intrinsically Disordered Domain of Kinesin-3 Kif14 Enables Unique Functional Diversity

In addition to their force-generating motor domains, kinesin motor proteins feature various accessory domains enabling them to fulfill a variety of functions in the cell. Human kinesin-3, Kif14, localizes to the midbody of the mitotic spindle and is involved in the progression of cytokinesis. The specific motor properties enabling Kif14's cellular functions, however, remain unknown. Here, we show in vitro that the intrinsically disordered N-terminal domain of Kif14 enables unique functional diversity of the kinesin. Using single molecule TIRF microscopy, we found that Kif14 exists either as a diffusible monomer or as processive dimer and that the disordered domain (1) enables diffusibility of the monomeric Kif14, (2) renders the dimeric Kif14 super-processive and enables the kinesin to pass through highly crowded areas, (3) enables robust, autonomous Kif14 tracking of growing microtubule tips, independent of microtubule end-binding (EB) proteins, and (4) is sufficient to enable crosslinking of parallel microtubules and necessary to enable Kif14-driven sliding of antiparallel ones. We explain these features of Kif14 by the observed diffusible interaction of the disordered domain with the microtubule lattice and the observed increased affinity of the disordered domain for GTP-bound tubulin. We suggest that the disordered domain tethers the motor domain to the microtubule providing a diffusible foothold and a regulatory hub, tuning the kinesin's interaction with microtubules. Our findings thus exemplify pliable protein tethering as a fundamental mechanism of molecular motor regulation.

Liquid-liquid phase separation in biology: mechanisms, physiological functions and human diseases

Cells are compartmentalized by numerous membrane-enclosed organelles and membraneless compartments to ensure that a wide variety of cellular activities occur in a spatially and temporally controlled manner. The molecular mechanisms underlying the dynamics of membrane-bound organelles, such as their fusion and fission, vesicle-mediated trafficking and membrane contactmediated inter-organelle interactions, have been extensively characterized. However, the molecular details of the assembly and functions of membraneless compartments remain elusive. Mounting evidence has emerged recently that a large number of membraneless compartments, collectively called biomacromolecular condensates, are assembled via liquid-liquid phase separation (LLPS). Phase-separated condensates participate in various biological activities, including higher-order chromatin organization, gene expression, triage of misfolded or unwanted proteins for autophagic degradation, assembly of signaling clusters and actin- and microtubule-based cytoskeletal networks, asymmetric segregations of cell fate determinants and formation of pre- and post-synaptic density signaling assemblies. Biomacromolecular condensates can transition into different material states such as gel-like structures and solid aggregates. The material properties of condensates are crucial for fulfilment of their distinct functions, such as biochemical reaction centers, signaling hubs and supporting architectures. Cells have evolved multiple mechanisms to ensure that biomacromolecular condensates are assembled and disassembled in a tightly controlled manner. Aberrant phase separation and transition are causatively associated with a variety of human diseases such as neurodegenerative diseases and cancers. This review summarizes recent major progress in elucidating the roles of LLPS in various biological pathways and diseases.

These motors were made for walking

Kinesins are a diverse group of adenosine triphosphate (ATP)-dependent motor proteins that transport cargos along microtubules (MTs) and change the organization of MT networks. Shared among all kinesins is a ~40 kDa motor domain that has evolved an impressive assortment of motility and MT remodeling mechanisms as a result of subtle tweaks and edits within its sequence. Several elegant studies of different kinesin isoforms have exposed the purpose of structural changes in the motor domain as it engages and leaves the MT. However, few studies have compared the sequences and MT contacts of these kinesins systematically. Along with clever strategies to trap kinesin-tubulin complexes for X-ray crystallography, new advancements in cryo-electron microscopy have produced a burst of high-resolution structures that show kinesin-MT interfaces more precisely than ever. This review considers the MT interactions of kinesin subfamilies that exhibit significant differences in speed, processivity, and MT remodeling activity. We show how their sequence variations relate to their tubulin footprint and, in turn, how this explains the molecular activities of previously characterized mutants. As more high-resolution structures become available, this type of assessment will quicken the pace toward establishing each kinesin's design-function relationship.

Tau Ser208 phosphorylation promotes aggregation and reveals neuropathologic diversity in Alzheimer's disease and other tauopathies

Tau protein abnormally aggregates in tauopathies, a diverse group of neurologic diseases that includes Alzheimer’s disease (AD). In early stages of disease, tau becomes hyperphosphorylated and mislocalized, which can contribute to its aggregation and toxicity. We demonstrate that tau phosphorylation at Ser208 (pSer208) promotes microtubule dysfunction and tau aggregation in cultured cells. Comparative assessment of the epitopes recognized by antibodies AT8, CP13, and 7F2 demonstrates that CP13 and 7F2 are specific for tau phosphorylation at Ser202 and Thr205, respectively, independently of the phosphorylation state of adjacent phosphorylation sites. Supporting the involvement of pSer208 in tau pathology, a novel monoclonal antibody 3G12 specific for tau phosphorylation at Ser208 revealed strong reactivity of tau inclusions in the brains of PS19 and rTg4510 transgenic mouse models of tauopathy. 3G12 also labelled neurofibrillary tangles in brains of patients with AD but revealed differential staining compared to CP13 and 7F2 for other types of tau pathologies such as in neuropil threads and neuritic plaques in AD, tufted astrocytes in progressive supranuclear palsy and astrocytic plaques in corticobasal degeneration. These results support the hypothesis that tau phosphorylation at Ser208 strongly contributes to unique types of tau aggregation and may be a reliable marker for the presence of mature neurofibrillary tangles.

Cytoskeletal organization through multivalent interactions

The cytoskeleton consists of polymeric protein filaments with periodic lattices displaying identical binding sites, which establish a multivalent platform for the binding of a plethora of filament-associated ligand proteins. Multivalent ligand proteins can tether themselves to the filaments through one of their binding sites, resulting in an enhanced reaction kinetics for the remaining binding sites. In this Opinion, we discuss a number of cytoskeletal phenomena underpinned by such multivalent interactions, namely (1) generation of entropic forces by filament crosslinkers, (2) processivity of molecular motors, (3) spatial sorting of proteins, and (4) concentration-dependent unbinding of filament-associated proteins. These examples highlight that cytoskeletal filaments constitute the basis for the formation of microenvironments, which cytoskeletal ligand proteins can associate with and, once engaged, can act within at altered reaction kinetics. We thus argue that multivalency is one of the properties crucial for the functionality of the cytoskeleton.

Unsaturated Fatty Acid-Induced Conformational Transitions and Aggregation of the Repeat Domain of Tau

Background: The intrinsically disordered, amyloidogenic protein Tau associates with diverse classes of molecules, including proteins, nucleic acids, and lipids. Mounting evidence suggests that fatty acid molecules could play a role in the dysfunction of this protein, however, their interaction with Tau remains poorly characterized. Methods: In a bid to elucidate the association of Tau with unsaturated fatty acids at the sub-molecular level, we carried out a variety of solution NMR experiments in combination with circular dichroism and fluorescence measurements. Our study shows that Tau^4RD, the highly basic four-repeat domain of Tau, associates strongly with arachidonic and oleic acid assemblies in a high lipid/protein ratio, perturbing their supramolecular states and itself undergoing time-dependent structural adaptation. The structural signatures of Tau^4RD/fatty acid aggregates appear similar for arachidonic acid and oleic acid, however, they are distinct from those of another prototypical intrinsically disordered protein, α-synuclein, when bound to these lipids, revealing protein-specific conformational adaptations. Both fatty acid molecules are found to invariably promote the self-aggregation of Tau^4RD and of α-synuclein. Conclusions: This study describes the reciprocal influence that Tau^4RD and fatty acids exert on their conformational states, contributing to our understanding of fundamental aspects of Tau/lipid co-assembly.

Extracellular vesicles derived from human ES-MSCs protect retinal ganglion cells and preserve retinal function in a rodent model of optic nerve injury

Background

Retinal and/or optic nerve injury is one of the leading causes of blindness due to retinal ganglion cell (RGC) degeneration. There have been extensive efforts to suppress this neurodegeneration. Various somatic tissue-derived mesenchymal stem cells (MSCs) demonstrated significant neuroprotective and axogenic effects on RGCs. An alternative source of MSCs could be human embryonic stem cells (ES-MSCs), which proliferate faster, express lower levels of inflammatory cytokines, and are capable of immune modulation. It has been demonstrated that MSCs secrete factors or extracellular vesicles that may heal the injury. However, possible therapeutic effects and underlying mechanism of human ES-MSC extracellular vesicles (EVs) on optic nerve injury have not been assessed.

Methods

EVs were isolated from human ES-MSCs. Then, ES-MSC EV was applied to an optic nerve crush (ONC) mouse model. Immunohistofluorescence, retro- and anterograde tracing of RGCs, Western blot, tauopathy in RGCs, and function assessments were performed during 2-month post-treatment to evaluate ONC improvement and underlying mechanism of human ES-MSC EV in in vivo.

Results

We found that the ES-MSC EV significantly improved Brn3a+ RGCs survival and retro- and anterograde tracing of RGCs, while preventing retinal nerve fiber layer (RNFL) degenerative thinning compared to the vehicle group. The EVs also significantly promoted GAP43+ axon counts in the optic nerve and improved cognitive visual behavior. Furthermore, cis p-tau, a central mediator of neurodegeneration in the injured RGCs, is detectable after the ONC at the early stages demonstrated tauopathy in RGCs. Notably, after EV treatment cis p-tau was downregulated.

Conclusions

Our findings propose that human ES-MSC EVs, as an off-the-shelf and cell-free product, may have profound clinical implications in treating injured RGCs and degenerative ocular disease. Moreover, the possible mechanisms of human ES-MSC EV are related to the rescue of tauopathy process of RGC degeneration.

Protein Diffusion on Charged Biopolymers: DNA versus Microtubule

Protein diffusion in lower-dimensional spaces is used for various cellular functions. For example, sliding on DNA is essential for proteins searching for their target sites, and protein diffusion on microtubules is important for proper cell division and neuronal development. On the one hand, these linear diffusion processes are mediated by long-range electrostatic interactions between positively charged proteins and negatively charged biopolymers and have similar characteristic diffusion coefficients. On the other hand, DNA and microtubules have different structural properties. Here, using computational approaches, we studied the mechanism of protein diffusion along DNA and microtubules by exploring the diffusion of both protein types on both biopolymers. We found that DNA-binding and microtubule-binding proteins can diffuse on each other's substrates; however, the adopted diffusion mechanism depends on the molecular properties of the diffusing proteins and the biopolymers. On the protein side, only DNA-binding proteins can perform rotation-coupled diffusion along DNA, with this being due to their higher net charge and its spatial organization at the DNA recognition helix. By contrast, the lower net charge on microtubule-binding proteins enables them to diffuse more quickly than DNA-binding proteins on both biopolymers. On the biopolymer side, microtubules possess intrinsically disordered, negatively charged C-terminal tails that interact with microtubule-binding proteins, thus supporting their diffusion. Thus, although both DNA-binding and microtubule-binding proteins can diffuse on the negatively charged biopolymers, the unique molecular features of the biopolymers and of their natural substrates are essential for function.

Resurrecting the Mysteries of Big Tau

Tau, a microtubule-associated protein that modifies the dynamic properties and organization of microtubules in neurons and affects axonal transport, shows remarkable heterogeneity, with multiple isoforms (45-65 kDa) generated by alternative splicing. A high-molecular-weight (HMW) isoform (110 kDa) that contains an additional large exon termed 4a was discovered more than 25 years ago. This isoform, called Big tau, is expressed mainly in the adult peripheral nervous system (PNS), but also in adult neurons of the central nervous system (CNS) that extend processes into the periphery. Surprisingly little has been learned about Big tau since its initial characterization, leaving a significant gap in knowledge about how the dramatic switch to Big tau affects the properties of neurons in the context of development, disease, or injury. Here we review what was learned about the structure and distribution of Big tau in those earlier studies, and add contemporary insights to resurrect interest in the mysteries of Big tau and thereby set a path for future studies.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

Mitochondrial dysfunction plays a key role in the development of neurodegenerative diseases in diabetes

Mitochondria have an essential function in cell survival due to their role in bioenergetics, reactive oxygen species generation, calcium buffering, and other metabolic activities. Mitochondrial dysfunctions are commonly found in neurodegenerative diseases (NDs), and diabetes is a risk factor for NDs. However, the role of mitochondria in diabetic neurodegeneration is still unclear. In the present study, we review the latest evidence on the role of mitochondrial dysfunctions in the development of diabetes-related NDs and the underlying molecular mechanisms. Hypoglycemic agents, especially metformin, have been proven to have neuroprotective effects in the treatment of diabetes, in which mitochondria could act as one of the underlying mechanisms. Other hypoglycemic agents, including thiazolidinediones (TZDs), dipeptidyl peptidase 4 (DPP-4) inhibitors, and glucagon-like peptide 1 (GLP-1) receptor agonists, have gained more attention because of their beneficial effects on NDs, presumably by improving mitochondrial function. Our review highlights the notion that mitochondria could be a promising therapeutic target in the treatment of NDs in patients with diabetes.

Oligomerization and Conformational Change Turn Monomeric β-Amyloid and Tau Proteins Toxic: Their Role in Alzheimer's Pathogenesis

The structural polymorphism and the physiological and pathophysiological roles of two important proteins, β-amyloid (Aβ) and tau, that play a key role in Alzheimer's disease (AD) are reviewed. Recent results demonstrate that monomeric Aβ has important physiological functions. Toxic oligomeric Aβ assemblies (AβOs) may play a decisive role in AD pathogenesis. The polymorph fibrillar Aβ (fAβ) form has a very ordered cross-β structure and is assumed to be non-toxic. Tau monomers also have several important physiological actions; however, their oligomerization leads to toxic oligomers (TauOs). Further polymerization results in probably non-toxic fibrillar structures, among others neurofibrillary tangles (NFTs). Their structure was determined by cryo-electron microscopy at atomic level. Both AβOs and TauOs may initiate neurodegenerative processes, and their interactions and crosstalk determine the pathophysiological changes in AD. TauOs (perhaps also AβO) have prionoid character, and they may be responsible for cell-to-cell spreading of the disease. Both extra- and intracellular AβOs and TauOs (and not the previously hypothesized amyloid plaques and NFTs) may represent the novel targets of AD drug research.

Axonal transport: Driving synaptic function

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

Common Functions of Disordered Proteins across Evolutionary Distant Organisms

Intrinsically disordered proteins and regions typically lack a well-defined structure and thus fall outside the scope of the classic sequence–structure–function relationship. Hence, classic sequence- or structure-based bioinformatic approaches are often not well suited to identify homology or predict the function of unknown intrinsically disordered proteins. Here, we give selected examples of intrinsic disorder in plant proteins and present how protein function is shared, altered or distinct in evolutionary distant organisms. Furthermore, we explore how examining the specific role of disorder across different phyla can provide a better understanding of the common features that protein disorder contributes to the respective biological mechanism.

What Could Go Wrong? A Practical Guide to Single-Particle Cryo-EM: From Biochemistry to Atomic Models

Cryo-electron microscopy (cryo-EM) has enjoyed explosive recent growth due to revolutionary advances in hardware and software, resulting in a steady stream of long-awaited, high-resolution structures with unprecedented atomic detail. With this comes an increased number of microscopes, cryo-EM facilities, and scientists eager to leverage the ability to determine protein structures without crystallization. However, numerous pitfalls and considerations beset the path toward high-resolution structures and are not necessarily obvious from literature surveys. Here, we detail the most common misconceptions when initiating a cryo-EM project and common technical hurdles, as well as their solutions, and we conclude with a vision for the future of this exciting field.

Site-Specific Hyperphosphorylation Inhibits, Rather than Promotes, Tau Fibrillization, Seeding Capacity, and Its Microtubule Binding

The consistent observation of phosphorylated tau in the pathology of Alzheimer's disease has contributed to the emergence of a model where hyperphosphorylation triggers both tau disassociation from microtubules and its subsequent aggregation. Herein, we applied a total chemical synthetic approach to site-specifically phosphorylate the microtubule binding repeat domain of tau (K18) at single (pS356) or multiple (pS356/pS262 and pS356/pS262/pS258) residues. We show that hyperphosphorylation of K18 inhibits 1) its aggregation in vitro, 2) its seeding activity in cells, 3) its binding to microtubules, and 4) its ability to promote microtubule polymerization. The inhibition increased with increasing the number of phosphorylated sites, with phosphorylation at S262 having the strongest effect. Our results argue against the hyperphosphorylation hypothesis and underscore the importance of revisiting the role of site-specific hyperphosphorylation in regulating tau functions in health and disease.

Taxanes convert regions of perturbed microtubule growth into rescue sites

Microtubules are polymers of tubulin dimers, and conformational transitions in the microtubule lattice drive microtubule dynamic instability and affect various aspects of microtubule function. The exact nature of these transitions and their modulation by anticancer drugs such as Taxol and epothilone, which can stabilize microtubules but also perturb their growth, are poorly understood. Here, we directly visualize the action of fluorescent Taxol and epothilone derivatives and show that microtubules can transition to a state that triggers cooperative drug binding to form regions with altered lattice conformation. Such regions emerge at growing microtubule ends that are in a pre-catastrophe state, and inhibit microtubule growth and shortening. Electron microscopy and in vitro dynamics data indicate that taxane accumulation zones represent incomplete tubes that can persist, incorporate tubulin dimers and repeatedly induce microtubule rescues. Thus, taxanes modulate the material properties of microtubules by converting destabilized growing microtubule ends into regions resistant to depolymerization.

The mechanism of Hsp90-induced oligomerizaton of Tau

Aggregation of the microtubule-associated protein Tau is a hallmark of Alzheimer's disease with Tau oligomers suspected as the most toxic agent. Tau is a client of the molecular chaperone Hsp90, although it is unclear whether and how the chaperone massages the structure of intrinsically disordered Tau. Using electron paramagnetic resonance, we extract structural information from the very broad conformational ensemble of Tau: Tau in solution is highly dynamic and polymorphic, although "paper clip"-shaped by long-range contacts. Interaction with Hsp90 promotes an open Tau conformation, which we identify as the molecular basis for the formation of small Tau oligomers by exposure of the aggregation-prone repeat domain to other Tau molecules. At the same time, formation of Tau fibrils is inhibited. We therefore provide the nanometer-scale zoom into chaperoning an amyloid client, highlighting formation of oligomers as the consequence of this biologically relevant interaction.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Potential interference with microtubule assembly by graphene: a tug-of-war

With the ever-increasing demand for graphene-based materials and their promising applications in numerous nanotechnologies, the biological effects of graphene on living systems have become crucial and ought to be well understood. Previously, both the cytotoxicity of graphene towards biological cells and its potential application as a nanomedicine have been revealed experimentally and theoretically. Besides many existing anticancer drugs that target microtubules, here we investigate the possibility of using graphene as a nanomedicine, which could alter the dynamic assembly and disassembly of a microtubule. We found that when a graphene nanosheet is at the hydrophilic interface of two neighboring heterodimers (containing α and β tubulins), it can pull one dimer away from the other through a "tug-of-war" mechanism, driven by the strong dispersive interaction exerted by the surface of the graphene nanosheet. This work demonstrates that based on the existing methods for mitigating graphene's cytotoxicity (already developed in this field), a graphene-based nanomedicine could be designed to target microtubules of cancer cells and induce cell apoptosis.

A Combinatorial MAP Code Dictates Polarized Microtubule Transport

Many eukaryotic cells distribute their intracellular components asymmetrically through regulated active transport driven by molecular motors along microtubule tracks. While intrinsic and extrinsic regulation of motor activity exists, what governs the overall distribution of activated motor-cargo complexes within cells remains unclear. Here, we utilize in vitro reconstitution of purified motor proteins and non-enzymatic microtubule-associated proteins (MAPs) to demonstrate that MAPs exhibit distinct influences on the motility of the three main classes of transport motors: kinesin-1, kinesin-3, and cytoplasmic dynein. Further, we dissect how combinations of MAPs affect motors and unveil MAP9 as a positive modulator of kinesin-3 motility. From these data, we propose a general "MAP code" that has the capacity to strongly bias directed movement along microtubules and helps elucidate the intricate intracellular sorting observed in highly polarized cells such as neurons.

Phase Separation and Cytotoxicity of Tau are Modulated by Protein Disulfide Isomerase and S-nitrosylation of this Molecular Chaperone

Cells have evolved molecular chaperones that modulate phase separation and misfolding of amyloidogenic proteins to prevent neurodegenerative diseases. Protein disulfide isomerase (PDI), mainly located at the endoplasmic reticulum and also present in the cytosol, acts as both an enzyme and a molecular chaperone. PDI is observed to be S-nitrosylated in the brain of Alzheimer's disease patients, but the mechanism has remained elusive. We herein report that both wild-type PDI and its quadruple cysteine mutant only having chaperone activity, significantly inhibit pathological phosphorylation and abnormal aggregation of Tau in cells, and significantly decrease the mitochondrial damage and Tau cytotoxicity resulting from Tau aberrant aggregation, highlighting the chaperone property of PDI. More importantly, we show that wild-type PDI is selectively recruited by liquid droplets of Tau, which significantly inhibits phase separation and stress granule formation of Tau, whereas S-nitrosylation of PDI abrogates the recruitment and inhibition. These findings demonstrate how phase separation of Tau is physiologically regulated by PDI and how S-nitrosylation of PDI, a perturbation in this regulation, leads to disease.

Pre-clinical characterisation of E2814, a high-affinity antibody targeting the microtubule-binding repeat domain of tau for passive immunotherapy in Alzheimer's disease

Tau deposition in the brain is a pathological hallmark of many neurodegenerative disorders, including Alzheimer's disease (AD). During the course of these tauopathies, tau spreads throughout the brain via synaptically-connected pathways. Such propagation of pathology is thought to be mediated by tau species ("seeds") containing the microtubule binding region (MTBR) composed of either three repeat (3R) or four repeat (4R) isoforms. The tau MTBR also forms the core of the neuropathological filaments identified in AD brain and other tauopathies. Multiple approaches are being taken to limit tau pathology, including immunotherapy with anti-tau antibodies. Given its key structural role within fibrils, specifically targetting the MTBR with a therapeutic antibody to inhibit tau seeding and aggregation may be a promising strategy to provide disease-modifying treatment for AD and other tauopathies. Therefore, a monoclonal antibody generating campaign was initiated with focus on the MTBR. Herein we describe the pre-clinical generation and characterisation of E2814, a humanised, high affinity, IgG1 antibody recognising the tau MTBR. E2814 and its murine precursor, 7G6, as revealed by epitope mapping, are antibodies bi-epitopic for 4R and mono-epitopic for 3R tau isoforms because they bind to sequence motif HVPGG. Functionally, both antibodies inhibited tau aggregation in vitro. They also immunodepleted a variety of MTBR-containing tau protein species. In an in vivo model of tau seeding and transmission, attenuation of deposition of sarkosyl-insoluble tau in brain could also be observed in response to antibody treatment. In AD brain, E2814 bound different types of tau filaments as shown by immunogold labelling and recognised pathological tau structures by immunohistochemical staining. Tau fragments containing HVPGG epitopes were also found to be elevated in AD brain compared to PSP or control. Taken together, the data reported here have led to E2814 being proposed for clinical development.

Role of Post-translational Modifications in Alzheimer's Disease

The global burden of Alzheimer's disease (AD) is growing. Valiant efforts to develop clinical candidates for treatment have continuously met with failure. Currently available palliative treatments are temporary and there is a constant need to search for reliable disease pathways, biomarkers and drug targets for developing diagnostic and therapeutic tools to address the unmet medical needs of AD. Challenges in drug-discovery efforts raise further questions about the strategies of current conventional diagnosis; drug design; and understanding of disease pathways, biomarkers and targets. In this context, post-translational modifications (PTMs) regulate protein trafficking, function and degradation, and their in-depth study plays a significant role in the identification of novel biomarkers and drug targets. Aberrant PTMs of disease-relevant proteins could trigger pathological pathways, leading to disease progression. Advancements in proteomics enable the generation of patterns or signatures of such modifications, and thus, provide a versatile platform to develop biomarkers based on PTMs. In addition, understanding and targeting the aberrant PTMs of various proteins provide viable avenues for addressing AD drug-discovery challenges. This review highlights numerous PTMs of proteins relevant to AD and provides an overview of their adverse effects on the protein structure, function and aggregation propensity that contribute to the disease pathology. A critical discussion offers suggestions of methods to develop PTM signatures and interfere with aberrant PTMs to develop viable diagnostic and therapeutic interventions in AD.

Structural Characterization of Tau in Fuzzy Tau:Tubulin Complexes

Tau is a neuronal microtubule (MT)-associated protein of significant interest due to its association with several neurodegenerative disorders. Tau's intrinsic disorder and the dynamic nature of its interactions with tubulin and MTs make its structural characterization challenging. Here, we use an environmentally sensitive fluorophore as a site-specific probe of tau bound to soluble tubulin. Comparison of our results with a recently published tau:MT cryoelectron microscopy model reveals structural similarities between tubulin- and MT-bound tau. Analysis of residues across the repeat regions reveals a hierarchy in tubulin occupancy, which may be relevant to tau's ability to differentiate between tubulin and MTs. As binding to soluble tubulin is a critical first step in MT polymerization, our characterization of the structural features of tau in dynamic, fuzzy tau:tubulin assemblies advances our understanding of how tau functions in the cell and how function may be disrupted in disease.

The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications

Microtubules are cytoskeletal structures involved in stability, transport and organization in the cell. The building blocks, the α- and β-tubulin heterodimers, form protofilaments that associate laterally into the hollow microtubule. Microtubule also exists as highly stable doublet microtubules in the cilia where stability is needed for ciliary beating and function. The doublet microtubule maintains its stability through interactions at its inner and outer junctions where its A- and B-tubules meet. Here, using cryo-electron microscopy, bioinformatics and mass spectrometry of the doublets of Chlamydomonas reinhardtii and Tetrahymena thermophila, we identified two new inner junction proteins, FAP276 and FAP106, and an inner junction-associated protein, FAP126, thus presenting the complete answer to the inner junction identity and localization. Our structural study of the doublets shows that the inner junction serves as an interaction hub that involves tubulin post-translational modifications. These interactions contribute to the stability of the doublet and hence, normal ciliary motility.

Direct observation of dynamic protein interactions involving human microtubules using solid-state NMR spectroscopy

Microtubules are important components of the eukaryotic cytoskeleton. Their structural organization is regulated by nucleotide binding and many microtubule-associated proteins (MAPs). While cryo-EM and X-ray crystallography have provided detailed views of interactions between MAPs with the microtubule lattice, little is known about how MAPs and their intrinsically disordered regions interact with the dynamic microtubule surface. NMR carries the potential to directly probe such interactions but so far has been precluded by the low tubulin yield. We present a protocol to produce [13C, 15N]-labeled, functional microtubules (MTs) from human cells for solid-state NMR studies. This approach allowed us to demonstrate that MAPs can differently modulate the fast time-scale dynamics of C-terminal tubulin tails, suggesting distinct interaction modes. Our results pave the way for in-depth NMR studies of protein dynamics involved in MT assembly and their interactions with other cellular components.

Study on the Multitarget Synergistic Effects of Kai-Xin-San against Alzheimer's Disease Based on Systems Biology

Kai-Xin-San (KXS), a classical Chinese traditional prescription, was widely applied in the treatment of Alzheimer's disease (AD), while its functional mechanisms still remain unclear. By using systems biology approaches at animal, cellular, and molecular levels, the improvement of KXS on cognitive impairment was achieved by inhibiting abnormal acetylcholinesterase. The function on the nerve skeleton was performed by regulating the Tau phosphorylation pathway. Its antioxidant, anti-inflammatory, and antiapoptotic effects by modulating the aberrant upregulation of ROS, proinflammatory factors, and apoptosis-related proteins in the brain were studied to reveal the synergistic therapeutic efficacy of KXS. Then, formula dismantling in vitro indicated that ginseng was the principal herb, whereas three other herbs served adjuvant roles to achieve the best effect. After that, the in vivo analysis of components into plasma and brain of AD rats showed that 8 of 23 components in blood and 4 of 10 components in brain were from ginseng, respectively, further verifying the principal status of ginseng and the synergistic effects of the formula. Thus, the anti-AD effects of KXS were achieved by multitargets and multichannels. The systems biology approaches presented here provide a novel way in traditional herbal medicine research.

Structural determinants of microtubule minus end preference in CAMSAP CKK domains

CAMSAP/Patronins regulate microtubule minus-end dynamics. Their end specificity is mediated by their CKK domains, which we proposed recognise specific tubulin conformations found at minus ends. To critically test this idea, we compared the human CAMSAP1 CKK domain (HsCKK) with a CKK domain from Naegleria gruberi (NgCKK), which lacks minus-end specificity. Here we report near-atomic cryo-electron microscopy structures of HsCKK- and NgCKK-microtubule complexes, which show that these CKK domains share the same protein fold, bind at the intradimer interprotofilament tubulin junction, but exhibit different footprints on microtubules. NMR experiments show that both HsCKK and NgCKK are remarkably rigid. However, whereas NgCKK binding does not alter the microtubule architecture, HsCKK remodels its microtubule interaction site and changes the underlying polymer structure because the tubulin lattice conformation is not optimal for its binding. Thus, in contrast to many MAPs, the HsCKK domain can differentiate subtly specific tubulin conformations to enable microtubule minus-end recognition.

Independent tubulin binding and polymerization by the proline-rich region of Tau is regulated by Tau's N-terminal domain

Tau is an intrinsically disordered, microtubule-associated protein that has a role in regulating microtubule dynamics. Despite intensive research, the molecular mechanisms of Tau-mediated microtubule polymerization are poorly understood. Here we used single-molecule fluorescence to investigate the role of Tau's N-terminal domain (NTD) and proline-rich region (PRR) in regulating interactions of Tau with soluble tubulin. We assayed both full-length Tau isoforms and truncated variants for their ability to bind soluble tubulin and stimulate microtubule polymerization. We found that Tau's PRR is an independent tubulin-binding domain that has tubulin polymerization capacity. In contrast to the relatively weak interactions with tubulin mediated by sites distributed throughout Tau's microtubule-binding region (MTBR), resulting in heterogeneous Tau: tubulin complexes, the PRR bound tubulin tightly and stoichiometrically. Moreover, we demonstrate that interactions between the PRR and MTBR are reduced by the NTD through a conserved conformational ensemble. On the basis of these results, we propose that Tau's PRR can serve as a core tubulin-binding domain, whereas the MTBR enhances polymerization capacity by increasing the local tubulin concentration. Moreover, the NTD appears to negatively regulate tubulin-binding interactions of both of these domains. The findings of our study draw attention to a central role of the PRR in Tau function and provide mechanistic insight into Tau-mediated polymerization of tubulin.

Structure of the Decorated Ciliary Doublet Microtubule

The axoneme of motile cilia is the largest macromolecular machine of eukaryotic cells. In humans, impaired axoneme function causes a range of ciliopathies. Axoneme assembly, structure, and motility require a radially arranged set of doublet microtubules, each decorated in repeating patterns with non-tubulin components. We use single-particle cryo-electron microscopy to visualize and build an atomic model of the repeating structure of a native axonemal doublet microtubule, which reveals the identities, positions, repeat lengths, and interactions of 38 associated proteins, including 33 microtubule inner proteins (MIPs). The structure demonstrates how these proteins establish the unique architecture of doublet microtubules, maintain coherent periodicities along the axoneme, and stabilize the microtubules against the repeated mechanical stress induced by ciliary motility. Our work elucidates the architectural principles that underpin the assembly of this large, repetitive eukaryotic structure and provides a molecular basis for understanding the etiology of human ciliopathies.

A microtubule RELION-based pipeline for cryo-EM image processing

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MiRP is a pipeline for processing cryo-EM images of microtubules in RELION.

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MiRP manages microtubule heterogeneity and pseudo-symmetry.

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MiRP reduces errors in angular and translational alignment.

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MiRP improved reconstructions from three different microtubule datasets.

Microtubules are polar filaments built from αβ-tubulin heterodimers that exhibit a range of architectures in vitro and in vivo. Tubulin heterodimers are arranged helically in the microtubule wall but many physiologically relevant architectures exhibit a break in helical symmetry known as the seam. Noisy 2D cryo-electron microscopy projection images of pseudo-helical microtubules therefore depict distinct but highly similar views owing to the high structural similarity of α- and β-tubulin. The determination of the αβ-tubulin register and seam location during image processing is essential for alignment accuracy that enables determination of biologically relevant structures. Here we present a pipeline designed for image processing and high-resolution reconstruction of cryo-electron microscopy microtubule datasets, based in the popular and user-friendly RELION image-processing package, Microtubule RELION-based Pipeline (MiRP). The pipeline uses a combination of supervised classification and prior knowledge about geometric lattice constraints in microtubules to accurately determine microtubule architecture and seam location. The presented method is fast and semi-automated, producing near-atomic resolution reconstructions with test datasets that contain a range of microtubule architectures and binding proteins.

Multivalent electrostatic microtubule interactions of synthetic peptides are sufficient to mimic advanced MAP-like behavior

Microtubule-associated proteins (MAPs) are a functionally highly diverse class of proteins that help to adjust the shape and function of the microtubule cytoskeleton in space and time. For this purpose, MAPs structurally support microtubules, modulate their dynamic instability, or regulate the activity of associated molecular motors. The microtubule-binding domains of MAPs are structurally divergent, but often depend on electrostatic interactions with the negatively charged surface of the microtubule. This suggests that the surface exposure of positive charges rather than a certain structural fold is sufficient for a protein to associate with microtubules. Consistently, positively charged artificial objects have been shown to associate with microtubules and to diffuse along their lattice. Natural MAPs, however, show a more sophisticated functionality beyond lattice-diffusion. Here, we asked whether basic electrostatic interactions are sufficient to also support advanced MAP functionality. To test this hypothesis, we studied simple positively charged peptide sequences for the occurrence of typical MAP-like behavior. We found that a multivalent peptide construct featuring four lysine-alanine heptarepeats (starPEG-(KA7)₄)-but not its monovalent KA7-subunits-show advanced, biologically relevant MAP-like behavior: starPEG-(KA7)₄ binds microtubules in the low nanomolar range, diffuses along their lattice with the ability to switch between intersecting microtubules, and tracks depolymerizing microtubule ends. Further, starPEG-(KA7)₄ promotes microtubule nucleation and growth, mediates depolymerization coupled pulling at plus ends, and bundles microtubules without significantly interfering with other proteins on the microtubule lattice (as exemplified by the motor kinesin-1). Our results show that positive charges and multivalency are sufficient to mimic advanced MAP-like behavior.

Cellular Logistics: Unraveling the Interplay Between Microtubule Organization and Intracellular Transport

Microtubules are core components of the cytoskeleton and serve as tracks for motor protein–based intracellular transport. Microtubule networks are highly diverse across different cell types and are believed to adapt to cell type–specific transport demands. Here we review how the spatial organization of different subsets of microtubules into higher-order networks determines the traffic rules for motor-based transport in different animal cell types. We describe the interplay between microtubule network organization and motor-based transport within epithelial cells, oocytes, neurons, cilia, and the spindle apparatus.

The Evolution of Tau Phosphorylation and Interactions

Tau is a neuronal microtubule-associated protein (MAP) that is involved in the regulation of axonal microtubule assembly. However, as a protein with intrinsically disordered regions (IDRs), tau also interacts with many other partners in addition to microtubules. Phosphorylation at selected sites modulates tau's various intracellular interactions and regulates the properties of IDRs. In Alzheimer's disease (AD) and other tauopathies, tau exhibits pathologically increased phosphorylation (hyperphosphorylation) at selected sites and aggregates into neurofibrillary tangles (NFTs). By bioinformatics means, we tested the hypothesis that the sequence of tau has changed during the vertebrate evolution in a way that novel interactions developed and also the phosphorylation pattern was affected, which made tau prone to the development of tauopathies. We report that distinct regions of tau show functional specialization in their molecular interactions. We found that tau's amino-terminal region, which is involved in biological processes related to "membrane organization" and "regulation of apoptosis," exhibited a strong evolutionary increase in protein disorder providing the basis for the development of novel interactions. We observed that the predicted phosphorylation sites have changed during evolution in a region-specific manner, and in some cases the overall number of phosphorylation sites increased owing to the formation of clusters of phosphorylatable residues. In contrast, disease-specific hyperphosphorylated sites remained highly conserved. The data indicate that novel, non-microtubule related tau interactions developed during evolution and suggest that the biological processes, which are mediated by these interactions, are of pathological relevance. Furthermore, the data indicate that predicted phosphorylation sites in some regions of tau, including a cluster of phosphorylatable residues in the alternatively spliced exon 2, have changed during evolution. In view of the "antagonistic pleiotropy hypothesis" it may be worth to take disease-associated phosphosites with low evolutionary conservation as relevant biomarkers into consideration.

Tubulin lattice in cilia is in a stressed form regulated by microtubule inner proteins

Cilia, the hair-like protrusions that beat at high frequencies to propel a cell or move fluid around are composed of radially bundled doublet microtubules. In this study, we present a near-atomic resolution map of the Tetrahymena doublet microtubule by cryoelectron microscopy. The map demonstrates that the network of microtubule inner proteins weaves into the tubulin lattice and forms an inner sheath. From mass spectrometry data and de novo modeling, we identified Rib43a proteins as the filamentous microtubule inner proteins in the protofilament ribbon region. The Rib43a-tubulin interaction leads to an elongated tubulin dimer distance every 2 dimers. In addition, the tubulin lattice structure with missing microtubule inner proteins (MIPs) by sarkosyl treatment shows significant longitudinal compaction and lateral angle change between protofilaments. These results are evidence that the MIPs directly affect and stabilize the tubulin lattice. It suggests that the doublet microtubule is an intrinsically stressed filament and that this stress could be manipulated in the regulation of ciliary waveforms.

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.

Dynamical decoration of stabilized-microtubules by Tau-proteins

Tau is a microtubule-associated protein that regulates axonal transport, stabilizes and spatially organizes microtubules in parallel networks. The Tau-microtubule pair is crucial for maintaining the architecture and integrity of axons. Therefore, it is essential to understand how these two entities interact to ensure and modulate the normal axonal functions. Based on evidence from several published experiments, we have developed a two-dimensional model that describes the interaction between a population of Tau proteins and a stabilized microtubule at the scale of the tubulin dimers (binding sites) as an adsorption-desorption dynamical process in which Tau can bind on the microtubule outer surface via two distinct modes: a longitudinal (along a protofilament) and lateral (across adjacent protofilaments) modes. Such a process yields a dynamical distribution of Tau molecules on the microtubule surface referred to as microtubule decoration that we have characterized at the equilibrium using two observables: the total microtubule surface coverage with Tau's and the distribution of nearest neighbors Tau's. Using both analytical and numerical approaches, we have derived expressions and computed these observables as a function of key parameters controlling the binding reaction: the stoichiometries of the Taus in the two binding modes, the associated dissociation constants and the ratio of the Tau concentration to that of microtubule tubulin dimers.

RNA Secondary Structure-Based Design of Antisense Peptide Nucleic Acids for Modulating Disease-Associated Aberrant Tau Pre-mRNA Alternative Splicing

Alternative splicing of tau pre-mRNA is regulated by a 5' splice site (5'ss) hairpin present at the exon 10-intron 10 junction. Single mutations within the hairpin sequence alter hairpin structural stability and/or the binding of splicing factors, resulting in disease-causing aberrant splicing of exon 10. The hairpin structure contains about seven stably formed base pairs and thus may be suitable for targeting through antisense strands. Here, we used antisense peptide nucleic acids (asPNAs) to probe and target the tau pre-mRNA exon 10 5'ss hairpin structure through strand invasion. We characterized by electrophoretic mobility shift assay the binding of the designed asPNAs to model tau splice site hairpins. The relatively short (10-15 mer) asPNAs showed nanomolar binding to wild-type hairpins as well as a disease-causing mutant hairpin C+19G, albeit with reduced binding strength. Thus, the structural stabilizing effect of C+19G mutation could be revealed by asPNA binding. In addition, our cell culture minigene splicing assay data revealed that application of an asPNA targeting the 3' arm of the hairpin resulted in an increased exon 10 inclusion level for the disease-associated mutant C+19G, probably by exposing the 5'ss as well as inhibiting the binding of protein factors to the intronic spicing silencer. On the contrary, the application of asPNAs targeting the 5' arm of the hairpin caused an increased exon 10 exclusion for a disease-associated mutant C+14U, mainly by blocking the 5'ss. PNAs could enter cells through conjugation with amino sugar neamine or by cotransfection with minigene plasmids using a commercially available transfection reagent.