Tau gene mutations: dissecting the pathogenesis of FTDP-17

Simple model systems reveal conserved mechanisms of Alzheimer's disease and related tauopathies

The lack of effective therapies that slow the progression of Alzheimer's disease (AD) and related tauopathies highlights the need for a more comprehensive understanding of the fundamental cellular mechanisms underlying these diseases. Model organisms, including yeast, worms, and flies, provide simple systems with which to investigate the mechanisms of disease. The evolutionary conservation of cellular pathways regulating proteostasis and stress response in these organisms facilitates the study of genetic factors that contribute to, or protect against, neurodegeneration. Here, we review genetic modifiers of neurodegeneration and related cellular pathways identified in the budding yeast Saccharomyces cerevisiae, the nematode Caenorhabditis elegans, and the fruit fly Drosophila melanogaster, focusing on models of AD and related tauopathies. We further address the potential of simple model systems to better understand the fundamental mechanisms that lead to AD and other neurodegenerative disorders.

The self-association equilibrium of DNAJA2 regulates its interaction with unfolded substrate proteins and with Hsc70

J-domain proteins tune the specificity of Hsp70s, engaging them in precise functions. Despite their essential role, the structure and function of many J-domain proteins remain largely unknown. We explore human DNAJA2, finding that it reversibly forms highly-ordered, tubular structures that can be dissociated by Hsc70, the constitutively expressed Hsp70 isoform. Cryoelectron microscopy and mutational studies reveal that different domains are involved in self-association. Oligomer dissociation into dimers potentiates its interaction with unfolded client proteins. The J-domains are accessible to Hsc70 within the tubular structure. They allow binding of closely spaced Hsc70 molecules that could be transferred to the unfolded substrate for its cooperative remodelling, explaining the efficient recovery of DNAJA2-bound clients. The disordered C-terminal domain, comprising the last 52 residues, regulates its holding activity and productive interaction with Hsc70. These in vitro findings suggest that the association equilibrium of DNAJA2 could regulate its interaction with client proteins and Hsc70.

A comprehensive review of protein misfolding disorders, underlying mechanism, clinical diagnosis, and therapeutic strategies

INTRODUCTION

Proteins are the most common biological macromolecules in living system and are building blocks of life. They are extremely dynamic in structure and functions. Due to several modifications, proteins undergo misfolding, leading to aggregation and thereby developing neurodegenerative and systemic diseases. Understanding the pathology of these diseases and the techniques used to diagnose them is therefore crucial for their effective management . There are several techniques, currently being in use to diagnose them and those will be discussed in this review.

AIM/OBJECTIVES

Current review aims to discuss an overview of protein aggregation and the underlying mechanisms linked to neurodegeneration and systemic diseases. Also, the review highlights protein misfolding disorders, their clinical diagnosis, and treatment strategies.

METHODOLOGY

Literature related to neurodegenerative and systemic diseases was explored through PubMed, Google Scholar, Scopus, and Medline databases. The keywords used for literature survey and analysis are protein aggregation, neurodegenerative disorders, Alzheimer's disease, Parkinson's disease, systemic diseases, protein aggregation mechanisms, etc. DISCUSSION /CONCLUSION: This review summarises the pathogenesis of neurodegenerative and systemic disorders caused by protein misfolding and aggregation. The clinical diagnosis and therapeutic strategies adopted for the management of these diseases are also discussed to aid in a better understanding of protein misfolding disorders. Many significant concerns about the role, characteristics, and consequences of protein aggregates in neurodegenerative and systemic diseases are not clearly understood to date. Regardless of technological advancements, there are still great difficulties in the management and cure of these diseases. Therefore, for better understanding, diagnosis, and treatment of neurodegenerative and systemic diseases, more studies to identify novel drugs that may aid in their treatment and management are required.

Domain-specific modulatory effects of phosphomimetic substitutions on liquid-liquid phase separation of tau protein

Aggregation of tau is one of the major pathogenic events in Alzheimer's disease and several other neurodegenerative disorders. Recent reports demonstrated that tau can condense into liquid droplets that undergo time-dependent transition to a solid-like state, suggesting that liquid condensates may be on the pathway to pathological aggregation of tau. While hyperphosphorylation is a key feature of tau isolated from brains of patients with Alzheimer's disease and other tauopathies, the mechanistic role of phosphorylation in tau liquid-liquid phase separation (LLPS) remains largely unexplored. In an attempt to bridge this gap, here we performed systematic studies by introducing phosphomimetic substitutions of Ser/Thr residues with negatively charged Asp/Glu residues in different regions of the protein. Our data indicate that the phosphorylation patterns that increase the polarization of charge distribution in full-length tau (tau441) promote protein LLPS, whereas those that decrease charge polarization have an opposite effect. Overall, this study further supports the notion that tau LLPS is driven by attractive intermolecular electrostatic interactions between the oppositely charged domains. We also show that the phosphomimetic tau variants with low intrinsic propensity for LLPS can be efficiently recruited to droplets formed by the variants with high LLPS propensity. Furthermore, the present data demonstrate that phosphomimetic substitutions have a major effect on time-dependent material properties of tau droplets, generally slowing down their aging. The latter effect is most dramatic for the tau variant with substitutions within the repeat domain, which correlates with the decreased fibrillation rate of this variant.

Altered Anterior Insular Metabolic Connectivity in Asymptomatic MAPT P301L Carriers

BACKGROUND

The insula is the predominant brain region impaired in behavior variant frontotemporal dementia (bvFTD). However, structural and functional changes in the sub-insula in the asymptomatic stage of bvFTD are unknown.

OBJECTIVE

To describe structural and functional changes in insula subregions in asymptomatic carriers of the P301L mutation of the microtubule-associated protein tau (MAPT) gene and patients with bvFTD.

METHODS

Six asymptomatic MAPT P301L mutation carriers and 12 MAPT negative control subjects of the same pedigree were enrolled, along with 30 patients with a clinical diagnosis of bvFTD and 30 matched controls. All subjects underwent hybrid positron emission tomography/magnetic resonance imaging. Atlas-based parcellation using a fine-grained Brainnetome Atlas was conducted to assess gray matter (GM) volume, metabolism, and metabolic connectivity in the sub-insula (region of interest).

RESULTS

There was no significant GM atrophy or hypometabolism in insula subregions in asymptomatic MAPT P301L carriers, although decreased metabolic connectivity between vIa-middle temporal gyrus, vIa-temporal poles, dIa-middle temporal gyrus and dIa-temporal poles; and increased connectivity between vIa-orbitofrontal, vIa-dorsal lateral superior frontal gyrus, and dIa-orbitofrontal and dIa-dorsal lateral superior frontal gyrus were observed. Patients with bvFTD had significant atrophy and hypometabolism in all insula subregions and decreased metabolic connectivity in the whole brain, including vIa/dIa-middle temporal and vIa/dIa-temporal poles. The standardized uptake value ratios of vIa and dIa were negatively associated with behavioral disinhibition scale scores.

CONCLUSION

Metabolic connectivity is altered in vIa and dIa subregions of the sub-insula in MAPT P301L mutation carriers before the occurrence of atrophy, hypometabolism, and clinical symptoms.

Predicting functional riboSNitches in the context of alternative splicing

RNAs are the major regulators of gene expression, and their secondary structures play crucial roles at different levels. RiboSNitches are disease-associated SNPs that cause changes in the pre-mRNA secondary structural ensemble. Several riboSNitches have been detected in the 5' and 3' untranslated regions and lncRNA. Although cases of secondary structural elements playing a regulatory role in alternative splicing are known, regions specific to splicing events, such as splice junctions have not received much attention. We tested splice-site mutations for their efficiency in disrupting the secondary structure and hypothesized that these could play a crucial role in alternative splicing. Multiple riboSNitch prediction methods were applied to obtain overlapping results that are potentially more reliable. Putative riboSNitches were identified from aberrant 5' and 3' splice site mutations, cancer-causing somatic mutations, and genes that harbor the regulatory RNA secondary structural elements. Our workflow for predicting riboSNitches associated with alternative splicing is novel and paves the way for subsequent experimental validation.

Tau liquid-liquid phase separation in neurodegenerative diseases

Aggregation of the microtubule-associated protein tau plays a major role in Alzheimer's disease and several other neurodegenerative disorders. An exciting recent development is the finding that, akin to some other proteins associated with neurodegenerative disease, tau has a high propensity to condensate via the mechanism of liquid-liquid phase separation (LLPS). Here, we discuss the evidence for tau LLPS in vitro, the molecular mechanisms of this reaction, and the role of post-translational modifications and pathogenic mutations in tau phase separation. We also discuss recent studies on tau LLPS in cells and the insights these studies provide regarding the link between LLPS and neurodegeneration in tauopathies.

Genetically engineered MAPT 10+16 mutation causes pathophysiological excitability of human iPSC-derived neurons related to 4R tau-induced dementia

Human iPSC lines represent a powerful translational model of tauopathies. We have recently described a pathophysiological phenotype of neuronal excitability of human cells derived from the patients with familial frontotemporal dementia and parkinsonism (FTDP-17) caused by the MAPT 10+16 splice-site mutation. This mutation leads to the increased splicing of 4R tau isoforms. However, the role of different isoforms of tau protein in initiating neuronal dementia-related dysfunction, and the causality between the MAPT 10+16 mutation and altered neuronal activity have remained unclear. Here, we employed genetically engineered cells, in which the IVS10+16 mutation was introduced into healthy donor iPSCs to increase the expression of 4R tau isoform in exon 10, aiming to explore key physiological traits of iPSC-derived MAPT IVS10+16 neurons using patch-clamp electrophysiology and multiphoton fluorescent imaging techniques. We found that during late in vitro neurogenesis (from ~180 to 230 days) iPSC-derived cortical neurons of the control group (parental wild-type tau) exhibited membrane properties compatible with "mature" neurons. In contrast, MAPT IVS10+16 neurons displayed impaired excitability, as reflected by a depolarized resting membrane potential, an increased input resistance, and reduced voltage-gated Na+- and K+-channel-mediated currents. The mutation changed the channel properties of fast-inactivating Nav and decreased the Nav1.6 protein level. MAPT IVS10+16 neurons exhibited reduced firing accompanied by a changed action potential waveform and severely disturbed intracellular Ca2+ dynamics, both in the soma and dendrites, upon neuronal depolarization. These results unveil a causal link between the MAPT 10+16 mutation, hence overproduction of 4R tau, and a dysfunction of human cells, identifying a biophysical basis of changed neuronal activity in 4R tau-triggered dementia. Our study lends further support to using iPSC lines as a suitable platform for modelling tau-induced human neuropathology in vitro.

Fatal Attraction: The Case of Toxic Soluble Dimers of Truncated PQBP-1 Mutants in X-Linked Intellectual Disability

The frameshift mutants K192S^fs*7 and R153S^fs*41, of the polyglutamine tract-binding protein 1 (PQBP-1), are stable intrinsically disordered proteins (IDPs). They are each associated with the severe cognitive disorder known as the Renpenning syndrome, a form of X-linked intellectual disability (XLID). Relative to the monomeric wild-type protein, these mutants are dimeric, contain more folded contents, and have higher thermal stabilities. Comparisons can be drawn to the toxic oligomerisation in the “conformational diseases”, which collectively describe medical conditions involving a substantial protein structural transition in the pathogenic mechanism. At the molecular level, the end state of these diseases is often cytotoxic protein aggregation. The conformational disease proteins contain varying extents of intrinsic disorder, and the consensus pathogenesis includes an early oligomer formation. We reviewed the experimental characterisation of the toxic oligomers in representative cases. PQBP-1 mutant dimerisation was then compared to the oligomerisation of the conformational disease proteins. The PQBP-1 mutants are unique in behaving as stable soluble dimers, which do not further develop into higher oligomers or aggregates. The toxicity of the PQBP-1 mutant dimers lies in the native functions (in transcription regulation and possibly, RNA splicing) being compromised, rather than proceeding to aggregation. Other examples of stable IDP dimers were discussed and we speculated on the roles of IDP dimerisation in protein evolution.

Regulatory mechanisms of tau protein fibrillation under the conditions of liquid-liquid phase separation

One of the hallmarks of Alzheimer's disease and several other neurodegenerative disorders is the aggregation of tau protein into fibrillar structures. Building on recent reports that tau readily undergoes liquid-liquid phase separation (LLPS), here we explored the relationship between disease-related mutations, LLPS, and tau fibrillation. Our data demonstrate that, in contrast to previous suggestions, pathogenic mutations within the pseudorepeat region do not affect tau441's propensity to form liquid droplets. LLPS does, however, greatly accelerate formation of fibrillar aggregates, and this effect is especially dramatic for tau441 variants with disease-related mutations. Most important, this study also reveals a previously unrecognized mechanism by which LLPS can regulate the rate of fibrillation in mixtures containing tau isoforms with different aggregation propensities. This regulation results from unique properties of proteins under LLPS conditions, where total concentration of all tau variants in the condensed phase is constant. Therefore, the presence of increasing proportions of the slowly aggregating tau isoform gradually lowers the concentration of the isoform with high aggregation propensity, reducing the rate of its fibrillation. This regulatory mechanism may be of direct relevance to phenotypic variability of tauopathies, as the ratios of fast and slowly aggregating tau isoforms in brain varies substantially in different diseases.

Extensive Plasmid Library to Prepare Tau Protein Variants and Study Their Functional Biochemistry

Tau neurofibrillary tangles are key pathological features of Alzheimer's disease and other tauopathies. Recombinant protein technology is vital for studying the structure and function of tau in physiology and aggregation in pathophysiology. However, open-source and well-characterized plasmids for efficiently expressing and purifying different tau variants are lacking. We generated 44 sequence-verified plasmids including those encoding full length (FL) tau-441, its four-repeat microtubule-binding (K18) fragment, and their respective selected familial pathological variants (N279K, V337M, P301L, C291R, and S356T). Moreover, plasmids for expressing single (C291A), double (C291A/C322A), and triple (C291A/C322A/I260C) cysteine-modified variants were generated to study alterations in cysteine content and locations. Furthermore, protocols for producing representative tau forms were developed. We produced and characterized the aggregation behavior of the triple cysteine-modified tau-K18, often used in real-time cell internalization and aggregation studies because it can be fluorescently labeled on a cysteine outside the microtubule-binding core. Similar to the wild type (WT), triple cysteine-modified tau-K18 aggregated by progressive β-sheet enrichment, albeit at a slower rate. On prolonged incubation, cysteine-modified K18 formed paired helical filaments similar to those in Alzheimer's disease, sharing morphological phenotypes with WT tau-K18 filaments. Nonetheless, cysteine-modified tau-K18 filaments were significantly shorter (p = 0.002) and mostly wider than WT filaments, explainable by their different principal filament elongation pathways: vertical (end-to-end) and lateral growth for WT and cysteine-modified, respectively. Cysteine rearrangement may therefore induce filament polymorphism. Together, the plasmid library, the protein production methods, and the new insights into cysteine-dependent aggregation should facilitate further studies and the design of antiaggregation agents.

Autophagic Pathways to Clear the Tau Aggregates in Alzheimer's Disease

Tau is a microtubule-associated protein with an intrinsically unstructured conformation. Tau is subjected to several pathological post-translational modifications (PTMs), leading to its loss of interaction with microtubules and accumulation as neurofibrillary tangles (NFTs) in neurons. Tau aggregates impede functions of endoplasmic reticulum and mitochondria leading to the generation of oxidative stress and in turn amplifying the Tau aggregation. Tau is channelled to chaperones for folding into their native form, which otherwise causes its degradation and clearance. Cellular response triggers the activation of ubiquitin-proteasome system or autophagy to facilitate Tau degradation, based on the PTMs or mutations associated with Tau. Further, autophagy can be selective where Hsc70 interacts with Tau in monomeric, oligomeric and aggregated form and drives its clearance by chaperone-mediated autophagy pathway (CMA). Lysosome-associated membrane proteins-2A (LAMP-2A) is the key player of CMA that recognises Hsc70-Tau complex and triggers the downstream cascade. Thus, it becomes challenging for mutant Tau to be cleared by CMA as it loses its affinity for Hsc70 and LAMP-2A. In such a scenario, Tau might be degraded by macroautophagy otherwise sequestered by aggresomes. Henceforth, the degradation of Tau and its blockage that is associated with various PTMs of Tau would explain the dynamics of Tau degradation or accumulation in AD. Further, unveiling the role of accessory proteins involved in these degradation pathways would help in understanding their loss of function and preventing Tau clearance.

Application of yeast to studying amyloid and prion diseases

Amyloids are fibrous cross-β protein aggregates that are capable of proliferation via nucleated polymerization. Amyloid conformation likely represents an ancient protein fold and is linked to various biological or pathological manifestations. Self-perpetuating amyloid-based protein conformers provide a molecular basis for transmissible (infectious or heritable) protein isoforms, termed prions. Amyloids and prions, as well as other types of misfolded aggregated proteins are associated with a variety of devastating mammalian and human diseases, such as Alzheimer's, Parkinson's and Huntington's diseases, transmissible spongiform encephalopathies (TSEs), amyotrophic lateral sclerosis (ALS) and transthyretinopathies. In yeast and fungi, amyloid-based prions control phenotypically detectable heritable traits. Simplicity of cultivation requirements and availability of powerful genetic approaches makes yeast Saccharomyces cerevisiae an excellent model system for studying molecular and cellular mechanisms governing amyloid formation and propagation. Genetic techniques allowing for the expression of mammalian or human amyloidogenic and prionogenic proteins in yeast enable researchers to capitalize on yeast advantages for characterization of the properties of disease-related proteins. Chimeric constructs employing mammalian and human aggregation-prone proteins or domains, fused to fluorophores or to endogenous yeast proteins allow for cytological or phenotypic detection of disease-related protein aggregation in yeast cells. Yeast systems are amenable to high-throughput screening for antagonists of amyloid formation, propagation and/or toxicity. This review summarizes up to date achievements of yeast assays in application to studying mammalian and human disease-related aggregating proteins, and discusses both limitations and further perspectives of yeast-based strategies.

Imaging Biomarkers for Neurodegeneration in Presymptomatic Familial Frontotemporal Lobar Degeneration

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disorder characterized by behavioral changes, language abnormality, as well as executive function deficits and motor impairment. In about 30-50% of FTLD patients, an autosomal dominant pattern of inheritance was found with major mutations in the MAPT, GRN, and the C9orf72 repeat expansion. These mutations could lead to neurodegenerative pathology years before clinical symptoms onset. With potential disease-modifying treatments that are under development, non-invasive biomarkers that help determine the early brain changes in presymptomatic FTLD patients will be critical for tracking disease progression and enrolling the right participants into the clinical trials at the right time during the disease course. In recent years, there is increasing evidence that a number of imaging biomarkers show the abnormalities during the presymptomatic stage. Imaging biomarkers of presymptomatic familial FTLD may provide insight into the underlying neurodegenerative process years before symptom onset. Structural magnetic resonance imaging (MRI) has demonstrated cortical degeneration with a mutation-specific neurodegeneration pattern years before onset of clinical symptoms in presymptomatic familial FTLD mutation carriers. In addition, diffusion tensor imaging (DTI) has shown the loss of white matter microstructural integrity in the presymptomatic stage of familial FTLD. Furthermore, proton magnetic resonance spectroscopy (¹H MRS), which provides a non-invasive measurement of brain biochemistry, has identified early neurochemical abnormalities in presymptomatic MAPT mutation carriers. Positron emission tomography (PET) imaging with [¹⁸F]-fluorodeoxyglucose (FDG) has demonstrated the glucose hypometabolism in the presymptomatic stage of familial FTLD. Also, a novel PET ligand, ¹⁸F-AV-1451, has been used in this group to evaluate tau deposition in the brain. Promising imaging biomarkers for presymptomatic familial FTLD have been identified and assessed for specificity and sensitivity for accurate prediction of symptom onset and tracking disease progression during the presymptomatic stage when clinical measures are not useful. Furthermore, identifying imaging biomarkers for the presymptomatic stage is important for the design of disease-modifying trials. We review the recent progress in imaging biomarkers of the presymptomatic phase of familial FTLD and discuss the imaging techniques and analysis methods, with a focus on the potential implication of these imaging techniques and their utility in specific mutation types.

Update on the association between alpha-synuclein and tau with mitochondrial dysfunction: Implications for Parkinson's disease

The critical role of mitochondrial dysfunction in the pathological mechanisms of neurodegenerative disorders, particularly Parkinson's disease (PD), is well established. Compelling evidence indicates that Parkinson's proteins (e.g., α-synuclein, Parkin, PINK1, DJ-1, and LRRK2) are associated with mitochondrial dysfunction and oxidative stress in PD. Significantly, there is a possible central role of alpha-synuclein (α-Syn) in the occurrence of mitochondrial dysfunction and oxidative stress by the mediation of different signaling pathways. Also, tau, traditionally considered as the main component of neurofibrillary tangles, aggregates and amplifies the neurotoxic effects on mitochondria by interacting with α-Syn. Moreover, oxidative stress caused by mitochondrial dysfunction favors assembly of both α-Syn and tau and also plays a key role in the formation of protein aggregates. In this review, we provide an overview of the relationship between these two pathological proteins and mitochondrial dysfunction in PD, and also summarize the underlying mechanisms in the interplay of α-Syn aggregation and phosphorylated tau targeting the mitochondria, to find new strategies to prevent PD processing.

Aging and Alzheimer's disease pathology

The number of people with dementia worldwide is predicted to increase to 131.5 million by 2050. When studying dementia, understanding the basis of the neuropathological background is very important. Taking Alzheimer's disease (AD) neuropathology as an example, we know that the accumulation of abnormal structures such as senile plaques and neurofibrillary tangles is a hallmark. Macroscopic atrophy affects the entorhinal area and hippocampus, amygdala, and associative regions of the neocortex. Braak advocates the spread of tau deposits from the entorhinal to associative regions of the neocortex as the disease progresses. If the AD has only tau pathology, the degree and distribution of tau deposition may be associated with clinical symptoms. However, AD is also accompanied by amyloid-β deposition and even atrophy. Although it is possible to make a neuropathological diagnosis of AD from the spread of amyloid and tau depositions, neuropathological abnormal protein accumulation cannot explain all clinical symptoms of AD. There is an ambiguity between clinical symptoms and neuropathological findings. It is important to understand neuropathological findings while understanding that this ambiguity exists. So, for the reader's help, first we briefly explain the changes in the brain with age, and then describe AD as a typical disease of dementia; finally we will describe the diseases that mimic AD for neurologists who are not experts in neuropathology.

Tracking white matter degeneration in asymptomatic and symptomatic MAPT mutation carriers

Our aim was to investigate the patterns and trajectories of white matter (WM) diffusion abnormalities in microtubule-associated protein tau (MAPT) mutations carriers. We studied 22 MAPT mutation carriers (12 asymptomatic, 10 symptomatic) and 20 noncarriers from 8 families, who underwent diffusion tensor imaging (DTI) and a subset (10 asymptomatic, 6 symptomatic MAPT mutation carriers, and 10 noncarriers) were followed annually (median = 4 years). Cross-sectional and longitudinal changes in mean diffusivity (MD) and fractional anisotropy were analyzed. Asymptomatic MAPT mutation carriers had higher MD in entorhinal WM, which propagated to the limbic tracts and frontotemporal projections in the symptomatic stage compared with noncarriers. Reduced fractional anisotropy and increased MD in the entorhinal WM were associated with the proximity to estimated and actual age of symptom onset. The annualized change of entorhinal MD on serial DTI was accelerated in MAPT mutation carriers compared with noncarriers. Entorhinal WM diffusion abnormalities precede the symptom onset and track with disease progression in MAPT mutation carriers. Our cross-sectional and longitudinal data showed a potential clinical utility for DTI to track neurodegenerative disease progression for MAPT mutation carriers in clinical trials.

Yeast surface display of full-length human microtubule-associated protein tau

Microtubule-associated protein tau is an intrinsically disordered, highly soluble protein found primarily in neurons. Under normal conditions, tau regulates the stability of axonal microtubules and intracellular vesicle transport. However, in patients of neurodegeneration such as Alzheimer's disease (AD), tau forms neurofibrillary deposits, which correlates well with the disease progression. Identifying molecular signatures in tau, such as posttranslational modification, truncation, and conformational change has great potential to detect earliest signs of neurodegeneration and develop therapeutic strategies. Here, we show that full-length human tau, including the longest isoform found in the adult brain, can be robustly displayed on the surface of yeast Saccharomyces cerevisiae. Yeast-displayed tau binds to anti-tau antibodies that cover epitopes ranging from the N-terminus to the 4R repeat region. Unlike tau expressed in the yeast cytosol, surface-displayed tau was not phosphorylated at sites found in AD patients (probed by antibodies AT8, AT270, AT180, and PHF-1). However, yeast-displayed tau showed clear binding to paired helical filament (PHF) tau conformation-specific antibodies Alz-50, MC-1, and Tau-2. Although the tau possessed a conformation found in PHFs, oligomerization or aggregation into larger filaments was undetected. Taken together, yeast-displayed tau enables robust measurement of protein interactions and is of particular interest for characterizing conformational change.

Tauopathy in the periaqueductal gray, kölliker-fuse nucleus and nucleus retroambiguus is not predicted by ultrasonic vocalization in tau-P301L mice

Upper airway and vocalization control areas such as the periaqueductal gray (PAG), kölliker-fuse nucleus (KF) and nucleus retroambiguus (NRA) are prone to developing tauopathy in mice expressing the mutant human tau P301L protein. Consequently, impaired ultrasonic vocalization (USV) previously identified in tau-P301L mice at the terminal disease stage of 8-9 months of age, was attributed to the presence of tauopathy in these regions. Our aim was to establish whether the onset of USV disorders manifest prior to the terminal stage, and if USV disorders are predictive of the presence of tauopathy in the PAG, KF and NRA. USVs produced by tau-P301L and wildtype mice aged 3-4, 5-6 or 8-9 months were recorded during male-female interaction. Immunohistochemistry was then performed to assess the presence or degree of tauopathy in the PAG, KF and NRA of mice displaying normal or abnormal USV patterns. Comparing various USV measurements, including the number, duration and frequency of calls, revealed no differences between tau-P301L and wildtype mice across all age groups, and linear discriminant analysis also failed to identify separate USV populations. Finally, the presence of tauopathy in the PAG, KF and NRA in individual tau-P301L mice did not reliably associate with USV disorders. Our findings that tauopathy in designated mammalian vocalization centres, such as the PAG, KF and NRA, did not associate with USV disturbances in tau-P301L mice questions whether USV phenotypes in this transgenic mouse are valid for studying tauopathy-related human voice and speech disorders.

Brain MR Spectroscopy Changes Precede Frontotemporal Lobar Degeneration Phenoconversion in Mapt Mutation Carriers

BACKGROUND AND PURPOSE

The objective of this study was to longitudinally investigate the trajectory of change in 1 H MRS measurements in asymptomatic MAPT mutation carriers who became symptomatic during follow-up, and to determine the time at which the neurochemical alterations accelerated during disease progression.

METHODS

We identified eight MAPT mutations carriers who transitioned from asymptomatic to symptomatic disease during follow-up. All participants were longitudinally followed with an average of 7.75 years (range 4-11 years) and underwent two or more single voxel 1 H MRS examinations from the posterior cingulate voxel, with a total of 60 examinations. The rate of longitudinal change for each metabolite was estimated using linear mixed models. A flex point model was used to estimate the flex time point of the change in slope.

RESULTS

The decrease in the NAA/mI ratio accelerated 2.09 years prior to symptom onset, and continued to decline. A similar trajectory was observed in the presumed glial marker mI/Cr ratio accelerating 1.86 years prior to symptom onset.

CONCLUSIONS

Our findings support the potential use of longitudinal 1 H MRS for monitoring the neurodegenerative progression in MAPT mutation carriers starting from the asymptomatic stage.

Rates of lobar atrophy in asymptomatic MAPT mutation carriers

INTRODUCTION

The aim of this study was to investigate the rates of lobar atrophy in the asymptomatic microtubule-associated protein tau (MAPT) mutation carriers.

METHODS

MAPT mutation carriers (n = 14; 10 asymptomatic, 4 converters from asymptomatic to symptomatic) and noncarriers (n = 13) underwent structural magnetic resonance imaging and were followed annually with a median of 9.2 years. Longitudinal changes in lobar atrophy were analyzed using the tensor-based morphometry with symmetric normalization algorithm.

RESULTS

The rate of temporal lobe atrophy in asymptomatic MAPT mutation carriers was faster than that in noncarriers. Although the greatest rate of atrophy was observed in the temporal lobe in converters, they also had increased atrophy rates in the frontal and parietal lobes compared to noncarriers.

DISCUSSION

Accelerated decline in temporal lobe volume occurs in asymptomatic MAPT mutation carriers followed by the frontal and parietal lobe in those who have become symptomatic. The findings have implications for monitoring the progression of neurodegeneration during clinical trials in asymptomatic MAPT mutation carriers.

Frontal lobe 1H MR spectroscopy in asymptomatic and symptomatic MAPT mutation carriers

OBJECTIVE

To determine the frontal lobe proton magnetic resonance spectroscopy (¹H MRS) abnormalities in asymptomatic and symptomatic carriers of microtubule-associated protein tau (MAPT) mutations.

METHODS

We recruited patients with MAPT mutations from 5 individual families, who underwent single voxel ¹H MRS from the medial frontal lobe at 3T (n = 19) from the Longitudinal Evaluation of Familial Frontotemporal Dementia Subjects (LEFFTDS) Study at the Mayo Clinic site. Asymptomatic MAPT mutation carriers (n = 9) had Frontotemporal Lobar Degeneration Clinical Dementia Rating Sum of Boxes (FTLD-CDR SOB) score of zero, and symptomatic MAPT mutation carriers (n = 10) had a median FTLD-CDR SOB score of 5. Noncarriers from healthy first-degree relatives of the patients were recruited as controls (n = 25). The demographic aspects and ¹H MRS metabolite ratios were compared by use of the Fisher exact test for sex and linear mixed models to account for within-family correlations. We used Tukey contrasts for pair-wise comparisons.

RESULTS

Asymptomatic MAPT mutation carriers had lower neuronal marker N-acetylaspartate (NAA)/creatine (Cr) (p = 0.001) and lower NAA/myo-inositol (mI) (p = 0.026) than noncarriers after adjustment for age. Symptomatic MAPT mutation carriers had lower NAA/Cr (p = 0.01) and NAA/mI (p = 0.01) and higher mI/Cr (p = 0.02) compared to noncarriers after adjustment for age. Furthermore, NAA/Cr (p = 0.006) and NAA/mI (p < 0.001) ratios decreased, accompanied by an increase in mI/Cr ratio (p = 0.001), as the ages of carriers approached and passed the age at symptom onset.

CONCLUSION

Frontal lobe neurochemical alterations measured with ¹H MRS precede the symptom onset in MAPT mutation carriers. Frontal lobe ¹H MRS is a potential biomarker for early neurodegenerative processes in MAPT mutation carriers.

Tau and TDP-43 proteinopathies: kindred pathologic cascades and genetic pleiotropy

We review the literature on Tau and TDP-43 proteinopathies in aged human brains and the relevant underlying pathogenetic cascades. Complex interacting pathways are implicated in Alzheimer's disease and related dementias (ADRD), wherein multiple proteins tend to misfold in a manner that is "reactive," but, subsequently, each proteinopathy may contribute strongly to the clinical symptoms. Tau proteinopathy exists in brains of individuals across a broad spectrum of primary underlying conditions-e.g., developmental, traumatic, and inflammatory/infectious diseases. TDP-43 proteinopathy is also expressed in a wide range of clinical disorders.  Although TDP-43 proteinopathy was first described in the central nervous system of patients with amyotrophic lateral sclerosis (ALS) and in subtypes of frontotemporal dementia (FTD/FTLD), TDP-43 proteinopathy is also present in chronic traumatic encephalopathy, cognitively impaired persons in advanced age with hippocampal sclerosis, Huntington's disease, and other diseases. We list known Tau and TDP-43 proteinopathies.  There is also evidence of cellular co-localization between Tau and TDP-43 misfolded proteins, suggesting common pathways or protein interactions facilitating misfolding in one protein by the other. Multiple pleiotropic gene variants can alter risk for Tau or TDP-43 pathologies, and certain gene variants (e.g., APOE ε4, Huntingtin triplet repeats) are associated with increases of both Tau and TDP-43 proteinopathies. Studies of genetic risk factors have provided insights into multiple nodes of the pathologic cascades involved in Tau and TDP-43 proteinopathies. Variants from a specific gene can be either a low-penetrant risk factor for a group of diseases, or alternatively, a different variant of the same gene may be a disease-driving allele that is associated with a relatively aggressive and early-onset version of a clinically and pathologically specific disease type. Overall, a complex but enlightening paradigm has emerged, wherein both Tau and TDP-43 proteinopathies are linked to numerous overlapping upstream influences, and both are associated with multiple downstream pathologically- and clinically-defined deleterious effects.

Neuropathological changes and cognitive deficits in rats transgenic for human mutant tau recapitulate human tauopathy

The assembly of tau protein into abnormal filaments and brain cell degeneration are characteristic of a number of human neurodegenerative diseases, including Alzheimer's disease and frontotemporal dementia and parkinsonism linked to chromosome 17. Several murine models have been generated to better understand the mechanisms contributing to tau assembly and neurodegeneration. Taking advantage of the more elaborate central nervous system and higher cognitive abilities of the rat, we generated a model expressing the longest human tau isoform (2N4R) with the P301S mutation. This transgenic rat line, R962-hTau, exhibits the main features of human tauopathies, such as: age-dependent increase in inclusions comprised of aggregated-tau, neuronal loss, global neurodegeneration as reflected by brain atrophy and ventricular dilation, alterations in astrocytic and microglial morphology, and myelin loss. In addition, substantial deficits across multiple memory and learning paradigms, including novel object recognition, fear conditioning and Morris water maze tasks, were observed at the time of advanced tauopathy. These results support the concept that progressive tauopathy correlates with brain atrophy and cognitive impairment.

Neurodegenerative Diseases and Ageing

This chapter describes the main neuropathological features of the most common age associated neurodegenerative diseases including Alzheimer's disease, Lewy body diseases, vascular dementia and the various types of frontotemporal lobar degeneration. In addition, the more recent concepts of primary age-related tauopathy and ageing-related tau astrogliopathy as well as chronic traumatic encephalopathy are briefly described. One section is dedicated to cerebral multi-morbidity as it is becoming increasingly clear that the old brain is characterised by the presence of multiple pathologies (to varying extent) rather than by one single, disease specific pathology alone. The main aim of this chapter is to inform the reader about the neuropathological basics of age associated neurodegenerative diseases as we feel this is crucial to meaningfully interpret the vast literature that is published in the broad field of dementia research.

Differential effects of 14-3-3 dimers on Tau phosphorylation, stability and toxicity in vivo

Neurodegenerative dementias collectively known as Tauopathies involve aberrant phosphorylation and aggregation of the neuronal protein Tau. The largely neuronal 14-3-3 proteins are also elevated in the central nervous system (CNS) and cerebrospinal fluid of Tauopathy patients, suggesting functional linkage. We use the simplicity and genetic facility of the Drosophila system to investigate in vivo whether 14-3-3s are causal or synergistic with Tau accumulation in precipitating pathogenesis. Proteomic, biochemical and genetic evidence demonstrate that both Drosophila 14-3-3 proteins interact with human wild-type and mutant Tau on multiple sites irrespective of their phosphorylation state. 14-3-3 dimers regulate steady-state phosphorylation of both wild-type and the R406W mutant Tau, but they are not essential for toxicity of either variant. Moreover, 14-3-3 elevation itself is not pathogenic, but recruitment of dimers on accumulating wild-type Tau increases its steady-state levels ostensibly by occluding access to proteases in a phosphorylation-dependent manner. In contrast, the R406W mutant, which lacks a putative 14-3-3 binding site, responds differentially to elevation of each 14-3-3 isoform. Although excess 14-3-3ζ stabilizes the mutant protein, elevated D14-3-3ɛ has a destabilizing effect probably because of altered 14-3-3 dimer composition. Our collective data demonstrate the complexity of 14-3-3/Tau interactions in vivo and suggest that 14-3-3 attenuation is not appropriate ameliorative treatment of Tauopathies. Finally, we suggest that 'bystander' 14-3-3s are recruited by accumulating Tau with the consequences depending on the composition of available dimers within particular neurons and the Tau variant.

Tau Interacting Proteins: Gaining Insight into the Roles of Tau in Health and Disease

Tau is most intensely studied in relation to its executive role in Tauopathies, a family of neurodegenerative disorders characterized by the accumulation of Tau aggregates [15, 21, 38, 75, 89, 111, 121, 135, 175, 176, 192]. Tau aggregation in the different Tauopathies differs in the affected cell type, the structure of aggregates and Tau isoform composition. However, in all Tauopathies, accumulation of pathological Tau in well-characterized and well-defined brain regions, correlates strongly with symptoms associated with the dysfunction of this brain region. Hence, symptoms of neurodegenerative Tauopathies can range from motoric to cognitive and behavioral symptoms, even extending to deterioration of vital functions when the disease progresses, or combinations of different symptoms governed by the affected brain regions. The most common Tauopathies are corticobasal degeneration (CBD), Pick's disease, progressive supranuclear palsy (PSP) and frontotemporal dementias with parkinsonism linked to chromosome 17 (FTDP-17). However a growing number of diseases are characterized by Tau aggregation amounting to a large family of more than 20 disorders [176]. Most Tauopathies are sporadic, and are hence linked to a combination of environmental and genetic risk factors. However, mutations in MAPT have been identified which are autosomal dominantly linked to Tauopathies, including FTDP, PSP and CBD [94, 163, 185] (Alzforum, https://www.alzforum.org/mutations/mapt ). More than 80 mutations have been identified in MAPT, both in intronic and exonic regions of the human MAPT. These mutations can be classified as missense mutations or splicing mutations. Most missense mutations cluster in or near the microtubule binding site of Tau, while most splicing mutations affect the splicing of exon 10 (encoding the R2 domain), and hence affect the 3R/4R ratio. While Alzheimer's disease (AD), is the most prevalent Tauopathy, no mutations in MAPT associated with AD have been identified. Brains of AD patients are pathologically characterized by the combined presence of amyloid plaques and neurofibrillary tangles [171]. Familial forms of AD, termed early onset familial AD (EOFAD) with clinical mutations in APP or PS1/2, have an early onset, and are invariably characterized by the combined presence of amyloid and Tau pathology [24, 80, 170]. These EOFAD cases, identify a causal link between APP/PS1 misprocessing and the development of Tau pathology and neurodegeneration [80, 170]. Furthermore, combined genetic, pathological, biomarker and in vivo modelling data, indicate that amyloid pathology precedes Tau pathology, and support a role for Aβ as initiator and Tau as executor in the pathogenetic process of AD [80, 96, 97]. Hence, AD is often considered as a secondary Tauopathy (similar as for Down syndrome patients), in contrast to the primary Tauopathies described above. Tau aggregates in Tauopathies vary with respect to the ratio of different Tau isoforms (3R/4R), to the cell types displaying Tau aggregation and the structure of the aggregates. However, in all Tauopathies a strong correlation between progressive development of pathological Tau accumulation and the loss of the respective brain functions is observed.

Delta-secretase (AEP) mediates tau-splicing imbalance and accelerates cognitive decline in tauopathies

Wang et al. demonstrate that AEP cleaves SRPK2 in tauopathies and plays a functional role in mediating tau-splicing imbalance and accelerating cognitive decline in mouse models of tauopathy.

SRPK2 is abnormally activated in tauopathies including Alzheimer’s disease (AD). SRPK2 is known to play an important role in pre–mRNA splicing by phosphorylating SR-splicing factors. Dysregulation of tau exon 10 pre–mRNA splicing causes pathological imbalances in 3R- and 4R-tau, leading to neurodegeneration; however, the role of SRPK2 in these processes remains unclear. Here we show that delta-secretase (also known as asparagine endopeptidase; AEP), which is activated in AD, cleaves SRPK2 and increases its nuclear translocation as well as kinase activity, augmenting exon 10 inclusion. Conversely, AEP-uncleavable SRPK2 N342A mutant increases exon 10 exclusion. Lentiviral expression of truncated SRPK2 increases 4R-tau isoforms and accelerates cognitive decline in htau mice. Uncleavable SRPK2 N342A expression improves synaptic functions and prevents spatial memory deficits in tau intronic mutant FTDP-17 transgenic mice. Hence, AEP mediates tau-splicing imbalance in tauopathies via cleaving SRPK2.

New advances in tau imaging in parkinsonism

Currently, the differential diagnosis between atypical parkinsonisms and classical idiopathic Parkinson's disease can be quite difficult because of the significant overlap of clinical presentation and symptoms. Neurodegenerative conditions, including progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia (FTD), are primarily characterized by accumulation of tau protein in the brain. Recent imaging developments for tau pathology may provide a promising tool for the assessment of diagnosis, prognosis, and progression of these neurodegenerative disorders. This review will survey PET studies to describe the recent advances in the imaging of tau pathology in PSP, CBD, and FTD.

Tau can switch microtubule network organizations: from random networks to dynamic and stable bundles

Tau is a neuronal microtubule bundler that is known to stabilize microtubules by promoting their growth and inhibiting their shrinkage. This study reveals novel mechanisms by which tau is able to switch microtubule network organizations via the differential regulation of microtubule bundling and dynamics.

In neurons, microtubule networks alternate between single filaments and bundled arrays under the influence of effectors controlling their dynamics and organization. Tau is a microtubule bundler that stabilizes microtubules by stimulating growth and inhibiting shrinkage. The mechanisms by which tau organizes microtubule networks remain poorly understood. Here, we studied the self-organization of microtubules growing in the presence of tau isoforms and mutants. The results show that tau’s ability to induce stable microtubule bundles requires two hexapeptides located in its microtubule-binding domain and is modulated by its projection domain. Site-specific pseudophosphorylation of tau promotes distinct microtubule organizations: stable single microtubules, stable bundles, or dynamic bundles. Disease-related tau mutations increase the formation of highly dynamic bundles. Finally, cryo–electron microscopy experiments indicate that tau and its variants similarly change the microtubule lattice structure by increasing both the protofilament number and lattice defects. Overall, our results uncover novel phosphodependent mechanisms governing tau’s ability to trigger microtubule organization and reveal that disease-related modifications of tau promote specific microtubule organizations that may have a deleterious impact during neurodegeneration.

Intrafamilial phenotypic heterogeneity in a Taiwanese family with a MAPT p.R5H mutation: a case report and literature review

Background

Frontotemporal degeneration (FTD) is a clinically and genetically heterogeneous neurodegenerative disorder characterized by deficits in executive function that frequently overlaps with parkinsonism and motor neuron disorders. Several genes have been identified to cause autosomal dominant forms of FTD, including the gene coding for the protein associated with microtubule tau (MAPT). While most reported pathogenic mutations in MAPT occur in exons 9–13, few families have been reported with mutations outside of this region. Herein, we report a first Taiwanese family having the exon 1 p.Arg5His mutation in MAPT with intrafamilial phenotype heterogeneity.

Case presentation

A 63-year-old man presented with progressive non-fluent speech and impaired memory for 3 years. He then developed apraxia, myoclonus and parkinsonism feature at his right hand. Extensive neurologic and neurocognitive examination lead to a diagnosis of FTD mixed with corticobasal syndrome. Magnetic resonance imaging revealed asymmetric atrophy in the left frontal and temporal lobes and single-photon emission computed tomography indicated decreased metabolism in the same areas as well as the left basal ganglia. The patient’s mother had been diagnosed with amyotrophic lateral sclerosis (ALS) at the age of 60 and was deceased 10 years later due to respiratory failure. The patient’s younger sister had persistent depressive disorder in her early forties and did not have any prominent cognitive or motor dysfunctions. We performed genetic analysis applying a targeted next generation sequencing (NGS) panel covering MAPT, GRN, VCP, FUS, CHMP2B, and TARDBP on the proband, followed by Sanger sequencing of candidate genes in eight family members. Hexanucleotide repeat expansion of C9Orf72 was determined by repeat-primed PCR. We identified a missense mutation in exon 1 of MAPT gene, c.14G > A (p.R5H), which was previously reported in only two Japanese patients in a literature review. This substitution co-segregated with the disease phenotypes in the family.

Conclusions

This is the first report of the occurrence of the MAPT p.R5H mutation in the Taiwanese population. Our findings extend the current knowledge of phenotypic heterogeneity among family members carrying the MAPT p.R5H mutation.

Transactive response DNA-binding protein 43 (TDP-43) regulates alternative splicing of tau exon 10: Implications for the pathogenesis of tauopathies

Hyperphosphorylation and aggregation of the neuronal protein tau are responsible for neurodegenerative diseases called tauopathies. Dysregulation of the alternative splicing of tau exon 10 results in alterations of the ratio of two tau isoforms, 3R-tau and 4R-tau, which have been seen in several tauopathies. Transactive response DNA-binding protein of 43 kDa (TDP-43) is involved in the regulation of RNA processing, including splicing. Cytoplasmic aggregation of TDP-43 has been observed in the brains of individuals with chronic traumatic encephalopathy or Alzheimer's disease, diseases in which neurofibrillary tangles of hyperphosphorylated tau are hallmarks. Here, we investigated the role of TDP-43 in tau exon 10 splicing. We found that TDP-43 promoted tau exon 10 inclusion, which increased production of the 4R-tau isoform. Moreover, TDP-43 could bind to intron 9 of tau pre-mRNA. Deletion of the TDP-43 N or C terminus promoted its cytoplasmic aggregation and abolished or diminished TDP-43-promoted tau exon 10 inclusion. Several TDP-43 mutations associated with amyotrophic lateral sclerosis or frontotemporal lobar degeneration with ubiquitin inclusions promoted tau exon 10 inclusion more effectively than wild-type TDP-43 but did not affect TDP-43 cytoplasmic aggregation in cultured cells. The ratio of 3R-tau/4R-tau was decreased in transgenic mouse brains expressing human TDP-43 and increased in the brains expressing the disease-causing mutation TDP-43^M337V, in which cytoplasmic TDP-43 was increased. These findings suggest that TDP-43 promotes tau exon 10 inclusion and 4R-tau expression and that disease-related changes of TDP-43, truncations and mutations, affect its function in tau exon 10 splicing, possibly because of TDP-43 mislocalization to the cytoplasm.

Mutation Frequency of the Major Frontotemporal Dementia Genes, MAPT, GRN and C9ORF72 in a Turkish Cohort of Dementia Patients

‘Microtubule-associated protein tau’ (MAPT), ‘granulin’ (GRN) and ‘chromosome 9 open reading frame72’ (C9ORF72) gene mutations are the major known genetic causes of frontotemporal dementia (FTD). Recent studies suggest that mutations in these genes may also be associated with other forms of dementia. Therefore we investigated whether MAPT, GRN and C9ORF72 gene mutations are major contributors to dementia in a random, unselected Turkish cohort of dementia patients. A combination of whole-exome sequencing, Sanger sequencing and fragment analysis/Southern blot was performed in order to identify pathogenic mutations and novel variants in these genes as well as other FTD-related genes such as the ‘charged multivesicular body protein 2B’ (CHMP2B), the ‘FUS RNA binding protein’ (FUS), the ‘TAR DNA binding protein’ (TARDBP), the ‘sequestosome1’ (SQSTM1), and the ‘valosin containing protein’ (VCP). We determined one pathogenic MAPT mutation (c.1906C>T, p.P636L) and one novel missense variant (c.38A>G, p.D13G). In GRN we identified a probably pathogenic TGAG deletion in the splice donor site of exon 6. Three patients were found to carry the GGGGCC expansions in the non-coding region of the C9ORF72 gene. In summary, a complete screening for mutations in MAPT, GRN and C9ORF72 genes revealed a frequency of 5.4% of pathogenic mutations in a random cohort of 93 Turkish index patients with dementia.

Behind the curtain of tauopathy: a show of multiple players orchestrating tau toxicity

tau, a microtubule-associated protein, directly binds with microtubules to dynamically regulate the organization of cellular cytoskeletons, and is especially abundant in neurons of the central nervous system. Under disease conditions such as Pick's disease, progressive supranuclear palsy, frontotemporal dementia, parkinsonism linked to chromosome 17 and Alzheimer's disease, tau proteins can self-assemble to paired helical filaments progressing to neurofibrillary tangles. In these diseases, collectively referred to as "tauopathies", alterations of diverse tau modifications including phosphorylation, metal ion binding, glycosylation, as well as structural changes of tau proteins have all been observed, indicating the complexity and variability of factors in the regulation of tau toxicity. Here, we review our current knowledge and hypotheses from relevant studies on tau toxicity, emphasizing the roles of phosphorylations, metal ions, folding and clearance control underlining tau etiology and their regulations. A summary of clinical efforts and associated findings of drug candidates under development is also presented. It is hoped that a more comprehensive understanding of tau regulation will provide us with a better blueprint of tau networking in neuronal cells and offer hints for the design of more efficient strategies to tackle tau-related diseases in the future.

Whole-genome sequencing reveals important role for TBK1 and OPTN mutations in frontotemporal lobar degeneration without motor neuron disease

Frontotemporal lobar degeneration with TAR DNA-binding protein 43 inclusions (FTLD-TDP) is the most common pathology associated with frontotemporal dementia (FTD). Repeat expansions in chromosome 9 open reading frame 72 (C9ORF72) and mutations in progranulin (GRN) are the major known genetic causes of FTLD-TDP; however, the genetic etiology in the majority of FTLD-TDP remains unexplained. In this study, we performed whole-genome sequencing in 104 pathologically confirmed FTLD-TDP patients from the Mayo Clinic brain bank negative for C9ORF72 and GRN mutations and report on the contribution of rare single nucleotide and copy number variants in 21 known neurodegenerative disease genes. Interestingly, we identified 5 patients (4.8 %) with variants in optineurin (OPTN) and TANK-binding kinase 1 (TBK1) that are predicted to be highly pathogenic, including two double mutants. Case A was a compound heterozygote for mutations in OPTN, carrying the p.Q235* nonsense and p.A481V missense mutation in trans, while case B carried a deletion of OPTN exons 13-15 (p.Gly538Glufs*27) and a loss-of-function mutation (p.Arg117*) in TBK1. Cases C-E carried heterozygous missense mutations in TBK1, including the p.Glu696Lys mutation which was previously reported in two amyotrophic lateral sclerosis (ALS) patients and is located in the OPTN binding domain. Quantitative mRNA expression and protein analysis in cerebellar tissue showed a striking reduction of OPTN and/or TBK1 expression in 4 out of 5 patients supporting pathogenicity in these specific patients and suggesting a loss-of-function disease mechanism. Importantly, neuropathologic examination showed FTLD-TDP type A in the absence of motor neuron disease in 3 pathogenic mutation carriers. In conclusion, we highlight TBK1 as an important cause of pure FTLD-TDP, identify the first OPTN mutations in FTLD-TDP, and suggest a potential oligogenic basis for at least a subset of FTLD-TDP patients. Our data further add to the growing body of evidence linking ALS and FTD and suggest a key role for the OPTN/TBK1 pathway in these diseases.

Homozygous MAPT R406W mutation causing FTDP phenotype: A unique instance of a unique mutation

Frontotemporal dementia is a neurodegenerative disorder among adults. An autosomal-dominantly form of frontotemporal dementia and parkinsonism linked to chromosome 17q21.2 (FTDP-17) was defined in 1996. The MAPT gene is responsible for the major cases of FTDP-17, and tau also has a role in Alzheimer's disease. So far, different FTDP-17 causing mutations have been identified in the MAPT gene. Among different MAPT mutations, the R406W mutation has been reported with a phenotype resembling Alzheimer's disease. Nonetheless, in this study we have identified the first homozygous case of R406W mutation in an Iranian family which shows characteristics of FTDP, just like the other heterozygous mutations of MAPT. This study clearly indicates that homozygous R406W mutation could result in FTDP phenotype. Our family confirms heterogeneity in the clinical phenotype of MAPT mutations; moreover, in the R406W mutation, a dosage effect is likely to contribute to this clinical heterogeneity.

The extracellular matrix in plasticity and regeneration after CNS injury and neurodegenerative disease

Chondroitin sulfate proteoglycans (CSPGs) are involved in several processes relevant to recovery of function after CNS damage. They restrict axon regeneration through their presence in glial scar tissue and plasticity through their presence in perineuronal nets (PNNs), affect memory through their effect on dendritic spines, and influence the inflammatory reaction. Much of our knowledge of these CSPG effects comes from digestion of their glycosaminoglycan chains by the enzyme chondroitinase ABC (ChABC). ChABC after spinal cord injury permits some axon regeneration and greatly increases plasticity through increased sprouting and through digestion of PNNs. When combined with appropriate rehabilitation, ChABC treatment can lead to considerable restoration of function. ChABC treatment of the perirhinal cortex greatly increases retention of object recognition memory. When applied to tauopathy animals that model Alzheimer's disease, ChABC digestion can restore normal object recognition memory. CSPGs in the adult CNS are found throughout the ECM, but 2% is concentrated in PNNs that surround GABAergic parvalbumin interneurons and other neurons. Knockout of the PNN-organizing protein Crtl1 link protein attenuates PNNs and leads to continued plasticity into adulthood, demonstrating that the CSPGs in PNNs are the key components in the control of plasticity. CSPGs act mainly through their sulfated glycosaminoglycan chains. A disulfated CS-E motif in these chains is responsible for binding of Semaphorin 3A to PNNs where it affects ocular dominance plasticity and probably other forms of plasticity. In addition OTX2 binds to CS-E motifs, where it can act on parvalbumin interneurons to maintain the PNNs.

In vivo axonal transport deficits in a mouse model of fronto-temporal dementia

Background

Axonal transport is vital for neurons and deficits in this process have been previously reported in a few mouse models of Alzheimer's disease prior to the appearance of plaques and tangles. However, it remains to be determined whether axonal transport is defective prior to the onset of neurodegeneration. The rTg4510 mouse, a fronto-temporal dementia and parkinsonism-17 (FTDP-17) tauopathy model, over-express tau-P301L mutation found in familial forms of FTDP-17, in the forebrain driven by the calcium–calmodulin kinase II promoter. This mouse model exhibits tau pathology, neurodegeneration in the forebrain, and associated behavioral deficits beginning at 4–5 months of age.

Animal model

rTg4510 transgenic mice were used in these studies. Mice were given 2 μL of MnCl₂ in each nostril 1 h prior to Magnetic Resonance Imaging (MRI). Following MnCl₂ nasal lavage, mice were imaged using Manganese enhanced Magnetic Resonance Imaging (MEMRI) Protocol with TE = 8.5 ms, TR = 504 ms, FOV = 3.0 cm, matrix size = 128 × 128 × 128, number of cycles = 15 with each cycle taking approximately 2 min, 9 s, and 24 ms using Paravision software (BrukerBioSpin, Billerica, MA). During imaging, body temperature was maintained at 37.0 °C using an animal heating system (SA Instruments, Stony Brook, NY).

Data analysis

Resulting images were analyzed using Paravision software. Regions of interest (ROI) within the olfactory neuronal layer (ONL) and the water phantom consisting of one pixel (ONL) and 9 pixels (water) were selected and copied across each of the 15 cycles. Signal intensities (SI) of ONL and water phantom ROIs were measured. SI values obtained for ONL were then normalized the water phantom SI values. The correlation between normalized signal intensity in the ONL and time were assessed using Prism (GraphPad Software, San Diego, CA).

Results

Using the MEMRI technique on 1.5, 3, 5, and 10-month old rTg4510 mice and littermate controls, we found significant axonal transport deficits present in the rTg4510 mice beginning at 3 months of age in an age-dependent manner. Using linear regression analysis, we measured rates of axonal transport at 1.5, 3, 5, and 10 months of age in rTg4510 and WT mice. Axonal transport rates were observed in rTg4510 mice at 48% of WT levels at 3 months, 40% of WT levels at 5 months, and 30% of WT levels at 10 months of age. In order to determine the point at which tau appears in the cortex, we probed for phosphorylated tau levels, and found that pSer262 is present at 3 months of age, not earlier at 1.5 months of age, but observed no pathological tau species until 6 months of age, months after the onset of the transport deficits. In addition, we saw localization of tau in the ONL at 6 months of age.

Discussion

In our study, we identified the presence of age-dependent axonal transport deficits beginning at 3 months of age in rTg4510 mice. We correlated these deficits at 3 months to the presence of hyperphosphorylated tau in the brain and the presence within the olfactory epithelium. We observed tau pathology not only in the soma of these neurons but also within the axons and processes of these neurons. Our characterization of axonal transport in this tauopathy model provides a functional time point that can be used for future therapeutic interventions.

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We used MEMRI to define axonal transport rate changes in the rTg4510 mouse.

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We observed significant hyperphosphorylated tau starting at 3 months of age.

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We found an age-dependent decline in axonal transport rates.

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Declines in axonal transport correlated with increases in hyperphosphorylated tau.

Interactions between Tau and α-synuclein augment neurotoxicity in a Drosophila model of Parkinson's disease

Clinical and pathological studies have suggested considerable overlap between tauopathies and synucleinopathies. Several genome-wide association studies have identified alpha-Synuclein (SNCA) and Tau (MAPT) polymorphisms as common risk factors for sporadic Parkinson's disease (PD). However, the mechanisms by which subtle variations in the expression of wild-type SNCA and MAPT influence risk for PD and the underlying cellular events that effect neurotoxicity remain unclear. To examine causes of neurotoxicity associated with the α-Syn/Tau interaction, we used the fruit fly as a model. We utilized misexpression paradigms in three different tissues to probe the α-Syn/Tau interaction: the retina, dopaminergic neurons and the larval neuromuscular junction. Misexpression of Tau and α-Syn enhanced a rough eye phenotype and loss of dopaminergic neurons in fly tauopathy and synucleinopathy models, respectively. Our findings suggest that interactions between α-Syn and Tau at the cellular level cause disruption of cytoskeletal organization, axonal transport defects and aberrant synaptic organization that contribute to neuronal dysfunction and death associated with sporadic PD. α-Syn did not alter levels of Tau phosphorylated at the AT8 epitope. However, α-Syn and Tau colocalized in ubiquitin-positive aggregates in eye imaginal discs. The presence of Tau also led to an increase in urea soluble α-Syn. Our findings have important implications in understanding the cellular and molecular mechanisms underlying α-Syn/Tau-mediated synaptic dysfunction, which likely arise in the early asymptomatic phase of sporadic PD.

Biosensing MAPs as "roadblocks": kinesin-based functional analysis of tau protein isoforms and mutants using suspended microtubules (sMTs)

The concept of a reconstructed microtubule kinesin-based transport system was originally introduced for studies of underlying biophysical mechanisms of intracellular transport and its potential applications in bioengineering at micro- and nanoscale levels. However, several technically challenging shortcomings prohibit its use in practical applications. One of them is the propensity of microtubules to bind various protein molecules creating "roadblocks" for kinesin molecule movement and subsequently preventing efficient delivery of the molecular cargo. The interruption in kinesin movement strictly depends on the specific type of "roadblock", i.e. the microtubule associated protein (MAP). Therefore, we propose to use the "roadblock" effect as a molecular sensor that may be used for functional characterization of particular MAPs with respect to their role in MT-based transport and associated pathologies, such as neurodegeneration. Here, we applied a kinesin-based assay using a suspended MT design (sMT assay) to functionally characterize known MAP tau protein isoforms and common mutations found in familial frontotemporal dementia (FTD). The proposed sMT assay is compatible with an on-chip format and may be used for the routine characterization of MT associated molecules applicable to diagnostics and translational research.

Role of tau protein in neuronal damage in Alzheimer's disease and Down syndrome

Neurodegenerative disorders constitute a growing concern worldwide. Their incidence has increased steadily, in particular among the elderly, a high-risk population that is becoming an important segment of society. Neurodegenerative mechanisms underlie many ailments such as Parkinson's disease, Huntington's disease, Alzheimer's disease (AD) and Down syndrome (DS, trisomy 21). Interestingly, there is increasing evidence suggesting that many such diseases share pathogenic mechanisms at the cellular and subcellular levels. These include altered protein misfolding, impaired autophagy, mitochondrial dysfunction, membrane damage, and altered axonal transport. Regarding AD and DS, the first common link comes from observations that DS patients undergo AD-like pathology early in adulthood. Also, the gene encoding for the amyloid precursor protein is present in human autosome 21 and in murine chromosome 16, an animal model of DS. Important functions related to preservation of normal neuronal architecture are impaired in both conditions. In particular, the stable assembly of microtubules, which is critical for the cytoskeleton, is impaired in AD and DS. In this process, tau protein plays a pivotal role in controlling microtubule stability. Abnormal tau expression and hyperphosphorylation are common features in both conditions, yet the mechanisms leading to these phenomena remain obscure. In the present report we review possible common mechanisms that may alter tau expression and function, in particular in relation to the effect of certain overexpressed DS-related genes, using cellular models of human DS. The latter contributes to the identification of possible therapeutic targets that could aid in the treatment of both AD and DS.

Frontotemporal lobar degeneration: current knowledge and future challenges

Frontotemporal lobar degeneration (FTLD) is one of the most frequent neurodegenerative disorders with a presenile onset. It presents with a spectrum of clinical manifestations, ranging from behavioral and executive impairment to language disorders and motor dysfunction. New diagnostic criteria identified two main cognitive syndromes: behavioral variant frontotemporal dementia (bvFTD) and primary progressive aphasia. Regarding bvFTD, new criteria include the use of biomarkers. According to them, bvFTD can be classified in "possible" (clinical features only), "probable" (inclusion of imaging biomarkers) and "definite" (in the presence of a known causal mutation or at autopsy). Familial aggregation is frequently reported in FTLD, and about 10 % of cases have an autosomal dominant transmission. Microtubule-associated protein tau gene mutations have been the first ones identified, and are generally associated with early onset (40-50 years) and with the bvFTD phenotype. More recently, progranulin gene mutations were recognized in association with the familial form of FTLD and a hexanucleotide repetition in C9ORF72 has been shown to be responsible for familial FTLD and amyotrophic lateral sclerosis. In addition, other genes are linked to rare cases of familiar FTLD. Lastly, a number of genetic risk factors for sporadic forms have also been identified.