Tau pathology and neurodegeneration

Amplification of Tau fibrils from minute quantities of seeds

The propagation of Tau pathology in Alzheimer’s disease (AD) is thought to proceed through templated conversion of Tau protein into fibrils and cell-to-cell transfer of elongation-competent seeds. To investigate the efficiency of Tau conversion, we adapted the protein misfolding cyclic amplification assay used for the conversion of prions. Utilizing heparin as a cofactor and employing repetitive cycles of shearing and growth, synthetic Tau fibrils and Tau fibrils in AD brain extract are progressively amplified. Concurrently, self-nucleation is suppressed. The results highlight breakage-induced replication of Tau fibrils as a potential facilitator of disease spread.

Single-molecule tracking of tau reveals fast kiss-and-hop interaction with microtubules in living neurons

The microtubule-associated phosphoprotein tau regulates microtubule dynamics and is involved in neurodegenerative diseases collectively called tauopathies. It is generally believed that the vast majority of tau molecules decorate axonal microtubules, thereby stabilizing them. However, it is an open question how tau can regulate microtubule dynamics without impeding microtubule-dependent transport and how tau is also available for interactions other than those with microtubules. Here we address this apparent paradox by fast single-molecule tracking of tau in living neurons and Monte Carlo simulations of tau dynamics. We find that tau dwells on a single microtubule for an unexpectedly short time of ∼40 ms before it hops to the next. This dwell time is 100-fold shorter than previously reported by ensemble measurements. Furthermore, we observed by quantitative imaging using fluorescence decay after photoactivation recordings of photoactivatable GFP-tagged tubulin that, despite this rapid dynamics, tau is capable of regulating the tubulin-microtubule balance. This indicates that tau's dwell time on microtubules is sufficiently long to influence the lifetime of a tubulin subunit in a GTP cap. Our data imply a novel kiss-and-hop mechanism by which tau promotes neuronal microtubule assembly. The rapid kiss-and-hop interaction explains why tau, although binding to microtubules, does not interfere with axonal transport.

Pathogenic tau species drive a psychosis-like phenotype in a mouse model of Alzheimer's disease

Psychotic Alzheimer's disease (AD+P) is a rapidly progressive variant of AD associated with an increased burden of frontal tau pathology that affects up to 50% of those with AD, and is observed more commonly in females. To date, there are no safe and effective medication interventions with an indication for treatment in this condition, and there has been only very limited exploration of potential animal models for pre-clinical drug development. Pathogenic tau is over represented in the frontal cortex in AD+P, especially in females. In order to develop a candidate animal model of AD+P, we employed a tau mouse model with a heavy burden of frontal tau pathology, the rTg(tauP301L)4510 mouse, hereafter termed rTg4510. We explored deficits of prepulse inhibition of acoustic startle (PPI), a model of psychosis in rodents, and the correlation between pathogenic phospho-tau species associated with AD+P and PPI deficits in female mice. We found that female rTg4510 mice exhibit increasing PPI deficits relative to littermate controls from 4.5 to 5.5 months of age, and that these deficits are driven by insoluble fractions of the phospho-tau species pSer396/404, pSer202, and pThr231 found to be associated with human AD+P. This preliminary data suggests the utility of the rTg4510 mouse as a candidate disease model of human female AD+P. Further work expanded to include both genders and other behavioral outcome measures relevant to AD+P is necessary.

Thermodynamics of the interaction between Alzheimer's disease related tau protein and DNA

Tau hyperphosphorylation can be considered as one of the hallmarks of Alzheimer's disease and other tauophaties. Besides its well-known role as a microtubule associated protein, Tau displays a key function as a protector of genomic integrity in stress situations. Phosphorylation has been proven to regulate multiple processes including nuclear translocation of Tau. In this contribution, we are addressing the physicochemical nature of DNA-Tau interaction including the plausible influence of phosphorylation. By means of surface plasmon resonance (SPR) we measured the equilibrium constant and the free energy, enthalpy and entropy changes associated to the Tau-DNA complex formation. Our results show that unphosphorylated Tau binding to DNA is reversible. This fact is in agreement with the protective role attributed to nuclear Tau, which stops binding to DNA once the insult is over. According to our thermodynamic data, oscillations in the concentration of dephosphorylated Tau available to DNA must be the variable determining the extent of Tau binding and DNA protection. In addition, thermodynamics of the interaction suggest that hydrophobicity must represent an important contribution to the stability of the Tau-DNA complex. SPR results together with those from Tau expression in HEK cells show that phosphorylation induces changes in Tau protein which prevent it from binding to DNA. The phosphorylation-dependent regulation of DNA binding is analogous to the Tau-microtubules binding inhibition induced by phosphorylation. Our results suggest that hydrophobicity may control Tau location and DNA interaction and that impairment of this Tau-DNA interaction, due to Tau hyperphosphorylation, could contribute to Alzheimer's pathogenesis.

Activated human microglia stimulate neuroblastoma cells to upregulate production of beta amyloid protein and tau: implications for Alzheimer's disease pathogenesis

Neuroinflammation is hypothesized to be a major driving force behind Alzheimer's disease (AD) pathogenesis. This hypothesis predicts that activated microglial cells can stimulate neurons to produce excessive amounts of β-amyloid protein (Aβ₁₋₄₂) and tau. The excess Aβ₁₋₄₂ forms extracellular deposits which stimulate further microglial activation. The excess tau is partially released but also becomes phosphorylated forming intracellular neurofibrillary deposits. The end result is a positive feedback mechanism which drives the disease development. To test the viability of this hypothesis, we exposed differentiated SH-SY5Y and N-tera2/D1 (N-tera2) cells to conditioned medium (CM) from LPS/IFNγ-stimulated human microglia. We found that the CM caused a large increase in the production and release of Aβ and tau. The CM also caused SH-SY5Y cells to increase their expression of amyloid precursor protein and release of its β-secretase cleaved products (sAPPβs) as well as Aβ oligomers, but the CM reduced release of its α-secretase cleaved products (sAPPαs). Direct treatment of SH-SY5Y and N-tera2 cells with the inflammatory cytokines IL-6 and IL-1β as well as with Aβ₁₋₄₂, resulted in an increase in tau messenger RNA and protein expression. Pretreatment of LPS/IFNγ-stimulated human microglia cells with the nonsteroidal anti-inflammatory drugs ibuprofen and aspirin, the antioxidant GSH, the H₂S donor NaSH, and the anti-inflammatory cytokine IL-10, resulted in a CM with diminished ability to stimulate tau expression. There was no effect on the morphology of SH-SY5Y cells, or on their viability, following exposure to micromolar levels of Aβ₁₋₄₂. Our data indicate that reactive microglia play an important role in governing the expression of Aβ and tau, and therefore the progression of AD. They provide further evidence that appropriate anti-inflammatory treatment should be beneficial in AD.

Physiological and pathological phosphorylation of tau by Cdk5

Hyperphosphorylation of microtubule-associated protein tau is one of the major pathological events in Alzheimer’s disease (AD) and other related neurodegenerative diseases, including frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Mutations in the tau gene MAPT are a cause of FTDP-17, and the mutated tau proteins are hyperphosphorylated in patient brains. Thus, it is important to determine the molecular mechanism of hyperphosphorylation of tau to understand the pathology of these diseases collectively called tauopathy. Tau is phosphorylated at many sites via several protein kinases, and a characteristic is phosphorylation at Ser/Thr residues in Ser/Thr-Pro sequences, which are targeted by proline-directed protein kinases such as ERK, GSK3β, and Cdk5. Among these kinases, Cdk5 is particularly interesting because it could be abnormally activated in AD. Cdk5 is a member of the cyclin-dependent kinases (Cdks), but in contrast to the major Cdks, which promote cell cycle progression in proliferating cells, Cdk5 is activated in post-mitotic neurons via the neuron-specific activator p35. Cdk5-p35 plays a critical role in brain development and physiological synaptic activity. In contrast, in disease brains, Cdk5 is thought to be hyperactivated by p25, which is the N-terminal truncated form of p35 and is generated by cleavage with calpain. Several reports have indicated that tau is hyperphosphorylated by Cdk5-p25. However, normal and abnormal phosphorylation of tau by Cdk5 is still not completely understood. In this article, we summarize the physiological and pathological phosphorylation of tau via Cdk5.

Role of PrP(C) Expression in Tau Protein Levels and Phosphorylation in Alzheimer's Disease Evolution

Alzheimer's disease (AD) is characterized by the presence of amyloid plaques mainly consisting of hydrophobic β-amyloid peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed principally of hyperphosphorylated tau. Aβ oligomers have been described as the earliest effectors to negatively affect synaptic structure and plasticity in the affected brains, and cellular prion protein (PrP(C)) has been proposed as receptor for these oligomers. The most widely accepted theory holds that the toxic effects of Aβ are upstream of change in tau, a neuronal microtubule-associated protein that promotes the polymerization and stabilization of microtubules. However, tau is considered decisive for the progression of neurodegeneration, and, indeed, tau pathology correlates well with clinical symptoms such as dementia. Different pathways can lead to abnormal phosphorylation, and, as a consequence, tau aggregates into paired helical filaments (PHF) and later on into NFTs. Reported data suggest a regulatory tendency of PrP(C) expression in the development of AD, and a putative relationship between PrP(C) and tau processing is emerging. However, the role of tau/PrP(C) interaction in AD is poorly understood. In this study, we show increased susceptibility to Aβ-derived diffusible ligands (ADDLs) in neuronal primary cultures from PrP(C) knockout mice, compared to wild-type, which correlates with increased tau expression. Moreover, we found increased PrP(C) expression that paralleled with tau at early ages in an AD murine model and in early Braak stages of AD in affected individuals. Taken together, these results suggest a protective role for PrP(C) in AD by downregulating tau expression, and they point to this protein as being crucial in the molecular events that lead to neurodegeneration in AD.

Parkinsonian syndrome in familial frontotemporal dementia

Parkinsonism in frontotemporal dementia (FTD) was first described in families with mutations in the microtubule-associated protein tau (MAPT) and progranulin (PRGN) genes. Since then, mutations in several other genes have been identified for FTD with parkinsonism, including chromosome 9 open reading frame 72 (C9ORF72), chromatin modifying protein 2B (CHMP2B), valosin-containing protein (VCP), fused in sarcoma (FUS) and transactive DNA-binding protein (TARDBP). The clinical presentation of patients with familial forms of FTD with parkinsonism is highly variable. The parkinsonism seen in FTD patients is usually characterized by akinetic-rigid syndrome and is mostly associated with the behavioral variant of FTD (bvFTD); however, some cases may present with classical Parkinson's disease. In other cases, atypical parkinsonism resembling progressive supranuclear palsy (PSP) or corticobasal syndrome (CBS) has also been described. Although rare, parkinsonism in FTD may coexist with motor neuron disease. Structural neuroimaging, which is crucial for the diagnosis of FTD, shows characteristic patterns of brain atrophy associated with specific mutations. Structural neuroimaging is not helpful in distinguishing among patients with parkinsonian features. Furthermore, dopaminergic imaging that shows nigrostriatal neurodegeneration in FTD with parkinsonism cannot discriminate parkinsonian syndromes that arise from different mutations. Generally, parkinsonism in FTD is levodopa unresponsive, but there have been cases where a temporary benefit has been reported, so dopaminergic treatment is worth trying, especially, when motor and non-motor manifestations can cause significant problems with daily functioning. In this review, we present an update on the clinical and genetic correlations of FTD with parkinsonism.

Cerebrospinal fluid β-amyloid and phospho-tau biomarker interactions affecting brain structure in preclinical Alzheimer disease

OBJECTIVE

To assess the relationships between core cerebrospinal fluid (CSF) biomarkers and cortical thickness (CTh) in preclinical Alzheimer disease (AD).

METHODS

In this cross-sectional study, normal controls (n = 145) from the Alzheimer's Disease Neuroimaging Initiative underwent structural 3T magnetic resonance imaging (MRI) and lumbar puncture. CSF β-amyloid1-42 (Aβ) and phospho-tau₁₈₁p (p-tau) levels were measured by Luminex assays. Samples were dichotomized using published cutoffs (Aβ(+) /Aβ(-) and p-tau(+) /ptau(-)). CTh was measured by Freesurfer. CTh difference maps were derived from interaction and correlation analyses. Clusters from the interaction analysis were isolated to analyze the directionality of the interaction by analysis of covariance.

RESULTS

We found a significant biomarker interaction between CSF Aβ and CSF p-tau levels affecting brain structure. Cortical atrophy only occurs in subjects with both Aβ(+) and p-tau(+). The stratified correlation analyses showed that the relationship between p-tau and CTh is modified by Aβ status and the relationship between Aβ and CTh is modified by p-tau status. p-Tau-dependent thinning was found in different cortical regions in Aβ(+) subjects but not in Aβ(-) subjects. Cortical thickening was related to decreasing CSF Aβ values in the absence of abnormal p-tau, but no correlations were found in p-tau(+) subjects.

INTERPRETATION

Our data suggest that interactions between biomarkers in AD result in a 2-phase phenomenon of pathological cortical thickening associated with low CSF Aβ, followed by atrophy once CSF p-tau becomes abnormal. These interactions should be considered in clinical trials in preclinical AD, both when selecting patients and when using MRI as a surrogate marker of efficacy.

Axotrophin/MARCH7 acts as an E3 ubiquitin ligase and ubiquitinates tau protein in vitro impairing microtubule binding

Tau is the major microtubule-associated protein in neurons involved in microtubule stabilization in the axonal compartment. Changes in tau gene expression, alternative splicing and posttranslational modification regulate tau function and in tauopathies can result in tau mislocalization and dysfunction, causing tau aggregation and cell death. To uncover proteins involved in the development of tauopathies, a yeast two-hybrid system was used to screen for tau-interacting proteins. We show that axotrophin/MARCH7, a RING-variant domain containing protein with similarity to E3 ubiquitin ligases interacts with tau. We defined the tau binding domain to amino acids 552–682 of axotrophin comprising the RING-variant domain. Co-immunoprecipitation and co-localization confirmed the specificity of the interaction. Intracellular localization of axotrophin is determined by an N-terminal nuclear targeting signal and a C-terminal nuclear export signal. In AD brain nuclear localization is lost and axotrophin is rather associated with neurofibrillary tangles. We find here that tau becomes mono-ubiquitinated by recombinant tau-interacting RING-variant domain, which diminishes its microtubule-binding. In vitro ubiquitination of four-repeat tau results in incorporation of up to four ubiquitin molecules compared to two molecules in three-repeat tau. In summary, we present a novel tau modification occurring preferentially on 4-repeat tau protein which modifies microtubule-binding and may impact on the pathogenesis of tauopathies.

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We search for tau-interacting proteins using a cytotrap yeast two-hybrid assay.

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MARCH7 was identified as a tau-binding protein and confirmed by several methods.

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Recombinant MARCH7 Ring-variant domain uses Ubc5 for E3 self-ubiquitinating activity.

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MARCH7 Ring-variant domain mono-ubiquitinates tau protein at multiple sites including the microtubule-binding domain.

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Mono-ubiquitination of tau protein diminishes its microtubule-binding.

The intersection of amyloid beta and tau at synapses in Alzheimer's disease

The collapse of neural networks important for memory and cognition, including death of neurons and degeneration of synapses, causes the debilitating dementia associated with Alzheimer's disease (AD). We suggest that synaptic changes are central to the disease process. Amyloid beta and tau form fibrillar lesions that are the classical hallmarks of AD. Recent data indicate that both molecules may have normal roles at the synapse, and that the accumulation of soluble toxic forms of the proteins at the synapse may be on the critical path to neurodegeneration. Further, the march of neurofibrillary tangles through brain circuits appears to take advantage of recently described mechanisms of transsynaptic spread of pathological forms of tau. These two key phenomena, synapse loss and the spread of pathology through the brain via synapses, make it critical to understand the physiological and pathological roles of amyloid beta and tau at the synapse.

Stages and conformations of the Tau repeat domain during aggregation and its effect on neuronal toxicity

Several neurodegenerative diseases are characterized by the aggregation and posttranslational modifications of Tau protein. Its “repeat domain” (TauRD) is mainly responsible for the aggregation properties, and oligomeric forms are thought to dominate the toxic effects of Tau. Here we investigated the conformational transitions of this domain during oligomerization and aggregation in different states of β-propensity and pseudo-phosphorylation, using several complementary imaging and spectroscopic methods. Although the repeat domain generally aggregates more readily than full-length Tau, its aggregation was greatly slowed down by phosphorylation or pseudo-phosphorylation at the KXGS motifs, concomitant with an extended phase of oligomerization. Analogous effects were observed with pro-aggregant variants of TauRD. Oligomers became most evident in the case of the pro-aggregant mutant TauRDΔK280, as monitored by atomic force microscopy, and the fluorescence lifetime of Alexa-labeled Tau (time-correlated single photon counting (TCSPC)), consistent with its pronounced toxicity in mouse models. In cell models or primary neurons, neither oligomers nor fibrils of TauRD or TauRDΔK280 had a toxic effect, as seen by assays with lactate dehydrogenase and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, respectively. However, oligomers of pro-aggregant TauRDΔK280 specifically caused a loss of spine density in differentiated neurons, indicating a locally restricted impairment of function.

CMT-associated mutations in glycyl- and tyrosyl-tRNA synthetases exhibit similar pattern of toxicity and share common genetic modifiers in Drosophila

Aminoacyl-tRNA synthetases are ubiquitously expressed proteins that charge tRNAs with their cognate amino acids. By ensuring the fidelity of protein synthesis, these enzymes are essential for the viability of every cell. Yet, mutations in six tRNA synthetases specifically affect the peripheral nerves and cause Charcot-Marie-Tooth (CMT) disease. The CMT-causing mutations in tyrosyl- and glycyl-tRNA synthetases (YARS and GARS, respectively) alter the activity of the proteins in a range of ways (some mutations do not impact charging function, while others abrogate it), making a loss of function in tRNA charging unlikely to be the cause of disease pathology. It is currently unknown which cellular mechanisms are triggered by the mutant enzymes and how this leads to neurodegeneration. Here, by expressing two pathogenic mutations (G240R, P234KY) in Drosophila, we generated a model for GARS-associated neuropathy. We observed compromised viability, and behavioral, electrophysiological and morphological impairment in flies expressing the cytoplasmic isoform of mutant GARS. Their features recapitulated several hallmarks of CMT pathophysiology and were similar to the phenotypes identified in our previously described Drosophila model of YARS-associated neuropathy. Furthermore, CG8316 and CG15599 - genes identified in a retinal degeneration screen to modify mutant YARS, also modified the mutant GARS phenotypes. Our study presents genetic evidence for common mutant-specific interactions between two CMT-associated aminoacyl-tRNA synthetases, lending support for a shared mechanism responsible for the synthetase-induced peripheral neuropathies.

The role of extracellular Tau in the spreading of neurofibrillary pathology

The microtubule-associated protein (MAP) tau plays a critical role in the pathogenesis of Alzheimer’s disease (AD) and several related disorders collectively known as tauopathies. Development of tau pathology is associated with progressive neuronal loss and cognitive decline. In the brains of AD patients, tau pathology spreads following an anatomically defined pattern. Mounting evidence strongly suggests that accumulation of abnormal tau is mediated through spreading of seeds of the protein from cell to cell and point at the involvement of extracellular tau species as the main agent in the interneuronal propagation of neurofibrillary lesions and spreading of tau toxicity throughout different brain regions in these disorders. That would support the concept that pathology initiates in a very small part of the brain many years before becoming symptomatic, spreading progressively to the whole brain within 10–20 years. Understanding the precise molecular mechanism underlying tau propagation is crucial for the development of therapeutics for this devastating disorder. In this work, we will discuss recent research on the role of extracellular tau in the spreading of tau pathology, through synaptic and non-synaptic mechanisms.

Common defects of mitochondria and iron in neurodegeneration and diabetes (MIND): a paradigm worth exploring

A popular, if not centric, approach to the study of an event is to first consider that of the simplest cause. When dissecting the underlying mechanisms governing idiopathic diseases, this generally takes the form of an ab initio genetic approach. To date, this genetic 'smoking gun' has remained elusive in diabetes mellitus and for many affected by neurodegenerative diseases. With no single gene, or even subset of genes, conclusively causative in all cases, other approaches to the etiology and treatment of these diseases seem reasonable, including the correlation of a systems' predisposed sensitivity to particular influence. In the cases of diabetes mellitus and neurodegenerative diseases, overlapping themes of mitochondrial influence or dysfunction and iron dyshomeostasis are apparent and relatively consistent. This mini-review discusses the influence of mitochondrial function and iron homeostasis on diabetes mellitus and neurodegenerative disease, namely Alzheimer's disease. Also discussed is the incidence of diabetes accompanied by neuropathy and neurodegeneration along with neurodegenerative disorders prone to development of diabetes. Mouse models containing multiple facets of this overlap are also described alongside current molecular trends attributed to both diseases. As a way of approaching the idiopathic and complex nature of these diseases we are proposing the consideration of a MIND (mitochondria, iron, neurodegeneration, and diabetes) paradigm in which systemic metabolic influence, iron homeostasis, and respective genetic backgrounds play a central role in the development of disease.

Minimal model of self-assembly: emergence of diversity and complexity

Molecular self-assembly is ubiquitous in nature, yet prediction of assembly pathways from fundamental interparticle interactions has yet to be achieved. Here, we introduce a minimal self-assembly model with two attractive and two repulsive beads bound into a tetrahedron. The model is associated with a single parameter η defined as the repulsive to attractive interaction ratio. We explore self-assembly pathways and resulting assembly morphologies for different η values by discrete molecular dynamics. Our results demonstrate that η governs the assembly dynamics and resulting assembly morphologies, revealing an unexpected diversity and complexity for 0.5 ≤ η < 1. One of the key processes that governs the assembly dynamics is assembly breakage, which emerges spontaneously at η > 0 with the breakage rate increasing with η. The observed assembly pathways display a broad variety of assembly structures characteristic of aggregation of amyloidogenic proteins, including quasi-spherical oligomers that coassemble into elongated protofibrils, followed by a conversion into ordered polymorphic fibril-like aggregates. We further demonstrate that η can be meaningfully mapped onto amyloidogenic protein sequences, with the majority of amyloidogenic proteins characterized by 0.5 ≤ η < 1. Prion proteins, which are known to form highly breakage-prone fibrils, are characterized by η > 1, consistent with the model predictions. Our model thus provides a theoretical basis for understanding the universal aspects of aggregation pathways of amyloidogenic proteins relevant to human disease. As the model is not specific to proteins, these findings represent an important step toward understanding and predicting assembly dynamics of not only proteins but also viruses, colloids, and nanoparticles.

A novel non-rapid-eye movement and rapid-eye-movement parasomnia with sleep breathing disorder associated with antibodies to IgLON5: a case series, characterisation of the antigen, and post-mortem study

BACKGROUND

Autoimmunity might be associated with or implicated in sleep and neurodegenerative disorders. We aimed to describe the features of a novel neurological syndrome associated with prominent sleep dysfunction and antibodies to a neuronal antigen.

METHODS

In this observational study, we used clinical and video polysomnography to identify a novel sleep disorder in three patients referred to the Sleep Unit of Hospital Clinic, University of Barcelona, Spain, for abnormal sleep behaviours and obstructive sleep apnoea. These patients had antibodies against a neuronal surface antigen, which were also present in five additional patients referred to our laboratory for antibody studies. These five patients had been assessed with polysomnography, which was done in our sleep unit in one patient and the recording reviewed in a second patient. Two patients underwent post-mortem brain examination. Immunoprecipitation and mass spectrometry were used to characterise the antigen and develop an assay for antibody testing. Serum or CSF from 298 patients with neurodegenerative, sleep, or autoimmune disorders served as control samples.

FINDINGS

All eight patients (five women; median age at disease onset 59 years [range 52-76]) had abnormal sleep movements and behaviours and obstructive sleep apnoea, as confirmed by polysomnography. Six patients had chronic progression with a median duration from symptom onset to death or last visit of 5 years (range 2-12); in four the sleep disorder was the initial and most prominent feature, and in two it was preceded by gait instability followed by dysarthria, dysphagia, ataxia, or chorea. Two patients had a rapid progression with disequilibrium, dysarthria, dysphagia, and central hypoventilation, and died 2 months and 6 months, respectively, after symptom onset. In five of five patients, video polysomnography showed features of obstructive sleep apnoea, stridor, and abnormal sleep architecture (undifferentiated non-rapid-eye-movement [non-REM] sleep or poorly structured stage N2, simple movements and finalistic behaviours, normalisation of non-REM sleep by the end of the night, and, in the four patients with REM sleep recorded, REM sleep behaviour disorder). Four of four patients had HLA-DRB1*1001 and HLA-DQB1*0501 alleles. All patients had antibodies (mainly IgG4) against IgLON5, a neuronal cell adhesion molecule. Only one of the 298 controls, who had progressive supranuclear palsy, had IgLON5 antibodies. Neuropathology showed neuronal loss and extensive deposits of hyperphosphorylated tau mainly involving the tegmentum of the brainstem and hypothalamus in the two patients studied.

INTERPRETATION

IgLON5 antibodies identify a unique non-REM and REM parasomnia with sleep breathing dysfunction and pathological features suggesting a tauopathy.

FUNDING

Fondo de Investigaciones Sanitarias, Centros de Investigación Biomédica en Red de enfermedades neurodegenerativas (CIBERNED) and Respiratorias (CIBERES), Ministerio de Economía y Competitividad, Fundació la Marató TV3, and the National Institutes of Health.

The dendritic hypothesis for Alzheimer's disease pathophysiology

Converging evidence indicates that processes occurring in and around neuronal dendrites are central to the pathogenesis of Alzheimer's disease. These data support the concept of a "dendritic hypothesis" of AD, closely related to the existing synaptic hypothesis. Here we detail dendritic neuropathology in the disease and examine how Aβ, tau, and AD genetic risk factors affect dendritic structure and function. Finally, we consider potential mechanisms by which these key drivers could affect dendritic integrity and disease progression. These dendritic mechanisms serve as a framework for therapeutic target identification and for efforts to develop disease-modifying therapeutics for Alzheimer's disease.

Glial epigenetics in neuroinflammation and neurodegeneration

Epigenetic regulation shapes the differentiation and response to stimuli of all tissues and cells beyond what genetics would dictate. Epigenetic regulation acts through covalent modifications of DNA and histones while leaving the nucleotide code intact. However, these chromatin modifications are known to be vital components of the regulation of cell fate and response. With regards to the central nervous system (CNS), little is known about how epigenetic regulation shapes the function of neural cell types. The focus of research so far has been on epigenetic regulation of neuronal function and the role of epigenetics in tumorigenesis. However, the glial cell compartment, which makes up 90 % of all CNS cells, has so far received scant attention as to how epigenetics shape their differentiation and function. Here, we highlight current knowledge about epigenetic changes in glial cells occurring during CNS injury, neuroinflammatory conditions and neurodegenerative disease. This review offers an overview of the current understanding of epigenetic regulation in glial cells in CNS disease.

Regional distribution of synaptic markers and APP correlate with distinct clinicopathological features in sporadic and familial Alzheimer's disease

Recent studies suggest that subcortical structures, including striatum, are vulnerable to amyloid-β accumulation and other neuropathological features in familial Alzheimer's disease due to autosomal dominant mutations. We explored differences between familial and sporadic Alzheimer's disease that might shed light on their respective pathogenic mechanisms. To this end, we analysed 12 brain regions, including neocortical, limbic and subcortical areas, from post-mortem brains of familial Alzheimer's disease (n = 10; age at death: 50.0 ± 8.6 years) with mutations in amyloid precursor protein (APP) or presenilin 1 (PSEN1), sporadic Alzheimer's disease (n = 19; age at death: 84.7 ± 7.8 years), neurologically normal elderly without amyloid-β accumulation (normal ageing; n = 13, age at death: 82.9 ± 10.8 years) and neurologically normal elderly with extensive cortical amyloid-β deposits (pathological ageing; n = 15; age at death: 92.7 ± 5.9 years). The levels of amyloid-β₄₀, amyloid-β₄₂, APP, apolipoprotein E, the synaptic marker PSD95 (now known as DLG4), the astrocyte marker GFAP, other molecules related to amyloid-β metabolism, and tau were determined by enzyme-linked immunosorbent assays. We observed that familial Alzheimer's disease had disproportionate amyloid-β₄₂ accumulation in subcortical areas compared with sporadic Alzheimer's disease, whereas sporadic Alzheimer's disease had disproportionate amyloid-β₄₂ accumulation in cortical areas compared to familial Alzheimer's disease. Compared with normal ageing, the levels of several proteins involved in amyloid-β metabolism were significantly altered in both sporadic and familial Alzheimer's disease; however, such changes were not present in pathological ageing. Among molecules related to amyloid-β metabolism, the regional distribution of PSD95 strongly correlated with the regional pattern of amyloid-β₄₂ accumulation in sporadic Alzheimer's disease and pathological ageing, whereas the regional distribution of APP as well as β-C-terminal fragment of APP were strongly associated with the regional pattern of amyloid-β₄₂ accumulation in familial Alzheimer's disease. Apolipoprotein E and GFAP showed negative regional association with amyloid-β (especially amyloid-β₄₀) accumulation in both sporadic and familial Alzheimer's disease. Familial Alzheimer's disease had greater striatal tau pathology than sporadic Alzheimer's disease. In a retrospective medical record review, atypical signs and symptoms were more frequent in familial Alzheimer's disease compared with sporadic Alzheimer's disease. These results suggest that disproportionate amyloid-β₄₂ accumulation in cortical areas in sporadic Alzheimer's disease may be mediated by synaptic processes, whereas disproportionate amyloid-β₄₂ accumulation in subcortical areas in familial Alzheimer's disease may be driven by APP and its processing. Region-specific amyloid-β₄₂ accumulation might account for differences in the relative amounts of tau pathology and clinical symptoms in familial and sporadic Alzheimer's disease.

Recent progress with microtubule stabilizers: new compounds, binding modes and cellular activities

Nature has yielded numerous classes of chemically distinct microtubule stabilizers. Several of these, including paclitaxel (Taxol) and docetaxel (Taxotere), are important drugs used in the treatment of cancer. New microtubule stabilizers and novel formulations of these agents continue to provide advances in cancer therapy. In this review we cover recent progress in the chemistry and biology of these diverse microtubule stabilizers focusing on the wide range of organisms that produce these compounds, their mechanisms of inhibiting microtubule-dependent processes, mechanisms of drug resistance, and their interactions with tubulin including their distinct binding sites and modes. A new potential role for microtubule stabilizers in neurodegenerative diseases is reviewed.

Single mutations in tau modulate the populations of fibril conformers through seed selection

Seeded conversion of tau monomers into fibrils is a central step in the progression of tau pathology in Alzheimer's disease and other neurodegenerative disorders. Self-assembly is mediated by the microtubule binding repeats in tau. There are either three or four repeats present depending on the protein isoform. Here, double electron-electron resonance spectroscopy was used to investigate the conformational ensemble of four-repeat tau fibrils. Single point mutations at key positions in the protein (ΔK280, P301S, P312I, D314I) markedly change the distribution of fibril conformers after template-assisted growth, whereas other mutations in the protein (I308M, S320F, G323I, G326I, Q336R) do not. These findings provide unprecedented insights into the seed selection of tau disease mutants and establish conformational compatibility as an important driving force in tau fibril propagation.

New perspectives on the role of tau in Alzheimer's disease. Implications for therapy

Alzheimer's disease (AD) and related dementias constitute a major public health issue due to an increasingly aged population as a consequence of generally improved medical care and demographic changes. Current drug treatment of AD, the most prevalent dementia, with cholinesterase inhibitors or NMDA antagonists have demonstrated very modest, symptomatic efficacy, leaving an unmet medical need for new, more effective therapies. While drug development efforts in the last two decades have primarily focused on the amyloid cascade hypothesis, so far with disappointing results, tau-based strategies have received little attention until recently despite that the presence of extensive tau pathology is central to the disease. The discovery of mutations within the tau gene that cause fronto-temporal dementia demonstrated that tau dysfunction, in the absence of amyloid pathology, was sufficient to cause neuronal loss and clinical dementia. Abnormal levels and hyperphosphorylation of tau protein have been reported to be the underlying cause of a group of neurodegenerative disorders collectively known as 'tauopathies'. The detrimental consequence is the loss of affinity between this protein and the microtubules, increased production of fibrillary aggregates and the accumulation of insoluble intracellular neurofibrillary tangles. However, it has become clear in recent years that tau is not only a microtubule interacting protein, but rather has additional roles in cellular processes. This review focuses on emerging therapeutic strategies aimed at treating the underlying causes of the tau pathology in tauopathies and AD, including some novel approaches on the verge of providing new treatment paradigms within the coming years.

Tau pathology is present in vivo and develops in vitro in sensory neurons from human P301S tau transgenic mice: a system for screening drugs against tauopathies

Intracellular tau aggregates are the neuropathological hallmark of several neurodegenerative diseases, including Alzheimer's disease, progressive supranuclear palsy, and cases of frontotemporal dementia, but the link between these aggregates and neurodegeneration remains unclear. Neuronal models recapitulating the main features of tau pathology are necessary to investigate the molecular mechanisms of tau malfunction, but current models show little and inconsistent spontaneous tau aggregation. We show that dorsal root ganglion (DRG) neurons in transgenic mice expressing human P301S tau (P301S-htau) develop tau pathology similar to that found in brain and spinal cord and a significant reduction in mechanosensation occurs before detectable fibrillar tau formation. DRG neuronal cultures established from adult P301S-htau mice at different ages retained the pattern of aberrant tau found in vivo. Moreover, htau became progressively hyperphosphorylated over 2 months in vitro beginning with nonsymptomatic neurons, while hyperphosphorylated P301S-htau-positive neurons from 5-month-old mice cultured for 2 months died preferentially. P301S-htau-positive neurons grew aberrant axons, including spheroids, typically found in human tauopathies. Neurons cultured at advanced stages of tau pathology showed a 60% decrease in the fraction of moving mitochondria. SEG28019, a novel O-GlcNAcase inhibitor, reduced steady-state pSer396/pSer404 phosphorylation over 7 weeks in a significant proportion of DRG neurons showing for the first time the possible beneficial effect of prolonged dosing of O-GlcNAcase inhibitor in vitro. Our system is unique in that fibrillar tau forms without external manipulation and provides an important new tool for understanding the mechanisms of tau dysfunction and for screening of compounds for treatment of tauopathies.

Neurofibrillary tangle-bearing neurons are functionally integrated in cortical circuits in vivo

Alzheimer's disease (AD) is pathologically characterized by the deposition of extracellular amyloid-β plaques and intracellular aggregation of tau protein in neurofibrillary tangles (NFTs) (1, 2). Progression of NFT pathology is closely correlated with both increased neurodegeneration and cognitive decline in AD (3) and other tauopathies, such as frontotemporal dementia (4, 5). The assumption that mislocalization of tau into the somatodendritic compartment (6) and accumulation of fibrillar aggregates in NFTs mediates neurodegeneration underlies most current therapeutic strategies aimed at preventing NFT formation or disrupting existing NFTs (7, 8). Although several disease-associated mutations cause both aggregation of tau and neurodegeneration, whether NFTs per se contribute to neuronal and network dysfunction in vivo is unknown (9). Here we used awake in vivo two-photon calcium imaging to monitor neuronal function in adult rTg4510 mice that overexpress a human mutant form of tau (P301L) and develop cortical NFTs by the age of 7-8 mo (10). Unexpectedly, NFT-bearing neurons in the visual cortex appeared to be completely functionally intact, to be capable of integrating dendritic inputs and effectively encoding orientation and direction selectivity, and to have a stable baseline resting calcium level. These results suggest a reevaluation of the common assumption that insoluble tau aggregates are sufficient to disrupt neuronal function.

LRRK2 phosphorylates novel tau epitopes and promotes tauopathy

Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) are the most frequent cause of familial Parkinson's disease (PD). The neuropathology of LRRK2-related PD is heterogeneous and can include aberrant tau phosphorylation or neurofibrillary tau pathology. Recently, LRRK2 has been shown to phosphorylate tau in vitro; however, the major epitopes phosphorylated by LRRK2 and the physiological or pathogenic consequences of these modifications in vivo are unknown. Using mass spectrometry, we identified multiple sites on recombinant tau that are phosphorylated by LRRK2 in vitro, including pT149 and pT153, which are phospho-epitopes that to date have been largely unexplored. Importantly, we demonstrate that expression of transgenic LRRK2 in a mouse model of tauopathy increased the aggregation of insoluble tau and its phosphorylation at T149, T153, T205, and S199/S202/T205 epitopes. These findings indicate that tau can be a LRRK2 substrate and that this interaction can enhance salient features of human disease.

Imaging tau deposits in vivo: progress in viewing more of the proteopathy picture

In this issue of Neuron, Maruyama et al. (2013) demonstrate the binding of a new class of selective tau ligands, termed PBBs, to tau deposits in transgenic mice and in human subjects with normal cognition, Alzheimer's disease, or a corticobasal syndrome.

The amyloid cascade-inflammatory hypothesis of Alzheimer disease: implications for therapy

The amyloid cascade hypothesis is widely accepted as the centerpiece of Alzheimer disease (AD) pathogenesis. It proposes that abnormal production of beta amyloid protein (Abeta) is the cause of AD and that the neurotoxicity is due to Abeta itself or its oligomeric forms. We suggest that this, in itself, cannot be the cause of AD because demonstrating such toxicity requires micromolar concentrations of these Abeta forms, while their levels in brain are a million times lower in the picomolar range. AD probably results from the inflammatory response induced by extracellular Abeta deposits, which later become enhanced by aggregates of tau. The inflammatory response, which is driven by activated microglia, increases over time as the disease progresses. Disease-modifying therapeutic attempts to date have failed and may continue to do so as long as the central role of inflammation is not taken into account. Multiple epidemiological and animal model studies show that NSAIDs, the most widely used antiinflammatory agents, have a substantial sparing effect on AD. These studies provide a proof of concept regarding the anti-inflammatory approach to disease modification. Biomarker studies have indicated that early intervention may be necessary. They have established that disease onset occurs more than a decade before it becomes clinically evident. By combining biomarker and pathological data, it is possible to define six phases of disease development, each separated by about 5 years. Phase one can be identified by decreases in Abeta in the CSF, phase 2 by increases of tau in the CSF plus clear evidence of Abeta brain deposits by PET scanning, phase 3 by slight decreases in brain metabolic rate by PET-FDG scanning, phase 4 by slight decreases in brain volume by MRI scanning plus minimal cognitive impairment, phase 5 by increased scanning abnormalities plus clinical diagnosis of AD, and phase 6 by advanced AD requiring institutional care. Utilization of antiinflammatory agents early in the disease process remains an overlooked therapeutic opportunity. Such agents, while not preventative, have the advantage of being able to inhibit the consequences of both Abeta and tau aggregation. Since there is more than a decade between disease onset and cognitive decline, a window of opportunity exists to introduce truly effective disease-modifying regimens. Taking advantage of this opportunity is the challenge for the future.

Functional screening in Drosophila identifies Alzheimer's disease susceptibility genes and implicates Tau-mediated mechanisms

Using a Drosophila model of Alzheimer's disease (AD), we systematically evaluated 67 candidate genes based on AD-associated genomic loci (P < 10(-4)) from published human genome-wide association studies (GWAS). Genetic manipulation of 87 homologous fly genes was tested for modulation of neurotoxicity caused by human Tau, which forms neurofibrillary tangle pathology in AD. RNA interference (RNAi) targeting 9 genes enhanced Tau neurotoxicity, and in most cases reciprocal activation of gene expression suppressed Tau toxicity. Our screen implicates cindr, the fly ortholog of the human CD2AP AD susceptibility gene, as a modulator of Tau-mediated disease mechanisms. Importantly, we also identify the fly orthologs of FERMT2 and CELF1 as Tau modifiers, and these loci have been independently validated as AD susceptibility loci in the latest GWAS meta-analysis. Both CD2AP and FERMT2 have been previously implicated with roles in cell adhesion, and our screen additionally identifies a fly homolog of the human integrin adhesion receptors, ITGAM and ITGA9, as a modifier of Tau neurotoxicity. Our results highlight cell adhesion pathways as important in Tau toxicity and AD susceptibility and demonstrate the power of model organism genetic screens for the functional follow-up of human GWAS.

Neurodegenerative lesions: seeding and spreading

Accumulation of specific proteins has replaced loss of specific populations of neurons in the definition of most neurodegenerative diseases. In some cases, the amino-acid sequence of the protein that accumulates is altered by a mutation in the gene that codes for it but most generally, the primary structure is normal. Much evidence from human neuropathology has been collected over the years indicating that the progression of the lesions in such neurodegenerative diseases as Alzheimer's disease, Parkinson's disease and progressive supranuclear palsy follow the neuroanatomical connections. More recently, injection of aggregates of the specific proteins in the brain of experimental animals has been attempted in various experimental settings. Brain homogenates containing Aβ aggregates induce the early development of Aβ deposits in APP transgenic mice. Brain homogenates from various human tauopathies induce tau aggregates in transgenic mice expressing normal human tau. Finally, synthetic preformed fibrils of alpha-synuclein initiate the development of alpha-synuclein accumulation resembling Parkinson's disease in wild-type mice. Experiments in cell cultures suggest that the protein has to be in some specific state of oligomerization or fibrillation to be endocytosed and transported by the neuron. These data suggest that the protein that accumulates in a specific disease is initially misfolded and that this misfolding contaminates normal protein in a prion-like manner - in some cases through the neuronal connections.