Genome-wide identification of genic and intergenic neuronal DNA regions bound by Tau protein under physiological and stress conditions

DNA Damage and Chromatin Rearrangement Work Together to Promote Neurodegeneration

Neurodegenerative diseases have a complex origin and are composed of genetic and environmental factors. Both DNA damage and chromatin rearrangement are important processes that occur under pathological conditions and in neurons functioning properly. While numerous studies have demonstrated the inseparable relationship between DNA damage and chromatin organization, understanding of this relationship, especially in neurodegenerative diseases, requires further study. Interestingly, recent studies revealed that known hallmark proteins involved in neurodegenerative diseases function in both DNA damage and chromatin reorganization, and this review discusses the current knowledge of this relationship. This review focused on hallmark proteins involved in various neurodegenerative diseases, such as the microtubule-associated protein tau, TAR DNA/RNA binding protein 43 (TDP-43), superoxide dismutase 1 (SOD1), fused in sarcoma (FUS), huntingtin (HTT), α-synuclein, and β-amyloid precursor protein (APP). Hence, DNA damage and chromatin rearrangement are associated with disease mechanisms in distinct neurodegenerative diseases. Targeting common modulators of DNA repair and chromatin reorganization may lead to promising therapies for treating neurodegeneration.

Cell redistribution of G quadruplex-structured DNA is associated with morphological changes of nuclei and nucleoli in neurons during tau pathology progression

While the double helical structure has long been its iconic representation, DNA is structurally dynamic and can adopt alternative secondary configurations. Specifically, guanine-rich DNA sequences can fold in guanine quadruplexes (G4) structures. These G4 play pivotal roles as regulators of gene expression and genomic stability, and influence protein homeostasis. Despite their significance, the association of G4 with neurodegenerative diseases such as Alzheimer's disease (AD) has been underappreciated. Recent findings have identified DNA sequences predicted to form G4 in sarkosyl-insoluble aggregates from AD brains, questioning the involvement of G4-structured DNA (G4 DNA) in the pathology. Using immunofluorescence coupled to confocal microscopy analysis we investigated the impact of tau pathology, a hallmark of tauopathies including AD, on the distribution of G4 DNA in murine neurons and its relevance to AD brains. In healthy neurons, G4 DNA is detected in nuclei with a notable presence in nucleoli. However, in a transgenic mouse model of tau pathology (THY-Tau22), early stages of the disease exhibit an impairment in the nuclear distribution of G4 DNA. In addition, G4 DNA accumulates in the cytoplasm of neurons exhibiting oligomerized tau and oxidative DNA damage. This altered distribution persists in the later stage of the pathology when larger tau aggregates are present. Still cytoplasmic deposition of G4 DNA does not appear to be a critical factor in the tau aggregation process. Similar patterns are observed in neurons from the AD cortex. Furthermore, the disturbance in G4 DNA distribution is associated with various changes in the size of neuronal nuclei and nucleoli, indicative of responses to stress and the activation of pro-survival mechanisms. Our results shed light on a significant impact of tau pathology on the dynamics of G4 DNA and on nuclear and nucleolar mechanobiology in neurons. These findings reveal new dimensions in the etiopathogenesis of tauopathies.

Phosphorylation regulates tau's phase separation behavior and interactions with chromatin

Tau is a microtubule-associated protein often found in neurofibrillary tangles (NFTs) in the brains of patients with Alzheimer's disease. Beyond this context, mounting evidence suggests that tau localizes into the nucleus, where it may play a role in DNA protection and heterochromatin regulation. The molecular mechanisms behind these observations are currently unclear. Using in vitro biophysical experiments, here we demonstrate that tau can undergo liquid-liquid phase separation (LLPS) with DNA, mononucleosomes, and reconstituted nucleosome arrays under low salt conditions. Low concentrations of tau promote chromatin compaction and protect DNA from digestion. While the material state of samples at physiological salt is dominated by chromatin oligomerization, tau can still associate strongly and reversibly with nucleosome arrays. These properties are driven by tau's strong interactions with linker and nucleosomal DNA. In addition, tau co-localizes into droplets formed by nucleosome arrays and phosphorylated HP1α, a key heterochromatin constituent thought to function through an LLPS mechanism. Importantly, LLPS and chromatin interactions are disrupted by aberrant tau hyperphosphorylation. These biophysical properties suggest that tau may directly impact DNA and chromatin accessibility and that loss of these interactions could contribute to the aberrant nuclear effects seen in tau pathology.

Phosphorylation regulates tau's phase separation behavior and interactions with chromatin

Tau is a microtubule-associated protein often found in neurofibrillary tangles (NFTs) in the brains of patients with Alzheimer's disease (AD). Beyond this context, mounting evidence suggests that tau localizes into the nucleus, where it may play a role in DNA protection and heterochromatin regulation. Models of tau depletion or pathology show loss of genetically silent heterochromatin, aberrant expression of heterochromatic genes, and transposable element activation. The molecular mechanisms behind these observations are currently unclear. Using in vitro biophysical experiments, here we demonstrate that tau can undergo liquid-liquid phase separation (LLPS) with DNA, mononucleosomes, and reconstituted nucleosome arrays under low salt conditions. Low concentrations of tau promote chromatin compaction and protect DNA from digestion. While the material state of samples at physiological salt is dominated by chromatin oligomerization, tau can still associate strongly and reversibly with nucleosome arrays. These properties are driven by tau's strong interactions with linker and nucleosomal DNA, while magic angle spinning (MAS) solid-state NMR experiments show that tau binding does not drastically alter nucleosome structure and dynamics. In addition, tau co-localizes into droplets formed by nucleosome arrays and phosphorylated HP1α, a key heterochromatin constituent thought to function through an LLPS mechanism. Importantly, LLPS and chromatin interactions are disrupted by aberrant tau hyperphosphorylation. These biophysical properties suggest that tau may directly impact DNA and chromatin accessibility and that loss of these interactions could contribute to the aberrant nuclear effects seen in tau pathology.

Nuclear face of Tau: an inside player in neurodegeneration

Tau (Tubulin associated unit) protein is a major hallmark of Alzheimer's disease (AD) and tauopathies. Tau is predominantly an axonal protein with a crucial role in the stabilization and dynamics of the microtubules. Since the discovery of Tau protein in 1975, research efforts were concentrated on the pathophysiological role of Tau protein in the context of the microtubules. Although, for more than three decades, different localizations of Tau protein have been discovered e.g., in the nuclear compartments. Discovery of the role of Tau protein in various cellular compartments especially in the nucleus opens up a new fold of complexity in tauopathies. Data from cellular models, animal models, and the human brain indicate that nuclear Tau is crucial for genome stability and to cope with cellular distress. Moreover, it's nature of nuclear translocation, its interactions with the nuclear DNA/RNA and proteins suggest it could play multiple roles in the nucleus. To comprehend Tau pathophysiology and efficient Tau-based therapies, there is an urgent need to understand whole repertoire of Tau species (nuclear and cytoplasmic) and their functional relevance. To complete the map of Tau repertoire, understanding of various species of Tau in the nucleus and cytoplasm, identification if specific transcripts of Tau, isoforms and post-translational modifications could foretell Tau's localizations and functions, and how they are modified in neurodegenerative diseases like AD, is urgently required. In this review, we explore the nuclear face of Tau protein, its nuclear localizations and functions and its linkage with Alzheimer's disease.

Tau-dependent HDAC1 nuclear reduction is associated with altered VGluT1 expression

During AD pathology, Tau protein levels progressively increase from early pathological stages. Tau altered expression causes an unbalance of Tau subcellular localization in the cytosol and in the nuclear compartment leading to synaptic dysfunction, neuronal cell death and neurodegeneration as a consequence. Due to the relevant role of epigenetic remodellers in synaptic activity in physiology and in neurodegeneration, in particular of TRIM28 and HDAC1, we investigated the relationship between Tau and these epigenetic factors. By molecular, imaging and biochemical approaches, here we demonstrate that Tau altered expression in the neuronal cell line SH-SY5y does not alter TRIM28 and HDAC1 expression but it induces a subcellular reduction of HDAC1 in the nuclear compartment. Remarkably, HDAC1 reduced activity modulates the expression of synaptic genes in a way comparable to that observed by Tau increased levels. These results support a competitive relationship between Tau levels and HDAC1 subcellular localization and nuclear activity, indicating a possible mechanism mediating the alternative role of Tau in the pathological alteration of synaptic genes expression.

Whole-genome sequencing reveals an association between small genomic deletions and an increased risk of developing Parkinson's disease

Single-nucleotide variants (SNVs) associated with Parkinson's disease (PD) have been investigated mainly through genome-wide association studies. However, other genomic alterations, including copy number variations, remain less explored. In this study, we conducted whole-genome sequencing of primary (310 PD patients and 100 healthy individuals) and independent (100 PD patients and 100 healthy individuals) cohorts from the Korean population to identify high-resolution small genomic deletions, gains, and SNVs. Global small genomic deletions and gains were found to be associated with an increased and decreased risk of PD development, respectively. Thirty significant locus deletions were identified in PD, with most being associated with an increased PD risk in both cohorts. Small genomic deletions in clustered loci located in the GPR27 region had high enhancer signals and showed the closest association with PD. GPR27 was found to be expressed specifically in brain tissue, and GPR27 copy number loss was associated with upregulated SNCA expression and downregulated dopamine neurotransmitter pathways. Clustering of small genomic deletions on chr20 in exon 1 of the GNAS isoform was detected. In addition, we found several PD-associated SNVs, including one in the enhancer region of the TCF7L2 intron, which exhibited a cis-acting regulatory mode and an association with the beta-catenin signaling pathway. These findings provide a global, whole-genome view of PD and suggest that small genomic deletions in regulatory domains contribute to the risk of PD development.

Tau Isoforms: Gaining Insight into MAPT Alternative Splicing

Tau microtubule-associated proteins, encoded by the MAPT gene, are mainly expressed in neurons participating in axonal transport and synaptic plasticity. Six major isoforms differentially expressed during cell development and differentiation are translated by alternative splicing of MAPT transcripts. Alterations in the expression of human Tau isoforms and their aggregation have been linked to several neurodegenerative diseases called tauopathies, including Alzheimer's disease, progressive supranuclear palsy, Pick's disease, and frontotemporal dementia with parkinsonism linked to chromosome 17. Great efforts have been dedicated in recent years to shed light on the complex regulatory mechanism of Tau splicing, with a perspective to developing new RNA-based therapies. This review summarizes the most recent contributions to the knowledge of Tau isoform expression and experimental models, highlighting the role of cis-elements and ribonucleoproteins that regulate the alternative splicing of Tau exons.

Pathological Nuclear Hallmarks in Dentate Granule Cells of Alzheimer’s Patients: A Biphasic Regulation of Neurogenesis

The dentate gyrus (DG) of the human hippocampus is a complex and dynamic structure harboring mature and immature granular neurons in diverse proliferative states. While most mammals show persistent neurogenesis through adulthood, human neurogenesis is still under debate. We found nuclear alterations in granular cells in autopsied human brains, detected by immunohistochemistry. These alterations differ from those reported in pyramidal neurons of the hippocampal circuit. Aging and early AD chromatin were clearly differentiated by the increased epigenetic markers H3K9me3 (heterochromatin suppressive mark) and H3K4me3 (transcriptional euchromatin mark). At early AD stages, lamin B2 was redistributed to the nucleoplasm, indicating cell-cycle reactivation, probably induced by hippocampal nuclear pathology. At intermediate and late AD stages, higher lamin B2 immunopositivity in the perinucleus suggests fewer immature neurons, less neurogenesis, and fewer adaptation resources to environmental factors. In addition, senile samples showed increased nuclear Tau interacting with aged chromatin, likely favoring DNA repair and maintaining genomic stability. However, at late AD stages, the progressive disappearance of phosphorylated Tau forms in the nucleus, increased chromatin disorganization, and increased nuclear autophagy support a model of biphasic neurogenesis in AD. Therefore, designing therapies to alleviate the neuronal nuclear pathology might be the only pathway to a true rejuvenation of brain circuits.

Oligomeropathies, inflammation and prion protein binding

The central role of oligomers, small soluble aggregates of misfolded proteins, in the pathogenesis of neurodegenerative disorders is recognized in numerous experimental conditions and is compatible with clinical evidence. To underline this concept, some years ago we coined the term oligomeropathies to define the common mechanism of action of protein misfolding diseases like Alzheimer, Parkinson or prion diseases. Using simple experimental conditions, with direct application of synthetic β amyloid or α-synuclein oligomers intraventricularly at micromolar concentrations, we could detect differences and similarities in the biological consequences. The two oligomer species affected cognitive behavior, neuronal dysfunction and cerebral inflammatory reactions with distinct mechanisms. In these experimental conditions the proposed mediatory role of cellular prion protein in oligomer activities was not confirmed. Together with oligomers, inflammation at different levels can be important early in neurodegenerative disorders; both β amyloid and α-synuclein oligomers induce inflammation and its control strongly affects neuronal dysfunction. This review summarizes our studies with β-amyloid or α-synuclein oligomers, also considering the potential curative role of doxycycline, a well-known antibiotic with anti-amyloidogenic and anti-inflammatory activities. These actions are analyzed in terms of the therapeutic prospects.

Failure of DNA double-strand break repair by tau mediates Alzheimer's disease pathology in vitro

DNA double-strand break (DSB) is the most severe form of DNA damage and accumulates with age, in which cytoskeletal proteins are polymerized to repair DSB in dividing cells. Since tau is a microtubule-associated protein, we investigate whether DSB is involved in tau pathologies in Alzheimer's disease (AD). First, immunohistochemistry reveals the frequent coexistence of DSB and phosphorylated tau in the cortex of AD patients. In vitro studies using primary mouse cortical neurons show that non-p-tau accumulates perinuclearly together with the tubulin after DSB induction with etoposide, followed by the accumulation of phosphorylated tau. Moreover, the knockdown of endogenous tau exacerbates DSB in neurons, suggesting the protective role of tau on DNA repair. Interestingly, synergistic exposure of neurons to microtubule disassembly and the DSB strikingly augments aberrant p-tau aggregation and apoptosis. These data suggest that DSB plays a pivotal role in AD-tau pathology and that the failure of DSB repair leads to tauopathy.

Tau Modulates mRNA Transcription, Alternative Polyadenylation Profiles of hnRNPs, Chromatin Remodeling and Spliceosome Complexes

Tau protein is a known contributor in several neurodegenerative diseases, including Alzheimer’s disease (AD) and frontotemporal dementia (FTD). It is well-established that tau forms pathological aggregates and fibrils in these diseases. Tau has been observed within the nuclei of neurons, but there is a gap in understanding regarding the mechanism by which tau modulates transcription. We are interested in the P301L mutation of tau, which has been associated with FTD and increased tau aggregation. Our study utilized tau-inducible HEK (iHEK) cells to reveal that WT and P301L tau distinctively alter the transcription and alternative polyadenylation (APA) profiles of numerous nuclear precursors mRNAs, which then translate to form proteins involved in chromatin remodeling and splicing. We isolated total mRNA before and after over-expressing tau and then performed Poly(A)-ClickSeq (PAC-Seq) to characterize mRNA expression and APA profiles. We characterized changes in Gene Ontology (GO) pathways using EnrichR and Gene Set Enrichment Analysis (GSEA). We observed that P301L tau up-regulates genes associated with reactive oxygen species responsiveness as well as genes involved in dendrite, microtubule, and nuclear body/speckle formation. The number of genes regulated by WT tau is greater than the mutant form, which indicates that the P301L mutation causes loss-of-function at the transcriptional level. WT tau up-regulates genes contributing to cytoskeleton-dependent intracellular transport, microglial activation, microtubule and nuclear chromatin organization, formation of nuclear bodies and speckles. Interestingly, both WT and P301L tau commonly down-regulate genes responsible for ubiquitin-proteosome system. In addition, WT tau significantly down-regulates several genes implicated in chromatin remodeling and nucleosome organization. Although there are limitations inherent to the model systems used, this study will improve understanding regarding the nuclear impact of tau at the transcriptional and post-transcriptional level. This study also illustrates the potential impact of P301L tau on the human brain genome during early phases of pathogenesis.

Tau Stabilizes Chromatin Compaction

An extensive body of literature suggested a possible role of the microtubule-associated protein Tau in chromatin functions and/or organization in neuronal, non-neuronal, and cancer cells. How Tau functions in these processes remains elusive. Here we report that Tau expression in breast cancer cell lines causes resistance to the anti-cancer effects of histone deacetylase inhibitors, by preventing histone deacetylase inhibitor-inducible gene expression and remodeling of chromatin structure. We identify Tau as a protein recognizing and binding to core histone when H3 and H4 are devoid of any post-translational modifications or acetylated H4 that increases the Tau's affinity. Consistent with chromatin structure alterations in neurons found in frontotemporal lobar degeneration, Tau mutations did not prevent histone deacetylase-inhibitor-induced higher chromatin structure remodeling by suppressing Tau binding to histones. In addition, we demonstrate that the interaction between Tau and histones prevents further histone H3 post-translational modifications induced by histone deacetylase-inhibitor treatment by maintaining a more compact chromatin structure. Altogether, these results highlight a new cellular role for Tau as a chromatin reader, which opens new therapeutic avenues to exploit Tau biology in neuronal and cancer cells.

Phospho-Tau and Chromatin Landscapes in Early and Late Alzheimer's Disease

Cellular identity is determined through complex patterns of gene expression. Chromatin, the dynamic structure containing genetic information, is regulated through epigenetic modulators, mainly by the histone code. One of the main challenges for the cell is maintaining functionality and identity, despite the accumulation of DNA damage throughout the aging process. Replicative cells can remain in a senescent state or develop a malign cancer phenotype. In contrast, post-mitotic cells such as pyramidal neurons maintain extraordinary functionality despite advanced age, but they lose their identity. This review focuses on tau, a protein that protects DNA, organizes chromatin, and plays a crucial role in genomic stability. In contrast, tau cytosolic aggregates are considered hallmarks of Alzheimer´s disease (AD) and other neurodegenerative disorders called tauopathies. Here, we explain AD as a phenomenon of chromatin dysregulation directly involving the epigenetic histone code and a progressive destabilization of the tau–chromatin interaction, leading to the consequent dysregulation of gene expression. Although this destabilization could be lethal for post-mitotic neurons, tau protein mediates profound cellular transformations that allow for their temporal survival.

Non-Canonical Roles of Tau and Their Contribution to Synaptic Dysfunction

Tau plays a central role in a group of neurodegenerative disorders collectively named tauopathies. Despite the wide range of diverse symptoms at the onset and during the progression of the pathology, all tauopathies share two common hallmarks, namely the misfolding and aggregation of Tau protein and progressive synaptic dysfunctions. Tau aggregation correlates with cognitive decline and behavioural impairment. The mechanistic link between Tau misfolding and the synaptic dysfunction is still unknown, but this correlation is well established in the human brain and also in tauopathy mouse models. At the onset of the pathology, Tau undergoes post-translational modifications (PTMs) inducing the detachment from the cytoskeleton and its release in the cytoplasm as a soluble monomer. In this condition, the physiological enrichment in the axon is definitely disrupted, resulting in Tau relocalization in the cell soma and in dendrites. Subsequently, Tau aggregates into toxic oligomers and amyloidogenic forms that disrupt synaptic homeostasis and function, resulting in neuronal degeneration. The involvement of Tau in synaptic transmission alteration in tauopathies has been extensively reviewed. Here, we will focus on non-canonical Tau functions mediating synapse dysfunction.

miR-135a-5p mediates memory and synaptic impairments via the Rock2/Adducin1 signaling pathway in a mouse model of Alzheimer's disease

Aberrant regulation of microRNAs (miRNAs) has been implicated in the pathogenesis of Alzheimer's disease (AD), but most abnormally expressed miRNAs found in AD are not regulated by synaptic activity. Here we report that dysfunction of miR-135a-5p/Rock2/Add1 results in memory/synaptic disorder in a mouse model of AD. miR-135a-5p levels are significantly reduced in excitatory hippocampal neurons of AD model mice. This decrease is tau dependent and mediated by Foxd3. Inhibition of miR-135a-5p leads to synaptic disorder and memory impairments. Furthermore, excess Rock2 levels caused by loss of miR-135a-5p plays an important role in the synaptic disorder of AD via phosphorylation of Ser726 on adducin 1 (Add1). Blocking the phosphorylation of Ser726 on Add1 with a membrane-permeable peptide effectively rescues the memory impairments in AD mice. Taken together, these findings demonstrate that synaptic-related miR-135a-5p mediates synaptic/memory deficits in AD via the Rock2/Add1 signaling pathway, illuminating a potential therapeutic strategy for AD.

Tau Oligomers Neurotoxicity

Although the mechanisms of toxic activity of tau are not fully recognized, it is supposed that the tau toxicity is related rather not to insoluble tau aggregates but to its intermediate forms. It seems that neurofibrillar tangles (NFTs) themselves, despite being composed of toxic tau, are probably neither necessary nor sufficient for tau-induced neuronal dysfunction and toxicity. Tau oligomers (TauOs) formed during the early stages of tau aggregation are the pathological forms that play a key role in eliciting the loss of neurons and behavioral impairments in several neurodegenerative disorders called tauopathies. They can be found in tauopathic diseases, the most common of which is Alzheimer's disease (AD). Evidence of co-occurrence of b-amyloid, α-synuclein, and tau into their most toxic forms, i.e., oligomers, suggests that these species interact and influence each other's aggregation in several tauopathies. The mechanism responsible for oligomeric tau neurotoxicity is a subject of intensive investigation. In this review, we summarize the most recent literature on the damaging effect of TauOs on the stability of the genome and the function of the nucleus, energy production and mitochondrial function, cell signaling and synaptic plasticity, the microtubule assembly, neuronal cytoskeleton and axonal transport, and the effectiveness of the protein degradation system.

Tau and DNA Damage in Neurodegeneration

Neurodegenerative disorders are a family of incurable conditions. Among them, Alzheimer's disease and tauopathies are the most common. Pathological features of these two disorders are synaptic loss, neuronal cell death and increased DNA damage. A key pathological protein for the onset and progression of the conditions is the protein tau, a microtubule-binding protein highly expressed in neurons and encoded by the MAPT (microtubule-associated protein tau) gene. Tau is predominantly a cytosolic protein that interacts with numerous other proteins and molecules. Recent findings, however, have highlighted new and unexpected roles for tau in the nucleus of neuronal cells. This review summarizes the functions of tau in the metabolism of DNA, describing them in the context of the disorders.

Alterations in Tau Metabolism in ALS and ALS-FTSD

There is increasing acceptance that amyotrophic lateral sclerosis (ALS), classically considered a neurodegenerative disease affecting almost exclusively motor neurons, is syndromic with both clinical and biological heterogeneity. This is most evident in its association with a broad range of neuropsychological, behavioral, speech and language deficits [collectively termed ALS frontotemporal spectrum disorder (ALS-FTSD)]. Although the most consistent pathology of ALS and ALS-FTSD is a disturbance in TAR DNA binding protein 43 kDa (TDP-43) metabolism, alterations in microtubule-associated tau protein (tau) metabolism can also be observed in ALS-FTSD, most prominently as pathological phosphorylation at Thr¹⁷⁵ (pThr¹⁷⁵tau). pThr¹⁷⁵ has been shown to promote exposure of the phosphatase activating domain (PAD) in the tau N-terminus with the consequent activation of GSK3β mediated phosphorylation at Thr²³¹ (pThr²³¹tau) leading to pathological oligomer formation. This pathological cascade of tau phosphorylation has been observed in chronic traumatic encephalopathy with ALS (CTE-ALS) and in both in vivo and in vitro experimental paradigms, suggesting that it is of critical relevance to the pathobiology of ALS-FTSD. It is also evident that the co-existence of alterations in the metabolism of TDP-43 and tau acts synergistically in a rodent model to exacerbate the pathology of either.

The physiological roles of tau and Aβ: implications for Alzheimer's disease pathology and therapeutics

Tau and amyloid beta (Aβ) are the prime suspects for driving pathology in Alzheimer's disease (AD) and, as such, have become the focus of therapeutic development. Recent research, however, shows that these proteins have been highly conserved throughout evolution and may have crucial, physiological roles. Such functions may be lost during AD progression or be unintentionally disrupted by tau- or Aβ-targeting therapies. Tau has been revealed to be more than a simple stabiliser of microtubules, reported to play a role in a range of biological processes including myelination, glucose metabolism, axonal transport, microtubule dynamics, iron homeostasis, neurogenesis, motor function, learning and memory, neuronal excitability, and DNA protection. Aβ is similarly multifunctional, and is proposed to regulate learning and memory, angiogenesis, neurogenesis, repair leaks in the blood-brain barrier, promote recovery from injury, and act as an antimicrobial peptide and tumour suppressor. This review will discuss potential physiological roles of tau and Aβ, highlighting how changes to these functions may contribute to pathology, as well as the implications for therapeutic development. We propose that a balanced consideration of both the physiological and pathological roles of tau and Aβ will be essential for the design of safe and effective therapeutics.

Nuclear Transport Deficits in Tau-Related Neurodegenerative Diseases

Tau is a cytosolic microtubule binding protein that is highly abundant in the axons of the central nervous system. However, alternative functions of tau also in other cellular compartments are suggested, for example, in the nucleus, where interactions of tau with specific nuclear entities such as DNA, the nucleolus, and the nuclear envelope have been reported. We would like to review the current knowledge about tau-nucleus interactions and lay out possible neurotoxic mechanisms that are based on the (pathological) interactions of tau with the nucleus.

Perinuclear Lamin A and Nucleoplasmic Lamin B2 Characterize Two Types of Hippocampal Neurons through Alzheimer's Disease Progression

BACKGROUND

Recent reports point to a nuclear origin of Alzheimer's disease (AD). Aged postmitotic neurons try to repair their damaged DNA by entering the cell cycle. This aberrant cell cycle re-entry involves chromatin modifications where nuclear Tau and the nuclear lamin are involved. The purpose of this work was to elucidate their participation in the nuclear pathological transformation of neurons at early AD.

METHODOLOGY

The study was performed in hippocampal paraffin embedded sections of adult, senile, and AD brains at I-VI Braak stages. We analyzed phospho-Tau, lamins A, B1, B2, and C, nucleophosmin (B23) and the epigenetic marker H4K20me3 by immunohistochemistry.

RESULTS

Two neuronal populations were found across AD stages, one is characterized by a significant increase of Lamin A expression, reinforced perinuclear Lamin B2, elevated expression of H4K20me3 and nuclear Tau loss, while neurons with nucleoplasmic Lamin B2 constitute a second population.

CONCLUSIONS

The abnormal cell cycle reentry in early AD implies a fundamental neuronal transformation. This implies the reorganization of the nucleo-cytoskeleton through the expression of the highly regulated Lamin A, heterochromatin repression and building of toxic neuronal tangles. This work demonstrates that nuclear Tau and lamin modifications in hippocampal neurons are crucial events in age-related neurodegeneration.

Hippocampal tau oligomerization early in tau pathology coincides with a transient alteration of mitochondrial homeostasis and DNA repair in a mouse model of tauopathy

Insoluble intracellular aggregation of tau proteins into filaments and neurodegeneration are histopathological hallmarks of Alzheimer disease (AD) and other tauopathies. Recently, prefibrillar, soluble, oligomeric tau intermediates have emerged as relevant pathological tau species; however, the molecular mechanisms of neuronal responses to tau oligomers are not fully understood. Here, we show that hippocampal neurons in six-month-old transgenic mouse model of tauopathy, THY-Tau22, are enriched with oligomeric tau, contain elongated mitochondria, and display cellular stress, but no overt cytotoxicity compared to the control mice. The levels of several key mitochondrial proteins were markedly different between the THY-Tau22 and control mice hippocampi including the mitochondrial SIRT3, PINK1, ANT1 and the fission protein DRP1. DNA base excision repair (BER) is the primary defense system against oxidative DNA damage and it was elevated in six-month-old transgenic mice. DNA polymerase β, the key BER DNA polymerase, was enriched in the cytoplasm of hippocampal neurons in six-month-old transgenic mice and localized with and within mitochondria. Polβ also co-localized with mitochondria in human AD brains in neurons containing oligomeric tau. Most of these altered mitochondrial and DNA repair events were specific to the transgenic mice at 6 months of age and were not different from control mice at 12 months of age when tau pathology reaches its maximum and oligomeric forms of tau are no longer detectable. In summary, our data suggests that we have identified key cellular stress responses at early stages of tau pathology to preserve neuronal integrity and to promote survival. To our knowledge, this work provides the first description of multiple stress responses involving mitochondrial homeostasis and BER early during the progression of tau pathology, and represents an important advance in the etiopathogenesis of tauopathies.

Altered Levels and Isoforms of Tau and Nuclear Membrane Invaginations in Huntington's Disease

Since the early reports of neurofibrillary Tau pathology in brains of some Huntington’s disease (HD) patients, mounting evidence of multiple alterations of Tau in HD brain tissue has emerged in recent years. Such Tau alterations range from increased total levels, imbalance of isoforms generated by alternative splicing (increased 4R-/3R-Tau ratio) or by post-translational modifications such as hyperphosphorylation or truncation. Besides, the detection in HD brains of a new Tau histopathological hallmark known as Tau nuclear rods (TNRs) or Tau-positive nuclear indentations (TNIs) led to propose HD as a secondary Tauopathy. After their discovery in HD brains, TNIs have also been reported in hippocampal neurons of early Braak stage AD cases and in frontal and temporal cortical neurons of FTD-MAPT cases due to the intronic IVS10+16 mutation in the Tau gene (MAPT) which results in an increased 4R-/3R-Tau ratio similar to that observed in HD. TNIs are likely pathogenic for contributing to the disturbed nucleocytoplasmic transport observed in HD. A key question is whether correction of any of the mentioned Tau alterations might have positive therapeutic implications for HD. The beneficial effect of decreasing Tau expression in HD mouse models clearly implicates Tau in HD pathogenesis. Such beneficial effect might be exerted by diminishing the excess total levels of Tau or specifically by diminishing the excess 4R-Tau, as well as any of their downstream effects. In any case, since gene silencing drugs are under development to attenuate both Huntingtin (HTT) expression for HD and MAPT expression for FTD-MAPT, it is conceivable that the combined therapy in HD patients might be more effective than HTT silencing alone.

From the prion-like propagation hypothesis to therapeutic strategies of anti-tau immunotherapy

The term “propagon” is used to define proteins that may transmit misfolding in vitro, in tissues or in organisms. Among propagons, misfolded tau is thought to be involved in the pathogenic mechanisms of various “tauopathies” that include Alzheimer's disease, progressive supranuclear palsy, and argyrophilic grain disease. Here, we review the available data in the literature and point out how the prion-like tau propagation has been extended from Alzheimer's disease to tauopathies. First, in Alzheimer’s disease, the progression of tau aggregation follows stereotypical anatomical stages which may be considered as spreading. The mechanisms of the propagation are now subject to intensive and controversial research. It has been shown that tau may be secreted in the interstitial fluid in an active manner as reflected by high and constant concentration of extracellular tau during Alzheimer’s pathology. Animal and cell models have been devised to mimic tau seeding and propagation, and despite their limitations, they have further supported to the prion-like propagation hypothesis. Finally, such new ways of thinking have led to different therapeutic strategies in anti-tau immunotherapy among tauopathies and have stimulated new clinical trials. However, it appears that the prion-like propagation hypothesis mainly relies on data obtained in Alzheimer’s disease. From this review, it appears that further studies are needed (1) to characterize extracellular tau species, (2) to find the right pathological tau species to target, (3) to follow in vivo tau pathology by brain imaging and biomarkers and (4) to interpret current clinical trial results aimed at reducing the progression of these pathologies. Such inputs will be essential to have a comprehensive view of these promising therapeutic strategies in tauopathies.

Developmental Pathogenicity of 4-Repeat Human Tau Is Lost with the P301L Mutation in Genetically Matched Tau-Transgenic Mice

Tau is a microtubule-associated protein that becomes dysregulated in a group of neurodegenerative diseases called tauopathies. Differential tau isoforms, expression levels, promoters, and disruption of endogenous genes in transgenic mouse models of tauopathy make it difficult to draw definitive conclusions about the biological role of tau in these models. We addressed this shortcoming by characterizing the molecular and cognitive phenotypes associated with the pathogenic P301L tau mutation (rT2 mice) in relation to a genetically matched transgenic mouse overexpressing nonmutant (NM) 4-repeat (4R) human tau (rT1 mice). Both male and female mice were included in this study. Unexpectedly, we found that 4R NM human tau (hTau) exhibited abnormal dynamics in young mice that were lost with the P301L mutation, including elevated protein stability and hyperphosphorylation, which were associated with cognitive impairment in 5-month-old rT1 mice. Hyperphosphorylation of NM hTau was observed as early as 4 weeks of age, and transgene suppression for the first 4 or 12 weeks of life prevented abnormal molecular and cognitive phenotypes in rT1, demonstrating that NM hTau pathogenicity is specific to postnatal development. We also show that NM hTau exhibits stronger binding to microtubules than P301L hTau, and is associated with mitochondrial abnormalities. Overall, our genetically matched mice have revealed that 4R NM hTau overexpression is pathogenic in a manner distinct from classical aging-related tauopathy, underlining the importance of assaying the effects of transgenic disease-related proteins at appropriate stages in life.SIGNIFICANCE STATEMENT Due to differences in creation of transgenic lines, the pathological properties of the P301L mutation confers to the tau protein in vivo have remained elusive, perhaps contributing to the lack of disease-modifying therapies for tauopathies. In an attempt to characterize P301L-specific effects on tau biology and cognition in novel genetically matched transgenic mouse models, we surprisingly found that nonmutant human tau has development-specific pathogenic properties of its own. Our findings indicate that overexpression of 4-repeat human tau during postnatal development is associated with excessive microtubule binding, which may disrupt important cellular processes, such as mitochondrial dynamics, leading to elevated stability and hyperphosphorylation of tau, and eventual cognitive impairments.

The elusive tau molecular structures: can we translate the recent breakthroughs into new targets for intervention?

Insights into tau molecular structures have advanced significantly in recent years. This field has been the subject of recent breakthroughs, including the first cryo-electron microscopy structures of tau filaments from Alzheimer’s and Pick’s disease inclusions, as well as the structure of the repeat regions of tau bound to microtubules. Tau structure covers various species as the tau protein itself takes many forms. We will here address a range of studies that help to define the many facets of tau protein structures and how they translate into pathogenic forms. New results shed light on previous data that need now to be revisited in order to up-date our knowledge of tau molecular structure. Finally, we explore how these data can contribute the important medical aspects of this research - diagnosis and therapeutics.