Structural and functional damage to neuronal nuclei caused by extracellular tau oligomers

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.

Internalization of extracellular Tau oligomers in Alzheimer's disease

A key factor in the progression of Alzheimer's disease (AD) is internalization of extracellular Tau oligomers (ecTauOs) by neuroglial cells. Aberrant hyperphosphorylation of Tau results in their dissociation from microtubules and formation of toxic intracellular Tau oligomers (icTauOs). These are subsequently released to the extracellular space following neuronal dysfunction and death. Although receptor mediated internalization of these ecTauOs by other neurons, microglia and astrocytes can facilitate elimination, incomplete degradation thereof promotes inflammation, exacerbates pathologic spread and accelerates neurodegeneration. Targeting Tau oligomer degradation pathways, blocking internalization receptors, and mitigating neuroinflammation are proposed as therapeutic strategies to control Tau propagation and toxicity. This review highlights the urgent need for innovative approaches to prevent the spread of Tau pathology, emphasizing its implications for AD and related neurodegenerative diseases.

Defining and characterizing neuronal senescence, 'neurescence', as GX arrested cells

Cellular senescence is a cell state characterized by resistance to apoptosis and stable cell cycle arrest. Senescence was first observed in mitotic cells in vitro. Recent evidence from in vivo studies and human tissue indicates that postmitotic cells, including neurons, may also become senescent. The quiescent cell state of neurons and inconsistent descriptions of neuronal senescence across studies, however, have caused confusion in this burgeoning field. We summarize evidence demonstrating that exit from G0 quiescence may protect neurons against apoptosis and predispose them toward senescence. Additionally, we propose the term 'neurescent' for senescent neurons and introduce the cell state, GX, to describe cell cycle arrest achieved by passing through G0 quiescence. Criteria are provided to identify neurescent cells, distinguish them from G0 quiescent neurons, and compare neurescent phenotypes with classic replicative senescence.

Mitochondrial NADK2-dependent NADPH controls Tau oligomer uptake in human neurons

Alterations in NADH and NADPH metabolism are associated with aging, cancer, and Alzheimer's Disease. Using 2P-FLIM imaging of the mitochondrial NAD(P)H in live human neurons and PS19 mouse brains, we show that tau oligomers (TauO) upregulate the mitochondrial de novo NADPH synthesis through NADK2. This process controls LRP1-mediated internalization of TauO, setting a vicious cycle for further TauO internalization. Thus, mitochondrial NADK2-dependent NADPH controls a key step in TauO toxicity.

Calcineurin inhibition prevents synaptic plasticity deficit induced by brain-derived tau oligomers

Compelling evidence suggests that cognitive decline in Alzheimer's disease is associated with the accumulation and aggregation of tau protein, with the most toxic aggregates being in the form of oligomers. This underscores the necessity for direct isolation and analysis of brain-derived tau oligomers from patients with Alzheimer's disease, potentially offering novel perspectives into tau toxicity. Alzheimer's brain-derived tau oligomers are potent inhibitors of synaptic plasticity; however, the involved mechanism is still not fully understood. We previously reported a significantly reduced incidence of Alzheimer's disease in ageing humans chronically treated with a Food and Drug Administration-approved calcineurin inhibitor, FK506 (tacrolimus), used as an immunosuppressant after solid organ transplant. Using a combination of electrophysiological and RNA-sequencing techniques, we provide here evidence that FK506 has the potential to block the acute toxic effect of brain-derived tau oligomers on synaptic plasticity, as well as to restore the levels of some key synaptic mRNAs. These results further support FK506 as a promising novel therapeutic strategy for the treatment of Alzheimer's disease.

Tau in neurodegenerative diseases: molecular mechanisms, biomarkers, and therapeutic strategies

The deposition of abnormal tau protein is characteristic of Alzheimer's disease (AD) and a class of neurodegenerative diseases called tauopathies. Physiologically, tau maintains an intrinsically disordered structure and plays diverse roles in neurons. Pathologically, tau undergoes abnormal post-translational modifications and forms oligomers or fibrous aggregates in tauopathies. In this review, we briefly introduce several tauopathies and discuss the mechanisms mediating tau aggregation and propagation. We also describe the toxicity of tau pathology. Finally, we explore the early diagnostic biomarkers and treatments targeting tau. Although some encouraging results have been achieved in animal experiments and preclinical studies, there is still no cure for tauopathies. More in-depth basic and clinical research on the pathogenesis of tauopathies is necessary.

Nuclear transport protein suppresses Tau neurodegeneration

The nuclear pore complex, a large multimeric structure consists of numerous protein components, serves as a crucial gatekeeper for the transport of macromolecules across the nuclear envelope in eukaryotic cells. Dysfunction of the NPC has been implicated in various neurodegenerative diseases, including Alzheimer's disease. In AD, Tau aggregates interact with NPC proteins, known as nucleoporins, leading to disruptions in nuclear transport. Hyperphosphorylated Tau, a hallmark of AD pathology, interacts with central channel NUPs such as Nup62 and Nup98, causing cytoplasmic mis-localization of these proteins and impairing nuclear transport. Furthermore, Tau-NUP interactions promote Tau aggregation and the formation of neurofibrillary tangles, exacerbating neurodegeneration. Oligomeric Tau adheres to the lamin B receptor as well as nuclear lamin, preventing nucleocytoplasmic transport and resulting in heterochromatin unwinding, DNA damage, and neuronal death. The decrease in lamin B and increasing levels of lamin A along with C in AD-affected brain areas highlight the disease's intricacy. Furthermore, Tau internalization in the nucleus and interaction with nuclear pore complexes worsen NPC dysfunction, which contributes to neurotoxicity. Tau-DNA interactions suggest a chaperone-like role for Tau in DNA organization and repair, highlighting its involvement in maintaining genomic integrity. This review explores the intricate relationships between Tau, NPC components, and nuclear lamin in the context of AD, providing insights into the mechanisms underlying Tau-induced neurodegeneration and potential therapeutic targets.

Nuclear Tau accumulation in Alzheimer's disease

Tau is a well-known microtubule-associated protein and is located in the cytoplasm of neurons, which play a crucial role in Alzheimer's diseases. Due to its preferred binding to DNA sequences found in the nucleolus and pericentromeric heterochromatin, Tau has been found within the cell nucleus, where it may be a nucleic acid-associated protein. Tau has the ability to directly interact with nuclear pore complex nucleoporins, influencing both their structural and functional integrity. The interaction between Tau and NUPs highlights a potential mechanism underlying NPC dysfunction in AD pathogenesis. Pathological Tau hinders the import and export of nucleus through RAN mediated cascades. Nuclear Tau aggregates colocalize with membrane less organelles called nuclear speckles, which are involved in pre-mRNA splicing, and modify their dynamics, composition, and structure. Additionally, SRRM2 and other nuclear speckle proteins including MSUT2 and PABPN1 mislocalize to cytosolic Tau aggregates, and causes propagation of Tau aggregates. Research highlights, Extracellular Tau Oligomers induce significant nuclear invagination. They act as a key player in the transformation of healthy neurons into sick neurons in AD. The mechanism behind this phenomenon depends on intracellular Tau and is linked to changes in chromatin structure, nucleocytoplasmic transport, and gene transcription. This review highlights the vital roles of nuclear Tau protein in the context of nuclear pore complex functioning and, modulation of nuclear speckles in Alzheimer's diseases. Addressing these pathways is essential for formulating focused therapeutics intended to alleviate Tau-induced neurodegeneration.