Altered Cytoskeletal Composition and Delayed Neurite Elongation in tau45-230-Expressing Hippocampal Neurons

Complicated Role of Post-translational Modification and Protease-Cleaved Fragments of Tau in Alzheimer's Disease and Other Tauopathies

Tau, a microtubule-associated protein predominantly localized in neuronal axons, plays a crucial role in promoting microtubule assembly, stabilizing their structure, and participating in axonal transport. Perturbations in tau's structure and function are implicated in the pathogenesis of neurodegenerative diseases collectively known as tauopathies, the most common disorder of which is Alzheimer's disease (AD). In tauopathies, it has been found that tau has a variety of post-translational modification (PTM) abnormalities and/or tau is cleaved into a variety of fragments by some specific proteolytic enzymes; however, the precise contributions of these abnormal modifications and fragments to disease onset and progression remain incompletely understood. Herein, we provide an overview about the involvement of distinctive abnormal tau PTMs and different tau fragments in the pathogenesis of AD and other tauopathies and discuss the involvement of proteolytic enzymes such as caspases, calpains, and asparagine endopeptidase in mediating tau cleavage while also addressing the intercellular transmission role played by tau. We anticipate that further exploration into PTMs and fragmented forms of tau will yield valuable insights for diagnostic approaches and therapeutic interventions targeting AD and other related disorders.

Controlled Tau Cleavage in Cells Reveals Abnormal Localizations of Tau Fragments

In several forms of dementia, such as Alzheimer's disease, the cytoskeleton-associated protein tau undergoes proteolysis, giving rise to fragments that have a toxic impact on neuronal homeostasis. How these fragments interact with cellular structures, in particular with the cytoskeleton, is currently incompletely understood. Here, we developed a method, derived from a Tobacco Etch Virus (TEV) protease system, to induce controlled cleavage of tau at specific sites. Five tau proteins containing specific TEV recognition sites corresponding to pathological proteolytic sites were engineered, and tagged with GFP at one end and mCherry at the other. Following controlled cleavage to produce GFP-N-terminal and C-terminal-mCherry fragments, we followed the fate of tau fragments in cells. Our results showed that whole engineered tau proteins associate with the cytoskeleton similarly to the non-modified tau, whereas tau fragments adopted different localizations with respect to the actin and microtubule cytoskeletons. These distinct localizations were confirmed by expressing each separate fragment in cells. Some cleavages - in particular cleavages at amino-acid positions 124 or 256 - displayed a certain level of cellular toxicity, with an unusual relocalization of the N-terminal fragments to the nucleus. Based on the data presented here, inducible cleavage of tau by the TEV protease appears to be a valuable tool to reproduce tau fragmentation in cells and study the resulting consequences on cell physiology.

Glutamate excitotoxicity: Potential therapeutic target for ischemic stroke

Glutamate-mediated excitotoxicity is an important mechanism leading to post ischemic stroke damage. After acute stroke, the sudden reduction in cerebral blood flow is most initially followed by ion transport protein dysfunction and disruption of ion homeostasis, which in turn leads to impaired glutamate release, reuptake, and excessive N-methyl-D-aspartate receptor (NMDAR) activation, promoting neuronal death. Despite extensive evidence from preclinical studies suggesting that excessive NMDAR stimulation during ischemic stroke is a central step in post-stroke damage, NMDAR blockers have failed to translate into clinical stroke treatment. Current treatment options for stroke are very limited, and there is therefore a great need to develop new targets for neuroprotective therapeutic agents in ischemic stroke to extend the therapeutic time window. In this review, we highlight recent findings on glutamate release, reuptake mechanisms, NMDAR and its downstream cellular signaling pathways in post-ischemic stroke damage, and review the pathological changes in each link to help develop viable new therapeutic targets. We then also summarize potential neuroprotective drugs and therapeutic approaches for these new targets in the treatment of ischemic stroke.

Flavones 7,8-DHF, Quercetin, and Apigenin Against Tau Toxicity via Activation of TRKB Signaling in ΔK280 TauRD-DsRed SH-SY5Y Cells

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with memory loss and cognitive decline. Neurofibrillary tangles (NFTs) formed by hyperphosphorylated Tau protein are one of the pathological hallmarks of several neurodegenerative diseases including AD. Heat shock protein family B (small) member 1 (HSPB1) is a molecular chaperone that promotes the correct folding of other proteins in response to environmental stress. Nuclear factor erythroid 2-like 2 (NRF2), a redox-regulated transcription factor, is the master regulator of the cellular response to excess reactive oxygen species. Tropomyosin-related kinase B (TRKB) is a membrane-bound receptor that, upon binding brain-derived neurotrophic factor (BDNF), phosphorylates itself to initiate downstream signaling for neuronal survival and axonal growth. In this study, four natural flavones such as 7,8-dihydroxyflavone (7,8-DHF), wogonin, quercetin, and apigenin were evaluated for Tau aggregation inhibitory activity and neuroprotection in SH-SY5Y neuroblastoma. Among the tested flavones, 7,8-DHF, quercetin, and apigenin reduced Tau aggregation, oxidative stress, and caspase-1 activity as well as improved neurite outgrowth in SH-SY5Y cells expressing ΔK280 Tau_RD-DsRed folding reporter. Treatments with 7,8-DHF, quercetin, and apigenin rescued the reduced HSPB1 and NRF2 and activated TRKB-mediated extracellular signal-regulated kinase (ERK) signaling to upregulate cAMP-response element binding protein (CREB) and its downstream antiapoptotic BCL2 apoptosis regulator (BCL2). Knockdown of TRKB attenuated the neuroprotective effects of these three flavones. Our results suggest 7,8-DHF, quercetin, and apigenin targeting HSPB1, NRF2, and TRKB to reduce Tau aggregation and protect cells against Tau neurotoxicity and may provide new treatment strategies for AD.

Caspase-cleaved tau is senescence-associated and induces a toxic gain of function by putting a brake on axonal transport

The microtubule-associated protein tau plays a central role in tauopathies such as Alzheimer's disease (AD). The exact molecular mechanisms underlying tau toxicity are unclear, but aging is irrefutably the biggest risk factor. This raises the question of how cellular senescence affects the function of tau as a microtubule regulator. Here we report that the proportion of tau that is proteolytically cleaved at the caspase-3 site (TauC3) doubles in the hippocampus of senescent mice. TauC3 is also elevated in AD patients. Through quantitative live-cell imaging, we show that TauC3 has a drastically reduced dynamics of its microtubule interaction. Single-molecule tracking of tau confirmed that TauC3 has a longer residence time on axonal microtubules. The reduced dynamics of the TauC3-microtubule interaction correlated with a decreased transport of mitochondria, a reduced processivity of APP-vesicle transport and an induction of region-specific dendritic atrophy in CA1 neurons of the hippocampus. The microtubule-targeting drug Epothilone D normalized the interaction of TauC3 with microtubules and modulated the transport of APP-vesicles dependent on the presence of overexpressed human tau. The results indicate a novel toxic gain of function, in which a post-translational modification of tau changes the dynamics of the tau-microtubule interaction and thus leads to axonal transport defects and neuronal degeneration. The data also introduce microtubule-targeting drugs as pharmacological modifiers of the tau-microtubule interaction with the potential to restore the physiological interaction of pathologically altered tau with microtubules.

The formation of small aggregates contributes to the neurotoxic effects of tau45-230

Intracellular deposits of hyperphosphorylated tau are commonly detected in tauopathies. Furthermore, these aggregates seem to play an important role in the pathobiology of these diseases. In the present study, we determined whether the recently identified neurotoxic tau_45-230 fragment also formed aggregates in neurodegenerative disorders. The presence of such aggregates was examined in brain samples obtained from Alzheimer's disease (AD) subjects by means of Western blot analysis performed under non-denaturing conditions. Our results showed that a mixture of tau_45-230 oligomers of different sizes was easily detectable in brain samples obtained from AD subjects. Our data also suggested that tau_45-230 oligomers could be internalized by cultured hippocampal neurons, mainly through a clathrin-mediated mechanism, triggering their degeneration. In addition, in vitro aggregation studies showed that tau_45-230 modulated full-length tau aggregation thereby inducing the formation of smaller, and potentially more toxic, aggregates of this microtubule-associated protein. Together, these data identified alternative mechanisms underlying the toxic effects of tau_45-230.

The detyrosination/re-tyrosination cycle of tubulin and its role and dysfunction in neurons and cardiomyocytes

Among the variety of post-translational modifications to which microtubules are subjected, the detyrosination/re-tyrosination cycle is specific to tubulin. It is conserved by evolution and characterized by the enzymatic removal and re-addition of a gene-encoded tyrosine residue at the C-terminus of α-tubulin. Detyrosinated tubulin can be further converted to Δ2-tubulin by the removal of an additional C-terminal glutamate residue. Detyrosinated and Δ2-tubulin are carried by stable microtubules whereas tyrosinated microtubules are present on dynamic polymers. The cycle regulates trafficking of many cargo transporting molecular motors and is linked to the microtubule dynamics via regulation of microtubule interactions with specific cellular effectors such as kinesin-13. Here, we give an historical overview of the general features discovered for the cycle. We highlight the recent progress toward structure and functioning of the enzymes that keep the levels of tyrosinated and detyrosinated tubulin in cells, the long-known tubulin tyrosine ligase and the recently discovered vasohibin-SVBP complexes. We further describe how the cycle controls microtubule functions in healthy neurons and cardiomyocytes and how deregulations of the cycle are involved in dysfunctions of these highly differentiated cells, leading to neurodegeneration and heart failure in humans.

Generation and Role of Calpain-Cleaved 17-kDa Tau Fragment in Acute Ischemic Stroke

Stroke is the leading cause of permanent disability and death in the world. The therapy for acute stroke is still limited due to the complex mechanisms underlying stroke-induced neuronal death. The generation of a 17-kDa neurotoxic tau fragment was reported in Alzheimer's disease but it has not been well studied in stroke. In this study, we observed the accumulation of 17-kDa tau fragment in cultured primary neurons and media after oxygen-glucose deprivation/reperfusion (OGD/R) treatment that could be diminished by the presence of a calpain inhibitor. This calpain-mediated proteolytic tau fragment was also detected in brain tissues from middle cerebral artery occlusion-injured rats and acute ischemic stroke patients receiving strokectomy, and human plasma samples collected within 48 h after the onset of stroke. The mass spectrometry analysis of this 17-kDa fragment identified 2 peptide sequences containing 195-224 amino acids of tau, which agrees with the previously reported tau_45-230 or tau_125-230 as the calpain-cleaved tau fragment. Ectopic expression of tau_45-230-GFP but not tau_125-230-GFP in cultured neurons induced the formation of tortuous processes without evident cell death. In summary, the 17-kDa tau fragment is a novel stroke biomarker and may play a pathophysiological role to affect post-stroke neuronal health.