Atypical role of proximal caspase-8 in truncated Tau-induced neurite regression and neuronal cell death

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.

Tau truncation in the pathogenesis of Alzheimer's disease: a narrative review

ABSTRACT

Alzheimer's disease is characterized by two major neuropathological hallmarks-the extracellular β-amyloid plaques and intracellular neurofibrillary tangles consisting of aggregated and hyperphosphorylated Tau protein. Recent studies suggest that dysregulation of the microtubule-associated protein Tau, especially specific proteolysis, could be a driving force for Alzheimer's disease neurodegeneration. Tau physiologically promotes the assembly and stabilization of microtubules, whereas specific truncated fragments are sufficient to induce abnormal hyperphosphorylation and aggregate into toxic oligomers, resulting in them gaining prion-like characteristics. In addition, Tau truncations cause extensive impairments to neural and glial cell functions and animal cognition and behavior in a fragment-dependent manner. This review summarizes over 60 proteolytic cleavage sites and their corresponding truncated fragments, investigates the role of specific truncations in physiological and pathological states of Alzheimer's disease, and summarizes the latest applications of strategies targeting Tau fragments in the diagnosis and treatment of Alzheimer's disease.

Caspase-6-cleaved Tau fails to induce Tau hyperphosphorylation and aggregation, neurodegeneration, glial inflammation, and cognitive deficits

Active Caspase-6 (Casp6) and Tau cleaved by Casp6 at amino acids 402 (Tau∆D402) and 421 (Tau∆D421) are present in early Alzheimer disease intraneuronal neurofibrillary tangles, which are made primarily of filamentous Tau aggregates. To assess whether Casp6 cleavage of Tau contributes to Tau pathology and Casp6-mediated age-dependent cognitive impairment, we generated transgenic knock-in mouse models that conditionally express full-length human Tau (hTau) 0N4R only (CTO) or together with human Casp6 (hCasp6) (CTC). Region-specific hippocampal and cortical hCasp6 and hTau expression were confirmed with western blot and immunohistochemistry in 2-25-month-old brains. Casp6 activity was confirmed with Tau∆D421 and Tubulin cleaved by Casp6 immunopositivity in 3-25-month-old CTC, but not in CTO, brains. Immunoprecipitated Tau∆D402 was detected in both CTC and CTO brains, but was more abundant in CTC brains. Intraneuronal hippocampal Tau hyperphosphorylation at S202/T205, S422, and T231, and Tau conformational change were absent in both CTC and CTO brains. A slight accumulation of Tau phosphorylated at S396/404 and S202 was observed in Cornu Ammonis 1 (CA1) hippocampal neuron soma of CTC compared to CTO brains. Eighteen-month-old CTC brains showed rare argentophilic deposits that increased by 25 months, whereas CTO brains only displayed them sparsely at 25 months. Tau microtubule binding was equivalent in CTC and CTO hippocampi. Episodic and spatial memory measured with novel object recognition and Barnes maze, respectively, remained normal in 3-25-month-old CTC and CTO mice, in contrast to previously observed impairments in ACL mice expressing equivalent levels of hCasp6 only. Consistently, the CTC and CTO hippocampal CA1 region displayed equivalent dendritic spine density and no glial inflammation. Together, these results reveal that active hCasp6 co-expression with hTau generates Tau cleavage and rare age-dependent argentophilic deposits but fails to induce cognitive deficits, neuroinflammation, and Tau pathology.

Inflammasome-Mediated Inflammation in Neurodegenerative Diseases

Inflammasomes are protein platforms consisting of multiple proteins. The biological function includes the activation of caspase-1, leading to the maturation of IL-1β and IL-18. These pro-inflammatory cytokines promote fundamental inflammatory processes in numerous infectious diseases. The inflammasome-mediated inflammation has become increasingly important in central nervous system disorders. In neurodegenerative disorders, significant contributors to disease progression include neuroinflammation and inflammatory cascades initiated by the inflammasome protein complex. This review discusses the recent progress of research on inflammasome associated with neurodegenerative disorders.

Differential protein expression of hippocampal cells associated with heavy metals (Pb, As, and MeHg) neurotoxicity: Deepening into the molecular mechanism of neurodegenerative diseases

Chronic exposure to heavy metals such as Pb, As, and MeHg can be associated with an increased risk of developing neurodegenerative diseases. Our in vitro bioassays results showed the potency of heavy metals in the order of Pb < As < MeHg on hippocampal cells. The main objective of this study was combining in vitro label free proteomics and systems biology approach for elucidating patterns of biological response, discovering underlying mechanisms of Pb, As, and MeHg toxicity in hippocampal cells. The omics data was refined by using different filters and normalization and multilevel analysis tools were employed to explore the data visualization. The functional and pathway visualization was performed by using Gene ontology and PathVisio tools. Using these all integrated approaches, we identified significant proteins across treatments within the mitochondrial dysfunction, oxidative stress, ubiquitin proteome dysfunction, and mRNA splicing related to neurodegenerative diseases. The systems biology analysis revealed significant alterations in proteins implicated in Parkinson's disease (PD) and Alzheimer's disease (AD). The current proteomics analysis of three metals support the insight into the proteins involved in neurodegeneration and the altered proteins can be useful for metal-specific biomarkers of exposure and its adverse effects.

SIGNIFICANCE

The proteomics techniques have been claimed to be more sensitive than the conventional toxicological assays, facilitating the measurement of responses to heavy metals (Pb, As, and MeHg) exposure before obvious harm has occurred demonstrating their predictive value. Also, proteomics allows for the comparison of responses between Pb, As, and MeHg metals, permitting the evaluation of potency differences hippocampal cells of the brain. Hereby, the molecular information provided by pathway and gene functional analysis can be used to develop a more thorough understanding of each metal mechanism at the protein level for different neurological adverse outcomes (e.g. Parkinson's disease, Alzheimer's diseases). Efforts are put into developing proteomics based toxicity testing methods using in vitro models for improving human risk assessment. Some of the key proteins identified can also potentially be used as biomarkers in epidemiologic studies. These heavy metal response patterns shed new light on the mechanisms of mRNA splicing, ubiquitin pathway role in neurodegeneration, and can be useful for the development of molecular biomarkers of heavy metals exposure.

Modifications of tau protein after cerebral ischemia and reperfusion in rats are similar to those occurring in Alzheimer's disease - Hyperphosphorylation and cleavage of 4- and 3-repeat tau

Epidemiological studies have suggested a close relationship between cerebral ischemia and Alzheimer's disease (AD). To clarify the pathological association of tau dynamics in both diseases, we performed comprehensive studies on the posttranslational modification of tau in cerebral ischemia and reperfusion (I/R) in rats. The present study suggests that both 4-repeat and 3-repeat tau isoforms are hyperphosphorylated in cerebral I/R, similar to the case in AD. The generation of a 60-kDa Asp⁴²¹-truncated tau in cerebral I/R preceded the emergence of a 17-kDa 3-repeat tau fragment and a 25-kDa 4-repeat tau fragment. The regional redistribution of tau from the neuropil to neuronal perikarya in our stroke model is thought to share similarity with that occurring in AD. In addition, immunofluorescence staining revealed the formation of axonal varicosities in cerebral I/R. Altered tau distribution may influence microtubule stability, disturbances in axonal transport, and the resulting formation of axonal varicosities. The staining profiles of granules in the ischemic cortex that were immunopositive for RD3, RD4, and AT8 in neuronal perikarya and that were argyrophilic on Gallyas-Braak staining were similar to those in AD. These findings suggest that transient cerebral ischemia shares a common pathology with AD, in the modification of tau protein.

Quercetin Protects against Okadaic Acid-Induced Injury via MAPK and PI3K/Akt/GSK3β Signaling Pathways in HT22 Hippocampal Neurons

Increasing evidence shows that oxidative stress and the hyperphosphorylation of tau protein play essential roles in the progression of Alzheimer’s disease (AD). Quercetin is a major flavonoid that has anti-oxidant, anti-cancer and anti-inflammatory properties. We investigated the neuroprotective effects of quercetin to HT22 cells (a cell line from mouse hippocampal neurons). We found that Okadaic acid (OA) induced the hyperphosphorylation of tau protein at Ser199, Ser396, Thr205, and Thr231 and produced oxidative stress to the HT22 cells. The oxidative stress suppressed the cell viability and decreased the levels of lactate dehydrogenase (LDH), superoxide dismutase (SOD), mitochondria membrane potential (MMP) and Glutathione peroxidase (GSH-Px). It up-regulated malondialdehyde (MDA) production and intracellular reactive oxygen species (ROS). In addition, phosphoinositide 3 kinase/protein kinase B/Glycogen synthase kinase3β (PI3K/Akt/GSK3β) and mitogen activated protein kinase (MAPK) were also involved in this process. We found that pre-treatment with quercetin can inhibited OA-induced the hyperphosphorylation of tau protein and oxidative stress. Moreover, pre-treatment with quercetin not only inhibited OA-induced apoptosis via the reduction of Bax, and up-regulation of cleaved caspase 3, but also via the inhibition of PI3K/Akt/GSK3β, MAPKs and activation of NF-κB p65. Our findings suggest the therapeutic potential of quercetin to treat AD.

3'poly-G-tailed ODNs inhibit F-spondin to induce cell death and neurite retraction in rat embryonic neurons

The effects and mechanism of action of oligodeoxyribonucleotides containing CpG motif (CpG-ODNs) on neuron cells are largely unexamined. Here, we found that CpG-A ODNs but not other types of CpG-ODNs induced neurite retraction and cell apoptosis of rat embryonic neurons in a TLR9-independent manner. These effects of CpG-A ODNs were primarily due to the poly-guanosine at the 3' terminus (3'G-ODNs). Pull-down analysis showed that 3'G-ODNs associated with transcription factor Y-BOX1 (YB-1) to facilitate the translocation of YB-1 into the nucleus via the nuclear localizing sequence of YB-1. YB-1 then interacted with the promoter of F-spondin directly at -45 and -1,375 sites as demonstrated by chromatin immunoprecipitation (ChIP) analysis. Binding of YB-1 to F-spondin promoter resulted in downregulation of F-spondin expression. Overexpression of F-spondin rescued the cell death and neurite retraction induced by 3'G-ODNs in embryonic neuron cells. Taken together, these findings suggest that 3'G-ODNs enhance nucleus YB-1 to inhibit F-spondin leading to cell death and neurite retraction of embryonic neuron cells.

The Parkinson disease protein leucine-rich repeat kinase 2 transduces death signals via Fas-associated protein with death domain and caspase-8 in a cellular model of neurodegeneration

Neurodegenerative illnesses such as Parkinson and Alzheimer disease are an increasingly prevalent problem in aging societies, yet no therapies exist that retard or prevent neurodegeneration. Dominant missense mutations in leucine-rich repeat kinase 2 (LRRK2) are the most common genetic cause of Parkinson disease (PD), but the mechanisms by which mutant forms of LRRK2 disrupt neuronal function and cause cell death remain poorly understood. We report that LRRK2 interacts with the death adaptor Fas-associated protein with death domain (FADD), and that in primary neuronal culture LRRK2-mediated neurodegeneration is prevented by the functional inhibition of FADD or depletion of caspase-8, two key elements of the extrinsic cell death pathway. This pathway is activated by disease-triggering mutations, which enhance the LRRK2-FADD association and the consequent recruitment and activation of caspase-8. These results establish a direct molecular link between a mutant PD gene and the activation of programmed cell death signaling, and suggest that FADD/caspase-8 signaling contributes to LRRK2-induced neuronal death.

Suppression of receptor-mediated apoptosis by death effecter domain recruiting domain binding peptide aptamer

FLASH protein is a component of death-inducing signaling complex and might be involved in death receptor-mediated extrinsic apoptosis. Here we developed the peptide aptamer against death effecter domain recruiting domain (DRD) of FLASH protein and showed that the peptide bound to FLASH protein in vitro. Intracellular expression of the DRD-binding peptide aptamer specifically suppressed receptor-mediated extrinsic apoptosis but not intrinsic pathway, which was recapitulated by the antisense oligonucleotides for FLASH. These data suggest that DRD-binding peptide is not only a novel inhibitor modulating receptor-mediated apoptosis but also a tool for elucidating the roles of FLASH in apoptosis.

Progressive neurodegeneration in C. elegans model of tauopathy

Discovery of various mutations in the tau gene among frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) families suggests gain-of-toxic function of wild-type or mutant tau as the mechanism for extensive neuronal loss. We thus generated transgenic nematode (Caenorhabditis elegans) expressing wild-type or mutant (P301L and R406W) tau in the touch (mechanosensory) neurons. Whereas the worm expressing wild-type tau showed a small decrease in the touch response across the lifespan, the worm expressing mutant tau displayed a large and progressive decrease. When the touch neurons lost their function, neuritic abnormalities were found prominent, and microtubular loss became remarkable in the later stage. A substantial fraction of degenerating neurons developed tau accumulation in the cell body and neuronal processes. This neuronal dysfunction is not related to the apoptotic process because little recovery from touch abnormality was observed in the ced-3 or ced-4-deficient background. Expression of GSK3 brought about slight deterioration in the touch response, while expression of HSP70 led to some improvement.

Caspase-cleavage of tau is an early event in Alzheimer disease tangle pathology

Neurofibrillary tangles (NFTs) are composed of abnormal aggregates of the cytoskeletal protein tau. Together with amyloid beta (Abeta) plaques and neuronal and synaptic loss, NFTs constitute the primary pathological hallmarks of Alzheimer disease (AD). Recent evidence also suggests that caspases are activated early in the progression of AD and may play a role in neuronal loss and NFT pathology. Here we demonstrate that tau is cleaved at D421 (DeltaTau) by executioner caspases. Following caspase-cleavage, DeltaTau facilitates nucleation-dependent filament formation and readily adopts a conformational change recognized by the early pathological tau marker MC1. DeltaTau can be phosphorylated by glycogen synthase kinase-3beta and subsequently recognized by the NFT antibody PHF-1. In transgenic mice and AD brains, DeltaTau associates with both early and late markers of NFTs and is correlated with cognitive decline. Additionally, DeltaTau colocalizes with Abeta(1-42) and is induced by Abeta(1-42) in vitro. Collectively, our data imply that Abeta accumulation triggers caspase activation, leading to caspase-cleavage of tau, and that this is an early event that may precede hyperphosphorylation in the evolution of AD tangle pathology. These results suggest that therapeutics aimed at inhibiting tau caspase-cleavage may prove beneficial not only in preventing NFT formation, but also in slowing cognitive decline.