DNA damage-inducing agents elicit gamma-secretase activation mediated by oxidative stress

Oxidative Stress and Beta Amyloid in Alzheimer's Disease. Which Comes First: The Chicken or the Egg?

The pathogenesis of Alzheimer's disease involves β amyloid (Aβ) accumulation known to induce synaptic dysfunction and neurodegeneration. The brain's vulnerability to oxidative stress (OS) is considered a crucial detrimental factor in Alzheimer's disease. OS and Aβ are linked to each other because Aβ induces OS, and OS increases the Aβ deposition. Thus, the answer to the question "which comes first: the chicken or the egg?" remains extremely difficult. In any case, the evidence for the primary occurrence of oxidative stress in AD is attractive. Thus, evidence indicates that a long period of gradual oxidative damage accumulation precedes and results in the appearance of clinical and pathological AD symptoms, including Aβ deposition, neurofibrillary tangle formation, metabolic dysfunction, and cognitive decline. Moreover, oxidative stress plays a crucial role in the pathogenesis of many risk factors for AD. Alzheimer's disease begins many years before its symptoms, and antioxidant treatment can be an important therapeutic target for attacking the disease.

Subtle genomic DNA damage induces intraneuronal production of amyloid-β (1-42) by increasing β-secretase activity

Aberrant accumulation of amyloid-β (Aβ) in brain is the major trigger for pathogenesis in Alzheimer's disease (AD). It is imperative to understand how Aβ attains such toxic levels in the brain parenchyma. We detected that a subtle and tolerable amount of DNA damage, related to aging, increased intraneuronal Aβ_1-42 production both in cultured neuron and in cortex of rodent brain. Strikingly, we also observed elevated levels of mitochondrial fusion and of its major driver protein, MFN2. Hyperfusion of mitochondria may be seen as an adaptive stress response resulting from the induction of ER stress since we detected the activation of both PERK and IRE1α arms of unfolded protein response of ER stress. We found increased phosphorylation of PERK substrate eukaryotic initiation factor 2 α (eIF2α), and upregulation of the downstream effector proteins, ATF4 and CHOP. Concomitantly, increased XBP1 level, the direct effecter protein of IRE-1α, was observed. Reports suggest that eIF2α phosphorylation can increase BACE1 activity, the rate limiting enzyme in Aβ production. Here, we show that inhibiting PERK, decreased Aβ_1-42 level while direct BACE1 inhibition, reduced the mitochondrial fusion. We found increased MFN2 expression in young 5xFAD mice when Aβ plaques and neurodegeneration were absent. Thus, our study indicates that mild DNA damage leads to increased Aβ_1-42 production almost as a consequence of an initial ER stress-directed protective mitochondrial fusion in brain. We propose that an age-related subtle genomic DNA damage may trigger enhanced intraneuronal Aβ_1-42 production in an apparently healthy neuron way before the appearance of clinical symptoms in AD.

Phloroglucinol ameliorates cognitive impairments by reducing the amyloid β peptide burden and pro-inflammatory cytokines in the hippocampus of 5XFAD mice

Among the various causative factors involved in the pathogenesis of Alzheimer's disease (AD), oxidative stress has emerged as an important factor. Phloroglucinol is a polyphenol component of phlorotannin, which is found at sufficient levels in Ecklonia cava (E. cava). Phloroglucinol has been reported to exert antioxidant activities in various tissues. Previously, we reported that the stereotaxic injection of phloroglucinol regulated synaptic plasticity in an AD mouse model. In this study, we aimed to investigate the effects of oral administration of phloroglucinol in AD. The oral administration of phloroglucinol for 2 months attenuated the impairments in cognitive function observed in 6-month-old 5X familial AD (5XFAD) mice, as assessed with the T-maze and Y-maze tests. The administration of phloroglucinol for 2 months in 5XFAD mice caused a reduction in the number of amyloid plaques and in the protein level of BACE1, a major amyloid precursor protein cleavage enzyme, together with γ-secretase. Phloroglucinol also restored the reduction in dendritic spine density and the number of mature spines in the hippocampi of 5XFAD mice. In addition, phloroglucinol-administered 5XFAD mice displayed lower protein levels of GFAP and Iba-1 and mRNA levels of TNF-α and IL-6 compared with vehicle-administered 5XFAD mice. These results demonstrated that phloroglucinol alleviated the neuropathological features and behavioral phenotypes in the 5XFAD mouse model. Taken together, our results suggest that phloroglucinol has therapeutic potential for AD treatment.

Increased acetylation of Peroxiredoxin1 by HDAC6 inhibition leads to recovery of Aβ-induced impaired axonal transport

Background

Reduction or inhibition of histone deacetylase 6 (HDAC6) has been shown to rescue memory in mouse models of Alzheimer’s disease (AD) and is recently being considered a possible therapeutic strategy. However, the restoring mechanism of HDAC6 inhibition has not been fully understood.

Methods and results

Here, we found that an anti-oxidant protein Peroxdiredoxin1 (Prx1), a substrate of HDAC6, malfunctions in Aβ treated cells, the brains of 5xFAD AD model mice and AD patients. Malfunctioning Prx1, caused by reduced Prx1 acetylation levels, was recovered by HDAC6 inhibition. Increasing acetylation levels of Prx1 by HDAC6 inhibition recovered elevated reactive oxygen species (ROS) levels, elevated Ca²⁺ levels and impaired mitochondrial axonal transport, sequentially, even in the presence of Aβ. Prx1 mutant studies on the K197 site for an acetylation mimic or silencing mutation support the results showing that HDAC6 inhibitor restores Aβ-induced disruption of ROS, Ca²⁺ and axonal transport.

Conclusions

Taken together, increasing acetylation of Prx1 by HDAC6 inhibition has several beneficial effects in AD pathology. Here, we present the novel mechanism by which elevated acetylation of Prx1 rescues mitochondrial axonal transport impaired by Aβ. Therefore, our results suggest that modulation of Prx1 acetylation by HDAC6 inhibition has great therapeutic potential for AD and has further therapeutic possibilities for other neurodegenerative diseases as well.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-017-0164-1) contains supplementary material, which is available to authorized users.

Inhibition of Cholesterol Biosynthesis Reduces γ-Secretase Activity and Amyloid-β Generation

Amyloid-β (Aβ) is one of major molecules contributing to the pathogenesis of Alzheimer's disease (AD). Aβ is derived from amyloid-β protein precursor (AβPP) through sequential cleavages by β- and γ-secretases. Regulation of these components is thought to be an important factor in Aβ generation during the pathogenesis of AD. AβPP, β-secretase, and γ-secretase reside in lipid rafts, where cholesterol regulates the integrity and flexibility of membrane proteins and Aβ is generated. However, the relationship between cholesterol and Aβ generation is controversial. In this study, we aimed to elucidate the direct effects of cholesterol depletion on AβPP processing using AY9944, which blocks the last step of cholesterol biosynthesis and thus minimizes the unknown side effects of upstream inhibitors, such as HMG-CoA reductase inhibitors. Treatment with AY9944 decreased γ-secretase activity and Aβ generation. These results suggested that changes in membrane composition by lowering cholesterol with AY9944 affected γ-secretase activity and Aβ generation, which is associated with AD pathogenesis.

The arsenic exposure hypothesis for Alzheimer disease

Prior research has shown that arsenic exposure induces changes that coincide with most of the developmental, biochemical, pathologic, and clinical features of Alzheimer disease (AD) and associated disorders. On the basis of this literature, we propose the Arsenic Exposure Hypothesis for AD that is inclusive of and cooperative with the existing hypotheses. Arsenic toxicity induces hyperphosphorylation of protein tau and overtranscription of the amyloid precursor protein, which are involved in the formation of neurofibrillary tangles and brain amyloid plaques, consistent with the amyloid hypothesis of AD. Arsenic exposure has been associated with cardiovascular diseases and associated risk factors, which is in agreement with the vascular hypothesis of AD. Arsenic exposure invokes brain inflammatory responses, which resonates with the inflammatory hypotheses of AD. Arsenic exposure has been linked to reduced memory and intellectual abilities in children and adolescents, which provides a biologic basis for the developmental origin of health and disease hypothesis for AD. Arsenic and its metabolites generate free radicals causing oxidative stress and neuronal death, which fits the existing oxidative stress hypothesis. Taken together, the arsenic exposure hypothesis for AD provides a parsimonious testable hypothesis for the development and progression of this devastating disease at least for some subsets of individuals.

Chemotherapeutic agents subvert tumor immunity by generating agonists of platelet-activating factor

Oxidative stress suppresses host immunity by generating oxidized lipid agonists of the platelet-activating factor receptor (PAF-R). Because many classical chemotherapeutic drugs induce reactive oxygen species (ROS), we investigated whether these drugs might subvert host immunity by activating PAF-R. Here, we show that PAF-R agonists are produced in melanoma cells by chemotherapy that is administered in vitro, in vivo, or in human subjects. Structural characterization of the PAF-R agonists induced revealed multiple oxidized glycerophosphocholines that are generated nonenzymatically. In a murine model of melanoma, chemotherapeutic administration could augment tumor growth by a PAF-R-dependent process that could be blocked by treatment with antioxidants or COX-2 inhibitors or by depletion of regulatory T cells. Our findings reveal how PAF-R agonists induced by chemotherapy treatment can promote treatment failure. Furthermore, they offer new insights into how to improve the efficacy of chemotherapy by blocking its heretofore unknown impact on PAF-R activation.

UV irradiation accelerates amyloid precursor protein (APP) processing and disrupts APP axonal transport

Overexpression and/or abnormal cleavage of amyloid precursor protein (APP) are linked to Alzheimer's disease (AD) development and progression. However, the molecular mechanisms regulating cellular levels of APP or its processing, and the physiological and pathological consequences of altered processing are not well understood. Here, using mouse and human cells, we found that neuronal damage induced by UV irradiation leads to specific APP, APLP1, and APLP2 decline by accelerating their secretase-dependent processing. Pharmacological inhibition of endosomal/lysosomal activity partially protects UV-induced APP processing implying contribution of the endosomal and/or lysosomal compartments in this process. We found that a biological consequence of UV-induced γ-secretase processing of APP is impairment of APP axonal transport. To probe the functional consequences of impaired APP axonal transport, we isolated and analyzed presumptive APP-containing axonal transport vesicles from mouse cortical synaptosomes using electron microscopy, biochemical, and mass spectrometry analyses. We identified a population of morphologically heterogeneous organelles that contains APP, the secretase machinery, molecular motors, and previously proposed and new residents of APP vesicles. These possible cargoes are enriched in proteins whose dysfunction could contribute to neuronal malfunction and diseases of the nervous system including AD. Together, these results suggest that damage-induced APP processing might impair APP axonal transport, which could result in failure of synaptic maintenance and neuronal dysfunction.

Critical role of presenilin-dependent γ-secretase activity in DNA damage-induced promyelocytic leukemia protein expression and apoptosis

Promyelocytic leukemia (PML) is a major component of macromolecular multiprotein complexes called PML nuclear-bodies (PML-NBs). These PML-NBs recruit numerous proteins including CBP, p53 and HIPK2 in response to DNA damage, senescence and apoptosis. In this study, we investigated the effect of presenilin (PS), the main component of the γ-secretase complex, in PML/p53 expression and downstream consequences during DNA damage-induced cell death using camptothecin (CPT). We found that the loss of PS in PS knockout (KO) MEFs (mouse embryonic fibroblasts) results in severely blunted PML expression and attenuated cell death upon CPT exposure, a phenotype that is fully reversed by re-expression of PS1 in PS KO cells and recapitulated by γ-secretase inhibitors in hPS1 MEFs. Interestingly, the γ-secretase cleavage product, APP intracellular domain (AICD), together with Fe65-induced PML expression at the protein and transcriptional levels in PS KO cells. PML and p53 reciprocally positively regulated each other during CPT-induced DNA damage, both of which were dependent on PS. Finally, elevated levels of PML-NB, PML protein and PML mRNA were detected in the brain tissues from Alzheimer's disease (AD) patients, where γ-secretase activity is essential for pathogenesis. Our data provide for the first time, a critical role of the PS/AICD-PML/p53 pathway in DNA damage-induced apoptosis, and implicate this pathway in AD pathogenesis.

Amyloid precursor protein binding protein Fe65 is cleaved by caspases during DNA damage-induced apoptosis

Caspases cleave several cellular proteins to execute cell death by apoptosis. The identification of novel substrates of caspases could provide an important clue for elucidation of new apoptosis signaling pathways. In this study, we tested whether an amyloid precursor protein (APP) binding protein Fe65 is proteolytically degraded in neuronal cell death by apoptosis, using a neuron-like cell line, human neuroblastoma SH-SY5Y cells. When treated with DNA damaging agents, etoposide (ETP) and camptothecin (CPT), SH-SY5Y cells underwent apoptosis in a dose-dependent manner. Interestingly, Fe65 (97 kDa) was cleaved to a 65 kDa product during DNA damage-induced apoptosis. Furthermore, the cleavage of Fe65 was accompanied by activation of caspases-9 and -3. The restriction cleavage of Fe65 was completely suppressed by the treatment with a pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp(OMe) fluoromethylketone (z-VAD-fmk). These results reveal the restriction cleavage of Fe65 by caspases during DNA damage-induced apoptosis. Since Fe65 has been shown to suppress APP processing to amyloid β (Aβ) production, our findings may provide a new insight into the molecular mechanism by which DNA damage induces Aβ production and subsequent neuronal cell death in Alzheimer's disease (AD).

L1CAM regulates DNA damage checkpoint response of glioblastoma stem cells through NBS1

Glioblastomas (GBMs) are highly lethal brain tumours with current therapies limited to palliation due to therapeutic resistance. We previously demonstrated that GBM stem cells (GSCs) display a preferential activation of DNA damage checkpoint and are relatively resistant to radiation. However, the molecular mechanisms underlying the preferential checkpoint response in GSCs remain undefined. Here, we show that L1CAM (CD171) regulates DNA damage checkpoint responses and radiosensitivity of GSCs through nuclear translocation of L1CAM intracellular domain (L1-ICD). Targeting L1CAM by RNA interference attenuated DNA damage checkpoint activation and repair, and sensitized GSCs to radiation. L1CAM regulates expression of NBS1, a critical component of the MRE11-RAD50-NBS1 (MRN) complex that activates ataxia telangiectasia mutated (ATM) kinase and early checkpoint response. Ectopic expression of NBS1 in GSCs rescued the decreased checkpoint activation and radioresistance caused by L1CAM knockdown, demonstrating that L1CAM signals through NBS1 to regulate DNA damage checkpoint responses. Mechanistically, nuclear translocation of L1-ICD mediates NBS1 upregulation via c-Myc. These data demonstrate that L1CAM augments DNA damage checkpoint activation and radioresistance of GSCs through L1-ICD-mediated NBS1 upregulation and the enhanced MRN-ATM-Chk2 signalling.

Caspases-2 and -8 are involved in the presenilin1/gamma-secretase-dependent cleavage of amyloid precursor protein after the induction of apoptosis

The presenilin/gamma-secretase protease cleaves many type-I membrane proteins, including the amyloid beta-protein (Abeta) precursor (APP). Previous studies have shown that apoptosis induces alterations in Abeta production in a caspase-dependent manner. Here, we report that staurosporine (STS)-induced apoptosis induces caspase-8 and/or-2-dependent gamma-secretase activation. Blocking of caspase activity with caspase-8 inhibitor z-IETD-fmk, and caspase-2 inhibitor z-VDVAD-fmk reduced Abeta production by STS in H4 cells expressing the Swedish mutant of APP (HSW) or APP-C99 (H4-C99). There was no inhibitory effect of other caspases (-1, -3, -5, -6, -9) on Abeta production by STS. This finding was further supported by evidence that siRNA transfection, depleting caspase-2 or -8 levels, lowered Abeta production in HSW and H4-C99 cells without affecting expression of APP or gamma-secretase complex. In addition, Abeta production by STS was decreased by JNK inhibitors, SP600125. These results suggest that caspase-2 and/or -8 is involved in presenilin/gamma-secretase activation and Abeta production in apoptosis.

Pro-oxidant diet enhances β/γ secretase-mediated APP processing in APP/PS1 transgenic mice

The etiology of Alzheimer's disease (AD) is complex with oxidative stress being a possible contributory factor to pathogenesis and disease progression. TASTPM transgenic mice expressing familial AD-associated amyloid precursor protein (APPswe) and presenilin transgenes (PS1M146V) show increased brain amyloid beta (Aβ) levels and Aβ plaques from 3 months. We tested if enhancing oxidative stress through diet would accelerate Aβ-related pathology. TASTPM were fed a pro-oxidant diet for 3 months resulting in increased brain levels of protein carbonyls, increased Nrf2, and elevated concentrations of glutathione (GSH). The diet increased both amyloid precursor protein (APP) and Aβ in the cortex of TASTPM but did not alter Aβ plaque load, presenilin 1, or β-secretase (BACE1) expression. TASTPM cortical neurons were cultured under similar pro-oxidant conditions resulting in increased levels of APP and Aβ likely as a result of enhanced β/γ secretase processing of APP. Thus, pro-oxidant conditions increase APP levels and enhance BACE1-mediated APP processing and in doing so might contribute to pathogenesis in AD.

Induction of amyloid precursor protein by the neurotoxic peptide, amyloid-beta 25-35, causes retinal ganglion cell death

Patients with Alzheimer's disease (AD) show a significantly increased incidence of glaucoma. AD is also associated with the occurrence of the neurotoxic peptide amyloid beta (Abeta). Therefore, we investigated whether Abeta is associated with retinal cell death in a retinal ganglion cell line (RGC-5). Treatment with Abeta(25-35), a neurotoxic fragment of Abeta, induced cell death in RGC-5 in both a concentration- and time-dependent manner. The amount of amyloid precursor protein was increased by treatment of RGC-5 and primary culture of mouse cortical neurons with fibril Abeta(25-35) and Abeta(1-42), which is a putative physiological neurotoxic fragment of Abeta present in AD. Amyloid precursor protein knockdown inhibited the cell death induced by Abeta(25-35). Treatment with Abeta(25-35) increased the amount of intracellular Abeta(1-40) and Abeta(1-42), while beta- and gamma-secretase inhibitors reduced cell death. Thus, the regulation of Abeta can be viewed as a new therapeutic target for glaucoma, especially in patients with coincident AD.

The metabolic enhancer piracetam ameliorates the impairment of mitochondrial function and neurite outgrowth induced by beta-amyloid peptide

BACKGROUND AND PURPOSE

beta-Amyloid peptide (Abeta) is implicated in the pathogenesis of Alzheimer's disease by initiating a cascade of events from mitochondrial dysfunction to neuronal death. The metabolic enhancer piracetam has been shown to improve mitochondrial dysfunction following brain aging and experimentally induced oxidative stress.

EXPERIMENTAL APPROACH

We used cell lines (PC12 and HEK cells) and murine dissociated brain cells. The protective effects of piracetam in vitro and ex vivo on Abeta-induced impairment of mitochondrial function (as mitochondrial membrane potential and ATP production), on secretion of soluble Abeta and on neurite outgrowth in PC12 cells were investigated.

KEY RESULTS

Piracetam improves mitochondrial function of PC12 cells and acutely dissociated brain cells from young NMRI mice following exposure to extracellular Abeta(1-42). Similar protective effects against Abeta(1-42) were observed in dissociated brain cells from aged NMRI mice, or mice transgenic for mutant human amyloid precursor protein (APP) treated with piracetam for 14 days. Soluble Abeta load was markedly diminished in the brain of those animals after treatment with piracetam. Abeta production by HEK cells stably transfected with mutant human APP was elevated by oxidative stress and this was reduced by piracetam. Impairment of neuritogenesis is an important consequence of Abeta-induced mitochondrial dysfunction and Abeta-induced reduction of neurite growth in PC12 cells was substantially improved by piracetam.

CONCLUSION AND IMPLICATIONS

Our findings strongly support the concept of improving mitochondrial function as an approach to ameliorate the detrimental effects of Abeta on brain function.