HMGCS2-Induced Autophagic Degradation of Tau Involves Ketone Body and ANKRD24.

BACKGROUND: Accumulation of hyperphosphorylated Tau (pTau) contributes to the formation of neurofibrillary tangles in Alzheimer's disease (AD), and targeting Tau/pTau metabolism has emerged as a therapeutic approach. We have previously reported that mitochondrial 3-hydroxy-3-methylglutaryl-COA synthase 2 (HMGCS2) is involved in AD by promoting autophagic clearance of amyloid-ß protein precursor via ketone body-associated mechanism, whether HMGCS2 may also regulate Tau metabolism remains elusive. OBJECTIVE: The present study was to investigate the role of HMGCS2 in Tau/p degradation. METHODS: The protein levels of Tau and pTau including pT217 and pT181, as well as autophagic markers LAMP1 and LC3-II were assessed by western blotting. The differentially regulated genes by HMGCS2 were analyzed by RNA sequencing. Autophagosomes were assessed by transmission electron microscopy. RESULTS: HMGCS2 significantly decreased Tau/pTau levels, which was paralleled by enhanced formation of autophagic vacuoles and prevented by autophagic regulators chloroquine, bafilomycin A1, 3-methyladenine, and rapamycin. Moreover, HMGCS2-induced alterations of LAMP1/LC3-II and Tau/pTau levels were mimicked by ketone body acetoacetate or ß-hydroxybutyrate. Further RNA-sequencing identified ankyrin repeat domain 24 (ANKRD24) as a target gene of HMGCS2, and silencing of ANKRD24 reduced LAMP1/LC3-II levels, which was accompanied by the altered formation of autophagic vacuoles, and diminished the effect of HMGCS2 on Tau/pTau. CONCLUSION: HMGCS2 promoted autophagic clearance of Tau/pTau, in which ketone body and ANKRD24 played an important role.

Sulfuretin exerts diversified functions in the processing of amyloid precursor protein.

Sulfuretin is a flavonoid that protects cell from damage induced by reactive oxygen species and inflammation. In this study, we investigated the role of sulfuretin in the processing of amyloid precursor protein (APP), in association with the two catalytic enzymes the α-secretase a disintegrin and metalloproteinase (ADAM10), and the beta-site APP cleaving enzyme 1 (BACE1) that play important roles in the generation of ß amyloid protein (Aß) in Alzheimer's disease (AD). We found that sulfuretin increased the levels of the immature but not the mature form of ADAM10 protein. The enhanced ADAM10 transcription by sulfuretin was mediated by the nucleotides -444 to -300 in the promoter region, and was attenuated by silencing or mutation of transcription factor retinoid X receptor (RXR) and by GW6471, a specific inhibitor of peroxisome proliferator-activated receptor α (PPAR-α). We further found that sulfuretin preferentially increased protein levels of the immature form of APP (im-APP) but significantly reduced those of BACE1, sAPPß and ß-CTF, whereas Aß1-42 levels were slightly increased. Finally, the effect of sulfuretin on BACE1 and im-APP was selectively attenuated by the translation inhibitor cycloheximide and by lysosomal inhibitor chloroquine, respectively. Taken together, (1) RXR/PPAR-α signaling was involved in sulfuretin-mediated ADAM10 transcription. (2) Alteration of Aß protein level by sulfuretin was not consistent with that of ADAM10 and BACE1 protein levels, but was consistent with the elevated level of im-APP protein, suggesting that im-APP, an isoform mainly localized to trans-Golgi network, plays an important role in Aß generation.

TUFM is involved in Alzheimer's disease-like pathologies that are associated with ROS.

Mitochondrial Tu translation elongation factor (TUFM or EF-Tu) is part of the mitochondrial translation machinery. It is reported that TUFM expression is reduced in the brain of Alzheimer's disease (AD), suggesting that TUFM might play a role in the pathophysiology. In this study, we found that TUFM protein level was decreased in the hippocampus and cortex especially in the aged APP/PS1 mice, an animal model of AD. In HEK cells that stably express full-length human amyloid-ß precursor protein (HEK-APP), TUFM knockdown or overexpression increased or reduced the protein levels of ß-amyloid protein (Aß) and ß-amyloid converting enzyme 1 (BACE1), respectively. TUFM-mediated reduction of BACE1 was attenuated by translation inhibitor cycloheximide (CHX) or α-[2-[4-(3,4-Dichlorophenyl)-2-thiazolyl]hydrazinylidene]-2-nitro-benzenepropanoic acid (4EGI1), and in cells overexpressing BACE1 constructs deleting the 5' untranslated region (5'UTR). TUFM silencing increased the half-life of BACE1 mRNA, suggesting that RNA stability was affected by TUFM. In support, transcription inhibitor Actinomycin D (ActD) and silencing of nuclear factor κB (NFκB) failed to abolish TUFM-mediated regulation of BACE1 protein and mRNA. We further found that the mitochondria-targeted antioxidant TEMPO diminished the effects of TUFM on BACE1, suggesting that reactive oxygen species (ROS) played an important role. Indeed, cellular ROS levels were affected by TUFM knockdown or overexpression, and TUFM-mediated regulation of apoptosis and Tau phosphorylation at selective sites was attenuated by TEMPO. Collectively, TUFM protein levels were decreased in APP/PS1 mice. TUFM is involved in AD pathology by regulating BACE1 translation, apoptosis, and Tau phosphorylation, in which ROS plays an important role.

Mitochondrial TXN2 attenuates amyloidogenesis via selective inhibition of BACE1 expression.

Thioredoxin-2 (TXN2) is a mitochondrial protein and represents one of the intrinsic antioxidant enzymes. It has long been recognized that mitochondrial dysfunction and oxidative stress contribute to the pathogenesis of Alzheimer's disease (AD). We hypothesized that mitochondrial TXN2 might play a role in AD-like pathology. In this study, we found that in SH-SY5Y and HEK cells stably express full-length human amyloid-ß precursor protein (HEK-APP), TXN2 silencing or over-expression selectively increased or decreased the transcription of beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), respectively, without altering the protein levels of others enzymes involved in the catalytic processing of APP. As a result, ß-amyloid protein (Aß) levels were significantly decreased by TXN2. In addition, in cells treated with 3-nitropropionic acid (3-NP) that is known to increase reactive oxygen species (ROS) and promote mitochondrial dysfunction, TXN2 silencing resulted in further enhancement of BACE1 protein levels, suggesting a role of TXN2 in ROS removal. The downstream signaling might involve NFκB, as TXN2 reduced the phosphorylation of p65 and IκBα; and p65 knockdown significantly attenuated TXN2-mediated regulation of BACE1. Concomitantly, the levels of cellular ROS, apoptosis-related proteins and cell viability were altered by TXN2 silencing or over-expression. In APPswe/PS1E9 mice, an animal model of AD, the cortical and hippocampal TXN2 protein levels were decreased at 12 months but not at 6 months, suggesting an age-dependent decline. Collectively, TXN2 regulated BACE1 expression and amyloidogenesis via cellular ROS and NFκB signaling. TXN2 might serve as a potential target especially for early intervention of AD.

HMGCS2 promotes autophagic degradation of the amyloid-ß precursor protein through ketone body-mediated mechanisms.

HMGCS2 (mitochondrial 3-hydroxy-3-methylglutaryl-COA synthase 2) is a control enzyme in ketogenesis. The mitochondrial localization and interaction with APP (ß-amyloid precursor protein) suggest that HMGCS2 may play a role in the pathophysiology of AD (Alzheimer's disease). Here we report that overexpression of HMGCS2 decreased levels of APP and related CTFs (carboxy-terminal fragments), which was largely prevented by an autophagic inhibitor chloroquine. In addition, HMGCS2 enhancement of autophagic marker LC3II was diminished by rapamycin, an inhibitor of mechanistic target of rapamycin. Moreover, deprivation of EBSS (Earle's Balanced Salt Solution) significantly augmented the effect of HMGCS2 on LC3II, while acetoacetate reversed the reduction of LC3II, APP and CTFs which was induced by HMGCS2 knockdown. In the presence of acetoacetate, rapamycin failed to induce further increase of LC3II, which mimicked the effect of HMGCS2 overexpression. Finally, HMGCS2 enhanced the antioxidant response. Collectively, HMGCS2 shares with ketone bodies common features in autophagic clearance of APP and CTFs, suggesting that ketone bodies play an important role in HMGCS2 regulation of the autophagy.