trans-(−)-ϵ-Viniferin Increases Mitochondrial Sirtuin 3 (SIRT3), Activates AMP-activated Protein Kinase (AMPK), and Protects Cells in Models of Huntington Disease*

Background: Mitochondrial dysfunction is a key event mediating mutant Htt-induced neurotoxicity. Results: trans-(−)-ϵ-Viniferin attenuates mutant Htt-induced SIRT3 depletion, activates AMPK, and preserves mitochondrial function. Conclusion: Increasing SIRT3 protects cells in HD. Significance: The result suggests a promising new target for development of HD therapeutics. Huntington disease (HD) is an inherited neurodegenerative disorder caused by an abnormal polyglutamine expansion in the protein Huntingtin (Htt). Currently, no cure is available for HD. The mechanisms by which mutant Htt causes neuronal dysfunction and degeneration remain to be fully elucidated. Nevertheless, mitochondrial dysfunction has been suggested as a key event mediating mutant Htt-induced neurotoxicity because neurons are energy-demanding and particularly susceptible to energy deficits and oxidative stress. SIRT3, a member of sirtuin family, is localized to mitochondria and has been implicated in energy metabolism. Notably, we found that cells expressing mutant Htt displayed reduced SIRT3 levels. trans-(−)-ϵ-Viniferin (viniferin), a natural product among our 22 collected naturally occurring and semisynthetic stilbenic compounds, significantly attenuated mutant Htt-induced depletion of SIRT3 and protected cells from mutant Htt. We demonstrate that viniferin decreases levels of reactive oxygen species and prevents loss of mitochondrial membrane potential in cells expressing mutant Htt. Expression of mutant Htt results in decreased deacetylase activity of SIRT3 and further leads to reduction in cellular NAD+ levels and mitochondrial biogenesis in cells. Viniferin activates AMP-activated kinase and enhances mitochondrial biogenesis. Knockdown of SIRT3 significantly inhibited viniferin-mediated AMP-activated kinase activation and diminished the neuroprotective effects of viniferin, suggesting that SIRT3 mediates the neuroprotection of viniferin. In conclusion, we establish a novel role for mitochondrial SIRT3 in HD pathogenesis and discovered a natural product that has potent neuroprotection in HD models. Our results suggest that increasing mitochondrial SIRT3 might be considered as a new therapeutic approach to counteract HD, as well as other neurodegenerative diseases with similar mechanisms.

ing hyperacetylation of mitochondrial proteins (16). These hyperacetylated proteins include key energy metabolic enzymes as well as antioxidant enzymes, such as Mn-SOD. The hyperacetylation results in dysfunction of these enzymes, eventually leading to mitochondrial dysfunction. Therefore, compounds that increase mitochondrial SIRT3 would improve mitochondrial function and might have therapeutic potential for HD.
Natural products and their synthetic derivatives are vital resources for the discovery of biologically active small organic molecules, and they exhibit numerous biophysical attributes that make them outstanding candidates for development of new drugs (17). Moreover, natural products are unique in terms of their chemical diversity when compared with most combinatorial chemical libraries (18,19). Stilbenoids, constituting a class of resveratrol and its derivatives, naturally occur in several plant families, such as Cyperaceae, Dipterocarpaceae, Gnetaceae, and Vitaceae, and have recently been characterized chemically (20,21). Among available stilbenoids, resveratrol has shown promise in treatment and prevention of neurodegenerative disorders, including Huntington disease (22)(23)(24). However, recent work in HD transgenic mice demonstrated that resveratrol protected against peripheral deficits, but was not effective in the central nervous system in HD mice, probably due to instability of resveratrol (25). Moreover, concentrations of resveratrol required for neuroprotective actions were in the range of 10 -100 M (26).
To develop more potent neuroprotective agents from natural products, we have screened a unique collection of 22 stilbenic compounds that consists of naturally occurring resveratrol monomers and oligomers, as well as semisynthetic resveratrol derivatives. Here we demonstrate that one compound, trans-(Ϫ)-⑀-viniferin (viniferin), preserves mitochondrial membrane potential and reduces reactive oxygen species (ROS) levels induced by mutant Htt. Moreover, viniferin increases mitochondrial SIRT3 levels, activates AMP-activated protein kinase (AMPK), and replenishes cellular NAD ϩ levels. The increased NAD ϩ leads to activation of SIRT3 deacetylase activity and enhances the antioxidant activity of target substrate Mn-SOD. SIRT3 is required for the neuroprotection of viniferin as inhibition of SIRT3 abolished the protection.
Tet-Off PC12 cells expressing truncated mutant Htt N63-148Q were maintained as described previously (28). Mutant Htt was inducibly expressed when doxycycline was removed from the culture medium. Cells were differentiated in the presence of nerve growth factor (NGF, 50 ng/ml).
Primary cortical neurons were prepared from embryonic day 18 pregnant C57/BL6 mice. Neurons were cultured in Neurobasal medium supplemented with B-27. Myc-tagged mutant Htt (N63-148Q) or normal htt (N63-16Q) was transfected by Lipofectamine 2000 (Invitrogen) at days in vitro 5; the transfection efficacy was ϳ5%. Neurons were fixed and stained for transgene expression by anti-Myc antibody and chromatin condensation by the dye Hoechst 33342 to determine the neuronal toxicity induced by mutant Htt.
Caspase3/7 Assay-Caspase3/7 activity was detected by a luminescence assay in 96-well plates. Cells were dissolved with 50 l of Caspase-Glo 3/7 Reagent (Promega, G8092) and gently mixed using a plate shaker at 300 -500 rpm for 1 h. Then 100 l of the mixture was transferred to a white-walled 96-well plate, 100 l of medium with a corresponding concentration of compounds was used as a blank, and the luminescence intensity of each sample was measured in a luminometer (Fluoroskan Ascent FL, Thermo Scientific).
Intracellular ROS Measurements-Striatal cells were incubated with the fluorescent probe CM-H 2 DCFDA (1 M) for 30 min in Krebs-Ringer-Hepes buffer supplemented with 5 mM glucose. Samples were analyzed using a flow cytometer (FACSCalibur). The mean fluorescence intensity of 10,000 cells was analyzed in each sample and corrected for autofluorescence from unlabeled cells.
Mitochondrial Potential Determination in Live Cells-Mitochondrial membrane potential was determined by using the fluorescent probe TRME. Striatal cells were incubated with TMRE for 1 h. Samples were analyzed by using a flow cytometer (FACSCalibur). The mean fluorescence intensity of 10,000 cells was analyzed in each sample and corrected for autofluorescence from unlabeled cells.
NAD ϩ /NADH Assay-Intracellular NAD ϩ and NADH levels in striatal cells were measured with an NAD ϩ /NADH assay kit (Abcam) according to the manufacturer's instructions. Briefly, 5 ϫ 10 5 striatal cells were cultured in serum-free medium with or without viniferin for 24 h, and then the cells were washed with cold PBS and extracted with NADH/NAD extraction buffer by two freeze/thaw cycles (20 min on dry ice and then 10 min at room temperature). Total NAD (NADt) and NADH levels were detected in a 96-well plate, and color was developed and read at 450 nm. NADϩ/NADH ratio was calculated as: Mitochondrial Biogenesis-The mitochondrial copy numbers were calculated from the ratio of 12 S rRNA/18 S rRNA and Cox-2/cyclophilin A. The PCR primers for detecting the 12 S rRNA and Cox-2 gene of murine mitochondrial genome were designed on the basis of the GenBank TM nucleotide sequence. Each tube contained 15 ng of total DNA from the same extract as well as 10 l of reaction mixture consisting of 2ϫ SsoFast TM EvaGreen supermix (1ϫ final) and other pairs of the primers directed against either 18 S RNA or cyclophilin A (nuclearencoding gene), with 0.5 M primer in each case.
Quantitative RT-PCR-The levels of PGC-1␣ mRNA and SIRT3 mRNA were detected by quantitative RT-PCR as follows. Total RNA from cultured cells was extracted with an RNeasy mini kit (Qiagen). cDNA was then reverse-transcribed and amplified by PCR with a Transcriptor reverse transcriptase kit (Applied Biosystems). Quantitative RT-PCR was carried out with the SsoFast TM EvaGreen supermix in ABI 9700, and the results were normalized to ␤-actin.
Knockdown of SIRT3 in Striatal Cells-Stealth RNAi duplexes (Invitrogen) were designed to target mouse SIRT3 (NM_001177804.1; NM_ 022433.2). RNAi treatment of cells was as follows. 1 ϫ 10 7 cells were collected and electroporated with 400 pM siRNA at 450 V, 600 microfarads. Then 10 6 cells/ well were placed in 6-well plates. After 24 h of recovery in maintenance medium, cells were maintained in serum-free medium for 24 h and then assayed for toxicity or analyzed by Western blotting.
Immunoprecipitation-Cell extracts were prepared by resuspending PBS-washed cell pellets in 1 ml of Nonidet P-40 extraction buffer (50 mM Tris-HCl (pH 8.0), 150 mM NaCl, and 1% Nonidet P-40) supplemented with EDTA-free protease inhibitor mixture tablets (Roche Applied Science) with 10 mM nicotinamide and 5 M trichostatin A. After incubation for 30 min on ice, nonextractable material was removed by centrifugation at 17,000 ϫ g for 10 min at 4°C, and the cleared supernatant fractions were subjected to immunoprecipitation.
Statistical Analysis-Data are expressed as means of triplicates Ϯ S.D. from at least three independent experiments. Statistical significance was determined using a Student's t test, accepting a significance level of p Ͻ 0.05.

Viniferin Protects Neurons against Mutant Htt-induced Cell
Toxicity-To develop small molecular compounds that have high potential to treat HD, we screened our unique collection of 22 stilbenic natural products and their semisynthetic derivatives in an inducible PC12 cell model that we have successfully used for compound screening (28). Among these 22 compounds, we identified six compounds that had protective activ-ity in an inducible PC12 HD cell model. The EC 50 ranged from 30 nM to 10 M (Table 1). Viniferin was the most potent compound that exhibited consistent protection in different cell models of HD, including the transient transfection of mutant Htt N-terminal truncated form in N2a cells (Fig. 1A), immortalized striatal precursor cells expressing full-length huntingtin ( Fig. 1B), as well as a primary neuronal model ( Fig. 1, C and D). The neuroprotective effect of viniferin is specific to mutant Htt as viniferin had no effect on cell death in untransfected neurons (20 Ϯ 4% in DMSO-untransfected neurons versus 15.6 Ϯ 1.6% in viniferin-treated untransfected neurons).
Viniferin Reduces Mutant Htt-mediated Oxidative Stress in Striatal Cells-The mitochondrion is the major site of superoxide formation, and the accumulation of superoxide is believed to contribute to oxidative damage associated with neurodegenerative disease (29). Cells expressing mutant Htt exhibited higher levels of ROS in response to withdrawal of trophic factors (serum deprivation). To determine whether viniferin could attenuate mutant Htt-induced accumulation of ROS, striatal cells were incubated in the absence or presence of 1 M viniferin for 24 h prior to measurement of ROS levels. Intracellular ROS production was measured by fluorescenceactivated cell sorting (FACS) with use of the fluorescent probe CM-H 2 DCFDA (30). Fig. 2A shows a representative image of intracellular ROS levels in striatal cells expressing wildtype Htt (STHdh Q7/Q7 ), mutant Htt (STHdh Q111/Q111 ), or STHdh Q111/Q111 cells treated with viniferin. Cells expressing mutant Htt had a greater increase in ROS levels in comparison with cells expressing wild-type Htt (Fig. 2, A and B). Viniferin treatment significantly reduced intracellular ROS accumulation in cells expressing mutant Htt (Fig. 2, A-C).

Viniferin Prevents Mitochondrial Dysfunction, Promotes Mitochondrial Biogenesis, and Attenuates PGC-1␣ Depletion in Striatal Cells Expressing Mutant Htt-Mutant
Htt impaired mitochondrial function, as indicated by loss of mitochondrial membrane potential and lower NAD ϩ /NADH ratios in cells expressing mutant Htt when compared with those expressing wild-type Htt (Fig. 3, A-D). To determine whether viniferin prevents mitochondrial dysfunction, cells expressing wild-type or mutant Htt were treated with 1 M viniferin for 48 h and then loaded with the mitochondrial dye TRME, and changes in mitochondrial membrane potential were measured by FACS. Cells expressing mutant Htt exhibited lower mitochondrial membrane potential after serum withdrawal than did cells expressing wild-type Htt (Fig. 3A), and viniferin significantly attenuated the loss of mitochondrial potential that occurred in response to serum withdrawal in cells expressing mutant Htt (Fig. 3, B and C).
To gain insight into the mechanism by which viniferin regulates energy metabolism, we measured intracellular NAD ϩ / NADH levels. Consistent with our hypothesis, viniferin increased the NAD ϩ /NADH ratio (Fig. 3D).
We also evaluated the role of viniferin in mitochondrial biogenesis. To measure mitochondrial DNA directly, we isolated total DNA and determined the relative copy number of mitochondrial DNA by a quantitative PCR assay of the mitochondrial DNA-encoded COX II and 12 S rRNA in striatal cells. Mutant Htt resulted in a significant reduction of mitochondrial DNA, indicating reduced mitochondrial biogenesis (Fig. 3, E and F). Viniferin treatment significantly increased the number of mitochondrial DNA copies, indicating increased mitochondrial biogenesis (Fig. 3, E and F).
Viniferin Ameliorates Mutant Htt-induced SIRT3 Decline-To further understand the molecular mechanisms underlying protection by viniferin, we focused on mitochondria. SIRT3 is a mitochondrial sirtuin that plays an important role in energy metabolism and mitochondrial function. Murine SIRT3 includes a long inactive isoform (about 37 kDa) and a short active isoform (about 28 kDa) (34 -36). Both isoforms were significantly reduced in cells expressing mutant Htt (SThdh Q111/Q111 cells) (Fig. 4A), and viniferin significantly increased the levels of both isoforms of SIRT3 protein in SThdh Q111/Q111 cells (Fig. 4B). To determine whether SIRT3 decrease is the cause or consequence of mitochondria depolarization, we did a time course study. Our results indicated that mutant Htt- expressing SThdh Q111/Q111 cells displayed mitochondrial depolarization before detectable SIRT3 decrease (Fig. 4, C  and D), suggesting that the SIRT3 might represent a compensatory response to mitochondrial dysfunction. However, SIRT3 protein levels declined along with persistent mitochondrial dysfunction (Fig. 4C), viniferin preserved SIRT3 protein levels (particularly the 28-kDa active isoform), and this protection lasted longer than its analog resveratrol (Fig.  4, E and F).
We then determined whether viniferin also altered the SIRT3 deacetylase activity by measuring the acetylated Mn-SOD, a SIRT3 substrate. We incubated cells with viniferin for 4 h and  found that viniferin reduced acetylated Mn-SOD levels, indicating that viniferin increases SIRT3 deacetylase activity (Fig. 4G).
Viniferin Increases SIRT3-dependent AMPK Activation-AMPK is a ubiquitous heterotrimeric serine/threonine protein kinase, which functions as a fuel sensor. Importantly, activated AMPK stimulates ATP-generating catabolic pathways, such as cellular glucose uptake and fatty acid ␣-oxidation. AMPK activation also represses ATP-consuming processes to restore intracellular energy balance (37,38). To gain insight into the mechanism by which viniferin protects mitochondrial function, we treated striatal cells expressing mutant Htt with viniferin for 30 min and analyzed the levels of activated AMPK by Western blotting.
Viniferin robustly increased the levels of activated AMPK (phosphorylated AMPK) in SThdh Q111/Q111 cells, but total AMPK protein levels remained unchanged (Fig. 5A). This increase in AMPK activation in response to viniferin is more sensitive in SThdh Q111/Q111 cells as we only detected increased AMPK phosphorylation after treatment with high concentrations of viniferin in SThdh Q7/Q7 cells (data not shown). To determine whether activation of AMPK is due to increased SIRT3 mediated by viniferin, we blocked the increase of SIRT3 with SIRT3 siRNA, which abolished the AMPK activation induced by viniferin (Fig. 5B), suggesting that SIRT3 increase is upstream and is required for AMPK activation mediated by viniferin. Viniferin Attenuates Mutant Htt-induced Hyperacetylation of LKB1-AMPK is allosterically activated by AMP and by phosphorylation at Thr-172 in the catalytic ␣-subunit, mainly by an upstream AMPK kinase, LKB1 (39,40). Because LKB1 regulates AMPK activity (41), we first sought to determine whether LKB1 activity is altered by mutant Htt, and if so, whether viniferin modifies LKB1 activity, which is regulated by phosphorylation or acetylation. First, we measured the levels of phospho-LKB1 in cells expressing mutant Htt versus normal Htt and found that mutant Htt did not alter the levels of phosphorylated LKB1 (data not shown). Viniferin had no effect on LKB1 phosphorylation (data not shown). Remarkably, mutant Htt increased levels of acetylated LKB1, although total LKB1 levels were not altered in cells expressing mutant Htt (Fig. 6A). Viniferin significantly reduced acetylated LKB1 levels in striatal cells expressing mutant Htt (Fig. 6B). Because LKB1 is a substrate of SIRT3, to determine whether the increased acetylated LKB1 is due to the mutant Htt-induced decrease of SIRT3, we performed immunoprecipitation. SIRT3 interacts with LKB1, and viniferin did not alter this interaction (data not shown). Next, to determine whether SIRT3 mediated the effect of viniferin on LKB1 acetylation, we treated cells with SIRT3 siRNA. Inhibition of SIRT3 increased LKB1 acetylation and also abolished the effect of viniferin on the acetylation of LKB1 (Fig. 6C).

SIRT3 Is Required for Viniferin-mediated Neuroprotection against Mutant
Htt-Viniferin induced SIRT3 expression and protected cells against mutant Htt-induced neurotoxicity. We then asked whether SIRT3 is a critical mediator of neuroprotection by viniferin. Striatal cells were treated with SIRT3 siRNA before viniferin treatment, and we found that SIRT3 siRNAs completely abolished the protective effects of viniferin on mutant Htt (Fig. 7, A and B), suggesting that SIRT3 is indeed the critical mediator of the neuroprotection of viniferin against mutant Htt.

DISCUSSION
Our data support a working model for viniferin shown in Fig.  8. Viniferin attenuates mutant Htt-induced depletion of mitochondrial SIRT3, a soluble protein that controls global mitochondrial protein acetylation levels. For example, SIRT3 deacetylates its substrate Mn-SOD and enhances its antioxidant activity; SIRT3 also deacetylates LKB1, resulting in activation of AMPK, and activated AMPK promotes mitochondrial biogenesis and homeostasis of energy metabolism, thereby protecting cells from mutant Htt-mediated mitochondrial dysfunction. Activation of AMPK leads to increased levels of cellular NAD ϩ , which in turn activates SIRT3 and subsequently affects multiple pathways involved in energy metabolism. Our results demonstrate that the effect of viniferin on enhancing mitochondrial function is mediated through SIRT3.
SIRT3 has been shown to play important roles in various mitochondrial functions, such as maintaining basal ATP levels and regulating apoptosis and energy homeostasis (42). Human SIRT3 exists in two isoforms, a full-length protein (ϳ44 kDa) and a shorter, active form (ϳ28 kDa) lacking the N-terminal 142 amino acids (43). The molecular mass of mouse SIRT3 is controversial, however. Our data confirmed the presence of two forms of endogenous mouse SIRT3 (37 and 28 kDa).
Recent studies have indicated that neuronal SIRT3 protects cells against excitotoxicity (44). Notably, we found that mutant Htt depleted SIRT3 protein levels. Although the molecular mechanisms underlying this regulation remain to be investi-gated, our findings open a new avenue for therapeutic intervention for HD. In this study, we demonstrate that viniferin, a stilbene resveratrol dimer with a five-membered oxygen heterocyclic ring, increases SIRT3 protein levels and plays a neuroprotective role in HD at a range of 100 nM-1 M. The preservation of SIRT3 protein levels by viniferin is not at the transcriptional level; it might result from enhancing protein translation or slowing down protein degradation. The role of SIRT3 in the maintenance of ATP levels and in regulating mitochondrial electron transport was shown with SIRT3 knock-out mice (45). Constitutive expression of SIRT3 promotes the expression of mitochondrial genes, leading to enhanced mitochondrial electron transport activity (46). Viniferin increased levels of SIRT3, thereby promoting mitochondrial biogenesis and maintaining mitochondrial function.
The role of SIRT3 in regulating ROS production has been demonstrated in that overexpression of SIRT3 reduces ROS in adipocytes (46 -48), whereas embryonic fibroblasts from SIRT3 knock-out mice exhibited increased levels of superoxide (49). ROS accumulation is associated with the pathogenesis of HD. The increased Mn-SOD activity by viniferin enhanced the antioxidant state in mitochondria accompanied by a significant decrease in ROS levels. ROS can react with DNA, proteins, and lipids and play important roles in many physiological and pathophysiological conditions, including neurodegenerative diseases and aging (50). Although ROS are produced in multiple cell compartments, the majority of cellular ROS (about 90%) contribute to mitochondrial energy metabolism. The mitochondrial electron transport chain complex I and complex III are presumed to be major sites of ROS generation (50), where electrons escape the electron transport chain and react with molecular oxygen, leading to the generation of superoxide. There are two main ways in which ROS production is limited in vivo. The first is through the action of detoxifying enzymes, including glutathione peroxidase-1 (GPx1) and superoxide dismutases (SODs); the second is through the uncoupling proteins (51). Our data suggest that viniferin FIGURE 7. SIRT3 mediates the protective effects of viniferin on mutant Htt. A, STHdh Q111/Q111 cells were treated with SIRT3 siRNA (10 nM) or scrambled control RNA for 48 h and then collected and analyzed by Western blotting. Note that both the 37-kDa and the 28-kDa bands are decreased by SIRT3 siRNA treatment. B, STHdh Q111/Q111 cells were pretreated with SIRT3 siRNA or scrambled control RNA for 24 h and then exposed to different treatments, as indicated, for another 24 h; caspase 3/7 activity was measured in a toxicity assay. *, p Ͻ 0.05 when compared with the values of DMSO-treated cells in scrambled RNA-pretreated group; **, p Ͻ 0.05 when compared with the values of viniferin-treated cells in scrambled RNA-pretreated group by standard Student's t tests. RLU, relative luminescence units.
promotes deacetylation of Mn-SOD and enhances its antioxidant activity, thereby reducing ROS levels.
The mechanism(s) by which different stimuli modulate SIRT3 and activate AMPK in neurons remains to be fully elucidated. For example, activation of acetyl-CoA synthetase 2 (AceCS2) by SIRT3 may elevate the AMP/ATP ratio and consequently activate AMPK. Interestingly, it has also been shown that activation of AMPK causes an increase in the cellular NAD ϩ /NADH ratio, consistent with a positive feedback loop needed for prolonged activation of SIRT3. It is known that activated AMPK directly phosphorylates PGC-1␣ (52). SIRT3 promotes mitochondrial biogenesis via PGC-1␣ (53). We have found that viniferin also attenuates mutant Htt-mediated decline of PGC-1␣. Although PGC-1␣ stimulates mitochondrial biogenesis and electron transport activity, it suppresses ROS production and protects neural cells from oxidative stressor-induced death through the induction of several key ROSdetoxifying enzymes (54,55).
Continuous supply of energy is crucial for neurons because their survival requires large amounts of energy coupled with their inability to store energy. Therefore, neurons are extremely susceptible to insults that lead to energy depletion. NAD is an essential molecule that has a pivotal role in energy metabolism, cellular redox reactions, and mitochondrial function. Recent studies have revealed that maintaining intracellular NAD is important in promoting cell survival in various types of disease. Loss of NAD decreases the ability of NAD-dependent cell survival factors to carry out energy-dependent processes, leading to cell death. Recent evidence has shown that SIRT3 is involved in mitochondrial energy metabolism and biogenesis (56) and preservation of ATP biosynthetic capacity in the heart (45). SIRT3 knock-out resulted in a marked decrease in basal ATP in vivo (57) and eliminated the role of SIRT3 in protection of cells from oxidative stress (58). Our study suggests that viniferin increases SIRT3, stimulates AMPK activities, and preserves NAD levels, and therefore, could have a pivotal role in protecting neurons from mutant Htt, which promotes bioenergetic failure.
Substantial levels of SIRT3 are expressed in the brain (46). Nonetheless, its pathophysiological role remains unclear. In this study, we show, to our knowledge for the first time, that mutant Htt down-regulates SIRT3 protein levels and provide evidence that the neuroprotective role of viniferin is mediated by SIRT3 and that viniferin further enhances the activity of the antioxidant enzyme Mn-SOD and activates AMPK. Taken together, our results indicate that viniferin enhances mitochon- FIGURE 8. Diagram of the protective mechanism of viniferin. Viniferin counteracts mutant Htt-induced depletion of SIRT3 and facilitates its deacetylase activity. SIRT3 deacetylates its substrates Mn-SOD and then enhances its antioxidant activity. SIRT3 also deacetylates LKB1, resulting in activation of AMPK, and activated AMPK promotes mitochondrial biogenesis and homeostasis of energy metabolism, thereby protecting cells from mutant Htt-mediated mitochondrial dysfunction and toxicity. P indicates phosphorylation. drial function and antioxidant activity via an increase of SIRT3 and activation of AMPK and downstream signaling pathways. This mechanism may provide a promising target for development of HD therapeutics.