Nrf2 as a Potential Therapeutic Target for Treatment of Huntington’s Disease

Abstract

Oxidative stress-induced neuronal damage plays a significant role in pathogenesis of several neuro-degenerative disorders including Huntington’s disease. In Huntington’s disease, oxidative stress-induced neuronal damage is reported to be mediated by PGC-1α and microglial cells. This development led to various clinical trials that tested the efficacy of several exogenous antioxidants such as vitamin E, vitamin C, etc. to prevent the oxidative stress-induced cell damage in several neuro-degenerative disorders. But these randomized clinical trials did not find any significant beneficial effects of exogenous antioxidants in neuro-degenerative disorders. This forced scientists to search endogenous targets that would enhance the production of antioxidants. Nrf2 is one such ideal target that increases the transcription of genes involved in production of antioxidants. Nrf2 is a transcription factor that controls the expression of antioxidant genes that defend cells against oxidative stress. This chapter focuses on the role of oxidative stress in Huntington’s disease and explores the therapeutic benefits of Nrf2 activators.

1. Introduction

Huntington’s disease is an inherited autosomal dominant neurodegenerative disorder characterized by a triad of psychiatric, cognitive and motor symptoms. Every human has two copies of the huntingtin gene (HTT) that codes for huntingtin protein (htt) [1]. The exact functions of huntingtin protein still remain unclear, but it is believed to be involved in the development and formation of cortical and striatal excitatory synapses, surveillance and biogenesis of mitochondrial function, activation of glial cells, upregulation of the expression of brain derived neurotrophic factor, balance of histone acetylation and deacetylation, axonal transport, regulation of signaling pathways and autophagy [2, 3, 4, 5]. The HTT gene, also called as IT15 (‘interesting transcript 15’) gene, is located on the short arm (p) of chromosome number 4 at 4p16.3 [6]. The 5′ end of the HTT gene comprises a three-nucleotide base sequence, cytosine-adenine-guanine (CAG), which is repeated multiple times and codes for the amino acid glutamine. The number of CAG repetitions in a healthy individual varies between 7 and 35. This region of CAG repeats called the trinucleotide repeats varies in length from person to person and may vary in length from generation to generation. The length of the CAG region in the HTT gene is increased due to a hereditary mutation in the HTT gene. The length of the CAG repeats ranges between 36 and 120 in people with HD. Individuals with CAG repeats between 36 and 39 may or may not develop signs of Huntington’s disease, whereas individuals with 40 or more repeats always display the characteristic signs and symptoms of Huntington’s disease. This expansion of CAG repeats due to the inherited mutation in the HTT gene leads to the production of an unusually long version of huntingtin protein (mHtt) [7]. The mutant huntingtin protein is highly susceptible to cleavage, and this results in the creation of shorter fragments containing polyglutamine expansion. These protein fragments are susceptible to misfolding and aggregation, producing fibrillar aggregates in which non-native polyglutamine strands from different proteins are bonded together by hydrogen bonds. These aggregates share the same basic β-amyloid structure seen in other protein deposition diseases [8]. One of the pathways through which mHtt causes cell death is mitochondrial dysfunction [9]. The impairment of mitochondrial electron transport chain by mHtt increases the level of free radicals and oxidative stress [10]. Following the irrefutable role of oxidative stress and associated neuroinflammation in the pathogenesis of neurodegenerative disorders including Huntington’s disease, several exogenous antioxidants were expected to have protective and therapeutic benefits in these degenerative diseases of the brain. But large-scale randomized clinical trials failed to establish any conclusive data to support the hypothesis that exogenous antioxidants could possess neuroprotective or therapeutic benefit in neurodegenerative diseases. Nevertheless, these clinical trials do not refute the fact that oxidative stress and associated neuroinflammation play a key role in the pathogenesis of neurodegenerative diseases. So, it appears logical to stimulate endogenous targets that would reduce oxidative stress and associated neuroinflammation in diseases associated with oxidative stress. Nuclear factor-erythroid-2-related factor 2 (Nrf2) is one such target. Nrf2 is a transcription factor present in the cytoplasm of cells [11]. By upregulating the expression of almost 200 cytoprotective genes, Nrf2 assists cells adapt to inflammation and oxidative stress. Keap1, a repressor protein, controls the level of Nrf2 in the cytoplasm. Keap1 is a cysteine-rich protein that binds to Nrf2 and activates the ubiquitin-proteasome pathway to degrade Nrf2 in the cytoplasm. During oxidative stress, the degradation of Nrf2 by Keap1 is blocked. This results in an increased level of Nrf2 in the cytoplasm of cells. The free Nrf2 moves into the nucleus of cell and increases the transcription of many genes that code for detoxification enzymes and cytoprotective proteins [12]. The potential of Nrf2 to negate oxidative stress and associated neuroinflammation makes it an effective target in the prevention and treatment of Huntington’s disease. The focus of this chapter is to review the role of oxidative stress and associated neuroinflammation in Huntington’s disease and the potential beneficial effects of Nrf2 activators in Huntington’s disease.

2. Oxidative stress in Huntington’s disease

Reactive oxygen species (ROS) are highly reactive molecules or molecular fragments formed from oxygen through biochemical reactions that occur during cellular respiration. Reactive oxygen species and reactive nitrogen species exert both beneficial and harmful effects on the living systems [13]. At low to moderate cellular levels, free radicals play a physiological role in destroying the invading pathogenic microorganisms, regulation of signaling pathways and induction of mitogenic response. At high cellular concentrations, free radicals exert a deleterious effect on lipids, proteins, nucleic acids and other cellular structures [14]. In many pathological conditions including Huntington’s disease, an increase in the level of free radicals and cellular damage due to free radicals is observed. But it still remains unclear whether free radical induced damage in pathological conditions is a cause or downstream consequence of the underlying pathological process.

3. Antioxidants in Huntington’s disease

Antioxidants are substances that are capable of scavenging the free radicals and thereby counteracting the free radical induced oxidative damage and inflammation. There are two classes of antioxidants—enzymatic and non-enzymatic antioxidants. Enzymatic antioxidants include superoxide dismutase (SOD), glutathione peroxidase (Gpx) and catalase (CAT). Ascorbic acid (Vitamin C), carotenoids, α-tocopherol, glutathione (GSH), retinoic acid and flavonoids are examples of non-enzymatic antioxidants. Many of these antioxidants have proven their efficacy in several in vitro and animal models but not in randomized clinical trials. The following table summarizes the findings of the studies that evaluated the efficacy of antioxidants in Huntington’s disease (Table 1).

Compound	Summary	Reference
α-Tocopherol	α-Tocopherol treatment (50 and 100 mg/kg, p.o.) significantly reversed the various behavioral, biochemical and mitochondrial alterations in malonic acid treated animals.	[15]
Metalloporphyrins	Metalloporphyrins are a class of metallic antioxidants with a potential to scavenge free radicals. In an in vitro model of HD, a manganese porphyrin (manganese (III)tetrakis(4-benzoic acid) porphyrin) reduced significant cell death.	[16]
Grape seed Polyphenolic Extract (GSPE)	GSPE is a natural compound and a strong antioxidant that has been reported to inhibit polyQ aggregation in phaeochromocytoma (PC)-12 cell line.	[17]
Melatonin	In the kainic acid animal model of neurodegeneration, melatonin has shown to be neuroprotective. Melatonin increased neuronal survival while reducing DNA damage. Melatonin therapy effectively reduced the increase in lipid peroxidation, protein carbonyls, and SOD activity inside the striatum in another investigation employing the 3-NP model of HD.	[18, 19]
Selenium	In rats treated with quinolinic acid, selenium reduced lipid peroxidation and enhanced neuronal morphology in the striatum in a dose-dependent manner.	[20]
Pyruvate	Treatment with pyruvate protected against striatal neurodegeneration in a quinolinic acid striatal lesion model of HD. Although smaller dosages had no effect, higher doses had a substantial neuroprotective effect, lowering the striatal lesion area when compared with controls.	[21]
TUDCA	TUDCA (tauroursodeoxycholic acid) is an antioxidant-rich hydrophilic bile acid. In a 3-NP rat model of HD, TUDCA reduced striatal degeneration and improved locomotor and cognitive impairments.	[22]
NAC	Treatment of rats with N-acetylcysteine (NAC), a known glutathione precursor, before exposure to 3-NP protected them from oxidative damage caused by 3-NP, as determined by electron paramagnetic resonance (EPR) and protein carbonyl analyzes on a Western blot. Furthermore, NAC therapy prior to 3-NP delivery reduced striatal lesion volumes considerably.	[23]
Lycopene	In a 3-NP induced mouse model of HD, lycopene, a carotenoid pigment and phytochemical naturally found in fruits and vegetables, decreased oxidative stress markers and improved behavior.	[24]
α-Tocopherol	In patients with mild to moderate HD symptoms, a year-long placebo-controlled, double-blind research was conducted. Although α-tocopherol had no effect on neurologic or neuropsychiatric symptoms in the overall therapy group, post hoc analysis revealed that it had a substantial effect on neurologic symptoms in HD patients early in the disease’s course.	[25]
Idebenone	A double-blind, placebo-controlled trial of idebenone in 92 HD patients was performed and no effect on primary outcome measures when compared with placebo controls were detected.	[26]

Table 1.

List of antioxidants studied in Huntington’s disease.

4. PGC1α-mediated oxidative stress in Huntington’s disease

The peroxisome proliferator-activated receptor co-activator-1α (PGC1α) is a transcriptional regulator present in tissues that have a high energy demand such as the brain, liver, cardiomyocytes, adipocytes, skeletal muscles and the kidneys [27, 28]. PGC1α plays a key role in mitochondrial biogenesis, metabolism, peroxisomal remodeling and detoxification of reactive oxygen species [29]. An important and effective mechanism through which PGC1α confers neuroprotection is by its antioxidant activity. Oxidative stress is suppressed by PGC1α by inducing the formation of antioxidant enzymes such as SOD1, SOD2, Gpx-1 and mitochondrial uncoupling proteins [30]. PGC1α also regulates the expression of SIRT3 in mitochondria and SIRT3 in turn activates SOD2 via deacetylation and reduces the level of reactive oxygen species [31, 32]. In short, PGC1α plays a key role in improving mitochondrial function, biogenesis, expression of antioxidant enzymes and amelioration of oxidative stress induced neuronal damage. A deficiency of PGC1α in the brain affects the integrity of mitochondrial structure and increases the level of reactive oxygen species leading to cellular senescence and disorders related to aging [33]. PGC1α expression has been found to be disturbed in neurodegenerative diseases such as Huntington’s disease, Parkinson’s disease and multiple sclerosis, resulting in mitochondrial abnormalities and elevated ROS levels [34, 35, 36]. Therapeutic agents that can activate endogenous antioxidant systems such as Nrf2/ARE pathway leading to increased expression of antioxidant enzymes hold great promise as neuroprotective agents in Huntington’s disease. Transcriptional modification of Nrf2 pathway, therefore, is considered an excellent approach to counteract the oxidative stress-mediated neuronal damage in Huntington’s disease.

In Huntington’s disease, mHtt causes an increase in oxidative stress mediated by PGC1α. mHtt binds to the promoter sequence of PGC1α and reduces the transcriptional level of PGC1α [37]. mHtt also supresses the expression of mitochondrial uncoupling proteins and antioxidant enzymes by direct binding and inactivation of PGC1α [30]. mHtt disrupts the balance between mitochondrial fission-fusion process by interfering with the function of Drp1 [38]. mHtt induces leakage of calcium ions through the calcium channel ryanodine receptors, further resulting in opening of the mitochondrial permeability transition pore (mPTP), which contributes to mitochondrial oxidative stress [39]. PGC-1α transcription and activity impact the enzyme system that combats reactive oxygen species (ROS). As a result, ROS defense genes such as SOD1, SOD2 and glutathione peroxidase (GPx1) are downregulated, resulting in increased oxidative damage and neuronal death in Huntington’s disease (Figure 1).

Figure 1.

PGC1α-mediated oxidative stress in Huntington’s disease.

5. Microgliosis, oxidative stress and associated neuroinflammation in Huntington’s disease

Microglial cells are the resident immune cells of the central nervous system (CNS) and make up between 10 and 15% of all glial cells in the brain. Microglial cells develop from pro-erythromyeloid progenitor cells in the yolk sac during embryogenesis and go through three stages of development: early, pre and adult microglia. They then migrate into the CNS, using white matter tracts as guiding structures, until the blood-brain barrier is formed. Microglial cells, once inside the CNS, multiply and disseminate evenly to various regions of the brain and maintain a constant population through self-renewal [40]. Microglial cells have numerous slender and elongated processes branching from the small oval-shaped body, which makes them appear ramified. However, when the brain is exposed to potential dangers such as infection, trauma or other factors, these cells lose their branches and take on an amoeboid shape. Microglial cells in the CNS are involved in the establishment and remodeling of neural circuits, protection and repair of the brain, phagocytosis of apoptotic cells in the developing brain, organization of synapses, neurogenesis, control of myelin turnover, control of neuronal excitability and programmed cell death [41, 42]. Homeostatic microglial cells interact with practically every component of the CNS to maintain homeostasis, development and repair by continuously monitoring ongoing actions in the brain. When microglial cells detect a threat to the CNS’s homeostasis, they become activated and produce a variety of cytokines and pro-inflammatory mediators to neutralize the threat. Although this acute response of microglial cells is protective and necessary for maintaining CNS homeostasis, over-activation of microglial cells has been linked to a variety of neurodegenerative diseases [43]. Microglial cells, after activation, release pro-inflammatory mediators and several cytokines that lead to severe oxidative stress and neuroinflammation. According to recent research, activated microglial cells release cytokines and pro-inflammatory mediators, which are the main contributors to neuroinflammation in neurodegenerative diseases [44, 45, 46].

A significant increase in microgliosis has been observed in the autopsied brains of the patients with Huntington’s disease compared with the controls. Accumulation of glial cells has been observed in all grades of Huntington’s disease, and the density of microglial cells finely correlates with the degree of neuronal loss [47, 48]. A significant activation of microglial cells in the regions of the brain affected by Huntington’s disease has been reported in an in vivo positron emission tomography [49, 50]. In Huntington’s disease, microglial cells are activated by mHtt protein, and activated glial cells cause degeneration of neurons in the striatal region of the brain by releasing a variety of proinflammatory cytokines and free radicals [51, 52].

6. Structure of Nrf2

The Nrf2 protein contains 6 highly conserved regions called Nrf2-ECH (Neh) homology domains. The first domain (Neh1) carries the CNC-bZIP domain that mediates heterodimerization with Maf (musculoaponeurotic fibrosarcoma oncogene homolog) proteins. Two degrons called DLG and ETGF, present in the second domain (Neh2) specifically bind to Keap1 protein that leads to degradation of Nrf2 [53]. The third domain (Neh3) is considered to improve the stability of Nrf2 and also acts as the transactivation domain. The fourth (Neh4) and fifth (Neh5) domains of Nrf2 also act as transactivation domains by binding to cAMP response Element Binding Protein (CREB). The sixth domain (Neh6) plays a role in the degradation of Nrf2 by E3 ubiquitin ligase [54].

7. Structure of Keap1

‘Kelch-like ECH-associated protein 1(Keap 1) is a protein that interacts with Nrf2 leading to degradation of Nrf2. Keap1 is a protein of BTB-Kelch family, composed of four domains. The N-terminal domain—Broad complex, Tramtrack and Bric a Bric (BTB) control homodimerization of Keap 1 and its interaction with cul3. This domain also contains Cys-151 residue that plays an important role in sensing oxidative stress. The second domain called the intervening region (IVR) domain contains Cys-273 and Cys-288. These two cysteine residues play a secondary role in sensing oxidative stress. The third domain, double glycine repeat (DGR) and the fourth domain, C-terminal region (CTR) binds to ETGE and DLG motifs of Nrf2 and causing its degradation (Figure 2) [54].

Figure 2.

Structure of Nrf2 and Keap1.

8. Mechanism of action of Nrf2 activators

Nrf2 is a transcription factor that regulates the expression of many antioxidant enzymes, phase I and phase II enzymes and several anti-inflammatory mediators. Nrf2 acts as an important defense mechanism in the neurons and glial cells against oxidative stress, neuroinflammation and other pathological insults. Nrf2 dysregulation has been reported in many oxidative-stress-related diseases such as Huntington’s disease [54]. This makes Nrf2 activators excellent agents to increase antioxidant capacity, decrease neuroinflammation and alleviate pathology in Huntington’s disease (Figure 3).

Figure 3.

Mechanism of action of Nrf2 activators.

Nuclear factor E2-related factor (Nrf2) is a transcription factor composed of 605 amino acids that controls the expression of as many as 200 genes [55, 56, 57]. The proteins encoded by Nrf2 genes are control several functions such as anti-inflammation, antioxidant defense, apoptosis, detoxification, removal of oxidized protein by proteasome and DNA repair [58, 59, 60]. In physiological conditions, the half-life of Nrf2 is very short (<20 minutes) as it is continuously degraded by Kelch-like ECH-associated protein 1 (Keap 1) [61]. Keap 1 is a regulatory protein that regulates the levels of Nrf2 in the cytoplasm of cell. In basal conditions, the Neh2 domain of Nrf2 binds to the β-barrel structure of Keap-1. This is followed by binding of Cullin-3 to Keap-1-Nrf2 complex, and this results in the formation of ubiquitin 3-ligase complex. The ubiquitin 3-ligase complex bins to many ubiquitin molecules resulting in polyubiquitination of Nrf2, which serves as a signal for proteasomal degradation [62]. Keap-1 contain a lot of cysteines in their structure and the free sulfhydryl (∙SH) of cysteine help keap-1 to act as sensors of oxidative stress. During oxidative stress, electrophiles alkylate keap-1 and prevent keap-1 from degrading Nrf2. This leads to accumulation of recently synthesized Nrf2 that increases the antioxidant potential by promoting the transcription of antioxidant and detoxifying genes. In an alternative pathway, Nrf2 is degraded by phosphorylation by glycogen synthase kinase 3β (GSK3β). This degradation of Nrf2 by GSK3β is also blocked by elevated levels of oxidants that leads to accumulation of freshly synthesized Nrf2 [63]. In another pathway, Keap-1 itself is degraded by p62. In this pathway, p62 is phosphorylated by TANK-binding kinase 1 (TBK1) and mechanistic target of rapamycin complex 1 (mTORC1). The phosphorylated p62 makes a complex with keap-1, and this complex is degraded by autophagy in cells [63]. Activation of all these pathways by oxidants leads to accumulation of newly synthesized Nrf2. Nrf2 escapes breakdown into the nucleus and forms heterodimers with sMaf (Nrf2/sMaf). In the nucleus, the activity of Nrf2 is negatively regulated by BACH-1, which competes with Nrf2 to form heterodimers with sMaf [63]. The binding of Nrf2/sMaf to antioxidant response elements promotes the expression of as many as 200 cytoprotective genes.

9. Nrf2 activators in Huntington’s disease

Minhee Jang et al. have reported that gintonin, a ginseng-derived lysophosphatidic acid receptor ligand, alleviated the severity of neurological impairment and lethality following 3-nitropropionic acid treatment in laboratory animals through activation of Nrf2. The authors of this study conclude that gintonin might be an innovative therapeutic candidate to treat HD-like syndromes because of its potential to activate Nrf2 and decrease oxidative stress and neuroinflammation [64]. A similar study evaluated the effect of Sulforaphane in animal model of 3-NP acid-induced Huntington’s disease. The study revealed that pre-treatment with sulforaphane activated Nrf2 in animals and decreased the formation of a lesion area, neuronal death, succinate dehydrogenase activity, apoptosis, microglial activation and expression of inflammatory mediators, including tumor necrosis factor-alpha, interleukin (IL)-1β, IL-6, inducible nitric oxide synthase and cyclooxygenase-2 in the striatum after 3-NP treatment [65]. Similarly, curcumin is also reported to have beneficial effects in HD via activation of Nrf2 [66]. D. Moretti et al. have reported that compound 2, a covalent KEAP1 binder, demonstrated an ability to stimulate the expression of genes known to be regulated by Nrf2 in neurons and astrocytes separated from wild-type rat, wild-type mouse and zQ175 (an HD mouse model) embryo [67].

10. Challenges facing Nrf2 activators

One of the main challenges associated with Nrf2 activators is achieving effective therapeutic concentrations as these agents are metabolized faster leading to a low bioavailability in distal organs [68, 69, 70]. The second concern with Nrf2 activators is lack of selectivity as these agents have been reported to act on other signaling pathways and affect associated physiological processes. For instance, sulforophane, a widely reported Nrf2 activator, suppresses the activation of inflammosome [71, 72] and causes cell arrest [73]. Nrf2 activators have been reported to promote the development of cancer [74, 75, 76] and development of resistance to anti-cancer drugs [77, 78, 79, 80].

11. Current status of Nrf2 activators

Oxidative stress plays a significant role in pathophysiology of numerous diseases. Initially, exogenous antioxidants were expected to have a protective and therapeutic role in the management of diseases associated with oxidative stress. But randomized clinical trials failed to find any significant therapeutic benefits of exogenous antioxidants. This unexpected outcome led to a search for endogenous targets that would enhance the antioxidant potential of the cells and tissues to prevent oxidative stress-induced damage. This quest for an endogenous antioxidant target led to the discovery of Nrf2 in the year 1994 [81]. Five years later, in 1999, it was discovered that the levels of Nrf2 in the cytoplasm are controlled by a negative regulator, Keap-1 [61]. In recent years, many potential Nrf2 activators are in pre-clinical and different stages of clinical trials for various diseases associated with oxidative stress. Table 2 provides a list of potential Nrf2 activators in clinical trials and possible indications.

Compound	Indications	Clinical trial	Reference
Bordoxolone methyl	Pulmonary arterial hypertension; Alport syndrome; Type I diabetes,; Polycystic kidney disease; Nephropathy; Glomerulosclerosis	II/III	[82]
Omaveloxolone	Friedreich’s ataxia	II	[82]
Bordoxolone methyl	Type II diabetes; Chronic kidney disease		[82]
ALK8700	Multiple Sclerosis	III	[82]
OT551	Dry eye macular degeneration.	II	[82]
CXA10	Primary focal segmental glomerulosclerosis; Pulmonary arterial hypertension	II	[82]
XP23829	Psoriasis	II	[82]
SFX-01	Subarachnoid hemorrhage; ER+ metastatic breast cancer (in combination with tamoxifen and fulvestrant)	II	[82]
Compound A	Chronic obstructive pulmonary disease	Preclinical	[83]
KEAP1 inhibitors	Parkinson’s disease; Amyotrophic lateral sclerosis	Preclinical	[82]
M102	Amyotrophic lateral sclerosis; Neurodegenerative diseases.	Preclinical	[82]
RS9	Retinovascular disease	Preclinical	[84]
TFM735	Progression of experimental; autoimmune encephalitis	Preclinical	[85]
CAT4001	Friedreich ataxia; Amyotrophic lateral sclerosis	Preclinical	[82]
ML334	Type II diabetes; Chronic obstructive pulmonary disease.	Preclinical	[86]
HPP971	Blood, lung, eye, kidney and bone diseases.	Preclinical	[87]
VCB101	Multiple Sclerosis	Preclinical	[82]
VCB102	Psoriasis	Preclinical	[82]
Sulforaphane	Schizophrenia; Atopic asthmatics; Chronic obstructive pulmonary disease; Melanoma; Prostate cancer; Breast cancer; Lung cancer; Diabetes Mellitus; Rhinitis; Helicobacter pylori infection	Different stages of clinical trials	[88]
Curcumin	Acute kidney injury; Schizophrenia; Type 2 diabetes; Chronic kidney disease; Alzheimer’s disease; Crohn’s disease; Prostate cancer; Major depression abdominal aortic aneurysm	—	[88]
ITH12674	Brain ischemia	Preclinical	[88]
Resveratrol	Type 2 diabetes; Colon cancer; Chronic Obstructive Pulmonary Disease Endometriosis; Alzheimer’s disease; Huntington’s disease; Chronic renal insufficiency; Non-ischemic cardiomyopathy; Non-alcoholic fatty liver Friedreich ataxia	Clinical	[88]
CXA-10	Acute kidney injury; Pulmonary arterial hypertension; Primary focal segmental glomerulosclerosis.	I; II	[88]
RTA 408	Topical application	—	[89]
Fimasartan	Unilateral ureteral obstruction	Preclinical	[90]
Artesunate	Sepsis induced lung injury	Preclinical	[91]
Isovitexin	LPS-induced acute lung injury	—	[92]
Sappanone A	LPS-induced mortality	Preclinical	[93]
Bixin	Ventilation injured lung injury	Preclinical	[94, 95]
Eriodictyol	Cisplatin-induced kidney injury	Preclinical	[96]
Anthocyanin	Atherosclerosis	—

Table 2.

List of current Nrf2 activators.

12. Conclusion

As free radicals-induced oxidative stress has been proven to play a major role in the pathogenesis of several diseases, it is quintessential to develop antioxidant therapies to negate oxidative stress-induced damage. The initial expectation that exogenous antioxidants such as vitamin E, vitamin C might have a therapeutic benefit in diseases associated with oxidative stress has failed to find any significant beneficial proof in randomized clinical trials. So, it is time to find agents that activate endogenous antioxidant mechanisms such as Nrf2. Nrf2 activators might offer a beneficial action in diseases associated with oxidative stress such as Huntington’s disease.

Acknowledgments

The authors would like to thank the Department of Science and Technology—Fund for Improvement of Science and Technology Infrastructure in Universities and Higher Educational Institutions (DST-FIST), New Delhi, for their infrastructure support to our department. We acknowledge the generous research infrastructure and support from JSS College of Pharmacy, JSS Academy of Higher Education & Research, Rocklands, Ooty, The Nilgiris, Tamil Nadu, India.