Alzheimer’s disease (AD) is a neurodegenerative disorder, which is commonly seen in older individuals. This is characterized by cognitive dysfunction, which leads to dementia. Pharmacological treatments for AD are mainly targeted on its symptoms like memory loss and cognitive impairment. The pathophysiology involved in AD is intra-neuronal accumulation of hyper-phosphorylated tau protein as neurofibrillary tangle and extra cellular beta amyloid plaque deposition, which is due to oxidative stress. Here we review the neuro-protective effects of Resveratrol (RSV) and its treatment efficacy in AD. RSV is a naturally available polyphenolic compound, which has antioxidant, anti-cancerous, anti-inflammatory and anti-aging properties. RSV crosses blood brain barrier and exerts its antioxidant effect by enhancing the anti-oxidant enzymes. RSV is involved in Sirtuin (SIRT1) mediated lifespan extension activity. RSV has reduced glial activation and helped in increasing the hippocampal neurogenesis. RSV was able to decrease the expression of amyloid precursor protein, along with improvement of spatial working memory. Since RSV acts as an antioxidant, it can be safely used as oral drug.
AD is a neurodegenerative disorder, which is characterized by cognitive dysfunction leading to dementia. On an average, AD is leading to death, three to ten years after its initial diagnosis (1). According to World Health Organization (WHO), there are about 35.6 million people who are diagnosed with AD and this number may increase to 65.7 million by 2030. This increase in number will affect developing countries like India and will be due to increased aging population. The pathophysiology involved in AD is intra-neuronal accumulation of hyper-phosphorylated tau protein as neurofibrillary tangle and extra cellular beta amyloid plaque deposition, which is due to oxidative stress (2). Normal tau is an unfolded and highly soluble protein, which is accumulated in axons and dendrites of neurons (3). These tau proteins when associated with microtubules, help in maintaining the neuronal structure and neuronal transport (4). Though tau protein helps in stabilizing the microtubules, its phosphorylated form becomes cytotoxic (5). Hence the function of tau protein is dependent on its phosphorylation state. Tau conformation and balance between the phosphate and kinase will determine the binding of phosphate group into the tau protein. Altered tau conformation will lead to hyper-phosphorylation. In addition, the binding capacity of tau proteins into the microtubules is reduced. These are the two phenomenon which play cruciate role in the process of neuro-degeneration (3). The hyper activated phosphatases will lead to paired helices filament and neurofibrillary tangle formation (4). Hyperphosphorylation of tau reduces the stability of microtubules, which leads to disruption of microtubules, aggregation of oligomers and paired helices filaments (6). Several studies propose, oxidative stress as a pathogenic mechanism in AD. In AD, the evidences of oxidative stress in brain are increased iron, alteration in protective enzymes and markers of oxidative damage to proteins and lipids.
Pharmacological treatments for AD are mainly targeted on its symptoms like memory loss and cognitive impairment. There are two therapeutic strategies in the treatment of AD. One is to inhibit the acetylcholine esterase, which helps to keep the acetylcholine in synapse. Another strategy is to use antagonists to the N-methyl-D-aspartate (NMDA) receptors. Commonly used cholinesterase inhibitors are donepezil, rivastigmine, galantamine and tacrine, whereas memantine is non-competitive antagonists for NMDA. In mild to moderate AD, cholinesterase inhibitors are used, and in case of moderate to severe AD, memantine is used. Amongst these drugs, donepezil was first to be approved by US Food and Drug Administration (FDA) (7). Besides there are many investigations are working on the molecules to act on the pathophysiology of AD.
Since AD is a multifactorial syndrome, multi-target drugs would show more therapeutic potency. Zhang et al. (8), in their review, have highlighted on nine major targets associated with AD, which are acetylcholine esterase (AChE), beta-site amyloid precursor protein cleaving enzyme 1 (β-secretase, BACE-1), glycogen synthase kinase 3 beta (GSK-3β), monoamine oxidases (MAOs), metal ions in the brain, NMDA receptor, 5-hydroxytryptamine (5-HT) receptors, the third subtype of histamine receptor (H3 receptor), and phosphodiesterases (PDEs). Sahoo et al. (9) has also opined that one-molecule-one-target strategy is failed to treat the AD. They reported that “combinations-drugs-multiple-targets” (CDMT) should be considered in treatment of AD.
Since these therapies have failed to become a promising remedy for AD (10), there is a significant gap in improving the therapeutic modules. Oxidative stress, being major pathophysiology in AD, antioxidant components have shown their efficacy in various studies and clinical trials (11-13). Polyphenol compounds found in plants which reduce the oxidative stress in them, would provide antioxidant support in human brain also. RSV is a naturally occuring polyphenolic compound found majorly in grapes. RSV has gained attention in pharmacology field due to its antioxidant, anti-inflammatory, anti-cancerous and anti-aging properties (14-15).
RSV is a phytoalexin found in many plants like white hellebore, mulberry, skin of grapes, red wine extracts and peanuts. RSV belongs to a family polyphenolic compounds called stilbenes (16). RSV was isolated for the first time, from the root of white hellebore (Veratum grandiflorum) in 1940 (17). Later RSV was extracted from root of polygonum cupsidatum in 1963. These plants were used in traditional Chinese and Japanese medicine (17). RSV gained its popularity in pharmacology, when an in vivo study showed its anti-carcinogenic activity during 1997. Far ahead, several reports confirmed the advantageous properties of RSV due to its antioxidant, anti-inflammatory, anti- diabetic and anti-obesity effects (17-19). It is also observed that RSV reduces the incidence of cardiovascular diseases (20).
The chemical structure of RSV is: P-hydroxyl group in ring A and conjugated double bond system, which is responsible for its antioxidant property. Chemical structure of RSV is similar to oestrogen diethylstilbestrol (21) (Figure 1). RSV presents as two geometric isomers: cis- RSV and trans- RSV. Trans-RSV is synthesized in plants as the end product of phenylproponoid pathway. When trans-RSV is exposed to ultra-violet rays, it can get isomerized into cis-RSV (22). It is readily soluble in fat and stable in powder form. Other compounds which belong to stilbene group are Combrestatin A-4 and Pterostilbene etc. (23).
Figure 1showing the chemical structure of trans-RSV.
RSV was found in more than 70 plant species (24). RSV was used in traditional Japanese and Chinese medicine especially in cancer treatments. The major dietary source of RSV is red wine. In India also, the Ayurvedic preparation Drakshasava contains RSV (20). Though RSV is present in many plants, red wine is the important dietary source of RSV. Many review articles on RSV mention about the “French Paradox”. In France, the incidence of cardiovascular diseases are less in spite of consuming high fatty diets. This is believed to be due to the consumption of red wine, which contains RSV (17).
Reinisalo et al. (25), have mentioned the sources of stilbene compounds. Their list contains many sources of RSV and some of the plants also contain other stilbenes. The sources of RSV according to Reinisalo et al. (25) are represented in Table 1.
| Plant | Stilbene compounds |
|---|---|
| Cocoa (Theobroma cacao L) | RSV |
| Grape (Vitis vinifera L.) | RSV |
| Hop (Humulus lupulus L.) | RSV |
| Peanut (Arachis hypogaea L.) | RSV |
| Rhubarbs (Rheum L.) | RSV, Piceatannol, Rhapontigenin, |
| Strawberry (Fragaria x ananassa Duch.) | RSV |
| Sugar cane (Saccharum spp.) | RSV, Piceatannol |
| Bilberry (V. myrtillus) | RSV |
| Cranberry (V. macrocarpon) | RSV |
| Tomato (Lycopersicon esculentum Mill.) | RSV |
| Highbush blueberry (V. corymbosum) | RSV, Piceatanno, Pterostilbene |
| Red wine | RSV, Piceatannol |
| White wine | RSV |
Several in-vitro and in-vivo studies have revealed the neuroprotective effect of RSV. RSV crosses blood brain barrier and exerts its antioxidant effect by enhancing the anti-oxidant enzymes (26). RSV was stated to be beneficial against cell death of neurons (Table 2) and cell dysfunction and was found to be beneficial against ailments like AD, Huntington’s disease and epilepsy (27-28). It is well known that RSV is neuroprotective and helps in diseases like diabetes mellitus and cerebral ischemia as well. RSV is also known to reverse the motor as well as cognitive impairment induced by 3-intropropionic acid in Huntington’s model (29) and intra cerebro-ventricular colchicine induced cognitive impairment in Alzheimer’s model (30).
| Action | Mechanism | References |
|---|---|---|
| Inhibition of Aβ plaque | Remodelling soluble oligomers and fibrillary β-amyloids into non-toxic form | Ahmed et al, (22) |
| Reducing the microglial activation |
Supressing pro-inflammatory mediators | Yao et al, (37) |
| Supression of M1 polarization and activation of M2 polarization | Yang, (39) | |
| Prevention of oxidative stress | Activation of SIRT1 | Albani et al, (34) |
| Mitochondrial biogenesis |
Promotion of PGC-1α expression | Chuang et al,(28) |
| AMPK Activation | Dasgupta and Milbrandt (36) | |
| Prevention of cell death | Prevention in increase in ROS | Albani et al, (34) |
| Prevention of apoptotic cell death in hippocampus | Increased mitochondrial UCP2 expression, through activation of PPARϒ | Chuang et al (28), Lin et al, (16) |
RSV shows its neuroprotective activity by activating SIRT1 which is NAD+-dependent class III deacetylases (28, 31-32). SIRT1 up regulation is one of the major features of calorie restriction and it is a well-known fact that calorie restriction will prevent the neuronal degeneration. Howitz et al. (19) explained that RSV can increase the life of Saccharomyces cerevisiae, a yeast. This action of RSV appears to imitate the limiting of calories in diet and activates the sirtuin proteins. RSV is involved in SIRT1 mediated lifespan extension activity (19, 32-33).
Albani et al. (34) found that RSV reduces the oxidative stress (Table 2) and protein aggregation in in-vitro model using neuro-blastoma cell line SK-N–BE. RSV also shown its capacity to increase the expression of peroxisome proliferator-activated receptor gamma co-activator 1 alpha (PGC-1α), nuclear respiratory factor 1 (NRF1) and T fam, COX 1 level, thereby increasing the mitochondrial biogenesis in CA3 region of hippocampus in rat model of status epilepticus (28). In their study they observed changes in the mitochondrial biogenesis machinery, which up regulated the PGC-1α expression. RSV which is a known activator of PGC-1α, along with increased PGC-1α expression promoted the mitochondrial biogenesis. They also observed that RSV has the capacity to decrease the activity of caspase -3 activity thereby decreasing the hippocampal damage. These observations state that RSV can show endogenous protective mechanism against hippocampal damage. Supporting the above study, Wang et al. (35) also confirmed the enhanced expression and activity of SIRT1 in hippocampus in status epilepticus rat model treated with RSV. In addition there was increase in PGC-1α expression, mitochondrial antioxidant enzymes including SOD2 and uncoupling protein 2 (UCP2). They also observed the enhanced mitochondrial electron transport chain complex 1 activity with increased adenosine triphosphate (ATP). These studies conclude that RSV can minimize the oxidative stress by activating the SIRT1 in hippocampus.
Evidence suggesting the effect of RSV to mitigate the brain tissue damage and reestablish normal activity of the mitochondria (Table 2) is partly credited to its effect on SIRT1-dependent deacetylation of PGC-1α, a protein factor necessary for biogenesis of mitochondria and activation of peroxisome proliferator-activated receptor-γ (PPAR-γ) (36). This ability of RSV to intensify the action of SIRT1 and related enzyme activities could result in alteration in transcription profiles of the neurons and improved anti-apoptotic activities (Table 2). Altogether, these studies strongly specify that along with the anti-inflammatory and antioxidant property, RSV shows neuroprotective activity by initiation of SIRT1. By these evidences we can consider RSV as a therapeutic agent for neurodegenerative disorders.
Microglia are the chief immune cells found in the central nervous system. Microglia are known to show their inflammatory reactions by releasing pro-inflammatory mediators like tumour necrosis factor α (TNF-α), interleukin –β, nitric oxide (NO) and reactive oxygen species (ROS) with response to pathogen invasion and cell debris. In neurodegenerative disorders like AD, Parkinson’s disorder and Multiple sclerosis, the activated microglia are accumulated around the lesion. Though the role of activated microglia is to remove the necrotic neurons, over activated microglia may damage the neurons (37- 39). Microglia have two phenotypes, pro-inflammatory M1 and anti-inflammatory M2. RSV has shown its anti-inflammatory potential by supressing M1 polarization and activating M2 polarization both in-vitro and in vivo (39). The previous study by Yao et al. (37), also proved that, RSV has the potential to inhibit the Aβ-induced BV-2 microglial cells (Table 2).
Another study by Kodali et al. (40) showed that RSV has reduced glial activation and helped in increasing the hippocampal neurogenesis along with the prevention of age related memory and mood dysfunction. The study was conducted on 21 month old F344 rats. The immuno-histochemical characterization using OX-42 (CD11b) antibody showed ramified (resting) microglia in hippocampus of RSV treated old aged rat models. However in old aged rats without RSV treatment, there was increased number of activated microglia (41).
Accumulation of amyloid plaques in the brain is one of the characteristic features of neuro degenerative disorders like AD. Accumulation of Aβ in brain leads to release of iNOS, which increases production of NO which causes neurodegeneration. In AD brain, high level of NO is seen. In Aβ induced AD rat model, intra-cerebro-vascular injection of RSV has reduced the accumulation of Aβ (Table 2) in hippocampus (41).
Chen et al. (42) have observed the beneficial effect of RSV in Tg6799 mice, a transgenic model of AD. Their study showed RSV was able to decrease the expression of amyloid precursor protein, along with improvement of spatial working memory. RSV can bind directly to monomeric and fibrillary β- amyloids. The binding capacity of RSV is higher with fibril β- amyloids than that of monomers. RSV selectively remodels the soluble oligomers and fibrillary β-amyloids into non-toxic form. And it is also witnessed that incubation of Aβ 1-40 and 1-42 fragments when incubated with RSV, the length as well as number of these fibrils were reduced (22, 43-44). These observations clearly suggest the usage of RSV in preventing the progression of AD. The binding capacity of RSV may also help in the diagnosis of AD (22, 45).
Though RSV shows good oral absorption, its bio availability is less due to poor water solubility. This poor bioavailability results in shorter biological half-life further leading to rapid metabolism and rapid clearance, thereby limiting the capacity to accumulate in the target tissue (46). The stability and biotransformation may influence the antioxidant property of RSV (47). The oral dose of 25mg of RSV results in less than 5μg/ml in the serum and intravenous dose of 0.2 mg results in 16.4- 30.7 ng/ml in serum (48). Modification in the chemical structure and metabolism can increase the efficacy by changing the cellular and metabolic targets. Hence many experiments have been working on modifying the chemical structure of RSV in order to increase its bioavailability and potency (47).
The molecular structure of trans-RSV is a stilbene core with two phenyl rings which is linked by a double bond. Three hydroxy groups are present in both phenyl rings in position 3, 4 and 5. Trans-RSV can be easily converted into its bio active form, isomer E RSV by isomerization of double bond (49). Many researchers have worked on modifying the chemical structure of RSV to increase its bioavailability. Most commonly used RSV analogue is its methylated derivative, 3,4,5,4׳-tetramethoxy-stilbene, which has shown high accumulation in intestinal mucosa (48, 50). RSV when administered with bio-enhancers like piperine has showed better bio efficacy without showing its increase in bioavailability (51). Recent studies have showed that, nano-formulations of RSV can exhibit improved drug release and stability (52).
In animal models RSV dose ranges from 0.1mg/kg to 1000 mg/kg body weight. Low dose RSV (5mg/kg/day) has caused weight gain in mice, whereas high dose (400mg/kg/day) has resulted in weight loss (17). Johnson et al. (53) reported that, administration of high dose of RSV in rats has shown hepatomegaly, but there was no histological evidences for hepatotoxicity. Chow et al. (54) reported from their human study, the events of adverse effects like diarrhea, heart burn, increased appetite, mood alteration and menstrual changes in females, which are possibly or probably caused by RSV. It has been opined that 1gm/day dose of RSV is well tolerated in human trials (53, 55). Since RSV is an antioxidant, it can be safely used as an oral drug.
Though there are several reports (17, 53-54) available about the pharmacological benefits of RSV, there are no significant evidence of drug interaction with RSV. Sahoo et al,(56) has reported that RSV can reverse the cognitive impairment in prenatal stress induced rats. This suggests that RSV is safe in pregnant rats and it has neuroprotective effects. RSV has been used with curcumine (57) and piperine (53) in order to increase its bioavailability and RSV shown its efficacy without any drug interaction. However adverse effects have been reported with other phenolic compounds (58), hence further studies are required to prove that RSV is safe.
RSV has antioxidant, anti-aging and neuroprotective properties. RSV crosses blood brain barrier and exerts its antioxidant effect by enhancing the anti-oxidant enzymes. RSV has reduced glial activation and helped in increasing the hippocampal neurogenesis. It prevents age related mood dysfunction and memory loss. Though the bioavailability of RSV is less, it can be improved by modifying its chemical structure. The combinations-drugs-multiple-targets should be considered in treatment of AD. RSV if given along with other acetyl choline esterase inhibitors, can give better results in AD. There are no significant evidences to show the adverse effects of RSV. However adverse effects have been reported with other phenolic compounds, hence further studies are required to prove that RSV is safe.
AD
Alzheimer’s disease
Resveratrol
Sirtuin
N-methyl-D-aspartate
acetylcholine esterase
nicotinamide adenine dinucleotide
peroxisome proliferator-activated receptor gamma co-activator 1 alpha
cyclooxygenase 1
cornu ammonis 3
nitric oxide
reactive oxygen species
nitric oxide synthase