Resveratrol in Alzheimer’s disease: a review of pathophysiology and therapeutic potential

Background : Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by progressive and irreversible loss of cognitive function. The presence of senile plaques is one of the pathological markers of the disease and is associated with the onset of neuroinflammatory mechanisms. The exact pathophysiology of AD has not been completely understood, and there are no curative therapies yet. Resveratrol (3,5,4’-trihydroxy-trans-stilbene) is a polyphenol that is noted for its antioxidant and anti-inflammatory properties. Objective : To review the role of resveratrol in the pathophysiological aspects of AD. Methods : This study carried out a literature review using PubMed/Medline, Virtual Health Library (VHL), Web of Sciences, SCOPUS and the Cochrane Library databases. Original research articles, describing both in vitro and in vivo experiments, published between 2008 and 2018, were included. Results : We identified 667 articles, of which 619 were excluded because they were repeated or did not follow the inclusion criteria. The present study includes the remaining 48 articles. Discussion : Resveratrol demonstrates beneficial and protective effects in AD models and seems to provide a promising therapeutic alternative. Conclusion : Although resveratrol appears to mitigate some pathophysiological aspects of AD, further studies are needed to prove the safety and efficacy of this compound in humans.

Although there is currently no cure for AD 7 , adopting a healthier lifestyle has been associated with a reduction of cognitive impairment and of dementia risk. Strategies include caloric restriction, physical exercise, and ingestion of antioxidant foods, which are associated with neuroprotective mechanisms 1 .
Resveratrol (3,5,4'-trihydroxy-trans-stilbene) is a polyphenol produced by several plants, and the main food sources are grapes and red wine 8 . This compound has gained notoriety in scientific circles, due to its biological and pharmacological properties 9 . It seems to exert beneficial effects in vitro and in vivo, although the precise mechanisms are still poorly understood 10 .
Turner et al. observed that resveratrol supplementation in patients with mild to moderate dementia was safe and well tolerated. Despite the fast metabolism, resveratrol and its main metabolites were identified in the plasma and cerebrospinal fluid (CSF) of the individuals, demonstrating their ability to cross the blood-brain barrier (BBB) and stabilize Aβ levels 11 .
Given the impact of AD on the quality and life expectancy of the affected individuals and the lack of information regarding effective therapeutic alternatives, the present study aims to review the literature regarding the role of resveratrol in pathophysiological aspects of AD, especially the aggregation of Aβ peptides and consequent triggering of neuroinflammatory processes.

METHODS
A literature review of published articles was carried out between February and November of 2018 using the search terms "Alzheimer' s disease", "resveratrol", and "amyloid" on the PubMed/ Medline, Virtual Health Library (VHL), Web of Science, SCOPUS, and Cochrane Library databases. Original experimental articles that addressed the performance of resveratrol on the pathophysiological factors of AD, especially neuroinflammation and Aβ peptide aggregation, both in vitro and in vivo, published in English, were selected. We excluded articles that were literature reviews or that did not particularly focus on the study objective.

RESULTS
We identified 667 articles, of which 619 were excluded because they did not fit the inclusion criteria. The remaining 48 articles have been included in this study. Figure 1 outlines our study selection process in a flowchart.

Alzheimer's disease: clinical and pathophysiological aspects
The pathophysiology of AD has not been completely understood yet. However, histopathological characteristics are the focus of attention, especially the deposition of Aβ, which appears to trigger the inflammation and oxidative stress that leads to neurodegeneration 3 . As a result, most of the AD studies in recent years have been conducted based on the amyloid cascade hypothesis, which advocates that Aβ deposition is the primary event in disease pathogenesis 12,13,14 .
β-amyloid proteins are peptides consisting of 36 to 43 amino acids that have been derived from the sequential cleavage of the amyloid precursor protein (APP) 15 . β-secretase/BACE1 aspartic protease and γ-secretase proteolytic complex cleave APP to produce Aβ peptides 16,17 . An imbalance between the production and clearance of these peptides enhances their aggregation and triggers the formation of senile plaques 6 . However, the reason for excessive Aβ in the brain is not known 18 .
The deposition of Aβ peptides in brain regions related to memory and learning, such as the frontal cortex and hippocampus, is considered an early event in disease progression 19 . Even though there are several products of APP cleavage, the aggregation of peptides of 40-42 amino acids in length are important in the AD development 20 , especially those terminated at amino acid 42 (Aβ42) 12 .
Neuroinflammation is another important characteristic feature in AD, which is manifested through the microglia proliferation and activation 21 . Microglia are the resident macrophages of the brain and form dense clusters around β-amyloid plaques. These cells function as tracers for damage to the central nervous system and detect injuries to the cerebral parenchyma 20 .   Upon detection of damage or immune stimulus in the brain, such as that caused by Aβ peptides, microglia are activated and produce proinflammatory mediators, for instance, tumor necrosis factor (TNF), interleukin-1β (IL-1β) and nitric oxide (NO) in response to it. The accumulation of these mediators contributes to neuronal damage and disease progression 8 . In addition, the NF-κB transcription factor is considered the primary regulator of the inflammatory response in AD 20 .
Free radicals also seem to play a key role in brain aging 15 . Several studies have found an association between Aβ-promoted cytotoxicity and oxidative stress as a result from an imbalance between the production and removal of reactive oxygen species (ROS) 17 .
High levels of ROS may lead to the destruction of proteins, nucleic acids and lipid peroxidation, as well as impairments in the cell membrane integrity, reduction of mitochondrial membrane potential, and an increase in plasma membrane permeability to calcium ions. This increase in reactive species, due to the reduction of antioxidant defenses, is responsible for the neurodegeneration mediated by oxidative stress 15 .
Currently, the drugs available for treating AD are not completely effective and do not alter the disease progression 13,22,23 . It is, therefore, critical that new therapies aimed to prevent and manage this chronic and debilitating disease be developed 24 .

Resveratrol
Several foods and natural products, whose chemical components demonstrate interesting pharmacological properties, have been suggested as AD modifying agents 10,17 .
Polyphenols are natural compounds derived from plants, fruits and vegetables, whose biological properties have demonstrated several beneficial effects, such as antioxidant and anti-inflammatory activities 23 .
Resveratrol is a non-flavonoid polyphenol 8 part of the stilbene family 23 , which can be found in grape skin and seeds, blackberries, peanuts, and vegetables 25 . Red wine is therefore also a good source of resveratrol, to which the potential beneficial health effects of this beverage has been attributed 26 .
The biological activity inherent to resveratrol has been discussed in literature for some time, such as its antioxidant, anti-inflammatory, anticarcinogenic and anti-aging effects in different organisms. More recently, the neuroprotective potential has also started to gain interest 2 .

Disaggregation of Aβ-peptides
Since β-amyloid aggregates have been strongly associated with AD pathogenesis, compounds that promote the inhibition of formation and destruction of preformed aggregates and attenuate Aβ-promoted cytotoxicity may provide an important therapeutic strategy for AD, as demonstrated by Feng et al. Although it does not prevent oligomerization of Aβ42, resveratrol seems to stabilize preformed oligomers and to attenuate oligomer toxicity 26 . This outcome is similar to the findings of Ladiwala et al. and Rushworth et al., in which resveratrol was also able to selectively remodel Aβ oligomers to non-toxic structures 27,28 .
Aromatic interactions between compounds and Aβ peptides are a determining factor in the inhibition of fibrillar formation and reduction of amyloid toxicity 26 . As verified also by Ge et al., resveratrol was able to bind directly to the amyloid structure 12 . Fu et al. have also noted that resveratrol binds to the N-terminus (residue 5-20) of the Aβ42 monomer and caps the height of oligomers, resulting in a reduction in toxicity 29 . Other studies have also shown a reduction in amyloid aggregation, both at a concentration of 50 30 and 100 μM 31 .
In CHO-APPswe cells, administration of resveratrol (100 μM) reduced amyloid burden, APP expression, and cleavage 32 . Karuppagounder et al. reported that 300 mg/kg/day of resveratrol supplementation also decreased the amount and amyloid burden in brain regions of mice, but this difference was not statistically significant 33 .
Similarly, Varamini et al. found that resveratrol supplementation of 174 mg/kg/day in AβPP/PS1 mice showed no differences in amyloid burden or levels of tau phosphorylation. However, resveratrol was able to increase the levels of proteins involved in neuroprotection processes 34 . In AD/ TTR+/-mice, the same amount of resveratrol supplementation increased transthyretin levels (TTR) and stabilized TTR tetramers, which bind more strongly to Aβ peptides, preventing their aggregation 35 .
In addition, in AβPP/PS1 mice, the supplementation of lower doses (100 ppm by weight) of resveratrol, or its synthetic analog LD55, significantly decreased the density of Aβ plaques in different brain regions, including the cortex and hippocampus 20 . The effects of resveratrol on Aβ-peptide aggregation are summarized in Table 1.

Neuroinflammation
Activated microglia seem to be an important target of the neuroprotective activity of resveratrol, resulting in the reduction of pro-inflammatory factors 36 . Regarding the findings of Solberg et al., microglial activation was physically associated with amyloid plaques. In addition, resveratrol and its analogue reduced microglia activation, both of which are inhibitors of the NF-κB pathway 20 .
It is noteworthy that the hippocampus, which is the brain region mostly affected by neuroinflammation, presented a greater reduction in the density of activated microglia in response to the compounds. The LD55 analogue has no hydroxyl group, which is present in the resveratrol structure; therefore, it does not have antioxidant properties. However, the compound showed the same efficacy and anti-inflammatory potential of resveratrol, suggesting that the effect is independent of antioxidant function and a result of its ability to inhibit the NF-κB signaling pathway in activated microglia 20 .
In rat astrocytes (RA) and microglia N9 cell lines, resveratrol administration also promoted anti-inflammatory effects by inhibiting NF-κB signaling and release of inflammatory cytokines at different concentrations (5, 12.5, and 25 μM in RA cell line and 10, 20, and 40 μM in N9 cell line). In this experiment, cytotoxicity was found when the concentration of resveratrol was higher than 50 μM in the RA cell line and higher than 80 μM in the N9 microglia cell line 37 .
In another experiment, administration of resveratrol in BV-2 and RAW 264.7 cell lines and supplementation of 350 mg/kg/day in APP/PS1 mice also inhibited Aβ-mediated microglia activation and lipopolysaccharide (LPS)stimulated activation of NF-κB, reducing the expression of its target genes, such as TNF and IL-6 38 . In BV-2 cells activated by β-amyloid oligomers, resveratrol (1, 3, 10 or 30 µM) reduced the production of reactive species and inhibited the production of inflammatory mediators, among them NO in a dose-dependent manner 8 . Overproduction of NO has been associated with AD, resulting in damage to DNA and the mitochondrial structure 39 .
In HEK-AbPP cells treated with a γ-secretase inhibitor (LY450139), additional treatment with resveratrol (0.5 µM) ceased most of the neurotoxicity and pro-apoptotic effects, promoting maximum neuroprotection. The authors have also discussed the importance of associating different neuroprotective agents, such as antioxidants (resveratrol) with AD-specific drugs 40 .
Neuroprotective effects were also observed with the injection of resveratrol in rats (100 µM/5 µL), which reduced amyloid accumulation, protected animals against neuronal death, increased antioxidant enzyme heme oxygenase-1 (HO-1) expression, and suppressed lipid peroxidation in the hippocampus. In addition, resveratrol improved spatial memory in the animals, which had been impaired by Aβ 39 . Resveratrol intraperitoneal injection (10, 20 and 40 mg/kg/ day) in Sprague-Dawley rats also improved cognitive impairment and attenuated LPS-induced neuroinflammation in rats, by inhibiting the generation of TNF, APP, cyclooxygenase (COX)-2 and NF-κB phosphorylation in the hippocampus 41 .
In mouse neuroblastoma (N2a) cells incubated with Aβ, pretreatment with resveratrol (5 µM) prevented the abnormal expression of peroxiredoxins and mitochondrial structural genes and preserved mitochondrial function, protecting cells against Aβ toxicity 42 . Also in N2a cells, resveratrol (0, 2.5, 5, 10, 25 and 50 µM) significantly reduced formaldehyde-induced cytotoxicity and cellular apoptosis, and inhibited tau protein hyperphosphorylation in a dose-dependent manner 24 . Similar results were found in another experiment, in which resveratrol (0, 10, 20 and 40 µM) dose-dependently increased the viability of Aβ-treated PC12 cells and stimulated HO-1 production. The cytoprotective mechanism was mediated by the PI3K/Akt/Nrf2 pathway 43 , which has also been found responsible for the neuroprotective effect of resveratrol, resulting in a decrease of ROS 17 .
Several studies have shown that the activation of AMPK (adenosine monophosphate-activated protein kinase) suppresses inflammation by inhibiting NF-κB, preventing oxidative stress. Resveratrol is a potent activator of AMPK, thereby implicating another pathway through that its neuroprotective effects may be exerted 3 .
An experiment in human neural stem cells (hNSCs) demonstrated that Aβ-treated cells increase the expression of TNF and IL-1β, thereby decreasing cell viability. In addition to inhibiting such deleterious effects, resveratrol (10 µM) prevented the increase of NF-κB and normalized oxidative stress. These effects were attributed to the AMPK pathway 3 .
The antioxidant resveratrol effects were also verified by Rege et al. in an H19-7 neuronal cell line derived from rat hippocampus. Resveratrol (10 µM) attenuated lipid peroxidation and upregulated antioxidant enzyme levels, such as catalase, superoxide dismutase (SOD), and glutathione reductase (GR). An interesting finding of this experiment was the increased levels of non-enzymatic antioxidants attributed to resveratrol, such as ascorbic acid, α-tocopherol, and glutathione 15 .
The same study showed that resveratrol decreased the expression of insulin-degrading enzyme (IDE). Insulin regulates neuronal function after crossing the BBB, facilitating glucose uptake by neurons. Insulin resistance, which is a characteristic of type 2 diabetes mellitus, is an important risk factor for AD 15 .
Another association between glucose metabolism and AD is the presence of advanced glycation end products (AGEs) from the Maillard reaction between carbohydrates and proteins. Advanced glycation end products and their receptors (RAGEs) have been identified in neurons and hippocampus and have been associated with oxidative stress-induced neurotoxicity, as demonstrated by Ko et al., in which the administration of resveratrol (10 and 20 µM) reduced ROS production in cells treated with AGEs 44 .
In addition, RAGEs located at the BBB are the main gateway for Aβ peptide transport to the brain. In female Wistar rats, resveratrol (20, 40 and 80 mg/kg/day for 12 weeks) protects BBB integrity by reducing RAGE expression in the hippocampus and inhibiting the expression of matrix metalloproteinase-9 (MMP-9), which is responsible for the degradation of junction proteins. It also promotes the expression of Claudine protein-5, which is related to the tight junctions that regulate BBB permeability 45 .
Plasma and CSF analyses in patients with confirmed AD showed that previous treatment with resveratrol (initial dose was 500 mg/day and was increased every 13 weeks until a final dose of 1,000 mg twice a day) attenuated neuroinflammation, reduced proinflammatory markers, and decreased MMP-9 in the CSF. Decreased levels of MMP-9 suggest that treatment with resveratrol may reduce CNS permeability and limit the infiltration of leukocytes and other inflammatory agents into the brain, thus preserving the integrity of BBB. Resveratrol also attenuated the patients' cognitive and functional decline 46 .

The Sirtuin 1 (SIRT1) pathway
One of the possible mechanisms by which resveratrol mediates neuroprotection is through the activation of the sirtuin 1 (SIRT1) pathway, which in turn inhibits the activation of the NF-κB signaling pathway. Through suppression of this pathway, SIRT1 is also able to protect neurons against Aβ toxicity 36 .
SIRT1 is one of a class of NAD+-dependent histone deacetylases that play an essential role in the cellular functioning regulation 2,47 , by the deacetylation of substrates important in neurodegenerative diseases 48 . Table 2 shows the resveratrol effects on the neuroinflammation and activation of SIRT1.
Resveratrol (0, 2.5, 5, 7.5, 10 and 15 µM) protected SK-N-BE neuroblastoma cells against induced oxidative stress and increased cell viability. However, when SIRT1 was up regulated with administration of sirtinol, antioxidant activity of resveratrol was suppressed 47 . In PC12 cells, resveratrol (12.5, 25, 50 and 100 µM) increased cell viability, reduced apoptosis, and attenuated Aβ-induced neurotoxicity. The peptide-induced suppression of SIRT1 activity was reversed by resveratrol, indicating that the protective effects are mediated by the sirtuin pathway 2 .
In addition to SIRT1, sirtuin 2 (SIRT2) was also observed to participate in the regulation of neuronal survival, albeit through more diverse mechanisms. Activation of SIRT1 and inhibition of SIRT2 (through the administration of resveratrol and AGK-2, respectively) reduced the activation of RA and consequent production of proinflammatory mediators, emphasizing the role of sirtuins and their modulatory substances in strategies for AD treatment 49 .

Effects on cognitive aspects
It has been shown that SIRT1 is essential for synaptic plasticity and cognitive functioning and can modulate learning and memory by regulating cAMP response element   binding protein (CREB) protein expression. It is noteworthy that levels of SIRT1 and CREB protein are significantly reduced in AD brains, and this reduction is strongly associated with Aβ deposition in the cerebral cortex of these individuals. In light of this, the injection of resveratrol (0.5, 1.25, 5, 22, and 44 μM) in Sprague-Dawley rats prevents memory damage and Aβ-induced learning and restores SIRT1 levels and CREB phosphorylation in the hippocampus of these animals 50 . Similarly, in another experiment, CREB levels were decreased by Aβ42 peptides, and the oral administration of 20 and 40 mg/kg/day of resveratrol for three days of resveratrol reversed the condition 19 . In an animal model of AD, 100 mg/kg supplementation of resveratrol for 10 months protected against memory impairment, improved exploratory behavior, and reduced anxiety. Resveratrol also increased AMPK levels by stimulating SIRT1 and, consequently, CREB protein. The hippocampus is one of the areas selectively affected in AD and the deterioration of hippocampal circuits contributes significantly to some effects of the disease, such as memory loss. The positive action of resveratrol on spatial learning and memory of the mice was associated with an improvement in the functioning of hippocampal circuits 18 .
Also in an animal model of AD, Porquet et al. observed that 1 g/kg/day supplementation of resveratrol increased life expectancy, reduced cognitive impairment by preserving memory, and reduced amyloid deposition in the hippocampus, and tau protein levels in the cortex and hippocampus. In addition, resveratrol activated the AMPK and SIRT1 pathways 48 . Similar results were found in another study, in which the 16 mg/kg/day supplementation of resveratrol prevented loss of short-term memory and decreased the amount and load of amyloid plaques in the hippocampus and cortex of the animals 51 . In addition, Wang et al. found that administration of 20 and 40 mg/kg/day of resveratrol in mice was also effective in reducing Aβ-induced memory and learning impairment and decreasing the expression of proinflammatory cytokines (IL-1β and IL-6) 19 . Also, the resveratrol injection (4 mg/kg) in mice reduced the decline of different memory types, such as working, nonspatial and locomotor functions, induced by LPS, and increased both neprilysin (NEP) and estradiol levels. In turn, estradiol also increased the NEP levels, which are responsible for decreasing Aβ deposition 52 . The effects of resveratrol on cognitive aspects are summarized in Table 3.

Autophagic mechanisms
Among the mechanisms of neuronal protection, autophagy has been highlighted as a catabolic process related to the degradation and recycling of macromolecules and organelles 10 . There is strong evidence that, in the brain of AD patients, autophagic mechanisms are dysregulated and they participate in the intracellular degradation of Aβ in both in vitro and in vivo models 16 . Mechanisms responsible for the clearance of Aβ and tau proteins include the ubiquitin-proteasome system (UPS), the lysosomal autophagic system, and the actions of extracellular proteases 18 .
In order to determine the autophagy role in the anti-neurotoxic effect of resveratrol, PC12 cells were exposed to Aβ  and cell viability was reduced in a dose-dependent manner. Resveratrol (20 μM) attenuated this effect and was also responsible for regulating the expression of LC3-II and p62 proteins, which are autophagy markers. The resveratrol induced autophagic mechanism and was dependent on SIRT1 10 . Mitophagy is a specific autophagy form, which plays an important role in the mitochondria control. The selective removal of dysfunctional mitochondria by mitophagy is an effective way of limiting neuronal oxidative damage. In PC12 cells, resveratrol (1, 3, 10 and 30 μM) promoted mitophagy in Aβ-treated cells, in addition to reducing the oxidative state and attenuating peptide-induced apoptosis, indicating that resveratrol-induced mitophagy played a protective role against oxidative damage by removing dysfunctional mitochondria 53 .
The mammalian rapamycin target (mTOR) is a potent inhibitor of autophagy and is negatively regulated by AMPK, which in turn controls important mechanisms for protein degradation. Vingtdeux et al. showed that the anti-amyloidogenic resveratrol mechanism involves the activation of AMPK in different cell lines and in primary mouse neurons, a process that resulted in the inhibition of mTOR, initiation of autophagy, and lysosomal clearance of Aβ 16 .
UPS is the primary mechanism that maintains the balance between synthesis and protein degradation and is related to several neuronal functions, such as memory and plasticity. Functional changes in this system have been associated with early changes in AD. In mice, 100 mg/kg/day supplementation of resveratrol for 10 months decreased Aβ and tau levels, by reducing BACE1 enzyme and increasing neprilysin (Aβ-degrading enzyme), in addition to normalizing ubiquitin levels 18 .

Metal ions and ion channels
Another risk factor in AD onset and development in the elderly is the accumulation of metals. The metal ion homeostasis imbalance in the brain may exacerbate the oxidative properties of Aβ peptides and their toxicity through the production of ROS 6 . Recent evidence indicates that high concentrations of metal ions such as copper, zinc and iron can bind to Aβ peptides, which promotes not only amyloid aggregation, but also accelerates ROS formation and cerebral oxidative stress 54 .
A study conducted by Granzotto and Zatta in human neuroblastoma cells exposed to Aβ, Aβ-metal complexes, or metal ions concluded that treatment with resveratrol resulted in a neuroprotective effect, reduced the toxicity induced by Aβ-Iron and Aβ-Zinc complexes, and regulated levels of SOD antioxidant enzyme. Resveratrol concentration required to inhibit 50% (IC50) of cell viability was found to be 100 μM. A concentration of 15 μM was shown to be largely non-toxic 6 .
It was also shown that the association of Aβ with aluminum had the most potent effect on reducing cell viability 6 . Aluminum acts as a neurotoxin capable of enhancing neuroinflammatory processes and, as studies have demonstrated, is associated with exacerbation of oxidative stress, amyloid deposition, and plaque formation in the brain. Both Aβ peptides and aluminum are capable of potentiating the formation of ROS, leading to DNA damage 36 .
Zaky et al. demonstrated that oral administration of 0.5 mg/kg of resveratrol in Wistar rats for one month attenuated aluminum-induced neuroinflammation, inhibiting TNF, IL-6 and iNOS release in the animals' brains 36 .
Ion channels have become drug targets for neurodegenerative diseases treatment and voltage-gated potassium channels (VGPC), present in the hippocampus, and play a crucial role in neuronal activity. β-amyloid peptides can cause excitotoxicity in pyramidal neurons, as demonstrated by Yin et al., in which Aβ treatment increased the excitability of these cells in the CA1 region of the rat hippocampus. Treatment with resveratrol (10 μM) attenuated this effect by recovering the activity of two important VGPCs 55 .

Perspectives
The results from both in vitro and in vivo studies indicate that resveratrol is a promising and safe compound for use in the AD treatment 26 . However, as discussed by Moussa et al., one of the major impediments to current AD therapeutic approaches is the limited evidence demonstrating significant clinical benefits 46 . There is a lack of phase 3 clinical trials that test the clinical benefits of resveratrol in AD as a primary outcome.
Resveratrol seems to exert beneficial effects on AD due to its diverse pharmacological properties. However, applicability for disease treatment is limited by factors such as low solubility, photosensitivity, short half-life and rapid metabolism and excretion, contributing to low bioavailability 22 . In addition, resveratrol is poorly hydrosoluble and chemically unstable and is degraded when exposed to high temperature, pH changes, ultraviolet radiation, and some enzymes 25 .
The BBB represents a significant obstacle to the entry of drugs into the CNS, restricting the pharmacological options for neurodegenerative diseases 22 . However, studies show that resveratrol is able to cross the BBB 45 as discussed by Vingtdeux et al. and Capiralla et al., in which orally administered resveratrol in rats was able to reach the brain and reduce Aβ levels and amyloid deposition in the cerebral cortex. This shows that resveratrol is not only bioavailable, but also bioactive 16,38 .
The complexity of AD and lack of scientific understanding regarding the onset and progression make the search for therapeutic strategies difficult 13 . Treatment with only either antioxidants or anti-inflammatory drugs has been shown ineffective in preventing AD. Therefore, the development of multi-target drugs is an alternative approach 23 .
Authors of the studies reviewed herein seemingly agree that further research is needed to clarify the mechanisms by which resveratrol affects AD pathophysiology, and to demonstrate efficacy and safety in humans. Therefore, phase 3 clinical trials are required to test the clinical benefits of resveratrol. However, these studies provide an excellent foundation for future exploration.
In addition to resveratrol, other polyphenols have gained attention for playing important roles in the pathophysiological mechanisms of AD. Curcumin, isolated from the rhizome of Curcuma longa (turmeric), improved mitochondrial activity and cell viability in cells incubated with Aβ (66.3 mM final concentration) 56 , reduced Caveolin-1 (protein that participates in the cleavage of APP and the generation of Aβ) levels, potentially inactivating GSK-3β and inhibiting Tau hyperphosphorylation (5 μM in N2a/APP695swe cells and 1.0 g/kg in APP/PS1 mice) 57 . In mice, curcumin suppressed glia activation and neuroinflammation, thereby improving induced tau/amyloid pathology and cognitive impairment (4 g/kg) 58 . Epigallocatechin gallate (EGCG) polyphenol found in green tea reduced in vitro amyloid accumulation and improved cognitive decline in SAMP8 mice (intragastric administration of 5 and 15 mg/kg) 59 , and suppressed the transcription and translation of BACE1. This attenuated Aβ formation, which reduced pro-apoptotic protein expression, NF-κB activity and inhibited oxidative stress (5-100 μM) 60 . EGCG also suppressed TNF, IL-6, IL-1β and iNOS expression, restored intracellular antioxidant levels, inhibited NF-κB activation and cytotoxicity (5 to 20 μM) 61 . Prolonged consumption of EGCG at relatively high doses (15 mg/kg) by SAMP8 mice improved animals' memory, reduced their Aβ and BACE1 levels, prevented hyperphosphorylation of tau, and reversed the decreased synaptic protein marker 62 . Cocoa polyphenols have also been studied due to their possible neuroprotective effects. Cocoa extract was effective in reducing the oligomerization of Aβ and protected against Aβ-induced long-term potentiation deficit 63 . Cocoa polyphenols resulted in antioxidant effect and neuroprotection by brain-derived neurotrophic factor (BDNF) activation 64 .
The potential role of other polyphenols in the pathophysiological aspects of AD is remarkable. Chan et al., who determined the relative potencies of nine food constituents in relation to AD including curcumin and EGCG, observed resveratrol to be a weak chelating agent, with very high concentrations required for 50% metal chelation. Resveratrol was also the least antioxidant compound and 100 μM was required to inhibit 27% of the Aβ fibrillar formation 14 . These findings further highlight the need of studies combining polyphenols as a multi-target strategy for the prevention or treatment of AD.

CONCLUSIONS
Much effort has been employed in investigating the potential contribution of resveratrol to the treatment and attenuation of AD. Studies with varying scope and outcomes seemingly converge on the conclusion that this polyphenol provides a promising alternative therapeutic, capable of acting on various aspects of disease pathophysiology.
However, it is noteworthy that most of this research was conducted in cell culture or animal models, and there is a lack of human clinical studies that demonstrate the safety and efficacy of resveratrol for the treatment of AD. The exact pathophysiology of AD is poorly understood, and there are no models that accurately portray the multifactoriality of the disease; these are obstacles that present important challenges for successful outcomes of research in this area.
Although the antioxidant and anti-inflammatory function of resveratrol has been widely described in the literature, its metabolism and structural characteristics present challenges for application in therapeutic settings. The action mechanisms have not been completely understood, and the optimal dose and route of administration in AD patients are unclear. In addition, there is no consensus on the therapeutic impact of resveratrol obtained through the diet, combined with other nutrients in the food matrix, compared to its action alone, either in the form of extracts or synthetic analogues.
In view of these findings, we conclude that, although resveratrol appears to mitigate some pathophysiological aspects of AD, further studies are needed to prove the safety and efficacy of this compound in humans.