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Keywords:

  • astrocytes;
  • resveratrol;
  • glutamatergic metabolism;
  • neuroprotective;
  • oxidative stress

Abstract

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

Resveratrol, a redox active compound present in grapes and wine, has a wide range of biological effects, including cardioprotective, chemopreventive, and anti-inflammatory activities. The central nervous system is a target of resveratrol, which can pass the blood–brain barrier and induce neuroprotective effects. Astrocytes are one of the most functionally diverse groups of cells in the nervous system, intimately associated with glutamatergic metabolism, transmission, synaptic plasticity, and neuroprotection. In this review, we focus on the resveratrol properties and response to oxidative insult on important astroglial parameters involved in brain plasticity, such as glutamate uptake, glutamine synthetase activity, glutathione content, and secretion of the trophic factor S100B.


Resveratrol: from plants to mammal brain-targeting

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

Plants

Many natural components of diet have been investigated in recent years, in particular the antioxidants that have been shown to cause numerous biological effects in different cell types and tissues.1–3 The redox active compound resveratrol (3,4′,5-trihydroxy-trans-stilbene) is one of the most studied antioxidants; it was first found in roots of white hellebore and later in roots of Polygonum cuspidatum.4 In plants, resveratrol is a phytoalexin found mainly in grapes, grape juice, wine, and berries.5,6 In ancient medicine Hippocrates made observations on the medicinal properties of wine; Galen also reported that preparations of wine and herbs could be used as antidotes to poisons.7 Today, red wine has gained particular attention especially owing to the French paradox that describes, particularly in southern France, an inverse correlation between intake of a diet rich in lipids (and wine) and a low incidence of heart disease.8–10 Among the wines with the highest concentration of resveratrol, wines from the south of Brazil stand out; due to the high humidity of the soil, they naturally have a higher amount of the phytoalexin resveratrol.11

Mammals

Since the first reported detection of resveratrol in grapevines in 1976, a plethora of beneficial effects have been described in mammals, including cardioprotective, chemopreventive, and anti-inflammatory activities.5,6,10,12–15 Many papers locate the most diverse actions of resveratrol in its direct antioxidant and scavenger effects or by its ability to modulate and improve cellular antioxidant defenses.6,13 One of the signaling pathways modulated by resveratrol is that of the sirtuin protein family (SIRT1–7 in mammals). This is a conserved family of NAD+-dependent deacetylases (class III histone deacetylases) that exerts effects related to lifespan extension in diverse species.16 In mammalian cells, resveratrol induces SIRT1-dependent effects that are consistent with improved cellular function and implicated to play a role in a number of age-related human diseases. The effects of resveratrol on sirtuins may explain its positive effect on longevity.13,17

Brain-targeting

As mentioned, the central nervous system (CNS) is a target of resveratrol because this polyphenol can pass the blood–brain barrier.18 Among resveratrol's neuroprotective roles are benefits described in animal models of Alzheimer's and Parkinson's diseases19 and ischemia.20

Resveratrol, oxidative stress, and brain pathology

From epidemiological studies, resveratrol is recognized as a component that offers many health benefits: it may protect cell constituents against oxidative damage and, therefore, limit the risk of diseases such as atherosclerosis and cancer by directly acting on reactive oxygen species (ROS) or by stimulating endogenous defense systems.6,9,21–23 Oxidative stress has strong implications for many human diseases and has been connected with neurodegenerative disorders.24 Brain cells have the capacity to produce peroxides, particularly hydrogen peroxide (H2O2), in large amounts.25 H2O2 concentrations of up to 100 μM have been reported for brain in a microdialysis study.26 In this context, the defense of glial cells against peroxide-mediated oxidative damage would likely be essential for maintaining brain functions.

Astroglial plasticity

The last 25 years have seen an exponential increase in knowledge of the neuroglial plasticity.27 Astroglial cells have been implicated in numerous ways in brain metabolism, especially by the fact as they influence neuronal function, particularly at the level of synapses.27–31 Numerous studies demonstrated that astrocytes play a significant role in neurodegenerative disorders32–34 and exert a fundamental protective function against oxidative stress because of their effects on the metabolism of the antioxidant glutathione (GSH) and the defence against ROS.35

Primary astrocytes and C6 astroglial cell cultures are good models to study glial function, signaling pathways, and mechanisms of peroxide disposal by brain cells.36–41 In such cultures, however, the influence of other types of brain cells on the antioxidant potential is lacking, in contrast to the in vivo situation. Nevertheless, in spite of the fact that comparison of the in vitro results with the in vivo condition is limited, mainly because astrocyte cells in cultures are two-dimensional and the astroglia in situ exist in a three-dimensional matrix, an enormous amount of molecular information has been learned from the study of astroglial cultures (primary and lineage cells), particularly as pertains to the molecular mechanisms that underlie glutamate metabolism, much of which is applicable in vivo.

Glutamate is the major excitatory neurotransmitter in the CNS and plays an important role in neural plasticity and neurotoxicity.42 The modulation of extracellular glutamate determines its physiological and excitotoxic actions. The main mechanism responsible for the maintenance of low-extracellular concentrations of glutamate is performed by a family of glutamate transporter proteins, which use the electrochemical gradients across the plasma membranes as driving forces for uptake.27 In astrocytes, glutamate is converted into glutamine by the enzyme glutamine synthetase (EC 6.3.1.2).43 Glutamine is released by astrocytes and taken up by neurons to be again converted to glutamate; this system is called glutamate–glutamine cycle.44 The interaction between presynaptic and postsynaptic neurons together with astrocytes characterizes the tripartite synapse.45 Glutamate uptake is also important for maintaining levels of GSH, the major antioxidant of the brain. GSH is a tripeptide formed by amino acids cysteine, glutamate, and glycine, where the sulfhydryl group (SH) of cysteine serves as a proton donor and is responsible for the biological antioxidant effect of GSH.44 Moreover, GSH secreted from astrocytes serves as the basis for the synthesis of GSH neuronal.43,46 A large variety of neurological and psychiatric disorders, including depression, anxiety disorders, schizophrenia, chronic pain, epilepsy, and Alzheimer's and Parkinson's diseases, demonstrate pathophysiology impairments in the glutamatergic system.47

Resveratrol and astroglial plasticity

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

Resveratrol modulates glutamate metabolism

As astroglial cells are responsible for the uptake of extracellular glutamate, our group has studied the effect of resveratrol on glutamate metabolism in primary culture, cell lines, and acute hippocampal slices.38,48–51 First, we demonstrated in C6 astroglial cells that resveratrol increased glutamate uptake with doses ranging from 0.1 to 250 μM.48 Afterward, in agreement with these results, we demonstrated that resveratrol increases glutamate uptake in both primary astrocyte culture and hippocampal slices. In all designed studies the increase in glutamate uptake was around 50% compared to the control condition, except for an opposite effect obtained with the highest concentration of resveratrol (250 μM), which decreased glutamate uptake in primary cell culture, indicating a hormetic phenomenon.52,53 The concept of chemical hormesis states that chemicals are able to display opposite effects at low and higher levels.54

Astroglial effects of resveratrol are influenced by redox condition

The influence of redox condition of the milieu on the effect of resveratrol, summarized in Tables 1–3, was undertaken by two models of oxidative insult:38 (I) higher concentration of hydrogen peroxide (1 mM), but short-time of exposure (30 min/acute); and (II) lower concentrations of hydrogen peroxide (0.1 mM) and longest time of incubation (up to 6 h). We observed an interesting dual effect of 100 μM resveratrol mostly protecting cells against H2O2-induced damage in model I and potentiating it in model II, suggesting a pro-oxidant effect. In both models, H2O2 decreased glutamate uptake, and resveratrol completely prevented this effect in model I and strongly potentiated H2O2 insult in model II (Table 1).38 The beneficial effect of resveratrol on glutamate metabolism mediated by modulation of important astroglial cell activities is promising; however, this dual effect of resveratrol observed in vitro needs to be extended to in vivo conditions, under different animal models of stress, to be better clarified.

Table 1.  Effects of resveratrol on glutamate uptake Thumbnail image of
Table 2.  Effects of resveratrol on glutamine synthetase activity Thumbnail image of
Table 3.  Effects of resveratrol on glutathione content Thumbnail image of

As the glutamatergic system is involved in several brain pathologies,55,56 the modulation of glutamate uptake by resveratrol may represent an important pharmacological opportunity. Our studies suggest that resveratrol itself can also be influenced by the surrounding redox environment.

The effect of resveratrol against glutamate excitotoxicity shown in this review may explain the efficacy of resveratrol in protecting brain disorders such as Alzheimer's and Parkinson's diseases, stroke, and ischemia injury. In addition, resveratrol has been able to protect organotipic hippocampal culture against ischemia.57 Thus, resveratrol may represent new therapeutic potential in protecting brain disorders involving glutamate and oxidative stress.

Resveratrol modulates major glutamate destinations in astrocyte

As resveratrol increased glutamate uptake by astrocyte, we have been investigating two major destinations of glutamate in glial cells: (1) the conversion of glutamate to glutamine, by measuring the activity of the astrocyte marker enzyme GS; and (2) the amount of GSH, the main antioxidant defense of the CNS. The glutamate–glutamine cycle is defined as carrying glutamine from astrocytes to neurons and glutamate in the opposite direction.58,59 After uptake by astrocytes, glutamate is converted to glutamine, which in turn is returned to neurons to be reconverted into glutamate.59 GSH synthesis is a mainly astrocytic process, and astroglial GSH exported to the extracellular space is essential for providing neurons with the GSH precursors.60

Resveratrol was able to increase the activity of the enzyme GS in both C6 astroglial cells38,48 and primary culture of astrocytes,49 indicating an important role in glutamate–glutamine cycle (Table 2). Glutamine levels are related with cellular redox variations and have been decreased under catabolic stress.44 ROS appears to be a key pleiotropic modulator that may be involved in different pathways leading to modifications of macromolecules such as proteins and lipids.24,61 The activity of the enzyme GS was impaired by oxidative stress, and resveratrol was able to prevent this effect. As glutamine is an important source of glutamate, it also helps to maintain GSH levels after injuries in CNS.62,63 There are many models of study for understanding the pathophysiology of Alzheimer's disease, and one of them involves the administration of intracerebroventricular streptozotocin.64 In this model there is a decrease in GSH levels, and resveratrol was able to restore the amount of this antioxidant, displaying an important in vivo effect of resveratrol in dementia.

Resveratrol increased intracellular GSH in astroglial cell culture38 and hippocampal slices.49 However, under oxidative insult resveratrol also displays a dual effect that depends on the redox condition of the milieu (Table 3), similar to the effect observed with glutamate uptake. In an intense (1 mM H2O2) and acute (30 min) oxidative insult, resveratrol prevented H2O2-induced GSH decrease, but after 6 h of oxidative insult resveratrol displayed a pro-oxidant effect, potentiating GSH decrease.38

In summary, we have demonstrated that resveratrol may have important role in neuroprotection by increasing glutamate uptake, GS activity, and GSH levels. Neurons are unable to take up extracellular GSH, but they can make use of cysteinyl-glicine and cysteine, two molecules derived from extracellular GSH. Thus, the neurons need GSH from astrocytes to synthesize it, and resveratrol can modulate the glutamate–glutamine cycle through GS activity and GSH levels.

Hypothesis for the resveratrol on the tripartite synapse

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

There has been a lack of studies demonstrating the effect of polyphenolic compounds on neuroglial communication and signaling. Astrocytes are one of the most functionally diverse groups of cells in the nervous system, intimately associated with glutamatergic metabolism and transmission, S100B secretion, and, thus, with synaptic plasticity and neuroprotection.27,28,33,65 S100B is a trophic factor produced and secreted by astrocytes involved in neuronal survival and activity during brain injury and recovery.66–68 Emerging evidence indicates that signaling between perisynaptic astrocytes and neurons at the tripartite synapse plays an important role when neural circuits are formed and refined.45 Given the role of glutamate in CNS injury, it is important to develop strategies to reduce glutamate-mediated excitotoxicity in neurological disorders. Among neural cells, astrocytes are more resistant to oxidative stress and provide a protective role for neurons, mainly due to their higher GSH content. Hence, resveratrol modulation of glutamate metabolism in vitro is an important key to clarifing how effectively this polyphenol acts in vivo. As resveratrol was able to induce in vitro a significant increase in glutamate uptake, GS activity, GSH levels, and S100B secretion, this indicates that astrocytes may be targets of resveratrol in vivo to improve brain pathologies (Fig. 1).

image

Figure 1. Hypothesis for the influence of resveratrol on neuroglial cells. After the release of glutamate at the synaptic cleft 1, resveratrol may improve glutamate uptake by astrocytes 2. This process stimulates the enzyme GS to convert glutamate into glutamine 3, which in turn is able to be released into the extracellular fluid, which is taken up by neurons and reconverted into glutamate 4. Additionally, resveratrol may stimulate another important fate of glutamate in astrocytes, particularly the synthesis of the tripeptide l-γ-glutamyl-l-cysteinyl-glycine or glutathione (GSH) 5, and promote the secretion of the trophic factor S100B 6.

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Conclusions and future directions

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

In spite of the vast progress made in our understanding of resveratrol's effects on the brain, our knowledge in this area is still rudimentary because the majority of the experiments have been performed in cell culture or in brain slices. The number of unanswered questions generated by this scenario highlights the relevance of further studies regarding the effect of resveratrol on neuroglial plasticity. It is important that this knowledge be translated into effective treatments for neural pathologies, including Parkinson's and Alzheimer's diseases. The results found in vitro need to be extended to studies in vivo with different doses and redox conditions to get a clear picture of the effect of resveratrol on human health.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), FINEP/Rede IBN 01.06.0842-00, and INCT-EN National Institute of Science and Technology for Excitotoxicity and Neuroprotection.

References

  1. Top of page
  2. Abstract
  3. Resveratrol: from plants to mammal brain-targeting
  4. Resveratrol and astroglial plasticity
  5. Hypothesis for the resveratrol on the tripartite synapse
  6. Conclusions and future directions
  7. Acknowledgments
  8. Conflicts of interest
  9. References