Accumulating evidences from experimental and human studies suggest that oxidative stress and mitochondrial dysfunction are important causative factors in the development and progression of several neurodegenerative diseases including AD (Gibson et al., 2000
; Ojaimi et al., 1999
; Balaban et al., 2005
. Induction of mild impairment of oxidative metabolism, oxidative stress and inflammation induced by thiamine deficiency alters BACE1 levels and metabolism of APP and/or Aβ and promotes accumulation of plaques in Tg 19959 mice (Karuppagounder et al., 2008
). If oxidative stress is critical in the alteration of APP processing and the pathogenesis of AD, treatment with the appropriate antioxidants should be beneficial and diminish plaque formation.
The dosage of resveratrol that was used in the current studies (0.2% in diet) is a practical amount and was administered in a clinically relevant manner. Dietary resveratrol at the level administered in these studies had a striking effect on brain even though resveratrol was not measurable. Although resveratrol and its derivatives have been detected following gastric gavage, or intraperitoneal administration (Wang et al., 2002
), they have not been detected following dietary administration (current results) (Asensi et al., 2002
; Sale et al., 2005
). This is true even though the daily dosage was higher in our experiments (300 mg/kg) than bolus injections in other experiments. This same dosage protected the vasculature of mice fed a high calorie diet (Baur et al., 2006
). Thus, the data suggests that the striking biological effects of resveratrol in the diet result from trace levels entering the brain, an effect on brain vasculature or a peripheral change that affects brain.
The data demonstrate that resveratrol reduced plaque counts and plaque burden most effectively in medial cortex, striatum and hypothalamus. The causative factors or thresholds may differ in various brain regions. For example, thiamine deficiency (i.e., mild impairment of oxidative metabolism) exacerbates plaque formation in regions that will eventually get plaques and induces plaque formation in brain regions that do not normally contain plaques (Karuppagounder et al., 2008
). The current results demonstrate that resveratrol can reduce plaques, but only in select regions. Thus, a more effective paradigm is required to reduce plaque burden throughout the brain. The relation of these restricted changes in plaque formation to learning and memory is critical. However, additional studies are required to optimize the response of the resveratrol or its derivatives before examining the interaction of learning and memory in multiple mouse models of plaque formation.
The exact mechanism(s) underlying reduction of plaque pathology by resveratrol is (are) unknown. The reduction in plaque pathology did not appear to be due to altered APP processing towards the non-amyloidgenic pathway. Resveratrol did not alter levels of α CTF or β CTF (C99) or high molecular weight APP species (holo APP). In vitro,
resveratrol promotes Aβ clearance by increasing the intracellular proteosomal activity. To demonstrate the role of proteasomes in vitro
, Marambaud et al., used a variety of selective proteasomal inhibitors and siRNA. These approaches are far more equivocal for in vivo
studies. The effective concentrations about 40 μM (Marambaud et al., 2005
) exceed those in brain in the current study (<0.5 nM) by orders of magnitude. Resveratrol has been postulated to have its beneficial effects on life span, neurodegeneration and prevents impaired memory by activation of SIRT1 (Kim et al., 2007a
; Lagouge et al., 2006
). However, we did not see activation of SIRT-1. Other reports suggest that resveratrol has other SIRT 1 independent actions (Dasgupta and Milbrandt, 2007
In the current study, the surprising resveratrol-induced decline in glutathione and increase in cysteine is consistent with a modest oxidant effect in brain. The decline in GSH could reflect a decreased synthesis, increased degradation or altered redox status. GSH consists of glycine, cysteine, and glutamate. Thus, GSH serves as a storage form of cysteine. Addition of cysteinyl analogues to cells (e.g., N-acetylcysteine, 2-oxo-4-thiazolidine carboxylic acid) would normally increase glutathione. Thus, the decline in glutathione with increased cysteine suggests that brain glutathione is providing additional cysteine residues. Since the endogenous concentration of glutathione is much higher than that of cysteine in brain, a small decline in glutathione can lead to a large increase in cysteine. Further, an age-related decrease in plasma cysteine suggests that high cysteine may be important in diminishing plaque formation (Chen et al., 1989
). Increasing intracellular cysteine protects against multiple oxidant injuries. In vitro
and in vivo
experiments have used N-acetylcysteine (NAC) to increase cellular cysteine. NAC protects SHSY-5Y neuroblastoma cells from oxidative stress and cell toxicity due to H2
, UV light and amyloid-β1–42
. Also, NAC promotes release of amyloid-β1–42
from cells and diminishes oxidant induced tau phosphorylation (Olivieri et al., 2001
). NAC down regulates transcription of the amyloid precursor protein in human neuroblastoma cells (Studer et al., 2001
). Thus, the resveratrol induced increase in cysteine may be protective and prevent plaque formation in the same way as NAC.
An alternative tantalizing speculation is that the resveratrol-induced reduction in plaques may be through cysteine or resveratrol chelation of copper or zinc. Cysteine chelates copper (Baker and Czarnecki-Maulden, 1987
; Cakir et al., 2001
; Hallman et al., 1971
; Li and Manning, 1955
) and zinc (Baker and Czarnecki-Maulden, 1987
; Cakir et al., 2001
; Chen and Liao, 2003
; Hallman et al., 1971
; Li and Manning, 1955
). The chelation appears pharmacologically important. Resveratrol inhibits human low density lipoproteins (LDL) oxidation by chelating copper (Frankel et al., 1993
; Belguendouz et al., 1997
; Fremont et al., 1999
). Cu and Zn are enriched in amyloid β deposits in AD, which are solubilized by Cu/Zn-selective chelators in vitro
. A bioavailable Cu/Zn chelator decreases brain amyloid-β deposition by 49% (Cherny et al., 2001
). Thus, the resveratrol induced increase in cysteine may help to reduce plaques by chelation of copper in a manner shown for studies of Cu/Zn chelators.
In conclusion, our results demonstrate that resveratrol treatment for 45 days reduced the plaque pathology in a region specific manner, decreased brain glutathione and increased its precursor product cysteine. Further studies need to be conducted to elucidate the precise mechanism(s) by which resveratrol reduces Aβ pathology and alters brain glutathione status. This study supports an important concept that onset of neurodegenerative disease may be delayed or mitigated with use of dietary chemo-preventive agents that protect against β-amyloid plaque formation and oxidative stress.