Neuroprotective effects of polyphenols in cellular models of Parkinson's disease
Parkinson’s disease (PD) is a neurodegenerative disorder which disrupts the lives of 5 million people worldwide. PD is characterized by a loss of dopaminergic neurons in the substantia nigra, resulting in muscle rigidity, tremors, loss of physical movement and eventually cognitive dysfunction in most patients. Current therapies against PD alleviate symptoms without slowing the progression of the disease. The death of dopaminergic neurons is mainly caused by excessive production of reactive oxygen species (ROS) and down-regulation of cellular cytoprotective mechanisms. Hence, current efforts in the field of Parkinson’s-related research are directed towards discovering drug candidates with ROS scavenging/antioxidant properties. Dietary polyphenols are excellent candidates for neuroprotection in PD because they combine potent antioxidant activities with the ability to up-regulate endogenous cytoprotective responses. Moreover, epidemiological evidence suggests that a high intake of flavonoids (a type of polyphenol) is associated with lower risk of PD. Hence we hypothesized that polyphenols are neuroprotective in PD models. A previous study done in our lab showed that botanical extracts rich in anthocyanins (ANC) and proanthocyanidins (PAC) protected against dopaminergic cell loss elicited by rotenone (a mitochondrial toxin epidemiologically linked to PD) in primary midbrain cultures obtained from embryonic rats. Based on this study, we aimed to determine whether the extracts were also protective against neurotoxicity elicited by paraquat (PQ), another environmental toxin associated with high PD risk. PQ is a pro-oxidant that triggers a buildup of ROS by engaging in a redox cycling mechanism in the cytosol. Given that rotenone and PQ have different mechanisms of action, we were motivated to compare the extent of protection conferred by the botanical extracts against the two PD-related insults. Our data showed that botanical extracts rich in ANC, PAC and stilbenes (a subclass of polyphenols) protected against PQ neurotoxicity. Interestingly, a subset of extracts showed differential neuroprotective activity against rotenone and PQ. We hypothesized that these extracts alleviated neurotoxicity elicited by the two different insults by activating different cytoprotective mechanisms. In partial support of this hypothesis, we found that a subset of the botanical extracts induced an increase in the activity of Nrf-2 (nuclear factor erythroid 2-related factor 2), a transcription factor involved in regulating the expression of cellular antioxidant enzymes, whereas some of the extracts alleviated mitochondrial dysfunction elicited by rotenone. Taken together, these data identify botanical extracts with neuroprotective activity against two PD-related insults, and they shed light on differences in neuroprotective mechanisms that can alleviate toxicity elicited by rotenone versus PQ. Because a blueberry (BB) extract rich in ANC protected against both rotenone- and PQ-mediated neurotoxicity in the study outlined above, we further assessed the neuroprotective activity of this extract against dopaminergic cell death triggerd by an additional PD-related insult, and we investigated the underlying neuroprotective mechanisms. The BB extract was found to alleviate dopaminergic cell death and neurite loss in primary midbrain cultures exposed to virus encoding a mutant form of alpha-synuclein (αSyn), a presynaptic protein that is thought to form neurotoxic aggregates involved in the pathogenesis of familial and sporadic PD. Additionally, we found that the BB extract suppressed glial activation elicited by the bacterial endotoxin lipopolysaccharide (LPS), a classic inflammatory agent, and rotenone. Additional experiments revealed that the BB extract and individual ANC stimulated the transcriptional activity of Nrf2 and upregulated DJ-1, a redox-sensitive chaperone protein implicated in PD. Furthermore, we showed that the pro-oxidant activity of the BB extract was necessary for Nrf-2 activation and the extract’s neuroprotective activity. Furthermore, we showed that although DJ-1 was not necessary for BB-mediated activation of Nrf-2, DJ-1 knockdown abrogated BB-mediated neuroprotection against rotenone toxicity. Together, the data from this study suggest that BB ANC exert neuroprotection via dual action of Nrf2 and DJ-1. Having shown that an ANC-rich BB extract alleviated neurotoxicity induced by various PD-related insults, we further tested a ‘wild’ variety of BB for neuroprotective activity against PQ toxicity. Because the wild BB extract had a slightly different polyphenol profile compared to the ‘cultivated’ variety tested earlier, we hypothesized that the two varieties might have different neuroprotective activities. In support of this idea, we found that although extracts prepared from both BB varieties protected against PQ neurotoxicity, they possessed different potencies. Additionally, we tested different fractions of the wild BB extract enriched in different types of polyphenols. We found that the ANC-rich fraction alleviated PQ neurotoxicity and that synergies among various polyphenols apparently contributed to the neuroprotective activity of the wild BB extract. Collectively, our data indicate that polyphenols are protective against different PD-related insults. Our results also suggest that ANC and ANC-rich extracts are excellent candidates for PD therapy. Finally, our findings provide insight into neuroprotective mechanisms that could reduce PD risk and/or slow disease progression and identify Nrf-2 and DJ-1 as key therapeutic targets in PD.
Rochet, Purdue University.
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