Protection of Green Tea Polyphenols against Neurodegenerative Diseases: Evidence and Possible Mechanisms

Aging is a major risk factor for neurodegenerative diseases. With aging of the global population, the prevalence of neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), has increased worldwide. Unfortunately, the available therapeutic options for these neurodegenerative diseases are limited, most of which only provide symptomatic relief and have potentially serious side effects. Epidemiological studies have shown that green tea consumption is associated with a lower prevalence of cognitive decline and decreased risk of AD and PD, providing an attractive preventive and therapeutic option. Polyphenols are major bioactive components in green tea, which contribute to the beneficial effects of green tea. Accumulating data suggest that green tea polyphenols (GTPs) have neuroprotective properties that inhibit the pathological development of neurodegenerative diseases; however, the underlying mechanisms are not yet completely understood. This paper reviews both in vitro and in vivo evidence that demonstrates the neuroprotective effects of GTPs against neurodegenerative diseases, with the main focus on AD and PD, and summarizes the possible molecular mechanisms by which GTPs impede the progression of neurodegeneration. In particular, this review highlights the modulation of GTPs on the common mechanisms involved in pathogenesis of neurodegenerative diseases, including oxidative stress-mediated neuronal toxicity, impaired proteostasis, and metal ion dyshomeostasis. The potential of using GTPs in the intervention of neurodegenerative diseases is also discussed, hopefully, providing useful insights into novel preventive and therapeutic strategies for these diseases.