Protective Effects of Foods Containing Flavonoids on Age-Related Cognitive Decline

Flavonoids exert a multiplicity of neuroprotective actions within the brain, including a potential to protect neurons against injury induced by neurotoxins, an ability to suppress neuroinflammation, and the potential to promote memory, learning and cognitive function. These effects appear to be underpinned by two common processes. Firstly, they interact with critical protein and lipid kinase signalling cascades in the brain leading to an inhibition of apoptosis triggered by neurotoxic species and to a promotion of neuronal survival and synaptic plasticity. Secondly, they induce beneficial effects on the vascular system leading to changes in cerebrovascular blood flow capable of causing angiogenesis, neurogenesis and changes in neuronal morphology. Through these mechanisms, the consumption of flavonoid-rich foods throughout life holds the potential to limit neurodegeneration and to prevent or reverse age-dependent loses in cognitive performance. The intense interest in the development of drugs capable of enhancing brain function means that flavonoids may represent important precursor molecules in the quest to develop of a new generation of brain enhancing drugs.

The emerging roles of microglia are currently being investigated in the healthy and diseased brain with a growing interest in their diverse functions. In recent years, it has been demonstrated that microglia are not only immunocentric, but also neurobiological and can impact neural development and the maintenance of neuronal cell function in both healthy and pathological contexts. In the disease context, there is widespread consensus that microglia are dynamic cells with a potential to contribute to both central nervous system damage and repair. Indeed, a number of studies have found that microenvironmental conditions can selectively modify unique microglia phenotypes and functions. One novel mechanism that has garnered interest involves the regulation of microglial function by microRNAs, which has therapeutic implications such as enhancing microglia-mediated suppression of brain injury and promoting repair following inflammatory injury. Furthermore, recently published articles have identified molecular signatures of myeloid cells, suggesting that microglia are a distinct cell population compared to other cells of myeloid lineage that access the central nervous system under pathological conditions. Thus, new opportunities exist to help distinguish microglia in the brain and permit the study of their unique functions in health and disease.

Neuroinflammation is one of the main mechanisms involved in the progression of several neurodegenerative diseases, such as Parkinson, Alzheimer, multiple sclerosis, amyotrophic lateral sclerosis and others. The activation of microglia is the main feature of neuroinflammation, promoting the release of pro-inflammatory cytokines and resulting in the progressive neuronal cell death. Natural compounds, such as flavonoids, possess neuroprotective potential probably related to their ability to modulate the inflammatory responses involved in neurodegenerative diseases. In fact, pure flavonoids (e.g., quercetin, genistein, hesperetin, epigallocatechin-3-gallate) or enriched-extracts, can reduce the expression of pro-inflammatory cytokines (IL-6, TNF-α, IL-1β and COX-2), down-regulate inflammatory markers and prevent neural damage. This anti-inflammatory activity is primarily related to the regulation of microglial cells, mediated by their effects on MAPKs and NF-κB signalling pathways, as demonstrated by in vivo and in vitro data. The present work reviews the role of inflammation in neurodegenerative diseases, highlighting the potential therapeutic effects of flavonoids as a promising approach to develop innovative neuroprotective strategy.