Neuroprotective effects of blueberries extend beyond antioxidant activity
Blueberries are rich in polyphenolic compounds and have shown an ability to improve cognitive function in several clinical trials but according to a recently published study, the molecular basis of the neuronal protection of blueberries is not fully understood.
While there is a report that blueberry inhibits, no systematic studies have been done to identify how blueberries inhibit the key enzymes involved in neuronal degeneration: Acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), tyrosinase and COX-2 .
This in-vitro study aimed to quantify the main classes of the polyphenols of blueberry extract and understand their role in the protection of neuronal cells, by examining their effects on these enzymes along with their impact on amyloid fibril formation.
The team of researchers from University of Massachusetts Dartmouth, USA, found that blueberries directly inhibit the enzymatic activity of AChE, BuChE, tyrosinase, and COX-2, as well as delaying the amyloid fibril formation by 24 hours. The fruit was also found to reduce the synthesis of acetylcholinesterase synthesis in a cellular model.
The research team says these results suggest that the neuroprotection effects of blueberries may involve different pathways, including enhancing cholinergic signalling through their effect on cholinesterase, reducing neuroinflammation through inhibition of COX-2, and reducing amyloid formation.
"Collectively, blueberries may play a vital role in neuronal protection beyond their antioxidant activity and our results provide more molecular mechanisms for their neuroprotective effects, and support blueberries being nutraceutical to improve cognitive function," the authors conclude.
"Through inhibition of these enzymes, blueberries could enhance cholinergic signaling, and further modulate the oxidation stress and APP processing, reduce neuroinflammation and confer neurocognition.
"Furthermore, through the HEWL model, polyphenol-rich blueberries have shown the anti-amyloidogenic property by reducing the progression of the amyloid fibril formation, which could lead to reducing the toxicity of amyloid to neurons.
"These results provide more molecular mechanisms of the neuroprotective effects of blueberries beyond their antioxidant activities. AD (and other neurodegenerative diseases) is a complex disease and not caused by any single factor. Thus, targeting just one pathway may not be adequate to treat or prevent AD and other neurogenerative diseases.
"Blueberries, as demonstrated in this study, affect multiple enzymes and pathways. Therefore, consuming blueberries, as part of a healthy lifestyle, may play a pivotal role in the prevention and/or delaying of the progression of neurodegenerative diseases."
The total phenolic content of the blueberry extract was determined using the Folin–Ciocalteu method. The total flavonoids in the blueberry extract were determined based on the formation of the flavonoid-aluminum complex. To quantify the total anthocyanins content a pH differential method was employed.
AChE and BuChE activity was measured using the spectrophotometric method developed by Ellman et al. in a microplate format to evaluate the inhibition effect of blueberry extract. Galantamine, a selective inhibitor of AChE, was used as a positive control.
The effect of blueberry extract on the activity of tyrosinase was spectrophotometrically measured using L-DOPA (6.28 mM) as the substrate in 96-well microplate format as reported previously with modifications. Kojic acid (0.27 mg/mL, stock) a known inhibitor of tyrosinase was used as a positive control.
Inhibition of COX-2 by the blueberry extract was assessed by monitoring the quantity of prostaglandins using a competitive enzyme-linked immunosorbent assay (ELISA). Cayman’s protocol was followed. PG generated from the enzymatic reaction was quantitated using competitive ELISA with rabbit antiserum against PG.
Human neuroblastoma cell line M17 was used to examine the effects of blueberry extract on the synthesis of AChE. The AChE produced from the cells was measured using indirect ELISA with the monoclonal antibody against human AChE. Cells were treated with different concentrations of blueberry extract (0 to 0.5 mg/mL) in serum-free media for 48 h. The resulting number of cells was counted and the viability of cells was assessed.
Pathogenesis of Alzheimer's
While the causes of AD are still unknown, several hypotheses have been proposed: deficits in the cholinergic transmission, beta-amyloid plagues (Aβ), tau tangles, oxidative damage and mitochondrial dysfunction, neuronal inflammation, synapse loss, vascular changes; endosomal abnormalities, among others. Genetics also plays a role in AD.Cholinesterase (including acetylcholinesterase and butyrylcholinesterase) is a critical enzyme to regulate the level of the neurotransmitter and thus plays important role in cholinergic signalling. Both Aβ and abnormally hyperphosphorylated tau can increase acetylcholinesterase (AChE) expression.
The increased AChE further influences PS1 and tau-protein kinase GSK-3β. GSk-3β induces hyperphosphorylated tau (P-tau), while PS1 affects APP processing and Aβ production. Aβ polymerizes to form plaques that deposit in the brain disrupting neuronal cell function. This cascade induces neuroinflammation and oxidative stress, which leads to cognitive decline.
Like AChE, butyrylcholinesterase (BuChE) hydrolyzes the neurotransmitter acetylcholine (butyrylcholine, the preferred substrate for BuChE, is not existent naturally), but at a much slower rate. While it is less abundant in the brain and considered to play a minor role in regulating the acetylcholine level in the brain, BuChE is found to compensate for AChE when its levels deplete. BuChE activity has been found to progressively increase in AD patients. Further, BuChE is also found to involve the Aβ plaques maturation.
Therefore, the authors of the current study say both AChE and BuChE are valid targets for AD. In addition to AD, both AChE and BuChE are also found to play roles in multiple sclerosis, and higher levels of AChE and BuChE are observed in patients with Parkinson’s disease dementia compared with Parkinson’s disease.
Neuronal inflammation is another key factor leading to the damage of neuronal cells. Accumulation of aggregated proteins and damaged neurons causes inflammation, along with imbalances between pro- and anti-inflammatory processes. Pathogenic stimuli, such as the accumulation of abnormal proteins in cells or extracellular spaces (including Aβ), cause cellular stress responses resulting in the progressive dysfunction and deration of neurons. Cyclooxygenase-2 (COX-2) is involved in the biosynthesis of prostaglandins (PGs) under severe inflammation. Elevated PGs are reported to be involved in the pathogenesis of AD.
Tyrosinase oxidizes dopamine to form melanin pigments through the formation of dopamine quinone, a reaction that results in the formation of highly reactive oxygen or nitrogen species capable of inducing neuronal cell death. Oxidation stress links to both inflammation and endosomal abnormalities, which may cause damage to neurons and has been hypothesized as potential links to Parkinson’s disease.
The protection of the neurons from damage by different factors is the key to preserving the normal functions of neurons. This is difficult to be accomplished through a single drug or even multiple drugs. The authors of the current report therefore argue that diet may hold key to the prevention of AD, and potentially even decelerate the progression of the disease.
https://doi.org/10.3390/nutraceuticals3010004 (registering DOI)
"Neuroprotective Effects of Blueberries through Inhibition on Cholinesterase, Tyrosinase, Cyclooxygenase-2, and Amyloidogenesis"
Samani, P.; Costa, S.; Cai, S.