Polyphenols as Inhibitors of Tau Toxicity: Protecting Neuronal Network Function in Alzheimer's Disease.

One of the pathological hallmarks of Alzheimer's disease (AD) is the formation of intracellular tau tangles in the brain,
making tau an attractive target for the development of new therapeutics for AD. Tau is a natively unfolded, soluble,
microtubule-associated protein showing little tendency for aggregation. However, in AD, tau can misfold, enabling
aggregation and leading to the formation of toxic oligomeric species which can trigger neuronal dysfunction and seed
self-propagation. Several polyphenols have been shown to disrupt tau aggregation in vitro but these tend to be
limited in bioavailability, membrane penetrance, and potency. A recent study showed that oral administration of the
polyphenol (-)-Epicatechin ((-)-EC) to rTg4510 tau transgenic mice halted the appearance of high molecular weight
tau species within the brain. This, therefore, suggests that ingestion of polyphenols could provide benefit in AD.
However, further investigation is required to determine if a reduction in tau pathology blocks neurotoxicity or reverses
deficits in synaptic function.
The entire metabolome of (-)-EC, consisting of more than 20 key metabolites circulating in human plasma, has now
been identified. Our objective is to determine which of these metabolites elicit the beneficial phenotype with the aim
of identifying the precise pharmacophore capable of preventing tau aggregation and its mechanism of action. To do
this, cellular models of tau pathology will first be characterised. Mouse primary cortical neurons transfected with GFPtagged
human tau constructs, some with mutations know to lead to tau pathology (P301L), will be utilised.
Microscopy and biochemical assays will enable the identification and characterisation of morphological and
biochemical changes that occur with the expression of disease-linked mutant tau. For assays requiring increased
transfection efficiencies, N2A cells (mouse neuroblastoma) and SHSY5Y cells (human) will be used. These models
will then be used to screen (-)-EC metabolites for the ability to bind tau and inhibit its aggregation inside neurones.
Furthermore, the positive (-)-EC metabolite hits will be tested for their ability to inhibit tau mislocalisation to dendritic
spines by comparing distribution of human tau-GFP wild type and human tau-GFP P301L in neurones co-expressing
dsRed. Any alterations in glutamate receptor function will be determined by measuring AMPA and NMDA-evoked
calcium increases using FURA-2AM dual emission fluorescence imaging. While there is evidence that (-)-EC
metabolites reach the brain, cellular distribution is less clear. Therefore, the uptake and further metabolism of (-)-EC,
and four lead metabolites, will be determined in neurons using HPLC and high-resolution mass spectrometry. (-)-EC
and two lead metabolites will then be selected for whole transcriptome RNA sequencing in a single neuronal model to
establish an unbiased compound-response biomarker signature.
Following these studies, work will be undertaken in the rTg4510 tau transgenic mouse model with Jon Brown at
Exeter in order to study tau-induced network dysfunction and whether this can be ameliorated with the selected (-)-
EC metabolites.
Collectively the study will provide new mechanistic insight into the link between tau and synaptic dysfunction and will
signpost future rationale drug discovery aimed at disrupting tau toxicity to halt progression and provide symptomatic
benefit in AD.