Tau pathology as the key to understanding Alzheimer’s disease
Alzheimer’s disease (AD) is a neurodegenerative disorder and the most common type of dementia (60–70%), causing problems with memory, language, thinking and judgement.
However, no treatment yet exists to halt AD’s progression, delay its onset, or, better yet, prevent it. Currently, available drugs are able to treat the symptoms of the disease but have no effect on its underlying pathology and patients experience only temporary benefit.
Scientists are not sure what triggers the development of AD pathology but two key changes in the brain have attracted the most attention: the abnormal deposition of two proteins, amyloid and tau.
Interest in the role of amyloid and tau in AD started at approximately the same time in the 1980s. People with AD have brains characterised by significantly reduced numbers of nerve cells and synapses than in a healthy brain, plaques (which are abnormal clusters of protein fragments with heavy amyloid deposits), and dead and dying nerve cells containing tangles comprised of twisted strands of tau. Whether it was the plaques or the tangles that drive the pathology was unclear and two hypotheses began to develop to explain the development of AD. Although the amyloid and tau hypotheses were of equal interest in the beginning, amyloid research came to dominate the field for a number of reasons.
However, the amyloid hypothesis has always been haunted by the poor correlation between the presence and extent of amyloid plaques and the progression of dementia. This poor correlation is undoubtedly a leading factor in the failure of approximately 25 late-stage amyloid-focused clinical trials using different molecules and approaches to plaque clearance over the years.
In the last three to four years, the role of tau as the key driver in the development and spread of AD pathology has gained credibility and become the focus of a rapidly expanding body of research. Over three decades ago, Professor Claude Wischik, then a neuroscience researcher at the University of Cambridge, UK, discovered that the tangles seen in nerve cells of AD patients are made up of aggregated sub-units of tau protein.
Subsequently, Professor Wischik and his research team pioneered the early-stage research into tau aggregation inhibitors. The hypothesis was that as the spread of tau tangles is highly correlated with the clinical progression of dementia, the inhibition and clearance of these tau tangles could slow, and possibly halt, the progression of AD. This foundational research into tau aggregation inhibition became the basis for establishing TauRx Pharmaceuticals to develop promising drug candidates and commercialise a range of effective therapeutic products. TauRx’s mission is to discover, develop and commercialise innovative products for the diagnosis, treatment and cure of neurodegenerative diseases caused through protein aggregation.
The tau tangle pathway as a driver of AD
Tau proteins have been found to have the greatest expression in the central nervous system and are involved in the assembly and stabilisation of microtubules within nerve cells, which are critical to the healthy functioning of the brain. In AD, tau proteins abnormally aggregate inside nerve cells in the brain, leading to the formation of tau tangles. The increase in tau tangles eventually overpowers the normal functioning of the affected nerve cell and results in its death.
In this pathway, abnormal aggregation of tau protein damages synapses, permitting the pathology to spread to neighbouring nerve cells via a prion-like process. The tangles first destroy nerve cells in the part of the brain critical for memory and then gradually spread to nerve cells in other parts of the brain in a very well-characterised staging process.
Typically, tau tangles first appear some 20 years before the clinical symptoms of AD are apparent; these tangles are the earliest detectable stages of the disease at post mortem, but no wholly reliable method for their in vivo detection and measurement is yet available.
The focus of the AD research landscape is clearly beginning to shift, with scientists moving away from amyloid to tau. The reduction in the presence and spread of tau tangles in the brain is thus a potential focus for the development of medicines that could alter the course of AD. Building on promising data from Phase 3 trials completed in 2016 and supported by more than 30 years of research, TauRx plans to initiate additional clinical studies for patients with mild to moderate forms of the disease in the near future.
In addition to AD research, TauRx is developing a pipeline of new small-molecule tau aggregation inhibitors, immunotherapeutics and imaging ligands for use in the poorly-served field of neurodegenerative diseases, with a particular interest in Parkinson’s disease and the frontotemporal lobar degeneration syndromes including behavioural variant frontotemporal dementia.
 World Health Organization. 2016 Dementia Fact Sheet. Available at: http://www.who.int/mediacentre/factsheets/fs362/en/ Accessed May 2017;
 Alzheimer’s Association. 2016 Alzheimer’s Disease Facts and Figures. Available at: http://www.alz.org/documents_custom/2016-facts-and-figures.pdf Accessed May 2017
 Simic G, et al. Biomolecules. 2016:6;doi:10.3390/biom6010006;
 National Institute of Ageing. 2008 Alzheimer’s disease: Unravelling the mystery. Available at: https://www.nia.nih.gov/alzheimers/publication/alzheimers-disease-unraveling-mystery/preface Accessed May 2017