Researchers in the UK Dementia Research Institute at the University of Cambridge screened more than 1,400 clinically-approved drug compounds using zebrafish genetically engineered to make them mimic so-called tauopathies. They discovered that drugs known as carbonic anhydrase inhibitors — of which the glaucoma drug methazolamide is one — clear tau build-up and reduce signs of the disease in zebrafish and mice carrying the mutant forms of tau that cause human dementias.

Tauopathies are neurodegenerative diseases characterised by the build-up in the brain of tau protein ‘aggregates’ within nerve cells. These include forms of dementia, Pick’s disease and progressive supranuclear palsy, where tau is believed to be the primary disease driver, and Alzheimer’s disease and chronic traumatic encephalopathy (neurodegeneration caused by repeated head trauma, as has been reported in football and rugby players), where tau build-up is one consequence of disease but results in degeneration of brain tissue.

There has been little progress in finding effective drugs to treat these conditions. One option is to repurpose existing drugs. However, drug screening — where compounds are tested against disease models — usually takes place in cell cultures, but these do not capture many of the characteristics of tau build-up in a living organism.

To work around this, the Cambridge team turned to zebrafish models they had previously developed. Zebrafish grow to maturity and are able to breed within two to three months and produce large numbers of offspring. Using genetic manipulation, it is possible to mimic human diseases as many genes responsible for human diseases often have equivalents in the zebrafish.

In a study published today in Nature Chemical Biology, Professor David Rubinsztein, Dr Angeleen Fleming and colleagues modelled tauopathy in zebrafish and screened 1,437 drug compounds. Each of these compounds has been clinically approved for other diseases.

Dr Ana Lopez Ramirez from the Cambridge Institute for Medical Research, Department of Physiology, Development and Neuroscience and the UK Dementia Research Institute at the University of Cambridge, joint first author, said: “Zebrafish provide a much more effective and realistic way of screening drug compounds than using cell cultures, which function quite differently to living organisms. They also enable us to do so at scale, something that it not feasible or ethical in larger animals such as mice.”

Using this approach, the team showed that inhibiting an enzyme known as carbonic anhydrase — which is important for regulating acidity levels in cells — helped the cell rid itself of the tau protein build-up. It did this by causing the lysosomes — the ‘cell’s incinerators’ — to move to the surface of the cell, where they fused with the cell membrane and ‘spat out’ the tau.

When the team tested methazolamide on mice that had been genetically engineered to carry the P301S human disease-causing mutation in tau, which leads to the progressive accumulation of tau aggregates in the brain, they found that those treated with the drug performed better at memory tasks and showed improved cognitive performance compared with untreated mice.

Analysis of the mouse brains showed that they indeed had fewer tau aggregates, and consequently a lesser reduction in brain cells, compared with the untreated mice.

Fellow joint author Dr Farah Siddiqi, also from the Cambridge Institute for Medical Research and the UK Dementia Research Institute, said: “We were excited to see in our mouse studies that methazolamide reduces levels of tau in the brain and protects against its further build-up. This confirms what we had shown when screening carbonic anhydrase inhibitors using zebrafish models of tauopathies.”

Professor Rubinsztein from the UK Dementia Research Institute and Cambridge Institute for Medical Research at the University of Cambridge, said: “Methazolamide shows promise as a much-needed drug to help prevent the build-up of dangerous tau proteins in the brain. Although we’ve only looked at its effects in zebrafish and mice, so it is still early days, we at least know about this drug’s safety profile in patients. This will enable us to move to clinical trials much faster than we might normally expect if we were starting from scratch with an unknown drug compound.

“This shows how we can use zebrafish to test whether existing drugs might be repurposed to tackle different diseases, potentially speeding up significantly the drug discovery process.”

The team hopes to test methazolamide on different disease models, including more common diseases characterised by the build-up of aggregate-prone proteins, such as Huntington’s and Parkinson’s diseases.

The research was supported by the UK Dementia Research Institute (through UK DRI Ltd, principally funded through the Medical Research Council), Tau Consortium and Wellcome.