Tau Protein Triggers Neuronal Death Process in Alzheimer’s, Say Researchers

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The debate over whether the neuronal death process in Alzheimer’s is caused by tau or beta-amyloid structures has long been fought. However, researchers from Georgetown University Medical Center have boldly claimed that the event is triggered by accumulation of a protein called tau, not beta-amyloid.

Speaking to Fox News, Charbel E-H Moussa, an assistant professor of neuroscience at Georgetown University Medical Center, briefly summarized his team’s findings:

“For a very long time, we believed, for almost 100 years, that [amyloid-beta] plaques are the main culprit in Alzheimer’s disease. This study shows it’s another protein — a very, very important one, called tau, is basically the main guilty one.”

Classified as a microtubule-associated protein (MAP), tau is critical to a number of cellular processes. Tau is part of the normal cellular framework, providing structure to neurons and facilitating transportation of various substances, including movement of toxic proteins out of the cell. The protein also anchors various enzymes to the cell and modulates growth of neurites.

Neuronal death often occurs when the tau fails to function normally. These abnormal proteins, including variants of tau and beta-amyloid, then accumulate within the affected neurons, eventually leading to cell death. “The cells start to spit the proteins out, as best they can, into the extracellular space so that they cannot exert their toxic effects inside the cell. Because Abeta is ‘sticky,’ it clumps together into plaque,” explained Moussa in a recent press release.

Moussa went on to explain that the neurons are no longer able to remove the “garbage” of the cell, once the tau becomes abnormal. As a consequence, deposits of misfolded tau protein – called neurofibrillary tangles (NFTs) – and beta-amyloid often lead to cell death in Alzheimer’s patients. NFTs and beta-amyloid become physical obstructions that disrupt normal axonal transportation. While the cell attempts to expel as much of this toxic protein as possible, it eventually becomes overwhelmed and dies.

According to the researchers, the ejected protein, which form plaques around the outside of the cell, is not toxic; this finding, if true, conflicts with previous theories. When tau is not functioning normally, the researchers say the amyloid-beta cannot be “digested” and removed from the neuron. Instead, it builds up inside the cell and causes it to die.

Moussa and colleagues used animal models to investigate the role of tau in the pathology of Alzheimer’s. Mice that did not possess tau could not digest and remove amyloid-beta, leading to twice as many plaques compared to those mice that had fully functioning tau. However, the mice that had the highest intracellular build-up of protein, and the fewest plaques outside the cell, were more toxic.

On this basis, Moussa believes current immunotherapy treatments that attempt to destroy plaque formations outside of the neuron are likely to be as ineffective as a placebo. Instead, he believes research efforts should be focused on attempting to restore the normal function of tau, thereby ensuring intracellular amyloid-beta is removed.

With this in mind, the researchers decided to test an anti-cancer drug called nilotinib. While the drug can clear away some of the plaques, it also helps to clear intracellular garbage; however, it can only do this when some tau remains functional. Preliminary experiments involving nilotinib appear promising, and clinical trials are to begin in the coming weeks.

“The common culprit is tau, so a drug that helps tau do its job may help protect against progression of these diseases,” concludes Moussa.

The latest study was published in the Oct. 29 issue of the Journal Molecular Neurodegeneration.