If you’re a regular reader of AlzScience, you know that Alzheimer’s disease is believe to be caused by two toxic proteins that accumulate in the brain: amyloid-beta and tau. (For more background, see Alzheimer’s Disease: A General Overview.) Recently, it’s been shown that tau is actually a better predictor of Alzheimer’s disease progression than amyloid-beta, suggesting that this mysterious protein might have a larger role in the disease than we once thought.A study published last week in Nature provided deeper insight into tau. The scientists were interested in studying the ApoE gene, which is considered the strongest genetic risk factor for Alzheimer’s (see The Genetics of Alzheimer’s Disease.) Specifically, having two copies of the ApoE4 allele increases your risk of Alzheimer’s by nearly 15 times, and it’s been shown that people with this allele have greater buildup of amyloid-beta in their brains. However, the researchers in this study wanted to see whether ApoE could also affect tau accumulation.
To test this, they used genetically engineered mice that overexpress the tau gene, causing them to develop many of the symptoms of Alzheimer’s. They then tampered with these mice’s genes so that they would also overexpress ApoE4. (Note: Overexpressing the tau and ApoE4 genes means those genes were more active than they normally would be in the mice. Think of it like a light switch stuck in the “on” position.) They found that these mice had more tau in their brains, and also more severe brain shrinkage due to neuronal death.To figure out how ApoE4 might be causing more tau accumulation, the researchers looked at the mice’s microglia, the immune cells of the brain. The microglia overexpressing ApoE4 tended to overreact to infections, releasing high amounts of pro-inflammatory molecules called cytokines. Neurons and other brains cells are very sensitive to cytokines, and high levels might cause them to produce more tau.
Finally, the researchers turned to human research. They used postmortem brain tissues taken from people who had tauopathies, which are diseases caused by accumulation of tau (but not amyloid-beta) in the brain. The people possessing the ApoE4 allele had more severe neurodegeneration and greater tau buildup in certain areas of the brain.
Overall, this study demonstrates that ApoE4 does not only act on amyloid-beta, but tau as well. It gives strong support to the notion that tau may be as important as amyloid-beta in understanding the pathology of Alzheimer’s disease. In an interview with Science News, Harvard neurologist Dennis Selkoe described this deadly combination of amyloid-beta and tau as a “double whammy.” Yet this study provides hope that future therapies against ApoE4 could be capable of halting both of these toxic proteins at once.