Researchers here provide evidence for the aggregation of altered forms of tau protein in the aging brain, and the resulting death of neurons, to be driven by chronic inflammation. This is good news if true, given recent work carried out in animal models of tauopathy, in which clearance of inflammatory, senescent glial cells in the brain was achieved via the use of senolytic drugs. The result was a marked reduction in both inflammation and tau pathology. To the degree that senescent cells in the brain prove to be the major cause of the chronic inflammation of aging and neurodegenerative conditions, it may well turn out that senolytic drugs will do a great deal for Alzheimer's patients. Since the senolytic drug dasatinib is off-patent, crosses the blood brain barrier, and is well tested for human use, trials could in principle begin just as soon as a sponsoring organization emerges and chooses to start.
Tau proteins usually stabilize a neuron's cytoskeleton. However, in Alzheimer's disease, frontotemporal dementia (FTD), and other tauopathies these proteins are chemically altered, they detach from the cytoskeleton and stick together. As a consequence, the cell's mechanical stability is compromised to such an extent that it dies off. In essence, tau pathology gives neurons the deathblow. The current study provides new insights into why tau proteins are transformed. As it turns out, inflammatory processes triggered by the brain's immune system are a driving force.
A particular protein complex, the NLRP3 inflammasome, plays a central role for these processes. It is a molecular switch that can trigger the release of inflammatory substances. For the current study, the researchers examined tissue samples from the brains of deceased FTD patients, cultured brain cells, and mice that exhibited hallmarks of Alzheimer's and FTD. In particular, the researchers discovered that the inflammasome influences enzymes that induce a hyperphosphorylation of tau proteins. This chemical change ultimately causes them to separate from the scaffold of neurons and clump together. "It appears that inflammatory processes mediated by the inflammasome are of central importance for most, if not all, neurodegenerative diseases with tau pathology."
This especially applies to Alzheimer's disease. Here another molecule comes into play: amyloid beta (Aβ). In Alzheimer's, this protein also accumulates in the brain. In contrast to tau proteins, this does not happen within the neurons but between them. In addition, deposition of Aβ starts in early phases of the disease, while aggregation of tau proteins occurs later. The results of the current study support the amyloid cascade hypothesis for the development of Alzheimer's. According to this hypothesis, deposits of Aβ ultimately lead to the development of tau pathology and thus to cell death. The study shows that the inflammasome is the decisive and hitherto missing link in this chain of events, because it bridges the development from Aβ pathology to tau pathology. Thus, deposits of Aβ activate the inflammasome. As a result, formation of further deposits of Aβ is promoted. On the other hand, chemical changes occur to the tau proteins resulting into their aggregation.