Inflammation drives the progression of neurodegenerative brain diseases and plays a major role in the accumulation of tau proteins within neurons. An international research team led by the German Center for Neurodegenerative Diseases (DZNE) and the University of Bonn comes to this conclusion in the journal Nature. The findings are based on the analyses of human brain tissue and further lab studies. In the particular case of Alzheimer’s the results reveal a hitherto unknown connection between Amyloid Beta and tau pathology. Furthermore, the results indicate that inflammatory processes represent a potential target for future therapies.
Tau proteins usually stabilize a neuron’s skeleton. 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 led by Prof. Michael Heneka, director of the Department of Neurodegenerative Diseases and Gerontopsychiatry at the University of Bonn and a senior researcher at the DZNE, 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 Molecular Switch
A particular protein complex, the “NLRP3 inflammasome”, plays a central role for these processes, the researchers report in Nature. Heneka and colleagues already studied this macromolecule, which is located inside the brain’s immune cells, in previous studies. 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.
“Our results indicate that the inflammasome and the inflammatory reactions it triggers, play an important role in the emergence of tau pathology”, Heneka said. 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.”
A Link between Amyloid Beta and Tau
This especially applies to Alzheimer’s disease. Here another molecule comes into play: “amyloid beta” (Amyloid Beta). 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 Amyloid Beta starts in early phases of the disease, while aggregation of tau proteins occurs later.
In previous studies, Heneka and colleagues were able to show that the inflammasome can promote the aggregation of Amyloid Beta. Here is where the connection to the recent findings comes in. “Our results support the amyloid cascade hypothesis for the development of Alzheimer’s. According to this hypothesis, deposits of Amyloid Beta ultimately lead to the development of tau pathology and thus to cell death,” said Heneka. “Our current study shows that the inflammasome is the decisive and hitherto missing link in this chain of events, because it bridges the development from Amyloid Beta pathology to tau pathology. It passes the baton, so to speak.” Thus, deposits of Amyloid Beta activate the inflammasome. As a result, formation of further deposits of Amyloid Beta is promoted. On the other hand, chemical changes occur to the tau proteins resulting into their aggregation.
A Possible Starting Point for Therapies
“Inflammatory processes promote the development of Amyloid Beta pathology, and as we have now been able to show, of tau pathology as well. Thus, the inflammasome plays a key role in Alzheimer’s and other brain diseases,” said Heneka, who is involved in the Bonn-based “ImmunoSensation” cluster of excellence and who also teaches at the University of Massachusetts Medical School. With these findings, the neuroscientist sees opportunities for new treatment methods. “The idea of influencing tau pathology is obvious. Future drugs could tackle exactly this aspect by modulating the immune response. With the development of tau pathology, mental abilities decline more and more. Therefore, if tau pathology could be contained, this would be an important step towards a better therapy.”
Marcus Neitzert – DZNE
The image is in the public domain.
Original Research: Closed access
“NLRP3 inflammasome activation drives tau pathology”. Christina Ising et al.
NLRP3 inflammasome activation drives tau pathology
Alzheimer’s disease is characterized by the accumulation of amyloid-beta in plaques, aggregation of hyperphosphorylated tau in neurofibrillary tangles and neuroinflammation, together resulting in neurodegeneration and cognitive decline1. The NLRP3 inflammasome assembles inside of microglia on activation, leading to increased cleavage and activity of caspase-1 and downstream interleukin-1β release2. Although the NLRP3 inflammasome has been shown to be essential for the development and progression of amyloid-beta pathology in mice3, the precise effect on tau pathology remains unknown. Here we show that loss of NLRP3 inflammasome function reduced tau hyperphosphorylation and aggregation by regulating tau kinases and phosphatases. Tau activated the NLRP3 inflammasome and intracerebral injection of fibrillar amyloid-beta-containing brain homogenates induced tau pathology in an NLRP3-dependent manner. These data identify an important role of microglia and NLRP3 inflammasome activation in the pathogenesis of tauopathies and support the amyloid-cascade hypothesis in Alzheimer’s disease, demonstrating that neurofibrillary tangles develop downstream of amyloid-beta-induced microglial activation.