Tau aggregation, the formation of solid deposits of altered tau protein called neurofibrillary tangles, is thought to be the most damaging of the processes underlying Alzheimer's disease. The earlier accumulation of amyloid-β only sets the stage for the later accumulation of altered tau. When looking at why protein aggregates such as amyloid-β and tau accumulate only in later life, one of many candidate mechanisms is the decline of autophagy that takes place with aging. Autophagy is the name given to a collection of cellular maintenance processes responsible for clearing out damaged structures and other unwanted waste, such as protein aggregates. A range of interventions shown to slow aging in laboratory species involve raised levels of autophagy: if cells are more aggressively maintained, there is less of a chance for damage and dysfunction in cellular processes to spread and cause further harm. The other side of the coin is that lower levels of autophagy mean more metabolic waste, more damaged components, and more downstream consequences.
Early in the course of Alzheimer's disease, neurons in the brain become clogged with toxic tau proteins that impair and eventually kill the neurons. A new study found that tau accumulates in certain types of neurons, probably because the cellular housekeeping system of autophagy is less effective in these neurons. Researchers have long known that neurodegenerative diseases like Alzheimer's affect some neurons but not others, even leaving neighboring neurons unharmed. But the reasons for this selectivity have been difficult to identify.
The new study was only possible because of new techniques that allow researchers to probe individual cells in the brain. Researchers detected signs that the components of a cellular cleaning system were less abundant in the neurons that accumulate tau proteins. To confirm the connection between the cleaning system and tau buildup, the researchers manipulated BAG3, a regulatory protein in autophagy, in mouse neurons. When the researchers decreased BAG3 levels in mouse neurons, tau piled up. But when BAG3 expression was enhanced, the neurons were able to rid themselves of excess tau.
The researchers have tantalizing, still unpublished data that the same housekeeping deficiencies found in vulnerable neurons occur with aging, which might explain the link between advanced age and Alzheimer's disease. "If we can develop therapies to support these natural defense mechanisms and stop tau from accumulating, then we might be able to prevent, or at least slow, the development of Alzheimer's and other tau-related neurodegenerative diseases."