The Paradox of Tau and RNA-Binding Proteins: How Adaptive Stress Granule Regulation Becomes Pathological with Aging.

RNA-binding proteins (RBPs) are a large class of proteins that form biological condensates to facilitate their functions. Chronic stress, such as occurs in neurodegenerative diseases, stimulates persistent accumulation of particular RBP condensates as part of the translational stress response, termed stress granules (SGs). These persistent SGs serve as a nidus for aggregation of RBPs to form pathologies that appear in neurodegenerative diseases, such as the occurrence of Tar DNA Binding Protein (TDP-43) in Amyotrophic Lateral Sclerosis. Many of the RBPs that accumulate in SGs are also associated with mutations that are linked to neurodegenerative diseases. The microtubule-associated protein tau is the major intracellular pathology that occurs in Alzheimer's disease. Tau is phosphorylated with stress, whereupon it functions to regulate SG biology; conversely, SGs serve as a crucible for the accumulation of toxic oligomeric tau. The regulation of stress by tau is an inherent part of biology that normally occurs during development and hibernation; however, with aging it becomes pathological, possibly because of the reduced proteostasis associated with aging.