Increased brain cell activity boosts brain fluid levels of a protein linked to Alzheimer's disease, according to new research. Tau protein is the main component of neurofibrillary tangles, one of the hallmarks of Alzheimer's disease. It has been linked to other neurodegenerative disorders, including frontotemporal dementia, supranuclear palsy and corticobasal degeneration. The regular patterns of tau spread through brain networks have led scientists to speculate that dysfunctional tau travels to different brain regions via synapses -- the areas where individual nerve cells communicate with each other.