MAPT and Alzheimer disease: Research has also shown that accumulated waste products within the brain parenchyma and CSF are attributable to activation of microglia and astrocytes post-injury or within the diseased brain.25 A study examining increased secretion of apolipoprotein E4 (APOE4) in activated astrocytes and microglia found that APOE4 expression on glial cells increased Tau protein concentration, an extracellular protein known to be involved in late-onset Alzheimer's disease (AD).26