Interleukin-1 mediates pathological effects of microglia on tau phosphorylation and on synaptophysin synthesis in cortical neurons through a p38-MAPK pathway.

The presence of tangles of abnormally phosphorylated tau is a characteristic of Alzheimer's disease (AD), and the loss of synapses correlates with the degree of dementia. In addition, the overexpression of interleukin-1 (IL-1) has been implicated in tangle formation in AD. As a direct test of the requirement for IL-1 in tau phosphorylation and synaptophysin expression, IL-1 actions in neuron-microglia cocultures were manipulated. Activation of microglia with secreted beta-amyloid precursor protein or lipopolysaccharide elevated their expression of IL-1alpha, IL-1beta, and tumor necrosis factor alpha (TNFalpha) mRNA. When such activated microglia were placed in coculture with primary neocortical neurons, a significant increase in the phosphorylation of neuronal tau was accompanied by a decline in synaptophysin levels. Similar effects were evoked by treatment of neurons with recombinant IL-1beta. IL-1 receptor antagonist (IL-1ra) as well as anti-IL-1beta antibody attenuated the influence of activated microglia on neuronal tau and synaptophysin, but anti-TNFalpha antibody was ineffective. Some effects of microglial activation on neurons appear to be mediated by activation of p38 mitogen-activated protein kinase (p38-MAPK), because activated microglia stimulated p38-MAPK phosphorylation in neurons, and an inhibitor of p38-MAPK reversed the influence of IL-1beta on tau phosphorylation and synaptophysin levels. Our results, together with previous observations, suggest that activated microglia may contribute to neurofibrillary pathology in AD through their production of IL-1, activation of neuronal p38-MAPK, and resultant changes in neuronal cytoskeletal and synaptic elements.