Project Summary/Abstract Low Density Lipoprotein Receptor-related Protein-1 (LRP1) functions as an endocytic and cell-signaling receptor for the fibrinolysis protease, tissue-type plasminogen activator (tPA). Binding of tPA to LRP1 in macrophages generates an anti-inflammatory response in which the activity of multiple Toll-like Receptors is attenuated. This response requires LRP1 co-receptors, including the NMDA Receptor, which is essential. Because LRP1 has numerous structurally and functionally diverse ligands, it is extremely important to understand whether different ligands generate distinct signaling responses. The microtubule-associated protein, TAU, is a recently identified LRP1 ligand which accumulates in the CNS in various forms of neurodegeneration and is a major driver of the pathophysiology observed in Alzheimer's Disease (AD). Using cultured macrophages, we have shown that TAU elicits responses that are distinct from those elicited from tPA; in fact, TAU functions as an LRP1-dependent pro- inflammatory factor. Neuro-inflammation is highly important in AD and we hypothesize that interaction of TAU with microglial LRP1 in the brain is a major driver of microglial activation, neuro-inflammation, and AD pro- gression. Mechanistically, we have evidence that TAU activates pathways that release LRP1 from the cell surface, converting the anti-inflammatory membrane-anchored receptor into a highly pro-inflammatory soluble derivative (shed LRP1). The major goal of this supplement to parent grant R01 HL136395 is to test our hypo- thesis that the receptor system under study in the parent grant is subjugated by TAU in the CNS to drive neuro- inflammation in AD. Two specific aims are proposed. In Specific Aim 1, we will characterize the interaction of TAU with LRP1 in cultured microglia and test the hypothesis that this interaction stimulates LRP1 shedding, which activates microglia and promotes inflammation. Although our previous work suggests that the activities of LRP1 ligands are conserved in macrophages and microglia, it is imperative that we test this hypothesis directly in microglia. The effects of TAU on LRP1 shedding, microglial physiology, cell-signaling, and inflammatory mediator expression will be considered. Microglia will be isolated from adult conditional gene knock-out mice to confirm the role of LRP1 and test whether co-receptors are involved. In Specific Aim 2, we will study the effects of microglial LRP1 on neuro-inflammation and the pathophysiology that develops in transgenic mice that express the P301S mutant of human TAU. These mice develop spontaneous TAU aggregates that are eventually lethal and neuro-inflammation plays a central role in the pathogenesis of disease. Neutralization of LRP1 expression in microglia in these mice will be accomplished by breeding with mice in which Lrp1 is deleted conditionally under the control of promoter systems active in microglia. These studies represent an important extension of R01 HL1363...