Inflammation and microglial activation are associated with Alzheimer's disease (AD) pathology. Somewhat surprisingly, injection of a prototypical inflammatory agent, lipopolysaccharide (LPS) into brains of amyloid precursor protein (APP) transgenic mice clears some of the pre-existing amyloid deposits. It is less well understood how brain inflammation modulates tau pathology in the absence of Aβ. These studies examined the role of LPS-induced inflammation on tau pathology. We used transgenic rTg4510 mice, which express the P301L mutation (4R0N TauP301L) and initiate tau pathology between 3-5 months of age. First, we found an age-dependent increase in several markers of microglial activation as these rTg4510 mice aged and tau tangles accumulated. LPS injections into the frontal cortex and hippocampus induced significant activation of CD45 and arginase 1 in rTg4510 and non-transgenic mice. In addition, activation of YM1 by LPS was exaggerated in transgenic mice relative to non-transgenic animals. Expression of Ser199/202 and phospho-tau Ser396 was increased in rTg4510 mice that received LPS compared to vehicle injections. However, the numbers of silver-positive neurons, implying presence of more pre- and mature tangles, was not significantly affected by LPS administration. These data suggest that inflammatory stimuli can facilitate tau phosphorylation. Coupled with prior results demonstrating clearance of Aβ by similar LPS injections, these results suggest that brain inflammation may have opposing effects on amyloid and tau pathology, possibly explaining the failures (to date) of anti-inflammatory therapies in AD patients.

The role of microglia/ macrophages during neuroinflammation and neurodegenerative diseases remains controversial. To date, at least two activations states have been suggested consisting of a classical response (M1) and the alternative response (M2). Identifying selective biomarkers of microglia that representative their functional activation states may help elucidate disease course and understand repair mechanisms. Two cocktails containing either TNF-α, IL-12, and IL-1β (referred to as CKT-1) or IL-13 and IL-4 (referred to CKT-2) were injections into the hippocampus of mice aged 6, 12, or 24 months. Microarray analysis was performed on hippocampal tissue 3 days post injection. Gene transcripts were compared between CKT-1 versus CKT-2 stimulator cocktails. Several selective transcripts expressed for the CKT-1 included CXCL13, haptoglobin, MARCO, and calgranulin B, while a smaller subset of genes was selectively induced by the CKT-2 and consisted of FIZZ1, IGF-1, and EAR 11. Importantly, selective transcripts were induced at all ages by CKT-1, whereas selective gene transcripts induced by CKT-2 decreased with age suggesting an age-related reduction in the IL-4/ IL-13 signaling pathway.

The inflammation hypothesis of Alzheimer’s pathogenesis has directed much scientific effort towards ameliorating this disease. The development of mouse models of amyloid deposition permitted direct tests of the proposal that amyloid-activated microglia could cause neurodegeneration in vivo. Many approaches to manipulating microglial activation have been applied to these mouse models, and are the subject of this review. In general, these results do not support a direct neuricidal action of microglia in mouse amyloid models under any activation state. Some of the manipulations cause both a reduction in pathology, and a reduction in microglial activation. However, at least for agents like ibuprofen, this outcome may result from a direct action on amyloid production, and a reduction in the microglial provoking amyloid deposits, rather than from reduced microglial activation leading to a decline in amyloid deposition. Instead, a surprising number of the experimental manipulations which increase microglial activation lead to enhanced clearance of the amyloid deposits. Both the literature and new data presented here suggest that either classical or alternative activation of microglia can lead to enhanced amyloid clearance. However, a limited number of studies comparing the same treatments in amyloid-depositing vs tau-depositing mice find the opposite effects. Treatments that benefit amyloid pathology accelerate tau pathology. This observation argues strongly that potential treatments be tested for impact on both amyloid and tau pathology before consideration of testing in humans.