Review: experimental manipulations of microglia in mouse models of Alzheimer's pathology: activation reduces amyloid but hastens tau pathology

Abstract

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.

Figure 1

Effects of M1 or M2 cytokine cocktails on amyloid deposition in 22 mo old APP mice. Methods. APP Tg2576 mice on a mixed genetic background from our breeding colony [11] aged 22 mo were injected (unilaterally) with 2 μl of either a) cytokine cocktail [CKT-1: M1 cocktail containing TNF-α (333ng), IL-12 (13ng) and IL-1β (13ng) total mass, with b) CKT-2: M2 cytokine cocktail containing IL-4 (400ng) and IL-13 (120ng total mass), or with c) vehicle control (phosphate-buffered saline). Mice were injected into the right anterior cortex using convection enhanced delivery as described previously [114]. Five days post cytokine injection, mice were intracardially perfused with 25 ml of 0.9% saline. The brain was removed, immersed in 4% paraformaldehyde in 100 mM PO4 buffer (pH 7.4) for 24 hours. The tissue was cryoprotected in a series of 10%, 20% and 30% sucrose solutions. Horizontal sections were cut at 25 μm using a sliding microtome and processed for floating section immunocytochemistry as described previously [115]. Primary antibodies used were rat anti-mouse CD45, rat anti-CD68 (AbD Serotec, Raleigh, NC), rabbit anti-mouse chitinase 3-like-3 (YM1; StemCell Technologies, Vancouver, Canada), rabbit anti-Abeta (Total anti serum, gifted by Paul Gottschall), rabbit anti-Abeta 40, rabbit anti-Abeta 42 (Biosource/Invitrogen, Grand Island, NY). Immunoreaction product in the anterior cortex was quantified as described previously [115]. Sample size was 7–8 per group. Panel A indicates the region quantified by image analysis in the horizontal brain sections. Panels B, D and F show immunostaining using a polyclonal anti-Aß antibody and panels C, E and G show immunostaining for an antibody specific for Aß ending at position 42. Rows specify the treatment used. Panels H and I show the quantification of the Aß staining for total and Aß42 respectively, measured as the ratio of the injected to the uninjected side. Data presented are mean ± sem. * indicate P < 0.05 compared to vehicle treated mice. ** indicates P < 0.01.

Figure 2

Microglial activation after injection of M1 or M2 cocktails in 22mo old APP mice. Methods as in Fig 1. Panels A, C, and E are stained for CD-68. Panels B, D and F are stained for YM1. Rows specify the treatment conditions. Values are mean ± sem of the immunostained area for each marker in panels G and H.