TREM2 IN MICROGLIA BIOLOGY AND ALZHEIMER'S DISEASE

PROJECT SUMMARY Alzheimer's disease (AD) is the most common form of late-onset dementia, affecting more than 5.5 million Americans. Currently, there are no approved therapies that can halt or reverse AD. AD is initiated by amyloid beta (Aβ) peptides that form extracellular aggregates; these promote intraneuronal tau hyperphosphorylation and aggregation, which subsequently lead to neuronal death. These lesions elicit a secondary response by microglia, which undergo prominent changes manifest as a transcriptional signature known as disease- associated microglia (DAM). Genetic studies of human AD have suggested that microglia modulate disease course. Most notably, heterozygous hypomorphic variants of the microglial receptor TREM2 increase the risk of AD several fold. Supported by this grant, our mechanistic studies in various mouse models demonstrated that microglia require TREM2 to acquire the DAM profile and respond to AD pathology. Given these advances in our understanding of how TREM2 is relevant to microglial response, it is important to understand the TREM2 signaling pathway in detail and determine whether it can be exploited for AD therapy. In our recent snRNA-seq study of brain specimens from AD patients with and without TREM2 risk variants, we found that human microglia upregulated the expression of genes integral to the DAM signature together with “homeostatic” genes and the transcription factor IRF8. This signature echoed that of microglia in a mouse model of peripheral nerve injury that reflects a response to neuronal death and is driven by IRF8. We validated IRF8 expression in human AD microglia and demonstrated that IRF8 expression is reduced in patients carrying the TREM2 AD risk variant. Furthermore, in epigenetic studies of mouse microglia we have found that TREM2 promotes transcriptional activation of target genes through IRF8. Specific Aim 1 will test the hypothesis that IRF8 plays a key role in driving the microglial response to AD pathology downstream of TREM2 using mouse models that combine Aβ and tau aggregation with neuronal death and lack or overexpress Irf8 in microglia. In a second recent study, we investigated the effects of an agonistic anti-human TREM2 mAb in mice that accumulate Aβ and express human TREM2 variants rather than endogenous TREM2. A single injection of anti-human TREM2 mAb was sufficient to expand metabolically active and proliferating microglia. Moreover, prolonged mAb treatment partially reduced the neurotoxicity of Aβ plaques, although it did not change the Aβ load and only mildy modified behavior. Specific Aim 2 will test the hypothesis that changes in the dosing or initiation of mAb treatment can moderate the Aβ load and improve cognitive behavior through appropriate modulation of the TREM2-IRF8 axis. Since TREM2 gene dosage has been proposed to control the beneficial or detrimental effects of TREM2 on tau pathology, Specific Aim 3 will test the hypothesis that a mouse model of tau may benefit from appropriate mAb engagement of TREM2. Overall, this proposal will provide mechanistic insights into AD pathogenesis and validate TREM2 as promising target for AD prevention and/or therapy.