This open access paper is illustrative of present work on the role of microglial dysfunction and chronic inflammation in Alzheimer's disease. The central nervous system immune cells called microglia become inappropriately inflammatory with age. A new consensus on Alzheimer's disease is that initial amyloid-β accumulation causes far greater than usual chronic disarray and inflammatory signaling in the supporting cells of the brain, such as microglia, astrocytes, and oligodendrocytes. This in turn leads to the much more damaging tau aggregation and consequent damage and death of neurons.
Alzheimer's disease (AD) is characterized by typical biochemical lesions (β-amyloid peptide [Aβ] plaques and tau tangles) accompanied by extensive cellular changes (neuronal dystrophic alterations, neuronal cell loss, astrogliosis, and microgliosis). Rare mutations in amyloid precursor protein (APP), presenilin 1 and presenilin 2 trigger Aβ plaque accumulation and are sufficient to induce the full biochemical and morphological signature of AD. While this clearly indicates a major role for Aβ in AD pathology even in these genetic forms, a decades-long asymptomatic phase is present. Thus, in addition to Aβ plaques, other pathological processes, either in response to or in parallel to Aβ accumulation, need activation to cause neurodegenerative disease.
The search for the genetic risk determinants in sporadic AD has highlighted the central role of non-neuronal genes in pathways that do not appear directly related to Aβ metabolism. Most of the genes associated with the ∼40 loci identified by genome-wide association (GWA) analysis or by rare variant sequencing studies are expressed in glial cells. Moreover, analysis of available single-cell transcriptome datasets for human brain cells reported an association between AD GWA signals and microglia as well as astrocytes. Analysis of regulatory networks of genes differentially expressed in AD patients indicates that immune- and microglia-specific gene modules are key contributors to AD pathology.
Thus, genetic and molecular evidence suggest that Aβ accumulation is the trigger of a series of pathogenic processes in which microglia play a central role. No consistent hypothesis, however, links the causality implied by the mutations in the amyloid pathway genes to the genetic risk linking sporadic AD to inflammatory pathways. One possible resolution is that amyloid pathology acts only as a trigger in sporadic AD; i.e., Aβ accumulation is necessary but insufficient to cause full-blown disease. The cellular response, determined by the genetic makeup of the patients, tilts the table from a rather benign Aβ proteopathy to the severe neurodegeneration with inflammation and tau pathology that characterizes AD. In this regard, further understanding of the microglia response to amyloid pathology and the role of risk factors for AD in this response is key.
Here, we set out to address in a systematic way the question of how microglia respond over time, in cortex and hippocampus, to progressive Aβ deposition and whether this is affected by the three major risk factors for AD, i.e., age, sex, and genetics. We use an App knockin mouse model, which displays progressive amyloidosis and microgliosis. We show that the microglial responses to Aβ pathology are complex but, surprisingly, largely reproducible cell states that are also appearing during normal aging, albeit slower and quantitatively more limited. Moreover, we show that microglia in female mice tend to react earlier and in a more pronounced way than microglia in male mice, particularly in older mice. Interestingly, the major response of microglia to amyloid pathology is enriched for AD risk genes, with Apoe expression, in particular, becoming highly upregulated. This is partially confirmed in human tissue.