Microglia targeted interventions in prodromal Alzheimer's disease stage - Project Summary
Genome-wide association studies for late-onset Alzheimer’s disease (LOAD) found that >70% loci are localized
to genes that are enriched in microglia, the resident immune cells of the brain. The most significant risk variant
for LOAD is apolipoprotein E ε4 (APOE4) and its homozygosity increasing AD risk >15-fold. APOE is
predominantly expressed in astrocytes and significantly upregulated in microglia near amyloid plaques and by
neurodegenerative environments. While microglia protect against development of AD by clearing toxic cellular
debris and compacting amyloid plaques, environmental and genetic factors can cause microglia to enter
persistent reactive states in which they escalate disease via excessive inflammation and neurotoxicity. Given
the role of microglia in AD pathogenesis, microglia-targeted pharmacological and cell therapeutic interventions
may be able to protect against AD progression in selectively vulnerable brain regions such as the entorhinal
cortex and hippocampus. In the APPNL-G-F AD mouse model, both regions show amyloidosis, reactive gliosis,
and synaptic loss in prodromal stages, especially the entorhinal cortex shows electrophysiological impairment
prior to cognitive impairment. Thus, these regions are critical targets for preclinical development of novel
therapeutics for prodromal AD. Acute pharmacological inhibition of colony stimulating factor 1 receptor with the
brain-penetrant drug PLX5622 has been shown to selectively induce “turnover” of microglia (near complete
removal of existing microglia and self-renewal of survived microglia), which rescues abnormal hippocampal
activity, promotes brain repair, and ameliorates cognitive deficits in aging, traumatic brain injury, and maternal
immune activation models. Replacement of AD risk (proinflammatory or APOE4) microglia with APOE3 neutral
or APOE2 protective human iPSC-derived microglia may be a promising microglia-targeted AD therapeutic.
Therefore, we hypothesize that electrophysiological, molecular, and cellular dysfunctions in the entorhinal cortex
and hippocampus at prodromal disease stages is driven by proinflammatory reactive microglia signatures (Aim
1) and APOE4 genotype microglia (Aim 2). It can potentially be treated in adulthood by pharmacologically forced-
turnover of AD microglia (Aim 1) and AD microglia replacement with APOE 33 or 22 microglia-like cells (Aim 3).
In Aim 1, we will demonstrate molecular and functional changes in forced turnover of microglia on the entorhinal
cortex and hippocampus of prodromal AD mice using neuron-like electronic probes during spatial navigation
virtual reality. In Aim 2, we will determine APOE genotype effects of human microglia on vulnerability of the
entorhinal and hippocampal neurons in chimeric human/mouse AD model. In Aim 3, we will establish cell
replacement therapeutics in adulthood by replacing AD microglia to APOE3 or APOE2 cells in adult mice. The
goal of this research is to demonstrate molecular mechanisms by which AD and APOE isoform microglia affect
neuronal network and to determine the efficacy by which microglia-targeted pharmacological and human iPSC-
based therapeutic strategies protect neurons and memory circuits that are selectively vulnerable in AD patients.
