Targeted microglial miR-155 inhibition restores homeostatic microglial phenotypes and attenuates vasculopathy in retinas of Alzheimer's models

Abstract

Purpose : Recent single cell RNA sequencing studies identified the neurodegeneration-associated microglial phenotype (MGnD/DAM), typically observed surrounding cerebral amyloid plaques in Alzheimer's disease (AD) murine models. Further analyses revealed that the MGnD phenotype expression was regulated by the TREM2/APOE/microRNA-155 (miR-155) signaling pathway. However, this microglial phenotype has never been investigated in any retinal disease, including in the AD-afflicted retina. Mounting evidence demonstrates that the retina mirrors pathological changes that occur in AD brains. Here, we investigated populations of MGnD and their impact on the retinas of double transgenic APP/PS1 mouse models of AD in response to conditional knock-out of microglial miR-155.

Methods : APP_SWE/PS1_L166P, miR155^fl/fl, and Cx3cr1Cre^ERT2 mice were crossed to generate an Alzheimer-like mice genotype that can be targeted for conditional knock-out (cKO) of miR-155, specifically in microglia. Mice were sacrificed at 4 and 8 months of age. Retinas or whole eyes were extracted for analyses of retinal flat-mounts, cross-sections, protein homogenates (e.g., MSD, western-blot, mass-spectrometry), and isolated retinal vasculature.

Results : Immunostaining of retinal flat-mounts and cross-sections revealed a substantial population of clec7a⁺ and galectin-3⁺ MGnD in retinas from APP/PS1 mice. Targeting microglial miR-155 diminished MGnD and upregulated homeostatic microglial marker P2ry12; these findings were associated with anti-inflammatory cytokine profiles. Global proteome analysis by mass spectrometry identified enhanced PI3K-Akt signaling cascades due to microglial miR-155 inhibition. Importantly, such immune intervention protected tight junction integrity and reduced vascular amyloidosis in retinas of APP/PS1 mice.

Conclusions : Targeting neurodegeneration-associated microglia restored the homeostatic retinal-immune milieu in murine models of AD. Our data uncover more complex microglial phenotypes, distinct from the traditional M0, M1 and M2 microglia, and their tight link to the neurovascular unit integrity. The robust protective effects of microglial miR-155 ablation may shed light onto novel innate immune-based treatments for retinal degeneration and AD.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.