Relinquishing microglia of a pesky 22-nucleotide microRNA freed the cells to respond more vigorously to amyloidosis, and to protect nearby neurons. That was the upshot of a study published in Nature Neuroscience on June 8, which was spurred by previous findings implicating this particular microRNA—miR155—in microglial function. When the scientists, led by Oleg Butovsky of Brigham and Women’s Hospital in Boston and Tsuneya Ikezu of the Mayo Clinic in Jacksonville, Florida, conditionally deleted miR-155 from microglia in APP/PS1 mice, the cells heightened their responses to IFN-γ. This bolstered phagocytosis, boosted containment of Aβ plaques, and ultimately protected synapses and spared memory loss. Notably, these benefits depended upon the interferon, a cytokine produced primarily by T cells. The findings underscore the complexity—and rich opportunities for therapeutic targets—embedded within the multi-pronged inflammatory response to amyloidosis.
- Sans microRNA-155, microglia respond better to interferon-γ.
- They more readily compact plaques.
- Synapses were spared, and memory preserved, in miR-155 knockouts.
Despite their teeny stature, microRNAs pack a gene expression punch, dousing translation of myriad target genes. Butovsky’s group had previously pegged miR-155 as part of a neurodegenerative microglial (MgND) signature, or what others have called a disease associated (DAM) signature, based on mouse models of amyloidosis and ALS (Sep 2017 news, Feb 2015 news). Knocking out the microRNA from an ALS model locked microglia in a homeostatic state, which proved beneficial in that context (Butovsky et al., 2015). For the new study, co-first authors Zhuoran Yin and Shawn Herron, and their colleagues, used an inducible expression system to conditionally knock out miR-155 from microglia in six-week-old APP/PS1 mice, then analyzed their microglia at four months of age. What they found surprised them, Ikezu said. Instead of being locked in a homeostatic state as in the global miR-155 knockout, microglia in the conditional KO had morphed into a responsive state. They revved up expression of multiple MGnD genes, including ApoE, Axl, Cst7, and Clec7a, as well as a host of interferon response genes. Many cell types throughout the body express miR-155, including immune cells that likely influence microglial states within the brain, and this might explain the discrepancies between the global and conditional knockouts, Ikezu speculated (reviewed in Mahesh and Biswas, 2019).
A finer-grained inspection of these transcriptional effects using single-cell RNA sequencing of brain tissue from APP/PS1 mice with or without microglial miR-155 revealed three major microglial transcriptional subtypes, based on their relative expression of homeostatic and MGnD signature genes: homeostatic, “pre-MGnD,” and MGnD. Looking closer at the pre-MGnD cells, the researchers observed both “early,” and “late” transcriptomes, based on a pseudo timeline between homeostatic and MGnD signatures. In the early phase, microglia retained expression of many homeostatic genes, while ramping up expression of a suite of IFN-response genes, such as Stat1, Isg15, Ifi204, Irf7, Ifit1 and Ifit3. In the late stage, microglia quelched homeostatic genes, continued to express IFN-response genes, and started expressing MGnD genes, including Clec7a, Axl, and ApoE. Finally, MGnD microglia downregulated both homeostatic and IFN-response genes, expressing the full suite of MGnD/DAM signature genes. Ablation of miR-155 led to the proliferation of both early and late pre-MGnD clusters in APP/PS1 mice, while reducing the relative proportion of homeostatic or mature MGnD cells.
In short, the findings suggested that removal of miR-155 shifts microglial into an interferon-responsive state. But how? Yin, Herron, and colleagues found a plausible mechanism, reporting that miR-155 suppressed expression of Stat1, a critical downstream mediator of IFN-γ receptor signaling. Removing this microRNA rendered the cells exquisitely responsive to IFN-γ.
How did this unfettered IFN-γ response affect microglial function? For one, it stoked their appetite for apoptotic neurons injected into the brain. Notably, this boost was not observed in mice treated with an IFN-γ blocking antibody. The conditional knockouts also had a slightly lower A burden compared to APP/PS1 controls at four months of age, and the plaques they did have were more spherical and surrounded by fewer dystrophic neurites (image above). This suggested that without miR-155, microglia were more proficient at compacting plaques, thus lessening damage to nearby neurons. In line with this, the conditional knockouts lost fewer synapses, and performed like wild-type mice on memory tests. Curiously, a previous study led by Michelle Ehrlich and Joel Dudley at the Icahn School of Medicine at Mount Sinai, New York, reported that full, constitutive knockout of miR-155 exacerbated amyloid accumulation in APP/PS1 mice, yet still protected synapses and memory, again suggesting that other cells besides microglia may rely on this micro RNA for controlling amyloid (Readhead et al., 2020).
In all, the findings indicate that at least in the APP/PS1 mouse model, miR-155 holds microglia back from responding too vigorously to IFN-γ. When this inhibition is relieved, IFN-γ rouses the cells into a phagocytic, plaque-compacting state, which attenuates neuronal damage.
In contrast, scientists led by Gwenn Garden of the University of North Carolina at Chapel Hill, found a dark side to the conditional miR-155 KO/APPPS1 model. Yes, microglia contained amyloid, but they also noshed on synapses, triggered neuronal hyperexcitability, and ramped up deadly seizures in the mice (Alois et al., 2023). The authors reported that removal of miR-155 upped microglial appetite for excitatory synapses, which, counterintuitively, can lead to a synaptic imbalance that sparks epilepsy. Garden told Alzforum that the APP/PS1 mice are prone to seizures, and that differences in background strains of mice used in the two studies may have influenced this electrical imbalance. Ikezu agreed, noting that while they did not observe the same increase in overt seizures among their conditional knockouts, they did not perform electrophysiological studies, as Garden did, to look for changes in neuronal activity.
Butovsky and Ikezu think miR-155 could be a potential therapeutic target for AD, particularly since this microRNA has been found to be elevated in the AD brain (Sierksma et al., 2018). Yet, herein lies another complexity. While the researchers found that miR-155 is predominantly expressed by microglia in the mouse brain, it also appears to be expressed by neurons and other cell types in the human brain, suggesting it may have many different effects across cell types. Because microRNAs are difficult to analyze via single-cell RNA sequencing, more work is needed to nail down the expression patterns of miR-155 in the human brain, Ikezu said.
Evgenia Salta of the Netherlands Institute for Neuroscience noted that even within a single cell type, each microRNA exerts its influence over hundreds of target genes, casting the RNA snippets as promising multi-targeted approaches to therapy (comment below). “However, this exact advantage is the same that may—on the flip side—jeopardize microRNA therapeutic applications,” she noted, because the many genes regulated by each microRNA may make them unsuitable for selective therapeutic targeting. (Walgrave et al., 2021).
David Holtzman of Washington University in St. Louis noted that while removal of miR-155 from microglia appeared to benefit mice in the early stages of amyloidosis, the ablation could have different effects in people, depending on disease stage (comment below). “When other changes begin to occur within the AD brain, such as tau pathology that is linked with cognitive decline and neurodegeneration, as well as a different inflammatory environment, the microglial state induced by mIR-155 or IFN-γ may not have the same effects that are associated with neuroprotection as shown here,” he wrote. “Work in other model systems that develop neurodegeneration, such as occurs with tau pathology, may be helpful to sort out when to try to target these pathways during different stages of AD.”—Jessica Shugart
- ApoE and Trem2 Flip a Microglial Switch in Neurodegenerative Disease
- Microglia in Disease: Innocent Bystanders, or Agents of Destruction?
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