Delayed Galectin-3-Mediated Reprogramming of Microglia After Stroke is Protective.

Galectin-3 (Gal-3), a ß-galactoside-binding lectin, has recently emerged as a molecule with immunoregulatory functions. We investigated the effects of Gal-3 on microglia morphology, migration, and secretory profile under physiological conditions and in the context of ischemic injury. We show that in the control conditions, exposure to recombinant Gal-3 increases microglial ramification and motility in vitro and in vivo via an IL-4-dependent mechanism. Importantly, after stroke, Gal-3 exerted marked immune-modulatory properties. Delivery of Gal-3 at 24 h after middle cerebral artery occlusion (MCAO) was associated with an increase in Ym1-positive microglia and decrease in iNOS. Analysis of cytokine profiles at the protein level revealed downregulation of pro-inflammatory cytokines and a marked upregulation of the anti-inflammatory cytokine, IL-4, 24 h after i.c.v. injection of Gal-3. Importantly, the observed shift in cytokines in microglia was associated with a significant decrease in the infarct size. Taken together, our results suggest that when delivered well after ischemic injury, Gal-3 might fine tune innate immunity and induce a therapeutic shift in microglia polarization.

IL-10 Controls Early Microglial Phenotypes and Disease Onset in ALS Caused by Misfolded Superoxide Dismutase 1.

While reactive microgliosis is a hallmark of advanced stages of amyotrophic lateral sclerosis (ALS), the role of microglial cells in events initiating and/or precipitating disease onset is largely unknown. Here we provide novel in vivo evidence of a distinct adaptive shift in functional microglial phenotypes in preclinical stages of superoxide dismutase 1 (SOD1)-mutant-mediated disease. Using a mouse model for live imaging of microglial activation crossed with SOD1(G93A) and SOD1(G37R) mouse models, we discovered that the preonset phase of SOD1-mediated disease is characterized by development of distinct anti-inflammatory profile and attenuated innate immune/TLR2 responses to lipopolysaccharide (LPS) challenge. This microglial phenotype was associated with a 16-fold overexpression of anti-inflammatory cytokine IL-10 in baseline conditions followed by a 4.5-fold increase following LPS challenge. While infusion of IL-10R blocking antibody, initiated at day 60, caused a significant increase in markers of microglial activation and precipitated clinical onset of disease, a targeted overexpression of IL-10 in microglial cells, delivered via viral vectors expressed under CD11b promoter, significantly delayed disease onset and increased survival of SOD1(G93A) mice. We propose that the high IL-10 levels in resident microglia in early ALS represent a homeostatic and compensatory "adaptive immune escape" mechanism acting as a nonneuronal determinant of clinical onset of disease. Significance statement: We report here for the first time that changing the immune profile of brain microglia may significantly affect clinical onset and duration of disease in ALS models. We discovered that in presymptomatic disease microglial cells overexpress anti-inflammatory cytokine IL-10. Given that IL-10 is major homeostatic cytokine and its production becomes deregulated with aging, this may suggest that the capacity of microglia to adequately produce IL-10 may be compromised in ALS. We show that blocking IL-10 increased inflammation and precipitated clinical disease onset, whereas overexpression of IL-10 in microglia using a gene therapy approach significantly delayed disease onset and increased survival of ALS mice. Based on our results, we propose that targeted overexpression of IL-10 in microglia may have therapeutic potential in ALS.

Galectin-3 is required for resident microglia activation and proliferation in response to ischemic injury.

Growing evidence suggests that galectin-3 is involved in fine tuning of the inflammatory responses at the periphery, however, its role in injured brain is far less clear. Our previous work demonstrated upregulation and coexpression of galectin-3 and IGF-1 in a subset of activated/proliferating microglial cells after stroke. Here, we tested the hypothesis that galectin-3 plays a pivotal role in mediating injury-induced microglial activation and proliferation. By using a galectin-3 knock-out mouse (Gal-3KO), we demonstrated that targeted disruption of the galectin-3 gene significantly alters microglia activation and induces ∼4-fold decrease in microglia proliferation. Defective microglia activation/proliferation was further associated with significant increase in the size of ischemic lesion, ∼2-fold increase in the number of apoptotic neurons, and a marked deregulation of the IGF-1 levels. Next, our results revealed that contrary to WT cells, the Gal3-KO microglia failed to proliferate in response to IGF-1. Moreover, the IGF-1-mediated mitogenic microglia response was reduced by N-glycosylation inhibitor tunicamycine while coimmunoprecipitation experiments revealed galectin-3 binding to IGF-receptor 1 (R1), thus suggesting that interaction of galectin-3 with the N-linked glycans of receptors for growth factors is involved in IGF-R1 signaling. While the canonical IGF-1 signaling pathways were not affected, we observed an overexpression of IL-6 and SOCS3, suggesting an overactivation of JAK/STAT3, a shared signaling pathway for IGF-1/IL-6. Together, our findings suggest that galectin-3 is required for resident microglia activation and proliferation in response to ischemic injury.