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Depletion of Arg1-Positive Microglia/Macrophages Exacerbates Cerebral Ischemic Damage by Facilitating the Inflammatory Response - PubMed

  • ️Sat Jan 01 2022

Depletion of Arg1-Positive Microglia/Macrophages Exacerbates Cerebral Ischemic Damage by Facilitating the Inflammatory Response

Ting Li et al. Int J Mol Sci. 2022.

Abstract

Stroke is a serious worldwide disease that causes death and disability, more than 80% of which is ischemic stroke. The expression of arginase 1 (Arg1), a key player in regulating nitrogen homeostasis, is altered in the peripheral circulation after stroke. Growing evidence indicates that ischemic stroke also induces upregulated Arg1 expression in the central nervous system, especially in activated microglia and macrophages. This implies that Arg1 may affect stroke progression by modulating the cerebral immune response. To investigate the effect of Arg1+ microglia/macrophages on ischemic stroke, we selectively eliminated cerebral Arg1+ microglia/macrophages by mannosylated clodronate liposomes (MCLs) and investigated their effects on behavior, neurological deficits, and inflammatory responses in mice after ischemic stroke. More than half of Arg1+ cells, mainly Arg1+ microglia/macrophages, were depleted after MCLs administration, resulting in a significant deterioration of motility in mice. After the elimination of Arg1+ microglia/macrophages, the infarct volume expanded and neuronal degenerative lesions intensified. Meanwhile, the absence of Arg1+ microglia/macrophages significantly increased the production of pro-inflammatory cytokines and suppressed the expression of anti-inflammatory factors, thus profoundly altering the immune microenvironment at the lesion site. Taken together, our data demonstrate that depletion of Arg1+ microglia/macrophages exacerbates neuronal damage by facilitating the inflammatory response, leading to more severe ischemic injury. These results suggest that Arg1+ microglia/macrophages, as a subpopulation regulating inflammation, is beneficial in controlling the development of ischemia and promoting recovery from injury. Regulation of Arg1 expression on microglia/macrophages at the right time may be a potential target for the treatment of ischemic brain injury.

Keywords: Arg1; depletion; inflammation; ischemia; microglia/macrophages.

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Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Figure 1
Figure 1

Expression of Arg1+ cells within the first week after acute ischemic stroke. (A) Distribution of Arg1+ cells in the ischemic zone at different days after ischemic stroke. (B) The density of Arg1+ cells at different days after stroke onset (n = 3, F(3, 8) = 116.7, ** p < 0.01).

Figure 2
Figure 2

MCLs selectively deplete Arg1+ microglia/macrophages four days after ischemic stroke. (A) Timeline of drug administration and tissue processing (lightning bolt: ischemic stroke; syringe: intracerebroventricular injection of MLs or MCLs). (B) Representative confocal images showing the distribution of Arg1+ cells in brain slices administrated with MLs or MCLs after stroke. (C) The density of Arg1+ cells after MLs or MCLs treatment. MCLs depleted more than half of Arg1+ cells after stroke (n = 3, ** p < 0.01). (D) Representative confocal images showing Arg1+ microglia/macrophages at the lesion site administrated with MLs after stroke. (E) Representative confocal images showing Arg1+ microglia/macrophages at the lesion site administrated with MCLs after stroke. (F) The density of microglia/macrophages in the accumulation zone after MLs or MCLs treatment (n = 3, *p < 0.05). (G) The width of microglia/macrophages’ accumulation zone after MLs or MCLs treatment. MCLs caused a significant reduction in activated microglia/macrophages (n = 3, * p < 0.05). (H) The percentage of Arg1+ microglia/macrophages after MLs or MCLs treatment. MCLs caused a decrease in Arg1+ microglia/macrophages (MG/MP, microglia/macrophages; n = 3, ** p < 0.01).

Figure 3
Figure 3

Effects of depleting Arg1+ microglia/macrophages on behavioral performance and neuronal deficit in mice 4 days after ischemic stroke. (A–C) Depletion of Arg1+ microglia/macrophages significantly decreased the body weight (A), walking time on the rotarod (B), and forelimb grip force (C) of mice 4 days after stroke. Ischemic stroke itself caused weight loss and worse behavioral performance (n = 3, * p < 0.05, ** p < 0.01). (D) Images of Nissl stained brain slices treated with MLs or MCLs after stroke. (E) Brain infarct volumes 4 days after stroke. Depletion of Arg1+ microglia/macrophages significantly enlarged the infarct size (n = 3, * p < 0.05). (F) Confocal images of Fluoro-Jade C-labeled degenerating neurons in brain slices after MLs or MCLs treatment. (G) Densities of degenerating neurons four days after stroke. Depleting Arg1+ microglia/macrophages significantly exacerbated neurodegeneration (n = 3, * p < 0.05).

Figure 4
Figure 4

The mRNA expression of key inflammatory factors and iNOS+ cells after depletion of Arg1+ microglia/macrophages or not. (A) Relative mRNA levels of anti-inflammatory factors Arg1, IL-10, and TGF-β. The mRNA expression of Arg1 and IL-10 decreased when Arg1+ microglia/macrophages were depleted, while no significant differences were found in TGF-β. However, ischemia itself induced a great increase of inflammatory factors (n = 3). (B) Relative mRNA levels of pro-inflammatory factors iNOS, IL-1β, and TNF. As Arg1+ microglia/macrophages were depleted, the mRNA expression of pro-inflammatory factors significantly elevated four days after stroke. (C) Representative confocal images showing iNOS+ cells in brain slices administrated with MLs or MCLs after stroke. (D) The density of iNOS+ cells after MLs or MCLs treatment. MCLs caused a rise of iNOS+ cells (n = 3, * p < 0.05, ** p < 0.01).

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