Fig 2: Working model: stroke activates microglia inflammatory response via Hmgb2. Stroke induces the expression of Hmgb2 in microglia and in turn causes microglia proliferation and morphoslogical changes. Hmgb2 in the nucleus of microglia binds to a promoter region of Ctss and activates Ctss transcription, resulting in the expression and secretion of Ctss in the extracellular space. Ctss is a cycteine protease and breaks blood brain barrier, leading to the entrance of blood cells into the brain, which causes the secondary brain infarction.

Fig 3: Inhibition of Hmgb2 protects against stroke damages. (A) the experimental schedules (top) and representative images (bottom) show MRI imaging from sham and stroke mice without (control) or with the expression of Hmgb2-SI or Hmgb2-I. (B) the representative images show FJ labeling from sham and stroke mice without (control) or with the expression of Hmgb2-SI or Hmgb2-I. (C) the infarction size (mean ± SEM, n = 9 mice per group, F(3, 32) = 129, ns = no significantly differences, ***p < 0.0001; BF ANOVA) and the number of FJ+ cells (mean ± SEM, n = 9 mice per group, F(3, 32) = 134.5, ns = no significantly differences, ***p < 0.0001; BF ANOVA) in the individual mice (circles) and the averages per group (triangles) are plotted. (D) inhibition of Hmgb2 improves the neurological functions of stroke mice. The neurological scores (N.S) of the individual mice (circles) and the averages per group (triangles) are plotted. Data are mean ± SEM (n = 9 mice per group, F(3, 32) = 23.9, ns = no significantly differences, ***p < 0.0001; BF ANOVA).

Fig 4: Hmgb2 mediates microglia pro-inflammatory response in stroke. (A) representative images show the expression of Hmgb2-SI (red, SI-tdT) and Hmgb2-I (red, I-tdT) in the cortex at 18 days after the injection of the AAV-PHP.eB-Hmgb2-SI/tdT or the AAV-PHP.eB-DIO-Hmgb2-I/tdT virus particles into the tail vein of the Cx3cr1-Cre mice. The sections are stained with anti-Iba1 (green). (B) the numbers and the soma size of the Iba1-labeted cells in the indivudal mice (circles) and their averages per group (triangles)at 18 days after the injection of AAV are plotted (ns = no significantly differences). (C) Hmgb2-I inhibits the Hmgb2 expression in microglia. The microglia lysates are prepared from mice expressing Hmgb2-SI or Hmgb2-I at 1, 2, or 3 days after operation with sham or stroke and blotted with anti-Hmgb2 or anti-α-tubulin, as indicated. (D) Relative expression (R.E) levels (defined by normalizing the band intensities of anti-Hmgb2 blots to the respective α-tubulin) in the individual mice (circles) and their averages per group (triangles) are plotted. Data are mean ± SEM (n = 5 mice per group, ns = no significantly differences, *p = 0.040; **p = 0.003; ***p < 0.0001 between Hmgb2-SI and Hmgb2-I, t-tests). (E) AAV-PHP.eB -DIO-Hmgb2-SI/tdT virus (green symbols) or AAV-PHP.eB -DIO-Hmgb2-I/tdT (blue symbols) virus particles or saline (pink symbols) were injected into the tail vein of the Cx3cr1-Cre mice. 18 days after the injection, mice were operated with sham or stroke. 24 or 72 hours after the operation, the ratios of IL-1, Cscl-16, Ctss, IL-6 and TNF-a in the extracellular space of stroke mice versus sham mice (stroke/Sham) are analyzed and plotted by the individual mice (circles) and their averages per group (triangles). Data are mean ± SEM (n = 5 mice per group, **p = 0.0041, ***p < 0.0001 between Hmgb2-SI and Hmgb2-I, t-tests). (F) the ratios of IL-10 in the extracellular space of stroke mice versus sham mice (stroke/Sham) are analyzed and plotted by the individual mice (circles) and their averages per group (triangles). Data are mean ± SEM (n = 5 mice per group, F (3,16) = 9.506, ns = no significantly differences, **p = 0.0038, 0.0011, 0.0054, BF ANOVA).

Fig 5: Validation of HMGA1, HMGA2, HMGB1 and HMGB2 by western blot analysis. The expression of HMGA1 and HMGA2 in the ET group was higher compared with the LPS group at 4 h after high-dose LPS stimulation), while HMGB1 and HMGB2 exhibited the opposite expression trend under the same conditions. LPS, lipopolysaccharide; ET, endotoxin tolerance; HMG, high mobility group.