Glioblastoma-Associated Microglia Reprogramming Is Mediated by Functional Transfer of Extracellular miR-21

Abstract

Gliomas are primary, diffusely infiltrating brain tumors. Microglia are innate immune cells in the CNS and make up a substantial portion of the tumor mass. Glioma cells shape their microenvironment, communicating with and reprogramming surrounding cells, resulting in enhanced angiogenesis, immune suppression, and remodeling of the extracellular matrix. Glioma cells communicate with microglia, in part by releasing extracellular vesicles (EVs). Mouse glioma cells stably expressing a palmitoylated GFP to label EVs were implanted intracranially into syngeneic miR-21-null mice. Here, we demonstrate functional delivery of miR-21, regulating specific downstream mRNA targets in microglia after uptake of tumor-derived EVs. These findings attest to EV-dependent microRNA delivery as studied in an in vivo-based model and provide insight into the reprograming of microglial cells by tumor cells to create a favorable microenvironment for cancer progression.

Figure 1.. miR-21 Is Abundantly Present in GL261 Tumor Cells and Isolated EVs

(A) GL261 cells were transduced to stably express palmitoylated GFP (palmGFP; lower left panel) as a membrane marker and the H2B.mRFP (upper right panel) as a nuclear marker that co-localized with DAPI (lower right panel). Scale bar, 50 μm. (B) Schematic overview of EV isolation using differential centrifugation. Pellets acquired after first round of ultracentrifugation were concentrated by second round of ultracentrifugation to obtain purer population of EVs. (C) Western blot demonstrates vesicle markers (ALIX and Flotillin-1) excluding TSG101, enriched in EV lysates. GFP was detected in all lysates (equal protein amount loaded). (D) Expression level of miR-21 analyzed using qRT-PCR, as plotted in Ct value normalized to spike-in (UniSp6), shows similar levels of miR-21 in cells and 2000 × g fraction. (E) Expression level of miR-21 analyzed using qRT-PCR, as plotted in Ct value normalized to spike-in (UniSp6), shows higher levels of miR-21 in EVs compared with cells. Data represent three independent experiments and are presented as the mean with SEM (error bars). p < 0.05, unpaired t test.

Figure 2.. miR-21 Is Transferred to Microglia after Uptake of Tumor-Derived EVs

(A) Schematic illustration of experimental setup using C57BL/6.miR-21-null mice implanted with GL261.palmGFP.H2B.mRFP glioma cells that release palmGFP fluorescent EVs. Brains were harvested 21 days after implantation. Tissue was dissociated using enzymatic and mechanic digestion and microglia sorted on the basis of cell markers and EV-GFP uptake. (B) Representative FACS plots showing gating strategy in which RFP expression is used to exclude tumor cells in downstream analysis, and subsequently microglia were identified as CD11b^high/CD45^med cells (blue gate). Microglia were then sorted on the basis of the GFP signal, detected as the upper limit in control (no tumor). (C) In mice implanted with GL261.palmGFP.H2B.mRFP, similar analysis (as in B) revealed a population of GFP-positive microglia (green gate in the microglia plot). (D) Uptake of EV-GFP results in the elevated levels of miR-21 in microglia, compared with control microglia (blue), with Ct > 40 considered baseline. (E) EV-GFP uptake visualized by imaging flow cytometry using ImageStream. Five representative cells presented showing EV-GFP co-localized with anti-GFP Alexa Fluor 647 within the contours of microglia as show by membrane marker CD11B and CD45. Scale bar, 10 μm. Data represent four independent experiments and are presented as the mean with SEM (error bars). *p < 0.05 and **p < 0.01, one-way ANOVA with Tukey’s multiple-comparisons test.

Figure 3.. miR-21 Is Transferred to Monocytes and/or Macrophages after Uptake of Tumor-Derived EVs

(A) Monocytes and/or macrophages (MOs and/or macrophages) from miR-21-null mice were identified by FACS as CD11b^high/CD45^high cells (magenta gate). MOs and/or macrophages were then sorted on the basis of the GFP signal detected as the upper limit in control (no tumor). (B) In mice implanted with GL261.palmGFP.H2B.mRFP, a population of GFP-positive MOs and/or macrophages was identified (green gate in the GFP/RFP plot). (C) Uptake of EV-GFP resulted in the presence of miR-21 in MOs and/or macrophages. (D) EV-GFP uptake was visualized by imaging flow cytometry using ImageStream. Scale bar, 10 μm. Data represent three independent experiments and are presented as the mean with SEM (error bars). One-way ANOVA with Tukey’s multiple-comparisons test.

Figure 4.. miR-21 Downregulates Target mRNAs in Tumor-Associated Microglia

(A) In unsupervised clustering analysis, the top 750 most differentially expressed genes microglia clustered on the basis of tumor status and EV-GFP uptake status. (B) MA-plot shows 441 significantly upregulated and 359 downregulated genes (plotted in red) when comparing EV-GFP^pos with EV-GFP^neg microglia. (C) Heatmap shows relative gene expression for 59 validated miR-21 gene targets. Bold gene names indicate genes with p-adj < 0.05 in differential expression analysis EV-GFP^pos versus EV-GFP^neg microglia.

Figure 5.. miR-21 Regulates Selected Target Genes In Vitro in Primary Neonatal Microglia

(A) In comparison with scrambled control, transfection of miR-21-mimic in primary neonatal miR-21-null mice microglia results in significant downregulation of miR-21 target genes (Btg2, Nfat5, and Pcdc4) normalized to β-Actin. (B) Transfection of miR-21-mimic in primary neonatal miR-21-null mice microglia did not affect a control gene Gaphd (not targeted by miR-21). (C) Schematic overview of EV isolation using differential centrifugation. EVs from conditioned media cultured with GL261 cells (EV) and unconditioned media (UcM) were subjected to differential centrifugation. Collected EV pellet was resuspended in PBS and added to miR-21-null microglia. (D) EVs from conditioned media (EV) and unconditioned media (UcM) were isolated and added to miR-21-null primary microglia followed by 24 h incubation. Increased levels of miR-21 were detected in microglia exposed to GL261-derived EV, compared with control media, with Ct > 40 considered baseline. (E and F) Fold expression of miR-21 target genes (Bmpr2, Btg2, Kbtbd2, Nfat5, Pdcd4, Pten, Rhob, Smad7, and Ski) (E) and (F) Gapdh, a gene not targeted by miR-21, normalized to β-Actin in miR-21-null microglia exposed to GL261-derived EVs, compared with UcM. Data represent three independent experiments and are presented as the mean with SEM (error bars). **p < 0.01 and ***p < 0.001, unpaired t test and multiple t test.

Figure 6.. EV-GFP Uptake after EV Intracranial Injection

(A and B) PBS (A) or GL261.palmGFP.H2B.mRFP (B) EVs were injected intracranially. After 16 h brains were dissociated and microglia were sorted on the basis of EV-GFP uptake. (C) Uptake of EV-GFP resulted in elevated levels of miR-21 in microglia; levels in EV-GFP^neg microglia were not detectable (n.d.), with Ct > 40 considered baseline. (D) Gene expression analyzed using ddPCR showing that Pdcd4 and Btg2 expression is reduced after EV-GFP uptake. Data represent three independent experiments and are presented as the mean with SEM (error bars). *p < 0.05, multiple t test.

Figure 7.. Btg2 Downregulation Leads to Increased Microglia Proliferation

(A) Normalized read count of genes involved in microglia proliferation (Hammond et al., 2019). (B) Representative images of in vitro culture miR-21-null microglia transfected with miR-21 mimic and control showing DAPI and Ki67 staining 24 h after transfection. Scale bar, 100 μm. (C) Quantification of proliferation 24 h after transfection with miR-21 mimic or control as measured by Ki67-positive cells per total number of DAPI-positive cells. (D) Representative images of in vitro culture miR-21-null microglia transfected with siRNA against Btg2 and siRNA control showing DAPI and Ki67 staining 24 h after transfection. Scale bar, 100 μm. (E) Quantification of proliferation 24 h after transfection with siRNA against Btg2 or control as measured by Ki67-positive cells per total number of DAPI-positive cells. Data represent three independent experiments and are presented as the mean with SEM (error bars). ***p < 0.001, multiple testing adjusted p value differential expression in (A), and ***p < 0.001, unpaired t test in (C), (E), and (F).