Targeted delivery of siRNA to macrophages for anti-inflammatory treatment

Abstract

Inflammation mediated by tumor necrosis factor-alpha (TNF-alpha) and the associated neuronal apoptosis characterizes a number of neurologic disorders. Macrophages and microglial cells are believed to be the major source of TNF-alpha in the central nervous system (CNS). Here, we show that suppression of TNF-alpha by targeted delivery of small interfering RNA (siRNA) to macrophage/microglial cells dramatically reduces lipopolysaccharide (LPS)-induced neuroinflammation and neuronal apoptosis in vivo. Because macrophage/microglia express the nicotinic acetylcholine receptor (AchR) on their surface, we used a short AchR-binding peptide derived from the rabies virus glycoprotein (RVG) as a targeting ligand. This peptide was fused to nona-D-arginine residues (RVG-9dR) to enable siRNA binding. RVG-9dR was able to deliver siRNA to induce gene silencing in macrophages and microglia cells from wild type, but not AchR-deficient mice, confirming targeting specificity. Treatment with anti-TNF-alpha siRNA complexed to RVG-9dR achieved efficient silencing of LPS-induced TNF-alpha production by primary macrophages and microglia cells in vitro. Moreover, intravenous injection with RVG-9dR-complexed siRNA in mice reduced the LPS-induced TNF-alpha levels in blood as well as in the brain, leading to a significant reduction in neuronal apoptosis. These results demonstrate that RVG-9dR provides a tool for siRNA delivery to macrophages and microglia and that suppression of TNF-alpha can potentially be used to suppress neuroinflammation in vivo.

Figure 1

RVG-9dR peptide allows delivery of siRNA to primary splenic macrophages in vitro. (a) AchR expression in CD11b gated primary macrophages from the spleens of wild type (WT) and AchR knockout (KO) mice was tested using flow cytometry. Filled histogram, isotype control; thick open histogram, wild-type mice; thin open histogram, AchR knockout mice. (b) Spleen cells from wild type and AchR knockout mice were incubated with a FITC-labeled RVG peptide (RVG-FITC) or the control scrambled RVG-FITC peptide along with CD11b antibody for 1 hour, then analyzed by flow cytometry. FITC positivity on CD11b gated cells is shown. Filled histograms represent cells without peptide treatment. (c) Spleen from wild type and AchR knockout mice were treated with FITC-labeled siRNA complexed with RVG-9dR or control RV-MAT-9dR peptides. Twenty-four hours later, cells were stained with CD11b antibody and analyzed by flow cytometry. FITC positivity on CD11b gated cells is shown. Gray histograms represent cells without siRNA treatment. Each figure is representative of at least three experiments with similar results. AchR, acetylcholine receptor; FITC, fluorescein isothiocyanate; RVG, rabies virus glycoprotein; siRNA, small interfering RNA.

Figure 2

RVG-9dR-mediated in vivo siRNA delivery in macrophages and microglial cells. (a) Mice were i.v. injected with RVG-FITC and FITC uptake by CD11b⁺ cells in the spleen, peripheral blood, and brain determined by flow cytometry 1 hour after injection. Representative histograms (upper panel) and cumulative data (lower panel) from two independent experiments with three mice each are shown. Mean values were normalized to control. Gray, wild-type mice without RVG-FITC injection; black, wild-type mice injected with RVG-FITC; red, AchR knockout mice injected with RVG-FITC. Error bars indicate SD; *P < 0.05. (b,c) Mice were i.v. injected with cyclophilin B siRNA (siCyPB) complexed with RVG-9dR and CD11b⁺ macrophages and microglia cells were immunomagnetically isolated from the spleen and brain 24 and 48 hours after siRNA treatment and tested for the presence of (b) specific siRNA and (c) cyclophilin B gene silencing by qRT-PCR. Mean values were normalized to U6B snRNA in b and to β-actin and expressed as percentage of no siRNA control in c. ND, not detected. Error bars indicate SD; *P < 0.05. AchR, acetylcholine receptor; FITC, fluorescein isothiocyanate; qRT-PCR, quantitative reverse transcription-PCR; RVG, rabies virus glycoprotein; siRNA, small interfering RNA.

Figure 3

LPS-induced neuronal cell death in mice brain is TNF-α dependent. (a) LPS-induced production of TNF-α was measured by ELISA in the serum of TNF-α knockout (KO) and wild-type (WT) mice 1 hour and 24 hours after administration of LPS (5 mg/kg, i.p.). Open bars, 1 hour; solid bars, 24 hours. Error bars indicate SD. N = 3. (b) Representative images of TUNEL staining of brain tissue sections from TNF-α knockout and wild-type mice 24 hours after LPS injection. PBS was injected as a control. DAPI (blue), TUNEL (red). n = 3. DAPI, 4′,6-diamidino-2-phenylindole; ELISA, enzyme-linked immunosorbent assay; i.p., intraperitoneal; LPS, lipopolysaccharide; PBS, phosphate-buffered saline; TNF-α, tumor necrosis factor-α TUNEL, terminal dUTP nick-end labeling.

Figure 4

In vitro and in vivo TNF-α silencing using RVG-9dR/siTNF. (a) N9 and (b) Raw 264.7 cells were transduced with siTNF or siLuc complexed with RVG-9dR peptide followed by LPS treatment (5 µg/ml) 24 hours after transduction. TNF-α mRNA levels in cells (upper panels) and secreted TNF-α protein in culture medium (lower panels) were monitored 10 hours after LPS treatment. Error bars indicate SD. N = 3. (c) Mice were i.v. injected with siTNF or siLuc complexed to RVG-9dR peptide followed by LPS (5 mg/kg, i.p.) 24 hours later. TNF-α mRNA levels in microglia and macrophages isolated 1 hour (upper panel) and 24 hours (lower panel) after LPS injection were tested by qRT-PCR. Error bars indicate SD. N = 3. (d) CD86 expression in microglia in the siRNA treated mice 24 hours after LPS injection. Far left histogram: isotype control, far right histogram: siLuc-treated, Middle histogram: siTNF-treated (e) Splenic macrophages and microglia cells isolated from siTNF/RVG-9dR treated mice were tested for the physical presence of siRNA by qRT-PCR. i.p., intraperitoneal; i.v., intravenous; LPS, lipopolysaccharide; ND, not detected; qRT-PCR, quantitative reverse transcription-PCR; RVG, rabies virus glycoprotein; TNF-α, tumor necrosis factor-α.

Figure 5

RVG-9dR-mediated delivery of siTNF protects mice from LPS-induced neuronal apoptosis. Mice were injected i.v. with siLuc or siTNF complexed to RVG-9dR and 24 hours later administered LPS. Twenty-four hours after LPS treatment, neuronal apoptosis was monitored by staining of brain sections with Nissl1 and cleaved caspase-3 antibody. Representative staining in untreated and siRNA treated mice is shown in a and cumulative data from six mice in two independent experiments is shown in b. In (a), red staining indicates Nissl1 and green staining indicates cleaved caspase B staining. In (b), cleaved caspase-3 positive apoptotic neurons were counted from fluorescence microscope images. (c,d) Twenty-four hours after LPS injection, TNF-α mRNA levels were measured from isolated brain cells by (c) qRT-PCR and TNF-α protein levels in brain homogenates tested by ELISA. (d) Error bars indicate SD. ELISA, enzyme-linked immunosorbent assay; LPS, lipopolysaccharide; qRT-PCR, quantitative reverse transcription-PCR; RVG, rabies virus glycoprotein; TNF-α, tumor necrosis factor-α.