Abstract

Microglia are the major innate immune cells in the brain and are essential for maintaining homeostasis in a neuronal microenvironment. Currently, a genetic tool to modify microglial gene expression in specific brain regions is not available. In this report, we introduce a tailor-designed method that uses lipid and polymer hybridized nanoparticles (LPNPs) for the local delivery of small interfering RNAs (siRNAs), allowing the silencing of specific microglial genes in the hypothalamus. Our physical characterization proved that this LPNP-siRNA was uniform and stable. We demonstrated that, due to their natural phagocytic behavior, microglial cells are the dominant cell type taking up these LPNPs in the hypothalamus of rats. We then tested the silencing efficiency of LPNPs carrying a cluster of differentiation molecule 11b (CD11b) or Toll-like receptor 4 (TLR4) siRNA using different in vivo and in vitro approaches. In cultured microglial cells treated with LPNP-CD11b siRNA or LPNP-TLR4 siRNA, we found a silencing efficiency at protein expression levels of 65 or 77%, respectively. In line with this finding, immunohistochemistry and western blotting results from in vivo experiments showed that LPNP-CD11b siRNA significantly inhibited microglial CD11b protein expression in the hypothalamus. Furthermore, following lipopolysaccharide (LPS) stimulation of cultured microglial cells, gene expression of the TLR4 downstream signaling component myeloid differentiation factor 88 and its associated cytokines was significantly inhibited in LPNP-TLR4 siRNA-treated microglial cells compared with cells treated with LPNP-scrambled siRNA. Finally, after LPNP-TLR4 siRNA injection into the rat hypothalamus, we observed a significant reduction in microglial activation in response to LPS compared with the control rats injected with LPNP-scrambled siRNA. Our results indicate that LPNP-siRNA is a promising tool to manipulate microglial activity locally in the brain and may serve as a prophylactic approach to prevent microglial dysfunction-associated diseases.

Highlights

  • Microglia are long-surviving and self-renewing innate immune cells in the central nervous system

  • The negatively charged cluster of differentiation molecule 11b (CD11b) small interfering RNAs (siRNAs), Toll-like receptor 4 (TLR4) siRNA, or scrambled control siRNA was absorbed onto the surface of the NPs

  • We found that CD11b immunoreactivity in the microglial cells surrounding the injection spot was downregulated by 90% compared with the microglial cells in lipid and polymer hybridized nanoparticles (LPNPs)-scrambled siRNAinjected rats (Figure 4E−G)

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Summary

Introduction

Microglia are long-surviving and self-renewing innate immune cells in the central nervous system. The major functions of microglia are clearing debris and invading pathogens to maintain a healthy microenvironment that enables adequate neuronal activity.[1−5] Recent genome-wide analyses have revealed that microglia show a distinctive and brain regiondependent transcriptional identity with clear differences in bioenergetic and immune-regulatory pathways.[6] Current approaches available for genetic manipulation of microglia largely depend on the inducible Cre−loxP recombination system, in which the Cre recombinase gene is mostly driven by the monocyte promoter Cx3cr[1]. Viral approaches that have proven to be effective in manipulating genes in neurons and astrocytes have substantial difficulties with transducing microglial cells, likely due to the natural immune protection behavior of microglial cells.[14] far, an approach enabling brain region-specific modulation of microglial activity is not available

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