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WIREs Nanomed Nanobiotechnol
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Insight into the siRNA transmembrane delivery—From cholesterol conjugating to tagging

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Abstract Small interfering RNA (siRNA), combining the features of unprecedented potency, target‐specificity, and the unique sequence‐based disease‐intervention model, has received immense considerations over the past decades in the academia and pharmaceutical industry. siRNA fits the criteria of being drug‐likely enough to meet with the therapeutic purpose, but its clinical translation has been impeded for a long time by the poor efficiency of in vivo delivery. To reach the cytosol where the RNA interference (RNAi) takes place, siRNA delivery faces a serial of systemic and cellular barriers, especially the endosomal sequestration that would prevent the majority of siRNA from cytosol entry. Transmembrane delivery of siRNA represents a new avenue for efficient delivery by bypassing the endosomal pathway. This rationale is bolstered by the high efficiency of viral entry by membrane fusion, but rarely pursued by artificial siRNA delivery systems. Here, this article provides an opinion of transmembrane delivery by hydrophobic modulation of siRNA. We give a brief introduction of the current siRNA delivery modes, including the hydrophobic cholesterol siRNA conjugates. The cholesterol tagging technology is design on the rationale of hydrophobic siRNAs approach, but hydrophobic modulation throughout the whole siRNA backbone for efficient membrane fusion and transmembrane delivery. The challenge and potential of this technology for preclinical development are also discussed. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies Biology‐Inspired Nanomaterials > Lipid‐Based Structures Biology‐Inspired Nanomaterials > Nucleic Acid‐Based Structures
The comparison of small interfering RNA (siRNA) delivery modes. (a) siRNA is unable to cross the cell membrane due to the repulsive force from the hydrophobic and anionic lipid bilayer. (b) The cationic siRNA nanoparticle is uptake by cells through the endocytic pathway, but most of the cargoes are degraded during lysosome maturation. (c) Some viruses are able to directly fuse their membrane with cell membrane without being internalized by endocytosis. (d) Chol‐siRNA inserts its cholesterol tail into membrane and triggers the endocytosis. (e) The tagged siRNA is able to fuse with cell membrane because of the hydrophobic modulation throughout the siRNA surface. The tagged siRNA eventually slips into the cytosol and leave the tags on the cell membrane
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The cholesterol tagging technology for cytosol delivery of small interfering RNA (siRNA). (a) The tagging technology decorates siRNA with six copies of cholesterol molecules by chelation. It eventually transforms the hydrophilic siRNA into membrane compatible and permeable. (b) The chemical structure of tags. (c) The amphiphilic chol‐siRNA fails to fuse into cell membrane after it inserts its cholesterol tail into lipid bilayer, because the hydrophobic interior of lipid bilayer is repulsive to the hydrophilic and polyanionic siRNA block (upper panel). Membrane fusion of the tagged siRNA is initiated by the insertion of its first cholesterol tail into cell membrane. Other tails outside of membrane provide a driving force to pull siRNA into cells because of the attraction of these cholesterol tails to lipid bilayer (bottom panel). The driving force disappears until the whole siRNA is slipping into the cytosol
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Hydrophobic modification of small interfering RNA (siRNA). (a) Structures of chol‐siRNA (upper panel) and sd‐rxRNA (bottom panel). (b) siRNA conjugated with α‐tocopherol (left), palmitic acid (middle) and PC docosahexaenoic acid (right). (c) siRNA prodrug by bioreversible modification on phosphate groups of siRNA backbone. (Reprinted with permission from Meade et al. (). Copyright 2014 Nature Publishing Group) (d) The phosphate groups of siRNA are unmasked by thioesterase in the cytoplasm
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Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures
Biology-Inspired Nanomaterials > Lipid-Based Structures
Therapeutic Approaches and Drug Discovery > Emerging Technologies
Nanotechnology Approaches to Biology > Nanoscale Systems in Biology

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