Small interfering RNAs, or siRNA, are a novel approach in RNA-based therapies, offering a new way to deliver gene silencing drugs to cells that are very specific and effective. molecular biology and medicine – siRNA can turn genes off by causing their messenger RNAs (mRNAs) to be destroyed. SiRNA-based therapies have a great deal of promise, but poor circulation stability, cell loss and immune system activation have prevented their clinical development. To combat these barriers, scientists have looked to cell membrane encapsulation to increase the pharmacokinetics and therapeutic activity of siRNA. The method embeds siRNA molecules in cell membrane vesicles made from cell membranes to make new delivery media that use the inherent characteristics of the cell membrane to optimise drug delivery. Cell membrane-encapsulated siRNA is based on the hypothesis that the lipid bilayer, as a natural part of living cells, can be significantly better than conventional synthetic carriers. Encapsulation of siRNA in cell membrane vesicles – exosomes or cell-derived liposomes – protects the RNA from degradation by nucleases and enzymatic reaction, which is a typical problem for siRNA treatments. And even more significantly, cell-based membrane vesicles often have cell-specific marker and protein on their surface to enable targeted localisation in the tissues or cells. This organic targeting ability makes siRNA treatments more selective and less off-target. Furthermore, because such vesicles can co-opt the target cell and eject the payload without inducing a significant immune response, cell membrane-encapsulated siRNA can be a useful alternative to the classical drug delivery route. This could be controlled release and very low toxicity further making this approach desirable for a range of therapeutic uses such as cancer, genetic disorders and viral infection. The strongest point of cell membrane-encapsulated siRNA is that it leverages the very structure of cell membranes to get siRNA to where it needs to go. Every cell type – from cancer cells to immune cells – has its own surface proteins and receptors that can be directed at by the enclosing cell membrane vesicle. Tumor cell membrane proteins, for example, can be used to design siRNA-rich vesicles that cancer cells recognise and then the siRNA gets transported directly to the tumour. Such selective therapy not only increases therapeutic concentration of siRNA in the target site, but it also reduces systemic exposure and the risk of unwanted side effects. Moreover, cell membrane-encapsulated siRNA was found to promote the endocytosis process so that the siRNA payload could be better internalised. This extracellular accumulation is key to gene silencing because the siRNA has to get into the cytoplasm to meet the RNA-induced silencing complex (RISC) and catalyse gene silencing. These biocompatibility and flexibility make cell membrane carriers a promising candidate for RNA therapeutics, that can change the course of treatments for many different illnesses.
Figure 1. Characterization of siRNA-loaded tMNV. (Ishiguro K, et al.; 2020)
The different ways that cell membrane-encapsulated siRNA works provide various different advantages for drug delivery. For one thing, trapping siRNA in cell membrane vesicles stabilizes it when released into the bloodstream. Free siRNA is very vulnerable to ribonuclease degraded in the extracellular environment and so becomes rapidly ineffective. If researchers can package siRNA in cell membrane vesicles, they can offer a barrier against enzymatic degradation that gives the RNA a longer half-life and a better chance of making it to the target cells intact. Its cell membrane encapsulation also makes the siRNA resistant to detection and clearance by the immune system, as most synthetic delivery systems are prone to be. They could construct the vesicles with erythrocytes, dendritic cells, even cancer cells using their own natural characteristics to suppress immune response and delay clearing by the MPS. One of the biggest advantages of cell membrane-encapsulated siRNA is targeting efficiency. Surface of cell membranes is dense with receptors and proteins of a specific cell type, thus forming an inbuilt target. For instance, membrane vesicles from cancer cells can go after cancer cells that have receptors and deliver the siRNA directly to the tumour. This selective targeting is important for reducing off-target effects, which is a big problem with most gene therapies, and boosting the therapeutic index of siRNA treatments. Furthermore, cell membrane vesicles can be modified even further by tailoring the membrane to contain ligands or antibodies targeting receptors on target cells' surface. This targeting precision fine-tuning results in the targeted siRNA getting infused into the target tissue or organ and so increasing the therapeutic potency of the treatment as a whole. Cellular uptake efficiency is the other big strength of cell membrane-encapsulated siRNA. Cell membranes are actually wired to join with the other cell membranes, an integral part of cell-to-cell signalling and transport. Such fusion is activated when cell membrane-based vesicles are used to transport siRNA into target cells. When the vesicles get to the target cell, they can bond to the cell membrane and transport the siRNA in its encapsulated form into the cytoplasm where it can bind to the RNA-induced silencing complex (RISC) and kill the target gene. This precise molecule delivery is critical for the optimal therapeutic effects of siRNA therapies. Additionally, cell membrane-encapsulated siRNA can promote endocytosis — when cells take in extracellular matter. Having cell-specific marks on the vesicle surface allows the target cell to identify it, which further boosts the internalization rate and makes sure the siRNA gets to the right cellular location.
While cell membrane-encapsulated siRNA is a great way to deliver drugs, there are a few obstacles to overcome before this technology can be fully scaled up for clinical use. There's a great difficulty in producing these vesicles in sufficient quantities and with the same quality. Isolation of cell membranes and maintaining their function – receptor-targeting and fusion capacity, for example – can be difficult and costly. Additionally, the efficiency of siRNA's encapsulation within these vesicles should be optimised so that sufficient siRNA gets to the target cells. There are currently membrane encapsulation methods such as sonication, extrusion or electroporation that could be improved to be more reproducible and scaleable for mass production. The immunogenicity threat is another. Even though membrane-bound vesicles of cells are normally biocompatible, proteins and lipids that coat the surface might be seen by the immune system, especially if the membrane elements are from non-human cells or if the siRNA inside the vesicles causes an immune response. This risk can be reduced by using methods like PEGylation or human membranes to decrease immune activation and make the delivery vehicle biocompatible. It's also important to consider the stability of the siRNA in storage and circulation, since RNA molecules are unstable and degrade over time. Enhancing the performance of these delivery systems will require advancements in encapsulation technologies and more stable siRNA formulations. Future possibilities with cell membrane-encapsulated siRNA for gene therapy and personalized medicine are staggering. Nanotechnology, membrane engineering and RNA engineering will bring even more efficient and specific forms of delivery. We will be able to load siRNA into cell membrane vesicles that target specific cancerous cells or tissues, and use this to cure everything from genetic diseases to cancers, viral infections to neurological disorders. And, by using siRNA in conjunction with other treatments – for example, chemotherapy or immune modulators – in the same delivery system, we may even be able to maximize the therapeutic benefit by targeting multiple aspects of a disease at once. At the end of the day, cell membrane-encapsulated siRNA therapies will only be successful if they continue to be optimized for their production, targeted activity and long-term stability and safety in vivo.
Alternate Names:
siRNA-Loaded Exosome-Mimetic Nanoparticles
siRNA-Encapsulated Exosome-Mimetic Nanovesicles
siRNA-Loaded EMNs
Exosome-Mimetic siRNA Nanovesicles
Exosome-Like siRNA Nanoparticles
Exosome-Mimetic siRNA Delivery Vesicles
siRNA-Exosome-Mimicking Nanoparticles
siRNA Encapsulated Exosome-Mimetic Vesicles
siRNA-Mimicking Nanovesicles
References:
1. Ishiguro K, et al. Targeting Liver Cancer Stem Cells Using Engineered Biological Nanoparticles for the Treatment of Hepatocellular Cancer. Hepatol Commun. 2020, 4(2):298-313.
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