Endosome Escape

DNA vectors entering the cell via endocytosis must escape the membrane-bound vesicle prior to gene uptake by the nucleus. Methods to enhance endosome escape often lead to significant improvements in transfection efficiency, thereby highlighting this barrier as one of the crucial bottlenecks in the gene delivery process for some cell type/gene carrier combinations. One method of endosome escape involves DNA vector fusion to the endosomal lipid bilayer, leading to the release of its cargo into the cytoplasm. This mechanism has been hypothesized for lipid-based delivery systems [185,186].

Another possible method of endosome escape is the "proton-sponge" hypothesis [74]. Nonviral DNA vectors that are able to accept protons at physiological pHs, such as polyethylenimine (PEI), act as a buffering agent within endosomes. As a result, more protons are pumped into the endosome, accompanied by an influx of chloride ions (to maintain the appropriate membrane potential) and water (due to osmotic pressure), which may lead to endosome rupture.

Lysosomotropic agents, such as chloroquine and sucrose, have been used to enhance DNA/vector release into the cytoplasm in vitro [187]. Viral DNA vectors have evolved the ability to escape acidified endosomes. To mimic this property, replication-defective adenoviruses were administered with transferrin-polylysine DNA vectors, resulting in greater gene expression [188,189]. More recently, endosomolytic structures have been attached to DNA vectors in lieu of using whole viruses. HA-2, the fusogenic peptide of the influenza virus hemagglutinin, has been used with transferrin-polylysine [190] and Lipo-fectamineĀ® [191] to enhance the escape of these DNA vectors from endosomes. Similarly, melittin, the component in bee sting venom that destabilizes membranes, has been conjugated to PEI and shown to improve endosome escape [192].

Synthetic polymers, such as poly(propylacrylic acid), have also been designed with the ability to disrupt membranes [193-195]. These polymers are pH-sensitive and were designed to destabilize membranes when within acidic environments such as endosomes.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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