Endosome escape

Fig. 1 Bioresponsive polyplexes. (a) Systemic circulation of shielded polyplexes in blood stream and attachment to cell surface receptor (b) endocytosis into endosomes, deshielding by cleavage of PEG linkers and activation of membrane-destabilizing component by acidic pH or other means (c) endosomal escape into cytosol (d) siRNA transfer to form a cytosolic RNA-induced silencing complex complex (e) cytosolic migration and intranuclear import of pDNA (/) presentation of pDNA in accessible form to the transcription machinery...

Shen H, Ackerman AL, Cody V et al (2006) Enhanced and prolonged cross-presentation following endosomal escape of exogenous antigens encapsulated in biodegradable nanoparticles. Immunology 117 78-88... 

Yessine MA, Leroux JC (2004) Membrane-destabilizing polyanions interaction with lipid bilayers and endosomal escape of biomacromolecules. Adv Drug Deliv Rev 56 999-1021... 

Some attempts have been made to rationally increase the efficiency of endosomal escape. One such avenue entails the incorporation of selected hydrophobic (viral) peptides into the gene delivery systems. Many viruses naturally enter animal cells via receptor-mediated endocytosis. These viruses have evolved efficient means of endosomal escape, usually relying upon membrane-disrupting peptides derived from the viral coat proteins. 

Figure 1 Potential points for the enhancement of liposome-mediated gene transfer. The above diagram illustrates the characteristic lipofection pathway demonstrating the four key stages bold, underlined), complex formation, targeting and internalization, endosomal escape, and nuclear translocation. Indicated alongside (italic) are the peptides that can be used to augment the transfection potential of the liposome. Abbreviation pDNA, plasmid DNA.

The inhibition of endosomal degradation or enhancement of endosomal escape by liposomes is an established strategy to enhance lipid-mediated transfection (Fig. 3). 

Figure 3 Endosomal escape assisted by fusogenic peptides. These peptides assist the release of DNA from the endosome, avoiding degradative damage from the binding with the lysosome.

Varkouhi AK, Scholte M, Storm G, Haisma HJ (2011) Endosomal escape pathways for delivery of biologicals. J Control Release 151 220-228... 

Modification of depolymerization kinetics and release Endosomal escape Nonviral gene delivery Boron neutron capture therapy Fusogenic liposomes, increase transfection efficiency... 

Electroporation, 294, 389, 397 Endosomal escape, 337 Endosomal release EBV, Epstein-Barr virus, 10 EBNA-1, 11... 

For lamellar CL-DNA complexes, endosomal escape via activated fusion limits TE and strongly depends on aM, whereas the inverted hexagonal phase promotes... 

The results described above show that adding cholesterol and certain analogs increases TE more than the resulting increase in membrane charge density would predict. Previous work has demonstrated that CL-DNA complexes at low aM transfect poorly due to inefficient endosomal escape (which involves fusion) [21, 25]. Thus, our findings suggest that cholesterol and certain analogs facilitate fusion of the membranes of the complex and the endosome, independent of their effect on aM. A possible explanation for this is the overall reduction of the hydration repulsion layer of the membrane. 

In summary, our findings suggest that cholesterol and certain analogs are a highly valuable neutral lipid component ( helper lipid ) for CL-DNA complexes because they facilitate endosomal escape by reducing the repulsive hydration and protrusion forces. They are thus able to lower the kinetic barrier for fusion of the cationic membranes of CL-DNA complexes with the anionic membrane of the endosome and increase TE, in addition to their beneficial effect on aM. 

Similar to cell attachment, endosomal escape of simple lamellar complexes is a process driven by electrostatics [21, 25, 83] (see Sect. 2) and therefore inhibited by PEGylation. A strategy to recover efficient endosomal escape is to prepare PEG-lipids in which the PEG chains are attached via bonds that are quickly cleaved as the endosomal pH is lowered in the course of maturation. This practically converts the shielded complex back into an unshielded complex. Several acid-labile moieties have been investigated for similar purposes, e.g., hydrazones [92], vinyl ethers [93] and orthoesters [87, 94]. 

Bioresponsive Polyplex Shielding and Endosomal Escape 3.1 Reversible Polyplex Shielding... 

Fig. 2 Masked endosomolytic agents for pH-triggered endosomal escape. The polymers designed by Meyer et al. [69] (a) and Rozema et al. [71] (b) contain endosomolytic compounds whose lytic potential is activated by endosomal cleavage of masking groups coupled by acid-sensitive linkages. Furthermore, disulfide bonds are embedded which release the nucleic acid after endosomal escape by cleavage in the reducing cytosolic environment...

Oliveira S, Fretz MM, Hogset A et al (2007) Photochemical internalization enhances silencing of epidermal growth factor receptor through improved endosomal escape of siRNA. Biochim Biophys Acta 1768 1211-1217... 

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