Polyplexes intracellular trafficking

Elouahabi A, Ruysschaert JM. Eormation and intracellular trafficking of lipoplexes and polyplexes. Mol Ther 2005 ll(3) 336-347. 

Thibault M, Nimesh S et al (2010) Intracellular trafficking and decondensation kinetics of chitosan-pDNA polyplexes. Mol Ther 18 1787-1795... 

Fig. 2. Intracellular trafficking of polymer gene delivery vectors. 1, Internalization (often by adsorptive pinocytosis or receptor-mediated endoc3d sis) 2, escape from endocytic vesicles 3, transport through the cytosol 4, transport across the nuclear membrane. In addition, polyplexes must be unpackaged at some point in the process, although it is not known where in the process unpackaging occurs.

Concerning the intracellular trafficking of polyplexes, it begins in early endosomal vesicles. These early endosomes subsequently fuse with sorting endosomes, which in turn transfer their contents to the late endosomes. Late endosomal vesicles are acidified (pH 5-6) by membrane-bound proton-pump ATPases. The normal process is that the endosomal content is then relocated to... 

Scheme 13 Intracellular trafficking of pol5 plexes. The size of a polyplex is genoally a few hundred nanometers (100-200 nm). Reprinted with permission fi-om [100]. Copyright 2012...

Fig. 1 Polymer characteristics for in vivo application, a The polymer should be nontoxic and non-immunogenic, form polyplexes which are stable and inert in blood, protect DNA against extracellular and intracellular degradation, allow transport to the target tissue, specific internalization into the target cell, and intracellular trafficking into the cell nucleus, b To introduce specificity and enhance efficiency, delivery functions may be conjugated to the polymer which facilitate the different steps...

This is in contrast to viruses, where the virus particles also show active transport when present in the cytosol after fusion with the plasma membrane or endosomal membrane [60-62], This is due to the ability of specific proteins of the virus particle to bind motor proteins. Single-particle tracking reveals that the quantitative intracellular transport properties of internalized non-viral gene vectors (e.g., polyplexes) are similar to that of viral vectors (e.g., adenovirus) [63]. Suk et al. showed that over 80% of polyplexes and adenoviruses in neurons are subdiffusive and 11-13% are actively transported. However, their trafficking pathways are substantially different. Polyplexes colocalized with endosomal compartments whereas adenovirus particles quickly escaped endosomes after endocytosis. Nevertheless, both exploit the intracellular transport machinery to be actively transported. 

Once the cell has been targeted, the vector particle has to be internahzed by the cellular uptake machinery via endocytotic or phagocytotic uptake processes. This step can be taken rather easily by polyplexes, as appropriate receptor-binding ligands and/or cationic charges may enhance intracellular uptake of particles into endosomal vesicles. Several intracellular barriers then have to be overcome for successful transgene expression. Endosomal release was found to be a major bottleneck for many non-viral vectors [151,167]. The vector particle needs to survive and escape from the endosomal vesicular compartment, traffick the cytoplasmic environment, target the nucleus, enter the nucleus, and expose the carried nucleic acid to the cellular transcription machinery. 

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