Endosomes acidification

FIG. 15 Cellular entry and intracellular kinetics of the cationic lipid-DNA complexes. Cationic lipid-DOPE liposomes form electrostatic complexes with DNA, and, in this case, also transferrin (Tf) is incorporated. Cellular uptake by endoc5dosis and endosomal acidification can be blocked with cytochaiasin B and bafilomycin Aj, respectively. DNA is proposed to be released at the level of endosomal wall after fusion of the carrier lipids with endosomal bilayer. This process is facilitated by the formation of inverted hexagonal DOPE phase as illustrated in the lower corner on the right. After its release to the C5doplasm DNA may enter the nucleus. (From Ref. 253. By permission of Nature Publishing Group.)... 

Sipe, D.M., Jesurum, A., and Murphy, R.F. (1991) Absence of Na+, K+-ATPase regulation of endosomal acidification in K562 erythroleukemia cells./. Biol. Chem. 266, 3469. 

In addition to extracellular degradation in tissues, endosomal acidification might also trigger PEG-lipid cleavage. We showed that despite the presence of the PEG, which slightly reduces lipoplex internalization into the cells, DNA transfection level almost reaches the level of the cationic lipo-plex (31). Cholesterol PEG incorporation into lipoplexes not only reduces lipoplex internalization, but also inhibits the transfection efficiency. 

The transition from coated vesicle to early endosome is accompanied by acidification of the vesicular lumen that continues into the late endosomal and lysosomal compartments, reaching a final pH in the perinuclear lysosome of approximately 4.5. Such acidification associated with endosome maturation provides the means by which certain viruses gain access to the cytosol. Acid-induced conformational changes in the viral proteins trigger translocation across the endosomal membrane via a fusion process. By taking advantage of the endosomal acidification, pH-sensitive liposomes, adenovirus and endosomolytic peptides have been used to facilitate the release of plasmids into the cytoplasm prior to lysosomal degradation. 

TfR-mediated endocytosis is a well-known uptake system Tf binds one or two Fe atoms, but only diferric Tf (Fe2Tf) has a high affinity for TfR to be taken up by the receptor-mediated endocytosis. This system uses a mobilization pathway that involves endosomal acidification, reduction of ferric Fe, and ferrous Fe transport [8]. Recently, it was clarified that divalent cation/ metal ion transporter (DCT1) or Nramp2 involves iron transport from the endosome to the cytosol [9, 10]. Al resembles Fe in chemical characteristics ionic radius, charge density, and coordination number [11]. Therefore, Al binds with Tf to form di—Al—Tf. Al bound to Tf even passes through the blood-brain barrier to enter the neuronal cells via Tf receptor-mediated endocytosis [12]. 

The redox system does not depend on endosomal acidification but needs TfR. Fe2Tf first binds to TfR which is located in close proximity to the proton-and electron-pumping NADHiTf oxidoreductase. The Fe—Tf bond is destabilized by proton efflux, making Fe3+ susceptible to reduction. Fe2+ is trapped by a plasma membrane binder and can be transported by a translocator [4]. As Al is a simple trivalent cation incapable of redox changes, it may be theoretically impossible that Al bound to Tf is taken up by a redox mechanism. Actually, no reports on a redox-mediated process of Al bound to Tf have been made. 

In order to evidence the influence of endosome acidification on transfection, experiments were also performed in the presence of bafilomycin Al, an inhibitor of endosome acidification (20). 

Kielian, M. C. Marsh, M. Helenius, A. Kinetics of endosome acidification detected by mutant and wild-type Semliki Forest virus. EMBO J. 1986, 5, 3103-3109. 

Yamashiro, D. J. Maxfield, E. R. Kinetics of endosome acidification in mutant and wUd-type Chinese hamster ovary cells. J. CeU Biol. 1987, 105, 2713-2721. 

Becker, S., Reinehr, R., Grether-Beck, S., Eberle, A., and Haussinger, D., Hydrophobic bile salts trigger ceramide formation through endosomal acidification, Biol Chem, 388 (2007) 185-196. 

Diaz, R., and Stahl, P. D. (1989). Digitonin permeabilization procedures for the study of endosome acidification and function. Methods Cell Biol. 31, 25-43. 

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