Fusogenic activity

This fusogenic activity of influenza hemagglutinin is frequently exploited in the laboratory. If, for example, the virus is bound to cells at a temperature too low for endocytosis and then the pH of the external medium is lowered, the hemagglutinin causes direct fusion of the viral envelope with the plasma membrane infection is achieved without endocytosis. Similarly, artificial vesicles with hemagglutinin in their membrane and other molecules in their lumen can be caused to fuse with cells by first allowing the vesicles to bind to the plasma membrane via the hemagglutinin and then lowering the pH of the medium. In this way the contents of the vesicles are delivered to the recipient cell s cytoplasm. 

Lescar, J., Roussel, A., Wein, M. W., Navaza, J., Fuller, S. D., Wengler, G., Wengler, G., and Rey, F. A. (2001). The fusion glycoprotein shell of Semliki Forest virus An icosahedral assembly primed for fusogenic activation at endosomal pH. Cell 105, 137-148. 

Figure 24 (a) A proposed new carrier for switching the release of an anticancer drug by early endosomal pH from polymeric micelles and (b) the polymeric micelles for effective anticancer drug delivery to MDR cells with receptor-mediated endocytosis, switching release rate, and fusogenic activity of polymeric micelle components. 

More recent approaches to enhance transfection efficiency use fusogenic peptides to improve the performance. Such peptides help the DNA to escape lysosomic digestion in a similar manner as the capsid proteins do in the case of viruses. A conformational change is induced at low pH, which triggers a fusogenic activity. Typically such peptides contain virus derived amino acid sequences. 

The sensitivity of CL-DNA complex-mediated gene transfer to serum proteins may be due to the degradation of DNA by nucleases contained in the semm, because DNA formed an unstable complex with CL but was not encapsulated in the CL membrane. In contrast, DNA in FL was encapsulated in the membranous stmcture and protected from nucleases. Another possible explanation is that the absorption to the cell surface and/or the fusogenic activity of CL may be interfered by serum lipoproteins. The... 

Bongartz, J.P., Aubertin, A.M., Milhaud, P.G., Lebleu, B. (1994). Improved biological activity of antisense oligonucleotides conjugated to a fusogenic peptide. Nucleic Acids Res., 22, 4681 1688. 

Due to its ability to form inverted hexagonal phase, DOPE is believed to impart fusogenicity to lipoplexes, thus facilitating fusion followed by destabilization of the endosomal membrane, lipoplex escape from the endosomes, and eventually the DNA release. Indeed, inclusion of DOPE into lipoplexes was shown to enhance considerably the transfection activity of some of the cationic lipid carriers [35,120, 121]. For example, formulations of oxypropyl quaternary ammonium cationic lipids with 50 mol% DOPE have been reported to exhibit 2-5 times higher transfection activity in COS7 cells than formulations with pure cationic lipid (Fig. 29) [35]. Recently, a triple-bond dialkynoyl analog of DOPE has been... 

Koynova R, Tarahovsky Y, Wang L et al (2007) Lipoplex formulation of superior efficacy exhibits high surface activity and fusogenicity, and readily releases DNA. Biochim Biophys Acta-Biomembranes 1768 375-386... 

Considerable effort has been dedicated to identifying the fusion-active state of influenza HA. Because the fusion of two opposing membranes to become a single bilayer is a dynamic process, the fusogenic species is most likely to be a transition between discrete structures rather than a single conformation. If such a transition occurs in a manner uncoupled from membrane fusion, the resulting structure would not be fusion active. The... 

The cellular membrane is a hydrophobic barrier that surrounds the cytoplasm of every cell and is involved in complex cellular processes, such as signaling and transport, which are essential to maintain the normal life cycle of a cell major components of this cellular membrane are lipids, proteins, peptides, and carbohydrates. Peptides that interact with cellular membranes are referred to as membrane-active peptides and can be broadly divided into three major classes antimicrobial, cell-penetrating, and fusogenic peptides. Any of these may have variable lengths, hydrophobicities, and secondary structures, but they often exhibit similar effects on membranes. For example, some antimicrobial peptides have cell-penetrating properties, and vice versa [23,24] these peptides usually cause some degree of membrane destabilization. 

Cationic lipids can destabilize a cellular membrane because of its intrinsic detergent property. Therefore, destabilization of endosomal and/or lysosomal membrane may be a contribution from the cationic lipids itself In the same context, it was shown that the cationic lipid/DOPE or cationic lipid/cholesterol liposome formulation exhibit surface anisotropies in terms of increased liposomal surface pH (161,162). The surface pH of the liposomal formulations exhibits at least two pH units higher than the pH of the solution at which they are made. Therefore, a liposomal solution made at physiological pH may in reality exhibit a surface pH > 9, which is detrimental for both the stability of endosome and activity of lysosomal enzymes. Endosomal disruptions were also done with fusogenic peptides, which promote pH-dependent fusion of small liposomes when associated with lipid bilayer. When these peptides were co-delivered with lipid/DNA complex, they imparted formidable endosomal disruption by changing its usual random coil conformation into amphipathic a-helix conformation at lower pH, resulting in consequent cytoplasmic delivery of DNA (163). 

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