Surface of liposomes

Covalent attachment of proteins to the surface of liposomal bilayers is done through reactive sites created on the head groups of phospholipids with the intermediary use of a crosslinker or other activating agent. The lipid functional groups described in Section 1 of this chapter are modified according to the methods discussed in Section 2 to be reactive toward specific target... 

Coupling of Peptides to the Surface of Liposomes—Application to Liposome-Based Synthetic Vaccines... 

Numerous methods have been developed for attaching ligands to the surface of liposomes for reviews, see Refs. (3,6 ). For peptides, they fall into two major categories (i) covalent coupling of the peptides to preformed liposomes that contain functionalized hydrophobic anchors such as, e.g., derivatives of phos-phatidylethanolamine (PE) or (ii) incorporation of lipopeptides, obtained by conjugation of peptides to hydrophobic anchors (fatty acids, phospholipids), into liposomes either during the preparation of the vesicles or by postinsertion into preformed vesicles. In this section, we will briefly discuss these techniques and focus on the ones we have been using in our own work. 

A frequently used strategy to couple peptides to the surface of liposomes consists in the use hydrophobic/amphipathic anchors that are functionalized with maleimide or bromoacetyl groups, i.e., thiol-reactive functions, which give by reaction with HS-peptides very stable thioether linkages. These functions are conveniently introduced into hydrophobic anchors such as phospholipids, e.g., PE (9,10), the adjuvant PamsCAG (11) or cholesterol... 

Amide and carbamate bonds. Unprotected peptides can be attached to the surface of liposomes by engaging, for example, their N-terminus into an amide... 

Because conformational epitopes are not easily mimicked with linear peptides, which can elicit nonspecific antibodies, several alternative strategies such as synthetic cyclic peptides have been developed [see e.g., (18)]. A similar conformational restriction was seemingly achieved with a P-amyloid peptide that was anchored to the surface of liposomes via hydrophobic tails introduced at its both N- and C-termini. The reconstituted peptide proved highly immunogenic and elicited antibodies that could significantly prevent amyloid plaque formation in a model system (70). 

Torchilin VP, et al. TAT peptide on the surface of liposomes affords their efficient intracellular delivery even at low temperature and in the presence of metabolic inhibitors. Proc Natl Acad Sci USA 2001 98 8786. 

Torchilin VP, Goldmacher VS, Smirnov VN. Comperative study on covalent and non-covalent immobilization of enzymes on the surface of liposomes. Biochem Biophys Res Commun 1978 85 983-990. 

This cooperative effect of several ligands at the surface of a liposome was recently demonstrated by the inhibition of agglutination of erythrocytes (natural vesicles covered with sialic acids) with the influenza virus by means of sialyl gangliosides. The lowest concentration of sialyl derivatives required for the inhibition was found to be 10 pM in solution whereas it was 20 nM at the surface of a vesicle. Actually, arrays of sialic acid at the surface of liposomes were found to be moderately more effective than sialic acid groups linked to a soluble polymer but as good or better than the best known naturd inhibitors of hemagglutination (i.e. mucins and macroglobulins) [150]. 

Also the surface of liposomes can be utilized for facilitating chemical reactions. One example is the liposome-aided synthesis of peptides already mentioned in Chapter 4 (Blocher et al., 2000 2001). In this case, the binding of hydrophobic... 

The formation of a coating layer on the surface of liposomes was confirmed and detected by measuring the zeta potential. As shown in Fig. 10.2, the zeta potential of liposomes was changed by increasing the concentration of polymers, because the surface charge of liposomes was neutralized by the opposite charge of the coating polymer. 

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