Liposome energy dependence

Several aspects of intracellular trafficking should be kept in mind in the intracellular trafficking section. The first is the dependence of acidification of endosomes on the uptake of liposomes. This aspect is sometimes discussed when analyzing clathrin uptake. However, several other pathways are also in need of acidic compartments as a destination of uptake so, we list this factor as an individual aspect. Other aspects of intracellular trafficking that are of interest are the transport from early endosomes to late endosomes, the dependence of actin filaments and dynamin, and/or microtubules. Furthermore, the energy dependence of liposome uptake is discussed. 

Energy Dependence on Liposome Uptake and Fusion with Cell Membranes... 

Incubation at 4°C (see section Energy Dependence on Liposome Uptake and Fusion with Cell Membranes ) and a block of metabolic activity (see section Metabolic Activity ) might also be used to block endocytosis and to detect cellular association or fusion. 

The exact mechanism responsible for the uptake of liposomes by KCs and SECs is not clear. Most studies confirm internalization of whole liposomes in an energy-dependent phagocytic process in which the liposomes are delivered to the lysosomes. The liposomal lipids are completely degraded and the encapsulated solutes released. Neutral liposomes consisting of lipids such as cholesterol and phosphatidylcholine are probably cleared by receptor-mediated mechanisms, due to the adsorption of opsonizing proteins onto the hpid bi-... 

Already in 1965, Ryser and Hancock provided evidence that histones and polyamino acids could greatly enhance albumin uptake by cultured tumor cells (6). More recently, several polybasic peptides (so-called protein transduction domains, PTDs or cell-penetrating peptides, CPPs) have been shown to efficiently mediate uptake of nucleic acids, bioactive peptides, phage particles, and liposomes into a wide variety of mammalian cells. The initially proposed ability of CPPs to penetrate plasma membranes via a temperature-independent, non-endocytotic pathway was later shown to be a fixation artifact, and it is currently widely accepted that CPP-mediated macromolecular delivery follows energy-dependent endocytotic pathways that in most cases depend on the expression of cell-surface heparan sulfate proteoglycans (HSPGs) (7). 

FIGURE 2.1 Energy of the 0-0 vibrational transition in the principal electronic absorption spectrum of violaxanthin (l Ag-—>1 BU+), recorded in different organic solvents, versus the polarizability term, dependent on the refraction index of the solvent (n). The dashed line corresponds to the position of the absorption band for violaxanthin embedded into the liposomes formed with DMPC (Gruszecki and Sielewiesiuk, 1990) and the arrow corresponds to the polarizability term of the hydrophobic core of the membrane (n = 1.44). 

The top-down approach involves size reduction by the application of three main types of force — compression, impact and shear. In the case of colloids, the small entities produced are subsequently kinetically stabilized against coalescence with the assistance of ingredients such as emulsifiers and stabilizers (Dickinson, 2003a). In this approach the ultimate particle size is dependent on factors such as the number of passes through the device (microfluidization), the time of emulsification (ultrasonics), the energy dissipation rate (homogenization pressure or shear-rate), the type and pore size of any membranes, the concentrations of emulsifiers and stabilizers, the dispersed phase volume fraction, the charge on the particles, and so on. To date, the top-down approach is the one that has been mainly involved in commercial scale production of nanomaterials. For example, the approach has been used to produce submicron liposomes for the delivery of ferrous sulfate, ascorbic acid, and other poorly absorbed hydrophilic compounds (Vuillemard, 1991 ... 

The isolated CFq-CFi has been incorporated into phospholipid liposomes and shown to carry in this form most of the energy-transducing functions which it catalyses within the thylakoid membranes. Thus, the reconstituted ATP synthase carries out ATP-dependent proton translocation resulting in both a 4pH and a developing across the reconstituted liposomes [72,73] an uncoupler-sensitive ATP-Pj exchange reaction [39] and ATP formation driven by artificially imposed 4pH and Ail/ [39,74,75], or by electric field pulses [56]. The ATP synthase proteolipo-somes provide the simplest system available today for the study of electrochemical-gradient-driven phosphorylation. 

Fig. 9. (A) Schematic representation of photophosphoryiation in a reconstituted iiposome containing photosystem-l reaction centers and CFo Fi ATP synthase, both purified from spinach chloroplasts, using PMS as the electron donor/acoeptor system. (B) Rate of photophosphoryiation as a function of light intensity (left) and time dependence of reconstituted photophosphoryiation (right). Figure source (B) Hauska, Samoray, Orlich and Nelson (1980) Reconstitution of photosynthetic energy conservation. II. Photophosphoryiation in liposomes containing photosystem-l reaction center and chloroplast coupling-factor complex. Eur J Biochem 111 540.

Aggregation of liposomes upon storage also depends on membrane components. Liposomes that included taurine as an isotonic solute were most stable at an optimal content of benzalkonium chloride.721 Repulsive energy forces between particles described by the Deryaguin-Verwey-Overbeek theory appeared to account for the stabilization. 

The liposomes are formed from bilayers by applying energy during the mixing process. In liposomes the core, an aqueous interior is separated by one or more phospholipid bilayers from the aqueous exterior. The size of the liposomes depends upon the manufacturing techniqne, intensity of mixing, etc. and is stable for a defined period of time. Liposomal properties and functionality depend on external parameters, such as pH and ionic strength of the medium. ... 

Amphiphilic block copolymers self-assemble in selective solvents such as water. Depending on parameters like overall average molecular weight, volume fraction of each block, or effective interaction energy between monomers in the blocks, vesicles with a bilayer shell and a solvent interior volume similar to liposomes are formed. They are often called polymersomes [241]. Such polymersomes in aqueous media have attracted increasing interest due to their enhanced stability compared to classical liposomes and due to the potential to control vesicle properties like bilayer thickness, permeability, or surface functionalities by appropriate chemical copolymer adjustment [242]. 

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