Spontaneous fusion of rat liver lysosomes in vitro

Reconstitution of an endocytic fusion event in a cell-free system

Using a cell-free system we have obtained fusion of vesicles from the endocytic pathway. The fusion is rapid, efficient, and requires ATP. Only vesicles derived from certain positions along the endocytic pathway are capable of fusing. Lysosomes and vesicles derived from the plasma membrane do not fuse.

In vitro fusion of Acanthamoeba phagolysosomes. I. Demonstration and quantitation of vacuole fusion in Acanthamoeba homogenates

Fusion of phagolysosomes (PLs) has been demonstrated to occur in vitro. Two separate cell homogenates of the ameba Acanthamoeba sp. (Neff) were prepared, each rich in PLs labeled with distinctive particulate markers. Portions of each were incubated together in vitro and fusion occurred as evidenced by the appearance of PLs containing both types of markers. Fusion was confirmed by electron microscopy, including serial sectioning. The membranes of fused vacuoles excluded the dye eosin Y. Surviving cells in the homogenates were not responsible for the observed fusion. Fusion was obtained using either synthetic markers (polystyrene and polyvinyltoluene latex) or biological markers (autoclaved yeast cells and glutaraldehyde-fixed goat red blood cells), or a combination of both. The specificity of PL fusion in vivo appeared to be maintained in vitro. As determined by light and electron microscopy, the fusion reaction was dependent on time and temperature, and on the initial presence of membrane around both marker particles. A minimum of 10% of the vacuoles fused by 10 min of incubation at 30 degrees C, and no rupture of the vacuoles was detected during this time. After 10 min of incubation, vacuole rupture began and fusion ceased. At a constant initial vacuole concentration, the extent of PL fusion in vitro was quantitatively reproducible. This appears to be a promising system for further investigation of membrane fusion in the lysosomal system.

Interaction of phospholipid vesicles with cultured mammalian cells. II. Studies of mechanism

The mechanism of interaction of artificially generated lipid vesicles (approximately 500 A diameter) with Chinese hamster V79 cells bathed in a simple balanced salt solution was investigated. The major pathways of exogenous lipid incorporation in vesicle-treated cells are vesicle-cell fusion and vesicle-cell lipid exchange. At 37 degrees C, the fusion process is dominant, while at 2 degrees C or with energy depleted cells, exchange of lipids between vesicles and cells is important. The fusion mechanism was demonstrated using vesicles of [14C]lecithin containing trapped [13H]inulin. Consistent with a fusion hypothesis, both components became cell associated at 37 degrees C in nearly the same proportions as they were present in the applied vesicles. Additional arguments in favor of vesicle-cell fusion and against phagocytosis or adsorption of intact vesicles are presented. At 2 degrees C or with inhibitor-treated cells, the [3H]inulin uptake was largely suppressed, while the lipid uptake was reduced to a lesser extent. Evidence for vesicle-cell lipid exchange was obtained using V79 cells grown on 3H precursors for cellular lipids. [14C]lecithin vesicles, incubated with such cells, showed no change in their elution properties when subjected to molecular sieve chromatography on Sepharose 4B. However, radioactivity and thin-layer chromatographic analyses revealed that a variety of cell lipiids had been exchanged into the uniamellar vesicles. Further evidence for the fusion and exchange processes was obtained using vesicles prepared from mixtures of [3H]lecithin and [14C]cholesterol. A two-step fusion mechanism consistent with the present findings is proposed as a working model for other fusion studies.

Interaction of phospholipid vesicles with cells. Endocytosis and fusion as alternate mechanisms for the uptake of lipid-soluble and water-soluble molecules

Depending on their phospholipid composition, liposomes are endocytosed by, or fuse with, the plasma membrane, of Acanthamoeba castellanii. Unilamellar egg lecithin vesicles are endocytosed by amoeba at 28 degrees C with equal uptake of the phospholipid bilayer and the contents of the internal aqueous space of the vesicles. Uptake is inhibited almost completely by incubation at 4 degrees C or in the presence of dinitrophenol. After uptake at 28 degrees C, the vesicle phospholipid can be visualized by electron microscope autoradiography within cytoplasmic vacuoles. In contrast, uptake of unilamellar dipalmitoyl lecithin vesicles and multilamellar dipalmitoyl lecithin liposomes is only partially inhibited at 4 degrees C, by dinitrophenol and by prior fixation of the amoebae with glutaraldehyde, each of which inhibits pinocytosis. Vesicle contents are taken up only about 40% as well as the phospholipid bilayer. Electron micrographs are compatible with the interpretation that dipalmitoyl lecithin vesicles fuse with the amoeba plasma membrane, adding their phospholipid to the cell surface, while their contents enter the cell cytoplasm. Dimyristoyl lecithin vesicles behave like egg lecithin vesicles while distearoyl lecithin vesicles behave like dipalmitoyl lecithin vesicles.