Electron microscopy of cardiolipin vesicles

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Preservation of ultrastructure in phosphatidylcholine vesicles by tannic acid and OsO4

To improve preservation and visualization of membrane structure in artificial phosphatidylcholine vesicles (PC liposomes), the following three fixation formulas including tannic acid (TA) and osmium (OsO4) steps were tested: (1) TA-OsO4, (2) OsO4-TA, and (3) OsO4-TA-OsO4. The TA-OsO4 method, known to stabilize PC against conventional dehydration, was inadequate for characterizing liposome structure because it induced a serious artifact that the liposomes were aggregated and fused. On the other hand, the OsO4-TA method did not cause the artifact but only a dull EM image was obtained for the membrane structure. The best results with egg PC liposomes have been obtained by the OsO4-TA-OsO4 method which gives improved preservation and enhanced contrast of membrane structure.

Effect of phospholipids on the transformation of amorphous calcium phosphate to hydroxapatite in vitro

The conversion of amorphous calcium phosphate (ACP) to crystalline hydroxyapatite (HA) was studied in vitro in the presence or absence of phosphatidyl serine (PS) and other phospholipids. ACP transformation and HA crystal growth were monitored by electron microscopy, selected-area electron diffraction and X-ray diffraction, and by measuring supernatant calcium and phosphate. PS, and other acidic phospholipids, had a significant stablizing effect on ACP. With performed ACP at ratios of only 1 lipid molecule per 30-50 Ca atoms, PS markedly delayed HA crystal formation. When PS was present during ACP precipitation, inhibition of conversion to HA was less pronounced, but crystal habit and aggregation were greatly altered resulting in stacks of thin, membrane-like sheets approximately 38-42 A thick. PS appeared to be most effective in blocking ACP to HA conversion when oriented primarily on the surface; it most affected subsequent crystal formation when distributed throughout the amorphous precursor. Phospholipids possessing anionic, and hence Ca-binding properties, were effective in stabilizing ACP; neutral zwitterion lipids, which have amphipathic properties but do not bind Ca, were not. In view of the presence of anionic lipids in matrix vesicles and their association with early mineral deposits, the current findings add further evidence that lipids may play a role in the control of normal mineralization in vivo.