VVO Formation

Given the similarities between caveolae and VVO vesicles and vacuoles, it is possible that VVOs might form from the linking together of individual caveolae. It is also possible that the larger vesicles and vacuoles of VVOs might form from the fusion of two or more caveola-sized vesicles. We did ultrastructural preembedding immunoperoxidase to localize caveolin in VVOs in vitro and in vivo [5] to investigate the first possibility. These studies showed, both in vivo and in vitro and as in caveolae, that endothelial cell VVOs were caveolin-positive, lending support for formation of caveolin-positive VVO clusters from the joining of individual caveolin-positive caveolae. We used an ultrastructural morphometric approach [3] to investigate the second possibility. For example, a fusion mechanism has been proposed in the generation of several types of cytoplasmic secretory granules in which small progranules of unit size fuse with each other in varying combinations to form larger mature granules whose volumes represent multiples of the volume of the unit progranule [6]. By analogy, the volume of the various VVO vesicles and vacuoles would not be expected to fall on a continuum but instead would represent multiples of the volume of the smaller unit vesicle. Measurements of the volumes of vesicles and vacuoles comprising VVOs revealed a heterogeneous distribution, where the volumes of individual vesicles and vacuoles were not continuous but exhibited a periodic modal distribution [3]. The most frequently occurring vesicle-vacuole had a unit volume of ~0.00015 mm3; this value corresponds to a spheroid of diameter ~60nm, that is, the size of typical capillary caveolae. Vacuoles corresponding to the fusion of as many as 10 unit vesicles were detected. The data, therefore, are consistent with the hypothesis that larger VVO vesicles and vacuoles arise from the fusion of different numbers of caveola-sized unit vesicles.

We also have investigated the substructural localization of other proteins of importance to vesicular trafficking and fusion, generally. One such protein, vesicle-associated membrane protein (VAMP), is localized to the cytoplasmic side of caveolar and VVO membranes [3]. The presence of this vesicle-SNARE protein in these locations lends support to the proposed construction of VVOs from fused caveolae.

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