Tubular invaginations with caveolae and coated pits in the sinus endothelial cells of the rat spleen

Ultrastructural Analysis of Vesicular Transport in Electrotransfection

Emerging evidence from various studies indicates that plasmid DNA (pDNA) is internalized by cells through an endocytosis-like process when it is used for electrotransfection. To provide morphological evidence of the process, we investigated ultrastructures in cells that were associated with the electrotransfected pDNA, using immunoelectron microscopy. The results demonstrate that four endocytic pathways are involved in the uptake of the pDNA, including caveolae- and clathrin-mediated endocytosis, macropinocytosis, and the clathrin-independent carrier/glycosylphosphatidylinositol-anchored protein-enriched early endosomal compartment (CLIC/GEEC) pathway. Among them, macropinocytosis is the most common pathway utilized by cells having various pDNA uptake capacities, and the CLIC/GEEC pathway is observed primarily in human umbilical vein endothelial cells. Quantitatively, the endocytic pathways are more active in easy-to-transfect cells than in hard-to-transfect ones. Taken together, our data provide ultrastructural evidence showing that endocytosis plays an important role in cellular uptake and intracellular transport of electrotransfected pDNA.

P2Y1, P2Y6, and P2Y12 receptors in rat splenic sinus endothelial cells: an immunohistochemical and ultrastructural study

Localization of three P2X and six P2Y receptors in sinus endothelial cells of the rat spleen was examined by immunofluorescent microscopy, and ultrastructural localization of the detected receptors was examined by immunogold electron microscopy. In immunofluorescent microscopy, labeling for anti-P2Y1, P2Y6, and P2Y12 receptors was detected in endothelial cells, but P2X1, P2X2, P2X4, P2Y2, P2Y4, and P2Y13 receptors was not detected. P2Y1 and P2Y12 receptors were prominently localized in the basal parts of endothelial cells. P2Y6 receptor was not only predominantly localized in the basal parts of endothelial cells, but also in the superficial layer. Triple immunofluorescent staining for a combination of two P2Y receptors and actin filaments showed that P2Y1, P2Y6, and P2Y12 receptors were individually localized in endothelial cells. Phospholipase C-β3, phospholipase C- γ2, and inositol-1,4,5-trisphosphate receptors, related to the release of the intracellular Ca(2+) from the endoplasmic reticulum, were also predominantly localized in the basal parts of endothelial cells. In immunogold electron microscopy, labeling for P2Y1, P2Y6, and P2Y12 receptors were predominantly localized in the basal part of endothelial cells and, in addition, in the junctional membrane, basal plasma membrane, and caveolae in the basal part of endothelial cells. Labeling for phospholipase C-β3 and phospholipase C-γ2 was dominantly localized in the basal parts and in close proximity to the plasma membranes of endothelial cells. The possible functional roles of these P2Y receptors in splenic sinus endothelial cells are discussed.

Vimentin intermediate filaments: the central base in sinus endothelial cells of the rat spleen

The ultrastructural distribution of vimentin intermediate filaments (IFs) and localizations of the related proteins in sinus endothelial cells of the rat spleen was examined by confocal laser scanning and electron microscopy with detergent extraction, myosin-fragment 1 decoration, and immunogold labeling to elucidate their functions in endothelial cells. Vimentin IFs were extremely abundant over stress fibers in the basal part of the endothelial cells. Some of them were intermingled with actin filaments in stress fibers, and were associated with coated vesicles. Plectin was predominantly localized in the layers of vimentin and stress fibers of the endothelial cells, but rarely in the vicinity of adherens junctions in the lateral part and focal adhesions in the basal part of the cells. Neither plakoglobin nor desmoplakin, which is coupled VE-cadherin to vimentin IFs, was detected in sinus endothelial cells. Vinculin was localized in the basal membranes of the endothelial cells. These data suggest that abundant vimentin IFs are associated with stress fibers by plectin in the basal part of the cells and form cytoskeletal cores of sinus endothelial cells only partially supported by the ring-shaped basal lamina to have roles in scaffolding and the mechanical stabilization of the endothelial cells. Furthermore, taken in connection with recently revealed functions of vimentin and plectin, vimentin might play a cytoskeletal core of sinus endothelial cells.

Structure of caveolae

The introduction of the electron microscope to the study of the biological materials in the second half of the last century has dramatically expanded our view and understanding of the inner workings of cells by enabling the discovery and study of subcellular organelles. A population of flask-shaped or spherical invaginations of the plasma membrane were described and named plasmalemmal vesicles or caveolae. Until the discovery of caveolin-1 as their first molecular marker in early 1990s, the study of caveolae was the exclusive domain of electron microscopists that demonstrated caveolae at different surface densities in most mammalian cells with few exceptions. Electron microscopy techniques in combination with other approaches have also revealed the structural features of caveolae as well as some of their protein and lipid residents. This review summarizes the data on the structure and components of caveolae and their stomatal diaphragms.

Localization of TRPC1 channel in the sinus endothelial cells of rat spleen

The ultrastructural localization of transient receptor potential C1 (TRPC1) channels in the sinus endothelial cells of rat spleen was examined by confocal laser scanning and electron microscopy. In addition, the localization of the closely associated proteins and channels, VE-cadherin, calreticulin, inositol-1,4,5-trisphosphate receptors type 1 (IP3R1), and ryanodine receptor (RyR), was also examined. Immunofluorescence microscopy of tissue cryosections revealed TRPC1 channels to be localized within the cytoplasm, in the superficial layer of the apical and basal parts of the cells, and in the junctional area of the adjacent endothelial cells. The distribution of Ca2+-storing tubulovesicular structures within endothelial cells was established by using tissue sections treated with osmium ferricyanide. Electron microscopy revealed densely stained tubulovesicular structures closely apposed to the plasma membrane and that occasionally ran closely parallel to the plasma membrane and near the caveolae and junctional apparatus. Immunolocalization analysis at the electron microscopy level using immunogold bound to the secondary antibody confirmed that TRPC1 channels were localized in the plasma membrane, caveolae, and vesicular structures in the subplasmalemmal cytoplasm of sinus endothelial cells. Calreticulin was predominantly localized in endoplasmic reticulum. IP3R1 and RyR, considered to be type 3, were colocalized in endoplasmic reticulum in proximity to the plasma membrane and caveolae. Thus, TRPC1 channels in sinus endothelial cells of the spleen might play an important role in controlling blood cell passage through phenomena including cytoskeletal reorganization, cell retraction, and disassembly of adherens junctions.

Cell surface orifices of caveolae and localization of caveolin to the necks of caveolae in adipocytes

Caveolae are noncoated invaginations of the plasma membrane that form in the presence of the protein caveolin. Caveolae are found in most cells, but are especially abundant in adipocytes. By high-resolution electron microscopy of plasma membrane sheets the detailed structure of individual caveolae of primary rat adipocytes was examined. Caveolin-1 and -2 binding was restricted to the membrane proximal region, such as the ducts or necks attaching the caveolar bulb to the membrane. This was confirmed by transfection with myc-tagged caveolin-1 and -2. Essentially the same results were obtained with human fibroblasts. Hence caveolin does not form the caveolar bulb in these cells, but rather the neck and may thus act to retain the caveolar constituents, indicating how caveolin participates in the formation of caveolae. Caveolae, randomly distributed over the plasma membrane, were very heterogeneous, varying in size between 25 and 150 nm. There was about one million caveolae in an adipocyte, which increased the surface area of the plasma membrane by 50%. Half of the caveolae, those larger than 50 nm, had access to the outside of the cell via ducts and 20-nm orifices at the cell surface. The rest of the caveolae, those smaller than 50 nm, were not open to the cell exterior. Cholesterol depletion destroyed both caveolae and the cell surface orifices.

The role of caveolae and caveolin in vesicle-dependent and vesicle-independent trafficking

Caveolae can mediate endocytosis, transcytosis, and potocytosis. Our understanding of these processes as well as the elucidation of the molecular machinery involved has greatly expanded. In addition, caveolin, a 22 kDa protein often associated with caveolae, can promote the trafficking of sterol through the cytoplasm independent of vesicles. Caveolin also influences the formation, morphology, and function of caveolae. The ability of caveolae and caveolin to mediate macromolecular transport directly impacts a variety of physiological and pathophysiological processes.