Relationship of actin filament assembly to clearance of fibrinogen gold, GPIIb-IIIa complexes on spread platelets

Ultrastructural characterization of surface-induced platelet activation on artificial materials by transmission electron microscopy

Platelet adhesion is one of the most pivotal events of blood clotting for artificial surfaces. However, the mechanisms of surface-induced platelet activation have not been fully been elucidated or visualized so far. In this study, we attempted to observe the internal structures and adhesion interfaces of human platelets attached to artificial surfaces by transmission electron microscopy (TEM) during the platelet activation process. We prepared observation samples by a conventional embedding method using EPON 812 resin. The sectioning was sliced perpendicular to the a-platelet/material interface. Observation by TEM indicates that internal granules coalesce in the center of the platelet accompanied by pseudopodial growth in the early stage of platelet activation. Pseudopodia from a platelet attach to the material interface not along a plane but at a point. In addition, along with the process of platelet activation, the gap between the platelet membrane and the material surface at the interface disappeared and a-platelet/material adhesion became much tighter. In the fully activated platelet stage, the platelet becomes thinner and tightly adheres to the substrate. As a result of comparative observation of an adherent platelet on polycarbonate (PC) and on amorphous carbon (a-C:H), it was found that internal granules release was inhibited more remarkably on a-C:H coating rather than on PC. Despite numerous technical difficulties in preparing sectional samples, such a study might prove the essential mechanism of biomaterial-related thrombosis, and it might become possible to modify the surfaces of materials to minimize material-related thrombosis.

Annexin V binds to the actin-based cytoskeleton at the plasma membrane of activated platelets

Immunocytochemical studies demonstrate that annexin V relocates to the plasma membranes of intact stimulated blood platelets. Anti-annexin V antibodies label the cytoplasmic side of the substrate-adherent plasma membrane of mechanically unroofed, glass-activated platelets and colocalize with actin. In addition, crosslinking experiments using detergent-solubilized membranes of activated platelets have identified an 85-kDa complex containing annexin V. The 85-kDa complex is also recognized by antibodies against actin, suggesting that annexin V interacts with actin. In addition, annexin V was found to associate with filamentous actin in the presence of millimolar Ca(2+). Annexin V was also shown by immunofluorescence microscopy to be associated with platelet cytoskeletons, colocalizing with actin in the presence of micromolar Ca(2+). These findings provide the first evidence for annexin V binding to the plasma membrane and to the actin-based cytoskeleton in activated platelets and indicate that annexin V may function in both cytoskeletal and membrane domains.

Characterization of patocytosis: endocytosis into macrophage surface-connected compartments

Previously, we described a unique macrophage endocytosis pathway in which aggregated low density lipoproteins and microcrystalline cholesterol induce and enter a labyrinth of membrane-bound compartments that remain connected to the cell surface. We now show that certain types of non-lipid particles such as polystyrene microspheres and colloidal gold also induce and enter macrophage surface-connected compartments (SCC), a process we call patocytosis. A common property among particles that stimulate patocytosis is their hydrophobic nature. Both aggregated LDL and microcrystalline cholesterol that we showed previously to stimulate patocytosis are hydrophobic. We now show that hydrophobic polystyrene microspheres and gold particles but not their hydrophilic counterparts triggered patocytosis. Uptake by patocytosis was limited to hydrophobic polystyrene microsphere particles less than 0.5 micron in diameter. Hydrophobic polystyrene microspheres greater than this size entered macrophages by phagocytosis. Actin-independent capping of hydrophobic polystyrene microspheres on the plasma membrane preceded actin-dependent uptake of the microspheres into SCC. Sequential rounds of microsphere uptake into SCC over two successive days could occur. There was some mixing of initial and subsequently accumulated microspheres in SCC. SCC formed from plasma membrane invaginations that connected with spaces created by unfolding of stacks of internal microvilli. Microsphere transport from plasma membrane invaginations into these spaces was inhibited by primaquine. Patocytosis is a unique endocytic process in macrophages triggered by small hydrophobic particles that provides a mechanism to sequester large amounts of these materials within a labyrinth of SCC.

Uptake of vWF-anti-vWF complexes by platelets in suspension

Efforts to identify the translocation of glycoprotein (GP) Ib/IX receptors, either bound to von Willebrand factor (vWF) or not, from exposed surfaces to interior membranes of thrombin-activated platelets in suspension have been unsuccessful. To observe vWF uptake by platelets, we added an anti-vWF antibody and staphylococcal protein A-gold (to act as a marker for the antibody) to an incubation medium containing washed platelets and bovine plasma vWF or ristocetin-activated human vWF. Thin sections of platelets incubated for 10, 20, or 30 minutes with vWF but without antibody revealed no internalization and minimal changes in the original discoid form. Over the same 30-minute period with anti-vWF, however, GPIb/IX-vWF-anti-vWF complexes were cleared from cell exteriors to channels of the open canalicular system. Engorgement of the open canalicular system with vWF multimers resulted in changes in shape, internal transformation, and degranulation. Physical changes associated with anti-vWF-induced uptake of vWF are not seen in platelets that are involved in hemostatic plug formation or clot retraction.

Redistribution of GPIb/IX and GPIIb/IIIa during spreading of discoid platelets

The purpose of the present study was to prelabel mobile receptors on discoid platelets with specific ligands identifiable in the electron microscope and follow their redistribution during spreading. Platelets were incubated in suspension with cytochalasin E (CE) to preserve discoid form, chilled and mounted on cold formvar grids or glass slide fragments to inhibit receptor movement, covered with cold bovine or ristocetin-activated human plasmas as sources of vWF to bind GPIb/IX, fibrinogen-coated gold particles (Fgn/Au) to couple GPIIb/IIIa, or both probes simultaneously, washed to remove CE and rewarmed to 37 degrees C for intervals up to 30 min to stimulate spreading. After brief fixation grids and glass fragments were incubated with anti-vWF antibody and, subsequently, staphylococcal protein A coupled to 5 nm and 10 nm gold particles to detect vWF multimers. Virtually all of the CE-treated chilled platelets retained their discoid shape. Half of the discs (53.3%) bound Fgn/Au, and all bound vWF. Receptors for both ligands were randomly dispersed on discoid cells from edge to edge. During rewarming discoid platelets expanded into spread forms. Fgn/Au-GPIIb/IIIa complexes moved into caps over cell centres and into residual channels of the open canalicular system (OCS). vWF bound to GPIb/IX moved with the cell membrane as the surface expanded during spreading. Discoid platelets prelabelled with both ligands demonstrated similar findings. During rewarming Fgn/Au-GPIIb/IIIa complexes moved to cell centres and into OCS channels. vWF multimers bound to GPIb/IX moved apart from each other toward peripheral margins of the spread cells. Thus, surface activation resulting in conversion of discoid platelets to spread forms does not cause clearance of GPIb/IX receptors to cell centres and channels of the OCS in the manner that GPIIb/IIIa receptors coupled to Fgn/Au are simultaneously translocated and concentrated in OCS channels.