The relation between the Golgi apparatus, cell surface, and cytoplasmic vesicles in amoebae studied by electron microscope radioautography

Induced pinocytosis and endosomal pathways in Amoeba proteus

The mode of endosome formation and processing was studied in Amoeba proteus by using bovine serum albumin (BSA) either coupled to the fluorochrome TRITC or to 12 nm gold particles (Au(12)) as a pinocytosis inducer. The intraendosomal modifications of BSA-TRITC and BSA-Au(12) start with the separation of the ligand complex from receptor sites of the endosomal membrane obviously caused by acidification (1-15 min). Subsequently, the delivery with lysosomal enzymes and the disintegration of the ligand complex into individual components (BSA and Au(12)) occurs (15-30 min). The intracytotic transport of the ligand components to distinct cellular compartments leads to separate endosome populations with a common fate, i.e. fusion with the cell membrane and exocytosis of the lysosomal content. However, a certain amount of vesicles containing the hydrolyzable ligand BSA escapes exocytosis and is delivered to the digestive apparatus thus contributing to the nutrient pool of the cell (30-40 min). The application of antibodies against human or Tetrahymena digestive enzymes resulted in the detection of a 22 kD protein which shows a distinct cross-reactivity with anti-cathepsin D. The protein is localized in the Golgi apparatus, in small vesicles and in secondary lysosomes containing ingested material. In general, induced pinocytosis can mainly be viewed as a survival mechanism developed by A. proteus to overcome extracellular stress situations. The consecutive endocytotic pathways involve the lysosomal system as well as some other intracellular compartments and are comparable to the situation in higher organisms.

Localization of lipophosphonoglycan in membranes of Acanthamoeba by using specific antibodies

Antibodies against two electrophoretically distinct forms of lipophosphonoglycan (LPG) were produced in rabbits. Antibody specificity was demonstrated by the coupled antibody 125I-protein A assay (Adair et al., J. Cell Biol. 79:281-285, 1978). Indirect immunofluorescent labeling of intact Acanthamoeba showed that antibodies to both LPG components had the same uniform distribution on the cell surface. Both antibodies also bound to the cytoplasmic surface of isolated phagosomes. The location of LPG in other membranes of the amoeba was demonstrated on sections by the unlabeled antibody method. Although LPG was absent from the nuclear membrane, virtually all of the internal vacuole membranes were labeled, including the contractile vacuole. Antibodies directed against LPG were utilized to label lipophosphonoglycan in the plasma membrane of living amoebae. Labeled membrane was internalized and then localized by immunofluorescence in cytoplasmic vacuoles within 10 min of incubation. Although these results are evidence for exchange between plasma and cytoplasmic vacuolar membranes, the contractile vacuole remained unlabeled and can be considered, therefore, a separate membrane compartment. Concanavalin A also was bound and internalized by the amoeba, but electron microscopy showed that this label caused pronounced membrane perturbation, limiting its usefulness as a membrane marker in this system.

Distribution of phospholipids labeled with 3H-choline and relationship between membranous organelles in amoebae, as studies by electron-microscopic radioautography

Amoebae were injected with a solution of 3H-choline, and samples were prepared for electron-microscopic radioautography at intervals between 15 min and 24 h thereafter. At the earliest interval, the rough endoplasmic reticulum was the most heavily labeled organelle. At subsequent intervals, the proportion of silver grains over the rough endoplasmic reticulum decreased rapidly, while that associated with other membranes increased. Most notably this involved a rapid rise in labeling of vacuoles, up to 1 h, and a more gradual increase in plasma membrane labeling up to 2 h. The results suggest that phospholipids are synthesized in the rough endoplasmic reticulum and are then transferred to other cellular membranes. A sequence of transfer steps suggested by the order of increases in labeling of different types of membranes is rough endoplasmic reticulum, smooth membranes and nuclear membranes, vacuoles, and the plasma membrane.

Binding of [125I]concanavalin A by interspecific amoeba hybrids

Two types of interspecific amoeba hybrids, viz. PnIc (Amoeba proteus nucleus in A. indica cytoplasm) and InPc (A. indica nucleus in A. proteus cytoplasm), were tested for their [125I]Concanavalin A (Con A) binding activity at different periods of time. The cell surface binding of the labelled Con A was reduced to approx. 40 and 75% in the PnIc and InPc cells, respectively, 96 h after the cells were made hybrids. A significant increase in the binding of radioactive Con A was observed after the homologous nucleus was back transferred into InPc cells, but not in PnIc cells. No significant difference in the binding of the radioactive Con A was found between the progeny of the hybrid amoebae and the control amoebae. The Con A binding activity of the interspecific amoebae has been discussed in relation to the changes in the cytoplasmic membrane structures which presumably play a role in the formation of cell surface in the amoebae.

Glycoprotein containing vesicles in the surface epithelial cells of the ascending colon of the rat

Recently, radioautographic studies have shown that cell coat glycoproteins are transported to the cell surface by vesicles both in the amoeba (Flickinger, '75) and in the epithelial cells of the ascending colon of the mouse (Michaels and Leblond, '76). In the current morphological and cytochemical study of the surface epithelial cells of the rat ascending colon, it is shown that filamentous material, resembling the cell coat, is contained in saccules toward the mature face of the Golgi apparatus and vesicles close to the apparatus and near the terminal web. The vesicles are limited by a unit membrane composed of asymmetric osmiophilic leaflets and similar to the plasma membrane. When stained by the periodic acid-chromic acid-silver methenamine technique, silver was precipitated on the cell components containing the filamentous material indicating the presence of glycoproteins. Narrow invaginations from the cell surface that may correspond to vesicles undergoing exocytosis were also positive for glycoproteins. The distribution of the filamentous material that was glycoprotein positive parallels the pathway followed by material that had been found to be labeled with a tritiated glycoprotein precursor (3H-fucose) in the epithelial cells of the ascending colon of the mouse. It is suggested that the system of vesicles in the rat colon cells is acting in a manner similar to the vesicles in the mouse cells to transport cell coat glycoproteins from the Golgi apparatus to the cell surface.