Variation in intramembrane components of Trypanosoma brucei from intact and x-irradiated rats: a freeze-cleave study

Subpellicular microtubules associate with an intramembranous particle lattice in the protozoan parasite Toxoplasma gondii

Application of Fourier analysis techniques to images of isolated, frozen-hydrated subpellicular microtubules from the protozoan parasite Toxoplasma gondii demonstrates a distinctive 32 nm periodicity along the length of the microtubules. A 32 nm longitudinal repeat is also observed in the double rows of intramembranous particles seen in freeze-fracture images of the parasite's pellicle; these rows are thought to overlie the subpellicular microtubules. Remarkably, the 32 nm intramembranous particle periodicity is carried over laterally to the single rows of particles that lie between the microtubule-associated double rows. This creates a two-dimensional particle lattice, with the second dimension at an angle of approximately 75 degrees to the longitudinal rows (depending on position along the length of the parasite). Drugs that disrupt known cytoskeletal components fail to destroy the integrity of the particle lattice. This intramembranous particle organization suggests the existence of multiple cytoskeletal filaments of unknown identity. Filaments associated with the particle lattice provide a possible mechanism for motility and shape change in Toxoplasma: distortion of the lattice may mediate the twirling motility seen upon host-cell lysis, and morphological changes observed during invasion.

Structural organization of the cell surface of pathogenic protozoa

The surface of parasitic protozoa plays an important role in the process of their interaction with cells from the host. The present review analyzes the structural organization of the surface of sporozoa, trypanosomatids, Entamoeba and Trichomonas, as evaluated by conventional transmission electron microscopy, cytochemical techniques and freeze-fracture. In most protozoa, no special region of surface membrane is detected. In others, however, special membrane domains have been described. As examples, we can mention the cytostome found in epimastigote forms of Trypanosoma cruzi, the region of attachment of the flagellum to the protozoon body in Trypanosomatidae and Trichomonadidae, and the inner membrane complex of Apicomplexa.

Mechanisms for the elimination of potentially lytic complement-fixing variable surface glycoprotein antibody-complexes in Trypanosoma brucei

Live antibody-coated Trypanosoma brucei parasites remove variable surface glycoprotein (VSG)-antibody complexes from their surface upon warming to 37 degrees C and evade antibody-activated complement lysis by both protein synthesis-dependent and protein synthesis-independent mechanisms. The protein synthesis-dependent process follows antibody-mediated trypanosome agglutination, whereas the protein synthesis-independent mechanism can occur in the absence of trypanosome agglutination. The latter process leads to a more rapid elimination of complement-fixing VSG-antibody complexes.

Deletion of an immunodominant Trypanosoma cruzi surface glycoprotein disrupts flagellum-cell adhesion

Null mutants of the Trypanosoma cruzi insect stage-specific glycoprotein GP72 were created by targeted gene replacement. Targeting plasmids were constructed in which the neomycin phosphotransferase and hygromycin phosphotransferase genes were flanked by GP72 sequences. These plasmids were sequentially transfected into T. cruzi epimastigotes by electroporation. Southern blot analyzes indicated that precise replacement of the two genes had occurred. No aberrant rearrangements occurred at the GP72 locus and no GP72 gene sequences had been translocated elsewhere in the genome. Western blots confirmed that GP72 is not expressed in these null mutants. The morphology of the mutants is dramatically different from wild-type. In both mutant and wild-type parasites, the flagellum emerges from the flagellar pocket. In the null mutant the normal attachment of the flagellum to the cell membrane of the parasite is lost.

The Cytoskeleton of trypanosomes

From the concept of cells as mere bags full of enzymes, cell biology has come a long way towards understanding the highly complex structural organization of eukaryotic cells. The cytoskeleton, ie. the complex of fibrous elements that are crucial for cell shape, motility and the structural organization of cytoplasm and cell membranes, is now recognized as vital for supporting many critical functions in eukaryotic cells. Surprisingly, this subject, which has provided scores of cell biologists with excitement and fascination, has been largely overlooked with respect to parasitic protozoa. A notable change of perception has taken place over the past few years as the cytoskeleton of parasitic protozoa has been increasingly recognized as a potential target for antiparasitic intervention. The following article by Thomas Seebeck, Andrew Hemphill and Durward Lawson highlights some recent developments in the analysis of what is presently the best-studied parasite cytoskeleton, that of the trypanosome.

Freeze-fracture study of the bloodstream form of Trypanosoma brucei gambiense

The ultrastructure of Trypanosoma brucei gambiense was investigated by the freeze-fracture method. Three different regions of the continuous plasma membrane; cell body proper, flagellar pocket, and flagellum were compared in density and distribution of the intramembranous particles (IMP's). The IMP-density was highest in the flagellar pocket membrane and lowest in flagellum. Intra membranous particles of the cell body membrane were distributed uniformly on both the protoplasmic (P) and exoplasmic (E) faces. On the P face of the flagellar membrane, a single row of IMP-clusters was seen along the juncture of the flagellum to the cell body. Since the spacing of the IMP-clusters was almost equal to the spacing of the paired rivet structures observed in thin section, these clusters likely are related to the junction of flagellum and cell body. At the neck of the flagellar pocket, several linear arrays of IMP's were found on the P face of the flagellar membrane, while on the E face rows of depressions were seen. At the flagellar base, the clusters of IMP's were only seen on the P face. On the flagellar pocket membrane, particle-rich depressions and linear particle arrays were also found on the P face, while on the E face such special particle arrangements were not recognized. These particle-rich depressions may correspond to the sites of pinocytosis of the bloodstream forms which have been demonstrated in thin sections.

Formation of filopodia in Trypanosoma congolense by crosslinking the variant surface antigen

Trypanosoma congolense was exposed to various substances binding to the variant surface antigen (VSG). All methods of crosslinking VSG molecules caused the rapid accumulation of ligands along the line of flagellar attachment and their shedding by formation of coat-covered vesicles and filopodia. This phenomenon was observed after treatment of the parasites with concanavalin A (Con A), anti-VSG-IgG plus protein A-gold, attachment of the cells to surfaces coated with poly-L-lysine and Con A and to Formvar films before negative staining. Moreover, trypanosomes aggregated by primary antibodies formed vesicles and filopodia at the points of contact. Those antibodies bound to the remaining cell surface, however, remained distributed uniformly. This indicates that primary antibodies alone do not cause crosslinking of VSG on the surface of T.congolense.

Leishmania chagasi: in vitro differentiation of promastigotes monitored by flow cytometry

A sequential development from a less infective to an infective stage of Leishmania promastigotes growing in culture has been previously reported. The aim of this work was to investigate whether freeze-fracture electron microscopy and flow cytometry would be able to provide some reliable morphological markers of in vitro differentiation of Leishmania chagasi promastigotes. The flow cytometry technique discriminates between the L. chagasi promastigotes from the different stages of their in vitro differentiation. The "forward scatter" intensity of the parasite, very high 15 hr after seeding when the parasites were very condensed and with a high DNA content per particle, strongly decreased during the culture course. Parallel experiments have shown a striking correlation between forward scatter intensity, growth curves, and infectivity of promastigote populations. By contrast, freeze-fracture techniques showed that in either less infective or infective promastigote plasma membranes, the intramembrane particles density in protoplasmic fracture faces (about 2800/micron 2) and in exoplasmic fracture faces (about 1000/micron 2) was independent of the time of cultivation. The amount of filipin lesions, which reflects the cholesterol content within the plasma membrane, was also constant throughout the culture course. Both data suggest that the architecture of the plasma membrane is an intrinsic characteristic of the promastigote stage. This study shows that whereas freeze-fracture electron microscopy does not provide markers for the differentiation of Leishmania promastigotes, flow cytometry may on the other hand be of value as a screening test for promastigote populations allowing the characterization of their developmental stages in in vitro cultures.

Leishmania mexicana: distribution of intramembranous particles and filipin sterol complexes in amastigotes and promastigotes

The density and distribution of intramembranous particles was analyzed in freeze fracture replicas of the plasma membrane of amastigotes, and infective as well as noninfective promastigotes of Leishmania mexicana amazonensis. The density of intramembranous particles on both protoplasmic and extracellular faces was higher in infective than in noninfective promastigotes and it was lower in amastigotes than in promastigotes. Amastigotes purified immediately after tissue homogenization were surrounded by a membrane which corresponded to the membrane which lined the endocytic vacuoles where the parasites were located within the tissue macrophages. Aggregation of the particles was seen in the flagellar membrane at the point of emergence of the flagellum from the flagellar pocket. Differences in the organization of the particles were seen in the membrane which lined the flagellar pocket of amastigotes and promastigotes. The polyene antibiotic, filipin, was used as a probe for the detection of sterols in the plasma membrane of L. m. amazonensis. The effect of filipin in the parasite's structure was analyzed by scanning electron microscopy and by transmission electron microscopy of thin sections and freeze fracture replicas. Filipin sterol complexes were distributed throughout the membrane which lined the cell body, the flagellar pocket, and the flagellum. No filipin sterol complexes were seen in the cell body-flagellar adhesion zone. The density of filipin sterol complexes was lower in the membrane lining the flagellum than in that lining the cell body of promastigotes.

A freeze-fracture electron microscope study of Trichomonas vaginalis Donné and Tritrichomonas foetus (Riedmüller)

Two strains of Trichomonas vaginalis, JH162A , with low pathogenicity, and Balt 44, with high pathogenicity, as well as one highly pathogenic strain, KV-1, of Tritrichomonas foetus were studied by freeze-fracture electron microscopy. The protoplasmic faces ( PFs ) of the cell membranes of all three strains of both species had similar numbers of intramembranous particles (IMPs); however, the particles in the external faces (EFs) of these membranes were least abundant in Trichomonas vaginalis strain Balt 44 and most numerous in those of strain JH162A of this species. In Tritrichomonas foetus strain KV-1 the number of IMPs in the EF was close to but somewhat lower than that in the mild strain of the human urogenital trichomonad . In both species, the anterior, but not the recurrent, flagella had rosette-like formations, consisting of approximately 9 to 12 IMPs on both the PFs and EFs. The numbers and distribution of the rosettes appeared to vary among different flagella and in different areas of individual flagella of a single organism belonging to either species. The freeze-fracture electron micrographs provided a more complete understanding of the fine structure of undulating membranes of Trichomonadinae , as represented by Trichomonas vaginalis, and of Tritrichomonadinae (the Tritrichomonas augusta -type), as exemplified by Tritrichomonas foetus, than was gained from previous transmission and scanning electron microscope studies. Typically three longitudinal rows of IMPs on the PF of the recurrent flagellum of Trichomonas vaginalis were noted in the area of attachment of this flagellum to the undulating membrane. The functional aspects of the various structures and differences between certain organelles revealed in the two trichomonad species by the freeze-fracture method are discussed.

Tritrichomonas foetus: fine structure of freeze-fractured membranes

Freeze-fracture techniques reveal differences in fine structure between the anterior three flagella of Tritrichomonas foetus and its recurrent flagellum. The anterior flagella have rosettes of 9-12 intramembranous particles on both the P and E faces. The recurrent flagellum lacks rosettes but has ribbon-like arrays of particles along the length of the flagellum, which may be involved in the flagellum's attachment to the cell body. This flagellum is attached to the membrane of the cell body along a distinct groove that contains few discernible particles. Some large intramembranous particles are visible on the P face of the cell body membrane at the point where the flagellum emerges from the cell body. The randomly distributed particles on the P and E faces of the plasma membrane have a particle density of 919/micron2 and 468/micron2 respectively, and there are areas on both faces that are devoid of particles. Freeze-fracture techniques also reveal numerous fenestrations in the membrane of the Golgi complex and about 24 pores per micron2 in the nuclear membrane.

Identification and partial characterization of plasma membrane polypeptides of Trypanosoma brucei

A plasma membrane-enriched vesicle fraction has been prepared from Trypanosoma brucei by sonication and differential centrifugation on sucrose gradients. This fraction is enriched 5-fold in the plasma membrane marker enzymes adenyl cyclase (EC 4.6.1.1) and ouabain-inhibitable, (Na+ +K+)-dependent adenosine triphosphatase (EC 3.6.1.3). It is also enriched up to 14-fold in iodinated surface proteins, and up to 4-fold in (3H-mannose-labeled glycoproteins, of which the major variable surface coat glycoprotein is the main constituent. Proteins of the plasma membrane fraction and other subcellular fractions have been identified by electrophoretic analysis in sodium dodecyl sulfate-polyacrylamide gradient slab gels. Several high molecular weight surface glycopeptides have been selectively investigated and partially characterized by a combination of metabolic labeling with [3H]mannose, lactoperoxidase-catalyzed surface iodination, and affinity chromatography on Con A-Sepharose. In addition to the major variable surface coat glycoprotein (estimated Mr = 58000), there are several minor surface glycopeptides (Mr = 76000, 86000 and 92000-100000) which are apparent extrinsic membrane components, and two surface glycopeptides (Mr = 42000 and 130000) which are intrinsic membrane components.

Transmission and scanning electron microscopic studies of the micronemata of Trypanosoma gambiense

The structure of micronemata arising from the surface of the bloodstream form of Trypanosoma gambiense was studied by electron microscopy. In order to produce micronemata, trypanosomes were incubated in either 1) phosphate buffered saline supplemented with glucose (PBSG), 2) immune mouse serum or 3) PBSG after passage through a DEAE-cellulose column. Electron microscopic examination of the parasite revealed the presence of thread-like micronemata arising from the anterior end and from the flagellar pocket regardless of the incubation conditions. Negative staining revealed a distinct peripheral fringe layer with nodular protrusions covering the entire surface of the micronema. The distribution and number of intramembrane particles (IMP) on the P and E faces of the micronema were similar to those of the flagellum of T. gambiense, indicating a close relationship between the membrane structure of the micronema and the flagellum. Micronemata became fragmented and adhered to each other after incubation of the parasite in the media for 12 h. Since micronemata tend to have the characteristics of adhesiveness and fragmentation, fragments of these structures might adhere to various host organs. Dispersal of potential antigenic material might be responsible, in part, for the induction of the host immune response.

Electron microscopy observations on Trypanosoma brucei: freeze-cleaving and thin-sectioning study of the apical part of the flagellar pocket

Observations on the apical part of the flagellar pocket of Trypanosoma brucei using the freeze-cleaving and thin-sectioning techniques of electron microscopy are reported. The flagellar pocket is shown as a flask-shaped depression in the body, continuous with the pellicle and flagellar sheath. The membranes of the apical part of the pocket on the flanged side of the body compress the flagellar sheath, forming part of a "neck region". This region is completed on the broad body side of the flagellar pocket when it embraces the flagellar sheath. The possible role of the neck region in the dynamic function of pinocytosis is discussed.