Differentiation in Trypanosoma brucei: host-parasite cell junctions and their persistence during acquisition of the variable antigen coat

1985 ◽  
Vol 74 (1) ◽  
pp. 1-19 ◽  
Author(s):  
L. Tetley ◽  
K. Vickerman
Keyword(s):  
Electron Microscopy ◽  
Trypanosoma Brucei ◽  
Morphological Changes ◽  
Freeze Fracture ◽  
Tsetse Fly ◽  
Cell Junctions ◽  
Surface Coat ◽  
Thin Sections ◽  
Variable Antigen ◽  
Host Parasite

Acquisition of the variable antigen-containing surface coat of Trypanosoma brucei occurs at the metacyclic stage in the salivary glands of the tsetse fly vector. The differentiation of the metacyclic trypanosome in the gland has been studied by scanning electron microscopy and by transmission electron microscopy of thin sections and freeze-fracture replicas. The uncoated epimastigote trypanosomes (with a prenuclear kinetoplast) divide while attached to the salivary gland epithelium brush border by elaborate branched flagellar outgrowths, which ramify between the host cell microvilli and form punctate hemidesmosome-like attachment plaques where they are indented by the microvilli. These outgrowths become reduced as the epimastigotes transform to uncoated trypomastigotes (with postnuclear kinetoplast), which remain attached and capable of binary fission. The flagellar outgrowths disappear but the attachment plaques persist as the uncoated trypomastigotes (premetacyclics) stop dividing and acquire the surface coat to become ‘nascent metacyclics’. Coat acquisition therefore occurs in the attached trypanosome and not, as previously believed, after detachment. Coating is accompanied by morphological changes in the glycosomes and mitochondrion of the parasite. Freeze-fracture replicas of the host-parasite junctional complexes show membrane particle aggregates on the host membrane but not on the parasite membrane. It is suggested that disruption of the complex occurs when maximum packing of the glycoprotein molecules has been achieved in the trypanosome surface coat, releasing the metacyclic trypanosome into the lumen of the gland.

Trypanosome sociology and antigenic variation

Parasitology ◽  
1989 ◽  
Vol 99 (S1) ◽  
pp. S37-S47 ◽  
Author(s):  
K. Vickerman
Keyword(s):  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Growth Characteristics ◽  
Tsetse Fly ◽  
Antigenic Specificity ◽  
Surface Coat ◽  
Humoral Immune ◽  
Variable Antigen ◽  
Minority Variants ◽  
Stimulation Of

SUMMARYSurvival of the trypanosome (Trypanosoma brucei) population in the mammalian body depends upon paced stimulation of the host's humoral immune response by different antigenic variants and serial sacrifice of the dominant variant (homotype) so that minority variants (heterotypes) can continue the infection and each become a homotype in its turn. New variants are generated by a spontaneous switch in gene expression so that the trypanosome puts on a surface coat of a glycoprotein differing in antigenic specificity from its predecessor. Homotypes appear in a characteristic order for a given trypanosome clone but what determines this order and the pacing of homotype generation so that the trypanosome does not quickly exhaust its repertoire of variable antigens, is not clear. The tendency of some genes to be expressed more frequently than others may reflect the location within the genome and mode of expression of the genes concerned and may influence homotype succession. Differences in the doubling time of different variants or in the rate at which trypanosomes belonging to a particular variant differentiate into non-dividing (vector infective) stumpy forms have also been invoked to explain how a heterotype's growth characteristics may determine when it becomes a homotype. Recent estimations of the frequency of variable antigen switching in trypanosome populations after transmission through the tsetse fly vector, however, suggest a much higher figure (0·97–2·2 × 10−3switches per cell per generation) than that obtained for syringe-passed infections (10−5–10−7switches per cell per generation) and it seems probable that most of the variable antigen genes are expressed as minority variable antigen types very early in the infection. Instability of expression is a feature of trypanosome clones derived from infective tsetse salivary gland (metacyclic) trypanosomes and it is suggested that high switching rates in tsetse-transmitted infections may delay the growth of certain variants to homotype status until later in the infection.

scholarly journals Keith Vickerman. 21 March 1933—28 June 2016

2021 ◽  
Author(s):  
Francis Cox ◽  
Keith Gull
Keyword(s):  
Antigenic Variation ◽  
Morphological Changes ◽  
Tsetse Fly ◽  
Mammalian Host ◽  
Surface Coat ◽  
African Trypanosomes ◽  
Causative Agents ◽  
University College London ◽  
Variable Antigen ◽  
Commercially Important

Keith Vickerman was a parasitologist and protozoologist who made major contributions to our understanding of the biology of African trypanosomes, the causative agents of human sleeping sickness and nagana in cattle. His first academic post was at University College London, where he quickly mastered the techniques of electron microscopy (EM) and produced some of the best electron micrographs of parasitic protozoa at that time. He was a great believer in observation and deduction, and what began as an exercise in EM led him to investigate two of the then outstanding problems of trypanosome biology: how the parasites manage the transition from the tsetse fly vector to its mammalian host, and how they evade the host's immune response. Morphological changes, he found, were correlated with changes in the single mitochondrion and ensuring biochemical changes during the transition from a glucose-rich environment in mammalian blood to the glucose-poor tsetse gut. It was while comparing bloodstream and tsetse forms that he observed that the trypanosomes possessed a thick surface coat in the blood, which he subsequently identified as the variable antigen that was repeatedly formed and reformed and that this was the basis of antigenic variation—findings that stimulated a vast amount of interest among immunologists, biochemists and geneticists. In his later career a new problem emerged, and he found that a disease devastating stocks of the commercially important Norway lobster, Nephrops norvegicus , thought to be caused by a virus was actually caused by a protozoan, Hematodinium . Keith will always be remembered as one of the founders of modern parasitology.

scholarly journals Development of cell junctions in sea-urchin embryos

1983 ◽  
Vol 62 (1) ◽  
pp. 27-48
Author(s):  
E. Spiegel ◽  
L. Howard
Keyword(s):  
Epithelial Cells ◽  
Basement Membrane ◽  
Digestive Tract ◽  
Sea Urchin ◽  
Freeze Fracture ◽  
Cell Junctions ◽  
Septate Junctions ◽  
Thin Sections ◽  
Sea Urchin Embryos ◽  
Lanthanum Tracer

The development of cell junctions in sea-urchin embryos has been investigated using thin sections, lanthanum-tracer and freeze-fracture techniques. Three types of desmosomes are present: belt desmosomes and spot desmosomes, which attach cells to each other, and hemi-desmosomes, which attach cells to the basement membrane. Two types of septate junctions are present: the straight, unbranched, double-septum septate, which is present in epithelial cells throughout embryogenesis, and the pleated, anastomosing, single-septum septate. The latter is formed only on cells that have invaginated to the interior of the embryo to form the digestive tract. The pleated junctions are shown to replace the straight junctions that were originally present before the cells migrated to the interior. It is suggested that these pleated septates may be specialized for digestive processes, since they are developed just prior to feeding and are retained in the adult intestine. Tricellular junctions, which join the bicellular junctions of three adjoining cells, have been identified in the embryo and in the adult intestine. Evidence for the presence of gap junctions was not obtained, but there are indications of their presence.

Ultrastructural changes in chloroplasts resulting from fluctuations in NaCl concentration: freeze-fracture of thylakoid membranes in Dunaliella salina

1975 ◽  
Vol 18 (2) ◽  
pp. 287-299 ◽  
Author(s):  
A.O. Pfeifhofer ◽  
J.C. Belton
Keyword(s):  
Salt Concentration ◽  
Dunaliella Salina ◽  
Unit Area ◽  
Morphological Changes ◽  
Membrane Surface ◽  
Freeze Fracture ◽  
Thylakoid Membranes ◽  
Ultrastructural Changes ◽  
Thin Sections ◽  
Number Of Particles

The structure of chloroplasts isolated from Dunaliella salina has been studied with respect to changing concentrations of sodium chloride in the culture medium. Freeze-fracture replicas and thin sections of intact chloroplasts do not exhibit any noticeable changes in structure at concentrations ranging between 3.5 and 25% NaCl. Chloroplasts isolated from algal cells that have been acclimatized to the higher salt concentration show a change in the thylakoid membranes. The thylakoid membranes appear compressed over a major portion of the membrane surface, with only the end of the thylakoid membranes unappressed. The number of particles per unit area on the B face is also altered by the salt concentration. The chloroplasts acclimatized to 25% NaCl have about 3 times the number of particles per unit area on a B face of end-membranes as on a comparable face of thylakoid membranes acclimatized to low (3.5% NaCl) salt concentration. These morphological changes can be reversed if the chloroplasts acclimatized to high or low salt concentrations are returned to a medium of different salt concentration prior to freeze-fracturing.

Plasma membranes, cell junctions and cuticle of the rectal chloride epithelia of the larval dragonfly Aeshna cyanea

1983 ◽  
Vol 59 (1) ◽  
pp. 159-182
Author(s):  
J. Kukulies ◽  
H. Komnick
Keyword(s):  
Gap Junctions ◽  
Structural Control ◽  
Plasma Membranes ◽  
Freeze Fracture ◽  
Septate Junction ◽  
Cell Junctions ◽  
Junctional Complex ◽  
Apical Region ◽  
Thin Sections ◽  
Aeshna Cyanea

The cell membranes and cell junctions of the rectal chloride epithelia of the larval dragonfly Aeshna cyanea were examined in thin sections and by freeze-fracture. These epithelia function in active ion absorption and maintain a high concentration gradient between the haemolymph and the fresh-water environment. Freeze-fracturing reveals fine-structural differences in the intramembraneous particles of the luminal and contraluminal plasma membranes of these epithelia, reflecting the functional diversity of the two membranes, which are separated by the junctional complex. The particle frequency of the basolateral plasma membranes is reduced after transfer of the larvae into high concentrations of environmental salinity. The junctional complex is located in the apical region and composed of three types of cell junctions: the zonula adhaerens, seen in freeze-fracture as a nearly particle-free zone; the extended and highly convoluted pleated septate junction and randomly interspersed gap junctions of the inverted type. Gap junctions also occur between the basolateral plasma membranes. They provide short-cuts in the diffusion pathway for direct and rapid co-ordination of the interconnected cell processes. Colloidal and ionic lanthanum tracer solutions applied in vivo from the luminal side penetrate through the cuticle via epicuticular depressions, but invade only the apical portion of the junctional complex. This indicates that the pleated septate junction constitutes a structural control of the paracellular pathway across the chloride epithelia, which are devoid of tight junctions. The structure of the pleated septate junctions is interpreted as a device for the extension of the diffusion distance, which is inversely related to the net diffusion. A conservative estimate of the total length of the junction, and the number and extension of septa reveals that the paracellular route exceeds the transcellular route by a factor of 50.

Onset of expression of the variant surface glycoproteins of Trypanosoma brucei in the tsetse fly studied using immunoelectron microscopy

1987 ◽  
Vol 87 (2) ◽  
pp. 363-372 ◽  
Author(s):  
L. Tetley ◽  
C.M. Turner ◽  
J.D. Barry ◽  
J.S. Crowe ◽  
K. Vickerman
Keyword(s):  
Trypanosoma Brucei ◽  
Tsetse Fly ◽  
Surface Glycoprotein ◽  
Variant Surface Glycoprotein ◽  
Fixed Sequence ◽  
Individual Variant ◽  
Variable Antigen ◽  
Variant Surface Glycoproteins ◽  
Scanning Electron

The acquisition of the variant surface glycoprotein (variable antigen) coat by metacyclic stage Trypanosoma brucei in the salivary glands of the tsetse fly, Glossina morsitans, has been studied in situ by transmission and scanning electron microscopy using monoclonal antibodies raised against metacyclic variable antigen types and complexed with horseradish peroxidase or colloidal gold. The coat is acquired after binary fission has ceased but while the parasite is still attached to the gland epithelium, i.e. before the mature metacyclic is released into the gland lumen. The variable antigen type heterogeneity previously observed in discharged mature metacyclics is here demonstrated in the nascent (attached) metacyclic population. The variant surface glycoprotein genes are thus not expressed in a fixed sequence since different metacyclic variable antigen types are present ab initio. The distribution of immunogold-marked nascent metacyclics of a particular variable antigen type, as shown by quadrat analysis of a scanning electron micrograph montage of the infected salivary gland epithelium, conforms to a Poisson series. This provides evidence that individual variant surface glycoprotein genes are stochastically activated and suggests that selective activation occurs after trypanosome division has ceased.

A freeze-fracture study of the cuticle of adult Nippostrongylus brasiliensis (Nematoda)

Parasitology ◽  
1993 ◽  
Vol 107 (5) ◽  
pp. 545-552 ◽  
Author(s):  
D. L. Lee ◽  
K. A. Wright ◽  
R. R. Shivers
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Freeze Fracture ◽  
Surface Coat ◽  
Nippostrongylus Brasiliensis ◽  
Freeze Fracturing ◽  
Transmission Electron ◽  
Fracture Study ◽  
20 Nm

SUMMARYThe surface of the cuticle of adult Nippostrongylus brasiliensis has been studied by means of the freeze-fracture technique and by transmission electron microscopy. Some of the surface coat appears to have been shed from the surface of the cuticle of adults fixed in situ in the intestine of its host and from the surface of individuals removed from the intestine and freeze-fractured. Freeze-fracturing the cuticle of individuals removed from the host has shown that this surface coat varies in thickness from 30 to 90 nm. The epicuticle is about 20 nm thick and cleaves readily to expose E- and P-faces. The P-face of the epicuticle possesses a small number of particles, similar to intra-membranous particles, whilst the E-face possesses a few, widely scattered depressions. Despite the presence of these particles the epicuticle is not considered to be a true membrane. Freeze-fracturing the remainder of the cuticle has confirmed its structure as described by conventional transmission electron microscopy. Clusters of particles on the P-face of the outer epidermal (hypodermal) membrane and corresponding depressions on the E-face of the membrane are thought to be associated with points of attachment of the cuticle to the epidermis (hypodermis). No differences in appearance of the cuticle and its surface layers were observed in individuals taken from 7-, 10-, 13- and 15-day infections.

scholarly journals GPI-anchored Proteins and Free GPI Glycolipids of Procyclic Form Trypanosoma brucei Are Nonessential for Growth, Are Required for Colonization of the Tsetse Fly, and Are Not the Only Components of the Surface Coat

2006 ◽  
Vol 17 (12) ◽  
pp. 5265-5274 ◽  
Author(s):  
Maria Lucia Sampaio Güther ◽  
Sylvia Lee ◽  
Laurence Tetley ◽  
Alvaro Acosta-Serrano ◽  
Michael A.J. Ferguson
Keyword(s):  
Cell Surface ◽  
Trypanosoma Brucei ◽  
Current Model ◽  
Apparent Molecular Weight ◽  
Tsetse Fly ◽  
Surface Coat ◽  
Procyclic Form ◽  
Cell Surface Biotinylation ◽  
A Cell

The procyclic form of Trypanosoma brucei exists in the midgut of the tsetse fly. The current model of its surface glycocalyx is an array of rod-like procyclin glycoproteins with glycosylphosphatidylinositol (GPI) anchors carrying sialylated poly-N-acetyllactosamine side chains interspersed with smaller sialylated poly-N-acetyllactosamine–containing free GPI glycolipids. Mutants for TbGPI12, deficient in the second step of GPI biosynthesis, were devoid of cell surface procyclins and poly-N-acetyllactosamine–containing free GPI glycolipids. This major disruption to their surface architecture severely impaired their ability to colonize tsetse fly midguts but, surprisingly, had no effect on their morphology and growth characteristics in vitro. Transmission electron microscopy showed that the mutants retained a cell surface glycocalyx. This structure, and the viability of the mutants in vitro, prompted us to look for non-GPI–anchored parasite molecules and/or the adsorption of serum components. Neither were apparent from cell surface biotinylation experiments but [3H]glucosamine biosynthetic labeling revealed a group of previously unidentified high apparent molecular weight glycoconjugates that might contribute to the surface coat. While characterizing GlcNAc-PI that accumulates in the TbGPI12 mutant, we observed inositolphosphoceramides for the first time in this organism.

Antigenic variation in Trypanosoma brucei infections: an holistic view

1999 ◽  
Vol 112 (19) ◽  
pp. 3187-3192
Author(s):  
C.M. Turner
Keyword(s):  
Immune Responses ◽  
Trypanosoma Brucei ◽  
Antigenic Variation ◽  
Tsetse Fly ◽  
High Rate ◽  
T Helper 1 ◽  
Holistic View ◽  
Humoral Immune ◽  
Humoral Immune Responses ◽  
Variable Antigen

Trypanosoma brucei parasites undergo clonal phenotypic (antigenic) variation to promote their transmission between mammals and tsetse-fly vectors. This process is classically considered to be a mechanism for evading humoral immune responses, but such an explanation cannot account for the high rate of switching between variable antigens or for their hierarchical (i.e. non-random) expression. I suggest that these anomalies can be explained by a new model: that antigenic variation has evolved as a bifunctional, rather than as a unifunctional, strategy that not only evades humoral immune responses but also enables competition between parasite strains in concomitantly infected hosts. This competition causes a depression of cellular responses. My proposal gives rise to a number of testable predictions. First, low numbers of trypanosomes should express some variable antigen types (VATs) in infections several weeks before these VATs are detectable. Second, as an infection progresses, the number of VATs expressed simultaneously in the population should decrease. Third, immunisation to generate a T helper 1 response against those VATs that are expressed most frequently should lower parasitaemias and reduce virulence.

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