Quantitative ultrastructural differences between strains of the Tryponasoma brucei subgroup during transformation in blood

FLAgellum Member 8 modulates extravascular distribution of African trypanosomes

In the mammalian host, the biology of tissue-dwelling Trypanosoma brucei parasites is not completely understood, especially the mechanisms involved in their extravascular colonization. The trypanosome flagellum is an essential organelle in multiple aspects of the parasites' development. The flagellar protein termed FLAgellar Member 8 (FLAM8) acts as a docking platform for a pool of cyclic AMP response protein 3 (CARP3) that is involved in signaling. FLAM8 exhibits a stage-specific distribution suggesting specific functions in the mammalian and vector stages of the parasite. Analyses of knockdown and knockout trypanosomes in their mammalian forms demonstrated that FLAM8 is not essential in vitro for survival, growth, motility and stumpy differentiation. Functional investigations in experimental infections showed that FLAM8-deprived trypanosomes can establish and maintain an infection in the blood circulation and differentiate into insect transmissible forms. However, quantitative bioluminescence imaging and gene expression analysis revealed that FLAM8-null parasites exhibit a significantly impaired dissemination in the extravascular compartment, that is restored by the addition of a single rescue copy of FLAM8. In vitro trans-endothelial migration assays revealed significant defects in trypanosomes lacking FLAM8. FLAM8 is the first flagellar component shown to modulate T. brucei distribution in the host tissues, possibly through sensing functions, contributing to the maintenance of extravascular parasite populations in mammalian anatomical niches, especially in the skin.

A comparative molecular and 3-dimensional structural investigation into cross-continental and novel avian Trypanosoma spp. in Australia

BACKGROUND

Molecular and structural information on avian Trypanosoma spp. throughout Australia is limited despite their intrinsic value in understanding trypanosomatid evolution, diversity, and structural biology. In Western Australia tissue samples (n = 429) extracted from 93 birds in 25 bird species were screened using generic PCR primers to investigate the diversity of Trypanosoma spp. To investigate avian trypanosome structural biology the first 3-dimensional ultrastructural models of a Trypanosoma spp. (Trypanosoma sp. AAT) isolated from a bird (currawong, Strepera spp.) were generated using focussed ion beam milling combined with scanning electron microscopy (FIB-SEM).

RESULTS

Here, we confirm four intercontinental species of avian trypanosomes in native Australian birds, and identify a new avian Trypanosoma. Trypanosome infection was identified in 18 birds from 13 different bird species (19%). A single new genotype was isolated and found to be closely related to T. culicavium (Trypanosoma sp. CC2016 B002). Other Trypanosoma spp. identified include T. avium, T. culicavium, T. thomasbancrofti, Trypanosoma sp. TL.AQ.22, Trypanosoma sp. AAT, and an uncharacterised Trypanosoma sp. (group C-III sensu Zidková et al. (Infect Genet Evol 12:102-112, 2012)), all previously identified in Australia or other continents. Serially-sectioning Trypanosoma sp. AAT epimastigotes using FIB-SEM revealed the disc-shaped kinetoplast pocket attached perpendicular to the branching mitochondrion. Additionally, the universal minicircle sequence within the kinetoplast DNA and the associated binding protein were determined in Trypanosoma sp. AAT.

CONCLUSIONS

These results indicate that bird trypanosomes are relatively conserved across continents, while being locally diverse, which supports the hypothesis that bird trypanosomes exist as fewer species than described in the literature. Evidence exists that avian Trypanosoma spp. are infecting mammals and could be transmitted by haemadipsid leeches. Trypanosoma sp. AAT is most likely a separate species currently found only in Australia and the first 3-dimentional ultrastructural analysis of an avian trypanosome provides interesting information on their morphology and organelle arrangement.

Biology of human pathogenic trypanosomatids: epidemiology, lifecycle and ultrastructure

Leishmania and Trypanosoma belong to the Trypanosomatidae family and cause important human infections such as leishmaniasis, Chagas disease, and sleeping sickness. Leishmaniasis, caused by protozoa belonging to Leishmania, affects about 12 million people worldwide and can present different clinical manifestations, i.e., visceral leishmaniasis (VL), cutaneous leishmaniasis (CL), mucocutaneous leishmaniasis (MCL), diffuse cutaneous leishmaniasis (DCL), and post-kala-azar dermal leishmaniasis (PKDL). Chagas disease, also known as American trypanosomiasis, is caused by Trypanosoma cruzi and is mainly prevalent in Latin America but is increasingly occurring in the United States, Canada, and Europe. Sleeping sickness or human African trypanosomiasis (HAT), caused by two sub-species of Trypanosoma brucei (i.e., T. b. rhodesiense and T. b. gambiense), occurs only in sub-Saharan Africa countries. These pathogenic trypanosomatids alternate between invertebrate and vertebrate hosts throughout their lifecycles, and different developmental stages can live inside the host cells and circulate in the bloodstream or in the insect gut. Trypanosomatids have a classical eukaryotic ultrastructural organization with some of the same main organelles found in mammalian host cells, while also containing special structures and organelles that are absent in other eukaryotic organisms. For example, the mitochondrion is ramified and contains a region known as the kinetoplast, which houses the mitochondrial DNA. Also, the glycosomes are specialized peroxisomes containing glycolytic pathway enzymes. Moreover, a layer of subpellicular microtubules confers mechanic rigidity to the cell. Some of these structures have been investigated to determine their function and identify potential enzymes and metabolic pathways that may constitute targets for new chemotherapeutic drugs.

The heart of darkness: growth and form of Trypanosoma brucei in the tsetse fly

The first description of African trypanosomes was made over a century ago. The importance of the tsetse in transmission and cyclic development of trypanosomes was discovered soon afterwards, and has been the focus of numerous studies since. However, investigation of trypanosomes in tsetse flies requires high resource investment and unusual patience; hence, many facets of trypanosome biology in the tsetse remain to be characterised despite the long history of research. Here, current knowledge and questions about some of the developmental changes in trypanosomes that occur in tsetse flies are summarised, along with recent technical advances that can now be used to provide some answers.

Particularities of mitochondrial structure in parasitic protists (Apicomplexa and Kinetoplastida)

Without mitochondria, eukaryotic cells would depend entirely on anaerobic glycolysis for ATP generation. This also holds true for protists, both free-living and parasitic. Parasitic protists include agents of human and animal diseases that have a huge impact on world populations. In the phylum Apicomplexa, several species of Plasmodium cause malaria, whereas Toxoplasma gondii is a cosmopolite parasite found on all continents. Flagellates of the order Kinetoplastida include the genera Leishmania and Trypanosoma causative agents of human leishmaniasis and (depending on the species) African trypanosomiasis and Chagas disease. Although clearly distinct in many aspects, the members of these two groups bear a single and usually well developed mitochondrion. The single mitochondrion of Apicomplexa has a dense matrix and many cristae with a circular profile. The organelle is even more peculiar in the order Kinetoplastida, exhibiting a condensed network of DNA at a specific position, always close to the flagellar basal body. This arrangement is known as Kinetoplast and the name of the order derived from it. Kinetoplastids also bear glycosomes, peroxisomes that concentrate enzymes of the glycolytic cycle. Mitochondrial volume and activity is maximum when glycosomal is low and vice versa. In both Apicomplexa and trypanosomatids, mitochondria show particularities that are absent in other eukaryotic organisms. These peculiar features make them an attractive target for therapeutic drugs for the diseases they cause.

Accumulation of a GPI-anchored protein at the cell surface requires sorting at multiple intracellular levels

Proteins modified by glycosylphosphatidylinositol membrane anchors have become popular for investigating the role of membrane lipid microdomains in cellular sorting processes. To this end, trypanosomatids offer the advantage that they express these molecules in high abundance. The parasitic protozoan Trypanosoma brucei is covered by a dense and nearly homogeneous coat composed of a glycosylphosphatidylinositol-anchored protein, the variant surface glycoprotein, which is essential for survival of the parasite in the mammalian blood. Therefore, T. brucei must possess mechanisms to selectively and efficiently deliver variant surface glycoprotein to the cell surface. In this study, we have quantified the steady-state distribution of variant surface glycoprotein by differential biotinylation, by fluorescence microscopy and by immunoelectron microscopy on high-pressure frozen and freeze-substituted samples. These three techniques provide very similar estimates of the fraction of variant surface glycoprotein located on the cell surface, on average 89.4%. The intracellular variant surface glycoprotein (10.6%) is predominantly located in the endosomal compartment (75%), while 25% are associated with the endoplasmic reticulum, Golgi apparatus and lysosomes. The density of variant surface glycoprotein in the plasma membrane including the membrane of the flagellar pocket, the only site for endo- and exocytosis in this organism, is 48-52 times higher than the density in endoplasmic reticulum membranes. The relative densities of the Golgi complex and of the endosomes are 2.7 and 10.8, respectively, compared to the endoplasmic reticulum. This data set provides the basis for an analysis of the dynamics of sorting. Depending on the intracellular itinerary of newly formed variant surface glycoprotein, the high surface density is achieved in two (endoplasmic reticulum --> Golgi complex --> cell surface) or three enrichment steps (endoplasmic reticulum --> Golgi complex --> endosomes --> cell surface), suggesting sorting between several membrane compartments.

The reservosomes of epimastigote forms of Trypanosoma cruzi: occurrence during in vitro cultivation

Reservosomes are large membrane-bound structures found mainly at the posterior end of epimastigote forms of Trypanosoma cruzi, the agent of Chagas' disease. We screened 5-day-old culture forms of the following strains at the ultrastructural level for the presence of reservosomes: T. cruzi strains Y and YuYu as well as Trypanosoma (Schizotrypanum) spp. strains M431, M504, and M519 isolated from bats (Epitesicus sp.) in Santa Catarina, Brazil. The organelles could be found in all strains analyzed, supporting a previous hypothesis that they are a marker of trypanosomes of the Schizotrypanum subgenus. Stereological analysis was carried out with the Y strain to follow the development of reservosomes and lipid droplets during in vitro cultivation of the parasites. Reservosomes were partitioned to the daughter cells during cell division such that the organelles were present in newly formed parasites. Estimation of the volume density after 3, 5, 7, 9, and 12 days of cultivation showed that it was lower in younger cultures, becoming maximal at day 9 (8.0%), but decreased in older cultures (5.9% at day 12). Morphological changes also occurred: type I reservosomes presented an electron-dense matrix with lipid droplets and were characteristic of younger cultures, whereas type II reservosomes presented a homogeneous matrix without lipid inclusions and predominated in older cultures. These organelles were absent in bloodstream trypomastigote forms isolated from infected mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of G1 to S cell cycle progression of Trypanosoma brucei S427cl1 organisms under axenic conditions

Trypanosoma brucei S427cl1 organisms made 6 divisions in modified minimal essential medium (BMEM) supplemented with fetal bovine serum (FBS)-low or high density lipoprotein (LDL, HDL) and fatty acid-free bovine serum albumin (FAF-BSA). Omission of lipoproteins or FAF-BSA from the medium caused the parasites to accumulate in G1 of the cell cycle and to lose the ability to replicate at 37 degrees C. Proteinase K-treated LDL or HDL, which did not have detectable apolipoprotein, supported the G1 to S cell cycle transition of T. brucei S427cl1 organisms in BMEM supplemented with FAF-BSA. Addition of C6:0, C7:0 or fatty C8:0 fatty acid (1 mol fatty acid mol-1 FAF-BSA in the incubation mixture) to serum-free medium supplemented with LDL or HDL and FAF-BSA prevented T. brucei S427cl1 organisms from progressing through G1 into S of the cell cycle. T. brucei S427cl1 organisms became stumpy-like forms during plateau phase growth under axenic conditions. Stumpy-like T. brucei S427cl1 organisms were mainly in G1 of the cell cycle, expressed raised levels of NAD diaphorase activity, were unable to replicate at 37 degrees C, but were able to differentiate to replicating procyclic organisms. Medium collected from plateau phase cultures of T. brucei S427cl1 did not support the G1 to S cell cycle transition of exponentially growing T. brucei organisms. The capacity of plateau phase medium to support G1 to S transition of T. brucei S427cl1 organisms was restored by addition of FAF-BSA and its capacity to support 4 cycles of replication of the parasites was restored by addition of FAF-BSA and LDL or HDL.

A stereological study of the differentiation process in Trypanosoma cruzi

When epimastigote forms of Trypanosoma cruzi grown in a rich medium (LIT) are transferred to a simple, chemically defined medium (TAU3AAG, containing Ca2+, Mg2+, K+, Na+, L-proline, L-glutamate, and L-aspartate in phosphate buffer) they transform into trypomastigote forms. Morphometric analysis of transmission electron micrographs of thin sections of parasites collected at different steps of the transformation process showed that no changes occurred in the volume density of mitochondria and cytoplasmic vacuoles. However, a significant increase in the volume density of the kinetoplast DNA network as well as the lipid inclusions and a decrease in that of the reservosome (a special type of endosome) was observed. These observations suggest that during differentiation, T. cruzi accumulates lipids and uses molecules contained in the reservosome as its main energy source.

Receptor-mediated endocytosis in the bloodstream form of Trypanosoma brucei

The uptake of various host plasma proteins by the bloodstream form of Trypanosoma brucei was studied both biochemically, using radiolabeled proteins, and with the electron microscope, using colloidal gold particles as molecular tracers onto which plasma proteins had been adsorbed. Total plasma proteins and serum albumin were taken up by a mechanism of fluid endocytosis with low clearance (0.1 microliter [mg cell protein]-1 h-1), while low-density lipoprotein (LDL) and transferrin were taken up by a receptor-mediated process with a clearance of two to three orders of magnitude higher than that of serum albumin. Binding prior to uptake of LDL and transferrin was saturable, depended on the presence of Ca2+, and the labeled ligand could be displaced by the homologous but not by heterologous protein. Binding of gold-labeled proteins was seen only to the membrane of the flagellar pocket and not elsewhere on the plasma membrane. After 1 h of incubation at 30 degrees C with gold-labeled LDL and transferrin, labeled cellular structures represented respectively half and one-third of the total volume of all single-membrane bounded endocytotic and electron-dense vacuoles within the cell.

Trypanosoma brucei: cis-aconitate and temperature reduction as triggers of synchronous transformation of bloodstream to procyclic trypomastigotes in vitro

Synchronous transformation of the monomorphic Trypanosoma brucei 427 variant clone MITat 1.4 (117) from bloodstream to procyclic trypomastigotes was studied in modified minimum essential medium plus 15% inactivated horse serum. Repression of variant surface glycoprotein synthesis, subsequent morphological transformation, and growth of procyclic cells was triggered by the simultaneous action of two signals: a reduction in temperature from 37 to 27 C and the addition of cis-aconitate. Repression of variant surface glycoprotein synthesis initiated by these two signals is reversible during the first hours, but becomes irreversible after about 1 day. Thereafter, cells are committed to differentiation at 27 C.

The effect of citrate/cis-aconitate on oxidative metabolism during transformation of Trypanosoma brucei

Monomorphic bloodstream forms of Trypanosoma brucei, grown in the mammal, are deficient in aconitase and 2-oxoglutarate dehydrogenase and they do not respire in the presence of the substrates citrate, cis-aconitate, succinate, proline or 2-oxoglutarate. When grown in vitro low levels of aconitase, succinate oxidase and proline oxidase are detected. Addition of citrate/cis-aconitate at 37 degrees C to bloodstream forms leads to the formation of aconitase and proline oxidase. Most cells undergo an 'abortive' transformation to non-dividing procyclic-like cells while some cells adapt to the presence of the citric acid cycle intermediates and continue to multiply as bloodstream forms. At 27 degrees C and in the presence of citrate/cis-aconitate bloodstream forms transform synchronously to dividing procyclic cells. Within 72 h the rate of respiration with proline, succinate and 2-oxoglutarate becomes similar to that in established procyclic cells while the rate of glucose oxidation decreases. The possible role of citric acid cycle intermediates in determining whether a trypanosome will retain the properties of a bloodstream trypomastigote or differentiate to a procyclic trypomastigote is discussed.

Three dimensional structure of the Leishmania amastigote as revealed by computer-aided reconstruction from serial sections

Computer-aided reconstruction from serial sections has been used to analyse the 3-dimensional structure of entire amastigotes of Leishmania mexicana and to determine the number, arrangement and volume of each organelle. In two reconstructions, the lysosome-like 'megasomes' were the most numerous organelle, there being 34 in one amastigote, and they comprised as much as 15% of the total cell volume. In contrast, as few as 9 glycosomes were present, accounting for less than 1% of the cell volume. The unitary nature of the mitochondrion was confirmed and its complex basket-like structure was revealed. The spatial arrangement of the cell organelles is here displayed in stereo-pairs.

Absence of detectable alteration in the kinetoplast DNA of a Trypanosoma brucei clone following loss of ability to infect the insect vector (Glossina morsitans)

A monomorphic bloodstream population of Trypanosoma brucei EATRO 1244 was derived from a cloned pleomorphic parental population by 77 rapid passages through mice. Loss of pleomorphism was accompanied by increased virulence of trypanosomes towards the mammal, by loss of ability to infect the tsetse fly, Glossina morsitans, loss of ability to transform to the procyclic stage in vitro at 26 degrees C, and by loss of oligomycin-sensitive ATPase activity in trypanosome homogenates. No differences in the maxicircle component of the kinetoplast DNAs (kDNA) of the two populations were detected by electron microscopy of kDNA network spreads or by electrophoretic analysis of restriction endonuclease digests. It appears, therefore, that loss of transmissibility and associated ability of the trypanosomes to activate the mitochondrion need not necessarily be the result of deletions in the mitochondrial (maxicircle) genome. We suggest that point mutations in critical mitochondrial genes, undetectable using out methods, or mutations of nuclear genes coding for important mitochondrial enzymes, may account for the observed changes in phenotype.

Role of glycerol permeation in the bloodstream form of Trypanosoma brucei

Under aerobic conditions, we have determined glycerol uptake in the long slender (LS) bloodstream form of Trypanosoma (Trypanozoon) brucei brucei by studying glycerophosphate accumulation in the parasites. The coupled enzyme theory applies to the permeation-phosphorylation sequence. Glycerol passage through the plasma membrane is asymmetric, the efflux process being favored over the influx process. No free diffusion of glycerol can be detected even under conditions under which free glycerol accumulates within the cells; most probably, glycerol permeation is mediated by a specific transport system. In the absence of respiratory activities, glycerol is known to be an end-product of T. brucei glycolysis; its production from glycerophosphate should allow ATP synthesis. The observed efflux of free glycerol following intracellular accumulation of glycerophosphate confirms the hypothesis that glycerol production occurs through reversal of glycerol kinase activity. We conclude that in vivo the role of the carrier-mediated asymmetric permeation process is to prevent inhibition of the reversal of the glycerol kinase-mediated reaction by removing free glycerol.

Characterization of Trypanosoma brucei isolated from lymph nodes of rats

Forms of Trypanosoma brucei isolated from lymph nodes of cyclically or intravenously infected rats, at different stages of infection, were morphologically compared with the corresponding bloodstream forms. Unlike the pleomorphic bloodstream trypanosome population, the lymph node trypanosome population was essentially monomorphic and resembled the slender forms of the bloodstream. Values for the morphometric parameters indicated a position between the slender and the stumpy forms. The number of trypanosomes/ml of blood showed the well-known periodic fluctuations while the number of trypanosomes/g of lymph node remained fairly constant. Serology revealed that lymph node trypanosome populations differed antigenically from the trypanosome populations isolated simultaneously frrom the bloodstream.

D-Glucose transport in Trypanosoma brucei. D-Glucose transport is the rate-limiting step of its metabolism

D-Glucose and 2-deoxyglucose enter the 'long-slender' bloodstream form of Trypanosoma brucei only by means of a carrier-mediated process; no free diffusion can be observed. Permeation is not energy-dependent. The uptake is driven by the downhill concentration gradient of free substrate. The latter is maintained by the continual removal of sugar, due to the extremely high activities of the glycolytic enzymes. The permeation process is the rate-limiting step of glucose consumption, because permeation proceeds at a rate slower than metabolism. The inhibition of sugar uptake by glycerol was tested. Interactions at the carrier site can be ruled out since glucose and its 2-deoxy analog exhibit different inhibition kinetics.

Identification of economically important parasites

The taxonomy of a parasite can be an important guide to its pathogenic characteristics. A wide range of anatomical, biochemical and behavioural tests is now being developed to define different strains and subspecies of the main tropical parasites.

Quantitative ultrastructural investigations of the life cycle of Trypanosoma brucei: a morphometric analysis

The quantitative ultrastructure of the developmental stages of Trypanosoma brucei brucei in its vector Glossina morsitans was studied by morphometric analysis. Values from ectoperitrophic midgut forms, proventricular forms, epimastigote and metacyclic forms in the salivary gland are compared with results from bloodstream forms, published previously. Significant differences in the volume densities of the trypanosome's single mitochondrion, of microbody-like organelles and in the surface densities of inner and outer mitochondrial membranes were found throughout the whole life cycle. A great increase in volume density of the mitochondrion was observed after transfer to the insect host; reduction took place during metacyclic development. Parallel to the biogenesis of the mitochondrion a reduction of microbodies was found in proventricular forms and there was a great increase in metacyclic forms concomitant with the regression of the mitochondrion. Metacyclic forms had a close quantitative morphologic similarity to bloodstream forms. The results are discussed in connection with changes in structure and in oxidative metabolism.