The requirement for epimastigote attachment during division and metacyclogenesis in Trypanosoma congolense

A sticky situation: When trypanosomatids attach to insect tissues

Transmission of trypanosomatids to their mammalian hosts requires a complex series of developmental transitions in their insect vectors, including stable attachment to an insect tissue. While there are many ultrastructural descriptions of attached cells, we know little about the signaling events and molecular mechanisms involved in this process. Each trypanosomatid species attaches to a specific tissue in the insect at a particular stage of its life cycle. Attachment is mediated by the flagellum, which is modified to accommodate a filament-rich plaque within an expanded region of the flagellar membrane. Attachment immediately precedes differentiation to the mammal-infectious stage and in some cases a direct mechanistic link has been demonstrated. In this review, we summarize the current state of knowledge of trypanosomatid attachment in insects, including structure, function, signaling, candidate molecules, and changes in gene expression. We also highlight remaining questions about this process and how the field is poised to address them through modern approaches.

The Haptomonad Stage of Crithidia acanthocephali in Apis mellifera Hindgut

Crithidia acanthocephali is a trypanosomatid species that was initially described in the digestive tract of Hemiptera. However, this parasite was recently detected in honey bee colonies in Spain, raising the question as to whether bees can act as true hosts for this species. To address this issue, worker bees were experimentally infected with choanomastigotes from the early stationary growth phase and after 12 days, their hindgut was extracted for analysis by light microscopy and TEM. Although no cellular lesions were observed in the honey bee’s tissue, trypanosomatids had differentiated and adopted a haptomonad morphology, transforming their flagella into an attachment pad. This structure allows the protozoa to remain attached to the gut walls via hemidesmosomes-such as junctions. The impact of this species on honey bee health, as well as the pathogenic mechanisms involved, remains unknown. Nevertheless, these results suggest that insect trypanosomatids may have a broader range of hosts than initially thought.

First description of Lotmaria passim and Crithidia mellificae haptomonad stages in the honeybee hindgut

The remodelling of flagella into attachment structures is a common and important event in the trypanosomatid life cycle. Lotmaria passim and Crithidia mellificae can parasitize Apis mellifera, and as a result they might have a significant impact on honeybee health. However, there are details of their life cycle and the mechanisms underlying their pathogenicity in this host that remain unclear. Here we show that both L. passim promastigotes and C. mellificae choanomastigotes differentiate into haptomonad stages covering the ileum and rectum of honeybees. These haptomonad cells remain attached to the host surface via zonular hemidesmosome-like structures, as revealed by transmission electron microscopy. This work describes for the first known time the haptomonad morphotype of these species and their hemidesmosome-like attachments in A. mellifera, a key trait used by other trypanosomatid species to proliferate in the insect host hindgut.

Dramatic changes in gene expression in different forms of Crithidia fasciculata reveal potential mechanisms for insect-specific adhesion in kinetoplastid parasites

Kinetoplastids are a group of parasites that includes several medically-important species. These human-infective species are transmitted by insect vectors in which the parasites undergo specific developmental transformations. For each species, this includes a stage in which parasites adhere to insect tissue via a hemidesmosome-like structure. Although this structure has been described morphologically, it has never been molecularly characterized. We are using Crithidia fasciculata, an insect parasite that produces large numbers of adherent parasites inside its mosquito host, as a model kinetoplastid to investigate both the mechanism of adherence and the signals required for differentiation to an adherent form. An advantage of C. fasciculata is that adherent parasites can be generated both in vitro, allowing a direct comparison to cultured swimming forms, as well as in vivo within the mosquito. Using RNAseq, we identify genes associated with adherence in C. fasciculata. As almost all of these genes have orthologs in other kinetoplastid species, our findings may reveal shared mechanisms of adherence, allowing investigation of a crucial step in parasite development and disease transmission. In addition, dual-RNAseq allowed us to explore the interaction between the parasites and the mosquito. Although the infection is well-tolerated, anti-microbial peptides and other components of the mosquito innate immune system are upregulated. Our findings indicate that C. fasciculata is a powerful model system for probing kinetoplastid-insect interactions.

Kinetoplastids are single-celled parasites that cause devastating human diseases worldwide. Although this group includes many species that infect a variety of hosts, they have a great deal of shared biology. One relatively unexplored aspect of the kinetoplastid life cycle is their ability to adhere to insect tissue. For pathogenic species, adherence is critical for transmission by insect vectors. We have used an insect parasite called Crithidia fasciculata as a model kinetoplastid to reveal shared mechanisms of insect adherence. We have compared gene expression profiles of motile, non-adherent C. fasciculata to those of C. fasciculata adhered to non-living substrates and those attached to the hindgut of mosquitoes. Through this analysis, we have identified a large number of candidate proteins that may mediate adhesion in these and related parasites. In addition, our findings suggest that the mosquito immune system is responding to the presence of parasites in the gut. These results establish a new, robust system to explore the interaction between kinetoplastids and their insect hosts.

Trypanosoma congolense: In Vitro Culture and Transfection

Trypanosoma congolense, together with T. vivax and T. brucei, causes African animal trypanosomiasis (AAT), or nagana, a livestock disease carried by bloodsucking tsetse flies in sub-Saharan Africa. These parasitic protists cycle between two hosts: mammal and tsetse fly. The environment offered by each host to the trypanosome is markedly different, and hence the metabolism of stages found in the mammal differs from that of insect stages. For research on new diagnostics and therapeutics, it is appropriate to use the mammalian life cycle stage, bloodstream forms. Insect stages such as procyclics are useful for studying differentiation and also serve as a convenient source of easily cultured, non-infective organisms. Here, we present protocols in current use in our laboratory for the in vitro culture of different life cycle stages of T. congolense-procyclics, epimastigotes, and bloodstream forms-together with methods for transfection enabling the organism to be genetically modified. © 2019 by John Wiley & Sons, Inc.

Expression profiling of Trypanosoma congolense genes during development in the tsetse fly vector Glossina morsitans morsitans

Background

The tsetse transmitted parasitic flagellate Trypanosoma congolense causes animal African trypanosomosis (AAT) across sub-Saharan Africa. AAT negatively impacts agricultural, economic, nutritional and subsequently, health status of the affected populace. The molecular mechanisms that underlie T. congolense’s developmental program within tsetse are largely unknown due to considerable challenges with obtaining sufficient parasite cells to perform molecular studies.

Methods

In this study, we used RNA-seq to profile T. congolense gene expression during development in two distinct tsetse tissues, the cardia and proboscis. Indirect immunofluorescent antibody test (IFA) and confocal laser scanning microscope was used to localize the expression of a putative protein encoded by the hypothetical protein (TcIL3000_0_02370).

Results

Consistent with current knowledge, genes coding several variant surface glycoproteins (including metacyclic specific VSGs), and the surface coat protein, congolense epimastigote specific protein, were upregulated in parasites in the proboscis (PB-parasites). Additionally, our results indicate that parasites in tsetse’s cardia (C-parasites) and PB employ oxidative phosphorylation and amino acid metabolism for energy. Several genes upregulated in C-parasites encoded receptor-type adenylate cyclases, surface carboxylate transporter family proteins (or PADs), transport proteins, RNA-binding proteins and procyclin isoforms. Gene ontology analysis of products of genes upregulated in C-parasites showed enrichment of terms broadly associated with nucleotides, microtubules, cell membrane and its components, cell signaling, quorum sensing and several transport activities, suggesting that the parasites colonizing the cardia may monitor their environment and regulate their density and movement in this tissue. Additionally, cell surface protein (CSP) encoding genes associated with the Fam50 ‘GARP’, ‘iii’ and ‘i’ subfamilies were also significantly upregulated in C-parasites, suggesting that they are important for the long non-dividing trypomastigotes to colonize tsetse’s cardia. The putative products of genes that were upregulated in PB-parasites were linked to nucleosomes, cytoplasm and membrane-bound organelles, which suggest that parasites in this niche undergo cell division in line with prior findings. Most of the CSPs upregulated in PB-parasites were hypothetical, thus requiring further functional characterization. Expression of one such hypothetical protein (TcIL3000_0_02370) was analyzed using immunofluorescence and confocal laser scanning microscopy, which together revealed preferential expression of this protein on the entire surface coat of T. congolense parasite stages that colonize G. m. morsitans’ proboscis.

Conclusion

Collectively, our results provide insight into T. congolense gene expression profiles in distinct niches within the tsetse vector. Our results show that the hypothetical protein TcIL3000_0_02370, is expressed on the entire surface of the trypanosomes inhabiting tsetse’s proboscis. We discuss our results in terms of their relevance to disease transmission processes.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2964-8) contains supplementary material, which is available to authorized users.

Shape, form, function and Leishmania pathogenicity: from textbook descriptions to biological understanding

The shape and form of protozoan parasites are inextricably linked to their pathogenicity. The evolutionary pressure associated with establishing and maintaining an infection and transmission to vector or host has shaped parasite morphology. However, there is not a 'one size fits all' morphological solution to these different pressures, and parasites exhibit a range of different morphologies, reflecting the diversity of their complex life cycles. In this review, we will focus on the shape and form of Leishmania spp., a group of very successful protozoan parasites that cause a range of diseases from self-healing cutaneous leishmaniasis to visceral leishmaniasis, which is fatal if left untreated.

Trypanosoma congolense: Molecular Toolkit and Resources for Studying a Major Livestock Pathogen and Model Trypanosome

The African trypanosomiases are diseases of humans and their livestock caused by trypanosomes carried by bloodsucking tsetse flies. Although the human pathogen Trypanosoma brucei is the best known, other trypanosome species are of greater concern for animal health in sub-Saharan Africa. In particular, Trypanosomacongolense is a major cattle pathogen, which is as amenable to laboratory culture as T. brucei, with the advantage that its whole life cycle can be recapitulated in vitro. Thus, besides being worthy of study in its own right, T. congolense could be useful as a model of trypanosome development. Here we review the biology of T. congolense, highlighting significant and intriguing differences from its sister, T. brucei. An up-to-date compilation of methods for cultivating and genetically manipulating T. congolense in the laboratory is provided, based on published work and current development of methods in our lab, as well as a description of available molecular resources.

Trypanosoma congolense: appearance and distribution of variable antigen types during metacyclic differentiation in vitro

Differentiation of epimastigotes and production of infective metacyclic forms of Trypanosoma congolense were examined in a culture system which enabled manipulation of the population density of insect forms. Scanning electron microscopy of cultures revealed the attachment sites of epimastigotes in detail, showing them to be attached as ‘clusters’ or ‘bundles’ and having associated fibrillar structures. Dividing epimastigotes were observed either within individual bundles or in association with two bundles. Metacyclic forms were detected by an immunofluorescence antibody test (IFAT) using metacyclic variable-antigen type (M-VAT) specific monoclonal antibodies, by day 7 after seeding cultures. Trypanosomes expressing M-VATs appeared singly in bundles, observed by both IFAT and an immunogold labelling method. Statistical analysis using Poisson calculations suggested that, in general, the distribution of metacyclics expressing individual M-VATs was random throughout cultures.

The life cycle of Trypanosoma (Nannomonas) congolense in the tsetse fly

Background

The tsetse-transmitted African trypanosomes cause diseases of importance to the health of both humans and livestock. The life cycles of these trypanosomes in the fly were described in the last century, but comparatively few details are available for Trypanosoma (Nannomonas) congolense, despite the fact that it is probably the most prevalent and widespread pathogenic species for livestock in tropical Africa. When the fly takes up bloodstream form trypanosomes, the initial establishment of midgut infection and invasion of the proventriculus is much the same in T. congolense and T. brucei. However, the developmental pathways subsequently diverge, with production of infective metacyclics in the proboscis for T. congolense and in the salivary glands for T. brucei. Whereas events during migration from the proventriculus are understood for T. brucei, knowledge of the corresponding developmental pathway in T. congolense is rudimentary. The recent publication of the genome sequence makes it timely to re-investigate the life cycle of T. congolense.

Methods

Experimental tsetse flies were fed an initial bloodmeal containing T. congolense strain 1/148 and dissected 2 to 78 days later. Trypanosomes recovered from the midgut, proventriculus, proboscis and cibarium were fixed and stained for digital image analysis. Trypanosomes contained in spit samples from individually caged flies were analysed similarly. Mensural data from individual trypanosomes were subjected to principal components analysis.

Results

Flies were more susceptible to infection with T. congolense than T. brucei; a high proportion of flies infected with T. congolense established a midgut and subsequent proboscis infection, whereas many T. brucei infections were lost in the migration from foregut to salivary glands. In T. congolense, trypomastigotes ceased division in the proventriculus and became uniform in size. The trypanosomes retained trypomastigote morphology during migration via the foregut to the mouthparts and we confirmed that the trypomastigote-epimastigote transition occurred in the proboscis. We found no equivalent to the asymmetric division stage in T. brucei that mediates transition of proventricular trypomastigotes to epimastigotes. In T. congolense extremely long epimastigotes with remarkably elongated posterior ends were observed in both the proboscis and cibarium; no difference was found in the developmental stages in these two organs. Dividing trypomastigotes and epimastigotes were recovered from the proboscis, some of which were in transition from trypomastigote to epimastigote and vice versa. It remains uncertain whether these morphological transitions are mediated by cell division, since we also found non-dividing cells with a variously positioned, juxta-nuclear kinetoplast.

Conclusions

We have presented a detailed description of the life cycle of T. congolense in its tsetse fly vector. During development in the fly T. congolense shares a common migratory pathway with its close relative T. brucei, culminating in the production of small metacyclic trypanosomes that can be inoculated with the saliva. Despite this outward similarity in life cycle, the transitional developmental stages in the foregut and mouthparts are remarkably different in the two trypanosome species.

Social motility in african trypanosomes

African trypanosomes are devastating human and animal pathogens that cause significant human mortality and limit economic development in sub-Saharan Africa. Studies of trypanosome biology generally consider these protozoan parasites as individual cells in suspension cultures or in animal models of infection. Here we report that the procyclic form of the African trypanosome Trypanosoma brucei engages in social behavior when cultivated on semisolid agarose surfaces. This behavior is characterized by trypanosomes assembling into multicellular communities that engage in polarized migrations across the agarose surface and cooperate to divert their movements in response to external signals. These cooperative movements are flagellum-mediated, since they do not occur in trypanin knockdown parasites that lack normal flagellum motility. We term this behavior social motility based on features shared with social motility and other types of surface-induced social behavior in bacteria. Social motility represents a novel and unexpected aspect of trypanosome biology and offers new paradigms for considering host-parasite interactions.

African trypanosomes, e.g. Trypanosoma brucei, and related kinetoplastid parasites cause morbidity and mortality in several million people worldwide. Trypanosomes are protists and are thus generally considered to behave as single-celled microorganisms. In other microorganisms, social interactions among individuals lead to development of multicellular communities with specialized and advantageous capabilities versus single cells. The concept of bacteria acting as groups of cells communicating and cooperating with one another has had a major impact on our understanding of bacterial physiology and pathogenesis, but this paradigm has not been applied to parasitic protozoa. Here we report that T. brucei is capable of social behavior when exposed to semisolid surfaces. This behavior, termed social motility, is characterized by the assembly of parasites into multicellular communities with emergent properties that are not evident in single cells. Parasites within communities exhibit polarized movements and cooperate to coordinate their movements in response to an external stimulus. Social motility offers many potential advantages, such as facilitating colonization and navigation through host tissues. The identification of social behavior in T. brucei reveals a novel and unexpected aspect of parasite biology and provides new concepts for considering host-parasite interactions.

Identification and molecular characterization of a novel stage-specific surface protein of Trypanosoma congolense epimastigotes

The cattle pathogen Trypanosoma congolense expresses life cycle stage-specific surface molecules involved in adaptation to different host and vector environments. Here we report the discovery and molecular characterization of a novel stage-specific GPI-anchored surface glycoprotein that is selectively expressed in the epimastigote (EMF) life cycle stage of T. congolense. Culture supernatants of EMF but not of procyclic culture forms (PCFs) promoted adhesion of PCF parasites in an in vitro assay. Biosynthetic labeling experiments showed that these EMF culture supernatants contained a 100kDa trypanosome-derived protein that was not present in supernatants from PCF. We named this molecule "congolense epimastigote-specific protein" (CESP). The gene encoding CESP was isolated from an EMF cDNA library after immunoscreening. The multicopy gene had a 2070-bp open reading frame that encodes a polypeptide of 689 amino acids with a predicted mass of 72.9kDa. The discrepancy between the predicted (72.9kDa) and observed (100kDa) masses may be explained partially by glycosylation of the molecule which has six potential N-glycosylation sites and a predicted GPI anchor. Indeed, metabolic labeling of CESP with [(3)H] ethanolamine revealed that CESP was a GPI-anchored protein. Confocal laser scanning microscopy showed that CESP was expressed only on the surface of the EMF stage of the parasite. The identification of CESP as a unique component of culture supernatants from EMF and that such supernatants can confer plastic-adhesive ability on PCF suggest that CESP is worth further investigation as an adhesion molecule that perhaps allows T. congolense EMF to adhere to the tsetse proboscis.

Growth-phase dependent substrate adhesion in Crithidia fasciculata

Crithidia fasciculata is a trypanosomatid flagellate that parasitizes several species of mosquito. Within the alimentary tract of its host, C. fasciculata exists in two forms: one is a non-motile form, attached in clusters to the lining of the gut, the other a more elongated form swimming freely in the gut lumen. We have developed an in vitro culture system that reproduces the appearance of these two distinct morphological forms. Using two different cultivation methods, shaking and stationary incubations, we have demonstrated that adherence phenotypes are growth-phase dependent. Organisms in the logarithmic phase of growth possess the ability to adhere to substrates; this ability is lost when the organism enters a stationary growth phase. Parasite adherence was independent of cultivation method or substrate. Furthermore, adherent forms of Crithidia maintained their adhesive properties following their removal from substrates. Our data reveal a growth-phase-regulated process of cell attachment that may influence the transmission and dissemination of this parasitic flagellate.

Flagellar attachment of Leishmania promastigotes to plastic film in vitro

Trypanosomatid parasites are able to use their flagella for attachment to cuticular surfaces within their arthropod hosts. In this study the attachment mechanism of Leishmania promastigotes was investigated using a new and quantifiable in vitro assay system. The results showed that hemidesmosomal flagellar attachment to three different plastic substrates occurred (Melinex, Polyvinyl, Thermanox). Attachment density was increased by scratching the surface of the substrate or by coating with the hydrocarbons n-octacosane and paraffin. Variation in attachment density was observed, depending on the culture medium and the parasite isolate used. All four species examined, L. braziliensis, L. donovani, L. major and L. mexicana, were capable of flagellar attachment in vitro. Collectively, these data indicate that flagellar attachment is mediated by a non-specific hydrophobic interaction in Leishmania species.

The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex

Okuda, K., Esteva, M., Segura, E. L., and Bijovsky, A. T. 1999. The cytostome of Trypanosoma cruzi epimastigotes is associated with the flagellar complex. Experimental Parasitology 92, 223-231. Proliferative forms of Trypanosoma cruzi, amastigotes and epimastigotes, have a cytostome, a specialized structure formed by an invagination of the flagellar pocket's membrane surrounded by microtubules and frequently followed by a row of vesicles. All this assemblage penetrates deeply into the cytoplasm overpassing the nucleus. This structure, together with the flagellar pocket, appears to play an important role in the nutrition of the parasite. We demonstrated that the monoclonal antibody 2C4, made-up against isolated flagellar complex of T. cruzi epimastigotes, recognizes a protein doublet of 76 and 87 kDa in total epimastigotes homogenate. The 76-kDa polypeptide is enriched in the detergent-soluble fraction whereas the 87-kDa polypeptide is highly represented in the insoluble fractions and the purified flagella. Immuno-fluorescence assays show the antigen as a small spot at the flagellar pocket region. Immunogold labeling of ultrathin sections of epimastigote forms reveals gold particles at the opening of flagellar pocket, concentrated in the cytostome region. Immunocytochemistry of epimastigote whole-mount cytoskeletons reveals the labeling on an array of three to four microtubules that appears attached to flagellum, running in the direction of the nucleus. Ultrastructural observations have shown that the posterior region of isolated flagella, corresponding to the level of the flagellar pocket, possesses a microtubular structure compatible with that from the cytostome. The relationship between the cytostome, an endocytic organelle, and the flagellum is here described for the first time.

Attachment of Trypanosoma cruzi epimastigotes to hydrophobic substrates and use of this property to separate stages and promote metacyclogenesis

In vivo, epimastigotes of Trypanosoma cruzi colonize a lipidic superficial layer of the rectal cuticle of the vector Triatoma infestans. In vitro, epimastigotes of four cultured strains and one strain from reduviids use a terminal area of the flagellum to attach to a variety of artificial hydrophobic substances, such as hydrocarbons and a range of synthetic plastics. Trypomastigotes did not attach to these substrates. Hydrophilic molecules, such as neutral or negatively charged polysaccharides, did not facilitate binding. Epimastigotes and trypomastigotes were artificially bound by electrostatic forces to positively charged chitosan or DEAE-Sephacel over their entire surface. Tween 20 and lipid-binding serum albumin effectively inhibited the hydrophobic attachment. Based on this hydrophobic interaction of epimastigotes, a new chromatography technique has been devised to gently separate trypomastigotes from epimastigotes using octacosane-coated beads. Furthermore, the in vitro transformation of epimastigotes to trypomastigotes was enhanced if epimastigotes were permitted to attach to hydrophobic, wax-coated culture vessels.

Stage-specific adhesion of Leishmania promastigotes to the sandfly midgut

Although leishmaniasis is transmitted to humans almost exclusively by the bite of infected phlebotomine sandflies, little is known about the molecules controlling the survival and development of Leishmania parasites in their insect vectors. Adhesion of Leishmania promastigotes to the midgut epithelial cells of the sandfly was found to be an inherent property of noninfective-stage promastigotes, which was lost during their transformation to metacyclic forms, thus permitting the selective release of infective-stage parasites for subsequent transmission by bite. Midgut attachment and release was found to be controlled by specific developmental modifications in terminally exposed saccharides on lipophosphoglycan, the major surface molecule on Leishmania promastigotes.

Ultrastructural aspects of Trypanosoma (Megatrypanum) freitasi in co-cultivation with mammalian cells

Cultures of Trypanosoma (Megatrypanum) freitasi with L929 mouse fibroblasts at 27.5 degrees C were examined by scanning and transmission electron microscopy in an attempt to clarify the processes of colony formation by the epimastigotes and of their attachment to substrata. It was seen that the flagellates occupy intercellular spaces and do not associate with intact fibroblasts. As the trypanosome population increases, ever larger portions of the substratum are cleared of fibroblasts and occupied by conglomerates of epimastigote colonies consisting of about a dozen organisms that attach to the substratum by their anterior extremities and form pyramidal clusters. Attachment of the epimastigotes involves the flagellar membrane, which becomes extraordinarily enlarged and assumes various aspects of broad sheets, filaments and loops over the substratum or along the flagellum, which exhibits a shortened axoneme. Desmosome or hemidesmosome plaques are present when the attachment takes place between membranes or between the membrane and the substratum.

Variability of in vitro culture characteristics, including metacyclic trypomastigote production, in different stocks of Trypanosoma congolense

Six cloned stocks of Trypanosoma congolense, isolated from the same area of Eastern Zambia, were maintained in vitro as insect form cultures producing infective metacyclic trypanosomes. Although the same general culture conditions were applied, different handling regimes were required for optimum growth of each stock. During primary isolation, many differences were found in the culture characteristics of the stocks. The time taken for cytoadherence to occur varied from 14 to 62 days, while the interval between attachment and the appearance of infective metacyclic trypanosomes ranged from 9 to 94 days. There was a 10-fold difference in the numbers of metacyclic forms produced by different stocks. Time in culture appeared to have little effect on the production of metacyclic forms, and it is probable that in vitro characteristics of T. congolense depend on the genetic constitution of individual stocks or clones.

Continuous growth and differentiation of Trypanosoma (Megatrypanum) freitasi Rego, Magalhães & Siqueira, 1957, in vitro

Trypanosoma (Megatrypanum) freitasi, a parasite of didelphid opossum, was known to be very difficult to cultivate in conventional media. Co-cultivation with L929 cell line in Baltz's medium at 27.5 degrees C resulted in luxuriant growth of the trypanosome with the production of epimastigote colonies that adhered to the surface of culture flasks or tubes, and transformation into metacyclics. Further transformation was stimulated by raising the incubation temperature. At 37 degrees C the population was of the bloodstream type and resistant to lysis by complement.