Mapping diversity in African trypanosomes using high resolution spatial proteomics

African trypanosomes are dixenous eukaryotic parasites that impose a significant human and veterinary disease burden on sub-Saharan Africa. Diversity between species and life-cycle stages is concomitant with distinct host and tissue tropisms within this group. Here, the spatial proteomes of two African trypanosome species, Trypanosoma brucei and Trypanosoma congolense, are mapped across two life-stages. The four resulting datasets provide evidence of expression of approximately 5500 proteins per cell-type. Over 2500 proteins per cell-type are classified to specific subcellular compartments, providing four comprehensive spatial proteomes. Comparative analysis reveals key routes of parasitic adaptation to different biological niches and provides insight into the molecular basis for diversity within and between these pathogen species.

In silico identification of Theileria parva surface proteins

East Coast Fever is a devastating African cattle disease caused by the apicomplexan parasite, Theileria parva. Little is known about the cell surface, and few proteins have been identified. Here, we take an in silico approach to identify novel cell surface proteins, and predict the structure of four key proteins.

An atypical DYRK kinase connects quorum-sensing with posttranscriptional gene regulation in Trypanosoma brucei

The sleeping sickness parasite, Trypanosoma brucei, uses quorum sensing (QS) to balance proliferation and transmission potential in the mammal bloodstream. A signal transduction cascade regulates this process, a component of which is a divergent member of the DYRK family of protein kinases, TbDYRK. Phylogenetic and mutational analysis in combination with activity and phenotypic assays revealed that TbDYRK exhibits a pre-activated conformation and an atypical HxY activation loop motif, unlike DYRK kinases in other eukaryotes. Phosphoproteomic comparison of TbDYRK null mutants with wild-type parasites identified molecules that operate on both the inhibitory 'slender retainer' and activatory 'stumpy inducer' arms of the QS control pathway. One of these molecules, the RNA-regulator TbZC3H20, regulates parasite QS, this being dependent on the integrity of its TbDYRK phosphorylation site. This analysis reveals fundamental differences to conventional DYRK family regulation and links trypanosome environmental sensing, signal transduction and developmental gene expression in a coherent pathway.

A receptor for the complement regulator factor H increases transmission of trypanosomes to tsetse flies

Persistent pathogens have evolved to avoid elimination by the mammalian immune system including mechanisms to evade complement. Infections with African trypanosomes can persist for years and cause human and animal disease throughout sub-Saharan Africa. It is not known how trypanosomes limit the action of the alternative complement pathway. Here we identify an African trypanosome receptor for mammalian factor H, a negative regulator of the alternative pathway. Structural studies show how the receptor binds ligand, leaving inhibitory domains of factor H free to inactivate complement C3b deposited on the trypanosome surface. Receptor expression is highest in developmental stages transmitted to the tsetse fly vector and those exposed to blood meals in the tsetse gut. Receptor gene deletion reduced tsetse infection, identifying this receptor as a virulence factor for transmission. This demonstrates how a pathogen evolved a molecular mechanism to increase transmission to an insect vector by exploitation of a mammalian complement regulator.

A single dose of antibody-drug conjugate cures a stage 1 model of African trypanosomiasis

Infections of humans and livestock with African trypanosomes are treated with drugs introduced decades ago that are not always fully effective and often have severe side effects. Here, the trypanosome haptoglobin-haemoglobin receptor (HpHbR) has been exploited as a route of uptake for an antibody-drug conjugate (ADC) that is completely effective against Trypanosoma brucei in the standard mouse model of infection. Recombinant human anti-HpHbR monoclonal antibodies were isolated and shown to be internalised in a receptor-dependent manner. Antibodies were conjugated to a pyrrolobenzodiazepine (PBD) toxin and killed T. brucei in vitro at picomolar concentrations. A single therapeutic dose (0.25 mg/kg) of a HpHbR antibody-PBD conjugate completely cured a T. brucei mouse infection within 2 days with no re-emergence of infection over a subsequent time course of 77 days. These experiments provide a demonstration of how ADCs can be exploited to treat protozoal diseases that desperately require new therapeutics.

Non-linear hierarchy of the quorum sensing signalling pathway in bloodstream form African trypanosomes

Trypanosoma brucei, the agents of African trypanosomiasis, undergo density-dependent differentiation in the mammalian bloodstream to prepare for transmission by tsetse flies. This involves the generation of cell-cycle arrested, quiescent, stumpy forms from proliferative slender forms. The signalling pathway responsible for the quorum sensing response has been catalogued using a genome-wide selective screen, providing a compendium of signalling protein kinases phosphatases, RNA binding proteins and hypothetical proteins. However, the ordering of these components is unknown. To piece together these components to provide a description of how stumpy formation arises we have used an extragenic suppression approach. This exploited a combinatorial gene knockout and overexpression strategy to assess whether the loss of developmental competence in null mutants of pathway components could be compensated by ectopic expression of other components. We have created null mutants for three genes in the stumpy induction factor signalling pathway (RBP7, YAK, MEKK1) and evaluated complementation by expression of RBP7, NEK17, PP1-6, or inducible gene silencing of the proposed differentiation inhibitor TbTOR4. This indicated that the signalling pathway is non-linear. Phosphoproteomic analysis focused on one pathway component, a putative MEKK, identified molecules with altered expression and phosphorylation profiles in MEKK1 null mutants, including another component in the pathway, NEK17. Our data provide a first molecular dissection of multiple components in a signal transduction cascade in trypanosomes.

African trypanosome parasites respond to density sensing information in the bloodstream of their mammalian hosts to generate their transmission stage, the stumpy form. Components of this ‘quorum sensing’ signalling cascade are known but their interactions and ordering are not. Here we have dissected the dependency relationships between molecules in the pathway by combinatorial gene knockout and ectopic expression, as well as by detailed phosphoproteomic analysis of one component. Our results provide a first analysis of the signal pathway architecture, revealing that it is non-linear. Moreover, phosphoproteome analysis reveals pathway hierarchy through identifying that the phosphorylation of a NEK kinase component of the pathway is reduced when a predicted upstream kinase is absent. This provides a framework for the coherent dissection of a signal transduction cascade in these parasites that use quorum sensing to control disease spread.

A Receptor's Tale: An Eon in the Life of a Trypanosome Receptor

African trypanosomes have complex life cycles comprising at least ten developmental forms, variously adapted to different niches in their tsetse fly vector and their mammalian hosts. Unlike many other protozoan pathogens, they are always extracellular and have evolved intricate surface coats that allow them to obtain nutrients while also protecting them from the immune defenses of either insects or mammals. The acquisition of macromolecular nutrients requires receptors that function within the context of these surface coats. The best understood of these is the haptoglobin-hemoglobin receptor (HpHbR) of Trypanosoma brucei, which is used by the mammalian bloodstream form of the parasite, allowing heme acquisition. However, in some primates it also provides an uptake route for trypanolytic factor-1, a mediator of innate immunity against trypanosome infection. Recent studies have shown that during the evolution of African trypanosome species the receptor has diversified in function from a hemoglobin receptor predominantly expressed in the tsetse fly to a haptoglobin-hemoglobin receptor predominantly expressed in the mammalian bloodstream. Structural and functional studies of homologous receptors from different trypanosome species have allowed us to propose an evolutionary history for how one receptor has adapted to different roles in different trypanosome species. They also highlight the challenges that a receptor faces in operating on the complex trypanosome surface and show how these challenges can be met.

Evolutionary diversification of the trypanosome haptoglobin-haemoglobin receptor from an ancestral haemoglobin receptor

The haptoglobin-haemoglobin receptor of the African trypanosome species, Trypanosoma brucei, is expressed when the parasite is in the bloodstream of the mammalian host, allowing it to acquire haem through the uptake of haptoglobin-haemoglobin complexes. Here we show that in Trypanosoma congolense this receptor is instead expressed in the epimastigote developmental stage that occurs in the tsetse fly, where it acts as a haemoglobin receptor. We also present the structure of the T. congolense receptor in complex with haemoglobin. This allows us to propose an evolutionary history for this receptor, charting the structural and cellular changes that took place as it adapted from a role in the insect to a new role in the mammalian host.

DOI:http://dx.doi.org/10.7554/eLife.13044.001

Trypanosomes are single-celled parasites that infect a range of animal hosts. These parasites need a molecule called haem to grow properly and are mostly spread by insects that feed on the blood of mammals. Most haem in mammals is found in red blood cells and is bound to a protein called haemoglobin. When it is released from these cells, haemoglobin forms a complex with another protein called haptoglobin as well.

The best-studied trypanosomes from Africa have a receptor protein on their surface that recognizes the haptoglobin-haemoglobin complex and allows the parasites to obtain haem from their hosts. An African trypanosome called T. brucei causes sleeping sickness in humans, and has a receptor that can only recognize haemoglobin when it is in complex with haptoglobin. However, few trypanosome receptors have been studied to date, and so it was not clear if they all work in the same way.

Trypanosoma congolense is a trypanosome that has a big impact on livestock farmers in sub-Saharan Africa and infects cattle, pigs and goats. Lane-Serff, MacGregor et al. now report that the receptor protein from T. congolense can bind to haemoglobin on its own. A technique called X-ray crystallography was used to reveal the three-dimensional structure of the T. congolense receptor and haemoglobin in fine detail. Further experiments then confirmed that the receptor actually binds more strongly to haemoglobin than it does to the haptoglobin-haemoglobin complex.

Experiments with living parasites showed that T. congolense produces its receptor when it is in the mouthparts of its insect host, the tsetse fly. This is unlike what occurs in T. brucei, which only produces its receptor while it is in the bloodstream of its mammalian host. Lane-Serff, MacGregor et al. suggest that T. congolense’s receptor is more like the receptor found in ancestor of the trypanosomes. This means that, at least once during the evolution of these parasites, this receptor evolved from being a haemoglobin receptor produced in the tsetse fly to a haptoglobin-haemoglobin receptor produced in an infected mammal.

The next step is to investigate the details of the role played by the T. congolense receptor when the parasite is in the tsetse fly. It will also be important to understand how this parasite is still able to grow in the mammalian host’s bloodstream even though it does not produce much of the receptor during this stage.

DOI:http://dx.doi.org/10.7554/eLife.13044.002

How Does the VSG Coat of Bloodstream Form African Trypanosomes Interact with External Proteins?

Variations on the statement “the variant surface glycoprotein (VSG) coat that covers the external face of the mammalian bloodstream form of Trypanosoma brucei acts a physical barrier” appear regularly in research articles and reviews. The concept of the impenetrable VSG coat is an attractive one, as it provides a clear model for understanding how a trypanosome population persists; each successive VSG protects the plasma membrane and is immunologically distinct from previous VSGs. What is the evidence that the VSG coat is an impenetrable barrier, and how do antibodies and other extracellular proteins interact with it? In this review, the nature of the extracellular surface of the bloodstream form trypanosome is described, and past experiments that investigated binding of antibodies and lectins to trypanosomes are analysed using knowledge of VSG sequence and structure that was unavailable when the experiments were performed. Epitopes for some VSG monoclonal antibodies are mapped as far as possible from previous experimental data, onto models of VSG structures. The binding of lectins to some, but not to other, VSGs is revisited with more recent knowledge of the location and nature of N-linked oligosaccharides. The conclusions are: (i) Much of the variation observed in earlier experiments can be explained by the identity of the individual VSGs. (ii) Much of an individual VSG is accessible to antibodies, and the barrier that prevents access to the cell surface is probably at the base of the VSG N-terminal domain, approximately 5 nm from the plasma membrane. This second conclusion highlights a gap in our understanding of how the VSG coat works, as several plasma membrane proteins with large extracellular domains are very unlikely to be hidden from host antibodies by VSG.

African trypanosomes have evolved two key strategies to prevent killing by the host immune response and, thus, maintain a long-term infection in a mammal. Both are based on a densely packed coat of a single protein, the variant surface glycoprotein (VSG), which covers the entire extracellular surface of the cell. The first strategy is antigenic variation, through which individual cells switch the identity of the expressed VSG at a low frequency and are selected by the host immune response. If the VSG is novel, the trypanosome proliferates, maintaining the infection; if it doesn't switch, or if the new VSG is not novel, it will be killed. In the second strategy, the VSG acts as a protective barrier, shielding the cell from innate and adaptive immune factors until there is an overwhelming titre of antibodies recognising the expressed VSG. In this review, the VSG coat is modelled, and past experiments that investigated how it protected the trypanosome are revisited using current knowledge of VSG sequence and structure. The conclusions are: (i) the identity of the individual VSGs explains early experimental variation; (ii) most of the VSG molecule is accessible to antibodies. This second conclusion highlights a gap in our understanding of how the VSG coat works, as several plasma membrane proteins with large extracellular domains are very unlikely to be hidden from host antibodies by VSG.

Structural basis for ligand and innate immunity factor uptake by the trypanosome haptoglobin-haemoglobin receptor

The haptoglobin-haemoglobin receptor (HpHbR) of African trypanosomes allows acquisition of haem and provides an uptake route for trypanolytic factor-1, a mediator of innate immunity against trypanosome infection. In this study, we report the structure of Trypanosoma brucei HpHbR in complex with human haptoglobin-haemoglobin (HpHb), revealing an elongated ligand-binding site that extends along its membrane distal half. This contacts haptoglobin and the β-subunit of haemoglobin, showing how the receptor selectively binds HpHb over individual components. Lateral mobility of the glycosylphosphatidylinositol-anchored HpHbR, and a ∼50° kink in the receptor, allows two receptors to simultaneously bind one HpHb dimer. Indeed, trypanosomes take up dimeric HpHb at significantly lower concentrations than monomeric HpHb, due to increased ligand avidity that comes from bivalent binding. The structure therefore reveals the molecular basis for ligand and innate immunity factor uptake by trypanosomes and identifies adaptations that allow efficient ligand uptake in the context of the complex trypanosome cell surface.

Genome-wide dissection of the quorum sensing signalling pathway in Trypanosoma brucei

The protozoan parasites Trypanosoma brucei spp. cause important human and livestock diseases in sub Saharan Africa. In the mammalian blood, two developmental forms of the parasite exist: proliferative ‘slender’ forms and arrested ‘stumpy’ forms that are responsible for transmission to tsetse flies. The slender to stumpy differentiation is a density-dependent response that resembles quorum sensing (QS) in microbial systems and is crucial for the parasite life cycle, ensuring both infection chronicity and disease transmission¹. This response is triggered by an elusive ‘stumpy induction factor’ (SIF) whose intracellular signaling pathway is also uncharacterized. Laboratory-adapted (monomorphic) trypanosome strains respond inefficiently to SIF but can generate forms with stumpy characteristics when exposed to cell permeable cAMP and AMP analogues. Exploiting this, we have used a genome-wide RNAi library screen to identify the signaling components driving stumpy formation. In separate screens, monomorphic parasites were exposed to 8-(4-chlorophenylthio)-cAMP (pCPTcAMP) or 8-pCPT-2′-O-Me-5′-AMP to select cells that were unresponsive to these signals and hence remained proliferative. Genome-wide ion torrent-based RNA interference Target sequencing identified cohorts of genes implicated in each step of the signaling pathway, from purine metabolism, through signal transducers (kinases, phosphatases) to gene expression regulators. Genes at each step were independently validated in cells naturally capable of stumpy formation, confirming their role in density sensing in vivo, whilst the putative RNA-binding protein, RBP7, was required for normal QS and promoted cell-cycle arrest and transmission competence when overexpressed. This study reveals that QS signaling in trypanosomes shares similarities to fundamental quiescence pathways in eukaryotic cells, its components providing targets for QS-interference based therapeutics.

Stable transformation of pleomorphic bloodstream form Trypanosoma brucei

African trypanosomes differentiate between various developmental stages both in mammalian hosts and their tsetse vector to adapt to and survive in the different environments they encounter. In the bloodstream, trypanosomes naturally exist as either proliferative slender-forms or non-proliferative stumpy-forms, the latter being responsible for both prolonged infection and transmission. However, most trypanosome studies are carried out on laboratory-adapted monomorphic cell lines, incapable of differentiating to stumpy-forms or completing the life cycle through the tsetse fly. Partly, this has been due to the inefficiency of transfection of pleomorphic strains which have retained the ability to generate stumpy-forms. Recently, Amaxa Nucleofector® technology was shown to increase transfection efficiency for monomorphic bloodstream forms. Using this technology we have optimised a similar method for pleomorphic bloodstream form transfection, generating transfection efficiencies of 10(-7)-10(-6). This permits routine genetic manipulation of pleomorphic lines, which have the most biological relevance for trypanosomes in the field.

Identification of the regulatory elements controlling the transmission stage-specific gene expression of PAD1 in Trypanosoma brucei

Trypanosomatid parasites provide an extreme model for the posttranscriptional control of eukaryotic gene expression. However, most analysis of their differential gene regulation has focussed on comparisons between life-cycle stages that exist in the blood of mammalian hosts and tsetse flies, the parasite's vector. These environments differ acutely in their temperature, and nutritional, metabolic and molecular composition. In the bloodstream, however, a more exquisitely regulated developmental step occurs: the production of transmissible stumpy forms from proliferative slender forms. This transition occurs in the relatively homogenous bloodstream environment, with stumpy-specific gene expression being repressed until accumulation of a proposed parasite-derived signal, stumpy induction factor. Here, we have dissected the regulatory signals that repress the expression of the stumpy-specific surface transporter PAD1 in slender forms. Using transgenic parasites capable of stumpy formation we show that PAD1-repression is mediated by its 3'-untranslated region. Dissection of this region in monomorphic slender forms and pleomorphic slender and stumpy forms has revealed that two regulatory regions co-operate to repress PAD1 expression, this being alleviated on exposure to SIF in pleomorphs or cAMP analogues that act as stumpy induction factor mimics in monomorphs. These studies identify elements that regulate trypanosome gene expression during development in their mammalian host.

Trypanosomal immune evasion, chronicity and transmission: an elegant balancing act

During their life cycle, trypanosomes must overcome conflicting demands to ensure their survival and transmission. First, they must evade immunity without overwhelming the host. Second, they must generate and maintain transmission stages at sufficient levels to allow passage into their tsetse vector. Finally, they must rapidly commit to onward development when they enter the tsetse fly. On the basis of recent quantification and modelling of Trypanosoma brucei infection dynamics, we propose that the interplay between immune evasion and development achieves both infection chronicity and transmissibility. Moreover, we suggest that a novel form of bistable regulation ensures developmental commitment on entry into the tsetse fly midgut.

Transmission stages dominate trypanosome within-host dynamics during chronic infections

Sleeping sickness is characterized by waves of the extracellular parasite Trypanosoma brucei in host blood, with infections continuing for months or years until inevitable host death. These waves reflect the dynamic conflict between the outgrowth of a succession of parasite antigenic variants and their control by the host immune system. Although a contributor to these dynamics is the density-dependent differentiation from proliferative “slender forms” to transmissible “stumpy forms,” an absence of markers discriminating stumpy forms has prevented accurate parameterization of this component. Here, we exploit the stumpy-specific PAD1 marker, which functionally defines transmission competence, to quantitatively monitor stumpy formation during chronic infections. This allows reconstruction of the temporal events early in infection. Mathematical modeling of these data describes the parameters controlling trypanosome within-host dynamics and provides strong support for a quorum-sensing-like mechanism. Our data reveal the dominance of transmission stages throughout infection, a consequence being austere use of the parasite's antigen repertoire.

New discoveries in the transmission biology of sleeping sickness parasites: applying the basics

The sleeping sickness parasite, Trypanosoma brucei, must differentiate in response to the changing environments that it encounters during its complex life cycle. One developmental form, the bloodstream stumpy stage, plays an important role in infection dynamics and transmission of the parasite. Recent advances have shed light on the molecular mechanisms by which these stumpy forms differentiate as they are transmitted from the mammalian host to the insect vector of sleeping sickness, tsetse flies. These molecular advances now provide improved experimental tools for the study of stumpy formation and function within the mammalian bloodstream. They also offer new routes to therapy via high-throughput screens for agents that accelerate parasite development. Here, we shall discuss the recent advances that have been made and the prospects for future research now available.

Modelling trypanosome chronicity: VSG dynasties and parasite density

A new mathematical model developed by Lythgoe et al. shows that the semi-predictable order of trypanosome antigenic variation can be generated by two parasite-intrinsic factors. The first is the different probabilities of antigen-gene activation that result from the different molecular mechanisms by which the genes become expressed. The second is the density-dependent differentiation of slender to stumpy cells. The study has important implications for understanding the dynamics of antigenic variation and for modelling the consequences of therapeutic strategies directed against trypanosomes.

Grief: the unrecognized parental response to mental illness in a child

The normal grief response of parents to the death of a child of any age is well documented in the literature. This response has some unique qualities because of the special nature of the parent-child relationship. Parents whose son or daughter develops a serious mental illness experience this same grief, but often it is not recognized or validated by society and either is not addressed by mental health professionals or is misperceived as evidence of pathology. As a result, healthy expression of parental grief over a child's mental illness is seriously inhibited, and problems, both for parents and the patient, are exacerbated. This article describes some common elements of parental bereavement, losses experienced with mental illness, the consequences of ignoring grief, and appropriate interventions for mental health professionals.

Renal transplantation in end stage renal disease patients with existing urinary diversion

From 1971 to 1984 renal transplantation was performed in 20 patients with end stage renal disease who presented with an existing form of urinary diversion. These patients were evaluated with a cystometrogram, voiding cystourethrogram and cystoscopy. In some cases bladder function was studied further by cycling through a suprapubically placed catheter. The bladder was considered unstable in 13 patients and undiversion was done at transplantation. The period of prior diversion ranged from 3 to 20 years (mean 12.7 years). There were no surgical complications postoperatively and normal bladder function returned in all patients. Currently, 8 patients have a functioning renal allograft 16 months to 9 years after transplantation (mean 4.2 years). Seven patients were considered to have a nonusable bladder owing to severe neurogenic disease or refractory contracture. In these patients transplantation was done into a pre-fashioned intestinal conduit (5) or cutaneous ureterostomy (2). Currently, 4 patients have a functioning renal allograft 16 months to 6.2 years after transplantation (mean 3.8 years). Transplantation candidates who present with an existing form of urinary diversion should be evaluated carefully, since many will have a usable bladder. Regardless of whether the bladder is usable, transplantation can be performed safely with no increased surgical or immunological risk.