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More distantly related human-infective kinetoplastids include Trypanosoma cruzi, which causes Chagas disease in South America and over 20 Leishmania species that cause leishmaniasis. Following tick bite, the parasites invade host red blood cells, causing anaemia and fever, and infection can also lead to cerebral babesiosis (B. bovis only) and death of cattle. Efforts to control parasitic disease are hindered by the challenge of implementing vector control strategies across the vast expanses of tsetse and tick-populated land; emerging drug resistance and the prevalence of counterfeit drugs; and a lack of suitable vaccination programmes. Vector control is complex and comes with many limitations, including the following: (i) Tsetse- and tick-infected regions are vast; therefore, traps can only provide a local level of protection, which needs to be ongoing; (ii) ticks and related insects are a valuable source of nutrition to reptiles and birds and so large-scale insecticide use is not feasible; (iii) cattle plunge-dipping into toxic organophosphates or synthetic pyrethroids can cause significant illness to the farmer and the environment and is not a widely available control option; and (iv) the choice of insecticides used are key, due to selective toxicity and resistance [13].

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. The molecular diversity between parasite species and life-stages correlate with diversity in the hosts they infect or the pathologies they cause. Here, Moloney et al., map the spatial proteomes of two African trypanosome species across two life stages and identify key routes of parasitic adaptation.

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.

The sleeping sickness parasite Trypanosoma brucei has a complex life cycle, alternating between a mammalian host and the tsetse fly vector. A tightly controlled developmental programme ensures parasite transmission between hosts as well as survival within them and involves strict regulation of mitochondrial activities. In the glucose-rich bloodstream, the replicative 'slender' stage is thought to produce ATP exclusively via glycolysis and uses the mitochondrial F1FO-ATP synthase as an ATP hydrolysis-driven proton pump to generate the mitochondrial membrane potential (ΔΨm). The 'procyclic' stage in the glucose-poor tsetse midgut depends on mitochondrial catabolism of amino acids for energy production, which involves oxidative phosphorylation with ATP production via the F1FO-ATP synthase. Both modes of the F1FO enzyme critically depend on FO subunit a, which is encoded in the parasite's mitochondrial DNA (kinetoplast or kDNA). Comparatively little is known about mitochondrial function and the role of kDNA in non-replicative 'stumpy' bloodstream forms, a developmental stage essential for disease transmission. Here we show that the L262P mutation in the nuclear-encoded F1 subunit γ that permits survival of 'slender' bloodstream forms lacking kDNA ('akinetoplastic' forms), via FO-independent generation of ΔΨm, also permits their differentiation into stumpy forms. However, these akinetoplastic stumpy cells lack a ΔΨm and have a reduced lifespan in vitro and in mice, which significantly alters the within-host dynamics of the parasite. We further show that generation of ΔΨm in stumpy parasites and their ability to use α-ketoglutarate to sustain viability depend on F1-ATPase activity. Surprisingly, however, loss of ΔΨm does not reduce stumpy life span. We conclude that the L262P γ subunit mutation does not enable FO-independent generation of ΔΨm in stumpy cells, most likely as a consequence of mitochondrial ATP production in these cells. In addition, kDNA-encoded genes other than FO subunit a are important for stumpy form viability.

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.

Key Points African trypanosomes generate chronic infections, extending over months or years, in mammalian hosts. The infection profile of trypanosomes is influenced by their rate of immune evasion by antigenic variation. A further contributor to the infection profile is the rate of differentiation from proliferative slender forms to non-proliferative transmission stages called stumpy forms. Recent mathematical models have demonstrated the importance of stumpy formation for infection chronicity and, together with cytological analyses, have provided a description of the temporal order of events that generate stumpy forms. We propose that the interplay between antigenic variation and parasite development optimizes the ability to establish and be sustained in mammalian hosts in the field. In the tsetse fly vector, trypanosomal commitment to onward differentiation and establishment is governed by a phosphatase signalling cascade that forms a novel bistable switch. We propose that density-sensing mechanisms in the mammalian host are sustained by an evolutionary selection for transmission competence. During their life cycle, trypanosomes must overcome conflicting demands to ensure their survival and transmission. In this Opinion article, Matthews and colleagues suggest that infection chronicity and transmissibility result from an interplay between programmes of antigenic variation and development. 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.

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. African trypanosome infections can persist for years, but immune evasion mechanisms are not fully understood. Here, Macleod et al . identify a trypanosome receptor for mammalian factor H, a negative regulator of the alternative complement pathway, that increases parasite transmission to tsetse flies.

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.

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 ∼50o 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. African Trypanosomes are a group of single-celled parasites that are a major concern for livestock farmers in sub-Saharan Africa. They are carried by the tsetse fly and can cause disease in domestic livestock that diminishes productivity through reduced growth, and may ultimately lead to death. The parasites are coated in a dense layer of protein that help them evade the host’s immune system by preventing immune cells from identifying them. Humans have evolved immunity to many trypanosome species by exploiting a weakness in their lifestyle. Trypanosomes need to get haem—a molecule found in the protein haemoglobin—from their host to survive. In blood plasma, haemoglobin is found associated with a carrier protein called haptoglobin. To acquire haem, the parasites have a protein called HpHbR that binds to these haptoglobin-haemoglobin ‘complexes’. However, in humans there are two complexes of proteins called TLFs that contain haemoglobin and a protein related to haptoglobin. The TLFs are also able to bind to HpHbR and are taken into the parasite. Once inside, TLFs cause internal compartments called lysosomes to swell, which leads to the death of the trypanosome. Two subspecies of Trypanosoma brucei are the only trypanosomes that infect humans as they can overcome the TLF1 defense. However, the details of how TLFs cause cell death at the molecular level are not clear. Lane-Serff et al. used a technique called x-ray crystallography to generate 3-D images of the HpHbR protein from T. brucei bound to the haptoglobin-haemoglobin complexes. These images show that HpHbR is elongated so that it only binds to haemoglobin and haptoglobin when they are together as a complex. The images also reveal that the shape of HpHbR enables it to hold apart the proteins in the protective layer that coats the trypanosome. This allows the haptoglobin-haemoglobin complex to bind to HpHbR, but in humans also makes HpHbR more likely to bind to TLF1. These findings may help to guide future efforts to protect humans and livestock from the diseases caused by trypanosomes.

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.