A major step towards defining the elusive stumpy inducing factor in Trypanosoma brucei

Trypanosoma brucei stumpy forms are the only stage that can transmit from human to tsetse fly. Stumpy formation is regulated by a quorum sensing mechanism that depends on parasite density and an unknown stumpy induction factor (SIF). Recently, an elegant study by Matthews and colleagues (Cell 176, 1-12) has identified several crucial components of this pathway, including the putative SIF and its receptor

Plasmodium asexual growth and sexual development in the haematopoietic niche of the host

Plasmodium spp. parasites are the causative agents of malaria in humans and animals, and they are exceptionally diverse in their morphology and life cycles. They grow and develop in a wide range of host environments, both within blood-feeding mosquitoes, their definitive hosts, and in vertebrates, which are intermediate hosts. This diversity is testament to their exceptional adaptability and poses a major challenge for developing effective strategies to reduce the disease burden and transmission. Following one asexual amplification cycle in the liver, parasites reach high burdens by rounds of asexual replication within red blood cells. A few of these blood-stage parasites make a developmental switch into the sexual stage (or gametocyte), which is essential for transmission. The bone marrow, in particular the haematopoietic niche (in rodents, also the spleen), is a major site of parasite growth and sexual development. This Review focuses on our current understanding of blood-stage parasite development and vascular and tissue sequestration, which is responsible for disease symptoms and complications, and when involving the bone marrow, provides a niche for asexual replication and gametocyte development. Understanding these processes provides an opportunity for novel therapies and interventions

An ancestral secretory apparatus in the protozoan parasite Giardia intestinalis

The protozoan parasite Giardia intestinalis belongs to one of the earliest diverged eukaryotic lineages. This is also reflected in a simple intracellular organization, as Giardia lacks common subcellular compartments such as mitochondria, peroxisomes, and apparently also a Golgi apparatus. During encystation, developmentally regulated formation of large secretory compartments containing cyst wall material occurs. Despite the lack of any morphological similarities, these encystation-specific vesicles (ESVs) show several biochemical characteristics of maturing Golgi cisternae. Previous studies suggested that Golgi structure and function are induced only during encystation in Giardia, giving rise to the hypothesis that ESVs, as a Giardia Golgi equivalent, are generated de novo. Alternatively, ESV compartments could be built on the template structure of a cryptic Golgi in trophozoites in response to ER export of cyst wall material during encystation. We addressed this question by defining the molecular framework of the Giardia secretory apparatus using a comparative genomic approach. Analysis of the corresponding transcriptome during growth and encystation revealed surprisingly little stage-specific regulation. A panel of antibodies was generated against selected marker proteins to investigate the developmental dynamics of the endomembrane system. We show evidence that Giardia accommodates the export of large amounts of cyst wall material through re-organization of membrane compartment(s) in trophozoites with biochemical similarities to ESVs. This suggests that ESVs are selectively stabilized Golgi-like compartments in a unique and archetypical secretory system, which arise from a structural template in trophozoites rather than being generated de novo

Re-defining the Golgi complex in Plasmodium falciparum using the novel Golgi marker PfGRASP

Plasmodium falciparum, the causative agent of malaria, relies on a sophisticated protein secretion system for host cell invasion and transformation. Although the parasite displays a secretory pathway similar to those of all eukaryotic organisms, a classical Golgi apparatus has never been described. We identified and characterised the putative Golgi matrix protein PfGRASP, a homologue of the Golgi re-assembly stacking protein (GRASP) family. We show that PfGRASP is expressed as a 70 kDa protein throughout the asexual life cycle of the parasite. We generated PfGRASP-GFP-expressing transgenic parasites and showed that this protein is localised to a single, juxtanuclear compartment in ring-stage parasites. The PfGRASP compartment is distinct from the ER, restricted within the boundaries of the parasite and colocalises with the cis-Golgi marker ERD2. Correct subcellular localisation of this Golgi matrix protein depends on a cross-species conserved functional myristoylation motif and is insensitive to Brefeldin A. Taken together our results define the Golgi apparatus in Plasmodium and depict the morphological organisation of the organelle throughout the asexual life cycle of the parasite

Signal-mediated export of proteins from the malaria parasite to the host erythrocyte

Intracellular parasites from the genus Plasmodium reside and multiply in a variety of cells during their development. After invasion of human erythrocytes, asexual stages from the most virulent malaria parasite, P. falciparum, drastically change their host cell and export remodelling and virulence proteins. Recent data demonstrate that a specific NH2-terminal signal conserved across the genus Plasmodium plays a central role in this export process

Probing Plasmodium falciparum sexual commitment at the single-cell level

Background: Malaria parasites go through major transitions during their complex life cycle, yet the underlying differentiation pathways remain obscure. Here we apply single cell transcriptomics to unravel the program inducing sexual differentiation in Plasmodium falciparum. Parasites have to make this essential life-cycle decision in preparation for human-to-mosquito transmission. Methods: By combining transcriptional profiling with quantitative imaging and genetics, we defined a transcriptional signature in sexually committed cells. Results: We found this transcriptional signature to be distinct from general changes in parasite metabolism that can be observed in response to commitment-inducing conditions. Conclusions: This proof-of-concept study provides a template to capture transcriptional diversity in parasite populations containing complex mixtures of different life-cycle stages and developmental programs, with important implications for our understanding of parasite biology and the ongoing malaria elimination campaign

Revisiting gametocyte biology in malaria parasites

Gametocytes are the only form of the malaria parasite that is transmissible to the mosquito vector. They are present at low levels in blood circulation and significant knowledge gaps exist in their biology. Recent reductions in the global malaria burden have brought the possibility of elimination and eradication, with renewed focus on malaria transmission biology as a basis for interventions. This review discusses recent insights into gametocyte biology in the major human malaria parasite, Plasmodium falciparum and related species

No evidence for Ago2 translocation from the host erythrocyte into the Plasmodium parasite

Background: Plasmodium parasites rely on various host factors to grow and replicate within red blood cells (RBC). While many host proteins are known that mediate parasite adhesion and invasion, few examples of host enzymes co-opted by the parasite during intracellular development have been described. Recent studies suggested that the host protein Argonaute 2 (Ago2), which is involved in RNA interference, can translocate into the parasite and affect its development. Here, we investigated this hypothesis. Methods: We used several different monoclonal antibodies to test for Ago2 localisation in the human malaria parasite, P. falciparum and rodent P. berghei parasites. In addition, we biochemically fractionated infected red blood cells to localize Ago2. We also quantified parasite growth and sexual commitment in the presence of the Ago2 inhibitor BCI-137. Results: Ago2 localization by fluorescence microscopy produced inconclusive results across the three different antibodies, suggesting cross-reactivity with parasite targets. Biochemical separation of parasite and RBC cytoplasm detected Ago2 only in the RBC cytoplasm and not in the parasite. Inhibition of Ago2 using BCl-137 did not result in altered parasite development. Conclusion: Ago2 localization in infected RBCs by microscopy is confounded by non-specific binding of antibodies. Complementary results using biochemical fractionation and Ago2 detection by western blot did not detect the protein in the parasite cytosol, and growth assays using a specific inhibitor demonstrated that its catalytical activity is not required for parasite development. We therefore conclude that previous data localising Ago2 to parasite ring stages are due to antibody cross reactivity, and that Ago2 is not required for intracellular Plasmodium development

From protease to decarboxylase: the molecular metamorphosis of phosphatidylserine decarboxylase

Background: Phosphatidylserine decarboxylase (PSD) undergoes an autoendoproteolytic cleavage but the mechanism has not been defined. Results: PSD proenzyme processing occurs by a canonical serine protease mechanism catalyzed by a conserved aspartate-histidine-serine triad. Conclusion: PSD proenzyme executes a proteolytic reaction in cis that creates the active site of the decarboxylase. Significance: The mechanism of autoendoproteolyic processing of PSDs across phyla has been elucidated