Sexual stages of the human malaria parasite Plasmodium falciparum use the hematopoietic system of the bone marrow as a developmental niche.

Despite over a century of study of malaria parasites, parts of the Plasmodium falciparum life cycle remain virtually unknown. One of these is the early gametocyte stage, a round shaped cell morphologically similar to an asexual trophozoite in which major cellular transformations ensure subsequent development of the elongated gametocyte. We developed a protocol to obtain for the first time highly purified preparations of early gametocytes using a transgenic line expressing a green fluorescent protein from the onset of gametocytogenesis. We determined the cellular proteome (1427 proteins) of this parasite stage by high accuracy tandem mass spectrometry and newly determined the proteomes of asexual trophozoites and mature gametocytes, identifying altogether 1090 previously undetected parasite proteins. Quantitative label-free comparative proteomics analysis determined enriched protein clusters for the three parasite developmental stages. Gene set enrichment analysis on the 251 proteins enriched in the early gametocyte proteome revealed that proteins putatively exported and involved in erythrocyte remodeling are the most overrepresented protein set in these stages. One-tenth of the early gametocyte-enriched proteome is constituted of putatively exported proteins, here named PfGEXPs (P. falciparum gametocyte-exported proteins). N-terminal processing and N-acetylation at a conserved leucine residue within the Plasmodium export element pentamotif were detected by mass spectrometry for three such proteins in the early but not in the mature gametocyte sample, further supporting a specific role in protein export in early gametocytogenesis. Previous reports and results of our experiments confirm that the three proteins are indeed exported in the erythrocyte cytoplasm. This work indicates that protein export profoundly marks early sexual differentiation in P. falciparum, probably contributing to host cell remodeling in this phase of the life cycle, and that gametocyte-enriched molecules are recruited to modulate this process in gametocytogenesis.

A major cause of the paucity of new starting points for drug discovery is the lack of interaction between academia and industry. Much of the global resource in biology is present in universities, whereas the focus of medicinal chemistry is still largely within industry. Open source drug discovery, with sharing of information, is clearly a first step towards overcoming this gap. But the interface could especially be bridged through a scale-up of open sharing of physical compounds, which would accelerate the finding of new starting points for drug discovery. The Medicines for Malaria Venture Malaria Box is a collection of over 400 compounds representing families of structures identified in phenotypic screens of pharmaceutical and academic libraries against the Plasmodium falciparum malaria parasite. The set has now been distributed to almost 200 research groups globally in the last two years, with the only stipulation that information from the screens is deposited in the public domain. This paper reports for the first time on 236 screens that have been carried out against the Malaria Box and compares these results with 55 assays that were previously published, in a format that allows a meta-analysis of the combined dataset. The combined biochemical and cellular assays presented here suggest mechanisms of action for 135 (34%) of the compounds active in killing multiple life-cycle stages of the malaria parasite, including asexual blood, liver, gametocyte, gametes and insect ookinete stages. In addition, many compounds demonstrated activity against other pathogens, showing hits in assays with 16 protozoa, 7 helminths, 9 bacterial and mycobacterial species, the dengue fever mosquito vector, and the NCI60 human cancer cell line panel of 60 human tumor cell lines. Toxicological, pharmacokinetic and metabolic properties were collected on all the compounds, assisting in the selection of the most promising candidates for murine proof-of-concept experiments and medicinal chemistry programs. The data for all of these assays are presented and analyzed to show how outstanding leads for many indications can be selected. These results reveal the immense potential for translating the dispersed expertise in biological assays involving human pathogens into drug discovery starting points, by providing open access to new families of molecules, and emphasize how a small additional investment made to help acquire and distribute compounds, and sharing the data, can catalyze drug discovery for dozens of different indications. Another lesson is that when multiple screens from different groups are run on the same library, results can be integrated quickly to select the most valuable starting points for subsequent medicinal chemistry efforts. Malaria leads to the loss of over 440,000 lives annually; accelerating research to discover new candidate drugs is a priority. Medicines for Malaria Venture (MMV) has distilled over 25,000 compounds that kill malaria parasites in vitro into a group of 400 representative compounds, called the "Malaria Box". These Malaria Box sets were distributed free-of-charge to research laboratories in 30 different countries that work on a wide variety of pathogens. Fifty-five groups compiled >290 assay results for this paper describing the many activities of the Malaria Box compounds. The collective results suggest a potential mechanism of action for over 130 compounds against malaria and illuminate the most promising compounds for further malaria drug development research. Excitingly some of these compounds also showed outstanding activity against other disease agents including fungi, bacteria, other single-cellular parasites, worms, and even human cancer cells. The results have ignited over 30 drug development programs for a variety of diseases. This open access effort was so successful that MMV has begun to distribute another set of compounds with initial activity against a wider range of infectious agents that are of public health concern, called the Pathogen Box, available now to scientific labs all over the world (www.PathogenBox.org).

SUMMARY: Blood-stage malaria parasites in the vertebrate host can develop either into the asexual, multiplying forms, called schizonts, or into gametocytes, the sexual stages of the parasite. In the present work we studied the differentiation into asexual parasites or gametocytes of the progeny of single, isolated schizonts of the clone 3D7A of Plasinodium falciparum, using monoclonal antibodies specific for the sexual or asexual stages of the parasite. We observed that schizonts obtained from a continuous culture undergoing serial cycles of growth and dilution with fresh red blood cells produced either only gametocytes or only asexual parasites, showing a high degree of commitment to one or the other developmental pathway.The relative proportion of schizonts which produced gametocytes was very low at low parasite densities in culture, while at high parasite densities a much greater proportion of schizonts produced gametocytes. Nevertheless, at both low and high parasite densities individual schizonts were almost always fully committed to producing only gametocytes or only asexual parasites.

Achievement of malaria elimination requires development of novel strategies interfering with parasite transmission, including targeting the parasite sexual stages (gametocytes). The formation of Plasmodium falciparum gametocytes in the human host takes several days during which immature gametocyte-infected erythrocytes (GIEs) sequester in host tissues. Only mature stage GIEs circulate in the peripheral blood, available to uptake by the Anopheles vector. Mechanisms underlying GIE sequestration and release in circulation are virtually unknown. We show here that mature GIEs are more deformable than immature stages using ektacytometry and microsphiltration methods, and that a switch in cellular deformability in the transition from immature to mature gametocytes is accompanied by the deassociation of parasite-derived STEVOR proteins from the infected erythrocyte membrane. We hypothesize that mechanical retention contributes to sequestration of immature GIEs and that regained deformability of mature gametocytes is associated with their release in the bloodstream and ability to circulate. These processes are proposed to play a key role in P falciparum gametocyte development in the host and to represent novel and unconventional targets for interfering with parasite transmission.

Malaria parasites invade erythrocytes of their host both for asexual multiplication and for differentiation to male and female gametocytes – the precursor cells of Plasmodium gametes. For further development the parasite is dependent on efficient release of the asexual daughter cells and of the gametes from the host erythrocyte. How malarial parasites exit their host cells remains largely unknown. We here report the characterization of a Plasmodium berghei protein that is involved in egress of both male and female gametes from the host erythrocyte. Protein MDV-1/PEG3, like its Plasmodium falciparum orthologue, is present in gametocytes of both sexes, but more abundant in the female, where it is associated with dense granular organelles, the osmiophilic bodies. Δmdv-1/peg3 parasites in which MDV-1/PEG3 production was abolished by gene disruption had a strongly reduced capacity to form zygotes resulting from a reduced capability of both the male and female gametes to disrupt the surrounding parasitophorous vacuole and to egress from the host erythrocyte. These data demonstrate that emergence from the host cell of male and female gametes relies on a common, MDV-1/PEG3-dependent mechanism that is distinct from mechanisms used by asexual parasites.