Plasmodium falciparum is the causative agent of the most burdensome form of human malaria, affecting 200–300 million individuals per year worldwide. The recently sequenced genome of P. falciparum revealed over 5,400 genes, of which 60% encode proteins of unknown function. Insights into the biochemical function and regulation of these genes will provide the foundation for future drug and vaccine development efforts toward eradication of this disease. By analyzing the complete asexual intraerythrocytic developmental cycle (IDC) transcriptome of the HB3 strain of P. falciparum, we demonstrate that at least 60% of the genome is transcriptionally active during this stage. Our data demonstrate that this parasite has evolved an extremely specialized mode of transcriptional regulation that produces a continuous cascade of gene expression, beginning with genes corresponding to general cellular processes, such as protein synthesis, and ending with Plasmodium-specific functionalities, such as genes involved in erythrocyte invasion. The data reveal that genes contiguous along the chromosomes are rarely coregulated, while transcription from the plastid genome is highly coregulated and likely polycistronic. Comparative genomic hybridization between HB3 and the reference genome strain (3D7) was used to distinguish between genes not expressed during the IDC and genes not detected because of possible sequence variations. Genomic differences between these strains were found almost exclusively in the highly antigenic subtelomeric regions of chromosomes. The simple cascade of gene regulation that directs the asexual development of P. falciparum is unprecedented in eukaryotic biology. The transcriptome of the IDC resembles a “just-in-time” manufacturing process whereby induction of any given gene occurs once per cycle and only at a time when it is required. These data provide to our knowledge the first comprehensive view of the timing of transcription throughout the intraerythrocytic development of P. falciparum and provide a resource for the identification of new chemotherapeutic and vaccine candidates.

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The Transcriptome of the Intraerythrocytic Developmental Cycle of Plasmodium falciparum

The relevance of the antibody-dependent cellular inhibition (ADCI) of Plasmodium falciparum to clinical protection has been previously established by in vitro studies of material obtained during passive transfer of protection by immunoglobulin G in humans. We here report further in vitro investigations aimed at elucidating the mechanisms underlying this ADCI effect. Results obtained so far suggest that (a) merozoite uptake by monocytes (MN) as well as by polymorphonuclear cells has little influence on the course of parasitemia; (b) the ADCI effect is mediated by a soluble factor released by MN; (c) this or these factors are able to block the division of surrounding intraerythrocytic parasites at the one nucleus stage; (d) the critical triggering antigen(s) targeted by effective Abs would appear to be associated with the surface of merozoites, as opposed to that of infected red blood cells; (e) the MN receptor for Abs effective in ADCI is apparently Fc gamma RII, and not RI; (f) MN function is up- and down-regulated by interferon-gamma and interleukin 4, respectively; and (g) of several potential mediators released by MN, only tumor necrosis factor (TNF) proved of relevance. The involvement of TNF in defense may explain the recently described increased frequency of the TNF-2 high-expression promoter in individuals living in endemic regions despite its compromising role in severe malaria.

https://rupress.org/jem/article-pdf/182/2/409/1107275/409.pdf

Mechanisms underlying the monocyte-mediated antibody-dependent killing of Plasmodium falciparum asexual blood stages.

When merozoites of the malaria parasite Plasmodium falciparum are released from infected erythrocytes and invade new red cells, a component of a protein complex derived from the merozoite surface protein 1 (MSP-1) precursor undergoes a single proteolytic cleavage known as secondary processing. This releases the complex from the parasite surface, except for a small membrane-bound fragment consisting of two epidermal growth factor (EGF)-like domains, which is the only part of MSP-1 to be carried into invaded erythrocytes. We report that, a group of monoclonal antibodies specific for epitopes within the EGF-like domains, some interfere with secondary processing whereas others do not. Those that most effectively inhibit processing have previously been shown to prevent invasion. Other antibodies, some of which can block this inhibition, not only do not prevent invasion but are carried into the host cell bound to the merozoite surface. These observations unequivocally demonstrate that the binding of antibody to the COOH-terminal region of MSP-1 on the merozoite surface may not be sufficient to prevent erythrocyte invasion, and show that the interaction of different antibodies with adjacent epitopes within the EGF-like domains of MSP-1 can have distinct biochemical effects on the molecule. Inhibition of MSP-1 processing on merozoites may be a mechanism by which protective antibodies interrupt the asexual cycle of the malaria parasite.

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Antibodies inhibit the protease-mediated processing of a malaria merozoite surface protein.

Merozoite surface protein-3: a malaria protein inducing antibodies that promote Plasmodium falciparum killing by cooperation with blood monocytes

Malaria elimination has recently been reinstated as a global health priority but current therapies seem to be insufficient for the task. Elimination efforts require new drug classes that alleviate symptoms, prevent transmission and provide a radical cure. To develop these next-generation medicines, public-private partnerships are funding innovative approaches to identify compounds that target multiple parasite species at multiple stages of the parasite life cycle. In this Review, we discuss the cell-, chemistry- and target-based approaches used to discover new drug candidates that are currently in clinical trials or undergoing preclinical testing.

/pdf/antimalarial-drug-discovery-approaches-and-progress-towards-45ei1id94i.pdf

Hans-G. Heidrich

Papers

Enzymes of purine and pyrimidine metabolism from the human malaria parasite, Plasmodium falciparum.

Plasmodium falciparum: isolation and purification of spontaneously released merozoites by nylon membrane sieves.