Less Lipid, More Commitment

Regulation of sexual commitment in malaria parasites - a complex affair

Malaria blood stage parasites commit to either one of two distinct cellular fates while developing within erythrocytes of their mammalian host: they either undergo another round of asexual replication or they differentiate into nonreplicative transmissible gametocytes. Depending on the state of infection, either path may support or impair the ultimate goal of human-to-human transmission via the mosquito vector. Malaria parasites therefore evolved strategies to control investments into asexual proliferation versus gametocyte formation. Recent work provided fascinating molecular insight into shared and unique mechanisms underlying the control and environmental modulation of sexual commitment in the two most widely studied malaria parasite species, Plasmodium falciparum and P. berghei. With this review, we aim at placing these findings into a comparative mechanistic context.

The patatin-like phospholipase PfPNPLA2 is involved in the mitochondrial degradation of phosphatidylglycerol during Plasmodium falciparum blood stage development

Plasmodium falciparum is an Apicomplexa responsible for human malaria, a major disease causing more than ½ million deaths every year, against which there is no fully efficient vaccine. The current rapid emergence of drug resistances emphasizes the need to identify novel drug targets. Increasing evidences show that lipid synthesis and trafficking are essential for parasite survival and pathogenesis, and that these pathways represent potential points of attack. Large amounts of phospholipids are needed for the generation of membrane compartments for newly divided parasites in the host cell. Parasite membrane homeostasis is achieved by an essential combination of parasite de novo lipid synthesis/recycling and massive host lipid scavenging. Latest data suggest that the mobilization and channeling of lipid resources is key for asexual parasite survival within the host red blood cell, but the molecular actors allowing lipid acquisition are poorly characterized. Enzymes remodeling lipids such as phospholipases are likely involved in these mechanisms. P. falciparum possesses an unusually large set of phospholipases, whose functions are largely unknown. Here we focused on the putative patatin-like phospholipase PfPNPLA2, for which we generated an glmS-inducible knockdown line and investigated its role during blood stages malaria. Disruption of the mitochondrial PfPNPLA2 in the asexual blood stages affected mitochondrial morphology and further induced a significant defect in parasite replication and survival, in particular under low host lipid availability. Lipidomic analyses revealed that PfPNPLA2 specifically degrades the parasite membrane lipid phosphatidylglycerol to generate lysobisphosphatidic acid. PfPNPLA2 knockdown further resulted in an increased host lipid scavenging accumulating in the form of storage lipids and free fatty acids. These results suggest that PfPNPLA2 is involved in the recycling of parasite phosphatidylglycerol to sustain optimal intraerythrocytic development when the host resources are scarce. This work strengthens our understanding of the complex lipid homeostasis pathways to acquire lipids and allow asexual parasite survival.

Plasmodium falciparum S-Adenosylmethionine Synthetase Is Essential for Parasite Survival through a Complex Interaction Network with Cytoplasmic and Nuclear Proteins

S-adenosylmethionine synthetase (SAMS) is a key enzyme for the synthesis of the lone methyl donor S-adenosyl methionine (SAM), which is involved in transmethylation reactions and hence required for cellular processes such as DNA, RNA, and histone methylation, but also polyamine biosynthesis and proteostasis. In the human malaria parasite Plasmodium falciparum, PfSAMS is encoded by a single gene and has been suggested to be crucial for malaria pathogenesis and transmission; however, to date, PfSAMS has not been fully characterized. To gain deeper insight into the function of PfSAMS, we generated a conditional gene knockdown (KD) using the glmS ribozyme system. We show that PfSAMS localizes to the cytoplasm and the nucleus of blood-stage parasites. PfSAMS-KD results in reduced histone methylation and leads to impaired intraerythrocytic growth and gametocyte development. To further determine the interaction network of PfSAMS, we performed a proximity-dependent biotin identification analysis. We identified a complex network of 1114 proteins involved in biological processes such as cell cycle control and DNA replication, or transcription, but also in phosphatidylcholine and polyamine biosynthesis and proteasome regulation. Our findings highlight the diverse roles of PfSAMS during intraerythrocytic growth and sexual stage development and emphasize that PfSAMS is a potential drug target.

Malaria parasites do respond to heat

The capacity of malaria parasites to respond to changes in their environment at the transcriptional level has been the subject of debate, but recent evidence has unambiguously demonstrated that Plasmodium spp. can produce adaptive transcriptional responses when exposed to some specific types of stress. These include metabolic conditions and febrile temperature. The Plasmodium falciparum protective response to thermal stress is similar to the response in other organisms, but it is regulated by a transcription factor evolutionarily unrelated to the conserved transcription factor that drives the heat shock (HS) response in most eukaryotes. Of the many genes that change expression during HS, only a subset constitutes an authentic response that contributes to parasite survival.

Stressed Out About Plasmodium falciparum Gametocytogenesis

Blocking malaria transmission is critical to malaria control programs but remains a major challenge especially in endemic regions with high levels of asymptomatic infections. New strategies targeting the transmissible sexual stages of the parasite, called gametocytes, are needed. This review focuses on P. falciparum gametocytogenesis in vivo and in vitro. Highlighting advances made elucidating genes required for gametocyte production and identifying key questions that remain unanswered such as the factors and regulatory mechanisms that contribute to gametocyte induction, and the mechanism of sequestration. Tools available to begin to address these issues are also described to facilitate advances in our understanding of this important stage of the life cycle.

The private life of malaria parasites: Strategies for sexual reproduction

Malaria parasites exhibit a complex lifecycle, requiring extensive asexual replication in the liver and blood of the vertebrate host, and in the haemocoel of the insect vector. Yet, they must also undergo a single round of sexual reproduction, which occurs in the vector's midgut upon uptake of a blood meal. Sexual reproduction is obligate for infection of the vector and thus, is essential for onwards transmission to new hosts. Sex in malaria parasites involves several bottlenecks in parasite number, making the stages involved attractive targets for blocking disease transmission. Malaria parasites have evolved a suite of adaptations ("strategies") to maximise the success of sexual reproduction and transmission, which could undermine transmission-blocking interventions. Yet, understanding parasite strategies may also reveal novel opportunities for such interventions. Here, we outline how evolutionary and ecological theories, developed to explain reproductive strategies in multicellular taxa, can be applied to explain two reproductive strategies (conversion rate and sex ratio) expressed by malaria parasites within the vertebrate host.

A patatin-like phospholipase functions during gametocyte induction in the malaria parasite Plasmodium falciparum

Patatin-like phospholipases (PNPLAs) are highly conserved enzymes of prokaryotic and eukaryotic organisms with major roles in lipid homeostasis. The genome of the malaria parasite Plasmodium falciparum encodes four putative PNPLAs with predicted functions during phospholipid degradation. We here investigated the role of one of the plasmodial PNPLAs, a putative PLA₂ termed PNPLA1, during blood stage replication and gametocyte development. PNPLA1 is present in the asexual and sexual blood stages and here localizes to the cytoplasm. PNPLA1-deficiency due to gene disruption or conditional gene-knockdown had no effect on intraerythrocytic growth, gametocyte development and gametogenesis. However, parasites lacking PNPLA1 were impaired in gametocyte induction, while PNPLA1 overexpression promotes gametocyte formation. The loss of PNPLA1 further leads to transcriptional down-regulation of genes related to gametocytogenesis, including the gene encoding the sexual commitment regulator AP2-G. Additionally, lipidomics of PNPLA1-deficient asexual blood stage parasites revealed overall increased levels of major phospholipids, including phosphatidylcholine (PC), which is a substrate of PLA₂ . PC synthesis is known to be pivotal for erythrocytic replication, while the reduced availability of PC precursors drives the parasite into gametocytogenesis; we thus hypothesize that the higher PC levels due to PNPLA1-deficiency prevent the blood stage parasites from entering the sexual pathway.

Role of phospholipid synthesis in the development and differentiation of malaria parasites in the blood

The life cycle of malaria parasites in both their mammalian host and mosquito vector consists of multiple developmental stages that ensure proper replication and progeny survival. The transition between these stages is fueled by nutrients scavenged from the host and fed into specialized metabolic pathways of the parasite. One such pathway is used by Plasmodium falciparum, which causes the most severe form of human malaria, to synthesize its major phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. Much is known about the enzymes involved in the synthesis of these phospholipids, and recent advances in genetic engineering, single-cell RNA-Seq analyses, and drug screening have provided new perspectives on the importance of some of these enzymes in parasite development and sexual differentiation and have identified targets for the development of new antimalarial drugs. This Minireview focuses on two phospholipid biosynthesis enzymes of P. falciparum that catalyze phosphoethanolamine transmethylation (PfPMT) and phosphatidylserine decarboxylation (PfPSD) during the blood stages of the parasite. We also discuss our current understanding of the biochemical, structural, and biological functions of these enzymes and highlight efforts to use them as antimalarial drug targets.

1,2-Diphenoxiethane salts as potent antiplasmodial agents

In this article we present a series of non-cytotoxic potent human choline kinase (CK) inhibitors that exhibit nanomolar antiplasmodial activity in vitro. The most active antiplasmodial compounds, 10a-b, bearing a pyridinium cationic head were inactive against CK, while compounds 10g and 10j with a quinolinium moiety exhibit moderate inhibition of both the parasite and the enzyme. The results point towards an additional mechanism of action unrelated to CK inhibition that remains to be established.