Identification of Plasmodium falciparum Translation Initiation eIF2β Subunit: Direct Interaction with Protein Phosphatase Type 1

Plasmodium berghei leucine-rich repeat protein 1 downregulates protein phosphatase 1 activity and is required for efficient oocyst development

Protein phosphatase 1 (PP1) is a key enzyme for Plasmodium development. However, the detailed mechanisms underlying its regulation remain to be deciphered. Here, we report the functional characterization of the Plasmodium berghei leucine-rich repeat protein 1 (PbLRR1), an orthologue of SDS22, one of the most ancient and conserved PP1 interactors. Our study shows that PbLRR1 is expressed during intra-erythrocytic development of the parasite, and up to the zygote stage in mosquitoes. PbLRR1 can be found in complex with PbPP1 in both asexual and sexual stages and inhibits its phosphatase activity. Genetic analysis demonstrates that PbLRR1 depletion adversely affects the development of oocysts. PbLRR1 interactome analysis associated with phospho-proteomics studies identifies several novel putative PbLRR1/PbPP1 partners. Some of these partners have previously been characterized as essential for the parasite sexual development. Interestingly, and for the first time, Inhibitor 3 (I3), a well-known and direct interactant of Plasmodium PP1, was found to be drastically hypophosphorylated in PbLRR1-depleted parasites. These data, along with the detection of I3 with PP1 in the LRR1 interactome, strongly suggest that the phosphorylation status of PbI3 is under the control of the PP1-LRR1 complex and could contribute (in)directly to oocyst development. This study provides new insights into previously unrecognized PbPP1 fine regulation of Plasmodium oocyst development through its interaction with PbLRR1.

Deciphering the Role of Protein Phosphatases in Apicomplexa: The Future of Innovative Therapeutics?

Parasites belonging to the Apicomplexa phylum still represent a major public health and world-wide socioeconomic burden that is greatly amplified by the spread of resistances against known therapeutic drugs. Therefore, it is essential to provide the scientific and medical communities with innovative strategies specifically targeting these organisms. In this review, we present an overview of the diversity of the phosphatome as well as the variety of functions that phosphatases display throughout the Apicomplexan parasites’ life cycles. We also discuss how this diversity could be used for the design of innovative and specific new drugs/therapeutic strategies.

Mapping PP1c and Its Inhibitor 2 Interactomes Reveals Conserved and Specific Networks in Asexual and Sexual Stages of Plasmodium

Malaria parasites require multiple phosphorylation and dephosphorylation steps to drive signaling pathways for proper differentiation and transformation. Several protein phosphatases, including protein phosphatase 1 (PP1), one of the main dephosphorylation enzymes, have been shown to be indispensable for the Plasmodium life cycle. The catalytic subunit of PP1 (PP1c) participates in cellular processes via dynamic interactions with a vast number of binding partners that contribute to its diversity of action. In this study, we used Plasmodium berghei transgenic parasite strains stably expressing PP1c or its inhibitor 2 (I2) tagged with mCherry, combined with the mCherry affinity pulldown of proteins from asexual and sexual stages, followed by mass spectrometry analyses. Mapped proteins were used to identify interactomes and to cluster functionally related proteins. Our findings confirm previously known physical interactions of PP1c and reveal enrichment of common biological processes linked to cellular component assembly in both schizonts and gametocytes to biosynthetic processes/translation in schizonts and to protein transport exclusively in gametocytes. Further, our analysis of PP1c and I2 interactomes revealed that nuclear export mediator factor and peptidyl-prolyl cis-trans isomerase, suggested to be essential in P. falciparum, could be potential targets of the complex PP1c/I2 in both asexual and sexual stages. Our study emphasizes the adaptability of Plasmodium PP1 and provides a fundamental study of the protein interaction landscapes involved in a myriad of events in Plasmodium, suggesting why it is crucial to the parasite and a source for alternative therapeutic strategies.

Metabolic Survival Adaptations of Plasmodium falciparum Exposed to Sublethal Doses of Fosmidomycin

The malaria parasite Plasmodium falciparum contains the apicoplast organelle that synthesizes isoprenoids, which are metabolites necessary for posttranslational modification of Plasmodium proteins. We used fosmidomycin, an antibiotic that inhibits isoprenoid biosynthesis, to identify mechanisms that underlie the development of the parasite's adaptation to the drug at sublethal concentrations. We first determined a concentration of fosmidomycin that reduced parasite growth by ∼50% over one intraerythrocytic developmental cycle (IDC). At this dose, we maintained synchronous parasite cultures for one full IDC and collected metabolomic and transcriptomic data at multiple time points to capture global and stage-specific alterations. We integrated the data with a genome-scale metabolic model of P. falciparum to characterize the metabolic adaptations of the parasite in response to fosmidomycin treatment. Our simulations showed that, in treated parasites, the synthesis of purine-based nucleotides increased, whereas the synthesis of phosphatidylcholine during the trophozoite and schizont stages decreased. Specifically, the increased polyamine synthesis led to increased nucleotide synthesis, while the reduced methyl-group cycling led to reduced phospholipid synthesis and methyltransferase activities. These results indicate that fosmidomycin-treated parasites compensate for the loss of prenylation modifications by directly altering processes that affect nucleotide synthesis and ribosomal biogenesis to control the rate of RNA translation during the IDC. This also suggests that combination therapies with antibiotics that target the compensatory response of the parasite, such as nucleotide synthesis or ribosomal biogenesis, may be more effective than treating the parasite with fosmidomycin alone.

Interaction of Plasmodium falciparum apicortin with α- and β-tubulin is critical for parasite growth and survival

Cytoskeletal structures of Apicomplexan parasites are important for parasite replication, motility, invasion to the host cell and survival. Apicortin, an Apicomplexan specific protein appears to be a crucial factor in maintaining stability of the parasite cytoskeletal assemblies. However, the function of apicortin, in terms of interaction with microtubules still remains elusive. Herein, we have attempted to elucidate the function of Plasmodium falciparum apicortin by monitoring its interaction with two main components of parasite microtubular structure, α-tubulin-I and β-tubulin through in silico and in vitro studies. Further, a p25 domain binding generic drug Tamoxifen (TMX), was used to disrupt PfApicortin-tubulin interactions which led to the inhibition in growth and progression of blood stage life cycle of P. falciparum.

The Multifaceted Role of Protein Phosphatase 1 in Plasmodium

Protein phosphatase type 1 (PP1) forms a wide range of Ser/Thr-specific phosphatase holoenzymes which contain one catalytic subunit (PP1c), present in all eukaryotic cells, associated with variable subunits known as regulatory proteins. It has recently been shown that regulators take a leading role in the organization and the control of PP1 functions. Many studies have addressed the role of these regulators in diverse organisms, including humans, and investigated their link to diseases. In this review we summarize recent advances on the role of PP1c in Plasmodium, its interactome and regulators. As a proof of concept, peptides interfering with the regulator binding capacity of PP1c were shown to inhibit the growth of P. falciparum, suggesting their potential as drug precursors.

Essential role of GEXP15, a specific Protein Phosphatase type 1 partner, in Plasmodium berghei in asexual erythrocytic proliferation and transmission

The essential and distinct functions of Protein Phosphatase type 1 (PP1) catalytic subunit in eukaryotes are exclusively achieved through its interaction with a myriad of regulatory partners. In this work, we report the molecular and functional characterization of Gametocyte EXported Protein 15 (GEXP15), a Plasmodium specific protein, as a regulator of PP1. In vitro interaction studies demonstrated that GEXP15 physically interacts with PP1 through the RVxF binding motif in P. berghei. Functional assays showed that GEXP15 was able to increase PP1 activity and the mutation of the RVxF motif completely abolished this regulation. Immunoprecipitation assays of tagged GEXP15 or PP1 in P. berghei followed by immunoblot or mass spectrometry analyses confirmed their interaction and showed that they are present both in schizont and gametocyte stages in shared protein complexes involved in the spliceosome and proteasome pathways and known to play essential role in parasite development. Phenotypic analysis of viable GEXP15 deficient P. berghei blood parasites showed that they were unable to develop lethal infection in BALB/c mice or to establish experimental cerebral malaria in C57BL/6 mice. Further, although deficient parasites produced gametocytes they did not produce any oocysts/sporozoites indicating a high fitness cost in the mosquito. Global proteomic and phosphoproteomic analyses of GEXP15 deficient schizonts revealed a profound defect with a significant decrease in the abundance and an impact on phosphorylation status of proteins involved in regulation of gene expression or invasion. Moreover, depletion of GEXP15 seemed to impact mainly the abundance of some specific proteins of female gametocytes. Our study provides the first insight into the contribution of a PP1 regulator to Plasmodium virulence and suggests that GEXP15 affects both the asexual and sexual life cycle.

In the absence of an effective vaccine and the emerging resistance to artemisinin combination therapy, malaria is still a significant threat to human health. Increasing our understanding of the specific mechanisms of the biology of Plasmodium is essential to propose new strategies to control this infection. Here, we demonstrated that GEXP15, a specific protein in Plasmodium, was able to interact with the Protein Phosphatase 1 and regulate its activity. We showed that both proteins are implicated in common protein complexes involved in the mRNA splicing and proteasome pathways. We reported that the deletion of GEXP15 leads to a loss of parasite virulence during asexual stages and a total abolishment of the capacity of deficient parasites to develop in the mosquito. We also found that this deletion affects both protein phosphorylation status and significantly decreases the expression of essential proteins in schizont and gametocyte stages. This study characterizes for the first time a novel molecular pathway through the control of PP1 by an essential and specific Plasmodium regulator, which may contribute to the discovery of new therapeutic targets to control malaria.

Plasmodium pseudo-Tyrosine Kinase-like binds PP1 and SERA5 and is exported to host erythrocytes

Pseudokinases play key roles in many biological processes but they are poorly understood compared to active kinases. Eight putative pseudokinases have been predicted in Plasmodium species. We selected the unique pseudokinase belonging to tyrosine kinase like (TKL) family for detailed structural and functional analysis in P. falciparum and P. berghei. The primary structure of PfpTKL lacks residues critical for kinase activity, supporting its annotation as a pseudokinase. The recombinant pTKL pseudokinase domain was able to bind ATP, but lacked catalytic activity as predicted. The sterile alpha motif (SAM) and RVxF motifs of PfpTKL were found to interact with the P. falciparum proteins serine repeat antigen 5 (SERA5) and protein phosphatase type 1 (PP1) respectively, suggesting that pTKL has a scaffolding role. Furthermore, we found that PP1c activity in a heterologous model was modulated in an RVxF-dependent manner. During the trophozoite stages, PbpTKL was exported to infected erythrocytes where it formed complexes with proteins involved in cytoskeletal organization or host cell maturation and homeostasis. Finally, genetic analysis demonstrated that viable strains obtained by genomic deletion or knocking down PbpTKL did not affect the course of parasite intra-erythrocytic development or gametocyte emergence, indicating functional redundancy during these parasite stages.

Characterization of a Protein Phosphatase Type-1 and a Kinase Anchoring Protein in Plasmodium falciparum

With its multiple regulatory partners, the conserved Protein Phosphatase type-1 (PP1) plays a central role in many functions of the biology of eukaryotic cells, including Plasmodium falciparum. Here, we characterized a protein named PfRCC-PIP, as a major partner of PfPP1. We established its direct interaction in vitro and its presence in complex with PfPP1 in the parasite. The use of Xenopus oocyte model revealed that RCC-PIP can interact with the endogenous PP1 and act in synergy with suboptimal doses of progesterone to trigger oocyte maturation, suggesting a regulatory effect on PP1. Reverse genetic studies suggested an essential role for RCC-PIP since no viable knock-out parasites could be obtained. Further, we demonstrated the capacity of protein region containing RCC1 motifs to interact with the parasite kinase CDPK7. These data suggest that this protein is both a kinase and a phosphatase anchoring protein that could provide a platform to regulate phosphorylation/dephosphorylation processes.

Transgenic Eimeria magna Pérard, 1925 Displays Similar Parasitological Properties to the Wild-type Strain and Induces an Exogenous Protein-Specific Immune Response in Rabbits (Oryctolagus cuniculus L.)

Rabbit coccidiosis causes great economic losses to world rabbitries. Little work has been done considering genetic manipulation on the etiological agents, rabbit Eimeria spp. In this study, we constructed a transgenic line of Eimeria magna (EmagER) expressing enhanced yellow fluorescent protein (EYFP) and red fluorescent protein (RFP) using regulatory sequences of Eimeria tenella and Toxoplasma gondii. We observed the life cycle of EmagER and confirmed that the transgenic parasites express exogenous proteins targeted to different cellular compartments throughout the entire life cycle. EYFP was expressed mainly in the nucleus and RFP both in the nucleus and cytoplasm. Then, coccidia-free, laboratory-reared 40-day-old rabbits were primarily infected with either EmagER or wild-type strain oocysts and challenged with the wild-type strain. EmagER showed similar reproductivity and immunogenicity to the wild-type strain. Finally, we examined the foreign protein-specific immune response elicited by EmagER. Rabbits were immunized with either transgenic or wild-type oocysts. Immune response against parasite-soluble antigen, EYFP and RFP in spleen, and mesenteric lymph nodes were detected by quantitative real-time PCR. The relative expression level of IFN-γ, IL-2, and TNF-α were higher in EmagER-immunized rabbits than wild-type parasites-immunized rabbits after stimulation with EYFP and RFP. Our study confirmed that a specific immune response was induced by the exogenous protein expressed by EmagER and favored future studies on application of transgenic rabbit coccidia as recombinant vaccine vectors.