The plasmodium receptor for activated C kinase protein inhibits Ca(2+) signaling in mammalian cells

Combining IP3 affinity chromatography and bioinformatics reveals a novel protein-IP3 binding site on Plasmodium falciparum MDR1 transporter

Intracellular Ca²⁺ mobilization induced by second messenger IP₃ controls many cellular events in most of the eukaryotic groups. Despite the increasing evidence of IP₃-induced Ca²⁺ in apicomplexan parasites like Plasmodium, responsible for malaria infection, no protein with potential function as an IP₃-receptor has been identified. The use of bioinformatic analyses based on previously known sequences of IP₃-receptor failed to identify potential IP₃-receptor candidates in any Apicomplexa. In this work, we combine the biochemical approach of an IP₃ affinity chromatography column with bioinformatic meta-analyses to identify potential vital membrane proteins that present binding with IP₃ in Plasmodium falciparum. Our analyses reveal that PF3D7_0523000, a gene that codes a transport protein associated with multidrug resistance as a potential target for IP₃. This work provides a new insight for probing potential candidates for IP₃-receptor in Apicomplexa.

Mechanisms underlying the effect of an oral antihyperglycaemic agent glyburide on calcium ion (Ca2+ ) movement and its related cytotoxicity in prostate cancer cells

Glyburide is an agent commonly used to treat type 2 diabetes and also affects various physiological responses in different models. However, the effect of glyburide on Ca²⁺ movement and its related cytotoxicity in prostate cancer cells is unclear. This study examined whether glyburide altered Ca²⁺ signalling and viability in PC3 human prostate cancer cells and investigated those underlying mechanisms. Intracellular Ca²⁺ concentrations ([Ca²⁺ ]_i ) in suspended cells were measured by using the fluorescent Ca²⁺ -sensitive dye fura-2. Cell viability was examined by WST-1 assay. Glyburide at concentrations of 100-1000 μM induced [Ca²⁺ ]_i rises. Ca²⁺ removal reduced the signal by approximately 60%. In Ca²⁺ -containing medium, glyburide-induced Ca²⁺ entry was inhibited by 60% by protein kinase C (PKC) activator (phorbol 12-myristate 13 acetate, PMA) and inhibitor (GF109203X), and modulators of store-operated Ca²⁺ channels (nifedipine, econazole and SKF96365). Furthermore, glyburide induced Mn²⁺ influx suggesting of Ca²⁺ entry. In Ca²⁺ -free medium, inhibition of phospholipase C (PLC) with U73122 significantly inhibited glyburide-induced [Ca²⁺ ]_i rises. Treatment with the endoplasmic reticulum (ER) Ca²⁺ pump inhibitor 2,5-di-tert-butylhydroquinone (BHQ) abolished glyburide-evoked [Ca²⁺ ]_i rises. Conversely, treatment with glyburide abolished BHQ-evoked [Ca²⁺ ]_i rises. Glyburide at 100-500 μM decreased cell viability, which was not reversed by pretreatment with the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM). Together, in PC3 cells, glyburide induced [Ca²⁺ ]_i rises by Ca²⁺ entry via PKC-sensitive store-operated Ca²⁺ channels and Ca²⁺ release from the ER in a PLC-dependent manner. Glyburide also caused Ca²⁺ -independent cell death. This study suggests that glyburide could serve as a potential agent for treatment of prostate cancer.

Molecular Features and mRNA Expression of the Receptor for Activated C Kinase 1 from Symbiodinium microadriaticum ssp. microadriaticum During Growth and the Light/Dark cycle

Two genes of the RACK1 homolog from the photosynthetic dinoflagellate Symbiodinium microadriaticum ssp. microadriaticum (SmicRACK1), termed SmicRACK1A and SmicRACK1B, were found tandemly arrayed and displayed a single synonymous substitution (T/C) encoding threonine. They included two exons of 942 bp each, encoding 313 amino acids with seven WD-40 repeats and two PKC-binding motifs. The protein theoretical mass and pI were 34,200 Da and 5.9, respectively. SmicRACK1 showed maximum identities with RACK1 homologs at the amino acid and nucleotide level, respectively, of 92 and 84% with S. minutum, and phylogenetic analysis revealed clustered related RACK1 sequences from the marine dinoflagellates S. minutum, Heterocapsa triquetra, Karenia brevis, and Alexandrium tamarense. Interestingly, light-dependent regulatory elements were found both within the 282 bp SmicRACK1A promotor sequence, and within an intergenic sequence of 359 nucleotides that separated both genes, which strongly suggest light-related functions. This was further supported by mRNA accumulation analysis, which fluctuated along the light and dark phases of the growth cycle showing maximum specific peaks under either condition. Finally, qRT-PCR analysis revealed differential SmicRACK1 mRNA accumulation with maxima at 6 and 20 d of culture. Our SmicRACK1 characterization suggests roles in active growth and proliferation, as well as light/dark cycle regulation in S. microadriaticum.

Receptor for Activated C-Kinase 1 (PfRACK1) is required for Plasmodium falciparum intra-erythrocytic proliferation

Emerging resistance to current anti-malarials necessitates a more detailed understanding of the biological processes of Plasmodium falciparum proliferation, thus allowing identification of new drug targets. The well-conserved protein Receptor for Activated C-Kinase 1 (RACK1) was originally identified in mammalian cells as an anchoring protein for protein kinase C (PKC) and has since been shown to be important for cell migration, cytokinesis, transcription, epigenetics, and protein translation. The P. falciparum ortholog, PfRACK1, is expressed in blood stages of the parasite and is diffusely localized in the parasite cytoplasm. Using a destabilizing domain to allow inducible knockdown of the endogenous protein level, we evaluated the requirement for PfRACK1 during blood-stage replication. Following destabilization, the parasites demonstrate a nearly complete growth arrest at the trophozoite stage. The essential nature of PfRACK1 suggests that the protein itself or the pathways regulated by the protein are potential targets for novel anti-malarial therapeutics.

Roles of Rack1 Proteins in Fungal Pathogenesis

Pathogenic fungi cause diseases on various organisms. Despite their differences in life cycles, fungal pathogens use well-conserved proteins and pathways to regulate developmental and infection processes. In this review, we focus on Rack1, a multifaceted scaffolding protein involved in various biological processes. Rack1 is well conserved in eukaryotes and plays important roles in fungi, though limited studies have been conducted. To accelerate the study of Rack1 proteins in fungi, we review the functions of Rack1 proteins in model and pathogenic fungi and summarize recent progress on how Rack1 proteins are involved in fungal pathogenesis.

Translational regulation in blood stages of the malaria parasite Plasmodium spp.: systems-wide studies pave the way

The malaria parasite Plasmodium spp. varies the expression profile of its genes depending on the host it resides in and its developmental stage. Virtually all messenger RNA (mRNA) is expressed in a monocistronic manner, with transcriptional activation regulated at the epigenetic level and by specialized transcription factors. Furthermore, recent systems-wide studies have identified distinct mechanisms of post-transcriptional and translational control at various points of the parasite lifecycle. Taken together, it is evident that 'just-in-time' transcription and translation strategies coexist and coordinate protein expression during Plasmodium development, some of which we review here. In particular, we discuss global and specific mechanisms that control protein translation in blood stages of the human malaria parasite Plasmodium falciparum, once a cytoplasmic mRNA has been generated, and its crosstalk with mRNA decay and storage. We also focus on the widespread translational delay observed during the 48-hour blood stage lifecycle of P. falciparum-for over 30% of transcribed genes, including virulence factors required to invade erythrocytes-and its regulation by cis-elements in the mRNA, RNA-processing enzymes and RNA-binding proteins; the first-characterized amongst these are the DNA- and RNA-binding Alba proteins. More generally, we conclude that translational regulation is an emerging research field in malaria parasites and propose that its elucidation will not only shed light on the complex developmental program of this parasite, but may also reveal mechanisms contributing to drug resistance and define new targets for malaria intervention strategies. WIREs RNA 2016, 7:772-792. doi: 10.1002/wrna.1365 For further resources related to this article, please visit the WIREs website.

A WD40-repeat protein unique to malaria parasites associates with adhesion protein complexes and is crucial for blood stage progeny

Background

During development in human erythrocytes, Plasmodium falciparum parasites display a remarkable number of adhesive proteins on their plasma membrane. In the invasive merozoites, these include members of the PfMSP1 and PfAMA1/RON complexes, which facilitate contact between merozoites and red blood cells. In gametocytes, sexual precursor cells mediating parasite transmission to the mosquito vector, plasma membrane-associated proteins primarily belong to the PfCCp and 6-cys families with roles in fertilization. This study describes a newly identified WD40-repeat protein unique to Plasmodium species that associates with adhesion protein complexes of both merozoites and gametocytes.

Methods

The WD40-repeat protein-like protein PfWLP1 was identified via co-immunoprecipitation assays followed by mass spectrometry and characterized using biochemical and immunohistochemistry methods. Reverse genetics were employed for functional analysis.

Results

PfWLP1 is expressed both in schizonts and gametocytes. In mature schizonts, the protein localizes underneath the merozoite micronemes and interacts with PfAMA1, while in gametocytes PfWLP1 primarily accumulates underneath the plasma membrane and associates with PfCCp1 and Pfs230. Reverse genetics failed to disrupt the pfwlp1 gene, while haemagglutinin-tagging was feasible, suggesting a crucial function for PfWLP1 during blood stage replication.

Conclusions

This is the first report on a plasmodial WD40-repeat protein associating with cell adhesion proteins. Since WD40 domains are known to mediate protein–protein contact by serving as a rigid scaffold for protein interactions, the presented data suggest that PfWLP1 supports the stability of adhesion protein complexes of the plasmodial blood stages.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0967-x) contains supplementary material, which is available to authorized users.

Dynamical features of the Plasmodium falciparum ribosome during translation

Plasmodium falciparum, the mosquito-transmitted Apicomplexan parasite, causes the most severe form of human malaria. In the asexual blood-stage, the parasite resides within erythrocytes where it proliferates, multiplies and finally spreads to new erythrocytes. Development of drugs targeting the ribosome, the site of protein synthesis, requires specific knowledge of its structure and work cycle, and, critically, the ways they differ from those in the human host. Here, we present five cryo-electron microscopy (cryo-EM) reconstructions of ribosomes purified from P. falciparum blood-stage schizonts at sub-nanometer resolution. Atomic models were built from these density maps by flexible fitting. Significantly, our study has taken advantage of new capabilities of cryo-EM, in visualizing several structures co-existing in the sample at once, at a resolution sufficient for building atomic models. We have discovered structural and dynamic features that differentiate the ribosomes of P. falciparum from those of mammalian system. Prompted by the absence of RACK1 on the ribosome in our and an earlier study we confirmed that RACK1 does not specifically co-purify with the 80S fraction in schizonts. More extensive studies, using cryo-EM methodology, of translation in the parasite will provide structural knowledge that may lead to development of novel anti-malarials.

Predicting and exploring network components involved in pathogenesis in the malaria parasite via novel subnetwork alignments

BACKGROUND

Malaria is a major health threat, affecting over 40% of the world's population. The latest report released by the World Health Organization estimated about 207 million cases of malaria infection, and about 627,000 deaths in 2012 alone. During the past decade, new therapeutic targets have been identified and are at various stages of characterization, thanks to the emerging omics-based technologies. However, the mechanism of malaria pathogenesis remains largely unknown. In this paper, we employ a novel neighborhood subnetwork alignment approach to identify network components that are potentially involved in pathogenesis.

RESULTS

Our module-based subnetwork alignment approach identified 24 functional homologs of pathogenesis-related proteins in the malaria parasite P. falciparum, using the protein-protein interaction networks in Escherichia coli as references. Eighteen out of these 24 proteins are associated with 418 other proteins that are related to DNA replication, transcriptional regulation, translation, signaling, metabolism, cell cycle regulation, as well as cytoadherence and entry to the host.

CONCLUSIONS

The subnetwork alignments and subsequent protein-protein association network mining predicted a group of malarial proteins that may be involved in parasite development and parasite-host interaction, opening a new systems-level view of parasite pathogenesis and virulence.

Genome-Wide Collation of the Plasmodium falciparum WDR Protein Superfamily Reveals Malarial Parasite-Specific Features

Despite a significant drop in malaria deaths during the past decade, malaria continues to be one of the biggest health problems around the globe. WD40 repeats (WDRs) containing proteins comprise one of the largest and functionally diverse protein superfamily in eukaryotes, acting as scaffolds for assembling large protein complexes. In the present study, we report an extensive in silico analysis of the WDR gene family in human malaria parasite Plasmodium falciparum. Our genome-wide identification has revealed 80 putative WDR genes in P. falciparum (PfWDRs). Five distinct domain compositions were discovered in Plasmodium as compared to the human host. Notably, 31 PfWDRs were annotated/re-annotated on the basis of their orthologs in other species. Interestingly, most PfWDRs were larger as compared to their human homologs highlighting the presence of parasite-specific insertions. Fifteen PfWDRs appeared specific to the Plasmodium with no assigned orthologs. Expression profiling of PfWDRs revealed a mixture of linear and nonlinear relationships between transcriptome and proteome, and only nine PfWDRs were found to be stage-specific. Homology modeling identified conservation of major binding sites in PfCAF-1 and PfRACK. Protein-protein interaction network analyses suggested that PfWDRs are highly connected proteins with ~1928 potential interactions, supporting their role as hubs in cellular networks. The present study highlights the roles and relevance of the WDR family in P. falciparum, and identifies unique features that lay a foundation for further experimental dissection of PfWDRs.

Protein phosphorylation during Plasmodium berghei gametogenesis

Plasmodium gametogenesis within the mosquito midgut is a complex differentiation process involving signaling mediated by phosphorylation, which modulate metabolic routes and protein synthesis required to complete this development. However, the mechanisms leading to gametogenesis activation are poorly understood. We analyzed protein phosphorylation during Plasmodium berghei gametogenesis in vitro in serum-free medium using bidimensional electrophoresis (2-DE) combined with immunoblotting (IB) and antibodies specific to phosphorylated serine, threonine and tyrosine. Approximately 75 protein exhibited phosphorylation changes, of which 23 were identified by mass spectrometry. These included components of the cytoskeleton, heat shock proteins, and proteins involved in DNA synthesis and signaling pathways among others. Novel phosphorylation events support a role for these proteins during gametogenesis. The phosphorylation sites of six of the identified proteins, HSP70, WD40 repeat protein msi1, enolase, actin-1 and two isoforms of large subunit of ribonucleoside reductase were investigated using TiO2 phosphopeptides enrichment and tandem mass spectrometry. In addition, transient exposure to hydroxyurea, an inhibitor of ribonucleoside reductase, impaired male gametocytes exflagellation in a dose-dependent manner, and provides a resource for functional studies.

Membrane active chelators as novel anti-African trypanosome and anti-malarial drugs

Malaria (Plasmodium spp.) and human African trypanosomiasis (Trypanosoma brucei spp.) are vector borne, deadly parasitic diseases. While chemotherapeutic agents for both diseases are available, difficulty in disease eradication and development of drug resistance require that new therapies targeting unexplored pathways or exploiting novel modes of action be developed. Intracellular Plasmodium and extracellular Trypanosoma brucei may have unique and essential requirements for divalent metal ions, beyond that deemed physiological for the host. Membrane Active Chelators (MACs), biologically active only in a hydrophobic lipid environment, are able to bind metal ions at elevated non-physiological concentrations in the vicinity of cell membranes. A dose-response relationship study using validated viability assays revealed that two MAC drugs, DP-b99 and DP-460, were cytotoxic for these parasites in vitro. The 50% effective concentration (EC50) values for DP-b99 and DP-460 were 87 μM and 39 μM for Trypanosoma brucei brucei and 21 μM and 28 μM for erythrocytic Plasmodium falciparum, respectively. Furthermore, drug potency was maintained for at least 24h in serum containing medium at 37°C. While the exact mechanism of action of MACs against intracellular malaria and extracellular African trypanosome parasites has yet to be determined, their potential as antiparasitic agents warrants further investigation.

Signal transduction in Plasmodium-Red Blood Cells interactions and in cytoadherence

Malaria is responsible for more than 1.5 million deaths each year, especially among children (Snow et al. 2005). Despite of the severity of malaria situation and great effort to the development of new drug targets (Yuan et al. 2011) there is still a relative low investment toward antimalarial drugs. Briefly there are targets classes of antimalarial drugs currently being tested including: kinases, proteases, ion channel of GPCR, nuclear receptor, among others (Gamo et al. 2010). Here we review malaria signal transduction pathways in Red Blood Cells (RBC) as well as infected RBCs and endothelial cells interactions, namely cytoadherence. The last process is thought to play an important role in the pathogenesis of severe malaria. The molecules displayed on the surface of both infected erythrocytes (IE) and vascular endothelial cells (EC) exert themselves as important mediators in cytoadherence, in that they not only induce structural and metabolic changes on both sides, but also trigger multiple signal transduction processes, leading to alteration of gene expression, with the balance between positive and negative regulation determining endothelial pathology during a malaria infection.

Generation of second messengers in Plasmodium

Signalling in malaria parasites is a field of growing interest as its components may prove to be valuable drug targets, especially when one considers the burden of a disease that is responsible for up to 500 million infections annually. The scope of this review is to discuss external stimuli in the parasite life cycle and the upstream machinery responsible for translating them into intracellular responses, focussing particularly on the calcium signalling pathway.

In vivo uptake of a haem analogue Zn protoporphyrin IX by the human malaria parasite P. falciparum-infected red blood cells

The cellular traffic of haem during the development of the human malaria parasite Plasmodium falciparum, through the stages R (ring), T (trophozoite) and S (schizonts), was investigated within RBC (red blood cells). When Plasmodium cultures were incubated with a fluorescent haem analogue, ZnPPIX (Zn protoporphyrin IX) the probe was seen at the cytoplasm (R stage), and the vesicle-like structure distribution pattern was more evident at T and S stages. The temporal sequence of ZnPPIX uptake by P. falciparum-infected erythrocytes shows that at R and S stages, a time-increase acquisition of the porphyrin reaches the maximum fluorescence distribution after 60 min; in contrast, at the T stage, the maximum occurs after 120 min of ZnPPIX uptake. The difference in time-increase acquisition of the porphyrin is in agreement with a maximum activity of haem uptake at the T stage. To gain insights into haem metabolism, recombinant PfHO (P. falciparum haem oxygenase) was expressed, and the conversion of haem into BV (biliverdin) was detected. These findings point out that, in addition to haemozoin formation, the malaria parasite P. falciparum has evolved two distinct mechanisms for dealing with haem toxicity, namely, the uptake of haem into a cellular compartment where haemozoin is formed and HO activity. However, the low Plasmodium HO activity detected reveals that the enzyme appears to be a very inefficient way to scavenge the haem compared with the Plasmodium ability to uptake the haem analogue ZnPPIX and delivering it to the food vacuole.