The central role of cAMP in regulating Plasmodium falciparum merozoite invasion of human erythrocytes

RNA polymerase III is involved in regulating Plasmodium falciparum virulence

While often undetected and untreated, persistent seasonal asymptomatic malaria infections remain a global public health problem. Despite the presence of parasites in the peripheral blood, no symptoms develop. Disease severity is correlated with the levels of infected red blood cells (iRBCs) adhering within blood vessels. Changes in iRBC adhesion capacity have been linked to seasonal asymptomatic malaria infections, however how this is occurring is still unknown. Here, we present evidence that RNA polymerase III (RNA Pol III) transcription in Plasmodium falciparum is downregulated in field isolates obtained from asymptomatic individuals during the dry season. Through experiments with in vitro cultured parasites, we have uncovered an RNA Pol III-dependent mechanism that controls pathogen proliferation and expression of a major virulence factor in response to external stimuli. Our findings establish a connection between P. falciparum cytoadhesion and a non-coding RNA family transcribed by Pol III. Additionally, we have identified P. falciparum Maf1 as a pivotal regulator of Pol III transcription, both for maintaining cellular homeostasis and for responding adaptively to external signals. These results introduce a novel perspective that contributes to our understanding of P. falciparum virulence. Furthermore, they establish a connection between this regulatory process and the occurrence of seasonal asymptomatic malaria infections.

Leishmania exploits host cAMP/EPAC/calcineurin signaling to induce an IL-33-mediated anti-inflammatory environment for the establishment of infection

Host anti-inflammatory responses are critical for the progression of visceral leishmaniasis, and the pleiotropic cytokine interleukin (IL)-33 was found to be upregulated in infection. Here, we documented that IL-33 induction is a consequence of elevated cAMP-mediated exchange protein activated by cAMP (EPAC)/calcineurin-dependent signaling and essential for the sustenance of infection. Leishmania donovani-infected macrophages showed upregulation of IL-33 and its neutralization resulted in decreased parasite survival and increased inflammatory responses. Infection-induced cAMP was involved in IL-33 production and of its downstream effectors PKA and EPAC, only the latter was responsible for elevated IL-33 level. EPAC initiated Rap-dependent phospholipase C activation, which triggered the release of intracellular calcium followed by calcium/calmodulin complex formation. Screening of calmodulin-dependent enzymes affirmed involvement of the phosphatase calcineurin in cAMP/EPAC/calcium/calmodulin signaling-induced IL-33 production and parasite survival. Activated calcineurin ensured nuclear localization of the transcription factors, nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha required for IL-33 transcription, and we further confirmed this by chromatin immunoprecipitation assay. Administering specific inhibitors of nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha in BALB/c mouse model of visceral leishmaniasis decreased liver and spleen parasite burden along with reduction in IL-33 level. Splenocyte supernatants of inhibitor-treated infected mice further documented an increase in tumor necrosis factor alpha and IL-12 level with simultaneous decrease of IL-10, thereby indicating an overall disease-escalating effect of IL-33. Thus, this study demonstrates that cAMP/EPAC/calcineurin signaling is crucial for the activation of IL-33 and in effect creates anti-inflammatory responses, essential for infection.

Apicomplexan phosphodiesterases in cyclic nucleotide turnover: conservation, function, and therapeutic potential

Apicomplexa encompasses a large number of intracellular parasites infecting a wide range of animals. Cyclic nucleotide signaling is crucial for a variety of apicomplexan life stages and cellular processes. The cyclases and kinases that synthesize and respond to cyclic nucleotides (i.e., 3',5'-cyclic guanosine monophosphate and 3',5'-cyclic adenosine monophosphate) are highly conserved and essential throughout the parasite phylum. Growing evidence indicates that phosphodiesterases (PDEs) are also critical for regulating cyclic nucleotide signaling via cyclic nucleotide hydrolysis. Here, we discuss recent advances in apicomplexan PDE biology and opportunities for therapeutic interventions, with special emphasis on the major human apicomplexan parasite genera Plasmodium, Toxoplasma, Cryptosporidium, and Babesia. In particular, we show a highly flexible repertoire of apicomplexan PDEs associated with a wide range of cellular requirements across parasites and lifecycle stages. Despite this phylogenetic diversity, cellular requirements of apicomplexan PDEs for motility, host cell egress, or invasion are conserved. However, the molecular wiring of associated PDEs is extremely malleable suggesting that PDE diversity and redundancy are key for the optimization of cyclic nucleotide turnover to respond to the various environments encountered by each parasite and life stage. Understanding how apicomplexan PDEs are regulated and integrating multiple signaling systems into a unified response represent an untapped avenue for future exploration.

Coordination of apicoplast transcription in a malaria parasite by internal and host cues

The malaria parasite Plasmodium falciparum has a nonphotosynthetic plastid called the apicoplast, which contains its own genome. Regulatory mechanisms for apicoplast gene expression remain poorly understood, despite this organelle being crucial for the parasite life cycle. Here, we identify a nuclear-encoded apicoplast RNA polymerase σ subunit (sigma factor) which, along with the α subunit, appears to mediate apicoplast transcript accumulation. This has a periodicity reminiscent of parasite circadian or developmental control. Expression of the apicoplast subunit gene, apSig, together with apicoplast transcripts, increased in the presence of the blood circadian signaling hormone melatonin. Our data suggest that the host circadian rhythm is integrated with intrinsic parasite cues to coordinate apicoplast genome transcription. This evolutionarily conserved regulatory system might be a future target for malaria treatment.

Phosphodiesterase delta governs the mechanical properties of erythrocytes infected with Plasmodium falciparum gametocytes

To persist in the blood circulation and to be available for mosquitoes, Plasmodium falciparum gametocytes modify the deformability and the permeability of their erythrocyte host via cyclic AMP (cAMP) signaling pathway. Cyclic nucleotide levels are tightly controlled by phosphodiesterases (PDE), however in Plasmodium these proteins are poorly characterized. Here, we characterize the P. falciparum phosphodiesterase delta (PfPDEδ) and we investigate its role in the cAMP signaling-mediated regulation of gametocyte-infected erythrocyte mechanical properties. Our results revealed that PfPDEδ is a dual-function enzyme capable of hydrolyzing both cAMP and cGMP, with a higher affinity for cAMP. We also show that PfPDEδ is the most expressed PDE in mature gametocytes and we propose that it is located in parasitophorous vacuole at the interface between the host cell and the parasite. We conclude that PfPDEδ is the master regulator of both the increase in deformability and the inhibition of channel activity in mature gametocyte stages, and may therefore play a crucial role in the persistence of mature gametocytes in the bloodstream.

Dynamic Palmitoylation of Red Cell Membrane Proteins Governs Susceptibility to Invasion by the Malaria Parasite, Plasmodium falciparum

Phosphorylation and other post-translational modifications of red blood cell (RBC) proteins govern membrane function and have a role in the invasion of RBCs by the malaria parasite, Plasmodium falciparum. Furthermore, a percentage of RBC proteins are palmitoylated, although the functional consequences are unknown. We establish dynamic palmitoylation of 118 RBC membrane proteins using click chemistry and acyl biotin exchange (ABE)-coupled LC-MS/MS and characterize their involvement in controlling membrane organization and parasite invasion. RBCs were treated with a generic palmitoylation inhibitor, 2-bromopalmitate (2-BMP), and then analyzed using ABE-coupled LC-MS/MS. Only 42 of the 118 palmitoylated proteins detected were palmitoylated in the 2-BMP-treated sample, indicating that palmitoylation is dynamically regulated. Interestingly, membrane receptors such as semaphorin 7A, CR1, and ABCB6, which are known to be involved in merozoite interaction with RBCs and parasite invasion, were found to be dynamically palmitoylated, including the blood group antigen, Kell, whose antigenic abundance was significantly reduced following 2-BMP treatment. To investigate the involvement of Kell in merozoite invasion of RBCs, a specific antibody to its extracellular domain was used. The antibody targeting Kell inhibited merozoite invasion of RBCs by 50%, implying a role of Kell, a dynamically palmitoylated potent host-derived receptor, in parasite invasion. Furthermore, a significant reduction in merozoite contact with the RBC membrane and a consequent decrease in parasite invasion following 2-BMP treatment demonstrated that palmitoylation does indeed regulate RBC susceptibility to parasite invasion. Taken together, our findings revealed the dynamic palmitoylome of RBC membrane proteins and its role in P. falciparum invasion.

Plasmodium Egress Across the Parasite Life Cycle

Human malaria, caused by infection with Plasmodium parasites, remains one of the most important global public health problems, with the World Health Organization reporting more than 240 million cases and 600,000 deaths annually as of 2020 (World malaria report 2021). Our understanding of the biology of these parasites is critical for development of effective therapeutics and prophylactics, including both antimalarials and vaccines. Plasmodium is a protozoan organism that is intracellular for most of its life cycle. However, to complete its complex life cycle and to allow for both amplification and transmission, the parasite must egress out of the host cell in a highly regulated manner. This review discusses the major pathways and proteins involved in the egress events during the Plasmodium life cycle-merozoite and gametocyte egress out of red blood cells, sporozoite egress out of the oocyst, and merozoite egress out of the hepatocyte. The similarities, as well as the differences, between the various egress pathways of the parasite highlight both novel cell biology and potential therapeutic targets to arrest its life cycle.

Functional Analysis of the Expanded Phosphodiesterase Gene Family in Toxoplasma gondii Tachyzoites

Toxoplasma motility is both activated and suppressed by 3′,5′-cyclic nucleotide signaling. Cyclic GMP (cGMP) signaling through Toxoplasma gondii protein kinase G (TgPKG) activates motility, whereas cyclic AMP (cAMP) signaling through TgPKAc1 inhibits motility. Despite their importance, it remains unclear how cGMP and cAMP levels are maintained in Toxoplasma. Phosphodiesterases (PDEs) are known to inactivate cyclic nucleotides and are highly expanded in the Toxoplasma genome. Here, we analyzed the expression and function of the 18-member TgPDE family in tachyzoites, the virulent life stage of Toxoplasma. We detected the expression of 11 of 18 TgPDEs, confirming prior expression studies. A knockdown screen of the TgPDE family revealed four TgPDEs that contribute to lytic Toxoplasma growth (TgPDE1, TgPDE2, TgPDE5, and TgPDE9). Depletion of TgPDE1 or TgPDE2 caused severe growth defects, prompting further investigation. While TgPDE1 was important for extracellular motility, TgPDE2 was important for host cell invasion, parasite replication, host cell egress, and extracellular motility. TgPDE1 displayed a plasma membrane/cytomembranous distribution, whereas TgPDE2 displayed an endoplasmic reticulum/cytomembranous distribution. Biochemical analysis of TgPDE1 and TgPDE2 purified from Toxoplasma lysates revealed that TgPDE1 hydrolyzes both cGMP and cAMP, whereas TgPDE2 was cAMP specific. Interactome studies of TgPDE1 and TgPDE2 indicated that they do not physically interact with each other or other TgPDEs but may be regulated by kinases and proteases. Our studies have identified TgPDE1 and TgPDE2 as central regulators of tachyzoite cyclic nucleotide levels and enable future studies aimed at determining how these enzymes are regulated and cooperate to control Toxoplasma motility and growth.

IMPORTANCE Apicomplexan parasites require motility to actively infect host cells and cause disease. Cyclic nucleotide signaling governs apicomplexan motility, but it is unclear how cyclic nucleotide levels are maintained in these parasites. In search of novel regulators of cyclic nucleotides in the model apicomplexan Toxoplasma, we identified and characterized two catalytically active phosphodiesterases, TgPDE1 and TgPDE2, that are important for Toxoplasma’s virulent tachyzoite life cycle. Enzymes that generate, sense, or degrade cyclic nucleotides make attractive targets for therapies aimed at paralyzing and killing apicomplexan parasites.

Contribution of Transcriptome to Elucidate the Biology of Plasmodium spp

In the present review, we discuss some of the new technologies that have been applied to elucidate how Plasmodium spp escape from the immune system and subvert the host physiology to orchestrate the regulation of its biological pathways. Our manuscript describes how techniques such as microarray approaches, RNA-Seq and single-cell RNA sequencing have contributed to the discovery of transcripts and changed the concept of gene expression regulation in closely related malaria parasite species. Moreover, the text highlights the contributions of high-throughput RNA sequencing for the current knowledge of malaria parasite biology, physiology, vaccine target and the revelation of new players in parasite signaling.

Evidence of G-Protein-Coupled Receptors (GPCR) in the Parasitic Protozoa Plasmodium falciparum-Sensing the Host Environment and Coupling within Its Molecular Signaling Toolkit

Throughout evolution, the need for single-celled organisms to associate and form a single cluster of cells has had several evolutionary advantages. In complex, multicellular organisms, each tissue or organ has a specialty and function that make life together possible, and the organism as a whole needs to act in balance and adapt to changes in the environment. Sensory organs are essential for connecting external stimuli into a biological response, through the senses: sight, smell, taste, hearing, and touch. The G-protein-coupled receptors (GPCRs) are responsible for many of these senses and therefore play a key role in the perception of the cells' external environment, enabling interaction and coordinated development between each cell of a multicellular organism. The malaria-causing protozoan parasite, Plasmodium falciparum, has a complex life cycle that is extremely dependent on a finely regulated cellular signaling machinery. In this review, we summarize strong evidence and the main candidates of GPCRs in protozoan parasites. Interestingly, one of these GPCRs is a sensor for K+ shift in Plasmodium falciparum, PfSR25. Studying this family of proteins in P. falciparum could have a significant impact, both on understanding the history of the evolution of GPCRs and on finding new targets for antimalarials.

Calcium in the Backstage of Malaria Parasite Biology

The calcium ion (Ca2+) is a ubiquitous second messenger involved in key biological processes in prokaryotes and eukaryotes. In Plasmodium species, Ca2+ signaling plays a central role in the parasite life cycle. It has been associated with parasite development, fertilization, locomotion, and host cell infection. Despite the lack of a canonical inositol-1,4,5-triphosphate receptor gene in the Plasmodium genome, pharmacological evidence indicates that inositol-1,4,5-triphosphate triggers Ca2+ mobilization from the endoplasmic reticulum. Other structures such as acidocalcisomes, food vacuole and mitochondria are proposed to act as supplementary intracellular Ca2+ reservoirs. Several Ca2+-binding proteins (CaBPs) trigger downstream signaling. Other proteins with no EF-hand motifs, but apparently involved with CaBPs, are depicted as playing an important role in the erythrocyte invasion and egress. It is also proposed that a cross-talk among kinases, which are not members of the family of Ca2+-dependent protein kinases, such as protein kinases G, A and B, play additional roles mediated indirectly by Ca2+ regulation. This statement may be extended for proteins directly related to invasion or egress, such as SUB1, ERC, IMC1I, IMC1g, GAP45 and EBA175. In this review, we update our understanding of aspects of Ca2+-mediated signaling correlated to the developmental stages of the malaria parasite life cycle.

Evidences of G Coupled-Protein Receptor (GPCR) Signaling in the human Malaria Parasite Plasmodium falciparum for Sensing its Microenvironment and the Role of Purinergic Signaling in Malaria Parasites

The nucleotides were discovered in the early 19th century and a few years later, the role of such molecules in energy metabolism and cell survival was postulated. In 1972, a pioneer work by Burnstock and colleagues suggested that ATP could also work as a neurotransmitter, which was known as the "purinergic hypothesis". The idea of ATP working as a signaling molecule faced initial resistance until the discovery of the receptors for ATP and other nucleotides, called purinergic receptors. Among the purinergic receptors, the P2Y family is of great importance because it comprises of G proteincoupled receptors (GPCRs). GPCRs are widespread among different organisms. These receptors work in the cells' ability to sense the external environment, which involves: to sense a dangerous situation or detect a pheromone through smell; the taste of food that should not be eaten; response to hormones that alter metabolism according to the body's need; or even transform light into an electrical stimulus to generate vision. Advances in understanding the mechanism of action of GPCRs shed light on increasingly promising treatments for diseases that have hitherto remained incurable, or the possibility of abolishing side effects from therapies widely used today.

cAMP-Dependent Signaling Pathways as Potential Targets for Inhibition of Plasmodium falciparum Blood Stages

We review the role of signaling pathways in regulation of the key processes of merozoite egress and red blood cell invasion by Plasmodium falciparum and, in particular, the importance of the second messengers, cAMP and Ca²⁺, and cyclic nucleotide dependent kinases. cAMP-dependent protein kinase (PKA) is comprised of cAMP-binding regulatory, and catalytic subunits. The less well conserved cAMP-binding pockets should make cAMP analogs attractive drug leads, but this approach is compromised by the poor membrane permeability of cyclic nucleotides. We discuss how the conserved nature of ATP-binding pockets makes ATP analogs inherently prone to off-target effects and how ATP analogs and genetic manipulation can be useful research tools to examine this. We suggest that targeting PKA interaction partners as well as substrates, or developing inhibitors based on PKA interaction sites or phosphorylation sites in PKA substrates, may provide viable alternative approaches for the development of anti-malarial drugs. Proximity of PKA to a substrate is necessary for substrate phosphorylation, but the P. falciparum genome encodes few recognizable A-kinase anchor proteins (AKAPs), suggesting the importance of PKA-regulatory subunit myristylation and membrane association in determining substrate preference. We also discuss how Pf14-3-3 assembles a phosphorylation-dependent signaling complex that includes PKA and calcium dependent protein kinase 1 (CDPK1) and how this complex may be critical for merozoite invasion, and a target to block parasite growth. We compare altered phosphorylation levels in intracellular and egressed merozoites to identify potential PKA substrates. Finally, as host PKA may have a critical role in supporting intracellular parasite development, we discuss its role at other stages of the life cycle, as well as in other apicomplexan infections. Throughout our review we propose possible new directions for the therapeutic exploitation of cAMP-PKA-signaling in malaria and other diseases caused by apicomplexan parasites.

Plasmodium sporozoites on the move: Switching from cell traversal to productive invasion of hepatocytes

Parasites of the genus Plasmodium, the etiological agent of malaria, are transmitted through the bite of anopheline mosquitoes, which deposit sporozoites into the host skin. Sporozoites migrate through the dermis, enter the bloodstream, and rapidly traffic to the liver. They cross the liver sinusoidal barrier and traverse several hepatocytes before switching to productive invasion of a final one for replication inside a parasitophorous vacuole. Cell traversal and productive invasion are functionally independent processes that require proteins secreted from specialized secretory organelles known as micronemes. In this review, we summarize the current understanding of how sporozoites traverse through cells and productively invade hepatocytes, and discuss the role of environmental sensing in switching from a migratory to an invasive state. We propose that timely controlled secretion of distinct microneme subsets could play a key role in successful migration and infection of hepatocytes. A better understanding of these essential biological features of the Plasmodium sporozoite may contribute to the development of new strategies to fight against the very first and asymptomatic stage of malaria.

The protozoan parasite Toxoplasma gondii encodes a gamut of phosphodiesterases during its lytic cycle in human cells

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Toxoplasma genome harbors at least 18 phosphodiesterases encoded by distinct genes.

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Most parasite PDEs lack regulatory modules and are quite divergent from their human orthologs.

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Acutely-infectious tachyzoite stage of T. gondii expresses 11 PDEs with varied localizations.

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PDE8 and PDE9 are closely-related dual-substrate specific proteins residing in the apical pole.

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Homology modeling of PDE8 and PDE9 reveals a conserved 3D topology and substrate pocket.

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PDE9 is dispensable in tachyzoites, signifying a functional redundancy with PDE8.

Cyclic nucleotide signaling is pivotal to the asexual reproduction of Toxoplasma gondii, however little do we know about the phosphodiesterase enzymes in this widespread obligate intracellular parasite. Here, we identified 18 phosphodiesterases (TgPDE1-18) in the parasite genome, most of which form apicomplexan-specific clades and lack archetypal regulatory motifs often found in mammalian PDEs. Genomic epitope-tagging in the tachyzoite stage showed the expression of 11 phosphodiesterases with diverse subcellular distributions. Notably, TgPDE8 and TgPDE9 are located in the apical plasma membrane to regulate cAMP and cGMP signaling, as suggested by their dual-substrate catalysis and structure modeling. TgPDE9 expression can be ablated with no apparent loss of growth fitness in tachyzoites. Likewise, the redundancy in protein expression, subcellular localization and predicted substrate specificity of several other PDEs indicate significant plasticity and spatial control of cyclic nucleotide signaling during the lytic cycle. Our findings shall enable a rational dissection of signaling in tachyzoites by combinatorial mutagenesis. Moreover, the phylogenetic divergence of selected Toxoplasma PDEs from human counterparts can be exploited to develop parasite-specific inhibitors and therapeutics.

An Update on Development of Small-Molecule Plasmodial Kinase Inhibitors

Malaria control relies heavily on the small number of existing antimalarial drugs. However, recurring antimalarial drug resistance necessitates the continual generation of new antimalarial drugs with novel modes of action. In order to shift the focus from only controlling this disease towards elimination and eradication, next-generation antimalarial agents need to address the gaps in the malaria drug arsenal. This includes developing drugs for chemoprotection, treating severe malaria and blocking transmission. Plasmodial kinases are promising targets for next-generation antimalarial drug development as they mediate critical cellular processes and some are active across multiple stages of the parasite's life cycle. This review gives an update on the progress made thus far with regards to plasmodial kinase small-molecule inhibitor development.

Targeting malaria protein kinases

Malaria is one of the most impacting public health problems in tropical and subtropical areas of the globe, with approximately 200 million cases worldwide annually. In the absence of an effective vaccine, rapid treatment is vital for effective malaria control. However, parasite resistance to currently available drugs underscores the urgent need for identifying new antimalarial therapies with new mechanisms of action. Among potential drug targets for developing new antimalarial candidates, protein kinases are attractive. These enzymes catalyze the phosphorylation of several proteins, thereby regulating a variety of cellular processes and playing crucial roles in the development of all stages of the malaria parasite life cycle. Moreover, the large phylogenetic distance between Plasmodium species and its human host is reflected in marked differences in structure and function of malaria protein kinases between the homologs of both species, indicating that selectivity can be attained. In this review, we describe the functions of the different types of Plasmodium kinases and highlight the main recent advances in the discovery of kinase inhibitors as potential new antimalarial drug candidates.

Phosphorylation of Rhoptry Protein RhopH3 Is Critical for Host Cell Invasion by the Malaria Parasite

Host cell invasion by the malaria parasite is critical for establishing infection in human host and is dependent on discharge of key ligands from organelles like rhoptry and microneme, and these ligands interact with host RBC receptors. In the present study, we demonstrate that phosphorylation of a key rhoptry protein, RhopH3, is critical for host invasion. Phosphorylation regulates its localization to rhoptries and discharge from the parasite.

Merozoites formed after asexual division of the malaria parasite invade the host red blood cells (RBCs), which is critical for initiating malaria infection. The process of invasion involves specialized organelles like micronemes and rhoptries that discharge key proteins involved in interaction with host RBC receptors. RhopH complex comprises at least three proteins, which include RhopH3. RhopH3 is critical for the process of red blood cell (RBC) invasion as well as intraerythrocytic development of human malaria parasite Plasmodium falciparum. It is phosphorylated at serine 804 (S804) in the parasite; however, it is unclear if phosphorylation regulates its function. To address this, a CRISPR-CAS9-based approach was used to mutate S804 to alanine (A) in P. falciparum. Using this phosphomutant (R3_S804A) of RhopH3, we demonstrate that the phosphorylation of S804 is critical for host RBC invasion by the parasite but not for its intraerythrocytic development. Importantly, the phosphorylation of RhopH3 regulates its localization to the rhoptries and discharge from the parasite, which is critical for RBC invasion. We also identified P. falciparum CDPK1 (PfCDPK1) as a possible candidate kinase for RhopH3-S804 phosphorylation and found that it regulates RhopH3 secretion from the parasite. These findings provide novel insights into the role of phosphorylation in rhoptry release and invasion, which is poorly understood.

cAMP signalling and its role in host cell invasion by malaria parasites

Cyclic adenosine monophosphate (cAMP) is an important signalling molecule across evolution, but until recently there was little information on its role in malaria parasites. Advances in gene editing - in particular conditional genetic approaches and mass spectrometry have paved the way for characterisation of the key components of the cAMP signalling pathway in malaria parasites. This has revealed that cAMP signalling plays a critical role in invasion of host red blood cells by Plasmodium falciparum merozoites through regulating the phosphorylation of key parasite proteins by the cAMP-dependent protein kinase (PKA). These insights will help us to investigate parasite cAMP signalling as a target for novel antimalarial drugs.

Uncovering the Cyclic AMP Signaling Pathway of the Protozoan Parasite Entamoeba histolytica and Understanding Its Role in Phagocytosis

Second messenger signaling controls a surprisingly diverse range of processes in several eukaryotic pathogens. Molecular machinery and pathways involving these messengers thus hold tremendous opportunities for therapeutic interventions. Relative to Ca²⁺ signaling, the knowledge of a crucial second messenger cyclic AMP (cAMP) and its signaling pathway is very scant in the intestinal parasite Entamoeba histolytica. In the current study, mining the available genomic resources, we have for the first time identified the cAMP signal transduction pathway of E. histolytica. Three heptahelical proteins with variable G-protein-coupled receptor domains, heterotrimeric G-proteins (Gα, Gβ, and Gγ subunits), soluble adenylyl cyclase, cyclase-associated protein, and enzyme carbonic anhydrase were identified in its genome. We could also identify several putative candidate genes for cAMP downstream effectors such as protein kinase A, A-kinase anchoring proteins, and exchange protein directly activated by the cAMP pathway. Using specific inhibitors against key identified targets, we could observe changes in the intracellular cAMP levels as well as defect in the rate of phagocytosis of red blood cells by the parasite E. histolytica. We thus strongly believe that characterization of some of these unexplored crucial signaling determinants will provide a paradigm shift in understanding the pathogenicity of this organism.

Phosphorylation-Dependent Assembly of a 14-3-3 Mediated Signaling Complex during Red Blood Cell Invasion by Plasmodium falciparum Merozoites

Red blood cell (RBC) invasion by Plasmodium merozoites requires multiple steps that are regulated by signaling pathways. Exposure of P. falciparum merozoites to the physiological signal of low K⁺, as found in blood plasma, leads to a rise in cytosolic Ca²⁺, which mediates microneme secretion, motility, and invasion. We have used global phosphoproteomic analysis of merozoites to identify signaling pathways that are activated during invasion. Using quantitative phosphoproteomics, we found 394 protein phosphorylation site changes in merozoites subjected to different ionic environments (high K⁺/low K⁺), 143 of which were Ca²⁺ dependent. These included a number of signaling proteins such as catalytic and regulatory subunits of protein kinase A (PfPKAc and PfPKAr) and calcium-dependent protein kinase 1 (PfCDPK1). Proteins of the 14-3-3 family interact with phosphorylated target proteins to assemble signaling complexes. Here, using coimmunoprecipitation and gel filtration chromatography, we demonstrate that Pf14-3-3I binds phosphorylated PfPKAr and PfCDPK1 to mediate the assembly of a multiprotein complex in P. falciparum merozoites. A phospho-peptide, P1, based on the Ca²⁺-dependent phosphosites of PKAr, binds Pf14-3-3I and disrupts assembly of the Pf14-3-3I-mediated multiprotein complex. Disruption of the multiprotein complex with P1 inhibits microneme secretion and RBC invasion. This study thus identifies a novel signaling complex that plays a key role in merozoite invasion of RBCs. Disruption of this signaling complex could serve as a novel approach to inhibit blood-stage growth of malaria parasites.IMPORTANCE Invasion of red blood cells (RBCs) by Plasmodium falciparum merozoites is a complex process that is regulated by intricate signaling pathways. Here, we used phosphoproteomic profiling to identify the key proteins involved in signaling events during invasion. We found changes in the phosphorylation of various merozoite proteins, including multiple kinases previously implicated in the process of invasion. We also found that a phosphorylation-dependent multiprotein complex including signaling kinases assembles during the process of invasion. Disruption of this multiprotein complex impairs merozoite invasion of RBCs, providing a novel approach for the development of inhibitors to block the growth of blood-stage malaria parasites.

Mechanism of Action of an EPAC1-Selective Competitive Partial Agonist

The exchange protein activated by cAMP (EPAC) is a promising drug target for a wide disease range, from neurodegeneration and infections to cancer and cardiovascular conditions. A novel partial agonist of the EPAC isoform 1 (EPAC1), I942, was recently discovered, but its mechanism of action remains poorly understood. Here, we utilize NMR spectroscopy to map the I942-EPAC1 interactions at atomic resolution and propose a mechanism for I942 partial agonism. We found that I942 interacts with the phosphate binding cassette (PBC) and base binding region (BBR) of EPAC1, similar to cyclic adenosine monophosphate (cAMP). These results not only reveal the molecular basis for the I942 vs cAMP mimicry and competition, but also suggest that the partial agonism of I942 arises from its ability to stabilize an inhibition-incompetent activation intermediate distinct from both active and inactive EPAC1 states. The mechanism of action of I942 may facilitate drug design for EPAC-related diseases.

Protein S-Palmitoylation Is Responsive to External Signals and Plays a Regulatory Role in Microneme Secretion in Plasmodium falciparum Merozoites

Protein S-palmitoylation is an important post-translational modification (PTM) in blood stages of the malaria parasite, Plasmodium falciparum. S-palmitoylation refers to reversible covalent modification of cysteine residues of proteins by saturated fatty acids. In vivo, palmitoylation is regulated by concerted activities of DHHC palmitoyl acyl transferases (DHHC PATs) and acyl protein thioesterases (APTs), which are enzymes responsible for protein palmitoylation and depalmitoylation, respectively. Here, we investigate the role of protein palmitoylation in red blood cell (RBC) invasion by P. falciparum merozoites. We demonstrate for the first time that free merozoites require PAT activity for microneme secretion in response to exposure to the physiologically relevant low [K⁺] environment, characteristic of blood plasma. We have adapted copper catalyzed alkyne azide chemistry (CuAAC) to image palmitoylation in merozoites and found that exposure to low [K⁺] activates PAT activity in merozoites. Moreover, using acyl biotin exchange chemistry (ABE) and confocal imaging, we demonstrate that a calcium dependent protein kinase, PfCDPK1, an essential regulator of key invasion processes such as motility and microneme secretion, undergoes dynamic palmitoylation and localizes to the merozoite membrane. Treatment of merozoites with the PAT inhibitor, 2-bromopalmitate (2-BP), effectively inhibits microneme secretion and RBC invasion by the parasite, thus opening the possibility of targeting P. falciparum PATs for antimalarial drug discovery to inhibit blood stage growth of malaria parasites.

Cell invasion by intracellular parasites - the many roads to infection

Intracellular parasites from the genera Toxoplasma, Plasmodium, Trypanosoma, Leishmania and from the phylum Microsporidia are, respectively, the causative agents of toxoplasmosis, malaria, Chagas disease, leishmaniasis and microsporidiosis, illnesses that kill millions of people around the globe. Crossing the host cell plasma membrane (PM) is an obstacle these parasites must overcome to establish themselves intracellularly and so cause diseases. The mechanisms of cell invasion are quite diverse and include (1) formation of moving junctions that drive parasites into host cells, as for the protozoans Toxoplasma gondii and Plasmodium spp., (2) subversion of endocytic pathways used by the host cell to repair PM, as for Trypanosoma cruzi and Leishmania, (3) induction of phagocytosis as for Leishmania or (4) endocytosis of parasites induced by specialized structures, such as the polar tubes present in microsporidian species. Understanding the early steps of cell entry is essential for the development of vaccines and drugs for the prevention or treatment of these diseases, and thus enormous research efforts have been made to unveil their underlying biological mechanisms. This Review will focus on these mechanisms and the factors involved, with an emphasis on the recent insights into the cell biology of invasion by these pathogens.

A network-based approach reveals novel invasion and Maurer's clefts-related proteins in Plasmodium falciparum

Malaria continues to be a major concern in developing countries despite continuous efforts to find a cure for the disease. Understanding the pathogenesis mechanism is necessary to identify more effective drug targets against malaria. Many years of experimental research have generated a large amount of data for the malarial parasite, Plasmodium falciparum. These data are useful to understand the importance of certain parasite proteins, but it often remains unclear how these proteins come together, interact with other proteins and carry out their function. Identification of all proteins involved in pathogenesis is an important step towards understanding the molecular mechanism of pathogenesis. In this study, dynamic stage-specific protein-protein interaction networks were created based on gene expression data during the parasite's intra-erythrocytic stages and static protein-protein interaction data. Using previously known proteins of a biological event as seed proteins, the random walk with restart (RWR) method was used on the dynamic protein-protein interaction networks to identify novel proteins related to that event. Two screening procedures namely, permutation test and GO enrichment test were performed to increase the reliability of the RWR predictions. The proposed method was first validated on Plasmodium falciparum proteins related to invasion, where it could reproduce the existing knowledge from a small set of seed proteins. It was then used to identify novel Maurer's clefts resident proteins, where it could identify 152 parasite proteins. We show that the current approach can annotate conserved proteins with unknown function. The predicted proteins can help build a mechanistic model for disease pathogenesis, which will be useful in identifying new drug targets.

Recent Advances in EPAC-Targeted Therapies: A Biophysical Perspective

The universal second messenger cAMP regulates diverse intracellular processes by interacting with ubiquitously expressed proteins, such as Protein Kinase A (PKA) and the Exchange Protein directly Activated by cAMP (EPAC). EPAC is implicated in multiple pathologies, thus several EPAC-specific inhibitors have been identified in recent years. However, the mechanisms and molecular interactions underlying the EPAC inhibition elicited by such compounds are still poorly understood. Additionally, being hydrophobic low molecular weight species, EPAC-specific inhibitors are prone to forming colloidal aggregates, which result in non-specific aggregation-based inhibition (ABI) in aqueous systems. Here, we review from a biophysical perspective the molecular basis of the specific and non-specific interactions of two EPAC antagonists—CE3F4R, a non-competitive inhibitor, and ESI-09, a competitive inhibitor of EPAC. Additionally, we discuss the value of common ABI attenuators (e.g., TX and HSA) to reduce false positives at the expense of introducing false negatives when screening aggregation-prone compounds. We hope this review provides the EPAC community effective criteria to evaluate similar compounds, aiding in the optimization of existing drug leads, and informing the development of the next generation of EPAC-specific inhibitors.

Protein Kinase A Is Essential for Invasion of Plasmodium falciparum into Human Erythrocytes

Understanding the mechanisms behind host cell invasion by Plasmodium falciparum remains a major hurdle to developing antimalarial therapeutics that target the asexual cycle and the symptomatic stage of malaria. Host cell entry is enabled by a multitude of precisely timed and tightly regulated receptor-ligand interactions. Cyclic nucleotide signaling has been implicated in regulating parasite invasion, and an important downstream effector of the cAMP-signaling pathway is protein kinase A (PKA), a cAMP-dependent protein kinase. There is increasing evidence that P. falciparum PKA (PfPKA) is responsible for phosphorylation of the cytoplasmic domain of P. falciparum apical membrane antigen 1 (PfAMA1) at Ser610, a cAMP-dependent event that is crucial for successful parasite invasion. In the present study, CRISPR-Cas9 and conditional gene deletion (dimerizable cre) technologies were implemented to generate a P. falciparum parasite line in which expression of the catalytic subunit of PfPKA (PfPKAc) is under conditional control, demonstrating highly efficient dimerizable Cre recombinase (DiCre)-mediated gene excision and complete knockdown of protein expression. Parasites lacking PfPKAc show severely reduced growth after one intraerythrocytic growth cycle and are deficient in host cell invasion, as highlighted by live-imaging experiments. Furthermore, PfPKAc-deficient parasites are unable to phosphorylate PfAMA1 at Ser610. This work not only identifies an essential role for PfPKAc in the P. falciparum asexual life cycle but also confirms that PfPKAc is the kinase responsible for phosphorylating PfAMA1 Ser610.IMPORTANCE Malaria continues to present a major global health burden, particularly in low-resource countries. Plasmodium falciparum, the parasite responsible for the most severe form of malaria, causes disease through rapid and repeated rounds of invasion and replication within red blood cells. Invasion into red blood cells is essential for P. falciparum survival, and the molecular events mediating this process have gained much attention as potential therapeutic targets. With no effective vaccine available, and with the emergence of resistance to antimalarials, there is an urgent need for the development of new therapeutics. Our research has used genetic techniques to provide evidence of an essential protein kinase involved in P. falciparum invasion. Our work adds to the current understanding of parasite signaling processes required for invasion, highlighting PKA as a potential drug target to inhibit invasion for the treatment of malaria.

Targeting malaria parasite invasion of red blood cells as an antimalarial strategy

Plasmodium spp. parasites that cause malaria disease remain a significant global-health burden. With the spread of parasites resistant to artemisinin combination therapies in Southeast Asia, there is a growing need to develop new antimalarials with novel targets. Invasion of the red blood cell by Plasmodium merozoites is essential for parasite survival and proliferation, thus representing an attractive target for therapeutic development. Red blood cell invasion requires a co-ordinated series of protein/protein interactions, protease cleavage events, intracellular signals, organelle release and engagement of an actin-myosin motor, which provide many potential targets for drug development. As these steps occur in the bloodstream, they are directly susceptible and exposed to drugs. A number of invasion inhibitors against a diverse range of parasite proteins involved in these different processes of invasion have been identified, with several showing potential to be optimised for improved drug-like properties. In this review, we discuss red blood cell invasion as a drug target and highlight a number of approaches for developing antimalarials with invasion inhibitory activity to use in future combination therapies.

Phosphatidic acid homeostasis regulated by a type-2 phosphatidic acid phosphatase represents a novel druggable target in malaria intervention

Type-2 phosphatidic acid phosphatase (PAP2) a member of PAP2 superfamily mediates the conversion of phosphatidic acid (PA) to diacylglycerol (DAG) and thus plays a pivotal role in numerous cellular signaling processes in diverse organisms. An elevated level of intracellular PA is detrimental for the cell and induces cell death. In this study we identified and characterized a PAP2 homologue in Plasmodium falciparum, PfPAP2 and further elucidated its significance in regulation of PA homeostasis in parasite life cycle. PfPAP2 is expressed in the blood stage and harbors the canonical acid phosphatase domain (APD) with signature motifs. PfPAP2 catalyzes the dephosphorylation of PA to produce DAG and inorganic phosphate (P_i). Propranolol, a generic inhibitor of PAP2, inhibited the phosphatase activity of PfPAP2 by binding to the active site of APD domain as evident by in silico docking and confirmed by surface plasmon resonance (SPR) analysis. Inhibition of native PfPAP2 by propranolol led to rise in intracellular PA mediating disruption of intracellular PA homeostasis in parasites. The propranolol mediated inhibition of PfPAP2 directed early secretion of a micronemal Perforin like Protein, PfPLP1 leading to untimely permeabilization and host cell egress. The merozoites following premature egress were non-invasive and were attenuated to invade erythrocytes and cannot continue next cycle growth. This study demonstrates that disruption of PA homeostasis can cause growth retardation in malaria parasites, and thus its master regulator, PfPAP2, can serve as a very good molecular target for antimalarial chemotherapeutic interventions.

A pan-apicomplexan phosphoinositide-binding protein acts in malarial microneme exocytosis

Invasion of human red blood cells by the malaria parasite Plasmodium falciparum is an essential step in the development of the disease. Consequently, the molecular players involved in host cell invasion represent important targets for inhibitor design and vaccine development. The process of merozoite invasion is a succession of steps underlined by the sequential secretion of the organelles of the apical complex. However, little is known with regard to how their contents are exocytosed. Here, we identify a phosphoinositide-binding protein conserved in apicomplexan parasites and show that it is important for the attachment and subsequent invasion of the erythrocyte by the merozoite. Critically, removing the protein from its site of action by knock sideways preferentially prevents the secretion of certain types of micronemes. Our results therefore provide evidence for a role of phosphoinositide lipids in the malaria invasion process and provide further insight into the secretion of microneme organelle populations, which is potentially applicable to diverse apicomplexan parasites.

PKAc is not required for the preerythrocytic stages of Plasmodium berghei

The mutant salivary gland sporozoites lacking PKAc are able to glide, invade hepatocytes, and mature into hepatic merozoites, which release successfully from the merosome, however, fail to initiate blood stage infection when inoculated into mice.

Plasmodium sporozoites invade hepatocytes to initiate infection in the mammalian host. In the infected hepatocytes, sporozoites undergo rapid expansion and differentiation, resulting in the formation and release of thousands of invasive merozoites into the bloodstream. Both sporozoites and merozoites invade their host cells by activation of a signaling cascade followed by discharge of micronemal content. cAMP-dependent protein kinase catalytic subunit (PKAc)–mediated signaling plays an important role in merozoite invasion of erythrocytes, but its role during other stages of the parasite remains unknown. Becaused of the essentiality of PKAc in blood stages, we generated conditional mutants of PKAc by disrupting the gene in Plasmodium berghei sporozoites. The mutant salivary gland sporozoites were able to glide, invaded hepatocytes, and matured into hepatic merozoites which were released successfully from merosome, however failed to initiate blood stage infection when inoculated into mice. Our results demonstrate that malaria parasite complete preerythrocytic stages development without PKAc, raising the possibility that the PKAc independent signaling operates in preerythrocytic stages of P. berghei.

Cyclic AMP signalling controls key components of malaria parasite host cell invasion machinery

Cyclic AMP (cAMP) is an important signalling molecule across evolution, but its role in malaria parasites is poorly understood. We have investigated the role of cAMP in asexual blood stage development of Plasmodium falciparum through conditional disruption of adenylyl cyclase beta (ACβ) and its downstream effector, cAMP-dependent protein kinase (PKA). We show that both production of cAMP and activity of PKA are critical for erythrocyte invasion, whilst key developmental steps that precede invasion still take place in the absence of cAMP-dependent signalling. We also show that another parasite protein with putative cyclic nucleotide binding sites, Plasmodium falciparum EPAC (PfEpac), does not play an essential role in blood stages. We identify and quantify numerous sites, phosphorylation of which is dependent on cAMP signalling, and we provide mechanistic insight as to how cAMP-dependent phosphorylation of the cytoplasmic domain of the essential invasion adhesin apical membrane antigen 1 (AMA1) regulates erythrocyte invasion.

Studying the rigidity of red blood cells induced by Plasmodium falciparum infection

We study the effect of different chemical moieties on the rigidity of red blood cells (RBCs) induced by Plasmodium falciparum infection, and the bystander effect previously found. The infected cells are obtained from a culture of parasite-infected RBCs grown in the laboratory. The rigidity of RBCs is measured by looking at the Brownian fluctuations of individual cells in an optical-tweezers trap. The results point towards increased intracellular cyclic adenosine monophosphate (cAMP) levels as being responsible for the increase in rigidity.

An apically located hybrid guanylate cyclase-ATPase is critical for the initiation of Ca2+ signaling and motility in Toxoplasma gondii

Protozoan parasites of the phylum Apicomplexa actively move through tissue to initiate and perpetuate infection. The regulation of parasite motility relies on cyclic nucleotide-dependent kinases, but how these kinases are activated remains unknown. Here, using an array of biochemical and cell biology approaches, we show that the apicomplexan parasite Toxoplasma gondii expresses a large guanylate cyclase (TgGC) protein, which contains several upstream ATPase transporter-like domains. We show that TgGC has a dynamic localization, being concentrated at the apical tip in extracellular parasites, which then relocates to a more cytosolic distribution during intracellular replication. Conditional TgGC knockdown revealed that this protein is essential for acute-stage tachyzoite growth, as TgGC-deficient parasites were defective in motility, host cell attachment, invasion, and subsequent host cell egress. We show that TgGC is critical for a rapid rise in cytosolic [Ca²⁺] and for secretion of microneme organelles upon stimulation with a cGMP agonist, but these deficiencies can be bypassed by direct activation of signaling by a Ca²⁺ ionophore. Furthermore, we found that TgGC is required for transducing changes in extracellular pH and [K⁺] to activate cytosolic [Ca²⁺] flux. Together, the results of our work implicate TgGC as a putative signal transducer that activates Ca²⁺ signaling and motility in Toxoplasma.

Nucleoside analogue activators of cyclic AMP-independent protein kinase A of Trypanosoma

Protein kinase A (PKA), the main effector of cAMP in eukaryotes, is a paradigm for the mechanisms of ligand-dependent and allosteric regulation in signalling. Here we report the orthologous but cAMP-independent PKA of the protozoan Trypanosoma and identify 7-deaza-nucleosides as potent activators (EC₅₀ ≥ 6.5 nM) and high affinity ligands (K_D ≥ 8 nM). A co-crystal structure of trypanosome PKA with 7-cyano-7-deazainosine and molecular docking show how substitution of key amino acids in both CNB domains of the regulatory subunit and its unique C-terminal αD helix account for this ligand swap between trypanosome PKA and canonical cAMP-dependent PKAs. We propose nucleoside-related endogenous activators of Trypanosoma brucei PKA (TbPKA). The existence of eukaryotic CNB domains not associated with binding of cyclic nucleotides suggests that orphan CNB domains in other eukaryotes may bind undiscovered signalling molecules. Phosphoproteome analysis validates 7-cyano-7-deazainosine as powerful cell-permeable inducer to explore cAMP-independent PKA signalling in medically important neglected pathogens.

Phosphodiesterase beta is the master regulator of cAMP signalling during malaria parasite invasion

Cyclic nucleotide signalling is a major regulator of malaria parasite differentiation. Phosphodiesterase (PDE) enzymes are known to control cyclic GMP (cGMP) levels in the parasite, but the mechanisms by which cyclic AMP (cAMP) is regulated remain enigmatic. Here, we demonstrate that Plasmodium falciparum phosphodiesterase β (PDEβ) hydrolyses both cAMP and cGMP and is essential for blood stage viability. Conditional gene disruption causes a profound reduction in invasion of erythrocytes and rapid death of those merozoites that invade. We show that this dual phenotype results from elevated cAMP levels and hyperactivation of the cAMP-dependent protein kinase (PKA). Phosphoproteomic analysis of PDEβ-null parasites reveals a >2-fold increase in phosphorylation at over 200 phosphosites, more than half of which conform to a PKA substrate consensus sequence. We conclude that PDEβ plays a critical role in governing correct temporal activation of PKA required for erythrocyte invasion, whilst suppressing untimely PKA activation during early intra-erythrocytic development.

Protein kinase A negatively regulates Ca2+ signalling in Toxoplasma gondii

The phylum Apicomplexa comprises a group of obligate intracellular parasites that alternate between intracellular replicating stages and actively motile extracellular forms that move through tissue. Parasite cytosolic Ca²⁺ signalling activates motility, but how this is switched off after invasion is complete to allow for replication to begin is not understood. Here, we show that the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A catalytic subunit 1 (PKAc1) of Toxoplasma is responsible for suppression of Ca²⁺ signalling upon host cell invasion. We demonstrate that PKAc1 is sequestered to the parasite periphery by dual acylation of PKA regulatory subunit 1 (PKAr1). Upon genetic depletion of PKAc1 we show that newly invaded parasites exit host cells shortly thereafter, in a perforin-like protein 1 (PLP-1)-dependent fashion. Furthermore, we demonstrate that loss of PKAc1 prevents rapid down-regulation of cytosolic [Ca²⁺] levels shortly after invasion. We also provide evidence that loss of PKAc1 sensitises parasites to cyclic GMP (cGMP)-induced Ca²⁺ signalling, thus demonstrating a functional link between cAMP and these other signalling modalities. Together, this work provides a new paradigm in understanding how Toxoplasma and related apicomplexan parasites regulate infectivity.

Central to pathogenesis and infectivity of Toxoplasma and related parasites is their ability to move through tissue, invade host cells, and establish a replicative niche. Ca²⁺-dependent signalling pathways are important for activating motility, host cell invasion, and egress, yet how this signalling is turned off after invasion is unclear. Here, we show that a cAMP-dependent protein kinase A (PKA) is essential for rapid suppression of Ca²⁺ signalling upon completion of host cell invasion. Parasites lacking this kinase rapidly invoke an egress program to re-exit host cells, thus preventing the establishment of a stable infection. This finding therefore highlights the first factor required for Toxoplasma (and any related apicomplexan parasite) to switch from invasive to the replicative forms.

Mechanism of Selective Enzyme Inhibition through Uncompetitive Regulation of an Allosteric Agonist

Classical uncompetitive inhibitors are potent pharmacological modulators of enzyme function. Since they selectively target enzyme-substrate complexes (E:S), their inhibitory potency is amplified by increasing substrate concentrations. Recently, an unconventional uncompetitive inhibitor, called CE3F4R, was discovered for the exchange protein activated by cAMP isoform 1 (EPAC1). Unlike conventional uncompetitive inhibitors, CE3F4R is uncompetitive with respect to an allosteric effector, cAMP, as opposed to the substrate (i.e., CE3F4R targets the E:cAMP rather than the E:S complex). However, the mechanism of CE3F4R as an uncompetitive inhibitor is currently unknown. Here, we elucidate the mechanism of CE3F4R's action using NMR spectroscopy. Due to limited solubility and line broadening, which pose major challenges for traditional structural determination approaches, we resorted to a combination of protein- and ligand-based NMR experiments to comparatively analyze EPAC mutations, inhibitor analogs, and cyclic nucleotide derivatives that trap EPAC at different stages of activation. We discovered that CE3F4R binds within the EPAC cAMP-binding domain (CBD) at a subdomain interface distinct from the cAMP binding site, acting as a wedge that stabilizes a cAMP-bound mixed-intermediate. The mixed-intermediate includes attributes of both the apo/inactive and cAMP-bound/active states. In particular, the intermediate targeted by CE3F4R traps a CBD's hinge helix in its inactive conformation, locking EPAC into a closed domain topology that restricts substrate access to the catalytic domain. The proposed mechanism of action also explains the isoform selectivity of CE3F4R in terms of a single EPAC1 versus EPAC2 amino acid difference that destabilizes the active conformation of the hinge helix.

Cyclic nucleotide signalling in malaria parasites

The cyclic nucleotides 3′, 5′-cyclic adenosine monophosphate (cAMP) and 3′, 5′-cyclic guanosine monophosphate (cGMP) are intracellular messengers found in most animal cell types. They usually mediate an extracellular stimulus to drive a change in cell function through activation of their respective cyclic nucleotide-dependent protein kinases, PKA and PKG. The enzymatic components of the malaria parasite cyclic nucleotide signalling pathways have been identified, and the genetic and biochemical studies of these enzymes carried out to date are reviewed herein. What has become very clear is that cyclic nucleotides play vital roles in controlling every stage of the complex malaria parasite life cycle. Our understanding of the involvement of cyclic nucleotide signalling in orchestrating the complex biology of malaria parasites is still in its infancy, but the recent advances in our genetic tools and the increasing interest in signalling will deliver more rapid progress in the coming years.

Plasmodial Kinase Inhibitors: License to Cure?

Advances in the genetics, function, and stage-specificity of Plasmodium kinases has driven robust efforts to identify targets for the design of antimalarial therapies. Reverse genomics following phenotypic screening against Plasmodia or related parasites has uncovered vulnerable kinase targets including PI4K, PKG, and GSK-3, an approach bolstered by access to human disease-directed kinase libraries. Alternatively, screening compound libraries against Plasmodium kinases has successfully led to inhibitors with antiplasmodial activity. As with other therapeutic areas, optimizing compound ADMET and PK properties in parallel with target inhibitory potency and whole cell activity becomes paramount toward advancing compounds as clinical candidates. These and other considerations will be discussed in the context of progress achieved toward deriving important, novel mode-of-action kinase-inhibiting antimalarial medicines.

The Molecular Basis of Erythrocyte Invasion by Malaria Parasites

Plasmodium species cause malaria by proliferating in human erythrocytes. Invasion of immunologically privileged erythrocytes provides a relatively protective niche as well as access to a rich source of nutrients. Plasmodium spp. target erythrocytes of different ages, but share a common mechanism of invasion. Specific engagement of erythrocyte receptors defines target cell tropism, activating downstream events and resulting in the physical penetration of the erythrocyte, powered by the parasite's actinomyosin-based motor. Here we review the latest in our understanding of the molecular composition of this highly complex and fascinating biological process.

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Genome-wide analysis of the human malaria parasite Plasmodium falciparum transcription factor PfNF-YB shows interaction with a CCAAT motif

Little is known about transcription factor regulation during the Plasmodium falciparum intraerythrocytic cycle. In order to elucidate the role of the P. falciparum (Pf)NF-YB transcription factor we searched for target genes in the entire genome. PfNF-YB mRNA is highly expressed in late trophozoite and schizont stages relative to the ring stage. In order to determine the candidate genes bound by PfNF-YB a ChIP-on-chip assay was carried out and 297 genes were identified. Ninety nine percent of PfNF-YB binding was to putative promoter regions of protein coding genes of which only 16% comprise proteins of known function. Interestingly, our data reveal that PfNF-YB binding is not exclusively to a canonical CCAAT box motif. PfNF-YB binds to genes coding for proteins implicated in a range of different biological functions, such as replication protein A large subunit (DNA replication), hypoxanthine phosphoribosyltransferase (nucleic acid metabolism) and multidrug resistance protein 2 (intracellular transport).

Human Cyclophilin B forms part of a multi-protein complex during erythrocyte invasion by Plasmodium falciparum

Invasion of human erythrocytes by Plasmodium falciparum merozoites involves multiple interactions between host receptors and their merozoite ligands. Here we report human Cyclophilin B as a receptor for PfRhopH3 during merozoite invasion. Localization and binding studies show that Cyclophilin B is present on the erythrocytes and binds strongly to merozoites. We demonstrate that PfRhopH3 binds to the RBCs and their treatment with Cyclosporin A prevents merozoite invasion. We also show a multi-protein complex involving Cyclophilin B and Basigin, as well as PfRhopH3 and PfRh5 that aids the invasion. Furthermore, we report identification of a de novo peptide CDP3 that binds Cyclophilin B and blocks invasion by up to 80%. Collectively, our data provide evidence of compounded interactions between host receptors and merozoite surface proteins and paves the way for developing peptide and small-molecules that inhibit the protein−protein interactions, individually or in toto, leading to abrogation of the invasion process.

Invasion of red blood cells by Plasmodium falciparum is a complex process and relies on several receptor-ligand interactions. Here, the authors show that human cyclophilin B binds Plasmodium surface protein PfRhopH3 and that interruption of this interaction reduces invasion by 80%.

Crosstalk between PKA and PKG controls pH-dependent host cell egress of Toxoplasma gondii

Toxoplasma gondii encodes three protein kinase A catalytic (PKAc1-3) and one regulatory (PKAr) subunits to integrate cAMP-dependent signals. Here, we show that inactive PKAc1 is maintained at the parasite pellicle by interacting with acylated PKAr. Either a conditional knockdown of PKAr or the overexpression of PKAc1 blocks parasite division. Conversely, down-regulation of PKAc1 or stabilisation of a dominant-negative PKAr isoform that does not bind cAMP triggers premature parasite egress from infected cells followed by serial invasion attempts leading to host cell lysis. This untimely egress depends on host cell acidification. A phosphoproteome analysis suggested the interplay between cAMP and cGMP signalling as PKAc1 inactivation changes the phosphorylation profile of a putative cGMP-phosphodiesterase. Concordantly, inhibition of the cGMP-dependent protein kinase G (PKG) blocks egress induced by PKAc1 inactivation or environmental acidification, while a cGMP-phosphodiesterase inhibitor circumvents egress repression by PKAc1 or pH neutralisation. This indicates that pH and PKAc1 act as balancing regulators of cGMP metabolism to control egress. These results reveal a crosstalk between PKA and PKG pathways to govern egress in T. gondii.

Shelph2, a bacterial-like phosphatase of the malaria parasite Plasmodium falciparum, is dispensable during asexual blood stage

During the erythrocytic cycle of the malaria parasite Plasmodium falciparum, egress and invasion are essential steps finely controlled by reversible phosphorylation. In contrast to the growing number of kinases identified as key regulators, phosphatases have been poorly studied, and calcineurin is the only one identified so far to play a role in invasion. PfShelph2, a bacterial-like phosphatase, is a promising candidate to participate in the invasion process, as it was reported to be expressed late during the asexual blood stage and to reside within an apical compartment, yet distinct from rhoptry bulb, micronemes, or dense granules. It was also proposed to play a role in the control of the red blood cell membrane deformability at the end of the invasion process. However, genetic studies are still lacking to support this hypothesis. Here, we take advantage of the CRISPR-Cas9 technology to tag shelph2 genomic locus while retaining its endogenous regulatory regions. This new strain allows us to follow the endogenous PfShelph2 protein expression and location during asexual blood stages. We show that PfShelph2 apical location is also distinct from the rhoptry neck or exonemes. We further demonstrate PfShelph2 dispensability during the asexual blood stage by generating PfShelph2-KO parasites using CRISPR-Cas9 machinery. Analyses of the mutant during the course of the erythrocytic development indicate that there are no detectable phenotypic consequences of Pfshelph2 genomic deletion. As this lack of phenotype might be due to functional redundancy, we also explore the likelihood of PfShelph1 (PfShelph2 paralog) being a compensatory phosphatase. We conclude that despite its cyclic expression profile, PfShelph2 is a dispensable phosphatase during the Plasmodium falciparum asexual blood stage, whose function is unlikely substituted by PfShelph1.

Molecular mechanisms that mediate invasion and egress of malaria parasites from red blood cells

PURPOSE OF REVIEW

Malaria parasites invade and multiply in diverse host cells during their complex life cycle. Some blood stage parasites transform into male and female gametocytes that are transmitted by female anopheline mosquitoes. The gametocytes are activated in the mosquito midgut to form male and female gametes, which egress from RBCs to mate and form a zygote. Here, we will review our current understanding of the molecular mechanisms that mediate invasion and egress by malaria parasites at different life cycle stages.

RECENT FINDINGS

A number of key effector molecules such as parasite protein ligands for receptor-engagement during invasion as well as proteases and perforin-like proteins that mediate egress have been identified. Interestingly, these parasite-encoded effectors are located in internal, vesicular organelles and are secreted in a highly regulated manner during invasion and egress. Here, we will review our current understanding of the functional roles of these effectors as well as the signaling pathways that regulate their timely secretion with accurate spatiotemporal coordinates.

SUMMARY

Understanding the molecular basis of key processes such as host cell invasion and egress by malaria parasites could provide novel targets for development of inhibitors to block parasite growth and transmission.

Molecular Signaling Involved in Entry and Exit of Malaria Parasites from Host Erythrocytes

During the blood stage, Plasmodium spp. merozoites invade host red blood cells (RBCs), multiply, exit, and reinvade uninfected RBCs in a continuing cycle that is responsible for all the clinical symptoms associated with malaria. Entry into (invasion) and exit from (egress) RBCs are highly regulated processes that are mediated by an array of parasite proteins with specific functional roles. Many of these parasite proteins are stored in specialized apical secretory vesicles, and their timely release is critical for successful invasion and egress. For example, the discharge of parasite protein ligands to the apical surface of merozoites is required for interaction with host receptors to mediate invasion, and the timely discharge of proteases and pore-forming proteins helps in permeabilization and dismantling of limiting membranes during egress. This review focuses on our understanding of the signaling mechanisms that regulate apical organelle secretion during host cell invasion and egress by malaria parasites. The review also explores how understanding key signaling mechanisms in the parasite can open opportunities to develop novel strategies to target Plasmodium parasites and eliminate malaria.

PfCDPK1 mediated signaling in erythrocytic stages of Plasmodium falciparum

Calcium Dependent Protein Kinases are key effectors of calcium signaling in malaria parasite. PfCDPK1 is critical for asexual development of Plasmodium falciparum, but its precise function and substrates remain largely unknown. Using a conditional knockdown strategy, we here establish that this kinase is critical for the invasion of host erythrocytes. Furthermore, using a multidisciplinary approach involving comparative phosphoproteomics we gain insights into the underlying molecular mechanisms. We identify substrates of PfCDPK1, which includes proteins of Inner Membrane Complex and glideosome-actomyosin motor assembly. Interestingly, PfCDPK1 phosphorylates PfPKA regulatory subunit (PfPKA-R) and regulates PfPKA activity in the parasite, which may be relevant for the process of invasion. This study delineates the signaling network of PfCDPK1 and sheds light on mechanisms via which it regulates invasion.Calcium dependent protein kinase 1 (CDPK1) plays an important role in asexual development of Plasmodium falciparum. Using phosphoproteomics and conditional knockdown of CDPK1, the authors here identify CDPK1 substrates and a cross-talk between CDPK1 and PKA, and show the role of CDPK1 in parasite invasion.

Red Blood Cell Invasion by the Malaria Parasite Is Coordinated by the PfAP2-I Transcription Factor

Obligate intracellular parasites must efficiently invade host cells in order to mature and be transmitted. For the malaria parasite Plasmodium falciparum, invasion of host red blood cells (RBCs) is essential. Here we describe a parasite-specific transcription factor PfAP2-I, belonging to the Apicomplexan AP2 (ApiAP2) family, that is responsible for regulating the expression of genes involved in RBC invasion. Our genome-wide analysis by ChIP-seq shows that PfAP2-I interacts with a specific DNA motif in the promoters of target genes. Although PfAP2-I contains three AP2 DNA-binding domains, only one is required for binding of the target genes during blood stage development. Furthermore, we find that PfAP2-I associates with several chromatin-associated proteins, including the Plasmodium bromodomain protein PfBDP1 and that complex formation is associated with transcriptional regulation. As a key regulator of red blood cell invasion, PfAP2-I represents a potential new antimalarial therapeutic target.