A unique profilin-actin interface is important for malaria parasite motility

Leishmania profilin interacts with actin through an unusual structural mechanism to control cytoskeletal dynamics in parasites

Diseases caused by Leishmania and Trypanosoma parasites are a major health problem in tropical countries. Because of their complex life cycle involving both vertebrate and insect hosts, and >1 billion years of evolutionarily distance, the cell biology of trypanosomatid parasites exhibits pronounced differences to animal cells. For example, the actin cytoskeleton of trypanosomatids is divergent when compared with other eukaryotes. To understand how actin dynamics are regulated in trypanosomatid parasites, we focused on a central actin-binding protein profilin. Co-crystal structure of Leishmania major actin in complex with L. major profilin revealed that, although the overall folds of actin and profilin are conserved in eukaryotes, Leishmania profilin contains a unique α-helical insertion, which interacts with the target binding cleft of actin monomer. This insertion is conserved across the Trypanosomatidae family and is similar to the structure of WASP homology-2 (WH2) domain, a small actin-binding motif found in many other cytoskeletal regulators. The WH2-like motif contributes to actin monomer binding and enhances the actin nucleotide exchange activity of Leishmania profilin. Moreover, Leishmania profilin inhibited formin-catalyzed actin filament assembly in a mechanism that is dependent on the presence of the WH2-like motif. By generating profilin knockout and knockin Leishmania mexicana strains, we show that profilin is important for efficient endocytic sorting in parasites, and that the ability to bind actin monomers and proline-rich proteins, and the presence of a functional WH2-like motif, are important for the in vivo function of Leishmania profilin. Collectively, this study uncovers molecular principles by which profilin regulates actin dynamics in trypanosomatids.

Crystal structure determination, molecular docking and dynamics of arylidene cyanoacetates as potential JNK-3 inhibitors for Ischemia reperfusion injury

Ischemia reperfusion injury is a cardiovascular condition which causes hypoxia by means of obstruction of arterial blood flow eventually leads to reduced synthesis of adenosine tri-phosphate in the mitochondria. c-Jun N-terminal kinase-3 are related to several cascade of events like apoptosis, oxidative stress and mitochondrial dysfunction which can be further related to Ischemia-reperfusion injury. The present study was aimed at determining crystal structure of the ligand by x-ray methods and to perform molecular docking and molecular dynamics studies of the arylidene cyano-acetates with c-Jun N-terminal kinase-3. The binding energy of Ethyl (2E)-2-cyano-3-(4-methoxyphenyl)prop-2-enoate is -4.462 kcal/mol and ethyl (2E)-2-cyano-3-phenylprop-2-enoate is -6.135 kcal/mol. This has created a new rational approach to drug design, where the structure of drug is designed, based on its fit to structures of receptor site, rather than basing it on analogies to other active structures. The above compounds are binding strongly with c-Jun N-terminal kinase-3 protein.[Figure: see text]Communicated by Ramaswamy H. Sarma.

Sulfonylpiperazine compounds prevent Plasmodium falciparum invasion of red blood cells through interference with actin-1/profilin dynamics

With emerging resistance to frontline treatments, it is vital that new antimalarial drugs are identified to target Plasmodium falciparum. We have recently described a compound, MMV020291, as a specific inhibitor of red blood cell (RBC) invasion, and have generated analogues with improved potency. Here, we generated resistance to MMV020291 and performed whole genome sequencing of 3 MMV020291-resistant populations. This revealed 3 nonsynonymous single nucleotide polymorphisms in 2 genes; 2 in profilin (N154Y, K124N) and a third one in actin-1 (M356L). Using CRISPR-Cas9, we engineered these mutations into wild-type parasites, which rendered them resistant to MMV020291. We demonstrate that MMV020291 reduces actin polymerisation that is required by the merozoite stage parasites to invade RBCs. Additionally, the series inhibits the actin-1-dependent process of apicoplast segregation, leading to a delayed death phenotype. In vitro cosedimentation experiments using recombinant P. falciparum proteins indicate that potent MMV020291 analogues disrupt the formation of filamentous actin in the presence of profilin. Altogether, this study identifies the first compound series interfering with the actin-1/profilin interaction in P. falciparum and paves the way for future antimalarial development against the highly dynamic process of actin polymerisation.

In silico identification of potential phytochemical inhibitors targeting farnesyl diphosphate synthase of cotton bollworm (Helicoverpa armigera)

Helicoverpa armigera (Ha), a polyphagous pest, causes significant damage to several crop plants, including cotton. The control of this cosmopolitan pest is largely challenging due to the development of resistance to existing management practices. The Juvenile Hormone (JH) plays a pivotal role in the life cycle of insects by regulating their morphogenetic and gonadotropic development. Hence, enzymes involved in JH biosynthesis are an attractive target for the development of selective insecticides. Farnesyl diphosphate synthase (FPPS), a member protein of (E)-prenyl-transferases, is one of the most crucial enzymes in the biosynthetic pathway of JHs. It catalyzes the condensation of isopentenyl diphosphate (IPP) with dimethylallyl diphosphate (DMAPP), forming farnesyl diphosphate (FPP), a precursor of JH. The study was designed to identify an effective small inhibitory molecule that could inhibit the activity of Helicoverpa armigera - FPPS (HaFPPS) for an effective pest control intervention. Therefore, a 3D model of FPPS protein was generated using homology modeling. The FooDB database library of small molecules was selected for virtual screening, following which binding affinities were evaluated using docking studies. Three top-scored molecules were analyzed for various pharmacophore properties. Further, molecular dynamics (MD) simulation analysis showed that the identified molecules (mitraphylline-ZINC1607834, chlorogenic acid-ZINC2138728 and llagate-ZINC3872446) had a reasonably acceptable binding affinity for HaFPPS and resulted in the formation of a stable HaFPPS-inhibitor(s) complex. The identified phytochemical molecules may be used as potent inhibitors of HaFPPS thus, paving the way for further developing environment-friendly insect growth regulator(s). Communicated by Ramaswamy H. Sarma.

A Microtubule-Associated Protein Is Essential for Malaria Parasite Transmission

Mature gametocytes of Plasmodium falciparum display a banana (falciform) shape conferred by a complex array of subpellicular microtubules (SPMT) associated with the inner membrane complex (IMC). Microtubule-associated proteins (MAPs) define MT populations and modulate interaction with pellicular components. Several MAPs have been identified in Toxoplasma gondii, and homologues can be found in the genomes of Plasmodium species, but the function of these proteins for asexual and sexual development of malaria parasites is still unknown. Here, we identified a novel subpellicular MAP, termed SPM3, that is conserved within the genus Plasmodium, especially within the subgenus Laverania, but absent in other Apicomplexa. Conditional knockdown and targeted gene disruption of Pfspm3 in Plasmodium falciparum cause severe morphological defects during gametocytogenesis, leading to round, nonfalciform gametocytes with an aberrant SPMT pattern. In contrast, Pbspm3 knockout in Plasmodium berghei, a species with round gametocytes, caused no defect in gametocytogenesis, but sporozoites displayed an aberrant motility and a dramatic defect in invasion of salivary glands, leading to a decreased efficiency in transmission. Electron microscopy revealed a dissociation of the SPMT from the IMC in Pbspm3 knockout parasites, suggesting a function of SPM3 in anchoring MTs to the IMC. Overall, our results highlight SPM3 as a pellicular component with essential functions for malaria parasite transmission. IMPORTANCE A key structural feature driving the transition between different life cycle stages of the malaria parasite is the unique three-membrane pellicle, consisting of the parasite plasma membrane (PPM) and a double membrane structure underlying the PPM termed the inner membrane complex (IMC). Additionally, there are numerous linearly arranged intramembranous particles (IMPs) linked to the IMC, which likely link the IMC to the subpellicular microtubule cytoskeleton. Here, we identified, localized, and characterized a novel subpellicular microtubule-associated protein unique to the genus Plasmodium. The knockout of this protein in the human-pathogenic species P. falciparum resulted in malformed gametocytes and aberrant microtubules. We confirmed the microtubule association in the P. berghei rodent malaria homologue and show that its knockout results in a perturbed microtubule architecture, aberrant sporozoite motility, and decreased transmission efficiency.

Divergent Plasmodium actin residues are essential for filament localization, mosquito salivary gland invasion and malaria transmission

Actin is one of the most conserved and ubiquitous proteins in eukaryotes. Its sequence has been highly conserved for its monomers to self-assemble into filaments that mediate essential cell functions such as trafficking, cell shape and motility. The malaria-causing parasite, Plasmodium, expresses a highly sequence divergent actin that is critical for its rapid motility at different stages within its mammalian and mosquito hosts. Each of Plasmodium actin’s four subdomains have divergent regions compared to canonical vertebrate actins. We previously identified subdomains 2 and 3 as providing critical contributions for parasite actin function as these regions could not be replaced by subdomains of vertebrate actins. Here we probed the contributions of individual divergent amino acid residues in these subdomains on parasite motility and progression. Non-lethal changes in these subdomains did not affect parasite development in the mammalian host but strongly affected progression through the mosquito with striking differences in transmission to and through the insect. Live visualization of actin filaments showed that divergent amino acid residues in subdomains 2 and 4 enhanced localization associated with filaments, while those in subdomain 3 negatively affected actin filaments. This suggests that finely tuned actin dynamics are essential for efficient organ entry in the mosquito vector affecting malaria transmission. This work provides residue level insight on the fundamental requirements of actin in highly motile cells.

Actin is one of the most abundant and conserved proteins known. Actin monomers can join together to form long filaments. The malaria-causing parasite is transmitted by mosquitoes and needs actin to move very rapidly. An actin from the parasite is different to other actins: its amino acid sequence has relatively high amounts of changes compared to animal species and the actin tends to form only short filaments. We previously identified two large parts of the protein that were critical for the parasite since these large parts could not be exchanged with the equivalent regions of other species. In this study, we focused in on these regions by making more discrete mutations. Most mutations of the actin sequence were tolerated by the parasite in the blood stages. However, these mutants has striking defects in progressing through mosquitoes, especially in invading its salivary glands. We used a new filament labeler to visualize how these mutations affect the actin filaments and found surprisingly different effects. Taken together, small changes to the sequence can have large consequences for the parasite, which ultimately affects its ability to transmit to a new host.

Phosphorylation of myosin A regulates gliding motility and is essential for Plasmodium transmission

Malaria‐causing parasites rely on an actin–myosin‐based motor for the invasion of different host cells and tissue traversal in mosquitoes and vertebrates. The unusual myosin A of Plasmodium spp. has a unique N‐terminal extension, which is important for red blood cell invasion by P. falciparum merozoites in vitro and harbors a phosphorylation site at serine 19. Here, using the rodent‐infecting P. berghei we show that phosphorylation of serine 19 increases ookinete but not sporozoite motility and is essential for efficient transmission of Plasmodium by mosquitoes as S19A mutants show defects in mosquito salivary gland entry. S19A along with E6R mutations slow ookinetes and salivary gland sporozoites in both 2D and 3D environments. In contrast to data from purified proteins, both E6R and S19D mutations lower force generation by sporozoites. Our data show that the phosphorylation cycle of S19 influences parasite migration and force generation and is critical for optimal migration of parasites during transmission from and to the mosquito.

Virtual screening of phytochemicals from Indian medicinal plants against the endonuclease domain of SFTS virus L polymerase

Severe fever with thrombocytopenia syndrome virus (SFTSV) causes a highly infectious disease with reported mortality in the range 2.8% to 47%. The replication and transcription of the SFTSV genome is performed by L polymerase, which has both an RNA dependent RNA polymerase domain and an N-terminal endonuclease (endoN) domain. Due to its crucial role in the cap-snatching mechanism required for initiation of viral RNA transcription, the endoN domain is an ideal antiviral drug target. In this virtual screening study for the identification of potential inhibitors of the endoN domain of SFTSV L polymerase, we have used molecular docking and molecular dynamics (MD) simulation to explore the natural product space of 14 011 phytochemicals from Indian medicinal plants. After generating a heterogeneous ensemble of endoN domain structures reflecting conformational diversity of the corresponding active site using MD simulations, ensemble docking of the phytochemicals was performed against the endoN domain structures. Apart from the ligand binding energy from docking, our virtual screening workflow imposes additional filters such as drug-likeness, non-covalent interactions with key active site residues, toxicity and chemical similarity with other hits, to identify top 5 potential phytochemical inhibitors of endoN domain of SFTSV L polymerase. Further, the stability of the protein–ligand docked complexes for the top 5 potential inhibitors was analyzed using MD simulations. The potential phytochemical inhibitors, predicted in this study using contemporary computational methods, are expected to serve as lead molecules in future experimental studies towards development of antiviral drugs against SFTSV.

Virtual screening of a large phytochemical library from Indian medicinal plants to identify potential endonuclease inhibitors against emerging virus SFTSV.

Structure and function of an atypical homodimeric actin capping protein from the malaria parasite

Apicomplexan parasites, such as Plasmodium spp., rely on an unusual actomyosin motor, termed glideosome, for motility and host cell invasion. The actin filaments are maintained by a small set of essential regulators, which provide control over actin dynamics in the different stages of the parasite life cycle. Actin filament capping proteins (CPs) are indispensable heterodimeric regulators of actin dynamics. CPs have been extensively characterized in higher eukaryotes, but their role and functional mechanism in Apicomplexa remain enigmatic. Here, we present the first crystal structure of a homodimeric CP from the malaria parasite and compare the homo- and heterodimeric CP structures in detail. Despite retaining several characteristics of a canonical CP, the homodimeric Plasmodium berghei (Pb)CP exhibits crucial differences to the canonical heterodimers. Both homo- and heterodimeric PbCPs regulate actin dynamics in an atypical manner, facilitating rapid turnover of parasite actin, without affecting its critical concentration. Homo- and heterodimeric PbCPs show partially redundant activities, possibly to rescue actin filament capping in life cycle stages where the β-subunit is downregulated. Our data suggest that the homodimeric PbCP also influences actin kinetics by recruiting lateral actin dimers. This unusual function could arise from the absence of a β-subunit, as the asymmetric PbCP homodimer lacks structural elements essential for canonical barbed end interactions suggesting a novel CP binding mode. These findings will facilitate further studies aimed at elucidating the precise actin filament capping mechanism in Plasmodium.

The multiple functions of actin in apicomplexan parasites

The cytoskeletal protein actin is highly abundant and conserved in eukaryotic cells. It occurs in two different states- the globular (G-actin) form, which can polymerise into the filamentous (F-actin) form, fulfilling various critical functions including cytokinesis, cargo trafficking and cellular motility. In higher eukaryotes, there are several actin isoforms with nearly identical amino acid sequences. Despite the high level of amino acid identity, they display regulated expression patterns and unique non-redundant roles. The number of actin isoforms together with conserved sequences may reflect the selective pressure exerted by scores of actin binding proteins (ABPs) in higher eukaryotes. In contrast, in many protozoans such as apicomplexan parasites which possess only a few ABPs, the regulatory control of actin and its multiple functions are still obscure. Here, we provide a summary of the regulation and biological functions of actin in higher eukaryotes and compare it with the current knowledge in apicomplexans. We discuss future experiments that will help us understand the multiple, critical roles of this fascinating system in apicomplexans.

Response of Leucine-Rich Repeat Domain-Containing Protein in Haemaphysalis longicornis to Babesia microti Infection and Its Ligand Identification

Haemaphysalis longicornis is a blood-feeding hard tick known for transmitting a variety of pathogens, including Babesia How the parasites in the imbibed blood become anchored in the midgut of ticks is still unknown. Leucine-rich repeat domain (LRR)-containing protein, which is associated with the innate immune reaction and conserved in many species, has been detected in H. longicornis and has previously been indicated in inhibiting the growth of Babesia gibsoni However, the detailed mechanism is unknown. In this study, one of the ligands for LRR from H. longicornis (HlLRR) was identified in Babesia microti, designated BmActin, using glutathione transferase (GST) pulldown experiments and immunofluorescence assays. Moreover, RNA interference of HlLRR led to a decrease in the BmActin mRNA expression in the midgut of fully engorged ticks which fed on B. microti-infected mice. We also found that the expression level of the innate immune molecules in H. longicornis, defensin, antimicrobial peptides (AMPs), and lysozyme, were downregulated after the knockdown of HlLRR. However, subolesin expression was upregulated. These results indicate that HlLRR not only recognizes BmActin but may also modulate innate immunity in ticks to influence Babesia growth, which will further benefit the development of anti-Babesia vaccines or drugs.

Malaria parasites differentially sense environmental elasticity during transmission

Transmission of malaria-causing parasites to and by the mosquito relies on active parasite migration and constitutes bottlenecks in the Plasmodium life cycle. Parasite adaption to the biochemically and physically different environments must hence be a key evolutionary driver for transmission efficiency. To probe how subtle but physiologically relevant changes in environmental elasticity impact parasite migration, we introduce 2D and 3D polyacrylamide gels to study ookinetes, the parasite forms emigrating from the mosquito blood meal and sporozoites, the forms transmitted to the vertebrate host. We show that ookinetes adapt their migratory path but not their speed to environmental elasticity and are motile for over 24 h on soft substrates. In contrast, sporozoites evolved more short-lived rapid gliding motility for rapidly crossing the skin. Strikingly, sporozoites are highly sensitive to substrate elasticity possibly to avoid adhesion to soft endothelial cells on their long way to the liver. Hence, the two migratory stages of Plasmodium evolved different strategies to overcome the physical challenges posed by the respective environments and barriers they encounter.

Characterization of a profilin-like protein from Fasciola hepatica

Fasciola hepatica is the causative agent of fasciolosis, an important disease of humans and livestock around the world. There is an urgent requirement for novel treatments for F. hepatica due to increasing reports of drug resistance appearing around the world. The outer body covering of F. hepatica is referred to as the tegument membrane which is of crucial importance for the modulation of the host response and parasite survival; therefore, tegument proteins may represent novel drug or vaccine targets. Previous studies have identified a profilin-like protein in the tegument of F. hepatica. Profilin is a regulatory component of the actin cytoskeleton in all eukaryotic cells, and in some protozoan parasites, profilin has been shown to drive a potent IL-12 response. This study characterized the identified profilin form F. hepatica (termed FhProfilin) for the first time. Recombinant expression of FhProfilin resulted in a protein approximately 14 kDa in size which was determined to be dimeric like other profilins isolated from a range of eukaryotic organisms. FhProfilin was shown to bind poly-L-proline (pLp) and sequester actin monomers which is characteristic of the profilin family; however, there was no binding of FhProfilin to phosphatidylinositol lipids. Despite FhProfilin being a component of the tegument, it was shown not to generate an immune response in experimentally infected sheep or cattle.

Multiplexed quantitative proteomics provides mechanistic cues for malaria severity and complexity

Management of severe malaria remains a critical global challenge. In this study, using a multiplexed quantitative proteomics pipeline we systematically investigated the plasma proteome alterations in non-severe and severe malaria patients. We identified a few parasite proteins in severe malaria patients, which could be promising from a diagnostic perspective. Further, from host proteome analysis we observed substantial modulations in many crucial physiological pathways, including lipid metabolism, cytokine signaling, complement, and coagulation cascades in severe malaria. We propose that severe manifestations of malaria are possibly underpinned by modulations of the host physiology and defense machinery, which is evidently reflected in the plasma proteome alterations. Importantly, we identified multiple blood markers that can effectively define different complications of severe falciparum malaria, including cerebral syndromes and severe anemia. The ability of our identified blood markers to distinguish different severe complications of malaria may aid in developing new clinical tests for monitoring malaria severity.

Targeting Imd pathway receptor in Drosophila melanogaster and repurposing of phyto-inhibitors: structural modulation and molecular dynamics

Dysbiosis is a major cause of disease in an individual, generally initiated in the gastrointestinal tract. The gut, also known as the second brain, constitutes a major role in immune signaling. To study the immunity cascade, the Drosophila model was considered targeting the Imd pathway receptor (2F2L) located in the midgut. This receptor further initiates the immune signaling mechanism influenced by bacteria. To inhibit the Imd pathway, the crystal structure of Imd with PDB: 2F2L was considered for the screening of suitable ligand/inhibitor. In light of our previous studies, repurposing of anti-diabetic ligands from the banana plant namely lupeol (LUP), stigmasterol (STI), β-sitosterol (BST) and umbelliferone (UMB) were screened. This study identifies the potential inhibitor along with the tracheal toxin (TCT), a major peptidoglycan constituent of microbes. The molecular docking and molecular dynamics simulation of complexes 2F2L-MLD, 2F2L- CAP, 2F2L-LUP, 2F2L-BST, 2F2L-STI and 2F2L-UMB elucidates the intermolecular interaction into the inhibitory property of ligands. The results of this study infer LUP and UMB as better ligands with high stability and functionality among the screened candidates. This study provides insights into the dysbiosis and its amelioration by plant-derived molecules. The identified drugs (LUP & UMB) will probably act as an inhibitor against microbial dysbiosis and other related pathogenesis (diabetes and diabetic neuropathy). Further, this study will widen avenues in fly biology research and which could be used as a therapeutic model in the rapid, reliable and reproducible screening of phytobiologics in complementary and alternative medicine for various lifestyle associated complications.Communicated by Ramaswamy H. Sarma.

Actomyosin forces and the energetics of red blood cell invasion by the malaria parasite Plasmodium falciparum

All symptoms of malaria disease are associated with the asexual blood stages of development, involving cycles of red blood cell (RBC) invasion and egress by the Plasmodium spp. merozoite. Merozoite invasion is rapid and is actively powered by a parasite actomyosin motor. The current accepted model for actomyosin force generation envisages arrays of parasite myosins, pushing against short actin filaments connected to the external milieu that drive the merozoite forwards into the RBC. In Plasmodium falciparum, the most virulent human malaria species, Myosin A (PfMyoA) is critical for parasite replication. However, the precise function of PfMyoA in invasion, its regulation, the role of other myosins and overall energetics of invasion remain unclear. Here, we developed a conditional mutagenesis strategy combined with live video microscopy to probe PfMyoA function and that of the auxiliary motor PfMyoB in invasion. By imaging conditional mutants with increasing defects in force production, based on disruption to a key PfMyoA phospho-regulation site, the absence of the PfMyoA essential light chain, or complete motor absence, we define three distinct stages of incomplete RBC invasion. These three defects reveal three energetic barriers to successful entry: RBC deformation (pre-entry), mid-invasion initiation, and completion of internalisation, each requiring an active parasite motor. In defining distinct energetic barriers to invasion, these data illuminate the mechanical challenges faced in this remarkable process of protozoan parasitism, highlighting distinct myosin functions and identifying potential targets for preventing malaria pathogenesis.

Overlapping and distinct roles of CDPK family members in the pre-erythrocytic stages of the rodent malaria parasite, Plasmodium berghei

Invasion of hepatocytes by Plasmodium sporozoites initiates the pre-erythrocytic step of a malaria infection. Subsequent development of the parasite within hepatocytes and exit from them is essential for starting the disease-causing erythrocytic cycle. Identification of signaling pathways that operate in pre-erythrocytic stages provides insight into a critical step of infection and potential targets for chemoprotection from malaria. We demonstrate that P. berghei homologs of Calcium Dependent Protein Kinase 1 (CDPK1), CDPK4 and CDPK5 play overlapping but distinct roles in sporozoite invasion and parasite egress from hepatocytes. All three kinases are expressed in sporozoites. All three are required for optimal motility of sporozoites and consequently their invasion of hepatocytes. Increased cGMP can compensate for the functional loss of CDPK1 and CDPK5 during sporozoite invasion but cannot overcome loss of CDPK4. CDPK1 and CDPK5 expression is downregulated after sporozoite invasion. CDPK5 reappears in a subset of late stage liver stages and is present in all merosomes. Chemical inhibition of CDPK4 and depletion of CDPK5 in liver stages implicate these kinases in the formation and/or release of merosomes from mature liver stages. Furthermore, depletion of CDPK5 in merosomes significantly delays initiation of the erythrocytic cycle without affecting infectivity of hepatic merozoites. These data suggest that CDPK5 may be required for the rupture of merosomes. Our work provides evidence that sporozoite invasion requires CDPK1 and CDPK5, and suggests that CDPK5 participates in the release of hepatic merozoites.

The malaria-parasite Plasmodium begins its mammalian cycle by infecting hepatocytes in the liver. A single parasite differentiates into tens of thousands of hepatic merozoites which exit the host cell in vesicles called merosomes. Hepatic merozoites initiate the first round of erythrocytic infection that eventually causes disease. We show that optimal invasion of liver cells by Plasmodium requires the action of three closely-related parasite kinases, CDPK1, 4 and 5. Loss of any of the three enzymes in the parasite significantly reduces infection of liver cells. Furthermore, CDPK5 is likely required for release of hepatic merozoites from merosomes and therefore for initiation of the erythrocytic cycle. A better understanding of how these kinases function could lead to drugs that prevent malaria.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Investigating the role of N-terminal domain in phosphodiesterase 4B-inhibition by molecular dynamics simulation

Phosphodiesterase 4B (PDE4B) is a potential therapeutic target for the inflammatory respiratory diseases such as congestive obstructive pulmonary disease (COPD) and asthma. The sequence identity of ∼88% with its isoform PDE4D is the key barrier in developing selective PDE4B inhibitors which may help to overcome associated side effects. Despite high sequence identity, both isoforms differ in few residues present in N-terminal (UCR2) and C-terminal (CR3) involved in catalytic site formation. Previously, we designed and tested specific PDE4B inhibitors considering N-terminal residues as a part of the catalytic cavity. In continuation, current work thoroughly presents an MD simulation-based analysis of N-terminal residues and their role in ligand binding. The various parameters viz. root mean square deviation (RMSD), radius of gyration (Rg), root mean square fluctuation (RMSF), principal component analysis (PCA), dynamical cross-correlation matrix (DCCM) analysis, secondary structure analysis and residue interaction mapping were investigated to establish rational. Results showed that UCR2 reduced RMSF values for the metal binding pocket (31.5 ± 11 to 13.12 ± 6 Ų) and the substrate-binding pocket (38.8 ± 32 to 17.3 ± 11 Ų). UCR2 enhanced anti-correlated motion at the active site region that led to the improved ligand-binding affinity of PDE4B from -24.57 ± 3 to -35.54 ± 2 kcal/mol. Further, the atomic-level analysis indicated that T-π and π-π interactions between inhibitors and residues are vital forces that regulate inhibitor association to PDE4B with high affinity. In conclusion, UCR2, the N-terminal domain, embraces the dynamics of PDE4B active site and stabilizes PDE4B inhibitor interactions. Therefore the N-terminal domain needs to be considered while designing next-generation, selective PDE4B-inhibitors as potential anti-inflammatory drugs. Communicated by Ramaswamy H. Sarma.

Malaria.tools-comparative genomic and transcriptomic database for Plasmodium species

Malaria is a tropical parasitic disease caused by the Plasmodium genus, which resulted in an estimated 219 million cases of malaria and 435 000 malaria-related deaths in 2017. Despite the availability of the Plasmodium falciparum genome since 2002, 74% of the genes remain uncharacterized. To remedy this paucity of functional information, we used transcriptomic data to build gene co-expression networks for two Plasmodium species (P. falciparum and P. berghei), and included genomic data of four other Plasmodium species, P. yoelii, P. knowlesi, P. vivax and P. cynomolgi, as well as two non-Plasmodium species from the Apicomplexa, Toxoplasma gondii and Theileria parva. The genomic and transcriptomic data were incorporated into the resulting database, malaria.tools, which is preloaded with tools that allow the identification and cross-species comparison of co-expressed gene neighbourhoods, clusters and life stage-specific expression, thus providing sophisticated tools to predict gene function. Moreover, we exemplify how the tools can be used to easily identify genes relevant for pathogenicity and various life stages of the malaria parasite. The database is freely available at www.malaria.tools.

A function of profilin in force generation during malaria parasite motility that is independent of actin binding

During transmission of malaria-causing parasites from mosquito to mammal, Plasmodium sporozoites migrate at high speed within the skin to access the bloodstream and infect the liver. This unusual gliding motility is based on retrograde flow of membrane proteins and highly dynamic actin filaments that provide short tracks for a myosin motor. Using laser tweezers and parasite mutants, we previously suggested that actin filaments form macromolecular complexes with plasma membrane-spanning adhesins to generate force during migration. Mutations in the actin-binding region of profilin, a near ubiquitous actin-binding protein, revealed that loss of actin binding also correlates with loss of force production and motility. Here, we show that different mutations in profilin, that do not affect actin binding in vitro, still generate lower force during Plasmodium sporozoite migration. Lower force generation inversely correlates with increased retrograde flow suggesting that, like in mammalian cells, the slow down of flow to generate force is the key underlying principle governing Plasmodium gliding motility.

Functional homo- and heterodimeric actin capping proteins from the malaria parasite

Actin capping proteins belong to the core set of proteins minimally required for actin-based motility and are present in virtually all eukaryotic cells. They bind to the fast-growing barbed end of an actin filament, preventing addition and loss of monomers, thus restricting growth to the slow-growing pointed end. Actin capping proteins are usually heterodimers of two subunits. The Plasmodium orthologs are an exception, as their α subunits are able to form homodimers. We show here that, while the β subunit alone is unstable, the α subunit of the Plasmodium actin capping protein forms functional homo- and heterodimers. This implies independent functions for the αα homo- and αβ heterodimers in certain stages of the parasite life cycle. Structurally, the homodimers resemble canonical αβ heterodimers, although certain rearrangements at the interface must be required. Both homo- and heterodimers bind to actin filaments in a roughly equimolar ratio, indicating they may also bind other sites than barbed ends.

Synthetic DNA Vaccines Adjuvanted with pIL-33 Drive Liver-Localized T Cells and Provide Protection from Plasmodium Challenge in a Mouse Model

The need for a malaria vaccine is indisputable. A single vaccine for Plasmodium pre-erythrocytic stages targeting the major sporozoite antigen circumsporozoite protein (CSP) has had partial success. Additionally, CD8+ T cells targeting liver-stage (LS) antigens induced by live attenuated sporozoite vaccines were associated with protection in human challenge experiments. To further evaluate protection mediated by LS antigens, we focused on exported pre-erythrocytic proteins (exported protein 1 (EXP1), profilin (PFN), exported protein 2 (EXP2), inhibitor of cysteine proteases (ICP), transmembrane protein 21 (TMP21), and upregulated in infective sporozoites-3 (UIS3)) expressed in all Plasmodium species and designed optimized, synthetic DNA (synDNA) immunogens. SynDNA antigen cocktails were tested with and without the molecular adjuvant plasmid IL-33. Immunized animals developed robust T cell responses including induction of antigen-specific liver-localized CD8+ T cells, which were enhanced by the co-delivery of plasmid IL-33. In total, 100% of mice in adjuvanted groups and 71%–88% in non-adjuvanted groups were protected from blood-stage disease following Plasmodium yoelii sporozoite challenge. This study supports the potential of synDNA LS antigens as vaccine components for malaria parasite infection.

The Actomyosin Systems in Apicomplexa

The phylum of Apicomplexa groups obligate intracellular parasites that exhibit unique classes of unconventional myosin motors. These parasites also encode a limited repertoire of actins, actin-like proteins, actin-binding proteins and nucleators of filamentous actin (F-actin) that display atypical properties. In the last decade, significant progress has been made to visualize F-actin and to unravel the functional contribution of actomyosin systems in the biology of Toxoplasma and Plasmodium, the most genetically-tractable members of the phylum. In addition to assigning specific roles to each myosin, recent biochemical and structural studies have begun to uncover mechanistic insights into myosin function at the atomic level. In several instances, the myosin light chains associated with the myosin heavy chains have been identified, helping to understand the composition of the motor complexes and their mode of regulation. Moreover, the considerable advance in proteomic methodologies and especially in assignment of posttranslational modifications is offering a new dimension to our understanding of the regulation of actin dynamics and myosin function. Remarkably, the actomyosin system contributes to three major processes in Toxoplasma gondii: (i) organelle trafficking, positioning and inheritance, (ii) basal pole constriction and intravacuolar cell-cell communication and (iii) motility, invasion, and egress from infected cells. In this chapter, we summarize how the actomyosin system harnesses these key events to ensure successful completion of the parasite life cycle.

Systematic analysis of Plasmodium myosins reveals differential expression, localisation, and function in invasive and proliferative parasite stages

The myosin superfamily comprises of actin-dependent eukaryotic molecular motors important in a variety of cellular functions. Although well studied in many systems, knowledge of their functions in Plasmodium, the causative agent of malaria, is restricted. Previously, six myosins were identified in this genus, including three Class XIV myosins found only in Apicomplexa and some Ciliates. The well characterized MyoA is a Class XIV myosin essential for gliding motility and invasion. Here, we characterize all other Plasmodium myosins throughout the parasite life cycle and show that they have very diverse patterns of expression and cellular location. MyoB and MyoE, the other two Class XIV myosins, are expressed in all invasive stages, with apical and basal locations, respectively. Gene deletion revealed that MyoE is involved in sporozoite traversal, MyoF and MyoK are likely essential in the asexual blood stages, and MyoJ and MyoB are not essential. Both MyoB and its essential light chain (MCL-B) are localised at the apical end of ookinetes but expressed at completely different time points. This work provides a better understanding of the role of actomyosin motors in Apicomplexan parasites, particularly in the motile and invasive stages of Plasmodium during sexual and asexual development within the mosquito.

In silico analysis for development of epitopes-based peptide vaccine against Alkhurma hemorrhagic fever virus

Alkhurma hemorrhagic fever virus (ALKV) causes a fatal clinical disease in human beings of different tropical and sub-tropical regions. Recently, the ALKV epidemics have raised a great public health concern with the room for improvement in the essential therapeutic interventions. Despite increased realistic clinical cases of ALKV infection, the efficient vaccine or immunotherapy is not yet available to-date. Therefore, the current study aimed to analyze the envelope glycoprotein of ALKV for the development of B-cells and T-cells epitope-based peptide vaccine using the computational in silico method. Utilizing various immunoinformatics approaches, a total of 5 B-cells and 25 T-cells (MHC-I = 17, MHC-II = 8) epitope-based peptides were predicted in the current study. All predicted peptides had highest antigenicity and immunogenicity scores along with high binding affinity to human leukocyte antigen (HLA) class II alleles. Among 25T-cell epitopes, three peptides were found alike to have affinity to bind both MHC-I and MHC-II alleles. These outcomes suggested that these predicted epitopes could potentially be used in the development of an efficient vaccine against ALKV, which may enable to elicit both humoral and cell-mediated immunity. Although, these predicted peptides could be useful in designing a candidate vaccine for the prevention of ALKV; however, it's in vitro and in vivo assessments are prerequisite.Communicated by Ramaswamy H. Sarma.

Microtubule number and length determine cellular shape and function in Plasmodium

Microtubules are cytoskeletal filaments essential for many cellular processes, including establishment and maintenance of polarity, intracellular transport, division and migration. In most metazoan cells, the number and length of microtubules are highly variable, while they can be precisely defined in some protozoan organisms. However, in either case the significance of these two key parameters for cells is not known. Here, we quantitatively studied the impact of modulating microtubule number and length in Plasmodium, the protozoan parasite causing malaria. Using a gene deletion and replacement strategy targeting one out of two α-tubulin genes, we show that chromosome segregation proceeds in the oocysts even in the absence of microtubules. However, fewer and shorter microtubules severely impaired the formation, motility and infectivity of Plasmodium sporozoites, the forms transmitted by the mosquito, which usually contain 16 microtubules. We found that α-tubulin expression levels directly determined the number of microtubules, suggesting a high nucleation barrier as supported by a mathematical model. Infectious sporozoites were only formed in parasite lines featuring at least 10 microtubules, while parasites with 9 or fewer microtubules failed to transmit.

An endocytic-secretory cycle participates in Toxoplasma gondii in motility

Apicomplexan parasites invade host cells in an active process involving their ability to move by gliding motility. While the acto-myosin system of the parasite plays a crucial role in the formation and release of attachment sites during this process, there are still open questions regarding the involvement of other mechanisms in parasite motility. In many eukaryotes, a secretory-endocytic cycle leads to the recycling of receptors (integrins), necessary to form attachment sites, regulation of surface area during motility, and generation of retrograde membrane flow. Here, we demonstrate that endocytosis operates during gliding motility in Toxoplasma gondii and appears to be crucial for the establishment of retrograde membrane flow, because inhibition of endocytosis blocks retrograde flow and motility. We demonstrate that extracellular parasites can efficiently incorporate exogenous material, such as labelled phospholipids, nanogold particles (NGPs), antibodies, and Concanavalin A (ConA). Using labelled phospholipids, we observed that the endocytic and secretory pathways of the parasite converge, and endocytosed lipids are subsequently secreted, demonstrating the operation of an endocytic-secretory cycle. Together our data consolidate previous findings, and we propose an additional model, working in parallel to the acto-myosin motor, that reconciles parasite motility with observations in other eukaryotes: an apicomplexan fountain-flow-model for parasite motility.

Regulation of Plasmodium sporozoite motility by formulation components

BACKGROUND

The protective efficacy of the most promising malaria whole-parasite based vaccine candidates critically depends on the parasite's potential to migrate in the human host. Key components of the parasite motility machinery (e.g. adhesive proteins, actin/myosin-based motor, geometrical properties) have been identified, however the regulation of this machinery is an unknown process.

METHODS

In vitro microscopic live imaging of parasites in different formulations was performed and analysed, with the quantitative analysis software SMOOTIn vitro, their motility; their adherence capacity, movement pattern and velocity during forward locomotion.

RESULTS

SMOOTIn vitro enabled the detailed analysis of the regulation of the motility machinery of Plasmodium berghei in response to specific (macro)molecules in the formulation. Albumin acted as an essential supplement to induce parasite attachment and movement. Glucose, salts and other whole serum components further increased the attachment rate and regulated the velocity of the movement.

CONCLUSIONS

Based on the findings can be concluded that a complex interplay of albumin, glucose and certain salts and amino acids regulates parasite motility. Insights in parasite motility regulation by supplements in solution potentially provide a way to optimize the whole-parasite malaria vaccine formulation.

A synthetic promoter for multi-stage expression to probe complementary functions of Plasmodium adhesins

Gene expression of malaria parasites is mediated by the apicomplexan Apetala2 (ApiAP2) transcription factor family. Different ApiAP2s control gene expression at distinct stages in the complex life cycle of the parasite, ensuring timely expression of stage-specific genes. ApiAP2s recognize short cis-regulatory elements that are enriched in the upstream/promoter region of their target genes. This should, in principle, allow the generation of 'synthetic' promoters that drive gene expression at desired stages of the Plasmodium life cycle. Here we test this concept by combining cis-regulatory elements of two genes expressed successively within the mosquito part of the life cycle. Our tailored 'synthetic' promoters, named Spooki 1.0 and Spooki 2.0, activate gene expression in early and late mosquito stages, as shown by the expression of a fluorescent reporter. We used these promoters to address the specific functionality of two related adhesins that are exclusively expressed either during the early or late mosquito stage. By modifying the expression profile of both adhesins in absence of their counterpart we were able to test for complementary functions in gliding and invasion. We discuss the possible advantages and drawbacks of our approach.This article has an associated First Person interview with the first author of the paper.

Inter-subunit interactions drive divergent dynamics in mammalian and Plasmodium actin filaments

Cell motility is essential for protozoan and metazoan organisms and typically relies on the dynamic turnover of actin filaments. In metazoans, monomeric actin polymerises into usually long and stable filaments, while some protozoans form only short and highly dynamic actin filaments. These different dynamics are partly due to the different sets of actin regulatory proteins and partly due to the sequence of actin itself. Here we probe the interactions of actin subunits within divergent actin filaments using a comparative dynamic molecular model and explore their functions using Plasmodium, the protozoan causing malaria, and mouse melanoma derived B16-F1 cells as model systems. Parasite actin tagged to a fluorescent protein (FP) did not incorporate into mammalian actin filaments, and rabbit actin-FP did not incorporate into parasite actin filaments. However, exchanging the most divergent region of actin subdomain 3 allowed such reciprocal incorporation. The exchange of a single amino acid residue in subdomain 2 (N41H) of Plasmodium actin markedly improved incorporation into mammalian filaments. In the parasite, modification of most subunit–subunit interaction sites was lethal, whereas changes in actin subdomains 1 and 4 reduced efficient parasite motility and hence mosquito organ penetration. The strong penetration defects could be rescued by overexpression of the actin filament regulator coronin. Through these comparative approaches we identified an essential and common contributor, subdomain 3, which drives the differential dynamic behaviour of two highly divergent eukaryotic actins in motile cells.

Actin is one of the most abundant and conserved proteins across eukaryotes. Its ability to assemble from individual monomers into dynamic polymers is essential for many cellular functions, including division and motility. In most cells, actin is able to form long and stable filaments. However, an actin of the malaria-causing parasite Plasmodium, while having a very similar monomer structure to actins from other eukaryotes, forms only short and unstable filaments. These short and dynamic filaments are crucial in allowing the parasite to move very rapidly in tissue. Here we investigated the basis of these differences. We used molecular dynamics simulations of actin filaments to investigate the actin–actin interfaces in filaments from Plasmodium and rabbit. We next engineered parasites to express chimeric actins that contained different parts of rabbit and parasite actin and thereby identified actin residues important for parasite viability and progression across the life cycle. We could rescue the most prominent defect specifically with overexpression of the actin binding protein coronin. This suggests that the more stable actin harms the parasite and that coronin helps in recycling filaments. By screening the effects of actin chimeras in mammalian cells, we also identified regions that allow these different actins to efficiently interact with each other. Taken together, our results improve our understanding of the interactions required for actin to incorporate into filaments across divergent eukaryotes.

Immunization efficacy of cryopreserved genetically attenuated Plasmodium berghei sporozoites

Malaria is transmitted through the injection of Plasmodium sporozoites into the skin by Anopheles mosquitoes. The parasites first replicate within the liver before infecting red blood cells, which leads to the symptoms of the disease. Experimental immunization with attenuated sporozoites that arrest their development in the liver has been extensively investigated in rodent models and humans. Recent technological advances have included the capacity to cryopreserve sporozoites for injection, which has enabled a series of controlled studies on human infection with sporozoites. Here, we used a cryopreservation protocol to test the efficiency of genetically attenuated cryopreserved sporozoites for immunization of mice in comparison with freshly isolated controls. This showed that cryopreserved sporozoites are highly viable as judged by their capacity to migrate in vitro but show only 20% efficiency in liver infection, which impacts their capacity to generate protection of animals in immunization experiments.

Potential role of salt-bridges in the hinge-like movement of apicomplexa specific β-hairpin of Plasmodium and Toxoplasma profilins: A molecular dynamics simulation study

Profilin is one of the actin-binding proteins that regulate dynamics of actin polymerization. It plays a key role in cell motility and invasion. It also interacts with several other proteins notably through its poly-L-proline (PLP) binding site. Profilin in apicomplexa is characterized by a unique mini-domain consisting of a large β-hairpin extension and an acidic loop which is relatively longer in Plasmodium species. Profilin is essential for the invasive blood stages of Plasmodium falciparum. In the current study, unbound profilins from Plasmodium falciparum (Pf), Toxoplasma gondii (Tg), and Homo sapiens (Hs) were subjected to molecular dynamics (MD) simulations for a timeframe of 100 ns each to understand the conformational dynamics of these proteins. It was found that the β-hairpin of profilins from Pf and Tg shows a hinge-like movement. This movement in Pf profilin may possibly be driven by the loss of a salt-bridge within profilin. The impact of this conformational change on actin binding was assessed by docking three dimensional (3D) structures of profilin from Pf and Tg with their corresponding actins using ClusPro2.0. The stability of docked Pf profilin-actin complex was assessed through a 50 ns MD simulation. As Hs profilin I does not have the apicomplexa specific mini-domain, MD simulation was performed for this protein and its dynamics was compared to that of profilins from Pf and Tg. Using an immunoinformatics approach, potential epitope regions were predicted for Pf profilin. This has a potential application in the design of vaccines as they mapped to its unique mini-domain.

Apicomplexan actin polymerization depends on nucleation

Filamentous actin is critical for apicomplexan motility and host cell invasion. Yet, parasite actin filaments are short and unstable. Their kinetic characterization has been hampered by the lack of robust quantitative methods. Using a modified labeling method, we carried out thorough biochemical characterization of malaria parasite actin. In contrast to the isodesmic polymerization mechanism suggested for Toxoplasma gondii actin, Plasmodium falciparum actin I polymerizes via the classical nucleation-elongation pathway, with kinetics similar to canonical actins. A high fragmentation rate, governed by weak lateral contacts within the filament, is likely the main reason for the short filament length. At steady state, Plasmodium actin is present in equal amounts of short filaments and dimers, with a small proportion of monomers, representing the apparent critical concentration of ~0.1 µM. The dimers polymerize but do not serve as nuclei. Our work enhances understanding of actin evolution and the mechanistic details of parasite motility, serving as a basis for exploring parasite actin and actin nucleators as drug targets against malaria and other apicomplexan parasitic diseases.