Aspartic proteases of Plasmodium falciparum and other parasitic protozoa as drug targets

Plasmodium falciparum proteases as new drug targets with special focus on metalloproteases

Malaria poses a significant global health threat particularly due to the prevalence of Plasmodium falciparum infection. With the emergence of parasite resistance to existing drugs including the recently discovered artemisinin, ongoing research seeks novel therapeutic avenues within the malaria parasite. Proteases are promising drug targets due to their essential roles in parasite biology, including hemoglobin digestion, merozoite invasion, and egress. While exploring the genomic landscape of Plasmodium falciparum, it has been revealed that there are 92 predicted proteases, with only approximately 14 of them having been characterized. These proteases are further distributed among 26 families grouped into five clans: aspartic proteases, cysteine proteases, metalloproteases, serine proteases, and threonine proteases. Focus on metalloprotease class shows further role in organelle processing for mitochondria and apicoplasts suggesting the potential of metalloproteases as viable drug targets. Holistic understanding of the parasite intricate life cycle and identification of potential drug targets are essential for developing effective therapeutic strategies against malaria and mitigating its devastating global impact.

Fragment-based virtual screening identifies novel leads against Plasmepsin IX (PlmIX) of Plasmodium falciparum: Homology modeling, molecular docking, and simulation approaches

Despite continuous efforts to develop safer and efficient medications, malaria remains a major threat posing great challenges for new drug discovery. The emerging drug resistance, increased toxicities, and impoverished pharmacokinetic profiles exhibited by conventional drugs have hindered the search for new entities. Plasmepsins, a group of Plasmodium-specific, aspartic acid protease enzymes, are involved in many key aspects of parasite biology, and this makes them interesting targets for antimalarial chemotherapy. Among different isoforms, PlmIX serves as an unexplored antimalarial drug target that plays a crucial role along with PlmV and X in the parasite's survival by digesting hemoglobin in the host's erythrocytes. In this study, fragment-based virtual screening was performed by modeling the three-dimensional structure of PlmIX and predicting its ligand-binding pocket by using the Sitemap tool. Screening identified the fragments with the XP docking score ≤ -3 kcal/mol from the OTAVA General Fragment Library (≈16,397 fragments), and the selected fragments were chosen for ligand breeding. The resulting ligands (≈69,858 ligands) were subsequently subjected to filtering based on the QikProp properties along with carcinogenicity testing performed using CarcinoPred-EL and then docked in the SP (≈14,078 ligands) as well as XP mode (≈3,104 ligands), and compared with that of control ligands 49C and I0L. The top-ranked ligands were taken further for the calculation of the free energy of binding using Prime MM-GBSA. Overall, a total of six complexes were taken further for MD simulation studies performed at 100 ns to attain a better understanding of the binding mechanisms, and compounds 3 and 4 were found to be the most efficient ones in silico. The analysis of compound 3 revealed that the carbonyl group present in position 1 on the isoindoline moiety (Arg554) was responsible for inhibitory activity against PlmIX. However, the analysis of compound 4 revealed that the amide linkage sandwiched between the phenyl ring and isoquinoline moiety (Lys555 and Ser226) as well as carbonyl oxygen of the carbamoyl group present at position 2 of the pyrazole ring (Gln222) were responsible for PlmIX inhibitory activity, owing to their crucial interactions with key amino acid residues.

Comparative genomics of Ascetosporea gives new insight into the evolutionary basis for animal parasitism in Rhizaria

BACKGROUND

Ascetosporea (Endomyxa, Rhizaria) is a group of unicellular parasites infecting aquatic invertebrates. They are increasingly being recognized as widespread and important in marine environments, causing large annual losses in invertebrate aquaculture. Despite their importance, little molecular data of Ascetosporea exist, with only two genome assemblies published to date. Accordingly, the evolutionary origin of these parasites is unclear, including their phylogenetic position and the genomic adaptations that accompanied the transition from a free-living lifestyle to parasitism. Here, we sequenced and assembled three new ascetosporean genomes, as well as the genome of a closely related amphizoic species, to investigate the phylogeny, origin, and genomic adaptations to parasitism in Ascetosporea.

RESULTS

Using a phylogenomic approach, we confirm the monophyly of Ascetosporea and show that Paramyxida group with Mikrocytida, with Haplosporida being sister to both groups. We report that the genomes of these parasites are relatively small (12-36 Mb) and gene-sparse (~ 2300-5200 genes), while containing surprisingly high amounts of non-coding sequence (~ 70-90% of the genomes). Performing gene-tree aware ancestral reconstruction of gene families, we demonstrate extensive gene losses at the origin of parasitism in Ascetosporea, primarily of metabolic functions, and little gene gain except on terminal branches. Finally, we highlight some functional gene classes that have undergone expansions during evolution of the group.

CONCLUSIONS

We present important new genomic information from a lineage of enigmatic but important parasites of invertebrates and illuminate some of the genomic innovations accompanying the evolutionary transition to parasitism in this lineage. Our results and data provide a genetic basis for the development of control measures against these parasites.

Inhibition of Plasmodium falciparum plasmepsins by drugs targeting HIV-1 protease: A way forward for antimalarial drug discovery

Plasmodium species are causative agents of malaria, a disease that is a serious global health concern. FDA-approved HIV-1 protease inhibitors (HIV-1 PIs) have been reported to be effective in reducing the infection by Plasmodium parasites in the population co-infected with both HIV-1 and malaria. However, the mechanism of HIV-1 PIs in mitigating Plasmodium pathogenesis during malaria/HIV-1 co-infection is not fully understood. In this study we demonstrate that HIV-1 drugs ritonavir (RTV) and lopinavir (LPV) exhibit the highest inhibition activity against plasmepsin II (PMII) and plasmepsin X (PMX) of P. falciparum. Crystal structures of the complexes of PMII with both drugs have been determined. The inhibitors interact with PMII via multiple hydrogen bonding and hydrophobic interactions. The P4 moiety of RTV forms additional interactions compared to LPV and exhibits conformational flexibility in a large S4 pocket of PMII. Our study is also the first to report inhibition of P. falciparum PMX by RTV and the mode of binding of the drug to the PMX active site. Analysis of the crystal structures implies that PMs can accommodate bulkier groups of these inhibitors in their S4 binding pockets. Structurally similar active sites of different vacuolar and non-vacuolar PMs suggest the potential of HIV-1 PIs in targeting these enzymes with differential affinities. Our structural investigations and biochemical data emphasize PMs as crucial targets for repurposing HIV-1 PIs as antimalarial drugs.

Hemoglobin uptake and utilization by human protozoan parasites: a review

The protozoan disease is a major global health concern. Amoebiasis, leishmaniasis, Chagas disease, and African sleeping sickness affect several million people worldwide, leading to millions of deaths annually and immense social and economic problems. Iron is an essential nutrient for nearly all microbes, including invading pathogens. The majority of iron in mammalian hosts is stored intracellularly in proteins, such as ferritin and hemoglobin (Hb). Hb, present in blood erythrocytes, is a very important source of iron and amino acids for pathogenic microorganisms ranging from bacteria to eukaryotic pathogens, such as worms, protozoa, yeast, and fungi. These organisms have developed adequate mechanisms to obtain Hb or its byproducts (heme and globin) from the host. One of the major virulence factors identified in parasites is parasite-derived proteases, essential for host tissue degradation, immune evasion, and nutrient acquisition. The production of Hb-degrading proteases is a Hb uptake mechanism that degrades globin in amino acids and facilitates heme release. This review aims to provide an overview of the Hb and heme-uptake mechanisms utilized by human pathogenic protozoa to survive inside the host.

Comparative Degradome Analysis of the Bovine Piroplasmid Pathogens Babesia bovis and Theileria annulata

Babesia bovis and Theileria annulata are tick-borne hemoprotozoans that impact bovine health and are responsible for considerable fatalities in tropical and subtropical regions around the world. Both pathogens infect the same vertebrate host, are closely related, and contain similar-sized genomes; however, they differ in invertebrate host specificity, absence vs. presence of a schizont stage, erythrocyte invasion mechanism, and transovarial vs. transstadial transmission. Phylogenetic analysis and bidirectional best hit (BBH) identified a similar number of aspartic, metallo, and threonine proteinases and nonproteinase homologs. In contrast, a considerably increased number of S54 serine rhomboid proteinases and S9 nonproteinase homologs were identified in B. bovis, whereas C1A cysteine proteinases and A1 aspartic nonproteinase homologs were found to be expanded in T. annulata. Furthermore, a single proteinase of families S8 (subtilisin-like protein) and C12 (ubiquitin carboxyl-terminal hydrolase), as well as four nonproteinase homologs, one with dual domains M23-M23 and three with S9-S9, were exclusively present in B. bovis. Finally, a pronounced difference in species-specific ancillary domains was observed between both species. We hypothesize that the observed degradome differences represent functional correlates of the dissimilar life history features of B. bovis and T. annulata. The presented improved classification of piroplasmid proteinases will facilitate an informed choice for future in-depth functional studies.

Computational insights for predicting the binding and selectivity of peptidomimetic plasmepsin IV inhibitors against cathepsin D

Plasmepsins (Plms) are aspartic proteases involved in the degradation of human hemoglobin by P. falciparum and are essential for the survival and growth of the parasite. Therefore, Plm enzymes are reported as an important antimalarial drug target. Herein, we have applied molecular docking, molecular dynamics (MD) simulations, and binding free energy with the Linear Interaction Energy (LIE) approach to investigate the binding of peptidomimetic PlmIV inhibitors with a particular focus on understanding their selectivity against the human Asp protease cathepsin D (CatD). The residual decomposition analysis results suggest that amino acid differences in the subsite S3 of PlmIV and CatD are responsible for the higher selectivity of the 5a inhibitor. These findings yield excellent agreement with experimental binding data and provide new details regarding van der Waals and electrostatic interactions of subsite residues as well as structural properties of the PlmIV and CatD systems.

Structure- and ligand-based drug design methods for the modeling of antimalarial agents: a review of updates from 2012 onwards

Malaria still persists as one of the deadliest infectious disease having a huge morbidity and mortality affecting the higher population of the world. Structure and ligand-based drug design methods like molecular docking and MD simulations, pharmacophore modeling, QSAR and virtual screening are widely used to perceive the accordant correlation between the antimalarial activity and property of the compounds to design novel dominant and discriminant molecules. These modeling methods will speed-up antimalarial drug discovery, selection of better drug candidates for synthesis and to achieve potent and safer drugs. In this work, we have extensively reviewed the literature pertaining to the use and applications of various ligand and structure-based computational methods for the design of antimalarial agents. Different classes of molecules are discussed along with their target interactions pattern, which is responsible for antimalarial activity. Communicated by Ramaswamy H. Sarma.

Targeting the Plasmodium falciparum proteome and organelles for potential antimalarial drug candidates

There is an unmet need to unearth alternative treatment options for malaria, wherein this quest is more pressing in recent times due to high morbidity and mortality data arising mostly from the endemic countries coupled with partial diversion of attention from the disease in view of the SARS-Cov-2 pandemic. Available therapeutic options for malaria have been severely threatened with the emergence of resistance to almost all the antimalarial drugs by the Plasmodium falciparum parasite in humans, which is a worrying situation. Artemisinin combination therapies (ACT) that have so far been the mainstay of malaria have encountered resistance by malaria parasite in South East Asia, which is regarded as a notorious ground zero for the emergence of resistance to antimalarial drugs. This review analyzes a few key druggable targets for the parasite and the potential of specific inhibitors to mitigate the emerging antimalarial drug resistance problem by providing a concise assessment of the essential proteins of the malaria parasite that could serve as targets. Moreover, this work provides a summary of the advances made in malaria parasite biology and the potential to leverage these findings for antimalarial drug production.

Exploring Aspartic Protease Inhibitor Binding to Design-Selective Antimalarials

Selectivity is a major issue in the development of drugs targeting pathogen aspartic proteases. Here, we explore the selectivity-determining factors by studying specifically designed malaria aspartic protease (plasmepsin) open-flap inhibitors. Metadynamics simulations are used to uncover the complex binding/unbinding pathways of these inhibitors and describe the critical transition states in atomistic resolution. The simulation results are compared with experimentally determined enzymatic activities. Our findings demonstrate that plasmepsin inhibitor selectivity can be achieved by targeting the flap loop with hydrophobic substituents that enable ligand binding under the flap loop, as such a behavior is not observed for several other aspartic proteases. The ability to estimate the selectivity of compounds before they are synthesized is of considerable importance in drug design; therefore, we expect that our approach will be useful in selective inhibitor designs against not only aspartic proteases but also other enzyme classes.

Structures of plasmepsin X from Plasmodium falciparum reveal a novel inactivation mechanism of the zymogen and molecular basis for binding of inhibitors in mature enzyme

Plasmodium falciparum plasmepsin X (PfPMX), involved in the invasion and egress of this deadliest malarial parasite, is essential for its survival and hence considered as an important drug target. We report the first crystal structure of PfPMX zymogen containing a novel fold of its prosegment. A unique twisted loop from the prosegment and arginine 244 from the mature enzyme is involved in zymogen inactivation; such mechanism, not previously reported, might be common for apicomplexan proteases similar to PfPMX. The maturation of PfPMX zymogen occurs through cleavage of its prosegment at multiple sites. Our data provide thorough insights into the mode of binding of a substrate and a potent inhibitor 49c to PfPMX. We present molecular details of inactivation, maturation, and inhibition of PfPMX that should aid in the development of potent inhibitors against pepsin-like aspartic proteases from apicomplexan parasites.

Proteases as Therapeutic Targets Against the Parasitic Cnidarian Ceratonova shasta: Characterization of Molecules Key to Parasite Virulence In Salmonid Hosts

Proteases and their inhibitors play critical roles in host-parasite interactions and in the outcomes of infections. Ceratonova shasta is a myxozoan pathogen that causes enteronecrosis in economically important salmonids from the Pacific Northwest of North America. This cnidarian parasite has host-specific genotypes with varying virulence, making it a powerful system to decipher virulence mechanisms in myxozoans. Using C. shasta genome and transcriptome, we identified four proteases of different catalytic types: cathepsin D (aspartic), cathepsin L and Z-like (cysteine) and aminopeptidase-N (metallo); and a stefin (cysteine protease inhibitor), which implied involvement in virulence and hence represent target molecules for the development of therapeutic strategies. We characterized, annotated and modelled their 3D protein structure using bioinformatics and computational tools. We quantified their expression in C. shasta genotype 0 (low virulence, no mortality) and IIR (high virulence and mortality) in rainbow trout Oncorhynchus mykiss, to demonstrate that there are major differences between the genotypes during infection and parasite development. High proliferation of genotype IIR was associated with high expression of the cathepsin D and the stefin, likely correlated with high nutrient demands and to regulate cell metabolism, with upregulation preceding massive proliferation and systemic dispersion. In contrast, upregulation of the cathepsin L and Z-like cysteine proteases may have roles in host immune evasion in genotype 0 infections, which are associated with low proliferation, low inflammation and non-destructive development. In contrast to the other proteases, C. shasta aminopeptidase-N appears to have a prominent role in nematocyst formation in both genotypes, but only during sporogenesis. Homology searches of C. shasta proteases against other myxozoan transcriptomes revealed a high abundance of cathepsin L and aminopeptidase homologs suggesting common gene requirements across species. Our study identified molecules of potential therapeutic significance for aquaculture and serves as a baseline for future research aimed at functional characterisation of these targets.

Plasmepsin-like Aspartyl Proteases in Babesia

Apicomplexan genomes encode multiple pepsin-family aspartyl proteases (APs) that phylogenetically cluster to six independent clades (A to F). Such diversification has been powered by the function-driven evolution of the ancestral apicomplexan AP gene and is associated with the adaptation of various apicomplexan species to different strategies of host infection and transmission through various invertebrate vectors. To estimate the potential roles of Babesia APs, we performed qRT-PCR-based expressional profiling of Babesia microti APs (BmASP2, 3, 5, 6), which revealed the dynamically changing mRNA levels and indicated the specific roles of individual BmASP isoenzymes throughout the life cycle of this parasite. To expand on the current knowledge on piroplasmid APs, we searched the EuPathDB and NCBI GenBank databases to identify and phylogenetically analyse the complete sets of APs encoded by the genomes of selected Babesia and Theileria species. Our results clearly determine the potential roles of identified APs by their phylogenetic relation to their homologues of known function—Plasmodium falciparum plasmepsins (PfPM I–X) and Toxoplasma gondii aspartyl proteases (TgASP1–7). Due to the analogies with plasmodial plasmepsins, piroplasmid APs represent valuable enzymatic targets that are druggable by small molecule inhibitors—candidate molecules for the yet-missing specific therapy for babesiosis.

A target safety assessment of the potential toxicological risks of targeting plasmepsin IX/X for the treatment of malaria

The aspartic proteases plasmepsin IX/X are important antimalarial drug targets due to their specificity to the malaria parasite and their vital role as mediators of disease progression. Focusing on parasite-specific targets where no human homologue exists reduces the possibility of on-target drug toxicity. However, there is a risk of toxicity driven by inadequate selectivity for plasmepsins IX/X in Plasmodium over related mammalian aspartic proteases. Of these, CatD/E may be of most toxicological relevance as CatD is a ubiquitous lysosomal enzyme present in most cell types and CatE is found in the gut and in erythrocytes, the clinically significant site of malarial infection. Based on mammalian aspartic protease physiology and adverse drug reactions (ADRs) to FDA-approved human immunodeficiency virus (HIV) aspartic protease inhibitors, we predicted several potential toxicities including β-cell and congenital abnormalities, hypotension, hypopigmentation, hyperlipidaemia, increased infection risk and respiratory, renal, gastrointestinal, dermatological, and other epithelial tissue toxicities. These ADRs to the HIV treatments are likely to be a result of host aspartic protease inhibition due a lack of specificity for the HIV protease; plasmepsins are much more closely related to human CatD than to HIV proteinase. Plasmepsin IX/X inhibition presents an opportunity to specifically target Plasmodium as an effective antimalarial treatment, providing adequate selectivity can be obtained. Potential plasmepsin IX/X inhibitors should be assayed for inhibitory activity against the main human aspartic proteases and particularly CatD/E. An investigative rodent study conducted early in drug discovery would serve as an initial risk assessment of the potential hazards identified.

Metalloaminopeptidases of the Protozoan Parasite Plasmodium falciparum as Targets for the Discovery of Novel Antimalarial Drugs

Malaria poses a significant threat to approximately half of the world's population with an annual death toll close to half a million. The emergence of resistance to front-line antimalarials in the most lethal human parasite species, Plasmodium falciparum (Pf), threatens progress made in malaria control. The prospect of losing the efficacy of antimalarial drugs is driving the search for small molecules with new modes of action. Asexual reproduction of the parasite is critically dependent on the recycling of amino acids through catabolism of hemoglobin (Hb), which makes metalloaminopeptidases (MAPs) attractive targets for the development of new drugs. The Pf genome encodes eight MAPs, some of which have been found to be essential for parasite survival. In this article, we discuss the biological structure and function of each MAP within the Pf genome, along with the drug discovery efforts that have been undertaken to identify novel antimalarial candidates of therapeutic value.

Identification of the natural compound inhibitors against Plasmodium falciparum plasmepsin-II via common feature based screening and molecular dynamics simulations

Malaria is counted amongst the deadly disease caused by Plasmodium falciparum. Recently, plasmepsin-II enzyme has gained much importance as an attractive drug target for the exploration of antimalarials. Therefore, the common feature pharmacophore models were generated from the crystallized complexes of the plasmepsin-II proteome. These models were subjected to a series of validation procedures, i.e. test set and Güner Henry studies to enlist the representative models. The selected representative hypotheses incorporating the most essential chemical features (common ZHHA) were screened against the natural product database to retrieve the potential candidates. To ensure the selection of the drug-like candidates, prior to screening, filtering steps (Drug-likeness and ADMET filters) were employed on the selected database. To study the interaction pattern of the candidates within the protein, these molecules were advanced to the molecular docking studies. Subsequently, based on the selected cut-off criteria obtained via redocking of the reference (4Z22), 15 compounds showed higher docking score (> -16.05 kcal/mol), and displayed the presence of hydrogen bonding with the crucial amino acids, i.e. Asp34 and Asp214. Further, the stability of the docked molecules was scrutinized via molecular dynamics simulations, and the results were compared with the reference compound 4Z22. All the docked compounds showed stable dynamics behaviour. Thus, in the present contribution, the combination of screening and stability procedures resulted in the identification of 15 hits that can serve as a new chemical space in the designing of the novel antimalarials.Communicated by Ramaswamy H. Sarma.

Malaria parasite plasmepsins: More than just plain old degradative pepsins

Plasmepsins are a group of diverse aspartic proteases in the malaria parasite Plasmodium Their functions are strikingly multifaceted, ranging from hemoglobin degradation to secretory organelle protein processing for egress, invasion, and effector export. Some, particularly the digestive vacuole plasmepsins, have been extensively characterized, whereas others, such as the transmission-stage plasmepsins, are minimally understood. Some (e.g. plasmepsin V) have exquisite cleavage sequence specificity; others are fairly promiscuous. Some have canonical pepsin-like aspartic protease features, whereas others have unusual attributes, including the nepenthesin loop of plasmepsin V and a histidine in place of a catalytic aspartate in plasmepsin III. We have learned much about the functioning of these enzymes, but more remains to be discovered about their cellular roles and even their mechanisms of action. Their importance in many key aspects of parasite biology makes them intriguing targets for antimalarial chemotherapy. Further consideration of their characteristics suggests that some are more viable drug targets than others. Indeed, inhibitors of invasion and egress offer hope for a desperately needed new drug to combat this nefarious organism.

Activation mechanism of plasmepsins, pepsin-like aspartic proteases from Plasmodium, follows a unique trans-activation pathway

Plasmodium parasites that cause malaria produce plasmepsins (PMs), pepsin-like aspartic proteases that are important antimalarial drug targets due to their role in host hemoglobin degradation. The enzymes are synthesized as inactive zymogens (pro-PMs), and the mechanism of their conversion to the active, mature forms has not been clearly elucidated. Our structural investigations of vacuolar pro-PMs with truncated prosegment (pro-tPMs) reveal that the formation of the S-shaped dimer is their innate property. Further structural studies, biochemical analysis, and molecular dynamics simulations indicate that disruption of the Tyr-Asp loop (121p-4), coordinated with the movement of the loop L1 (237-247) and helix H2 (101p-113p), is responsible for the extension of the pro-mature region (harboring the cleavage site). Consequently, under acidic pH conditions, these structural changes result in the dissociation of the dimers to monomers and the protonation of the residues in the prosegment prompts its unfolding. Subsequently, we demonstrated that the active site of the monomeric pro-tPMs with the unfolded prosegment is accessible for peptide substrate binding; in contrast, the active site is blocked in folded prosegment form of pro-tPMs. Thus, we propose a novel mechanism of auto-activation of vacuolar pro-tPMs that under acidic conditions can form a catalytically competent active site. One monomer cleaves the prosegment of the other one through a trans-activation process, resulting in formation of mature enzyme. As a result, once a mature enzyme is generated, it leads to the complete conversion of all the inactive pro-tPMs to their mature form. DATABASE: Atomic coordinates and structure factors have been submitted in the Protein Data Bank (PDB) under the PDB IDs 6KUB, 6KUC, and 6KUD.

Current progress in antimalarial pharmacotherapy and multi-target drug discovery

Discovery and development of antimalarial drugs have long been dominated by single-target therapy. Continuous effort has been made to explore and identify different targets in malaria parasite crucial for the malaria treatment. The single-target drug therapy was initially successful, but it was later supplanted by combination therapy with multiple drugs to overcome drug resistance. Emergence of resistant strains even against the combination therapy has warranted a review of current antimalarial pharmacotherapy. This has led to the development of the new concept of covalent biotherapy, in which two or more pharmacophores are chemically bound to produce hybrid antimalarial drugs with multi-target functionalities. Herein, the review initially details the current pharmacotherapy for malaria as well as the conventional and novel targets of importance identified in the malaria parasite. Then, the rationale of multi-targeted therapy for malaria, approaches taken to develop the multi-target antimalarial hybrids, and the examples of hybrid molecules are comprehensively enumerated and discussed.

Molecular docking and dynamics studies on novel benzene sulfonamide substituted pyrazole-pyrazoline analogues as potent inhibitors of Plasmodium falciparum Histo aspartic protease

Malaria is the major health issue in African, Asian and Mediterranean regions of the world. Due to the emerging resistance by the parasites and mosquitoes for the current medications and insecticides, respectively, the malaria free human world can be attained only by the novel design and development of new anti-malarial drugs. Hence, we attempted to carry out in silico screening of benzene sulfonamide substituted pyrazole-pyrazoline series against Histo aspartic protease. Our results reveal that the 65% of the data set with the free binding energy in the range of -11.58 to -11.21 kcal/mol, which is categorized as 'high scoring'. Ligands are docked with the catalytic residues Asp 215, Ser 75, Thr 33 and Ala 217, respectively. Molecular dynamic simulation study of free enzyme and the enzyme complex with 4-(5-(4-methoxyphenyl)-1'phenyl-3'-(p-tolyl)-3,4-1'H,2H-[3,4'-bipyrazol]-2-yl)benezenesulfonamide indicated structural stability. The trajectory analysis of complex reveals that the HAP-ligand complex is more stable than the free HAP. We are of the opinion that our results will be useful for designing potential anti-malarial compounds. AbbreviationsADTauto dock toolsBSPPbenzene sulfonamide substituted pyrazole-pyrazolineCQchloroquineHAPhisto aspartic proteaseKKelvinMDmolecular dockingMM/PBSAmolecular mechanics/Poisson Boltzmann surface areaNVTnormal volume and temperatureNPTnormal pressure and temperatureNsnanosecondsPDBprotein data bank.pdbprogram data base formatP. falciparumPlasmodium falciparumPspicosecondsPMsplasmepsinsP. vivaxPlasmodium vivaxRgradius of gyrationRMSDroot mean square deviationRMSFroot mean square fluctuationWHOWorld Health OrganizationCommunicated by Ramaswamy H. Sarma.

A dual target of Plasmepsin IX and X: Unveiling the atomistic superiority of a core chemical scaffold in malaria therapy

Plasmepsin IX and X, members of the prominent aspartic family of proteases whose function were hitherto unknown have only recently been established as key mediators of erythrocyte invasion and egress of the virulent malarial parasite. Inhibitor 49c, a potent antimalarial peptidomimetic inhibitor initially developed to target Plasmepsin II has lately been proven to exhibit potent inhibitory activity against Plasmepsin IX and X. However, the molecular and structural dynamics supporting its inhibitory activity remain inconclusive. Hindering the motion of the flap and hinge region of an aspartic protease remains essential for disabling the catalytic activity of the enzyme. Integrating molecular dynamic simulations coupled with other advanced biocomputational tools, we reveal the enhanced structural mechanistic competence of 49c in complex with Plasmepsin IX and X relative to Pepstatin. Pepstatin, a known aspartic protease inhibitor which actively hinders the opening and closing of the flap tip and flexible loop and consequently limits access to the catalytic aspartic residues, however, its administration has been related to elevated levels of toxicity. Thermodynamic calculations reveal a higher relative binding free energy associated with Plasmepsin IX and X in complex with 49c as opposed to Pepstatin. A relatively compact and structurally rigid 49c bound complexes sequel into the restriction of the flap and hinge residues by restraining cohesive movement, consequently hindering their "twisting motion" from transpiring. Findings unveil an atomistic perspective into the structural superiority of 49c in complex with Plasmepsin IX and X.

Structural Insights Into Key Plasmodium Proteases as Therapeutic Drug Targets

Malaria, caused by protozoan of genus Plasmodium, remains one of the highest mortality infectious diseases. Malaria parasites have a complex life cycle, easily adapt to their host’s immune system and have evolved with an arsenal of unique proteases which play crucial roles in proliferation and survival within the host cells. Owing to the existing knowledge of enzymatic mechanisms, 3D structures and active sites of proteases, they have been proven to be opportune for target based drug development. Here, we discuss in depth the crucial roles of essential proteases in Plasmodium life cycle and particularly focus on highlighting the atypical “structural signatures” of key parasite proteases which have been exploited for drug development. These features, on one hand aid parasites pathogenicity while on the other hand could be effective in designing targeted and very specific inhibitors for counteracting them. We conclude that Plasmodium proteases are suitable as multistage targets for designing novel drugs with new modes of action to combat malaria.

Overexpression of plasmepsin II and plasmepsin III does not directly cause reduction in Plasmodium falciparum sensitivity to artesunate, chloroquine and piperaquine

Artemisinin derivatives and their partner drugs in artemisinin combination therapies (ACTs) have played a pivotal role in global malaria mortality reduction during the last two decades. The loss of artemisinin efficacy due to evolving drug-resistant parasites could become a serious global health threat. Dihydroartemisinin-piperaquine is a well tolerated and generally highly effective ACT. The implementation of a partner drug in ACTs is critical in the control of emerging artemisinin resistance. Even though artemisinin is highly effective in parasite clearance, it is labile in the human body. A partner drug is necessary for killing the remaining parasites when the pulses of artemisinin have ceased. A population of Plasmodium falciparum parasites in Cambodia and adjacent countries has become resistant to piperaquine. Increased copy number of the genes encoding the haemoglobinases Plasmepsin II and Plasmepsin III has been linked with piperaquine resistance by genome-wide association studies and in clinical trials, leading to the use of increased plasmepsin II/plasmepsin III copy number as a molecular marker for piperaquine resistance. Here we demonstrate that overexpression of plasmepsin II and plasmepsin III in the 3D7 genetic background failed to change the susceptibility of P. falciparum to artemisinin, chloroquine and piperaquine by both a standard dose-response analysis and a piperaquine survival assay. Whilst plasmepsin copy number polymorphism is currently implemented as a molecular surveillance resistance marker, further studies to discover the molecular basis of piperaquine resistance and potential epistatic interactions are needed.

Activity-Based Protein Profiling for the Study of Parasite Biology

Parasites exist within most ecological niches, often transitioning through biologically and chemically complex host environments over the course of their parasitic life cycles. While the development of technologies for genetic engineering has revolutionised the field of functional genomics, parasites have historically been less amenable to such modification. In light of this, parasitologists have often been at the forefront of adopting new small-molecule technologies, repurposing drugs into biological tools and probes. Over the last decade, activity-based protein profiling (ABPP) has evolved into a powerful and versatile chemical proteomic platform for characterising the function of enzymes. Central to ABPP is the use of activity-based probes (ABPs), which covalently modify the active sites of enzyme classes ranging from serine hydrolases to glycosidases. The application of ABPP to cellular systems has contributed vastly to our knowledge on the fundamental biology of a diverse range of organisms and has facilitated the identification of potential drug targets in many pathogens. In this chapter, we provide a comprehensive review on the different forms of ABPP that have been successfully applied to parasite systems, and highlight key biological insights that have been enabled through their application.

Deciphering the mechanism of potent peptidomimetic inhibitors targeting plasmepsins - biochemical and structural insights

Malaria is a deadly disease killing worldwide hundreds of thousands people each year and the responsible parasite has acquired resistance to the available drug combinations. The four vacuolar plasmepsins (PMs) in Plasmodium falciparum involved in hemoglobin (Hb) catabolism represent promising targets to combat drug resistance. High antimalarial activities can be achieved by developing a single drug that would simultaneously target all the vacuolar PMs. We have demonstrated for the first time the use of soluble recombinant plasmepsin II (PMII) for structure-guided drug discovery with KNI inhibitors. Compounds used in this study (KNI-10742, 10743, 10395, 10333, and 10343) exhibit nanomolar inhibition against PMII and are also effective in blocking the activities of PMI and PMIV with the low nanomolar K_i values. The high-resolution crystal structures of PMII-KNI inhibitor complexes reveal interesting features modulating their differential potency. Important individual characteristics of the inhibitors and their importance for potency have been established. The alkylamino analog, KNI-10743, shows intrinsic flexibility at the P2 position that potentiates its interactions with Asp132, Leu133, and Ser134. The phenylacetyl tripeptides, KNI-10333 and KNI-10343, accommodate different ρ-substituents at the P3 phenylacetyl ring that determine the orientation of the ring, thus creating novel hydrogen-bonding contacts. KNI-10743 and KNI-10333 possess significant antimalarial activity, block Hb degradation inside the food vacuole, and show no cytotoxicity on human cells; thus, they can be considered as promising candidates for further optimization. Based on our structural data, novel KNI derivatives with improved antimalarial activity could be designed for potential clinical use. DATABASE: Structural data are available in the PDB under the accession numbers 5YIE, 5YIB, 5YID, 5YIC, and 5YIA.

In vitro and in silico studies of naphthoquinones and peptidomimetics toward Plasmodium falciparum plasmepsin V

Plasmodium proteases play both regulatory and effector roles in essential biological processes in this important pathogen and have long been investigated as drug targets. Plasmepsin V from P. falciparum (PfPMV) is an essential protease that processes proteins for export into the host erythrocyte and is a focus of ongoing drug development efforts. In the present study, recombinant protein production, inhibition assays, binding studies as well as molecular docking and molecular dynamics simulation studies were used to investigate the mode of binding of a PEXEL-based peptidomimetic and naphthoquinone compounds to PfPMV. Consistent with our previous study, refolded PfPMVs were produced with functional characteristics similar to the soluble counterpart. Naphthoquinone compounds inhibited PfPMV activity by 50% at 50 μM but did not affect pepsin activity. The IC₅₀ values of compounds 31 and 37 against PfPMV were 22.25 and 68.94 μM, respectively. Molecular dynamics simulations revealed that PEXEL peptide interacted with PfPMV active site residues via electrostatic interactions while naphthoquinone binding preferred van der Waal interactions. P₁'-Ser of the PfEMP2 substrate formed an additional H-bond with Asp365 promoting the catalytic efficiency. Additionally, the effect of metal ions on the secondary structure of PfPMV was examined. Our results confirmed that Hg²⁺ ions reversibly induced the changes in secondary structure of the protein whereas Fe³⁺ ions induced irreversibly. No change was observed in the presence of Ca²⁺ ions. Overall, the results here suggested that naphthoquinone derivatives may represent another source of antimalarial inhibitors targeting aspartic proteases but further chemical modifications are required.

Drug targets for resistant malaria: Historic to future perspectives

New antimalarial targets are the prime need for the discovery of potent drug candidates. In order to fulfill this objective, antimalarial drug researches are focusing on promising targets in order to develop new drug candidates. Basic metabolism and biochemical process in the malaria parasite, i.e. Plasmodium falciparum can play an indispensable role in the identification of these targets. But, the emergence of resistance to antimalarial drugs is an escalating comprehensive problem with the progress of antimalarial drug development. The development of resistance has highlighted the need for the search of novel antimalarial molecules. The pharmaceutical industries are committed to new drug development due to the global recognition of this life threatening resistance to the currently available antimalarial therapy. The recent developments in the understanding of parasite biology are exhilarating this resistance issue which is further being ignited by malaria genome project. With this background of information, this review was aimed to highlights and provides useful information on various present and promising treatment approaches for resistant malaria, new progresses, pursued by some innovative targets that have been explored till date. This review also discusses modern and futuristic multiple approaches to antimalarial drug discovery and development with pictorial presentations highlighting the various targets, that could be exploited for generating promising new drugs in the future for drug resistant malaria.

Plasmepsins IX and X are essential and druggable mediators of malaria parasite egress and invasion

Proteases of the malaria parasite Plasmodium falciparum have long been investigated as drug targets. The P. falciparum genome encodes 10 aspartic proteases called plasmepsins, which are involved in diverse cellular processes. Most have been studied extensively but the functions of plasmepsins IX and X (PMIX and PMX) were unknown. Here we show that PMIX is essential for erythrocyte invasion, acting on rhoptry secretory organelle biogenesis. In contrast, PMX is essential for both egress and invasion, controlling maturation of the subtilisin-like serine protease SUB1 in exoneme secretory vesicles. We have identified compounds with potent antimalarial activity targeting PMX, including a compound known to have oral efficacy in a mouse model of malaria.

Modeling and resistant alleles explain the selectivity of antimalarial compound 49c towards apicomplexan aspartyl proteases

Toxoplasma gondii aspartyl protease 3 (TgASP3) phylogenetically clusters with Plasmodium falciparum Plasmepsins IX and X (PfPMIX, PfPMX). These proteases are essential for parasite survival, acting as key maturases for secreted proteins implicated in invasion and egress. A potent antimalarial peptidomimetic inhibitor (49c) originally developed against Plasmepsin II selectively targets TgASP3, PfPMIX, and PfPMX To unravel the molecular basis for the selectivity of 49c, we constructed homology models of PfPMIX, PfPMX, and TgASP3 that were first validated by identifying the determinants of microneme and rhoptry substrate recognition. The flap and flap-like structures of several reported Plasmepsins are highly flexible and critically modulate the access to the binding cavity. Molecular docking of 49c to TgASP3, PfPMIX, and PfPMX models predicted that the conserved phenylalanine residues in the flap, F344, F291, and F305, respectively, account for the sensitivity toward 49c. Concordantly, phenylalanine mutations in the flap of the three proteases increase twofold to 15-fold the IC₅₀ values of 49c. Compellingly the selection of mutagenized T. gondii resistant strains to 49c reproducibly converted F344 to a cysteine residue.

Sequence, Structural Analysis and Metrics to Define the Unique Dynamic Features of the Flap Regions Among Aspartic Proteases

Aspartic proteases are a class of hydrolytic enzymes that have been implicated in a number of diseases such as HIV, malaria, cancer and Alzheimer's. The flap region of aspartic proteases is a characteristic unique structural feature of these enzymes; and found to have a profound impact on protein overall structure, function and dynamics. Flap dynamics also plays a crucial role in drug binding and drug resistance. Therefore, understanding the structure and dynamic behavior of this flap regions is crucial in the design of potent and selective inhibitors against aspartic proteases. Defining metrics that can describe the flap motion/dynamics has been a challenging topic in literature. This review is the first attempt to compile comprehensive information on sequence, structure, motion and metrics used to assess the dynamics of the flap region of different aspartic proteases in "one pot". We believe that this review would be of critical importance to the researchers from different scientific domains.

Profiling invasive Plasmodium falciparum merozoites using an integrated omics approach

The symptoms of malaria are brought about by blood-stage parasites, which are established when merozoites invade human erythrocytes. Our understanding of the molecular events that underpin erythrocyte invasion remains hampered by the short-period of time that merozoites are invasive. To address this challenge, a Plasmodium falciparum gamma-irradiated long-lived merozoite (LLM) line was developed and investigated. Purified LLMs invaded erythrocytes by an increase of 10-300 fold compared to wild-type (WT) merozoites. Using an integrated omics approach, we investigated the basis for the phenotypic difference. Only a few single nucleotide polymorphisms within the P. falciparum genome were identified and only marginal differences were observed in the merozoite transcriptomes. By contrast, using label-free quantitative mass-spectrometry, a significant change in protein abundance was noted, of which 200 were proteins of unknown function. We determined the relative molar abundance of over 1100 proteins in LLMs and further characterized the major merozoite surface protein complex. A unique processed MSP1 intermediate was identified in LLM but not observed in WT suggesting that delayed processing may be important for the observed phenotype. This integrated approach has demonstrated the significant role of the merozoite proteome during erythrocyte invasion, while identifying numerous unknown proteins likely to be involved in invasion.

Different intra- and inter-molecular hydrogen-bonding patterns in (3S,4aS,8aS)-2-[(2R,3S)-3-(2,5-X2-benzamido)-2-(2,5-X2-benzo-yloxy)-4-phenyl-butyl]-N-tert-butyldeca-hydro-iso-quinoline-3-carboxamides (X = H or Cl): compounds with moderate aspartyl protease inhibition activity

The crystal structures of (3S,4aS,8aS)-2-[(2R,3S)-3-benzamido-2-benzo-yloxy-4-phenyl-but-yl]-N-tert-butyldeca-hydro-iso-quinoline-3-carboxamide, C38H47N3O4, (I), and (3S,4aS,8aS)-2-[(2R,3S)-3-(2,5-di-chloro-benzamido)-2-(2,5-di-chloro-benzo-yloxy)-4-phenyl-but-yl]-N-tert-butyldeca-hydro-iso-quinoline-3-carboxamide, C38H43Cl4N3O4, (II), are described. Despite their chemical similarity, they adopt different conformations in the solid state: (I) features a bifurcated intra-molecular N-H⋯(N,O) hydrogen bond from the tert-butylamide NH group to the piperidine N atom and the benzoate O atom, whereas (II) has an intra-molecular N-H⋯O link from the benzamide NH group to the tert-butyl-amide O atom. In the crystal of (I), mol-ecules are linked by C(4) amide N-H⋯O hydrogen bonds into chains propagating in the [010] direction, with both donor and acceptor parts of the benzamide group. In the extended structure of (II), C(11) N-H⋯O chains propagating in the [010] direction arise, with the donor being the tert-butylamide NH group and the acceptor being the O atom of the benzamide group.

Plasmodium berghei plasmepsin VIII is essential for sporozoite gliding motility

Plasmodium aspartic proteases, termed plasmepsins (PMs) play many critical roles such as haemoglobin degradation, cleavage of PEXEL proteins and sporozoite development in the parasite life cycle. Most of the plasmepsins are well characterized, however the role of PM VIII in Plasmodium remains unknown. Here, we elucidate the functions of PM VIII (PBANKA_132910) in the rodent malaria parasite Plasmodium berghei (Pb). By targeted gene deletion, we show that PbPM VIII is critical for sporozoite egress from an oocyst and gliding motility, which is a prerequisite for the invasion of salivary glands and subsequent transmission to the vertebrate host.

Altered Plasmodium falciparum Sensitivity to the Antiretroviral Protease Inhibitor Lopinavir Associated with Polymorphisms in pfmdr1

The HIV protease inhibitor lopinavir inhibits Plasmodium falciparum aspartic proteases (plasmepsins) and parasite development, and children receiving lopinavir-ritonavir experienced fewer episodes of malaria than those receiving other antiretroviral regimens. Resistance to lopinavir was selected in vitro over ∼9 months, with ∼4-fold decreased sensitivity. Whole-genome sequencing of resistant parasites showed a mutation and increased copy number in pfmdr1 and a mutation in a protein of unknown function, but no polymorphisms in plasmepsin genes.

Crystal structure of Plasmodium falciparum proplasmepsin IV: the plasticity of proplasmepsins

Plasmepsin IV from Plasmodium falciparum (PM IV) is a promising target for the development of novel antimalarial drugs. Here, the crystal structure of the truncated zymogen of PM IV (pPM IV), consisting of the mature enzyme plus a prosegment of 47 residues, has been determined at 1.5 Å resolution. pPM IV presents the fold previously described for studied proplasmepsins, displaying closer similarities to proplasmepin IV from P. vivax (pPvPM) than to the other two proplasmepsins from P. falciparum. The study and comparison of the pPM IV structure with the proplasmepsin structures described previously provide information about the similarities and differences in the inactivation-activation mechanisms among the plasmepsin zymogens.

Understanding the structural basis of substrate recognition by Plasmodium falciparum plasmepsin V to aid in the design of potent inhibitors

Plasmodium falciparum plasmepsin V (PfPMV) is an essential aspartic protease required for parasite survival, thus, considered as a potential drug target. This study reports the first detailed structural analysis and molecular dynamics simulation of PfPMV as an apoenzyme and its complexes with the substrate PEXEL as well as with the inhibitor saquinavir. The presence of pro-peptide in PfPMV may not structurally hinder the formation of a functionally competent catalytic active site. The structure of PfPMV-PEXEL complex shows that the unique positions of Glu179 and Gln222 are responsible for providing the specificity of PEXEL substrate with arginine at P3 position. The structural analysis also reveals that the S4 binding pocket in PfPMV is occupied by Ile94, Ala98, Phe370 and Tyr472, and therefore, does not allow binding of pepstatin, a potent inhibitor of most pepsin-like aspartic proteases. Among the screened inhibitors, the HIV-1 protease inhibitors and KNI compounds have higher binding affinities for PfPMV with saquinavir having the highest value. The presence of a flexible group at P2 and a bulky hydrophobic group at P3 position of the inhibitor is preferred in the PfPMV substrate binding pocket. Results from the present study will aid in the design of potent inhibitors of PMV.

Parasite Cathepsin D-Like Peptidases and Their Relevance as Therapeutic Targets

Inhibition of aspartic cathepsin D-like peptidases (APDs) has been often discussed as an antiparasite intervention strategy. APDs have been considered as virulence factors of Trypanosoma cruzi and Leishmania spp., and have been demonstrated to have important roles in protein trafficking mechanisms of apicomplexan parasites. APDs also initiate blood digestion as components of multienzyme proteolytic complexes in malaria, platyhelminths, nematodes, and ticks. Increasing DNA and RNA sequencing data indicate that parasites express multiple APD isoenzymes of various functions that can now be specifically evaluated using new functional-genomic and biochemical tools, from which we can further assess the potential of APDs as targets for novel effective intervention strategies against parasitic diseases that still pose an alarming threat to mankind.

Pepsin is a positive regulator of Ac-cathB-2 involved in the rat gut penetration of Angiostrongylus cantonensis

Background

Angiostrongyliasis caused by the rat lungworm, Angiostrongylus cantonensis (A. cantonensis), has globally spread from the traditional epidemic areas. The small intestine is the site where the third-stage larvae (L3) of this worm entered the host body, and parasite proteases are involved in this process. Ac-cathB-2, a cathepsin B-like cysteine of A. cantonensis, was formerly isolated from the insoluble part of fragmentised Escherichia coli without activity. The unplanned low activity of prokaryotic expression proteins and difficulties in genetic modification hindered understanding the function of this protein.

Methods

The recombinant Ac-cathB-2 was expressed and harvested from 293 T cells and the enzymatic property and the effects of processing on the activity of the recombinant protease were investigated in vitro. The expression of Ac-cathB-2 in response to external stimulation was assessed, and the function of this protease during host gut penetration was observed by using antiserum for inhibition.

Results

Of the life-cycle stages studied, L3 expressed the highest level of Ac-cathB-2 gene and released the corresponding gene product from the body. The expression of this gene was rapidly upregulated after incubating L3 in small intestine homogenate of rat. Recombinant Ac-cathB-2 was harvested from 293 T cell culture medium. This protease was activated by pepsin-HCl and the enabled Ac-cathB-2 could subsequently digest laminin and fibronectin readily. Moreover, the small intestine isolated from rat was disrupted after incubating with the activated Ac-cathB-2, resulting in the detachment of epithelial cells. Antiserum treatment inhibited the hydrolytic ability of recombinant Ac-cathB-2 by 82.7 %, and also reduced the tissue penetration of activated L3 by 41.2 %. Additionally, pre-incubation of L3 with artificial gastric acid increased the number of penetrating larvae by 53.2 %, and this alteration could be partly blocked by antiserum treatment.

Conclusion

We believe that Ac-cathB-2 from A. cantonensis might help the worm to penetrate the rat gut, because the protease was able to degrade the tissue components of host. Nevertheless, our results further indicated that host pepsin played a beneficial role in this process by cleaving Ac-cathB-2 for activation. Thus, Ac-cathB-2 may probably represent an important target for the control of A. cantonensis infection.

Flap Dynamics in Aspartic Proteases: A Computational Perspective

Recent advances in biochemistry and drug design have placed proteases as one of the critical target groups for developing novel small-molecule inhibitors. Among all proteases, aspartic proteases have gained significant attention due to their role in HIV/AIDS, malaria, Alzheimer's disease, etc. The binding cleft is covered by one or two β-hairpins (flaps) which need to be opened before a ligand can bind. After binding, the flaps close to retain the ligand in the active site. Development of computational tools has improved our understanding of flap dynamics and its role in ligand recognition. In the past decade, several computational approaches, for example molecular dynamics (MD) simulations, coarse-grained simulations, replica-exchange molecular dynamics (REMD) and metadynamics, have been used to understand flap dynamics and conformational motions associated with flap movements. This review is intended to summarize the computational progress towards understanding the flap dynamics of proteases and to be a reference for future studies in this field.

Plasmodium falciparum ookinete expression of plasmepsin VII and plasmepsin X

BACKGROUND

Plasmodium invasion of the mosquito midgut is a population bottleneck in the parasite lifecycle. Interference with molecular mechanisms by which the ookinete invades the mosquito midgut is one potential approach to developing malaria transmission-blocking strategies. Plasmodium aspartic proteases are one such class of potential targets: plasmepsin IV (known to be present in the asexual stage food vacuole) was previously shown to be involved in Plasmodium gallinaceum infection of the mosquito midgut, and plasmepsins VII and plasmepsin X (not known to be present in the asexual stage food vacuole) are upregulated in Plasmodium falciparum mosquito stages. These (and other) parasite-derived enzymes that play essential roles during ookinete midgut invasion are prime candidates for transmission-blocking vaccines.

METHODS

Reverse transcriptase PCR (RT-PCR) was used to determine timing of P. falciparum plasmepsin VII (PfPM VII) and plasmepsin X (PfPM X) mRNA transcripts in parasite mosquito midgut stages. Protein expression was confirmed by western immunoblot and immunofluorescence assays (IFA) using anti-peptide monoclonal antibodies (mAbs) against immunogenic regions of PfPM VII and PfPM X. These antibodies were also used in standard membrane feeding assays (SMFA) to determine whether inhibition of these proteases would affect parasite transmission to mosquitoes. The Mann-Whitney U test was used to analyse mosquito transmission assay results.

RESULTS

RT-PCR, western immunoblot and immunofluorescence assay confirmed expression of PfPM VII and PfPM X in mosquito stages. Whereas PfPM VII was expressed in zygotes and ookinetes, PfPM X was expressed in gametes, zygotes, and ookinetes. Antibodies against PfPM VII and PfPM X decreased P. falciparum invasion of the mosquito midgut when used at high concentrations, indicating that these proteases play a role in Plasmodium mosquito midgut invasion. Failure to generate genetic knockouts of these genes limited determination of the precise role of these proteases in parasite transmission but suggests that they are essential during the intraerythrocytic life cycle.

CONCLUSIONS

PfPM VII and PfPM X are present in the mosquito-infective stages of P. falciparum. Standard membrane feeding assays demonstrate that antibodies against these proteins reduce the infectivity of P. falciparum for mosquitoes, suggesting their viability as transmission-blocking vaccine candidates. Further study of the role of these plasmepsins in P. falciparum biology is warranted.

Structures of plasmepsin II from Plasmodium falciparum in complex with two hydroxyethylamine-based inhibitors

Plasmepsin II (PMII) is one of the ten plasmepsins (PMs) identified in the genome of Plasmodium falciparum, the causative agent of the most severe and deadliest form of malaria. Owing to the emergence of P. falciparum strains that are resistant to current antimalarial agents such as chloroquine and sulfadoxine/pyrimethamine, there is a constant pressure to find new and lasting chemotherapeutic drug therapies. Previously, the crystal structure of PMII in complex with NU655, a potent antimalarial hydroxyethylamine-based inhibitor, and the design of new compounds based on it have been reported. In the current study, two of these newly designed hydroxyethylamine-based inhibitors, PG418 and PG394, were cocrystallized with PMII and their structures were solved, analyzed and compared with that of the PMII-NU655 complex. Structural analysis of the PMII-PG418 complex revealed that the flap loop can adopt a fully closed conformation, stabilized by interactions with the inhibitor, and a fully open conformation, causing an overall expansion in the active-site cavity, which in turn causes unstable binding of the inhibitor. PG418 also stabilizes the flexible loop Gln275-Met286 of another monomer in the asymmetric unit of PMII, which is disordered in the PMII-NU655 complex structure. The crystal structure of PMII in complex with the inhibitor PG418 demonstrates the conformational flexibility of the active-site cavity of the plasmepsins. The interactions of the different moieties in the P1' position of PG418 and PG394 with Thr217 have to be taken into account in the design of new potent plasmepsin inhibitors.

Protein profiling of mefloquine resistant Plasmodium falciparum using mass spectrometry-based proteomics

Malaria is a mosquito borne infectious disease caused by protozoa of genus Plasmodium. There are five species of Plasmodium that are found to infect humans. Plasmodium falciparum can cause severe malaria leading to higher morbidity and mortality of malaria than the other four species. Antimalarial resistance is the major obstacle to control malaria. Mefloquine was used in combination with Artesunate for uncomplicated P. falciparum in South East Asia and it has developed and established mefloquine resistance in this region. Here, gel-enhanced liquid chromatography/tandem mass spectrometry (GeLC-MS/MS)-based proteomics and label-free quantification were used to explore the protein profiles of mefloquine-sensitive and -induced resistant P. falciparum. A Thai P. falciparum isolate (S066) was used as a model in this research. Our data revealed for the first time that 69 proteins exhibited at least 2-fold differences in their expression levels between the two parasite lines. Of these, 36 were up-regulated and 33 were down-regulated in the mefloquine-resistant line compared with the mefloquine-sensitive line. These findings are consistent with those of past studies, where the multidrug resistance protein Pgh1 showed an up-regulation pattern consistent with that expected from its average 3-copy pfmdr1 gene number. Pgh1 and eight other up-regulated proteins (i.e., histo-aspartyl protease protein, exportin 1, eukaryotic translation initiation factor 3 subunit 8, peptidyl-prolyl cis-trans isomerase, serine rich protein homologue, exported protein 1, ATP synthase beta chain and phospholipid scramblase 1) were further validated for their expression levels using reverse transcriptase quantitative real-time PCR. The data support the up-regulation status in the mefloquine-resistant parasite line of all the candidate genes referred to above. Therefore, GeLC-MS/MS-based proteomics combined with label-free quantification is a reliable approach for exploring mefloquine resistance biomarkers in P. falciparum. Identification of these proteins leads to better understanding of mefloquine resistant mechanisms in malaria parasites.

Enzymatic Characterization of Recombinant Food Vacuole Plasmepsin 4 from the Rodent Malaria Parasite Plasmodium berghei

The rodent malaria parasite Plasmodium berghei is a practical model organism for experimental studies of human malaria. Plasmepsins are a class of aspartic proteinase isoforms that exert multiple pathological effects in malaria parasites. Plasmepsins residing in the food vacuole (FV) of the parasite hydrolyze hemoglobin in red blood cells. In this study, we cloned PbPM4, the FV plasmepsin gene of P. berghei that encoded an N-terminally truncated pro-segment and the mature enzyme from genomic DNA. We over-expressed this PbPM4 zymogen as inclusion bodies (IB) in Escherichia coli, and purified the protein following in vitro IB refolding. Auto-maturation of the PbPM4 zymogen to mature enzyme was carried out at pH 4.5, 5.0, and 5.5. Interestingly, we found that the PbPM4 zymogen exhibited catalytic activity regardless of the presence of the pro-segment. We determined the optimal catalytic conditions for PbPM4 and studied enzyme kinetics on substrates and inhibitors of aspartic proteinases. Using combinatorial chemistry-based peptide libraries, we studied the active site preferences of PbPM4 at subsites S1, S2, S3, S1’, S2’ and S3’. Based on these results, we designed and synthesized a selective peptidomimetic compound and tested its inhibition of PbPM4, seven FV plasmepsins from human malaria parasites, and human cathepsin D (hcatD). We showed that this compound exhibited a >10-fold selectivity to PbPM4 and human malaria parasite plasmepsin 4 orthologs versus hcatD. Data from this study furthesr our understanding of enzymatic characteristics of the plasmepsin family and provides leads for anti-malarial drug design.