Synthetic peptides derived from the C-terminal 6kDa region of Plasmodium falciparum SERA5 inhibit the enzyme activity and malaria parasite development

An Overview on Antimalarial Peptides: Natural Sources, Synthetic Methodology and Biological Properties

Peptide compounds play a significant role in medicinal chemistry as they can inhibit the activity of species that cause malaria. This literature review summarizes the isolation of antimalarial peptides, the synthesis method with the detailed structure and sequences of each peptide, and discusses the biological activity of the isolated and synthesized compounds. The synthetic routes and reactions for cyclic and linear antimalarial peptides are systematically highlighted in this review including preparing building blocks, protection and deprotection, coupling and cyclization reactions until the target compound is obtained. Based on the literature data and the results, this review's aim is to provide information to discover and synthesize more antimalarial peptide for future research.

The Nϵ-Rule for Serine, but Not Cysteine Catalytic Triads

Catalytic triads, composed of a serine or cysteine nucleophile, a histidine, and a third triad residue (typically Asp/Glu/Asn), are common in enzyme active sites and catalyze a wide variety of chemical reactions. Two types of triads can be distinguished: We refer to them as Nδ- or Nϵ-configured, depending on whether the histidine imidazole Nδ or Nϵ atom is close to the nucleophile Oγ/Sγ. In this study, we have analyzed triad configuration. In structural triads, the more stable Nδ-configuration predominates. For catalytic triads, the configuration depends on the nucleophile. When it is a cysteine residue, both configuration types occur, depending on the family. However, when the nucleophile is a serine residue, the less stable Nϵ-configuration is almost exclusively found. We posit that the energetically less favored conformation is selected for in serine triads to facilitate the otherwise difficult proton transfer from the nucleophile to the histidine residue.

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.

Title: A Comprehensive Review on Classifying Fast-acting and Slow-acting Antimalarial Agents Based on Time of Action and Target Organelle of Plasmodium sp

The clinical resistance towards malarial parasites has rendered many antimalarials ineffective, likely due to a lack of understanding of time of action and stage specificity of all life stages. Therefore, to tackle this problem a more incisive comprehensive analysis of the fast and slow-acting profile of antimalarial agents relating to parasite time-kill kinetics and the target organelle on the progression of blood-stage parasites was carried out. It is evident from numerous findings that drugs targeting food vacuole, nuclear components, and endoplasmic reticulum mainly exhibit a fast-killing phenotype within 24h affecting first-cycle activity. Whereas drugs targeting mitochondria, apicoplast, microtubules, parasite invasion and egress exhibit a largely slow-killing phenotype within 96-120h, affecting second-cycle activity with few exemptions as moderately fast-killing. It is essential to understand the susceptibility of drugs on rings, trophozoites, schizonts, merozoites, and the appearance of organelle at each stage of 48h intraerythrocytic parasite cycle. Therefore, these parameters may facilitate the paradigm for understanding the timing of antimalarials action in deciphering its precise mechanism linked with time. Thus, classifying drugs based on the time of killing may promote designing new combination regimens against varied strains of P. falciparum and evaluating potential clinical resistance.

Antimalarial effect of cell penetrating peptides derived from the junctional region of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase

Dihydrofolate reductase-thymidylate synthase of Plasmodium falciparum (PfDHFR-TS) is an important target of antifolate antimalarial drugs. However, drug resistant parasites are widespread in malaria endemic regions. The unique bifunctional property of PfDHFR-TS could be exploited for the design of allosteric inhibitors that interfere with the active dimer conformation. In this study, peptides were derived from the junctional region (JR) of PfDHFR-TS amino acid sequence in the α_j1 helix (JR-helix) and the DHFR domain that is necessary for interaction with α_j1 helix (JR21). Five peptides were synthesized and tested for inhibition of PfDHFR-TS enzyme by Bacterial inhibition assay (BIA) based on the growth of an E. coli DHFR and TS knockout complemented with a recombinant plasmid expressing PfDHFR-TS enzyme. Significant inhibition was observed for JR21 and JR21 conjugated to cell-penetrating octa-arginine peptide (rR8-JR21) with 50 % inhibitory concentration (IC₅₀) of 3.87 and 1.53 μM, respectively. The JR-helix and rR8-JR-helix peptides were inactive. JR21 and rR8-JR21 peptides showed similar growth inhibitory effects on P. falciparum NF54 parasites cultured in vitro. Treatment with rR8-JR21 delayed parasite development, in which an accumulation of ring stage parasites was observed after 12 h of culture. Minimal red blood cell (RBC) hemolysis was observed at the highest dose of peptide tested. The most potent peptide rR8-JR21 not only compromised the development of the P. falciparum, but also inhibited the parasite growth and has low hemolytic effect on human RBCs.

Metacaspase-3 of Plasmodium falciparum: An atypical trypsin-like serine protease

Metacaspases are clan CD cysteine peptidases found in plants, fungi and protozoa that possess a conserved Peptidase_C14 domain, homologous to the human caspases and a catalytic His/Cys dyad. Earlier reports have indicated the role of metacaspases in cell death; however, metacaspases of human malaria parasite remains poorly understood. In this study, we aimed to functionally characterize a novel malarial protease, P. falciparum metacaspase-3 (PfMCA3). Unlike other clan CD peptidases, PfMCA3 has an atypical active site serine (Ser1865) residue in place of canonical cysteine and it phylogenetically forms a distinct branch across the species. To investigate whether this domain retains catalytic activity, we expressed, purified and refolded the Peptidase_C14 domain of PfMCA3 which was found to express in all asexual stages. PfMCA3 exhibited trypsin-like serine protease activity with ser1865 acting as catalytic residue to cleave trypsin oligopeptide substrate. PfMCA3 is inhibited by trypsin-like serine protease inhibitors. Our study found that PfMCA3 enzymatic activity was abrogated when catalytic serine1865 (S1865A) was mutated. Moreover, PfMCA3 was found to be inactive against caspase substrate. Overall, our study characterizes a novel metacaspase of P. falciparum, different from human caspases and not responsible for the caspase-like activity, therefore, could be considered as a potential chemotherapeutic target.

Plasmodium falciparum MSP3 Exists in a Complex on the Merozoite Surface and Generates Antibody Response during Natural Infection

Plasmodium falciparum merozoite surface protein 3 (MSP3) is an abundantly expressed secreted merozoite surface protein and a leading malaria vaccine candidate antigen. However, it is unclear how MSP3 is retained on the surface of merozoites without a glycosylphosphatidylinositol (GPI) anchor or a transmembrane domain. In the present study, we identified an MSP3-associated network on the Plasmodium merozoite surface by immunoprecipitation of Plasmodium merozoite lysate using antibody to the N terminus of MSP3 (anti-MSP3N) followed by mass spectrometry analysis. The results suggested the association of MSP3 with other merozoite surface proteins: MSP1, MSP6, MSP7, RAP2, and SERA5. Protein-protein interaction studies by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR) analysis showed that MSP3 complex consists of MSP1, MSP6, and MSP7 proteins. Immunological characterization of MSP3 revealed that MSP3N is strongly recognized by hyperimmune serum from African and Asian populations. Furthermore, we demonstrate that human antibodies, affinity purified against recombinant MSP3N (rMSP3N), promote opsonic phagocytosis of merozoites in cooperation with monocytes. At nonphysiological concentrations, anti-MSP3N antibodies inhibited the growth of P. falciparum in vitro Together, the data suggest that MSP3 and especially its N-terminal region containing known B/T cell epitopes are targets of naturally acquired immunity against malaria and also comprise an important candidate for a multisubunit malaria vaccine.

Calcium-dependent phosphorylation of Plasmodium falciparum serine repeat antigen 5 triggers merozoite egress

The human malaria parasite Plasmodium falciparum proliferates in red blood cells following repeated cycles of invasion, multiplication, and egress. P. falciparum serine repeat antigen 5 (PfSERA5), a putative serine protease, plays an important role in merozoite egress. However, regulation of its activity leading to merozoite egress is poorly understood. In this study, we show that PfSERA5 undergoes phosphorylation prior to merozoite egress. Immunoprecipitation of parasite lysates using anti-PfSERA5 serum followed by MS analysis identified calcium-dependent protein kinase 1 (PfCDPK1) as an interacting kinase. Association of PfSERA5 with PfCDPK1 was corroborated by co-sedimentation, co-immunoprecipitation, and co-immunolocalization analyses. Interestingly, PfCDPK1 phosphorylated PfSERA5 in vitro in the presence of Ca²⁺ and enhanced its proteolytic activity. A PfCDPK1 inhibitor, purfalcamine, blocked the phosphorylation and activation of PfSERA5 both in vitroas well as in schizonts, which, in turn, blocked merozoite egress. Together, these results suggest that phosphorylation of PfSERA5 by PfCDPK1 following a rise in cytosolic Ca²⁺ levels activates its proteolytic activity to trigger merozoite egress.

A novel Pfs38 protein complex on the surface of Plasmodium falciparum blood-stage merozoites

Background

The Plasmodium genome encodes for a number of 6-Cys proteins that contain a module of six cysteine residues forming three intramolecular disulphide bonds. These proteins have been well characterized at transmission as well as hepatic stages of the parasite life cycle. In the present study, a large complex of 6-Cys proteins: Pfs41, Pfs38 and Pfs12 and three other merozoite surface proteins: Glutamate-rich protein (GLURP), SERA5 and MSP-1 were identified on the Plasmodium falciparum merozoite surface.

Methods

Recombinant 6-cys proteins i.e. Pfs38, Pfs12, Pfs41 as well as PfMSP-1₆₅ were expressed and purified using Escherichia coli expression system and antibodies were raised against each of these proteins. These antibodies were used to immunoprecipitate the native proteins and their associated partners from parasite lysate. ELISA, Far western, surface plasmon resonance and glycerol density gradient fractionation were carried out to confirm the respective interactions. Furthermore, erythrocyte binding assay with 6-cys proteins were undertaken to find out their possible role in host-parasite infection and seropositivity was assessed using Indian and Liberian sera.

Results

Immunoprecipitation of parasite-derived polypeptides, followed by LC–MS/MS analysis, identified a large Pfs38 complex comprising of 6-cys proteins: Pfs41, Pfs38, Pfs12 and other merozoite surface proteins: GLURP, SERA5 and MSP-1. The existence of such a complex was further corroborated by several protein–protein interaction tools, co-localization and co-sedimentation analysis. Pfs38 protein of Pfs38 complex binds to host red blood cells (RBCs) directly via glycophorin A as a receptor. Seroprevalence analysis showed that of the six antigens, prevalence varied from 40 to 99%, being generally highest for MSP-1₆₅ and GLURP proteins.

Conclusions

Together the data show the presence of a large Pfs38 protein-associated complex on the parasite surface which is involved in RBC binding. These results highlight the complex molecular interactions among the P. falciparum merozoite surface proteins and advocate the development of a multi-sub-unit malaria vaccine based on some of these protein complexes on merozoite surface.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1716-0) contains supplementary material, which is available to authorized users.

Functionally relevant proteins in Plasmodium falciparum host cell invasion

A totally effective, antimalarial vaccine must involve sporozoite and merozoite proteins (or their fragments) to ensure complete parasite blocking during critical invasion stages. This Special Report examines proteins involved in critical biological functions for parasite survival and highlights the conserved amino acid sequences of the most important proteins involved in sporozoite invasion of hepatocytes and merozoite invasion of red blood cells. Conserved high activity binding peptides are located in such proteins’ functionally strategic sites, whose functions are related to receptor binding, nutrient and protein transport, enzyme activity and molecule–molecule interactions. They are thus excellent targets for vaccine development as they block proteins binding function involved in invasion and also their biological function.

In silico approach to ascertain the calcium dependent role of Plasmodium falciparum SERA5

The P. falciparum serine repeat antigen (PfSERA5) is the most abundantly expressed protein in the parasitophorous vacuole during the asexual blood stage and serves as both drug and vaccine target. The processed central fragment (56 KDa) of PfSERA5 is implicated to play an important role in parasite exit (egress) during schizont rupture from erythrocytes. Structural characterization of its enzymatic domain supports protease-like function for this central domain. The understanding of exact functional role of PfSERA5 in parasite egress remains unconfirmed as recent studies also indicate an indispensable non-catalytic role for PfSERA5 putative enzyme domain in the blood stage. No structural insight into PfSERA5 prodomain is available. Structure prediction of PfSERA5 prodomain using in silico approach in our study, showed it to have structural similarity with calcium-binding proteins. An earlier observation of steep rise in intracellular calcium concentration as an important factor in egress makes the prodomain calcium-binding role significant. The implication of calcium on structure and activity of PfSERA5 putative enzyme domain is also unknown, and such information would aid to substantiating any calcium-dependent effects on PfSERA5. To understand this, we performed molecular dynamic (MD) simulation both in the presence and absence of calcium. MD results show secondary structure conformational differences in local regions of protein structure. Our results support calcium to be an important parameter for stability and function of PfSERA5. This computational assessment suggest a need to design future experiments like calcium-dependent inhibition studies to reveal exact functional role of PfSERA5 in parasite egress.

Recent advances in recombinant protein-based malaria vaccines

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Protein-based vaccines remain the cornerstone approach for B cell and antibody induction against leading target malaria antigens.

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Advances in antigen selection, immunogen design and epitope-focusing are advancing the field.

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New heterologous expression platforms are enabling cGMP production of next-generation protein vaccines.

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Next-generation antigens, protein-based immunogens and virus-like particle (VLP) delivery platforms are in clinical development.

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Protein-based vaccines will form part of a highly effective multi-component/multi-stage/multi-antigen subunit formulation against malaria.

Plasmodium parasites are the causative agent of human malaria, and the development of a highly effective vaccine against infection, disease and transmission remains a key priority. It is widely established that multiple stages of the parasite's complex lifecycle within the human host and mosquito vector are susceptible to vaccine-induced antibodies. The mainstay approach to antibody induction by subunit vaccination has been the delivery of protein antigen formulated in adjuvant. Extensive efforts have been made in this endeavor with respect to malaria vaccine development, especially with regard to target antigen discovery, protein expression platforms, adjuvant testing, and development of soluble and virus-like particle (VLP) delivery platforms. The breadth of approaches to protein-based vaccines is continuing to expand as innovative new concepts in next-generation subunit design are explored, with the prospects for the development of a highly effective multi-component/multi-stage/multi-antigen formulation seeming ever more likely. This review will focus on recent progress in protein vaccine design, development and/or clinical testing for a number of leading malaria antigens from the sporozoite-, merozoite- and sexual-stages of the parasite's lifecycle–including PfCelTOS, PfMSP1, PfAMA1, PfRH5, PfSERA5, PfGLURP, PfMSP3, Pfs48/45 and Pfs25. Future prospects and challenges for the development, production, human delivery and assessment of protein-based malaria vaccines are discussed.

Plasmodium falciparum SERA5 plays a non-enzymatic role in the malarial asexual blood-stage lifecycle

The malaria parasite Plasmodium falciparum replicates in an intraerythrocytic parasitophorous vacuole (PV). The most abundant P. falciparum PV protein, called SERA5, is essential in blood stages and possesses a papain-like domain, prompting speculation that it functions as a proteolytic enzyme. Unusually however, SERA5 possesses a Ser residue (Ser596) at the position of the canonical catalytic Cys of papain-like proteases, and the function of SERA5 or whether it performs an enzymatic role is unknown. In this study, we failed to detect proteolytic activity associated with the Ser596-containing parasite-derived or recombinant protein. However, substitution of Ser596 with a Cys residue produced an active recombinant enzyme with characteristics of a cysteine protease, demonstrating that SERA5 can bind peptides. Using targeted homologous recombination in P. falciparum, we substituted Ser596 with Ala with no phenotypic consequences, proving that SERA5 does not perform an essential enzymatic role in the parasite. We could also replace an internal segment of SERA5 with an affinity-purification tag. In contrast, using almost identical targeting constructs, we could not truncate or C-terminally tag the SERA5 gene, or replace Ser596 with a bulky Arg residue. Our findings show that SERA5 plays an indispensable but non-enzymatic role in the P. falciparum blood-stage life cycle.

Antimicrobial peptides: a new class of antimalarial drugs?

A range of antimicrobial peptides (AMP) exhibit activity on malaria parasites, Plasmodium spp., in their blood or mosquito stages, or both. These peptides include a diverse array of both natural and synthetic molecules varying greatly in size, charge, hydrophobicity, and secondary structure features. Along with an overview of relevant literature reports regarding AMP that display antiplasmodial activity, this review makes a few considerations about those molecules as a potential new class of antimalarial drugs.