Antibodies reactive with the N-terminal domain of Plasmodium falciparum serine repeat antigen inhibit cell proliferation by agglutinating merozoites and schizonts

Genetic diversity and in silico analysis of Plasmodium knowlesi Serine Repeat Antigen (SERA) 3 antigen 2 in Malaysia

Plasmodium knowlesi is the leading cause of malaria in Malaysia. Serine Repeat Antigens (SERAs) have an essential role in the parasite life cycle. However, genetic characterization on P. knowlesi SERA3 Ag2 (PkSERA3 Ag2) is lacking. In the present study, nucleotide diversity, natural selection, and haplotypes of PkSERA3 Ag2 in clinical samples from Peninsular Malaysia and Malaysian Borneo were investigated. A total of 50 P. knowlesi clinical samples were collected from Peninsular Malaysia and Malaysian Borneo. The PkSERA3 Ag2 gene was amplified using PCR, and subsequently cloned and sequenced. Genetic diversity, haplotype, natural selection as well as genetic structure and differentiation of PkSERA3 Ag2 were analysed. In addition, in silico analyses were performed to identify repeat motifs, B-cell epitopes, and antigenicity indices of the protein. Analysis of 114 PkSERA3 Ag2 sequences revealed high nucleotide diversity of the gene in Malaysia. A codon-based Z-test indicated that the gene underwent purifying selection. Haplotype and population structure analyses identified two distinct PkSERA3 Ag2 clusters (K = 2, ΔK = 721.14) but no clear genetic distinction between PkSERA3 Ag2 from Peninsular Malaysia and Malaysian Borneo. FST index indicated moderate differentiation of the gene. In silico analyses revealed unique repeat motifs among PkSERA3 Ag2 isolates. Moreover, the amino acid sequence of PkSERA3 Ag2 exhibited potential B-cell epitopes and possessed high antigenicity indices. These findings enhance the understanding of PkSERA3 Ag2 gene as well as its antigenic properties. Further validation is necessary to ascertain the utility of PkSERA3 Ag2 as a serological marker for P. knowlesi infection.

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

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

African-specific polymorphisms in Plasmodium falciparum serine repeat antigen 5 in Uganda and Burkina Faso clinical samples do not interfere with antibody response to BK-SE36 vaccination

BK-SE36, based on Plasmodium falciparum serine repeat antigen 5 (SERA5), is a blood-stage malaria vaccine candidate currently being evaluated in clinical trials. Phase 1 trials in Uganda and Burkina Faso have demonstrated promising safety and immunogenicity profiles. However, the genetic diversity of sera5 in Africa and the role of allele/variant-specific immunity remain a major concern. Here, sequence analyses were done on 226 strains collected from the two clinical trial/follow-up studies and 88 strains from two cross-sectional studies in Africa. Compared to other highly polymorphic vaccine candidate antigens, polymorphisms in sera5 were largely confined to the repeat regions of the gene. Results also confirmed a SERA5 consensus sequence with African-specific polymorphisms. Mismatches with the vaccine-type SE36 (BK-SE36) in the octamer repeat, serine repeat, and flanking regions, and single-nucleotide polymorphisms in non-repeat regions could compromise vaccine response and efficacy. However, the haplotype diversity of SERA5 was similar between vaccinated and control participants. There was no marked bias or difference in the patterns of distribution of the SE36 haplotype and no statistically significant genetic differentiation among parasites infecting BK-SE36 vaccinees and controls. Results indicate that BK-SE36 does not elicit an allele-specific immune response.

Progress in the Development of Subunit Vaccines against Malaria

Malaria is a life-threatening disease and one of the main causes of morbidity and mortality in the human population. The disease also results in a major socio-economic burden. The rapid spread of malaria epidemics in developing countries is exacerbated by the rise in drug-resistant parasites and insecticide-resistant mosquitoes. At present, malaria research is focused mainly on the development of drugs with increased therapeutic effects against Plasmodium parasites. However, a vaccine against the disease is preferable over treatment to achieve long-term control. Trials to develop a safe and effective immunization protocol for the control of malaria have been occurring for decades, and continue on today; still, no effective vaccines are available on the market. Recently, peptide-based vaccines have become an attractive alternative approach. These vaccines utilize short protein fragments to induce immune responses against malaria parasites. Peptide-based vaccines are safer than traditional vaccines, relatively inexpensive to produce, and can be composed of multiple T- and B-cell epitopes integrated into one antigenic formulation. Various combinations, based on antigen choice, peptide epitope modification and delivery mechanism, have resulted in numerous potential malaria vaccines candidates; these are presently being studied in both preclinical and clinical trials. This review describes the current landscape of peptide-based vaccines, and addresses obstacles and opportunities in the production of malaria vaccines.

Characteristic features of the SERA multigene family in the malaria parasite

Serine repeat antigen (SERA) is conserved among species of the genus Plasmodium. Sera genes form a multigene family and are generally tandemly clustered on a single chromosome. Although all Plasmodium species encode multiple sera genes, the number varies between species. Among species, the members share similar sequences and gene organization. SERA possess a central papain-like cysteine protease domain, however, in some members, the active site cysteine residue is substituted with a serine. Recent studies implicate this gene family in a number of aspects in parasite biology and induction of protective immune response. This review summarizes the current understanding on this important gene family in several Plasmodium species. The Plasmodium falciparum (Pf)-sera family, for example, consists of nine gene members. Unlike other multigene families in Plasmodium species, Pf-sera genes do not exhibit antigenic variation. Pf-sera5 nucleotide diversity is also low. Moreover, although Pf-sera5 is highly transcribed during the blood stage of malaria infection, and a large amount is released into the host blood following schizont rupture, in malaria endemic countries the sero-positive rates for Pf-SERA5 are low, likely due to Pf-SERA5 binding of host proteins to avoid immune recognition. As an antigen, the N-terminal 47 kDa domain of Pf-SERA5 is a promising vaccine candidate currently undergoing clinical trials. Pf-SERA5 and Pf-SERA6, as well as P. berghei (Pb)-SERA3, and Pb-SERA5, have been investigated for their roles in parasite egress. Two P. yoelii SERA, which have a serine residue at the protease active center, are implicated in parasite virulence. Overall, these studies provide insight that during the evolution of the Plasmodium parasite, the sera gene family members have increased by gene duplication, and acquired various functions that enable the parasite to survive and successfully maintain infection in the host.

Immunomic Identification of Malaria Antigens Associated With Protection in Mice

Efforts to develop vaccines against malaria represent a major research target. The observations that 1) sterile protection can be obtained when the host is exposed to live parasites and 2) the immunity against blood stage parasite is principally mediated by protective antibodies suggest that a protective vaccine is feasible. However, only a small number of proteins have been investigated so far and most of the Plasmodium proteome has yet to be explored. To date, only few immunodominant antigens have emerged for testing in clinical trials but no formulation has led to substantial protection in humans. The nature of parasite molecules associated with protection remains elusive. Here, immunomic screening of mice immune sera with different protection efficiencies against the whole parasite proteome allowed us to identify a large repertoire of antigens validated by screening a library expressing antigens. The calculation of weighted scores reflecting the likelihood of protection of each antigen using five predictive criteria derived from immunomic and proteomic data sets, highlighted a priority list of protective antigens. Altogether, the approach sheds light on conserved antigens across Plasmodium that are amenable to targeting by the host immune system upon merozoite invasion and blood stage development. Most of these antigens have preliminary protection data but have not been widely considered as candidate for vaccine trials, opening new perspectives that overcome the limited choice of immunodominant, poorly protective vaccines currently being the focus of malaria vaccine researches.

Molecular Camouflage of Plasmodium falciparum Merozoites by Binding of Host Vitronectin to P47 Fragment of SERA5

The malaria parasite Plasmodium falciparum proliferates in the blood stream where the host immune system is most active. To escape from host immunity, P. falciparum has developed a number of evasion mechanisms. Serine repeat antigen 5 (SERA5) is a blood stage antigen highly expressed at late trophozoite and schizont stages. The P47 N-terminal domain of SERA5, the basis of SE36 antigen of the blood stage vaccine candidate under clinical trials, covers the merozoite surface. Exploring the role of the P47 domain, screening of serum proteins showed that vitronectin (VTN) directly binds to 20 residues in the C-terminal region of SE36. VTN co-localized with P47 domain in the schizont and merozoite stages. Phagocytosis assay using THP-1 cells demonstrated that VTN bound to SE36 prevented engulfment of SE36-beads. In addition, several serum proteins localized on the merozoite surface, suggesting that host proteins camouflage merozoites against host immunity via binding to VTN.

Mechanisms of naturally acquired immunity to P. falciparum and approaches to identify merozoite antigen targets

Malaria is one the most serious infectious diseases with over 200 million clinical cases annually. Most cases of the severe disease are caused by Plasmodium falciparum. The blood stage of Plasmodium parasite is entirely responsible for malaria-associated pathology. The population most susceptible to severe malaria are children under the age of 5, with low levels of immunity. It is only after many years of repeated exposure that individuals living in endemic areas develop clinical immunity. This form of protection prevents clinical episodes by substantially reducing parasite burden. Naturally acquired immunity predominantly targets blood-stage parasites with antibody responses being the main mediators of protection. The targets of clinical immunity are the extracellular merozoite and the infected erythrocyte surface, with the extremely diverse PfEMP1 proteins the main target here. This observation provides a strong rationale that an effective anti-malaria vaccine targeting blood-stage parasites is achievable. Thus the identification of antigenic targets of naturally acquired immunity remains an important step towards the formulation of novel vaccine combinations before testing their efficacy in clinical trials. This review summarizes the main findings to date defining antigenic targets present on the extracellular merozoite associated with naturally acquired immunity to P. falciparum malaria.

Parasitophorous vacuole poration precedes its rupture and rapid host erythrocyte cytoskeleton collapse in Plasmodium falciparum egress

In the asexual blood stages of malarial infection, merozoites invade erythrocytes and replicate within a parasitophorous vacuole to form daughter cells that eventually exit (egress) by sequential rupture of the vacuole and erythrocyte membranes. The current model is that PKG, a malarial cGMP-dependent protein kinase, triggers egress, activating malarial proteases and other effectors. Using selective inhibitors of either PKG or cysteine proteases to separately inhibit the sequential steps in membrane perforation, combined with video microscopy, electron tomography, electron energy loss spectroscopy, and soft X-ray tomography of mature intracellular Plasmodium falciparum parasites, we resolve intermediate steps in egress. We show that the parasitophorous vacuole membrane (PVM) is permeabilized 10-30 min before its PKG-triggered breakdown into multilayered vesicles. Just before PVM breakdown, the host red cell undergoes an abrupt, dramatic shape change due to the sudden breakdown of the erythrocyte cytoskeleton, before permeabilization and eventual rupture of the erythrocyte membrane to release the parasites. In contrast to the previous view of PKG-triggered initiation of egress and a gradual dismantling of the host erythrocyte cytoskeleton over the course of schizont development, our findings identify an initial step in egress and show that host cell cytoskeleton breakdown is restricted to a narrow time window within the final stages of egress.

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.

Antibody titres and boosting after natural malaria infection in BK-SE36 vaccine responders during a follow-up study in Uganda

The malaria vaccine BK-SE36 is a recombinant protein (SE36) based on the Honduras 1 serine repeat antigen-5 of Plasmodium falciparum, adsorbed to aluminium hydroxide gel. The phase Ib trial in Uganda demonstrated the safety and immunogenicity of BK-SE36. Ancillary analysis in the follow-up study of 6–20 year-old volunteers suggest significant differences in time to first episodes of clinical malaria in vaccinees compared to placebo/control group. Here, we aimed to get further insights into the association of anti-SE36 antibody titres and natural P. falciparum infection. Children who received BK-SE36 and whose antibody titres against SE36 increased by ≥1.92-fold after vaccination were categorised as responders. Most responders did not have or only had a single episode of natural P. falciparum infection. Notably, responders who did not experience infection had relatively high anti-SE36 antibody titres post-second vaccination compared to those who were infected. The anti-SE36 antibody titres of the responders who experienced malaria were boosted after infection and they had lower risk of reinfection. These findings show that anti-SE36 antibody titres induced by BK-SE36 vaccination offered protection against malaria. The vaccine is now being evaluated in a phase Ib trial in children less than 5 years old.

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.

Protective epitopes of the Plasmodium falciparum SERA5 malaria vaccine reside in intrinsically unstructured N-terminal repetitive sequences

The malaria vaccine candidate antigen, SE36, is based on the N-terminal 47 kDa domain of Plasmodium falciparum serine repeat antigen 5 (SERA5). In epidemiological studies, we have previously shown the inhibitory effects of SE36 specific antibodies on in vitro parasite growth and the negative correlation between antibody level and malaria symptoms. A phase 1 b trial of the BK-SE36 vaccine in Uganda elicited 72% protective efficacy against symptomatic malaria in children aged 6–20 years during the follow-up period 130–365 days post–second vaccination. Here, we performed epitope mapping with synthetic peptides covering the whole sequence of SE36 to identify and map dominant epitopes in Ugandan adult serum presumed to have clinical immunity to P. falciparum malaria. High titer sera from the Ugandan adults predominantly reacted with peptides corresponding to two successive N-terminal regions of SERA5 containing octamer repeats and serine rich sequences, regions of SERA5 that were previously reported to have limited polymorphism. Affinity purified antibodies specifically recognizing the octamer repeats and serine rich sequences exhibited a high antibody-dependent cellular inhibition (ADCI) activity that inhibited parasite growth. Furthermore, protein structure predictions and structural analysis of SE36 using spectroscopic methods indicated that N-terminal regions possessing inhibitory epitopes are intrinsically unstructured. Collectively, these results suggest that strict tertiary structure of SE36 epitopes is not required to elicit protective antibodies in naturally immune Ugandan adults.

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

BACKGROUND

Plasmodium falciparum serine repeat antigen 5 (PfSERA5) is an abundant blood stage protein that plays an essential role in merozoite egress and invasion. The native protein undergoes extensive proteolytic cleavage that appears to be tightly regulated. PfSERA5 N-terminal fragment is being developed as vaccine candidate antigen. Although PfSERA5 belongs to papain-like cysteine protease family, its catalytic domain has a serine in place of cysteine at the active site.

METHODS

In the present study, we synthesized a number of peptides from the N- and C-terminal regions of PfSERA5 active domain and evaluated their inhibitory potential.

RESULTS

The final proteolytic step of PfSERA5 involves removal of a C-terminal ~6kDa fragment that results in the generation of a catalytically active ~50kDa enzyme. In the present study, we demonstrate that two of the peptides derived from the C-terminal ~6kDa region inhibit the parasite growth and also cause a delay in the parasite development. These peptides reduced the enzyme activity of the recombinant protein and co-localized with the PfSERA5 protein within the parasite, thereby indicating the specific inhibition of PfSERA5 activity. Molecular docking studies revealed that the inhibitory peptides interact with the active site of the protein. Interestingly, the peptides did not have an effect on the processing of PfSERA5.

CONCLUSIONS

Our observations indicate the temporal regulation of the final proteolytic cleavage step that occurs just prior to egress.

GENERAL SIGNIFICANCE

These results reinforce the role of PfSERA5 for the intra-erythrocytic development of malaria parasite and show the role of carboxy terminal ~6kDa fragments in the regulation of PfSERA5 activity. The results also suggest that final cleavage step of PfSERA5 can be targeted for the development of new anti-malarials.

How Should Antibodies against P. falciparum Merozoite Antigens Be Measured?

Immunity against malaria develops slowly and only after repeated exposure to the parasite. Many of those that die of the disease are children under five years of age. Antibodies are an important part of immunity, but which antibodies that are protective and how these should be measured are still unclear. We discuss the pros and cons of ELISA, invasion inhibition assays/ADCI, and measurement of affinity of antibodies and what can be done to improve these assays, thereby increasing the knowledge about the immune status of an individual, and to perform better evaluation of vaccine trials.

The role of serine-type serine repeat antigen in Plasmodium yoelii blood stage development

A key step for the survival of the malaria parasite is the release from and subsequent invasion of erythrocytes by the merozoite. Differences in the efficiency of these two linked processes have a direct impact on overall parasite burden in the host and thereby virulence. A number of parasite proteases have recently been shown to play important roles during both merozoite egress as well as merozoite invasion. The rodent malaria parasite Plasmodium yoelii has been extensively used to investigate the mechanisms of parasite virulence in vivo and a number of important proteins have been identified as being key contributors to pathology. Here we have utilized transcriptional comparisons to identify two protease-like SERAs as playing a potential role in virulence. We show that both SERAs are non-essential for blood stage development of the parasite though they provide a subtle but important growth advantage in vivo. In particular SERA2 appears to be an important factor in enabling the parasite to fully utilize the whole age repertoire of circulating erythrocytes. This work for the first time demonstrates the subtle contributions different protease-like SERAs make to provide the parasite with a maximal capacity to successfully maintain an infection in the host.

TLR9 adjuvants enhance immunogenicity and protective efficacy of the SE36/AHG malaria vaccine in nonhuman primate models

The SE36 antigen, derived from serine repeat antigen 5 (SERA5) of Plasmodium falciparum, is a promising blood stage malaria vaccine candidate. Ongoing clinical trials suggest the efficacy of the SE36 vaccine could be increased by the incorporation of more effective adjuvants into the vaccine formulation. In this study, we assessed the safety, immunogenicity and protective efficacy of SE36/AHG formulated with TLR9 ligand adjuvants K3 CpG oligodeoxyribonucleotides (CpG ODNs) (K3 ODN), D3 ODN or synthetic hemozoin, in two non-human primate models. SE36/AHG with or without each adjuvant was administrated to cynomolgus monkeys. A combination of TLR9 ligand adjuvant with SE36/AHG induced higher humoral and cellular immune response compared with SE36/AHG alone. Administration of a crude extract of P. falciparum parasite resulted in the induction of more SE36-specific IgG antibodies in monkeys vaccinated with a combination of SE36/AHG and adjuvant, as opposed to vaccination with SE36/AHG alone. The most effective TLR9 ligand, K3 ODN, was chosen for further vaccine trials in squirrel monkeys, in combination with SE36/AHG. All monkeys immunized with the combined SE36/AHG and K3 ODN formulation effectively suppressed parasitemia and symptoms of malaria following challenge infections. Furthermore, no serious adverse events were observed. Our results show that the novel vaccine formulation of K3 ODN with SE36/AHG demonstrates safety, potent immunogenicity and efficacy in nonhuman primates, and this vaccine formulation may form the basis of a more effective malaria vaccine.

Inhibitory potential of prodomain of Plasmodium falciparum protease serine repeat antigen 5 for asexual blood stages of parasite

Plasmodium falciparum serine repeat antigen 5 (SERA5) is a target for both drug and vaccine intervention against malaria. SERA5 is secreted in the parasitophorous vacuole where it is proteolytically processed before schizont rupture. Among the processed products is a 50.8-kDa central domain of the protease, which possesses chymotrypsin-like activity and consists of a 28.9-kDa catalytic domain with a 21.9-kDa N-terminal prodomain, which remain attached together. Because SERA5 has been implicated in merozoite egress from host erythrocytes, the effect of the prodomain and a heptapeptide derived from its C-terminus spanning from D(560) to F(566) (DNSDNMF) on parasite growth was studied. When E. coli-expressed prodomain was incubated with parasite culture, a significant delay in transition from schizont to ring stages was observed up to nanomolar concentrations. The peptide, DNSDNMF also showed similar effects but at nearly 1000-fold higher concentrations. The peptide was also found to interact with the catalytic domain. These data demonstrate the crucial role of SERA5 prodomain for the egress process. Given the inhibitory potential of the prodomain for the parasite, we suggest that peptidomimetic inhibitors based on SERA5 prodomain sequences can be developed as future therapeutics against malaria.

Geographic differentiation of polymorphism in the Plasmodium falciparum malaria vaccine candidate gene SERA5

SERA5 is regarded as a promising malaria vaccine candidate of the most virulent human malaria parasite Plasmodium falciparum. SERA5 is a 120 kDa abundantly expressed blood-stage protein containing a papain-like protease. Since substantial polymorphism in blood-stage vaccine candidates may potentially limit their efficacy, it is imperative to fully investigate polymorphism of the SERA5 gene (sera5). In this study, we performed evolutionary and population genetic analysis of sera5. The level of inter-species divergence (kS=0.076) between P. falciparum and Plasmodium reichenowi, a closely related chimpanzee malaria parasite is comparable to that of housekeeping protein genes. A signature of purifying selection was detected in the proenzyme and enzyme domains. Analysis of 445 near full-length P. falciparum sera5 sequences from nine countries in Africa, Southeast Asia, Oceania and South America revealed extensive variations in the number of octamer repeat (OR) and serine repeat (SR) regions as well as substantial level of single nucleotide polymorphism (SNP) in non-repeat regions (2562 bp). Remarkably, a 14 amino acid sequence of SERA5 (amino acids 59-72) that is known to be the in vitro target of parasite growth inhibitory antibodies was found to be perfectly conserved in all 445 worldwide isolates of P. falciparum evaluated. Unlike other major vaccine target antigen genes such as merozoite surface protein-1, apical membrane antigen-1 or circumsporozoite protein, no strong evidence for positive selection was detected for SNPs in the non-repeat regions of sera5. A biased geographical distribution was observed in SNPs as well as in the haplotypes of the sera5 OR and SR regions. In Africa, OR- and SR-haplotypes with low frequency (<5%) and SNPs with minor allele frequency (<5%) were abundant and were mostly continent-specific. Consistently, significant genetic differentiation, assessed by the Wright's fixation index (Fst) of inter-population variance in allele frequencies, was detected for SNPs and both OR- and SR-haplotypes among almost all parasite populations. The exception was parasite populations between Tanzania and Ghana, suggesting frequent gene flow in Africa. The present study points to the importance of investigating whether biased geographical distribution for SNPs and repeat variants in the OR and SR regions affect the reactivity of human serum antibodies to variants.

Proteases as regulators of pathogenesis: examples from the Apicomplexa

The diverse functional roles that proteases play in basic biological processes make them essential for virtually all organisms. Not surprisingly, proteolysis is also a critical process required for many aspects of pathogenesis. In particular, obligate intracellular parasites must precisely coordinate proteolytic events during their highly regulated life cycle inside multiple host cell environments. Advances in chemical, proteomic and genetic tools that can be applied to parasite biology have led to an increased understanding of the complex events centrally regulated by proteases. In this review, we outline recent advances in our knowledge of specific proteolytic enzymes in two medically relevant apicomplexan parasites: Plasmodium falciparum and Toxoplasma gondii. Efforts over the last decade have begun to provide a map of key proteotolyic events that are essential for both parasite survival and propagation inside host cells. These advances in our molecular understanding of proteolytic events involved in parasite pathogenesis provide a foundation for the validation of new networks and enzyme targets that could be exploited for therapeutic purposes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Global identification of multiple substrates for Plasmodium falciparum SUB1, an essential malarial processing protease

The protozoan pathogen responsible for the most severe form of human malaria, Plasmodium falciparum, replicates asexually in erythrocytes within a membrane-bound parasitophorous vacuole (PV). Following each round of intracellular growth, the PV membrane (PVM) and host cell membrane rupture to release infectious merozoites in a protease-dependent process called egress. Previous work has shown that, just prior to egress, an essential, subtilisin-like parasite protease called PfSUB1 is discharged into the PV lumen, where it directly cleaves a number of important merozoite surface and PV proteins. These include the essential merozoite surface protein complex MSP1/6/7 and members of a family of papain-like putative proteases called SERA (serine-rich antigen) that are implicated in egress. To determine whether PfSUB1 has additional, previously unrecognized substrates, we have performed a bioinformatic and proteomic analysis of the entire late asexual blood stage proteome of the parasite. Our results demonstrate that PfSUB1 is responsible for the proteolytic processing of a range of merozoite, PV, and PVM proteins, including the rhoptry protein RAP1 (rhoptry-associated protein 1) and the merozoite surface protein MSRP2 (MSP7-related protein-2). Our findings imply multiple roles for PfSUB1 in the parasite life cycle, further supporting the case for considering the protease as a potential new antimalarial drug target.

Erythrocyte invasion and functionally inhibitory antibodies in Plasmodium falciparum malaria

Malaria is a disease that kills several million people every year. P. falciparum merozoites invade new erythrocytes every 48 h, causing fever, anemia and cerebral malaria. Effective immunity against malaria develops slowly and only after repeated exposure. Antibodies are an important part of this immunity. However, the antigens that mediate immunity by inducing functionally imperative antibodies have not yet been identified. This review gives an overview of the erythrocyte invasion process, which has been described to include several different antigens. Invasion inhibitory antibodies can inhibit merozoite penetration of new erythrocytes, and different methods for measurement of the presence of functionally important antibodies have been employed. ELISA, Invasion inhibition assays and ADCI are some of the methods discussed.

Evidences of protection against blood-stage infection of Plasmodium falciparum by the novel protein vaccine SE36

An effective malaria vaccine is a public health priority. Proteins expressed during the blood-stage of the parasite life cycle have been proposed as good vaccine candidates. No such blood-stage vaccine, however, is available against Plasmodium falciparum, the deadliest Plasmodium species. We show here that P. falciparum serine repeat antigen 5 (SERA5) is a potential vaccine immunogen. We have constructed a new recombinant molecule of SERA5, namely SE36, based on previously reported SE47' molecule by removing the serine repeats. Epidemiological study in the holo-endemic population of Solomon Islands shows highly significant correlation of sero-conversion and malaria protective immunity against this antigen. Animal experiments using non-human primates, and a human phase 1a clinical trial assessed SE36 vaccine immunogenicity. Vaccination of squirrel monkeys with SE36 protein and aluminum hydroxyl gel (SE36/AHG) conferred protection against high parasitemia and boosted serum anti-SE36 IgG after P. falciparum parasite challenge. SE36/AHG was highly immunogenic in chimpanzees, where serum anti-SE36 IgG titers last more than one year. Phase 1a clinical trial (current controlled trials, ISRCTN78679862) demonstrated the safety and immunogenicity of SE36/AHG with 30 healthy adults and 10 placebo controls. Three subcutaneous administrations of 50 and 100microg dose of SE36/AHG were well-tolerated, with no severe adverse events; and resulted in 100% sero-conversion in both dose arms. The current research results for SE36/AHG provide initial clinical validation for future trials and suggest clues/strategies for further vaccine development.

The Plasmodium serine-type SERA proteases display distinct expression patterns and non-essential in vivo roles during life cycle progression of the malaria parasite

Parasite proteases play key roles in several fundamental steps of the Plasmodium life cycle, including haemoglobin degradation, host cell invasion and parasite egress. Plasmodium exit from infected host cells appears to be mediated by a class of papain-like cysteine proteases called ‘serine repeat antigens’ (SERAs). A SERA subfamily, represented by Plasmodium falciparum SERA5, contains an atypical active site serine residue instead of a catalytic cysteine. Members of this SERAser subfamily are abundantly expressed in asexual blood stages, rendering them attractive drug and vaccine targets. In this study, we show by antibody localization and in vivo fluorescent tagging with the red fluorescent protein mCherry that the two P. berghei serine-type family members, PbSERA1 and PbSERA2, display differential expression towards the final stages of merozoite formation. Via targeted gene replacement, we generated single and double gene knockouts of the P. berghei SERAser genes. These loss-of-function lines progressed normally through the parasite life cycle, suggesting a specialized, non-vital role for serine-type SERAs in vivo. Parasites lacking PbSERAser showed increased expression of the cysteine-type PbSERA3. Compensatory mechanisms between distinct SERA subfamilies may thus explain the absence of phenotypical defect in SERAser disruptants, and challenge the suitability to develop potent antimalarial drugs based on specific inhibitors of Plasmodium serine-type SERAs.

Post-translational modifications in Plasmodium: more than you think!

Recent evidences indicate that transcription in Plasmodium may be hard-wired and rigid, deviating from the classical model of transcriptional gene regulation. Thus, it is important that other regulatory pathways be investigated as a comprehensive effort to curb the deadly malarial parasite. Research in post-translational modifications in Plasmodium is an emerging field that may provide new venues for drug discovery and potential new insights into how parasitic protozoans regulate their life cycle. Here, we discuss the recent findings of post-translational modifications in Plasmodium.

Structural insights into the protease-like antigen Plasmodium falciparum SERA5 and its noncanonical active-site serine

The sera genes of the malaria-causing parasite Plasmodium encode a family of unique proteins that are maximally expressed at the time of egress of parasites from infected red blood cells. These multi-domain proteins are unique, containing a central papain-like cysteine-protease fragment enclosed between the disulfide-linked N- and C-terminal domains. However, the central fragment of several members of this family, including serine repeat antigen 5 (SERA5), contains a serine (S596) in place of the active-site cysteine. Here we report the crystal structure of the central protease-like domain of Plasmodium falciparum SERA5, revealing a number of anomalies in addition to the putative nucleophilic serine: (1) the structure of the putative active site is not conducive to binding substrate in the canonical cysteine-protease manner; (2) the side chain of D594 restricts access of substrate to the putative active site; and (3) the S(2) specificity pocket is occupied by the side chain of Y735, reducing this site to a small depression on the protein surface. Attempts to determine the structure in complex with known inhibitors were not successful. Thus, despite having revealed its structure, the function of the catalytic domain of SERA5 remains an enigma.

A multifunctional serine protease primes the malaria parasite for red blood cell invasion

The malaria parasite Plasmodium falciparum replicates within an intraerythrocytic parasitophorous vacuole (PV). Rupture of the host cell allows release (egress) of daughter merozoites, which invade fresh erythrocytes. We previously showed that a subtilisin-like protease called PfSUB1 regulates egress by being discharged into the PV in the final stages of merozoite development to proteolytically modify the SERA family of papain-like proteins. Here, we report that PfSUB1 has a further role in ‘priming' the merozoite prior to invasion. The major protein complex on the merozoite surface comprises three proteins called merozoite surface protein 1 (MSP1), MSP6 and MSP7. We show that just before egress, all undergo proteolytic maturation by PfSUB1. Inhibition of PfSUB1 activity results in the accumulation of unprocessed MSPs on the merozoite surface, and erythrocyte invasion is significantly reduced. We propose that PfSUB1 is a multifunctional processing protease with an essential role in both egress of the malaria merozoite and remodelling of its surface in preparation for erythrocyte invasion.

Inhibition of malaria parasite development by a cyclic peptide that targets the vital parasite protein SERA5

The serine repeat antigen (SERA) proteins of the malaria parasites Plasmodium spp. contain a putative enzyme domain similar to that of papain family cysteine proteases. In Plasmodium falciparum parasites, more than half of the SERA family proteins, including the most abundantly expressed form, SERA5, have a cysteine-to-serine substitution within the putative catalytic triad of the active site. Although SERA5 is required for blood-stage parasite survival, the occurrence of a noncanonical catalytic triad casts doubt on the importance of the enzyme domain in this function. We used phage display to identify a small (14-residue) disulfide-bonded cyclic peptide (SBP1) that targets the enzyme domain of SERA5. Biochemical characterization of the interaction shows that it is dependent on the conformation of both the peptide and protein. Addition of this peptide to parasite cultures compromised development of late-stage parasites compared to that of control parasites or those incubated with equivalent amounts of the carboxymethylated peptide. This effect was similar in two different strains of P. falciparum as well as in a transgenic strain where the gene encoding the related serine-type parasitophorous vacuole protein SERA4 was deleted. In compromised parasites, the SBP1 peptide crosses both the erythrocyte and parasitophorous vacuole membranes and accumulates within the parasitophorous vacuole. In addition, both SBP1 and SERA5 were identified in the parasite cytosol, indicating that the plasma membrane of the parasite was compromised as a result of SBP1 treatment. These data implicate an important role for SERA5 in the regulation of the intraerythrocytic development of late-stage parasites and as a target for drug development.

Malarial proteases and host cell egress: an 'emerging' cascade

Malaria is a scourge of large swathes of the globe, stressing the need for a continuing effort to better understand the biology of its aetiological agent. Like all pathogens of the phylum Apicomplexa, the malaria parasite spends part of its life inside a host cell or cyst. It eventually needs to escape (egress) from this protective environment to progress through its life cycle. Egress of Plasmodium blood-stage merozoites, liver-stage merozoites and mosquito midgut sporozoites relies on protease activity, so the enzymes involved have potential as antimalarial drug targets. This review examines the role of parasite proteases in egress, in the light of current knowledge of the mechanics of the process. Proteases implicated in egress include the cytoskeleton-degrading malarial proteases falcipain-2 and plasmepsin II, plus a family of putative papain-like proteases called SERA. Recent revelations have shown that activation of the SERA proteases may be triggered by regulated secretion of a subtilisin-like serine protease called SUB1. These findings are discussed in the context of the potential for development of new chemotherapeutics targeting this stage in the parasite's life cycle.

Plasmodium falciparum malaria vaccines in development

The development and implementation of a malaria vaccine would constitute a major breakthrough for global health. Recently, numerous new candidates have entered clinical testing, following strategies that are as diverse as the malaria cycle is complex. While promising results have been obtained, some candidate vaccines have not fulfilled expectations. The challenges are not merely scientific; further progresses will require the development of competent investigator networks, partnerships between academics, industry and funding agencies, and continuous political commitment. In this review, we present the developmental status of all malaria vaccine candidates that are currently in human clinical testing against Plasmodium falciparum, as well as selected malaria vaccine candidates at preclinical development stage, and discuss the main challenges facing the field of malaria vaccine development.

Subcellular discharge of a serine protease mediates release of invasive malaria parasites from host erythrocytes

The most virulent form of malaria is caused by waves of replication of blood stages of the protozoan pathogen Plasmodium falciparum. The parasite divides within an intraerythrocytic parasitophorous vacuole until rupture of the vacuole and host-cell membranes releases merozoites that invade fresh erythrocytes to repeat the cycle. Despite the importance of merozoite egress for disease progression, none of the molecular factors involved are known. We report that, just prior to egress, an essential serine protease called PfSUB1 is discharged from previously unrecognized parasite organelles (termed exonemes) into the parasitophorous vacuole space. There, PfSUB1 mediates the proteolytic maturation of at least two essential members of another enzyme family called SERA. Pharmacological blockade of PfSUB1 inhibits egress and ablates the invasive capacity of released merozoites. Our findings reveal the presence in the malarial parasitophorous vacuole of a regulated, PfSUB1-mediated proteolytic processing event required for release of viable parasites from the host erythrocyte.

Identification of proteases that regulate erythrocyte rupture by the malaria parasite Plasmodium falciparum

Newly replicated Plasmodium falciparum parasites escape from host erythrocytes through a tightly regulated process that is mediated by multiple classes of proteolytic enzymes. However, the identification of specific proteases has been challenging. We describe here a forward chemical genetic screen using a highly focused library of more than 1,200 covalent serine and cysteine protease inhibitors to identify compounds that block host cell rupture by P. falciparum. Using hits from the library screen, we identified the subtilisin-family serine protease PfSU B1 and the cysteine protease dipeptidyl peptidase 3 (DPAP3) as primary regulators of this process. Inhibition of both DPAP3 and PfSUB1 caused a block in proteolytic processing of the serine repeat antigen (SERA) protein SERA5 that correlated with the observed block in rupture. Furthermore, DPAP3 inhibition reduced the levels of mature PfSUB1. These results suggest that two mechanistically distinct proteases function to regulate processing of downstream substrates required for efficient release of parasites from host red blood cells.

Roles of proteases during invasion and egress by Plasmodium and Toxoplasma

Apicomplexan pathogens replicate exclusively within the confines of a host cell. Entry into (invasion) and exit from (egress) these cells requires an array of specialized parasite molecules, many of which have long been considered to have potential as targets of drug or vaccine-based therapies. In this chapter the authors discuss the current state of knowledge regarding the role of parasite proteolytic enzymes in these critical steps in the life cycle of two clinically important apicomplexan genera, Plasmodium and Toxoplasma. At least three distinct proteases of the cysteine mechanistic class have been implicated in egress of the malaria parasite from cells of its vertebrate and insect host. In contrast, the bulk of the evidence indicates a prime role for serine proteases of the subtilisin and rhomboid families in invasion by both parasites. Whereas proteases involved in egress may function predominantly to degrade host cell structures, proteases involved in invasion probably act primarily as maturases and 'sheddases', required to activate and ultimately remove ligands involved in interactions with the host cell.

Post-immune UV irradiation induces Tr1-like regulatory T cells that suppress humoral immune responses

It is well documented that UV radiation present in sunlight suppresses immune responses, especially T(h)1-driven cellular immune responses, resulting in the exacerbation of skin cancer and infectious diseases. However, the effects of UV irradiation on humoral immune responses remain less clearly defined. In addition, the majority of studies documenting immunosuppressive effects of UV irradiation has been demonstrated in animals exposed to UV radiation before immunization. In the present study, therefore, we examined the effects of UV irradiation on humoral immune responses in mice that had been immunized before UV irradiation. Both T(h)1- and T(h)2-associated Ig responses were significantly suppressed by UV irradiation given 7 days after immunization in an antigen-specific manner. Adoptive transfer experiments revealed that CD4(+) T cells from UV-irradiated mice are responsible for the UV-induced suppression of antibody responses. These CD4(+) regulatory T cells suppressed proliferation of conventional CD4(+) T cells in vivo and in vitro and contained IL-10-producing cells that did not express Foxp3. Mice depleted of CD25(+) cells also exhibited reduced antibody responses by UV irradiation. Finally, we showed that CD4(+) T cells from UV-irradiated mice treated with anti-IL-10 mAb failed to suppress antibody responses upon transfer. These results indicate that UV irradiation after immunization suppresses T(h)1- and T(h)2-mediated humoral immunity via the generation of Tr1-like regulatory T cells, in the process of which IL-10 appears to be important. Possible detrimental effects of UV irradiation after vaccination are also discussed.

Evidence for a common role for the serine-type Plasmodium falciparum serine repeat antigen proteases: implications for vaccine and drug design

Serine repeat antigens (SERAs) are a family of secreted "cysteine-like" proteases of Plasmodium parasites. Several SERAs possess an atypical active-site serine residue in place of the canonical cysteine. The human malaria parasite Plasmodium falciparum possesses six "serine-type" (SERA1 to SERA5 and SERA9) and three "cysteine-type" (SERA6 to SERA8) SERAs. Here, we investigate the importance of the serine-type SERAs to blood-stage parasite development and examine the extent of functional redundancy among this group. We attempted to knock out the four P. falciparum serine-type SERA genes that have not been disrupted previously. SERA1, SERA4, and SERA9 knockout lines were generated, while only SERA5, the most strongly expressed member of the SERA family, remained refractory to genetic deletion. Interestingly, we discovered that while SERA4-null parasites completed the blood-stage cycle normally, they exhibited a twofold increase in the level of SERA5 mRNA. The inability to disrupt SERA5 and the apparent compensatory increase in SERA5 expression in response to the deletion of SERA4 provides evidence for an important blood-stage function for the serine-type SERAs and supports the notion of functional redundancy among this group. Such redundancy is consistent with our phylogenetic analysis, which reveals a monophyletic grouping of the serine-type SERAs across the genus Plasmodium and a predominance of postspeciation expansion. While SERA5 is to some extent further validated as a target for vaccine and drug development, our data suggest that the expression level of other serine-type SERAs is the only barrier to escape from anti-SERA5-specific interventions.

Phylogeny and evolution of the SERA multigene family in the genus Plasmodium

The serine repeat antigen gene family of Plasmodium falciparum (Pf-SERA) consists of nine gene members. By sequence similarity search, 45 genes were identified to be homologous to the Pf-SERA genes in the ongoing seven Plasmodium genome sequencing project databases for the species: P. reichenowi, P. vivax, P. knowlesi, P. yoelii, P. berghei, P. chabaudi, and P. gallinaceum. In combination with additional PCR-based sequencing, we found that almost all SERA genes in each species were aligned in a tandem cluster and sandwiched between two conserved hypothetical protein genes, except for P. reichenowi, which could not be confirmed. The minimum and maximum numbers of clustered genes were 2 and 12 for P. gallinaceum and P. vivax, respectively. The best tree of the maximum likelihood analysis demonstrated that all Plasmodium SERA homologues, except for SERA1 of P. gallinaceum (Pg-SERA1), can be classified into four groups, represented by Pf-SERA5, Pf-SERA6, Pf-SERA7, and Pf-SERA8. Genes in the Pf-SERA8 group, although highly divergent and distantly related to the sequences of other groups, were not pseudogenes. P. berghei SERA5, the counterpart of Pf-SERA8, was expressed in the mosquito stage. P. gallinaceum lacks the orthologues to Pf-SERA5, Pf-SERA6, and Pf-SERA7, suggesting that P. gallinaceum diverged from a common ancestor of all eight Plasmodium species examined before gene duplication(s) occurred to generate these paralogous groups. Here, we reveal an evolutionary trail of SERA gene cluster in the genus Plasmodium and discuss a phylogeny of Plasmodium species from the viewpoint of the evolution of a multigene family.

Production of high-affinity human monoclonal antibody fab fragments to the 19-kilodalton C-terminal merozoite surface protein 1 of Plasmodium falciparum

A combinatorial immunoglobulin gene library was constructed from peripheral blood lymphocytes of eight patients infected with Plasmodium falciparum and was screened for the production of human monoclonal antibody Fab fragments to the C-terminal 19-kDa fragment of P. falciparum merozoite surface protein 1 (MSP-1(19)). Three Fab clones recognized recombinant MSP-1(19) under nonreducing conditions. Indirect immunofluorescence microscopy demonstrated that three Fab clones stained the surfaces of late trophozoites/schizonts and merozoites of the FCR3 and 3D7 strains, suggesting the Fabs' reactivities to a conserved epitope. Sequence analysis of the heavy-chain genes revealed that the closest germ line V segments were VH1-8 and VH7-81, with 91% to 98% homology. The closest germ line D segment was D3-10, and the closest germ line J segment was JH4 or JH5, with 90% to 97% homology. In the light-chain genes, the closest germ line V segment was A27 for the Jkappa2, Jkappa4, and Jkappa5 segments. The dissociation constants of these Fab fragments for recombinant MSP-1(19) ranged from 1.09 x 10(-9) to 2.66 x 10(-9) M. The binding of the three Fab fragments to MSP-1(19) was competitively inhibited by the anti-MSP-1(19) mouse monoclonal antibody 12.8, which inhibits erythrocyte invasion by merozoites. However, the human Fab fragment with the highest affinity did not inhibit in vitro growth of P. falciparum. This is the first report of gene analysis and bacterial expression of human monoclonal antibodies to P. falciparum MSP-1(19). The combinatorial immunoglobulin gene library derived from malaria patients provides a potential tool for producing high-affinity human antibodies specific for P. falciparum.

A review of human vaccine research and development: malaria

The last several years have seen significant progress in the development of vaccines against malaria. Most recently, proof-of-concept of vaccine-induced protection from malaria infection and disease was demonstrated in African children. Pursued by various groups and on many fronts, several other candidate vaccines are in early clinical trials. Yet, despite the optimism and promise, an effective malaria vaccine is not yet available, in part because of the lack of understanding of the types of immune responses needed for protection, added to the difficulty of identifying, selecting and producing the appropriate protective antigens from a parasite with a genome of well over five thousand genes and to the frequent need to enhance the immunogenicity of purified antigens through the use of novel adjuvants or delivery systems. Insufficient clinical trial capacity and normative research functions such as local ethical committee reviews also contribute to slow down the development process. This article attempts to summarize the state of the art of malaria vaccine development.

Apoptosis-like death as a feature of malaria infection in mosquitoes

Malaria parasites of the genusPlasmodiummake a hazardous journey through their mosquito vectors. The majority die in the process, many as a result of the action of mosquito defence mechanisms. The mosquito too is not unscathed by the encounter with these parasites. Tissue damage occurs as a result of mid-gut invasion and reproductive fitness is lost when many developing ovarian follicles are resorbed. Here we discuss some of the mechanisms that are involved in killing the parasite and in the self-defence mechanisms employed by the mosquito to repair the mid-gut epithelium and to manipulate resources altering the trade-off position that balances reproduction and survival. In all cases, cells die by apoptotic-like mechanisms. In the midgut cells, apoptosis-induction pathways are being elucidated, the molecules involved in apoptosis are being recognised andDrosophilahomologues sought. The death of ookinetes in the mosquito mid-gut lumen is associated with caspase-like activity and, although homologues of mammalian caspases are not present in the malaria genome, other cysteine proteases that are potential candidates have been discussed. In the ovary, apoptosis of patches of follicular epithelial cells is followed by resorption of the developing follicle and a subsequent loss of egg production in that follicle.

Plasmodium falciparum serine-repeat antigen (SERA) forms a homodimer through disulfide bond

Plasmodium falciparum serine-repeat antigen (SERA) is one potential blood-stage vaccine candidate and is expressed as a protomer that is subsequently processed into four fragments (P47, P50, P6, and P17). Although recent evidence shows that P50 exhibits chymotrypsin-like protease activity, the function of SERA is still largely unknown. Here, we found that apart from cathepsin L-like cysteine protease, P50 showed significant homology to silicatein-alpha and testin which were shown to bind to cellular components, suggesting that SERA may have similar function. Immunoprecipitation of schizont lysate and molecular assignment of its precipitate by mass spectrometry provided evidence that SERA forms a homodimer through disulfide bond. Moreover, analysis of the fate of SERA using cell-free system revealed that the kinetics of conversion of SERA dimer into monomer is faster than that of processing of SERA monomer into various fragments. These findings may contribute to elucidate a possible function of SERA other than a protease.

Apical expression of three RhopH1/Clag proteins as components of the Plasmodium falciparum RhopH complex

The Plasmodium falciparum high molecular mass rhoptry protein ('PfRhopH') complex is important for parasite growth and comprises three distinct gene products: RhopH1, RhopH2 and RhopH3. We have previously shown that P. falciparum RhopH1 is encoded by either PFC0110w (clag3.2) or PFC0120w (clag3.1), members of the previously-named clag (cytoadherence-linked asexual gene) multigene family. In this report, we have further characterized rhoph1/clag members in terms of gene structure, transcription and protein expression. The cDNA sequences for all five rhoph1/clag members were determined, confirming previous in silico predictions of intron-exon boundaries. All member genes were transcribed in HB3 and 3D7 parasite lines, but clag3.2 was not transcribed in Dd2 parasites. The peak abundance of transcripts for all genes was observed during the late schizont stage. Antisera specific to Clag2 and Clag3.1 localized these proteins to the apical end of merozoites in segmented schizonts, and both proteins are found to be components of the PfRhopH complex. PfRhopH complex that was immunoprecipitated with anti-Clag9 antibody contained neither Clag2 nor Clag3.1, thereby suggesting that PfRhopH complexes contain only individual rhoph1/clag gene products. Since the PfRhopH complex binds the erythrocyte surface, and RhopH2 and RhopH3 are encoded by single copy genes, the RhopH1/Clag proteins may serve to confer some degree of specificity to the roles of the individual complexes.

Proteases in host cell invasion by the malaria parasite

The life cycle of the malaria parasite contains three distinct invasive forms, or zoites. For at least two of these--the sporozoite and the blood-stage merozoite--invasion into their respective host cell requires the activity of parasite proteases. This review summarizes the evidence for this, discusses selected well-described proteolytic modifications linked to invasion, and describes recent progress towards identifying the proteases involved.

Developmental-stage-specific triacylglycerol biosynthesis, degradation and trafficking as lipid bodies in Plasmodium falciparum-infected erythrocytes

Triacylglycerol (TAG) serves as a major energy storage molecule in eukaryotes. In Plasmodium, however, this established function of TAG appears unlikely, despite detecting previously considerable amount of TAG associated with intraerythrocytic parasites, because plasmodial cells have very little capacity to oxidize fatty acids. Thus, it is plausible that TAG and its biosynthesis in Plasmodium have other functions. As a first step in understanding the biological significance of TAG and its biosynthesis to the intraerythrocytic proliferation of Plasmodium falciparum, we performed detailed characterization of TAG metabolism and trafficking in parasitized erythrocyte. Metabolic labeling using radiolabeled-oleic and palmitic acids in association with serum albumin, which have been shown to be among the serum essential factors for intraerythrocytic proliferation of P. falciparum, revealed that accumulation of TAG was strikingly pronounced from trophozoite to schizont, whereas TAG degradation became active from schizont to segmented schizont; the consequent products, free fatty acids, were released into the medium during schizont rupture and/or merozoite release. These results were further supported by visualization of lipid bodies through immunofluorescence and electron microscopy. At the schizont stages, there is some evidence that the lipid bodies are partly localized in the parasitophorous vacuole. Interestingly, the discrete formation and/or trafficking of lipid bodies are inhibited by brefeldin A and trifluoperazine. Inhibition by trifluoperazine hints at least that a de novo TAG biosynthetic pathway via phosphatidic acid contributes to lipid body formation. Indeed, biochemical analysis reveals a higher activity of acyl-CoA:diacylglycerol acyltransferase, the principal enzyme in the sn-glycerol-3-phosphate pathway for TAG synthesis, at trophozoite and schizont stages. Together, these results establish that TAG metabolism and trafficking in P. falciparum-infected erythrocyte occurs in a stage-specific manner during the intraerythrocytic cycle and we propose that these unique and dynamic cellular events participate during schizont rupture and/or merozoite release.

Production of the subdomains of the Plasmodium falciparum apical membrane antigen 1 ectodomain and analysis of the immune response

The apical membrane antigen 1 of Plasmodium falciparum is one of the leading candidate antigens being developed as a vaccine to prevent malaria. This merozoite transmembrane protein has an ectodomain that can be divided into three subdomains (D I, D II, and D III). We have previously expressed a major portion of this ectodomain and have shown that it can induce antibodies that prevent merozoite invasion into red blood cells in an in vitro growth and invasion assay. To analyze the antibody responses directed against the individual subdomains, we constructed six different genes that express each of the domains separately (D I, D II, or D III) or in combination with another domain (D I+II, D II+III, or D I+III). These proteins were purified and used to immunize rabbits to raise construct-specific antibodies. We demonstrated that D I+II induced a significant amount of the growth-inhibitory antibodies active in the growth and invasion assay.

Enzymic, phylogenetic, and structural characterization of the unusual papain-like protease domain of Plasmodium falciparum SERA5

Serine repeat antigen 5 (SERA5) is an abundant antigen of the human malaria parasite Plasmodium falciparum and is the most strongly expressed member of the nine-gene SERA family. It appears to be essential for the maintenance of the erythrocytic cycle, unlike a number of other members of this family, and has been implicated in parasite egress and/or erythrocyte invasion. All SERA proteins possess a central domain that has homology to papain except in the case of SERA5 (and some other SERAs), where the active site cysteine has been replaced with a serine. To investigate if this domain retains catalytic activity, we expressed, purified, and refolded a recombinant form of the SERA5 enzyme domain. This protein possessed chymotrypsin-like proteolytic activity as it processed substrates downstream of aromatic residues, and its activity was reversed by the serine protease inhibitor 3,4-diisocoumarin. Although all Plasmodium SERA enzyme domain sequences share considerable homology, phylogenetic studies revealed two distinct clusters across the genus, separated according to whether they possess an active site serine or cysteine. All Plasmodia appear to have at least one member of each group. Consistent with separate biological roles for members of these two clusters, molecular modeling studies revealed that SERA5 and SERA6 enzyme domains have dramatically different surface properties, although both have a characteristic papain-like fold, catalytic cleft, and an appropriately positioned catalytic triad. This study provides impetus for the examination of SERA5 as a target for antimalarial drug design.