Effect of heat-shock on Plasmodium falciparum viability, growth and expression of the heat-shock protein 'PFHSP70-I' gene.

Cultures of the human malaria parasite Plasmodium falciparum were subjected to heat-shock for varying times and temperatures and then tested for their viability, growth and expression of heat-shock protein. Results show that the majority of parasites remained viable after heat-shock but their growth was affected. However, the expression of the heat-shock protein 'PFHSP70-I' gene was enhanced after heat-shock. We conclude that malarial parasites are able to survive in vivo during fever probably due to the overexpression of the heat-shock protein gene.     

Recurrent fever promotes Plasmodium falciparum development in human erythrocytes

The human malarial parasite Plasmodium falciparum (Pf) is exposed to wide temperature fluctuations during its life cycle, ranging from 25 degrees C in the mosquito vector and 37 degrees C in humans to 41 degrees C during febrile episodes in the patient. The repeated occurrence of fever at regular intervals is a characteristic of human malaria. We have examined the influence of repeated exposure to elevated temperatures encountered during fever on the intraerythrocytic development of the parasite. Using flow cytometry, we show that repeated exposure to temperatures mimicking febrile episodes promotes parasite development in human erythrocytes. Heat shock-mediated cytoprotection and growth promotion is dependent on the heat shock protein 90 (PfHsp90) multi-chaperone complex. Inhibition of PfHsp90 function using geldanamycin attenuates temperature-dependent progression from the ring to the trophozoite stage. Geldanamycin inhibits parasite development by disrupting the PfHsp90 complex consisting of PfHsp70, PfPP5, and tubulin, among other proteins. While explaining the contribution of febrile episodes to the pathogenesis of malaria, our results implicate temperature as an important environmental cue used by the parasite to coordinate its development in humans.     

Structure and possible function of heat-shock proteins in Falciparum malaria.

Like many prokaryotes and eukaryotes, the malaria parasite also synthesizes several stress proteins. Most widely studied stress proteins of this parasite are the heat-shock proteins (hsps). Their discovery in malaria is a gift of recombinant DNA technology. Five hsp genes from Plasmodium falciparum have been identified which are located on different chromosomes. Thus the inheritance and expression of hsp genes are independent of each other. They share a large amount of sequence homology at N-terminus with the hsps of other organisms. Their gene regulatory sequences and other elements, important for gene expression, are yet to be determined. The biological role of these proteins in malaria is not fully understood but it is possible that they provide protection to the parasite from various stresses encountered in the host. In this process hsps probably bind to the toxic molecules as well as damaged proteins to flush them out of the parasite. Their involvement in the stage-specific parasite transformation to increase the infectivity and virulence, as observed in other parasites, remains to be determined. Malarial hsps are antigenic in humans. This antigenicity could be attributed to the non-homologous sequences in the C-terminus region. The potential of one of them (pfhsp 70I) for a future malaria vaccine and immunodiagnostics requires re-evaluation of the data.     

Clinical and molecular aspects of malaria fever

Although clinically benign, malaria fever is thought to have significant relevance in terms of parasite growth and survival and its virulence which in turn may alter the clinical course of illness. In this article, the historical literature is reviewed, providing some evolutionary perspective on the genesis and biological relevance of malaria fever, and the available molecular data on the febrile-temperature-inducible parasite factors that may contribute towards the regulation of parasite density and alteration of virulence in the host is also discussed. The potential molecular mechanisms that could be responsible for the induction and regulation of cyclical malaria fevers caused by different species of Plasmodium are also discussed.     

Antibodies and reactive T cells against the malaria heat-shock protein Pf72/Hsp70-1 and derived peptides in individuals continuously exposed to Plasmodium falciparum.

Pf72/Hsp70-1, a heat-shock protein of m.w. 72 kDa from Plasmodium falciparum is one of the Ag of interest to be included in a polyvalent vaccine against malaria. It is one of the major immunogens present in a fraction of purified blood stage parasites that elicited protection against experimental infection of Saimiri monkeys with blood stages of P. falciparum. It is present at all blood stages and one of its B cell epitopes is also detected on the surface of the infected hepatocyte. Moreover, Pf72 appears to be well conserved among different isolates of P. falciparum. We have examined the immune response against Pf72/Hsp70-1 in individuals from different age groups living in a holoendemic area (West Africa). The immune response against the native Ag (purified from schizonts and called Pf/Hsp70) was analyzed both at the humoral level by ELISA and at the cellular level by assessing in vitro proliferation and IFN-gamma production of PBMC. Of the individuals studied 52% had a statistically significant level of anti-Pf/Hsp70 antibodies as compared with unexposed individuals. These positive individuals showed a heterogeneous distribution because significant levels of antibodies were found in 70% of the adults but in only 26% of the children. The presence of Pf/Hsp70-specific reactive T cells in the blood was detected in 32% of the individuals. The total anti-Pf/Hsp70 antibody level (IgG+IgM) appeared strongly age related and correlated positively with parasite exposure, whereas the T cell response failed to correlate either with the antibody level or with age. Moreover, PBMC of donors responded to the Pf/Hsp70 in a dissociated way, namely, by either T cell proliferation or IFN-gamma production. Ten synthetic peptides based on sequences found in the C-terminal part of Pf72/Hsp70-1 were further tested as potential T cell epitopes. The proliferative response of PBMC from individuals continuously exposed to the parasite showed that three peptides more frequently trigger significant T cell proliferation (in 21% to 27% of the individuals) and three others less frequently (10%). None of these peptides allowed detection of reactive T cells in PBMC of Europeans with no previous exposure to malaria. Some of the stimulating peptides are highly similar to human heat-shock Hsc and Hsp70 with large stretches of identical amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)     

Plasmodium falciparum encodes a single cytosolic type I Hsp40 that functionally interacts with Hsp70 and is upregulated by heat shock

Heat shock protein 70 (Hsp70) and heat shock protein 40 (Hsp40) function as molecular chaperones during the folding and trafficking of proteins within most cell types. However, the Hsp70-Hsp40 chaperone partnerships within the malaria parasite, Plasmodium falciparum, have not been elucidated. Only one of the 43 P. falciparum Hsp40s is predicted to be a cytosolic, canonical Hsp40 (termed PfHsp40) capable of interacting with the major cytosolic P. falciparum-encoded Hsp70, PfHsp70. Consistent with this hypothesis, we found that PfHsp40 is upregulated under heat shock conditions in a similar pattern to PfHsp70. In addition, PfHsp70 and PfHsp40 reside mainly in the parasite cytosol, as assessed using indirect immunofluorescence microscopy. Recombinant PfHsp40 stimulated the ATP hydrolytic rates of both PfHsp70 and human Hsp70 similar to other canonical Hsp40s of yeast (Ydj1) and human (Hdj2) origin. In contrast, the Hsp40-stimulated plasmodial and human Hsp70 ATPase activities were differentially inhibited in the presence of pyrimidinone-based small molecule modulators. To further probe the chaperone properties of PfHsp40, protein aggregation suppression assays were conducted. PfHsp40 alone suppressed protein aggregation, and cooperated with PfHsp70 to suppress aggregation. Together, these data represent the first cellular and biochemical evidence for a PfHsp70-PfHsp40 partnership in the malaria parasite, and furthermore that the plasmodial and human Hsp70-Hsp40 chaperones possess unique attributes that are differentially modulated by small molecules.     

Improved In Vitro Culture of Plasmodium falciparum Permits Establishment of Clinical Isolates with Preserved Multiplication,Invasion and Rosetting Phenotypes

To be able to robustly propagate P. falciparum at optimal conditions in vitro is of fundamental importance for genotypic and phenotypic studies of both established and fresh clinical isolates. Cryo-preserved P. falciparum isolates from Ugandan children with severe or uncomplicated malaria were investigated for parasite phenotypes under different in vitro growth conditions or studied directly from the peripheral blood. The parasite cultures showed a minimal loss of parasite-mass and preserved percentage of multiple infected pRBCs to that in peripheral blood, maintained adhesive phenotypes and good outgrowth and multiplication rates when grown in suspension and supplemented with gas. In contrast, abnormal and greatly fluctuating levels of multiple infections were observed during static growth conditions and outgrowth and multiplication rates were inferior. Serum, as compared to Albumax, was found necessary for optimal presentation of PfEMP1 at the pRBC surface and/or for binding of serum proteins (immunoglobulins). Optimal in vitro growth conditions of P. falciparum therefore include orbital shaking (50 rev/min), human serum (10%) and a fixed gas composition (5% O₂, 5% CO₂, 90% N₂). We subsequently established 100% of 76 frozen patient isolates and found rosetting with schizont pRBCs in every isolate (>26% schizont rosetting rate). Rosetting during schizogony was often followed by invasion of the bound RBC as seen by regular and time-lapse microscopy as well as transmission electron microscopy. The peripheral parasitemia, the level of rosetting and the rate of multiplication correlated positively to one another for individual isolates. Rosetting was also more frequent with trophozoite and schizont pRBCs of children with severe versus uncomplicated malaria (p<0.002; p<0.004). The associations suggest that rosetting enhances the ability of the parasite to multiply within the human host.     

Determination of the Plasmodium vivax schizont stage proteome

With the genome of the malaria parasite Plasmodium vivax sequenced, it is important to determine the proteomes of the parasite in order to assist efforts in antigen and drug target discovery. Since a method for continuous culture of P. vivax parasite is not available, we tried to study the proteome of the erythrocytic stages using fresh parasite isolates from patients. In schizont-enriched samples, 316 proteins were confidently identified by tandem mass spectrometry. Almost 50% of the identified proteins were hypothetical, while other major categories include proteins with binding function, protein fate, protein synthesis, metabolism and cellular transport. To identify proteins that are recognized by host humoral immunity, parasite proteins were separated by two-dimensional gel electrophoresis and screened by Western blot using an immune serum from a P. vivax patient. Mass spectrometry analysis of protein spots recognized by the serum identified four potential antigens including PV24. The recombinant protein PV24 was recognized by antibodies from vivax malaria patients even during the convalescent period, indicating that PV24 could elicit long-lasting antibody responses in P. vivax patients.     

15-deoxyspergualin primarily targets the trafficking of apicoplast proteins in Plasmodium falciparum

15-Deoxyspergualin, an immunosuppressant with tumoricidal and antimalarial properties, has been implicated in the inhibition of a diverse array of cellular processes including polyamine synthesis and protein synthesis. Endeavoring to identify the mechanism of antimalarial action of this molecule, we examined its effect on Plasmodium falciparum protein synthesis, polyamine biosynthesis, and transport. 15-Deoxyspergualin stalled protein synthesis in P. falciparum through Hsp70 sequestration and subsequent phosphorylation of the eukaryotic initiation factor eIF2alpha. However, protein synthesis inhibition as well as polyamine depletion were invoked only by high micromolar concentrations of 15-deoxyspergualin, in contrast to the submicromolar concentrations sufficient to inhibit parasite growth. Further investigations demonstrated that 15-deoxyspergualin in the malaria parasite primarily targets the hitherto underexplored process of trafficking of nucleus-encoded proteins to the apicoplast. Our finding that 15-deoxyspergualin kills the malaria parasite by interfering with targeting of nucleus-encoded proteins to the apicoplast not only exposes a chink in the armor of the malaria parasite, but also reveals new realms in our endeavors to study this intriguing biological process.     

Why is malaria fever periodic? A hypothesis

For poorly understood reasons, malaria parasites tend to develop in synchrony with each other in the asexual erythrocytic phase of infection, and this synchronization determines the periodic nature of malaria fever. There is evidence to suggest that fever might help to protect the host, while synchronization might provide counter-protection for the parasite. Dominic Kwiatkowski and Brian Greenwood propose that malaria fever may be of mutual benefit for parasite and host.     

Characterisation of the <Emphasis Type="Italic">Plasmodium falciparum</Emphasis> Hsp70–Hsp90 organising protein (PfHop)

Malaria is caused by Plasmodium species, whose transmission to vertebrate hosts is facilitated by mosquito vectors. The transition from the cold blooded mosquito vector to the host represents physiological stress to the parasite, and additionally malaria blood stage infection is characterised by intense fever periods. In recent years, it has become clear that heat shock proteins play an essential role during the parasite's life cycle. Plasmodium falciparum expresses two prominent heat shock proteins: heat shock protein 70 (PfHsp70) and heat shock protein 90 (PfHsp90). Both of these proteins have been implicated in the development and pathogenesis of malaria. In eukaryotes, Hsp70 and Hsp90 proteins are functionally linked by an essential adaptor protein known as the Hsp70–Hsp90 organising protein (Hop). In this study, recombinant P. falciparum Hop (PfHop) was heterologously produced in E. coli and purified by nickel affinity chromatography. Using specific anti-PfHop antisera, the expression and localisation of PfHop in P. falciparum was investigated. PfHop was shown to co-localise with PfHsp70 and PfHsp90 in parasites at the trophozoite stage. Gel filtration and co-immunoprecipitation experiments suggested that PfHop was present in a complex together with PfHsp70 and PfHsp90. The association of PfHop with both PfHsp70 and PfHsp90 suggests that this protein may mediate the functional interaction between the two chaperones.     

Differential expression of heat-shock proteins and spontaneous synthesis of HSP70 during the life cycle of Blastocladiella emersonii

The heat-shock response in Blastocladiella emersonii is dependent on the developmental stage. Cells exposed to elevated temperatures at different stages of the life cycle (sporulation, germination or growth) show a differential synthesis of heat-shock proteins (hsps). Of a total of 22 polypeptides induced, particular subsets of hsps appear in each phase, demonstrating a non-coordinate heat-shock gene expression. In contrast, heat-shock-related proteins (hsp76, hsp70, hsp39a) are spontaneously expressed at a high level during sporulation. By the criteria of two-dimensional gel electrophoresis and partial proteolysis mapping, the 70,000-Da protein, whose synthesis is induced spontaneously during sporulation, is indistinguishable from the heat-inducible hsp70. The techniques of in vitro translation, and Northern analysis using a Drosophila hsp70 probe, demonstrated that enhanced synthesis of hsp70, which occurs during heat-shock treatment and spontaneously during sporulation, is associated with an accumulation of hsp70 mRNA. These observations suggest that hsp70 gene expression is induced during sporulation.     

Structural and antigenic polymorphism of the 35- to 48-kilodalton merozoite surface antigen (MSA-2) of the malaria parasite Plasmodium falciparum.

Merozoite surface antigen MSA-2 of the human parasite Plasmodium falciparum is being considered for the development of a malaria vaccine. The antigen is polymorphic, and specific monoclonal antibodies differentiate five serological variants of MSA-2 among 25 parasite isolates. The variants are grouped into two major serogroups, A and B. Genes encoding two different variants from serogroup A have been sequenced, and their DNA together with deduced amino acid sequences were compared with sequences encoded by other alleles. The comparison shows that the serological classification reflects differences in DNA sequences and deduced primary structure of MSA-2 variants and serogroups. Thus, the overall homologies of DNA and amino acid sequences are over 95% among variants in the same serogroup. In contrast, similarities between the group A variants and a group B variant are only 70 and 64% for DNA and amino acid sequences, respectively. We propose that the MSA-2 protein is encoded by two highly divergent groups of alleles, with limited additional polymorphism displayed within each group.     

Identification and characterisation of a Leishmania donovani antigen belonging to the 70-kDa heat-shock protein family

A Leishmania donovani promastigote cDNA library was screened with serum obtained from a patient infected with visceral leishmaniasis. Sequence analysis of a clone obtained from this library revealed that the 600-bp insert corresponded to the carboxy-terminal region of an antigen related to the 70-kDa heat-shock protein family. The full-length sequence of the corresponding gene (1959 nucleotides) was determined after isolation of genomic clones. Genes encoding the antigen are present on a single chromosome as a series of approximately twelve 3.7-kb direct tandem repeats. The antigen can be identified as a 70-kDa heat-shock cognate protein by virtue of its molecular mass, sequence and constitutive expression during heat shock. It is expressed at all stages of the parasite life-cycle. Antibodies against the lambda gt11 fusion protein were detected in more than 50% of serum samples obtained from patients with visceral leishmaniasis, but were not detected in sera from patients with cutaneous leishmaniasis or Chagas' disease.     

Characterization of Precursor PfHsp60 in Plasmodium falciparum Cytosol during Its Asexual Development in Human Erythrocytes

Mitochondrial heat shock protein 60 (Hsp60) is a nuclear encoded gene product that gets post-translationally translocated into the mitochondria. Using multiple approaches such as immunofluorescence experiments, isoelectric point analysis with two-dimensional gel electrophoresis, and mass spectrometric identification of the signal peptide, we show that Hsp60 from Plasmodium falciparum (PfHsp60) accumulates in the parasite cytoplasm during the ring, trophozoite, and schizont stages of parasite development before being imported into the parasite mitochondria. Using co-immunoprecipitation experiments with antibodies specific to cytoplasmic PfHsp90, PfHsp70-1, and PfHsp60, we show association of precursor PfHsp60 with cytoplasmic chaperone machinery. Metabolic labeling involving pulse and chase indicates translocation of the precursor pool into the parasite mitochondrion during chase. Analysis of results obtained with Geldanamycin treatment confirmed precursor PfHsp60 to be one of the clients for PfHsp90. Cytosolic chaperones bind precursor PfHsp60 prior to its import into the mitochondrion of the parasite. Our data suggests an inefficient co-ordination in the synthesis and translocation of mitochondrial PfHsp60 during asexual growth of malaria parasite in human erythrocytes.     

Plasmodial surface anion channel-independent phloridzin resistance in Plasmodium falciparum

The plasmodial surface anion channel (PSAC) is an unusual ion channel induced on the human red blood cell membrane after infection with the malaria parasite, Plasmodium falciparum. Because PSAC is permeant to small metabolic precursors essential for parasite growth and is present on red blood cells infected with geographically divergent parasite isolates, it may be an ideal target for future antimalarial development. Here, we used chemically induced mutagenesis and known PSAC antagonists that inhibit in vitro parasite growth to examine whether resistance mutations in PSAC can be readily induced. Stable mutants resistant to phloridzin were generated and selected within 3 weeks after treatment with 1-methyl-3-nitro-1-nitrosoguanidine. These mutants were evaluated with osmotic lysis and electrophysiological transport assays, which indicate that PSAC inhibition by phloridzin is complex with at least two different modes of inhibition. Mutants resistant to the growth inhibitory effects of phloridzin expressed PSAC activity indistinguishable from that on sensitive parasites, indicating selection of resistance via mutations in one or more other parasite targets. Failure to induce mutations in PSAC activity is consistent with a highly constrained channel protein less susceptible to resistance mutations; whether this protein is parasite- or host-encoded remains to be determined.