Cytoadherence and the Plasmodium falciparum-infected erythrocyte

Functional analysis of the exported type IV HSP40 protein PfGECO in Plasmodium falciparum gametocytes

During Plasmodium falciparum infection, host red blood cell (RBC) remodeling is required for the parasite's survival. Such modifications are mediated by the export of parasite proteins into the RBC that alter the architecture of the RBC membrane and enable cytoadherence. It is probable that some exported proteins also play a protective role against the host defense response. This may be of particular importance for the gametocyte stage of the life cycle that is responsible for malaria transmission, since the gametocyte remains in contact with blood as it proceeds through five morphological stages (I to V) during its 12-day maturation. Using microarray analysis, we identified several genes with encoded secretory or export sequences that were differentially expressed during early gametocytogenesis. One of these, PfGECO, encodes a predicted type IV heat shock protein 40 (HSP40) that we show is expressed in gametocyte stages I to IV and is exported to the RBC cytoplasm. HSPs are traditionally induced under stressful conditions to maintain homeostasis, but PfGECO expression was not increased upon heat shock, suggesting an alternate function. Targeted disruption of PfGECO indicated that the gene is not essential for gametocytogenesis in vitro, and quantitative reverse transcriptase PCR (RT-PCR) showed that there was no compensatory expression of the other type IV HSP40 genes. Although P. falciparum HSP40 members are implicated in the trafficking of proteins to the RBC surface, removal of PfGECO did not affect the targeting of other exported gametocyte proteins. This work has expanded the repertoire of known gametocyte-exported proteins to include a type IV HSP40, PfGECO.

Malaria parasite proteins that remodel the host erythrocyte

Exported proteins of the malaria parasite Plasmodium falciparum interact with proteins of the erythrocyte membrane and induce substantial changes in the morphology, physiology and function of the host cell. These changes underlie the pathology that is responsible for the deaths of 1-2 million children every year due to malaria infections. The advent of molecular transfection technology, including the ability to generate deletion mutants and to introduce fluorescent reporter proteins that track the locations and dynamics of parasite proteins, has increased our understanding of the processes and machinery for export of proteins in P. falciparum-infected erythrocytes and has provided us with insights into the functions of the parasite protein exportome. We review these developments, focusing on parasite proteins that interact with the erythrocyte membrane skeleton or that promote delivery of the major virulence protein, PfEMP1, to the erythrocyte membrane.

Host chaperones are recruited in membrane-bound complexes by Plasmodium falciparum

The ability of malarial parasite to deploy proteins at the surface of infected erythrocytes is well known. After their synthesis within the parasite, the cargo proteins are exported from the parasite and carried across the erythrocyte cytoplasm to be delivered at the erythrocyte surface. Our knowledge about the mechanisms involved in this complex trafficking path is limited. We have addressed the involvement of chaperones in traffic across erythrocyte cytoplasm. Our analyses of the chaperones available to the parasite indicated that none of the reported chaperones of the parasite origin are present in the erythrocyte cytoplasm. The chaperones of the host (Hsp70, Hsp90, Hop60), on the other hand, were readily detected in the erythrocyte cytosol. Hypotonic lysis and detergent solubilization experiments indicated that unlike their soluble nature in normal erythrocytes, host chaperones are recruited in membrane-bound, detergent-resistant complexes in infected cells. The association of host-Hsp70 with detergent-resistant complexes was ATP-dependent. Importantly, host chaperones could be detected in knob-enriched fractions and could be cross-linked to the knob subunit, PfHRP1, in a large complex at the surface of the infected erythrocytes. Our results implicate host chaperones in the assembly of parasite proteins such as knob subunits at the erythrocyte surface.

A high capacity in vitro assay for measuring the cytoadherence of Plasmodium falciparum-infected erythrocytes

A simple, efficient, sensitive, reproducible and high throughput assay for measuring the cytoadhesion of Plasmodium falciparum-infected red blood cells (human malaria) is described. The assay format uses 96-well microplates, with the number of P. falciparum parasitized erythrocytes bound determined by measuring Plasmodium specific lactic dehydrogenase activity colorimetrically (absorbance at 655 nm) using the 3-acetylpyridine analog of nicotinamide adenine dinucleotide, nitro blue tetrazolium and diaphorase. The results of the described microplate assay were found to be comparable to those using microscopic analysis but much less time consuming.