Host receptors in malaria merozoite invasion

Human erythrocyte band 3 functions as a receptor for the sialic acid-independent invasion of Plasmodium falciparum. Role of the RhopH3-MSP1 complex

Plasmodium falciparum takes advantage of two broadly defined alternate invasion pathways when infecting human erythrocytes: one that depends on and the other that is independent of host sialic acid residues on the erythrocyte surface. Within the sialic acid-dependent (SAD) and sialic acid-independent (SAID) invasion pathways, several alternate host receptors are used by P. falciparum based on its particular invasion phenotype. Earlier, we reported that two putative extracellular regions of human erythrocyte band 3 termed 5C and 6A function as host invasion receptor segments binding parasite proteins MSP1 and MSP9 via a SAID mechanism. In this study, we developed two mono-specific anti-peptide chicken IgY antibodies to demonstrate that the 5C and 6A regions of band 3 are exposed on the surface of human erythrocytes. These antibodies inhibited erythrocyte invasion by the P. falciparum 3D7 and 7G8 strains (SAID invasion phenotype), and the blocking effect was enhanced in sialic acid-depleted erythrocytes. In contrast, the IgY antibodies had only a marginal inhibitory effect on FCR3 and Dd2 strains (SAD invasion phenotype). A direct biochemical interaction between erythrocyte band 3 epitopes and parasite RhopH3, identified by the yeast two-hybrid screen, was established. RhopH3 formed a complex with MSP119 and the 5ABC region of band 3, and a recombinant segment of RhopH3 inhibited parasite invasion in human erythrocytes. Together, these findings provide evidence that erythrocyte band 3 functions as a major host invasion receptor in the SAID invasion pathway by assembling a multi-protein complex composed of parasite ligands RhopH3 and MSP1.

Uncertainty in mapping malaria epidemiology: implications for control

Malaria is a location-specific, dynamic infectious disease transmitted by mosquitoes to humans and is influenced by environmental, vector, parasite, and host factors. The principal purposes of malarial epidemiology are 1) to describe the malarial distribution in space and time along with the physical, biologic, and social etiologic factors and 2) to guide control objectives for either modeling impact or measuring progress of control tactics. Mapping malaria and many of its causative factors has been achieved on many different levels from global distribution to biologic quantitative trait localization in humans, parasites, and mosquitoes. Despite these important achievements, a large degree of uncertainty still exists on the annual burden of malarial cases. Accurate, sensitive detection and treatment of asymptomatic reservoirs important to infectious transmission are additional components necessary for future control measures. Presently spurred by the leadership and funding of Bill and Melinda Gates, the malarial community is developing and implementing plans for elimination of malaria. The challenge for malariologists is to digitally integrate and map epidemiologic factors and intervention measures in space and time to target effective, sustainable control alongside research efforts.

Receptor-based identification of an inhibitory peptide against blood stage malaria

Plasmodium falciparum uses multiple host receptors to attach and invade human erythrocytes. Glycophorins have been implicated as receptors for parasite invasion in human erythrocytes. Here, we screened a phage display cDNA library of P. falciparum (FCR3, a sialic acid-dependent strain) using purified glycophorins and erythrocytes as bait. Several phage clones were identified that bound to immobilized glycophorins and contained the same 74 bp insert encoding the 7-amino acids sequence ETTLKSF. A similar screen using intact human erythrocytes in solution identified additional phage clones containing the same 7-amino acids sequence. Using ELISA and immunofluorescence, direct binding of ETTLKSF peptide to glycophorins and erythrocytes was confirmed. Pull-down and protease treatment assays suggest that ETTLKSF peptide specifically interacts with glycophorin C. The synthetic ETTLKSF peptide partially blocks merozoite invasion in human erythrocytes. Further characterization of ETTLKSF peptide could lead to the development of a novel class of inhibitors against the blood stage malaria.

A Presenilin-like protease associated with Plasmodium falciparum micronemes is involved in erythrocyte invasion

We describe identification of a Plasmodium falciparum microneme protease involved in RBC invasion. From the yeast two-hybrid screening of a P. falciparum cDNA library, we have identified a 47 kDa membrane protein that interacted with the 5ABC domain of human RBC band 3. This protein shared homology with a Presenilin-type aspartyl protease, the signal peptide peptidase (SPP). An antibody raised against a predicted exposed region of this protein reacted specifically to a single band of approximately 47 kDa in the P. falciparum protein extract. Immunofluorescence microscopy suggested that this protein co-localized with the microneme protein EBA-175 in schizonts, and immunoelectron microscopy established that it is primarily localized to micronemes in merozoites. Functional characterization of Plasmodium falciparum signal peptide peptidase (PfSPP), demonstrates that an antibody to PfSPP blocks RBC invasion by P. falciparumin vitro. Native and recombinant PfSPP bound directly to the 5ABC domain of band 3 in solution and the binding of PfSPP to RBCs was chymotrypsin-sensitive, but trypsin and neuraminidase-resistant. Together, these results suggest that host band 3 interacts with PfSPP during RBC invasion presumably following parasite microneme discharge. PfSPP is the first microneme-associated intramembrane aspartyl protease identified in the apicomplexan parasites that interacts with a major transmembrane receptor on host erythrocytes.