Parasite hijacks red cell membrane proteins

In this issue of Blood, Baro et al explored the role of CD44, a glycoprotein expressed on the surface of human red cells, in regulating malarial parasite invasion of red cells.¹ Using CRISPR/Cas9 genome editing, they abrogated the surface expression of CD44 in erythroid cells and found that the lack of CD44 had little effect on in vitro erythropoiesis and on the enucleation of orthochromatic erythroblasts. Interestingly, however, they observed that the rate of Plasmodium falciparum invasion was reduced in these CD44-null culture-derived red cells, validating CD44 as an important host factor for parasite invasion of red cells. Molecularly, 2 previously well-characterized parasite invasion ligands, erythrocyte-binding antigen 175 (EBA-175) and EBA-140, identified as the ligands for glycophorin A (GYPA) and glycophorin C (GYPC), respectively, were shown to be binding partners for CD44, and, finally, the authors demonstrated that EBA-175–induced phosphorylation of erythrocyte cytoskeletal proteins following invasion is CD44 dependent. These findings imply that CD44 is a coreceptor during invasion of human erythrocytes, which, by stimulating CD44-dependent phosphorylation of host cytoskeletal proteins, alters host cell deformability and facilitates parasite entry (see figure).

Malaria due to the parasite P falciparum is a leading cause of morbidity and mortality in the developing world, with ≈600 000 deaths annually, despite implementation of various control strategies. The clinical symptoms of malaria occur following invasion of red cells by the malarial parasite. During the ≈48 hours of intraerythrocytic development, the parasites replicate in exponential cycles and induce significant morphologic, structural, and functional changes in infected red cells, which account for various clinical manifestations, including anemia. As asexual replication is critical to disease pathogenesis, major efforts in development of new interventional strategies are focused on developing detailed molecular and mechanistic understanding of the invasion process. The present study represents a significant contribution to such efforts.

Plasmodium falciparum follows a multistep process for successful invasion of red cells, the first stage of intraerythrocytic parasite development. Amazingly, this multistep process that requires the coordinated execution of diverse events occurs during a period of ≈2 minutes. The process requires numerous ligand-receptor interactions and a complex machinery engaging several distinct cytoskeletal proteins and signaling interventions.^2-5 Distinct features of the invasion process include the apical reorientation of the merozoite, host cell deformation, the formation of a moving junction, and the creation and maintenance of a vacuole that surrounds the intracellular organism in which the intracellular parasite proliferates. The driving force for internalization is powered by the parasite’s actinomyosin motor. Like other invasive parasites of the phylum Apicomplexa, P falciparum merozoites harbor 2 major secretory organelles at their apical tip, the micronemes and the rhoptries, that release an impressive array of proteins to facilitate the invasion process. Secreted ligands during the invasion process include the erythrocyte binding-like (EBL) and reticulocyte-binding protein homolog (PfRh) families of proteins that bind to specific receptors on the erythrocyte, including glycophorin A, B, C, and CR1. EBA-175 binds to GYPA, whereas EBA-140 binds to GYPC on the red cell surface. The EBL and rhesus (Rh) ligands are considered functionally redundant. They mediate apical reorientation of the merozoite and induce host cell deformation during attachment. Following apical reorientation, the P falciparum ligand PfRh5 binds to its receptor, basigin, on the red cell surface.⁶ This interaction is believed to result in pore formation in the red cell membrane and is required for rhoptry discharge and formation of the moving junction critical for completion of the invasion process.

Although significant progress has been made in our understanding of the role of erythrocyte host factors during P falciparum invasion, gaps still exist, in part, because of the genetic intractability of the enucleated red cells. Recent advances in our ability to generate enucleated cells in vitro from hematopoietic stem/progenitor cells⁷ in combination with CRISPR/Cas9 genome editing have enabled access to enucleated red cells with specific deficiencies of various red cell proteins; and they, thus, facilitate the identification of host proteins involved in the invasion process.^8,9 This significant methodological development has previously enabled the identification of CD44, CD55, and other red cell proteins as host factors involved in parasite invasion of red cells.

Using these state-of-the-art methods, Baro et al show in the present study that CD44 acts a coreceptor working in conjunction with well-established parasite ligands in regulating the complex process of parasite invasion of red cells. The identification of host cofactors in regulating parasite invasion is a significant advancement. Of course, several questions remain unanswered, including the manner of the spatial and temporal assembly of the various components during the invasion process. The mechanistic basis of the identified role of the posttranslational modification of host proteins in the invasion process also remains unclear. It is hoped that future work using genetic tools and state-of-the-art imaging techniques used in the present study will provide more comprehensive insights into this fascinating biological process.

Finally, the work of Baro et al provides us with a valuable impetus to further explore the complex role of various red cell proteins in malarial invasion. Such findings are important for development of novel antimalaria drugs that target host proteins and as such will not lead to acquired drug resistance, a major concern of the current antimalarial drugs.

Conflict-of-interest disclosure: The authors declare no competing financial interests.