A traffic jam to reduce morbidity in malaria

In this issue of Blood, Rug et al report that genetic disruption of a single malaria parasite protein disturbs recruitment of host actin and protein trafficking, thus disabling parasite adherence to the host endothelial lining, which is the major cause of malaria pathology.¹ 

Malaria caused by Plasmodium falciparum still elicits a substantial burden of disease mainly in sub-Saharan countries, where malaria is still endemic despite many efforts to curb the disease. P falciparum is an aggressive invader of cells, beginning with the invasion of hepatocytes upon the infectious bite of an Anopheles mosquito. Subsequently, after a massive cellular amplification, the P falciparum blood forms emerge and invade mature erythrocytes. These blood forms can cause severe anemia, but the true damages inflicted by this deadly disease come from structural modifications of the infected erythrocyte caused by the parasite.

It has long been known that the parasite exports a massive number of proteins into the host cell cytosol and beyond, but there has been little evidence of how this works (see figure). In their paper, Rug and colleagues¹  share information on the function of exported proteins of P falciparum and their involvement in the remodeling of the host cell.

Why is this important? Parasite-induced modifications change the form and shape of the infected erythrocyte; they also change both the osmotic regulation of the host cell and the membrane rigidity. Most importantly, the induced modifications convey the ability of infected erythrocytes to adhere to the endothelial lining of the capillaries.²  Most of this adherence is mediated by a single parasite protein called P falciparum erythrocyte membrane protein 1 (PfEMP1). This molecule, encoded by a variable parasite gene family, is embedded into the erythrocyte membrane and conveys adherence to a variety of endothelial receptors.²  This cytoadherence protects the parasite from splenic clearance and is considered the major virulence factor in malaria tropica.

Therefore, understanding how PfEMP1 migrates to the surface of the infected cell is extremely important and may lead to innovative interventions that help to eliminate the parasite and reduce morbidity in infected persons. Previously, parts of the export machinery had already been identified, starting from an export signal called PEXEL,³  followed by the discovery of a translocon complex that transports proteins to be exported through the parasitophorous vacuolar membrane.⁴  This subsequently led to the identification of a small number of proteins that seemed to be essential for PfEMP1 translocation to the cell surface.⁵  One that had been identified in a genetic screen was PfEMP1 trafficking protein 1 (PfPTP1), which the current manuscript convincingly shows is essential for this translocation.¹  The essentiality of a number of proteins for PfEMP1 translocation has been demonstrated previously^5-7 ; however, in this manuscript, the authors not only show that parasite-derived structures known as Maurer's clefts are completely distorted but that recruitment of host proteins is also disturbed. It is fascinating to see how the parasite not only modifies the cell by its own proteins but how it overtakes essential host-cell proteins such as actin for the transport of vesicles to the host-cell surface. The first indications of the importance of correctly assembled actin filaments were seen in studies using P falciparum–infected hemoglobin S host cells, in which Cyrklaff and colleagues elegantly demonstrated that actin filaments were incorrectly assembled. This prevented vesicles, apparently transporting PfEMP1, to move to the erythrocyte membrane.⁸  These findings provide a sound explanation for why the sickle cell trait protects against severe malaria. Rug et al mimic a similar phenotype with the genetic disruption of PfPTP1, and it must be assumed that, similar to sickle cells, these parasite-infected cells can no longer bind to endothelial receptors.

Interestingly, PfPTP1 knockout parasites also showed an impaired transport of a second ligand called STEVOR, which has been implicated in a process called rosetting, in which infected erythrocytes bind to noninfected red blood cells. STEVOR was also no longer translocated in the PfPTP1 knockout parasites, indicating that this dangerous phenomenon would also be eliminated. This is in contrast to findings from a genetic deletion of MAHRP1, another Maurer's cleft protein also essential for PfEMP1 translocation,⁷  where only PfEMP1 trafficking was impaired.⁹  This difference raises the question of whether there are different mechanisms and pathways of translocation.

In conclusion, the paper by Rug and colleagues provides a further piece to the puzzle and augments our understanding of how P falciparum modifies the host erythrocyte upon infection to permit its intracellular survival. The authors provide fair evidence of vesicular transport to and from the parasite-derived Maurer's clefts, show convincingly how the parasite overtakes host proteins, and show clearly that genetic disruption of a single protein (here PfPTP1) results in a loss of surface-exposed PfEMP1 and STEVOR. The similarity between phenotypes of a host genetic trait (sickle cell anemia) and a genetic disruption of a single parasite gene (PfPTP1) is striking, both with disrupted Maurer's clefts and with completely disorganized actin filaments essential for PfEMP1 translocation. These findings should engage us to think about new approaches to inhibit these processes in our endeavors to develop innovative strategies to eliminate malaria.