Hemoglobin S and C affect protein export in Plasmodium falciparum-infected erythrocytes

Abstract

Malaria is a potentially deadly disease. However, not every infected person develops severe symptoms. Some people are protected by naturally occurring mechanisms that frequently involve inheritable modifications in their hemoglobin. The best studied protective hemoglobins are the sickle cell hemoglobin (HbS) and hemoglobin C (HbC) which both result from a single amino acid substitution in β-globin: glutamic acid at position 6 is replaced by valine or lysine, respectively. How these hemoglobinopathies protect from severe malaria is only partly understood. Models currently proposed in the literature include reduced disease-mediating cytoadherence of parasitized hemoglobinopathic erythrocytes, impaired intraerythrocytic development of the parasite, dampened inflammatory responses, or a combination thereof. Using a conditional protein export system and tightly synchronized Plasmodium falciparum cultures, we now show that export of parasite-encoded proteins across the parasitophorous vacuolar membrane is delayed, slower, and reduced in amount in hemoglobinopathic erythrocytes as compared to parasitized wild type red blood cells. Impaired protein export affects proteins targeted to the host cell cytoplasm, Maurer's clefts, and the host cell plasma membrane. Impaired protein export into the host cell compartment provides a mechanistic explanation for the reduced cytoadherence phenotype associated with parasitized hemoglobinopathic erythrocytes.

Oxidative insult can induce malaria-protective trait of sickle and fetal erythrocytes

“…It is known that actin is highly susceptible to oxidative stress and that oxidation impacts on actin polymerization and dynamics10303132. If host actin reorganization is impaired, then the actin network that connects the Maurer's clefts with the knobs cannot form and vesicular trafficking of adhesins to the host cell surface is compromised13. Moreover, Maurer's clefts do not properly develop because their morphology depends on a functional actin network, as previously shown for parasitized HbAA erythrocytes treated with cytochalasin D, an inhibitor of actin polymerization10.…”

“…P. falciparum FCR3 CSA were cultured with different haemoglobinopathic erythrocytes (HbAA, HbAS, HbAC and HbF) or pretreated erythrocytes for at least two cycles, and 5 × 10 6 infected erythrocytes (trophozoite stage) were applied onto these spots for 1 h. After multiple washing steps, the adherent cells were fixed with 2% glutaraldehyde for at least 2 h. After staining for 10 min with 10% Giemsa, three randomly selected areas from each spot were imaged using a Zeiss Axiovert 200M microscope (objective × 10/0.25 air differential interference contrast (DIC)). The amount of adhesive parasites were quantified using ImageJ as previously described13. Namely, the Giemsa-stained infected erythrocytes per image were automatically counted, using thresholding and watershed functions.…”

“…Recent findings have suggested that reduced cytoadherence results from impaired intracellular trafficking and sorting of parasite-encoded adhesins to the host cell surface1314. According to our current understanding, adhesins are usually trafficked across the host cell cytoplasm to the erythrocyte surface via a complex pathway that involves transient association with parasite-generated unilamellar membrane profiles, termed Maurer's clefts, and vesicular transport along actin-containing filaments from the Maurer's clefts to knob-like protrusions in the host cell membrane, where the adhesins are presented101516.…”

Critical Steps in Protein Export of Plasmodium falciparum Blood Stages

“…In addition, vesicle-like structures positive for PfEMP1 were seen both on these actin filaments and on structures termed 'tethers' that were observed to attach Maurer's clefts to the red blood cell periphery [29,84,87,88]. However, newer work also showed impaired trafficking across the PVM in parasites growing in HbC or HbS red blood cells and this might be a further reason for the reduced cytoadherence of these red blood cells [89].…”

Heterozygous HbAC but not HbAS is associated with higher newborn birthweight among women with pregnancy-associated malaria

“…In support of this latter hypothesis, two recently-published studies have shown that in static adhesion assays, VAR2CSA-expressing FCR3 parasites in HbAS and HbAC erythrocytes exhibit significant reductions in adherence to CSA compared to those grown in HbAA erythrocytes, this decreased binding being much greater for parasitized HbAC than HbAS erythrocytes 26, 27 . However, these studies assessed only a single parasite strain in static but not flow adhesion assays.…”