Of membranes and malaria: phospholipid asymmetry in Plasmodium falciparum-infected red blood cells

Abstract

Malaria is a vector-borne parasitic disease with a vast impact on human history, and according to the World Health Organisation, Plasmodium parasites still infect over 200 million people per year. Plasmodium falciparum, the deadliest parasite species, has a remarkable ability to undermine the host immune system and cause life-threatening disease during blood infection. The parasite's host cells, red blood cells (RBCs), generally maintain an asymmetric distribution of phospholipids in the two leaflets of the plasma membrane bilayer. Alterations to this asymmetry, particularly the exposure of phosphatidylserine (PS) in the outer leaflet, can be recognised by phagocytes. Because of the importance of innate immune defence numerous studies have investigated PS exposure in RBCs infected with P. falciparum, but have reached different conclusions. Here we review recent advancements in our understanding of the molecular mechanisms which regulate asymmetry in RBCs, and whether infection with the P. falciparum parasite results in changes to PS exposure. On the balance of evidence, it is likely that membrane asymmetry is disrupted in parasitised RBCs, though some methodological issues need addressing. We discuss the potential causes and consequences of altered asymmetry in parasitised RBCs, particularly for in vivo interactions with the immune system, and the role of host-parasite co-evolution. We also examine the potential asymmetric state of parasite membranes and summarise current knowledge on the parasite proteins, which could regulate asymmetry in these membranes. Finally, we highlight unresolved questions at this time and the need for interdisciplinary approaches to uncover the machinery which enables P. falciparum parasites to hide in mature erythrocytes.

Keywords: Annexin V; Host-Parasite Interactions; Malaria; Phosphatidylserine exposure; Plasmodium falciparum; Red blood cells.

Fig. 1

Membrane asymmetry in RBCs is regulated by flippase, floppase, and scramblase enzymes. Flippase and floppase hydrolyse ATP to move PS and PE to the inner layer (flippase) and PC to the outer layer (floppase). Scramblase moves lipids indiscriminately when activated by cytoplasmic Ca²⁺. Text indicates the molecular identity of the proteins in RBCs, with number of each protein per cell based on proteomic data [45]. Blue question mark uncertain functional assignment; purple question mark uncertain presence in RBCs. PS phosphatidylserine, PE phosphatidylethanolamine, PC phosphatidylcholine, ATP adenosine triphosphate, ADP adenosine diphosphate, ABC ATP binding cassette. UniProt: ATP11C (Q8NB49); ATP11A (P98196), ATP11B (Q9Y2G3); ABCC1 (P33527); ABCA7 (Q8IZY2); ABCG2 (Q9UNQ0); ABCC5 (O15440); ABCB4 (P21439); PLSCR1 (O15162); PLSCR4 (Q9NRQ2); TMEM16F (Q4KMQ2)

Fig. 2

Percentages of uninfected and infected RBCs with PS exposure, measured by Annexin V binding. Percentages were calculated from tables or graphs using ImageJ when not specified in the text. Only results from RBCs not treated with a modifying agent (drug, peptide, heat stress etc.) are shown. Where multiple percentages (e.g. multiple stages) were available, the highest and lowest have been included. Studies are grouped by major methodological considerations. Error bars are shown where available in the original publications. * = used iRBCs enriched by density gradient centrifugation rather than fluorescent DNA stain

Fig. 3

Potential candidates for non-opsonic recognition signals between iRBCs and macrophages/monocytes. The scavenger receptor CD36 recognises PfEMP1, exposed PS, and potentially other ligands. Treatment with antibodies against CD36 decreases phagocytosis by 50–60%, while removal of iRBC surface proteins (including PfEMP1) with trypsin decreases phagocytosis by ~ 75–85% [–147]. Exposed PS can also be recognised by other receptors including BAI-1, STAB-2, TIM-1, and TIM-4, some of which may be present on monocytes and macrophages [16]. List is not exhaustive. PS phosphatidylserine, iRBC infected red blood cell, PV parasitophorous vacuole

Fig. 4

Simplified lifecycle of Plasmodium, showing stages where PS exposure has been investigated at the host or parasite membrane. Since not all life-cycle stages of the human malaria parasite P. falciparum are easily accessible for study, information on the murine malaria parasite P. berghei is also included, serving as an in vivo model for human malaria. PS phosphatidylserine, RBC red blood cell