Manipulating Eryptosis of Human Red Blood Cells: A Novel Antimalarial Strategy?

Abstract

Malaria is a major global health burden, affecting over 200 million people worldwide. Resistance against all currently available antimalarial drugs is a growing threat, and represents a major and long-standing obstacle to malaria eradication. Like many intracellular pathogens, Plasmodium parasites manipulate host cell signaling pathways, in particular programmed cell death pathways. Interference with apoptotic pathways by malaria parasites is documented in the mosquito and human liver stages of infection, but little is known about this phenomenon in the erythrocytic stages. Although mature erythrocytes have lost all organelles, they display a form of programmed cell death termed eryptosis. Numerous features of eryptosis resemble those of nucleated cell apoptosis, including surface exposure of phosphatidylserine, cell shrinkage and membrane ruffling. Upon invasion, Plasmodium parasites induce significant stress to the host erythrocyte, while delaying the onset of eryptosis. Many eryptotic inducers appear to have a beneficial effect on the course of malaria infection in murine models, but major gaps remain in our understanding of the underlying molecular mechanisms. All currently available antimalarial drugs have parasite-encoded targets, which facilitates the emergence of resistance through selection of mutations that prevent drug-target binding. Identifying host cell factors that play a key role in parasite survival will provide new perspectives for host-directed anti-malarial chemotherapy. This review focuses on the interrelationship between Plasmodium falciparum and the eryptosis of its host erythrocyte. We summarize the current knowledge in this area, highlight the different schools of thoughts and existing gaps in knowledge, and discuss future perspectives for host-directed therapies in the context of antimalarial drug discovery.

Keywords: Plasmodium; apoptosis; eryptosis; host-directed therapy; host-pathogen interaction; malaria; programmed cell death.

Figure 1

The asexual proliferation cycle of Plasmodium falciparum in human erythrocytes. Extracellular merozoites invade red blood cells to establish the erythrocytic asexual cycle. Each intracellular merozoite develops into an intra-erythrocytic ring stage, matures into a trophozoite stage, and subsequently forms a multi-nucleated schizont. Forty-eight hours post-merozoite infection, 8–32 new merozoites egress from each schizont-infected erythrocyte and a new erythrocytic cycle begins. Repeated cycles of erythrocyte invasion by Plasmodium falciparum parasites lead to all aspects of malaria pathogenesis.

Figure 2

Proposed model of eryptosis mechanisms. Induction of eryptosis (energy depletion, oxidative stress or exposure to xenobiotics) leads to an increase of intracellular calcium concentration. (i) Intracellular calcium activates scramblases (membrane proteins that transport lipids non-specifically and bidirectionally) and inactivates flippases (membrane proteins that actively maintain phosphatidylserine (PS) in the membrane inner leaflet). This leads to exposure of PS at the surface of the cell, a signal that is recognized by macrophages, which then mediate clearance of eryptotic cells. (ii) Intracellular calcium activates calcium-sensitive potassium channels known as Gardos channels. This is followed by exit of potassium and chlorine, leading to loss of water by osmosis and subsequent cell shrinkage. (iii) Increased concentration of free intracellular calcium activates calpains (calcium-activated proteases). Calpains degrade cytoskeleton proteins which leads to membrane ruffling and blebbing.

Figure 3

Proposed model of the interactions between Plasmodium and its host red blood cell. Upon erythrocyte invasion, Plasmodium digests host cell hemoglobin in its digestive vacuole (1). In this process, heme is detoxified by polymerisation into hemozoin and amino acids are utilized for parasite development. The surplus of amino acids is exported to the RBC cytosol (2), and further secreted to the extracellular milieu to decrease colloid concentration (3), which is proposed to enhance survival of the host cell. Digestion of hemoglobin also produces reactive oxygen species (ROS) (2), which enhances eryptosis. New Permeability Pathways (NPPs) and/or other transporters allow Ca²⁺ entry into the red cell cytosol, a key player in triggering eryptosis (4). However, it has been suggested that Plasmodium uptakes most of the intracellular calcium in its own cytosol, thus achieving delay of host cell death (4). Nevertheless, active parasite sphingomyelinase has been proposed to break down sphingomyelin, producing ceramide (5), which enhances exposure of phosphatidylserine (PS), and hence recognition and clearance by macrophages.