Host Cell Invasion by Apicomplexan Parasites: The Junction Conundrum

Apicomplexans form a large phylum of parasitic protists, some of which cause severe diseases in humans. Most notorious is Plasmodium, the agent of malaria, which kills around a million people each year, mostly young children in Africa. Most successful is Toxoplasma, which parasitizes nearly a third of the human population, making those people at risk of life-threatening complications, primarily encephalitis or pneumonia, in case of immunosuppression. Other apicomplexans of human importance include Cryptosporidium, Isospora, and Sarcocystis, which are opportunistic pathogens that cause severe diarrhea often associated with AIDS. Several apicomplexan parasites cause heavy losses in livestock, particularly Theileria and Babesia in cattle and Eimeria in poultry. Most apicomplexans are obligate intracellular parasites. Their extracellular stages, called zoites, display several conserved features: they are elongated and polarized cells, their shape is maintained by a set of microtubules running longitudinally, and their anterior pole contains secretory vesicles, called micronemes and rhoptries, which secrete their content at the anterior tip of the parasite. Most zoites also share two unique properties among eukaryotic cells. They move on substrate by a gliding type of motility, i.e., without overt deformation of the cell shape, at speeds of several microns per second. They also typically invade host cells by forming a ring-like junction with the host cell membrane. Zoites slide through the junction into an invagination of the host cell surface that becomes the parasitophorous vacuole (PV) after pinching off from the host cell plasma membrane, in a process that takes less than a minute. Once inside the PV niche, the zoite can multiply into multiple new zoites that eventually egress the infected cell to infect new host cells. Much work has been performed since the late 1970s to understand the cellular and molecular bases of host cell invasion by apicomplexans, using various zoites as models. The overall invasion process encompasses several steps, including loose followed by intimate attachment, reorientation relative to the host cell surface, and organelle discharge with junction formation. The ultimate step, sliding through the junction inside the PV and called here internalization, is commonly viewed as powered by the zoite submembrane actin-myosin motor. The junction is thought to act as a traction point for the motor, to bridge the cortical cytoskeletons in the two cells, and to be made of parasite proteins conserved in the apicomplexan phylum. In this review, we confront these established notions with genetic data recently obtained in Plasmodium and Toxoplasma parasites. The Junction: From “Moving” to Stationary The first observation of a junction between an apicomplexan zoite and its host cell was made using Plasmodium merozoites and their target cells, erythrocytes [1]. Electron microscopy showed that the merozoite, after initial random binding, reorients so that its apical tip faces the erythrocyte surface, and then induces a circumferential zone of close apposition of the zoite and erythrocyte membranes over ∼250 nm and the thickening of the inner leaflet of the erythrocyte membrane [1]. This junctional area was described as “actively moving down the body of the merozoite,” since the poorly motile merozoite was not thought to be capable of actively moving inside the cell, and was thus termed “moving junction” [1]. Studies in the 1980s focused on the highly motile Eimeria sporozoites. They showed that several activities at the zoite surface were dependent on parasite actin, including the posterior translocation (capping) of various surface ligands and beads [2]. Videomicroscopic studies revealed that host cell invasion by Eimeria sporozoites was continuous with extracellular gliding [3]. This led to the proposal that the zoite actin-based system would power both gliding motility and host cell invasion by capping substrate-binding ligands or the junction, respectively, which implied that the zoite actively moved inside the host cell [3], [4]. After myosins were identified in Toxoplasma [5] and in Plasmodium [6], it was assumed that an actin-myosin motor powered the zoite motile processes. The role of the host cell during zoite invasion has been studied mainly with Toxoplasma tachyzoites, which can be made to invade host cells at high frequency and synchronicity. The host cell was initially described as displaying no detectable actin reorganization and playing no active role during tachyzoite invasion [7], [8]. More recent work found that Toxoplasma tachyzoites induced, specifically at the junction, host actin polymerization and recruitment of the Arp2/3 complex, an actin-nucleating factor, which is important for tachyzoite entry [9]. Videomicroscopic studies showed a stationary ring of host F-actin at the parasite constriction, in agreement with the junction acting as an anchor for zoite traction inside the cell. In addition to de novo actin polymerization at the junction, tachyzoite invasion also requires disorganization of the host cortical actin meshwork. This activity is in part dependent on Toxofilin [10], a Toxoplasma protein that sequesters actin monomers in vitro [11] and promotes actin turnover at the leading edge of the cell [10]. Localized actin disassembly might thus release G-actin necessary to feed actin reassembly at the junction, regulated by recruited Arp2/3 complex, to anchor the junction to the host cortical cytoskeleton.