Plasmodium translocon component EXP2 facilitates hepatocyte invasion

Abstract

Plasmodium parasites possess a translocon that exports parasite proteins into the infected erythrocyte. Although the translocon components are also expressed during the mosquito and liver stage of infection, their function remains unexplored. Here, using a combination of genetic and chemical assays, we show that the translocon component Exported Protein 2 (EXP2) is critical for invasion of hepatocytes. EXP2 is a pore-forming protein that is secreted from the sporozoite upon contact with the host cell milieu. EXP2-deficient sporozoites are impaired in invasion, which can be rescued by the exogenous administration of recombinant EXP2 and alpha-hemolysin (an S. aureus pore-forming protein), as well as by acid sphingomyelinase. The latter, together with the negative impact of chemical and genetic inhibition of acid sphingomyelinase on invasion, reveals that EXP2 pore-forming activity induces hepatocyte membrane repair, which plays a key role in parasite invasion. Overall, our findings establish a novel and critical function for EXP2 that leads to an active participation of the host cell in Plasmodium sporozoite invasion, challenging the current view of the establishment of liver stage infection.

Transport mechanisms at the malaria parasite-host cell interface

“…Functional analysis of EXP2 in the liver-stage by a stage-specific conditional knockdown revealed EXP2 is important for transition from the sporozoite to the blood-stage, but it is unclear if this involves a function in protein export and/or small molecule transport within the hepatocyte [ 152 ]. Surprisingly, this stage transition defect was recently tied to an unexpected role for EXP2 in hepatocyte invasion [ 155 ]. In this study, extracellular sporozoites were shown to secrete EXP2 into the supernantant upon serum stimulation, and reexamination of the same conditional EXP2 knockdown parasites revealed a specific impact on invasion but not intracellular development.…”

“…The apparent nonessential role of EXP2 during intracellular liver-stage development [ 155 ] is suprising given that even if liver-stage protein export relies on a distinct translocon, EXP2 might still be expected to serve an important role in small molecule transport across the PVM. This would parallel the situation in Toxoplasma , where EXP2 orthologs GRA17 and GRA23 (the only PTEX-related components present; several additional HSP100s are encoded but are not found in the PV [ 156 , 157 ]) are only important for small molecule transport [ 59 ], while a distinct set of proteins is required for effector translocation into the host cell.…”

Secretory Organelle Function in the Plasmodium Sporozoite

“…Lysosome exocytosis is also triggered by Trypanosoma cruzi infection [59]. Moreover, as CT triggers wounding in the traversed cells, the cell wound-healing process elicits modifications at the plasma membrane that could facilitate invasion [60].…”

“…Moreover, a recent study highlights the role of EXP2 in the influx of intracellular calcium in the parasite and/or the host. It has been suggested that EXP2 could act either at the entry phase or during the early phase of EEF transformation [60]. EXP2 is secreted by merozoite DGs after erythrocyte invasion [90].…”

Maria M. Mota: Bringing Plasmodium Liver Infection to the Centre Stage of Malaria Research

“…This work showed that the pore-forming activity of the parasite’s exported protein 2 (EXP2) triggers a response from the hepatic host cell, which leads to the production of acid sphingomyelinase. This enzyme then plays a critical role in the repair of the host cell membrane, which is key for sporozoite invasion and establishment in hepatocytes ( Mello-Vieira et al., 2020 ). This finding is a perfect illustration of Maria’s ability to push the boundaries of knowledge beyond the status quo , by unveiling an active role for the hepatic host cell in a process long thought to depend solely on the parasite.…”