PfPI3K, a phosphatidylinositol-3 kinase from Plasmodium falciparum, is exported to the host erythrocyte and is involved in hemoglobin trafficking

Abstract

Polyphosphorylated phosphoinositides (PIPs) are potent second messengers, which trigger a wide variety of signaling and trafficking events in most eukaryotic cells. However, the role and metabolism of PIPs in malaria parasite Plasmodium have remained largely unexplored. Our present studies suggest that PfPI3K, a novel phosphatidylinositol-3-kinase (PI3K) in Plasmodium falciparum, is exported to the host erythrocyte by the parasite in an active form. PfPI3K is a versatile enzyme as it can generate various 3'-phosphorylated PIPs. In the parasite, PfPI3K was localized in vesicular compartments near the membrane and in its food vacuole. PI3K inhibitors wortmannin and LY294002 were effective against PfPI3K and were used to study PfPI3K function. We found that PfPI3K is involved in endocytosis from the host and trafficking of hemoglobin in the parasite. The inhibition of PfPI3K resulted in entrapment of hemoglobin in vesicles in the parasite cytoplasm, which prevented its transport to the food vacuole, the site of hemoglobin catabolism. As a result, hemoglobin digestion, which is a source of amino acids necessary for parasite growth, was attenuated and caused the inhibition of parasite growth.

Figure 1

PfPI3K, a PI3 kinase from P falciparum. (A) Schematic diagram showing domain architecture of PfPI3K. The helical and catalytic domain of PfPI3K is separated by a linker. In addition, a C2 domain is present near its N-terminal end. (B) Equal amounts of cell lysates prepared either from either uninfected erythrocytes (E left panel) or trophozoite stage parasites (Pf right panel) were electrophoresed on 7% SDS-polyacrylamide gel electrophoresis gel, and Western blot analyses were performed using anti-PfPI3K antisera on both erythrocyte as well as the parasite lysate. A control Western blot with preimmune antisera was performed to probe the parasite lysate (middle panel). (C) PfPI3K was immunoprecipitated from trophozoite lysates, and PfPI3K-IP was used in a lipid kinase assay wherein either PI or PI3P (left panel), PI4P, or PI(4,5)P2 (right panel) was used as substrate. Phospholipids were separated on a TLC along with phosphoinositide standards, and radiolabeled lipid products were detected using a phosphorimager. (D) The activity of PfPI3K-IP from trophozoite lysates was assayed using PI4P as substrate (as described in panel C) in the presence of 2.5μM wortmannin (left panel), or 50μM LY294002 (right panel) or DMSO (−).

Figure 2

Intracellular localization of PfPI3K and its trafficking to the host. (A) IFA was performed for localizing PfPI3K, FCP (i) and VARC (ii). FCP (red) is located inside the food vacuole of mature trophozoites. PfPI3K (green) exhibits “vesicular” staining on PVM/PM (black arrows) and the host erythrocyte (white arrows) and in the food vacuole, which can be identified by the presence of black hemozoin. (ii) PfPI3K colocalizes with VARC at the erythrocyte surface. (B) Parasites were treated either with 5μg/mL BFA or DMSO, and immunofluorescence was performed using anti-PfPI3K antisera. BFA treatment blocked the transport of PfPI3K to the erythrocyte. (C) Trophozoite stage parasites were treated with either streptolysin (SLO) or saponin (SAP). The soluble (S) and pellet (P) fractions were used for Western blot (bottom panel) and immunoprecipitation of PfPI3K. PfPI3K-IP was assayed for activity using PI4P as the substrate; the radiolabeled product PI(3,4)P2 was detected by phosphorimaging of TLC plates. Anti-PfHSP70 was used for a control Western blot.

Figure 3

Hemoglobin accumulation in malaria parasites as result of PfPI3K inhibition. (A) Late ring stage parasites were incubated with DMSO (control), wortmannin, or LY294002 for approximately 5 hours. After releasing the parasite from the erythrocytes, parasite lysates were prepared and Western blotting performed using antihemoglobin antiserum or anti-actin antibody. (i) A representative Western blot from each experiment is shown. Actin was used as a loading control. (ii) The hemoglobin band intensity from Western blots performed on 3 independent experiments in which the parasite was treated with 10μM wortmannin or LY294002 was quantified by densitometry using Kodak 1D image analysis software and is represented by bar graphs. The intensity levels were normalized to the control set at 100, and comparisons were made. The error bars represent SEM. (B) Erythrocytes were preloaded with HRP, infected with parasites, and treated with 10μM wortmannin for 14 hours. After releasing parasites from infected erythrocytes by saponin treatment, HRP levels associated with parasites were determined with a colorimetric enzyme assay. Absorbance values were normalized to the solvent controls set at 100. Error bars represent SEM.

Figure 4

The effect of PfPI3K inhibition on hemoglobin trafficking. (A) Parasites were treated with DMSO or wortmannin. Saponin treatment was used to release excess nonparasitic hemoglobin, followed by attachment of parasites onto poly-lysine-coated glass cover slips. After fixation in paraformaldehyde/glutaraldehyde and Triton X-100 permeabilization, IFA was performed using anti-hemoglobin antibody, and 4,6-diamidino-2-phenylindole was used to stain the parasite nucleus. A single erythrocyte-free parasite can be seen in each phase-contrast image (PC) with its food vacuole. The thick arrow represents the collection of hemozoin crystals, which indicates the position of the food vacuole. Corresponding hemoglobin immunofluorescence images (Hb) contain punctate vesicle-like structures indicated by the thin arrow. (B) Average transport vesicle counts per parasite were compared between wortmannin and DMSO control. Error bars represent SEM. (C) Transmission electron micrographs of DMSO and 10μM wortmannin-treated trophozoite stage malaria parasite. Each micrograph shows a single parasite located inside a host erythrocyte. The cytoplasm of the host cell is darkly stained because of the preponderance of electron-dense hemoglobin. Labeled structures inside the parasites are the parasite nucleus (N), food vacuole (Fv), and hemoglobin transport vesicles (V).

Figure 5

PfPI3K inhibition stalls parasite growth. Parasite cultures in complete medium (A,C) or Ile medium (B,D) were incubated with DMSO/MeOH (control), indicated concentrations of wortmannin (A-B), LY294002 (C-D), or 30μM pepstatin A (Peps). Parasite growth was monitored by counting the parasite infected erythrocytes every 48 hours, the data obtained after 4 (C-D) or 6 (A-B) days of treatment are shown. The percentage growth of parasite in drug-treated cultures compared with control (100%) is indicated. Error bars represent SEM.

Figure 6

A model for the role and regulation of PfPI3K in malaria parasite. PfPI3K is trafficked to vesicular compartments at PM/PVM and the food vacuole, and exported to the host RBCs. It may regulate the function of FCP, a PI3P binding protein, which is present in the food vacuole. PfPI3K inhibition blocks endocytosis of hemoglobin to the food vacuole and blocks parasite growth. Because PfPI3K exported to the host erythrocyte is active, it may trigger signaling and trafficking pathways that may regulate additional events.