Shedding of TRAP by a Rhomboid Protease from the Malaria Sporozoite Surface Is Essential for Gliding Motility and Sporozoite Infectivity

Plasmodium sporozoites, the infective stage of the malaria parasite, move by gliding motility, a unique form of locomotion required for tissue migration and host cell invasion. TRAP, a transmembrane protein with extracellular adhesive domains and a cytoplasmic tail linked to the actomyosin motor, is central to this process. Forward movement is achieved when TRAP, bound to matrix or host cell receptors, is translocated posteriorly. It has been hypothesized that these adhesive interactions must ultimately be disengaged for continuous forward movement to occur. TRAP has a canonical rhomboid-cleavage site within its transmembrane domain and mutations were introduced into this sequence to elucidate the function of TRAP cleavage and determine the nature of the responsible protease. Rhomboid cleavage site mutants were defective in TRAP shedding and displayed slow, staccato motility and reduced infectivity. Moreover, they had a more dramatic reduction in infectivity after intradermal inoculation compared to intravenous inoculation, suggesting that robust gliding is critical for dermal exit. The intermediate phenotype of the rhomboid cleavage site mutants suggested residual, albeit inefficient cleavage by another protease. We therefore generated a mutant in which both the rhomboid-cleavage site and the alternate cleavage site were altered. This mutant was non-motile and non-infectious, demonstrating that TRAP removal from the sporozoite surface functions to break adhesive connections between the parasite and extracellular matrix or host cell receptors, which in turn is essential for motility and invasion

Disruption of the rhomboid motif impairs TRAP cleavage.

<p>(A) Primary structure of TRAP expressed in TRAP-VAL and TRAP-FFF mutants, with the point mutations introduced to disrupt the putative rhomboid substrate motif indicated above each transmembrane domain. (B) Western blot analysis of recombinant control TRAP-rWT (rWT) and rhomboid cleavage mutant salivary gland sporozoites TRAP-VAL and TRAP-FFF (VAL and FFF) probed with TRAP anti-repeat antisera. As a loading control the bottom half of the membrane was probed with mAb 3D11 which recognizes the repeat region of CSP. (C) Pulse-chase metabolic labeling of TRAP-rWT and rhomboid cleavage site mutants. Salivary gland sporozoites were metabolically labeled for 1 hr and either placed on ice for 4 hrs (Time = 0) or chased for 4 hrs (Time = 4). Sporozoites were then centrifuged and TRAP was immunoprecipitated from either the pellet (P) or supernatant (S) using anti-repeat antisera and analyzed by SDS-PAGE and autoradiography. Molecular weight markers shown on left of top panel. Top panel: 6 day exposure. Bottom panel: 14 day exposure, arrows show location of processed VAL and FFF mutant TRAP. (D) Immunofluorescence analysis of surface TRAP staining in TRAP-rWT and rhomboid cleavage site mutants. Shown are representative fluorescence and phase contrast images of the TRAP staining pattern after fixation with paraformaldehyde. Microscope and camera settings were identical for all photographs. (E) Box plot of fluorescence intensity of TRAP surface staining in TRAP-rWT and rhomboid cleavage site mutants. Unpermeabilized sporozoites were stained with anti-TRAP repeat antisera and intensity of staining was measured using NIS Elements software. Identical camera and microscope settings were used for all measurements. Boxes contain 50% of the data around its median (black line in box). Whiskers show the range of data within the 10^th and 90^th percentiles and outliers are shown individually. Results are pooled from 2 to 4 independent experiments. There was a statistically significant difference in staining intensity between TRAP-rWT and TRAP-VAL sporozoites (p<.0001) and between TRAP-rWT and TRAP-FFF sporozoites (p<.0001).</p

Impaired TRAP processing of rhomboid cleavage site mutants leads to impaired host cell invasion.

<p>(A) In vitro invasion. Salivary gland sporozoites were incubated with Hepa 1–6 cells and fixed after 1 hr (data on left) or 6 hrs (data on right). Cells fixed after 1 hr were stained with a double staining assay that distinguishes between extracellular and intracellular sporozoites and the percent of total sporozoites that were intracellular was determined (left axis). Cells fixed after 6 hrs were stained with UIS4 antisera to determine the number of sporozoites that had entered in a vacuole (right axis). For both experiments at least 50 fields per well were counted and shown are the means ± SD of duplicate wells. (B) Kinetics of entry into hepatocytes. Salivary gland sporozoites were incubated with Hepa 1–6 cells for 15, 30 and 45 mins before being washed, fixed and stained with a double-staining assay that distinguishes extracellular and intracellular sporozoites. Shown is the percent of total sporozoites that were in the process of entering host cells, i.e. partially inside and partially outside. 50 fields per coverslip were counted and the means of duplicates ±SD are shown. (C) EEF development. Salivary gland sporozoites were added to Hepa 1,6 cells and incubated for 48 hrs at which time they were fixed and stained. The number of EEFs in 50 fields per coverslip were counted and shown are the means ± SD of duplicate wells. (D) Cell traversal. Salivary gland sporozoites were incubated with Hepa 1,6 cells for 1 hr, in the presence of the nucleic acid dye TOTO-1. Controls were pre-incubated and kept in the presence of cytochalasin D (CD), which inhibits motility. The number of TOTO-1 positive cells in 50 fields was counted and the means ±SD of duplicate wells are shown. All experiments were performed at least twice and representative experiments are shown.</p

ROM4 is expressed on the sporozoite surface and antibodies against the extracellular tail of ROM4 inhibit hepatocyte invasion.

<p>(A) Western blot analysis of <i>P. berghei</i> erythrocytic stage schizont lysate (sch) and salivary gland sporozoites (spz) probed with anti-PfROM4 C-terminal IgG. Molecular weight markers are in kDa. (B) Immunofluorescence of <i>P. berghei</i> salivary gland sporozoites fixed with cold methanol and stained with anti-CSP antibodies and anti-PfROM4 C-terminal IgG.</p

TRAP-DMut sporozoites are severely impaired in gliding motility and host cell invasion.

<p>(A) Gliding motility of TRAP-JMD and TRAP-DMut sporozoites. Salivary gland sporozoites were incubated on slides for 1 hr and trails were visualized and counted. The percentage of sporozoites with and without trails is shown in the pie charts. For those sporozoites associated with trails, the number of circles produced by each sporozoite was counted and shown is their distribution for each parasite line. Over 100 sporozoites per well were counted and shown are the means of triplicate wells ± SD. (B) Representative images of the types of trails produced by each mutant. (C) Hepatocyte invasion. Salivary gland sporozoites were incubated with Hepa 1–6 cells for 1 hr, fixed and stained with a double staining assay that distinguishes extracellular and intracellular sporozoites. Percent invasion was determined and shown are the means ± SD of duplicate wells. All experiments were performed at least twice and shown is a representative experiment.</p

In vivo infectivity of TRAP mutant sporozoites as determined by prepatent period.

<p>In vivo infectivity of TRAP mutant sporozoites as determined by prepatent period.</p

Impaired TRAP processing leads to aberrant gliding motility.

<p>(A) Gliding motility of rhomboid cleavage site mutants. Salivary gland sporozoites were incubated on slides for 1 hr and trails were visualized and counted. The percentage of sporozoites with and without trails is shown in the pie charts. For those sporozoites associated with trails, the number of circles produced by each sporozoite was counted and shown is their distribution for each parasite line. Asterisks indicate that none of the TRAP-VAL and TRAP-FFF mutants were associated with over 50 circles. Over 100 sporozoites per well were counted and shown are the means of triplicate wells ± SD. (B) Live imaging of gliding motility of rhomboid cleavage site mutant sporozoites. Sporozoites were observed and recorded using a Leica laser scanning confocal microscope. Time lapse images of sporozoites gliding on glass bottom dishes are shown with the maximum intensity projection on the right. (C) For each parasite line the average speed of ten sporozoites was determined for 60 s. All experiments were performed at least twice and a representative experiment is shown.</p

Sporozoite numbers and localization in mosquitoes infected with control and mutant parasites.

<p>*Mosquitoes were infected with the indicated parasite clones and midguts, hemolymph and salivary glands were harvested from 15 mosquitoes at days 14, 16 and 18 post-infective blood meal respectively, pooled and sporozoites were counted. Shown are the means of three independent experiments ± SD.</p>#<p>Sporozoites in the supernatant and pellet of trypsin-treated salivary glands were counted and the percentage inside was calculated. There were 15 mosquitoes per group. This experiment was performed twice with similar results.</p

TRAP is proteolytically processed and shed from the sporozoite surface by a serine protease.

<p>(A) Primary Structure of TRAP: Shown are the extracellular adhesive domains, namely the A-domain and the type I thrombospondin repeat (TSR), as well as the repeat region, the juxtamembrane region (JMD), the transmembrane domain (TM) and cytoplasmic tail (CT). Anti-TRAP antibodies used in this study recognize either the repeat region (α-Rep) or the cytoplasmic tail of TRAP (α-CT). (B) Pulse-chase metabolic labeling and TRAP immunoprecipitation using anti-repeat or anti-cytoplasmic tail antisera. Salivary gland sporozoites were metabolically labeled and placed on ice for 2 hrs (Time = 0) or chased at 28°C for 2 hrs (Time = 2). Sporozoites were then centrifuged and TRAP was immunoprecipitated from either the pellet (P) or supernatant (S) using antibodies against the repeat region of TRAP (left panel) or antibodies against the cytoplasmic tail (right panel) and analyzed by SDS-PAGE and autoradiography. Supernatants from control sporozoites kept on ice did not contain any TRAP (data not shown). (C) Effect of protease inhibitors on TRAP cleavage. Salivary gland sporozoites were metabolically labeled and chased at 28°C for 2 hrs in the presence of the indicated protease inhibitors. Sporozoites were then centrifuged and TRAP was immunoprecipitated from either the pellet (P) or supernatant (S) using anti-repeat antisera and analyzed by SDS-PAGE and autoradiography. The following inhibitors were used: 10 µM E64, 1 mM PMSF, 1 µM pepstatin (Pep), 0.3 µM aprotinin (Apr), 100 µM 3,4 DCI, 100 µM TLCK, 75 µM leupeptin (Leu), and 5 mM EDTA. (D) Effect of protease inhibitors on gliding motility. Salivary gland sporozoites were pre-incubated with the indicated protease inhibitors and then added to slides in the continued presence of the inhibitor for 1 hr at 37°C. Sporozoite trails were visualized and the number of sporozoites with and without trails was counted. Inhibition of motility was calculated based on the motility of sporozoites pre-treated with media alone. Each inhibitor was tested in triplicate and 50 fields per well were counted. The means ± SD are shown. DCI-R indicates that DCI was replenished every 20 min. All inhibitors were tested in at least two independent experiments however DCI and PMSF were tested in 3 or more independent experiments. A representative experiment is shown.</p

Inhibition by stabilization: Targeting the Plasmodium falciparum aldolase-TRAP complex

Background: Emerging resistance of the malaria parasite Plasmodium to current therapies underscores the critical importance of exploring novel strategies for disease eradication. Plasmodium species are obligate intracellular protozoan parasites. They rely on an unusual form of substrate-dependent motility for their migration on and across host-cell membranes and for host cell invasion. This peculiar motility mechanism is driven by the 'glideosome', an actin-myosin associated, macromolecular complex anchored to the inner membrane complex of the parasite. Myosin A, actin, aldolase, and thrombospondin-related anonymous protein (TRAP) constitute the molecular core of the glideosome in the sporozoite, the mosquito stage that brings the infection into mammals. Methods: Virtual library screening of a large compound library against the PfAldolase-TRAP complex was used to identify candidate compounds that stabilize and prevent the disassembly of the glideosome. The mechanism of these compounds was confirmed by biochemical, biophysical and parasitological methods. Results: A novel inhibitory effect on the parasite was achieved by stabilizing a protein-protein interaction within the glideosome components. Compound 24 disrupts the gliding and invasive capabilities of Plasmodium parasites in in vitro parasite assays. A high-resolution, ternary X-ray crystal structure of PfAldolase-TRAP in complex with compound 24 confirms the mode of interaction and serves as a platform for future ligand optimization. Conclusion: This proof-of-concept study presents a novel approach to anti-malarial drug discovery and design. By strengthening a protein-protein interaction within the parasite, an avenue towards inhibiting a previously "undruggable" target is revealed and the motility motor responsible for successful invasion of host cells is rendered inactive. This study provides new insights into the malaria parasite cell invasion machinery and convincingly demonstrates that liver cell invasion is dramatically reduced by 95 % in the presence of the small molecule stabilizer compound 24.Fil: Nemetski, Sondra Maureen. New York University School of Medicine; Estados Unidos. New York-Presbyterian Hospital-Weill Cornell Medical College; Estados UnidosFil: Cardozo, Timothy J.. New York University School of Medicine; Estados UnidosFil: Bosch, Gundula. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. Johns Hopkins Malaria Research Institute ; Estados UnidosFil: Weltzer, Ryan. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. Johns Hopkins Malaria Research Institute ; Estados UnidosFil: O'Malley, Kevin. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. Johns Hopkins Malaria Research Institute ; Estados UnidosFil: Ejigiri, Ijeoma. New York University School of Medicine; Estados UnidosFil: Kumar, Kota Arun. New York University School of Medicine; Estados Unidos. University of Hyderabad; Estados UnidosFil: Buscaglia, Carlos Andres. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas ; ArgentinaFil: Nussenzweig, Victor. New York University School of Medicine; Estados UnidosFil: Sinnis, Photini. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. New York University School of Medicine; Estados Unidos. Johns Hopkins Malaria Research Institute; Estados UnidosFil: Levitskaya, Jelena. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. Johns Hopkins Malaria Research Institute; Estados UnidosFil: Bosch, Jürgen. Johns Hopkins University Bloomberg School of Public Health; Estados Unidos. Johns Hopkins Malaria Research Institute; Estados Unido