Plasmodium Merozoite TRAP Family Protein Is Essential for Vacuole Membrane Disruption and Gamete Egress from Erythrocytes

Abstract

Surface-associated TRAP (thrombospondin-related anonymous protein) family proteins are conserved across the phylum of apicomplexan parasites. TRAP proteins are thought to play an integral role in parasite motility and cell invasion by linking the extracellular environment with the parasite submembrane actomyosin motor. Blood stage forms of the malaria parasite Plasmodium express a TRAP family protein called merozoite-TRAP (MTRAP) that has been implicated in erythrocyte invasion. Using MTRAP-deficient mutants of the rodent-infecting P. berghei and human-infecting P. falciparum parasites, we show that MTRAP is dispensable for erythrocyte invasion. Instead, MTRAP is essential for gamete egress from erythrocytes, where it is necessary for the disruption of the gamete-containing parasitophorous vacuole membrane, and thus for parasite transmission to mosquitoes. This indicates that motor-binding TRAP family members function not just in parasite motility and cell invasion but also in membrane disruption and cell egress.

Graphical abstract

Figure 1

Generation of PbMTRAP^KO Clones (A) Illustration of the strategy used for replacing the coding sequence of MTRAP by a cassette for expression of the selection marker human dihydrofolate reductase (hDHFR), that confers resistance to pyrimethamine, and a cassette for expression of mCherry (red fluorescence). The primers (arrowheads) and probes (green bars) used for genotyping are shown. The expected fragment sizes after digestion of the loci with MfeI are also shown. (B) PCR analysis of the mtrap locus in wild-type (WT) or mutant (B4 and B8) parasites. P1/P2 pair of primers is specific to the WT locus, and P1/P3 pair is specific to integration of the targeting sequence. (C) Southern blot detecting the mtrap locus in wild-type (WT) or mutant (B4 and B8) parasites after digestion of genomic DNA with MfeI. The probe used is illustrated in (A) (green bars). (D) Growth curves assessed daily in mouse blood after infection with wild-type (black line) or the two clones of PbMTRAP^KO parasites (blue and red lines). Results are shown as mean ± SD and are representative of three independent experiments. N = 5 for each group. (E) Fluorescence microscopy with anti-PbMTRAP (green), anti-AMA1 (red), and DAPI (blue) in wild-type P. berghei merozoites. BF, brightfield. Scale bar, 5 μm. (F) Fluorescence microscopy with anti-PbMTRAP (green), anti-MDV-1/PEG3 (red) and DAPI (blue) in a wild-type P. berghei sexual stage isolated from infected mouse blood. BF, brightfield. Scale bar, 5 μm. See also Figure S1. (G) Fluorescence microscopy with anti-PbMTRAP (green), anti-MDV-1/PEG3 (red), and DAPI (blue) in nonactivated or 10 min activated wild-type P. berghei sexual stages isolated from infected mouse blood. BF, brightfield. Scale bar, 5 μm. (H) Fluorescence microscopy with anti-PbMTRAP (green) and DAPI (blue) in MTRAP knockout (PbMTRAP^KO) and wild-type parasites. BF, brightfield. Scale bar, 5 μm. (I) Western blot analysis of the gametocyte extract of PbMTRAP^KO (gKO) with a specific antibody recognizing the MTRAP C-terminal region. Total extract (tWT) or gametocyte extract (gWT) of wild-type P. berghei ANKA strain was used as control. Anti-aldolase (ALD) was used as loading control. The anti-MTRAP recognizes two specific bands in tWT and one specific band in gWT parasites. No bands are recognized in the three gKO extract.

Figure 2

PbMTRAP^KO Are Blocked in Mosquito Transmission (A) P. berghei oocysts in the midgut of mosquitoes fed onto mice infected with wild-type or PbMTRAP^KO. Oocysts are visualized by mercurochrome staining of mosquito midguts 7 days after mosquito feeding. Scale bar, 100 μm. Quantification is shown on the left. N = 100 mosquitoes for each group. (B) Quantification of P. berghei male gametocytes (MG, blue) and female gametocytes (FG, pink) circulating in mouse blood infected with either wild-type (WT) or PbMTRAP^KO parasites. (C) Quantification of in vitro ookinete formation from gametocytes circulating in mouse blood infected with either wild-type (WT) or PbMTRAP^KO parasites. (D) Quantification of green, red, and yellow (green + red) P. berghei oocyst numbers by fluorescence microscopy of mosquito midguts 7 days after mosquito feeding onto mice infected with a control mixture of green and red wild-type parasites (GFP⁺WT and RFP⁺WT, respectively), or with a mixture of PbMTRAP^KO (red, mCh⁺PbMTRAP^KO) and wild-type green (GFP⁺WT) parasites. N = 100 mosquitoes for each group. The gametocytemia of green and red parasites were comparable in infected mice of the different groups used for mosquito feeding (data not shown). For all panels, data are shown as mean ± SD and are representative of three independent experiments.

Figure 3

PbMTRAP^KO Male Gametocytes Do Not Make Exflagellation Centers but Form Motile Axonemes (A) Quantification of exflagellation centers per 10× field formed by in vitro-activated wild-type P. berghei (WT), PbMTRAP^KO male gametocytes, or PbMTRAP^KO carrying either a control episome (contComp) or an episome with the promoter and coding sequence of mtrap cloned upstream the 3′UTR of trap, a centromeric sequence and a cassette for GFP (green) expression (mtrapComp). The results are shown as mean ± SD and are representative of four independent experiments. (B) Time-lapse microscopy of an activated PbMTRAP^KO male gametocyte. The time in seconds is shown in each image. The results are representative of five independent experiments. (C) Quantification of oocysts per midgut of mosquitoes fed onto mice infected with PbMTRAP^KO carrying either the control episome (contComp) or the episome with mtrap (mtrapComp). The results are shown as mean ± SD and are representative of two independent experiments. (D) Fluorescence microscopy of mosquito midgut 7 days after mosquito feeding onto mice infected with PbMTRAP^KO (red) electroporated with the mtrapComp episome. The presence of the episome is depicted by green fluorescence, and parasites are red fluorescent. Single color oocysts were never seen. N = 100 mosquitoes. Scale bar, 100 μm.

Figure 4

PbMTRAP^KO Gametes Are Trapped inside the PV Membrane (A) Micrographs of wild-type P. berghei (WT) or PbMTRAP^KO gametocytes isolated from infected mice blood and immediately fixed (nonactivated) or fixed after activation in vitro for 15 min (15 min p.a.) in ookinete medium. Ultrastructures are indicated with arrows. IMC, inner membrane complex; PPM, parasite plasma membrane; PVM, parasitophorous vacuole membrane; EM, erythrocyte membrane; N, nucleus; G, Golgi complex. Results are representative of three independent experiments. N = 6 for WT and 19 for PbMTRAP^KO. Scale bars, 1 μm. (B) Micrograph of a male PbMTRAP^KO gametocyte activated in vitro for 15 min in ookinete medium. Ultrastructures are shown as in (A), except for Ax, axonemes. Scale bars, 1 μm.

Figure 5

MTRAP Is Dispensable for P. falciparum Asexual Stages (A) Illustration of the strategy used to target P. falciparum mtrap for disruption. Two plasmids were transfected in the P. falciparum 3D7 strain, one plasmid carrying a guide DNA sequence (GAATGGTCAGAATGTAAAGA) and a hDHFR cassette flanked by two homology regions with the 5′and 3′ sequences of the mtrap coding sequence as indicated in the figure, and the second plasmid bearing a cassette for Cas9 expression. Double homologous recombination replaces 935 base pairs of the mtrap coding sequence by the hDHFR cassette, creating a disrupted locus. Primers used for PCR specific detection of the genomic or the disrupted loci are shown. See also Figure S2. (B) Western blot analysis of the PfMTRAP^KO clones C3, C8, and C18 with a specific antibody recognizing the MTRAP C-terminal region (α-MTRAP-Tail). Wild-type P. falciparum 3D7 strain (WT) was used as control. The α-MTRAP-Tail recognizes three specific bands in WT parasites, FL as the full-length protein, cleavage as a processed fragment, and Tail as the C-terminal region after processing. No bands are recognized in the three PfMTRAP^KO clones. Actin (α-actin) was used as loading controls. (C) Growth curves assessed every 48 hr by flow cytometry in blood cultures of P. falciparum wild-type (3D7, black line) or the three PfMTRAP^KO clones (colored lines). The experiment was performed in triplicate and the data are presented as mean ± SD.

Figure 6

MTRAP Is Expressed in Sexual Stages of P. falciparum (A) Fluorescence microscopy with anti-PfMTRAP (green), anti-Pfg377 (red), and DAPI (blue) in wild-type P. falciparum sexual stages matured in vitro. Stages III, IV, and V gametocytes are shown. BF, brightfield. Scale bar, 5 μm. See also Figure S3. (B) Fluorescence microscopy with anti-PfMTRAP (green), anti-Band3 (red), and DAPI (blue) in wild-type P. falciparum sexual stages matured in vitro. A gametocyte nonactivated (preactivation), a gametocyte activated for 30 s in vitro, and an egressed gamete after 600 s of activation in vitro are shown. BF, brightfield. Scale bar, 5 μm. See also Figure S7A.

Figure 7

PPLP2 Secretion in PfMTRAP^KO Gametocytes (A) Fluorescence microscopy with anti-Band3 (green), anti-PPLP2 (red), and DAPI (blue) in wild-type and MTRAP^KO NF54 P. falciparum sexual stages matured in vitro nonactivated or 2.5 hr postactivation. BF, brightfield. Scale bar, 5 μm. (B) Fluorescence microscopy with anti-tubulin (green), anti-PPLP2 (red), and DAPI (blue) in wild-type and MTRAP^KO NF54 P. falciparum sexual stages matured in vitro 25 min postactivation. BF, brightfield. Scale bar, 5 μm.