Cytoplasmic free Ca2+ is essential for multiple steps in malaria parasite egress from infected erythrocytes
© Glushakova et al.; licensee BioMed Central Ltd. 2013
Received: 17 November 2012
Accepted: 19 January 2013
Published: 30 January 2013
Egress of Plasmodium falciparum, from erythrocytes at the end of its asexual cycle and subsequent parasite invasion into new host cells, is responsible for parasite dissemination in the human body. The egress pathway is emerging as a coordinated multistep programme that extends in time for tens of minutes, ending with rapid parasite extrusion from erythrocytes. While the Ca2+ regulation of the invasion of P. falciparum in erythrocytes is well established, the role of Ca2+ in parasite egress is poorly understood. This study analysed the involvement of cytoplasmic free Ca2+ in infected erythrocytes during the multistep egress programme of malaria parasites.
Live-cell fluorescence microscopy was used to image parasite egress from infected erythrocytes, assessing the effect of drugs modulating Ca2+ homeostasis on the egress programme.
A steady increase in cytoplasmic free Ca2+ is found to precede parasite egress. This increase is independent of extracellular Ca2+ for at least the last two hours of the cycle, but is dependent upon Ca2+ release from internal stores. Intracellular BAPTA chelation of Ca2+ within the last 45 minutes of the cycle inhibits egress prior to parasitophorous vacuole swelling and erythrocyte membrane poration, two characteristic morphological transformations preceding parasite egress. Inhibitors of the parasite endoplasmic reticulum (ER) Ca2+-ATPase accelerate parasite egress, indicating that Ca2+ stores within the ER are sufficient in supporting egress. Markedly accelerated egress of apparently viable parasites was achieved in mature schizonts using Ca2+ ionophore A23187. Ionophore treatment overcomes the BAPTA-induced block of parasite egress, confirming that free Ca2+ is essential in egress initiation. Ionophore treatment of immature schizonts had an adverse effect inducing parasitophorous vacuole swelling and killing the parasites within the host cell.
The parasite egress programme requires intracellular free Ca2+ for egress initiation, vacuole swelling, and host cell cytoskeleton digestion. The evidence that parasitophorous vacuole swelling, a stage of unaffected egress, is dependent upon a rise in intracellular Ca2+ suggests a mechanism for ionophore-inducible egress and a new target for Ca2+ in the programme liberating parasites from the host cell. A regulatory pathway for egress that depends upon increases in intracellular free Ca2+ is proposed.
KeywordsPlasmodium falciparum Asexual cycle of replication Parasite egress Free calcium Swelling of parasitophorous vacuole
Malaria parasite replicates within its own plasma membrane, positioned inside a parasitophorous vacuole that is located within the host human erythrocyte. Both the vacuolar and erythrocyte plasma membranes must be breached by the parasites to exit the host cell (see reviews by V Lew [1, 2]. Experimental evidence supports a multistep process [3–9] involving at least two Ca2+-dependent enzymes: protein kinase PfCDPK5  and the host protease calpain . The emerging complexity of the egress mechanism suggests a high degree of spatial and temporal coordination. Cytosolic Ca2+ is a universal secondary messenger for intracellular signalling in eukaryotic cells (for review see ). Ca2+ signalling in malaria parasites is well recognized but poorly understood; several signalling pathways have been described [11–14]; for a recent review see . The total erythrocyte calcium content increases with infection [16, 17]. However, it is harder to determine the free calcium concentration, [Ca2+, in the various compartments of an infected erythrocyte at the schizont stage of the asexual cycle. Divided and tightly packed parasites occupying approximately half of the erythrocyte volume , are situated within a series of concentric membrane spheroids; the individual parasites have additional levels of membrane compartments (organelles) internal to their own plasma membrane. Literature estimates of [Ca2+ in different compartments range from low nM in the erythrocyte cytoplasm  to ~ 40 μM in the parasitophorous vacuole – the space between the PV membrane and the parasite plasma membrane . Presumably, there are compartments within the parasite that maintain even higher Ca2+ concentrations and can act as intracellular Ca2+ stores . Thus, there is a complex set of potential gradients for Ca2+ translocations between compartments with differing [Ca2+. An influx of extracellular Ca2+ into erythrocytes is required for parasite invasion [20–22], however, the role Ca2+ plays in the multistep egress programme is not clear [21, 23]. The role of intracellular Ca2+ is intensely studied in the egress programme of the related Apicomplexan parasites, Toxoplasma[24–29]. However, sufficient differences in the molecular mechanisms of the egress pathway between this species and the malaria parasites are emerging [28, 30]. This study reports the measurement of intracellular fluorescent probes for [Ca2+ during the egress pathway of Plasmodium falciparum from infected erythrocytes. Perturbation with calcium homeostasis reagents together with quantitative measures of egress that decouple egress from invasion allowed a determination of one source for the calcium fluxes that must occur for [Ca2+ to change. Finally, to determine which of the observable stages of the parasite egress programme  are sensitive to changes in [Ca2+, the morphology of treated cells was analysed.
Culture of Plasmodium falciparum
Plasmodium falciparum strain 3D7 (ATCC, Manassas, VA, USA) was cultured, according to the Trager–Jensen method , with a modification that uses a controlled gas mixture (5% CO2, 5% O2, 90% N2) instead of a candle jar, in human erythrocytes in RPMI 1640 medium (Invitrogen) supplemented with 25 mM Hepes (Invitrogen), 4.5 mg ml−1 glucose (Sigma), 0.1 mM hypoxanthine (Invitrogen), 25 μg ml−1 gentamicin (Invitrogen) and 0.5% AlbuMax II (Invitrogen). Schizonts were isolated from infected cultures using Percoll enrichment and used to initiate a new 4 h-span, synchronized infection [32, 33] in erythrocytes from donor blood drawn within three days of the procedure.
Parasite egress assay
The detailed description of the parasite egress assay is published . Briefly, experimental and control parasite-infected cells, approaching the end of the cycle, were injected together with their experimental or control solutions into separate microscopy chambers and incubated at 37°C for an interval of time specified for each experiment, before cooling chambers to 15°C to end egress. Egress was calculated as the fraction of schizonts ruptured in the chamber during the incubation time. Treatment effect on parasite egress was evaluated by comparing the fraction of ruptured schizonts in treated and control cultures. Calcium ionophore A23187, EDTA, EGTA, staurosporine, cyclopiazonic acid, and thapsigargin was purchased from Sigma-Aldrich (St. Louis, MO, USA); BAPTA AM, calcein AM, Fluo-4 AM, Fura Red AM and ethidium homodimer were purchased from Invitrogen (Eugene, OR, USA). Cell suspensions of infected erythrocytes at 0.5% haematocrit in AlbuMax II-containing medium were used.
Live cell microscopy
Laser scanning microscopy (LSM 510, Zeiss; 100x or 63x 1.4 NA oil objectives) was used to follow the morphology of parasite-infected erythrocytes, to detect labelled viable or dead parasites, and the origin of swelled membranes in ionophore-treated cultures. To discriminate between the erythrocyte and vacuolar membranes, abundant proteins associated with the erythrocyte membrane were detected using fluorescence microscopy. Glycophorin A was detected using a fluorescent antibody (Allophycocyanin anti-CD235a, BD Biosciences, San Jose, CA, USA) and the cytoskeletal protein F-actin was detected using fluorescent phalloidin (Alexa Fluor 488 phalloidin, Invitrogen, Eugene, OR, USA) as described in . Allophycocyanin and Alexa Fluor 488 were excited at 514 nm and 488 nm respectively. Calcein was excited at 488 nm; ethidium homodimer was excited at 514 nm. Differential interference contrast microscopy was performed using 488 nm laser illumination. To detect free calcium, infected red blood cells were labelled with the calcium indicator Fluo-4 AM (5 μM) and were imaged in time using 488 nm laser illumination; acquisition times varied from ~0.7–31 seconds per image frame. Using ImageJ (NIH) the image sequences were background subtracted (Rolling Ball, 75 pixels), a region of interest that included only the infected cell defined, and the mean fluorescence intensity per pixel calculated as a function of time. The resulting time series were normalized by the initial value: (F(t)-F(t = 0))/F(t = 0) or ΔF/F.
Long-term experiments with Fura Red AM-labelled cells to detect changes in free calcium level were performed in an environmental chamber at 37°C using an inverted fluorescence microscope (Nikon Ti) equipped with 60× 1.49 NA objective, TIRF-illumination arm, and custom-built laser combiner (405, 488, 561 and 640, Coherent). Cells were labelled 30 min with 5 μM Fura Red AM at 37°C in the medium supplemented with 40 μM of probenecid, washed and resuspended to 0.5% haematocrit in medium with 40 μM probenecid. Sequential illumination using 405 nm and 488 nm lasers was used to excite Fura Red. The incident angle of the laser beam was set by a motorized TIRF-unit to wide-field illumination (90 degrees). Fluorescence signals were separated from the excitation light using a quad-band dichroic and emission filter set (405/488/561/640, Semrock) and digitized using an Andor Ixon EMCCD camera. Twenty-four to 48 frames were acquired for each fluorescence channel using a 200 ms exposure and a 5-min interval between exposures to avoid cell photodamage. Microscope, laser, and camera were controlled using Micro-Manager 1.4.10 . Fluorescence images were processed using a custom ratio-metric macro (available upon request) in ImageJ (NIH). In brief, the image sequences were background subtracted (Rolling Ball, 75 pixels), and aligned to correct for cell motion and drift. A region of interest that included only the infected cell was selected, and the mean fluorescence intensity per pixel was calculated as a function of time. The ratio of fluorescence F405/F488 was calculated for each infected cell from background-subtracted images. The F405/F488 ratio rate of change (dRatio405/488/dt) was estimated as the difference between final and initial ratio values divided by the duration of the growth period. Calcium estimates of the ratio measurements were derived using Monte Carlo simulations. The relationship [Ca2+ = Kd*F488([Ca2+min)/F488([Ca2+max)*(R-Rmin)/(Rmax-R) was used to estimate the ratio R using average values and standard deviations for F488([Ca2+min), F488([Ca2+max), Rmin and Rmax where the lowest and highest ratio values and fluorescence intensities from early and late stage schizonts were used as approximations in lieu of direct calibration. Each ratio value at a specific calcium concentration was the average of 100,000 simulations of normal random variables calculated using the population averages and standard deviations, repeated 100 times. A Kd of 0.7 μM was used for Fura Red and represents the expected increase in Kd in the cellular environment [36, 37] compared to the saline value of 0.14 μM.
Results and discussion
Changes in intracellular [Ca2+] in late-stage infected cells
To test the hypothesis that a quasi-static Ca2+ concentration is necessary to activate Ca2+-sensitive proteins regulating or affecting the egress pathway, another protocol to probe [Ca2+] at longer time scales was adopted. To avoid cell photodamage, the time of observation was extended but the interval between frames was increased.
Buffers to intracellular, but not extracellular Ca2+ prevent parasite egress from erythrocytes
The independence on external Ca2+ during the last two hours of the parasite cycle, coupled with the finding of increases in intracellular [Ca2+ during that same time period suggests two possibilities: that Ca2+ is not required for the observable parasite egress stages, i e, swelling of parasitophorous vacuole, poration of host cell membrane and digestion of erythrocyte cytoskeleton, or that intracellular Ca2+ regulates the entire egress pathway. The role of internal Ca2+ stores in parasite egress was tested by treating cells with the membrane permeant Ca2+ chelator BAPTA AM to minimize intracellular increases in [Ca2+. Unlike external Ca2+ chelation, BAPTA AM inhibited egress in a dose-dependent manner over a micro-molar concentration range (15–60 μM) in the bathing media (Figure 3B). To exclude a known nonspecific effects of BAPTA AM on cell physiology, ATP depletion following the production of formaldehyde upon hydrolysis of AM esters  and the finding that this effect is minimized by the addition of exogenous pyruvate , parasite egress was assessed in the presence of both BAPTA AM (30 μM) and pyruvate (4–12 mM). BAPTA AM again inhibited parasite egress in the presence of pyruvate (Additional file 2, B). Moreover, calcein AM, a vital dye that labels both parasite and erythrocyte cytosol following AM ester hydrolysis, did not affect parasite egress when added to the culture medium in the same concentrations as BAPTA AM (Additional file 2, C). The results of these control experiments indicate that the effect of BAPTA AM on parasite egress is due to the Ca2+ affinity of BAPTA, and support the hypothesis that increases in intracellular [Ca2+ are required for parasite egress. To further test this hypothesis, using the Ca2+ ionophore A23187, a permeability pathway for exogenous Ca2+ to enter schizonts saturated with BAPTA was created. This treatment rescued BAPTA-blockage of parasite egress at all times during the last 45–60 min of the parasite cycle (Figure 3C the first bar on the right, Additional file 2, D) confirming that intracellular free Ca2+ is needed for egress.
If only one stage of egress was inhibited, one would expect a pile-up of schizonts at that one stage of egress, and no effect of Ca2+ chelation on schizonts whose egress pathway had progressed beyond the putative Ca2+-sensitive stage. To test this reasoning, live-cell microscopy was used to examine the morphology of infected cells treated with BAPTA AM. Schizonts whose cycle was blocked by Ca2+ chelation were morphologically indistinguishable from untreated schizonts, suggesting that their morphogenesis was not affected by BAPTA AM. They did not enter into the stage of shape transformations that precedes egress and produces a “flower” form of schizonts, i e, swelling of the parasitophorous vacuole, shrinkage of the erythrocyte cytoplasm, and erythrocyte membrane poration [6, 7, 45]. The duration of pre-egress shape-transformations in control schizonts lasted less than 10 min (6.8 ± 0.6 min; mean ± SEM, n = 55). However, in eight of 10 mature schizonts treated with 60 μM BAPTA AM and randomly selected for light microscopy observation the expected cycle progression towards parasite egress was not detected over relatively long observation times (up to 37 min of observation; Additional file 3). This data suggests that Ca2+ chelation blocks the egress pathway upstream of vacuole swelling. Based on the shortest effective inhibitory time of BAPTA treatment (45 min, Additional file 2, D), and prevention of the flower stage in BAPTA-treated schizonts (the last 10 min of the cycle), it is reasonable to conclude that the earliest Ca2+-dependent stages in the egress programme occurs between the last 45 and 10 minutes of the parasite cycle.
The source of Ca2+ for egress
Next, the hypothesis that membrane-bound compartments within the schizont were the source of intracellular Ca2+ needed to increase cytoplasmic [Ca2+ to the threshold levels required for egress initiation and progression was tested. The prediction was that increasing the Ca2+ permeability of internal membranes in the absence of extracellular Ca2+ would prematurely increase cytoplasmic [Ca2+ and prematurely initiate egress. Synchronized cultures of schizonts were exposed to the Ca2+ ionophore A23187 in the presence or absence of extracellular Ca2+. Indeed, in both cases significantly accelerated egress of parasites was detected (Figure 3C). Morphological analysis of cultures treated with ionophore did not reveal any differences in the sites of parasite egress, i e, limited areas in microscopy chambers where scattered parasites, food vacuole, fragmented host membrane are localized [6, 34]. There was no significant difference in the viability of parasites released in the presence of A23187 compared to control (2.07 ± 0.48% of dead cells vs 0.85 ± 0.28% respectively, ethidium homodimer assay, mean ± SEM, n = 369 and 367 respectively; P = 0.1). The accelerated egress was dose-dependent in the 1–10 μM ionophore concentration range (Additional file 4, A). Note, the active concentration of A23187 may be even lower because this hydrophobic ionophore is known to bind to serum albumin in the medium . The finding that accelerated egress occurs in the absence of extracellular Ca2+ (Figure 3C) indicates that an intracellular Ca2+ source is sufficient for initiating egress. To confirm that egress initiation by A23187 is Ca2+-dependent, cells were treated with ionophore in Ca2+-free medium (medium with 3 mM EDTA) following chelation of intracellular Ca2+ stores with 30 μΜ BAPTA AM. Parasite egress was inhibited (Figure 3C), indicating the finite nature of the chelatable Ca2+ pool within the internal stores since A23187 plus exogenous Ca2+ could rescue BAPTA AM treated cells (Figure 3C). Thus, finite internal stores of Ca2+ can initiate and regulate the egress pathway.
One of the suggested targets for intracellular Ca2+ in the egress programme is the Ca2+-dependent protein kinase PfCDPK5  that is activated downstream of another kinase involved in egress, a cyclic guanosine monophosphate (cGMP)–dependent kinase PfPKG . If PfCDPK5 were a required transducer of the rise in [Ca2+ to affect release, then one would predict that inhibiting kinase activity during the sensitivity time to increased [Ca2+ would prevent egress. To test this prediction, schizonts were treated with 1.0-2.5 μM staurosporine, a promiscuous inhibitor of protein kinases  during the last hour of the parasite cycle. Staurosporine strongly inhibited parasite egress (82.8 ± 1.7% inhibition; mean ± SEM, n = 4). This pharmacological data suggests that the Ca2+-sensitive stage of parasite egress and the stage of egress that requires kinase activities are temporally overlapping events.
Initiation of vacuole swelling is a newly discovered Ca2+ target in egress execution
Plasmodium thus is similar to other Apicomplexan parasites with respect to the involvement of Ca2+ in its egress. A23187 is a potent trigger of Toxoplasma parasite egress, where the effect is also independent of external Ca2+, but it is dependent on internal Ca2+ that activates two kinases involved in egress, TgCDPK1 and TgCDPK3  . In addition, A23187 inducible egress is also seen in two other Apicomplexan parasites of veterinary significance: Neospora caninum and Eimeria bovis. A distinct difference between Toxoplasma gondii, N. caninum, E. bovis and results presented in this study with P. falciparum is the timing of the ionophore-induced egress: in the first three organisms egress could be induced as early as two hours after parasite invasion [52, 53], while in malaria parasite egress required maturation – full completion of merozoites morphogenesis within the schizonts – to prevent premature death in the host cell. This difference in parasite biology allowed the establishment of an underlying mechanism for A23187-induced parasite egress, which is the swelling of the parasitophorous vacuole. There are other indications that Plasmodium and Toxoplasma egress pathways differ: Toxoplasma species initiate egress by production of the plant hormone abscisic acid (ABA)  and the progression of unaffected egress in Toxoplasma depends on Ca2+-binding protein DOC2 . However, the ABA pathway has not been confirmed for P. falciparum (S. Glushakova, unpublished observation) and the DOC2 protein is not involved in the egress pathway of Plasmodium. These differences in parasite biology highlight the divergence between Plasmodium and Toxoplasma and stress the importance of cautious extrapolation of data on the egress mechanism of Toxoplasma to Plasmodium and vice versa.
Ca2+-dependent digestion of host-cell cytoskeleton
The ionophore-induced acceleration of malaria parasite egress could not be accomplished without dismantling the host erythrocyte cytoskeleton [4, 54]. Cytoskeleton digestion precedes constitutive egress  and could be blocked by inhibitors of cysteine proteases [45, 54]. Presented here findings that the sites of parasite egress in the ionophore-treated cultures are indistinguishable from those in untreated egress and the presence of fragmented erythrocyte membranes supports the hypothesis that the erythrocyte cytoskeleton is dismantled by Ca2+-activation of calpain . In addition, the observation of blebbed erythrocyte membranes shed from damaged, ionophore treated, immature schizonts suggests that Ca2+ may have activated calpain in these cells as well (Additional file 4, C).
Further investigations are required to establish the source of free Ca2+ for initiation of vacuole swelling and erythrocyte cytoskeleton digestion and the pathway by which this second messenger arrives at the site of action. At the onset of egress, both host and parasite cells, each have their individual cytoplasm low in [Ca2+. Since putative targets for [Ca2+ are in both cell’s cytoplasm, it is reasonable to think that each cytoplasm can receive calcium. The parasite compartment includes two potential stores, parasite’s internal stores and the parasitophorous vacuole with a relatively high [Ca2+. The host erythrocyte cytoplasm borders the parasitophorous vacuole, with its irregular extensions into the erythrocyte cytoplasm . In this speculation the parasitophorous vacuole is associated with the calcium storage function usually seen in the endoplasmic reticulum. Another unknown is the nature of the effector proteins involved in vacuolar membrane swelling. Usually, it is a change in the permeability of a limiting membrane that drives biological swelling via the resulting osmotic imbalance, implicating ion channels and transporters of either parasite or host cell origin. If involved, these proteins are attractive therapeutic target molecules and may be the basis for the design of new anti-malarial drugs.
In this study an increase in cytoplasmic [Ca2+] that appears to be essential for progression of the emerging release process of P. falciparum was detected. Intracellular [Ca2+] has multiple targets in the malaria parasite egress programme: initiation of egress (signalling) within the last hour of the cycle through Ca2+ release from the ER stores of mature schizonts and following activation of Ca2+-dependent PfCDPK5, induction of Ca2+-dependent vacuole swelling within the last 10–20 min of cycle, and activation of Ca2+-dependent host-cell calpain for destabilization of the erythrocyte cytoskeleton finalized prior to egress (Additional file 5). The finding that malaria parasite egress from erythrocytes could be pharmacologically induced by modulators of cellular Ca2+ homeostasis was demonstrated for the first time and thus these reagents now could be used to study egress pathway in a controlled manner.
A modified egress pathway is now presented. Parasites replicate within a single plasma membrane. This membrane invaginates wrapping newly synthesized material and creating distinct merozoites. Upon completion of morphogenesis, Ca2+ signalling from intracellular stores initiates a cascade of biochemical and physiological events that initiate parasite departure from the host cell. This Ca2+-dependent cascade includes the activation of a protease cascade (reviewed in ) and at least one Ca2+-dependent protein kinase . For the execution of egress, increases in [Ca2+ activates vacuole swelling, presumably via Ca2+- activated channels to alter internal membrane permeability needed to rupture the parasitophorus vacuole. The same intracellular Ca2+ must activate calpain and the suggested perforin in the erythrocyte cytoplasmic compartment to destabilize and perforate the host cell membrane [4, 7, 45, 54]. Cytoskeleton digestion eliminates erythrocyte membrane resistance to host cell shrinkage and permeation of erythrocyte membrane reduces the haemoglobin concentration and minimizes erythrocyte cytoplasm viscosity. Egress of parasites from the bursting vacuole through the haemoglobin-depleted erythrocyte compartment and weakened erythrocyte membrane into the bloodstream finalizes the erythrocyte cycle of this human pathogen.
Glycine N,N’-[1,2-ethanediylbis(oxy-2,1-phenylene)]bis[N-[2-[(acetyloxy)methoxy]-2-oxoethyl]] bis[(acetyloxy)methyl] ester
Cyclic guanosine monophosphate
Ethylene diamine tetraacidic acid
Ethylene glycol tetraacidic acid
Electron multiplying charge coupled device
Laser scanning microscope
National Institutes of Health
Plasmodium falciparum Ca2+-ATP-ase 6
Plasmodium falciparum calcium-dependent protein kinase 5
Plasmodium falciparum cGMP-dependent protein kinase
Total internal reflection fluorescence microscopy
Standard error of the mean
Sarco/endoplasmic reticulum Ca2+-ATP-ase.
We would like to thank Amanda Balaban for her contribution to the initial stages of this project and Dr Rea Ravin for critical discussions. This work was supported by the intramural programme of the NICHD, NIH.
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