Different Roles of Mitochondrial Calcium Uniporter Complex Subunits in Growth and Infectivity of Trypanosoma cruzi

ABSTRACT

Trypanosoma cruzi is the agent of Chagas disease, and the finding that this parasite possesses a mitochondrial calcium uniporter (TcMCU) with characteristics similar to that of mammalian mitochondria was fundamental for the discovery of the molecular nature of MCU in eukaryotes. We report here that ablation of TcMCU, or its paralog TcMCUb, by clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 led to a marked decrease in mitochondrial Ca²⁺ uptake without affecting the membrane potential of these cells, whereas overexpression of each gene caused a significant increase in the ability of mitochondria to accumulate Ca²⁺. While TcMCU-knockout (KO) epimastigotes were viable and able to differentiate into trypomastigotes, infect host cells, and replicate normally, ablation of TcMCUb resulted in epimastigotes having an important growth defect, lower rates of respiration and metacyclogenesis, more pronounced autophagy changes under starvation, and significantly reduced infectivity. Overexpression of TcMCUb, in contrast to what was proposed for its mammalian ortholog, did not result in a dominant negative effect on TcMCU.

IMPORTANCE

The finding of a mitochondrial calcium uniporter (MCU) in Trypanosoma cruzi was essential for the discovery of the molecular nature of this transporter in mammals. In this work, we used the CRISPR/Cas9 technique that we recently developed for T. cruzi to knock out two components of the uniporter: MCU, the pore subunit, and MCUb, which was proposed as a negative regulator of MCU in human cells. In contrast to what occurs in human cells, MCU is not essential, while MCUb is essential for growth, differentiation, and infectivity; has a bioenergetic role; and does not act as a dominant negative subunit of MCU.

INTRODUCTION

Trypanosoma cruzi is the etiologic agent of Chagas disease, an enormous burden on human health in the American continent, which has four major developmental stages that alternate between an insect vector and a mammalian host. Two are replicative forms, the epimastigote found in the insect vector intestine and the intracellular mammalian form or amastigote, and two are nonreplicative, the metacyclic trypomastigote found in the rectum and urine of the vector and the bloodstream trypomastigote found in the mammalian host. All these forms appear to have functional mitochondria with an active oxidative metabolism (1, 2).

The finding that mitochondrial Ca²⁺ transport in T. cruzi is electrogenic, has low affinity and high capacity, and is inhibited by ruthenium red, as occurs with vertebrate mitochondria, identified the presence of a mitochondrial calcium uniporter (MCU) in trypanosomatids (3, 4). The presence of MCU in trypanosomes together with its absence in yeast (5) led to the identification, first, of the gene encoding an MCU modulator (mitochondrial calcium uptake 1 [MICU1]) (6) and then of the gene encoding the MCU of mammalian cells (7–9). In recent work, we demonstrated that the MCU of Trypanosoma brucei, which belongs to the group of parasites that produce African trypanosomiasis or sleeping sickness, is essential for the regulation of the bioenergetics of the parasite and its growth and infectivity (10).

After the discovery of the molecular identity of MCU, other components of the MCU complex were described (11, 12). One of them, MCUb, was shown to exert a dominant negative effect in HeLa cell mitochondria, reducing the mitochondrial Ca²⁺ increase evoked by agonist stimulation (13).

In this work, we report that knockout of the T. cruzi MCU (TcMCU) gene by clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 in T. cruzi epimastigotes (14) abolishes mitochondrial calcium uptake without affecting their mitochondrial membrane potential (ΔΨ_m) and reduces growth in low-glucose medium. However, epimastigotes conserve their ability to differentiate into metacyclic trypomastigotes and infect mammalian cells. In contrast to a previous report on HeLa cells (13), overexpression of TcMCUb does not have a dominant negative effect but increases mitochondrial Ca²⁺ uptake without affecting the ΔΨ_m. Knockout of TcMCUb by CRISPR/Cas9 abolishes mitochondrial Ca²⁺ transport, reduces respiration, has a significant effect on epimastigote growth, and increases autophagy. These cells have a reduced ability to differentiate into metacyclic trypomastigotes and are unable to efficiently infect cells, underscoring the relevance of TcMCUb for the parasite life cycle.

RESULTS

Ca²⁺ uptake by TcMCU and TcMCUb knockouts.

After the recent characterization of MCU as the channel-forming subunit of the mitochondrial calcium uniporter complex (15), several pore regulators were reported, among them mitochondrial calcium uptake 1 (MICU1), MICU2, MCUb, essential MCU regulator (EMRE), and MCU regulator 1 (MCUR1) (11). It has been suggested that MCUb is a dominant negative regulator of the uniporter complex (13). However, evidence of its influence on MCU regulation is lacking. MCUb has been identified in the T. cruzi genome, and similarly to its paralog MCU, it has two predicted transmembrane domains (16). Therefore, we aimed at investigating the effect of downregulation and overexpression of TcMCUb on the physiological role of the MCU complex in T. cruzi.

We used the recently developed CRISPR/Cas9 system with a DNA donor cassette for DNA repair (14) to knock out TcMCU (Fig. 1A and B) and TbMCUb (Fig. 1E and F) in epimastigotes. After 5 to 6 weeks of selection under G418 and blasticidin, we obtained resistant populations of epimastigotes transfected with specific single guide RNAs (sgRNAs) and blasticidin cassettes. These were constructed with long (~500-bp) flanking untranslated regions (UTRs) cloned in pGEM-T Easy vector to obtain TcMCU-KO cells or using long oligonucleotides (ultramers) with shorter homology regions (100 bp) to obtain the TcMCUb-KO cell line. Primers (see Table S1 in the supplemental material) were used in PCR experiments to check TcMCU and TcMCUb disruption in G418/blasticidin-resistant cells. As shown in Fig. 1C, TcMCU was ablated and replaced by the blasticidin resistance gene in TcMCU-KO cells. Disruption of the TcMCUb gene was also demonstrated using specific primers (Fig. 1G). In addition, Southern blot analyses demonstrated that TcMCU (Fig. 1D) and TcMCUb (Fig. 1H) were absent in genomic DNA of the KO cell lines.

FIG 1 .

Ca²⁺ uptake by TcMCU and TcMCUb knockouts. (A) Schematic representation of the strategy used to generate a TcMCU-KO mutant by CRISPR/Cas9-induced homologous recombination. A double-stranded gDNA break was produced by Cas9 at nt +115 of the TcMCU ORF. DNA was repaired with a blasticidin S-deaminase (Bsd) cassette containing ~500-bp homologous regions from the TcMCU locus. (B) Primers (arrows) that were used to verify gene disruption by PCR. The intact locus generates a PCR product of 1,890 bp, while the replaced locus generates a fragment of 1,370 bp. (C) TcMCU was ablated at its genomic locus and replaced in genomic DNA of the KO cell line. Lanes: L, 1-kb plus ladder; 1, wild type; 2, pTREX-n control cells; 3, Cas9/pTREX-n control cells; 4, TcMCU-KO mutant cell line; 5, scrambled sgRNA/Cas9/pTREX-n control cells; 6, PCR negative control. (D) Southern blot analysis of wild-type (WT) and TcMCU-KO (KO) epimastigotes. The blot was hybridized with a 100-bp radiolabeled region of TcMCU. (E) Schematic representation of the strategy used to generate a TcMCUb-KO mutant by CRISPR/Cas9-induced homologous recombination. A double-stranded gDNA break was produced by Cas9 at nt +310 of the TcMCUb ORF (792 bp). DNA was repaired with a blasticidin S-deaminase (Bsd) cassette containing 100-bp homologous regions spanning from nt −20 to +80 and from nt +510 to +610 of the TcMCUb locus. (F) Primers (arrows) used to verify gene disruption by PCR. The intact locus generates a PCR product of 650 bp, while the disrupted locus generates a fragment of 620 bp. (G) TcMCUb was disrupted at its genomic locus where the Bsd gene replaced a 430-bp fragment in the KO cell line. Lanes: L, 1-kb plus ladder; 1, wild type; 2, TcMCUb-KO mutant cell line; 3, PCR negative control. (H) Southern blot analysis of wild-type (WT) and TcMCUb-KO (KO) epimastigotes. The blot was hybridized with a biotin-labeled probe corresponding to 430 bp of TcMCUb (nt +80 to +510). (I) Ca²⁺ uptake by digitonin-permeabilized epimastigotes in relative fluorescence units (RFU). MCU-KO, TcMCU-KO cells; Scrambled, scrambled control cells in absence or presence (+ RR) of 5 µM ruthenium red. The reaction was started after adding 50 µM digitonin in the presence of 20 µM CaCl₂. Where indicated, 4 µM FCCP was added. (J) Quantification of data in panel I. Relative Ca²⁺ uptake at 400 s compared with epimastigotes transfected with scrambled control. (K) Ca²⁺ uptake by digitonin-permeabilized epimastigotes in relative fluorescence units (RFU). MCUb-KO, TcMCU-KO cells; Scrambled, scrambled control cells in absence or presence (+ RR) of 5 µM ruthenium red. The reaction was started after adding 50 µM digitonin in the presence of 20 µM CaCl₂. Where indicated, 4 µM FCCP was added. (L) Quantification of data in panel K. Relative Ca²⁺ uptake at 400 s compared with epimastigotes transfected with scrambled control. Values in panels J and L are means ± SD (n = 3). ***, P < 0.001.

TABLE S1 

Oligonucleotides used in this work. Download TABLE S1, DOCX file, 0.1 MB ^{(110.9KB, docx)} .

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To determine the capacity of TcMCU-KO and TcMCUb-KO cell lines to take up Ca²⁺, we monitored Ca²⁺ uptake with calcium green-5N probe in digitonin-permeabilized epimastigotes. A decrease in fluorescence indicates decreasing medium Ca²⁺ or increasing vesicular Ca²⁺. Figures 1I and K show that addition of 50 µM digitonin in the presence of 5 mM succinate and 20 µM Ca²⁺ produced a fast decrease in Ca²⁺ concentration starting after a period of about 60 s, which continued until addition of the uncoupler p-trifluoromethoxyphenylhydrazone (FCCP), which released the mitochondrial Ca²⁺ taken up. This Ca²⁺-transporting activity was fully eliminated by the addition of 5 µM ruthenium red, indicating that it is due to the uniporter. Knockout of TcMCU (Fig. 1I and J) or TcMCUb (Fig. 1K and L) abolished the T. cruzi mitochondrial ability to take up Ca²⁺.

Coimmunoprecipitation of TcMCU and TcMCUb and Ca²⁺ uptake by TcMCU- and TcMCUb-overexpressing cells.

We also generated cell lines overexpressing TcMCU (TcMCU-OE) or TcMCUb (TcMCUb-OE) cloned in pTREX-n vector, as described in Materials and Methods. We recently demonstrated that endogenously tagged TcMCU localizes to the mitochondria of T. cruzi (17). TcMCU-OE protein also localized to the mitochondria, as demonstrated by immunofluorescence microscopy (see Fig. S1A). Antibody to T. brucei MCU (10) colocalized with antibody against the voltage-dependent anion channel (TbVDAC), which is a major protein of the outer mitochondrial membrane of eukaryotes. Western blot analysis detected a band of the expected size (32 kDa) in the overexpressing cell line (Fig. 2A). The band was not visible in wild-type (WT) cells, probably as a consequence of its low expression levels. TcMCU-OE and TcMCUb-OE (Fig. S1B) cells exhibited the same growth rate as control cells transfected with the pTREX-n empty vector. Western blot analysis of TcMCUb-OE cells using antibodies against its hemagglutinin (HA) tag confirmed its expression (Fig. 2B), and immunofluorescence analysis demonstrated its mitochondrial localization (Fig. S1C).

FIG 2 .

Coimmunoprecipitation of TcMCU and TcMCUb and Ca²⁺ uptake by TcMCU- and TcMCUb-overexpressing cells. (A) Western blot analysis of TcMCU in total protein extracts of wild-type (WT) and TcMCU-OE (MCU-OE) epimastigotes, using anti-TbMCU antibodies. Anti-α-tubulin antibodies were used as a loading control. (B) Western blot analysis of total protein extracts of wild-type (WT) and TcMCUb-3×HA-OE (MCUb OE) epimastigotes using anti-HA antibodies. Anti-α-tubulin antibodies were used as a loading control. (C) Anti-c-Myc and anti-HA immunoprecipitations (IP) using lysates from epimastigotes overexpressing TcMCUb-3×HA and TcMCU-3×c-Myc. Immunoblot assays (IB) were done with anti-c-Myc and anti-HA antibodies. Analyzed fractions were as follows: Lys, total lysate; PC, precleared lysate; FT, flowthrough; W, last wash; E, eluate; C, control agarose. (D) Ca²⁺ uptake by digitonin-permeabilized empty vector (pTREX-n) and TcMCU-OE (MCU-OE) cells. Other additions were as in Fig. 1I. EGTA (4 µM) and Ca²⁺ (2 µM pulses) were added where indicated. (E) Quantification of data in panel D. Relative Ca²⁺ uptake at 400 s compared with empty vector (EV). (F) Similar conditions as in panel D, except that reaction buffer contained 0.2% bovine serum albumin and further CaCl₂ pulses (6 µM each time) were added to empty vector and MCU-OE cells to show the high mitochondrial capacity to take up Ca²⁺. (G) Quantification of data in panel F. Relative Ca²⁺ uptake capacity (number of Ca²⁺ pulses) compared with that of empty vector cells. (H) Ca²⁺ uptake by digitonin-permeabilized empty vector (pTREX-n) and MCUb-OE cells. Other additions were as in Fig. 1K. EGTA (4 µM) and Ca²⁺ (2 µM pulses) were added where indicated. (I) Quantification of data in panel H. Relative Ca²⁺ uptake at 400 s compared with empty vector. (J) Similar conditions as in panel H except that reaction buffer contained 0.2% bovine serum albumin and further CaCl₂ additions (6 µM each time) were added to empty vector and MCUb-OE cells to show the high mitochondrial capacity to take up Ca²⁺. (K) Quantification of data in panel J. Relative Ca²⁺ uptake capacity (number of Ca²⁺ pulses) compared with that of empty vector cells. Values in panels E, G, I, and K are means ± SD (n = 3). *, P < 0.05; **, P < 0.01.

FIG S1 

Localization and growth of TcMCU-OE and TcMCUb-OE epimastigotes. (A) Fluorescence microscopy of TcMCU-OE epimastigotes (MCU) using anti-TbMCU antibodies (green) and anti-TbVDAC (VDAC) antibodies as a mitochondrial marker (red). Nuclei and kinetoplast were labeled with DAPI (blue). Colocalization of TcMCU-OE and TcVDAC is shown in yellow (merge). A differential interference contrast (DIC) image is shown in the left panel. (B) Growth curve of TcMCU-OE (MCU-OE) epimastigotes, TcMCUb-OE (MCUb-OE) epimastigotes, and epimastigotes transfected with pTREX-n empty vector. (C) Colocalization of TcMCUb-3×HA-OE using anti-HA antibodies (HA) with MitoTracker (MT). (D) Western blot analysis of WT, TcMCU-3×c-Myc, and TcMCU-3×c-Myc/pTREX-n-TcMCUb-3×HA cell lines. Anti-c-Myc and anti-HA antibodies detect TcMCU-3×c-Myc (predicted size, 35 kDa) and TcMCUb-3×HA (predicted size, 31 kDa), respectively, whereas both bands are absent in WT parasites. Anti-α-tubulin antibody was used as a loading control. The TcMCU-3×c-Myc cell line was obtained by CRISPR/Cas9-mediated endogenous C-terminal tagging as described previously (15). Antibodies are indicated on the right side of the blots, and molecular weights are on the left side. There was no significant difference in intensity of bands corresponding to TcMCU-3×c-Myc detected with anti-c-Myc antibodies between TcMCU-3×c-Myc and TcMCU-3×c-Myc/pTREX-n-TcMCUb-3×HA cell lines. Bars in panels A and C, 10 µm. Values in panel B are means ± SD (n = 3; no significant difference). Download FIG S1, TIF file, 1.4 MB ^{(1.5MB, tif)} .

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To investigate whether TcMCU and TcMCUb could form oligomers as previously described with their orthologs in HeLa cells (13), we used a T. cruzi cell line that coexpresses the c-Myc-tagged TcMCU (TcMCU-3×c-Myc) and the HA-tagged TcMCUb (TcMCUb-3×HA), as described in Materials and Methods. Cells were lysed, and immunoprecipitation was done with either anti-c-Myc or anti-HA antibody. Western blot analyses showed that the anti-HA antibody immunoprecipitated TcMCU-3×c-Myc and the anti-c-Myc antibody immunoprecipitated TcMCUb-3×HA, revealing the MCU-MCUb interaction in situ (Fig. 2C).

TcMCU-OE (Fig. 2D and E) and TcMCUb-OE (Fig. 2H and I) epimastigotes showed increased Ca²⁺ uptake. The mitochondria of permeabilized control epimastigotes were capable of buffering multiple pulses of exogenously added Ca²⁺, and overexpression of TcMCU (Fig. 2F and G) or TcMCUb (Fig. 2J and K) increased significantly the ability of their mitochondria to accumulate Ca²⁺ in response to Ca²⁺ pulses. Overexpression of TcMCUb in epimastigotes possessing endogenously tagged TcMCU did not induce its overexpression (Fig. S1D).

Analysis of the mitochondrial membrane potential (ΔΨ_m) of mutant cell lines.

Safranin O was used to measure ΔΨ_m in digitonin-permeabilized epimastigotes in the presence of succinate as the mitochondrial substrate. To be certain that the ΔΨ_m was not affected in the mutants, we calibrated the ΔΨ_m using valinomycin and potassium as we described before (18) and expressed the changes in millivolts instead of fluorescence arbitrary units. The magnitude of this membrane potential of epimastigote mitochondria respiring on succinate was 177 ± 2.0, 179.6 ± 2.8, 177.6 ± 2.3, 177.7 ± 1.6, and 177.9 ± 1.7 mV for wild-type, TcMCU-KO, TcMCU-OE, TcMCUb-KO, and TcMCUb-OE cells, respectively (n = 3). When using safranin O, an increase in fluorescence after addition of digitonin indicated stacking of the dye to the energized inner mitochondrial membrane (Fig. S2A to C). Addition of ADP produced the expected small decrease in membrane potential, indicating ADP phosphorylation. ΔΨ_m returned to its initial level after addition of the adenine nucleotide translocator inhibitor carboxyatractyloside (CAT). Addition of FCCP collapsed the membrane potential. Neither knockout nor overexpression of either TcMCU (Fig. S2A to C) or TcMCUb (Fig. S2D to F) affected the ΔΨ_m at the steady state or ADP phosphorylation.

FIG S2 

Mitochondrial membrane potential in mutant epimastigotes (A and B). Changes in mitochondrial membrane potential (ΔΨ_m) of digitonin-permeabilized epimastigotes as detected by changes in safranin O fluorescence in epimastigotes transfected with scrambled sgRNA/Cas9/pTREX-n (Scrambled) or TcMCU-KO (MCU-KO) (A) and empty vector or TcMCU-OE (MCU-OE) (B). Cells (5 × 10⁷) were added to the reaction buffer (2 ml) containing 0.2% bovine serum albumin, 2 mM succinate, and 5 µM safranin O. The reaction was started with 50 µM digitonin, and 250 µM ADP, 20 µM carboxyatractyloside (CAT), and 4 µM carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) were added where indicated. (C) Quantification of changes in ΔΨ_m in panels A and B. (D and E) Changes in ΔΨ_m in digitonin-permeabilized epimastigotes as detected by changes in safranin O fluorescence in epimastigotes transfected with scrambled sgRNA/Cas9/pTREX-n (Scrambled) or TcMCUb-KO (MCUb-KO) (D) and empty vector or TcMCUb-OE (MCUb-OE) (E). Other conditions are as in panels A and B. (F) Quantification of changes in ΔΨ_m in panels D and E. Values in panels C and F are means ± SD (n = 3; no significant differences, Student’s t test). Download FIG S2, TIF file, 1.4 MB ^{(1.5MB, tif)} .

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Growth of TcMCU and TcMCUb mutants.

We initially evaluated the growth rate of epimastigotes in liver infusion tryptose (LIT) medium. TcMCU-KO cells grew at a slightly lower initial rate than wild-type cells or cells transfected with a scrambled sgRNA (Fig. 3A). We then tested their growth in low-glucose LIT medium. In this case, the initial growth rate of TcMCU-KO cells was significantly lower. Wild-type epimastigotes reached the stationary phase earlier (day 8) and then started dying, while TcMCU-KO cells reached the stationary phase later (day 11) and continued at steady state for a longer period (Fig. 3B). The longer survival of TcMCU-KO cells in low-glucose medium suggested that these cells could have access to an energy reserve. Amino acids or fatty acids derived from triglycerides have been proposed as a possible source of energy of epimastigotes under low-glucose conditions (1). We therefore investigated the presence of lipid droplets in TcMCU-KO cells under these conditions. Lipid droplets were more abundant in TcMCU-KO cells grown in LIT medium or in low-glucose LIT medium than in wild-type cells at the beginning of the stationary phase, suggesting triglycerides as a potential energy reserve of these cells (Fig. S3A and B).

FIG 3 .

Phenotypic changes in TcMCU and TcMCUb mutant epimastigotes. (A) Growth of wild-type (WT) and TcMCU-KO epimastigotes (MCU-KO) and epimastigotes transfected with scrambled sgRNA (Scrambled) in LIT medium. (B) Growth of WT and MCU-KO epimastigotes in low-glucose LIT medium. (C) Growth curve of wild-type (WT) and TcMCUb-KO (MCUb-KO) epimastigotes and epimastigotes transfected with a scrambled sgRNA grown in LIT medium (n = 3). (D) Relative Rhod-2 fluorescence in control cells (WT) compared with MCU-OE or MCUb-OE cells. (E) Mitochondrial oxidative stress as measured with MitoSOX Red in control (WT), TcMCU-OE (MCU-OE), and TcMCUb-OE (MCUb-OE) epimastigotes permeabilized with 50 μM digitonin in the presence or absence of 0.5 mM Ca²⁺. Controls were treated with 2 mM antimycin A₁ (WT + AA1) or were epimastigotes exposed to 0.5 mM Ca²⁺ in the absence of digitonin [WT + Ca²⁺ (-Dig)]. (F) Quantification of changes in panel E at 1,200 s. (G) Percentage of metacyclic trypomastigotes in epimastigote cultures after incubation in TAU 3AAG medium. Differentiation of epimastigotes to metacyclic trypomastigotes was quantified by staining with DAPI to distinguish the position of the kinetoplast by fluorescence microscopy. (H and I) Effect of TcMCU knockout on trypomastigote infection of Vero cells. There were no significant differences in percentages of infected Vero cells (H) or in the number of intracellular parasites per 100 host cells (I). (J) Effect of TcMCU knockout in amastigote replication after 48 h. Differences were not significant. Values in panels A to D and F to J are means ± SD (n = 3). *, P < 0.05; **, P < 0.01; ***, P < 0.001; n.s., not significant.

FIG S3 

Lipid droplets in epimastigotes and PCR analysis of TcMCU-KO trypomastigotes. (A) Staining with Nile red (red) of lipid droplets in wild-type (WT) and TcMCU-KO epimastigotes cultured in LIT or low-glucose LIT medium. T. cruzi epimastigotes were incubated in PBS containing 1.5 μg of Nile red (Sigma)/ml for 30 min at 28°C. Fluorescence optical images were captured with excitation at 510 nm and emission at 580 nm. Nuclei and kinetoplasts were labeled with DAPI (blue). Bars, 10 µm. (B) Number of lipid droplets detected per cell in LIT or low-glucose medium. At least 200 cells from three experiments with 20 random fields/experiment were analyzed (means ± SD; n = 3; ***, P < 0.001, Student’s t test). (C) PCR used to confirm TcMCU-KO genotype in trypomastigotes isolated after host cell infection, as described in Materials and Methods. The intact locus generates a PCR product of 909 bp in wild-type cells that is absent in TcMCU-KO trypomastigotes. Download FIG S3, TIF file, 2.2 MB ^{(2.2MB, tif)} .

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In contrast to TcMCU-KO cells, TcMCUb-KO cells grew in LIT medium at a significantly lower rate than wild-type cells or cells transfected with a scrambled sgRNA (Fig. 3C).

Oxidative stress in TcMCU-OE and TcMCUb-OE cells.

Overexpression of TcMCU or TcMCUb resulted in higher basal mitochondrial Ca²⁺ concentration, as measured by Rhod-2 fluorescence (Fig. 3D). Mitochondrial Ca²⁺ overload is known to generate oxidative stress. Accordingly, reactive oxygen species (ROS) generation was increased in digitonin-permeabilized TcMCU-OE or TcMCUb-OE epimastigotes compared with control cells (wild type [WT]) when in the presence of 0.5 mM Ca²⁺ (Fig. 3E and F). They reached levels comparable to those produced by addition of antimycin A₁, a known complex III inhibitor of the electron transport chain (Fig. 3E and F).

Differentiation and infectivity of mutant cells.

To induce the differentiation of mutant epimastigotes into infective metacyclic trypomastigotes (metacyclogenesis), we incubated these cells in triatome artificial urine (TAU) medium as described in Materials and Methods. TcMCU-KO cells were able to differentiate to metacyclic trypomastigotes in a higher proportion than wild-type or TcMCU-OE cells (Fig. 3G). The TcMCU-KO metacyclic trypomastigotes were able to infect host cells. After several cycles of infection to obtain a sufficient amount of culture-derived trypomastigotes, we conducted in vitro infection assays to determine the percentage of infected cells after incubation with wild-type or TcMCU-KO trypomastigotes. Figures 3H and I show that there was no significant difference between infections with wild-type and TcMCU-KO trypomastigotes, and Fig. 3J shows that replication of amastigotes was not significantly affected by TcMCU knockout, demonstrating that TcMCU is not essential for differentiation, infectivity, or intracellular replication. PCR analysis of the TcMCU-KO trypomastigotes obtained from these tissue cultures confirmed the absence of TcMCU in these cells (Fig. S3C). In contrast, induction of metacyclogenesis in TcMCUb-OE cells was similar to that of wild-type cells but greatly diminished in TcMCUb-KO cells (Fig. 3G). Several attempts (n = 4) to infect tissue culture cells with TcMCUb-KO differentiated cells failed to recover significant amounts of trypomastigotes, suggesting that TcMCUb is important for infectivity.

Complementation of the KO mutants.

To exclude the possibility that the TcMCU silencing phenotype is due to off-target effects, we investigated whether an exogenous TcMCU gene could complement the ablation of endogenous TcMCU. First, we demonstrated that the exogenous TcMCU (with changes in the PAM sequence to prevent disruption by CRISPR/Cas9), bearing a hemagglutinin (HA) epitope tag (TcMCU_-PAM-HA), was targeted to the epimastigote mitochondrion. Immunofluorescence assays in Fig. S4A show that exogenous TcMCU-HA colocalizes with MitoTracker in the mitochondria. Then, we observed that mitochondrial Ca²⁺ transport in permeabilized TcMCU-KO epimastigotes expressing exogenous TcMCU-HA was significantly higher than that of wild-type cells (Fig. 4A and B). The expression of TcMCU-HA was confirmed by Western blot analysis (Fig. 4C). Then, we introduced mutations in highly conserved residues to evaluate their ability to restore Ca²⁺ transport in TcMCU-KO cells (Fig. S4B). Complementation with a TcMCU^R214W,D219V mutant, but not with the TcMCU^D223N,E226Q mutant of the DIME region of TcMCU, was able to restore mitochondrial Ca²⁺ uptake (Fig. 4D and E), indicating the importance of Asp²²³ and Glu²²⁶ of the DIME domain for Ca²⁺ transport. TcMCU-KO epimastigotes overexpressing TcMCUb failed to rescue Ca²⁺ uptake, suggesting that TcMCUb cannot replace TcMCU. Western blot analysis confirmed the expression of these proteins (Fig. 4F). We also performed the complementation of TcMCU-KO cells with human MCU (Homo sapiens MCU [HsMCU]), and we found that it was not effective in restoring mitochondrial Ca²⁺ transport (Fig. 4G and H), although the protein colocalized with MitoTracker in the mitochondria of epimastigotes (Fig. S4C) and Western blot analysis confirmed its expression (Fig. 4I).

FIG 4 .

Complementation of the endogenous TcMCU and reconstitution of Ca²⁺ transport in yeast. (A) Ca²⁺ uptake reconstitution in digitonin-permeabilized TcMCU-KO epimastigotes transfected with pTREX-h and pTREX-h-TcMCU_-PAM-HA (TcMCU). Experimental conditions were as in Fig. 1I. (B) Quantification of data in panel A. Relative Ca²⁺ uptake at 600 s compared with WT. (C) Western blot analysis of total protein extracts of wild-type (WT), TcMCU-KO plus pTREX-h empty vector (KO + EV), and TcMCU-KO plus TcMCU_-PAM-HA (KO + MCU) epimastigotes, using anti-HA antibodies. Anti-α-tubulin antibodies were used as a loading control. (D) Ca²⁺ uptake reconstitution in digitonin-permeabilized TcMCU-KO epimastigotes transfected with TcMCU^R214W,D219V (MCU-KO + R214W,D219V), TcMCU^D223N,E226Q (MCU-KO + D223N,E226Q), or TcMCUb (MCU-KO + TcMCUb). Experimental conditions were as in Fig. 1I. (E) Quantification of data in panel D. Relative Ca²⁺ uptake at 600 s compared with WT (TcMCU^R214W,D219V [shown as R214W,D219V] and TcMCU^D223N,E226Q [shown as D223N,E226Q]). (F) Western blot analysis of total protein extracts of wild-type (WT), TcMCU-KO (MCU-KO), TcMCU-KO plus pTREX-h empty vector (KO + EV), TcMCU^D223N,E226Q (D223N,E226Q), TcMCU^R214W,D219V (R214W,D219V), and TcMCUb (KO + MCUb) epimastigotes, using anti-c-Myc antibodies. Anti-α-tubulin antibodies were used as a loading control. (G) Ca²⁺ uptake reconstitution in digitonin-permeabilized TcMCU-KO epimastigotes transfected with HsMCU or pTREX-p. Experimental conditions were as in Fig. 1I. (H) Quantification of data in panel G. Relative Ca²⁺ uptake at 600 s compared with WT. (I) Western blot analysis of total protein extracts of WT, TcMCU-KO (MCU-KO), TcMCU-KO plus pTREX-p empty vector (KO + EV), and TcMCU-KO plus HsMCU (KO + HsMCU) epimastigotes, using anti-c-Myc antibodies. Anti-α-tubulin antibodies were used as a loading control. (J) Ca²⁺ uptake by spheroplasts from yeast transformed with pACT2 empty vector, TcMCU, ^ScMTSTcMCU, or DdMCU. Experimental conditions were as described in Materials and Methods. (K) Quantification of data in panel J. Relative Ca²⁺ uptake at 350 s compared with DdMCU. (L) Western blot analysis of lysates and mitochondrial fractions of yeast complemented with empty vector, TcMCU-HA (TcMCU), or ^ScMTSTcMCU-HA (^ScMTSTcMCU) using anti-HA antibodies. Anti-cytochrome c antibodies were used as a loading control. (M) Western blot analysis of lysates and mitochondrial fractions of yeast complemented with DdMCU-Flag (DdMCU) using anti-Flag antibodies. Anti-cytochrome c antibodies were used as a loading control. Values in panels B, E, H, and K are means ± SD (n = 3). *, P < 0.05; ***, P < 0.001.

FIG S4 

Mitochondrial localization of TcMCU-HA and HsMCU-3×c-Myc in epimastigotes. (A) Fluorescence microscopy of exogenous TcMCU-HA expressed in TcMCU-KO epimastigotes using antibodies against HA (green) and MitoTracker as a mitochondrial marker (red). Nuclei and kinetoplast were labeled with DAPI (blue). Colocalization is shown in yellow (merge). (B) Schematic representation of MCU subunit domains. Sequence alignment of the DIME motif and flanking residues from MCU and MCUb. Numbers of critical conserved and amino acid substitutions are shown. TM, transmembrane domain; DIME, functional DIME motif. (C) Colocalization of HsMCU-3×c-Myc with MitoTracker using antibodies to c-Myc. Bars in panels A and C, 10 µm. Download FIG S4, TIF file, 1.6 MB ^{(1.7MB, tif)} .

Copyright © 2017 Chiurillo et al.

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Reconstitution of the mitochondrial calcium uniporter in Saccharomyces cerevisiae.

Saccharomyces cerevisiae does not possess an MCU, which makes it an ideal in vivo reconstitution system for studying the uniporter in a physiologically relevant organellar membrane (5, 19). Because the minimal components sufficient for in vivo MCU activity in trypanosomatids are unknown, we transformed S. cerevisiae with TcMCU to assess whether this protein is able to reconstitute yeast mitochondrial calcium uniporter activity on its own. We transformed yeast with MCU from Dictyostelium discoideum (DdMCU) to use as a positive control, as this protein was reported to be sufficient alone to reconstitute mitochondrial calcium uniporter activity in yeast (19). To ensure expression of TcMCU in yeast mitochondria, we created a chimeric gene, consisting of the mitochondrial targeting sequence (MTS) of yeast cytochrome oxidase 4 (ScCox4p) fused to TcMCU lacking its predicted MTS (^ScMTSTcMCU) to obtain the expression of TcMCU in the yeast mitochondrial inner membrane. Extramitochondrial calcium levels were monitored with calcium green-5N, and we observed that mitochondria of yeast spheroplasts were able to take up Ca²⁺ in the presence of mitochondrial substrates (5 mM succinate, 5 mM malate, 5 mM pyruvate, 5 mM α-ketoglutarate, and 1 mM glutamate) only when complemented with DdMCU (Fig. 4J and K). However, neither TcMCU- nor ^ScMTSTcMCU-transformed cells were able to take up Ca²⁺ under the experimental conditions tested, although the cells were able to take up Ca²⁺ after addition of the membrane-potential-dependent calcium ionophore ETH-129 (19) (Fig. 4J). The enrichment of these proteins in the mitochondrial fraction was confirmed by Western blot analysis (Fig. 4L and M). Our results suggest that TcMCU alone is unable to reconstitute mitochondrial Ca²⁺ uptake in yeast.

Respiration of TcMCU-KO and TcMCUb-KO mutants.

We measured oxygen uptake (Fig. 5A) and oxygen consumption rate (OCR) (Fig. 5B) under basal (state 2), ADP-stimulated (state 3), oligomycin-inhibited (state 4), and FCCP-stimulated (state 3u) conditions in wild-type and mutant digitonin-permeabilized cells in the presence of succinate. Wild-type and mutant mitochondria showed well-coupled respiration, although OCRs in the presence of ADP, oligomycin, and FCCP were significantly higher in TcMCU-KO and significantly lower in TcMCUb-KO mitochondria (Fig. 5C to E). Respiratory control rates (state 3/state 4) were 1.93 ± 0.05, 1.79 ± 0.09, and 1.92 ± 0.07 for WT, TcMCU-KO, and TcMCUb-KO cells, respectively (n = 4). The lower OCR of TcMCUb-KO mitochondria correlated with lower staining of the mitochondria with MitoTracker Deep Red FM, a dye currently used to estimate mitochondrial mass (20) (Fig. 5F and G), as well as with lower activity of citrate synthase, another mitochondrial mass indicator (21) (Fig. 5H), suggesting a lower mitochondrial mass in these cells.

FIG 5 .

Mitochondrial changes in mutant epimastigotes. (A and B) Representative traces of oxygen uptake (A) and oxygen consumption rate (OCR) (B) by digitonin-permeabilized wild-type (WT), TcMCU-KO (MCU-KO), and TcMCUb-KO (MCUb-KO) epimastigotes. Additions were 100 μM ADP, 1 µg/ml oligomycin (Oligo), and 1.0 µM FCCP. (C to E) OCR after addition of ADP (respiration stimulated by oxidative phosphorylation [OXPHOS]), oligomycin (minimal respiratory rate), and FCCP (maximal respiratory capacity), respectively (n = 4), using the test system as in panels A and B. (F) Fluorescence microscopy of representative wild-type (WT), TcMCU-KO (MCU-KO), and TcMCUb-KO (MCUb-KO) epimastigotes labeled with MitoTracker Deep Red FM for estimation of mitochondrial mass. (G) Quantification of data in panel F. Relative fluorescence of MitoTracker Deep Red FM-labeled cells. Over 200 cells from three experiments with 20 random fields/experiment were analyzed (n = 3). (H) Relative citrate synthase activity of wild-type and mutant parasites, measured for enzymatic estimation of mitochondrial mass, considering V_max values of WT to be 1.0 (n = 5). Values in panels C to E, G, and H are means ± SD. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

Autophagy in TcMCUb-KO cells.

Autophagy has been reported as a starvation response in different trypanosomatids (22, 23). In addition, block of mitochondrial Ca²⁺ uptake was shown to increase autophagy as a survival mechanism (24, 25). Huang et al. (10) recently showed that knockdown of MCU in T. brucei procyclic forms increased the expression of autophagy markers under starvation condition. We evaluated autophagy in these cells by immunofluorescence microscopy (Fig. 6A) using antibodies against the autophagy marker Atg8.1, which is the ortholog of LC3-II in mammalian cells (26). Then, we quantified the number of autophagosomes per cell (Fig. 6B) and the number of cells containing autophagosomes (Fig. 6C). TcMCUb-KO cells had significantly increased numbers of autophagosomes per cell when incubated under starvation conditions, while lower levels were found in TcMCU-KO cells than in wild-type cells (Fig. 6A to C). Moreover, TcMCUb-KO cells showed an increased number of autophagosomes/cell and a higher percentage of cells with autophagosomes even when cultured in rich medium. It has been proposed that in vertebrate cells a lower mitochondrial Ca²⁺ transport capacity leads to an increase of the AMP/ATP ratio and stimulation of the AMP-dependent protein kinase (AMPK) signaling axis that stimulates autophagy (24). However, knockout of both TcMCU and TcMCUb abolished mitochondrial Ca²⁺ transport while only TcMCUb-KO cells showed increased autophagy. In agreement with these results, there was no correlation between the AMP/ATP ratio of these cells (Fig. 6D) and autophagy (Fig. 6A to C), ruling out this potential mechanism of autophagy stimulation in T. cruzi.

FIG 6 .

Autophagy changes in mutant epimastigotes. (A) Representative fluorescence microscopy images of wild-type (WT), TcMCU-KO (MCU-KO), and TcMCUb-KO (MCUb-KO) epimastigotes labeled with anti-TcATG8.1 antibody (red) after incubation in LIT medium or PBS for 16 h. DAPI staining (blue) and merged images on a differential interference contrast (DIC) background (Merge) are also shown. Bars, 10 µm. (B) Number of autophagosomes per cell under different conditions shown in panel A. (C) Percentage of cells with autophagosomes under conditions shown in panel A. (D) Comparison of AMP/ATP ratios between WT, MCU-KO, and MCUb-KO epimastigotes incubated in LIT medium or PBS for 16 h. For panels B to D, over 200 cells from wild-type (WT), TcMCU-KO (MCU-KO), and TcMCUb-KO (MCUb-KO) epimastigotes from three experiments with 20 random fields/experiment were analyzed. Values in panels B to D are means ± SD (n = 3). *, P < 0.05; **, P < 0.01; ***, P < 0.001.

DISCUSSION

Our studies have shown that the mitochondrial calcium uniporter (TcMCU) is not essential for T. cruzi epimastigote growth in rich medium, for differentiation (metacyclogenesis) under well-established starvation conditions, and for trypomastigote host cell infection and intracellular replication, although it is needed for optimal exponential growth of epimastigotes. No mitochondrial Ca²⁺ transport was detected in TcMCU-KO epimastigotes, while the mitochondrial membrane potential (ΔΨ_m) was unaffected in these cells. In contrast to these results, TcMCUb was required for optimal growth in rich medium, for metacyclogenesis under starvation conditions, and for trypomastigote host cell infection. However, as occurs with TcMCU-KO cells, mitochondria of TcMCUb-KO epimastigotes were unable to transport Ca²⁺ while their ΔΨ_m was not altered. Additional differences between these KO cells were also found. TcMCUb-KO cells had lower respiratory activity, a higher AMP/ATP ratio in rich medium, reduced mitochondrial mass, and increased autophagy, suggesting that MCUb has additional bioenergetic roles independent of Ca²⁺ transport. Overexpression of TcMCUb or TcMCU resulted in higher rates of Ca²⁺ transport, no effects on ΔΨ_m, and no dominant negative effect of TcMCUb on mitochondrial Ca²⁺ transport, in contrast to previous results described for HeLa cells (13). The two proteins interact, as shown by their coimmunoprecipitation.

MICU1 has been shown to have a gatekeeping effect on MCU (27, 28), and the increased matrix Ca²⁺ observed after overexpression of TcMCU could be the result of a change in the stoichiometry of MCU/MICU1. A similar situation could be occurring in TcMCUb-overexpressing cells.

The lack of essentiality of MCU in epimastigotes grown in rich medium and their increased survival in stationary phase under low-glucose conditions is interesting. It has been previously reported that epimastigotes use glucose from the medium during their exponential phase of growth and that once glucose is depleted and cells enter the stationary phase, they use tricarboxylic acid (TCA) cycle intermediates (29). All the main stages of T. cruzi have a high endogenous rate of respiration, suggesting that the organisms contain an energy reserve. Early reports ruled out the presence of a carbohydrate polymer (1, 30). Oliveira et al. (31) first suggested the possibility that triglycerides could be the energy reserve, which would be similar to the situation in another stercorarian trypanosome, Trypanosoma lewisi, which also has a high endogenous rate of respiration and high triglyceride content (32). However, we found that lipid droplets increase, not decrease, in TcMCU-KO cells. As the presence of lipid droplets was evaluated at the beginning of the stationary phase, one possibility is that they could allow TcMCU-KO cells to survive longer because they were not consumed yet at that point. On the other hand, it is known that epimastigotes have a large pool of free amino acids that they use to maintain their osmolarity (33) and that protein degradation occurs under starvation conditions (1). Increased amino acid oxidation through the activity of the TCA cycle would bypass the need for the Ca²⁺-regulated formation of acetyl coenzyme A (CoA) from pyruvate (10). The higher ATP production via this pathway would explain the better survival of the cells when grown in low-glucose medium. The intracellular amastigotes appear to use fatty acids as an energy source, taking into account the increased levels of fatty acid oxidation enzymes in these stages (34). Amastigotes could then also bypass the need for the Ca²⁺-regulated pyruvate dehydrogenase to obtain energy through the TCA cycle. The predominance of fatty acid oxidation over glycolysis in the intracellular stages would explain the nonessential role of TcMCU for infection and replication.

Metacyclogenesis is usually stimulated under glucose-depleted conditions, and we detected a higher rate in TcMCU-KO cells. This higher rate of differentiation could be related to the lower ability of these cells to use endogenous substrates in low-glucose medium in contrast to TcMCU-OE cells. The increased metacyclogenesis observed in TcMCU-KO cells could also explain its longer survival during the stationary phase in low-glucose medium. On the other hand, the reduced metacyclogenesis of TcMCUb-KO cells correlated well with their inefficient ability to invade host cells.

To our knowledge, we report for the first time the complementation of a gene knockout (TcMCU-KO) obtained by CRISPR/Cas9 in T. cruzi. Complementation of TcMCU-KO cells with an exogenous copy of TcMCU but not with the gene mutated in the nucleotides encoding amino acids Asp²²³ and Glu²²⁶ of the DIME domain was able to restore mitochondrial Ca²⁺ transport, confirming that as demonstrated in mammals, these two conserved acidic residues of the DIME motif are also key for TcMCU-mediated Ca²⁺ uptake (8, 13). However, a mutant including a substitution at the acidic residue Asp²¹⁹ (TcMCU^R214W/D219V; Glu²⁵⁷ in HsMCU), which is highly conserved within MCU orthologs and has been proposed as a critical residue for Ca²⁺ transport (8, 13), rescued Ca²⁺ transport in TcMCU-KO epimastigotes. Similarly, a recent report showed that the HsMCU^E257A mutant was able to rescue Ca²⁺ transport in HEK-293T MCU-KO cells, which is consistent with the predicted nuclear magnetic resonance (NMR)/electron microscopy (EM) structure of the MCU that exposes this residue outside the channel entrance (35).

Moreover, human MCU was unable to restore mitochondrial Ca²⁺ transport in TcMCU-KO cells. TcMCU alone, in contrast to DdMCU (19), could not reconstitute mitochondrial Ca²⁺ transport in yeast mitochondria. As T. cruzi lacks orthologs of EMRE (36) or MCUR1 (37, 38), this result suggests that, as in the case of human MCU (19), other, still-unknown subunits of the complex might be necessary for full reconstitution of mitochondrial Ca²⁺ transport in yeast by TcMCU.

An increase in autophagy was observed in TcMCUb-KO but not in TcMCU-KO cells when incubated under either rich or starvation conditions. In vertebrate cells, the lack of MCU-mediated Ca²⁺ uptake promotes autophagy (37, 38). An increase in the AMP/ATP ratio produced by downregulation of the TCA cycle by inhibition of the MCU could stimulate the phosphorylation of the AMP-dependent kinase (AMPK) and promote autophagy (24). However, we did not find such a correlation in T. cruzi, as TcMCU-KO cells have higher AMP/ATP ratios under starvation conditions and no difference in autophagy while TcMCUb-KO cells do not have differences in AMP/ATP ratios but exhibit higher autophagy than wild-type cells. These results suggest that this pathway might not be operative in T. cruzi and are in agreement with a reported AMPK-independent autophagy pathway reported recently in T. brucei (39).

It is possible that phenotypic changes in the TcMCUb-KO mutants may have been caused indirectly by adaptation to the loss of the gene that were not seen in the TcMCU-KO mutants because of the occurrence of more beneficial compensatory mutations in these cells. However, our attempts to complement TcMCUb-KO mutants with an exogenous TcMCUb gene devoid of the PAM motif did not result in selected parasites, probably due to the fragility of these cells and their inability to resist the electroporation stress (data not shown).

The mitochondria of TbMCU-OE epimastigotes have an increased ability for Ca²⁺ uptake, which results in Ca²⁺ overload and oxidative stress, and similar results were observed in TcMCUb-OE cells. These results agree with those obtained in HeLa cells (7) and T. brucei (10). Mitochondrial Ca²⁺ overload has also been proposed as the link between complement deposition and triggering of cell death in T. cruzi epimastigotes (40). Accumulation of Ca²⁺ in the mitochondrion leads to a decrease in cell respiration, dissipation of the inner membrane potential, and increased reactive oxygen species production (40).

In contrast with the results obtained in HeLa cells (13), where overexpression of MCUb has a dominant negative effect on mitochondrial Ca²⁺ transport, overexpression of TcMCUb led to increased Ca²⁺ uptake without affecting the mitochondrial membrane potential. Knockout of TcMCUb abolished mitochondrial Ca²⁺ transport, and the cells had a lower growth rate than control cells. Moreover, as mentioned above, we introduced in TcMCU amino acid substitutions (R214W/D219V) that are conserved in its paralog TcMCUb and considered critical to determine the dominant negative effect of the mammalian protein (13). However, we found that TcMCU^R214W/D219V, but not TcMCUb, was able to restore mitochondrial Ca²⁺ transport in TcMCU-KO cells, suggesting that these substitutions are not enough to abolish Ca²⁺ transport.

In summary, our results indicate that TcMCU is not essential for growth, differentiation, infectivity, and intracellular replication of T. cruzi, although it could be important under stress situations, such as for growth in low-glucose medium. This could be relevant for epimastigotes grown in the insect’s gut, in which glucose is supposed to be scarce (41). In contrast to what has been described in some mammalian cells, where MCUb acts as a dominant negative subunit of the MCU complex (13), mitochondrial Ca²⁺ transport is enhanced in TcMCUb-OE epimastigotes and abolished in TcMCUb-KO cells. TcMCUb-KO cells have additional phenotypic alterations that suggest other bioenergetic roles besides mitochondrial Ca²⁺ transport.

MATERIALS AND METHODS

Culture methods.

T. cruzi epimastigotes (Y strain) were grown at 28°C in liver infusion tryptose (LIT) medium (42) (5.4 mM KCl, 150 mM NaCl, 24 mM glucose, 5% [vol/vol] liver extract, 0.02% [wt/vol] hemin, 2% [wt/vol] yeast extract, 1.5% [wt/vol] tryptose), supplemented with 10% heat-inactivated fetal bovine serum (FBS). To obtain a low-glucose LIT medium, glucose was not added. We determined the growth rate of T. cruzi epimastigotes by counting cells in a Neubauer chamber with a starting culture of 1 × 10⁶ epimastigotes. Tissue culture cell-derived trypomastigotes were obtained from Vero cells infected with metacyclic trypomastigotes obtained as described below. T. cruzi trypomastigote forms were collected from the culture medium of infected host cells, using a modification of the method of Schmatz and Murray (43) as described previously (44). Vero cells were grown in RPMI supplemented with 10% fetal bovine serum and maintained at 37°C with 5% CO₂.

Metacyclogenesis.

We followed the protocol described by Bourguignon et al. (45) with minor modifications. Epimastigotes were obtained after 4 days in LIT medium and submitted to a stress (incubation for 2 h in a medium containing 190 mM NaCl, 17 mM KCl, 2 mM MgCl₂, 2 mM CaCl₂, 0.035% sodium bicarbonate, 8 mM phosphate, pH 6.9, at room temperature; triatome artificial urine [TAU] medium). After this stress, parasites were incubated for 96 h in TAU 3AAG medium (which consists of the above-described TAU medium supplemented with 10 mM l-proline, 50 mM sodium l-glutamate, 2 mM sodium l-aspartate, and 10 mM glucose). To increase the number of metacyclic forms to infect Vero cells, the contents of the flask were collected and resuspended in medium containing fresh fetal bovine serum and incubated at 37°C for 20 h. The complement in the FBS kills epimastigotes while metacyclic trypomastigotes survive. Samples were harvested from the TAU 3AAG plus FBS-containing medium at days 5 and 10 of cultivation.

In vitro infection assay.

Gamma-irradiated (2,000-rad) Vero cells (4.5 × 10⁵ cells) were plated onto sterile coverslips in a 12-well plate and incubated overnight at 35°C, 7% CO₂, in RPMI medium plus 10% fresh fetal bovine serum. Tissue culture-derived trypomastigote collections were incubated at 4°C overnight to allow amastigotes to settle from swimming trypomastigotes. Trypomastigotes from the supernatants of these collections were counted and used to infect the coverslips at a ratio of 50 parasites to 1 host cell. At 4 h postinfection, coverslips were washed extensively with Dulbecco’s Hanks’ solution, followed by phosphate-buffered saline (PBS), pH 7.4, to remove any extracellular parasites. Coverslips were fixed immediately in 4% paraformaldehyde in PBS, pH 7.4, at 4°C for 30 min. Coverslips were washed once with PBS and mounted onto glass slides in Fluoromount G containing 15 µg/ml of 2-(4-aminophenyl)-1H-indole-6-carboxamidine (DAPI), which stains host and parasite DNA. Coverslips were viewed on an Olympus BX60 microscope to quantify the number of host cells that contained intracellular parasites and the number of intracellular parasites per cell in randomly selected fields. Three hundred host cells were counted per sample in three independent experiments. To quantify amastigote replication, the following modifications were used: host cells were infected at a ratio of 10 parasites to 1 host cell, and coverslips were allowed to incubate for 48 h postinfection at 35°C, 7% CO₂, prior to fixation and DAPI staining.

TcMCU-KO.

Chimera single guide RNA (sgRNA) sequences to target the TcMCU gene (TryTripDB identifier [ID] TcCLB.503893.120) were PCR amplified from plasmid pUC_sgRNA, containing the sgRNA backbone sequence (82 bp) (46). One specific protospacer was included in the forward primer (primer 1 [see Table S1 in the supplemental material]) while using a common reverse primer (primer 2 [Table S1]) for sgRNA amplification. These primers also contained a BamHI restriction site for cloning into Cas9/pTREX-n (14) upstream of the HX1 transsplicing signal (47) to generate the TcMCU-sgRNA/Cas9/pTREX-n construct. The sgRNA orientation was verified by PCR using the specific TcMCU-sgRNA forward primer and the HX1 reverse primer (14), which annealed on the HX1 region of the vector (primer 3 [Table S1]). Positive clones that generate a 190-bp PCR fragment were also sequenced. A scrambled sgRNA (Sc-sgRNA/Cas9/pTREX-n), obtained with primers 2 and 4 (Table S1), was used as a control. The Cas9/pTREX-n vector contains the Streptococcus pyogenes Cas9 sequence with a twice-repeated simian virus 40 (SV40) nuclear localization signal (2×NLS) and green fluorescent protein (GFP) (14), thus generating mutant cell lines with green fluorescent nuclei after transfection. The molecular construct (DNA donor) to promote homologous directed repair and ablation of the target gene after the double-strand break (DSB) induced by Cas9 was obtained using a recombinant PCR strategy. Briefly, the 5′ and 3′ UTRs of the TcMCU gene (490 and 529 bp, respectively) were amplified by PCR using T. cruzi genomic DNA as the template (primers 7 to 10 [Table S1]). The open reading frame (ORF) of the blasticidin S-deaminase gene (Bsd) was also amplified by PCR from pTREX-b vector as the template using primers containing 20 nucleotides (nt) overlapping with the 5′ and 3′ flanking sequences of the TcMCU gene (primers 5 and 6 [Table S1]). The three PCR fragments (5′-TcMCU flanking sequence, blasticidin-resistant gene, and 3′-TcMCU flanking sequence) were linked together by sequential PCR using primers 7 and 10 (Table S1). The final PCR product was cloned into pGEM-T Easy vector (Promega). After sequencing verification of positive clones, circular constructs TcMCU-sgRNA/Cas9/pTREX-n and the Bsd cassette in pGEM-T Easy were used to cotransfect T. cruzi epimastigotes. After 5 weeks of selection with 250 µg/ml G418 and 10 µg/ml blasticidin, TcMCU gene ablation was verified by PCR using primers 11 and 12 (Table S1). Alternatively, to confirm TcMCU gene KO in trypomastigotes obtained from infected cells, we amplified the entire TcMCU ORF (909 bp) by PCR (primers 15 and 19 [Table S1]).

TcMCUb-KO.

An sgRNA to target the sequence coding for the hypothetical TcMCUb protein (TryTripDB ID TcCLB.416883.9) was amplified by PCR (primers 31 and 2 [Table S1]). Following the same strategy mentioned above for TcMCU, we obtained the TcMCUb-sgRNA/Cas9/pTREX-n construct. The DNA donor cassette designed to promote homologous directed repair and disruption of TcMCUb was obtained by PCR using a set of long primers (ultramers) containing 120 nucleotides, from which 100 nucleotides correspond to the region from nt −20 to +80 (forward ultramer) and nt +510 to +610 (reverse ultramer) of the TcMCUb ORF. Each one of the ultramers had 20 nt annealing on the Bsd gene (primers 32 and 33 [Table S1]). Gene disruption of TcMCUb was verified by PCR using primers 34 and 35 (Table S1).

Generation of TcMCU/TcMCUb-overexpressing cell lines in T. cruzi.

For gain-of-function analysis of TcMCU/TcMCUb, T. cruzi epimastigotes were transfected with the trypanosome expression vector pTREX-n (47, 48) containing the corresponding genes. The molecular construct for TcMCU overexpression was made as follows: the full sequence of TcMCU was PCR amplified using primers 15 and 16 (Table S1) and T. cruzi gDNA as the template. The purified fragment of 909 bp was digested with EcoRI and HindIII restriction enzymes and cloned into the pTREX-n vector linearized with the same enzymes. The molecular construct for TcMCUb overexpression was made as follows: the full sequence of TcMCUb was PCR amplified using primers 38 and 39 (Table S1) and T. cruzi gDNA as the template. For this purpose, we used the genomic sequence (KB205522 from nucleotide 139,788 to 140,579, TryTripDB) from T. cruzi strain Esmeraldo to design the reverse primer, because the MCUb gene 3′ end and 3′ UTRs are missing in the TcCLB.416883.9 sequence. The purified fragment of 792 bp was digested with XbaI and SalI restriction enzymes to be cloned into a version of the pTREX-n vector that includes a 3×HA epitope tag coding sequence. The pTREX-n vector was linearized with XbaI and XhoI enzymes, and the TcMCUb gene was cloned in frame with the 3×HA sequence for C-terminal tagging, allowing the detection of the overexpressed protein with anti-HA antibody. For this purpose, we previously cloned the 3×HA tag sequence, excised from pMOTag-4H vector (49) with XhoI and SalI enzymes, into pTREX-n by using the XhoI restriction site.

Molecular constructs to complement TcMCU-KO mutants.

To revert the phenotype exhibited by TcMCU-knockout epimastigotes, we obtained the TcMCU_-PAM-HA construct to reconstitute the ablated TcMCU gene. Following a two-step-PCR strategy, we eliminated the PAM sequence (CGG to CCT) specific for the TcMCU-sgRNA used to obtain the TcMCU-KO cells, therefore avoiding constitutively expressed Cas9 to target the inserted sequence (Table S1, primers 15 and 17 to 19). The fragment obtained by overlap extension PCR was digested with EcoRI and HindIII and then cloned into pTREX-h (which confers hygromycin resistance) digested with the same enzymes. We also included a C-terminal HA tag in order to detect the overexpressed protein using anti-HA antibody.

Furthermore, we attempted to reconstitute the ablated TcMCU gene in TcMCU-KO mutants with the following. For TcMCUb, primers 38 and 39 (see Table S1 in the supplemental material) were used to amplify from T. cruzi gDNA the TcMCUb gene that was digested with XbaI and SalI enzymes and cloned in frame with the 3×c-Myc tag sequence into pTREX-h digested with XbaI and XhoI. For TcMCU^D223N,E226Q and TcMCU^R14W,D219N, the generation of pTREX-h-TcMCU^D223N,E226Q-3×c-Myc was done by overlap extension PCR using the TcMCU_-PAM-HA plasmid as a template and primers 20 to 23 (Table S1). Following a similar strategy, the pTREX-h-TcMCU^R14W,D219N-3×c-Myc was obtained using primers 20, 21, 24, and 25 (Table S1). For the human MCU gene, we purchased from GenScript (USA) a cDNA clone corresponding to the Homo sapiens MCU variant 1 (GenBank accession number NM_138357) of 1,100 bp. The HsMCU fragment amplified with primers 26 and 27 (Table S1) was digested and cloned into pTREX-p (which confers puromycin resistance) by EcoRI and XhoI restriction sites in frame with the 3×c-Myc tag sequence.

For this purpose, we first generated pTREX-h and pTREX-p vectors that were not available (Table S1, primers 40 to 48). As templates to amplify GAPDH UTRs and hygromycin and puromycin genes, we used pTREX-n, pMOTag-4H (49), and pMOTag-23M (49) plasmids, respectively. The 3×c-Myc tag was excised from the pMOTag-23M vector (49) with XhoI and SalI enzymes and subsequently cloned into pTREX-h and pTREX-p by the XhoI restriction site.

Statistical analysis.

Values indicated in the figure legends are means ± standard deviations (SD) of n biological experiments (n is indicated in the figure legends), and the significance of their differences was evaluated by the Student t test.

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