Toxoplasma gondii is an important food and waterborne pathogen causing toxoplasmosis, a potentially severe disease in immunocompromised or congenitally infected humans. Available therapeutic agents are limited by suboptimal efficacy and frequent side effects that can lead to treatment discontinuation. Here we report that the benzoxaborole AN3661 had potent in vitro activity against T. gondii. Parasites selected to be resistant to AN3661 had mutations in TgCPSF3, which encodes a homologue of cleavage and polyadenylation specificity factor subunit 3 (CPSF‐73 or CPSF3), an endonuclease involved in mRNA processing in eukaryotes. Point mutations in TgCPSF3 introduced into wild‐type parasites using the CRISPR/Cas9 system recapitulated the resistance phenotype. Importantly, mice infected with T. gondii and treated orally with AN3661 did not develop any apparent illness, while untreated controls had lethal infections. Therefore, TgCPSF3 is a promising novel target of T. gondii that provides an opportunity for the development of anti‐parasitic drugs.
Benzoxaboroles are boron‐containing compounds effective against a wide range of infectious pathogens. The benzoxaborole AN3661 is a novel drug candidate against acute toxoplasmosis. The compound blocks an unprecedented target of Toxoplasma, the endonuclease CPSF3 that cleaves 3′‐mRNAs in eukaryotes.
AN3661 inhibits Toxoplasma growth in vitro at low micromolar concentrations, with potency comparable to that of the clinically relevant drug pyrimethamine.
AN3661 orally administered to mice controls otherwise lethal infections with comparable efficacy to sulfadiazine, the standard of care to treat toxoplasmosis.
Mice treated with AN3661 developed protective immunity to subsequent Toxoplasma infections.
Forward genetics and CRISPR/Cas9 editing validated CPSF3 as the main target of AN3661 in Toxoplasma.
Structural modelling allowed to place the resistant mutations at the Toxoplasma CPSF3 catalytic site and to identify the putative binding site of AN3661.
Toxoplasma gondii chronically infects about 30–50% of the human population (Pappas et al, 2009; Flegr et al, 2014; Parlog et al, 2015). Toxoplasmosis is usually an unapparent or mild disease in immunocompetent individuals, but it is a serious threat in immunocompromised patients, who can experience lethal or chronic cardiac, pulmonary or cerebral pathologies. Moreover, congenital toxoplasmosis can cause a range of problems including foetal malformations and retinochoroiditis. Current therapies for toxoplasmosis are reasonably effective, but they require long durations of treatment, often with toxic side effects (Farthing et al, 1992; Fung & Kirschenbaum, 1996), underlining the need for new classes of drugs to treat this infection (Neville et al, 2015).
Benzoxaboroles are boron‐containing compounds that have demonstrated efficacy in a number of clinical indications in recent years (Baker et al, 2009; Liu et al, 2014). Notably, Kerydin is an FDA‐approved benzoxaborole that inhibits fungal leucyl‐tRNA synthetase (LeuRS) and is used for the treatment of onychomycosis. Related compounds are being developed as LeuRS inhibitors of other human pathogens (Hernandez et al, 2013; Palencia et al, 2016a,b). Other benzoxaboroles inhibit phosphodiesterase‐4 (Freund et al, 2012), Rho kinase (Akama et al, 2013) and bacterial β‐lactamases (Xia et al, 2011). Overall, these compounds are synthetically tractable and show excellent drug‐like properties without significant safety liabilities.
In this study, we report that the benzoxaborole AN3661 inhibits T. gondii growth in human cells at low micromolar concentrations. Resistant parasites had mutations in a previously unexploited protein target, TgCPSF3. Importantly, all mice treated orally with AN3661 survived an otherwise lethal T. gondii infection and developed protective immunity to subsequent infections. Our results suggest TgCPSF3 is a promising novel target for the generation of new drugs to treat toxoplasmosis.
A benzoxaborole with potent in vitro activity against Toxoplasma gondii
Human foreskin fibroblasts (HFFs) were infected with tachyzoites of the virulent RH strain and treated with benzoxaboroles, pyrimethamine, the standard of care to treat toxoplasmosis, or vehicle (DMSO). We screened a group of 20 representative benzoxaboroles that were previously shown to have activity against bacteria, fungi or other eukaryotic parasites (Rock et al, 2007; Xia et al, 2011; Hernandez et al, 2013; Zhang et al, 2013; Palencia et al, 2016b). Some of these compounds were known to target leucyl‐tRNA synthetase (LeuRS). From this group of benzoxaboroles, only two compounds, AN6426 and AN3661, showed activity against Toxoplasma. AN6426 is a LeuRS inhibitor with moderate activity against Toxoplasma and is described in a separated article (Palencia et al, 2016a). However, AN3661 demonstrated very good activity (IC50 = 0.9 μM), with potency comparable to that of pyrimethamine, and without apparent detrimental effects to host cells (Fig 1).
Selection of Toxoplasma gondii parasite lines resistant to AN3661 and target identification
To explore the mechanism of action of AN3661, resistant parasites were generated with 7 mM ethyl methanesulphonate (EMS) in four independent chemical mutagenesis experiments, followed by selection in the presence of 5 μM AN3661 (> sixfold the IC50 value) over approximately 4 weeks. This is a useful approach to increase the frequency of mutations in Toxoplasma, which is otherwise very low (Farrell et al, 2014). The resistant parasite lines were then cloned by serial dilution. In a concomitant study, Plasmodium falciparum parasites that were resistant to AN3661 harboured mutations in two genes, pfcpsf3 and pfmdr1 (Sonoiki et al, 2017). CPSF3 encodes a homologue of the metal‐dependent endonuclease, subunit 3, of the mammalian cleavage and polyadenylation specificity factor complex (CPSF‐73) (Ryan et al, 2004; Xiang et al, 2014), and pfmdr1 encodes for an ABC transporter. Based on previous benzoxaboroles binding to proteins containing bimetal centres, we first decided to sequence Toxoplasma CPSF3 (TGGT1_285200; TgCPSF3), because it has a putative MBL domain with bimetal centre (two zinc ions). In all the AN3661‐resistant T. gondii lines that we isolated, we invariably found three single nucleotide polymorphisms (SNPs) leading to one of the following amino acid substitutions: E545K, Y328C and Y483N (Fig 2A).
In humans, CPSF‐73 co‐assembles in the nucleus into a large complex, including other cleavage/polyadenylation or stimulatory factors and polyadenylate polymerase (PAP). The complex cleaves the 3′‐end of pre‐mRNAs, which is subsequently polyadenylated (Xiang et al, 2014; Schönemann et al, 2014) before the mRNA is exported into the cytoplasm for translation (Fig EV1A). CPSF‐73 provides the endonuclease activity for this complex (Ryan et al, 2004; Dominski et al, 2005; Mandel et al, 2006). When we monitored TgCPSF3 in a T. gondii line expressing the endogenous protein tagged with an HA‐FLAG, we found that TgCPSF3 accumulates in the parasite nucleus (Fig EV1B), consistent with a similar function to that of its human counterpart.
CRISPR/Cas9‐mediated point mutations in TgCPSF3 confer resistance to AN3661
To confirm that TgCPSF3 mutations account for resistance to AN3661, we introduced each of the mutations identified in AN3661‐resistant parasites into the T. gondii parental strain using CRISPR/Cas9 gene editing (Fig 2B). After co‐transfection with oligonucleotides containing the desired mutations, resistant parasites were selected in the presence of 5 μM AN3661 (> sixfold the IC50 value). Emergent resistant parasites were cloned, and DNA sequencing confirmed that the mutations were correctly introduced into TgCPSF3 (Figs 2B and EV2). No resistant parasite lines emerged following transfection with the CRISPR/Cas9 control vectors alone. Compared to wild‐type parasites, mutant lines (each containing only one of the above mutations) had markedly decreased susceptibility to AN3661 (Fig 1B). To corroborate that TgCPSF3 is the bona fide target of AN3661, we expressed a mutated copy of TgCPSF3 (TgCPSF3E545K) in wild‐type parasites and evaluated whether the transgene would restore parasite growth in the presence of AN3661. The TgCPSF3E545K cassette was inserted by homologous recombination into the locus coding for the surface antigen protein 1 (SAG1), a non‐essential gene, using CRISPR/Cas9 gene editing (Fig EV3A). All the resultant transgenic lines contained the TgCPSF3E545K cassette correctly inserted into the SAG1 locus, as confirmed by both immunofluorescence and genomic analysis (Fig EV3A and B). This extra copy efficiently restored parasite growth in the presence of 5 μM AN3661, indicating that the ectopic expression of mutant TgCPSF3E545K conferred resistance to AN3661.
Mutations conferring resistance are clustered at the endonucleolytic site of TgCPSF3
We built a structural homology model of TgCPSF3 with a pre‐mRNA substrate bound into the catalytic site using structures of mammalian CPSF‐73 and bacterial J/Z RNases that contained a metallo‐β‐lactamase (MBL) domain (Fig 3A). Similar to eukaryotic homologues, TgCPSF3 contained an MBL domain, a β‐CASP domain and a C‐terminal domain with a putative endonuclease site at the interface between the MBL and β‐CASP domains (Fig 3A). TgCPSF3 showed strict conservation of the catalytic motifs, including highly conserved histidine and aspartic/glutamic acid residues, which coordinate the two zinc atoms involved in the cleavage of the 3′‐end of pre‐mRNAs (Fig EV4). The three mutated residues associated with AN3661 resistance (Y328C, Y483N and E545K) clustered to one side of the catalytic site, which, by homology to other CPSF‐73 or bacterial RNases, binds the 3′‐end of pre‐mRNAs (Fig 3A–C). In fact, one of the mutated residues, Y483, was described as important for positioning of the 3′‐end of the pre‐mRNA by forming the closing gate at the catalytic site of human CPSF‐73 (Mandel et al, 2006). To investigate the binding of the inhibitor, we performed in silico molecular docking of AN3661 into the homology model of TgCPSF3, and found that AN3661 favourably fits (docking Glide score ~6 kcal/mol) into the catalytic site of TgCPSF3, and the placement mimics the position of the 3′‐end of the mRNA substrate (Fig 3C and D). More specifically, the tetrahedral boron atom of AN3661 occupies the position of the cleavage site phosphate (second to last, Pi−1) of the mRNA substrate near the catalytic site, with one hydroxyl group interacting with a zinc atom (Fig 3C and D). This is consistent with structures for other benzoxaboroles that were shown to bind to the bimetal centres of beta‐lactamases and phosphodiesterase‐4 (Xia et al, 2011; Freund et al, 2012). In this conformation, the aromatic ring AN3661 favourably binds into the inverted V‐shape pocket formed by the aromatic residues Y328 and Y483. In addition, the carboxylic group of AN3661 establishes hydrogen bonds to the side chains of Y366 and S519, and to the main chain backbone atoms of Y483 and A520. Considering the clustered position of the mutations conferring resistance to AN3661 in the catalytic site of TgCPSF3, it is likely that the compound binds into this site and perturbs the pre‐mRNA processing activity that is essential for parasite growth.
We then extended this structural analysis to understand how the mutations confer resistance to AN3661. This analysis shows that rather than clashing with AN3661, the mutations Y483N and Y328C distort the geometry of the drug binding pocket and would lead to loss of contacts between the protein and AN3661 that decrease the affinity (Fig 3D–F). The mutation E545K has an indirect effect on the drug binding pocket that is mediated by Y483, again likely via the perturbation of the drug binding pocket (Fig 3D–G). The side chain of E545 is in a favourable conformation at 4.6 Å of Y483. The change to lysine introduces a positive charge and a bulky side chain that would clash to Y483 as it gets as close as 1.8 Å. Therefore, it is likely that the rearrangement of Y483 to prevent the clash with K545 impacts negatively on the affinity of AN3661. As the mutated residues in Plasmodium (Y406, D470) are equivalent to the ones we found in T. gondii (Fig 3B), it is very much possible that the resistance mechanism is shared. It is also interesting to note that all the mutated residues are conserved among other apicomplexan parasites (Fig 3B).
In vivo efficacy of AN3661 in a murine model of acute toxoplasmosis and development of protective immunity
In mice, type I T. gondii strains are virulent and lethal whereas type II strains are less virulent, but can establish chronic infections (Sibley & Ajioka, 2008). We studied type I and II strains to gain insights into the efficacy of AN3661 in both acute and chronic murine models of toxoplasmosis. Mice were infected intraperitoneally with the highly virulent type I (RH) strain and then treated once a day for 7 days with AN3661 or sulphadiazine starting 24 h after infection. Untreated mice succumbed and were euthanized within 7 days (Fig 4A), while all mice administered AN3661 at 20 mg/kg orally for 7 days demonstrated no apparent signs of illness, such as lethargy, ruffled fur or hunched posture. In contrast, after infection with the resistant TgCPSF3E545K line, both untreated mice and mice treated with the same regimen of AN3661 described above succumbed to infection (Fig 4A), consistent with the conclusion that AN3661 acts in vivo by inhibiting TgCPSF3.
We also tested AN3661 against T. gondii type II strains by infecting mice intraperitoneally with a type II (76K) strain containing a luciferase‐encoding reporter gene that is ubiquitously expressed in both acute (tachyzoite) and chronic (bradyzoite) T. gondii stages. We found a drastic reduction in the population size of parasites imaged in AN3661‐treated mice compared to untreated mice. Differences between treated and control mice were seen as early as day 2 post‐infection and persisted over 7 days of monitoring (Fig 4B). We next explored whether AN3661 treatment could confer protective immunity to secondary infection. First, serologic analyses of animals treated with AN3661 revealed enriched levels of anti‐Toxoplasma‐specific antibodies (30 days after infection), suggesting the development of immunity to toxoplasmosis (Fig 4C). Then, by providing lethal challenges of the hypervirulent RH and GT1 strains to the same mice, we confirmed that the initial 7‐day treatment with AN3661 enhanced an immune response in 100% of mice, which was capable of protecting them from subsequent Toxoplasma infections (Fig 4C and D).
Taken together, AN3661 possesses all of the attributes of a highly promising future drug against T. gondii. Indeed, the compound is rapidly parasiticidal after oral administration, prevents dissemination of parasites in deep tissues and enables protective immunity against toxoplasmosis in mice.
Available drugs to treat toxoplasmosis have limitations in efficacy and can cause serious adverse effects. Moreover, toxoplasmosis is now recognized as a leading cause of foodborne illness in the United States (Scallan et al, 2011). Since a Toxoplasma vaccine for use in humans is not currently available, new classes of drugs, preferably directed against novel targets, are needed. Here we report that the benzoxaborole AN3661 inhibits Toxoplasma growth in vitro and, when orally administered to mice, is not only effective against otherwise lethal infections but also enables protective immunity against subsequent Toxoplasma infections. Genetic evidence reported herein supports the conclusion that AN3661 acts via the inhibition of a novel target of T. gondii, TgCPSF3, which is homologous to the endonuclease subunit (CPSF‐73) within the human CPSF complex that cleaves 3′‐mRNAs (Ryan et al, 2004; Mandel et al, 2006).
In another study, it is shown that AN3661 is also active against the human malaria parasite P. falciparum (Sonoiki et al, 2017). Interestingly, T. gondii and P. falciparum parasite lines selected for resistance to AN3661 both harboured mutations in residues located in the active site of CPSF3. Indeed, two out of the three SNPs selected in T. gondii, Y483N and E545K, correspond to SNPs selected in P. falciparum, Y408S and D470N. Importantly, knock‐in of these mutations using CRISPR/Cas9 gene editing recapitulated resistance to AN3661 in T. gondii and P. falciparum parasites in vitro. In addition, mice infected with the resistant TgCPSF3E545K line and then treated with AN3661 did not survive to infection, whereas mice infected with wild‐type strain and treated did, thus validating CPSF3 as the Toxoplasma target of AN3661 in vivo. Combined, these studies strongly support the conclusion that AN3661 acts via the inhibition of CPSF3 in these apicomplexan parasites.
To our knowledge, CPSF3 has not previously been proposed as a target for drug discovery; however, in very recent study on T. gondii genome‐wide gene‐knockout screenings, CPSF3 was identified as an essential gene for the life of the parasite (Sidik et al, 2016). Given that CPSF3 has an essential function during 3′‐mRNA processing and is conserved among eukaryotes, structural and mechanistic studies to identify key differences in CPSF3 between humans and pathogens are warranted. Similar to its human homologue, TgCPSF3 accumulates in the nucleus of T. gondii (Fig EV1B) and presumably forms a complex with other CPSF subunits whose genes are present in the Toxoplasma genome. Indeed, genes encoding CPSF3 homologues are also conserved in other protozoans (Fig 3D), providing an opportunity to inhibit multiple parasites with the same or related compounds.
After acute infection characterized by fast asexual reproduction, T. gondii parasites disseminate into tissues (including brain, lungs and heart) and form cysts where they reproduce slowly, often for the lifetime of the host. In immunocompromised hosts, dormant infections can reactivate, causing encephalitis, pneumonitis, myocarditis and other serious sequelae for which available therapies offer suboptimal efficacy (Crespo et al, 2000). Our in vivo experiments demonstrating activity of AN3661 against Toxoplasma infections in murine models of infection suggest a new avenue in toxoplasmosis drug discovery and should encourage new studies to evaluate the activity of this CPSF3 inhibitor and related compounds against slow‐growing bradyzoites in cysts, which are so far untreatable. Furthermore, the appearance of full protective immunity against Toxoplasma in mice that were treated with AN3661 is noteworthy and could have implications for vaccine development or other preventive therapies.
Materials and Methods
Parasite strains and cell culture
The parasites used were T. gondii type I RH strain wild type or mutant ku80 for gene editing by homologous recombination, and the T. gondii type II 76K strain expressing green fluorescent protein (GFP) and luciferase (provided by Michael E. Grigg, National Institute of Health, Bethesda). The T. gondii strains were maintained by serial passage in HFF monolayers in Dulbecco's modified Eagle's medium (DMEM, Invitrogen) supplemented with 10% (v/v) FBS (Invitrogen), 25 mM Hepes buffer, pH 7.2, and 50 mg/ml each of penicillin and streptomycin. Cells were incubated at 37°C with 5% CO2 in humidified air.
Toxoplasma gondii random mutagenesis
Parasites were chemically mutagenized according to a previously published protocol (Coleman & Gubbels, 2012; Farrell et al, 2014). Briefly, ~107 tachyzoites growing intracellularly in HFF cells (18–25 h post‐infection) in a T25 flask were incubated at 37°C for 4 h in 0.1% FBS DMEM growth medium containing either 7 mM ethyl methanesulphonate (EMS, Sigma, diluted from a 1 M stock solution in DMSO) or the appropriate vehicle controls. Plaque assays revealed that 7 mM EMS induced ~70% killing of the parasite population, theoretically leading to ~20 SNPs per genome (Farrell et al, 2014). After exposure to mutagen, parasites were washed three times with phosphate‐buffered saline (PBS), and the mutagenized population was allowed to recover in a fresh T25 flask containing an HFF monolayer in the absence of drug for 3–5 days. Released tachyzoites were then inoculated into fresh cell monolayers in medium containing 5 μM AN3661 and incubated until viable extracellular tachyzoites emerged 8–10 days later. Surviving parasites were passaged once more under continued AN3661 treatment and cloned by limiting dilution. Four cloned mutants were isolated each from four independent mutagenesis experiments. Each flask therefore contained unique SNP pools. TgCPSF3 (TGGT1_285200) was amplified by PCR using primers TGGT1_285200_F and TGGT1_285200_R (Appendix Table S1). The resulting PCR products were sequenced to identify putative SNPs.
The bicistronic vectors expressing the Cas9 genome editing enzyme and specific sgRNAs targeting the CPSF3 coding sequence were constructed as described previously (Curt‐Varesano et al, 2016). Briefly, oligonucleotides CPSF3E545K‐CRISPR‐FWD and CPSF3E545K‐CRISPR‐REV, CPSF3Y328C‐CRISPR‐FWD and CPSF3Y328C‐CRISPR‐REV, and CPSF3Y483N‐CRISPR‐FWD and CPSF3Y483N‐CRISPR‐REV (Appendix Table S1) were annealed and ligated into the pTOXO_Cas9CRISPR plasmid to create vectors used for construction of T. gondii recombinant for CPSF3E545K, CPSF3Y328C and CPSF3Y483N, respectively. The plasmid used for targeting SAG1 coding sequence was constructed as above using the oligonucleotides SAG1‐CRISPR‐FWD and SAG1‐CRISP‐REV.
Toxoplasma gondii genome editing
For construction of the recombinant parasites harbouring allelic replacement for CPSF3Y328C, CPSF3Y483N and CPSF3E545K, the T. gondii RH strain was transfected by electroporation using parameters established previously (Bougdour et al, 2013) with pTOXO_Cas9CRISPR vectors targeting the CPSF3 coding sequence (sgCPSF3Y328C, sgCPSF3Y483N and sgCPSF3E545K) and their respective 120‐base donor oligonucleotides (CPSF3Y328C_(s), CPSF3Y483N_(s) and CPSF3E545K_(s); Appendix Table S1) for homology‐directed repair. Recombinant parasites were selected with 5 μM AN3661 prior to subcloning by limited dilution, and allelic replacement was verified by sequencing of TgCPSF3 genomic DNA as described above.
For construction of the SAG1 insertional mutant, T. gondii RH ku80 parasites were co‐transfected with a mixture of the pTOXO_Cas9CRISPR vector targeting the SAG1 coding sequence with a repair template corresponding to a purified amplicon containing the CPSF3E545K cassette with homology arms of 60 nucleotides flanking the site of alteration targeted by sgSAG1 and cleaved by Cas9 (5:1 mass ratio). These amplicons were generated by PCR amplification of the CPSF3E545K cassette using primers HRSAG1‐5UTR‐CPSF3_F and HRSAG1‐3UTR‐CPSF3_R (Appendix Table S1) and genomic DNA extracted from the CPSF3E545K recombinant parasites as template. As a negative control, a PCR amplicon was generated using genomic DNA from wild‐type RH parasites. Stable recombinants were selected in the presence of 5 μM AN3661, single cells were cloned by limiting dilution, and sequences were verified by PCR analysis as described in Fig EV3 using the primers 5UTR‐SAG1_F and SAG1_R (Appendix Table S1). The constructed TgCPSF3E545K line represents a new and highly efficient selection cassette that expands the current genome editing toolbox for T. gondii.
Cells grown on coverslips were fixed in 3% formaldehyde for 20 min at room temperature, permeabilized with 0.1% (v/v) Triton X‐100 for 5 min and blocked in PBS containing 3% (w/v) BSA. The cells were then incubated for 1 h with primary antibodies (mouse anti‐IMC1, IMC1 = inner membrane complex protein 1; and rabbit anti‐toxofilin) followed by the addition of secondary antibodies conjugated to Alexa Fluor 488 or 594 (Molecular Probes) to detect intracellular parasites. Nuclei were stained for 10 min at room temperature with Hoechst 33258. Coverslips were mounted on a glass slide with Mowiol mounting medium, and images were acquired with an Axio Imager M2 fluorescence microscope with ApoTome 2 module (Carl Zeiss, Inc.).
Determination of IC50s against Toxoplasma gondii by fluorescence imaging assays
The in vitro inhibitory activity of AN3661 on T. gondii proliferation was determined by high‐content fluorescence imaging as follows; HFFs at a density of 10,000 cells per well in 96‐well plates were infected with 4 × 104 parasites. Invasion was synchronized by briefly centrifuging the plate at 400 rpm, and plates were placed at 37°C for 2 h. Infected cells were then washed three times with PBS followed immediately by the addition of the test compounds diluted at the indicated final concentrations in culture medium. Pyrimethamine was used as a positive control. After 24 h of growth, nuclei were stained with Hoechst 33342 at 5 μg/ml for 20 min. Cells were washed with PBS and fixed with pre‐warmed 3.7% formaldehyde for 10 min at 37°C. Fixed cells were permeabilized in PBS supplemented with 0.1% Triton X‐100. Parasite vacuoles were immunostained by incubating in blocking solution (3% BSA in PBS) for 1 h followed by incubation with anti‐GRA1 primary antibody (specific to Toxoplasma) and Alexa Fluor 488‐conjugated secondary antibodies (Thermo Fisher). Images were automatically acquired using the ScanR microscope system (Olympus) with a 20× objective. Twenty fields per well were acquired and analysed using the ScanR analysis module. Parasitophorous vacuoles were analysed as follows: a background subtraction was applied in all sets of images and an intensity algorithm module was used with a minimum of 50 pixels size to segment the smallest vacuoles corresponding to those containing a single tachyzoite. Parasite nuclei were discriminated from host cell nuclei by using an edge segmentation module by ScanR analysis. A gating procedure was used to hierarchically filter the selected data points with precise boundaries (e.g. number of vacuoles vs. number of parasites/vacuole). Experiments were done in triplicate, and data were processed using the GraphPad Prism software to determine the IC50s.
The TgCPSF3 model was built by homology modelling using the X‐ray crystallographic coordinates of eukaryotic/archaeal CPSF homologues (PDB codes: 3AF5, 2I7V) and bacterial RNases Z/J [Protein Data Bank (PDB) accession codes: 3A4Y, 3IEK and 3AF5] with I‐TASSER (Zhang, 2008). The position of the zinc atoms was modelled by aligning to the metallo‐β‐lactamase and β‐CASP domains of RNase J (PDB code: 5A0T) and manually adjusted in Coot using local refinements (Emsley & Cowtan, 2004). The 3′‐mRNA substrate was modelled in the catalytic site of TgCPSF3 using as templates bacterial RNase complexes with RNA (PDB codes: 3IEM and 5A0T). Manual adjustments, mutagenesis and local energy refinements were carried out in Coot. The PyMOL Molecular Graphics System (v.1.6.0, Schrodinger, LLC) was used to prepare figures.
In silico docking of AN3661 into the catalytic site of TgCPSF3
The three‐dimensional structure of AN3661, with the boron atom in tetrahedral configuration, was built using Maestro interface in the Schrodinger suite (Maestro, version 9.5, Schrodinger, LLC, New York, NY, USA, 2013). The ligand was prepared using the LigPrep module of Maestro (LigPrep, version 2.7, Schrodinger, LLC) and was minimized using OPLS‐2005 force field.
The three‐dimensional structure of a TgCPSF3 protein model, built using the PBD entry 3IEM as a template, was refined using the Protein Preparation Wizard of Maestro. All of the crystallographic water molecules, except that in the binding site, were removed from the three‐dimensional structure of the protein. Following these steps, a minimization step of the protein was carried out in Maestro. In order to perform virtual docking experiments, the receptor grid for CPSF3 was set up and generated from a Receptor Grid Generation panel: a cubic box of 10 Å per side was built and centred on the zinc ion cluster to define the active site of the protein. Docking of AN3661 was performed using the extra precision (XP) method of Glide with default settings (Friesner et al, 2006).
In vivo mouse therapeutic efficacy assays
All animal procedures were conducted under pathogen‐free conditions in compliance with established institutional guidance and approved protocols from the European Directive 2010/63/EU. We used randomization and blinding to treatment assignment to reduce bias in mice selection and outcome assessment. Three independent experiments were performed with three mice per treatment group (female CBA/JRj mice, Janvier, Le Genest‐Dt‐Isle, France; 7–9 weeks old). Mice were infected intraperitoneally with 103 tachyzoites of the type I virulent RH wild‐type and CPSF3E545K mutant strains, 103 tachyzoites of the type I GT1, or with 105 tachyzoites of the type II 76K GFP Luc strain. These inocula routinely resulted in high mortality in control mice at 6–12 days post‐infection. All treatments were initiated at day 1 post‐infection and were continued for seven consecutive days. Treated mice were orally administered 20 mg/kg AN3661 or 200 mg/kg sulphadiazine (Sigma), as previously described (Romand et al, 1993), both suspended in 1% (w/v) methylcellulose and 0.1% (v/v) Tween‐80. Parasitemias of mice infected by the type II 76K GFP Luc strain were monitored using the in vivo Imaging System (IVIS Kinetic; Perkinelmer, USA) to acquire bioluminescence signals at days 0, 2, 5 and 7 after infection. Day 0 corresponds to the signal at 12 h after infection, which is considered the optimal time point 0 because parasites are not detectable before 12–24 h after infection. Five minutes before imaging, mice received an intraperitoneal injection of 150 μg/g of D‐luciferin (Promega, France) and were then anesthetized (isoflurane 4% for induction and 1.5% thereafter) and placed in the optical imaging system for image acquisition. This allowed localization of luciferase‐positive parasites and evaluation of the parasite load. Bioluminescence signals were expressed as photons/seconds (p/s). The mice serological status for toxoplasmosis was determined by Western blot analysis using the LDBio‐Toxo II IgG test (LDBIO Diagnostics, Lyon, France) with an anti‐mouse IgG–alkaline phosphatase conjugate. Mice sera were extracted at days 30 and 88 after infection.
The paper explained
Toxoplasmosis is a widespread foodborne infection in humans that poses significant public health problems. Caused by the protozoan apicomplexa parasite T. gondii, toxoplasmosis, a usually mild disease, can turn into a major threat during pregnancy or in immunocompromised patients who experience lethal or chronic cardiac, pulmonary or cerebral pathologies. Current drugs to treat toxoplasmosis including sulphadiazine and pyrimethamine require long courses of therapy and are often limited by side effects. Therefore, new classes of drugs, preferably directed against novel targets, are needed.
Benzoxaboroles are boron‐containing compounds effective against a wide range of infectious pathogens. This study reports the potent activity of the benzoxaborole AN3661 against Toxoplasma growth in vitro. Importantly, when orally administered to mice, the compound is not only effective against otherwise lethal infections but also enables protective immunity against subsequent Toxoplasma infections and without signs of toxicity. AN3661‐resistant parasites have been readily selected in vitro and the resistance‐causing mutations were identified in residues of the active site of CPSF3, the endonuclease that cleaves the 3′‐end of immature mRNAs in eukaryotes. Knock‐in of the mutations using CRISPR/Cas9 gene editing recapitulated the phenotype resistance to AN3661 both in vitro and in the murine model of toxoplasmosis, thus validating CPSF3 as the primary target of AN3661.
The discovery of CPSF3 as the target of AN3361 opens a new avenue to rationally design inhibitors with improved drug‐like properties against Toxoplasma and related parasites of the phylum including Plasmodium spp that cause malaria.
AP, AB, SC and M‐AH conceived and designed the experiments; AP, AB, M‐PB‐P, BT, R‐LB, CS, JV, VJ, GG, EE, HP and M‐AH acquired data. AP, AB and M‐AH analysed and interpreted data; AP and AB prepared the manuscript with main input by PJR, YRF, SC and M‐AH; all authors provided comments and approved the final version of the manuscript.
Conflict of interest
E.E. and Y.R.F. are employees of Anacor Pharmaceuticals, Inc. The other authors declare that they have no conflict of interest.
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This work was supported by the Laboratoire d'Excellence (LabEx) ParaFrap [ANR‐11‐LABX‐0024], the European Research Council [ERC Consolidator Grant No. 614880 Hosting TOXO to M.A.H.], the grant ANR Blanc 2012 TOXOHDAC [ANR‐12‐BSV3‐0009‐01], and the grant ANR Jeune Chercheur 2012 ToxoEffect [ANR‐ 12‐JSV3‐0004‐01].
FundingLaboratoire d'Excellence (LabEx) ParaFraphttp://dx.doi.org/10.13039/501100004100
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