Plasmodium falciparum: direct cloning and expression of pyrimethamine-sensitive and pyrimethamine-resistant dihydrofolate reductase domains

Antimalarial effect of cell penetrating peptides derived from the junctional region of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase

Dihydrofolate reductase-thymidylate synthase of Plasmodium falciparum (PfDHFR-TS) is an important target of antifolate antimalarial drugs. However, drug resistant parasites are widespread in malaria endemic regions. The unique bifunctional property of PfDHFR-TS could be exploited for the design of allosteric inhibitors that interfere with the active dimer conformation. In this study, peptides were derived from the junctional region (JR) of PfDHFR-TS amino acid sequence in the α_j1 helix (JR-helix) and the DHFR domain that is necessary for interaction with α_j1 helix (JR21). Five peptides were synthesized and tested for inhibition of PfDHFR-TS enzyme by Bacterial inhibition assay (BIA) based on the growth of an E. coli DHFR and TS knockout complemented with a recombinant plasmid expressing PfDHFR-TS enzyme. Significant inhibition was observed for JR21 and JR21 conjugated to cell-penetrating octa-arginine peptide (rR8-JR21) with 50 % inhibitory concentration (IC₅₀) of 3.87 and 1.53 μM, respectively. The JR-helix and rR8-JR-helix peptides were inactive. JR21 and rR8-JR21 peptides showed similar growth inhibitory effects on P. falciparum NF54 parasites cultured in vitro. Treatment with rR8-JR21 delayed parasite development, in which an accumulation of ring stage parasites was observed after 12 h of culture. Minimal red blood cell (RBC) hemolysis was observed at the highest dose of peptide tested. The most potent peptide rR8-JR21 not only compromised the development of the P. falciparum, but also inhibited the parasite growth and has low hemolytic effect on human RBCs.

Insights into the role of the junctional region of Plasmodium falciparum dihydrofolate reductase-thymidylate synthase

Background

Plasmodium falciparum dihydrofolate reductase-thymidylate synthase (pfDHFR-TS) is a well-defined target of anti-malarial drug, such as pyrimethamine and cycloguanil. Emergence of malaria parasites resistant to these drugs has been shown to be associated with point mutations of the gene coding for the target enzymes. Although the 3D-structure of P. falciparum bifunctional pfDHFR-TS has been reported previously, relatively little is known about the interactions between the pfDHFR and pfTS domains and the roles of the junctional region that links the two domains together. Therefore, a thorough understanding of the interaction of the two domains and the role of the junctional region of this target is important as the knowledge could assist the development of new effective anti-malarial drugs aimed at overcoming drug-resistant malaria.

Methods

A system was developed to investigate the interaction between pfDHFR and pfTS domains and the role of the junctional region on the activity of the recombinant pfTS. Based on the ability of co-transformed plasmids coding for pfDHFR and pfTS with truncated junctional region to complement the growth of TS-deficient Escherichia coli strain χ2913recA(DE3) on minimum media without thymidine supplementation, active pfTS mutants with minimal length of junctional region were identified. Interactions between active pfDHFR and the pfTS domains were demonstrated by using a bacterial two-hybrid system.

Results

Using TS-deficient E. coli strain χ2913recA(DE3), the authors have shown for the first time that in P. falciparum a junctional region of at least 44 amino acids or longer was necessary for the pfTS domain to be active for the synthesis of thymidylate for the cells. Truncation of the junctional region of the bifunctional pfDHFR-TS further confirmed the above results, and suggested that a critical length of the junctional peptide of pfDHFR-TS would be essential for the activity of TS to catalyze the synthesis of thymidylate.

Conclusion

The present study demonstrated the interactions between the pfDHFR and pfTS domains of the bifunctional pfDHFR-TS, and revealed that the junctional region linking the two protein domains is essential for the expression of catalytically active pfTS domain. The findings could be useful since inhibition of the pfDHFR-TS domain-domain interaction could form a basis for the development of new anti-malarial drugs based on targeting the non-active site region of this important enzyme.

Novel triazine JPC-2067-B inhibits Toxoplasma gondii in vitro and in vivo

BACKGROUND AND METHODOLOGY

Toxoplasma gondii causes substantial morbidity, mortality, and costs for healthcare in the developed and developing world. Current medicines are not well tolerated and cause hypersensitivity reactions. The dihydrotriazine JPC-2067-B (4, 6-diamino-1, 2-dihydro-2, 2-dimethyl-1-(3'(2-chloro-, 4-trifluoromethoxyphenoxy)propyloxy)-1, 3, 5-triazine), which inhibits dihydrofolate reductase (DHFR), is highly effective against Plasmodium falciparum, Plasmodium vivax, and apicomplexans related to T. gondii. JPC-2067-B is the primary metabolite of the orally active biguanide JPC-2056 1-(3'-(2-chloro-4-trifluoromethoxyphenyloxy)propyl oxy)- 5-isopropylbiguanide, which is being advanced to clinical trials for malaria. Efficacy of the prodrug JPC-2056 and the active metabolite JPC-2067-B against T. gondii and T. gondii DHFR as well as toxicity toward mammalian cells were tested.

PRINCIPAL FINDINGS AND CONCLUSIONS

Herein, we found that JPC-2067-B is highly effective against T. gondii. We demonstrate that JPC-2067-B inhibits T. gondii growth in culture (IC50 20 nM), inhibits the purified enzyme (IC50 6.5 nM), is more efficacious than pyrimethamine, and is cidal in vitro. JPC-2067-B administered parenterally and the orally administered pro-drug (JPC-2056) are also effective against T. gondii tachyzoites in vivo. A molecular model of T. gondii DHFR-TS complexed with JPC-2067-B was developed. We found that the three main parasite clonal types and isolates from South and Central America, the United States, Canada, China, and Sri Lanka have the same amino acid sequences preserving key binding sites for the triazine.

SIGNIFICANCE

JPC-2056/JPC-2067-B have potential to be more effective and possibly less toxic treatments for toxoplasmosis than currently available medicines.

Mechanisms of resistance of malaria parasites to antifolates

Antifolate antimalarial drugs interfere with folate metabolism, a pathway essential to malaria parasite survival. This class of drugs includes effective causal prophylactic and therapeutic agents, some of which act synergistically when used in combination. Unfortunately, the antifolates have proven susceptible to resistance in the malaria parasite. Resistance is caused by point mutations in dihydrofolate reductase and dihydropteroate synthase, the two key enzymes in the folate biosynthetic pathway that are targeted by the antifolates. Resistance to these drugs arises relatively rapidly in response to drug pressure and is now common worldwide. Nevertheless, antifolate drugs remain first-line agents in several sub-Saharan African countries where chloroquine resistance is widespread, at least partially because they remain the only affordable, effective alternative. New antifolate combinations that are more effective against resistant parasites are being developed and in one case, recently introduced into use. Combining these antifolates with drugs that act on different targets in the parasite should greatly enhance their effectiveness as well as deter the development of resistance. Molecular epidemiological techniques for monitoring parasite drug resistance may contribute to development of strategies for prolonging the useful therapeutic life of this important class of drugs.

Divergent regulation of dihydrofolate reductase between malaria parasite and human host

For half a century, successful antifolate therapy against Plasmodium falciparum malaria has been attributed to host-parasite differences in drug binding to dihydrofolate reductase-thymidylate synthase (DHFR-TS). Selectivity may also arise through previously unappreciated differences in regulation of this drug target. The DHFR-TS of Plasmodium binds its cognate messenger RNA (mRNA) and inhibits its own translation. However, unlike translational regulation of DHFR or TS in humans, DHFR-TS mRNA binding is not coupled to enzyme active sites. Thus, antifolate treatment does not relieve translational inhibition and parasites cannot replenish dead enzyme.

Novel alleles of the Plasmodium falciparum dhfr highly resistant to pyrimethamine and chlorcycloguanil, but not WR99210

We have expressed dhfr alleles of Plasmodium falciparum in the budding yeast, Saccharomyces cerevisiae, and used this yeast model to identify single amino acid substitutions that confer high level pyrimethamine resistance on the background of the triple mutant dhfr (I51+R59+N108). Mutations in three clusters were identified: codons 50-57, 187-193, and 213-214. Several mutations previously identified in field samples were also isolated, including codons 50 and 164. The I164L mutation is of particular interest, because the quadruple mutant genotype (N51I+C59R+S108N+I164L) encodes an enzyme that is no longer inhibited by pyrimethamine, rendering sulfadoxine/pyrimethamine (SP; Fansidar) clinically ineffective. Thirty-six novel alleles were tested to determine their sensitivity to chlorcycloguanil and WR99210, two DHFR inhibitors that are in clinical and pre-clinical development, respectively. Chlorcycloguanil is effective against parasites that carry the triple mutant allele, but in vitro analysis has suggested that chlorcycloguanil will be clinically ineffective against parasites that carry the quadruple mutant allele of dhfr. In our screen, 23 of 36 novel strains were as resistant to chlorcycloguanil as the quadruple mutant, and one strain was 10-fold more resistant. WR99210 is still effective in the nM range against parasites that carry the quadruple mutant allele. In the preliminary screen, 31 of 36 novel alleles were as sensitive to WR99210 as the quadruple mutant. Detailed analysis of the remaining five showed that four of the five had IC(50) values in the same range as the quadruple mutant, and one, N51I+C59R+S108N+E192G, had an IC(50) value about fivefold higher. This result suggests that WR99210 and related compounds will be clinically effective against quadruple mutants currently found in Southeast Asia and South America and against most novel alleles that could be selected on the background of the triple mutant genotype now prevalent in East Africa.

Kinetic properties of dihydrofolate reductase from wild-type and mutant Plasmodium vivax expressed in Escherichia coli

Antifolate drugs inhibit malarial dihydrofolate reductase (DHFR). In Plasmodium falciparum, antifolate resistance has been associated with point mutations in the gene encoding DHFR. Recently, mutations at homologous positions have been observed in the P. vivax gene. Since P. vivax cannot be propagated in a continuous in vitro culture for drug sensitivity assays, the kinetic properties of DHFR were studied by expression of the DHFR domain in Escherichia coli. Induced expression yielded a protein product that precipitated as an inclusion body in E. coli. The soluble, active DHFR recovered after denaturation and renaturation was purified to homogeneity by affinity chromatography. Kinetic properties of the recombinant P. vivax DHFR showed that the wild-type DHFR (Ser-58 and Ser-117) and double mutant DHFR (Arg-58 and Asn-117) have similar K(m) values for dihydrofolate and NADPH. Antifolate drugs (pyrimethamine, cycloguanil, trimethoprim, and methotrexate), but not proguanil (parent compound of cycloguanil) inhibit DHFR activity, as expected. The kinetics of enzyme inhibition indicated that point mutations (Ser58Arg and Ser117Asn) are associated with lower affinity between the mutant enzyme and pyrimethamine and cycloguanil, which may be the origin of antifolate resistance.

Essential protein-protein interactions between Plasmodium falciparum thymidylate synthase and dihydrofolate reductase domains

In Plasmodium falciparum, dihydrofolate reductase and thymidylate synthase activities are conferred by a single 70-kDa bifunctional polypeptide (DHFR-TS, dihydrofolate reductase-thymidylate synthase) which assembles into a functional 140-kDa homodimer. In mammals, the two enzymes are smaller distinct molecules encoded on different genes. A 27-kDa amino domain of malarial DHFR-TS is sufficient to provide DHFR activity, but the structural requirements for TS function have not been established. Although the 3'-end of DHFR-TS has high homology to TS sequences from other species, expression of this protein fragment failed to yield active TS enzyme, and it failed to complement TS(-) Escherichia coli. Unexpectedly, even partial 5'-deletion of full-length DHFR-TS gene abolished TS function on the 3'-end. Thus, it was hypothesized that the amino end of the bifunctional parasite protein plays an important role in TS function. When the 27-kDa amino domain (DHFR) was provided in trans, a previously inactive 40-kDa carboxyl-domain from malarial DHFR-TS regained its TS function. Physical characterization of the "split enzymes" revealed that the 27- and the 40-kDa fragments of DHFR-TS had reassembled into a 140-kDa hybrid complex. Thus, in malarial DHFR-TS, there are physical interactions between the DHFR domain and the TS domain, and these interactions are necessary to obtain a catalytically active TS. Interference with these essential protein-protein interactions could lead to new selective strategies to treat malaria resistant to traditional DHFR-TS inhibitors.

A biochemical and genetic model for parasite resistance to antifolates. Toxoplasma gondii provides insights into pyrimethamine and cycloguanil resistance in Plasmodium falciparum

We have exploited the experimental accessibility of the protozoan parasite Toxoplasma gondii and its similarity to Plasmodium falciparum to investigate the influence of specific dihydrofolate reductase polymorphisms known from field isolates of drug-resistant malaria. By engineering appropriate recombinant shuttle vectors, it is feasible to examine mutations by transient or stable transformation of T. gondii parasites, in bacterial and yeast complementation assays, and through biochemical analysis of purified enzyme. A series of mutant alleles that mirror P. falciparum variants reveals that the key mutation Asn-108 (Asn-83 in T. gondii) probably confers resistance to pyrimethamine by affecting critical interactions in the ternary complex. Mutations such as Arg-59 (T. gondii 36) have limited effect in isolation, but in combination with other mutations they enhance the competitive ability of folate by increasing the speed of product turnover. Val-16 (T. gondii 10) confers low level resistance to cycloguanil but hypersensitivity to pyrimethamine. This mutation precludes Asn-108, probably because compression of the folate binding pocket introduced by this combination is incompatible with enzyme function. These studies permit detailed biochemical, kinetic, and structural analysis of drug resistance mutations and reconstruction of the probable phylogeny of antifolate resistance in malaria.

Dihydrofolate reductase and antifolate resistance in malaria

The dihydrofolate reductase (DHFR, EC 1.5.1.3) domain of Plasmodium falciparum bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is an attractive target of two important antifolate antimalarials: pyrimethamine (Pyr) and cycloguanil (Cyc). Over recent years, knowledge of malarial DHFR and mechanism(s) of antifolate resistance have increased substantially. These observations have provided an important framework for better understanding the molecular basis of antifolate resistance in malaria. This article provides a brief review and update on molecular aspects relevant to antifolate resistance in malaria.

Plasmodium falciparum: kinetic interactions of WR99210 with pyrimethamine-sensitive and pyrimethamine-resistant dihydrofolate reductase

With emerging drug resistance in Plasmodium falciparum, novel antifolates effective against pyrimethamine-resistant and cycloguanil-resistant dihydrofolate reductase (DHFR) are in demand. Based on structural similarity to cycloguanil, it has been proposed that WR99210, and its metabolic precursor PS-15, exerts selective antimalarial activity by binding tightly to both drug-sensitive and drug-resistant DHFR. In the present study, Linweaver-Burk plots and Ackermann-Potter plots reveal that both forms of malarial DHFR bind WR99210 at subnanomolar concentrations. It is not necessary to invoke an alternate target for WR99210 in P. falciparum. The present studies confirm that malarial DHFRs offer potential binding interactions in the folate-binding pocket distinct from those exploited by pyrimethamine and cycloguanil. These kinetic studies also provide a useful framework for the design and interpretation of future structural studies on drug-resistant DHFR from P. falciparum.