The purification of dihydrofolate reductase from Drosophila melanogaster

Genotoxicity assessment of four novel quinazoline-derived trypanocidal agents in the Drosophila wing somatic mutation and recombination test

Chagas disease, caused by the protozoan Trypanosoma cruzi, has increased in the world due to migration, travelling and climate change; at present, the principal problem is that common trypanocidal agents have resulted in toxic or inconvenient side effects. We tested for genotoxicity in the standard (ST) and high bioactivation (HB) crosses of Drosophila wing somatic mutation and recombination test, four novel trypanocidal agents derived from 2, 4, 6-triaminquinazoline (TAQ): 2,4-diamino-6 nitro-1,3 diazonaftalene (S-1QN2-1), 2,4-diacetamino-6-amino 1,3 diazonaftalene (D-1), N6-(4,methoxybenzyl)quinazoline-2,4,6-triamine (GHPM) and N6-[4-(trifluoromethoxy)benzyl]quinazoline-2,4,6-triamine (GHPMF) at 1.9, 3.9, 7.9 and 15 µM, respectively. Also, high-pressure liquid chromatography (HPLC) analysis was run to determine the remanence of either drug in flare, and Oregon R(R)-flare flies emerged from treated larvae. S-1QN2-1 showed genotoxicity only in the ST cross, increasing the small, large and total spot frequencies at all concentrations and twin spots only at 1.9 µM; D-1 and GHPM showed significant increments of large spots only at 15 µM in the ST cross; GHPMF was not genotoxic at any concentration or either cross. In the mwh clones accumulated distribution frequencies analysis, associated with disrupted cell division, S-1QN2-1 caused alterations in the ST cross at all concentrations but only at 15 µM in the HB cross; D-1 caused alterations at 3.9, 7.9 and 15 µM in the ST cross and at 1.9 and 15 µM in the HB cross; GHPM caused alterations at 7.9 and 15 µM in the ST cross and also at 1.9, 3.9 and 7.9 µM in the HB cross; GHPMF caused those alterations at all concentrations in the ST cross and at 1.9, 3.9 and 7.9 µM in the HB cross. The HPLC results indicated no traces of either agent in the flare and Oregon R(R)-flare flies. We conclude that S-1QN2-1 is clearly genotoxic, D-1 and GHPM have an unclear genotoxicity and GHPMF was not genotoxic; all quinazoline derivatives disrupted cell division. GHPMF is a good candidate to be tested in other genotoxicity and cytotoxic bioassays. The differences in the genotoxic activity of these trypanocidal agents are correlated with differences in their chemical structure.

The effects of folate intake on DNA and single-carbon pathway metabolism in the fruit fly Drosophila melanogaster compared to mammals

Mechanisms of vitamin function in non-mammals are poorly understood, despite being essential for development. Folate and cobalamin are B-vitamin cofactors with overlapping roles in transferring various single-carbon units. In mammals, one or both is needed for nucleotide synthesis, DNA methylation, amino acid conversions and other reactions. However, there has been little investigation of the response to folate or cobalamin in insects. Here, we manipulated folate intake and potentially cobalamin levels in the fruit fly Drosophila melanogaster with chemically-defined diets, an antibiotic to reduce bacterially-derived vitamins, and the folate-interfering pharmaceutical methotrexate, to see if single-carbon metabolites and DNA synthesis rates would be affected. We found that similar to mammals with low folate intake, fruit fly larvae had significantly slower growth and DNA synthesis rates. But changes to single carbon-metabolites did not mirror that of mammals with abnormal folate or given MTX. Five of the nine metabolites measured were not significantly affected (methionine, serine, glycine, methylglycine, and dimethylglycine) and three (cystathionine, methylgycine, and methylmalonic acid) were only decreased in larvae consuming methotrexate. Metabolites expected to be elevated if flies used cobalamin from microbial symbionts were not affected by dietary sulfaquinoxaline. Our data support the role of folate in nucleotide synthesis in D. melanogaster and that microbial symbionts provide functioning folates. We could not confirm how folate intake affects single carbon pathway metabolites, nor whether Drososphila use microbially-derived cobalamin. Further work should explore which cofactors are used in fruit flies in these important and potentially novel pathways.

Molecular cloning and expression of the dihydrofolate reductase (DHFR) gene from adult buffalo fly (Haematobia irritans exigua): effects of antifolates

The folate analogues methotrexate, aminopterin and pyrimethamine were toxic when fed in a blood meal to adult buffalo flies (Haematobia irritans exigua), but aminopterin caused greater mortality than methotrexate, while trimethoprim was not toxic to adult flies. This is the first recorded instance of mortality in adult insects caused by ingestion of folate analogues. In order to investigate the mechanism of this toxicity, the dihydrofolate reductase (DHFR) gene was cloned from adult buffalo fly cDNA using a PCR-based approach. The full-length DHFR coding sequence (BF-DHFR) was 887 bp and contained an open reading frame encoding a protein of 188 amino acids. The deduced protein sequence identities between BF-DHFR and the other known insect DHFR sequences were: Drosophila melanogaster, 75%; Aedes albopictus, 54%; Heliothis virescens, 43%. The BF-DHFR gene has a single 52 bp intron, an organization more similar to Dipteran species (Drosophila and Aedes). The cDNA encoding BF-DHFR was inserted into an Escherichia coli expression vector and the recombinant protein was expressed to levels representing about 25% of total cell protein. The active enzyme was purified by affinity chromatography on methotrexate-agarose and displayed a relatively low affinity (IC50 = 30 nm) for methotrexate.

Tobacco budworm dihydrofolate reductase is a promising target for insecticide discovery

Structural differences in dihydrofolate reductases from different species have been exploited to develop specific inhibitory molecules, such as chemotherapeutic agents, antibiotics or antihelminthics, that show species specificity or selectivity. As dihydrofolate reductase (DHFR) is a crucial enzyme for the synthesis of purines, pyrimidines and some amino acids, and also because developing insects show a remarkably rapid rate of cell division, DHFR is a potentially promising target for the discovery of novel insecticides. We have thus isolated and characterized the enzyme from a serious agricultural pest, Heliothis (Helicoverpa) virescens, the tobacco budworm. Sequencing tryptic peptides of the 35 000-fold purified DHFR allowed the subsequent isolation of a partial cDNA, with the full Dhfr gene sequence obtained from a genomic library. The H. virescens Dhfr spans 4 kb, with three introns, and encodes 185 amino acids. The enzyme shows an overall similarity of approximately 68% with DHFR from other metazoans, which has facilitated the molecular modeling of the protein. DHFRs from insects appear to have strikingly reduced sensitivity to inhibition by methotrexate, compared with the vertebrate enzymes, and this reduction was also reflected in the total binding energy seen after modeling experiments. Four residues that may be characteristic of insect DHFR, as well as a unique cysteine in the H. virescens DHFR active site, offer insight into the nature of inhibitor selectivity and provide suitable target sites for insecticide discovery.

An amplified insect dihydrofolate reductase gene contains a single intron

We have used methotrexate-resistant mosquito (Aedes albopictus) cells as the source of DNA for cloning an 8.5-kb EcoRI fragment containing an amplified dihydrofolate reductase (DHRF) gene. An estimated 1200 copies of the DHFR gene were represented in nuclear DNA from Mtx-5011-256 cells, which were 3000-fold more resistant to methotrexate than wild-type cells. Southern blot analysis indicated that all of the amplified DHFR genes were contained within a 1.8-kb AccI fragment represented in the cloned DNA. In contrast to mammalian DHFR genes which span approximately 30 kb, the complete amino acid coding sequence of the mosquito DHFR gene spanned 614 nucleotides, including a single 56-nucleotide intron that interrupted a conserved Arg codon at amino acid position 27. Additional introns characteristic of mammalian DHFR genes were absent; conservation of the first intron in the mosquito DHFR gene supports a regulatory role for this intron. The mosquito DHFR gene coded for a 186-amino-acid protein with 43-48% similarity to vertebrate DHFR.