The Tryptophan oxygenase gene of Anopheles gambiae

Targeting TDO in cancer immunotherapy

Tryptophan-2,3-dioxygenase (TDO) is a homotetrameric heme-containing protein catalyzing the initial step in the kynurenine pathway, which oxidates the 2,3-double bond of the indole ring in L-tryptophan and catalyzes it into kynurenine (KYN). The upregulation of TDO results in a decrease in tryptophan and the accumulation of KYN and its metabolites. These metabolites can affect the proliferation of T cells. Increasing evidence demonstrates that TDO is a promising therapeutic target in the anti-tumor process. Despite its growing popularity, there are only a few reviews focusing on TDO in tumors. Hence, we herein review the biological features and regulatory mechanisms of TDO. Additionally, we focus on the role of TDO in the anti-tumor immune response in different tumors. Finally, we also provide our viewpoint regarding the future developmental directions of TDO in cancer research, especially in relation to the development and application of TDO inhibitors as novel cancer treatments.

TDO as a therapeutic target in brain diseases

Tryptophan-2, 3-dioxygenase (TDO) is a heme-containing protein catalyzing the first reaction in the kynurenine pathway, which incorporates oxygen into the indole moiety of tryptophan and catalyzes it into kynurenine (KYN). The activation of TDO results in the depletion of tryptophan and the accumulation of kynurenine and its metabolites. These metabolites can affect the function of neurons and inhibit the proliferation of T cells. Increasing evidence demonstrates that TDO is a potential therapeutic target in the treatment of brain diseases as well as in the antitumor and transplant fields. Despite its growing popularity, there are few reviews only focusing on TDO. Hence, we herein review TDO by providing a comprehensive overview of TDO, including its biological functions as well as the evolution, structure and catalytic process of TDO. Additionally, this review will focus on the role of TDO in the pathology of three groups of brain diseases: Schizophrenia, Alzheimer's disease (AD) and Glioma. Finally, we will also provide an opinion regarding the future developmental directions of TDO in brain diseases, especially whether TDO has a potential role in other brain diseases as well as the development and applications of TDO inhibitors as treatments.

The ABCs of eye color in Tribolium castaneum: orthologs of the Drosophila white, scarlet, and brown Genes

In Drosophila melanogaster, each of the three paralogous ABC transporters, White, Scarlet and Brown, is required for normal pigmentation of the compound eye. We have cloned the three orthologous genes from the beetle Tribolium castaneum. Conceptual translations of Tribolium white (Tcw), scarlet (Tcst), and brown (Tcbw) are 51, 48, and 32% identical to their respective Drosophila counterparts. We have identified loss-of-eye-pigment strains that bear mutations in Tcw and Tcst: the Tcw gene in the ivory (i) strain carries a single-base transversion, which leads to an E → D amino-acid substitution in the highly conserved Walker B motif, while the Tcst gene in the pearl (p) strain has a deletion resulting in incorporation of a premature stop codon. In light of these findings, the mutant strains i and p are herein renamed white(ivory) (w(i)) and scarlet(pearl) (st(p)), respectively. In addition, RNA inhibition of Tcw and Tcst recapitulates the mutant phenotypes, confirming the roles of these genes in normal eye pigmentation, while RNA interference of Tcbw provides further evidence that it has no role in eye pigmentation in Tribolium. We also consider the evolutionary implications of our findings.

Exploring the origin and degree of genetic isolation of Anopheles gambiae from the islands of São Tomé and Príncipe, potential sites for testing transgenic-based vector control

The evolutionary processes at play between island and mainland populations of the malaria mosquito vector Anopheles gambiae sensu stricto are of great interest as islands may be suitable sites for preliminary application of transgenic-based vector control strategies. São Tomé and Príncipe, located off the West African coast, have received such attention in recent years. This study investigates the degree of isolation of An. gambiae s.s. populations between these islands and the mainland based on mitochondrial and ribosomal DNA molecular data. We identify possible continental localities from which these island populations derived. For these purposes, we used F_ST values, haplotype networks, and nested clade analysis to estimate migration rates and patterns. Haplotypes from both markers are geographically widespread across the African continent. Results indicate that the populations from São Tomé and Príncipe are relatively isolated from continental African populations, suggesting they are promising sites for test releases of transgenic individuals. These island populations are possibly derived from two separate continental migrations. This result is discussed in the context of the history of the African slave trade with respect to São Tomé and Príncipe.

Purification and biochemical characterization of a recombinant Anopheles gambiae tryptophan 2,3-dioxygenase expressed in Escherichia coli

In the malaria vector Anopheles gambiae, tryptophan 2,3-dioxygenase (TDO) is the only enzyme able to initiate l-tryptophan degradation through the kynurenine pathway. TDO converts l-tryptophan to N-formylkynurenine by catalyzing the heme-dependent oxidative opening of the substrate indole ring. Despite the central role exerted by kynurenines in the physiology of living organisms, only a few insect TDOs have been subjected to biochemical characterization in vitro. We performed a RT-PCR-based analysis of the tissue distribution of TDO mRNA in A. gambiae that revealed a ubiquitous expression of the gene, thus further underlining the importance of the enzyme in the mosquito biology. We developed an expression/purification procedure yielding pure and active recombinant A. gambiae TDO. Spectral analyses showed that the enzyme was purified in its heme-ferric form that was subsequently used to determining the Michaelis-Menten constants of the TDO catalyzed reaction in the presence of reducing agents. The screening of a number of compounds as potential TDO modulators showed that several kynurenines and other Tryptophan-derived molecules interfere with the enzyme activity in vitro. Our study could contribute to understanding TDO regulation in vivo and to the identification of inhibitors to be used to alter Tryptophan homeostasis in the malaria vector.

Molecular cloning and analysis of the tryptophan oxygenase gene in the silkworm, Bombyx mori

A Bombyx mori L. (Lepidoptera: Bombycidae) gene encoding tryptophan oxygenase has been molecularly cloned and analyzed. The tryptophan oxygenase cDNA had 1374 nucleotides that encoded a 401 amino acid protein with an estimated molecular mass of 46.47 kDa and a PI of 5.88. RT-PCR analysis showed that the B. mori tryptophan oxygenase gene was transcribed in all examined stages. Tryptophan oxygenase proteins are relatively well conserved among different orders of arthropods.

The tryptophan oxidation pathway in mosquitoes with emphasis on xanthurenic acid biosynthesis

Oxidation of tryptophan to kynurenine and 3-hydroxykynurenine (3-HK) is the major catabolic pathway in mosquitoes. However, 3-HK is oxidized easily under physiological conditions, resulting in the production of reactive radical species. To overcome this problem, mosquitoes have developed an efficient mechanism to prevent 3-HK from accumulating by converting this chemically reactive compound to the chemically stable xanthurenic acid. Interestingly, 3-HK is a precursor for the production of compound eye pigments during the pupal and early adult stages; consequently, mosquitoes need to preserve and transport 3-HK for compound eye pigmentation in pupae and adults. This review summarizes the tryptophan oxidation pathway, compares and contrasts the mosquito tryptophan oxidation pathway with other model species, and discusses possible driving forces leading to the functional adaptation and evolution of enzymes involved in the mosquito tryptophan oxidation pathway.

Do molluscs possess indoleamine 2,3-dioxygenase?

Indoleamine 2,3-dioxygenase (IDO)-like myoglobin (Mb) was discovered in 1989 in the buccal mass of the abalone Sulculus diversicolor, and it has since been isolated from several archaegastropods. The amino acid sequences and genomic structures of IDO-like Mbs show significant homology with those of mammalian IDOs, suggesting that they have evolved from a common ancestral gene. However, details of the evolutionary relationships between them remain unknown. Here, we isolated a novel multicopy gene from Sulculus named molluscan IDO-like protein (MIP). The amino acid sequences of MIPs show the highest homology (about 60% identity) with Sulculus IDO-like Mb, and their exon/intron structures are also highly homologous. However, MIPs are mainly expressed in the gut whereas IDO-like Mb was found only in the buccal mass, suggesting that MIPs are not simply isoforms of IDO-like Mb. A bacterial expression study showed that MIP is a heme-binding protein, and that His335 is the proximal ligand of heme. Although we could not detect IDO activity using a recombinant glutathione S-transferase (GST)-MIP fusion protein in the present study, MIP should have some function other than that of an oxygen carrier like myoglobin, and it might in fact be molluscan IDO.

Construction of a BAC library and generation of BAC end sequence-tagged connectors for genome sequencing of the African malaria mosquito Anopheles gambiae

A Bacterial Artificial Chromosome (BAC) genomic DNA library of Anopheles gambiae, the major human malaria vector in sub-Saharan Africa, was constructed and characterized. This library (ND-TAM) is composed of 30,720 BAC clones in eighty 384-well plates. The estimated average insert size of the library is 133 kb, with an overall genome coverage of approximately 14-fold. The ends of approximately two-thirds of the clones in the library were sequenced, yielding 32,340 pair-mate ends. A statistical analysis (G-test) of the results of PCR screening of the library indicated a random distribution of BACs in the genome, although one gap encompassing the white locus on the X-chromosome was identified. Furthermore, combined with another previously constructed BAC library (ND-1), ~2,000 BACs have been physically mapped by polytene chromosomal in situ hybridization. These BAC end pair mates and physically mapped BACs have been useful for both the assembly of a fully sequenced A. gambiae genome and for linking the assembled sequence to the three polytene chromosomes. This ND-TAM library is now publicly available at both http://www.malaria.mr4.org/mr4pages/index.html/ and http://hbz.tamu.edu/, providing a valuable resource to the mosquito research community.

Structure and evolution of mtanga, a retrotransposon actively expressed on the Y chromosome of the African malaria vector Anopheles gambiae

Here we report the discovery of a novel family of long terminal repeat (LTR)-retrotransposons designated mtanga-Y, specific to the Y chromosome of the African malaria vector, Anopheles gambiae. mtanga-Y elements represent the first Y-linked sequences and the first members of the Ty1-copia superfamily of retrotransposons described from this mosquito. Analysis of a full-length 4,284-bp element revealed the presence of two intact overlapping open reading frames bounded by LTRs of 119 bp. Evidence suggests that the elements are capable of retrotransposition, as transcripts and potential replication intermediates (one-LTR circles) were detected. However, the approximately 12 copies of mtanga-Y appear to be clustered rather than dispersed on the Y chromosome. Absent from the Y chromosome of four sibling species (A. arabiensis, A. quadriannulatus, A. melas, and A. merus), similar, but often defective, mtanga elements are present elsewhere in these genomes, as well as in A. gambiae. These data are consistent with a relatively recent invasion of the A. gambiae Y chromosome by an intact element. The presence of functional mtanga-Y elements suggests that the Y chromosome may be a source, not just a sink, for retrotransposons.

Patterns of DNA sequence variation in chromosomally recognized taxa of Anopheles gambiae: evidence from rDNA and single-copy loci

Patterns of DNA sequence variation in the ribosomal DNA (rDNA) second internal transcribed spacer (ITS2) and five unlinked single-copy nuclear loci were examined for evidence of reproductive isolation among four chromosomally recognized taxa of Anopheles gambiae from West Africa: Savanna, Bamako, Mopti and Forest, as well as sibling species An. arabiensis and An. merus. Included among the single-copy loci were three sequence-tagged random amplified polymorphic DNA (RAPD) loci, two of which (R15 and R37) had been reported as discriminating between Mopti and other chromosomal forms. Each of the five single-copy sequences were highly polymorphic in most samples. However, the R15 and R37 loci had no diagnostic value, and therefore are not recommended as tools in recognition of field-collected An. gambiae chromosomal forms. Although pairwise comparisons between species generally revealed significant levels of differentiation at all five loci, variation was not partitioned by chromosomal form within An. gambiae at any single-copy locus examined. The few exceptions to these trends appear related to a location either inside or nearby chromosomal inversions. At the tryptophan oxygenase locus inside inversion 2Rb, variation was structured only by inversion orientation and not by taxonomic designation even between An. gambiae and An. arabiensis, providing the first molecular evidence that the 2Rb inversion was transferred between species by introgressive hybridization. By contrast, the rDNA showed fixed differences between species and a difference diagnostic for Mopti, consistent with effective, if not complete, reproductive isolation. The apparent disagreement between the data from this locus and multiple single-copy loci within An. gambiae may be explained by the much lower effective population size of rDNA, owing to concerted evolution, which confers increased sensitivity at much shorter divergence times. Taken together with the accompanying reports by della Torre et al. (2001), Favia et al. (2001) and Gentile et al. (2001), our data suggest that neutral molecular markers may not have the sensitivity required to detect isolation between these recently established taxa.

The Anopheles gambiae tryptophan oxygenase gene expressed from a baculovirus promoter complements Drosophila melanogaster vermilion

An Anopheles gambiae cDNA encoding tryptophan oxygenase was placed under the control of the constitutive baculovirus promoter, ie-1. The chimeric construct, expressed transiently in vermilion (tryptophan oxygenase) mutants of Drosophila melanogaster, partially rescued adult eye color. The successful genetic complementation by this construct demonstrated both the proper function of the tryptophan oxygenase product and the effectiveness of the ie-1 promoter in directing expression of foreign genes in live insects. The functionality of An. gambiae tryptophan oxygenase in a higher fly fulfils predictions based on its structural conservation throughout millions of years of independent evolution.