Knockout of 'metal-responsive transcription factor' MTF-1 in Drosophila by homologous recombination reveals its central role in heavy metal homeostasis

The parkin mutant phenotype in the fly is largely rescued by metal-responsive transcription factor (MTF-1)

The gene for Parkin, an E3 ubiquitin ligase, is mutated in some familial forms of Parkinson's disease, a severe neurodegenerative disorder. A homozygous mutant of the Drosophila ortholog of human parkin is viable but results in severe motoric impairment including an inability to fly, female and male sterility, and a decreased life span. We show here that a double mutant of the genes for Parkin and the metal-responsive transcription factor 1 (MTF-1) is not viable. MTF-1, which is conserved from insects to mammals, is a key regulator of heavy metal homeostasis and detoxification and plays additional roles in other stress conditions, notably oxidative stress. In contrast to the synthetic lethality of the double mutant, elevated expression of MTF-1 dramatically ameliorates the parkin mutant phenotype, as evidenced by a prolonged life span, motoric improvement including short flight episodes, and female fertility. At the cellular level, muscle and mitochondrial structures are substantially improved. A beneficial effect is also seen with a transgene encoding human MTF-1. We propose that Parkin and MTF-1 provide complementary functions in metal homeostasis, oxidative stress and other cellular stress responses. Our findings also raise the possibility that MTF-1 gene polymorphisms in humans could affect the severity of Parkinson's disease.

Mechanisms underlying the protective effect of zinc and selenium against cadmium-induced oxidative stress in zebrafish Danio rerio

The present study was designed to elucidate the protective effect mechanism of Zinc (Zn) and Selenium (Se) on cadmium (Cd)-induced oxidative stress in zebrafish. For this purpose we investigate the response of oxidative stress markers, metallothionein accumulation and gene expression in liver and ovary of female zebrafish exposed to 0,4 mg/l Cd in water and supplemented with Zn (5 mg kg(-1)) and/or Se (2 mg kg(-1)) for 21 days in their diet. Liver and ovary Cd uptake was evaluated after the exposure period. Cd exposure significantly inhibited the antioxidant enzyme activities termed as catalase (CAT), superoxide dismutase (SOD) and glutathione peroxydase (GPx) and caused a pronounced malondialdehyde (MDA) accumulation in both organs. Co-administration of Zn and Se reversed the Cd-induced toxicity in liver and ovary measured as MDA accumulation. Interestingly, gene expression patterns of Cat, CuZnSod and Gpx were up-regulated when related enzymatic activities were altered. Zebrafish metallothionein transcripts (zMt) significantly decreased in tissues of fish supplemented with Zn and/or Se when compared to Cd-exposed fish. Our data would suggest that Zn and Se protective mechanism against Cd-induced oxidative stress is more depending on the correction of the proteins biological activities rather than on the transcriptional level of related genes.

Syntaxin 5 is required for copper homeostasis in Drosophila and mammals

Copper is essential for aerobic life, but many aspects of its cellular uptake and distribution remain to be fully elucidated. A genome-wide screen for copper homeostasis genes in Drosophila melanogaster identified the SNARE gene Syntaxin 5 (Syx5) as playing an important role in copper regulation; flies heterozygous for a null mutation in Syx5 display increased tolerance to high dietary copper. The phenotype is shown here to be due to a decrease in copper accumulation, a mechanism also observed in both Drosophila and human cell lines. Studies in adult Drosophila tissue suggest that very low levels of Syx5 result in neuronal defects and lethality, and increased levels also generate neuronal defects. In contrast, mild suppression generates a phenotype typical of copper-deficiency in viable, fertile flies and is exacerbated by co-suppression of the copper uptake gene Ctr1A. Reduced copper uptake appears to be due to reduced levels at the plasma membrane of the copper uptake transporter, Ctr1. Thus Syx5 plays an essential role in copper homeostasis and is a candidate gene for copper-related disease in humans.

Induction of FPN1 transcription by MTF-1 reveals a role for ferroportin in transition metal efflux

Ferroportin (Fpn) is the only known iron exporter in vertebrate cells and plays a critical role in iron homeostasis regulating cytosolic iron levels and exporting iron to plasma. Ferroportin1 (FPN1) expression can be transcriptionally regulated by iron as well as other transition metals. Fpn can also be posttranslationally regulated by hepcidin-mediated internalization and degradation. We demonstrate that zinc and cadmium induce FPN1 transcription through the action of Metal Transcription Factor-1 (MTF-1). These transition metals induce MTF-1 translocation into the nucleus. Zinc leads to MTF-1 binding to the FPN1 promoter, while iron does not. Silencing of MTF-1 reduces FPN1 transcription in response to zinc but not in response to iron. The mouse FPN1 promoter contains 2 MTF-1 binding sites and mutation of those sites affects the zinc and cadmium-dependent expression of a FPN1 promoter reporter construct. We demonstrate that Fpn can transport zinc and can protect zinc sensitive cells from high zinc toxicity.

TRPM channels mediate zinc homeostasis and cellular growth during Drosophila larval development

TRPM channels have emerged as key mediators of diverse physiological functions. However, the ionic permeability relevant to physiological function in vivo remains unclear for most members. We report that the single Drosophila TRPM gene (dTRPM) generates a conductance permeable to divalent cations, especially Zn(2+) and in vivo a loss-of-function mutation in dTRPM disrupts intracellular Zn(2+) homeostasis. TRPM deficiency leads to profound reduction in larval growth resulting from a decrease in cell size and associated defects in mitochondrial structure and function. These phenotypes are cell-autonomous and can be recapitulated in wild-type animals by Zn(2+) depletion. Both the cell size and mitochondrial defect can be rescued by extracellular Zn(2+) supplementation. Thus our results implicate TRPM channels in the regulation of cellular Zn(2+) in vivo. We propose that regulation of Zn(2+) homeostasis through dTRPM channels is required to support molecular processes that mediate class I PI3K-regulated cell growth.

Characterization and localization of metal-responsive-element-binding transcription factors from tilapia

Two isoforms of MTF-1, MTF-1L (long form) and MTF-1S (short form), were cloned in tilapia (Ti) and characterized in a tilapia liver cell line, Hepa-T1. The cloned tiMTF-1L has the characteristics of all of the tiMTF-1S identified so far with the zinc finger domain having six fingers, the acidic-rich, proline-rich, and serine/threonine-rich domains; however, the short form encodes for the zinc finger domain with five zinc fingers only and no other domains. The transient transfection of tiMTF-1L into human HepG2 cells showed both constitutive and zinc-induced metal-responsive-element (MRE)-driven reporter gene expression. However, the transfection of tiMTF-1S (which lacks all three transactivation domains) into a human cell line showed reduced transcriptional activities compared with an endogenous control in both basal- and Zn(2+)-induced conditions. The tiMTF-1 isoforms were tagged with GFP and transfected into Hepa-T1 cells (tilapia hepatocytes). The nuclear translocation of tiMTF-1L was observed when the cells were exposed to a sufficient concentration of metals for 6h. However, tiMTF-1S, was localized in the nucleus with or without metal treatment. Electrophoretic mobility shift assay (EMSA) confirmed that both of the isoforms were able to bind to the MRE specifically in vitro. Tissue distribution studies showed that tiMTF-1L was more abundant than tiMTF-1S in all of the tissues tested.

Overexpression of metal-responsive transcription factor (MTF-1) in Drosophila melanogaster ameliorates life-span reductions associated with oxidative stress and metal toxicity

Heavy metals are essential components of many biological processes but are toxic at high concentrations. Our results illustrate that when metal homeostasis is compromised by a mutation in the metal-responsive transcription factor (MTF-1), the life-span is shortened. In contrast, MTF-1 overexpression results in resistant flies with prolonged longevity on iron or cadmium-supplemented media but shortened life-span on zinc-supplemented medium. This effect was mediated by the overexpression of MTF-1 in specific tissues, such as the gut, hemocytes and in particular in neurons, indicating that these tissues are particularly sensitive to the perturbance of metal homeostasis. Further, MTF-1 overexpression in a neuron-specific manner protects flies against hyperoxia and prolongs the life-span of Cu/Zn superoxide dismutase-deficient flies, suggesting the presence of a common mechanism for protection against both oxidative stress and metal toxicity. Finally, normal life-span is extended up to 40% upon MTF-1 overexpression in either the peripheral nervous system or motorneurons. These results document the tissue-specific import of heavy metal toxicity and oxidative damage in aging and life-span determination.

Drosophila RecQ5 is required for efficient SSA repair and suppression of LOH in vivo

RecQ5 in mammalian cells has been suggested to suppress inappropriate homologous recombination. However, the specific pathway(s) in which it is involved and the underlining mechanism(s) remain poorly understood. We took advantage of genetic tools in Drosophila to investigate how Drosophila RecQ5 (dRecQ5) functions in vivo in homologous recombination-mediated double strand break (DSB) repair. We generated null alleles of dRecQ5 using the targeted recombination technique. The mutant animals are homozygous viable, but with growth retardation during development. The mutants are sensitive to both exogenous DSB-inducing treatment, such as gamma-irradiation, and endogenously induced double strand breaks (DSBs) by I-Sce I endonuclease. In the absence of dRecQ5, single strand annealing (SSA)-mediated DSB repair is compromised with compensatory increases in either inter-homologous gene conversion, or non-homologous end joining (NHEJ) when inter-chromosomal homologous sequence is unavailable. Loss of function of dRecQ5 also leads to genome instability in loss of heterozygosity (LOH) assays. Together, our data demonstrate that dRecQ5 functions in SSA-mediated DSB repair to achieve its full efficiency and in suppression of LOH in Drosophila.

Zinc accumulation in heterozygous mutants of fumble, the pantothenate kinase homologue of Drosophila

Coenzyme A (CoA) functions in the intracellular trafficking of acetyl groups. In humans, mutations in the pantothenate kinase-2 gene, which encodes a key enzyme in CoA biosynthesis, are associated with neurodegeneration and premature death. Diagnosis is based on iron accumulation in the globus pallidus observed by magnetic resonance imaging. We investigated the elemental composition of the fumble mutant, a model of the human disease. Surprisingly, flies carrying a fumble loss-of-function allele had a three-fold increase in total zinc levels per dry weight when compared to control strains, but no change in total iron, copper or manganese levels. Accordingly, zinc supplementation had an adverse impact on the development of fumble mutant larvae, but zinc chelation failed to protect.

Zinc supplement greatly improves the condition of parkin mutant Drosophila

Parkinson's disease (PD) is a progressive neurodegenerative disorder in which oxidative stress is implicated as a major causative factor. Mutations in the gene encoding Parkin, a ubiquitin ligase, are responsible for a familial form of PD. In a Drosophila disease model lacking Parkin (park(25) null mutant), we tested the effect of zinc supplementation. Zinc is an essential trace metal and a component of many enzymes and transcriptional regulators. Unlike copper and iron, zinc is not redox-active and under most conditions serves as an antioxidant. We find that the condition of parkin mutants raised on zinc-supplemented food is greatly improved. At zinc concentrations where controls begin to show adverse effects as a result of the metal supplement, parkin mutants perform best, as manifested in a higher frequency of reaching adulthood, extended lifespan and improved motoric abilities.

The Drosophila copper transporter Ctr1C functions in male fertility

Living organisms have evolved intricate systems to harvest trace elements from the environment, to control their intracellular levels, and to ensure adequate delivery to the various organs and cellular compartments. Copper is one of these trace elements. It is at the same time essential for life but also highly toxic, not least because it facilitates the generation of reactive oxygen species. In mammals, copper uptake in the intestine and copper delivery into other organs are mediated by the copper importer Ctr1. Drosophila has three Ctr1 homologs: Ctr1A, Ctr1B, and Ctr1C. Earlier work has shown that Ctr1A is an essential gene that is ubiquitously expressed throughout development, whereas Ctr1B is responsible for efficient copper uptake in the intestine. Here, we characterize the function of Ctr1C and show that it functions as a copper importer in the male germline, specifically in maturing spermatocytes and mature sperm. We further demonstrate that loss of Ctr1C in a Ctr1B mutant background results in progressive loss of male fertility that can be rescued by copper supplementation to the food. These findings hint at a link between copper and male fertility, which might also explain the high Ctr1 expression in mature mammalian spermatozoa. In both mammals and Drosophila, the X chromosome is known to be inactivated in the male germline. In accordance with such a scenario, we provide evidence that in Drosophila, the autosomal Ctr1C gene originated as a retrogene copy of the X-linked Ctr1A, thus maintaining copper delivery during male spermatogenesis.

Orthologs of the class A4 heat shock transcription factor HsfA4a confer cadmium tolerance in wheat and rice

Cadmium (Cd) is a widespread soil pollutant; thus, the underlying molecular controls of plant Cd tolerance are of substantial interest. A screen for wheat (Triticum aestivum) genes that confer Cd tolerance to a Cd hypersensitive yeast strain identified Heat shock transcription factor A4a (HsfA4a). Ta HsfA4a is most similar to the class A4 Hsfs from monocots. The most closely related rice (Oryza sativa) homolog, Os HsfA4a, conferred Cd tolerance in yeast, as did Ta HsfA4a, but the second most closely related rice homolog, Os HsfA4d, did not. Cd tolerance was enhanced in rice plants expressing Ta HsfA4a and decreased in rice plants with knocked-down expression of Os HsfA4a. An analysis of the functional domain using chimeric proteins constructed from Ta HsfA4a and Os HsfA4d revealed that the DNA binding domain (DBD) of HsfA4a is critical for Cd tolerance, and within the DBD, Ala-31 and Leu-42 are important for Cd tolerance. Moreover, Ta HsfA4a-mediated Cd resistance in yeast requires metallothionein (MT). In the roots of wheat and rice, Cd stress caused increases in HsfA4a expression, together the MT genes. Our findings thus suggest that HsfA4a of wheat and rice confers Cd tolerance by upregulating MT gene expression in planta.

dRecQ4 is required for DNA synthesis and essential for cell proliferation in Drosophila

BACKGROUND

The family of RecQ DNA helicases plays an important role in the maintenance of genomic integrity. Mutations in three of the five known RecQ family members in humans, BLM, WRN and RecQ4, lead to disorders that are characterized by predisposition to cancer and premature aging.

METHODOLOGY/PRINCIPAL FINDINGS

To address the in vivo functions of Drosophila RecQ4 (dRecQ4), we generated mutant alleles of dRecQ4 using the targeted gene knock-out technique. Our data show that dRecQ4 mutants are homozygous lethal with defects in DNA replication, cell cycle progression and cell proliferation. Two sets of experiments suggest that dRecQ4 also plays a role in DNA double strand break repair. First, mutant animals exhibit sensitivity to gamma irradiation. Second, the efficiency of DsRed reconstitution via single strand annealing repair is significantly reduced in the dRecQ4 mutant animals. Rescue experiments further show that both the N-terminal domain and the helicase domain are essential to dRecQ4 function in vivo. The N-terminal domain is sufficient for the DNA repair function of dRecQ4.

CONCLUSIONS/SIGNIFICANCE

Together, our results show that dRecQ4 is an essential gene that plays an important role in not only DNA replication but also DNA repair and cell cycle progression in vivo.

Dietary zinc absorption is mediated by ZnT1 in Drosophila melanogaster

Zinc is an essential nutritional factor involved in many key biological processes. However, the physiological function of zinc transporters at the organismal level is not well characterized. Early embryonic lethality of Znt1 knockout mice precludes functional analysis of the role of ZnT1 in dietary zinc absorption. Here, we report the identification and characterization of the Drosophila ZnT1 orthologue, dZnT1, for its role in Drosophila dietary zinc absorption. In cell culture, dZnT1 promoted zinc transport to reduce cytoplasmic zinc levels. Ubiquitous RNA interference of dZnT1 in Drosophila resulted in developmental arrest under restriction of dietary zinc, while dZnT1-overexpressing flies exhibited hypersensitivity to zinc. dZnT1 was prominently expressed in restricted regions of the midgut and exhibited a distribution on the basolateral membrane of the enterocytes. Gut-specific silencing of dZnT1 was sufficient to evoke lethality under zinc scarcity. Human ZnT1, but not ZnT7 or ZnT4, could rescue the zinc-acquiring defects caused by dZnT1 silencing. Taken together, our results proved that dZnT1 is a key zinc transporter in dietary zinc absorption, functioning by pumping zinc out of the enterocytes across the basolateral membrane. This study will be helpful in understanding the fundamental process of acquiring dietary zinc in higher eukaryotes.

Mercury and cadmium trigger expression of the copper importer Ctr1B, which enables Drosophila to thrive on heavy metal-loaded food

Organisms from insects to mammals respond to heavy metal load (copper, zinc, cadmium, and mercury) by activating the metal-responsive transcription factor 1 (MTF-1). MTF-1 binds to short DNA sequence motifs, termed metal response elements, and boosts transcription of a number of genes, notably those for metallothioneins. In Drosophila, MTF-1 somewhat counter-intuitively also activates transcription of a copper importer gene (Ctr1B) in response to copper starvation. Here, we report that mutant flies lacking Ctr1B are extremely sensitive to cadmium and mercury treatment, but can be rescued by excess copper in the food. We thus propose that copper, by competing for binding sites on cellular proteins, alleviates the toxic effects of mercury and cadmium. Such a scenario also explains a seemingly fortuitous metal response, namely, that cadmium and mercury strongly induce the expression of a Ctr1B reporter gene. Thus, the transcription enhancer/promoter region of the Ctr1B copper importer gene is subject to three modes of regulation. All of them depend on MTF-1 and all make biological sense, namely, (i) induction by copper starvation, (ii) repression by copper abundance, and (iii), as shown here, induction by cadmium or mercury at normal copper supply.

Metallothioneins in Diptera

Dipterean insecta owe their relevance to the well-known model organism D. melanogaster, and extensively to the Drosophila genus. In the frame of the study of metallothioneins (MTs), they constitute a bizarre exception, since Drosophila is the only metazoan organism synthesizing only MTs similar to the yeast (S. cerevisiae) Cup1 paradigmatic copper-thionein. D. melanogaster MTs are optimized for copper binding and metabolism, and no MT analogous to the mammalian MTs has been identified, unlike in the cases of other Arthropoda and invertebrates analyzed so far (Chapters 7 and 8 of this book). The D. melanogaster genome contains four MT genes (MtnA, MtnB, MtnC, and MtnD), which are clustered in the same genomic region, probably arising from amplifying duplications. The main isoforms are MtnA and MtnB, encoding peptides 40- and 43-amino acids long, comprising 10 and 12 cysteines, respectively. MtnC and MtnD probably arise from recent MtnB duplications and play a minor role in flies. Expression of the four genes is dependent on dMTF-1, a transcriptional activator homologous to the mammalian MTF-1. Major MT synthesis in larvae and adults is observed in the digestive tract, mainly in the midgut, although with a different distribution between MtnA and MtnB. Other tissues of Mtn gene expression are salivary glands, ventricula, Malpighian tubules, and hemocytes. Metal induction increases the rate of MT synthesis in these tissues, but not normally in MT gene-silent organs, and invariably, the best response is obtained for copper, followed by cadmium, and very poorly for zinc. In conclusion, all the available data suggests a definitive role of D. melanogaster MT in copper metabolism (from ingestion to distribution, storage, delivery, and detoxification in the organism) as well as in cadmium tolerance (by digestive assimilation blockage). This is readily confirmed by studies of population genetics that show that both spontaneous MtnA genomic duplications, independently isolated in worldwide locations, and fly strains containing supernumerary Mtn genes constructed in the laboratory are able to tolerate increased copper and cadmium concentrations; while Mtn and dMTF-1 knock-outs are extremely sensitive to these conditions.

Regulation of Metallothionein Gene Expression

Organisms from bacteria to humans use elaborate systems to regulate levels of bioavailable zinc, copper, and other essential metals. An excess of them, or even traces of non-essential metals such as cadmium and mercury, can be highly toxic. Metallothioneins (MTs), short, cysteine-rich proteins, play pivotal roles in metal homeostasis and detoxification. With their sulfhydryl groups they avidly bind toxic metals and also play a role in cellular redox balance and radical scavenging. The intracellular concentration of MTs is adjusted to cellular demand primarily via regulated transcription. Especially upon heavy metal load, metallothionein gene transcription is strongly induced. From insects to mammals, the major regulator of MT transcription is MTF-1 (metal-responsive transcription factor 1), a zinc finger protein that binds to specific DNA sequence motifs (MREs) in the promoters of MT genes and other metal-regulated genes. This chapter provides an overview of our current knowledge on the expression and regulation of MT genes in higher eukaryotes, with some reference also to fungi which apparently have independently evolved their own regulatory systems.

Differential proteome expression associated with urokinase plasminogen activator receptor (uPAR) suppression in malignant epithelial cancer

Dysregulation of the plasminogen activation cascade is a prototypic feature in many malignant epithelial cancers. Principally, this is thought to occur through activation of overexpressed urokinase plasminogen activator (uPA) concomitant with binding to its high specificity cell surface receptor urokinase plasminogen activator receptor (uPAR). Up-regulation of uPA and uPAR in cancer appears to potentiate the malignant phenotype, either (i) directly by triggering plasmin-mediated degradation or activation of uPA's or plasmin's proteolytic targets (e.g., extracellular matrix zymogen proteases or nascent growth factors) or indirectly by simultaneously altering a range of downstream functions including signal transduction pathways ( Romer, J. ; Nielsen, B. S. ; Ploug, M. The urokinase receptor as a potential target in cancer therapy Curr. Pharm. Des. 2004, 10 ( 19), 235976 ). Because many malignant epithelial cancers express high levels of uPAR, uPA or other components of the plasminogen activation cascade and because these are often associated with poor prognosis, characterizing how uPAR changes the downstream cellular "proteome" is fundamental to understanding any role in cancer. This study describes a carefully designed proteomic study of the effects of antisense uPAR suppression in a previously studied colon cancer cell line (HCT116). The study utilized replicate 2DE gels and two independent gel image analysis software packages to confidently identify 64 proteins whose expression levels changed (by > or =2 fold) coincident with a moderate ( approximately 40%) suppression of cell-surface uPAR. Not surprisingly, many of the altered proteins have previously been implicated in the regulation of tumor progression (e.g., p53 tumor suppressor protein and c-myc oncogene protein among many others). In addition, through a combination of proteomics and immunological methods, this study demonstrates that stathmin 1alpha, a cytoskeletal protein implicated in tumor progression, undergoes a basic isoelectric point shift (p I) following uPAR suppression, suggesting that post-translational modification of stathmin occur secondary to uPAR suppression. Overall, these results shed new light on the molecular mechanisms involved in uPAR signaling and how it may promulgate the malignant phenotype.

Expression of copper-related genes in response to copper load

Copper is an essential micronutrient for all biological systems. Multiple proteins require one or more atoms of copper for proper structure and function, but excess of copper is toxic. To prevent the consequences of copper deficiency and overload, living organisms have evolved molecular mechanisms that regulate its uptake, intracellular traffic, storage, and efflux. Underlying some of the cellular responses to variations in copper levels are changes in the expression of genes encoding molecular components of copper metabolism. In recent years, genome-scale expression analysis in several eukaryotic models has allowed the identification of copper-responsive genes involved in copper homeostasis. Characterization of the transcriptional changes in response to varying copper levels include both genes directly involved in copper homeostasis and genes involved in different cellular process that, even though they are not directly connected to copper metabolism, change their expression during the cellular adaptation to copper availability. Evaluation of these gene expression patterns could aid in the identification of biologically relevant markers to monitor copper status in humans.

Xenobiotic-activated receptors: from transcription to drug metabolism to disease

Xenobiotic-activated receptors (XARs) are a group of ligand-activated transcription factors that are evolutionally specialized to regulate genomic programs to protect the body against innumerable chemicals from the environment. XARs share unique properties, such as promiscuous ligand binding, conserved structural motifs, common protein partners, and overlapping target genes. These unique features of XARs clearly distinguish them from receptors that are activated by endogenous chemicals to regulate energy metabolism, reproduction, and growth and differentiation. XARs regulate xenobiotic metabolism and disposition by controlling the expression and induction of drug-metabolizing enzymes and transporters. Furthermore, XARs integrate a broad range of protective mechanisms, such as antioxidative response and immune/inflammatory functions, to antagonize foreign chemicals. As the primary means of xenobiotic sensing and defense, XARs are intimately involved in drug disposition, polymorphic drug clearance, drug-drug interaction, and pathogenesis of some chemically induced cancers and chronic diseases. As a consequence, some XAR characteristics have been exploited in drug development and safety evaluation of drugs and environmental carcinogens and toxicants. In this perspective, common features and recent advances in the structures, modes of action, and implications in disease and drug development of XARs are discussed.

Dumpy-30 family members as determinants of male fertility and interaction partners of metal-responsive transcription factor 1 (MTF-1) in Drosophila

BACKGROUND

Metal-responsive transcription factor 1 (MTF-1), which binds to metal response elements (MREs), plays a central role in transition metal detoxification and homeostasis. A Drosophila interactome analysis revealed two candidate dMTF-1 interactors, both of which are related to the small regulatory protein Dumpy-30 (Dpy-30) of the worm C. elegans. Dpy-30 is the founding member of a protein family involved in chromatin modifications, notably histone methylation. Mutants affect mating type in yeast and male mating in C. elegans.

RESULTS

Constitutive expression of the stronger interactor, Dpy-30L1 (CG6444), in transgenic flies inhibits MTF-1 activity and results in elevated sensitivity to Cd(II) and Zn(II), an effect that could be rescued by co-overexpression of dMTF-1. Electrophoretic mobility shift assays (EMSA) suggest that Dpy-30L1 interferes with the binding of MTF-1 to its cognate MRE binding site. Dpy-30L1 is expressed in the larval brain, gonads, imaginal discs, salivary glands and in the brain, testes, ovaries and salivary glands of adult flies. Expression of the second interactor, Dpy-30L2 (CG11591), is restricted to larval male gonads, and to the testes of adult males. Consistent with these findings, dpy-30-like transcripts are also prominently expressed in mouse testes. Targeted gene disruption by homologous recombination revealed that dpy-30L1 knockout flies are viable and show no overt disruption of metal homeostasis. In contrast, the knockout of the male-specific dpy-30L2 gene results in male sterility, as does the double knockout of dpy-30L1 and dpy-30L2. A closer inspection showed that Dpy-30L2 is expressed in elongated spermatids but not in early or mature sperm. Mutant sperm had impaired motility and failed to accumulate in sperm storage organs of females.

CONCLUSION

Our studies help to elucidate the physiological roles of the Dumpy-30 proteins, which are conserved from yeast to humans and typically act in concert with other nuclear proteins to modify chromatin structure and gene expression. The results from these studies reveal an inhibitory effect of Dpy-30L1 on MTF-1 and an essential role for Dpy-30L2 in male fertility.

Malvolio is a copper transporter in Drosophila melanogaster

Divalent metal ion transporter 1 (DMT1; also known as SLC11A2) can transport several metals including Fe and Cu in mammalian systems. We set out to determine whether Malvolio (Mvl), the Drosophila melanogaster orthologue of DMT1, can also transport Cu. Overexpression of Mvl caused Cu accumulation in Drosophila S2 cultured cells and conversely dsRNAi knockdown of endogenous Mvl reduced cellular Cu levels. Cell viability under Cu limiting conditions was reduced following dsRNAi knockdown. A homozygous viable Mvl loss-of-function mutant (Mvl(97f)) was sensitive to excess Cu and female Mvl(97f) flies were also sensitive to Cu limitation. An MtnA-EYFP reporter was used as a proxy measure of Cu distribution within Mvl(97f/+) larvae. Under basal conditions Cu levels were reduced in the anterior midgut and proventriculus relative to control larvae. These results demonstrate Mvl is a functional Cu transporter and that despite partial functional redundancy with the Ctr1 proteins, Cu uptake through this pathway is necessary for optimal viability at the cellular and organismal levels.

Copper sensing function of Drosophila metal-responsive transcription factor-1 is mediated by a tetranuclear Cu(I) cluster

Drosophila melanogaster MTF-1 (dMTF-1) is a copper-responsive transcriptional activator that mediates resistance to Cu, as well as Zn and Cd. Here, we characterize a novel cysteine-rich domain which is crucial for sensing excess intracellular copper by dMTF-1. Transgenic flies expressing mutant dMTF-1 containing alanine substitutions of two, four or six cysteine residues within the sequence (547)CNCTNCKCDQTKSCHGGDC(565) are significantly or completely impaired in their ability to protect flies from copper toxicity and fail to up-regulate MtnA (metallothionein) expression in response to excess Cu. In contrast, these flies exhibit wild-type survival in response to copper deprivation thus revealing that the cysteine cluster domain is required only for sensing Cu load by dMTF-1. Parallel studies show that the isolated cysteine cluster domain is required to protect a copper-sensitive S. cerevisiae ace1Delta strain from copper toxicity. Cu(I) ligation by a Cys-rich domain peptide fragment drives the cooperative assembly of a polydentate [Cu(4)-S(6)] cage structure, characterized by a core of trigonally S(3) coordinated Cu(I) ions bound by bridging thiolate ligands. While reminiscent of Cu(4)-L(6) (L = ligand) tetranuclear clusters in copper regulatory transcription factors of yeast, the absence of significant sequence homology is consistent with convergent evolution of a sensing strategy particularly well suited for Cu(I).

Expression and localisation of the essential copper transporter DmATP7 in Drosophila neuronal and intestinal tissues

Copper homeostasis is achieved by a combination of regulated uptake, efflux and sequestration and is essential for animal health and viability. Transmembrane copper transport proteins of the P-type ATPase family play key roles in cellular copper efflux. Here, the transcriptional and post-translational regulation of DmATP7, the sole Drosophila melanogaster ortholog of the human MNK and WND copper transport genes, is examined. An enhancer element with sufficient regulatory information to rescue DmATP7 mutant flies to adulthood is identified. This regulatory element drives expression in all neuronal tissues examined and demonstrates copper-inducible, Mtf-1 dependent expression in the larval midgut. These results support an important functional role for copper transport in neuronal tissues and indicate that regulation of DmATP7 expression is not used to limit copper absorption in toxic copper conditions. Localisation of a functional EYFP-DmATP7 fusion protein is also examined. This fusion protein localises at or proximal to the basolateral membrane of DmATP7 expressing midgut cells supporting a role for DmATP7 in export of copper from midgut cells.

Methods for homologous recombination in Drosophila

We present detailed protocols for two methods of gene targeting in Drosophila. The first, ends-out targeting, is identical in concept to gene replacement techniques used routinely in mammalian and yeast cells. In Drosophila, the targeted gene is replaced by the marker gene white + (although options exist to generate unmarked targeted alleles). This approach is simple in both the molecular cloning and the genetic manipulations. Ends-out will likely serve most investigators' purposes to generate simple gene deletions or reporter gene "knock-ins." The second method, ends-in targeting, targets a wild-type gene with an engineered mutated copy and generates a duplication structure at the target locus. This duplication can subsequently be reduced to one copy, removing the wild-type gene and leaving only the introduced mutation. Although more complicated in the cloning and genetic manipulations (see Note 1), this approach has the benefit that the mutations may be introduced with no other remnant of the targeting procedure. This "surgical" approach will appeal to investigators who desire minimal perturbation to the genome, such as single nucleotide mutation. Although both approaches appear to be approximately equally efficient (see Note 2), each method has separate strengths and drawbacks. The choice of which approach is best depends on the researcher's goal.

Understanding the mechanisms of zinc-sensing by metal-response element binding transcription factor-1 (MTF-1)

The regulation of divalent zinc has been observed in a wide range of organisms. Since this metal is an essential nutrient, but also toxic in excess, zinc homeostasis is crucial for normal cellular functioning. The metal-responsive-element-binding transcription factor-1 (MTF-1) is a key regulator of zinc in higher eukaryotes ranging from insects to mammals. MTF-1 controls the expression of metallothioneins (MTs) and a number of other genes directly involved in the intracellular sequestration and transport of zinc. Although the diverse functions of MTF-1 extend well beyond zinc homeostasis to include stress-responses to heavy metal toxicity, oxidative stress, and selected chemical agents, in this review we focus on the recent advances in understanding the mechanisms whereby MTF-1 regulates MT gene expression to protect the cell from fluctuations in environmental zinc. Particular emphasis is devoted to recent studies involving the Cys2His2 zinc finger DNA-binding domain of MTF-1, which is an important contributor to the zinc-sensing and metal-dependent transcriptional activation functions of this protein.

Know thy fly

The generation and analysis of mutants is central to studies of gene function in model organisms. Methods for random mutagenesis in Drosophila melanogaster have been available for many years, but an alternative approach--targeted mutagenesis using homologous recombination--has only recently been developed. This approach has the advantage of specificity, because genes of interest can be altered. One might expect with a gene-targeting approach that the frequency of background mutations would be minimal. Unfortunately, we have found that this is not the case. Although the possibility of background mutations arising during homologous-recombination-based gene targeting has been raised in the literature, it is not routinely taken into account when using this technique. Our experience suggests that it can be a considerable problem but that it has a relatively simple solution.

Copper homeostasis in Drosophila by complex interplay of import, storage and behavioral avoidance

Copper is an essential but potentially toxic trace element. In Drosophila, the metal-responsive transcription factor (MTF-1) plays a dual role in copper homeostasis: at limiting copper concentrations, it induces the Ctr1B copper importer gene, whereas at high copper concentrations, it mainly induces the metallothionein genes. Here we find that, despite the downregulation of the Ctr1B gene at high copper concentrations, the protein persists on the plasma membrane of intestinal cells for many hours and thereby fills the intracellular copper stores. Drosophila may risk excessive copper accumulation for the potential benefit of overcoming a period of copper scarcity. Indeed, we find that copper-enriched flies donate a vital supply to their offspring, allowing the following generation to thrive on low-copper food. We also describe two additional modes of copper handling: behavioral avoidance of food containing high (>or=0.5 mM) copper levels, as well as the ability of DmATP7, the Drosophila homolog of Wilson/Menkes disease copper exporters, to counteract copper toxicity. Regulated import, storage, export, and avoidance of high-copper food establish an adequate copper homeostasis under variable environmental conditions.

Copper homeostasis gene discovery in Drosophila melanogaster

Recent studies have shown a high level of conservation between Drosophila melanogaster and mammalian copper homeostasis mechanisms. These studies have also demonstrated the efficiency with which this species can be used to characterize novel genes, at both the cellular and whole organism level. As a versatile and inexpensive model organism, Drosophila is also particularly useful for gene discovery applications and thus has the potential to be extremely useful in identifying novel copper homeostasis genes and putative disease genes. In order to assess the suitability of Drosophila for this purpose, three screening approaches have been investigated. These include an analysis of the global transcriptional response to copper in both adult flies and an embryonic cell line using DNA microarray analysis. Two mutagenesis-based screens were also utilized. Several candidate copper homeostasis genes have been identified through this work. In addition, the results of each screen were carefully analyzed to identify any factors influencing efficiency and sensitivity. These are discussed here with the aim of maximizing the efficiency of future screens and the most suitable approaches are outlined. Building on this information, there is great potential for the further use of Drosophila for copper homeostasis gene discovery.

CsoR is a novel Mycobacterium tuberculosis copper-sensing transcriptional regulator

Copper is an essential element that becomes highly cytotoxic when concentrations exceed the capacity of cells to sequester the ion. Here, we identify a new copper-specific repressor (CsoR) of a copper-sensitive operon (cso) in Mycobacterium tuberculosis (Mtb) that is representative of a large, previously uncharacterized family of proteins (DUF156). Electronic and X-ray absorption spectroscopies reveal that CsoR binds a single-monomer mole equivalent of Cu(I) to form a trigonally coordinated (S(2)N) Cu(I) complex. The 2.6-A crystal structure of copper-loaded CsoR shows a homodimeric antiparallel four-helix bundle architecture that represents a novel DNA-binding fold. The Cu(I) is coordinated by Cys36, Cys65' and His61' in a subunit bridging site. Cu(I) binding negatively regulates the binding of CsoR to a DNA fragment encompassing the operator-promoter region of the Mtb cso operon; this results in derepression of the operon in Mtb and the heterologous host Mycobacterium smegmatis. Substitution of Cys36 or His61 with alanine abolishes Cu(I)- and CsoR-dependent regulation in vivo and in vitro. Potential roles of CsoR in Mtb pathogenesis are discussed.

Transcriptome response to heavy metal stress in Drosophila reveals a new zinc transporter that confers resistance to zinc

All organisms are confronted with external variations in trace element abundance. To elucidate the mechanisms that maintain metal homeostasis and protect against heavy metal stress, we have determined the transcriptome responses in Drosophila to sublethal doses of cadmium, zinc, copper, as well as to copper depletion. Furthermore, we analyzed the transcriptome of a metal-responsive transcription factor (MTF-1) null mutant. The gene family encoding metallothioneins, and the ABC transporter CG10505 that encodes a homolog of 'yeast cadmium factor' were induced by all three metals. Zinc and cadmium responses have similar features: genes upregulated by both metals include those for glutathione S-transferases GstD2 and GstD5, and for zinc transporter-like proteins designated ZnT35C and ZnT63C. Several of the metal-induced genes that emerged in our study are regulated by the transcription factor MTF-1. mRNA studies in MTF-1 overexpressing or null mutant flies and in silico search for metal response elements (binding sites for MTF-1) confirmed novel MTF-1 regulated genes such as ferritins, the ABC transporter CG10505 and the zinc transporter ZnT35C. The latter was analyzed in most detail; biochemical and genetic approaches, including targeted mutation, indicate that ZnT35C is involved in cellular and organismal zinc efflux and plays a major role in zinc detoxification.

The four members of the Drosophila metallothionein family exhibit distinct yet overlapping roles in heavy metal homeostasis and detoxification

Four metallothionein genes are present in the Drosophila melanogaster genome, designated MtnA, MtnB, MtnC, MtnD, all of which are transcriptionally induced by heavy metals through the same metal-responsive transcription factor, MTF-1. Here we show, by targeted mutagenesis, that the four metallothionein genes exhibit distinct, yet overlapping, roles in heavy metal homeostasis and toxicity prevention. Among the individual metallothionein mutants, the most prominent distinction between them was that MtnA-defective flies were the most sensitive to copper load, while MtnB-defective flies were the most sensitive to cadmium. Using various reporter gene constructs and mRNA quantification, we show that the MtnA promoter is preferentially induced by copper, while the MtnB promoter is preferentially induced by cadmium. Such a metal preference is also observed at the protein level as the stoichiometric, spectrometric and spectroscopic features of the copper and cadmium complexes with MtnA and MtnB correlate well with a greater stability of copper-MtnA and cadmium-MtnB. Finally, MtnC and MtnD, both of which are very similar to MtnB, display lower copper and cadmium binding capabilities compared to either MtnA or MtnB. In accordance with these binding studies, Drosophila mutants of MtnC or MtnD have a near wild type level of resistance against copper or cadmium load. Furthermore, eye-specific over-expression of MtnA and MtnB, but not of MtnC or MtnD, can rescue a "rough eye" phenotype caused by copper load in the eye. Taken together, while the exact roles of MtnC and MtnD remain to be determined, the preferential protection against copper and cadmium toxicity by MtnA and MtnB, respectively, are the result of a combination of promoter preference and metal binding.

Metallothionein response to cadmium and zinc exposures compared in two freshwater bivalves, Dreissena polymorpha and Corbicula fluminea

Metallothionein (MT) response to cadmium (Cd) and zinc (Zn) bioaccumulation after single or combined direct exposure was compared in two freshwater bivalves, Dreissena polymorpha (zebra mussel) and Corbicula fluminea (Asiatic clam). Bivalves were exposed to 0.133 microM Cd and/or 15.3 microM Zn, with metal and MT concentrations analysed in the whole soft body after 1, 3, 10 and 24 days of exposure and compared with controls. Results showed significant increase in MT concentrations in both species exposed to Cd and Cd+Zn with a higher accumulation of the protein compared to the control in D. polymorpha for nevertheless similar Cd levels accumulated with time. Exposure to Zn alone led to a significant increase in MT concentrations only in C. fluminea, whereas there was a lack of MT gene induction in the zebra mussels which was confirmed by MT mRNA quantification in gills (RT-PCR). Mussel mortality after 10 days of exposure to Zn and Cd + Zn is discussed with regard to detoxification mechanisms, which include metallothioneins.

Copper homeostasis in eukaryotes: Teetering on a tightrope

The transition metal copper is an essential trace element for both prokaryotes and eukaryotes. However, intracellular free copper has to be strictly limited due to its toxic side effects, not least the generation of reactive oxygen species (ROS) via redox cycling. Thus, all organisms have sophisticated copper homeostasis mechanisms that regulate uptake, distribution, sequestration and export of copper. From insects to mammals, metal-responsive transcription factor (MTF-1), a zinc finger transcription factor, controls expression of metallothioneins and other components involved in heavy metal homeostasis. In the fruit fly Drosophila, MTF-1 paradoxically acts as an activator under both high and low copper concentrations. Namely, under high copper conditions, MTF-1 activates metallothioneins in order to protect the cell, while under low copper conditions MTF-1 activates the copper importer Ctr1B in order to acquire scarce copper from the surroundings. This review highlights the current knowledge of copper homeostasis in eukaryotes with a focus on Drosophila and the role of MTF-1.

Coactivator cross-talk specifies transcriptional output

Cells often fine-tune gene expression at the level of transcription to generate the appropriate response to a given environmental or developmental stimulus. Both positive and negative influences on gene expression must be balanced to produce the correct level of mRNA synthesis. To this end, the cell uses several classes of regulatory coactivator complexes including two central players, TFIID and Mediator (MED), in potentiating activated transcription. Both of these complexes integrate activator signals and convey them to the basal apparatus. Interestingly, many promoters require both regulatory complexes, although at first glance they may seem to be redundant. Here we have used RNA interference (RNAi) in Drosophila cells to selectively deplete subunits of the MED and TFIID complexes to dissect the contribution of each of these complexes in modulating activated transcription. We exploited the robust response of the metallothionein genes to heavy metal as a model for transcriptional activation by analyzing direct factor recruitment in both heterogeneous cell populations and at the single-cell level. Intriguingly, we find that MED and TFIID interact functionally to modulate transcriptional response to metal. The metal response element-binding transcription factor-1 (MTF-1) recruits TFIID, which then binds promoter DNA, setting up a "checkpoint complex" for the initiation of transcription that is subsequently activated upon recruitment of the MED complex. The appropriate expression level of the endogenous metallothionein genes is achieved only when the activities of these two coactivators are balanced. Surprisingly, we find that the same activator (MTF-1) requires different coactivator subunits depending on the context of the core promoter. Finally, we find that the stability of multi-subunit coactivator complexes can be compromised by loss of a single subunit, underscoring the potential for combinatorial control of transcription activation.

A family knockout of all four Drosophila metallothioneins reveals a central role in copper homeostasis and detoxification

Metallothioneins are ubiquitous, small, cysteine-rich proteins with the ability to bind heavy metals. In spite of their biochemical characterization, their in vivo function remains elusive. Here, we report the generation of a metallothionein gene family knockout in Drosophila melanogaster by targeted disruption of all four genes (MtnA to -D). These flies are viable if raised in standard laboratory food. During development, however, they are highly sensitive to copper, cadmium, and (to a lesser extent) zinc load. Metallothionein expression is particularly important for male viability; while copper load during development affects males and females equally, adult males lacking metallothioneins display a severely reduced life span, possibly due to copper-mediated oxidative stress. Using various reporter gene constructs, we find that different metallothioneins are expressed with virtually the same tissue specificity in larvae, notably in the intestinal tract at sites of metal accumulation, including the midgut's "copper cells." The same expression pattern is observed with a synthetic minipromoter consisting only of four tandem metal response elements. From these and other experiments, we conclude that tissue specificity of metallothionein expression is a consequence, rather than a cause, of metal distribution in the organism. The bright orange luminescence of copper accumulated in copper cells of the midgut is severely reduced in the metallothionein gene family knockout, as well as in mutants of metal-responsive transcription factor 1 (MTF-1), the main regulator of metallothionein expression. This indicates that an in vivo metallothionein-copper complex forms the basis of this luminescence. Strikingly, metallothionein mutants show an increased, MTF-1-dependent induction of metallothionein promoters in response to copper, cadmium, silver, zinc, and mercury. We conclude that free metal, but not metallothionein-bound metal, triggers the activation of MTF-1 and that metallothioneins regulate their own expression by a negative feedback loop.

Superinduction of metallothionein I by inhibition of protein synthesis: role of a labile repressor in MTF-1 mediated gene transcription

Induction of metallothioneins (MTs) through the metal-activated transcription factor-1 (MTF-1) provides a model response for analyzing transcriptional gene regulation by heavy metals. Here, we report inhibition of protein synthesis by cycloheximide (CHX) increases induction of Mt1 by approximately five-fold, a phenomenon designated as "superinduction." Characterization of superinduction revealed it is time- and concentration-dependent of CHX, requires the presence of an MTF-1 activator, and occurs at a transcriptional level, suggesting a labile repressor in the control of Mt1 induction. Genetic analyses using Mtf1 null cells and a metal response element (MRE)-driven reporter construct showed that superinduction of Mt1 is mediated through MTF-1 and MRE-dependent transcription. Analyses of intracellular zinc content by inductively coupled plasma emission spectroscopy and fluorescence imaging demonstrated that treatment with CHX alone or CHX plus an inducer does not increase the total zinc accumulation or the concentration of free zinc in cells under the conditions in which superinduction occurs. Moreover, superinduction was observed in cells cultured in a zinc-depleted medium, suggesting that superinduction does not involve elevation of intracellular zinc concentration. Northern blotting showed that Cd, CHX, or Cd + CHX does not affect the expression of the mRNA of MTF-1. Immunoblotting using antibodies specific for MTF-1 demonstrated that Cd induces a down-regulation of the MTF-1 protein, whereas cotreatment with Cd and CHX blocked the Cd-induced degradation of MTF-1. The findings reveal a new mechanistic aspect of the superinduction of Mt1, in which a labile repressor negatively controls agonist-induced turnover of the MTF-1 protein.

Essential roles in development and pigmentation for the Drosophila copper transporter DmATP7

Defects in the mammalian Menkes and Wilson copper transporting P-type ATPases cause severe copper homeostasis disease phenotypes in humans. Here, we find that DmATP7, the sole Drosophila orthologue of the Menkes and Wilson genes, is vital for uptake of copper in vivo. Analysis of a DmATP7 loss-of-function allele shows that DmATP7 is essential in embryogenesis, early larval development, and adult pigmentation and is probably required for copper uptake from the diet. These phenotypes are analogous to those caused by mutation in the mouse and human Menkes genes, suggesting that like Menkes, DmATP7 plays at least two roles at the cellular level: delivering copper to cuproenzymes required for pigmentation and neuronal function and removing excess cellular copper via facilitated efflux. DmATP7 displays a dynamic and unexpected expression pattern in the developing embryo, implying novel functions for this copper pump and the lethality observed in DmATP7 mutant flies is the earliest seen for any copper homeostasis gene.

Two major branches of anti-cadmium defense in the mouse: MTF-1/metallothioneins and glutathione

Metal-responsive transcription factor 1 (MTF-1) regulates expression of its target genes in response to various stress conditions, notably heavy metal load, via binding to metal response elements (MREs) in the respective enhancer/promoter regions. Furthermore, it serves a vital function in embryonic liver development. However, targeted deletion of Mtf1 in the liver after birth is no longer lethal. For this study, Mtf1 conditional knockout mice and control littermates were both mock- or cadmium-treated and liver-specific transcription was analyzed. Besides the well-characterized metallothionein genes, several new MTF-1 target genes with MRE motifs in the promoter region emerged. MTF-1 is required for the basal expression of selenoprotein W, muscle 1 gene (Sepw1) that encodes a glutathione-binding and putative antioxidant protein, supporting a role of MTF-1 in the oxidative stress response. Furthermore, MTF-1 mediates the cadmium-induced expression of N-myc downstream regulated gene 1 (Ndrg1), which is induced by several stress conditions and is overexpressed in many cancers. MTF-1 is also involved in the cadmium response of cysteine- and glycine-rich protein 1 gene (Csrp1), which is implicated in cytoskeletal organization. In contrast, MTF-1 represses the basal expression of Slc39a10, a putative zinc transporter. In a pathway independent of MTF-1, cadmium also induced the transcription of genes involved in the synthesis and regeneration of glutathione, a cadmium-binding antioxidant. These data provide strong evidence for two major branches of cellular anti-cadmium defense, one via MTF-1 and its target genes, notably metallothioneins, the other via glutathione, with an apparent overlap in selenoprotein W.

Genomic deletions of the Drosophila melanogaster Hsp70 genes

Homologous recombination can produce directed mutations in the genomes of a number of model organisms, including Drosophila melanogaster. One of the most useful applications has been to delete target genes to generate null alleles. In Drosophila, specific gene deletions have not yet been produced by this method. To test whether such deletions could be produced by homologous recombination in D. melanogaster we set out to delete the Hsp70 genes. Six nearly identical copies of this gene, encoding the major heat-shock protein in Drosophila, are found at two separate but closely linked loci. This arrangement has thwarted standard genetic approaches to generate an Hsp70-null fly, making this an ideal test of gene targeting. In this study, ends-out targeting was used to generate specific deletions of all Hsp70 genes, including one deletion that spanned approximately 47 kb. The Hsp70-null flies are viable and fertile. The results show that genomic deletions of varied sizes can be readily generated by homologous recombination in Drosophila.

Gene deletions by ends-in targeting in Drosophila melanogaster

Following the advent of a gene targeting technique in Drosophila, different methods have been developed to modify the Drosophila genome. The initial demonstration of gene targeting in flies used an ends-in method, which generates a duplication of the target locus. The duplicated locus can then be efficiently reduced to a single copy by generating a double-strand break between the duplicated segments. This method has been used to knock out target genes by introducing point mutations. A derivative of this method is reported here. By using different homologous regions for the targeting and reduction steps, a complete deletion of the target gene can be generated to produce a definitive null allele. The breakpoints of the deletion can be precisely controlled. Unlike ends-out targeting, this method does not leave exogenous sequence at the deleted locus. Three endogenous genes, Sir2, Sirt2, and p53 have been successfully deleted using this method.

Metal-responsive transcription factor (MTF-1) handles both extremes, copper load and copper starvation, by activating different genes

From insects to mammals, metallothionein genes are induced in response to heavy metal load by the transcription factor MTF-1, which binds to short DNA sequence motifs, termed metal response elements (MREs). Here we describe a novel and seemingly paradoxical role for MTF-1 in Drosophila in that it also mediates transcriptional activation of Ctr1B, a copper importer, upon copper depletion. Activation depends on the same type of MRE motifs in the upstream region of the Ctr1B gene as are normally required for metal induction. Thus, a single transcription factor, MTF-1, plays a direct role in both copper detoxification and acquisition by inducing the expression of metallothioneins and of a copper importer, respectively.

Emerging technologies for gene manipulation in Drosophila melanogaster

The popularity of Drosophila melanogaster as a model for understanding eukaryotic biology over the past 100 years has been accompanied by the development of numerous tools for manipulating the fruitfly genome. Here we review some recent technologies that will allow Drosophila melanogaster to be manipulated more easily than any other multicellular organism. These developments include the ability to create molecularly designed deletions, improved genetic mapping technologies, strategies for creating targeted mutations, new transgenic approaches and the means to clone and modify large fragments of DNA.

Quantitative PCR analysis of two molluscan metallothionein genes unveils differential expression and regulation

The mRNA levels of two components of the mussel (Mytilus galloprovincialis) metallothionein (MT) gene families, MT10 and MT20, were evaluated using real-time quantitative-PCR and Sybr Green I chemistry in animals exposed to heavy metals in vivo and in primary cell cultures. This method was highly specific in detecting the expression of the two genes over a widely dynamic range of starting DNA amounts, showing that the basal level of MT expression is mostly due to MT10 mRNA. Basal MT expression reflected the intracellular concentration of heavy metal as indicated by the use of the heavy metal chelator TPEN on primary cells. MT10 was observed to be inducible by Cd, Zn, and Cu ions, and to a lesser extent by Hg. By contrast, the MT20 expression level was very low under basal conditions, while its mRNA increased dramatically in response to Cd exposure, and to a lesser extend to Hg, leading to levels of expression similar to those of the MT10 gene. The essential metals Cu and Zn had a very small effect on the MT20 gene, whereas the concomitant exposure to Cu and H(2)O(2) produced a rapid rise of expression. In summary, data indicate that the MT isogenes are differentially regulated by heavy metals, while hydroxyl radicals may have a role in MT20 gene activation. Also, protein expression showed metal inducibility only after Cd exposure, suggesting the occurrence of posttranscriptional control mechanisms.

Regulation of malignant progression by the hypoxia-sensitive transcription factors HIF-1alpha and MTF-1

Solid tumors are known to develop microenvironmental hypoxia or anoxia due to malfunction and malformation of blood vessels and the energy demands of the highly proliferative tumor cells. Oxygen deprivation can cause aberrant modifications of signaling pathways and their downstream transcription factors that are believed to contribute to malignancy. Here, we review the latest studies related to the involvement of hypoxia-inducible transcription factor-1alpha (HIF-1alpha), the first known mammalian intracellular hypoxia sensor, in tumor development. We propose that a second far less studied protein, metal transcription factor-1 (MTF-1), acts as a more general oxygen sensor, responding to both hypoxia and oxidative stress, and is also intimately involved in malignant progression. Existing evidence suggests that activation of these two ubiquitous proteins, by hypoxia and genetic modifications, modulate the expression patterns of a number of important proteins involved in tumorigenesis.