Drosophila Ctr1A Functions as a Copper Transporter Essential for Development

Distinct function of Chlamydomonas CTRA-CTR transporters in Cu assimilation and intracellular mobilization

Successful acclimation to copper (Cu) deficiency involves a fine balance between Cu import and export. In the green alga Chlamydomonas reinhardtii, Cu import is dependent on a transcription factor, Copper Response Regulator 1 (CRR1), responsible for activating genes in Cu-deficient cells. Among CRR1 target genes are two Cu transporters belonging to the CTR/COPT gene family (CTR1 and CTR2) and a related soluble protein (CTR3). The ancestor of these green algal proteins was likely acquired from an ancient chytrid and contained conserved cysteine-rich domains (named the CTR-associated domains, CTRA) that are predicted to be involved in Cu acquisition. We show by reverse genetics that Chlamydomonas CTR1 and CTR2 are canonical Cu importers albeit with distinct affinities, while loss of CTR3 did not result in an observable phenotype under the conditions tested. Mutation of CTR1, but not CTR2, recapitulates the poor growth of crr1 in Cu-deficient medium, consistent with a dominant role for CTR1 in high-affinity Cu(I) uptake. On the other hand, the overaccumulation of Cu(I) (20 times the quota) in zinc (Zn) deficiency depends on CRR1 and both CTR1 and CTR2. CRR1-dependent activation of CTR gene expression needed for Cu over-accumulation can be bypassed by the provision of excess Cu in the growth medium. Over-accumulated Cu is sequestered into the acidocalcisome but can become remobilized by restoring Zn nutrition. This mobilization is also CRR1-dependent, and requires activation of CTR2 expression, again distinguishing CTR2 from CTR1 and consistent with the lower substrate affinity of CTR2.

ONE SENTENCE SUMMARY

Regulation of Cu uptake and sequestration by members of the CTR family of proteins in Chlamydomonas.

Gene expression variation underlying tissue-specific responses to copper stress in Drosophila melanogaster

Copper is one of a handful of biologically necessary heavy metals that is also a common environmental pollutant. Under normal conditions, copper ions are required for many key physiological processes. However, in excess, copper results in cell and tissue damage ranging in severity from temporary injury to permanent neurological damage. Because of its biological relevance, and because many conserved copper-responsive genes respond to nonessential heavy metal pollutants, copper resistance in Drosophila melanogaster is a useful model system with which to investigate the genetic control of the heavy metal stress response. Because heavy metal toxicity has the potential to differently impact specific tissues, we genetically characterized the control of the gene expression response to copper stress in a tissue-specific manner in this study. We assessed the copper stress response in head and gut tissue of 96 inbred strains from the Drosophila Synthetic Population Resource using a combination of differential expression analysis and expression quantitative trait locus mapping. Differential expression analysis revealed clear patterns of tissue-specific expression. Tissue and treatment specific responses to copper stress were also detected using expression quantitative trait locus mapping. Expression quantitative trait locus associated with MtnA, Mdr49, Mdr50, and Sod3 exhibited both genotype-by-tissue and genotype-by-treatment effects on gene expression under copper stress, illuminating tissue- and treatment-specific patterns of gene expression control. Together, our data build a nuanced description of the roles and interactions between allelic and expression variation in copper-responsive genes, provide valuable insight into the genomic architecture of susceptibility to metal toxicity, and highlight candidate genes for future functional characterization.

Distinct function of Chlamydomonas CTRA-CTR transporters in Cu assimilation and intracellular mobilization

Successful acclimation to copper (Cu) deficiency involves a fine balance between Cu import and export. In the unicellular green alga Chlamydomonas reinhardtii , Cu import is dependent on C opper R esponse R egulator 1 (CRR1), the master regulator of Cu homeostasis. Among CRR1 target genes are two Cu transporters belonging to the CTR/COPT gene family ( CTR1 and CTR2 ) and a related soluble cysteine-rich protein (CTR3). The ancestor of these green algal proteins was likely acquired from an ancient chytrid and contained conserved cysteine-rich domains (named the CTR-associated domains, CTRA) that are predicted to be involved in Cu acquisition. We show by reverse genetics that Chlamydomonas CTR1 and CTR2 are canonical Cu importers albeit with distinct affinities, while loss of CTR3 did not result in an observable phenotype under the conditions tested. Mutation of CTR1 , but not CTR2 , recapitulate the poor growth of crr1 in Cu-deficient medium, consistent with a dominant role for CTR1 in high affinity Cu(I) uptake. Notably, the over-accumulation of Cu(I) in Zinc (Zn)-deficiency (20 times the quota) depends on CRR1 and both CTR1 and CTR2. CRR1-dependent activation of CTR gene expression needed for Cu over-accumulation can be bypassed by the provision of excess Cu in the growth medium. Over-accumulated Cu is sequestered into the acidocalcisome but can become remobilized by restoring Zn nutrition. This mobilization is also CRR1-dependent, and requires activation of CTR2 expression, again distinguishing CTR2 from CTR1 and is consistent with the lower substrate affinity of CTR2.

Gene expression variation underlying tissue-specific responses to copper stress in Drosophila melanogaster

Copper is one of a handful of biologically necessary heavy metals that is also a common environmental pollutant. Under normal conditions, copper ions are required for many key physiological processes. However, in excess, copper quickly results in cell and tissue damage that can range in severity from temporary injury to permanent neurological damage. Because of its biological relevance, and because many conserved copper-responsive genes also respond to other non-essential heavy metal pollutants, copper resistance in Drosophila melanogaster is a useful model system with which to investigate the genetic control of the response to heavy metal stress. Because heavy metal toxicity has the potential to differently impact specific tissues, we genetically characterized the control of the gene expression response to copper stress in a tissue-specific manner in this study. We assessed the copper stress response in head and gut tissue of 96 inbred strains from the Drosophila Synthetic Population Resource (DSPR) using a combination of differential expression analysis and expression quantitative trait locus (eQTL) mapping. Differential expression analysis revealed clear patterns of tissue-specific expression, primarily driven by a more pronounced gene expression response in gut tissue. eQTL mapping of gene expression under control and copper conditions as well as for the change in gene expression following copper exposure (copper response eQTL) revealed hundreds of genes with tissue-specific local cis-eQTL and many distant trans-eQTL. eQTL associated with MtnA, Mdr49, Mdr50, and Sod3 exhibited genotype by environment effects on gene expression under copper stress, illuminating several tissue- and treatment-specific patterns of gene expression control. Together, our data build a nuanced description of the roles and interactions between allelic and expression variation in copper-responsive genes, provide valuable insight into the genomic architecture of susceptibility to metal toxicity, and highlight many candidate genes for future functional characterization.

The genomic basis of copper tolerance in Drosophila is shaped by a complex interplay of regulatory and environmental factors

BACKGROUND

Escalation in industrialization and anthropogenic activity have resulted in an increase of pollutants released into the environment. Of these pollutants, heavy metals such as copper are particularly concerning due to their bio-accumulative nature. Due to its highly heterogeneous distribution and its dual nature as an essential micronutrient and toxic element, the genetic basis of copper tolerance is likely shaped by a complex interplay of genetic and environmental factors.

RESULTS

In this study, we utilized the natural variation present in multiple populations of Drosophila melanogaster collected across Europe to screen for variation in copper tolerance. We found that latitude and the degree of urbanization at the collection sites, rather than any other combination of environmental factors, were linked to copper tolerance. While previously identified copper-related genes were not differentially expressed in tolerant vs. sensitive strains, genes involved in metabolism, reproduction, and protease induction contributed to the differential stress response. Additionally, the greatest transcriptomic and physiological responses to copper toxicity were seen in the midgut, where we found that preservation of gut acidity is strongly linked to greater tolerance. Finally, we identified transposable element insertions likely to play a role in copper stress response.

CONCLUSIONS

Overall, by combining genome-wide approaches with environmental association analysis, and functional analysis of candidate genes, our study provides a unique perspective on the genetic and environmental factors that shape copper tolerance in natural D. melanogaster populations and identifies new genes, transposable elements, and physiological traits involved in this complex phenotype.

Mitochondrial Cytochrome c Oxidase Defects Alter Cellular Homeostasis of Transition Metals

The redox activity of cytochrome c oxidase (COX), the terminal oxidase of the mitochondrial respiratory chain (MRC), depends on the incorporation of iron and copper into its catalytic centers. Many mitochondrial proteins have specific roles for the synthesis and delivery of metal-containing cofactors during COX biogenesis. In addition, a large set of different factors possess other molecular functions as chaperones or translocators that are also necessary for the correct maturation of these complexes. Pathological variants in genes encoding structural MRC subunits and these different assembly factors produce respiratory chain deficiency and lead to mitochondrial disease. COX deficiency in Drosophila melanogaster, induced by downregulated expression of three different assembly factors and one structural subunit, resulted in decreased copper content in the mitochondria accompanied by different degrees of increase in the cytosol. The disturbances in metal homeostasis were not limited only to copper, as some changes in the levels of cytosolic and/or mitochondrial iron, manganase and, especially, zinc were observed in several of the COX-deficient groups. The altered copper and zinc handling in the COX defective models resulted in a transcriptional response decreasing the expression of copper transporters and increasing the expression of metallothioneins. We conclude that COX deficiency is generally responsible for an altered mitochondrial and cellular homeostasis of transition metals, with variations depending on the origin of COX assembly defect.

"A fly appeared": sable, a classic Drosophila mutation, maps to Yippee, a gene affecting body color, wings, and bristles

Insect body color is an easily assessed and visually engaging trait that is informative on a broad range of topics including speciation, biomaterial science, and ecdysis. Mutants of the fruit fly Drosophila melanogaster have been an integral part of body color research for more than a century. As a result of this long tenure, backlogs of body color mutations have remained unmapped to their genes, all while their strains have been dutifully maintained, used for recombination mapping, and part of genetics education. Stemming from a lesson plan in our undergraduate genetics class, we have mapped sable1, a dark body mutation originally described by Morgan and Bridges, to Yippee, a gene encoding a predicted member of the E3 ubiquitin ligase complex. Deficiency/duplication mapping, genetic rescue, DNA and cDNA sequencing, RT-qPCR, and 2 new CRISPR alleles indicated that sable1 is a hypomorphic Yippee mutation due to an mdg4 element insertion in the Yippee 5'-UTR. Further analysis revealed additional Yippee mutant phenotypes including curved wings, ectopic/missing bristles, delayed development, and failed adult emergence. RNAi of Yippee in the ectoderm phenocopied sable body color and most other Yippee phenotypes. Although Yippee remains functionally uncharacterized, the results presented here suggest possible connections between melanin biosynthesis, copper homeostasis, and Notch/Delta signaling; in addition, they provide insight into past studies of sable cell nonautonomy and of the genetic modifier suppressor of sable.

Oral Elesclomol Treatment Alleviates Copper Deficiency in Animal Models

Copper (Cu) is an essential trace element for key biochemical reactions. Dietary or genetic copper deficiencies are associated with anemia, cardiomyopathy, and neurodegeneration. The essential requirement for copper in humans is illustrated by Menkes disease, a fatal neurodegenerative disorder of early childhood caused by mutations in the ATP7A copper transporter. Recent groundbreaking studies have demonstrated that a copper delivery small molecule compound, elesclomol (ES), is able to substantially ameliorate pathology and lethality in a mouse model of Menkes disease when injected as an ES-Cu2+ complex. It is well appreciated that drugs administered through oral means are more convenient with better efficacy than injection methods. Here we show, using genetic models of copper-deficient C. elegans and mice, that dietary ES supplementation fully rescues copper deficiency phenotypes. Worms lacking either the homolog of the CTR1 copper importer or the ATP7 copper exporter showed normal development when fed ES. Oral gavage with ES rescued intestine-specific Ctr1 knockout mice from early postnatal lethality without additional copper supplementation. Our findings reveal that ES facilitates copper delivery from dietary sources independent of the intestinal copper transporter CTR1 and provide insight into oral administration of ES as an optimal therapeutic for Menkes disease and possibly other disorders of copper insufficiency.

Drosophila melanogaster as a Model Organism to Study Lithium and Boron Bioactivity

The fruit fly Drosophila melanogaster has become a valuable model organism in nutritional science, which can be applied to elucidate the physiology and the biological function of nutrients, including trace elements. Importantly, the application of chemically defined diets enables the supply of trace elements for nutritional studies under highly standardized dietary conditions. Thus, the bioavailability and bioactivity of trace elements can be systematically monitored in D. melanogaster. Numerous studies have already revealed that central aspects of trace element homeostasis are evolutionary conserved among the fruit fly and mammalian species. While there is sufficient evidence of vital functions of boron (B) in plants, there is also evidence regarding its bioactivity in animals and humans. Lithium (Li) is well known for its role in the therapy of bipolar disorder. Furthermore, recent findings suggest beneficial effects of Li regarding neuroprotection as well as healthy ageing and longevity in D. melanogaster. However, no specific essential function in the animal kingdom has been found for either of the two elements so far. Here, we summarize the current knowledge of Li and B bioactivity in D. melanogaster in the context of health and disease prevention.

The copper chaperone CCS facilitates copper binding to MEK1/2 to promote kinase activation

Normal physiology relies on the precise coordination of intracellular signal transduction pathways that respond to nutrient availability to balance cell growth and cell death. The canonical MAPK pathway consists of the RAF-MEK-ERK signaling cascade and represents one of the most well-defined axes within eukaryotic cells to promote cell proliferation, which underscores its frequent mutational activation in the majority of human cancers. Our recent studies illuminated a function for the redox-active micronutrient copper (Cu) as an intracellular mediator of signaling by connecting Cu to the amplitude of MAPK signaling via a direct interaction between Cu and the kinases MEK1 and MEK2. Given the large quantities of molecules like glutathione and metallothionein that limit cellular toxicity from free Cu ions, evolutionarily conserved Cu chaperones facilitate the efficient delivery of Cu to cuproenzymes. Thus, a dedicated cellular delivery mechanism of Cu to MEK1/2 is likely to exist. Using surface plasmon resonance and proximity-dependent biotin ligase studies, we report here that the Cu chaperone CCS selectively bound to and facilitated Cu transfer to MEK1. Mutations in CCS that disrupt Cu(I) acquisition and exchange or a CCS small molecule inhibitor were employed and resulted in reduced Cu-stimulated MEK1 kinase activity. Our findings indicate that the Cu chaperone CCS provides fidelity within a complex biological system to achieve appropriate installation of Cu within the MEK1 kinase active site that in turn modulates kinase activity and support the development of novel MEK1/2 inhibitors that target the Cu structural interface or blunt dedicated Cu delivery mechanisms via CCS.

The Effects of Essential and Non-Essential Metal Toxicity in the Drosophila melanogaster Insect Model: A Review

The biological effects of environmental metal contamination are important issues in an industrialized, resource-dependent world. Different metals have different roles in biology and can be classified as essential if they are required by a living organism (e.g., as cofactors), or as non-essential metals if they are not. While essential metal ions have been well studied in many eukaryotic species, less is known about the effects of non-essential metals, even though essential and non-essential metals are often chemically similar and can bind to the same biological ligands. Insects are often exposed to a variety of contaminated environments and associated essential and non-essential metal toxicity, but many questions regarding their response to toxicity remain unanswered. Drosophila melanogaster is an excellent insect model species in which to study the effects of toxic metal due to the extensive experimental and genetic resources available for this species. Here, we review the current understanding of the impact of a suite of essential and non-essential metals (Cu, Fe, Zn, Hg, Pb, Cd, and Ni) on the D. melanogaster metal response system, highlighting the knowledge gaps between essential and non-essential metals in D. melanogaster. This review emphasizes the need to use multiple metals, multiple genetic backgrounds, and both sexes in future studies to help guide future research towards better understanding the effects of metal contamination in general.

The transfer of trace metals in the soil-plant-arthropod system

Essential and non-essential trace metals are capable of causing toxicity to organisms above a threshold concentration. Extensive research has assessed the behaviour of trace metals in biological and ecological systems, but has typically focused on single organisms within a trophic level and not on multi-trophic transfer through terrestrial food chains. This reinforces the notion of metal toxicity as a closed system, failing to consider one trophic level as a pollution source to another; therefore, obscuring the full extent of ecosystem effects. Given the relatively few studies on trophic transfer of metals, this review has taken a compartment-based approach, where transfer of metals through trophic pathways is considered as a series of linked compartments (soil-plant-arthropod herbivore-arthropod predator). In particular, we consider the mechanisms by which trace metals are taken up by organisms, the forms and transformations that can occur within the organism and the consequences for trace metal availability to the next trophic level. The review focuses on four of the most prevalent metal cations in soil which are labile in terrestrial food chains: Cd, Cu, Zn and Ni. Current knowledge of the processes and mechanisms by which these metals are transformed and moved within and between trophic levels in the soil-plant-arthropod system are evaluated. We demonstrate that the key factors controlling the transfer of trace metals through the soil-plant-arthropod system are the form and location in which the metal occurs in the lower trophic level and the physiological mechanisms of each organism in regulating uptake, transformation, detoxification and transfer. The magnitude of transfer varies considerably depending on the trace metal concerned, as does its toxicity, and we conclude that biomagnification is not a general property of plant-arthropod and arthropod-arthropod systems. To deliver a more holistic assessment of ecosystem toxicity, integrated studies across ecosystem compartments are needed to identify critical pathways that can result in secondary toxicity across terrestrial food-chains.

Characterizing the genetic basis of copper toxicity in Drosophila reveals a complex pattern of allelic, regulatory, and behavioral variation

A range of heavy metals are required for normal cell function and homeostasis. However, the anthropogenic release of metal compounds into soil and water sources presents a pervasive health threat. Copper is one of many heavy metals that negatively impacts diverse organisms at a global scale. Using a combination of quantitative trait locus (QTL) mapping and RNA sequencing in the Drosophila Synthetic Population Resource, we demonstrate that resistance to the toxic effects of ingested copper in D. melanogaster is genetically complex and influenced by allelic and expression variation at multiple loci. QTL mapping identified several QTL that account for a substantial fraction of heritability. Additionally, we find that copper resistance is impacted by variation in behavioral avoidance of copper and may be subject to life-stage specific regulation. Gene expression analysis further demonstrated that resistant and sensitive strains are characterized by unique expression patterns. Several of the candidate genes identified via QTL mapping and RNAseq have known copper-specific functions (e.g., Ccs, Sod3, CG11825), and others are involved in the regulation of other heavy metals (e.g., Catsup, whd). We validated several of these candidate genes with RNAi suggesting they contribute to variation in adult copper resistance. Our study illuminates the interconnected roles that allelic and expression variation, organism life stage, and behavior play in copper resistance, allowing a deeper understanding of the diverse mechanisms through which metal pollution can negatively impact organisms.

Cuticle darkening correlates with increased body copper content in Drosophila melanogaster

Insect epidermal cells secrete a cuticle that serves as an exoskeleton providing mechanical rigidity to each individual, but also insulation, camouflage or communication within their environment. Cuticle deposition and hardening (sclerotization) and pigment synthesis are parallel processes requiring tyrosinase activity, which depends on an unidentified copper-dependent enzyme component in Drosophila melanogaster. We determined the metallomes of fly strains selected for lighter or darker cuticles in a laboratory evolution experiment, asking whether any specific element changed in abundance in concert with pigment deposition. The results showed a correlation between total iron content and strength of pigmentation, which was further corroborated by ferritin iron quantification. To ask if the observed increase in iron body content along with increased pigment deposition could be generalizable, we crossed yellow and ebony alleles causing light and dark pigmentation, respectively, into similar genetic backgrounds and measured their metallomes. Iron remained unaffected in the various mutants providing no support for a causative link between pigmentation and iron content. In contrast, the combined analysis of both experiments suggested instead a correlation between pigment deposition and total copper body content, possibly due to increased demand for epidermal tyrosinase activity.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

Copper and Zinc Homeostasis: Lessons from Drosophila melanogaster

Maintenance of metal homeostasis is crucial for many different enzymatic activities and in turn for cell function and survival. In addition, cells display detoxification and protective mechanisms against toxic accumulation of metals. Perturbation of any of these processes normally leads to cellular dysfunction and finally to cell death. In the last years, loss of metal regulation has been described as a common pathological feature in many human neurodegenerative diseases. However, in most cases, it is still a matter of debate whether such dyshomeostasis is a primary or a secondary downstream defect. In this review, we will summarize and critically evaluate the contribution of Drosophila to model human diseases that involve altered metabolism of metals or in which metal dyshomeostasis influence their pathobiology. As a prerequisite to use Drosophila as a model, we will recapitulate and describe the main features of core genes involved in copper and zinc metabolism that are conserved between mammals and flies. Drosophila presents some unique strengths to be at the forefront of neurobiological studies. The number of genetic tools, the possibility to easily test genetic interactions in vivo and the feasibility to perform unbiased genetic and pharmacological screens are some of the most prominent advantages of the fruitfly. In this work, we will pay special attention to the most important results reported in fly models to unveil the role of copper and zinc in cellular degeneration and their influence in the development and progression of human neurodegenerative pathologies such as Parkinson's disease, Alzheimer's disease, Huntington's disease, Friedreich's Ataxia or Menkes, and Wilson's diseases. Finally, we show how these studies performed in the fly have allowed to give further insight into the influence of copper and zinc in the molecular and cellular causes and consequences underlying these diseases as well as the discovery of new therapeutic strategies, which had not yet been described in other model systems.

Gene duplication and neo-functionalization in the evolutionary and functional divergence of the metazoan copper transporters Ctr1 and Ctr2

Copper is an essential element for proper organismal development and is involved in a range of processes, including oxidative phosphorylation, neuropeptide biogenesis, and connective tissue maturation. The copper transporter (Ctr) family of integral membrane proteins is ubiquitously found in eukaryotes and mediates the high-affinity transport of Cu+ across both the plasma membrane and endomembranes. Although mammalian Ctr1 functions as a Cu+ transporter for Cu acquisition and is essential for embryonic development, a homologous protein, Ctr2, has been proposed to function as a low-affinity Cu transporter, a lysosomal Cu exporter, or a regulator of Ctr1 activity, but its functional and evolutionary relationship to Ctr1 is unclear. Here we report a biochemical, genetic, and phylogenetic comparison of metazoan Ctr1 and Ctr2, suggesting that Ctr2 arose over 550 million years ago as a result of a gene duplication event followed by loss of Cu+ transport activity. Using a random mutagenesis and growth selection approach, we identified amino acid substitutions in human and mouse Ctr2 proteins that support copper-dependent growth in yeast and enhance copper accumulation in Ctr1-/- mouse embryonic fibroblasts. These mutations revert Ctr2 to a more ancestral Ctr1-like state while maintaining endogenous functions, such as stimulating Ctr1 cleavage. We suggest key structural aspects of metazoan Ctr1 and Ctr2 that discriminate between their biological roles, providing mechanistic insights into the evolutionary, biochemical, and functional relationships between these two related proteins.

Drosophila melanogaster Models of Metal-Related Human Diseases and Metal Toxicity

Iron, copper and zinc are transition metals essential for life because they are required in a multitude of biological processes. Organisms have evolved to acquire metals from nutrition and to maintain adequate levels of each metal to avoid damaging effects associated with its deficiency, excess or misplacement. Interestingly, the main components of metal homeostatic pathways are conserved, with many orthologues of the human metal-related genes having been identified and characterized in Drosophila melanogaster. Drosophila has gained appreciation as a useful model for studying human diseases, including those caused by mutations in pathways controlling cellular metal homeostasis. Flies have many advantages in the laboratory, such as a short life cycle, easy handling and inexpensive maintenance. Furthermore, they can be raised in a large number. In addition, flies are greatly appreciated because they offer a considerable number of genetic tools to address some of the unresolved questions concerning disease pathology, which in turn could contribute to our understanding of the metal metabolism and homeostasis. This review recapitulates the metabolism of the principal transition metals, namely iron, zinc and copper, in Drosophila and the utility of this organism as an experimental model to explore the role of metal dyshomeostasis in different human diseases. Finally, a summary of the contribution of Drosophila as a model for testing metal toxicity is provided.

Evidence for a role for the putative Drosophila hGRX1 orthologue in copper homeostasis

Glutaredoxins are a family of small molecular weight proteins that have a central role in cellular redox regulation. Human GRX1 (hGRX1) has also been shown to play an integral role in copper homeostasis by regulating the redox activity of the metalated sites of copper chaperones such as ATOX1 and SOD1, and the copper efflux proteins ATP7A and ATP7B. To further elucidate the role of hGRX1 in copper homeostasis, we examined the impact of RNA interference-mediated knockdown of CG6852, a putative Drosophila orthologue of hGRX1. CG6852 shares ~41 % amino acid identity with hGRX1 and key functional domains including the metal-binding CXXC motif are conserved between the two proteins. Knockdown of CG6852 in the adult midline caused a thoracic cleft and reduced scutellum, phenotypes that were exacerbated by additional knockdown of copper uptake transporters Ctr1A and Ctr1B. Knockdown of CG6852 in the adult eye enhanced a copper-deficiency phenotype caused by Ctr1A knockdown while ubiquitous knockdown of CG6852 resulted a mild systemic copper deficiency. Therefore we conclude that CG6852 is a putative orthologue of hGRX1 and may play an important role in Drosophila copper homeostasis.

Cathepsin Protease Controls Copper and Cisplatin Accumulation via Cleavage of the Ctr1 Metal-binding Ectodomain

Copper is an essential metal ion for embryonic development, iron acquisition, cardiac function, neuropeptide biogenesis, and other critical physiological processes. Ctr1 is a high affinity Cu(+) transporter on the plasma membrane and endosomes that exists as a full-length protein and a truncated form of Ctr1 lacking the methionine- and histidine-rich metal-binding ectodomain, and it exhibits reduced Cu(+) transport activity. Here, we identify the cathepsin L/B endolysosomal proteases functioning in a direct and rate-limiting step in the Ctr1 ectodomain cleavage. Cells and mice lacking cathepsin L accumulate full-length Ctr1 and hyper-accumulate copper. As Ctr1 also transports the chemotherapeutic drug cisplatin via direct binding to the ectodomain, we demonstrate that the combination of cisplatin with a cathepsin L/B inhibitor enhances cisplatin uptake and cell killing. These studies identify a new processing event and the key protease that cleaves the Ctr1 metal-binding ectodomain, which functions to regulate cellular Cu(+) and cisplatin acquisition.

Elemental mapping of the entire intact Drosophila gastrointestinal tract

The main role of the animal gastrointestinal (GI) tract is the selective absorption of dietary nutrients from ingested food sources. One class of vital micronutrients are the essential biometals such as copper, zinc and iron, which participate in a plethora of biological process, acting as enzymatic or structural co-factors for numerous proteins and also as important cellular signalling molecules. To help elucidate the mechanisms by which biometals are absorbed from the diet, we mapped elemental distribution in entire, intact Drosophila larval GI tracts using synchrotron X-ray fluorescence microscopy. Our results revealed distinct regions of the GI tract enriched for specific metals. Copper was found to be concentrated in the copper cell region but also in the region directly anterior to the copper cells and unexpectedly, in the middle midgut/iron cell region as well. Iron was observed exclusively in the iron cell region, confirming previous work with iron-specific histological stains. Zinc was observed throughout the GI tract with an increased accumulation in the posterior midgut region, while manganese was seen to co-localize with calcium specifically in clusters in the distal Malpighian tubules. This work simultaneously reveals distribution of a number of biologically important elements in entire, intact GI tracts. These distributions revealed not only a previously undescribed Ca/Mn co-localization, but also the unexpected presence of additional Cu accumulations in the iron cell region.

Vacuolar-type H(+)-ATPase subunits and the neurogenic protein big brain are required for optimal copper and zinc uptake

Copper and zinc homeostasis in polarized epithelial cells requires the correct localization and regulation of membrane-bound transport proteins at the apical and basolateral cell membranes. We have identified a subunit of the vacuolar-type H(+)-ATPase (V-ATPase) complex, vhaPPA1-2, and the Drosophila aquaporin homolog big brain (bib), as being required for the correct localization of the copper uptake transporters Ctr1A and Ctr1B and the zinc uptake protein dZip89B and hence necessary for optimal copper and zinc accumulation in vivo. Knockdown of vhaPPA1-2 or bib resulted in cuticle hypo-pigmentation phenotypes typical of copper deficiency in the fly and induction of midgut Ctr1B expression, a known response to low cellular copper levels. Furthermore, midgut-specific knockdown of bib increased tolerance to elevated dietary zinc levels. Ctr1A, Ctr1B and dZip89B are normally localized to the apical plasma membrane. Upon knockdown of vhaPPA1-2 or bib, this localization was strongly disrupted as was that of the generic plasma membrane marker CD8-GFP, indicating that these two genes are not acting specifically on metal ion homeostasis but rather are necessary for general apical membrane protein localization in polarized epithelial cells. These results suggest that metal ion transport is particularly sensitive to disturbances in cellular protein localization processes.

Genome-wide RNAi screen for nuclear actin reveals a network of cofilin regulators

Nuclear actin plays an important role in many processes that regulate gene expression. Cytoplasmic actin dynamics are tightly controlled by numerous actin-binding proteins, but regulation of nuclear actin has remained unclear. Here, we performed a genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins that influence either nuclear polymerization or import of actin. We validate 19 factors as specific hits, and show that Chinmo (known as Bach2 in mammals), SNF4Aγ (Prkag1 in mammals) and Rab18 play a role in nuclear localization of actin in both fly and mammalian cells. We identify several new regulators of cofilin activity, and characterize modulators of both cofilin kinases and phosphatase. For example, Chinmo/Bach2, which regulates nuclear actin levels also in vivo, maintains active cofilin by repressing the expression of the kinase Cdi (Tesk in mammals). Finally, we show that Nup98 and lamin are candidates for regulating nuclear actin polymerization. Our screen therefore reveals new aspects of actin regulation and links nuclear actin to many cellular processes.

Identification of high-affinity copper transporters in Aspergillus fumigatus

We investigated the copper metabolism of Aspergillus fumigatus, which has not been characterized well. We cloned the putative copper transporters ctrA2 and ctrC from A. fumigatus and investigated the functions of these transporters in copper metabolism. Four putative copper transporters were identified in the A. fumigatus genome; ctrA2 and ctrC complemented CTR1 functionally and localized to the plasma membrane in Saccharomyces cerevisiae. ctrA2 and ctrC single-deletion mutants and a double-deletion mutant of ctrA2 and ctrC were constructed in A. fumigatus. The ctrA2 and ctrC double-deletion mutant exhibited a growth defect on Aspergillus minimal medium (AMM) supplemented with bathocuproine disulfonic acid (BCS) and was sensitive to H2O2. Furthermore, the deletion of ctrA2 and ctrC reduced superoxide dismutase (SOD) activity, laccase activity, and intracellular copper contents. The activities of the ctrA2 and ctrC genes were up-regulated by BCS treatment. In addition, the deletion of ctrA2 up-regulated ctrC and vice versa. ctrA2 and ctrC were localized to the A. fumigatus plasma membrane. Although ctrA2 and ctrC failed to affect the mouse survival rate, these genes affected conidial killing activity. Taken together, these results indicate that ctrA2 and ctrC may function as membrane transporters and that the involvement of these genes in pathogenicity merits further investigation.

Oral magnetite nanoparticles disturb the development of Drosophila melanogaster from oogenesis to adult emergence

The potential impacts of nanomaterials (NMs) on fetal development have attracted great concerns because of the increased potential exposure to NMs during pregnancy. Drosophila melanogaster oogenesis and developmental transitions may provide an attractive system to study the biological and environmental effects of NMs on the embryonic development. In this study, the effects of three types of magnetite (Fe3O4) nanoparticles (MNPs): UN-MNPs (pristine), CA-MNPs (citric acid modified) and APTS-MNPs (3-aminopropyltriethoxylsilane coated) on the development of Drosophila at 300 and 600 μg/g dosage were studied. The uptake of MNPs by female and male flies caused obvious reduction in the female fecundity, and the developmental delay at the egg-pupae and pupae-adult transitions, especially in those treated by the positive APTS-MNPs. Further investigation demonstrates that the parental uptake of MNPs disturbs the oogenesis period, induces ovarian defect, reduces the length of eggs, decreases the number of nurse cells and delays egg chamber development, which may contribute to the decrease of fecundity of female Drosophila and the development delay of their offspring. Using the synchrotron radiation-based micro-X-ray fluorescence (SR-μXRF), the dyshomeostasis of trace elements such as Fe, Ca and Cu along the anterior-posterior axis of the fertilized eggs was found, which may be an important reason for the development delay of Drosophila.

Sub-lethal metal stress response of larvae of Aedes aegypti

Aedes aegypti (Diptera: Culicidae) has adapted to urban environments; the urbanisation process provides suitable habitats for this disease vector subsequently increasing the probability of the transmission of pathogens in high-density environments. Urban environments provide metal stressed larval habitats. However, little is known about the physiological cost of metal stress or how this might affect the performance of this mosquito species. This study aims to characterise the sub-lethal physiological consequences of metal stress in Aedes aegypti. Various parameters of mosquito physiology under larval metal stress are assessed including larval metallothionein expression and the effects of larval metal stress on adult performance and their progeny. Results show that environmentally relevant larval metal stress compromises larval and adult development and performance, and results in larval metal tolerance along with an increase in lipid consumption. These performance costs are coupled to a dramatic increase in metallothionein expression in the midgut. Metal stress results in lowered adult body mass and neutral storage lipids at emergence, starvation tolerance, fecundity and starvation tolerance of offspring compared to non-metal stressed individuals. Ironically, larval metal stress results in increased adult longevity. Together, these findings indicate that even low levels of environmentally relevant larval metal stress have considerable physiological consequences for this important disease vector.

What can flies tell us about copper homeostasis?

Copper (Cu) is an essential redox active metal that is potentially toxic in excess. Multicellular organisms acquire Cu from the diet and must regulate uptake, storage, distribution and export of Cu at both the cellular and organismal levels. Systemic Cu deficiency can be fatal, as seen in Menkes disease patients. Conversely Cu toxicity occurs in patients with Wilson disease. Cu dyshomeostasis has also been implicated in neurodegenerative disorders such as Alzheimer's disease. Over the last decade, the fly Drosophila melanogaster has become an important model organism for the elucidation of eukaryotic Cu regulatory mechanisms. Gene discovery approaches with Drosophila have identified novel genes with conserved protein functions relevant to Cu homeostasis in humans. This review focuses on our current understanding of Cu uptake, distribution and export in Drosophila and the implications for mammals.

Ctr2 regulates biogenesis of a cleaved form of mammalian Ctr1 metal transporter lacking the copper- and cisplatin-binding ecto-domain

Copper is an essential catalytic cofactor for enzymatic activities that drive a range of metabolic biochemistry including mitochondrial electron transport, iron mobilization, and peptide hormone maturation. Copper dysregulation is associated with fatal infantile disease, liver, and cardiac dysfunction, neuropathy, and anemia. Here we report that mammals regulate systemic copper acquisition and intracellular mobilization via cleavage of the copper-binding ecto-domain of the copper transporter 1 (Ctr1). Although full-length Ctr1 is critical to drive efficient copper import across the plasma membrane, cleavage of the ecto-domain is required for Ctr1 to mobilize endosomal copper stores. The biogenesis of the truncated form of Ctr1 requires the structurally related, previously enigmatic copper transporter 2 (Ctr2). Ctr2(-/-) mice are defective in accumulation of truncated Ctr1 and exhibit increased tissue copper levels, and X-ray fluorescence microscopy demonstrates that copper accumulates as intracellular foci. These studies identify a key regulatory mechanism for mammalian copper transport through Ctr2-dependent accumulation of a Ctr1 variant lacking the copper- and cisplatin-binding ecto-domain.

Inhibition of human high-affinity copper importer Ctr1 orthologous in the nervous system of Drosophila ameliorates Aβ42-induced Alzheimer's disease-like symptoms

Disruption of copper homeostasis has been implicated in Alzheimer's disease (AD) during the last 2 decades; however, whether copper is a friend or a foe is controversial. Within a genetically tractable Drosophila AD model, we manipulated the expression of human high-affinity copper importer orthologous in Drosophila to explore the in vivo roles of copper ions in the development of AD. We found that inhibition of Ctr1C expression by RNAi in Aβ-expressing flies significantly reduced copper accumulation in the brains of the flies as well as ameliorating neurodegeneration, enhancing climbing ability, and prolonging lifespan. Interestingly, Ctr1C inhibition led to a significant increase in higher-molecular-weight Aβ42 forms in brain lysates, whereas it was accompanied by a trend of decreased expression of amyloid-β degradation proteases (including NEP1-3 and IDE) with age and reduced Cu-Aβ interaction-induced oxidative stress in Ctr1C RNAi flies. Similar results were obtained from inhibiting another copper importer Ctr1B and overexpressing a copper exporter DmATP7 in the nervous system of AD flies. These results imply that copper may play a causative role in developing AD, as either Aβ oligomers or aggregates were less toxic in a reduced copper environment or one with less copper binding. Early manipulation of brain copper uptake can have a great effect on Aβ pathology.

Mechanism of silver nanoparticles action on insect pigmentation reveals intervention of copper homeostasis

Silver nanoparticles (AgNPs), like almost all nanoparticles, are potentially toxic beyond a certain concentration because the survival of the organism is compromised due to scores of pathophysiological abnormalities past that concentration. However, the mechanism of AgNP toxicity remains undetermined. Instead of applying a toxic dose, we attempted to monitor the effects of AgNPs at a nonlethal concentration on wild type Drosophila melanogaster by exposing them throughout their development. All adult flies raised in AgNP doped food showed that up to 50 mg/L concentration AgNP has no negative influence on median survival; however, these flies appeared uniformly lighter in body color due to the loss of melanin pigments in their cuticle. Additionally, fertility and vertical movement ability were compromised due to AgNP feeding. Determination of the amount of free ionic silver (Ag⁺) led us to claim that the observed biological effects have resulted from the AgNPs and not from Ag⁺. Biochemical analysis suggests that the activity of copper dependent enzymes, namely tyrosinase and Cu-Zn superoxide dismutase, are decreased significantly following the consumption of AgNPs, despite the constant level of copper present in the tissue. Consequently, we propose a mechanism whereby consumption of excess AgNPs in association with membrane bound copper transporter proteins cause sequestration of copper, thus creating a condition that resembles copper starvation. This model also explains the cuticular demelanization effect resulting from AgNP since tyrosinase activity is essential for melanin biosynthesis. Finally, we claim that Drosophila, an established genetic model system, can be well utilized for further understanding of the biological effects of nanoparticles.

A Plasmodium falciparum copper-binding membrane protein with copper transport motifs

Background

Copper is an essential catalytic co-factor for metabolically important cellular enzymes, such as cytochrome-c oxidase. Eukaryotic cells acquire copper through a copper transport protein and distribute intracellular copper using molecular chaperones. The copper chelator, neocuproine, inhibits Plasmodium falciparum ring-to-trophozoite transition in vitro, indicating a copper requirement for malaria parasite development. How the malaria parasite acquires or secretes copper still remains to be fully elucidated.

Methods

PlasmoDB was searched for sequences corresponding to candidate P. falciparum copper-requiring proteins. The amino terminal domain of a putative P. falciparum copper transport protein was cloned and expressed as a maltose binding fusion protein. The copper binding ability of this protein was examined. Copper transport protein-specific anti-peptide antibodies were generated in chickens and used to establish native protein localization in P. falciparum parasites by immunofluorescence microscopy.

Results

Six P. falciparum copper-requiring protein orthologs and a candidate P. falciparum copper transport protein (PF14_0369), containing characteristic copper transport protein features, were identified in PlasmoDB. The recombinant amino terminal domain of the transport protein bound reduced copper in vitro and within Escherichia coli cells during recombinant expression. Immunolocalization studies tracked the copper binding protein translocating from the erythrocyte plasma membrane in early ring stage to a parasite membrane as the parasites developed to schizonts. The protein appears to be a PEXEL-negative membrane protein.

Conclusion

Plasmodium falciparum parasites express a native protein with copper transporter characteristics that binds copper in vitro. Localization of the protein to the erythrocyte and parasite plasma membranes could provide a mechanism for the delivery of novel anti-malarial compounds.

Deletion of Siah-interacting protein gene in Drosophila causes cardiomyopathy

Drosophila is a useful model organism in which the genetics of human diseases, including recent advances in identification of the genetics of heart development and disease in the fly, can be studied. To identify novel genes that cause cardiomyopathy, we performed a deficiency screen in adult Drosophila. Using optical coherence tomography to phenotype cardiac function in awake adult Drosophila, we identified Df(1)Exel6240 as having cardiomyopathy. Using a number of strategies including customized smaller deletions, screening of mutant alleles, and transgenic rescue, we identified CG3226 as the causative gene for this deficiency. CG3226 is an uncharacterized gene in Drosophila possessing homology to the mammalian Siah-interacting protein (SIP) gene. Mammalian SIP functions as an adaptor protein involved in one of the β-catenin degradation complexes. To investigate the effects of altering β-catenin/Armadillo signaling in the adult fly, we measured heart function in flies expressing either constitutively active Armadillo or transgenic constructs that block Armadillo signaling, specifically in the heart. While, increasing Armadillo signaling in the heart did not have an effect on adult heart function, decreasing Armadillo signaling in the fly heart caused the significant reduction in heart chamber size. In summary, we show that deletion of CG3226, which has homology to mammalian SIP, causes cardiomyopathy in adult Drosophila. Alterations in Armadillo signaling during development lead to important changes in the size and function of the adult heart.

A novel role for copper in Ras/mitogen-activated protein kinase signaling

Copper (Cu) is essential for development and proliferation, yet the cellular requirements for Cu in these processes are not well defined. We report that Cu plays an unanticipated role in the mitogen-activated protein (MAP) kinase pathway. Ablation of the Ctr1 high-affinity Cu transporter in flies and mouse cells, mutation of Ctr1, and Cu chelators all reduce the ability of the MAP kinase kinase Mek1 to phosphorylate the MAP kinase Erk. Moreover, mice bearing a cardiac-tissue-specific knockout of Ctr1 are deficient in Erk phosphorylation in cardiac tissue. in vitro investigations reveal that recombinant Mek1 binds two Cu atoms with high affinity and that Cu enhances Mek1 phosphorylation of Erk in a dose-dependent fashion. Coimmunoprecipitation experiments suggest that Cu is important for promoting the Mek1-Erk physical interaction that precedes the phosphorylation of Erk by Mek1. These results demonstrate a role for Ctr1 and Cu in activating a pathway well known to play a key role in normal physiology and in cancer.

Membrane lipid peroxidation in copper alloy-mediated contact killing of Escherichia coli

Copper alloy surfaces are passive antimicrobial sanitizing agents that kill bacteria, fungi, and some viruses. Studies of the mechanism of contact killing in Escherichia coli implicate the membrane as the target, yet the specific component and underlying biochemistry remain unknown. This study explores the hypothesis that nonenzymatic peroxidation of membrane phospholipids is responsible for copper alloy-mediated surface killing. Lipid peroxidation was monitored with the thiobarbituric acid-reactive substances (TBARS) assay. Survival, TBARS levels, and DNA degradation were followed in cells exposed to copper alloy surfaces containing 60 to 99.90% copper or in medium containing CuSO(4). In all cases, TBARS levels increased with copper exposure levels. Cells exposed to the highest copper content alloys, C11000 and C24000, exhibited novel characteristics. TBARS increased immediately at a very rapid rate but peaked at about 30 min. This peak was associated with the period of most rapid killing, loss in membrane integrity, and DNA degradation. DNA degradation is not the primary cause of copper-mediated surface killing. Cells exposed to the 60% copper alloy for 60 min had fully intact genomic DNA but no viable cells. In a fabR mutant strain with increased levels of unsaturated fatty acids, sensitivity to copper alloy surface-mediated killing increased, TBARS levels peaked earlier, and genomic DNA degradation occurred sooner than in the isogenic parental strain. Taken together, these results suggest that copper alloy surface-mediated killing of E. coli is triggered by nonenzymatic oxidative damage of membrane phospholipids that ultimately results in the loss of membrane integrity and cell death.

Detection of genetically altered copper levels in Drosophila tissues by synchrotron x-ray fluorescence microscopy

Tissue-specific manipulation of known copper transport genes in Drosophila tissues results in phenotypes that are presumably due to an alteration in copper levels in the targeted cells. However direct confirmation of this has to date been technically challenging. Measures of cellular copper content such as expression levels of copper-responsive genes or cuproenzyme activity levels, while useful, are indirect. First-generation copper-sensitive fluorophores show promise but currently lack the sensitivity required to detect subtle changes in copper levels. Moreover such techniques do not provide information regarding other relevant biometals such as zinc or iron. Traditional techniques for measuring elemental composition such as inductively coupled plasma mass spectroscopy are not sensitive enough for use with the small tissue amounts available in Drosophila research. Here we present synchrotron x-ray fluorescence microscopy analysis of two different Drosophila tissues, the larval wing imaginal disc, and sectioned adult fly heads and show that this technique can be used to detect changes in tissue copper levels caused by targeted manipulation of known copper homeostasis genes.

Distorted copper homeostasis with decreased sensitivity to cisplatin upon chaperone Atox1 deletion in Drosophila

Copper is an integral part of a number of proteins and thus an essential trace metal. However, free copper ions can be highly toxic and every organism has to carefully control its bioavailability. Eukaryotes contain three copper chaperones; Atx1p/Atox1 which delivers copper to ATP7 transporters located in the trans-Golgi network, Cox17 which provides copper to the mitochondrial cytochrome c oxidase, and CCS which is a copper chaperone for superoxide dismutase 1. Here we describe the knockout phenotype of the Drosophila homolog of mammalian Atox1 (ATX1 in yeast). Atox1-/- flies develop normally, though at reduced numbers, and the eclosing flies are fertile. However, the mutants are unable to develop on low-copper food. Furthermore, the intestinal copper importer Ctr1B, which is regulated by copper demand, fails to be induced upon copper starvation in Atox1-/- larvae. At the same time, intestinal metallothionein is upregulated. This phenotype, which resembles the one of the ATP7 mutant, is best explained by intestinal copper accumulation, combined with insufficient delivery to the rest of the body. In addition, compared to controls, Drosophila Atox1 mutants are relatively insensitive to the anticancer drug cisplatin, a compound which is also imported via Ctr1 copper transporters and was recently found to bind mammalian Atox1.

Iron depletion in the intestines of Malvolio mutant flies does not occur in the absence of a multicopper oxidase

Malvolio (Mvl) encodes the sole Drosophila melanogaster homologue of divalent metal transporter-1 (DMT1). The Drosophila transporter has been implicated in iron, manganese and copper cellular import. Indeed, the extent of metal specificity for this family of transporters is still under investigation in many eukaryotic species. Here, we revisit metal accumulation in Mvl mutants raised under normal and metal-supplemented diets. We found iron deficiency in Mvl mutant flies, whereas whole body copper and manganese concentrations remained unaltered. Iron supplementation restored total body iron concentrations in Mvl mutants, but without replenishing iron stores in the middle midgut, suggesting a role for Mvl in systemic iron trafficking, in addition to a role in intestinal iron absorption. Interestingly, dietary copper sulphate supplementation further exacerbated the iron deficiency. We investigated whether dietary copper affected iron storage through the function of an insect multicopper oxidase (MCO), because the mammalian MCO ceruloplasmin is known to regulate iron storage in the liver. We identified a Drosophila MCO mutant that suppressed aspects of the Mvl mutant phenotype and most notably Mvl, MCO3 double mutants showed normal intestinal iron storage. Therefore, MCO3 may encode an insect ferroxidase. Intriguingly, MCO3 mutants had a mild accumulation of copper, which was suppressed in Mvl mutants, revealing a reciprocal genetic interaction between the two 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.

Cardiac copper deficiency activates a systemic signaling mechanism that communicates with the copper acquisition and storage organs

Copper (Cu) is an essential cofactor for a variety of metabolic functions, and the regulation of systemic Cu metabolism is critical to human health. Dietary Cu is absorbed through the intestine, stored in the liver, and mobilized into the circulation; however, systemic Cu homeostasis is poorly understood. We generated mice with a cardiac-specific knockout of the Ctr1 Cu transporter (Ctr1(hrt/hrt)), resulting in cardiac Cu deficiency and severe cardiomyopathy. Unexpectedly, Ctr1(hrt/hrt) mice exhibited increased serum Cu levels and a concomitant decrease in hepatic Cu stores. Expression of the ATP7A Cu exporter, thought to function predominantly in intestinal Cu acquisition, was strongly increased in liver and intestine of Ctr1(hrt/hrt) mice. These studies identify ATP7A as a candidate for hepatic Cu mobilization in response to peripheral tissue demand, and illuminate a systemic regulation in which the Cu status of the heart is signaled to organs that take up and store Cu.

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.

Tryptophan scanning analysis of the membrane domain of CTR-copper transporters

Membrane proteins of the CTR family mediate cellular copper uptake in all eukaryotic cells and have been shown to participate in uptake of platinum-based anticancer drugs. Despite their importance for life and the clinical treatment of malignancies, directed biochemical studies of CTR proteins have been difficult because high-resolution structural information is missing. Building on our recent 7A structure of the human copper transporter hCTR1, we present the results of an extensive tryptophan-scanning analysis of hCTR1 and its distant relative, yeast CTR3. The comparative analysis supports our previous assignment of the transmembrane helices and shows that most functionally and structurally important residues are clustered around the threefold axis of CTR trimers or engage in helix packing interactions. The scan also identified residues that may play roles in interactions between CTR trimers and suggested that the first transmembrane helix serves as an adaptor that allows evolutionarily diverse CTRs to adopt the same overall structure. Together with previous biochemical and biophysical data, the results of the tryptophan scan are consistent with a mechanistic model in which copper transport occurs along the center of the trimer.

New developments in the regulation of intestinal copper absorption

The transition metal copper is an essential trace element involved in many enzymatic processes that require redox-chemistry. The redox-activity of copper is potentially harmful. Severe imbalance of copper homeostasis can occur with some hereditary disorders of copper metabolism. Copper is acquired from the diet by intestinal absorption and is subsequently distributed throughout the body. The regulation of intestinal copper absorption to maintain whole-body copper homeostasis is currently poorly understood. This review evaluates novel findings regarding the molecular mechanism of intestinal copper uptake. The role of recently identified transporters in enterocyte copper uptake and excretion into the portal circulation is described, and the regulation of dietary copper uptake during physiological and pathophysiological conditions is discussed.

Human copper transporter Ctr1 is functional in Drosophila, revealing a high degree of conservation between mammals and insects

Living cells have to carefully control the intracellular concentration of trace metals, especially of copper, which is at the same time essential but owing to its redox activity can also facilitate generation of reactive oxygen species. Mammals have two related copper transporters, Ctr1 and Ctr2, with Ctr1 playing the major role. The fruit fly Drosophila has three family members, termed Ctr1A, Ctr1B, and Ctr1C. Ctr1A is expressed throughout development, and a null mutation causes lethality at an early stage. Ctr1B ensures efficient copper uptake in the intestinal tract, whereas Ctr1C is mainly expressed in male gonads. Ectopic expression of Ctr1 transporters in Drosophila causes toxic effects due to excessive copper uptake. Here, we compare the effects of human Ctr1 (hCtr1) with those of the Drosophila homologs Ctr1A and Ctr1B in two overexpression assays. Whereas the overexpression of Drosophila Ctr1A and Ctr1B results in strong phenotypes, expression of hCtr1 causes only a very mild phenotype, indicating a low copper-import efficiency in the Drosophila system. However, this can be boosted by coexpressing the human copper chaperone CCS. Surprisingly, hCtr1 complements a lethal Ctr1A mutation at least as well as Ctr1A and Ctr1B transgenes. These findings reveal a high level of conservation between the mammalian and insect Ctr1-type copper importers, and they also demonstrate that the Drosophila Ctr1 proteins are functionally interchangeable.