Copper-dependent interaction of glutaredoxin with the N termini of the copper-ATPases (ATP7A and ATP7B) defective in Menkes and Wilson diseases

Roles of Copper Transport Systems Members in Breast Cancer

BACKGROUND

The occurrence and progression of breast cancer are closely linked to copper ion homeostasis. Both copper deficiency and excess can inhibit breast cancer growth, while copper transport systems may contribute to its progression by regulating copper ion transport and the activity of associated proteins. However, a comprehensive review of the roles and applications of copper transport systems in breast cancer remains limited. In this study, we summarize the workflow of copper transport systems and the dual role of copper in cancer, highlighting the contributions of specific members of the copper transport system to breast cancer.

METHODS

A comprehensive search of the PubMed database was conducted to identify articles published over the past 30 years that focus on the relationship between copper transport system members and breast cancer. The findings were synthesized to elucidate the roles and mechanisms of these transporters in the onset and progression of breast cancer.

RESULTS

We identified 13 members of the copper transport system associated with the occurrence, progression, and mortality of breast cancer, including SLC31A1, DMT1, ATP7A, ATP7B, MTs, GSH, ATOX1, CCS, COX17, SCO1, SCO2, and COX11. Our findings revealed that, apart from STEAP, the remaining 12 members were overexpressed in breast cancer. These members influence the onset, progression, and cell death of breast cancer by modulating biological pathways such as intracellular copper ion levels and ROS. Notably, we observed for the first time that depletion of the copper storage protein GSH leads to increased copper ion accumulation, resulting in cuproptosis in breast cancer cells.

CONCLUSION

By integrating the members of the copper transport system in breast cancer, we offer novel insights for the treatment of breast cancer and copper-related therapies.

Iron-tracking strategies: Chaperones capture iron in the cytosolic labile iron pool

Cells express hundreds of iron-dependent enzymes that rely on the iron cofactors heme, iron-sulfur clusters, and mono-or di-nuclear iron centers for activity. Cells require systems for both the assembly and the distribution of iron cofactors to their cognate enzymes. Proteins involved in the binding and trafficking of iron ions in the cytosol, called cytosolic iron chaperones, have been identified and characterized in mammalian cells. The first identified iron chaperone, poly C-binding protein 1 (PCBP1), has also been studied in mice using genetic models of conditional deletion in tissues specialized for iron handling. Studies of iron trafficking in mouse tissues have necessitated the development of new approaches, which have revealed new roles for PCBP1 in the management of cytosolic iron. These approaches can be applied to investigate use of other nutrient metals in mammals.

Pyrene Functionalized Luminescent Phenylalanine for Selective Detection of Copper (II) Ions in Aqueous Media

A novel pyrene-based fluorescent chemosensor 1 (pyren-1-ylmethyl)-L-phenylalanine was designed and synthesized by combining 1-pyrenecarboxyaldehyde and L-phenylalanine. 1 was characterized by several analytical methods and used as a fluorescent chemosensor for the selective and sensitive detection of Cu²⁺ ions through "turn-off" mechanism with a detection limit of 2 × 10^-8 M. 1 can also be used to detect Cu²⁺ ions in a natural water sample and exhibits gelation properties with high thermal stability.

The role of metal ions in the Golgi apparatus

The Golgi apparatus is a membrane-bound organelle that functions as a central role in the secretory pathway. Since the discovery of the Golgi apparatus, its structure and function have attracted ever-increasing attention from researchers. Recently, it has been demonstrated that metal ions are necessary for the Golgi apparatus to maintain its proper structure and functions. Given that metal ions play an important role in various biological processes, their abnormal homeostasis is related to many diseases. Therefore, in this paper, we reviewed the uptake and release mechanisms of the Golgi apparatus Ca²⁺ , Cu, and Zn²⁺ . Furthermore, we describe the diseases associated with Golgi apparatus Ca²⁺ , Cu, and Zn²⁺ imbalance.

Do glutathione and copper interact to modify Alzheimer's disease pathogenesis?

Alzheimer's disease (AD) is a progressive neurodegenerative disorder first described in 1906 that is currently estimated to impact ∼40 million people worldwide. Extensive research activities have led to a wealth of information on the pathogenesis, hallmarks, and risk factors of AD; however, therapeutic options remain extremely limited. The large number of pathogenic factors that have been reported to potentially contribute to AD include copper dyshomeostasis as well as increased oxidative stress, which is related to alterations to molecular antioxidants like glutathione (GSH). While the individual roles of GSH and copper in AD have been studied by many research groups, their interactions have received relatively little attention, although they appear to interact and affect each other's regulation. Existing knowledge on how GSH-copper interactions may affect AD is sparse and lacks focus. This review first highlights the most relevant individual roles that GSH and copper play in physiology and AD, and then collects and assesses research concerning their interactions, in an effort to provide a more accessible and understandable picture of the role of GSH, copper, and their interactions in AD.

Aza-Crown-Based Macrocyclic Probe Design for "PET-off" Multi-Cu2+ Responsive and "CHEF-on" Multi-Zn2+ Sensor: Application in Biological Cell Imaging and Theoretical Studies

The work represents a rare example of an aza-crown-based macrocyclic chemosensor, H2DTC (H2DTC = 1,16-dihydroxy-tetraaza-30-crown-8) for the selective detection of both Zn2+ and Cu2+ in HEPES buffer medium (pH 7.4). H2DTC exhibits a fluorescence response for both Zn2+ and Cu2+ ions. The reversibility of the chemosensor in its binding with Zn2+ and Cu2+ ions is also examined using a Na2EDTA solution. H2DTC exhibits a chelation-enhanced fluorescence (CHEF) effect in the presence of Zn2+ ions and a quenching effect (CHEQ) in the presence of paramagnetic Cu2+ ions. Furthermore, the geometry and spectral properties of H2DTC and the chemosensor bound to Zn2+ have been studied by DFT and TDDFT calculations. The limit of detection (LOD) values are 0.11 × 10-9 and 0.27 × 10-9 M for Cu2+ and Zn2+, respectively. The formation constants for the Zn2+ and Cu2+ complexes have been measured by pH-potentiometry in 0.15 M NaCl in 70:30 (v:v) water:ethanol at 298.1 K. UV-vis absorption and fluorometric spectral data and pH-potentiometric titrations indicate 1:1 and 2:1 metal:chemosensor species. In the solid state H2DTC is able to accommodate up to four metal ions, as proved by the crystal structures of the complexes [Zn4(DTC)(OH)2(NO3)4] (1) and {[Cu4(DTC)(OCH3)2(NO3)4]·H2O}n (2). H2DTC can be used as a potential chemosensor for monitoring Zn2+ and Cu2+ ions in biological and environmental media with outstanding accuracy and precision. The propensity of H2DTC to detect intracellular Cu2+ and Zn2+ ions in the triple negative human breast cancer cell line MDA-MB-468 and in HeLa cells has been determined by fluorescence cell imaging.

Redox-Active Metal Ions and Amyloid-Degrading Enzymes in Alzheimer's Disease

Redox-active metal ions, Cu(I/II) and Fe(II/III), are essential biological molecules for the normal functioning of the brain, including oxidative metabolism, synaptic plasticity, myelination, and generation of neurotransmitters. Dyshomeostasis of these redox-active metal ions in the brain could cause Alzheimer’s disease (AD). Thus, regulating the levels of Cu(I/II) and Fe(II/III) is necessary for normal brain function. To control the amounts of metal ions in the brain and understand the involvement of Cu(I/II) and Fe(II/III) in the pathogenesis of AD, many chemical agents have been developed. In addition, since toxic aggregates of amyloid-β (Aβ) have been proposed as one of the major causes of the disease, the mechanism of clearing Aβ is also required to be investigated to reveal the etiology of AD clearly. Multiple metalloenzymes (e.g., neprilysin, insulin-degrading enzyme, and ADAM10) have been reported to have an important role in the degradation of Aβ in the brain. These amyloid degrading enzymes (ADE) could interact with redox-active metal ions and affect the pathogenesis of AD. In this review, we introduce and summarize the roles, distributions, and transportations of Cu(I/II) and Fe(II/III), along with previously invented chelators, and the structures and functions of ADE in the brain, as well as their interrelationships.

Human Amniotic Epithelial Cells as a Tool to Investigate the Effects of Cyanidin 3-O-Glucoside on Cell Differentiation

Cyanidin, a kind of anthocyanin, has been reported to have chemotherapeutic activities in humans. Human amniotic epithelial cells (hAECs) are considered a potential source of pluripotent stem cells. hAECs have been used as a novel tool in regenerative cellular therapy and cell differentiation studies. In this study, to explore the effects of cyanidin-3-O-glucoside (Cy3G) on hAECs and their mechanisms, we investigated the transcriptomic changes in the Cy3G-treated cells using microarray analysis. Among the differentially expressed genes (Fold change > 1.1; p-value < 0.05), 109 genes were upregulated and 232 were downregulated. Ratios of upregulated and downregulated genes were 0.22% and 0.47% of the total expressed genes, respectively. Next, we explored the enriched gene ontology, i.e., the biological process, molecular function, and cellular component of the 37 upregulated (>1.3-fold change) and 124 downregulated (<1.3-fold change) genes. Significantly enriched biological processes by the upregulated genes included "response to muscle activity," and the genes involved in this gene ontology (GO) were Metrnl and SRD5A1, which function in the adipocyte. On the other hand, the cell cycle biological process was significantly enriched by the downregulated genes, including some from the SMC gene family. An adipogenesis-associated gene DDX6 was also included in the cell cycle biological process. Thus, our findings suggest the prospects of Cy3G in modulating adipocyte differentiation in hAECs.

Highly selective detection of Cu2+ in aqueous media based on Tb3+-functionalized metal-organic framework

In this work, a metal-organic framework UiO-66-(COOH)₂ has been synthesized and is further functionalized with Tb³⁺ through coordination interactions. The functionalized MOF, denoted as Tb³⁺@UiO-66-(COOH)₂, is fully characterized and further developed as an excellent fluorescent probe to monitor Cu²⁺ ions in aqueous media by fluorescence quenching effect. Tb³⁺@UiO-66-(COOH)₂ exhibits high selectivity and sensitivity, broad linear concentration range (0-200 μM), low detection limits (0.23 μM), fast response speed (within 1 min), as well as in situ naked eye observation under UV light for sensing Cu²⁺ ion. Furthermore, this probe was successfully employed to detect Cu²⁺ ion in real water with good recovery. Hence, this work developed a very excellent fluorescent sensor with high potential practical applications for detection of Cu²⁺ ion in environmental water samples.

Mammalian copper homeostasis requires retromer-dependent recycling of the high-affinity copper transporter 1

The concentration of essential micronutrients, such as copper (used here to describe both Cu⁺ and Cu²⁺), within the cell is tightly regulated to avoid their adverse deficiency and toxicity effects. Retromer-mediated sorting and recycling of nutrient transporters within the endo-lysosomal network is an essential process in regulating nutrient balance. Cellular copper homeostasis is regulated primarily by two transporters: the copper influx transporter copper transporter 1 (CTR1; also known as SLC31A1), which controls the uptake of copper, and the copper-extruding ATPase ATP7A, a recognised retromer cargo. Here, we show that in response to fluctuating extracellular copper, retromer controls the delivery of CTR1 to the cell surface. Following copper exposure, CTR1 is endocytosed to prevent excessive copper uptake. We reveal that internalised CTR1 localises on retromer-positive endosomes and, in response to decreased extracellular copper, retromer controls the recycling of CTR1 back to the cell surface to maintain copper homeostasis. In addition to copper, CTR1 plays a central role in the trafficking of platinum. The efficacy of platinum-based cancer drugs has been correlated with CTR1 expression. Consistent with this, we demonstrate that retromer-deficient cells show reduced sensitivity to the platinum-based drug cisplatin.

Summary: CTR1 (SLC31A1) is the only known mammalian importer of copper. We show that CTR1 is a retromer complex cargo protein, and that retromer is required for cellular sensitivity to extracellular copper.

Human glutaredoxin-1 can transfer copper to isolated metal binding domains of the P1B-type ATPase, ATP7B

Intracellular copper (Cu) in eukaryotic organisms is regulated by homeostatic systems, which rely on the activities of soluble metallochaperones that participate in Cu exchange through highly tuned protein-protein interactions. Recently, the human enzyme glutaredoxin-1 (hGrx1) has been shown to possess Cu metallochaperone activity. The aim of this study was to ascertain whether hGrx1 can act in Cu delivery to the metal binding domains (MBDs) of the P_1B-type ATPase ATP7B and to determine the thermodynamic factors that underpin this activity. hGrx1 can transfer Cu to the metallochaperone Atox1 and to the MBDs 5-6 of ATP7B (WLN5-6). This exchange is irreversible. In a mixture of the three proteins, Cu is delivered to the WLN5-6 preferentially, despite the presence of Atox1. This preferential Cu exchange appears to be driven by both the thermodynamics of the interactions between the proteins pairs and of the proteins with Cu(I). Crucially, protein-protein interactions between hGrx1, Atox1 and WLN5-6 were detected by NMR spectroscopy both in the presence and absence of Cu at a common interface. This study augments the possible activities of hGrx1 in intracellular Cu homeostasis and suggests a potential redundancy in this system, where hGrx1 has the potential to act under cellular conditions where the activity of Atox1 in Cu regulation is attenuated.

Visualization of the Redox Status of Cytosolic Glutathione Using the Organelle- and Cytoskeleton-Targeted Redox Sensors

Glutathione is a small thiol-containing peptide that plays a central role in maintaining cellular redox homeostasis. Glutathione serves as a physiologic redox buffer by providing thiol electrons for catabolizing harmful oxidants and reversing oxidative effects on biomolecules. Recent evidence suggests that the balance of reduced and oxidized glutathione (GSH/GSSG) defines the redox states of Cys residues in proteins and fine-tunes their stabilities and functions. To elucidate the redox balance of cellular glutathione at subcellular resolution, a number of redox-sensitive green fluorescent protein (roGFP) variants have been developed. In this study, we constructed and functionally validated organelle- and cytoskeleton-targeted roGFP and elucidated the redox status of the cytosolic glutathione at a subcellular resolution. These new redox sensors firmly established a highly reduced redox equilibrium of cytosolic glutathione, wherein significant deviation was observed among cells. By targeting the sensor to the cytosolic and lumen sides of the Golgi membrane, we identified a prominent redox gradient across the biological membrane at the Golgi body. The results demonstrated that organelle- and cytoskeleton-targeted sensors enable the assessment of glutathione oxidation near the cytosolic surfaces of different organelle membranes.

From and to the Golgi - defining the Wilson disease protein road map

Recent studies highlight the continued growth in the identification of a variety of cellular functions that involve the Golgi apparatus. Apart from well-known membrane sorting/trafficking and glycosylation machineries, the Golgi harbors molecular platforms operating in intracellular signaling, cytoskeleton organization, and protein quality control mechanisms. One of new emerging Golgi functions consists in the regulation of copper homeostasis by coordinating the relocation and activity of copper transporters. Of these, the Cu-transporting ATPase ATP7B (known as Wilson disease protein) plays a key role in the maintenance of the Cu balance in the body via the supply of essential Cu to the systemic circulation and via elimination of excess Cu into the bile. These activities require tightly regulated shuttling of ATP7B between the Golgi and different post-Golgi compartments. Despite significant progress over recent years, a number of issues regarding ATP7B trafficking remain to be clarified. This review summarizes current views on ATP7B trafficking pathways from and to the Golgi and underscores the challenges that should be addressed to define the ATP7B trafficking routes and mechanisms in health and disease.

Trafficking mechanisms of P-type ATPase copper transporters

Copper is an essential micronutrient required for oxygen-dependent enzymes, yet excess of the metal is a toxicant. The tug-of-war between these copper activities is balanced by chaperones and membrane transporters, which control copper distribution and availability. The P-type ATPase transporters, ATP7A and ATP7B, regulate cytoplasmic copper by pumping copper out of cells or into the endomembrane system. Mutations in ATP7A and ATP7B cause diseases that share neuropsychiatric phenotypes, which are similar to phenotypes observed in mutations affecting cytoplasmic trafficking complexes required for ATP7A/B dynamics. Here, we discuss evidence indicating that phenotypes associated to genetic defects in trafficking complexes, such as retromer and the adaptor complex AP-1, result in part from copper dyshomeostasis due to mislocalized ATP7A and ATP7B.

Molecular Mechanisms of Glutaredoxin Enzymes: Versatile Hubs for Thiol-Disulfide Exchange between Protein Thiols and Glutathione

The tripeptide glutathione (GSH) and its oxidized form glutathione disulfide (GSSG) constitute a key redox couple in cells. In particular, they partner protein thiols in reversible thiol-disulfide exchange reactions that act as switches in cell signaling and redox homeostasis. Disruption of these processes may impair cellular redox signal transduction and induce redox misbalances that are linked directly to aging processes and to a range of pathological conditions including cancer, cardiovascular diseases and neurological disorders. Glutaredoxins are a class of GSH-dependent oxidoreductase enzymes that specifically catalyze reversible thiol-disulfide exchange reactions between protein thiols and the abundant thiol pool GSSG/GSH. They protect protein thiols from irreversible oxidation, regulate their activities under a variety of cellular conditions and are key players in cell signaling and redox homeostasis. On the other hand, they may also function as metal-binding proteins with a possible role in the cellular homeostasis and metabolism of essential metals copper and iron. However, the molecular basis and underlying mechanisms of glutaredoxin action remain elusive in many situations. This review focuses specifically on these aspects in the context of recent developments that illuminate some of these uncertainties.

Cellular copper homeostasis: current concepts on its interplay with glutathione homeostasis and its implication in physiology and human diseases

Copper is a trace element essential for almost all living organisms. But the level of intracellular copper needs to be tightly regulated. Dysregulation of cellular copper homeostasis leading to various diseases demonstrates the importance of this tight regulation. Copper homeostasis is regulated not only within the cell but also within individual intracellular compartments. Inactivation of export machinery results in excess copper being redistributed into various intracellular organelles. Recent evidence suggests the involvement of glutathione in playing an important role in regulating copper entry and intracellular copper homeostasis. Therefore interplay of both homeostases might play an important role within the cell. Similar to copper, glutathione balance is tightly regulated within individual cellular compartments. This review explores the existing literature on the role of glutathione in regulating cellular copper homeostasis. On the one hand, interplay of glutathione and copper homeostasis performs an important role in normal physiological processes, for example neuronal differentiation. On the other hand, perturbation of the interplay might play a key role in the pathogenesis of copper homeostasis disorders.

A glimpse into the regulation of the Wilson disease protein, ATP7B, sheds light on the complexity of mammalian apical trafficking pathways

Wilson disease (WD), a Mendelian disorder of copper metabolism caused by mutations in the ATP7B gene, manifests a large spectrum of phenotypic variability. This phenomenon of extensive symptom variation is not frequently associated with a monogenic disorder. We hypothesize that the phenotypic variability in WD is primarily driven by the variations in interacting proteins that regulate the ATP7B function and localization in the cell. Based on existing literature, we delineated a potential molecular mechanism for ATP7B mediated copper transport in the milieu of its interactome, its dysfunction in WD and the resulting variability in the phenotypic manifestation. Understanding the copper-induced apical trafficking of ATP7B also significantly contributes to the appreciation of the complexities of the ligand-induced transport pathway. We believe that this holistic view of WD will pave the way for a better opportunity for rational drug design and therapeutics.

Rare Disease Mechanisms Identified by Genealogical Proteomics of Copper Homeostasis Mutant Pedigrees

Rare neurological diseases shed light onto universal neurobiological processes. However, molecular mechanisms connecting genetic defects to their disease phenotypes are elusive. Here, we obtain mechanistic information by comparing proteomes of cells from individuals with rare disorders with proteomes from their disease-free consanguineous relatives. We use triple-SILAC mass spectrometry to quantify proteomes from human pedigrees affected by mutations in ATP7A, which cause Menkes disease, a rare neurodegenerative and neurodevelopmental disorder stemming from systemic copper depletion. We identified 214 proteins whose expression was altered in ATP7A^-/y fibroblasts. Bioinformatic analysis of ATP7A-mutant proteomes identified known phenotypes and processes affected in rare genetic diseases causing copper dyshomeostasis, including altered mitochondrial function. We found connections between copper dyshomeostasis and the UCHL1/PARK5 pathway of Parkinson disease, which we validated with mitochondrial respiration and Drosophila genetics assays. We propose that our genealogical "omics" strategy can be broadly applied to identify mechanisms linking a genomic locus to its phenotypes.

The six metal binding domains in human copper transporter, ATP7B: molecular biophysics and disease-causing mutations

Wilson Disease (WD) is a hereditary genetic disorder, which coincides with a dysfunctional copper (Cu) metabolism caused by mutations in ATP7B, a membrane-bound P1B-type ATPase responsible for Cu export from hepatic cells. The N-terminal part (~ 600 residues) of the multi-domain 1400-residue ATP7B constitutes six metal binding domains (MBDs), each of which can bind a copper ion, interact with other ATP7B domains as well as with different proteins. Although the ATP7B's MBDs have been investigated in vitro and in vivo intensively, it remains unclear how these domains modulate overall structure, dynamics, stability and function of ATP7B. The presence of six MBDs is unique to mammalian ATP7B homologs, and many WD causing missense mutations are found in these domains. Here, we have summarized previously reported in vitro biophysical data on the MBDs of ATP7B and WD point mutations located in these domains. Besides the demonstration of where the research field stands today, this review showcasts the need for further biophysical investigation about the roles of MBDs in ATP7B function. Molecular mechanisms of ATP7B are important not only in the development of new WD treatment but also for other aspects of human physiology where Cu transport plays a role.

Thiol-based copper handling by the copper chaperone Atox1

Human antioxidant protein 1 (Atox1) plays a crucial role in cellular copper homeostasis. Atox1 captures cytosolic copper for subsequent transfer to copper pumps in trans Golgi network, thereby facilitating copper supply to various copper-dependent oxidereductases matured within the secretory vesicles. Atox1 and other copper chaperones handle cytosolic copper using Cys thiols which are ideal ligands for coordinating Cu(I). Recent studies demonstrated reversible oxidation of these Cys residues in copper chaperones, linking cellular redox state to copper homeostasis. Highlighted in this review are unique redox properties of Atox1 and other copper chaperones. Also, summarized are the redox nodes in the cytosol which potentially play dominant roles in the redox regulation of copper chaperones. © 2016 IUBMB Life, 69(4):246-254, 2017.

Orchestration of dynamic copper navigation – new and missing pieces

A general principle in all cells in the body is that an essential metal – here copper – is taken up at the plasma membrane, directed through cellular compartments for use in specific enzymes and pathways, stored in specific scavenging molecules if in surplus, and finally expelled from the cells.

The Role of Copper Chaperone Atox1 in Coupling Redox Homeostasis to Intracellular Copper Distribution

Human antioxidant protein 1 (Atox1) is a small cytosolic protein with an essential role in copper homeostasis. Atox1 functions as a copper carrier facilitating copper transfer to the secretory pathway. This process is required for activation of copper dependent enzymes involved in neurotransmitter biosynthesis, iron efflux, neovascularization, wound healing, and regulation of blood pressure. Recently, new cellular roles for Atox1 have emerged. Changing levels of Atox1 were shown to modulate response to cancer therapies, contribute to inflammatory response, and protect cells against various oxidative stresses. It has also become apparent that the activity of Atox1 is tightly linked to the cellular redox status. In this review, we summarize biochemical information related to a dual role of Atox1 as a copper chaperone and an antioxidant. We discuss how these two activities could be linked and contribute to establishing the intracellular copper balance and functional identity of cells during differentiation.

Metallo-pathways to Alzheimer's disease: lessons from genetic disorders of copper trafficking

Copper is an essential metal ion that provides catalytic function to numerous enzymes and also regulates neurotransmission and intracellular signaling. Conversely, a deficiency or excess of copper can cause chronic disease in humans. Menkes and Wilson disease are two rare heritable disorders of copper transport that are characterized by copper deficiency and copper overload, respectively. Changes to copper status are also a common feature of several neurodegenerative disorders including Alzheimer's disease (AD), Parkinson's disease (PD) and Amyotrophic lateral sclerosis (ALS). In the case of AD, which is characterized by brain copper depletion, changes in the distribution of copper has been linked with various aspects of the disease process; protein aggregation, defective protein degradation, oxidative stress, inflammation and mitochondrial dysfunction. Although AD is a multifactorial disease that is likely caused by a breakdown in multiple cellular pathways, copper and other metal ions such as iron and zinc play a central role in many of these cellular processes. Pioneering work by researchers who have studied relatively rare copper transport diseases has shed light on potential metal ion related disease mechanisms in other forms of neurodegeneration such as AD.

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.

Myosin Vb mediates Cu+ export in polarized hepatocytes

The cellular machinery responsible for Cu(+)-stimulated delivery of the Wilson-disease-associated protein ATP7B to the apical domain of hepatocytes is poorly understood. We demonstrate that myosin Vb regulates the Cu(+)-stimulated delivery of ATP7B to the apical domain of polarized hepatic cells, and that disruption of the ATP7B-myosin Vb interaction reduces the apical surface expression of ATP7B. Overexpression of the myosin Vb tail, which competes for binding of subapical cargos to myosin Vb bound to subapical actin, disrupted the surface expression of ATP7B, leading to reduced cellular Cu(+) export. The myosin-Vb-dependent targeting step occurred in parallel with hepatocyte-like polarity. If the myosin Vb tail was expressed acutely in cells just prior to the establishment of polarity, it appeared as part of an intracellular apical compartment, centered on γ-tubulin. ATP7B became selectively arrested in this compartment at high [Cu(+)] in the presence of myosin Vb tail, suggesting that these compartments are precursors of donor-acceptor transfer stations for apically targeted cargos of myosin Vb. Our data suggest that reduced hepatic Cu(+) clearance in idiopathic non-Wilsonian types of disease might be associated with the loss of function of myosin Vb.

Rhodamine-modified upconversion nanoprobe for distinguishing Cu2+ from Hg2+ and live cell imaging

A new organic–inorganic hybrid nanoprobe based on luminescence resonance energy transfer (LRET) from mesoporous silica coated upconversion nanoparticles to a rhodamine B derivative was prepared for distinguishing Cu²⁺ from Hg²⁺ and live cell imaging applications.

Redox sulfur chemistry of the copper chaperone Atox1 is regulated by the enzyme glutaredoxin 1, the reduction potential of the glutathione couple GSSG/2GSH and the availability of Cu(I)

Glutaredoxins have been characterised as enzymes regulating the redox status of protein thiols via cofactors GSSG/GSH. However, such a function has not been demonstrated with physiologically relevant protein substrates in in vitro experiments. Their active sites frequently feature a Cys-xx-Cys motif that is predicted not to bind metal ions. Such motifs are also present in copper-transporting proteins such as Atox1, a human cytosolic copper metallo-chaperone. In this work, we present the first demonstration that: (i) human glutaredoxin 1 (hGrx1) efficiently catalyses interchange of the dithiol and disulfide forms of the Cys(12)-xx-Cys(15) fragment in Atox1 but does not act upon the isolated single residue Cys(41); (ii) the direction of catalysis is regulated by the GSSG/2GSH ratio and the availability of Cu(I); (iii) the active site Cys(23)-xx-Cys(26) in hGrx1 can bind Cu(I) tightly with femtomolar affinity (K(D) = 10(-15.5) M) and possesses a reduction potential of E(o)' = -118 mV at pH 7.0. In contrast, the Cys(12)-xx-Cys(15) motif in Atox1 has a higher affinity for Cu(I) (K(D) = 10(-17.4) M) and a more negative potential (E(o)' = -188 mV). These differences may be attributed primarily to the very low pKa of Cys23 in hGrx1 and allow rationalisation of conclusion (ii) above: hGrx1 may catalyse the oxidation of Atox1(dithiol) by GSSG, but not the complementary reduction of the oxidised Atox1(disulfide) by GSH unless Cu(aq)(+) is present at a concentration that allows binding of Cu(I) to reduced Atox1 but not to hGrx1. In fact, in the latter case, the catalytic preferences are reversed. Both Cys residues in the active site of hGrx1 are essential for the high affinity Cu(I) binding but the single Cys(23) residue only is required for the redox catalytic function. The molecular properties of both Atox1 and hGrx1 are consistent with a correlation between copper homeostasis and redox sulfur chemistry, as suggested by recent cell experiments. These proteins appear to have evolved the features necessary to fill multiple roles in redox regulation, Cu(I) buffering and Cu(I) transport.

An expanding range of functions for the copper chaperone/antioxidant protein Atox1

SIGNIFICANCE

Antioxidant protein 1 (Atox1 in human cells) is a copper chaperone for the copper export pathway with an essential role in cellular copper distribution. In vitro, Atox1 binds and transfers copper to the copper-transporting ATPases, stimulating their catalytic activity. Inactivation of Atox1 in cells inhibits maturation of secreted cuproenzymes as well as copper export from cells.

RECENT ADVANCES

Accumulating data suggest that cellular functions of Atox1 are not limited to its copper-trafficking role and may include storage of labile copper, modulation of transcription, and antioxidant defense. The conserved metal binding site of Atox1, CxGC, differs from the metal-binding sites of copper-transporting ATPases and has a physiologically relevant redox potential that equilibrates with the GSH:GSSG pair.

CRITICAL ISSUES

Tight relationship appears to exist between intracellular copper levels and glutathione (GSH) homeostasis. The biochemical properties of Atox1 place it at the intersection of cellular networks that regulate copper distribution and cellular redox balance. Mechanisms through which Atox1 facilitates copper export and contributes to oxidative defense are not fully understood.

FUTURE DIRECTIONS

The current picture of cellular redox homeostasis and copper physiology will be enhanced by further mechanistic studies of functional interactions between the GSH:GSSG pair and copper-trafficking machinery.

Role of the P-Type ATPases, ATP7A and ATP7B in brain copper homeostasis

Over the past two decades there have been significant advances in our understanding of copper homeostasis and the pathological consequences of copper dysregulation. Cumulative evidence is revealing a complex regulatory network of proteins and pathways that maintain copper homeostasis. The recognition of copper dysregulation as a key pathological feature in prominent neurodegenerative disorders such as Alzheimer's, Parkinson's, and prion diseases has led to increased research focus on the mechanisms controlling copper homeostasis in the brain. The copper-transporting P-type ATPases (copper-ATPases), ATP7A and ATP7B, are critical components of the copper regulatory network. Our understanding of the biochemistry and cell biology of these complex proteins has grown significantly since their discovery in 1993. They are large polytopic transmembrane proteins with six copper-binding motifs within the cytoplasmic N-terminal domain, eight transmembrane domains, and highly conserved catalytic domains. These proteins catalyze ATP-dependent copper transport across cell membranes for the metallation of many essential cuproenzymes, as well as for the removal of excess cellular copper to prevent copper toxicity. A key functional aspect of these copper transporters is their copper-responsive trafficking between the trans-Golgi network and the cell periphery. ATP7A- and ATP7B-deficiency, due to genetic mutation, underlie the inherited copper transport disorders, Menkes and Wilson diseases, respectively. Their importance in maintaining brain copper homeostasis is underscored by the severe neuropathological deficits in these disorders. Herein we will review and update our current knowledge of these copper transporters in the brain and the central nervous system, their distribution and regulation, their role in normal brain copper homeostasis, and how their absence or dysfunction contributes to disturbances in copper homeostasis and neurodegeneration.

Molecular events initiating exit of a copper-transporting ATPase ATP7B from the trans-Golgi network

The copper-transporting ATPase ATP7B has a dual intracellular localization: the trans-Golgi network (TGN) and cytosolic vesicles. Changes in copper levels, kinase-mediated phosphorylation, and mutations associated with Wilson disease alter the steady-state distribution of ATP7B between these compartments. To identify a primary molecular event that triggers ATP7B exit from the TGN, we characterized the folding, activity, and trafficking of the ATP7B variants with mutations within the regulatory N-terminal domain (N-ATP7B). We found that structural changes disrupting the inter-domain contacts facilitate ATP7B exit from the TGN. Mutating Ser-340/341 in the N-ATP7B individually or together to Ala, Gly, Thr, or Asp produced active protein and shifted the steady-state localization of ATP7B to vesicles, independently of copper levels. The Ser340/341G mutant had a lower kinase-mediated phosphorylation under basal conditions and no copper-dependent phosphorylation. Thus, negative charges introduced by copper-dependent phosphorylation are not obligatory for ATP7B trafficking from the TGN. The Ser340/341A mutation did not alter the overall fold of N-ATP7B, but significantly decreased interactions with the nucleotide-binding domain, mimicking consequences of copper binding to N-ATP7B. We propose that structural changes that specifically alter the inter-domain contacts initiate exit of ATP7B from the TGN, whereas increased phosphorylation may be needed to maintain an open interface between the domains.

The CXXC motifs in the metal binding domains are required for ATP7B to mediate resistance to cisplatin

The copper (Cu) exporter ATP7B mediates resistance to cisplatin (cDDP) but details of the mechanism are unknown. We explored the role of the CXXC motifs in the metal binding domains (MBDs) of ATP7B by investigating binding of cDDP to the sixth metal binding domain (MBD6) or a variant in which the CXXC motif was converted to SXXS. Platinum measurement showed that cDDP bound to wild type MBD6 but not to the SXXS variant. Wild type ATP7B rendered ovarian 2008 cells resistant to cDDP. In 2008 and in HEK293T cells, wild type ATP7B trafficked from TGN to peripheral locations in response to Cu or cDDP. A variant in which the CXXC motifs in all 6 MBDs were converted to SXXS localized correctly to the TGN but failed to traffic when exposed to either Cu or cDDP. Deletion of either the first 5 MBDs or all 6 MBDs resulted in failure to localize to the TGN. Neither the SXXS variant nor the deletion variant was able to mediate resistance to cDDP. We conclude that cDDP binds to the CXXC motifs of ATP7B and that this interaction is essential to the trafficking of ATP7B and to its ability to mediate resistance to cDDP.

Roles of COMM-domain-containing 1 in stability and recruitment of the copper-transporting ATPase in a mouse hepatoma cell line

A novel function of COMMD1 {COMM [copper metabolism MURR1 (mouse U2af1-rs1 region 1)]-domain-containing 1}, a protein relevant to canine copper toxicosis, was examined in the mouse hepatoma cell line Hepa 1-6 with multi-disciplinary techniques consisting of molecular and cellular biological techniques, speciation and elemental imaging. To clarify the function of COMMD1, COMMD1-knockdown was accomplished by introducing siRNA (small interfering RNA) into the cells. Although COMMD1-knockdown did not affect copper incorporation, it inhibited copper excretion, resulting in copper accumulation, which predominantly existed in the form bound to MT (metallothionein). It is known that the liver copper transporter Atp7b (ATP-dependent copper transporter 7beta), localizes on the trans-Golgi network membrane under basal copper conditions and translocates to cytoplasmic vesicles to excrete copper when its concentration exceeds a certain threshold, with the vesicles dispersing in the periphery of the cell. COMMD1-knockdown reduced the expression of Atp7b, and abolished the relocation of Atp7b back from the periphery to the trans-Golgi network membrane when the copper concentration was reduced by treatment with a Cu(I) chelator. The same phenomena were observed during COMMD1-knockdown when another Atp7b substrate, cis-diamminedichloroplatinum, and its sequestrator, glutathione ethylester, were applied. These results suggest that COMMD1 maintains the amount of Atp7b and facilitates recruitment of Atp7b from cytoplasmic vesicles to the trans-Golgi network membrane, i.e. COMMD1 is required to shuttle Atp7b when the intracellular copper level returns below the threshold.

Role of glutaredoxin1 and glutathione in regulating the activity of the copper-transporting P-type ATPases, ATP7A and ATP7B

The copper-transporting P-type ATPases (Cu-ATPases), ATP7A and ATP7B, are essential for the regulation of intracellular copper homeostasis. In this report we describe new roles for glutathione (GSH) and glutaredoxin1 (GRX1) in Cu homeostasis through their regulation of Cu-ATPase activity. GRX1 is a thiol oxidoreductase that catalyzes the reversible reduction of GSH-mixed disulfides to their respective sulfhydryls (deglutathionylation). Here, we demonstrated that glutathionylation of the Cu-ATPases and their interaction with GRX1 were affected by alterations in Cu levels. The data support our hypothesis that the Cu-ATPases serve as substrates for Cu-dependent GRX1-mediated deglutathionylation. This in turn liberates the Cu-ATPase cysteinyl thiol groups for Cu binding and transport. GSH depletion experiments led to reversible inhibition of the Cu-ATPases that correlated with effects on intracellular Cu levels and GRX1 activity. Finally, knockdown of GRX1 expression resulted in an increase in intracellular Cu accumulation. Together, these data directly implicate GSH and GRX1 with important new roles in redox regulation of the Cu-ATPases, through modulation of Cu binding by the Cu-ATPase cysteine motifs.

Interactions between copper-binding sites determine the redox status and conformation of the regulatory N-terminal domain of ATP7B

Copper-transporting ATPase ATP7B is essential for human copper homeostasis and normal liver function. ATP7B has six N-terminal metal-binding domains (MBDs) that sense cytosolic copper levels and regulate ATP7B. The mechanism of copper sensing and signal integration from multiple MBDs is poorly understood. We show that MBDs communicate and that this communication determines the oxidation state and conformation of the entire N-terminal domain of ATP7B (N-ATP7B). Mutations of copper-coordinating Cys to Ala in any MBD (2, 3, 4, or 6) change the N-ATP7B conformation and have distinct functional consequences. Mutating MBD2 or MBD3 causes Cys oxidation in other MBDs and loss of copper binding. In contrast, mutation of MBD4 and MBD6 does not alter the redox status and function of other sites. Our results suggest that MBD2 and MBD3 work together to regulate access to other metal-binding sites, whereas MBD4 and MBD6 receive copper independently, downstream of MBD2 and MBD3. Unlike Ala substitutions, the Cys-to-Ser mutation in MBD2 preserves the conformation and reduced state of N-ATP7B, suggesting that hydrogen bonds contribute to interdomain communications. Tight coupling between MBDs suggests a mechanism by which small changes in individual sites (induced by copper binding or mutation) result in stabilization of distinct conformations of the entire N-ATP7B and altered exposure of sites for interactions with regulatory proteins.

Novel perspectives in mammalian copper metabolism through the use of genome-wide approaches

The transition metal copper plays an essential role in many biological processes but is highly toxic in excess. Recent studies have characterized a highly conserved set of proteins that mediate cellular copper import, distribution, sequestration, utilization, and export. Nevertheless, the pathogenesis of copper overload and copper deficiency disorders is not well understood, and we are only beginning to comprehend the results of mild copper overload or deficiency in relation to nutritional uptake and common diseases at the population level. Technological advances open the possibility to dissect the complete genome for genetic variants predisposing to copper overload or depletion and for variations in gene expression generated by either reduced or excessive copper intake. We discuss the potential of integrated genome-wide applications to advance our knowledge of copper homeostasis and to develop molecular biomarker profiles as indicators of copper status.

Expressional control of a cadmium-transporting P1B-type ATPase by a metal sensing degradation signal

Cadmium is a highly toxic environmental contaminant implicated in various diseases. Our previous data demonstrated that Pca1, a P1B-type ATPase, plays a critical role in cadmium resistance in yeast S. cerevisiae by extruding intracellular cadmium. This illustrates the first cadmium-specific efflux pump in eukaryotes. In response to cadmium, yeast cells rapidly enhance expression of Pca1 by a post-transcriptional mechanism. To gain mechanistic insights into the cadmium-dependent control of Pca1 expression, we have characterized the pathway for Pca1 turnover and the mechanism of cadmium sensing that leads to up-regulation of Pca1. Pca1 is a short-lived protein (t1/2 < 5 min) and is subject to ubiquitination when cells are growing in media lacking cadmium. Distinct from many plasma membrane transporters targeted to the vacuole for degradation via endocytosis, cells defective in this pathway did not stabilize Pca1. Rather, Pca1 turnover was dependent on the proteasome. These data suggest that, in the absence of cadmium, Pca1 is targeted for degradation before reaching the plasma membrane. Mapping of the N terminus of Pca1 identified a metal-responding degradation signal encompassing amino acids 250-350. Fusion of this domain to a stable protein demonstrated that it functions autonomously in a metal-responsive manner. Cadmium sensing by cysteine residues within this domain circumvents ubiquitination and degradation of Pca1. These data reveal a new mechanism for substrate-mediated control of P1B-type ATPase expression. Cells have likely evolved this mode of regulation for a rapid and specific cellular response to cadmium.

Mitochondrial copper(I) transfer from Cox17 to Sco1 is coupled to electron transfer

The human protein Cox17 contains three pairs of cysteines. In the mitochondrial intermembrane space (IMS) it exists in a partially oxidized form with two S-S bonds and two reduced cysteines (HCox17(2S-S)). HCox17(2S-S) is involved in copper transfer to the human cochaperones Sco1 and Cox11, which are implicated in the assembly of cytochrome c oxidase. We show here that Cu(I)HCox17(2S-S), i.e., the copper-loaded form of the protein, can transfer simultaneously copper(I) and two electrons to the human cochaperone Sco1 (HSco1) in the oxidized state, i.e., with its metal-binding cysteines forming a disulfide bond. The result is Cu(I)HSco1 and the fully oxidized apoHCox17(3S-S), which can be then reduced by glutathione to apoHCox17(2S-S). The HSco1/HCox17(2S-S) redox reaction is thermodynamically driven by copper transfer. These reactions may occur in vivo because HSco1 can be found in the partially oxidized state within the IMS, consistent with the variable redox properties of the latter compartment. The electron transfer-coupled metallation of HSco1 can be a mechanism within the IMS for an efficient specific transfer of the metal to proteins, where metal-binding thiols are oxidized. The same reaction of copper-electron-coupled transfer does not occur with the human homolog of Sco1, HSco2, for kinetic reasons that may be ascribed to the lack of a specific metal-bridged protein-protein complex, which is instead observed in the Cu(I)HCox17(2S-S)/HSco1 interaction.

Sequence variation in the ATP-binding domain of the Wilson disease transporter, ATP7B, affects copper transport in a yeast model system

ATP7B is a copper transporting P-type ATPase defective in the autosomal recessive copper storage disorder, Wilson disease (WND). Functional assessment of variants helps to distinguish normal from disease-causing variants and provides information on important amino acid residues. A total of 11 missense variants of ATP7B, originally identified in WND patients, were examined for their capacity to functionally complement a yeast mutant strain in which the yeast gene ortholog, CCC2, was disrupted. Solution structures of ATP7B domains were used to predict the effects of each variant on ATP7B structure. Three variants lie within the copper-binding domain and eight within the ATP-binding domain of ATP7B. All three ATP7B variants within the copper-binding domain and four within the ATP-binding domain showed full complementation of the yeast ccc2 phenotype. For the remaining four located in the ATP-binding domain, p.Glu1064Lys and p.Val1106Asp were unable to complement the yeast ccc2 high-affinity iron uptake deficiency phenotype, apparently due to mislocalization and/or change in conformation of the variant protein. p.Leu1083Phe exhibited a temperature-sensitive phenotype with partial complementation at 30 degrees C and a severe deficit at 37 degrees C. p.Met1169Val only partially complemented the ccc2 phenotype at 30 degrees C and 37 degrees C. Therefore, four variant positions were identified as important for copper transport and as disease-causing changes. Since the yeast assay specifically evaluates copper transport function, variants with normal transport could be defective in some other aspect of ATP7B function, particularly trafficking in mammalian cells. Functional assessment is critical for reliable use of mutation analysis as an aid to diagnosis of this clinically variable condition.

Function and regulation of human copper-transporting ATPases

Copper-transporting ATPases (Cu-ATPases) ATP7A and ATP7B are evolutionarily conserved polytopic membrane proteins with essential roles in human physiology. The Cu-ATPases are expressed in most tissues, and their transport activity is crucial for central nervous system development, liver function, connective tissue formation, and many other physiological processes. The loss of ATP7A or ATP7B function is associated with severe metabolic disorders, Menkes disease, and Wilson disease. In cells, the Cu-ATPases maintain intracellular copper concentration by transporting copper from the cytosol across cellular membranes. They also contribute to protein biosynthesis by delivering copper into the lumen of the secretory pathway where metal ion is incorporated into copper-dependent enzymes. The biosynthetic and homeostatic functions of Cu-ATPases are performed in different cell compartments; targeting to these compartments and the functional activity of Cu-ATPase are both regulated by copper. In recent years, significant progress has been made in understanding the structure, function, and regulation of these essential transporters. These studies raised many new questions related to specific physiological roles of Cu-ATPases in various tissues and complex mechanisms that control the Cu-ATPase function. This review summarizes current data on the structural organization and functional properties of ATP7A and ATP7B as well as their localization and functions in various tissues, and discusses the current models of regulated trafficking of human Cu-ATPases.