Biochemical characterization of P-type copper ATPases

Structure and mechanism of the human copper transporting ATPases: Fitting the pieces into a moving puzzle

Human copper transporters ATP7B and ATP7A deliver copper to biosynthetic pathways and maintain copper homeostasis in the cell. These enzymes combine several challenges for structural biology because they are large low abundance membrane proteins with many highly mobile domains and long disordered loops. No method has yet succeeded in solving the structure of the complete fully functional protein. Still, X-ray crystallography, Cryo-EM and NMR helped to piece together a structure based model of the enzyme activity and regulation by copper. We review the structures of ATP7B and ATP7A with an emphasis on the mechanistic insights into the unique aspects of the transport function and regulation of the human copper ATPases that have emerged from more than twenty years of research.

Copper in Gynecological Diseases

Copper (Cu) is an essential micronutrient for the correct development of eukaryotic organisms. This metal plays a key role in many cellular and physiological activities, including enzymatic activity, oxygen transport, and cell signaling. Although the redox activity of Cu is crucial for enzymatic reactions, this property also makes it potentially toxic when found at high levels. Due to this dual action of Cu, highly regulated mechanisms are necessary to prevent both the deficiency and the accumulation of this metal since its dyshomeostasis may favor the development of multiple diseases, such as Menkes' and Wilson's diseases, neurodegenerative diseases, diabetes mellitus, and cancer. As the relationship between Cu and cancer has been the most studied, we analyze how this metal can affect three fundamental processes for tumor progression: cell proliferation, angiogenesis, and metastasis. Gynecological diseases are characterized by high prevalence, morbidity, and mortality, depending on the case, and mainly include benign and malignant tumors. The cellular processes that promote their progression are affected by Cu, and the mechanisms that occur may be similar. We analyze the crosstalk between Cu deregulation and gynecological diseases, focusing on therapeutic strategies derived from this metal.

Structures of the human Wilson disease copper transporter ATP7B

The P-type ATPase ATP7B exports cytosolic copper and plays an essential role in the regulation of cellular copper homeostasis. Mutants of ATP7B cause Wilson disease (WD), an autosomal recessive disorder of copper metabolism. Here, we present cryoelectron microscopy (cryo-EM) structures of human ATP7B in the E1 state in the apo, the putative copper-bound, and the putative cisplatin-bound forms. In ATP7B, the N-terminal sixth metal-binding domain (MBD6) binds at the cytosolic copper entry site of the transmembrane domain (TMD), facilitating the delivery of copper from the MBD6 to the TMD. The sulfur-containing residues in the TMD of ATP7B mark the copper transport pathway. By comparing structures of the E1 state human ATP7B and E2-P_i state frog ATP7B, we propose the ATP-driving copper transport model of ATP7B. These structures not only advance our understanding of the mechanisms of ATP7B-mediated copper export but can also guide the development of therapeutics for the treatment of WD.

The Redox Active [2Fe-2S] Clusters: Key-Components of a Plethora of Enzymatic Reactions—Part I: Archaea

The earliest forms of life (i.e., Archaea, Bacteria, and Eukarya) appeared on our planet about ten billion years after its formation. Although Archaea do not seem to possess the multiprotein machinery constituted by the NIF (Nitrogen Fixation), ISC (Iron Sulfur Cluster), SUF (sulfur mobilization) enzymes, typical of Bacteria and Eukarya, some of them are able to encode Fe-S proteins. Here we discussed the multiple enzymatic reactions triggered by the up-to-date structurally characterized members of the archaeal family that require the crucial presence of structurally characterized [2Fe-2S] assemblies, focusing on their biological functions and, when available, on their electrochemical behavior.

Plasmids Bring Additional Capabilities to Caulobacter Isolates

Caulobacter is a well-studied bacterial genus, but little is known about the plasmids that are found in some wild Caulobacter isolates. We used bioinformatic approaches to identify nine plasmids from seven different Caulobacter strains and grouped them based on their size and the similarity of their repABC, parAB, and mobAB genes. Protein pathway analysis of the genes on the K31p1 and K31p2 plasmids showed many metabolic pathways that would enhance the metabolic versatility of the host strain. In contrast, the CB4 plasmid contained 21 heavy metal resistance genes with the majority coding for proteins that enhance copper resistance. Growth assays of C. henricii CB4 demonstrated increased copper resistance and quantitative PCR showed an increase in the expression of eight heavy metal genes when induced with copper.

Characterization of a fluorescent 1,8-naphthalimide-functionalized PAMAM dendrimer and its Cu(ii) complexes as cytotoxic drugs: EPR and biological studies in myeloid tumor cells

The activity and interacting ability of a polyamidoamine (PAMAM) dendrimer modified with 4-N-methylpiperazine-1,8-naphthalimide units (termed D) and complexed by Cu(ii) ions, towards healthy and cancer cells were studied. Comparative electron paramagnetic resonance (EPR) studies of the Cu(ii)-D complex are presented: coordination mode, chemical structure, flexibility and stability of these complexes, in the absence and presence of myeloid cancer cells and peripheral blood mononuclear cells (PBMC). The interactions of Cu(ii) ions in the biological media at different equilibrium times were studied, highlighting different stability and interacting conditions with the cells. Furthermore, flow cytometry and confocal analysis, trace the peculiar properties of the dendrimers in PBMC and U937 cells. Indeed, a new probe (Fly) was used as a potential fluorescent tool for biological imaging of Cu(ii). The study highlights that dendrimer and, mainly, the Cu(ii) metallodendrimer are cytotoxic agents for the cells, specifically for U937 tumor cells, inducing mitochondrial dysfunction, ROS increase and lysosome involvement. The metallodendrimer shows antitumor selectivity, fewer affecting healthy PBMC, inducing a massive apoptotic cell death on U937 cells, in line with the high stability of this complex, as verified by EPR studies. The results underline the potentiality of this metallodendrimer to be used as anticancer drug.

Crossroads between membrane trafficking machinery and copper homeostasis in the nerve system

Imbalanced copper homeostasis and perturbation of membrane trafficking are two common symptoms that have been associated with the pathogenesis of neurodegenerative and neurodevelopmental diseases. Accumulating evidence from biophysical, cellular and in vivo studies suggest that membrane trafficking orchestrates both copper homeostasis and neural functions-however, a systematic review of how copper homeostasis and membrane trafficking interplays in neurons remains lacking. Here, we summarize current knowledge of the general trafficking itineraries for copper transporters and highlight several critical membrane trafficking regulators in maintaining copper homeostasis. We discuss how membrane trafficking regulators may alter copper transporter distribution in different membrane compartments to regulate intracellular copper homeostasis. Using Parkinson's disease and MEDNIK as examples, we further elaborate how misregulated trafficking regulators may interplay parallelly or synergistically with copper dyshomeostasis in devastating pathogenesis in neurodegenerative diseases. Finally, we explore multiple unsolved questions and highlight the existing challenges to understand how copper homeostasis is modulated through membrane trafficking.

Cu transporter protein CrpF protects against Cu-induced toxicity in Fusarium oxysporum

Cu is an essential trace element for cell growth and proliferation. However, excess of Cu accumulation leads to cellular toxicity. Thus, precise and tight regulation of Cu homeostasis processes, including transport, delivery, storage, detoxification, and efflux machineries, is required. Moreover, the maintenance of Cu homeostasis is critical for the survival and virulence of fungal pathogens. Cu homeostasis has been extensively studied in mammals, bacteria, and yeast, but it has not yet been well documented in filamentous fungi. In the present work, we investigated Cu tolerance in the filamentous fungus Fusarium oxysporum by analysing the Cu transporter coding gene crpF, previously studied in Aspergillus fumigatus. The expression studies demonstrated that crpF is upregulated in the presence of Cu and its deletion leads to severe sensitivity to low levels of CuSO₄ in F. oxysporum. Targeted deletion of crpF did not significantly alter the resistance of the fungus to macrophage killing, nor its pathogenic behaviour on the tomato plants. However, the targeted deletion mutant ΔcrpF showed increased virulence in a murine model of systemic infection compared to wild-type strain (wt).

Targeting the Copper Transport System to Improve Treatment Efficacies of Platinum-Containing Drugs in Cancer Chemotherapy

The platinum (Pt)-containing antitumor drugs including cisplatin (cis-diamminedichloroplatinum II, cDDP), carboplatin, and oxaliplatin, have been the mainstay of cancer chemotherapy. These drugs are effective in treating many human malignancies. The major cell-killing target of Pt drugs is DNA. Recent findings underscored the important roles of Pt drug transport system in cancer therapy. While many mechanisms have been proposed for Pt-drug transport, the high-affinity copper transporter (hCtr1), Cu chaperone (Atox1), and Cu exporters (ATP7A and ATP7B) are also involved in cDDP transport, highlighting Cu homeostasis regulation in Pt-based cancer therapy. It was demonstrated that by reducing cellular Cu bioavailable levels by Cu chelators, hCtr1 is transcriptionally upregulated by transcription factor Sp1, which binds the promoters of Sp1 and hCtr1. In contrast, elevated Cu poisons Sp1, resulting in suppression of hCtr1 and Sp1, constituting the Cu-Sp1-hCtr1 mutually regulatory loop. Clinical investigations using copper chelator (trientine) in carboplatin treatment have been conducted for overcoming Pt drug resistance due in part to defective transport. While results are encouraging, future development may include targeting multiple steps in Cu transport system for improving the efficacies of Pt-based cancer chemotherapy. The focus of this review is to delineate the mechanistic interrelationships between Cu homeostasis regulation and antitumor efficacy of Pt drugs.

Interference between copper transport systems and platinum drugs

Cisplatin, or cis-diamminedichloridoplatinum(II) cis-[PtCl₂(NH₃)₂], is a platinum-based anticancer drug largely used for the treatment of various types of cancers, including testicular, ovarian and colorectal carcinomas, sarcomas, and lymphomas. Together with other platinum-based drugs, cisplatin triggers malignant cell death by binding to nuclear DNA, which appears to be the ultimate target. In addition to passive diffusion across the cell membrane, other transport systems, including endocytosis and some active or facilitated transport mechanisms, are currently proposed to play a pivotal role in the uptake of platinum-based drugs. In this review, an updated view of the current literature regarding the intracellular transport and processing of cisplatin will be presented, with special emphasis on the plasma membrane copper permease CTR1, the Cu-transporting ATPases, ATP7A and ATP7B, located in the trans-Golgi network, and the soluble copper chaperone ATOX1. Their role in eliciting cisplatin efficacy and their exploitation as pharmacological targets will be addressed.

Analysis of Wilson disease mutations revealed that interactions between different ATP7B mutants modify their properties

Wilson disease (WD) is an autosomal-recessive disorder caused by mutations in the copper (Cu)-transporter ATP7B. Thus far, studies of WD mutations have been limited to analysis of ATP7B mutants in the homozygous states. However, the majority of WD patients are compound-heterozygous, and how different mutations on two alleles impact ATP7B properties is unclear. We characterized five mutations identified in Indian WD patients, first by expressing each alone and then by co-expressing two mutants with dissimilar properties. Mutations located in the regulatory domains of ATP7B-A595T, S1362A, and S1426I-do not affect ATP7B targeting to the trans-Golgi network (TGN) but reduce its Cu-transport activity. The S1362A mutation also inhibits Cu-dependent trafficking from the TGN. The G1061E and G1101R mutations, which are located within the ATP-binding domain, cause ATP7B retention in the endoplasmic reticulum, inhibit Cu-transport, and lower ATP7B protein abundance. Co-expression of the A595T and G1061E mutations, which mimics the compound-heterozygous state of some WD patients, revealed an interaction between these mutants that altered their intracellular localization and trafficking under both low and high Cu conditions. These findings highlight the need to study WD variants in both the homozygous and compound-heterozygous states to better understand the genotype-phenotype correlations and incomplete penetrance observed in WD.

Correlations Among mRNA Expression Levels of ATP7A, Serum Ceruloplasmin Levels, and Neuronal Metabolism in Unmedicated Major Depressive Disorder

BACKGROUND

Previous studies have found that elevated copper levels induce oxidation, which correlates with the occurrence of major depressive disorder (MDD). However, the mechanism of abnormal cerebral metabolism of MDD patients remains ambiguous. The main function of the enzyme ATPase copper-transporting alpha (ATP7A) is to transport copper across the membrane to retain copper homeostasis, which is closely associated with the onset of mental disorders and cognitive impairment. However, less is known regarding the association of ATP7A expression in MDD patients.

METHODS

A total of 31 MDD patients and 21 healthy controls were recruited in the present study. Proton magnetic resonance spectroscopy was used to assess the concentration levels of N-acetylaspartate, choline (Cho), and creatine (Cr) in brain regions of interest, including prefrontal white matter (PWM), anterior cingulate cortex (ACC), thalamus, lentiform nucleus, and cerebellum. The mRNA expression levels of ATP7A were measured using polymerase chain reaction (SYBR Green method). The correlations between mRNA expression levels of ATP7A and/or ceruloplasmin levels and neuronal biochemical metabolite ratio in the brain regions of interest were evaluated.

RESULTS

The decline in the mRNA expression levels of ATP7A and the increase in ceruloplasmin levels exhibited a significant correlation in MDD patients. In addition, negative correlations were noted between the decline in mRNA expression levels of ATP7A and the increased Cho/Cr ratios of the left PWM, right PWM, and right ACC in MDD patients. A positive correlation between elevated ceruloplasmin levels and increased Cho/Cr ratio of the left PWM was noted in MDD patients.

CONCLUSIONS

The findings suggested that the decline in the mRNA expression levels of ATP7A and the elevated ceruloplasmin levels induced oxidation that led to the disturbance of neuronal metabolism in the brain, which played important roles in the pathophysiology of MDD. The decline in the mRNA expression levels of ATP7A and the elevated ceruloplasmin levels affected neuronal membrane metabolic impairment in the left PWM, right PWM, and right ACC of MDD patients.

Expression and copper binding properties of the N-terminal domain of copper P-type ATPases of African trypanosomes

Copper is an essential component of cuproproteins but can be toxic to cells, therefore copper metabolism is very carefully regulated within cells. To gain insight into trypanosome copper metabolism, Trypanosoma spp. genomic databases were screened for the presence of copper-containing and -transporting proteins. Among other genes encoding copper-binding proteins, a copper-transporting P-type ATPase (CuATPase) gene was identified. Sequence and phylogenetic analyses suggest that the gene codes for a Cu⁺ transporter belonging to the P_1B-1 ATPase subfamily that has an N-terminal domain with copper binding motifs. The N-terminal cytosolic domains of the proteins from Trypanosoma congolense and Trypanosoma brucei brucei were recombinantly expressed in Escherichia coli as maltose binding protein (MBP) fusion proteins. These N-terminal domains bound copper in vitro and within E. coli cells, more than the control MBP fusion partner alone. The copper binding properties of the recombinant proteins were further confirmed when they inhibited copper catalysed ascorbate oxidation. Native CuATPases were detected in a western blot of lysates of T. congolense IL3000 and T. b. brucei ILTat1.1 bloodstream form parasites using affinity purified IgY antibodies against N-terminal domain peptides. The CuATPase was also detected by immunofluorescence in T. b. brucei bloodstream form parasites where it was associated with subcellular vesicles. In conclusion, Trypanosoma species express a copper-transporting P_1B-1-type ATPase and together with other copper-binding proteins identified in the genomes of kinetoplastid parasites may constitute potential targets for anti-trypanosomal drug discovery.

Copper homeostasis as target of both consolidated and innovative strategies of anti-tumor therapy

BACKGROUND

Copper was reported to be involved in the onset and progression of cancer. Proteins in charge of copper uptake and distribution, as well as cuproenzymes, are altered in cancer. More recently, proteins involved in signaling cascades, regulating cell proliferation, and anti-apoptotic protein factors were found to interact with copper. Therefore, therapeutic strategies using copper complexing molecules have been proposed for cancer therapy and used in clinical trials.

OBJECTIVES

This review will focus on novel findings about the involvement of copper and cupro-proteins in cancer dissemination process, epithelium to mesenchymal transition and vascularization. Particularly, implication of well-established (e.g. lysil oxidase) or newly identified copper-binding proteins (e.g. MEMO1), as well as their interplay, will be discussed. Moreover, we will describe recently synthesized copper complexes, including plant-derived ones, and their efficacy in contrasting cancer development.

CONCLUSIONS

The research on the involvement of copper in cancer is still an open field. Further investigation is required to unveil the mechanisms involved in copper delivery to the novel copper-binding proteins, which may identify other possible gene and protein targets for cancer therapy.

Copper trafficking in eukaryotic systems: current knowledge from experimental and computational efforts

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The main copper transporter, Ctr1, can transfer Cu(I) in the cell, through two different intracellular domains.

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Conformational flexibility of the copper metallochaperone Atox1 controls copper transfer mechanism in the cell.

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Each metal binding domain in ATP7B has a specific role.

Copper plays a vital role in fundamental cellular functions, and its concentration in the cell must be tightly regulated, as dysfunction of copper homeostasis is linked to severe neurological diseases and cancer. This review provides a compendium of current knowledge regarding the mechanism of copper transfer from the blood system to the Golgi apparatus; this mechanism involves the copper transporter hCtr1, the metallochaperone Atox1, and the ATPases ATP7A/B. We discuss key insights regarding the structural and functional properties of the hCtr1-Atox1-ATP7B cycle, obtained from diverse studies relying on distinct yet complementary biophysical, biochemical, and computational methods. We further address the mechanistic aspects of the cycle that continue to remain elusive. These knowledge gaps must be filled in order to be able to harness our understanding of copper transfer to develop therapeutic approaches with the capacity to modulate copper metabolism.

Genetics and epigenetic factors of Wilson disease

Wilson disease (WD) is a complex condition due to copper accumulation mainly in the liver and brain. The genetic base of WD is represented by pathogenic mutations of the copper-transporting gene ATP7B with consequent lack of copper excretion through the biliary tract. ATP7B is the only gene so far identified and known to be responsible for the development of the disease. Our understanding of the disease has been evolving as functional studies have associated specific disease-causing mutations with specific copper-transporter impairments. The most frequent variant in patients of European descent is the H1069Q missense mutation and it has been associated with protein misfolding, aberrant phosphorylation of the P-domain, and altered ATP binding orientation and affinity. Conversely, there is much less understanding of the relation between the genotype and the clinical manifestations of WD. WD is characterized by a highly varied and unpredictable presentation with different combined hepatic, neurological, and psychiatric symptoms. Several studies have attempted to correlate genotype and phenotype but the most recent evidences on larger populations failed to identify a relation between genotype and clinical presentations. Given that so far also modifier genes have not shown convincing association with WD, there is growing interest to identify epigenetic mechanisms of gene expression regulation as underlying the onset and progression of WD phenotype. Evidence from animal models indicated changes in methionine metabolism regulation with possible effects on DNA methylation. Mouse models of WD have indicated transcript level changes of genes related to DNA methylation in fetal and adult livers. And finally, evidence is accumulating regarding DNA methylation changes in patients with WD. It is unexplored how ATP7B genetic mutations combine with epigenetic changes to affect the phenotype. In conclusion, WD is a genetic disease with a complex regulation of its phenotype that includes molecular genetics and epigenetic mechanisms.

The Cu(II) affinity of the N-terminus of human copper transporter CTR1: Comparison of human and mouse sequences

Copper Transporter 1 (CTR1) is a homotrimeric membrane protein providing the main route of copper transport into eukaryotic cells from the extracellular milieu. Its N-terminal extracellular domain, rich in His and Met residues, is considered responsible for directing copper into the transmembrane channel. Most of vertebrate CTR1 proteins contain the His residue in position three from N-terminus, creating a well-known Amino Terminal Cu(II)- and Ni(II)-Binding (ATCUN) site. CTR1 from humans, primates and many other species contains the Met-Asp-His (MDH) sequence, while some rodents including mouse have the Met-Asn-His (MNH) N-terminal sequence. CTR1 is thought to collect Cu(II) ions from blood copper transport proteins, including albumin, but previous reports indicated that the affinity of N-terminal peptide/domain of CTR1 is significantly lower than that of albumin, casting serious doubt on this aspect of CTR1 function. Using potentiometry and spectroscopic techniques we demonstrated that MDH-amide, a tripeptide model of human CTR1 N-terminus, binds Cu(II) with K of 1.3 × 10¹³ M^-1 at pH 7.4, ~13 times stronger than Human Serum Albumin (HSA), and MNH-amide is even stronger, K of 3.2 × 10¹⁴ M^-1 at pH 7.4. These results indicate that the N-terminus of CTR1 may serve as intermediate binding site during Cu(II) transfer from blood copper carriers to the transporter. MDH-amide, but not MNH-amide also forms a low abundance complex with non-ATCUN coordination involving the Met amine, His imidazole and Asp carboxylate. This species might assist Cu(II) relay down the peptide chain or its reduction to Cu(I), both steps necessary for the CTR1 function.

Copper signalling: causes and consequences

Copper-containing enzymes perform fundamental functions by activating dioxygen (O2) and therefore allowing chemical energy-transfer for aerobic metabolism. The copper-dependence of O2 transport, metabolism and production of signalling molecules are supported by molecular systems that regulate and preserve tightly-bound static and weakly-bound dynamic cellular copper pools. Disruption of the reducing intracellular environment, characterized by glutathione shortage and ambient Cu(II) abundance drives oxidative stress and interferes with the bidirectional, copper-dependent communication between neurons and astrocytes, eventually leading to various brain disease forms. A deeper understanding of of the regulatory effects of copper on neuro-glia coupling via polyamine metabolism may reveal novel copper signalling functions and new directions for therapeutic intervention in brain disorders associated with aberrant copper metabolism.

Copper transporters and copper chaperones: roles in cardiovascular physiology and disease

Copper (Cu) is an essential micronutrient but excess Cu is potentially toxic. Its important propensity to cycle between two oxidation states accounts for its frequent presence as a cofactor in many physiological processes through Cu-containing enzymes, including mitochondrial energy production (via cytochrome c-oxidase), protection against oxidative stress (via superoxide dismutase), and extracellular matrix stability (via lysyl oxidase). Since free Cu is potentially toxic, the bioavailability of intracellular Cu is tightly controlled by Cu transporters and Cu chaperones. Recent evidence reveals that these Cu transport systems play an essential role in the physiological responses of cardiovascular cells, including cell growth, migration, angiogenesis and wound repair. In response to growth factors, cytokines, and hypoxia, their expression, subcellular localization, and function are tightly regulated. Cu transport systems and their regulators have also been linked to various cardiovascular pathophysiologies such as hypertension, inflammation, atherosclerosis, diabetes, cardiac hypertrophy, and cardiomyopathy. A greater appreciation of the central importance of Cu transporters and Cu chaperones in cell signaling and gene expression in cardiovascular biology offers the possibility of identifying new therapeutic targets for cardiovascular disease.

Modulating Chemosensitivity of Tumors to Platinum-Based Antitumor Drugs by Transcriptional Regulation of Copper Homeostasis

Platinum (Pt)-based antitumor agents have been effective in treating many human malignancies. Drug importing, intracellular shuffling, and exporting—carried out by the high-affinity copper (Cu) transporter (hCtr1), Cu chaperone (Ato x1), and Cu exporters (ATP7A and ATP7B), respectively—cumulatively contribute to the chemosensitivity of Pt drugs including cisplatin and carboplatin, but not oxaliplatin. This entire system can also handle Pt drugs via interactions between Pt and the thiol-containing amino acid residues in these proteins; the interactions are strongly influenced by cellular redox regulators such as glutathione. hCtr1 expression is induced by acute Cu deprivation, and the induction is regulated by the transcription factor specific protein 1 (Sp1) which by itself is also regulated by Cu concentration variations. Copper displaces zinc (Zn) coordination at the zinc finger (ZF) domains of Sp1 and inactivates its DNA binding, whereas Cu deprivation enhances Sp1-DNA interactions and increases Sp1 expression, which in turn upregulates hCtr1. Because of the shared transport system, chemosensitivity of Pt drugs can be modulated by targeting Cu transporters. A Cu-lowering agent (trientine) in combination with a Pt drug (carboplatin) has been used in clinical studies for overcoming Pt-resistance. Future research should aim at further developing effective Pt drug retention strategies for improving the treatment efficacy.

Drug Interactions With the Ca2+-ATPase From Sarco(Endo)Plasmic Reticulum (SERCA)

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) is an intracellular membrane transporter that utilizes the free energy provided by ATP hydrolysis for active transport of Ca²⁺ ions from the cytoplasm to the lumen of sarco(endo)plasmic reticulum. SERCA plays a fundamental role for cell calcium homeostasis and signaling in muscle cells and also in cells of other tissues. Because of its prominent role in many physiological processes, SERCA dysfunction is associated to diseases displaying various degrees of severity. SERCA transport activity can be inhibited by a variety of compounds with different chemical structures. Specific SERCA inhibitors were identified which have been instrumental in studies of the SERCA catalytic and transport mechanism. It has been proposed that SERCA inhibition may represent a novel therapeutic strategy to cure certain diseases by targeting SERCA activity in pathogens, parasites and cancer cells. Recently, novel small molecules have been developed that are able to stimulate SERCA activity. Such SERCA activators may also offer an innovative and promising therapeutic approach to treat diseases, such as heart failure, diabetes and metabolic disorders. In the present review the effects of pharmacologically relevant compounds on SERCA transport activity are presented. In particular, we will discuss the interaction of SERCA with specific inhibitors and activators that are potential therapeutic agents for different diseases.

Ammonium tetrathiomolybdate treatment targets the copper transporter ATP7A and enhances sensitivity of breast cancer to cisplatin

Cisplatin is an effective breast cancer drug but resistance often develops over prolonged chemotherapy. Therefore, we performed a candidate approach RNAi screen in combination with cisplatin treatment to identify molecular pathways conferring survival advantages. The screen identified ATP7A as a therapeutic target. ATP7A is a copper ATPase transporter responsible for intercellular movement and sequestering of cisplatin. Pharmaceutical replacement for ATP7A by ammonium tetrathiomolybdate (TM) enhanced cisplatin treatment in breast cancer cells. Allograft and xenograft models in athymic nude mice treated with cisplatin/TM exhibited retarded tumor growth, reduced accumulation of cancer stem cells and decreased cell proliferation as compared to mono-treatment with cisplatin or TM. Cisplatin/TM treatment of cisplatin-resistant tumors reduced ATP7A protein levels, attenuated cisplatin sequestering by ATP7A, increased nuclear availability of cisplatin, and subsequently enhanced DNA damage and apoptosis. Microarray analysis of gene ontology pathways that responded uniquely to cisplatin/TM double treatment depicted changes in cell cycle regulation, specifically in the G1/S transition. These findings offer the potential to combat platinum-resistant tumors and sensitize patients to conventional breast cancer treatment by identifying and targeting the resistant tumors' unique molecular adaptations.

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.

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.

Identification of novel ATP7A mutations and prenatal diagnosis in Chinese patients with Menkes disease

Menkes disease (MD) is a fatal X-linked multisystem disease caused by mutations in ATP7A. In this study, clinical and genetic analysis was performed in 24 male MD patients. Development delay, seizures, kinky coarse hair, and dystonia were found in 24, 22, 24, and 24 patients, respectively. Serum ceruloplasmin/copper tested in 19 patients was low. Abnormal classic features of MD presented in the MRI/MRA of 19 patients. Seventeen mutations of ATP7A were identified in 22 patients. Twelve were novel mutations including three small deletion/insertion, one missense mutation, two nonsense mutations, three splicing-site mutations, and three gross deletions. Twenty-two patients were genetically diagnosed; neither point mutation nor deletion/duplication was found in two of them. c.2179G > A found in five patients might be a hot-spot mutation. Prenatal molecular diagnosis was performed for five unrelated fetuses (1 female and 4 male), which found four fetuses to be wild type and one male carried the same mutation as the proband. This study of the largest sample of Chinese MD patients examined to date discovered the unique phenotype and genotype spectrum in Chinese patients with 12 novel mutations of ATP7A, and that c.2179G > A might be a hot-spot mutation in MD patients. Five successful prenatal diagnosis contributed important information for MD families.

Copper Resistance in Aspergillus nidulans Relies on the PI-Type ATPase CrpA, Regulated by the Transcription Factor AceA

Copper homeostasis has been extensively studied in mammals, bacteria, and yeast, but it has not been well-documented in filamentous fungi. In this report, we investigated the basis of copper tolerance in the model fungus Aspergillus nidulans. Three genes involved in copper homeostasis have been characterized. First, crpA the A. nidulans ortholog of Candida albicans CaCRP1 gene encoding a P_I-type ATPase was identified. The phenotype of crpA deletion led to a severe sensitivity to Cu⁺² toxicity and a characteristic morphological growth defect in the presence of high copper concentration. CrpA displayed some promiscuity regarding metal species response. The expression pattern of crpA showed an initial strong elevation of mRNA and a low continuous gene expression in response to long term toxic copper levels. Coinciding with maximum protein expression level, CrpA was localized close to the cellular surface, however protein distribution across diverse organelles suggests a complex regulated trafficking process. Secondly, aceA gene, encoding a transcription factor was identified and deleted, resulting in an even more extreme copper sensitivity than the ΔcrpA mutant. Protein expression assays corroborated that AceA was necessary for metal inducible expression of CrpA, but not CrdA, a putative metallothionein the function of which has yet to be elucidated.

Mechanisms of charge transfer in human copper ATPases ATP7A and ATP7B

ATP7A and ATP7B are Cu⁺ -transporting ATPases of subclass IB and play a fundamental role in intracellular copper homeostasis. ATP7A/B transfer Cu⁺ ions across the membrane from delivery to acceptor proteins without establishing a free Cu⁺ gradient. Transfer of copper across the membrane is coupled to ATP hydrolysis. Current measurements on solid supported membranes (SSM) were performed to investigate the mechanism of copper-related charge transfer across ATP7A and ATP7B. SSM measurements demonstrated that electrogenic copper displacement occurs within ATP7A/B following addition of ATP and formation of the phosphorylated intermediate. Comparison of the time constants for cation displacement in ATP7A/B and sarcoplasmic reticulum Ca²⁺ -ATPase is consistent with the slower phosphoenzyme formation in copper ATPases. Moreover, ATP-dependent copper transfer in ATP7A/B is not affected by varying the pH, suggesting that net proton counter-transport may not occur in copper ATPases. Platinum anticancer drugs activate ATP7A/B and are subjected to ATP-dependent vectorial displacement with a mechanism analogous to that of copper. © 2016 IUBMB Life, 69(4):218-225, 2017.

New silver nanoparticles induce apoptosis-like process in E. coli and interfere with mammalian copper metabolism

Silver nanoparticles (SNPs) are new functional materials that are widely used in biomedical and industrial technologies. Two main features that make SNPs valuable are their strong antibacterial effects and low toxicity to eukaryotes. In this study, SNPs were synthesized using a modified method of reducing the metal ions to their atomic state followed by crystallization. SNPs were characterized by UV/vis spectroscopy, X-ray diffractometry, atomic force microscopy, and transmission electron microscopy (TEM). The SNPs were spherically shaped with an average linear dimension of 20 nm. In aqueous solution, the SNPs were beige-yellow in color, and they formed a black color in bacteria-rich growth media. The toxicity and bioavailability of the SNPs were tested using Escherichia coli cells and C57Bl/6 mice. Although the SNPs displayed bactericidal activity, an E. coli cell strain transformed with an expression plasmid carrying a human CTR1 ectodomain with three motives that bind Cu(II), Cu(I), and Ag(I) demonstrated increased resistance to treatment with SNPs. TEM showed that the SNPs were absorbed by the E. coli cell, and flow cytometry showed that the SNPs induced apoptosis-like death. In mice treated with SNPs (daily intraperitoneal injection of 10 μg SNPs/g body weight over 4 days), the ceruloplasmin (Cp) oxidase activity in the blood serum decreased. However, level of Cp gene expression, the relative contents of the Cp protein in the Golgi complex and in the serum did not change. Treatment with SNPs did not influence the activity of superoxide dismutase 1 in the liver and had no apparent toxic effects in mice. These findings expand the scope of application for the use of new SNPs. The data are discussed in a paradigm, in which the effects of SNPs are caused by the interference of silver ions with copper metabolism.

Molecular features of copper binding proteins involved in copper homeostasis

Copper has a wide and important role in biological systems, determining conformation and activity of many metalloproteins and enzymes, such as cytochrome oxidase and superoxide dismutase . Furthermore, due to its possible reactivity with nonspecific proteins and toxic effects, elaborate systems of absorption, concentration buffering, delivery to specific protein sites and elimination, require a complex system including small carriers, chaperones and active transporters. The P-type copper ATPases ATP7A and ATP7B provide an important system for acquisition, active transport, distribution and elimination of copper. Relevance of copper metabolism to human diseases and therapy is already known. It is quite certain that further studies will reveal detailed and useful information on biochemical mechanisms and relevance to diseases. © 2016 IUBMB Life, 69(4):211-217, 2017.

Novel Metal Cation Resistance Systems from Mutant Fitness Analysis of Denitrifying Pseudomonas stutzeri

Metal ion transport systems have been studied extensively, but the specificity of a given transporter is often unclear from amino acid sequence data alone. In this study, predicted Cu(2+) and Zn(2+) resistance systems in Pseudomonas stutzeri strain RCH2 are compared with those experimentally implicated in Cu(2+) and Zn(2+) resistance, as determined by using a DNA-barcoded transposon mutant library. Mutant fitness data obtained under denitrifying conditions are combined with regulon predictions to yield a much more comprehensive picture of Cu(2+) and Zn(2+) resistance in strain RCH2. The results not only considerably expand what is known about well-established metal ion exporters (CzcCBA, CzcD, and CusCBA) and their accessory proteins (CzcI and CusF), they also reveal that isolates with mutations in some predicted Cu(2+) resistance systems do not show decreased fitness relative to the wild type when exposed to Cu(2+) In addition, new genes are identified that have no known connection to Zn(2+) (corB, corC, Psest_3226, Psest_3322, and Psest_0618) or Cu(2+) resistance (Mrp antiporter subunit gene, Psest_2850, and Psest_0584) but are crucial for resistance to these metal cations. Growth of individual deletion mutants lacking corB, corC, Psest_3226, or Psest_3322 confirmed the observed Zn-dependent phenotypes. Notably, to our knowledge, this is the first time a bacterial homolog of TMEM165, a human gene responsible for a congenital glycosylation disorder, has been deleted and the resulting strain characterized. Finally, the fitness values indicate Cu(2+)- and Zn(2+)-based inhibition of nitrite reductase and interference with molybdenum cofactor biosynthesis for nitrate reductase. These results extend the current understanding of Cu(2+) and Zn(2+) efflux and resistance and their effects on denitrifying metabolism.

IMPORTANCE

In this study, genome-wide mutant fitness data in P. stutzeri RCH2 combined with regulon predictions identify several proteins of unknown function that are involved in resisting zinc and copper toxicity. For zinc, these include a member of the UPF0016 protein family that was previously implicated in Ca(2+)/H(+) antiport and a human congenital glycosylation disorder, CorB and CorC, which were previously linked to Mg(2+) transport, and Psest_3322 and Psest_0618, two proteins with no characterized homologs. Experiments using mutants lacking Psest_3226, Psest_3322, corB, corC, or czcI verified their proposed functions, which will enable future studies of these little-characterized zinc resistance determinants. Likewise, Psest_2850, annotated as an ion antiporter subunit, and the conserved hypothetical protein Psest_0584 are implicated in copper resistance. Physiological connections between previous studies and phenotypes presented here are discussed. Functional and mechanistic understanding of transport proteins improves the understanding of systems in which members of the same protein family, including those in humans, can have different functions.

Copper trafficking to the secretory pathway

Copper (Cu) is indispensible for growth and development of human organisms. It is required for such fundamental and ubiquitous processes as respiration and protection against reactive oxygen species. Cu also enables catalytic activity of enzymes that critically contribute to the functional identity of many cells and tissues. Pigmentation, production of norepinephrine by the adrenal gland, the key steps in the formation of connective tissue, neuroendocrine signaling, wound healing - all these processes require Cu and depend on Cu entering the secretory pathway. To reach the Cu-dependent enzymes in a lumen of the trans-Golgi network and various vesicular compartments, Cu undertakes a complex journey crossing the extracellular and intracellular membranes and staying firmly on course while traveling in a cytosol. The proteins that assist Cu in this journey by mediating its entry, distribution, and export, have been identified. The accumulating data also indicate that the current model of cellular Cu homeostasis is still a "skeleton" that has to be fleshed out with many new details. This review summarizes recent data on the mechanisms responsible for Cu transfer to the secretory pathway. The emerging new concepts and gaps in our knowledge are discussed.

Ammonia-Oligotrophic and Diazotrophic Heavy Metal-Resistant Serratia liquefaciens Strains from Pioneer Plants and Mine Tailings

Mine tailings are man-made environments characterized by low levels of organic carbon and assimilable nitrogen, as well as moderate concentrations of heavy metals. For the introduction of nitrogen into these environments, a key role is played by ammonia-oligotrophic/diazotrophic heavy metal-resistant guilds. In mine tailings from Zacatecas, Mexico, Serratia liquefaciens was the dominant heterotrophic culturable species isolated in N-free media from bulk mine tailings as well as the rhizosphere, roots, and aerial parts of pioneer plants. S. liquefaciens strains proved to be a meta-population with high intraspecific genetic diversity and a potential to respond to these extreme conditions. The phenotypic and genotypic features of these strains reveal the potential adaptation of S. liquefaciens to oligotrophic and nitrogen-limited mine tailings with high concentrations of heavy metals. These features include ammonia-oligotrophic growth, nitrogen fixation, siderophore and indoleacetic acid production, phosphate solubilization, biofilm formation, moderate tolerance to heavy metals under conditions of diverse nitrogen availability, and the presence of zntA, amtB, and nifH genes. The acetylene reduction assay suggests low nitrogen-fixing activity. The nifH gene was harbored in a plasmid of ∼60 kb and probably was acquired by a horizontal gene transfer event from Klebsiella variicola.

The promiscuous phosphomonoestearase activity of Archaeoglobus fulgidus CopA, a thermophilic Cu+ transport ATPase

Membrane transport P-type ATPases display two characteristic enzymatic activities: a principal ATPase activity provides the driving force for ion transport across biological membranes, whereas a promiscuous secondary activity catalyzes the hydrolysis of phosphate monoesters. This last activity is usually denoted as the phosphatase activity of P-ATPases. In the present study, we characterize the phosphatase activity of the Cu(+)-transport ATPase from Archaeglobus fulgidus (Af-CopA) and compare it with the principal ATPase activity. Our results show that the phosphatase turnover number was 20 times higher than that corresponding to the ATPase activity, but it is compensated by a high value of Km, producing a less efficient catalysis for pNPP. This secondary activity is enhanced by Mg(2+) (essential activator) and phospholipids (non-essential activator), and inhibited by salts and Cu(+). Transition state analysis of the catalyzed and noncatalyzed hydrolysis of pNPP indicates that Af-CopA enhances the reaction rates by a factor of 10(5) (ΔΔG(‡)=38 kJ/mol) mainly by reducing the enthalpy of activation (ΔΔH(‡)=30 kJ/mol), whereas the entropy of activation is less negative on the enzyme than in solution. For the ATPase activity, the decrease in the enthalpic component of the barrier is higher (ΔΔH(‡)=39 kJ/mol) and the entropic component is small on both the enzyme and in solution. These results suggest that different mechanisms are involved in the transference of the phosphoryl group of p-nitrophenyl phosphate and ATP.

The role of transition metal transporters for iron, zinc, manganese, and copper in the pathogenesis of Yersinia pestis

Yersinia pestis, the causative agent of bubonic, septicemic and pneumonic plague, encodes a multitude of Fe transport systems. Some of these are defective due to frameshift or IS element insertions, while others are functional in vitro but have no established role in causing infections. Indeed only 3 Fe transporters (Ybt, Yfe and Feo) have been shown to be important in at least one form of plague. The yersiniabactin (Ybt) system is essential in the early dermal/lymphatic stages of bubonic plague, irrelevant in the septicemic stage, and critical in pneumonic plague. Two Mn transporters have been characterized (Yfe and MntH). These two systems play a role in bubonic plague but the double yfe mntH mutant is fully virulent in a mouse model of pneumonic plague. The same in vivo phenotype occurs with a mutant lacking two (Yfe and Feo) of four ferrous transporters. A role for the Ybt siderophore in Zn acquisition has been revealed. Ybt-dependent Zn acquisition uses a transport system completely independent of the Fe-Ybt uptake system. Together Ybt components and ZnuABC play a critical role in Zn acquisition in vivo. Single mutants in either system retain high virulence in a mouse model of septicemic plague while the double mutant is completely avirulent.

Mechanism of Copper Uptake from Blood Plasma Ceruloplasmin by Mammalian Cells

Ceruloplasmin, the main copper binding protein in blood plasma, has been of particular interest for its role in efflux of iron from cells, but has additional functions. Here we tested the hypothesis that it releases its copper for cell uptake by interacting with a cell surface reductase and transporters, producing apoceruloplasmin. Uptake and transepithelial transport of copper from ceruloplasmin was demonstrated with mammary epithelial cell monolayers (PMC42) with tight junctions grown in bicameral chambers, and purified human (64)Cu-labeled ceruloplasmin secreted by HepG2 cells. Monolayers took up virtually all the (64)Cu over 16h and secreted half into the apical (milk) fluid. This was partly inhibited by Ag(I). The (64)Cu in ceruloplasmin purified from plasma of (64)Cu-injected mice accumulated linearly in mouse embryonic fibroblasts (MEFs) over 3-6h. Rates were somewhat higher in Ctr1+/+ versus Ctr1-/- cells, and 3-fold lower at 2 °C. The ceruloplasmin-derived (64)Cu could not be removed by extensive washing or trypsin treatment, and most was recovered in the cytosol. Actual cell copper (determined by furnace atomic absorption) increased markedly upon 24h exposure to holoceruloplasmin. This was accompanied by a conversion of holo to apoceruloplasmin in the culture medium and did not occur during incubation in the absence of cells. Four different endocytosis inhibitors failed to prevent 64Cu uptake from ceruloplasmin. High concentrations of non-radioactive Cu(II)- or Fe(III)-NTA (substrates for cell surface reductases), or Cu(I)-NTA (to compete for transporter uptake) almost eliminated uptake of (64)Cu from ceruloplasmin. MEFs had cell surface reductase activity and expressed Steap 2 (but not Steaps 3 and 4 or dCytB). However, six-day siRNA treatment was insufficient to reduce activity or uptake. We conclude that ceruloplasmin is a circulating copper transport protein that may interact with Steap2 on the cell surface, forming apoceruloplasmin, and Cu(I) that enters cells through CTR1 and an unknown copper uptake transporter.

Targeting copper in cancer therapy: 'Copper That Cancer'

Copper is an essential micronutrient involved in fundamental life processes that are conserved throughout all forms of life. The ability of copper to catalyze oxidation-reduction (redox) reactions, which can inadvertently lead to the production of reactive oxygen species (ROS), necessitates the tight homeostatic regulation of copper within the body. Many cancer types exhibit increased intratumoral copper and/or altered systemic copper distribution. The realization that copper serves as a limiting factor for multiple aspects of tumor progression, including growth, angiogenesis and metastasis, has prompted the development of copper-specific chelators as therapies to inhibit these processes. Another therapeutic approach utilizes specific ionophores that deliver copper to cells to increase intracellular copper levels. The therapeutic window between normal and cancerous cells when intracellular copper is forcibly increased, is the premise for the development of copper-ionophores endowed with anticancer properties. Also under investigation is the use of copper to replace platinum in coordination complexes currently used as mainstream chemotherapies. In comparison to platinum-based drugs, these promising copper coordination complexes may be more potent anticancer agents, with reduced toxicity toward normal cells and they may potentially circumvent the chemoresistance associated with recurrent platinum treatment. In addition, cancerous cells can adapt their copper homeostatic mechanisms to acquire resistance to conventional platinum-based drugs and certain copper coordination complexes can re-sensitize cancer cells to these drugs. This review will outline the biological importance of copper and copper homeostasis in mammalian cells, followed by a discussion of our current understanding of copper dysregulation in cancer, and the recent therapeutic advances using copper coordination complexes as anticancer agents.

Molecular basis of neurodegeneration and neurodevelopmental defects in Menkes disease

ATP7A mutations impair copper metabolism resulting in three distinct genetic disorders in humans. These diseases are characterized by neurological phenotypes ranging from intellectual disability to neurodegeneration. Severe ATP7A loss-of-function alleles trigger Menkes disease, a copper deficiency condition where systemic and neurodegenerative phenotypes dominate clinical outcomes. The pathogenesis of these manifestations has been attributed to the hypoactivity of a limited number of copper-dependent enzymes, a hypothesis that we refer as the oligoenzymatic pathogenic hypothesis. This hypothesis, which has dominated the field for 25 years, only explains some systemic Menkes phenotypes. However, we argue that this hypothesis does not fully account for the Menkes neurodegeneration or neurodevelopmental phenotypes. Here, we propose revisions of the oligoenzymatic hypothesis that could illuminate the pathogenesis of Menkes neurodegeneration and neurodevelopmental defects through unsuspected overlap with other neurological conditions including Parkinson's, intellectual disability, and schizophrenia.