Identification of an ATP-dependent copper transport system in endoplasmic reticulum vesicles isolated from rat liver

The coming of age of the mitochondria-ER contact: a matter of thickness

The sites of near-contact between the mitochondrion and the endoplasmic reticulum (ER) have earned a lot of attention due to their key role in the maintenance of lipid and calcium (Ca(2+)) homeostasis, in the initiation of autophagy and mitochondrial division, and in sensing metabolic shifts. At these sites, typically called MAMs (mitochondria-associated ER membranes) or MERCs (mitochondria-ER contacts), the organelles juxtapose at a distance that can range from ~10 to ~50 nm. The multifunctional role of this subcellular compartment is puzzling; further, recent studies have shown that mitochondria-ER contacts are highly plastic structures that remodel upon metabolic transitions and that their activity in controlling lipid homeostasis could be involved in Alzheimer's disease pathogenesis. This review aims at integrating the functions of this subcellular compartment to its most characterizing and unexplored structural parameter, their 'thickness': that is, the width of the cleft that separates the cytosolic face of the outer mitochondrial membrane from that of the ER. We describe and discuss the reasons why the thickness of a MERC should be considered a regulated structural parameter of the cell that defines and controls its function. Further, we propose a MERC classification that will help organize the expanding field of MERCs biology and of their role in cell physiology and human disease.

Molecular pathogenesis of Wilson and Menkes disease: correlation of mutations with molecular defects and disease phenotypes

The trace metal copper is essential for a variety of biological processes, but extremely toxic when present in excessive amounts. Therefore, concentrations of this metal in the body are kept under tight control. Central regulators of cellular copper metabolism are the copper-transporting P-type ATPases ATP7A and ATP7B. Mutations in ATP7A or ATP7B disrupt the homeostatic copper balance, resulting in copper deficiency (Menkes disease) or copper overload (Wilson disease), respectively. ATP7A and ATP7B exert their functions in copper transport through a variety of interdependent mechanisms and regulatory events, including their catalytic ATPase activity, copper-induced trafficking, post-translational modifications and protein-protein interactions. This paper reviews the extensive efforts that have been undertaken over the past few years to dissect and characterise these mechanisms, and how these are affected in Menkes and Wilson disease. As both disorders are characterised by an extensive clinical heterogeneity, we will discus how the underlying genetic defects correlate with the molecular functions of ATP7A and ATP7B and with the clinical expression of these disorders.

A partially purified putative iron P type-ATPase mediates Fe3+-transport into proteoliposome

We report that two fractions containing proteins from rat hepatocyte nuclei, obtained by nondenaturing gel electrophoresis, were able to bind iron and ATP, and to hydrolyze ATP. Electroelution of these two active fractions followed by SDS-PAGE analysis showed an identical protein pattern, each one containing four proteins in a range of 62-80 kDa. Phosphorylated protein bands were also detected in acid gel and disappeared after treatment with hydroxylamine/acetate or KOH, and upon chasing with cold ATP. A proteoliposome system, made by the incorporation of these partially purified protein fractions into phosphatidylcholine vesicles, carried out Fe(3+)-citrate uptake in a Mg(2+)-ATP-dependent way; Fe(3+) accumulation increased with time reaching a plateau in 30 min. Iron uptake was not supported by AMP-PNP, was partially inhibited by orthovanadate and was not affected by a mix of specific inhibitors of known ATPases. These results support our previous hypothesis that a putative nuclear membrane Fe(3+)-ATPase is involved in nuclear iron homeostasis.

Conformation-dependent post-translational glycosylation of tyrosinase. Requirement of a specific interaction involving the CuB metal binding site

Tyrosinase, the rate-limiting enzyme in mammalian melanogenesis, is a copper-containing transmembrane glycoprotein. Tyrosinase undergoes a complex post-translational processing before reaching the melanosomal membrane. This processing involves N-glycosylation in several sites, including one located in the CuB copper binding site, movement from the endoplasmic reticulum (ER) to the Golgi, copper binding, and sorting to the melanosome. Aberrant processing is causally related to the depigmented phenotype of human melanomas. Moreover, some forms of albinism and several other pigmentary syndromes are considered ER retention diseases or trafficking defects. A critical step in tyrosinase maturation is the acquisition of an ER export-competent conformation recognized positively by the ER quality control system. However, the minimal structural requirements allowing exit from the ER to the Golgi have not yet been identified for tyrosinase or other melanosomal proteins. We addressed this question by analyzing the enzymatic activity and glycosylation pattern of mouse tyrosinase point mutants and chimeric constructs, where selected portions of tyrosinase were replaced by the homologous fragments of the highly similar tyrosinase-related protein 1. We show that a completely inactive tyrosinase point mutant lacking a critical histidine residue involved in copper binding is nevertheless able to exit from the ER and undergo further processing. Moreover, we demonstrate that tyrosinase displays at least two sites whose glycosylation is post-translational and most likely conformation-dependent and that a highly specific interaction involving the CuB site is essential not only for correct glycosylation but also for exit from the ER and enzymatic activity.

Role of a copper-specific metallothionein of the blue crab, Callinectes sapidus, in copper metabolism associated with degradation and synthesis of hemocyanin

We have identified three MT encoding genes in the blue crab: MT-I, inducible by cadmium, zinc and copper; MT-II, inducible by cadmium and zinc; and MT-III, inducible by copper only [Syring et al., Comp. Biochem. Physiol. C, 125 (2000) 325-332]. To examine the role of the CuMT-I and CuMT-III isoforms in copper metabolism associated with the synthesis and degradation of the oxygen-binding copper protein, hemocyanin, we (1) cloned and sequenced hemocyanin cDNA, (2) examined interaction of the CuMTs with endoplasmic reticulum (ER) vesicles and (3) measured changes in levels of hemocyanin, MT-I, MT-III protein and mRNA that occur in crabs during different stages of the molt cycle. The cDNA-derived hemocyanin amino-acid sequence revealed the presence of a leader peptide indicating that hemocyanin is a secretory protein that is synthesized on the ER. Copper uptake studies show that ER vesicles take up both Cu1+ and Cu2+ in an ATP-independent process. The copper transporter has a Km of 10.8+/-2.4 microM copper and a Vmax of 6.1+/-0.5 nmol Cu/mg protein/10 min. ER vesicles contain hemocyanin, and bind CuMT-I and, preferentially, CuMT-III. However, binding does not result in copper transfer to the ER. There are statistically significant changes in hepatopancreas MT-III and hemocyanin mRNA, and in hemolymph hemocyanin concentrations during the molt cycle. MT-I mRNA remains constant. Changes in MT-III mRNA are positively correlated with changes in hemocyanin mRNA and hemocyanin protein, which points to coordinate control of MT-III and hemocyanin transcription. No CuMT-III protein is observed in hepatopancreas of intermolt crabs when levels of both MT-III and hemocyanin mRNA are high, suggesting rapid utilization of copper bound to MT-III when cells are actively synthesizing hemocyanin. CuMT-III is present in premolt and softshell crabs, and its emergence appears to coincide with a decrease in hemocyanin synthesis and increase in hemocyanin degradation. These results support the hypothesis that the copper-specific metallothionein is intricately involved in copper homeostasis associated with both the synthesis and degradation of hemocyanin.

Copper transportion of WD protein in hepatocytes from Wilson disease patients in vitro

AIM

To study the effect of copper transporting P-type ATPase in copper metabolism of hepatocyte and pathogenesis of Wilson disease (WD).

METHODS

WD copper transporting properties in some organelles of the cultured hepatocytes were studied from WD patients and normal controls.These cultured hepatocytes were incubated in the media of copper 15 mg x L(-1) only, copper 15 mg x L(-1) with vincristine (agonist of P-type ATPase) 0.5mg x L(-1), or copper 15 mg x L(-1) with vanadate (antagonist of P-type ATPase) 18.39 mg x L(-1) separately. Microsome (endoplasmic reticulum and Golgi apparatus), lysosome, mitochondria, and cytosol were isolated by differential centrifugation. Copper contents in these organelles were measured with atomic absorption spectrophotometer, and the influence in copper transportion of these organelles by vanadate and vincristine were comparatively analyzed between WD patients and controls. WD copper transporting P-type ATPase was detected by SDS-PAGE in conjunction with Western blot in liver samples of WD patients and controls.

RESULTS

The specific WD proteins (M(r)155,000 lanes) were expressed in human hepatocytes, including the control and WD patients. After incubation with medium containing copper for 2 h or 24 h, the microsome copper concentration in WD patients was obviously lower than that of controls, and the addition of vanadate or vincristine would change the copper transporting of microsomes obviously. When incubated with vincristine, levels of copper in microsome were significantly increased, while incubated with vanadate, the copper concentrations in microsome were obviously decreased. The results indicated that there were WD proteins, the copper transportion P-type ATPase in the microsome of hepatocytes. WD patients possessed abnormal copper transporting function of WD protein in the microsome, and the agonist might correct the defect of copper transportion by promoting the activity of copper transportion P-type ATPase.

CONCLUSION

Copper transportion P-type ATPase plays an important role in hepatocytic copper metabolism. Dysfunction of hepatocytic WD protein copper transportion might be one of the most important factors for WD.

Proper folding and endoplasmic reticulum to golgi transport of tyrosinase are induced by its substrates, DOPA and tyrosine

Tyrosinase is essential for pigmentation and is a source of tumor-derived antigenic peptides and cellular immune response. Wild type tyrosinase in melanoma cells and certain albino mutants in untransformed melanocytes are targeted to proteolytic degradation by the 26 S proteasome due to retention of the misfolded protein in the endoplasmic reticulum and its subsequent retranslocation to the cytosol. Here, we demonstrate that the substrates DOPA and tyrosine induced in melanoma cells a transition of misfolded wild type tyrosinase to the native form that is resistant to proteolysis, competent to exit the endoplasmic reticulum, and able to produce melanin. Because the enzymatic activity of tyrosinase is induced by DOPA, we propose that proper folding of the wild type protein, just like mutant forms, is tightly linked to its catalytic state. Loss of pigmentation, therefore, in tyrosinase-positive melanoma cells is a consequence of tumor-induced metabolic changes that suppress tyrosinase activity and DOPA production within these cells.

Cellular copper transport and metabolism

The transport and cellular metabolism of Cu depends on a series of membrane proteins and smaller soluble peptides that comprise a functionally integrated system for maintaining cellular Cu homeostasis. Inward transport across the plasma membrane appears to be a function of integral membrane proteins that form the channels that select Cu ions for passage. Two membrane-bound Cu-transporting ATPase enzymes, ATP7A and ATP7B, the products of the Menkes and Wilson disease genes, respectively, catalyze an ATP-dependent transfer of Cu to intracellular compartments or expel Cu from the cell. ATP7A and ATP7B work in concert with a series of smaller peptides, the copper chaperones, that exchange Cu at the ATPase sites or incorporate the Cu directly into the structure of Cu-dependent enzymes such as cytochrome c oxidase and Cu, Zn superoxide dismutase. These mechanisms come into play in response to a high influx of Cu or during the course of normal Cu metabolism.

Coordinated regulation of ceruloplasmin and metallothionein mRNA by interleukin-1 and copper in HepG2 cells

During the acute phase response, cytokines induce hepatic metallothionein and ceruloplasmin synthesis and the uptake of metals. We have investigated how copper and cytokines may interact in controlling ceruloplasmin (CP) and metallothionein mRNA in liver cells. We found that IL-1alpha, IL-1beta and IL-6 increased both metallothionein-1 (MT-1) and metallothionein-2 (MT-2) mRNA in HepG2 cells. The time and pattern of induction was different, both IL-1alpha and IL-1beta inducing two peaks of MT-1 and MT-2, with that of MT-2 being much larger. IL-6 induced only low levels of both MT-1 and MT-2 mRNA. CP mRNA was also increased after 16 h by IL-1beta, whereas IL-1alpha induced two CP peaks at 8 and 20 h, while IL-6 had little effect. Copper administration gave rise to substantially increased MT-1 mRNA, a slightly lower increase in MT-2 and also a significant increase in CP mRNA with similar kinetics. These parallel increases in MT and CP mRNA suggest that the coordinated expression of these proteins may be important for their synthesis during the acute phase response.

Cu metabolism in the liver

This paper has, given some idea of our concepts of the processes involved in the transport of Cu across cell membranes in the liver, which we have summarised in Fig 1. Cu(II)His2 is reduced to Cu(I). This is transported across the membrane, re-oxidised, either before or after binding to glutathione (Freedman et al., 1989) or HAH1 (Klomp et al., 1997), binds to SAHH, and donates Cu(II) to the ATPase. It is very interesting that cells which are very diverse from an evolutionary point of view still use very similar methods to handle the metal. Whether regulation of transport is also the sam remains to be seen. We would guess that, although there will be strong similarities, there will also be very significant differences, reflecting the different environments seen by different tissues in mammalian cells and given the different requirements of the tissues.

Characterization of a heavy metal ion transporter in the lysosomal membrane

Lysosomes are thought to play a role in various aspects of heavy metal metabolism. In the present study we demonstrate for the first time the presence of a heavy metal ion transport protein in the lysosomal membrane. Uptake of radioactive silver both in highly purified lysosomal membrane vesicles and in purified intact lysosomes showed the typical kinetics of a carrier-mediated process. This transport was stimulated by ATP hydrolysis, and showed specificity for Ag+, Cu2+, and Cd2+. All biochemical properties of this lysosomal metal ion transporter could classify it as a heavy metal transporting P-type ATPase. Long Evans Cinnamon (LEC) rats, an animal model for the copper transport disorder Wilson disease, showed normal lysosomal silver transport.

ATP-dependent copper transport by the Menkes protein in membrane vesicles isolated from cultured Chinese hamster ovary cells

The Menkes (MNK) protein is a vital component of copper homeostasis in mammalian cells. In this paper we provide the first biochemical evidence that the MNK protein functions as a copper-translocating P-type ATPase in mammalian cells. The enzyme activity in membrane vesicles prepared from Chinese hamster ovary cells overexpressing MNK was ATP-dependent, correlated with the amount of MNK and followed Michaelis-Menten kinetics with respect to copper. The copper transport was observed only under reducing conditions suggesting MNK transports Cu(I). This study opens the way to detailed structure-function studies and assessment of functional MNK derived from patients with Menkes disease.

Two forms of Wilson disease protein produced by alternative splicing are localized in distinct cellular compartments

Copper is an essential trace element in prokaryotes and eukaryotes and is strictly regulated by biological mechanisms. Menkes and Wilson diseases are human disorders that arise from disruption of the normal process of copper export from the cytosol to the extracellular environment. Recently a gene for Wilson disease (WD)(also named the ATP7B gene) was cloned. This gene encodes a copper transporter of the P-type ATPase. We prepared monoclonal and polyclonal anti-(WD protein) antibodies and characterized the full-length WD protein as well as a shorter form that is produced by alternative splicing in the human brain. We found that the WD protein is localized mainly in the Golgi apparatus, whereas the shorter form is present in the cytosol. These results suggest that the alternative WD proteins act as key regulators of copper metabolism, perhaps by performing distinct roles in the intracellular transport and export of copper.

Adenosine triphosphate-dependent copper transport in human liver

BACKGROUND/AIM

The recent cloning and sequencing of the Wilson disease gene indicates that hepatic copper (Cu) transport is mediated by a P-type ATPase. The location of this Cu-transporting protein within the hepatocyte is not known; in view of its proposed function and current concepts of hepatic Cu transport, it may reside in intracellular membranes (endoplasmic reticulum (ER), lysosomes) and/or in the bile canalicular membrane. The objective of this study was to establish characteristics and localization of ATP-dependent Cu transport in human liver.

METHODS

We have investigated Cu transport in vesicles of human liver plasma membranes showing a gradual increase in enrichment of canalicular domain markers: i.e. basolateral liver plasma membranes (blLPM), a mixed population of basolateral and canalicular (XLPM) and canalicular liver plasma membranes (cLPM).

RESULTS

In the presence of ATP (4 mM) and an ATP-regenerating system, uptake of radiolabeled Cu (64Cu, 10 microM) into cLPM vesicles and, to a lesser extent, into blLPM and XLPM was clearly stimulated when compared to control AMP values. Initial uptake rates of ATP-dependent Cu transport were 5.6, 7.8 and 13.7 nmol.min-1.mg-1 protein for blLPM, XLPM and cLPM, respectively, and showed no relationship with marker enzyme activity of ER and lysosomes (glucose-6-phosphatase and acid-phosphatase, respectively). Leucine aminopeptidase activity, as a marker for the cLPM, significantly correlated with ATP-dependent uptake rates measured in different membrane preparations: r = 0.70 (n = 9, p < 0.05). Estimated K(m) and Vmax values of ATP-dependent Cu uptake were 49.5 microM and 36.9 nmol.min-1.mg-1 protein, respectively.

CONCLUSION

This study provides biochemical evidence for the presence of an ATP-dependent Cu transport system in human liver (cCOP), mainly localized at the canalicular domain of the hepatocytic plasma membrane.

An ATP-dependent iron transport system in isolated rat liver nuclei

A concerted translational control is responsible for maintaining an iron level in the cytosol that is both adequate for the synthesis of iron-containing proteins and does not represent a danger to the cell. However, little is known about how iron level is controlled in the nucleus. Nuclei of rat liver take up iron from ferric citrate by a process that is dependent on ATP. This system shares several properties with known P-type ATPases, suggesting that a P-type ATPase in the nuclear membrane is responsible for iron transport. (i) Adenosine 5'-(beta,gamma-iminodiphosphate), a non-hydrolyzable ATP analogue, does not support iron uptake; (ii) the uptake is strongly inhibited by vanadate; (iii) there is an absolute requirement for Mg2+; and (iv) reagents that oxidize SH groups inhibit uptake, and this inhibition can be prevented by dithiothreitol. The energy of activation for the uptake (11.5 kcal/mol) and the Km for ATP (0.4 mM) are similar to values for other known cation transport ATPases. Inhibitors of Na+,K+-ATPase, sarcoplasmic reticulum Ca2+-ATPase, proton V-ATPase, and nuclear Ca2+-ATPase have no effect on uptake. Ferric citrate can be replaced by Fe-ATP as a source of iron for the transport system; however, two other stronger iron chelators, Tiron and desferrioxamine, completely inhibit the uptake. Taken together, these data strongly suggest that an Fe-ATPase, distinct from other known P-type ATPases, is responsible for iron transport in the nucleus.