Correction of the copper transport defect of Menkes patient fibroblasts by expression of two forms of the sheep Wilson ATPase

ATP7B detoxifies silver in ciliated airway epithelial cells

Silver is a centuries-old antibiotic agent currently used to treat infected burns. The sensitivity of a wide range of drug-resistant microorganisms to silver killing suggests that it may be useful for treating refractory lung infections. Toward this goal, we previously developed a methylated caffeine silver acetate compound, SCC1, that exhibits broad-spectrum antimicrobial activity against clinical strains of bacteria in vitro and when nebulized to lungs in mouse infection models. Preclinical testing of high concentrations of SCC1 in primary culture mouse tracheal epithelial cells (mTEC) showed selective ciliated cell death. Ciliated cell death was induced by both silver- and copper-containing compounds but not by the methylated caffeine portion of SCC1. We hypothesized that copper transporting P-type ATPases, ATP7A and ATP7B, play a role in silver detoxification in the airway. In mTEC, ATP7A was expressed in non-ciliated cells, whereas ATP7B was expressed only in ciliated cells. The exposure of mTEC to SCC1 induced the trafficking of ATP7B, but not ATP7A, suggesting the presence of a cell-specific silver uptake and detoxification mechanisms. Indeed, the expression of the copper uptake protein CTR1 was also restricted to ciliated cells. A role of ATP7B in silver detoxification was further substantiated when treatment of SCC1 significantly increased cell death in ATP7B shRNA-treated HepG2 cells. In addition, mTEC from ATP7B(-/-) mice showed enhanced loss of ciliated cells compared to wild type. These studies are the first to demonstrate a cell type-specific expression of the Ag+/Cu+ transporters ATP7A, ATP7B, and CTR1 in airway epithelial cells and a role for ATP7B in detoxification of these metals in the lung.

Localization of the Wilson disease protein in murine intestine

Wilson disease is an inherited disorder of human copper metabolism, characterized by gradual accumulation of copper in tissues, predominantly liver and brain. The gene defect lies in the Wilson disease protein ATP7B, a copper transporting ATPase highly active in hepatocytes. In the liver, ATP7B is essential for excretion of excess copper into the bile and for copper loading of ceruloplasmin in the Golgi apparatus. The extrahepatic role of ATP7B is not yet completely understood. We analysed the intestinal expression of ATP7B in mice using RT-PCR, Western blot and indirect immunofluorescence. We found abundant expression of ATP7B in stomach and small intestine, but not in colon. Using confocal microscopy we demonstrate a Golgi localization of ATP7B in enterocytes. In response to elevated copper, the Wilson disease protein shows an intracellular trafficking pattern in the intestinal polarized cell line CaCo-2, moving away from the Golgi apparatus to dispersed vesicles. This suggests a role for intestinal ATP7B in sequestration of copper in intracellular vesicles for maintenance of copper homeostasis in the enterocyte. In conclusion, the expression of ATP7B in the small intestine might represent an additional regulatory mechanism to fine-tune intestinal copper absorption.

Copper-transporting ATPases ATP7A and ATP7B: cousins, not twins

Copper plays an essential role in human physiology and is indispensable for normal growth and development. Enzymes that are involved in connective tissue formation, neurotransmitter biosynthesis, iron transport, and others essential physiological processes require copper as a cofactor to mediate their reactions. The biosynthetic incorporation of copper into these enzymes takes places within the secretory pathway and is critically dependent on the activity of copper-transporting ATPases ATP7A or ATP7B. In addition, ATP7A and ATP7B regulate intracellular copper concentration by removing excess copper from the cell. These two transporters belong to the family of P(1)-type ATPases, share significant sequence similarity, utilize the same general mechanism for their function, and show partial colocalization in some cells. However, the distinct biochemical characteristics and dissimilar trafficking properties of ATP7A and ATP7B in cells, in which they are co-expressed, indicate that specific functions of these two copper-transporting ATPases are not identical. Immuno-detection studies in cells and tissues have begun to suggest specific roles for ATP7A and ATP7B. These experiments also revealed technical challenges associated with quantitative detection of copper-transporting ATPases in tissues, as illustrated here by comparing the results of ATP7A and ATP7B immunodetection in mouse cerebellum.

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.

Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis

Copper is essential for human health and copper imbalance is a key factor in the aetiology and pathology of several neurodegenerative diseases. The copper-transporting P-type ATPases, ATP7A and ATP7B are key molecules required for the regulation and maintenance of mammalian copper homeostasis. Their absence or malfunction leads to the genetically inherited disorders, Menkes and Wilson diseases, respectively. These proteins have a dual role in cells, namely to provide copper to essential cuproenzymes and to mediate the excretion of excess intracellular copper. A unique feature of ATP7A and ATP7B that is integral to these functions is their ability to sense and respond to intracellular copper levels, the latter manifested through their copper-regulated trafficking from the transGolgi network to the appropriate cellular membrane domain (basolateral or apical, respectively) to eliminate excess copper from the cell. Research over the last decade has yielded significant insight into the enzymatic properties and cell biology of the copper-ATPases. With recent advances in elucidating their localization and trafficking in human and animal tissues in response to physiological stimuli, we are progressing rapidly towards an integrated understanding of their physiological significance at the level of the whole animal. This knowledge in turn is helping to clarify the biochemical and cellular basis not only for the phenotypes conferred by individual Menkes and Wilson disease patient mutations, but also for the clinical variability of phenotypes associated with each of these diseases. Importantly, this information is also providing a rational basis for the applicability and appropriateness of certain diagnostic markers and therapeutic regimes. This overview will provide an update on the current state of our understanding of the localization and trafficking properties of the copper-ATPases in cells and tissues, the molecular signals and posttranslational interactions that govern their trafficking activities, and the cellular basis for the clinical phenotypes associated with disease-causing mutations.