Menkes' disease is a fatal, X-linked, copper deficiency disorder that results from defective copper efflux from intestinal cells and inadequate copper delivery to other tissues, leading to deficiencies of critical copper-dependent enzymes. Wilson's disease is an autosomally inherited, copper toxicosis disorder resulting from defective biliary excretion of copper, which leads to copper accumulation in the liver. The ATP7A and ATP7B genes that are defective in patients with Menkes' and Wilson's diseases, respectively, encode transmembrane, P-type ATPase proteins (ATP7A or MNK and ATP7B or WND, respectively) that function to translocate copper across cellular membranes. In this study, the cDNAs derived from a normal human ATP7A gene and the murine ATP7B homologue, Atp7b, were separately transfected into an immortalized fibroblast cell line obtained from a Menkes' disease patient. Both MNK and WND expressed from plasmid constructs were able to correct the copper accumulation and copper retention phenotype of these cells. However, the two proteins responded differently to elevated extracellular copper levels. Although MNK showed copper-induced trafficking from the trans-Golgi network to the plasma membrane, in the same cell line the intracellular location of WND did not appear to be affected by elevated copper.

Wilson disease (WD) is an autosomal recessive defect of copper transport characterized by massive accumulation of copper in the liver, which can lead to liver failure. Mutations in a copper transporting ATPase (WND or ATP7B) have been shown to cause the disease. The toxic milk mouse mutant (tx) accumulates copper in the liver in a manner similar to that observed in patients with WD. However, some physiological differences between tx mice and human WD patients have cast doubts on whether this mutant mouse is a valid model for WD. In this paper we report the isolation of cDNA clones encoding the murine homologue of WND. The predicted amino acid sequence is 1462 amino acids and contains the same functional domains identified in human and rat WND. As in the rat, the fourth metal binding domain is apparently non-functional. Similar levels of a 7.5 kb WND mRNA were detected in liver and kidney from normal and tx mice, indicating that transcription of this gene was unaffected in the mutant mice. The coding sequence of WND cDNA from the tx mouse liver identified a single nucleotide difference between the normal DL mouse and the tx which is predicted to change methionine 1356 in the eighth transmembrane domain to valine. This methionine is conserved in all copper ATPases including those from bacteria and yeast. The conclusion that this is the causative mutation is supported by the recent mapping of tx and WND to the same region of mouse chromosome 8. Thus the tx mouse is presented as a valid model for studies of the role of WND in copper transport and for investigation of different treatment strategies for WD.

Wilson disease is an autosomal recessive copper transport disorder resulting from defective biliary excretion of copper and subsequent hepatic copper accumulation and liver failure if not treated. The disease is caused by mutations in the ATP7B (WND) gene, which is expressed predominantly in the liver and encodes a copper-transporting P-type ATPase that is structurally and functionally similar to the Menkes protein (MNK), which is defective in the X-linked copper transport disorder Menkes disease. The toxic milk (tx) mouse has a clinical phenotype similar to Wilson disease patients and, recently, the tx mutation within the murine WND homologue (WND:) of this mouse was identified, establishing it as an animal model for Wilson disease. In this study, cDNA constructs encoding the wild-type (Wnd-wt) and mutant (Wnd-tx) Wilson proteins (Wnd) were generated and expressed in Chinese hamster ovary (CHO) cells. The tx mutation disrupted the copper-induced relocalization of Wnd in CHO cells and abrogated Wnd-mediated copper resistance of transfected CHO cells. In addition, co-localization experiments demonstrated that while Wnd and MNK are located in the trans-Golgi network in basal copper conditions, with elevated copper, these proteins are sorted to different destinations within the same cell. Ultrastructural studies showed that with elevated copper levels, Wnd accumulated in large multi-vesicular structures resembling late endosomes that may represent a novel compartment for copper transport. The data presented provide further support for a relationship between copper transport activity and the copper-induced relocalization response of mammalian copper ATPases, and an explanation at a molecular level for the observed phenotype of tx mice.

The biological functions of the structural protein sigma C, from avian reovirus strain RAM-1, were investigated in this study. A putative fusion peptide in sigma C was recognized in the deduced amino acid sequence by homology with Pneumovirus fusion sequences, and it was thus postulated that this protein may be involved in the formation of syncytia in cells infected with RAM-1. The sigma C gene was cloned and expressed in mammalian (COS7) cells and the sigma C protein was found to induce syncytia. It was therefore concluded that this protein is indeed responsible for avian reovirus-induced cell fusion. It was also found that sigma C caused condensation of the nuclei within a syncytium, as observed in RAM-1-infected cells. On the basis that this represented condensation of the chromatin, the inhibition of cellular DNA synthesis by the virus and by the sigma C protein was measured. It was found that the virus caused a 50% reduction in cellular DNA synthesis, but the sigma C protein did not inhibit DNA synthesis. Therefore pyknosis of the nuclei and inhibition of cellular DNA synthesis by RAM-1 are likely to be separate events.

No abstract