Of mice and men, metals and mutations

Combined Metallomics/Transcriptomics Profiling Reveals a Major Role for Metals in Wound Repair

Endogenous metals are required for all life, orchestrating the action of diverse cellular processes that are crucial for tissue function. The dynamic wound healing response is underpinned by a plethora of such cellular behaviours, occurring in a time-dependent manner. However, the importance of endogenous metals for cutaneous repair remains largely unexplored. Here we combine ICP-MS with tissue-level RNA-sequencing to reveal profound changes in a number of metals, and corresponding metal-regulated genes, across temporal healing in mice. Wound calcium, magnesium, iron, copper and manganese are elevated at 7 days post-wounding, while magnesium, iron, aluminium, manganese and cobalt increase at 14 days post-wounding. At the level of transcription, wound-induced pathways are independently highly enriched for metal-regulated genes, and vice versa. Moreover, specific metals are linked to distinct wound-induced biological processes and converge on key transcriptional regulators in mice and humans. Finally, we reveal a potential role for one newly identified transcriptional regulator, TNF, in calcium-induced epidermal differentiation. Together, these data highlight potential new and diverse roles for metals in cutaneous wound repair, paving the way for further studies to elucidate the contribution of metals to cellular processes in the repair of skin and other tissues.

Animal Models of Normal and Disturbed Iron and Copper Metabolism

Research on the interplay between iron and copper metabolism in humans began to flourish in the mid-20th century, and diseases associated with dysregulated homeostasis of these essential trace minerals are common even today. Iron deficiency is the most frequent cause of anemia worldwide, leading to significant morbidity, particularly in developing countries. Iron overload is also quite common, usually being the result of genetic mutations which lead to inappropriate expression of the iron-regulatory hormone hepcidin. Perturbations of copper homeostasis in humans have also been described, including rare genetic conditions which lead to severe copper deficiency (Menkes disease) or copper overload (Wilson disease). Historically, the common laboratory rat (Rattus norvegicus) was the most frequently utilized species to model human physiology and pathophysiology. Recently, however, the development of genetic-engineering technology combined with the worldwide availability of numerous genetically homogenous (i.e., inbred) mouse strains shifted most research on iron and copper metabolism to laboratory mice. This created new opportunities to understand the function of individual genes in the context of a living animal, but thoughtful consideration of whether mice are the most appropriate models of human pathophysiology was not necessarily involved. Given this background, this review is intended to provide a guide for future research on iron- and copper-related disorders in humans. Generation of complementary experimental models in rats, swine, and other mammals is now facile given the advent of newer genetic technologies, thus providing the opportunity to accelerate the identification of pathogenic mechanisms and expedite the development of new treatments to mitigate these important human disorders.

Biometals in rare neurodegenerative disorders of childhood

Copper, iron, and zinc are just three of the main biometals critical for correct functioning of the central nervous system (CNS). They have diverse roles in many functional processes including but not limited to enzyme catalysis, protein stabilization, and energy production. The range of metal concentrations within the body is tightly regulated and when the balance is perturbed, debilitating effects ensue. Homeostasis of brain biometals is mainly controlled by various metal transporters and metal sequestering proteins. The biological roles of biometals are vastly reviewed in the literature with a large focus on the connection to neurological conditions associated with ageing. Biometals are also implicated in a variety of debilitating inherited childhood disorders, some of which arise soon following birth or as the child progresses into early adulthood. This review acts to highlight what we know about biometals in childhood neurological disorders such as Wilson's disease (WD), Menkes disease (MD), neuronal ceroid lipofuscinoses (NCLs), and neurodegeneration with brain iron accumulation (NBIA). Also discussed are some of the animal models available to determine the pathological mechanisms in these childhood disorders, which we hope will aid in our understanding of the role of biometals in disease and in attaining possible therapeutics in the future.

The adaptive response to dietary zinc in mice involves the differential cellular localization and zinc regulation of the zinc transporters ZIP4 and ZIP5

The ZIP5 gene encodes a protein closely related to ZIP4, a zinc transporter mutated in the human genetic disorder acrodermatitis enteropathica. Herein, we demonstrate that mouse ZIP5 and ZIP4 genes are co-expressed in several tissues involved in zinc homeostasis (intestine, pancreas, embryonic yolk sac). However, unlike expression of the ZIP4 gene, which is induced during periods of zinc deficiency, ZIP5 gene expression is unaltered by dietary zinc. Immunohistochemistry localizes ZIP5 to the basolateral surfaces of enterocytes, acinar cells, and visceral endoderm cells in mice fed a zinc-adequate diet. However, this protein is removed from these cell surfaces and internalized during dietary zinc deficiency. In contrast, ZIP4 is induced and recruited to the apical surface of enterocytes and endoderm cells during zinc deficiency. In the pancreas, ZIP4 is expressed in beta-cells, whereas ZIP5 is expressed in acinar cells. These results suggest that the function of ZIP5 is antagonistic to that of ZIP4 in the control of zinc homeostasis; rather than functioning in the acquisition of dietary zinc, as does ZIP4, ZIP5 may function in the removal of zinc from the body. Thus, during periods when dietary zinc is replete, ZIP5 may function to remove zinc from the blood via the pancreas and intestine, the major sites of zinc excretion in mammals, whereas the acquisition of dietary zinc by intestinal ZIP4 would be minimal. In contrast, during periods of dietary zinc deficiency when secretion of zinc by the pancreas and intestine is minimized, ZIP5 is removed from the cell surface, and the intestinal uptake of zinc is augmented by induction of ZIP4.

Intragenic deletions at Atp7a in mouse models for Menkes disease

Mottled mice have mutations in the copper-transporting ATPase Atp7a. They are proven models for the human disorder Menkes disease (MD), which results from mutations in a homologous gene. Mottled mice can be divided into three classes: class 1, in which affected males die before birth; class 2, in which affected males die in the early postnatal period; and class 3, in which affected males survive to adulthood. In humans, it has been shown that mutations that lead to a complete absence of functional protein cause classical MD, which is characterized by death of boys in early childhood. We hypothesized that the most severely affected mottled alleles would be the most likely to carry mutations equivalent to those causing classical MD and therefore undertook mutational analysis of several class 1 mottled alleles to assess whether these were appropriate models for the disease at the molecular level. Two novel mutations, a deletion of exons 11-14 in mottled spot and an insertion in exon 10 leading to missplicing in mottled candy, were identified. However, these are both "in-frame" mutations, as are the other eight Atp7a mutations reported to date, and therefore no frameshift or nonsense mutations have yet been associated with the mottled phenotype. This contrasts with the mutation spectrum associated with MD, emphasizing the need for caution when mottled mice are used as models for the clinical disorder.

Molecular mechanisms of copper metabolism and the role of the Menkes disease protein

Menkes disease is an X-linked, recessive disorder of copper metabolism that occurs in approximately 1 in 200,000 live births. The condition is characterized by skeletal abnormalities, severe mental retardation, neurologic degeneration, and patient mortality in early childhood. The symptoms of Menkes disease result from a deficiency of serum copper and copper-dependent enzymes. A candidate gene for the disease has been isolated and designated MNK. The MNK gene codes for a P-type cation transporting ATPase, based on homology to known P-type ATPases and in vitro experimentation. cDNA clones of MNK in Menkes patients show diminished or absented hybridization in northern blot experiments. The Menkes protein functions to export excess intracellular copper and activates upon Cu(I) binding to the six metal-binding repeats in the amino-terminal domain. The loss of Menkes protein activity blocks the export of dietary copper from the gastrointestinal tract and causes the copper deficiency associated with Menkes disease. Each of the Menkes protein amino-terminal repeats contains a conserved -X-Met-X-Cys-X-X-Cys- motif (where X is any amino acid). These metal-binding repeats are conserved in other cation exporting ATPases involved in metal metabolism and in proteins involved in cellular defense against heavy metals in both prokaryotes and eukaryotes. An overview of copper metabolism in humans and a discussion of our understanding of the molecular basis of cellular copper homeostasis is presented. This forms the basis for a discussion of Menkes disease and the protein deficit in this disease.

A phenotype map of the mouse X chromosome: models for human X-linked disease

The identification of many of the transcribed genes in man and mouse is being achieved by large scale sequencing of expressed sequence tags (ESTs). Attention is now being turned to elucidating gene function and many laboratories are looking to the mouse as a model system for this phase of the genome project. Mouse mutants have long been used as a means of investigating gene function and disease pathogenesis, and recently, several large mutagenesis programs have been initiated to fulfill the burgeoning demand of functional genomics research. Nevertheless, there is a substantial existing mouse mutant resource that can be used immediately. This review summarizes the available information about the loci encoding X-linked phenotypic mutants and variants, including 40 classical mutants and 40 that have arisen from gene targeting.

Expression profile of the copper homeostasis gene, rAtox1, in the rat brain

In humans the regulation of cellular copper homeostasis is essential for proper organ development and function. A novel cytosolic protein, named Atox 1, was recently identified in yeast that functions in shuttling intracellular mononuclear copper [Cu(I)] to copper-requiring proteins. Atox 1 and its human homolog, hAtox1, are members of an emerging family of proteins termed copper chaperones that are involved in the maintenance of copper homeostasis. Northern blot analysis demonstrates that Atox 1 is widely expressed at varying levels in a variety of rat tissues including brain. Using in situ hybridization histochemistry, we characterized the expression profile for the rat homolog of Atox1 (rAtox1) in the normal adult rat brain. There is widespread expression within the brain that appears to be primarily neuronal. The highest levels of Atox1 message consists of distinct neuronal subtypes that are also characterized by their high levels of metals like copper, iron, and zinc, which include the pyramidal neurons of the cerebral cortex and hippocampus in addition to the neurons of the locus coeruleus. The high levels of a metal chaperone like Atox1 in subsets of neurons that also sequester metals suggests that Atox1 may be important in maintaining the functionality of metal requiring enzymes. A detailed analysis of the restricted expression profile for a novel copper chaperone, rAtox1, is described in the adult rat CNS. Further analysis shows that Atoxl expression is associated with neuronal populations that sequester copper.

Menkes disease: recent advances and new aspects

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Expression of the Wilson disease gene is deficient in the Long-Evans Cinnamon rat

Long-Evans Cinnamon rats develop a necrotizing hepatitis characterized by excessive hepatic copper accumulation, defective holoceruloplasmin biosynthesis and impaired biliary copper excretion. To elucidate the molecular basis of this defect, a cDNA clone encoding the rat Wilson disease gene was isolated and used to examine gene expression in selected tissues from normal and Long-Evans Cinnamon rats. Although this cDNA readily detects Wilson transcripts in liver and other tissues from normal rats, such transcripts are entirely absent from tissues derived from the Long-Evans Cinnamon rat strain. These data therefore identify the Long-Evans Cinnamon rat as the first bona fide animal model of Wilson disease and suggest that this rat strain may be a valuable resource in the study of this genetic disorder.

A mutant mouse (tx) with increased hepatic metallothionein stability and accumulation

Metallothioneins (MTs) are low-molecular-mass cysteine-rich proteins implicated in metal homoeostasis and resistance to toxicity induced by heavy metals and alkylating agents. We report high hepatic MT protein accumulation (greater than 100-fold compared with wild-type mice) in toxic milk (tx) mice, along with markedly higher cytosol copper and zinc levels. Increased MT-gene transcription alone could not account for the high constitutive MT protein levels, since MT mRNA levels were not increased in tx mouse livers. However, hepatic MT was significantly more stable in adult tx mice: MT half-life (t1/2) was 79 or 77% greater than in wild-type mice before and after Cd induction respectively. Cd or Zn treatment increased MT mRNA, but not MT protein, accumulation in tx mouse livers: Cd displaced MT-bound Zn and Cu in preexisting MT. Thus tx mice appear to accumulate hepatic MT as a result of decreased protein degradation. These animals may provide a useful model to study the physiological role of MT, and human diseases (such as Wilson's disease) with abnormal copper metabolism.

Menkes disease: an X-linked neurological disorder of the copper metabolism

Menkes disease is an X-linked, recessive disturbance of copper metabolism associated with a progressive clinical course and abnormal hair. The disease is dominated by neurological symptoms combined with connective tissue manifestations, most of which can be explained by the lack of important copper enzymes. Despite excessive accumulation of the metal in various tissues, a functional copper deficiency is evident, probably caused by a defective intracellular copper transport protein of unknown nature. The molecular basis of the copper disturbance has proven difficult to define and will most likely have to await cloning of the gene. The chromosomal region of interest has now been narrowed down to a sub-band on the long arm of the chromosome (Xq13.3), and positional cloning is in progress in a number of laboratories including our own. Identification of the Menkes gene will be of importance for our understanding of the cellular handling of copper and other trace elements.

Mechanisms of caeruloplasmin biosynthesis in normal and copper-deficient rats

To examine the mechanisms of holo-caeruloplasmin biosynthesis, we measured the serum caeruloplasmin concentration and oxidase activity, hepatic caeruloplasmin mRNA content and hepatocyte caeruloplasmin biosynthesis and secretion in normal and copper-deficient rats. Copper deficiency resulted in a near-complete loss of serum caeruloplasmin oxidase activity, yet only a 60% reduction in serum caeruloplasmin concentration and no change in the abundance of hepatic caeruloplasmin mRNA or the rate of caeruloplasmin biosynthesis. Both interleukin-1 alpha and lipopolysaccharide increased hepatic caeruloplasmin mRNA content and caeruloplasmin biosynthesis in normal and copper-deficient animals, but neither mediator increased caeruloplasmin oxidase activity in the copper-deficient group. Pulse-chase studies in primary hepatocytes from normal and copper-deficient rats revealed that the secretory rates for newly synthesized caeruloplasmin were identical, despite little or no holo-caeruloplasmin synthesis in hepatocytes of copper-deficient rats. We conclude that hepatocyte copper content has no effect on hepatic caeruloplasmin-gene expression or caeruloplasmin biosynthesis and that the incorporation of copper into newly synthesized caeruloplasmin is not a rate-limiting step in the biosynthesis or secretion of the apoprotein from rat hepatocytes.

Correction of cerebrospinal fluid copper in Menkes kinky hair disease

A patient with Menkes Kinky Hair disease was treated with infusions of copper-histidine which resulted in normal copper values in the cerebrospinal fluid. This tends to confirm the in vitro data that copper is transported into the central nervous system complexed with histidine or other similar ligands.