The effect of the acrodermatitis enteropathica mutation on zinc uptake in human fibroblasts

Molecular Insight into the Regulation of Vimentin by Cysteine Modifications and Zinc Binding

The intermediate filament protein vimentin is involved in essential cellular processes, including cell division and stress responses, as well as in the pathophysiology of cancer, pathogen infection, and autoimmunity. The vimentin network undergoes marked reorganizations in response to oxidative stress, in which modifications of vimentin single cysteine residue, Cys328, play an important role, and is modulated by zinc availability. However, the molecular basis for this regulation is not fully understood. Here, we show that Cys328 displays a low pK_a, supporting its reactivity, and is readily alkylated and oxidized in vitro. Moreover, combined oxidation and crosslinking assays and molecular dynamics simulations support that zinc ions interact with Cys328 in its thiolate form, whereas Glu329 and Asp331 stabilize zinc coordination. Vimentin oxidation can induce disulfide crosslinking, implying the close proximity of Cys328 from neighboring dimers in certain vimentin conformations, supported by our computational models. Notably, micromolar zinc concentrations prevent Cys328 alkylation, lipoxidation, and disulfide formation. Moreover, zinc selectively protects vimentin from crosslinking using short-spacer cysteine-reactive but not amine-reactive agents. These effects are not mimicked by magnesium, consistent with a lower number of magnesium ions hosted at the cysteine region, according to molecular dynamics simulations. Importantly, the region surrounding Cys328 is involved in interaction with several drugs targeting vimentin and is conserved in type III intermediate filaments, which include glial fibrillary acidic protein and desmin. Altogether, our results identify this region as a hot spot for zinc binding, which modulates Cys328 reactivity. Moreover, they provide a molecular standpoint for vimentin regulation through the interplay between cysteine modifications and zinc availability.

Vimentin filament organization and stress sensing depend on its single cysteine residue and zinc binding

The vimentin filament network plays a key role in cell architecture and signalling, as well as in epithelial-mesenchymal transition. Vimentin C328 is targeted by various oxidative modifications, but its role in vimentin organization is not known. Here we show that C328 is essential for vimentin network reorganization in response to oxidants and electrophiles, and is required for optimal vimentin performance in network expansion, lysosomal distribution and aggresome formation. C328 may fulfil these roles through interaction with zinc. In vitro, micromolar zinc protects vimentin from iodoacetamide modification and elicits vimentin polymerization into optically detectable structures; in cells, zinc closely associates with vimentin and its depletion causes reversible filament disassembly. Finally, zinc transport-deficient human fibroblasts show increased vimentin solubility and susceptibility to disruption, which are restored by zinc supplementation. These results unveil a critical role of C328 in vimentin organization and open new perspectives for the regulation of intermediate filaments by zinc.

Zinc: The Metal of Life

The importance of zinc was 1st reported for Aspergillus niger. It took over 75 y to realize that zinc is also an essential trace element for rats, and an additional 30 y went by before it was recognized that this was also true for humans. The adult body contains about 2 to 3 g of zinc. Zinc is found in organs, tissues, bones, fluids, and cells. It is essential for many physiological functions and plays a significant role in a number of enzyme actions in the living systems. Bioinformatics estimates report that 10% of the human proteome contains zinc-binding sites. Based on its role in such a plethora of cellular components, zinc has diverse biological functions from enzymatic catalysis to playing a crucial role in cellular neuronal systems. Thus, based on the various published studies and reports, it is pertinent to state that zinc is one of the most important essential trace metals in human nutrition and lifestyle. Its deficiency may severely affect the homeostasis of a biological system. This review compiles the role of zinc in prophylaxis/therapeutics and provides current information about its effect on living beings.

A mouse model of acrodermatitis enteropathica: loss of intestine zinc transporter ZIP4 (Slc39a4) disrupts the stem cell niche and intestine integrity

Mutations in the human Zip4 gene cause acrodermatitis enteropathica, a rare, pseudo-dominant, lethal genetic disorder. We created a tamoxifen-inducible, enterocyte-specific knockout of this gene in mice which mimics this human disorder. We found that the enterocyte Zip4 gene in mice is essential throughout life, and loss-of-function of this gene rapidly leads to wasting and death unless mice are nursed or provided excess dietary zinc. An initial effect of the knockout was the reprogramming of Paneth cells, which contribute to the intestinal stem cell niche in the crypts. Labile zinc in Paneth cells was lost, followed by diminished Sox9 (sex determining region Y-box 9) and lysozyme expression, and accumulation of mucin, which is normally found in goblet cells. This was accompanied by dysplasia of the intestinal crypts and significantly diminished small intestine cell division, and attenuated mTOR1 activity in villus enterocytes, indicative of increased catabolic metabolism, and diminished protein synthesis. This was followed by disorganization of the absorptive epithelium. Elemental analyses of small intestine, liver, and pancreas from Zip4-intestine knockout mice revealed that total zinc was dramatically and rapidly decreased in these organs whereas iron, manganese, and copper slowly accumulated to high levels in the liver as the disease progressed. These studies strongly suggest that wasting and lethality in acrodermatitis enteropathica patients reflects the loss-of-function of the intestine zinc transporter ZIP4, which leads to abnormal Paneth cell gene expression, disruption of the intestinal stem cell niche, and diminished function of the intestinal mucosa. These changes, in turn, cause a switch from anabolic to catabolic metabolism and altered homeostasis of several essential metals, which, if untreated by excess dietary zinc, leads to dramatic weight loss and death.

Loss-of-function of the zinc transporter ZIP4 in the mouse intestine mimics the lethal human disease acrodermatitis enteropathica. This is a rare disease in humans that is not well understood. Our studies demonstrate the paramount importance of ZIP4 in the intestine in this disease and reveal that a root cause of lethality is disruption of the intestine stem cell niche and impaired function of the small intestine. This, in turn, leads to dramatic weight loss and death unless treated with exogenous zinc.

[Pustular diseases of the newborn]

In the newborn, there exists a wide spectrum of pustular skin diseases. These range from transitory, benign adaptation disorders up to systemic, life threatening illnesses. In 30-60% of newborns pustules are observed in association with the relatively harmless Erythema toxicum, the origin of which is still unknown today. It is necessary to differentiate this from the pustular diseases which may be of infectious or non-infectious nature and which require therapy. Typical pathogens include Malassezia furfur, Staphylococci, Streptococci, Candida spp. and the herpes virus group.

The Mammalian Zip5 Protein Is a Zinc Transporter That Localizes to the Basolateral Surface of Polarized Cells

The mouse and human Zip5 proteins are members of the ZIP family of metal ion transporters. In this study, we present evidence that mouse Zip5 is a zinc uptake transporter that is specific for Zn(II) over other potential metal ion substrates. We also show that, unlike many other mammalian ZIP proteins, the endocytic removal of mZip5 from the plasma membrane is not triggered by zinc treatment. Thus, the activity of mZip5 does not appear to be down-regulated by zinc repletion. Zip5 expression is restricted to many tissues important for zinc homeostasis, including the intestine, pancreas, liver, and kidney. Zip5 is similar in sequence to the Zip4 protein, which is involved in the uptake of dietary zinc. Co-expression of Zip4 and Zip5 in the intestine led to the hypothesis that these proteins play overlapping roles in the uptake of dietary zinc across the apical membrane of intestinal enterocytes. Surprisingly, however, we found that mZip5 localizes specifically to the basolateral membrane of polarized Madin-Darby canine kidney cells. These observations suggest that Zip5 plays a novel role in polarized cells by carrying out serosal-to-mucosal zinc transport. Furthermore, given its expression in tissues important to zinc homeostasis, we propose that Zip5 plays a central role in controlling organismal zinc status.

Zn2+-stimulated endocytosis of the mZIP4 zinc transporter regulates its location at the plasma membrane

Zinc is an essential nutrient for all organisms. Its requirement in humans is illustrated dramatically by the genetic disorder acrodermatitis enteropathica (AE). AE is caused by the reduced uptake of dietary zinc by enterocytes, and the ensuing systemic zinc deficiency leads to dermatological lesions and immune and reproductive dysfunction. The gene responsible for AE, SLC39A4, encodes a member of the ZIP family of metal transporters, hZIP4. The mouse ZIP4 protein, mZIP4, stimulates zinc uptake in cultured cells, and studies in mice have demonstrated that zinc treatment decreases mZIP4 mRNA levels in the gut. In this study, we demonstrated using transfected cultured cells that the mZIP4 protein is also regulated at a post-translational level in response to zinc availability. Zinc deficiency increased mZIP4 protein levels at the plasma membrane, and this was associated with increased zinc uptake. Significantly, treating cells with low micromolar zinc concentrations stimulated the rapid endocytosis of the transporter. Zinc-regulated localization of the human ZIP4 protein was also demonstrated in cultured cells. These findings suggest that zinc-regulated trafficking of human and mouse ZIP4 is a key mechanism controlling dietary zinc absorption and cellular zinc homeostasis.

The acrodermatitis enteropathica gene ZIP4 encodes a tissue-specific, zinc-regulated zinc transporter in mice

The human ZIP4 gene (SLC39A4) is a candidate for the genetic disorder of zinc metabolism acrodermatitis enteropathica. To understand its role in zinc homeostasis, we examined the function and expression of mouse ZIP4. This gene encodes a well conserved eight-transmembrane protein that can specifically increase the influx of zinc into transfected cells. Expression of this gene is robust in tissues involved in nutrient uptake, such as the intestines and embryonic visceral yolk sac, and is dynamically regulated by zinc. Dietary zinc deficiency causes a marked increase in the accumulation of ZIP4 mRNA in these tissues, whereas injection of zinc or increasing zinc content of the diet rapidly reduces its abundance. Zinc can also regulate the accumulation of ZIP4 protein at the apical surface of enterocytes and visceral endoderm cells. These results provide compelling evidence that ZIP4 is a zinc transporter that plays an important role in zinc homeostasis, a process that is defective in acrodermatitis enteropathica in humans.

Functional expression of the human hZIP2 zinc transporter

Zinc is an essential nutrient for humans, yet we know little about how this metal ion is taken up by mammalian cells. In this report, we describe the characterization of hZip2, a human zinc transporter identified by its similarity to zinc transporters recently characterized in fungi and plants. hZip2 is a member of the ZIP family of eukaryotic metal ion transporters that includes two other human genes, hZIP1 and hZIP3, and genes in mice and rats. To test whether hZip2 is a zinc transporter, we examined (65)Zn uptake activity in transfected K562 erythroleukemia cells expressing hZip2 from the CMV promoter. hZip2-expressing cells accumulated more zinc than control cells because of an increased initial zinc uptake rate. This activity was time-, temperature-, and concentration-dependent and saturable with an apparent K(m) of 3 microM. hZip2 zinc uptake activity was inhibited by several other transition metals, suggesting that this protein may transport other substrates as well. hZip2 activity was not energy-dependent, nor did it require K(+) or Na(+) gradients. Zinc uptake by hZip2 was stimulated by HCO(3)(-) treatment, suggesting a Zn(2+)-HCO(3)(-) cotransport mechanism. Finally, hZip2 was exclusively localized in the plasma membrane. These results indicate that hZip2 is a zinc transporter, and its identification provides one of the first molecular tools to study zinc uptake in mammalian cells.

Zinc uptake into fibroblasts is inhibited by probenecid

Cellular zinc transport has not been fully characterized. The role of an anion carrier was investigated by treating normal human fibroblasts, and those carrying a mutation which affects zinc transport, acrodermatitis enteropathica (AE), with the anion carrier inhibitor, probenecid. Zinc uptake (2, 10, or 20 micromol 1(-1) 65zinc) was determined during initial rates of uptake (15 min) following treatment with 0, 10 or 20 mmol 1(-1) probenecid. Probenecid stimulated extracellular zinc binding in normal and AE fibroblasts. Probenecid inhibited the internalization of zinc in normal, but not AE, fibroblasts. Normal fibroblasts exhibited an apparent Km which was reduced by 53% and 44% in the 10 and 20 mmol 1(-1) probenecid treated cells. The Vmax was also reduced in the normal fibroblasts by 51% and 50% in the 10 and 20 mmol 1(-1) probenecid treated cells. The results suggest that a probenecid-sensitive anion carrier is involved in the internalization of zinc in human fibroblasts. The lack of an effect of probenecid on the internalization of zinc in the AE fibroblasts suggests that the mutation involves a probenecid-sensitive anion transport system, and that there may be a secondary mechanism for zinc transport in these cells.

Differences in the cellular zinc content and 5'-nucleotidase activity of normal and acrodermatitis enteropathica (AE) fibroblasts

The acrodermatitis enteropathica (AE) mutation affects zinc (Zn) metabolism in human fibroblasts. We hypothesize that the mutation affects the cell Zn content, which subsequently affects the activity of various zinc-dependent enzymes, such as 5'-nucleotidase. Therefore, normal and AE fibroblasts were grown in normal medium containing physiological levels of Zn (16 micromol/L) for approximately 24 h. The medium was replaced by normal medium (16 micromol/L Zn), Zn-depleted medium (1.5 micromol/L Zn), or Zn-supplemented medium (200 micromol/L Zn) for another 24 h. Regardless of the Zn concentration of the growth medium, the AE fibroblasts contained significantly less Zn than normal fibroblasts grown in comparable medium. Nevertheless, growth of the fibroblasts in 200 micromol/L Zn medium significantly increased the cell Zn content fourfold of both normal and AE fibroblasts. The activity of 5'-nucleotidase in the AE fibroblasts grown in 16 micromol/L Zn or 1.5 micromol/L Zn medium was also significantly lower than in normal fibroblasts. Changing the growth medium from 16 micromol/L Zn to 1.5 micromol/L Zn medium did not affect the activity of the enzyme in either genotype. Cells grown in 200 micromol/L Zn medium exhibited threefold greater 5'-nucleotidase activity in AE fibroblasts, but had no affect on enzyme activity in normal cells. In summary, altering the cell Zn content of normal fibroblasts did not result in a significant change in their 5'-nucleotidase activity. However, AE fibroblasts grown in 200 micromol/L Zn medium exhibited recovery of their 5'-nucleotidase activity to normal levels. These results support the hypothesis that the AE mutation affects the cellular Zn content. The lower cell Zn content subsequently affects the activity of 5'-nucleotidase.

Nystatin affects zinc uptake in human fibroblasts

The mechanism(s) by which zinc is transported into cells has not been identified. Since zinc uptake is inhibited by reducing the temperature, zinc uptake may depend on the movement of plasma membrane micoenvironments, such as endocytosis or potocytosis. We investigated the potential role of potocytosis in cellular zinc uptake by incubating normal and acrodermatitis enteropathica fibroblasts with nystatin, a sterol-binding drug previously shown to inhibit potocytosis. Zinc uptake was determined during initial rates of uptake (10 min) following incubation of the fibroblasts in 50 micrograms nystatin/mL or 0.1% dimethylsulfoxide for 10 min at 37 degrees C. The cells were then incubated with 1 to 30 microM 65zinc. Michaelis-Menten kinetics were observed for zinc uptake. Nystatin inhibited zinc uptake in both the normal and AE fibroblasts. Reduced cellular uptake of zinc was associated with its internalization, not its external binding. In normal fibroblasts, nystatin significantly reduced the K(m) 56% and the Vmax 69%. In the AE fibroblasts, nystatin treatment significantly reduced the Vmax 59%, but did not significantly affect the K(m). The AE mutation alone affected the Vmax for cellular zinc uptake. The control AE fibroblasts exhibited a 40% reduction in Vmax compared to control normal fibroblasts. We conclude that nystatin exerts its effect on zinc uptake by reducing the velocity at which zinc traverses the cell membrane, possibly through potocytosis. Furthermore, the AE mutation also affects zinc transport by reducing zinc transport.