The emerging roles and therapeutic potential of cyclin M/CorC family of Mg2+ transporters.

Cyclin M (CNNM) and its prokaryotic ortholog CorC belong to a family of proteins that function as Mg2+-extruding transporters by stimulating Na+/Mg2+ exchange, and thereby control intracellular Mg2+ levels. The Mg2+-extruding function of CNNM is inhibited by the direct binding of an oncogenic protein, phosphatase of regenerating liver (PRL), and this inhibition is responsible for the PRL-driven malignant progression of cancers. Studies with mouse strains deficient for the CNNM gene family revealed the importance of CNNM4 and CNNM2 in maintaining organismal Mg2+ homeostasis by participating in intestinal Mg2+ absorption and renal reabsorption, respectively. Moreover, CNNM proteins are involved in various diseases, and gene mutations in CNNM2 and CNNM4 cause dominant familial hypomagnesemia and Jalili syndrome, respectively. Genome wide association studies have also revealed the importance of CNNM2 in multiple major diseases, such as hypertension and schizophrenia. Collectively, the molecular and biological characterizations of CNNM/CorC show that they are an intriguing therapeutic target; the current status of drug development targeting these proteins is also discussed.

CNNM proteins selectively bind to the TRPM7 channel to stimulate divalent cation entry into cells.

Magnesium is essential for cellular life, but how it is homeostatically controlled still remains poorly understood. Here, we report that members of CNNM family, which have been controversially implicated in both cellular Mg2+ influx and efflux, selectively bind to the TRPM7 channel to stimulate divalent cation entry into cells. Coexpression of CNNMs with the channel markedly increased uptake of divalent cations, which is prevented by an inactivating mutation to the channel's pore. Knockout (KO) of TRPM7 in cells or application of the TRPM7 channel inhibitor NS8593 also interfered with CNNM-stimulated divalent cation uptake. Conversely, KO of CNNM3 and CNNM4 in HEK-293 cells significantly reduced TRPM7-mediated divalent cation entry, without affecting TRPM7 protein expression or its cell surface levels. Furthermore, we found that cellular overexpression of phosphatases of regenerating liver (PRLs), known CNNMs binding partners, stimulated TRPM7-dependent divalent cation entry and that CNNMs were required for this activity. Whole-cell electrophysiological recordings demonstrated that deletion of CNNM3 and CNNM4 from HEK-293 cells interfered with heterologously expressed and native TRPM7 channel function. We conclude that CNNMs employ the TRPM7 channel to mediate divalent cation influx and that CNNMs also possess separate TRPM7-independent Mg2+ efflux activities that contribute to CNNMs' control of cellular Mg2+ homeostasis.

Identification of NRAMP4 from Arabis paniculata enhance cadmium tolerance in transgenic Arabidopsis.

Arabis paniculata has been reported as a hyperaccumulator and functions in cadmium (Cd) tolerance and accumulation. However, the genes involved in Cd stress resistance in A. paniculata are still unknown. In this work, genes of the natural resistanceassociated macrophage proteins (NRAMPs) were characterized in A. paniculata, and their evolutionary relationship and expression patterns were analysed. Expression profiles indicated that ApNRAMPs showed large differences in response to Cd stress. It was highly induced by Cd in root and shoot tissues. To investigate the function of ApNRAMP4 under Cd stress, ApNRAMP4 was cloned and expressed in yeast and Arabidopsis. The results indicated that yeast and Arabidopsis expressing ApNRAMP4 showed normal growth under Cd stress. In addition, transgenic yeast and Arabidopsis showed the ability to concentrate Cd. Under 20 µM CdCl2, Cd concentrations in wild type (WT) and transgenic yeast were 3.11 and 5.92 mg/kg, respectively. Cd concentrations in root tissues of WTand transgenic Arabidopsis were 0.18 and 0.54 mg/kg, respectively. In shoot tissues of WT and transgenic Arabidopsis, Cd concentrations were 0.13 and 0.49 mg/kg, respectively. This report provides genomic information on hyperaccumulator A. paniculata. In addition, the present work identified key NRAMP genes that may serve as resources for heavy metal phytoremediation.

A role for zinc transporter gene SLC39A12 in the nervous system and beyond.

The SLC39A12 gene encodes the zinc transporter protein ZIP12, which is expressed across many tissues and is highly abundant in the vertebrate nervous system. As a zinc transporter, ZIP12 functions to transport zinc across cellular membranes, including cellular zinc influx across the plasma membrane. Genome-wide association and exome sequencing studies have shown that brain susceptibility-weighted magnetic resonance imaging (MRI) intensity is associated with ZIP12 polymorphisms and rare mutations. ZIP12 is required for neural tube closure and embryonic development in Xenopus tropicalis. Frog embryos depleted of ZIP12 by antisense morpholinos develop an anterior neural tube defect and lack viability. ZIP12 is also necessary for neurite outgrowth and mitochondrial function in mouse neural cells. ZIP12 mRNA is increased in brain regions of schizophrenic patients. Outside of the nervous system, hypoxia induces ZIP12 expression in multiple mammalian species, including humans, which leads to endothelial and smooth muscle thickening in the lung and contributes towards pulmonary hypertension. Other studies have associated ZIP12 with other diseases such as cancer. Given that ZIP12 is highly expressed in the brain and that susceptibility-weighted MRI is associated with brain metal content, ZIP12 may affect neurological diseases and psychiatric illnesses such as Parkinson's disease, Alzheimer's disease, and schizophrenia. Furthermore, the induction of ZIP12 and resultant zinc uptake under pathophysiological conditions may be a critical component of disease pathology, such as in pulmonary hypertension. Drug compounds that bind metals like zinc may be able to treat diseases associated with impaired zinc homeostasis and altered ZIP12 function.

Functional overlap of two major facilitator superfamily transporter, ZIF1, and ZIFL1 in zinc and iron homeostasis.

Zinc and iron are essential micronutrients for plant growth, and their homeostasis must be tightly regulated. Previously, it has been shown that Zinc-Induced Facilitator 1 (ZIF1) is involved in basal Zn tolerance by controlling the vacuolar storage of nicotianamine (NA). However, knowledge of the functional roles of two ZIF1 paralogs, ZIF-LIKE1 (ZIFL1) and ZIFL2, in metal homeostasis remains limited. Here, we functionally characterized the roles of ZIF1, ZIFL1, and ZIFL2 in Zn and Fe homeostasis. Expression of ZIF1 and ZIFL1 was induced by both excess Zn and Fe-deficiency, and their loss-of-function led to hypersensitivity under excess Zn and Fe-deficiency, suggesting functional overlap between ZIF1 and ZIFL1. By contrast, the disruption of ZIFL2 resulted in no obvious phenotypic alteration under both conditions. Additionally, the expression of ZIFL1, but not that of ZIFL2, in the zif1 mutant partially restored the phenotype under excess Zn, suggesting that ZIF1 and ZIFL1 perform functionally redundant roles in Zn homeostasis.

[Roles of Zinc Transporters That Control the Essentiality and Toxicity of Manganese and Cadmium].

Cellular transport systems for both essential and toxic trace elements remain elusive. In our studies on the transport systems for cadmium (Cd), we found that the cellular uptake of Cd is mediated by the transporter for manganese (Mn). We identified ZIP8 and ZIP14, members of the ZIP zinc (Zn) transporter family, as transporters having high affinities for both Cd and Mn. Notably, the uptake of Cd into rice root from soil is mediated by a transporter for Mn as well. We found that ZIP8 is highly expressed at the S3 segment of the kidney proximal tubule and can transport glomerulus-filtered Cd and Mn ions in the lumen into epithelial cells of the proximal tubule, suggesting that ZIP8 has an important role in the renal reabsorption of both toxic Cd and essential Mn. Mutations in ZIP8 and ZIP14 genes were found in humans having congenital disorders associated with the disturbed transport of Mn, although ZIP8 mutation causes whole-body Mn deficiency while ZIP14 mutation causes Mn accumulation in the brain. Mutations in ZnT10, a Zn transporter responsible for Mn excretion, also cause hyperaccumulation of Mn in the brain. Results of genome-wide association studies have indicated that ZIP8 SNPs are involved in a variety of common diseases. Thus, ZIP8, ZIP14, and ZnT10 play crucial roles in the transport of Mn and thereby control Mn- and Cd-related biological events in the body.

Knockdown of Circ_SLC39A8 protects against the progression of osteoarthritis by regulating miR-591/IRAK3 axis.

BACKGROUND: The dysregulation of circular RNAs (circRNAs) has been identified in various human diseases, including osteoarthritis (OA). The purpose of this study was to identify the role and mechanism of circ_SLC39A8 in regulating the progression of OA. METHODS: The expression levels of circ_SLC39A8, miR-591, and its potential target gene, interleukin-1-receptor-associated kinase 3 (IRAK3), were identified by quantitative real-time polymerase chain reaction (qRT-PCR). Cell viability and apoptosis were determined by Cell Counting Kit-8 (CCK-8) assay and flow cytometry, respectively. The relationship between miR-591 and circ_SLC39A8 or IRAK3 was predicted by bioinformatics tools and verified by dual-luciferase reporter. RESULTS: Circ_SLC39A8 and IRAK3 were upregulated and miR-591 was downregulated in OA cartilage tissues. Knockdown of circ_SLC39A8 inhibited apoptosis and inflammation in OA chondrocytes, while these effects were reversed by downregulating miR-591. Promotion cell viability effects of miR-591 were partially reversed by IRAK3 overexpression. CONCLUSION: Our findings indicated that knockdown of circ_SLC39A8 delayed the progression of OA via modulating the miR-591-IRAK3 axis, providing new insight into the molecular mechanisms of OA pathogenesis.

Marginally reduced maternal hepatic and splenic ferroportin under severe nutritional iron deficiency in pregnancy maintains systemic iron supply.

The demand for iron is high in pregnancy to meet the increased requirements for erythropoiesis. Even pregnant females with initially iron-replete stores develop iron-deficiency anemia, due to inadequate iron absorption. In anemic females, the maternal iron supply is dedicated to maintaining iron metabolism in the fetus and placenta. Here, using a mouse model of iron deficiency in pregnancy, we show that iron recycled from senescent erythrocytes becomes a predominant source of this microelement that can be transferred to the placenta in females with depleted iron stores. Ferroportin is a key protein in the molecular machinery of cellular iron egress. We demonstrate that under iron deficiency in pregnancy, levels of ferroportin are greatly reduced in the duodenum, placenta and fetal liver, but not in maternal liver macrophages and in the spleen. Although low expression of both maternal and fetal hepcidin predicted ferroportin up-regulation in examined locations, its final expression level was very likely correlated with tissue iron status. Our results argue that iron released into the circulation of anemic females is taken up by the placenta, as evidenced by high expression of iron importers on syncytiotrophoblasts. Then, a substantial decrease in levels of ferroportin on the basolateral side of syncytiotrophoblasts, may be responsible for the reduced transfer of iron to the fetus. As attested by the lowest decrease in iron content among analyzed tissues, some part is retained in the placenta. These findings confirm the key role played by ferroportin in tuning iron turnover in iron-deficient pregnant mouse females and their fetuses.

The grapevine CAX-interacting protein VvCXIP4 is exported from the nucleus to activate the tonoplast Ca2+/H+ exchanger VvCAX3.

MAIN CONCLUSION: The nuclear-localized CAX-interacting protein VvCXIP4 is exported to the cytosol after a Ca2+ pulse, to activate the tonoplast-localized Ca2+/H+ exchanger VvCAX3. Vacuolar cation/H+ exchangers (CAXs) have long been recognized as 'housekeeping' components in cellular Ca2+ and trace metal homeostasis, being involved in a range of key cellular and physiological processes. However, the mechanisms that drive functional activation of the transporters are largely unknown. In the present study, we investigated the function of a putative grapevine CAX-interacting protein, VvCXIP4, by testing its ability to activate VvCAX3, previously characterized as a tonoplast-localized Ca2+/H+ exchanger. VvCAX3 contains an autoinhibitory domain that drives inactivation of the transporter and thus, is incapable of suppressing the Ca2+-hypersensitive phenotype of the S. cerevisiae mutant K667. In this study, the co-expression of VvCXIP4 and VvCAX3 in this strain efficiently rescued its growth defect at high Ca2+ levels. Flow cytometry experiments showed that yeast harboring both proteins effectively accumulated higher Ca2+ levels than cells expressing each of the proteins separately. Bimolecular fluorescence complementation (BiFC) assays allowed visualization of the direct interaction between the proteins in tobacco plants and in yeast, and also showed the self-interaction of VvCAX3 but not of VvCXIP4. Subcellular localization studies showed that, despite being primarily localized to the nucleus, VvCXIP4 is able to move to other cell compartments upon a Ca2+ stimulus, becoming prone to interaction with the tonoplast-localized VvCAX3. qPCR analysis showed that both genes are more expressed in grapevine stems and leaves, followed by the roots, and that the steady-state transcript levels were higher in the pulp than in the skin of grape berries. Also, both VvCXIP4 and VvCAX3 were upregulated by Ca2+ and Na+, indicating they share common regulatory mechanisms. However, VvCXIP4 was also upregulated by Li+, Cu2+ and Mn2+, and its expression increased steadily throughout grape berry development, contrary to VvCAX3, suggesting additional physiological roles for VvCXIP4, including the regulation of VvCAXs not yet functionally characterized. The main novelty of the present study was the demonstration of physical interaction between CXIP and CAX proteins from a woody plant model by BiFC assays, demonstrating the intracellular mobilization of CXIPs in response to Ca2+.

Role of Fermented Goat Milk on Liver Gene and Protein Profiles Related to Iron Metabolism during Anemia Recovery.

Despite the crucial role of the liver as the central regulator of iron homeostasis, no studies have directly tested the modulation of liver gene and protein expression patterns during iron deficiency instauration and recovery with fermented milks. Fermented goat milk consumption improves the key proteins of intestinal iron metabolism during iron deficiency recovery, enhancing the digestive and metabolic utilization of iron. The aim of this study was to assess the influence of fermented goat or cow milk consumption on liver iron homeostasis during iron-deficiency anemia recovery with normal or iron-overload diets. Analysis included iron status biomarkers, gene and protein expression in hepatocytes. In general, fermented goat milk consumption either with normal or high iron content up-regulated liver DMT1, FPN1 and FTL1 gene expression and DMT1 and FPN1 protein expression. However, HAMP mRNA expression was lower in all groups of animals fed fermented goat milk. Additionally, hepcidin protein expression decreased in control and anemic animals fed fermented goat milk with normal iron content. In conclusion, fermented goat milk potentiates the up-regulation of key genes coding for proteins involved in iron metabolism, such as DMT1, and FPN1, FTL1 and down-regulation of HAMP, playing a key role in enhanced iron repletion during anemia recovery, inducing a physiological adaptation of the liver key genes and proteins coordinated with the fluctuation of the cellular iron levels, favoring whole-body iron homeostasis.

Segment-specific and direction-dependent transport of cadmium and manganese in immortalized S1, S2, and S3 cells derived from mouse kidney proximal tubules.

The kidney proximal tubule is a target of many renal toxicants, including cadmium (Cd), and also a place of reabsorption of essential metals in glomerular filtrate to systemic circulation. Although the mechanisms of metal transport in the convoluted proximal tubule (S1 and S2 segments) and the straight proximal tubule (S3 segment) may differ, little is known about the segment-specific modes of metal transport. Here, we utilized immortalized cell lines derived from the S1, S2, and S3 segments of mouse kidney proximal tubules, and examined the segment-specific and direction-dependent transport of Cd and manganese (Mn) using a trans-well culture system. The results showed that the uptakes of Cd2+ and Mn2+ from apical sides were the highest in S3 cells, and Cd2+, Mn2+, and Zn2+ mutually inhibited the apical uptake of each metal. As the expression of ZIP8, a zinc transporter having affinities for Cd2+ and Mn2+, was the highest in S3 cells, ZIP8 may contribute largely to the apical uptakes of these metals. The efficient uptake of Mn2+ from apical side of S3 cells may suggest an important role of ZIP8 in proximal tubule in reabsorption of Mn, an essential metal. Our study demonstrated that S1, S2, and S3 cells provide a useful tool for studying the segment-specific and direction-dependent transport of both toxic and essential metals in the kidney's proximal tubules.

Sideroflexin 3 is a Mitochondrial Protein Enriched in Neurons.

Sideroflexin 1 (Sfxn1) is a mitochondrial serine transporter involved in one-carbon metabolism in blood and cancer cell lines. The expression of other Sfxn homologs varies across tissues implying that each homolog may have tissue-specific functions. RNA databases suggest that among the Sfxns, Sfxn3 may have a specific function in the brain. Here, we systematically analyzed the level, cellular distribution, and subcellular localization of Sfxn3 protein in the developing and adult rodent brain. We found that, in the cortex and hippocampus, Sfxn3 protein level is low at birth but increases during development and remains at a high level in the mature brains. Similarly, in cultured hippocampal neurons, Sfxn3 protein level is low in young neurons but increases as neurons mature. Sfxn3 protein level is much higher in neurons than in astrocytes. Within neurons, Sfxn3 localizes to mitochondria in all major neuronal compartments. Our results establish that Sfxn3 is a mitochondrial protein enriched in neurons wherein it is developmentally expressed. These findings provide a foundation for future research aimed at understanding the functions of Sfxn3 and one-carbon metabolism in neurons.

Targeting the Zinc Transporter ZIP7 in the Treatment of Insulin Resistance and Type 2 Diabetes.

Type 2 diabetes mellitus (T2DM) is a disease associated with dysfunctional metabolic processes that lead to abnormally high levels of blood glucose. Preceding the development of T2DM is insulin resistance (IR), a disorder associated with suppressed or delayed responses to insulin. The effects of this response are predominately mediated through aberrant cell signalling processes and compromised glucose uptake into peripheral tissue including adipose, liver and skeletal muscle. Moreover, a major factor considered to be the cause of IR is endoplasmic reticulum (ER) stress. This subcellular organelle plays a pivotal role in protein folding and processes that increase ER stress, leads to maladaptive responses that result in cell death. Recently, zinc and the proteins that transport this metal ion have been implicated in the ER stress response. Specifically, the ER-specific zinc transporter ZIP7, coined the "gate-keeper" of zinc release from the ER into the cytosol, was shown to be essential for maintaining ER homeostasis in intestinal epithelium and myeloid leukaemia cells. Moreover, ZIP7 controls essential cell signalling pathways similar to insulin and activates glucose uptake in skeletal muscle. Accordingly, ZIP7 may be essential for the control of ER localized zinc and mechanisms that disrupt this process may lead to ER-stress and contribute to IR. Accordingly, understanding the mechanisms of ZIP7 action in the context of IR may provide opportunities to develop novel therapeutic options to target this transporter in the treatment of IR and subsequent T2DM.

Discovery of a ZIP7 inhibitor from a Notch pathway screen.

The identification of activating mutations in NOTCH1 in 50% of T cell acute lymphoblastic leukemia has generated interest in elucidating how these mutations contribute to oncogenic transformation and in targeting the pathway. A phenotypic screen identified compounds that interfere with trafficking of Notch and induce apoptosis via an endoplasmic reticulum (ER) stress mechanism. Target identification approaches revealed a role for SLC39A7 (ZIP7), a zinc transport family member, in governing Notch trafficking and signaling. Generation and sequencing of a compound-resistant cell line identified a V430E mutation in ZIP7 that confers transferable resistance to the compound NVS-ZP7-4. NVS-ZP7-4 altered zinc in the ER, and an analog of the compound photoaffinity labeled ZIP7 in cells, suggesting a direct interaction between the compound and ZIP7. NVS-ZP7-4 is the first reported chemical tool to probe the impact of modulating ER zinc levels and investigate ZIP7 as a novel druggable node in the Notch pathway.

Cnnm4 deficiency suppresses Ca2+ signaling and promotes cell proliferation in the colon epithelia.

CNNM4 is a Mg2+ transporter highly expressed in the colon epithelia. Its importance in regulating intracellular Mg2+ levels and cancer development has been documented, but how CNNM4 function affects the dynamic homeostasis of the epithelial tissue remains unclear. Here, we show that Cnnm4 deficiency promotes cell proliferation and partly suppresses cell differentiation in the colon epithelia, making them vulnerable to cancer development. Such phenotypic characteristics are highly similar to those of mice lacking Trpv1, which encodes the cation channel involved in capsaicin-stimulated Ca2+ influx. Indeed, Ca2+-imaging analyses using the organoid culture reveal that Ca2+ influx stimulated by capsaicin is greatly impaired by Cnnm4 deficiency. Moreover, EGF receptor signaling is constitutively activated in the colon epithelia of Cnnm4-deficient mice, as is the case with Trpv1-deficient mice. The administration of gefitinib, a clinically available inhibitor of EGF receptor, cancels the augmented proliferation of cells observed in Cnnm4-deficient mice. Collectively, these results establish the functional interplay between Mg2+ and Ca2+ in the colon epithelia, which is crucial for maintaining the dynamic homeostasis of the epithelial tissue.

Exosomal zinc transporter ZIP4 promotes cancer growth and is a novel diagnostic biomarker for pancreatic cancer.

Pancreatic cancer is one of the deadliest cancers with rapid disease progression. Further elucidation of its underlying molecular mechanisms and novel biomarkers for early detection is necessary. Exosomes are small extracellular vesicles that are released by multiple cell types acting as message carriers during intercellular communication and are promising biomarker candidates. However, the role of pancreatic cancer cell-derived exosomes in cancer progression and the application of these vesicles as novel diagnostic biomarkers have not been fully studied. In this study, we found that PC-1.0 (a highly malignant pancreatic cell line) cell-derived exosomes could be taken up by and enhance PC-1 (a moderately malignant pancreatic cell line) cell proliferation, migration and invasion abilities. We identified ZIP4 as the most upregulated exosomal protein in PC-1.0 cells from our proteomic analysis. In vitro and in vivo (a subcutaneous BALB/c nude mouse model) studies showed that exosomal ZIP4 can significantly promote pancreatic cancer growth. Using clinical blood samples, we compared the diagnostic values of serum exosomal ZIP4 levels between malignant pancreatic cancer patients (n = 24) and benign pancreatic disease patients (n = 32, AUC = .89), and between biliary disease patients (n = 32, AUC = .8112) and healthy controls (n = 46, AUC = .8931). In conclusion, exosomal ZIP4 promotes cancer growth and is a novel diagnostic biomarker for pancreatic cancer.

Role of ZIP8 in regulating cell morphology and NF-κB/Snail2 signaling.

ZIP8 is a recently identified membrane transporter which facilitates uptake of many substrates including both essential and toxic divalent metals (e.g. zinc, manganese, iron, cadmium) and inorganic selenium. Many ZIP8 regulated downstream signals and pathways remain to be elucidated. In this study, we investigated ZIP8 regulatory roles in downstream targets in ZIP8-gain and loss cells and in ZIP8 overexpressed lungs. Our results show that the overexpression of ZIP8 in mouse fibroblast cells (MEF) induces significant morphological change and re-organization of filament actin (F-actin), along with increased cell proliferation and migration rate. In ZIP8 knockout chronic myelogenous leukemia HAP1 cells, significant clonal morphological change with increased cell-cell adhesion was observed. In the ZIP8 overexpressed lung, F-actin was aberrantly enriched around the tracheal branch. In these ZIP8 gain and loss cell lines and ZIP8 transgenic lungs, we identified two relevant transcription factors, NF-κB and Snail2, whose activation is dependent on the ZIP8 level. They were both significantly upregulated in ZIP8 overexpressed cells and lungs. Expression of NF-κB and Snail2 targets, COL1A2 and E-cadherin, was also correspondingly elevated. Taken together, our results suggest that ZIP8 is a new regulator for cell morphology and cytoskeleton which involves NF-κB and Snail2.

The Multiple Faces of the Metal Transporter ZIP14 (SLC39A14).

The SLC39A family of metal transporters was identified through homologies with the Zrt- and Irt-like (ZIP) proteins from yeast and plants. Of all the ZIP transporters, ZIP14 is arguably the most robustly characterized in terms of function at the integrative level. Mice with a global knockout of Zip14 are viable, thus providing the opportunity to conduct physiologic experiments. In mice, Zip14 expression is highly tissue specific, with the greatest abundance in the jejunum > liver > heart > kidney > white adipose tissue > skeletal muscle > spleen > pancreas. A unique feature of Zip14 is its upregulation by proinflammatory conditions, particularly increased interleukin 6 (IL-6) and nitric oxide. The transcription factors AP-1, ATF4, and ATF6α are involved in Zip14 regulation. ZIP14 does not appear to be zinc-regulated. The Zip14 knockout phenotype shows multiple sites of ZIP14 function, including the liver, adipose tissue, brain, pancreas, and bone. A prominent feature of the Zip14 ablation is a reduction in intestinal barrier function and onset of metabolic endotoxemia. Many aspects of the phenotype are accentuated with age and accompany increased circulating IL-6. Studies with 65Zn, 59Fe [nontransferrin-bound iron (NTBI)] and 54Mn show that ZIP14 transports these metals. At a steady state, the plasma concentrations of zinc, NTBI, and manganese are such that zinc ions are the major substrate available for ZIP14 at the cell surface. Upregulation of ZIP14 accounts for the hypozincemia and hepatic zinc accumulation associated with acute inflammation and sepsis and is required for liver regeneration and resistance to endoplasmic reticulum (ER) stress. Zip14 ablation in mice produces a defect in manganese excretion that leads to excess manganese accumulation in the brain that produces characteristics of Parkinsonism.

Protein synthesis controls phosphate homeostasis.

Phosphorus is an essential element assimilated largely as orthophosphate (Pi). Cells respond to Pi starvation by importing Pi from their surroundings. We now report that impaired protein synthesis alone triggers a Pi starvation response even when Pi is plentiful in the extracellular milieu. In the bacterium Salmonella enterica serovar Typhimurium, this response entails phosphorylation of the regulatory protein PhoB and transcription of PhoB-dependent Pi transporter genes and is eliminated upon stimulation of adenosine triphosphate (ATP) hydrolysis. When protein synthesis is impaired due to low cytoplasmic magnesium (Mg2+), Salmonella triggers the Pi starvation response because ribosomes are destabilized, which reduces ATP consumption and thus free cytoplasmic Pi. This response is transient because low cytoplasmic Mg2+ promotes an uptake in Mg2+ and a decrease in ATP levels, which stabilizes ribosomes, resulting in ATP consumption and Pi increase, thus ending the response. Notably, pharmacological inhibition of protein synthesis also elicited a Pi starvation response in the bacterium Escherichia coli and the yeast Saccharomyces cerevisiae Our findings identify a regulatory connection between protein synthesis and Pi homeostasis that is widespread in nature.

The P-type ATPase CtpG preferentially transports Cd2+ across the Mycobacterium tuberculosis plasma membrane.

P1B-type ATPases are involved in heavy metal transport across the plasma membrane. Some Mycobacterium tuberculosis P-type ATPases are induced during infection, suggesting that this type of transporter could play a critical role in mycobacterial survival. To date, the ion specificity of M. tuberculosis heavy metal-transporting P1B-ATPases is not well understood. In this work, we observed that, although divalent heavy metal cations such as Cu2+, Co2+, Ni2+, Zn2+ Cd2+ and Pb2+ stimulate the ATPase activity of the putative P1B-type ATPase CtpG in the plasma membrane, whole cells of M. smegmatis expressing CtpG only tolerate high levels of Cd2+ and Cu2+. As indicator of the catalytic constant, Michaelis-Menten kinetics showed that CtpG embedded in the mycobacterial cell membrane has a V max/K m ratio 7.4-fold higher for Cd2+ than for Cu2+ ions. Thus, although CtpG can accept different substrates in vitro, this P-type ATPase transports Cd2+ more efficiently than other heavy metal cations across the mycobacterial plasma membrane.