Study of the interactions of cadmium and zinc ions with cellular calcium homoeostasis using 19F-NMR spectroscopy

The role of calcium, Akt and ERK signaling in cadmium-induced hair cell death

Exposure to heavy metals has been shown to cause damage to a variety of different tissues and cell types including hair cells, the sensory cells of our inner ears responsible for hearing and balance. Elevated levels of one such metal, cadmium, have been associated with hearing loss and shown to cause hair cell death in multiple experimental models. While the mechanisms of cadmium-induced cell death have been extensively studied in other cell types they remain relatively unknown in hair cells. We have found that calcium signaling, which is known to play a role in cadmium-induced cell death in other cell types through calmodulin and CaMKII activation as well as IP3 receptor and mitochondrial calcium uniporter mediated calcium flow, does not appear to play a significant role in cadmium-induced hair cell death. While calmodulin inhibition can partially protect hair cells this may be due to impacts on mechanotransduction activity. Removal of extracellular calcium, and inhibiting CaMKII, the IP3 receptor and the mitochondrial calcium uniporter all failed to protect against cadmium-induced hair cell death. We also found cadmium treatment increased pAkt levels in hair cells and pERK levels in supporting cells. This activation may be protective as inhibiting these pathways enhances cadmium-induced hair cell death rather than protecting cells. Thus cadmium-induced hair cell death appears distinct from cadmium-induced cell death in other cell types where calcium, Akt and ERK signaling all promote cell death.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Essential metals in health and disease

In total, twenty elements appear to be essential for the correct functioning of the human body, half of which are metals and half are non-metals. Among those metals that are currently considered to be essential for normal biological functioning are four main group elements, sodium (Na), potassium (K), magnesium (Mg), and calcium (Ca), and six d-block transition metal elements, manganese (Mn), iron (Fe), cobalt (Co), copper (Cu), zinc (Zn) and molybdenum (Mo). Cells have developed various metallo-regulatory mechanisms for maintaining a necessary homeostasis of metal-ions for diverse cellular processes, most importantly in the central nervous system. Since redox active transition metals (for example Fe and Cu) may participate in electron transfer reactions, their homeostasis must be carefully controlled. The catalytic behaviour of redox metals which have escaped control, e.g. via the Fenton reaction, results in the formation of reactive hydroxyl radicals, which may cause damage to DNA, proteins and membranes. Transition metals are integral parts of the active centers of numerous enzymes (e.g. Cu,Zn-SOD, Mn-SOD, Catalase) which catalyze chemical reactions at physiologically compatible rates. Either a deficiency, or an excess of essential metals may result in various disease states arising in an organism. Some typical ailments that are characterized by a disturbed homeostasis of redox active metals include neurological disorders (Alzheimer's, Parkinson's and Huntington's disorders), mental health problems, cardiovascular diseases, cancer, and diabetes. To comprehend more deeply the mechanisms by which essential metals, acting either alone or in combination, and/or through their interaction with non-essential metals (e.g. chromium) function in biological systems will require the application of a broader, more interdisciplinary approach than has mainly been used so far. It is clear that a stronger cooperation between bioinorganic chemists and biophysicists - who have already achieved great success in understanding the structure and role of metalloenzymes in living systems - with biologists, will access new avenues of research in the systems biology of metal ions. With this in mind, the present paper reviews selected chemical and biological aspects of metal ions and their possible interactions in living systems under normal and pathological conditions.

The Cardiac Ryanodine Receptor Provides a Suitable Pathway for the Rapid Transport of Zinc (Zn2+)

The sarcoplasmic reticulum (SR) in cardiac muscle is suggested to act as a dynamic storage for Zn²⁺ release and reuptake, albeit it is primarily implicated in the Ca²⁺ signaling required for the cardiac cycle. A large Ca²⁺ release from the SR is mediated by the cardiac ryanodine receptor (RYR2), and while this has a prominent conductance for Ca²⁺ in vivo, it also conducts other divalent cations in vitro. Since Zn²⁺ and permeant Mg²⁺ have similar physical properties, we tested if the RYR2 channel also conducts Zn²⁺. Using the method of planar lipid membranes, we evidenced that the RYR2 channel is permeable to Zn²⁺ with a considerable conductance of 81.1 ± 2.4 pS, which was significantly lower than the values for Ca²⁺ (127.5 ± 1.8 pS) and Mg²⁺ (95.3 ± 1.4 pS), obtained under the same asymmetric conditions. Despite similar physical properties, the intrinsic Zn²⁺ permeability (P_Ca/P_Zn = 2.65 ± 0.19) was found to be ~2.3-fold lower than that of Mg²⁺ (P_Ca/P_Mg = 1.146 ± 0.071). Further, we assessed whether the channel itself could be a direct target of the Zn²⁺ current, having the Zn²⁺ finger extended into the cytosolic vestibular portion of the permeation pathway. We attempted to displace Zn²⁺ from the RYR2 Zn²⁺ finger to induce its structural defects, which are associated with RYR2 dysfunction. Zn²⁺ chelators were added to the channel cytosolic side or strongly competing cadmium cations (Cd²⁺) were allowed to permeate the RYR2 channel. Only the Cd²⁺ current was able to cause the decay of channel activity, presumably as a result of Zn²⁺ to Cd²⁺ replacement. Our findings suggest that the RYR2 channel can provide a suitable pathway for rapid Zn²⁺ escape from the cardiac SR; thus, the channel may play a role in local and/or global Zn²⁺ signaling in cardiomyocytes.

Exposure to multiple metals and the risk of hypertension in adults: A prospective cohort study in a local area on the Yangtze River, China

BACKGROUND

Exposure to multiple metals is recognized as a common and real scenario in daily life. However, limited prospective studies have assessed associations between multiple metals exposure and hypertension.

METHODS

In total, 2625 adults in a local area on the Yangtze River were investigated at baseline from 2014 to 2015 and followed up in 2019. We measured baseline urine levels of 22 metals and used multivariate logistic analysis and Bayesian kernel machine regression (BKMR) to explore associations between multiple metals exposure and the risk of hypertension.

RESULTS

A total of 385 individuals (29.6%) were diagnosed with hypertension. Five metals (cadmium, copper, magnesium, molybdenum and zinc) were positively associated with hypertension in single-metal models. Cadmium and zinc remained significantly positive associations after adjusting for these five metals, with the odds ratio (OR) in the highest quartiles of 1.49 (95% CI: 1.01, 2.21; p-trend = 0.05) and 1.60 (95% CI: 1.08, 2.38; p-trend = 0.02), respectively. BKMR analysis showed a significant joint effect of multiple metals on hypertension when the concentrations of five metals were at or above their 55th percentile compared with their median values. A potential interaction between cadmium and zinc in increasing the risk of hypertension was observed with the OR_int of 1.41 (95%CI: 1.05, 1.89).

CONCLUSIONS

We identified the joint effect of multiple metals on hypertension and observed a significant interaction between cadmium and zinc. Further cohort studies are needed to clarify the health effects of multiple metals exposure in a larger population.

Cellular and physiological approaches to evaluate the chelating effect of Chlorella on metal ion stressed lymphocytes

Chlorella is a green alga consumed as dietary food supplement in pulverized form. In addition to its high nutritional value, it is reported as an excellent detoxifying agent. The pulverized Chlorella is partially soluble in water and insoluble portion has been reported for removal of mercury, cadmium and radioactive strontium from body. Chlorella contains a variety of metal-binding functional groups such as carboxyl, amino, phosphoryl, hydroxyl and carbonyl groups, which has high affinity towards various metal ions. The present study was envisaged to evaluate the chelating effect of water soluble fraction of Chlorella powder (AqCH) on metal ions. Fura-2 fluorescence ratio (F340/F380) was measured by fluorescence spectrometer (FS) after the exposure of chloride salt of metals viz., strontium, cobalt, barium, cesium, thallium and mercury to lymphocytes. Pretreatment of AqCH (0.1-20 mg mL^-1) was given to evaluate the attenuating effect on fura-2 fluorescence ratio induced by metal ions. The intracellular levels of these metal ions were analyzed by atomic absorption spectrophotometer (AAS) and fluorescence microscopy (FM). Pretreatment with AqCH significantly attenuated the metal induced fluorescence ratio in dose-dependent manner. The results of AAS and FM were found in coherence with fura-2 fluorescence ratio which emphasized that AqCH significantly prevented the metal ions internalization. The present study suggests AqCH chelates with these metal ions and prevents its interaction with cells thereby reducing the intracellular mobilization of Ca²⁺.

Metallation pathway of a plant metallothionein: Cicer arietinum MT2

The plant metallothionein 2 protein from Cicer arietinum (cicMT2) is a typical member of the plant MT subfamily p2 that is characterized by an N- and C-terminal cysteine- (Cys-)rich, metal binding sequence connected by a long cysteine-free linker region. cicMT2 coordinates up to five Zn^II or Cd^II ions by its 14 cysteine thiolate groups forming a single metal-thiolate cluster. While MTs from other phyla are considerably well-studied, many details about plant MTs are missing. In this study the metallation pathway of cicMT2 is investigated using mass spectrometry. To evaluate the influence of the linker region as well as the interplay of the two Cys-rich stretches, the full-length cicMT2 protein as well as the individual Cys-rich domains with and without the linker region were analysed. Up to three Cd^II ions can be coordinated by the eight Cys residues of the N-terminal part and up to two Cd^II ions by the six Cys residues of the C-terminal sequence. However, no preferential binding to either of the two sequences is observed, which is in-line with the closely similar apparent binding constants of the individual domains obtained from competition reactions with the chelator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid. The combination of limited proteolytic digestion, mass spectrometry, dynamic light scattering, size-exclusion chromatography, and ¹⁹F NMR spectroscopy enables us to draw conclusions about the overall protein-fold and the cluster formation process.

First Crystal Structure of a Nonstructural Hepatitis E Viral Protein Identifies a Putative Novel Zinc-Binding Protein

Hepatitis E virus (HEV) is a 7.2-kb positive-sense, single-stranded RNA virus containing three partially overlapping reading frames, ORF1 to ORF3. All nonstructural proteins required for viral replication are encoded by ORF1 and are transcribed as a single transcript. Computational analysis of the complete ORF1 polyprotein identified a previously uncharacterized region of predicted secondary structure bordered by two disordered regions coinciding partially with a region predicted as a putative cysteine protease. Following successful cloning, expression, and purification of this region, the crystal structure of the identified protein was determined and identified to have considerable structural homology to a fatty acid binding domain. Further analysis of the structure revealed a metal binding site, shown unambiguously to specifically bind zinc via a nonclassical, potentially catalytic zinc-binding motif. Based on the structural homology of the HEV protein with known structures, along with the presence of a catalytic zinc-binding motif, it is possible that the identified protein corresponds to the HEV protease, which could require activation or repression through the binding of a fatty acid. This represents a significant step forward in the characterization and the understanding of the molecular mechanisms of the HEV genome. We present analysis for the first time of this identified nonstructural protein, expanding the knowledge and understanding of the complex mechanisms of HEV biology.IMPORTANCE Hepatitis E virus (HEV) is an emerging virus found predominately in developing countries; it causes an estimated 20 million infections, which result in approximately 57,000 deaths a year. Although it is known that the nonstructural proteins of HEV ORF1 are expressed as a single transcript, there is debate as to whether ORF1 functions as a single polyprotein or if it is processed into separate domains via a viral or endogenous cellular protease. Here we present the first structural and biophysical characterization of an HEV nonstructural protein using a construct that has partially overlapping boundaries with the predicted putative cysteine protease.

Environmental exposure to metals and the risk of hypertension: A cross-sectional study in China

Metal pollution is a severe environmental issue in China, which has been recently linked with the risk of hypertension. However, relevant epidemiological studies are limited. The present exploratory study was conducted to assess the associations of environmental exposure to metals with the odds of hypertension as well as blood pressure (BP) levels using urine samples in a Chinese general population. From May 2016 to April 2017, a total of 823 eligible participants were consecutively enrolled in our study in Wuhan, China. Hypertension was defined as systolic BP (SBP) of ≥140 mmHg or diastolic BP (DBP) of ≥90 mmHg, a self-reported physician diagnosis, or current use of antihypertensive medication. We used urine samples as biomarkers to reflect the levels of environmental exposure to 20 metals. Multivariable regression models were applied to assess the potential association. Multi-metal models were conducted to investigate the impacts of co-exposure to various metals. Based on the results from various models, positive trends for increased odds of hypertension with increasing quartiles of vanadium (V), iron (Fe), zinc (Zn) and selenium (Se) were suggested. Compared with those in the lowest quartiles, participants in the highest quartiles of V, Fe, Zn and Se had a 4.4-fold, 4.9-fold, 4.2-fold and 2.5-fold increased odds of having hypertension, respectively. High urinary Hg level was found to increase the levels of DBP. Individuals in the highest group of Hg were found to have a 4.3 mmHg higher level of DBP. Our findings suggest that environmental exposure to V, Fe, Zn, Se and Hg might increase the risk of hypertension or elevate the levels of BP. These findings warrant further prospective studies in a larger population.

Evaluating electronic structure methods for accurate calculation of 19 F chemical shifts in fluorinated amino acids

The ability of electronic structure methods (11 density functionals, HF, and MP2 calculations; two basis sets and two solvation models) to accurately calculate the ¹⁹ F chemical shifts of 31 structures of fluorinated amino acids and analogues with known experimental ¹⁹ F NMR spectra has been evaluated. For this task, BHandHLYP, ωB97X, and Hartree-Fock with scaling factors (provided within) are most accurate. Additionally, the accuracy of methods to calculate relative changes in fluorine shielding across 23 sets of structural variants, such as zwitterionic amino acids versus side chains only, was also determined. This latter criterion may be a better indicator of reliable methods for the ultimate goal of assigning and interpreting chemical shifts of fluorinated amino acids in proteins. It was found that MP2 and M062X calculations most accurately assess changes in shielding among analogues. These results serve as a guide for computational developments to calculate ¹⁹ F chemical shifts in biomolecular environments. © 2017 Wiley Periodicals, Inc.

Dysregulated Zn2+ homeostasis impairs cardiac type-2 ryanodine receptor and mitsugumin 23 functions, leading to sarcoplasmic reticulum Ca2+ leakage

Aberrant Zn²⁺ homeostasis is associated with dysregulated intracellular Ca²⁺ release, resulting in chronic heart failure. In the failing heart a small population of cardiac ryanodine receptors (RyR2) displays sub-conductance-state gating leading to Ca²⁺ leakage from sarcoplasmic reticulum (SR) stores, which impairs cardiac contractility. Previous evidence suggests contribution of RyR2-independent Ca²⁺ leakage through an uncharacterized mechanism. We sought to examine the role of Zn²⁺ in shaping intracellular Ca²⁺ release in cardiac muscle. Cardiac SR vesicles prepared from sheep or mouse ventricular tissue were incorporated into phospholipid bilayers under voltage-clamp conditions, and the direct action of Zn²⁺ on RyR2 channel function was examined. Under diastolic conditions, the addition of pathophysiological concentrations of Zn²⁺ (≥2 nm) caused dysregulated RyR2-channel openings. Our data also revealed that RyR2 channels are not the only SR Ca²⁺-permeable channels regulated by Zn²⁺. Elevating the cytosolic Zn²⁺ concentration to 1 nm increased the activity of the transmembrane protein mitsugumin 23 (MG23). The current amplitude of the MG23 full-open state was consistent with that previously reported for RyR2 sub-conductance gating, suggesting that in heart failure in which Zn²⁺ levels are elevated, RyR2 channels do not gate in a sub-conductance state, but rather MG23-gating becomes more apparent. We also show that in H9C2 cells exposed to ischemic conditions, intracellular Zn²⁺ levels are elevated, coinciding with increased MG23 expression. In conclusion, these data suggest that dysregulated Zn²⁺ homeostasis alters the function of both RyR2 and MG23 and that both ion channels play a key role in diastolic SR Ca²⁺ leakage.

Protein kinase CK2 regulates metal toxicity in neuronal cells

Protein kinase CK2 is a pleiotropic tetrameric enzyme, regulating numerous biological processes from cell proliferation to stress response. This study demonstrates for the first time that CK2 is involved in the regulation of metal uptake and toxicity in neuronal cells. After the determination of inhibitory concentrations (IC50) for a range of metal salts (ZnSO4, Al(mal)3, CoCl2, CrO3, NaAsO2 and CaCl2) in Neuro-2a mouse neuroblastoma cells, the effect of CK2 on metal toxicity was investigated by three lines of experiments using CK2 inhibitors, metal ion specific fluorophores and siRNA-mediated knockdown of CK2 expression. The results showed that both CK2 inhibitors, 4,5,6,7-tetrabromobenzotriazole (TBB) and quinalizarin, markedly reduced the toxicity of Zn(ii), Al(iii), Co(ii), Cr(vi) and As(iii). Confocal microscopy imaging revealed that Zn(ii) uptake was accompanied by the increase of intracellular Ca(ii) in Neuro-2a cells treated with IC50 of ZnSO4 (240 μM), and such concurrent elevation of intracellular Zn(ii) and Ca(ii) was blocked by TBB and quinalizarin. The role of CK2 in metal uptake was further characterised using specific siRNA against each of the three subunits (CK2α, α' and β) and the data demonstrate that CK2α' is the prominent subunit regulating the metal toxicity. Finally, the role of CK2 in metal toxicity was found to be conserved in the distant species-Saccharomyces cerevisiae by employing the complete deletion mutants of CK2 (cka1Δ, cka2Δ, ckb1Δ and ckb2Δ). Taken together, these findings shed light on a new facet of CK2 functionality and provide a basis for further research on the regulation of Zn(ii) and Ca(ii) homeostasis by CK2.

Zinc flexes its muscle: Correcting a novel analysis of calcium for zinc interference uncovers a method to measure zinc

The divalent cation chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), often used to buffer physiological changes in cytosolic Ca(2+), also binds Zn(2+) with high affinity. In a recently published method (Lamboley et al. 2015. J. Gen. Physiol. http://dx.doi.org/10.1085/jgp.201411250), the absorbance shift of BAPTA at 292 nm was successfully used to determine the total calcium concentrations of various skeletal muscle tissues. In the present study, we show that endogenous Zn(2+) in rat skeletal muscle tissue can be unknowingly measured as "Ca(2+)," unless appropriate measures are taken to eliminate Zn(2+) interference. We analyzed two rat skeletal muscle tissues, soleus and plantaris, for total calcium and zinc using either inductively coupled plasma mass spectrometry (ICP-MS) or the BAPTA method described above. ICP-MS analysis showed that total zinc contents in soleus and plantaris were large enough to affect the determination of total calcium by the BAPTA method (calcium = 1.72 ± 0.31 and 1.96 ± 0.14, and zinc = 0.528 ± 0.04 and 0.192 ± 0.01; mean ± standard error of the mean [SEM]; n = 5; mmole/kg, respectively). We next analyzed total calcium using BAPTA but included the Zn(2+)-specific chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) that buffers Zn(2+) without affecting Ca(2+)/BAPTA binding. We found that estimated concentrations of total calcium ([CaT]WM) in soleus and plantaris were reduced after TPEN addition ([CaT]WM = 3.71 ± 0.62 and 3.57 ± 0.64 without TPEN and 3.39 ± 0.64 and 3.42 ± 0.62 with TPEN; mean ± SEM; n = 3; mmole/kg, respectively). Thus, we show that a straightforward correction can be applied to the BAPTA method to improve the accuracy of the determination of total calcium that should be applicable to most any tissue studied. In addition, we show that using TPEN in combination with the BAPTA method allows one to make reasonable estimates of total zinc concentration that are in agreement with the direct determination of zinc concentration by ICP-MS.

Mobilization and removing of cadmium from kidney by GMDTC utilizing renal glucose reabsorption pathway

Chronic exposure to cadmium compounds (Cd(2+)) is one of the major public health problems facing humans in the 21st century. Cd(2+) in the human body accumulates primarily in the kidneys which leads to renal dysfunction and other adverse health effects. Efforts to find a safe and effective drug for removing Cd(2+) from the kidneys have largely failed. We developed and synthesized a new chemical, sodium (S)-2-(dithiocarboxylato((2S,3R,4R,5R)-2,3,4,5,6 pentahydroxyhexyl)amino)-4-(methylthio) butanoate (GMDTC). Here we report that GMDTC has a very low toxicity with an acute lethal dose (LD50) of more than 10,000mg/kg or 5000mg/kg body weight, respectively, via oral or intraperitoneal injection in mice and rats. In in vivo settings, up to 94% of Cd(2+) deposited in the kidneys of Cd(2+)-laden rabbits was removed and excreted via urine following a safe dose of GMDTC treatment for four weeks, and renal Cd(2+) level was reduced from 12.9μg/g to 1.3μg/g kidney weight. We observed similar results in the mouse and rat studies. Further, we demonstrated both in in vitro and in animal studies that the mechanism of transporting GMDTC and GMDTC-Cd complex into and out of renal tubular cells is likely assisted by two glucose transporters, sodium glucose cotransporter 2 (SGLT2) and glucose transporter 2 (GLUT2). Collectively, our study reports that GMDTC is safe and highly efficient in removing deposited Cd(2+) from kidneys assisted by renal glucose reabsorption system, suggesting that GMDTC may be the long-pursued agent used for preventive and therapeutic purposes for both acute and chronic Cd(2+) exposure.

Characterization of Zinc Influx Transporters (ZIPs) in Pancreatic β Cells: ROLES IN REGULATING CYTOSOLIC ZINC HOMEOSTASIS AND INSULIN SECRETION

Zinc plays an essential role in the regulation of pancreatic β cell function, affecting important processes including insulin biosynthesis, glucose-stimulated insulin secretion, and cell viability. Mutations in the zinc efflux transport protein ZnT8 have been linked with both type 1 and type 2 diabetes, further supporting an important role for zinc in glucose homeostasis. However, very little is known about how cytosolic zinc is controlled by zinc influx transporters (ZIPs). In this study, we examined the β cell and islet ZIP transcriptome and show consistent high expression of ZIP6 (Slc39a6) and ZIP7 (Slc39a7) genes across human and mouse islets and MIN6 β cells. Modulation of ZIP6 and ZIP7 expression significantly altered cytosolic zinc influx in pancreatic β cells, indicating an important role for ZIP6 and ZIP7 in regulating cellular zinc homeostasis. Functionally, this dysregulated cytosolic zinc homeostasis led to impaired insulin secretion. In parallel studies, we identified both ZIP6 and ZIP7 as potential interacting proteins with GLP-1R by a membrane yeast two-hybrid assay. Knock-down of ZIP6 but not ZIP7 in MIN6 β cells impaired the protective effects of GLP-1 on fatty acid-induced cell apoptosis, possibly via reduced activation of the p-ERK pathway. Therefore, our data suggest that ZIP6 and ZIP7 function as two important zinc influx transporters to regulate cytosolic zinc concentrations and insulin secretion in β cells. In particular, ZIP6 is also capable of directly interacting with GLP-1R to facilitate the protective effect of GLP-1 on β cell survival.

Synthesis and properties of Asante Calcium Red--a novel family of long excitation wavelength calcium indicators

Although many synthetic calcium indicators are available, a search for compounds with improved characteristics continues. Here, we describe the synthesis and properties of Asante Calcium Red-1 (ACR-1) and its low affinity derivative (ACR-1-LA) created by linking BAPTA to seminaphthofluorescein. The indicators combine a visible light (450-540 nm) excitation with deep-red fluorescence (640 nm). Upon Ca2+ binding, the indicators raise their fluorescence with longer excitation wavelengths producing higher responses. Although the changes occur without any spectral shifts, it is possible to ratio Ca(2+)-dependent (640 nm) and quasi-independent (530 nm) emission when using visible (< 490 nm) or multiphoton (∼780 nm) excitation. Therefore, both probes can be used as single wavelength or, less dynamic, ratiometric indicators. Long indicator emission might allow easy [Ca2+]i measurement in GFP expressing cells. The indicators bind Ca2+ with either high (Kd = 0.49 ± 0.07 μM; ACR-1) or low affinity (Kd = 6.65 ± 0.13 μM; ACR-1-LA). Chelating Zn2+ (Kd = 0.38 ± 0.02 nM) or Mg2+ (Kd∼5mM) slightly raises and binding Co2+ quenches dye fluorescence. New indicators are somewhat pH-sensitive (pKa = 6.31 ± 0.07), but fairly resistant to bleaching. The probes are rather dim, which combined with low AM ester loading efficiency, might complicate in situ imaging. Despite potential drawbacks, ACR-1 and ACR-1-LA are promising new calcium indicators.

Effect of complexing ligands on the surface adsorption, internalization, and bioresponse of copper and cadmium in a soil bacterium, Pseudomonas putida

Environmental quality criteria for metals toxic to soil and water organisms, using the free ion activity model or the biotic ligand model, are based on the concept that the major form of the metal available to the organism is the free metal ion, yet various metal complexes are bioavailable to a variety of soil and water organisms. We test here whether neutral copper or cadmium sulfates, negatively-charged copper or cadmium citrates and positively-charged copper acetate and cadmium chloride are bioavailable to a soil bacterium, Pseudomonas putida. Adsorption onto the cell surface and uptake into the periplasm and cytoplasm of this Gram-negative root colonizing bacterium was studied by adding a single concentration of Cu or Cd and varying the concentration of the ligands to complex 10-100% of the metal. Metal association from the complexes on and within the cell was defined using selective extraction procedures and compared with free ion controls using the Langmuir isotherm. Cellular responses also were assessed using a P. putida biosensor. Both uptake and bioresponse methodologies showed that P. putida was sensitive to the metal complexes. In particular, the bioresponse to Cu and Cd supplied as a citrate complex occurred with activities of free metal ions two orders of magnitude lower than for the control. We concluded that the tested metal complexes for Cu and Cd are taken up into the cell, where they trigger a bioresponse. We also discuss the implications of these findings on interactions between soil and water organisms and nanoparticles that release metal ions.

Effects of heavy metals on mitogen-activated protein kinase pathways

The signaling pathways leading to cellular protection or cell death following exposure to heavy metals have not been fully clarified. Mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated protein kinase (ERK), c-Jun NH(2)-terminal kinase (JNK) and p38 MAPK transmit extracellular signals into the nucleus, and have been shown to participate in a diverse array of cellular functions such as cell growth, differentiation and apoptosis. Treatment with cadmium, inorganic mercury or tributyltin can activate ERK, JNK and p38 MAPK, and induces the expression of c-fos and c-jun genes prior to the development of apoptosis. However, the members of the MAPK family appear to be differentially activated depending on the heavy metal and the cell type exposed. Consequently, various cellular responses may be caused by the distinct pattern of MAPKs activation. MAPKs may be one of the important cellular signal transduction pathways affected by various environmental pollutants, including heavy metals.

Effects of chronic cadmium poisoning on Zn, Cu, Fe, Ca, and metallothionein in liver and kidney of rats

An experiment was conducted to invest effects of chronic cadmium poisoning on Zn, Cu, Fe, Ca, and metallothionein gene expression and protein synthesis in liver and kidney in rats. Forty rats, 6 weeks old, were randomly allocated into two groups. A group was given CdCl(2) (1 mg/KgCd(2+)) by intraperitoneal injection once a day. The other group was treated with normal saline in the same way. Liver and kidney were collected for analysis at the end of the third week. Results showed that Cd exposure increased Cd (P<0.01) and Zn (P<0.01) content both in liver and kidney. Fe and Ca concentration had a considerable increase in kidney (P<0.01), while both had different degree reduction in liver. Discrepancies between MT mRNA and protein were observed in liver and kidney. In liver, both MT mRNA and protein had a significant increase (P<0.01), while in kidney, only MT gene increase was checked. Meanwhile, the expression levels of MT-1 mRNA and MT-2 mRNA were distinct between liver and kidney. The present study indicated that changes in tissue Cd and Zn levels tended to reflect MT mRNA expression, but bear no clear relationship with MT protein. There did not have a strict dose-dependent relationship among Cd content, MT gene expression, and MT protein synthesis. What is more, changes of Zn, Fe, Cu, and Ca had a certain interaction with both MT mRNA and protein.

2, 2'-(Phenylazanediyl) diacetic acid modified Fe3O4@PEI for selective removal of cadmium ions from blood

A water-dispersible and supermagnetic nanocomposite (PAD-PEG-Fe(3)O(4)@PEI) has been successfully synthesized using polyethylenimine (PEI, Mol MW = 10000) coated supermagnetic Fe(3)O(4)-NH(2) which was modified with 2, 2'-(phenylazanediyl) diacetic acid (PAD) through the bridge of poly(ethylene glycol) (PEG, Mol MW = 2000). The average particle size of PAD-PEG-Fe(3)O(4)@PEI was determined by TEM, and was about 50 nm. From magnetic hysteresis cycles for PAD-PEG-Fe(3)O(4)@PEI at room temperature, the saturation magnetization (Ms) was shown to be 58.14 emu g(-1). Inductively coupled plasma spectrometry (ICP) analysis showed that the designed magnetic nanocomposite can remove 98% and 80% of Cd(2+) from water and blood, respectively.

Advances in metal-induced oxidative stress and human disease

Detailed studies in the past two decades have shown that redox active metals like iron (Fe), copper (Cu), chromium (Cr), cobalt (Co) and other metals undergo redox cycling reactions and possess the ability to produce reactive radicals such as superoxide anion radical and nitric oxide in biological systems. Disruption of metal ion homeostasis may lead to oxidative stress, a state where increased formation of reactive oxygen species (ROS) overwhelms body antioxidant protection and subsequently induces DNA damage, lipid peroxidation, protein modification and other effects, all symptomatic for numerous diseases, involving cancer, cardiovascular disease, diabetes, atherosclerosis, neurological disorders (Alzheimer's disease, Parkinson's disease), chronic inflammation and others. The underlying mechanism of action for all these metals involves formation of the superoxide radical, hydroxyl radical (mainly via Fenton reaction) and other ROS, finally producing mutagenic and carcinogenic malondialdehyde (MDA), 4-hydroxynonenal (HNE) and other exocyclic DNA adducts. On the other hand, the redox inactive metals, such as cadmium (Cd), arsenic (As) and lead (Pb) show their toxic effects via bonding to sulphydryl groups of proteins and depletion of glutathione. Interestingly, for arsenic an alternative mechanism of action based on the formation of hydrogen peroxide under physiological conditions has been proposed. A special position among metals is occupied by the redox inert metal zinc (Zn). Zn is an essential component of numerous proteins involved in the defense against oxidative stress. It has been shown, that depletion of Zn may enhance DNA damage via impairments of DNA repair mechanisms. In addition, Zn has an impact on the immune system and possesses neuroprotective properties. The mechanism of metal-induced formation of free radicals is tightly influenced by the action of cellular antioxidants. Many low-molecular weight antioxidants (ascorbic acid (vitamin C), alpha-tocopherol (vitamin E), glutathione (GSH), carotenoids, flavonoids, and other antioxidants) are capable of chelating metal ions reducing thus their catalytic activity to form ROS. A novel therapeutic approach to suppress oxidative stress is based on the development of dual function antioxidants comprising not only chelating, but also scavenging components. Parodoxically, two major antioxidant enzymes, superoxide dismutase (SOD) and catalase contain as an integral part of their active sites metal ions to battle against toxic effects of metal-induced free radicals. The aim of this review is to provide an overview of redox and non-redox metal-induced formation of free radicals and the role of oxidative stress in toxic action of metals.

Endoplasmic reticulum is a major target of cadmium toxicity in yeast

Cadmium (Cd(2+)) is a very toxic metal that causes DNA damage, oxidative stress and apoptosis. Despite many studies, the cellular and molecular mechanisms underlying its high toxicity are not clearly understood. We show here that very low doses of Cd(2+) cause ER stress in Saccharomyces cerevisiae as evidenced by the induction of the unfolded protein response (UPR) and the splicing of HAC1 mRNA. Furthermore, mutant strains (Delta ire1 and Delta hac1) unable to induce the UPR are hypersensitive to Cd(2+), but not to arsenite and mercury. The full functionality of the pathways involved in ER stress response is required for Cd(2+) tolerance. The data also suggest that Cd(2+)-induced ER stress and Cd(2+) toxicity are a direct consequence of Cd(2+) accumulation in the ER. Cd(2+) does not inhibit disulfide bond formation but perturbs calcium metabolism. In particular, Cd(2+) activates the calcium channel Cch1/Mid1, which also contributes to Cd(2+) entry into the cell. The results reinforce the interest of using yeast as a cellular model to study toxicity mechanisms in eukaryotic cells.

Cytosolic zinc buffering and muffling: their role in intracellular zinc homeostasis

Our knowledge of the molecular mechanisms of intracellular homeostatic control of zinc ions is now firmly grounded on experimental findings gleaned from the study of zinc proteomes and metallomes, zinc transporters, and insights from the use of computational approaches. A cell's repertoire of zinc homeostatic molecules includes cytosolic zinc-binding proteins, transporters localized to cytoplasmic and organellar membranes, and sensors of cytoplasmic free zinc ions. Under steady state conditions, a primary function of cytosolic zinc-binding proteins is to buffer the relatively large zinc content found in most cells to a cytosolic zinc(ii) ion concentration in the picomolar range. Under non-steady state conditions, zinc-binding proteins and transporters act in concert to modulate transient changes in cytosolic zinc ion concentration in a process that is called zinc muffling. For example, if a cell is challenged by an influx of zinc ions, muffling reactions will dampen the resulting rise in cytosolic zinc ion concentration and eventually restore the cytosolic zinc ion concentration to its original value by shuttling zinc ions into subcellular stores or by removing zinc ions from the cell. In addition, muffling reactions provide a potential means to control changes in cytosolic zinc ion concentrations for purposes of cell signalling in what would otherwise be considered a buffered environment not conducive for signalling. Such intracellular zinc ion signals are known to derive from redox modifications of zinc-thiolate coordination environments, release from subcellular zinc stores, and zinc ion influx via channels. Recently, it has been discovered that metallothionein binds its seven zinc ions with different affinities. This property makes metallothionein particularly well positioned to participate in zinc buffering and muffling reactions. In addition, it is well established that metallothionein is a source of zinc ions under conditions of redox signalling. We suggest that the biological functions of transient changes in cytosolic zinc ion concentrations (presumptive zinc signals) complement those of calcium ions in both spatial and temporal dimensions.

The role of endoplasmic reticulum in cadmium-induced mesangial cell apoptosis

Cd is an industrial and environmental pollutant that affects many organs in humans and other mammals. However, the molecular mechanisms of Cd-induced nephrotoxicity are unclear. In this study, we show that endoplasmic reticula (ER) played a pivotal role in Cd-induced apoptosis in mesangial cells. Using Fluo-3 AM, the intracellular concentration of calcium ([Ca(2+)](i)) was detected as being elevated as time elapsed after Cd treatment. Co-treatment with BAPTA-AM, a calcium chelator, was able to significantly suppress Cd-induced apoptosis. Calcineurin is a cytosolic phosphatase, which was able to dephosphorylate the inositol-1,4,5-triphosphate receptor (IP(3)R) calcium channel to prevent the release of calcium from ER. Cyclosporine A, a calcineurin inhibitor, increased both [Ca(2+)](i) and the percentage of Cd-induced apoptosis. However, EGTA and the IP(3)R inhibitor, 2-APB, were able to partially modulate Cd cytotoxicity. These results led us to suggest that the extracellular and ER-released calcium plays a crucial role in Cd-induced apoptosis in mesangial cells. Following this line, we further detected the ER stress after Cd treatment since ER is one of the major calcium storage organelles. After Cd exposure, GADD153, a hallmark of ER stress, was upregulated (at 4h of exposure), followed by activation of ER-specific caspase-12 and its downstream molecule caspase-3 (at 16h of exposure). The pan caspase inhibitor, Z-VAD, and BAPTA-AM were able to reverse the Cd-induced cell death and ER stress, respectively. Furthermore, the mitochondrial membrane potential (DeltaPsi(m)) was depolarized significantly and cytochrome c was released after 24h of exposure to Cd and followed by mild activation of caspase-9 at the 36-h time point, indicating that mitochondria stress is a late event. Therefore, we concluded that ER is the major killer organelle in Cd-induced mesangial cell apoptosis and that calcium oscillation plays a pivotal role.

Heavy metal and growth hormone pathways in metallothionein regulation in fish RTH-149 cell line

Interference between heavy metals and growth hormone (GH) on cell signaling has been previously demonstrated in fish cells. This study was aimed at assessing their effects on expression of the metallothionein isoforms MT-A and MT-B. The results indicate that all heavy metals induce MT-A more markedly than MT-B, but differences appeared when metals were combined with GH. For MT-B induction, a positive interference between metals and GH was observed for Zn(2+)/GH and Cd(2+)/GH, a negative interference for Hg(2+)/GH. With regards to MT-A, no interference was observed for Zn(2+)/GH and Hg(2+)/GH, while a negative interference occurred with Cu(2+)/GH and a positive interference with Cd(2+)/GH. The possible mechanisms underlying the differential regulation of metallothioneins include different signaling pathways. The results show that STAT5 and ERKs responded differently to different combinations, and Zn(2+)/GH and Cd(2+)/GH exerted a slight positive interference on ERK activation. On the other hand, a synergic rise in [Ca(2+)](i) occurred for all combinations except for Cu(2+)/GH. Our data suggest that the cross-talk between heavy metals and GH resulting in MT transcription modulation does not strictly depend on Ca(2+) signalling; (ii)ERK activation may represent the point of cross-talk between Zn(2+) or Cd(2+) and GH, converging on MT-B transcription, probably through a differential recruitment of transcription factors.

Mobile trap algorithm for zinc detection using protein sensors

We present a mobile trap algorithm to sense zinc ions using protein-based sensors such as carbonic anhydrase (CA). Zinc is an essential biometal required for mammalian cellular functions although its intracellular concentration is reported to be very low. Protein-based sensors like CA molecules are employed to sense rare species like zinc ions. In this study, the zinc ions are mobile targets, which are sought by the mobile traps in the form of sensors. Particle motions are modeled using random walk along with the first passage technique for efficient simulations. The association reaction between sensors and ions is incorporated using a probability (p1) upon an ion-sensor collision. The dissociation reaction of an ion-bound CA molecule is modeled using a second, independent probability (p2). The results of the algorithm are verified against the traditional simulation techniques (e.g., Gillespie's algorithm). This study demonstrates that individual sensor molecules can be characterized using the probability pair (p1,p2), which, in turn, is linked to the system level chemical kinetic constants, kon and koff. Further investigations of CA-Zn reaction using the mobile trap algorithm show that when the diffusivity of zinc ions approaches that of sensor molecules, the reaction data obtained using the static trap assumption differ from the reaction data obtained using the mobile trap formulation. This study also reveals similar behavior when the sensor molecule has higher dissociation constant. In both the cases, the reaction data obtained using the static trap formulation reach equilibrium at a higher number of complex molecules (ion-bound sensor molecules) compared to the reaction data from the mobile trap formulation. With practical limitations on the number sensors that can be inserted/expressed in a cell and stochastic nature of the intracellular ionic concentrations, fluorescence from the number of complex sensor molecules at equilibrium will be the measure of the intracellular ion concentration. For reliable detection of zinc ions, it is desirable that the sensors must not bind all the zinc ions tightly, but should rather bind and unbind. Thus for a given fluorescence and with association-dissociation reactions between ions and sensors, the static trap approach will underestimate the number of zinc ions present in the system.

Ca(2+) -dependent plant response to Pb(2+) is regulated by LCT1

Tobacco plants transformed with TaLCT1 were cultured on Knop's medium with modified calcium concentrations (0.01-3 mM) in the presence of Pb(2+), and in soil contaminated by lead. A 4-5 microM Pb(2+) administered in the presence of 1 mM Ca(2+) inhibited the root growth of transgenic plants to much lesser degree than of control plants, whereas in the presence of 3mM Ca(2+) no differences were found between the studied lines. The reduction of Pb(2+) toxicity in the presence of 1 mM Ca(2+) was not accompanied by a change in the lead tissue concentration. However, when Ca(2+) level in the medium was lowered to 0.01 mM, several fold higher root/shoot Pb ratio in transgenic plants was observed, twofold increase in the total amount of metal accumulated, and lower concentration of Pb in the xylem sap. Results suggest the involvement of TaLCT1 in the regulation of Ca-dependent Pb-detoxification, and under conditions of low calcium in lead uptake and distribution.

The zinc/thiolate redox biochemistry of metallothionein and the control of zinc ion fluctuations in cell signaling

Free zinc ions are potent effectors of proteins. Their tightly controlled fluctuations ("zinc signals") in the picomolar range of concentrations modulate cellular signaling pathways. Sulfur (cysteine) donors generate redox-active coordination environments in proteins for the redox-inert zinc ion and make it possible for redox signals to induce zinc signals. Amplitudes of zinc signals are determined by the cellular zinc buffering capacity, which itself is redox-sensitive. In part by interfering with zinc and redox buffering, reactive species, drugs, toxins, and metal ions can elicit zinc signals that initiate physiological and pathobiochemical changes or lead to cellular injury when free zinc ions are sustained at higher concentrations. These interactions establish redox-inert zinc as an important factor in redox signaling. At the center of zinc/redox signaling are the zinc/thiolate clusters of metallothionein. They can transduce zinc and redox signals and thereby attenuate or amplify these signals.

Efects of growth hormone and cadmium on the transcription regulation of two metallothionein isoforms

The effect of growth hormone (GH) and cadmium (Cd) on metallothionein (MT) expression was investigated in hepatoma cells. In fish the constitutive isoform MT-B and the metal-responsive MT-A are expressed. Real-time RT-PCR revealed that: Cd up-regulates mostly MT-A, GH slightly induces MT-B and the GH/Cd combination induces synergistically both MTs. Perturbations in Ca2+ levels suppressed or reduced the Cd-induction of MTs and abolished the GH/Cd synergy. Similar results were obtained by inhibition of tyrosine kinases. Also the signaling molecules recruited by the GH receptor responded differently to GH and Cd, with ERKs showing a synergistic activation upon GH/Cd. The following conclusions can be drawn: (1) cytosolic Ca2+ is mainly involved in MT-A regulation; (2) both Ca2+ and tyrosine phosphorylation are essential for Cd-induction and GH/Cd synergy on MTs. The synergy could depend on interactions in different signaling pathways, leading to a differential recruitment of MTF-1 and AP-1 transcription factors.

Cadmium Toxicity on Arterioles Vascular Smooth Muscle Cells of Spontaneously Hypertensive Rats

Cadmium (Cd) is frequently used in various industrial applications and is a ubiquitous environmental toxicant, also present in tobacco smoke. An important route of exposure is the circulatory system whereas blood vessels are considered to be main stream organs of Cd toxicity. Our previous results indicate that cadmium chloride (CdCl2) affects mean arterial blood pressure in hypertensive rats. We hypothesized that Cd alters the intracellular calcium transient mechanism, by cadmium-induced stimulation of MAPKs (ERK 1 & 2) which is mediated partially through calcium-dependent PKC mechanism. To investigate this hypothesis, we exposed primary cultures of vascular smooth muscle cells (VSMCs) from wistar kyoto (WKY) and spontaneously hypertensive rats (SHR) to increased concentrations of CdCl2 on cell viability, expression of mitogen-activated protein kinases (MAPKs/ERK 1 & 2), and protein kinase C (PKC) which are activated by Cd in several cell types. The results from these studies indicate that CdCl2 decreased cell viability of both SHR and WKY VSMCs in a concentration dependent-manner. Viability of both cell types decreased 33+/-5.3 (SHR) and 39+/-2.3% (WKY) when exposed to 1 microM CdCl2, whereas, 8 and 16 microM reduced viability by 66+/-3.1 and 62+/-4.5% in SHR cells. CdCl2 increased ERK 1 & 2 in a biphasic manner with maximum increase occurring when cells are exposed to 1 and 4 muM in SHR VSMCs, whereas, a reduction in ERK 1 and 2 is observed when WKY cells are treated with 2 microM. The results also indicate that CdCl2 increased PKC a/Beta in both SHR and WKY VSMCs with a greater increase in expression in SHR VSMCs. In addition, the [Ca2+]i chelator, BAPTA, suppressed the CdCl2 effect, whereas, the PKC inhibitor, GF109203X, reduced the CdCl2 induced-effect on PKC expression. The present studies support the hypothesis that Cd can be a risk factor of hypertension through dysfunction of vascular smooth muscle cells under certain conditions.

Zinc-buffering capacity of a eukaryotic cell at physiological pZn

In spite of the paramount importance of zinc in biology, dynamic aspects of cellular zinc metabolism remain poorly defined at the molecular level. Investigations with human colon cancer (HT-29) cells establish a total cellular zinc concentration of 264 microM. Remarkably, about 10% of the potential high-affinity zinc-binding sites are not occupied by zinc, resulting in a surplus of 28 muM ligands (average Kd(c) = 83 pM) that ascertain cellular zinc-buffering capacity and maintain the "free" zinc concentration in proliferating cells at picomolar levels (784 pM, pZn = 9.1). This zinc-buffering capacity allows zinc to fluctuate only with relatively small amplitudes (DeltapZn = 0.3; below 1 nM) without significantly perturbing physiological pZn. Thus, the "free" zinc concentrations in resting and differentiated HT-29 cells are 614 pM and 1.25 nM, respectively. The calculation of these "free" zinc concentrations is based on measurements at different concentrations of the fluorogenic zinc-chelating agent and extrapolation to a zero concentration of the agent. It depends on the state of the cell, its buffering capacity, and the zinc dissociation constant of the chelating agent. Zinc induction of thionein (apometallothionein) ensures a surplus of unbound ligands, increases zinc-buffering capacity and the availability of zinc (DeltapZn = 0.8), but preserves the zinc-buffering capacity of the unoccupied high-affinity zinc-binding sites, perhaps for crucial physiological functions. Jointly, metallothionein and thionein function as the major zinc buffer under conditions of increased cellular zinc.

Zinc coordination environments in proteins as redox sensors and signal transducers

Zinc/cysteine coordination environments in proteins are redox-active. Oxidation of the sulfur ligands mobilizes zinc, while reduction of the oxidized ligands enhances zinc binding, providing redox control over the availability of zinc ions. Some zinc proteins are redox sensors, in which zinc release is coupled to conformational changes that control varied functions such as enzymatic activity, binding interactions, and molecular chaperone activity. Whereas the released zinc ion in redox sensors has no known function, the redox signal is transduced to specific and sensitive zinc signals in redox transducers. Released zinc can bind to sites on other proteins and modulate signal transduction, generation of metabolic energy, mitochondrial function, and gene expression. The paradigm of such redox transducers is the zinc protein metallothionein, which, together with its apoprotein, thionein, functions at a central node in cellular signaling by redistributing cellular zinc, presiding over the availability of zinc, and interconverting redox and zinc signals. In this regard, the transduction of nitric oxide (NO) signals into zinc signals by metallothionein has received particular attention. It appears that redox-inert zinc has been chosen to control some aspects of cellular thiol/disulfide redox metabolism. Tight control of zinc is essential for redox homeostasis because both increases and decreases of cellular zinc elicit oxidative stress. Depending on its availability, zinc can be cytoprotective as a pro-antioxidant or cytotoxic as a pro-oxidant. Any condition with acute or chronic oxidative stress is expected to perturb zinc homeostasis.

Cadmium induces the expression of Grp78, an endoplasmic reticulum molecular chaperone, in LLC-PK1 renal epithelial cells

To reveal the effects of cadmium exposure on the endoplasmic reticulum (ER) stress response, we examined the expression and function of 78-kDa glucose-regulated protein (Grp78) , an ER-resident molecular chaperone, in LLC-PK1 cells. In cells treated with 10 microM cadmium chloride, Grp78 protein levels increased after 6 hr and remained elevated at 24 hr. When cells were incubated with 1-20 microM CdCl2 for 6 hr, Grp78 increased in a dose-dependent manner. In addition, Grp78 mRNA levels were elevated in response to CdCl2 exposure. After exposure to 10 microM CdCl2, the levels of activating transcription factor 4 (ATF4) were increased at 2 hr, with a further enhancement after that ; this accumulation followed the transient but marked phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2(alpha)) on serine 51. Although ATF4 mRNA levels increased mildly by CdCl2 exposure, treatment with actinomycin D did not suppress CdCl2-induced accumulation of ATF4 protein, suggesting the involvement of posttranscriptional and, in part, transcriptional mechanisms. Compared with other heavy-metal compounds such as manganese chloride, zinc chloride, mercuric chloride, and lead chloride, CdCl2 could increase the levels of Grp78, ATF4, and the phosphorylated form of eIF2(alpha) more markedly without definite cellular damage. The silencing of Grp78 expression using short-interference RNA enhanced CdCl2-induced cellular damage. These results show that cadmium induces the expression of Grp78 probably via phosphorylation of eIF2(alpha) and resultant translation of ATF4, and this ER stress response plays a role in protection against cadmium cytotoxicity in this renal epithelial cell.

Flow cytometric measurement of labile zinc in peripheral blood mononuclear cells

Labile (i.e., free or loosely bound) zinc has the potential to modulate cellular function. Therefore, a flow cytometric assay for the measurement of labile zinc was developed to facilitate the investigation of the physiological roles of zinc. The zinc-sensitive fluorescent probe FluoZin-3 was used to quantify the amount of labile zinc in peripheral blood mononuclear cells isolated from human blood. Maximal fluorescence and autofluorescence of the probe were measured after the addition of zinc in the presence of the ionophore pyrithione, or the membrane-permeant chelator N,N,N',N'-tetrakis-(2-pyridyl-methyl)ethylenediamine, respectively. In this way, the intracellular concentrations of labile zinc in resting cells were estimated to be 0.17 nM in monocytes and 0.35 nM in lymphocytes. The method was successfully employed to monitor phorbol 12-myristate 13-acetate-induced zinc release, which occurred in monocytes but not lymphocytes, and the displacement of protein-bound zinc by the mercury-containing compounds HgCl(2) and thimerosal. Costaining with dyes that emit at higher wavelengths than FluoZin-3 allows multiparameter measurements. Two combinations with other dyes are shown: loading with propidium iodide to measure cellular viability and labeling with antibodies against the surface antigen CD4. This method allows measurement of the concentration of biologically active labile zinc in distinct cell populations.

The neurobiology of zinc in health and disease

The use of zinc in medicinal skin cream was mentioned in Egyptian papyri from 2000 BC (for example, the Smith Papyrus), and zinc has apparently been used fairly steadily throughout Roman and modern times (for example, as the American lotion named for its zinc ore, 'Calamine'). It is, therefore, somewhat ironic that zinc is a relatively late addition to the pantheon of signal ions in biology and medicine. However, the number of biological functions, health implications and pharmacological targets that are emerging for zinc indicate that it might turn out to be 'the calcium of the twenty-first century'.

Aequorin chimeras as valuable tool in the measurement of Ca2+ concentration during cadmium injury

The ability of cadmium to disrupt calcium homeostasis has been known since a long time, but the precise cellular targets of its toxic action are still debated. A great problem in the interpretation of data has been associated with the ability of cadmium to strongly bind traditional calcium probes. Aequorin, the well-characterized calcium-sensitive photoprotein, was used as intracellular calcium indicator during cadmium injury in NIH 3T3 murine fibroblasts. NIH 3T3 cells were transfected with a cDNA construct containing aequorin fused to a truncated glutamate receptor, which directs the probe to the outer surface of intracellular membranes. At first, we tested if different cadmium concentrations were able to modify the rate of light emission by aequorin showing that cadmium concentrations <15 microM were ineffective on aequorin luminescence. Hence, aequorin chimeras revealed as a useful tool in the analyses of Cd2+/Ca2+ interference. To directly investigate the role of Cd2+ in Ca2+ homeostasis, we have started to selectively measure the free Ca2+ concentration in different cell compartments. Here, we report that cadmium reduces the transient free calcium signal after stimulation of cells with bradykinin. Further studies are in progress to clarify the role of mitochondria and endoplasmic reticulum in cadmium-induced alterations of Ca2+ homeostasis in order to link signal transduction modifications with the onset of apoptosis induced by cadmium exposure.