Transition metals in control of gene expression

Maternally-derived zinc transporters ZIP6 and ZIP10 drive the mammalian oocyte-to-egg transition

Rapid cellular zinc influx regulates early mammalian development during the oocyte-to-egg transition through modulation of the meiotic cell cycle. Despite the physiological necessity of this zinc influx, the molecular mechanisms that govern such accumulation are unknown. Here we show that the fully grown mammalian oocyte does not employ a transcriptionally based mechanism of zinc regulation involving metal response element-binding transcription factor-1 (MTF-1), as demonstrated by a lack of MTF-1 responsiveness to environmental zinc manipulation. Instead, the mammalian oocyte controls zinc uptake through two maternally derived and cortically distributed zinc transporters, ZIP6 and ZIP10. Targeted disruption of these transporters using several approaches during meiotic maturation perturbs the intracellular zinc quota and results in a cell cycle arrest at a telophase I-like state. This arrest phenocopies established models of zinc insufficiency during the oocyte-to-egg transition, indicating the essential function of these maternally expressed transporters. Labile zinc localizes to punctate cytoplasmic structures in the human oocyte, and ZIP6 and ZIP10 are enriched in the cortex. Altogether, we demonstrate a mechanism of metal regulation required for female gamete development that may be evolutionarily conserved.

Structural metal sites in nonclassical zinc finger proteins involved in transcriptional and translational regulation

Zinc finger (ZF) proteins are a large family of metalloproteins that utilize zinc for structural purposes. Zinc coordinates to a combination of cysteine thiol and histidine imidazole residues within the ZF polypeptide sequence resulting in a folded and functional protein. Initially, a single class of ZFs were identified. These ZFs, now referred to as the "classical" ZFs, utilize a Cys2His2 (CCHH) ligand set to bind zinc. Upon Zn coordination, the classical ZFs fold into a structure made up of an α helix and an antiparallel β sheet. When folded, classical ZFs recognize and bind to specific DNA targets and function as transcription factors. With the advent of genome sequencing and proteomics, many additional classes of ZFs were identified based upon their primary amino acid sequences. At least 13 additional classes of ZFs are known, and collectively these "nonclassical" ZFs differ in the ligand set involved in Zn(II) coordination, the organization of the ligands within the polypeptide sequence and the macromolecular targets. Some nonclassical ZFs are DNA binding "transcription factors", while others are involved in RNA regulation and protein recognition. Much less is known about these nonclassical ZFs with regards to the roles of metal coordination in fold and function. This Account focuses on our laboratory's efforts to characterize two families of "nonclassical" ZFs: the Cys3His (or CCCH) ZF family and the Cys2His2Cys (or CCHHC) ZF family. Our work on the CCCH ZF family has focused on the protein Tristetraprolin (TTP), which is a key protein in regulating inflammation. TTP contains two CCCH domains that were proposed to be ZFs based upon their sequence. We have shown that while this protein can coordinate Zn(II) at the CCCH sites, it can also coordinate Fe(II) and Fe(III). Moreover, the zinc and iron bound forms of TTP are equally adept at discriminating between RNA targets, which we have demonstrated via a fluorescence anisotropy based approach. Thus, CCCH type ZFs appear to be promiscuous with respect to metal preference and a role for iron coordination in CCCH ZF function is proposed. The CCHHC family of ZFs is a small family of nonclassical ZFs that are essential for the development of the central nervous system. There are three ZFs in this family: neural zinc finger factor-1 (NZF-1), myelin transcription factor-1 (MyT1), and suppressor of tumorgenicity 18 (ST18). All three proteins contain multiple clusters of "CCHHC" domains, which are all predicted to be Zn binding domains. We have focused on a tandem-CCHHC domain construct of NZF-1, which recognizes β-RARE DNA, and we have identified key residues required for DNA recognition. Unlike classical ZFs, for which a few conserved residues are required for DNA recognition, the CCHHC class of ZFs utilize a few nonconserved residues to drive DNA recognition leading us to propose a new paradigm for ZF/DNA binding.

Disrupted calcium homeostasis is involved in elevated zinc ion-induced photoreceptor cell death

Zinc (Zn), the second abundant trace element in living organisms, plays an important role in regulating cell metabolism, signaling, proliferation, gene expression and apoptosis. Meanwhile, the overload of Zn will disrupt the intracellular calcium homeostasis via impairing mitochondrial function. However, the specific molecular mechanism underlying zinc-induced calcium regulation remains poorly understood. In the present study, using zinc chloride (ZnCl2) as a stressor, we investigated the effect of exogenous Zn(2+) in regulating murine photoreceptor cell viability, reactive oxygen species (ROS), cell cycle distribution and calcium homeostasis as well as plasma membrane calcium ATPase (PMCA) isoforms (PMCA1 and PMCA2, i.e., ATP2B1, ATP2B2) expression. We found that the exogenous Zn(2+) in the exposure range (31.25-125.0 μmol/L) results in the overgeneration of ROS, cell cycle arrest at G2/M phases, elevation of cytosolic [Ca(2+)], inactivation of Ca(2+)-ATPase and reduction of both PMCA1 and PMCA2 in 661 W cells, and thus induces cell death. In conclusion, ZnCl2 exposure can elevate the cytosolic [Ca(2+)], disrupt the intracellular calcium homeostasis, further initiate Ca(2+)-dependent signaling pathway in 661 W cells, and finally cause cell death. Our results will facilitate the understanding of cell death induced by the zinc ion-mediated calcium homeostasis disruption.

A novel dual-emission ratiometric fluorescent nanoprobe for sensing and intracellular imaging of Zn2+

The integration of unique characteristics of nanomaterials with highly specific recognition elements, such as biomolecules and organic molecules, are the foundation of many novel nanoprobes for bio/chemical sensing and imaging. In the present report, branched polyethylenimine (PEI) was grafted with 8-chloroacetyl-aminoquinoline to synthesize a water-soluble and biocompatible quinoline-based Zn(2+) probe PEIQ. Then the PEIQ was covalently conjugated to [Ru(bpy)3](2+)-encapsulated SiNPs to obtain the ratiometric fluorescence nanoprobe which exhibits a strong fluorescence emission at 600 nm and a negligible fluorescence emission at 500 nm in the absence of Zn(2+) upon a single wavelength excitation. After the addition of different amounts of Zn(2+), the fluorescence intensity at 500 nm increased continuously while the fluorescence intensity at 600 nm remained stable, thus changing the dual emission intensity ratios and displaying continuous color changes from red to green which can be clearly observed by the naked eye. The nanoprobe exhibits good water dispersivity, biocompatibility and cell permeability, high selectivity over competing metal ions, and high sensitivity with a detection limit as low as 0.5 μM. Real-time imaging of Zn(2+) in A549 cells has also been realized using this novel nanoprobe.

Fluorescence lifetime imaging of physiological free Cu(II) levels in live cells with a Cu(II)-selective carbonic anhydrase-based biosensor

Copper is a required trace element that plays key roles in a number of human enzymes, such that copper deficiency or genetic defects in copper transport lead to serious or fatal disease. Rae, et al., had famously predicted that free copper ion levels in the cell cytoplasm were extremely low, typically too low to be observable. We recently developed a variant of human apocarbonic anhydrase II for sensing metal ions that exhibits 25-fold better selectivity for Cu(II) over Zn(II) than the wild type protein, enabling us to accurately measure Cu(II) in the presence of ordinary cellular (picomolar) concentrations of free zinc. We inserted a fluorescent labeled Cu(II)-specific variant of human apocarbonic anhydrase into PC-12 cells and found that the levels are indeed extremely low (in the femtomolar range). We imaged the free Cu(II) levels in living cells by means of frequency-domain fluorescence lifetime microscopy. Implications of this finding are discussed.

Structural analysis and insight into metal-ion activation of the iron-dependent regulator fromThermoplasma acidophilum

The iron-dependent regulator (IdeR) is a metal ion-activated transcriptional repressor that regulates the expression of genes encoding proteins involved in iron uptake to maintain metal-ion homeostasis. IdeR is a functional homologue of the diphtheria toxin repressor (DtxR), and both belong to the DtxR/MntR family of metalloregulators. The structure of Fe2+-bound IdeR (TA0872) fromThemoplasma acidophilumwas determined at 2.1 Å resolution by X-ray crystallography using single-wavelength anomalous diffraction. The presence of Fe2+, which is the true biological activator of IdeR, in the metal-binding site was ascertained by the use of anomalous difference electron-density maps using diffraction data collected at the Fe absorption edge. Each DtxR/IdeR subunit contains two metal ion-binding sites separated by 9 Å, labelled the primary and ancillary sites, whereas the crystal structures of IdeR fromT. acidophilumshow a binuclear iron cluster separated by 3.2 Å, which is novel toT. acidophilumIdeR. The metal-binding site analogous to the primary site in DtxR was unoccupied, and the ancillary site was occupied by binuclear clustered ions. This difference suggests thatT. acidophilumIdeR and its closely related homologues are regulated by a mechanism distinct from that of either DtxR or MntR.T. acidophilumIdeR was also shown to have a metal-dependent DNA-binding property by electrophoretic mobility shift assay.

Fluxes in "free" and total zinc are essential for progression of intraerythrocytic stages of Plasmodium falciparum

Dynamic fluxes in the concentration of ions and small molecules are fundamental features of cell signaling, differentiation, and development. Similar roles for fluxes in transition metal concentrations are less well established. Here, we show that massive zinc fluxes are essential in the infection cycle of an intracellular eukaryotic parasite. Using single-cell quantitative imaging, we show that growth of the blood-stage Plasmodium falciparum parasite requires acquisition of 30 million zinc atoms per erythrocyte before host cell rupture, corresponding to a 400% increase in total zinc concentration. Zinc accumulates in a freely available form in parasitophorous compartments outside the food vacuole, including mitochondria. Restriction of zinc availability via small molecule treatment causes a drop in mitochondrial membrane potential and severely inhibits parasite growth. Thus, extraordinary zinc acquisition and trafficking are essential for parasite development.

Application of immobilized metal affinity chromatography in proteomics

It has been proved that the progress of proteomics is mostly determined by the development of advanced and sensitive protein separation technologies. Immobilized metal affinity chromatography (IMAC) is a powerful protein fractionation method used to enrich metal-associated proteins and peptides. In proteomics, IMAC has been widely employed as a prefractionation method to increase the resolution in protein separation. The combination of IMAC with other protein analytical technologies has been successfully utilized to characterize metalloproteome and post-translational modifications. In the near future, newly developed IMAC integrated with other proteomic methods will greatly contribute to the revolution of expression, cell-mapping and structural proteomics.

Quinoline derivative-functionalized carbon dots as a fluorescent nanosensor for sensing and intracellular imaging of Zn2+

Functionalization of carbon nanodots (C-dots) with quinoline derivatives enables a highly sensitive and specific nanosensor for Zn²⁺ sensing in aqueous solution and Zn²⁺ imaging in vivo.

A novel chemosensor with visible light excitability for sensing Zn2+in physiological medium and in HeLa cells

A highly sensitive, fluorescent, non-cytotoxic turn-on chemosensor with visible light excitability for the detection of intracellular Zn²⁺ions.

FeON-FeOFF: the Helicobacter pylori Fur regulator commutates iron-responsive transcription by discriminative readout of opposed DNA grooves

Most transcriptional regulators bind nucleotide motifs in the major groove, although some are able to recognize molecular determinants conferred by the minor groove of DNA. Here we report a transcriptional commutator switch that exploits the alternative readout of grooves to mediate opposite output regulation for the same input signal. This mechanism accounts for the ability of the Helicobacter pylori Fur regulator to repress the expression of both iron-inducible and iron-repressible genes. When iron is scarce, Fur binds to DNA as a dimer, through the readout of thymine pairs in the major groove, repressing iron-inducible transcription (FeON). Conversely, on iron-repressible elements the metal ion acts as corepressor, inducing Fur multimerization with consequent minor groove readout of AT-rich inverted repeats (FeOFF). Our results provide first evidence for a novel regulatory paradigm, in which the discriminative readout of DNA grooves enables to toggle between the repression of genes in a mutually exclusive manner.

Conformational and thermodynamic hallmarks of DNA operator site specificity in the copper sensitive operon repressor from Streptomyces lividans

Metal ion homeostasis in bacteria relies on metalloregulatory proteins to upregulate metal resistance genes and enable the organism to preclude metal toxicity. The copper sensitive operon repressor (CsoR) family is widely distributed in bacteria and controls the expression of copper efflux systems. CsoR operator sites consist of G-tract containing pseudopalindromes of which the mechanism of operator binding is poorly understood. Here, we use a structurally characterized CsoR from Streptomyces lividans (CsoR^Sl) together with three specific operator targets to reveal the salient features pertaining to the mechanism of DNA binding. We reveal that CsoR^Sl binds to its operator site through a 2-fold axis of symmetry centred on a conserved 5′-TAC/GTA-3′ inverted repeat. Operator recognition is stringently dependent not only on electropositive residues but also on a conserved polar glutamine residue. Thermodynamic and circular dichroic signatures of the CsoR^Sl–DNA interaction suggest selectivity towards the A-DNA-like topology of the G-tracts at the operator site. Such properties are enhanced on protein binding thus enabling the symmetrical binding of two CsoR^Sl tetramers. Finally, differential binding modes may exist in operator sites having more than one 5′-TAC/GTA-3′ inverted repeat with implications in vivo for a mechanism of modular control.

Single-molecule dynamics and mechanisms of metalloregulators and metallochaperones

Understanding how cells regulate and transport metal ions is an important goal in the field of bioinorganic chemistry, a frontier research area that resides at the interface of chemistry and biology. This Current Topic reviews recent advances from the authors' group in using single-molecule fluorescence imaging techniques to identify the mechanisms of metal homeostatic proteins, including metalloregulators and metallochaperones. It emphasizes the novel mechanistic insights into how dynamic protein-DNA and protein-protein interactions offer efficient pathways via which MerR-family metalloregulators and copper chaperones can fulfill their functions. This work also summarizes other related single-molecule studies of bioinorganic systems and provides an outlook toward single-molecule imaging of metalloprotein functions in living cells.

In vitro protection by pyruvate against cadmium-induced cytotoxicity in hippocampal HT-22 cells

Cadmium is a toxic metal with no biological function in higher-order mammals. Humans are exposed to cadmium environmental contamination and the mechanism underlying the cadmium's cytotoxicity is unclear. To better understand this mechanism, we employed murine hippocampal HT-22 cells to test the in vitro effects of cadmium toxicity. Our study indicated that cadmium inhibits both mitochondria oxidative phosphorylation and glycolysis. In turn, this causes depolarization of mitochondrial membrane potential, increase of superoxide production and decrease of ATP generation. Furthermore, we demonstrated that the detrimental action of cadmium in bioenergetics could be mitigated by pyruvate, an intermediate metabolic product. Pyruvate decreased superoxide production, maintained mitochondrial membrane potential, restored glycolysis, mitigated the decrease in cellular ATP and attenuated cadmium cytotoxicity. Our study provides the first evidence that pyruvate might offer promising therapy for cadmium poisoning.

Zinc at sub-cytotoxic concentrations induces heme oxygenase-1 expression in human cancer cells

BACKGROUND/AIMS

This study investigated the effects of zinc on heme oxygenase-1 (HO-1) expression in human cancer cells.

METHODS/RESULTS

Zinc at sub-cytotoxic concentrations (50-100 μM) induces HO-1 expression in the MDA-MB-231 (human breast cancer) and A2780 (human ovarian cancer) cell lines in a concentration- and time-dependent manner. The induction of HO-1 by zinc was detected after 4-6 hours of treatment, reached maximal level at 8 hours, and declined thereafter. Using a human HO-1 gene promoter reporter construct, we identified two antioxidant response elements (AREs) that mediated the zinc-induced increase in HO-1 gene transcription, indicating that the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signaling pathway is involved in this event. This assumption was supported by the observations that knockdown of Nrf2 expression compromised the zinc-induced increase in HO-1 gene transcription, and that zinc increased Nrf2 protein expression and the Nrf2 binding to the AREs. Additionally, we found that the zinc-induced HO-1 gene transcription can be enhanced by clioquinol, a zinc ionophore, and reversed by pretreatment with TPEN, a known zinc chelator, indicating that an increase in intracellular zinc levels is responsible for this induction.

CONCLUSION

These findings demonstrate that zinc at sub-cytotoxic concentrations induces HO-1 expression in human cancer cells. The biological significance of this induction merits further investigation.

Zinc modulation of calcium activity at the photoreceptor terminal: a calcium imaging study

There is abundant experimental evidence that zinc ions (Zn(2+)) are present in the synaptic vesicles of vertebrate photoreceptors, and that they are co-released with glutamate. Here we show that increasing the concentration of extracellular zinc (2 μM-2 mM) suppresses the entry of calcium into the synaptic terminals of isolated salamander double cones. The resultant dose-dependent curve was fit by an inverse Hill equation having an IC50 of 38 μM, and Hill coefficient of 1.1. Because there is currently no reliable way to measure the concentration of extracellular zinc, it is not known whether the zinc released under normal circumstances is of physiological significance. In an attempt to circumvent this problem we used zinc chelators to reduce the available pool of endogenous zinc. This enabled us to determine how the absence of zinc affected calcium entry. We found that when intra- or extra-cellular zinc was chelated by 250 μM of membrane-permeable TPEN or 500 μM of membrane-impermeable histidine, there was a significant rise in the depolarization-induced intracellular calcium level within photoreceptor terminals. This increase in internal [Ca(2+)] will undoubtedly lead to a concomitant increase in glutamate release. In addition, we found that blocking the L-type calcium channels that are expressed on the synaptic terminals of photoreceptors with 50 μM nicardipine or 100 μM verapamil abolished the effects of zinc chelation. These findings are a good indication that, when released in vivo, the zinc concentration is sufficient to suppress voltage-gated calcium channels, and reduce the rate of glutamate release from photoreceptor terminals.

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Effects of exogenous zinc on the cellular zinc distribution and cell cycle of A549 cells

As the second most abundant transition metal in humans, zinc plays essential roles in normal cellular biological functions, including metabolism, signalling, proliferation, gene expression and apoptosis. We use ZnSO(4) as a stressor in this study to investigate for the first time the effects of exogenous Zn(2+) on both the cellular distribution of zinc and zinc-related proteins and the cell cycle of human lung adenocarcinoma (A549) cells. The cellular distribution of zinc and soluble proteins was determined in the whole cell as well as in the cytoplasmic and nuclear fractions. Exogenous zinc in the tested exposure range (0-100 µM) resulted in an altered cellular distribution of both zinc and the soluble proteins, together with total glutathione (GSx), the ratio of glutathione (GSH) to glutathione disulfide (GSSG) and non-protein sulphydryl (NPSH). Surprisingly, a turning point was observed in the re-distribution trend at a concentration of approximately 50 µM ZnSO(4). It is concluded that there exists a regulatory system in A549 cells that maintains the cellular zinc content stable in the presence of a certain range of extracellular zinc concentration. In addition, an MTT assay and flow cytometric analysis showed that the ZnSO(4) treatment led to a bi-phasic variation in viability and a slight fluctuation in the apoptosis of A549 cells. Our results will help to further elucidate zinc-related cell biology and biochemistry.

The zinc-responsive regulon of Neisseria meningitidis comprises 17 genes under control of a Zur element

Zinc is a bivalent cation essential for bacterial growth and metabolism. The human pathogen Neisseria meningitidis expresses a homologue of the Zinc uptake regulator Zur, which has been postulated to repress the putative zinc uptake protein ZnuD. In this study, we elucidated the transcriptome of meningococci in response to zinc by microarrays and quantitative real-time PCR (qRT-PCR). We identified 15 genes that were repressed and two genes that were activated upon zinc addition. All transcription units (genes and operons) harbored a putative Zur binding motif in their promoter regions. A meningococcal Zur binding consensus motif (Zur box) was deduced in silico, which harbors a conserved central palindrome consisting of hexameric inverted repeats separated by three nucleotides (TGTTATDNHATAACA). In vitro binding of recombinant meningococcal Zur to this Zur box was shown for the first time using electrophoretic mobility shift assays. Zur binding to DNA depended specifically on the presence of zinc and was sensitive to mutations in the palindromic sequence. The Zur regulon among genes of unknown function comprised genes involved in zinc uptake, tRNA modification, and ribosomal assembly. In summary, this is the first study of the transcriptional response to zinc in meningococci.

The complex architecture of mycobacterial promoters

The genus Mycobacterium includes a variety of species with differing phenotypic properties, including growth rate, pathogenicity and environment- and host-specificity. Although many mycobacterial species have been extensively studied and their genomes sequenced, the reasons for phenotypic variation between closely related species remain unclear. Variation in gene expression may contribute to these characteristics and enable the bacteria to respond to changing environmental conditions. Gene expression is controlled primarily at the level of transcription, where the main element of regulation is the promoter. Transcriptional regulation and associated promoter sequences have been studied extensively in E. coli. This review describes the complex structure and characteristics of mycobacterial promoters, in comparison to the classical E. coli prokaryotic promoter structure. Some components of mycobacterial promoters are similar to those of E. coli. These include the predominant guanine residue at the transcriptional start point, conserved -10 hexamer, similar interhexameric distances, the use of ATG as a start codon, the guanine- and adenine-rich ribosome binding site and the presence of extended -10 (TGn) motifs in strong promoters. However, these components are much more variable in sequence in mycobacterial promoters and no conserved -35 hexamer sequence (clearly defined in E. coli) can be identified. This may be a result of the high G+C content of mycobacterial genomes, as well as the large number of sigma factors present in mycobacteria, which may recognise different promoter sequences. Mycobacteria possess a complex transcriptional regulatory network. Numerous regulatory motifs have been identified in mycobacterial promoters, predominantly in the interhexameric region. These are bound by specific transcriptional regulators in response to environmental changes. The combination of specific promoter sequences, transcriptional regulators and a variety of sigma factors enables rapid and specific responses to diverse conditions and different stages of infection. This review aims to provide an overview of the complex architecture of mycobacterial transcriptional regulation.

Nonspecific interactions between Escherichia coli NikR and DNA are critical for nickel-activated DNA binding

The Escherichia coli transcription factor NikR is responsible for nickel-mediated repression of the operon encoding the Nik uptake transporter. The crystal structure of Ni(II)-NikR bound to the nik operator sequence revealed that residues in the loop preceding helix α3 in the metal-binding domain, which becomes structurally ordered upon stoichiometric nickel binding, interact with the DNA backbone. Here, we show that mutating both of these residues that make the nonspecific contacts, K64 and R65, abolishes DNA binding in vitro and nickel-responsive transcriptional repression of the nik promoter in vivo. In contrast, mutation of Q118, which forms a bridge between R65 and a potassium site, does not impact the activities of NikR. These data support the model that the nonspecific interactions between the metal-binding domain of the protein and the DNA phosphodiester backbone are critical for the Ni(II)-responsive activity of E. coli NikR.

Ultrastructural and morphometrical changes of mice ovaries following experimentally induced copper poisoning

BACKGROUND

Copper (Cu) is an essential trace element involved in normal reproduction but its overexposure may produce some detrimental effects. The aim of this study was to investigate the effects of copper sulfate poisoning on morphometery of mice ovarian structures and probable intracellular changes.

METHODS

Thirty mature female mice were randomly allocated to control and two treatment groups. In treatment groups, two different doses of copper sulfate including 100 mg/kg and 200 mg/kg in 0.2 cc were applied once a day for 35 consecutive days by gavage. Control animals received normal saline using the same volume and similar method. Animals from each experimental group were sacrificed 14 and 35 days after the beginning of drug administration and the left ovaries were removed for stereological evaluations by light microscopy and right ovaries were obtained for preparing electron microscopic sections.

RESULTS

The morphometrical results showed that only the number of antral follicles was decreased by 100 mg/kg copper sulfate on day 14 compared to the control group (P=0.043). Hence, higher copper dose or longer consumption period significantly reduced different classes of follicles and corpora lutea. With 100 mg/kg copper sulfate some mild ultrastructural cell damages such as decrease of zona pellucida thickness, limited vacuolated areas and nuclear envelop dilation were seen on day 14. Higher or longer Cu administration produced more detrimental effects including more vacuolated areas, presence of secondary lysosomes, irregularity in cell shape and segmented nuclei with condensed and marginated chromatin and more enlarged and damaged mitochondria.

CONCLUSION

New evidences of early as well as late intracellular damages of copper has been presented by accurate stereological and ultrastructural methods. Antral follicles was the most susceptible cells with the lower and shorter copper consumption and long term or higher dose of copper affected the whole of ovarian structures.

Role of morphological growth state and gene expression in Desulfovibrio africanus strain Walvis Bay mercury methylation

The biogeochemical transformations of mercury are a complex process, with the production of methylmercury, a potent human neurotoxin, repeatedly demonstrated in sulfate- and Fe(III)-reducing as well as methanogenic bacteria. However, little is known regarding the morphology, genes, or proteins involved in methylmercury generation. Desulfovibrio africanus strain Walvis Bay is a Hg-methylating δ-proteobacterium with a sequenced genome and has unusual pleomorphic forms. In this study, a relationship between the pleomorphism and Hg methylation was investigated. Proportional increases in the sigmoidal (regular) cell form corresponded with increased net MeHg production but decreased when the pinched cocci (persister) form became the major morphotype. D. africanus microarrays indicated that the ferrous iron transport genes (feoAB), as well as ribosomal genes and several genes whose products are predicted to have metal binding domains (CxxC), were up-regulated during exposure to Hg in the exponential phase. Whereas no specific methylation pathways were identified, the finding that Hg may interfere with iron transport and the correlation of growth-phase-dependent morphology with MeHg production are notable. The identification of these relationships between differential gene expression, morphology, and the growth-phase dependence of Hg transformations suggests that actively growing cells are primarily responsible for methylation, and so areas with ample carbon and electron-acceptor concentrations may also generate a higher proportion of methylmercury than more oligotrophic environments. The observation of increased iron transporter expression also suggests that Hg methylation may interfere with iron biogeochemical cycles.

A zinc-dependent mechanism regulates meiotic progression in mammalian oocytes

Precise coordination of meiotic progression is a critical determinant of an egg's capacity to be fertilized successfully, and zinc has emerged as a key regulatory element in this process. An early manifestation of a regulatory role for this transition metal is the significant increase in total intracellular zinc. This accumulation is essential for meiotic progression beyond telophase I and the establishment of meiotic arrest at metaphase II. The subsequent developmental event, fertilization, induces a rapid expulsion of labile zinc that is a hallmark event in meiotic resumption. In the present study, we show that the zinc fluxes work, in part, by altering the activity of the cytostatic factor (CSF), the cellular activity required for the establishment and maintenance of metaphase II arrest in the mature, unfertilized egg. We propose a model in which zinc exerts concentration-dependent regulation of meiosis through the CSF component EMI2, a zinc-binding protein. Together, the data support the conclusion that zinc itself, through its interaction with EMI2, is a central component of the CSF.

Zinc at cytotoxic concentrations affects posttranscriptional events of gene expression in cancer cells

Zinc at cytotoxic concentrations has been shown to regulate gene transcription in cancer cells, though zinc's involvement in posttranscriptional regulation is less characterized. In this study, we investigated the involvement of cytotoxic zinc in the posttranscriptional steps of gene expression. Clioquinol, a well-established zinc ionophore, was used to raise intracellular zinc to reported cytotoxic levels. The MCF-7 human cancer cell line was applied as a cell model system. Several parameters were used as indictors of posttranscriptional regulation, including p-body formation, microRNA profiling, expression level of proteins known to regulate mRNA degradation, microRNA processing, and protein translation. p-body formation was observed in MCF-7 cells using several molecules known as p-body components. Clioquinol plus zinc enhanced p-body assembly in MCF-7 cells. This enhancement was zinc-specific and could be blocked by a high affinity zinc chelator. The enhancement does not seem to be due to a stress response, as paclitaxel, a commonly used chemotherapeutic, did not cause enhanced p-body formation at a highly cytotoxic concentration. microRNA profiling indicated that clioquinol plus zinc globally down-regulates microRNA expression in this model system, which is associated with the reduced expression of Dicer, an enzyme key to microRNA maturation, and Ago2, a protein essential for microRNA stability. This study demonstrates that ionophoric zinc can induce cytotoxicity in cancer cells by globally regulating posttranscriptional events.

Simulations of allosteric motions in the zinc sensor CzrA

The zinc sensing transcriptional repressor Staphylococcus aureus CzrA represents an excellent model system to understand how metal sensor proteins maintain cellular metal homeostasis. Zn(II) binding induces a quaternary structural switch from a "closed" conformation to a more "open" conformation, reducing the DNA binding affinity by 4 orders of magnitude. In this study, we use classical molecular dynamics and quantum mechanical/molecular mechanical molecular dynamics simulations to investigate the molecular basis for the large conformational motions and allosteric coupling free energy (~6 kcal/mol) associated with Zn(II) binding. Our simulations successfully capture the closed to open allosteric switching in DNA bound CzrA on Zn(II) binding. They reveal that zinc binding quenches global conformational sampling by CzrA, whereas DNA binding enhances the mobility of residues in the allosteric metal binding sites. These findings are in close agreement with experiments. We also identify networks of residues involved in correlated and anticorrelated motions that connect the metal binding and DNA binding sites. Our analysis of the essential dynamics shows metal ion binding to be the primary driving force for the quaternary structural change in CzrA. We also show that Zn(II) binding limits the conformational space sampled by CzrA and causes the electrostatic surface potential at the DNA binding interface to become less favorable toward DNA binding. Finally, our simulations provide strong support for a proposed hydrogen-bonding pathway that physically connects the metal binding residue, His97, to the DNA binding interface through the αR helix that is present only in the Zn(II)-bound state. Overall, our simulations provide molecular-level insights into the mechanism of allosteric regulation by CzrA and demonstrate the importance of protein motion in its biological activity.

Allosteric coupling between transition metal-binding sites in homooligomeric metal sensor proteins

Intracellular concentrations of transition metal ions are controlled at the transcriptional level by a panel of metalloregulatory proteins that collectively allow the cell to respond to changes in bioavailable metal concentration to elicit the appropriate cellular response, e.g., upregulation of genes coding for metal export or detoxification proteins in the event of metal excess. These proteins represent a specialized class of allosteric regulators that are ideal for studying ligand-mediated allostery in a comprehensive way due to the size, stability, reactivity, and the spectroscopic properties of transition metal ions as allosteric ligands. In addition to the commonly studied heterotropic regulation of metal binding and DNA binding, many of these proteins exhibit homotropic allostery, i.e., communication between two or more identical metal (ligand) binding sites on an oligomer. This chapter aims to guide the reader through the design and execution of experiments that allow quantification of the thermodynamic driving forces (ΔG (C), ΔH (C), and ΔS (C)) that govern both homotropic and heterotropic allosteric interactions in metal sensor proteins as well as the steps required to remove the influence of complex speciation from the measured parameter values.