Development and Application of an Atomic Absorption Spectrometry-Based Method to Quantify Magnesium in Leaves of Dioscorea polystachya

The Chinese yam (Dioscorea polystachya, DP) is known for the nutritional value of its tuber. Nevertheless, DP also has promising pharmacological properties. Compared with the tuber, the leaves of DP are still very little studied. However, it may be possible to draw conclusions about the plant quality based on the coloration of the leaves. Magnesium, as a component of chlorophyll, seems to play a role. Therefore, the aim of this research work was to develop an atomic absorption spectrometry-based method for the analysis of magnesium (285.2125 nm) in leaf extracts of DP following the graphite furnace sub-technique. The optimization of the pyrolysis and atomization temperatures resulted in 1500 °C and 1800 °C, respectively. The general presence of flavonoids in the extracts was detected and could explain the high pyrolysis temperature due to the potential complexation of magnesium. The elaborated method had linearity in a range of 1-10 µg L-1 (R2 = 0.9975). The limits of detection and quantification amounted to 0.23 µg L-1 and 2.00 µg L-1, respectively. The characteristic mass was 0.027 pg, and the recovery was 96.7-102.0%. Finally, the method was applied to extracts prepared from differently colored leaves of DP. Similar magnesium contents were obtained for extracts made of dried and fresh leaves. It is often assumed that the yellowing of the leaves is associated with reduced magnesium content. However, the results indicated that yellow leaves are not due to lower magnesium levels. This stimulates the future analysis of DP leaves considering other essential minerals such as molybdenum or manganese.

Density functional theory study of crown ether-magnesium complexes: from a solvated ion to an ion trap

Metal ion detection rests on host-guest recognition. We propose a theoretical protocol for designing an optimal trap for a desired metal cation. A host for magnesium ions was sought for among derivatives of crown ethers 12-crown-4, 15-crown-5, and 18-crown-6. Mg-crown complexes and their hydrated counterparts with water molecules bound to the cation were optimized using density functional theory. Based on specific geometric criteria, Interacting quantum atoms analysis and density functional theory-based molecular dynamics of Mg-crown complexes immersed in water, crown ethers for optimal accommodation of Mg2+ in aqueous solution were identified. Selectivity of the chosen crowns towards Na+, K+, and Ca2+ ions is addressed.

Ratiometric Fluorescent Sensors Illuminate Cellular Magnesium Imbalance in a Model of Acetaminophen-Induced Liver Injury

Magnesium(II) plays catalytic, structural, regulatory, and signaling roles in living organisms. Abnormal levels of this metal have been associated with numerous pathologies, including cardiovascular disease, diabetes, metabolic syndrome, immunodeficiency, cancer, and, most recently, liver pathologies affecting humans. The role of Mg2+ in the pathophysiology of liver disease, however, has been occluded by concomitant changes in concentration of interfering divalent cations, such as Ca2+, which complicates the interpretation of experiments conducted with existing molecular Mg2+ indicators. Herein, we introduce a new quinoline-based fluorescent sensor, MagZet1, that displays a shift in its excitation and emission wavelengths, affording ratiometric detection of cellular Mg2+ by both fluorescence microscopy and flow cytometry. The new sensor binds the target metal with a submillimolar dissociation constant─well suited for detection of changes in free Mg2+ in cells─and displays a 10-fold selectivity against Ca2+. Furthermore, the fluorescence ratio is insensitive to changes in pH in the physiological range, providing an overall superior performance over existing indicators. We provide insights into the metal selectivity profile of the new sensor based on computational modeling, and we apply it to shed light on a decrease in cytosolic free Mg2+ and altered expression of metal transporters in cellular models of drug-induced liver injury caused by acetaminophen overdose.

Restoration of metal homeostasis: a potential strategy against neurodegenerative diseases

Metal homeostasis is critical to normal neurophysiological activity. Metal ions are involved in the development, metabolism, redox and neurotransmitter transmission of the central nervous system (CNS). Thus, disturbance of homeostasis (such as metal deficiency or excess) can result in serious consequences, including neurooxidative stress, excitotoxicity, neuroinflammation, and nerve cell death. The uptake, transport and metabolism of metal ions are highly regulated by ion channels. There is growing evidence that metal ion disorders and/or the dysfunction of ion channels contribute to the progression of neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS). Therefore, metal homeostasis-related signaling pathways are emerging as promising therapeutic targets for diverse neurological diseases. This review summarizes recent advances in the studies regarding the physiological and pathophysiological functions of metal ions and their channels, as well as their role in neurodegenerative diseases. In addition, currently available metal ion modulators and in vivo quantitative metal ion imaging methods are also discussed. Current work provides certain recommendations based on literatures and in-depth reflections to improve neurodegenerative diseases. Future studies should turn to crosstalk and interactions between different metal ions and their channels. Concomitant pharmacological interventions for two or more metal signaling pathways may offer clinical advantages in treating the neurodegenerative diseases.

Lanthanide-based luminescent probes for biological magnesium: accessing polyphosphate-bound Mg2

Biomolecule-bound Mg2+ species, particularly polyphosphate complexes, represent a large and dynamic fraction of the total cellular magnesium that is essential for cellular function but remains invisible to most indicators. Here we report a new family of Eu(III)-based indicators, the MagQEu family, functionalized with a 4-oxo-4H-quinolizine-3-carboxylic acid metal recognition group/sensitization antenna for turn-on, luminescence-based detection of biologically relevant Mg2+ species.

Inhibition of Mg2+ Extrusion Attenuates Glutamate Excitotoxicity in Cultured Rat Hippocampal Neurons

Magnesium plays important roles in the nervous system. An increase in the Mg²⁺ concentration in cerebrospinal fluid enhances neural functions, while Mg²⁺ deficiency is implicated in neuronal diseases in the central nervous system. We have previously demonstrated that high concentrations of glutamate induce excitotoxicity and elicit a transient increase in the intracellular concentration of Mg²⁺ due to the release of Mg²⁺ from mitochondria, followed by a decrease to below steady-state levels. Since Mg²⁺ deficiency is involved in neuronal diseases, this decrease presumably affects neuronal survival under excitotoxic conditions. However, the mechanism of the Mg²⁺ decrease and its effect on the excitotoxicity process have not been elucidated. In this study, we demonstrated that inhibitors of Mg²⁺ extrusion, quinidine and amiloride, attenuated glutamate excitotoxicity in cultured rat hippocampal neurons. A toxic concentration of glutamate induced both Mg²⁺ release from mitochondria and Mg²⁺ extrusion from cytosol, and both quinidine and amiloride suppressed only the extrusion. This resulted in the maintenance of a higher Mg²⁺ concentration in the cytosol than under steady-state conditions during the ten-minute exposure to glutamate. These inhibitors also attenuated the glutamate-induced depression of cellular energy metabolism. Our data indicate the importance of Mg²⁺ regulation in neuronal survival under excitotoxicity.

Structure and Lateral Organization of Phosphatidylinositol 4,5-bisphosphate

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is a minor but ubiquitous component of the inner leaflet of the plasma membrane of eukaryotic cells. However, due to its particular complex biophysical properties, it stands out from its neighboring lipids as one of the most important regulators of membrane-associated signaling events. Despite its very low steady-state concentration, PI(4,5)P₂ is able to engage in a multitude of simultaneous cellular functions that are temporally and spatially regulated through the presence of localized transient pools of PI(4,5)P₂ in the membrane. These pools are crucial for the recruitment, activation, and organization of signaling proteins and consequent regulation of downstream signaling. The present review showcases some of the most important PI(4,5)P2 molecular and biophysical properties as well as their impact on its membrane dynamics, lateral organization, and interactions with other biochemical partners.

Magnesium Is a Key Player in Neuronal Maturation and Neuropathology

Magnesium (Mg) is the second most abundant cation in mammalian cells, and it is essential for numerous cellular processes including enzymatic reactions, ion channel functions, metabolic cycles, cellular signaling, and DNA/RNA stabilities. Because of the versatile and universal nature of Mg²⁺, the homeostasis of intracellular Mg²⁺ is physiologically linked to growth, proliferation, differentiation, energy metabolism, and death of cells. On the cellular and tissue levels, maintaining Mg²⁺ within optimal levels according to the biological context, such as cell types, developmental stages, extracellular environments, and pathophysiological conditions, is crucial for development, normal functions, and diseases. Hence, Mg²⁺ is pathologically involved in cancers, diabetes, and neurodegenerative diseases, such as Parkinson's disease, Alzheimer's disease, and demyelination. In the research field regarding the roles and mechanisms of Mg²⁺ regulation, numerous controversies caused by its versatility and complexity still exist. As Mg²⁺, at least, plays critical roles in neuronal development, healthy normal functions, and diseases, appropriate Mg²⁺ supplementation exhibits neurotrophic effects in a majority of cases. Hence, the control of Mg²⁺ homeostasis can be a candidate for therapeutic targets in neuronal diseases. In this review, recent results regarding the roles of intracellular Mg²⁺ and its regulatory system in determining the cell phenotype, fate, and diseases in the nervous system are summarized, and an overview of the comprehensive roles of Mg²⁺ is provided.

Design and synthesis of cell-permeable fluorescent nitrilotriacetic acid derivatives

Fluorescence labeling of the target molecules using a small molecule-based probe is superior than a method using genetically expressed green fluorescence protein (GFP) in terms of convenience in its preparation and functionalization. Fluorophore-nitrilotriacetic acid (NTA) conjugates with several ester protecting groups were synthesized and evaluated for their cell membrane permeability by fluorescence microscopy analysis. One of the derivatives, acetoxymethyl (AM)-protected NTA conjugate is hydrolyzed, resulting in intracellular accumulation, thus providing localized fluorescence intensity in cells. This modification is expected as an effective method for converting a non-cell membrane permeable NTA-BODIPY conjugates to a cell membrane permeable derivatives.

Multiparametric quantification of heterogeneity of metal ion concentrations, as demonstrated for [Mg2+] by way of 31P MRS

Magnesium(II) is the second most abundant intracellular cation in mammals. Non-invasive ³¹P MRS is currently used to measure intracellular free Mg²⁺ levels in studies of magnesium deficiency disorders. However, this technique only provides one [Mg²⁺] value for a given tissue volume (or voxel), based on the chemical shift of the ATP-β (or NTP-β) resonance. We present here an approach for quantifying tissue heterogeneity in regard to [Mg²⁺], by way of multiple ³¹P MRS-derived descriptors characterizing the statistical intra-volume distribution of free [Mg²⁺] values. Our novel paradigm exploits the fact that the lineshape of the ATP-β ³¹P MRS resonance reflects the statistical distribution of [Mg²⁺] values within the observed volume (or voxel). Appropriate lineshape analysis reveals multiple quantitative statistical parameters (descriptors) characterizing the [Mg²⁺] distribution. First, the ATP-β ³¹P MRS resonance is transformed into a [Mg²⁺] curve that is used to construct a histogram with our specially developed algorithms. From this histogram, at least eight [Mg²⁺] descriptors are computed: weighted mean concentration and median concentration, standard deviation of concentration, range of concentration, concentration mode(s), concentration kurtosis, concentration skewness, and concentration entropy. Comprehensive evaluation based on in silico and experimental models demonstrates the validity of this new method. This basic feasibility study should open new avenues for future in vivo studies in physiology and medicine.

Characterization of unusual MgCa particles involved in the formation of foraminifera shells using a novel quantitative cryo SEM/EDS protocol

Quantifying ion concentrations and mapping their intracellular distributions at high resolution can provide much insight into the formation of biomaterials. The key to achieving this goal is cryo-fixation, where the biological materials, tissues and associated solutions are rapidly frozen and preserved in a vitreous state. We developed a correlative cryo-Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) protocol that provides quantitative elemental analysis correlated with spatial imaging of cryo-immobilized specimens. We report the accuracy and sensitivity of the cryo-EDS method, as well as insights we derive on biomineralization pathways in a foraminifer. Foraminifera are marine protozoans that produce Mg-containing calcitic shells and are major calcifying organisms in the oceans. We use the cryo-SEM/EDS correlative method to characterize unusual Mg and Ca-rich particles in the cytoplasm of a benthic foraminifer. The Mg/Ca ratio of these particles is consistently lower than that of seawater, the source solution for these ions. We infer that these particles are involved in Ca ion supply to the shell. We document the internal structure of the MgCa particles, which in some cases include a separate Si rich core phase. This approach to mapping ion distribution in cryo-preserved specimens may have broad applications to other mineralized biomaterials.

STATEMENT OF SIGNIFICANCE

Ions are an integral part of life, and some ions play fundamental roles in cell metabolism. Determining the concentrations of ions in cells and between cells, as well as their distributions at high resolution can provide valuable insights into ion uptake, storage, functions and the formation of biomaterials. Here we present a new cryo-SEM/EDS protocol that allows the mapping of different ion distributions in solutions and biological samples that have been cryo-preserved. We demonstrate the value of this novel approach by characterizing a novel biogenic mineral phase rich in Mg found in foraminifera, single celled marine organisms. This method has wide applicability in biology, and especially in understanding the formation and function of mineral-containing hard tissues.

Structural and spectroscopic insight into the metal binding properties of the o-aminophenol-N,N,O-triacetic acid (APTRA) chelator: implications for design of metal indicators

The o-aminophenol-N,N,O-triacetic acid (APTRA) chelator is employed extensively as a metal-recognition moiety in fluorescent indicators for biological free Mg(2+), as well as in low-affinity indicators for the detection of high levels of cellular Ca(2+). Despite its widespread use in sensor design, the limited metal selectivity of this chelating moiety can lead to binding of competing cations that complicate the fluorescence-based detection of metals of interest in complex samples. Reported herein are the structural characterization of APTRA complexes with various biologically relevant cations, and the thermodynamic analysis of complex formation with Mg(2+), Ca(2+) and Zn(2+). Our results indicate that the low affinity of APTRA for Mg(2+), which makes it a suitable metal-recognition moiety for sensitive analysis of typical millimolar levels of this metal in cells, stems from a much higher enthalpic cost of Mg(2+) binding compared to that of other cations. The results are discussed in the context of indicator design, highlighting the aspects that may aid the future development of fluorescent sensors with enhanced metal selectivity profiles.

Visualization of long-term Mg2+ dynamics in apoptotic cells using a novel targetable fluorescent probe

Long-term Mg²⁺ imaging during apoptosis using a HaloTag-coupled Mg²⁺ probe demonstrated a Mg²⁺ concentration increase caused by dissociation of Mg²⁺ from ATP.

Mg²⁺ plays important roles in many physiological processes. However, the underlying molecular mechanisms, especially in the apoptotic pathway, remain unclear due to the diffusion of Mg²⁺ probes, which hinders long-term imaging in specific organelles. We developed an immobilized Mg²⁺ probe, MGH, which is covalently conjugated with the HaloTag protein in various organelles. HaloTag-coupled MGH enabled long-term imaging of intracellular local Mg²⁺ dynamics for 24 h. To exploit this remarkable property, MGH was applied to the investigation of intracellular Mg²⁺ dynamics during apoptosis. Time-lapse imaging revealed an increase in the Mg²⁺ concentration after apoptotic cell shrinkage. Combined imaging analyses of intracellular Mg²⁺ and ATP concentrations strongly suggested that this Mg²⁺ concentration increase was caused by the dissociation of Mg²⁺ from ATP, along with a decrease in the intracellular ATP concentration. Thus, this protein-coupled Mg²⁺ probe could be a new chemical tool to elucidate intracellular Mg²⁺ dynamics with high spatiotemporal resolution.

Where is it and how much? Mapping and quantifying elements in single cells

The biological function of a chemical element in cells not only requires the determination of its intracellular quantity, but also the spatial distribution of its concentration. Different strategies can be employed to quantify and map the intracellular concentration of elements in single cells. The assessment of the intracellular elemental concentration, which is the relevant information, requires the measurement of cell volume. This challenging and demanding task requires combining different techniques allowing gathering of both morphological and compositional information on the same cell. Moreover, the need to analyse samples more similar to their natural state requires complex hardware equipment, and supplementary efforts in preparation protocols. Nevertheless, the response to the question: "where is it and how much?" is worth all these efforts. This review aims at providing an insight into the recent and most advanced techniques and strategies for quantifying and mapping chemical elements in single cells. We describe and discuss indirect detection techniques (label based) which make use of fluorescent dyes, and direct ones (label free), such as particle induced X-ray emission, proton backscattering spectrometry, scanning transmission ion spectrometry, nano-secondary ion mass spectrometry, X-ray fluorescence microscopy, complemented by X-ray imaging.

Recognition of Mg²⁺ by a new fluorescent "turn-on" chemosensor based on pyridyl-hydrazono-coumarin

A new fluoroionophore PyHC bearing 2-pyridylhydrazone and 7-hydroxycoumarin moieties for selective detection of Mg(2+) was synthesized and characterized. This chemosensor exhibited "turn-on" fluorescence behavior and was sensitive to Mg(2+) concentrations as low as 105 nmol L(-1) in ethanol-water solution. Detailed spectroscopic studies revealed the binding mode of a 1:1 complex between PyHC and Mg(2+) that leads to a fluorescence enhancement.

Bright, red emitting fluorescent sensor for intracellular imaging of Mg2+

A new fluorescent sensor with excellent turn-on ratio, low energy excitation and emission over 600 nm enables Mg²⁺detection in live cells.

Highly sensitive fluorescent sensor for Mg2+ and Ca2+ based on a multi-addressable diarylethene

A new photochromic diarylethene bearing 8-aminoquinoline unit was designed and synthesized, and the multi-addressable behaviors were investigated. It was highly sensitive towards Mg²⁺ and Ca²⁺ with different fluorescence emission and color change.

Visualizing changes in mitochondrial Mg2+ during apoptosis with organelle-targeted triazole-based ratiometric fluorescent sensors

Magnesium is one of the most abundant metals in cells and is essential for a wide range of cellular processes. Magnesium imbalance has been linked to a variety of diseases, but the scarcity of sensors suitable for detection of Mg²⁺ with subcellular resolution has hampered the study of compartmentalization and mobilization of this ion in the context of physiological and pathological processes. We report herein a family of fluorescent probes for targeted detection of free Mg²⁺ in specific intracellular organelles, and its application in the study of programmed cell death. The new sensors feature a triazole unit that plays both structural and electronic roles by serving as an attachment group for targeting moieties, and modulating a possible internal charge transfer process for ratiometric ion sensing. A probe decorated with an alkylphosphonium group was employed for the detection of mitochondrial Mg²⁺ in live HeLa cells, providing the first direct observation of an increase in free Mg²⁺ levels in this organelle in the early stages of Staurosporine-induced apoptosis.

Intracellular magnesium level determines cell viability in the MPP(+) model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder resulting from mitochondrial dysfunction in dopaminergic neurons. Mitochondria are believed to be responsible for cellular Mg²⁺ homeostasis. Mg²⁺ is indispensable for maintaining ordinal cellular functions, hence perturbation of the cellular Mg²⁺ homeostasis may be responsible for the disorders of physiological functions and diseases including PD. However, the changes in intracellular Mg²⁺ concentration ([Mg²⁺]i) and the role of Mg²⁺ in PD have still been obscure. In this study, we investigated [Mg²⁺]i and its effect on neurodegeneration in the 1-methyl-4-phenylpyridinium (MPP⁺) model of PD in differentiated PC12 cells. Application of MPP⁺ induced an increase in [Mg²⁺]i immediately via two different pathways: Mg²⁺ release from mitochondria and Mg²⁺ influx across cell membrane, and the increased [Mg²⁺]i sustained for more than 16 h after MPP⁺ application. Suppression of Mg²⁺ influx decreased the viability of the cells exposed to MPP⁺. The cell viability correlated highly with [Mg²⁺]i. In the PC12 cells with suppressed Mg²⁺ influx, ATP concentration decreased and the amount of reactive oxygen species (ROS) increased after an 8h exposure to MPP⁺. Our results indicate that the increase in [Mg²⁺]i inhibited cellular ROS generation and maintained ATP production, which resulted in the protection from MPP⁺ toxicity.

Solvent effect on the fluorescence response of hydroxycoumarin bearing a dipicolylamine binding site to metal ions

The fluorescence behavior of a probe dpa-HC, which has a coumarin derivative that acts as a fluorophore and a dipicolylamine (DPA) unit that functions as a metal ion-recognition site, was investigated with various metal ions in aqueous and several non-aqueous solvents. In aqueous solution, the fluorescence of dpa-HC was enhanced by Zn(2+) and Cd(2+), but was quenched by other metal ions. On the other hand, in an acetonitrile solution, only Mg(2+) enhanced the fluorescence, and the addition of a small amount of water quenched this fluorescence. This dramatic selectivity change is explained by stabilization of a metal-dpa-HC complex due to acetonitrile coordination and ON-OFF switching of the intramolecular photoinduced electron transfer (PET) from the nitrogen lone pair of DPA to the coumarin derivative.

Enhanced ratiometric fluorescent indicators for magnesium based on azoles of the heavier chalcogens

Red-shifted fluorescent indicators for magnesium were developed by incorporation of sulfur or selenium in the azole moiety of 'fura' fluorophores. Single atom replacement in the acceptor of these ITC probes affords longer excitation and emission wavelengths as well as greater separation between excitation bands, valuable for ratiometric intracellular Mg(2+) imaging.

Disruption of an M. tuberculosis membrane protein causes a magnesium-dependent cell division defect and failure to persist in mice

The identification of Mycobacterium tuberculosis genes necessary for persistence in vivo provides insight into bacterial biology as well as host defense strategies. We show that disruption of M. tuberculosis membrane protein PerM (Rv0955) resulted in an IFN-γ-dependent persistence defect in chronic mouse infection despite the mutant's near normal growth during acute infection. The perM mutant required increased magnesium for replication and survival; incubation in low magnesium media resulted in cell elongation and lysis. Transcriptome analysis of the perM mutant grown in reduced magnesium revealed upregulation of cell division and cell wall biosynthesis genes, and live cell imaging showed PerM accumulation at the division septa in M. smegmatis. The mutant was acutely sensitive to β-lactam antibiotics, including specific inhibitors of cell division-associated peptidoglycan transpeptidase FtsI. Together, these data implicate PerM as a novel player in mycobacterial cell division and pathogenesis, and are consistent with the hypothesis that immune activation deprives M. tuberculosis of magnesium.

A real-time fluorescent sensor specific to Mg2+: crystallographic evidence, DFT calculation and its use for quantitative determination of magnesium in drinking water

An “off-the-shelf” fluorescence “turn-on” Mg²⁺ chemosensor BCSA has been developed for real-time quantitative monitoring of magnesium in drinking water.

Formation of ternary complexes with MgATP: effects on the detection of Mg2+ in biological samples by bidentate fluorescent sensors

Fluorescent indicators based on β-keto-acid bidentate coordination motifs display superior metal selectivity profiles compared to current o-aminophenol-N,N,O-triacetic acid (APTRA) based chelators for the study of biological magnesium. These low denticity chelators, however, may allow for the formation of ternary complexes with Mg(2+) and common ligands present in the cellular milieu. In this work, absorption, fluorescence, and NMR spectroscopy were employed to study the interaction of turn-on and ratiometric fluorescent indicators based on 4-oxo-4H-quinolizine-3-carboxylic acid with Mg(2+) and ATP, the most abundant chelator of biological magnesium, thus revealing the formation of ternary complexes under conditions relevant to fluorescence imaging. The formation of ternary species elicits comparable or greater optical changes than those attributed to the formation of binary complexes alone. Dissociation of the fluorescent indicators from both ternary and binary species have apparent equilibrium constants in the low millimolar range at pH 7 and 25 °C. These results suggest that these bidentate sensors are incapable of distinguishing between free Mg(2+) and MgATP based on ratio or intensity-based steady-state fluorescence measurements, thus posing challenges in the interpretation of results from fluorescence imaging of magnesium in nucleotide-rich biological samples.

A fluorescent probe for the detection of Mg(ii) and Cu(ii) and its application for imaging in living cells

A novel fluorescent probe 7-[4′-hydroxy-3′-(5′′-methyl-1H-benzo[d]imidazole-2-yl)styryl]nalidixic acid (HBIN) was synthesized that contains two independent fluorophores and acts as a very sensitive and selective probe for Mg²⁺ and Cu²⁺ ions.

A novel fluorescent chemosensor allows the assessment of intracellular total magnesium in small samples

Remarkable features of a novel fluorescent Mg dye: high fluorescence intensity and intracellular retention.

MagFRET: the first genetically encoded fluorescent Mg2+ sensor

Magnesium has important structural, catalytic and signaling roles in cells, yet few tools exist to image this metal ion in real time and at subcellular resolution. Here we report the first genetically encoded sensor for Mg²⁺, MagFRET-1. This sensor is based on the high-affinity Mg²⁺ binding domain of human centrin 3 (HsCen3), which undergoes a transition from a molten-globular apo form to a compactly-folded Mg²⁺-bound state. Fusion of Cerulean and Citrine fluorescent domains to the ends of HsCen3, yielded MagFRET-1, which combines a physiologically relevant Mg²⁺ affinity (K_d = 148 µM) with a 50% increase in emission ratio upon Mg²⁺ binding due to a change in FRET efficiency between Cerulean and Citrine. Mutations in the metal binding sites yielded MagFRET variants whose Mg²⁺ affinities were attenuated 2- to 100-fold relative to MagFRET-1, thus covering a broad range of Mg²⁺ concentrations. In situ experiments in HEK293 cells showed that MagFRET-1 can be targeted to the cytosol and the nucleus. Clear responses to changes in extracellular Mg²⁺ concentration were observed for MagFRET-1-expressing HEK293 cells when they were permeabilized with digitonin, whereas similar changes were not observed for intact cells. Although MagFRET-1 is also sensitive to Ca²⁺, this affinity is sufficiently attenuated (K_d of 10 µM) to make the sensor insensitive to known Ca²⁺ stimuli in HEK293 cells. While the potential and limitations of the MagFRET sensors for intracellular Mg²⁺ imaging need to be further established, we expect that these genetically encoded and ratiometric fluorescent Mg²⁺ sensors could prove very useful in understanding intracellular Mg²⁺ homeostasis and signaling.

The structure and regulation of magnesium selective ion channels

The magnesium ion (Mg(2+)) is the most abundant divalent cation within cells. In man, Mg(2+)-deficiency is associated with diseases affecting the heart, muscle, bone, immune, and nervous systems. Despite its impact on human health, little is known about the molecular mechanisms that regulate magnesium transport and storage. Complete structural information on eukaryotic Mg(2+)-transport proteins is currently lacking due to associated technical challenges. The prokaryotic MgtE and CorA magnesium transport systems have recently succumbed to structure determination by X-ray crystallography, providing first views of these ubiquitous and essential Mg(2+)-channels. MgtE and CorA are unique among known membrane protein structures, each revealing a novel protein fold containing distinct arrangements of ten transmembrane-spanning α-helices. Structural and functional analyses have established that Mg(2+)-selectivity in MgtE and CorA occurs through distinct mechanisms. Conserved acidic side-chains appear to form the selectivity filter in MgtE, whereas conserved asparagines coordinate hydrated Mg(2+)-ions within the selectivity filter of CorA. Common structural themes have also emerged whereby MgtE and CorA sense and respond to physiologically relevant, intracellular Mg(2+)-levels through dedicated regulatory domains. Within these domains, multiple primary and secondary Mg(2+)-binding sites serve to staple these ion channels into their respective closed conformations, implying that Mg(2+)-transport is well guarded and very tightly regulated. The MgtE and CorA proteins represent valuable structural templates to better understand the related eukaryotic SLC41 and Mrs2-Alr1 magnesium channels. Herein, we review the structure, function and regulation of MgtE and CorA and consider these unique proteins within the expanding universe of ion channel and transporter structural biology.

Ethanol promotes rat aortic vascular smooth muscle cell proliferation via increase of homocysteine and oxidized-low-density lipoprotein

BACKGROUND

Increased levels of homocysteine and oxidized low-density lipoprotein (Ox-LDL) are considered independent risk factors for atherosclerosis. However, no previous study has examined the effects of ethanol-induced increase of homocysteine and Ox-LD on aortic vascular smooth muscle cell (VSMC) proliferation. The aim of the present study was to investigate the relationship between ethanol consumption, increase in homocysteine, Ox-LDL, and aortic VSMC proliferation in rats.

METHODS AND RESULTS

To address this issue, 24 male Wistar rats were randomly divided into three groups: control, sham, and ethanol-treated. Homocysteine, Ox-LDL, lipid profile, and aortic VSMC proliferation were assessed after 42 days. The results revealed a concurrent, significant increase in homocysteine and Ox-LDL levels, lipid profile levels, and aortic VSMC proliferation in the ethanol-treated group compared with the control and sham groups.

CONCLUSION

Based on these results, we conclude that ethanol apparently exerts aortic VSMC proliferation through increase in homocysteine and Ox-LDL-mediated oxidative stress, which in turn trigger proatherogenic changes in the aortic wall.

NO/cGMP/PKG signaling pathway induces magnesium release mediated by mitoKATP channel opening in rat hippocampal neurons

Intracellular Mg²⁺ concentration ([Mg²⁺]i) and NO regulate cell survival and death. To reveal the involvement of NO in intracellular Mg²⁺ regulation, we visualized intracellular Mg²⁺ using the fluorescent Mg²⁺ indicator KMG-104-AM in rat hippocampal neurons. Pharmacological experiments using SNAP, 8-Br-cGMP, diazoxide and several inhibitors revealed that the NO/cGMP/Protein kinsase G (PKG) signaling pathway triggers an increase in [Mg²⁺]i, and that Mg²⁺ mobilization is due to Mg²⁺ release from mitochondria induced by mitoKATP channel opening. In addition, Mg²⁺ release is potentiated by the positive feedback loop including mitoKATP channel opening, mitochondrial depolarization and PKC activation.