Magnesium homeostasis in cardiac myocytes of Mg-deficient rats

Beyond Ion Homeostasis: Hypomagnesemia, Transient Receptor Potential Melastatin Channel 7, Mitochondrial Function, and Inflammation

As the second most abundant intracellular divalent cation, magnesium (Mg2+) is essential for cell functions, such as ATP production, protein/DNA synthesis, protein activity, and mitochondrial function. Mg2+ plays a critical role in heart rhythm, muscle contraction, and blood pressure. A significant decline in Mg2+ intake has been reported in developed countries because of the increased consumption of processed food and filtered/deionized water, which can lead to hypomagnesemia (HypoMg). HypoMg is commonly observed in cardiovascular diseases, such as heart failure, hypertension, arrhythmias, and diabetic cardiomyopathy, and HypoMg is a predictor for cardiovascular and all-cause mortality. On the other hand, Mg2+ supplementation has shown significant therapeutic effects in cardiovascular diseases. Some of the effects of HypoMg have been ascribed to changes in Mg2+ participation in enzyme activity, ATP stabilization, enzyme kinetics, and alterations in Ca2+, Na+, and other cations. In this manuscript, we discuss new insights into the pathogenic mechanisms of HypoMg that surpass previously described effects. HypoMg causes mitochondrial dysfunction, oxidative stress, and inflammation. Many of these effects can be attributed to the HypoMg-induced upregulation of a Mg2+ transporter transient receptor potential melastatin 7 channel (TRMP7) that is also a kinase. An increase in kinase signaling mediated by HypoMg-induced TRPM7 transcriptional upregulation, independently of any change in Mg2+ transport function, likely seems responsible for many of the effects of HypoMg. Therefore, Mg2+ supplementation and TRPM7 kinase inhibition may work to treat the sequelae of HypoMg by preventing increased TRPM7 kinase activity rather than just altering ion homeostasis. Since many diseases are characterized by oxidative stress or inflammation, Mg2+ supplementation and TRPM7 kinase inhibition may have wider implications for other diseases by acting to reduce oxidative stress and inflammation.

Rescuing Cardiac Cells and Improving Cardiac Function by Targeted Delivery of Oxygen-Releasing Nanoparticles after or Even before Acute Myocardial Infarction

Myocardial infarction (MI) causes massive cell death due to restricted blood flow and oxygen deficiency. Rapid and sustained oxygen delivery following MI rescues cardiac cells and restores cardiac function. However, current oxygen-generating materials cannot be administered during acute MI stage without direct injection or suturing methods, both of which risk rupturing weakened heart tissue. Here, we present infarcted heart-targeting, oxygen-releasing nanoparticles capable of being delivered by intravenous injection at acute MI stage, and specifically accumulating in the infarcted heart. The nanoparticles can also be delivered before MI, then gather at the injured area after MI. We demonstrate that the nanoparticles, delivered either pre-MI or post-MI, enhance cardiac cell survival, stimulate angiogenesis, and suppress fibrosis without inducing substantial inflammation and reactive oxygen species overproduction. Our findings demonstrate that oxygen-delivering nanoparticles can provide a nonpharmacological solution to rescue the infarcted heart during acute MI and preserve heart function.

Chronic magnesium deficiency causes reversible mitochondrial permeability transition pore opening and impairs hypoxia tolerance in the rat heart

Chronic magnesium (Mg) deficiency induces and exacerbates various cardiovascular diseases. We previously investigated the mechanisms underlying decline in cardiac function caused by chronic Mg deficiency and the effectiveness of Mg supplementation on this decline using the Langendorff-perfused isolated mouse heart model. Herein, we used the Langendorff-perfused isolated rat heart model to demonstrate the chronic Mg-deficient rats (Mg-deficient group) had lower the heart rate (HR) and left ventricular pressure (LVDP) than rats with normal Mg levels (normal group). Furthermore, decline in cardiac function due to hypoxia/reoxygenation injury was significantly greater in the Mg-deficient group than in the normal group. Experiments on mitochondrial permeability transition pore (mPTP) using isolated mitochondria revealed that mitochondrial membrane was fragile in the Mg-deficient group, implying that cardiac function decline through hypoxia/reoxygenation injury is associated with mitochondrial function. Mg supplementation for chronic Mg-deficient rats not only improved hypomagnesemia but also almost completely restored cardiac and mitochondrial functions. Therefore, proactive Mg supplementation in pathological conditions induced by Mg deficiency or for those at risk of developing hypomagnesemia may suppress the development and exacerbation of certain disease states.

Magnesium Deficiency Causes a Reversible, Metabolic, Diastolic Cardiomyopathy

Background Dietary Mg intake is associated with a decreased risk of developing heart failure, whereas low circulating Mg level is associated with increased cardiovascular mortality. We investigated whether Mg deficiency alone could cause cardiomyopathy. Methods and Results C57BL/6J mice were fed with a low Mg (low-Mg, 15-30 mg/kg Mg) or a normal Mg (nl-Mg, 600 mg/kg Mg) diet for 6 weeks. To test reversibility, half of the low-Mg mice were fed then with nl-Mg diet for another 6 weeks. Low-Mg diet significantly decreased mouse serum Mg (0.38±0.03 versus 1.14±0.03 mmol/L for nl-Mg; P<0.0001) with a reciprocal increase in serum Ca, K, and Na. Low-Mg mice exhibited impaired cardiac relaxation (ratio between mitral peak early filling velocity E and longitudinal tissue velocity of the mitral anterior annulus e, 21.1±1.1 versus 15.4±0.4 for nl-Mg; P=0.011). Cellular ATP was decreased significantly in low-Mg hearts. The changes were accompanied by mitochondrial dysfunction with mitochondrial reactive oxygen species overproduction and membrane depolarization. cMyBPC (cardiac myosin-binding protein C) was S-glutathionylated in low-Mg mouse hearts. All these changes were normalized with Mg repletion. In vivo (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride treatment during low-Mg diet improved cardiac relaxation, increased ATP levels, and reduced S-glutathionylated cMyBPC. Conclusions Mg deficiency caused a reversible diastolic cardiomyopathy associated with mitochondrial dysfunction and oxidative modification of cMyBPC. In deficiency states, Mg supplementation may represent a novel treatment for diastolic heart failure.

Magnesium: Biochemistry, Nutrition, Detection, and Social Impact of Diseases Linked to Its Deficiency

Magnesium plays an important role in many physiological functions. Habitually low intakes of magnesium and in general the deficiency of this micronutrient induce changes in biochemical pathways that can increase the risk of illness and, in particular, chronic degenerative diseases. The assessment of magnesium status is consequently of great importance, however, its evaluation is difficult. The measurement of serum magnesium concentration is the most commonly used and readily available method for assessing magnesium status, even if serum levels have no reliable correlation with total body magnesium levels or concentrations in specific tissues. Therefore, this review offers an overview of recent insights into magnesium from multiple perspectives. Starting from a biochemical point of view, it aims at highlighting the risk due to insufficient uptake (frequently due to the low content of magnesium in the modern western diet), at suggesting strategies to reach the recommended dietary reference values, and at focusing on the importance of detecting physiological or pathological levels of magnesium in various body districts, in order to counteract the social impact of diseases linked to magnesium deficiency.

Dietary Mg2+ Intake and the Na+/Mg2+ Exchanger SLC41A1 Influence Components of Mitochondrial Energetics in Murine Cardiomyocytes

Cardiomyocytes are among the most energy-intensive cell types. Interplay between the components of cellular magnesium (Mg) homeostasis and energy metabolism in cardiomyocytes is poorly understood. We have investigated the effects of dietary Mg content and presence/functionality of the Na+/Mg2+ exchanger SLC41A1 on enzymatic functions of selected constituents of the Krebs cycle and complexes of the electron transport chain (ETC). The activities of aconitate hydratase (ACON), isocitrate dehydrogenase (ICDH), α-ketoglutarate dehydrogenase (KGDH), and ETC complexes CI-CV have been determined in vitro in mitochondria isolated from hearts of wild-type (WT) and Slc41a1-/- mice fed a diet with either normal or low Mg content. Our data demonstrate that both, the type of Mg diet and the Slc41a1 genotype largely impact on the activities of enzymes of the Krebs cycle and ETC. Moreover, a compensatory effect of Slc41a1-/- genotype on the effect of low Mg diet on activities of the tested Krebs cycle enzymes has been identified. A machine-learning analysis identified activities of ICDH, CI, CIV, and CV as common predictors of the type of Mg diet and of CII as suitable predictor of Slc41a1 genotype. Thus, our data delineate the effect of dietary Mg content and of SLC41A1 functionality on the energy-production in cardiac mitochondria.

TRPM7 silencing attenuates Mg2+ influx in cardiac myoblasts, H9c2 cells

TRPM7, a member of the melastatin subfamily of transient receptor potential channels, is suggested to be a potential candidate for a physiological Mg2+ channel. However, there is no direct evidence of Mg2+ permeation through endogenous TRPM7. To determine the physiological roles of TRPM7 in intracellular Mg2+ homeostasis, we measured the cytoplasmic free Mg2+ concentration ([Mg2+]i) in TRPM7-silenced H9c2 cells. [Mg2+]i was measured in a cluster of 8-10 cells using the fluorescent indicator, furaptra. TRPM7 silencing did not change [Mg2+]i in Ca2+-free Tyrode's solution containing 1 mM Mg2+. Increasing the extracellular Mg2+ to 92.5 mM raised [Mg2+]i in control cells (1.56 ± 0.19 mM) at 30 min, while this effect was significantly attenuated in TRPM7-silenced cells (1.12 ± 0.07 mM). The Mg2+ efflux driven by Na+ gradient was unaffected by TRPM7 silencing. These results suggest that TRPM7 regulates the rate of Mg2+ influx in H9c2 cells, although cytoplasmic Mg2+ homeostasis at basal conditions is unaffected by TRPM7 silencing.

Magnesium, Oxidative Stress, Inflammation, and Cardiovascular Disease

Hypomagnesemia is commonly observed in heart failure, diabetes mellitus, hypertension, and cardiovascular diseases. Low serum magnesium (Mg) is a predictor for cardiovascular and all-cause mortality and treating Mg deficiency may help prevent cardiovascular disease. In this review, we discuss the possible mechanisms by which Mg deficiency plays detrimental roles in cardiovascular diseases and review the results of clinical trials of Mg supplementation for heart failure, arrhythmias and other cardiovascular diseases.

Magnesium: A Magic Bullet for Cardiovascular Disease in Chronic Kidney Disease?

Magnesium is essential for many physiological functions in the human body. Its homeostasis involves dietary intake, absorption, uptake and release from bone, swifts between the intra- and extracellular compartment, and renal excretion. Renal excretion is mainly responsible for regulation of magnesium balance. In chronic kidney disease (CKD), for a long time the general policy has been limiting magnesium intake. However, this may not be appropriate for many patients. The reference ranges for magnesium are not necessarily optimal concentrations, and risks for insufficient magnesium intake exist in patients with CKD. In recent years, many observational studies have shown that higher (in the high range of “normal” or slightly above) magnesium concentrations are associated with better survival in CKD cohorts. This review gives an overview of epidemiological associations between magnesium and overall and cardiovascular survival in patients with CKD. In addition, potential mechanisms explaining the protective role of magnesium in clinical cardiovascular outcomes are described by reviewing evidence from in vitro studies, animal studies, and human intervention studies with non-clinical endpoints. This includes the role of magnesium in cardiac arrhythmia, heart failure, arterial calcification, and endothelial dysfunction. Possible future implications will be addressed, which will need prospective clinical trials with relevant clinical endpoints before these can be adopted in clinical practice.

Novel insights into TRPM7 function in fibrotic diseases: a potential therapeutic target

"Transient receptor potential (TRP) channels are cellular sensors for a wide spectrum of physical and chemical stimuli. Activation of TRP channels changes the membrane potential, translocates important signaling ions crossing the cell membrane, alters enzymatic activity, and initiates endocytosis/exocytosis (Zheng, 2013)." Fibrosis is the leading cause of organ dysfunction in diseases, which is characterized by an imbalance in the turnover of extracellular matrix components. Accumulating evidence has demonstrated that TRPM7, a member of TRP channels superfamily, participates in the development and pathogenesis of fibrotic diseases, such as hepatic, pulmonary and cardiac fibrosis. In this review, we discuss the comprehensive role of TRPM7 in modulating profibrotic response and its potential as therapeutic target for fibrotic diseases.

Magnesium in man: implications for health and disease

Magnesium (Mg(2+)) is an essential ion to the human body, playing an instrumental role in supporting and sustaining health and life. As the second most abundant intracellular cation after potassium, it is involved in over 600 enzymatic reactions including energy metabolism and protein synthesis. Although Mg(2+) availability has been proven to be disturbed during several clinical situations, serum Mg(2+) values are not generally determined in patients. This review aims to provide an overview of the function of Mg(2+) in human health and disease. In short, Mg(2+) plays an important physiological role particularly in the brain, heart, and skeletal muscles. Moreover, Mg(2+) supplementation has been shown to be beneficial in treatment of, among others, preeclampsia, migraine, depression, coronary artery disease, and asthma. Over the last decade, several hereditary forms of hypomagnesemia have been deciphered, including mutations in transient receptor potential melastatin type 6 (TRPM6), claudin 16, and cyclin M2 (CNNM2). Recently, mutations in Mg(2+) transporter 1 (MagT1) were linked to T-cell deficiency underlining the important role of Mg(2+) in cell viability. Moreover, hypomagnesemia can be the consequence of the use of certain types of drugs, such as diuretics, epidermal growth factor receptor inhibitors, calcineurin inhibitors, and proton pump inhibitors. This review provides an extensive and comprehensive overview of Mg(2+) research over the last few decades, focusing on the regulation of Mg(2+) homeostasis in the intestine, kidney, and bone and disturbances which may result in hypomagnesemia.

Physiological pathway of magnesium influx in rat ventricular myocytes

Cytoplasmic free Mg(2+) concentration ([Mg(2+)]i) was measured in rat ventricular myocytes with a fluorescent indicator furaptra (mag-fura-2) introduced by AM-loading. By incubation of the cells in a high-K(+) (Ca(2+)- and Mg(2+)-free) solution, [Mg(2+)]i decreased from ? 0.9 mM to 0.2 to 0.5 mM. The lowered [Mg(2+)]i was recovered by perfusion with Ca(2+)-free Tyrode's solution containing 1 mM Mg(2+). The time course of the [Mg(2+)]i recovery was fitted by a single exponential function, and the first derivative at time 0 was analyzed as being proportional to the initial Mg(2+) influx rate. The Mg(2+) influx rate was inversely related to [Mg(2+)]i, being higher at low [Mg(2+)]i. The Mg(2+) influx rate was augmented by the high extracellular Mg(2+) concentration (5 mM), whereas it was greatly reduced by cell membrane depolarization caused by high K(+). Known inhibitors of TRPM7 channels, 2-aminoethoxydiphenyl borate (2-APB), NS8593, and spermine reduced the Mg(2+) influx rate with half inhibitory concentrations (IC50) of, respectively, 17 ?M, 2.0 ?M, and 22 ?M. We also studied Ni(2+) influx by fluorescence quenching of intracellular furaptra by Ni(2+). The Ni(2+) influx was activated by lowering intra- and extracellular Mg(2+) concentrations, and it was inhibited by 2-APB and NS8593 with IC50 values comparable with those for the Mg(2+) influx. Intracellular alkalization (caused by pulse application of NH4Cl) enhanced, whereas intracellular acidification (induced after the removal of NH4Cl) slowed the Mg(2+) influx. Under the whole-cell patch-clamp configuration, the removal of intracellular and extracellular divalent cations induced large inward and outward currents, MIC (Mg-inhibited cation) currents or IMIC, carried by monovalent cations likely via TRPM7 channels. IMIC measured at -120 mV was diminished to ? 50% by 100 ?M 2-APB or 10 ?M NS8593. These results suggest that TRPM7/MIC channels serve as a major physiological pathway of Mg(2+) influx in rat ventricular myocytes.

Membrane protein CNNM4-dependent Mg2+ efflux suppresses tumor progression

Intracellular Mg(2+) levels are strictly regulated; however, the biological importance of intracellular Mg(2+) levels and the pathways that regulate them remain poorly understood. Here, we determined that intracellular Mg(2+) is important in regulating both energy metabolism and tumor progression. We determined that CNNM4, a membrane protein that stimulates Mg(2+) efflux, binds phosphatase of regenerating liver (PRL), which is frequently overexpressed in malignant human cancers. Biochemical analyses of cultured cells revealed that PRL prevents CNNM4-dependent Mg(2+) efflux and that regulation of intracellular Mg(2+) levels by PRL and CNNM4 is linked to energy metabolism and AMPK/mTOR signaling. Indeed, treatment with the clinically available mTOR inhibitor rapamycin suppressed the growth of cancer cells in which PRL was overexpressed. In ApcΔ(14/+) mice, which spontaneously form benign polyps in the intestine, deletion of Cnnm4 promoted malignant progression of intestinal polyps to adenocarcinomas. IHC analyses of tissues from patients with colon cancer demonstrated an inverse relationship between CNNM4 expression and colon cancer malignancy. Together, these results indicate that CNNM4-dependent Mg(2+) efflux suppresses tumor progression by regulating energy metabolism.

SLC41A1 knockdown inhibits angiotensin II-induced cardiac fibrosis by preventing Mg(2+) efflux and Ca(2+) signaling in cardiac fibroblasts

Na(+)/Mg(2+) exchanger plays an important role in cardiovascular system, but the molecular mechanisms still largely remain unknown. The Solute Carrier family 41A1 (SLC41A1), a novel Mg(2+) transporter, recently was found to function as Na(+)/Mg(2+) exchanger, which mainly regulates the intracellular Mg(2+) ([Mg(2+)]i) homeostasis. Our present studies were designed to investigate whether SLC41A1 impacts on the fibrogenesis of cardiac fibroblasts under Ang II stimulation. Our results showed that quinidine, a prototypical inhibitor of Na(+)/Mg(2+) exchanger, inhibited Ang II-induced cardiac fibrosis via attenuating the overexpression of vital biomarkers of fibrosis, including connective tissue growth factor (CTGF), fibronectin (FN) and α-smooth muscle actin (α-SMA). In addition, quinidine also decreased the Ang II-mediated elevation of concentration of intracellular Ca(2+) ([Ca(2+)]i) and extrusion of intracellular Mg(2+). Meanwhile, silencing SLC41A1 by RNA interference also impaired the elevation of [Ca(2+)]i, [Mg(2+)]i efflux and the upregulation of CTGF, FN and α-SMA provoked by Ang II. Furthermore, we found that Ang II-mediated activation of NFATc4 translocation decreased in SLC41A1-siRNA cells. These results support the notion that rapid extrusion of intracellular Mg(2+) is mediated by SLC41A1 and provide the evidence that the intracellular free Ca(2+) concentration is influenced by extrusion of intracellular Mg(2+) which facilitates fibrosis reaction in cardiac fibroblasts.