In cardiac dysfunction, intracellular Ca2+-dynamics are disrupted leading to leakage of Ca2+ from the sarcoplasmic reticulum (SR). This results in diminished cardiac contractility and impaired cardiac function. In cardiac tissue, the underlying molecular mechanisms responsible for RyR2-independent Ca2+-leak are poorly understood. Mitsugumin 23 (MG23) is an intracellular Ca2+-conducting ion channel located on endoplasmic/sarcoplasmic reticulum (ER/SR) and nuclear membranes. We propose that MG23 contributes to regulation of intracellular Ca2+-homeostasis, and that altered MG23 function may drive progression of cardiac dysfunction. The aim of this research was to investigate the role of MG23 in SR Ca2+-leak, and whether knockout of Mg23 protects the heart against pressure-overload induced left ventricular hypertrophy. Cardiac pressure-overload was induced in wild type (WT) and Mg23-knockout (KO) mice through subcutaneous Angiotensin II (AngII, 1.1 mg/kg/day) infusion via osmotic pump. After 10-day infusion, in vivo pressure-volume dynamics were measured by insertion of a pressure-volume catheter into the left ventricle. MG23 protein expression was assessed through Western blot analysis. Ventricular fibrosis and cardiomyocyte size were measured using histological and immunofluorescence approaches. Cardiomyocytes were isolated from WT and Mg23-KO hearts and intracellular Ca2+-dynamics assessed through live cell imaging using the Ca2+ indicator Fluo-4. AngII-induced cardiac pressure-overload increased expression of MG23 in WT mouse hearts. Knockout of Mg23 protected hearts against AngII-induced cardiac hypertrophy. Compared to WT animals, AngII treated Mg23-KO mice displayed a significant reduction in left ventricular fibrosis and displayed normal cardiac functioning. Overexpression of MG23 in the ventricular cell line H9C2, resulted in reduced SR Ca2+ store levels. In Mg23-KO hearts, no alteration in expression of key Ca2+-handling proteins was identified, but cardiomyocytes displayed altered Ca2+-spark profiles consistent with a role for MG23 in SR Ca2+-leak. MG23 plays a key role in driving Ca2+-dysregulation observed in the early pathological stages of pressure-overload induced heart failure.

Mitsugumin 23 (MG23) is a transmembrane protein expressed in the nuclear membrane and endo/sarcoplasmic reticulum (ER/SR) of various tissues, including skeletal and cardiac muscle. MG23 is a non-selective cation channel that has been implicated in the leakage of calcium ions (Ca2+) under diverse pathophysiological conditions. SR Ca2+ leak is considered to be a contributing factor of skeletal muscle weakness and is also implicated in the progression of heart failure. The absence of MG23 has been reported to alleviate negative outcomes associated with SR Ca2+ leak. Targeting MG23 could represent a new therapeutic strategy against muscle disorders. This review discusses the potential role of MG23 in skeletal and cardiac muscle, and highlights MG23 as both a regulator of basal SR Ca2+-handling and a mediator of pathophysiological remodelling in muscle.

Delivery of antibacterial effector proteins into competitor cells using the Type VI secretion system (T6SS) is a widespread strategy for inter-bacterial competition. While many enzymatic T6SS effectors have been described, relatively few which form pores in target cell membranes have been reported. Here, we describe a widely-occurring family of T6SS-dependent pore-forming effectors, exemplified by Ssp4 of Serratia marcescens Db10. We show in vitro that Ssp4 forms regulated pores with high selectivity for cations, and use structural models and molecular dynamics simulations to predict how these pores conduct ions. Ssp4 has a broader phylogenetic distribution and is active against a wider range of bacterial species than Ssp6, the other pore-forming effector delivered by the same T6SS, with the two effectors displaying distinct ion selectivities and impacts on intoxicated cells. Finally, identification of Ssp4-resistant mutants revealed that a mucA mutant of Pseudomonas fluorescens is protected against T6SS attacks. We propose that deployment of two distinct T6SS-dependent pore-forming toxins is a common strategy to ensure effective de-energisation of closely- and distantly-related competitors.

Intracellular zinc and calcium dynamics are important for both platelet activation and function. We have recently shown that the Ca2+ permeable cation channel mitsugumin 23 (MG23), found in the endoplasmic/sarcoplasmic reticulum of other cell types, is Zn2+-sensitive and may facilitate crosstalk between the two metal ions. The role of MG23 in platelet responses has never been explored but given its suggested role as a Ca2+ leak channel we suggest that MG23 may contribute to the elevation of intracellular Ca2+ resulting in hyperactive platelets. The aim of this study was to confirm the presence of MG23 in platelets, identify its subcellular localization and evaluate the role of MG23 in platelet aggregation by comparing the aggregatory responses of platelets prepared from wild type and MG23-KO mice. Expression of MG23 was assessed using RT-PCR and western blotting using mRNA and protein extracted from healthy donor platelets and megakaryocyte-like cells (MEG-01 cell line). Platelet aggregation assays were carried out on washed platelets from both WT and MG23-KO mice to evaluate the effect of MG23 knockout on platelet clotting. The institutional ethics committee at the University of St. Andrews approved the study. Work was carried out under project licence P82006EDF. Subcellular localization of MG23 within platelets was assessed by confocal microscopy using fixed MEG-01 cells. MG23 mRNA and protein were identified in both platelets and MEG-01 cells. The staining of platelets with antibodies directed against SERCA2 ATPase and MG23 yielded colocalized fluorescence, indicative of MG23 localization to the dense tubular system. Both WT and MG23-KO murine platelets aggregated in response to various platelet agonists. MG23 expression was confirmed within platelets and is present in the dense tubular system. Though it has been proposed to serve as a Ca2+ leak channel in other cell types, its specific role in platelets is less clear.

EDITORIAL article Front. Endocrinol., 04 August 2023Sec. Cardiovascular Endocrinology Volume 14 - 2023 | https://doi.org/10.3389/fendo.2023.1266173

The main non-malignant cause of death in cancer survivors is heart disease caused by side effects from many chemotherapy agents, including anthracycline doxorubicin. Approximately 1-in-10 cancer survivors suffer irreversible heart damage, but the cause remains unknown. We have recently suggested that Mitsugumin 23 (MG23) is a Ca2+-leak channel located in sarcoplasmic reticulum (SR) membranes (Reilly-O’Donnell et al., 2017). Leakage of Ca2+ from SR stores is a hallmark of heart failure (Bers, 2014). We hypothesize that doxorubicin modulates MG23 resulting in Ca2+ cycle dysfunction. To study the impact of doxorubicin on intrinsic Ca2+ cycling, cardiomyocytes were isolated from wild type (WT) and MG23 knockout (KO) mice using a Langendorff-free method (Ackers-Johnson et al., 2016). Cells were treated with 2.5 µM doxorubicin for 24 hours, cells were loaded with 2 µM Fluo-4 and SR Ca2+ store levels assessed by the addition of 10 mM caffeine. Doxorubicin reduced SR Ca2+ store levels in cardiomyocytes isolated from both WT and MG23 KO mice but had a markedly greater effect on store levels in WT cells. To investigate the direct effect of doxorubicin on MG23 channel function we incorporated mouse cardiac SR vesicles into artificial bilayers under voltage-clamp conditions (Pitt et al., 2010). We show that luminal addition of doxorubicin to MG23 increases channel activity. Human model translation was assessed by demonstrating that doxorubicin increased the activity of recombinant human MG23 channels. This work implicates MG23 as a potential new therapeutic target in the treatment of cardiac dysfunction as a side effect of anthracycline chemotherapies.

Increasing evidence suggests that Zn2+ acts as a second messenger capable of transducing extracellular stimuli into intracellular signaling events. The importance of Zn2+ as a signaling molecule in cardiovascular functioning is gaining traction. In the heart, Zn2+ plays important roles in excitation–contraction (EC) coupling, excitation–transcription coupling, and cardiac ventricular morphogenesis. Zn2+ homeostasis in cardiac tissue is tightly regulated through the action of a combination of transporters, buffers, and sensors. Zn2+ mishandling is a common feature of various cardiovascular diseases. However, the precise mechanisms controlling the intracellular distribution of Zn2+ and its variations during normal cardiac function and during pathological conditions are not fully understood. In this review, we consider the major pathways by which the concentration of intracellular Zn2+ is regulated in the heart, the role of Zn2+ in EC coupling, and discuss how Zn2+ dyshomeostasis resulting from altered expression levels and efficacy of Zn2+ regulatory proteins are key drivers in the progression of cardiac dysfunction.

Magnesium (Mg2+) has many physiological functions within the body. These include important roles in maintaining cardiovascular functioning, where it contributes to the regulation of cardiac excitation-contraction coupling, endothelial functioning and haemostasis. The haemostatic roles of Mg2+ impact upon both the protein and cellular arms of coagulation. In this review, we examine how Mg2+ homeostasis is maintained within the body and highlight the various molecular roles attributed to Mg2+ in the cardiovascular system. In addition, we describe how nutritional and/or disease-associated magnesium deficiency, seen in some metabolic conditions, has the potential to influence cardiac and vascular outcomes. Finally, we also examine the potential for magnesium supplements to be employed in the prevention and treatment of cardiovascular disorders and in the management of cardiometabolic health.

The function of ion channels is essential in the infectious cycle of many viruses. To facilitate viral uptake, maturation and export, viruses must modify the ionic balance of their host cells, in particular of calcium ions (Ca 2+ ). Viroporins encoded in the viral genome play a key part in altering the cell’s ionic homeostasis. In SARS-Coronavirus-2 (SARS-CoV-2) – the causative agent of Covid-19 – the envelope (E) protein is considered to form ion channels in ERGIC organellar membranes, whose function is closely linked to disease progression and lethality. Deletion, blockade, or loss-of-function mutation of coronaviral E proteins results in propagation-deficient or attenuated virus variants. The exact physiological function of the E protein, however, is not sufficiently understood. Since one of the key features of the ER is its function as a Ca 2+ storage compartment, we investigated the activity of E in the context of this cation. Molecular dynamics simulations and voltage-clamp electrophysiological measurements show that E exhibits ion channel activity that is regulated by increased luminal Ca 2+ concentration, membrane voltage, post-translational protein modification, and negatively charged ERGIC lipids. Particularly, calcium ions bind to a distinct region at the ER-luminal channel entrance, where they activate the channel and maintain the pore in an open state. Also, alongside monovalent ions, the E protein is highly permeable to Ca 2+ . Our results suggest that the physiological role of the E protein is the release of Ca 2+ from the ER, and that the distinct Ca 2+ activation site may serve as a promising target for channel blockers, potentially inhibiting the infectious cycle of coronaviruses.

At least 5% of children present unexpected difficulties in expressing and understanding spoken language. This condition is highly heritable and often co-occurs with other neurodevelopmental disorders such as dyslexia and ADHD. Through an exome sequencing analysis, we identified a rare missense variant (chr16:84405221, GRCh38.p12) in the ATP2C2 gene. ATP2C2 was implicated in language disorders by linkage and association studies, and exactly the same variant was reported previously in a different exome sequencing study for language impairment (LI). We followed up this finding by genotyping the mutation in cohorts selected for LI and comorbid disorders. We found that the variant had a higher frequency in LI cases (1.8%, N = 360) compared with cohorts selected for dyslexia (0.8%, N = 520) and ADHD (0.7%, N= 150), which presented frequencies comparable to reference databases (0.9%, N = 24 046 gnomAD controls). Additionally, we observed that carriers of the rare variant identified from a general population cohort (N= 42, ALSPAC cohort) presented, as a group, lower scores on a range of reading and language-related measures compared to controls (N = 1825; minimum P = 0.002 for non-word reading). ATP2C2 encodes for an ATPase (SPCA2) that transports calcium and manganese ions into the Golgi lumen. Our functional characterization suggested that the rare variant influences the ATPase activity of SPCA2. Thus, our results further support the role of ATP2C2 locus in language-related phenotypes and pinpoint the possible effects of a specific rare variant at molecular level.

Zn2+ is an essential regulator of coagulation and is released from activated platelets. In plasma, the free Zn2+ concentration is fine-tuned through buffering by human serum albumin (HSA). Importantly, the ability of HSA to bind/buffer Zn2+ is compromised by co-transported non-esterified fatty acids (NEFAs). Given the role of Zn2+ in blood clot formation, we hypothesise that Zn2+ displacement from HSA by NEFAs in certain conditions (such as type 2 diabetes mellitus, T2DM) impacts on the cellular and protein arms of coagulation. To test this hypothesis, we assessed the extent to which increasing concentrations of a range of medium- and long-chain NEFAs reduced Zn2+-binding ability of HSA. Amongst the NEFAs tested, palmitate (16 : 0) and stearate (18 : 0) were the most effective at suppressing zinc-binding, whilst the mono-unsaturated palmitoleate (16 : 1c9) was markedly less effective. Assessment of platelet aggregation and fibrin clotting parameters in purified systems and in pooled plasma suggested that the HSA-mediated impact of the model NEFA myristate on zinc speciation intensified the effects of Zn2+ alone. The effects of elevated Zn2+ alone on fibrin clot density and fibre thickness in a purified protein system were mirrored in samples from T2DM patients, who have derranged NEFA metabolism. Crucially, T2DM individuals had increased total plasma NEFAs compared to controls, with the concentrations of key saturated (myristate, palmitate, stearate) and mono-unsaturated (oleate, cis-vaccenate) NEFAs positively correlating with clot density. Collectively, these data strongly support the concept that elevated NEFA levels contribute to altered coagulation in T2DM through dysregulation of plasma zinc speciation.

Characterizing single cell contractility in the beating heart is strongly limited by light scattering and extreme tissue dynamics. Here, we use tissue-integrated microlasers to measure contractility in live zebrafish and living myocardial slices at a depth several times deeper than multiphoton microscopy-based techniques.

Type-1 diabetes mellitus (T1DM) is associated with metabolic changes leading to alterations in glucose and lipid handling. While T1DM-associated effects on many major plasma lipids have been characterised, such effects on plasma free fatty acids (FFA) have not been fully examined. Using gas chromatography-mass spectrometry, we measured the plasma concentrations of FFA species in individuals with T1DM (n = 44) and age/sex-matched healthy controls (n = 44). Relationships between FFA species and various parameters were evaluated. Plasma concentrations of myristate (14:0), palmitoleate (16:1), palmitate (16:0), linoleate (18:2), oleate (18:1c9), cisvaccenate (18:1c11), eicosapentaenoate (20:5), arachidonate (20:4) and docosahexanoate (22:6) were reduced in the T1DM group (p < 0.0001 for all, except p = 0.0020 for eicosapentaenoate and p = 0.0068 for arachidonate); alpha-linolenate (18:3) and dihomo-gamma-linolenate (20:3) concentrations were unchanged. The saturated/unsaturated FFA ratio, n-3/n-6 ratio, de novo lipogenesis index (palmitate (main lipogenesis product)/linoleate (only found in diet)) and elongase index (oleate/palmitoleate) were increased in the T1DM group (p = 0.0166, p = 0.0089, p < 0.0001 and p = 0.0008 respectively). The stearoyl-CoA desaturase 1 (SCD1) index 1 (palmitoleate/palmitate) and index 2 (oleate/stearate) were reduced in T1DM (p < 0.0001 for both). The delta-(5)-desaturase (D5D) index (arachidonate/dihomo-gamma-linolenate) was unchanged. Age and sex had no effect on plasma FFA concentrations in T1DM, while SCD1 index 1 was positively correlated (p = 0.098) and elongase index negatively correlated with age (p = 0.0363). HbA1c was negatively correlated with all plasma FFA concentrations measured except alpha-linolenate and dihomo-gamma-linolenate. Correlations were observed between plasma FFA concentrations and cholesterol and HDL concentrations, but not LDL concentration or diabetes duration. Collectively, these results aid our understanding of T1DM and its effects on lipid metabolism.

Levothyroxine (LT4) is used to treat frequently encountered endocrinopathies such as thyroid diseases. It is regularly used in clinical (overt) hypothyroidism cases and subclinical (latent) hypothyroidism cases in the last decade. Suppressive LT4 therapy is also part of the medical regimen used to manage thyroid malignancies after a thyroidectomy. LT4 treatment possesses dual effects: substituting new-onset thyroid hormone deficiency and suppressing the local and distant malignancy spreading in cancer. It is the practice to administer LT4 in less-than-high suppressive doses for growth control of thyroid nodules and goiter, even in patients with preserved thyroid function. Despite its approved safety for clinical use, LT4 can sometimes induce side-effects, more often recorded with patients under treatment with LT4 suppressive doses than in unintentionally LT4-overdosed patients. Cardiac arrhythmias and the deterioration of osteoporosis are the most frequently documented side-effects of LT4 therapy. It also lowers the threshold for the onset or aggravation of cardiac arrhythmias for patients with pre-existing heart diseases. To improve the quality of life in LT4-substituted patients, clinicians often prescribe higher doses of LT4 to reach low normal TSH levels to achieve cellular euthyroidism. In such circumstances, the risk of cardiac arrhythmias, particularly atrial fibrillation, increases, and the combined use of LT4 and triiodothyronine further complicates such risk. This review summarizes the relevant available data related to LT4 suppressive treatment and the associated risk of cardiac arrhythmia.

The contractility of cardiac cells is a key parameter that describes the biomechanical characteristics of the beating heart, but functional monitoring of three-dimensional cardiac tissue with single-cell resolution remains a major challenge. Here, we introduce microscopic whispering-gallery-mode lasers into cardiac cells to realize all-optical recording of transient cardiac contraction profiles with cellular resolution. The brilliant emission and high spectral sensitivity of microlasers to local changes in refractive index enable long-term tracking of individual cardiac cells, monitoring of drug administration, accurate measurements of organ-scale contractility in live zebrafish, and robust contractility sensing through hundreds of micrometres of rat heart tissue. Our study reveals changes in sarcomeric protein density as an underlying factor to cardiac contraction. More broadly, the use of novel micro- and nanoscopic lasers as non-invasive, biointegrated optical sensors brings new opportunities to monitor a wide range of physiological parameters with cellular resolution. The incorporation of microsphere lasers into heart cells allows all-optical recording of cardiac contraction with cellular resolution. [This summary has been amended from ‘microdisk’ to ‘microsphere’ lasers.]

Individuals with type-1 diabetes mellitus (T1DM) have a higher risk of thrombosis and low plasma magnesium concentrations. As magnesium is a known regulator of fibrin network formation, we investigated potential associations between fibrin clot properties and plasma magnesium concentrations in 45 individuals with T1DM and 47 age- and sex-matched controls without diabetes. Fibrin clot characteristics were assessed using a validated turbidimetric assay and associations with plasma magnesium concentration were examined. Plasma concentrations of fibrinogen, plasminogen activator inhibitor-1 (PAI-1), and lipids were measured and fibrin fiber diameters assessed using scanning electron microscopy. Fibrin clot maximum absorbance was unchanged in subjects with T1DM compared with controls, while lysis time was prolonged ( p = 0.0273). No differences in fibrin fiber diameters or in lipid profile were observed between T1DM and controls. PAI-1 concentration was lower in the T1DM group compared with the controls ( p = 0.0232) and positively correlated with lysis time ( p = 0.0023). Plasma magnesium concentration was lower in the T1DM group compared with controls ( p < 0.0001). Magnesium concentration negatively correlated with clot maximum absorbance ( p = 0.0215) and lysis time ( p = 0.0464). A turbidimetric fibrin clot lysis assay performed in a purified system that included PAI-1 and 0 to 3.2 mM Mg (2+) showed a shortening of lysis time with increasing Mg (2+) concentrations ( p = 0.0004). Our findings reveal that plasma magnesium concentration is associated with changes in fibrin clot and lysis parameters.

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