Regulation of intracellular and mitochondrial sodium in health and disease

The potential anti-arrhythmic effect of SGLT2 inhibitors

Sodium-glucose cotransporter type 2 inhibitors (SGLT2i) were initially recommended as oral anti-diabetic drugs to treat type 2 diabetes (T2D), by inhibiting SGLT2 in proximal tubule and reduce renal reabsorption of sodium and glucose. While many clinical trials demonstrated the tremendous potential of SGLT2i for cardiovascular diseases. 2022 AHA/ACC/HFSA guideline first emphasized that SGLT2i were the only drug class that can cover the entire management of heart failure (HF) from prevention to treatment. Subsequently, the antiarrhythmic properties of SGLT2i have also attracted attention. Although there are currently no prospective studies specifically on the anti-arrhythmic effects of SGLT2i. We provide clues from clinical and fundamental researches to identify its antiarrhythmic effects, reviewing the evidences and mechanism for the SGLT2i antiarrhythmic effects and establishing a novel paradigm involving intracellular sodium, metabolism and autophagy to investigate the potential mechanisms of SGLT2i in mitigating arrhythmias.

The influence of cation exchange on the possible mechanism of erionite toxicity: A synchrotron-based micro-X-ray fluorescence study on THP-1-derived macrophages exposed to erionite-Na

Fibrous erionite is the only zeolite classified as Group 1 carcinogen by the International Agency for Research on Cancer (IARC). Carcinogenesis induced by erionite is thought to involve several factors as biopersistence, the iron role and cation exchange processes. To better understand these mechanisms, a detailed investigation at the micro scale was performed, collecting elemental information on iron and cation release and their distribution in biological systems by synchrotron micro-X-ray fluorescence mapping (SR-micro-XRF) and synchrotron micro-X-ray absorption spectroscopy (SR-micro-XANES) at the TwinMic beamline (Elettra synchrotron) and at the ID21 beamline of the European Synchrotron Radiation Facility (ESRF). By microscopy and chemical mapping, highly detailed maps of the chemical and morphological interaction of biological systems with fibres could be produced. In detail, THP-1 cell line derived macrophages, used as in vitro model, were analysed during erionite-Na phagocytosis at different time intervals, after single dose exposure. For comparison, cellular fluorescent probes were also used to evaluate the intracellular free sodium and calcium concentrations. Synchrotron analyses visualised the spatial distribution of both fibre and mineral particle associated metals during the phagocytosis, describing the mechanism of internalisation of erionite-Na and its accessory mineral phases. The intracellular distribution of metals and other cations was mapped to evaluate metal release, speciation changes and/or cation exchange during phagocytosis. The fluorescent probes complemented microchemical data clarifying, and confirming, the cation distribution observed in the SR-micro-XRF maps. The significant cytoplasmic calcium decrease, and the concomitant sodium increase, after the fibre phagocytosis seemed due to activation of plasma membrane cations exchangers triggered by the internalisation while, surprisingly, the ion-exchange capacity of erionite-Na could play a minor role in the disruption of the two cations intracellular homeostasis. These results help to elucidate the role of cations in the toxicity of erionite-treated THP-1 macrophages and add knowledge to its carcinogenicity process.

Aging as a loss of morphostatic information: A developmental bioelectricity perspective

Maintaining order at the tissue level is crucial throughout the lifespan, as failure can lead to cancer and an accumulation of molecular and cellular disorders. Perhaps, the most consistent and pervasive result of these failures is aging, which is characterized by the progressive loss of function and decline in the ability to maintain anatomical homeostasis and reproduce. This leads to organ malfunction, diseases, and ultimately death. The traditional understanding of aging is that it is caused by the accumulation of molecular and cellular damage. In this article, we propose a complementary view of aging from the perspective of endogenous bioelectricity which has not yet been integrated into aging research. We propose a view of aging as a morphostasis defect, a loss of biophysical prepattern information, encoding anatomical setpoints used for dynamic tissue and organ homeostasis. We hypothesize that this is specifically driven by abrogation of the endogenous bioelectric signaling that normally harnesses individual cell behaviors toward the creation and upkeep of complex multicellular structures in vivo. Herein, we first describe bioelectricity as the physiological software of life, and then identify and discuss the links between bioelectricity and life extension strategies and age-related diseases. We develop a bridge between aging and regeneration via bioelectric signaling that suggests a research program for healthful longevity via morphoceuticals. Finally, we discuss the broader implications of the homologies between development, aging, cancer and regeneration and how morphoceuticals can be developed for aging.

Replenishment of mitochondrial Na+ and H+ by ionophores potentiates cutaneous wound healing in diabetes

Diabetic foot ulcer (DFU) is a highly morbid complication in patients with diabetes mellitus, necessitating the development of innovative pharmaceuticals to address unmet medical needs. Sodium ion (Na+) is a well-established mediator for membrane potential and osmotic equilibrium. Recently, Na+ transporters have been identified as a functional regulator of regeneration. However, the role of Na+ in the intricate healing process of mammalian wounds remains elusive. Here, we found that the skin wounds in hyponatremic mice display a hard-to-heal phenotype. Na+ ionophores that were employed to increase intracellular Na+ content could facilitate keratinocyte proliferation and migration, and promote angiogenesis, exhibiting diverse biological activities. Among of them, monensin A emerges as a promising agent for accelerating the healing dynamics of skin wounds in diabetes. Mechanistically, the elevated mitochondrial Na+ decelerates inner mitochondrial membrane fluidity, instigating the production of reactive oxygen species (ROS), which is identified as a critical effector on the monensin A-induced improvement of wound healing. Concurrently, Na+ ionophores replenish H+ to the mitochondrial matrix, causing an enhancement of mitochondrial energy metabolism to support productive wound healing programs. Our study unfolds a new role of Na+, which is a pivotal determinant in wound healing. Furthermore, it directs a roadmap for developing Na+ ionophores as innovative pharmaceuticals for treating chronic dermal wounds in diabetic patients.

The interplay between sodium/glucose cotransporter type 2 and mitochondrial ionic environment

Mitochondrial volume is maintained through the permeability of the inner mitochondrial membrane by a specific aquaporin and the osmotic balance between the mitochondrial matrix and cellular cytoplasm. Various electrolytes, such as calcium and hydrogen ions, potassium, and sodium, as well as other osmotic substances, affect the swelling of mitochondria. Intracellular glucose levels may also affect mitochondrial swelling, although the relationship between mitochondrial ion homeostasis and intracellular glucose is poorly understood. This article reviews what is currently known about how the Sodium-Glucose transporter (SGLT) may impact mitochondrial sodium (Na+) homeostasis. SGLTs regulate intracellular glucose and sodium levels and, therefore, interfere with mitochondrial ion homeostasis because mitochondrial Na+ is closely linked to cytoplasmic calcium and sodium dynamics. Recently, a large amount of data has been available on the effects of SGLT2 inhibitors on mitochondria in different cell types, including renal proximal tubule cells, endothelial cells, mesangial cells, podocytes, neuronal cells, and cardiac cells. The current evidence suggests that SGLT inhibitors (SGLTi) may affect mitochondrial dynamics regarding intracellular Sodium and hydrogen ions. Although the regulation of mitochondrial ion channels by SGLTs is still in its infancy, the evidence accumulated thus far of the effect of SGLTi on mitochondrial functions certainly will foster further research in this direction.

Mitochondrial Dysfunction in Kidney Tubulopathies

Mitochondria play a key role in kidney physiology and pathology. They produce ATP to fuel energy-demanding water and solute reabsorption processes along the nephron. Moreover, mitochondria contribute to cellular health by the regulation of autophagy, (oxidative) stress responses, and apoptosis. Mitochondrial abundance is particularly high in cortical segments, including proximal and distal convoluted tubules. Dysfunction of the mitochondria has been described for tubulopathies such as Fanconi, Gitelman, and Bartter-like syndromes and renal tubular acidosis. In addition, mitochondrial cytopathies often affect renal (tubular) tissues, such as in Kearns-Sayre and Leigh syndromes. Nevertheless, the mechanisms by which mitochondrial dysfunction results in renal tubular diseases are only scarcely being explored. This review provides an overview of mitochondrial dysfunction in the development and progression of kidney tubulopathies. Furthermore, it emphasizes the need for further mechanistic investigations to identify links between mitochondrial function and renal electrolyte reabsorption.

Na+ Binding and Transport: Insights from Light-Driven Na+-Pumping Rhodopsin

Na+ plays a vital role in numerous physiological processes across humans and animals, necessitating a comprehensive understanding of Na+ transmembrane transport. Among the various Na+ pumps and channels, light-driven Na+-pumping rhodopsin (NaR) has emerged as a noteworthy model in this field. This review offers a concise overview of the structural and functional studies conducted on NaR, encompassing ground/intermediate-state structures and photocycle kinetics. The primary focus lies in addressing key inquiries: (1) unraveling the translocation pathway of Na+; (2) examining the role of structural changes within the photocycle, particularly in the O state, in facilitating Na+ transport; and (3) investigating the timing of Na+ uptake/release. By delving into these unresolved issues and existing debates, this review aims to shed light on the future direction of Na+ pump research.

Structure and function of the human mitochondrial MRS2 channel

The human Mitochondrial RNA Splicing 2 protein (MRS2) has been implicated in Mg2+ transport across mitochondrial inner membranes, thus playing an important role in Mg2+ homeostasis critical for mitochondrial integrity and function. However, the molecular mechanisms underlying its fundamental channel properties such as ion selectivity and regulation remain unclear. Here, we present structural and functional investigation of MRS2. Cryo-electron microscopy structures in various ionic conditions reveal a pentameric channel architecture and the molecular basis of ion permeation and potential regulation mechanisms. Electrophysiological analyses demonstrate that MRS2 is a Ca2+-regulated, non-selective channel permeable to Mg2+, Ca2+, Na+ and K+, which contrasts with its prokaryotic ortholog, CorA, operating as a Mg2+-gated Mg2+ channel. Moreover, a conserved arginine ring within the pore of MRS2 functions to restrict cation movements, likely preventing the channel from collapsing the proton motive force that drives mitochondrial ATP synthesis. Together, our results provide a molecular framework for further understanding MRS2 in mitochondrial function and disease.

Multi-nuclear sodium, diffusion, and perfusion MRI in human gliomas

PURPOSE

There is limited knowledge about the associations between sodium and proton MRI measurements in brain tumors. The purpose of this study was to quantify intra- and intertumoral correlations between sodium, diffusion, and perfusion MRI in human gliomas.

METHODS

Twenty glioma patients were prospectively studied on a 3T MRI system with multinuclear capabilities. Three mutually exclusive tumor volumes of interest (VOIs) were segmented: contrast-enhancing tumor (CET), T2/FLAIR hyperintense non-enhancing tumor (NET), and necrosis. Median and voxel-wise associations between apparent diffusion coefficient (ADC), normalized relative cerebral blood volume (nrCBV), and normalized sodium measurements were quantified for each VOI.

RESULTS

Both relative sodium concentration and ADC were significantly higher in areas of necrosis compared to NET (P = 0.003 and P = 0.008, respectively) and CET (P = 0.02 and P = 0.02). Sodium concentration was higher in CET compared to NET (P = 0.04). Sodium and ADC were higher in treated compared to treatment-naïve gliomas within NET (P = 0.006 and P = 0.01, respectively), and ADC was elevated in CET (P = 0.03). Median ADC and sodium concentration were positively correlated across patients in NET (r = 0.77, P < 0.0001) and CET (r = 0.84, P < 0.0001), but not in areas of necrosis (r = 0.45, P = 0.12). Median nrCBV and sodium concentration were negatively correlated across patients in areas of NET (r=-0.63, P = 0.003). Similar associations were observed when examining voxel-wise correlations within VOIs.

CONCLUSION

Sodium MRI is positively correlated with proton diffusion MRI measurements in gliomas, likely reflecting extracellular water. Unique areas of multinuclear MRI contrast may be useful in future studies to understand the chemistry of the tumor microenvironment.

Lipoxin A4 attenuated dexamethasone-induced muscle atrophy via activation of PGC-1α/Nrf2/TFAM pathway

Prolonged dexamethasone (DEX) administration causes skeletal muscle atrophy through induction of both oxidative stress and mitochondrial dysfunction. Lipoxin A4 (LXA4) is a recognized antioxidant but its effect against DEX-induced muscle atrophy has not been studied yet. This study aimed to assess the potential ameliorating effect of LXA4 on DEX-induced muscle atrophy and investigate the possible involvement of the mitochondrial dynamics pathway and the redox state in this effect. Forty male rats were divided into four groups; normal control, LXA4-treated, DEX-treated, and LXA4 plus DEX-treated. At the end of the experiment, LXA4 counteracted the effect of DEX on different parameters including muscle weight, muscle strength, serum creatine kinase activity, malondialdehyde and protein carbonyl contents, Na/K-ATPase and citrate synthase activities, mitochondrial transmembrane potential, mitochondrial transcription factor (TFAM), peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), and nuclear factor erythroid 2-related factor 2 (Nrf2). These findings signify the promising therapeutic effect of LXA4 against DEX-induced skeletal muscle atrophy and indicate the possible involvement of LXA4-induced mitochondrial activation in addition to its well-known antioxidant effects.

Cardiac Hypertrophy and Related Dysfunctions in Cushing Syndrome Patients-Literature Review

The survival rate of adrenal Cushing syndrome patients has been greatly increased because of the availability of appropriate surgical and pharmacological treatments. Nevertheless, increased possibility of a heart attack induced by a cardiovascular event remains a major risk factor for the survival of affected patients. In experimental studies, hypercortisolemia has been found to cause cardiomyocyte hypertrophy via glucocorticoid receptor activation, including the possibility of cross talk among several hypertrophy signals related to cardiomyocytes and tissue-dependent regulation of 11β-hydroxysteroid dehydrogenase type 1. However, the factors are more complex in clinical cases, as both geometric and functional impairments leading to heart failure have been revealed, and their associations with a wide range of factors such as hypertension are crucial. In addition, knowledge regarding such alterations in autonomous cortisol secretion, which has a high risk of leading to heart attack as well as overt Cushing syndrome, is quite limited. When considering the effects of treatment, partial improvement of structural alterations is expected, while functional disorders are controversial. Therefore, whether the normalization of excess cortisol attenuates the risk related to cardiac hypertrophy has yet to be fully elucidated.

Prokaryotic Na+/H+ Exchangers—Transport Mechanism and Essential Residues

Na⁺/H⁺ exchangers are essential for Na⁺ and pH homeostasis in all organisms. Human Na⁺/H⁺ exchangers are of high medical interest, and insights into their structure and function are aided by the investigation of prokaryotic homologues. Most prokaryotic Na⁺/H⁺ exchangers belong to either the Cation/Proton Antiporter (CPA) superfamily, the Ion Transport (IT) superfamily, or the Na⁺-translocating Mrp transporter superfamily. Several structures have been solved so far for CPA and Mrp members, but none for the IT members. NhaA from E. coli has served as the prototype of Na⁺/H⁺ exchangers due to the high amount of structural and functional data available. Recent structures from other CPA exchangers, together with diverse functional information, have allowed elucidation of some common working principles shared by Na⁺/H⁺ exchangers from different families, such as the type of residues involved in the substrate binding and even a simple mechanism sufficient to explain the pH regulation in the CPA and IT superfamilies. Here, we review several aspects of prokaryotic Na⁺/H⁺ exchanger structure and function, discussing the similarities and differences between different transporters, with a focus on the CPA and IT exchangers. We also discuss the proposed transport mechanisms for Na⁺/H⁺ exchangers that explain their highly pH-regulated activity profile.

Sodium in Mitochondrial Redox Signaling

Background: Mitochondrial Na⁺ has been discovered as a new second messenger regulating inner mitochondrial membrane (IMM) fluidity and reactive oxygen species (ROS) production by complex III (CIII). However, the roles of mitochondrial Na⁺ in mitochondrial redox signaling go beyond what was initially expected. Significance: In this review, we systematize the current knowledge on mitochondrial Na⁺ homeostasis and its implications on different modes of ROS production by mitochondria. Na⁺ behaves as a positive modulator of forward mitochondrial ROS production either by complex III (CIII) or by decreasing antioxidant capacity of mitochondria and as a potential negative modulator of reverse electron transfer (RET) by complex I (CI). Such duality depends on the bioenergetic status, cation and redox contexts, and can either lead to potential adaptations or cell death. Future Directions: Direct Na⁺ interaction with phospholipids, proven in the IMM, allows us to hypothesize its potential role in the existence and function of lipid rafts in other biological membranes regarding redox homeostasis, as well as the potential role of other monovalent cations in membrane biology. Thus, we provide the reader an update on the emerging field of mitochondrial Na⁺ homeostasis and its relationship with mitochondrial redox signaling. Antioxid. Redox Signal. 37, 290-300.

Ischemic damage to every segment of the oxidative phosphorylation cascade elevates ETC driving force and ROS production in cardiac mitochondria

Myocardial ischemia has long-lasting negative impacts on cardiomyocyte mitochondrial ATP production. However, the location(s) of damage to the oxidative phosphorylation pathway responsible for altered mitochondrial function is unclear. Mitochondrial reactive oxygen species (ROS) production increases following ischemia, but the specific factors controlling this increase are unknown. To determine how ischemia affects the mitochondrial energy conversion cascade and ROS production, mitochondrial driving forces [redox potential and membrane potential (ΔΨ)] were measured at resting, intermediate, and maximal respiration rates in mitochondria isolated from rat hearts after 60 min of control flow (control) or no-flow ischemia (ischemia). The effective activities of the dehydrogenase enzymes, the electron transport chain (ETC), and ATP synthesis and transport were computed using the driving forces and flux. Ischemia lowered maximal mitochondrial respiration rates and diminished the responsiveness of respiration to both redox potential and ΔΨ. Ischemia decreased the activities of every component of the oxidative phosphorylation pathway: the dehydrogenase enzymes, the ETC, and ATP synthesis and transport. ROS production was linearly related to driving force down the ETC; however, ischemia mitochondria demonstrated a greater driving force down the ETC and higher ROS production. Overall, results indicate that ischemia ubiquitously damages the oxidative phosphorylation pathway, reduces mitochondrial sensitivity to driving forces, and augments the propensity for electrons to leak from the ETC. These findings underscore that strategies to improve mitochondrial function following ischemia must target the entire mitochondrial energy conversion cascade.

NEW & NOTEWORTHY This integrative analysis is the first to assess how myocardial ischemia alters the mitochondrial driving forces and the degree to which individual segments of the mitochondrial energy transduction pathway contribute to diminished function following ischemia. This investigation demonstrates that increased reactive oxygen species production following ischemia is related to a lower effective activity of the electron transport chain and a greater driving force down the electron transport chain.

Genetic Ablation of Na,K-ATPase α4 Results in Sperm Energetic Defects

The Na,K-ATPase alpha 4 isoform (NKAα4) is expressed specifically in the male germ cells of the testes and is particularly abundant in mature spermatozoa. Genetic deletion of NKAα4 in mice (NKAα4 KO mice) results in complete infertility of male, but not female mice. The reduced fecundity of NKAα4 KO male mice is due to a series of defects, including a severe impairment in total and hyperactive sperm motility. In this work, we show that deletion of NKAα4 also leads to major defects in sperm metabolism and energetics. Thus, compared to wild-type sperm, sperm from NKAα4 KO mice display a significant reduction in the extracellular acidification rate (ECAR), indicative of impaired glycolytic flux. In addition, mitochondrial function is disrupted in sperm lacking NKAα4, as indicated by a reduction in the mitochondrial membrane potential and lower oxygen consumption rate (OCR). Moreover, the ratio between the oxidized and reduced forms of nicotinamide adenine dinucleotide (NAD/NADH) is increased in NKAα4 KO sperm, indicating a shift in the cellular redox state. These metabolic changes are associated with augmented reactive oxygen species (ROS) production and increased lipid peroxidation in NKAα4 KO sperm. Altogether, these findings reveal a novel link between NKAα4 activity and sperm energetics, highlighting the essential role of this ion transporter in sperm physiology.

Simultaneous 3D acquisition of 1 H MRF and 23 Na MRI

PURPOSE

To develop a 3D MR technique to simultaneously acquire proton multiparametric maps (T₁ , T₂ , and proton density) and sodium density weighted images over the whole brain.

METHODS

We implemented a 3D stack-of-stars MR pulse sequence which consists of interleaved proton (¹ H) and sodium (²³ Na) excitations, tailored slice encoding gradients that can encode the same slice for both nuclei, and simultaneous readout with different radial trajectories (¹ H, full-radial; ²³ Na, center-out radial). The receive chain of our 7T scanner was modified to enable simultaneous acquisition of ¹ H and ²³ Na signal. A heuristically optimized flip angle train was implemented for proton MR fingerprinting (MRF). The SNR and the accuracy of proton T₁ and T₂ were evaluated in phantoms. Finally, in vivo application of the method was demonstrated in five healthy subjects.

RESULTS

The SNR for the simultaneous measurement was almost identical to that for the single-nucleus measurements (<2% change). The proton T₁ and T₂ maps remained similar to the results from a reference 2D MRF technique (normalized RMS error in T₁ ≈ 4.2% and T₂ ≈ 11.3%). Measurements in healthy subjects corroborated these results and demonstrated the feasibility of our method for in vivo application. The in vivo T₁ values measured using our method were lower than the results measured by other conventional techniques.

CONCLUSIONS

With the 3D simultaneous implementation, we were able to acquire sodium and proton density weighted images in addition to proton T₁ , T₂ , and B1+ from ¹ H MRF that covers the whole brain volume within 21 min.

Mineral requirements for mitochondrial function: A connection to redox balance and cellular differentiation

Professor Bruce Ames demonstrated that nutritional recommendations should be adjusted in order to 'tune-up' metabolism and reduce mitochondria decay, a hallmark of aging and many disease processes. A major subset of tunable nutrients are the minerals, which despite being integral to every aspect of metabolism are often deficient in the typical Western diet. Mitochondria are particularly rich in minerals, where they function as essential cofactors for mitochondrial physiology and overall cellular health. Yet substantial knowledge gaps remain in our understanding of the form and function of these minerals needed for metabolic harmony. Some of the minerals have known activities in the mitochondria but with incomplete regulatory detail, whereas other minerals have no established mitochondrial function at all. A comprehensive metallome of the mitochondria is needed to fully understand the patterns and relationships of minerals within metabolic processes and cellular development. This brief overview serves to highlight the current progress towards understanding mineral homeostasis in the mitochondria and to encourage more research activity in key areas. Future work may likely reveal that adjusting the amounts of specific nutritional minerals has longevity benefits for human health.

Regulation of aged skeletal muscle regeneration by circulating extracellular vesicles

Heterochronic blood exchange (HBE) has demonstrated that circulating factors restore youthful features to aged tissues. However, the systemic mediators of those rejuvenating effects remain poorly defined. We show here that the beneficial effect of young blood on aged muscle regeneration was diminished when serum was depleted of extracellular vesicles (EVs). Whereas EVs from young animals rejuvenate aged cell bioenergetics and skeletal muscle regeneration, aging shifts EV subpopulation heterogeneity and compromises downstream benefits on recipient cells. Machine learning classifiers revealed that aging shifts the nucleic acid, but not protein, fingerprint of circulating EVs. Alterations in sub-population heterogeneity were accompanied by declines in transcript levels of the pro-longevity protein, α-Klotho, and injection of EVs improved muscle regeneration in a Klotho mRNA-dependent manner. These studies demonstrate that EVs play a key role in the rejuvenating effects of HBE and that Klotho transcripts within EVs phenocopy the effects of young serum on aged skeletal muscle.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

A red-emissive two-photon fluorescent probe for mitochondrial sodium ions in live tissue

A cyclocyanine (CC)-based organic small molecule two-photon (TP) fluorescent probe (CCNa1) was developed for mitochondrial sodium ion sensing. CCNa1 exhibits a low solvatochromic shift and strong TP fluorescence enhancement at 575 nm upon binding to Na⁺ and is insensitive to other metal ions and to pH. CCNa1 demonstrated fast cell loading ability, biocompatibility, and sensitive response to mitochondrial Na⁺ influx in live cells and mouse brain tissue.

Elevated miR-34a expression and altered transcriptional profile are associated with adverse electromechanical remodeling in the heart of male rats exposed to social stress

This study investigated epigenetic risk factors that may contribute to stress-related cardiac disease in a rodent model. Experiment 1 was designed to evaluate the expression of microRNA-34a (miR-34a), a known modulator of both stress responses and cardiac pathophysiology, in the heart of male adult rats exposed to a single or repeated episodes of social defeat stress. Moreover, RNA sequencing was conducted to identify transcriptomic profile changes in the heart of repeatedly stressed rats. Experiment 2 was designed to assess cardiac electromechanical changes induced by repeated social defeat stress that may predispose rats to cardiac dysfunction. Results indicated a larger cardiac miR-34a expression after repeated social defeat stress compared to a control condition. This molecular modification was associated with increased vulnerability to pharmacologically induced arrhythmias and signs of systolic left ventricular dysfunction. Gene expression analysis identified clusters of differentially expressed genes in the heart of repeatedly stressed rats that are mainly associated with morphological and functional properties of the mitochondria and may be directly regulated by miR-34a. These results suggest the presence of an association between miR-34a overexpression and signs of adverse electromechanical remodeling in the heart of rats exposed to repeated social defeat stress, and point to compromised mitochondria efficiency as a potential mediator of this link. This rat model may provide a useful tool for investigating the causal relationship between miR-34a expression, mitochondrial (dys)function, and cardiac alterations under stressful conditions, which could have important implications in the context of stress-related cardiac disease.

With a grain of salt: Sodium elevation and metabolic remodelling in heart failure

Elevated intracellular Na (Na_i) and metabolic impairment are interrelated pathophysiological features of the failing heart (HF). There have been a number of studies showing that myocardial sodium elevation subtly affects mitochondrial function. During contraction, mitochondrial calcium (Ca_mito) stimulates a variety of TCA cycle enzymes, thereby providing reducing equivalents to maintain ATP supply. Na_i elevation has been shown to impact Ca_mito; however, whether metabolic remodelling in HF is caused by increased Na_i has only been recently demonstrated. This novel insight may help to elucidate the contribution of metabolic remodelling in the pathophysiology of HF, the lack of efficacy of current HF therapies and a rationale for the development of future metabolism-targeting treatments. Here we review the relationship between Na pump inhibition, elevated Na_i, and altered metabolic profile in the context of HF and their link to metabolic (in)flexibility and mitochondrial reprogramming.

Mitochondrial Metal Ion Transport in Cell Metabolism and Disease

Mitochondria are vital to life and provide biological energy for other organelles and cell physiological processes. On the mitochondrial double layer membrane, there are a variety of channels and transporters to transport different metal ions, such as Ca²⁺, K⁺, Na⁺, Mg²⁺, Zn²⁺ and Fe²⁺/Fe³⁺. Emerging evidence in recent years has shown that the metal ion transport is essential for mitochondrial function and cellular metabolism, including oxidative phosphorylation (OXPHOS), ATP production, mitochondrial integrity, mitochondrial volume, enzyme activity, signal transduction, proliferation and apoptosis. The homeostasis of mitochondrial metal ions plays an important role in maintaining mitochondria and cell functions and regulating multiple diseases. In particular, channels and transporters for transporting mitochondrial metal ions are very critical, which can be used as potential targets to treat neurodegeneration, cardiovascular diseases, cancer, diabetes and other metabolic diseases. This review summarizes the current research on several types of mitochondrial metal ion channels/transporters and their functions in cell metabolism and diseases, providing strong evidence and therapeutic strategies for further insights into related diseases.

Effects of molidustat, a hypoxia-inducible factor prolyl hydroxylase inhibitor, on sodium dynamics in hypertensive subtotally nephrectomized rats

Hypoxia-inducible factor prolyl hydroxylase (HIF-PHD) inhibitors were developed for treatment of renal anemia. Patients applicable for HIF-PHD inhibitor treatment experience complications such as chronic kidney disease, whereby water and electrolyte homeostasis is disrupted. The effects of hypoxia-inducible factor stabilization on salt accumulation in the setting of reduced renal function remain unclear. In the present study, we investigated the effect of a HIF-PHD inhibitor, molidustat, on salt distribution and excretion in rats with subtotal nephrectomy-induced chronic kidney disease. Male Wistar rats were subjected to 5/6 nephrectomy. After confirming blood pressure elevation (>150 mmHg, at 4 weeks after surgery), rats were treated with molidustat. After 1 week of treatment, molidustat did not significantly improve blood cell volume or blood pressure. Distribution of sodium, potassium, and water in skin, carcass, and bone samples was not affected by molidustat. Furthermore, molidustat had no significant effect on urinary sodium excretion or concentration in response to acute oral salt loading (1 g/kg). In conclusion, molidustat did not affect distribution or excretion of salt in rats subjected to a model of nephron loss.

Molecular machinery regulating mitochondrial calcium levels: The nuts and bolts of mitochondrial calcium dynamics

Mitochondria play vital role in regulating the cellular energetics and metabolism. Further, it is a signaling hub for cell survival and apoptotic pathways. One of the key determinants that calibrate both cellular energetics and survival functions is mitochondrial calcium (Ca2+) dynamics. Mitochondrial Ca2+ regulates three Ca2+-sensitive dehydrogenase enzymes involved in tricarboxylic acid cycle (TCA) cycle thereby directly controlling ATP synthesis. On the other hand, excessive Ca2+ concentration within the mitochondrial matrix elevates mitochondrial reactive oxygen species (mROS) levels and causes mitochondrial membrane depolarization. This leads to opening of the mitochondrial permeability transition pore (mPTP) and release of cytochrome c into cytosol eventually triggering apoptosis. Therefore, it is critical for cell to maintain mitochondrial Ca2+ concentration. Since cells can neither synthesize nor metabolize Ca2+, it is the dynamic interplay of Ca2+ handling proteins involved in mitochondrial Ca2+ influx and efflux that take the center stage. In this review we would discuss the key molecular machinery regulating mitochondrial Ca2+ concentration. We would focus on the channel complex involved in bringing Ca2+ into mitochondrial matrix i.e. Mitochondrial Ca2+ Uniporter (MCU) and its key regulators Mitochondrial Ca2+ Uptake proteins (MICU1, 2 and 3), MCU regulatory subunit b (MCUb), Essential MCU Regulator (EMRE) and Mitochondrial Ca2+ Uniporter Regulator 1 (MCUR1). Further, we would deliberate on major mitochondrial Ca2+ efflux proteins i.e. Mitochondrial Na+/Ca2+/Li+ exchanger (NCLX) and Leucine zipper EF hand-containing transmembrane1 (Letm1). Moreover, we would highlight the physiological functions of these proteins and discuss their relevance in human pathophysiology. Finally, we would highlight key outstanding questions in the field.

Myocardial Impact of NHE1 Regulation by Sildenafil

The cardiac Na⁺/H⁺ exchanger (NHE1) is a membrane glycoprotein fundamental for proper cell functioning due its multiple housekeeping tasks, including regulation of intracellular pH, Na⁺ concentration, and cell volume. In the heart, hyperactivation of NHE1 has been linked to the development of different pathologies. Several studies in animal models that reproduce the deleterious effects of ischemia/reperfusion injury or cardiac hypertrophy have conclusively demonstrated that NHE1 inhibition provides cardioprotection. Unfortunately, NHE1 inhibitors failed to reproduce these effects in the clinical arena. The reasons for those discrepancies are not apparent yet. However, a reasonable clue to consider would be that drugs that completely abolish the exchanger activity, including that its essential housekeeping function may not be the best therapeutic approach. Therefore, interventions tending to specifically reduce its hyperactive state without affecting its basal activity emerge as a novel potential gold standard. In this regard, a promising goal seems to be the modulation of the phosphorylation state of the cytosolic tail of the exchanger. Recent own experiments demonstrated that Sildenafil, a phosphodiesterase 5A inhibitor drug that has been widely used for the treatment of erectile dysfunction is able to decrease NHE1 phosphorylation, and hence reduce its hyperactivity. In connection, growing evidence demonstrates cardioprotective properties of Sildenafil against different cardiac pathologies, with the distinctive characteristic of directly affecting cardiac tissue without altering blood pressure. This mini-review was aimed to focus on the regulation of NHE1 activity by Sildenafil. For this purpose, experimental data reporting Sildenafil effects in different animal models of heart disease will be discussed.

Regulation of Mitochondrial Ca2+ Uptake

Mitochondria are responsible for ATP production but are also known as regulators of cell death, and mitochondrial matrix Ca2+ is a key modulator of both ATP production and cell death. Although mitochondrial Ca2+ uptake and efflux have been studied for over 50 years, it is only in the past decade that the proteins responsible for mitochondrial Ca2+ uptake and efflux have been identified. The identification of the mitochondrial Ca2+ uniporter (MCU) led to an explosion of studies identifying regulators of the MCU. The levels of these regulators vary in a tissue- and disease-specific manner, providing new insight into how mitochondrial Ca2+ is regulated. This review focuses on the proteins responsible for mitochondrial transport and what we have learned from mouse studies with genetic alterations in these proteins.

Structural and Functional Characterization of a Nav1.5-Mitochondrial Couplon

RATIONALE

The cardiac sodium channel NaV1.5 has a fundamental role in excitability and conduction. Previous studies have shown that sodium channels cluster together in specific cellular subdomains. Their association with intracellular organelles in defined regions of the myocytes, and the functional consequences of that association, remain to be defined.

OBJECTIVE

To characterize a subcellular domain formed by sodium channel clusters in the crest region of the myocytes and the subjacent subsarcolemmal mitochondria.

METHODS AND RESULTS

Through a combination of imaging approaches including super-resolution microscopy and electron microscopy we identified, in adult cardiac myocytes, a NaV1.5 subpopulation in close proximity to subjacent subsarcolemmal mitochondria; we further found that subjacent subsarcolemmal mitochondria preferentially host the mitochondrial NCLX (Na+/Ca2+ exchanger). This anatomic proximity led us to investigate functional changes in mitochondria resulting from sodium channel activity. Upon TTX (tetrodotoxin) exposure, mitochondria near NaV1.5 channels accumulated more Ca2+ and showed increased reactive oxygen species production when compared with interfibrillar mitochondria. Finally, crosstalk between NaV1.5 channels and mitochondria was analyzed at a transcriptional level. We found that SCN5A (encoding NaV1.5) and SLC8B1 (which encode NaV1.5 and NCLX, respectively) are negatively correlated both in a human transcriptome data set (Genotype-Tissue Expression) and in human-induced pluripotent stem cell-derived cardiac myocytes deficient in SCN5A.

CONCLUSIONS

We describe an anatomic hub (a couplon) formed by sodium channel clusters and subjacent subsarcolemmal mitochondria. Preferential localization of NCLX to this domain allows for functional coupling where the extrusion of Ca2+ from the mitochondria is powered, at least in part, by the entry of sodium through NaV1.5 channels. These results provide a novel entry-point into a mechanistic understanding of the intersection between electrical and structural functions of the heart.

Comparison of [18F]Fluoroethyltyrosine PET and Sodium MRI in Cerebral Gliomas: a Pilot Study

PURPOSE

Positron emission tomography (PET) using O-(2-[¹⁸F]fluoroethyl)-L-tyrosine ([¹⁸F]FET) improves the diagnostics of cerebral gliomas compared with conventional magnetic resonance imaging (MRI). Sodium MRI is an evolving method to assess tumor metabolism. In this pilot study, we explored the relationship of [¹⁸F]FET-PET and sodium MRI in patients with cerebral gliomas in relation to the mutational status of the enzyme isocitrate dehydrogenase (IDH).

PROCEDURES

Ten patients with untreated cerebral gliomas and one patient with a recurrent glioblastoma (GBM) were investigated by dynamic [¹⁸F]FET-PET and sodium MRI using an enhanced simultaneous single-quantum- and triple-quantum-filtered imaging of ²³Na (SISTINA) sequence to estimate total (NaT), weighted non-restricted (NaNR, mainly extracellular), and restricted (NaR, mainly intracellular) sodium in tumors and normal brain tissue. [¹⁸F]FET uptake and sodium parameters in tumors with a different IDH mutational status were compared. After biopsy or resection, histology and the IDH mutational status were determined neuropathologically.

RESULTS

NaT (p = 0.05), tumor-to-brain ratios (TBR) of NaT (p = 0.02), NaNR (p = 0.003), and the ratio of NaT/NaR (p < 0.001) were significantly higher in IDH-mutated than in IDH-wild-type gliomas (n = 5 patients each) while NaR was significantly lower in IDH-mutated gliomas (p = 0.01). [¹⁸F]FET parameters (TBR, time-to-peak) were not predictive of IDH status in this small cohort of patients. There was no obvious relationship between sodium distribution and [¹⁸F]FET uptake. The patient with a recurrent GBM exhibited an additional radiation injury with strong abnormalities in sodium MRI.

CONCLUSIONS

Sodium MRI appears to be more strongly related to the IDH mutational status than are [¹⁸F]FET-PET parameters. A further evaluation of the combination of the two methods in a larger group of high- and low-grade gliomas seems promising.

PDE5 inhibition improves cardiac morphology and function in SHR by reducing NHE1 activity: Repurposing Sildenafil for the treatment of hypertensive cardiac hypertrophy

Previously, we have shown that an increased cGMP-activated protein Kinase (PKG) activity after phosphodiesterase 5 (PDE5) inhibition by Sildenafil (SIL), leads to myocardial Na+/H+ exchanger (NHE1) inhibition preserving its basal homeostatic function. Since NHE1 is hyperactive in the hypertrophied myocardium of spontaneous hypertensive rats (SHR), while its inhibition was shown to prevent and revert this pathology, the current study was aimed to evaluate the potential antihypertrophic effect of SIL on adult SHR myocardium. We initially tested the inhibitory capability of SIL on NHE1 in isolated cardiomyocytes of SHR by comparing H+ efflux during the recovery from an acid load. After confirmed that effect, eight-month-old SHR were chronically treated for one month with SIL through drinking water. Compared to their littermate controls, SIL-treated rats presented a decreased NHE1 activity, which correlated with a reduction in its phosphorylation level assigned to activation of a PKG-p38 MAP kinase-PP2A signaling pathway. Moreover, treated animals showed a decreased oxidative stress that appears to be a consequence of a decreased mitochondrial NHE1 phosphorylation. Treated SHR showed a significant reduction in the pro-hypertrophic phosphatase calcineurin, despite slight tendency to decrease hypertrophy was detected. When SIL treatment was prolonged to three months, a significant decrease in myocardial hypertrophy and interstitial fibrosis that correlated with a lower myocardial stiffness was observed. In conclusion, the current study provides evidence concerning the ability of SIL to revert established cardiac hypertrophy in SHR, a clinically relevant animal model that resembles human essential hypertension.

Maneb alters central carbon metabolism and thiol redox status in a toxicant model of Parkinson's disease

The dithiocarbamate fungicide maneb (MB) has attracted interest due to increasing concern of the negative health effects of pesticides, as well as its association with Parkinson's disease (PD). Our laboratory has previously reported distinct phenotypic changes of neuroblastoma cells exposed to acute, sub-toxic levels of MB, including decreased mitochondrial respiration, altered lactate dynamics, and metabolic stress. In this study, we aimed to further define the specific molecular mechanisms of MB toxicity through the comparison of several thiol-containing compounds and their effects on cellular energy metabolism and thiol redox nodes. Extracellular flux analyses and stable isotope labeled tracer metabolomics were employed to evaluate alterations in energy metabolism of SK-N-AS human neuroblastoma cells after acute exposure of an array of compounds, including dithiocarbamates (maneb, nabam, zineb) and other thiol-containing small molecules (glutathione, N-acetylcysteine). These studies revealed MB and its methylated form (MeDTC) as unique toxicants with significant alterations to mitochondrial respiration, proliferation, and glycolysis. We observed MB to significantly impact cellular thiol redox status by oxidizing cellular glutathione and altering the thiol redox status of peroxiredoxin 3 (Prx3, mitochondrial) after acute exposure. Redox Western blotting revealed a MB-specific modification of cellular Prx3, strengthening the argument that MB can preferentially target mitochondrial enzymes containing reactive cysteine thiols. Further, stable isotope tracer metabolomics confirmed our energetics assessments, and demonstrated that MB exposure results in acute derangement of central carbon metabolism. Specifically, we observed shunting of cellular glucose into the pentose-phosphate pathway and reduction of TCA intermediates derived from glucose and glutamine. Also, we report novel lactate utilization for TCA enrichment and glutathione synthesis after MB exposure. In summary, our results further confirm that MB exerts its toxic effects via thiol modification, and significantly transforms central carbon metabolism.

Cellular and mitochondrial mechanisms of atrial fibrillation

The molecular mechanisms underlying atrial fibrillation (AF), the most common form of arrhythmia, are poorly understood and therefore target-specific treatment options remain an unmet clinical need. Excitation–contraction coupling in cardiac myocytes requires high amounts of adenosine triphosphate (ATP), which is replenished by oxidative phosphorylation in mitochondria. Calcium (Ca²⁺) is a key regulator of mitochondrial function by stimulating the Krebs cycle, which produces nicotinamide adenine dinucleotide for ATP production at the electron transport chain and nicotinamide adenine dinucleotide phosphate for the elimination of reactive oxygen species (ROS). While it is now well established that mitochondrial dysfunction plays an important role in the pathophysiology of heart failure, this has been less investigated in atrial myocytes in AF. Considering the high prevalence of AF, investigating the role of mitochondria in this disease may guide the path towards new therapeutic targets. In this review, we discuss the importance of mitochondrial Ca²⁺ handling in regulating ATP production and mitochondrial ROS emission and how alterations, particularly in these aspects of mitochondrial activity, may play a role in AF. In addition to describing research advances, we highlight areas in which further studies are required to elucidate the role of mitochondria in AF.

Arthropod toxins inhibiting Ca2+ and Na+ channels prevent AC-1001 H3 peptide-induced apoptosis

Peptide toxins of arthropods are one of the potential sources of bioactive substances. Toxins are able to bind to calcium channels and block them. Ca²⁺ ions play an important role in many cell processes, in particular, in apoptosis. In this work, we study the effect of some arthropod toxins on intracellular processes associated with the induction of apoptosis. Synthetic analogs of U₅ -scytotoxin-Sth1a, ω-hexatoxin-Hv1a, ω-theraphotoxin-Hhn2a, and μ-agatoxin-Aa1a toxins-inhibitors of calcium L, P, and Q channels and sodium channels were used in the study. Apoptosis was induced by AC-1001 H3 peptide. We study the effect of toxins on the level of apoptosis, ROS, mitochondrial potential, GSH, and ATP in CHO-K1 cells. We show that all the tested toxins are able to dose dependently block the induction of apoptosis triggered by AC-1001 H3 and reduce the level of natural apoptosis in CHO-K1 cells. Cell incubation with apoptosis inducer AC-1001 H3 in the presence and absence of toxins causes an increase in the intracellular concentrations of ROS, ATP, and mitochondrial potential and decreases the GSH concentration. The present study reveals the antiapoptotic effect of a number of arthropod peptide toxins. The toxins studied can represent a novel approach used in the treatment of pathologies associated with the activation of apoptotic mechanisms.

Simultaneous proton magnetic resonance fingerprinting and sodium MRI

PURPOSE

The goal of this work is to demonstrate a method for the simultaneous acquisition of proton multiparametric maps (T1 , T2 , and proton density) and sodium density images in 1 single scan. We hope that the development of such capabilities will help to ease the implementation of sodium MRI in clinical trials and provide more opportunities for researchers to investigate metabolism through sodium MRI.

METHODS

We developed a sequence based on magnetic resonance fingerprinting (MRF), which contains interleaved proton (1 H) and sodium (23 Na) excitations followed by a simultaneous center-out radial readout for both nuclei. The receive chain of a 7T scanner was modified to enable simultaneous acquisition of 1 H and 23 Na signal. The obtained signal-to-noise ratio (SNR) was evaluated, and the accuracy of both proton T1 , T2 , and B 1 + and sodium density maps were verified in phantoms. Finally, the method was demonstrated in 2 healthy subjects.

RESULTS

The SNR obtained using the simultaneous measurement was almost identical to single-nucleus measurements (<1% change). Similarly, the proton T1 and T2 maps remained stable (normalized root mean square error in T1 ≈ 2.2%, in T2 ≈ 1.4%, and B 1 + ≈ 5.4%), which indicates that the proposed sequence and hardware have no significant effects on the signal from either nucleus. In vivo measurements corroborated these results and demonstrated the feasibility of our method for in vivo application.

CONCLUSIONS

With the proposed approach, we were able to simultaneously acquire sodium density images in addition to proton T1 , T2 , and B 1 + maps as well as proton density images.

Autophagy-dependent and -independent modulation of oxidative and organellar stress in the diabetic heart by glucose-lowering drugs

Autophagy is a lysosome-dependent intracellular degradative pathway, which mediates the cellular adaptation to nutrient and oxygen depletion as well as to oxidative and endoplasmic reticulum stress. The molecular mechanisms that stimulate autophagy include the activation of energy deprivation sensors, sirtuin-1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK). These enzymes not only promote organellar integrity directly, but they also enhance autophagic flux, which leads to the removal of dysfunctional mitochondria and peroxisomes. Type 2 diabetes is characterized by suppression of SIRT1 and AMPK signaling as well as an impairment of autophagy; these derangements contribute to an increase in oxidative stress and the development of cardiomyopathy. Antihyperglycemic drugs that signal through insulin may further suppress autophagy and worsen heart failure. In contrast, metformin and SGLT2 inhibitors activate SIRT1 and/or AMPK and promote autophagic flux to varying degrees in cardiomyocytes, which may explain their benefits in experimental cardiomyopathy. However, metformin and SGLT2 inhibitors differ meaningfully in the molecular mechanisms that underlie their effects on the heart. Whereas metformin primarily acts as an agonist of AMPK, SGLT2 inhibitors induce a fasting-like state that is accompanied by ketogenesis, a biomarker of enhanced SIRT1 signaling. Preferential SIRT1 activation may also explain the ability of SGLT2 inhibitors to stimulate erythropoiesis and reduce uric acid (a biomarker of oxidative stress)—effects that are not seen with metformin. Changes in both hematocrit and serum urate are the most important predictors of the ability of SGLT2 inhibitors to reduce the risk of cardiovascular death and hospitalization for heart failure in large-scale trials. Metformin and SGLT2 inhibitors may also differ in their ability to mitigate diabetes-related increases in intracellular sodium concentration and its adverse effects on mitochondrial functional integrity. Differences in the actions of SGLT2 inhibitors and metformin may reflect the distinctive molecular pathways that explain differences in the cardioprotective effects of these drugs.

Membrane current evoked by mitochondrial Na+-Ca2+ exchange in mouse heart

The electrogenicity of mitochondrial Na+-Ca2+ exchange (NCXm) had been controversial and no membrane current through it had been reported. We succeeded for the first time in recording NCXm-mediated currents using mitoplasts derived from mouse ventricle. Under conditions that K+, Cl-, and Ca2+ uniporter currents were inhibited, extra-mitochondrial Na+ induced inward currents with 1 μM Ca2+ in the pipette. The half-maximum concentration of Na+ was 35.6 mM. The inward current was diminished without Ca2+ in the pipette, and was augmented with 10 μM Ca2+. The Na+-induced inward currents were largely inhibited by CGP-37157, an NCXm blocker. However, the reverse mode of NCXm, which should be detected as an outward current, was hardly induced by extra-mitochondrial application of Ca2+ with Na+ in the pipette. It was concluded that NCXm is electrogenic. This property may be advantageous for facilitating Ca2+ extrusion from mitochondria, which has large negative membrane potential.

Role of Impaired Nutrient and Oxygen Deprivation Signaling and Deficient Autophagic Flux in Diabetic CKD Development: Implications for Understanding the Effects of Sodium-Glucose Cotransporter 2-Inhibitors

Growing evidence indicates that oxidative and endoplasmic reticular stress, which trigger changes in ion channels and inflammatory pathways that may undermine cellular homeostasis and survival, are critical determinants of injury in the diabetic kidney. Cells are normally able to mitigate these cellular stresses by maintaining high levels of autophagy, an intracellular lysosome-dependent degradative pathway that clears the cytoplasm of dysfunctional organelles. However, the capacity for autophagy in both podocytes and renal tubular cells is markedly impaired in type 2 diabetes, and this deficiency contributes importantly to the intensity of renal injury. The primary drivers of autophagy in states of nutrient and oxygen deprivation-sirtuin-1 (SIRT1), AMP-activated protein kinase (AMPK), and hypoxia-inducible factors (HIF-1α and HIF-2α)-can exert renoprotective effects by promoting autophagic flux and by exerting direct effects on sodium transport and inflammasome activation. Type 2 diabetes is characterized by marked suppression of SIRT1 and AMPK, leading to a diminution in autophagic flux in glomerular podocytes and renal tubules and markedly increasing their susceptibility to renal injury. Importantly, because insulin acts to depress autophagic flux, these derangements in nutrient deprivation signaling are not ameliorated by antihyperglycemic drugs that enhance insulin secretion or signaling. Metformin is an established AMPK agonist that can promote autophagy, but its effects on the course of CKD have been demonstrated only in the experimental setting. In contrast, the effects of sodium-glucose cotransporter-2 (SGLT2) inhibitors may be related primarily to enhanced SIRT1 and HIF-2α signaling; this can explain the effects of SGLT2 inhibitors to promote ketonemia and erythrocytosis and potentially underlies their actions to increase autophagy and mute inflammation in the diabetic kidney. These distinctions may contribute importantly to the consistent benefit of SGLT2 inhibitors to slow the deterioration in glomerular function and reduce the risk of ESKD in large-scale randomized clinical trials of patients with type 2 diabetes.

Mitochondrial Calcium Regulation of Redox Signaling in Cancer

Calcium (Ca²⁺) uptake into the mitochondria shapes cellular Ca²⁺ signals and acts as a key effector for ATP generation. In addition, mitochondria-derived reactive oxygen species (mROS), produced as a consequence of ATP synthesis at the electron transport chain (ETC), modulate cellular signaling pathways that contribute to many cellular processes. Cancer cells modulate mitochondrial Ca²⁺ ([Ca²⁺]m) homeostasis by altering the expression and function of mitochondrial Ca²⁺ channels and transporters required for the uptake and extrusion of mitochondrial Ca²⁺. Regulated elevations in [Ca²⁺]m are required for the activity of several mitochondrial enzymes, and this in turn regulates metabolic flux, mitochondrial ETC function and mROS generation. Alterations in both [Ca²⁺]m and mROS are hallmarks of many tumors, and elevated mROS is a known driver of pro-tumorigenic redox signaling, resulting in the activation of pathways implicated in cellular proliferation, metabolic alterations and stress-adaptations. In this review, we highlight recent studies that demonstrate the interplay between [Ca²⁺]m and mROS signaling in cancer.

Sodium-glucose co-transporter 2 inhibition as a mitochondrial therapy for atrial fibrillation in patients with diabetes?

While patients with type 2 diabetes mellitus (T2DM) are at increased risk to develop atrial fibrillation (AF), the mechanistic link between T2DM and AF-susceptibility remains unclear. Common co-morbidities of T2DM, particularly hypertension, may drive AF in the setting of T2DM. But direct mechanisms may also explain this relation, at least in part. In this regard, recent evidence suggests that mitochondrial dysfunction drives structural, electrical and contractile remodelling of atrial tissue in patients T2DM. Mitochondrial dysfunction may therefore be the mechanistic link between T2DM and AF and could also serve as a therapeutic target. An elegant series of experiments published in Cardiovascular Diabetology provide compelling new evidence to support this hypothesis. Using a model of high fat diet (HFD) and low-dose streptozotocin (STZ) injection, Shao et al. provide data that demonstrate a direct association between mitochondrial dysfunction and the susceptibility to develop AF. But the authors also demonstrated that the sodium-glucose co-transporter 2 inhibitors (SGLT2i) empagliflozin has the capacity to restore mitochondrial function, ameliorate electrical and structural remodelling and prevent AF. These findings provide a new horizon in which mitochondrial targeted therapies could serve as a new class of antiarrhythmic drugs.

Sodium homeostasis in the tumour microenvironment

The concentration of sodium ions (Na+) is raised in solid tumours and can be measured at the cellular, tissue and patient levels. At the cellular level, the Na+ gradient across the membrane powers the transport of H+ ions and essential nutrients for normal activity. The maintenance of the Na+ gradient requires a large proportion of the cell's ATP. Na+ is a major contributor to the osmolarity of the tumour microenvironment, which affects cell volume and metabolism as well as immune function. Here, we review evidence indicating that Na+ handling is altered in tumours, explore our current understanding of the mechanisms that may underlie these alterations and consider the potential consequences for cancer progression. Dysregulated Na+ balance in tumours may open opportunities for new imaging biomarkers and re-purposing of drugs for treatment.

In vitro study of interaction of 17β-hydroxysteroid dehydrogenase type 10 and cyclophilin D and its potential implications for Alzheimer's disease

In early stages of Alzheimer's disease (AD), amyloid-β (Aβ) accumulates in neuronal mitochondria where it interacts with a number of biomolecules including 17beta-hydroxysteroide dehydrogenase 10 (17β-HSD10) and cyclophilin D (cypD). It has been hypothesized that 17β-HSD10 interacts with cypD preventing it from opening mitochondrial permeability transition pores and that its regulation during AD may be affected by the accumulation of Aβ. In this work, we demonstrate for the first time that 17β-HSD10 and cypD form a stable complex in vitro. Furthermore, we show that factors, such as pH, ionic environment and the presence of Aβ, affect the ability of 17β-HSD10 to bind cypD. We demonstrate that K+ and Mg2+ ions present at low levels may facilitate this binding. We also show that different fragments of Aβ (Aβ1-40 and Aβ1-42) affect the interaction between 17β-HSD10 and cypD differently and that Aβ1-42 (in contrast to Aβ1-40) is capable of simultaneously binding both 17β-HSD10 and cypD in a tri-complex.

Quantitative determination of cellular [Na+] by fluorescence lifetime imaging with CoroNaGreen

Meyer et al. establish the suitability of the sodium-sensitive indicator dye CoroNaGreen for fluorescence lifetime imaging inside cells. This approach represents a valuable tool for quantitative and dynamic determination of intracellular sodium concentrations independent of dye concentration.

Fluorescence lifetime imaging microscopy (FLIM) with fluorescent ion sensors enables the measurement of ion concentrations based on the detection of photon emission events after brief excitation with a pulsed laser source. In contrast to intensity-based imaging, it is independent of dye concentration, photobleaching, or focus drift and has thus been successfully employed for quantitative analysis of, e.g., calcium levels in different cell types and cellular microdomains. Here, we tested the suitability of CoroNaGreen for FLIM-based determination of sodium concentration ([Na⁺]) inside cells. In vitro measurements confirmed that fluorescence lifetimes of CoroNaGreen (CoroNaFL) increased with increasing [Na⁺]. Moreover, CoroNaFL was largely independent of changes in potassium concentration or viscosity. Changes in pH slightly affected FL in the acidic range (pH ≤ 5.5). For intracellular determination of [Na⁺], HEK293T cells were loaded with the membrane-permeable form of CoroNaGreen. Fluorescence decay curves of CoroNaGreen, derived from time-correlated single-photon counting, were approximated by a bi-exponential decay. In situ calibrations revealed a sigmoidal dependence of CoroNaFL on [Na⁺] between 0 and 150 mM, exhibiting an apparent K_d of ∼80 mM. Based on these calibrations, a [Na⁺] of 17.6 mM was determined in the cytosol. Cellular nuclei showed a significantly lower [Na⁺] of 13.0 mM, whereas [Na⁺] in perinuclear regions was significantly higher (26.5 mM). Metabolic inhibition or blocking the Na⁺/K⁺-ATPase by removal of extracellular K⁺ caused significant [Na⁺] increases in all cellular subcompartments. Using an alternative approach for data analysis (“Ratio FLIM”) increased the temporal resolution and revealed a sequential response to K⁺ removal, with cytosolic [Na⁺] increasing first, followed by the nucleus and finally the perinuclear regions. Taken together, our results show that CoroNaGreen is suitable for dynamic, FLIM-based determination of intracellular [Na⁺]. This approach thus represents a valuable tool for quantitative determination of [Na⁺] and changes thereof in different subcellular compartments.

Na+ Selective Fluorescent Tools Based on Fluorescence Intensity Enhancements, Lifetime Changes, and on a Ratiometric Response

Over the years, we developed highly selective fluorescent probes for K+ in water, which show K+ -induced fluorescence intensity enhancements, lifetime changes, or a ratiometric behavior at two emission wavelengths (cf. Scheme 1, K1-K4). In this paper, we introduce selective fluorescent probes for Na+ in water, which also show Na+ induced signal changes, which are analyzed by diverse fluorescence techniques. Initially, we synthesized the fluorescent probes 2, 4, 5, 6 and 10 for a fluorescence analysis by intensity enhancements at one wavelength by varying the Na+ responsive ionophore unit and the fluorophore moiety to adjust different Kd values for an intra- or extracellular Na+ analysis. Thus, we found that 2, 4 and 5 are Na+ selective fluorescent tools, which are able to measure physiologically important Na+ levels at wavelengths higher than 500 nm. Secondly, we developed the fluorescent probes 7 and 8 to analyze precise Na+ levels by fluorescence lifetime changes. Herein, only 8 (Kd =106 mm) is a capable fluorescent tool to measure Na+ levels in blood samples by lifetime changes. Finally, the fluorescent probe 9 was designed to show a Na+ induced ratiometric fluorescence behavior at two emission wavelengths. As desired, 9 (Kd =78 mm) showed a ratiometric fluorescence response towards Na+ ions and is a suitable tool to measure physiologically relevant Na+ levels by the intensity change of two emission wavelengths at 404 nm and 492 nm.

Beyond T2 and 3T: New MRI techniques for clinicians

Technological advances in Magnetic Resonance Imaging (MRI) in terms of field strength and hybrid MR systems have led to improvements in tumor imaging in terms of anatomy and functionality. This review paper discusses the applications of such advances in the field of radiation oncology with regards to treatment planning, therapy guidance and monitoring tumor response and predicting outcome.

Determining sodium diffusion through acoustic impedance measurements using 80 MHz Scanning Acoustic Microscopy: Agarose phantom verification

The purpose of this study is to explore the feasibility of time-dependent acoustic impedance measurement by Scanning Acoustic Microscopy (SAM) for analyzing the sodium diffusion. The purpose is motivated by the fact that sodium monitoring is challenging and still in the area of exploratory analysis despite its biological importance. To our knowledge, this is the first study in which sodium diffusion has been investigated by time-dependent acoustic impedance measurements provided by SAM. We first tested the idea in an agarose phantom as a proof-of-concept. Accordingly, we designed the agarose phantom which initially contains a well of sodium chloride (NaCl) solution moving radially into the phantom. By using NaCl diffusion in the phantom, we obtained two-dimensional (2D) acoustic impedance (Z) maps over time through SAM operating with 80 MHz ultrasonic transducer having a lateral resolution of 20 μm. A linear correlation between the changes in the concentration profile of the phantom and its acoustic impedance was introduced. Analysis of experimental data proved that spatially changing acoustic impedance could be ascribed to the diffusion process and produced a diffusion coefficient in the order of 10^-5 cm²/s which matches well with the literature. Our results showed that SAM could monitor the time-dependent alterations in acoustic impedance resulting from the diffusion of sodium inside the agarose phantom. With this study, SAM shows a promise as a monitoring tool not only to obtain static images but also to perform dynamic investigations of sodium ions with the advantages of providing images in micrometer resolution with a scanning time no longer than 2 min for an image area of 4.8 mm × 4.8 mm.

SR-mitochondria communication in adult cardiomyocytes: A close relationship where the Ca2+ has a lot to say

In adult cardiomyocytes, T-tubules, junctional sarcoplasmic reticulum (jSR), and mitochondria juxtapose each other and form a unique and highly repetitive functional structure along the cell. The close apposition between jSR and mitochondria creates high Ca2+ microdomains at the contact sites, increasing the efficiency of the excitation-contraction-bioenergetics coupling, where the Ca2+ transfer from SR to mitochondria plays a critical role. The SR-mitochondria contacts are established through protein tethers, with mitofusin 2 the most studied SR-mitochondrial "bridge", albeit controversial. Mitochondrial Ca2+ uptake is further optimized with the mitochondrial Ca2+ uniporter preferentially localized in the jSR-mitochondria contact sites and the mitochondrial Na+/Ca2+ exchanger localized away from these sites. Despite all these unique features facilitating the privileged transport of Ca2+ from SR to mitochondria in adult cardiomyocytes, the question remains whether mitochondrial Ca2+ concentrations oscillate in synchronicity with cytosolic Ca2+ transients during heartbeats. Proper Ca2+ transfer controls not only the process of mitochondrial bioenergetics, but also of mitochondria-mediated cell death, autophagy/mitophagy, mitochondrial fusion/fission dynamics, reactive oxygen species generation, and redox signaling, among others. Our review focuses specifically on Ca2+ signaling between SR and mitochondria in adult cardiomyocytes. We discuss the physiological and pathological implications of this SR-mitochondrial Ca2+ signaling, research gaps, and future trends.

Potential of Sodium MRI as a Biomarker for Neurodegeneration and Neuroinflammation in Multiple Sclerosis

In multiple sclerosis (MS), experimental and ex vivo studies indicate that pathologic intra- and extracellular sodium accumulation may play a pivotal role in inflammatory as well as neurodegenerative processes. Yet, in vivo assessment of sodium in the microenvironment is hard to achieve. Here, sodium magnetic resonance imaging (²³NaMRI) with its non-invasive properties offers a unique opportunity to further elucidate the effects of sodium disequilibrium in MS pathology in vivo in addition to regular proton based MRI. However, unfavorable physical properties and low in vivo concentrations of sodium ions resulting in low signal-to-noise-ratio (SNR) as well as low spatial resolution resulting in partial volume effects limited the application of ²³NaMRI. With the recent advent of high-field MRI scanners and more sophisticated sodium MRI acquisition techniques enabling better resolution and higher SNR, ²³NaMRI revived. These studies revealed pathologic total sodium concentrations in MS brains now even allowing for the (partial) differentiation of intra- and extracellular sodium accumulation. Within this review we (1) demonstrate the physical basis and imaging techniques of ²³NaMRI and (2) analyze the present and future clinical application of ²³NaMRI focusing on the field of MS thus highlighting its potential as biomarker for neuroinflammation and -degeneration.

Functional properties and mode of regulation of the mitochondrial Na+/Ca2+ exchanger, NCLX

Mitochondrial Ca²⁺ transient is the earliest discovered organellar Ca²⁺ signaling pathway. It consist of a Ca²⁺ influx, mediated by mitochondrial Ca²⁺ uniporter (MCU), and mitochondrial Ca²⁺ efflux mediated by a Na⁺/Ca²⁺ exchanger (NCLX). Mitochondrial Ca²⁺ signaling machinery plays a fundamental role in linking metabolic activity to cellular Ca²⁺ signaling, and in controlling local Ca²⁺ concertation in distinct cellular compartments. Impaired balance between mitochondrial Ca²⁺ influx and efflux leads to mitochondrial Ca²⁺ overload, an early and key event in ischemic or neurodegenerative syndromes. Molecular identification of NCLX and MCU happened only recently. Surprisingly, MCU knockout yielded a relatively mild phenotype while conditional knockout of NCLX led to a rapid fatal heart failure. Here we will focus on recent functional and molecular studies on NCLX structure and its mode of regulation. We will describe the unique crosstalk of this exchanger with Na⁺ and Ca²⁺ signaling pathways in the cell membrane and the endoplasmic reticulum, and with protein kinases that posttranslationally modulate NCLX activity. We will critically compare selectivity of pharmacological blockers versus molecular control of NCLX expression and activity. Finally we will discuss why this exchanger is essential for survival and can serve as an attractive therapeutic target.