Mitochondria Na(+)-Ca (2+) exchange in cardiomyocytes and lymphocytes

The mitochondrial sodium/calcium exchanger NCLX (Slc8b1) in B lymphocytes

Antigen receptor stimulation triggers cytosolic Ca2+ signals, which activate transcriptional and metabolic programs critical for immune function. B-cell receptor (BCR) engagement causes rapid cytosolic Ca2+ rise through the ubiquitous store-operated calcium entry (SOCE) pathway. Slc8b1, which encodes the mitochondrial Na+/Ca2+ exchanger (NCLX), extrudes Ca2+ out of the mitochondria and maintains optimal SOCE activity. Inhibition of NCLX in DT40 and A20 B lymphocyte lines was recently shown to impair cytosolic Ca2+ transients in response to antigen-receptor stimulation, however the downstream functional consequences of this impairment remain unclear. Here, we generated Slc8b1 knockout A20 B-cell lines using CRISPR/Cas9 technology and B-cell specific Slc8b1 knockout mice. Surprisingly, while loss of Slc8b1 in B lymphocytes led to reduction in SOCE, it had a marginal effect on mitochondrial Ca2+ extrusion, suggesting that NCLX is not the major mitochondrial Ca2+ extrusion mechanism in B cells. Furthermore, endoplasmic reticulum (ER) Ca2+ content and rates of ER depletion and refilling remained unaltered in Slc8b1 knockout B cells. Slc8b1 deficiency increased mitochondrial production of oxidants, reduced mitochondrial bioenergetics and altered mitochondrial ultrastructure. B-cell specific Slc8b1 knockout mice showed reduced germinal center B cell responses following foreign antigen and pathogen driven immune responses. Our studies provide novel insights into the function of Slc8b1 in germinal center B cells and its contribution to B-cell signaling and effector function.

Sepsis-induced myocardial dysfunction: the role of mitochondrial dysfunction

INTRODUCTION

Sepsis-induced myocardial dysfunction (SIMD) is a condition manifested by an intrinsic myocardial systolic and diastolic dysfunction during sepsis, which is associated with worse clinical outcomes and a higher mortality.

MATERIALS AND METHODS

Several pathophysiological mechanisms including mitochondrial dysfunction, abnormal body immune reaction, metabolic reprogramming, excessive production of reactive oxygen species (ROS), and disorder of calcium regulation have been involved in SIMD. Mitophagy has potential role in protecting myocardial cells in sepsis, especially in survivors.

CONCLUSION

In the current review, we focus on the role of mitochondrial dysfunction and other mitochondria-related mechanisms including immunologic imbalance, energetic reprogramming, mitophagy, and pyroptosis in the mechanisms of SIMD.

Cytoplasmic and Mitochondrial Calcium Signaling: A Two-Way Relationship

Intracellular Ca²⁺ signals are well organized in all cell types, and trigger a variety of vital physiological processes. The temporal and spatial characteristics of cytosolic Ca²⁺ increases are mainly governed by the fluxes of this ion across the membrane of the endoplasmic/sarcoplasmic reticulum and the plasma membrane. However, various Ca²⁺ transporters also allow for Ca²⁺ exchanges between the cytoplasm and mitochondria. Increases in mitochondrial Ca²⁺ stimulate the production of ATP, which allows the cells to cope with the increased energy demand created by the stimulus. Less widely appreciated is the fact that Ca²⁺ handling by mitochondria also shapes cytosolic Ca²⁺ signals. Indeed, the frequency, amplitude, and duration of cytosolic Ca²⁺ increases can be altered by modifying the rates of Ca²⁺ transport into, or from, mitochondria. In this review, we focus on the interplay between mitochondria and Ca²⁺ signaling, highlighting not only the consequences of cytosolic Ca²⁺ changes on mitochondrial Ca²⁺, but also how cytosolic Ca²⁺ dynamics is controlled by modifications of the Ca²⁺-handling properties and the metabolism of mitochondria.

[Role of mitochondrial permeability transition pore in mediating the inhibitory effect of gastrodin on oxidative stress in cardiac myocytes in vitro]

OBJECTIVE

To explore the role of mitochondrial permeability transition pore (mPTP) in mediating the protective effect of gastrodin against oxidative stress damage in H9c2 cardiac myocytes.

METHODS

H9c2 cardiac myocytes were treated with H₂O₂, gastrodin, gastrodin+H₂O₂, cyclosporin A (CsA), or CsA+gas+H₂O₂ group. MTT assay was used to detect the survival ratio of H9c2 cells, and flow cytometry with Annexin V-FITC/PI double staining was used to analyze the early apoptosis rate after the treatments. The concentration of ATP and level of reactive oxygen species (ROS) in the cells were detected using commercial kits. The mitochondrial membrane potential of the cells was detected with laser confocal microscopy. The expression of cytochrome C was detected with Western blotting, and the activity of caspase-3 was also assessed in the cells.

RESULTS

Gastrodin pretreatment could prevent oxidative stress-induced reduction of mitochondrial membrane potential, and this effect was inhibited by the application of CsA. Gastrodin significantly lowered the levels of ROS and apoptosis-related factors in H₂O₂-exposed cells, and such effects were reversed by CsA. CsA significantly antagonized the protective effect of gastrodin against apoptosis in H₂O₂-exposed cells.

CONCLUSIONS

Gastrodin prevents oxidative stress-induced injury in H9c2 cells by inhibiting mPTP opening to reduce the cell apoptosis.

Mitochondrial pore opening and loss of Ca2+ exchanger NCLX levels occur after frataxin depletion

Frataxin-deficient neonatal rat cardiomyocytes and dorsal root ganglia neurons have been used as cell models of Friedreich ataxia. In previous work we show that frataxin depletion resulted in mitochondrial swelling and lipid droplet accumulation in cardiomyocytes, and compromised DRG neurons survival. Now, we show that these cells display reduced levels of the mitochondrial calcium transporter NCLX that can be restored by calcium-chelating agents and by external addition of frataxin fused to TAT peptide. Also, the transcription factor NFAT3, involved in cardiac hypertrophy and apoptosis, becomes activated by dephosphorylation in both cardiomyocytes and DRG neurons. In cardiomyocytes, frataxin depletion also results in mitochondrial permeability transition pore opening. Since the pore opening can be inhibited by cyclosporin A, we show that this treatment reduces lipid droplets and mitochondrial swelling in cardiomyocytes, restores DRG neuron survival and inhibits NFAT dephosphorylation. These results highlight the importance of calcium homeostasis and that targeting mitochondrial pore by repurposing cyclosporin A, could be envisaged as a new strategy to treat the disease.

Roles of the mitochondrial Na(+)-Ca(2+) exchanger, NCLX, in B lymphocyte chemotaxis

Lymphocyte chemotaxis plays important roles in immunological reactions, although the mechanism of its regulation is still unclear. We found that the cytosolic Na(+)-dependent mitochondrial Ca(2+) efflux transporter, NCLX, regulates B lymphocyte chemotaxis. Inhibiting or silencing NCLX in A20 and DT40 B lymphocytes markedly increased random migration and suppressed the chemotactic response to CXCL12. In contrast to control cells, cytosolic Ca(2+) was higher and was not increased further by CXCL12 in NCLX-knockdown A20 B lymphocytes. Chelating intracellular Ca(2+) with BAPTA-AM disturbed CXCL12-induced chemotaxis, suggesting that modulation of cytosolic Ca(2+) via NCLX, and thereby Rac1 activation and F-actin polymerization, is essential for B lymphocyte motility and chemotaxis. Mitochondrial polarization, which is necessary for directional movement, was unaltered in NCLX-knockdown cells, although CXCL12 application failed to induce enhancement of mitochondrial polarization, in contrast to control cells. Mouse spleen B lymphocytes were similar to the cell lines, in that pharmacological inhibition of NCLX by CGP-37157 diminished CXCL12-induced chemotaxis. Unexpectedly, spleen T lymphocyte chemotaxis was unaffected by CGP-37157 treatment, indicating that NCLX-mediated regulation of chemotaxis is B lymphocyte-specific, and mitochondria-endoplasmic reticulum Ca(2+) dynamics are more important in B lymphocytes than in T lymphocytes. We conclude that NCLX is pivotal for B lymphocyte motility and chemotaxis.

Mitochondrial ion channels/transporters as sensors and regulators of cellular redox signaling

SIGNIFICANCE

Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently.

RECENT ADVANCES

Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K+ channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling.

CRITICAL ISSUES

Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation.

FUTURE DIRECTIONS

Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters.

The destiny of Ca(2+) released by mitochondria

Mitochondrial Ca²⁺ is known to regulate diverse cellular functions, for example energy production and cell death, by modulating mitochondrial dehydrogenases, inducing production of reactive oxygen species, and opening mitochondrial permeability transition pores. In addition to the action of Ca²⁺ within mitochondria, Ca²⁺ released from mitochondria is also important in a variety of cellular functions. In the last 5 years, the molecules responsible for mitochondrial Ca²⁺ dynamics have been identified: a mitochondrial Ca²⁺ uniporter (MCU), a mitochondrial Na⁺–Ca²⁺ exchanger (NCLX), and a candidate for a mitochondrial H⁺–Ca²⁺ exchanger (Letm1). In this review, we focus on the mitochondrial Ca²⁺ release system, and discuss its physiological and pathophysiological significance. Accumulating evidence suggests that the mitochondrial Ca²⁺ release system is not only crucial in maintaining mitochondrial Ca²⁺ homeostasis but also participates in the Ca²⁺ crosstalk between mitochondria and the plasma membrane and between mitochondria and the endoplasmic/sarcoplasmic reticulum.

The mitochondrial Na+-Ca2+ exchanger, NCLX, regulates automaticity of HL-1 cardiomyocytes

Mitochondrial Ca²⁺ is known to change dynamically, regulating mitochondrial as well as cellular functions such as energy metabolism and apoptosis. The NCLX gene encodes the mitochondrial Na⁺-Ca²⁺ exchanger (NCX_mit), a Ca²⁺ extrusion system in mitochondria. Here we report that the NCLX regulates automaticity of the HL-1 cardiomyocytes. NCLX knockdown using siRNA resulted in the marked prolongation of the cycle length of spontaneous Ca²⁺ oscillation and action potential generation. The upstrokes of action potential and Ca²⁺ transient were markedly slower, and sarcoplasmic reticulum (SR) Ca²⁺ handling were compromised in the NCLX knockdown cells. Analyses using a mathematical model of HL-1 cardiomyocytes demonstrated that blocking NCX_mit reduced the SR Ca²⁺ content to slow spontaneous SR Ca²⁺ leak, which is a trigger of automaticity. We propose that NCLX is a novel molecule to regulate automaticity of cardiomyocytes via modulating SR Ca²⁺ handling.