Physiological functions of mitochondrial Na+-Ca2+ exchanger, NCLX, in lymphocytes

The pathological mechanisms and potential therapeutic drugs for myocardial ischemia reperfusion injury

BACKGROUND

Cardiovascular disease is the main cause of death and disability, with myocardial ischemia being the predominant type that poses a significant threat to humans. Reperfusion, an essential therapeutic approach, promptly reinstates blood circulation to the ischemic myocardium and stands as the most efficacious clinical method for myocardial preservation. Nevertheless, the restoration of blood flow associated with this process can potentially induce myocardial ischemia-reperfusion injury (MIRI), thereby diminishing the effectiveness of reperfusion and impacting patient prognosis. Therefore, it is of great significance to prevent and treat MIRI.

PURPOSE

MIRI is an important factor affecting the prognosis of patients, and there is no specific in-clinic treatment plan. In this review, we have endeavored to summarize its pathological mechanisms and therapeutic drugs to provide more powerful evidence for clinical application.

METHODS

A comprehensive literature review was conducted using PubMed, Web of Science, Embase, Medline and Google Scholar with a core focus on the pathological mechanisms and potential therapeutic drugs of MIRI.

RESULTS

Accumulated evidence revealed that oxidative stress, calcium overload, mitochondrial dysfunction, energy metabolism disorder, ferroptosis, inflammatory reaction, endoplasmic reticulum stress, pyroptosis and autophagy regulation have been shown to participate in the process, and that the occurrence and development of MIRI are related to plenty of signaling pathways. Currently, a range of chemical drugs, natural products, and traditional Chinese medicine (TCM) preparations have demonstrated the ability to mitigate MIRI by targeting various mechanisms.

CONCLUSIONS

At present, most of the research focuses on animal and cell experiments, and the regulatory mechanisms of each signaling pathway are still unclear. The translation of experimental findings into clinical practice remains incomplete, necessitating further exploration through large-scale, multi-center randomized controlled trials. Given the absence of a specific drug for MIRI, the identification of therapeutic agents to reduce myocardial ischemia is of utmost significance. For the future, it is imperative to enhance our understanding of the pathological mechanism underlying MIRI, continuously investigate and develop novel pharmaceutical agents, expedite the clinical translation of these drugs, and foster innovative approaches that integrate TCM with Western medicine. These efforts will facilitate the emergence of fresh perspectives for the clinical management of MIRI.

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.

Spatial and Functional Crosstalk between the Mitochondrial Na+-Ca2+ Exchanger NCLX and the Sarcoplasmic Reticulum Ca2+ Pump SERCA in Cardiomyocytes

The mitochondrial Na⁺-Ca²⁺ exchanger, NCLX, was reported to supply Ca²⁺ to sarcoplasmic reticulum (SR)/endoplasmic reticulum, thereby modulating various cellular functions such as the rhythmicity of cardiomyocytes, and cellular Ca²⁺ signaling upon antigen receptor stimulation and chemotaxis in B lymphocytes; however, there is little information on the spatial relationships of NCLX with SR Ca²⁺ handling proteins, and their physiological impact. Here we examined the issue, focusing on the interaction of NCLX with an SR Ca²⁺ pump SERCA in cardiomyocytes. A bimolecular fluorescence complementation assay using HEK293 cells revealed that the exogenously expressed NCLX was localized in close proximity to four exogenously expressed SERCA isoforms. Immunofluorescence analyses of isolated ventricular myocytes showed that the NCLX was localized to the edges of the mitochondria, forming a striped pattern. The co-localization coefficients in the super-resolution images were higher for NCLX–SERCA2, than for NCLX–ryanodine receptor and NCLX–Na⁺/K⁺ ATPase α-1 subunit, confirming the close localization of endogenous NCLX and SERCA2 in cardiomyocytes. The mathematical model implemented with the spatial and functional coupling of NCLX and SERCA well reproduced the NCLX inhibition-mediated modulations of SR Ca²⁺ reuptake in HL-1 cardiomyocytes. Taken together, these results indicated that NCLX and SERCA are spatially and functionally coupled in cardiomyocytes.

Physiological and Pathophysiological Roles of Mitochondrial Na+-Ca2+ Exchanger, NCLX, in Hearts

It has been over 10 years since SLC24A6/SLC8B1, coding the Na⁺/Ca²⁺/Li⁺ exchanger (NCLX), was identified as the gene responsible for mitochondrial Na⁺-Ca²⁺ exchange, a major Ca²⁺ efflux system in cardiac mitochondria. This molecular identification enabled us to determine structure–function relationships, as well as physiological/pathophysiological contributions, and our understandings have dramatically increased. In this review, we provide an overview of the recent achievements in relation to NCLX, focusing especially on its heart-specific characteristics, biophysical properties, and spatial distribution in cardiomyocytes, as well as in cardiac mitochondria. In addition, we discuss the roles of NCLX in cardiac functions under physiological and pathophysiological conditions—the generation of rhythmicity, the energy metabolism, the production of reactive oxygen species, and the opening of mitochondrial permeability transition pores.

Impact of mitochondria on local calcium release in murine sinoatrial nodal cells

Roles of mitochondria in sinoatrial nodal cells (SANCs) have not been fully clarified. We have previously demonstrated that mitochondrial Ca²⁺ efflux through the Na⁺-Ca²⁺ exchanger, NCXm, modulates sarcoplasmic reticulum (SR) Ca²⁺ content and automaticity of HL-1 cardiomyocytes. In this study, we extended this line of investigation to clarify the spatial and functional association between mitochondria and local calcium release (LCR) from the SR in murine SANCs. High-speed two dimensional (2D) and confocal line-scan imaging of SANCs revealed that LCRs in the early phase of the Ca²⁺ transient cycle length (CL) appeared with a higher probability near mitochondria. Although LCR increased toward the late phase of CL, no significant difference was noted in the occurrence of late LCRs near and distant from mitochondria. LCRs, especially in the late phase of CL, induced temporal and spatial heterogeneity of the Ca²⁺ transient amplitude. Attenuating mitochondrial Ca²⁺ efflux using an NCXm inhibitor, CGP-37157 (1 μM), reduced the amplitude, duration and size of LCR. It also attenuated early LCR occurrence, and simultaneously prolonged LCR period and CL. Additionally, CGP-37157 reduced caffeine-induced Ca²⁺ transient. Therefore, the inhibitory effect on LCR was attributable to the reduction of the SR Ca²⁺ content through NCXm inhibition. No obvious off-target effects of 1 μM CGP-37157 were found on T- and L-type voltage-gated Ca²⁺ currents and hyperpolarization-activated inward current. Taken together, these results suggest that mitochondria are involved in LCR generation by modulating the SR Ca²⁺ content through NCXm-mediated Ca²⁺ efflux in murine SANCs.

Lockdown of mitochondrial Ca2+ extrusion and subsequent resveratrol treatment kill HeLa cells by Ca2+ overload

Anticancer effect of resveratrol and the role of sodium/lithium/calcium exchanger in context with calcium ions are studied in human cervical cancer cell line. This therapeutic approach using siNCLX mediated gene silencing and drug therapy with resveratrol indicates the disruption of calcium homeostasis, increase in caspase (-3, 8, 9) mRNA expressions and DNA damage leading to apoptotic cell death. Monitoring the intracellular Ca²⁺ changes using fluo-4AM indicates highest rise in [Ca²⁺] level in sodium/lithium/calcium exchanger silenced group with five different stages, that is distinguishable based on the fluorescence intensity. In resveratrol treated and siNCLX + resveratrol treated groups no such cell staging differences were observed, despite uniform Ca²⁺ rise followed by decrease in the intensity. Integrating RNAi gene silencing of sodium/lithium/calcium exchanger with resveratrol can form the most interesting, efficient and promising therapeutic strategy in the treatment of cancer.

Minor contribution of NCX to Na+-Ca2+ exchange activity in brain mitochondria

NCLX was identified as a mitochondrial Na⁺-Ca²⁺ exchanger. However, contribution of NCLX to overall mitochondrial Na⁺-Ca²⁺ exchange activity remains unclear, especially in brain mitochondria where plasma membrane Na⁺-Ca²⁺ exchanger NCX also exists. We studied the issue using isolated mouse brain mitochondria. The Na⁺- as well as Li⁺-dependent Ca²⁺ efflux from mitochondria was significantly inhibited by a NCLX blocker, but was insensitive to NCX blockers, suggesting that NCLX comprises a major part in forward mode of mitochondrial Na⁺-Ca²⁺ exchange activity. On the other hand, the Na⁺-dependent Ca²⁺ influx into mitochondria, the reverse mode, was insensitive to all the blockers tested, suggesting unidentified Ca²⁺ transport systems.

Calcium regulation of T cell metabolism

T cells are an essential component of the immune system that provide antigen-specific acute and long lasting immune responses to infections and tumors, ascertain the maintenance of immunological tolerance and, on the flipside, mediate autoimmunity in a variety of diseases. The activation of T cells through antigen recognition by the T cell receptor (TCR) results in transient and sustained Ca²⁺ signals that are shaped by the opening of Ca²⁺ channels in the plasma membrane and cellular organelles. The dynamic regulation of intracellular Ca²⁺ concentrations controls a variety of T cell functions on the timescale of seconds to days after signal initiation. Among the more recently identified roles of Ca²⁺ signaling in T cells is the regulation of metabolic pathways that control the function of many T cell subsets. In this review, we discuss how Ca²⁺ regulates several metabolic programs in T cells such as the activation of AMPK and the PI3K-AKT-mTORC1 pathway, aerobic glycolysis, mitochondrial metabolism including tricarboxylic acid (TCA) cycle function and oxidative phosphorylation (OXPHOS), as well as lipid metabolism.

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.