Beyond the TCA cycle: new insights into mitochondrial calcium regulation of oxidative phosphorylation

Rhodiola crenulata alleviates hypobaric hypoxia-induced brain injury by maintaining BBB integrity and balancing energy metabolism dysfunction

BACKGROUND/PURPOSE

Rhodiola crenulata (Hook. f. et Thoms.) H. Ohba (R. crenulate), a famous and characteristic Tibetan medicine, has been demonstrated to exert an outstanding brain protection role in the treatment of high-altitude hypoxia disease. However, the metabolic effects of R. crenulate on high-altitude hypoxic brain injury (HHBI) are still incompletely understood. Herein, the anti-hypoxic effect and associated mechanisms of R. crenulate were explored through both in vivo and in vitro experiments.

STUDY DESIGN/METHODS

The mice model of HHBI was established using an animal hypobaric and hypoxic chamber. R. crenulate extract (RCE, 0.5, 1.0 and 2.0 g/kg) and salidroside (Sal, 25, 50 and 100 mg/kg) was given by gavage for 7 days. Pathological changes and neuronal apoptosis of mice hippocampus and cortex were evaluated using H&E and TUNEL staining, respectively. The effects of RCE and Sal on the permeability of blood brain barrier (BBB) were detected by Evans blue staining and NIR-II fluorescence imaging. Meanwhile, the ultrastructural BBB and cerebrovascular damages were observed using a transmission electron microscope (TEM). The levels of tight junction proteins Claudin-1, ZO-1 and occludin were detected by immunofluorescence. Additionally, the metabolites in mice serum and brain were determined using UHPLC-MS and MALDI-MSI analysis. The cell viability of Sal on hypoxic HT22 cells induced by CoCl2 was investigated by cell counting kit-8. The contents of LDH, MDA, SOD, GSH-PX and SDH were detected by using commercial biochemical kits. Meanwhile, intracellular ROS, Ca2+ and mitochondrial membrane potential were determined by corresponding specific labeled probes. The intracellular metabolites of HT22 cells were performed by the targeted metabolomics analysis of the Q300 kit. The cell apoptosis and necrosis were examined by YO-PRO-1/PI, Annexin V/PI and TUNEL staining. In addition, mitochondrial morphology was tested by Mito-tracker red with confocal microscopy and TEM. Real-time ATP production, oxygen consumption rate, and proton efflux rate were measured using a Seahorse analyzer. Subsequently, MCU, OPA1, p-Drp1ser616, p-AMPKα, p-AMPKβ and Sirt1 were determined by immunofluorescent and western blot analyses.

RESULTS

The results demonstrated that R. crenulate and Sal exert anti-hypoxic brain protection from inhibiting neuronal apoptosis, maintaining BBB integrity, increasing tight junction protein Claudin-1, ZO-1 and occludin and improving mitochondrial morphology and function. Mechanistically, R. crenulate and Sal alleviated HHBI by enhancing the tricarboxylic acid cycle to meet the demand of energy of brain. Additionally, experiments in vitro confirmed that Sal could ameliorate the apoptosis of HT22 cells, improve mitochondrial morphology and energy metabolism by enhancing mitochondrial respiration and glycolysis. Meanwhile, Sal-mediated MCU inhibited the activation of Drp1 and enhanced the expression of OPA1 to maintain mitochondrial homeostasis, as well as activation of AMPK and Sirt1 to enhance ATP production.

CONCLUSION

Collectively, the findings suggested that RCE and Sal may afford a protective intervention in HHBI through maintaining BBB integrity and improving energy metabolism via balancing MCU-mediated mitochondrial homeostasis by activating the AMPK/Sirt1 signaling pathway.

Unravelling the complexity of the mitochondrial Ca2+ uniporter: regulation, tissue specificity, and physiological implications

Calcium (Ca2+) signalling acts a pleiotropic message within the cell that is decoded by the mitochondria through a sophisticated ion channel known as the Mitochondrial Ca2+ Uniporter (MCU) complex. Under physiological conditions, mitochondrial Ca2+ signalling is crucial for coordinating cell activation with energy production. Conversely, in pathological scenarios, it can determine the fine balance between cell survival and death. Over the last decade, significant progress has been made in understanding the molecular bases of mitochondrial Ca2+ signalling. This began with the elucidation of the MCU channel components and extended to the elucidation of the mechanisms that regulate its activity. Additionally, increasing evidence suggests molecular mechanisms allowing tissue-specific modulation of the MCU complex, tailoring channel activity to the specific needs of different tissues or cell types. This review aims to explore the latest evidence elucidating the regulation of the MCU complex, the molecular factors controlling the tissue-specific properties of the channel, and the physiological and pathological implications of mitochondrial Ca2+ signalling in different tissues.

Functional interplay of lipid droplets and mitochondria

Our body stores energy mostly in form of fatty acids (FAs) in lipid droplets (LDs). From there the FAs can be mobilized and transferred to peroxisomes and mitochondria. This transfer is dependent on close opposition of LDs and mitochondria and peroxisomes and happens at membrane contact sites. However, the composition and the dynamics of these contact sites is not well understood, which is in part due to the dependence on the metabolic state of the cell and on the cell- and tissue-type. Here, we summarize the current knowledge on the contacts between lipid droplets and mitochondria both in mammals and in the yeast Saccharomyces cerevisiae, in which various contact sites are well studied. We discuss possible functions of the contact site and their implication in disease.

Antimicrobial cetylpyridinium chloride causes functional inhibition of mitochondria as potently as canonical mitotoxicants, nanostructural disruption of mitochondria, and mitochondrial Ca2+ efflux in living rodent and primary human cells

People are exposed to high concentrations of antibacterial agent cetylpyridinium chloride (CPC) via food and personal care products, despite little published information regarding CPC effects on eukaryotes. Here, we show that low-micromolar CPC exposure, which does not cause cell death, inhibits mitochondrial ATP production in primary human keratinocytes, mouse NIH-3T3 fibroblasts, and rat RBL-2H3 immune mast cells. ATP inhibition via CPC (EC50 1.7 μM) is nearly as potent as that caused by canonical mitotoxicant CCCP (EC50 1.2 μM). CPC inhibition of oxygen consumption rate (OCR) tracks with that of ATP: OCR is halved due to 1.75 μM CPC in RBL-2H3 cells and 1.25 μM in primary human keratinocytes. Mitochondrial [Ca2+] changes can cause mitochondrial dysfunction. Here we show that CPC causes mitochondrial Ca2+ efflux from mast cells via an ATP-inhibition mechanism. Using super-resolution microscopy (fluorescence photoactivation localization) in live cells, we have discovered that CPC causes mitochondrial nanostructural defects in live cells within 60 min, including the formation of spherical structures with donut-like cross section. This work reveals CPC as a mitotoxicant despite widespread use, highlighting the importance of further research into its toxicological safety.

High-Throughput Identification of Calcium Regulated Proteins Across Diverse Proteomes

Calcium ions play important roles in nearly every biological process, yet whole-proteome analysis of calcium effectors has been hindered by lack of high-throughput, unbiased, and quantitative methods to identify proteins-calcium engagement. To address this, we adapted protein thermostability assays in the budding yeast, human cells, and mouse mitochondria. Based on calcium-dependent thermostability, we identified 2884 putative calcium-regulated proteins across human, mouse, and yeast proteomes. These data revealed calcium engagement of novel signaling hubs and cellular processes, including metabolic enzymes and the spliceosome. Cross-species comparison of calcium-protein engagement and mutagenesis experiments identified residue-specific cation engagement, even within well-known EF-hand domains. Additionally, we found that the dienoyl-CoA reductase DECR1 binds calcium at physiologically-relevant concentrations with substrate-specific affinity, suggesting direct calcium regulation of mitochondrial fatty acid oxidation. These unbiased, proteomic analyses of calcium effectors establish a key resource to dissect cation engagement and its mechanistic effects across multiple species and diverse biological processes.

Dihydroartemisinin inhibits liver cancer cell migration and invasion by reducing ATP synthase production through CaMKK2/NCLX

Calcium/calmodulin-dependent protein kinase kinase 2 (CaMKK2) and mitochondrial sodium/calcium exchanger protein (NCLX) are key regulatory factors in calcium homeostasis. Finding natural drugs that target regulators of calcium homeostasis is critical. Dihydroartemisinin (DHA) is considered to have anticancer effects. The present study aimed to investigate the mechanism of DHA in regulating liver cancer migration and invasion. The present study used HepG2 and HuH-7 cells and overexpressed CaMKK2 and knocked down CaMKK2 and NCLX. The antiproliferative activity of DHA on liver cancer cells was assessed through colony formation and EdU assays. Cell apoptosis was detected through YO-PRO-1/PI staining. The levels of reactive oxygen species (ROS) were measured using a ROS detection kit (DCFH-DA fluorescent probe). Cell migratory and invasive abilities were examined using wound healing and Transwell assays. The ATP production of liver cancer cells was detected using ATP fluorescent probes. Cell microfilaments were monitored for changes using Actin-Tracker Green-488. The effects of DHA on the expression of CaMKK2, NCLX, sodium/potassium-transporting ATPase subunit α-1 (ATP1A1) and ATP synthase subunit d, mitochondrial (ATP5H) were determined by western blotting and reverse transcription-quantitative PCR. The results revealed that DHA significantly inhibited proliferation, reduced ROS levels and promoted apoptosis in liver cancer cells. CaMKK2 overexpression significantly enhanced the invasive and migratory ability of liver cancer cells, whereas DHA inhibited the pro-migratory effects of CaMKK2 overexpression. DHA significantly reduced the mitochondrial ATP production and altered the arrangement of microfilaments in liver cancer cells. In addition, DHA significantly decreased the expression of CaMKK2, NCLX, ATP1A1 and ATP5H. Furthermore, by knockdown experiments of NCLX the results demonstrated that CaMKK2 downregulated the expression of ATP1A1 and ATP5H in liver cancer cells through NCLX. In conclusion, DHA may reduce ATP synthase production via the CaMKK2/NCLX signaling pathway to inhibit the invasive phenotype of liver cancer cells. It is essential to further investigate the effectiveness of DHA in the anticancer mechanism of liver cancer cells.

Cardioprotective Effect against Ischemia-Reperfusion Injury of PAK-200, a Dihydropyridine Analog with an Inhibitory Effect on Cl- but Not Ca2+ Current

We examined the effects of a dihydropyridine analog, PAK-200, on guinea pig myocardium during experimental ischemia and reperfusion. In isolated ventricular cardiomyocytes, PAK-200 (1 μM) had no effect on the basal peak inward and steady-state currents but inhibited the isoprenaline-induced time-independent Cl- current. In the right atria, PAK-200 had no effect on the beating rate and the chronotropic response to isoprenaline. In an ischemia-reperfusion model with coronary-perfused right ventricular tissue, a decrease in contractile force and a rise in tension were observed during a period of 30-min no-flow ischemia. Upon reperfusion, contractile force returned to less than 50% of preischemic values. PAK-200 had no effect on the decline in contractile force during the no-flow ischemia but reduced the rise in resting tension. PAK-200 significantly improved the recovery of contractile force after reperfusion to about 70% of the preischemic value. PAK-200 was also shown to attenuate the decrease in tissue ATP during ischemia. Treatment of ventricular myocytes with an ischemia-mimetic solution resulted in depolarization of the mitochondrial membrane potential and an increase in cytoplasmic and mitochondrial Ca2+ concentrations. PAK-200 significantly delayed these changes. Thus, PAK-200 inhibits the cAMP-activated chloride current in cardiac muscle and may have protective effects against ischemia-reperfusion injury through novel mechanisms.

PITX2 gain-of-function mutation associated with atrial fibrillation alters mitochondrial activity in human iPSC atrial-like cardiomyocytes

Atrial fibrillation (AF) is the most common cardiac arrhythmia worldwide; however, the underlying causes of AF initiation are still poorly understood, particularly because currently available models do not allow in distinguishing the initial causes from maladaptive remodeling that induces and perpetuates AF. Lately, the genetic background has been proven to be important in the AF onset. iPSC-derived cardiomyocytes, being patient- and mutation-specific, may help solve this diatribe by showing the initial cell-autonomous changes underlying the development of the disease. Transcription factor paired-like homeodomain 2 (PITX2) has been identified as a key regulator of atrial development/differentiation, and the PITX2 genomic locus has the highest association with paroxysmal AF. PITX2 influences mitochondrial activity, and alterations in either its expression or function have been widely associated with AF. In this work, we investigate the activity of mitochondria in iPSC-derived atrial cardiomyocytes (aCMs) obtained from a young patient (24 years old) with paroxysmal AF, carrying a gain-of-function mutation in PITX2 (rs138163892) and from its isogenic control (CTRL) in which the heterozygous point mutation has been reverted to WT. PITX2 aCMs show a higher mitochondrial content, increased mitochondrial activity, and superoxide production under basal conditions when compared to CTRL aCMs. However, increasing mitochondrial workload by FCCP or β-adrenergic stimulation allows us to unmask mitochondrial defects in PITX2 aCMs, which are incapable of responding efficiently to the higher energy demand, determining ATP deficiency.