SR-mitochondria communication in adult cardiomyocytes: A close relationship where the Ca 2+ has a lot to say (original) (raw)
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The SR-mitochondria interaction: a new player in cardiac pathophysiology
Cardiovascular Research, 2010
Mitochondria are essential for energy supply and cell signalling and may be triggers and effectors of cell death. Mitochondrial respiration is tightly controlled by the matrix Ca 2+ concentration, which is beat-to-beat regulated by uptake and release mainly through the mitochondrial Ca 2+ uniporter and Na + /Ca 2+ exchanger, respectively. Recent studies demonstrate that mitochondrial Ca 2+ uptake is more dependent on anatomo-functional microdomains established with the sarcoplasmic reticulum (SR) than on cytosolic Ca 2+. This privileged communication between SR and mitochondria is not restricted to Ca 2+ but may involve ATP and reactive oxygen species, which has important implications in cardiac pathophysiology. The disruption of the SR-mitochondria interaction caused by cell remodelling has been implicated in the deterioration of excitation-contraction coupling of the failing heart. The SR-mitochondria interplay has been suggested to be involved in the depressed Ca 2+ transients and mitochondrial dysfunction observed in diabetic hearts as well as in the genesis of certain arrhythmias, and it may play an important role in myocardial reperfusion injury. During reperfusion, re-energization in the presence of cytosolic Ca 2+ overload results in SR-driven Ca 2+ oscillations that may promote mitochondrial permeability transition (MPT). The relationship between MPT and Ca 2+ oscillations is bidirectional, as recent data show that the induction of MPT in Ca 2+-overloaded cardiomyocytes may result in mitochondrial Ca 2+ release that aggravates Ca 2+ handling and favours hypercontracture. A more complete characterization of the structural arrangements responsible for SR-mitochondria interplay will allow better understanding of cardiac (patho)physiology but also, and no less important, should serve as a basis for the development of new treatments for cardiac diseases.
Mitochondrial Ca2+ uptake contributes to buffering cytoplasmic Ca2+ peaks in cardiomyocytes
Proceedings of the National Academy of Sciences of the United States of America, 2012
Mitochondrial ability of shaping Ca 2+ signals has been demonstrated in a large number of cell types, but it is still debated in heart cells. Here, we take advantage of the molecular identification of the mitochondrial Ca 2+ uniporter (MCU) and of unique targeted Ca 2+ probes to directly address this issue. We demonstrate that, during spontaneous Ca 2+ pacing, Ca 2+ peaks on the outer mitochondrial membrane (OMM) are much greater than in the cytoplasm because of a large number of Ca 2+ hot spots generated on the OMM surface. Cytoplasmic Ca 2+ peaks are reduced or enhanced by MCU overexpression and siRNA silencing, respectively; the opposite occurs within the mitochondrial matrix. Accordingly, the extent of contraction is reduced by overexpression of MCU and augmented by its down-regulation. Modulation of MCU levels does not affect the ATP content of the cardiomyocytes. Thus, in neonatal cardiac myocytes, mitochondria significantly contribute to buffering the amplitude of systolic Ca 2+ rises.
The role of Ca2+ signaling in the coordination of mitochondrial ATP production with cardiac work
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2009
The heart is capable of balancing the rate of mitochondrial ATP production with utilization continuously over a wide range of activity. This results in a constant phosphorylation potential despite a large change in metabolite turnover. The molecular mechanisms responsible for generating this energy homeostasis are poorly understood. The best candidate for a cytosolic signaling molecule reflecting ATP hydrolysis is Ca 2+. Since Ca 2+ initiates and powers muscle contraction as well as serves as the primary substrate for SERCA, Ca 2+ is an ideal feed-forward signal for priming ATP production. With the sarcoplasmic reticulum to cytosolic Ca 2+ gradient near equilibrium with the free energy of ATP, cytosolic Ca 2+ release is exquisitely sensitive to the cellular energy state providing a feedback signal. Thus, Ca 2+ can serve as a feed-forward and feedback regulator of ATP production. Consistent with this notion is the correlation of cytosolic and mitochondrial Ca 2+ with work in numerous preparations as well as the localization of mitochondria near Ca 2+ release sites. How cytosolic Ca 2+ signaling might regulate oxidative phosphorylation is a focus of this review. The relevant Ca 2+ sensitive sites include several dehydrogenases and substrate transporters together with a post-translational modification of F1-FO-ATPase and cytochrome oxidase. Thus, Ca 2+ apparently activates both the generation of the mitochondrial membrane potential as well as utilization to produce ATP. This balanced activation extends the energy homeostasis observed in the cytosol into the mitochondria matrix in the never resting heart.
Journal of Molecular and Cellular Cardiology, 2009
In this study a Ca 2+ sensitive protein was targeted to the mitochondria of adult rabbit ventricular cardiomyocytes using an adenovirus transfection technique. The probe (Mitycam) was a Ca 2+ -sensitive inverse pericam fused to subunit VIII of human cytochrome c oxidase. Mitycam expression pattern and Ca 2+ sensitivity was characterized in HeLa cells and isolated adult rabbit cardiomyocytes. Cardiomyocytes expressing Mitycam were voltage-clamped and depolarized at regular intervals to elicit a Ca 2+ transient. Cytoplasmic (Fura-2) and mitochondrial Ca 2+ (Mitycam) fluorescence were measured simultaneously under a range of cellular Ca 2+ loads. After 48 h post-adenoviral transfection, Mitycam expression showed a characteristic localization pattern in HeLa cells and cardiomyocytes. The Ca 2+ sensitive component of Mitycam fluorescence was 12% of total fluorescence in HeLa cells with a K d of ∼ 220 nM. In cardiomyocytes, basal and beat-to-beat changes in Mitycam fluorescence were detected on initiation of a train of depolarizations. Time to peak of the mitochondrial Ca 2+ transient was slower, but the rate of decay was faster than the cytoplasmic signal. During spontaneous Ca 2+ release the relative amplitude and the time course of the mitochondrial and cytoplasmic signals were comparable. Inhibition of mitochondrial respiration decreased the mitochondrial transient amplitude by ∼ 65% and increased the time to 50% decay, whilst cytosolic Ca 2+ transients were unchanged. The mitochondrial Ca 2+ uniporter (mCU) inhibitor Ru360 prevented both the basal and transient components of the rise in mitochondrial Ca 2+ . The mitochondrialtargeted Ca 2+ probe indicates sustained and transient phases of mitochondrial Ca 2+ signal, which are dependent on cytoplasmic Ca 2+ levels and require a functional mCU.
Circulation Research, 2012
Rationale: Mitochondrial Ca 2؉ uptake is essential for the bioenergetic feedback response through stimulation of Krebs cycle dehydrogenases. Close association of mitochondria to the sarcoplasmic reticulum (SR) may explain efficient mitochondrial Ca 2؉ uptake despite low Ca 2؉ affinity of the mitochondrial Ca 2؉ uniporter. However, the existence of such mitochondrial Ca 2؉ microdomains and their functional role are presently unresolved. Mitofusin (Mfn) 1 and 2 mediate mitochondrial outer membrane fusion, whereas Mfn2 but not Mfn1 tethers endoplasmic reticulum to mitochondria in noncardiac cells.
Pflügers Archiv - European Journal of Physiology, 2021
Ca 2+ cycling plays a critical role in regulating cardiomyocyte (CM) function under both physiological and pathological conditions. Mitochondria have been implicated in Ca 2+ handling in adult cardiomyocytes (ACMs). However, little is known about their role in the regulation of Ca 2+ dynamics in human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs). In the present study, we developed a multifunctional genetically-encoded Ca 2+ probe capable of simultaneously measuring cytosolic and mitochondrial Ca 2+ in real time. Using this novel probe, we determined and compared mitochondrial Ca 2+ activity and the coupling with cytosolic Ca 2+ dynamics in hiPSC-CMs and ACMs. Our data showed that while ACMs displayed a highly coordinated beat-by-beat response in mitochondrial Ca 2+ in sync with cytosolic Ca 2+ ,whereas hiPSC-CMs showed high cell-wide variability in mitochondrial Ca 2+ activity that is poorly coordinated with cytosolic Ca 2+. We then revealed that mitochondrial-sarcoplasmic reticulum (SR) tethering, as well as the inter-mitochondrial network connection, are underdeveloped in hiPSC-CM compared to ACM, which may underlie the observed spatiotemporal decoupling between cytosolic and mitochondrial Ca 2+ dynamics. Finally, we showed that knockdown of mitofusin-2 (Mfn2), a protein tethering mitochondria and SR, led to reduced cytosolic-mitochondrial Ca 2+ Terms of use and reuse: academic research for non-commercial purposes, see here for full terms.
Inhibition by Sr2+ of specific mitochondrial Ca2+-efflux pathways
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1983
The effect of Sr 2+ on the set point for external Ca 2+ was studied in rat heart and liver mitochondria with the aid of a Ca2+-sensitive electrode. In respiring mitochondria the set point is determined by the rates of Ca 2+ influx on the Ca 2+ uniporter and efflux by various mechanisms. We studied the Ca2+-Na + exchange pathway in heart mitochondria and the A~-modulated efflux pathway in liver mitochondria. Prior accumulation of Sr 2+ was found to shift the set points towards lower external Ca 2+ both in heart mitochondria under conditions of Ca2+-Na + exchange and in liver mitochondria under conditions that should promote opening of the A~k-modulated pathway. The effect on the set point was found to be due to inhibition of Ca 2+ efflux by Sr 2+ taken up by the mitochondria, while Sr 2+ efflux was too slow to be measurable.
Cardiovascular Research, 2010
Mitochondrial cardiomyopathy is associated with deleterious remodelling of cardiomyocyte Ca 2+ signalling that is partly due to the suppressed expression of the sarcoplasmic reticulum (SR) Ca 2+ buffer calsequestrin (CASQ2). This study was aimed at determining whether CASQ2 downregulation is directly caused by impaired mitochondrial function. Methods and results Mitochondrial stress was induced in cultured neonatal rat cardiomyocytes by means of the mitochondrial uncoupler carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP). Ca 2+ transients and reactive oxygen species (ROS) were measured by confocal microscopy using the indicators fluo-4 and MitoSOX red, respectively. Mitochondrial stress led to concentration-dependent downregulation of calsequestrin (CASQ2) and changes in the Ca 2+ signals of the cardiomyocytes that were accompanied by reduction in SR Ca 2+ content and amplitude and duration of Ca 2+ sparks. Caspase 3, p38, and p53 inhibitors had no effect on FCCP-induced CASQ2 downregulation; however, it was attenuated by the ROS scavenger N-acetylcysteine (NAC). Importantly, NAC not only decreased FCCP-induced ROS production, but it also restored the Ca 2+ signals, SR Ca 2+ content, and Ca 2+ spark properties to control levels. Conclusion Mitochondrial uncoupling results in fast transcriptional changes in CASQ2 expression that manifest as compromised Ca 2+ signalling, and these changes can be prevented by ROS scavengers. As impaired mitochondrial function has been implicated in several cardiac pathologies as well as in normal ageing, the mechanisms described here might be involved in a wide spectrum of cardiac conditions.
Circulation Research, 2012
Rationale: Mitochondrial [Ca 2+ ] ([Ca 2+ ] mito ) regulates mitochondrial energy production, provides transient Ca 2+ buffering under stress, and can be involved in cell death. Mitochondria are near the sarcoplasmic reticulum (SR) in cardiac myocytes, and evidence for crosstalk exists. However, quantitative measurements of [Ca 2+ ] mito are limited, and spatial [Ca 2+ ] mito gradients have not been directly measured. Objective: To directly measure local [Ca 2+ ] mito during normal SR Ca release in intact myocytes, and evaluate potential subsarcomeric spatial [Ca 2+ ] mito gradients. Methods and Results: Using the mitochondrially targeted inverse pericam indicator Mitycam, calibrated in situ, we directly measured [Ca 2+ ] mito during SR Ca 2+ release in intact rabbit ventricular myocytes by confocal microscopy. During steady state pacing, Δ[Ca 2+ ] mito amplitude was 29±3 nmol/L, rising rapidly (similar to cytosolic free [Ca 2+ ]) but declining much more slowly. Taking advantage of ...