Switch from ER-mitochondrial to SR-mitochondrial calcium coupling during muscle differentiation

Supralinear Dependence of the IP3 Receptor-to-Mitochondria Local Ca2+ Transfer on the Endoplasmic Reticulum Ca2+ Loading

Calcium signal propagation from endoplasmic reticulum (ER) to mitochondria regulates a multitude of mitochondrial and cell functions, including oxidative ATP production and cell fate decisions. Ca2+ transfer is optimal at the ER-mitochondrial contacts, where inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) can locally expose the mitochondrial Ca2+ uniporter (mtCU) to high [Ca2+] nanodomains. The Ca2+ loading state of the ER (Ca2 + ER) can vary broadly in physiological and pathological scenarios, however, the correlation between Ca2 + ER and the local Ca2+ transfer is unclear. Here, we studied IP3-induced Ca2+ transfer to mitochondria at different Ca2 + ER in intact and permeabilized RBL-2H3 cells via fluorescence measurements of cytoplasmic [Ca2+] ([Ca2+]c) and mitochondrial matrix [Ca2+] ([Ca2+]m). Preincubation of intact cells in high versus low extracellular [Ca2+] caused disproportionally greater increase in [Ca2+]m than [Ca2+]c responses to IP3-mobilizing agonist. Increasing Ca2 + ER by small Ca2+ boluses in suspensions of permeabilized cells supralinearly enhanced the mitochondrial Ca2+ uptake from IP3-induced Ca2+ release. The IP3-induced local [Ca2+] spikes exposing the mitochondrial surface measured using a genetically targeted sensor appeared to linearly correlate with Ca2 + ER, indicating that amplification happened in the mitochondria. Indeed, overexpression of an EF-hand deficient mutant of the mtCU gatekeeper MICU1 reduced the cooperativity of mitochondrial Ca2+ uptake. Interestingly, the IP3-induced [Ca2+]m signal plateaued at high Ca2 + ER, indicating activation of a matrix Ca2+ binding/chelating species. Mitochondria thus seem to maintain a "working [Ca2+]m range" via a low-affinity and high-capacity buffer species, and the ER loading steeply enhances the IP3R-linked [Ca2+]m signals in this working range.

Characterization of differential distribution patterns between mitofusin isoforms and their interaction in developing skeletal muscles of rat

Skeletal muscle during postnatal development undergoes several structural and biochemical modifications. It is proposed that these changes are closely intertwined with the increase in load-bearing capacity of the muscle (i.e., myofibrils) and molecular machinery to support the energy demand (i.e., mitochondria). Concomitant establishment of the sarcoplasmic reticulum (SR) and mitochondrial network seems to be a major developmental adjustment of skeletal muscle leading to adult phenotype. Here, we have studied oxidativeness, vascularization, and the changes in mitofusins (Mfn) 1-Mfn 2 expression and interaction in the due course of muscle development. Toward this, we used a series of histochemical techniques to compare neonatal and adult limb muscles (Gastrocnemius and Quadriceps) of Wistar rat (Rattus norvegicus). Additionally, we probed the proximity between Mfn 1 and Mfn 2 using a highly sensitive antibody-based proximity ligation assay indicating the change in mitochondrial fusion pattern or mitochondria-SR interaction. The results show that neonatal fibers bear a uniform distribution of mitochondria while a differential pattern of distribution is seen in adults. The distribution of the blood vessels is also quite distinct in adult muscles with a well-formed capillary network but in neonates, only central blood vessels are seen. Interestingly, our Mfn 1-Mfn 2 interaction data show that this interaction is uniformly distributed throughout the neonatal fibers, while it becomes peripherally localized in fibers of adult muscles. This peripheralization of Mfn 1-Mfn 2 interaction must be an important event of muscle development and might be critical to cater to the metabolic needs of adult muscle.

The retinoic acid response is a minor component of the cardiac phenotype in H9c2 myoblast differentiation

The H9c2 myoblast cell line, isolated from the left ventricular tissue of rat, is currently used in vitro as a mimetic for skeletal and cardiac muscle due to its biochemical, morphological, and electrical/hormonal signaling properties. During culture, H9c2 cells acquire a myotube phenotype, where a critical component is the inclusion of retinoic acid (RA). The results from some authors on H9c2 suggested that thousands of genes respond to RA stimuli, while others report hundreds of genes responding to RA over different cell types. In this article, using a more appropriate experimental design, we first confirm the H9c2 cardiac phenotype with and without RA and report transcriptomic and physiological changes regarding calcium handling, bioenergetics, and other biological concepts. Interestingly, of the 2360 genes showing a transcriptional change, 622 genes were statistically associated with the RA response. Of these genes, only 305 were RA-specific, and the rest also showed a culture-time component. Thus, the major expression changes (from 74 to 87%) were indeed due to culture conditions over time. Unexpectedly, only a few components of the retinol pathway in KEGG responded to RA. Our results show the role of RA in the H9c2 cultures impacting the interpretation using H9c2 as an in vitro model.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Simultaneous Measurement of Changes in Mitochondrial and Endoplasmic Reticulum Free Calcium in Pancreatic Beta Cells

The free calcium (Ca2+) levels in pancreatic beta cell organelles have been the subject of many recent investigations. Under pathophysiological conditions, disturbances in these pools have been linked to altered intracellular communication and cellular dysfunction. To facilitate studies of subcellular Ca2+ signaling in beta cells and, particularly, signaling between the endoplasmic reticulum (ER) and mitochondria, we designed a novel dual Ca2+ sensor which we termed DS-1. DS-1 encodes two stoichiometrically fluorescent proteins within a single plasmid, G-CEPIA-er, targeted to the ER and R-CEPIA3-mt, targeted to mitochondria. Our goal was to simultaneously measure the ER and mitochondrial Ca2+ in cells in real time. The Kds of G-CEPIA-er and R-CEPIA3-mt for Ca2+ are 672 and 3.7 μM, respectively. Confocal imaging of insulin-secreting INS-1 832/13 expressing DS-1 confirmed that the green and red fluorophores correctly colocalized with organelle-specific fluorescent markers as predicted. Further, we tested whether DS-1 exhibited the functional properties expected by challenging an INS-1 cell to glucose concentrations or drugs having well-documented effects on the ER and mitochondrial Ca2+ handling. The data obtained were consistent with those seen using other single organelle targeted probes. These results taken together suggest that DS-1 is a promising new approach for investigating Ca2+ signaling within multiple organelles of the cell.

Calcium and Reactive Oxygen Species Signaling Interplays in Cardiac Physiology and Pathologies

Mitochondria are key players in energy production, critical activity for the smooth functioning of energy-demanding organs such as the muscles, brain, and heart. Therefore, dysregulation or alterations in mitochondrial bioenergetics primarily perturb these organs. Within the cell, mitochondria are the major site of reactive oxygen species (ROS) production through the activity of different enzymes since it is one of the organelles with the major availability of oxygen. ROS can act as signaling molecules in a number of different pathways by modulating calcium (Ca²⁺) signaling. Interactions among ROS and calcium signaling can be considered bidirectional, with ROS regulating cellular Ca²⁺ signaling, whereas Ca²⁺ signaling is essential for ROS production. In particular, we will discuss how alterations in the crosstalk between ROS and Ca²⁺ can lead to mitochondrial bioenergetics dysfunctions and the consequent damage to tissues at high energy demand, such as the heart. Changes in Ca²⁺ can induce mitochondrial alterations associated with reduced ATP production and increased production of ROS. These changes in Ca²⁺ levels and ROS generation completely paralyze cardiac contractility. Thus, ROS can hinder the excitation-contraction coupling, inducing arrhythmias, hypertrophy, apoptosis, or necrosis of cardiac cells. These interplays in the cardiovascular system are the focus of this review.

The mitochondrial calcium homeostasis orchestra plays its symphony: Skeletal muscle is the guest of honor

Skeletal muscle mitochondria are placed in close proximity of the sarcoplasmic reticulum (SR), the main intracellular Ca²⁺ store. During muscle activity, excitation of sarcolemma and of T-tubule triggers the release of Ca²⁺ from the SR initiating myofiber contraction. The rise in cytosolic Ca²⁺ determines the opening of the mitochondrial calcium uniporter (MCU), the highly selective channel of the inner mitochondrial membrane (IMM), causing a robust increase in mitochondrial Ca²⁺ uptake. The Ca²⁺-dependent activation of TCA cycle enzymes increases the synthesis of ATP required for SERCA activity. Thus, Ca²⁺ is transported back into the SR and cytosolic [Ca²⁺] returns to resting levels eventually leading to muscle relaxation. In recent years, thanks to the molecular identification of MCU complex components, the role of mitochondrial Ca²⁺ uptake in the pathophysiology of skeletal muscle has been uncovered. In this chapter, we will introduce the reader to a general overview of mitochondrial Ca²⁺ accumulation. We will tackle the key molecular players and the cellular and pathophysiological consequences of mitochondrial Ca²⁺ dyshomeostasis. In the second part of the chapter, we will discuss novel findings on the physiological role of mitochondrial Ca²⁺ uptake in skeletal muscle. Finally, we will examine the involvement of mitochondrial Ca²⁺ signaling in muscle diseases.

The yin and yang of mitochondrial Ca2+ signaling in cell physiology and pathology

Mitochondria are autonomous and dynamic cellular organelles orchestrating a diverse range of cellular activities. Numerous cell-signaling pathways target these organelles and Ca²⁺ is one of the most significant. Mitochondria are able to rapidly and transiently take up Ca²⁺, thanks to the mitochondrial Ca²⁺ uniporter complex, as well as to extrude it through the Na⁺/Ca²⁺ and H⁺/Ca²⁺ exchangers. The transient accumulation of Ca²⁺ in the mitochondrial matrix impacts on mitochondrial functions and cell pathophysiology. Here we summarize the role of mitochondrial Ca²⁺ signaling in both physiological (yang) and pathological (yin) processes and the methods that can be used to investigate mitochondrial Ca²⁺ homeostasis. As an example of the pivotal role of mitochondria in pathology, we described the state of the art of mitochondrial Ca²⁺ alterations in different pathological conditions, with a special focus on Alzheimer's disease.

PDK4 Augments ER-Mitochondria Contact to Dampen Skeletal Muscle Insulin Signaling During Obesity

Mitochondria-associated endoplasmic reticulum membrane (MAM) is a structural link between mitochondria and endoplasmic reticulum (ER). MAM regulates Ca2+ transport from the ER to mitochondria via an IP3R1-GRP75-VDAC1 complex-dependent mechanism. Excessive MAM formation may cause mitochondrial Ca2+ overload and mitochondrial dysfunction. However, the exact implication of MAM formation in metabolic syndromes remains debatable. Here, we demonstrate that PDK4 interacts with and stabilizes the IP3R1-GRP75-VDAC1 complex at the MAM interface. Obesity-induced increase in PDK4 activity augments MAM formation and suppresses insulin signaling. Conversely, PDK4 inhibition dampens MAM formation and improves insulin signaling by preventing MAM-induced mitochondrial Ca2+ accumulation, mitochondrial dysfunction, and ER stress. Furthermore, Pdk4-/- mice exhibit reduced MAM formation and are protected against diet-induced skeletal muscle insulin resistance. Finally, forced formation and stabilization of MAMs with synthetic ER-mitochondria linker prevented the beneficial effects of PDK4 deficiency on insulin signaling. Overall, our findings demonstrate a critical mediatory role of PDK4 in the development of skeletal muscle insulin resistance via enhancement of MAM formation.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

Calcium, mitochondria and cell metabolism: A functional triangle in bioenergetics

The versatility of mitochondrial metabolism and its fine adjustments to specific physiological or pathological conditions regulate fundamental cell pathways, ranging from proliferation to apoptosis. In particular, Ca²⁺ signalling has emerged as a key player exploited by mitochondria to tune their activity according with cell demand. The functional interaction between mitochondria and endoplasmic reticulum (ER) deeply impacts on the correct mitochondrial Ca²⁺ signal, thus modulating cell bioenergetics and functionality. Indeed, Ca²⁺ released by the ER is taken up by mitochondria where, both in the intermembrane space and in the matrix, it regulates the activity of transporters, enzymes and proteins involved in organelles' metabolism. In this review, we will briefly summarize Ca²⁺-dependent mechanisms involved in the regulation of mitochondrial activity. Moreover, we will discuss some recent reports, in which alterations in mitochondrial Ca²⁺ signalling have been associated with specific pathological conditions, such as neurodegeneration and cancer.

Mitochondrial fusion dynamics is robust in the heart and depends on calcium oscillations and contractile activity

Mitochondrial fusion is thought to be important for supporting cardiac contractility, but is hardly detectable in cultured cardiomyocytes and is difficult to directly evaluate in the heart. We overcame this obstacle through in vivo adenoviral transduction with matrix-targeted photoactivatable GFP and confocal microscopy. Imaging in whole rat hearts indicated mitochondrial network formation and fusion activity in ventricular cardiomyocytes. Promptly after isolation, cardiomyocytes showed extensive mitochondrial connectivity and fusion, which decayed in culture (at 24-48 h). Fusion manifested both as rapid content mixing events between adjacent organelles and slower events between both neighboring and distant mitochondria. Loss of fusion in culture likely results from the decline in calcium oscillations/contractile activity and mitofusin 1 (Mfn1), because (i) verapamil suppressed both contraction and mitochondrial fusion, (ii) after spontaneous contraction or short-term field stimulation fusion activity increased in cardiomyocytes, and (iii) ryanodine receptor-2-mediated calcium oscillations increased fusion activity in HEK293 cells and complementing changes occurred in Mfn1. Weakened cardiac contractility in vivo in alcoholic animals is also associated with depressed mitochondrial fusion. Thus, attenuated mitochondrial fusion might contribute to the pathogenesis of cardiomyopathy.

Inhibiting c-Jun N-terminal kinase partially attenuates caffeine-dependent cell death without alleviating the caffeine-induced reduction in mitochondrial respiration in C2C12 skeletal myotubes

Caffeine is a widely consumed stimulant that has previously been shown to promote cytotoxic stress and even cell death in numerous mammalian cell lines. Thus far there is little information available regarding the toxicity of caffeine in skeletal muscle cells. Our preliminary data revealed that treating C2C12 myotubes with 5 mM caffeine for 6 h increased nuclear fragmentation and reduced basal and maximal oxygen consumption rate (OCR) in skeletal myotubes. The purpose of this study was to further elucidate the pathways by which caffeine increased cell death and reduced mitochondrial respiration. We specifically examined the role of c-Jun N-terminal kinase (JNK), which has previously been shown to simultaneously increase caspase-dependent cell death and reduce mitochondrial respiration in other mammalian cell lines. We found that caffeine promoted a dose-dependent increase in cell death in multinucleated myotubes but did not in mononucleated myoblasts. The addition of 10 μM Z-DEVD-FMK, a specific inhibitor of executioner caspases, completely inhibited caffeine-dependent cell death. Further, the addition of 400 μM dantrolene, a specific ryanodine receptor (RYR) inhibitor, prevented the caffeine-dependent increase in cell death and the reduction in basal and maximal OCR. We also discovered that caffeine treatment significantly increased the phosphorylation of JNK and that the addition of 30 μM SP600125 (JNKi), a specific JNK inhibitor, partially attenuated caffeine-induced cell death without preventing the caffeine-dependent reduction in basal and maximal OCR. Our results suggest that JNK partially mediates the increase in caspase-dependent cell death but does not contribute to reduced mitochondrial respiration in caffeine-treated skeletal muscle cells. We conclude that caffeine increased cell death and reduced mitochondrial respiration in a calcium-dependent manner by activating the RYR and promoting reticular calcium release.

Junctophilin 3 expresses in pancreatic beta cells and is required for glucose-stimulated insulin secretion

It is well accepted that junctophilin (JPHs) isoforms act as a physical bridge linking plasma membrane and endoplasmic reticulum (ER) for channel crosstalk in excitable cells. Our purpose is to investigate whether JPHs are involved in the proper communication between Ca(2+) influx and subsequent Ca(2+) amplification in pancreatic beta cells, thereby participating in regulating insulin secretion. The expression of JPH isoforms was examined in human and mouse pancreatic tissues, and JPH3 expression was found in both the beta cells. In mice, knockdown of Jph3 (si-Jph3) in islets decreased glucose-stimulated insulin secretion (GSIS) accompanied by mitochondrial function impairment. Si-Jph3 lowered the insulin secretory response to Ca(2+) signaling in the presence of glucose, and reduced [Ca(2+)]c transient amplitude triggered by caffeine. Si-Jph3 also attenuated mitofusin 2 expression, thereby disturbing the spatial organization of ER-mitochondria contact in islets. These results suggest that the regulation of GSIS by the KATP channel-independent pathways is partly impaired due to decrease of JPH3 expression in mouse islets. JPH3 also binds to type 2 ryanodine receptors (RyR2) in mouse and human pancreatic tissues, which might contribute to Ca(2+) release amplification in GSIS. This study demonstrates some previously unrecognized findings in pancreatic tissues: (1) JPH3 expresses in mouse and human beta cells; (2) si-Jph3 in mouse primary islets impairs GSIS in vitro; (3) impairment in GSIS in si-Jph3 islets is due to changes in RyR2-[Ca(2+)]c transient amplitude and ER-mitochondria contact.

Analysis of mitochondrial structure and function in the Drosophila larval musculature

Mitochondria are dynamic organelles that change their architecture in normal physiological conditions. Mutations in genes that control mitochondrial fission or fusion, such as dynamin-related protein (Drp1), Mitofusins 1 (Mfn1) and 2 (Mfn2), and Optic atrophy 1 (Opa1), result in neuropathies or neurodegenerative diseases. It is increasingly clear that altered mitochondrial dynamics also underlie the pathology of other degenerative diseases, including Parkinson's disease (PD). Thus, understanding mitochondrial distribution, shape, and dynamics in all cell types is a prerequisite for developing and defining treatment regimens that may differentially affect tissues. The majority of Drosophila genes implicated in mitochondrial dynamics have been studied in the adult indirect flight muscle (IFM). Here, we discuss the utility of Drosophila third instar larvae (L3) as an alternative model to analyze and quantify mitochondrial behaviors. Advantages include large muscle cell size, a stereotyped arrangement of mitochondria that is conserved in mammalian muscles, and the ability to analyze muscle-specific gene function in mutants that are lethal prior to adult stages. In particular, we highlight methods for sample preparation and analysis of mitochondrial morphological features.

In vivo calcium regulation in diabetic skeletal muscle

In skeletal muscle, dysfunctional contractile activity has been linked to impaired intracellular Ca(2+) concentration ([Ca(2+)]i) regulation. Muscle force production is impaired and fatigability and muscle fragility deteriorate with diabetes. Use of a novel in vivo model permits investigation of [Ca(2+)]i homeostasis in diabetic skeletal muscle. Within this in vivo environment we have shown that diabetes perturbs the Ca(2+) regulatory system such that resting [Ca(2+)]i homeostasis following muscle contractions is compromised and elevations of [Ca(2+)]i are exacerbated. This review considers the impact of diabetes on the capacity of skeletal muscle to regulate [Ca(2+)]i, following muscle contractions and, in particular, the relationship between muscle fatigue and elevated [Ca(2+)]i in a highly ecologically relevant circulation-intact environment. Importantly, the role of mitochondria in calcium sequestration and the possibility that diabetes impacts this process is explored. Given the profound microcirculatory dysfunction in diabetes this preparation offers the unique opportunity to study the interrelationships among microvascular function, blood-myocyte oxygen flux and [Ca(2+)]i as they relate to enhanced muscle fatigability and exercise intolerance.

Mitochondrial fusion is frequent in skeletal muscle and supports excitation-contraction coupling

Mitochondrial fusion is frequent in skeletal muscle, and its disruption jeopardizes excitation–contraction coupling and may contribute to the pathology of myopathies.

Genetic targeting experiments indicate a fundamental role for mitochondrial fusion proteins in mammalian physiology. However, owing to the multiple functions of fusion proteins, their related phenotypes are not necessarily caused by altered mitochondrial fusion. Perhaps the biggest mystery is presented by skeletal muscle, where mostly globular-shaped mitochondria are densely packed into the narrow intermyofilamental space, limiting the interorganellar interactions. We show here that mitochondria form local networks and regularly undergo fusion events to share matrix content in skeletal muscle fibers. However, fusion events are less frequent and more stable in the fibers than in nondifferentiated myoblasts. Complementation among muscle mitochondria was suppressed by both in vivo genetic perturbations and chronic alcohol consumption that cause myopathy. An Mfn1-dependent pathway is revealed whereby fusion inhibition weakens the metabolic reserve of mitochondria to cause dysregulation of calcium oscillations during prolonged stimulation. Thus, fusion dynamically connects skeletal muscle mitochondria and its prolonged loss jeopardizes bioenergetics and excitation–contraction coupling, providing a potential pathomechanism contributing to myopathies.

New functions of mitochondria associated membranes in cellular signaling

In all eukaryotic cells, the endoplasmic reticulum (ER) and the mitochondria establish a tight interplay, which is structurally and functionally modulated through a proteinaceous tether formed at specific subdomains of the ER membrane, designated mitochondria-associated membranes or MAMs. The tethering function of the MAMs allows the regulation of lipid synthesis and rapid transmission of calcium (Ca(2+)) signals between the ER and mitochondria, which is crucial to shape intracellular Ca(2+) signaling and regulate mitochondrial bioenergetics. Research on the molecular characterization and function of MAMs has boomed in the last few years and the list of signaling and structural proteins dynamically associated with the ER-mitochondria contact sites in physiological and pathological conditions, is rapidly increasing along with the realization of an unprecedented complexity underlying the functional role of MAMs. Besides their established role as a signaling hub for Ca(2+) and lipid transfer between ER and mitochondria, MAMs have been recently shown to regulate mitochondrial shape and motility, energy metabolism and redox status and to be central to the modulation of various key processes like ER stress, autophagy and inflammasome signaling. In this review we will discuss some emerging cell-autonomous and cell non-autonomous roles of the MAMs in mammalian cells and their relevance for important human diseases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Overexpression of ryanodine receptor type 1 enhances mitochondrial fragmentation and Ca2+-induced ATP production in cardiac H9c2 myoblasts

Ca(+) influx to mitochondria is an important trigger for both mitochondrial dynamics and ATP generation in various cell types, including cardiac cells. Mitochondrial Ca(2+) influx is mainly mediated by the mitochondrial Ca(2+) uniporter (MCU). Growing evidence also indicates that mitochondrial Ca(2+) influx mechanisms are regulated not solely by MCU but also by multiple channels/transporters. We have previously reported that skeletal muscle-type ryanodine receptor (RyR) type 1 (RyR1), which expressed at the mitochondrial inner membrane, serves as an additional Ca(2+) uptake pathway in cardiomyocytes. However, it is still unclear which mitochondrial Ca(2+) influx mechanism is the dominant regulator of mitochondrial morphology/dynamics and energetics in cardiomyocytes. To investigate the role of mitochondrial RyR1 in the regulation of mitochondrial morphology/function in cardiac cells, RyR1 was transiently or stably overexpressed in cardiac H9c2 myoblasts. We found that overexpressed RyR1 was partially localized in mitochondria as observed using both immunoblots of mitochondrial fractionation and confocal microscopy, whereas RyR2, the main RyR isoform in the cardiac sarcoplasmic reticulum, did not show any expression at mitochondria. Interestingly, overexpression of RyR1 but not MCU or RyR2 resulted in mitochondrial fragmentation. These fragmented mitochondria showed bigger and sustained mitochondrial Ca(2+) transients compared with basal tubular mitochondria. In addition, RyR1-overexpressing cells had a higher mitochondrial ATP concentration under basal conditions and showed more ATP production in response to cytosolic Ca(2+) elevation compared with nontransfected cells as observed by a matrix-targeted ATP biosensor. These results indicate that RyR1 possesses a mitochondrial targeting/retention signal and modulates mitochondrial morphology and Ca(2+)-induced ATP production in cardiac H9c2 myoblasts.

Role of nociceptor αCaMKII in transition from acute to chronic pain (hyperalgesic priming) in male and female rats

We have previously shown that activation of protein kinase Cε (PKCε) in male rats induces a chronic, long-lasting change in nociceptors such that a subsequent exposure to proinflammatory mediators produces markedly prolonged mechanical hyperalgesia. This neuroplastic change, hyperalgesic priming, is dependent on activation of cytoplasmic polyadenylation element-binding protein (CPEB), downstream of PKCε, and consequent translation of mRNAs in the peripheral terminal of the nociceptor. Since α calmodulin-dependent protein kinase II (αCaMKII), a molecule implicated in neuroplasticity, is a target of CPEB and can also affect CPEB function, we investigated its role in the transition from acute to chronic pain. Priming induced by direct activation of PKCε can be prevented by inhibition of αCaMKII. In addition, direct activation of αCaMKII induces priming, which was not prevented by pretreatment with PKCε antisense, suggesting that αCaMKII is downstream of PKCε in the induction of priming. Activation of ryanodine receptors (RyRs), which can lead to activation of αCaMKII, also induced priming, in a calcium- and αCaMKII-dependent manner. Similarly, inhibition of the RyR and a calcium buffer prevented induction of priming by PKCε. Unlike activation of PKCε, ryanodine and αCaMKII induced priming in female as well as male rats. Our results demonstrate a contribution of αCaMKII to induction of hyperalgesic priming, a phenomenon implicated in the transition from acute to chronic pain.

In vivo imaging of intracellular Ca2+ after muscle contractions and direct Ca2+ injection in rat skeletal muscle in diabetes

The effects of muscle contractions on the profile of postcontraction resting intracellular Ca2+ ([Ca2+]i) accumulation in Type 1 diabetes are unclear. We tested the hypothesis that, following repeated bouts of muscle contractions, the rise in resting [Ca2+]i evident in healthy rats would be increased in diabetic rats and that these changes would be associated with a decreased cytoplasmic Ca2+ -buffering capacity. Adult male Wistar rats were divided randomly into diabetic (DIA; streptozotocin, ip) and healthy control (CONT) groups. Four weeks later, animals were anesthetized and spinotrapezius muscle contractions (10 sets of 50 contractions) were elicited by electrical stimulation (100 Hz). Ca2+ imaging was achieved using Fura-2 AM in the spinotrapezius muscle in vivo (i.e., circulation intact). The ratio (340/380 nm) was determined from fluorescence images following each set of contractions for estimation of [Ca2+]i. Also, muscle Ca2+ buffering was studied in individual myocytes microinjected with 2 mM Ca2+ solution. After muscle contractions, resting [Ca2+]i in DIA increased earlier and more rapidly than in CONT (P < 0.05 vs. precontraction). Peak [Ca2+]i in response to the Ca2+ injection was significantly higher in CONT (25.8 ± 6.0% above baseline) than DIA (10.2 ± 1.1% above baseline). Subsequently, CONT [Ca(2+)]i decreased rapidly (<15 s) to plateau 9-10% above baseline, whereas DIA remained elevated throughout the 60-s measurement window. No differences in SERCA1 and SERCA2 (Ca2+ uptake) protein levels were evident between CONT and DIA, whereas ryanodine receptor (Ca2+ release) protein level and mitochondrial oxidative enzyme activity (succinate dehydrogenase) were decreased in DIA (P < 0.05). In conclusion, diabetes impairs resting [Ca2+]i homeostasis following muscle contractions. Markedly different responses to Ca2+ injection in DIA vs. CONT suggest fundamentally deranged Ca2+ handling.

Interactions between sarco-endoplasmic reticulum and mitochondria in cardiac and skeletal muscle - pivotal roles in Ca²⁺ and reactive oxygen species signaling

Mitochondria are strategically and dynamically positioned in the cell to spatially coordinate ATP production with energy needs and to allow the local exchange of material with other organelles. Interactions of mitochondria with the sarco-endoplasmic reticulum (SR/ER) have been receiving much attention owing to emerging evidence on the role these sites have in cell signaling, dynamics and biosynthetic pathways. One of the most important physiological and pathophysiological paradigms for SR/ER-mitochondria interactions is in cardiac and skeletal muscle. The contractile activity of these tissues has to be matched by mitochondrial ATP generation that is achieved, at least in part, by propagation of Ca(2+) signals from SR to mitochondria. However, the muscle has a highly ordered structure, providing only limited opportunity for mitochondrial dynamics and interorganellar interactions. This Commentary focuses on the latest advances in the structure, function and disease relevance of the communication between SR/ER and mitochondria in muscle. In particular, we discuss the recent demonstration of SR/ER-mitochondria tethers that are formed by multiple proteins, and local Ca(2+) transfer between SR/ER and mitochondria.