A bibliometric analysis: Ca2+ fluxes and inflammatory phenotyping by flow cytometry in peripheral blood mononuclear cells.

Background: The immune system, composed of organs, tissues, cells, and proteins, is the key to protecting the body from external biological attacks and inflammation. The latter occurs in several pathologies, such as cancers, type 1 diabetes, and human immunodeficiency virus infection. Immunophenotyping by flow cytometry is the method of choice for diagnosing these pathologies. Under inflammatory conditions, the peripheral blood mononuclear cells (PBMCs) are partially activated and generate intracellular pathways involving Ca2+-dependent signaling cascades leading to transcription factor expression. Ca2+ signaling is typically studied by microscopy in cell lines but can present some limitations to explore human PBMCs, where flow cytometry can be a good alternative. Objective: In this review, we dived into the research field of inflammation and Ca2+ signaling in PBMCs. We aimed to investigate the structure and evolution of this field in a physio-pathological context, and then we focused our review on flow cytometry analysis of Ca2+ fluxes in PBMCs. Methods: From 1984 to 2022, 3865 articles on inflammation and Ca2+ signaling in PBMCs were published, according to The Clarivate Web of Science (WOS) database used in this review. A bibliometric study was designed for this collection and consisted of a co-citation and bibliographic coupling analysis. Results: The co-citation analysis was performed on 133 articles: 4 clusters highlighted the global context of Ca2+ homeostasis, including chemical probe development, identification of the leading players in Ca2+ signaling, and the link with chemokine production in immune cell function. Next, the bibliographic coupling analysis combined 998 articles in 8 clusters. This analysis outlined the mechanisms of PBMC activation, from signal integration to cellular response. Further explorations of the bibliographic coupling network, focusing on flow cytometry, revealed 21 articles measuring cytosolic Ca2+ in PBMCs, with only 5 since 2016. This final query showed that Ca2+ signaling analysis in human PBMCs using flow cytometry is still underdeveloped and investigates mainly the cytosolic Ca2+ compartment. Conclusion: Our review uncovers remaining knowledge gaps of intracellular players involved in Ca2+ signaling in PBMCs, such as reticulum and mitochondria, and presents flow cytometry as a solid option to supplement gold-standard microscopy studies.

Aging is associated with impaired triggering of TRPV3-mediated cutaneous vasodilation: a crucial process for local heat exposure.

Sensing temperature is vitally important to adapt our body to environmental changes. Local warm detection is required to initiate regulation of cutaneous blood flow, which is part of the peripheral thermoregulatory mechanisms, and thus avoid damage to surrounding tissues. The mechanisms mediating cutaneous vasodilation during local heat stress are impaired with aging. However, the impact of aging on the ability of the skin to detect subtle thermal changes is unknown. Among heat-activated cation channels, transient receptor potential vanilloid 3 (TRPV3) is a thermo-sensor predominantly expressed on keratinocytes and involved in local vascular thermoregulatory mechanisms of the skin in young mice. In the present study, using a murine in vivo model of local heat exposure of the skin, we showed that heat-induced vasodilation was reduced in old mice associated with reduced expression of TRPV3 channels. We also found a decrease in expression and activity of TRPV3 channel, as well as reduced TRPV3-dependent adenosine tri-phosphate release in human primary keratinocytes from old donors. This study shows that aging alters the epidermal TRPV3 channels, which might delay the detection of changes in skin temperature, thereby limiting the mechanisms triggered for local vascular thermoregulation in the old skin.

TRPV1 Channels Are New Players in the Reticulum-Mitochondria Ca2+ Coupling in a Rat Cardiomyoblast Cell Line.

The Ca2+ release in microdomains formed by intercompartmental contacts, such as mitochondria-associated endoplasmic reticulum membranes (MAMs), encodes a signal that contributes to Ca2+ homeostasis and cell fate control. However, the composition and function of MAMs remain to be fully defined. Here, we focused on the transient receptor potential vanilloid 1 (TRPV1), a Ca2+-permeable ion channel and a polymodal nociceptor. We found TRPV1 channels in the reticular membrane, including some at MAMs, in a rat cardiomyoblast cell line (SV40-transformed H9c2) by Western blotting, immunostaining, cell fractionation, and proximity ligation assay. We used chemical and genetic probes to perform Ca2+ imaging in four cellular compartments: the endoplasmic reticulum (ER), cytoplasm, mitochondrial matrix, and mitochondrial surface. Our results showed that the ER Ca2+ released through TRPV1 channels is detected at the mitochondrial outer membrane and transferred to the mitochondria. Finally, we observed that prolonged TRPV1 modulation for 30 min alters the intracellular Ca2+ equilibrium and influences the MAM structure or the hypoxia/reoxygenation-induced cell death. Thus, our study provides the first evidence that TRPV1 channels contribute to MAM Ca2+ exchanges.

SERCA2 phosphorylation at serine 663 is a key regulator of Ca2+ homeostasis in heart diseases.

Despite advances in cardioprotection, new therapeutic strategies capable of preventing ischemia-reperfusion injury of patients are still needed. Here, we discover that sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA2) phosphorylation at serine 663 is a clinical and pathophysiological event of cardiac function. Indeed, the phosphorylation level of SERCA2 at serine 663 is increased in ischemic hearts of patients and mouse. Analyses on different human cell lines indicate that preventing serine 663 phosphorylation significantly increases SERCA2 activity and protects against cell death, by counteracting cytosolic and mitochondrial Ca2+ overload. By identifying the phosphorylation level of SERCA2 at serine 663 as an essential regulator of SERCA2 activity, Ca2+ homeostasis and infarct size, these data contribute to a more comprehensive understanding of the excitation/contraction coupling of cardiomyocytes and establish the pathophysiological role and the therapeutic potential of SERCA2 modulation in acute myocardial infarction, based on the hotspot phosphorylation level of SERCA2 at serine 663 residue.

Expression of the Calcium-Binding Protein CALB1 Is Induced and Controls Intracellular Ca2+ Levels in Senescent Cells.

In response to many stresses, such as oncogene activation or DNA damage, cells can enter cellular senescence, a state of proliferation arrest accompanied by a senescence-associated secretory phenotype (SASP). Cellular senescence plays a key role in many physiopathological contexts, including cancer, aging and aging-associated diseases, therefore, it is critical to understand how senescence is regulated. Calcium ions (Ca2+) recently emerged as pivotal regulators of cellular senescence. However, how Ca2+ levels are controlled during this process is barely known. Here, we report that intracellular Ca2+ contents increase in response to many senescence inducers in immortalized human mammary epithelial cells (HMECs) and that expression of calbindin 1 (CALB1), a Ca2+-binding protein, is upregulated in this context, through the Ca2+-dependent calcineurin/NFAT pathway. We further show that overexpression of CALB1 buffers the rise in intracellular Ca2+ levels observed in senescent cells. Finally, we suggest that increased expression of Ca2+-binding proteins calbindins is a frequent mark of senescent cells. This work thus supports that, together with Ca2+channels, Ca2+-binding proteins modulate Ca2+ levels and flux during cellular senescence. This opens potential avenues of research to better understand the role of Ca2+ and of Ca2+-binding proteins in regulating cellular senescence.

NF-κB-dependent secretome of senescent cells can trigger neuroendocrine transdifferentiation of breast cancer cells.

Cellular senescence is characterized by a stable proliferation arrest in response to stresses and the acquisition of a senescence-associated secretory phenotype, called SASP, composed of numerous factors including pro-inflammatory molecules, proteases, and growth factors. The SASP affects the environment of senescent cells, especially during aging, by inducing and modulating various phenotypes such as paracrine senescence, immune cell activity, and extracellular matrix deposition and organization, which critically impact various pathophysiological situations, including fibrosis and cancer. Here, we uncover a novel paracrine effect of the SASP: the neuroendocrine transdifferentiation (NED) of some epithelial cancer cells, evidenced both in the breast and prostate. Mechanistically, this effect is mediated by NF-κB-dependent SASP factors, and leads to an increase in intracellular Ca2+ levels. Consistently, buffering Ca2+ by overexpressing the CALB1 buffering protein partly reverts SASP-induced NED, suggesting that the SASP promotes NED through a SASP-induced Ca2+ signaling. Human breast cancer dataset analyses support that NED occurs mainly in p53 WT tumors and in older patients, in line with a role of senescent cells and its secretome, as they are increasing during aging. In conclusion, our work, uncovering SASP-induced NED in some cancer cells, paves the way for future studies aiming at better understanding the functional link between senescent cell accumulation during aging, NED and clinical patient outcome.

Impaired aerobic capacity and premature fatigue preceding muscle weakness in the skeletal muscle Tfam-knockout mouse model.

Mitochondrial diseases are genetic disorders that lead to impaired mitochondrial function, resulting in exercise intolerance and muscle weakness. In patients, muscle fatigue due to defects in mitochondrial oxidative capacities commonly precedes muscle weakness. In mice, deletion of the fast-twitch skeletal muscle-specific Tfam gene (Tfam KO) leads to a deficit in respiratory chain activity, severe muscle weakness and early death. Here, we performed a time-course study of mitochondrial and muscular dysfunctions in 11- and 14-week-old Tfam KO mice, i.e. before and when mice are about to enter the terminal stage, respectively. Although force in the unfatigued state was reduced in Tfam KO mice compared to control littermates (wild type) only at 14 weeks, during repeated submaximal contractions fatigue was faster at both ages. During fatiguing stimulation, total phosphocreatine breakdown was larger in Tfam KO muscle than in wild-type muscle at both ages, whereas phosphocreatine consumption was faster only at 14 weeks. In conclusion, the Tfam KO mouse model represents a reliable model of lethal mitochondrial myopathy in which impaired mitochondrial energy production and premature fatigue occur before muscle weakness and early death.

Reduced reticulum-mitochondria Ca2+ transfer is an early and reversible trigger of mitochondrial dysfunctions in diabetic cardiomyopathy.

Type 2 diabetic cardiomyopathy features Ca2+ signaling abnormalities, notably an altered mitochondrial Ca2+ handling. We here aimed to study if it might be due to a dysregulation of either the whole Ca2+ homeostasis, the reticulum-mitochondrial Ca2+ coupling, and/or the mitochondrial Ca2+ entry through the uniporter. Following a 16-week high-fat high-sucrose diet (HFHSD), mice developed cardiac insulin resistance, fibrosis, hypertrophy, lipid accumulation, and diastolic dysfunction when compared to standard diet. Ultrastructural and proteomic analyses of cardiac reticulum-mitochondria interface revealed tighter interactions not compatible with Ca2+ transport in HFHSD cardiomyocytes. Intramyocardial adenoviral injections of Ca2+ sensors were performed to measure Ca2+ fluxes in freshly isolated adult cardiomyocytes and to analyze the direct effects of in vivo type 2 diabetes on cardiomyocyte function. HFHSD resulted in a decreased IP3R-VDAC interaction and a reduced IP3-stimulated Ca2+ transfer to mitochondria, with no changes in reticular Ca2+ level, cytosolic Ca2+ transients, and mitochondrial Ca2+ uniporter function. Disruption of organelle Ca2+ exchange was associated with decreased mitochondrial bioenergetics and reduced cell contraction, which was rescued by an adenovirus-mediated expression of a reticulum-mitochondria linker. An 8-week diet reversal was able to restore cardiac insulin signaling, Ca2+ transfer, and cardiac function in HFHSD mice. Therefore, our study demonstrates that the reticulum-mitochondria Ca2+ miscoupling may play an early and reversible role in the development of diabetic cardiomyopathy by disrupting primarily the mitochondrial bioenergetics. A diet reversal, by counteracting the MAM-induced mitochondrial Ca2+ dysfunction, might contribute to restore normal cardiac function and prevent the exacerbation of diabetic cardiomyopathy.

The Contractile Phenotype of Skeletal Muscle in TRPV1 Knockout Mice is Gender-Specific and Exercise-Dependent.

The transient receptor potential vanilloid 1 (TRPV1) belongs to the transient receptor potential superfamily of sensory receptors. TRPV1 is a non-selective cation channel permeable to Ca2+ that is capable of detecting noxious heat temperature and acidosis. In skeletal muscles, TRPV1 operates as a reticular Ca2+-leak channel and several TRPV1 mutations have been associated with two muscle disorders: malignant hyperthermia (MH) and exertional heat stroke (EHS). Although TRPV1-/- mice have been available since the 2000s, TRPV1's role in muscle physiology has not been thoroughly studied. Therefore, the focus of this work was to characterize the contractile phenotype of skeletal muscles of TRPV1-deficient mice at rest and after four weeks of exercise. As MS and EHS have a higher incidence in men than in women, we also investigated sex-related phenotype differences. Our results indicated that, without exercise, TRPV1-/- mice improved in vivo muscle strength with an impairment of skeletal muscle in vitro twitch features, i.e., delayed contraction and relaxation. Additionally, exercise appeared detrimental to TRPV1-/- slow-twitch muscles, especially in female animals.

Acute Induction of Translocon-Mediated Ca2+ Leak Protects Cardiomyocytes Against Ischemia/Reperfusion Injury.

During myocardial infarction, dysregulation of Ca2+ homeostasis between the reticulum, mitochondria, and cytosol occurs in cardiomyocytes and leads to cell death. Ca2+ leak channels are thought to be key regulators of the reticular Ca2+ homeostasis and cell survival. The present study aimed to determine whether a particular reticular Ca2+ leak channel, the translocon, also known as translocation channel, could be a relevant target against ischemia/reperfusion-mediated heart injury. To achieve this objective, we first used an intramyocardial adenoviral strategy to express biosensors in order to assess Ca2+ variations in freshly isolated adult mouse cardiomyocytes to show that translocon is a functional reticular Ca2+ leak channel. Interestingly, translocon activation by puromycin mobilized a ryanodine receptor (RyR)-independent reticular Ca2+ pool and did not affect the excitation-concentration coupling. Second, puromycin pretreatment decreased mitochondrial Ca2+ content and slowed down the mitochondrial permeability transition pore (mPTP) opening and the rate of cytosolic Ca2+ increase during hypoxia. Finally, this translocon pre-activation also protected cardiomyocytes after in vitro hypoxia reoxygenation and reduced infarct size in mice submitted to in vivo ischemia-reperfusion. Altogether, our report emphasizes the role of translocon in cardioprotection and highlights a new paradigm in cardioprotection by functionally uncoupling the RyR-dependent Ca2+ stores and translocon-dependent Ca2+ stores.

Variations in the TRPV1 gene are associated to exertional heat stroke.

OBJECTIVES: Exertional Heat Stroke (EHS) is one of the top three causes of sudden death in athletes. Extrinsic and intrinsic risk factors have been identified but the genetic causes still remain unclear. Our aim was to identify genes responsible for EHS, which is a necessary step to identify patients at risk and prevent crises. DESIGN: Genetic and functional laboratory studies METHODS: Whole Exome Sequencing (WES) was performed to search for candidate genes in a cohort of 15 soldiers who had a documented EHS episode. In silico and in vitro functional studies were performed to evaluate the effect of mutations identified in the candidate gene TRPV1. RESULTS: WES led to the identification of two missense variations in the TRPV1 gene. These variations were very rare or unreported in control databases and located in critical domains of the protein. In vitro functional studies revealed that both variations induce a strong modification of the channel response to one of its natural agonist, the capsaicin. CONCLUSIONS: We evidenced mutations altering channel properties of the TRPV1 gene and demonstrated that TRPV1, which is involved in thermoregulation and nociception, is a new candidate gene for EHS. Our data provide the bases to explore genetic causes and molecular mechanisms governing the pathophysiology of EHS.

Differential Effect of Glucose on ER-Mitochondria Ca2+ Exchange Participates in Insulin Secretion and Glucotoxicity-Mediated Dysfunction of ß-Cells.

Glucotoxicity-induced ß-cell dysfunction in type 2 diabetes is associated with alterations of mitochondria and the endoplasmic reticulum (ER). Both organelles interact at contact sites, defined as mitochondria-associated membranes (MAMs), which were recently implicated in the regulation of glucose homeostasis. The role of MAMs in ß-cells is still largely unknown, and their implication in glucotoxicity-associated ß-cell dysfunction remains to be defined. Here, we report that acute glucose treatment stimulated ER-mitochondria interactions and calcium (Ca2+) exchange in INS-1E cells, whereas disruption of MAMs altered glucose-stimulated insulin secretion (GSIS). Conversely, chronic incubations with high glucose of either INS-1E cells or human pancreatic islets altered GSIS and concomitantly reduced ER Ca2+ store, increased basal mitochondrial Ca2+, and reduced ATP-stimulated ER-mitochondria Ca2+ exchanges, despite an increase of organelle interactions. Furthermore, glucotoxicity-induced perturbations of Ca2+ signaling are associated with ER stress, altered mitochondrial respiration, and mitochondria fragmentation, and these organelle stresses may participate in increased organelle tethering as a protective mechanism. Last, sustained induction of ER-mitochondria interactions using a linker reduced organelle Ca2+ exchange, induced mitochondrial fission, and altered GSIS. Therefore, dynamic organelle coupling participates in GSIS in ß-cells, and over time, disruption of organelle Ca2+ exchange might be a novel mechanism contributing to glucotoxicity-induced ß-cell dysfunction.

TRPV1 variants impair intracellular Ca2+ signaling and may confer susceptibility to malignant hyperthermia.

PURPOSE: Malignant hyperthermia (MH) is a pharmacogenetic disorder arising from uncontrolled muscle calcium release due to an abnormality in the sarcoplasmic reticulum (SR) calcium-release mechanism triggered by halogenated inhalational anesthetics. However, the molecular mechanisms involved are still incomplete. METHODS: We aimed to identify transient receptor potential vanilloid 1 (TRPV1) variants within the entire coding sequence in patients who developed sensitivity to MH of unknown etiology. In vitro and in vivo functional studies were performed in heterologous expression system, trpv1-/- mice, and a murine model of human MH. RESULTS: We identified TRPV1 variants in two patients and their heterologous expression in muscles of trpv1-/- mice strongly enhanced calcium release from SR upon halogenated anesthetic stimulation, suggesting they could be responsible for the MH phenotype. We confirmed the in vivo significance by using mice with a knock-in mutation (Y524S) in the type I ryanodine receptor (Ryr1), a mutation analogous to the Y522S mutation associated with MH in humans. We showed that the TRPV1 antagonist capsazepine slows the heat-induced hypermetabolic response in this model. CONCLUSION: We propose that TRPV1 contributes to MH and could represent an actionable therapeutic target for prevention of the pathology and also be responsible for MH sensitivity when mutated.

The Role of the Anti-Aging Protein Klotho in IGF-1 Signaling and Reticular Calcium Leak: Impact on the Chemosensitivity of Dedifferentiated Liposarcomas.

By inhibiting Insulin-Like Growth Factor-1-Receptor (IGF-1R) signaling, Klotho (KL) acts like an aging- and tumor-suppressor. We investigated whether KL impacts the aggressiveness of liposarcomas, in which IGF-1R signaling is frequently upregulated. Indeed, we observed that a higher KL expression in liposarcomas is associated with a better outcome for patients. Moreover, KL is downregulated in dedifferentiated liposarcomas (DDLPS) compared to well-differentiated tumors and adipose tissue. Because DDLPS are high-grade tumors associated with poor prognosis, we examined the potential of KL as a tool for overcoming therapy resistance. First, we confirmed the attenuation of IGF-1-induced calcium (Ca2+)-response and Extracellular signal-Regulated Kinase 1/2 (ERK1/2) phosphorylation in KL-overexpressing human DDLPS cells. KL overexpression also reduced cell proliferation, clonogenicity, and increased apoptosis induced by gemcitabine, thapsigargin, and ABT-737, all of which are counteracted by IGF-1R-dependent signaling and activate Ca2+-dependent endoplasmic reticulum (ER) stress. Then, we monitored cell death and cytosolic Ca2+-responses and demonstrated that KL increases the reticular Ca2+-leakage by maintaining TRPC6 at the ER and opening the translocon. Only the latter is necessary for sensitizing DDLPS cells to reticular stressors. This was associated with ERK1/2 inhibition and could be mimicked with IGF-1R or MEK inhibitors. These observations provide a new therapeutic strategy in the management of DDLPS.

Exosome-like vesicles released from lipid-induced insulin-resistant muscles modulate gene expression and proliferation of beta recipient cells in mice.

AIMS/HYPOTHESIS: The crosstalk between skeletal muscle (SkM) and beta cells plays a role in diabetes aetiology. In this study, we have investigated whether SkM-released exosome-like vesicles (ELVs) can be taken up by pancreatic beta cells and can deliver functional cargoes. METHODS: Mice were fed for 16 weeks with standard chow diet (SCD) or with standard diet enriched with 20% palmitate (HPD) and ELVs were purified from quadriceps muscle. Fluorescent ELVs from HPD or SCD quadriceps were injected i.v. or intramuscularly (i.m.) into mice to determine their biodistributions. Micro (mi)RNA quantification in ELVs was determined using quantitative real-time RT-PCR (qRT-PCR)-based TaqMan low-density arrays. Microarray analyses were performed to determine whether standard diet ELVs (SD-ELVs) and high palmitate diet ELVs (HPD-ELVs) induced specific transcriptional signatures in MIN6B1 cells. RESULTS: In vivo, muscle ELVs were taken up by pancreas, 24 h post-injection. In vitro, both SD-ELVs and HPD-ELVs transferred proteins and miRNAs to MIN6B1 cells and modulated gene expressions whereas only HPD-ELVs induced proliferation of MIN6B1 cells and isolated islets. Bioinformatic analyses suggested that transferred HPD-ELV miRNAs may participate in these effects. To validate this, we demonstrated that miR-16, which is overexpressed in HPD-ELVs, was transferred to MIN6B1 cells and regulated Ptch1, involved in pancreas development. In vivo, islets from HPD mice showed increased size and altered expression of genes involved in development, including Ptch1, suggesting that the effect of palm oil on islet size in vivo was reproduced in vitro by treating beta cells with HPD-ELVs. CONCLUSIONS/INTERPRETATION: Our data suggest that muscle ELVs might have an endocrine effect and could participate in adaptations in beta cell mass during insulin resistance.

Losartan, an angiotensin II type 1 receptor blocker, protects human islets from glucotoxicity through the phospholipase C pathway.

As shown in a large clinical prospective trial, inhibition of the renin-angiotensin system (RAS) can delay the onset of type 2 diabetes in high-risk individuals. We evaluated the beneficial effects of RAS inhibition on ß-cell function under glucotoxic conditions. Human islets from 13 donors were cultured in 5.5 mM (controls) or 16.7 mM glucose [high glucose (HG)] for 4 d with or without losartan (5 µM), a selective AT1R blocker, and/or U73122 (2 µM), a selective PLC inhibitor, during the last 2 d. HG induced RAS activation with overexpression of AT1R (P<0.05) and angiotensinogen (P<0.001) mRNAs. HG increased endoplasmic reticulum (ER) stress markers (P<0.001) such as GRP78, sXBP1, and ATF4 mRNAs and Grp78 protein levels (P<0.01). HG also decreased reticular calcium concentration (P<0.0001) and modified protein expressions of ER calcium pumps with reduction of SERCA2b (P<0.01) and increase of IP3R2 (P<0.05). Losartan prevented these deleterious effects and was associated with improved insulin secretion despite HG exposure. AT1R activation triggers the PLC-IP3-calcium pathway. Losartan prevented the increase of PLC ß1 and γ1 protein levels induced by HG (P<0.05). U73122 reproduced all the protective effects of losartan. AT1R blockade protects human islets from the deleterious effects of glucose through inhibition of the PLC-IP3-calcium pathway.

Respective contribution of mitochondrial superoxide and pH to mitochondria-targeted circularly permuted yellow fluorescent protein (mt-cpYFP) flash activity.

Superoxide flashes are transient bursts of superoxide production within the mitochondrial matrix that are detected using the superoxide-sensitive biosensor, mitochondria-targeted circularly permuted YFP (mt-cpYFP). However, due to the pH sensitivity of mt-cpYFP, flashes were suggested to reflect transient events of mitochondrial alkalinization. Here, we simultaneously monitored flashes with mt-cpYFP and mitochondrial pH with carboxy-SNARF-1. In intact cardiac myocytes and purified skeletal muscle mitochondria, robust mt-cpYFP flashes were accompanied by only a modest increase in SNARF-1 ratio (corresponding to a pH increase of <0.1), indicating that matrix alkalinization is minimal during an mt-cpYFP flash. Individual flashes were also accompanied by stepwise increases of MitoSOX signal and decreases of NADH autofluorescence, supporting the superoxide origin of mt-cpYFP flashes. Transient matrix alkalinization induced by NH4Cl only minimally influenced flash frequency and failed to alter flash amplitude. However, matrix acidification modulated superoxide flash frequency in a bimodal manner. Low concentrations of nigericin (< 100 nM) that resulted in a mild dissipation of the mitochondrial pH gradient increased flash frequency, whereas a maximal concentration of nigericin (5 µm) collapsed the pH gradient and abolished flash activity. These results indicate that mt-cpYFP flash events reflect a burst in electron transport chain-dependent superoxide production that is coincident with a modest increase in matrix pH. Furthermore, flash activity depends strongly on a combination of mitochondrial oxidation and pH gradient.

Characterization of functional TRPV1 channels in the sarcoplasmic reticulum of mouse skeletal muscle.

TRPV1 represents a non-selective cation channel activated by capsaicin, acidosis and high temperature. In the central nervous system where TRPV1 is highly expressed, its physiological role in nociception is clearly identified. In skeletal muscle, TRPV1 appears implicated in energy metabolism and exercise endurance. However, how as a Ca(2+) channel, it contributes to intracellular calcium concentration ([Ca(2+)]i) maintenance and muscle contraction remains unknown. Here, as in rats, we report that TRPV1 is functionally expressed in mouse skeletal muscle. In contrast to earlier reports, our analysis show TRPV1 presence only at the sarcoplasmic reticulum (SR) membrane (preferably at the longitudinal part) in the proximity of SERCA1 pumps. Using intracellular Ca(2+) imaging, we directly accessed to the channel functionality in intact FDB mouse fibers. Capsaicin and resiniferatoxin, both agonists as well as high temperature (45°C) elicited an increase in [Ca(2+)]i. TRPV1-inhibition by capsazepine resulted in a strong inhibition of TRPV1-mediated functional responses and abolished channel activation. Blocking the SR release (with ryanodine or dantrolene) led to a reduced capsaicin-induced Ca(2+) elevation suggesting that TRPV1 may participate to a secondary SR Ca(2+) liberation of greater amplitude. In conclusion, our experiments point out that TRPV1 is a functional SR Ca(2+) leak channel and may crosstalk with RyR1 in adult mouse muscle fibers.

Modulation of ER stress and apoptosis by endoplasmic reticulum calcium leak via translocon during unfolded protein response: involvement of GRP78.

The endoplasmic reticulum (ER) is involved in many cellular functions, including protein folding and Ca(2+) homeostasis. The ability of cells to respond to the ER stress is critical for cell survival, and disruption in such regulation can lead to apoptosis. ER stress is accompanied by alterations in Ca(2+) homeostasis, and the ER Ca(2+) store depletion by itself can induce ER stress and apoptosis. Despite that, the ER Ca(2+) leak channels activated in response to the ER stress remain poorly characterized. Here we demonstrate that ER Ca(2+) depletion during the ER stress occurs via translocon, the ER protein complex involved in translation. Numerous ER stress inducers stimulate the ER Ca(2+) leak that can be prevented by translocon inhibitor, anisomycin. Expression of GRP78, an ER stress marker, increased following treatment with puromycin (a translocon opener) and was suppressed by anisomycin, confirming a primary role of translocon in ER stress induction. Inhibition of ER store depletion by anisomycin significantly reduces apoptosis stimulated by the ER stress inducers. We suggest that translocon opening is physiologically modulated by GRP78, particularly during the ER stress. The ability to modulate the ER Ca(2+) permeability and subsequent ER stress can lead to development of a novel therapeutic approach.

Inverse regulation of the cytosolic Ca²âº buffer parvalbumin and mitochondrial volume in muscle cells via SIRT1/PGC-1α axis.

Skeletal muscles show a high plasticity to cope with various physiological demands. Different muscle types can be distinguished by the force, endurance, contraction/relaxation kinetics (fast-twitch vs. slow-twitch muscles), oxidative/glycolytic capacity, and also with respect to Ca²âº-signaling components. Changes in Ca²âº signaling and associated Ca²âº-dependent processes are thought to underlie the high adaptive capacity of muscle fibers. Here we investigated the consequences and the involved mechanisms caused by the ectopic expression of the Ca²âº-binding protein parvalbumin (PV) in C2C12 myotubes in vitro, and conversely, the effects caused by its absence in in fast-twitch muscles of parvalbumin null-mutant (PV⁻/⁻) mice in vivo. The absence of PV in fast-twitch muscle tibialis anterior (TA) resulted in an increase in the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and of its positive regulator, the deacetylase sirtuin 1 (SIRT1). TA muscles from PV⁻/⁻ mice also have an increased mitochondrial volume. Mild ionophore treatment of control (PV-devoid) C2C12 myotubes causing a moderate elevation in [Ca²âº](c) resulted in an increase in mitochondrial volume, together with elevated PGC-1α and SIRT1 expression levels, whilst it increased PV expression levels in myotubes stably transfected with PV. In PV-expressing myotubes the mitochondrial volume, PGC-1α and SIRT1 were significantly lower than in control C2C12 myotubes already at basal conditions and application of ionophore had no effect on either one. SIRT1 activation causes a down-regulation of PV in transfected myotubes, whilst SIRT1 inhibition has the opposite effect. We conclude that PV expression and mitochondrial volume in muscle cells are inversely regulated via a SIRT1/PGC-1α signaling axis.