Sarcoplasmic Reticulum Stores Research Articles

Abstract P2164: Expression Of Engineered Bacterial Sodium Channel Improves Cardiomyocyte Contractility

Introduction: Our recent studies in non-human primates have demonstrated adeno-associated virus (AAV) delivery of prokaryotic sodium channel (BacNa v ) can improve the contractile function of infarcted hearts. We hypothesized that BacNa v expression increases cardiomyocyte (CM) contractile function by indirectly augmenting Ca 2+ transient amplitude via increase in sarcoplasmic reticulum (SR) Ca 2+ stores. Methods: We developed an in vitro model of ischemia-reperfusion (I/R) injury in engineered heart tissues (cardiopatches) made from human iPSC-derived CMs (hiPSC-CMs). Prior to formation, tissues were transduced with lentivirus conferring a doxycycline (Dox)-inducible expression of BacNa v . Upon I/R injury, the tissues were treated with either vehicle or Dox for 48h followed by force testing and Ca 2+ imaging. The effect of BacNa v expression on Ca 2+ handling was additionally studied using neonatal rat ventricular myocytes (NRVMs). Results: The tissue injury from I/R was evident from increases in cleaved-caspase 3 area (0.25% vs 1.53%), ROS generation (1.2 fold), LDH release (4.1 fold), and percent dead cells (0.97% vs 4.67%). Immediately following the I/R injury, tissues showed decrease in contractile force (2.78mN vs 0.74mN) and conduction velocity (20.7cm/s vs 7.9cm/s). At 72hr post-injury, BacNa v -expressing tissues exhibited significantly higher contractile force (1.67mN vs 1.32mN) and Ca 2+ transient (1.8 fold) amplitudes compared to the vehicle control. Furthermore, in uninjured NRVM monolayers, BacNa v expression resulted in higher Ca 2+ transient amplitude (1.31 fold), as well as increased SR calcium stores (1.3 fold) and slower sodium/calcium exchanger (NCX) extrusion rate (1.03s -1 vs 0.82s -1 ) in the presence of caffeine. Blocking NCX with 10M ORM-10962 eliminated the difference in Ca 2+ transient amplitude between BacNa v -expressing and control CMs. Conclusion: Our results suggest BacNa v expression augments CM contractility and Ca 2+ transient amplitude, at least in part through the suppression of Ca 2+ efflux mode of the NCX. In addition to its antiarrhythmic effects, BacNa v gene delivery may represent an effective therapeutic strategy to improve cardiac contractility in congestive heart failure.

Increased Ca2+ Transient Underlies RyR2-Related Left Ventricular Noncompaction.

A loss-of-function cardiac ryanodine receptor (RyR2) mutation, I4855M+/-, has recently been linked to a new cardiac disorder termed RyR2 Ca2+ release deficiency syndrome (CRDS) as well as left ventricular noncompaction (LVNC). The mechanism by which RyR2 loss-of-function causes CRDS has been extensively studied, but the mechanism underlying RyR2 loss-of-function-associated LVNC is unknown. Here, we determined the impact of a CRDS-LVNC-associated RyR2-I4855M+/- loss-of-function mutation on cardiac structure and function. We generated a mouse model expressing the CRDS-LVNC-associated RyR2-I4855M+/- mutation. Histological analysis, echocardiography, ECG recording, and intact heart Ca2+ imaging were performed to characterize the structural and functional consequences of the RyR2-I4855M+/- mutation. As in humans, RyR2-I4855M+/- mice displayed LVNC characterized by cardiac hypertrabeculation and noncompaction. RyR2-I4855M+/- mice were highly susceptible to electrical stimulation-induced ventricular arrhythmias but protected from stress-induced ventricular arrhythmias. Unexpectedly, the RyR2-I4855M+/- mutation increased the peak Ca2+ transient but did not alter the L-type Ca2+ current, suggesting an increase in Ca2+-induced Ca2+ release gain. The RyR2-I4855M+/- mutation abolished sarcoplasmic reticulum store overload-induced Ca2+ release or Ca2+ leak, elevated sarcoplasmic reticulum Ca2+ load, prolonged Ca2+ transient decay, and elevated end-diastolic Ca2+ level upon rapid pacing. Immunoblotting revealed increased level of phosphorylated CaMKII (Ca2+-calmodulin dependent protein kinases II) but unchanged levels of CaMKII, calcineurin, and other Ca2+ handling proteins in the RyR2-I4855M+/- mutant compared with wild type. The RyR2-I4855M+/- mutant mice represent the first RyR2-associated LVNC animal model that recapitulates the CRDS-LVNC overlapping phenotype in humans. The RyR2-I4855M+/- mutation increases the peak Ca2+ transient by increasing the Ca2+-induced Ca2+ release gain and the end-diastolic Ca2+ level by prolonging Ca2+ transient decay. Our data suggest that the increased peak-systolic and end-diastolic Ca2+ levels may underlie RyR2-associated LVNC.

Compensatory calcium handling may underlie increased arrhythmia susceptibility of sarcospan-deficient mice after ischemia-reperfusion injury.

The protein sarcospan (SSPN) is an integral member of the dystrophin glycoprotein complex (DGC) and has been shown important in the heart during development and periods of acute stress. In this study, ex vivo hearts obtained from WT and SSPN-deficient (SSPN-/-) mice were used to investigate the role of SSPN in protecting against arrhythmia after acute ischemia-reperfusion (IR) injury. Measurements of sinus heart rates in ex vivo hearts showed that SSPN-/- hearts trended toward slower cardiac conduction post-IR injury than WT hearts. Premature ventricular contractions (PVCs) were increased in SSPN-/- hearts post-IR injury during reperfusion. The regulation of intracellular Ca2+ was examined in Fura-2AM loaded WT and SSPN-/- cardiomyocytes. Acute beta-adrenergic stimulation of cardiomyocytes with 1 μM isoproterenol resulted in reduced sarcomere length, increased diastolic Ca2+ levels of WT but not SSPN-/- cardiomyocytes. After isoproterenol exposure changes in systolic Ca2+ levels similar between WT and SSPN-/- cardiomyocytes. Cardiomyocytes were treated with caffeine (10 mM) to deplete the sarcoplasmic reticulum (SR) Ca2+ store. It was found that WT cardiomyocytes had greater Ca2+ SR store than SSPN-/-. However, the total Ca2+ release from WT and SSPN-/- cardiomyocytes was similar. To better understand these results in context of IR injury, qRT-PCR was utilized to examine changes in Ca2+ handling genes. Overall, SSPN-/- hearts exhibited significantly increased expression of major Ca2+ handling genes suggesting compensation for perturbed Ca2+ handling dynamics. Interestingly, after acute IR injury the expression of L-type Ca2+-channel (LTCC) and phospholamban (PLN) were reduced in SSPN-/- hearts but not WT hearts. These results suggest that increased arrhythmia susceptibility in SSPN-/- hearts post-IR injury may arise from disruption of compensatory Ca2+ handling mechanisms in the SSPN-/- heart.

Abstract 13579: TAX1BP3 -Encoding TAX1-Binding Protein 3 is a Novel Genetic Cause of Autosomal Recessive Arrhythmogenic Cardiomyopathy and Causes Pro-Arrhythmic Ryanodine Receptor 2-Mediated Calcium Leak

Introduction: Arrhythmogenic cardiomyopathy (AC) is a heart muscle disease where fibrofatty tissue replaces healthy myocardium. AC predisposes to ventricular arrhythmias and sudden cardiac death. We identified biallelic variants in the TAX1BP3- encoded tax1-binding protein 3 (TAX1BP3) in a genetically elusive family with AC. TAX1BP3 has putative roles in canonical Wnt signaling, but its function in the heart is unknown. Our goal is to identify novel mechanisms of AC development to develop therapeutics to mitigate or reverse disease progression. Hypothesis: We hypothesize that genetic loss of TAX1BP3 suppresses normal canonical Wnt-signaling in the heart, allowing pathologic remodeling and arrhythmogenesis associated with AC development. Methods: Whole exome sequencing of an AC genotype-negative family was performed. Induced pluripotent stem cells generated from peripheral blood mononuclear cells (PBMCs) from were differentiated into cardiac myocytes (iPSC-CMs) for 3 kindred. Line scans using CAL-520-labeled calcium were conducted with a Zeiss confocal microscope to capture calcium sparks after pacing iPSC-CMs. Sarcoplasmic reticulum store was measured after adding 10mM caffeine to iPSC-CMs. Adipogenesis in iPSC-CMs was visualized by immunofluorescence with Nile Red stain. Results: TAX1BP3 M78T/del variant co-segregates with AC in an autosomal recessive inheritance mode in an AC genotype-negative family. TAX1BP3 M78T/del PBMCs had reduced proliferation and qPCR from whole blood identified decreased canonical Wnt signaling. iPSC-CMs M78T/del have decreased intracellular calcium store, increased adipogenesis, and increased calcium leak associated with spontaneous calcium release events. Small molecule Wnt-activator BML-284 normalized calcium leak in iPSC-CMs M78T/del . Conclusions: Our results demonstrate that loss-of-function of TAX1BP3 is pro-arrhythmic and driven by calcium leak in iPSC-CMs which can be rescued by Wnt activation. We identified a new AC-associated gene, TAX1BP3, that has not been studied in the heart. Further studies are needed to explore the impact of TAX1BP3 on cardiac Wnt signaling, but our findings implicate Wnt activation as a possible treatment for AC - currently an untreatable, deadly disease.

Ablation of Calsequestrin-1, Ca2+ unbalance, and susceptibility to heat stroke.

Introduction: Ca2+ levels in adult skeletal muscle fibers are mainly controlled by excitation-contraction (EC) coupling, a mechanism that translates action potentials in release of Ca2+ from the sarcoplasmic reticulum (SR) release channels, i.e. the ryanodine receptors type-1 (RyR1). Calsequestrin (Casq) is a protein that binds large amounts of Ca2+ in the lumen of the SR terminal cisternae, near sites of Ca2+ release. There is general agreement that Casq is not only important for the SR ability to store Ca2+, but also for modulating the opening probability of the RyR Ca2+ release channels. The initial studies: About 20 years ago we generated a mouse model lacking Casq1 (Casq1-null mice), the isoform predominantly expressed in adult fast twitch skeletal muscle. While the knockout was not lethal as expected, lack of Casq1 caused a striking remodeling of membranes of SR and of transverse tubules (TTs), and mitochondrial damage. Functionally, CASQ1-knockout resulted in reduced SR Ca2+ content, smaller Ca2+ transients, and severe SR depletion during repetitive stimulation. The myopathic phenotype of Casq1-null mice: After the initial studies, we discovered that Casq1-null mice were prone to sudden death when exposed to halogenated anaesthetics, heat and even strenuous exercise. These syndromes are similar to human malignant hyperthermia susceptibility (MHS) and environmental-exertional heat stroke (HS). We learned that mechanisms underlying these syndromes involved excessive SR Ca2+ leak and excessive production of oxidative species: indeed, mortality and mitochondrial damage were significantly prevented by administration of antioxidants and reduction of oxidative stress. Though, how Casq1-null mice could survive without the most important SR Ca2+ binding protein was a puzzling issue that was not solved. Unravelling the mystery: The mystery was finally solved in 2020, when we discovered that in Casq1-null mice the SR undergoes adaptations that result in constitutively active store-operated Ca2+ entry (SOCE). SOCE is a mechanism that allows skeletal fibers to use external Ca2+ when SR stores are depleted. The post-natal compensatory mechanism that allows Casq1-null mice to survive involves the assembly of new SR-TT junctions (named Ca2+ entry units) containing Stim1 and Orai1, the two proteins that mediate SOCE.

Metabolic Syndrome and SIRT1 Mutation Impair Ca2+Channels in Coronary Smooth Muscle Cells of Ossabaw Miniature Swine

BackgroundNormal metabolic health and sirtuin1 (SIRT1) by its deacetylase activity maintain voltage‐gated Ca2+ channel (VGCC) function that promotes gene expression to maintain contractile coronary smooth muscle (CSM) phenotype. MetS impairs VGCC function and sarcoplasmic reticulum (SR) Ca2+ release, contributing to dedifferentiation of CSM to proliferative and osteogenic phenotypes.HypothesisMetS and impaired SIRT1 will impair VGCC and SR Ca2+ release channels in CSM.MethodsUsing CRISPR/Cas9 methodology a point mutation (SIRT1L100P) was made in Ossabaw miniature swine to mimic the naturally occurring mutation in humans and decrease SIRT1 activity, thereby resulting in hyperacetylation (Ac) of Ca2+transporters and impaired function. Four groups of pigs were used to analyze genotype and diet interactions: wild type lean, SIRT1 lean, wild type MetS, SIRT1 MetS. Pigs were age 4 months at the start and fed normal chow (lean) or atherogenic diet (MetS) for 7 months. CSM cells were enzymatically dispersed and Ca2+ measured with fura‐2. Depolarization with 80 mM K+ assessed Ca2+ influx through VGCC and the peak Ca2+ response to 5 mM caffeine to open SR Ca2+ release channels assessed the caffeine‐sensitive SR Ca2+ store.ResultsTwo‐way ANOVA showed SIRT1 mutation (p=0.02) and MetS diet (p<0.0001) significantly decreased VGCC function independently, but not additively or synergistically. Interaction of SIRT1 genotype and MetS diet was significant (p<0.0001). No significant effect of genotype or diet was observed on SR calcium store release.ConclusionSIRT1L100P mutation is likely to contribute to coronary atherosclerosis and SIRT1 activators may be effective therapies.

Endurance exercise attenuates juvenile irradiation-induced skeletal muscle functional decline and mitochondrial stress

BackgroundRadiotherapy is commonly used to treat childhood cancers and can have adverse effects on muscle function, but the underlying mechanisms have yet to be fully elucidated. We hypothesized that endurance exercise following radiation treatment would improve skeletal muscle function.MethodsWe utilized the Small Animal Radiation Research Platform (SARRP) to irradiate juvenile male mice with a clinically relevant fractionated dose of 3× (every other day over 5 days) 8.2 Gy X-ray irradiation locally from the knee to footpad region of the right hindlimb. Mice were then singly housed for 1 month in cages equipped with either locked or free-spinning voluntary running wheels. Ex vivo muscle contractile function, RT-qPCR analyses, resting cytosolic and sarcoplasmic reticulum (SR) store Ca2+ levels, mitochondrial reactive oxygen species levels (MitoSOX), and immunohistochemical and biochemical analyses of muscle samples were conducted to assess the muscle pathology and the relative therapeutic impact of voluntary wheel running (VWR).ResultsIrradiation reduced fast-twitch extensor digitorum longus (EDL) muscle-specific force by 27% compared to that of non-irradiated mice, while VWR post-irradiation improved muscle-specific force by 37%. Radiation treatment similarly reduced slow-twitch soleus muscle-specific force by 14% compared to that of non-irradiated mice, while VWR post-irradiation improved specific force by 18%. We assessed intracellular Ca2+ regulation, oxidative stress, and mitochondrial homeostasis as potential mechanisms of radiation-induced pathology and exercise-mediated rescue. We found a significant reduction in resting cytosolic Ca2+ concentration following irradiation in sedentary mice. Intriguingly, however, SR Ca2+ store content was increased in myofibers from irradiated mice post-VWR compared to mice that remained sedentary. We observed a 73% elevation in the overall protein oxidization in muscle post-irradiation, while VWR reduced protein nitrosylation by 35% and mitochondrial reactive oxygen species (ROS) production by 50%. Finally, we found that VWR significantly increased the expression of PGC1α at both the transcript and protein levels, consistent with an exercise-dependent increase in mitochondrial biogenesis.ConclusionsJuvenile irradiation stunted muscle development, disrupted proper Ca2+ handling, damaged mitochondria, and increased oxidative and nitrosative stress, paralleling significant deficits in muscle force production. Exercise mitigated aberrant Ca2+ handling, mitochondrial homeostasis, and increased oxidative and nitrosative stress in a manner that correlated with improved skeletal muscle function after radiation.

Metabolic Syndrome and SIRT1 Mutation Impair Ca2+ Channels in Coronary Smooth Muscle Cells of Ossabaw Miniature Swine

Background:
 Changes in Ca2+ regulation have been implicated in various pathologies such as coronary artery disease and metabolic syndrome (MetS), thereby potentiating these diseases. Our lab has shown that MetS decreases voltage-gated Ca2+ channel (VGCC) activity and sarcoplasmic reticulum (SR) Ca2+ release in coronary smooth muscle cells and increases coronary artery disease in Ossabaw miniature swine. Furthermore, decreased SIRT1 enzyme function can impair Ca2+ signaling and increase coronary disease and MetS. We hypothesized that impaired SIRT1 and MetS would decrease VGCC function and SR calcium store.
 Methods:
 CRISPR/Cas9 methods delivered a leucine to proline point mutation in SIRT1 (SIRT1L100P) into the Ossabaw swine genome to compare to wild type (WT), mimicking the naturally occurring mutation in humans which decreases SIRT1 activity. Four treatment groups of juvenile swine were based on genotype and diet: WT Lean, SIRT1 Lean, WT MetS, and SIRT1 MetS. Lean swine were fed normal chow and MetS were fed a hypercaloric, atherogenic diet for 7 months. The left anterior descending coronary artery was harvested and enzymatically digested to obtain cells. Fluorescence microscopy measured the Ca2+ indicator fura-2 in single cells. Depolarization of cells with perfusion of 80 mM K+ was used to elicit Ca2+ influx through VGCC. Caffeine (5 mM) exposure activated the Ca2+ release channel (ryanodine receptor) on the SR.
 Results:
 MetS was confirmed by increased body weight, impaired glucose tolerance, hyperinsulinemia, and hypercholesterolemia. Coronary atherosclerosis was shown by angiography, intravascular ultrasound, and gross imaging. A two-way analysis of variance revealed statistically significant overall effects of genotype (p=0.02), diet (p<0.0001), and an interaction (p<0.0001) between these variables to decrease VGCC function. In contrast, no effect was observed on SR Ca2+ release.
 Conclusion and Potential Impact:
 SIRT1 inhibition and MetS decreased VGCC function independently, but not additively or synergistically. (Support: NIH T35HL110854, DK120240, DK09751.)

Acetazolamide prevents hypoxia-induced reactive oxygen species generation and calcium release in pulmonary arterial smooth muscle.

Upon sensing a reduction in local oxygen partial pressure, pulmonary vessels constrict, a phenomenon known as hypoxic pulmonary vasoconstriction. Excessive hypoxic pulmonary vasoconstriction can occur with ascent to high altitude and is a contributing factor to the development of high-altitude pulmonary edema. The carbonic anhydrase inhibitor, acetazolamide, attenuates hypoxic pulmonary vasoconstriction through stimulation of alveolar ventilation via modulation of acid–base homeostasis and by direct effects on pulmonary vascular smooth muscle. In pulmonary arterial smooth muscle cells (PASMCs), acetazolamide prevents hypoxia-induced increases in intracellular calcium concentration ([Ca2+]i), although the exact mechanism by which this occurs is unknown. In this study, we explored the effect of acetazolamide on various calcium-handling pathways in PASMCs. Using fluorescent microscopy, we tested whether acetazolamide directly inhibited store-operated calcium entry or calcium release from the sarcoplasmic reticulum, two well-documented sources of hypoxia-induced increases in [Ca2+]i in PASMCs. Acetazolamide had no effect on calcium entry stimulated by store-depletion, nor on calcium release from the sarcoplasmic reticulum induced by either phenylephrine to activate inositol triphosphate receptors or caffeine to activate ryanodine receptors. In contrast, acetazolamide completely prevented Ca2+-release from the sarcoplasmic reticulum induced by hypoxia (4% O2). Since these results suggest the acetazolamide interferes with a mechanism upstream of the inositol triphosphate and ryanodine receptors, we also determined whether acetazolamide might prevent hypoxia-induced changes in reactive oxygen species production. Using roGFP, a ratiometric reactive oxygen species-sensitive fluorescent probe, we found that hypoxia caused a significant increase in reactive oxygen species in PASMCs that was prevented by 100 μM acetazolamide. Together, these results suggest that acetazolamide prevents hypoxia-induced changes in [Ca2+]i by attenuating reactive oxygen species production and subsequent activation of Ca2+-release from sarcoplasmic reticulum stores.

NEW GENES AND DISEASES: EP.304 Early onset lower limp assymmetry: a new clinical phenotype of calsequestrin mutation

Calsequestrin is a protein that handles calcium in sarcoplasmic reticulum (SR),Ca2+ transients are crucial in skeletal muscle excitation-contraction coupling. Synchronous Ca2+ release from the SR terminal cisternae into the cytoplasm and its reuptake into the SR store are granted by a structural and functional unit and their defect causes myopathy. In an 11-year-old girl, affected by lower limb asymmetry (R limb shorter of 3 cm), upper limb hypotrophy and winging scapulae were found; she had high CK:2900 U/l, her sister presented myalgias and CK:1500 U/l, the mother CK 500 U/l and grandmother had both congenital club foot. We investigated the causes of the high CK by EMG that was myopathic, muscle biopsy that showed few atrophic fibres, in ATPase rare 2C fibres, some increased NADH-TR stain at the periphery, increased central nuclei and rare basophilic fibres. We performed immunohistochemistry for dystrophin, caveolin-3, alpha-sarcoglycan, western blot for dysferlin: all normal. The girl was operated at 15 years for elongation of right femur and by epiphysiodesis left femur and the limb asymmetry was reduced to only half a centimeter: however she complained of fatiguability, especially after physical education at school, and high CK (2186 U). She presented with scoliosis, difficulty walking with calf hypertrophy. A search for the causes for hyperCKemia was done with WGS at TIGEM and resulted in detection of a pathogenetic variant of CASQ1 gene (p.ASP244Gly), coding for calsequestrin, which allowed the solution of this puzzling familial case. In 7 members of 4 Italian families with vacuolar myopathy with CASQ1 mutation calsequestrin aggregates and a similar variant was found. In CASQ1 myopathy in several cases hyperCKemia, myalgia or other vague symptoms are present: in 22 CASQ1-mutated patients from 12 families 21 sharing the same variant of our case presented symptoms in the sixth decade with exercise intolerance and myalgias and later developed mild to moderate, slowly progressive proximal weakness with quadriceps atrophy and scapular winging. In the present family, clubfoot, early onset, limb asymmetry were the predominant clinical symptoms, they could be the effect of somatic mosaicism. In this respect our patient presents an early onset and a new clinical phenotype of lower limb asymmetry that is unique. The present familial case expands the clinical phenotype of CASQ1-myopathy.

SERCA Inhibition as a Potential Therapeutic Target for Breast Cancer

Chemotherapy is both a palliative treatment for many cancers and adjuvant treatment before, during, or after local treatment (surgery and/or radiation therapy) in single or combination regimens. Despite current treatments, approximately 1 in 39 women (3%) still die from breast cancer. A major complication with systemic therapy is drug resistance which limits the efficacy of clinical chemotherapy. Hence, current research aims to uncover novel targets for breast cancer therapy. Various studies have shown altered calcium signaling in breast cancer cells, therefore, calcium modulators may be potential therapeutic targets. Studies have also shown that Thapsigargin (TG) and cyclopiazonic acid (CPZ), two potent and specific inhibitors of the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA), can impair cancer cell growth. However, TG and CPZ also impair non-malignant cells, which results in significant toxicity, thus preventing use in chemotherapy. Using spectrometry UV-visible measurements to assess calcium loading and ATPase activity in SR microsomes, our lab has found that various drugs which display cell protective effects in ischemia also inhibit SERCA. They include novel benzothiazepines (BZTs; K201, CGP37157, PH00095) and have shown to inhibit SERCA in a calcium dependent mechanism. They decreased SR uptake when cytosolic Ca2+ levels ([Ca2+]c) are low, similar to competitive inhibition of Ca2+ uptake, which produces mild store depletion. Contrarily, TG and CPZ block Ca2+ uptake at all [Ca2+]c and largely deplete SR stores. Using various SERCA inhibitors, we sought out to determine their effects on breast cancer cell lines of different subtypes. MTS assay studies show that TG and CPZ significantly reduce cell viability in MCF7 and MCF10A cell lines while having no significant effects on triple negative breast cancer (TNBC) cell line MDAMB231. Preliminary studies also show no significant effect by TG on BT20 cells, another TNBC cell line. BZT K201 also significantly reduced cell viability in MCF7 cells but not MDAMB231. Immunoblot shows MCF7 cells have significantly higher SERCA2 expression than MDAMB231. We observe no significant differences in plasma membrane calcium channel ATPase (PMCA) protein expression. Preliminary RT-PCR studies estimate MCF7 cells have a higher fold change in SERCA genes. Thus, targeting SERCA may serve as a potential therapeutic target for breast cancer. Here, we find that SERCA inhibitors TG, CPZ, and BZT K201 reduce cell viability in MCF7 cells. Still, these studies indicate that SERCA inhibition may only be efficacious in certain breast cancer cell types which do not express a TNBC phenotype. TNBC cell lines (MDAMB231, BT20) are not significantly affected by SERCA inhibition, which may be due to higher dependence for signaling on plasmalemma calcium channels and transporters. Additional studies are required to confirm the expression profiles of calcium transporters among cell lines, including those in the plasmalemma (PMCA, calcium channels), which may help to understand if alternative sources of Ca2+ for intracellular signaling make TNBC cell lines more resistant to SERCA inhibitors.

TRPV1 channels regulate the automaticity of embryonic stem cell-derived cardiomyocytes through stimulating the Na+ /Ca2+ exchanger current.

Calcium controls the excitation-contraction coupling in cardiomyocytes. Embryonic stem cell-derived cardiomyocytes (ESC-CMs) are an important cardiomyocyte source for regenerative medicine and drug screening. Transient receptor potential vanilloid 1 (TRPV1) channels are nonselective cation channels that permeate sodium and calcium. This study aimed to investigate whether TRPV1 channels regulate the electrophysiological characteristics of ESC-CMs. If yes, what is the mechanism behind? By immunostaining and subcellular fractionation, followed by western blotting, TRPV1 was found to locate intracellularly. The staining pattern of TRPV1 was found to largely overlap with that of the sarco/endoplasmic reticulum Ca2+ -ATPase, the sarcoplasmic reticulum (SR) marker. By electrophysiology and calcium imaging, pharmacological blocker of TRPV1 and the molecular tool TRPV1β (which could functionally knockdown TRPV1) were found to decrease the rate and diastolic depolarization slope of spontaneous action potentials, and the amplitude and frequency of global calcium transients. By calcium imaging, in the absence of external calcium, TRPV1-specific opener increased intracellular calcium; this increase was abolished by preincubation with caffeine, which could deplete SR calcium store. The results suggest that TRPV1 controls calcium release from the SR. By electrophysiology, TRPV1 blockade and functional knockdown of TRPV1 decreased the Na+ /Ca2+ exchanger (NCX) currents from both the forward and reverse modes, suggesting that sodium and calcium through TRPV1 stimulate the NCX activity. Our novel findings suggest that TRPV1 activity is important for regulating the spontaneous activity of ESC-CMs and reveal a novel interplay between TRPV1 and NCX in regulating the physiological functions of ESC-CMs.

Sarcoplasmic reticulum calcium mishandling: central tenet in heart failure?

Excitation-contraction coupling links excitation of the sarcolemmal surface membrane to mechanical contraction. In the heart this link is established via a Ca2+-induced Ca2+ release process, which, following sarcolemmal depolarisation, prompts Ca2+ release from the sarcoplasmic reticulum (SR)though the ryanodine receptor (RyR2). This substantially raises the cytoplasmic Ca2+ concentration to trigger systole. In diastole, Ca2+ is removed from the cytoplasm, primarily via the sarcoplasmic-endoplasmic reticulum Ca2+-dependent ATPase (SERCA) pump on the SRmembrane, returning Ca2+ to the SR store. Ca2+ movement across the SR is thus fundamental to the systole/diastole cycle and plays an essential role in maintaining cardiac contractile function. Altered SR Ca2+ homeostasis (due to disrupted Ca2+ release, storage, and reuptake pathways) is a central tenet of heart failure and contributes to depressed contractility, impaired relaxation, and propensity to arrhythmia. This review will focus on the molecular mechanisms that underlie asynchronous Ca2+ cycling around the SR in the failing heart. Further, this review will illustrate that the combined effects of expression changes and disruptions to RyR2 and SERCA2a regulatory pathways are critical to the pathogenesis of heart failure.

"Phosphorylation of cardiac ryanodine receptors by protein kinase G: Evaluation of its impact on store overload-induced calcium release

Cardiac Ryanodine receptors (RyR2) are Ca2+ channels located within the sarcoplasmic reticulum (SR) membrane of cardiac myocytes and play a key role in excitation-contraction coupling. RyR2 can also open spontaneously in a SR store dependent manner, a process known as Store Overload-Induced Ca2+ Release (SOICR). We and others previously demonstrated PKG-dependent phosphorylation of RyR2 at Ser2808 in cardiac myocytes but its functional impact on SOICR remains unknown. Our objective is to study the effects promoted by PKG-dependent phosphorylation of the RyR2, with respect to SOICR frequency and amplitude.

RyR1-targeted drug discovery pipeline integrating FRET-based high-throughput screening and human myofiber dynamic Ca2+ assays

Elevated cytoplasmic [Ca2+] is characteristic in severe skeletal and cardiac myopathies, diabetes, and neurodegeneration, and partly results from increased Ca2+ leak from sarcoplasmic reticulum stores via dysregulated ryanodine receptor (RyR) channels. Consequently, RyR is recognized as a high-value target for drug discovery to treat such pathologies. Using a FRET-based high-throughput screening assay that we previously reported, we identified small-molecule compounds that modulate the skeletal muscle channel isoform (RyR1) interaction with calmodulin and FK506 binding protein 12.6. Two such compounds, chloroxine and myricetin, increase FRET and inhibit [3H]ryanodine binding to RyR1 at nanomolar Ca2+. Both compounds also decrease RyR1 Ca2+ leak in human skinned skeletal muscle fibers. Furthermore, we identified compound concentrations that reduced leak by > 50% but only slightly affected Ca2+ release in excitation-contraction coupling, which is essential for normal muscle contraction. This report demonstrates a pipeline that effectively filters small-molecule RyR1 modulators towards clinical relevance.

Overactivation of the sodium-calcium exchanger and transient receptor potential in anesthesia-induced malignant hyperthermia.

Malignant hyperthermia is a pharmacogenetic disorder, which is an uncommon but frequently fatal intricacy of inhalation anesthesia in man. It causes a quick rise in body temperature to highly irreversible levels, which causes death in around three of four cases. The trigger anesthetics cause an anomalous, continued ascent in myoplasmic calcium levels. Possible mechanisms by which continuous release of sodium, calcium from skeletal muscle plasma membrane and sarcoplasmic reticulum stores respectively can produce the profound hyperthermia are discussed.

Interactions of Dichlorodiphenyltrichloroethane (DDT) and Dichlorodiphenyldichloroethylene (DDE) With Skeletal Muscle Ryanodine Receptor Type 1.

Dichlorodiphenyltrichloroethane (DDT) and its metabolite dichlorodiphenyldichloroethylene (DDE) are ubiquitous in the environment and detected in tissues of living organisms. Although DDT owes its insecticidal activity to impeding closure of voltage-gated sodium channels, it mediates toxicity in mammals by acting as an endocrine disruptor (ED). Numerous studies demonstrate DDT/DDE to be EDs, but studies examining muscle-specific effects mediated by nonhormonal receptors in mammals are lacking. Therefore, we investigated whether o,p'-DDT, p,p'-DDT, o,p'-DDE, and p,p'-DDE (DDx, collectively) alter the function of ryanodine receptor type 1 (RyR1), a protein critical for skeletal muscle excitation-contraction coupling and muscle health. DDx (0.01-10 µM) elicited concentration-dependent increases in [3H]ryanodine ([3H]Ry) binding to RyR1 with o,p'-DDE showing highest potency and efficacy. DDx also showed sex differences in [3H]Ry-binding efficacy toward RyR1, where [3H]Ry-binding in female muscle preparations was greater than male counterparts. Measurements of Ca2+ transport across sarcoplasmic reticulum (SR) membrane vesicles further confirmed DDx can selectively engage with RyR1 to cause Ca2+ efflux from SR stores. DDx also disrupts RyR1-signaling in HEK293T cells stably expressing RyR1 (HEK-RyR1). Pretreatment with DDx (0.1-10 µM) for 100 s, 12 h, or 24 h significantly sensitized Ca2+-efflux triggered by RyR agonist caffeine in a concentration-dependent manner. o,p'-DDE (24 h; 1 µM) significantly increased Ca2+-transient amplitude from electrically stimulated mouse myotubes compared with control and displayed abnormal fatigability. In conclusion, our study demonstrates DDx can directly interact and modulate RyR1 conformation, thereby altering SR Ca2+-dynamics and sensitize RyR1-expressing cells to RyR1 activators, which may ultimately contribute to long-term impairments in muscle health.

The ERG1a K+ Channel Increases Intracellular Calcium and Calpain Activity in C2C12 Myotubes

In skeletal muscle increased intracellular calcium concentration ([Ca2+]i; uM range) is necessary for excitation‐contraction coupling; however, smaller increases in [Ca2+]i (nM range) can modulate other physiological processes in non‐contracting muscle. Indeed, fluctuations in localized calcium concentration can serve as a second messenger. In contrast, inappropriate changes in [Ca2+]i can have detrimental effects on muscle tissue and are associated with numerous skeletal muscle pathologies, for example muscular dystrophy, amyotrophic lateral sclerosis, malignant hyperthermia, cancer cachexia, and atrophy. Therefore, [Ca2+]i must be tightly regulated in terms of time, space and amplitude for cellular processes to occur in a properly coordinated fashion. In earlier reports, we showed that the ERG1a K+ channel is upregulated in atrophying skeletal muscle and contributes to increased ubiquitin proteasome proteolysis. Here, using the ratiometric Ca2+ indicator fura‐2AM, we show that ERG1a expression in C2C12 myotubes produces an increase in basal [Ca2+]i as well as a transient increase in [Ca2+]i as a consequence of depolarization. We explored this transient increase in [Ca2+]i using pharmacological agents. The data demonstrate that the ERG1a‐induced increase is not sensitive to the L‐type calcium channel blocker nifedipine, suggesting that it does not result from modulation of Cav1.1 channels. To further support this data, immunoblots reveal that there is no change in Cav1.1 channel abundance. However, the data do demonstrate that the increase in [Ca2+]i is sensitive to the SERCA blocking agent thapsigargin, suggesting that the source of the calcium is the sarcoplasmic reticulum stores. Additionally, the data reveal that ERG1a expression also increases basal calpain activity. In summary, to date the data show that ERG1a increases [Ca2+]i levels and suggest that this increase could occur, at least in part, as a result of release of calcium from sarcoplasmic reticulum stores. Additionally, the data reveal that ERG1a expression also increases calpain activity, suggesting that the increase in intracellular calcium results in increased calpain activity and possibly contributes to the increased proteolysis that occurs in atrophic skeletal muscle.Support or Funding InformationU.S. Department of Defense, PRMRP Grant# PR170326This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

2D+T Imaging of Calcium Signaling Microdomains in Cardiac Myocytes

Myocyte contraction is coupled with electrical depolarization via the process of excitation contraction coupling (ECC). During ECC, CM depolarization results in opening of voltage-gated Ca channels and calcium influx, which in turn triggers a larger calcium release via RyR calcium release channels on the sarcoplasmic reticulum (SR) store. Central to this process is the close apposition of the SR localized RyRs and plasma membrane voltage-gated calcium channels. This close coupling relies on membranous invaginations of the sarcolemma termed t-tubules, which ensure homogenous calcium elevations throughout the cytosol. In large mammals and in disease, t-tubules are not regularly distributed. Consequently, not all calcium release units (RyR clusters) are directly engaged by calcium influx, leading to potential inhomogeneities in cytosolic calcium. Here we optimized a multi-beam array confocal microscope to image signaling microdomains at high spatial (215 nm pixel size) and temporal resolution (125 fps) in intact pig myocytes. We fitted pixel-by-pixel the time evolution of calcium transients, extracted local key functional parameters and registered these pixel-based calcium changes with WGA stained cell membranes using a descriptor-based algorithm. When correlating functional parameters to the distance to closest membrane, we observed gradients in local calcium handling. Such differences were smaller during prolonged stimulation, suggesting a potential role for functional modulation, possibly through phosphorylation. We complemented live cells analysis with immunofluorescence staining of cells that were fixed after varying the duration of stimulation. We examined location and distribution of RyR clusters, their phosphorylation state and their distance to the closest membrane. We observed a site-specific RyR cluster density as well as the setting of local phosphorylation gradients that evolve with stimulation parameters and over time. Together, these data suggest that spatio-temporal patterns of phosphorylation contribute to the synchrony of calcium signals throughout the cell volume.

Ca2+ signalling in mouse urethral smooth muscle in situ: role of Ca2+ stores and Ca2+ influx mechanisms.

Contraction of urethral smooth muscle cells (USMCs) contributes to urinary continence. Ca2+ signalling in USMCs was investigated in intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs were spontaneously active in situ, firing intracellular Ca2+ waves that were asynchronous at different sites within cells and between adjacent cells. Spontaneous Ca2+ waves in USMCs were myogenic but enhanced by adrenergic or purinergic agonists and decreased by nitric oxide. Ca2+ waves arose from inositol trisphosphate type 1 receptors and ryanodine receptors, and Ca2+ influx by store-operated calcium entry was required to maintain Ca2+ release events. Ca2+ release and development of Ca2+ waves appear to be the primary source of Ca2+ for excitation-contraction coupling in the mouse urethra, and no evidence was found that voltage-dependent Ca2+ entry via L-type or T-type channels was required for responses to α adrenergic responses. Urethral smooth muscle cells (USMCs) generate myogenic tone and contribute to urinary continence. Currently, little is known about Ca2+ signalling in USMCs in situ, and therefore little is known about the source(s) of Ca2+ required for excitation-contraction coupling. We characterized Ca2+ signalling in USMCs within intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs fired spontaneous intracellular Ca2+ waves that did not propagate cell-to-cell across muscle bundles. Ca2+ waves increased dramatically in response to the α1 adrenoceptor agonist phenylephrine (10μm) and to ATP (10μm). Ca2+ waves were inhibited by the nitric oxide donor DEA NONOate (10μm). Ca2+ influx and release from sarcoplasmic reticulum stores contributed to Ca2+ waves, as Ca2+ free bathing solution and blocking the sarcoplasmic Ca2+ -ATPase abolished activity. Intracellular Ca2+ release involved cooperation between ryanadine receptors and inositol trisphosphate receptors, as tetracaine and ryanodine (100μm) and xestospongin C (1μm) reduced Ca2+ waves. Ca2+ waves were insensitive to L-type Ca2+ channel modulators nifedipine (1μm), nicardipine (1μm), isradipine (1μm) and FPL 64176 (1μm), and were unaffected by the T-type Ca2+ channel antagonists NNC-550396 (1μm) and TTA-A2 (1μm). Ca2+ waves were reduced by the store operated Ca2+ entry blocker SKF 96365 (10μm) and by an Orai antagonist, GSK-7975A (1μm). The latter also reduced urethral contractions induced by phenylephrine, suggesting that Orai can function effectively as a receptor-operated channel. In conclusion, Ca2+ waves in mouse USMCs are a source of Ca2+ for excitation-contraction coupling in urethral muscles.