Effects of rapamycin on ryanodine receptor/Ca(2+)-release channels from cardiac muscle

Calcium handling abnormalities increase arrhythmia susceptibility in DMSXL myotonic dystrophy type 1 mice

BACKGROUND

Myotonic dystrophy type 1 (DM1) is a multiorgan disorder with significant cardiac involvement. ECG abnormalities, including arrhythmias, occur in 80 % of DM1 patients and are the second-most common cause of death after respiratory complications; however, the mechanisms underlying the arrhythmogenesis remain unclear. The objective of this study was to investigate the basis of the electrophysiological abnormalities in DM1 using the DMSXL mouse model.

METHODS

ECG parameters were evaluated at baseline and post flecainide challenge. Calcium transient and action potential parameters were evaluated in Langendorff-perfused hearts using fluorescence optical mapping. Calcium transient/sparks were evaluated in ventricular myocytes via confocal microscopy. Protein and mRNA levels for calcium handling proteins were evaluated using western blot and RT-qPCR, respectively.

RESULTS

DMSXL mice showed arrhythmic events on ECG including premature ventricular contractions and sinus block. DMSXL mice showed increased calcium transient time to peak without any change to voltage parameters. Calcium alternans and both sustained and non-sustained ventricular tachyarrhythmias were readily inducible in DMSXL mice. The confocal experiments also showed calcium transient alternans and increased frequency of calcium sparks in DMSXL cardiomyocytes. These calcium abnormalities were correlated with increased RyR2 phosphorylation without changes to the other calcium handling proteins.

CONCLUSIONS

The DMSXL mouse model of DM1 exhibited enhanced arrhythmogenicity associated with abnormal intracellular calcium handling due to hyperphosphorylation of RyR2, pointing to RyR2 as a potential new therapeutic target in DM1 treatment.

Abnormal phosphorylation / dephosphorylation and Ca2+ dysfunction in heart failure

Heart failure (HF) can be caused by a variety of causes characterized by abnormal myocardial systole and diastole. Ca2+ current through the L-type calcium channel (LTCC) on the membrane is the initial trigger signal for a cardiac cycle. Declined systole and diastole in HF are associated with dysfunction of myocardial Ca2+ function. This disorder can be correlated with unbalanced levels of phosphorylation / dephosphorylation of LTCC, endoplasmic reticulum (ER), and myofilament. Kinase and phosphatase activity changes along with HF progress, resulting in phased changes in the degree of phosphorylation / dephosphorylation. It is important to realize the phosphorylation / dephosphorylation differences between a normal and a failing heart. This review focuses on phosphorylation / dephosphorylation changes in the progression of HF and summarizes the effects of phosphorylation / dephosphorylation of LTCC, ER function, and myofilament function in normal conditions and HF based on previous experiments and clinical research. Also, we summarize current therapeutic methods based on abnormal phosphorylation / dephosphorylation and clarify potential therapeutic directions.

Pharmacological mTOR inhibitors in ameliorating Alzheimer's disease: current review and perspectives

Traditionally, pharmacological mammalian/mechanistic targets of rapamycin (mTOR) kinase inhibitors have been used during transplantation and tumor treatment. Emerging pre-clinical evidence from the last decade displayed the surprising effectiveness of mTOR inhibitors in ameliorating Alzheimer's Disease (AD), a common neurodegenerative disorder characterized by progressive cognitive function decline and memory loss. Research shows mTOR activation as an early event in AD development, and inhibiting mTOR may promote the resolution of many hallmarks of Alzheimer's. Aberrant protein aggregation, including amyloid-beta (Aβ) deposition and tau filaments, and cognitive defects, are reversed upon mTOR inhibition. A closer inspection of the evidence highlighted a temporal dependence and a hallmark-specific nature of such beneficial effects. Time of administration relative to disease progression, and a maintenance of a functional lysosomal system, could modulate its effectiveness. Moreover, mTOR inhibition also exerts distinct effects between neurons, glial cells, and endothelial cells. Different pharmacological properties of the inhibitors also produce different effects based on different blood-brain barrier (BBB) entry capacities and mTOR inhibition sites. This questions the effectiveness of mTOR inhibition as a viable AD intervention strategy. In this review, we first summarize the different mTOR inhibitors available and their characteristics. We then comprehensively update and discuss the pre-clinical results of mTOR inhibition to resolve many of the hallmarks of AD. Key pathologies discussed include Aβ deposition, tauopathies, aberrant neuroinflammation, and neurovascular system breakdowns.

Isolated Cardiac Ryanodine Receptor Function Varies Between Mammals

Concerted robust opening of cardiac ryanodine receptors' (RyR2) Ca2+ release 1oplasmic reticulum (SR) is fundamental for normal systolic cardiac function. During diastole, infrequent spontaneous RyR2 openings mediate the SR Ca2+ leak that normally constrains SR Ca2+ load. Abnormal large diastolic RyR2-mediated Ca2+ leak events can cause delayed after depolarizations (DADs) and arrhythmias. The RyR2-associated mechanisms underlying these processes are being extensively studied at multiple levels utilizing various model animals. Since there are well-described species-specific differences in cardiac intracellular Ca2+ handing in situ, we tested whether or not single RyR2 function in vitro retains this species specificity. We isolated RyR2-rich heavy SR microsomes from mouse, rat, rabbit, and human ventricular muscle and quantified RyR2 function using identical solutions and methods. The single RyR2 cytosolic Ca2+ sensitivity was similar across these species. However, there were significant species differences in single RyR2 mean open times in both systole and diastole-like solutions. In diastole-like solutions, single rat/mouse RyR2 open probability and frequency of long openings (> 6 ms) were similar, but these values were significantly greater than those of either single rabbit or human RyR2s. We propose these in vitro single RyR2 functional differences across species stem from the species-specific RyR2 regulatory environment present in the source tissue. Our results show the single rabbit RyR2 functional attributes, particularly in diastole-like conditions, replicate those of single human RyR2 best among the species tested.

Dantrolene inhibition of ryanodine channels (RyR2) in artificial lipid bilayers depends on FKBP12.6

Dantrolene is a neutral hydantoin that is clinically used as a skeletal muscle relaxant to prevent overactivation of the skeletal muscle calcium release channel (RyR1) in response to volatile anesthetics. Dantrolene has aroused considerable recent interest as a lead compound for stabilizing calcium release due to overactive cardiac calcium release channels (RyR2) in heart failure. Previously, we found that dantrolene produces up to a 45% inhibition RyR2 with an IC50 of 160 nM, and that this inhibition requires the physiological association between RyR2 and CaM. In this study, we tested the hypothesis that dantrolene inhibition of RyR2 in the presence of CaM is modulated by RyR2 phosphorylation at S2808 and S2814. Phosphorylation was altered by incubations with either exogenous phosphatase (PP1) or kinases; PKA to phosphorylate S2808 or endogenous CaMKII to phosphorylate S2814. We found that PKA caused selective dissociation of FKBP12.6 from the RyR2 complex and a loss of dantrolene inhibition. Rapamycin-induced FKBP12.6 dissociation from RyR2 also resulted in the loss of dantrolene inhibition. Subsequent incubations of RyR2 with exogenous FKBP12.6 reinstated dantrolene inhibition. These findings indicate that the inhibitory action of dantrolene on RyR2 depends on RyR2 association with FKBP12.6 in addition to CaM as previously found.

Targeting ryanodine receptors to treat human diseases

This Review provides an update on ryanodine receptors (RyRs) and their role in human diseases of heart, muscle, and brain. Calcium (Ca2+) is a requisite second messenger in all living organisms. From C. elegans to mammals, Ca2+ is necessary for locomotion, bodily functions, and neural activity. However, too much of a good thing can be bad. Intracellular Ca2+ overload can result in loss of function and death. Intracellular Ca2+ release channels evolved to safely provide large, rapid Ca2+ signals without exposure to toxic extracellular Ca2+. RyRs are intracellular Ca2+ release channels present throughout the zoosphere. Over the past 35 years, our knowledge of RyRs has advanced to the level of atomic-resolution structures revealing their role in the mechanisms underlying the pathogenesis of human disorders of heart, muscle, and brain. Stress-induced RyR-mediated intracellular Ca2+ leak in the heart can promote heart failure and cardiac arrhythmias. In skeletal muscle, RyR1 leak contributes to muscle weakness in inherited myopathies, to age-related loss of muscle function and cancer-associated muscle weakness, and to impaired muscle function in muscular dystrophies, including Duchenne. In the brain, leaky RyR channels contribute to cognitive dysfunction in Alzheimer's disease, posttraumatic stress disorder, and Huntington's disease. Novel therapeutics targeting dysfunctional RyRs are showing promise.

Regulation of Ryanodine Receptor-Dependent Neurotransmitter Release by AIP, Calstabins, and Presenilins

Ryanodine receptors (RyRs) are Ca2+ release channels located in the endoplasmic reticulum membrane. Presynaptic RyRs play important roles in neurotransmitter release and synaptic plasticity. Recent studies suggest that the proper function of presynaptic RyRs relies on several regulatory proteins, including aryl hydrocarbon receptor-interacting protein, calstabins, and presenilins. Dysfunctions of these regulatory proteins can greatly impact neurotransmitter release and synaptic plasticity by altering the function or expression of RyRs. This chapter aims to describe the interaction between these proteins and RyRs, elucidating their crucial role in regulating synaptic function.

HIV-Tat Exacerbates the Actions of Atazanavir, Efavirenz, and Ritonavir on Cardiac Ryanodine Receptor (RyR2)

The incidence of sudden cardiac death (SCD) in people living with HIV infection (PLWH), especially those with inadequate viral suppression, is high and the reasons for this remain incompletely characterized. The timely opening and closing of type 2 ryanodine receptor (RyR2) is critical for ensuring rhythmic cardiac contraction-relaxation cycles, and the disruption of these processes can elicit Ca²⁺ waves, ventricular arrhythmias, and SCD. Herein, we show that the HIV protein Tat (HIV-Tat: 0-52 ng/mL) and therapeutic levels of the antiretroviral drugs atazanavir (ATV: 0-25,344 ng/mL), efavirenz (EFV: 0-11,376 ng/mL), and ritonavir (RTV: 0-25,956 ng/mL) bind to and modulate the opening and closing of RyR2. Abacavir (0-14,315 ng/mL), bictegravir (0-22,469 ng/mL), Rilpivirine (0-14,360 ng/mL), and tenofovir disoproxil fumarate (0-18,321 ng/mL) did not alter [³H]ryanodine binding to RyR2. Pretreating RyR2 with low HIV-Tat (14 ng/mL) potentiated the abilities of ATV and RTV to bind to open RyR2 and enhanced their ability to bind to EFV to close RyR2. In silico molecular docking using a Schrodinger Prime protein-protein docking algorithm identified three thermodynamically favored interacting sites for HIV-Tat on RyR2. The most favored site resides between amino acids (AA) 1702-1963; the second favored site resides between AA 467-1465, and the third site resides between AA 201-1816. Collectively, these new data show that HIV-Tat, ATV, EFV, and RTV can bind to and modulate the activity of RyR2 and that HIV-Tat can exacerbate the actions of ATV, EFV, and RTV on RyR2. Whether the modulation of RyR2 by these agents increases the risk of arrhythmias and SCD remains to be explored.

Role of oxidation of excitation-contraction coupling machinery in age-dependent loss of muscle function in C. elegans

Age-dependent loss of body wall muscle function and impaired locomotion occur within 2 weeks in C. elegans; however, the underlying mechanism has not been fully elucidated. In humans, age-dependent loss of muscle function occurs at about 80 years of age and has been linked to dysfunction of ryanodine receptor (RyR)/intracellular calcium (Ca²⁺) release channels on the sarcoplasmic reticulum (SR). Mammalian skeletal muscle RyR1 channels undergo age-related remodeling due to oxidative overload, leading to loss of the stabilizing subunit calstabin1 (FKBP12) from the channel macromolecular complex. This destabilizes the closed state of the channel resulting in intracellular Ca²⁺ leak, reduced muscle function, and impaired exercise capacity. We now show that the C. elegans RyR homolog, UNC-68, exhibits a remarkable degree of evolutionary conservation with mammalian RyR channels and similar age-dependent dysfunction. Like RyR1 in mammals UNC-68 encodes a protein that comprises a macromolecular complex which includes the calstabin1 homolog FKB-2 and is immunoreactive with antibodies raised against the RyR1 complex. Further, as in aged mammals, UNC-68 is oxidized and depleted of FKB-2 in an age-dependent manner, resulting in 'leaky' channels, depleted SR Ca²⁺ stores, reduced body wall muscle Ca²⁺ transients, and age-dependent muscle weakness. FKB-2 (ok3007)-deficient worms exhibit reduced exercise capacity. Pharmacologically induced oxidization of UNC-68 and depletion of FKB-2 from the channel independently caused reduced body wall muscle Ca²⁺ transients. Preventing FKB-2 depletion from the UNC-68 macromolecular complex using the Rycal drug S107 improved muscle Ca²⁺ transients and function. Taken together, these data suggest that UNC-68 oxidation plays a role in age-dependent loss of muscle function. Remarkably, this age-dependent loss of muscle function induced by oxidative overload, which takes ~2 years in mice and ~80 years in humans, occurs in less than 2-3 weeks in C. elegans, suggesting that reduced antioxidant capacity may contribute to the differences in life span amongst species.

Ryanodine receptor remodeling in cardiomyopathy and muscular dystrophy caused by lamin A/C gene mutation

Mutations in the lamin A/C gene (LMNA), which encodes A-type lamins, cause several diseases called laminopathies, the most common of which is dilated cardiomyopathy with muscular dystrophy. The role of Ca2+ regulation in these diseases remain poorly understood. We now show biochemical remodeling of the ryanodine receptor (RyR)/intracellular Ca2+ release channel in heart samples from human subjects with LMNA mutations, including protein kinase A-catalyzed phosphorylation, oxidation and depletion of the stabilizing subunit calstabin. In the LmnaH222P/H222P murine model of Emery-Dreifuss muscular dystrophy caused by LMNA mutation, we demonstrate an age-dependent biochemical remodeling of RyR2 in the heart and RyR1 in skeletal muscle. This RyR remodeling is associated with heart and skeletal muscle dysfunction. Defective heart and muscle function are ameliorated by treatment with a novel Rycal small molecule drug (S107) that fixes 'leaky' RyRs. SMAD3 phosphorylation is increased in hearts and diaphragms of LmnaH222P/H222P mice, which enhances NADPH oxidase binding to RyR channels, contributing to their oxidation. There is also increased generalized protein oxidation, increased calcium/calmodulin-dependent protein kinase II-catalyzed phosphorylation of RyRs and increased protein kinase A activity in these tissues. Our data show that RyR remodeling plays a role in cardiomyopathy and skeletal muscle dysfunction caused by LMNA mutation and identify these Ca2+ channels as a potential therapeutic target.

Mitochondrial Dysfunction Increases Arrhythmic Triggers and Substrates; Potential Anti-arrhythmic Pharmacological Targets

Incidence of cardiac arrhythmias increases significantly with age. In order to effectively stratify arrhythmic risk in the aging population it is crucial to elucidate the relevant underlying molecular mechanisms. The changes underlying age-related electrophysiological disruption appear to be closely associated with mitochondrial dysfunction. Thus, the present review examines the mechanisms by which age-related mitochondrial dysfunction promotes arrhythmic triggers and substrate. Namely, via alterations in plasmalemmal ionic currents (both sodium and potassium), gap junctions, cellular Ca²⁺ homeostasis, and cardiac fibrosis. Stratification of patients' mitochondrial function status permits application of appropriate anti-arrhythmic therapies. Here, we discuss novel potential anti-arrhythmic pharmacological interventions that specifically target upstream mitochondrial function and hence ameliorates the need for therapies targeting downstream changes which have constituted traditional antiarrhythmic therapy.

FK506 regulates Ca2+ release evoked by inositol 1,4,5-trisphosphate independently of FK-binding protein in endothelial cells

Background and Purpose

FK506 and rapamycin are modulators of FK‐binding proteins (FKBP) that are used to suppress immune function after organ and hematopoietic stem cell transplantations. The drugs share the unwanted side‐effect of evoking hypertension that is associated with reduced endothelial function and nitric oxide production. The underlying mechanisms are not understood. FKBP may regulate IP₃ receptors (IP₃R) and ryanodine receptors (RyR) to alter Ca²⁺ signalling in endothelial cells.

Experimental Approach

We investigated the effects of FK506 and rapamycin on Ca²⁺ release via IP₃R and RyR in hundreds of endothelial cells, using the indicator Cal‐520, in intact mesenteric arteries from male Sprague‐Dawley rats. IP₃Rs were activated by acetylcholine or localised photo‐uncaging of IP₃, and RyR by caffeine.

Key Results

While FKBPs were present, FKBP modulation with rapamycin did not alter IP₃‐evoked Ca²⁺ release. Conversely, FK506, which modulates FKBP and blocks calcineurin, increased IP₃‐evoked Ca²⁺ release. Inhibition of calcineurin (okadiac acid or cypermethrin) also increased IP₃‐evoked Ca²⁺ release and blocked FK506 effects. When calcineurin was inhibited, FK506 reduced IP₃‐evoked Ca²⁺ release. These findings suggest that IP₃‐evoked Ca²⁺ release is not modulated by FKBP, but by FK506‐mediated calcineurin inhibition. The RyR modulators caffeine and ryanodine failed to alter Ca²⁺ signalling suggesting that RyR is not functional in native endothelium.

Conclusion and Implications

The hypertensive effects of the immunosuppressant drugs FK506 and rapamycin, while mediated by endothelial cells, do not appear to be exerted at the documented cellular targets of Ca²⁺ release and altered FKBP binding to IP₃ and RyR.

Unbalance Between Sarcoplasmic Reticulum Ca2 + Uptake and Release: A First Step Toward Ca2 + Triggered Arrhythmias and Cardiac Damage

The present review focusses on the regulation and interplay of cardiac SR Ca²⁺ handling proteins involved in SR Ca²⁺ uptake and release, i.e., SERCa2/PLN and RyR2. Both RyR2 and SERCA2a/PLN are highly regulated by post-translational modifications and/or different partners' proteins. These control mechanisms guarantee a precise equilibrium between SR Ca²⁺ reuptake and release. The review then discusses how disruption of this balance alters SR Ca²⁺ handling and may constitute a first step toward cardiac damage and malignant arrhythmias. In the last part of the review, this concept is exemplified in different cardiac diseases, like prediabetic and diabetic cardiomyopathy, digitalis intoxication and ischemia-reperfusion injury.

Influence of mTOR signaling pathway on ketamine-induced injuries in the hippocampal neurons of rats

OBJECTIVE

To explore the influences of mammalian target of rapamycin (mTOR) signaling pathway on ketamine-induced apoptosis, oxidative stress and Ca²⁺ concentration in the hippocampal neurons of rats.

METHODS

The primary hippocampal neurons isolated from fetal Sprague Dawley rats were treated with ketamine (0, 50, 100 and 500 μM) for 4 days to observe its effect on mTOR signaling pathway and apoptosis of rat hippocampal neurons. Then, the hippocampal neurons were divided into C (Control), R (Rapamycin, an inhibitor of mTOR signaling pathway), K (Ketamine) and R + K (Rapamycin + Ketamine) groups to detect the apoptosis, reactive oxygen species (ROS) production, and Ca²⁺ concentration via the terminal transferase uridyl nick end labelling (TUNEL) assay, dichloro-dihydro-fluorescein diacetate (DCFH-DA) method and Fluo-3 acetoxymethyl ester (Fluo-3AM) staining, respectively. The expressions of mTOR signaling pathway and apoptosis-related proteins in hippocampal neurons were examined by qRT-PCR and Western blot.

RESULTS

Ketamine could dose-dependently promote the apoptosis of rat hippocampal neurons with upregulation of p-mTOR and its downstream regulators (p-4E-BP-1 and p-p70S6K). However, ketamine-induced apoptosis in hippocampal neurons was reversed significantly by the administration of rapamycin, as evident by the decrease in expressions of pro-apoptotic proteins (Bax and cleaved Caspase-3) and the increase in anti-apoptotic protein (Bcl-2). Meanwhile, the ROS generation and Ca²⁺ concentration was inhibited accompanied with reduced malonildialdehyde levels but elevated superoxide and glutathione peroxidase activities.

CONCLUSION

Inhibition of mTOR signaling pathway protected rat hippocampal neurons from ketamine-induced injuries via reducing apoptosis, oxidative stress, as well as Ca²⁺ concentration.

ABBREVIATIONS

mTOR: mammalian target of rapamycin; SD: Sprague-Dawley; SPF: Specific-pathogen free; ROS: reactive oxygen species; TUNEL: terminal transferase uridyl nick end labelling; DCFH-DA: Dichloro-dihydro-fluorescein diacetate; Fluo-3A: Fluo-3 acetoxymethyl ester; NMDAR: non-competitive N-methyl-D-aspartame glutamate receptor; 4E-BP1: 4E binding protein 1; p70S6K: p70 S6 Kinase; PCR: Polymerase chain reaction; MDA: malonildialdehyde; GSH-PX: glutathione peroxidase; ANOVA: One-way Analysis of Variance.

Toll-like receptor 4-induced ryanodine receptor 2 oxidation and sarcoplasmic reticulum Ca2+ leakage promote cardiac contractile dysfunction in sepsis

Studies suggest the potential role of a sarcoplasmic reticulum (SR) Ca²⁺ leak in cardiac contractile dysfunction in sepsis. However, direct supporting evidence is lacking, and the mechanisms underlying this SR leak are poorly understood. Here, we investigated the changes in cardiac Ca²⁺ handling and contraction in LPS-treated rat cardiomyocytes and a mouse model of polymicrobial sepsis produced by cecal ligation and puncture (CLP). LPS decreased the systolic Ca²⁺ transient and myocyte contraction as well as SR Ca²⁺ content. Meanwhile, LPS increased Ca²⁺ spark-mediated SR Ca²⁺ leak. Preventing the SR leak with ryanodine receptor (RyR) blocker tetracaine restored SR load and increased myocyte contraction. Similar alterations in Ca²⁺ handling were observed in cardiomyocytes from CLP mice. Treatment with JTV-519, an anti-SR leak drug, restored Ca²⁺ handling and improved cardiac function. In the LPS-treated cardiomyocytes, mitochondrial reactive oxygen species and oxidative stress in RyR2 were increased, whereas the levels of the RyR2-associated FK506-binding protein 1B (FKBP12.6) were decreased. The Toll-like receptor 4 (TLR4)-specific inhibitor TAK-242 reduced the oxidative stress in LPS-treated cells, decreased the SR leak, and normalized Ca²⁺ handling and myocyte contraction. Consistently, TLR4 deletion significantly improved cardiac function and corrected abnormal Ca²⁺ handling in the CLP mice. This study provides evidence for the critical role of the SR Ca²⁺ leak in the development of septic cardiomyopathy and highlights the therapeutic potential of JTV-519 by preventing SR leak. Furthermore, it reveals that TLR4 activation-induced mitochondrial reactive oxygen species production and the resulting oxidative stress in RyR2 contribute to the SR Ca²⁺ leak.

Role of FK506-binding protein in Ca2+ spark regulation

The elementary Ca²⁺ release events, Ca²⁺ sparks, has been found for a quarter of century. However, the molecular regulation of the spark generator, the ryanodine receptor (RyR) on the sarcoplasmic reticulum, remains obscure. Although each subunit of the RyR homotetramer has a site for FK506-binding protein (FKBP), the role of FKBPs in modifying RyR Ca²⁺ sparks has been debated for long. One of the reasons behind the controversy is that most previous studies detect spontaneous sparks, where the mixture with out-of-focus events and local wavelets prevents an accurate characterization of Ca²⁺ sparks. In the present study, we detected Ca²⁺ sparks triggered by single L-type Ca²⁺ channels (LCCs) under loose-seal patch clamp conditions in FK506-treated or FKBP12.6 knockout cardiomyocytes. We found that FKBP dissociation both by FK506 and by rapamycin decreased the Ca²⁺ spark amplitude in ventricular cardiomyocytes. This change was neither due to decreased releasable Ca²⁺ in the sarcoplasmic reticulum, nor explained by changed RyR sensitivity. Actually FK506 increased the LCC-RyR coupling probability and curtailed the latency for an LCC to trigger a RyR Ca²⁺ spark. FKBP12.6 knockout had similar effects as FK506/rapamycin treatment, indicating that the decreased spark amplitude was attributable to the dissociation of FKBP12.6 rather than FKBP12. We also explained how decreased amplitude of spontaneous sparks after FKBP dissociation sometimes appears to be increased or unchanged due to inappropriate data processing. Our results provided firm evidence that without the inter-RyR coordination by functional FKBP12.6, the RyR recruitment during a Ca²⁺ spark would be compromised despite the sensitization of individual RyRs.

Sensitized signalling between L-type Ca2+ channels and ryanodine receptors in the absence or inhibition of FKBP12.6 in cardiomyocytes

Aims

The heart contraction is controlled by the Ca2+-induced Ca2+ release (CICR) between L-type Ca2+ channels and ryanodine receptors (RyRs). The FK506-binding protein FKBP12.6 binds to RyR subunits, but its role in stabilizing RyR function has been debated for long. Recent reports of high-resolution RyR structure show that the HD2 domain that binds to the SPRY2 domain of neighbouring subunit in FKBP-bound RyR1 is detached and invisible in FKBP-null RyR2. The present study was to test the consequence of FKBP12.6 absence on the in situ activation of RyR2.

Methods and results

Using whole-cell patch-clamp combined with confocal imaging, we applied a near threshold depolarization to activate a very small fraction of LCCs, which in turn activated RyR Ca2+ sparks stochastically. FKBP12.6-knockout and FK506/rapamycin treatments increased spark frequency and LCC-RyR coupling fidelity without altering LCC open probability. Neither FK506 nor rapamycin further altered LCC-RyR coupling fidelity in FKBP12.6-knockout cells. In loose-seal patch-clamp experiments, the LCC-RyR signalling kinetics, indexed by the delay for a LCC sparklet to trigger a RyR spark, was accelerated after FKBP12.6 knockout and FK506/rapamycin treatments. These results demonstrated that RyRs became more sensitive to Ca2+ triggers without FKBP12.6. Isoproterenol (1 μM) further accelerated the LCC-RyR signalling in FKBP12.6-knockout cells. The synergistic sensitization of RyRs by catecholaminergic signalling and FKBP12.6 dysfunction destabilized the CICR system, leading to chaotic Ca2+ waves and ventricular arrhythmias.

Conclusion

FKBP12.6 keeps the RyRs from over-sensitization, stabilizes the potentially regenerative CICR system, and thus may suppress the life-threatening arrhythmogenesis.

Targeting deregulated AMPK/mTORC1 pathways improves muscle function in myotonic dystrophy type I

Myotonic dystrophy type I (DM1) is a disabling multisystemic disease that predominantly affects skeletal muscle. It is caused by expanded CTG repeats in the 3'-UTR of the dystrophia myotonica protein kinase (DMPK) gene. RNA hairpins formed by elongated DMPK transcripts sequester RNA-binding proteins, leading to mis-splicing of numerous pre-mRNAs. Here, we have investigated whether DM1-associated muscle pathology is related to deregulation of central metabolic pathways, which may identify potential therapeutic targets for the disease. In a well-characterized mouse model for DM1 (HSALR mice), activation of AMPK signaling in muscle was impaired under starved conditions, while mTORC1 signaling remained active. In parallel, autophagic flux was perturbed in HSALR muscle and in cultured human DM1 myotubes. Pharmacological approaches targeting AMPK/mTORC1 signaling greatly ameliorated muscle function in HSALR mice. AICAR, an AMPK activator, led to a strong reduction of myotonia, which was accompanied by partial correction of misregulated alternative splicing. Rapamycin, an mTORC1 inhibitor, improved muscle relaxation and increased muscle force in HSALR mice without affecting splicing. These findings highlight the involvement of AMPK/mTORC1 deregulation in DM1 muscle pathophysiology and may open potential avenues for the treatment of this disease.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Structural Basis for Gating and Activation of RyR1

The type-1 ryanodine receptor (RyR1) is an intracellular calcium (Ca(2+)) release channel required for skeletal muscle contraction. Here, we present cryo-EM reconstructions of RyR1 in multiple functional states revealing the structural basis of channel gating and ligand-dependent activation. Binding sites for the channel activators Ca(2+), ATP, and caffeine were identified at interdomain interfaces of the C-terminal domain. Either ATP or Ca(2+) alone induces conformational changes in the cytoplasmic assembly ("priming"), without pore dilation. In contrast, in the presence of all three activating ligands, high-resolution reconstructions of open and closed states of RyR1 were obtained from the same sample, enabling analyses of conformational changes associated with gating. Gating involves global conformational changes in the cytosolic assembly accompanied by local changes in the transmembrane domain, which include bending of the S6 transmembrane segment and consequent pore dilation, displacement, and deformation of the S4-S5 linker and conformational changes in the pseudo-voltage-sensor domain.

Bcl-2 and FKBP12 bind to IP3 and ryanodine receptors at overlapping sites: the complexity of protein-protein interactions for channel regulation

The 12- and 12.6-kDa FK506-binding proteins, FKBP12 (12-kDa FK506-binding protein) and FKBP12.6 (12.6-kDa FK506-binding protein), have been implicated in the binding to and the regulation of ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP3Rs), both tetrameric intracellular Ca2+-release channels. Whereas the amino acid sequences responsible for FKBP12 binding to RyRs are conserved in IP3Rs, FKBP12 binding to IP3Rs has been questioned and could not be observed in various experimental models. Nevertheless, conservation of these residues in the different IP3R isoforms and during evolution suggested that they could harbour an important regulatory site critical for IP3R-channel function. Recently, it has become clear that in IP3Rs, this site was targeted by B-cell lymphoma 2 (Bcl-2) via its Bcl-2 homology (BH)4 domain, thereby dampening IP3R-mediated Ca2+ flux and preventing pro-apoptotic Ca2+ signalling. Furthermore, vice versa, the presence of the corresponding site in RyRs implied that Bcl-2 proteins could associate with and regulate RyR channels. Recently, the existence of endogenous RyR-Bcl-2 complexes has been identified in primary hippocampal neurons. Like for IP3Rs, binding of Bcl-2 to RyRs also involved its BH4 domain and suppressed RyR-mediated Ca2+ release. We therefore propose that the originally identified FKBP12-binding site in IP3Rs is a region critical for controlling IP3R-mediated Ca2+ flux by recruiting Bcl-2 rather than FKBP12. Although we hypothesize that anti-apoptotic Bcl-2 proteins, but not FKBP12, are the main physiological inhibitors of IP3Rs, we cannot exclude that Bcl-2 could help engaging FKBP12 (or other FKBP isoforms) to the IP3R, potentially via calcineurin.

mTOR signaling in mice with dysfunctional cardiac ryanodine receptor ion channel

Simultaneous substitution of three amino acid residues in the calmodulin binding domain (W3587A/L3591D/F3603A, ADA) of the cardiac ryanodine receptor ion channel (RyR2) impairs calmodulin inhibition of RyR2 and causes cardiac hypertrophy and early death of Ryr2^ADA/ADA mice. To determine the physiological significance of growth promoting signaling molecules, the protein and phosphorylation levels of Ser/Thr kinase mTOR and upstream and downstream signaling molecules were determined in hearts of wild-type and Ryr2^ADA/ADA mice. Phosphorylation of mTOR at Ser-2448, and mTOR downstream targets p70S6 kinase at Thr-389, S6 ribosomal protein at Ser-240/244, and 4E-BP1 at Ser-65 were increased. However, there was no increased phosphorylation of mTOR upstream kinases PDK1 at Ser-241, AKT at Thr-308, AMPK at Thr-172, and ERK1/2 at Thr-202/Tyr204. To confirm a role for mTOR signaling in the development of cardiac hypertrophy, rapamycin, an inhibitor of mTOR, was injected into wild-type and mutant mice. Rapamycin decreased mouse heart-to-body weight ratio, improved cardiac performance, and decreased phosphorylation of mTOR and downstream targets p70S6K and S6 in 10-day-old Ryr2^ADA/ADA mice but did not extend longevity. Taken together, the results link a dysfunctional RyR2 to an altered activity of signaling molecules that regulate cardiac growth and function.

Microdomain-specific localization of functional ion channels in cardiomyocytes: an emerging concept of local regulation and remodelling

Cardiac excitation involves the generation of action potential by individual cells and the subsequent conduction of the action potential from cell to cell through intercellular gap junctions. Excitation of the cellular membrane results in opening of the voltage-gated L-type calcium ion (Ca2+) channels, thereby allowing a small amount of Ca2+ to enter the cell, which in turn triggers the release of a much greater amount of Ca2+ from the sarcoplasmic reticulum, the intracellular Ca2+ store, and gives rise to the systolic Ca2+ transient and contraction. These processes are highly regulated by the autonomic nervous system, which ensures the acute and reliable contractile function of the heart and the short-term modulation of this function upon changes in heart rate or workload. It has recently become evident that discrete clusters of different ion channels and regulatory receptors are present in the sarcolemma, where they form an interacting network and work together as a part of a macro-molecular signalling complex which in turn allows the specificity, reliability and accuracy of the autonomic modulation of the excitation-contraction processes by a variety of neurohormonal pathways. Disruption in subcellular targeting of ion channels and associated signalling proteins may contribute to the pathophysiology of a variety of cardiac diseases, including heart failure and certain arrhythmias. Recent methodological advances have made it possible to routinely image the topography of live cardiomyocytes, allowing the study of clustering functional ion channels and receptors as well as their coupling within a specific microdomain. In this review we highlight the emerging understanding of the functionality of distinct subcellular microdomains in cardiac myocytes (e.g. T-tubules, lipid rafts/caveolae, costameres and intercalated discs) and their functional role in the accumulation and regulation of different subcellular populations of sodium, Ca2+ and potassium ion channels and their contributions to cellular signalling and cardiac pathology.

FKBP12.6 activates RyR1: investigating the amino acid residues critical for channel modulation

We have previously shown that FKBP12 associates with RyR2 in cardiac muscle and that it modulates RyR2 function differently to FKBP12.6. We now investigate how these proteins affect the single-channel behavior of RyR1 derived from rabbit skeletal muscle. Our results show that FKBP12.6 activates and FKBP12 inhibits RyR1. It is likely that both proteins compete for the same binding sites on RyR1 because channels that are preactivated by FKBP12.6 cannot be subsequently inhibited by FKBP12. We produced a mutant FKBP12 molecule (FKBP12E31Q/D32N/W59F) where the residues Glu(31), Asp(32), and Trp(59) were converted to the corresponding residues in FKBP12.6. With respect to the functional regulation of RyR1 and RyR2, the FKBP12E31Q/D32N/W59F mutant lost all ability to behave like FKBP12 and instead behaved like FKBP12.6. FKBP12E31Q/D32N/W59F activated RyR1 but was not capable of activating RyR2. In conclusion, FKBP12.6 activates RyR1, whereas FKBP12 activates RyR2 and this selective activator phenotype is determined within the amino acid residues Glu(31), Asp(32), and Trp(59) in FKBP12 and Gln(31), Asn(32), and Phe(59) in FKBP12.6. The opposing but different effects of FKBP12 and FKBP12.6 on RyR1 and RyR2 channel gating provide scope for diversity of regulation in different tissues.

Dissociation of FK506-binding protein 12.6 kD from ryanodine receptor in bronchial smooth muscle cells in airway hyperresponsiveness in asthma

Airway hyperresponsiveness (AHR) in asthma is predominantly caused by increased sensitivity of bronchial smooth muscle cells (BSMCs) to stimuli. The sarcoplasmic reticulum (SR)-Ca(2+) release channel, known as ryanodine receptor (RyR), mediates the contractive response of BSMCs to stimuli. FK506-binding protein 12.6 kD (FKBP12.6) stabilizes the RyR2 channel in a closed state. However, the interaction of FKBP12.6 with RyR2 in AHR remains unknown. This study examined the interaction of FKBP12.6 with RyR2 in BSMCs in AHR of asthma. The interaction of FKBP12.6 with RyR2 and FKBP12.6 expression was determined in a rat asthma model and in BSMCs treated with inflammatory cytokines. The calcium responses to contractile agonists were determined in BSMCs with overexpression and knockdown of FKBP12.6. Asthmatic serum, IL-5, IL-13, and TNF-α enhance the calcium response of BSMCs to contractile agonists and cause dissociation of FKBP12.6 from RyR2 and a decrease in FKBP12.6 gene expression in BSMCs in culture and in ovalbumin (OVA)-sensitized and -challenged rats. Knockdown of FKBP12.6 in BSMCs causes a decrease in the association of RyR2 with FKBP12.6 and an increase in the calcium response of BSMCs. Overexpression of FKBP12.6 increases the association of FKBP12.6 with RyR2, decreases the calcium response of BSMCs, and normalizes airway responsiveness in OVA-sensitized and -challenged rats. Dissociation of FKBP12.6 from RyR2 in BSMCs is responsible for the increased calcium response contributing to AHR in asthma. Manipulating the interaction of FKBP12.6 with RyR2 might be a novel and useful treatment for asthma.

Dysfunctional ryanodine receptors in the heart: new insights into complex cardiovascular diseases

Calcium dependent signaling is highly regulated in cardiomyocytes and determines the force of cardiac muscle contraction. The cardiac ryanodine receptors (RyR2) play important roles in health and disease. Modulation of RyR2 by phosphorylation is required for sympathetic regulation of cardiac function. Abnormal regulation of RyR2 contributes to heart failure, and atrial and ventricular arrhythmias. RyR2 channels are oxidized, nitrosylated, and hyperphosphorylated by protein kinase A (PKA) in heart failure, resulting in "leaky" channels. These leaky RyR2 channels contribute to depletion of calcium from the sarcoplasmic reticulum, resulting in defective cardiac excitation-contraction coupling. In this review, we discuss both the importance of PKA and calcium/calmodulin-dependent kinase II (CaMKII) regulation of RyR2 in health, and how altered phosphorylation, nitrosylation and oxidation of RyR2 channels lead to cardiac disease. Correcting these defects using either genetic manipulation (knock-in) in mice, or specific and novel small molecules ameliorates the RyR2 dysfunction, reducing the progression to heart failure and the incidence of arrhythmias.

Stabilization of the skeletal muscle ryanodine receptor ion channel-FKBP12 complex by the 1,4-benzothiazepine derivative S107

Activation of the skeletal muscle ryanodine receptor (RyR1) complex results in the rapid release of Ca²⁺ from the sarcoplasmic reticulum and muscle contraction. Dissociation of the small FK506 binding protein 12 subunit (FKBP12) increases RyR1 activity and impairs muscle function. The 1,4-benzothiazepine derivative JTV519, and the more specific derivative S107 (2,3,4,5,-tetrahydro-7-methoxy-4-methyl-1,4-benzothiazepine), are thought to improve skeletal muscle function by stabilizing the RyR1-FKBP12 complex. Here, we report a high degree of nonspecific and specific low affinity [³H]S107 binding to SR vesicles. SR vesicles enriched in RyR1 bound ∼48 [³H]S107 per RyR1 tetramer with EC₅₀ ∼52 µM and Hillslope ∼2. The effects of S107 and FKBP12 on RyR1 were examined under conditions that altered the redox state of RyR1. S107 increased FKBP12 binding to RyR1 in SR vesicles in the presence of reduced glutathione and the NO-donor NOC12, with no effect in the presence of oxidized glutathione. Addition of 0.15 µM FKBP12 to SR vesicles prevented FKBP12 dissociation; however, in the presence of oxidized glutathione and NOC12, FKBP12 dissociation was observed in skeletal muscle homogenates that contained 0.43 µM myoplasmic FKBP12 and was attenuated by S107. In single channel measurements with FKBP12-depleted RyR1s, in the absence and presence of NOC12, S107 augmented the FKBP12-mediated decrease in channel activity. The data suggest that S107 can reverse the harmful effects of redox active species on SR Ca²⁺ release in skeletal muscle by binding to RyR1 low affinity sites.

Beyond the cardiac myofilament: hypertrophic cardiomyopathy- associated mutations in genes that encode calcium-handling proteins

Traditionally regarded as a genetic disease of the cardiac sarcomere, hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disease and a significant cause of sudden cardiac death. While the most common etiologies of this phenotypically diverse disease lie in a handful of genes encoding critical contractile myofilament proteins, approximately 50% of patients diagnosed with HCM worldwide do not host sarcomeric gene mutations. Recently, mutations in genes encoding calcium-sensitive and calcium-handling proteins have been implicated in the pathogenesis of HCM. Among these are mutations in TNNC1- encoded cardiac troponin C, PLN-encoded phospholamban, and JPH2-encoded junctophilin 2 which have each been associated with HCM in multiple studies. In addition, mutations in RYR2-encoded ryanodine receptor 2, CASQ2-encoded calsequestrin 2, CALR3-encoded calreticulin 3, and SRI-encoded sorcin have been associated with HCM, although more studies are required to validate initial findings. While a relatively uncommon cause of HCM, mutations in genes that encode calcium-handling proteins represent an emerging genetic subset of HCM. Furthermore, these naturally occurring disease-associated mutations have provided useful molecular tools for uncovering novel mechanisms of disease pathogenesis, increasing our understanding of basic cardiac physiology, and dissecting important structure-function relationships within these proteins.

Mammalian target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) control the dendritic arbor morphology of hippocampal neurons

Dendrites are the main site of information input into neurons. Their development is a multistep process controlled by mammalian target of rapamycin (mTOR) among other proteins. mTOR is a serine/threonine protein kinase that forms two functionally distinct complexes in mammalian cells: mTORC1 and mTORC2. However, the one that contributes to mammalian neuron development remains unknown. This work used short hairpin RNA against Raptor and Rictor, unique components of mTORC1 and mTORC2, respectively, to dissect mTORC involvement in this process. We provide evidence that both mTOR complexes are crucial for the proper dendritic arbor morphology of hippocampal neurons. These two complexes are required for dendritic development both under basal conditions and upon the induction of mTOR-dependent dendritic growth. We also identified Akt as a downstream effector of mTORC2 needed for proper dendritic arbor morphology, the action of which required mTORC1 and p70S6K1.

FKBP12 activates the cardiac ryanodine receptor Ca2+-release channel and is antagonised by FKBP12.6

Changes in FKBP12.6 binding to cardiac ryanodine receptors (RyR2) are implicated in mediating disturbances in Ca(2+)-homeostasis in heart failure but there is controversy over the functional effects of FKBP12.6 on RyR2 channel gating. We have therefore investigated the effects of FKBP12.6 and another structurally similar molecule, FKBP12, which is far more abundant in heart, on the gating of single sheep RyR2 channels incorporated into planar phospholipid bilayers and on spontaneous waves of Ca(2+)-induced Ca(2+)-release in rat isolated permeabilised cardiac cells. We demonstrate that FKBP12 is a high affinity activator of RyR2, sensitising the channel to cytosolic Ca(2+), whereas FKBP12.6 has very low efficacy, but can antagonise the effects of FKBP12. Mathematical modelling of the data shows the importance of the relative concentrations of FKBP12 and FKBP12.6 in determining RyR2 activity. Consistent with the single-channel results, physiological concentrations of FKBP12 (3 µM) increased Ca(2+)-wave frequency and decreased the SR Ca(2+)-content in cardiac cells. FKBP12.6, itself, had no effect on wave frequency but antagonised the effects of FKBP12.We provide a biophysical analysis of the mechanisms by which FK-binding proteins can regulate RyR2 single-channel gating. Our data indicate that FKBP12, in addition to FKBP12.6, may be important in regulating RyR2 function in the heart. In heart failure, it is possible that an alteration in the dual regulation of RyR2 by FKBP12 and FKBP12.6 may occur. This could contribute towards a higher RyR2 open probability, 'leaky' RyR2 channels and Ca(2+)-dependent arrhythmias.

Disparities in the association of the ryanodine receptor and the FK506-binding proteins in mammalian heart

The FK506-binding proteins (FKBP12 and FKBP12.6; also known as FKBP1A and FKBP1B, respectively) are accessory subunits of the ryanodine receptor (RyR) Ca(2+) release channel. Aberrant RyR2-FKBP12.6 interactions have been proposed to be the underlying cause of channel dysfunction in acquired and inherited cardiac disease. However, the stoichiometry of the RyR2 association with FKBP12 or FKBP12.6 in mammalian heart is currently unknown. Here, we describe detailed quantitative analysis of cardiac stoichiometry between RyR2 and FKBP12 or FKBP12.6 using immunoblotting and [(3)H]ryanodine-binding assays, revealing striking disparities between four mammalian species. In mouse and pig heart, RyR2 is found complexed with both FKBP12 and FKBP12.6, although the former is the most abundant isoform. In rat heart, RyR2 is predominantly associated with FKBP12.6, whereas in rabbit it is associated with FKBP12 only. Co-immunoprecipitation experiments demonstrate RyR2-specific interaction with both FKBP isoforms in native cardiac tissue. Assuming four FKBP-binding sites per RyR2 tetramer, only a small proportion of available sites are occupied by endogenous FKBP12.6. FKBP interactions with RyR2 are very strong and resistant to drug (FK506, rapamycin and cyclic ADPribose) and redox (H(2)O(2) and diamide) treatment. By contrast, the RyR1-FKBP12 association in skeletal muscle is readily disrupted under oxidative conditions. This is the first study to directly assess association of endogenous FKBP12 and FKBP12.6 with RyR2 in native cardiac tissue. Our results challenge the widespread perception that RyR2 associates exclusively with FKBP12.6 to near saturation, with important implications for the role of the FK506-binding proteins in RyR2 pathophysiology and cardiac disease.

Rapamycin (sirolimus) protects against hypoxic damage in primary heart cultures via Na+/Ca2+ exchanger activation

AIMS

Rapamycin (sirolimus) is an antibiotic that inhibits protein synthesis through mammalian targeting of rapamycin (mTOR) signaling, and is used as an immunosuppressant in the treatment of organ rejection in transplant recipients. Rapamycin confers preconditioning-like protection against ischemic-reperfusion injury in isolated mouse heart cultures. Our aim was to further define the role of rapamycin in intracellular Ca(2+) homeostasis and to investigate the mechanism by which rapamycin protects cardiomyocytes from hypoxic damage.

MAIN METHODS

We demonstrate here that rapamycin protects rat heart cultures from hypoxic-reoxygenation (H/R) damage, as revealed by assays of lactate dehydrogenase (LDH) and creatine kinase (CK) leakage to the medium, by MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) measurements, and desmin immunostaining. As a result of hypoxia, intracellular calcium levels ([Ca(2+)](i)) were elevated. However, treatment of heart cultures with rapamycin during hypoxia attenuated the increase of [Ca(2+)](i). Rapamycin also attenuated (45)Ca(2+) uptake into the sarcoplasmic reticulum (SR) of skinned heart cultures in a dose- and time-dependent manner. KB-R7943, which inhibits the "reverse" mode of Na(+)/Ca(2+) exchanger (NCX), protected heart cultures from H/R damage with or without the addition of rapamycin. Rapamycin decreased [Ca(2+)](i) following its elevation by extracellular Ca(2+) ([Ca(2+)](o)) influx, thapsigargin treatment, or depolarization with KCl.

KEY FINDINGS

We suggest that rapamycin induces cardioprotection against hypoxic/reoxygenation damage in primary heart cultures by stimulating NCX to extrude Ca(2+) outside the cardiomyocytes.

SIGNIFICANCE

According to our findings, rapamycin preserves Ca(2+) homeostasis and prevents Ca(2+) overload via extrusion of Ca(2+) surplus outside the sarcolemma, thereby protecting the cells from hypoxic stress.

Is ryanodine receptor phosphorylation key to the fight or flight response and heart failure?

In situations of stress the heart beats faster and stronger. According to Marks and colleagues, this response is, to a large extent, the consequence of facilitated Ca²+ release from intracellular Ca²+ stores via ryanodine receptor 2 (RyR2), thought to be due to catecholamine-induced increases in RyR2 phosphorylation at serine 2808 (S2808). If catecholamine stimulation is sustained (for example, as occurs in heart failure), RyR2 becomes hyperphosphorylated and "leaky," leading to arrhythmias and other pathology. This "leaky RyR2 hypothesis" is highly controversial. In this issue of the JCI, Marks and colleagues report on two new mouse lines with mutations in S2808 that provide strong evidence supporting their theory. Moreover, the experiments revealed an influence of redox modifications of RyR2 that may account for some discrepancies in the field.

The ryanodine receptor in cardiac physiology and disease

According to the American Heart Association it is estimated that the United States will spend close to $39 billion in 2010 to treat over five million Americans suffering from heart failure. Patients with heart failure suffer from dyspnea and decreased exercised tolerance and are at increased risk for fatal ventricular arrhythmias. Food and Drug Administration -approved pharmacologic therapies for heart failure include diuretics, inhibitors of the renin-angiotensin system, and β-adrenergic receptor antagonists. Over the past 20 years advances in the field of ryanodine receptor (RyR2)/calcium release channel research have greatly advanced our understanding of cardiac physiology and the pathogenesis of heart failure and arrhythmias. Here we review the key observations, controversies, and discoveries that have led to the development of novel compounds targeting the RyR2/calcium release channel for treating heart failure and for preventing lethal arrhythmias.

Fixing ryanodine receptor Ca leak - a novel therapeutic strategy for contractile failure in heart and skeletal muscle

A critical component in regulating cardiac and skeletal muscle contractility is the release of Ca(2+) via ryanodine receptor (RyR) Ca(2+) release channels in the sarcoplasmic reticulum (SR). In heart failure and myopathy, the RyR has been found to be excessively phosphorylated or nitrosylated and depleted of the RyR-stabilizing protein calstabin (FK506 binding protein 12/12.6). This remodeling of the RyR channel complex results in an intracellular SR Ca(2+) leak and impaired contractility. Despite recent advances in heart failure treatment, there are still devastatingly high mortality rates with this disease. Moreover, pharmacological treatment for muscle weakness and myopathy is nearly nonexistent. A novel class of RyR-stabilizing drugs, rycals, which reduce Ca(2+) leak by stabilizing the RyR channels due to preservation of the RyR-calstabin interaction, have recently been shown to improve contractile function in both heart and skeletal muscle. This opens up a novel therapeutic strategy for the treatment of contractile failure in the cardiac and skeletal muscle.

Kinetics of FKBP12.6 binding to ryanodine receptors in permeabilized cardiac myocytes and effects on Ca sparks

RATIONALE

FK506-binding proteins FKBP12.6 and FKBP12 are associated with cardiac ryanodine receptors (RyR2), and cAMP-dependent protein kinase A (PKA)-dependent phosphorylation of RyR2 was proposed to interrupt FKBP12.6-RyR2 association and activate RyR2. However, the function of FKBP12.6/12 and role of PKA phosphorylation in cardiac myocytes are controversial.

OBJECTIVE

To directly measure in situ binding of FKBP12.6/12 to RyR2 in ventricular myocytes, with simultaneous Ca sparks measurements as a RyR2 functional index.

METHODS AND RESULTS

We used permeabilized rat and mouse ventricular myocytes, and fluorescently-labeled FKBP12.6/12. Both FKBP12.6 and FKBP12 concentrate at Z-lines, consistent with RyR2 and Ca spark initiation sites. However, only FKBP12.6 inhibits resting RyR2 activity. Assessment of fluorescent FKBP binding in myocyte revealed a high FKBP12.6-RyR2 affinity (K(d)=0.7+/-0.1 nmol/L) and much lower FKBP12-RyR2 affinity (K(d)=206+/-70 nmol/L). Fluorescence recovery after photobleach confirmed this K(d) difference and showed that it is mediated by k(off). RyR2 phosphorylation by PKA did not alter binding kinetics or affinity of FKBP12.6/12 for RyR2. Using quantitative immunoblots, we determined endogenous [FKBP12] in intact myocytes is approximately 1 micromol/L (similar to [RyR]), whereas [FKBP12.6] is <or=150 nmol/L.

CONCLUSIONS

Only 10% to 20% of endogenous myocyte RyR2s have FKBP12.6 associated, but virtually all myocyte FKBP12.6 is RyR2-bound (because of very high affinity). FKBP12.6 but not FKBP12 inhibits basal RyR2 activity. PKA-dependent RyR2 phosphorylation has no significant effect on binding of either FKBP12 or 12.6 to RyR2 in myocytes.

Ion channels, transporters, and pumps as targets for heart failure therapy

Congestive heart failure is a leading cause of morbidity and mortality. Congestive heart failure is marked by atrial and ventricular enlargements and reduced cardiac contractility and an association with an increased incidence of atrial and ventricular arrhythmias and sudden cardiac death. Dysfunctional ion channel function is one of the major underlying mechanisms of the reduced contractility and arrhythmias. In this review, we explore the utility of ion channels, transporters, and pumps as targets for the treatment of heart failure, focusing predominantly on the treatment for reduced contractility and arrhythmias.

Regulation by FK506 and rapamycin of Ca2+ release from the sarcoplasmic reticulum in vascular smooth muscle: the role of FK506 binding proteins and mTOR

BACKGROUND AND PURPOSE

The sarcoplasmic reticulum (SR), regulates the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)) in vascular smooth muscle. Release from the SR is controlled by two intracellular receptor/channel complexes, the ryanodine receptor (RyR) and the inositol 1,4,5-trisphosphate receptor (IP(3)R). These receptors may be regulated by the accessory FK506-binding protein (FKBP) either directly, by binding to the channel, or indirectly via FKBP modulation of two targets, the phosphatase, calcineurin or the kinase, mammalian target of rapamycin (mTOR).

EXPERIMENTAL APPROACH

Single portal vein myocytes were voltage-clamped in whole cell configuration and [Ca(2+)](cyto) measured using fluo-3. IP(3)Rs were activated by photolysis of caged IP(3) and RyRs activated by hydrostatic application of caffeine.

KEY RESULTS

FK506 which displaces FKBP from each receptor (to inhibit calcineurin) increased the [Ca(2+)](cyto) rise evoked by activation of either RyR or IP(3)R. Rapamycin which displaces FKBP (to inhibit mTOR) also increased the amplitude of the caffeine-evoked, but reduced the IP(3)-evoked [Ca(2+)](cyto) rise. None of the phosphatase inhibitors, cypermethrin, okadaic acid or calcineurin inhibitory peptide, altered either caffeine- or IP(3)-evoked [Ca(2+)](cyto) release; calcineurin did not contribute to FK506-mediated potentiation of RyR- or IP(3)R-mediated Ca(2+) release. The mTOR inhibitor LY294002, like rapamycin, decreased IP(3)-evoked Ca(2+) release.

CONCLUSIONS AND IMPLICATIONS

Ca(2+) release in portal vein myocytes, via RyR, was modulated directly by FKBP binding to the channel; neither calcineurin nor mTOR contributed to this regulation. However, IP(3)R-mediated Ca(2+) release, while also modulated directly by FKBP may be additionally regulated by mTOR. Rapamycin inhibition of IP(3)-mediated Ca(2+) release may be explained by mTOR inhibition.

The ER and ageing II: calcium homeostasis

Increase in intracellular Ca(2+) concentration occurs by Ca(2+) influx through the plasma membrane and by Ca(2+) release from intracellular stores. The ER is the most important Ca(2+) store. Its stress, characterized by the impairment of Ca(2+) homeostasis and by the accumulation of misfolded proteins, can be induced by different factors. In turn, it induces defense mechanisms such as unfolded protein response, and when it is severe and prolonged, activation of the apoptotic pathway. Damage to the ER, impairment of its function, and a decreased level of its Ca(2+)-handling proteins might all play a role in physiological ageing by handicapping the ER stress response. Thus, healthy ageing is accompanied by subtle alterations of Ca(2+) homeostasis and signaling, including alterations in the ER Ca(2+) load and release. The expression and/or function of ryanodine receptors, IP3 receptors, and SERCA Ca(2+) pumps located in the ER membrane, and Ca(2+)-binding proteins within ER lumen all seem to be affected in aged cells. Data are presented on age-dependent, tissue-specific changes in ER-related Ca(2+) homeostasis in skeletal, cardiac and smooth muscles, as well as in the nervous and immune systems. Disturbances of Ca(2+) homeostasis and of signaling are potential targets for intervention in aged humans.

AKAP-scaffolding proteins and regulation of cardiac physiology

A kinase anchoring proteins (AKAPs) compose a growing list of diverse but functionally related proteins defined by their ability to bind to the regulatory subunit of protein kinase A. AKAPs perform an integral role in the spatiotemporal modulation of a multitude of cellular signaling pathways. This review highlights the extensive role of AKAPs in cardiac excitation/contraction coupling and cardiac physiology. The literature shows that particular AKAPs are involved in cardiac Ca(2+) influx, release, reuptake, and myocyte repolarization. Studies have also suggested roles for AKAPs in cardiac remodeling. Transgenic studies show functional effects of AKAPs, not only in the cardiovascular system but in other organ systems as well.

Dysregulated sarcoplasmic reticulum calcium release: potential pharmacological target in cardiac disease

In the heart, Ca(2+) released from the intracellular Ca(2+) storage site, the sarcoplasmic reticulum (SR), is the principal determinant of cardiac contractility. SR Ca(2+) release is controlled by dedicated molecular machinery, composed of the cardiac ryanodine receptor (RyR2) and a number of accessory proteins, including FKBP12.6, calsequestrin (CASQ2), triadin (TRD) and junctin (JN). Acquired and genetic defects in the components of the release channel complex result in a spectrum of abnormal Ca(2+) release phenotypes ranging from arrhythmogenic spontaneous Ca(2+) releases and Ca(2+) alternans to the uniformly diminished systolic Ca(2+) release characteristic of heart failure. In this article, we will present an overview of the structure and molecular components of the SR and Ca(2+) release machinery and its modulation by different intracellular factors, such as Ca(2+) levels inside the SR as well as phosphorylation and redox modification of RyR2s. We will also discuss the relationships between abnormal SR Ca(2+) release and various cardiac disease phenotypes, including, arrhythmias and heart failure, and consider SR Ca(2+) release as a potential therapeutic target.

Leaky Ca2+ release channel/ryanodine receptor 2 causes seizures and sudden cardiac death in mice

The Ca2+ release channel ryanodine receptor 2 (RyR2) is required for excitation-contraction coupling in the heart and is also present in the brain. Mutations in RyR2 have been linked to exercise-induced sudden cardiac death (catecholaminergic polymorphic ventricular tachycardia [CPVT]). CPVT-associated RyR2 mutations result in "leaky" RyR2 channels due to the decreased binding of the calstabin2 (FKBP12.6) subunit, which stabilizes the closed state of the channel. We found that mice heterozygous for the R2474S mutation in Ryr2 (Ryr2-R2474S mice) exhibited spontaneous generalized tonic-clonic seizures (which occurred in the absence of cardiac arrhythmias), exercise-induced ventricular arrhythmias, and sudden cardiac death. Treatment with a novel RyR2-specific compound (S107) that enhances the binding of calstabin2 to the mutant Ryr2-R2474S channel inhibited the channel leak and prevented cardiac arrhythmias and raised the seizure threshold. Thus, CPVT-associated mutant leaky Ryr2-R2474S channels in the brain can cause seizures in mice, independent of cardiac arrhythmias. Based on these data, we propose that CPVT is a combined neurocardiac disorder in which leaky RyR2 channels in the brain cause epilepsy, and the same leaky channels in the heart cause exercise-induced sudden cardiac death.

Calcium cycling and signaling in cardiac myocytes

Calcium (Ca) is a universal intracellular second messenger. In muscle, Ca is best known for its role in contractile activation. However, in recent years the critical role of Ca in other myocyte processes has become increasingly clear. This review focuses on Ca signaling in cardiac myocytes as pertaining to electrophysiology (including action potentials and arrhythmias), excitation-contraction coupling, modulation of contractile function, energy supply-demand balance (including mitochondrial function), cell death, and transcription regulation. Importantly, although such diverse Ca-dependent regulations occur simultaneously in a cell, the cell can distinguish distinct signals by local Ca or protein complexes and differential Ca signal integration.

Single-channel characterization of the rabbit recombinant RyR2 reveals a novel inactivation property of physiological concentrations of ATP

Ryanodine receptor 2 (RyR2) cDNA has been available for more than 15 years; however, due to the complex nature of ligand gating in this channel, many aspects of recombinant RyR2 function have been unresearched. We established a stable, inducible HEK 293 cell line expressing full-length rabbit RyR2 cDNA and assessed the single-channel properties of the recombinant RyR2, with particular reference to ligand regulation with Ca2+ as the permeant ion. We found that the single-channel conductances of recombinant RyR2 and RyR2 isolated from cardiac muscle are essentially identical, as is irreversible modification by ryanodine. Although it is known that RyR2 expressed in HEK 293 cells is not associated with FKBP12.6, we demonstrate that these channels do not exhibit any discernable disorganized gating characteristics or subconductance states. We also show that the gating of recombinant RyR2 is indistinguishable from that of channels isolated from cardiac muscle when activated by cytosolic Ca2+, caffeine or suramin. The mechanisms underlying ATP activation are also similar; however, the experiments highlighted a novel effect of ATP at physiologically relevant concentrations of 5-10 mM. With Ca2+ as permeant ion, 5-10 mM ATP consistently inactivated recombinant channels (15/16 experiments). Such inactivation was rarely observed with native RyR2 isolated from cardiac muscle (1 in 16 experiments). However, if the channels were purified, inactivation by ATP was then revealed in all experiments. This action of ATP may be relevant for inactivation of sarcoplasmic reticulum Ca2+ release during cardiac excitation-contraction coupling or may represent unnatural behavior that is revealed when RyR2 is purified or expressed in noncardiac systems.

Cyclic ADP-ribose mediates formyl methionyl leucyl phenylalanine (fMLP)-induced intracellular Ca(2+) rise and migration of human neutrophils

Although cyclic ADP-ribose (cADPR), a novel Ca(2+)-mobilizing mediator, is suggested to be involved in the functions of neutrophils in rodents, its role in human neutrophils remains unclear. The present study examined the ability of cADPR to mobilize Ca(2+) and mediate formyl methionyl leucyl phenylalanine (fMLP)-stimulated increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and migration in human neutrophils. cADPR induced Ca(2+) release from digitonin-permeabilized neutrophils, and the release was blocked by 8Br-cADPR, an antagonist of cADPR. Immunophilin ligands, FK506 and rapamycin, but not cyclosporine A, inhibited cADPR-induced Ca(2+) release. 8Br-cADPR partially reduced fMLP-induced [Ca(2+)](i) rise and abolished the rise in combination with 2APB, an IP(3)-receptor antagonist. Anti-CD38Ab and NADase that interfere with cADPR formation, reduced the fMLP-induced [Ca(2+)](i) rise. When beta-NAD(+), a substrate of ADP-ribosyl cyclase, and cADPR were added to the medium, the former gradually increased [Ca(2+)](i) and the latter potentiated the fMLP-induced [Ca(2+)](i) rise. The beta-NAD(+)-induced [Ca(2+)](i) rise in Ca(2+)-free medium was inhibited by anti-CD38Ab, 8Br-cADPR, FK506, ruthenium red, and thapsigargin. mRNAs of nucleoside transporter (NT), ENT1, ENT2, CNT, and CNT3 were expressed in neutrophils; and their inhibitors, inosine, uridine, and s-(4-nitrobenzyl)-6-thioinosine, reduced the [Ca(2+)](i) rise induced by beta-NAD(+) and fMLP. fMLP-timulated migration was inhibited by the removal of Ca(2+) from the medium or by the addition of 8Br-cADPR, anti-CD38Ab, NADase, and NT inhibitors. These results suggest that cADPR was synthesized extracellularly by CD38, transported into the cells through NTs, and then Ca(2+) was mobilized by FK506-binding protein-dependent process. This process may be involved in fMLP-induced intracellular Ca(2+) signaling and migration in human neutrophils.

Mechanisms and potential therapeutic targets for ventricular arrhythmias associated with impaired cardiac calcium cycling

The close relationship between life-threatening ventricular arrhythmias and contractile dysfunction in the heart implicates intracellular calcium cycling as an important underlying mechanism of arrhythmogenesis. Despite this close association, however, the mechanisms of arrhythmogenesis attributable to impaired calcium cycling are not fully appreciated or understood. In this report we review some of the current thinking regarding arrhythmia mechanisms associated with either abnormal impulse initiation (i.e. arrhythmia triggers) or impulse propagation (i.e. arrhythmia substrates). In all cases, the mechanisms are primarily related to dysfunction of calcium regulatory proteins associated with the sarcomere. These findings highlight the broad scope of arrhythmias associated with abnormal calcium cycling, and provide a basis for a causal relationship between cardiac electrical instability and contractile dysfunction. Moreover, calcium cycling proteins may provide much needed targets for novel antiarrhythmic therapies.