Dihydropyridine Receptor Research Articles

P.11.5 PMO-mediated dystrophin exon 23 skipping restores mitochondrial function in the mdx mouse heart

Approximately 20% of boys with Duchenne Muscular Dystrophy will die of dilated cardiomyopathy. The cardiomyopathy is characterised by disrupted structure and function of cardiac muscle cells and reduced energy production. However, the mechanisms responsible for the altered energy metabolism have been poorly understood. We have previously sought to identify the mechanisms for metabolic inhibition in mdx mouse cardiomyopathy. Calcium influx through the L-type Ca2+ channel (also known as the dihydropyridine receptor) in cardiac myocytes is essential for contraction. Calcium is also important for the regulation of mitochondrial function and production of ATP that is required to meet the energy demands of the heart. We have shown that the L-type Ca2+ channel can regulate mitochondrial function and metabolic activity in cardiac myocytes. In mdx heart, the communication between the L-type Ca2+ channel and the mitochondria is altered as a result of disruption of the cytoskeletal architecture. This contributes to metabolic inhibition in the mdx heart. We demonstrate that treatment of mdx mice with a phosphorodiamidate morpholino oligomer, designed to induce skipping of dystrophin exon 23, “restored” the increase in mitochondrial membrane potential in mdx cardiomyocytes after activation of the L-type Ca2+ channel with the dihydropyridine receptor agonist BayK (−). These results confirm that metabolic inhibition occurs as a result of the absence of dystrophin, and oligomer therapy may be able to normalise metabolic activity and restore contractility in mdx mouse heart.

Type 2 Ryanodine Receptor Domain A Contains a Unique and Dynamic α-Helix That Transitions to a β-Strand in a Mutant Linked with a Heritable Cardiomyopathy

Ryanodine receptors (RyRs) are large tetrameric calcium (Ca2+) release channels found on the sarcoplasmic reticulum that respond to dihydropyridine receptor activity through a direct conformational interaction and/or indirect Ca2+ sensitivity, propagating sarcoplasmic reticulum luminal Ca2+ release in the process of excitation–contraction coupling. There are three human RyR subtypes, and several debilitating diseases are linked to heritable mutations in RyR1 and RyR2 including malignant hypothermia, central core disease, catecholaminergic polymorphic ventricular tachycardia (CPVT) and arrhythmogenic right ventricular dysplasia type 2 (ARVD2). Despite the recent appreciation that many disease-associated mutations within the N-terminal RyRABC domains (i.e., residues 1–559) are located in the putative interfaces mediating tetrameric channel assembly, the precise structural and dynamical consequences of the mutations are not well understood. We used solution nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography to examine the effect of ARVD2-associated (i.e., R176Q) and CPVT-associated [i.e., P164S, R169Q and delta exon 3 (Δ3)] mutations on the structure and dynamics of RyR2A. Our solution NMR data exposed a mobile α-helix, unique to type 2; further, this α2 helix rescues the β-strand lost in RyR2A Δ3 but remains dynamic in the hot-spot loop (HS-loop) P164S, R169Q and R176Q mutant proteins. Docking of our X-ray crystal/NMR hybrid structure into the RyR1 cryo-electron microscopy map revealed that this RyR2A α2 helix is in close proximity to dense “columns” projecting toward the channel pore. This is in contrast to the HS-loop mutations that cause structural changes largely localized to the intersubunit interface between adjacent ABC domains. Taken together, our data suggest that ARVD2 and CPVT mutations have at least two distinct structural consequences linked to channel dysfunction: perturbation of the HS-loop (i.e., domain A):domain B intersubunit interface and disruption of the communication between the N-terminal region and the channel domain.

Reduced DHPRα1S and RyR1 expression levels are associated with diaphragm contractile dysfunction during sepsis

Sepsis often causes diaphragm contractile dysfunction. Dihydropyridine receptors (DHPRα1s and DHPRα1c) and ryanodine receptors (RyR1, RyR2, and RyR3) are essential for excitation-contraction coupling in striated muscles. However, their expression in diaphragm during sepsis have not been explored. Eight rats received endotoxin, and 8 more rats received placebo. After 24 hours, 3) diaphragm isometric contractile force was measured. The mRNA and protein levels of DHPRs and RyRs in diaphragm muscles were determined. Sepsis weakened diaphragm contractile function. The expression levels of DHPRα1s and RyR1 were significantly lower in septic rats than in control rats. The expression levels of DHPRα1c and RyR3 were unaffected by sepsis. RyR2 was undetectable at both mRNA and protein levels in the control and sepsis groups. Weakened diaphragm contraction in the septic rats was associated with reduced mRNA and protein expression of DHPRα1s and RyR1, the isoforms of skeletal muscles.

V876E mutation in CACNA1S gene associated with severe hypokalemic periodic paralysis in a Chinese woman

V876E mutation in CACNA1S gene associated with severe hypokalemic periodic paralysis in a Chinese woman

Association between genetic polymorphisms in Ca(v)2.3 (R-type) Ca2+ channels and fentanyl sensitivity in patients undergoing painful cosmetic surgery.

Individual differences in the sensitivity to fentanyl, a widely used opioid analgesic, lead to different proper doses of fentanyl, which can hamper effective pain treatment. Voltage-activated Ca2+ channels (VACCs) play a crucial role in the nervous system by controlling membrane excitability and calcium signaling. Cav2.3 (R-type) VACCs have been especially thought to play critical roles in pain pathways and the analgesic effects of opioids. However, unknown is whether single-nucleotide polymorphisms (SNPs) of the human CACNA1E (calcium channel, voltage-dependent, R type, alpha 1E subunit) gene that encodes Cav2.3 VACCs influence the analgesic effects of opioids. Thus, the present study examined associations between fentanyl sensitivity and SNPs in the human CACNA1E gene in 355 Japanese patients who underwent painful orofacial cosmetic surgery, including bone dissection. We first conducted linkage disequilibrium (LD) analyses of 223 SNPs in a region that contains the CACNA1E gene using genomic samples from 100 patients, and a total of 13 LD blocks with 42 Tag SNPs were observed within and around the CACNA1E gene region. In the preliminary study using the same 100 genomic samples, only the rs3845446 A/G SNP was significantly associated with perioperative fentanyl use among these 42 Tag SNPs. In a confirmatory study using the other 255 genomic samples, this SNP was also significantly associated with perioperative fentanyl use. Thus, we further analyzed associations between genotypes of this SNP and all of the clinical data using a total of 355 samples. The rs3845446 A/G SNP was associated with intraoperative fentanyl use, 24 h postoperative fentanyl requirements, and perioperative fentanyl use. Subjects who carried the minor G allele required significantly less fentanyl for pain control compared with subjects who did not carry this allele. Although further validation is needed, the present findings show the possibility of the involvement of CACNA1E gene polymorphisms in fentanyl sensitivity.

Oxygen-coupled Redox Regulation of the Skeletal Muscle Ryanodine Receptor/Ca2+ Release Channel (RyR1): SITES AND NATURE OF OXIDATIVE MODIFICATION

In mammalian skeletal muscle, Ca(2+) release from the sarcoplasmic reticulum (SR) through the ryanodine receptor/Ca(2+)-release channel RyR1 can be enhanced by S-oxidation or S-nitrosylation of separate Cys residues, which are allosterically linked. S-Oxidation of RyR1 is coupled to muscle oxygen tension (pO2) through O2-dependent production of hydrogen peroxide by SR-resident NADPH oxidase 4. In isolated SR (SR vesicles), an average of six to eight Cys thiols/RyR1 monomer are reversibly oxidized at high (21% O2) versus low pO2 (1% O2), but their identity among the 100 Cys residues/RyR1 monomer is unknown. Here we use isotope-coded affinity tag labeling and mass spectrometry (yielding 93% coverage of RyR1 Cys residues) to identify 13 Cys residues subject to pO2-coupled S-oxidation in SR vesicles. Eight additional Cys residues are oxidized at high versus low pO2 only when NADPH levels are supplemented to enhance NADPH oxidase 4 activity. pO2-sensitive Cys residues were largely non-overlapping with those identified previously as hyperreactive by administration of exogenous reagents (three of 21) or as S-nitrosylated. Cys residues subject to pO2-coupled oxidation are distributed widely within the cytoplasmic domain of RyR1 in multiple functional domains implicated in RyR1 activity-regulating interactions with the L-type Ca(2+) channel (dihydropyridine receptor) and FK506-binding protein 12 as well as in "hot spot" regions containing sites of mutation implicated in malignant hyperthermia and central core disease. pO2-coupled disulfide formation was identified, whereas neither S-glutathionylated nor sulfenamide-modified Cys residues were observed. Thus, physiological redox regulation of RyR1 by endogenously generated hydrogen peroxide is exerted through dynamic disulfide formation involving multiple Cys residues.

Junctophilins and μ‐calpain: partners in excitation–contraction uncoupling

Junctophilins and μ‐calpain: partners in excitation–contraction uncoupling

The transverse-axial tubular system of cardiomyocytes.

A characteristic histological feature of striated muscle cells is the presence of deep invaginations of the plasma membrane (sarcolemma), most commonly referred to as T-tubules or the transverse-axial tubular system (TATS). TATS mediates the rapid spread of the electrical signal (action potential) to the cell core triggering Ca(2+) release from the sarcoplasmic reticulum, ultimately inducing myofilament contraction (excitation-contraction coupling). T-tubules, first described in vertebrate skeletal muscle cells, have also been recognized for a long time in mammalian cardiac ventricular myocytes, with a structure and a function that in recent years have been shown to be far more complex and pivotal for cardiac function than initially thought. Renewed interest in T-tubule function stems from the loss and disorganization of T-tubules found in a number of pathological conditions including human heart failure (HF) and dilated and hypertrophic cardiomyopathies, as well as in animal models of HF, chronic ischemia and atrial fibrillation. Disease-related remodeling of the TATS leads to asynchronous and inhomogeneous Ca(2+)-release, due to the presence of orphan ryanodine receptors that have lost their coupling with the dihydropyridine receptors and are either not activated or activated with a delay. Here, we review the physiology of the TATS, focusing first on the relationship between function and structure, and then describing T-tubular remodeling and its reversal in disease settings and following effective therapeutic approaches.

Shotgun proteomic analysis of sarcoplasmic reticulum preparations from rabbit skeletal muscle

To obtain a comprehensive understanding of proteins involved in excitation-contraction coupling, a catalog of proteins from sarcoplasmic reticulum (SR) membrane fractions of New Zealand white rabbit skeletal muscle was analyzed by an optimized shotgun proteomic method. Light and heavy SR membrane fractions were obtained by nonlinear sucrose gradient centrifugation and separated by 1DE followed by a highly reproducible, automated LC-MS/MS on the hybrid linear ion trap (LTQ) Orbitrap mass spectrometer. By integrating as low as 1% false discovery rate as one of the features for quality control method, 483 proteins were identified from both of the two independent SR preparations. Proteins involved in calcium release unit complex, including ryanodine receptor 1, dihydropyridine receptor, calmodulin, triadin, junctin, and calsequestrin, were all detected, which offered validation for this protein identification method. Rigorous bioinformatics analysis was performed. Protein pI value, molecular weight range, hydrophobicity index, and transmembrane region were calculated using bioinformatics softwares. Eighty-three proteins were classified as hydrophobic proteins and 175 proteins were recognized as membrane proteins. Based on the proteomic analysis results, we found as the first time that not only transverse tubule but also mitochondrion physically connected to SR. The complete mapping of these proteomes may help in the elucidation of the process of excitation-contraction coupling and excitation-metabolism coupling.

STIM1 negatively regulates Ca2+ release from the sarcoplasmic reticulum in skeletal myotubes

STIM1 (stromal interaction molecule 1) mediates SOCE (store-operated Ca²⁺ entry) in skeletal muscle. However, the direct role(s) of STIM1 in skeletal muscle, such as Ca²⁺ release from the SR (sarcoplasmic reticulum) for muscle contraction, have not been identified. The times required for the maximal expression of endogenous STIM1 or Orai1, or for the appearance of puncta during the differentiation of mouse primary skeletal myoblasts to myotubes, were all different, and the formation of puncta was detected with no stimulus during differentiation, suggesting that, in skeletal muscle, the formation of puncta is a part of the differentiation. Wild-type STIM1 and two STIM1 mutants (Triple mutant, missing Ca²⁺-sensing residues but possessing the intact C-terminus; and E136X, missing the C-terminus) were overexpressed in the myotubes. The wild-type STIM1 increased SOCE, whereas neither mutant had an effect on SOCE. It was interesting that increases in the formation of puncta were observed in the Triple mutant as well as in wild-type STIM1, suggesting that SOCE-irrelevant puncta could exist in skeletal muscle. On the other hand, overexpression of wild-type or Triple mutant, but not E136X, attenuated Ca²⁺ releases from the SR in response to KCl [evoking ECC (excitation-contraction coupling) via activating DHPR (dihydropyridine receptor)] in a dominant-negative manner. The attenuation was removed by STIM1 knockdown, and STIM1 was co-immunoprecipitated with DHRP in a Ca²⁺-independent manner. These results suggest that STIM1 negatively regulates Ca²⁺ release from the SR through the direct interaction of the STIM1 C-terminus with DHPR, and that STIM1 is involved in both ECC and SOCE in skeletal muscle.

Renoprotective Effects of the L-/T-type Calcium Channel Blocker Benidipine in Patients with Hypertension

The renoprotective effects of benidipine, a calcium channel blocker (CCB) developed in Japan, are reviewed herein. Benidipine has a sustained antihypertensive effect independent of its blood concentration since it binds to dihydropyridine (DHP) receptors via a "membrane approach" (approach to the cell membrane followed by long retention at the DHP binding site). Benidipine dilates glomerular afferent and efferent arterioles equally through inhibition of Ttype Ca channels. Thus, it may cause a decrease of intraglomerular pressure and is superior to CCBs (capable of inhibiting only L-type Ca channels) in terms of suppression of proteinuria. Additionally, benidipine suppresses worsening of renal function more powerfully than CCBs (suppressing only L-type Ca channels), allowing better prognosis as to renal function. The inhibitory effect of benidipine on T-type calcium channels results in the suppression of aldosterone formation in the adrenal glands and of oxidative stress induced by aldosterone. Thus, the aldosterone-inhibitory and antioxidant activities of benidipine mediated by inhibition of T-type calcium channels would result in renoprotection and suppression of disease progression in hypertensive patients with chronic kidney disease (CKD). If such patients have proteinuria, renin-angiotensin system (RAS) inhibitors are used as first-line drugs, but benidipine, as an L-/T-type CCB, is recommended when they require some concomitant drugs. Moreover, the superiority of RAS inhibitors has not been demonstrated in hypertensive patients with CKD and without proteinuria. Thus, in such patients, benidipine should be considered as a first-line antihypertensive drug.

RyR1 Deficiency in Congenital Myopathies Disrupts Excitation-Contraction Coupling

In skeletal muscle, excitation-contraction (EC) coupling is the process whereby the voltage-gated dihydropyridine receptor (DHPR) located on the transverse tubules activates calcium release from the sarcoplasmic reticulum by activating ryanodine receptor (RyR1) Ca(2+) channels located on the terminal cisternae. This subcellular membrane specialization is necessary for proper intracellular signaling and any alterations in its architecture may lead to neuromuscular disorders. In this study, we present evidence that patients with recessive RYR1-related congenital myopathies due to primary RyR1 deficiency also exhibit downregulation of the alfa 1 subunit of the DHPR and show disruption of the spatial organization of the EC coupling machinery. We created a cellular RyR1 knockdown model using immortalized human myoblasts transfected with RyR1 siRNA and confirm that knocking down RyR1 concomitantly downregulates not only the DHPR but also the expression of other proteins involved in EC coupling. Unexpectedly, this was paralleled by the upregulation of inositol-1,4,5-triphosphate receptors; functionally however, upregulation of the latter Ca(2+) channels did not compensate for the lack of RyR1-mediated Ca(2+) release. These results indicate that in some patients, RyR1 deficiency concomitantly alters the expression pattern of several proteins involved in calcium homeostasis and that this may influence the manifestation of these diseases.

Domain cooperativity in the β1a subunit is essential for dihydropyridine receptor voltage sensing in skeletal muscle.

The dihydropyridine receptor (DHPR) β1a subunit is crucial for enhancement of DHPR triad expression, assembly of DHPRs in tetrads, and elicitation of DHPRα1S charge movement--the three prerequisites of skeletal muscle excitation-contraction coupling. Despite the ability to fully target α1S into triadic junctions and tetradic arrays, the neuronal isoform β3 was unable to restore considerable charge movement (measure of α1S voltage sensing) upon expression in β1-null zebrafish relaxed myotubes, unlike the other three vertebrate β-isoforms (β1a, β2a, and β4). Thus, we used β3 for chimerization with β1a to investigate whether any of the five distinct molecular regions of β1a is dominantly involved in inducing the voltage-sensing function of α1S. Surprisingly, systematic domain swapping between β1a and β3 revealed a pivotal role of the src homology 3 (SH3) domain and C terminus of β1a in charge movement restoration. More interestingly, β1a SH3 domain and C terminus, when simultaneously engineered into β3 sequence background, were able to fully restore charge movement together with proper intracellular Ca(2+) release, suggesting cooperativity of these two domains in induction of the α1S voltage-sensing function in skeletal muscle excitation-contraction coupling. Furthermore, substitution of a proline by alanine in the putative SH3-binding polyproline motif in the proximal C terminus of β1a (also of β2a and β4) fully obstructed α1S charge movement. Consequently, we postulate a model according to which β subunits, probably via the SH3-C-terminal polyproline interaction, adapt a discrete conformation required to modify the α1S conformation apt for voltage sensing in skeletal muscle.

The ATP required for potentiation of skeletal muscle contraction is released via pannexin hemichannels

During repetitive stimulation of skeletal muscle, extracellular ATP levels raise, activating purinergic receptors, increasing Ca2+ influx, and enhancing contractile force, a response called potentiation. We found that ATP appears to be released through pannexin1 hemichannels (Panx1 HCs). Immunocytochemical analyses and function were consistent with pannexin1 localization to T-tubules intercalated with dihydropyridine and ryanodine receptors in slow (soleus) and fast (extensor digitorum longus, EDL) muscles. Isolated myofibers took up ethidium (Etd+) and released small molecules (as ATP) during electrical stimulation. Consistent with two glucose uptake pathways, induced uptake of 2-NBDG, a fluorescent glucose derivative, was decreased by inhibition of HCs or glucose transporter (GLUT4), and blocked by dual blockade. Adult skeletal muscles apparently do not express connexins, making it unlikely that connexin hemichannels contribute to the uptake and release of small molecules. ATP release, Etd+ uptake, and potentiation induced by repetitive electrical stimulation were blocked by HC blockers and did not occur in muscles of pannexin1 knockout mice. MRS2179, a P2Y1R blocker, prevented potentiation in EDL, but not soleus muscles, suggesting that in fast muscles ATP activates P2Y1 but not P2X receptors. Phosphorylation on Ser and Thr residues of pannexin1 was increased during potentiation, possibly mediating HC opening. Opening of Panx1 HCs during repetitive activation allows efflux of ATP, influx of glucose and possibly Ca2+ too, which are required for potentiation of contraction.This article is part of the Special Issue Section entitled ‘Current Pharmacology of Gap Junction Channels and Hemichannels’.

Ryanodine receptor type 1 gene variants in the malignant hyperthermia-susceptible population of the United States.

Mutations in the ryanodine receptor type 1 gene (RYR1) that encodes the skeletal muscle-specific intracellular calcium (Ca(2+)) release channel are a cause of malignant hyperthermia (MH). In this study, we examined RYR1 mutations in a large number of North American MH-susceptible (MHS) subjects without prior genetic diagnosis. RYR1 was examined in 120 unrelated MHS subjects from the United States in a tiered manner. The α-1 subunit of the dihydropyridine receptor gene (CACNA1S) was screened for 4 variants in subjects in whom no abnormality was found in ≥ 100 exons of RYR1. Ten known causative MH mutations were found in 26 subjects. Variants of uncertain significance in RYR1 were found in 36 subjects, 16 of which are novel. Novel variants in both RYR1 and CACNA1S were found in the 1 subject who died of MH. Two RYR1 variants were found in 4 subjects. Variants of uncertain significance were found outside and inside the hotspots of RYR1. Maximal contractures in the caffeine-halothane contracture test were greater in those who had a known MH mutation or variant of uncertain significance in RYR1 than in those who did not. The identification of novel RYR1 variants and previously observed RYR1 variants of uncertain significance in independent MHS families is necessary for demonstrating the significance of these variants for MH susceptibility and supports the need for functional studies of these variants. Continued reporting of the clinical phenotypes of MH is necessary for interpretation of genetic findings, especially because the pathogenicity of most of these genetic variants associated with MHS remains to be elucidated.

A Role for the Excess Ryanodine Receptors in Excitation‐Contraction Coupling

A close interaction between the plasma membrane bound DHPR (dihydropyridine receptor, the sensor of electrical changes in the membrane) and the endoplasmic reticulum bound RyR (ryanodine receptor, the Ca2+ release channel to the cytosol) is central to excitation‐contraction coupled Ca2+ release (ECC) by skeletal muscle. We hypothesized that the 2:1 ratio of RyR:DHPR may reflect relative movement of DHPR and RyR accompanied by bursts of Ca2+ release. We tested the hypothesis by measuring the sensitivity of the RyR and DHPR to modulators of these proteins. The response of RyR (measured as Ca2+ appearance in the cytosol) to its namesake modulator ryanodine (Ry) was shifted to lower concentrations of Ry when cells were subjected to field depolarization. In a similar way, Ca2+ release in response to the DHPR activator BayK was also shifted to lower concentrations of BayK when cells were depolarized. However, sensitivity of Ca2+ release to these agents was not altered when either the direct RyR activator CMC (a caffeine‐like agent) or increased medium Ca2+ was applied. While some studies have suggested that RyR may not be essential in ECC, this was based on very high concentrations of Ry (above 100 μM). Yet, we found that cells incubated with Ry at 10 μM completely blocked ECC. Ry at concentrations greater than 50 μM compromised cell viability. Our findings support an interaction model of DHPR and RyR in ECC, a role for the excess number of RyR over DHPR, and indicate Ca2+ release for ECC is exclusively through the RyR.

Targeted delivery of siRNA to differentiated murine myotubes in culture by a conjugate of cationic oligopeptide with FS2 venom

A conjugate of the ligand of FS2 venom dihydropyridine receptors with a cationic arginine-containing oligopeptide was synthesized. It was found that the conjugate provides siRNA delivery to murine myotubes differentiated in vitro. The effect of RNA interference with the use of siRNA complexes with the conjugate was observed when siRNA concentrations were an order of magnitude lower than those used in the case of siRNA complexes with a non-conjugated oligopeptide.

Enhanced dihydropyridine receptor calcium channel activity restores muscle strength in JP45/CASQ1 double knockout mice

Muscle strength declines with age in part due to a decline of Ca(2+) release from sarcoplasmic reticulum calcium stores. Skeletal muscle dihydropyridine receptors (Ca(v)1.1) initiate muscle contraction by activating ryanodine receptors in the sarcoplasmic reticulum. Ca(v)1.1 channel activity is enhanced by a retrograde stimulatory signal delivered by the ryanodine receptor. JP45 is a membrane protein interacting with Ca(v)1.1 and the sarcoplasmic reticulum Ca(2+) storage protein calsequestrin (CASQ1). Here we show that JP45 and CASQ1 strengthen skeletal muscle contraction by modulating Ca(v)1.1 channel activity. Using muscle fibres from JP45 and CASQ1 double knockout mice, we demonstrate that Ca(2+) transients evoked by tetanic stimulation are the result of massive Ca(2+) influx due to enhanced Ca(v)1.1 channel activity, which restores muscle strength in JP45/CASQ1 double knockout mice. We envision that JP45 and CASQ1 may be candidate targets for the development of new therapeutic strategies against decay of skeletal muscle strength caused by a decrease in sarcoplasmic reticulum Ca(2+) content.

Ultrastructural and Functional Alterations of EC Coupling Elements in mdx Cardiomyocytes: An Analysis from Membrane Surface to Depth

A dilated cardiomyopathy (DCM) is associated with Duchenne muscular dystrophy (DMD). The loss of dystrophin leads to membrane instability and calcium dysregulation in skeletal muscle but effects of such a loss are not elucidated at cardiomyocytes level. We sought to examine whether membrane and transverse tubules damages occur in ventricular myocytes from mdx mouse model of DMD and how they impact the function of single excitation-contraction coupling elements. Scanning ion conductance microscopy (SICM) was used to characterize the integrity loss of living mdx cardiomyocytes surface. 2D Fourier transform analysis of labeled internal networks (transverse tubules, alpha-actinin, dihydropyridine receptors, ryanodine receptors) was performed to evaluate internal alterations. During calcium measurements, "smart microperfusions" of depolarizing solutions were applied through SICM nanopipette, stimulating single tubules elements. These approaches revealed structural membrane surface (39% decrease for Z-groove ratio) and transverse tubules disorganization (21% transverse tubules ratio decrease) in mdx as compared to control. These disruptions were associated with functional alterations (sixfold increase of calcium signal duration and twofold increase of sparks frequency). In DCM associated with DMD, myocytes display evident membrane alterations at the surface level but also in the cell depth with a disruption of transverse tubules network as observed in other cases of heart failure. These ultrastructural changes are associated with changes in the function of some coupling elements. Thus, these profound disruptions may play a role in calcium dysregulation through excitation-contraction coupling elements perturbation and suggest a transverse tubules stabilizing role for dystrophin.

Mechanical unloading reverses transverse tubule remodelling and normalises local calcium-induced calcium release in a rodent model of heart failure

Calcium-induced calcium release (CICR) is crucial for contraction in cardiomyocytes. The transverse (t)-tubule system guarantees the proximity of the triggers for calcium release (L-type calcium channel, dihydropyridine receptors) and the sarcoplasmic reticulum calcium-release channels (ryanodine receptors). Transverse tubule disruption occurs early in heart failure. Clinical studies of left ventricular assist devices in heart failure indicate that mechanical unloading induces reverse remodelling. We hypothesise that unloading of failing hearts normalises t-tubule structure and improves CICR.Heart failure was induced in Lewis rats by left coronary artery ligation for 12 weeks; sham-operated animals were used as controls. Failing hearts were mechanically unloaded for 4 weeks by heterotopic abdominal heart transplantation (HF-UN). Heart failure reduced the t-tubule density as measured by di-8-ANEPPS staining in isolated left ventricular myocytes, and this was reversed by unloading. The deterioration in the regularity of the t-tubule system in heart failure was also reversed in HF-UN. Scanning ion conductance microscopy showed the reappearance of normal surface striations in HF-UN. Electron microscopy revealed recovery of normal t-tubule microarchitecture in HF-UN. L-type calcium current density, measured with whole-cell patch clamping, was reduced in heart failure but unaffected by unloading. The variance of the time-to-peak of the calcium transient, an index of CICR dyssynchrony, was increased in heart failure and normalised by unloading. The increased calcium spark frequency observed in heart failure was reduced in HF-UN. These results could be explained by the recoupling of orphaned ryanodine receptors in heart failure, as indicated by immunofluorescence.Our data show that mechanical unloading of the failing heart reverses the pathological remodelling of the t-tubule system and improves CICR. FundingBritish Heart Foundation.