Sarcoplasmic Reticulum Proteins Research Articles

Impact of functional gene groups on outcomes after ablation of ventricular tachycardia in patients with inherited left ventricular cardiomyopathy: an international multicentre study

Abstract Background Advances in genetic testing has enabled identification of pathogenic variants of different functional gene groups associated with non-dilated left ventricular cardiomyopathy (NDLCM)/dilated cardiomyopathy (DCM) and ventricular tachycardia (VT). However, there is limited data on the impact of functional groups on VT ablation outcome. Aim We aimed to assess the impact of likely pathogenic/pathogenic variants (LP/Pv) of different functional gene groups on heart failure (HF) outcomes and VT recurrence in patients with NDLCM/DCM after VT ablation. Methods Patients with NDLCM/DCM who underwent genetic testing (next-generation sequencing) and were referred for VT ablation were enrolled at 6 participating centers. Patients were followed for VT recurrence, HF-related adverse outcomes (death, left ventricular assist device [LVAD], or heart transplantation due to HF), and all-cause mortality. Results A total of 239 patients (age 55±14yrs; male n=193; LVEF 40±12%; amiodarone at admission n=103; ICD n=180) were included. LP/Pv were identified in 128 patients (54%), predominantly found in genes encoding sarcomeric (n=36/128, [28%]; titin n=20, myosin binding protein C n=12), followed by nuclear membrane (n=27/128, [22%]; lamin A/C; n=26), desmosomal (n=23/128, [18%]; desmoplakin n=11, plakophilin n=9), cytoskeleton (n=12/128, [9%]; filamin C n=8), and sarcoplasmic reticulum proteins (n=12/128, [9%]; phospholamban n=10). Variants of unknown significance (n=32) were grouped with patients who tested negative. Acute complete ablation success, defined as non-VT inducibility of any sustained monomorphic VT, was achieved in 113 patients (47%), 63 (56%) with LP/Pv. During a median follow-up of 32 months, 108 patients (45%) had VT recurrence, 41 (17%) had HF-related adverse outcomes (death with/without LVAD in 34, LVAD in 2, heart transplantation in 5) and 47 (20%) died. Patients with LP/Pv were more likely to have VT recurrence, HF-related adverse outcomes and had a higher mortality compared to those without (VT, 68 [53%] vs. 40 [36%]; HF-related adverse outcomes, 30 [23%] vs. 11 [10%]; mortality, 30 [23%] vs. 17 [15%]; log-rank, P<0.01 for all). As compared to patients without LP/Pv, those with LP/Pv in genes encoding nuclear membrane or sarcomeric proteins had significantly worse (P<0.01 for both) and those with LP/Pv in genes encoding desmosomal proteins had similar HF and VT recurrence outcomes (Figure). Of interest, patients with LP/Pv in genes encoding cytoskeleton proteins had favorable HF-related adverse outcomes, but worse VT recurrence (P=0.02) compared to those without LP/Pv (Figure). Conclusion In patients with NDLCM/DCM and VT, the impact of LP/Pv on HF and VT-recurrence outcomes after ablation differs across functional groups. Integrating genetic testing in the comprehensive evaluation of patients with NDLCM/DCM undergoing VT ablation may facilitate personalized ablation strategies and early consideration of alternative HF treatments.

Induced degeneration and regeneration in aged muscle reduce tubular aggregates but not muscle function.

Tubular aggregates (TA) are skeletal muscle structures that arise from the progressive accumulation of sarcoplasmic reticulum proteins. Cytoplasmic aggregates in muscle fibers have already been observed in mice and humans, mainly during aging and muscle disease processes. However, the effects of muscle regeneration on TA formation have not yet been reported. This study aimed to investigate the relationship between degeneration/regeneration and TA in aged murine models. We investigated the presence and quantity of TA in old males from two murine models with intense muscle degeneration and regeneration. One murine lineage was a Dmdmdx model of Duchenne muscular dystrophy (n = 6). In the other model, muscle damage was induced by electroporation in C57BL/6J wild-type mice, and analyzed after 5, 15, and 30 days post-electroporation (dpe; n = 15). Regeneration was evaluated based on the quantity of developmental myosin heavy chain (dMyHC)-positive fibers. The frequency of fibers containing TA was higher in aged C57BL/6J (26 ± 8.3%) than in old dystrophic Dmdmdx mice (2.4 ± 2%). Comparing the data from induced degeneration/regeneration in normal mice revealed a reduced proportion of TA-containing fibers after 5 and 30 dpe. Normal aged muscle was able to regenerate and form dMyHC+ fibers, mainly at 5 dpe (0.1 ± 0.1 vs. 16.5 ± 2.6%). However, there was no difference in force or resistance between normal and 30 dpe animals, except for the measurements by the Actimeter device, which showed the worst parameters in the second group. Our results suggest that TA also forms in the Dmdmdx muscle but in smaller amounts. The intense degeneration and regeneration of the old dystrophic model resulted in the generation of new muscle fibers with a lower quantity of TA. Data from electroporated wild-type mice support the idea that muscle regeneration leads to a reduction in the amount of TA. We suggest that TA accumulates in muscle fibers throughout physiological aging and that regeneration leads to the formation of new fibers without these structures. In addition, these new fibers do not confer functional benefits to the muscle.

Abstract 17305: Diet-Induced Obesity Drives Phospholamban Acetylation and Promotes Cardiomyocyte Calcium Reuptake Into the Sarcoplasmic Reticulum

Background: The rising incidence of obesity and metabolic syndrome in the US has profound implications for the pathology of heart disease in coming years. Excessive fat and sugar intake is linked to elevated acetyl-CoA, which in turn is linked to protein acetylation. Although an increasing body of work has determined that non-nuclear lysine acetylation can alter the activity and stability of key proteins involved in cardiac health, the impact of acetylation on calcium handling sarcoplasmic reticulum (SR) proteins, particularly the SERCA2a regulator phospholamban, remains poorly understood. Hypothesis: Metabolic dysregulation and obesity lead to the acetylation of phospholamban, relieving its inhibition on SERCA2a and increasing calcium uptake into the SR. APPROACH: C57BL/6J Mice were subjected to high fat diet-induced obesity (DIO). Left ventricle (LV) tissue was collected and analyzed using immunoblotting and acetylation was quantified by immunoprecipitation with acetyl-lysine antibodies. Human LV tissue from control and diabetic patients was similarly processed and evaluated. Adult ventricular cardiomyocytes were isolated from DIO and control mice, and DIO cells were incubated in media containing excess lipid substrates (1% Lipid Mix 1, Millipore Sigma) and acutely administered HDAC activator ITSA-1 to promote deacetylation. Changes in stimulated calcium dynamics were assessed using Fura-2AM and Fluo-4AM imaging. Results: Phospholamban acetylation significantly increased in LV tissue from both DIO mice (p=0.003) and human diabetic patients (p=0.034) compared to their respective controls. Primary cardiomyocytes isolated from DIO mice and incubated for 24hrs in a high-fat environment exhibited increased calcium transient amplitude and reuptake time (tau) into the SR, while incubation with pan HDAC activators significantly reduced these parameters (p=0.0004, and p=0.003 respectively). Conclusions: Obesity increases the acetylation of phospholamban and enhances the reuptake of calcium into the SR, while deacetylation reverses these effects. These data directly tie nutritive status to a novel phospholamban post-translational modification that may directly impact myocardial calcium handling and contractile function.

Activation of Ca2+ transport in cardiac microsomes enriches functional sets of ER and SR proteins

The importance of sarcoplasmic reticulum (SR) Ca2+-handling in heart has led to detailed understanding of Ca2+-release and re-uptake protein complexes, while less is known about other endoplasmic reticulum (ER) functions in the heart. To more fully understand cardiac SR and ER functions, we analyzed cardiac microsomes based on their increased density through the actions of the SR Ca2+-ATPase (SERCA) and the ryanodine receptor that are highly active in cardiomyocytes. Crude cardiac microsomal vesicles loaded with Ca oxalate produced two higher density subfractions, MedSR and HighSR. Proteins from 20.0 μg of MV, MedSR, and HighSR protein were fractionated using SDS-PAGE, then trypsinized from 20 separate gel pieces, and analyzed by LC–MS/MS to determine protein content. From 62,000 individual peptide spectra obtained, we identified 1105 different proteins, of which 354 were enriched ≥ 2.0-fold in SR fractions compared to the crude membrane preparation. Previously studied SR proteins were all enriched, as were proteins associated with canonical ER functions. Contractile, mitochondrial, and sarcolemmal proteins were not enriched. Comparing the levels of SERCA-positive SR proteins in MedSR versus HighSR vesicles produced a range of SR subfraction enrichments signifying differing levels of Ca2+ leak co-localized in the same membrane patch. All known junctional SR proteins were more enriched in MedSR, while canonical ER proteins were more enriched in HighSR membrane. Proteins constituting other putative ER/SR subdomains also exhibited average Esub enrichment values (mean ± S.D.) that spanned the range of possible Esub values, suggesting that functional sets of proteins are localized to the same areas of the ER/SR membrane. We conclude that active Ca2+ loading of cardiac microsomes, reflecting the combined activities of Ca2+ uptake by SERCA, and Ca2+ leak by RyR, permits evaluation of multiple functional ER/SR subdomains. Sets of proteins from these subdomains exhibited similar enrichment patterns across membrane subfractions, reflecting the relative levels of SERCA and RyR present within individual patches of cardiac ER and SR.

Sarcoplasmic Reticulum Ca2+ Buffer Proteins: A Focus on the Yet-To-Be-Explored Role of Sarcalumenin in Skeletal Muscle Health and Disease.

Sarcalumenin (SAR) is a luminal Ca2+ buffer protein with high capacity but low affinity for calcium binding found predominantly in the longitudinal sarcoplasmic reticulum (SR) of fast- and slow-twitch skeletal muscles and the heart. Together with other luminal Ca2+ buffer proteins, SAR plays a critical role in modulation of Ca2+ uptake and Ca2+ release during excitation-contraction coupling in muscle fibers. SAR appears to be important in a wide range of other physiological functions, such as Sarco-Endoplasmic Reticulum Calcium ATPase (SERCA) stabilization, Store-Operated-Calcium-Entry (SOCE) mechanisms, muscle fatigue resistance and muscle development. The function and structural features of SAR are very similar to those of calsequestrin (CSQ), the most abundant and well-characterized Ca2+ buffer protein of junctional SR. Despite the structural and functional similarity, very few targeted studies are available in the literature. The present review provides an overview of the role of SAR in skeletal muscle physiology, as well as of its possible involvement and dysfunction in muscle wasting disorders, in order to summarize the current knowledge on SAR and drive attention to this important but still underinvestigated/neglected protein.

Low aerobic capacity in McArdle disease: A role for mitochondrial network impairment?

BackgroundMcArdle disease is caused by myophosphorylase deficiency and results in complete inability for muscle glycogen breakdown. A hallmark of this condition is muscle oxidation impairment (e.g., low peak oxygen uptake (VO2peak)), a phenomenon traditionally attributed to reduced glycolytic flux and Krebs cycle anaplerosis. Here we hypothesized an additional role for muscle mitochondrial network alterations associated with massive intracellular glycogen accumulation. MethodsWe analyzed in depth mitochondrial characteristics-content, biogenesis, ultrastructure-and network integrity in skeletal-muscle from McArdle/control mice and two patients. We also determined VO2peak in patients (both sexes, N = 145) and healthy controls (N = 133). ResultsBesides corroborating very poor VO2peak values in patients and impairment in muscle glycolytic flux, we found that, in McArdle muscle: (a) damaged fibers are likely those with a higher mitochondrial and glycogen content, which show major disruption of the three main cytoskeleton components-actin microfilaments, microtubules and intermediate filaments-thereby contributing to mitochondrial network disruption in skeletal muscle fibers; (b) there was an altered subcellular localization of mitochondrial fission/fusion proteins and of the sarcoplasmic reticulum protein calsequestrin-with subsequent alteration in mitochondrial dynamics/function; impairment in mitochondrial content/biogenesis; and (c) several OXPHOS-related complex proteins/activities were also affected. ConclusionsIn McArdle disease, severe muscle oxidative capacity impairment could also be explained by a disruption of the mitochondrial network, at least in those fibers with a higher capacity for glycogen accumulation. Our findings might pave the way for future research addressing the potential involvement of mitochondrial network alterations in the pathophysiology of other glycogenoses.

Dysferlin facilitates the de novo biogenesis of tubular membranes in hypertrophic remodelling

Abstract Introduction Ferlins are transmembrane proteins with multiple C2 domains that may mediate Ca2+ dependent membrane fusion and repair events. In ventricles, Dysferlin expression is dominant, and patient mutations have been associated with cardiomyopathies. Hence, we hypothesized that Dysferlin is essential for the functional integrity of the tubular endomembrane system in ventricular myocytes (VM), and may facilitate the cellular hypertrophic remodelling in pressure-overload. Results In 30 weeks old Dysferlin knockout (KO) mice, echocardiography revealed dilative cardiomyopathy with reduced left-ventricular ejection fractions (difference between means KO vs. wildtype (WT) −13.45±2.17%, p<0.001, n=16/20 mice). Asking for the subcellular relevance of Dysferlin, we investigated VM nanodomains by immunolabelling in STimulated Emission Depletion (STED) nanoscopy and visualized punctual Dysferlin clusters decorating the T-tubule system of mouse and human VM. Precisely, Dysferlin clusters localized to junctions of the tubular system and the sarcoplasmic reticulum (SR) in nanometric proximity to RyR2 Ca2+ release units. Complexome profiling (LC-MS/MS) also suggested interactions of Dysferlin with RyR2 and the SR membrane-tethering protein Junctophilin-2 (JP2) in high molecular weight complexes. In JP2-overexpressing mice, live-cell STED membrane imaging captured polyadic junctional membrane structures in VM, specifically surrounded by dense Dysferlin clusters, which proposes Dysferlin as stabilizing protein of tubule-SR junctions. Further, we studied the ultrastructural remodelling of the tubular system in a mouse model of cardiac hypertrophy and heart failure induced by transaortic constriction (TAC). Interestingly, mortality four weeks post TAC was not increased in KO mice. However, echocardiography uncovered that Dysf-KO prevents left-ventricular hypertrophy in response to pressure-overload, which was confirmed by single cell measurements (VM area: WT 3559±120 μm2 vs. KO 2671±87 μm2, n=163/158 VM, p<0.001). Thus, we assumed Dysferlin to mediate tubular network proliferation during hypertrophic remodelling. Indeed, WT VM showed an augmented Dysferlin expression post TAC (158±0,125%, p=0,005, n=7/7 mice), and immunofluorescence STED nanoscopy detected an increased Dysferlin network complexity. In detail, we visualized highly abundant Dysferlin clusters decorating newly shaped axial tubules, which were quantified by component-specific tubule network analysis. Conclusion Nanoscale imaging proposes Dysferlin as stabilizer of tubule-SR junctions, thereby promoting functional excitation-contraction coupling, and demonstrates the crucial importance of Dysferlin in pressure-overload induced hypertrophic remodelling, facilitating the de novo biogenesis of axial tubule membranes and tubule-SR junctions necessary for VM growth. Thus, Dysferlin may emerge as potential novel therapeutic rationale to control subcellular membrane remodelling in cardiac diseases. Funding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Multiscale Bioimaging Cluster of Excellence: From Molecular Machines to Networks of Excitable Cells;German Research Foundation (DFG) - Collaborative Research Centre 1002 (SFB 1002): Modulatory Units in Heart Failure

Impaired Dynamic Sarcoplasmic Reticulum Ca Buffering in Autosomal Dominant CPVT2.

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal cardiac arrhythmia syndrome triggered by catecholamines released during exercise, stress, or sudden emotion. Variants in the calsequestrin-2 gene (CASQ2), encoding the major calcium (Ca) binding protein in the sarcoplasmic reticulum (SR), are the second most common cause of CPVT. Recently, several CASQ2 gene variants, such as CASQ2-K180R, have been linked to an autosomal dominant form of Casq2-linked CPVT (CPVT2), but the underlying mechanism is not known. A K180R mouse model was generated using CRIPSR/Cas9. Heterozygous and homozygous K180R mice were studied using telemetry ECG recordings in vivo. Ventricular cardiomyocytes were isolated and studied using fluorescent Ca indicators and patch clamp. Expression levels and localization of SR Ca-handling proteins were evaluated using Western blotting and immunostaining. Intra-SR Ca kinetics were quantified using low-affinity Ca indicators. K180R mice exhibit an autosomal dominant CPVT phenotype following exercise or catecholamine stress. Upon catecholamine stress, K180R ventricular cardiomyocytes exhibit increased spontaneous SR Ca release events, triggering delayed afterdepolarizations and spontaneous beats. K180R had no effect on levels of Casq2, Casq2 polymers, or other SR Ca-handling proteins. Intra-SR Ca measurements revealed that K180R impaired dynamic intra-SR Ca buffering, resulting in a more rapid rise of free Ca in the SR during diastole. Steady-state SR Ca buffering and total SR Ca content were not changed. Consistent with the reduced dynamic intra-SR buffering, K180R causes reduced SR Ca release refractoriness. CASQ2-K180R causes CPVT2 via a heretofore unknown mechanism that differs from CASQ2 variants associated with autosomal recessive CPVT2. Unlike autosomal recessive CASQ2 variants, K180R impairs the dynamic buffering of Ca within the SR without affecting total SR Ca content or Casq2 protein levels. Our data provide insight into the molecular mechanism underlying autosomal dominant CPVT2.

CMYA5 establishes cardiac dyad architecture and positioning

Cardiac excitation-contraction coupling requires dyads, the nanoscopic microdomains formed adjacent to Z-lines by apposition of transverse tubules and junctional sarcoplasmic reticulum. Disruption of dyad architecture and function are common features of diseased cardiomyocytes. However, little is known about the mechanisms that modulate dyad organization during cardiac development, homeostasis, and disease. Here, we use proximity proteomics in intact, living hearts to identify proteins enriched near dyads. Among these proteins is CMYA5, an under-studied striated muscle protein that co-localizes with Z-lines, junctional sarcoplasmic reticulum proteins, and transverse tubules in mature cardiomyocytes. During cardiac development, CMYA5 positioning adjacent to Z-lines precedes junctional sarcoplasmic reticulum positioning or transverse tubule formation. CMYA5 ablation disrupts dyad architecture, dyad positioning at Z-lines, and junctional sarcoplasmic reticulum Ca2+ release, leading to cardiac dysfunction and inability to tolerate pressure overload. These data provide mechanistic insights into cardiomyopathy pathogenesis by demonstrating that CMYA5 anchors junctional sarcoplasmic reticulum to Z-lines, establishes dyad architecture, and regulates dyad Ca2+ release.

An Investigation of the Altered Textural Property in Woody Breast Myopathy Using an Integrative Omics Approach.

Woody breast (WB) is a myopathy observed in broiler Pectoralis major (PM) characterized by its tough and rubbery texture with greater level of calcium content. The objective of this study was to investigate the functionality/integrity of WB sarcoplasmic reticulum (SR), which may contribute to the elevated calcium content observed in WB and other factors that may influence WB texture. Fourteen Ross line broiler PM [7 severe WB and 7 normal (N)] were selected, packaged, and frozen at −20°C at 8 h postmortem from a commercial processing plant. Samples were used to measure pH, sarcomere length, proteolysis, calpain activity, collagenase activity, collagen content, collagen crosslinks density, and connective tissue peak transitional temperature. Exudate was also collected from each sample to evaluate free calcium concentration. The SR fraction of the samples was separated and utilized for proteomic and lipidomic analysis. The WB PM had a higher pH, shorter sarcomeres, lower % of intact troponin-T, more autolyzed μ/m calpain, more activated collagenase, greater collagen content, greater mature collagen crosslinks density, and higher connective tissue peak transitional temperature than the N PM (p ≤ 0.05). Exudate from WB PM had higher levels of free calcium than those from N PM (p < 0.05). Proteomics data revealed an upregulation of calcium transport proteins and a downregulation of proteins responsible for calcium release (p < 0.05) in WB SR. Interestingly, there was an upregulation of phospholipase A2 (PLA2), and cholinesterase exhibited a 7.6-fold increase in WB SR (p < 0.01). Lipidomics data revealed WB SR had less relative % of phosphatidylcholine (PC) and more lysophosphatidylcholine (LPC; p < 0.05). The results indicated that upregulation of calcium transport proteins and downregulation of calcium-release proteins in WB SR may be the muscle’s attempt to regulate this proposed excessive signaling of calcium release due to multiple factors, such as upregulation of PLA2 resulting in PC hydrolysis and presence of cholinesterase inhibitors in the system prolonging action potential. In addition, the textural abnormality of WB may be the combined effects of shorter sarcomere length and more collagen with greater crosslink density being deposited in the broiler PM.

Effects of 6-week aerobic exercise on calcium regulation in skeletal muscle of ApoE knockout mice fed by high-fat diet

Objective: To investigate the effects of 6 weeks of aerobic exercise on the sarcoplasmic reticulum calcium regulatory proteins in skeletal muscle of apolipoprotein E (ApoE) knockout mice fed by high-fat diet. Methods: There were a total of twenty five 9-week-old ApoE knockout mice (ApoE knockout mice, ApoE KO), five of which were selected randomly for the maximum running speed test. The running speed would be increased by 1.2 m/min every 3 min after a 5-min duration of initial speed of 4.8 m/min without slope until exhaustion, then the final speed was set as maximal speed, and the test result of the maximum running speed was (27.0±2.4)m/min. The remaining 20 ApoE KO mice were randomly divided into ApoE KO mouse high-fat diet group (KO) and ApoE KO mouse high-fat diet + aerobic exercise group (KE), 10 mice per group. Ten 9-week-old wild-type C57BL/6J mice were used as a blank control group (wild-type, WT). High fat diet composition: fat content was 21% (w/w) and cholesterol content was 1.5% (w/w). Exercise intervention was initiated 1 week after adaptive training in the KE group with an exercise protocol consisting of 40% maximal running speed (10.8 m/min), 40 min/d and a frequency of 3 d/w for a total of 6 weeks. All mice were anesthetized and sacrificed by cardiac puncture then bilateral gastrocnemius muscles were isolated immediately 48 h after the final exercise. Skeletal muscle Ca2+ concentration was detected by visible light colorimetry; the expression levels of RyR, CaM, CaMK Ⅱ, SERCA1 and SERCA2, which are calcium regulated proteins of sarcoplasmic reticulum, in mouse skeletal muscle, was detected by Western blotting. Results: Compared with WT mice, the Ca2+ concentration, the expression levels of the sarcoplasmic reticulum calcium releasing proteins RyR, CaMKⅡ, and the calcium recycling proteins SERCA1 and SERCA2 were decreased significantly in skeletal muscle of the KO mice (P＜0.05, P＜0.01), while the expression of CaM protein did not change. Skeletal muscle Ca2+ concentration and the levels of sarcoplasmic reticulum calcium recycling proteins SERCA1 and SERCA2 were increased significantly (P＜0.05) in KE mice compared with KO mice, but there were no significant differences in the expressions of the sarcoplasmic reticulum calcium release proteins RyR, CaM and CaMK II. Conclusion: High fat diet can reduce the concentration of Ca2+ in skeletal muscle and weaken the release and recovery of sarcoplasmic reticulum calcium in ApoE knockout mice. 6-week aerobic exercise training can significantly increase its Ca2+concentration and promote the recovery of sarcoplasmic reticulum calcium.

Multiple regions within junctin drive its interaction with calsequestrin-1 and its localization to triads in skeletal muscle.

Junctin is a transmembrane protein of striated muscles, located at the junctional sarcoplasmic reticulum (SR). It is characterized by a luminal C-terminal tail, through which it functionally interacts with calsequestrin and the ryanodine receptor (RyR). Interaction with calsequestrin was ascribed to the presence of stretches of charged amino acids (aa). However, the regions able to bind calsequestrin have not been defined in detail. We report here that, in non-muscle cells, junctin and calsequestrin assemble in long linear regions within the endoplasmic reticulum, mirroring the formation of calsequestrin polymers. In differentiating myotubes, the two proteins colocalize at triads, where they assemble with other proteins of the junctional SR. By performing GST pull-down assays with distinct regions of the junctin tail, we identified two KEKE motifs that can bind calsequestrin. In addition, stretches of charged aa downstream these motifs were found to also bind calsequestrin and the RyR. Deletion of even one of these regions impaired the ability of junctin to localize at the junctional SR, suggesting that interaction with other proteins at this site represents a key element in junctin targeting.

Sarcopenia in pulmonary diseases is associated with elevated sarcoplasmic reticulum stress and myonuclear disorganization.

Chronic obstructive pulmonary disease (COPD) is frequently associated with age-related muscle loss or sarcopenia. However, the exact molecular mechanism of muscle loss in COPD remains elusive. We investigated the association of chronic dysregulation of sarcoplasmic reticulum (SR) protein homeostasis (a condition called SR stress) and myonuclear disorganization with sarcopenia in patients with COPD. Markers of SR stress and their downstream consequences, including apoptosis and inflammation, were upregulated in patients with COPD. The maximal SR Ca2+ ATPase (SERCA) activity was significantly reduced in advanced COPD as compared to healthy controls. Single muscle fiber diameter and cytoplasmic domain per myonucleus were significantly smaller in patients with advanced COPD than in healthy controls. Increased disruption of myonuclear organization was found in the COPD patients as compared to healthy controls. These changes in SR dysfunction were accompanied by elevated global levels of oxidative stress, including lipid peroxidation and mitochondrial reactive oxygen species (ROS) production. Altogether, our data suggest that muscle weakness in advanced COPD is in part associated with the disruption of SR protein and calcium homeostasis and their pathological consequences.

Myospryn deficiency leads to impaired cardiac structure and function and schizophrenia-associated symptoms.

The desmin-associated protein myospryn, encoded by the cardiomyopathy-associated gene 5 (CMYA5), is a TRIM-like protein associated to the BLOC-1 (Biogenesis of Lysosomes Related Organelles Complex 1) protein dysbindin. Human myospryn mutations are linked to both cardiomyopathy and schizophrenia; however, there is no evidence of a direct causative link of myospryn to these diseases. Therefore, we sought to unveil the role of myospryn in heart and brain. We have genetically inactivated the myospryn gene by homologous recombination and demonstrated that myospryn null hearts have dilated phenotype and compromised cardiac function. Ultrastructural analyses revealed that the sarcomere organization is not obviously affected; however, intercalated disk (ID) integrity is impaired, along with mislocalization of ID and sarcoplasmic reticulum (SR) protein components. Importantly, cardiac and skeletal muscles of myospryn null mice have severe mitochondrial defects with abnormal internal vacuoles and extensive cristolysis. In addition, swollen SR and T-tubules often accompany the mitochondrial defects, strongly implying a potential link of myospryn together with desmin to SR- mitochondrial physical and functional cross-talk. Furthermore, given the reported link of human myospryn mutations to schizophrenia, we performed behavioral studies, which demonstrated that myospryn-deficient male mice display disrupted startle reactivity and prepulse inhibition, asocial behavior, decreased exploratory behavior, and anhedonia. Brain neurochemical and ultrastructural analyses revealed prefrontal-striatal monoaminergic neurotransmitter defects and ultrastructural degenerative aberrations in cerebellar cytoarchitecture, respectively, in myospryn-deficient mice. In conclusion, myospryn is essential for both cardiac and brain structure and function and its deficiency leads to cardiomyopathy and schizophrenia-associated symptoms.

Loss of α-actinin-3 during human evolution provides superior cold resilience and muscle heat generation

SummaryThe protein α-actinin-3 expressed in fast-twitch skeletal muscle fiber is absent in 1.5 billion people worldwide due to homozygosity for a nonsense polymorphism in ACTN3 (R577X). The prevalence of the 577X allele increased as modern humans moved to colder climates, suggesting a link between α-actinin-3 deficiency and improved cold tolerance. Here, we show that humans lacking α-actinin-3 (XX) are superior in maintaining core body temperature during cold-water immersion due to changes in skeletal muscle thermogenesis. Muscles of XX individuals displayed a shift toward more slow-twitch isoforms of myosin heavy chain (MyHC) and sarcoplasmic reticulum (SR) proteins, accompanied by altered neuronal muscle activation resulting in increased tone rather than overt shivering. Experiments on Actn3 knockout mice showed no alterations in brown adipose tissue (BAT) properties that could explain the improved cold tolerance in XX individuals. Thus, this study provides a mechanism for the positive selection of the ACTN3 X-allele in cold climates and supports a key thermogenic role of skeletal muscle during cold exposure in humans.

Local Synthesis of Sarcolemma and Sarcoplasmic Reticulum Membrane Proteins in Cardiac Myocytes

Classically, cardiac sarcolemma (SL) and sarcoplasmic reticulum (SR) membrane proteins are thought to be synthesized and processed on the rough ER, in the perinuclear region and then transported to locations of employment in the SR and SL. However, this view is largely based on studies in non-myocyte cell types. Therefore, we investigated the localization and regulation of synthesis of several key sarcolemmal SL and SR membrane proteins in adult cardiac myocytes. in contrast to the canonical view, in mouse ventricular myocytes we identified synthesis machinery and translation for such proteins, namely SERCA2a, RyR2, Cav1.2 and Nav1.5 localized not only at the perinuclear area, but spread throughout the entire cell volume, particularly in the vicinity of surface/T-tubule and SR membranes. inhibition of microtubular trafficking by colchicine resulted in redistribution of the mRNA signal for these proteins towards the nuclei, consistent with microtubule trafficking providing a dedicated pool of mRNA for local protein translation and synthesis. Active translation of SERCA2a mRNA throughout the cardiac myocyte space was confirmed through visualization of active translation sites using a novel in-situ hybridization assay, which we developed. Collectively, these results suggest that synthesis of SR and SL proteins in cardiomyocytes occurs locally at sites of utilization from dedicated, local mRNA pools.

Relationship between the expression level of genes of Ca(2+)-transporting proteins of the cardiomyocytes' sarcoplasmic reticulum with the severity of heart failure

Abstract Introduction The sarcoplasmic reticulum of cardiomyocytes is the main Ca(2+) ion depot, the main functional proteins of which are Ca(2+)-ATPase SERCA2a, which re-traps ions from myoplasma, calsequestrin CASQ2, which binds most of Ca(2+), and ryanodine receptors RyR2 releasing Ca(2+) from the intracellular depot. A change in their functional activity can determine the progression of contractile myocardial dysfunction. The effectiveness of the Ca(2+)-transporting system and, consequently, the risk of development and progression of heart failure, may depend on the expression of the corresponding genes. Purpose To evaluate the association between the level of relative expression of the ATP2A2, CASQ2, RYR2 genes in the myocardium and the clinical parameters of heart failure in patients with coronary heart disease. Methods The study was carried out on the material of 90 patients with chronic heart failure developed on the background of coronary heart disease. Myocardial samples (a fragment of the right atrium appendage) obtained by connecting the cardiopulmonary bypass during planned coronary bypass surgery. The level of expression of the Ca(2+)-ATPase ATP2A2 gene, the ryanodine receptor RYR2 gene, and the calsequestrin CASQ2 gene was estimated. Samples were homogenized, RNA was isolated and cDNA was synthesized, and real-time PCR was performed. As reference, the glyceraldehyde-3-phosphate dehydrogenase GAPDH gene, the beta-actin ACTB gene, and the 18S gene were used. The level of gene expression was calculated automatically using the software of the thermocycler. For the analysis of quantitative data, the Kruskel-Wallis test or the Mann-Whitney test was used. The analysis of the strength of the linear relationship was carried out using the Spearman rank correlation coefficient. Results ATP2A2 gene expression was reduced in the myocardium of patients with left ventricular (LV) hypertrophy (p=0.027 for ATP2A2/GAPDH, p=0.043 for ATP2A2/ACTB, p=0.039 for ATP2A2/18S). A correlation was observed between an increase in the functional class of heart failure (NYHA) and a decrease in the expression level of the RYR2/GAPDH gene (p=0.023). Among patients with heart failure, there was a weak negative linear relationship between the expression level of CASQ2/18S and the LV ejection fraction (r=−0.288, p=0.047). A significant (p=0.040) increase in CASQ2/ACTB gene expression was also found in patients with diastolic dysfunction compared to individuals without it, as well as a similar tendency for CASQ2/GAPDH (p=0.068) and CASQ2/18S (p=0.090). Conclusion In patients with heart failure due to coronary heart disease with LV hypertrophy, there was a decrease in the expression of the Ca (2+)-ATPase ATP2A2 gene. A decrease in the expression of the ryanodine receptor RYR2 gene was found as the heart failure class worsened. However, CASQ2 expression increased with diastolic dysfunction and a decrease in LV ejection fraction. Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): Russian Foundation for Basic Research

Phylogenetic and biochemical analysis of calsequestrin structure and association of its variants with cardiac disorders

Calsequestrin is among the most abundant proteins in muscle sarcoplasmic reticulum and displays a high capacity but a low affinity for Ca2+ binding. In mammals, calsequestrin is encoded by two genes, CASQ1 and CASQ2, which are expressed almost exclusively in skeletal and cardiac muscles, respectively. Phylogenetic analysis indicates that calsequestrin is an ancient gene in metazoans, and that the duplication of the ancestral calsequestrin gene took place after the emergence of the lancelet. CASQ2 gene variants associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) in humans are positively correlated with a high degree of evolutionary conservation across all calsequestrin homologues. The mutations are distributed in diverse locations of the calsequestrin protein and impart functional diversity but remarkably manifest in a similar phenotype in humans.

Calsequestrin, a key protein in striated muscle health and disease.

Calsequestrin (CASQ) is the most abundant Ca2+ binding protein localized in the sarcoplasmic reticulum (SR)of skeletal and cardiac muscle. The genome of vertebrates contains two genes, CASQ1 and CASQ2. CASQ1 and CASQ2 have a high level of homology, but show specific patterns of expression. Fast-twitch skeletal muscle fibers express only CASQ1, both CASQ1 and CASQ2 are present in slow-twitch skeletal musclefibers, while CASQ2 is the only protein present in cardiomyocytes. Depending on the intraluminal SR Ca2+ levels, CASQ monomers assemble to form large polymers, which increase their Ca2+ binding ability. CASQ interacts with triadin and junctin, two additional SR proteins which contribute to localize CASQ to the junctional region of the SR (j-SR) and also modulate CASQ ability to polymerize into large macromolecular complexes. In addition to its ability to bind Ca2+ in the SR, CASQ appears also to be able to contribute to regulation of Ca2+ homeostasis in muscle cells. Both CASQ1 and CASQ2 are able to either activate and inhibit the ryanodine receptors (RyRs) calcium release channels, likely through their interactions with junctin and triadin. Additional evidence indicates that CASQ1 contributes to regulate the mechanism of store operated calcium entry in skeletal muscle via a direct interaction with the Stromal Interaction Molecule 1 (STIM1). Mutations in CASQ2 and CASQ1 have been identified, respectively, in patients with catecholamine-induced polymorphic ventricular tachycardia and in patients with some forms of myopathy. This review will highlight recent developments in understanding CASQ1 and CASQ2 in health and diseases.

SUN-559 Involvement of Sarco/Endoplasmic Reticulum Ca- ATPase (SERCA) in Membrane Progesterone Receptor Alpha (PAQR7)-Mediated Progesterone Induction of Vascular Smooth Muscle Relaxation

Progesterone (P4) exerts multiple beneficial effects on the human cardiovascular system through its actions on vascular endothelial cells and also by acting directly on vascular smooth muscle cells (VSMCs). Membrane progesterone receptor alpha (mPRα) has been shown to mediate the rapid P4-induction of human VSMC relaxation through activation of MAPK, Akt/Pi3k and RhoA/ROCK signaling pathways and the resulting reduction of calcium influx through calcium channels. In this study, we demonstrate that treatment of cultured human VSMCs with P4 for 1-2 hours increases both the mRNA and protein expression of sarco/endoplasmic reticulum Ca- ATPase (SERCA), the major transporter of calcium from the cytosol into the sarcoplasmic reticulum (SR) during muscle relaxation. Knockdown of mPRα with siRNA completely blocked this stimulatory effect of P4 as well as that of OD 02-0, a mPR selective agonist, on SERCA protein expression. In contrast, expression levels of phospholamban (PLB), a SR protein that reversibly inhibits SERCA were downregulated by this P4 treatment, and mRNA expression of a channel that releases calcium from the SR, inositol trisphosphate receptor (IP3R), was unaltered after treatment with P4. Moreover, treatments with P4 and OD 02-0, but not with R5020, a nuclear PR agonist, increased PLB phosphorylation, which would result in disinhibition of SERCA function. P4 and OD 02-0 significantly increased calcium levels in the SR detected with Fluo-5N, a specific SR calcium indicator, and caused VSMC relaxation. These effects were blocked by cyclopiazonic acid (CPA, a SERCA inhibitor), suggesting that SERCA plays a critical role in P4 induction of VSMC relaxation. Similarly, the effects of P4 and OD 02-0 on relaxation of umbilical artery rings measured with a myograph were significantly attenuated by CPA, which confirms the critical role of SERCA in the rapid action of P4 and 02-0 on vascular muscle relaxation. P4 has previously been shown to activate MAPK and Akt signaling pathways to induce VSMC relaxation. The P4- and OD 02-0-induced increases in calcium in the SR were blocked by MAPK and Akt/Pi3k signaling inhibitors, AZD6244 and wortmannin. Taken together, these results suggest that the direct, rapid effects of P4 on relaxation of VSMCs through mPRα involves regulation of the expression and function of the SR proteins SERCA and PLB through MAPK and Akt signaling pathways.