The regulation of SERCA-type pumps by phospholamban and sarcolipin

Looking into a conceptual framework of ROS-miRNA-atrial fibrillation

Atrial fibrillation (AF) has been recognized as a major cause of cardiovascular-related morbidity and mortality. MicroRNAs (miRNAs) represent recent additions to the collection of biomolecules involved in arrhythmogenesis. Reactive oxygen species (ROS) have been independently linked to both AF and miRNA regulation. However, no attempts have been made to investigate the possibility of a framework composed of ROS–miRNA–AF that is related to arrhythmia development. Therefore, this review was designed as an attempt to offer a new approach to understanding AF pathogenesis. The aim of this review was to find and to summarize possible connections that exist among AF, miRNAs and ROS to understand the interactions among the molecular entities underlying arrhythmia development in the hopes of finding unappreciated mechanisms of AF. These findings may lead us to innovative therapies for AF, which can be a life-threatening heart condition. A systemic literature review indicated that miRNAs associated with AF might be regulated by ROS, suggesting the possibility that miRNAs translate cellular stressors, such as ROS, into AF pathogenesis. Further studies with a more appropriate experimental design to either prove or disprove the existence of an ROS–miRNA–AF framework are strongly encouraged.

Clinical interpretation of genetic variants in arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy is an inherited cardiac entity characterized by right ventricular, or biventricular, fibrofatty replacement of myocardium. Structural alterations may lead to sudden cardiac death, mainly in young males during exercise. Autosomal dominant pattern of inheritance is reported in most parts of pathogenic genetic variations identified. Currently, 13 genes have been associated with the disease but nearly 40 % of clinically diagnosed cases remain without a genetic diagnosis. New genetic technologies allow further genetic analysis, generating a significant amount of genetic data in novel genes, which is often classified as of ambiguous significance. We focus on genetic advances of arrhythmogenic right ventricular cardiomyopathy, helping clinicians to interpret and translate genetic data into clinical practice.

Fiber-type-specific sensitivities and phenotypic adaptations to dietary fat overload differentially impact fast- versus slow-twitch muscle contractile function in C57BL/6J mice

High-fat diets (HFDs) have been shown to interfere with skeletal muscle energy metabolism and cause peripheral insulin resistance. However, understanding of HFD impact on skeletal muscle primary function, i.e., contractile performance, is limited. Male C57BL/6J mice were fed HFD containing lard (HFL) or palm oil (HFP), or low-fat diet (LFD) for 5weeks. Fast-twitch (FT) extensor digitorum longus (EDL) and slow-twitch (ST) soleus muscles were characterized with respect to contractile function and selected biochemical features. In FT EDL muscle, a 30%-50% increase in fatty acid (FA) content and doubling of long-chain acylcarnitine (C14-C18) content in response to HFL and HFP feeding were accompanied by increase in protein levels of peroxisome proliferator-activated receptor-γ coactivator-1α, mitochondrial oxidative phosphorylation complexes and acyl-CoA dehydrogenases involved in mitochondrial FA β-oxidation. Peak force of FT EDL twitch and tetanic contractions was unaltered, but the relaxation time (RT) of twitch contractions was 30% slower compared to LFD controls. The latter was caused by accumulation of lipid intermediates rather than changes in the expression levels of proteins involved in calcium handling. In ST soleus muscle, no evidence for lipid overload was found in any HFD group. However, particularly in HFP group, the peak force of twitch and tetanic contractions was reduced, but RT was faster than LFD controls. The latter was associated with a fast-to-slow shift in troponin T isoform expression. Taken together, these data highlight fiber-type-specific sensitivities and phenotypic adaptations to dietary lipid overload that differentially impact fast- versus slow-twitch skeletal muscle contractile function.

Dysregulation of calcium homeostasis in muscular dystrophies

Muscular dystrophies are a group of diseases characterised by the primary wasting of skeletal muscle, which compromises patient mobility and in the most severe cases originate a complete paralysis and premature death. Existing evidence implicates calcium dysregulation as an underlying crucial event in the pathophysiology of several muscular dystrophies, such as dystrophinopathies, calpainopathies or myotonic dystrophy among others. Duchenne muscular dystrophy is the most frequent myopathy in childhood, and calpainopathy or LGMD2A is the most common form of limb-girdle muscular dystrophy, whereas myotonic dystrophy is the most frequent inherited muscle disease worldwide. In this review, we summarise recent advances in our understanding of calcium ion cycling through the sarcolemma, the sarcoplasmic reticulum and mitochondria, and its involvement in the pathogenesis of these dystrophies. We also discuss some of the clinical implications of recent findings regarding Ca2+ handling as well as novel approaches to treat muscular dystrophies targeting Ca2+ regulatory proteins.

Co-expression of SERCA isoforms, phospholamban and sarcolipin in human skeletal muscle fibers

Sarcolipin (SLN) and phospholamban (PLN) inhibit the activity of sarco(endo)plasmic reticulum Ca²⁺-ATPases (SERCAs) by reducing their apparent affinity for Ca²⁺. A ternary complex between SLN, PLN, and SERCAs results in super-inhibition of SERCA activity. Analysis of skeletal muscle homogenate has limited our current understanding of whether SLN and PLN regulate SERCA1a, SERCA2a, or both in skeletal muscle and whether SLN and PLN are co-expressed in skeletal muscle fibers. Biopsies from human vastus lateralis were analyzed through single fiber Western blotting and immunohisto/fluorescence staining to circumvent this limitation. With a newly generated SLN antibody, we report for the first time that SLN protein is present in human skeletal muscle. Addition of the SLN antibody (50 µg) to vastus lateralis homogenates increased the apparent Ca²⁺ affinity of SERCA (K_Ca, pCa units) (-Ab, 5.85 ± 0.02 vs. +Ab, 5.95 ± 0.02) and maximal SERCA activity (μmol/g protein/min) (-Ab, 122 ± 6.4 vs. +Ab, 159 ± 11) demonstrating a functional interaction between SLN and SERCAs in human vastus lateralis. Specifically, our results suggest that although SLN and PLN may preferentially regulate SERCA1a, and SERCA2a, respectively, physiologically they both may regulate either SERCA isoform. Furthermore, we show that SLN and PLN co-immunoprecipitate in human vastus lateralis homogenate and are simultaneously expressed in 81% of the fibers analyzed with Western blotting which implies that super-inhibition of SERCA may exist in human skeletal muscle. Finally, we demonstrate unequivocally that mouse soleus contains PLN protein suggesting that super-inhibition of SERCA may also be important physiologically in rodent skeletal muscle.

Mechanisms of stress (takotsubo) cardiomyopathy

Stress cardiomyopathy is a form of reversible systolic dysfunction of the mid and apical left ventricle with pathologic changes of the electrocardiogram in the absence of an obstructive coronary artery disease. The prevalence of stress cardiomyopathy among patients with symptoms suggestive of myocardial infarction is 0.7% to 2.5%, and it is found predominantly in postmenopausal women (90%). No large studies have confirmed the cause of stress cardiomyopathy. Published data suggest that substantially elevated plasma catecholamine levels, due to emotional or physical stress, may be relevant.

Ablation of sarcolipin decreases the energy requirements for Ca2+ transport by sarco(endo)plasmic reticulum Ca2+-ATPases in resting skeletal muscle

The purpose of this study was to examine the effects of sarcolipin (SLN) on sarco(endo) plasmic reticulum Ca(2+)-ATPase (SERCA pump) energetics in vivo and resting skeletal muscle metabolic rate. Using SLN knockout (Sln(-/-)) mice we show that SLN ablation increases the transport stoichiometry of SERCA pumps (Ca(2+) uptake/Ca(2+)-ATPase activity) and decreases the relative contribution of SERCA pumps to resting oxygen consumption (VO2) in soleus without affecting soleus or whole body VO2. These data suggest that the lower energy requirements for Ca(2+) cycling in resting skeletal muscle of Sln(-/-) mice do not impact significantly either skeletal muscle or whole body metabolic rate.

Sarcolipin protein interaction with sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is distinct from phospholamban protein, and only sarcolipin can promote uncoupling of the SERCA pump

Sarco(endo)plasmic reticulum Ca(2+)ATPase (SERCA) pump activity is modulated by phospholamban (PLB) and sarcolipin (SLN) in cardiac and skeletal muscle. Recent data suggest that SLN could play a role in muscle thermogenesis by promoting uncoupling of the SERCA pump (Lee, A.G. (2002) Curr. Opin. Struct. Biol. 12, 547-554 and Bal, N. C., Maurya, S. K., Sopariwala, D. H., Sahoo, S. K., Gupta, S. C., Shaikh, S. A., Pant, M., Rowland, L. A., Bombardier, E., Goonasekera, S. A., Tupling, A. R., Molkentin, J. D., and Periasamy, M. (2012) Nat. Med. 18, 1575-1579), but the mechanistic details are unknown. To better define how binding of SLN to SERCA promotes uncoupling of SERCA, we compared SLN and SERCA1 interaction with that of PLB in detail. The homo-bifunctional cross-linker (1,6-bismaleimidohexane) was employed to detect dynamic protein interaction during the SERCA cycle. Our studies reveal that SLN differs significantly from PLB: 1) SLN primarily affects the Vmax of SERCA-mediated Ca(2+) uptake but not the pump affinity for Ca(2+); 2) SLN can bind to SERCA in the presence of high Ca(2+), but PLB can only interact to the ATP-bound Ca(2+)-free E2 state; and 3) unlike PLB, SLN interacts with SERCA throughout the kinetic cycle and promotes uncoupling of the SERCA pump. Using SERCA transmembrane mutants, we additionally show that PLB and SLN can bind to the same groove but interact with a different set of residues on SERCA. These data collectively suggest that SLN is functionally distinct from PLB; its ability to interact with SERCA in the presence of Ca(2+) causes uncoupling of the SERCA pump and increased heat production.

Heat-shock proteins attenuate SERCA inactivation by the anti-apoptotic protein Bcl-2: possible implications for the ER Ca2+-mediated apoptosis

We have demonstrated previously that Bcl-2 and Bcl-2Δ21, a C-terminally truncated Bcl-2 sequence, inactivate SERCA (sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase) 1 in isolated SR (sarcoplasmic reticulum), accompanied by a translocation from CRDs (caveolae-related domains) of the SR. In the present study, we obtained evidence for the interaction of Bcl-2 with SERCA2b in C2C12 myoblasts and HEK (human embryonic kidney)-293 cells. Bcl-2 and SERCA2b co-immunoprecipitated from lysate and microsomal fractions of Bcl-2-overexpressing cells. However, Bcl-2 overexpression resulted only in a slight translocation from the CRDs and no significant SERCA inactivation. In isolated HEK-293 cell microsomes, incubation with Bcl-2Δ21 afforded SERCA2b inactivation and some translocation. HSP (heat-shock protein) 70, HSP90, HSP27 and α-crystallin attenuated Bcl-2Δ21-dependent SERCA2b inactivation. An in vitro mechanistic study with the SERCA1 isoform shows that HSP70 (i) protects SERCA1 from the inactivation by Bcl-2Δ21, (ii) inhibits SERCA1 translocation from CRD fractions, and (iii) prevents the Bcl-2Δ21-dependent loss of FITC labelling. Our data demonstrate that the mechanism of SERCA inactivation by Bcl-2 established in vitro for the SERCA1 isoform can be extended to the main housekeeping SERCA2b isoform, and that functional interactions of SERCA2b and Bcl-2 in the cell may be modulated by HSP70 and other chaperones and stress-regulated proteins.

Dynamic regulation of sarcoplasmic reticulum Ca(2+) stores by stromal interaction molecule 1 and sarcolipin during muscle differentiation

During muscle development, the sarco/endoplasmic reticulum (SR/ER) undergoes remodeling to establish a specialized internal Ca(2+) store for muscle contraction. We hypothesized that store operated Ca(2+) entry (SOCE) is required to fill Ca(2+) stores and is, therefore, critical to creating a mature SR/ER. Stromal interaction molecule 1 (STIM1) functions as a sensor of internal Ca(2+) store content and an activator of SOCE channels. Myocytes lacking STIM1 display reduced SR Ca(2+) content and altered expression of key SR proteins. Sarcolipin (SLN), an inhibitor of the SR calcium pump, was markedly increased in the muscle of mutant STIM1 mice. SLN opposes the actions of STIM1 by limiting SOCE, reducing SR Ca(2+) content and delaying muscle differentiation. During mouse muscle development SLN is highly expressed in embryonic muscle, while the expression of STIM1 is up-regulated postnatally. These results suggest that SOCE regulates SR/ER specialization and that SLN and STIM1 act in opposing fashions to govern SOCE during myogenesis.

SRP-35, a newly identified protein of the skeletal muscle sarcoplasmic reticulum, is a retinol dehydrogenase

In the present study we provide evidence that SRP-35, a protein we identified in rabbit skeletal muscle sarcoplasmic reticulum, is an all-trans-retinol dehydrogenase. Analysis of the primary structure and tryptic digestion revealed that its N-terminus encompasses a short hydrophobic sequence bound to the sarcoplasmic reticulum membrane, whereas its C-terminal catalytic domain faces the myoplasm. SRP-35 is also expressed in liver and adipocytes, where it appears in the post-microsomal supernatant; however, in skeletal muscle, SRP-35 is enriched in the longitudinal sarcoplasmic reticulum. Sequence comparison predicts that SRP-35 is a short-chain dehydrogenase/reductase belonging to the DHRS7C [dehydrogenase/reductase (short-chain dehydrogenase/reductase family) member 7C] subfamily. Retinol is the substrate of SRP-35, since its transient overexpression leads to an increased production of all-trans-retinaldehyde. Transfection of C2C12 myotubes with a fusion protein encoding SRP-35-EYFP (enhanced yellow fluorescent protein) causes a decrease of the maximal Ca²⁺ released via RyR (ryanodine receptor) activation induced by KCl or 4-chloro-m-chresol. The latter result could be mimicked by the addition of retinoic acid to the C2C12 cell tissue culture medium, a treatment which caused a significant reduction of RyR1 expression. We propose that in skeletal muscle SRP-35 is involved in the generation of all-trans-retinaldehyde and may play an important role in the generation of intracellular signals linking Ca2+ release (i.e. muscle activity) to metabolism.

Ca(2+) signaling: an outlook on the characterization of Ca(2+) channels and their importance in cellular functions

Calcium (Ca(2+)) is essential in regulating a plethora of cellular functions that includes cell proliferation and differentiation, axonal guidance and cell migration, neuro/enzyme secretion and exocytosis, development/maintenance of neural circuits, cell death and many more. Since Ca(2+) regulates so many fundamental processes, it could be anticipated that numerous Ca(2+) channels and transporters will assist in regulating Ca(2+) entry across the plasma membrane. Towards this several Ca(2+) channels such as voltage-gated channels, store-operated Ca(2+) entry (SOCE) channels, NMDA, AMPA and other ligand gated channels have been identified. In recent years research focus has been targeted towards identification of the precise function of these essential channels. Furthermore, characterization of these individual Ca(2+) channels has also gained much attention, since specific Ca(2+) channels have been shown to influence a particular cellular response. Moreover, perturbations in these Ca(2+) channels have also been implicated in a spectrum of pathological conditions. Hence, understanding the precise involvement of these Ca(2+) channels in disease conditions would presumably unveil avenues for plausible therapeutic interventions. We thus review the role of Ca(2+) signaling in select -disease conditions and also provide experimental evidence as how they can be characterized in a given cell.

Improving cardiac Ca⁺² transport into the sarcoplasmic reticulum in heart failure: lessons from the ubiquitous SERCA2b Ca⁺² pump

As a major Ca2+ pump in the sarcoplasmic reticulum of the cardiomyocyte, SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) controls the relaxation and contraction of the cardiomyocyte. It is meticulously regulated by adapting its expression levels and affinity for Ca2+ ions to the physiological demand of the heart. Dysregulation of the SERCA2a activity entails poor cardiomyocyte contractility, resulting in heart failure. Conversely, improving cardiac SERCA2a activity, e.g. by boosting its expression level or by increasing its affinity for Ca2+, is a promising strategy to rescue contractile dysfunction of the failing heart. The structures of the related SERCA1a Ca2+ pump and the Na+/K+-ATPase of the plasma membrane exposed the pumping mechanism and conserved domain architecture of these ion pumps. However, how the Ca2+ affinity of SERCA2a is regulated at the molecular level remained unclear. A structural and functional analysis of the closely related SERCA2b Ca2+ pump, i.e. the housekeeping Ca2+ pump found in the endoplasmic reticulum and the only SERCA isoform characterized by a high Ca2+ affinity, aimed to fill this gap. We demonstrated the existence of a novel and highly conserved site on the SERCA2 pump mediating Ca2+ affinity regulation by the unique C-terminus of SERCA2b (2b-tail). It differs from the earlier-described target site of the affinity regulator phospholamban. Targeting this novel site may provide a new approach to improve SERCA2a function in the failing heart. Strikingly, the intramembrane interaction site of the 2b-tail in SERCA2b shares sequence and structural homology with the binding site of the β-subunit on the α Na+/K+-ATPase. Thus P-type ATPases seem to have developed related mechanisms of regulation, and it is a future challenge for us to discover these general principles of P-type regulation.

The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump

The sarcoplasmic (SERCA 1a) Ca2+-ATPase is a membrane protein abundantly present in skeletal muscles where it functions as an indispensable component of the excitation-contraction coupling, being at the expense of ATP hydrolysis involved in Ca2+/H+ exchange with a high thermodynamic efficiency across the sarcoplasmic reticulum membrane. The transporter serves as a prototype of a whole family of cation transporters, the P-type ATPases, which in addition to Ca2+ transporting proteins count Na+, K+-ATPase and H+, K+-, proton- and heavy metal transporting ATPases as prominent members. The ability in recent years to produce and analyze at atomic (2·3-3 Å) resolution 3D-crystals of Ca2+-transport intermediates of SERCA 1a has meant a breakthrough in our understanding of the structural aspects of the transport mechanism. We describe here the detailed construction of the ATPase in terms of one membraneous and three cytosolic domains held together by a central core that mediates coupling between Ca2+-transport and ATP hydrolysis. During turnover, the pump is present in two different conformational states, E1 and E2, with a preference for the binding of Ca2+ and H+, respectively. We discuss how phosphorylated and non-phosphorylated forms of these conformational states with cytosolic, occluded or luminally exposed cation-binding sites are able to convert the chemical energy derived from ATP hydrolysis into an electrochemical gradient of Ca2+ across the sarcoplasmic reticulum membrane. In conjunction with these basic reactions which serve as a structural framework for the transport function of other P-type ATPases as well, we also review the role of the lipid phase and the regulatory and thermodynamic aspects of the transport mechanism.

The Ca2+ pumps of the endoplasmic reticulum and Golgi apparatus

The various splice variants of the three SERCA- and the two SPCA-pump genes in higher vertebrates encode P-type ATPases of the P(2A) group found respectively in the membranes of the endoplasmic reticulum and the secretory pathway. Of these, SERCA2b and SPCA1a represent the housekeeping isoforms. The SERCA2b form is characterized by a luminal carboxy terminus imposing a higher affinity for cytosolic Ca(2+) compared to the other SERCAs. This is mediated by intramembrane and luminal interactions of this extension with the pump. Other known affinity modulators like phospholamban and sarcolipin decrease the affinity for Ca(2+). The number of proteins reported to interact with SERCA is rapidly growing. Here, we limit the discussion to those for which the interaction site with the ATPase is specified: HAX-1, calumenin, histidine-rich Ca(2+)-binding protein, and indirectly calreticulin, calnexin, and ERp57. The role of the phylogenetically older and structurally simpler SPCAs as transporters of Ca(2+), but also of Mn(2+), is also addressed.

Quantification of calsequestrin 2 (CSQ2) in sheep cardiac muscle and Ca2+-binding protein changes in CSQ2 knockout mice

Calsequestrin 2 (CSQ2) is generally regarded as the primary Ca2+-buffering molecule present inside the sarcoplasmic reticulum (SR) in cardiac cells, but findings from CSQ2 knockout experiments raise major questions about its role and necessity. This study determined the absolute amount of CSQ2 present in cardiac ventricular muscle to gauge its likely influence on SR free Ca2+ concentration ([Ca2+]) and maximal Ca2+ capacity. Ventricular tissue from hearts of freshly killed sheep was examined by SDS-PAGE without any fractionation, and CSQ2 was detected by Western blotting; this method avoided the >90% loss of CSQ2 occurring with usual fractionation procedures. Band intensities were compared against those for purified CSQ2 run on the same blots. Fidelity of quantification was verified by demonstrating that CSQ2 added to homogenates was detected with equal efficacy as purified CSQ2 alone. Ventricular tissue from sheep (n=8) contained 24±2 μmol CSQ2/kg wet wt. Total Ca2+ content of the ventricular tissue, measured by atomic absorption spectroscopy, was 430±20 μmol/kg (with SR Ca2+ likely<250 μmol/kg) and displayed a linear correlation with CSQ2 content, with gradient of ∼10 Ca2+ per CSQ2. The large amount of CSQ2 bestows the SR with a high theoretical maximal Ca2+-binding capacity (∼1 mmol Ca2+/kg ventricular tissue, assuming a maximum of ∼40 Ca2+ per CSQ2) and would keep free [Ca2+] within the SR relatively low, energetically favoring Ca2+ uptake and reducing SR leak. In mice with CSQ2 ablated, histidine-rich Ca2+-binding protein was upregulated ∼35% in ventricular tissue, possibly in compensation.

Endoplasmic reticulum protein targeting of phospholamban: a common role for an N-terminal di-arginine motif in ER retention?

BACKGROUND

Phospholamban (PLN) is an effective inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase, which transports Ca(2+) into the SR lumen, leading to muscle relaxation. A mutation of PLN in which one of the di-arginine residues at positions 13 and 14 was deleted led to a severe, early onset dilated cardiomyopathy. Here we were interested in determining the cellular mechanisms involved in this disease-causing mutation.

METHODOLOGY/PRINCIPAL FINDING

Mutations deleting codons for either or both Arg13 or Arg14 resulted in the mislocalization of PLN from the ER. Our data show that PLN is recycled via the retrograde Golgi to ER membrane traffic pathway involving COP-I vesicles, since co-immunoprecipitation assays determined that COP I interactions are dependent on an intact di-arginine motif as PLN RDelta14 did not co-precipitate with COP I containing vesicles. Bioinformatic analysis determined that the di-arginine motif is present in the first 25 residues in a large number of all ER/SR Gene Ontology (GO) annotated proteins. Mutations in the di-arginine motif of the Sigma 1-type opioid receptor, the beta-subunit of the signal recognition particle receptor, and Sterol-O-acyltransferase, three proteins identified in our bioinformatic screen also caused mislocalization of these known ER-resident proteins.

CONCLUSION

We conclude that PLN is enriched in the ER due to COP I-mediated transport that is dependent on its intact di-arginine motif and that the N-terminal di-arginine motif may act as a general ER retrieval sequence.

Structural divergence between the two subgroups of P5 ATPases

Evolution of P5 type ATPases marks the origin of eukaryotes but still they remain the least characterized pumps in the superfamily of P-type ATPases. Phylogenetic analysis of available sequences suggests that P5 ATPases should be divided into at least two subgroups, P5A and P5B. P5A ATPases have been identified in the endoplasmic reticulum and seem to have basic functions in protein maturation and secretion. P5B ATPases localize to vacuolar/lysosomal or apical membranes and in animals play a role in hereditary neuronal diseases. Here we have used a bioinformatical approach to identify differences in the primary sequences between the two subgroups. P5A and P5B ATPases appear have a very different membrane topology from other P-type ATPases with two and one, respectively, additional transmembrane segments inserted in the N-terminal end. Based on conservation of residues in the transmembrane region, the two P5 subgroups most likely have different substrate specificities although these cannot be predicted from their sequences. Furthermore, sequence differences between P5A and P5B ATPases are identified in the catalytic domains that could influence key kinetic properties differentially. Together these findings indicate that P5A and P5B ATPases are structurally and functionally different.

PI3K kinase and scaffold functions in heart

Signal transduction events are key modulators of cellular function and, in the cardiovascular system, an emerging role is played by phosphoinositide 3-kinases (PI3Ks), a family of enzymes containing a 3-phosphorylated phosphoinositide that produce lipid second messengers. In the heart, multiple PI3K isoforms are expressed, but play potentially distinct roles. Among cardiac PI3Ks, PI3Kalpha is triggered by tyrosine kinase receptors and plays a role in adaptive hypertrophy, while PI3Kgamma is triggered by G protein-coupled receptors and is involved in maladaptive heart remodeling. This view has been recently complicated by the finding that PI3Ks can also be involved in protein-protein interactions and affect signaling independently of their kinase activity. This review will thus focus on the effects of these multiple signaling events, with particular emphasis on their involvement in cardiac hypertrophy and failure.

Threonine-5 at the N-terminus can modulate sarcolipin function in cardiac myocytes

Sarcolipin (SLN) has emerged as an important regulator of the atrial sarcoplasmic reticulum (SR) Ca2+ transport. The inhibitory effect of SLN on cardiac SR Ca2+ ATPase (SERCA) pump can be relieved by beta-adrenergic stimulation, which indicates that SLN is a reversible inhibitor. However, the mechanism of this reversible regulation of SERCA pump by SLN is yet to be determined. In the current study using adult rat ventricular myocytes we provide evidence that the threonine 5 (T5) residue at the N-terminus of SLN which is conserved among various species, critically regulates the SLN function. Point mutation of T5-->alanine exerts an inhibitory effect on myocyte contractility and calcium transients similar to that of wild-type SLN, whereas mutation of T5-->glutamic acid which mimics the phosphorylation abolished the inhibitory function of SLN. Our results showed that T5 can be phosphorylated in vitro by calcium-calmodulin dependent protein kinase II (CaMKII). Blocking the CaMKII activity in WT-SLN overexpressing myocytes using autocamtide inhibitory peptide completely abolished the beta-adrenergic response. Taken together, our data suggest that T5 is the key amino acid which modulates SLN function via phosphorylation/dephosphorylation mechanisms through CaMKII pathway.

The decay phase of Ca2+ transients in skeletal muscle: regulation and physiologyThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process

Cytosolic Ca2+ transients associated with contraction and relaxation cycles in skeletal muscle are primarily dependent on the kinetics of Ca2+ release and Ca2+ uptake by the sarcoplasmic reticulum (SR). In humans, sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) are solely responsible for the removal of Ca2+ from the cytosol following muscle contraction. There are several signalling systems involved in the acute regulation of SERCAs required to achieve a given Ca2+ transient during muscle contraction–relaxation cycles. Cyclic-AMP-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase signalling activate SERCAs through the regulation of the endogenous SERCA-regulatory proteins, phospholamban and sarcolipin, both of which are highly expressed in human skeletal muscle. Recent studies on the regulation of SERCA2b in arterial smooth muscle and work from my laboratory on the interaction between SERCAs and the inducible 70-kDa heat shock protein suggests a novel role for redox signalling in regulating SERCA activity. In the absence of fatigue, activation of these signalling systems in response to repeated muscle activity serves to increase the rate of cytosolic free Ca2+ ([Ca2+]f) decay (i.e., SR Ca2+ uptake) and the speed of muscle relaxation.

Determination of apparent calcium affinity for endogenously expressed human sarco(endo)plasmic reticulum calcium-ATPase isoform SERCA3

The sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) play a crucial role in regulating free cytosolic Ca2+ concentration in diverse cell types. It has been shown that recombinant SERCA3, when measured in heterologous systems, exhibits low apparent affinity for Ca2+; however, Ca2+ affinity of native SERCA3 in an endogenous setting has not been examined. Such a measurement is complicated, because SERCA3 is always coexpressed with the housekeeping isoform SERCA2b. We used a fluorescence-based assay for monitoring continuous Ca2+ uptake into microsomes to examine the properties of endogenous human SERCA3 and SERCA2b. The kinetic parameters were derived using a cooperative two-component uptake model for Ca2+ activation, and the values assigned to SERCA3 were confirmed using the highly specific human SERCA3 inhibitory antibody PL/IM430. First, using recombinant human SERCA3 and SERCA2b proteins transiently expressed in HEK-293 cells, we confirmed the previously observed low apparent Ca2+ affinity for SERCA3 compared with SERCA2b (1.10 ± 0.04 vs. 0.26 ± 0.01 μM), and using mixtures of recombinant protein isoforms, we validated the two-component uptake model. Then we determined apparent Ca2+ affinity for SERCA proteins present endogenously in cultured Jurkat T lymphocytes and freshly isolated human tonsil lymphocytes. The apparent Ca2+ affinity in these two preparations was 1.04 ± 0.07 and 1.1 ± 0.2 μM for SERCA3 and 0.27 ± 0.02 and 0.26 ± 0.01 μM for SERCA2b, respectively. Our data demonstrate, for the first time, that affinity for Ca2+ is inherently lower for SERCA3 expressed in situ than for other SERCA isoforms.

Minor sarcoplasmic reticulum membrane components that modulate excitation-contraction coupling in striated muscles

In striated muscle, activation of contraction is initiated by membrane depolarisation caused by an action potential, which triggers the release of Ca(2+) stored in the sarcoplasmic reticulum by a process called excitation-contraction coupling. Excitation-contraction coupling occurs via a highly sophisticated supramolecular signalling complex at the junction between the sarcoplasmic reticulum and the transverse tubules. It is generally accepted that the core components of the excitation-contraction coupling machinery are the dihydropyridine receptors, ryanodine receptors and calsequestrin, which serve as voltage sensor, Ca(2+) release channel, and Ca(2+) storage protein, respectively. Nevertheless, a number of additional proteins have been shown to be essential both for the structural formation of the machinery involved in excitation-contraction coupling and for its fine tuning. In this review we discuss the functional role of minor sarcoplasmic reticulum protein components. The definition of their roles in excitation-contraction coupling is important in order to understand how mutations in genes involved in Ca(2+) signalling cause neuromuscular disorders.

Abnormal sarcoplasmic reticulum Ca2+-sequestering properties in skeletal muscle in chronic obstructive pulmonary disease

The objective of this study was to investigate the hypothesis that alterations in sarcoplasmic reticulum (SR) Ca2+-cycling properties would occur in skeletal muscle in patients with moderate to severe chronic obstructive pulmonary disease (COPD). To investigate this hypothesis, tissue samples were obtained from the vastus lateralis of 8 patients with COPD [age 65.6 ± 3.2 yr; forced expiratory volume in 1 s (FEV1)/forced vital capacity (FVC) = 44 ± 2%; mean ± SE] and 10 healthy age-matched controls (CON, age 67.5 ± 2.5 yr; FEV1/FVC = 77 ± 2%), and homogenates were analyzed for a wide range of SR properties. Compared with CON, COPD displayed (in μmol·g protein−1·min−1) a 16% lower maximal Ca2+-ATPase activity [maximal velocity ( Vmax), 158 ± 10 vs. 133 ± 7, P < 0.05] and a 17% lower Ca2+uptake (4.65 ± 0.039 vs. 3.85 ± 0.26, P < 0.05) that occurred in the absence of differences in Ca2+release. The lower Vmaxin COPD was also accompanied by an 11% lower ( P < 0.05) Ca2+sensitivity, as measured by the Hill coefficient (defined as the relationship between Ca2+-ATPase activity and free cytosolic Ca2+concentration for 10–90% Vmax). For the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) isoforms, SERCA1a was 16% higher ( P < 0.05) and SERCA2a was 14% lower ( P < 0.05) in COPD. It is concluded that moderate to severe COPD results in abnormalities in SR Ca2+-ATPase properties that cannot be explained by changes in the SERCA isoform phenotypes. The reduced catalytic properties of SERCA in COPD suggest a disturbance in Ca2+cycling, possibly resulting in impairment in Ca2+-mediated mechanical function and/or second messenger regulated processes.

Genomics, heart failure and sudden cardiac death

Sudden cardiac death (SCD) is among the most common causes of death in developed countries throughout the world. Despite decreased overall cardiac mortality, SCD rates appear to be increasing in concert with escalating global prevalence of coronary disease and heart failure, the two major conditions predisposing to SCD. This unfavorable trend is a consequence of our inability to identify those who will die suddenly from lethal ventricular arrhythmias and to develop effective therapies for all populations at risk. The known risk factors for SCD lack the predictive power needed to generate preventive strategies for the large number of fatal arrhythmic events that occur among lower-risk subsets of the population. Even among recognized high-risk subsets, prediction of SCD remains challenging. With the exception of the implantable cardioverter defibrillator (ICD) there are few effective strategies for the prevention and treatment of SCD. This article discusses the prospect of genomic science as an approach to the identification of patients at high-risk for SCD. While the final common pathway for SCD is malignant ventricular arrhythmias, there are many potential contributors, pathways, and mechanisms by which common genetic variants (polymorphisms) could affect initiation and propagation of life-threatening cardiac arrhythmias. Recent advances in genomic medicine now provide us with novel approaches to both identify candidate genes/pathways and relatively common polymorphisms which may predispose patients to increased risk for SCD. Improved understanding of the relationship between common polymorphisms and SCD will not only improve risk stratification such that ICDs can be targeted to those patients most likely to benefit from them but also provide new insight into the pathophysiology of SCD.

Sarcoplasmic reticulum calcium uptake and speed of relaxation are depressed in nebulin-free skeletal muscle

Previous work suggested that altered Ca(2+) homeostasis might contribute to dysfunction of nebulin-free muscle, as gene expression analysis revealed that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-inhibitor sarcolipin (SLN) is up-regulated >70-fold in nebulin knockout mice, and here we tested this proposal. We investigated SLN protein expression in nebulin-free and wild-type skeletal muscle, as well as expression of other Ca(2+)-handling proteins. Ca(2+) uptake capacity was determined in isolated sarcoplasmic reticulum vesicles and in intact myofibers by measuring Ca(2+) transients. Muscle contractile performance was determined in skinned muscle activated with exogenous Ca(2+), as well as in electrically stimulated intact muscle. We found profound up-regulation of SLN protein in nebulin-free skeletal muscle, whereas expression of other Ca(2+)-handling proteins was not (calsequestrin and phospholamban) or was minimally (SERCA) affected. Speed of Ca(2+) uptake was >3-fold decreased in sarcoplasmic reticulum vesicles isolated from nebulin-free muscle as well as in nebulin-free intact myofibers. Ca(2+)-activated stress in skinned muscle and stress produced by intact nebulin-free muscle were reduced to a similar extent compared with wild type. Half-relaxation time was significantly longer in nebulin-free compared with wild-type muscle. Thus, the present study demonstrates for the first time that nebulin might also be involved in physiological Ca(2+) handling of the SR-myofibrillar system.

The non-excitable smooth muscle: calcium signaling and phenotypic switching during vascular disease

Calcium (Ca(2+)) is a highly versatile second messenger that controls vascular smooth muscle cell (VSMC) contraction, proliferation, and migration. By means of Ca(2+) permeable channels, Ca(2+) pumps and channels conducting other ions such as potassium and chloride, VSMC keep intracellular Ca(2+) levels under tight control. In healthy quiescent contractile VSMC, two important components of the Ca(2+) signaling pathways that regulate VSMC contraction are the plasma membrane voltage-operated Ca(2+) channel of the high voltage-activated type (L-type) and the sarcoplasmic reticulum Ca(2+) release channel, Ryanodine Receptor (RyR). Injury to the vessel wall is accompanied by VSMC phenotype switch from a contractile quiescent to a proliferative motile phenotype (synthetic phenotype) and by alteration of many components of VSMC Ca(2+) signaling pathways. Specifically, this switch that culminates in a VSMC phenotype reminiscent of a non-excitable cell is characterized by loss of L-type channels expression and increased expression of the low voltage-activated (T-type) Ca(2+) channels and the canonical transient receptor potential (TRPC) channels. The expression levels of intracellular Ca(2+) release channels, pumps and Ca(2+)-activated proteins are also altered: the proliferative VSMC lose the RyR3 and the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase isoform 2a pump and reciprocally regulate isoforms of the ca(2+)/calmodulin-dependent protein kinase II. This review focuses on the changes in expression of Ca(2+) signaling proteins associated with VSMC proliferation both in vitro and in vivo. The physiological implications of the altered expression of these Ca(2+) signaling molecules, their contribution to VSMC dysfunction during vascular disease and their potential as targets for drug therapy will be discussed.

Molecular basis of diastolic dysfunction

Diastolic dysfunction is characterized by prolonged relaxation, increased filling pressure, decreased contraction velocity, and reduced cardiac output. Phenotypical features of diastolic dysfunction can be observed at the level of the isolated myocyte. This article reviews the cellular mechanisms that control relaxation at the level of the myocyte in the healthy situation and discusses the alterations that can affect physiologic function during disease. It focuses specifically on the mechanisms that regulate intracellular calcium handling, and the response of the myofilaments to calcium, including the changes in these components that can contribute to diastolic dysfunction.

Muscle metabolic, SR Ca2+-cycling responses to prolonged cycling, with and without glucose supplementation

This study investigated the effects of prolonged exercise, with and without glucose supplementation, on metabolism and sarcoplasmic reticulum (SR) Ca2+-handling properties in working vastus lateralis muscle. Fifteen untrained volunteers [peak O2consumption (V̇o2peak) = 3.45 ± 0.17 l/min; mean ± SE] cycled at ∼60% V̇o2peakon two occasions, during which they were provided with either an artificially sweetened placebo beverage (NG) or a 6% glucose (G) beverage (∼1.00 g carbohydrate/kg body mass). Beverage supplementation started at 30 min of exercise and continued every 15 min thereafter. SR Ca2+handling, metabolic, and substrate responses were assessed in tissue extracted from the vastus lateralis at rest, after 30 min and 90 min of exercise, and at fatigue in both conditions. Plasma glucose during G was 15–23% higher ( P < 0.05) than those observed during NG following 60 min of exercise until fatigue. Cycle time to fatigue was increased ( P < 0.05) by ∼19% during G (137 ± 7 min) compared with NG (115 ± 6 min). Prolonged exercise reduced ( P < 0.05) maximal Ca2+-ATPase activity (−18.4%), SR Ca2+uptake (−27%), and both Phase 1 (−22.2%) and Phase 2 (−34.2%) Ca2+-release rates during NG. The exercise-induced reductions in SR Ca2+-cycling properties were not altered during G. The metabolic responses to exercise were all unaltered by glucose supplementation, since no differences in respiratory exchange ratios, carbohydrate and lipid oxidation rates, and muscle metabolite and glycogen contents were observed between NG and G. These results indicate that the maintenance of blood glucose homeostasis by glucose supplementation is without effect in modifying the muscle metabolic, endogenous glycogen, or SR Ca2+-handling responses.

Muscle-specific overexpression of IGF-I improves E-C coupling in skeletal muscle fibers from dystrophic mdx mice

Duchenne muscular dystrophy (DMD) is a lethal X-linked disease caused by the absence of functional dystrophin. Abnormal excitation-contraction (E-C) coupling has been reported in dystrophic muscle fibers from mdx mice, and alterations in E-C coupling components may occur as a direct result of dystrophin deficiency. We hypothesized that muscle-specific overexpression of insulin-growth factor-1 (IGF-I) would reduce E-C coupling failure in mdx muscle. Mechanically skinned extensor digitorum longus muscle fibers from mdx mice displayed a faster decline in depolarization-induced force responses (DIFR); however, there were no differences in sarcoplasmic reticulum (SR)-mediated Ca(2+) resequestration or in the properties of the contractile apparatus when compared with nondystrophic controls. The rate of DIFR decline was restored to control levels in fibers from transgenic mdx mice that overexpressed IGF-I in skeletal muscle (mdx/IGF-I mice). Dystrophic muscles have a lower transcript level of a specific dihydropyridine receptor (DHPR) isoform, and IGF-I-mediated changes in E-C coupling were associated with increased transcript levels of specific DHPR isoforms involved in Ca(2+) regulation. Importantly, IGF-I overexpression also increased the sensitivity of the contractile apparatus to Ca(2+). The results demonstrate that IGF-I can ameliorate fundamental aspects of E-C coupling failure in dystrophic muscle fibers and that these effects are important for the improvements in cellular function induced by this growth factor.

Phospholamban overexpression in transgenic rabbits

There has been considerable interest in pursuing phospholamban as a putative therapeutic target for overcoming depressed calcium handling in human heart failure. Studies predominantly done in mice have shown that phospholamban is a key regulator of sarcoplasmic reticulum calcium cycling and cardiac function. However, mice differ significantly from humans in how they regulate calcium, whereas rabbits better recapitulate human cardiac function and calcium handling. To investigate phospholamban's role in the rabbit heart, transgenic rabbits that overexpressed wild-type phospholamban in the ventricular cardiomyocytes and slow-twitch skeletal muscles were generated. Rabbits expressing high levels of phospholamban were not viable due to severe skeletal muscle wasting, the onset of cardiac pathology and early death. A viable transgenic line exhibited a 30% increase in PLN protein levels in the heart. These animals showed isolated foci of cardiac pathology, but cardiac function as well as the response to beta-adrenergic stimulation were normal. SR-calcium uptake measurements showed that the transgenic hearts had the expected reduced affinity for calcium. The data show that phospholamban-overexpressing transgenic rabbits differ markedly in phenotype from analogous transgenic mice in that rabbits are quite sensitive to alterations in phospholamban levels. Exceeding a relatively narrow window of phospholamban expression results in significant morbidity and early death.

Differential expression of sarcolipin protein during muscle development and cardiac pathophysiology

Sarcolipin (SLN) is a small molecular weight sarcoplasmic reticulum (SR) membrane protein expressed both in cardiac and skeletal muscle tissues. Recent studies using transgenic mouse models have demonstrated that SLN is an important regulator of cardiac SR Ca2+ ATPase 2a (SERCA2a). However, there is a paucity of information regarding the SLN protein expression in small versus larger mammals and its regulation during development and cardiac pathophysiology. Therefore, the major goal of this study was to generate an SLN specific antibody and perform detailed analyses of SLN protein expression during muscle development and in the diseased myocardium. The important findings of the present study are: (i) in small mammals, SLN expression is predominant in the atria but low in the ventricle and in skeletal muscle tissues, whereas in large mammals, SLN is quite abundant in skeletal muscle tissues than the atria, (ii) SLN and SERCA2a are co-expressed in all striated muscle tissues studied except ventricle and co-ordinately regulated during muscle development and (iii) SLN protein levels are approximately 3 fold upregulated in the atria of heart failure dogs and approximately 30% decreased in the atria of hearts prone to myocardial ischemia. In addition we found that in the phospholamban null atria, SLN protein levels are upregulated.

Essential roles of the bHLH transcription factor Hrt2 in repression of atrial gene expression and maintenance of postnatal cardiac function

The basic helix-loop-helix transcriptional repressor Hairy-related transcription factor 2 (Hrt2) is expressed in ventricular, but not atrial, cardiomyocytes, and in endothelial and vascular smooth muscle cells. Mice homozygous for a null mutation of Hrt2 die perinatally from a spectrum of cardiac abnormalities, raising questions about the specific functions of this transcriptional regulator in individual cardiac cell lineages. Using a conditional Hrt2 null allele, we show that cardiomyocyte-specific deletion of Hrt2 in mice results in ectopic activation of atrial genes in ventricular myocardium with an associated impairment of cardiac contractility and a unique distortion in morphology of the right ventricular chamber. Consistent with the atrialization of ventricular gene expression in Hrt2 mutant mice, forced expression of Hrt2 in atrial cardiomyocytes is sufficient to repress atrial cardiac genes. These findings reveal a ventricular myocardial cell-autonomous function for Hrt2 in the suppression of atrial cell identity and the maintenance of postnatal cardiac function.

Sarcolipin and phospholamban as regulators of cardiac sarcoplasmic reticulum Ca2+ ATPase

The cardiac sarcoplasmic reticulum calcium ATPase (SERCA2a) plays a critical role in maintaining the intracellular calcium homeostasis during cardiac contraction and relaxation. It has been well documented over the years that altered expression and activity of SERCA2a can lead to systolic and diastolic dysfunction. The activity of SERCA2a is regulated by two structurally similar proteins, phospholamban (PLB) and sarcolipin (SLN). Although, the relevance of PLB has been extensively studied over the years, the role SLN in cardiac physiology is an emerging field of study. This review focuses on the advances in the understanding of the regulation of SERCA2a by SLN and PLB. In particular, it highlights the similarities and differences between the two proteins and their roles in cardiac patho-physiology.

A musculoskeletal model of low grade connective tissue inflammation in patients with thyroid associated ophthalmopathy (TAO): the WOMED concept of lateral tension and its general implications in disease

Background

Low level connective tissue inflammation has been proposed to play a role in thyroid associated ophthalmopathy (TAO). The aim of this study was to investigate this postulate by a musculoskeletal approach together with biochemical parameters.

Methods

13 patients with TAO and 16 controls were examined. Erythrocyte levels of Zn, Cu, Ca²⁺, Mg, and Fe were determined. The musculoskeletal evaluation included observational data on body posture with emphasis on the orbit-head region. The angular foot position in the frontal plane was quantified following gait observation. The axial orientation of the legs and feet was evaluated in an unloaded supine position. Functional propioceptive tests based on stretch stimuli were done by using foot inversion and foot rotation.

Results

Alterations in the control group included neck tilt in 3 cases, asymmetrical foot angle during gait in 2, and a reaction to foot inversion in 5 cases. TAO patients presented facial asymmetry with displaced eye fissure inclination (mean 9.1°) as well as tilted head-on-neck position (mean 5.7°). A further asymmetry feature was external rotation of the legs and feet (mean 27°). Both foot inversion as well as foot rotation induced a condition of neuromuscular deficit. This condition could be regulated by gentle acupressure either on the lateral abdomen or the lateral ankle at the acupuncture points gall bladder 26 or bladder 62, respectively. In 5 patients, foot rotation produced a phenomenon of moving toes in the contra lateral foot. In addition foot rotation was accompanied by an audible tendon snapping. Lower erythrocyte Zn levels and altered correlations between Ca²⁺, Mg, and Fe were found in TAO.

Conclusion

This whole body observational study has revealed axial deviations and body asymmetry as well as the phenomenon of moving toes in TAO. The most common finding was an arch-like displacement of the body, i.e. eccentric position, with foot inversion and head tilt to the contra lateral side and tendon snapping. We propose that eccentric muscle action over time can be the basis for a low grade inflammatory condition. The general implications of this model and its relations to Zn and Se will be discussed.

SERCA pumps and human diseases

Sarco(endo)plasmic reticulum (SER) Ca2+ ATPases represent a highly conserved family of Ca2+ pumps which actively transport Ca2+ from the cytosol to the SER against a large concentration gradient. In humans, 3 genes (ATP2A1-3) generate multiple isoforms (SERCAla,b, SERCA2a-c, SECA3a-f) by developmental or tissue-specific alternative splicing. These pumps differ by their regulatory and kinetic properties, allowing for optimized function in the tissue where they are expressed. They play a central role in calcium signalling through regenerating SER Ca2+ stores, maintaining appropriate Ca2+ levels in this organelle and shaping cytosolic and nuclear Ca2+ variations which govern cell response. Defects in ATP2A1 encoding SERCA1 cause recessive Brody myopathy, mutations in ATP2A2 coding for SERCA2 underlie a dominant skin disease, Darier disease and its clinical variants. SERCA2a expression is reduced in heart failure in human and in mice models. Gene-targeting studies in mouse confirmed the expected function of these isoforms in some cases, but also resulted in unexpected phenotypes: SERCA1 null mutants die from respiratory failure, SERCA2 heterozygous mutant mice develop skin cancer with age and SERCA3 null mice display no diabetes. These unique phenotypes have provided invaluable information on the role of these pumps in specific tissues and species, and have improved our understanding of Ca2+ regulated processes in muscles, the heart and the skin in human and in mice. Although the understanding of the pathogenesis of these diseases is still incomplete, these recent advances hold the promise of improved knowledge on the disease processes and the identification of new targets for therapeutic interventions.

Cytoplasmic tail of phospholemman interacts with the intracellular loop of the cardiac Na+/Ca2+ exchanger

Phospholemman (PLM), a member of the FXYD family of small ion transport regulators, inhibits cardiac Na+/Ca2+ exchanger (NCX1). NCX1 is made up of N-terminal domain consisting of the first five transmembrane segments (residues 1-217), a large intracellular loop (residues 218-764), and a C-terminal domain comprising the last four transmembrane segments (residues 765-938). Using glutathione S-transferase (GST) pull-down assay, we demonstrated that the intracellular loop, but not the N- or C-terminal transmembrane domains of NCX1, was associated with PLM. Further analysis using protein constructs of GST fused to various segments of the intracellular loop of NCX1 suggest that PLM bound to residues 218-371 and 508-764 but not 371-508. Split Na+/Ca2+ exchangers consisting of N- or C-terminal domains with different lengths of the intracellular loop were co-expressed with PLM in HEK293 cells that are devoid of endogenous PLM and NCX1. Although expression of N-terminal but not C-terminal domain alone resulted in correct membrane targeting, co-expression of both N- and C-terminal domains was required for correct membrane targeting and functional exchange activity. NCX1 current measurements indicate that PLM decreased NCX1 current only when the split exchangers contained residues 218-358 of the intracellular loop. Co-immunoprecipitation experiments with PLM and split exchangers suggest that PLM associated with the N-terminal domain of NCX1 when it contained intracellular loop residues 218-358. TM43, a PLM mutant with its cytoplasmic tail truncated, did not co-immunoprecipitate with wild-type NCX1 when co-expressed in HEK293 cells, confirming little to no interaction between the transmembrane domains of PLM and NCX1. We conclude that PLM interacted with the intracellular loop of NCX1, most likely at residues 218-358.

Quantitative mapping of oxidation-sensitive cysteine residues in SERCA in vivo and in vitro by HPLC-electrospray-tandem MS: selective protein oxidation during biological aging

The selective reversible S-glutathiolation of specific SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) cysteine residues represents a novel physiologic pathway of NO (nitric oxide)-dependent arterial smooth muscle relaxation [Adachi, Weisbrod, Pimentel, Ying, Sharov, Schöneich and Cohen (2004) Nat. Med. 10, 1200-1207]. This mechanism may be impaired through the irreversible oxidation of functionally important cysteine residues as a consequence of oxidative stress and aging. To establish whether in vivo aging and in vitro oxidation by peroxynitrite result in the loss of such functionally important cysteine residues of SERCA, we have developed and optimized a quantitative method to monitor the oxidation state of the individual SERCA cysteine residues using a maleimide-based fluorescence dye, TG1 (ThioGlo 1), as a label for cysteine residues that have not been altered by oxidation and are not involved in disulphide bridges. A high efficiency for TG1 labelling of such residues and the chemical structure of cysteine-TG1 adducts were validated by MS analysis of model peptides, model proteins and rat skeletal muscle SERCA1. Tryptic peptides containing 18 out of a total of 24 cysteine residues were identified by HPLC-ESI (electrospray ionization)-MS/MS (tandem MS). Two cysteine residues, at positions 344 and 349, were detected in the form of an internal disulphide bridge, and another 16 were found to be labelled with TG1. Using HPLC-ESI-MS, we quantitatively mapped peroxynitrite oxidation of eight cysteine residues (positions 364, 417, 420, 498, 525, 674, 675 and 938), some of which are involved in the control of SERCA activity. Biological aging resulted in the partial modification of cysteine residues 377, 498, 525, 561, 614, 636, 674, 675, 774 and 938. Neither peroxynitrite exposure nor biological aging affected the apparent SERCA1 ATP affinity. Our data show an age-dependent loss of cysteine residues (approx. 2.8 mol of cysteine/mol of SERCA1), which may be partially responsible for the age-dependent decrease in the specific Ca2+-ATPase activity (by 40%).

The variation of the sarcolipin gene (SLN) in atrial fibrillation, long QT syndrome and sudden arrhythmic death syndrome

BACKGROUND

Mutations in genes responsible for the cardiac action potential and control of intracellular Ca(2+)-distribution are associated with cardiac arrhythmia and sudden death. Sarcolipin is a 31-amino acid protein that inhibits the sarcoplasmic reticulum Ca(2+) ATPase pump (SERCA2). The sarcolipin gene, SLN, is expressed in the heart and a candidate gene for cardiomyopathy as well as atrial fibrillation (AF), long QT syndrome (LQTS) or sudden arrhythmic death syndrome (SADS). We examined the genetic variation of SLN in patients with the arrhythmic disorders AF, LQTS and SADS.

METHODS

We screened the coding region of SLN for mutations using single strand conformation polymorphism/heteroduplex analysis on PCR-amplified genomic DNA from 95 unrelated LQTS patients, 59 SADS cases and 147 patients with atrial fibrillation (AF) and 92 controls. Aberrant conformers were sequenced.

RESULTS

No mutations or polymorphisms were found in the coding sequence. A G>C transition in the highly conserved position +1 of the 3'untranslated region (3'UTR) was found in two SADS cases. A polymorphism, a G>C transition at position -65 in the 5'untranslated region (5'UTR), was found with a G allele frequency of 0.48. A borderline significant difference in genotype distribution of the latter polymorphism was found between the AF group and controls.

CONCLUSION

Mutations in the coding region of SLN are not frequently involved in LQTS, SADS or AF. Whether the described 3'- and 5'UTR variants have functional significance must await further studies.

Integration of calcium with the signaling network in cardiac myocytes

Calcium has evolved as global intracellular messenger for signal transduction in the millisecond time range by reversibly binding to calcium-sensing proteins. In the cardiomyocyte, ion pumps, ion exchangers and channels keep the cytoplasmic calcium level at rest around approximately 100 nM which is more than 10,000-fold lower than outside the cell. Intracellularly, calcium is mainly stored in the sarcoplasmic reticulum, which comprises the bulk of calcium available for the heartbeat. Regulation of cardiac function including contractility and energy production relies on a three-tiered control system, (i) immediate and fast feedback in response to mechanical load on a beat-to-beat basis (Frank-Starling relation), (ii) more sustained regulation involving transmitters and hormones as primary messengers, and (iii) long-term adaptation by changes in the gene expression profile. Calcium signaling over largely different time scales requires its integration with the protein kinase signaling network which is governed by G-protein-coupled receptors, growth factor and cytokine receptors at the surface membrane. Short-term regulation is dominated by the beta-adrenergic system, while long-term regulation with phenotypic remodeling depends on sustained signaling by growth factors, cytokines and calcium. Mechanisms and new developments in intracellular calcium handling and its interrelation with the MAPK signaling pathways are discussed in detail.

Post-transcriptional downregulation of sarcolipin mRNA by triiodothyronine in the atrial myocardium

Thyroid hormone-mediated positive cardiotropic effects are differently regulated between the atria and ventricles. This regulation is, at least in part, dependent on sarcoplasmic reticulum (SR) proteins. Sarcolipin, a homologue of phospholamban, has been recently identified as an atrium-specific SR protein. The expression of sarcolipin mRNA was significantly decreased in the atria of mice with hyperthyroidism and in 3,5,3'-triiodo-l-thyronine-treated neonatal rat atrial myocytes. Promoter activity and mRNA stability analyses revealed that thyroid hormone post-transcriptionally down regulated the expression of sarcolipin mRNA. The atrium-specific effect of thyroid hormone may occur in part through the regulation of atrial sarcolipin gene expression.

Defining the intramembrane binding mechanism of sarcolipin to calcium ATPase using solution NMR spectroscopy

Sarcolipin (SLN) is an integral membrane protein that is expressed in both skeletal and cardiac muscle, where it inhibits SERCA (calcium ATPase) by lowering its apparent Ca2+ affinity in a manner similar to that of its homologue phospholamban (PLN). We use solution NMR to map the structural changes occurring within SLN upon interaction with the regulatory target, SERCA, co-reconstituting the two proteins in dodecylphosphocholine (DPC) detergent micelles, a system that preserves the native structure of SLN and the activity of SERCA, with the goal of comparing these interactions with those of the previously studied PLN-SERCA complex. Our analysis of the structural dynamics of SLN in DPC micelles shows this polypeptide to be partitioned into four subdomains: a short unstructured N terminus (residues 1-6), a short dynamic helix (residues 7-14), a more rigid helix (residues 15-26), and an unstructured C terminus (residues 27-31). Upon addition of SERCA, the different domains behave according to their dynamics, molding onto the surface of the enzyme. Remarkably, each domain of SLN behaves in a manner similar to that of the corresponding domains in PLN, supporting the hypothesis that both SLN and PLN bind SERCA in the same groove and with similar mechanisms.

Targeted Overexpression of Sarcolipin in the Mouse Heart Decreases Sarcoplasmic Reticulum Calcium Transport and Cardiac Contractility

The role of sarcolipin (SLN) in cardiac physiology was critically evaluated by generating a transgenic (TG) mouse model in which the SLN to sarco(endoplasmic)reticulum (SR) Ca(2+) ATPase (SERCA) ratio was increased in the ventricle. Overexpression of SLN decreases SR calcium transport function and results in decreased calcium transient amplitude and rate of relaxation. SLN TG hearts exhibit a significant decrease in rates of contraction and relaxation when assessed by ex vivo work-performing heart preparations. Similar results were also observed with muscle preparations and myocytes from SLN TG ventricles. Interestingly, the inhibitory effect of SLN was partially relieved upon high dose of isoproterenol treatment and stimulation at high frequency. Biochemical analyses show that an increase in SLN level does not affect PLB levels, monomer to pentamer ratio, or its phosphorylation status. No compensatory changes were seen in the expression of other calcium-handling proteins. These studies suggest that the SLN effect on SERCA pump is direct and is not mediated through increased monomerization of PLB or by a change in PLB phosphorylation status. We conclude that SLN is a novel regulator of SERCA pump activity, and its inhibitory effect can be reversed by beta-adrenergic agonists.

Structural interactions between FXYD proteins and Na+,K+-ATPase: alpha/beta/FXYD subunit stoichiometry and cross-linking

Interactions of rat FXYD4 (corticosteroid hormone-induced factor (CHIF)), FXYD2 (gamma), or FXYD1 (phospholemman (PLM)) proteins with rat alpha1 subunits of Na(+),K(+)-ATPase have been analyzed by co-immunoprecipitation and covalent cross-linking. In detergent-solubilized membranes from HeLa cells expressing both gamma and CHIF or CHIF and hemagglutinin A-tagged CHIF, mixed complexes of CHIF and gamma or CHIF and hemagglutinin A-tagged CHIF with alpha/beta subunits are undetectable. This implies that the alpha/beta/FXYD protomer is the major species in detergent solution. A lipid-soluble cysteine-cysteine bifunctional reagent, dibromobimane, cross-links CHIF to alpha in colonic membranes but not gamma or PLM to alpha in kidney or heart membranes, respectively. Sequence comparisons of the FXYD proteins suggested that Cys-49 in the trans-membrane segment of CHIF could be involved. In detergent-solubilized HeLa cell membranes, dibromobimane cross-links wild-type CHIF to alpha but not the C49F mutant, and also the corresponding F36C mutant but not wild-type gammab, and F48C but not wild-type PLM. C140S, C338A, C804A, and C966S mutants of the alpha subunit have been expressed. Only the C140S mutant prevents cross-linking with CHIF. The data demonstrated the proximity of trans-membrane segments of CHIF, gamma, and PLM to M2 of alpha. Molecular modeling is consistent with location of the trans-membrane segment of all FXYD proteins between M2, M6, and M9 and the proximity of Cys-49 of CHIF or Phe-36 of gamma with Cys-140 of M2. Cross-linking also demonstrated CHIF-alpha and CHIF-beta proximities in extra-membrane regions, similar to the evidence for gamma-alpha and gamma-beta cross-links.

Sarcolipin and phospholamban mRNA and protein expression in cardiac and skeletal muscle of different species

The widely held view that SLN (sarcolipin) would be the natural inhibitor of SERCA1 (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase 1), and PLB (phospholamban) its counterpart for SERCA2 inhibition is oversimplified and partially wrong. The expression of SLN and PLB mRNA and protein relative to SERCA1 or SERCA2 was assessed in ventricle, atrium, soleus and EDL (extensor digitorum longus) of mouse, rat, rabbit and pig. SLN protein levels were quantified by means of Western blotting using what appears to be the first successfully generated antibody directed against SLN. Our data confirm the co-expression of PLB and SERCA2a in cardiac muscle and the very low levels (in pig and rabbit) or the absence (in rat and mouse) of PLB protein in the slow skeletal muscle. In larger animals, the SLN mRNA and protein expression in the soleus and EDL correlates with SERCA1a expression, but, in rodents, SLN mRNA and protein show the highest abundance in the atria, which are devoid of SERCA1. In the rodent atria, SLN could therefore potentially interact with PLB and SERCA2a. No SLN was found in the ventricles of the different species studied, and there was no compensatory SLN up-regulation for the loss of PLB in PLB(-/-) mouse. In addition, we found that SLN expression was down-regulated at the mRNA and protein level in the atria of hypertrophic hearts of SERCA2(b/b) mice. These data suggest that superinhibition of SERCA by PLB-SLN complexes could occur in the atria of the smaller rodents, but not in those of larger animals.

BnNHL18A shows a localization change by stress-inducing chemical treatments

The two genes, named BnNHL18A and BnNHL18B, showing sequence homology with Arabidopsis NDR1/HIN1-like (NHL) genes, were isolated from cDNA library prepared with oilseed rape (Brassica napus) seedlings treated with NaCl. The transcript level of BnNHL18A was increased by sodium chloride, ethephon, hydrogen peroxide, methyl jasmonate, or salicylic acid treatment. The coding regions of BnNHL18A and BnNHL18B contain a sarcolipin (SLN)-like sequence. Analysis of the localization of smGFP fusion proteins showed that BnNHL18A is mainly localized to endoplasmic reticulum (ER). This result suggests that the SLN-like sequence plays a role in retaining proteins in ER membrane in plants. In response to NaCl, hydrogen peroxide, ethephon, and salicylic acid treatments, the protein localization of BnNHL18A was changed. Our findings suggest a common function of BnNHL18A in biotic and abiotic stresses, and demonstrate the presence of the shared mechanism of protein translocalization between the responses to plant pathogen and to osmotic stress.