The regulation of SERCA-type pumps by phospholamban and sarcolipin

Unsupervised ensemble-based phenotyping enhances discoverability of genes related to left-ventricular morphology

Recent genome-wide association studies have successfully identified associations between genetic variants and simple cardiac morphological parameters derived from cardiac magnetic resonance images. However, the emergence of large databases, including genetic data linked to cardiac magnetic resonance facilitates the investigation of more nuanced patterns of cardiac shape variability than those studied so far. Here we propose a framework for gene discovery coined unsupervised phenotype ensembles. The unsupervised phenotype ensemble builds a redundant yet highly expressive representation by pooling a set of phenotypes learnt in an unsupervised manner, using deep learning models trained with different hyperparameters. These phenotypes are then analysed via genome-wide association studies, retaining only highly confident and stable associations across the ensemble. We applied our approach to the UK Biobank database to extract geometric features of the left ventricle from image-derived three-dimensional meshes. We demonstrate that our approach greatly improves the discoverability of genes that influence left ventricle shape, identifying 49 loci with study-wide significance and 25 with suggestive significance. We argue that our approach would enable more extensive discovery of gene associations with image-derived phenotypes for other organs or image modalities.

Basic science of cardiac contractility modulation therapy: Molecular and electrophysiological mechanisms

In heart failure with reduced ejection fraction and heart failure with preserved ejection fraction, profound cellular and molecular changes have recently been documented in the failing myocardium. These changes include altered calcium handling and metabolic efficiency of the cardiac myocyte, reactivation of the fetal gene program, changes in the electrophysiological properties of the heart, and accumulation of collagen (fibrosis) at the interstitial level. Cardiac contractility modulation therapy is an innovative device-based therapy currently approved for heart failure with reduced ejection fraction in patients with narrow QRS complex and under investigation for the treatment of heart failure with preserved ejection fraction. This therapy is based on the delivery of high-voltage biphasic electrical signals to the septal wall of the right ventricle during the absolute refractory period of the myocardium. At the cellular level, in patients with heart failure with reduced ejection fraction, cardiac contractility modulation therapy has been shown to restore calcium handling and improve the metabolic status of cardiac myocytes, reverse the heart failure-associated fetal gene program, and reduce the extent of interstitial fibrosis. This review summarizes the preclinical literature on the use of cardiac contractility modulation therapy in heart failure with reduced and preserved ejection fraction, correlating the molecular and electrophysiological effects with the clinical benefits demonstrated by this therapy.

The Meeting of Micropeptides with Major Ca2+ Pumps in Inner Membranes-Consideration of a New Player, SERCA1b

Calcium is a major signalling bivalent cation within the cell. Compartmentalization is essential for regulation of calcium mediated processes. A number of players contribute to intracellular handling of calcium, among them are the sarco/endoplasmic reticulum calcium ATP-ases (SERCAs). These molecules function in the membrane of ER/SR pumping Ca²⁺ from cytoplasm into the lumen of the internal store. Removal of calcium from the cytoplasm is essential for signalling and for relaxation of skeletal muscle and heart. There are three genes and over a dozen isoforms of SERCA in mammals. These can be potentially influenced by small membrane peptides, also called regulins. The discovery of micropeptides has increased in recent years, mostly because of the small ORFs found in long RNAs, annotated formerly as noncoding (lncRNAs). Several excellent works have analysed the mechanism of interaction of micropeptides with each other and with the best known SERCA1a (fast muscle) and SERCA2a (heart, slow muscle) isoforms. However, the array of tissue and developmental expressions of these potential regulators raises the question of interaction with other SERCAs. For example, the most abundant calcium pump in neonatal and regenerating skeletal muscle, SERCA1b has never been looked at with scrutiny to determine whether it is influenced by micropeptides. Further details might be interesting on the interaction of these peptides with the less studied SERCA1b isoform.

Micropeptide hetero-oligomerization adds complexity to the calcium pump regulatory network

The sarco(endo)plasmic reticulum calcium ATPase (SERCA) is an ion transporter that creates and maintains intracellular calcium stores. SERCA is inhibited or stimulated by several membrane micropeptides including another-regulin, dwarf open reading frame, endoregulin, phospholamban (PLB), and sarcolipin. We previously showed that these micropeptides assemble into homo-oligomeric complexes with varying affinity. Here, we tested whether different micropeptides can interact with each other, hypothesizing that coassembly into hetero-oligomers may affect micropeptide bioavailability to regulate SERCA. We quantified the relative binding affinity of each combination of candidates using automated fluorescence resonance energy transfer microscopy. All pairs were capable of interacting with good affinity, similar to the affinity of micropeptide self-binding (homo-oligomerization). Testing each pair at a 1:5 ratio and a reciprocal 5:1 ratio, we noted that the affinity of hetero-oligomerization of some micropeptides depended on whether they were the minority or majority species. In particular, sarcolipin was able to join oligomers when it was the minority species but did not readily accommodate other micropeptides in the reciprocal experiment when it was expressed in fivefold excess. The opposite was observed for endoregulin. PLB was a universal partner for all other micropeptides tested, forming avid hetero-oligomers whether it was the minority or majority species. Increasing expression of SERCA decreased PLB-dwarf open reading frame hetero-oligomerization, suggesting that SERCA-micropeptide interactions compete with micropeptide-micropeptide interactions. Thus, micropeptides populate a regulatory network of diverse protein assemblies. The data suggest that the complexity of this interactome increases exponentially with the number of micropeptides that are coexpressed in a particular tissue.

Multiphasic changes in smooth muscle Ca2+ transporters during the progression of coronary atherosclerosis

Ischemic heart disease due to macrovascular atherosclerosis and microvascular dysfunction is the major cause of death worldwide and the unabated increase in metabolic syndrome is a major reason why this will continue. Intracellular free Ca²⁺ ([Ca²⁺]_i) regulates a variety of cellular functions including contraction, proliferation, migration, and transcription. It follows that studies of vascular Ca²⁺ regulation in reductionist models and translational animal models are vital to understanding vascular health and disease. Swine with metabolic syndrome (MetS) develop the full range of coronary atherosclerosis from mild to severe disease. Intravascular imaging enables quantitative measurement of atherosclerosis in vivo, so viable coronary smooth muscle (CSM) cells can be dispersed from the arteries to enable Ca²⁺ transport studies in native cells. Transition of CSM from the contractile phenotype in the healthy swine to the proliferative phenotype in mild atherosclerosis was associated with increases in SERCA activity, sarcoplasmic reticulum Ca²⁺, and voltage-gated Ca²⁺ channel function. In vitro organ culture confirmed that SERCA activation induces CSM proliferation. Transition from the proliferative to a more osteogenic phenotype was associated with decreases in all three Ca²⁺ transporters. Overall, there was a biphasic change in Ca²⁺ transporters over the progression of atherosclerosis in the swine model and this was confirmed in CSM from failing explanted hearts of humans. A major determinant of endolysosome content in human CSM is the severity of atherosclerosis. In swine CSM endolysosome Ca²⁺ release occurred through the TPC2 channel. We propose a multiphasic change in Ca²⁺ transporters over the progression of coronary atherosclerosis.

Copper binding leads to increased dynamics in the regulatory N-terminal domain of full-length human copper transporter ATP7B

ATP7B is a human copper-transporting P_1B-type ATPase that is involved in copper homeostasis and resistance to platinum drugs in cancer cells. ATP7B consists of a copper-transporting core and a regulatory N-terminal tail that contains six metal-binding domains (MBD1-6) connected by linker regions. The MBDs can bind copper, which changes the dynamics of the regulatory domain and activates the protein, but the underlying mechanism remains unknown. To identify possible copper-specific structural dynamics involved in transport regulation, we constructed a model of ATP7B spanning the N-terminal tail and core catalytic domains and performed molecular dynamics (MD) simulations with (holo) and without (apo) copper ions bound to the MBDs. In the holo protein, MBD2, MBD3 and MBD5 showed enhanced mobilities, which resulted in a more extended N-terminal regulatory region. The observed separation of MBD2 and MBD3 from the core protein supports a mechanism where copper binding activates the ATP7B protein by reducing interactions among MBD1-3 and between MBD1-3 and the core protein. We also observed an increased interaction between MBD5 and the core protein that brought the copper-binding site of MBD5 closer to the high-affinity internal copper-binding site in the core protein. The simulation results assign specific, mechanistic roles to the metal-binding domains involved in ATP7B regulation that are testable in experimental settings.

Living organisms depend upon active transport against gradients across biological membranes for survival. Such transport can be accomplished by ATP-dependent membrane protein transporters for which the activity must be regulated to maintain optimal concentrations in the cellular compartments. The regulatory mechanisms often involve structural responses inherent to the protein structure, which because of their dynamic nature can be hard to assess experimentally. A prime example is regulation of cellular copper levels by a copper-binding tail in the human copper transporter ATP7B. Dysregulation can cause severe diseases, for example the copper metabolism disorder Wilson’s disease, which is caused by mutations in ATP7B regulation machinery. Due to the practical difficulties in working with membrane proteins, most studies of ATP7B have been conducted in the absence of the membrane-bound protein core. Here, we used computer simulations of full-length ATP7B to study how structural dynamics in the regulatory tail differ between copper-bound and copper-free states. Copper induced increased dynamics in the tail, resulting in an overall movement towards the ion-binding site in the protein core. The simulations identified several, hitherto not reported, interactions between the regulatory tail and the protein core that can be targeted experimentally to enhance our understanding of this medically relevant regulatory mechanism.

Natural Polyphenols as SERCA Activators: Role in the Endoplasmic Reticulum Stress-Related Diseases

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a key protein responsible for transporting Ca²⁺ ions from the cytosol into the lumen of the sarco/endoplasmic reticulum (SR/ER), thus maintaining Ca²⁺ homeostasis within cells. Accumulating evidence suggests that impaired SERCA function is associated with disruption of intracellular Ca²⁺ homeostasis and induction of ER stress, leading to different chronic pathological conditions. Therefore, appropriate strategies to control Ca²⁺ homeostasis via modulation of either SERCA pump activity/expression or relevant signaling pathways may represent a useful approach to combat pathological states associated with ER stress. Natural dietary polyphenolic compounds, such as resveratrol, gingerol, ellagic acid, luteolin, or green tea polyphenols, with a number of health-promoting properties, have been described either to increase SERCA activity/expression directly or to affect Ca²⁺ signaling pathways. In this review, potential Ca²⁺-mediated effects of the most studied polyphenols on SERCA pumps or related Ca²⁺ signaling pathways are summarized, and relevant mechanisms of their action on Ca²⁺ regulation with respect to various ER stress-related states are depicted. All data were collected using scientific search tools (i.e., Science Direct, PubMed, Scopus, and Google Scholar).

Dietary nitrate increases submaximal SERCA activity and ADP transfer to mitochondria in slow-twitch muscle of female mice

Rapid oscillations in cytosolic calcium (Ca²⁺) coordinate muscle contraction, relaxation, and physical movement. Intriguingly, dietary nitrate decreases ATP cost of contraction, increases force production, and increases cytosolic Ca²⁺, which would seemingly necessitate a greater demand for sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) to sequester Ca²⁺ within the sarcoplasmic reticulum (SR) during relaxation. As SERCA is highly regulated, we aimed to determine the effect of 7-day nitrate supplementation (1 mM via drinking water) on SERCA enzymatic properties and the functional interaction between SERCA and mitochondrial oxidative phosphorylation. In soleus, we report that dietary nitrate increased force production across all stimulation frequencies tested, and throughout a 25 min fatigue protocol. Mice supplemented with nitrate also displayed an ∼25% increase in submaximal SERCA activity and SERCA efficiency (P = 0.053) in the soleus. To examine a possible link between ATP consumption and production, we established a methodology coupling SERCA and mitochondria in permeabilized muscle fibers. The premise of this experiment is that the addition of Ca²⁺ in the presence of ATP generates ADP from SERCA to support mitochondrial respiration. Similar to submaximal SERCA activity, mitochondrial respiration supported by SERCA-derived ADP was increased by ∼20% following nitrate in red gastrocnemius. This effect was fully attenuated by the SERCA inhibitor cyclopiazonic acid and was not attributed to differences in mitochondrial oxidative capacity, ADP sensitivity, protein content, or reactive oxygen species emission. Overall, these findings suggest that improvements in submaximal SERCA kinetics may contribute to the effects of nitrate on force production during fatigue.NEW & NOTEWORTHY We show that nitrate supplementation increased force production during fatigue and increased submaximal SERCA activity. This was also evident regarding the high-energy phosphate transfer from SERCA to mitochondria, as nitrate increased mitochondrial respiration supported by SERCA-derived ADP. Surprisingly, these observations were only apparent in muscle primarily expressing type I (soleus) but not type II fibers (EDL). These findings suggest that alterations in SERCA properties are a possible mechanism in which nitrate increases force during fatiguing contractions.

Roles of ATP and SERCA in the Regulation of Calcium Turnover in Unloaded Skeletal Muscles: Current View and Future Directions

A decrease in skeletal muscle contractile activity or its complete cessation (muscle unloading or disuse) leads to muscle fibers’ atrophy and to alterations in muscle performance. These changes negatively affect the quality of life of people who, for one reason or another, are forced to face a limitation of physical activity. One of the key regulatory events leading to the muscle disuse-induced changes is an impairment of calcium homeostasis, which leads to the excessive accumulation of calcium ions in the sarcoplasm. This review aimed to analyze the triggering mechanisms of calcium homeostasis impairment (including those associated with the accumulation of high-energy phosphates) under various types of muscle unloading. Here we proposed a hypothesis about the regulatory mechanisms of SERCA and IP3 receptors activity during muscle unloading, and about the contribution of these mechanisms to the excessive calcium ion myoplasmic accumulation and gene transcription regulation via excitation–transcription coupling.

Heterozygous SOD2 deletion selectively impairs SERCA function in the soleus of female mice

The sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA) restores intracellular Ca²⁺ ([Ca²⁺ ]_i ) to resting levels after muscle contraction, ultimately eliciting relaxation. SERCA pumps are highly susceptible to tyrosine (T)-nitration, impairing their ability to take up Ca²⁺ resulting in reduced muscle function and increased [Ca²⁺ ]_i and cellular damage. The mitochondrial antioxidant enzyme, superoxide dismutase 2 (SOD2), converts superoxide radicals into less reactive H₂ O₂ . Heterozygous deletion of SOD2 (Sod2^+/- ) in mice increases mitochondrial oxidative stress; however, the consequences of reduced SOD2 expression in skeletal and cardiac muscle, specifically the effect on SERCA pumps, has yet to be investigated. We obtained soleus, extensor digitorum longus (EDL), and left ventricle (LV) muscles from 6 to 7 month-old wild-type (WT) and Sod2^+/- female C57BL/6J mice. Ca²⁺ -dependent SERCA activity assays were performed to assess SERCA function. Western blotting was conducted to examine the protein content of SERCA, phospholamban, and sarcolipin; and immunoprecipitation experiments were done to assess SERCA2a- and SERCA1a-specific T-nitration. Heterozygous SOD2 deletion did not alter SERCA1a or SERCA2a expression in the soleus or LV but reduced SERCA2a in the EDL compared with WT, though this was not statistically significant. Soleus muscles from Sod2^+/- mice showed a significant reduction in SERCA's apparent affinity for Ca²⁺ when compared to WT, corresponding with significantly elevated SERCA2a T-nitration in the soleus. No effect was seen in the EDL or the LV. This is the first study to investigate the effects of SOD2 deficiency on muscle SERCA function and shows that it selectively impairs SERCA function in the soleus.

Microproteins in skeletal muscle: hidden keys in muscle physiology

Recent advances in the transcriptomics, translatomics, and proteomics have led us to the exciting new world of functional endogenous microproteins. These microproteins have a small size and are derived from small open reading frames (smORFs) of RNAs previously annotated as non-coding (e.g. lncRNAs and circRNAs) as well as from untranslated regions and canonical mRNAs. The presence of these microproteins reveals a much larger translatable portion of the genome, shifting previously defined dogmas and paradigms. These findings affect our view of organisms as a whole, including skeletal muscle tissue. Emerging evidence demonstrates that several smORF-derived microproteins play crucial roles during muscle development (myogenesis), maintenance, and regeneration, as well as lipid and glucose metabolism and skeletal muscle bioenergetics. These microproteins are also involved in processes including physical activity capacity, cellular stress, and muscular-related diseases (i.e. myopathy, cachexia, atrophy, and muscle wasting). Given the role of these small proteins as important key regulators of several skeletal muscle processes, there are rich prospects for the discovery of new microproteins and possible therapies using synthetic microproteins.

Maternal exercise intergenerationally drives muscle-based thermogenesis via activation of apelin-AMPK signaling

BACKGROUND

Sarcolipin and uncoupling protein 3 (UCP3) mediate muscle-based non-shivering thermogenesis (NST) to improve metabolic homeostasis. The impacts of maternal obesity (MO) and maternal exercise (ME) on NST in offspring muscle remain unexamined.

METHODS

Female mice were fed with a control diet or high fat diet to induce obesity. Then, obese mice were further separated into two groups: obesity only (OB) and OB plus daily exercise (OB/Ex). Fetal muscle was collected at embryonic day 18.5 and offspring mice at 3-month-old. Apelin administration during pregnancy and apelin receptor (APJ) knockout mouse were further used for investigating the mediatory role of APJ on muscle-based thermogenesis. To explore the direct effects of exercise on AMP-activated protein kinase (AMPK) downstream targets, AMPK knockout mouse was used.

FINDINGS

MO inhibited while ME activated AMPK and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) in fetal muscle. AMPK activation increased sarcolipin expression, which inhibited the uptake of calcium ions into sarcoplasmic reticulum, thereby activating CaMKK2. Consistently, the expression of UCP3 and sarcolipin was suppressed due to MO but activated in ME fetal muscle. Importantly, changes of UCP3 and sarcolipin maintained in offspring muscle, showing the transgenerational effects. Furthermore, apelin administration during pregnancy mimicked the effects of ME on AMPK and CaMKK2 activation, and UCP3 and sarcolipin expression, underscoring the mediatory roles of apelin-AMPK signaling in improving fetal muscle development.

INTERPRETATION

ME, via activation of apelin signaling-AMPK axis, enhances NST gene expression in fetal and offspring muscle impaired due to MO, which intergenerationally protects offspring from diet-induced obesity and metabolic disorders.

FUNDING

This work was supported by National Institutes of Health Grant R01-HD067449.

TUNAR lncRNA Encodes a Microprotein that Regulates Neural Differentiation and Neurite Formation by Modulating Calcium Dynamics

Long noncoding RNAs (lncRNAs) are regulatory molecules which have been traditionally considered as “non-coding”. Strikingly, recent evidence has demonstrated that many non-coding regions, including lncRNAs, do in fact contain small-open reading frames that code for small proteins that have been called microproteins. Only a few of them have been characterized so far, but they display key functions in a wide variety of cellular processes. Here, we show that TUNAR lncRNA encodes an evolutionarily conserved microprotein expressed in the nervous system that we have named pTUNAR. pTUNAR deficiency in mouse embryonic stem cells improves their differentiation potential towards neural lineage both in vitro and in vivo. Conversely, pTUNAR overexpression impairs neuronal differentiation by reduced neurite formation in different model systems. At the subcellular level, pTUNAR is a transmembrane protein that localizes in the endoplasmic reticulum and interacts with the calcium transporter SERCA2. pTUNAR overexpression reduces cytoplasmatic calcium, consistent with a possible role of pTUNAR as an activator of SERCA2. Altogether, our results suggest that our newly discovered microprotein has an important role in neural differentiation and neurite formation through the regulation of intracellular calcium. From a more general point of view, our results provide a proof of concept of the role of lncRNAs-encoded microproteins in neural differentiation.

Function and regulation of phosphatase 1 in healthy and diseased heart

Reversible phosphorylation of ion channels and calcium-handling proteins provides precise post-translational regulation of cardiac excitation and contractility. Serine/threonine phosphatases govern dephosphorylation of the majority of cardiac proteins. Accordingly, dysfunction of this regulation contributes to the development and progression of heart failure and atrial fibrillation. On the molecular level, these changes include alterations in the expression level and phosphorylation status of Ca²⁺ handling and excitation-contraction coupling proteins provoked by dysregulation of phosphatases. The serine/threonine protein phosphatase PP1 is one a major player in the regulation of cardiac excitation-contraction coupling. PP1 essentially impacts on cardiac physiology and pathophysiology via interactions with the cardiac ion channels Cav1.2, NKA, NCX and KCNQ1, sarcoplasmic reticulum-bound Ca²⁺ handling proteins such as RyR2, SERCA and PLB as well as the contractile proteins MLC2, TnI and MyBP-C. PP1 itself but also PP1-regulatory proteins like inhibitor-1, inhibitor-2 and heat-shock protein 20 are dysregulated in cardiac disease. Therefore, they represent interesting targets to gain more insights in heart pathophysiology and to identify new treatment strategies for patients with heart failure or atrial fibrillation. We describe the genetic and holoenzymatic structure of PP1 and review its role in the heart and cardiac disease. Finally, we highlight the importance of the PP1 regulatory proteins for disease manifestation, provide an overview of genetic models to study the role of PP1 for the development of heart failure and atrial fibrillation and discuss possibilities of pharmacological interventions.

Oxidative Stress in Takotsubo Syndrome-Is It Essential for an Acute Attack? Indirect Evidences Support Multisite Impact Including the Calcium Overload-Energy Failure Hypothesis

Indirect evidences in reviews and case reports on Takotsubo syndrome (TTS) support the fact that the existence of oxidative stress (OS) might be its common feature in the pre-acute stage. The sources of OS are exogenous (environmental factors including pharmacological and toxic influences) and endogenous, the combination of both may be present, and they are being discussed in detail. OS is associated with several pathological conditions representing TTS comorbidities and triggers. The dominant source of OS electrones are mitochondria. Our analysis of drug therapy related to acute TTS shows many interactions, e.g., cytostatics and glucocorticoids with mitochondrial cytochrome P450 and other enzymes important for OS. One of the most frequently discussed mechanisms in TTS is the effect of catecholamines on myocardium. Yet, their metabolic influence is neglected. OS is associated with the oxidation of catecholamines leading to the synthesis of their oxidized forms - aminochromes. Under pathological conditions, this pathway may dominate. There are evidences of interference between OS, catecholamine/aminochrome effects, their metabolism and antioxidant protection. The OS offensive may cause fast depletion of antioxidant protection including the homocystein-methionine system, whose activity decreases with age. The alteration of effector subcellular structures (mitochondria, sarco/endoplasmic reticulum) and subsequent changes in cellular energetics and calcium turnover may also occur and lead to the disruption of cellular function, including neurons and cardiomyocytes. On the organ level (nervous system and heart), neurocardiogenic stunning may occur. The effects of OS correspond to the effect of high doses of catecholamines in the experiment. Intensive OS might represent "conditio sine qua non" for this acute clinical condition. TTS might be significantly more complex pathology than currently perceived so far.

The effect of Mitoquinol (MitoQ) on heat stressed skeletal muscle from pigs, and a potential confounding effect of biological sex

Heat stress (HS) poses a major threat to human health and agricultural production. Oxidative stress and mitochondrial dysfunction appear to play key roles in muscle injury caused by HS. We hypothesized that mitoquinol (MitoQ), would alleviate oxidative stress and cellular dysfunction in skeletal muscle during HS. To address this, crossbred barrows (male pigs) were treated with placebo or MitoQ (40 mg/d) and were then exposed to thermoneutral (TN; 20 °C) or HS (35 °C) conditions for 24 h. Pigs were euthanized following the environmental challenge and the red portion of the semitendinosus (STR) was collected for analysis. Unexpectedly, malondialdehyde concentration, an oxidative stress marker, was similar between environmental and supplement treatments. Heat stress decreased LC3A/B-I (p < 0.05) and increased the ratio of LC3A/B-II/I (p < 0.05), while p62 was similar among groups suggesting increased degradation of autophagosomes during HS. These outcomes were in disagreement with our previous results in muscle from gilts (female pigs). To probe the impact of biological sex on HS-mediated injury in skeletal muscle, we compared STR from these barrows to archived STR from gilts subjected to a similar environmental intervention. We confirmed our previous findings of HS-mediated dysfunction in muscle from gilts but not barrows. These data also raise the possibility that muscle from gilts is more susceptible to environment-induced hyperthermia than muscle from barrows.

Deciphering the Mechanism of Inhibition of SERCA1a by Sarcolipin Using Molecular Simulations

SERCA1a is an ATPase calcium pump that transports Ca²⁺ from the cytoplasm to the sarco/endoplasmic reticulum lumen. Sarcolipin (SLN), a transmembrane peptide, regulates the activity of SERCA1a by decreasing its Ca²⁺ transport rate, but its mechanism of action is still not well-understood. To decipher this mechanism, we have performed normal mode analysis in the all-atom model, with the SERCA1a-SLN complex, or the isolated SERCA1a, embedded in an explicit membrane. The comparison of the results allowed us to provide an explanation at the atomic level for the action of SLN that is in good agreement with experimental observations. In our analyses, the presence of SLN locally perturbs the TM6 transmembrane helix and as a consequence modifies the position of D800, one of the key metal-chelating residues. Additionally, it reduces the flexibility of the gating residues, V304, and E309 in TM4, at the entrance of the Ca²⁺ binding sites, which would decrease the affinity for Ca²⁺. Unexpectedly, SLN has also an effect on the ATP binding site more than 35 Å away, due to the straightening of TM5, a long helix considered as the spine of the protein. The straightening of TM5 modifies the structure of the P-N linker that sits above it, and which comprises the ³⁵¹DKTG³⁵⁴ conserved motif, resulting in an increase of the distance between ATP and the phosphorylation site. As a consequence, the turn-over rate could be affected. All this gives SERCA1a the propensity to go toward a Ca²⁺ low-affinity E2-like state in the presence of SLN and toward a Ca²⁺ high-affinity E1-like state in the absence of SLN. In addition to a general mechanism of inhibition of SERCA1a regulatory peptides, this study also provides an insight into the conformational transition between the E2 and E1 states.

LncRNA-Encoded Peptide: Functions and Predicting Methods

Long non-coding RNA (lncRNA) was originally defined as the representative of the non-coding RNAs and unable to encode. However, recent reports suggest that some lncRNAs actually contain open reading frames that encode peptides. These coding products play important roles in the pathogenesis of many diseases. Here, we summarize the regulatory pathways of mammalian lncRNA-encoded peptides in influencing muscle function, mRNA stability, gene expression, and so on. We also address the promoting and inhibiting functions of the peptides in different cancers and other diseases. Then we introduce the computational predicting methods and data resources to predict the coding ability of lncRNA. The intention of this review is to provide references for further coding research and contribute to reveal the potential prospects for targeted tumor therapy.

Mechanisms underlying pathological Ca2+ handling in diseases of the heart

Cardiomyocyte contraction relies on precisely regulated intracellular Ca²⁺ signaling through various Ca²⁺ channels and transporters. In this article, we will review the physiological regulation of Ca²⁺ handling and its role in maintaining normal cardiac rhythm and contractility. We discuss how inherited variants or acquired defects in Ca²⁺ channel subunits contribute to the development or progression of diseases of the heart. Moreover, we highlight recent insights into the role of protein phosphatase subunits and striated muscle preferentially expressed protein kinase (SPEG) in atrial fibrillation, heart failure, and cardiomyopathies. Finally, this review summarizes current drug therapies and new advances in genome editing as therapeutic strategies for the cardiac diseases caused by aberrant intracellular Ca²⁺ signaling.

Phospholamban and sarcolipin prevent thermal inactivation of sarco(endo)plasmic reticulum Ca2+-ATPases

Na+-K+-ATPase from mice lacking the γ subunit exhibits decreased thermal stability. Phospholamban (PLN) and sarcolipin (SLN) are small homologous proteins that regulate sarco(endo)plasmic reticulum Ca2+-ATPases (SERCAs) with properties similar to the γ subunit, through physical interactions with SERCAs. Here, we tested the hypothesis that PLN and SLN may protect against thermal inactivation of SERCAs. HEK-293 cells were co-transfected with different combinations of cDNAs encoding SERCA2a, PLN, a PLN mutant (N34A) that cannot bind to SERCA2a, and SLN. One-half of the cells were heat stressed at 40°C for 1 h (HS), and one-half were maintained at 37°C (CTL) before harvesting the cells and isolating microsomes. Compared with CTL, maximal SERCA activity was reduced by 25-35% following HS in cells that expressed either SERCA2a alone or SERCA2a and mutant PLN (N34A) whereas no change in maximal SERCA2a activity was observed in cells that co-expressed SERCA2a and either PLN or SLN following HS. Increases in SERCA2a carbonyl group content and nitrotyrosine levels that were detected following HS in cells that expressed SERCA2a alone were prevented in cells co-expressing SERCA2a with PLN or SLN, whereas co-expression of SERCA2a with mutant PLN (N34A) only prevented carbonyl group formation. In other experiments using knock-out mice, we found that thermal inactivation of SERCA was increased in cardiac left ventricle samples from Pln-null mice and in diaphragm samples from Sln-null mice, compared with WT littermates. Our results show that both PLN and SLN form a protective interaction with SERCA pumps during HS, preventing nitrosylation and oxidation of SERCA and thus preserving its maximal activity.

Structural Changes of Sarco/Endoplasmic Reticulum Ca2+-ATPase Induced by Rutin Arachidonate: A Molecular Dynamics Study

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) maintains the level of calcium concentration in cells by pumping calcium ions from the cytoplasm to the lumen while undergoing substantial conformational changes, which can be stabilized or prevented by various compounds. Here we attempted to clarify the molecular mechanism of action of new inhibitor rutin arachidonate, one of the series of the acylated rutin derivatives. We performed molecular dynamics simulations of SERCA1a protein bound to rutin arachidonate positioned in a pure dipalmitoylphosphatidylcholine bilayer membrane. Our study predicted the molecular basis for the binding of rutin arachidonate towards SERCA1a in the vicinity of the binding site of calcium ions and near the location of the well-known inhibitor thapsigargin. The stable hydrogen bond between Glu771 and rutin arachidonate plays a key role in the binding. SERCA1a is kept in the E2 conformation preventing the formation of important salt bridges between the side chains of several residues, primarily Glu90 and Lys297. All in all, the structural changes induced by the binding of rutin arachidonate to SERCA1a may shift proton balance near the titrable residues Glu771 and Glu309 into neutral species, hence preventing the binding of calcium ions to the transmembrane binding sites and thus affecting calcium homeostasis. Our results could lead towards the design of new types of inhibitors, potential drug candidates for cancer treatment, which could be anchored to the transmembrane region of SERCA1a by a lipophilic fatty acid group.

The sarcoplasmic reticulum and SERCA: a nexus for muscular adaptive thermogenesis

We are currently facing an "obesity epidemic" worldwide. Promoting inefficient metabolism in muscle represents a potential treatment for obesity and its complications. Sarco(endo)plasmic reticulum (SR) Ca²⁺-ATPase (SERCA) pumps in muscle are responsible for maintaining low cytosolic Ca²⁺ concentration through the ATP-dependent pumping of Ca²⁺ from the cytosol into the SR lumen. SERCA activity has the potential to be a critical regulator of body mass and adiposity given that it is estimated to contribute upwards of 20% of daily energy expenditure. More interestingly, this fraction can be modified physiologically in the face of stressors, such as ambient temperature and diet, through its physical interaction with several regulators known to inhibit Ca²⁺ uptake and muscle function. In this review, we discuss advances in our understanding of Ca²⁺-cycling thermogenesis within skeletal muscle, focusing on SERCA and its protein regulators, which were thought previously to only modulate muscular contractility. Novelty ATP consumption by SERCA pumps comprises a large proportion of resting energy expenditure in muscle and is dynamically regulated through interactions with small SERCA regulatory proteins. SERCA efficiency correlates significantly with resting metabolism, such that individuals with a higher resting metabolic rate have less energetically efficient SERCA Ca²⁺ pumping in muscle (i.e., lower coupling ratio). Futile Ca²⁺ cycling is a versatile heat generating mechanism utilized by both skeletal muscle and beige fat.

Muscle nonshivering thermogenesis in a feral mammal

Muscle nonshivering thermogenesis (NST) was recently suggested to play an important role in thermoregulation of species lacking brown adipose tissue (BAT). The mechanism, which is independent of muscle contractions, produces heat based on the activity of an ATPase pump in the sarcoplasmic reticulum (SERCA1a) and is controlled by the protein sarcolipin. To evaluate whether muscle NST could indeed play an important role in thermoregulation in species lacking BAT, we investigated the thermogenic capacities of newborn wild boar piglets. During cold exposure over the first 5 days of life, total heat production was improved while shivering intensity decreased, indicating an increasing contribution of NST. Sampling skeletal muscle tissue for analyses of SERCA activity as well as gene expression of SERCA1a and sarcolipin, we found an age-related increase in all three variables as well as in body temperature. Hence, the improved thermogenesis during the development of wild boars is not due to shivering but explained by the observed increase in SERCA activity. Our results suggest that muscle NST may be the primary mechanism of heat production during cold stress in large mammals lacking BAT, strengthening the hypothesis that muscle NST has likely played an important role in the evolution of endothermy.

Therapeutic implications of novel peptides targeting ER-mitochondria Ca2+-flux systems

Intracellular Ca²⁺-flux systems located at the ER-mitochondrial axis govern mitochondrial Ca²⁺ balance and cell fate. Multiple yet incurable pathologies are characterized by insufficient or excessive Ca²⁺ fluxes toward the mitochondria, in turn leading to aberrant cell life or death dynamics. The discovery and ongoing molecular characterization of the main interorganellar Ca²⁺ gateways have resulted in a novel class of peptide tools able to regulate relevant protein-protein interactions (PPIs) underlying this signaling scenario. Here, we review peptides, molecularly derived from Ca²⁺-flux systems or their accessory proteins. We discuss how they alter Ca²⁺-signaling protein complexes and modulate cell survival in light of their forthcoming therapeutic applications.

Role of Endoplasmic Reticulum-Mediated Ca2+ Signaling in Neuronal Cell Death

SIGNIFICANCE

Properly controlled intracellular Ca²⁺ dynamics is crucial for regulation of neuronal function and survival in the central nervous system. The endoplasmic reticulum (ER), a major intracellular Ca²⁺ store, plays a critical role as a source and sink for neuronal Ca²⁺. Recent Advances: Accumulating evidence indicates that disrupted ER Ca²⁺ signaling is involved in neuronal cell death under various pathological conditions, providing novel insight into neurodegenerative disease mechanisms.

CRITICAL ISSUES

We summarize current knowledge concerning the relationship between abnormal ER Ca²⁺ dynamics and neuronal cell death. We also introduce recent technical advances for probing ER intraluminal Ca²⁺ dynamics with unprecedented spatiotemporal resolution.

FUTURE DIRECTIONS

Further studies on ER Ca²⁺ signaling are expected to provide progress for unmet medical needs in neurodegenerative disease. Antioxid. Redox Signal. 29, 1147-1157.

Role of defective Ca2+ signaling in skeletal muscle weakness: Pharmacological implications

The misbehaving attitude of Ca²⁺ signaling pathways could be the probable reason in many muscular disorders such as myopathies, systemic disorders like hypoxia, sepsis, cachexia, sarcopenia, heart failure, and dystrophy. The present review throws light upon the calcium flux regulating signaling channels like ryanodine receptor complex (RyR1), SERCA (Sarco-endoplasmic Reticulum Calcium ATPase), DHPR (Dihydropyridine Receptor) or Cav1.1 and Na+/Ca²⁺ exchange pump in detail and how remodelling of these channels contribute towards disturbed calcium homeostasis. Understanding these pathways will further provide an insight for establishing new therapeutic approaches for the prevention and treatment of muscle atrophy under stress conditions, targeting calcium ion channels and associated regulatory proteins.

Effects of chronic hypoxia on diaphragm function in deer mice native to high altitude

AIM

We examined the effects of chronic hypoxia on diaphragm function in high- and low-altitude populations of Peromyscus mice.

METHODS

Deer mice (P. maniculatus) native to high altitude and congeneric mice native to low altitude (P. leucopus) were born and raised in captivity to adulthood and were acclimated to normoxia or hypobaric hypoxia (12 or 9 kPa, simulating hypoxia at 4300 and 7000 m) for 6-8 weeks. We then measured indices of mitochondrial respiration capacity, force production, and fatigue resistance in the diaphragm.

RESULTS

Mitochondrial respiratory capacities (assessed using permeabilized fibres with single or multiple inputs to the electron transport system), citrate synthase activity (a marker of mitochondrial volume), twitch force production, and muscle fatigue resistance increased after exposure to chronic hypoxia in both populations. These changes were not well explained by variation in the fibre-type composition of the muscle. However, there were several differences in diaphragm function in high-altitude mice compared to low-altitude mice. Exposure to a deeper level of hypoxia (9 kPa vs 12 kPa) was needed to elicit increases in mitochondrial respiration rates in highlanders. Chronic hypoxia did not increase the emission of reactive oxygen species from permeabilized fibres in highlanders, in contrast to the pronounced increases that occurred in lowlanders. In general, the diaphragm of high-altitude mice had greater capillary length densities, produced less force in response to stimulation and had shorter relaxation times. The latter was associated with higher activity of sarcoplasmic reticulum Ca²⁺ -ATPase (SERCA) activity in the diaphragm of high-altitude mice.

CONCLUSION

Overall, our work suggests that exposure to chronic hypoxia increases the capacities for mitochondrial respiration, force production and fatigue resistance of the diaphragm. However, many of these effects are opposed by evolved changes in diaphragm function in high-altitude natives, such that highlanders in chronic hypoxia maintain similar diaphragm function to lowlanders in sea level conditions.

Congenital myopathies: disorders of excitation-contraction coupling and muscle contraction

The congenital myopathies are a group of early-onset, non-dystrophic neuromuscular conditions with characteristic muscle biopsy findings, variable severity and a stable or slowly progressive course. Pronounced weakness in axial and proximal muscle groups is a common feature, and involvement of extraocular, cardiorespiratory and/or distal muscles can implicate specific genetic defects. Central core disease (CCD), multi-minicore disease (MmD), centronuclear myopathy (CNM) and nemaline myopathy were among the first congenital myopathies to be reported, and they still represent the main diagnostic categories. However, these entities seem to belong to a much wider phenotypic spectrum. To date, congenital myopathies have been attributed to mutations in over 20 genes, which encode proteins implicated in skeletal muscle Ca²⁺ homeostasis, excitation-contraction coupling, thin-thick filament assembly and interactions, and other mechanisms. RYR1 mutations are the most frequent genetic cause, and CCD and MmD are the most common subgroups. Next-generation sequencing has vastly improved mutation detection and has enabled the identification of novel genetic backgrounds. At present, management of congenital myopathies is largely supportive, although new therapeutic approaches are reaching the clinical trial stage.

The SarcoEndoplasmic Reticulum Calcium ATPase

The calcium pump (a.k.a. Ca²⁺-ATPase or SERCA) is a membrane transport protein ubiquitously found in the endoplasmic reticulum (ER) of all eukaryotic cells. As a calcium transporter, SERCA maintains the low cytosolic calcium level that enables a vast array of signaling pathways and physiological processes (e.g. synaptic transmission, muscle contraction, fertilization). In muscle cells, SERCA promotes relaxation by pumping calcium ions from the cytosol into the lumen of the sarcoplasmic reticulum (SR), the main storage compartment for intracellular calcium. X-ray crystallographic studies have provided an extensive understanding of the intermediate states that SERCA populates as it progresses through the calcium transport cycle. Historically, SERCA is also known to be regulated by small transmembrane peptides, phospholamban (PLN) and sarcolipin (SLN). PLN is expressed in cardiac muscle, whereas SLN predominates in skeletal and atrial muscle. These two regulatory subunits play critical roles in cardiac contractility. While our understanding of these regulatory mechanisms are still developing, SERCA and PLN are one of the best understood examples of peptide-transporter regulatory interactions. Nonetheless, SERCA appeared to have only two regulatory subunits, while the related sodium pump (a.k.a. Na⁺, K⁺-ATPase) has at least nine small transmembrane peptides that provide tissue specific regulation. The last few years have seen a renaissance in our understanding of SERCA regulatory subunits. First, structures of the SERCA-SLN and SERCA-PLN complexes revealed molecular details of their interactions. Second, an array of micropeptides concealed within long non-coding RNAs have been identified as new SERCA regulators. This chapter will describe our current understanding of SERCA structure, function, and regulation.

Calcium signaling at the endoplasmic reticulum: fine-tuning stress responses

Endoplasmic reticulum (ER) calcium signaling is implicated in a myriad of coordinated cellular processes. The ER calcium content is tightly regulated as it allows a favorable environment for protein folding, in addition to operate as a major reservoir for fast and specific release of calcium. Altered ER homeostasis impacts protein folding, activating the unfolded protein response (UPR) as a rescue mechanism to restore proteostasis. ER calcium release impacts mitochondrial metabolism and also fine-tunes the threshold to undergo apoptosis under chronic stress. The global coordination between UPR signaling and energetic demands takes place at mitochondrial associated membranes (MAMs), specialized subdomains mediating interorganelle communication. Here we discuss current models explaining the functional relationship between ER homeostasis and various cellular responses to coordinate proteostasis and metabolic maintenance.

Interactions between small ankyrin 1 and sarcolipin coordinately regulate activity of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA1)

SERCA1, the sarco(endo)plasmic reticulum Ca2+-ATPase of skeletal muscle, is essential for muscle relaxation and maintenance of low resting Ca2+ levels in the myoplasm. We recently reported that small ankyrin 1 (sAnk1) interacts with the sarco(endo)plasmic reticulum Ca2+-ATPase in skeletal muscle (SERCA1) to inhibit its activity. We also showed that this interaction is mediated at least in part through sAnk1's transmembrane domain in a manner similar to that of sarcolipin (SLN). Earlier studies have shown that SLN and phospholamban, the other well studied small SERCA-regulatory proteins, oligomerize either alone or together. As sAnk1 is coexpressed with SLN in muscle, we sought to determine whether these two proteins interact with one another when coexpressed exogenously in COS7 cells. Coimmunoprecipitation (coIP) and anisotropy-based FRET (AFRET) assays confirmed this interaction. Our results indicated that sAnk1 and SLN can associate in the sarcoplasmic reticulum membrane and after exogenous expression in COS7 cells in vitro but that their association did not require endogenous SERCA2. Significantly, SLN promoted the interaction between sAnk1 and SERCA1 when the three proteins were coexpressed, and both coIP and AFRET experiments suggested the formation of a complex consisting of all three proteins. Ca2+-ATPase assays showed that sAnk1 ablated SLN's inhibition of SERCA1 activity. These results suggest that sAnk1 interacts with SLN both directly and in complex with SERCA1 and reduces SLN's inhibitory effect on SERCA1 activity.

Proteomic-Biostatistic Integrated Approach for Finding the Underlying Molecular Determinants of Hypertension in Human Plasma

Despite advancements in lowering blood pressure, the best approach to lower it remains controversial because of the lack of information on the molecular basis of hypertension. We, therefore, performed plasma proteomics of plasma from patients with hypertension to identify molecular determinants detectable in these subjects but not in controls and vice versa. Plasma samples from hypertensive subjects (cases; n=118) and controls (n=85) from the InGenious HyperCare cohort were used for this study and performed mass spectrometric analysis. Using biostatistical methods, plasma peptides specific for hypertension were identified, and a model was developed using least absolute shrinkage and selection operator logistic regression. The underlying peptides were identified and sequenced off-line using matrix-assisted laser desorption ionization orbitrap mass spectrometry. By comparison of the molecular composition of the plasma samples, 27 molecular determinants were identified differently expressed in cases from controls. Seventy percent of the molecular determinants selected were found to occur less likely in hypertensive patients. In cross-validation, the overall R² was 0.434, and the area under the curve was 0.891 with 95% confidence interval 0.8482 to 0.9349, P<0.0001. The mean values of the cross-validated proteomic score of normotensive and hypertensive patients were found to be -2.007±0.3568 and 3.383±0.2643, respectively, P<0.0001. The molecular determinants were successfully identified, and the proteomic model developed shows an excellent discriminatory ability between hypertensives and normotensives. The identified molecular determinants may be the starting point for further studies to clarify the molecular causes of hypertension.

Regulation of Calcium Homeostasis by ER Redox: A Close-Up of the ER/Mitochondria Connection

Calcium signaling plays an important role in cell survival by influencing mitochondria-related processes such as energy production and apoptosis. The endoplasmic reticulum (ER) is the main storage compartment for cell calcium (Ca²⁺; ~60-500μM), and the Ca²⁺ released by the ER has a prompt effect on the homeostasis of the juxtaposed mitochondria. Recent findings have highlighted a close connection between ER redox and Ca²⁺ signaling that is mediated by Ca²⁺-handling proteins. This paper describes the redox-regulated mediators and mechanisms that orchestrate Ca²⁺ signals from the ER to mitochondria.

Widespread control of calcium signaling by a family of SERCA-inhibiting micropeptides

Micropeptides function as master regulators of calcium-dependent signaling in muscle. Sarco/endoplasmic reticulum Ca²⁺ ATPase (SERCA), the membrane pump that promotes muscle relaxation by taking up Ca²⁺ into the sarcoplasmic reticulum, is directly inhibited by three muscle-specific micropeptides: myoregulin (MLN), phospholamban (PLN), and sarcolipin (SLN). The widespread and essential function of SERCA across diverse cell types has raised questions as to how SERCA is regulated in cells that lack MLN, PLN, and SLN. We identified two transmembrane micropeptides, endoregulin (ELN) and another-regulin (ALN), that share key amino acids with their muscle-specific counterparts and function as direct inhibitors of SERCA pump activity. The distribution of transcripts encoding ELN and ALN mirrored that of SERCA isoform-encoding transcripts in nonmuscle cell types. Our findings identify additional members of the SERCA-inhibitory micropeptide family, revealing a conserved mechanism for the control of intracellular Ca²⁺ dynamics in both muscle and nonmuscle cell types.

Disulfide-activated protein kinase G Iα regulates cardiac diastolic relaxation and fine-tunes the Frank-Starling response

The Frank–Starling mechanism allows the amount of blood entering the heart from the veins to be precisely matched with the amount pumped out to the arterial circulation. As the heart fills with blood during diastole, the myocardium is stretched and oxidants are produced. Here we show that protein kinase G Iα (PKGIα) is oxidant-activated during stretch and this form of the kinase selectively phosphorylates cardiac phospholamban Ser16—a site important for diastolic relaxation. We find that hearts of Cys42Ser PKGIα knock-in (KI) mice, which are resistant to PKGIα oxidation, have diastolic dysfunction and a diminished ability to couple ventricular filling with cardiac output on a beat-to-beat basis. Intracellular calcium dynamics of ventricular myocytes isolated from KI hearts are altered in a manner consistent with impaired relaxation and contractile function. We conclude that oxidation of PKGIα during myocardial stretch is crucial for diastolic relaxation and fine-tunes the Frank–Starling response.

The stroke volume of the heart increases in response to an increase in the blood volume filling the heart. Here the authors reveal that this coordinated process is mediated in part by oxidative activation of the protein kinase G Iα, which phosphorylates phospholamban to enhance diastolic relaxation in mice.

Inhibitory action of linoleamide and oleamide toward sarco/endoplasmic reticulum Ca2+-ATPase

BACKGROUND

SERCA maintains intracellular Ca²⁺ homeostasis by sequestering cytosolic Ca²⁺ into SR/ER stores. Two primary fatty acid amides (PFAAs), oleamide (18:1^9-cis) and linoleamide (18:2^9,12-cis), induce an increase in intracellular Ca²⁺ levels, which might be caused by their inhibition of SERCA.

METHODS

Three major SERCA isoforms, rSERCA1a, hSERCA2b, and hSERCA3a, were individually overexpressed in COS-1 cells, and the inhibitory action of PFAAs on Ca²⁺-ATPase activity of SERCA was examined.

RESULTS

The Ca²⁺-ATPase activity of each SERCA was inhibited in a concentration-dependent manner strongly by linoleamide (IC₅₀ 15-53μM) and partially by oleamide (IC₅₀ 8.3-34μM). Inhibition by other PFAAs, such as stearamide (18:0) and elaidamide (18:1^9-trans), was hardly or slightly observed. With increasing dose, linoleamide decreased the apparent affinity for Ca²⁺ and the apparent maximum velocity of Ca²⁺-ATPase activity of all SERCAs tested. Oleamide also lowered these values for hSERCA3a. Meanwhile, oleamide uniquely reduced the apparent Ca²⁺ affinity of rSERCA1a and hSERCA2b: the reduction was considerably attenuated above certain concentrations of oleamide. The dissociation constants for SERCA interaction varied from 6 to 45μM in linoleamide and from 1.6 to 55μM in oleamide depending on the isoform.

CONCLUSIONS

Linoleamide and oleamide inhibit SERCA activity in the micromolar concentration range, and in a different manner. Both amides mainly suppress SERCA activity by lowering the Ca²⁺ affinity of the enzyme.

GENERAL SIGNIFICANCE

Our findings imply a novel role of these PFAAs as modulators of intracellular Ca²⁺ homeostasis via regulation of SERCA activity.

Genomic correction of familial cardiomyopathy in human engineered cardiac tissues

In this study, we used three-dimensional human engineered cardiac tissue technology to directly show that phospholamban (PLN) R14del mutation impairs cardiac contractility and to demonstrate restoration of contractile properties with targeted genetic correction of this inheritable form of dilated cardiomyopathy.

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

The physiological process by which Ca²⁺ is released from the sarcoplasmic reticulum is called excitation-contraction coupling; it is initiated by an action potential which travels deep into the muscle fiber where it is sensed by the dihydropyridine receptor, a voltage sensing L-type Ca²⁺channel localized on the transverse tubules. Voltage-induced conformational changes in the dihydropyridine receptor activate the ryanodine receptor Ca²⁺ release channel of the sarcoplasmic reticulum. The released Ca²⁺ binds to troponin C, enabling contractile thick-thin filament interactions. The Ca²⁺ is subsequently transported back into the sarcoplasmic reticulum by specialized Ca²⁺ pumps (SERCA), preparing the muscle for a new cycle of contraction. Although other proteins are involved in excitation-contraction coupling, the mechanism described above emphasizes the unique role played by the two Ca²⁺ channels (the dihydropyridine receptor and the ryanodine receptor), the SERCA Ca²⁺ pumps and the exquisite spatial organization of the membrane compartments endowed with the proteins responsible for this mechanism to function rapidly and efficiently. Research over the past two decades has uncovered the fine details of excitation-contraction coupling under normal conditions while advances in genomics have helped to identify mutations in novel genes in patients with neuromuscular disorders. While it is now clear that many patients with congenital muscle diseases carry mutations in genes encoding proteins directly involved in Ca²⁺ homeostasis, it has become apparent that mutations are also present in genes encoding for proteins not thought to be directly involved in Ca²⁺ regulation. Ongoing research in the field now focuses on understanding the functional effect of individual mutations, as well as understanding the role of proteins not specifically located in the sarcoplasmic reticulum which nevertheless are involved in Ca²⁺ regulation or excitation-contraction coupling. The principal challenge for the future is the identification of drug targets that can be pharmacologically manipulated by small molecules, with the ultimate aim to improve muscle function and quality of life of patients with congenital muscle disorders. The aim of this review is to give an overview of the most recent findings concerning Ca²⁺ dysregulation and its impact on muscle function in patients with congenital muscle disorders due to mutations in proteins involved in excitation-contraction coupling and more broadly on Ca²⁺ homeostasis.

Phospholamban degradation is induced by phosphorylation-mediated ubiquitination and inhibited by interaction with cardiac type Sarco(endo)plasmic reticulum Ca(2+)-ATPase

Phospholamban (PLN) regulates cardiac type sarco (endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) via Ser(16)-phosphorylation. During heart failure, PLN expression is downregulated with SERCA2a; however, the mechanism of its regulation is not fully understood. Phosphorylation triggers protein degradation and because PLN phosphorylation is upregulated in failing hearts, we examined whether PLN is degraded by Ser(16)-phosphorylation. Cells overexpressing PLN exhibited its degradation post isoproterenol (Iso), forskolin, or 3-isobutyl-1-methylxanthine (IBMX) addition. Moreover, this degradation was inhibited by a cAMP-dependent protein kinase (PKA) inhibitor--H89. Co-immunoprecipitation revealed that Lys(3) of PLN was oligo-ubiquitinated when ubiquitin was overexpressed, and was degraded by Iso treatment. However, when co-expressed with SERCA2a, oligo-ubiquitinated PLN at Lys(3) was not degraded by Iso treatment. In failing hearts from 16 week-old TgPLN(R9C) mice, oligo-ubiquitinated PLN levels increased and PLN expression was downregulated. Furthermore, SERCA2a mRNA levels in TgPLN(R9C) mice hearts were lower than that in wild type mice; however, PLN mRNA levels showed no changes. In another heart failure model, MG132 treatment reversed PLN degradation. These data suggest that PLN is, at least partially, oligo-ubiquitinated at Lys(3) and degraded through Ser(16)-phosphorylation-mediated poly-ubiquitination during heart failure.

The role of Ca(2+) signaling on the self-renewal and neural differentiation of embryonic stem cells (ESCs)

Embryonic stem cells (ESCs) are promising resources for both scientific research and clinical regenerative medicine. With regards to the latter, ESCs are especially useful for treating several neurodegenerative disorders. Two significant characteristics of ESCs, which make them so valuable, are their capacity for self-renewal and their pluripotency, both of which are regulated by the integration of various signaling pathways. Intracellular Ca(2+) signaling is involved in several of these pathways. It is known to be precisely controlled by different Ca(2+) channels and pumps, which play an important role in a variety of cellular activities, including proliferation, differentiation and apoptosis. Here, we provide a review of the recent work conducted to investigate the function of Ca(2+) signaling in the self-renewal and the neural differentiation of ESCs. Specifically, we describe the role of intracellular Ca(2+) mobilization mediated by RyRs (ryanodine receptors); by cADPR (cyclic adenosine 5'-diphosphate ribose) and CD38 (cluster of differentiation 38/cADPR hydrolase); and by NAADP (nicotinic acid adenine dinucleotide phosphate) and TPC2 (two pore channel 2). We also discuss the Ca(2+) influx mediated by SOCs (store-operated Ca(2+) channels), TRPCs (transient receptor potential cation channels) and LTCC (L-type Ca(2+) channels) in the pluripotent ESCs as well as in neural differentiation of ESCs. Moreover, we describe the integration of Ca(2+) signaling in the other signaling pathways that are known to regulate the fate of ESCs.

A peptide encoded by a transcript annotated as long noncoding RNA enhances SERCA activity in muscle

Muscle contraction depends on release of Ca(2+) from the sarcoplasmic reticulum (SR) and reuptake by the Ca(2+)adenosine triphosphatase SERCA. We discovered a putative muscle-specific long noncoding RNA that encodes a peptide of 34 amino acids and that we named dwarf open reading frame (DWORF). DWORF localizes to the SR membrane, where it enhances SERCA activity by displacing the SERCA inhibitors, phospholamban, sarcolipin, and myoregulin. In mice, overexpression of DWORF in cardiomyocytes increases peak Ca(2+) transient amplitude and SR Ca(2+) load while reducing the time constant of cytosolic Ca(2+) decay during each cycle of contraction-relaxation. Conversely, slow skeletal muscle lacking DWORF exhibits delayed Ca(2+) clearance and relaxation and reduced SERCA activity. DWORF is the only endogenous peptide known to activate the SERCA pump by physical interaction and provides a means for enhancing muscle contractility.

Maturing human pluripotent stem cell-derived cardiomyocytes in human engineered cardiac tissues

Engineering functional human cardiac tissue that mimics the native adult morphological and functional phenotype has been a long held objective. In the last 5 years, the field of cardiac tissue engineering has transitioned from cardiac tissues derived from various animal species to the production of the first generation of human engineered cardiac tissues (hECTs), due to recent advances in human stem cell biology. Despite this progress, the hECTs generated to date remain immature relative to the native adult myocardium. In this review, we focus on the maturation challenge in the context of hECTs, the present state of the art, and future perspectives in terms of regenerative medicine, drug discovery, preclinical safety testing and pathophysiological studies.

Calcium Uptake in Crude Tissue Preparation

The various isoforms of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) are responsible for the Ca(2+) uptake from the cytosol into the endoplasmic or sarcoplasmic reticulum (ER/SR). In some tissues, the activity of SERCA can be modulated by binding partners, such as phospholamban and sarcolipin. The activity of SERCA can be characterized by its apparent affinity for Ca(2+) as well as maximal enzymatic velocity. Both parameters can be effectively determined by the protocol described here. Specifically, we describe the measurement of the rate of oxalate-facilitated (45)Ca uptake into the SR of crude mouse ventricular homogenates. This protocol can easily be adapted for different tissues and animal models as well as cultured cells.

Identification of Small Ankyrin 1 as a Novel Sarco(endo)plasmic Reticulum Ca2+-ATPase 1 (SERCA1) Regulatory Protein in Skeletal Muscle

Small ankyrin 1 (sAnk1) is a 17-kDa transmembrane (TM) protein that binds to the cytoskeletal protein, obscurin, and stabilizes the network sarcoplasmic reticulum in skeletal muscle. We report that sAnk1 shares homology in its TM amino acid sequence with sarcolipin, a small protein inhibitor of the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Here we investigate whether sAnk1 and SERCA1 interact. Our results indicate that sAnk1 interacts specifically with SERCA1 in sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle, and in COS7 cells transfected to express these proteins. This interaction was demonstrated by co-immunoprecipitation and an anisotropy-based FRET method. Binding was reduced ~2-fold by the replacement of all of the TM amino acids of sAnk1 with leucines by mutagenesis. This suggests that, like sarcolipin, sAnk1 interacts with SERCA1 at least in part via its TM domain. Binding of the cytoplasmic domain of sAnk1 to SERCA1 was also detected in vitro. ATPase activity assays show that co-expression of sAnk1 with SERCA1 leads to a reduction of the apparent Ca(2+) affinity of SERCA1 but that the effect of sAnk1 is less than that of sarcolipin. The sAnk1 TM mutant has no effect on SERCA1 activity. Our results suggest that sAnk1 interacts with SERCA1 through its TM and cytoplasmic domains to regulate SERCA1 activity and modulate sequestration of Ca(2+) in the sarcoplasmic reticulum lumen. The identification of sAnk1 as a novel regulator of SERCA1 has significant implications for muscle physiology and the development of therapeutic approaches to treat heart failure and muscular dystrophies linked to Ca(2+) misregulation.

A micropeptide encoded by a putative long noncoding RNA regulates muscle performance

Functional micropeptides can be concealed within RNAs that appear to be noncoding. We discovered a conserved micropeptide, which we named myoregulin (MLN), encoded by a skeletal muscle-specific RNA annotated as a putative long noncoding RNA. MLN shares structural and functional similarity with phospholamban (PLN) and sarcolipin (SLN), which inhibit SERCA, the membrane pump that controls muscle relaxation by regulating Ca(2+) uptake into the sarcoplasmic reticulum (SR). MLN interacts directly with SERCA and impedes Ca(2+) uptake into the SR. In contrast to PLN and SLN, which are expressed in cardiac and slow skeletal muscle in mice, MLN is robustly expressed in all skeletal muscle. Genetic deletion of MLN in mice enhances Ca(2+) handling in skeletal muscle and improves exercise performance. These findings identify MLN as an important regulator of skeletal muscle physiology and highlight the possibility that additional micropeptides are encoded in the many RNAs currently annotated as noncoding.