Ole Mathias Sejersted

• Lindinger, Michael I.; Cairns, Simeon P. & Sejersted, Ole Mathias (2024). Resting membrane potential and intracellular [Na⁺] at rest, during fatigue and during recovery in rat soleus muscle fibres in situ. Journal of Physiology. ISSN 0022-3751. 602(14), p. 3469–3487. doi: 10.1113/JP285870. Full text in Research Archive
• Li, Jia; Sundnes, Joakim; Hou, Yufeng; Laasmaa, Martin; Ruud, Marianne & Unger, Andreas [Show all 19 contributors for this article] (2023). Stretch Harmonizes Sarcomere Strain Across the Cardiomyocyte. Circulation Research. ISSN 0009-7330. 133(3), p. 255–270. doi: 10.1161/CIRCRESAHA.123.322588. Full text in Research Archive Show summary

  BACKGROUND: Increasing cardiomyocyte contraction during myocardial stretch serves as the basis for the Frank-Starling mechanism in the heart. However, it remains unclear how this phenomenon occurs regionally within cardiomyocytes, at the level of individual sarcomeres. We investigated sarcomere contractile synchrony and how intersarcomere dynamics contribute to increasing contractility during cell lengthening. METHODS: Sarcomere strain and Ca2+ were simultaneously recorded in isolated left ventricular cardiomyocytes during 1 Hz field stimulation at 37 °C, at resting length and following stepwise stretch. RESULTS: We observed that in unstretched rat cardiomyocytes, differential sarcomere deformation occurred during each beat. Specifically, while most sarcomeres shortened during the stimulus, ≈10% to 20% of sarcomeres were stretched or remained stationary. This nonuniform strain was not traced to regional Ca2+ disparities but rather shorter resting lengths and lower force production in systolically stretched sarcomeres. Lengthening of the cell recruited additional shortening sarcomeres, which increased contractile efficiency as less negative, wasted work was performed by stretched sarcomeres. Given the known role of titin in setting sarcomere dimensions, we next hypothesized that modulating titin expression would alter intersarcomere dynamics. Indeed, in cardiomyocytes from mice with titin haploinsufficiency, we observed greater variability in resting sarcomere length, lower recruitment of shortening sarcomeres, and impaired work performance during cell lengthening. CONCLUSIONS: Graded sarcomere recruitment directs cardiomyocyte work performance, and harmonization of sarcomere strain increases contractility during cell stretch. By setting sarcomere dimensions, titin controls sarcomere recruitment, and its lowered expression in haploinsufficiency mutations impairs cardiomyocyte contractility.

• Edwards, Andrew; Mørk, Halvor; Stokke, Mathis; Lipsett, David; Sjaastad, Ivar & Richard, Sylvain [Show all 8 contributors for this article] (2021). Sarcoplasmic Reticulum Calcium Release Is Required for Arrhythmogenesis in the MouseData_Sheet_1.pdf. Frontiers in Physiology. 129247113307196118101757330290951073619410426311810310255549995881112965921565931388107972921061031069987209102119109419102782. doi: 10.3389/fphys.2021.74473010.3389/fphys.2021.744730.s001. Full text in Research Archive
• Wanichawan, Pimthanya; Skogestad, Jonas; Lunde, Marianne; Støle, Thea Parsberg; Stensland, Maria & Nyman, Tuula Anneli [Show all 10 contributors for this article] (2021). Design of a proteolytically stable sodium-calcium exchanger 1 activator peptide for in vivo studies. Frontiers in Pharmacology. 12(638646). doi: 10.3389/fphar.2021.638646. Full text in Research Archive Show summary

  The cardiac sodium–calcium exchanger (NCX1) is important for normal Na + - and Ca 2+ -homeostasis and cardiomyocyte relaxation and contraction. It has been suggested that NCX1 activity is reduced by phosphorylated phospholemman (pSer68-PLM); however its direct interaction with PLM is debated. Disruption of the potentially inhibitory pSer68-PLM-NCX1 interaction might be a therapeutic strategy to increase NCX1 activity in cardiac disease. In the present study, we aimed to analyze the binding affinities and kinetics of the PLM-NCX1 and pSer68-PLM-NCX1 interactions by surface plasmon resonance (SPR) and to develop a proteolytically stable NCX1 activator peptide for future in vivo studies. The cytoplasmic parts of PLM (PLM cyt ) and pSer68-PLM (pSer68-PLM cyt ) were found to bind strongly to the intracellular loop of NCX1 (NCX1 cyt ) with similar K D values of 4.1 ± 1.0 nM and 4.3 ± 1.9 nM, but the PLM cyt -NCX1 cyt interaction showed higher on/off rates. To develop a proteolytically stable NCX1 activator, we took advantage of a previously designed, high-affinity PLM binding peptide (OPT) that was derived from the PLM binding region in NCX1 and that reverses the inhibitory PLM (S68D)-NCX1 interaction in HEK293. We performed N- and C-terminal truncations of OPT and identified PYKEIEQLIELANYQV as the minimum sequence required for pSer68-PLM binding. To increase peptide stability in human serum, we replaced the proline with an N-methyl-proline (NOPT) after identification of N-terminus as substitution tolerant by two-dimensional peptide array analysis. Mass spectrometry analysis revealed that the half-life of NOPT was increased 17-fold from that of OPT. NOPT pulled down endogenous PLM from rat left ventricle lysate and exhibited direct pSer68-PLM binding in an ELISA-based assay and bound to pSer68-PLM cyt with a K D of 129 nM. Excess NOPT also reduced the PLM cyt -NCX1 cyt interaction in an ELISA-based competition assay, but in line with that NCX1 and PLM form oligomers, NOPT was not able to outcompete the physical interaction between endogenous full length proteins. Importantly, cell-permeable NOPT-TAT increased NCX1 activity in cardiomyocytes isolated from both SHAM-operated and aorta banded heart failure (HF) mice, indicating that NOPT disrupted the inhibitory pSer68-PLM-NCX1 interaction. In conclusion, we have developed a proteolytically stable NCX1-derived PLM binding peptide that upregulates NCX1 activity in SHAM and HF cardiomyocytes.

• Edwards, Andrew G.; Mørk, Halvor Kjeang; Stokke, Mathis Korseberg; Lipsett, David Bruce; Sjaastad, Ivar & Richard, Sylvain [Show all 8 contributors for this article] (2021). Sarcoplasmic Reticulum Calcium Release Is Required for Arrhythmogenesis in the Mouse. Frontiers in Physiology. 12, p. 1–13. doi: 10.3389/fphys.2021.744730. Full text in Research Archive Show summary

  Dysfunctional sarcoplasmic reticulum Ca 2+ handling is commonly observed in heart failure, and thought to contribute to arrhythmogenesis through several mechanisms. Some time ago we developed a cardiomyocyte-specific inducible SERCA2 knockout mouse, which is remarkable in the degree to which major adaptations to sarcolemmal Ca 2+ entry and efflux overcome the deficit in SR reuptake to permit relatively normal contractile function. Conventionally, those adaptations would also be expected to dramatically increase arrhythmia susceptibility. However, that susceptibility has never been tested, and it is possible that the very rapid repolarization of the murine action potential (AP) allows for large changes in sarcolemmal Ca 2+ transport without substantially disrupting electrophysiologic stability. We investigated this hypothesis through telemetric ECG recording in the SERCA2-KO mouse, and patch-clamp electrophysiology, Ca 2+ imaging, and mathematical modeling of isolated SERCA2-KO myocytes. While the SERCA2-KO animals exhibit major (and unique) electrophysiologic adaptations at both the organ and cell levels, they remain resistant to arrhythmia. A marked increase in peak L-type calcium ( I CaL ) current and slowed I CaL decay elicited pronounced prolongation of initial repolarization, but faster late repolarization normalizes overall AP duration. Early afterdepolarizations were seldom observed in KO animals, and those that were observed exhibited a mechanism intermediate between murine and large mammal dynamical properties. As expected, spontaneous SR Ca 2+ sparks and waves were virtually absent. Together these findings suggest that intact SR Ca 2+ handling is an absolute requirement for triggered arrhythmia in the mouse, and that in its absence, dramatic changes to the major inward currents can be resisted by the substantial K + current reserve, even at end-stage disease.

• Carlson, Cathrine Rein; Aronsen, Jan Magnus; Dahl, Anna Bergan; Moutty, Marie Christine; Lunde, Marianne & Lunde, Per Kristian [Show all 22 contributors for this article] (2021). AKAP18δ Anchors and Regulates CaMKII Activity at Phospholamban-SERCA2 and RYR. Circulation Research. ISSN 0009-7330. 130(1), p. 27–44. doi: 10.1161/CIRCRESAHA.120.317976. Full text in Research Archive Show summary

  Background: The sarcoplasmic reticulum (SR) Ca2+-ATPase 2 (SERCA2) mediates Ca2+ reuptake into SR and thereby promotes cardiomyocyte relaxation, whereas the ryanodine receptor (RYR) mediates Ca2+ release from SR and triggers contraction. Ca2+/CaMKII (CaM [calmodulin]-dependent protein kinase II) regulates activities of SERCA2 through phosphorylation of PLN (phospholamban) and RYR through direct phosphorylation. However, the mechanisms for CaMKIIδ anchoring to SERCA2-PLN and RYR and its regulation by local Ca2+ signals remain elusive. The objective of this study was to investigate CaMKIIδ anchoring and regulation at SERCA2-PLN and RYR. Methods: A role for AKAP18δ (A-kinase anchoring protein 18δ) in CaMKIIδ anchoring and regulation was analyzed by bioinformatics, peptide arrays, cell-permeant peptide technology, immunoprecipitations, pull downs, transfections, immunoblotting, proximity ligation, FRET-based CaMKII activity and ELISA-based assays, whole cell and SR vesicle fluorescence imaging, high-resolution microscopy, adenovirus transduction, adenoassociated virus injection, structural modeling, surface plasmon resonance, and alpha screen technology. Results: Our results show that AKAP18δ anchors and directly regulates CaMKIIδ activity at SERCA2-PLN and RYR, via 2 distinct AKAP18δ regions. An N-terminal region (AKAP18δ-N) inhibited CaMKIIδ through binding of a region homologous to the natural CaMKII inhibitor peptide and the Thr17-PLN region. AKAP18δ-N also bound CaM, introducing a second level of control. Conversely, AKAP18δ-C, which shares homology to neuronal CaMKIIα activator peptide (N2B-s), activated CaMKIIδ by lowering the apparent Ca2+ threshold for kinase activation and inducing CaM trapping. While AKAP18δ-C facilitated faster Ca2+ reuptake by SERCA2 and Ca2+ release through RYR, AKAP18δ-N had opposite effects. We propose a model where the 2 unique AKAP18δ regions fine-tune Ca2+-frequency-dependent activation of CaMKIIδ at SERCA2-PLN and RYR. Conclusions: AKAP18δ anchors and functionally regulates CaMKII activity at PLN-SERCA2 and RYR, indicating a crucial role of AKAP18δ in regulation of the heartbeat. To our knowledge, this is the first protein shown to enhance CaMKII activity in heart and also the first AKAP (A-kinase anchoring protein) reported to anchor a CaMKII isoform, defining AKAP18δ also as a CaM-KAP.

• Danielsen, Tore Kristian; Sadredini, Mani; Manotheepan, Ravinea; Aronsen, Jan Magnus; Frisk, Michael & Haugsten, Marie Hansen [Show all 13 contributors for this article] (2021). Exercise training stabilizes RyR2-dependent Ca2+ release in post-infarction heart failure. Frontiers in Cardiovascular Medicine. 7. doi: 10.3389/fcvm.2020.623922. Full text in Research Archive Show summary

  Aim: Dysfunction of the cardiac ryanodine receptor (RyR2) is an almost ubiquitous finding in animal models of heart failure (HF) and results in abnormal Ca 2+ release in cardiomyocytes that contributes to contractile impairment and arrhythmias. We tested whether exercise training (ET), as recommended by current guidelines, had the potential to stabilize RyR2-dependent Ca 2+ release in rats with post-myocardial infarction HF. Materials and Methods: We subjected male Wistar rats to left coronary artery ligation or sham operations. After 1 week, animals were characterized by echocardiography and randomized to high-intensity interval ET on treadmills or to sedentary behavior (SED). Running speed was adjusted based on a weekly VO 2max test. We repeated echocardiography after 5 weeks of ET and harvested left ventricular cardiomyocytes for analysis of RyR2-dependent systolic and spontaneous Ca 2+ release. Phosphoproteins were analyzed by Western blotting, and beta-adrenoceptor density was quantified by radioligand binding. Results: ET increased VO 2max in HF-ET rats to 127% of HF-SED ( P < 0.05). This coincided with attenuated spontaneous SR Ca 2+ release in left ventricular cardiomyocytes from HF-ET but also reduced Ca 2+ transient amplitude and slowed Ca 2+ reuptake during adrenoceptor activation. However, ventricular diameter and fractional shortening were unaffected by ET. Analysis of Ca 2+ homeostasis and major proteins involved in the regulation of SR Ca 2+ release and reuptake could not explain the attenuated spontaneous SR Ca 2+ release or reduced Ca 2+ transient amplitude. Importantly, measurements of beta-adrenoceptors showed a normalization of beta 1 -adrenoceptor density and beta 1 :beta 2 -adrenoceptor ratio in HF-ET. Conclusion: ET increased aerobic capacity in post-myocardial infarction HF rats and stabilized RyR2-dependent Ca 2+ release. Our data show that these effects of ET can be gained without major alterations in SR Ca 2+ regulatory proteins and indicate that future studies should include upstream parts of the sympathetic signaling pathway.

• Tazmini, Kiarash; Frisk, Michael; Lewalle, Alexandre; Laasmaa, Martin; Morotti, Stefano & Lipsett, David Bruce [Show all 16 contributors for this article] (2020). Hypokalemia promotes arrhythmia by distinct mechanisms in atrial and ventricular myocytes. Circulation Research. ISSN 0009-7330. 126(7), p. 889–906. doi: 10.1161/CIRCRESAHA.119.315641. Full text in Research Archive Show summary

  Rationale: Hypokalemia occurs in up to 20% of hospitalized patients and is associated with increased incidence of ventricular and atrial fibrillation. It is unclear whether these differing types of arrhythmia result from direct and perhaps distinct effects of hypokalemia on cardiomyocytes. Objective: To investigate proarrhythmic mechanisms of hypokalemia in ventricular and atrial myocytes. Methods and Results: Experiments were performed in isolated rat myocytes exposed to simulated hypokalemia conditions (reduction of extracellular [K+] from 5.0 to 2.7 mmol/L) and supported by mathematical modeling studies. Ventricular cells subjected to hypokalemia exhibited Ca2+ overload and increased generation of both spontaneous Ca2+ waves and delayed afterdepolarizations. However, similar Ca2+-dependent spontaneous activity during hypokalemia was only observed in a minority of atrial cells that were observed to contain t-tubules. This effect was attributed to close functional pairing of the Na+-K+ ATPase and Na+-Ca2+ exchanger proteins within these structures, as reduction in Na+ pump activity locally inhibited Ca2+ extrusion. Ventricular myocytes and tubulated atrial myocytes additionally exhibited early afterdepolarizations during hypokalemia, associated with Ca2+ overload. However, early afterdepolarizations also occurred in untubulated atrial cells, despite Ca2+ quiescence. These phase-3 early afterdepolarizations were rather linked to reactivation of nonequilibrium Na+ current, as they were rapidly blocked by tetrodotoxin. Na+ current-driven early afterdepolarizations in untubulated atrial cells were enabled by membrane hyperpolarization during hypokalemia and short action potential configurations. Brief action potentials were in turn maintained by ultra-rapid K+ current (IKur); a current which was found to be absent in tubulated atrial myocytes and ventricular myocytes. Conclusions: Distinct mechanisms underlie hypokalemia-induced arrhythmia in the ventricle and atrium but also vary between atrial myocytes depending on subcellular structure and electrophysiology.

• Røe, Åsmund Treu; Ruud, Marianne; Espe, Emil Knut Stenersen; Manfra, Ornella; Longobardi, S & Aronsen, Jan Magnus [Show all 16 contributors for this article] (2019). Regional diastolic dysfunction in post-infarction heart failure: role of local mechanical load and SERCA expression. Cardiovascular Research (CVR). ISSN 0008-6363. 115(4), p. 752–764. doi: 10.1093/cvr/cvy257. Full text in Research Archive Show summary

  Aims: Regional heterogeneities in contraction contribute to heart failure with reduced ejection fraction (HFrEF). We aimed to determine whether regional changes in myocardial relaxation similarly contribute to diastolic dysfunction in post-infarction HFrEF, and to elucidate the underlying mechanisms. Methods and results: Using the magnetic resonance imaging phase-contrast technique, we examined local diastolic function in a rat model of post-infarction HFrEF. In comparison with sham-operated animals, post-infarction HFrEF rats exhibited reduced diastolic strain rate adjacent to the scar, but not in remote regions of the myocardium. Removal of Ca2+ within cardiomyocytes governs relaxation, and we indeed found that Ca2+ transients declined more slowly in cells isolated from the adjacent region. Resting Ca2+ levels in adjacent zone myocytes were also markedly elevated at high pacing rates. Impaired Ca2+ removal was attributed to a reduced rate of Ca2+ sequestration into the sarcoplasmic reticulum (SR), due to decreased local expression of the SR Ca2+ ATPase (SERCA). Wall stress was elevated in the adjacent region. Using ex vivo experiments with loaded papillary muscles, we demonstrated that high mechanical stress is directly linked to SERCA down-regulation and slowing of relaxation. Finally, we confirmed that regional diastolic dysfunction is also present in human HFrEF patients. Using echocardiographic speckle-tracking of patients enrolled in the LEAF trial, we found that in comparison with controls, post-infarction HFrEF subjects exhibited reduced diastolic train rate adjacent to the scar, but not in remote regions of the myocardium. Conclusion: Our data indicate that relaxation varies across the heart in post-infarction HFrEF. Regional diastolic dysfunction in this condition is linked to elevated wall stress adjacent to the infarction, resulting in down-regulation of SERCA, disrupted diastolic Ca2+ handling, and local slowing of relaxation.

• Skogestad, Jonas; Aronsen, Jan Magnus; Tovsrud, Nils; Wanichawan, Pimthanya; Hougen, Karina & Stokke, Mathis Korseberg [Show all 10 contributors for this article] (2019). Coupling of the Na+/K+-ATPase to Ankyrin B controls Na+/Ca2+ exchanger activity in cardiomyocytes. Cardiovascular Research (CVR). ISSN 0008-6363. 116(1), p. 78–90. doi: 10.1093/cvr/cvz087. Full text in Research Archive
• Skogestad, Jonas; Lines, Glenn; Louch, William Edward; Sejersted, Ole M; Sjaastad, Ivar & Aronsen, Jan Magnus (2019). Evidence for heterogeneous subsarcolemmal Na+ Levels in rat ventricular myocytes. American Journal of Physiology. Heart and Circulatory Physiology. ISSN 0363-6135. 316(5). doi: 10.1152/ajpheart.00637.2018. Full text in Research Archive
• Kolstad, Terje; Brink, Jonas van den; MacQuaide, Niall; Lunde, Per; Frisk, Michael & Aronsen, Jan [Show all 12 contributors for this article] (2018). Ryanodine Receptor Dispersion Disrupts Ca2+-Release in Failing Cardiac Myocytes. eLIFE. 7. doi: 10.7554/eLife.39427. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani; Leren, Ida Skrinde; Edwards, Andrew & Vincent, Kevin P. [Show all 11 contributors for this article] (2018). Arrhythmia initiation in catecholaminergic polymorphic ventricular tachycardia type 1 depends on both heart rate and sympathetic stimulation. PLOS ONE. 13:e0207100(11), p. 1–21. doi: 10.1371/journal.pone.0207100. Full text in Research Archive Show summary

  Aims: Catecholaminergic polymorphic ventricular tachycardia type 1 (CPVT1) predisposes to ventricular tachyarrhythmias (VTs) during high heart rates due to physical or psychological stress. The essential role of catecholaminergic effects on ventricular cardiomyocytes in this situation is well documented, but the importance of heart rate per se for arrhythmia initiation in CPVT1 is largely unexplored. Methods and results: Sixteen CPVT1 patients performed a bicycle stress-test. Occurrence of VT triggers, i.e. premature ventricular complexes (PVC), depended on high heart rate, with individual thresholds. Atrial pacing above the individual PVC threshold in three patients did not induce PVCs. The underlying mechanism for the clinical observation was explored using cardiomyocytes from mice with the RyR2-R2474S (RyR2-RS) mutation, which exhibit exercise-induced VTs. While rapid pacing increased the number of Ca2+ waves in both RyR2-RS and wild-type (p<0.05), β-adrenoceptor (βAR) stimulation induced more Ca2+ waves in RyR2-RS (p<0.05). Notably, Ca2+ waves occurred despite decreased sarcoplasmic reticulum (SR) Ca2+ content in RyR2-RS (p<0.05), suggesting increased cytosolic RyR2 Ca2+ sensitivity. A computational model of mouse ventricular cardiomyocyte electrophysiology reproduced the cellular CPVT1 phenotype when RyR2 Ca2+ sensitivity was increased. Importantly, diastolic fluctuations in phosphorylation of RyR2 and SR Ca2+ content determined Ca2+ wave initiation. These factors were modulated towards increased propensity for arrhythmia initiation by increased pacing rates, but even more by βAR stimulation. Conclusion: In CPVT1, VT propensity depends on individual heart rate thresholds for PVCs. Through converging data from clinical exercise stress-testing, cellular studies and computational modelling, we confirm the heart rate-independent pro-arrhythmic effects of βAR stimulation in CPVT1, but also identify an independent and synergistic contribution from effects of high heart rate.

• Kolstad, Terje R Selnes; Brink, Jonas van Den; Macquaide, Niall; Lunde, Per Kristian; Frisk, Michael & Aronsen, Jan Magnus [Show all 13 contributors for this article] (2018). Ryanodine receptor dispersion disrupts Ca2+ release in failing cardiac myocytes. eLIFE. 7. doi: 10.7554/eLife.39427. Full text in Research Archive Show summary

  Reduced cardiac contractility during heart failure (HF) is linked to impaired Ca2+ release from Ryanodine Receptors (RyRs). We investigated whether this deficit can be traced to nanoscale RyR reorganization. Using super-resolution imaging, we observed dispersion of RyR clusters in cardiomyocytes from post-infarction HF rats, resulting in more numerous, smaller clusters. Functional groupings of RyR clusters which produce Ca2+ sparks (Ca2+ release units, CRUs) also became less solid. An increased fraction of small CRUs in HF was linked to augmented ‘silent’ Ca2+ leak, not visible as sparks. Larger multi-cluster CRUs common in HF also exhibited low fidelity spark generation. When successfully triggered, sparks in failing cells displayed slow kinetics as Ca2+ spread across dispersed CRUs. During the action potential, these slow sparks protracted and desynchronized the overall Ca2+ transient. Thus, nanoscale RyR reorganization during HF augments Ca2+ leak and slows Ca2+ release kinetics, leading to weakened contraction in this disease.

• Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Eriksen, Morten; Sejersted, Ole M; Zhang, Lili & Sjaastad, Ivar (2017). Regional dysfunction after myocardial infarction in rats. Circulation Cardiovascular Imaging. ISSN 1941-9651. 10(9), p. e005997–e005997. doi: 10.1161/CIRCIMAGING.116.005997. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Wanichawan, Pimthanya; Manfra, Ornella; Souza, Gustavo Antonio De; Lunde, Marianne & Martinsen, Marita [Show all 9 contributors for this article] (2017). Mapping the in vitro interactome of cardiac sodium (Na+ )-calcium (Ca2+ ) exchanger 1 (NCX1). Proteomics. ISSN 1615-9853. 17(17-18). doi: 10.1002/pmic.201600417. Full text in Research Archive
• Røe, Åsmund Treu; Aronsen, Jan Magnus; Young-Halvorsen, Kristine Skårdal; Hamdani, Nazha; Linke, Wolfgang A. & Danielsen, Håvard Emil [Show all 9 contributors for this article] (2017). Increased passive stiffness promotes diastolic dysfunction despite improved Ca2+ handling during left ventricular concentric hypertrophy. Cardiovascular Research (CVR). ISSN 0008-6363. 113(10), p. 1161–1172. doi: 10.1093/cvr/cvx087. Full text in Research Archive
• Wanichawan, Pimthanya; Hodne, Kjetil; Hafver, Tandekile Lubelwana; Lunde, Marianne; Martinsen, Marita & Louch, William Edward [Show all 8 contributors for this article] (2016). Development of a high affinity peptide that prevents phospholemman (PLM) inhibition of the sodium-calcium exchanger 1 (NCX1). Biochemical Journal. ISSN 0264-6021. 473(15), p. 2413–2423. doi: 10.1042/BCJ20160465. Full text in Research Archive
• Logantha, Sunil Jit R.; Stokke, Mathis Korseberg; Atkinson, Andrew J.; Kharche, Sanjay R.; Parveen, Sajida & Saeed, Yawer [Show all 9 contributors for this article] (2016). Ca2+ -clock-dependent pacemaking in the sinus node is impaired in mice with a cardiac specific reduction in SERCA2 abundance. Frontiers in Physiology. 7:197. doi: 10.3389/fphys.2016.00197. Full text in Research Archive
• Hortemo, Kristin Halvorsen; Lunde, Per Kristian; Anonsen, Jan Haug; Kvaløy, Heidi; Munkvik, Morten & Rehn, Tommy Aune [Show all 10 contributors for this article] (2016). Exercise training increases protein O-GlcNAcylation in rat skeletal muscle. Physiological Reports. 4(18), p. e12896–e12896. doi: 10.14814/phy2.12896. Full text in Research Archive Show summary

  Protein O‐GlcNAcylation has emerged as an important intracellular signaling system with both physiological and pathophysiological functions, but the role of protein O‐GlcNAcylation in skeletal muscle remains elusive. In this study, we tested the hypothesis that protein O‐GlcNAcylation is a dynamic signaling system in skeletal muscle in exercise and disease. Immunoblotting showed different protein O‐GlcNAcylation pattern in the prototypical slow twitch soleus muscle compared to fast twitch EDL from rats, with greater O‐GlcNAcylation level in soleus associated with higher expression of the modulating enzymes O‐GlcNAc transferase (OGT), O‐GlcNAcase (OGA), and glutamine fructose‐6‐phosphate amidotransferase isoforms 1 and 2 (GFAT1, GFAT2). Six weeks of exercise training by treadmill running, but not an acute exercise bout, increased protein O‐GlcNAcylation in rat soleus and EDL. There was a striking increase in O‐GlcNAcylation of cytoplasmic proteins ~50 kDa in size that judged from mass spectrometry analysis could represent O‐GlcNAcylation of one or more key metabolic enzymes. This suggests that cytoplasmic O‐GlcNAc signaling is part of the training response. In contrast to exercise training, postinfarction heart failure (HF) in rats and humans did not affect skeletal muscle O‐GlcNAcylation level, indicating that aberrant O‐GlcNAcylation cannot explain the skeletal muscle dysfunction in HF. Human skeletal muscle displayed extensive protein O‐GlcNAcylation that by large mirrored the fiber‐type‐related O‐GlcNAcylation pattern in rats, suggesting O‐GlcNAcylation as an important signaling system also in human skeletal muscle.

• Hafver, Tandekile Lubelwana; Hodne, Kjetil; Wanichawan, Pimthanya; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Per Kristian [Show all 14 contributors for this article] (2016). Protein phosphatase 1c associated with the cardiac sodium calcium exchanger 1 regulates its activity by dephosphorylating Serine 68-phosphorylated phospholemman. Journal of Biological Chemistry. ISSN 0021-9258. 291(9), p. 4561–4579. doi: 10.1074/jbc.M115.677898. Full text in Research Archive Show summary

  The sodium (Na+)-calcium (Ca2+) exchanger 1 (NCX1) is an important regulator of intracellular Ca2+ homeostasis. Serine 68 phosphorylated phospholemman (pSer-68-PLM) inhibits NCX1 activity. In the context of Na+/K+ ATPase (NKA) regulation, pSer-68-PLM is dephosphorylated by protein phosphatase 1 (PP1). PP1 also associates with NCX1, however, the molecular basis of this association is unknown. In the present study, we aimed to analyze the mechanisms of PP1 targeting to the NCX1-pSer-68-PLM complex and hypothesized that a direct and functional NCX1-PP1 interaction is a prerequisite for pSer-68-PLM dephosphorylation. Using a variety of molecular techniques, we show that PP1 catalytic subunit (PP1c) co-localized, co-fractionated, and co-immunoprecipitated with NCX1 in rat cardiomyocytes, left ventricle lysates and HEK293 cells. Bioinformatic analysis, immunoprecipitations, mutagenesis, pull-down experiments and peptide arrays constrained PP1c anchoring to the K-I/V-F-F motif in the first Ca2+ binding domain (CBD1) in NCX1. This binding site is also partially in agreement with the extended PP1 binding motif; K-I/V-F-F-X5-8-Φ1Φ2-X8-9-R. The cytosolic loop of NCX1, containing the K-I/V-F-F motif, had no effect on PP1 activity in an in vitro assay. Dephosphorylation of pSer-68-PLM in HEK293 cells was not observed when NCX1 was absent, when the K-I/V-F-F motif was mutated or when the PLM and PP1c binding sites were separated (mimicking calpain cleavage of NCX1). Co-expression of PLM and NCX1 inhibited NCX1 current (both modes). Moreover, co-expression of PLM with NCX1-(F407P) (mutated K-I/V-F-F motif) resulted in NCX1 current being completely abolished. In conclusion, NCX1 is a substrate-specifying PP1c regulator protein, indirectly regulating NCX1 activity through pSer-68-PLM dephosphorylation.

• Frisk, Michael; Ruud, Marianne; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Røe, Åsmund Treu & Zhang, Lili [Show all 11 contributors for this article] (2016). Elevated ventricular wall stress disrupts cardiomyocyte t-tubule structure and calcium homeostasis. Cardiovascular Research (CVR). ISSN 0008-6363. doi: 10.1093/cvr/cvw111. Full text in Research Archive Show summary

  Aims Invaginations of the cellular membrane called t-tubules are essential for maintaining efficient excitation–contraction coupling in ventricular cardiomyocytes. Disruption of t-tubule structure during heart failure has been linked to dyssynchronous, slowed Ca2+ release and reduced power of the heartbeat. The underlying mechanism is, however, unknown. We presently investigated whether elevated ventricular wall stress triggers remodelling of t-tubule structure and function. Methods and results MRI and blood pressure measurements were employed to examine regional wall stress across the left ventricle of sham-operated and failing, post-infarction rat hearts. In failing hearts, elevated left ventricular diastolic pressure and ventricular dilation resulted in markedly increased wall stress, particularly in the thin-walled region proximal to the infarct. High wall stress in this proximal zone was associated with reduced expression of the dyadic anchor junctophilin-2 and disrupted cardiomyocyte t-tubular structure. Indeed, local wall stress measurements predicted t-tubule density across sham and failing hearts. Elevated wall stress and disrupted cardiomyocyte structure in the proximal zone were also associated with desynchronized Ca2+ release in cardiomyocytes and markedly reduced local contractility in vivo. A causative role of wall stress in promoting t-tubule remodelling was established by applying stretch to papillary muscles ex vivo under culture conditions. Loads comparable to wall stress levels observed in vivo in the proximal zone reduced expression of junctophilin-2 and promoted t-tubule loss. Conclusion Elevated wall stress reduces junctophilin-2 expression and disrupts t-tubule integrity, Ca2+ release, and contractile function. These findings provide new insight into the role of wall stress in promoting heart failure progression.

• Hortemo, Kristin Halvorsen; Aronsen, Jan Magnus; Lunde, Ida Gjervold; Sjaastad, Ivar; Lunde, Per Kristian & Sejersted, Ole M (2015). Exhausting treadmill running causes dephosphorylation of sMLC2 and reduced level of myofilament MLCK2 in slow twitch rat soleus muscle. Physiological Reports. 3(2). doi: 10.14814/phy2.12285. Full text in Research Archive Show summary

  Myosin light chain 2 (MLC2) is a small protein in the myosin complex, regulating muscle contractile function by modulating Ca2+ sensitivity of myofilaments. MLC2 can be modified by phosphorylation and O-GlcNAcylation, two reversible and dynamic posttranslational modifications. The slow isoform of MLC2 (sMLC2) is dephosphorylated in soleus muscle during in situ loaded shortening contractions, which correlates with reduction in shortening capacity. Here, we hypothesize that exhausting in vivo treadmill running induces dephosphorylation of MLC2 in slow twitch soleus, but not in fast twitch EDL muscle, and that there are reciprocal changes in MLC2 O-GlcNAcylation. At rest, both phosphorylation and O-GlcNAcylation of MLC2 were lower in slow than fast twitch muscles. One bout of exhausting treadmill running induced dephosphorylation of sMLC2 in soleus, paralleled by reduced levels of the kinase MLCK2 associated to myofilaments, suggesting that the acute reduction in phosphorylation is mediated by dissociation of MLCK2 from myofilaments. O-GlcNAcylation of MLC2 did not change significantly, and seems of limited importance in the regulation of MLC2 phosphorylation during in vivo running. After 6 weeks of treadmill running, the dephosphorylation of sMLC2 persisted in soleus along with reduction in MLCK2 both in myofilament- and total protein fraction. In EDL on the contrary, phosphorylation of MLC2 was not altered after one exercise bout or after 6 weeks of treadmill running. Thus, in contrast to fast twitch muscle, MLC2 dephosphorylation occurs in slow twitch muscle during in vivo exercise and may be linked to reduced myofilament-associated MLCK2 and reduced shortening capacity.

• Clancy, Colleen E; Chen-Izu, Ye; Bers, Donald M; Belardinelli, Luiz; Boyden, Penelope A & Csernoch, Laszlo [Show all 23 contributors for this article] (2015). Deranged sodium to sudden death. Journal of Physiology. ISSN 0022-3751. 593(6), p. 1331–1345. doi: 10.1113/jphysiol.2014.281204. Full text in Research Archive
• Aronsen, Jan Magnus; Skogestad, Jonas; Lewalle, Alexandre; Louch, William Edward; Hougen, Karina & Stokke, Mathis Korseberg [Show all 11 contributors for this article] (2015). Hypokalaemia induces Ca2+ overload and Ca2+ waves in ventricular myocytes by reducing Na+ ,K+-ATPase α2 activity. Journal of Physiology. ISSN 0022-3751. 593(6), p. 1509–1521. doi: 10.1113/jphysiol.2014.279893. Full text in Research Archive
• Boardman, Neoma Tove; Aronsen, Jan Magnus; Louch, William Edward; Sjaastad, Ivar; Willoch, Frode & Christensen, Geir Arve [Show all 8 contributors for this article] (2014). Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2. American Journal of Physiology. Heart and Circulatory Physiology. ISSN 0363-6135. 306(7), p. H1018–H1024. doi: 10.1152/ajpheart.00741.2013. Full text in Research Archive
• Boardman, Neoma Tove; Aronsen, Jan Magnus; Louch, William Edward; Sjaastad, Ivar; Willoch, Frode & Christensen, Geir Arve [Show all 8 contributors for this article] (2014). Impaired left ventricular mechanical and energetic function in mice after cardiomyocyte-specific excision of Serca2. American Journal of Physiology. Heart and Circulatory Physiology. ISSN 0363-6135. doi: 10.1152/ajpheart.00741.2013. Full text in Research Archive Show summary

  Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2 transports Ca2+ from the cytosol into the sarcoplasmic reticulum of cardiomyocytes and is essential for maintaining myocardial Ca2+ handling and thus the mechanical function of the heart. SERCA2 is a major ATP consumer in excitation-contraction coupling but is regarded to contribute to energetically efficient Ca2+ handling in the cardiomyocyte. Previous studies using cardiomyocyte-specific SERCA2 knockout (KO) mice have demonstrated that decreased SERCA2 activity reduces the Ca2+ transient amplitude and induces compensatory Ca2+ transport mechanisms that may lead to more inefficient Ca2+ transport. In this study, we examined the relationship between left ventricular (LV) function and myocardial O2 consumption (MV̇O2) in ex vivo hearts from SERCA2 KO mice to directly measure how SERCA2 elimination influences mechanical and energetic features of the heart. Ex vivo hearts from SERCA2 KO hearts developed mechanical dysfunction at 4 wk and demonstrated virtually no working capacity at 7 wk. In accordance with the reported reduction in Ca2+ transient amplitude in cardiomyocytes from SERCA2 KO mice, work-independent MV̇O2 was decreased due to a reduced energy cost of excitation-contraction coupling. As these hearts also showed a marked impairment in the efficiency of chemomechanical energy transduction (contractile efficiency, i.e, work-dependent MV̇O2), hearts from SERCA2 KO mice were found to be mechanically inefficient. This ex vivo evaluation of mechanical and energetic function in hearts from SERCA2 KO mice brings together findings from previous experimental and mathematical modeling-based studies and demonstrates that reduced SERCA2 activity not only leads to mechanical dysfunction but also to energetic dysfunction.

• Wanichawan, Pimthanya; Lubelwana, Tandekile; Hodne, Kjetil; Aronsen, Jan Magnus; Lunde, Ida Gjervold & Dalhus, Bjørn [Show all 13 contributors for this article] (2014). Molecular basis of calpain cleavage and inactivation of the sodium-calcium exchanger 1 in heart failure. Journal of Biological Chemistry. ISSN 0021-9258. 289(49), p. 33984–33998. doi: 10.1074/jbc.M114.602581. Full text in Research Archive Show summary

  Cardiac sodium (Na+)-calcium (Ca2+) exchanger 1 (NCX1) is central to the maintenance of normal Ca2+ homeostasis and contraction. Studies indicate that the Ca2+-activated protease calpain cleaves NCX1. We hypothesized that calpain is an important regulator of NCX1 in response to pressure overload and aimed to identify molecular mechanisms and functional consequences of calpain binding and cleavage of NCX1 in the heart. NCX1 full-length protein and a 75-kDa NCX1 fragment along with calpain were up-regulated in aortic stenosis patients and rats with heart failure. Patients with coronary artery disease and sham-operated rats were used as controls. Calpain co-localized, co-fractionated, and co-immunoprecipitated with NCX1 in rat cardiomyocytes and left ventricle lysate. Immunoprecipitations, pull-down experiments, and extensive use of peptide arrays indicated that calpain domain III anchored to the first Ca2+ binding domain in NCX1, whereas the calpain catalytic region bound to the catenin-like domain in NCX1. The use of bioinformatics, mutational analyses, a substrate competitor peptide, and a specific NCX1-Met369 antibody identified a novel calpain cleavage site at Met369. Engineering NCX1-Met369 into a tobacco etch virus protease cleavage site revealed that specific cleavage at Met369 inhibited NCX1 activity (both forward and reverse mode). Finally, a short peptide fragment containing the NCX1-Met369 cleavage site was modeled into the narrow active cleft of human calpain. Inhibition of NCX1 activity, such as we have observed here following calpain-induced NCX1 cleavage, might be beneficial in pathophysiological conditions where increased NCX1 activity contributes to cardiac dysfunction.

• Frisk, Michael; Koivumaki, Jussi; Norseng, Per Andreas; Maleckar, Mary; Sejersted, Ole M & Louch, William Edward (2014). Variable t-tubule organization and Ca2+ homeostasis across the atria. American Journal of Physiology. Heart and Circulatory Physiology. ISSN 0363-6135. 307(4), p. H609–H620. doi: 10.1152/ajpheart.00295.2014. Full text in Research Archive
• Louch, William Edward; Hake, Johan Elon; Mørk, Halvor Kjeang; Hougen, Karina; Skrbic, Biljana & Ursu, Daniel [Show all 9 contributors for this article] (2013). Slow Ca2+ sparks de-synchronize Ca2+ release in failing cardiomyocytes: Evidence for altered configuration of Ca2+ release units? Journal of Molecular and Cellular Cardiology. ISSN 0022-2828. 58, p. 41–52. doi: 10.1016/j.yjmcc.2013.01.014. Full text in Research Archive
• Hortemo, Kristin Halvorsen; Munkvik, Morten; Lunde, Per Kristian & Sejersted, Ole M (2013). Multiple Causes of Fatigue during Shortening Contractions in Rat Slow Twitch Skeletal Muscle. PLOS ONE. 8(8). doi: 10.1371/journal.pone.0071700. Full text in Research Archive Show summary

  Fatigue in muscles that shorten might have other causes than fatigue during isometric contractions, since both cross-bridge cycling and energy demand are different in the two exercise modes. While isometric contractions are extensively studied, the causes of fatigue in shortening contractions are poorly mapped. Here, we investigate fatigue mechanisms during shortening contractions in slow twitch skeletal muscle in near physiological conditions. Fatigue was induced in rat soleus muscles with maintained blood supply by in situ shortening contractions at 37°C. Muscles were stimulated repeatedly (1 s on/off at 30 Hz) for 15 min against a constant load, allowing the muscle to shorten and perform work. Fatigue and subsequent recovery was examined at 20 s, 100 s and 15 min exercise. The effects of prior exercise were investigated in a second exercise bout. Fatigue developed in three distinct phases. During the first 20 s the regulatory protein Myosin Light Chain-2 (slow isoform, MLC-2s) was rapidly dephosphorylated in parallel with reduced rate of force development and reduced shortening. In the second phase there was degradation of high-energy phosphates and accumulation of lactate, and these changes were related to slowing of muscle relengthening and relaxation, culminating at 100 s exercise. Slowing of relaxation was also associated with increased leak of calcium from the SR. During the third phase of exercise there was restoration of high-energy phosphates and elimination of lactate, and the slowing of relaxation disappeared, whereas dephosphorylation of MLC-2s and reduced shortening prevailed. Prior exercise improved relaxation parameters in a subsequent exercise bout, and we propose that this effect is a result of less accumulation of lactate due to more rapid onset of oxidative metabolism. The correlation between dephosphorylation of MLC-2s and reduced shortening was confirmed in various experimental settings, and we suggest MLC-2s as an important regulator of muscle shortening. Copyright 2013 Hortemo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License.

• Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Skrbic, Biljana; Skulberg, Vidar Magne; Schneider, Jürgen & Sejersted, Ole M [Show all 8 contributors for this article] (2013). Improved MR phase contrast velocimetry utilizing a novel nine-point balanced motion encoding scheme with increased robustness to eddy current effects. Magnetic Resonance in Medicine. ISSN 0740-3194. 69(1), p. 48–61. doi: 10.1002/mrm.24226. Full text in Research Archive
• Land, Sander; Louch, William Edward; Niederer, Steven A.; Aronsen, Jan Magnus; Christensen, Geir Arve & Sjaastad, Ivar [Show all 8 contributors for this article] (2013). Beta-Adrenergic Stimulation Maintains Cardiac Function in Serca2 Knockout Mice. Biophysical Journal. ISSN 0006-3495. 104(6), p. 1349–1356. doi: 10.1016/j.bpj.2013.01.042. Full text in Research Archive
• Stokke, Mathis Korseberg; Tovsrud, Nils; Louch, William Edward; Øyehaug, Leiv; Hougen, Karina & Sejersted, Ole M [Show all 8 contributors for this article] (2013). I-CaL inhibition prevents arrhythmogenic Ca2+ waves caused by abnormal Ca2+ sensitivity of RyR or SR Ca2+ accumulation. Cardiovascular Research (CVR). ISSN 0008-6363. 98(2), p. 315–325. doi: 10.1093/cvr/cvt037. Full text in Research Archive
• Land, Sander; Niederer, Steven A.; Louch, William Edward; Sejersted, Ole M & Smith, Nicolas P. (2013). Integrating multi-scale data to create a virtual physiological mouse heart. Interface Focus. ISSN 2042-8898. 3(2). doi: 10.1098/rsfs.2012.0076. Full text in Research Archive
• Øyehaug, Leiv; Loose, Kristian Øglænd; Jølle, Guro Five; Røe, Åsmund Treu; Sjaastad, Ivar & Christensen, Geir Arve [Show all 8 contributors for this article] (2013). Synchrony of Cardiomyocyte Ca2+ Release is Controlled by t-tubule Organization, SR Ca2+ Content, and Ryanodine Receptor Ca2+ Sensitivity. Biophysical Journal. ISSN 0006-3495. 104(8), p. 1685–1697. doi: 10.1016/j.bpj.2013.03.022. Full text in Research Archive

View all works in NVA

• Graver, Hans Petter; Lohndal, Terje & Sejersted, Ole M (2020). Finansiering og styring. Khrono.no. ISSN 1894-8995. Full text in Research Archive
• Graver, Hans Petter; Lohndal, Terje & Sejersted, Ole M (2020). Kriser og behov for vitenråd. Khrono.no. ISSN 1894-8995. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Aronsen, Jan Magnus; Sejersted, Ole M; Sjaastad, Ivar & Christensen, Geir Arve [Show all 7 contributors for this article] (2016). Sarcolemmal structure and function revert to an immature phenotype in failing cardiomyocytes. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Aronsen, Jan Magnus; Dahl, Christen Peder; Jølle, Guro Five & Aakhus, Svend [Show all 15 contributors for this article] (2016). Mechanical load regulates cardiomyocyte t-tubular structure and Ca2+ homeostasis. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Aronsen, Jan Magnus; Singh, Neha; Sejersted, Ole M & Sjaastad, Ivar [Show all 9 contributors for this article] (2016). BIN1 triggers t-tubule development in cardiomyocytes. Full text in Research Archive
• Kolstad, Terje R Selnes; Macquaide, Niall; Hake, Johan; Aronsen, Jan Magnus; Frisk, Michael & Sjaastad, Ivar [Show all 8 contributors for this article] (2016). Dispersion of ryanodine receptor clusters disrupts calcium sparks in failing cardiomyocyte. Full text in Research Archive
• Røe, Åsmund Treu; Ruud, Marianne; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Sejersted, Ole M & Sjaastad, Ivar [Show all 7 contributors for this article] (2016). Elevated ventricular wall stress disrupts diastolic Calcium homeostasis of the cardiomyocyte. Full text in Research Archive
• Wanichawan, Pimthanya; Hafver, Tandekile Lubelwana; Lunde, Marianne; Martinsen, Marita; Louch, William Edward & Sejersted, Ole M [Show all 7 contributors for this article] (2016). The N-terminal membrane occupation and recognition nexus (MORN) domains of junctophilin-2 bind to Na+/Ca2+ exchanger. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani; Leren, I.S.; Edwards, Andrew & Vincent, K. [Show all 11 contributors for this article] (2016). Mechanisms underlying the frequency-dependence of triggered arrhythmias in catecholaminergic polymorphic ventricular tachycardia type 1. Full text in Research Archive
• Kolstad, Terje R Selnes; Macquaide, Niall; Hake, Johan; Aronsen, Jan Magnus; Frisk, Michael & Sjaastad, Ivar [Show all 8 contributors for this article] (2016). Super-resolution imaging reveals dispersion of ryanodine receptor clusters during heart failure. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Souza, Gustavo Antonio De; Wanichawan, Pimthanya; Lunde, Marianne; Martinsen, Marita & Sejersted, Ole M [Show all 7 contributors for this article] (2016). In vivo protein-protein interaction map of cardiac NCX1. Full text in Research Archive
• Skogestad, Jonas; Albert, Ingrid; Marshall, Serena; Børstad, Mira Halvorsen; Lothe, Gustav B & Sejersted, Ole M [Show all 8 contributors for this article] (2016). Phosphodiesterase 2 regulates Na+/K+-ATPase activity in adult cardiomyocytes. Full text in Research Archive
• Li, Jia; Frisk, Michael; Norseng, Per Andreas; Sjaastad, Ivar; Sejersted, Ole M & Louch, William Edward (2016). Regional activation of the Frank-Starling mechanism within the cardiomyocyte. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Zhang, Lili & Norseng, Per Andreas [Show all 10 contributors for this article] (2016). Elevated ventricular wall stress disrupts cardiomyocyte t-tubular structure and Ca2+ homeostasis. Full text in Research Archive
• Frisk, Michael; Aronsen, Jan Magnus; Dahl, Christen Peder; Lunde, Ida Gjervold; Jølle, Guro Five & Almaas, Victor [Show all 13 contributors for this article] (2015). Maintained cardiomyocyte t-tubule structure and function in patients with non-dilated hypertrophy. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Souza, Gustavo Antonio De; Lunde, Marianne; Martinsen, Marita; Sejersted, Ole M & Carlson, Cathrine Rein (2015). The in vitro NCX1 interactome. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Souza, Gustavo Antonio De; Lunde, Marianne; Martinsen, Marita; Sejersted, Ole M & Carlson, Cathrine Rein (2015). Identification of the in vitro NCX1 interactome. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Norseng, Per Andreas; Røe, Åsmund Treu; Espe, Emil Knut Stenersen & Aronsen, Jan Magnus [Show all 10 contributors for this article] (2015). Regulation of cardiomyocyte t-tubule organization and density by ventricular wall stress. Biophysical Journal. ISSN 0006-3495. 108, p. 443a–443a. Full text in Research Archive
• Wanichawan, Pimthanya; Hodne, Kjetil; Hafver, Tandekile Lubelwana; Lunde, Marianne; Martinsen, Marita & Louch, William Edward [Show all 8 contributors for this article] (2015). Development of a high affinity phospholemman binding peptide that reverses phosphorylated serine 68 phospholemman inhibition of the cardiac sodium-calcium exchanger 1. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Aronsen, Jan Magnus; Singh, Neha; Sejersted, Ole M & Sjaastad, Ivar [Show all 9 contributors for this article] (2015). BIN1 regulates t-tubule growth during cardiac development and disease. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Aronsen, Jan Magnus; Singh, Neha; Sejersted, Ole M & Sjaastad, Ivar [Show all 9 contributors for this article] (2015). BIN1 regulates t-tubule growth during cardiac development and disease. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Norseng, Per Andreas & Sjaastad, Ivar [Show all 9 contributors for this article] (2015). Elevated ventricular wall stress disrupts cardiomyocyte t-tubular structure and Ca2+ homeostasis. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Norseng, Per Andreas & Sjaastad, Ivar [Show all 9 contributors for this article] (2015). Elevated ventricular wall stress disrupts cardiomyocyte t-tubular structure and Ca2+ homeostasis. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Aronsen, Jan Magnus; Singh, Neha; Sejersted, Ole M & Sjaastad, Ivar [Show all 9 contributors for this article] (2015). BIN1 regulates t-tubule growth during cardiac development and diseases. Full text in Research Archive
• Ruud, Marianne; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Norseng, Per Andreas; Sjaastad, Ivar & Sejersted, Ole M [Show all 8 contributors for this article] (2015). Elevated ventricular wall stress disrupts cardiomyocyte t-tubular structure and Ca 2+ homeostasis. Full text in Research Archive
• Frisk, Michael; Ruud, Marianne; Espe, Emil Knut Stenersen; Aronsen, Jan Magnus; Røe, Åsmund Treu & Zhang, Lili [Show all 11 contributors for this article] (2015). Ventricular wall stress predicts disruption of cardiomyocyte t-tubule structure and Ca2+ homeostasis across the infarcted heart. Biophysical Journal. ISSN 0006-3495. 108, p. 106a–106a. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani Nikoo; Gaur, Namit; Leren, IS & Edwards, Andrew [Show all 11 contributors for this article] (2015). Mechanisms for the frequency-dependence of triggered activity in catecholaminergic polymorphic ventricular tachycardia type 1. Full text in Research Archive
• Kolstad, Terje R Selnes; Stokke, Mathis Korseberg; Stang, Espen; Brorson, Sverre Henning; Louch, William Edward & Sejersted, Ole M (2015). Serca located in the junctional SR shapes calcium release in cardiac myocytes. Biophysical Journal. ISSN 0006-3495. 108, p. 503a–503a. Full text in Research Archive
• Hodne, Kjetil; Wanichawan, Pimthanya; Lubelwana, Tandekile; Louch, William Edward; Lunde, Marianne & Mathisen, Marita [Show all 8 contributors for this article] (2015). Liberation of Pser68-Plm inhibition of NCX1 by an optimized anchoring disruptor peptide. Biophysical Journal. ISSN 0006-3495. 108, p. 434a–434a. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Singh, Neha; Aronsen, Jan Magnus; Marszalec, William & Sejersted, Ole M [Show all 10 contributors for this article] (2015). Bridging integrator 1 (BIN1) initiates t-tubule Growth during Cardiac Development and disease. Biophysical Journal. ISSN 0006-3495. 108, p. 130a–130a. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Souza, Gustavo Antonio De; Lunde, Marianne; Martinsen, Marita; Sejersted, Ole M & Carlson, Cathrine Rein (2015). Mapping the in vitro interactome of NCX1. Full text in Research Archive
• Kolstad, Terje R Selnes; Macquaide, Niall; Hake, Johan; Aronsen, Jan Magnus; Frisk, Michael & Sjaastad, Ivar [Show all 8 contributors for this article] (2015). Super-resolution imaging of ryanodine receptor cluster organization in normal and failing rat cardiac myocytes. Full text in Research Archive
• Kolstad, Terje R Selnes; MacQuaide, Niall; Hake, Johan; Aronsen, Jan Magnus; Frisk, Michael & Sjaastad, Ivar [Show all 8 contributors for this article] (2015). Super-resolution imaging of ryanodine receptor cluster organization in normal and failing rat cardiac myocytes. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Wanichawan, Pimthanya; Hodne, Kjetil; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Marianne [Show all 12 contributors for this article] (2015). Anchoring onto NCX1 facilitates dephosphorylation of P-SER68-PLM. Biophysical Journal. ISSN 0006-3495. 108, p. 584a–584a. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Hodne, Kjetil; Wanichawan, Pimthanya; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Per Kristian [Show all 12 contributors for this article] (2015). PP1 associated with cardiac NCX1 regulates exchange activity by dephosphorylating pSer-68-PLM. Full text in Research Archive
• Hortemo, Kristin Halvorsen; Aronsen, Jan Magnus; Sjaastad, Ivar; Lunde, Per Kristian & Sejersted, Ole M (2014). Phospho-GlcNAc modulation of MLC2 is different in slow- and fast twitch muscle during treadmill running. Full text in Research Archive
• Frisk, Michael; Espe, Emil Knut Stenersen; Røe, Åsmund Treu; Aronsen, Jan Magnus; Enger, Ulla Helene & Sejersted, Ole M [Show all 8 contributors for this article] (2014). In vivo strain measurements predict regional disruption of cardiomyocyte structure and Ca2+ homeostasis during heart failure. Full text in Research Archive
• Røe, Åsmund Treu; Aronsen, Jan Magnus; Frisk, Michael; Sejersted, Ole M; Sjaastad, Ivar & Louch, William Edward (2014). Active and passive stiffness in a hypertrophic rat model of diastolic dysfunction. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Wanichawan, Pimthanya; Hodne, Kjetil; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Marianne [Show all 10 contributors for this article] (2014). Molecular analysis of PP1 anchoring to NCX1 macromolecular complex. Full text in Research Archive Show summary

  Introduction: The cardiac Na+/Ca2+ exchanger 1 (NCX1) modulates excitation-contraction coupling and contributes to Ca2+ removal in cardiomyocytes. Altered expression and activity of NCX1 is linked to dysfunctional Ca2+ handling in chronic heart disease. Consequently, modulation of NCX1 activity has been proposed as a therapeutic target, but detailed understanding of NCX1 regulation is warranted. It has been shown that NCX1 exists in a macromolecular complex which would allow for regulation of its activity. NCX1 is also regulated by its accessory protein phospholemman (PLM) which inhibits NCX1 activity when phosphorylated at serine 68 (pSer68-PLM). Protein phosphatase 1 (PP1) dephosphorylates pSer68-PLM, and thus indirectly regulates NCX1 activity. Although no anchoring site is known, PP1 co-precipitates NCX1. We hypothesised that PP1 regulates NCX1 activity and that a direct and functional NCX1-PP1 interaction is a prerequisite for pSer68-PLM dephosphorylation. We set out to identify PP1 binding sites to the pPLM-NCX1 complex and assess the biological function of the interaction. Methods and results: NCX1 co-precipitated PP1 and PLM in wild-type rat left ventricular lysates and PLM-NCX1 co transfected HEK 293 cells. NCX1 protein levels were upregulated, PP1/NCX1 protein levels were downregulated while pSer68-PLM/total PLM protein levels were upregulated in our pressure overload model. The proximity ligation assay showed that cardiac NCX1 and PP1 colocalize within <40nm in isolated cardiomyocytes. Bioinformatic analysis revealed three putative PP1 binding sites on NCX1. Overlay assays of NCX1 and PP1α showed that PP1α bound directly to the consensus sequence R/KVxF in CBD1 (calcium binding domain 1) in NCX1, which is preserved across isoforms. The reciprocal NCX1 binding site was identified within residues 235-260 in PP1α, a region which harbours the aspartic acid, leucine and arginine docking motif. Binding data were confirmed by immunoprecipitations, mutation analysis of NCX1-GFP and FLAG-HIS-PP1α deletion mutants expressed in HEK 293 cells and in pulldown assays using NCX1 biotinylated peptides. We have generated a peptide docking model of the R/KVxF motif bound close to the PP1α active site. Single and double mutations of residues in the docking motif confirmed the interaction. Biacore analysis indicated that the NCX1-PPIα binding is strong. In competition assay using NCX1, PP1 and PLM co transfected HEK cells a NCX1 peptide disruptor was able to displace the interaction, p<0.05, indicating that PP1 dephosphorylates pSer68-PLM when anchored to NCX1. Conclusion: The R/KVxF motif in CBD1 of NCX1 facilitates PP1 anchoring to NCX1 and is directly and functionally required for pSer68-PLM dephosphorylation.

• Hafver, Tandekile Lubelwana; Wanichawan, Pimthanya; Hodne, Kjetil; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Marianne [Show all 10 contributors for this article] (2014). Molecular analysis of the pPLM-NCX1-PP1 complex. Full text in Research Archive Show summary

  Introduction: The cardiac Na+/Ca2+ exchanger (NCX1) is an ion transporting membrane protein which modulates excitation-contraction coupling and contributes to Ca2+ removal in cardiomyocytes. Altered expression and activity of NCX1 has been linked to dysfunctional Ca2+ handling in chronic heart disease. Consequently, modulation of NCX1 activity has been proposed as a therapeutic target, but detailed understanding of NCX1 regulation is warranted. In the rat heart, NCX1 exists in a macromolecular complex comprising of kinases, phosphatases, anchoring and adaptor proteins, which enables fine tuning of signals which converge on NCX1. NCX1 is also regulated by its accessory protein phospholemman (PLM). When phosphorylated at Serine 68 by PKA and PKC, PLM binds to and inhibits NCX1 activity. It has been shown that phospho Serine 68-PLM (pSer68-PLM) is dephosphorylated by protein phosphatase 1 (PP1) and furthermore PP1 co-precipitate with NCX1, however, no anchoring site is known. We hypothesised that PP1 regulates NCX1 activity and that a direct and functional NCX1-PP1 interaction is a prerequisite for pSer68-PLM dephosphorylation. We set out to identify PP1 binding sites to the pPLM-NCX1 complex and assess the biological function of the interaction. Methods and results: We have shown that NCX1 exists in a macromolecular complex with PP1 and PLM in wild-type rat left ventricular lysates and in transfected HEK cells. The proximity ligation assay was used to show that cardiac NCX1 and PP1 colocalize within <40nm in isolated cardiomyocytes. Bioinformatic analysis revealed three putative PP1 binding sites on NCX1. Overlay assays of NCX1 and PP1α showed that PP1α bound directly to the consensus sequence R/KVxF in CBD1 (calcium binding region 1) in NCX1, which is preserved across isoforms. The reciprocal NCX1 binding site was identified within residues 235-260 in PP1, a region which harbours the aspartic acid (Asp), leucine (Leu) and arginine (Arg) docking motif. Immunoprecipitations and mutation analysis of NCX1-GFP and FLAG-HIS-PP1α deletion mutants expressed in HEK 293 cells confirmed this interaction. A peptide docking model of the R/KVxF motif bound to the PP1α active site showed that the Asp 242 and Leu 243, identified from the PP1α overlay, make contact with the Valine (Val) and Phenylalanine (Phe) in the R/KVxF motif. Single and double mutations of Asp 242, Leu 243 and Arg 257 confirmed the interaction. Biacore analysis also showed that recombinant NCX1 cytoplasmic part bound to GST-PP1α recombinant protein. Conclusion: PP1 anchored directly to NCX1 through an R/KVxF motif in CBD1, whereas NCX1 bound to a DL motif within residues 235-260 in PP1. The biological role of the R/KVxF motif in PLM and NCX1 regulation is currently being assessed.

• Røe, Åsmund Treu; Aronsen, Jan Magnus; Frisk, Michael; Sejersted, Ole M; Sjaastad, Ivar & Louch, William Edward (2014). Compensatory alterations in cardiomyocytes Ca2+ homeostasis during aortic stenosis. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani Nikoo; Aronsen, Jan Magnus; Hougen, Karina & Sejersted, Ole M [Show all 8 contributors for this article] (2014). The effect of exercise training on Ca2+ wave probability after myocardial infarction varies with cardiac function at onset of training. Full text in Research Archive
• Kolstad, Terje R Selnes; Stokke, Mathis Korseberg; Stang, Espen; Brorson, Sverre Henning; Christensen, Geir Arve & Louch, William Edward [Show all 7 contributors for this article] (2014). SERCA localized near the dyad regulates calcium release in cardiac myocytes. Full text in Research Archive
• Wanichawan, Pimthanya; Hodne, Kjetil; Hafver, Lubelwana; Lunde, Marianne & Sejersted, Ole M (2014). Identification and optimization of PLM binding sequences in NCX1. Full text in Research Archive
• Kolstad, Terje R Selnes; Stokke, Mathis Korseberg; Stang, Espen; Brorson, Sverre Henning; Louch, William Edward & Sejersted, Ole M (2014). SERCA localized near the dyad regulates calcium release in cardiac myocytes. Full text in Research Archive
• Lipsett, David Bruce; Frisk, Michael; Hodne, Kjetil; Singh, Neha; Nygård, Ståle & Aronsen, Jan Magnus [Show all 11 contributors for this article] (2014). B1N1 controls longitudinal T-tubule growth in cardiomyocytes during development and disease. Full text in Research Archive
• Hodne, Kjetil; Wanichawan, Pimthanya; Hafver, Tandekile Lubelwana; Aronsen, Jan Magnus; Lunde, Ida Gjervold & Lunde, Marianne [Show all 12 contributors for this article] (2014). Identification and functional role of calpain cleavage site in Na+-Ca2+ exchanger 1 (NCX1). Biophysical Journal. ISSN 0006-3495. 106, p. 762a–762a. Full text in Research Archive
• Ruud, Marianne; Frisk, Michael; Norseng, Per Andreas; Røe, Åsmund Treu; Sejersted, Ole M & Christensen, Geir Arve [Show all 7 contributors for this article] (2014). Regulation of cardiomyocyte t-tubule organization by preload and afterload. Full text in Research Archive
• Sjåland, Cecilie; Aronsen, Jan Magnus; Ericsson, Madelene; Qu, Hong; Ellingsen, Øyvind & Sjaastad, Ivar [Show all 10 contributors for this article] (2014). Oxygen consumption of brown adipose tissue (BAT) and skeletal muscle is inversely related. Biophysical Journal. ISSN 0006-3495. 106, p. 776a–776a. Full text in Research Archive
• Lipsett, David Bruce; Singh, Neha; Frisk, Michael; Aronsen, Jan Magnus; Sejersted, Ole M & Sjaastad, Ivar [Show all 9 contributors for this article] (2014). Bridging integrator 1 (BIN1)-induced t-tubule formation in cardiomyocytes. Biophysical Journal. ISSN 0006-3495. 106(2), p. 772a–772a. Full text in Research Archive
• Frisk, Michael; Espe, Emil Knut Stenersen; Røe, Åsmund Treu; Aronsen, Jan Magnus; Enger, Ulla Helene & Sejersted, Ole M [Show all 8 contributors for this article] (2014). Regional left ventricular strain and wall stress predict cardiomyocyte T-tubular structure and Ca2+ homeostasis following myocardial infarction. Full text in Research Archive
• Hafver, Lubelwana; Wanichawan, Pimthanya; Hodne, Kjetil; Aronsen, Jan Magnus; Dalhus, Bjørn & Lunde, Marianne [Show all 10 contributors for this article] (2014). Molecular analysis of the phospho-PLM-NCX1-PP1 complex. Full text in Research Archive
• Kolstad, Terje R Selnes; Stokke, Mathis Korseberg; Stang, Espen; Brorson, Sverre Henning; Louch, William Edward & Sejersted, Ole M (2014). SERCA localized near the dyad regulates calcium release in cardiac myoctyes. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani Nikoo; Aronsen, Jan Magnus; Hougen, Karina & Sejersted, Ole M [Show all 8 contributors for this article] (2013). The effect of exercise training on ca2+ wave probability after myocardial infarction depends on cardiac function at onset of training. Heart Rhythm. ISSN 1547-5271. 10, p. 1749–1749. Full text in Research Archive
• Wanichawan, Pimthanya; Hafver, Tandekile Lubelwana; Lunde, Ida Gjervold; Aronsen, Jan Magnus; Lunde, Marianne & Kvaløy, Heidi [Show all 10 contributors for this article] (2013). Calpain induces cleavage of the cardiac Na+ - Ca2+ exchanger 1 (NCX1) in pressure-overloaded hearts: Molecular interactions and identification of a calpain cleavage site. Full text in Research Archive
• Røe, Åsmund Treu; Aronsen, Jan Magnus; Frisk, Michael; Sejersted, Ole M; Sjaastad, Ivar & Louch, William Edward (2013). Compensated Cardiomyocyte Ca2+ Homeostasis in a Rat Model of Diastolic Heart Failure. Biophysical Journal. ISSN 0006-3495. 104(2), p. 603A–603A. Full text in Research Archive
• Røe, Åsmund Treu; Aronsen, Jan Magnus; Frisk, Michael; Sejersted, Ole M; Sjaastad, Ivar & Louch, William Edward (2013). Contribution of active and passive stiffness to diastolic dysfunction in rats following aortic bonding. Full text in Research Archive
• Hafver, Tandekile Lubelwana; Wanichawan, Pimthanya; Aronsen, Jan Magnus; Lunde, Marianne; Mathisen, Marita & Sjaastad, Ivar [Show all 8 contributors for this article] (2013). Molecular analysis of the pPLm-NCX1-PP1 complex. Full text in Research Archive
• Frisk, Michael; Norseng, Per Andreas; Sejersted, Ole M & Louch, William Edward (2013). Variable t-tubule density and Ca2+ homeostasis across the atria. Full text in Research Archive
• Frisk, Michael; Jølle, Guro Five; Lunde, Ida Gjervold; Skrbic, Biljana; Sejersted, Ole M & Tønnessen, Theis [Show all 8 contributors for this article] (2013). Aortic stenosis triggers t-tubule growth in human and murine cardiomyocytes. Full text in Research Archive
• Wanichawan, Pimthanya; Hafver, Tandekile; Aronsen, Jan Magnus; Lunde, Ida Gjervold; Lunde, Marianne & Kvaløy, Heidi [Show all 10 contributors for this article] (2013). Modulation of the Sodium-calcium Exchanger 1 (NCX1) by Calpain; Molecular Interactions and Identification of a Calpain Cleavage Site. Circulation. ISSN 0009-7322. 128. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani Nikoo; Aronsen, Jan Magnus; Hougen, Karina & Sejersted, Ole M [Show all 8 contributors for this article] (2013). The effect of exercise training on Ca2+ wave probability after myocardial infarction depends on cardiac function at onset of training. Full text in Research Archive
• Skogestad, Jonas; Aronsen, Jan Magnus; Hougen, Karina; Stokke, Mathis Korseberg; Swift, Fredrik & Sejersted, Ole M [Show all 7 contributors for this article] (2013). Extracellular potassium regulates Ca2+ homeostasis through interaction between NCX and NKA. Full text in Research Archive
• Danielsen, Tore Kristian; Manotheepan, Ravinea; Sadredini, Mani Nikoo; Aronsen, Jan Magnus; Hougen, Karina & Sejersted, Ole M [Show all 8 contributors for this article] (2013). The effect of exercise training on Ca2+ wave probability after myocardial infarction depends on cardiac function at onset of training. Full text in Research Archive

View all works in NVA