Frequency-dependent changes in calcium cycling and contractile activation in SERCA2a transgenic mice

Sarco/endoplasmic reticulum Ca2+-ATPase is a more effective calcium remover than sodium-calcium exchanger in human embryonic stem cell-derived cardiomyocytes

Human pluripotent stem cell (hPSCs)-derived ventricular (V) cardiomyocytes (CMs) display immature Ca2+–handing properties with smaller transient amplitudes and slower kinetics due to such differences in crucial Ca2+-handling proteins as the poor sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump but robust Na+-Ca2+ exchanger (NCX) activities in human embryonic stem cell (ESC)-derived VCMs compared with adult. Despite their fundamental importance in excitation-contraction coupling, the relative contribution of SERCA and NCX to Ca2+-handling of hPSC-VCMs remains unexplored. We systematically altered the activities of SERCA and NCX in human embryonic stem cell-derived ventricular cardiomyocytes (hESC-VCMs) and their engineered microtissues, followed by examining the resultant phenotypic consequences. SERCA overexpression in hESC-VCMs shortened the decay of Ca2+ transient at low frequencies (0.5 Hz) without affecting the amplitude, SR Ca2+ content and Ca2+ baseline. Interestingly, short hairpin RNA-based NCX suppression did not prolong the transient decay, indicating a compensatory response for Ca2+ removal. Although hESC-VCMs and their derived microtissues exhibited negative frequency-transient/force responses, SERCA overexpression rendered them less negative at high frequencies (>2 Hz) by accelerating Ca2+ sequestration. We conclude that for hESC-VCMs and their microtissues, SERCA, rather than NCX, is the main Ca2+ remover during diastole; poor SERCA expression is the leading cause for immature negative-frequency/force responses, which can be partially reverted by forced expression. Combinatorial approach to mature calcium handling in hESC-VCMs may help shed further mechanistic insights.

NEW & NOTEWORTHY In this study of human pluripotent stem cell-derived cardiomyocytes, we studied the role of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and Na+-Ca2+ exchanger (NCX) in Ca2+ handling. Our data support the notion that SERCA is more effective in cytosolic calcium removal than the NCX.

Endocardial endothelium is a key determinant of force-frequency relationship in rat ventricular myocardium

We tested the hypothesis that removing endocardial endothelium (EE) negatively impacts the force-frequency relationship (FFR) of ventricular myocardium and dissected the signaling that underlies this phenomenon. EE of rat trabeculae was selectively damaged by brief (<1 s) exposure to 0.1% Triton X-100. Force, intracellular Ca(2+) transient (iCa(2+)), and activity of protein kinase A (PKA) and protein kinase C (PKC) were determined. In control muscles, force and iCa(2+) increased as the stimulation frequency increased in steps of 0.5 Hz up to 3.0 Hz. However, EE-denuded (EED) muscles exhibited a markedly blunted FFR. Neither isoproterenol (ISO; 0.1-5 nmol/l) nor endothelin-1 (ET-1; 10-100 nmol/l) alone restored the slope of FFR in EED muscles. Intriguingly, however, a positive FFR was restored in EED preparations by combining low concentrations of ISO (0.1 nmol/l) and ET-1 (20 nmol/l). In intact muscles, PKA and PKC activity increased proportionally with the increase in frequency. This effect was completely lost in EED muscles. Again, combining ISO and ET-1 fully restored the frequency-dependent rise in PKA and PKC activity in EED muscles. In conclusion, selective damage of EE leads to significantly blunted FFR. A combination of low concentrations of ISO and ET-1 successfully restores FFR in EED muscles. The interdependence of ISO and ET-1 in this process indicates cross-talk between the β1-PKA and ET-1-PKC pathways for a normal (positive) FFR. The results also imply that dysfunction of EE and/or EE-myocyte coupling may contribute to flat (or even negative) FFR in heart failure.

Effects of increased systolic Ca²⁺ and phospholamban phosphorylation during β-adrenergic stimulation on Ca²⁺ transient kinetics in cardiac myocytes

Previous studies demonstrated higher systolic intracellular Ca(2+) concentration ([Ca(2+)](i)) amplitudes result in faster [Ca(2+)](i) decline rates, as does β-adrenergic (β-AR) stimulation. The purpose of this study is to determine the major factor responsible for the faster [Ca(2+)](i) decline rate with β-AR stimulation, the increased systolic Ca(2+) concentration levels, or phosphorylation of phospholamban. Mouse myocytes were perfused under basal conditions [1 mM extracellular Ca(2+) concentration ([Ca(2+)](o))], followed by high extracellular Ca(2+) (3 mM [Ca(2+)](o)), washout with 1 mM [Ca(2+)](o), followed by 1 μM isoproterenol (ISO) with 1 mM [Ca(2+)](o). ISO increased Ser(16) phosphorylation compared with 3 mM [Ca(2+)](o), whereas Thr(17) phosphorylation was similar. Ca(2+) transient (CaT) (fluo 4) data were obtained from matched CaT amplitudes with 3 mM [Ca(2+)](o) and ISO. [Ca(2+)](i) decline was significantly faster with ISO compared with 3 mM [Ca(2+)](o). Interestingly, the faster decline with ISO was only seen during the first 50% of the decline. CaT time to peak was significantly faster with ISO compared with 3 mM [Ca(2+)](o). A Ca(2+)/calmodulin-dependent protein kinase (CAMKII) inhibitor (KN-93) did not affect the CaT decline rates with 3 mM [Ca(2+)](o) or ISO but normalized ISO's time to peak with 3 mM [Ca(2+)](o). Thus, during β-AR stimulation, the major factor for the faster CaT decline is due to Ser(16) phosphorylation, and faster time to peak is due to CAMKII activation.

Fibronectin increases the force production of mouse papillary muscles via α5β1 integrin

The extracellular matrix (ECM) protein-integrin-cytoskeleton axis plays a central role as a mechanotransducing protein assemblage in many cell types. However, how the process of mechanotransduction and the mechanically generated signals arising from this axis affect myofilament function in cardiac muscle are not completely understood. We hypothesize that ECM proteins can regulate cardiac function through integrin binding, and thereby alter the intracellular calcium concentration ([Ca(2+)](i)) and/or modulate myofilament activation processes. Force measurements made in mouse papillary muscle demonstrated that in the presence of the soluble form of the ECM protein, fibronectin (FN), active force was increased significantly by 40% at 1 Hz, 54% at 2 Hz, 35% at 5 Hz and 16% at 9 Hz stimulation frequencies. Furthermore, increased active force in the presence of FN was associated with 12-33% increase in [Ca(2+)](i) and 20-50% increase in active force per unit Ca(2+). A function blocking antibody for α5 integrin prevented the effects of the FN on the changes in force and [Ca(2+)](i), whereas a function blocking α3 integrin antibody did not reverse the effects of FN. The effects of FN were reversed by an L-type Ca(2+) channel blocker, verapamil or PKA inhibitor. Freshly isolated cardiomyocytes exhibited a 39% increase in contraction force and a 36% increase in L-type Ca(2+) current in the presence of FN. Fibers treated with FN showed a significant increase in the phosphorylation of phospholamban; however, the phosphorylation of troponin I was unchanged. These results demonstrate that FN acts via α5β1 integrin to increase force production in myocardium and that this effect is partly mediated by increases in [Ca(2+)](i) and Ca(2+) sensitivity, PKA activation and phosphorylation of phospholamban.

Calcium sensitivity, force frequency relationship and cardiac troponin I: critical role of PKA and PKC phosphorylation sites

Transgenic models with pseudo phosphorylation mutants of troponin I, PKA sites at Ser 22 and 23 (cTnIDD(22,23) mice) or PKC sites at Ser 42 and 44 (cTnIAD(22,23)DD(42,44)) displayed differential force-frequency relationships and afterload relaxation delay in vivo. We hypothesized that cTnI PKA and PKC phosphomimics impact cardiac muscle rate-related developed twitch force and relaxation kinetics in opposite directions. cTnIDD(22,23) transgenic mice produce a force frequency relationship (FFR) equivalent to control NTG albeit at lower peak [Ca(2+)](i), while cTnIAD(22,23)DD(42,44) TG mice had a flat FFR with normal peak systolic [Ca(2+)](i), thus suggestive of diminished responsiveness to [Ca(2+)](i) at higher frequencies. Force-[Ca(2+)](i) hysteresis analysis revealed that cTnIDD(22,23) mice have a combined enhanced myofilament calcium peak response with an enhanced slope of force development and decline per unit of [Ca(2+)](i), whereas cTnIAD(22,23)DD(42,44) transgenic mice showed the opposite. The computational ECME model predicts that the TG lines may be distinct from each other due to different rate constants for association/dissociation of Ca(2+) at the regulatory site of cTnC. Our data indicate that cTnI phosphorylation at PKA sites plays a critical role in the FFR by increasing relative myofilament responsiveness, and results in a distinctive transition between activation and relaxation, as displayed by force-[Ca(2+)](i) hysteresis loops. These findings may have important implications for understanding the specific contribution of cTnI to beta-adrenergic inotropy and lusitropy and to adverse contractile effects of PKC activation, which is relevant during heart failure development.

Effects of halothane, sevoflurane and desflurane on the force-frequency relation in the dog heart in vivo

PURPOSE

Frequency potentiation is the increase in force of contraction induced by an increased heart rate (HR). This positive staircase phenomenon has been attributed to changes in Ca2+ entry and loading of intracellular Ca2+ stores. Volatile anesthetics interfere with Ca2+ homeostasis of cardiomyocytes. We hypothesized that frequency potentiation is altered by volatile anesthetics and investigated the influence of halothane (H), sevoflurane (S) and desflurane (D) on the positive staircase phenomenon in dogsin vivo.

METHODS

Dogs were chronically instrumented for measurement of left ventricular (LV) pressure and cardiac output. Heart rate was increased by atrial pacing from 120 to 220 beats·min^-1 and the LV maximal rate of pressure increase (dP/ dt_max) was determined as an index of myocardial performance. Measurements were performed in conscious dogs and during anesthesia with 1.0 minimal alveolar concentrations of each of the three inhaled anesthetics.

RESULTS

Increasing HR from 120 to 220 beats·min^-1 increased dP/dt_max from 3394 ± 786 (mean ± SD) to 3798 ± 810 mmHg sec-1 in conscious dogs. All anesthetics reduced dP/dt_max during baseline (at 120 beatss·min^-1: H, 1745 ± 340 mmHgs·sec^-1; S, 1882 ± 418; D, 1928 ± 454, allP < 0.05vs awake) but did not influence the frequency potentiation of dP/dtmax (at 220 beatss·min^-1: H, 1981 ± 587 mmHgs·sec^-1; S, 2187 ± 787; D, 2307 ± 691). The slope of the regression line correlating dP/dt_max and HR was not different between awake and anesetized dogs. Increasing HR did not influence cardiac output in awake or anesthetized dogs.

CONCLUSION

These results indicate that volatile anesthetics do not alter the force-frequency relation in dogs in vivo.

Force-frequency relationship in intact mammalian ventricular myocardium: physiological and pathophysiological relevance

The force-frequency relationship (FFR) is an important intrinsic regulatory mechanism of cardiac contractility. The FFR in most mammalian ventricular myocardium is positive; that is, an increase in contractile force in association with an increase in the amplitude of Ca(2+) transients is induced by elevation of the stimulation frequency, which reflects the cardiac contractile reserve. The relationship is different depending on the range of frequency and species of animal. In some species, including rat and mouse, a 'primary-phase' negative FFR is induced over the low-frequency range up to approximately 0.5-1 Hz (rat) and 1-2 Hz (mouse). Even in these species, the FFR over the frequency range close to the physiological heart rate is positive and qualitatively similar to that in larger mammalian species, although the positive FFR is less prominent. The integrated dynamic balance of the intracellular Ca(2+) concentration ([Ca(2+)](i)) is the primary cellular mechanism responsible for the FFR and is determined by sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) flux through the sarcolemma via L-type Ca(2+) channels and the Na(+)-Ca(2+) exchanger. Intracellular Na(+) concentration is also an important factor in [Ca(2+)](i) regulation. In isolated rabbit papillary muscle, over a lower frequency range (<0.5 Hz), an increase in duration rather than amplitude of Ca(2+) transients appears to be responsible for the increase in contractile force, while over an intermediate frequency range (0.5-2.0 Hz), the amplitude of Ca(2+) transients correlates well with the increase in contractile force. Over a higher frequency range (>2.5 Hz), the contractile force is dissociated from the amplitude of Ca(2+) transients probably due to complex cellular mechanisms, including oxygen limitation in the central fibers of isolated muscle preparations, while the amplitude of Ca(2+) transients increases further with increasing frequency ('secondary-phase' negative FFR). Calmodulin (CaM) may contribute to a positive FFR and the frequency-dependent acceleration of relaxation, although the role of calmodulin has not yet been established unequivocally. In failing ventricular myocardium, the positive FFR disappears or is inverted and becomes negative. The activation and overexpression of cardiac sarcoplasmic reticulum Ca(2+) ATPase (SERCA2a) is able to reverse these abnormalities. Frequency-dependent alterations of systolic and diastolic force in association with those of Ca(2+) transients and diastolic [Ca(2+)](i) levels are excellent indicators for analysis of cardiac excitation-contraction coupling, and for evaluating the severity of cardiac contractile dysfunction, cardiac reserve capacity and the effectiveness of therapeutic agents in congestive heart failure.

Roles of phosphorylation of myosin binding protein-C and troponin I in mouse cardiac muscle twitch dynamics

A normal heart increases its contractile force with increasing heart rate. Although calcium handling and myofibrillar proteins have been implicated in maintaining this positive force-frequency relationship (FFR), the exact mechanisms by which it occurs have not been addressed. In this study, we have developed an analytical method to define the calcium-force loop data, which characterizes the function of the contractile proteins in response to calcium that is independent of the calcium handling proteins. Results demonstrate that increasing the stimulation frequency causes increased force production per unit calcium concentration and decreased frequency-dependent calcium sensitivity during the relaxation phase. We hypothesize that phosphorylation of myosin binding protein-C (MyBP-C) and troponin I (TnI) acts coordinately to change the rates of force generation and relaxation, respectively. To test this hypothesis, we performed simultaneous calcium and force measurements on stimulated intact mouse papillary bundles before and after inhibition of MyBP-C and TnI phosphorylation using the calcium/calmodulin kinase II (CaMK2) inhibitor autocamtide-2 related inhibitory peptide, or the protein kinase A (PKA) inhibitor 14-22 amide. CaMK2 inhibition reduced both MyBP-C and TnI phosphorylation and decreased active force without changing the magnitude of the [Ca(2+)](i) transient. This reduced the normalized change in force per change in calcium by 19-39%. Data analyses demonstrated that CaMK2 inhibition changed the myofilament characteristics via a crossbridge feedback mechanism. These results strongly suggest that the phosphorylation of MyBP-C and TnI contributes significantly to the rates of force development and relaxation.

Altered dose response to beta-agonists in SERCA1a-expressing hearts ex vivo and in vivo

In this study we evaluated the contractile characteristics of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)1a-expressing hearts ex vivo and in vivo and in particular their response to beta-adrenergic stimulation. Analysis of isolated, work-performing hearts revealed that transgenic (TG) hearts develop much higher maximal rates of contraction and relaxation than wild-type (WT) hearts. Addition of isoproterenol only moderately increased the maximal rate of relaxation (+20%) but did not increase contractility or decrease relaxation time in TG hearts. Perfusion with varied buffer Ca(2+) concentrations indicated an altered dose response to Ca(2+). In vivo TG hearts displayed fairly higher maximal rates of contraction (+ 25%) but unchanged relaxation parameters and a blunted but significant response to dobutamine. Our study also shows that the phospholamban (PLB) level was decreased (-40%) and its phosphorylation status modified in TG hearts. This study clearly demonstrates that increases in SERCA protein level alter the beta-adrenergic response and affect the phosphorylation of PLB. Interestingly, the overall cardiac function in the live animal is only slightly enhanced, suggesting that (neuro)hormonal regulations may play an important role in controlling in vivo heart function.

Sarcoplasmic reticulum Ca(2+) atpase (SERCA) 1a structurally substitutes for SERCA2a in the cardiac sarcoplasmic reticulum and increases cardiac Ca(2+) handling capacity

Ectopic expression of the sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA) 1a pump in the mouse heart results in a 2.5-fold increase in total SERCA pump level. SERCA1a hearts show increased rates of contraction/relaxation and enhanced Ca(2+) transients; however, the cellular mechanisms underlying altered Ca(2+) handling in SERCA1a transgenic (TG) hearts are unknown. In this study, using confocal microscopy, we demonstrate that SERCA1a protein traffics to the cardiac SR and structurally substitutes for the endogenous SERCA2a isoform. SR Ca(2+) load measurements revealed that TG myocytes have significantly enhanced SR Ca(2+) load. Confocal line-scan images of field-stimulated SR Ca(2+) release showed an increased rate of Ca(2+) removal in TG myocytes. On the other hand, ryanodine receptor binding activity was decreased by approximately 30%. However, TG myocytes had a greater rate of spontaneous ryanodine receptor opening as measured by spark frequency. Whole-cell L-type Ca(2+) current density was reduced by approximately 50%, whereas the time course of inactivation was unchanged in TG myocytes. These studies provide important evidence that SERCA1a can substitute both structurally and functionally for SERCA2a in the heart and that SERCA1a overexpression can be used to enhance SR Ca(2+) transport and cardiac contractility.