Basal Spontaneous Firing of Rabbit Sinoatrial Node Cells Is Regulated by Dual Activation of PDEs (Phosphodiesterases) 3 and 4

BACKGROUND

Spontaneous firing of sinoatrial node cells (SANCs) is regulated by cAMP-mediated, PKA (protein kinase A)-dependent (cAMP/PKA) local subsarcolemmal Ca²⁺ releases (LCRs) from RyRs (ryanodine receptors). LCRs occur during diastolic depolarization and activate an inward Na⁺/Ca²⁺ exchange current that accelerates diastolic depolarization rate prompting the next action potential. PDEs (phosphodiesterases) regulate cAMP-mediated signaling; PDE3/PDE4 represent major PDE activities in SANC, but how they modulate LCRs and basal spontaneous SANC firing remains unknown.

METHODS

Real-time polymerase chain reaction, Western blot, immunostaining, cellular perforated patch clamping, and confocal microscopy were used to elucidate mechanisms of PDE-dependent regulation of cardiac pacemaking.

RESULTS

PDE3A, PDE4B, and PDE4D were the major PDE subtypes expressed in rabbit SANC, and PDE3A was colocalized with α-actinin, PDE4D, SERCA (sarcoplasmic reticulum Ca²⁺ ATP-ase), and PLB (phospholamban) in Z-lines. Inhibition of PDE3 (cilostamide) or PDE4 (rolipram) alone increased spontaneous SANC firing by ≈20% (P<0.05) and ≈5% (P>0.05), respectively, but concurrent PDE3+PDE4 inhibition increased spontaneous firing by ≈45% (P<0.01), indicating synergistic effect. Inhibition of PDE3 or PDE4 alone increased L-type Ca²⁺ current (I_Ca,L) by ≈60% (P<0.01) or ≈5% (P>0.05), respectively, and PLB phosphorylation by ≈20% (P>0.05) each, but dual PDE3+PDE4 inhibition increased I_Ca,L by ≈100% (P<0.01) and PLB phosphorylation by ≈110% (P<0.05). Dual PDE3+PDE4 inhibition increased the LCR number and size (P<0.01) and reduced the SR (sarcoplasmic reticulum) Ca²⁺ refilling time (P<0.01) and the LCR period (time from action potential-induced Ca²⁺ transient to subsequent LCR; P<0.01), leading to decrease in spontaneous SANC cycle length (P<0.01). When RyRs were disabled by ryanodine and LCRs ceased, dual PDE3+PDE4 inhibition failed to increase spontaneous SANC firing.

CONCLUSIONS

Basal cardiac pacemaker function is regulated by concurrent PDE3+PDE4 activation which operates in a synergistic manner via decrease in cAMP/PKA phosphorylation, suppression of LCR parameters, and prolongation of the LCR period and spontaneous SANC cycle length.

An Aptamer-Based Near-Infrared Fluorescence Nanoprobe for Detecting and Imaging of Phospholamban Micropeptide in Cardiomyocytes

A growing body of evidence indicates that micropeptides encoded by long noncoding RNAs (lncRNAs) act independently or as regulators of larger proteins in fundamental biological processes, especially in the maintenance of cellular homeostasis. However, due to their small size and low intracellular expression, visual monitoring of micropeptides in living cells is still a challenge. In this work, we have designed and synthesized an aptamer-based near-infrared fluorescence nanoprobe for fluorescence imaging of phospholamban (PLN), which is an intracellular micropeptide that affects calcium homeostasis, and is closely associated with human heart failure in the clinic. The nanoprobe could respond specifically to PLN with excellent selectivity, high sensitivity, good nuclease stability, and biocompatibility, and it was successfully applied for imaging of changes in PLN levels in cardiomyocytes and in frozen sections of heart tissues. Further combined with clinical myocardial biopsy, we believe that the developed nanoprobe should be of great significance in later molecular pathology study of heart failure, which may help with diagnosis of early heart failure in the future. More importantly, for the first time nanoprobes were applied to visually monitor the changes of micropeptides in living cells and in frozen tissue sections, and the design concept of the aptamer-based nanoprobe can be extended to fluorescence detection of other micropeptides.

Differential localizations of protein phosphatase 1 isoforms determine their physiological function in the heart

Protein phosphatase 1 isoforms α, β, and γ (PP1α, PP1β, and PP1γ) are highly homologous in the catalytic domains but have distinct subcellular localizations. In this study, we utilized both primary cell culture and knockout mice to investigate the isoform-specific roles of PP1s in the heart. In both neonatal and adult cardiac myocytes, PP1β was mainly localized in the nucleus, compared to the predominant presence of PP1α and PP1γ in the cytoplasm. Adenovirus-mediated overexpression of PP1α led to decreased phosphorylation of phospholamban, which was not influenced by overexpression of either PP1β or PP1γ. Interestingly, only cardiac-specific knockout of PP1β resulted in increased HDAC7 phosphorylation, consistent with the predominant nuclear localization of PP1β. Functionally, deletion of either PP1 isoform resulted in reduced fractional shortening in aging mice, however only PP1β deletion resulted in interstitial fibrosis in mice as early as 3 weeks of age. Deletion of neither PP1 isoform had any effect on pathological cardiac hypertrophy induced by 2 weeks of pressure overload stimulation. Together, our data suggest that PP1 isoforms have differential localizations to regulate the phosphorylation of their specific substrates for the physiological function in the heart.

Phospholamban deficiency does not alter skeletal muscle SERCA pumping efficiency or predispose mice to diet-induced obesity

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pump is a major contributor to skeletal muscle Ca²⁺ homeostasis and metabolic rate. SERCA activity can become adaptively uncoupled by its regulator sarcolipin (SLN) to increase the energy demand of Ca²⁺ pumping, preventing excessive obesity and glucose intolerance in mice. Several other SERCA regulators bear structural and functional resemblance to SLN, including phospholamban (PLN). Here, we sought to examine whether endogenous levels of skeletal muscle PLN control SERCA Ca²⁺ pumping efficiency and whole body metabolism. Using PLN-null mice ( Pln^-/-), we found that soleus (SOL) muscle's SERCA pumping efficiency (measured as an apparent coupling ratio: Ca²⁺ uptake/ATP hydrolysis) was unaffected by PLN. Expression of Ca²⁺-handling proteins within the SOL, including SLN, were comparable between Pln^-/- and wild-type (WT) littermates, as were fiber-type characteristics. Not surprisingly then, Pln^-/- mice developed a similar degree of diet-induced obesity and glucose intolerance as WT controls when given a "Western" high-fat diet. Lack of an excessively obesogenic phenotype of Pln^-/- could not be explained by compensation from skeletal muscle SLN or brown adipose tissue uncoupling protein-1 content. In agreement with several other reports, our study lends support to the notion that PLN serves a functionally distinct role from that of SLN in skeletal muscle physiology.

The intramembrane protease SPPL2c promotes male germ cell development by cleaving phospholamban

Signal peptide peptidase (SPP) and the four homologous SPP-like (SPPL) proteases constitute a family of intramembrane aspartyl proteases with selectivity for type II-oriented transmembrane segments. Here, we analyse the physiological function of the orphan protease SPPL2c, previously considered to represent a non-expressed pseudogene. We demonstrate proteolytic activity of SPPL2c towards selected tail-anchored proteins. Despite shared ER localisation, SPPL2c and SPP exhibit distinct, though partially overlapping substrate spectra and inhibitory profiles, and are organised in different high molecular weight complexes. Interestingly, SPPL2c is specifically expressed in murine and human testis where it is primarily localised in spermatids. In mice, SPPL2c deficiency leads to a partial loss of elongated spermatids and reduced motility of mature spermatozoa, but preserved fertility. However, matings of male and female SPPL2c ^-/- mice exhibit reduced litter sizes. Using proteomics we identify the sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA2)-regulating protein phospholamban (PLN) as a physiological SPPL2c substrate. Accumulation of PLN correlates with a decrease in intracellular Ca²⁺ levels in elongated spermatids that likely contribute to the compromised male germ cell differentiation and function of SPPL2c ^-/- mice.

Chronic inhibition of nitric oxide synthase modulates calcium handling in rat heart 1

Systemic infusion of nitric oxide synthase (NOS) inhibitors increases peripheral vascular resistance due to inhibition of endothelial NOS leading to the activation of the arterial baroreceptor mechanisms and inhibition of central sympathetic outflow. In the current study, we explored that systemic NOS blockage activates protein kinase A (PKA)-mediated signaling pathway through maintained cGMP-dependent protein kinase (PKG) activation. Rats were treated with 3 different concentrations of N(ω)-nitro-l-arginine methyl ester (L-NAME) for 14 days. Systemic L-NAME treatment induced a dose-dependent increase in blood pressure and increased mRNA levels of atrial natriuretic peptide (ANP) and phosphorylation levels of p44/42 MAPK without any change in cardiac mass. The cardiac cGMP levels and PKG-mediated phosphorylation of vasodilator-stimulated phosphoprotein (VASP) (Ser239) did not alter in any group. At the highest dose of treatment (100 mg/kg per day), PKA-mediated phosphorylations of VASP (Ser157) and troponin I (TnI) (Ser23/24) were enhanced significantly indicating the increase in PKA activation in response to chronic NOS blockage. Alterations in both phosphorylated phospholamban (Ser16/Thr17) and sarcoplasmic/endoplasmic Ca²⁺-ATPase (SERCA2) levels can increase cytosolic Ca²⁺ load and impair Ca²⁺ handling. Our data suggest that the increased PKA activation in response to chronic NOS blockage appears to be responsible for cardiac abnormalities that occur due to prolonged L-NAME treatment.

The cardiac ryanodine receptor, but not sarcoplasmic reticulum Ca2+-ATPase, is a major determinant of Ca2+ alternans in intact mouse hearts

Sarcoplasmic reticulum (SR) Ca²⁺ cycling is governed by the cardiac ryanodine receptor (RyR2) and SR Ca²⁺-ATPase (SERCA2a). Abnormal SR Ca²⁺ cycling is thought to be the primary cause of Ca²⁺ alternans that can elicit ventricular arrhythmias and sudden cardiac arrest. Although alterations in either RyR2 or SERCA2a function are expected to affect SR Ca²⁺ cycling, whether and to what extent altered RyR2 or SERCA2a function affects Ca²⁺ alternans is unclear. Here, we employed a gain-of-function RyR2 variant (R4496C) and the phospholamban-knockout (PLB-KO) mouse model to assess the effect of genetically enhanced RyR2 or SERCA2a function on Ca²⁺ alternans. Confocal Ca²⁺ imaging revealed that RyR2-R4496C shortened SR Ca²⁺ release refractoriness and markedly suppressed rapid pacing-induced Ca²⁺ alternans. Interestingly, despite enhancing RyR2 function, intact RyR2-R4496C hearts exhibited no detectable spontaneous SR Ca²⁺ release events during pacing. Unlike for RyR2, enhancing SERCA2a function by ablating PLB exerted a relatively minor effect on Ca²⁺ alternans in intact hearts expressing RyR2 WT or a loss-of-function RyR2 variant, E4872Q, that promotes Ca²⁺ alternans. Furthermore, partial SERCA2a inhibition with 3 μm 2,5-di-tert-butylhydroquinone (tBHQ) also had little impact on Ca²⁺ alternans, whereas strong SERCA2a inhibition with 10 μm tBHQ markedly reduced the amplitude of Ca²⁺ transients and suppressed Ca²⁺ alternans in intact hearts. Our results demonstrate that enhanced RyR2 function suppresses Ca²⁺ alternans in the absence of spontaneous Ca²⁺ release and that RyR2, but not SERCA2a, is a key determinant of Ca²⁺ alternans in intact working hearts, making RyR2 an important therapeutic target for cardiac alternans.

Structural basis for relief of phospholamban-mediated inhibition of the sarcoplasmic reticulum Ca2+-ATPase at saturating Ca2+ conditions

Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) is critical for cardiac Ca²⁺ transport. Reversal of phospholamban (PLB)-mediated SERCA inhibition by saturating Ca²⁺ conditions operates as a physiological rheostat to reactivate SERCA function in the absence of PLB phosphorylation. Here, we performed extensive atomistic molecular dynamics simulations to probe the structural mechanism of this process. Simulation of the inhibitory complex at superphysiological Ca²⁺ concentrations ([Ca²⁺] = 10 mm) revealed that Ca²⁺ ions interact primarily with SERCA and the lipid headgroups, but not with PLB's cytosolic domain or the cytosolic side of the SERCA-PLB interface. At this [Ca²⁺], a single Ca²⁺ ion was translocated from the cytosol to the transmembrane transport sites. We used this Ca²⁺-bound complex as an initial structure to simulate the effects of saturating Ca²⁺ at physiological conditions ([Ca²⁺]_total ≈ 400 μm). At these conditions, ∼30% of the Ca²⁺-bound complexes exhibited structural features consistent with an inhibited state. However, in ∼70% of the Ca²⁺-bound complexes, Ca²⁺ moved to transport site I, recruited Glu⁷⁷¹ and Asp⁸⁰⁰, and disrupted key inhibitory contacts involving the conserved PLB residue Asn³⁴ Structural analysis showed that Ca²⁺ induces only local changes in interresidue inhibitory interactions, but does not induce repositioning or changes in PLB structural dynamics. Upon relief of SERCA inhibition, Ca²⁺ binding produced a site I configuration sufficient for subsequent SERCA activation. We propose that at saturating [Ca²⁺] and in the absence of PLB phosphorylation, binding of a single Ca²⁺ ion in the transport sites rapidly shifts the equilibrium toward a noninhibited SERCA-PLB complex.

Insertional Mutagenesis Confounds the Mechanism of the Morbid Phenotype of a PLNR9C Transgenic Mouse Line

BACKGROUND

A mouse line with heterozygous transgenic expression of phospholamban carrying a substitution of cysteine for arginine 9 (TgPLN^R9C) under the control of α-myosin heavy chain (αMHC) promoter features dilated cardiomyopathy, heart failure, and premature death.

METHODS AND RESULTS

Determination of transgene chromosomal localization by conventional methods shows that in this line the transgenic array of 13 PLN^R9C expression cassettes, arranged in a head-to-tail tandem orientation, have integrated into the bidirectional promoter of the αMHC (Myh6) gene and the gene for the regulatory noncoding RNA Myheart (Mhrt), both of which are known to be involved in cardiac development and pathology. Expression of the noncoding RNA Mhrt in TgPLN^R9C mice exhibits profound deregulation, despite the presence of the second, intact allele.

CONCLUSIONS

The TgPLN^R9C mouse strain is, in the best case, a functionally ambiguous phenocopy of the human PLN^R9C heterozygote, because a similar constellation of genetically programmed events can not occur in a patient. Publications featuring "cardiac-specific overexpression" are focused on the phenotype and typically forgo the definition of the transgene integration site or transgene temporal expression profile, so caution should be exercised in attributing clinical relevance to pathologic phenomena observed in αMHC-driven transgenes.

The antiapoptotic protein HAX-1 mediates half of phospholamban's inhibitory activity on calcium cycling and contractility in the heart

The antiapoptotic protein HAX-1 (HS-associated protein X-1) localizes to sarcoplasmic reticulum (SR) in the heart and interacts with the small membrane protein phospholamban (PLN), inhibiting the cardiac sarco/endoplasmic reticulum calcium ATPase 2a (SERCA2a) in the regulation of overall calcium handling and heart muscle contractility. However, because global HAX-1 deletion causes early lethality, how much endogenous HAX-1 contributes to PLN's inhibitory activity on calcium cycling is unknown. We therefore generated a cardiac-specific and inducible knock-out mouse model. HAX-1 ablation in the adult heart significantly increased contractile parameters and calcium kinetics, associated with increased SR calcium load. These changes occurred without any changes in the protein expression of SERCA2a, PLN, and ryanodine receptor or in the PLN phosphorylation status. The enhanced calcium cycling in the HAX-1-depleted heart was mediated through increases in the calcium affinity of SERCA2a and reduced PLN-SERCA2a binding. Comparison of the HAX-1 deletion-induced stimulatory effects with those elicited by PLN ablation indicated that HAX-1 mediates ∼50% of the PLN-associated inhibitory effects in the heart. Stimulation with the inotropic and lusitropic agent isoproterenol eliminated the differences among wild-type, HAX-1-deficient, and PLN-deficient hearts, and maximally stimulated contractile and calcium kinetic parameters were similar among these three groups. Furthermore, PLN overexpression in the HAX-1-null cardiomyocytes did not elicit any inhibitory effects, indicating that HAX-1 may limit PLN activity. These findings suggest that HAX-1 is a major mediator of PLN's inhibitory activity and a critical gatekeeper of SR calcium cycling and contractility in the heart.

Conformational memory in the association of the transmembrane protein phospholamban with the sarcoplasmic reticulum calcium pump SERCA

The sarcoplasmic reticulum Ca²⁺-ATPase SERCA promotes muscle relaxation by pumping calcium ions from the cytoplasm into the sarcoplasmic reticulum. SERCA activity is regulated by a variety of small transmembrane peptides, most notably by phospholamban in cardiac muscle and sarcolipin in skeletal muscle. However, how phospholamban and sarcolipin regulate SERCA is not fully understood. In the present study, we evaluated the effects of phospholamban and sarcolipin on calcium translocation and ATP hydrolysis by SERCA under conditions that mimic environments in sarcoplasmic reticulum membranes. For pre-steady-state current measurements, proteoliposomes containing SERCA and phospholamban or sarcolipin were adsorbed to a solid-supported membrane and activated by substrate concentration jumps. We observed that phospholamban altered ATP-dependent calcium translocation by SERCA within the first transport cycle, whereas sarcolipin did not. Using pre-steady-state charge (calcium) translocation and steady-state ATPase activity under substrate conditions (various calcium and/or ATP concentrations) promoting particular conformational states of SERCA, we found that the effect of phospholamban on SERCA depends on substrate preincubation conditions. Our results also indicated that phospholamban can establish an inhibitory interaction with multiple SERCA conformational states with distinct effects on SERCA's kinetic properties. Moreover, we noted multiple modes of interaction between SERCA and phospholamban and observed that once a particular mode of association is engaged it persists throughout the SERCA transport cycle and multiple turnover events. These observations are consistent with conformational memory in the interaction between SERCA and phospholamban, thus providing insights into the physiological role of phospholamban and its regulatory effect on SERCA transport activity.

Phospholamban Is Downregulated by pVHL-Mediated Degradation through Oxidative Stress in Failing Heart

The E3 ubiquitin ligase, von Hippel–Lindau (VHL), regulates protein expression by polyubiquitination. Although the protein VHL (pVHL) was reported to be involved in the heart function, the underlying mechanism is unclear. Here, we show that pVHL was upregulated in hearts from two types of genetically dilated cardiomyopathy (DCM) mice models. In comparison with the wild-type mouse, both DCM mice models showed a significant reduction in the expression of phospholamban (PLN), a potent inhibitor of sarco(endo)plasmic reticulum Ca²⁺-ATPase, and enhanced interaction between pVHL and PLN. To clarify whether pVHL is involved in PLN degradation in failing hearts, we used carbonylcyanide m-chlorophenylhydrazone (CCCP), a mitochondrial membrane potential (MMP)-lowering reagent, to mimic the heart failure condition in PLN-expressing HEK293 cells and found that CCCP treatment resulted in PLN degradation and increased interaction between PLN and pVHL. However, these effects were reversed with the addition of N-acetyl-l-cysteine. Furthermore, the co-transfection of VHL and PLN in HEK293 cells decreased PLN expression under oxidative stress, whereas knockdown of VHL increased PLN expression both under normal and oxidative stress conditions. Together, we propose that oxidative stress upregulates pVHL expression to induce PLN degradation in failing hearts.

Metabolic Stress-Induced Activation of AMPK and Inhibition of Constitutive Phosphoproteins Controlling Smooth Muscle Contraction: Evidence for Smooth Muscle Fatigue?

Metabolic stress diminishes smooth muscle contractile strength by a poorly defined mechanism. To test the hypothesis that metabolic stress activates a compensatory cell signaling program to reversibly downregulate contraction, arterial rings and bladder muscle strips in vitro were deprived of O₂ and glucose for 30 and 60 min (“starvation”) to induce metabolic stress, and the phosphorylation status of proteins involved in regulation of contraction and metabolic stress were assessed in tissues under basal and stimulated conditions. A 15–30 min recovery period (O₂ and glucose repletion) tested whether changes induced by starvation were reversible. Starvation decreased basal phosphorylation of myosin regulatory light chain (MLC-pS19) and of the rho kinase (ROCK) downstream substrates cofilin (cofilin-pS3) and myosin phosphatase targeting subunit MYPT1 (MYPT1-pT696 and MYPT1-pT853), and abolished the ability of contractile stimuli to cause a strong, sustained contraction. Starvation increased basal phosphorylation of AMPK (AMPK-pT172) and 3 downstream AMPK substrates, acetyl-CoA carboxylase (ACC-pS79), rhoA (rhoA-pS188), and phospholamban (PLB-pS16). Increases in rhoA-pS188 and PLB-pS16 would be expected to inhibit contraction. Recovery restored basal AMPK-pT172 and MLC-pS19 to control levels, and restored contraction. In AMPKα₂ deficient mice (AMPKα2-/-), the basal level of AMPK-pT172 was reduced by 50%, and MLC-pS19 was elevated by 50%, but AMPKα2-/- did not prevent starvation-induced contraction inhibition nor enhance recovery from starvation. These results indicate that constitutive AMPK activity participates in constitutive regulation of contractile proteins, and suggest that AMPK activation is necessary, but may not be sufficient, to cause smooth muscle contraction inhibition during metabolic stress.

Prostaglandin E2 Reduces Cardiac Contractility via EP3 Receptor

BACKGROUND

Prostaglandin E2 (PGE2) EP receptors EP3 and EP4 signal via decreased and increased cAMP production, respectively. Previously, we reported that cardiomyocyte-specific EP4 knockout mice develop dilated cardiomyopathy with reduced ejection fraction. Thus, we hypothesized that PGE2 increases contractility via EP4 but decreases contractility via EP3.

METHODS AND RESULTS

The effects of PGE2 and the EP1/EP3 agonist sulprostone on contractility were examined in the mouse Langendorff preparation and in adult mouse cardiomyocytes. Isolated hearts of adult male C57Bl/6 mice were perfused with PGE2 (10(-6) M) or sulprostone (10(-6) M) and compared with vehicle. Both PGE2 and sulprostone decreased +dp/dt (P<0.01) and left ventricular developed pressure (P<0.001) with reversal by an EP3 antagonist. In contrast, the EP4 agonist had the opposite effect. Adult mouse cardiomyocytes contractility was also reduced after treatment with either PGE2 or sulprostone for 10 minutes. We then examined the acute effects of PGE2, sulprostone, and the EP4 agonist on expression of phosphorylated phospholamban and sarcoendoplasmic reticulum Ca(2+)-ATPase 2a in adult mouse cardiomyocytes using Western blot. Treatment with either PGE2 or sulprostone decreased expression of phosphorylated phospholamban corrected to total phospholamban, whereas treatment with the EP4 agonist had the opposite effect. Sarcoendoplasmic reticulum Ca(2+)-ATPase 2a expression was unaffected. Finally, we examined the effect of these compounds in vivo using pressure-volume loops. Both PGE2 and sulprostone decreased +dp/dt, whereas the EP4 agonist increased +dp/dt.

CONCLUSIONS

Contractility is reduced via the EP3 receptor but increased via EP4. These effects may be mediated through changes in phospholamban phosphorylation and has relevance to detrimental effects of inflammation.

Deregulated Ca2+ cycling underlies the development of arrhythmia and heart disease due to mutant obscurin

Obscurins are cytoskeletal proteins with structural and regulatory roles encoded by OBSCN. Mutations in OBSCN are associated with the development of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Specifically, the R4344Q mutation present in immunoglobulin domain 58 (Ig58) was the first to be linked with the development of HCM. To assess the effects of R4344Q in vivo, we generated the respective knock-in mouse model. Mutant obscurins are expressed and incorporated normally into sarcomeres. The expression patterns of sarcomeric and Ca2+-cycling proteins are unaltered in sedentary 1-year-old knock-in myocardia, with the exception of sarco/endoplasmic reticulum Ca2+ adenosine triphosphatase 2 (SERCA2) and pentameric phospholamban whose levels are significantly increased and decreased, respectively. Isolated cardiomyocytes from 1-year-old knock-in hearts exhibit increased Ca2+-transients and Ca2+-load in the sarcoplasmic reticulum and faster contractility kinetics. Moreover, sedentary 1-year-old knock-in animals develop tachycardia accompanied by premature ventricular contractions, whereas 2-month-old knock-in animals subjected to pressure overload develop a DCM-like phenotype. Structural analysis revealed that the R4344Q mutation alters the distribution of electrostatic charges over the Ig58 surface, thus interfering with its binding capabilities. Consistent with this, wild-type Ig58 interacts with phospholamban modestly, and this interaction is markedly enhanced in the presence of R4344Q. Together, our studies demonstrate that under sedentary conditions, the R4344Q mutation results in Ca2+ deregulation and spontaneous arrhythmia, whereas in the presence of chronic, pathological stress, it leads to cardiac remodeling and dilation. We postulate that enhanced binding between mutant obscurins and phospholamban leads to SERCA2 disinhibition, which may underlie the observed pathological alterations.

Molecular Changes in Children with Heart Failure Undergoing Left Ventricular Assist Device Therapy

OBJECTIVE

To determine whether left ventricular assist device (LVAD) treatment in children with heart failure would result in the modification of molecular pathways involved in heart failure pathophysiology.

STUDY DESIGN

Forty-seven explanted hearts from children were studied (16 nonfailing control, 20 failing, and 11 failing post-LVAD implantation [F-LVAD]). Protein expression and phosphorylation states were determined by receptor binding assays and Western blots. mRNA expression was measured with real-time quantitative polymerase chain reaction. To evaluate for interactions and identify correlations, 2-way ANOVA and regression analysis were performed.

RESULTS

Treatment with LVAD resulted in recovery of total β-adrenergic receptor expression and β₁-adrenergic receptor (β₁-AR) in failing hearts to normal levels (β-adrenergic receptor expression : 67.2 ± 11.5 fmol/mg failing vs 99.5 ± 27.7 fmol/mg nonfailing, 104 ± 38.7 fmol/mg F-LVAD, P ≤ .01; β₁-AR: 52.2 ± 10.3 fmol/mg failing vs 83.0 ± 23 fmol/mg non-failing, 76.5 ± 32.1 fmol/mg F-LVAD P ≤ .03). The high levels of G protein-coupled receptor kinase-2 were returned to nonfailing levels after LVAD treatment (5.6 ± 9.0 failing vs 1.0 ± 0.493 nonfailing, 1.0 ± 1.3 F-LVAD). Interestingly, β₂-adrenergic receptor expression was significantly greater in F-LVAD (27.5 ± 12; P < .005) hearts compared with nonfailing (16.4 ± 6.1) and failing (15.1 ± 4.2) hearts. Phospholamban phosphorylation at serine 16 was significantly greater in F-LVAD (7.7 ± 11.7) hearts compared with nonfailing (1.0 ± 1.2, P = .02) and failing (0.8 ± 1.0, P = .01) hearts. Also, atrial natriuretic factor (0.6 ± 0.8) and brain natriuretic peptide (0.1 ± 0.1) expression in F-LVAD was significantly lower compared with failing hearts (2.8 ± 3.6, P = .01 and 0.6 ± 0.7, P = .02).

CONCLUSION

LVAD treatment in children with heart failure results in reversal of several pathologic myocellular processes, and G protein-coupled receptor kinase-2 may regulate β₁-AR but not β₂-adrenergic receptor expression in children with heart failure.

Phospholamban Inhibition by a Single Dose of Locked Nucleic Acid Antisense Oligonucleotide Improves Cardiac Contractility in Pressure Overload-Induced Systolic Dysfunction in Mice

BACKGROUND

Phospholamban (PLN) inhibition enhances calcium cycling and is a potential novel therapy for heart failure (HF). Antisense oligonucleotides (ASOs) are a promising tool for unmet medical needs. Nonviral vector use of locked nucleic acid (LNA)-modified ASOs (LNA-ASOs), which shows strong binding to target RNAs and is resistant to nuclease, is considered to have a potential for use in novel therapeutics in the next decades. Thus, the efficacy of a single-dose injection of LNA-ASO for cardiac disease needs to be elucidated. We assessed the therapeutic efficacy of a single-dose LNA-ASO injection targeting PLN in pressure overload-induced cardiac dysfunction.

METHODS AND RESULTS

Mice intravenously injected with Cy3-labeled LNA-ASO displayed Cy3 fluorescence in the liver and heart 24 hours after injection. Subsequently, male C57BL/6 mice were subjected to sham or transverse aortic constriction surgery; after 3 weeks, these were treated with PLN-targeting LNA-ASO (0.3 mg/kg) or scrambled LNA-ASO. Cardiac function was measured by echocardiography before and 1 week after injection. Phospholamban-targeting LNA-ASO treatment significantly improved fractional shortening (FS) by 6.5%, whereas administration of the scrambled LNA-ASO decreased FS by 4.0%.

CONCLUSION

Our study revealed that a single-dose injection of PLN-targeting LNA-ASO improved contractility in pressure overload-induced cardiac dysfunction, suggesting that LNA-ASO is a promising tool for hypertensive HF treatment.

Global phosphoproteomic profiling reveals perturbed signaling in a mouse model of dilated cardiomyopathy

Phospholamban (PLN) plays a central role in Ca²⁺ homeostasis in cardiac myocytes through regulation of the sarco(endo)plasmic reticulum Ca²⁺-ATPase 2A (SERCA2A) Ca²⁺ pump. An inherited mutation converting arginine residue 9 in PLN to cysteine (R9C) results in dilated cardiomyopathy (DCM) in humans and transgenic mice, but the downstream signaling defects leading to decompensation and heart failure are poorly understood. Here we used precision mass spectrometry to study the global phosphorylation dynamics of 1,887 cardiac phosphoproteins in early affected heart tissue in a transgenic R9C mouse model of DCM compared with wild-type littermates. Dysregulated phosphorylation sites were quantified after affinity capture and identification of 3,908 phosphopeptides from fractionated whole-heart homogenates. Global statistical enrichment analysis of the differential phosphoprotein patterns revealed selective perturbation of signaling pathways regulating cardiovascular activity in early stages of DCM. Strikingly, dysregulated signaling through the Notch-1 receptor, recently linked to cardiomyogenesis and embryonic cardiac stem cell development and differentiation but never directly implicated in DCM before, was a prominently perturbed pathway. We verified alterations in Notch-1 downstream components in early symptomatic R9C transgenic mouse cardiomyocytes compared with wild type by immunoblot analysis and confocal immunofluorescence microscopy. These data reveal unexpected connections between stress-regulated cell signaling networks, specific protein kinases, and downstream effectors essential for proper cardiac function.

The Autonomic Nervous System Regulates the Heart Rate through cAMP-PKA Dependent and Independent Coupled-Clock Pacemaker Cell Mechanisms

Sinoatrial nodal cells (SANCs) generate spontaneous action potentials (APs) that control the cardiac rate. The brain modulates SANC automaticity, via the autonomic nervous system, by stimulating membrane receptors that activate (adrenergic) or inactivate (cholinergic) adenylyl cyclase (AC). However, these opposing afferents are not simply additive. We showed that activation of adrenergic signaling increases AC-cAMP/PKA signaling, which mediates the increase in the SANC AP firing rate (i.e., positive chronotropic modulation). However, there is a limited understanding of the underlying internal pacemaker mechanisms involved in the crosstalk between cholinergic receptors and the decrease in the SANC AP firing rate (i.e., negative chronotropic modulation). We hypothesize that changes in AC-cAMP/PKA activity are crucial for mediating either decrease or increase in the AP firing rate and that the change in rate is due to both internal and membrane mechanisms. In cultured adult rabbit pacemaker cells infected with an adenovirus expressing the FRET sensor AKAR3, PKA activity and AP firing rate were tightly linked in response to either adrenergic receptor stimulation (by isoproterenol, ISO) or cholinergic stimulation (by carbachol, CCh). To identify the main molecular targets that mediate between PKA signaling and pacemaker function, we developed a mechanistic computational model. The model includes a description of autonomic-nervous receptors, post- translation signaling cascades, membrane molecules, and internal pacemaker mechanisms. Yielding results similar to those of the experiments, the model simulations faithfully reproduce the changes in AP firing rate in response to CCh or ISO or a combination of both (i.e., accentuated antagonism). Eliminating AC-cAMP-PKA signaling abolished the core effect of autonomic receptor stimulation on the AP firing rate. Specifically, disabling the phospholamban modulation of the SERCA activity resulted in a significantly reduced effect of CCh and a failure to increase the AP firing rate under ISO stimulation. Directly activating internal pacemaker mechanisms led to a similar extent of changes in the AP firing rate with respect to brain receptor stimulation. Thus, Ca²⁺ and cAMP/PKA-dependent phosphorylation limits the rate and magnitude of chronotropic changes in the spontaneous AP firing rate.

Malonate in the nucleotide-binding site traps human AKAP18γ/δ in a novel conformational state

A-kinase anchoring proteins (AKAPs) are a family of proteins that provide spatiotemporal resolution of protein kinase A (PKA) phosphorylation. In the myocardium, PKA and AKAP18γ/δ are found in complex with sarcoendoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) and phospholamban (PLB). This macromolecular complex provides a means by which anchored PKA can dynamically regulate cytoplasmic Ca(2+) release and re-uptake. For this reason, AKAP18γ/δ presents an interesting drug target with therapeutic potential in cardiovascular disease. The crystal structure of the central domain of human AKAP18γ has been determined at the atomic resolution of 1.25 Å. This first structure of human AKAP18γ is trapped in a novel conformation by a malonate molecule bridging the important R-loop with the 2H phosphoesterase motif. Although the physiological substrate of AKAP18γ is currently unknown, a potential proton wire deep in the central binding crevice has been indentified, leading to bulk solvent below the R-loop. Malonate complexed with AKAP18γ at atomic resolution provides an excellent starting point for structure-guided drug design.

CaMKII-dependent phosphorylation regulates basal cardiac pacemaker function via modulation of local Ca2+ releases

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) due to gradual change of the membrane potential called diastolic depolarization (DD). Spontaneous, submembrane local Ca(2+) releases (LCR) from ryanodine receptors (RyR) occur during late DD and activate an inward Na(+)/Ca(2+)exchange current to boost the DD rate and fire an action potential (AP). Here we studied the extent of basal Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation and the role of basal CaMKII-dependent protein phosphorylation in generation of LCRs and regulation of normal automaticity of intact rabbit SANC. The basal level of activated (autophosphorylated) CaMKII in rabbit SANC surpassed that in ventricular myocytes (VM) by approximately twofold, and this was accompanied by high basal level of protein phosphorylation. Specifically, phosphorylation of phospholamban (PLB) at the CaMKII-dependent Thr(17) site was approximately threefold greater in SANC compared with VM, and RyR phosphorylation at CaMKII-dependent Ser(2815) site was ∼10-fold greater in the SA node, compared with that in ventricle. CaMKII inhibition reduced phosphorylation of PLB and RyR, decreased LCR size, increased LCR periods (time from AP-induced Ca(2+) transient to subsequent LCR), and suppressed spontaneous SANC firing. Graded changes in CaMKII-dependent phosphorylation (indexed by PLB phosphorylation at the Thr(17)site) produced by CaMKII inhibition, β-AR stimulation or phosphodiesterase inhibition were highly correlated with changes in SR Ca(2+) replenishment times and LCR periods and concomitant changes in spontaneous SANC cycle lengths (R(2) = 0.96). Thus high basal CaMKII activation modifies the phosphorylation state of Ca(2+) cycling proteins PLB, RyR, L-type Ca(2+) channels (and likely others), adjusting LCR period and characteristics, and ultimately regulates both normal and reserve cardiac pacemaker function.

Myocardial Response to Milrinone in Single Right Ventricle Heart Disease

OBJECTIVES

Empiric treatment with milrinone, a phosphodiesterase (PDE) 3 inhibitor, has become increasingly common in patients with single ventricle heart disease of right ventricular (RV) morphology (SRV); our objective was to characterize the myocardial response to PDE3 inhibition (PDE3i) in the pediatric population with SRV.

STUDY DESIGN

Cyclic adenosine monophosphate levels, PDE activity, and phosphorylated phospholamban (PLN) were determined in explanted human ventricular myocardium from nonfailing pediatric donors (n = 10) and pediatric patients transplanted secondary to SRV. Subjects with SRV were further classified by PDE3i treatment (n = 13 with PDE3i and n = 12 without PDE3i).

RESULTS

In comparison with nonfailing RV myocardium (n = 8), cyclic adenosine monophosphate levels are lower in patients with SRV treated with PDE3i (n = 12, P = .021). Chronic PDE3i does not alter total PDE or PDE3 activity in SRV myocardium. Compared with nonfailing RV myocardium, SRV myocardium (both with and without PDE3i) demonstrates equivalent phosphorylated PLN at the protein kinase A phosphorylation site.

CONCLUSIONS

As evidenced by preserved phosphorylated PLN, the molecular adaptation associated with SRV differs significantly from that demonstrated in pediatric heart failure because of dilated cardiomyopathy. These alterations support a pathophysiologically distinct mechanism of heart failure in pediatric patients with SRV, which has direct implications regarding the presumed response to PDE3i treatment in this population.

A Single Protein Kinase A or Calmodulin Kinase II Site Does Not Control the Cardiac Pacemaker Ca2+ Clock

BACKGROUND

Fight or flight heart rate (HR) increases depend on protein kinase A (PKA)- and calmodulin kinase II (CaMKII)-mediated enhancement of Ca(2+) uptake and release from sarcoplasmic reticulum (SR) in sinoatrial nodal cells (SANC). However, the impact of specific PKA and CaMKII phosphorylation sites on HR is unknown.

METHODS AND RESULTS

We systematically evaluated validated PKA and CaMKII target sites on phospholamban and the ryanodine receptor using genetically modified mice. We found that knockin alanine replacement of ryanodine receptor PKA (S2808) or CaMKII (S2814) target sites failed to affect HR responses to isoproterenol or spontaneous activity in vivo or in SANC. Similarly, selective mutation of phospholamban amino acids critical for enhancing SR Ca(2+) uptake by PKA (S16) or CaMKII (T17) to alanines did not affect HR in vivo or in SANC. In contrast, CaMKII inhibition by expression of AC3-I has been shown to slow SANC rate responses to isoproterenol and decrease SR Ca(2+) content. Phospholamban deficiency rescued SR Ca(2+) content and SANC rate responses to isoproterenol in mice with AC3-I expression, suggesting that CaMKII affects HR by modulation of SR Ca(2+) content. Consistent with this, mice expressing a superinhibitory phospholamban mutant had low SR Ca(2+) content and slow HR in vivo and in SANC.

CONCLUSIONS

SR Ca(2+) depletion reduces HR and SR Ca(2+) repletion restores physiological SANC rate responses, despite CaMKII inhibition. PKA and CaMKII do not affect HR by a unique target site governing SR Ca(2+) uptake or release. HR acceleration may require an SR Ca(2+) content threshold.

Diaphragm assessment in mice overexpressing phospholamban in slow-twitch type I muscle fibers

AIMS

Phospholamban (PLN) and sarcolipin (SLN) are small inhibitory proteins that regulate the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) pump. Previous work from our laboratory revealed that in the soleus and gluteus minimus muscles of mice overexpressing PLN (Pln (OE)), SERCA function was impaired, dynamin 2 (3-5 fold) and SLN (7-9 fold) were upregulated, and features of human centronuclear myopathy (CNM) were observed. Here, we performed structural and functional experiments to evaluate whether the diaphragm muscles of the Pln (OE) mouse would exhibit CNM pathology and muscle weakness.

METHODS

Diaphragm muscles from Pln (OE) and WT mice were subjected to histological/histochemical/immunofluorescent staining, Ca(2+)-ATPase and Ca(2+) uptake assays, Western blotting, and in vitro electrical stimulation.

RESULTS

Our results demonstrate that PLN overexpression reduced SERCA's apparent affinity for Ca(2+) but did not reduce maximal SERCA activity or rates of Ca(2+) uptake. SLN was upregulated 2.5-fold, whereas no changes in dynamin 2 expression were found. With respect to CNM, we did not observe type I fiber predominance, central nuclei, or central aggregation of oxidative activity in diaphragm, although type I fiber hypotrophy was present. Furthermore, in vitro contractility assessment of Pln (OE) diaphragm strips revealed no reductions in force-generating capacity, maximal rates of relaxation or force development, but did indicate that ½ relaxation time was prolonged.

CONCLUSIONS

Therefore, the effects of PLN overexpression on skeletal muscle phenotype differ between diaphragm and the postural soleus and gluteus minimus muscles. Our findings here point to differences in SLN expression and type I fiber distribution as potential contributing factors.