Preservation of the in vivo phosphorylation status of phospholamban in the heart: evidence for a site-specific difference in the dephosphorylation of phospholamban

Synthetic phosphopeptides enable quantitation of the content and function of the four phosphorylation states of phospholamban in cardiac muscle

We have studied the differential effects of phospholamban (PLB) phosphorylation states on the activity of the sarcoplasmic reticulum Ca-ATPase (SERCA). It has been shown that unphosphorylated PLB (U-PLB) inhibits SERCA and that phosphorylation of PLB at Ser-16 or Thr-17 relieves this inhibition in cardiac sarcoplasmic reticulum. However, the levels of the four phosphorylation states of PLB (U-PLB, P16-PLB, P17-PLB, and doubly phosphorylated 2P-PLB) have not been measured quantitatively in cardiac tissue, and their functional effects on SERCA have not been determined directly. We have solved both problems through the chemical synthesis of all four PLB species. We first used the synthetic PLB as standards for a quantitative immunoblot assay, to determine the concentrations of all four PLB phosphorylation states in pig cardiac tissue, with and without left ventricular hypertrophy (LVH) induced by aortic banding. In both LVH and sham hearts, all phosphorylation states were significantly populated, but LVH hearts showed a significant decrease in U-PLB, with a corresponding increase in the ratio of total phosphorylated PLB to U-PLB. To determine directly the functional effects of each PLB species, we co-reconstituted each of the synthetic peptides in phospholipid membranes with SERCA and measured calcium-dependent ATPase activity. SERCA inhibition was maximally relieved by P16-PLB (the most highly populated PLB state in cardiac tissue homogenates), followed by 2P-PLB, then P17-PLB. These results show that each PLB phosphorylation state uniquely alters Ca(2+) homeostasis, with important implications for cardiac health, disease, and therapy.

Accurate quantitation of phospholamban phosphorylation by immunoblot

We have developed a quantitative immunoblot method to measure the mole fraction of phospholamban (PLB) phosphorylated at Ser16 (X(p)) in biological samples. In cardiomyocytes, PLB phosphorylation activates the sarcoplasmic reticulum calcium ATPase (SERCA), which reduces cytoplasmic Ca(2+) to relax the heart during diastole. Unphosphorylated PLB (uPLB) inhibits SERCA at low [Ca(2+)] but phosphorylated PLB (pPLB) is less inhibitory, so myocardial physiology and pathology depend critically on X(p). Current methods of X(p) determination by immunoblot provide moderate precision but poor accuracy. We have solved this problem using purified uPLB and pPLB standards produced by solid-phase peptide synthesis. In each assay, a pair of blots is performed with identical standards and unknowns using antibodies partially selective for uPLB and pPLB, respectively. When performed on mixtures of uPLB and pPLB, the assay measures both total PLB (tPLB) and X(p) with accuracy of 96% or better. We assayed pig cardiac sarcoplasmic reticulum (SR) and found that X(p) varied widely among four animals, from 0.08 to 0.38, but there was remarkably little variation in the ratios of X(p)/tPLB and uPLB/SERCA, suggesting that PLB phosphorylation is tuned to maintain homeostasis in SERCA regulation.

Insulin-dependent rescue from cardiogenic shock is not mediated by phospholamban phosphorylation

INTRODUCTION

We used immunoblots to determine whether inotropic and lusitropic effects of high-dose insulin (HDI) in cardiogenic shock, induced by a beta-blocker (BB) or a calcium channel blocker (CCB), are mediated by phosphorylation of phospholamban (PLB). PLB is a membrane protein that regulates calcium uptake into the sarcoplasmic reticulum (SR) by inhibition of the cardiac calcium pump (SERCA2a). Phosphorylation of PLB relieves SERCA inhibition, thus enhancing diastolic relaxation and preload.

METHODS

Our Institutional Animal Care and Use Committee approved this research. Swine myocardia from six groups were flash frozen immediately upon death or sacrifice. Groups 1-6 received: (1) no medications, (2) HDI and glucose only, (3) toxic propranolol infusions and saline resuscitation, (4) toxic propranolol infusions and HDI resuscitation, (5) toxic verapamil infusions and saline resuscitation, and (6) toxic verapamil infusions and HDI resuscitation. Groups 3-6 were resuscitated for 4 h. Tissue samples from all six groups were analyzed by quantitative immunoblots, using antibodies to both unphosphorylated PLB (uPLB) and phosphorylated PLB (pPLB), to determine the total PLB content and the fraction of PLB phosphorylated.

RESULTS

There were no differences in either pPLB or total PLB in cardiac tissue among any of the six groups. However, infusion of a pig with the beta-adrenergic agonist, isoproterenol, produced enhanced PLB phosphorylation.

CONCLUSION

The mechanism by which HDI produces its inotropic and lusitropic effects in CCB- and BB-induced cardiovascular toxicity, resulting in resuscitation, is not due to changes in phosphorylation of PLB or a change in the total PLB in the SR.

Compartmentalisation of cAMP-dependent signalling by caveolae in the adult cardiac myocyte

Cyclic AMP exhibits local (sarcolemmal) and global (cytosolic) patterns of signalling, allowing receptor-specific signals to be generated by a single second messenger. Here we determine whether caveolae, invaginated lipid rafts, are responsible for confining the beta(2) adrenoceptor (AR) cAMP signal to the sarcolemmal compartment. Myocytes were treated with the cholesterol-depleting agent methyl-beta-cyclodextrin (M beta C) to disrupt caveolae. Caveolae-containing membrane fractions were isolated by detergent-free sucrose gradient fractionation. Cell shortening and phosphorylation of the sarcoplasmic reticular protein phospholamban (PLB) and the myofilament protein troponin I (TnI) were measured in response to beta(2) AR stimulation (with salbutamol in the presence of 1 microM atenolol). Ser(16) phosphorylation of PLB (pPLB), Ser(22,23) phosphorylation of TnI (pTnI), and positive lusitropy were used as indices of global cAMP signals. The ability of M beta C to disrupt caveolae was confirmed by selective depletion of the buoyant membrane fractions of cholesterol and caveolin 3, the 2 essential components of caveolae. In control cells, no change in pPLB, pTnI or time to half relaxation was recorded with beta(2) AR stimulation (P>0.05), but following caveolar disruption a 60-70% increase in phosphorylation of both proteins was seen, accompanied by positive lusitropy (P<0.05). These data show for the first time that disrupting caveolae converts the sarcolemmal-confined cAMP signal associated with beta(2) AR stimulation to a global signal that targets proteins of the sarcoplasmic reticulum and myofilaments, with functional sequelae. The role of caveolae in spatial control of cAMP may be relevant to perturbation of beta AR signalling in cardiovascular disease.

Direct inhibition of type 5 adenylyl cyclase prevents myocardial apoptosis without functional deterioration

Adenylyl cyclase, a major target enzyme of beta-adrenergic receptor signals, is potently and directly inhibited by P-site inhibitors, classic inhibitors of this enzyme, when the enzyme catalytic activity is high. Unlike beta-adrenergic receptor antagonists, this is a non- or uncompetitive inhibition with respect to ATP. We have examined whether we can utilize this enzymatic property to regulate the effects of beta-adrenergic receptor stimulation differentially. After screening multiple new and classic compounds, we found that some compounds, including 1R,4R-3-(6-aminopurin-9-yl)-cyclopentanecarboxylic acid hydroxyamide, potently inhibited type 5 adenylyl cyclase, the major cardiac isoform, but not other isoforms. In normal mouse cardiac myocytes, contraction induced by low beta-adrenergic receptor stimulation was poorly inhibited with this compound, but the induction of cardiac myocyte apoptosis by high beta-adrenergic receptor stimulation was effectively prevented by type 5 adenylyl cyclase inhibitors. In contrast, when cardiac myocytes from type 5 adenylyl cyclase knock-out mice were examined, beta-adrenergic stimulation poorly induced apoptosis. Our data suggest that the inhibition of beta-adrenergic signaling at the level of the type 5 adenylyl cyclase isoform by P-site inhibitors may serve as an effective method to prevent cardiac myocyte apoptosis induced by excessive beta-adrenergic stimulation without deleterious effect on cardiac myocyte contraction.

Critical Evaluation of Cardiac Ca2+-ATPase Phosphorylation on Serine 38 Using a Phosphorylation Site-specific Antibody

The phosphorylation of the cardiac muscle isoform of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) on serine 38 has been described as a regulatory event capable of very significant enhancement of enzyme activity (Hawkins, C., Xu, A., and Narayanan, N. (1994) J. Biol. Chem. 269, 31198-31206). Independent confirmation of these observations has not been forthcoming. This study has utilized a polyclonal antibody specific for the phosphorylated serine 38 epitope on the Ca(2+)-ATPase to evaluate the phosphorylation of SERCA2a in isolated sarcoplasmic reticulum vesicles and isolated rat ventricular myocytes. A quantitative Western blot approach failed to detect serine 38-phosphorylated Ca(2+)-ATPase in either kinase-treated sarcoplasmic reticulum vesicles or suitably stimulated cardiac myocytes. Calibration standards confirmed that the detection sensitivity of assays was adequate to detect Ser-38 phosphorylation if it occurred on at least 1% of Ca(2+)-ATPase molecules in SR vesicle experiments or on at least 0.1% of Ca(2+)-ATPase molecules in cardiac myocytes. The failure to detect a phosphorylated form of the Ca(2+)-ATPase in either preparation (isolated myocyte, purified sarcoplasmic reticulum vesicles) suggests that Ser-38 phosphorylation of the Ca(2+)-ATPase is not a significant regulatory feature of cardiac Ca(2+) homeostasis.

Phosphorylation of phospholamban at threonine-17 in the absence and presence of beta-adrenergic stimulation in neonatal rat cardiomyocytes

The site-specific phospholamban phosphorylation was studied with respect to the interplay of cAMP- and Ca(2+)signaling in neonatal rat cardiomyocytes. To elucidate the signal pathway(s) for the activation of Ca(2+)/calmodulin-dependent protein kinase (CaMKII) we studied Thr17 phosphorylation of phospholamban in dependence of Ca(2+)channel activation by S(-)-Bay K8644 and in dependence of the depletion of the sarcoplasmic reticulum Ca(2+)stores by ryanodine or thapsigargin in the absence or presence of beta -adrenergic stimulation. The isoproterenol (0.1 microM)-induced Thr17 phosphorylation was potentiated 2.5-fold in presence of 1 microM S(-)-Bay K8644. Interestingly, S(-)-Bay K8644 alone was also able to induce Thr17 phosphorylation in a dose- and time-dependent fashion. Ryanodine (1.0 microM) reduced both the isoproterenol (0.1 microM) and S(-)-Bay K8644-(1 microM) mediated Thr17 phosphorylation by about 90%. Thapsigargin (1 microM) diminished the S(-)-Bay K8644 and isoproterenol-associated Thr17 phosphorylation by 53.5+/-6.3% and 92. 5+/-11.1%, respectively. Ser16 phosphorylation was not affected under these conditions. KN-93 reduced the Thr17 phosphorylation by S(-)-Bay K8644 and isoproterenol to levels of 1.1+/-0.3% and 8.6+/-2. 1%, respectively. However, the effect of KN-93 was attenuated (47. 8+/-3.6%) in isoproterenol prestimulated cells. Protein phosphatase inhibition by okadaic acid increased exclusively the Ser16 phosphorylation. In summary, our results reflect a cross-talk between beta -adrenoceptor stimulation and intracellular Ca(2+)at the level of CaMKII-mediated phospholamban phosphorylation in neonatal rat cardiomyocytes. We report conditions which exclusively produce Thr17 or Ser16 phosphorylation. We postulate that Ca(2+)transport systems of the sarcoplasmic reticulum are critical determinants for the activation of CaMKII that catalyzes phosphorylation of phospholamban.

Characterization and quantitation of phospholamban and its phosphorylation state using antibodies

Quantitative immunoassays to discriminate and quantitate phospholamban and its phosphorylation states in heart homogenates were developed using known amounts of protein determined by amino acid analysis. Synthetic 1-52 phospholamban, the hydrophilic 1-25 peptide, and 1-25 phosphopeptides containing P-Ser(16), P-Thr(17), and dually phosphorylated (P-Ser(16), P-Thr(17)) were used to calibrate immunoblot systems. In addition, synthetic 1-52 peptide was phosphorylated using cAMP-dependent protein kinase (P-Ser(16)) or Ca(2+)-calmodulin protein kinase (P-Thr(17)) and then separated from unphosphorylated 1-52 by HPLC prior to quantitation. Further, canine cardiac sarcoplasmic reticulum was phosphorylated in vitro using [gamma-(32)P]-ATP with cAMP-dependent protein kinase and/or Ca(2+)-calmodulin-dependent protein kinase as well as sequential phosphorylation in both orders to assess the veracity of antibody recognition of phosphorylated forms. Western blots proved useful in characterizing the reactivity of the different antibodies to phospholamban and phosphorylated phospholamban, but were inefficient for accurate quantitation and problems with antibody recognition of dually phosphorylated phospholamban were found. mAb 1D11 recognized all forms of phospholamban, polyclonal antibodies 285 and PS-16 were highly selective for P-Ser(16) phospholamban but had diminished reactivity to diphosphorylated (P-Ser(16), P-Thr(17)) phospholamban, and polyclonal antibody PT-17, although selective for P-Thr(17) phospholamban, generated very weak signals on Western blots and reacted poorly with diphosphorylated phospholamban. Results in quantitative immunodot blot experiments were even more compelling. None of the phosphorylation specific antibodies reacted with the diphospho 1-25 phospholamban peptide. Transgenic mouse hearts expressing varying levels of PLB and ferret heart biopsy samples taken before and after isoproterenol perfusion were analyzed. In all samples containing phospholamban, a basal level of Ser(16) phosphorylation (about 4% of the total PLB population) and a lesser amount of Thr(17) phosphorylation was observed. Upon isoproterenol perfusion, Ser(16) phosphorylation increased only to 17% of the total phospholamban population with a similar change in Thr(17) phosphorylation. This suggests that phospholamban phosphorylation may serve as an electrostatic switch that dissociates inactive calcium pump complexes into catalytically active units. Thus, direct correlations between phospholamban phosphorylation state and contractile parameters may not be valid.