Published online before print June 26, 2008, doi: 10.1161/CIRCRESAHA.107.169953
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© 2008 American Heart Association, Inc.
Integrative Physiology |
Glycogen Synthase Kinase-3 Inactivation Is Not Required for Ischemic Preconditioning or Postconditioning in the Mouse
From the King’s College London BHF Centre (Y.N., I.G.W., A.I.A., J.E.C., S.J., A.M.S., M.A., M.S.M.), Cardiovascular Division, The Rayne Institute, St. Thomas’ Hospital, UK; The Hatter Cardiovascular Institute (S.M.D., D.M.Y.), University College London Hospital and Medical School, UK; and Second Department of Internal Medicine (T.M.), Sapporo Medical University School of Medicine, Japan.
Correspondence to Prof Michael S. Marber, Department of Cardiology, King’s College London, The Rayne Institute, St. Thomas’ Hospital, Lambeth Palace Rd, London SE1 7EH, United Kingdom. E-mail mike.marber{at}kcl.ac.uk
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Abstract |
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The inactivation of glycogen synthase kinase-3β (GSK-3β) is proposed as the event integrating protective pathways initiated by preconditioning and other interventions. The inactivation of GSK-3 is thought to decrease the probability of opening of the mitochondrial permeability transition pore. The aim of this study was to verify the role of GSK-3 using a targeted mouse line lacking the critical N-terminal serine within GSK-3β (Ser9) and the highly homologous GSK-3
(Ser21), which when phosphorylated results in kinase inactivation. Postconditioning with 10 cycles of 5 seconds of reperfusion/5 seconds of ischemia and preconditioning with 6 cycles of 4 minutes of ischemia/6 minutes of reperfusion, similarly reduced infarction of the isolated perfused mouse heart in response to 30 minutes of global ischemia and 120 minutes of reperfusion. Preconditioning caused noticeable inactivating phosphorylation of GSK-3. However, both preconditioning and postconditioning still protected hearts of homozygous GSK-3 double knockin mice. Moreover, direct pharmacological inhibition of GSK-3 catalytic activity with structurally diverse inhibitors before or after ischemia failed to recapitulate conditioning protection. Nonetheless, cyclosporin A, a direct mitochondrial permeability transition pore inhibitor, reduced infarction in hearts from both wild-type and homozygous GSK-3 double knockin mice. Furthermore, in adult cardiac myocytes from GSK-3 double knockin mice, insulin exposure was still as effective as cyclosporin A in delaying mitochondrial permeability transition pore opening. Our results, which include a novel genetic approach, suggest that the inhibition of GSK-3 is unlikely to be the key determinant of cardioprotective signaling in either preconditioning or postconditioning in the mouse.
Key Words: postconditioniong • preconditioning • GSK-3 • mPTP
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Introduction |
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Ischemic preconditioning is a powerful and established modulator of intrinsic myocardial resistance to lethal ischemia.1 Its clinical application in patients with acute myocardial infarction is limited as a result of the inability to predict the moment of coronary artery occlusion. More recently, Zhao et al described the concept of postconditioning as a novel strategy for protecting the heart against reperfusion-injury.2 They demonstrated that a similar regimen of brief periods of ischemia after a lethal index ischemic insult was as protective as preconditioning. This phenomenon has subsequently been confirmed in a variety of animal models, both in vivo and in isolated heart preparations,3 as well as in preliminary clinical interventional studies.4,5
The endogenous cellular and molecular mechanisms involved in cardioprotection against ischemia have been extensively investigated in the field of preconditioning.6 Because pre- and postconditioning are temporally remote, the signaling pathways might be expected to differ considerably.2,7 However, recent evidence suggests that the early reperfusion phase after lethal ischemia is crucial in mediating the cardioprotective effects of preconditioning.8,9 Furthermore, many of the mimetics and kinases of preconditioning have also been implicated in postconditioning.10,11 Finally, there is increasing evidence that the final step of both signaling pathways is inhibition of the mitochondrial permeability transition pore (mPTP),12–14 whereby pore opening results in cell death. Glycogen synthase kinase-3β (GSK-3β) has been reported as a common target of converging protective signals in preconditioning, immediately proximal to the mPTP, that inhibits pore opening in cardiomyocytes.15 Using both adult rat cardiomyocytes and neonatal rat cardiomyocytes, Juhaszova et al15 demonstrated that numerous preconditioning pathways converged to inactivate GSK-3β by Ser9 phosphorylation. Although the importance of GSK-3 in preconditioning is supported by the literature,16,17 these data predominantly rely on pharmacological manipulation, with all the inherent problems of target selectivity. There is, therefore, a need to verify the role of GSK-3 in both ischemic preconditioning and postconditioning in the whole heart by using genetic manipulation of the signaling axis.
GSK-3 exists as 2 isoforms, β and , which are highly homologous, similarly inhibited by protein kinase (PK)B/Akt-mediated phosphorylation of a critical N-terminal serine residue, and share common substrates and sensitivity to pharmacological inhibitors.18,19 Thus, the primary aim of this study was to clarify the role of GSK-3 in ischemic pre- and postconditioning in the whole heart, through the use of mice in which all GSK-3 alleles encode inactivation resistant kinases lacking the PKB/Akt-targeted Ser21() and Ser9 (β).
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Materials and Methods |
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Detailed methodology is provided in the online data supplement, available at http://circres.ahajournals.org. Key techniques involved adaptations of previously published protocols, including those for the perfusion and assessment of infarction in isolated murine hearts,20,21 immunoblot analysis,20,21 isolation of murine ventricular myocytes,22 and assay of the mitochondrial permeability transition.23
GSK-3 Knockin Mice
The targeting strategy used to generate GSK-3/β knockin (KI) mice, in which the PKB/Akt phosphorylation sites on GSK-3 (Ser 21) and GSK-3β (Ser9) are changed to Ala, has been described previously.18
Pre- and Postconditioning Experiments
Protocol 1
Our objective in this protocol was to assess the potential for postconditioning (PostC) to limit infarct size and compare it to preconditioning (PreC) in our model. C57BL/6 mice were divided into 4 study groups, as shown in Figure 1A. In the control group, there was no additional intervention. Hearts in the PreC group underwent 4 cycles of 4 minutes of ischemia/6 minutes of reperfusion before the 30 minutes of global ischemia. Hearts in the PostC group were subjected to 1 of 2 protocols to determine the optimum strategy: in the 30 seconds PostC and 5 seconds PostC groups, 4 cycles of 30 seconds of reperfusion/30 seconds of ischemia and 10 cycles of 5 seconds of reperfusion/5 seconds of ischemia were performed at the end of 30 minutes of index ischemia, respectively.
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Protocol 2
GSK-3/β KI mice were used to determine the dependence of pre- and postconditioning cardioprotection on GSK-3 inhibition. GSK-3 wild-type (WT) and KI mice were assigned to control, PreC, and PostC with repetitive 5 seconds of reperfusion/ischemia groups as described in protocol 1.
Protocol 3
In these experiments, the effects on infarct size of direct pharmacological inhibition of GSK-3 activity before and after ischemia were examined (Figure 1B). Two inhibitors, SB216763 (3 µmol/L) (Sigma) and 6-bromoindirubin-3'-oxime (BIO) (100 nmol/L) (Calbiochem), were used based on concentrations required to achieve dephosphorylation of glycogen synthase. C57BL/6 mice were divided into five groups (Figure 1B): (1) standard ischemia/reperfusion group (control); pretreatment with (2) 3 µmol/L SB216763 (Pre SB) or (3) 100 nmol/L BIO (Pre BIO) for the last 15 minutes of stabilization; and treatment postischemia with (4) 3 µmol/L SB216763 for the first 15 minutes of reperfusion (Post SB15) or (5) 3 µmol/L SB216763 for 2 hours (Post SB120). SB216763 and BIO were dissolved in dimethyl sulfoxide (DMSO) and then diluted with perfusion buffer so that the final concentration of DMSO was 0.05%.
Protocol 4
These experiments set out to determine whether the cardioprotective effect of direct pharmacological inhibition of mPTP opening at reperfusion is preserved in the GSK-3/β KI mouse. After 30 minutes of global ischemia, GSK-3 WT and KI hearts were subjected to either infusion of 0.2 µmol/L cyclosporin A (CsA) (Sigma) for 10 minutes at the moment of reperfusion or normal buffer (control) (Figure 1B). CsA was dissolved in DMSO at a final concentration of less than 0.05%. Similar experiments were also performed in isolated adult murine myocytes to visualize mPTP opening directly.
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Results |
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Pre- and Postconditioning the Isolated Murine Heart: Protocol 1
The PreC protocol was as used previously in this model.20 Postconditioning was examined using 2 reperfusion patterns to determine the optimum strategy (30 seconds PostC and 5 seconds PostC). The protocols, described in Figure 1A, resulted in significant reductions of infarction only in the case of PreC and 5 seconds PostC (see protocol 1 in Table II in the online data supplement), which was reflected by improved contractile recovery in these groups (see protocol 1 of supplemental Table I).
Despite improved recovery in the PreC and 5 seconds PostC groups, there was only a slight effect on GSK-3/β phosphorylation (see Figure 2A and 2B, respectively). On close examination GSK-3β, but not GSK-3, was phosphorylated after preconditioning (phospho-/total GSK-3β 335.3±38.1% of control baseline; P<0.05, n=5) and then dephosphorylated during ischemia to a level similar to that of control (150.5±82.4% of control baseline and 95.1±52.1% of time-matched control ischemia; P=NS, n=5 for both). During reperfusion, GSK-3β demonstrated some rephosphorylation that was comparable in preconditioned and control-reperfused hearts (Figure 2A) and far less than that seen with insulin. Postconditioning did not significantly alter GSK-3/β Ser21/9 phosphorylation compared to control (Figure 2B). Furthermore, we examined the phosphorylation status of Tyr 279/216 of GSK-3/β and found these sites similarly phosphorylated under all conditions (data not shown).
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Thus, robust methods exist to pre- and postcondition the isolated mouse heart on the C57BL/6 background. These forms of conditioning were associated with variable, but low levels, of inhibitory phosphorylation of GSK-3/β. We next examined whether this phosphorylation, albeit at a low level, had any influence on conditioning of WT and KI mice from the GSK-3 colony, ostensibly on the same C57BL/6 background.
Characterization of GSK-3/β-Dependent Signaling in Inactivation-Resistant Hearts
GSK-3 KI mice were phenotypically similar to WT animals from the same colony. In particular, there were no differences in baseline developed pressure or coronary flow (see protocol 2 of supplemental Table I) or body weight and heart size (see protocol 2 of supplemental Table II). Myocardial content of PKB/Akt, GSK-3, and glycogen synthase protein was also comparable between genotypes (see Figure 3), consistent with existing data.24 The and β isoforms of GSK-3 seem approximately equally abundant and unchanged by targeting. Insulin induced robust phosphorylation of PKB/Akt at Ser473 in both genotypes (Figure 3). As expected, GSK-3/β was phosphorylated at serines 21 and 9, respectively, but only in WT hearts. However, because these epitopes are disrupted in the KI hearts, we examined glycogen synthase phosphorylation as an index of GSK-3 activity. Under basal conditions, glycogen synthase remains phosphorylated at serine 641 and 645 in both WT and KI hearts because of constitutive GSK-3 activity. Importantly, GSK-3 inhibition by insulin-induced activation of PKB/Akt results in glycogen synthase dephosphorylation, presumably through unopposed phosphatase activity in WT, but not in KI, hearts. Moreover, the serial nature of the GSK-3 consensus sites on glycogen synthase results in noticeable changes in migration between phospho- and dephosphoforms recognized by the total glycogen synthase antibody (see lowermost bands). In conclusion, GSK-3 and -β are both resistant to inactivation by PKB/Akt in the KI, but not WT, heart.
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P<0.05 KI vs WT in each protocol. N/D indicates not detectable.
Pre- and Postconditioning of GSK-3/β Inactivation-Resistant Hearts: Protocol 2
Having determined that GSK-3 within KI hearts is resistant to inactivation even in response to insulin (see Figure 3), they were next subjected to pre- and postconditioning. We used the protocols (PreC and 5 seconds PostC within protocol 1 of supplemental Tables I and II) optimized to reduce infarction, which were associated with residual GSK-3 phosphorylation (albeit at a much lower level than occurs with insulin; see Figure 2).
Surprisingly, both pre- and postconditioning protected hearts with inactivation-resistant (KI) forms of GSK-3 just as well as within colony controls containing inactivation-sensitive forms of GSK-3 (WT) (see protocol 2 of supplemental Tables I and II). The protection by both forms of conditioning was evident in infarction size on both WT and KI backgrounds. However, the WT mice were more sensitive to infarction than the KI mice and also probably outsourced C57BL/6 controls.
Based on these data, it seems that inhibition of GSK-3/β through phosphorylation of the regulatory N-terminal serines is not essential to either pre- or postconditioning. However, given the indirect supporting evidence within the literature, it is possible that GSK-3 inhibition by another means contributed to protection. To examine this possibility we determined the effect of pharmacological inhibition of GSK-3 on infarction and postischemic recovery.
Pharmacological Inhibition of GSK-3/β: Protocol 3
To determine whether direct inhibition of GSK-3 mimics conditioning, we used 2 structurally diverse ATP-competitive small molecule inhibitors. As seen in Figure 2, comparing insulin to other lanes, the majority of GSK-3 is in a nonphosphorylated active form and thereby amenable to inhibition.
In Figure 4, we used the same readout of GSK-3 activity as in Figure 3, ie, the maintenance of glycogen synthase phosphorylation. Exposure to 3 µmol/L SB216763 or 100 nmol/L BIO for 15 minutes resulted in complete dephosphorylation of the GSK-3 sites within glycogen synthase, suggesting that these concentrations completely inhibit GSK-3 activity. We therefore used these concentrations and exposure times immediately before global ischemia to determine the relevance of the GSK-3 phosphorylation seen with preconditioning (see Figure 2A). Neither regime had a consistent effect on postischemic recovery or infarction size (see protocol 3 Pre BIO and Pre SB of supplemental Tables I and II). Next, we examined the relevance of the low levels of GSK-3 phosphorylation seen during reperfusion with postconditioning in Figure 2B by determining whether protection could be mimicked by GSK-3 inhibition during reperfusion. Because it was uncertain for how long the low levels of GSK-3 phosphorylation persisted during reperfusion, we examined infarction in response to 3 µmol/L SB216763 for the first 15 minutes (Post SB15) and throughout (Post SB120) reperfusion. As can be seen from the results in protocol 3 of supplemental Tables I and II, neither duration of GSK-3 inhibition recapitulated the protection seen with postconditioning.
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The Effect of CsA: Protocol 4
Although we were able to pre- and postcondition the mouse heart, none of our pharmacological or genetic manipulations of GSK-3 activity had an effect on postischemic recovery or infarction. The mouse heart has been used by a variety of investigators to elucidate cardioprotective signaling, but we remained concerned that the GSK-3
mPTP signaling axis was not operative in our model and in particular on the background of the GSK-3 colony. We therefore examined the effect of CsA. Once again, the WT hearts had greater infarcts than KI hearts (see protocol 4 of supplemental Table II). However, CsA reduced infarction and improved contractile recovery similarly in both genotypes (see protocol 4 of supplemental Tables I and II), suggesting mPTP opening at reperfusion does contribute to infarction in this model but is not influenced by varied manipulations of GSK-3 activity.
To confirm more directly that GSK-3 inactivation did not influence mPTP, the timing of opening was assayed in cardiomyocytes isolated from adult KI mice using confocal laser analysis, as previously described.23 The time to mPTP opening was significantly increased in cells exposed to CsA in both GSK-3 WT cardiomyocytes (from 91.6±6.6 to 144.2±6.4 seconds; P<0.05) and KI cardiomyocytes (from 127.4±10.3 to 196.3±18.7 seconds; P<0.05). Insulin exposure was as effective as CsA in delaying mPTP opening in both genotypes (WT, 136.0±6.8 seconds; KI, 194.5±17.2 seconds; P<0.05 compared to control baseline). This demonstrates that, even in the absence of inactivatable GSK-3, signals downstream of insulin are still able to act on the mPTP. Furthermore, the time to mPTP opening in isolated cardiomyocytes subjected to laser-induced oxidative stress was longer in KI cells compared to WT cells. This was true at baseline and after treatment with insulin or CsA (P<0.05 in each study group). This is consistent with the results that the KI heart is more resistant to global ischemia than WT.
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Discussion |
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The purpose of the present study was to use a novel mouse line to confirm that the inhibition of GSK-3 is a necessary step in the signal transduction cascade that leads to protection in pre- and postconditioning. Contrary to our expectations, mouse hearts with only noninhibitable forms of GSK-3 could still be protected by either form of conditioning. Furthermore, in myocytes isolated from these mice, insulin was still able to delay mPTP opening. Moreover, documented pharmacological inhibition of GSK-3 with structurally diverse agents present before, or after, ischemia failed to recapitulate the protection observed with conditioning. Thus, in the murine heart, the inhibition of GSK-3 is neither necessary nor sufficient to cause pre- or postconditioning.
Notably, our results differ from those of Juhaszova et al15 who previously demonstrated that isolated cardiomyocytes from transgenic mice expressing a noninhibitable isoform of GSK-3β were resistant to the protective effects of ischemic and pharmacological preconditioning.15 Transgenic GSK-3β expression in these hearts was approximately 8-fold greater than the physiological endogenous level introducing the possibility of loss of physiological substrate selectivity and induction of a misfolded protein response, whereas allelically encoded GSK-3β still remained potentially inhibitable by N-terminal phosphorylation. Furthermore, in these mice at 4 weeks, transgene expression is accompanied by a 4- to 5-fold upregulation of both atrial natriuretic factor and brain natriuretic peptide,25 which are cardioprotective peptides.26–28 Moreover, transgenic overexpression of GSK-3β has been associated with metabolic derangement29 and myocardial contractile dysfunction.30 These factors may in part explain the divergence in findings between the 2 genetic approaches taken to examine the same hypothesis.
The reason we adopted a genetic approach that differed from that taken by Juhaszova et al15 included the concerns outlined above, as well as the fact that little is known about the action of GSK-3 in the heart. The most notable difference between the 2 isoforms is that GSK-3 possesses a glycine-rich N-terminal extension of unknown function that is absent in GSK-3β, resulting in 5-kDa difference in molecular mass between isoforms.31 However, the isoforms share more than 98% homology within their kinase domain,19 seem approximately equally abundant in cardiac myocytes,32 in keeping with our findings, are identically inhibited by PKB/Akt,31 cannot be selectively inhibited pharmacologically,31 and, most importantly, exhibit functional redundancy on systematic allelic disruption.31 Thus, although the emphasis to date has been on GSK-3β, it seems probable that inhibition of GSK-3 could similarly lead to protection or even that coincident inhibition of both kinases is required. To cover all of these possibilities, we chose to use GSK-3/β double KI mice in our initial experiments to confirm their role in conditioning, before progressing to mice homologous for targeted alleles encoding noninhibitable kinases of each individual isoform. Because conditioning still occurred on the double KI background, however, the additional experiments became redundant.
Despite the evidence of functional redundancy between GSK-3 and GSK-3β in early mammalian development,31 the situation in the heart maybe more complex. For example, in zebrafish, a morphilino-based knockdown of GSK-3 results in a cardiac phenotype that differs from GSK-3β knockdown and cannot be rescued by GSK-3β overexpression.33 Similarly, although cardiac-restricted expression of GSK-3 and GSK-3β in mice results in a similar phenotype, there are biochemical distinctions in the coupled downstream signals and in the response to hemodynamic stress.34 Thus, it is possible GSK-3 and GSK-3β have distinct, and perhaps opposing, roles during myocardial ischemia that complicate our observations in a mouse line in which both alleles have been targeted in all cell types and tissues. Thus, our findings may have differed if we had examined mice in which only GSK-3β had been targeted. However, in other species at least, it seems such opposition does not prevent the reduction in infarction seen with pharmacological inhibitors that do not discriminate between the GSK-3 isoforms.16,17,35,36
Downstream of GSK-3/β there are proposed links with certain components of the mPTP, including the voltage-dependent anion channel and adenine nucleotide translocator.15,37 GSK-3/β is suggested as the immediate point of convergence for multiple preconditioning signaling pathways.15 However, more recent studies have suggested that voltage-dependent anion channel and adenine nucleotide translocator are not essential to functional mPTP formation38,39 and that there is no mitochondrial GSK-3β detectable after preconditioning stimuli,14 raising more doubt regarding the dependency of mPTP function on upstream GSK-3 activity. CsA administered at the point of reperfusion is believed to inhibit mPTP opening by preventing binding of cyclophilin D to the adenine nucleotide translocator40 and has been shown significantly to reduce infarct size by numerous independent investigators.3,41–44 This correlates with the cardioprotection afforded by CsA in our study, demonstrated in both WT and KI animals, supporting the independence of mPTP function and its own candidacy as end effector in protective signaling.
Surprisingly, GSK-3 KI hearts appeared less sensitive to infarction than WT mice, both in response to global ischemia in the whole heart and at the cellular level in response to oxidative stress. One possible explanation for this may be attributable to compensatory signaling pathways other than GSK-3 in KI mice. However, we did not observe any difference between genotypes in Akt, p38, extracellular signal-regulated kinase, p70S6 kinase, and PKC-
signaling, all of which have been reported as important kinases in cardioprotective signaling (data not shown).10,11
Our results using pharmacological inhibitors of GSK-3 in the mouse heart differ from those previously reported in other species (rats16,17,35 or rabbit36). Most importantly, this may reflect species difference, because we observed a protective effect in parallel studies of the isolated ischemic rat heart pretreated with BIO and SB216763 (percentage infarct size; 29.7±2.9% [P<0.05] and 38.4±4.3% versus 43.6±3.9% in control, respectively) (Y Nishino, MS Marber, unpublished data, 2008). Other potential explanations for the discrepancy, including experimental preparations (global versus regional ischemia17,35,36 or ex vivo versus in vivo17) and inhibitor concentration (eg, 3 µmol/L versus 1 µmol/L SB21676336), should be considered and inhibitors titrated based on their effects on signaling in that specific model and species. However, the use of 2 structurally diverse pharmacological inhibitors, together with the persistence of the conditioning effects, in the double KI mice would support the validity of our results in the mouse.
In the present study, the isolated Langendorff-perfused murine heart model was used to obtain contractile parameters of cardiac function, without confounding influences of neurohormonal and other compensatory factors. We acknowledge the use of isolated hearts with relatively short durations of reperfusion as a limitation of the study which might affect 5-triphenyl tetrazolium chloride staining for infarct measurement. Two postconditioning regimens were tested against an established murine preconditioning protocol. Postconditioning with 5-second cycles of reperfusion/ischemia was equally effective as preconditioning in limiting infarct size and preserving hemodynamic parameters in response to global ischemia, whereas 30-second cycles of postconditioning, known to be protective in some species,2,45–47 proved ineffective. These findings provide confirmatory evidence that the effectiveness of ischemic postconditioning is highly dependent on the number and duration of the interruptions to reperfusion, with a trend toward longer reperfusion-ischemia cycle lengths required in animals of increasing size (ie, 5 to 10 seconds in mice7,42 versus 1 minute in humans4).
In conclusion, our findings based on a combination of genetic and pharmacological interventions do not support the premise that GSK-3 inhibition is essential to pre- or postconditioning of isolated mouse hearts.
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Acknowledgments |
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We thank Dario Alessi (Medical Research Council Protein Phosphorylation Unit, School of Life Sciences, University of Dundee, Scotland) for providing the GSK-3–targeted mice.
Sources of Funding
This work was funded by a Wellcome Trust Project grant (074653) and British Heart Foundation fellowships 07/032 (to I.G.W.) and 06/026 (to J.E.C.).
Disclosures
None.
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Footnotes |
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Original received December 13, 2007; revision received June 9, 2008; accepted June 12, 2008.
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References |
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