Does inhibition of glycogen synthase kinase protect in mice?

Preconditioning (PreC) and postconditioning (PostC) have been shown to initiate a number of signaling cascades that reduce cell death. However, the mechanisms by which these signals reduce cell death have been elusive.1 PreC has been shown to phosphorylate and thereby inhibit glycogen synthase kinase (GSK)-3 , and perfusion with GSK inhibitors has been shown to reduce cell death induced by ischemia/reperfusion, when added before ischemia2 or when added at the start of reperfusion.3–5 These studies are consistent with data in other tissues showing that inhibition of GSK-3 reduces apoptosis. Information regarding the mechanism by which inhibition of GSK protects has been provided by Juhaszova et al,6 who report that inhibition of GSK-3 delays the opening of the mitochondrial permeability transition pore (MPT) (see the Figure). The MPT is a large-conductance pore in the inner mitochondrial membrane which is opened under conditions associated with ischemia/reperfusion, such as high matrix reactive oxygen species and high matrix calcium. Pharmacological inhibitors of the MPT have been shown to reduce ischemia/reperfusion injury, suggesting that activation of MPT might have a role in ischemia/reperfusion-mediated cell death. However the molecular components of the MPT have not been identified.7 Juhaszova et al6 showed that myocytes isolated from mice with cardiac specific overexpression of a constitutively active form of GSK-3 , in which the serine 9 is replaced with alanine, are not protected by PreC or diazoxide. Juhaszova et al also decreased GSK-3 using interfering RNA and showed that this was protective, whereas decreasing GSK-3 was without effect. These data agree with data from other groups showing that inhibitors of GSK protect and that many types of cardioprotection result in increased phosphorylation of GSK-3 .2–6 However, the obligatory role of phosphorylation and/or inhibition of GSK in cardioprotection has been questioned by Nishino et al in this issue of Circulation Research.8 Nishino et al used GSK-3 / knock-in (KI) mice in which the phosphorylation sites on GSK-3 (Ser21) and GSK-3 (Ser9) are changed to alanine. For controls, they used WT mice that were inbred from the same colony but were not littermates. In the GSK double KI mice, infarct size, measured in a Langendorff model of global ischemia and reperfusion, was significantly lower in PreC (21.9%) and PostC (22.2%) hearts compared to nonconditioned hearts (39.5%), calling into question whether phosphorylation or inhibition of GSK is required for protection in mice. The authors further tested the involvement of GSK inhibition in cardioprotection using pharmacological GSK inhibitors and found that GSK inhibitors were not protective in this species, even though they observed protection in rats. Thus, these data suggest a species difference in the role of GSK in ischemia/reperfusion injury. In contrast to the study by Nishino et al,8 others have found that GSK inhibition is critical for cardioprotection in mice.5,6 In addition to the study by Sollott and colleagues,6 a recent study by Gomez et al5 found that infarct size was markedly reduced by GSK inhibitors in mice and that PostC was not The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Vascular Medicine Branch (E.M.), National Heart, Lung, and Blood Institute, NIH, Bethesda; and Department of Pathology (C.S.), The Johns Hopkins Medical Institutions, Baltimore, Md. Correspondence to Elizabeth Murphy, Vascular Medicine Branch, NHLBI, NIH, 10 Center Dr, Bethesda, MD 20892. E-mail [email protected] (Circ Res. 2008;103:226-228.) © 2008 American Heart Association, Inc.