Abstract 17445: NF2 Regulates Mst1 Activation and Ischemia Reperfusion Injury

Abstract

Neurofibromin 2 (NF2)/merlin is a tumor suppressor that can regulate activation of the hippo pathway in Drosophila and the mammalian liver, thereby inhibiting cell growth and promoting apoptosis. We have shown previously that the mammalian homologue of hippo, Mst1, has a similar function in the heart. Therefore, we sought to determine whether NF2 can mediate activation of Mst1 in the heart and modulate cardiac ischemia/reperfusion (I/R) injury. Both NF2 and Mst1 are activated by oxidative stress in isolated cardiomyocytes in vitro and in the heart following I/R. Increased expression of NF2 in cardiomyocytes increased Mst1 phosphorylation (2.9±0.2-fold vs LacZ, p<0.05) and elicited apoptosis (4.8±1.7-fold vs LacZ, p<0.05) that was prevented by co-expression of kinase inactive Mst1 (61±12% decrease vs. NF2, p<0.05). To examine the importance of endogenous NF2 in I/R injury, we employed two different genetic models - systemic heterozygous nf2 deletion ( nf2 +/- ) and cardiomyocyte-specific disruption using nf2 floxed mice crossed with αMHC-Cre transgenic mice ( nf2 +/flox Cre). We performed global I/R (30/60min) injury on isolated perfused hearts using the Langendorff method. Functional parameters and infarct size were determined following the reperfusion phase. While nf2 +/- hearts showed no difference in function and infarct size (45±5% vs 48±7%, NS) compared to WT, nf2 +/flox Cre hearts had significantly smaller infarcts (22±5% vs 53±8%, p<0.05) and improved function versus nf2 +/flox controls. nf2 +/- hearts also failed to protect against I/R injury in vivo as Infarct/AAR (34±3% vs 32±2%, NS) and myocyte apoptosis (1.5±0.2% vs 1.7±0.3%, NS) were unchanged despite significant attenuation of Mst1 activation in ventricular homogenates (71±8% reduced vs WT, p<0.05). Preliminary experiments to determine the effect of cardiomyocyte nf2 deletion show a trend toward protection (Infarct/AAR, 23±6% vs 33±6%, n=4) in nf2 +/flox Cre versus nf2 +/flox control mice. Based on these data we conclude that nf2 disruption in cardiomyocytes is protective against I/R injury through inhibition of Mst1 activity. However, NF2 signaling in residential non-myocytes appears beneficial for the heart and obscures the detrimental function of NF2 in cardiomyocytes during I/R.