Abstract 16325: WW45, a Scaffold Protein, is a Positive Regulator of Mst1 and Stimulates Myocardial Injury Through the Suppression of YAP
The serine/threonine kinase Mst1 is a key component of the mammalian Hippo pathway. Activation of Mst1 promotes cardiomyocyte (CM) death during ischemia/reperfusion (I/R). The signaling mechanism mediating ischemic injury through Mst1 remains to be elucidated. In Drosophila, the scaffold protein Salvador is required for activation of Hippo, a homolog of Mst1. We investigated whether the mammalian homolog of Salvador, WW45, is required for activation of Mst1 and induction of apoptosis during I/R, and if so, which downstream mechanism mediates its effect upon I/R injury. After I/R (30 min/24 h), cardiac-specific WW45 KO mice (W-cKO) exhibited a smaller myocardial infarct (MI)/area at risk (20.4 ± 6.0 vs. 38.2 ± 4.5%; p<0.05) than control (CT) mice. WW45 physically interacted with Mst1, and W-cKO showed reduced Mst1 phosphorylation (Thr183)/activation after I/R (0.5 fold, p<0.05), indicating that WW45 is required for Mst1 activation. Transgenic mice with cardiac-specific Mst1-overexpression (Tg-Mst1) exhibited larger MIs than CT after I/R (60.6 ± 3.0 vs. 39.2 ± 5.4%, p<0.05), but Tg-Mst1 crossed with W-cKO exhibited significantly smaller MIs than Tg-Mst1 (31.3 ± 2.6%, p<0.05), indicating that WW45 also mediates the effect of Mst1 activation. Consistently, WW45 deletion disrupts the interaction between Mst1 and the serine/threonine kinase Lats2, the main downstream target of Mst1. W-cKO exhibited significant upregulation (4.2 fold vs. CT I/R, p<0.05), reduced phosphorylation (-75%, p<0.05), and increased nuclear localization of YAP, a transcription co-factor promoting cell survival which is exported from the nucleus and degraded upon Lats2-mediated phosphorylation. In order to test the effect of the normalization of cardiac YAP levels in W-cKO, we crossed W-cKO mice with cardiac-specific YAP KO mice (W/Y-cKO mice). Heterozygous YAP deletion in W-cKO mice significantly reduced the protection against ischemic injury observed in W-cKO, indicating that YAP activation partly mediates the protection (CT 40.7 ± 4.8%, W-cKO 23.8 ± 7.4%, W/Y-cKO 36.6 ± 8.3%, p<0.05). In conclusion, WW45 is both a positive regulator and an effector of Mst1, and disruption of the WW45/Mst1 pathway reduces ischemic injury through the activation of YAP.
- © 2013 by American Heart Association, Inc.