Mst1-mediated phosphorylation of Bcl-xL is required for myocardial reperfusion injury

Mst1 is a central Ser-Thr kinase in the Hippo pathway, which promotes apoptosis and inhibits cell proliferation. We have shown previously that, in cardiomyocytes, oxidative stress activates Mst1 at mitochondria, where Mst1 phosphorylates Bcl-xL at Ser14, inducing dissociation of Bcl-xL from Bax and thereby promoting apoptosis. However, the functional significance of Ser14 phosphorylation of endogenous Bcl-xL in vivo remains elusive. We generated knockin (KI) mice in which Ser14 of Bcl-xL is replaced with Ala. KI mice were born at the expected Mendelian ratio, and adult KI mice exhibited normal cardiac morphology and function at baseline. However, KI mice were protected from myocardial ischemia/reperfusion (I/R) injury and exhibited reduced cardiomyocyte apoptosis. Although suppression of endogenous Mst1 also reduced I/R injury, there was no additive protective effect when Mst1 was inhibited in KI mice. The development of dilated cardiomyopathy induced by cardiac-specific overexpression of Mst1 was also ameliorated in KI mice. Lats2 and YAP, two other key components of the Hippo pathway, were not affected in KI mice. These results suggest that Ser14 phosphorylation of Bcl-xL plays an essential role in mediating both cardiomyocyte apoptosis and myocardial injury by acting as a key downstream mediator of Mst1 independently of the canonical Hippo pathway.