Cardiac resynchronization sensitizes the sarcomere to calcium by reactivating GSK-3β

Introduction For the past 30 years or more, the quest to develop treatments that directly improve systolic performance of the failing heart while also conferring long-term survival benefits has been [...]
Cardiac resynchronization therapy (CRT), the application of biventricular stimulation to correct discoordinate contraction, is the only heart failure treatment that enhances acute and chronic systolic function, increases cardiac work, and reduces mortality. Resting myocyte function also increases after CRT despite only modest improvement in calcium transients, suggesting that CRT may enhance myofilament calcium responsiveness. To test this hypothesis, we examined adult dogs subjected to tachypacing-induced heart failure for 6 weeks, concurrent with ventricular dyssynchrony ([HF.sub.dys]) or CRT. Myofilament force-calcium relationships were measured in skinned trabeculae and/or myocytes. Compared with control, maximal calcium-activated force and calcium sensitivity declined globally in HFdys; however, CRT restored both. Phosphatase PP1 induced calcium desensitization in control and CRT-treated cells, while [HF.sub.dys] cells were unaffected, implying that CRT enhances myofilament phosphorylation. Proteomics revealed phosphorylation sites on Z-disk and M-band proteins, which were predicted to be targets of glycogen synthase kinase-3β (GSK-3β ). We found that GSK-3β was deactivated in [HF.sub.dys] and reactivated by CRT. Mass spectrometry of myofilament proteins from [HF.sub.dys] animals incubated with GSK-3β confirmed GSK-3β -dependent phosphorylation at many of the same sites observed with CRT. GSK-3β restored calcium sensitivity in HFdys, but did not affect control or CRT cells. These data indicate that CRT improves calcium responsiveness of myofilaments following [HF.sub.dys] through GSK-3β reactivation, identifying a therapeutic approach to enhancing contractile function.