AMP-activated Protein Kinase Phosphorylates Cardiac Troponin I at Ser-150 to Increase Myofilament Calcium Sensitivity and Blunt PKA-dependent Function

Background: Myofilament protein phosphorylation is central to cardiac muscle contractile regulation. Results: AMP-activated protein kinase (AMPK) phosphorylates troponin I (TnI) to increase cardiac contraction and blunt the effects of TnI protein kinase A phosphorylation. Conclusion: AMPK myofilament signaling represents a novel mechanism to regulate cardiac contraction. Significance: AMPK links metabolics to TnI regulation of cardiac muscle function and adrenergic regulation. AMP-activated protein kinase (AMPK) is an energy-sensing enzyme central to the regulation of metabolic homeostasis. In the heart AMPK is activated during cardiac stress-induced ATP depletion and functions to stimulate metabolic pathways that restore the AMP/ATP balance. Recently it was demonstrated that AMPK phosphorylates cardiac troponin I (cTnI) at Ser-150 in vitro. We sought to determine if the metabolic regulatory kinase AMPK phosphorylates cTnI at Ser-150 in vivo to alter cardiac contractile function directly at the level of the myofilament. Rabbit cardiac myofibrils separated by two-dimensional isoelectric focusing subjected to a Western blot with a cTnI phosphorylation-specific antibody demonstrates that cTnI is endogenously phosphorylated at Ser-150 in the heart. Treatment of myofibrils with the AMPK holoenzyme increased cTnI Ser-150 phosphorylation within the constraints of the muscle lattice. Compared with controls, cardiac fiber bundles exchanged with troponin containing cTnI pseudo-phosphorylated at Ser-150 demonstrate increased sensitivity of calcium-dependent force development, blunting of both PKA-dependent calcium desensitization, and PKA-dependent increases in length dependent activation. Thus, in addition to the defined role of AMPK as a cardiac metabolic energy gauge, these data demonstrate AMPK Ser-150 phosphorylation of cTnI directly links the regulation of cardiac metabolic demand to myofilament contractile energetics. Furthermore, the blunting effect of cTnI Ser-150 phosphorylation cross-talk can uncouple the effects of myofilament PKA-dependent phosphorylation from -ad-renergic signaling as a novel thin filament contractile regulatory signaling mechanism.