title: Troponin I in the murine myocardium: influence on length-dependent activation and interfilament spacing.
abstract: Cyclic AMP-dependent protein kinase (PKA) targets contractile proteins, troponin-I (TnI) and myosin binding protein C (MyBP-C) in the heart and induces a decrease in myofilament Ca2+ sensitivity. Yet, the effect of sarcomere length (SL) change on Ca2+ sensitivity (length-dependent activation: LDA) following PKA-dependent phosphorylation is not clear. To clarify the role of PKA-dependent phosphorylation of TnI and MyBP-C on LDA in the heart, we examined LDA in skinned myocytes from a non-transgenic (NTG) and a transgenic murine model in which the native cardiac isoform (cTnI) was completely replaced by the slow skeletal isoform of TnI (ssTnI-TG) lacking the phosphorylation sites for PKA, while retaining PKA sites on MyBP-C. In NTG myocytes, PKA treatment decreased Ca2+ sensitivity at each SL, but enhanced the impact of SL change on Ca2+ sensitivity. Despite a greater sensitivity to Ca2+ and a reduction in LDA, neither Ca2+ responsiveness nor LDA was affected by PKA treatment in ssTnI-TG myocytes. To determine whether the above observations could be explained by the lateral separation between thick and thin filaments, as suggested by others, we measured interfilament spacing by X-ray diffraction as a function of SL in skinned cardiac trabeculae in the passive state from both NTG and ssTnI-TG models before and following treatment with PKA. Phosphorylation by PKA increased lattice spacing at every SL in NTG trabeculae. However, the relationship between SL and myofilament lattice spacing in ssTnI-TG was markedly shifted downward to an overall decreased myofilament lattice spacing following PKA treatment. We conclude: (1) PKA-dependent phosphorylation enhances length-dependent activation in NTG hearts; (2) replacement of native TnI with ssTnI increases Ca2+ sensitivity of tension but reduces length-dependent activation; (3) MyBP-C phosphorylation by PKA does not alter calcium responsiveness and induces a decrease in myofilament lattice spacing at all sarcomere lengths and (4) length-dependent activation in the heart cannot be entirely explained by alterations in myofilament lattice spacing.
authors.affiliation: Program in Cardiovascular Sciences, Department of Physiology and Biophysics, Section of Cardiology, University of Illinois at Chicago, College of Medicine, Chicago, IL 60612, USA.
authors.forename: John P
authors.initials: JP
authors.lastname: Konhilas
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Irving
authors.forename: Beata M
authors.initials: BM
authors.lastname: Wolska
authors.forename: Eias E
authors.initials: EE
authors.lastname: Jweied
authors.forename: Anne F
authors.initials: AF
authors.lastname: Martin
authors.forename: R John
authors.initials: RJ
authors.lastname: Solaro
authors.forename: Pieter P
authors.initials: PP
authors.lastname: de Tombe
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000378:metabolism*; D010766:Phosphorylation; D012518:Sarcomeres / Q000502:physiology; D019210:Troponin I / Q000235:genetics* / Q000378:metabolism*; D000099163:Myosin Binding Protein C
chemical_list: D002118:Calcium; D002352:Carrier Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D019210:Troponin I; D000099163:Myosin Binding Protein C