title: Combined troponin I Ser-150 and Ser-23/24 phosphorylation sustains thin filament Ca(2+) sensitivity and accelerates deactivation in an acidic environment.
abstract: The binding of Ca(2+) to troponin C (TnC) in the troponin complex is a critical step regulating the thin filament, the actin-myosin interaction and cardiac contraction. Phosphorylation of the troponin complex is a key regulatory mechanism to match cardiac contraction to demand. Here we demonstrate that phosphorylation of the troponin I (TnI) subunit is simultaneously increased at Ser-150 and Ser-23/24 during in vivo myocardial ischemia. Myocardial ischemia decreases intracellular pH resulting in depressed binding of Ca(2+) to TnC and impaired contraction. To determine the pathological relevance of these simultaneous TnI phosphorylations we measured individual TnI Ser-150 (S150D), Ser-23/24 (S23/24D) and combined (S23/24/150D) pseudo-phosphorylation effects on thin filament regulation at acidic pH similar to that in myocardial ischemia. Results demonstrate that while acidic pH decreased thin filament Ca(2+) binding to TnC regardless of TnI composition, TnI S150D attenuated this decrease rendering it similar to non-phosphorylated TnI at normal pH. The dissociation of Ca(2+) from TnC was unaltered by pH such that TnI S150D remained slow, S23/24D remained accelerated and the combined S23/24/150D remained accelerated. This effect of the combined TnI Ser-150 and Ser-23/24 pseudo-phosphorylations to maintain Ca(2+) binding while accelerating Ca(2+) dissociation represents the first post-translational modification of troponin by phosphorylation to both accelerate thin filament deactivation and maintain Ca(2+) sensitive activation. These data suggest that TnI Ser-150 phosphorylation induced attenuation of the pH-dependent decrease in Ca(2+) sensitivity and its combination with Ser-23/24 phosphorylation to maintain accelerated thin filament deactivation may impart an adaptive role to preserve contraction during acidic ischemia pH without slowing relaxation.
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Benjamin R
authors.initials: BR
authors.lastname: Nixon
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Shane D
authors.initials: SD
authors.lastname: Walton
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Brundage
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Sean C
authors.initials: SC
authors.lastname: Little
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Mark T
authors.initials: MT
authors.lastname: Ziolo
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA.
authors.forename: Jonathan P
authors.initials: JP
authors.lastname: Davis
authors.affiliation: Department of Physiology and Cell Biology, Davis Heart and Lung Research Institute, The Ohio State University, Columbus, OH 43210, USA. Electronic address: biesiadecki.1@osu.edu.
authors.forename: Brandon J
authors.initials: BJ
authors.lastname: Biesiadecki
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D000222:Adaptation, Physiological; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D006352:Heart Ventricles / Q000378:metabolism* / Q000473:pathology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008810:Mice, Inbred C57BL; D009200:Myocardial Contraction; D009203:Myocardial Infarction / Q000378:metabolism* / Q000473:pathology; D009218:Myosins / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D011499:Protein Processing, Post-Translational; D019209:Troponin C / Q000378:metabolism; D019210:Troponin I / Q000378:metabolism*
chemical_list: D000199:Actins; D019209:Troponin C; D019210:Troponin I; D009218:Myosins; D002118:Calcium
keywords: Acidosis; Cardiac troponin I; Phosphorylation; Thin filament deactivation