title: Differential integration of Ca2+-calmodulin signal in intact ventricular myocytes at low and high affinity Ca2+-calmodulin targets.
abstract: Cardiac myocyte intracellular calcium varies beat-to-beat and calmodulin (CaM) transduces Ca2+ signals to regulate many cellular processes (e.g. via CaM targets such as CaM-dependent kinase and calcineurin). However, little is known about the dynamics of how CaM targets process the Ca2+ signals to generate appropriate biological responses in the heart. We hypothesized that the different affinities of CaM targets for the Ca2+-bound CaM (Ca2+-CaM) shape their actions through dynamic and tonic interactions in response to the repetitive Ca2+ signals in myocytes. To test our hypothesis, we used two fluorescence resonance energy transfer-based biosensors, BsCaM-45 (Kd = approximately 45 nm) and BsCaM-2 (Kd = approximately 2 nm), to monitor the real time Ca2+-CaM dynamics at low and high affinity CaM targets in paced adult ventricular myocytes. Compared with BsCaM-2, BsCaM-45 tracks the beat-to-beat Ca2+-CaM alterations more closely following the Ca2+ oscillations at each myocyte contraction. When pacing frequency is raised from 0.1 to 1.0 Hz, the higher affinity BsCaM-2 demonstrates significant elevation of diastolic Ca2+-CaM binding compared with the lower affinity BsCaM-45. Biochemically detailed computational models of Ca2+-CaM biosensors in beating cardiac myocytes revealed that the different Ca2+-CaM binding affinities of BsCaM-2 and BsCaM-45 are sufficient to predict their differing kinetics and diastolic integration. Thus, data from both experiments and computational modeling suggest that CaM targets with low versus high Ca2+-CaM affinities (like CaM-dependent kinase versus calcineurin) respond differentially to the same Ca2+ signal (phasic versus integrating), presumably tuned appropriately for their respective and distinct Ca2+ signaling pathways.
authors.affiliation: Department of Physiology, Loyola University Chicago, Maywood, Illinois 60153, USA.
authors.forename: Qiujing
authors.initials: Q
authors.lastname: Song
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Saucerman
authors.forename: Julie
authors.initials: J
authors.lastname: Bossuyt
authors.forename: Donald M
authors.initials: DM
authors.lastname: Bers
mesh_terms: D000375:Aging / Q000502:physiology; D000595:Amino Acid Sequence; D000818:Animals; D015374:Biosensing Techniques; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002147:Calmodulin / Q000378:metabolism*; D002148:Calmodulin-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002470:Cell Survival; D002478:Cells, Cultured; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D008954:Models, Biological; D008969:Molecular Sequence Data; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D013379:Substrate Specificity
chemical_list: D002147:Calmodulin; D002148:Calmodulin-Binding Proteins; D002118:Calcium
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