Diastolic dysfunction and thin filament dysregulation resulting from excitation-contraction uncoupling in a mouse model of restrictive cardiomyopathy.
Restrictive cardiomyopathy (RCM) has been linked to mutations in the thin filament regulatory protein cardiac troponin I (cTnI). As the pathogenesis of RCM from genotype to clinical phenotype is not fully understood, transgenic (Tg) mice were generated with cardiac specific expression of an RCM-linked missense mutation (R193H) in cTnI. R193H Tg mouse hearts with 15% stoichiometric replacement had smaller hearts and significantly elevated end diastolic pressures (EDP) in vivo. Using a unique carbon microfiber-based assay, membrane intact R193H adult cardiac myocytes generated higher passive tensions across a range of physiologic sarcomere lengths resulting in significant Ca(2+) independent cellular diastolic tone that was manifest in vivo as elevated organ-level EDP. Sarcomere relaxation and Ca(2+) decay was uncoupled in isolated R193H Tg adult myocytes due to the increase in myofilament Ca(2+) sensitivity of tension, decreased passive compliance of the sarcomere, and adaptive in vivo changes in which phospholamban (PLN) content was decreased. Further evidence of Ca(2+) and mechanical uncoupling in R193H Tg myocytes was demonstrated by the biphasic response of relaxation to increased pacing frequency versus the negative staircase seen with Ca(2+) decay. In comparison, non-transgenic myocyte relaxation closely paralleled the accelerated Ca(2+) decay. Ca(2+) transient amplitude was also significantly blunted in R193H Tg myocytes despite normal mechanical shortening resulting in myocyte hypercontractility when compared to non-transgenics. These results identify for the first time that a single point mutation in cTnI, R193H, directly causes elevated EDP due to a myocyte intrinsic loss of compliance independent of Ca(2+) cycling or altered cardiac morphology. The compound influence of impaired relaxation and elevated EDP represents a clinically severe form of diastolic dysfunction similar to the hemodynamic state documented in RCM patients.
Department of Integrative Biology and Physiology, University of Minnesota Medical School, Minneapolis, MN 55455, USA.
Jennifer
J
Davis
Soichiro
S
Yasuda
Nathan J
NJ
Palpant
Joshua
J
Martindale
Tamara
T
Stevenson
Kimber
K
Converso
Joseph M
JM
Metzger
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002313:Cardiomyopathy, Restrictive / Q000235:genetics / Q000378:metabolism* / Q000503:physiopathology*; D004195:Disease Models, Animal; D056966:Excitation Contraction Coupling; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation; D009200:Myocardial Contraction / Q000235:genetics; D032383:Myocytes, Cardiac / Q000378:metabolism; D012518:Sarcomeres / Q000378:metabolism; D019210:Troponin I / Q000235:genetics / Q000378:metabolism*
D019210:Troponin I; D002118:Calcium