Thin filament disinhibition by restrictive cardiomyopathy mutant R193H troponin I induces Ca2+-independent mechanical tone and acute myocyte remodeling.
Inherited restrictive cardiomyopathy (RCM) is a debilitating disease characterized by a stiff heart with impaired ventricular relaxation. Mutations in cardiac troponin I (cTnI) were identified as causal for RCM. Acute genetic engineering of adult cardiac myocytes was used to identify primary structure/function effects of mutant cTnI. Studies focused on R193H cTnI owing to the poor prognosis of this allele. Compared with wild-type cTnI, R193H mutant cTnI more effectively incorporated into the sarcomere, where it exerted dose-dependent effects on basal and dynamic contractile function. Under loaded conditions, permeabilized myocyte Ca(2+) sensitivity of tension was increased, whereas the passive tension-extension relationship was not altered by R193H cTnI. Normal rod-shaped myocyte morphology acutely transitioned to a "short-squat" phenotype in concert with progressive stoichiometric incorporation of R193H in the absence of altered diastolic Ca(2+). The specific myosin inhibitor blebbistatin fully blocked this transition. Heightened Ca(2+) buffering by the R193H myofilaments, and not alterations in Ca(2+) handling by the sarcoplasmic reticulum, slowed the decay rate of the Ca(2+) transient. Incomplete mechanical relaxation conferred by R193H was exacerbated at increasing pacing frequencies independent of elevated diastolic Ca(2+). R193H cTnI-dependent mechanical tone caused acute remodeling to a quasicontracted state not elicited by other Ca(2+)-sensitizing proteins and is a direct correlate of the stiff heart characteristic of RCM in vivo. These results point toward targets downstream of Ca(2+) handling, notably thin filament regulation and actin-myosin interaction, in designing therapeutic strategies to redress the primary cell morphological and mechanical underpinnings of RCM.
Davis
Jennifer
J
Department of Molecular and Integrative Physiology, University of Michigan, Ann Arbor, MI, USA.
Wen
Haitao
H
Edwards
Terri
T
Metzger
Joseph M
JM
D008841:Actin Cytoskeleton / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002313:Cardiomyopathy, Restrictive / Q000235:genetics* / Q000503:physiopathology; D003971:Diastole; D005260:Female; D009154:Mutation; D009200:Myocardial Contraction; D032383:Myocytes, Cardiac / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019210:Troponin I / Q000235:genetics*
D019210:Troponin I; D002118:Calcium