Increase in tension-dependent ATP consumption induced by cardiac troponin T mutation

Restrictive cardiomyopathy (RCM) is a rare disorder characterized by impaired ventricular filling with decreased diastolic volume. We are reporting the functional effects of the first cardiac troponin T (CTnT) mutation linked to infantile RCM resulting from a de novo deletion mutation of glutamic acid 96. The mutation was introduced into adult and fetal isoforms of human cardiac TnT (HCTnT3-⌬E96 and HCTnT1-⌬E106, respectively) and studied with either cardiac troponin I (CTnI) or slow skeletal troponin I (SSTnI). Skinned cardiac fiber measurements showed a large leftward shift in the Ca 2؉ sensitivity of force development with no differences in the maximal force. HCTnT1-⌬E106 showed a significant increase in the activation of actomyosin ATPase with either CTnI or SSTnI, whereas HCTnT3-⌬E96 was only able to increase the ATPase activity with CTnI. Both mutants showed an impaired ability to inhibit the ATPase activity. The capacity of the CTnI⅐CTnC and SSTnI⅐CTnC complexes to fully relax the fibers after TnT displacement was also compromised. Experiments performed using fetal troponin isoforms showed a less severe impact compared with the adult isoforms, which is consistent with the cardioprotective role of SSTnI and the rapid onset of RCM after birth following the isoform switch. These data indicate that troponin mutations related to RCM may have specific functional phenotypes, including large leftward shifts in the Ca 2؉ sensitivity and impaired abilities to inhibit ATPase and to relax skinned fibers. All of this would account for and contribute to the severe diastolic dysfunction seen in RCM.

Section: Discussionsupporting

confidence: 64%

Section: Discussionmentioning

confidence: 99%

A Troponin T Mutation That Causes Infantile Restrictive Cardiomyopathy Increases Ca2+ Sensitivity of Force Development and Impairs the Inhibitory Properties of Troponin

Pinto

Parvatiyar

Jones

et al. 2008

“…Although the cTnI-ND increased cycling will increase the energy consumed per time, at the same stroke volume the increased energy usage is offset by the concurrent decrease in Ca 2ϩ sensitivity to shorten the systolic portion of the cardiac cycle and maintain overall contractile energy usage. Unlike other genetic cardiomyopathy models that exhibit decreased energetic efficiency resulting from increases in both Ca 2ϩ sensitivity and tension cost (34,35), cTnI-ND Tg mice do not exhibit altered cardiac morphology or decreased contractile function further supporting the cTnI-ND modification as nondetrimental. We cannot rule out a beneficial effect of cTnI-ND expression as partially resulting from compensatory alterations in Ca 2ϩ handling, MyBP-C phosphorylation or the decreased ␤-MHC expression (Fig.…”

Section: Discussionmentioning

confidence: 88%

Removal of the Cardiac Troponin I N-terminal Extension Improves Cardiac Function in Aged Mice

Biesiadecki¹,

Tachampa

Yuan

et al. 2010

The cardiac troponin I (cTnI) isoform contains a unique N-terminal extension that functions to modulate activation of cardiac myofilaments. During cardiac remodeling restricted proteolysis of cTnI removes this cardiac specific N-terminal modulatory extension to alter myofilament regulation. We have demonstrated expression of the N-terminal-deleted cTnI (cTnI-ND) in the heart decreased the development of the cardiomyopathy like phenotype in a ␤-adrenergic-deficient transgenic mouse model. To investigate the potential beneficial effects of cTnI-ND on the development of naturally occurring cardiac dysfunction, we measured the hemodynamic and biochemical effects of cTnI-ND transgenic expression in the aged heart. Echocardiographic measurements demonstrate cTnI-ND transgenic mice exhibit increased systolic and diastolic functions at 16 months of age compared with age-matched controls. This improvement likely results from decreased Ca 2؉ sensitivity and increased cross-bridge kinetics as observed in skinned papillary bundles from young transgenic mice prior to the effects of aging. Hearts of cTnI-ND transgenic mice further exhibited decreased ␤ myosin heavy chain expression compared to age matched nontransgenic mice as well as altered cTnI phosphorylation. Finally, we demonstrated cTnI-ND expressed in the heart is not phosphorylated indicating the cTnI N-terminal is necessary for the higher level phosphorylation of cTnI. Taken together, our data suggest the regulated proteolysis of cTnI during cardiac stress to remove the unique cardiac N-terminal extension functions to improve cardiac contractility at the myofilament level and improve overall cardiac function.Troponin I (TnI), 2 the inhibitory domain of the troponin (Tn) complex, is an essential protein involved in regulation of the actin-myosin interaction responsible for striated muscle contraction. Troponin I is encoded by three muscle genes expressing fast skeletal, slow skeletal, and cardiac TnI (cTnI) isoforms (1), with the cTnI gene the most recently evolved (2). Unlike the fast and slow skeletal isoforms, the cTnI isoform has evolved to contain a unique 32 amino acid N-terminal extension with the function of modulating cardiac contraction. In response to ␤-adrenergic-induced stimulatory G-protein (G s ␣) activation of protein kinase A (PKA), Ser-23/24 of the cTnI N-terminal extension are bis-phosphorylated (3). Phosphorylation of these residues alters cTnI structure diminishing the cTnI N-terminal interaction with troponin C (TnC) to decrease Ca 2ϩ -sensitive activation of muscle contraction (4, 5) and increase myosin cross-bridge kinetics (6 -8). Functionally, this structural change in the modulatory cTnI N-terminal extension is essential to increase the rate of myocardial relaxation and maintain adequate diastolic filling during the increased heart rate of ␤-adrenergic stimulation to meet increased cardiac demand. Non-physiological truncations removing the cTnI N-terminal extension result in a cTnI molecule that exhibits similar structural and functional al...

“…2B and Table 1). There were no significant changes in the WT (plus and minus the F27W mutation) pCa 50 values obtained by actomyosin ATPase (see Fig. 2A).…”

Section: Effects Of the Ctnc Mutations On The Ca 2ϩ Sensitivity And Mmentioning

confidence: 80%

“…In addition to the disease classification, it has been seen that mutations that cause large changes in cooperativity tend to manifest a more severe phenotype (12,50). Changes in the cooperativity of force activation (n Hill ) are typically displayed as an altered slope of the force-pCa relationship in skinned fibers (51).…”

Section: Discussionmentioning

confidence: 99%

Predicting Cardiomyopathic Phenotypes by Altering Ca2+ Affinity of Cardiac Troponin C

Parvatiyar

Pinto

Liang

et al. 2010

Cardiac diseases associated with mutations in troponin subunits include hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and restrictive cardiomyopathy (RCM). Altered calcium handling in these diseases is evidenced by changes in the Ca 2؉ sensitivity of contraction. Mutations in the Ca 2؉ sensor, troponin C (TnC), were generated to increase/ decrease the Ca 2؉ sensitivity of cardiac skinned fibers to create the characteristic effects of DCM, HCM, and RCM. We also used a reconstituted assay to determine the mutation effects on ATPase activation and inhibition. One mutant (A23Q) was found with HCM-like properties (increased Ca 2؉ sensitivity of force and normal levels of ATPase inhibition). Three mutants (S37G, V44Q, and L48Q) were identified with RCM-like properties (a large increase in Ca 2؉ sensitivity, partial loss of ATPase inhibition, and increased basal force). Two mutations were identified (E40A and I61Q) with DCM properties (decreased Ca 2؉ sensitivity, maximal force recovery, and activation of the ATPase at high [Ca 2؉ ]). Steady-state fluorescence was utilized to assess Ca 2؉ affinity in isolated cardiac (c)TnCs containing F27W and did not necessarily mirror the fiber Ca 2؉ sensitivity. Circular dichroism of mutant cTnCs revealed a trend where increased ␣-helical content correlated with increased Ca 2؉ sensitivity in skinned fibers and vice versa. The main findings from this study were as follows: 1) cTnC mutants demonstrated distinct functional phenotypes reminiscent of bona fide HCM, RCM, and DCM mutations; 2) a region in cTnC associated with increased Ca 2؉ sensitivity in skinned fibers was identified; and 3) the F27W reporter mutation affected Ca 2؉ sensitivity, maximal force, and ATPase activation of some mutants.