Cardiomyopathy mutation (F88L) in troponin T abolishes length dependency of myofilament Ca2+ sensitivity

Implications of S-glutathionylation of sarcomere proteins in cardiac disorders, therapies, and diagnosis

The discovery that cardiac sarcomere proteins are substrates for S-glutathionylation and that this post-translational modification correlates strongly with diastolic dysfunction led to new concepts regarding how levels of oxidative stress affect the heartbeat. Major sarcomere proteins for which there is evidence of S-glutathionylation include cardiac myosin binding protein C (cMyBP-C), actin, cardiac troponin I (cTnI) and titin. Our hypothesis is that these S-glutathionylated proteins are significant factors in acquired and familial disorders of the heart; and, when released into the serum, provide novel biomarkers. We consider the molecular mechanisms for these effects in the context of recent revelations of how these proteins control cardiac dynamics in close collaboration with Ca2+ fluxes. These revelations were made using powerful approaches and technologies that were focused on thin filaments, thick filaments, and titin filaments. Here we integrate their regulatory processes in the sarcomere as modulated mainly by neuro-humoral control of phosphorylation inasmuch evidence indicates that S-glutathionylation and protein phosphorylation, promoting increased dynamics and modifying the Frank-Starling relation, may be mutually exclusive. Earlier studies demonstrated that in addition to cTnI as a well-established biomarker for cardiac disorders, serum levels of cMyBP-C are also a biomarker for cardiac disorders. We describe recent studies approaching the question of whether serum levels of S-glutathionylated-cMyBP-C could be employed as an important clinical tool in patient stratification, early diagnosis in at risk patients before HFpEF, determination of progression, effectiveness of therapeutic approaches, and as a guide in developing future therapies.

Hypertrophic and Dilated Cardiomyopathy-Associated Troponin T Mutations R130C and ΔK210 Oppositely Affect Length-Dependent Calcium Sensitivity of Force Generation

Length-dependent activation of calcium-dependent myocardial force generation provides the basis for the Frank-Starling mechanism. To directly compare the effects of mutations associated with hypertrophic cardiomyopathy and dilated cardiomyopathy, the native troponin complex in skinned trabecular fibers of guinea pigs was exchanged with recombinant heterotrimeric, human, cardiac troponin complexes containing different human cardiac troponin T subunits (hcTnT): hypertrophic cardiomyopathy-associated hcTnT^R130C, dilated cardiomyopathy-associated hcTnT^ΔK210 or the wild type hcTnT (hcTnT^WT) serving as control. Force-calcium relations of exchanged fibers were explored at short fiber length defined as 110% of slack length (L₀) and long fiber length defined as 125% of L₀ (1.25 L₀). At short fiber length (1.1 L₀), calcium sensitivity of force generation expressed by −log [Ca²⁺] required for half-maximum force generation (pCa₅₀) was highest for the hypertrophic cardiomyopathy-associated mutation R130C (5.657 ± 0.019), intermediate for the wild type control (5.580 ± 0.028) and lowest for the dilated cardiomyopathy-associated mutation ΔK210 (5.325 ± 0.038). Lengthening fibers from 1.1 L₀ to 1.25 L₀ increased calcium sensitivity in fibers containing hcTnT^R130C (delta-pCa₅₀ = +0.030 ± 0.010), did not alter calcium sensitivity in the wild type control (delta-pCa₅₀ = −0.001 ± 0.010), and decreased calcium sensitivity in fibers containing hcTnT^ΔK210 (delta-pCa₅₀ = −0.034 ± 0.013). Length-dependent activation indicated by the delta-pCa₅₀ was highly significantly (P < 0.001) different between the two mutations. We hypothesize that primary effects of mutations on length-dependent activation contribute to the development of the diverging phenotypes in hypertrophic and dilated cardiomyopathy.

Dilated cardiomyopathy mutation (R174W) in troponin T attenuates the length-mediated increase in cross-bridge recruitment and myofilament Ca2+ sensitivity

Alterations in length-dependent activation (LDA) may constitute a mechanism by which cardiomyopathy mutations lead to deleterious phenotypes and compromised heart function, because LDA underlies the molecular basis by which the heart tunes myocardial force production on a beat-to-beat basis (Frank-Starling mechanism). In this study, we investigated the effect of DCM-linked mutation (R173W) in human cardiac troponin T (TnT) on myofilament LDA. R173W mutation is associated with left ventricular dilatation and systolic dysfunction and is found in multiple families. R173W mutation is in the central region (residues 80-180) of TnT, which is known to be important for myofilament cooperativity and cross-bridge (XB) recruitment. Steady-state and dynamic contractile parameters were measured in detergent-skinned guinea pig left ventricular muscle fibers reconstituted with recombinant guinea pig wild-type TnT (TnT_WT) or mutant TnT (TnT_R174W; guinea pig analog of human R173W mutation) at two different sarcomere lengths (SL): short (1.9 µm) and long (2.3 µm). TnT_R174W decreased pCa₅₀ (-log [Ca²⁺]_free required for half-maximal activation) to a greater extent at long than at short SL; for example, pCa₅₀ decreased by 0.12 pCa units at long SL and by 0.06 pCa units at short SL. Differential changes in pCa₅₀ at short and long SL attenuated the SL-dependent increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) in TnT_R174W fibers; ΔpCa₅₀ was 0.10 units in TnT_WT fibers but only 0.04 units in TnT_R174W fibers. Furthermore, TnT_R174W blunted the SL-dependent increase in the magnitude of XB recruitment. Our observations suggest that the R173W mutation in human cardiac TnT may impair Frank-Starling mechanism.NEW & NOTEWORTHY This work characterizes the effect of dilated cardiomyopathy mutation in cardiac troponin T (TnT_R174W) on myofilament length-dependent activation. TnT_R174W attenuates the length-dependent increase in cross-bridge recruitment and myofilament Ca²⁺ sensitivity.

Developmental increase in β-MHC enhances sarcomere length-dependent activation in the myocardium

The expression of β-myosin heavy chain (β-MHC) in the guinea pig heart increases during postnatal development. Reda et al. show that this increase in β-MHC enhances length-mediated increases in myofilament Ca²⁺ sensitivity and sarcomere length–dependent changes in contractile function.

Shifts in myosin heavy chain (MHC) isoforms in cardiac myocytes have been shown to alter cardiac muscle function not only in healthy developing hearts but also in diseased hearts. In guinea pig hearts, there is a large age-dependent shift in MHC isoforms from 80% α-MHC/20% β-MHC at 3 wk to 14% α-MHC/86% β-MHC at 11 wk. Because kinetic differences in α- and β-MHC cross-bridges (XBs) are known to impart different cooperative effects on thin filaments, we hypothesize here that differences in α- and β-MHC expression in guinea pig cardiac muscle impact sarcomere length (SL)–dependent contractile function. We therefore measure steady state and dynamic contractile parameters in detergent-skinned cardiac muscle preparations isolated from the left ventricles of young (3 wk old) or adult (11 wk old) guinea pigs at two different SLs: short (1.9 µm) and long (2.3 µm). Our data show that SL-dependent effects on contractile parameters are augmented in adult guinea pig cardiac muscle preparations. Notably, the SL-mediated increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) is twofold greater in adult guinea pig muscle preparations (ΔpCa₅₀ being 0.11 units in adult preparations but only 0.05 units in young preparations). Furthermore, adult guinea pig cardiac muscle preparations display greater SL-dependent changes than young muscle preparations in (1) the magnitude of length-mediated increase in the recruitment of new force-bearing XBs, (2) XB detachment rate, (3) XB strain-mediated effects on other force-bearing XBs, and (4) the rate constant of force redevelopment. Our findings suggest that increased β-MHC expression enhances length-dependent activation in the adult guinea pig cardiac myocardium.

Mechanistic complexity of contractile dysfunction in hypertrophic cardiomyopathy

Reflections on recent work providing mechanistic insight into the pathological effects of a cardiac troponin T mutation.