Ca ²⁺ -desensitizing effect of a deletion mutation ΔK210 in cardiac troponin T that causes familial dilated cardiomyopathy

Abstract

A deletion mutation ⌬K210 in cardiac troponin T (cTnT) was recently found to cause familial dilated cardiomyopathy (DCM). To explore the effect of this mutation on cardiac muscle contraction under physiological conditions, we determined the Ca 2؉ -activated force generation in permeabilized rabbit cardiac muscle fibers into which the mutant and wild-type cTnTs were incorporated by using our TnT exchange technique. The free Ca 2؉ concentrations required for the force generation were higher in the mutant cTnTexchanged fibers than in the wild-type cTnT-exchanged ones, with no statistically significant differences in maximal force-generating capability and cooperativity. Exchanging the mutant cTnT into isolated cardiac myofibrils also increased the free Ca 2؉ concentrations required for the activation of ATPase. In contrast, a deletion mutation ⌬E160 in cTnT that causes familial hypertrophic cardiomyopathy (HCM) decreased the free Ca 2؉ concentrations required for force generation, just as in the case of the other HCM-causing mutations in cTnT. The results indicate that cTnT mutations found in the two distinct forms of cardiomyopathy (i.e., HCM and DCM) change the Ca 2؉ sensitivity of cardiac muscle contraction in opposite directions. The present study strongly suggests that Ca 2؉ desensitization of force generation in sarcomere is a primary mechanism for the pathogenesis of DCM associated with the deletion mutation ⌬K210 in cTnT.C ontraction of the vertebrate-striated muscles (i.e., skeletal and cardiac muscles) is regulated by Ca 2ϩ through its binding to a specific regulatory protein complex, troponin (Tn), which is distributed at regular intervals along the entire thin filament (1, 2). Tn is a complex of three different proteins, troponin T (TnT; tropomyosin-binding component), troponin I (TnI; inhibitory component), and troponin C (TnC; Ca 2ϩ -binding component). On Ca 2ϩ binding to TnC, a Ca 2ϩ -induced interaction of TnC with TnI relieves the inhibitory action of TnI exerted on the thin filament and enables the myosin head to cyclically interact with actin in the thin filament and generate force. The Ca 2ϩ sensitivity of muscle contraction is determined by the Ca 2ϩ -binding affinity of TnC, which is dynamically altered through interaction with TnI and TnT in the myofilament lattice (3-8).Mutations in genes for cardiac troponin T (cTnT) and cardiac troponin I (cTnI) have been found to cause familial hypertrophic cardiomyopathy (HCM), an autosomal dominant heart disease characterized by asymmetrical ventricular hypertrophy with a high incidence of sudden death in young adults (9). We have already examined the effects of eight HCM-linked cTnT mutations (I79N, R92Q, ⌬E160, E244D, R278C, and two truncated mutants produced by a splice donor site mutation Int15G 1 3A) and six HCM-linked cTnI mutations (R145G, R145Q, R162W, ⌬K183, G203S, and K206Q) on the contractile functions of cardiac muscle by using a technique for exchanging the exogenous Tn complex into skinned muscle fibers and isolated myofibrils. We found t...

Myofibrillar remodelling in cardiac hypertrophy, heart failure and cardiomyopathies

“…The functional changes caused by cTnT mutation in DCM appear to be opposite from those induced by fHCM-related cTnT mutations. This suggests that the primary mechanism for the pathogenesis of cTnT mutation-associated DCM is a deficiency of force generation, which is in contrast to the enhancement of force generation in fHCM associated with other mutations in cTnT (228). However, the role of Ca 2+ sensitization/desensitization in myopathies is still not completely understood.…”

“…Morimoto et al (228) have also recently reported a similar decrease in Ca 2+ sensitivity in rabbit heart trabeculae when endogenous troponin was replaced with human cTnT (either wild type or mutant). The decreased Ca 2+ sensitivity, together with the decreased crossbridge turnover rate and reduced cooperativity, suggests that the expression and incorporation of this DCM-related cTnT mutant in vivo may result in myofilaments that are markedly less responsive to Ca 2+ , contract more slowly and thus are unable to produce sufficient force during activation (213,228).…”

Different Functional Properties of Troponin T Mutants That Cause Dilated Cardiomyopathy

“…2). Morimoto et al (11) postulated that a decrease in Ca 2ϩ sensitivity was the main cause of DCM. However, the results from the TnT-R141W mutant suggest that this may not be the case for all TnT-DCM mutations.…”

“…In this study, we have investigated the functional consequences of the DCM-causing TnT mutations R141W, ⌬K210, and K273E on the regulatory properties of the thin filament. Previously, Morimoto et al (11) investigated TnT-⌬K210 and hypothesized that the Ca 2ϩ desensitizing effect of this mutation is the primary cause of dilated cardiomyopathy. Because Ca 2ϩ is the primary activator of the thin filament, we investigated how this mutation affects the Ca 2ϩ sensitivity of force development.…”

On Mice, Rabbits, and Human Heart Failure

“…14,15 It is noteworthy that mutations in the components of thin filaments that cause DCM are associated with decreased Ca 2ϩ sensitivity of the myofibrillar ATPase activity, force generation, or both. 16,17 It is intriguing to postulate that the impact of the thin filament mutations on Ca 2ϩ sensitivity of myofibrillar force generation, ATPase activity, or both is the main determinant of the ensuing phenotype being DCM or HCM. Ca 2ϩ sensitivity of myofibrils is probably determined by the impact of mutation on the affinity of the inhibitory arm of cTnI for the ␣-tropomyosin-actin complex.…”