Cardiac Troponin T Isoforms Affect the Ca2+ Sensitivity of Force Development in the Presence of Slow Skeletal Troponin I

Abstract

In this study we investigated the physiological role of the cardiac troponin T (cTnT) isoforms in the presence of human slow skeletal troponin I (ssTnI). ssTnI is the main troponin I isoform in the fetal human heart. In reconstituted fibers containing the cTnT isoforms in the presence of ssTnI, cTnT1-containing fibers showed increased Ca 2؉ sensitivity of force development compared with cTnT3-and cTnT4-containing fibers. The maximal force in reconstituted skinned fibers was significantly greater for the cTnT1 (predominant fetal cTnT isoform) when compared with cTnT3 (adult TnT isoform) in the presence of ssTnI. Troponin (Tn) complexes containing ssTnI and reconstituted with cTnT isoforms all yielded different maximal actomyosin ATPase activities. Tn complexes containing cTnT1 and cTnT4 (both fetal isoforms) had a reduced ability to inhibit actomyosin ATPase activity when compared with cTnT3 (adult isoform) in the presence of ssTnI. The rate at which Ca 2؉ was released from site II of cTnC in the cTnI⅐cTnC complex (122/s) was 12.5-fold faster than for the ssTnI⅐cTnC complex (9.8/s). Addition of cTnT3 to the cTnI⅐cTnC complex resulted in a 3.6-fold decrease in the Ca 2؉ dissociation rate from site II of cTnC. Addition of cTnT3 to the ssTnI⅐cTnC complex resulted in a 1.9-fold increase in the Ca 2؉ dissociation rate from site II of cTnC. The rate at which Ca 2؉ dissociated from site II of cTnC in Tn complexes also depended on the cTnT isoform present. However, the TnI isoforms had greater effects on the Ca 2؉ dissociation rate of site II than the cTnT isoforms. These results suggest that the different N-terminal TnT isoforms would produce distinct functional properties in the presence of ssTnI when compared with cTnI and that each isoform would have a specific physiological role in cardiac muscle.

Mutations in Human Cardiac Troponin I That Are Associated with Restrictive Cardiomyopathy Affect Basal ATPase Activity and the Calcium Sensitivity of Force Development

“…Although the molecular mechanism of cooperativity is still not clearly understood, the Ca 2ϩ dependence of muscle contraction is known to be highly cooperative with Hill coefficients in cardiac fibers ranging from one to three depending on the system used (21,30,31). The cTnI R145G HCM mutation was previously shown to cause a decrease in the Hill coefficient when compared with wild-type cTnI (14).…”

Engineered Troponin C Constructs Correct Disease-related Cardiac Myofilament Calcium Sensitivity

“…TnC, TnI, and TnT were purified as described previously (15,16). Rabbit skeletal actin and bovine ventricular cardiac tropomyosin (Tm) were purified from acetone powders as described previously (34,35).…”

“…Different isoforms of the troponin complex (Tn) within an organism and between different species help to tune the response of TnC to Ca 2ϩ to meet developmental and environmental demands of the heart (11)(12)(13)(14)(15). In this regard, TnC does not behave like a simple switch because the Ca 2ϩ binding properties of the regulatory EF-hand of TnC are modulated by interactions with its protein binding partner, troponin I (TnI) (16,17).…”

Quantitative and evolutionary biology of alternative splicing: how changing the mix of alternative transcripts affects phenotypic plasticity and reaction norms

“…There is nearly as strong a correlation between the summed relative abundance of the same two transcripts and the power output of intact muscle (Figure 3d). Experiments in other animals have repeatedly shown that manipulation of Tnt genes and/or protein isoforms affects muscle calcium sensitivity and contractile performance (Ogut et al, 1999;Gomes et al, 2002Gomes et al, , 2004MacFarland et al, 2002;Nassar et al, 2005;Brotto et al, 2006;Chandra et al, 2006), and single amino-acid replacement mutations in human Tnt also have these effects (Hernandez et al, 2005).…”