Functional consequence of mutation in rat cardiac troponin T is affected differently by myosin heavy chain isoforms

Abstract

Cardiac troponin T (cTnT)is an essential component of the thin filament regulatory unit (RU) that regulates Ca 2+ activation of tension in the heart muscle. Because there is coupling between the RU and myosin crossbridges, the functional outcome of cardiomyopathy-related mutations in cTnT may be modified by the type of myosin heavy chain (MHC) isoform. Ca 2+ activation of tension and ATPase activity were measured in muscle fibres from normal rat hearts containing α-MHC isoform and propylthiouracil (PTU)-treated rat hearts containing β-MHC isoform. Muscle fibres from normal and PTU-treated rat hearts were reconstituted with two different mutations in rat cTnT; the deletion of Glu162 (cTnT E162DEL ) and the deletion of Lys211 (cTnT K211DEL ). α-MHC and β-MHC isoforms had contrasting impact on tension-dependent ATP consumption (tension cost) in cTnT E162DEL and cTnT K211DEL reconstituted muscle fibres. Significant increases in tension cost in α-MHC-containing muscle fibres corresponded to 17% (P < 0.01) and 23% (P < 0.001) when reconstituted with cTnT E162DEL and cTnT K211DEL , respectively. In contrast, tension cost decreased when these two cTnT mutants were reconstituted in muscle fibres containing β-MHC; by approximately 24% (P < 0.05) when reconstituted with cTnT E162DEL and by approximately 17% (P = 0.09) when reconstituted with cTnT K211DEL . Such differences in tension cost were substantiated by the mechano-dynamic analysis of cTnT mutant reconstituted muscle fibres from normal and PTU-treated rat hearts. Our observation demonstrates that qualitative changes in MHC isoform alters the nature of cardiac myofilament dysfunction induced by mutations in cTnT.

Interaction between myosin heavy chain and troponin isoforms modulate cardiac myofiber contractile dynamics

“…Furthermore, there is interplay between TnT and MHC such that the effect of mutant TnT on both tension-dependent ATP consumption and the fast XB distortion dynamic were significantly influenced by a shift from ␣-to ␤-MHC isoform (34). These findings suggest that coordinated expression of speciesspecific contractile regulatory protein isoforms may bring about a complementarity such that the resulting myofilament dynamics produce satisfactory function when the cardiac myofiber system is driven at different resting heart rates seen across different species.…”

“…This combination of higher ATPase activity and lower tension resulted in the higher tension cost in the mouse. We determined the tension cost from the mean slope of the tension-ATPase relationship of several muscle fibers (9,34). The tension cost in the mouse was 51% higher than that in the rat (P Ͻ 0.001; Table 1).…”

“…Both mouse cTnT and rat cTnT DNA constructs were tagged with the human c-myc epitope (33). Previous studies from our own and other laboratories have demonstrated that the presence of c-myc epitope in cTnT has no significant effect on cardiac muscle function (8,23,33,34).…”

“…The k tr in mouse and rat cardiac muscle fibers reconstituted ) was determined from the mean slopes of tension-ATPase relationship of several fibers (9,34). XB distortion rate constant (c, s Ϫ1 ) was determined at pCa 4.3 (5,9).…”

“…Furthermore, because it is relatively easy to shift the MHC isoform from ␣-to ␤-MHC in mouse and rat hearts, these animals are ideal for probing the interplay between Tn and different MHC isoforms. In the left ventricle of both mice and rats, propylthiouracil (PTU) treatment shifts the MHC isoform from predominantly ␣-to ␤-MHC without affecting the expression of other thin filament regulatory proteins (16,22,24,27,34). An ␣-to ␤-MHCinduced alteration in XB cycling kinetics changes cooperative interactions between XB-Tn and XB-XB, thus affecting contractile dynamics by modifying XB recruitment (3,5,6).…”