Effects of the N-Terminus of Cardiac Troponin T on Contractile Activation Depend on the Type of Myosin Heavy Chain Isoform

Abstract

We have previously shown that two specific regions (residues 1-43 and residues 44-73) in rat cardiac troponin T (RcTnT) have divergent effects on contractile function against an α-myosin heavy chain (α-MHC) background (Mamidi et al., Abstract: Biophys J, vol.100, Issue 3, p586a, 2010). Because the effects of Tn isoforms are modulated by an interplay with MHC, we hypothesized that the divergent effects of residues 1-43 and residues 44-73 in cTnT would be further modulated by a shift in MHC isoform to β-MHC. This has a direct bearing on understanding the effects of alterations in cTnT and MHC isoforms not only in health but also in disease. We generated two recombinant mutants (RcTnT1-43 and RcTnT44-73) by deleting residues 1-43 and 44-73 in RcTnT, respectively. We studied contractile function by reconstituting RcTnT1-43 and RcTnT44-73 recombinant proteins into detergent-skinned papillary muscle fibers from normal rat hearts (α-MHC) and propylthiouracil-treated rat hearts (β-MHC). Our data shows that functional alterations induced by RcTnT1-43 and RcTnT44-73 were different in α-MHC vs. β-MHC fibers. For example, RcTnT1-43 decreased both Ca2+-activated maximal tension (by ∼46%) and myofilament Ca2+-sensitivity (by 0.09 pCa units) in α-MHC fibers. However, RcTnT1-43 modestly decreased maximal tension (by ∼18%), with no effect on Ca2+-sensitivity in β-MHC fibers. Thus, the desensitizing effect of RcTnT1-43 was attenuated by β-MHC. Ca2+-activated maximal tension data were supported by myofiber dynamic stiffness measurements. Another major finding is that RcTnT44-73 abolished the sarcomere length-dependent increase in Ca2+-sensitivity in β-MHC, but not in α-MHC fibers. Thus, our data demonstrates that the functional outcome of the N-terminus of cTnT is modulated by the type of MHC isoform present.