Decreased energetics in murine hearts bearing the R92Q mutation in cardiac troponin T

Abstract

The thin filament protein cardiac troponin T (cTnT) is an important regulator of myofilament activation. Here we report a significant change in cardiac energetics in transgenic mice bearing the missense mutation R92Q within the tropomyosin-binding domain of cTnT, a mutation associated with a clinically severe form of familial hypertrophic cardiomyopathy. This functional domain of cTnT has recently been shown to be a crucial modulator of contractile function despite the fact that it does not directly interact with the ATP hydrolysis site in the myosin head. Simultaneous measurements of cardiac energetics using 31P NMR spectroscopy and contractile performance of the intact beating heart revealed both a decrease in the free energy of ATP hydrolysis available to support contractile work and a marked inability to increase contractile performance upon acute inotropic challenge in hearts from R92Q mice. These results show that alterations in thin filament protein structure and function can lead to significant defects in myocardial energetics and contractile reserve.

Figure 1

Sum of ³¹P NMR spectra from six 16-week-old NTG (left) and R92Q (right) hearts obtained during baseline perfusion with buffer containing either 11 mM glucose (top spectra) or 11 mM glucose with 0.5 mM pyruvate (bottom spectra). Resonance areas from left to right correspond to Pi, PCr, and γ-, α-, and β-phosphates of ATP. R29Q hearts had lower PCr and higher Pi resonance areas than did NTG hearts. PPM, parts per million.

Figure 2

Plot of the relationship between RPP and |ΔG_∼ATP| for NTG (squares, n = 9), WT-Tg (triangles, n = 4), and R92Q (circles, n = 8) hearts perfused with buffers containing glucose and pyruvate with 2.5 mM (open symbols) and 4.0 mM (filled symbols) Ca²⁺. The means of the NTG hearts (large squares), WT-Tg hearts (large triangles), and R92Q hearts (large circles) for each workload are connected. The relationships for NTG and WT-Tg mice are indistinguishable; the relationship for R92Q hearts differs from that of both NTG and WT-Tg hearts.

Figure 3

Relationship between MVO₂ and RPP in NTG (filled squares) and R92Q (open circles) hearts. Hearts were perfused with the same buffers described in Figure 2 legend. The equations for the linear regressions are similar in both groups: NTG (n = 4, seven data points), MVO₂ = 1.9x + 2.65; R92Q (n = 5, ten data points), MVO₂ = 1.4x + 5.94; however, the R92Q hearts fall on the lower part of the relationship.