L71F mutation in rat cardiac troponin T augments crossbridge recruitment and detachment dynamics against α-myosin heavy chain, but not against β-myosin heavy chain

Abstract

The N-terminal extension of human cardiac troponin T (TnT), which modulates myofilament Ca sensitivity, contains several hypertrophic cardiomyopathy (HCM)-causing mutations including S69F. However, the functional consequence of S69F mutation is unknown. The human analog of S69F in rat TnT is L71F (TnT). Because the functional consequences due to structural changes in the N-terminal extension are influenced by the type of myosin heavy chain (MHC) isoform, we hypothesized that the TnT-mediated effect would be differently modulated by α- and β-MHC isoforms. TnT and wild-type rat TnT were reconstituted into de-membranated muscle fibers from normal (α-MHC) and propylthiouracil-treated rat hearts (β-MHC) to measure steady-state and dynamic contractile parameters. The magnitude of the TnT-mediated attenuation of Ca-activated maximal tension was greater in α- than in β-MHC fibers. For example, TnT attenuated maximal tension by 31% in α-MHC fibers but only by 10% in β-MHC fibers. Furthermore, TnT reduced myofilament Ca sensitivity by 0.11 pCa units in α-MHC fibers but only by 0.05 pCa units in β-MHC fibers. TnT augmented rate constants of crossbridge recruitment and crossbridge detachment dynamics in α-MHC fibers but not in β-MHC fibers. Collectively, our data demonstrate that TnT induces greater contractile deficits against α-MHC than against β-MHC background.

Omecamtiv Mecarbil Abolishes Length-Mediated Increase in Guinea Pig Cardiac Myofiber Ca2+ Sensitivity

“…1 A), as described in the Supporting Material. We have previously shown that E D is correlated with maximal tension and that it is an approximation of the number of force-bearing XBs (24)(25)(26)(27). Two-way ANOVA of E D did not show a significant interaction effect (p ¼ 0.62), but it confirmed significant main effects of both OM (p < 0.05) and SL (p < 0.01).…”

Cardiomyopathy mutation (F88L) in troponin T abolishes length dependency of myofilament Ca2+ sensitivity

“…Previous studies have shown that mutations in TnT may alter the rate of XB detachment, g ( Chandra et al, 2015 ; Gollapudi and Chandra, 2016a ; Reda et al, 2016 ). Changes in SL have also been shown to impact g ( Chandra et al, 2006 ; Stelzer and Moss, 2006 ; Ford and Chandra, 2013 ), such that an increase in SL decreases g and vice versa.…”

Developmental increase in β-MHC enhances sarcomere length–dependent activation in the myocardium

“…S1; Ford et al, 2010). We have previously shown that tension cost and c are approximate measures of g (Campbell et al, 2004; Gollapudi et al, 2015; Reda et al, 2016). Two-way ANOVA of tension cost did not show a significant SL–MHC interaction effect (P = 0.29), but it demonstrated a significant main effect of SL (P < 0.001) and MHC (P < 0.001).…”

“…Such SL-mediated increase in myofilament Ca 2+ sensitivity (enhanced length-dependent activation) has been attributed to greater contribution from XB-based cooperative mechanisms that feedback to enhance thin filament Ca 2+ sensitivity (Wang and Fuchs, 1994; Fitzsimons and Moss, 1998; Smith et al, 2009). It is well documented that β-MHC XBs impart greater cooperative effects on myofilaments when compared with α-MHC XBs (Rundell et al, 2005; Ford and Chandra, 2013; Mamidi and Chandra, 2013; Michael et al, 2014; Chandra et al, 2015; Reda et al, 2016). This is presumably due to the slower cycling kinetics of β-MHC (VanBuren et al, 1995), which increases XB dwell time in the strongly bound state, thereby allowing for allosteric length-sensing mechanisms to induce greater cooperative effects on contractile function.…”

“…There is ample evidence to suggest that α- and β-MHC isoforms impart different effects on thin filaments in accordance with their intrinsic kinetic properties (Ford et al, 2012; Chandra et al, 2015; Gollapudi et al, 2015; Gollapudi and Chandra, 2016). The longer dwell time of β-MHC cross-bridges (XBs), which is ascribed to their slower XB cycling kinetics, may enhance XB-based cooperative feedback effect on thin filaments (Ford and Chandra, 2013; Mamidi and Chandra, 2013; Michael et al, 2014; Chandra et al, 2015; Reda et al, 2016). Because XB-mediated activation is known to play a role in cardiac length-dependent activation (Allen and Kentish, 1985; Fitzsimons and Moss, 1998; Moss and Fitzsimons, 2002; Smith et al, 2009), it is reasonable to posit that β-MHC may respond differently to SL changes when compared with α-MHC.…”