Role of cardiac troponin I carboxy terminal mobile domain and linker sequence in regulating cardiac contraction

Abstract

Inhibition of striated muscle contraction at resting Ca2+ depends on the C-terminal half of troponin I (TnI) in thin filaments. Much focus has been on a short inhibitory peptide (Ip) sequence within TnI, but structural studies and identification of disease-associated mutations broadened emphasis to include a larger mobile domain (Md) sequence at the C-terminus of TnI. For Md to function effectively in muscle relaxation, tight mechanical coupling to troponin’s core—and thus tropomyosin—is presumably needed. We generated recombinant, human cardiac troponins containing one of two TnI constructs: either an 8-amino acid linker between Md and the rest of troponin (cTnILink8), or an Md deletion (cTnI1-163). Motility assays revealed that Ca2+-sensitivity of reconstituted thin filament sliding was markedly increased with cTnILink8 (~0.9 pCa unit leftward shift of speed-pCa relation compared to WT), and increased further when Md was missing entirely (~1.4 pCa unit shift). Cardiac Tn’s ability to turn off filament sliding at diastolic Ca2+ was mostly (61%), but not completely eliminated with cTnI1-163. TnI’s Md is required for full inhibition of unloaded filament sliding, although other portions of troponin—presumably including Ip—are also necessary. We also confirm that TnI’s Md is not responsible for superactivation of actomyosin cycling by troponin.

Hypertrophic Cardiomyopathy Cardiac Troponin C Mutations Differentially Affect Slow Skeletal and Cardiac Muscle Regulation

“…These alterations in the cTnC N-domain may increase the affinity between cTnC-C84Y and the cTnI switch region and ultimately impact initiation of cardiac contraction. The switch region of cTnI (148–158) is immediately flanked by the inhibitory (136–147) and regulatory (161–209) regions, which anchors to actin during diastole (Tripet et al, 1997 ) and, in doing so, is largely responsible for inhibition of contractility at diastolic Ca 2+ levels (Meyer and Chase, 2016 ). Increased affinity between cTnC-C84Y and the cTnI switch region would therefore be expected to reduce interactions between the cTnI inhibitory and regulatory regions during diastole, permitting actomyosin ATPase activity at subthreshold Ca 2+ concentrations, thereby promoting greater ATPase activation at higher Ca 2+ concentrations.…”

Amino Acid Changes at Arginine 204 of Troponin I Result in Increased Calcium Sensitivity of Force Development

“…Certain regions of cTnI seem to be more important functionally than other regions (Mogensen et al, 2015; Wei and Jin, 2015; Meyer and Chase, 2016; Sheng and Jin, 2016). It may be that any or most amino acid changes at certain TnI residues that occur in regions functionally important for regulating Ca 2+ -sensitivity would all be associated with increased Ca 2+ -sensitivity of force development.…”

Myofilament Calcium Sensitivity: Consequences of the Effective Concentration of Troponin I

“…Interestingly, the vast majority of disease-associated mutations and even several of the phosphomimetics in TnI and TnT do not have much of an impact on the Ca 2+ binding properties of the isolated Tn complex, but do so on the thin filament (Nixon et al, 2012; Liu et al, 2012b, 2014). As the biochemical systems build and become more physiological, technical limitations necessitate having to follow other experimental outputs, which are merely transformations of the actual Ca 2+ binding event (i.e., fluorescence, actomyosin ATPase activity, motility or force; Davis et al, 2007; Tikunova et al, 2010; Sommese et al, 2013; Meyer and Chase, 2016). In these cases, it is difficult to discern intrinsic from extrinsic influences on the apparent Ca 2+ binding properties of TnC to even know what to “fix” or target (Davis J. P. et al, 2016).…”