Investigating the role of uncoupling of troponin I phosphorylation from changes in myofibrillar Ca2+-sensitivity in the pathogenesis of cardiomyopathy

Abstract

Contraction in the mammalian heart is controlled by the intracellular Ca2+ concentration as it is in all striated muscle, but the heart has an additional signaling system that comes into play to increase heart rate and cardiac output during exercise or stress. β-adrenergic stimulation of heart muscle cells leads to release of cyclic-AMP and the activation of protein kinase A which phosphorylates key proteins in the sarcolemma, sarcoplasmic reticulum and contractile apparatus. Troponin I (TnI) and Myosin Binding Protein C (MyBP-C) are the prime targets in the myofilaments. TnI phosphorylation lowers myofibrillar Ca2+-sensitivity and increases the speed of Ca2+-dissociation and relaxation (lusitropic effect). Recent studies have shown that this relationship between Ca2+-sensitivity and TnI phosphorylation may be unstable. In familial cardiomyopathies, both dilated and hypertrophic (DCM and HCM), a mutation in one of the proteins of the thin filament often results in the loss of the relationship (uncoupling) and blunting of the lusitropic response. For familial dilated cardiomyopathy in thin filament proteins it has been proposed that this uncoupling is causative of the phenotype. Uncoupling has also been found in human heart tissue from patients with hypertrophic obstructive cardiomyopathy as a secondary effect. Recently, it has been found that Ca2+-sensitizing drugs can promote uncoupling, whilst one Ca2+-desensitizing drug Epigallocatechin 3-Gallate (EGCG) can reverse uncoupling. We will discuss recent findings about the role of uncoupling in the development of cardiomyopathies and the molecular mechanism of the process.

Order–Disorder Transitions in the Cardiac Troponin Complex

“…28 Finally, it has been proposed that the IR interacts with TnI N when the latter region is phosphorylated by PKA, which has the potential to affect many dynamic interactions within the troponin complex (further discussion of transient post-translational modifications is outside the scope of this review; the reader is referred to references 41–44). 41; 42; 43; 44; 45; 46 Regardless of the mechanism the IR uses, its importance in the troponin complex is high: alanine scanning most residues in the IR, or replacing sections of the IR with an alanine-glycine linker, both have negative effects on the inhibition of myosin in the blocked state. 39; 47 …”

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

“…It is unclear whether the aberrant Ca 2+ sensitivity shift is caused by a direct change in the intrinsic Ca 2+ binding properties of TnC (as would be assumed in a simple two-state switch-like mechanism), and/or merely apparently shifting the Ca 2+ sensitivity by altering subsequent downstream events in how TnC interacts with, or is influenced by, its other regulatory subunits (such as TnI and TnT, and all their interacting proteins). Further complicating molecular insight, the disease-associated protein modifications can also impact the myocyte's innate ability to tune Ca 2+ sensitivity via phosphorylation, so that only under certain conditions might the apparent Ca 2+ sensitivity even appear altered (Biesiadecki et al, 2007; Messer and Marston, 2014). Depending on the system being studied, the apparent Ca 2+ sensitivity of both the biochemical and physiological systems can vary by over an order of magnitude (Biesiadecki et al, 2014).…”

Pim-1 Kinase Phosphorylates Cardiac Troponin I and Regulates Cardiac Myofilament Function

“…We speculate that once Pim-1 binds to cTnI, it will cause a conformational change of cTnI that favors the binding of Pim-1 to the site of Ser23/24. It is well established that phosphorylation of cTnI at different sites results in different myofilament function [34]. For example, both PKA and PKD have been shown to phosphorylate cTnI at Ser23/24, which resulted in a reduction of Ca 2+ sensitivity and acceleration of relaxation and crossbridge cycle kinetics [13-15].…”

“…Furthermore, altered phosphorylation of cTnI and other myofilament proteins have been shown to contribute causally to the cardiac dysfunction in the transition from compensated hypertrophy to heart failure [23]. Thus far, several protein kinases, including PKA, PKC, AMPK, and PKD, have been shown to phosphorylate cTnI and regulate myofilament activity [34, 35]. In the present study, we provide the compelling evidence implicating Pim-1 as a novel protein kinase that specifically interacts with and phosphorylates cTnI at Ser23/24 and Ser150 in the heart.…”