Emerging importance of oxidative stress in regulating striated muscle elasticity

Abstract

The contractile function of striated muscle cells is altered by oxidative/nitrosative stress, which can be observed under physiological conditions but also in diseases like heart failure or muscular dystrophy. Oxidative stress causes oxidative modifications of myofilament proteins and can impair myocyte contractility. Recent evidence also suggests an important effect of oxidative stress on muscle elasticity and passive stiffness via modifications of the giant protein titin. In this review we provide a short overview of known oxidative modifications in thin and thick filament proteins and then discuss in more detail those oxidative stress-related modifications altering titin stiffness directly or indirectly. Direct modifications of titin include reversible disulfide bonding within the cardiac-specific N2-Bus domain, which increases titin stiffness, and reversible S-glutathionylation of cryptic cysteines in immunoglobulin-like domains, which only takes place after the domains have unfolded and which reduces titin stiffness in cardiac and skeletal muscle. Indirect effects of oxidative stress on titin can occur via reversible modifications of protein kinase signalling pathways (especially the NO-cGMP-PKG axis), which alter the phosphorylation level of certain disordered titin domains and thereby modulate titin stiffness. Oxidative stress also activates proteases such as matrix-metalloproteinase-2 and (indirectly via increasing the intracellular calcium level) calpain-1, both of which cleave titin to irreversibly reduce titin-based stiffness. Although some of these mechanisms require confirmation in the in vivo setting, there is evidence that oxidative stress-related modifications of titin are relevant in the context of biomarker design and represent potential targets for therapeutic intervention in some forms of muscle and heart disease.

Fig. 1

Schematic overview of important sources and targets of oxidative stress, as well as protectors against it, in striated muscle cells. Sources of reactive oxygen/nitrogen species include xanthine oxidase (XO), NADPH oxidases (Nox), uncoupled endothelial nitric oxide synthase (eNOS), inducible nitric oxide synthase (iNOS), neuronal nitric oxide synthase (nNOS), and mitochondrial factors such as complex I or III and monoamine oxidase (MAO). Antioxidant enzymes include superoxide dismutase (SOD), catalase, and thioredoxins, whereas non-enzymatic antioxidants are vitamins C and E. Oxidative stress damages DNA, lipids, and proteins, and among others, causes oxidation of myofilament proteins and alterations to the ratio between oxidized (GSSG) and reduced forms (GSH) of glutathione. Among the sarcomere proteins biochemically modified by oxidative stress are actin, tropomyosin (Tm), troponin I (TnI) and troponin C (TnC), myosin light chains 1 and 2 (MLC1 and MLC2), myosin heavy chain (MHC), myosin-binding protein-C (MyBP-C), and titin

Fig. 2

Oxidative stress-related modifications of titin affecting titin-based passive stiffness. The top panel illustrates the different segments of the titin chain (N2BA isoform) in a half-sarcomere, focusing on the various regions making up the elastic I-band segment. Segments where oxidative modifications occur are marked by arrows; the letters correspond to the respective type of oxidative modification indicated in panels (a–d). a Oxidative stress induces hypo-phosphorylation of the titin N2-Bus as it impairs NO-cGMP-PKG signalling; this modification increases titin stiffness. b Oxidizing conditions promote the formation of disulfide bonds in the titin N2-Bus; this modification increases titin stiffness. c Under oxidative conditions, buried cysteines in titin immunoglobulin (Ig-)domains are S-glutathionylated after they become exposed by domain unfolding (triggered by sarcomere stretch); this modification prevents domain refolding and thus reduces titin stiffness. d Oxidative stress increases the activity of proteases such as matrix metalloproteinase-2 (MMP2) and (via a rise in intracellular Ca²⁺ concentration) calpain-1, which degrade titin; these alterations would decrease titin stiffness