Alterations to myofibrillar protein function in nonischemic regions of the heart early after myocardial infarction

Abstract

zone left ventricular dysfunction (LVD) contributes to global reductions in contractile function after localized myocardial infarction (MI). However, the molecular mechanisms underlying this form of LVD are not clear. This study tested the hypothesis that myofibrillar protein function is directly affected in remote-zone LVD early after MI. Cardiac myosin and native thin filaments were purified from mouse myocardium taken from both the nonnecrotic zone adjacent to and the nonischemic zone remote from an infarct induced by 1 h of coronary occlusion followed by 24 h of reperfusion. Thin filament velocities were measured using the in vitro motility assay. Results showed that overall function was significantly reduced in samples from both the adjacent (43 Ϯ 12% of control, n ϭ 7) and remote (53 Ϯ 8% of control, n ϭ 13) zones when compared with control proteins (P Ͻ 0.05). Myosin from the remote zone propelled control thin filaments at reduced velocities similar to those measured above. In contrast, the Ca 2ϩ sensitivity of remotezone thin filaments over control myosin was unchanged from control thin filaments (half-maximal at pCa 6.32 Ϯ 0.08 and 6.27 Ϯ 0.06, respectively) but showed a 20% increase in velocity at saturating Ca 2ϩ that parallels an increase in tropomyosin phosphorylation. Myosin dysfunction may be related to oxidation of cysteines in the myosin heavy chains or carbonylation of myosin binding protein-C. We hypothesize that phosphorylation of tropomyosin may serve a compensatory role, augmenting contraction during periods of oxidative stress when myosin function is compromised. thin filaments; calcium sensitivity; contractility; left ventricular dysfunction; motility assay LEFT VENTRICULAR DYSFUNCTION (LVD) as a result of myocardial ischemia may be classified as either irreversible (as in the case of infarct) or as potentially reversible (as in the cases of hibernation, stunning, and remote-zone LVD). Remote-zone LVD is unique in that it occurs in regions of the heart that never experienced ischemia. Remote-zone LVD has been documented in humans (3, 21) and in mice (7) using cardiac magnetic resonance imaging (MRI). The mechanisms underlying remote-zone dysfunction are unclear, but they contribute to global reductions in contractile function after localized myocardial infarction (MI). Compensatory hyperkinesis of the remote zone immediately after MI gives way to reduced contractility within 24 h (21), and this condition may persist for several days. This time course overlaps that of excess mortality resulting from pump failure in patients surviving initial MI, suggesting the possibility of a causal relationship. With nearly eight million MI occurring annually in the United States alone, the potential impact of remote-zone LVD is significant.Mechanisms that may play key roles in remote-zone LVD include mechanical tethering to the infarcted region, changes in mechanical load, reduced coronary vasodilator function, oxidative stress and inflammation, altered Ca 2ϩ handling, and alterations to the contractile...

Phosphorylation of tropomyosin extends cooperative binding of myosin beyond a single regulatory unit

“…Further, Tm phosphorylation increases 150% over control during remote zone left ventricular dysfunction (RZLVD), a contractile disorder that occurs after myocardial infarction, but in regions of the left ventricle that do not experience ischemia. This increase in phosphorylation coincides with a small but significant increase in the velocity of purified thin filaments over myosin in the in vitro motility assay [Rao et al, 2007]. Taken together, these results suggest that Tm phosphorylation may serve as a compensatory mechanism that attempts to upregulate contractile function when myosin activity is significantly reduced.…”

“…In vitro motility was performed at 30°C at 1 mM ATP as previously described [Guo and Guilford, 2004; Rao et al, 2007; Snook et al, 2008]. In brief, HMM in actin buffer (25 mM KCl, 25 mM Imidazole, 1 mM EGTA, 4 mM MgCl 2 , 10 mM DTT, pH 7.4) was flowed into a flow cell constructed from a nitrocellulose coverslip and glass slide spaced by two mylar shims.…”

Mitochondrial Dysfunction and Oxidative Damage to Sarcomeric Proteins

“…Importantly, the ATP content was maintained constant in this experimental system, implicating that the contractile dysfunction was due to oxidative damage to sarcomeric proteins and not to a reduction in energy availability. Rao et al [46] showed that I/R increased oxidative stress even in the remote areas of the heart and that cardiac myosin isolated from distal myocardium was less efficient in propelling thin filaments purified from either I/R or control mouse hearts. Both filament sliding velocity and actin-myosin binding were impaired, suggesting that posttranslational modifications of myosin play a role in the pathology of remote-zone left ventricular dysfunction.…”

Myocardial infarction in mice alters sarcomeric function via post-translational protein modification

“…After MI, hypertrophy of surviving cardiomyocytes combined with other remodeling effects in non-myocytes initiates the progression to clinical heart failure. Ventricular cells, including those unaffected by ischemia during the MI itself, may display altered or impaired function, a situation sometimes called “remote zone ventricular dysfunction” [1]. The physiology of the myocardium shortly after infarction when the heart has yet to begin maladaptive remodeling is therefore a clinically significant target of investigation.…”