Increased phosphorylation of Ca(2+) handling proteins as a proarrhythmic mechanism in myocarditis.

BACKGROUND: Because fatal arrhythmia is an important cause of death in patients with myocarditis, we investigated the proarrhythmic mechanisms of experimental autoimmune myocarditis. METHODS AND RESULTS: Myocarditis was induced by injection of 2 mg porcine cardiac myosin into the footpads of adult Lewis rats on days 1 and 8 (Myo, n=15) and the results compared with Control rats (Control, n=15). In an additional 15 rats, 6 mg/kg prednisolone was injected into the gluteus muscle before the injection of porcine cardiac myosin on days 1 and 8 (MyoS, n=15). Hearts with myocarditis had longer action potential duration (APD), slower conduction velocity (CV; P<0.01 vs. Control), higher CV heterogeneity, greater fibrosis, higher levels of immunoblotting of high-mobility group protein B1, interleukin 6 and tumor necrosis factor-α proteins. Steroid treatment partially reversed the translations for myocarditis, CV heterogeneity, reduced APD at 90% recovery to baseline, increased CV (P<0.01), and reversed fibrosis (P<0.05). Programmed stimulation triggered sustained ventricular tachycardia in Myo rats (n=4/5), but not in controls (n=0/5) or Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor (KN93) treated Myo rats (n=0/5, P=0.01). CaMKII autophosphorylation at Thr287 (201%), and RyR2 phosphorylation at Ser2808 (protein kinase A/CaMKII site, 126%) and Ser2814 (CaMKII site, 21%) were increased in rats with myocarditis and reversed by steroid. CONCLUSIONS: The myocarditis group had an increased incidence of arrhythmia caused by increased phosphorylation of Ca(2+)handling proteins. These changes were partially reversed by an antiinflammatory treatment and CaMKII inhibition.

Candesartan cilexetil protects from cardiac myosin induced cardiotoxicity via reduction of endoplasmic reticulum stress and apoptosis in rats: involvement of ACE2-Ang (1-7)-mas axis.

Candesartan cilexetil, an angiotensin (Ang) II receptor 1 blocker was reported to suppress the myocardial damage in various cardiovascular complications but the mode by which it is effective in preventing the progression of dilated cardiomyopathy (DCM) is unknown. Emerging evidences suggest that, at least, part of the benefits observed with the use of AT1 receptor blockers could be attributed to the increased Ang (1-7) levels observed during administration of these agents. Identification of the novel components of the RAS, ACE2 and Ang (1-7) receptor mas, provided essential elements for considering the existence of a vasodilator arm of the RAS, represented by the ACE2-Ang (1-7)-mas axis. In this study, rat model of DCM was prepared by injection with porcine cardiac myosin. Twenty-eight days after immunization, candesartan cilexetil was administered intraperitoneally at 1 or 10mg/kg/day to rats for four weeks. Myocardial expression of Ang receptors and markers of calcium homeostasis, endoplasmic reticulum (ER) stress and apoptosis were measured by Western blotting and histopathological staining techniques. Candesartan improved the functional markers in a dose-dependent manner and also upregulated Ang (1-7), ACE2 and mas1 in the myocardium of DCM rats. Various ER stress and apoptosis markers were attenuated and the number apoptotic cells were significantly lower in the candesartan treated rats compared with those of the vehicle group. These findings suggest that candesartan treatment prevented the progression of DCM by activation of the counter regulatory arm of the RAS and possibly through modulation of ER stress and subsequently, cardiac apoptosis.

The pathogenesis of postinfectious myocarditis.

Myocarditis is an important cause of heart failure among adolescents and young adults. A remarkable observation is the discrepancy between the limited overt evidence of myocyte injury and the global impairment of left ventricular function. This discrepancy has stimulated suggestions that immunological mechanisms contribute to cardiac damage. We have developed two murine models of myocarditis, one elicited by cardiotropic Coxsackie B3 (CB3) virus infection and the other by cardiac myosin immunization, to better analyze the pathogenetic mechanisms responsible for immune-mediated heart-muscle disease. Both virus infection and myosin immunization produce myocardial inflammation and elicit heart-reactive antibodies which bind to the myocardium in vivo and which recognize the cardiac myosin heavy chain. Each model offers unique advantages. The virus-induced disease more closely resembles human myocarditis; myosin immunization isolates the autoimmune components of the disease since no virus infection is involved. We have also distinguished strains of mice resistant to autoimmune myocarditis (such as B10.A) from those susceptible to the autoimmune phase of disease (such as A.CA and A/J). Mice from a resistant strain to virus-or myosin-induced autoimmune heart disease develop myocardial inflammation and myosin antibodies if co-treated with tumor necrosis factor (TNF)-alpha or interleukin (IL)-1 when infected or immunized. Thus, cytokines can modulate the outcome of cardiotropic virus infection and enhance its autoimmune sequela. We also found that blocking IL-1 receptor inhibits autoimmune myocarditis in genetically susceptible mice.

Myocarditis-inducing epitope of myosin binds constitutively and stably to I-Ak on antigen-presenting cells in the heart.

Immune interactions in the heart were studied using a murine model of myosin-induced autoimmune myocarditis. A T cell hybridoma specific for mouse cardiac myosin was generated from A/J mice and used to demonstrate that endogenous myosin/I-Ak complexes are constitutively expressed on antigen-presenting cells in the heart. This T cell hybridoma, Seu.5, was used as a functional probe to identify a myocarditis-inducing epitope of cardiac myosin. Overlapping peptides based on the cardiac myosin heavy chain alpha (myhc alpha) sequences were synthesized and tested for their ability to stimulate Seu.5 T cells. One peptide, myhc alpha (325-357) strongly stimulated the Seu.5 T cells, localizing the epitope to this region of the myhc alpha molecule. Using truncated peptides, the epitope was further localized to residues 334-352. The myhc alpha (334-352) peptide strongly induced myocarditis when administered to A/J mice, which was histologically indistinguishable from that induced by myosin. The myhc alpha (334-352) epitope was present in cardiac myosin and not skeletal muscle myosins, providing a biochemical basis for the cardiac specificity of this autoimmune disease. Induction of myocarditis by this epitope was restricted to the myhc alpha isoform and not the myhc beta isoform, suggesting there may be a difference in the efficiency of generating tolerance to these isoforms of cardiac myosin, which are differentially developmentally regulated. The myhc alpha (334-352) epitope bound to purified I-Ak molecules in a similar manner to other I-Ak-restricted immunogenic epitopes, HEL(48-61) and RNase(43-56). Importantly, the myhc alpha (334-352) epitope was able to bind to I-Ak molecules on the surface of antigen-presenting cells in a stable manner. These findings demonstrate that autoantigenic epitopes can behave in a dominant manner and constitutively bind to class II molecules in the target organ in a similar manner to foreign immunogenic epitopes.

Cardiac alpha-myosin heavy chains differ in their induction of myocarditis. Identification of pathogenic epitopes.

BALB/c mice develop autoimmune myocarditis after immunization with mouse cardiac myosin, whereas C57B/6 mice do not. To define the immunogenicity and pathogenicity of cardiac myosin in BALB/c mice, we immunized mice with different forms of cardiac myosin. These studies demonstrate the discordance of immunogenicity and pathogenicity of myosin heavy chains. The cardiac alpha-myosin heavy chains of BALB/c and C57B/6 mice differ by two residues that are near the junction of the head and rod in the S2 fragment of myosin. Myosin preparations from both strains are immunogenic in susceptible BALB/c as well as in nonsusceptible C57B/6 mice; however, BALB/c myosin induces a greater incidence of disease. To further delineate epitopes of myosin heavy chain responsible for immunogenicity and disease, mice were immunized with fragments of genetically engineered rat alpha cardiac myosin. Epitopes in the region of difference between BALB/c and C57B/6 (residues 735-1032) induce disease in both susceptible and nonsusceptible mice. The data presented here demonstrate that pathogenic epitopes of both mouse and rat myosin residue in the polymorphic region of the S2 subunit. In addition, these studies suggest that polymorphisms in the autoantigen may be part of the genetic basis for autoimmune myocarditis.