Lack of evidence for a pathogenic role of proteasome-directed autoimmunity in dilated cardiomyopathy

Ubiquitin-Like Protein ISG15 (Interferon-Stimulated Gene of 15 kDa) in Host Defense Against Heart Failure in a Mouse Model of Virus-Induced Cardiomyopathy

Background—

Common causative agents in the development of inflammatory cardiomyopathy include cardiotropic viruses such as coxsackievirus B3 (CVB3). Here, we investigated the role of the ubiquitin-like modifier interferon-stimulated gene of 15 kDa (ISG15) in the pathogenesis of viral cardiomyopathy.

Methods and Results—

In CVB3-infected mice, the absence of protein modification with ISG15 was accompanied by a profound exacerbation of myocarditis and by a significant increase in mortality and heart failure. We found that ISG15 in cardiomyocytes contributed significantly to the suppression of viral replication. In the absence of an intact ISG15 system, virus titers were markedly elevated by postinfection day 8, and viral RNA persisted in ISG15 −/− mice at postinfection day 28. Ablation of the ISG15 protein modification system in CVB3 infection predisposed mice to long-term disease with deposition of collagen fibers, all leading to inflammatory cardiomyopathy. We found that ISG15 acts as part of the intrinsic immunity in cardiomyocytes and detected no significant effects of ISG15 modification on the cellular immune response. ISG15 modification of CVB3 2A protease counterbalanced CVB3-induced cleavage of the host cell eukaryotic initiation factor of translation eIF4G in cardiomyocytes, thereby counterbalancing the shutoff of host cell translation in CVB3 infection. We demonstrate that ISG15 suppressed infectious virus yield in human cardiac myocytes and the induction of ISG15 in patients with viral cardiomyopathy.

Conclusions—

The ISG15 conjugation system represents a critical innate response mechanism in cardiomyocytes to fight the battle against invading pathogens, limiting inflammatory cardiomyopathy, heart failure, and death. Interference with the ISG15 system might be a novel therapeutic approach in viral cardiomyopathy.

Defective proteolytic systems in Mybpc3-targeted mice with cardiac hypertrophy

Several lines of evidence suggest that alterations of the ubiquitin-proteasome system (UPS) and autophagy-lysosome pathway (ALP) may be involved in cardiac diseases. Little is known, however, in hypertrophic cardiomyopathy (HCM). This study studied these pathways in two mouse models of HCM that mainly differ by the presence or absence of truncated mutant proteins. Analyses were performed in homozygous Mybpc3-targeted knock-in (KI) mice, carrying a HCM mutation and exhibiting low levels of mutant cardiac myosin-binding protein C (cMyBP-C), and in Mybpc3-targeted knock-out (KO) mice expressing no cMyBP-C, thus serving as a model of pure cMyBP-C insufficiency. In the early postnatal development of cardiac hypertrophy, both models showed higher levels of ubiquitinated proteins and greater proteasomal activities. To specifically monitor the degradation capacity of the UPS with age, mice were crossed with transgenic mice that overexpress Ub(G76V)-GFP. Ub(G76V)-GFP protein levels were fourfold higher in 1-year-old KI, but not KO mice, suggesting a specific UPS impairment in mice expressing truncated cMyBP-C. Whereas protein levels of key ALP markers were higher, suggesting ALP activation in both mutant mice, their mRNA levels did not differ between the groups, underlying rather defective ALP-mediated degradation. Analysis of key proteins regulated in heart failure did not reveal specific alterations in KI and KO mice. Our data suggest (1) UPS activation in early postnatal development of cardiac hypertrophy, (2) specific UPS impairment in old KI mice carrying a HCM mutation, and (3) defective ALP as a common mechanism in genetically engineered mice with cardiac hypertrophy.

Impact of troponin I-autoantibodies in chronic dilated and ischemic cardiomyopathy

The aim of this study was to investigate the prognostic value of circulating troponin I (TNI)-autoantibodies in plasma of patients with chronic heart failure. Sera of 390 heart failure patients were tested for the presence of anti-TNI antibodies by enzyme-linked immunosorbent assay (ELISA), including 249 (63.8% of total) patients with dilated cardiomyopathy (DCM) and 141 (36.2% of total) patients with ischemic cardiomyopathy (ICM). A total of 72 patients (18.5% of total) were female and 318 (81.5% of total) were male. Mean patient age was 54.6 ± 11.3 years and mean follow-up time was 3.8 ± 3.2 years. TNI-autoantibodies (titer of ≥1:40) were detected in 73 out of 390 patients (18.7% of total). In TNI-autoantibody positive patients mean left ventricular ejection fraction (LVEF) was 27.6 ± 5.8%, compared to 25.8 ± 5.9% in TNI-autoantibody negative patients, P = 0.03. The combined end-point of death (n = 118, 30.3% of total) or heart transplantation (HTX) (n = 44, 11.3% of total) was reached in 162 patients (41.5% of total). Kaplan-Meier analysis demonstrated superior survival (combined end-point of death or HTX) in patients with DCM versus ICM (P = 0.0198) and TNI-autoantibody positive patients versus TNI-autoantibody negative patients (P = 0.0348). Further subgroup analysis revealed a favorable outcome in TNI-positive patients with heart failure if the patients suffered from DCM (P = 0.0334), whereas TNI-autoantibody status in patients with ICM was not associated with survival (P = 0.8486). In subsequent multivariate Weibull-analysis, a positive TNI serostatus was associated with a significantly lower all-cause mortality in DCM patients (P = 0.0492). The presence of TNI-autoantibodies in plasma is associated with an improved survival in patients with chronic DCM, but not ICM. This might possibly indicate a prophylactic effect of TNI-autoantibodies in this subgroup of patients, encouraging further studies into possible protective effects of antibodies against certain cardiac target structures.