Mark Luedde

Mechanisms of disease: hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most-common monogenically inherited form of heart disease, characterized by thickening of the left ventricular wall, contractile dysfunction, and potentially fatal arrhythmias. HCM is also the most-common cause of sudden cardiac death in individuals younger than 35 years of age. Much progress has been made in the elucidation of the genetic basis of HCM, resulting in the identification of more than 900 individual mutations in over 20 genes. Interestingly, most of these genes encode sarcomeric proteins, such as myosin-7 (also known as cardiac muscle β-myosin heavy chain; MYH7), cardiac myosin-binding protein C (MYBPC3), and cardiac muscle troponin T (TNNT2). However, the molecular events that ultimately lead to the clinical phenotype of HCM are still unclear. We discuss several potential pathways, which include altered calcium cycling and sarcomeric calcium sensitivity, increased fibrosis, disturbed biomechanical stress sensing, and impaired cardiac energy homeostasis. An improved understanding of the pathological mechanisms involved will result in greater specificity and success of therapies for patients with HCM.

A positive feedback loop between RIP3 and JNK controls non-alcoholic steatohepatitis

Non‐alcoholic fatty liver disease (NAFLD) represents the most common liver disease in Western countries and often progresses to non‐alcoholic steatohepatitis (NASH) leading ultimately to liver fibrosis and liver cancer. The occurrence of hepatocyte cell death—so far characterized as hepatocyte apoptosis—represents a fundamental step from benign steatosis toward progressive steatohepatitis. In contrast, the function of RIP3‐dependent “necroptosis” in NASH and NASH‐induced fibrosis is currently unknown. We show that RIP3 is upregulated in human NASH and in a dietary mouse model of steatohepatitis. RIP3 mediates liver injury, inflammation, induction of hepatic progenitor cells/activated cholangiocytes, and liver fibrosis through a pathway suppressed by Caspase‐8. This function of RIP3 is mediated by a positive feedback loop involving activation of Jun‐(N)‐terminal Kinase (JNK). Furthermore, RIP3‐dependent JNK activation promotes the release of pro‐inflammatory mediators like MCP‐1, thereby attracting macrophages to the injured liver and further augmenting RIP3‐dependent signaling, cell death, and liver fibrosis. Thus, RIP3‐dependent necroptosis controls NASH‐induced liver fibrosis. This pathway might represent a novel and specific target for pharmacological strategies in patients with NASH.

RIP3, a kinase promoting necroptotic cell death, mediates adverse remodelling after myocardial infarction.

AIMS Programmed necrosis (necroptosis) represents a newly identified mechanism of cell death combining features of both apoptosis and necrosis. Like apoptosis, necroptosis is tightly regulated by distinct signalling pathways. A key regulatory role in programmed necrosis has been attributed to interactions of the receptor-interacting protein kinases, RIP1 and RIP3. However, the specific functional role of RIP3-dependent signalling and necroptosis in the heart is unknown. The aims of this study were thus to assess the significance of necroptosis and RIP3 in the context of myocardial ischaemia. METHODS AND RESULTS Immunoblots revealed strong expression of RIP3 in murine hearts, indicating potential functional significance of this protein in the myocardium. Consistent with a role in promoting necroptosis, adenoviral overexpression of RIP3 in neonatal rat cardiomyocytes and stimulation with TNF-α induced the formation of a complex of RIP1 and RIP3. Moreover, RIP3 overexpression was sufficient to induce necroptosis of cardiomyocytes. In vivo, cardiac expression of RIP3 was up-regulated upon myocardial infarction (MI). Conversely, mice deficient for RIP3 (RIP3(-/-)) showed a significantly better ejection fraction (45 ± 3.6 vs. 32 ± 4.4%, P < 0.05) and less hypertrophy in magnetic resonance imaging studies 30 days after experimental infarction due to left anterior descending coronary artery ligation. This was accompanied by a diminished inflammatory response of infarcted hearts and decreased generation of reactive oxygen species. CONCLUSION Here, we show that RIP3-dependent necroptosis modulates post-ischaemic adverse remodelling in a mouse model of MI. This novel signalling pathway may thus be an attractive target for future therapies that aim to limit the adverse consequences of myocardial ischaemia.

Circulating microRNA-150 serum levels predict survival in patients with critical illness and sepsis.

Background and Aims Down-regulation of miR-150 was recently linked to inflammation and bacterial infection. Furthermore, reduced serum levels of miR-150 were reported from a small cohort of patients with sepsis. We thus aimed at evaluating the diagnostic and prognostic value of miR-150 serum levels in patients with critically illness and sepsis. Methods miR-150 serum levels were analyzed in a cohort of 223 critically ill patients of which 138 fulfilled sepsis criteria and compared to 76 healthy controls. Results were correlated with clinical data and extensive sets of routine and experimental biomarkers. Results Measurements of miR-150 serum concentrations revealed only slightly reduced miR-150 serum levels in critically ill patients compared to healthy controls. Furthermore miR-150 levels did not significantly differ in critically ill patients with our without sepsis, indicating that miR-150 serum levels are not suitable for diagnostic establishment of sepsis. However, serum levels of miR-150 correlated with hepatic or renal dysfunction. Low miR-150 serum levels were associated with an unfavorable prognosis of patients, since low miR-150 serum levels predicted mortality with high diagnostic accuracy compared with established clinical scores and biomarkers. Conclusion Reduced miR-150 serum concentrations are associated with an unfavorable outcome in patients with critical illness, independent of the presence of sepsis. Besides a possible pathogenic role of miR-150 in critical illness, our study indicates a potential use of circulating miRNAs as a prognostic rather than diagnostic marker in critically ill patients.

U6 is unsuitable for normalization of serum miRNA levels in patients with sepsis or liver fibrosis

MicroRNA (miRNA) levels in serum have recently emerged as potential novel biomarkers for various diseases. miRNAs are routinely measured by standard quantitative real-time PCR (qPCR); however, the high sensitivity of qPCR demands appropriate normalization to correct for nonbiological variation. Presently, RNU6B (U6) is used for data normalization of circulating miRNAs in many studies. However, it was suggested that serum levels of U6 themselves might differ between individuals. Therefore, no consensus has been reached on the best normalization strategy in ‘circulating miRNA’. We analyzed U6 levels as well as levels of spiked-in SV40-RNA in sera of 44 healthy volunteers, 203 intensive care unit patients and 64 patients with liver fibrosis. Levels of U6 demonstrated a high variability in sera of healthy donors, patients with critical illness and liver fibrosis. This high variability could also be confirmed in sera of mice after the cecal ligation and puncture procedure. Most importantly, levels of circulating U6 were significantly upregulated in sera of patients with critical illness and sepsis compared with controls and correlated with established markers of inflammation. In patients with liver fibrosis, U6 levels were significantly downregulated. In contrast, levels of spiked-in SV40 displayed a significantly higher stability both in human cohorts (healthy, critical illness, liver fibrosis) and in mice. Thus, we conclude that U6 levels in the serum are dysregulated in a disease-specific manner. Therefore, U6 should not be used for data normalization of circulating miRNAs in inflammatory diseases and previous studies using this approach should be interpreted with caution. Further studies are warranted to identify specific regulatory processes of U6 levels in sepsis and liver fibrosis.

Micro-RNA profiling in human serum reveals compartment-specific roles of miR-571 and miR-652 in liver cirrhosis.

Background and Aims Micro-RNAs (miRNAs) have recently emerged as crucial modulators of molecular processes involved in chronic liver diseases. The few miRNAs with previously proposed roles in liver cirrhosis were identified in screening approaches on liver parenchyma, mostly in rodent models. Therefore, in the present study we performed a systematic screening approach in order to identify miRNAs with altered levels in the serum of patients with chronic liver disease and liver cirrhosis. Methods We performed a systematic, array-based miRNA expression analysis on serum samples from patients with liver cirrhosis. In functional experiments we evaluated the relationship between alterations of miRNA serum levels and their role in distinct cellular compartments involved in hepatic cirrhosis. Results The array analysis and the subsequent confirmation by qPCR in a larger patient cohort identified significant alterations in serum levels of miR-513-3p, miR-571 and miR-652, three previously uncharacterized miRNAs, in patients with alcoholic or hepatitis C induced liver cirrhosis. Of these, miR-571 serum levels closely correlated with disease stages, thus revealing potential as a novel biomarker for hepatic cirrhosis. Further analysis revealed that up-regulation of miR-571 in serum reflected a concordant regulation in cirrhotic liver tissue. In isolated primary human liver cells, miR-571 was up-regulated in human hepatocytes and hepatic stellate cells in response to the pro-fibrogenic cytokine TGF-β. In contrast, alterations in serum levels of miR-652 were stage-independent, reflecting a concordant down-regulation of this miRNA in circulating monocytes of patients with liver cirrhosis, which was inducible by proinflammatory stimuli like bacterial lipopolysaccharide. Conclusion Alterations of miR571 and miR-652 serum levels in patients with chronic liver disease reflect their putative roles in the mediation of fibrogenic and inflammatory processes in distinct cellular compartments involved in the pathogenesis of liver cirrhosis.

Levels of circulating miR-133a are elevated in sepsis and predict mortality in critically ill patients.

Objective:Serum levels of microRNA have been proposed as biomarkers in various inflammatory diseases. However, up to now, their clinical relevance in critical illness and sepsis is unclear. Design:Single-center clinical study. Setting:Fourteen-bed medical ICU of the University Hospital Aachen, university laboratory research unit. Subjects and Patients:Experimental sepsis model in C57Bl/6 mice; 223 critically ill patients in comparison with 76 healthy volunteers. Interventions:We used the model of cecal pole ligation and puncture for induction of polymicrobial sepsis in mice and measured alterations in serum levels of six different microRNAs with a known function in inflammatory diseases upon induction of septic disease. These results from mice were translated into a large and well-characterized cohort of critically ill patients admitted to the medical ICU. Measurements and Main Results:Serum miR-133a was then measured in 223 critically ill patients (138 with sepsis and 85 without sepsis) and 76 controls and associated with disease severity, organ failure, and prognosis. Significant alterations of miR-133a, miR-150, miR-155, and miR-193b* were found in mice after cecal pole ligation and puncture–induced sepsis. Among all regulated microRNAs, miR-133a displayed the most prominent and concordant up-regulation in sepsis, and this microRNA was therefore chosen for further investigation in the human. Here, significantly elevated miR-133a levels were found in critically ill patients at ICU admission, when compared with healthy controls, especially in patients with sepsis. Correlation analyses revealed significant correlations of miR-133a with disease severity, classical markers of inflammation and bacterial infection, and organ failure. Strikingly, high miR-133a levels were predictive for an unfavorable prognosis and represented a strong independent predictor for both ICU and long-term mortality in critically ill patients. Conclusions:miR-133a serum levels were significantly elevated in critical illness and sepsis. High miR-133a levels were associated with the severity of disease and predicted an unfavorable outcome of critically ill patients.

Regulation of Cardiac cAMP Synthesis and Contractility by Nucleoside Diphosphate Kinase B/G Protein βγ Dimer Complexes

Heterotrimeric G proteins are pivotal regulators of myocardial contractility. In addition to the receptor-induced GDP/GTP exchange, G protein &agr; subunits can be activated by a phosphate transfer via a plasma membrane-associated complex of nucleoside diphosphate kinase B (NDPK B) and G protein &bgr;&ggr;-dimers (G&bgr;&ggr;). To investigate the physiological role of this phosphate transfer in cardiomyocytes, we generated a G&bgr;1&ggr;2-dimer carrying a single amino acid exchange at the intermediately phosphorylated His-266 in the &bgr;1 subunit (G&bgr;1H266L&ggr;2). Recombinantly expressed G&bgr;1H266L&ggr;2 were integrated into heterotrimeric G proteins in rat cardiomyocytes but were deficient in intermediate G&bgr; phosphorylation. Compared with wild-type G&bgr;1&ggr;2 (G&bgr;1WT&ggr;2), overexpression of G&bgr;1H266L&ggr;2 suppressed basal cAMP formation up to 55%. A similar decrease in basal cAMP production occurred when the formation of NDPK B/G&bgr;&ggr; complexes was attenuated by siRNA-mediated NDPK B knockdown. In adult rat cardiomyocytes expressing G&bgr;1H266L&ggr;2, the basal contractility was suppressed by ≈50% which correlated to similarly reduced basal cAMP levels and reduced Ser16-phosphorylation of phospholamban. In the presence of the &bgr;-adrenoceptor agonist isoproterenol, the total cAMP formation and contractility were significantly lower in G&bgr;1H266L&ggr;2 than in G&bgr;1WT&ggr;2 expressing cardiomyocytes. However, the relative isoproterenol-induced increased was not affected by G&bgr;1H266L&ggr;2. We conclude that the receptor-independent activation of G proteins via NDPK B/G&bgr;&ggr; complexes requires the intermediate phosphorylation of G protein &bgr; subunits at His-266. Our results highlight the histidine kinase activity of NDPK B for G&bgr; and demonstrate its contribution to the receptor-independent regulation of cAMP synthesis and contractility in intact cardiomyocytes.

Severe familial left ventricular non-compaction cardiomyopathy due to a novel troponin T (TNNT2) mutation

AIMS Left ventricular non-compaction (LVNC) is caused by mutations in multiple genes. It is still unclear whether LVNC is the primary determinant of cardiomyopathy or rather a secondary phenomenon with intrinsic cardiomyocyte dysfunction being the actual cause of the disease. Here, we describe a family with LVNC due to a novel missense mutation, pE96K, in the cardiac troponin T gene (TNNT2). METHODS AND RESULTS The novel mutation was identified in the index patient and all affected relatives, but not in 430 healthy control individuals. Mutations in known LVNC-associated genes were excluded. To investigate the pathophysiological implications of the mutation, we generated transgenic mice expressing human wild-type cTNT (hcTNT) or a human troponin T harbouring the pE96K mutation (mut cTNT). Animals were characterized by echocardiography, histology, and gene expression analysis. Mut cTNT mice displayed an impaired left ventricular function and induction of marker genes of heart failure. Remarkably, left ventricular non-compaction was not observed. CONCLUSION Familial co-segregation and the cardiomyopathy phenotype of mut cTNT mice strongly support a causal relationship of the pE96K mutation and disease in our index patient. In addition, our data suggest that a non-compaction phenotype is not required for the development of cardiomyopathy in this specific TNNT2 mutation leading to LVNC.

Elevated miR-122 serum levels are an independent marker of liver injury in inflammatory diseases.

Serum concentrations of miR‐122 were proposed as a marker for various inflammatory diseases, but the mechanisms driving alterations in miR‐122 serum levels are unknown.