Bodo Levkau

Apoptosis overrides survival signals through a caspase-mediated dominant-negative NF-|[kgr]|B loop

The transcription factor NF-κB is an important regulator of gene expression during immune and inflammatory responses, and can also protect against apoptosis. Here we show that endothelial cells undergo apoptosis when deprived of growth factors. Surviving viable cells exhibit increased activity of NF-κB, whereas apoptotic cells show caspase-mediated cleavage of the NF-κB p65/RelA subunit. This cleavage leads to loss of carboxy-terminal transactivation domains and a transcriptionally inactive p65 molecule. The truncated p65 acts as a dominant-negative inhibitor of NF-κB, promoting apoptosis, whereas an uncleavable, caspase-resistant p65 protects the cells from apoptosis. The generation of a dominant-negative fragment of p65 during apoptosis may be an efficient pro-apoptotic feedback mechanism between caspase activation and NF-κB inactivation.

Dynamic regulation of MEK/Erks and Akt/GSK-3β in human end-stage heart failure after left ventricular mechanical support: myocardial mechanotransduction-sensitivity as a possible molecular mechanism

OBJECTIVEnLeft ventricular assist devices (LVAD) are used to bridge patients with end-stage heart failure to transplantation. After long-term LVAD support, ventricular function may partially recover, a process called reverse remodeling. As several kinase-mediated signal transduction pathways have been implicated in the development of cardiac hypertrophy and failure, we examined the activities of the Erks, MEKs, Akt, GSK-3 beta, p70S6K, JNKs and p38 under LVAD support as well as during single myocyte strain and whole heart stretch.nnnMETHODSnWestern blotting and immunohistochemistry were performed using phospho-specific antibodies in matched samples from ten patients with end-stage heart failure before and after LVAD. Cyclic strain was performed in rat neonatal cardiac myocytes, and tensile stretch applied to Langendorff-perfused mouse hearts via a left ventricular balloon.nnnRESULTSnThe activity of Erks and Akt in failing hearts dramatically decreased after LVAD support, while that of GSK-3 beta increased. There was an endo/epicardial gradient for Erk activity which persisted after LVAD despite the reduction of total Erk activity. TUNEL-positivity and myocyte size decreased after LVAD, but independently of changes in kinase activity. In cardiomyocytes and Langendorff-perfused mouse hearts both strain/stretch and its relief regulated the activities of Erks, Akt, and GSK-3 beta.nnnCONCLUSIONnErks and Akt/GSK-3 beta are highly responsive to myocyte stretch in vitro and in vivo, and may be sensitive molecular parameters of reverse remodeling under LVAD support.

Reversible activation of nuclear factor-κB in human end-stage heart failure after left ventricular mechanical support

OBJECTIVEnLeft ventricular assist devices (LVAD) have been used to bridge patients with end-stage heart failure to transplantation. Although several reports have suggested that the native ventricular function recovers after long-term LVAD support, a process called reverse remodeling, the underlying biological mechanisms are still unknown. As the transcription factor nuclear factor-kappaB (NF-kappaB) has been shown to be active in the failing human heart, we examined whether its activity is altered under LVAD support, and may thus contribute to the dynamic process of reverse remodeling.nnnMETHODSnThe activity of NF-kappaB was studied in 16 patients with end-stage heart failure (eight with dilated cardiomyopathy, six with ischemic heart disease, one with myocarditis, and one with congenital heart disease) before and after LVAD support by immunohistochemistry using an antibody against active NF-kappaB. Gel-shifts for NF-kappaB DNA-binding activity were performed with paired human myocardial tissue from four patients. The mean cardiomyocyte diameter before and after mechanical unloading was measured with an image analyzer system.nnnRESULTSn15 patients out of 16 showed a significant decrease in the number of NF-kappaB positive cardiomyocyte nuclei after LVAD support in the left ventricular myocardium. The NF-kappaB DNA-binding activity also decreased after LVAD support as measured by gel-shift analysis. While the number of positive cardiomyocytes was significantly higher in the subendocardium than in the subepicardium at the time of LVAD implantation, this difference was no longer present at the time of LVAD explantation. The diameter of cardiomyocytes in the left ventricle decreased significantly as a parameter of structural reverse remodeling.nnnCONCLUSIONnLVAD support decreases the extent of NF-kappaB activation in failing human hearts, suggesting that NF-kappaB may be involved in the process of reverse remodeling.

Reduction of hypoxia‐inducible heme oxygenase‐1 in the myocardium after left ventricular mechanical support

Left ventricular assist devices (LVAD) may improve cardiac function. The pathogenesis of this phenomenon, called ‘reverse remodelling’, is not completely elucidated. To examine the hypothesis that LVAD support eliminates tissue stress by reducing local hypoxia, the distribution of heme oxygenase‐1 (HO‐1), a stress protein inducible by hypoxia, was examined in vivo and in vitro. The immunoreactivity for HO‐1 was semi‐quantitatively analysed in left ventricular tissue of 23patients (14 dilated cardiomyopathy (DCM), six ischaemic heart disease (IHD), three myocarditis/congenital heart disease) with end‐stage heart failure before and after LVAD support, while two unused donor hearts served as controls. Control hearts stained almost negative for HO‐1, while failing hearts showed immunoreactivity mainly in cardiomyocytes, but also in endothelial cells, some smooth muscle cells and fibroblasts. Hearts with IHD showed significantly higher HO‐1 immunoreactivity than hearts with DCM or myocarditis/congenital heart disease. After LVAD support, the HO‐1 content decreased significantly in the DCM and IHD group and was significantly higher in the subendocardium than in the subepicardium. In vitro, under hypoxic conditions, neonatal rat cardiomyocytes showed an increase of HO‐1 protein content up to sixfold above the normal level, which returned to normal values after normoxic cultivation. Mechanical support reduces the HO‐1 content of the failing heart and HO‐1 is inducible in vitro under hypoxia and is reversible under normoxia. This supports the concept that restoration of cardiac normoxia by mechanical unloading, particularly in the subendocardium, may be in part responsible for the phenomenon of ‘reverse remodelling’. Copyright

Molecular Mechanisms of Inherited Ventricular Arrhythmias

Background: Inherited ventricular arrhythmias such as the long QT syndrome (LQTS), Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia (CPVT), idiopathic ventricular fibrillation (VF), and arrhythmogenic right ventricular cardiomyopathy (ARVC) account for a relevant proportion of sudden cardiac death cases in young patients cohorts. The detailed pathogenetic mechanisms of inherited ventricular arrhythmias are still poorly understood because systematic investigations are difficult to perform due to low patient numbers and the lack of appropriate experimental models. However, recent advances in research and science have identified a genetic background for many of these diseases.nPresent Knowledge: In LQTS, various mutations in different genes encoding for cardiac potassium and sodium channel proteins have been identified (“channelopathy”), and initial progress in genotype-phenotype correlation is made. Mutations in the cardiac sodium channel gene have also been identified in a subset of patients with Brugada syndrome, whereas a genetic background has not yet been demonstrated in idiopathic VF and right ventricular outflow-tract tachycardia (RVO-VT). Very recently, mutations in the cardiac ryanodine receptor gene have been identified in CPVT and in a subgroup of patients with ARVC. Although several chromosomal loci were suggested, no other responsible genes or mutations have been found in autosomal dominant forms of ARVC. However, in Naxos disease, a recessive form of ARVC with coexpression of palmoplantar keratoderma and woolly hair, a mutation in the plakoglobin gene has recently been discovered, thus underscoring the potential role of genetic alterations in cytoskeletal proteins in ARVC.nFuture Perspectives: In the next years, significant progress in the genetic diagnosis pathophysiologic understanding of disease mechanisms, genotype-phenotype correlation, and the development of gene- or target-directed treatment strategies can be expected in the field of inherited ventricular arrhythmias.nConclusion: This review summarizes the current knowledge of the molecular mechanisms, including aspects of pathoanatomy, autonomic innervation, genetics, and genotype-phenotype correlations with their potential implications for diagnosis and treatment of inherited ventricular arrhythmias.Hintergrund: Angeborene ventrikuläre Arrhythmien wie QT-Syndrom (LQTS), Brugada-Syndrom, katecholamingerge polymorphe ventrikuläre Tachykardie (CPVT), idiopathisches Kammerflimmern sowie arrhythmogene rechtsventrikuläre Kardiomyopathie (ARVC) sind wesentliche Ursachen plötzlicher Herztodesfälle bei jungen Patienten. Die detaillierten pathogenetischen Mechanismen sind bislang nur in Anfängen aufgeklärt, da bei geringen Patientenzahlen und fehlenden Tiermodellen systematische Untersuchungen erschwert sind. Dennoch konnten jüngste Fortschritte in klinischer und experimenteller Forschung neue Erkenntnisse zum genetischen Hintergrund angeborener ventrikulärer Arrhythmien beitragen.nAktuelle Erkentnisse: Bei LQTS wurden in den letzten Jahren zahlreiche Mutationen in verschiedenen Genen nachgewiesen, die für Proteine kardialer Kalium- und Natriumkanäle kodieren (“Ionenkanalerkrankung”). Auch wurden erste Ergebnisse bei der Genotyp-Phänotyp-Korrelation erzielt. Mutationen im kardialen Natriumkanalgen wurden auch beim Brugada-Syndrom nachgewiesen, während bei Patienten mit idiopathischem Kammerflimmern und rechtsventrikulärer Ausflusstrakttachykardie bislang keine genetischen Veränderungen gefunden werden. Kürzlich konnten Mutationen im Gen des kardialen Ryanodinrezeptors bei Patienten mit CPVT und einer Untergruppe der ARVC nachgewiesen werden. Dagegen konnte bei anderen Formen autosomal-dominant vererbter ARVC trotz Lokalisation mehrerer chromosomaler Loci bislang kein Gendefekt identifiziert werden. Bei der rezessiv vererbten Naxos-Erkrankung, einer Sonderform der ARVC mit Koexpression von palmoplantarer Keratose, wurde unlängst eine Mutation im Plakoglobingen nachgewiesen, wodurch die potentielle Rolle genetischer Veränderungen in zytoskelettären Proteinen bei ARVC unterstrichen wird.nPerspektiven: Auch in den kommenden Jahren sind entscheidende Fortschritte in der genetischen Diagnostik, dem Verständnis pathogenetischer Mechanismen, der Genotyp-Phänotyp-Korrelation und der Entwicklung von gentyporientierten Therapiestrategien bei angeborenen ventrikulären Arrythmien zu erwarten.nSchlussfolgerung: Diese Übersicht fasst den aktuellen Wissensstand der molekularen Mechanismen zusammen und diskutiert dabei Aspekte von Pathoanatomie, autonomer Innervation, Genetik, und Genotyp-Phänotyp-Korrelation mit ihren potentiellen Implikationen für die Diagnostik und Therapie angeborener ventrikulärer Arrhythmien.

Structural basis of sterol binding and transport by a yeast StARkin domain

The StARkin superfamily comprises proteins with steroidogenic acute regulatory protein–related lipid transfer (StART) domains that are implicated in intracellular, non-vesicular lipid transport. A new family of membrane-anchored StARkins was recently identified, including six members, Lam1–Lam6, in the yeast Saccharomyces cerevisiae. Lam1–Lam4 are anchored to the endoplasmic reticulum (ER) membrane at sites where the ER is tethered to the plasma membrane and proposed to be involved in sterol homeostasis in yeast. To better understand the biological roles of these proteins, we carried out a structure-function analysis of the second StARkin domain of Lam4, here termed Lam4S2. NMR experiments indicated that Lam4S2 undergoes specific conformational changes upon binding sterol, and fluorescence-based assays revealed that it catalyzes sterol transport between vesicle populations in vitro, exhibiting a preference for vesicles containing anionic lipids. Using such vesicles, we found that sterols are transported at a rate of ∼50 molecules per Lam4S2 per minute. Crystal structures of Lam4S2, with and without bound sterol, revealed a largely hydrophobic but surprisingly accessible sterol-binding pocket with the 3-OH group of the sterol oriented toward its base. Single or multiple alanine or aspartic acid replacements of conserved lysine residues in a basic patch on the surface of Lam4S2 near the likely sterol entry/egress site strongly attenuated sterol transport. Our results suggest that Lam4S2 engages anionic membranes via a basic surface patch, enabling “head-first” entry of sterol into the binding pocket followed by partial closure of the entryway. Reversal of these steps enables sterol egress.

Molecular Changes of the Myocardium after Mechanical Circulatory Support

Left ventricular assist devices (LVAD) have been used to “bridge” patients with end-stage heart failure to transplantation. Although several reports have suggested that the native ventricular function recovers after long-term LVAD support, a process called “reverse remodeling”, the underlying biological mechanisms are still unclarified.