Ubiquitylation and SUMOylation of Cardiac Ion Channels

Cardiac ion channels play an essential role in the generation of the action potential of cardiomyocytes. Over the past 15 years, a new field of research called channelopathies has emerged; it regroups all diseases caused by ion channel dysfunction. Investigators have largely determined the physiological roles of cardiac ion channels, but little is known about the molecular determinants of their regulation. Two post-translational mechanisms that are crucial in determining the fate of proteins are ubiquitylation and the SUMOylation pathways, which lead to the degradation and/or regulation of modified proteins. Recently, several groups have investigated the physiological impacts of these mechanisms on the regulation of different classes of cardiac ion channels. The objective of this review is to summarize and briefly discuss these results.

Soziale Ungleichheit von Bildungschancen und Chancengleichheit - eine Reanalyse mit bildungspolitischen Implikationen

Zweifelsohne war und ist das Prinzip der Chancengleichheit – gleiches Recht auf Entfaltung ungleicher Anlagen – eine Maxime für die Bildungspolitik und Gestal-tung von Bildungssystemen (vgl. Müller 1998; Friedeburg 1992; Baumert 1991: 333). Diese Prämisse wird nicht zuletzt durch Artikel 3, Absatz 3 des Grundgeset-zes, wonach niemand wegen seines Geschlechts, seiner Abstammung, seiner Ras-se, seiner Sprache, seiner Heimat und Herkunft, seines Glaubens, seiner religiösen oder politischen Anschauungen benachteiligt oder bevorzugt werden darf, inhalt-lich vorgegeben. So lag ein Hauptteil der Zielsetzungen von Bildungsreformen seit den 1960er Jahren darin, die Rahmenbedingungen für den Bildungszugang – für die Chancennutzung – in der Weise zu setzen, dass weder das strukturelle Angebot an Bildungsgelegenheiten noch sozialstrukturelle Eigenschaften von Schulkindern und ihres Elternhauses systematische Einflüsse auf den Bildungsweg und den Erwerb von Bildungszertifikaten haben (Friedeburg 1992). Mit dem Ausbau des Schul- und Hochschulwesens und den institutionellen Reformen seit den 1960er Jahren ist das Ziel auch größtenteils erreicht worden, dass neben institutionellen Barrieren auch ökonomische und geografische Barrieren beim Bildungszugang weitgehend an Bedeutung verloren haben (Müller 1998; Krais 1996). Bildungs-disparitäten nach sozialstrukturellen Merkmalen wie etwa Klassenlage des Eltern-hauses haben sich ebenfalls bis zu einem bestimmten Grad abgeschwächt (Müller und Haun 1994; Schimpl-Neimanns 2000). Insbesondere konnten Mädchen ihre Bildungsdefizite gegenüber den Jungen mehr als ausgleichen (Rodax und Rodax 1996; Rodax 1995), sodass nunmehr von einer Bildungsungleichheit zu Unguns-ten von Jungen auszugehen ist (Diefenbach und Klein 2002; Becker 2007). Einige der beabsichtigten wie unbeabsichtigten Folgen dieser Reformbemühungen kön-nen am Wandel der Bildungsbeteiligung, insbesondere beim Übergang von der Grundschule auf das Gymnasium, abgelesen werden (Becker 2006).

Acid-sensing channels in human bladder: expression, function and alterations during bladder pain syndrome

We examined the possible role of H(+) activated acid-sensing ion channels in pain perception. We characterized expression in bladder dome biopsies from patients with bladder pain syndrome and controls, in cultured human urothelium and in urothelial TEU-2 cells.

Nedd4-2-dependent ubiquitylation and regulation of the cardiac potassium channel hERG1

The voltage-gated cardiac potassium channel hERG1 (human ether-à-gogo-related gene 1) plays a key role in the repolarization phase of the cardiac action potential (AP). Mutations in its gene, KCNH2, can lead to defects in the biosynthesis and maturation of the channel, resulting in congenital long QT syndrome (LQTS). To identify the molecular mechanisms regulating the density of hERG1 channels at the plasma membrane, we investigated channel ubiquitylation by ubiquitin ligase Nedd4-2, a post-translational regulatory mechanism previously linked to other ion channels. We found that whole-cell hERG1 currents recorded in HEK293 cells were decreased upon neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2) co-expression. The amount of hERG1 channels in total HEK293 lysates and at the cell surface, as assessed by Western blot and biotinylation assays, respectively, were concomitantly decreased. Nedd4-2 and hERG1 interact via a PY motif located in the C-terminus of hERG1. Finally, we determined that Nedd4-2 mediates ubiquitylation of hERG1 and that deletion of this motif affects Nedd4-2-dependent regulation. These results suggest that ubiquitylation of the hERG1 protein by Nedd4-2, and its subsequent down-regulation, could represent an important mechanism for modulation of the duration of the human cardiac action potential.

Identification of a novel loss-of-function calcium channel gene mutation in short QT syndrome (SQTS6)

Short QT syndrome (SQTS) is a genetically determined ion-channel disorder, which may cause malignant tachyarrhythmias and sudden cardiac death. Thus far, mutations in five different genes encoding potassium and calcium channel subunits have been reported. We present, for the first time, a novel loss-of-function mutation coding for an L-type calcium channel subunit.

Neuronal precursor cell-expressed developmentally down-regulated 4-1 (NEDD4-1) controls the sorting of newly synthesized Ca(V)1.2 calcium channels

Neuronal precursor cell-expressed developmentally down-regulated 4 (Nedd4) proteins are ubiquitin ligases, which attach ubiquitin moieties to their target proteins, a post-translational modification that is most commonly associated with protein degradation. Nedd4 ubiquitin ligases have been shown to down-regulate both potassium and sodium channels. In this study, we investigated whether Nedd4 ubiquitin ligases also regulate Ca(v) calcium channels. We expressed three Nedd4 family members, Nedd4-1, Nedd4-2, and WWP2, together with Ca(v)1.2 channels in tsA-201 cells. We found that Nedd4-1 dramatically decreased Ca(v) whole-cell currents, whereas Nedd4-2 and WWP2 failed to regulate the current. Surface biotinylation assays revealed that Nedd4-1 decreased the number of channels inserted at the plasma membrane. Western blots also showed a concomitant decrease in the total expression of the channels. Surprisingly, however, neither the Ca(v) pore-forming α1 subunit nor the associated Ca(v)β and Ca(v)α(2)δ subunits were ubiquitylated by Nedd4-1. The proteasome inhibitor MG132 prevented the degradation of Ca(v) channels, whereas monodansylcadaverine and chloroquine partially antagonized the Nedd4-1-induced regulation of Ca(v) currents. Remarkably, the effect of Nedd4-1 was fully prevented by brefeldin A. These data suggest that Nedd4-1 promotes the sorting of newly synthesized Ca(v) channels for degradation by both the proteasome and the lysosome. Most importantly, Nedd4-1-induced regulation required the co-expression of Ca(v)β subunits, known to antagonize the retention of the channels in the endoplasmic reticulum. Altogether, our results suggest that Nedd4-1 interferes with the chaperon role of Ca(v)β at the endoplasmic reticulum/Golgi level to prevent the delivery of Ca(v) channels at the plasma membrane.

SAP97 and dystrophin macromolecular complexes determine two pools of cardiac sodium channels Nav1.5 in cardiomyocytes

The cardiac sodium channel Na(v)1.5 plays a key role in excitability and conduction. The 3 last residues of Na(v)1.5 (Ser-Ile-Val) constitute a PDZ-domain binding motif that interacts with the syntrophin-dystrophin complex. As dystrophin is absent at the intercalated discs, Na(v)1.5 could potentially interact with other, yet unknown, proteins at this site.

Regulation of the cardiac sodium channel Nav1.5 by utrophin in dystrophin-deficient mice

Duchenne muscular dystrophy (DMD) is a severe striated muscle disease due to the absence of dystrophin. Dystrophin deficiency results in dysfunctional sodium channels and conduction abnormalities in hearts of mdx mice. Disease progression in the mdx mouse only modestly reflects that of DMD patients, possibly due to utrophin up-regulation. Here, we investigated mice deficient in both dystrophin and utrophin [double knockout (DKO)] to assess the role of utrophin in the regulation of the cardiac sodium channel (Na(v)1.5) in mdx mice.

Intravital microscopy reveals endothelial dysfunction in resistance arterioles in Angiotensin II-induced hypertension

It is known that hypertension is associated with endothelial dysfunction and that Angiotensin II (Ang II) is a key player in the pathogenesis of hypertension. We aimed to elucidate whether endothelial dysfunction is a specific feature of Ang II-mediated hypertension or a common finding of hypertension, independently of underlying etiology. We studied endothelial-dependent vasorelaxation in precapillary resistance arterioles and in various large-caliber conductance arteries in wild-type mice with Ang II-dependent hypertension (2-kidney 1-clip (2K1C) model) or Ang II-independent (volume overload) hypertension (1-kidney 1-clip model (1K1C)). Normotensive sham mice were used as controls. Aortic mechanical properties were also evaluated. Intravital microscopy of precapillary arterioles revealed a significantly impaired endothelium-dependent vasorelaxation in 2K1C mice compared with sham mice, as quantified by the ratio of acetylcholine (ACh)-induced over S-nitroso-N-acetyl-D,L-penicillamine (SNAP)-induced vasorelaxation (2K1C: 0.49±0.12 vs. sham: 0.87±0.11, P=0.018). In contrast, the ACh/SNAP ratio in volume-overload hypertension 1K1C mice was not significantly different from sham mice, indicating no specific endothelial dysfunction (1K1C: 0.77±0.27 vs. sham: 0.87±0.11, P=0.138). Mechanical aortic wall properties and endothelium-dependent vasorelaxation, assessed ex vivo in rings of large-caliber conductance (abdominal and thoracic aorta, carotid and femoral arteries), were not different between 2K1C, 1K1C and sham mice. Endothelial dysfunction is an early feature of Ang II- but not volume-overload-mediated hypertension. This occurs exclusively at the level of precapillary arterioles and not in conduit arteries. Our findings, if confirmed in clinical studies, will provide a better understanding of the pathophysiological mechanisms of hypertension.

ImmunoChip study implicates antigen presentation to T cells in narcolepsy

Recent advances in the identification of susceptibility genes and environmental exposures provide broad support for a post-infectious autoimmune basis for narcolepsy/hypocretin (orexin) deficiency. We genotyped loci associated with other autoimmune and inflammatory diseases in 1,886 individuals with hypocretin-deficient narcolepsy and 10,421 controls, all of European ancestry, using a custom genotyping array (ImmunoChip). Three loci located outside the Human Leukocyte Antigen (HLA) region on chromosome 6 were significantly associated with disease risk. In addition to a strong signal in the T cell receptor alpha (TRA@), variants in two additional narcolepsy loci, Cathepsin H (CTSH) and Tumor necrosis factor (ligand) superfamily member 4 (TNFSF4, also called OX40L), attained genome-wide significance. These findings underline the importance of antigen presentation by HLA Class II to T cells in the pathophysiology of this autoimmune disease.