2009-11-15, Schildknecht, Stefan, Poeltl, Dominik, Nagel, Daniel M., Matt, Florian, Scholz, Diana, Lotharius, Julie, Schmieg, Nathalie, Salvo-Vargas, Alberto, Leist, Marcel

LUHMES cells are conditionally-immortalized non-transformed human fetal cells that can be differentiated to acquire a dopaminergic neuron-like phenotype under appropriate growth conditions. After differentiation by GDNF and cyclic adenosine monophosphate, LUHMES were sensitive to 1-methyl-4-pheriylpyridinium (MPP+) toxicity at ≤5 μM, but resistant to the parental compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The high homogeneity and purity of the cultures allowed the detection of metabolic changes during the degeneration. Cellular ATP dropped in two phases after 24 and 48 h; cellular glutathione (GSH) decreased continuously, paralleled by an increase in lipid peroxidation. These events were accompanied by a time-dependent degeneration of neurites. Block of the dopamine transporter by GBR 12909 or mazindol completely abrogated MPP+ toxicity. Inhibition of de novo dopamine synthesis by α-methyl-L-tyrosine or 3-iodo-L-tyrosine attenuated toxicity, but did not reduce the initial drop in ATP. Inhibition of mixed lineage kinases by CEP1347 completely prevented the MPP+-induced loss of viability and intracellular GSH, but failed to attenuate the initial drop of ATP. For the quantitative assessment of neurite degeneration, an automated imaging-based high content screening approach was applied and confirmed the findings made by pharmacological interventions in this study. Our data indicate that inhibition of mitochondrial ATP svnthesis is not sufficient to trigger cell death in MPP+-treated LUHMES.

2009-04-15, Schildknecht, Stefan, Ullrich, Volker

Prostanoids and nitric oxide ((.)NO) are essential modulators of cardiovascular function in health and disease. Among the (.)NO-derived species formed in cells, peroxynitrite (ONOO(-)) is generally associated with its role as nitrating agent under severe pathophysiological conditions. This review, however, highlights a physiological role of peroxynitrite as endogenously formed regulator of prostanoid synthesis in the cardiovascular system. Prostaglandin endoperoxide H2 synthase (PGHS)(1), the central enzyme in the prostanoid pathway was observed to be nitrated and inactivated by high fluxes of peroxynitrite. In contrast, low nanomolar levels, that are formed endogenously in cardiovascular cells, turned out to activate PGHS and therefore prostanoid formation. A further increase in the rates of (.)NO and superoxide ((.)O2(-)) generation, that can be observed after exposure of vascular endothelial cells to endotoxin, results in enhanced levels of peroxynitrite that were shown to selectively nitrate and inactivate prostacyclin (PGI(2))-synthase as one of the dominating terminal prostanoid synthases in the cardiovascular system. As a consequence, accumulation of the intermediate PGH(2) occurs that is capable to activate the thromboxane A(2) (TxA(2)) receptor on the surface of smooth muscle cells to promote vasoconstriction. The nitration of PGI(2)-synthase thus functions as endogenous posttranslational switch that shuts off the PGI(2)-mediated vasodilatory, anti-aggregatory, and anti-adhesive conditions in order to support the transmigration of immune cells from the blood to the sites of an infection. As a third type of interaction between the (.)NO and the prostanoid pathways, an activation of nitrite by the endogenous peroxidase activity of PGHS can lead to an autocatalytic nitration and inactivation of PGHS under conditions of high nitrite and low arachidonic acid levels that mostly prevail in progressive activation stages in cell types that express inducible NOS-2 such as macrophages.

2009, Volbracht, Christiane, Penzkofer, Stephan, Mansson, David, Vielsted Christensen, Kenneth, Fog, Karina, Schildknecht, Stefan, Leist, Marcel, Nielsen, Jacob

Amyloid-β peptide (Aβ), a putatively causative agent of Alzheimer s disease (AD), is proteolytically derived from β-amyloid precursor protein (APP). Here we describe cellular assays to detect the activity of the key protease β-site of APP cleaving enzyme 1 (BACE1) based on an artificial reporter construct containing the BACE1 cleavage site of APP. These methods allow identification of inhibitors and indirect modulators of BACE1. In primary neuronal cultures transfected with human APP constructs (huAPP), Aβ production was modified by BACE1 inhibitors similarly to the production of endogenous murine Aβ in wild-type cells and to that of different transgenic neurons. To further improve the assay, we substituted the extracellular domain of APP by secreted alkaline phosphatase (SEAP). SEAP was easily quantified in the cell culture supernatants after cleavage of SEAP APP by BACE1 or α-secretases. To render the assay specific for BACE1, the α-secretase cleavage site of SEAP APP was eliminated either by site-directed mutagenesis or by substituting the transmembrane part of APP by the membrane domain of the erythropoietin receptor (EpoR). The pharmacology of these constructs was characterized in detail in HEK293 cells (human embryonic kidney cell line), and the SEAP APP EpoR construct was also introduced into primary murine neurons and there allowed specific measurement of BACE1 activity

2008, Schildknecht, Stefan, Daiber, Andreas, Ghisla, Sandro, Cohen, Richard A., Bachschmid, Markus Michael

The primary pharmacological target of acetaminophen is prostaglandin endoperoxide H2 synthase (PGHS). The enzymatic catalytic mechanism is radical-based, initiated, and maintained by the persistent presence of peroxides, particularly peroxynitrite, which is termed "peroxide tone". Whereas the prevailing concept assumes a direct reduction of the active, oxidized enzyme by acetaminophen, here we show that acetaminophen is a potent scavenger of peroxynitrite (peroxynitrite-mediated phenol nitration, IC50≈72µM; Sin-1-mediated DHR123 oxidation, IC50≈11µM) and thus inhibits PGHS by eliminating the peroxide tone. Nanomolar concentrations of peroxynitrite increased the activity of isolated PGHS and prostacyclin formation by aortic endothelial cells. This elevated activity was efficiently inhibited by pharmacologically relevant concentrations of acetaminophen (IC50{approx}10µM for 6-keto-PGF1α) and other free radical scavengers. However, when the peroxide tone was provided by H2O2 or tert-butyl-OOH, acetaminophen had only negligible inhibitory effects. Our concept could help to explain the efficacy of acetaminophen to inhibit PGHS in cell types with moderate oxidant formation. However, high levels of peroxynitrite or other peroxides such as lipid peroxides formed at inflammatory sites might overwhelm the ability of acetaminophen to decrease PGHS activation. The concept presented herein provides a molecular basis to explain the excellent analgesic and antipyretic properties of acetaminophen together with its minimal anti-inflammatory effects. Schildknecht, S., Daiber, A., Ghisla, S., Cohen, R. A., Bachschmid, M. M. Acetaminophen inhibits prostanoid synthesis by scavenging the PGHS-activator peroxynitrite.

2009-08-15, Daiber, Andreas, Schildknecht, Stefan, Müller, Johanna, Kamuf, Jens, Bachschmid, Markus M., Ullrich, Volker

S-nitros(yl)ation belongs to the redox-based posttranslational modifications of proteins but the underlying chemistry is controversial. In contrast to current concepts involving the autoxidation of nitric oxide ((.)NO, nitrogen monoxide), we and others have proposed the formation of peroxynitrite (oxoperoxonitrate (1(-))as an essential intermediate. This requires low cellular fluxes of (.)NO and superoxide (UO2(-)), for which model systems have been introduced. We here propose two new systems for nitros(yl)ation that avoid the shortcomings of previous models. Based on the thermal decomposition of 3-morpholinosydnonimine,equal fluxes of (.)NO and UO2(-) were generated and modulated by the addition of (.)NO donors or Cu,Zn superoxide dismutase. As reactants for S-nitros(yl)ation, NADP+-dependent isocitrate dehydrogenase and glutathione were employed, for which optimal S-nitros(yl)ation was observed at nanomolar fluxes of (.)NO and UO2(-) at a ratio of about 3:1. The previously used reactants phenol and diaminonaphthalene (C- and Nnitrosation)demonstrated potential participation of multiple pathways for nitros(yl)ation. According to our data, neither peroxynitrite nor autoxidation of UNO was as efficient as the 3 (.)NO/1 UO2(-) system in mediating S-nitros(yl)ation. In theory this could lead to an elusive nitrosonium (nitrosyl cation)-like species in the first step and to N2O3 in the subsequent reaction. Which of these two species or whether both together will participate in biological S-nitros(yl)ation remains to be elucidated. Finally, we developed several hypothetical scenarios to which the described (.)NO/UO2-flux model could apply, providing conditions that allow either direct electrophilic substitution at a thiolate or S-nitros(yl)ation via transnitrosation from S-nitrosoglutathione.

2009-03, Van Der Loo, Bernd, Schildknecht, Stefan, Zee, Rebecca, Bachschmid, Markus M.

Ageing is an important risk factor for the development of cardiovascular diseases. Vascular ageing is mainly characterized by endothelial dysfunction, an alteration of endothelium-dependent signalling processes and vascular remodelling. The underlying mechanisms comprise increased production of reactive oxygen species (ROS), inactivation of nitric oxide (.NO) and subsequent formation of peroxynitrite (ONOO(-)). Elevated ONOO(-) may exhibit new messenger functions by post-translational oxidative modification of intracellular regulatory proteins. Mitochondria are a major source of age-associated superoxide formation, as electrons are misdirected from the respiratory chain. Manganese superoxide dismutase (MnSOD), a mitochondrial antioxidant enzyme, is an integral part of the nucleoids and may protect mitochondrial DNA from ROS. A model linking .NO, mitochondria, MnSOD and its acetylation/deacetylation by sirtuins (NAD+-dependent class III histone deacetylases) may be the basis for a potentially new powerful therapeutic intervention in the ageing process.

2009, Schildknecht, Stefan, Nagel, Daniel M., Leist, Marcel

2009-07, Wenzel, Philip, Schulz, Eberhard, Gori, Tommaso, Ostad, Mir A., Mäthner, Falk, Schildknecht, Stefan, Göbel, Sebastian, Oelze, Matthias, Stalleicken, Dirk, Warnholtz, Ascan

Recent animal data suggest that reduced lipoic acid (LA) prevents oxidative inhibition of the nitrate bioactivating enzyme, the mitochondrial aldehyde dehydrogenase (ALDH-2), and that pentaerythritol tetranitrate (PETN) does not induce nitrate tolerance because of its intrinsic antioxidative properties, thereby preserving ALDH-2 activity. We sought to determine whether ALDH-2 activity in circulating white blood cells (WBCs) can be used to monitor nitrate tolerance and whether LA can prevent nitroglycerin tachyphylaxis in humans. Eight healthy male volunteers received, in randomized order, a single dose of glyceryl trinitrate (GTN; 0.8 mg), PETN (80 mg), or GTN plus LA (600 mg) orally. GTN (30 min) and PETN (120 min) administration lead to a comparable dilation of the brachial artery (15 +/- 1%). In contrast to PETN, acute GTN treatment resulted in a 60% decrease in WBC ALDH-2 activity (high-performance liquid chromatography), 30% reduction of nitrate bioactivation, and 25% decrease in serum antioxidant capacity (fluorescence assay), which all were prevented by pretreatment with LA. Mechanistic studies in rats identified oxidative stress, ALDH-2 inactivation, and vascular dysfunction as common features in acute and chronic nitrate tolerance. Treatment with GTN, but not PETN, acutely inhibits ALDH-2 activity and nitrate bioactivation in healthy volunteers. These effects were prevented by LA pretreatment, emphasizing the role of oxidative stress-triggered ALDH-2 dysfunction. Assessment of WBC ALDH-2 activity could be used as an easily accessible marker for the detection of nitroglycerin-induced tachyphylaxis in humans and may be of high clinical interest because recent data suggest that ALDH-2 activity correlates with protection from ischemic heart damage in infarct models.

2009, Nagel, Daniel M., Schildknecht, Stefan, Matt, Florian, Leist, Marcel

2008-08-15, Schildknecht, Stefan, Van Der Loo, Bernd, Weber, Klaus, Tiefenthaler, Katja, Daiber, Andreas, Bachschmid, Markus Michael

Aggregation of activated platelets is considerably mediated by the autocrine action of thromboxane A2 (TxA2) which is formed in a prostaglandin endoperoxide H2 synthase-1 (PGHS-1 or COX-1)-dependent manner. The activity of PGHS-1 can be stimulated by peroxides, an effect termed "peroxide tone", that renders PGHS-1 the key regulatory enzyme in the formation of TxA2. Activated platelets release nitric oxide (*NO) and superoxide (O*2) but their interactions with the prostanoid pathway have been controversially discussed in platelet physiology and pathophysiology. The current study demonstrates that endogenously formed peroxynitrite at nanomolar concentrations, originating from the interaction of *NO and *O2, potently activated PGHS-1, which parallels TxA2 formation and aggregation in human platelets. Inhibition of the endogenous formation of either *NO or O*2 resulted in a concentration-dependent decline of PGHS-1 activity, TxA2 release, and aggregation. The concept of peroxynitrite as modulator of TxA2 formation and aggregation explains the interaction of *NO and O*2 with the PGHS pathway and suggests a mechanism by which antioxidants can regulate PGHS-1-dependent platelet aggregation. This may provide a molecular explanation for the clinically observed hyperreactivity of platelets in high-risk patients and serve as a basis for novel therapeutic interventions.