Bernd Moosmann

Antioxidants as treatment for neurodegenerative disorders

Oxidative stress is a ubiquitously observed hallmark of neurodegenerative disorders. Neuronal cell dysfunction and cell death due to oxidative stress may causally contribute to the pathogenesis of progressive neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease, as well as acute syndromes of neurodegeneration, such as ischaemic and haemorrhagic stroke. Neuroprotective antioxidants are considered a promising approach to slowing the progression and limiting the extent of neuronal cell loss in these disorders. The clinical evidence demonstrating that antioxidant compounds can act as protective drugs in neurodegenerative disease, however, is still relatively scarce. In the following review, the available data from clinical, animal and cell biological studies regarding the role of antioxidant neuroprotection in progressive neurodegenerative disease will be summarised, focussing particularly on Alzheimer’s disease, Parkinson’s disease, Huntington’s disease and amyotrophic lateral sclerosis. The general complications in developing potent neuroprotective antioxidant drugs directed against these long-term degenerative conditions will also be discussed. The major challenges for drug development are the slow kinetics of disease progression, the unsolved mechanistic questions concerning the final causalities of cell death, the necessity to attain an effective permeation of the blood–brain barrier and the need to reduce the high concentrations currently required to evoke protective effects in cellular and animal model systems. Finally, an outlook as to which direction antioxidant drug development and clinical practice may be leading to in the near future will be provided.

Mitochondrially encoded cysteine predicts animal lifespan.

The role of genetic factors in the determination of lifespan is undisputed. However, numerous successful efforts to identify individual genetic modulators of longevity have not yielded yet a quantitative measure to estimate the lifespan of a species from scratch, merely based on its genomic constitution. Here, we report on a meta‐examination of genome sequences from 248 animal species with known maximum lifespan, including mammals, birds, fish, insects, and helminths. Our analysis reveals that the frequency with which cysteine is encoded by mitochondrial DNA is a specific and phylogenetically ubiquitous molecular indicator of aerobic longevity: long‐lived species synthesize respiratory chain complexes which are depleted of cysteine. Cysteine depletion was also found on a proteome‐wide scale in aerobic versus anaerobic bacteria, archaea, and unicellular eukaryotes; in mitochondrial versus hydrogenosomal sequences; and in the mitochondria of free‐living, aerobic versus anaerobic–parasitic worms. The association of longevity with mitochondrial cysteine depletion persisted after correction for body mass and phylogenetic interdependence, but it was uncoupled in helminthic species with predominantly anaerobic lifestyle. We conclude that protein‐coding genes on mitochondrial DNA constitute a quantitative trait locus for aerobic longevity, wherein the oxidation of mitochondrially translated cysteine mediates the coupling of trait and locus. These results provide distinct support for the free radical theory of aging.

Oxidation of the cysteine-rich regions of parkin perturbs its E3 ligase activity and contributes to protein aggregation

BackgroundAccumulation of aberrant proteins to form Lewy bodies (LBs) is a hallmark of Parkinsons disease (PD). Ubiquitination-mediated degradation of aberrant, misfolded proteins is critical for maintaining normal cell function. Emerging evidence suggests that oxidative/nitrosative stress compromises the precisely-regulated network of ubiquitination in PD, particularly affecting parkin E3 ligase activity, and contributes to the accumulation of toxic proteins and neuronal cell death.ResultsTo gain insight into the mechanism whereby cell stress alters parkin-mediated ubiquitination and LB formation, we investigated the effect of oxidative stress. We found significant increases in oxidation (sulfonation) and subsequent aggregation of parkin in SH-SY5Y cells exposed to the mitochondrial complex I inhibitor 1-methyl-4-phenlypyridinium (MPP+), representing an in vitro cell-based PD model. Exposure of these cells to direct oxidation via pathological doses of H2O2 induced a vicious cycle of increased followed by decreased parkin E3 ligase activity, similar to that previously reported following S-nitrosylation of parkin. Pre-incubation with catalase attenuated H2O2 accumulation, parkin sulfonation, and parkin aggregation. Mass spectrometry (MS) analysis revealed that H2O2 reacted with specific cysteine residues of parkin, resulting in sulfination/sulfonation in regions of the protein similar to those affected by parkin mutations in hereditary forms of PD. Immunohistochemistry or gel electrophoresis revealed an increase in aggregated parkin in rats and primates exposed to mitochondrial complex I inhibitors, as well as in postmortem human brain from patients with PD with LBs.ConclusionThese findings show that oxidative stress alters parkin E3 ligase activity, leading to dysfunction of the ubiquitin-proteasome system and potentially contributing to LB formation.

Adaptive antioxidant methionine accumulation in respiratory chain complexes explains the use of a deviant genetic code in mitochondria

Humans and most other animals use 2 different genetic codes to translate their hereditary information: the standard code for nuclear-encoded proteins and a modern variant of this code in mitochondria. Despite the pivotal role of the genetic code for cell biology, the functional significance of the deviant mitochondrial code has remained enigmatic since its first description in 1979. Here, we show that profound and functionally beneficial alterations on the encoded protein level were causative for the AUA codon reassignment from isoleucine to methionine observed in most mitochondrial lineages. We demonstrate that this codon reassignment leads to a massive accumulation of the easily oxidized amino acid methionine in the highly oxidative inner mitochondrial membrane. This apparently paradoxical outcome can yet be smoothly settled if the antioxidant surface chemistry of methionine is taken into account, and we present direct experimental evidence that intramembrane accumulation of methionine exhibits antioxidant and cytoprotective properties in living cells. Our results unveil that methionine is an evolutionarily selected antioxidant building block of respiratory chain complexes. Collective protein alterations can thus constitute the selective advantage behind codon reassignments, which authenticates the “ambiguous decoding” hypothesis of genetic code evolution. Oxidative stress has shaped the mitochondrial genetic code.

Protective activity of aromatic amines and imines against oxidative nerve cell death.

Abstract Oxidative stress is a widespread phenomenon in the pathology of neurodegenerative diseases such as Alzheimers disease, Parkinsons disease, and amyotrophic lateral sclerosis. Neuronal cell death due to oxidative stress may causally contribute to the pathogeneses of these diseases. Therefore, neuroprotective antioxidants are considered to be a promising approach to slow down disease progression. We have investigated different aromatic amine and imine compounds for neuroprotective antioxidant functions in cell culture, and found that these compounds possess excellent cytoprotective potential in diverse paradigms of oxidative neuronal cell death, including clonal cell lines, primary cerebellar neurons, and organotypic hippocampal slice cultures. Aromatic amines and imines are effective against oxidative glutamate toxicity, glutathione depletion, and hydrogen peroxide toxicity. Their mode of action as direct antioxidants was experimentally confirmed by electron spin resonance spectroscopy, cellfree brain lipid peroxidation assays, and intracellular peroxide measurements. With halfmaximal effective concentrations of 2075 nM in different neuroprotection experiments, the aromatic imines phenothiazine, phenoxazine, and iminostilbene proved to be about two orders of magnitude more effective than common phenolic antioxidants. This remarkable efficacy could be directly correlated to calculated properties of the compounds by means of a novel, quantitative structureactivity relationship model. We conclude that bridged bisarylimines with a single free NHbond, such as iminostilbene, are superior neuroprotective antioxidants, and may be promising lead structures for rational drug development.

Dietary Phenols: Antioxidants for the Brain?

Human diet contains numerous phenolic compounds which have been shown to exert protective antioxidant effects in cellular paradigms of oxidative cell death relevant to neurodegenerative disorders. Since reliable in vivo data are scarce, the question whether dietary phenols may act as beneficial neuroprotective agents in the human brain can only be estimated from the chemical composition of the diet with respect to phenolic compounds, their resorption, their metabolic fate, and their ability to cross the blood-brain barrier. We conclude that antioxidant neuroprotection by natural phenolic compounds is highly questionable. Therefore, dietary supplementation with specifically designed phenolic antioxidants has to be in the center of interest. We outline some chemical structural principles of such designer molecules, focusing on a decreased impact on hormone receptors and the absence of prooxidant side-effects.