Many approaches have been undertaken to understand Alzheimer's disease (AD) but the heterogeneity of the etiologic factors makes it difficult to define the clinically most important factor determining the onset and progression of the disease. However, there is increasing evidence that the previously so-called "secondary factors" such as a disturbed glucose metabolism, oxidative stress and formation of "advanced glycation endproducts" (AGEs) and their interaction in a vicious cycle are also important for the onset and progression of AD. AGEs are protein modifications that contribute to the formation of the histopathological and biochemical hallmarks of AD: amyloid plaques, neurofibrillary tangles and activated microglia. Oxidative modifications are formed by a complex cascade of dehydration, oxidation and cyclisation reactions, subsequent to a non-enzymatic reaction of sugars with amino groups of proteins. Accumulation of AGE-crosslinked proteins throughout life is a general phenomenon of ageing. However, AGEs are more than just markers of ageing since they can also exert adverse biologic effects on tissues and cells, including the activation of intracellular signal transduction pathways, leading to the upregulation of cytokine and free radical production (oxidative stress). Oxidative stress is involved in various divergent events leading to cell damage, including an increase in membrane rigidity, DNA strand breaks and an impairment in glucose uptake. In addition, other age-related metabolic changes such as depletion of antioxidants or decreased energy production by a disturbed glucose metabolism diminish the ability of the cell to cope with the effects of radical-induced membrane, protein and DNA damage. With our improving understanding of the molecular basis for the clinical symptoms of dementia, it is hoped that the elucidation of the etiologic causes, particularly the positive feedback loops involving radical damage and a reduced glucose metabolism, will help to develop novel "neuroprotective" treatment strategies able to interrupt this vicious cycle of oxidative stress and energy shortage in AD.

Non-enzymatic glycation of proteins with reducing sugars and subsequent transition metal catalysed oxidations leads to the formation of protein bound "advanced glycation endproducts" (AGEs). They accumulate on long-lived proteins and are for example structural components of the beta-amyloid plaques in Alzheimer's disease. Since the oxidation of glycated proteins as well as the interaction of AGEs with cell surface receptors produces superoxide radicals, it was tested in BHK 21 hamster fibroblast cells and SH-SY5Y human neuroblastoma cells if AGEs can exert cytotoxic effects on cells. Cell viability was assessed with three independent tests: MTT-assay (activity of the mitochondrial respiratory chain), lactate dehydrogenase assay (release of cytoplasmatic enzymes, membrane integrity) and Neutral Red assay (active uptake of a hydrophilic dye). Two model AGEs, chicken egg albumin-AGE and BSA-AGE, both caused significant cell death in a dose-dependent manner. The cytotoxic effects of AGEs could be attenuated by alpha-ketoglutarate and pyruvate, by antioxidants such as thioctic acid and N-acetylcysteine, and by aminoguanidine, an inhibitor of nitric oxide synthase. This suggests that reactive oxygen species as well as reactive nitrogen species contribute to AGE mediated cytotoxicity. Since AGEs accumulate on beta-amyloid plaques in AD over time, they may additionally contribute to oxidative stress, cell damage, functional loss and even neuronal cell death in the Alzheimer's disease brain.

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b-Amyloid deposits, hallmarks of Alzheimer's disease, contain both sugar-derived`advanced glycation end products' (AGEs) and copper and iron ions. Our in vitro experiments using synthetic b-amyloid peptide and glucose or fructose show that formation of covalently cross-linked high-molecular-mass b-amyloid peptide oligomers is accelerated by micromolar amounts of copper (Cu 1 , Cu 21 ) and iron (Fe 21 , Fe 31 ) ions. Formation of these covalent AGE cross-links can be inhibited by capping agents of amino groups, redox-inactive metal chelators and antioxidants, suggesting that these drugs may be able to slow down the formation of insoluble b-amyloid deposits in vivo and possibly the progression of Alzheimer's disease.Keywords: advanced glycation end products; Alzheimer's disease; b-amyloid peptide; transition metals.Alzheimer's disease (AD) is a progressive dementia affecting a large proportion of the ageing population. Histological hallmarks of the disease are a regionally progressive loss of synaptic integrity and neuronal cell death, accompanied by the formation of amyloid plaques and neurofibrillary tangles [1]. The major proteinaceous component of the amyloid deposits is the b-amyloid peptide, which consists of 39±42 amino acids and is formed by proteolytic degradation of its precursor, the amyloid b precursor protein, in lysosomes and the endoplasmic reticulum [2,3].Although most patients with AD develop the sporadic and late-onset form, some show an obvious family history of the disease. In these families, mutations in three genes (the gene encoding the amyloid precursor protein and the presenilin 1 and 2 genes) have been discovered that account for nearly all earlyonset cases. These mutations influence b-amyloid peptide aggregation parameters, its concentration or the ratio of the 1±42 to 1±40 form, thus determining the rate of amyloid plaque formation [4,5]. It is commonly believed that the time course of amyloid plaque formation is one of the parameters that influences the time of onset or progression of the disease. Methods of minimizing b-amyloid peptide production and aggregation or interfering with its neurotoxic or pro-inflammatory effects are therefore expected to be of value in the prevention or treatment of AD [6,7].There is increasing evidence that the insolubility of b-amyloid plaques is caused by extensive covalent protein cross-linking [8]. One mechanism by which long-lived proteins can be cross-linked involves`advanced glycation end products' (AGEs) [9,10]. Formation of AGEs starts with non-enzymatic addition of a sugar or a sugar-fragmentation product to a protein, followed by rearrangement to a Schiff-base adduct and finally to a protein-bound Amadori product. AGEs are then formed through subsequent oxidations and dehydrations, including free-radical intermediates, and consist of a broad range of heterogeneous fluorescent and yellow-brown products including nitrogen-containing and oxygen-containing heterocycles (Fig. 1) [11,12].In particular, extracellular AGE formation in AD has been demon...

Advanced glycation end products (AGEs) have been identified in age-related intracellular protein deposits of Alzheimer's disease (amyloid plaques and neurofibrillary tangles) and Parkinson disease (Lewy bodies), suggesting that these protein deposits have been exposed to AGE precursors such as the reactive dicarbonyl compound methylglyoxal. In ageing tissue and under diabetic pseudohypoxia, intracellular methylglyoxal levels rise through an impairment of triosephosphate utilization. Furthermore, methylglyoxal detoxification is impaired when reduced glutathione levels are low, conditions, which have all been described in Alzheimer's disease. However, there is less known about the toxicity of methylglyoxal, particularly about therapeutic strategies to scavenge such dicarbonyl compounds and attenuate their toxicity. In our study, extracellularly applied methylglyoxal was shown to be toxic to human neuroblastoma cells in a dose-dependent manner above concentrations of 150 microM with a LD50 of approximately 1.25 mM. Pre-incubation of methylglyoxal with a variety of carbonyl scavengers such as aminoguanidine or tenilsetam and the thiol antioxidant lipoic acid significantly reduced its toxicity. In summary, carbonyl scavengers might offer a promising therapeutic strategy to reduce the neurotoxicity of reactive carbonyl compounds, providing a potential benefit for patients with age-related neurodegenerative diseases.