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: Deposits of amyloid β‐peptide (Aβ), reduced glucose uptake into brain cells, oxidative damage to cellular proteins and lipids, and excitotoxic mechanisms have all been suggested to play roles in the neurodegenerative process in Alzheimer's disease. Synapse loss is closely correlated with cognitive impairments in Alzheimer's disease, suggesting that the synapse may be the site at which degenerative mechanisms are initiated and propagated. We report that Aβ causes oxyradical‐mediated impairment of glucose transport, glutamate transport, and mitochondrial function in rat neocortical synaptosomes. Aβ induced membrane lipid peroxidation in synaptosomes that occurred within 1 h of exposure; significant decreases in glucose transport occurred within 1 h of exposure to Aβ and decreased further with time. The lipid peroxidation product 4‐hydroxynonenal conjugated to synaptosomal proteins and impaired glucose transport; several antioxidants prevented Aβ‐induced impairment of glucose transport, indicating that lipid peroxidation was causally linked to this adverse action of Aβ. FeSO4 (an initiator of lipid peroxidation), Aβ, and 4‐hydroxynonenal each induced accumulation of mitochondrial reactive oxygen species, caused concentration‐dependent decreases in 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide reduction, and reduced cellular ATP levels significantly. Aβ also impaired glutamate transport, an effect blocked by antioxidants. These data suggest that Aβ induces membrane lipid peroxidation, which results in impairment of the function of membrane glucose and glutamate transporters, altered mitochondrial function, and a deficit in ATP levels; 4‐hydroxynonenal appears to be a mediator of these actions of Aβ. These data suggest that oxidative stress occurring at synapses may contribute to the reduced glucose uptake and synaptic degeneration that occurs in Alzheimer's disease patients. They further suggest a sequence of events whereby oxidative stress promotes excitotoxic synaptic degeneration and neuronal cell death in a variety of different neurodegenerative disorders.

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Proteolytic cleavage of~-amyloid precursor protein (/IAPP) by cs-secretase results in release of one secreted form (sAPP) of APP (SAPPa), whereas cleavage by~3-secretase releases a C-terminally truncated 5APP

Telomerase is a protein-RNA enzyme complex that adds a six-base DNA sequence (TTAGGG) to the ends of chromosomes and thereby prevents their shortening. Reduced telomerase activity is associated with cell differentiation and accelerated cellular senescence, whereas increased telomerase activity is associated with cell transformation and immortalization. Because many types of cancer have been associated with reduced apoptosis, whereas cell differentiation and senescence have been associated with increased apoptosis, we tested the hypothesis that telomerase activity is mechanistically involved in the regulation of apoptosis. Levels of telomerase activity in cultured pheochromocytoma cells decreased prior to cell death in cells undergoing apoptosis. Treatment of cells with the oligodeoxynucleotide TTAGGG or with 3,3-diethyloxadicarbocyanine, agents that inhibit telomerase activity in a concentration-dependent manner, significantly enhanced mitochondrial dysfunction and apoptosis induced by staurosporine, Fe 2؉ (an oxidative insult), and amyloid ␤-peptide (a cytotoxic peptide linked to neuronal apoptosis in Alzheimer's disease). Overexpression of Bcl-2 and the caspase inhibitor zVAD-fmk protected cells against apoptosis in the presence of telomerase inhibitors, suggesting a site of action of telomerase prior to caspase activation and mitochondrial dysfunction. Telomerase activity decreased in cells during the process of nerve growth factor-induced differentiation, and such differentiated cells exhibited increased sensitivity to apoptosis. Our data establish a role for telomerase in suppressing apoptotic signaling cascades and suggest a mechanism whereby telomerase may suppress cellular senescence and promote tumor formation.Telomeres consist of repeats of the sequence TTAGGG/ CCCTAA at the ends of chromosomes. These DNA repeats are synthesized by enzymatic activity associated with an RNAprotein complex called telomerase (1-3). Telomerase activity decreases dramatically during the processes of growth arrest and cell differentiation (4 -6). Indeed, in most somatic cells, telomerase activity is low or nonexistent, and telomere length decreases with increasing cell divisions; telomere shortening has therefore been proposed to play a role in cellular senescence (7-9). The latter hypothesis recently gained strong support from studies showing that the life span of normal human fibroblasts can be extended if they are transfected with a vector encoding the telomerase catalytic subunit (10). The mechanism whereby telomerase activity suppresses cellular senescence has not been established.Apoptosis is a stereotyped form of cell death that occurs in a variety of physiological and pathological settings (11,12). Apoptosis is characterized by cell shrinkage, plasma membrane blebbing, and nuclear chromatin condensation and DNA fragmentation. Biochemical features of apoptosis include loss of plasma membrane phospholipid asymmetry, activation of one or more cysteine proteases of the caspase family, mitochondrial dysfunction, and relea...

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Traumatic injury to the spinal cord initiates a host of pathophysiological events that are secondary to the initial insult. One such event is the accumulation of free radicals that damage lipids, proteins, and nucleic acids. A major reactive product formed following lipid peroxidation is the aldehyde, 4‐hydroxynonenal (HNE), which cross‐links to side chain amino acids and inhibits the function of several key metabolic enzymes. In the present study, we used immunocytochemical and immunoblotting techniques to examine the accumulation of protein‐bound HNE, and synaptosomal preparations to study the effects of spinal cord injury and HNE formation on glutamate uptake. Protein‐bound HNE increased in content in the damaged spinal cord at early times following injury (1–24 h) and was found to accumulate in myelinated fibers distant to the site of injury. Immunoblots revealed that protein‐bound HNE levels increased dramatically over the same postinjury interval. Glutamate uptake in synaptosomal preparations from injured spinal cords was decreased by 65% at 24 h following injury. Treatment of control spinal cord synaptosomes with HNE was found to decrease significantly, in a dose‐dependent fashion, glutamate uptake, an effect that was mimicked by inducers of lipid peroxidation. Taken together, these findings demonstrate that the lipid peroxidation product HNE rapidly accumulates in the spinal cord following injury and that a major consequence of HNE accumulation is a decrease in glutamate uptake, which may potentiate neuronal cell dysfunction and death through excitotoxic mechanisms.