TRANSGENIC MICE OVEREXPRESSING COPPER-ZINC SUPEROXIDE-DISMUTASE AS MODELS FOR THE STUDY OF FREE-RADICALS METABOLISM AND AGING

Oxidants are ubiquitous in our aerobic environment and could play an etiological role in aging and neurodegenerative diseases such as Alzheimer's disease. All cells contain several antioxydant enzymes, most importantly, superoxide dismutases (MnSOD and CuZnSOD), glutathione peroxydase (GSH-Px), glutathione reductase and catalase. The individual contribution of these antioxidant enzymes in neuronal protection during aging and under in vivo conditions remains unknown. We feel that the use of genetic manipulations to construct cells and/or transgenic mice that specifically overexpress or lack a single function represent a way to an understanding of the role of the individual antioxidant enzymes in neuronal aging. Copper-zinc superoxide dismutase (CuZnSOD) is one of the genes encoded by chromosome 21. As a consequence of gene dosage excess, CuZnSOD activity and protein are increased by 50 % in all tissues of Down syndrome (DS) patients. It has been suggested that this increment, by accelerating hydrogen peroxide formation, might promote oxidative damage within DS cells and might be involved in the various neurobiological abnormalities found in DS such as premature aging and Alzheimer-type neurological lesions. Moreover, the level of CuZnSOD protein and mRNA is particularly high in pyramidal hippocampal neurons susceptible to degenerative processes in Alzheimer's disease, and in dopaminergic melanized-neurons vulnerable in Parkinson's disease. In order to test this hypothesis, we have created transfected cells and transgenic mice which express human CuZnSOD gene. An oversupply of this enzyme is not beneficial to the brain of transgenic mice and causes increased thiobarbituric-reactive substances (TBARS), an index of lipid peroxidation, and may be due to peroxides generated by an imbalance between enzymatic activities of CuZnSOD and GSH-Px. Unlike what has been observed in transfected cells with the human CuZnSOD gene, but similar to what was found in the DS fetal brain, the GSH-Px activity was not increased in the brain of transgenic mice. One possibility to explain this discrepancy could be the differential cellular localization of these two enzymes in the brain (CuZnSOD in neurons and GSH-Px in glial cells). This heterogeneous cellular distribution of the enzymes implicated in oxygen-free radicals detoxification could participate to a selective neuronal degeneration. Interestlingly, overexpression of cuZnSOD in the brain of transgenic mice is associated with an increased MnSOD activity, the mitochondrial form of the enzyme. This increased MnSOD might be a defense response to protect mitochondria from oxidative damage. Alternatively, this MnSOD increase could lead to enhanced production of hydrogen peroxide inside the mitochondria leading to peroxidative damage and possibly to alterations of the mitochondrial genome, this latter appears to be more vulnerable to mutagenesis than the nuclear genome since it is not protected by histones or by competent repair mechanisms. Immunohistochemical and in situ hybridization analysis of brain sections reveals that human CuZnSOD protein and mRNA were preferentially expressed in neurons, particularly in the stratum pyramidale of Ammon's horn thoughout the CA1-CA4 fields of the hippocampus and in the granule cell layer of the dentate gyrus. Since expression of endogeneous CuZnSOD gene and human CuZnSOD gene is present in the age-vulnerable neurons of transgenic mice, we can know investigate whether there is a connection between the cell-specific overexpression, an accelerated aging, a neuronal pathology and mithochondrial damages by extended biochemical and neuroanatomical studies of the brain of transgenic mice, specially with regard to DS and Alzheimer-like neuropathology.