Objective-The enzyme telomerase and its catalytic subunit the telomerase reverse transcriptase (TERT) are important for maintenance of telomere length in the nucleus. Recent studies provided evidence for a mitochondrial localization of TERT. Therefore, we investigated the exact localization of TERT within the mitochondria and its function. Methods and Results-Here, we demonstrate that TERT is localized in the matrix of the mitochondria. TERT binds to mitochondrial DNA at the coding regions for ND1 and ND2. Binding of TERT to mitochondrial DNA protects against ethidium bromide-induced damage. TERT increases overall respiratory chain activity, which is most pronounced at complex I and dependent on the reverse transcriptase activity of the enzyme. Moreover, mitochondrial reactive oxygen species are increased after genetic ablation of TERT by shRNA. Mitochondrially targeted TERT and not wild-type TERT revealed the most prominent protective effect on H 2 O 2 -induced apoptosis. Lung fibroblasts from 6-month-old TERT Ϫ/Ϫ mice (F2 generation) showed increased sensitivity toward UVB radiation and heart mitochondria exhibited significantly reduced respiratory chain activity already under basal conditions, demonstrating the protective function of TERT in vivo. Key Words: aging Ⅲ apoptosis Ⅲ mitochondrial functions Ⅲ mitochondrial DNA Ⅲ reactive oxygen species Ⅲ telomerase reverse transcriptase T o date several theories exist to explain the phenomenon of normal and pathological aging. The free radical theory of aging 1 proposes that reactive oxygen species (ROS) in biological systems attack molecules and thereby cause functional decline of organ systems that eventually leads to death. This damage accumulates over time and may contribute to diseases associated with aging like atherosclerosis, neurodegeneration, or cataracts. 2 Recently, Schriner et al produced transgenic mice that overexpressed human catalase localized to the peroxisome, the nucleus, and the mitochondria. Only mice overexpressing mitochondrially targeted catalase showed a significant increase in life span and a reduction in oxidative damage to DNA and consequently in apoptosis. 3 Thus, these data define the mitochondria as compartment of ROS formation, which contributes to aging processes. Further evidence supporting the importance of mitochondria and formation of ROS in the mitochondria comes from findings that overexpression of mitochondrially localized antioxidant enzymes lengthens lifespan of Drosophila 4,5 and that deletion of manganese superoxide dismutase results in the age-related decline of mitochondrial function, culminating in increased apoptosis. 6 Recent studies using isolated complex I of the respiratory chain clearly demonstrated that superoxide production into the mitochondrial matrix is predominantly dependent on flavine-mononucleotide within complex I. 7,8 The enzyme telomerase counteracts the shortening of the physical ends of chromosomes and, thereby, prevents the onset of replicative senescence and genetic instability. 9 -12 The c...

A type of retroviral gene trap vectors has been developed that can induce conditional mutations in most genes expressed in mouse embryonic stem (ES) cells. The vectors rely on directional site-specific recombination systems that can repair and reinduce gene trap mutations when activated in succession. After the gene traps are inserted into the mouse genome, genetic mutations can be produced at a particular time and place in somatic cells. In addition to their conditional features, the vectors create multipurpose alleles amenable to a wide range of postinsertional modifications. Here we have used these directional recombination vectors to assemble the largest library of ES cell lines with conditional mutations in single genes yet assembled, presently totaling 1,000 unique genes. The trapped ES cell lines, which can be ordered from the German Gene Trap Consortium, are freely available to the scientific community.conditional mutagenesis ͉ ES cells ͉ gene trapping ͉ site-specific recombination ͉ insertional mutagenesis