Mitochondrial Telomerase Reverse Transcriptase Binds to and Protects Mitochondrial DNA and Function From Damage

Abstract

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...

Extra-Telomeric Effects of Telomerase (hTERT) in Cell Death

“…Additional studies demonstrated the ability of TERT to antagonize apoptosis induced by topoisomerase inhibitors in PC12 cell line or by oxidative stress in lymphocytes CD4+ model (Lu et al, 2001;Luiten et al, 2003). Such results illustrating the protective role of TERT against ROS were confirmed later in other models (Ahmed et al, 2008;Haendeler et al, 2009;Indran et al, 2011). Although most of these early investigations did not explore the involvement of the reverse transcriptase activity of TERT in this anti-apoptotic mechanism, these results had already outlined a potential function of TERT unrelated to its enzymatic activity and ability to lengthen telomeres (Sung et al, 2005).…”

“…Following this hypothesis it was then demonstrated that telomerase, or more specifically the catalytic subunit TERT is able to translocate from the nucleus to the mitochondria following drug treatments or increase of oxidative stress (Ahmed et al, 2008;Haendeler et al, 2009;Santos et al, 2006;Saretzki, 2009) (Figure 2). This new interesting finding was linked to the discovery of mitochondrial targeting sequence at the N-terminal extremity of TERT (Santos et al, 2004).…”

“…In 2009 it was reported that TERT translocation to mitochondria follows a classical pathway of proteins imported into mitochondria. Indeed it was observed that TERT translocated to mitochondrial matrix through the translocase of outer membrane (TOM) and translocase of inner membrane (TIM) complexes (Haendeler et al, 2009). Once in the matrix, it was shown that TERT can bind to mtDNA through the coding regions of the NADH:ubiquinone oxidoreductase subunits 1 and 2.…”

“…Moreover this mechanism of protection is directly correlated with the ability of TERT to localize in the mitochondria given that a construct of TERT targeted specifically to mitochondria enhanced the protective effect seen previously with TERT wild-type. The authors also have shown that the reverse transcriptase of TERT seems to be required in order to fulfill this protective role against oxidative stress (Haendeler et al, 2009). However the requirement of the reverse transcriptase activity of TERT in this protective role still remains highly controversial as no detailed mechanism has been clearly demonstrated.…”

Differentiation-dependent telomeric long non-coding transcription in a model of skeletal myogenesis

“…http://dx.doi.org/10.1101/000679 doi: bioRxiv preprint first posted online Nov. 18, 2013; against oxidative stress-induced damage (Haendeler et al 2009). Proteomic analysis of C2C12 cells indicates that respiratory chain proteins are up-regulated by 10 days postdifferentiation (Kislinger et al 2005), resulting in increased potential for generation of oxidative stress.…”