Effects of Aging on Mitochondrial DNA Copy Number and Cytochromec Oxidase Gene Expression in Rat Skeletal Muscle, Liver, and Heart

Abstract

Mitochondrial DNA (mtDNA) deletions and mutations have been reported to occur with aging in various tissues. To determine the functional impact of these changes, we measured mtDNA copy number, mitochondria-encoded cytochrome c oxidase (COX) subunit I and III transcript levels, and COX enzyme activity in skeletal muscles (medial and lateral gastrocnemius and soleus), liver, and heart in 6-and 27-month-old rats. Substantial age-related reductions of mtDNA copy number occurred in skeletal muscle groups (؊23-40%, p < 0.03) and liver (؊50%, p < 0.01) but not in the heart. The decline in mtDNA was not associated with reduced COX transcript levels in tissues with high oxidative capacities such as red soleus muscle or liver, while transcript levels were reduced with aging in the less oxidative mixed fiber gastrocnemius muscle (؊17-22%, p < 0.05). Consistent with transcript levels, COX activity also remained unchanged in aging liver and heart but declined with age in the lateral gastrocnemius (؊32%, p < 0.05). Thus, the effects of aging on mitochondrial gene expression are tissue-specific. A substantial age-related decline in mtDNA copy number proportional to tissue oxidative capacities is demonstrated in skeletal muscle and liver. mtDNA levels are in contrast preserved in the aging heart muscle, presumably due to its incessant aerobic activity. Reduced mtDNA copy number has no major effects on mitochondrial encoded transcript levels and enzyme activities in various tissues under these baseline study conditions. In contrast, maintenance of mitochondrial transcript levels that may be linked to oxidative metabolism and energy demand appears to be the main determinant of mitochondrial oxidative capacity in aging tissues.Aging is associated with deletions and mutations of mitochondrial DNA (mtDNA) 1 resulting from the combined effects of intense oxidative damage (1-4) and low efficiency of mitochondrial DNA repair systems (5). Age-associated mtDNA mutations have been proposed as a possible cause of senescence (1, 2) because of their potential negative impact on mitochondrial gene expression and oxidative capacity. However, whether, to what extent, and in which tissues age-related oxidative alterations affect mitochondrial gene expression remains largely to be defined. In highly oxidative organs such as liver and heart, the effects of aging on specific steps of mitochondrial gene expression and respiratory chain activity are indeed controversial (6 -11). Impaired mitochondrial enzyme activity (12) and reduced mitochondrial protein fractional synthetic rate (13) are associated with reduced muscle mass and endurance capacity in aging skeletal muscle (14 -16), but the molecular level at which these changes occur is not known. Moreover, skeletal muscle groups are highly heterogeneous with respect to oxidative metabolism and function (17). In particular muscle fiber composition and oxidative metabolism have been shown to influence skeletal muscle mitochondrial gene expression in adult animal models (18). However the impact of thes...

Age- and calorie restriction-related changes in rat brain mitochondrial DNA and TFAM binding

“…In addition, our data reporting mtDNA loss with aging in the rat frontal cortex are in agreement with previous reports, such as that by McInerny et al (2009), who described mtDNA decrease in rat cerebral cortex, and with several other reports in various rat tissues such as skeletal muscle and liver (Barazzoni et al 2000) as well as in humans (Short et al 2005).…”

“…As a consequence of oxidative stress, mtDNA can undergo quantitative and qualitative alterations (bases modifications, abasic sites, single-and doublestrand breaks, point mutations, and large-sized deletions) that affect its structure and function. Age-related changes in the mtDNA content have often been found in various mammalian tissues (Barazzoni et al 2000;Barrientos et al 1997;Dinardo et al 2003;Lee et al 1998;McInerny et al 2009;Pesce et al 2001Pesce et al , 2005Short et al 2005), as well as in single cells (Blokhin et al 2008). Large-sized deletions are among the most investigated qualitative alterations of mtDNA.…”

Population-level expression variability of mitochondrial DNA-encoded genes in humans

“…[22][23][24][25] For a given chromosomal region, short reads mapped to the region were retrieved from the BAM file at the FTP site of the 1000 Genomes Project 26 using samtools. 27 This was done for the whole mtDNA region and the autosomal regions; the numbers of short reads in the two regions were used to compute mtDNA/nDNA.…”

Is defective electron transport at the hub of aging?

“…Some laboratories report no attenuation of respiratory complexes with age (Torii et al, 1992;Barazzoni et al, 2000;Kwong & Sohal, 2000), whereas others have observed decreased complex I and IV activities (Sugiyama et al, 1993;Andreu et al, 1998), transcript levels (Andreu et al, 1998;Hudson et al, 1998) and protein synthesis (Hudson et al, 1998). In rat models of congestive heart failure, declines in transcripts for subunits of complex IV and enzyme activity are associated with decreased levels of the nuclear factors PGC-1α, Tfam and NRF-2 (Garnier et al, 2003).…”

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