Mitochondrial biogenesis is reduced in skeletal muscle of HFHSD mice.

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Next, we investigated the impact of HFHSD on muscle mitochondrial density. As shown in Figure 1A, the mitochondrial DNA (mtDNA)/nuclear DNA ratio was significantly decreased in skeletal muscle of 16 week HFHSD mice compared to 16 week SD mice (3%, p<0.05), whereas no change was observed after 4 weeks of diet. In agreement, the mRNA levels of the subunits 1 and 3 of cytochrome c oxidase (COX), two mitochondria-encoded genes, were significantly decreased in skeletal muscle of 16 week HFHSD mice (Figure 1B). Using transmission electron microscopy, we found that 16 week HFHSD resulted in a decrease of both subsarcolemmal (31%, p<0.05) and intermyofibrillar (41%, p<0.01) mitochondria amounts in oxidative fibers when compared to SD mice (Figure 1C and 1D). These alterations were not observed after 4 weeks of HFHSD diet (Figure 1C). As shown in Figure 1E, citrate synthase (CS) activity was slightly decreased in isolated mitochondria from HFHSD mice, both after 4 weeks and 16 weeks of HFHSD feeding (13% and 16%, respectively, p<0.05).

            To clarify the mechanisms involved in the reduction of mitochondrial density in the muscle of 16 week HFHSD mice, we measured the mRNA levels of genes implicated in mitochondrial biogenesis, such as PGC1a, PGC1b, NRF1, NRF2, the mitochondrial transcription factor (mtTFA), the estrogen-related receptor a (ERRa)  and mitofusin 2 (Mfn2). Only the mRNA levels of PGC1a and Mfn2 were decreased in skeletal muscle of 16 week HFHSD mice compared to SD mice (~ 50% for both transcripts) (Figure 2A). This difference was not seen after 4 weeks of diet (Figure 2A). The protein levels of PGC1a were also significantly decreased in skeletal muscle of 16 week HFHSD mice (data not shown). Concerning mtDNA replication and repair, we have investigated both gamma DNA polymerase (POLG1, the catalytic subunit and POLG2, the accessory subunit) and the single stranded DNA binding protein 1 (SSBP1), which play a key role in this process (16). As illustrated in Figure 2B, 16 weeks of HFHSD feeding induced a decrease of POLG2 and SSBP1 mRNA levels in skeletal muscle, whereas no effect was observed after 4 weeks of diet. POLG1 expression was not affected by HFHSD feeding.