Alterations of mitochondria biogenesis and structure in skeletal muscle of streptozotocin-treated mice.

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To test whether ROS production is a key feature in HFHSD-induced mitochondrial dysfunctions, we investigated mitochondrial structure and functions in streptozotocin (STZ)-treated mice, a model of hyperglycaemia-associated oxidative stress without insulin-resistance and obesity. Eleven days after STZ administration, mice were hyperglycaemic (p<0.001), and hypoinsulinaemic (p<0.001) and showed unaltered plasma FFA levels  and a reduced body weight (p<0.005) (Table 1). Insulin injection (STZ+INS) rapidly decreased plasma glucose levels and 24 hours after insulin injections, plasma glucose were lower (p<0.001), body weight were higher (p<0.05) and FFA were undetected in STZ+INS mice compared to STZ mice (Table 1). Phlorizin injection (STZ+PHL) reproduced the effect of insulin on glycaemia (decrease of 25% compared to STZ mice, p<0.05).

In agreement with hyperglycaemia-induced oxidative stress, protein carbonylation levels were elevated in skeletal muscle of STZ mice and insulin treatment restored the extent of protein carbonylation close to the levels observed in control mice (Figure 5A). Furthermore, STZ treatment induced a release of cytochrome C from mitochondria and insulin treatment reversed this pro-apoptotic process (Figure 5B). Regarding mitochondrial density, the mtDNA/nuclear DNA ratio (Figure 5C) and mitochondria amount per area (Figure 5D) were reduced in the muscle of STZ mice compared to control mice. The morphology of both types of mitochondria was also affected in skeletal muscle of STZ mice, with altered cristae and reduced electron density of the matrix (Figure 5D). Importantly, density and structural abnormalities of mitochondria in the muscle of STZ mice were restored by insulin and phlorizin treatments (Figures 5C and 5D).

To verify whether mitochondrial alterations were related to ROS production, we treated STZ mice with N-acetylcysteine (NAC), a general antioxidant. NAC treatment, did not modify systemic oxidative stress (Figure 6A), but decreased muscle protein carbonylation to the levels of control mice (Figure 6B), and restored mitochondria density (Figure 6C) and structure (Figure 6D) in gastrocnemius muscle of STZ mice.

            Taken together, these results demonstrate that oxidative stress in hyperglycaemic mice is associated with altered mitochondrial structure and function in skeletal muscle and that both amelioration of glycaemia and antioxidant treatment restore mitochondrial structure.