Aifm1 (R200 del) mice show pathological features associated with OXPHOS deficiency in skeletal muscle. (A) Modified Gomori trichrome staining on transverse quadriceps muscle sections in Aifm1 (R200 del) KI, Hq mutant, and respective control animals. Scale bar = 50 μm. (B) At 6 months of age, Aifm1 (R200 del) KI mice showed a significantly higher number of muscle fibers with cytosolic inclusions compared to age-matched wt littermates (mean ± SEM, Student's t-test, n = 4–5 per genotype, **p < 0.01). (C) Schematic representation of the COX and SDH staining. Visualization of COX activity is based on the use of 3,3′-diaminobenzidine (DAB) as electron donor. The reaction product on oxidation of DAB occurs as brown pigmentation corresponding to the distribution of mitochondria. Visualization of SDH activity is based on the use of nitro blue tetrazolium (NBT) as an electron acceptor. (D) Histochemical analysis and (E) relative quantification showed a reduction of COX-positive fibers in quadriceps muscle sections of 3 month-old Aifm1 (R200 del) KI mice compared to controls. Scale bar = 50 μm. (F) SDH staining showed similar CII activity in skeletal muscle of wt and AIF deficient mice. Fibers relying on glycolytic metabolism stain light blue while highly oxidative fibers, that contain more mitochondria, show a dark blue staining. Scale bar = 50 μm. (G–H) Immunoblot analyses were performed using quadriceps muscles from wt, Aifm1 (R200 del) KI and Hq mutant mice at (G) 3 and (H) 6 months of age. Densitometry is relative to wt littermates and reported as mean ± SEM, Student's t-test, number of animals = 4–9 per genotype, ***p < 0.001, **p < 0.01, *p < 0.05. Color code is: black = wt (for KI); red = KI; dark grey = wt (for Hq); green = Hq.