Figure 4. Mitochondrial morphology in muscle fibers responds to changes in OXPHOS activity.

(A) Co-labeling of IIA fibers (MyH2-DsRed transgene; red) and mitochondria from IIX/IIB fibers (mito-Dendra2; green) in plantaris muscle from sedentary and exercised mice. The mito-Dendra2 marker is activated in IIA/IIB fibers by the Pvalb-Cre driver. Note that co-expression of these markers is found only in fibers from exercised animals. Scale bar, 10 μm. (B) Enlarged view of mitochondrial morphology from regions indicated in (A) (boxes). Scale bar, 5 μm. (C) Percentage of originally IIX/IIB fibers (Dendra2 positive) which are co-labeled with the IIA marker (MyH2-DsRed) in sedentary and exercised mice. At least 30 fibers were scored for 5 animals in each group. *, p<0.05 (z-test). (D) In exercised mice, percentage of originally IIX/IIB fibers (Dendra2 positive) which have elongated or fragmented mitochondria. Fibers were separated into two groups: those having switched to the IIA fiber type (MyH2-DsRed positive) and those that have not switched (MyH2-DsRed negative). *, p<0.01 (z-test). (E) Representative oxygen consumption rates (OCR) measured ex vivo in paired EDL muscles from the same animal (left). The chamber was closed at the time indicated by the arrowhead. Respective oxygen consumption rates (nmol/min/mL) are indicated for the time interval between the two arrows. (Right) Oxygen consumption rates, normalized to OCR in glucose-containing media. Experiments were performed in triplicate. (F) Percentage of IIX/IIB fibers having long tubular, short tubular or fragmented mitochondria, after overnight incubation in the indicated media. At least 30 fibers per muscle were counted, and experiments were performed in triplicate. *, p<0.01 (t-test). (G) Representative images of mitochondrial morphology from IIX/IIB fibers after overnight incubation in the indicated media. Mitochondria were visualized by expression of mito-Dendra2. Scale bar, 5 μm. Error bars indicate standard errors.