Tango2 deficiency results in abnormal SR and mitochondria in skeletal muscle. (A-H) Transmission electron microscopy images of the longitudinal sections of control and tango2 mutant larval fish (8 dpf). (A,B,I,J) No abnormalities were present in the sarcomeres in tango2 mutants (B) compared to controls (A). The SR in tango2 mutants exhibited small or collapsed SR (arrows, B). The tango2 mutant muscle also showed the presence of abnormal vesicular structures in the proximity of mitochondria (‘M’) and SR (white arrowhead, B). Quantification of the sarcomere length (I) and width (J) in control (+/+) and tango2 mutant sarcomeres showed no significant differences. (C,D,K) The enlarged panels show normal SR (arrow) in controls (C) on either side of the darkly stained t-tubule. In comparison, tango2 mutant SR (D) appears smaller or collapsed (arrow). Quantification of the abnormal SR (K) showed a significant increase in tango2 mutants compared to controls. (E,F) Mitochondria in the control muscle (E) exhibited normal organization of the inner and outer mitochondrial membranes. A number of myofibers in tango2 mutants (F) contained abnormal mitochondria with electron-dense whorled membrane structures. (G,H,L) Enlarged view of mitochondria in control myofibers (G) and tango2 mutant myofibers (H), which showed abnormal mitochondria with electron-dense whorled membrane structures (arrow, H). Quantification of the mitochondria (L) showed an increase in mitochondria with whorled membrane structures in the tango2 mutants. Data represent mean±s.e.m. from three different samples and 45-65 myofibers. ns, not significant; **P<0.01; two-tailed unpaired Student's t-test. Scale bars: 500 nm (A,B,E,F); 100 nm (C,D,G,H).
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