p21 is required for p53- and ATF4-mediated muscle fiber atrophy. A–D: In C57BL/6 mice, 1 TA muscle was transfected with 10 μg of p-wt-p53 plus either 20 μg of p-MiR-p21 #1 or 20 μg of p-MiR-p21 #2, as indicated; the contralateral TA (control) was transfected with 10 μg of p-wt-p53 plus 20 μg of p-miR-Control. Bilateral TAs were harvested 7 days later for histological analysis. A and C: representative images. B and D: quantification of muscle fiber size. D, left: fiber size distributions; each distribution represents ≥1,500 fibers from 6 muscles. D, right: mean fiber diameters; each data point represents the mean diameter of ≥250 fibers from 1 muscle. Horizontal bars denote means ± SE. E–H: in C57BL/6 mice, 1 TA muscle was transfected with 10 μg of p-ATF4-FLAG plus either 20 μg of p-MiR-p21 #1 or 20 μg of p-MiR-p21 #2, as indicated; the contralateral TA (control) was transfected with 10 μg of p-ATF4-FLAG plus 20 μg of p-miR-Control. Bilateral TAs were harvested 7 days later for histological analysis. E and G: representative images. F and H: quantification of muscle fiber size. F, left: fiber size distributions; each distribution represents ≥1,600 fibers from 6 muscles. F, right: mean fiber diameters; each data point represents the mean diameter of ≥250 fibers from 1 muscle. Horizontal bars denote means ± SE. B, D, F, and H: P values were determined with a paired t-test.