Right hind limb of WT mice were subjected to denervation by transecting the sciatic nerve, whereas the left hind limb was sham operated. After 7 days, TA muscle was isolated and processed for biochemical analysis. (A) Representative gel showing phosphorylated MKK6 protein in in vitro kinase assay (KA) in undenervated and denervated muscle. Immunoblots showing levels of phosphorylated TAK1 and total TAK1, MAFbx, MuRF1, and GAPDH protein. (B) Densitometry quantification of TAK1 enzymatic activity, phosphorylated vs. total TAK1 levels, MAFbx, and MuRF1 protein in skeletal muscle of Tak1^fl/fl and Tak1^mKO mice. n = 4 in each group. Error bars represent ± SD. *P < 0.05 values significantly different from sham-operated muscle by unpaired t test. In a separate experiment, 14-week-old Tak1^fl/fl and Tak1^mKO mice were given i.p. injections of tamoxifen, and immediately thereafter, the sciatic nerve of the right hind limb was transected to induce muscle atrophy, while the left side was sham operated. Fourteen days after denervation surgery, the mice were euthanized and TA and soleus muscles were isolated and processed for histological analysis. Representative photomicrographs of TA muscle sections of Tak1^fl/fl and Tak1^mKO mice after (C) anti-dystrophin staining and (D) H&E staining. Scale bars: 20 μm. Quantification of (E) average myofiber CSA, and (F) average minimal Feret’s diameter in anti-dystrophin-stained TA muscle sections. Quantification of (G) average myofiber CSA, and (H) average minimal Feret’s diameter in anti-dystrophin-stained soleus muscle sections of Tak1^fl/fl and Tak1^mKO. n = 4 or 5 in each group. Error bars represent ± SEM. *P < 0.05, values significantly different from sham-operated muscle of Tak1^fl/fl mice by 1-way ANOVA with post hoc Bonferroni’s multiple comparison test. ^#P < 0.05, values significantly different from denervated muscle of Tak1^fl/fl mice by 1-way ANOVA with post hoc Bonferroni’s multiple comparison test.