A) TIMP-2−/− myotube area was measured on live cells after 1 and 3 days in differentiation media in the absence or presence of recombinant active mouse MMP-2 (5, 10, 20 nM). Both at 1 and 3 days, 10 nM MMP-2 is most effective at rescuing the reduced TIMP-2−/− myotube size. B) Western blot analysis (25 μg whole cell lysates) shows that MMP-2 treatment has no effect on the proteins analyzed. C) Western blot and zymographic analysis (25 μl conditioned medium) reveals a dose-dependent increase in active MMP-9 secretion in response to MMP-2 treatment both at 1 and 3 days. In addition, MMP-2 treatment (5 and 10 nM) decreases active MMP-2 secretion at 3 days. D) Immunocytochemistry of Triton permeabilized untreated and MMP-2 treated (10 nM) cells. Note that the number of myoballs (arrows) is increased in MMP-2 treated cultures at 3 days. *p < 0.05, **p ≤ 0.01, #p ≤ 0.001. Scale bar = 100 μm.