Fig. 3

Myostatin, latent complex, and precursor forms bound by REGN1033 in vivo. a Antibody complexes present in serum samples from C17-SCID mice that had received multiple REGN1033 or isotype-matched control antibody injections over 28 days were collected by immunoprecipitation, separated by non-reducing SDS-PAGE, and immunoblotted with a myostatin antibody. The first lane contains conditioned medium from CHO cells transiently transfected with myostatin precursor expression vector to serve as a positive control; two independent serum immunoprecipitations from control or REGN1033-treated mice are shown. Arrows indicate mature myostatin homodimer, unprocessed precursor, and myostatin monomer (M). Asterisk indicates myostatin form believed to be partially processed precursor. All samples were resolved on the same gel and appropriate lanes are juxtaposed for clarity. b Duplicate samples as described in a were probed with a myostatin propeptide antibody. Arrow indicates cleaved prodomain which co-precipitates in complex with mature myostatin, arrowhead indicates unprocessed precursor, and asterisks indicate partially processed precursor forms. c Sera from WT and Mstn ^−/− mice was probed with the myostatin antibody to demonstrate specificity. Arrows indicate myostatin precursor and mature myostatin homodimer, and the asterisk indicates partially processed precursor, all of which are absent in the Mstn ^−/− sample