The effect of an ActRIIB ligand trapping agent on the number of satellite cells and myonuclei in dystrophic (mdx) muscle fibers

Abstract

Duchenne muscular dystrophy is characterized by repeated cycles of degeneration and regeneration ultimately leading to respiratory failure and death. Muscle growth is largely under the control of myostatin, which negatively influences fiber growth and regeneration by binding to the ActRIIB receptor. Ligand trapping agents for the ActRIIB receptor (RAP‐435) are therefore currently being tested as therapeutic agents using the mdx mouse model for Duchenne muscular dystrophy. The present study utilized confocal immunochemical techniques to examine the effects of in vivo RAP‐435 treatment on the number of satellite cells and myonuclei associated with isolated mdx costal diaphragm fibers. One month old mdx mice were treated with RAP435 or vehicle for one month. Following treatment, the isolated muscles were exposed to 0.2% collagenase. Satellite cells were identified with a PAX 7 primary and an AlexaFluor 488 secondary antibody. Myonuclei were identified by DAPI. Mdx costal diaphragm fibers exhibited approximately 700 myonuclei and 5 satellite cells per muscle fiber. Experiments comparing specific age‐matched nondystrophic and mdx respiratory muscles will determine if the regeneration characteristic of dystrophic muscle is associated with corresponding increases in the number of myonuclei per fiber. RAP‐435 treatment influenced neither the number of myonuclei nor satellite cells per muscle fiber. Morphometric analyses are being performed in conjunction with these confocal studies to determine if ActRIIB inhibition specifically influences myonuclear domain in promoting fiber growth and regeneration.