The α7β1-integrin accelerates fiber hypertrophy and myogenesis following a single bout of eccentric exercise

The α ₇β ₁-integrin is a heterodimeric transmembrane protein that adheres to laminin in the extracellular matrix, representing a critical link that maintains structure in skeletal muscle. In addition to preventing exerciseinduced skeletal muscle injury, the α7-integrin has been proposed to act as an intrinsic mechanosensor, initiating cellular growth in response to mechanical strain. The purpose of this study was to determine the extent to which the α ₇-integrin regulates muscle hypertrophy following eccentric exercise. Wild-type (WT) and α ₇-integrin transgenic (α ₇Tg) mice completed a single bout of downhill running exercise (-20°, 17 m/min, 60 min), and gastrocnemius-soleus complexes were collected 1, 2, 4, and 7 days (D) postexercise (PE). Maximal isometric force was maintained and macrophage accumulation was suppressed in α ₇Tg muscle 1D PE. Mean fiber cross-sectional area was unaltered in WT mice but increased 40% in α ₇Tg mice 7D PE. In addition, a rapid and striking fivefold increase in embryonic myosin heavy chain-positive fibers appeared in α ₇Tg mice 2D PE. Although Pax7-positive satellite cells were increased in α ₇Tg muscle 1D PE, the number of nuclei per myofiber was not altered 7D PE. Phosphorylation of mammalian target of rapamycin (mTOR) was significantly elevated in α ₇Tg 1D PE. This study provides the first demonstration that the presence of the α ₇ β 1-integrin in skeletal muscle increases fiber hypertrophy and new fiber synthesis in the early time course following a single bout of eccentric exercise. Further studies are necessary to elucidate the precise mechanism by which the α 7-integrin can enhance muscle hypertrophy following exercise.