Role of muscle-derived growth factors in bone formation

Abstract

Muscle and bone anabolism and catabolism are tightly coupled during growth, development, and aging, yet the cellular and molecular mechanisms linking these two tissues are not well understood. Here we show that FGF-2 and IGF-1, two growth factors known to play a major role in regulating bone formation, are localized to muscle fibers along the muscle-bone interface of the mouse forelimb. Likewise, receptors for these growth factors are also abundant in periosteum adjacent to fleshy muscle attachments along the diaphysis of long bones. Growth factor levels were quantified from homogenized mouse forelimb muscles and IGF-1 was found to be the most abundant factor with FGF-2 also detected. Growth factor levels were also analyzed in conditioned medium from cultured myotubes, and IGF-1 and FGF-2 were again detected at significant levels. Mechanically wounding C2C12 myotubes increased the release of FGF-2 into conditioned medium, whereas IGF-1 was secreted at lower concentrations than FGF-2 following injury. Together these findings suggest that muscle is an important, local source of growth factors for bone tissue. Hence, the integrated growth and development of bone and muscle is likely to be regulated in part by paracrine mechanisms at the muscle-bone interface involving growth factor signaling.

Fig. 1

Van Gieson-stained sections of the mouse forelimb demonstrating microanatomy of the muscle-bone interface along the mid-diaphysis of the radius (r) and ulna (u). Arrow a indicates region of image shown in 1B and arrow b indicates region of image shown in 1C. B. Asterisks indicate relatively thick and fibrous peristeum where it merges with muscle fibers, whereas in other locations (C) the periosteum is only a single cell-layer thick and muscle fibers appear to attach directly onto the bone surface. m=skeletal muscle, oc=osteocytes, CB=cortical bone.

Fig. 2

Immunofluorescence using antibodies to IGF-1 (A) and the IGF-1 receptor (IGF-1R; B) imaged with confocal microscopy show (arrows) IGF-1 in myofibers (myo) adjacent to the IGF-1 receptor in periosteum (p). Likewise, antibodies to FGF-2 localize this growth factor in myofibers along the muscle-bone interphase (C), and the FGF-2 receptor (FGF-R2) is abundant in muscle fibers (asterisk) and in periosteum (arrows) (D).

Fig. 3

Protein levels of IGF-1 are significantly higher than FGF-2 levels in homogenates from forelimb muscles of normal mice mice (A). Protein levels of FGF-2 and IGF-1 detected using ELISA assays in conditioned medium from C2C12 cells (B). FGF-2 is abundant in injured myofibers (myo) of dystrophin-deficient mdx mice in vivo (asterisks) (C), and injury of mouse C2C12 myotubes increases the concentration of FGF-2 secreted into culture medium (D).

Fig. 4

Generalized model showing paracrine interactions at the muscle-bone interface. Muscle growth, development, and hypertrophy lead to the secretion of IGF-1, which stimulates bone formation by osteoprogenitor cells in the periosteum that express IGF-1R. Muscle injury during strenuous exercise or traumatic extremity injury promotes the release of FGF-2, which induces bone formation and stimulates fracture healing by periosteal osteoprogenitor cells expressing FGF-R2.