Extracellular regulation of bone mass by transforming growth factor-ß-related ligands and their binding proteins - The focus of this project will be to investigate the role of the myostatin/GDF-11/activin branch of the transforming growth factor-ß (TGF-ß) superfamily of secreted signaling molecules in regulating bone mass and density. The important role that this signaling pathway plays in regulating bone homeostasis has been documented by both pharmacologic and genetic studies targeting receptors for this group of ligands. Work from several groups, including ours, has shown that systemic administration of soluble forms of either of the activin type 2 receptors, ACVR2 and ACVR2B, is capable of inducing significant increases in bone density. By genetically targeting these receptors in osteoblasts, we showed that at least part of this effect is due to inhibition of direct signaling to bone. Strikingly, however, we very recently showed that targeting the type 1 receptors, ALK4 and ALK5, in osteoblasts led to much more substantial effects, resulting in increases in bone mass and density by approximately 10-fold. These findings revealed the extraordinary capacity for bone accrual that is normally kept in check by this regulatory system and suggest that the potential for increasing bone mass and density by targeting this signaling pathway is substantially greater than previously appreciated. As a starting point for developing the most effective strategies to harness the potential of targeting this pathway for bone applications, we will elucidate the extracellular components that play key roles in this regulatory network in bone. In Specific Aim 1, we will examine the roles of known inhibitory binding proteins for this group of ligands in regulating bone structure. In our recent study, we carried out an extensive analysis of the role of one binding protein, namely follistatin (FST), using genetically-targeted mouse lines in which expression levels of FST were either up- or down-regulated. Here, we will examine the roles of the three other known binding proteins, FSTL-3, GASP-1, and GASP-2, utilizing targeted mouse lines that we have generated carrying both deletion and floxed alleles for each of these components. In Specific Aim 2, we will examine the roles of specific ligands in this subgroup of the TGF-ß superfamily in regulating bone structure. In our recent study, we showed that targeting two ligands simultaneously, namely myostatin and activin A, led to substantial increases in bone mass and density but that these increases were significantly less pronounced than the approximately 10-fold effects that we observed upon targeting their type 1 receptors. Here, we will use genetic approaches to examine the roles of a wider spectrum of ligands in this subgroup of the TGF-ß superfamily in regulating bone structure. The overall goal of this project will be to elucidate the specific extracellular signaling components that play key roles in regulating bone homeostasis with the long-term goal of developing the most effective strategies to target this signaling pathway to treat bone loss.
