Smad4/B-Catenin Signaling Cross-Talk for Osteoblastogenesis

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DESCRIPTION (provided by applicant): The Wnt/¿-catenin system is essential for skeletal development in embryogenesis and regulates bone mass in adult life. However, the mechanisms by which Wnt/¿-catenin signaling stimulates osteogenesis and bone formation in the post-natal skeleton remain nebulous. Recent data suggest that ¿-catenin provides critical regulatory cues at two points during the osteoblast differentiation program; in immature but committed osteoblast precursors ¿-catenin favors maturation into matrix secreting osteoblasts and expansion of the osteoblast precursor pool; in differentiated osteoblasts it inhibits osteoclastogenesis and perhaps terminal osteoblast differentiation. In previous studies, we demonstrated that ¿-catenin interacts with bone morphogenetic protein-2 and 4 (BMP-2/4) in producing new bone. Indeed, we find that BMP signaling is required for full osteogenic stimulation by ¿-catenin, whereas Tcf/Lef-dependent transcriptional activity is not. We also find that ¿-catenin is competitively recruited to either BMP or Wnt signaling pathways, and that the intersection of BMP and ¿-catenin signaling is, at least in part, mediated by Smad4/¿-catenin interactions. The central hypothesis of this project is that the pro-osteogenic action of ¿-catenin originates from its interactions with BMP signaling, and specifically Smad4, in immature osteoblasts, resulting in competitive recruitment of ¿-catenin to either canonical Tcf/Lef-dependent or Smad4-dependent signals. Thus, ¿-catenin can function as stimulator of either proliferation, via Tcf/Lef transcriptional activity, or maturation of immature osteoblasts, via recruitment into Smad4-containing transcriptional complexes. To test this hypothesis we propose to, 1: determine the dependency of ¿-catenin pro-osteogenic action on BMP signaling via Smad4; 2: analyze the role of Smad4 in modulating Wnt-dependent osteogenesis and Tcf/Lef signaling; 3 analyze the molecular interactions between Smad4 and ¿-catenin for osteogenesis. We will use in vivo and in vitro approaches based on inducible, conditional gene ablation or activation models to study whether interference with Smad4 expression alters ¿-catenin pro-osteogenic action and "canonical" Tcf/Lef-dependent activity. Considering the fundamental role of ¿-catenin in bone cell regulation, understanding the mechanisms by which ¿-catenin delivers either a mitogenic or a differentiation signal is essential to gain a full picture of the molecular network by which bone development and homeostasis are controlled. The proposed studies will also disclose the biologic importance of a molecular interaction that may be used as a new target for bone anabolism.