The runt related transcription factor CBFA1 (AML3/PEBP2alphaA/RUNX2) regulates expression of several bone- and cartilage-related genes and is required for bone formation in vivo. The gene regulatory mechanisms that control activation and repression of CBFA1 gene transcription during osteoblast differentiation and skeletal development are essential for proper execution of the osteogenic program. We have therefore defined functional contributions of 5' regulatory sequences conserved in rat, mouse and human CBFA1 genes to transcription. Deletion analysis reveals that 0.6 kB of the bone-related rat or mouse CBFA1 promoter (P1, MASNS protein isoform) is sufficient to confer transcriptional activation, and that there are multiple promoter domains which positively and negatively regulate transcription. Progressive deletion of promoter segments between nt -351 and -92 causes a striking 30- to 100-fold combined decrease in promoter activity. Additionally, 5' UTR sequences repress reporter gene transcription 2- to 3-fold. Our data demonstrate that CBFA1 is a principal DNA binding protein interacting with the 5' region of the CBFA1 gene in osseous cells, that there are at least three CBFA1 recognition motifs in the rat CBFA1 promoter, and that there are three tandemly repeated CBFA1 sites within the 5' UTR. We find that forced expression of CBFA1 protein downregulates CBFA1 promoter activity and that a single CBFA1 site is sufficient for transcriptional autosuppression. Thus, our data indicate that the CBFA1 gene is autoregulated in part by negative feedback on its own promoter to stringently control CBFA1 gene expression and function during bone formation.

The runt homology transcription factor Runx2/Cbfa1 is essential for bone development and osteoblast differentiation. Regulatory mechanisms that govern Runx2 transcription in osteoblasts define the osteogenic pathways that control skeletal development. In this study, we systematically examined transcription factor binding within the upstream Runx2 P1 promoter, which regulates expression of the bone-related Runx2 factor. We identified two novel protein/DNA interactions that are mediated by sequence specific factors, based on cross-competition experiments, point mutations, and gel-shift immunoassays. One complex recognizes a non-canonical Runx2 site, whereas the other factor binds to a palindromic sequence. Site-directed mutagenesis of the novel Runx2 motif (5'TCCCAC3') within the 0.6 kb rat Runx2 promoter reduces transcription by 2-fold, indicating that this site supports enhancement of Runx2 promoter activity. Mutation of the palindromic motif (5'AGTACT3') results in a 2-3-fold activation of the Runx2 promoter, demonstrating that the wild type sequence contributes to transcriptional repression. These studies, together with our previous findings of auto-suppression of the Runx2 promoter and negative regulation by 1,25(OH)(2) Vitamin D3, suggest that physiological control of Runx2 gene expression is mediated by a series of intricate regulatory mechanisms.

Development of the osteoblast phenotype requires transcriptional mechanisms that regulate induction of a program of temporally expressed genes. Key components of gene activation, repression, and responsiveness to physiologic mediators require remodeling of the chromatin structure of a gene that renders promoter elements competent for the assembly of macromolecular transcriptional complexes. Here we review evidence that the Runx transcription factors support tissue-specific gene expression and bone formation by contributing to promoter structure, chromatin remodeling, and the integration of independent signaling pathways. In addition, we discuss the role of Runx2 in both activation and negative regulation of gene promoters (osteocalcin, bone sialoprotein, and Runx2/Cbfa1) in relation to the interaction of Runx with co-regulatory proteins in distinct subnuclear foci. The modifications in chromatin organization and transcription of the osteocalcin gene that are influenced by the activities of Runx2/Cbfa1 mediated by interacting proteins (YAP, TLE, SMAD, C/EBP) are emphasized. These functional properties of Runx2 provide novel insights into the requirements for multiple levels of transcriptional control within the context of nuclear architecture to support the convergence of regulatory signals that control tissue-specific gene expression.

Age‐related delays in bone repair remains an important clinical issue that can prolong pain and suffering. It is now well established that inflammation increases with aging and that this exacerbated inflammatory response can influence skeletal regeneration. Recently, simple dietary supplementation with beneficial probiotic bacteria has been shown to influence fracture repair in young mice. However, the contribution of the gut microbiota to age‐related impairments in fracture healing remains unknown. Here, we sought to determine whether supplementation with a single beneficial probiotic species, Bifidobacterium longum (B. longum), would promote fracture repair in aged (18‐month‐old) female mice. We found that B. longum supplementation accelerated bony callus formation which improved mechanical properties of the fractured limb. We attribute these pro‐regenerative effects of B. longum to preservation of intestinal barrier, dampened systemic inflammation, and maintenance of the microbiota community structure. Moreover, B. longum attenuated many of the fracture‐induced systemic pathologies. Our study provides evidence that targeting the gut microbiota using simple dietary approaches can improve fracture healing outcomes and minimize systemic pathologies in the context of aging.