Cellular differentiation entails the coordination of cell cycle arrest and tissue-specific gene expression. We investigated the involvement of basic helix-loop-helix (bHLH) factors in differentiation of osteoblasts using the human osteoblastic cell line MG63. Serum starvation induced growth arrest at G₁ phase, accompanied by expression of cyclin-dependent kinase inhibitor p21^WAF1/Cip1. Reporter assays with the p21 gene promoter demonstrated that the combination of E2A (E12 or E47) and coactivator CBP was responsible for p21 induction independent of p53. Twist inhibited E2A-CBP-dependent activation of the exogenous and endogenous p21 promoters. Ids similarly inhibited the exogenously transfected p21 promoter; however less antagonistic effect on the endogenous p21 promoter was observed. Twist was predominantly present in nuclei in MG63 cells growing in complete medium, while it localized mainly in the cytoplasm after serum starvation. The fibroblast growth factor receptor 3 gene (FGFR3), which generates signals leading to differentiation of osteoblasts, was found to be controlled by the same transcriptional regulation as the p21 gene. E2A and Twist influenced alkaline phosphatase expression, a consensus marker of osteoblast differentiation. Expression of E2A and FGFR3 was seen at the location of osteoblast differentiation in the calvaria of mouse embryos, implicating bHLH molecules in physiological osteoblast differentiation. These results demonstrate that a common regulatory system is involved in at least two distinct steps in osteoblastic differentiation. Our results also provide the molecular basis of Saethre-Chotzen syndrome, caused by mutations of the TWISTand FGFR3 genes.

Members of the basic helix-loop-helix (bHLH) family of transcription factors regulate the specification and differentiation of numerous cell types during embryonic development. Hand1 and Hand2 are expressed by a subset of neural crest cells in the anterior branchial arches and are involved in craniofacial development. However, the precise mechanisms by which Hand proteins mediate biological actions and regulate downstream target genes in branchial arches is largely unknown. Here, we report that Hand2 negatively regulates intramembranous ossification of the mandible by directly inhibiting the transcription factor Runx2, a master regulator of osteoblast differentiation. Hand proteins physically interact with Runx2, suppressing its DNA binding and transcriptional activity. This interaction is mediated by the N-terminal domain of the Hand protein and requires neither dimerization with other bHLH proteins nor DNA binding. We observed partial colocalization of Hand2 and Runx2 in the mandibular primordium of the branchial arch, and downregulation of Hand2 precedes Runx2-driven osteoblast differentiation. Hand2 hypomorphic mutant mice display insufficient mineralization and ectopic bone formation in the mandible due to accelerated osteoblast differentiation, which is associated with the upregulation and ectopic expression of Runx2 in the mandibular arch. Here, we show that Hand2 acts as a novel inhibitor of the Runx2-DNA interaction and thereby regulates osteoblast differentiation in branchial arch development.

Cleft palate, the most frequent congenital craniofacial birth defect, is a multifactorial condition induced by the interaction of genetic and environmental factors. In addition to complete cleft palate, a large number of human cases involve soft palate cleft and submucosal cleft palate. However, the etiology of these forms of cleft palate has not been well understood. T-box transcriptional factor (Tbx) family of transcriptional factors has distinct roles in a wide range of embryonic differentiation or response pathways. Here, we show that genetic disruption of Tbx1, a major candidate gene for the human congenital disorder 22q11.2 deletion syndrome (Velo-cardio-facial/DiGeorge syndrome), led to abnormal epithelial adhesion between the palate and mandible in mouse, resulting in various forms of cleft palate similar to human conditions. We found that hyperproliferative epithelium failed to undergo complete differentiation in Tbx1-null mice (Tbx1^−/−). Inactivation of Tbx1 specifically in the keratinocyte lineage (Tbx1^KCKO) resulted in an incomplete cleft palate confined to the anterior region of the palate. Interestingly, Tbx1 overexpression resulted in decreased cell growth and promoted cell-cycle arrest in MCF7 epithelial cells. These findings suggest that Tbx1 regulates the balance between proliferation and differentiation of keratinocytes and is essential for palatal fusion and oral mucosal differentiation. The impaired adhesion separation of the oral epithelium together with compromised palatal mesenchymal growth is an underlying cause for various forms of cleft palate phenotypes in Tbx1^−/− mice. Our present study reveals new pathogenesis of incomplete and submucous cleft palate during mammalian palatogenesis.

Epicardin/capsulin/Pod-1, expressed in skeletal myoblasts within brachial arches and in the condensing mesenchyme, is a member of the basic helix-loop-helix (bHLH) transcription factor family that is involved in various cell differentiation processes. In this study, we examined the functional properties of epicardin/capsulin/Pod-1 in differentiation. The yeast and mammalian two-hybrid systems showed physical associations between epicardin/capsulin/Pod-1 and E2A, both of which were present in the nuclei. The bHLH domains mediated this association. Ectopic expression of epicardin/capsulin/Pod-1 inhibited E2A-dependent activation of the exogenous and endogenous expression of the cyclin-dependent kinase inhibitor,p21(WAF1/Cip1) gene, and the muscle creatine kinase gene that encodes the predominant creatine kinase isoform expressed in mammalian skeletal muscle. Transfection with epicardin/capsulin/Pod-1 small interfering RNA abolished the epicardin/capsulin/Pod-1-mediated suppression of E12-dependent activation of the p21 promoter. Chromatin immunoprecipitation assay showed that epicardin/capsulin/Pod-1 was physically associated with the muscle creatine kinase promoter in vivo. Moreover, terminal differentiation of C2C12 myoblasts was inhibited by exogenous introduction of epicardin/capsulin/Pod-1. These inhibitory functions of epicardin/capsulin/Pod-1 closely resemble those of the bHLH inhibitor Twist protein. These results indicate that epicardin/capsulin/Pod-1 functions as a negative regulator of differentiation of myoblasts through transcription in at least two distinct steps, cell growth arrest and lineage-specific differentiation.