0013-7227/05/$15.00/0 Endocrinology 146(3):1012–1017 Printed in U.S.A. Copyright © 2005 by The Endocrine Society doi: 10.1210/en.2004-1343 Minireview: Transcriptional Regulation in Development of Bone Tatsuya Kobayashi and Henry Kronenberg Endocrine Unit, Department of Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114 Downloaded from https://academic.oup.com/endo/article/146/3/1012/2499970 by guest on 25 September 2021 Regulation of gene expression by transcription factors is one distinct developmental programs. These cells go through mul- of the major mechanisms for controlling cellular functions. tiple differentiation stages, which are often regulated by spe- Recent advances in genetic manipulation of model animals cific transcription factors. In this minireview, we will discuss has allowed the study of the roles of various genes and their selected transcription factors that have been demonstrated to products in physiological settings and has demonstrated the critically affect bone cell development. Further study of these importance of specific transcription factors in bone develop- molecules will lead to deeper understanding in mechanisms that ment. Three lineages of bone cells, chondrocytes, osteoblasts, govern development of bone. (Endocrinology 146: 1012–1017, and osteoclasts, develop and differentiate according to their 2005) ONE DEVELOPMENT IS achieved through the use of signaling (4), likely play an important role. After the formation B two distinct pathways, intramembranous and endo- of mesenchymal condensations, cells start proliferating again chondral bone formation. In intramembranous bone for- and differentiate into chondrocytes (Fig. 1). mation, mesenchymal cells condense and directly differ- entiate into bone-forming osteoblasts. In contrast, in Sox9 (sex reversal Y-related high-mobility group box endochondral bone formation, mesenchymal cells con- protein) and chondrocyte commitment dense and then become chondrocytes. This cartilage mold then directs the formation of osteoblasts, which form ma- Analysis of genetically manipulated mice demonstrated that ture bone. This bone is continually remodeled by cycles of Sox transcription factors, members of the high mobility group bone formation and resorption. Osteoclasts, of hemato- superfamily, are necessary for conversion of condensed mes- poietic origin, are responsible for bone resorption. Thus, enchymal cells to chondrocytes. Studies of chimeric mice (5) three distinct cell types (chondrocytes, osteoblasts, and and conditional knockouts (6) have shown that Sox9 is required osteoclasts) from two cell lineages direct the formation and for formation of normal mesenchymal condensations, for con- remodeling of bone (1). version of mesenchymal cells to chondrocytes, for proliferation of chondrocytes, and for suppression of premature conversion Endochondral Bone Formation and Chondrocyte of these chondrocytes to hypertrophic chondrocytes. Sox9 is Differentiation also required for the production of L-Sox5 and Sox6, two related Sox family members required for normal chondrocyte function Formation of the cartilage anlage starts with mesen- (7). Sox9 is not only required for development of chondrocytes chymal condensation (2). The molecular mechanism that but also directly regulates expression of genes important for regulates this process at the transcriptional level is not fully chondrocyte function, such as Col2a1 (8), Col11a2 (9), Agc1 understood. However, because bone morphogenetic protein (aggrecan) (10), and Mia (Cdrap) (11). The function of Sox9 may (BMP) signaling appears crucial for this process (3), Smads be modulated by phosphorylation by the PTH-related peptide [a name derived from Sma and MADD (Mother against signaling pathway (12). It is, therefore, possible that Sox9 decapentaplegic)], transcription factors downstream of BMP mediates part of PTH-related peptide action to regulate hyper- trophic differentiation. First Published Online December 16, 2004 Expression of Sox9 begins in the mesenchymal condensation. Abbreviations: BMP, Bone morphogenetic protein; FGF, fibroblast BMPs can induce SOX9 expression, and noggin, a BMP antag- growth factor; Ihh, Indian hedgehog; LEF, lymphoid enhancer-binding onist, blocks expression of SOX9 in mesenchymal condensa- factor; M-CSF, macrophage colony-stimulating factor; MITF, microph- tions (13). Cytokines such as IL-1 and TNF-␣, and nuclear factor thalmia-associated transcription factors; NFATc1 nuclear factor of ac- ␬ ␬ tivated T cells, cytoplasmic, calcineurin-dependent 1; NF-␬B, nuclear B (NF- B), the transcription factor that mediates many actions factor ␬B; Osx, Osterix; RANK, receptor activator of nuclear factor ␬B; of these cytokines, suppress Sox9 expression, partly through RANKL, RANK ligand; Smad, a name derived from Sma and MADD induction of posttranscriptional mRNA degradation (14, 15); (Mother against decapentaplegic); Sox, sex reversal Y-related high- this suppression of SOX9 probably contributes to chondrocyte mobility group box protein; TCF, T-cell factor; TFE, transcription factor E; Wnt, a name that combines wingless and int. loss in inflammatory arthritis. How specific transcription fac- Endocrinology is published monthly by The Endocrine Society (http:// tors regulate Sox9 gene transcription is a central question, al- www.endo-society.org), the foremost professional society serving the though it appears complicated; genetic analysis of human and endocrine community. mouse Sox9 mutations demonstrated that cis elements quite 1012 Kobayashi and Kronenberg • Minireview Endocrinology, March 2005, 146(3):1012–1017 1013 Downloaded from https://academic.oup.com/endo/article/146/3/1012/2499970 by guest on 25 September 2021 FIG. 1. Differentiation of bone cells of three lineages and its regulation by transcription factors. Transcription factors important for bone cell differentiation are indicated by bold black letters with arrows at major differentiation steps. Signaling molecules are indicated by blue letters. Osteoblasts and chondrocytes are derived from common mesenchymal precursors. During endochondral bone formation, mesenchymal cells condense and differentiate into chondrocytes. BMP signaling is likely required for these processes. Sox9 and its related molecules L-Sox5 and Sox6 play a pivotal role in commitment and maintenance of chondrocyte phenotypes. In the growth plate, chondrocytes proliferate and further differentiate into column-forming cells, then into postmitotic hypertrophic chondrocytes. Runx2 expression stimulates terminal differentiation. Runx2 and Osx are both required for osteoblast differentiation. Runx2 expression starts before that of Osx. Its activity is regulated by interaction with other transcription factors such as members of the Twist family. Osteoclasts differentiate from hematopoietic lineage cells through multiple steps. M-CSF and RANLK are essential external stimuli for osteoclastogenesis. PU.1, MITFs, Nf-␬B, Fos/Fra1, and NFATc1 are all required for differentiation of mature osteoclasts. distant from the transcription start site of the Sox9 gene are become hypertrophic. Runx transcription factors are mem- critical for Sox9 transcription in cartilage (16). bers of the runt family, named for the DNA binding domain, Interestingly, overexpression of Sox9 in the cartilage conserved across species from Drosophila to humans (22). causes a decrease in chondrocyte proliferation and a delay in Mice missing Runx2 show a defect in chondrocyte matura- bone development. This decrease in proliferation may result tion, with lack of hypertrophic chondrocytes in many bones from binding of Sox9 to ␤-catenin, the essential component (23), and mice missing both Runx2 and Runx3 completely of the canonical Wnt (a name that combines wingless and int) lack chondrocytes (24). Furthermore, overexpression of signaling pathway (17). Some Wnt members are expressed in Runx2 hastens hypertrophy and even converts chondrocytes cartilage, and effects of Wnt signaling in cartilage have been in the trachea, which normally never hypertrophy, into hy- demonstrated (18–21). Along with the observation that over- pertrophic chondrocytes and then bone (25, 26). activation or deletion of ␤-catenin in chondrocytes resulted The stages of chondrocyte differentiation are regulated by a in severe skeletal dysplasias, these findings suggest that the complex series of signaling molecules and transcription factors transcription complex, lymphoid enhancer-binding factor in addition to Sox9 and Runx2/3. Differentiation of early peri- ␤ (LEF)/T-cell factor (TCF)/ -catenin, regulates some aspects articular chondrocytes into flat, columnar proliferating chon- of cartilage development (17). drocytes appears to be regulated by multiple signaling mole- cules such as Indian hedgehog (Ihh) and Wnts (19, 27). It is, Runx2 and chondrocyte hypertrophy therefore, likely that some of the downstream transcription Chondrocytes divide and produce a characteristic matrix factors triggered by Ihh and wnts, Gli family members, and the but then stop dividing, change the matrix they synthesize, ␤-catenin/TCF/LEF complex, play a role in this step. Chon- and become quite large (hypertrophic). Runx2 and, to a lesser drocyte differentiation is accompanied by changes in prolifer- extent, Runx3, are the major transcription factors controlling ation. Alterations in fibroblast growth factor (FGF) signaling the crucial steps because chondrocytes stop dividing and (28, 29), BMP signaling (30), and Ihh signaling