The mammalian HoxD cluster lies between two topologically associating domains (TADs) matching distinct enhancer-rich regulatory landscapes. During limb development, the telomeric TAD controls the early transcription of Hoxd genes in forearm cells, whereas the centromeric TAD subsequently regulates more posterior Hoxd genes in digit cells. Therefore, the TAD boundary prevents the terminal Hoxd13 gene from responding to forearm enhancers, thereby allowing proper limb patterning. To assess the nature and function of this CTCF-rich DNA region in embryos, we compared chromatin interaction profiles between proximal and distal limb bud cells isolated from mutant stocks where various parts of this boundary region were removed. The resulting progressive release in boundary effect triggered inter-TAD contacts, favored by the activity of the newly accessed enhancers. However, the boundary was highly resilient, and only a 400-kb deletion, including the whole-gene cluster, was eventually able to merge the neighboring TADs into a single structure. In this unified TAD, both proximal and distal limb enhancers nevertheless continued to work independently over a targeted transgenic reporter construct. We propose that the whole HoxD cluster is a dynamic TAD border and that the exact boundary position varies depending on both the transcriptional status and the developmental context.

The skeleton is a highly dynamic tissue whose structure relies on the balance between bone deposition and resorption. This equilibrium, which depends on osteoblast and osteoclast functions, is controlled by multiple factors that can be modulated post-translationally. Some of the modulators are Mitogen-activated kinases (MAPKs), whose role has been studied in vivo and in vitro. p38-MAPK modifies the transactivation ability of some key transcription factors in chondrocytes, osteoblasts and osteoclasts, which affects their differentiation and function. Several commercially available inhibitors have helped to determine p38 action on these processes. Although it is frequently mentioned in the literature, this chemical approach is not always as accurate as it should be. Conditional knockouts are a useful genetic tool that could unravel the role of p38 in shaping the skeleton. In this review, we will summarize the state of the art on p38 activity during osteoblast differentiation and function, and emphasize the triggers of this MAPK.

During vertebrate limb development, Hoxd genes are regulated following a bimodal strategy involving two topologically associating domains (TADs) located on either side of the gene cluster. These regulatory landscapes alternatively control different subsets of Hoxd targets, first into the arm and subsequently into the digits. We studied the transition between these two global regulations, a switch that correlates with the positioning of the wrist, which articulates these two main limb segments. We show that the HOX13 proteins themselves help switch off the telomeric TAD, likely through a global repressive mechanism. At the same time, they directly interact with distal enhancers to sustain the activity of the centromeric TAD, thus explaining both the sequential and exclusive operating processes of these two regulatory domains. We propose a model in which the activation of Hox13 gene expression in distal limb cells both interrupts the proximal Hox gene regulation and re-enforces the distal regulation. In the absence of HOX13 proteins, a proximal limb structure grows without any sign of wrist articulation, likely related to an ancestral fish-like condition.

Osterix, a zinc finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osx-null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that, in several mesenchymal and osteoblastic cell types, BMP-2 induces an increase in expression of the two isoforms of Osterix arising from two alternative promoters. We identified a consensus Sp1 sequence (GGGCGG) as Osterix binding regions in the fibromodulin and the bone sialoprotein promoters in vitro and in vivo. Furthermore, we show that Osterix is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-73 and Ser-77 are the regulatory sites phosphorylated by p38. Our data also demonstrate that Osterix is able to increase recruitment of p300 and Brg1 to the promoters of its target genes fibromodulin and bone sialoprotein in vivo and that it directly associates with these cofactors through protein-protein interactions. Phosphorylation of Osterix at Ser-73/77 increased its ability to recruit p300 and SWI/SNF to either fibromodulin or bone sialoprotein promoters. We therefore propose that Osterix binds to Sp1 sequences on target gene promoters and that its phosphorylation by p38 enhances recruitment of coactivators to form transcriptionally active complexes.Bone is a highly dynamic tissue that is constantly remodeled throughout life. Bone remodeling activity is dependent on a delicate balance between osteoclast resorption and osteoblast new bone formation. Deregulation of these two activities unleashes pathological states such as osteoporosis and osteosclerosis. Both endochondral and intramembranous ossification depends on osteoblasts that are derived from pluripotent mesenchymal stem cells that, in response to various cellular and environmental signals, commit to the osteoblast phenotype. Among them, BMPs 5 are essential for commitment and differentiation to the osteoblast lineage; they promote osteoblast differentiation in vitro and in vivo, bone regeneration, and ectopic bone formation in vivo (1-3). The BMP signal is transduced through the binding to its heteromeric cell membrane receptors (4, 5). BMP binding to receptors results in the activation of the Smad family of transcription factors, which directly regulate target gene expression (6). BMP target genes include a growing number of osteoblastdetermining transcription factors. For instance, in vivo genetic evidence as well as osteogenic induction of bone marrow mesenchymal stem cells in vitro has identified several types of transcription factors such as Id1, homeodomain proteins such as Dlx3 and Dlx5, ATF4, Runx2, and Osterix (Osx) (7-9). Runx2 and Osx have been widely accepted as master osteogenic factors because neither Runx2-nor Osx-null mice form mature osteoblasts (10, 11). Osx contains a proline-and serine-rich transactivation domain located in the N-terminal part of the protein and three zinc fingers with homology to the Sp1/Kruppel transcription factor family. Osx express...

The mammalian HoxD cluster lies between two topologically associating domains (TADs) matching distinct, enhancer-rich regulatory landscapes. During limb development, the telomeric TAD controls the early transcription of Hoxd gene in forearm cells, whereas the centromeric TAD subsequently regulates more posteriorHoxd genes in digit cells. Therefore, the TAD boundary prevents the terminal Hoxd13 gene to respond to forearm enhancers, thereby allowing proper limb patterning. To assess the nature and function of this CTCF-rich DNA region in embryo, we compared chromatin interaction profiles between proximal and distal limb bud cells isolated from mutant stocks where various parts or this boundary region were removed. The resulting progressive release in boundary effect triggered inter-TAD contacts, favored by the activity of the newly accessed enhancers. However, the boundary was highly resilient and only a 400kb large deletion including the whole gene cluster was eventually able to merge the neighboring TADs into a single structure. In this unified TAD, both proximal and distal limb enhancers nevertheless continued to work independently over a targeted transgenic reporter construct. We propose that the whole HoxD cluster is a dynamic TAD border and that the exact boundary position varies depending on both the transcriptional status and the developmental context.