Dlx5, a Positive Regulator of Osteoblastogenesis, is Essential for Osteoblast-Osteoclast Coupling

Abstract

The homeodomain protein Dlx5 is an activator of Runx2 (a key regulator of osteogenesis) and is thought to be an important regulator of bone formation. At present, however, the perinatal lethality of Dlx5-null mice has hampered the elucidation of its function in osteogenesis. Here we provide the first analysis of the effects of Dlx5 inactivation on bone development. Femurs of Dlx5-null mouse embryos at the end of gestation exhibit a reduction in both total and trabecular bone volume associated with increased trabecular separation and reduced trabecular number. These parameters are often associated with pathological conditions characterized by reduced osteoblast activity and increased bone resorption. Dlx5؊/؊ osteoblasts in culture display reduced proliferation and differentiation rate and reduction of Runx2, Osx, Osteocalcin and Bone Sialoprotein expression. In addition to impaired osteoblast function, Dlx5 ؊/؊ femurs exhibit significant increases in osteoclast number. As Dlx5 is not expressed by osteoclasts, we suggest that its osteoblastic expression might control osteoblast/osteoclast coupling. Cultured Dlx5 ؊/؊ osteoblasts displayed a higher RANKL/ OPG ratio. Furthermore, Dlx5 ؊/؊ osteoblasts induced a higher number of TRAP-positive multinucleated cells in normal spleen cultures with a globally increased resorption activity. These findings suggest that Dlx5 is a central regulator of bone turnover as it activates bone formation directly and bone resorption indirectly.

Bone Anabolic Agents for the Treatment of Multiple Myeloma

“…The type I receptor activates SMAD2, that induces the nuclear translocation of SMAD4 resulting in gene modulation, such as DLX5 inhibition [41,42]. DLX5 contributes to the regulation of osterix expression [43]. Lack of DLX5 gene expression induces abnormal osteogenesis.…”

Dlx genes and the maintenance of bone homeostasis and skeletal integrity

“…RNA-Seq analysis of Dlx3 OCN-cKO metaphysis shows upregulation of transcription factors essential for osteoblastogenesis, including Runx2, 16 its downstream osteoblast-specific target Sp7 17 and Dlx5/Dlx6, two positive regulators of chondrocyte and osteoblast differentiation (see Figure 1). 8,9,18,19 The fact that Dlx3 plays an important role in regulating osteoblast activity is further supported by the analysis of Dlx3-deleted bone marrow stroma cells (BMSCs), which also shows increased osteoblast differentiation and bone-forming activity associated with increased gene expression of Runx2 and Dlx5. The notion that DLX3 acts as a negative regulator of osteoblastogenesis by reducing Runx2, Sp7, Dlx5 and Dlx6 gene expression is further supported by ChIP analysis on BMSCs, which shows that DLX3 binds to the promoters of Sp7, Dlx5 and Dlx6 and Runx2, directly modulating their activity (see also Hassan et al 12 ).…”

“…7 Dlx transcription factors my well play different and synergic roles in the control of chondrogenesis and/or osteogenesis. 8,9 In particular, mutations in DLX3 result in Tricho-DentoOsseous syndrome, an autosomal dominant ectodermal dysplasia characterized by curly kinky hair, enamel hypoplasia, taurodontism and increased bone mineral density (BMD) in intramembranous and endochondral bones. 10 The importance of DLX3 in bone is also supported by its capacity to regulate directly in vitro critical determinants of bone differentiation such as osteocalcin (Ocn), Runx2 and osteoactivin, [11][12][13] and by the observation that overexpression of DLX3 in osteoprogenitors stimulates transcription of osteogenic markers.…”

“…: Samee et al 9 ) or in organ culture (e.g. : Hsu et al 14 ) as early lethality of null mutant mice has often hampered the elucidation of their postnatal role.…”

“…Indeed, Dlx5 À / À mice have an increased number of osteoclasts while Dlx5 À / À osteoblasts exhibit decreased Opg expression, resulting in a higher Rankl/Opg ratio. 9 It seems plausible that, besides Dlx5, 9 Dlx3 is also a central regulator of osteoblast/ osteoclast coupling. However, conversely to Dlx5, Dlx3 would inhibit osteoblast bone-forming activity via a negative transcriptional control of genes responsible for bone formation, while simultaneously activating bone resorption through osteoblast-activated regulation of osteoclastogenesis.…”