Distinct roles for Hedgehog and canonical Wnt signaling in specification,differentiation and maintenance of osteoblast progenitors

Abstract

DEVELOPMENT 3231 RESEARCH ARTICLE INTRODUCTIONThe mammalian skeleton forms from three distinct cell lineages (Nakashima and de Crombrugghe, 2003). Facial bones and the cranium are derived from the neural crest. The base of the skull, the parietal bones and the axial elements of the ribs and vertebrae are derived from paraxial mesoderm. The sternum and long bones are formed from lateral plate mesoderm. Although several cell lineages contribute to skeletal structures, each gives rise to a common bone matrix-secreting cell type, the osteoblast. The requirement of this cell type for the production of bone is clearly demonstrated by the targeted mutation of Runx2 (Komori et al., 1997;Ducy et al., 1997;Otto et al., 1997) and osterix1 (Osx1; Sp7 -Mouse Genome Informatics) (Nakashima et al., 2002), transcriptional regulators expressed in osteoblast progenitors where both mutants lack all bone.Bone can be produced by two distinct mechanisms, direct differentiation of osteoblasts from mesenchymal progenitors (intramembranous ossification, e.g. skull and face) or by formation of bone on a cartilage scaffold (endochondral ossification, e.g. remainder of the skeleton). Endochondral ossification, the principle focus of this study, is initiated by the condensation of multipotent mesenchymal progenitor cells into structures that anticipate skeletal elements of the adult (reviewed by Kronenberg, 2003). Chondrocytes are the first cell type to form, starting out as immature proliferative cells that express type 2 collagen (Col2a1) that subsequently mature into postmitotic type 10 collagen (Col10a1)-expressing hypertrophic chondrocytes. Osteoblast progenitors can first be identified in the inner layer of perichondrial cells that lie immediately adjacent to the zone of hypertrophic chondrocytes, the periosteum, where the first bone matrix is deposited. Death of hypertrophic chondrocytes and vascular invasion result in the formation of a new area of mineralization, the primary spongiosa, within the shaft of the long bones.Several lines of evidence implicate Hedgehog (Hh) and canonical Wnt signaling in the regulation of endochondral ossification (reviewed by Kronenberg, 2003;Karsenty, 2003). Indian hedgehog (Ihh) is expressed by prehypertrophic chondrocytes and plays an essential role in coordinating the growth and differentiation of chondrocytes, both directly and through the control of other factors, notably parathyroid hormone-related peptide (Pthrp; Pthlh -Mouse Genome Informatics). In addition, Ihh appears to act directly on perichondrial mesenchyme to initiate an osteogenic program in osteoblast progenitors; in the absence of an Ihh input, these cells adopt an alternate chondrogenic fate (Long et al., 2004). The failure of activation of Runx2, a crucial early determinant of the osteoblast lineage, indicates that Hh signaling acts to initiate an osteogenic program. Whether Hh signaling is required at later stages of the osteogenic program has not been addressed.Initial in vivo evidence for canonical Wnt activity in osteogenesis c...

Wnt signaling and skeletal development

“…The above data indicates that without a β-catenin signal osteoblasts can not differentiate to an osterix positive stage. When β-catenin was deleted in osterix positive osteoblasts, those cells failed to progress to mature osteoblasts (characterized by high osteocalcin expression) although collagen I, Runx2, and osterix were observed [52]. In summary, the above data suggest that canonical Wnt signaling is required for both osteoblast early differentiation and terminal differentiation.…”

“…In bone marrow stromal osteoblast cultures, Wnt7b showed a marked increase in its expression after induction of differentiation [85]. An initial report on the Wnt7b −/− E18.5 embryo did not show any clear skeletal phenotype [52]. However, ablation of Wnt7b from skeletal progenitors by using CreloxP technique with Dermo1-Cre results in a deficiency in embryonic bone formation, which is at least partly due to a deficit in osterix activation and subsequent osteoblast differentiation [86].…”

“…In these mutant mice, the limbs contained only tiny remnants of skeletal elements and there was also a loss of skull bone [43]. The stabilization of β-catenin in osterix positive osteoblast precursor cells led to a marked increase in proliferation and an accelerated progression to mature bone matrixsecreting osteoblasts [52]. However, these cells failed to differentiate into terminal osteocalcinpositive osteoblasts.…”

Bone mass is inversely proportional to Dkk1 levels in mice

“…The absence of β-catenin in early skeletal progenitor cells causes osteoblast precursors to become chondrocytes [13][14][15]. Inactivation of β-catenin in committed osteoblast progenitors prevented the expression of osteoblast-specific genes, indicating that β-catenin is also essential for osteoblast differentiation [15,16]. Mice with β-catenin removal in mature osteoblasts displayed severe osteopenia, which surprisingly was due to an increase in osteoclasts [17,18].…”

Constitutive Activation of Smoothened Leads to Impaired Developments of Postnatal Bone in Mice

“…It is known that osterix acts downstream of Runx2 (Franceschi et al, 2007). Considerable evidence has revealed that Wnt up-regulates the expression of both Runx2 and osterix (Bennett, 2005;Rodda and McMahon, 2006). Consequently, it is likely that suppression of osterix is associated with a reduction in postnatal development in mutant mice.…”