A Cbfa1-dependent genetic pathway controls bone formation beyond embryonic development

Abstract

The molecular mechanisms controlling bone extracellular matrix (ECM) deposition by differentiated osteoblasts in postnatal life, called hereafter bone formation, are unknown. This contrasts with the growing knowledge about the genetic control of osteoblast differentiation during embryonic development. Cbfa1, a transcriptional activator of osteoblast differentiation during embryonic development, is also expressed in differentiated osteoblasts postnatally. The perinatal lethality occurring in Cbfa1-deficient mice has prevented so far the study of its function after birth. To determine if Cbfa1 plays a role during bone formation we generated transgenic mice overexpressing Cbfa1 DNA-binding domain (⌬Cbfa1) in differentiated osteoblasts only postnatally. ⌬Cbfa1 has a higher affinity for DNA than Cbfa1 itself, has no transcriptional activity on its own, and can act in a dominant-negative manner in DNA cotransfection assays. ⌬Cbfa1-expressing mice have a normal skeleton at birth but develop an osteopenic phenotype thereafter. Dynamic histomorphometric studies show that this phenotype is caused by a major decrease in the bone formation rate in the face of a normal number of osteoblasts thus indicating that once osteoblasts are differentiated Cbfa1 regulates their function. Molecular analyses reveal that the expression of the genes expressed in osteoblasts and encoding bone ECM proteins is nearly abolished in transgenic mice, and ex vivo assays demonstrated that ⌬Cbfa1-expressing osteoblasts were less active than wild-type osteoblasts. We also show that Cbfa1 regulates positively the activity of its own promoter, which has the highest affinity Cbfa1-binding sites characterized. This study demonstrates that beyond its differentiation function Cbfa1 is the first transcriptional activator of bone formation identified to date and illustrates that developmentally important genes control physiological processes postnatally.

The p38α MAPK positively regulates osteoblast function and postnatal bone acquisition

“…Due to the increasing need for anabolic therapies to prevent age-related bone loss, research has been focused over the last decade on molecular mechanisms regulating osteoblast function in postnatal remodeling [20][21][22]. For instance, Runx2 and Osx have been found to regulate osteoblast function in adult bone in addition to their primarily established role in osteoblastogenesis [23,24]. Another transcription factor, Atf4 regulates bone matrix deposition by mature osteoblasts [25].…”

Growth defect in Grg5 null mice is associated with reduced Ihh signaling in growth plates

“…After careful dissection to remove soft tissues, the calvarial bones were rinsed with PBS. The osteoblasts were isolated as described (Ducy et al, 1999). Briefly, the bones were incubated for 20 min at 37°C in ␣-Minimal Essential Medium (␣ -MEM, Life Technologies, MD) containing 0.1 mg/ml collagenase P (Boehringer Mannheim, Germany) and 2% trypsin/EDTA.…”

Hertwig's epithelial root sheath cells do not transform into cementoblasts in rat molar cementogenesis

“…Runx2 (runt-related transcription factor2), also known as core-binding factor alpha 1(Cbfa1), was originally identified as an osteoblast-specific transcription factor essential for osteoblast differentiation (Ducy et al, 1999;Ducy, 2000). To date, Runx2 has been detected in both epithelial and mesenchymal cells in various kinds of mineralized and non-mineralized tissues (Bronckers et al, 2001).…”

“…To date, Runx2 has been detected in both epithelial and mesenchymal cells in various kinds of mineralized and non-mineralized tissues (Bronckers et al, 2001). In mineralized tissues, it has been established that Runx2 regulates the differentiation of mineralization-inducing cells, viz., osteoblasts (bone), ameloblasts (enamel), odontoblasts (dentin), and cementoblasts (cementum) (D'Souza et al, 1999;Ducy et al, 1999;Ducy, 2000;Bronckers et al, 2001;Zhao et al, 2002;Camilleri and McDonald, 2006). From these, we considered that Runx2 could be used as a marker for mineralization-inducing cells in cementogenesis.…”