Differential Roles for Bone Morphogenetic Protein (BMP) Receptor Type IB and IA in Differentiation and Specification of Mesenchymal Precursor Cells to Osteoblast and Adipocyte Lineages

Abstract

Cumulative evidence indicates that osteoblasts and adipocytes share a common mesenchymal precursor and that bone morphogenetic proteins (BMPs) can induce both osteoblast and adipocyte differentiation of this precursor. In the present study, we investigated the roles of BMP receptors in differentiation along these separate lineages using a well-characterized clonal cell line, 2T3, derived from the mouse calvariae. BMP-2 induced 2T3 cells to differentiate into mature osteoblasts or adipocytes depending upon culture conditions. To test the specific roles of the type IA and IB BMP receptor components, truncated and constitutively active type IA and IB BMP receptor cDNAs were stably expressed in these cells. Overexpression of truncated type IB BMP receptor (trBMPR-IB) in 2T3 cells completely blocked BMP-2–induced osteoblast differentiation and mineralized bone matrix formation. Expression of trBMPR-IB also blocked mRNA expression of the osteoblast specific transcription factor, Osf2/ Cbfa1, and the osteoblast differentiation-related genes, alkaline phosphatase (ALP) and osteocalcin (OC). BMP-2–induced ALP activity could be rescued by transfection of wild-type (wt) BMPR-IB into 2T3 clones containing trBMPR-IB. Expression of a constitutively active BMPR-IB (caBMPR-IB) induced formation of mineralized bone matrix by 2T3 cells without addition of BMP-2. In contrast, overexpression of trBMPR-IA blocked adipocyte differentiation and expression of caBMPR-IA induced adipocyte formation in 2T3 cells. Expression of the adipocyte differentiation-related genes, adipsin and PPARγ, correlated with the distinct phenotypic changes found after overexpression of the appropriate mutant receptors. These results demonstrate that type IB and IA BMP receptors transmit different signals to bone-derived mesenchymal progenitors and play critical roles in both the specification and differentiation of osteoblasts and adipocytes.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

“…Thus, effects at 10, 100 and 500 ng/ml MEPE are shown after day 13 (A) and day 20 (B), respectively. The mineralized bone matrix formation of 2T3 cells were quantitated by computer image analysis as previously described [14] and the data represent the mean for three samples (see Materials and methods). ANOVA non-parametric and Neuman-Keuls multiple comparison confirm highly significant inhibition of BMP2 mediated mineralization by Hu-MEPE (see graph).…”

“…A mouse osteoblast cell line (2T3 cells) was monitored using a mineralized matrix formation assay as described by Bhargava et al and cultured as described previously [7,14,105]. Mineralization of the murine osteoblastic 2T3 cell line has been extensively studied using physical techniques, electron microscopy and histological techniques.…”

“…Mineralization of the murine osteoblastic 2T3 cell line has been extensively studied using physical techniques, electron microscopy and histological techniques. This cell line (2T3) undergoes exogenous BMP2-induced mineralized matrix indistinguishable from normal bone-mineral hydroxyapatite in vitro [7,14,105]. The cells were plated in 24-well culture plates at a density of 2 × 10 4 cells/well and cultured with minimal essential medium (MEM) supplemented with 10% FCS.…”

“…The plates were washed with water, and then 2 × 95% ethanol and 2 × 100% ethanol, and dried for image analysis. The area of von Kossa-stained matrix was quantitated as previously described [14]. Briefly matrix was quantified by automated image analysis using a video analysis program (Jandel Scientific, San Rafael, CA) and also a Bio-Rad quantity 1 image analysis capture program.…”

Transforming growth factor‐β in liver cancer stem cells and regeneration

“…In embryonic stem cells (ESCs), another TGF‐β family member, bone morphogenetic protein (BMP4) is required for ESC self‐renewal through a balanced inhibition of ESC lineage commitment. In mesenchymal stem cells, the BMP signal induces osteoblastic differentiation through Bmpr1b but inhibits osteoblastic differentiation through Bmpr1a 45. BMP signaling inhibits stem cell activation and expansion in intestinal stem cells 46.…”