Bone Morphogenetic Protein Signaling Is Required for Maintenance of Differentiated Phenotype, Control of Proliferation, and Hypertrophy in Chondrocytes

Abstract

To examine the role of bone morphogenetic protein (BMP) signaling in chondrocytes during endochondral ossification, the dominant negative (DN) forms of BMP receptors were introduced into immature and mature chondrocytes isolated from lower and upper portions of chick embryo sternum, respectively. We found that control sternal chondrocyte populations expressed type IA, IB, and II BMP receptors as well as BMP-4 and -7. Expression of a DN-type II BMP receptor (termed DN-BMPR-II) in immature lower sternal (LS) chondrocytes led to a loss of differentiated functions; compared with control cells, the DN-BMPR- II–expressing LS chondrocytes proliferated more rapidly, acquired a fibroblastic morphology, showed little expression of type II collagen and aggrecan genes, and upregulated type I collagen gene expression. Expression of DN-BMPR-II in mature hypertrophic upper sternal (US) chondrocytes caused similar effects. In addition, the DN-BMPR-II–expressing US cells exhibited little alkaline phosphatase activity and type X collagen gene expression, while the control US cells produced both alkaline phosphatase and type X collagen. Both DN-BMPR-II–expressing US and LS chondrocytes failed to respond to treatment with BMP-2 . When we examined the effects of DN forms of types IA and IB BMP receptors, we found that DN-BMPR-IA had little effect, while DN-BMPR-IB had similar but weaker effects compared with those of DN-BMPR-II. We conclude that BMP signaling, particularly that mediated by the type II BMP receptor, is required for maintenance of the differentiated phenotype, control of cell proliferation, and expression of hypertrophic phenotype.

Modulating hedgehog signaling can attenuate the severity of osteoarthritis

“…Bmp-2 levels are found in both human and murine OA cartilage where they help to promote chondrocyte differentiation and hypertrophy in comparison to areas of least OA involvement 41 . In contrast, TGF-beta levels are observed initially to increase during the onset of OA, and then decrease during later stages, based on expression studies in human osteoarthritic cartilage samples 24 .…”

The BMP signaling and in vivo bone formation

“…In contrast, expression of Bmpr1b is restricted in early mesenchymal cells and differentiated chondrocytes (Ashique et al, 2002). Over-expression of a dominant-negative Bmpr1b but not Bmpr1a inhibits chondrocyte differentiation (Enomoto-Iwamoto et al, 1998) and overexpression of a constitutively active Alk-2 promotes chondrocyte maturation (Zhang et al, 2003a) in chicken sternal chondrocytes. Over-expression of a dominant-negative Bmpr1b also inhibits osteoblast differentiation in osteoblast precursor cells .…”

Small GTPase protein Rac-1 is activated with maturation and regulates cell morphology and function in chondrocytes

“…[22] Chondrocyte culture Chondrocytes were isolated from E17 chick embryos by enzymatic digestion of upper and lower sternal explants with 0.15% collagenase and 0.25% trypsin for 3 hours at 37°C as previously reported. [23] Chondrocytes were kept in DMEM/10% FBS except where indicated.…”

“…We had previously shown that LS chondrocytes remain immature for more than two weeks in culture, while US chondrocytes expressed a mature phenotype, marked by high levels of alkaline phosphatase activity and collagen type X expression from the beginning of culture. [23] Whole cell lysates were prepared from both LS and US culture and the endogenous, steady-state expression levels of RhoA, Rac-1, and Cdc42 proteins were examined by immunoblot analysis (Fig. 1A, Total).…”