Gradients in bone morphogenetic protein-related gene expression across the growth plate

Abstract

In the growth plate, stem-like cells in the resting zone differentiate into rapidly dividing chondrocytes of the proliferative zone and then terminally differentiate into the nondividing chondrocytes of the hypertrophic zone. To explore the molecular switches responsible for this two-step differentiation program, we developed a microdissection method to isolate RNA from the resting (RZ), proliferative (PZ), and hypertrophic zones (HZ) of 7-day-old male rats. Expression of approximately 29 000 genes was analyzed by microarray and selected genes verified by real-time PCR. The analysis identified genes whose expression changed dramatically during the differentiation program, including multiple genes functionally related to bone morphogenetic proteins (BMPs). BMP-2 and BMP-6 were upregulated in HZ compared with RZ and PZ (30-fold each, P!0 . 01 and 0 . 001 respectively). In contrast, BMP signaling inhibitors were expressed early in the differentiation pathway; BMP-3 and gremlin were differentially expressed in RZ (100-and 80-fold, compared with PZ, P! 0 . 001 and 0 . 005 respectively) and growth differentiation factor (GDF)-10 in PZ (160-fold compared with HZ, P!0 . 001). Our findings suggest a BMP signaling gradient across the growth plate, which is established by differential expression of multiple BMPs and BMP inhibitors in specific zones. Since BMPs can stimulate both proliferation and hypertrophic differentiation of growth plate chondrocytes, these findings suggest that low levels of BMP signaling in the resting zone may help maintain these cells in a quiescent state. In the lower RZ, greater BMP signaling may help induce differentiation to proliferative chondrocytes. Farther down the growth plate, even greater BMP signaling may help induce hypertrophic differentiation. Thus, BMP signaling gradients may be a key mechanism responsible for spatial regulation of chondrocyte proliferation and differentiation in growth plate cartilage.

Growth plate expression profiling: Large and small breed dogs provide new insights in endochondral bone formation

“…The higher expression of BMP‐2 and BMP‐6 in the hypertrophic zone of Great Danes resembles the BMP expression profile reported in growth plates of 7 days old rats and of mid‐gestation human fetuses 14, 38. Notably, BMP‐2 and BMP‐6 expression peaked in the proliferative zone of Miniature Poodles being more in line with the BMP expression profile found in older animals, that is, 6‐39 and 10‐week‐old rats 38.…”

“…The latter is further supported by the significantly higher alkaline phosphatase ( ALP ) in the PZ compared with the RZ. Furthermore, proper dissection of the HZ was supported by the significantly higher expressed IHH and Collagen X in the HZ compared with the RZ and Bone 14, 28…”

“…The micro array data was validated by RT‐qPCR of the whole growth plate tissue samples employed in the microarray ( n  = 5 per group) and on the separate growth plate zones separated by dissection as described by Nilsson et al14 …”

Cell biology of osteochondromas: Bone morphogenic protein signalling and heparan sulphates

“…Thus, HSPGs can regulate the cellular distribution of noggin, offering a possible mechanism through which HSPGs can regulate the responses of cells to BMPs in vivo [26]. Chordin is expressed predominantly in the epiphyseal regions of developing long bones and binds to BMP-2 and BMP-4 [24,30,42]. Chordin is capable of antagonising BMP-induced chondrocyte differentiation and is in turn proteolytically inactivated by BMP-1 [30].…”

Growth Factors and Signalling Molecules for Cartilage Tissue Engineering – from Embryology to Innovative Release Strategies for Guided Tissue Engineering

“…Furthermore, BMP pathways inhibit FGF signalling by constraining the expression of FGFR1 (fibroblast growth factor receptor 1) (Yoon et al, 2006). Within the growth plate a concentration gradient of BMP may be a key feature responsible for spatial regulation of chondrocyte proliferation and differentiation (Nilsson et al, 2007). Regarding tissue engineering strategies it is not surprising that recombinant human BMP's have already found their way into clinical practice for the treatment of critical sized bone defects.…”