BMP and Ihh/PTHrP signaling interact to coordinate chondrocyte proliferation and differentiation

During endochondral ossification, two secreted signals, Indian hedgehog (Ih h) and parathyroid hormone-related protein (PTHrP), have been shown to form a negative feedback loop regulating the onset of hypertrophic differentiat ion of chondrocytes. Bone morphogenetic proteins (BMPs), another family of secreted factors regulating bone formation, have been implicated as potenti al interactors of the Ihh/PTHrP feedback loop. To analyze the relationship between the two signaling pathways, we used an organ culture system for lim b explants of mouse and chick embryos. We manipulated chondrocyte different iation by supplementing these cultures either with BMP2, PTHrP and Sonic he dgehog as activators or with Noggin and cyclopamine as inhibitors of the BM P and Ihh/PTHrP signaling systems. Overexpression of Ihh in the cartilage e lements of transgenic mice results in an upregulation of PTHrP expression a nd a delayed onset of hypertrophic differentiation. Noggin treatment of lim bs from these mice did not antagonize the effects of Ihh overexpression. Co nversely, the promotion of chondrocyte maturation induced by cyclopamine, w hich blocks Ihh signaling, could not be rescued with BMP2. Thus BMP signali ng does not act as a secondary signal of Ihh to induce PTHrP expression or to delay the onset of hypertrophic differentiation. Similar results were ob tained using cultures of chick limbs. We further investigated the role of BMP signaling in regulating proliferati on and hypertrophic differentiation of chondrocytes and identified three fu nctions of BMP signaling in this process. First we found that maintaining a normal proliferation rate requires BMP and Ihh signaling acting in paralle l. We further identified a role for BMP signaling in modulating the express ion of Ihh. Finally, the application of Noggin to mouse limb explants; resu lted in advanced differentiation of terminally hypertrophic cells, implicat ing BMP signaling in delaying the process of hypertrophic differentiation i tself. This role of BMP signaling is independent of the Ihh/PTHrP pathway.