Editors' ChoicePhysiology

Keeping Bone Where Bone Belongs

Sci. Signal.  19 Nov 2013:
Vol. 6, Issue 302, pp. ec281
DOI: 10.1126/scisignal.2004918

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The skeleton is a dynamic and tightly regulated organ. Two rare genetic disorders, fibrodysplasia ossificans progressive (FOP) and progressive osseous heteroplasia (POH), cause extraskeletal bone formation. Whereas activating mutations in GNAS (encoding the heterotrimeric G protein subunit Gαs) induce Wnt signaling to cause fibrous dysplasia, Regard et al. found that inactivating mutations in GNAS specifically induced Hedgehog signaling to cause heterotopic bone formation in POH. In mice, deletion of Gnas in limb mesenchymal progenitor cells caused their osteogenic differentiation, webbing between the toes, fused joints, and heterotopic ossification in soft tissues that progressed until death for most mutant pups by postnatal day 20. Loss of Gnas induced the expression of Hedgehog target genes (Ptch1, Gli1, and Hhip) in the limbs of mutant mice and in isolated cultures of bone marrow stem cells (BMSCs) and subcutaneous mesenchymal progenitors (SMPs). However, the abundance of cyclic adenosine monophosphate (cAMP) and phosphorylated CREB [an indicator of protein kinase A (PKA) activity], as well as the expression of Wnt target genes, was lower in Gnas-deficient limbs, BMSCs, and SMPs. Pharmacologically increasing cAMP concentration rescued PKA and CREB activation in Gnas-deficient SMPs. Expression of a dominant-negative PKA mimicked the effects of Gnas deletion on Hedgehog target genes, suggesting that Gαs signaling suppressed Hedgehog signaling through activation of cAMP and PKA. In bud cultures from Gnas-deficient limbs, cyclopamine [an inhibitor of the Hedgehog receptor smoothened (Smo)] could not repress Hedgehog target gene expression, whereas either arsenic trioxide or GANT-58 (inhibitors of the Hedgehog-induced Gli transcription factors) could. Further, injection of Gli inhibitors or deletion of Gli2 in Gnas-deficient mice decreased the extent of soft tissue mineralization. Retroviral-activation of Smo in Gnas-deficient mice was sufficient to induce heterotopic ossification and osteoblast differentiation, indicating that Hedgehog signaling alone is sufficient to cause the POH phenotype and that it must be actively suppressed by Gαs to spatially restrict bone formation. These findings, combined with the previous observations in FOP, exemplify the central placement of Gαs in limiting osteoblast differentiation in mesenchymal stem cell pools through independent mechanisms.

J. B. Regard, D. Malhotra, J. Gvozdenovic-Jeremic, M. Josey, M. Chen, L. S. Weinstein, J. Lu, E. M. Shore, F. S. Kaplan, Y. Yang, Activation of Hedgehog signaling by loss of GNAS causes heterotopic ossification. Nat. Med. 19, 1505–1512 (2013). [PubMed]