Ciliary intraflagellar transport protein 80 balances canonical versus non-canonical hedgehog signaling for osteoblast differentiation

Authors: YUAN, XUE - University Of Buffalo; Cao, Jay; HE, XIAONING - University Of Buffalo; WANG, CHANGDONG - University Of Buffalo; SERRA, ROSA - University Of Alabama; QU, JUN - University Of Buffalo; CAO, XU - Johns Hopkins University School Of Medicine; YANG, SHUYING - University Of Buffalo

Submitted to: Nature Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2016
Publication Date: 3/21/2016
Citation: Yuan, X., Cao, J.J., He, X., Wang, C., Serra, R., Qu, J., Cao, X., Yang, S. 2016. Ciliary intraflagellar transport protein 80 balances canonical versus non-canonical hedgehog signaling for osteoblast differentiation. Nature Communications. doi:10.1038/ncomms11024.

Interpretive Summary: Intraflagellar transport (IFT) proteins play an important role in development of diseases in humans such as brain cancer, blindness, diabetes, severe cranial malformations. Some studies indicated that IFT proteins are highly involved in bone development and mutation of these proteins induced skeletal abnormalities. We successfully generated conditional knockout mice lacking osteoblast-specific IFT80, one of the IFT complex B proteins. Mice with deletion of IFT80 showed apparent growth retardation, decreased bone mass, and increased length of hypertrophic cartilage. IFT80 knockout mice lost osteogenic capacity with significantly decreased alkaline phosphatase activity and mineralization. Deletion of IFT80 also dramatically reduced the expression of bone formation markers such BMP2, Osx, osteocalcin, and RUNX2. Our results suggest that IFT80 is a potential therapeutic target for the treatment of bone diseases.

Technical Abstract: Mutation of different IFT proteins cause numerous different clinical bone disorders accompanied with or without the disruption of cilia formation. Currently, there is no any effective treatment for these disorders due to lack of understanding in the function and mechanism of these proteins. IFT80 is one of the IFT complex B proteins. Partial mutation of IFT80 can cause one of two diseases with severe bone abnormalities in humans - Jeune asphyxiating thoracic dystrophy or short rib polydactyly type III. To understand the role and mechanism by which IFT80 regulates osteoblast (OB) differentiation, and bone development in vivo, we generated IFT80 conditional knockout mice (IFT80f/f); specifically, we deleted IFT80 in osteoblast precursor cells (OPCs) using the osterix-Cre (Osx-cre) transgenic line. OB-specific deletion of IFT80 resulted in apparent growth retardation, markedly decreased bone mass, and increased length of hypertrophic cartilage. About 70% of OB progenitor cells derived from the OSX;IFT80f/f calvarial bone showed either loss of cilia or shortened cilia. These cells completely lost osteogenic capacity with significantly decreased ALP activity and mineralization compared to IFT80f/f control cells. Deletion of IFT80 dramatically reduced the expression of BMP2, Osx, osteocalcin, and RUNX2. Ectopic expression of BMP2 or RUNX2 partially restored osteogenic differentiation ability in IFT80-deficiency OPCs. Loss of IFT80 also significantly inhibited the transcription of the Hh target genes Gli1 and Patched, as well as Gli2 biological activity, a critical transcriptional factor in Hh canonical signaling pathway; but dramatically elevated the activity of RhoA, the important downstream effector of non-canonical Hh signaling pathway. Furthermore, overactive RhoA inhibited OB differentiation also through increasing stress fiber density, and phosphorylation of MLC2 and Cofilin. Reorganization of stress fibers with cytochalasin D, or inhibition of Gai or Phosphoinositide 3-kinase, ROCK activity with pertussis toxin, LY294002 and Y-27632 partially restored osteogenic differentiation in IFT80-deficiency OPCs by blocking RhoA activation. Our results revealed a new mechanism through which IFT80 balances the Hh-Gli2 canonical and Hh-Gai-RhoA non-canonical pathways in OB differentiation and bone development, suggesting IFT80 is a potential therapeutic target for the treatment of bone diseases.