The objective was to determine the role of Hedgehog signalling in UBR5-associated heterotopic tendon ossification. Our studies into the N-end Rule Ubiquitin-protein ligase UBR5 revealed its role in controlling heterotopic tendon ossification in the mouse limb and hypothesise that UBR5 regulates stem/progenitor behaviour to control tendon homeostasis. Spatiotemporal skeletal development is tightly regulated to maintain a functioning skeleton. However, in certain diseases heterotopic ossification (HO) can occur in soft tissue. Using Prx-Cre combined with a floxed UBR5 mutant allele (UBR5mt) we deleted UBR5 function in the developing murine embryonic limb bud. Micro-CT analysis revealed HO in homozygous UBR5mt adult animals and histological analysis identified numerous chondrocytes within the tendon midbody. HO was progressive, identified in multiple tendons and first detected at 6 weeks of age. Importantly, no HO was observed in the Prx-Cre control animals (n=18). Our work in other tissues indicates UBR5 as an important regulator of stem/progenitor cell function, with UBR5 being highly upregulated in pericytes, the progenitors of mesenchymal stem cells. Furthermore, we revealed that UBR5 regulates both Hedgehog (HH) ligand production and signal transduction. Indian Hedgehog signalling plays a central role in controlling stem/progenitor cell function in various tissues including the adult skeleton. UBR5mt animals treated with the HH pathway antagonist cyclopamine (n=3) resulted in an enhanced level of HO in comparison with those treated with an inactive cyclopamine analog (n=4). Cyclopamine treatment in a wild-type background did not promote HO, suggesting that a UBR5mt background sensitises tendons to HH pathway inhibition and ossification. We therefore conclude that UBR5 and HH signalling normally act to suppress heterotopic tendon ossification. In conclusion, UBR5 regulates heterotopic tendon ossification through regulating HH signalling.
Disclosure: The authors declared no competing interests. This work was supported by The University of Edinburgh.