Hedgehog (Hh) signalling underlies essential developmental, homeostatic and tumorigenic processes. Using a murine conditional mutant allele developed in our lab we revealed a role for the N-end rule E3 UBR5 in regulating Hh signalling, tissue homeostasis and oncogenic-Ras-mediated tumourigenesis.
Cre-mediated deletion of UBR5 in the limb mesenchyme resulted in progressive articular cartilage damage, calcification of tendons and ligaments and altered long bone morphometrics. All of the phenotypes are associated with defects in chondrogenesis and are associated with human diseases such as osteoarthritis, calcific tendonitis and heterotopic ossification. In agreement with its important role in regulating chondrogenesis, Hh signalling was significantly altered in UBR5 mutant tissue; with canonical activity increased and GPCR-associated non-canonical activity decreased. Pharmacological or genetic manipulation dramatically enhanced the skeletal-associated phenotypes, which included the formation of huge ectopic bony masses. Importantly, these studies challenge existing dogma and highlight important roles for UBR5 and the less-well studied non-canonical pathway in regulating tissue homeostasis.
Complimentary studies in gastrointestinal tissues supported an important role for UBR5 in normal tissue homeostasis as well as an ability to suppress oncogenic-Ras-mediated tumourigenesis. Intestinal epithelial loss of UBR5 resulted in dramatic perturbation in Hh signalling activity and apoptosis of intestinal crypt cell populations. Intriguingly, these effects were suppressed upon co-expression of oncogenic-Ras. Furthermore, loss of UBR5 function increased oncogenic-Ras-associated tumour burden, and lethality, in both intestinal and pancreatic tumour models.
In summary, our work provides the first in vivo study of mammalian UBR5 function and reveals important genetic interactions between UBR5, Hh and Ras signalling pathways relevant to human disease-associated processes.
Having established robust in vivo evidence, I now wish to investigate the underlying molecular and cellular mechanisms. A key molecular focus will establish how UBR5 is able to (i) suppress the kinase-regulated canonical Hh pathway and (ii) promote the GPCR-associated non-canonical Hh pathways. The plan involves following up physical interactions between UBR5 and key components of both Hh pathways and investigating UBR5’s role as an E3 enzyme. Well-established mesenchymal stem cell- and intestinal organoid-based system will help identify how UBR5 influence chondrogenesis and intestinal cell function, respectively.