The Hedgehog (Hh) signalling pathway is essential for embryogenesis and regulating cellular homeostasis in the adult, however much remains unknown about the molecular mechanisms that control ligand expression. In 2002, Lee et al. demonstrated that conditional mutation of the Drosophila hyperplastic discs gene (hyd) resulted in increased hh levels, suggesting that Hyd can act as a negative regulator of hh gene expression. Based on this evidence, the aim of this project was to investigate the hypothesis that UBR5, the murine homologue of Hyd, acts as a novel regulator of Hh gene expression in mammals. To investigate this hypothesis in vivo, I utilized the developing mouse limb as a model system that is highly sensitive to abnormal Hh expression. Morphological analysis of Ubr5 limb mutant embryos did not reveal an obvious phenotype, however quantitative analysis of Ihh gene expression and its downstream targets at E13.5 demonstrated a significant decrease in levels. In addition, changes in the expression of Runx2 and Msx2 were detected. Therefore, these data indicate that UBR5 can act as a positive regulator of Ihh expression, in addition to regulating other factors involved in chondrogenesis. The role for UBR5 as a positive regulator of Hh expression was also supported by in vitro investigations, demonstrating that UBR5 is required for Shh expression in mouse embryonic stem cells. Morphological analysis of adult Ubr5 limb mutant mice revealed the presence of significantly shorter long bones. These observations support previous reports that interference with IHH during early chondrogenesis can negatively affect long bone growth in the adult. Interestingly, adult Ubr5 limb mutant mice also possess osteophytes, a feature typically observed in osteoarthritis (OA), in addition to sites of ectopic mineralization (EM) near tendons of the knee and ankle. Based on these observations and evidence from the literature, I hypothesize that in addition to the role for UBR5 as a positive regulator of Ihh expression in the bone, UBR5 also plays a role in ligament/tendon development and/or maintenance, whereby its loss results in defective ligaments/tendons that are incapable of stabilizing the joints of the limb, culminating in joint deterioration, as observed in OA, in addition to EM. However, further investigation is required to determine whether this is also related to deregulated Ihh. These experiments suggest that Ubr5 limb mutant mice could provide a novel mouse model in the study of OA and prompt the investigation of the potential role for EDD, the human homologue of UBR5, in OA initiation and progression.

No description available.

Mammals are an extremely successful group of tetrapods, and the great diversity of limb morphologies and locomotor modes is underpinned by adaptations in their long bones. Elongation of long bones takes place at each end of the bone shaft, at the epiphyseal growth plate surface. Secondary ossification centres form bony epiphyses above these growth plates, eventually fusing with the plates and marking the end of bone elongation. The proximal growth plate surface in the humerus is not uniform, with varying distributions of peaks and grooves that appear to differ across Mammalia. Although the process of growth plate bone deposition is well understood, there has been limited research on the growth plate surface morphology and what factors may be influencing it, such as ecology, phylogeny, or ontogeny. Using synchrotron propagation phase-contrast X-ray microtomography to reconstruct 3D models of the growth plate surface of the proximal humerus. We applied a metric called ariaDNE to quantify and measure the curvatures of these surfaces. We studied several extant mammals and one fossil stem mammal. We found that there was a high diversity of growth plate morphologies across mammal species, but the overall morphology remained consistent within the same species. The morphology also appears to shift throughout different ontogenetic stages, with a trend towards higher complexity in later developmental stages. The fossil specimen, G. planiceps, has a growth plate morphology that is remarkably similar to the short-beaked echidna (Tachyglossus aculeatus). Preliminary statistical tests suggest that growth plate morphology may be influenced by phylogeny and locomotory mode, although our small dataset has likely introduced sampling bias and the sample size should be expanded to find significant patterns. Growth plate surface morphology should be further explored due to its potential palaeobiological applications for the interpretation of ecology and evolution of extinct taxa.

No description available.