Investigation of the E3 ubiquitin ligase UBR5; a novel regulator of Hh gene expression
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Date
04/07/2015Author
Kinsella, Elaine
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Abstract
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.