Ubr3, a Novel Modulator of Hh Signaling Affects the Degradation of Costal-2 and Kif7 through Poly-ubiquitination

Hedgehog (Hh) signaling regulates multiple aspects of metazoan development and tissue homeostasis, and is constitutively active in numerous cancers. We identified Ubr3, an E3 ubiquitin ligase, as a novel, positive regulator of Hh signaling in Drosophila and vertebrates. Hh signaling regulates the Ubr3-mediated poly-ubiquitination and degradation of Cos2, a central component of Hh signaling. In developing Drosophila eye discs, loss of ubr3 leads to a delayed differentiation of photoreceptors and a reduction in Hh signaling. In zebrafish, loss of Ubr3 causes a decrease in Shh signaling in the developing eyes, somites, and sensory neurons. However, not all tissues that require Hh signaling are affected in zebrafish. Mouse UBR3 poly-ubiquitinates Kif7, the mammalian homologue of Cos2. Finally, loss of UBR3 up-regulates Kif7 protein levels and decreases Hh signaling in cultured cells. In summary, our work identifies Ubr3 as a novel, evolutionarily conserved modulator of Hh signaling that boosts Hh in some tissues.
Author Summary Hedgehog signaling regulates many important biological processes and has been linked to developmental disorders, wound healing, and cancer. Although the major components in the pathway have been well studied in Drosophila and vertebrates, how the signaling is regulated by different modulators is not well understood. Here, we take advantage of a fly forward genetic screen to isolate Ubr3. We show that it is a novel modulator that regulates Hh signaling. Loss of ubr3 leads to Hh signaling defects in developing eyes of Drosophila, and affects eye, and somite and sensory neuron development in zebrafish. However, Hh signaling is not affected in all cells known to be dependent on Hh signaling as loss of ubr3 in the fly wing and zebrafish inner ear are not affected. This suggests that Ubr3 is a modulator that is only required in some Hh dependent organs/cells. We have shown that Ubr3 down-regulates the levels of Cos2 and its mammalian homolog Kif7, key negative regulators of Hh signaling, through poly-ubiquitination. The poly-ubiquitination of Cos2 by Ubr3 is enhanced by Hh activation, suggesting that it functions in a positive feedback that modulates Hh activation.