Identification of TULP3 as a negative regulator of Hedgehog signalling in the mouse
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The Hedgehog (Hh) family of secreted signalling factors play diverse roles in animal development. In mammals, the Hh ortholog Sonic hedgehog (Shh) is critical for the proper formation of the limbs, central nervous system, and axial skeleton, amoung other tissues. Mutations affecting the function of this pathway during development have severe consequences to the developing embryo and can cause birth defects in humans. Inappropriate activation of the pathway in adult tissues has also been implicated in several human cancers. In recent years several unexpected regulatory factors of the pathway during embryogenesis and in the adult have emerged through genetic studies in the mouse, such as proteins involved in vesicle transport and in the formation and function of primary (non-motile) cilia. Evidence is presented here that the mouse Tubby gene family member Tubby-like protein 3 (Tulp3) plays an important negative regulatory function in the Hh signalling pathway during embryogenesis, a role not previously associated with the Tubby proteins. Embryos lacking Tulp3 develop severe neural tube defects and polydactyly, along with ectopic activation of Shh target genes in the developing limbs and CNS, and altered Shh mediated axon guidance in the developing spinal cord. Moreover, Tulp3 was found to act largely independently of Shh, as compound Tulp3/Shh mutant embryos retain expression of Shh target genes and related abnormalities. Finally, the Tulp3 protein was found to localize to the primary cilium in cultured cells, implicating Tulp3, and possibly other Tubby proteins as regulators of cilium based signalling. These results have important implications in the understanding of the regulation of the Hh pathway, and in the emergence of Hh related birth defects and tumourigenesis.