Regulation Of Sonic Hedgehog Signaling Pathway By Sufu In The Nucleus

Hedgehog (Hh) gene was originally identified as a segment polarity gene in Drosophila. There are three Hh genes in vertebrates:Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). The classic Shh pathway is composed of Shh ligand, two transmembrane receptors Patched (Ptch) and Smoothened (Smo), and the downstream transcription factors Gli protein family (Gli1, Gli2, Gli3). During vertebrate embryonic development, Shh pathway plays an important role in development of neural tube, axial skeleton, and limb. Abnormal Shh pathway will lead embryo anamorphosis. Moreover, disorders in Hh pathway and mutations of Hh signaling pathway components will lead tumorigenesis and tumor development, such as basal cell carcinoma, lung, prostate, oral, and digestive tract cancers and so on. Strengthen the Shh signaling pathway detailed understanding will help us to regulate Shh signal better and understand the relationship between Shh pathway regulation and embryonic development, as well as tissue tumors.Suppressor of fused (Sufu) is one negative regulator of Hh signaling and play a central role in controlling the vertebrate Hh signaling pathway.With respect to the roles of Sufu in regulating the activity of Hh signaling pathway in mammalian, there are so far mainly two different models. One is based on the fact that Sufu can sequester Gli1 in the cytoplasm, thereby restricting the transcription activity of Gli1 in the nucleus. Some evidences suggest human Sufu normally shuttles between the nucleus and cytoplasm. All these data suggest that Sufu functions to tether Gli1 to a cytoplasmic anchor or it may facilitate exporting Gli1 from the nuclear to the cytoplasm, resulting in the repression of Gli1 transcriptional activity. Several nuclear export signals (NES) identified in the carboxyl region of Sufu seems support this. The second model is that Sufu may function as transcription corepressor of Glis. The nuclear protein SAP18 can interact with Sufu specifically and SAP 18 is a binding partner of mSin3 protein, which in association with histone deacetylase (HDAC) forms a corepressor of transcription. So it is possible that Sufu may repress Gli1 mediated transcription by recruiting mSin3-HDAC corepressor in the nucleus.In order to know further what effects on Hh signaling pathway activity and the mechanism when Sufu is in the nucleus, we added nuclear localization signal (NLS) to Sufu protein. When expressed the Sufu tagged NLS in Sufu null cells, Sufu protein is completely in the nucleus. The results suggest that Sufu is more able to repress Hh pathway activity when it is in the nucleus. This is not because Sufu stabilize the transcription factor Gli3 truncated form Gli3R, but it inhibit Glil transcriptional activity to suppress target gene expression. So we provide a new evidence that Sufu can regulate Hh pathway activity in the nucleus. Meanwhile, We constructed Sufu-5NLS-GFP strain of mice, providing a good model to study the physiological function of Sufu in the nucleus.