Capping Enzyme MRNA-cap/RNGTT Regulates Hedgehog Pathway Activity By Antagonizing Protein Kinase A

Hedgehog(Hh)was first identified as a segment polarity gene in affecting the segmentation pattern of the Drosophila larva with its deleted mutation.The Hh signaling plays pivotal roles in animal embryonic development including proliferation,cell-cell communication,pattern formation and adult tissue homeostasis in both vertebrates and invertebrates.In addition,deregulation of Hh signaling activity has been implicated in various human disorders including congenital anomalies and several types of cancers.Protein Kinase A(PKA)modulates Hh signaling activity through phosphorylating the transcription factor Cubitus interruptus(Ci)and G protein coupled receptor(GPCR)family protein Smoothened(Smo)in Drosophila,but how PKA activity is regulated remains elusive.In this study,we identify a novel regulator of the Hh pathway,the capping-enzyme mRNA-cap,which positively regulates Hh signaling activity through modulating PKA activity.We provide genetic and biochemical evidence that mRNA-cap inhibits PKA kinase activity to promote Hh signaling.Interestingly,regulation of Hh signaling by mRNA-cap depends on its cytoplasmic capping-enzyme activity.In addition,we show that the mammalian homolog of mRNA-cap,RNGTT,can replace mRNA-cap to play the same function in the Drosophila Hh pathway and that knockdown of Rngtt in cultured mammalian NIH/3T3 cells compromised Shh pathway activity,suggesting that RNGTT is functionally conserved.Our study makes an unexpected link between the mRNA capping machinery and the Hh signaling pathway,unveils a new facet of Hh signaling regulation,and reveals a potential drug target for modulating Hh signaling activity.