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Reading the Hedgehog morphogen gradient by measuring the ratio of bound to unbound Patched protein

Abstract

Morphogens are ‘form-generating’ substances that spread from localized sites of production and specify distinct cellular outcomes at different concentrations. A cell's perception of morphogen concentration is thought to be determined by the number of active receptors, with inactive receptors making little if any contribution1. Patched (Ptc)2,3,4,5, the receptor for the morphogen Hedgehog (Hh)6,7,8,9,10,11,12, is active in the absence of ligand and blocks the expression of target genes by inhibiting Smoothened (Smo), an essential transducer of the Hh signal3,13,14,15,16. Hh binding to Ptc abrogates the ability of Ptc to inhibit Smo, thereby unleashing Smo activity and inducing target gene expression2,3,12,13,14,15,16. Here, we show that a cell's measure of ambient Hh concentration is not determined solely by the number of active (unliganded) Ptc molecules. Instead, we find that Hh-bound Ptc can titrate the inhibitory action of unbound Ptc. Furthermore, we demonstrate that this effect is sufficient to allow normal reading of the Hh gradient in the presence of a form of Ptc that cannot bind the ligand12 but retains its ability to inhibit Smo. These results support a model in which the ratio of bound to unbound Ptc molecules determines the cellular response to Hh.

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Figure 1: Generation of clones expressing low (L > PΔ2), medium (M > PΔ2) and high (H > PΔ2) levels of PtcΔloop2.
Figure 2: Different amounts of PtcΔloop2 are required to inhibit the Hh pathway depending on the presence or absence of endogenous Ptc.
Figure 3: Inhibition of Hh transduction by PtcΔloop2 is blocked by co-expression of Ptc+ and Hh.
Figure 4: Estimating the change in the ratio of liganded to unliganded Ptc necessary to distinguish on and off states of the Hh transduction pathway.

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Acknowledgements

We thank A. Adachi for generating transgenic flies; X.-J. Qiu for technical assistance; and R. Axel, E. Gouaux, I. Greenwald, T. Jessell, L. Johnston, R. Mann and A. Tomlinson for discussion and advice on the manuscript. A.C. is a Postdoctoral Associate and G.S. is an Investigator of the Howard Hughes Medical Institute.

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Correspondence to Gary Struhl.

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Casali, A., Struhl, G. Reading the Hedgehog morphogen gradient by measuring the ratio of bound to unbound Patched protein. Nature 431, 76–80 (2004). https://doi.org/10.1038/nature02835

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