title: Absolute requirement of cholesterol binding for Hedgehog gradient formation in Drosophila.
abstract: How morphogen gradients are shaped is a major question in developmental biology, but remains poorly understood. Hedgehog (Hh) is a locally secreted ligand that reaches cells at a distance and acts as a morphogen to pattern the Drosophila wing and the vertebrate neural tube. The proper patterning of both structures relies on the precise control over the slope of Hh activity gradient. A number of hypotheses have been proposed to explain Hh movement and hence graded activity of Hh. A crux to all these models is that the covalent binding of cholesterol to Hh N-terminus is essential to achieve the correct slope of the activity gradient. Still, the behavior of cholesterol-free Hh (Hh-N) remains controversial: cholesterol has been shown to either increase or restrict Hh range depending on the experimental setting. Here, in fly embryos and wing imaginal discs, we show that cholesterol-free Hh diffuses at a long-range. This unrestricted diffusion of cholesterol-free Hh leads to an absence of gradient while Hh signaling strength remains uncompromised. These data support a model where cholesterol addition restricts Hh diffusion and can transform a leveled signaling activity into a gradient. In addition, our data indicate that the receptor Patched is not able to sequester cholesterol-free Hh. We propose that a morphogen gradient does not necessarily stem from the active transfer of a poorly diffusing molecule, but can be achieved by the restriction of a highly diffusible ligand.
authors.affiliation: LBMC, UMR5239 CNRS/Ecole Normale Supérieure de Lyon, SFR 128 Biosciences Lyon Gerland, Université de Lyon , 69364 Lyon Cedex 07 , France.
authors.forename: Antoine
authors.initials: A
authors.lastname: Ducuing
authors.forename: Bertrand
authors.initials: B
authors.lastname: Mollereau
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Axelrod
authors.forename: Stephane
authors.initials: S
authors.lastname: Vincent
keywords: Cholesterol; Drosophila; Gradient; Hedgehog; Patterning