The proteins of the Hedgehog (Hh) family function during development as morphogens, so their concentration and distribution must be tightly regulated. Using Drosophila melanogaster, two groups now report in Development that lipid modifications have an important role in Hh signalling by affecting multimerization, Hh spreading and signalling activity.

Establishing the molecular mechanisms that generate the Hh gradient is essential for our understanding of how the Hh signal elicits multiple responses in a temporally and spatially specific manner. The Hh spreading is particularly intriguing, because Hh is a lipid-modified molecule, and lipid-modified proteins are usually membrane tethered. Gallet et al. and Callejo et al. sought to investigate the contradictory findings that have been previously reported about the role of lipid modifications in Hh signalling, in particular between vertebrates and D. melanogaster.

Gallet et al. investigated whether the cholesterol group of Hh is important for its long-range activity. They compared the behaviour of lipidated Hh with that of a non-cholesterol-modified Hh (Hh-N) form in three different epithelial tissues and found that the absence of cholesterol affects the secretion of Hh, its multimerization and its long-range signalling activity. For example, distant cell types in the dorsal ectoderm, which require low Hh levels, are absent in Hh-N-expressing embryos, which indicates that the range of activity of Hh-N is limited. Based on these and other findings, the authors proposed that, as in vertebrates, cholesterol modification is required for the controlled planar movement of Hh, thereby preventing the unrestricted spreading of the protein in D. melanogaster.

In another study, Callejo and colleagues analysed the role of Hh lipid modifications (cholesterol and palmitic acid) during Hh-gradient formation in the D. melanogaster imaginal discs. They found that dual lipid modifications are essential for the interaction between Hh with heparan sulphate proteoglycans (HSPGs), and that this interaction retains and stabilizes wild-type Hh within the epithelium to control spreading and proper signalling. Unlipidated Hh formed more extensive gradients, spreading for many more cell diameters than wild-type Hh. Cholesterol-unmodified Hh induced the same targets that respond to low levels of Hh as wild-type Hh independently of HSPGs. However, the activation of the targets that respond to high Hh levels required both HSPGs function and lipid modifications. The Hh forms are also internalized differently — wild-type Hh is taken up by the Hh receptor Patched through the basolateral membrane, whereas unlipidated Hh is taken up mainly by an unknown receptor through the apical membrane.

Further work is now required to resolve how lipid modification affects the precise range and activity of Hh in different D. melanogaster, as well as mammalian tissues.