Patched, the receptor of Hedgehog, is a lipoprotein receptor

Abstract

The Hedgehog (Hh) family of secreted signaling proteins has a broad variety of functions during metazoan development and implications in human disease. Despite Hh being modified by two lipophilic adducts, Hh migrates far from its site of synthesis and programs cellular outcomes depending on its local concentrations. Recently, lipoproteins were suggested to act as carriers to mediate Hh transport in Drosophila. Here, we examine the role of lipophorins (Lp), the Drosophila lipoproteins, in Hh signaling in the wing imaginal disk, a tissue that does not express Lp but obtains it through the hemolymph. We use the up-regulation of the Lp receptor 2 (LpR2), the main Lp receptor expressed in the imaginal disk cells, to increase Lp endocytosis and locally reduce the amount of available free extracellular Lp in the wing disk epithelium. Under this condition, secreted Hh is not stabilized in the extracellular matrix. We obtain similar results after a generalized knock-down of hemolymph Lp levels. These data suggest that Hh must be packaged with Lp in the producing cells for proper spreading. Interestingly, we also show that Patched (Ptc), the Hh receptor, is a lipoprotein receptor; Ptc actively internalizes Lp into the endocytic compartment in a Hh-independent manner and physically interacts with Lp. Ptc, as a lipoprotein receptor, can affect intracellular lipid homeostasis in imaginal disk cells. However, by using different Ptc mutants, we show that Lp internalization does not play a major role in Hh signal transduction but does in Hh gradient formation.Drosophila ͉ LDLR ͉ morphogen

The cell-surface proteins Dally-like and Ihog differentially regulate Hedgehog signaling strength and range during development

“…These data argue that colocalization of Dlp with Ptc in endocytic vesicles is not essential for Dlp function in Hh signaling. Consistent with this view, several studies have shown that endocytosis is not essential for Hh signaling in Drosophila wing discs (Callejo et al, 2008;Han et al, 2004b;Torroja et al, 2004). Our conclusion differs from a recent publication arguing that the GPI anchor of Dlp is required for its function in Hh signaling (Gallet et al, 2008).…”

Patched regulates Smoothened trafficking using lipoprotein-derived lipids

“…Hh associates with the Drosophila lipoprotein particle Lipophorin (Lpp) (Panáková et al, 2005) and these particles are necessary for normal Hh signaling (Callejo et al, 2008;Panáková et al, 2005). Here, we show that specific lipids contained within Lpp are required to decrease Smo levels and Smo-dependent Ci 155 stabilization in the absence of Hh.…”

“…Indeed, Lpp knockdown narrows the range of Hh target gene activation (Fig. 1Q,R) (Callejo et al, 2008;Panáková et al, 2005). Despite the reduced range of target gene activation in LppRNAi discs, Smo actually accumulates on the basolateral membrane as it would if Ptc activity were reduced.…”

“…4A-C; see also Fig. S3K-M in the supplementary material) (Callejo et al, 2008). Ptc overexpression does not obviously alter the size or number of either Rab5-or Rab7-positive endosomes, suggesting that it does not generally retard progression from early to late endosomes (see Fig.…”

“…, also fails to cause Lpp accumulation when overexpressed (Callejo et al, 2008), suggesting that Ptc must be internalized to cause Lpp accumulation in endosomes. Although the SSD is required for Smo repression, it is not required for the accumulation of Lpp in endosomes with Ptc (Fig.…”