Sonic hedgehog (Shh), a vertebrate homologue of the Drosophila segment polarity gene Hedgehog, is required for patterning in the developing central nervous system, somite, and limb. It is produced as a 47-49 kDa secreted holoprotein that is autocatalytically cleaved to yield a 19 kDa N-terminal fragment that accounts for all known hedgehog biological activity (residues 25-198 in mouse Shh) and a 29-31 kDa C-terminal fragment that contains the autoprocessing machinery.1-3 Shh is believed to act both as a short range, contact-dependent inducer and as a long range, diffusible morphogen. A lipid tether, which anchors the 19 kDa fragment to the external surface of the plasma membrane, is critical for the short range biological activity of this molecule by restricting the tissue localization of the Shh signal. The lipid tether consists of cholesterol, which is concomitantly attached to the C-terminus of the 19 kDa fragment during autoproteolysis, (see Figure 1 panel A) and palmitic acid, which is attached to the N-terminal Cys residue (Cys25 in mShh).4 Lipid modification of Shh also increases the bioactivity of Shh relative to the unmodified protein in a cell-based signaling assay.4,5
Cholesterol modification of Shh is essential for mediating long range signaling.6,7 In a study by Zeng et al.,6 they provide evidence for a freely diffusible form of Shh (s-ShhNp) that is cholesterol modified, multimeric, and biologically potent (see Figure 1 panel B). s-ShhNp forms a gradient across the anterior-posterior axis of the chick limb. RT-PCR experiments confirm that Shh activity observed in the anterior and central fragments of limb tissue is not due to upregulation of endogenous Shh mRNA, thus suggesting that s-ShhNp diffuses from its site of synthesis providing long range patterning information. Cholesterol modification of Shh also influences Shh-mediated patterning of mouse digits.7 Comparison of biological activity of Shh with or without cholesterol indicates that signaling is restricted (i.e. posterior localization) with the cholesterol-lacking form of Shh.7
|Figure 1. A lipid tether, cholesterol, attached to the C-terminus of the 19 kDa fragment of Sonic hedgehog (Shh) anchors it to the plasma membrane and restricts the tissue localization of the Shh signal (panel A). A freely diffusible form of Shh (s-ShhNp) that is cholesterol modified, multimeric and biologically potent is represented in panel B.
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