Oxysterols are a class of endogenous signaling molecules that can activate the Hedgehog pathway, which plays critical roles in development, regeneration and cancer. However, it has been unclear how oxysterols influence Hedgehog signaling, including whether their effects are mediated through a protein target or indirectly through effects on membrane properties. To answer this question, we synthesized the enantiomer and an epimer of the most potent oxysterol, 20(S)-hydroxycholesterol. Using these molecules, we show that the effects of oxysterols on Hedgehog signaling are exquisitely stereoselective, consistent with their function through a specific protein target. We present several lines of evidence that this protein target is the 7-pass transmembrane protein Smoothened, a major drug target in oncology. Our work suggests that these enigmatic sterols, which have multiple effects on cell physiology, may act as ligands for signaling receptors and provides a generally applicable framework for probing their mechanism of action.

The Hedgehog (Hh) signal is transduced across the membrane by the heptahelical protein Smoothened (Smo), a developmental regulator, oncoprotein and drug target in oncology. We present the 2.3 Å crystal structure of the extracellular cysteine rich domain (CRD) of vertebrate Smo and show that it binds to oxysterols, endogenous lipids that activate Hh signaling. The oxysterol-binding groove in the Smo CRD is analogous to that used by Frizzled 8 to bind to the palmitoleyl group of Wnt ligands and to similar pockets used by other Frizzled-like CRDs to bind hydrophobic ligands. The CRD is required for signaling in response to native Hh ligands, showing that it is an important regulatory module for Smo activation. Indeed, targeting of the Smo CRD by oxysterol-inspired small molecules can block signaling by all known classes of Hh activators and by clinically relevant Smo mutants.DOI: http://dx.doi.org/10.7554/eLife.01340.001

The main focus of this perspective lies in the discussion of the recent mechanistic theories and supporting experimental evidences that have been put forth in an attempt to advance our understanding of the factors affecting chemical glycosylation.

The strategic placement of common protecting groups led to the discovery of a new method for "superarming" glycosyl donors. Conceptualized from our previous studies on the O-2/O-5 Cooperative Effect, it was determined that S-benzoxazolyl glycosyl donors possessing both a participating moiety at C-2 and an electronically armed lone pair at O-5, such as the superarmed glycosyl donor shown above, were exceptionally reactive.

A complementary concept for superarming glycosyl donors through the use of common protecting groups was previously discovered with S-benzoxazolyl (SBox) glycosyl donors. As this strategy can be of benefit to existing oligosaccharide methodologies, it has now been expanded to encompass a wide array of common, stable glycosyl donors. The versatility of this developed technique has been further illustrated in application to a sequential chemoselective oligosaccharide synthesis, wherein a superarmed ethyl thioglycoside was incorporated into the conventional armed-disarmed strategy.

Recently, we discovered a novel method for "superarming" glycosyl donors. Herein, this concept has been exemplified in one-pot oligosaccharide syntheses, whereby the superarmed glycosyl donor was chemoselectively activated over traditional "armed" and disarmed glycosyl acceptors. Direct side-by-side comparison of the reactivities of the classic armed and superarmed glycosyl donors further validates the credibility of the novel concept.

It is reported that stable glycosyl sulfonium salts can be generated via direct anomeric S-methylation of ethylthio glycosides. Mechanistically, this pathway represents the first step in the activation of thioglycosides for glycosidation, however, it can further allow for the synthesis and isolation of quasi-stable sulfonium ions, representing a new approach for studying these key intermediates.

The accumulation of psychosine (galactosyl sphingosine) has been associated with the pathogenesis of Krabbe disease, however, the exact mechanism of its cytotoxicity remains unclear. Herein, we describe the synthesis of the unnatural enantiomer of erythrosphingosine, psychosine, and related derivatives thereof that would allow for the mechanistic elucidation of the toxicity of psychosine.