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"Robotnikinin", a macrocycle, was discovered when a Harvard University team used a small-molecule microarray to screen for molecules that bind to Sonic hedgehog, the most prominent protein in the pathway.
The Harvard team, led by Dr Stuart Schreiber, decided to look for modulators that act before Smoothened in the hedgehog pathway. Using the small-molecule microarray, they found a new macrocyle that bound directly to the Sonic hedgehog protein. They tested it on human skin cells and a synthetic skin model and found that robotnikinin inhibits Sonic hedgehog signaling in a concentration-dependent manner.”
(a) The structure of robotnikinin, which is the compound that resulted from follow-up chemistry efforts to optimize potency. (b) SPR curve of robotnikinin showing concentration-dependent binding to purified ShhN. Normalized RUs are plotted over a time course. The concentrations plotted are 1.56 M, 3.13 M, 6.25 M, 12.5 M and 25 M, in order of increasing Rus. (c) Inhibition of Gli signaling by robotnikinin in Shh-LIGHT2 cells stimulated with medium containing ShhN palmitoylated at the N terminus, relative to 6.25 M cyclopamine (a small-molecule inhibitor of Smoothened). Shh-LIGHT2 cells stimulated with N-palmitoylated ShhN along with 3.6 M purmorphamine or 100 nM SAG (small-molecule activator of Smoothened) showed negligible inhibition at the indicated concentrations of inhibitor. (d) Robotnikinin lowers levels of endogenous Gli2 mRNA (analyzed by qPCR) in primary human keratinocytes in a dose-dependent manner; this effect is blocked by the co-administration of Smo agonists. Note that there is some Gli expression in the absence of exogenous Shh due to the presence of a basal amount of Shh in the growth medium. (e) When analyzed by qPCR, synthetic human skin displayed Gli1 and Gli2 transcriptional repression in the presence of varying concentrations of robotnikinin. (f) Robotnikinin inhibits the induction of the Shh pathway. Our experiments support a mechanism involving inhibition of the actions of Shh, either directly or indirectly by interfering with a precursor complex. All error bars show s.d.