Hh signaling and cholesterol metabolism
The Hedgehog (Hh) signaling pathway orchestrates key events in embryonic and post-natal development across the metazoans. Its dysregulation leads to the profound congenital deformities observed in holoprosencephaly and brachydactyly, and is responsible for several human cancers, including basal cell carcinoma and juvenile medulloblastoma. Multiple lines of biochemical, cellular, and patient-based data indicate that Hh pathway components use cholesterol and its oxygenated derivatives as currency to communicate with downstream effectors.
Hh signaling is initiated by the secreted hedgehog morphogens (HH)
Strikingly, Hedghog ligands are the only metazoan proteins known to be covalently attached to cholesterol. This modification is appended in an autoprocessing mechanism analogous to intein splicing in bacteria– a process that is unique among higher organisms. During embryogenesis, cholesteroylation dictates the spread of the Hh morphogen and its roles in anterior/porsterior patterning; however, the molecular mechanisms by which cholesterol controls the Hh signaling gradient remain uncharacterized.
Hh ligands are internalized by the receptor Patched (PTCH)
Patched is a 12-pass transmembrane protein that possesses a sterol-sensing domain and shows structural homology to cholesterol transporters. Its unique structural features and ability to repress pathway in a catalytic manner have led to the hypothesis that Ptch translocates an inhibitory cholesterol metabolite. Despite the central roles of Ptch as a developmental regulator and tumor suppressor, the identity of an endogenous sterol molecule transported by Ptch has not been elucidated.
Binding of HH ligands to PTCH releases catalytic suppression of Smoothened (SMO)
Smoothened is a GPCR-like protein that undergoes phosphorylation and traffics to to the primary cilium upon activation. A discrete collection of regio- and stereochemically defined oxysterols regulate Hh signaling by directly binding to Smo at a unique structural domain. Despite evidence for oxysterol-regulated Smo activity in normal and pathological Hh signaling, the links between oxysterol trafficking and Smo activation are poorly understood.
Graded activation of the Gli transcription factors coordinates gene expression involved in normal embryogenesis and Hh pathway-driven cancers. We use sophisticated chemical, genetic, and proteomic tools to define interactions between cholesterol metabolism and Hh signaling, offering new therapeutic strategies for Hh pathway-driven disease.