The evolutionarily conserved Hedgehog (Hh) pathway is essential for normal embryonic development and plays critical roles in adult tissue maintenance, renewal and regeneration. Secreted Hh proteins act in a concentration- and time-dependent manner to initiate a series of cellular responses that range from survival and proliferation to cell fate specification and differentiation.
Proper levels of Hh signaling require the regulated production, processing, secretion and trafficking of Hh ligands– in mammals this includes Sonic (Shh), Indian (Ihh) and Desert (Dhh). All Hh ligands are synthesized as precursor proteins that undergo autocatalytic cleavage and concomitant cholesterol modification at the carboxy terminus and palmitoylation at the amino terminus, resulting in a secreted, dually-lipidated protein. Hh ligands are released from the cell surface through the combined actions of Dispatched and Scube2, and subsequently trafficked over multiple cells through interactions with the cell surface proteins LRP2 and the Glypican family of heparan sulfate proteoglycans (GPC1-6).
Hh proteins initiate signaling through binding to the canonical receptor Patched (PTCH1) and to the co-receptors GAS1, CDON and BOC. Hh binding to PTCH1 results in derepression of the GPCR-like protein Smoothened (SMO) that results in SMO accumulation in cilia and phosphorylation of its cytoplasmic tail. SMO mediates downstream signal transduction that includes dissociation of GLI proteins (the transcriptional effectors of the Hh pathway) from kinesin-family protein, Kif7, and the key intracellular Hh pathway regulator SUFU.
GLI proteins also traffic through cilia and in the absence of Hh signaling are sequestered by SUFU and Kif7, allowing for GLI phosphorylation by PKA, GSK3β and CK1, and subsequent processing into transcriptional repressors (through cleavage of the carboxy-terminus) or targeting for degradation (mediated by the E3 ubiquitin ligase β-TrCP). In response to activation of Hh signaling, GLI proteins are differentially phopshorylated and processed into transcriptional activators that induce expression of Hh target genes, many of which are components of the pathway (e.g. PTCH1 and GLI1). Feedback mechanisms include the induction of Hh pathway antagonists (PTCH1, PTCH2 and Hhip1) that interfere with Hh ligand function, and GLI protein degradation mediated by the E3 ubiquitin ligase adaptor protein, SPOP.
In addition to vital roles during normal embryonic development and adult tissue homeostasis, aberrant Hh signaling is responsible for the initiation of a growing number of cancers including, classically, basal cell carcinoma, edulloblastoma, and rhabdomyosarcoma; more recently overactive Hh signaling has been implicated in pancreatic, lung, prostate, ovarian, and breast cancer. Thus, understanding the mechanisms that control Hh pathway activity will inform the development of novel therapeutics to treat a growing number of Hh-driven pathologies.
We would like to thank Prof. Hans Widlund, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, for reviewing this diagram.
created June 2006
revised July 2014