Sonic Hedgehog
Sonic hedgehog (SHH) is the most widely expressed of the three mammalian Hedgehog (Hh) family secreted morphogens—alongside Indian hedgehog (IHH), which regulates endochondral bone growth-plate chondrocyte proliferation, and Desert hedgehog (DHH), which directs male Leydig-cell and peripheral nerve perineurial differentiation.
Sonic hedgehog (SHH) is the most widely expressed of the three mammalian Hedgehog (Hh) family secreted morphogens—alongside Indian hedgehog (IHH), which regulates endochondral bone growth-plate chondrocyte proliferation, and Desert hedgehog (DHH), which directs male Leydig-cell and peripheral nerve perineurial differentiation. First discovered as the Drosophila melanogaster segment-polarity locus hedgehog (hh) by Christiane Nüsslein-Volhard and Eric Wieschaus in 1980, and cloned in vertebrates in 1993 by Clifford Tabin, Philip Ingham, and Andrew McMahon's laboratories, Sonic hedgehog acts as a graded, concentration-dependent morphogen that patterns the ventral neural tube, the anteroposterior axis of the developing tetrapod limb bud (from the posterior Zone of Polarizing Activity, ZPA), the bilateral separation of the prosencephalic forebrain and eye fields, and hair-follicle / tooth morphogenesis.
In vertebrates, canonical Sonic hedgehog signal transduction depends on the primary cilium and operates through a double-negative relief-of-repression circuit: in the absence of SHH, the 12-pass transmembrane sterol transporter Patched-1 (PTCH1) inhibits the 7-transmembrane GPCR-like transducer Smoothened (SMO), causing the transcription factors GLI2 and GLI3 to be proteolytically cleaved into C-terminally truncated transcriptional repressors (GLI3R). Binding of dual-lipidated SHH-Np to PTCH1 relieves SMO inhibition, driving SMO accumulation inside the primary cilium to convert full-length GLI2 and GLI3 into nuclear transcriptional activators (GLI2A/GLI3A).
Autoproteolytic Cholesterol Attachment and N-Terminal Palmitoylation
Sonic hedgehog is synthesized in the endoplasmic reticulum as a ~45 kDa precursor polypeptide consisting of an N-terminal signaling domain (SHH-N, ~19 kDa) and a C-terminal autoprocessing domain (SHH-C, ~25 kDa) that is structurally and mechanistically homologous to self-splicing protein inteins:
Because SHH-Np is covalently modified by both an N-terminal palmitate and a C-terminal cholesterol, it tethers tightly to the outer leaflet of the plasma membrane. Long-range morphogen gradient formation across 100–300 μm of embryonic tissue therefore requires the twelve-transmembrane Resistance-Nodulation-Division (RND) antiporter Dispatched-1 (DISP1) (driven by the transmembrane Na+ gradient) together with the secreted glycoprotein SCUBE2 and cell-surface heparan sulfate proteoglycans (Glypicans) or specialized filopodial extensions (cytonemes).
Primary Cilium Signal Transduction: Patched-1, Smoothened, and the GLI Switch
Classic Embryonic Morphogen Gradients
1. Ventral Neural Tube Patterning (The French Flag Model in Vertebrates)
During neurulation, Sonic hedgehog is secreted first by the axial mesoderm of the notochord and subsequently by the induced floor plate at the ventral midline of the neural tube, establishing a ventral-to-dorsal concentration gradient opposed by dorsal BMP4/7 and Wnt signals from the roof plate. As demonstrated by Thomas Jessell and James Briscoe, neural progenitor cells along the dorsoventral axis read out both the concentration and duration of SHH signaling via cross-repressive pairs of Class I (SHH-repressed: Pax6, Irx3, Dbx1/2, Pax7) and Class II (SHH-induced: Nkx2.2, Olig2, Nkx6.1, Nkx6.2) homeodomain/bHLH transcription factors to specify five distinct ventral progenitor domains:
- Floor Plate (highest/longest SHH exposure; FoxA2)
- p3 domain (Nkx2.2+) ──► V3 interneurons
- pMN domain (Olig2+) ──► Somatic α/γ-motor neurons and later oligodendrocyte precursor cells (OPCs)
- p2, p1, and p0 domains (Nkx6.1, Irx3, Dbx1) ──► V2, V1, and V0 locomotor interneurons
2. Limb-Bud Anteroposterior Digit Specification (The Zone of Polarizing Activity, ZPA)
In the vertebrate embryonic limb bud, SHH is expressed specifically in a posterior mesenchymal signaling center—the Zone of Polarizing Activity (ZPA)—where it opposes anterior GLI3R repressor levels to specify digit number and anteroposterior identity (from the anterior thumb/hallux [digit 1], which forms in the complete absence of SHH at maximal GLI3R, to the posterior little finger [digit 5] at maximal SHH exposure). Point mutations inside the ZRS (ZPA Regulatory Sequence)—a limb-specific long-range enhancer of SHH located ~1 megabase upstream inside intron 5 of the neighboring LMBR1 gene—cause ectopic anterior SHH expression in the limb bud, resulting in preaxial polydactyly (extra thumbs/toes in humans, Hemingway's polydactyl cats, and Silkie chickens).
Congenital Teratogenesis, Holoprosencephaly, and Cancer Pharmacology
| Condition / Drug | Molecular Target | Mechanism | Clinical Presentation / Indication |
|---|---|---|---|
| Holoprosencephaly (HPE) | SHH, PTCH1, GLI2, DISP1, or 7-Dehydrocholesterol Reductase (DHCR7) | Haploinsufficiency of prechordal plate SHH signaling (or cholesterol deficiency in Smith–Lemli–Opitz syndrome [DHCR7]) fails to split the embryonic prosencephalon into left and right cerebral hemispheres and eye fields | Spectrum from alobar HPE with cyclopia and proboscis to midline cleft lip/palate and a single central maxillary incisor |
| Greig Cephalopolysyndactyly vs. Pallister–Hall Syndrome | GLI3 (Chr 7p14.1) | Greig: Complete GLI3 loss-of-function haploinsufficiency (polydactyly + macrocephaly). Pallister–Hall: Frameshift truncations in the middle third of GLI3 produce a constitutive GLI3R repressor (hypothalamic hamartoma + central/postaxial polydactyly). | Distinct allelic morphogen-readout syndromes |
| Cyclopamine (Veratrum californicum Corn Lily Alkaloid) | Smoothened (SMO) 7-TM Heptahelical Pocket | Steroidal jerveratrum alkaloid ingested by pregnant ewes grazing on wild corn lily binds directly to the 7-TM pocket of SMO, locking it inactive during early gestation | Discovery through epidemic of one-eyed (cyclopic) lambs in Idaho in the 1950s (Binns & Keeler); prototype for all clinical SMO inhibitors |
| Gorlin Syndrome (Nevoid Basal Cell Carcinoma Syndrome) & Sporadic BCC / SHH-Medulloblastoma | PTCH1 (loss of function, ~70–85%), SMO (gain of function, ~10–15%), or SUFU | Biallelic inactivation of the PTCH1 tumor suppressor (or activating SMO Trp535Leu mutation) causes ligand-independent ciliary SMO/GLI1 activation | Multiple basal cell carcinomas (BCCs) of the skin, odontogenic keratocysts of the jaw, palmar/plantar pits, falx cerebri calcification, and cerebellar medulloblastoma |
| Vismodegib, Sonidegib, & Glasdegib | Smoothened (SMO) Antagonists | Orally bioavailable small molecules that bind the cyclopamine/cholesterol pocket in the 7-TM bundle of SMO | FDA-approved for locally advanced or metastatic basal cell carcinoma and acute myeloid leukemia |
References
- Echelard, Y., Epstein, D. J., St-Jacques, B., Shen, L., Mohler, J., McMahon, J. A., & McMahon, A. P. (1993). Sonic hedgehog, a member of a family of putative signaling molecules, is implicated in the regulation of CNS polarity. Cell, 75(7), 1417–1430. Google Scholar
- Briscoe, J., & Thérond, P. P. (2013). The mechanisms of Hedgehog signalling and its roles in development and disease. Nature Reviews Molecular Cell Biology, 14(7), 416–429. Google Scholar
- Qi, X., Schmiege, P., Coutavas, E., Wang, J., & Li, X. (2018). Structures of human Patched and its complex with native palmitoylated Sonic hedgehog. Nature, 560(7716), 128–132. Google Scholar
- Bangs, F., & Anderson, K. V. (2017). Primary cilia and mammalian Hedgehog signaling. Cold Spring Harbor Perspectives in Biology, 9(5), a028175. Google Scholar