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Updated: Feb 17, 2026
Quantitative PCR-based Assay to Measure Sonic Hedgehog Signaling in Cellular Model of Ciliogenesis
Published on: January 31, 2025
Rapid, direct activity assays for Smoothened reveal Hedgehog pathway regulation by membrane cholesterol and
Benjamin R Myers1,2,3,4, Lila Neahring5,2,3,4, Yunxiao Zhang5,2,3,4
1Institute for Stem Cell Biology and Regenerative Medicine, Stanford University School of Medicine, Stanford, CA 94305; benjamin.r.myers@gmail.com pbeachy@stanford.edu.
Abstract:
Hedgehog signaling specifies tissue patterning and renewal, and pathway components are commonly mutated in certain malignancies. Although central to ensuring appropriate pathway activity in all Hedgehog-responsive cells, how the transporter-like receptor Patched1 regulates the seven-transmembrane protein Smoothened remains mysterious, partially due to limitations in existing tools and experimental systems. Here we employ direct, real-time, biochemical and physiology-based approaches to monitor Smoothened activity in cellular and in vitro contexts. Patched1-Smoothened coupling is rapid, dynamic, and can be recapitulated without cilium-specific proteins or lipids. By reconstituting purified Smoothened in vitro, we show that cholesterol within the bilayer is sufficient for constitutive Smoothened activation. Cholesterol effects occur independently of the lipid-binding Smoothened extracellular domain, a region that is dispensable for Patched1-Smoothened coupling. Finally, we show that Patched1 specifically requires extracellular Na+ to regulate Smoothened in our assays, raising the possibility that a Na+ gradient provides the energy source for Patched1 catalytic activity. Our work suggests a hypothesis wherein Patched1, chemiosmotically driven by the transmembrane Na+ gradient common to metazoans, regulates Smoothened by shielding its heptahelical domain from cholesterol, or by providing an inhibitor that overrides this cholesterol activation.
Insights
Patched1 regulates Smoothened activity via cholesterol and extracellular sodium. This mechanism, independent of cilium proteins, offers new insights into Hedgehog pathway regulation and cancer.
Area of Science:
- Cell Biology
- Biochemistry
- Developmental Biology
Background:
- Hedgehog signaling is crucial for tissue development and is implicated in various cancers.
- The precise mechanism by which Patched1 (PTCH1) regulates Smoothened (SMO) activity is not fully understood.
- Existing experimental systems have limitations in studying this interaction.
Purpose of the Study:
- To investigate the real-time regulation of Smoothened activity by Patched1.
- To elucidate the molecular components and conditions required for Patched1-Smoothened coupling.
- To explore the role of cholesterol and ion gradients in this regulatory process.
Main Methods:
- Direct, real-time biochemical and physiological assays.
- Monitoring Smoothened activity in cellular and in vitro reconstituted systems.
- Investigating the effects of cholesterol and extracellular Na+ on Smoothened activity.
Main Results:
- Patched1-Smoothened coupling is rapid and dynamic, occurring independently of cilium-specific proteins.
- Cholesterol in the lipid bilayer is sufficient for constitutive Smoothened activation.
- Patched1 requires extracellular Na+ to regulate Smoothened, suggesting a role for Na+ gradients.
Conclusions:
- The interaction between Patched1 and Smoothened can be recapitulated in a simplified system, highlighting key regulatory factors.
- Cholesterol directly activates Smoothened, and this activation is independent of SMO's extracellular domain.
- A model is proposed where Patched1 utilizes a transmembrane Na+ gradient to regulate Smoothened, potentially by shielding it from cholesterol or acting as an inhibitor.
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