Cholesterol accessibility at the ciliary membrane controls hedgehog signaling

Area of Science:

• Cell biology
• Biochemistry
• Molecular signaling

Background:

• The Hedgehog (Hedgehog) signaling pathway is vital for development and implicated in cancer.
• Smoothened (SMO) protein mediates Hedgehog signal transmission across the plasma membrane, a process previously linked to cholesterol interaction.
• Regulation of membrane cholesterol levels is critical for controlling SMO activity and Hedgehog signaling.

Purpose of the Study:

• To investigate the specific role of distinct cholesterol pools in Hedgehog signal activation.
• To elucidate how extracellular ligands modulate membrane cholesterol organization to control primary cilia function.
• To identify mechanisms regulating cholesterol accessibility for SMO activation.

Main Methods:

• Utilized toxin-based sensors to differentiate and quantify specific membrane cholesterol pools.
• Manipulated sphingomyelin levels to alter cholesterol accessibility.
• Investigated the effect of Hedgehog ligands on Patched 1 (PTCH1) activity and ciliary membrane cholesterol.
• Assessed the impact of trapping accessible cholesterol on Hedgehog signal transmission.

Main Results:

• Hedgehog signaling and SMO activation are specifically dependent on a small fraction of membrane cholesterol termed accessible cholesterol.
• Depletion of sphingomyelin enhances Hedgehog signaling by increasing cholesterol accessibility.
• Hedgehog ligands increase cholesterol accessibility in the primary cilium by inhibiting PTCH1.
• Blocking accessible cholesterol prevents Hedgehog signal transmission across the membrane.

Conclusions:

• The organization of cholesterol within the ciliary membrane is a key regulatory mechanism for Hedgehog signaling.
• Extracellular ligands dynamically control cholesterol accessibility to modulate the activity of cilia-localized proteins like SMO.
• Targeting cholesterol organization in primary cilia presents a potential therapeutic strategy for diseases involving Hedgehog pathway dysregulation.