mTORC1 hampers Hedgehog signaling in Tsc2 deficient cells

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The mechanistic target of rapamycin complex 1 (mTORC1) is a key regulator of cell growth and metabolism.
• Hedgehog signaling pathway activation relies on the interaction between Smoothened and GLI2 within primary cilia.
• The precise mechanisms governing this interaction remain largely unknown.

Purpose of the Study:

• To investigate the role of mTORC1 in regulating Hedgehog signaling.
• To elucidate the interplay between mTORC1, Smoothened, and GLI2 in primary cilia.

Main Methods:

• Utilized mouse embryonic fibroblasts (MEFs) deficient for Tsc2.
• Manipulated mTORC1 activity through constitutive activation and inhibition.
• Assessed Smoothened localization via cell imaging.
• Quantified GLI2 concentration within cilia.
• Measured Hedgehog signaling by analyzing Gli1 gene expression.

Main Results:

• Constitutive mTORC1 activation caused Smoothened mis-localization and increased GLI2 ciliary concentration, leading to reduced Hedgehog signaling.
• Inhibition of mTORC1 restored Smoothened to the cilia, decreased GLI2 ciliary levels, and rescued Hedgehog pathway activity.
• Identified a two-step GLI2 activation process: stabilization/cilia localization and Smoothened interaction.

Conclusions:

• mTORC1 negatively regulates the second step of GLI2 activation, specifically the communication with cilia-localized Smoothened.
• This study provides the first evidence linking mTORC1 to the regulation of Hedgehog signaling.
• Discovered a novel regulatory axis where mTORC1 controls key steps in Hedgehog pathway activation.