Cilia and Hedgehog responsiveness in the mouse

Area of Science:

• Developmental Biology
• Cell Biology
• Genetics

Background:

• Intraflagellar transport (IFT) proteins, including Ift172/Wimple and Polaris/Ift88, and the kinesin-II motor are vital for cilia formation and maintenance.
• These IFT components are also implicated in the activation of Hedgehog (Hh) pathway targets by Gli transcription factors.
• Observed phenotypes in IFT mutants differ from those lacking Smoothened (Smo), suggesting a complex regulatory relationship.

Purpose of the Study:

• To elucidate the precise role of Ift172 within the Hedgehog signaling pathway.
• To investigate the relationship between IFT, Smo, and Gli processing in mouse embryonic development.
• To understand how IFT influences both positive and negative regulation of the Hh pathway.

Main Methods:

• Genetic analysis of mouse mutants lacking specific IFT proteins (Ift172, Dnchc2, Kif3a) and the Hh pathway component Smoothened (Smo).
• Phenotypic comparison of single and compound mutants to determine genetic interactions and pathway order.
• Assessment of Gli3 processing and Gli activator formation in different genetic backgrounds.

Main Results:

• Mouse embryos lacking both Ift172 and Smo exhibit phenotypes identical to Ift172 single mutants, indicating Ift172 acts downstream of Smo.
• Ift172 mutants display a milder neural patterning defect than Smo mutants due to Ift172's role in Gli3 repressor processing.
• IFT proteins are essential for both Gli activator formation and Gli3 repressor processing, leading to context-dependent Hh signaling outcomes (loss in neural tube, gain in limb).

Conclusions:

• Intraflagellar transport is essential for both the activation and repression of the Hedgehog signaling pathway.
• The findings suggest that cilia may function as signaling organelles integrating all aspects of mouse Hh pathway activity.
• IFT's dual role in Hh signaling explains opposing phenotypes observed in different embryonic tissues.