Context-dependent ciliary regulation of hedgehog pathway repression in tissue morphogenesis

Area of Science:

• Cell Biology
• Developmental Biology
• Genetics

Background:

• Tissue morphogenesis relies on subcellular signaling, with primary cilia playing a key role in compartmentalizing signaling pathways.
• The hedgehog pathway utilizes GLI transcription factors, producing both activators (GLI-A) and repressors (GLI-R), with cilia essential for their formation.
• Understanding how GLI-A/GLI-R coordinate cilia-regulated morphogenetic pathways is crucial but not well understood.

Purpose of the Study:

• To determine the specific requirements of GLI-A and GLI-R in phenotypes resulting from hedgehog pathway derepression during mouse development.
• To investigate the roles of GPR161 and ANKMY2 in cilia-mediated GLI-R generation via cAMP/protein kinase-A signaling.
• To elucidate the distinct modes of GLI effector regulation by primary cilia in tissue morphogenesis.

Main Methods:

• Genetic epistasis analysis involving mutants in Gpr161, Ankmy2, Gli2/Gli3, and Smoothened.
• Utilized a Gpr161 ciliary localization knock-in mutant to assess cAMP signaling competence independent of ciliary localization.
• Examined phenotypes in mouse neural tube and skeletal development.

Main Results:

• Cilia-dependent derepression phenotypes manifested in three distinct modes: exclusive GLI-R loss, exclusive GLI-A excess, or dual regulation.
• These modes were largely independent of the hedgehog pathway transducer Smoothened.
• A cAMP signaling-competent Gpr161 mutant recapitulated phenotypes solely through GLI-R loss, highlighting its critical role.

Conclusions:

• The study demonstrates complex, tissue-specific requirements for GLI effectors in morphogenesis.
• Primary cilia establish tissue-specific GLI-R thresholds, essential for hedgehog pathway repression.
• A framework is proposed for understanding diverse primary cilia-generated signaling modes in tissue development.