Three-dimensional architecture of epithelial primary cilia

Area of Science:

• Cell Biology
• Structural Biology
• Biophysics

Background:

• Primary cilia are crucial cellular organelles involved in various signaling pathways.
• The canonical 9+0 microtubule arrangement is widely accepted for primary cilia structure.
• Understanding primary cilia architecture is key to deciphering their roles in mechanosensing and disease.

Purpose of the Study:

• To generate a complete 3D structural model of typical epithelial primary cilia.
• To investigate the detailed architecture of the ciliary axoneme and its components.
• To explore the functional implications of primary cilia structure, including mechanosensing and cell communication.

Main Methods:

• Serial section electron tomography was employed to obtain high-resolution structural maps of full-length primary cilia.
• Advanced 3D reconstruction techniques were used to model the ciliary axoneme and associated protein networks.
• Detailed analysis of microtubule complexes (MtCs) and their arrangement was performed.

Main Results:

• The study reveals that primary cilia architecture significantly differs from the 9+0 paradigm.
• A novel fibrous protein network cross-linking axonemal microtubule complexes (MtCs) was identified, contributing to ciliary elasticity and mechanosensing.
• Axoneme structure exhibits base-to-tip evolution with decreasing MtCs and diameter, and varying MtC lengths suggest complex intraflagellar transport.
• Longitudinal gaps at the A-B tubule inner junction indicate the role of inner junction proteins in doublet-singlet transitions.
• Ciliary vesicles budding from kidney primary cilia were observed, supporting their role as ectosomes in cell-cell communication.

Conclusions:

• Primary cilia possess a complex and dynamic 3D structure distinct from the traditional 9+0 model.
• The identified structural features provide a basis for understanding primary cilia's mechanosensing abilities and elastic resilience.
• Emerging evidence suggests primary cilia are involved in sophisticated intraflagellar transport and intercellular communication via ciliary vesicles.