Design principles of ciliary signaling.

Primary cilia are microtubule-based sensory organelles that have been conserved throughout eukaryotic evolution. As discussed in this Review, a cilium is an elongated and highly specialized structure, and, together with its ability to selectively traffic and concentrate proteins, lipids and second messengers, it creates a signaling environment distinct from the cell body. Ciliary signaling pathways adopt a bow-tie network architecture, in which diverse inputs converge on shared effectors and second messengers before diverging to multiple outputs. Unlike other cellular bow-tie systems, cells exploit ciliary geometry, compartmentalization and infrastructure to enhance sensitivity at multiple scales, from individual molecular reactions to entire signaling pathways. In cilia, integration of the bow-tie network architecture with their specialized structure and unique environment confers robustness and evolvability, which enables cilia to acquire diverse signaling roles. However, this versatility comes with vulnerability - rare mutations that disrupt the features most essential for cilia robustness cause multisystem ciliopathies.