Itinerary of vesicles to primary cilia

Abstract

Primary cilia are evolutionarily conserved cellular organelles with a distinctive rod-like structure that protrudes from the cell surface. Primary cilia are non-motile and near-ubiquitous in animal body. Studies using model organisms have demonstrated that primary cilia are involved in signal transduction pathways that control embryonic development and adult homeostasis. Moreover, defects in the formation or function of primary cilia have been shown to underlie various genetic disorders in human. The biogenesis of cilia requires both the assembly of a microtubule-based core structure, called the axoneme, and the establishment of the ciliary membrane. Emerging evidence suggests that specialized vesicle trafficking is critical for the formation and maintenance of the ciliary membrane, whose composition is distinct from that of the surrounding plasma membrane. The signaling functions of primary cilia also depend on selective delivery of ciliary membrane proteins via vesicle trafficking routes to primary cilia. The list of genes that are thought to be required for ciliary membrane biogenesis increases at a steady pace. However, the sorting and trafficking mechanisms of cilium-bound transport vesicles remain elusive. Here, we review current findings on several key molecules that may regulate vesicle trafficking during and after ciliogenesis.