The perennial organelle: assembly and disassembly of the primary cilium

Primary cilia contain signaling receptors of diverse classes, and ciliary dysfunction results in a variety of developmental defects. Thus, primary cilia are thought to have an important role in sensing and transducing cellular signals. Although there is clear evidence demonstrating that these organelles are assembled and disassembled dynamically as cells progress through the cell cycle, the mechanisms by which the cell cycle controls the assembly and disassembly of the primary cilium remain poorly understood. In this Commentary, we review the basic cellular mechanisms that underlie the early stages of cilium assembly and discuss how the cell cycle communicates with the ciliation program. A commonly held view is that ciliation occurs exclusively in cells that have exited the cell cycle and entered quiescence or differentiation. However, this concept is at odds with the finding that, during development, many actively proliferating cells require cilia-mediated signaling pathways to instruct their developmental fate. Here, we reassess the quiescence-centric view of ciliation by reviewing historic and current literature. We discuss ample evidence that cilia are in fact present on many proliferating cells, and that a transient peak of ciliation before the G1-S transition might be tightly coupled to entry into the DNA replication phase. Finally, we touch on the relationship between the ciliation and cell-division cycles and the tissue distribution of primary cilia in order to highlight potential roles for the primary cilium in restraining cells from the hyperproliferative state that contributes to cancer.