Soluble levels of cytosolic tubulin regulate ciliary length control

The primary cilium is an evolutionarily conserved dynamic organelle important for regulating numerous signaling pathways, and, as such, mutations disrupting ciliogenesis result in a variety of developmental abnormalities and postnatal disorders. The length of the cilium is regulated by the cell through largely unknown mechanisms. Normal cilia length is important, as either shortened or elongated cilia have been associated with disease and developmental defects. Here we explore the importance of cytoskeletal dynamics in regulating cilia length. Using pharmacological approaches in different cell types, we demonstrate that actin depolymerization or stabilization and protein kinase A activation result in a rapid elongation of the primary cilium. The effects of pharmacological agents on cilia length are associated with a subsequent increase in soluble tubulin levels and can be impaired by depletion of soluble tubulin with taxol. In addition, subtle nocodazole treatment was able to induce ciliogenesis under conditions in which cilia are not normally formed and also increases cilia length on cells that have already established cilia. Together these data indicate that cilia length can be regulated through changes in either the actin or microtubule network and implicate a possible role for soluble tubulin levels in cilia length control.