Nek2 activation of Kif24 ensures cilium disassembly during the cell cycle

Abstract

Summary Many proteins are known to promote ciliogenesis, but mechanisms that promote primary cilia disassembly prior to mitosis are largely unknown. Here, we identify a mechanism that favors cilium disassembly and maintains the disassembled state. We show that co-localization of the S/G2 phase kinase, Nek2, and Kif24 triggers Kif24 phosphorylation, inhibiting cilia formation. We show that Kif24, a microtubule depolymerizing kinesin, is phosphorylated by Nek2, which stimulates its activity and prevents the outgrowth of cilia in proliferating cells, independent of Aurora A and HDAC6. Our data also suggest that cilium assembly and disassembly are in dynamic equilibrium, but Nek2 and Kif24 can shift the balance toward disassembly. Further, Nek2 and Kif24 are over-expressed in breast cancer cells, and ablation of these proteins restores ciliation in these cells, thereby reducing proliferation. Thus, Kif24 is a physiological substrate of Nek2, which regulates cilia disassembly through a concerted mechanism involving Kif24-mediated microtubule de-polymerization.

Nek2A phosphorylates and stabilizes SuFu: A new strategy of Gli2/Hedgehog signaling regulatory mechanism

“…Three possible mechanisms by which Nek2A-phosphorylated SuFu might inhibit Gli2 activity have been proposed: one mechanism is when stable SuFu tethers more Gli2 in the cytoplasm and prevents the latter from entering the nucleus; another mechanism is when phosphorylated SuFu translocates into the nucleus and inhibits Gli2 transcriptional activity in the nucleus; and the third mechanism is when phosphorylated SuFu recruits SAP18 to suppress Gli2 transcriptional activity in the nucleus. A recent study reported that Nek2 may phosphorylate Kif24, a microtubule depolymerizing kinesin, to prevent the outgrowth of cilia in proliferating cells [51]. In order to induce sufficient cilia for observation, we employed murine siRNA targeting NEK2 to address the localization ability of SuFu in primary cilia of NIH3T3 cells.…”

Cilium assembly and disassembly

“…Cilium disassembly requires the destabilization and de-polymerization of axonemal microtubules, and two members of the Kinesin-13 family of de-polymerizing kinesins, Kif2a and Kif24, are implicated in cilium disassembly 62–64 (Fig. 3).…”

“…As for CP110 loss, ablation of Kif24 results in inappropriate assembly of cilia in proliferating cells, although it does not regulate centriole length. The microtubule de-polymerizing activity of Kif24 is enhanced by Nek2- mediated phosphorylation 64 . Nek2 is expressed during S and G2 phase, ensuring that Kif24 is active in cells that lack cilia.…”

“…Nek2 is expressed during S and G2 phase, ensuring that Kif24 is active in cells that lack cilia. Notably, the ability of Nek2 and Kif24 to suppress cilium assembly can be temporally distinguished from the Aurora-A–HDAC6 pathway, suggesting that Nek2–Kif24 activation ensures the irreversibility of the disassembly process after S phase commences 64 . These studies suggest that Kif24 acts to safeguard against the extension of de-polymerized microtubules, preventing the aberrant assembly of cilia prior to mitosis.…”

“…As described above, the proteins that catalyse cilium disassembly may provide an avenue for drug discovery, since restoration of the primary cilium in tumour cells could promote quiescence 64 , especially if two or more pathways were simultaneously targeted. It will be important to distinguish the contributions of aberrant regulation of cell cycle proteins (such as Nek2, Plk1 and Aurora A) to mitotic defects in chromosome segregation, spindle pole behaviour and other centrosome-associated processes from their effects on cilium assembly and function.…”