Co-ordinate regulation of cilia and the cell cycle in Chlamydomonas reinhardtii

Cilia are multifunctional organelles implicated in many human diseases. Cilia project from the cell surface and thus are well-situated for roles both in motility and sensory reception. The green alga Chlamydomonas reinhardtii is a useful model system for studies of cilia due to its combination of genetics, molecular biology, and biochemistry. At the base of cilia are modified centrioles called basal bodies, which are connected with cilia via a transition zone. Ciliary assembly and disassembly must be co-ordinated with cell cycle progression because the basal bodies must be freed in order to organize the mitotic spindle. Ciliary disassembly can occur either by deflagellation, a drastic stress response, or resorption, a gradual process normally occurring before cell division. I hypothesized that there might be commonality between the two pathways for ciliary loss. I discovered that cells with mutations in ciliary assembly invoke deflagellation in some circumstances, and resorption in others. Deflagellation mutants also exhibited resorption defects, supporting my hypothesis. To further explore the relationship between deflagellation and cell cycle, Qasim Rasi and I examined katanin, a microtubule-severing protein implicated in deflagellation in Chlamydomonas, and in mitosis in mammalian cells. A reverse genetics approach revealed that the only cells that can survive knock-down of katanin are mutants which already lack cilia. This led us to form the hypothesis that the role of katanin in the cell cycle is related to its cilia-severing activity. In support of this idea, I observed that wild-type cells entering mitosis sever their cilia to free the basal bodies. Katanin-mediated deflagellation is defective in fa2 mutants, and FA2 is a member of the Nek family of mitotic kinases. The Neks are a large family of kinases and through evolutionary analyses, Brian Bradley and I discovered that the family expanded early in eukaryotic evolution and it is likely that Neks co-ordinate the cycle of ciliary assembly and dissasemby with cell cycle progression. This study also identified the Chlamydomonas Nek CNK3 as an orthologue of the human disease gene Nek8, suggesting an avenue to further disease-related research.