Tctex-1 promotes cell cycle re-entry by regulating branched actin remodeling, endocytosis and primary ciliary resorption

[Speaker] Masaki Saito:1
[Co-author] Wataru Otsu:2, Jen-Zen Chuang:2, Teruyuki Yanagisawa:1, Ching-Hwa Sung:2,3
1:Department of Molecular Pharmacology, Tohoku University School of Medicine, Japan, 2:Department of Ophthalmology, Margaret M. Dyson Vision Research Institute, Weill Medical College of Cornell University, USA, 3:Department of Cell and Developmental Biology, Weill Medical College of Cornell University, USA

The primary cilium is a non-motile sensory organelle, whose proximal area is surrounded by a specialized invaginated plasma membrane domain called the ciliary pocket. Emerging data suggest that ciliary formation and resorption have a correlation with cell cycle; the cilium is formed on cell surface at the G0/G1 phase, and is biphasically (early and late phases) resorbed by growth factor stimuli to undergo cell cycle re-entry. Thereby, the ciliary resorption is crucial for the subsequent cell cycle re-entry. The fact that dysregulation of the ciliary dynamics is linked with rare genetic deseases, ciliopathy, indicates that investigating mechanisms of the ciliary resorption is indispensable for medical treatment of ciliopathy. However, molecular links between the ciliary resorption and cell cycle progression has been unresolved. t-Complex testis expressed-1 (Tctex-1) is a light chain of cytoplasmic dynein. Phosphorylation of Tctex-1 at Thr94 is free from the dynein complex and possesses a dynein-independent activation of Rac1 GTPase and remodeling of branched actin. In the present study, we found that phospho(T94)-Tctex-1 was recruited to ciliary transition zone before S-phase entry and had a pivotal role in both early-phase ciliary resorption and cell cycle re-entry in human retinal pigment epithelial (RPE-1) cells. The neural progenitor radial glial cells in the developing neocortex display primary cilia on their apical surfaces. It was clarified that phospho(T94)-Tctex-1 recruited to the transition zone promoted cell cycle progression and cell proliferation in the radial glial cells. On the other hand, the ciliary resorption accelerated by overexpression of a phospho-mimic Tctex-1 mutant was suppressed in the presence of an actin polymerization inhibitor, cytochalasin D, in RPE-1 cells. Although both Cdc42 and Rac1 can organize branched actin, knockdown of Cdc42, but not Rac1, abrogated the ciliary resorption. It was also appeared that both branched actin remodeling and endocytosis were organized at the ciliary pocket membrane in early-phase ciliary resorption. Finally, we found that clathrin-dependent endocytosis was essential for the ciliary resorption. These data collectively demonstrate that the ciliary pocket is a central hub for branched actin remodeling and membrane trafficking, and these mechanisms are responsible for primary ciliary resorption and cell cycle progression.

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