Ciliary resorption modulates G₁ length and cell cycle progression

G₁ is the most variable phase in the cell cycle. What determines the duration of G₁? Emerging evidence suggests that primary cilia may provide the brake to retain cells in the resting state; the time required for the disassembly of cilia plays a critical role in determining the duration of G₁ and the timing of the G₁-S transition.^1,2

Primary cilia are hair-like sensory organelles present in G₁/G₀ cells, and a biphasic ciliary resorption takes place as the cell cycle progresses³ (Fig. 1A). Complete ciliary resorption prior to mitosis may be necessary to release centrioles from basal bodies to form mitotic poles, yet the significance of the ciliary disassembly prior to the S-phase re-entry remains enigmatic.³ Our recent data suggested that cilium resorption is a prerequisite for cells to proceed to S-phase entry.² We found that Tctex-1 has a coupled role in both cilium resorption and S-phase entry; silencing Tctex-1 blocked both cellular events. However, Tctex-1 suppression did not affect cell cycling in non-ciliated cells, implying that the inhibition of ciliary disassembly is not a consequence of blocking S-phase entry. Tctex-1 is a light chain subunit of cytoplasmic dynein, yet, the role of Tctex-1 in ciliary resorption is dynein-independent. Interestingly, the role of Tctex-1 in ciliary resorption requires phosphorylation at its Thr⁹⁴ residue. Phospho(T94)Tctex-1 is activated at the ciliary transition zone (i.e., between the distal region of the basal body and the ciliary axoneme) as the cell proceeds toward S phase. Infusion of cell-permeable phosphomimic-Tctex-1 peptides into ciliated cells not only triggered ciliary resorption by omitting the requirement of serum (i.e., proliferative stimuli), but also hastened the resorption process to minutes (vs. hours). These treated cells concomitantly exhibited an accelerated S-entry rate. Independent studies performed by Tsiokas and his colleagues showed that cells with longer cilia (via Nde1 suppression) indeed took a longer time to re-enter the cell cycle.¹ AuroraA kinase is also activated (phosphorylation at Thr²⁸⁸) at the basal body in the G₁/S border. Consistently, inactivation of AuroraA (and its upstream regulator Pitchfork) blocked ciliary resorption^4,5 and cell cycle re-entry.² Thus, we propose that ciliary resorption can be considered as a novel “checkpoint” for G₁-S transition. Upon receiving a sufficient proliferative signal on cilia, yet unidentified messenger(s) undergoes retrograde transport to the base of the cilium where it recruits the downstream effectors (e.g., AuroraA, Tctex-1), that in turn, promote ciliary axonemal disassembly (Fig. 1B). Although the signals that convey ciliary resorption status from the ciliary bases to the nuclei where DNA synthesis takes place are unknown, this model also fits well with the idea that the centrosome/basal body is a key hub for cells to prepare for S-phase entry.⁶ It remains unclear whether different mitogens use the same or distinct pathway to activate the ciliary disassembly.

Figure 1.

(A) Proposed model suggesting that the temporal correlation between ciliary resorption and cell cycle re-entry are causally linked. Phospho(T94)Tctex-1 is recruited at the ciliary bases of the G₁, but not G₀, cells.² (B) Signals sensed by cilia are retrogradely transduced to the sites proximal to the basal bodies, where the downstream signaling complexes and effectors (e.g., phospho-AuroraA, phospho-Tctex-1) are organized to prepare for the disassembly of ciliary axonemes. The mechanism that couples the ciliary resorption and DNA replication remains to be elucidated. (C) Schematic drawing of a ciliated cycling radial glial (RG), a mitotic (M) cell and a post-mitotic neuron (N) in developing neocortex. (D) Depletion of Tctex-1 induces premature cell cycle exit and neuronal differentiation. (E) Overexpression of phospho-mimic Tctex-1 increases the population of proliferative progenitors.

Cilium-regulated cell cycle control is likely to be relevant to mammalian brain development.² During early cortical development, neural progenitors (or radial glia, Fig. 1C) predominantly divide to produce identical daughters to expand the mitotic progenitor population. In contrast, the daughters of the later progenitors tend to adopt a neuronal fate.⁷ The early and later progenitors have shorter and longer G₁ phases, respectively. In fact, the length of G₁ has a causal link to the cell fate determination of radial glia.⁷ We found that inactivation of phospho-Tctex-1 promoted cell cycle exit and premature neuronal differentiation at the expense of proliferation (Fig. 1D). Forced expression of phosphomimic Tctex-1 significantly shortened G₁, accelerated S-phase entry, and caused the expansion of the mitotic progenitor population (Fig. 1E). Interestingly, phospho(T94)Tctex-1 was also specifically expressed at the bases of cilia that project from the apical endfeet of the radial glia and protrude into the ventricles. These findings imply that the proliferation cues sensed by the cilia from the cerebrospinal fluid activate phospho-Tctex-1 (and other machineries), and ciliary resorption gives a “green light” to the cells to undergo division right at the ventricular wall. This model may explain the significance of the ventricular attachment of radial glia. It also predicts that the daughter who inherits the apical cilia will remain as a progenitor during asymmetric division. Since the timing that switches from proliferative division to differentiation division plays a decisive role in determining the size of the cortex,⁷ it is interesting to note that several proteins including Nde1 whose mutations are responsible for human diseases associated with microcephaly have been localized to the centrosome/basal body.^8,9

Is cilia-dependent cell cycle control also physiologically relevant elsewhere? Tctex-1 is also particularly enriched in adult-born amplifying neural progenitors¹⁰ and the Tctex-1 gene has been mapped to a chromosomal region that is deleted in some patients with mental retardation.¹¹ Mental retardation is a prominent feature of several systemic disorders associated with dysfunctional cilia. Hence, the relationship between Tctex-1, adult neurogenesis and ciliary diseases with neurological manifestation warrants further investigation.

Comment on: Li A, et al. Nat Cell Biol. 2011;13:402–411. doi: 10.1038/ncb2218.