The primary cilium dampens proliferative signaling and represses a G2/M transcriptional network in quiescent myoblasts

Abstract

Background Reversible cell cycle arrest (quiescence/G0) is characteristic of adult stem cells and is actively controlled at multiple levels. Quiescent cells also extend a primary cilium, which functions as a signaling hub. Primary cilia have been shown to be important in multiple developmental processes, and are implicated in numerous developmental disorders. Although the association of the cilium with G0 is established, the role of the cilium in the control of the quiescence program is still poorly understood. Results Primary cilia are dynamically regulated across different states of cell cycle exit in skeletal muscle myoblasts: quiescent myoblasts elaborate a primary cilium in vivo and in vitro , but terminally differentiated myofibers do not. Myoblasts where ciliogenesis is ablated using RNAi against a key ciliary assembly protein (IFT88) can exit the cell cycle but display an altered quiescence program and impaired self-renewal. Specifically, the G0 transcriptome in IFT88 knockdown cells is aberrantly enriched for G2/M regulators, suggesting a focused repression of this network by the cilium. Cilium-ablated cells also exhibit features of activation including enhanced activity of Wnt and mitogen signaling and elevated protein synthesis via inactivation of the translational repressor 4E-BP1. Conclusions Taken together, our results show that the primary cilium integrates and dampens proliferative signaling, represses translation and G2/M genes, and is integral to the establishment of the quiescence program.

Cellular Mechanisms and Regulation of Quiescence

“…In proliferating cells, centrioles assemble into centrosomes-microtubule-organizing centers that regulate spindle assembly during mitosis (Breslow and Holland, 2019). In contrast, in nondividing cells, including quiescent cells, centrioles form the base for the primary cilium (Figure 4), a structure that assembles at the plasma membrane to act as a key sensory structure for cell signaling (Breslow and Holland, 2019;Jaafar Marican et al, 2016;Venugopal et al, 2020). Induction of quiescence triggers centriole migration to the apical surface to initiate formation of the primary cilium, whereas exit from quiescence is accompanied by the shortening and resorption of the cilium (Pitaval et al, 2017;Pugacheva et al, 2007).…”

“…Reciprocally, failure to dismantle the cilium can delay cell-cycle progression and act as a brake to retain cells in quiescence (Goto et al, 2017;Inaba et al, 2016). One way in which primary cilia have been proposed to regulate quiescence is by sequestering the centrioles, thus preventing centrosome formation in mitosis (Goto et al, 2017;Snell and Golemis, 2007;Venugopal et al, 2020). However, this model does not explain the failure of ciliated cells to progress through prior stages of the cell cycle where centrioles do not play functional roles.…”

“…However, this model does not explain the failure of ciliated cells to progress through prior stages of the cell cycle where centrioles do not play functional roles. The observed suppression of cell division by the primary cilium could occur through cilia-mediated dampening of proliferative signaling pathways (Venugopal et al, 2020), as the cilium functions as a key signaling hub for multiple pathways, including the Hedgehog, PDGF, mTOR, Notch, and Wnt signaling pathways (Breslow and Holland, 2019). Cilia may also prevent cell-cycle reentry by controlling the levels of the CDK inhibitor, p27, to maintain ciliated cells in a quiescent state (Izawa et al, 2015).…”

Histone acylation marks respond to metabolic perturbations and enable cellular adaptation

“…5c). These genes are involved in the cell fate of proliferation versus differentiation, depending on energy availability [37][38][39] . Additionally, we also found that genes associated with RNA synthesis (DNA transcription) were significantly downregulated under all three knockdown conditions (Fig.…”

Muscle stem cell function is impaired in absence of Talpid3 - a gene required for primary cilia formation

“…Both pathways are linked to primary ciliogenesis and depending on the context, they act cooperatively or antagonistically 13 14 15 16 Quiescent muscle stem cells (MuSC) have a primary cilium, which is disassembled upon activation and cell cycle entry and reassembled in self-renewing MuSCs 17 . Knock-down of IFT88 in C2C12 myoblasts leads to ablation of ciliogenesis and reduced expression of p27, a marker for quiescence 18 . In mice, conditional deletion of IFT88 in MuSCs, impairs recovery of overall muscle strength and alters the expression of cell-cycle-related genes 19 .…”