Role of primary cilia in non-dividing and post-mitotic cells

Abstract

The essential role of primary (non-motile) cilia during the development of multi-cellular tissues and organs is well established and is underlined by severe disease manifestations caused by mutations in cilia-associated molecules that are collectively termed ciliopathies. However, the role of primary cilia in non-dividing and terminally differentiated, post-mitotic cells is less well understood. Although the prevention of cells from re-entering the cell cycle may represent a major chore, primary cilia have recently been linked to DNA damage responses, autophagy and mitochondria. Given this connectivity, primary cilia in non-dividing cells are well positioned to form a signaling hub outside of the nucleus. Such a center could integrate information to initiate responses and to maintain cellular homeostasis if cell survival is jeopardized. These more discrete functions may remain undetected until differentiated cells are confronted with emergencies.

Keywords: Autophagy; Ciliopathy; DNA damage response; Mitochondria; mTOR.

Fig. 1

Proteins involved in cilia assembly and disassembly. This representation is an over-simplification of a dynamic process that controls (bottom) cilia assembly in cells exiting the cell-cycle and (top) cilia disassembly in proliferating cells. HEF1 activates Aurora A (AurA), which initiates ciliary disassembly by activating HDAC6. PIFO promotes ciliary disassembly, supporting AurA activation. Mitostatin supports AurA activation; its degradation is inhibited by NDEL1. Wnt5a-mediated activation of CK1e triggers phosphorylation of Dvl2 and interaction with PLK1, which in turn favors stabilization of HEF1. KIF2A, a kinesin family member that depolymerizes microtubules, is phosphorylated and activated by PLK1. INPP5E maintains PI4P levels to prevent the interaction of TTBK2 with CEP164 and ciliogenesis. APC-Cdc20 targets NEK1 for degradation, whereas Nek1 supports the stability and integrity of cilia. CEP97 recruits CP110 to the centrosome; CP110 suppresses ciliary assembly at high levels, whereas optimal levels of CP110 are required for ciliogenesis. DYNLT1 promotes ciliary disassembly before S phase. Control of centriole duplication is essential to prevent premature ciliogenesis. CDK1/Cyclin B bind STIL to prevent the premature formation of the STIL/SAS-6/PLK4 complex required for centriole duplication. CEP76 is phosphorylated by CDK2 to prevent the activation of PLK1 and centriole reduplication during the cell cycle. NDE1 acts upstream of DYNNL1 (LC8) and negatively controls ciliary length. The MSt1/2 component of the Hippo signaling cascade promotes ciliogenesis by dissociating the AurA/HDAC6 cilia-disassembly complex. NEK2 facilitates centriole separation but phosphorylates KIF24 to promote ciliary disassembly (see text for abbreviated protein names and further details). Note that the depicted interactions do not occur simultaneously but are precisely coordinated with the cell cycle (black lines indicate positive, red lines negative regulation).

Fig. 2

Representation of cilia-associated molecules that have been implicated in DNA damage responses (DDR). DNA damage recruits members of the poly-ADP-ribose polymerase (PARP) family and the MRN complex (MRE11, RAD40 and NBS1) to induce cell-intrinsic checkpoints, including p53. The ATM/CHK2 module is activated after DNA double-strand breaks (DSB), whereas the ATR/CHK1 pathway responds primarily to DNA signal-strand breaks. Both pathways converge on CDC25 phosphatase, a positive regulator of cell cycle progression upstream of CDK1/Cyclin B1. CEP164 interacts with ATM and ATR and is phosphorylated (P) by both kinases. CEP164 facilitates the activation of CHK1, MCD1 and PRA. Nek8 inhibits the cyclin-A-dependent activation of CDK1/2, pausing cell cycle progression. NPHP7 (Glis2) is associated with the activation of checkpoint kinase 1 (CHK1), the stabilization of p53 and the induction of senescence. OFD1 interacts with NPHP10 (SDCCAG8) and components of the TIP60 histone acetyltransferase complex. Cells with defective OFD1 exhibit reduced histone acetylation, impaired repair and prolonged arrest at the G2-M checkpoint after DNA DSB. In response to H₂O₂, ATM can undergo auto-phosphorylation to activate LKB1/AMPK and autophagy (for abbreviations, see text)

Fig. 3

Interactions of cilia with mitochondria, autophagy and differentiation. Anks6 interacts with several mitochondrial proteins; however, the significance of these interactions remains unknown. Mitochondria are not required for ciliogenesis, although ciliary proteins have recently been implicated in mitochondrial positioning in sperm. MNS1, by interacting with MFN2 and ODF2, anchors mitochondria in sperm. CFAP157, a FOXJ1 target gene and IFT22 are also involved in mitochondrial positioning in sperm. OFD2 recruits Mitostatin, which inhibits ciligenesis and regulates mitochondrial-endoplasmic reticulum (ER) contact sites through mitofusin. Autophagy targets IF20 for degradation, preventing ciliogenesis under nutrient-rich conditions. DNA damage can activate LKB1/AMPK/TSC2 via ATM to initiate autophagy. Autophagy in turn might provide the necessary building blocks (e.g., dNTPs) to facilitate DNA repair. Oxidative stress results in ATM activation and stress granule formation. RNA granules, containing GW182 and Ago2, localize to centrosomes or basal bodies of primary cilia. In human embryonal stem cells, cilia activate autophagy to degrade NRF2, promoting neuroectoderm development (for abbreviations, see text)