The regulation of cilium assembly and disassembly in development and disease

Abstract

The primary cilium is an antenna-like organelle assembled on most types of quiescent and differentiated mammalian cells. This immotile structure is essential for interpreting extracellular signals that regulate growth, development and homeostasis. As such, ciliary defects produce a spectrum of human diseases, termed ciliopathies, and deregulation of this important organelle also plays key roles during tumor formation and progression. Recent studies have begun to clarify the key mechanisms that regulate ciliary assembly and disassembly in both normal and tumor cells, highlighting new possibilities for therapeutic intervention. Here, we review these exciting new findings, discussing the molecular factors involved in cilium formation and removal, the intrinsic and extrinsic control of cilium assembly and disassembly, and the relevance of these processes to mammalian cell growth and disease.

Keywords: Cancer; Ciliopathies; Cilium assembly; Cilium disassembly; Primary cilia.

Fig. 1.

Structure of the primary cilium. The overall architecture and key structural elements of the primary cilium are shown. IFT, intraflagellar transport.

Fig. 2.

A tale of two cycles: cell cycle-linked control of cilium formation and disassembly. A newly formed daughter centriole matures into a MC in two consecutive cell cycles. In the first cell cycle, new daughter centrioles (+) are assembled from an existing (mother) and older (grandmother, #) centriole. In the next cell cycle, the newly formed daughter centriole (*, dark green cylinder) gradually matures into a MC (*, light green cylinder), beginning with the loss of daughter centriole proteins at the G1/S phase and followed by the acquisition of distal appendages and sub-distal appendages in late G2 phase. The primary cilium then assembles when the cell exits the cell cycle (to enter G0) or receives developmental cues (blue arrow). Disassembly of the primary cilium occurs in a biphasic manner (yellow arrows), with the first wave occurring in G1 (1) and a second wave occurring before mitosis (2).

Fig. 3.

The multiple phases and regulation of cilium assembly. The process of cilium assembly involves several successive stages. Cilium assembly is initiated upon cell cycle exit or after receiving developmental signals (1). PCVs are transported via microtubule-actin networks to the distal end of the MC and fuse into a larger CV (2). The process is accompanied by reorganization of the cytoskeleton, which drives the migration of centrioles from the cytoplasm to the cell membrane. CP110 is removed. Next, IFT complexes are continuously recruited to the ciliary base to allow axoneme elongation, while Rab8a is recruited to the MC to facilitate ciliary membrane extension (3). The transition zone is then assembled, and this is followed by axoneme elongation and membrane fusion. Inhibition of the ciliary disassembly pathway also allows outgrowth of the cilium (4). Key proteins that play a regulatory role at each stage of the assembly process are shown. PI(4)P, PtdIns(4)P.

Fig. 4.

The regulation of cilium disassembly. Cell cycle re-entry, accompanied by several signaling pathways and regulatory proteins (including non-canonical Wnt signaling, phosphoinositide signaling, calcium signaling, PDGFRβ signaling, HDAC2 and Pifo), can trigger cilium disassembly via the activation of Aurora A. Aurora A then phosphorylates and stimulates the histone deacetylase HDAC6, which de-acetylates and destabilizes microtubules within the axoneme. During cilium disassembly, Plk1 and Nek2 activate the kinesins Kif2a and Kif24, respectively, which are required for the depolymerization of microtubules. Plk1 is recruited by PCM1 with the help of CDK1. Cilium disassembly also requires the modulation of IFT transport: KDM3A, for example, inhibits entry of the IFT complex into the cilium, whereas Nde1 regulates retrograde IFT transport. Growth signaling also triggers the removal of IFT-B particles through ciliary ectosomes released from the ciliary tip, which is also under the control of Aurora A. Cilium disassembly also requires remodeling of the ciliary pocket, accompanied by the enhancement of clathrin-mediated endocytosis. This process is controlled by Tctex1 and associated proteins (such as ANXA2, Cdc42 and ARPC2) and can be induced by IGF1 signaling. PDGFRβ, platelet-derived growth factor receptor β.

Fig. 5.

The function of primary cilia in cancer. (A) Loss of cilia has been observed in several types of tumors and can lead to aberrant activation of many oncogenic pathways. After treatment with anti-tumor drugs, tumor cells with cilia, and hence aberrant activation of Hh pathway, survive and can further proliferate. Depletion of cilia or inhibition of Hh signal pathway can block the proliferation of these drug-resistant cells. (B) The persistence of cilia can also be observed in a range of tumors, in which they appear to help maintain the oncogenic Hh pathway. After treatment with an SMO inhibitor, tumor cells with cilia die and the surviving cells (without cilia) evolve a modified Hh pathway that confers resistance to SMO inhibition.