Cilium structure, assembly, and disassembly regulated by the cytoskeleton

Abstract

The cilium, once considered a vestigial structure, is a conserved, microtubule-based organelle critical for transducing extracellular chemical and mechanical signals that control cell polarity, differentiation, and proliferation. The cilium undergoes cycles of assembly and disassembly that are controlled by complex inter-relationships with the cytoskeleton. Microtubules form the core of the cilium, the axoneme, and are regulated by post-translational modifications, associated proteins, and microtubule dynamics. Although actin and septin cytoskeletons are not major components of the axoneme, they also regulate cilium organization and assembly state. Here, we discuss recent advances on how these different cytoskeletal systems- affect cilium function, structure, and organization.

Keywords: actin; cilia; cytoskeleton; microtubule; organelle biogenesis; signaling.

Figure 1.. Elements of the microtubule cytoskeleton within the cilium.

The axoneme consists of nine bundles of MTs that are anchored in the basal body. Free tubulin diffuses through the axoneme lumen [1] and accumulates at the ciliary base, transition zone, and cilium tip. Exchange of tubulin between cytoplasmic and ciliary pools regulates axoneme length [2]. IFT motors, kinesin and dynein, carry cargo along the axoneme in different directions. Microtubule-associated proteins localize along the cilium and at the basal body, including SAXO1 (green), MAP4 (orange), and septin family proteins (pink), which also associate with the membrane of the ciliary pocket.

Figure 2.. Composition and arrangement of axoneme microtubules.

(A) Cross-section of a generic non-motile axoneme is depicted, composed of nine microtubule doublets. (B) A doublet consists of a complete A-tubule and a partial B-tubule. The B-tubule binds the anterograde IFT motor protein kinesin-II, which transports cargos including tubulin subunits and the retrograde motor cytoplasmic dynein 2. The A-tubule binds the retrograde IFT motor dynein [12]. (C) AxoMTs are targets of microtubule PTMs, which affect MT stability and dynamics. K40 acetylation (orange) occurs on α-tubulin on the lumenal face of the tubule, catalyzed by αTAT1. Glycylation (green) and glutamylation (yellow) occur on the C-terminal tails of α- and β- tubulin.

Figure 3.. Functions of actin within and around cilia.

The ciliary pocket is a hub for actin dynamics and endocytosis. Ciliary ectosomes pinch off from the tip of the ciliary membrane in response to GPCR (purple) activation and blockage of intracellular retrieval machinery [18], prior to cilia disassembly [19], and in response to mating cues in Chlamydomonas [20]. Actin accumulates at the site of membrane pinching [19]. Actin-associated proteins drebrin (green), Myosin VI (blue), and α-actinin 4 (lavender) accumulate at the ciliary tip during excision [18].

Figure 4.. Regulators of cilia and the actin cytoskeleton.

Actin polymerization and dynamics generally inhibit ciliogenesis/promote cilia disassembly. Several cytoskeletal elements, kinases, and critical signaling pathways are involved in the link between cilia and actin. See the text for details. Abbreviations: ATAT-1, α-tubulin acetylatransferase 1; AurA, Aurora A; CTTN, cortactin; GSN, gelsolin; HDAC6, histone deacetylase 1; KDM3A, lysine demethylase 3A; KV10.1, voltage-dependent potassium channel 10.1; MTs, microtubules; Plk1, polo-like kinase 1; YAP, yes-associated protein.

Figure 5.. Diverse roles of cytoplasmic actin cytoskeleton in each stage of the ciliary life cycle.

Cellular actin and myosins mediate accumulation and fusion of vesicles at the PPC to form the ciliary vesicle (CV), basal body migration and docking at the apical surface, ciliary function and length maintenance, and disassembly. Ciliary signaling also regulates canonical functions of cytoplasmic actin, such as reorganization of actin and focal adhesions in cell migration.