Unraveling the mysteries of centriolar satellites: time to rewrite the textbooks about the centrosome/cilium complex

Abstract

Centriolar satellites are membraneless granules that localize and move around centrosomes and cilia. Once referred to as structures with no obvious function, research in the past decade has identified satellites as key regulators of a wide range of cellular and organismal processes. Importantly, these studies have revealed a substantial overlap between functions, proteomes, and disease links of satellites with centrosomes and cilia. Therefore, satellites are now accepted as the "third component" of the vertebrate centrosome/cilium complex, which profoundly changes the way we think about the assembly, maintenance, and remodeling of the complex at the cellular and organismal levels. In this perspective, we first provide an overview of the cellular and structural complexities of centriolar satellites. We then describe the progress in the identification of the satellite interactome, which have paved the way to a molecular understanding of their mechanism of action and assembly mechanisms. After exploring current insights into their functions as recently described by loss-of-function studies and comparative evolutionary approaches, we discuss major unanswered questions regarding their functional and compositional diversity and their functions outside centrosomes and cilia.

FIGURE 1:

Centriolar satellite structure and distribution during cell cycle and in a variety of different cell types. (A) The vertebrate centrosome/cilium complex is composed of centrosomes, cilia, and centriolar satellites. At the core of centrosomes are two centrioles, which recruit pericentriolar material and nucleate formation of the primary cilium. Satellites are an array of membraneless structures that localize and move around centrosomes and cilia in a microtubule and molecular motor–dependent manner. There is compositional heterogeneity among different satellite granules. (B) Satellites are membraneless macromolecular protein complexes. They have extensive interactions with centrosome proteins, microtubule-associated proteins, and enzymes such as kinases and ubiquitin ligases. They interact with the dynein/dynactin complex and multiple kinesin motors, which engage in a “tug-of-war.” Adaptor proteins mediate the interaction between motors and satellites. The residents of satellites are represented as circles within each satellite granule. (C) Satellites are dynamically regulated in mitosis. Human HeLa cells were fixed at different stages of mitosis and stained for satellites (anti-PCM1 antibody, magenta), microtubules (anti–α-tubulin antibody, green), and DNA (DAPI, blue). Satellites localize to duplicated centrosomes in prometaphase and spindle poles during early metaphase, dissolve as cells progress further in mitosis, and recondense back upon mitotic exit. Mitotic satellite dissolution is reflected as an increase in their cytoplasmic pool and decrease in the number of granules. Scale bar: 10 μm. (D–G) Cellular distribution of satellites in different cell types. Drawings are not to scale. (D) Majority of satellites concentrate around the basal body in epithelial cells that form primary cilia. (E) Satellites are remodeled during differentiation of in vitro mouse tracheal epithelial cells (MTEC) after induction with air–liquid interface (ALI). At the centriole amplification stage, satellites form fibrous granules in close proximity to nascent centrioles generated by parental centriole and deuterosome-mediated duplication pathways. In mature ciliated cells, a small pool of satellites is scattered below the basal bodies at the apical surface. (F) In neuronal cells, satellites are scattered throughout the cell body. (G) In muscle cells, satellites are concentrated around the nuclear envelope, the noncentrosomal MTOC. (H) Proliferating and differentiated mouse C2C12 cells, primary embryonic cortical neurons, in vitro MTEC cultures at different stages of differentiation, and retinal pigmental epithelial cells were fixed and stained for satellites (anti-PCM1 antibody, magenta), microtubules (anti–α-tubulin antibody) or centrioles (anti-Centrin antibody, green), apical junction markers (anti-ZO1 antibody) or cilia (anti–acetylated-tubulin antibody, cyan), and DNA (DAPI, blue). Scale bar: 5 μm.

FIGURE 2:

Comparative analysis of the centrosome, primary cilium, and centriolar satellite proteomes. (A) Comparison of shared and distinct protein numbers between satellite proteomes generated by different approaches. Venn diagram of the satellite proteome identified by affinity purification of PCM1 from sucrose gradient fractions enriched for satellites (Quarantotti et al., 2019) and by BioID proximity mapping of 22 satellite proteins (Gheiratmand et al., 2019). (B) Comparison of shared and distinct protein numbers between satellite, centrosome, and primary cilium proteomes. Venn diagram of the satellite proteome identified by affinity purifications (Quarantotti et al., 2019) and BioID-based proximity mapping (Gheiratmand et al., 2019), centrosome proteome identified by enrichment of centrosomes using sucrose gradients followed by protein correlation profiling (Jakobsen et al., 2011), and primary cilium proteome identified by APEX-based proximity mapping of the ciliary targeting domains of NPHP3 (Mick et al., 2015) and HTR6 (Kohli et al., 2017).