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Title: A novel Cep120-dependent mechanism inhibits centriole maturation in quiescent cells}
Article:
Abstract:
The two centrioles of the centrosome in quiescent cells are inherently asymmetric structures that differ in age, morphology and function. How these asymmetric properties are established and maintained during quiescence remains unknown. Here, we show that a daughter centriole-associated ciliopathy protein, Cep120, plays a critical inhibitory role at daughter centrioles. Depletion of Cep120 in quiescent mouse and human cells causes accumulation of pericentriolar material( PCM) components including pericentrin, Cdk5Rap2, ninein and Cep170. The elevated PCM levels result in increased microtubule-nucleation activity at the centrosome. Consequently, loss of Cep120 leads to aberrant dynein-dependent trafficking of centrosomal proteins, dispersal of centriolar satellites, and defective ciliary assembly and signaling. Our results indicate that Cep120 helps to maintain centrosome homeostasis by inhibiting untimely maturation of the daughter centriole, and defines a potentially new molecular defect underlying the pathogenesis of ciliopathies such as Jeune Asphyxiating Thoracic Dystrophy and Joubert syndrome.
Introduction:
The centrosome is the major microtubule-organizing center( MTOC) in animal cells, which helps regulate the assembly of the interphase microtubule array and mitotic spindle. The centrosome in a quiescent cell is composed of a pair of centrioles surrounded by a proteinaceous matrix referred to as pericentriolar material( PCM), involved in microtubule nucleation and anchoring. The two centrioles in quiescent cells differ in age, structure and function. The older of the two was assembled at least two cell cycles ago and is commonly called the ‘mother’ centriole( MC). The younger one, which was assembled in the previous cell cycle, is called the ‘daughter’ centriole( DC). Most activities at the centrosome are regulated and mediated by the mother centriole. For example, the MC acts as the template for the assembly of the primary cilium, an essential chemosensory organelle( Malicki and Johnson, 2017). The MC possesses distinct sets of projections called distal appendages that confer the ability to dock at the plasma membrane and initiate ciliogenesis, and subdistal appendages that help organize the PCM, nucleate and anchor microtubules( Garcia and Reiter, 2016). In contrast, the DC lacks distal and subdistal appendages, does not form a cilium, and possesses significantly less PCM and thus MTOC activity. How and why the DC is prohibited from recruiting these protein complexes remains a mystery. The predominant theory is that the DC undergoes structural and biochemical modifications during the cell cycle that confers the ability to acquire these traits, a multi-step process classically referred to as ‘maturation’( Hoyer-Fender, 2010). However, recent studies suggest that this model may be too simplistic( Kong et al., 2014; Wang et al., 2011). So, how is the DC prohibited from recruiting these protein complexes during quiescence? We previously identified Cep120 as a centrosomal protein that is preferentially enriched on the DC of quiescent cells( Mahjoub et al., 2010). This asymmetric localization between the mother and daughter centriole was relieved coincident with new procentriole assembly in S-phase, where Cep120 became enriched on the procentrioles([Mahjoub et al., 2010]; summarized in Figure 1A). Thus, Cep120 associates with the youngest generation of centrioles at all stages of the cell cycle, although some of the protein remains on the MC([Mahjoub et al., 2010]; Figure 1A). To determine the functional significance of Cep120 enrichment on procentrioles, we depleted the protein and analyzed the consequences on centriole assembly. We found that Cep120 was required for procentriole formation in S-phase of cycling cells, and for nascent centriole assembly in multiciliated cells( Mahjoub et al., 2010), revealing a conserved role for Cep120 in centriole formation. Subsequent studies helped to define the role of Cep120 in the molecular pathways regulating procentriole assembly( Comartin et al., 2013; Lin et al., 2013). More recently, mutations in Cep120 were identified in patients with ciliopathy phenotypes, namely Joubert syndrome and Jeune Asphyxiating Thoracic Dystrophy( Roosing et al., 2016; Shaheen et al., 2015), further highlighting a critical role in centriole-centrosome-cilia biology. However, one major question remains unanswered: what is the functional significance of the enrichment of Cep120 on the DC of quiescent cells, at a stage of the cell cycle when centriole duplication is not occurring?
Discussion:
In this study, we have demonstrated that Cep120 plays an important inhibitory role at the centrosome, by regulating PCM levels during quiescence. Loss of Cep120 initiates a series of changes in protein localization and abundance that ultimately result in defective ciliary assembly and function( Figure 7H). The first step involves changes in the maturation state of the daughter centriole, on which Cep120 is preferentially enriched. Centriole maturation is a multi-step process that includes accumulation of PCM material, assembly of appendage structures, recruitment of proteins essential for ciliary assembly, and elimination of inhibitory/licensing factors( such as CP110) of ciliogenesis. Although most of these components are asymmetrically localized on mother versus daughter centrioles, our results show that Cep120 mainly controls the PCM stage of the maturation process. Removal of Cep120 did not result in complete maturation of the daughter centriole, nor enhance its ability to build a primary cilium. This suggests that the centriole maturation pathway regulated by Cep120 during quiescence is different than the one controlled by Plk1, a kinase that triggers centrosome maturation as cells enter mitosis( Mahen and Venkitaraman, 2012). Indeed, over-expression of Plk1 in G1 can fully induce daughter centriole maturation independently of Cep120 loss, resulting in the assembly of appendages and the formation of a secondary primary cilium( Kong et al., 2014). An alternative possibility is that Cep120 may be one of many targets of Plk1 and thus its depletion would not be expected to phenocopy Plk1 overexpression. However, inducing expression of Plk1 in G1 did not cause removal of Cep120 from the maturing daughter centriole( Kong et al., 2014), suggesting that cells may have evolved an alternative pathway by which they can regulate centriole maturation during quiescence, controlled by protein( s) that localize to the daughter centriole. Depletion of Cep120 resulted in preferential accumulation of PCM components on the daughter centriole. However some PCM proteins, such as ninein and Cep170, showed higher levels of accumulation on the mother centriole. This indicates that the small pool of Cep120 that remains associated with the mother centriole also contains some inhibitory function, and suggests that the levels of certain PCM proteins on the mother centriole are also ‘fine-tuned’ during quiescence. One intriguing reason for this type of control may be to ensure that the mother centriole( and the centrosome in general) assembles a microtubule network that specifically helps target ciliary proteins to the mother centriole( Figure 7H). Indeed, we found that the elevated PCM negatively impacts centrosome homeostasis by disrupting the nucleation and organization of the microtubule cytoskeleton, potentially causing a ‘traffic jam’ that interrupts dynein-dependent movement of CS complexes. It is well established that the organization, localization and trafficking functions of CS are particularly sensitive to defects in the microtubule cytoskeleton( Hori and Toda, 2017). These changes in turn disrupted ciliary assembly and signaling, a process that requires CS-dependent delivery of essential factors to the centrosome. How does Cep120 regulate the asymmetric distribution of PCM proteins at the centrosome? Using immuno-electron microscopy, we previously showed that Cep120 resides on the outer wall of the centriolar microtubules( Mahjoub et al., 2010). Subsequent studies using superresolution microscopy also positioned Cep120 near the centriolar microtubules, placing it in a layer proximal to the centriole wall and surrounded by the PCM proteins( Lawo et al., 2012; Mennella et al., 2014). Moreover, Cep120 contains an N-terminal microtubule-binding domain that was shown to be essential for directly binding microtubules in vitro( Lin et al., 2013). Intriguingly, mutations in this microtubule-binding domain are pathogenic, and cause ciliopathies such as Jeune and Joubert syndrome( Roosing et al., 2016; Shaheen et al., 2015). Thus, one interesting possibility is that Cep120 binds to and occupies sites on the surface of the centriolar microtubule wall, which could inhibit the association of PCM proteins in those locations. The loss of Cep120 from the daughter centriole at G1/S could potentially ‘free’ these sites and permit the accumulation of PCM proteins. The mechanism( s) that establish the asymmetric localization of Cep120 on mother and daughter centrioles remain unknown, and will be the focus of our future studies. In sum, our results highlight a new function for daughter centriole-associated proteins that is independent of their role in centriole duplication and assembly. Furthermore, these data illuminate the potential role( s) of other daughter centriole-enriched proteins, such as centrobin and Neurl4( Li et al., 2012; Zou et al., 2005), in quiescent cells. Finally, our findings establish the important and exquisitely sensitive control of centrosome PCM homeostasis and its relationship to primary cilium assembly and signaling, and uncover a potentially new type of molecular defect that may underlie the pathogenesis of ciliopathies such as Jeune and Joubert syndrome.
Among the countless components of an animal cell , microtubules perform many important roles . These hollow filaments support the cell’s shape and help to transport different materials around within it . They also form a hair-like projection on the cell surface called the primary cilium , which helps the cell sense its environment . Most microtubules in an animal cell are organized by a structure called the centrosome , which has two smaller cylindrical structures called centrioles at its core . In cells that are not dividing , these two centrioles are different in age . The older of the two centrioles was assembled at least two cell divisions ago and is commonly called the “mother” centriole . The younger one , which was assembled the previous time the cell divided , is called the “daughter” centriole . Most activities at the centrosome are controlled by the mother centriole . For example , the mother centriole contains protein complexes called appendages that allow it to dock at the cell surface and build the cilium . The mother centriole also contains a complex of proteins called the pericentriolar material , which helps it assemble microtubules and anchor them in place . In contrast , the daughter centriole lacks appendages , does not form a cilium , has less pericentriolar material and so assembles fewer microtubules . Why the daughter centriole cannot recruit these protein complexes remains a mystery . One possibly important difference between mother and daughter centrioles is that daughter centrioles in non-dividing cells have much higher levels of a protein called Cep120 . Now , Betleja et al . have studied the role of this protein in more detail . Experiments with mouse and human cells show that Cep120 plays an important inhibitory role at the daughter centriole . When the production of Cep120 was blocked , more pericentriolar material associated with the daughter centriole , and more microtubules were assembled by the centrosome . This interfered with the movement of other proteins to the centrosome , which ultimately disrupted both the centrosome’s ability to assemble cilia and the cell’s ability to sense its environment . The findings of Betleja et al . show that a Cep120-dependent mechanism actively regulates the centrosome’s function in non-dividing cells . These experiments uncover a potentially new type of molecular defect that may be responsible for diseases caused by faulty cilia , such as Joubert Syndrome and Jeune Asphyxiating Thoracic Dystrophy . The next challenge will be to understand how Cep120 inhibits the levels of pericentriolar material only at the daughter centriole but not the mother centriole .