Asterless is a Polo-like kinase 4 substrate that both activates and inhibits kinase activity depending on its phosphorylation state

CCDC15 localizes to the centriole inner scaffold and controls centriole length and integrity

Centrioles are microtubule-based organelles responsible for forming centrosomes and cilia, which serve as microtubule-organizing, signaling, and motility centers. Biogenesis and maintenance of centrioles with proper number, size, and architecture are vital for their functions during development and physiology. While centriole number control has been well-studied, less is understood about their maintenance as stable structures with conserved size and architecture during cell division and ciliary motility. Here, we identified CCDC15 as a centriole protein that colocalizes with and interacts with the inner scaffold, a crucial centriolar subcompartment for centriole size control and integrity. Using ultrastructure expansion microscopy, we found that CCDC15 depletion affects centriole length and integrity, leading to defective cilium formation, maintenance, and response to Hedgehog signaling. Moreover, loss-of-function experiments showed CCDC15's role in recruiting both the inner scaffold protein POC1B and the distal SFI1/Centrin-2 complex to centrioles. Our findings reveal players and mechanisms of centriole architectural integrity and insights into diseases linked to centriolar defects.

A simple Turing reaction-diffusion model explains how PLK4 breaks symmetry during centriole duplication and assembly

Centrioles duplicate when a mother centriole gives birth to a daughter that grows from its side. Polo-like-kinase 4 (PLK4), the master regulator of centriole duplication, is recruited symmetrically around the mother centriole, but it then concentrates at a single focus that defines the daughter centriole assembly site. How PLK4 breaks symmetry is unclear. Here, we propose that phosphorylated and unphosphorylated species of PLK4 form the 2 components of a classical Turing reaction-diffusion system. These 2 components bind to/unbind from the surface of the mother centriole at different rates, allowing a slow-diffusing activator species of PLK4 to accumulate at a single site on the mother, while a fast-diffusing inhibitor species of PLK4 suppresses activator accumulation around the rest of the centriole. This "short-range activation/long-range inhibition," inherent to Turing systems, can drive PLK4 symmetry breaking on a either a continuous or compartmentalised Plk4-binding surface, with PLK4 overexpression producing multiple PLK4 foci and PLK4 kinase inhibition leading to a lack of symmetry-breaking and PLK4 accumulation-as observed experimentally.

Cep104 is a component of the centriole distal tip complex that regulates centriole growth and contributes to Drosophila spermiogenesis

Proper centrosome number and function relies on the accurate assembly of centrioles, barrel-shaped structures that form the core duplicating elements of the organelle. The growth of centrioles is regulated in a cell cycle-dependent manner; while new daughter centrioles elongate during the S/G2/M phase, mature mother centrioles maintain their length throughout the cell cycle. Centriole length is controlled by the synchronized growth of the microtubules that ensheathe the centriole barrel. Although proteins exist that target the growing distal tips of centrioles, such as CP110 and Cep97, these proteins are generally thought to suppress centriolar microtubule growth, suggesting that distal tips may also contain unidentified counteracting factors that facilitate microtubule polymerization. Currently, a mechanistic understanding of how distal tip proteins balance microtubule growth and shrinkage to either promote daughter centriole elongation or maintain centriole length is lacking. Using a proximity-labeling screen in Drosophila cells, we identified Cep104 as a novel component of a group of evolutionarily conserved proteins that we collectively refer to as the distal tip complex (DTC). We found that Cep104 regulates centriole growth and promotes centriole elongation through its microtubule-binding TOG domain. Furthermore, analysis of Cep104 null flies revealed that Cep104 and Cep97 cooperate during spermiogenesis to align spermatids and coordinate individualization. Lastly, we mapped the complete DTC interactome and showed that Cep97 is the central scaffolding unit required to recruit DTC components to the distal tip of centrioles.

The E3 ligase Poe promotes Pericentrin degradation

Centrosomes are essential parts of diverse cellular processes, and precise regulation of the levels of their constituent proteins is critical for their function. One such protein is Pericentrin (PCNT) in humans and Pericentrin-like protein (PLP) in Drosophila. Increased PCNT expression and its protein accumulation are linked to clinical conditions including cancer, mental disorders, and ciliopathies. However, the mechanisms by which PCNT levels are regulated remain underexplored. Our previous study demonstrated that PLP levels are sharply down-regulated during early spermatogenesis and this regulation is essential to spatially position PLP on the proximal end of centrioles. We hypothesized that the sharp drop in PLP protein was a result of rapid protein degradation during the male germ line premeiotic G2 phase. Here, we show that PLP is subject to ubiquitin-mediated degradation and identify multiple proteins that promote the reduction of PLP levels in spermatocytes, including the UBR box containing E3 ligase Poe (UBR4), which we show binds to PLP. Although protein sequences governing posttranslational regulation of PLP are not restricted to a single region of the protein, we identify a region that is required for Poe-mediated degradation. Experimentally stabilizing PLP, via internal PLP deletions or loss of Poe, leads to PLP accumulation in spermatocytes, its mispositioning along centrioles, and defects in centriole docking in spermatids.

Polo-like kinase 4 homodimerization and condensate formation regulate its own protein levels but are not required for centriole assembly

Polo-like kinase 4 (Plk4) is the master-regulator of centriole assembly, and cell cycle-dependent regulation of its activity maintains proper centrosome number. During most of the cell cycle, Plk4 levels are nearly undetectable due to its ability to autophosphorylate and trigger its own ubiquitin-mediated degradation. However, during mitotic exit, Plk4 forms a single aggregate on the centriole surface to stimulate centriole duplication. Whereas most Polo-like kinase family members are monomeric, Plk4 is unique because it forms homodimers. Notably, Plk4 trans-autophosphorylates a degron near its kinase domain, a critical step in autodestruction. While it is thought that the purpose of homodimerization is to promote trans-autophosphorylation, this has not been tested. Here, we generated separation-of-function Plk4 mutants that fail to dimerize and show that homodimerization creates a binding site for the Plk4 activator, Asterless. Surprisingly, however, Plk4 dimer mutants are catalytically active in cells, promote centriole assembly, and can trans-autophosphorylate through concentration-dependent condensate formation. Moreover, we mapped and then deleted the weak-interacting regions within Plk4 that mediate condensation and conclude that dimerization and condensation are not required for centriole assembly. Our findings suggest that Plk4 dimerization and condensation function simply to down-regulate Plk4 and suppress centriole overduplication.

Balancing the scales: fine-tuning Polo-like kinase 4 to ensure proper centriole duplication

Polo-like kinase 4 (Plk4) is the master regulator of centriole assembly. Several evolutionarily conserved mechanisms strictly regulate Plk4 abundance and activity to ensure cells maintain a proper number of centrioles. In this issue of Genes & Development, Phan et al. (pp. 718-736) add to this growing list by describing a new mechanism of control that restricts Plk4 translation through competitive ribosome binding at upstream open reading frames (uORFs) in the mature Plk4 mRNA. Fascinatingly, this mechanism is especially critical in the development of primordial germ cells in mice that are transcriptionally hyperactive and thus exquisitely sensitive to Plk4 mRNA regulation.

TEC kinase stabilizes PLK4 to promote liver cancer metastasis

Aberrated PLK4 expression has been reported in different malignancies and causes centrosome amplification, aneuploidy, and genomic instability. However, the mechanism by which PLK4 is regulated in carcinogenesis remains not fully characterised. Here, we showed that PLK4 was overexpressed in human HCC and overexpression of PLK4 predicted poorer patient prognosis. Unexpectedly, we found that induced expression of PLK4 promotes, but knockdown of PLK4 inhibits, HCC cell migration and invasion. Mechanistically, we found that TEC tyrosine kinase, which also promotes HCC cell migration, stabilizes PLK4 by phosphorylation. TEC directly phosphorylates PLK4 at tyrosine 86 residue, which not only stabilizes the protein but also enhances PLK4-mediated HCC cell invasion. Further investigation by transcriptome sequencing indicated that PLK4 promotes the phosphorylation of focal adhesion kinase to regulate the focal adhesion pathway in HCC cell migration. Taken together, our results demonstrated that PLK4 plays an important role in HCC metastasis and revealed for the first time the mechanism by which PLK4 promotes HCC metastasis via TEC phosphorylation.

Autonomous clocks that regulate organelle biogenesis, cytoskeletal organization, and intracellular dynamics

How do cells perceive time? Do cells use temporal information to regulate the production/degradation of their enzymes, membranes, and organelles? Does controlling biological time influence cytoskeletal organization and cellular architecture in ways that confer evolutionary and physiological advantages? Potential answers to these fundamental questions of cell biology have historically revolved around the discussion of 'master' temporal programs, such as the principal cyclin-dependent kinase/cyclin cell division oscillator and the circadian clock. In this review, we provide an overview of the recent evidence supporting an emerging concept of 'autonomous clocks,' which under normal conditions can be entrained by the cell cycle and/or the circadian clock to run at their pace, but can also run independently to serve their functions if/when these major temporal programs are halted/abrupted. We begin the discussion by introducing recent developments in the study of such clocks and their roles at different scales and complexities. We then use current advances to elucidate the logic and molecular architecture of temporal networks that comprise autonomous clocks, providing important clues as to how these clocks may have evolved to run independently and, sometimes at the cost of redundancy, have strongly coupled to run under the full command of the cell cycle and/or the circadian clock. Next, we review a list of important recent findings that have shed new light onto potential hallmarks of autonomous clocks, suggestive of prospective theoretical and experimental approaches to further accelerate their discovery. Finally, we discuss their roles in health and disease, as well as possible therapeutic opportunities that targeting the autonomous clocks may offer.

Plk4 triggers autonomous de novo centriole biogenesis and maturation

Centrioles form centrosomes and cilia. In most proliferating cells, centrioles assemble through canonical duplication, which is spatially, temporally, and numerically regulated by the cell cycle and the presence of mature centrioles. However, in certain cell types, centrioles assemble de novo, yet by poorly understood mechanisms. Herein, we established a controlled system to investigate de novo centriole biogenesis, using Drosophila melanogaster egg explants overexpressing Polo-like kinase 4 (Plk4), a trigger for centriole biogenesis. We show that at a high Plk4 concentration, centrioles form de novo, mature, and duplicate, independently of cell cycle progression and of the presence of other centrioles. Plk4 concentration determines the temporal onset of centriole assembly. Moreover, our results suggest that distinct biochemical kinetics regulate de novo and canonical biogenesis. Finally, we investigated which other factors modulate de novo centriole assembly and found that proteins of the pericentriolar material (PCM), and in particular γ-tubulin, promote biogenesis, likely by locally concentrating critical components.

Emerging insights into symmetry breaking in centriole duplication: updated view on centriole duplication theory

Centriole duplication occurs once per cell cycle. Since only a single daughter centriole is assembled adjacent to each mother centriole, symmetry around the mother centriole must be broken in the process of centriole duplication. Recent studies have established that Plk4, a master kinase for centriole duplication, can self-assemble into condensates, and have suggested that this Plk4 self-assembly is the key to symmetry breaking. Here, we present the current hypotheses for how Plk4 could break symmetry around the mother centriole via autonomous regulation. After this initial symmetry-breaking process, the ring-to-dot conversion of Plk4 around the mother centriole completes the selection of the site for procentriole formation. We also discuss how this dynamic transition contributes to the strict regulation of centriole duplication.

A self-assembled cylindrical platform for Plk4-induced centriole biogenesis

The centrosome, a unique membraneless multiprotein organelle, plays a pivotal role in various cellular processes that are critical for promoting cell proliferation. Faulty assembly or organization of the centrosome results in abnormal cell division, which leads to various human disorders including cancer, microcephaly and ciliopathy. Recent studies have provided new insights into the stepwise self-assembly of two pericentriolar scaffold proteins, Cep63 and Cep152, into a near-micrometre-scale higher-order structure whose architectural properties could be crucial for proper execution of its biological function. The construction of the scaffold architecture appears to be centrally required for tight control of a Ser/Thr kinase called Plk4, a key regulator of centriole duplication, which occurs precisely once per cell cycle. In this review, we will discuss a new paradigm for understanding how pericentrosomal scaffolds are self-organized into a new functional entity and how, on the resulting structural platform, Plk4 undergoes physico-chemical conversion to trigger centriole biogenesis.

Drosophila Sas-6, Ana2 and Sas-4 self-organise into macromolecular structures that can be used to probe centriole and centrosome assembly

Centriole assembly requires a small number of conserved proteins. The precise pathway of centriole assembly has been difficult to study, as the lack of any one of the core assembly proteins [Plk4, Ana2 (the homologue of mammalian STIL), Sas-6, Sas-4 (mammalian CPAP) or Asl (mammalian Cep152)] leads to the absence of centrioles. Here, we use Sas-6 and Ana2 particles (SAPs) as a new model to probe the pathway of centriole and centrosome assembly. SAPs form in Drosophila eggs or embryos when Sas-6 and Ana2 are overexpressed. SAP assembly requires Sas-4, but not Plk4, whereas Asl helps to initiate SAP assembly but is not required for SAP growth. Although not centrioles, SAPs recruit and organise many centriole and centrosome components, nucleate microtubules, organise actin structures and compete with endogenous centrosomes to form mitotic spindle poles. SAPs require Asl to efficiently recruit pericentriolar material (PCM), but Spd-2 (the homologue of mammalian Cep192) can promote some PCM assembly independently of Asl. These observations provide new insights into the pathways of centriole and centrosome assembly.

An Autonomous Oscillation Times and Executes Centriole Biogenesis

The accurate timing and execution of organelle biogenesis is crucial for cell physiology. Centriole biogenesis is regulated by Polo-like kinase 4 (Plk4) and initiates in S-phase when a daughter centriole grows from the side of a pre-existing mother. Here, we show that a Plk4 oscillation at the base of the growing centriole initiates and times centriole biogenesis to ensure that centrioles grow at the right time and to the right size. The Plk4 oscillation is normally entrained to the cell-cycle oscillator but can run autonomously of it-potentially explaining why centrioles can duplicate independently of cell-cycle progression. Mathematical modeling indicates that the Plk4 oscillation can be generated by a time-delayed negative feedback loop in which Plk4 inactivates the interaction with its centriolar receptor through multiple rounds of phosphorylation. We hypothesize that similar organelle-specific oscillations could regulate the timing and execution of organelle biogenesis more generally.

Evaluation of the Potential Phosphorylation Effect on Isocitrate Dehydrogenases from Saccharomyces cerevisiae and Yarrowia lipolytica

Escherichia coli isocitrate dehydrogenase (IDH) is regulated by reversible phosphorylation on Ser113. Latest phosphoproteomic studies revealed that eukaryotic IDHs can also be phosphorylated on the analogous Ser site. So as to understand the possible phosphorylation mechanism, the equivalent Ser of NADP-IDHs from yeast Saccharomyces cerevisiae (ScIDH) and Yarrowia lipolytica(YlIDH) were investigated by site-directed mutagenesis. ScIDH Ser110 and YlIDH Ser103 were replaced by Asp or Glu to mimic a continuous phosphorylation state. Meanwhile, the effects of another four amino acids (Thr, Tyr, Gly, Ala) with various side chain on IDH activity were determined as well. Enzymatic analysis showed that replacement of Ser with Asp or Glu nearly inactivated ScIDH and YlIDH. Four other mutant enzymes of ScIDH, S110T, S110G, S110A, and S110Y, retained 38.07%, 3.24%, 2.65%, and 0.01% of its original activity, and four other mutant enzymes of YlIDH, S103T, S103G, S103A, and S103Y retained 44.26%, 27.99%, 16.29%, and 0.01% of its original activity, respectively. These results suggested that phosphorylation on eukaryotic IDHs has identical consequence to that on the bacterial IDHs. We thus presume that phosphorylation on the substrate-binding Ser shall be a common regulatory mechanism among IDHs.

Centriole Biogenesis: Symmetry Breaking and Site Selection

Centrioles must duplicate as cells progress through the cell cycle but it is unclear how the site of duplication is selected. A recent computational study demonstrates that two separate but interacting feedback mechanisms (autocatalytic activation and substrate depletion) are capable of selecting a single site for centriole biogenesis.