NEDD1-S411 phosphorylation plays a critical function in the coordination of microtubule nucleation during mitosis

During mitosis, spindle assembly relies on centrosomal and acentrosomal microtubule nucleation pathways that all require the γ-Tubulin Ring Complex (γ-TuRC) and its adaptor protein NEDD1. The activity of these different pathways needs to be coordinated to ensure bipolar spindle assembly ( Cavazza et al., 2016) but the underlying mechanism is still unclear. Previous studies have identified three sites in NEDD1 (S377, S405 and S411) that when phosphorylated drive MT nucleation at the centrosomes, around the chromosomes and on pre-existing MTs respectively ( Lüders et al., 2006; Pinyol et al., 2013; Sdelci et al., 2012). Here we aimed at getting additional insights into the mechanism that coordinates the different MT nucleation pathways in dividing cells using a collection of HeLa stable inducible cell lines expressing NEDD1 phospho-variants at these three sites and Xenopus egg extracts. Our results provide further support for the essential role of phosphorylation at the three residues. Moreover, we directly demonstrate that S411 phosphorylation is essential for MT branching using TIRF microscopy in Xenopus egg extracts and we show that it plays a crucial role in ensuring the balance between centrosome and chromosome-dependent MT nucleation required for bipolar spindle assembly in mitotic cells.

Reconstitution and mechanistic dissection of the human microtubule branching machinery

Branching microtubule (MT) nucleation is mediated by the augmin complex and γ-tubulin ring complex (γ-TuRC). However, how these two complexes work together to promote this process remains elusive. Here, using purified components from native and recombinant sources, we demonstrate that human augmin and γ-TuRC are sufficient to reconstitute the minimal MT branching machinery, in which NEDD1 bridges between augmin holo complex and GCP3/MZT1 subcomplex of γ-TuRC. The single-molecule experiment suggests that oligomerization of augmin may activate the branching machinery. We provide direct biochemical evidence that CDK1- and PLK1-dependent phosphorylation are crucial for NEDD1 binding to augmin, for their synergistic MT-binding activities, and hence for branching MT nucleation. In addition, we unveil that NEDD1 possesses an unanticipated intrinsic affinity for MTs via its WD40 domain, which also plays a pivotal role in the branching process. In summary, our study provides a comprehensive understanding of the underlying mechanisms of branching MT nucleation in human cells.

Molecular insight into how γ-TuRC makes microtubules

As one of four filament types, microtubules are a core component of the cytoskeleton and are essential for cell function. Yet how microtubules are nucleated from their building blocks, the αβ-tubulin heterodimer, has remained a fundamental open question since the discovery of tubulin 50 years ago. Recent structural studies have shed light on how γ-tubulin and the γ-tubulin complex proteins (GCPs) GCP2 to GCP6 form the γ-tubulin ring complex (γ-TuRC). In parallel, functional and single-molecule studies have informed on how the γ-TuRC nucleates microtubules in real time, how this process is regulated in the cell and how it compares to other modes of nucleation. Another recent surprise has been the identification of a second essential nucleation factor, which turns out to be the well-characterized microtubule polymerase XMAP215 (also known as CKAP5, a homolog of chTOG, Stu2 and Alp14). This discovery helps to explain why the observed nucleation activity of the γ-TuRC in vitro is relatively low. Taken together, research in recent years has afforded important insight into how microtubules are made in the cell and provides a basis for an exciting era in the cytoskeleton field.

Microtubule nucleation by γ-tubulin complexes and beyond

In this short review, we give an overview of microtubule nucleation within cells. It is nearly 30 years since the discovery of γ-tubulin, a member of the tubulin superfamily essential for proper microtubule nucleation in all eukaryotes. γ-tubulin associates with other proteins to form multiprotein γ-tubulin ring complexes (γ-TuRCs) that template and catalyse the otherwise kinetically unfavourable assembly of microtubule filaments. These filaments can be dynamic or stable and they perform diverse functions, such as chromosome separation during mitosis and intracellular transport in neurons. The field has come a long way in understanding γ-TuRC biology but several important and unanswered questions remain, and we are still far from understanding the regulation of microtubule nucleation in a multicellular context. Here, we review the current literature on γ-TuRC assembly, recruitment, and activation and discuss the potential importance of γ-TuRC heterogeneity, the role of non-γ-TuRC proteins in microtubule nucleation, and whether γ-TuRCs could serve as good drug targets for cancer therapy.

Organization and Function of Non-dynamic Biomolecular Condensates

Cells compartmentalize biochemical reactions using organelles. Organelles can be either membrane-bound compartments or supramolecular assemblies of protein and ribonucleic acid known as 'biomolecular condensates'. Biomolecular condensates, such as nucleoli and germ granules, have been described as liquid like, as they have the ability to fuse, flow, and undergo fission. Recent experiments have revealed that some liquid-like condensates can mature over time to form stable gels. In other cases, biomolecular condensates solidify into amyloid-like fibers. Here we discuss the assembly, organization, and physiological roles of these more stable condensates in cells, focusing on Balbiani bodies, centrosomes, nuclear pores, and amyloid bodies. We discuss how the material properties of these condensates can be explained by the principles of liquid-liquid phase separation and maturation.

Divergent regulation of functionally distinct γ-tubulin complexes during differentiation

Differentiation induces the formation of noncentrosomal microtubule arrays in diverse tissues. The formation of these arrays requires loss of microtubule-organizing activity (MTOC) at the centrosome, but the mechanisms regulating this transition remain largely unexplored. Here, we use the robust loss of centrosomal MTOC activity in the epidermis to identify two pools of γ-tubulin that are biochemically and functionally distinct and differentially regulated. Nucleation-competent CDK5RAP2-γ-tubulin complexes were maintained at centrosomes upon initial epidermal differentiation. In contrast, Nedd1-γ-tubulin complexes did not promote nucleation but were required for anchoring of microtubules, a previously uncharacterized activity for this complex. Cell cycle exit specifically triggered loss of Nedd1-γ-tubulin complexes, providing a mechanistic link connecting MTOC activity and differentiation. Collectively, our studies demonstrate that distinct γ-tubulin complexes regulate different microtubule behaviors at the centrosome and show that differential regulation of these complexes drives loss of centrosomal MTOC activity.

Mechanisms of Mitotic Spindle Assembly

Life depends on cell proliferation and the accurate segregation of chromosomes, which are mediated by the microtubule (MT)-based mitotic spindle and ∼200 essential MT-associated proteins. Yet, a mechanistic understanding of how the mitotic spindle is assembled and achieves chromosome segregation is still missing. This is mostly due to the density of MTs in the spindle, which presumably precludes their direct observation. Recent insight has been gained into the molecular building plan of the metaphase spindle using bulk and single-molecule measurements combined with computational modeling. MT nucleation was uncovered as a key principle of spindle assembly, and mechanistic details about MT nucleation pathways and their coordination are starting to be revealed. Lastly, advances in studying spindle assembly can be applied to address the molecular mechanisms of how the spindle segregates chromosomes.

GCP-WD mediates γ-TuRC recruitment and the geometry of microtubule nucleation in interphase arrays of Arabidopsis

Many differentiated animal cells, and all higher plant cells, build interphase microtubule arrays of specific architectures without benefit of a central organizer, such as a centrosome, to control the location and geometry of microtubule nucleation. These acentrosomal arrays support essential cell functions such as morphogenesis, but the mechanisms by which the new microtubules are positioned and oriented are poorly understood. In higher plants, nucleation of microtubules arises from distributed γ-tubulin ring complexes (γ-TuRCs) at the cell cortex that are associated primarily with existing microtubules and from which new microtubules are nucleated in a geometrically bimodal fashion, either in parallel to the mother microtubule or as a branching event at a mean angle of approximately 40° to the mother microtubule. By imaging the dynamics of individual nucleation events in Arabidopsis, we found that a conserved peripheral protein of the γ-TuRC, GCP-WD/NEDD1, associated with motile γ-TuRCs and localized to nucleation events. Knockdown of this essential protein resulted in reduction of γ-TuRC recruitment to cortical microtubules and total nucleation frequency, showing that GCP-WD controls γ-TuRC positioning and function in these interphase arrays. Further, we discovered an unexpected role for GCP-WD in determining the geometry of microtubule-dependent microtubule nucleation, where it acts to increase the likelihood of branching over parallel nucleation. Cells with normally complex patterns of cortical array organization constructed simpler arrays with cell-wide ordering, suggesting that control of nucleation frequency, positioning, and geometry by GCP-WD allows plant cells to build alternative cortical array architectures.

Nek9 phosphorylation of NEDD1/GCP-WD contributes to Plk1 control of γ-tubulin recruitment to the mitotic centrosome

The accumulation of γ-tubulin at the centrosomes during maturation is a key mechanism that ensures the formation of two dense microtubule (MT) asters in cells entering mitosis, defining spindle pole positioning and ensuring the faithful outcome of cell division. Centrosomal γ-tubulin recruitment depends on the adaptor protein NEDD1/GCP-WD and is controlled by the kinase Plk1. Surprisingly, and although Plk1 binds and phosphorylates NEDD1 at multiple sites, the mechanism by which this kinase promotes the centrosomal recruitment of γ-tubulin has remained elusive. Using Xenopus egg extracts and mammalian cells, we now show that it involves Nek9, a NIMA-family kinase required for normal mitotic progression and spindle organization. Nek9 phosphorylates NEDD1 on Ser377 driving its recruitment and thereby that of γ-tubulin to the centrosome in mitotic cells. This role of Nek9 requires its activation by Plk1-dependent phosphorylation but is independent from the downstream related kinases Nek6 and Nek7. Our data contribute to understand the mechanism by which Plk1 promotes the recruitment of γ-tubulin to the centrosome in dividing cells and position Nek9 as a key regulator of centrosome maturation.

Novel NEDD1 phosphorylation sites regulate γ-tubulin binding and mitotic spindle assembly

During cell division, microtubules organize a bipolar spindle to drive accurate chromosome segregation to daughter cells. Microtubules are nucleated by the γ-TuRC, a γ-tubulin complex that acts as a template for microtubules with 13 protofilaments. Cells lacking γ-TuRC core components do nucleate microtubules; however, these polymers fail to form bipolar spindles. NEDD1 is a γ-TuRC-interacting protein whose depletion, although not affecting γ-TuRC stability, causes spindle defects similar to the inhibition of its core subunits, including γ-tubulin. Several residues of NEDD1 are phosphorylated in mitosis. However, previously identified phosphorylation sites only partially regulate NEDD1 function, as NEDD1 depletion has a much stronger phenotype than mutation of these residues. Using mass spectrometry, we have identified multiple novel phosphorylated sites in the serine (S)557-S574 region of NEDD1, close to its γ-tubulin-binding domain. Serine to alanine mutations in S565-S574 inhibit the binding of NEDD1 to γ-tubulin and perturb NEDD1 mitotic function, yielding microtubule organization defects equivalent to those observed in NEDD1-depleted cells. Interestingly, additional mutations in the S557-T560 region restore the capacity of NEDD1 to bind γ-tubulin and promote bipolar spindle assembly. All together, our data suggest that the NEDD1/γ-tubulin interaction is finely tuned by multiple phosphorylation events in the S557-S574 region and is critical for spindle assembly. We also found that CEP192, a centrosomal protein similarly required for spindle formation, associates with NEDD1 and modulates its mitotic phosphorylation. Thus CEP192 may regulate spindle assembly by modulating NEDD1 function.

Dgp71WD is required for the assembly of the acentrosomal Meiosis I spindle, and is not a general targeting factor for the γ-TuRC

Dgp71WD/Nedd1 proteins are essential for mitotic spindle formation. In human cells, Nedd1 targets γ-tubulin to both centrosomes and spindles, but in other organisms the function of Dgp71WD/Nedd1 is less clear. In Drosophila cells, Dgp71WD plays a major part in targeting γ-tubulin to spindles, but not centrosomes, while in Xenopus egg extracts, Nedd1 acts as a more general microtubule (MT) organiser that can function independently of γ-tubulin. The interpretation of these studies, however, is complicated by the fact that some residual Dgp71WD/Nedd1 is likely present in the cells/extracts analysed. Here we generate a Dgp71WD null mutant lacking all but the last 12 nucleotides of coding sequence. The complete loss of Dgp71WD has no quantifiable effect on γ-tubulin or Centrosomin recruitment to the centrosome in larval brain cells. The recruitment of γ-tubulin to spindle MTs, however, is severely impaired, and spindle MT density is reduced in a manner that is indistinguishable from cells lacking Augmin or γ-TuRC function. In contrast, the absence of Dgp71WD leads to defects in the assembly of the acentrosomal female Meiosis I spindle that are more severe than those seen in Augmin or γ-TuRC mutants, indicating that Dgp71WD has additional functions that are independent of these complexes in oocytes. Moreover, the localisation of bicoid RNA during oogenesis, which requires γ-TuRC function, is unperturbed in Dgp71WD(120) mutants. Thus, Dgp71WD is not simply a general cofactor required for γ-TuRC and/or Augmin targeting, and it appears to have a crucial role independent of these complexes in the acentrosomal Meiosis I spindle.

The where, when and how of microtubule nucleation – one ring to rule them all

The function of microtubules depends on their arrangement into highly ordered arrays. Spatio-temporal control over the formation of new microtubules and regulation of their properties are central to the organization of these arrays. The nucleation of new microtubules requires γ-tubulin, an essential protein that assembles into multi-subunit complexes and is found in all eukaryotic organisms. However, the way in which γ-tubulin complexes are regulated and how this affects nucleation and, potentially, microtubule behavior, is poorly understood. γ-tubulin has been found in complexes of various sizes but several lines of evidence suggest that only large, ring-shaped complexes function as efficient microtubule nucleators. Human γ-tubulin ring complexes (γTuRCs) are composed of γ-tubulin and the γ-tubulin complex components (GCPs) 2, 3, 4, 5 and 6, which are members of a conserved protein family. Recent work has identified additional unrelated γTuRC subunits, as well as a large number of more transient γTuRC interactors. In this Commentary, we discuss the regulation of γTuRC-dependent microtubule nucleation as a key mechanism of microtubule organization. Specifically, we focus on the regulatory roles of the γTuRC subunits and interactors and present an overview of other mechanisms that regulate γTuRC-dependent microtubule nucleation and organization.

Alcohol induced epigenetic perturbations during the inflammatory stage of fracture healing

It is well recognized by orthopedic surgeons that fractures of alcoholics are more difficult to heal successfully and have a higher incidence of non-union, but the mechanism of alcohol's effect on fracture healing is unknown. In order to give direction for the study of the effects of alcohol on fracture healing, we propose to identify gene expression and microRNA changes during the early stages of fracture healing that might be attributable to alcohol consumption. As the inflammatory stage appears to be the most critical for successful fracture healing, this paper focuses on the events at day three following fracture or the stage of inflammation. Sprague-Dawley rats were placed on an ethanol-containing or pair-fed Lieber and DeCarli diet for four weeks prior to surgical fracture. Following insertion of a medullary pin, a closed mid-diaphyseal fracture was induced using a Bonnarens and Einhorn fracture device. At three days' post-fracture, the region of the fracture calluses was harvested from the right hind-limb. RNA was extracted and microarray analysis was conducted against the entire rat genome. There were 35 genes that demonstrated significant increased expression due to alcohol consumption and 20 that decreased due to alcohol. In addition, the expression of 20 microRNAs was increased and six decreased. In summary, while it is recognized that mRNA levels may or may not represent protein levels successfully produced by the cell, these studies reveal changes in gene expression that support the hypothesis that alcohol consumption affects events involved with inflammation. MicroRNAs are known to modulate mRNA and these findings were consistent with much of what was seen with mRNA microarray analysis, especially the involvement of smad4 which was demonstrated by mRNA microarray, microRNA and polymerase chain reaction.

A direct interaction with NEDD1 regulates gamma-tubulin recruitment to the centrosome

The centrosome is the primary microtubule organizing centre of the cell. γ-tubulin is a core component of the centrosome and is required for microtubule nucleation and centrosome function. The recruitment of γ-tubulin to centrosomes is mediated by its interaction with NEDD1, a WD40-repeat containing protein. Here we demonstrate that NEDD1 is likely to be oligomeric in vivo and binds directly to γ-tubulin through a small region of just 62 residues at the carboxyl-terminus of the protein. This carboxyl-terminal domain that binds γ-tubulin has a helical structure and is a stable tetramer in solution. Mutation of residues in NEDD1 that disrupt binding to γ-tubulin result in a mis-localization of γ-tubulin away from the centrosome. Hence, this study defines the binding site on NEDD1 that is required for its interaction with γ-tubulin, and shows that this interaction is required for the correct localization of γ-tubulin.

An essential function for the centrosomal protein NEDD1 in zebrafish development

The centrosome is the primary microtubule organising centre of the cell. It is composed of many proteins, some of which make up the core of the centrosome, whereas others are used for specific functions. Although the cellular roles of many centrosomal proteins are well defined, much less is known about their functions and the role of the centrosome in development. In this study we investigated the function of NEDD1, a critical component of the centrosome essential for microtubule nucleation, in zebrafish (Danio rerio) development. The zebrafish homologue of NEDD1 (zNEDD1) was cloned and found to have a similar localisation and function to mammalian NEDD1. We show that zNEDD1 is essential for survival, as a high level of knockdown was embryonic lethal. Partial knockdown of zNEDD1 caused abnormalities including an increase in mitotic and apoptotic cells. Pronounced phenotypic defects were seen in the brain, with a lack of defined brain structures, incomplete neural tube formation and a disorganisation of neurons. In addition, we show that a reduction in zNEDD1 resulted in the loss of gamma-tubulin at the centrosome. Our data thus demonstrate that zNEDD1 is critical for the recruitment of gamma-tubulin to the centrosome, and is essential for the proper development of zebrafish.

NEDD1 is crucial for meiotic spindle stability and accurate chromosome segregation in mammalian oocytes

Defects in meiotic spindle structure contribute to chromosome segregation errors leading to genomic instability in oocytes and embryos upon fertilization. In this study, we analyzed the mechanisms that control spindle microtubule nucleation and stability in mammalian oocytes, and identified NEDD1/GCP-WD as a key regulator. NEDD1 specifically co-localizes with gamma-tubulin and pericentrin at microtubule-organizing centers (MTOCs) in mouse oocytes arrested at prophase-I. During metaphase-I and metaphase-II, the protein remains associated with MTOCs, in a pericentrin dependent manner. Notably, knockdown of Nedd1 transcripts using specific siRNAs resulted in a high incidence (65-70%) of metaphase-I arrest. The arrested oocytes were characterized by disrupted meiotic spindle structure, reduced microtubule density and significant chromosome misalignment. Detection of MAD2 at kinetochores indicated an absence of stable chromosome-microtubule attachment as well as activation of the spindle assembly checkpoint (SAC). Importantly, the disruption of meiotic spindle stability was associated with decreased gamma-tubulin at MTOCs in NEDD1-depleted oocytes, as well as a high frequency of chromosome non-disjunction errors leading to aneuploidy (50%) in the oocytes that did progress to metaphase-II. This study demonstrates that NEDD1 is an essential component of acentriolar oocyte MTOCs, which functions in the regulation of meiotic spindle stability. Moreover, it underscores that disruption of spindle stability in oocytes can lead to chromosomes segregation errors that are not fully resolved by SAC.

In vivo analysis of the functions of gamma-tubulin-complex proteins

To enhance our understanding of the function(s) of gamma-tubulin-complex proteins (GCPs), we identified and analyzed the functions of the Aspergillus nidulans homologs of GCP2-GCP6 (here designated GCPB-GCBF). The gamma-tubulin small complex (gamma-TuSC) components, gamma-tubulin, GCPB and GCPC, are essential for viability and mitotic spindle formation, whereas GCPD-GCPF are not essential for viability, spindle formation or sexual reproduction. GCPD-GCPF function in reducing the frequency of chromosome mis-segregation and in the assembly of large gamma-tubulin complexes. Deletion of any of the gamma-TuSC components eliminates the localization of all GCPs to the spindle pole body (SPB), whereas deletion of GCPD-GCPF does not affect localization of gamma-TuSC components. Thus, GCPD-GCPF do not tether the gamma-TuSC to the SPB, but, rather, the gamma-TuSC tethers them to the SPB. GCPD-GCPF exhibit a hierarchy of localization to the SPB. Deletion of GCPF eliminates GCPD-GCPE localization to the SPB, and deletion of GCPD eliminates GCPE (but not GCPF) localization. All GCPs localize normally in a GCPE deletion. We propose a model for the structure of the gamma-tubulin complex and its attachment to polar microtubule organizing centers.

Sequential phosphorylation of Nedd1 by Cdk1 and Plk1 is required for targeting of the gammaTuRC to the centrosome

Nedd1 is a new member of the gamma-tubulin ring complex (gammaTuRC) and targets the gammaTuRC to the centrosomes for microtubule nucleation and spindle assembly in mitosis. Although its role is known, its functional regulation mechanism remains unclear. Here we report that the function of Nedd1 is regulated by Cdk1 and Plk1. During mitosis, Nedd1 is firstly phosphorylated at T550 by Cdk1, which creates a binding site for the polo-box domain of Plk1. Then, Nedd1 is further phosphorylated by Plk1 at four sites: T382, S397, S637 and S426. The sequential phosphorylation of Nedd1 by Cdk1 and Plk1 promotes its interaction with gamma-tubulin for targeting the gammaTuRC to the centrosome and is important for spindle formation. Knockdown of Plk1 by RNAi decreases Nedd1 phosphorylation and attenuates Nedd1 accumulation at the spindle pole and subsequent gamma-tubulin recruitment at the spindle pole for microtubule nucleation. Taken together, we propose that the sequential phosphorylation of Nedd1 by Cdk1 and Plk1 plays a pivotal role in targeting gammaTuRC to the centrosome by promoting the interaction of Nedd1 with the gammaTuRC component gamma-tubulin, during mitosis.

The WD40 repeat protein NEDD1 functions in microtubule organization during cell division in Arabidopsis thaliana

Although cells of flowering plants lack a structurally defined microtubule-organizing center like the centrosome, organization of the spindles and phragmoplasts in mitosis is known to involve the evolutionarily conserved gamma-tubulin complex. We have investigated the function of Arabidopsis thaliana NEDD1, a WD40 repeat protein related to the animal NEDD1/GCP-WD protein, which interacts with the gamma-tubulin complex. The NEDD1 protein decorates spindle microtubules (MTs) preferentially toward spindle poles and phragmoplast MTs toward their minus ends. A T-DNA insertional allele of the single NEDD1 gene was isolated and maintained in heterozygous sporophytes, and NEDD1's function in cell division was analyzed in haploid microspores produced by the heterozygote. In approximately half of the dividing microspores exhibiting aberrant MT organization, spindles were no longer restricted to the cell periphery and became abnormally elongated. After mitosis, MTs aggregated between reforming nuclei but failed to appear in a bipolar configuration. Consequently, defective microspores did not form a continuous cell plate, and two identical nuclei were produced with no differentiation into generative and vegetative cells. Our results support the notion that the plant NEDD1 homolog plays a critical role in MT organization during mitosis, and its function is likely linked to that of the gamma-tubulin complex.