Main Microtubule-organizing Center Research Articles

Centrosome age breaks spindle size symmetry even in cells thought to divide symmetrically.

Centrosomes are the main microtubule-organizing centers in animal cells. Due to the semiconservative nature of centrosome duplication, the two centrosomes differ in age. In asymmetric stem cell divisions, centrosome age can induce an asymmetry in half-spindle lengths. However, whether centrosome age affects the symmetry of the two half-spindles in tissue culture cells thought to divide symmetrically is unknown. Here, we show that in human epithelial and fibroblastic cell lines centrosome age imposes a mild spindle asymmetry that leads to asymmetric cell daughter sizes. At the mechanistic level, we show that this asymmetry depends on a cenexin-bound pool of the mitotic kinase Plk1, which favors the preferential accumulation on old centrosomes of the microtubule nucleation-organizing proteins pericentrin, γ-tubulin, and Cdk5Rap2, and microtubule regulators TPX2 and ch-TOG. Consistently, we find that old centrosomes have a higher microtubule nucleation capacity. We postulate that centrosome age breaks spindle size symmetry via microtubule nucleation even in cells thought to divide symmetrically.

β-catenin mediates growth defects induced by centrosome loss in a subset of APC mutant colorectal cancer independently of p53.

Colorectal cancer is the third most common cancer and the second leading cause of cancer-related deaths worldwide. The centrosome is the main microtubule-organizing center in animal cells and centrosome amplification is a hallmark of cancer cells. To investigate the importance of centrosomes in colorectal cancer, we induced centrosome loss in normal and cancer human-derived colorectal organoids using centrinone B, a Polo-like kinase 4 (Plk4) inhibitor. We show that centrosome loss represses human normal colorectal organoid growth in a p53-dependent manner in accordance with previous studies in cell models. However, cancer colorectal organoid lines exhibited different sensitivities to centrosome loss independently of p53. Centrinone-induced cancer organoid growth defect/death positively correlated with a loss of function mutation in the APC gene, suggesting a causal role of the hyperactive WNT pathway. Consistent with this notion, β-catenin inhibition using XAV939 or ICG-001 partially prevented centrinone-induced death and rescued the growth two APC-mutant organoid lines tested. Our study reveals a novel role for canonical WNT signaling in regulating centrosome loss-induced growth defect/death in a subset of APC-mutant colorectal cancer independently of the classical p53 pathway.

Single-cell proteo-genomic reveals a comprehensive map of centrosome-associated spliceosome components

Ribonucleoprotein (RNP) condensates are crucial for controlling RNA metabolism and splicing events in animal cells. We used spatial proteomics and transcriptomic to elucidate RNP interaction networks at the centrosome, the main microtubule-organizing center in animal cells. We found a number of cell-type specific centrosome-associated spliceosome interactions localized in subcellular structures involved in nuclear division and ciliogenesis. A component of the nuclear spliceosome BUD31 was validated as an interactor of the centriolar satellite protein OFD1. Analysis of normal and disease cohorts identified the cholangiocarcinoma as target of centrosome-associated spliceosome alterations. Multiplexed single-cell fluorescent microscopy for the centriole linker CEP250 and spliceosome components including BCAS2, BUD31, SRSF2 and DHX35 recapitulated bioinformatic predictions on the centrosome-associated spliceosome components tissue-type specific composition. Collectively, centrosomes and cilia act as anchor for cell-type specific spliceosome components, and provide a helpful reference for explore cytoplasmic condensates functions in defining cell identity and in the origin of rare diseases.

Divergent Contribution of the Golgi Apparatus to Microtubule Organization in Related Cell Lines.

Membrane trafficking in interphase animal cells is accomplished mostly along the microtubules. Microtubules are often organized radially by the microtubule-organizing center to coordinate intracellular transport. Along with the centrosome, the Golgi often serves as a microtubule-organizing center, capable of nucleating and retaining microtubules. Recent studies revealed the role of a special subset of Golgi-derived microtubules, which facilitates vesicular traffic from this central transport hub of the cell. However, proteins essential for microtubule organization onto the Golgi might be differentially expressed in different cell lines, while many potential participants remain undiscovered. In the current work, we analyzed the involvement of the Golgi complex in microtubule organization in related cell lines. We studied two cell lines, both originating from green monkey renal epithelium, and found that they relied either on the centrosome or on the Golgi as a main microtubule-organizing center. We demonstrated that the difference in their Golgi microtubule-organizing activity was not associated with the well-studied proteins, such as CAMSAP3, CLASP2, GCC185, and GMAP210, but revealed several potential candidates involved in this process.

Emerging roles of centrosome cohesion.

The centrosome, consisting of centrioles and the associated pericentriolar material, is the main microtubule-organizing centre (MTOC) in animal cells. During most of interphase, the two centrosomes of a cell are joined together by centrosome cohesion into one MTOC. The most dominant element of centrosome cohesion is the centrosome linker, an interdigitating, fibrous network formed by the protein C-Nap1 anchoring a number of coiled-coil proteins including rootletin to the proximal end of centrioles. Alternatively, centrosomes can be kept together by the action of the minus end directed kinesin motor protein KIFC3 that works on interdigitating microtubules organized by both centrosomes and probably by the actin network. Although cells connect the two interphase centrosomes by several mechanisms into one MTOC, the general importance of centrosome cohesion, particularly for an organism, is still largely unclear. In this article, we review the functions of the centrosome linker and discuss how centrosome cohesion defects can lead to diseases.

The cytoplasmic microtubule array in Neurospora crassa depends on microtubule-organizing centers at spindle pole bodies and microtubule +end-depending pseudo-MTOCs at septa

γ-Tubulin ring complexes (γ-TuRC) mediate nucleation and anchorage of microtubules (MTs) to microtubule organizing centers (MTOCs). In fungi, the spindle pole body (SPB) is the functional equivalent of the centrosome, which is the main MTOC. In addition, non-centrosomal MTOCs (ncMTOCs) contribute to MT formation in some fungi like Schizosaccharomyces pombe and Aspergillus nidulans. In A. nidulans, MTOCs are anchored at septa (sMTOC) and share components of the outer plaque of the SPB. Here we show that the Neurospora crassa SPB is embedded in the nuclear envelope, with the γ-TuRC targeting proteins PCP-1Pcp1/PcpA located at the inner plaque and APS-2Mto1/ApsB located at the outer plaque of the SPB. PCP-1 was a specific component of nuclear MTOCs, while APS-2 was also present at the septal pore. Although γ-tubulin was only detected at the nucleus, spontaneous MT nucleation occurred in the apical and subapical cytoplasm during recovery from benomyl-induced MT depolymerization experiments. There was no evidence for MT nucleation at septa. However, without benomyl treatment MT plus-ends were organized in the septal pore through MTB-3EB1. Those septal MT plus ends polymerized MTs from septa in interphase cells Thus we conclude that the SPB is the only MT nucleation site in N. crassa, but the septal pore aids the MT network arrangement through the anchorage of the MT plus-ends through a pseudo-MTOC.

Actin network architecture can ensure robust centering or sensitive decentering of the centrosome.

The orientation of cell polarity depends on the position of the centrosome, the main microtubule-organizing center (MTOC). Microtubules (MTs) transmit pushing forces to the MTOC as they grow against the cell periphery. How the actin network regulates these forces remains unclear. Here, in a cell-free assay, we used purified proteins to reconstitute the interaction of a microtubule aster with actin networks of various architectures in cell-sized microwells. In the absence of actin filaments, MTOC positioning was highly sensitive to variations in microtubule length. The presence of a bulk actin network limited microtubule displacement, and MTOCs were held in place. In contrast, the assembly of a branched actin network along the well edges centered the MTOCs by maintaining an isotropic balance of pushing forces. An anisotropic peripheral actin network caused the MTOC to decenter by focusing the pushing forces. Overall, our results show that actin networks can limit the sensitivity of MTOC positioning to microtubule length and enforce robust MTOC centering or decentering depending on the isotropy of its architecture.

Global cellular response to chemical perturbation of PLK4 activity and abnormal centrosome number.

Centrosomes act as the main microtubule organizing center (MTOC) in metazoans. Centrosome number is tightly regulated by limiting centriole duplication to a single round per cell cycle. This control is achieved by multiple mechanisms, including the regulation of the protein kinase PLK4, the most upstream facilitator of centriole duplication. Altered centrosome numbers in mouse and human cells cause p53-dependent growth arrest through poorly defined mechanisms. Recent work has shown that the E3 ligase TRIM37 is required for cell cycle arrest in acentrosomal cells. To gain additional insights into this process, we undertook a series of genome-wide CRISPR/Cas9 screens to identify factors important for growth arrest triggered by treatment with centrinone B, a selective PLK4 inhibitor. We found that TRIM37 is a key mediator of growth arrest after partial or full PLK4 inhibition. Interestingly, PLK4 cellular mobility decreased in a dose-dependent manner after centrinone B treatment. In contrast to recent work, we found that growth arrest after PLK4 inhibition correlated better with PLK4 activity than with mitotic length or centrosome number. These data provide insights into the global response to changes in centrosome number and PLK4 activity and extend the role for TRIM37 in regulating the abundance, localization, and function of centrosome proteins.

Mitotic Maturation Compensates for Premature Centrosome Splitting and PCM Loss in Human cep135 Knockout Cells.

Centrosomes represent main microtubule organizing centers (MTOCs) in animal cells. Their duplication in S-phase enables the establishment of two MTOCs in M-phase that define the poles of the spindle and ensure equal distribution of chromosomes and centrosomes to the two daughter cells. While key functions of many centrosomal proteins have been addressed in RNAi experiments and chronic knockdown, knockout experiments with complete loss of function in all cells enable quantitative analysis of cellular phenotypes at all cell-cycle stages. Here, we show that the centriolar satellite proteins SSX2IP and WDR8 and the centriolar protein CEP135 form a complex before centrosome assembly in vertebrate oocytes and further functionally interact in somatic cells with established centrosomes. We present stable knockouts of SSX2IP, WDR8, and CEP135 in human cells. While loss of SSX2IP and WDR8 are compensated for, cep135 knockout cells display compromised PCM recruitment, reduced MTOC function, and premature centrosome splitting with imbalanced PCMs. Defective cep135 knockout centrosomes, however, manage to establish balanced spindle poles, allowing unperturbed mitosis and regular cell proliferation. Our data show essential functions of CEP135 in interphase MTOCs and demonstrate that loss of individual functions of SSX2IP, WDR8, and CEP135 are fully compensated for in mitosis.

3D-Structured Illumination Microscopy of Centrosomes in Human Cell Lines.

The centrosome is the main microtubule-organizing center of animal cells, and is composed of two barrel-shaped microtubule-based centrioles embedded in protein dense pericentriolar material. Compositional and architectural re-organization of the centrosome drives its duplication, and enables its microtubule-organizing activity and capability to form the primary cilium, which extends from the mature (mother) centriole, as the cell exits the cell cycle. Centrosomes and primary cilia are essential to human health, signified by the causal role of centrosome- and cilia-aberrations in numerous congenic disorders, as well as in the etiology and progression of cancer. The list of disease-associated centrosomal proteins and their proximitomes is steadily expanding, emphasizing the need for high resolution mapping of such proteins to specific substructures of the organelle. Here, we provide a detailed 3D-structured illumination microscopy (3D-SIM) protocol for comparative localization analysis of fluorescently labeled proteins at the centrosome in fixed human cell lines, at approximately 120 nm lateral and 300 nm axial resolution. The procedure was optimized to work with primary antibodies previously known to depend on more disruptive fixation reagents, yet largely preserves centriole and centrosome architecture, as shown by transposing acquired images of landmark proteins on previously published transmission electron microscopy (TEM) images of centrosomes. Even more advantageously, it is compatible with fluorescent protein tags. Finally, we introduce an internal reference to ensure correct 3D channel alignment. This protocol hence enables flexible, swift, and information-rich localization and interdependence analyses of centrosomal proteins, as well as their disorder-associated mutations.

The Transformation of the Centrosome into the Basal Body: Similarities and Dissimilarities between Somatic and Male Germ Cells and Their Relevance for Male Fertility.

The sperm flagellum is essential for the transport of the genetic material toward the oocyte and thus the transmission of the genetic information to the next generation. During the haploid phase of spermatogenesis, i.e., spermiogenesis, a morphological and molecular restructuring of the male germ cell, the round spermatid, takes place that includes the silencing and compaction of the nucleus, the formation of the acrosomal vesicle from the Golgi apparatus, the formation of the sperm tail, and, finally, the shedding of excessive cytoplasm. Sperm tail formation starts in the round spermatid stage when the pair of centrioles moves toward the posterior pole of the nucleus. The sperm tail, eventually, becomes located opposed to the acrosomal vesicle, which develops at the anterior pole of the nucleus. The centriole pair tightly attaches to the nucleus, forming a nuclear membrane indentation. An articular structure is formed around the centriole pair known as the connecting piece, situated in the neck region and linking the sperm head to the tail, also named the head-to-tail coupling apparatus or, in short, HTCA. Finally, the sperm tail grows out from the distal centriole that is now transformed into the basal body of the flagellum. However, a centriole pair is found in nearly all cells of the body. In somatic cells, it accumulates a large mass of proteins, the pericentriolar material (PCM), that together constitute the centrosome, which is the main microtubule-organizing center of the cell, essential not only for the structuring of the cytoskeleton and the overall cellular organization but also for mitotic spindle formation and chromosome segregation. However, in post-mitotic (G1 or G0) cells, the centrosome is transformed into the basal body. In this case, one of the centrioles, which is always the oldest or mother centriole, grows the axoneme of a cilium. Most cells of the body carry a single cilium known as the primary cilium that serves as an antenna sensing the cell’s environment. Besides, specialized cells develop multiple motile cilia differing in substructure from the immotile primary cilia that are essential in moving fluids or cargos over the cellular surface. Impairment of cilia formation causes numerous severe syndromes that are collectively subsumed as ciliopathies. This comparative overview serves to illustrate the molecular mechanisms of basal body formation, their similarities, and dissimilarities, in somatic versus male germ cells, by discussing the involved proteins/genes and their expression, localization, and function. The review, thus, aimed to provide a deeper knowledge of the molecular players that is essential for the expansion of clinical diagnostics and treatment of male fertility disorders.

The Golgi apparatus is the main microtubule-organizing center in differentiating skeletal muscle cells.

Studies in differentiating skeletal muscle cells in vitro have revealed that the microtubule-organizing center shifts from the centrosome to the perinuclear sites. As the Golgi apparatus surrounds the nucleus in a myotube, it is unclear whether microtubules are nucleated at the nuclear envelope or at the surrounding Golgi apparatus. In this study, we investigated the positional relationship between the microtubule nucleating sites and the Golgi apparatus in C2C12 myotubes and in primary cultured mouse skeletal myotubes. We focused on gaps in the perinuclear Golgi apparatus where the nuclear envelope was not covered with the Golgi apparatus. In microtubule regrowth assay, microtubule regrowth after cold-nocodazole depolymerization of preexisting microtubules was not found at the gap of the perinuclear Golgi apparatus. Most of the microtubule regrowth was detected at the CDK5RAP2 (CDK5 regulatory subunit-associated protein 2)-rich spots on the perinuclear Golgi apparatus. Disruption of the perinuclear Golgi apparatus with brefeldin A treatment eliminated the perinuclear microtubule regrowth. The Golgi apparatus of undifferentiated myoblasts and those at the cytoplasm of myotubes were also the microtubule nucleating sites. From these observations, we concluded that most of the perinuclear microtubule nucleation occurs on the Golgi apparatus surrounding the nucleus.

Superresolution characterization of core centriole architecture.

The centrosome is the main microtubule-organizing center in animal cells. It comprises of two centrioles and the surrounding pericentriolar material. Protein organization at the outer layer of the centriole and outward has been studied extensively; however, an overall picture of the protein architecture at the centriole core has been missing. Here we report a direct view of Drosophila centriolar proteins at ∼50-nm resolution. This reveals a Sas6 ring at the C-terminus, where it overlaps with the C-terminus of Cep135. The ninefold symmetrical pattern of Cep135 is further conveyed through Ana1-Asterless axes that extend past the microtubule wall from between the blades. Ana3 and Rcd4, whose termini are close to Cep135, are arranged in ninefold symmetry that does not match the above axes. During centriole biogenesis, Ana3 and Rcd4 are sequentially loaded on the newly formed centriole and are required for centriole-to-centrosome conversion through recruiting the Cep135-Ana1-Asterless complex. Together, our results provide a spatiotemporal map of the centriole core and implications of how the structure might be built.

Dynamic centriolar localization of Polo and Centrobin in early mitosis primes centrosome asymmetry.

Centrosomes, the main microtubule organizing centers (MTOCs) of metazoan cells, contain an older "mother" and a younger "daughter" centriole. Stem cells either inherit the mother or daughter-centriole-containing centrosome, providing a possible mechanism for biased delivery of cell fate determinants. However, the mechanisms regulating centrosome asymmetry and biased centrosome segregation are unclear. Using 3D-structured illumination microscopy (3D-SIM) and live-cell imaging, we show in fly neural stem cells (neuroblasts) that the mitotic kinase Polo and its centriolar protein substrate Centrobin (Cnb) accumulate on the daughter centriole during mitosis, thereby generating molecularly distinct mother and daughter centrioles before interphase. Cnb's asymmetric localization, potentially involving a direct relocalization mechanism, is regulated by Polo-mediated phosphorylation, whereas Polo's daughter centriole enrichment requires both Wdr62 and Cnb. Based on optogenetic protein mislocalization experiments, we propose that the establishment of centriole asymmetry in mitosis primes biased interphase MTOC activity, necessary for correct spindle orientation.

SAS-6 Association with γ-Tubulin Ring Complex Is Required for Centriole Duplication in Human Cells

Centrioles are essential components of centrosome, the main microtubule-organizing center of animal cellsrequired for robust spindle bipolarity [1, 2]. They are duplicated once during the cell cycle [3], and the duplication involves assembly of a cartwheel on the pre-existing centriole followed by assembly of triplet microtubules around the cartwheel [4, 5]. Although the molecular details of cartwheel formation are understood [6-13], the mechanisms initiating the formation of centriolar microtubules are not known. Here, we show that the central component of cartwheel, HsSAS-6 plays a crucial role in the formation of centriolar microtubules by interacting with the microtubule nucleation machinery, γ-tubulin ring complex (γ-TuRC) in human cells. The globular N terminus and the central coiled-coil domain of SAS-6 are required for formation of the cartwheel [7, 14], whereas the function of its C-terminal outer cartwheel region in centriole duplication remains unclear. We find that deletion of HsSAS-6 C terminus disrupts microtubule formation in daughter centriole, and as a result, cells fail to form the new centriole. Consequently, this results in mitotic cells having only two centrioles localized at a single site. Detailed molecular analyses showed that HsSAS-6 interacts with the γ-TuRC proteins and associates with the γ-TuRC at the centrosome, and furthermore, the C terminus isessential for this association. High-resolution microscopy revealed localization of the γ-TuRC protein, γ-tubulin as multiple lobes surrounding the HsSAS-6-containing central hub in the centriole. Together, theresults indicate that HsSAS-6 regulates centriolar microtubule assembly by anchoring γ-TuRCs to the pro-centriole at the onset of daughter centriole formation.

Proximity mapping of the microtubule plus-end tracking protein SLAIN2 using the BioID approach.

The centrosome is the main microtubule-organizing center of animal cells, which plays key roles in critical cellular processes ranging from cell division to cellular signaling. Accordingly, defects in the structure and function of centrosomes cause various human diseases such as cancer and primary microcephaly. To elucidate the molecular defects underlying these diseases, the biogenesis and functions of the centrosomes have to be fully understood. An essential step towards addressing these questions is the identification and functional dissection of the full repertoire of centrosome proteins. Here, we used high-resolution imaging and showed that the microtubule plus-end tracking protein SLAIN2 localizes to the pericentriolar material at the proximal end of centrioles. To gain insight into its cellular functions and mechanisms, we applied in vivo proximity-dependent biotin identification to SLAIN2 and generated its proximity interaction map. Gene ontology analysis of the SLAIN2 interactome revealed extensive interactions with centriole duplication, ciliogenesis, and microtubule-associated proteins, including previously characterized and uncharacterized interactions. Collectively, our results define SLAIN2 as a component of pericentriolar material and provide an important resource for future studies aimed at elucidating SLAIN2 functions at the centrosome.

Actin filaments regulate microtubule growth at thecentrosome.

The centrosome is the main microtubule-organizing centre. It also organizes a local network of actin filaments. However, the precise function of the actin network at the centrosome is not well understood. Here, we show that increasing densities of actin filaments at the centrosome of lymphocytes are correlated with reduced amounts of microtubules. Furthermore, lymphocyte activation resulted in disassembly of centrosomal actin and an increase in microtubule number. To further investigate the direct crosstalk between actin and microtubules at the centrosome, we performed invitro reconstitution assays based on (i) purified centrosomes and (ii) on the co-micropatterning of microtubule seeds and actin filaments. These two assays demonstrated that actin filaments constitute a physical barrier blocking elongation of nascent microtubules. Finally, we showed that cell adhesion and cell spreading lead to lower densities of centrosomal actin, thus resulting in higher microtubule growth. We therefore propose a novel mechanism, by which the number of centrosomal microtubules is regulated by cell adhesion and actin-network architecture.

A centrosomal view of CNS growth

Embryonic development of the central nervous system (CNS) requires the proliferation of neural progenitor cells to be tightly regulated, allowing the formation of an organ with the right size and shape. This includes regulation of both the spatial distribution of mitosis and the mode of cell division. The centrosome, which is the main microtubule-organizing centre of animal cells, contributes to both of these processes. Here, we discuss the impact that centrosome-mediated control of cell division has on the shape of the overall growing CNS. We also review the intrinsic properties of the centrosome, both in terms of its molecular composition and its signalling capabilities, and discuss the fascinating notion that intrinsic centrosomal asymmetries in dividing neural progenitor cells are instructive for neurogenesis. Finally, we discuss the genetic links between centrosome dysfunction during development and the aetiology of microcephaly.

Mid1ip1b modulates apical reorientation of non-centrosomal microtubule organizing center in epithelial cells

In most kinds of animal cells, the centrosome serves as the main microtubule organizing center (MTOC) that nucleates microtubule arrays throughout the cytoplasm to maintain cell structure, cell division and intracellular transport. Whereas in epithelial cells, non-centrosomal MTOCs are established in the apical domain for generating asymmetric microtubule fibers and cilia in epithelial cells for the organ morphogenesis during embryonic development. However, the mechanism by which MTOCs localize to the apical domain in epithelial cells remains largely unknown. Here, we show that Mid1ip1b has a close interaction with γ-tubulin protein, the central component of MTOC, and modulates lumen opening of the neural tube, gut, intestine, and kidney of zebrafish. Knockdown or dominant negative effect of Mid1ip1b resulted in failure of lumen formation of the organs as aforementioned. Moreover, the non-centrosomal MTOCs were unable to orientate to the apical domain in Mid1ip1b knockdown epithelial cells, and the centrosomal MTOCs were inaccurately placed in the apical domain, resulting in defective formation of asymmetric microtubules and misplacement of cilia in the apical domain. These data uncover a molecule that controls the proper localization of MTOCs in the apical domain in epithelial cells for organ morphogenesis during embryonic development.

Centrosomal Abnormalities in Pancreatic Cancer: Molecular Mechanisms and Clinical Implications.

The centrosome is the main microtubule-organizing center in human cells. It regulates normal cell-cycle progression and cell division. Aberrations in the number, structure and function of centrosomes have been found to drive genomic instability and tumorigenesis. Pancreatic cancer frequently displays centrosomal aberrations. Supernumerary and abnormal centrosomes are observed in the earliest stages of pancreatic tumor development, and the p53 pathway acts as an initial barrier to the proliferation of cells with extra centrosomes. In this review, we summarize recent advances in the understanding of centrosomal aberrations in pancreatic cancer, focusing on regulatory mechanisms and prospects for future anticancer treatment.