Centrosomes and the Art of Mitotic Spindle Maintenance

Roles of Tubulin Concentration during Prometaphase and Ran-GTP during Anaphase of Caenorhabditis elegans Meiosis

In many animal species, the oocyte meiotic spindle, which is required for chromosome segregation, forms without centrosomes. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in Caenorhabditis elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly occurred after auxin-induced degradation of Ran-GEF, but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown. We found that the concentration of soluble tubulin in the metaphase spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules. Measurement of the volume occupied by yolk granules and mitochondria indicated that volume exclusion would be sufficient to explain the concentration of tubulin in the spindle volume. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes.

Roles of Tubulin Concentration during Prometaphase and Ran-GTP during Anaphase of C. elegans meiosis

In many animal species, the oocyte meiotic spindle, which is required for chromosome segregation, forms without centrosomes. In some systems, Ran-GEF on chromatin initiates spindle assembly. We found that in C. elegans oocytes, endogenously-tagged Ran-GEF dissociates from chromatin during spindle assembly but re-associates during meiotic anaphase. Meiotic spindle assembly occurred after auxin-induced degradation of Ran-GEF but anaphase I was faster than controls and extrusion of the first polar body frequently failed. In search of a possible alternative pathway for spindle assembly, we found that soluble tubulin concentrates in the nuclear volume during germinal vesicle breakdown. We found that the concentration of soluble tubulin in the metaphase spindle region is enclosed by ER sheets which exclude cytoplasmic organelles including mitochondria and yolk granules. Measurement of the volume occupied by yolk granules and mitochondria indicated that volume exclusion would be sufficient to explain the concentration of tubulin in the spindle volume. We suggest that this concentration of soluble tubulin may be a redundant mechanism promoting spindle assembly near chromosomes.

DNAJA2 deficiency activates cGAS-STING pathway via the induction of aberrant mitosis and chromosome instability

Molecular chaperone HSP70s are attractive targets for cancer therapy, but their substrate broadness and functional non-specificity have limited their role in therapeutical success. Functioning as HSP70's cochaperones, HSP40s determine the client specificity of HSP70s, and could be better targets for cancer therapy. Here we show that tumors defective in HSP40 member DNAJA2 are benefitted from immune-checkpoint blockade (ICB) therapy. Mechanistically, DNAJA2 maintains centrosome homeostasis by timely degrading key centriolar satellite proteins PCM1 and CEP290 via HSC70 chaperone-mediated autophagy (CMA). Tumor cells depleted of DNAJA2 or CMA factor LAMP2A exhibit elevated levels of centriolar satellite proteins, which causes aberrant mitosis characterized by abnormal spindles, chromosome missegregation and micronuclei formation. This activates the cGAS-STING pathway to enhance ICB therapy response in tumors derived from DNAJA2-deficient cells. Our study reveals a role for DNAJA2 to regulate mitotic division and chromosome stability and suggests DNAJA2 as a potential target to enhance cancer immunotherapy, thereby providing strategies to advance HSPs-based cancer therapy.

Dynein localization and pronuclear movement in the C. elegans zygote

Centrosomes serve as a site for microtubule nucleation and these microtubules will grow and interact with the motor protein dynein at the cortex. The position of the centrosomes determines where the mitotic spindle will develop across all cell types. Centrosome positioning is achieved through dynein and microtubule-mediated force generation. The mechanism and regulation of force generation during centrosome positioning are not fully understood. Centrosome and pronuclear movement in the first cell cycle of the Caenorhabditis elegans early embryo undergoes both centration and rotation prior to cell division. The proteins LET-99 and GPB-1 have been postulated to have a role in force generation associated with pronuclear centration and rotation dynamics. When the expression of these proteins is perturbed, pronuclear positioning exhibits a movement defect characterized by oscillatory ("wobble") behavior of the pronuclear complex (PNC). To determine if this movement defect is due to an effect on cortical dynein distribution, we utilize RNAi-mediated knockdown of LET-99 and GPB-1 to induce wobble and assay for any effects on GFP-tagged dynein localization in the early C. elegans embryo. To compare and quantify the movement defect produced by the knockdown of LET-99 and GPB-1, we devised a quantification method that measures the strength of wobble ("wobble metric") observed under these experimental conditions. Our quantification of pronuclear complex dynamics and dynein localization shows that loss of LET-99 and GPB-1 induces a similar movement defect which is independent of cortical dynein localization in the early C. elegans embryo.

Centrioles and Ciliary Structures during Male Gametogenesis in Hexapoda: Discovery of New Models

Centrioles are-widely conserved barrel-shaped organelles present in most organisms. They are indirectly involved in the organization of the cytoplasmic microtubules both in interphase and during the cell division by recruiting the molecules needed for microtubule nucleation. Moreover, the centrioles are required to assemble cilia and flagella by the direct elongation of their microtubule wall. Due to the importance of the cytoplasmic microtubules in several aspects of the cell life, any defect in centriole structure can lead to cell abnormalities that in humans may result in significant diseases. Many aspects of the centriole dynamics and function have been clarified in the last years, but little attention has been paid to the exceptions in centriole structure that occasionally appeared within the animal kingdom. Here, we focused our attention on non-canonical aspects of centriole architecture within the Hexapoda. The Hexapoda is one of the major animal groups and represents a good laboratory in which to examine the evolution and the organization of the centrioles. Although these findings represent obvious exceptions to the established rules of centriole organization, they may contribute to advance our understanding of the formation and the function of these organelles.

From isolated structures to continuous networks: A categorization of cytoskeleton-based motile engineered biological microstructures

As technology at the small scale is advancing, motile engineered microstructures are becoming useful in drug delivery, biomedicine, and lab-on-a-chip devices. However, traditional engineering methods and materials can be inefficient or functionally inadequate for small-scale applications. Increasingly, researchers are turning to the biology of the cytoskeleton, including microtubules, actin filaments, kinesins, dyneins, myosins, and associated proteins, for both inspiration and solutions. They are engineering structures with components that range from being entirely biological to being entirely synthetic mimics of biology and on scales that range from isotropic continuous networks to single isolated structures. Motile biological microstructures trace their origins from the development of assays used to study the cytoskeleton to the array of structures currently available today. We define 12 types of motile biological microstructures, based on four categories: entirely biological, modular, hybrid, and synthetic, and three scales: networks, clusters, and isolated structures. We highlight some key examples, the unique functionalities, and the potential applications of each microstructure type, and we summarize the quantitative models that enable engineering them. By categorizing the diversity of motile biological microstructures in this way, we aim to establish a framework to classify these structures, define the gaps in current research, and spur ideas to fill those gaps. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Nanotechnology Approaches to Biology > Cells at the Nanoscale Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Characterization of Novel Murine and Human PDAC Cell Models: Identifying the Role of Intestine Specific Homeobox Gene ISX in Hypoxia and Disease Progression

Therapy failure and metastasis-associated mortality are stumbling blocks in the management of PDAC in patients. Failure of therapy is associated to intense hypoxic conditions of tumors. To develop effective therapies, a complete understanding of hypoxia-associated changes in genetic landscape of tumors during disease progression is needed. Because artificially immortalized cell lines do not rightly represent the disease progression, studying genetics of tumors in spontaneous models is warranted. In the current study, we generated a spectrum of spontaneous human (UM-PDC1; UM-PDC2) and murine (HI-PanL, HI-PancI, HI-PanM) models representing localized, invasive, and metastatic PDAC from a patient and transgenic mice (K-ras^G12D/Pdx^cre/Ink4a/p16^-/). These spontaneous models grow vigorously under hypoxia and exhibit activated K-ras signaling, progressive loss of PTEN, and tumorigenicity in vivo. Whereas UM-PDC1 form localized tumors, the UM-PDC2 metastasize to lungs in mice. In an order of progression, these models exhibit genomic instability marked by gross chromosomal rearrangements, centrosome-number variations, Aurora-kinase/H2AX colocalization, loss of primary cilia, and α-tubulin acetylation. The RNA sequencing of hypoxic models followed by qRT-PCR validation and gene-set enrichment identified Intestine-Specific Homeobox factor (ISX)–driven molecular pathway as an indicator PDAC aggressivness. TCGA-PAAD clinical data analysis showed high ISX expression correlation to poor survival of PDAC patients, particularly women. The functional studies showed ISX as a regulator of i) invasiveness and migratory potential and ii) VEGF, MMP2, and NFκB activation in PDAC cells. We suggest that ISX is a potential druggable target and newly developed spontaneous cell models are valuable tools for studying mechanism and testing therapies for PDAC.

The effect and mechanism of millepachine-disrupted spindle assembly in tumor cells

Millepachine (MIL) is a bioactive natural product that shows great potential for cancer treatment. Previous studies showed that MIL was a novel cancer drug candidate with a special structure. To provide reference for the research and development of MIL, we further investigated the mechanism of MIL inducing G2/M arrest and found MIL disrupted spindle assembly in tumor cells. In this study, we investigated the disrupting spindle assembly effects of MIL with a focus on its potential mechanism of action. First, we indicated that MIL did not inhibit microtubule polymerization from the results of in-vivo microtubule nucleation assay and microtubule polymerization in-vitro assay but delayed this process by inhibiting the production of ATP in tumor cells. Thereafter, we investigated the effect of MIL on the mitotic spindle. We found that MIL induced multipolar spindles by inhibiting the activity of Eg5 and inhibited mitotic spindle formation and chromatin condensation by the activation of the spindle assembly checkpoint (SAC) in tumor cells. These results established a novel function of MIL in regulating the assembly of mitotic spindle. As Eg5 and SAC are antitumor targets, effect of MIL on the Eg5 protein and SAC activation hinted that MIL has novel application in the development of antitumor drugs.

It takes two (centrioles) to tango

Cells that divide during embryo development require precisely two centrioles during interphase and four centrioles during mitosis. This precise number is maintained by allowing each centriole to nucleate only one centriole per cell cycle (i.e. centriole duplication). Yet, how the first cell of the embryo, the zygote, obtains two centrioles has remained a mystery in most mammals and insects. The mystery arose because the female gamete (oocyte) is thought to have no functional centrioles and the male gamete (spermatozoon) is thought to have only one functional centriole, resulting in a zygote with a single centriole. However, recent studies in fruit flies, beetles and mammals, including humans, suggest an alternative explanation: spermatozoa have a typical centriole and an atypical centriole. The sperm typical centriole has a normal structure but distinct protein composition, whereas the sperm atypical centriole is distinct in both. During fertilization, the atypical centriole is released into the zygote, nucleates a new centriole and participates in spindle pole formation. Thus, the spermatozoa's atypical centriole acts as a second centriole in the zygote. Here, we review centriole biology in general and especially in reproduction, we describe the discovery of the spermatozoon atypical centriole, and we provide an updated model for centriole inherence during sexual reproduction. While we focus on humans and other non-rodent mammals, we also provide a broader evolutionary perspective.

Isomer-specific effect of microRNA miR-29b on nuclear morphology

Targeting mRNAs via seed region pairing is the canonical mechanism by which microRNAs (miRNAs) regulate cellular functions and disease processes. Emerging evidence suggests miRNAs might also act through other mechanisms. miRNA isomers that contain identical seed region sequences, such as miR-29a and miR-29b, provide naturally occurring, informative models for identifying those miRNA effects that are independent of seed region pairing. miR-29a and miR-29b are both expressed in HeLa cells, and miR-29b has been reported to localize to the nucleus in early mitosis because of unique nucleotide sequences on its 3' end. Here, we sought to better understand the mechanism of miR-29b nuclear localization and its function in cell division. We hypothesized that its nuclear localization may be facilitated by protein-miRNA interactions unique to miR-29b. Specific blockade of miR-29b resulted in striking nuclear irregularities not observed following miR-29a blockade. We also observed that miR-29b, but not miR-29a, is enriched in the nucleus and perinuclear clusters during mitosis. Targeted proteomic analysis of affinity-purified samples identified several proteins interacting with synthetic oligonucleotides mimicking miR-29b, but these proteins did not interact with miR-29a. One of these proteins, ADP/ATP translocase 2 (ANT2), known to be involved in mitotic spindle formation, colocalized with miR-29b in perinuclear clusters independently of Argonaute 2. Of note, ANT2 knockdown resulted in nuclear irregularities similar to those observed following miR-29b blockade and prevented nuclear uptake of endogenous miR-29b. Our findings reveal that miR-29 regulates nuclear morphology during mitosis and that this critical function is unique to the miR-29b isoform.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.

Cardiac Regeneration: Lessons From Development

Palliative surgery for congenital heart disease has allowed patients with previously lethal heart malformations to survive and, in most cases, to thrive. However, these procedures often place pressure and volume loads on the heart, and over time, these chronic loads can cause heart failure. Current therapeutic options for initial surgery and chronic heart failure that results from failed palliation are limited, in part, by the mammalian heart's low inherent capacity to form new cardiomyocytes. Surmounting the heart regeneration barrier would transform the treatment of congenital, as well as acquired, heart disease and likewise would enable development of personalized, in vitro cardiac disease models. Although these remain distant goals, studies of heart development are illuminating the path forward and suggest unique opportunities for heart regeneration, particularly in fetal and neonatal periods. Here, we review major lessons from heart development that inform current and future studies directed at enhancing cardiac regeneration.

Parthenogenesis in Insects: The Centriole Renaissance

Building a new organism usually requires the contribution of two differently shaped haploid cells, the male and female gametes, each providing its genetic material to restore diploidy of the new born zygote. The successful execution of this process requires defined sequential steps that must be completed in space and time. Otherwise, development fails. Relevant among the earlier steps are pronuclear migration and formation of the first mitotic spindle that promote the mixing of parental chromosomes and the formation of the zygotic nucleus. A complex microtubule network ensures the proper execution of these processes. Instrumental to microtubule organization and bipolar spindle assembly is a distinct non-membranous organelle, the centrosome. Centrosome inheritance during fertilization is biparental, since both gametes provide essential components to build a functional centrosome. This model does not explain, however, centrosome formation during parthenogenetic development, a special mode of sexual reproduction in which the unfertilized egg develops without the contribution of the male gamete. Moreover, whereas fertilization is a relevant example in which the cells actively check the presence of only one centrosome, to avoid multipolar spindle formation, the development of parthenogenetic eggs is ensured, at least in insects, by the de novo assembly of multiple centrosomes.Here, we will focus our attention on the assembly of functional centrosomes following fertilization and during parthenogenetic development in insects. Parthenogenetic development in which unfertilized eggs are naturally depleted of centrosomes would provide a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation.

The interplay between centrosomes and the Hippo tumor suppressor pathway

Centrosome amplification is a common feature of both solid and hematological human malignancies. Extra centrosomes are not merely innocent bystanders in cancer cells, but rather promote tumor progression by disrupting normal cellular architecture and generating chromosome instability. Consequently, centrosome amplification correlates with advanced tumor grade and overall poor clinical prognosis. By contrast, extra centrosomes are adversely tolerated in non-transformed cells and hinder cell proliferation. This suggests that in addition to acquiring extra centrosomes, cancer cells must also adapt to overcome the deleterious consequences associated with them. Here, we review evidence that implicates core components of the Hippo tumor suppressor pathway as having key roles in both the direct and indirect regulation of centrosome number. Intriguingly, functional inactivation of the Hippo pathway, which is common across broad spectrum of human cancers, likely represents one key adaptation that enables cancer cells to tolerate extra centrosomes.

Identification of MAC1: A Small Molecule That Rescues Spindle Bipolarity in Monastrol-Treated Cells

The genetic integrity of each organism is intimately tied to the correct segregation of its genome during mitosis. Insights into the underlying mechanisms are fundamental for both basic research and the development of novel strategies to treat mitosis-relevant diseases such as cancer. Due to their fast mode of action, small molecules are invaluable tools to dissect mitosis. Yet, there is a great demand for novel antimitotic compounds. We performed a chemical genetic suppression screen to identify compounds that restore spindle bipolarity in cells treated with Monastrol, an inhibitor of the mitotic kinesin Eg5. We identified one compound-MAC1-that rescued spindle bipolarity in cells lacking Eg5 activity. Mechanistically, MAC1 induces the formation of additional microtubule nucleation centers, which allows kinesin Kif15-dependent bipolar spindle assembly in the absence of Eg5 activity. Thus, our chemical genetic suppression screen revealed novel unexpected insights into the mechanism of spindle assembly in mammalian cells.

Aurora-A Kinase as a Promising Therapeutic Target in Cancer

Mammalian Aurora family of serine/threonine kinases are master regulators of mitotic progression and are frequently overexpressed in human cancers. Among the three members of the Aurora kinase family (Aurora-A, -B, and -C), Aurora-A and Aurora-B are expressed at detectable levels in somatic cells undergoing mitotic cell division. Aberrant Aurora-A kinase activity has been implicated in oncogenic transformation through the development of chromosomal instability and tumor cell heterogeneity. Recent studies also reveal a novel non-mitotic role of Aurora-A activity in promoting tumor progression through activation of epithelial-mesenchymal transition reprograming resulting in the genesis of tumor-initiating cells. Therefore, Aurora-A kinase represents an attractive target for cancer therapeutics, and the development of small molecule inhibitors of Aurora-A oncogenic activity may improve the clinical outcomes of cancer patients. In the present review, we will discuss mitotic and non-mitotic functions of Aurora-A activity in oncogenic transformation and tumor progression. We will also review the current clinical studies, evaluating small molecule inhibitors of Aurora-A activity and their efficacy in the management of cancer patients.

Trial Watch-Immunostimulation with cytokines in cancer therapy

During the past decade, great efforts have been dedicated to the development of clinically relevant interventions that would trigger potent (and hence potentially curative) anticancer immune responses. Indeed, developing neoplasms normally establish local and systemic immunosuppressive networks that inhibit tumor-targeting immune effector cells, be them natural or elicited by (immuno)therapy. One possible approach to boost anticancer immunity consists in the (generally systemic) administration of recombinant immunostimulatory cytokines. In a limited number of oncological indications, immunostimulatory cytokines mediate clinical activity as standalone immunotherapeutic interventions. Most often, however, immunostimulatory cytokines are employed as immunological adjuvants, i.e., to unleash the immunogenic potential of other immunotherapeutic agents, like tumor-targeting vaccines and checkpoint blockers. Here, we discuss recent preclinical and clinical advances in the use of some cytokines as immunostimulatory agents in oncological indications.

Unique subcellular distribution of RPB1 with a phosphorylated C-terminal domain (CTD) in mouse oocytes during meiotic division and its relationship with chromosome separation

Polymerase (RNA) II (DNA directed) polypeptide A (RPB1) is the largest subunit of RNA polymerase II (RNAPII), and phosphorylation of its C-terminal domain (CTD) is required for transcription initiation, elongation and RNA processing. Little is known about the CTD phosphorylation pattern and potential function during cell division when transcription is silenced. In this study, we assessed the protein expression and subcellular distribution of RPB1 during mouse oocyte meiotic division. Western blot analysis revealed that the RPB1 CTD was highly phosphorylated on Ser2 (pRPB1^Ser2), Ser5 (pRPB1^Ser5) and Ser7 (pRPB1^Ser7). High and stable expression of pRPB1^Ser2 and pRPB1^Ser5 was detected from germinal vesicle (GV) to Metaphase II (MII) stage. In contrast, pRPB1^Ser7 only emerged after germinal vesicle breakdown (GVBD) and gradually increased to its peak level at metaphase I (MI) and MII. Immunofluorescence demonstrated that pRPB1^Ser2, pRPB1^Ser5 and pRPB1^Ser7 were pronouncedly aggregated within the nucleus of GV oocytes with a non-surrounded nucleolus (NSN) but very faintly labeled in oocytes with a surrounded nucleolus (SN). After meiotic resumption, pRPB1^Ser2 was again detected at spindle poles and co-localized with key microtubule organizing center (MTOC) components, pericentrin and γ-tubulin. pRPB1^Ser5 and pRPB1^Ser7 were assembled as filamentous aggregates and co-localized with microtubules throughout the spindle structure, responding to spindle-disturbing drugs, nocodazole or taxol, in pattern strongly similar to microtubules. pRPB1^Ser2 and pRPB1^Ser5 were constantly localized on chromosomes, with a relatively high concentration in centromere areas. Taken together, our data suggest that the CTD is highly phosphorylated and may be required for accurate chromosome segregation in mouse oocytes during meiosis.

Dissecting the function and assembly of acentriolar microtubule organizing centers in Drosophila cells in vivo

Acentriolar microtubule organizing centers (aMTOCs) are formed during meiosis and mitosis in several cell types, but their function and assembly mechanism is unclear. Importantly, aMTOCs can be overactive in cancer cells, enhancing multipolar spindle formation, merotelic kinetochore attachment and aneuploidy. Here we show that aMTOCs can form in acentriolar Drosophila somatic cells in vivo via an assembly pathway that depends on Asl, Cnn and, to a lesser extent, Spd-2--the same proteins that appear to drive mitotic centrosome assembly in flies. This finding enabled us to ablate aMTOC formation in acentriolar cells, and so perform a detailed genetic analysis of the contribution of aMTOCs to acentriolar mitotic spindle formation. Here we show that although aMTOCs can nucleate microtubules, they do not detectably increase the efficiency of acentriolar spindle assembly in somatic fly cells. We find that they are required, however, for robust microtubule array assembly in cells without centrioles that also lack microtubule nucleation from around the chromatin. Importantly, aMTOCs are also essential for dynein-dependent acentriolar spindle pole focusing and for robust cell proliferation in the absence of centrioles and HSET/Ncd (a kinesin essential for acentriolar spindle pole focusing in many systems). We propose an updated model for acentriolar spindle pole coalescence by the molecular motors Ncd/HSET and dynein in conjunction with aMTOCs.