RESUMO
S-phase entry and exit are regulated by hundreds of protein complexes that assemble "just in time," orchestrated by a multitude of distinct events. To help understand their interplay, we have created a tailored visualization based on the Minardo layout, highlighting over 80 essential events. This complements our earlier visualization of M-phase, and both can be displayed together, giving a comprehensive overview of the events regulating the cell division cycle. To view this SnapShot, open or download the PDF.
Assuntos
Ciclo Celular/genética , Mitose/genética , Complexos Multiproteicos/genética , Fase S/genética , Divisão Celular/genética , Ciclina B/genética , Ciclina D/genética , Quinases Ciclina-Dependentes/genética , Fase G2/genética , Humanos , Fosforilação/genética , Complexo de Endopeptidases do Proteassoma/genéticaRESUMO
Constitutive heterochromatin is a major component of the eukaryotic nucleus and is essential for the maintenance of genome stability. Highly concentrated at pericentromeric and telomeric domains, heterochromatin is riddled with repetitive sequences and has evolved specific ways to compartmentalize, silence, and repair repeats. The delicate balance between heterochromatin epigenetic maintenance and cellular processes such as mitosis and DNA repair and replication reveals a highly dynamic and plastic chromatin domain that can be perturbed by multiple mechanisms, with far-reaching consequences for genome integrity. Indeed, heterochromatin dysfunction provokes genetic turmoil by inducing aberrant repeat repair, chromosome segregation errors, transposon activation, and replication stress and is strongly implicated in aging and tumorigenesis. Here, we summarize the general principles of heterochromatin structure and function, discuss the importance of its maintenance for genome integrity, and propose that more comprehensive analyses of heterochromatin roles in tumorigenesis will be integral to future innovations in cancer treatment.
Assuntos
Reparo do DNA/genética , Instabilidade Genômica , Heterocromatina/genética , Mitose/genética , Centrômero/genética , Segregação de Cromossomos/genética , Genoma/genética , Histonas/genética , Humanos , Sequências Repetitivas de Ácido Nucleico/genética , Telômero/genéticaRESUMO
Intermediate filaments (IFs) are one of the three major elements of the cytoskeleton. Their stability, intrinsic mechanical properties, and cell type-specific expression patterns distinguish them from actin and microtubules. By providing mechanical support, IFs protect cells from external forces and participate in cell adhesion and tissue integrity. IFs form an extensive and elaborate network that connects the cell cortex to intracellular organelles. They act as a molecular scaffold that controls intracellular organization. However, IFs have been revealed as much more than just rigid structures. Their dynamics is regulated by multiple signaling cascades and appears to contribute to signaling events in response to cell stress and to dynamic cellular functions such as mitosis, apoptosis, and migration.
Assuntos
Biologia Celular/tendências , Citoplasma/genética , Filamentos Intermediários/genética , Microtúbulos/genética , Actinas/química , Actinas/genética , Citoplasma/química , Citoesqueleto/química , Citoesqueleto/genética , Proteína Glial Fibrilar Ácida/genética , Humanos , Filamentos Intermediários/química , Microtúbulos/química , Mitose/genética , Transdução de Sinais/genéticaRESUMO
The high incidence of whole-arm chromosome aneuploidy and translocations in tumors suggests instability of centromeres, unique loci built on repetitive sequences and essential for chromosome separation. The causes behind this fragility and the mechanisms preserving centromere integrity remain elusive. We show that replication stress, hallmark of pre-cancerous lesions, promotes centromeric breakage in mitosis, due to spindle forces and endonuclease activities. Mechanistically, we unveil unique dynamics of the centromeric replisome distinct from the rest of the genome. Locus-specific proteomics identifies specialized DNA replication and repair proteins at centromeres, highlighting them as difficult-to-replicate regions. The translesion synthesis pathway, along with other factors, acts to sustain centromere replication and integrity. Prolonged stress causes centromeric alterations like ruptures and translocations, as observed in ovarian cancer models experiencing replication stress. This study provides unprecedented insights into centromere replication and integrity, proposing mechanistic insights into the origins of centromere alterations leading to abnormal cancerous karyotypes.
Assuntos
Centrômero , Sequências Repetitivas de Ácido Nucleico , Humanos , Centrômero/genética , Mitose/genética , Instabilidade GenômicaRESUMO
Mitotic cell division is tightly monitored by checkpoints that safeguard the genome from instability. Failures in accurate chromosome segregation during mitosis can cause numerical aneuploidy, which was hypothesized by Theodor Boveri over a century ago to promote tumorigenesis. Recent interrogation of pan-cancer genomes has identified unexpected classes of chromosomal abnormalities, including complex rearrangements arising through chromothripsis. This process is driven by mitotic errors that generate abnormal nuclear structures that provoke extensive yet localized shattering of mis-segregated chromosomes. Here, we discuss emerging mechanisms underlying chromothripsis from micronuclei and chromatin bridges, as well as highlight how this mutational cascade converges on the DNA damage response. A fundamental understanding of these catastrophic processes will provide insight into how initial errors in mitosis can precipitate rapid cancer genome evolution.
Assuntos
Cromotripsia , Neoplasias , Humanos , Aberrações Cromossômicas , Mitose/genética , Instabilidade Genômica , Neoplasias/genéticaRESUMO
The DNA topological challenges generated by cellular manipulation of extremely long DNA fibers remain poorly understood. In this issue of Molecular Cell, Hildebrand et al.1 describe how mitotic chromosomes are self entangled and that disentanglement requires TOP2 activity in late mitosis.
Assuntos
Cromossomos , DNA Topoisomerases Tipo II , DNA Topoisomerases Tipo II/genética , Cromossomos/genética , DNA/genética , Mitose/genéticaRESUMO
The topological state of chromosomes determines their mechanical properties, dynamics, and function. Recent work indicated that interphase chromosomes are largely free of entanglements. Here, we use Hi-C, polymer simulations, and multi-contact 3C and find that, by contrast, mitotic chromosomes are self-entangled. We explore how a mitotic self-entangled state is converted into an unentangled interphase state during mitotic exit. Most mitotic entanglements are removed during anaphase/telophase, with remaining ones removed during early G1, in a topoisomerase-II-dependent process. Polymer models suggest a two-stage disentanglement pathway: first, decondensation of mitotic chromosomes with remaining condensin loops produces entropic forces that bias topoisomerase II activity toward decatenation. At the second stage, the loops are released, and the formation of new entanglements is prevented by lower topoisomerase II activity, allowing the establishment of unentangled and territorial G1 chromosomes. When mitotic entanglements are not removed in experiments and models, a normal interphase state cannot be acquired.
Assuntos
Cromossomos , DNA Topoisomerases Tipo II , DNA Topoisomerases Tipo II/genética , Cromossomos/genética , Mitose/genética , Interfase/genética , PolímerosRESUMO
The highly reproducible inheritance of chromosomes during mitosis in mammalian cells involves nuclear envelope breakdown, increased chromatin compaction, loss of long-range intrachromosomal interactions, loss of enhancer-promoter proximity, displacement of many transcription regulators from the chromatin and a marked decrease in RNA synthesis. Despite these dramatic changes in the mother cell, daughter cells are able to faithfully re-establish the parental chromatin and gene expression features characteristic of the cell type. Pioneering studies of mitotic chromatin signatures showed that despite global repression of transcription, the Hsp70 gene promoter retains an open chromatin conformation, which was proposed to allow the reactivation of the Hsp70 gene upon completion of mitosis - a phenomenon termed mitotic bookmarking. It was later shown that various cell-type-specific transcription factors, such as GATA-binding factor 1 (GATA1) in erythroblasts and forkhead box protein A1 (FOXA1) in hepatocytes, remain bound at a subset of their interphase binding sites in mitosis. Such bookmarking transcription factors remain on chromosomes in mitosis and have been shown to enable a subset of genes to be reactivated in a timely fashion upon mitotic exit. In addition, sensitive new methods to measure transcription revealed that mitotic cells retain residual transcription at a large number of genes. Furthermore, genes recover their interphase level of transcription in distinct waves. Thus, gene expression is precisely regulated as cells pass through mitosis to ensure faithful propagation of cell identity and function through cellular generations.
Assuntos
Memória/fisiologia , Mitose/genética , Transcrição Gênica/genética , Animais , Cromatina/genética , Cromossomos/genética , Regulação da Expressão Gênica/genética , Humanos , Fatores de Transcrição/genéticaRESUMO
Mitotic DNA synthesis (MiDAS) is an unusual form of DNA replication that occurs during mitosis. Initially, MiDAS was characterized as a process associated with intrinsically unstable loci known as common fragile sites that occurs after cells experience DNA replication stress (RS). However, it is now believed to be a more widespread "salvage" mechanism that is called upon to complete the duplication of any under-replicated genomic region. Emerging data suggest that MiDAS is a DNA repair process potentially involving two or more pathways working in parallel or sequentially. In this review, we introduce the causes of RS, regions of the human genome known to be especially vulnerable to RS, and the strategies used to complete DNA replication outside of S phase. Additionally, because MiDAS is a prominent feature of aneuploid cancer cells, we will discuss how targeting MiDAS might potentially lead to improvements in cancer therapy.
Assuntos
Reparo do DNA , Replicação do DNA , Humanos , Fase S/genética , Mitose/genética , Replicação ViralRESUMO
Epigenetic reprogramming resets parental epigenetic memories and differentiates primordial germ cells (PGCs) into mitotic pro-spermatogonia or oogonia. This process ensures sexually dimorphic germ cell development for totipotency1. In vitro reconstitution of epigenetic reprogramming in humans remains a fundamental challenge. Here we establish a strategy for inducing epigenetic reprogramming and differentiation of pluripotent stem-cell-derived human PGC-like cells (hPGCLCs) into mitotic pro-spermatogonia or oogonia, coupled with their extensive amplification (about >1010-fold). Bone morphogenetic protein (BMP) signalling is a key driver of these processes. BMP-driven hPGCLC differentiation involves attenuation of the MAPK (ERK) pathway and both de novo and maintenance DNA methyltransferase activities, which probably promote replication-coupled, passive DNA demethylation. hPGCLCs deficient in TET1, an active DNA demethylase abundant in human germ cells2,3, differentiate into extraembryonic cells, including amnion, with de-repression of key genes that bear bivalent promoters. These cells fail to fully activate genes vital for spermatogenesis and oogenesis, and their promoters remain methylated. Our study provides a framework for epigenetic reprogramming in humans and an important advance in human biology. Through the generation of abundant mitotic pro-spermatogonia and oogonia-like cells, our results also represent a milestone for human in vitro gametogenesis research and its potential translation into reproductive medicine.
Assuntos
Reprogramação Celular , Epigênese Genética , Células Germinativas , Técnicas In Vitro , Feminino , Humanos , Masculino , Âmnio/citologia , Proteínas Morfogenéticas Ósseas/metabolismo , Reprogramação Celular/genética , Metilação de DNA/genética , Células Germinativas/metabolismo , Células Germinativas/citologia , Sistema de Sinalização das MAP Quinases , Mitose/genética , Oxigenases de Função Mista/deficiência , Oogênese/genética , Oogônios/citologia , Oogônios/metabolismo , Células-Tronco Pluripotentes/citologia , Células-Tronco Pluripotentes/metabolismo , Regiões Promotoras Genéticas/genética , Espermatogênese/genética , Espermatogônias/citologia , Espermatogônias/metabolismo , Regulação da Expressão Gênica no DesenvolvimentoRESUMO
By sequencing sites of mitotic DNA synthesis in cells lacking homologous recombination, Groelly, Bhowmick, and colleagues show how conflicts between transcription and replication in early S phase can cause under-replicated DNA to persist into mitosis.
Assuntos
Replicação do DNA , Transcrição Gênica , DNA , Mitose/genéticaRESUMO
Aberrant replication causes cells lacking BRCA2 to enter mitosis with under-replicated DNA, which activates a repair mechanism known as mitotic DNA synthesis (MiDAS). Here, we identify genome-wide the sites where MiDAS reactions occur when BRCA2 is abrogated. High-resolution profiling revealed that these sites are different from MiDAS at aphidicolin-induced common fragile sites in that they map to genomic regions replicating in the early S-phase, which are close to early-firing replication origins, are highly transcribed, and display R-loop-forming potential. Both transcription inhibition in early S-phase and RNaseH1 overexpression reduced MiDAS in BRCA2-deficient cells, indicating that transcription-replication conflicts (TRCs) and R-loops are the source of MiDAS. Importantly, the MiDAS sites identified in BRCA2-deficient cells also represent hotspots for genomic rearrangements in BRCA2-mutated breast tumors. Thus, our work provides a mechanism for how tumor-predisposing BRCA2 inactivation links transcription-induced DNA damage with mitotic DNA repair to fuel the genomic instability characteristic of cancer cells.
Assuntos
Replicação do DNA , Mitose , Afidicolina/farmacologia , Proteína BRCA2/genética , Sítios Frágeis do Cromossomo/genética , DNA/genética , Dano ao DNA , Instabilidade Genômica , Humanos , Mitose/genéticaRESUMO
SMC protein complexes are molecular machines that provide structure to chromosomes. These complexes bridge DNA elements and by doing so build DNA loops in cis and hold together the sister chromatids in trans. We discuss how drastic conformational changes allow SMC complexes to build such intricate DNA structures. The tight regulation of these complexes controls fundamental chromosomal processes such as transcription, recombination, repair, and mitosis.
Assuntos
Proteínas de Ciclo Celular , Proteínas Cromossômicas não Histona , Proteínas de Ciclo Celular/metabolismo , Cromátides/metabolismo , Proteínas Cromossômicas não Histona/genética , Proteínas Cromossômicas não Histona/metabolismo , DNA/genética , Mitose/genéticaRESUMO
One of the major cell fate transitions in eukaryotes is entry into meiosis. While in single-celled yeast this decision is triggered by nutrient starvation, in multicellular eukaryotes, such as plants, it is under developmental control. In contrast to animals, plants have only a short germline and instruct cells to become meiocytes in reproductive organs late in development. This situation argues for a fundamentally different mechanism of how plants recruit meiocytes, and consistently, none of the regulators known to control meiotic entry in yeast and animals are present in plants. In recent years, several factors involved in meiotic entry have been identified, especially in the model plant Arabidopsis, and pieces of a regulatory network of germline control in plants are emerging. However, the corresponding studies also show that the mechanisms of meiotic entry control are diversified in flowering plants, calling for further analyses in different plant species.
Assuntos
Arabidopsis , Meiose , Animais , Arabidopsis/genética , Diferenciação Celular , Células Germinativas/fisiologia , Meiose/genética , Mitose/genéticaRESUMO
Complex genome rearrangements can be generated by the catastrophic pulverization of missegregated chromosomes trapped within micronuclei through a process known as chromothripsis1-5. As each chromosome contains a single centromere, it remains unclear how acentric fragments derived from shattered chromosomes are inherited between daughter cells during mitosis6. Here we tracked micronucleated chromosomes with live-cell imaging and show that acentric fragments cluster in close spatial proximity throughout mitosis for asymmetric inheritance by a single daughter cell. Mechanistically, the CIP2A-TOPBP1 complex prematurely associates with DNA lesions within ruptured micronuclei during interphase, which poises pulverized chromosomes for clustering upon mitotic entry. Inactivation of CIP2A-TOPBP1 caused acentric fragments to disperse throughout the mitotic cytoplasm, stochastically partition into the nucleus of both daughter cells and aberrantly misaccumulate as cytoplasmic DNA. Mitotic clustering facilitates the reassembly of acentric fragments into rearranged chromosomes lacking the extensive DNA copy-number losses that are characteristic of canonical chromothripsis. Comprehensive analysis of pan-cancer genomes revealed clusters of DNA copy-number-neutral rearrangements-termed balanced chromothripsis-across diverse tumour types resulting in the acquisition of known cancer driver events. Thus, distinct patterns of chromothripsis can be explained by the spatial clustering of pulverized chromosomes from micronuclei.
Assuntos
Cromossomos Humanos , Cromotripsia , Micronúcleos com Defeito Cromossômico , Mitose , Humanos , Centrômero , Cromossomos Humanos/genética , DNA/genética , DNA/metabolismo , Variações do Número de Cópias de DNA , Interfase , Mitose/genética , Neoplasias/genéticaRESUMO
For cells to initiate and sustain a differentiated state, it is necessary that a 'memory' of this state is transmitted through mitosis to the daughter cells1-3. Mammalian switch/sucrose non-fermentable (SWI/SNF) complexes (also known as Brg1/Brg-associated factors, or BAF) control cell identity by modulating chromatin architecture to regulate gene expression4-7, but whether they participate in cell fate memory is unclear. Here we provide evidence that subunits of SWI/SNF act as mitotic bookmarks to safeguard cell identity during cell division. The SWI/SNF core subunits SMARCE1 and SMARCB1 are displaced from enhancers but are bound to promoters during mitosis, and we show that this binding is required for appropriate reactivation of bound genes after mitotic exit. Ablation of SMARCE1 during a single mitosis in mouse embryonic stem cells is sufficient to disrupt gene expression, impair the occupancy of several established bookmarks at a subset of their targets and cause aberrant neural differentiation. Thus, SWI/SNF subunit SMARCE1 has a mitotic bookmarking role and is essential for heritable epigenetic fidelity during transcriptional reprogramming.
Assuntos
Diferenciação Celular , Proteínas Cromossômicas não Histona , Epigênese Genética , Mitose , Animais , Camundongos , Diferenciação Celular/genética , Cromatina/genética , Montagem e Desmontagem da Cromatina/genética , Proteínas Nucleares/metabolismo , Fatores de Transcrição/metabolismo , Mitose/genética , Proteínas Cromossômicas não Histona/deficiência , Proteínas Cromossômicas não Histona/genética , Proteínas Cromossômicas não Histona/metabolismo , Subunidades Proteicas/metabolismo , Células-Tronco Embrionárias Murinas/metabolismo , Elementos Facilitadores Genéticos/genética , Regiões Promotoras Genéticas/genética , Divisão Celular/genética , Epigênese Genética/genéticaRESUMO
Anniina Vihervaara and Lea Sistonen share with us some of their experiences, including the paths to setting up their own labs and some behind-the-scenes details about their paper, "Stress-induced transcriptional memory accelerates promoter-proximal pause-release and decelerates termination over mitotic divisions."
Assuntos
Transcrição Gênica/genética , Humanos , Pessoal de Laboratório , Mitose/genética , Regiões Promotoras Genéticas/genética , Pesquisa , Estresse Fisiológico/genéticaRESUMO
Heat shock instantly reprograms transcription. Whether gene and enhancer transcription fully recover from stress and whether stress establishes a memory by provoking transcription regulation that persists through mitosis remained unknown. Here, we measured nascent transcription and chromatin accessibility in unconditioned cells and in the daughters of stress-exposed cells. Tracking transcription genome-wide at nucleotide-resolution revealed that cells precisely restored RNA polymerase II (Pol II) distribution at gene bodies and enhancers upon recovery from stress. However, a single heat exposure in embryonic fibroblasts primed a faster gene induction in their daughter cells by increasing promoter-proximal Pol II pausing and by accelerating the pause release. In K562 erythroleukemia cells, repeated stress refined basal and heat-induced transcription over mitotic division and decelerated termination-coupled pre-mRNA processing. The slower termination retained transcripts on the chromatin and reduced recycling of Pol II. These results demonstrate that heat-induced transcriptional memory acts through promoter-proximal pause release and pre-mRNA processing at transcription termination.
Assuntos
Mitose/genética , Regiões Promotoras Genéticas/genética , Estresse Fisiológico/genética , Transcrição Gênica/genética , Linhagem Celular Tumoral , Cromatina/genética , Fibroblastos/fisiologia , Regulação da Expressão Gênica/genética , Genoma/genética , Resposta ao Choque Térmico/genética , Humanos , Células K562 , RNA Polimerase II/genética , RNA Mensageiro/genéticaRESUMO
During self-renewal, cell-type-defining features are drastically perturbed in mitosis and must be faithfully reestablished upon G1 entry, a process that remains largely elusive. Here, we characterized at a genome-wide scale the dynamic transcriptional and architectural resetting of mouse pluripotent stem cells (PSCs) upon mitotic exit. We captured distinct waves of transcriptional reactivation with rapid induction of stem cell genes and transient activation of lineage-specific genes. Topological reorganization at different hierarchical levels also occurred in an asynchronous manner and showed partial coordination with transcriptional resetting. Globally, rapid transcriptional and architectural resetting associated with mitotic retention of H3K27 acetylation, supporting a bookmarking function. Indeed, mitotic depletion of H3K27ac impaired the early reactivation of bookmarked, stem-cell-associated genes. However, 3D chromatin reorganization remained largely unaffected, suggesting that these processes are driven by distinct forces upon mitotic exit. This study uncovers principles and mediators of PSC molecular resetting during self-renewal.
Assuntos
Cromatina/genética , Código das Histonas/genética , Histonas/genética , Mitose/genética , Células-Tronco Pluripotentes/fisiologia , Acetilação , Animais , Linhagem Celular , Drosophila/genética , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Transcrição Gênica/genética , Ativação Transcricional/genéticaRESUMO
Aberrant cell proliferation is a hallmark of cancer, including glioblastoma (GBM). Here we report that protein arginine methyltransferase (PRMT) 6 activity is required for the proliferation, stem-like properties, and tumorigenicity of glioblastoma stem cells (GSCs), a subpopulation in GBM critical for malignancy. We identified a casein kinase 2 (CK2)-PRMT6-regulator of chromatin condensation 1 (RCC1) signaling axis whose activity is an important contributor to the stem-like properties and tumor biology of GSCs. CK2 phosphorylates and stabilizes PRMT6 through deubiquitylation, which promotes PRMT6 methylation of RCC1, which in turn is required for RCC1 association with chromatin and activation of RAN. Disruption of this pathway results in defects in mitosis. EPZ020411, a specific small-molecule inhibitor for PRMT6, suppresses RCC1 arginine methylation and improves the cytotoxic activity of radiotherapy against GSC brain tumor xenografts. This study identifies a CK2α-PRMT6-RCC1 signaling axis that can be therapeutically targeted in the treatment of GBM.