Old cells, new tricks: chromatin structure in senescence

NIH SenNet Consortium to map senescent cells throughout the human lifespan to understand physiological health

Cells respond to many stressors by senescing, acquiring stable growth arrest, morphologic and metabolic changes, and a proinflammatory senescence-associated secretory phenotype. The heterogeneity of senescent cells (SnCs) and senescence-associated secretory phenotype are vast, yet ill characterized. SnCs have diverse roles in health and disease and are therapeutically targetable, making characterization of SnCs and their detection a priority. The Cellular Senescence Network (SenNet), a National Institutes of Health Common Fund initiative, was established to address this need. The goal of SenNet is to map SnCs across the human lifespan to advance diagnostic and therapeutic approaches to improve human health. State-of-the-art methods will be applied to identify, define and map SnCs in 18 human tissues. A common coordinate framework will integrate data to create four-dimensional SnC atlases. Other key SenNet deliverables include innovative tools and technologies to detect SnCs, new SnC biomarkers and extensive public multi-omics datasets. This Perspective lays out the impetus, goals, approaches and products of SenNet.

Research on Werner Syndrome: Trends from Past to Present and Future Prospects

A rare and autosomal recessive premature aging disorder, Werner syndrome (WS) is characterized by the early onset of aging-associated diseases, including shortening stature, alopecia, bilateral cataracts, skin ulcers, diabetes, osteoporosis, arteriosclerosis, and chromosomal instability, as well as cancer predisposition. WRN, the gene responsible for WS, encodes DNA helicase with a 3′ to 5′ exonuclease activity, and numerous studies have revealed that WRN helicase is involved in the maintenance of chromosome stability through actions in DNA, e.g., DNA replication, repair, recombination, and epigenetic regulation via interaction with DNA repair factors, telomere-binding proteins, histone modification enzymes, and other DNA metabolic factors. However, although these efforts have elucidated the cellular functions of the helicase in cell lines, they have not been linked to the treatment of the disease. Life expectancy has improved for WS patients over the past three decades, and it is hoped that a fundamental treatment for the disease will be developed. Disease-specific induced pluripotent stem (iPS) cells have been established, and these are expected to be used in drug discovery and regenerative medicine for WS patients. In this article, we review trends in research to date and present some perspectives on WS research with regard to the application of pluripotent stem cells. Furthermore, the elucidation of disease mechanisms and drug discovery utilizing the vast amount of scientific data accumulated to date will be discussed.

Senescence: An Identity Crisis Originating from Deep Within the Nucleus

Cellular senescence is implicated in a wide range of physiological and pathological conditions throughout an organism's entire lifetime. In particular, it has become evident that senescence plays a causative role in aging and age-associated disorders. This is not due simply to the loss of function of senescent cells. Instead, the substantial alterations of the cellular activities of senescent cells, especially the array of secretory factors, impact the surrounding tissues or even entire organisms. Such non-cell-autonomous functionality is largely coordinated by tissue-specific genes, constituting a cell fate-determining state. Senescence can be viewed as a gain-of-function phenotype or a process of cell identity shift. Cellular functionality or lineage-specific gene expression is tightly linked to the cell type-specific epigenetic landscape, reinforcing the heterogeneity of senescence across cell types. Here, we aim to define the senescence cellular functionality and epigenetic features that may contribute to the gain-of-function phenotype.

Inflammasome activation: from molecular mechanisms to autoinflammation

Inflammasomes are assembled by innate immune sensors that cells employ to detect a range of danger signals and respond with pro-inflammatory signalling. Inflammasomes activate inflammatory caspases, which trigger a cascade of molecular events with the potential to compromise cellular integrity and release the IL-1β and IL-18 pro-inflammatory cytokines. Several molecular mechanisms, working in concert, ensure that inflammasome activation is tightly regulated; these include NLRP3 post-translational modifications, ubiquitination and phosphorylation, as well as single-domain proteins that competitively bind to key inflammasome components, such as the CARD-only proteins (COPs) and PYD-only proteins (POPs). These diverse regulatory systems ensure that a suitable level of inflammation is initiated to counteract any cellular insult, while simultaneously preserving tissue architecture. When inflammasomes are aberrantly activated can drive excessive production of pro-inflammatory cytokines and cell death, leading to tissue damage. In several autoinflammatory conditions, inflammasomes are aberrantly activated with subsequent development of clinical features that reflect the degree of underlying tissue and organ damage. Several of the resulting disease complications may be successfully controlled by anti-inflammatory drugs and/or specific cytokine inhibitors, in addition to more recently developed small-molecule inhibitors. In this review, we will explore the molecular processes underlying the activation of several inflammasomes and highlight their role during health and disease. We also describe the detrimental effects of these inflammasome complexes, in some pathological conditions, and review current therapeutic approaches as well as future prospective treatments.

Translation of Cellular Senescence to Novel Therapeutics: Insights From Alternative Tools and Models

Increasing chronological age is the greatest risk factor for human diseases. Cellular senescence (CS), which is characterized by permanent cell-cycle arrest, has recently emerged as a fundamental mechanism in developing aging-related pathologies. During the aging process, senescent cell accumulation results in senescence-associated secretory phenotype (SASP) which plays an essential role in tissue dysfunction. Although discovered very recently, senotherapeutic drugs have been already involved in clinical studies. This review gives a summary of the molecular mechanisms of CS and its role particularly in the development of cardiovascular diseases (CVD) as the leading cause of death. In addition, it addresses alternative research tools including the nonhuman and human models as well as computational techniques for the discovery of novel therapies. Finally, senotherapeutic approaches that are mainly classified as senolytics and senomorphics are discussed.

The methyltransferase enzymes, KMT2D, SETD1B, and ASH1L, are key mediators of both metabolic and epigenetic changes during cellular senescence

Cellular senescence is a terminal cell fate characterized by growth arrest and a metabolically active state characterized by high glycolytic activity. Human fibroblasts were placed in a unique metabolic state using a combination of methionine restriction (MetR) and rapamycin (Rapa). This combination induced a metabolic reprogramming that prevented the glycolytic shift associated with senescence. Surprisingly, cells treated in this manner did not undergo senescence but continued to divide at a slow rate even at high passage, in contrast with either Rapa treatment or MetR, both of which extended life span but eventually resulted in growth arrest. Transcriptome-wide analysis revealed a coordinated regulation of metabolic enzymes related to one-carbon metabolism including three methyltransferase enzymes (KMT2D, SETD1B, and ASH1L), key enzymes for both carnitine synthesis and histone modification. These enzymes appear to be involved in both the metabolic phenotype of senescent cells and the chromatin changes required for establishing the senescence arrest. Targeting one of these enzymes, ASH1L, produced both a glycolytic shift and senescence, providing proof of concept. These findings reveal a mechanistic link between a major metabolic hallmark of senescence and nuclear events required for senescence.

Cellular Senescence: Mechanisms and Therapeutic Potential

Cellular senescence is a complex and multistep biological process which cells can undergo in response to different stresses. Referring to a highly stable cell cycle arrest, cellular senescence can influence a multitude of biological processes-both physiologically and pathologically. While phenotypically diverse, characteristics of senescence include the expression of the senescence-associated secretory phenotype, cell cycle arrest factors, senescence-associated β-galactosidase, morphogenesis, and chromatin remodelling. Persistent senescence is associated with pathologies such as aging, while transient senescence is associated with beneficial programmes, such as limb patterning. With these implications, senescence-based translational studies, namely senotherapy and pro-senescence therapy, are well underway to find the cure to complicated diseases such as cancer and atherosclerosis. Being a subject of major interest only in the recent decades, much remains to be studied, such as regarding the identification of unique biomarkers of senescent cells. This review attempts to provide a comprehensive understanding of the diverse literature on senescence, and discuss the knowledge we have on senescence thus far.

A common signature of cellular senescence; does it exist?

Cellular senescence is a stress response, which can be evoked in all type of somatic cells by different stimuli. Senescent cells accumulate in the body and participate in aging and aging-related diseases mainly by their secretory activity, commonly known as senescence-associated secretory phenotype-SASP. Senescence is typically described as cell cycle arrest. This definition stems from the original observation concerning limited cell division potential of human fibroblasts in vitro. At present, the process of cell senescence is attributed also to cancer cells and to non-proliferating post-mitotic cells. Many cellular signaling pathways and specific and unspecific markers contribute to the complex, dynamic and heterogeneous phenotype of senescent cells. Considering the diversity of cells that can undergo senescence upon different inducers and variety of mechanisms involved in the execution of this process, we ask if there is a common signature of cell senescence. It seems that cell cycle arrest in G0, G1 or G2 is indispensable for cell senescence; however, to ensure irreversibility of divisions, the exit from the cell cycle to the state, which we call a GS (Gero Stage), is necessary. The DNA damage, changes in nuclear architecture and chromatin rearrangement are involved in signaling pathways leading to altered gene transcription and secretion of SASP components. Thus, nuclear changes and SASP are vital features of cell senescence that, together with temporal arrest in the cell cycle (G1 or/and G2), which may be followed by polyploidisation/depolyploidisation or exit from the cell cycle leading to permanent proliferation arrest (GS), define the signature of cellular senescence.

FGF-2 induces a failure of cell cycle progression in cells harboring amplified K-Ras, revealing new insights into oncogene-induced senescence

Paradoxically, oncogenes that drive cell cycle progression may also trigger pathways leading to senescence, thereby inhibiting the growth of tumorigenic cells. Knowledge of how these pathways operate, and how tumor cells may evade these pathways, is important for understanding tumorigenesis. The Y1 cell line, which harbors an amplification of the proto-oncogene Ras, rapidly senesces in response to the mitogen fibroblast growth factor-2 (FGF-2). To gain a more complete picture of how FGF-2 promotes senescence, we employed a multi-omics approach to analyze histone modifications, mRNA and protein expression, and protein phosphorylation in Y1 cells treated with FGF-2. Compared to control cells treated with serum alone, FGF-2 caused a delayed accumulation of acetylation on histone H4 and higher levels of H3K27me3. Sequencing analysis revealed decreased expression of cell cycle-related genes with concomitant loss of H3K27ac. At the same time, FGF-2 promoted the expression of p21, various cytokines, and MAPK-related genes. Nuclear envelope proteins, particularly lamin B1, displayed increased phosphorylation in response to FGF-2. Proteome analysis suggested alterations in cellular metabolism, as evident by modulated expression of enzymes involved in purine biosynthesis, tRNA aminoacylation, and the TCA cycle. We propose that Y1 cells senesce due to an inability to progress through the cell cycle, which may stem from DNA damage or TGFb signaling. Altogether, the phenotype of Y1 cells is consistent with rapidly established oncogene-induced senescence, demonstrating the synergy between growth factors and oncogenes in driving senescence and bringing additional insight into this tumor suppressor mechanism.

Senescence-induced inflammation: an important player and key therapeutic target in atherosclerosis

Inflammation is a hallmark and potent driver of pathological vascular remodelling in atherosclerosis. However, current anti-inflammatory therapeutic strategies have shown mixed results. As an alternative perspective on the conundrum of chronic inflammation emerging evidence points towards a small subset of senescent cells as a critical player and central node driving atherosclerosis. Senescent cells belonging to various cell types are a dominant and chronic source of a large array of pro-inflammatory cytokines and various additional plaque destabilizing factors, being involved with various aspects of atherosclerosis pathogenesis. Antagonizing these key agitators of local chronic inflammation and plaque instability may provide a causative and multi-purpose therapeutic strategy to treat atherosclerosis. Anti-senescence treatment options with translational potential are currently in development. However, several questions and challenges remain to be addressed before these novel treatment approaches may enter the clinical setting.

Hallmarks of aging and immunosenescence: Connecting the dots

Aging is a natural physiological process that features various and variable challenges, associated with loss of homeostasis within the organism, often leading to negative consequences for health. Cellular senescence occurs when cells exhaust the capacity to renew themselves and their tissue environment as the cell cycle comes to a halt. This process is influenced by genetics, metabolism and extrinsic factors. Immunosenescence, the aging of the immune system, is a result of the aging process, but can also in turn act as a secondary inducer of senescence within other tissues. This review aims to summarize the current state of knowledge regarding hallmarks of aging in relation to immunosenescence, with a focus on aging-related imbalances in the medullary environment, as well as the components of the innate and adaptive immune responses. Aging within the immune system alters its functionality, and has consequences for the person's ability to fight infections, as well as for susceptibility to chronic diseases such as cancer and cardiovascular disease. The senescence-associated secretory phenotype is described, as well as the involvement of this phenomenon in the paracrine induction of senescence in otherwise healthy cells. Inflammaging is discussed in detail, along with the comorbidities associated with this process. A knowledge of these processes is required in order to consider possible targets for the application of senotherapeutic agents - interventions with the potential to modulate the senescence process, thus prolonging the healthy lifespan of the immune system and minimizing the secondary effects of immunosenescence.

To Breathe or Not to Breathe: The Role of Oxygen in Bone Marrow-Derived Mesenchymal Stromal Cell Senescence

Stem cell-based cellular therapy is a promising tool for the treatment of pathological conditions with underlying severe tissue damage or malfunction like in chronic cardiovascular, musculoskeletal, or inflammatory conditions. One of the biggest technical challenges of the use of natural stem cells, however, is the prevention of their premature senescence during therapeutical manipulations. Culturing stem cells under hypoxic conditions is believed to be a possible route to fulfill this goal. Here, we review current literature data on the effects of hypoxia on bone marrow-derived mesenchymal stromal cells, one of the most popular tools of practical cellular therapy, in the context of their senescence.

Explore a novel function of human condensins in cellular senescence

There are two kinds of condensins in human cells, known as condensin I and condensin II. The canonical roles of condensins are participated in chromosome dynamics, including chromosome condensation and segregation during cell division. Recently, a novel function of human condensins has been found with increasing evidences that they play important roles in cellular senescence. This paper reviewed the research progress of human condensins involved in different types of cellular senescence, mainly oncogene-induced senescence (OIS) and replicative senescence (RS). The future perspectives of human condensins involved in cellular senescence are also discussed.

Senescence in RASopathies, a possible novel contributor to a complex pathophenoype

Senescence is a biological process that induces a permanent cell cycle arrest and a specific gene expression program in response to various stressors. Following studies over the last few decades, the concept of senescence has evolved from an antiproliferative mechanism in cancer (oncogene-induced senescence) to a critical component of physiological processes associated with embryonic development, tissue regeneration, ageing and its associated diseases. In somatic cells, oncogenic mutations in RAS-MAPK pathway genes are associated with oncogene-induced senescence and cancer, while germline mutations in the same pathway are linked to a group of monogenic developmental disorders generally termed RASopathies. Here, we consider that in these disorders, senescence induction may result in opposing outcomes, a tumour protective effect and a possible contributor to a premature ageing phenotype identified in Costello syndrome, which belongs to the RASopathy group. In this review, we will highlight the role of senescence in organismal homeostasis and we will describe the current knowledge about senescence in RASopathies. Additionally, we provide a perspective on examples of experimentally characterised RASopathy mutations that, alone or in combination with various stressors, may also trigger an age-dependent chronic senescence, possibly contributing to the age-dependent worsening of RASopathy pathophenotype and the reduction of lifespan.

Aging mechanisms-A perspective mostly from Drosophila

A mechanistic understanding of the natural aging process, which is distinct from aging-related disease mechanisms, is essential for developing interventions to extend lifespan or healthspan. Here, we discuss current trends in aging research and address conceptual and experimental challenges in the field. We examine various molecular markers implicated in aging with an emphasis on the role of heterochromatin and epigenetic changes. Studies in model organisms have been advantageous in elucidating conserved genetic and epigenetic mechanisms and assessing interventions that affect aging. We highlight the use of Drosophila, which allows controlled studies for evaluating genetic and environmental contributors to aging conveniently. Finally, we propose the use of novel methodologies and future strategies using Drosophila in aging research.

Rising from the ashes: cellular senescence in regeneration

Cellular senescence has recently become causally implicated in pathological ageing. Hence, a great deal of research is currently dedicated towards developing senolytic agents to selectively kill senescent cells. However, senescence also plays important roles in a range of physiological processes including during organismal development, providing a barrier to tumorigenesis and in limiting fibrosis. Recent evidence also suggests a role for senescence in coordinating tissue remodelling and in the regeneration of complex structures. Through its non-cell-autonomous effects, a transient induction of senescence may create a permissive environment for remodelling or regeneration through promoting local proliferation, cell plasticity, tissue patterning, balancing growth, or indirectly through finely tuned interactions with infiltrating immune mediators. A careful analysis of the beneficial roles of cellular senescence may provide insights into important physiological processes as well as informing strategies to counteract its detrimental consequences in ageing and disease.

An Appraisal on the Value of Using Nutraceutical Based Senolytics and Senostatics in Aging

The average human life expectancy has increased globally, and continues to rise, owing to the substantive progress made in healthcare, medicine, sanitation, housing and education. This ultimately enriches society with a greater proportion of elderly people. Sustaining a healthy aged population is key to diminish the societal and economic impact of age-related infirmities. This is especially challenging because tissue function, and thus wellbeing, naturally progressively decline as humans age. With age increasing the risk of developing diseases, one of the therapeutic options is to interfere with the molecular and cellular pathways involved in age-related tissue dysfunction, which is in part caused by the accumulation of senescent cells. One strategy to prevent this could be using drugs that selectively kill these cells (senolytics). In parallel, some compounds have been identified that prevent or slow down the progression of senescence or some of its features (senostatics). Senolytic and senostatic therapies have been shown to be efficient in vivo, but they also have unwanted dose-dependent side effects, including toxicity. Important advances might be made using bioactive compounds from plants and foods (nutraceuticals) if, as is proposed, they offer similar effectiveness with fewer side effects. The focus of this review is on the use of nutraceuticals in interfering with cellular senescence.

Elucidating Proteoform Dynamics Underlying the Senescence Associated Secretory Phenotype

Primary diploid cells exit the cell cycle in response to exogenous stress or oncogene activation through a process known as cellular senescence. This cell-autonomous tumor-suppressive mechanism is also a major mechanism operative in organismal aging. To date, temporal aspects of senescence remain understudied. Therefore, we use quantitative proteomics to investigate changes following forced HRAS^G12V expression and induction of senescence across 1 week in normal diploid fibroblasts. We demonstrate that global intracellular proteomic changes correlate with the emergence of the senescence-associated secretory phenotype and the switch to robust cell cycle exit. The senescence secretome reinforces cell cycle exit, yet is largely detrimental to tissue homeostasis. Previous studies of secretomes rely on ELISA, bottom-up proteomics or RNA-seq. To date, no study to date has examined the proteoform complexity of secretomes to elucidate isoform-specific, post-translational modifications or regulated cleavage of signal peptides. Therefore, we use a quantitative top-down proteomics approach to define the molecular complexity of secreted proteins <30 kDa. We identify multiple forms of immune regulators with known activities and affinities such as distinct forms of interleukin-8, as well as GROα and HMGA1, and temporally resolve secreted proteoform dynamics. Together, our work demonstrates the complexity of the secretome past individual protein accessions and provides motivation for further proteoform-resolved measurements of the secretome.

HMGA Genes and Proteins in Development and Evolution

HMGA (high mobility group A) (HMGA1 and HMGA2) are small non-histone proteins that can bind DNA and modify chromatin state, thus modulating the accessibility of regulatory factors to the DNA and contributing to the overall panorama of gene expression tuning. In general, they are abundantly expressed during embryogenesis, but are downregulated in the adult differentiated tissues. In the present review, we summarize some aspects of their role during development, also dealing with relevant studies that have shed light on their functioning in cell biology and with emerging possible involvement of HMGA1 and HMGA2 in evolutionary biology.

Proteostasis failure and cellular senescence in long-term cultured postmitotic rat neurons

Cellular senescence, a stress-induced irreversible cell cycle arrest, has been defined for mitotic cells and is implicated in aging of replicative tissues. Age-related functional decline in the brain is often attributed to a failure of protein homeostasis (proteostasis), largely in postmitotic neurons, which accordingly is a process distinct by definition from senescence. It is nevertheless possible that proteostasis failure and cellular senescence have overlapping molecular mechanisms. Here, we identify postmitotic cellular senescence as an adaptive stress response to proteostasis failure. Primary rat hippocampal neurons in long-term cultures show molecular changes indicative of both senescence (senescence-associated β-galactosidase, p16, and loss of lamin B1) and proteostasis failure relevant to Alzheimer's disease. In addition, we demonstrate that the senescent neurons exhibit resistance to stress. Importantly, treatment of the cultures with an mTOR antagonist, protein synthesis inhibitor, or chemical compound that reduces the amount of protein aggregates relieved the proteotoxic stresses as well as the appearance of senescence markers. Our data propose mechanistic insights into the pathophysiological brain aging by establishing senescence as a primary cell-autonomous neuroprotective response.

Cellular senescence in development, regeneration and disease

Cellular senescence is a state comprising an essentially irreversible proliferative arrest combined with phenotypic changes and pronounced secretory activity. Although senescence has long been linked with aging, recent studies have uncovered functional roles for senescence in embryonic development, regeneration and reprogramming, and have helped to advance our understanding of this process as a highly coordinated and programmed cellular state. In this Primer article, we summarize some of the key findings in the field and attempt to explain them in a simple model that reconciles the normal and pathological roles for senescence. We discuss how a primary role of cellular senescence is to contribute to normal development, cell plasticity and tissue repair, as a dynamic and tightly regulated cellular program. However, when this process is perturbed, the beneficial effects turn detrimental and can contribute to disease and aging.

Super-resolution binding activated localization microscopy through reversible change of DNA conformation

Methods of super-resolving light microscopy (SRM) have found an exponentially growing range of applications in cell biology, including nuclear structure analyses. Recent developments have proven that Single Molecule Localization Microscopy (SMLM), a type of SRM, is particularly useful for enhanced spatial analysis of the cell nucleus due to its highest resolving capability combined with very specific fluorescent labeling. In this commentary we offer a brief review of the latest methodological development in the field of SMLM of chromatin designated DNA Structure Fluctuation Assisted Binding Activated Localization Microscopy (abbreviated as fBALM) as well as its potential future applications in biology and medicine.

Expansion and Cell-Cycle Arrest: Common Denominators of Cellular Senescence

Cellular senescence is a major driver of age-related diseases, and senotherapies are being tested in clinical trials. Despite its popularity, cellular senescence is weakly defined and is frequently referred to as irreversible cell-cycle arrest. In this article we hypothesize that cellular senescence is a phenotype that results from the coordination of two processes: cell expansion and cell-cycle arrest. We provide evidence for the compatibility of the proposed model with recent findings showing senescence in postmitotic tissues, wound healing, obesity, and development. We believe our model also explains why some characteristics of senescence can be found in non-senescent cells. Finally, we propose new avenues for research from our model.

Characterization of a Unique Form of Arrhythmic Cardiomyopathy Caused by Recessive Mutation in LEMD2

Nuclear envelope proteins have been shown to play an important role in the pathogenesis of inherited dilated cardiomyopathy. Here, we present a remarkable cardiac phenotype caused by a homozygous LEMD2 mutation in patients of the Hutterite population with juvenile cataract. Mutation carriers develop arrhythmic cardiomyopathy with mild impairment of left ventricular systolic function but severe ventricular arrhythmias leading to sudden cardiac death. Affected cardiac tissue from a deceased patient and fibroblasts exhibit elongated nuclei with abnormal condensed heterochromatin at the periphery. The patient fibroblasts demonstrate cellular senescence and reduced proliferation capacity, which may suggest an involvement of LEM domain containing protein 2 in chromatin remodeling processes and premature aging.

Spatial Genome Organization: From Development to Disease

Every living organism, from bacteria to humans, contains DNA encoding anything from a few hundred genes in intracellular parasites such as Mycoplasma, up to several tens of thousands in many higher organisms. The first observations indicating that the nucleus had some kind of organization were made over a hundred years ago. Understanding of its significance is both limited and aided by the development of techniques, in particular electron microscopy, fluorescence in situ hybridization, DamID and most recently HiC. As our knowledge about genome organization grows, it becomes apparent that the mechanisms are conserved in evolution, even if the individual players may vary. These mechanisms involve DNA binding proteins such as histones, and a number of architectural proteins, some of which are very much conserved, with some others having diversified and multiplied, acquiring specific regulatory functions. In this review we will look at the principles of genome organization in a hierarchical manner, from DNA packaging to higher order genome associations such as TADs, and the significance of radial positioning of genomic loci. We will then elaborate on the dynamics of genome organization during development, and how genome architecture plays an important role in cell fate determination. Finally, we will discuss how misregulation can be a factor in human disease.

Lamina Associated Domains and Gene Regulation in Development and Cancer

The nuclear lamina (NL) is a thin meshwork of filaments that lines the inner nuclear membrane, thereby providing a platform for chromatin binding and supporting genome organization. Genomic regions contacting the NL are lamina associated domains (LADs), which contain thousands of genes that are lowly transcribed, and enriched for repressive histone modifications. LADs are dynamic structures that shift spatial positioning in accordance with cell-type specific gene expression changes during differentiation and development. Furthermore, recent studies have linked the disruption of LADs and alterations in the epigenome with the onset of diseases such as cancer. Here we focus on the role of LADs and the NL in gene regulation during development and cancer.

Short-term gain, long-term pain: the senescence life cycle and cancer

Originally thought of as a stress response end point, the view of cellular senescence has since evolved into one encompassing a wide range of physiological and pathological functions, including both protumorignic and antitumorigenic features. It has also become evident that senescence is a highly dynamic and heterogenous process. Efforts to reconcile the beneficial and detrimental features of senescence suggest that physiological functions require the transient presence of senescent cells in the tissue microenvironment. Here, we propose the concept of a physiological "senescence life cycle," which has pathological consequences if not executed in its entirety.

NOTCH-mediated non-cell autonomous regulation of chromatin structure during senescence

Senescent cells interact with the surrounding microenvironment achieving diverse functional outcomes. We have recently identified that NOTCH1 can drive 'lateral induction' of a unique senescence phenotype in adjacent cells by specifically upregulating the NOTCH ligand JAG1. Here we show that NOTCH signalling can modulate chromatin structure autonomously and non-autonomously. In addition to senescence-associated heterochromatic foci (SAHF), oncogenic RAS-induced senescent (RIS) cells exhibit a massive increase in chromatin accessibility. NOTCH signalling suppresses SAHF and increased chromatin accessibility in this context. Strikingly, NOTCH-induced senescent cells, or cancer cells with high JAG1 expression, drive similar chromatin architectural changes in adjacent cells through cell-cell contact. Mechanistically, we show that NOTCH signalling represses the chromatin architectural protein HMGA1, an association found in multiple human cancers. Thus, HMGA1 is involved not only in SAHFs but also in RIS-driven chromatin accessibility. In conclusion, this study identifies that the JAG1-NOTCH-HMGA1 axis mediates the juxtacrine regulation of chromatin architecture.

The Power Behind the Throne: Senescence and the Hallmarks of Cancer

Cellular senescence is a state of stable proliferative arrest triggered by various stimuli, including oncogenic and other cellular stress. Senescent cells are highly metabolically active and have diverse and profound nonautonomous effects through the senescence-associated secretory phenotype (SASP). It has become increasingly evident that senescent cells can have tumour suppressive or pro-oncogenic effects on adjacent cancer cells and other players in the tumor microenvironment such as the stroma, vasculature, and immune system. Thus, the last decade or so has witnessed a huge leap forward in our understanding of the biology of senescence, promoting it from an autonomous tumor suppressor to a complex, dynamic, and interactive phenotype. It is perhaps not a coincidence that the concept of the “hallmarks of cancer” has also evolved during this period, with the latest iteration ( Hanahan & Weinberg 2011 ) focusing more on the microenvironment. Here, we suggest that cellular senescence could underpin the biology of many of the hallmarks of cancer, making it the true power behind the throne.

Senescence and tumor suppression

Cellular senescence has emerged as a potent tumor suppression mechanism that restrains proliferation of cells at risk for malignant transformation. Although senescent cells have permanently exited the cell cycle, their presence can have detrimental effects on the surrounding tissue, largely due to the development of the senescence-associated secretory phenotype (SASP). Here, we review the tumor-suppressive and tumor-promoting consequences of the senescence response, focusing on the SASP as a key mediator of this dichotomy. Accumulating evidence suggests that the persistence of senescent cells can exacerbate the development of a pro-inflammatory, immunosuppressive microenvironment that can favor tumorigenesis. Given that senescence of tumor and stromal cells is a frequent outcome of anti-cancer therapy, approaches that harness the growth inhibitory effects of senescence while limiting its detrimental effects are likely to have great clinical potential.

Epithelial cell senescence: an adaptive response to pre-carcinogenic stresses?

Senescence is a cell state occurring in vitro and in vivo after successive replication cycles and/or upon exposition to various stressors. It is characterized by a strong cell cycle arrest associated with several molecular, metabolic and morphologic changes. The accumulation of senescent cells in tissues and organs with time plays a role in organismal aging and in several age-associated disorders and pathologies. Moreover, several therapeutic interventions are able to prematurely induce senescence. It is, therefore, tremendously important to characterize in-depth, the mechanisms by which senescence is induced, as well as the precise properties of senescent cells. For historical reasons, senescence is often studied with fibroblast models. Other cell types, however, much more relevant regarding the structure and function of vital organs and/or regarding pathologies, are regrettably often neglected. In this article, we will clarify what is known on senescence of epithelial cells and highlight what distinguishes it from, and what makes it like, replicative senescence of fibroblasts taken as a standard.

Roles of pRB in the Regulation of Nucleosome and Chromatin Structures

Retinoblastoma protein (pRB) interacts with E2F and other protein factors to play a pivotal role in regulating the expression of target genes that induce cell cycle arrest, apoptosis, and differentiation. pRB controls the local promoter activity and has the ability to change the structure of nucleosomes and/or chromosomes via histone modification, epigenetic changes, chromatin remodeling, and chromosome organization. Functional inactivation of pRB perturbs these cellular events and causes dysregulated cell growth and chromosome instability, which are hallmarks of cancer cells. The role of pRB in regulation of nucleosome/chromatin structures has been shown to link to tumor suppression. This review focuses on the ability of pRB to control nucleosome/chromatin structures via physical interactions with histone modifiers and chromatin factors and describes cancer therapies based on targeting these protein factors.

The Chromatin Landscape of Cellular Senescence

Cellular senescence, an irreversible growth arrest triggered by a variety of stressors, plays important roles in normal physiology and tumor suppression, but accumulation of senescent cells with age contributes to the functional decline of tissues. Senescent cells undergo dramatic alterations to their chromatin landscape that affect genome accessibility and their transcriptional program. These include the loss of DNA-nuclear lamina interactions, the distension of centromeres, and changes in chromatin composition that can lead to the activation of retrotransposons. Here we discuss these findings, as well as recent advances in microscopy and genomics that have revealed the importance of the higher-order spatial organization of the genome in defining and maintaining the senescent state.