The development of cell senescence

Cellular senescence of renal tubular epithelial cells in acute kidney injury

Cellular senescence represents an irreversible state of cell-cycle arrest during which cells secrete senescence-associated secretory phenotypes, including inflammatory factors and chemokines. Additionally, these cells exhibit an apoptotic resistance phenotype. Cellular senescence serves a pivotal role not only in embryonic development, tissue regeneration, and tumor suppression but also in the pathogenesis of age-related degenerative diseases, malignancies, metabolic diseases, and kidney diseases. The senescence of renal tubular epithelial cells (RTEC) constitutes a critical cellular event in the progression of acute kidney injury (AKI). RTEC senescence inhibits renal regeneration and repair processes and, concurrently, promotes the transition of AKI to chronic kidney disease via the senescence-associated secretory phenotype. The mechanisms underlying cellular senescence are multifaceted and include telomere shortening or damage, DNA damage, mitochondrial autophagy deficiency, cellular metabolic disorders, endoplasmic reticulum stress, and epigenetic regulation. Strategies aimed at inhibiting RTEC senescence, targeting the clearance of senescent RTEC, or promoting the apoptosis of senescent RTEC hold promise for enhancing the renal prognosis of AKI. This review primarily focuses on the characteristics and mechanisms of RTEC senescence, and the impact of intervening RTEC senescence on the prognosis of AKI, aiming to provide a foundation for understanding the pathogenesis and providing potentially effective approaches for AKI treatment.

Patterns of senescence and apoptosis during development of branchial arches, epibranchial placodes, and pharyngeal pouches

BACKGROUND

Many developmental processes are coregulated by apoptosis and senescence. However, there is a lack of data on the development of branchial arches, epibranchial placodes, and pharyngeal pouches, which harbor epibranchial signaling centers.

RESULTS

Using immunohistochemical, histochemical, and 3D reconstruction methods, we show that in mice, senescence and apoptosis together may contribute to the invagination of the branchial clefts and the deepening of the cervical sinus floor, in antagonism to the proliferation acting in the evaginating branchial arches. The concomitant apoptotic elimination of lateral line rudiments occurs in the absence of senescence. In the epibranchial placodes, senescence and apoptosis appear to (1) support invagination or at least indentation by immobilizing the margins of the centrally proliferating pit, (2) coregulate the number and fate of Pax8+ precursors, (3) progressively narrow neuroblast delamination sites, and (4) contribute to placode regression. Putative epibranchial signaling centers in the pharyngeal pouches are likely deactivated by rostral senescence and caudal apoptosis.

CONCLUSIONS

Our results reveal a plethora of novel patterns of apoptosis and senescence, some overlapping, some complementary, whose functional contributions to the development of the branchial region, including the epibranchial placodes and their signaling centers, can now be tested experimentally.

Inflammation and DNA damage: cause, effect or both

Inflammation is a biological response involving immune cells, blood vessels and mediators induced by endogenous and exogenous stimuli, such as pathogens, damaged cells or chemicals. Unresolved (chronic) inflammation is characterized by the secretion of cytokines that maintain inflammation and redox stress. Mitochondrial or nuclear redox imbalance induces DNA damage, which triggers the DNA damage response (DDR) that is orchestrated by ATM and ATR kinases, which modify gene expression and metabolism and, eventually, establish the senescent phenotype. DDR-mediated senescence is induced by the signalling proteins p53, p16 and p21, which arrest the cell cycle in G1 or G2 and promote cytokine secretion, producing the senescence-associated secretory phenotype. Senescence and inflammation phenotypes are intimately associated, but highly heterogeneous because they vary according to the cell type that is involved. The vicious cycle of inflammation, DNA damage and DDR-mediated senescence, along with the constitutive activation of the immune system, is the core of an evolutionarily conserved circuitry, which arrests the cell cycle to reduce the accumulation of mutations generated by DNA replication during redox stress caused by infection or inflammation. Evidence suggests that specific organ dysfunctions in apparently unrelated diseases of autoimmune, rheumatic, degenerative and vascular origins are caused by inflammation resulting from DNA damage-induced senescence.

Cellular senescence and developmental defects

Cellular senescence is a distinct state that is frequently induced in response to ageing and stress. Yet studies have also uncovered beneficial functions in development, repair and regeneration. Current opinion therefore suggests that timely and controlled induction of senescence can be beneficial, while misregulation of the senescence program, either through mis-timed activation, or chronic accumulation of senescent cells, contributes to many disease states and the ageing process. Whether atypical activation of senescence plays a role in the pathogenesis of developmental defects has been relatively underexplored. Here, we discuss three recent studies that implicate ectopic senescence in neurodevelopmental defects, with possible causative roles for senescence in these birth defects. In addition, we highlight how the examination of senescence in other birth defects is warranted, and speculate that aberrantly activated senescence may play a much broader role in developmental defects than currently appreciated.

Highly concentrated trehalose induces prohealing senescence-like state in fibroblasts via CDKN1A/p21

Trehalose is the nonreducing disaccharide of glucose, evolutionarily conserved in invertebrates. The living skin equivalent (LSE) is an organotypic coculture containing keratinocytes cultivated on fibroblast-populated dermal substitutes. We demonstrated that human primary fibroblasts treated with highly concentrated trehalose promote significantly extensive spread of the epidermal layer of LSE without any deleterious effects. The RNA-seq analysis of trehalose-treated 2D and 3D fibroblasts at early time points revealed the involvement of the CDKN1A pathway, the knockdown of which significantly suppressed the upregulation of DPT, ANGPT2, VEGFA, EREG, and FGF2. The trehalose-treated fibroblasts were positive for senescence-associated β-galactosidase. Finally, transplantation of the dermal substitute with trehalose-treated fibroblasts accelerated wound closure and increased capillary formation significantly in the experimental mouse wounds in vivo, which was canceled by the CDKN1A knockdown. These data indicate that high-concentration trehalose can induce the senescence-like state in fibroblasts via CDKN1A/p21, which may be therapeutically useful for optimal wound repair.

Towards AI-driven longevity research: An overview

While in the past technology has mostly been utilized to store information about the structural configuration of proteins and molecules for research and medical purposes, Artificial Intelligence is nowadays able to learn from the existing data how to predict and model properties and interactions, revealing important knowledge about complex biological processes, such as aging. Modern technologies, moreover, can rely on a broader set of information, including those derived from the next-generation sequencing (e.g., proteomics, lipidomics, and other omics), to understand the interactions between human body and the external environment. This is especially relevant as external factors have been shown to have a key role in aging. As the field of computational systems biology keeps improving and new biomarkers of aging are being developed, artificial intelligence promises to become a major ally of aging research.

Nutritional Niches of Cancer Therapy-Induced Senescent Cells

Therapy-induced senescence (TIS) is a state of stable proliferative arrest of both normal and neoplastic cells that is triggered by exposure to anticancer treatments. TIS cells acquire a senescence-associated secretory phenotype (SASP), which is pro-inflammatory and actively promotes tumor relapse and adverse side-effects in patients. Here, we hypothesized that TIS cells adapt their scavenging and catabolic ability to overcome the nutritional constraints in their microenvironmental niches. We used a panel of mechanistically-diverse TIS triggers (i.e., bleomycin, doxorubicin, alisertib, and palbociclib) and Biolog Phenotype MicroArrays to identify (among 190 different carbon and nitrogen sources) candidate metabolites that support the survival of TIS cells in limiting nutrient conditions. We provide evidence of distinguishable TIS-associated nutrient consumption profiles involving a core set of shared (e.g., glutamine) and unique (e.g., glucose-1-phosphate, inosine, and uridine) nutritional sources after diverse senescence-inducing interventions. We also observed a trend for an inverse correlation between the intensity of the pro-inflammatory SASP provoked by different TIS agents and diversity of compensatory nutritional niches utilizable by senescent cells. These findings support the detailed exploration of the nutritional niche as a new metabolic dimension to understand and target TIS in cancer.

Repair, regeneration and rejuvenation require un-entangling pluripotency from senescence

There is a notion that pluripotency and senescence, represent two extremes of life of cells. Pluripotent cells display epigenetic youth, unlimited proliferative capacity and pluripotent differentiating potential whereas cells that reach the Hayflick limit, transit to senescence, undergo permanent inhibition of cell replication and create an aging tissue landscape. However, pluripotency and senescence appear to be intimately linked and are jointly generated in many different contexts such as during embryogenesis or formation of tissue spheroids, in stem cell niches, cancer, or by induction of a pluripotent state (induced pluripotency). Tissue damage and senescence provide signals that are critical to generation of a pluripotent state and, in turn, pluripotency, induces senescence. Thus, it follows, that precisely timed control of senescence is required for harnessing the full benefits of both senescence and its associated pluripotency during tissue regeneration or rejuvenation.

Aberrant induction of p19Arf-mediated cellular senescence contributes to neurodevelopmental defects

Valproic acid (VPA) is a widely prescribed drug to treat epilepsy, bipolar disorder, and migraine. If taken during pregnancy, however, exposure to the developing embryo can cause birth defects, cognitive impairment, and autism spectrum disorder. How VPA causes these developmental defects remains unknown. We used embryonic mice and human organoids to model key features of VPA drug exposure, including exencephaly, microcephaly, and spinal defects. In the malformed tissues, in which neurogenesis is defective, we find pronounced induction of cellular senescence in the neuroepithelial (NE) cells. Critically, through genetic and functional studies, we identified p19Arf as the instrumental mediator of senescence and microcephaly, but, surprisingly, not exencephaly and spinal defects. Together, these findings demonstrate that misregulated senescence in NE cells can contribute to developmental defects.

The role of cellular senescence in tissue repair and regeneration

The capacity to regenerate damaged or lost tissue varies widely along the animal kingdom and generally declines with aging of the organism. The gradual accumulation of senescent cells in tissues during aging has been causally involved in their reduced function at old age, and to be at the basis of age-related diseases. Recently, however, cellular senescence has been shown to play a positive role as a morphogenetic force modelling and promoting tissue development during embryogenesis, and to be responsible for tissue wound healing and repair. Work done on organismal models ranging from fish and amphibians, with extraordinary regenerative capacities, to mammals, with a more restricted regenerative potential, is shedding light on a novel and unexpected function of cellular senescence. In this review, we will analyze the senescence phenotype and how could it be contributing or restricting tissue regeneration.

Possibility of inducing tumor cell senescence during therapy

The treatment options for cancer include surgery, radiotherapy and chemotherapy. However, the traditional approach of high-dose chemotherapy brings tremendous toxic side effects to patients, as well as potentially causing drug resistance. Drug resistance affects cell proliferation, cell senescence and apoptosis. Cellular senescence refers to the process in which cells change from an active proliferative status to a growth-arrested status. There are multiple factors that regulate this process and cellular senescence is activated by various pathways. Senescent cells present specific characteristics, such as an increased cell volume, flattened cell body morphology, ceased cell division and the expression of β-galactosidase. Tumor senescence can be categorized into replicative senescence and premature senescence. Cellular senescence may inhibit the occurrence and development of tumors, serving as an innovative strategy for the treatment of cancer. The present review mainly focuses on senescent biomarkers, methods for the induction of cellular senescence and its possible application in the treatment of cancer.

Mechanisms of Cellular Senescence: Cell Cycle Arrest and Senescence Associated Secretory Phenotype

Cellular senescence is a stable cell cycle arrest that can be triggered in normal cells in response to various intrinsic and extrinsic stimuli, as well as developmental signals. Senescence is considered to be a highly dynamic, multi-step process, during which the properties of senescent cells continuously evolve and diversify in a context dependent manner. It is associated with multiple cellular and molecular changes and distinct phenotypic alterations, including a stable proliferation arrest unresponsive to mitogenic stimuli. Senescent cells remain viable, have alterations in metabolic activity and undergo dramatic changes in gene expression and develop a complex senescence-associated secretory phenotype. Cellular senescence can compromise tissue repair and regeneration, thereby contributing toward aging. Removal of senescent cells can attenuate age-related tissue dysfunction and extend health span. Senescence can also act as a potent anti-tumor mechanism, by preventing proliferation of potentially cancerous cells. It is a cellular program which acts as a double-edged sword, with both beneficial and detrimental effects on the health of the organism, and considered to be an example of evolutionary antagonistic pleiotropy. Activation of the p53/p21^WAF1/CIP1 and p16^INK4A/pRB tumor suppressor pathways play a central role in regulating senescence. Several other pathways have recently been implicated in mediating senescence and the senescent phenotype. Herein we review the molecular mechanisms that underlie cellular senescence and the senescence associated growth arrest with a particular focus on why cells stop dividing, the stability of the growth arrest, the hypersecretory phenotype and how the different pathways are all integrated.

Sprouty1 Prevents Cellular Senescence Maintaining Proliferation and Differentiation Capacity of Human Adipose Stem/Progenitor Cells

The role of Ras-Mitogen-activated protein kinase (MAPK) signaling in cellular aging is not precisely understood. Recently, we identified Sprouty1 (SPRY1) as a weight-loss target gene in human adipose stem/progenitor cells (ASCs) and showed that Sprouty1 is important for proper regulation of adipogenesis. In the present study, we show that loss-of-function of Sprouty1 by CRISPR/Cas9-mediated genome editing in human ASCs leads to hyper-activation of MAPK signaling and a senescence phenotype. Sprouty1 knockout ASCs undergo an irreversible cell cycle arrest, become enlarged and stain positive for senescence-associated β-galactosidase. Sprouty1 down-regulation leads to DNA double strand breaks, a considerably increased number of senescence-associated heterochromatin foci and induction of p53 and p21^Cip1. In addition, we detect an increase of hypo-phosphorylated Retinoblastoma (Rb) protein in SPRY1 knockout ASCs. p16^Ink4A is not induced. Moreover, we show that Sprouty1 knockout leads to induction of a senescence-associated secretory phenotype as indicated by the activation of the transcription factors NFκB and C/EBPβ and a significant increase in mRNA expression and secretion of interleukin-8 (IL-8) and CXCL1/GROα. Finally, we demonstrate that adipogenesis is abrogated in senescent SPRY1 knockout ASCs. In conclusion, this study reveals a novel mechanism showing the importance of Sprouty1 for the prevention of senescence and the maintenance of the proliferation and differentiation capacity of human ASCs.

Is Senescence-Associated β-Galactosidase a Reliable in vivo Marker of Cellular Senescence During Embryonic Development?

During vertebrate embryonic development, cellular senescence occurs at multiple locations. To date, it has been accepted that when there has been induction of senescence in an embryonic tissue, β-galactosidase activity is detectable at a pH as high as 6.0, and this has been extensively used as a marker of cellular senescence in vivo in both whole-mount and cryosections. Such senescence-associated β-galactosidase (SA-β-GAL) labeling appears enhanced in degenerating regions of the vertebrate embryo that are also affected by programmed cell death. In this sense, there is a strong SA-β-GAL signal which overlaps with the pattern of cell death in the interdigital tissue of the developing limbs, and indeed, many of the labeled cells detected go on to subsequently undergo apoptosis. However, it has been reported that β-GAL activity at pH 6.0 is also enhanced in healthy neurons, and some retinal neurons are strongly labeled with this histochemical technique when they begin to differentiate during early embryonic development. These labeled early post-mitotic neurons also express other senescence markers such as p21. Therefore, the reliability of this histochemical technique in studying senescence in cells such as neurons that undergo prolonged and irreversible cell-cycle arrest is questionable because it is also expressed in healthy post-mitotic cells. The identification of new biomarkers of cellular senescence would, in combination with established markers, increase the specificity and efficiency of detecting cellular senescence in embryonic and healthy mature tissues.

Programmed Cell Senescence in the Mouse Developing Spinal Cord and Notochord

Programmed cell senescence is a cellular process that seems to contribute to embryo development, in addition to cell proliferation, migration, differentiation and programmed cell death, and has been observed in evolutionary distant organisms such as mammals, amphibians, birds and fish. Programmed cell senescence is a phenotype similar to stress-induced cellular senescence, characterized by the expression of the cell cycle inhibitors p21^CIP1/WAF and p16^INK4A, increased activity of a lysosomal enzyme with beta-galactosidase activity (coined senescence-associated beta-galactosidase) and secretion of growth factors, interleukins, chemokines, metalloproteases, etc., collectively known as a senescent-associated secretory phenotype that instructs surrounding tissue. How wide is the distribution of programmed cell senescence during mouse development and its specific mechanisms to shape the embryo are still poorly understood. Here, we investigated whether markers of programmed cell senescence are found in the developing mouse spinal cord and notochord. We found discrete areas and developmental windows with high senescence-associated beta galactosidase in both spinal cord and notochord, which was reduced in mice embryos developed ex-utero in the presence of the senolytic ABT-263. Expression of p21^CIP1/WAF was documented in epithelial cells of the spinal cord and the notochord, while p16^INK4A was observed in motoneurons. Treatment with the senolytic ABT-263 decreased the number of motoneurons, supporting their senescent phenotype. Our data suggest that a subpopulation of motoneurons in the developing spinal cord, as well as some notochord cells undergo programmed cell senescence.

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.

Developmentally-programmed cellular senescence is conserved and widespread in zebrafish

Cellular senescence is considered a stress response imposing a stable cell cycle arrest to restrict the growth of damaged cells. More recently however, cellular senescence was identified during mouse embryo development at particular structures during specific periods of time. This programmed cell senescence has been proposed to serve developmental and morphogenetic functions and to potentially represent an evolutionary origin of senescence. Cellular senescence has also been described to take place during bird (chick and quail) and amphibian (xenopus and axoltl) development. Fish however, have been described to show a very narrow and restricted pattern of developmental cell senescence. Here we carried out a detailed characterization of senescence during zebrafish development and found it to be conserved and widespread. Apart from yolk and cloaca, previously described structures, we also identified senescence in the developing central nervous system, intestine, liver, pronephric ducts, and crystalline. Interestingly, senescence at these developing structures disappeared upon treatment with senolytic compound ABT-263, supporting their senescent identity and opening the possibility of studying the contribution of this process to development. In summary, our findings extend the description of developmentally-programmed cell senescence to lower vertebrates contributing to the notion of the relevance of this process for embryo development.

Otic Neurogenesis Is Regulated by TGFβ in a Senescence-Independent Manner

Cellular senescence has classically been associated with aging. Intriguingly, recent studies have also unraveled key roles for senescence in embryonic development, regeneration, and reprogramming. Developmental senescence has been reported during embryonic development in different organisms and structures, such as the endolymphatic duct during inner ear development of mammals and birds. However, there is no study addressing the possible role of senescence on otic neurogenesis. TGFβ/SMAD is the best-known pathway linked to the induction of developmentally programmed cell senescence. Here, we studied if TGFβ2 induces cellular senescence during acoustic-vestibular-ganglion (AVG) formation. Using organotypic cultures of AVG, and characterizing different stages of otic neurogenesis in the presence of TGFβ2 and a selective TGF-β receptor type-I inhibitor, we show that TGFβ2 exerts a powerful action in inner ear neurogenesis but, contrary to what we recently observed during endolymphatic duct development, these actions are independent of cellular senescence. We show that TGFβ2 reduces proliferation, and induces differentiation and neuritogenesis of neuroblasts, without altering cell death. Our studies highlight the roles of TGFβ2 and cellular senescence in the precise regulation of cell fate within the developing inner ear and its different cell types, being their mechanisms of action highly cell-type dependent.

Senescence in Wound Repair: Emerging Strategies to Target Chronic Healing Wounds

Cellular senescence is a fundamental stress response that restrains tumour formation. Yet, senescence cells are also present in non-cancerous states, accumulating exponentially with chronological age and contributing to age- and diabetes-related cellular dysfunction. The identification of hypersecretory and phagocytic behaviours in cells that were once believed to be non-functional has led to a recent explosion of senescence research. Here we discuss the profound, and often opposing, roles identified for short-lived vs. chronic tissue senescence. Transiently induced senescence is required for development, regeneration and acute wound repair, while chronic senescence is widely implicated in tissue pathology. We recently demonstrated that sustained senescence contributes to impaired diabetic healing via the CXCR2 receptor, which when blocked promotes repair. Further studies have highlighted the beneficial effects of targeting a range of senescence-linked processes to fight disease. Collectively, these findings hold promise for developing clinically viable strategies to tackle senescence in chronic wounds and other cutaneous pathologies.

Renal Programming by Transient Postnatal Overfeeding: The Role of Senescence Pathways

Background

Early nutrition influences the risk of chronic kidney diseases (CKDs) development in adulthood. Mechanisms underlying the early programming of altered renal function remain incompletely understood. This study aims at characterizing the role of cell senescence pathways in early programming of CKD after transient postnatal overfeeding.

Materials and Methods

Reduced litters of 3 mice pups and standard litters of 9 mice pups were obtained to induce overfed animals during lactation and control animals, respectively. Animals were sacrificed at 24 days (weaning) or at 7 months of life (adulthood). Body weight, blood pressure, kidney weight, and glomerular count were assessed in both groups. Senescence pathways were investigated using β-Galactosidase staining and Western blotting of P16, P21, P53, P-Rb/Rb, and Sirtuin 1 (Sirt1) proteins.

Results

Early overfed animals had a higher body weight, a higher blood pressure at adulthood, and a higher glomerular number endowment compared to the control group. A higher β-Galactosidase activity, a significant increase in P53 protein expression (p = 0.0045) and a significant decrease in P-Rb/Rb ratio (p = 0.02), were observed at weaning in animals who underwent early postnatal overfeeding. Protein expression of Sirt1, a protective factor against accelerated stress-induced senescence, was significantly decreased (p = 0.03) at weaning in early overfed animals.

Conclusion

Early postnatal overfeeding by litter size reduction is associated with increased expression of factors involved in cellular senescence pathways, and decreased expression of Sirt 1 in the mouse kidney at weaning. These alterations may contribute to CKD programming after early postnatal overfeeding.

Cellular Senescence as a Therapeutic Target for Age-Related Diseases: A Review

Life expectancy has increased substantially over the last few decades, leading to a worldwide increase in the prevalence and burden of aging-associated diseases. Recent evidence has proven that cellular senescence contributes substantially to the development of these disorders. Cellular senescence is a state of cell cycle arrest with suppressed apoptosis and concomitant secretion of multiple bioactive factors (the senescence-associated secretory phenotype-SASP) that plays a physiological role in embryonic development and healing processes. However, DNA damage and oxidative stress that occur during aging cause the accumulation of senescent cells, which through their SASP bring about deleterious effects on multiple organ and systemic functions. Ablation of senescent cells through genetic or pharmacological means leads to improved life span and health span in animal models, and preliminary evidence suggests it may also have a positive impact on human health. Thus, strategies to reduce or eliminate the burden of senescent cells or their products have the potential to impact multiple clinical outcomes with a single intervention. In this review, we touch upon the basics of cell senescence and summarize the current state of development of therapies against cell senescence for human use.

Beyond Tumor Suppression: Senescence in Cancer Stemness and Tumor Dormancy

Here, we provide an overview of the importance of cellular fate in cancer as a group of diseases of abnormal cell growth. Tumor development and progression is a highly dynamic process, with several phases of evolution. The existing evidence about the origin and consequences of cancer cell fate specification (e.g., proliferation, senescence, stemness, dormancy, quiescence, and cell cycle re-entry) in the context of tumor formation and metastasis is discussed. The interplay between these dynamic tumor cell phenotypes, the microenvironment, and the immune system is also reviewed in relation to cancer. We focus on the role of senescence during cancer progression, with a special emphasis on its relationship with stemness and dormancy. Selective interventions on senescence and dormancy cell fates, including the specific targeting of cancer cell populations to prevent detrimental effects in aging and disease, are also reviewed. A new conceptual framework about the impact of synthetic lethal strategies by using senogenics and then senolytics is given, with the promise of future directions on innovative anticancer therapies.