Age- and oxidative stress-induced DNA damage in Drosophila intestinal stem cells as marked by Gamma-H2AX

Drosophila model systems reveal intestinal stem cells as key players in aging

The intestines play important roles in responding immediately and dynamically to food intake, environmental stress, and metabolic dysfunction, and they are involved in various human diseases and aging. A key part of their function is governed by intestinal stem cells (ISCs); therefore, understanding ISCs is vital. Dysregulation of ISC activity, which is influenced by various cell signaling pathways and environmental signals, can lead to inflammatory responses, tissue damage, and increased cancer susceptibility. Aging exacerbates these dynamics and affects ISC function and tissue elasticity. Additionally, proliferation and differentiation profoundly affect ISC behavior and gut health, highlighting the complex interplay between environmental factors and gut homeostasis. Drosophila models help us understand the complex regulatory networks in the gut, providing valuable insights into disease mechanisms and therapeutic strategies targeting human intestinal diseases.

Werner syndrome exonuclease promotes gut regeneration and causes age-associated gut hyperplasia in Drosophila

Human Werner syndrome (adult progeria, a well-established model of human aging) is caused by mutations in the Werner syndrome (WRN) gene. However, the expression patterns and functions of WRN in natural aging remain poorly understood. Despite the link between WRN deficiencies and progeria, our analyses of human colon tissues, mouse crypts, and Drosophila midguts revealed that WRN expression does not decrease but rather increases in intestinal stem cells (ISCs) with aging. Mechanistically, we found that the Drosophila WRN homologue (WRNexo) binds to Heat shock 70-kDa protein cognate 3 (Hsc70-3/Bip) to regulate the unfolded protein response of the endoplasmic reticulum (UPRER). Activation of the WRNexo-mediated UPRER in ISCs is required for ISC proliferation during injury repair. However, persistent DNA damage during aging leads to chronic upregulation of WRNexo in ISCs, where excessive WRNexo-induced ER stress drives age-associated gut hyperplasia in Drosophila. This study reveals how elevated WRNexo contributes to stem cell aging, providing new insights into organ aging and the pathogenesis of age-related diseases, such as colon cancer.

Cellular stress and epigenetic regulation in adult stem cells

Stem cells are a unique class of cells that possess the ability to differentiate and self-renew, enabling them to repair and replenish tissues. To protect and maintain the potential of stem cells, the cells and the environment surrounding these cells (stem cell niche) are highly responsive and tightly regulated. However, various stresses can affect the stem cells and their niches. These stresses are both systemic and cellular and can arise from intrinsic or extrinsic factors which would have strong implications on overall aging and certain disease states. Therefore, understanding the breadth of drivers, namely epigenetic alterations, involved in cellular stress is important for the development of interventions aimed at maintaining healthy stem cells and tissue homeostasis. In this review, we summarize published findings of epigenetic responses to replicative, oxidative, mechanical, and inflammatory stress on various types of adult stem cells.

The anti-aging effect of vitamin D and vitamin D receptor in Drosophila midgut

Adult stem cells are pivotal for maintaining tissue homeostasis, and their functional decline is linked to aging and its associated diseases, influenced by the niche cells' environment. Age- and cancer-related reduction of vitamin D and its receptor levels are well documented in human clinical studies. However, the mechanisms through which the vitamin D/vitamin D receptor pathway contributes to anti-aging and extends life expectancy are not well understood. In this study, we aimed to determine the protective role of the vitamin D/vitamin D receptor pathway in differentiated enterocytes (ECs) during intestinal stem cell (ISC) aging. By utilizing a well- established Drosophila midgut model for stem cell aging biology, we revealed that vitamin D receptor knockdown in ECs induced ISC proliferation, EC death, ISC aging, and enteroendocrine cell differentiation. Additionally, age- and oxidative stress-induced increases in ISC proliferation and centrosome amplification were reduced by vitamin D treatment. Our findings suggest a direct evidence of the anti-aging role of the vitamin D/vitamin D receptor pathway and provides insights into the molecular mechanisms underlying healthy aging in Drosophila.

Flavonoids-Natural Gifts to Promote Health and Longevity

The aging of mammals is accompanied by the progressive atrophy of tissues and organs and the accumulation of random damage to macromolecular DNA, protein, and lipids. Flavonoids have excellent antioxidant, anti-inflammatory, and neuroprotective effects. Recent studies have shown that flavonoids can delay aging and prolong a healthy lifespan by eliminating senescent cells, inhibiting senescence-related secretion phenotypes (SASPs), and maintaining metabolic homeostasis. However, only a few systematic studies have described flavonoids in clinical treatment for anti-aging, which needs to be explored further. This review first highlights the association between aging and macromolecular damage. Then, we discuss advances in the role of flavonoid molecules in prolonging the health span and lifespan of organisms. This study may provide crucial information for drug design and developmental and clinical applications based on flavonoids.

Balancing DNA repair to prevent ageing and cancer

DNA damage is a constant stressor to the cell. Persistent damage to the DNA over time results in an increased risk of mutation and an accumulation of mutations with age. Loss of efficient DNA damage repair can lead to accelerated ageing phenotypes or an increased cancer risk, and the trade-off between cancer susceptibility and longevity is often driven by the cell's response to DNA damage. High levels of mutations in DNA repair mutants often leads to excessive cell death and stem cell exhaustion which may promote premature ageing. Stem cells themselves have distinct characteristics that enable them to retain low mutation rates. However, when mutations do arise, stem cell clonal expansion can also contribute to age-related tissue dysfunction as well as heightened cancer risk. In this review, we will highlight increasing DNA damage and mutation accumulation as hallmarks common to both ageing and cancer. We will propose that anti-ageing interventions might be cancer preventative and discuss the mechanisms through which they may act.

An amuse-bouche of stem cell regulation: Underlying principles and mechanisms from adult Drosophila intestinal stem cells

Stem cells have essential functions in the development and maintenance of our organs. Improper regulation of adult stem cells and tissue homeostasis can result in cancers and age-dependent decline. Therefore, understanding how tissue-specific stem cells can accurately renew tissues is an important aim of regenerative medicine. The Drosophila midgut harbors multipotent adult stem cells that are essential to renew the gut in homeostatic conditions and upon stress-induced regeneration. It is now a widely used model system to decipher regulatory mechanisms of stem cell biology. Here, we review recent findings on how adult intestinal stem cells differentiate, interact with their environment, and change during aging.

Nutrient-Driven O-GlcNAcylation Controls DNA Damage Repair Signaling and Stem/Progenitor Cell Homeostasis

Stem/progenitor cells exhibit high proliferation rates, elevated nutrient uptake, altered metabolic flux, and stress-induced genome instability. O-GlcNAcylation is an essential post-translational modification mediated by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which act in a nutrient- and stress-responsive manner. The precise role of O-GlcNAc in adult stem cells and the relationship between O-GlcNAc and the DNA damage response (DDR) is poorly understood. Here, we show that hyper-O-GlcNacylation leads to elevated insulin signaling, hyperproliferation, and DDR activation that mimic the glucose- and oxidative-stress-induced response. We discover a feedback mechanism involving key downstream effectors of DDR, ATM, ATR, and CHK1/2 that regulates OGT stability to promote O-GlcNAcylation and elevate DDR. This O-GlcNAc-dependent regulatory pathway is critical for maintaining gut homeostasis in Drosophila and the DDR in mouse embryonic stem cells (ESCs) and mouse embryonic fibroblasts (MEFs). Our findings reveal a conserved mechanistic link among O-GlcNAc cycling, stem cell self-renewal, and DDR with profound implications for stem-cell-derived diseases including cancer.

Tissue-specific modulation of gene expression in response to lowered insulin signalling in Drosophila

Reduced activity of the insulin/IGF signalling network increases health during ageing in multiple species. Diverse and tissue-specific mechanisms drive the health improvement. Here, we performed tissue-specific transcriptional and proteomic profiling of long-lived Drosophila dilp2-3,5 mutants, and identified tissue-specific regulation of >3600 transcripts and >3700 proteins. Most expression changes were regulated post-transcriptionally in the fat body, and only in mutants infected with the endosymbiotic bacteria, Wolbachia pipientis, which increases their lifespan. Bioinformatic analysis identified reduced co-translational ER targeting of secreted and membrane-associated proteins and increased DNA damage/repair response proteins. Accordingly, age-related DNA damage and genome instability were lower in fat body of the mutant, and overexpression of a minichromosome maintenance protein subunit extended lifespan. Proteins involved in carbohydrate metabolism showed altered expression in the mutant intestine, and gut-specific overexpression of a lysosomal mannosidase increased autophagy, gut homeostasis, and lifespan. These processes are candidates for combatting ageing-related decline in other organisms.

Age-related changes in polycomb gene regulation disrupt lineage fidelity in intestinal stem cells

Tissue homeostasis requires long-term lineage fidelity of somatic stem cells. Whether and how age-related changes in somatic stem cells impact the faithful execution of lineage decisions remains largely unknown. Here, we address this question using genome-wide chromatin accessibility and transcriptome analysis as well as single-cell RNA-seq to explore stem-cell-intrinsic changes in the aging Drosophila intestine. These studies indicate that in stem cells of old flies, promoters of Polycomb (Pc) target genes become differentially accessible, resulting in the increased expression of enteroendocrine (EE) cell specification genes. Consistently, we find age-related changes in the composition of the EE progenitor cell population in aging intestines, as well as a significant increase in the proportion of EE-specified intestinal stem cells (ISCs) and progenitors in aging flies. We further confirm that Pc-mediated chromatin regulation is a critical determinant of EE cell specification in the Drosophila intestine. Pc is required to maintain expression of stem cell genes while ensuring repression of differentiation and specification genes. Our results identify Pc group proteins as central regulators of lineage identity in the intestinal epithelium and highlight the impact of age-related decline in chromatin regulation on tissue homeostasis.

Potential application of γ-H2AX as a biodosimetry tool for radiation triage

Radiation triage and biological dosimetry are two initial steps in the medical management of exposed individuals following radiological accidents. Well established biodosimetry methods such as the dicentric (DC) assay, micronucleus (MN) assay, and fluorescence in-situ hybridization (FISH) translocation assay (for residual damage) have been used for this purpose for several decades. Recent advances in scoring methodology and networking among established laboratories have increased triage capacity; however, these methods still have limitations in analysing large sample numbers, particularly because of the ∼ 48 h minimum culture time required prior to analysis. Hence, there is a need for simple, and high throughput markers to identify exposed individuals in case of radiological/nuclear emergencies. In recent years, a few markers were identified, one being phosphorylated histone 2AX (γ-H2AX), which measured a nuclear foci or nuclear staining intensity that was found to be suitable for triage. Measurement of γ-H2AX foci formed at and around the sites of DNA double-strand breaks is a rapid and sensitive biodosimetry method which does not require culturing and is thus promising for the analysis of a large number of samples. In this review, we have summarized the recent developments of γ-H2AX assay in radiation triage and biodosimetry, focusing chiefly on: i) the importance of baseline frequency and reported values among different laboratories, ii) the influence of known and unknown variables on dose estimation, iii) quality assurance such as inter-laboratory comparison between scorers and scoring methods, and iv) current limitations and potential for future development.

The role of the stem cell epigenome in normal aging and rejuvenative therapy

Adult stem cells are ultimately responsible for the lifelong maintenance of regenerating of tissues during both homeostasis and following injury. Hence, the functional attrition of adult stem cells is thought to be an important driving factor behind the progressive functional decline of tissues and organs that is observed during aging. The mechanistic cause underlying this age-associated exhaustion of functional stem cells is likely to be complex and multifactorial. However, it is clear that progressive remodeling of the epigenome and the resulting deregulation of gene expression programs can be considered a hallmark of aging, and is likely a key factor in mediating altered biological function of aged stem cells. In this review, we outline cell intrinsic and extrinsic mediators of epigenome remodeling during aging; discuss how such changes can impact on stem cell function; and describe how resetting the aged epigenome may rejuvenate some of the biological characteristics of stem cells.

Anti-Aging Effect of the Ketone Metabolite β-Hydroxybutyrate in Drosophila Intestinal Stem Cells

Age-related changes in tissue-resident adult stem cells may be closely linked to tissue aging and age-related diseases, such as cancer. β-Hydroxybutyrate is emerging as an important molecule for exhibiting the anti-aging effects of caloric restriction and fasting, which are generally considered to be beneficial for stem cell maintenance and tissue regeneration. The effects of β-hydroxybutyrate on adult stem cells remain largely unknown. Therefore, this study was undertaken to investigate whether β-hydroxybutyrate supplementation exerts beneficial effects on age-related changes in intestinal stem cells that were derived from the Drosophila midgut. Our results indicate that β-hydroxybutyrate inhibits age- and oxidative stress-induced changes in midgut intestinal stem cells, including centrosome amplification (a hallmark of cancers), hyperproliferation, and DNA damage accumulation. Additionally, β-hydroxybutyrate inhibits age- and oxidative stress-induced heterochromatin instability in enterocytes, an intestinal stem cells niche cells. Our results suggest that β-hydroxybutyrate exerts both intrinsic as well as extrinsic influence in order to maintain stem cell homeostasis.

Intestinal epithelial barrier functions in ageing

The intestinal epithelial barrier protects the mucosa of the gastrointestinal (GI)-tract and plays a key role in maintaining the host homeostasis. It encompasses several elements that include the intestinal epithelium and biochemical and immunological products, such as the mucus layer, antimicrobial peptides (AMPs) and secretory immunologlobulin A (sIgA). These components are interlinked with the large microbial community inhabiting the gut to form a highly sophisticated biological system that plays an important role on many aspects of human health both locally and systemically. Like any other organ and tissue, the intestinal epithelial barrier is affected by the ageing process. New insights have surfaced showing that critical functions, including intestinal stem cell regeneration and regulation of the intestinal crypt homeostasis, barrier integrity, production of regulatory cytokines, and epithelial innate immunity to pathogenic antigens change across life. Here we review the age-associated changes of the various components of the intestinal epithelial barrier and we highlight the necessity to elucidate further the mechanisms underlying these changes. Expanding our knowledge in this area is a goal of high medical relevance and it will help to define intervention strategies to ameliorate the quality of life of the ever-expanding elderly population.

DNA damage in aging, the stem cell perspective

DNA damage is one of the most consistent cellular process proposed to contribute to aging. The maintenance of genomic and epigenomic integrity is critical for proper function of cells and tissues throughout life, and this homeostasis is under constant strain from both extrinsic and intrinsic insults. Considering the relationship between lifespan and genotoxic burden, it is plausible that the longest-lived cellular populations would face an accumulation of DNA damage over time. Tissue-specific stem cells are multipotent populations residing in localized niches and are responsible for maintaining all lineages of their resident tissue/system throughout life. However, many of these stem cells are impacted by genotoxic stress. Several factors may dictate the specific stem cell population response to DNA damage, including the niche location, life history, and fate decisions after damage accrual. This leads to differential handling of DNA damage in different stem cell compartments. Given the importance of adult stem cells in preserving normal tissue function during an individual's lifetime, DNA damage sensitivity and accumulation in these compartments could have crucial implications for aging. Despite this, more support for direct functional effects driven by accumulated DNA damage in adult stem cell compartments is needed. This review will present current evidence for the accumulation and potential influence of DNA damage in adult tissue-specific stem cells and propose inquiry directions that could benefit individual healthspan.

Genetic identification and characterization of three genes that prevent accumulation of oxidative DNA damage in Drosophila adult tissues

Reactive oxygen species generated in the process of energy production represent a major cause of oxidative DNA damage. Production of the oxidized guanine base, 8-oxo-guanine (8-oxoG), results in mismatched pairing with adenine and subsequently leads to G:C to T:A transversions after DNA replication. Our previous study demonstrated that Drosophila CG1795 encodes an ortholog of Ogg1, which is essential for the elimination of 8-oxoG. Moreover, the Drosophila ribosomal protein S3 (RpS3) possesses N-glycosylase activity that eliminates 8-oxoG in vitro. In this study, we show that RpS3 heterozygotes hyper-accumulate 8-oxoG in midgut cell nuclei after oxidant feeding, suggesting thatRpS3 is required for the elimination of 8-oxoG in Drosophila adults. We further showed that several muscle-aging phenotypes were significantly accelerated in RpS3 heterozygotes. Ogg1 is localized in the nucleus, while RpS3 is in the cytoplasm, closely associated with endoplasmic reticulum networks. Results of genetic analyses also suggest that these two proteins operate similarly but independently in the elimination of oxidized guanine bases from genomic DNA. Next, we obtained genetic evidence suggesting that CG42813 functions as the Drosophila ortholog of mammalian Mth1 in the elimination of oxidized dGTP (8-oxo-dGTP) from the nucleotide pool. Depletion of this gene significantly increased the number of DNA damage foci in the nuclei of Drosophila midgut cells. Furthermore, several aging-related phenotypes such as age-dependent loss of adult locomotor activities and accumulation of polyubiquitylated proteins in adult muscles were also significantly accelerated in CG42813-depleted flies. Lastly, we investigated the phenotype of adults depleted of CG9272, which encodes a protein with homology to mammalian Nth1 that is essential for the elimination of oxidized thymine. Excessive accumulation of oxidized bases was observed in the epithelial cell nuclei after oxidant feeding. In conclusion, three genes that prevent accumulation of oxidative DNA damage were identified in Drosophila.

Drosophila PEBP1 inhibits intestinal stem cell aging via suppression of ERK pathway

The intestine is a high cellular turnover tissue largely dependent on the regenerative function of stem cell throughout life, and a signaling center for the health and viability of organisms. Therefore, better understanding of the mechanisms underlying the regulation of intestinal stem cell (ISC) regenerative potential is essential for the possible intervention of aging process and age-related diseases. Drosophila midgut is a well-established model system for studying the mechanisms underlying ISC regenerative potential during aging. Here, we report the requirement of Drosophila phosphatidylethanolamine binding protein 1 (PEBP1) in ISC regenerative potential. We showed that PEBP1 was strongly expressed in enterocytes (ECs) of guts and its decrease with age and oxidative stress. Furthermore, the downregulation of PEBP1 in ECs accelerates ISC aging, as evidenced by ISC hyper-proliferation, γH2AX accumulation, and centrosome amplification, and intestinal hyperplasia. The decrease in PEBP1 expression was associated with increased extracellular signal-regulated kinase (ERK) activity in ECs. All these phenotypes by EC-specific depletion of PEBP1 were rescued by the concomitant inhibition of ERK signaling. Our findings evidence that the age-related downregulation of PEBP1 in ECs is a novel cause accelerating ISC aging and that PEBP1 is an EC-intrinsic suppressor of epidermal growth factor receptor (EGFR)/ERK signaling. Our study provides molecular insights into the tight regulation of EGFR/ERK signaling in niches for stem cell regenerative potential.

Deficiency in DNA damage response of enterocytes accelerates intestinal stem cell aging in Drosophila

Stem cell dysfunction is closely linked to tissue and organismal aging and age-related diseases, and heavily influenced by the niche cells' environment. The DNA damage response (DDR) is a key pathway for tissue degeneration and organismal aging; however, the precise protective role of DDR in stem cell/niche aging is unclear. The Drosophila midgut is an excellent model to study the biology of stem cell/niche aging because of its easy genetic manipulation and its short lifespan. Here, we showed that deficiency of DDR in Drosophila enterocytes (ECs) accelerates intestinal stem cell (ISC) aging. We generated flies with knockdown of Mre11, Rad50, Nbs1, ATM, ATR, Chk1, and Chk2, which decrease the DDR system in ECs. EC-specific DDR depletion induced EC death, accelerated the aging of ISCs, as evidenced by ISC hyperproliferation, DNA damage accumulation, and increased centrosome amplification, and affected the adult fly's survival. Our data indicated a distinct effect of DDR depletion in stem or niche cells on tissue-resident stem cell proliferation. Our findings provide evidence of the essential role of DDR in protecting EC against ISC aging, thus providing a better understanding of the molecular mechanisms of stem cell/niche aging.

Deficiency of Atg6 impairs beneficial effect of metformin on intestinal stem cell aging in Drosophila

Age-related changes of adult stem cell are crucial for tissue aging and age-related diseases. Thus, clarifying mechanisms to prevent adult stem cell aging is indispensable for healthy aging. Metformin, a drug for type 2 diabetes, has been highlighted for its anti-aging and anti-cancer effect. In Drosophila intestinal stem cell (ISC), we previously reported the inhibitory effect of metformin on age-related phenotypes of ISC. Here, we showed that knockdown of Atg6, a crucial autophagy-related factor, in ISC induces age-related phenotypes of ISC such as hyperproliferation, centrosome amplification, and DNA damage accumulation. Then, we revealed that metformin inhibits ISC aging phenotypes in Atg6-dependent manner. Taken together, our study suggests that Atg6 is required for the inhibitory effect of metformin on ISC aging, providing an intervention mechanism of metformin on adult stem cell aging.

Ubiquitous overexpression of the DNA repair factor dPrp19 reduces DNA damage and extends Drosophila life span

Mechanisms that ensure and maintain the stability of genetic information are fundamentally important for organismal function and can have a large impact on disease, aging, and life span. While a multi-layered cellular apparatus exists to detect and respond to DNA damage, various insults from environmental and endogenous sources continuously affect DNA integrity. Over time this can lead to the accumulation of somatic mutations, which is thought to be one of the major causes of aging. We have previously found that overexpression of the essential human DNA repair and splicing factor SNEV, also called PRP19 or hPso4, extends replicative life span of cultured human endothelial cells and impedes accumulation of DNA damage. Here, we show that adult-specific overexpression of dPrp19, the D. melanogaster ortholog of human SNEV/PRP19/hPso4, robustly extends life span in female fruit flies. This increase in life span is accompanied by reduced levels of DNA damage and improved resistance to oxidative and genotoxic stress. Our findings suggest that dPrp19 plays an evolutionarily conserved role in aging, life span modulation and stress resistance, and support the notion that superior DNA maintenance is key to longevity.

Requirement of ATR for maintenance of intestinal stem cells in aging Drosophila

The stem cell genomic stability forms the basis for robust tissue homeostasis, particularly in high-turnover tissues. For the genomic stability, DNA damage response (DDR) is essential. This study was focused on the role of two major DDR-related factors, ataxia telangiectasia-mutated (ATM) and ATM- and RAD3-related (ATR) kinases, in the maintenance of intestinal stem cells (ISCs) in the adult Drosophila midgut. We explored the role of ATM and ATR, utilizing immunostaining with an anti-pS/TQ antibody as an indicator of ATM/ATR activation, γ-irradiation as a DNA damage inducer, and the UAS/GAL4 system for cell type-specific knockdown of ATM, ATR, or both during adulthood. The results showed that the pS/TQ signals got stronger with age and after oxidative stress. The pS/TQ signals were found to be more dependent on ATR rather than on ATM in ISCs/enteroblasts (EBs). Furthermore, an ISC/EB-specific knockdown of ATR, ATM, or both decreased the number of ISCs and oxidative stress-induced ISC proliferation. The phenotypic changes that were caused by the ATR knockdown were more pronounced than those caused by the ATM knockdown; however, our data indicate that ATR and ATM are both needed for ISC maintenance and proliferation; ATR seems to play a bigger role than does ATM.

Drosophila Ogg1 is required to suppress 8-oxo-guanine accumulation following oxidative stress

Reactive oxygen species (ROS) generated during energy production processes are a major cause of oxidative DNA damage. A DNA glycosylase encoded by the Ogg1 gene removes oxidized guanine bases and is widely conserved. However, the biological role of the gene in individual organisms has not yet been characterized in Drosophila, which is a suitable model to study the influence of oxidative damage on senescence. Here, we performed a genetic analysis to confirm that Ogg1 plays an essential role in the removal of 8-oxo-guanines from nuclei. We first confirmed by quantitative real-time PCR that Ogg1 mRNA expression was reduced by 30-55% in Ogg1 mutants and in flies expressing inducible Ogg1 dsRNA compared to control flies. We then showed that additional accumulation of 8-oxo-guanines occurred in the nuclei of epithelial midgut cells after paraquat feeding in flies with downregulated Ogg1 expression. We confirmed that a transposon possessing the UAS sequence was integrated in the 5'-UTR of the Ogg1 alleles and that it is oriented in the same transcriptional direction as the gene. Using the Gal4/UAS system, which enables us to induce ectopic expression in Drosophila, we induced overexpression of Ogg1 by 40-fold. We observed a lower amount of 8-oxo-guanine in the midgut epithelial cells of adults overexpressing Ogg1. These genetic data strongly suggest that the Drosophila Ogg1 ortholog CG1795 plays an essential role in the suppression of 8-oxo-guanines, consistent with its role in other organisms. Although adult flies with reduced Ogg1 expression failed to show elevated sensitivity to paraquat, those with Ogg1 overexpression showed resistance to oxidative stress by paraquat feeding and had a significantly longer lifespan in normal feeding conditions. These observations are consistent with the hypothesis that oxidative DNA damage by ROS accumulation is a major contributor to senescence.

Frequent Somatic Mutation in Adult Intestinal Stem Cells Drives Neoplasia and Genetic Mosaicism during Aging

Adult stem cells may acquire mutations that modify cellular behavior, leading to functional declines in homeostasis or providing a competitive advantage resulting in premalignancy. However, the frequency, phenotypic impact, and mechanisms underlying spontaneous mutagenesis during aging are unclear. Here, we report two mechanisms of genome instability in adult Drosophila intestinal stem cells (ISCs) that cause phenotypic alterations in the aging intestine. First, we found frequent loss of heterozygosity arising from mitotic homologous recombination in ISCs that results in genetic mosaicism. Second, somatic deletion of DNA sequences and large structural rearrangements, resembling those described in cancers and congenital diseases, frequently result in gene inactivation. Such modifications induced somatic inactivation of the X-linked tumor suppressor Notch in ISCs, leading to spontaneous neoplasias in wild-type males. Together, our findings reveal frequent genomic modification in adult stem cells and show that somatic genetic mosaicism has important functional consequences on aging tissues.

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The aging Drosophila intestine is genetically mosaic

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Somatic recombination, genomic deletions, and rearrangements occur in aging ISCs

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Somatic inactivation of the tumor-suppressor Notch causes male-specific neoplasia

Attenuation of radiation-induced gastrointestinal damage by epidermal growth factor and bone marrow transplantation in mice

PURPOSE

We examined the effect of epidermal growth factor (EGF) and bone marrow transplantation (BMT) on gastrointestinal damage after high-dose irradiation of mice.

MATERIAL AND METHODS

C57Black/6 mice were used. Two survival experiments were performed (12 and 13 Gy; (60)Co, 0.59-0.57 Gy/min). To evaluate BMT and EGF action, five groups were established - 0 Gy, 13 Gy, 13 Gy + EGF (at 2 mg/kg, first dose 24 h after irradiation and then every 48 h), 13 Gy + BMT (5 × 10(6) cells from green fluorescent protein [GFP] syngenic mice, 4 h after irradiation), and 13 Gy + BMT + EGF. Survival data, blood cell counts, gastrointestine and liver parameters and GFP positive cell migration were measured.

RESULTS

BMT and EGF (three doses, at 2 mg/kg, administered 1, 3 and 5 days after irradiation) significantly increased survival (13 Gy). In blood, progressive cytopenia was observed with BMT, EGF or their combination having no improving effect early after irradiation. In gastrointestinal system, BMT, EGF and their combination attenuated radiation-induced atrophy and increased regeneration during first week after irradiation with the combination being most effective. Signs of systemic inflammatory reaction were observed 30 days after irradiation.

CONCLUSIONS

Our data indicate that BMT together with EGF is a promising strategy in the treatment of high-dose whole-body irradiation damage.

Metformin inhibits age-related centrosome amplification in Drosophila midgut stem cells through AKT/TOR pathway

We delineated the mechanism regulating the inhibition of centrosome amplification by metformin in Drosophila intestinal stem cells (ISCs). Age-related changes in tissue-resident stem cells may be closely associated with tissue aging and age-related diseases, such as cancer. Centrosome amplification is a hallmark of cancers. Our recent work showed that Drosophila ISCs are an excellent model for stem cell studies evaluating age-related increase in centrosome amplification. Here, we showed that metformin, a recognized anti-cancer drug, inhibits age- and oxidative stress-induced centrosome amplification in ISCs. Furthermore, we revealed that this effect is mediated via down-regulation of AKT/target of rapamycin (TOR) activity, suggesting that metformin prevents centrosome amplification by inhibiting the TOR signaling pathway. Additionally, AKT/TOR signaling hyperactivation and metformin treatment indicated a strong correlation between DNA damage accumulation and centrosome amplification in ISCs, suggesting that DNA damage might mediate centrosome amplification. Our study reveals the beneficial and protective effects of metformin on centrosome amplification via AKT/TOR signaling modulation. We identified a new target for the inhibition of age- and oxidative stress-induced centrosome amplification. We propose that the Drosophila ISCs may be an excellent model system for in vivo studies evaluating the effects of anti-cancer drugs on tissue-resident stem cell aging.

Age-related change in γH2AX of Drosophila muscle: its significance as a marker for muscle damage and longevity

Muscle aging is closely related to unhealthy late-life and organismal aging. Recently, the state of differentiated cells was shown to be critical to tissue homeostasis. Thus, understanding how fully differentiated muscle cells age is required for ensuring healthy aging. Adult Drosophila muscle is a useful model for exploring the aging process of fully differentiated cells. In this study, we investigated age-related changes of γH2AX, an indicator of DNA strand breaks, in adult Drosophila muscle to document whether its changes are correlated with muscle degeneration and lifespan. The results demonstrate that γH2AX accumulation increases in adult Drosophila thoracic and leg muscles with age. Analyses of short-, normal-, and long-lived strains indicate that the age-related increase of γH2AX is closely associated with the extent of muscle degeneration, cleaved caspase-3 and poly-ubiquitin aggregates, and longevity. Further analysis of muscle-specific knockdown of heterochromatin protein 1a revealed that the excessive γH2AX accumulation in thoracic and leg muscles induces accelerated degeneration and decreases longevity. These data suggest a strong correlation between age-related muscle damage and lifespan in Drosophila. Our findings indicate that γH2AX may be a reliable biomarker for assessing muscle aging in Drosophila.

Gene expression profiling identifies the zinc-finger protein Charlatan as a regulator of intestinal stem cells in Drosophila

Intestinal stem cells (ISCs) in the adult Drosophila midgut can respond to tissue damage and support repair. We used genetic manipulation to increase the number of ISC-like cells in the adult midgut and performed gene expression profiling to identify potential ISC regulators. A detailed analysis of one of these potential regulators, the zinc-finger protein Charlatan, was carried out. MARCM clonal analysis and RNAi in precursor cells showed that loss of Chn function caused severe ISC division defects, including loss of EdU incorporation, phosphorylated histone 3 staining and expression of the mitotic protein Cdc2. Loss of Charlatan also led to a much reduced histone acetylation staining in precursor cells. Both the histone acetylation and ISC division defects could be rescued by the simultaneous decrease of the Histone Deacetylase 2. The overexpression of Charlatan blocked differentiation reversibly, but loss of Charlatan did not lead to automatic differentiation. The results together suggest that Charlatan does not simply act as an anti-differentiation factor but instead functions to maintain a chromatin structure that is compatible with stem cell properties, including proliferation.

The tumor suppressor PML specifically accumulates at RPA/Rad51-containing DNA damage repair foci but is nonessential for DNA damage-induced fibroblast senescence

The PML tumor suppressor has been functionally implicated in DNA damage response and cellular senescence. Direct evidence for such a role based on PML knockdown or knockout approaches is still lacking. We have therefore analyzed the irradiation-induced DNA damage response and cellular senescence in human and mouse fibroblasts lacking PML. Our data show that PML nuclear bodies (NBs) nonrandomly associate with persistent DNA damage foci in unperturbed human skin and in high-dose-irradiated cell culture systems. PML bodies do not associate with transient γH2AX foci after low-dose gamma irradiation. Superresolution microscopy reveals that all PML bodies within a nucleus are engaged at Rad51- and RPA-containing repair foci during ongoing DNA repair. The lack of PML (i) does not majorly affect the DNA damage response, (ii) does not alter the efficiency of senescence induction after DNA damage, and (iii) does not affect the proliferative potential of primary mouse embryonic fibroblasts during serial passaging. Thus, while PML NBs specifically accumulate at Rad51/RPA-containing lesions and senescence-derived persistent DNA damage foci, they are not essential for DNA damage-induced and replicative senescence of human and murine fibroblasts.

Mechanism of metformin: inhibition of DNA damage and proliferative activity in Drosophila midgut stem cell

Age-related changes in stem cells could have a profound impact on tissue aging and the development of age-related diseases such as cancer. However, the effects of metformin, a recently recognized anti-cancer drug, on stem cell aging remain largely unknown. In the present study, an experiment was set up to investigate the underlying mechanism of metformin's beneficial effects on age-related changes in intestinal stem cells (ISCs) derived from Drosophila midgut. Results showed that metformin reduced age- and oxidative stress-related accumulation of DNA damage marked by Drosophila γH2AX foci and 8-oxo-dG in ISCs and progenitor cells. Metformin also inhibited age and- oxidative stress-related ISC hyperproliferation as well as intestinal hyperplasia. Our study further revealed that the inhibitory effects of metformin on DNA damage accumulation may be due to the down-regulation of age-related and oxidative stress-induced AKT activity. These data indicate that metformin has beneficial effects on age-related changes in ISCs derived from Drosophila midgut. Further, our results suggest a possible impact of DNA damage on stem cell genomic instability, which leads to the development of age-related diseases. Additionally, our study suggests that Drosophila midgut stem cells can be a suitable model system for studying stem cell biology and stem cell aging.