DNA damage-induced Lok/CHK2 activation compromises germline stem cell self-renewal and lineage differentiation

Methods to Analyze Nutritional and Inter-Organ Control of Drosophila Ovarian Germline Stem Cells

Physiological status, particularly dietary input, has major impacts on the Drosophila melanogaster ovarian germline stem cell lineage. Moreover, several studies have shed light on the role that inter-organ communication plays in coordinating whole-organism responses to changes in physiology. For example, nutrient-sensing signaling pathways function within the fat body to regulate germline stem cells and their progeny in the ovary. Together with its incredible genetic and cell biological toolkits, Drosophila serves as an amenable model organism to use for uncovering molecular mechanisms that underlie physiological control of adult stem cells. In this methods chapter, we describe a general dietary manipulation paradigm, genetic manipulation of adult adipocytes, and whole-mount ovary immunofluorescence to investigate physiological control of germline stem cells.

Drosophila p53 isoforms have overlapping and distinct functions in germline genome integrity and oocyte quality control

p53 gene family members in humans and other organisms encode a large number of protein isoforms whose functions are largely undefined. Using Drosophila as a model, we find that a p53B isoform is expressed predominantly in the germline where it colocalizes with p53A into subnuclear bodies. It is only p53A, however, that mediates the apoptotic response to ionizing radiation in the germline and soma. In contrast, p53A and p53B are both required for the normal repair of meiotic DNA breaks, an activity that is more crucial when meiotic recombination is defective. We find that in oocytes with persistent DNA breaks p53A is also required to activate a meiotic pachytene checkpoint. Our findings indicate that Drosophila p53 isoforms have DNA lesion and cell type-specific functions, with parallels to the functions of mammalian p53 family members in the genotoxic stress response and oocyte quality control.

Chemical Genetic Screen in Drosophila Germline Uncovers Small Molecule Drugs That Sensitize Stem Cells to Insult-Induced Apoptosis

Cancer stem cells, in contrast to their more differentiated daughter cells, can endure genotoxic insults, escape apoptosis, and cause tumor recurrence. Understanding how normal adult stem cells survive and go to quiescence may help identify druggable pathways that cancer stem cells have co-opted. In this study, we utilize a genetically tractable model for stem cell survival in the Drosophila gonad to screen drug candidates and probe chemical-genetic interactions. Our study employs three levels of small molecule screening: (1) a medium-throughput primary screen in male germline stem cells (GSCs), (2) a secondary screen with irradiation and protein-constrained food in female GSCs, and (3) a tertiary screen in breast cancer organoids in vitro. Herein, we uncover a series of small molecule drug candidates that may sensitize cancer stem cells to apoptosis. Further, we have assessed these small molecules for chemical-genetic interactions in the germline and identified the NF-κB pathway as an essential and druggable pathway in GSC quiescence and viability. Our study demonstrates the power of the Drosophila stem cell niche as a model system for targeted drug discovery.

dRTEL1 is essential for the maintenance of Drosophila male germline stem cells

Stem cells have the potential to maintain undifferentiated state and differentiate into specialized cell types. Despite numerous progress has been achieved in understanding stem cell self-renewal and differentiation, many fundamental questions remain unanswered. In this study, we identify dRTEL1, the Drosophila homolog of Regulator of Telomere Elongation Helicase 1, as a novel regulator of male germline stem cells (GSCs). Our genome-wide transcriptome analysis and ChIP-Seq results suggest that dRTEL1 affects a set of candidate genes required for GSC maintenance, likely independent of its role in DNA repair. Furthermore, dRTEL1 prevents DNA damage-induced checkpoint activation in GSCs. Finally, dRTEL1 functions to sustain Stat92E protein levels, the key player in GSC maintenance. Together, our findings reveal an intrinsic role of the DNA helicase dRTEL1 in maintaining male GSC and provide insight into the function of dRTEL1.

Chk2-p53 and JNK in irradiation-induced cell death of hematopoietic progenitors and differentiated cells in Drosophila larval lymph gland

Ionizing radiation (IR) induces DNA double-strand breaks that activate the DNA damage response (DDR), which leads to cell cycle arrest, senescence, or apoptotic cell death. Understanding the DDR of stem cells is critical to tissue homeostasis and the survival of the organism. Drosophila hematopoiesis serves as a model system for sensing stress and environmental changes; however, their response to DNA damage remains largely unexplored. The Drosophila lymph gland is the larval hematopoietic organ, where stem-like progenitors proliferate and differentiate into mature blood cells called hemocytes. We found that apoptotic cell death was induced in progenitors and hemocytes after 40 Gy irradiation, with progenitors showing more resistance to IR-induced cell death compared to hemocytes at a lower dose. Furthermore, we found that Drosophila ATM (tefu), Chk2 (lok), p53, and reaper were necessary for IR-induced cell death in the progenitors. Notably, IR-induced cell death in mature hemocytes required tefu, Drosophila JNK (bsk), and reaper, but not lok or p53. In summary, we found that DNA damage induces apoptotic cell death in the late third instar larval lymph gland and identified lok/p53-dependent and -independent cell death pathways in progenitors and mature hemocytes, respectively.

Rapid and highly efficient genomic engineering with a novel iEditing device for programming versatile extracellular electron transfer of electroactive bacteria

The advances in synthetic biology bring exciting new opportunities to reprogram microorganisms with novel functionalities for environmental applications. For real-world applications, a genetic tool that enables genetic engineering in a stably genomic inherited manner is greatly desired. In this work, we design a novel genetic device for rapid and efficient genome engineering based on the intron-encoded homing-endonuclease empowered genome editing (iEditing). The iEditing device enables rapid and efficient genome engineering in Shewanella oneidensis MR-1, the representative strain of the electroactive bacteria group. Moreover, combining with the Red or RecET recombination system, the genome-editing efficiency was greatly improved, up to approximately 100%. Significantly, the iEditing device itself is eliminated simultaneously when genome editing occurs, thereby requiring no follow-up to remove the encoding system. Then, we develop a new extracellular electron transfer (EET) engineering strategy by programming the parallel EET systems to enhance versatile EET. The engineered strains exhibit sufficiently enhanced electron output and pollutant reduction ability. Furthermore, this device has demonstrated its great potential to be extended for genome editing in other important microbes. This work provides a useful and efficient tool for the rapid generation of synthetic microorganisms for various environmental applications.

Myc plays an important role in Drosophila P-M hybrid dysgenesis to eliminate germline cells with genetic damage

Genetic damage in the germline induced by P-element mobilization causes a syndrome known as P-M hybrid dysgenesis (HD), which manifests as elevated mutation frequency and loss of germline cells. In this study, we found that Myc plays an important role in eliminating germline cells in the context of HD. P-element mobilization resulted in downregulation of Myc expression in the germline. Myc knockdown caused germline elimination; conversely, Myc overexpression rescued the germline loss caused by P-element mobilization. Moreover, restoration of fertility by Myc resulted in the production of gametes with elevated mutation frequency and reduced ability to undergo development. Our results demonstrate that Myc downregulation mediates elimination of germline cells with accumulated genetic damage, and that failure to remove these cells results in increased production of aberrant gametes. Therefore, we propose that elimination of germline cells mediated by Myc downregulation is a quality control mechanism that maintains the genomic integrity of the germline.

Germline stem cell aging in the Drosophila ovary

The age-related decline of adult stem cells leads to loss of tissue homeostasis and contributes to organismal aging. Though the phenotypic hallmarks of aging are well-characterized at the organ or tissue level, the molecular processes that govern stem cell aging remain unclear. This review seeks to highlight recent research in stem cell aging in the Drosophila ovary and connect the discoveries in the fly to ongoing questions in stem cell aging.

Uninvited guests: how transposable elements take advantage of Drosophila germline stem cells, and how stem cells fight back

Transposable elements (TEs) are mobile genetic parasites that spread through host genomes by replicating in germline cells. New TE copies that arise in the genomes of germline stem cells (GSCs) are of particular value, because they are potentially transmitted to multiple offspring through the plethora of gametes arising from the same progenitor GSC. However, the fidelity of GSC genomes is also of utmost importance to the host in ensuring the production of abundant and fit offspring. Here we review tactics that TEs employ to replicate in Drosophila female GSCs, as well as mechanisms those cells use to defend against TEs. We also discuss the relationship between transposition and GSC loss, which is arbitrated through reduced signaling for self renewal, increased signaling for differentiation, and DNA damage response pathways.

Special vulnerability of somatic niche cells to transposable element activation in Drosophila larval ovaries

In the Drosophila ovary, somatic escort cells (ECs) form a niche that promotes differentiation of germline stem cell (GSC) progeny. The piRNA (Piwi-interacting RNA) pathway, which represses transposable elements (TEs), is required in ECs to prevent the accumulation of undifferentiated germ cells (germline tumor phenotype). The soma-specific piRNA cluster flamenco (flam) produces a substantial part of somatic piRNAs. Here, we characterized the biological effects of somatic TE activation on germ cell differentiation in flam mutants. We revealed that the choice between normal and tumorous phenotypes of flam mutant ovaries depends on the number of persisting ECs, which is determined at the larval stage. Accordingly, we found much more frequent DNA breaks in somatic cells of flam larval ovaries than in adult ECs. The absence of Chk2 or ATM checkpoint kinases dramatically enhanced oogenesis defects of flam mutants, in contrast to the germline TE-induced defects that are known to be mostly suppressed by сhk2 mutation. These results demonstrate a crucial role of checkpoint kinases in protecting niche cells against deleterious TE activation and suggest substantial differences between DNA damage responses in ovarian somatic and germ cells.

Germ Cell Lineage Homeostasis in Drosophila Requires the Vasa RNA Helicase

The conserved RNA helicase Vasa is required for germ cell development in many organisms. In Drosophila melanogaster loss of PIWI-interacting RNA pathway components, including Vasa, causes Chk2-dependent oogenesis arrest. However, whether the arrest is due to Chk2 signaling at a specific stage and whether continuous Chk2 signaling is required for the arrest is unknown. Here, we show that absence of Vasa during the germarial stages causes Chk2-dependent oogenesis arrest. Additionally, we report the age-dependent decline of the ovariole number both in flies lacking Vasa expression only in the germarium and in loss-of-function vasa mutant flies. We show that Chk2 activation exclusively in the germarium is sufficient to interrupt oogenesis and to reduce ovariole number in aging flies. Once induced in the germarium, Chk2-mediated arrest of germ cell development cannot be overcome by restoration of Vasa or by downregulation of Chk2 in the arrested egg chambers. These findings, together with the identity of Vasa-associated proteins identified in this study, demonstrate an essential role of the helicase in the germ cell lineage maintenance and indicate a function of Vasa in germline stem cell homeostasis.

The GATOR complex regulates an essential response to meiotic double-stranded breaks in Drosophila

The TORC1 regulator GATOR1/SEACIT controls meiotic entry and early meiotic events in yeast. However, how metabolic pathways influence meiotic progression in metazoans remains poorly understood. Here we examine the role of the TORC1 regulators GATOR1 and GATOR2 in the response to meiotic double-stranded breaks (DSB) during Drosophila oogenesis. We find that in mutants of the GATOR2 component mio, meiotic DSBs trigger the constitutive downregulation of TORC1 activity and a permanent arrest in oocyte growth. Conversely, in GATOR1 mutants, high TORC1 activity results in the delayed repair of meiotic DSBs and the hyperactivation of p53. Unexpectedly, we found that GATOR1 inhibits retrotransposon expression in the presence of meiotic DSBs in a pathway that functions in parallel to p53. Thus, our studies have revealed a link between oocyte metabolism, the repair of meiotic DSBs and retrotransposon expression.

Local and Physiological Control of Germline Stem Cell Lineages in Drosophila melanogaster

The long-term survival of any multicellular species depends on the success of its germline in producing high-quality gametes and maximizing survival of the offspring. Studies in Drosophila melanogaster have led our growing understanding of how germline stem cell (GSC) lineages maintain their function and adjust their behavior according to varying environmental and/or physiological conditions. This review compares and contrasts the local regulation of GSCs by their specialized microenvironments, or niches; discusses how diet and diet-dependent factors, mating, and microorganisms modulate GSCs and their developing progeny; and briefly describes the tie between physiology and development during the larval phase of the germline cycle. Finally, it concludes with broad comparisons with other organisms and some future directions for further investigation.

Role of p53 isoforms in the DNA damage response during Drosophila oogenesis

The tumor suppressor p53 is involved in the DNA damage response and induces cell cycle arrest or apoptosis upon DNA damage. Drosophila p53 encodes two isoforms, p53A and p53B, that induce apoptosis in somatic cells. To investigate the roles of Drosophila p53 isoforms in female germline cells, the DNA damage response was analyzed in the adult ovary. Early oogenesis was sensitive to irradiation and lok-, p53-, and hid-dependent cell death occurred rapidly after both low- and high-dose irradiation. Both p53 isoforms were responsible for this cell death. On the other hand, delayed cell death in mid-oogenesis was induced at a low level only after high-dose irradiation in a p53-independent manner. The daily egg production, which did not change after low-dose irradiation, was severely reduced after high-dose irradiation in p53 mutant females due to the loss of germline stem cells. When the p53A or p53B isoform was expressed in the germline cells in the p53 mutant females at levels that do not affect normal oogenesis, p53A, but not p53B, restored the fertility of the irradiated female. In summary, moderate expression of p53A is critical to maintain the function of germline stem cells during normal oogenesis as well as after high-dose irradiation.

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.

Loss of putzig in the germline impedes germ cell development by inducing cell death and new niche like microenvironments

Germline stem cell development and differentiation is tightly controlled by the surrounding somatic cells of the stem cell niche. In Drosophila females, cells of the niche emit various signals including Dpp and Wg to balance stem cell renewal and differentiation. Here, we show that the gene pzg is autonomously required in cells of the germline to sustain the interplay between niche and stem cells. Loss of pzg impairs stem cell differentiation and provokes the death of cells in the germarium. As a consequence of pzg loss, increased growth signalling activity predominantly of Dpp and Wg/Wnt, was observed, eventually disrupting the balance of germ cell self-renewal and differentiation. Whereas in the soma, apoptosis-induced compensatory growth is well established, the induction of self-renewal signals during oogenesis cannot compensate for dying germ cells, albeit inducing a new niche-like microenvironment. Instead, they impair the further development of germ cells and cause in addition a forward and feedback loop of cell death.

Expression of circular RNAs in gynecological tumors: A systematic review

BACKGROUND

The rapid development of bioinformatic technology is boosting the discovery of components hiding in the darkness. As a type of universal, conservative, tissue-specific and stable molecules, circular RNA (circRNA) is a class of endogenous non-coding RNA that has no 5' cap and 3' poly(A) tail and forms a covalently closed continuous loop. At present, 3 types of circRNAs including exonic circRNA (ecRNA), intronic circRNA (ciRNA), and axon-intronic circRNA have been reported. Nowadays informatic technology and high-throughput sequencing have verified the abundance of endogenous circRNAs in eukaryocytes, with predominantly expressed in the cell cytoplasm. Their unique sequences endow them with special functions, such as miRNA sponge, selective transcription or splicing, and attaching to RNA-binding proteins.

DATA SOURCES

This review was based on articles published in PubMed databases up to January, 2019, with the following keywords: "circular RNA", "database", and "reproductive tumor" (Flow chart).

OBJECTIVES

Original articles and reviews on the topics were selected.

RESULTS

Studies have uncovered the interplay between circRNAs and the development of ovarian epithelial tumors, ovarian carcinoma, and cervical carcinoma, which suggesting the potential of circRNAs as biomarkers or therapeutic targets for human diseases.

CONCLUSIONS

Circular RNA has been found to play a role in gynecological tumors diseases. Meanwhile, we reviewed the studies on how CircularRNA participate in gynecological tumors, which provides a basis for the study of CircularRNA in gynecological tumors.

Genome Damage Sensing Leads to Tissue Homeostasis in Drosophila

DNA repair is a critical cellular process required for the maintenance of genomic integrity. It is now well appreciated that cells employ several DNA repair pathways to take care of distinct types of DNA damage. It is also well known that a cascade of signals namely DNA damage response or DDR is activated in response to DNA damage which comprise cellular responses, such as cell cycle arrest, DNA repair and cell death, if the damage is irreparable. There is also emerging literature suggesting a cross-talk between DNA damage signaling and several signaling networks within a cell. Moreover, cell death players themselves are also well known to engage in processes outside their canonical function of apoptosis. This chapter attempts to build a link between DNA damage, DDR and signaling from the studies mainly conducted in mammals and Drosophila model systems, with a special emphasis on their relevance in overall tissue homeostasis and development.

QTL mapping of natural variation reveals that the developmental regulator bruno reduces tolerance to P-element transposition in the Drosophila female germline

Transposable elements (TEs) are obligate genetic parasites that propagate in host genomes by replicating in germline nuclei, thereby ensuring transmission to offspring. This selfish replication not only produces deleterious mutations—in extreme cases, TE mobilization induces genotoxic stress that prohibits the production of viable gametes. Host genomes could reduce these fitness effects in two ways: resistance and tolerance. Resistance to TE propagation is enacted by germline-specific small-RNA-mediated silencing pathways, such as the Piwi-interacting RNA (piRNA) pathway, and is studied extensively. However, it remains entirely unknown whether host genomes may also evolve tolerance by desensitizing gametogenesis to the harmful effects of TEs. In part, the absence of research on tolerance reflects a lack of opportunity, as small-RNA-mediated silencing evolves rapidly after a new TE invades, thereby masking existing variation in tolerance. We have exploited the recent historical invasion of the Drosophila melanogaster genome by P-element DNA transposons in order to study tolerance of TE activity. In the absence of piRNA-mediated silencing, the genotoxic stress imposed by P-elements disrupts oogenesis and, in extreme cases, leads to atrophied ovaries that completely lack germline cells. By performing quantitative trait locus (QTL) mapping on a panel of recombinant inbred lines (RILs) that lack piRNA-mediated silencing of P-elements, we uncovered multiple QTL that are associated with differences in tolerance of oogenesis to P-element transposition. We localized the most significant QTL to a small 230-kb euchromatic region, with the logarithm of the odds (LOD) peak occurring in the bruno locus, which codes for a critical and well-studied developmental regulator of oogenesis. Genetic, cytological, and expression analyses suggest that bruno dosage modulates germline stem cell (GSC) loss in the presence of P-element activity. Our observations reveal segregating variation in TE tolerance for the first time, and implicate gametogenic regulators as a source of tolerant variants in natural populations.

Transposable elements (TEs), or “jumping genes,” are mobile fragments of selfish DNA that leave deleterious mutations and DNA damage in their wake as they spread through host genomes. Their harmful effects are known to select for resistance by the host, in which the propagation of TEs is regulated and reduced. Here, we study for the first time whether host cells might also exhibit tolerance to TEs, by reducing their harmful effects without directly controlling their movement. By taking advantage of a panel of wild-type Drosophila melanogaster that lack resistance to P-element DNA transposons, we identified a small region of the genome that influences tolerance of P-element activity. We further demonstrate that a gene within that region, bruno, strongly influences the negative effects of P-element mobilization on the fly. When bruno dosage is reduced, the fertility of females carrying mobile P-elements is enhanced. The bruno locus encodes a protein with no known role in TE regulation but multiple well-characterized functions in oogenesis. We propose that bruno function reduces tolerance of the developing oocyte to DNA damage that is caused by P-elements.

Nuclear lamina dysfunction triggers a germline stem cell checkpoint

LEM domain (LEM-D) proteins are conserved components of the nuclear lamina (NL) that contribute to stem cell maintenance through poorly understood mechanisms. The Drosophila emerin homolog Otefin (Ote) is required for maintenance of germline stem cells (GSCs) and gametogenesis. Here, we show that ote mutants carry germ cell-specific changes in nuclear architecture that are linked to GSC loss. Strikingly, we found that both GSC death and gametogenesis are rescued by inactivation of the DNA damage response (DDR) kinases, ATR and Chk2. Whereas the germline checkpoint draws from components of the DDR pathway, genetic and cytological features of the GSC checkpoint differ from the canonical pathway. Instead, structural deformation of the NL correlates with checkpoint activation. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. Taken together, these data suggest that NL dysfunction caused by Otefin loss triggers a GSC-specific checkpoint that contributes to maintenance of gamete quality.

Otefin is a nuclear lamina protein required for survival of Drosophila germ stem cells. Here the authors show that nuclear lamina dysfunction resulting from loss of Otefin activates a DNA damage-independent germ stem cell-specific checkpoint, mediated by the ATR and Chk2 kinases, which ensures that healthy gametes are passed on to the next generation.

Enhancer of polycomb maintains germline activity and genome integrity in Drosophila testis

Tissue homeostasis depends on the ability of tissue-specific adult stem cells to maintain a balance between proliferation and differentiation, as well as ensure DNA damage repair. Here, we use the Drosophila male germline stem cell system to study how a chromatin factor, enhancer of polycomb [E(Pc)], regulates the proliferation-to-differentiation (mitosis-to-meiosis) transition and DNA damage repair. We identified two critical targets of E(Pc). First, E(Pc) represses CycB transcription, likely through modulating H4 acetylation. Second, E(Pc) is required for accumulation of an important germline differentiation factor, Bag-of-marbles (Bam), through post-transcriptional regulation. When E(Pc) is downregulated, increased CycB and decreased Bam are both responsible for defective mitosis-to-meiosis transition in the germline. Moreover, DNA double-strand breaks (DSBs) accumulate upon germline inactivation of E(Pc) under both physiological condition and recovery from heat shock-induced endonuclease expression. Failure of robust DSB repair likely leads to germ cell loss. Finally, compromising the activity of Tip60, a histone acetyltransferase, leads to germline defects similar to E(Pc) loss-of-function, suggesting that E(Pc) acts cooperatively with Tip60. Together, our data demonstrate that E(Pc) has pleiotropic roles in maintaining male germline activity and genome integrity. Our findings will help elucidate the in vivo molecular mechanisms of E(Pc).

Chromosome Healing Is Promoted by the Telomere Cap Component Hiphop in Drosophila

The addition of a new telomere onto a chromosome break, a process termed healing, has been studied extensively in organisms that utilize telomerase to maintain their telomeres. In comparison, relatively little is known about how new telomeres are constructed on broken chromosomes in organisms that do not use telomerase. Chromosome healing was studied in somatic and germline cells of Drosophila melanogaster, a nontelomerase species. We observed, for the first time, that broken chromosomes can be healed in somatic cells. In addition, overexpression of the telomere cap component Hiphop increased the survival of somatic cells with broken chromosomes, while the cap component HP1 did not, and overexpression of the cap protein HOAP decreased their survival. In the male germline, Hiphop overexpression greatly increased the transmission of healed chromosomes. These results indicate that Hiphop can stimulate healing of a chromosome break. We suggest that this reflects a unique function of Hiphop: it is capable of seeding formation of a new telomeric cap on a chromosome end that lacks a telomere.

Loss of foxo rescues stem cell aging in Drosophila germ line

Aging stem cells lose the capacity to properly respond to injury and regenerate their residing tissues. Here, we utilized the ability of Drosophila melanogaster germline stem cells (GSCs) to survive exposure to low doses of ionizing radiation (IR) as a model of adult stem cell injury and identified a regeneration defect in aging GSCs: while aging GSCs survive exposure to IR, they fail to reenter the cell cycle and regenerate the germline in a timely manner. Mechanistically, we identify foxo and mTOR homologue, Tor as important regulators of GSC quiescence following exposure to ionizing radiation. foxo is required for entry in quiescence, while Tor is essential for cell cycle reentry. Importantly, we further show that the lack of regeneration in aging germ line stem cells after IR can be rescued by loss of foxo.

Stem cells are unspecialized cells that have the unique ability to replace dead cells and repair damaged tissues. To give rise to new cells, stem cells need to divide. This process, known as the cell cycle, includes several stages and is regulated by many different genes.

For example, in many organisms, a gene called foxo helps cells respond to stress and to regulate the cell cycle and cell death. Defects in this gene have been linked to age-related diseases, such as cancer and Alzheimer’s disease. Previous research has shown that foxo can also regulate Tor – a gene that helps cells to divide and grow.

As we age, stem cells become less efficient at regenerating tissues, especially after exposure to toxins and radiation. However, until now, it was not known how stem cells control their division after injury and during aging, and what role these two genes play in injured and aging stem cells.

Now, Artoni, Kreipke et al. used germline stem cells from fly ovaries to investigate how young and old stem cells respond to injury. In young flies, foxo paused the cell cycle of the damaged stem cells. After 24 hours, Tor was able to overcome the action of foxo, and the stem cells resumed dividing and regenerating the damaged tissue. However, in old stem cells, foxo and Tor were misregulated and the stem cells could not restart dividing or repairing tissue after injury. When the levels of foxo in old stem cells were experimentally reduced, their ability to regenerate the tissue was restored.

These discoveries provide new insights into how stem cells respond to injury and suggest that stem cell aging may be a reversible process. A next step will be to investigate why foxo and Tor are misregulated during aging and how these two genes interact with each another. In future, this could help develop new anti-aging therapies that can restore the body’s natural ability to repair itself following injury. Moreover, since cancer cells can become resistant to conventional cancer treatment by withdrawing from the cell cycle, developing new treatments that target foxo and Tor could help beat cancer and prevent its reoccurrence.