Identification and characterization of a p53 homologue in Drosophila melanogaster

Latest News from the "Guardian": p53 Directly Activates Asymmetric Stem Cell Division Regulators

Since its discovery in 1979, the human tumor suppressor gene TP53-also known as the "guardian of the genome"-has been the subject of intense research. Mutated in most human cancers, TP53 has traditionally been considered a key fighter against stress factors by trans-activating a network of target genes that promote cell cycle arrest, DNA repair, or apoptosis. Intriguingly, over the past years, novel non-canonical functions of p53 in unstressed cells have also emerged, including the mode of stem cell division regulation. However, the mechanisms by which p53 modulates these novel functions remain incompletely understood. In a recent work, we found that Drosophila p53 controls asymmetric stem cell division (ASCD) in neural stem cells by transcriptionally activating core ASCD regulators, such as the conserved cell-fate determinants Numb and Brat (NUMB and TRIM3/TRIM2/TRIM32 in humans, respectively). In this short communication, we comment on this new finding, the mild phenotypes associated with Drosophila p53 mutants in this context, as well as novel avenues for future research.

Drosophila p53 tumor suppressor directly activates conserved asymmetric stem cell division regulators

p53 is the most mutated tumor suppressor gene in human cancers. Besides p53 classical functions inducing cell-cycle arrest and apoptosis in stressed cells, additional p53 non-canonical roles in unstressed cells have emerged over the past years, including the mode of stem cell division regulation. However, the mechanisms by which p53 impacts on this process remain elusive. Here, we show that Drosophila p53 controls asymmetric stem cell division (ASCD), a key process in development, cancer and adult tissue homeostasis, by transcriptionally activating Numb, Brat, and Traf4 ASCD regulators. p53 knockout caused failures in their localization in dividing neural stem cells, as well as a significant decrease in their expression levels. Moreover, p53 directly bound numb, brat, and Traf4 regulatory regions. Remarkably, human and mice genes related to Drosophila brat (TRIM32) and Traf4 (TRAF4) were recently identified in a meta-analysis of transcriptomic and ChIP-seq datasets as predicted conserved p53 targets.

Molecular Events in Response to Triclosan-Induced Oxidative Stress in CRISPR/Cas9-Mediated p53-Targeted Mutants in Daphnia magna

Triclosan (TCS), a widely used antimicrobial agent, has been implicated in the oxidative stress induction and disruption of cellular processes in aquatic organisms. As TCS is ubiquitous in the aquatic environment, many previous studies have documented the effects of exposure to TCS on aquatic organisms. Nevertheless, most of the research has concentrated on the molecular and physiological responses of TCS, but there are still limited studies on the function of specific genes and the consequences of their absence. In this study, we focused on p53, a gene that is crucial for molecular responses such as autophagy and apoptosis as a result of TCS exposure. In order to ascertain the role and impact of the p53 gene in TCS-induced molecular responses, we examined the molecular responses to TCS-induced oxidative stress in wild-type (WT) and CRISPR/Cas9-mediated p53 mutant (MT) water fleas. The result has been accomplished by examining changes in molecular mechanisms, including in vivo end points, enzyme activities, adenosine triphosphate release rate, and apoptosis, to determine the role and impact of the p53 gene on TCS-induced molecular responses. The results indicated that the sensitivity of MT water fleas to TCS was greater than that of WT water fleas; however, the difference in sensitivity was significant at short exposures within 48 h and decreased toward 48 h. Accordingly, when we confirmed the oxidative stress after 24 h of exposure, the oxidative stress to TCS exposure was stronger in the MT group, with an imbalance of redox. To identify the mechanisms of tolerance to TCS in WT and MT Daphnia magna, we checked mitochondrial and ER-stress-related biomarkers and found an increase in apoptosis and greater sensitivity to TCS exposure in the MT group than in the WT. Our results suggest that the absence of p53 caused alterations in molecular processes in response to TCS exposure, resulting in increased sensitivity to TCS, and that p53 plays a critical role in response to TCS exposure.

Dominant suppressor genes of p53-induced apoptosis in Drosophila melanogaster

One of the major functions of programmed cell death (apoptosis) is the removal of cells that suffered oncogenic mutations, thereby preventing cancerous transformation. By making use of a Double-Headed-EP (DEP) transposon, a P element derivative made in our laboratory, we made an insertional mutagenesis screen in Drosophila melanogaster to identify genes that, when overexpressed, suppress the p53-activated apoptosis. The DEP element has Gal4-activatable, outward-directed UAS promoters at both ends, which can be deleted separately in vivo. In the DEP insertion mutants, we used the GMR-Gal4 driver to induce transcription from both UAS promoters and tested the suppression effect on the apoptotic rough eye phenotype generated by an activated UAS-p53 transgene. By DEP insertions, 7 genes were identified, which suppressed the p53-induced apoptosis. In 4 mutants, the suppression effect resulted from single genes activated by 1 UAS promoter (Pka-R2, Rga, crol, and Spt5). In the other 3 (Orct2, Polr2M, and stg), deleting either UAS promoter eliminated the suppression effect. In qPCR experiments, we found that the genes in the vicinity of the DEP insertion also showed an elevated expression level. This suggested an additive effect of the nearby genes on suppressing apoptosis. In the eukaryotic genomes, there are coexpressed gene clusters. Three of the DEP insertion mutants are included, and 2 are in close vicinity of separate coexpressed gene clusters. This raises the possibility that the activity of some of the genes in these clusters may help the suppression of the apoptotic cell death.

Xrp1 governs the stress response program to spliceosome dysfunction

Co-transcriptional processing of nascent pre-mRNAs by the spliceosome is vital to regulating gene expression and maintaining genome integrity. Here, we show that the deficiency of functional U5 small nuclear ribonucleoprotein particles (snRNPs) in Drosophila imaginal cells causes extensive transcriptome remodeling and accumulation of highly mutagenic R-loops, triggering a robust stress response and cell cycle arrest. Despite compromised proliferative capacity, the U5 snRNP-deficient cells increased protein translation and cell size, causing intra-organ growth disbalance before being gradually eliminated via apoptosis. We identify the Xrp1-Irbp18 heterodimer as the primary driver of transcriptional and cellular stress program downstream of U5 snRNP malfunction. Knockdown of Xrp1 or Irbp18 in U5 snRNP-deficient cells attenuated JNK and p53 activity, restored normal cell cycle progression and growth, and inhibited cell death. Reducing Xrp1-Irbp18, however, did not rescue the splicing defects, highlighting the requirement of accurate splicing for cellular and tissue homeostasis. Our work provides novel insights into the crosstalk between splicing and the DNA damage response and defines the Xrp1-Irbp18 heterodimer as a critical sensor of spliceosome malfunction and mediator of the stress-induced cellular senescence program.

The antagonistic relationship between apoptosis and polyploidy in development and cancer

One of the important functions of regulated cell death is to prevent cells from inappropriately acquiring extra copies of their genome, a state known as polyploidy. Apoptosis is the primary cell death mechanism that prevents polyploidy, and defects in this apoptotic response can result in polyploid cells whose subsequent error-prone chromosome segregation are a major contributor to genome instability and cancer progression. Conversely, some cells actively repress apoptosis to become polyploid as part of normal development or regeneration. Thus, although apoptosis prevents polyploidy, the polyploid state can actively repress apoptosis. In this review, we discuss progress in understanding the antagonistic relationship between apoptosis and polyploidy in development and cancer. Despite recent advances, a key conclusion is that much remains unknown about the mechanisms that link apoptosis to polyploid cell cycles. We suggest that drawing parallels between the regulation of apoptosis in development and cancer could help to fill this knowledge gap and lead to more effective therapies.

Regulation and coordination of the different DNA damage responses in Drosophila

Cells have evolved mechanisms that allow them to respond to DNA damage to preserve genomic integrity and maintain tissue homeostasis. These responses include the activation of the cell cycle checkpoints and the repair mechanisms or the induction of apoptosis that eventually will eliminate damaged cells. These “life” vs. “death” decisions differ depending on the cell type, stages of development, and the proliferation status of the cell. The apoptotic response after DNA damage is of special interest as defects in its induction could contribute to tumorigenesis or the resistance of cancer cells to therapeutic agents such as radiotherapy. Multiples studies have elucidated the molecular mechanisms that mediate the activation of the DNA damage response pathway (DDR) and specifically the role of p53. However, much less is known about how the different cellular responses such as cell proliferation control and apoptosis are coordinated to maintain tissue homeostasis. Another interesting question is how the differential apoptotic response to DNA damage is regulated in distinct cell types. The use of Drosophila melanogaster as a model organism has been fundamental to understand the molecular and cellular mechanisms triggered by genotoxic stress. Here, we review the current knowledge regarding the cellular responses to ionizing radiation as the cause of DNA damage with special attention to apoptosis in Drosophila: how these responses are regulated and coordinated in different cellular contexts and in different tissues. The existence of intrinsic mechanisms that might attenuate the apoptotic pathway in response to this sort of DNA damage may well be informative for the differences in the clinical responsiveness of tumor cells after radiation therapy.

Structural diversity of p63 and p73 isoforms

Abstract

The p53 protein family is the most studied protein family of all. Sequence analysis and structure determination have revealed a high similarity of crucial domains between p53, p63 and p73. Functional studies, however, have shown a wide variety of different tasks in tumor suppression, quality control and development. Here we review the structure and organization of the individual domains of p63 and p73, the interaction of these domains in the context of full-length proteins and discuss the evolutionary origin of this protein family.

Facts

Open questions

Coordination between cell proliferation and apoptosis after DNA damage in Drosophila

Exposure to genotoxic stress promotes cell cycle arrest and DNA repair or apoptosis. These "life" or "death" cell fate decisions often rely on the activity of the tumor suppressor gene p53. Therefore, the precise regulation of p53 is essential to maintain tissue homeostasis and to prevent cancer development. However, how cell cycle progression has an impact on p53 cell fate decision-making is mostly unknown. In this work, we demonstrate that Drosophila p53 proapoptotic activity can be impacted by the G2/M kinase Cdk1. We find that cell cycle arrested or endocycle-induced cells are refractory to ionizing radiation-induced apoptosis. We show that p53 binding to the regulatory elements of the proapoptotic genes and its ability to activate their expression is compromised in experimentally arrested cells. Our results indicate that p53 genetically and physically interacts with Cdk1 and that p53 proapoptotic role is regulated by the cell cycle status of the cell. We propose a model in which cell cycle progression and p53 proapoptotic activity are molecularly connected to coordinate the appropriate response after DNA damage.

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.

Dpp/TGFβ-superfamily play a dual conserved role in mediating the damage response in the retina

Retinal homeostasis relies on intricate coordination of cell death and survival in response to stress and damage. Signaling mechanisms that coordinate this process in the adult retina remain poorly understood. Here we identify Decapentaplegic (Dpp) signaling in Drosophila and its mammalian homologue Transforming Growth Factor-beta (TGFβ) superfamily, that includes TGFβ and Bone Morphogenetic Protein (BMP) signaling arms, as central mediators of retinal neuronal death and tissue survival following acute damage. Using a Drosophila model for UV-induced retinal damage, we show that Dpp released from immune cells promotes tissue loss after UV-induced retinal damage. Interestingly, we find a dynamic response of retinal cells to this signal: in an early phase, Dpp-mediated stimulation of Saxophone/Smox signaling promotes apoptosis, while at a later stage, stimulation of the Thickveins/Mad axis promotes tissue repair and survival. This dual role is conserved in the mammalian retina through the TGFβ/BMP signaling, as supplementation of BMP4 or inhibition of TGFβ using small molecules promotes retinal cell survival, while inhibition of BMP negatively affects cell survival after light-induced photoreceptor damage and NMDA induced inner retinal neuronal damage. Our data identify key evolutionarily conserved mechanisms by which retinal homeostasis is maintained.

Understanding p53 tumour suppressor network

The mutation of TP53 gene affects half of all human cancers, resulting in impairment of the regulation of several cellular functions, including cell cycle progression and cell death in response to genotoxic stress. In the recent years additional p53-mediated tumour suppression mechanisms have been described, questioning the contribution of its canonical pathway for tumour suppression. These include regulation of alternative cell death modalities (i.e. ferroptosis), cell metabolism and the emerging role in RNA stability. Here we briefly summarize our knowledge on p53 “canonical DNA damage response” and discuss the most relevant recent findings describing potential mechanistic explanation of p53-mediated tumour suppression.

A Small Molecule with Bridged Carbonyl and Tri-fluoro-aceto-phenone Groups Impedes Microtubule Dynamics and Subsequently Triggers Cancer Cell Apoptosis

We identified a new microtubule targeted small molecule, which showed significant anticancer activity and induced apoptotic death of cancer cells. Precisely the central bridged carbonyl group and trifluoro-acetophenone group of a bis-benzothiazole molecule (BBT) interacts with tubulin close to the curcumin site and perturbs microtubule dynamics as well as causes microtubule depolymerization. We observed a significant enhancement of fluorescence while BBT interacts with the tubulin through bridged carbonyl moiety, a similar phenomenon to colchicine. Further, BBT activates tumor-suppressing bim and p53-puma axes to inhibit cancer survival. It also shows promising results against a tumor spheroid model. BBT is also capable of tumor regression, which shows that this molecule can serve as a potential template for the design of next-generation microtubule targeted anticancer drugs.

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

DNA Damaged Induced Cell Death in Oocytes

The production of haploid gametes through meiosis is central to the principle of sexual reproduction. The genetic diversity is further enhanced by exchange of genetic material between homologous chromosomes by the crossover mechanism. This mechanism not only requires correct pairing of homologous chromosomes but also efficient repair of the induced DNA double-strand breaks. Oocytes have evolved a unique quality control system that eliminates cells if chromosomes do not correctly align or if DNA repair is not possible. Central to this monitoring system that is conserved from nematodes and fruit fly to humans is the p53 protein family, and in vertebrates in particular p63. In mammals, oocytes are stored for a long time in the prophase of meiosis I which, in humans, can last more than 50 years. During the entire time of this arrest phase, the DNA damage checkpoint remains active. The treatment of female cancer patients with DNA damaging irradiation or chemotherapeutics activates this checkpoint and results in elimination of the oocyte pool causing premature menopause and infertility. Here, we review the molecular mechanisms of this quality control system and discuss potential therapeutic intervention for the preservation of the oocyte pool during chemotherapy.

Selective Killing of RAS-Malignant Tissues by Exploiting Oncogene-Induced DNA Damage

Several oncogenes induce untimely entry into S phase and alter replication timing and progression, thereby generating replicative stress, a well-known source of genomic instability and a hallmark of cancer. Using an epithelial model in Drosophila, we show that the RAS oncogene, which triggers G1/S transition, induces DNA damage and, at the same time, silences the DNA damage response pathway. RAS compromises ATR-mediated phosphorylation of the histone variant H2Av and ATR-mediated cell-cycle arrest in G2 and blocks, through ERK, Dp53-dependent induction of cell death. We found that ERK is also activated in normal tissues by an exogenous source of damage and that this activation is necessary to dampen the pro-apoptotic role of Dp53. We exploit the pro-survival role of ERK activation upon endogenous and exogenous sources of DNA damage to present evidence that its genetic or chemical inhibition can be used as a therapeutic opportunity to selectively eliminate RAS-malignant tissues.

ZBED1/DREF: A transcription factor that regulates cell proliferation

Maintenance of genomic diversity is critically dependent on gene regulation at the transcriptional level. This occurs via the interaction of regulatory DNA sequence motifs with DNA-binding transcription factors. The zinc finger, BED-type (ZBED) gene family contains major DNA-binding motifs present in human transcriptional factors. It encodes proteins that present markedly diverse regulatory functions. ZBED1 has similar structural and functional properties to its Drosophila homolog DNA replication-related element-binding factor (DREF) and plays a critical role in the regulation of transcription. ZBED1 regulates the expression of several genes associated with cell proliferation, including cell cycle regulation, chromatin remodeling and protein metabolism, and some genes associated with apoptosis and differentiation. In the present review, the origin, structure and functional role of ZBED1 were comprehensively assessed. In addition, the similarities and differences between ZBED1 and its Drosophila homolog DREF were highlighted, and future research directions, particularly in the area of clinical cancer, were discussed.

Towards reconstructing the dipteran demise of an ancient essential gene: E3 ubiquitin ligase Murine double minute

Genome studies have uncovered many examples of essential gene loss, raising the question of how ancient genes transition from essentiality to dispensability. We explored this process for the deeply conserved E3 ubiquitin ligase Murine double minute (Mdm), which is lacking in Drosophila despite the conservation of its main regulatory target, the cellular stress response gene p53. Conducting gene expression and knockdown experiments in the red flour beetle Tribolium castaneum, we found evidence that Mdm has remained essential in insects where it is present. Using bioinformatics approaches, we confirm the absence of the Mdm gene family in Drosophila, mapping its loss to the stem lineage of schizophoran Diptera and Pipunculidae (big-headed flies), about 95-85 million years ago. Intriguingly, this gene loss event was preceded by the de novo origin of the gene Companion of reaper (Corp), a novel p53 regulatory factor that is characterized by functional similarities to vertebrate Mdm2 despite lacking E3 ubiquitin ligase protein domains. Speaking against a 1:1 compensatory gene gain/loss scenario, however, we found that hoverflies (Syrphidae) and pointed-wing flies (Lonchopteridae) possess both Mdm and Corp. This implies that the two p53 regulators have been coexisting for ~ 150 million years in select dipteran clades and for at least 50 million years in the lineage to Schizophora and Pipunculidae. Given these extensive time spans of Mdm/Corp coexistence, we speculate that the loss of Mdm in the lineage to Drosophila involved further acquisitions of compensatory gene activities besides the emergence of Corp. Combined with the previously noted reduction of an ancestral P53 contact domain in the Mdm homologs of crustaceans and insects, we conclude that the loss of the ancient Mdm gene family in flies was the outcome of incremental functional regression over long macroevolutionary time scales.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

Decoupling of Apoptosis from Activation of the ER Stress Response by the Drosophila Metallopeptidase superdeath

Endoplasmic reticulum (ER) stress-induced apoptosis is a primary cause and modifier of degeneration in a number of genetic disorders. Understanding how genetic variation influences the ER stress response and subsequent activation of apoptosis could improve individualized therapies and predictions of outcomes for patients. In this study, we find that the uncharacterized, membrane-bound metallopeptidase CG14516 in Drosophila melanogaster, which we rename as SUPpressor of ER stress-induced DEATH (su per death), plays a role in modifying ER stress-induced apoptosis. We demonstrate that loss of su per death reduces apoptosis and degeneration in the Rh1^G69D model of ER stress through the JNK signaling cascade. This effect on apoptosis occurs without altering the activation of the unfolded protein response (IRE1 and PERK), suggesting that the beneficial prosurvival effects of this response are intact. Furthermore, we show that su per death functions epistatically upstream of CDK5-a known JNK-activated proapoptotic factor in this model of ER stress. We demonstrate that su per death is not only a modifier of this particular model, but affects the general tolerance to ER stress, including ER stress-induced apoptosis. Finally, we present evidence of Superdeath localization to the ER membrane. While similar in sequence to a number of human metallopeptidases found in the plasma membrane and ER membrane, its localization suggests that su per death is orthologous to ERAP1/2 in humans. Together, this study provides evidence that su per death is a link between stress in the ER and activation of cytosolic apoptotic pathways.

The Emerging Landscape of p53 Isoforms in Physiology, Cancer and Degenerative Diseases

p53, first described four decades ago, is now established as a master regulator of cellular stress response, the “guardian of the genome”. p53 contributes to biological robustness by behaving in a cellular-context dependent manner, influenced by several factors (e.g., cell type, active signalling pathways, the type, extent and intensity of cellular damage, cell cycle stage, nutrient availability, immune function). The p53 isoforms regulate gene transcription and protein expression in response to the stimuli so that the cell response is precisely tuned to the cell signals and cell context. Twelve isoforms of p53 have been described in humans. In this review, we explore the interactions between p53 isoforms and other proteins contributing to their established cellular functions, which can be both tumour-suppressive and oncogenic in nature. Evidence of p53 isoform in human cancers is largely based on RT-qPCR expression studies, usually investigating a particular type of isoform. Beyond p53 isoform functions in cancer, it is implicated in neurodegeneration, embryological development, progeroid phenotype, inflammatory pathology, infections and tissue regeneration, which are described in this review.

Natural Genetic Variation Screen in Drosophila Identifies Wnt Signaling, Mitochondrial Metabolism, and Redox Homeostasis Genes as Modifiers of Apoptosis

Apoptosis is the primary cause of degeneration in a number of neuronal, muscular, and metabolic disorders. These diseases are subject to a great deal of phenotypic heterogeneity in patient populations, primarily due to differences in genetic variation between individuals. This creates a barrier to effective diagnosis and treatment. Understanding how genetic variation influences apoptosis could lead to the development of new therapeutics and better personalized treatment approaches. In this study, we examine the impact of the natural genetic variation in the Drosophila Genetic Reference Panel (DGRP) on two models of apoptosis-induced retinal degeneration: overexpression of p53 or reaper (rpr). We identify a number of known apoptotic, neural, and developmental genes as candidate modifiers of degeneration. We also use Gene Set Enrichment Analysis (GSEA) to identify pathways that harbor genetic variation that impact these apoptosis models, including Wnt signaling, mitochondrial metabolism, and redox homeostasis. Finally, we demonstrate that many of these candidates have a functional effect on apoptosis and degeneration. These studies provide a number of avenues for modifying genes and pathways of apoptosis-related disease.

P53 and Apoptosis in the Drosophila Model

Human P53 (HsP53) is the most frequently mutated gene associated with cancers. Despite heightened research interest over the last four decades, a clear picture of how wild type HsP53 functions as the guardian against malignant transformation remains elusive. Studying the ortholog of P53 in the genetic model organism Drosophila melanogaster (DmP53) has revealed many interesting insights. This chapter focuses on recent findings that have shed light on how DmP53 -mediated apoptosis plays an important role in maintaining genome integrity, and how the immediate output of activated DmP53 is determined by the epigenetic landscape of individual cells.

Drosophila p53 integrates the antagonism between autophagy and apoptosis in response to stress

The tumor suppressor TP53/p53 is a known regulator of apoptosis and macroautophagy/autophagy. However, the molecular mechanism by which TP53 regulates 2 apparently incompatible processes remains unknown. We found that Drosophila lacking p53 displayed impaired autophagic flux, higher caspase activation and mortality in response to oxidative stress compared with wild-type flies. Moreover, autophagy and apoptosis were differentially regulated by the p53 (p53B) and ΔNp53 (p53A) isoforms: while the former induced autophagy in differentiated neurons, which protected against cell death, the latter inhibited autophagy by activating the caspases Dronc, Drice, and Dcp-1. Our results demonstrate that the differential use of p53 isoforms combined with the antagonism between apoptosis and autophagy ensures the generation of an appropriate p53 biological response to stress.

The animal nuclear factor Y: an enigmatic and important heterotrimeric transcription factor

Nuclear factor Y (NF-Y) is a heterotrimeric transcription factor with the ability to bind to CCAAT boxes in nearly all eukaryotes and has long been a topic of interest since it is first identified. In plants, due to each subunit of NF-Y is encoded by multiple gene families, there are a wide variety NF-Y complex combinations that fulfill many pivotal functions. However, the animal NF-Y complex usually has only one type of combination, as each subunit is generally encoded by a single gene. Even though, mounting evidence points to that the animal NF-Y complex is also essential for numerous biological processes involved in proliferation and apoptosis, cancer and tumor, stress responses, growth and development. Therefore, a relatively comprehensive functional dissection of animal NF-Y will enable a deeper comprehension of how lesser combinations of the NF-Y complex regulate diverse aspects of biology processes in animal. Here, we focus mainly on reviewing recent advances related to NF-Y in the animal field, including subunit structural characteristics, expression regulation models and biological functions, and we also discuss future directions.

Regulation and function of p53: A perspective from Drosophila studies

Tp53 is a central regulator of cellular responses to stress and one of the most frequently mutated genes in human cancers. P53 is activated by a myriad of stress signals and drives specific cellular responses depending on stress nature, cell type and cellular context. Additionally to its classical functions in regulating cell cycle arrest, apoptosis and senescence, newly described non-canonical functions of p53 are increasingly coming under the spotlight as important functions not only for its role as a tumour suppressor but also for its non-cancer associated activities. Drosophila melanogaster is a valuable model to study multiple aspects of normal animal physiology, stress response and disease. In this review, we discuss the contribution of Drosophila studies to the current knowledge on p53 and highlight recent evidences pointing to p53 novel roles in promoting tissue homeostasis and metabolic adaptation.

p53 is required for brain growth but is dispensable for resistance to nutrient restriction during Drosophila larval development

BACKGROUND

Animal growth is influenced by the genetic background and the environmental circumstances. How genes promote growth and coordinate adaptation to nutrient availability is still an open question. p53 is a transcription factor that commands the cellular response to different types of stresses. In adult Drosophila melanogaster, p53 regulates the metabolic adaptation to nutrient restriction that supports fly viability. Furthermore, the larval brain is protected from nutrient restriction in a phenomenon called 'brain sparing'. Therefore, we hypothesised that p53 may regulate brain growth and show a protective role over brain development under nutrient restriction.

RESULTS

Here, we studied the function of p53 during brain growth in normal conditions and in animals subjected to developmental nutrient restriction. We showed that p53 loss of function reduced animal growth and larval brain size. Endogenous p53 was expressed in larval neural stem cells, but its levels and activity were not affected by nutritional stress. Interestingly, p53 knockdown only in neural stem cells was sufficient to decrease larval brain growth. Finally, we showed that in p53 mutant larvae under nutrient restriction, the energy storage levels were not altered, and these larvae generated adults with brains of similar size than wild-type animals.

CONCLUSIONS

Using genetic approaches, we demonstrate that p53 is required for proper growth of the larval brain. This developmental role of p53 does not have an impact on animal resistance to nutritional stress since brain growth in p53 mutants under nutrient restriction is similar to control animals.

The p53 gene with emphasis on its paralogues in mosquitoes

The p53 gene is highly important in human cancers, as it serves as a tumor-suppressor gene. Subsequently, two p53 homologues, i.e., p73 and p63, with high identity of amino acids were identified, leading to construction of the p53 family. The p53 gene is highly important in human cancer because it usually transcribes genes that function by causing apoptosis in mammalian cells. In contrast, p63 and p73 tend to be more important in modulating development than inducing cell death, even though they share similar protein structures. Relatively recently, p53 was also identified in mosquitoes and many other insect species. Uniquely, its structure lacks the sterile alpha motif domain which is a putative protein-protein interaction domain and exclusively exists at the C-terminal region in p73 and p63 in mammals. A phylogenetic analysis revealed that the p53 gene derived from mosquitoes is composed of two paralogues, p53-1 and p53-2. Of these, only p53-2 is responsively upregulated by dengue 2 virus (DENV2) in C6/36 cells which usually survive the infection. This indicates that the p53 gene is closely related to DENV infection in mosquito cells. The specific significance of p53-2's involvement in cell survival from virus-induced stress is described and briefly discussed in this report.

p53 in the game of transposons

Throughout the animal kingdom, p53 genes function to restrain mobile elements and recent observations indicate that transposons become derepressed in human cancers. Together, these emerging lines of evidence suggest that cancers driven by p53 mutations could represent "transpospoathies," i.e. disease states linked to eruptions of mobile elements. The transposopathy hypothesis predicts that p53 acts through conserved mechanisms to contain transposon movement, and in this way, prevents tumor formation. How transposon eruptions provoke neoplasias is not well understood but, from a broader perspective, this hypothesis also provides an attractive framework to explore unrestrained mobile elements as inciters of late-onset idiopathic disease. Also see the video abstract here.

NF-Y in invertebrates

Both Drosophila melanogaster and Caenorhabditis elegans (C. elegans) are useful model organisms to study in vivo roles of NF-Y during development. Drosophila NF-Y (dNF-Y) consists of three subunits dNF-YA, dNF-YB and dNF-YC. In some tissues, dNF-YC-related protein Mes4 may replace dNF-YC in dNF-Y complex. Studies with eye imaginal disc-specific dNF-Y-knockdown flies revealed that dNF-Y positively regulates the sevenless gene encoding a receptor tyrosine kinase, a component of the ERK pathway and negatively regulates the Sensless gene encoding a transcription factor to ensure proper development of R7 photoreceptor cells together with proper R7 axon targeting. dNF-Y also controls the Drosophila Bcl-2 (debcl) to regulate apoptosis. In thorax development, dNF-Y is necessary for both proper Drosophila JNK (basket) expression and JNK signaling activity that is responsible for thorax development. Drosophila p53 gene was also identified as one of the dNF-Y target genes in this system. C. elegans contains two forms of NF-YA subunit, CeNF-YA1 and CeNF-YA2. C. elegans NF-Y (CeNF-Y) therefore consists of CeNF-YB, CeNF-YC and either CeNF-YA1 or CeNF-YA2. CeNF-Y negatively regulates expression of the Hox gene egl-5 (ortholog of Drosophila Abdominal-B) that is involved in tail patterning. CeNF-Y also negatively regulates expression of the tbx-2 gene that is essential for development of the pharyngeal muscles, specification of neural cell fate and adaptation in olfactory neurons. Negative regulation of the expression of egl-5 and tbx-2 by CeNF-Y provides new insight into the physiological meaning of negative regulation of gene expression by NF-Y during development. In addition, studies on NF-Y in platyhelminths are also summarized. This article is part of a Special Issue entitled: Nuclear Factor Y in Development and Disease, edited by Prof. Roberto Mantovani.

Male-killing symbiont damages host's dosage-compensated sex chromosome to induce embryonic apoptosis

Some symbiotic bacteria are capable of interfering with host reproduction in selfish ways. How such bacteria can manipulate host's sex-related mechanisms is of fundamental interest encompassing cell, developmental and evolutionary biology. Here, we uncover the molecular and cellular mechanisms underlying Spiroplasma-induced embryonic male lethality in Drosophila melanogaster. Transcriptomic analysis reveals that many genes related to DNA damage and apoptosis are up-regulated specifically in infected male embryos. Detailed genetic and cytological analyses demonstrate that male-killing Spiroplasma causes DNA damage on the male X chromosome interacting with the male-specific lethal (MSL) complex. The damaged male X chromosome exhibits a chromatin bridge during mitosis, and bridge breakage triggers sex-specific abnormal apoptosis via p53-dependent pathways. Notably, the MSL complex is not only necessary but also sufficient for this cytotoxic process. These results highlight symbiont's sophisticated strategy to target host's sex chromosome and recruit host's molecular cascades toward massive apoptosis in a sex-specific manner.

Symbiotic bacteria are able to interfere with host reproduction in ways that are detrimental to the host organism. Here the authors show that Spiroplasma induces DNA damage on the male X chromosome in Drosophila, causing sex-specific apoptosis.

p53 Isoforms: Key Regulators of the Cell Fate Decision

It is poorly understood how a single protein, p53, can be responsive to so many stress signals and orchestrates very diverse cell responses to maintain/restore cell/tissue functions. The uncovering that TP53 gene physiologically expresses, in a tissue-dependent manner, several p53 splice variants (isoforms) provides an explanation to its pleiotropic biological activities. Here, we summarize a decade of research on p53 isoforms. The clinical studies and the diverse cellular and animal models of p53 isoforms (zebrafish, Drosophila, and mouse) lead us to realize that a p53-mediated cell response is, in fact, the sum of the intrinsic activities of the coexpressed p53 isoforms and that unbalancing expression of different p53 isoforms leads to cancer, premature aging, (neuro)degenerative diseases, inflammation, embryo malformations, or defects in tissue regeneration. Cracking the p53 isoforms' code is, thus, a necessary step to improve cancer treatment. It also opens new exciting perspectives in tissue regeneration.

Third Chromosome Balancer Inversions Disrupt Protein-Coding Genes and Influence Distal Recombination Events in Drosophila melanogaster

Balancer chromosomes are multiply inverted chromosomes that suppress meiotic crossing over and prevent the recovery of crossover products. Balancers are commonly used in Drosophila melanogaster to maintain deleterious alleles and in stock construction. They exist for all three major chromosomes, yet the molecular location of the breakpoints and the exact nature of many of the mutations carried by the second and third chromosome balancers has not been available. Here, we precisely locate eight of 10 of the breakpoints on the third chromosome balancer TM3, six of eight on TM6, and nine of 11 breakpoints on TM6B. We find that one of the inversion breakpoints on TM3 bisects the highly conserved tumor suppressor gene p53—a finding that may have important consequences for a wide range of studies in Drosophila. We also identify evidence of single and double crossovers between several TM3 and TM6B balancers and their normal-sequence homologs that have created genetic diversity among these chromosomes. Overall, this work demonstrates the practical importance of precisely identifying the position of inversion breakpoints of balancer chromosomes and characterizing the mutant alleles carried by them.

Molecular cloning, characterization, and expression analysis of p53 from the oriental river prawn, Macrobrachium nipponense, in response to hypoxia

The tumor suppressor gene p53 plays a critical role in safeguarding the integrity of the genome in mammalian cells. It acts as a sequence-specific transcription factor. Once p53 is activated by a variety of cellular stresses, it transactivates downstream target genes and regulates the cell cycle and apoptosis. However, little is known about the functions of the p53 pathway in prawns in response to hypoxia. In this study, the cDNA of p53 from the oriental river prawn, Macrobrachium nipponense, (Mnp53) was cloned using a combination of homology cloning and rapid amplification of cDNA ends. The full-length cDNA of Mnp53 has 2130 bp, including an open reading frame of 1125 bp that encodes a polypeptide of 374 amino acids with a predicted molecular weight of 41.9 kDa and a theoretical isoelectric point of 6.9. Quantitative real-time (qRT)-PCR assays revealed that Mnp53 was ubiquitously expressed in all examined tissues, but at high levels in the hepatopancreas. In addition, we studied respiratory bursts and reactive oxygen species (ROS) production in the hepatopancreas of M. nipponense. Our results suggest that oxidative stress occurred in prawns in response to hypoxia and that apoptosis was associated with an increase in caspase-3 mRNA expression. qRT-PCR and western blot results confirmed that hypoxic stress induced the upregulation of Mnp53 at mRNA and protein levels. Furthermore, immunohistochemistry showed remarkable changes in immunopositive staining after the same hypoxic treatment. These results suggest that hypoxia-induced oxidative stress may cause apoptosis and cooperatively stimulate the expression of Mnp53.

Transgenic Drosophila for Investigating DUX4 and FRG1, Two Genes Associated with Facioscapulohumeral Muscular Dystrophy (FSHD)

Facioscapulohumeral muscular dystrophy (FSHD) is typically an adult onset dominant myopathy. Epigenetic changes in the chromosome 4q35 region linked to both forms of FSHD lead to a relaxation of repression and increased somatic expression of DUX4-fl (DUX4-full length), the pathogenic alternative splicing isoform of the DUX4 gene. DUX4-fl encodes a transcription factor expressed in healthy testis and pluripotent stem cells; however, in FSHD, increased levels of DUX4-fl in myogenic cells lead to aberrant regulation of target genes. DUX4-fl has proven difficult to study in vivo; thus, little is known about its normal and pathogenic roles. The endogenous expression of DUX4-fl in FSHD-derived human muscle and myogenic cells is extremely low, exogenous expression of DUX4-fl in somatic cells rapidly induces cytotoxicity, and, due in part to the lack of conservation beyond primate lineages, viable animal models based on DUX4-fl have been difficult to generate. By contrast, the FRG1 (FSHD region gene 1), which is linked to FSHD, is evolutionarily conserved from invertebrates to humans, and has been studied in several model organisms. FRG1 expression is critical for the development of musculature and vasculature, and overexpression of FRG1 produces a myopathic phenotype, yet the normal and pathological functions of FRG1 are not well understood. Interestingly, DUX4 and FRG1 were recently linked when the latter was identified as a direct transcriptional target of DUX4-FL. To better understand the pathways affected in FSHD by DUX4-fl and FRG1, we generated transgenic lines of Drosophila expressing either gene under control of the UAS/GAL4 binary system. Utilizing these lines, we generated screenable phenotypes recapitulating certain known consequences of DUX4-fl or FRG1 overexpression. These transgenic Drosophila lines provide resources to dissect the pathways affected by DUX4-fl or FRG1 in a genetically tractable organism and may provide insight into both muscle development and pathogenic mechanisms in FSHD.

The function of Drosophila p53 isoforms in apoptosis

The p53 protein is a major mediator of the cellular response to genotoxic stress and is a crucial suppressor of tumor formation. In a variety of organisms, p53 and its paralogs, p63 and p73, each encode multiple protein isoforms through alternative splicing, promoters, and translation start sites. The function of these isoforms in development and disease are still being defined. Here, we evaluate the apoptotic potential of multiple isoforms of the single p53 gene in the genetic model Drosophila melanogaster. Most previous studies have focused on the p53A isoform, but it has been recently shown that a larger p53B isoform can induce apoptosis when overexpressed. It has remained unclear, however, whether one or both isoforms are required for the apoptotic response to genotoxic stress. We show that p53B is a much more potent inducer of apoptosis than p53A when overexpressed. Overexpression of two newly identified short isoforms perturbed development and inhibited the apoptotic response to ionizing radiation. Analysis of physiological protein expression indicated that p53A is the most abundant isoform, and that both p53A and p53B can form a complex and co-localize to sub-nuclear compartments. In contrast to the overexpression results, new isoform-specific loss-of-function mutants indicated that it is the shorter p53A isoform, not full-length p53B, that is the primary mediator of pro-apoptotic gene transcription and apoptosis after ionizing radiation. Together, our data show that it is the shorter p53A isoform that mediates the apoptotic response to DNA damage, and further suggest that p53B and shorter isoforms have specialized functions.

Effect of Low Doses (5-40 cGy) of Gamma-irradiation on Lifespan and Stress-related Genes Expression Profile in Drosophila melanogaster

Studying of the effects of low doses of γ-irradiation is a crucial issue in different areas of interest, from environmental safety and industrial monitoring to aerospace and medicine. The goal of this work is to identify changes of lifespan and expression stress-sensitive genes in Drosophila melanogaster, exposed to low doses of γ-irradiation (5 – 40 cGy) on the imaginal stage of development. Although some changes in life extensity in males were identified (the effect of hormesis after the exposure to 5, 10 and 40 cGy) as well as in females (the effect of hormesis after the exposure to 5 and 40 cGy), they were not caused by the organism “physiological” changes. This means that the observed changes in life expectancy are not related to the changes of organism physiological functions after the exposure to low doses of ionizing radiation. The identified changes in gene expression are not dose-dependent, there is not any proportionality between dose and its impact on expression. These results reflect nonlinear effects of low dose radiation and sex-specific radio-resistance of the postmitotic cell state of Drosophila melanogaster imago.

Corp Regulates P53 in Drosophila melanogaster via a Negative Feedback Loop

The tumor suppressor P53 is a critical mediator of the apoptotic response to DNA double-strand breaks through the transcriptional activation of pro-apoptotic genes. This mechanism is evolutionarily conserved from mammals to lower invertebrates, including Drosophila melanogaster. P53 also transcriptionally induces its primary negative regulator, Mdm2, which has not been found in Drosophila. In this study we identified the Drosophila gene companion of reaper (corp) as a gene whose overexpression promotes survival of cells with DNA damage in the soma but reduces their survival in the germline. These disparate effects are shared by p53 mutants, suggesting that Corp may be a negative regulator of P53. Confirming this supposition, we found that corp negatively regulates P53 protein level. It has been previously shown that P53 transcriptionally activates corp; thus, Corp produces a negative feedback loop on P53. We further found that Drosophila Corp shares a protein motif with vertebrate Mdm2 in a region that mediates the Mdm2:P53 physical interaction. In Corp, this motif mediates physical interaction with Drosophila P53. Our findings implicate Corp as a functional analog of vertebrate Mdm2 in flies.

Drosophila p53 controls Notch expression and balances apoptosis and proliferation

A balance between cell proliferation and apoptosis is important for normal development and tissue homeostasis. Under stress conditions, the conserved tumor suppressor and transcription factor Dp53 induces apoptosis to contribute to the maintenance of homeostasis. However, in some cases Dp53-induced apoptosis results in the proliferation of surrounding non-apoptotic cells. To gain insight into the Dp53 function in the control of apoptosis and proliferation, we studied the interaction between the Drosophila Dp53 and Notch genes. We present evidence that simultaneous reduction of Dp53 and Notch function synergistically increases the wing phenotype of Notch heterozygous mutant flies. Further, we found that a Notch cis-regulatory element is responsive to loss and gain of Dp53 function and that over-expression of Dp53 up-regulates Notch mRNA and protein expression. These findings suggest not only that Dp53 and Notch act together to control wing development but also indicate that Dp53 transcriptionally regulates Notch expression. Moreover, using Notch  gain and loss of function mutations we examined the relevance of Dp53 and Notch interactions in the process of Dp53-apoptosis induced proliferation. Results show that proliferation induced by Dp53 over-expression is dependent on Notch, thus identifying Notch as a new player in Dp53-induced proliferation. Interestingly, we found that Dp53-induced Notch activation and proliferation occurs even under conditions where apoptosis was inhibited. Our findings highlight the conservation between flies and vertebrates of the Dp53 and Notch cross-talk and suggest that Dp53 has a dual role regulating cell death and proliferation gene networks to control the homeostatic balance between apoptosis and proliferation.

The p53 control of apoptosis and proliferation: lessons from Drosophila

The canonical role of p53 in preserving genome integrity and limiting carcinogenesis has been well established. In the presence of acute DNA-damage, oncogene deregulation and other forms of cellular stress, p53 orchestrates a myriad of pleiotropic processes to repair cellular damages and maintain homeostasis. Beside these well-studied functions of p53, recent studies in Drosophila have unraveled intriguing roles of Dmp53 in promoting cell division in apoptosis-induced proliferation, enhancing fitness and proliferation of the winner cell in cell competition and coordinating growth at the organ and organismal level in the presence of stress. In this review, we describe these new functions of Dmp53 and discuss their relevance in the context of carcinogenesis.

Acentrosomal Drosophila epithelial cells exhibit abnormal cell division, leading to cell death and compensatory proliferation

Mitotic spindles are critical for accurate chromosome segregation. Centrosomes, the primary microtubule nucleating centers of animal cells, play key roles in forming and orienting mitotic spindles. However, the survival of Drosophila without centrosomes suggested they are dispensable in somatic cells, challenging the canonical view. We used fly wing disc epithelia as a model to resolve these conflicting hypotheses, revealing that centrosomes play vital roles in spindle assembly, function, and orientation. Many acentrosomal cells exhibit prolonged spindle assembly, chromosome missegregation, DNA damage, misoriented divisions, and eventual apoptosis. We found that multiple mechanisms buffer the effects of centrosome loss, including alternative microtubule nucleation pathways and the spindle assembly checkpoint. Apoptosis of acentrosomal cells is mediated by JNK signaling, which also drives compensatory proliferation to maintain tissue integrity and viability. These data reveal the importance of centrosomes in fly epithelia and demonstrate the robust compensatory mechanisms at the cellular and organismal level.

Low levels of p53 protein and chromatin silencing of p53 target genes repress apoptosis in Drosophila endocycling cells

Apoptotic cell death is an important response to genotoxic stress that prevents oncogenesis. It is known that tissues can differ in their apoptotic response, but molecular mechanisms are little understood. Here, we show that Drosophila polyploid endocycling cells (G/S cycle) repress the apoptotic response to DNA damage through at least two mechanisms. First, the expression of all the Drosophila p53 protein isoforms is strongly repressed at a post-transcriptional step. Second, p53-regulated pro-apoptotic genes are epigenetically silenced in endocycling cells, preventing activation of a paused RNA Pol II by p53-dependent or p53-independent pathways. Over-expression of the p53A isoform did not activate this paused RNA Pol II complex in endocycling cells, but over-expression of the p53B isoform with a longer transactivation domain did, suggesting that dampened p53B protein levels are crucial for apoptotic repression. We also find that the p53A protein isoform is ubiquitinated and degraded by the proteasome in endocycling cells. In mitotic cycling cells, p53A was the only isoform expressed to detectable levels, and its mRNA and protein levels increased after irradiation, but there was no evidence for an increase in protein stability. However, our data suggest that p53A protein stability is regulated in unirradiated cells, which likely ensures that apoptosis does not occur in the absence of stress. Without irradiation, both p53A protein and a paused RNA pol II were pre-bound to the promoters of pro-apoptotic genes, preparing mitotic cycling cells for a rapid apoptotic response to genotoxic stress. Together, our results define molecular mechanisms by which different cells in development modulate their apoptotic response, with broader significance for the survival of normal and cancer polyploid cells in mammals.

In order to maintain genome integrity, eukaryotic cells have evolved multiple ways to respond to DNA damage stress. One of the major cellular responses is apoptosis, during which the cell undergoes programmed cell death in order to prevent the propagation of the damaged genome to daughter cells. Although clinical observations and other studies have shown that tissues can differ in their apoptotic response, the molecular mechanisms underlying these differences are largely unknown. We have shown in our model system, Drosophila, that endocycling cells do not initiate cell death in response to DNA damage. The endocycle is a cell cycle variation that is widely found in nature and conserved from plant to animals. During the endocycle, cells duplicate their genomic DNA but do not enter mitosis to segregate chromosomes, resulting in a polyploid genome content. In this study, we investigate how the apoptotic response to DNA damage is repressed in endocycling cells. We find that the Drosophila ortholog of the human p53 tumor suppressor protein is expressed at very low levels in endocycling cells. Moreover, the downstream pro-apoptotic genes that are regulated by p53 are epigenetically silenced in endocycling cells. Our results provide important insights into tissue-specific apoptotic responses in development, with possible broader impact on understanding radiation therapy response and cancer of different tissues.

MicroRNA-mediated regulation of Dp53 in the Drosophila fat body contributes to metabolic adaptation to nutrient deprivation

Multiple conserved mechanisms sense nutritional conditions and coordinate metabolic changes in the whole organism. We unravel a role for the Drosophila homolog of p53 (Dp53) in the fat body (FB; a functional analog of vertebrate adipose and hepatic tissues) in starvation adaptation. Under nutrient deprivation, FB-specific depletion of Dp53 accelerates consumption of major energy stores and reduces survival rates of adult flies. We show that Dp53 is regulated by the microRNA (miRNA) machinery and miR-305 in a nutrition-dependent manner. In well-fed animals, TOR signaling contributes to miR-305-mediated inhibition of Dp53. Nutrient deprivation reduces the levels of miRNA machinery components and leads to Dp53 derepression. Our results uncover an organism-wide role for Dp53 in nutrient sensing and metabolic adaptation and open up avenues toward understanding the molecular mechanisms underlying p53 activation under nutrient deprivation.

p53 activity is selectively licensed in the Drosophila stem cell compartment

Oncogenic stress provokes tumor suppression by p53 but the extent to which this regulatory axis is conserved remains unknown. Using a biosensor to visualize p53 action, we find that Drosophila p53 is selectively active in gonadal stem cells after exposure to stressors that destabilize the genome. Similar p53 activity occurred in hyperplastic growths that were triggered either by the Ras^V12 oncoprotein or by failed differentiation programs. In a model of transient sterility, p53 was required for the recovery of fertility after stress, and entry into the cell cycle was delayed in p53^- stem cells. Together, these observations establish that the stem cell compartment of the Drosophila germline is selectively licensed for stress-induced activation of the p53 regulatory network. Furthermore, the findings uncover ancestral links between p53 and aberrant proliferation that are independent of DNA breaks and predate evolution of the ARF/Mdm2 axis.

DOI:http://dx.doi.org/10.7554/eLife.01530.001

The most common genetic change seen in cancer patients produces a faulty version of the p53 protein, which normally restricts tissue growth. This change promotes cancer because cells can now divide faster and fail to die when they should. Much remains to be learned about how p53 functions to restrain growth. As p53 is found in primitive organisms, and cancer is unlikely to have significantly influenced evolution, suppressing tumor formation was almost certainly not the original function of this gene. Furthermore, p53 works in a different way compared to many other tumour suppressors. Therefore, prevention of cancer is likely to have evolved as a side effect derived from more ancient functions.

Recently, a link between p53 and stem cells has been uncovered. Stem cells are special because they can develop into many different types of cells, and they are crucial for the growth and repair of tissues. To form a particular type of cell, the stem cell divides to create two daughter cells. Commonly, one daughter cell stays in the stem state, whereas the other becomes a particular type of cell, such as a nerve cell or muscle cell. Because of this special property, scientists hypothesize that stem cells have special mechanisms to protect them from DNA damage that might partially depend on p53. This would prevent the spread of damaged genomes that would otherwise occur among daughter cells.

To learn more about how p53 influences stem cells, Wylie, Lu et al. monitored its activity in the gonads of fruit flies, which are a powerful genetic model. They found that damaging DNA activates p53 in stem cells and their daughter cells, but not in other types of cells that have been damaged. In addition, p53 is activated by the uncontrolled growth and division of stem cells in the gonad, even when DNA is not damaged. This is unexpected since molecules linking inappropriate growth to p53 were thought to be present only in mammals. Therefore, it appears that the tumor-suppressing behavior of p53 in mammals was adapted from its more ancient ability to regulate stem cell growth—an ability that evolved before organisms divided into vertebrates and invertebrates.

DOI:http://dx.doi.org/10.7554/eLife.01530.002

Inhibitors of the p53-Mdm2 interaction increase programmed cell death and produce abnormal phenotypes in the placozoon Trichoplax adhaerens (F.E. Schulze)

Recent identification of genes homologous to human p53 and Mdm2 in the basal phylum Placozoa raised the question whether the network undertakes the same functions in the most primitive metazoan organism as it does in more derived animals. Here, we describe inhibition experiments on p53/Mdm2 interaction in Trichoplax adhaerens by applying the inhibitors nutlin-3 and roscovitine. Both inhibitors had a strong impact on the animals' survival by significantly increasing programmed cell death (cf. apoptosis, measured via terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling assay). Treatment with roscovitine decreased cell proliferation (visualized by means of bromodeoxyuridine incorporation), which is likely reducible to its function as cyclin-dependent kinase inhibitor. Obvious phenotypic abnormalities have been observed during long-term application of both inhibitors, and either treatment is highly lethal in T. adhaerens. The findings of this study suggest a conserved role of the p53/Mdm2 network for programmed cell death since the origin of the Metazoa and advocate the deployment of Placozoa as a model for p53, apoptosis, and possibly cancer research.

Chk2 and p53 regulate the transmission of healed chromosomes in the Drosophila male germline

When a dicentric chromosome breaks in mitosis, the broken ends cannot be repaired by normal mechanisms that join two broken ends since each end is in a separate daughter cell. However, in the male germline of Drosophila melanogaster, a broken end may be healed by de novo telomere addition. We find that Chk2 (encoded by lok) and P53, major mediators of the DNA damage response, have strong and opposite influences on the transmission of broken-and-healed chromosomes: lok mutants exhibit a large increase in the recovery of healed chromosomes relative to wildtype control males, but p53 mutants show a strong reduction. This contrasts with the soma, where mutations in lok and p53 have the nearly identical effect of allowing survival and proliferation of cells with irreparable DNA damage. Examination of testes revealed a transient depletion of germline cells after dicentric chromosome induction in the wildtype controls, and further showed that P53 is required for the germline to recover. Although lok mutant males transmit healed chromosomes at a high rate, broken chromosome ends can also persist through spermatogonial divisions without healing in lok mutants, giving rise to frequent dicentric bridges in Meiosis II. Cytological and genetic analyses show that spermatid nuclei derived from such meiotic divisions are eliminated during spermiogenesis, resulting in strong meiotic drive. We conclude that the primary responsibility for maintaining genome integrity in the male germline lies with Chk2, and that P53 is required to reconstitute the germline when cells are eliminated owing to unrepaired DNA damage.

p53 is involved in shrimp survival via its regulation roles on MnSOD and GPx in response to acute environmental stresses

The tumor suppressor gene p53 plays a critical role in safeguarding the integrity of genome in mammalian cells. It acts as a sequence-specific transcription factor. Once activated by a variety of cellular stresses, p53 transactivates downstream target genes, through which it regulates cell cycle and apoptosis. However, little is known about p53 as well as its downstream target genes in invertebrates. A full length cDNA that encodes a 453-amino-acid p53 protein (Lvp53) was characterized in the Pacific white shrimp (Litopenaeus vannamei) to explore the potential relationships between p53 and two antioxidant enzyme genes: Mn-superoxide dismutase (MnSOD) and glutathione peroxidase (GPx) in eliminating cell stresses in L. vannamei. Sequence analysis revealed a close phylogenetic relationship between Lvp53 and that of Marsupenaeus japonicus, and a high degree of conservation in critical amino acids residues is involved in DNA and zinc binding among species. Quantitative real-time PCR showed that Lvp53 was expressed with varied levels in all the 11 tissues under investigation. In response to acute pH challenge, the relative expression of Lvp53 was induced in a pH- and time-dependent manner, with the peak observed at pH 6.1 and after 24 h of treatment, in which condition, both the relative mRNA expressions and the enzymatic activities of LvMnSOD and LvGPx were increased correspondingly. In response to acute cadmium (Cd) exposure, the relative expression of Lvp53 was upregulated in a time- and concentration-dependent manner, with the maximum detected at Cd 6.6 μM and after 48 h of exposure, in which case, both the transcripts and the enzymatic activities of LvMnSOD and LvGPx were also induced. After Lvp53 transcripts were declined by double-strand RNA injection, the relative mRNA expressions of LvMnSOD and LvGPx were decreased correspondingly. Meanwhile, pH 6.1 or 6.6 μM Cd could not induce the transcripts or the enzymatic activities of LvMnSOD or LvGPx any more in Lvp53-silenced shrimp, but increased shrimp mortalities. These results indicated the involvement of Lvp53, LvMnSOD and LvGPx in mediating cell stress caused by suboptimal pH and elevated levels of Cd in L. vannamei, and that the expressions of LvMnSOD and LvGPx were positively regulated by Lvp53, which is a potential mechanism for shrimp to survive the oxidative stress that occurs during short-term exposure to Cd or challenge with acidic pH. This finding will contribute to better understanding of p53 signaling pathways and redox regulation in invertebrate organisms.

Aneuploidy, cell delamination and tumorigenesis in Drosophila epithelia

Chromosomal instability (CIN) is a common feature in human cancer, and highly aneuploid tumors are frequently associated with poor prognosis; however, the molecular and cellular mechanisms underlying CIN-induced tumorigenesis are poorly understood. Here we review recent findings about the role of CIN in driving tumor-like growth and host invasiveness in Drosophila epithelia and discuss the commonalities of CIN-induced tumors with other Drosophila-based cancer models. We also discuss possible scenarios that can account for the participation of CIN in tumorigenesis and propose that, alternatively to the classical role of aneuploidy in promoting the accumulation of mutations in cancer cells, aneuploidy can be a source of stress that may contribute to cancer initiation and/or progression.

Evaluating Drosophila p53 as a model system for studying cancer mutations

The transcription factor p53 is a key tumor suppressor protein. In about half of human cancers, p53 is inactivated directly through mutation in its sequence-specific DNA-binding domain. Drosophila p53 (Dmp53) has similar apoptotic functions as its human homolog and is therefore an attractive model system for studying cancer pathways. To probe the structure and function of Dmp53, we studied the effect of point mutations, corresponding to cancer hot spot mutations in human p53 (Hp53), on the stability and DNA binding affinity of the full-length protein. Despite low sequence conservation, the Hp53 and Dmp53 proteins had a similar melting temperature and generally showed a similar energetic and functional response to cancer-associated mutations. We also found a correlation between the thermodynamic stability of the mutant proteins and their rate of aggregation. The effects of the mutations were rationalized based on homology modeling of the Dmp53 DNA-binding domain, suggesting that the drastically different effects of a cancer mutation in the loop-sheet-helix motif (R282W in Hp53 and R268W in Dmp53) on stability and DNA binding affinity of the two proteins are related to conformational differences in the L1 loop adjacent to the mutation site. On the basis of these data, we discuss the advantages and limitations of using Dmp53 as a model system for studying p53 function and testing p53 rescue drugs.