Gene name - E2F transcription factor 1
Cytological map position - 93E8--93E9
Function - transcription factor
Keywords - cell cycle
Symbol - E2f1
Genetic map position -
Classification - multifunctional
Cellular location - nuclear
|Recent literature||Bradley-Gill, M.R., Kim, M., Feingold, D., Yergeau, C., Houde, J. and Moon, N.S. (2016). Alternate transcripts of the Drosophila "activator" E2F are necessary for maintenance of cell cycle exit during development Dev Biol [Epub ahead of print]. PubMed ID: 26859702
The E2F family of transcription factors are evolutionarily conserved regulators of the cell cycle that can be divided into two groups based on their ability to either activate or repress transcription. In Drosophila, there is only one "activator" E2F, dE2F1, which provides all of the pro-proliferative activity of E2F during development. Interestingly, the de2f1 gene can be transcribed from multiple promoters resulting in six alternate transcripts. This study investigated the biological significance of the alternate transcriptional start sites. The de2f1 promoter region shows tissue and cell-type specific enhancer activities at the larval stage. While a genomic deletion of this region, de2f1ΔRA, decreases the overall expression level of dE2F1, flies develop normally with no obvious proliferation defects. However, a detailed analysis of the de2f1ΔRA mutant eye imaginal discs revealed that dE2F1 is needed for proper cell cycle exit. dE2F1 expression during G1 arrest prior to the differentiation process of the developing eye is important for maintaining cell cycle arrest at a later stage of the eye development. Overall, this study suggests that specific alternate transcripts of "activator" E2F, dE2F1, may have a dual function on cell cycle progression and cannot simply be viewed as a pro-proliferative transcription factor.
|Zappia, M.P. and Frolov, M.V. (2016). E2F function in muscle growth is necessary and sufficient for viability in Drosophila. Nat Commun 7: 10509. PubMed ID: 26823289
The E2F transcription factor is a key cell cycle regulator. However, the inactivation of the entire E2F family in Drosophila is permissive throughout most of animal development until pupation when lethality occurs. This study shows that E2F function in the adult skeletal muscle is essential for animal viability since providing E2F function in muscles rescues the lethality of the whole-body E2F-deficient animals. Muscle-specific loss of E2F results in a significant reduction in muscle mass and thinner myofibrils. It was demonstrated that E2F is dispensable for proliferation of muscle progenitor cells, but is required during late myogenesis to directly control the expression of a set of muscle-specific genes. Interestingly, E2f1 provides a major contribution to the regulation of myogenic function, while E2f2 appears to be less important. These findings identify a key function of E2F in skeletal muscle required for animal viability, and illustrate how the cell cycle regulator is repurposed in post-mitotic cells.
|Flegel, K., Grushko, O., Bolin, K., Griggs, E. and Buttitta, L. (2016). The role of the histone modifying and exchange complex NuA4 in cell cycle progression in Drosophila melanogaster. Genetics [Epub ahead of print]. PubMed ID: 27184390
Robust and synchronous repression of E2F-dependent gene expression is critical to the proper timing of cell cycle exit when cells transition to a post-mitotic state. Previously histone Modifying and Exchange Complex NuA4 was suggested to act as a barrier to proliferation in Drosophila, by repressing E2F-dependent gene expression. This study shows that NuA4 activity is required for proper cell cycle exit and the repression of cell cycle genes during the transition to a post-mitotic state in vivo However, the delay of cell cycle exit caused by compromising NuA4 is not due to additional proliferation or effects on E2F activity. Instead NuA4 inhibition results in slowed cell cycle progression through late S and G2 phases due to aberrant activation of an intrinsic p53-independent DNA damage response. A reduction in NuA4 function ultimately produces a paradoxical cell cycle gene expression program, where certain cell cycle genes become de-repressed in cells that are delayed during the G2 phase of the final cell cycle. Bypassing the G2 delay when NuA4 is inhibited leads to abnormal mitoses and results in severe tissue defects. NuA4 physically and genetically interacts with components of the E2F complex termed DREAM/MMB (Rbf, E2F and Myb/Multi-vulva class B), and modulates a DREAM/MMB-dependent ectopic neuron phenotype in the posterior wing margin. However, this effect is also likely due to the cell cycle delay, as simply reducing Cdk1 is sufficient to generate a similar phenotype. This work reveals that the major requirement for NuA4 in the cell cycle in vivo is to suppress an endogenous DNA damage response, which is required to coordinate proper S and G2 cell cycle progression with differentiation and cell cycle gene expression.
|Hinnant, T. D., Alvarez, A. A. and Ables, E. T. (2017). Temporal remodeling of the cell cycle accompanies differentiation in the Drosophila germline. Dev Biol 429(1): 118-131. PubMed ID: 28711427
During Drosophila oogenesis, mature oocytes are created through a series of precisely controlled division and differentiation steps, originating from a single tissue-specific stem cell. To describe how the cell cycle is remodeled in germ cells as they differentiate in situ, the Drosophila Fluorescence Ubiquitin-based Cell Cycle Indicator (Fly-FUCCI) system was used, in which degradable versions of GFP::E2f1 and RFP::CycB fluorescently label cells in each phase of the cell cycle. The lengths of the G1, S, and G2 phases of the cell cycle were found to change dramatically over the course of differentiation, and the 4/8-cell cyst was identified as a key developmental transition state in which cells prepare for specialized cell cycles. The data suggest that the transcriptional activator E2f1, which controls the transition from G1 to S phase, is a key regulator of mitotic divisions in the early germline. These data support the model that E2f1 is necessary for proper GSC proliferation, self-renewal, and daughter cell development. In contrast, while E2f1 degradation by the Cullin 4 (Cul4)-containing ubiquitin E3 ligase (CRL4) is essential for developmental transitions in the early germline, the data do not support a role for E2f1 degradation as a mechanism to limit GSC proliferation or self-renewal. Taken together, these findings provide further insight into the regulation of cell proliferation and the acquisition of differentiated cell fate, with broad implications across developing tissues.
|Zhang, P., Pei, C., Wang, X., Xiang, J., Sun, B. F., Cheng, Y., Qi, X., Marchetti, M., Xu, J. W., Sun, Y. P., Edgar, B. A. and Yuan, Z. (2017). A balance of Yki/Sd activator and E2F1/Sd repressor complexes controls cell survival and affects organ size. Dev Cell 43(5): 603-617.e605. PubMed ID: 29207260
The Hippo/Yki and RB/E2F pathways both regulate tissue growth by affecting cell proliferation and survival, but interactions between these parallel control systems are poorly defined. This study demonstrates that interaction between Drosophila E2F1 and Sd disrupts Yki/Sd complex formation and thereby suppresses Yki target gene expression. RBF modifies these effects by reducing E2F1/Sd interaction. This regulation has significant effects on apoptosis, organ size, and progenitor cell proliferation. Using a combination of DamID-seq and RNA-seq, this study identified a set of Yki targets that play a diversity of roles during development and are suppressed by E2F1. Further, it was found that human E2F1 competes with YAP for TEAD1 binding, affecting YAP activity, indicating that this mode of cross-regulation is conserved. In sum, this study uncovers a previously unknown mechanism in which RBF and E2F1 modify Hippo signaling responses to modulate apoptosis, organ growth, and homeostasis.
|Kim, M., Tang, J. P. and Moon, N. S. (2018). An alternatively spliced form affecting the Marked Box domain of Drosophila E2F1 is required for proper cell cycle regulation. PLoS Genet 14(2): e1007204. PubMed ID: 29420631
Across metazoans, cell cycle progression is regulated by E2F family transcription factors that can function as either transcriptional activators or repressors. For decades, the Drosophila E2F family has been viewed as a streamlined RB/E2F network, consisting of one activator (dE2F1) and one repressor (dE2F2). This study reports that an uncharacterized isoform of dE2F1, hereon called dE2F1b, plays an important function during development and is functionally distinct from the widely-studied dE2F1 isoform, dE2F1a. dE2F1b contains an additional exon that inserts 16 amino acids to the evolutionarily conserved Marked Box domain. Analysis of de2f1b-specific mutants generated via CRISPR/Cas9 indicates that dE2F1b is a critical regulator of the cell cycle during development. This is particularly evident in endocycling salivary glands in which a tight control of dE2F1 activity is required. Interestingly, close examination of mitotic tissues such as eye and wing imaginal discs suggests that dE2F1b plays a repressive function as cells exit from the cell cycle. Evidence is also provided demonstrating that dE2F1b differentially interacts with RBF1 and alters the recruitment of RBF1 and dE2F1 to promoters. Collectively, these data suggest that dE2F1b is a novel member of the E2F family, revealing a previously unappreciated complexity in the Drosophila RB/E2F network.
The E2F protein is a critical component for normal cell cycle regulation. As a transcription factor, it positively regulates many of the genes required for initiation of S phase (the DNA synthetic phase). The complexity of E2F regulation is fairly well understood in human biology, while the availability of mutations makes it a prime target for investigation in Drosophila.
Drosophila E2F mutants complete early cell cycles, using maternal gene products, but DNA synthesis fails in cycle 17. Instead of undergoing their normal proliferation during a rapid cycle lasting about 40 minutes, cell cycles in the CNS of E2F mutants gradually decrease DNA synthesis after stage 12. Abdominal histoblasts also cease DNA replication in E2F mutants. Messenger RNAs coding for proteins required for DNA synthesis disappear in E2F mutants (Duronio, 1995a).
Interactions with cyclin E are especially complex. Cyclin E, which is the regulatory subunit of the cyclin E/cdc2c kinase heterodimer, regulates entry in S by phosphorylation of target substrates. Cyclin E expression at G1-S requires E2F. Activation of E2F without cyclin E is not sufficient for S phase. Early in G1, ectopic expression of cyclin E alone can bypass E2F and induce S phase. Thus cyclin E is a downstream target of E2F, coupling E2F activity to G1 control (Duronio, 1995b).
How is E2F regulated? In mammalian systems E2F is held inactive by the retinoblastoma (Rb) family of pocket proteins (See Drosophila Retinoblastoma-family protein). Hypophosphorylated Rb can interact with E2F during G1. This complex can bind to DNA and repress transcription of E2F target genes. A chain of events leads first to sequestration and ultimately the release of E2F. The sequestering of E2F is actively carried out by Rb family members until Rb is inactivated by hyperphosphorylation, carried out by cyclin dependent kinases. At this point, E2F is released allowing it to activate genes required for DNA synthesis and entry into S (Zhu, 1995 a and b).
An additional factor conspires to hold E2F inactive in G1 phase: the association of E2F with p53. As a consequence of its ability to physically associate with E2F, the expression of wild-type p53 can inhibit transcriptional activation by E2F. The expression of both E2F1 and DP1 (E2F's dimerization partner) can also downregulate p53-dependent transcriptional activation (O'Conner, 1995).
Equally important as the G1 phase regulation of E2F is the inactivation of E2F during S phase, when it is no longer required for gene activation. In mammalian systems the inactivation of E2F is again carried out by cyclin A dependent kinase activity. Just as there is a physical association of E2F with the Retinoblastoma protein in G1, there is an association of cyclin A with E2F in S phase (Kitagawa, 1995). These G1 and S phase interactions of E2B have yet to be documented in Drosophila. In Drosophila cyclin A is not synthesized until G2, too late to inactivate E2F in S phase.
E2F is capable of driving the G1-S transition of the cell cycle. However, mice in which the E2F-1 gene has been disrupted develop tumors, suggesting a negative role for E2F in controlling cell proliferation in some tissues. The consequences of disrupting the DP genes have not been reported. A screen was carried out for mutations that disrupt G1-S transcription late in Drosophila embryogenesis and five mutations in the dDP gene (DP transcription factor) were identified. Sequencing of dDP reveals the presence of several important motifs, including the DNA-binding region, the DEF box that is predicted to be required for DP/E2F heterodimerization, and three other highly homologous regions named DP-conserved box 1 (DCB1), DCB2, and negatively charged box (NCB). Although mutations in dDP or dE2F nearly eliminate E2F-dependent G1-S transcription, S-phase still occurs. Cyclin E has been shown to be essential for S-phase in late embryogenesis, but in dDP and dE2F mutants the peaks of G1-S transcription of cyclin E are missing. Thus, greatly reduced levels of cyclin E transcript suffice for DNA replication until late in development. Both dDP and dE2F are necessary for viability, and mutations in the genes cause lethality at the late larval/pupal stage. The mutant phenotypes reveal that both genes promote progression of the cell cycle (Royzman, 1997).
Although the dE2F mutant animals survive through larval life, a dramatic delay is observed in larval growth. It takes between 288 and 432 hr for the dE2F mutant larvae to pupate, compared to 120 hr for heterozygous sibling controls. Five days after egg laying (AEL) the dE2F mutant larvae are very sluggish and much smaller in size than their wild-type counterparts. The polytene salivary gland and diploid imaginal discs can not be identified in the 5-day-old dE2F mutant larvae, presumably because they are so small. The brains are also greatly reduced in size as compared to wild type. The size of the dE2F mutant larvae increases over time, and the internal tissues approached wild-type size. Therefore, DNA replication can occur during this larval period, but it is slow. Replication in the absence of dE2F is further evidenced by the formation of banded polytene salivary gland chromosomes in some of the 12- to 18-day larvae. Although the polytene chromosomes from the dE2F mutant larvae are smaller and more fragile than normal they are clearly visible. Thus, it is concluded that S phase occurs in the absence of dE2F, but dE2F is necessary for timely replication and growth. In addition to the growth delay, the dE2F mutant larvae had another striking phenotype: melanotic pseudotumors are formed. Melanotic tumors are groups of cells within the larvae that are recognized by the immune system and encapsulated in melanized cuticle. They are referred to as pseudotumors to emphasize that they are not necessarily the consequence of hyperproliferation but can be abnormal cells recognized by the immune cells. Small pseudotumors were first observed in the dE2F mutants 7 days AEL, and these early pseudotumors grow and darken as the larvae age. In the dE2F mutants that initiate pupation, numerous additional small pseudotumors form (Royzman, 1997).
Approximately half of the dDP mutant pupae reach adulthood in the pupal case. These adults struggled to eclose but ultimately die. Organisms dissected from the pupal case have essentially normal heads and thoraxes. However, their abdominal defects are severe. This is informative as the head and thorax are derived from imaginal discs, whereas the abdomen arises from the abdominal histoblast nests. The imaginal discs proliferate during larval stages, but the abdominal histoblast nests proliferate during pupal development. Thus, pupal lethality may result from a defect in abdomen formation that occurs during pupal development. Having shown that heat shock dDP rescues the dDP mutants, the developmental period during which ectopic dDP expression is capable of rescuing the lethality of the dDP mutants was defined. Ectopic expression of dDP results in 100% rescue of dDP mutant animals. Thus, the late lethality of dDP mutants is not a manifestation of a defect in the early development of the organism, but rather it stems from defects in larval/pupal life (Royzman, 1997).
The striking observation from the Drosophila dDP and dE2F mutants is that although cyclic transcription of cyclin E, PCNA, and ribonucleotide reductase 2 (RNR2) is not detectable, S phase still occurs. Although the possibility that cyclic transcription of these genes occurs at a low level driven by maternal pools of dDP and dE2F cannot be excluded, the bursts of transcription that normally precede S phase are not essential for the G1-S transition. In these mutants the cell cycle may be driven by basal levels of transcripts and post-transcriptional regulation. The maternal pools of components of the replication machinery can persist until late in development, as evidenced by the fact that mutations in PCNA and MCM2 cause late larval lethality (Royzman, 1997 and references).
E2F positively regulates many of the genes required for initiation of S phase (the DNA synthetic phase). In mammals, the tumor suppressor RB interacts with, and negatively regulates, E2F, but it is not clear whether the function of pRB is solely mediated by E2F. In addition, E2F has been shown to mediate both transcription activation and repression; it remains to be tested which function of E2F is critical for normal development. Drosophila homologs of the RB and E2F family of proteins Rbf and E2f have been identified. The genetic interactions between Rbf and E2f were analyzed during Drosophila development, and the results show that Rbf is required at multiple stages of development. Unexpectedly, Rbf null mutants can develop until late pupae stage when the activity of E2f is experimentally reduced, and can develop into viable adults with normal adult appendages in the presence of an E2f mutation that retains the DNA binding domain but lacks the transactivation domain. These results indicate that most, if not all, of the function of Rbf during development is mediated through E2f. In turn, the genetic interactions shown here also suggest that E2f functions primarily as a transcription activator rather than a co-repressor of Rbf during Drosophila development. Analysis of the expression of an E2F target gene Pcna in eye discs shows that the expression of PCNA is activated by E2f in the second mitotic wave and repressed in the morphogenetic furrow and posterior to the second mitotic wave by Rbf. Interestingly, reducing the level of Rbf restores the normal pattern of cell proliferation in E2f mutant eye discs but not the expression of E2f target genes, suggesting that the coordinated transcription of E2f target genes does not significantly affect the pattern of cell proliferation (Du, 2000).
Given that E2f is just one of many potential targets of Rbf, the dramatic suppression of the rbf mutant phenotypes by E2f mutants is very unexpected. (1) Lowering the activity of E2f can suppress the early larval lethality of the rbf mutants as well as the developmental phenotypes observed in the adult eyes and bristles. (2) An allele of E2f with an intact DNA binding domain but with no transactivation domain or Rbf binding domain can suppress the lethality of rbf null mutants, allowing the double mutant flies to develop into viable adults. Furthermore, these suppressed rbf null adults show normal adult structures. Thus the uninhibited E2f in rbf mutants mediates both the lethality as well as the observed eye and bristle phenotypes in adults. These observations provide strong evidence that E2F mediates most, if not all, of the phenotypes of rbf during development (Du, 2000).
Interestingly, lowering the activity of Rbf can also partially suppress the E2f null phenotypes. There are at least two possible explanations for this observation. One possibility is that Rbf has a function downstream of E2f; the other possibility is that Rbf can affect the E2f mutant phenotypes through a parallel pathway (for example Rbf may be able to regulate the expression of E2f target genes through another target such as dE2F2). The second explanation seems to be more likely for the following reasons: (1) in the suppressed E2f null mutants, the expression of E2f target genes is not restored, and the larvae growth is still greatly retarded, suggesting that reducing the level of Rbf bypasses rather than restores the function lost by E2f mutation; (2) the rbf null mutant phenotype is fully suppressed by an E2f mutant that lacks transcription activation and an Rbf binding domain, demonstrating that Rbf functions upstream of E2f. In summary, these results do not point to a role for Rb downstream of E2f. In contrast, loss of Rbf indeed causes deregulation of the expression of PCNA even in the absence of transcription activation by E2f, supporting the notion that Rbf can regulate the expression of PCNA by targets other than E2f (Du, 2000).
E2F transcription factors can function both to activate transcription and to repress transcription by recruiting RB family proteins to specific promoters. Although analysis of E2f mutants shows that E2f is required for the coordinated expression of replication functions such as PCNA and Ribonucleoside diphosphate reductase small subunit (RnrS), it is not clear whether the lack of transcription of these set of genes is the cause of the larval lethality. It is formally possible that the lethality of E2f mutant is caused by the failure to repress certain critical E2f target genes. Depending on the function of E2f as a transcription activator or a co-repressor of Rbf, completely different predications are expected regarding the genetic interaction between Rbf and E2f. The observation that lowering the level of Rbf can suppress the larval lethality of E2f mutants and allow the E2f mutants to develop into pharate adults, suggests that during Drosophila development, the function of E2f is mainly to activate transcription and not to recruit Rbf to repress transcription (Du, 2000).
In addition to the first E2f to be identified in Drosophila, a second, termed E2f2, has been identified (Sawado, 1998). Interestingly, E2f2 can bind to E2F binding sites, but the function of E2f2 appears to be distinct from that of E2f. Cotransfection of E2f2 represses the expression from the PCNA gene promoter while cotransfection of E2f activates the expression (Sawado, 1998). Similar findings are also observed in transfection experiments in which E2f strongly activates transcription, while E2f2 fails to activate a reporter with E2F binding sites. These results suggest that E2f2 may function mainly to repress transcription while E2f functions mainly to activate transcription. Taken together, these data suggest a model for the function of Rbf, E2f and E2f2. In this model, E2f functions mainly to activate transcription of the E2f target genes. Rbf negatively regulates the activity of E2f to inhibit the expression of E2f target genes. In addition, Rbf can also repress the expression of E2F targets genes through other targets of Rbf such as E2f2. Thus the expression of E2F target genes will have three different states: activated, when there is free E2f/Dp to activate transcription; repressed, when there is E2f2/Dp/Rbf (and possibly E2f/Dp/Rbf) to repress transcription; and basal, when there is neither activation nor repression. At present, it is not clear whether E2f also has a function to repress transcription during development, nor is it is clear about the function of free E2f2/Dp (Du, 2000).
Precise control of cell cycle regulators is critical for normal development and tissue homeostasis. E2F transcription factors are activated during G1 to drive the G1-S transition and are then inhibited during S phase by a variety of mechanisms. The single Drosophila activator E2F (E2f1) was genetically manipulate to explore the developmental requirement for S phase-coupled E2F down-regulation. Expression of an E2f1 mutant that is not destroyed during S phase drives cell cycle progression and causes apoptosis. Interestingly, this apoptosis is not exclusively the result of inappropriate cell cycle progression, because a stable E2f1 mutant that cannot function as a transcription factor or drive cell cycle progression also triggers apoptosis. This observation suggests that the inappropriate presence of E2f1 protein during S phase can trigger apoptosis by mechanisms that are independent of E2F acting directly at target genes. The ability of S phase-stabilized E2f1 to trigger apoptosis requires an interaction between E2f1 and the Drosophila pRb homolog, Rbf1, and involves induction of the pro-apoptotic gene, hid. Simultaneously blocking E2f1 destruction during S phase and inhibiting the induction of apoptosis results in tissue overgrowth and lethality. It is proposed that inappropriate accumulation of E2f1 protein during S phase triggers the elimination of potentially hyperplastic cells via apoptosis in order to ensure normal development of rapidly proliferating tissues (Davidson, 2012).
Thus stabilizing the single Drosophila activator E2f1 in S phase results in apoptosis is necessary to prevent hypertrophy of wing imaginal discs. It is concluded from these data that hyper-accumulation of E2f1 during S phase represents a form of proliferative stress during development that is sensed by the apoptotic machinery and results in the elimination of cells with excess E2f1 activity to maintain homeostasis during tissue growth (Davidson, 2012).
What might be the function of a Drosophila cell's ability to detect abnormal accumulation of E2f1 protein during S phase and subsequently trigger apoptosis? One possibility is that accumulation of E2f1 during S phase resembles instances of abnormally high E2f1 activity that might occur sporadically during rapid growth of a population of precursor cells such as those in the wing imaginal disc. These events could be caused by stochastic or even genetic changes that affect either E2f1 gene transcription or the ability of the CRL4Cdt2/PCNA pathway to destroy E2f1 after replication factor genes are activated in late G1. The cell's ability to detect E2f1 accumulation in S phase clears these potentially hyperplastic cells from developing tissues via apoptosis, consequently contributing to the balance between cell proliferation and cell death that is necessary for normal tissue growth (Davidson, 2012).
Growing Drosophila imaginal discs possess another mechanism of homeostasis in which a process of compensatory proliferation is activated in order to achieve normal tissue development when as many as 50% of cells are killed by external stimuli like radiation-induced DNA damage. Indeed, in spite of high levels of apoptosis (15% of the cells), 50% of en-Gal4>E2f1Stable progeny survive until adulthood with about 2/3 of these surviving flies containing wings with somewhat mild morphological defects. Blocking apoptosis with baculovirus p35 when E2f1Stable is expressed shifts the cell proliferation/apoptosis balance too strongly in favor of cell proliferation, resulting in massive hypertrophy and 100% lethality (Davidson, 2012).
p35 is a broad caspase inhibitor that blocks effector caspase activity at a step downstream of their proteolytic activation. Therefore, cells expressing p35 can initiate apoptosis, but lack the capacity to complete it and are referred to as 'undead cells.' These undead cells produce signals that stimulate unaffected neighboring cells to proliferate. Thus, the dramatic hypertrophy seen in E2f1Stable/p35 wing discs might be the result of two synergizing growth signals: hyper-active E2f1 and compensatory proliferation from undead cells. The current experiments cannot precisely discern the relative contribution of these two inputs, but E2f1 activity appears to make a larger contribution because E2f1Stable/DBD Mut expression does not cause dramatic overgrowth (Davidson, 2012).
What might explain the 32% of en-Gal4>E2f1Stable discs that displayed a reduced posterior compartment rather than an overgrown one? The DNA damage observed in eye discs experiments provides a possible answer. Perhaps early in development the 'arrest' class of wing discs sustained enough genomic damage to prevent proliferation, resulting in too small a pool of cells that could respond to the hyper-active E2f1 and undead cell signals to support disc overgrowth. Thus, the wide range of phenotypes that were observed in E2f1Stable/p35 wing discs may result from multiple influences that act stochastically within the population (Davidson, 2012).
Because endogenous E2f1 is quantitatively destroyed only in S phase, the relative amount of hyper-accumulation of E2f1Stable is greater during S phase than during any other stage of the cell cycle. Therefore, one possibility is that E2f1Stable-induced phenotypes result from the stability of E2f1 protein in S phase, and not from general over-expression throughout the cell cycle. Failure to detect E2f1Stable induced apoptosis in G1-arrested embryonic cells is consistent with this possibility. However, another difference between these embryonic cells and wing discs cells is that the former are cell cycle arrested and the latter are continuingly proliferating during larval development. Thus, another possibility is that S phase-destruction of E2f1 modulates the levels of E2f1 in proliferating cells, and cells that fail to destroy E2f1 during S phase have an increased chance of activating apoptosis at any point in the cell cycle. In either model, S phase E2f1 destruction is not essential for proliferation per se. In marked contrast, E2f1Stable expression blocks endocycle progression, suggesting that knocking in E2f1Stable to the endogenous locus would be lethal during development, perhaps even dominant lethal (Davidson, 2012).
E2f1Stable induces apoptosis at least in part through expression of the pro-apoptotic gene hid. Surprisingly, these events still occur after expression of an E2f1Stable variant that cannot bind DNA and therefore lacks the ability to stimulate transcription of replication factor genes or cell cycle progression. Instead, E2f1Stable requires the ability to bind Rbf1 to induce hid gene expression and apoptosis. Genetic disruption of Rbf1 is well known to result in increased hid expression. It is therefore proposed that the inappropriate accumulation of E2f1 in S phase disrupts some aspect of Rbf1 function leading to hid expression and apoptosis (Davidson, 2012).
The data do not discern either the function of Rbf1 that is disrupted by E2f1Stable or the mechanism of hid induction. While the mechanism connecting Rbf1/E2f1 function and hid may be indirect, some studies suggest that Rbf1 and/or E2f1 could regulate hid directly. It has been demonstrated that Drosophila wing disc cells undergo apoptosis in response to ionizing radiation independently of p53 and that this response requires E2f1 and is triggered by hid expression. In eye discs, loss of Rbf1 function in the MF results in apoptosis that requires E2f1 transactivation function and is accompanied by hid expression. However, whether these effects represent a direct induction of hid by E2f1 is not clear. E2f1 binding at the hid locus has been observed, but the binding site is located ~1.4 kb upstream of the of the start of hid transcription, which is more distal than in well characterized E2F-regulated promoters. When located this far upstream the hid E2f1 binding site fails to activate gene expression in S2 cell reporter assays. hid is also a target of p53, and so any DNA damage resulting from stabilizing E2f1 during S phase, as was observed in eye discs, may also contribute to the activation of hid expression via p53-mediated DNA damage response pathways (Davidson, 2012).
Another possibility is that E2f1, in combination with Rbf1, plays primarily a repressive role at the hid locus. In this model, the result that E2f1Stable or E2f1Stable/DBD Mut both induce apoptosis would be explained by disruption of Rbf1/E2f1 repressive complexes at the hid locus causing de-repression of hid expression. This model has interesting caveats: what protects the Rbf1/E2f1 complex at the hid locus from being disrupted by Cyclin E/Cdk2, which is active during S phase and inactivates Rbf1-mediated repression of E2f1, or by CRL4Cdt2-mediate destruction of E2f1? Recent data indicate that the dREAM/MMB complex is required for the stability of E2F/Rbf1 repressive complexes in S phase, and acts to protect these complexes from CDK-mediated phosphorylation at non-cell cycle-regulated genes. While there is yet no evidence that dREAM/MMB regulates hid , this work provides precedent for gene specific Rbf1 regulation during S phase (Davidson, 2012).
Finally, while hid might be a critical player in the response to E2f1Stable, there are likely other mechanisms responsible for sensing and modulating the apoptotic response to E2f1 levels. For instance, it has been demonstrated that a micro-RNA, mir-11, which is located within the last intron of the Drosophila E2f1 gene, acts to dampen expression of pro-apoptotic E2f1 target genes following DNA damage. In this way, the normal controls of E2f1 gene expression modulate apoptosis. In addition, transgenic constructs lack the normal E2f1 3' UTR, which serves as a site for suppression of E2f1 expression by pumilio translational repressor complexes. Thus, several modes of E2f1 regulation have been bypassed via transgenic expression of E2f1Stable (Davidson, 2012).
The finding that stabilized Drosophila E2f1 can induce apoptosis independently of transcription and cell cycle progression parallels previous observations made in mammalian cells, albeit with important differences. In mammalian cells, E2F1 can induce apoptosis independently of transcription and cell cycle progression, but apoptosis required E2F1 DNA binding activity, unlike in the current experiments. These studies suggested that DNA binding by E2F1 prevented pro-apoptotic promoters from binding repressor E2F family members (Davidson, 2012).
This comparison of results highlights the way similar phenotypic outcomes in different species can arise from different mechanisms. While mammalian activator E2Fs are also inhibited during S phase, they are not subject to CRL4Cdt2-mediated, S phase-coupled destruction like Drosophila E2f1. Instead, mammalian activator E2Fs are inhibited by direct Cyclin A/Cdk2 phosphorylation, targeted for destruction by SCFSkp2, and functionally antagonized by E2F7 and E2F8. The regulation provided by E2F7 and E2F8 is of particular note, as it is essential for mouse development. These atypical E2Fs homo and hetero-dimerize and act redundantly to repress E2F1 target genes independently of pRb family proteins, thus blocking E2F1 from inducing apoptosis. Moreover, the E2F7 and E2F8 genes are E2F1 targets, consequently creating a negative feedback loop that limits E2F1 activity after the G1/S transition. A similar negative feedback loop among factors that regulate G1/S transcription exists in yeast. The analogous Drosophila negative feedback loop involves E2f1 inducing its own destruction by stimulating Cyclin E transcription, which triggers S phase. Therefore, the evolution of eukaryotes has resulted in the use of different molecular mechanism to achieve negative feedback regulation of G1/S-regulated transcription, and in the case of activator E2Fs this regulation is essential for normal development (Davidson, 2012).
Bases 5' UTR -701
Bases in 3' UTR - 1261
Although Drosophila and human proteins share three regions with a high degree of homology, the fly protein is much larger, in part due to the insertion of a 300-aa block with a unique sequence between the two homologous blocks closest to the C-terminal. The Drosophila gene is equally related to each of the human family members. Amino acids 249-318 of the fly protein share a striking homology with DNA-binding domains of the human E2F genes. In addition, a region termed the "marked box" is highly conserved. This domain may function in protein dimerization. The C-terminal Rb-binding domain has also been largely conserved in the fly protein, sharing 56% similarity to the human proteins (Dynlacht, 1994 and Ohtani, 1994).
date revised: 11 August 97
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