Mre11-Rad50-Nbs complex is required to cap telomeres during Drosophila embryogenesis

Drosophila ring chromosomes interact with sisters and homologs to produce anaphase bridges in mitosis

Ring chromosomes are known in many eukaryotic organisms, including humans. They are typically associated with a variety of maladies, including abnormal development and lethality. Underlying these phenotypes are anaphase chromatin bridges that can lead to chromosome loss, nondisjunction and breakage. By cytological examination of ring chromosomes in Drosophila melanogaster we identified five causes for anaphase bridges produced by ring chromosomes. Catenation of sister chromatids appears to be the most common cause and these bridges frequently resolve during anaphase, presumably by the action of topoisomerase II. Sister chromatid exchange and chromosome breakage followed by sister chromatid union also produce anaphase bridges. Mitotic recombination with the homolog was rare, but was another route to generation of anaphase bridges. Most surprising, was the discovery of homolog capture, where the ring chromosome was connected to its linear homolog in anaphase. We hypothesize that this is a remnant of mitotic pairing and that the linear chromosome is connected to the ring by multiple wraps produced through the action of topoisomerase II during establishment of homolog pairing. In support, we showed that in a ring/ring homozygote the two rings are frequently catenated in mitotic metaphase, a configuration that requires breaking and rejoining of at least one chromosome.

Drosophila ring chromosomes interact with sisters and homologs to produce anaphase bridges in mitosis

Ring chromosomes are known in many eukaryotic organisms, including humans. They are typically associated with a variety of maladies, including abnormal development and lethality. Underlying these phenotypes are anaphase chromatin bridges that can lead to chromosome loss, nondisjunction and breakage. By cytological examination of ring chromosomes in Drosophila melanogaster we identified five causes for anaphase bridges produced by ring chromosomes. Catenation of sister chromatids is the most common cause and these bridges frequently resolve during anaphase, presumably by the action of topoisomerase II. Sister chromatid exchange and chromosome breakage followed by sister chromatid union also produce anaphase bridges. Mitotic recombination with the homolog was rare, but was another route to generation of anaphase bridges. Most surprising, was the discovery of homolog capture, where the ring chromosome was connected to its linear homolog in anaphase. We hypothesize that this is a remnant of mitotic pairing and that the linear chromosome is connected to the ring by multiple wraps produced through the action of topoisomerase II during establishment of homolog pairing. In support, we showed that in a ring/ring homozygote the two rings are frequently catenated in mitotic metaphase, a configuration that requires breaking and rejoining of at least one chromosome.

Telomere Checkpoint in Development and Aging

The maintenance of genome integrity through generations is largely determined by the stability of telomeres. Increasing evidence suggests that telomere dysfunction may trigger changes in cell fate, independently of telomere length. Telomeric multiple tandem repeats are potentially highly recombinogenic. Heterochromatin formation, transcriptional repression, the suppression of homologous recombination and chromosome end protection are all required for telomere stability. Genetic and epigenetic defects affecting telomere homeostasis may cause length-independent internal telomeric DNA damage. Growing evidence, including that based on Drosophila research, points to a telomere checkpoint mechanism that coordinates cell fate with telomere state. According to this scenario, telomeres, irrespective of their length, serve as a primary sensor of genome instability that is capable of triggering cell death or developmental arrest. Telomeric factors released from shortened or dysfunctional telomeres are thought to mediate these processes. Here, we discuss a novel signaling role for telomeric RNAs in cell fate and early development. Telomere checkpoint ensures genome stability in multicellular organisms but aggravates the aging process, promoting the accumulation of damaged and senescent cells.

Taming active transposons at Drosophila telomeres: The interconnection between HipHop's roles in capping and transcriptional silencing

Drosophila chromosomes are elongated by retrotransposon attachment, a process poorly understood. Here we characterized a mutation affecting the HipHop telomere-capping protein. In mutant ovaries and the embryos that they produce, telomere retrotransposons are activated and transposon RNP accumulates. Genetic results are consistent with that this hiphop mutation weakens the efficacy of HP1-mediated silencing while leaving piRNA-based mechanisms largely intact. Remarkably, mutant females display normal fecundity suggesting that telomere de-silencing is compatible with germline development. Moreover, unlike prior mutants with overactive telomeres, the hiphop stock does not over-accumulate transposons for hundreds of generations. This is likely due to the loss of HipHop’s abilities both to silence transcription and to recruit transposons to telomeres in the mutant. Furthermore, embryos produced by mutant mothers experience a checkpoint activation, and a further loss of maternal HipHop leads to end-to-end fusion and embryonic arrest. Telomeric retroelements fulfill an essential function yet maintain a potentially conflicting relationship with their Drosophila host. Our study thus showcases a possible intermediate in this arm race in which the host is adapting to over-activated transposons while maintaining genome stability. Our results suggest that the collapse of such a relationship might only occur when the selfish element acquires the ability to target non-telomeric regions of the genome. HipHop is likely part of this machinery restricting the elements to the gene-poor region of telomeres. Lastly, our hiphop mutation behaves as a recessive suppressor of PEV that is mediated by centric heterochromatin, suggesting its broader effect on chromatin not limited to telomeres.

Transposons are selfish elements that multiply by inserting extra copies of themselves into the host genome. Active transposons thus threaten the stability of the host genome, while the host responses by transcriptionally silencing the selfish elements or targeting their insertions towards gene-poor regions of the genome. Chromosome ends (telomeres) in the fruit fly Drosophila are elongated by active transposition of retrotransposons. Although much is known about how these elements are silenced, little is known about the remarkable accuracy by which they are targeted to telomeres. Prime candidates through which the host mounts such defenses are members of the protein complexes that protect telomeres. Here we characterized a hypomorphic mutation of the HipHop protein, and showed that active telomeric transcription in the mutant germline persists for generations without leading to runaway telomere elongation, that embryos laid by the mutant female suffer rampant end-to-end fusions, and that telomeric targeting of the transposon machinery is defective in the mutant soma. Collectively our data suggest that HipHop is essential for preventing telomere fusions, silencing telomeric transposons, and recruiting transposon machinery to telomeres. Our study thus identifies a factor essential for the host control over active transposons and a paradigm for studying such control mechanisms.

E2F/Dp inactivation in fat body cells triggers systemic metabolic changes

The E2F transcription factors play a critical role in controlling cell fate. In Drosophila, the inactivation of E2F in either muscle or fat body results in lethality, suggesting an essential function for E2F in these tissues. However, the cellular and organismal consequences of inactivating E2F in these tissues are not fully understood. Here, we show that the E2F loss exerts both tissue-intrinsic and systemic effects. The proteomic profiling of E2F-deficient muscle and fat body revealed that E2F regulates carbohydrate metabolism, a conclusion further supported by metabolomic profiling. Intriguingly, animals with E2F-deficient fat body had a lower level of circulating trehalose and reduced storage of fat. Strikingly, a sugar supplement was sufficient to restore both trehalose and fat levels, and subsequently rescued animal lethality. Collectively, our data highlight the unexpected complexity of E2F mutant phenotype, which is a result of combining both tissue-specific and systemic changes that contribute to animal development.

Drosophila telomere capping protein HOAP interacts with DSB sensor proteins Mre11 and Nbs

In eukaryotes, specific DNA-protein structures called telomeres exist at linear chromosome ends. Telomere stability is maintained by a specific capping protein complex. This capping complex is essential for the inhibition of the DNA damage response (DDR) at telomeres and contributes to genome integrity. In Drosophila, the central factors of telomere capping complex are HOAP and HipHop. Furthermore, a DDR protein complex Mre11-Rad50-Nbs (MRN) is known to be important for the telomere association of HOAP and HipHop. However, whether MRN interacts with HOAP and HipHop, and the telomere recognition mechanisms of HOAP and HipHop are poorly understood. Here, we show that Nbs interacts with Mre11 and transports the Mre11-Rad50 complex from the cytoplasm to the nucleus. In addition, we report that HOAP interacts with both Mre11 and Nbs. The N-terminal region of HOAP is essential for its co-localization with HipHop. Finally, we reveal that Nbs interacts with the N-terminal region of HOAP.

The end protection problem-an unexpected twist in the tail

In this perspective, we introduce shelterin and the mechanisms of ATM activation and NHEJ at telomeres, before discussing the following questions: How are t-loops proposed to protect chromosome ends and what is the evidence for this model? Can other models explain how TRF2 mediates end protection? Could t-loops be pathological structures? How is end protection achieved in pluripotent cells? What do the insights into telomere end protection in pluripotent cells mean for the t-loop model of end protection? Why might different cell states have evolved different mechanisms of end protection? Finally, we offer support for an updated t-loop model of end protection, suggesting that the data is supportive of a critical role for t-loops in protecting chromosome ends from NHEJ and ATM activation, but that other mechanisms are involved. Finally, we propose that t-loops are likely dynamic, rather than static, structures.

Silence at the End: How Drosophila Regulates Expression and Transposition of Telomeric Retroelements

The maintenance of chromosome ends in Drosophila is an exceptional phenomenon because it relies on the transposition of specialized retrotransposons rather than on the activity of the enzyme telomerase that maintains telomeres in almost every other eukaryotic species. Sequential transpositions of Het-A, TART, and TAHRE (HTT) onto chromosome ends produce long head-to-tail arrays that are reminiscent to the long arrays of short repeats produced by telomerase in other organisms. Coordinating the activation and silencing of the HTT array with the recruitment of telomere capping proteins favors proper telomere function. However, how this coordination is achieved is not well understood. Like other Drosophila retrotransposons, telomeric elements are regulated by the piRNA pathway. Remarkably, HTT arrays are both source of piRNA and targets of gene silencing thus making the regulation of Drosophila telomeric transposons a unique event among eukaryotes. Herein we will review the genetic and molecular mechanisms underlying the regulation of HTT transcription and transposition and will discuss the possibility of a crosstalk between piRNA-mediated regulation, telomeric chromatin establishment, and telomere protection.

Drosophila Morgana is an Hsp90-interacting protein with a direct role in microtubule polymerisation

Morgana (Mora, also known as CHORD in flies) and its mammalian homologue, called CHORDC1 or CHP1, is a highly conserved cysteine and histidine-rich domain (CHORD)-containing protein that has been proposed to function as an Hsp90 co-chaperone. Morgana deregulation promotes carcinogenesis in both mice and humans while, in Drosophila, loss of mora causes lethality and a complex mitotic phenotype that is rescued by a human morgana transgene. Here, we show that Drosophila Mora localises to mitotic spindles and co-purifies with the Hsp90-R2TP-TTT supercomplex and with additional well-known Hsp90 co-chaperones. Acute inhibition of Mora function in the early embryo results in a dramatic reduction in centrosomal microtubule stability, leading to small spindles nucleated from mitotic chromatin. Purified Mora binds to microtubules directly and promotes microtubule polymerisation in vitro, suggesting that Mora directly regulates spindle dynamics independently of its Hsp90 co-chaperone role.

NBS1 interacts with HP1 to ensure genome integrity

Heterochromatin Protein 1 (HP1) and the Mre11-Rad50-Nbs1 (MRN) complex are conserved factors that play crucial role in genome stability and integrity. Despite their involvement in overlapping cellular functions, ranging from chromatin organization, telomere maintenance to DNA replication and repair, a tight functional relationship between HP1 and the MRN complex has never been elucidated. Here we show that the Drosophila HP1a protein binds to the MRN complex through its chromoshadow domain (CSD). In addition, loss of any of the MRN members reduces HP1a levels indicating that the MRN complex acts as regulator of HP1a stability. Moreover, overexpression of HP1a in nbs (but not in rad50 or mre11) mutant cells drastically reduces DNA damage associated with the loss of Nbs suggesting that HP1a and Nbs work in concert to maintain chromosome integrity in flies. We have also found that human HP1α and NBS1 interact with each other and that, similarly to Drosophila, siRNA-mediated inhibition of NBS1 reduces HP1α levels in human cultured cells. Surprisingly, fibroblasts from Nijmegen Breakage Syndrome (NBS) patients, carrying the 657del5 hypomorphic mutation in NBS1 and expressing the p26 and p70 NBS1 fragments, accumulate HP1α indicating that, differently from NBS1 knockout cells, the presence of truncated NBS1 extends HP1α turnover and/or promotes its stability. Remarkably, an siRNA-mediated reduction of HP1α in NBS fibroblasts decreases the hypersensitivity to irradiation, a characteristic of the NBS syndrome. Overall, our data provide an unanticipated evidence of a close interaction between HP1 and NBS1 that is essential for genome stability and point up HP1α as a potential target to counteract chromosome instability in NBS patient cells.

A Drosophila cell-free system that senses DNA breaks and triggers phosphorylation signalling

Preblastoderm Drosophila embryo development is characterized by fast cycles of nuclear divisions. Extracts from these embryos can be used to reconstitute complex chromatin with high efficiency. We now discovered that this chromatin assembly system contains activities that recognize unprotected DNA ends and signal DNA damage through phosphorylation. DNA ends are initially bound by Ku and MRN complexes. Within minutes, the phosphorylation of H2A.V (homologous to γH2A.X) initiates from DNA breaks and spreads over tens of thousands DNA base pairs. The γH2A.V phosphorylation remains tightly associated with the damaged DNA and does not spread to undamaged DNA in the same reaction. This first observation of long-range γH2A.X spreading along damaged chromatin in an in vitro system provides a unique opportunity for mechanistic dissection. Upon further incubation, DNA ends are rendered single-stranded and bound by the RPA complex. Phosphoproteome analyses reveal damage-dependent phosphorylation of numerous DNA-end-associated proteins including Ku70, RPA2, CHRAC16, the exonuclease Rrp1 and the telomer capping complex. Phosphorylation of spindle assembly checkpoint components and of microtubule-associated proteins required for centrosome integrity suggests this cell-free system recapitulates processes involved in the regulated elimination of fatally damaged syncytial nuclei.

Multiple Arginine Residues Are Methylated in Drosophila Mre11 and Required for Survival Following Ionizing Radiation

Mre11 is a key player for DNA double strand break repair. Previous studies have shown that mammalian Mre11 is methylated at multiple arginines in its C-terminal Glycine-Arginine-Rich motif (GAR) by protein arginine methyltransferase PRMT1. Here, we found that the Drosophila Mre11 is methylated at arginines 559, 563, 565, and 569 in the GAR motif by DART1, the Drosophila homolog of PRMT1. Mre11 interacts with DART1 in S2 cells, and this interaction does not require the GAR motif. Arginines methylated Mre11 localizes exclusively in the nucleus as soluble nuclear protein or chromatin-binding protein. To study the in vivo functions of methylation, we generated the single Arg-Ala and all Arginines mutated flies. We found these mutants were sensitive to ionizing radiation. Furthermore, Arg-Ala mutated flies had no irradiation induced G2/M checkpoint defect in wing disc and eye disc. Thus, we provided evidence that arginines in Drosophila Mre11 are methylated by DART1 methytransferase and flies loss of arginine methylation are sensitive to irradiation.

E2F/DP Prevents Cell-Cycle Progression in Endocycling Fat Body Cells by Suppressing dATM Expression

To understand the consequences of the complete elimination of E2F regulation, we profiled the proteome of Drosophila dDP mutants that lack functional E2F/DP complexes. The results uncovered changes in the larval fat body, a differentiated tissue that grows via endocycles. We report an unexpected mechanism of E2F/DP action that promotes quiescence in this tissue. In the fat body, dE2F/dDP limits cell-cycle progression by suppressing DNA damage responses. Loss of dDP upregulates dATM, allowing cells to sense and repair DNA damage and increasing replication of loci that are normally under-replicated in wild-type tissues. Genetic experiments show that ectopic dATM is sufficient to promote DNA synthesis in wild-type fat body cells. Strikingly, reducing dATM levels in dDP-deficient fat bodies restores cell-cycle control, improves tissue morphology, and extends animal development. These results show that, in some cellular contexts, dE2F/dDP-dependent suppression of DNA damage signaling is key for cell-cycle control and needed for normal development.

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

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

Maternal Haploid, a Metalloprotease Enriched at the Largest Satellite Repeat and Essential for Genome Integrity in Drosophila Embryos

The incorporation of the paternal genome into the zygote during fertilization requires chromatin remodeling. The maternal haploid (mh) mutation in Drosophila affects this process and leads to the formation of haploid embryos without the paternal genome. mh encodes the Drosophila homolog of SPRTN, a conserved protease essential for resolving DNA-protein cross-linked products. Here we characterize the role of MH in genome maintenance. It is not understood how MH protects the paternal genome during fertilization, particularly in light of our finding that MH is present in both parental pronuclei during zygote formation. We showed that maternal chromosomes in mh mutant embryos experience instabilities in the absence of the paternal genome, which suggests that MH is generally required for chromosome stability during embryogenesis. This is consistent with our finding that MH is abundantly present on chromatin throughout the cell cycle. Remarkably, MH is prominently enriched at the 359-bp satellite repeats during interphase, which becomes unstable without MH. This dynamic localization and specific enrichment of MH at the 359 repeats resemble that of Topoisomerase 2 (Top2), suggesting that MH regulates Top2, possibly as a protease for the resolution of Top2-DNA intermediates. We propose that maternal MH removes proteins specifically enriched on sperm chromatin. In the absence of that function, paternal chromosomes are precipitously lost. This mode of paternal chromatin remodeling is likely conserved and the unique phenotype of the Drosophila mh mutants represents a rare opportunity to gain insights into the process that has been difficult to study.

Recurrent Innovation at Genes Required for Telomere Integrity in Drosophila

Telomeres are nucleoprotein complexes at the ends of linear chromosomes. These specialized structures ensure genome integrity and faithful chromosome inheritance. Recurrent addition of repetitive, telomere-specific DNA elements to chromosome ends combats end-attrition, while specialized telomere-associated proteins protect naked, double-stranded chromosome ends from promiscuous repair into end-to-end fusions. Although telomere length homeostasis and end-protection are ubiquitous across eukaryotes, there is sporadic but building evidence that the molecular machinery supporting these essential processes evolves rapidly. Nevertheless, no global analysis of the evolutionary forces that shape these fast-evolving proteins has been performed on any eukaryote. The abundant population and comparative genomic resources of Drosophila melanogaster and its close relatives offer us a unique opportunity to fill this gap. Here we leverage population genetics, molecular evolution, and phylogenomics to define the scope and evolutionary mechanisms driving fast evolution of genes required for telomere integrity. We uncover evidence of pervasive positive selection across multiple evolutionary timescales. We also document prolific expansion, turnover, and expression evolution in gene families founded by telomeric proteins. Motivated by the mutant phenotypes and molecular roles of these fast-evolving genes, we put forward four alternative, but not mutually exclusive, models of intra-genomic conflict that may play out at very termini of eukaryotic chromosomes. Our findings set the stage for investigating both the genetic causes and functional consequences of telomere protein evolution in Drosophila and beyond.

The Deadbeat Paternal Effect of Uncapped Sperm Telomeres on Cell Cycle Progression and Chromosome Behavior in Drosophila melanogaster

Telomere-capping complexes (TCCs) protect the ends of linear chromosomes from illegitimate repair and end-to-end fusions and are required for genome stability. The identity and assembly of TCC components have been extensively studied, but whether TCCs require active maintenance in nondividing cells remains an open question. Here we show that Drosophila melanogaster requires Deadbeat (Ddbt), a sperm nuclear basic protein (SNBP) that is recruited to the telomere by the TCC and is required for TCC maintenance during genome-wide chromatin remodeling, which transforms spermatids to mature sperm. Ddbt-deficient males produce sperm lacking TCCs. Their offspring delay the initiation of anaphase as early as cycle 1 but progress through the first two cycles. Persistence of uncapped paternal chromosomes induces arrest at or around cycle 3. This early arrest can be rescued by selective elimination of paternal chromosomes and production of gynogenetic haploid or haploid mosaics. Progression past cycle 3 can also occur if embryos have reduced levels of the maternally provided checkpoint kinase Chk2. The findings provide insights into how telomere integrity affects the regulation of the earliest embryonic cell cycles. They also suggest that other SNBPs, including those in humans, may have analogous roles and manifest as paternal effects on embryo quality.

Hypothesis: Paralog Formation from Progenitor Proteins and Paralog Mutagenesis Spur the Rapid Evolution of Telomere Binding Proteins

Through elegant studies in fungal cells and complex organisms, we propose a unifying paradigm for the rapid evolution of telomere binding proteins (TBPs) that associate with either (or both) telomeric DNA and telomeric proteins. TBPs protect and regulate telomere structure and function. Four critical factors are involved. First, TBPs that commonly bind to telomeric DNA include the c-Myb binding proteins, OB-fold single-stranded binding proteins, and G-G base paired Hoogsteen structure (G4) binding proteins. Each contributes independently or, in some cases, cooperatively, to provide a minimum level of telomere function. As a result of these minimal requirements and the great abundance of homologs of these motifs in the proteome, DNA telomere-binding activity may be generated more easily than expected. Second, telomere dysfunction gives rise to genome instability, through the elevation of recombination rates, genome ploidy, and the frequency of gene mutations. The formation of paralogs that diverge from their progenitor proteins ultimately can form a high frequency of altered TBPs with altered functions. Third, TBPs that assemble into complexes (e.g., mammalian shelterin) derive benefits from the novel emergent functions. Fourth, a limiting factor in the evolution of TBP complexes is the formation of mutually compatible interaction surfaces amongst the TBPs. These factors may have different degrees of importance in the evolution of different phyla, illustrated by the apparently simpler telomeres in complex plants. Selective pressures that can utilize the mechanisms of paralog formation and mutagenesis to drive TBP evolution along routes dependent on the requisite physiologic changes.

The hnRNP A1 homolog Hrb87F/Hrp36 is important for telomere maintenance in Drosophila melanogaster

Unlike the telomerase-dependent mammalian telomeres, HeT-A, TART, and TAHRE (HTT) retroposon arrays regulate Drosophila telomere length. Cap prevents telomeric associations (TAs) and telomeric fusions (TFs). Our results suggest important roles of Hrb87F in telomeric HTT array and cap maintenance in Drosophila. All chromosome arms, except 2L, in Df(3R)Hrb87F homozygotes (Hrb87F-null) displayed significantly elongated telomeres with amplified HTT arrays and high TAs, all of which resolved without damage. Presence of FLAG-tagged Hrb87F (FLAG-Hrb87F) on cap and subtelomeric regions following hsFLAG-Hrb87F transgene expression in Df(3R)Hrb87F homozygotes suppressed TAs without affecting telomere length. A normal X-chromosome telomere expanded within five generations in Hrb87F-null background and displayed high TAs, but not when hsFLAG-Hrb87F was co-expressed. Tel (1) /Gaiano line or HP1 loss-of-function mutant-derived expanded telomeres carry Hrb87F on cap and HTT arrays while Hrb87F-null telomeres have HP1 and HOAP on caps and expanded HTT arrays. ISWI, seen only on cap on normal telomeres, was abundant on Hrb87F-null expanded HTT arrays. Extended telomeres derived from Tel (1) (Gaiano) or HP1-null mutation background interact with those from Hrb87F-null, since while the end association frequency was negligible in Df(3R)Hrb87F/+ nuclei, it increased significantly in co-presence of Tel (1) or HP1-null-based expanded telomere/s. Together, these suggest complex interactions between members of the proteome of telomere so that absence of any key member leads to telomere expansion and/or enhanced TAs/TFs. HTT expansion in Hrb87F-null condition is not developmental but a germline event presumably because absence of Hrb87F in germline may deregulate HTT retroposition/replication leading to telomere elongation.

Telomeric repeat silencing in germ cells is essential for early development in Drosophila

The germline-specific role of telomeres consists of chromosome end elongation and proper chromosome segregation during early developmental stages. Despite the crucial role of telomeres in germ cells, little is known about telomere biology in the germline. We analyzed telomere homeostasis in the Drosophila female germline and early embryos. A novel germline-specific function of deadenylase complex Ccr4-Not in the telomeric transcript surveillance mechanism is reported. Depletion of Ccr4-Not complex components causes strong derepression of the telomeric retroelement HeT-A in the germ cells, accompanied by elongation of the HeT-A poly(A) tail. Dysfunction of transcription factors Woc and Trf2, as well as RNA-binding protein Ars2, also results in the accumulation of excessively polyadenylated HeT-A transcripts in ovaries. Germline knockdowns of Ccr4-Not components, Woc, Trf2 and Ars2, lead to abnormal mitosis in early embryos, characterized by chromosome missegregation, centrosome dysfunction and spindle multipolarity. Moreover, the observed phenotype is accompanied by the accumulation of HeT-A transcripts around the centrosomes in early embryos, suggesting the putative relationship between overexpression of telomeric transcripts and mitotic defects. Our data demonstrate that Ccr4-Not, Woc, Trf2 and Ars2, components of different regulatory pathways, are required for telomere protection in the germline in order to guarantee normal development.

Brca2-Pds5 complexes mobilize persistent meiotic recombination sites to the nuclear envelope

Homologous recombination is required for reciprocal exchange between homologous chromosome arms during meiosis. Only select meiotic recombination events become chromosomal crossovers; the majority of recombination outcomes are noncrossovers. Growing evidence suggests that crossovers are repaired after noncrossovers. Here, I report that persisting recombination sites are mobilized to the nuclear envelope of Drosophila pro-oocytes during mid-pachytene. Their number correlates with the average crossover rate per meiosis. Proteomic and interaction studies reveal that the recombination mediator Brca2 associates with lamin and the cohesion factor Pds5 to secure persistent recombination sites at the nuclear envelope. In Rad51(-/-) females, all persistent DNA breaks are directed to the nuclear envelope. By contrast, a reduction of Pds5 or Brca2 levels abolishes the movement and has a negative impact on crossover rates. The data suggest that persistent meiotic DNA double-strand breaks might correspond to crossovers, which are mobilized to the nuclear envelope for their repair. The identification of Brca2-Pds5 complexes as key mediators of this process provides a first mechanistic explanation for the contribution of lamins and cohesins to meiotic recombination.

Crumbs interacts with Xpd for nuclear division control in Drosophila

Crumbs (Crb) family proteins are crucial for cell polarity. Recent studies indicate that they are also involved in growth regulation and cancer. However, it is not well-understood how Crb participates in mitotic processes. Here, we report that Drosophila Crb is critically involved in nuclear division by interacting with Xeroderma pigmentosum D (XPD). A novel gene named galla-1 was identified from a genetic screen for crb modifiers. Galla-1 protein shows homology to MIP18, a subunit of the mitotic spindle-associated MMS19-XPD complex. Loss-of-function galla-1 mutants show abnormal chromosome segregation, defective centrosome positions and branched spindles during nuclear division in early embryos. Embryos with loss-of-function or overexpression of crb show similar mitotic defects and genetic interaction with galla-1. Both Galla-1 and Crb proteins show overlapping localization with spindle microtubules during nuclear division. Galla-1 physically interacts with the intracellular domain of Crb. Interestingly, Galla-1 shows little binding to the Drosophila homolog of XPD, but a related protein Galla-2 binds both Crb and Xpd. Loss-of-function galla-2 mutants show similar mitotic defects as galla-1 and strong genetic interaction with crb. Xpd can form a physical complex with Crb. In imaginal disc, Crb overexpression causes tissue overgrowth as well as DNA damages marked by H2Av phosphorylation. These phenotypes are suppressed by reduction of Xpd. Taken together, this study identifies a novel Crb-Galla-Xpd complex and its function for proper chromosome segregation during nuclear division, implicating a potential link between Crb and Xpd-related genome instability.

TCTP directly regulates ATM activity to control genome stability and organ development in Drosophila melanogaster

Translationally controlled tumour protein (TCTP) is implicated in growth regulation and cancer. Recently, human TCTP has been suggested to play a role in the DNA damage response by forming a complex with ataxia telangiectasia-mutated (ATM) kinase . However, the exact nature of this interaction and its roles in vivo remained unclear. Here, we utilize Drosophila as an animal model to study the nuclear function of Drosophila TCTP (dTCTP). dTCTP mutants show increased radiation sensitivity during development as well as strong genetic interaction with dATM mutations, resulting in severe defects in developmental timing, organ size and chromosome stability. We identify Drosophila ATM (dATM) as a direct binding partner of dTCTP and describe a mechanistic basis for dATM activation by dTCTP. Altogether, this study provides the first in vivo evidence for direct modulation of dATM activity by dTCTP in the control of genome stability and organ development.

Multiple Mechanisms Contribute To Telomere Maintenance

The unlimited growth potential of tumors depends on telomere maintenance and typically depends on telomerase, an RNA-dependent DNA polymerase, which reverse transcribes the telomerase RNA template, synthesizing telomere repeats at the ends of chromosomes. Studies in various model organisms genetically deleted for telomerase indicate that several recombination-based mechanisms also contribute to telomere maintenance. Understanding the molecular basis of these mechanisms is critical since some human tumors form without telomerase, yet the sequence is maintained at the telomeres. Recombination-based mechanisms also likely contribute at some frequency to telomere maintenance in tumors expressing telomerase. Preventing telomere maintenance is predicted to impact tumor growth, yet inhibiting telomerase may select for the recombination-based mechanisms. Telomere recombination mechanisms likely involve altered or unregulated pathways of DNA repair. The use of some DNA damaging agents may encourage the use of these unregulated pathways of DNA repair to be utilized and may allow some tumors to generate resistance to these agents depending on which repair pathways are altered in the tumors. This review will discuss the various telomere recombination mechanisms and will provide rationale regarding the possibility that L1 retrotransposition may contribute to telomere maintenance in tumors lacking telomerase.

Clustering and protein dynamics of Drosophila melanogaster telomeres

Telomeres are obligatory chromosomal landmarks that demarcate the ends of linear chromosomes to distinguish them from broken ends and can also serve to organize the genome. In both budding and fission yeast, they cluster at the periphery of the nucleus, potentially to establish a compartment of silent chromatin. To gain insight into telomere organization in higher organisms, we investigated their distribution in interphase nuclei of Drosophila melanogaster. We focused on the syncytial blastoderm, an excellent developmental stage for live imaging due to the synchronous division of the nuclei at this time. We followed the EGFP-labeled telomeric protein HOAP in vivo and found that the 16 telomeres yield four to six foci per nucleus, indicative of clustering. Furthermore, we confirmed clustering in other somatic tissues. Importantly, we observed that HOAP signal intensity in the clusters increases in interphase, potentially due to loading of HOAP to newly replicated telomeres. To determine the rules governing clustering, we used in vivo imaging and fluorescence in situ hybridization to test several predictions. First, we inspected mutant embryos that develop as haploids and found that clustering is not mediated by associations between homologs. Second, we probed specifically for a telomere of novel sequence and found strong evidence against DNA sequence identity and homology as critical factors. Third, we ruled out predominance of intrachromosomal interactions by marking both ends of a chromosome. Based on these results, we propose that clustering is independent of sequence and is likely maintained by an as yet undetermined factor.

A hypomorphic mutation reveals a stringent requirement for the ATM checkpoint protein in telomere protection during early cell division in Drosophila

Using Drosophila as a model system, we identified a stringent requirement for the conserved function of Ataxia Telangiectasia Mutated (ATM) in telomere protection during early embryonic development. Animals homozygous for a hypomorphic mutation in atm develop normally with minimal telomere dysfunction. However, mutant females produce inviable embryos that succumb to mitotic failure caused by covalent fusions of telomeric DNA. Interestingly, although the atm mutation encodes a premature stop codon, it must not have eliminated the production of the mutant protein, and the mutant protein retains kinase activity upon DNA damage. Moreover, although the embryonic phenotype of this mutation resembles that of hypomorphic mutations in the MRN complex, the function of MRN appears normal in the atm embryos. In contrast, there is a prominent reduction of the level of HipHop, an essential member of the Drosophila capping complex. How ATM functions in telomere protection remains poorly understood. The amenability of Drosophila embryos to molecular and biochemical investigations ensures that this newly identified mutation will facilitate future studies of ATM in telomere maintenance.

Multiple pathways suppress telomere addition to DNA breaks in the Drosophila germline

Telomeres protect chromosome ends from being repaired as double-strand breaks (DSBs). Just as DSB repair is suppressed at telomeres, de novo telomere addition is suppressed at the site of DSBs. To identify factors responsible for this suppression, we developed an assay to monitor de novo telomere formation in Drosophila, an organism in which telomeres can be established on chromosome ends with essentially any sequence. Germline expression of the I-SceI endonuclease resulted in precise telomere formation at its cut site with high efficiency. Using this assay, we quantified the frequency of telomere formation in different genetic backgrounds with known or possible defects in DNA damage repair. We showed that disruption of DSB repair factors (Rad51 or DNA ligase IV) or DSB sensing factors (ATRIP or MDC1) resulted in more efficient telomere formation. Interestingly, partial disruption of factors that normally regulate telomere protection (ATM or NBS) also led to higher frequencies of telomere formation, suggesting that these proteins have opposing roles in telomere maintenance vs. establishment. In the ku70 mutant background, telomere establishment was preceded by excessive degradation of DSB ends, which were stabilized upon telomere formation. Most strikingly, the removal of ATRIP caused a dramatic increase in telomeric retrotransposon attachment to broken ends. Our study identifies several pathways that suppress telomere addition at DSBs, paving the way for future mechanistic studies.

Drosophila genes that affect meiosis duration are among the meiosis related genes that are more often found duplicated

Using a phylogenetic approach, the examination of 33 meiosis/meiosis-related genes in 12 Drosophila species, revealed nine independent gene duplications, involving the genes cav, mre11, meiS332, polo and mtrm. Evidence is provided that at least eight out of the nine gene duplicates are functional. Therefore, the rate at which Drosophila meiosis/meiosis-related genes are duplicated and retained is estimated to be 0.0012 per gene per million years, a value that is similar to the average for all Drosophila genes. It should be noted that by using a phylogenetic approach the confounding effect of concerted evolution, that is known to lead to overestimation of the duplication and retention rate, is avoided. This is an important issue, since even in our moderate size sample, evidence for long-term concerted evolution (lasting for more than 30 million years) was found for the meiS332 gene pair in species of the Drosophila subgenus. Most striking, in contrast to theoretical expectations, is the finding that genes that encode proteins that must follow a close stoichiometric balance, such as polo, mtrm and meiS332 have been found duplicated. The duplicated genes may be examples of gene neofunctionalization. It is speculated that meiosis duration may be a trait that is under selection in Drosophila and that it has different optimal values in different species.

Paternal imprint essential for the inheritance of telomere identity in Drosophila

Chromatin remodeling during sperm maturation could erase epigenetic landmarks on the paternal genome, creating a challenge for its reestablishment on fertilization. Here, we show that selective retention of a chromosomal protein in mature sperm protects the identity of paternal telomeres in Drosophila. The ms(3)k81 (k81) gene is a duplication of hiphop that encodes a telomeric protein. Although HipHop protects telomeres in somatic cells, K81 is produced exclusively in males and localizes to telomeres in postmitotic cells, including mature sperm. In embryos fathered by k81 mutants, the maternal supplies fail to reestablish a protective cap on paternal telomeres, leading to their fusions. These fusions hinder the segregation of the paternal genome and result in haploid embryos with maternal chromosomes. The functional divergence between hiphop and k81 manifests not only in their expression patterns but also in the protein functions that they encode. By swapping the two coding regions, we show that K81 can replace HipHop for somatic protection; however, HipHop cannot replace K81 in the germ line to specify telomere identity, because HipHop ectopically expressed in the testis is removed from chromatin during sperm maturation. HipHop lacks a short motif in K81 that is essential for K81 to survive the remodeling process. We show that the combined functions of HipHop and K81 are likely fulfilled by the single ancestral hiphop locus in other Drosophila species, supporting the hypothesis that the evolutionary process of subfunctionalization was responsible for the preservation of the hiphop-k81 duplicate.

Specialization of a Drosophila capping protein essential for the protection of sperm telomeres

BACKGROUND

A critical function of telomeres is to prevent fusion of chromosome ends by the DNA repair machinery. In Drosophila somatic cells, assembly of the protecting capping complex at telomeres notably involves the recruitment of HOAP, HP1, and their recently identified partner, HipHop. We previously showed that the hiphop gene was duplicated before the radiation of the melanogaster subgroup of species, giving birth to K81, a unique paternal effect gene specifically expressed in the male germline.

RESULTS

Here we show that K81 specifically associates with telomeres during spermiogenesis, along with HOAP and HP1, and is retained on paternal chromosomes until zygote formation. In K81 mutant testes, capping proteins are not maintained at telomeres in differentiating spermatids, resulting in the transmission of uncapped paternal chromosomes that fail to properly divide during the first zygotic mitosis. Despite the apparent similar capping roles of K81 and HipHop in their respective domain of expression, we demonstrate by in vivo reciprocal complementation analyses that they are not interchangeable. Strikingly, HipHop appeared to be unable to maintain capping proteins at telomeres during the global chromatin remodeling of spermatid nuclei.

CONCLUSIONS

Our data demonstrate that K81 is essential for the maintenance of capping proteins at telomeres in postmeiotic male germ cells. In species of the melanogaster subgroup, HipHop and K81 have not only acquired complementary expression domains, they have also functionally diverged following the gene duplication event. We propose that K81 specialized in the maintenance of telomere protection in the highly peculiar chromatin environment of differentiating male gametes.

Evolution of species-specific promoter-associated mechanisms for protecting chromosome ends by Drosophila Het-A telomeric transposons

The non-LTR retrotransposons forming Drosophila telomeres constitute a robust mechanism for telomere maintenance, one which has persisted since before separation of the extant Drosophila species. These elements in D. melanogaster differ from nontelomeric retrotransposons in ways that give insight into general telomere biology. Here, we analyze telomere-specific retrotransposons from D. virilis, separated from D. melanogaster by 40 to 60 million years, to evaluate the evolutionary divergence of their telomeric traits. The telomeric retrotransposon HeT-A from D. melanogaster has an unusual promoter near its 3' terminus that drives not the element in which it resides, but the adjacent downstream element in a head-to-tail array. An obvious benefit of this promoter is that it adds nonessential sequence to the 5' end of each transcript, which is reverse transcribed and added to the chromosome. Because the 5' end of each newly transposed element forms the end of the chromosome until another element transposes onto it, this nonessential sequence can buffer erosion of sequence essential for HeT-A. Surprisingly, we have now found that HeT-A in D. virilis has a promoter typical of non-LTR retrotransposons. This promoter adds no buffering sequence; nevertheless, the complete 5' end of the element persists in telomere arrays, necessitating a more precise processing of the extreme end of the telomere in D. virilis.