TGS1 impacts snRNA 3'-end processing, ameliorates survival motor neuron-dependent neurological phenotypes in vivo and prevents neurodegeneration.

Trimethylguanosine synthase 1 (TGS1) is a highly conserved enzyme that converts the 5'-monomethylguanosine cap of small nuclear RNAs (snRNAs) to a trimethylguanosine cap. Here, we show that loss of TGS1 in Caenorhabditis elegans, Drosophila melanogaster and Danio rerio results in neurological phenotypes similar to those caused by survival motor neuron (SMN) deficiency. Importantly, expression of human TGS1 ameliorates the SMN-dependent neurological phenotypes in both flies and worms, revealing that TGS1 can partly counteract the effects of SMN deficiency. TGS1 loss in HeLa cells leads to the accumulation of immature U2 and U4atac snRNAs with long 3' tails that are often uridylated. snRNAs with defective 3' terminations also accumulate in Drosophila Tgs1 mutants. Consistent with defective snRNA maturation, TGS1 and SMN mutant cells also exhibit partially overlapping transcriptome alterations that include aberrantly spliced and readthrough transcripts. Together, these results identify a neuroprotective function for TGS1 and reinforce the view that defective snRNA maturation affects neuronal viability and function.

The S-adenosylmethionine analog sinefungin inhibits the trimethylguanosine synthase TGS1 to promote telomerase activity and telomere lengthening.

Mutations in many genes that control the expression, the function, or the stability of telomerase cause telomere biology disorders (TBDs), such as dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia. Mutations in a subset of the genes associated with TBDs cause reductions of the telomerase RNA moiety hTR, thus limiting telomerase activity. We have recently found that loss of the trimethylguanosine synthase TGS1 increases both hTR abundance and telomerase activity and leads to telomere elongation. Here, we show that treatment with the S-adenosylmethionine analog sinefungin inhibits TGS1 activity, increases the hTR levels, and promotes telomere lengthening in different cell types. Our results hold promise for restoring telomere length in stem and progenitor cells from TBD patients with reduced hTR levels.

Loss of Human TGS1 Hypermethylase Promotes Increased Telomerase RNA and Telomere Elongation.

Biogenesis of the human telomerase RNA (hTR) involves a complex series of posttranscriptional modifications, including hypermethylation of the 5' mono-methylguanosine cap to a tri-methylguanosine cap (TMG). How the TMG cap affects hTR maturation is unknown. Here, we show that depletion of trimethylguanosine synthase 1 (TGS1), the enzyme responsible for cap hypermethylation, increases levels of hTR and telomerase. Diminished trimethylation increases hTR association with the cap-binding complex (CBC) and with Sm chaperone proteins. Loss of TGS1 causes an increase in accumulation of mature hTR in both the nucleus and the cytoplasm compared with controls. In TGS1 mutant cells, increased hTR assembles with telomerase reverse transcriptase (TERT) protein to yield elevated active telomerase complexes and increased telomerase activity, resulting in telomere elongation in cultured human cells. Our results show that TGS1-mediated hypermethylation of the hTR cap inhibits hTR accumulation, restrains levels of assembled telomerase, and limits telomere elongation.

Disruption of Telomerase RNA Maturation Kinetics Precipitates Disease.

Mutations in RNA-processing enzymes are increasingly linked to human disease. Telomerase RNA and related noncoding RNAs require 3' end-processing steps, including oligoadenylation. Germline mutations in poly(A)ribonuclease (PARN) cause accumulation of extended human telomerase RNA (hTR) species and precipitate dyskeratosis congenita and pulmonary fibrosis. Here, we develop nascent RNAend-seq to measure processing rates of RNA precursors. We find that mature hTR derives from extended precursors but that in PARN-mutant cells hTR maturation kinetically stalls and unprocessed precursors are degraded. Loss of poly(A)polymerase PAPD5 in PARN-mutant cells accelerates hTR maturation and restores hTR processing, indicating that oligoadenylation and deadenylation set rates of hTR maturation. The H/ACA domain mediates hTR maturation by precisely defining the 3' end, recruiting poly(A)polymerase activity, and conferring sensitivity to PARN regulation. These data reveal a feedforward circuit in which post-transcriptional oligoadenylation controls RNA maturation kinetics. Similar alterations in RNA processing rates may contribute to mechanisms of RNA-based human disease.

AKTIP/Ft1, a New Shelterin-Interacting Factor Required for Telomere Maintenance.

Telomeres are nucleoprotein complexes that protect the ends of linear chromosomes from incomplete replication, degradation and detection as DNA breaks. Mammalian telomeres are protected by shelterin, a multiprotein complex that binds the TTAGGG telomeric repeats and recruits a series of additional factors that are essential for telomere function. Although many shelterin-associated proteins have been so far identified, the inventory of shelterin-interacting factors required for telomere maintenance is still largely incomplete. Here, we characterize AKTIP/Ft1 (human AKTIP and mouse Ft1 are orthologous), a novel mammalian shelterin-bound factor identified on the basis of its homology with the Drosophila telomere protein Pendolino. AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes and has been previously implicated in the control of apoptosis and in vesicle trafficking. RNAi-mediated depletion of AKTIP results in formation of telomere dysfunction foci (TIFs). Consistent with these results, AKTIP interacts with telomeric DNA and binds the shelterin components TRF1 and TRF2 both in vivo and in vitro. Analysis of AKTIP- depleted human primary fibroblasts showed that they are defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint. Accordingly, AKTIP physically interacts with PCNA and the RPA70 DNA replication factor. Ft1-depleted p53-/- MEFs did not arrest in the S phase but displayed significant increases in multiple telomeric signals (MTS) and sister telomere associations (STAs), two hallmarks of defective telomere replication. In addition, we found an epistatic relation for MST formation between Ft1 and TRF1, which has been previously shown to be required for replication fork progression through telomeric DNA. Ch-IP experiments further suggested that in AKTIP-depleted cells undergoing the S phase, TRF1 is less tightly bound to telomeric DNA than in controls. Thus, our results collectively suggest that AKTIP/Ft1 works in concert with TRF1 to facilitate telomeric DNA replication.

The Analysis of Pendolino (peo) Mutants Reveals Differences in the Fusigenic Potential among Drosophila Telomeres.

Drosophila telomeres are sequence-independent structures that are maintained by transposition to chromosome ends of three specialized retroelements (HeT-A, TART and TAHRE; collectively designated as HTT) rather than telomerase activity. Fly telomeres are protected by the terminin complex (HOAP-HipHop-Moi-Ver) that localizes and functions exclusively at telomeres and by non-terminin proteins that do not serve telomere-specific functions. Although all Drosophila telomeres terminate with HTT arrays and are capped by terminin, they differ in the type of subtelomeric chromatin; the Y, XR, and 4L HTT are juxtaposed to constitutive heterochromatin, while the XL, 2L, 2R, 3L and 3R HTT are linked to the TAS repetitive sequences; the 4R HTT is associated with a chromatin that has features common to both euchromatin and heterochromatin. Here we show that mutations in pendolino (peo) cause telomeric fusions (TFs). The analysis of several peo mutant combinations showed that these TFs preferentially involve the Y, XR and 4th chromosome telomeres, a TF pattern never observed in the other 10 telomere-capping mutants so far characterized. peo encodes a non-terminin protein homologous to the E2 variant ubiquitin-conjugating enzymes. The Peo protein directly interacts with the terminin components, but peo mutations do not affect telomeric localization of HOAP, Moi, Ver and HP1a, suggesting that the peo-dependent telomere fusion phenotype is not due to loss of terminin from chromosome ends. peo mutants are also defective in DNA replication and PCNA recruitment. However, our results suggest that general defects in DNA replication are unable to induce TFs in Drosophila cells. We thus hypothesize that DNA replication in Peo-depleted cells results in specific fusigenic lesions concentrated in heterochromatin-associated telomeres. Alternatively, it is possible that Peo plays a dual function being independently required for DNA replication and telomere capping.

GOLPH3 is essential for contractile ring formation and Rab11 localization to the cleavage site during cytokinesis in Drosophila melanogaster.

The highly conserved Golgi phosphoprotein 3 (GOLPH3) protein has been described as a Phosphatidylinositol 4-phosphate [PI(4)P] effector at the Golgi. GOLPH3 is also known as a potent oncogene, commonly amplified in several human tumors. However, the molecular pathways through which the oncoprotein GOLPH3 acts in malignant transformation are largely unknown. GOLPH3 has never been involved in cytokinesis. Here, we characterize the Drosophila melanogaster homologue of human GOLPH3 during cell division. We show that GOLPH3 accumulates at the cleavage furrow and is required for successful cytokinesis in Drosophila spermatocytes and larval neuroblasts. In premeiotic spermatocytes GOLPH3 protein is required for maintaining the organization of Golgi stacks. In dividing spermatocytes GOLPH3 is essential for both contractile ring and central spindle formation during cytokinesis. Wild type function of GOLPH3 enables maintenance of centralspindlin and Rho1 at cell equator and stabilization of Myosin II and Septin rings. We demonstrate that the molecular mechanism underlying GOLPH3 function in cytokinesis is strictly dependent on the ability of this protein to interact with PI(4)P. Mutations that abolish PI(4)P binding impair recruitment of GOLPH3 to both the Golgi and the cleavage furrow. Moreover telophase cells from mutants with defective GOLPH3-PI(4)P interaction fail to accumulate PI(4)P-and Rab11-associated secretory organelles at the cleavage site. Finally, we show that GOLPH3 protein interacts with components of both cytokinesis and membrane trafficking machineries in Drosophila cells. Based on these results we propose that GOLPH3 acts as a key molecule to coordinate phosphoinositide signaling with actomyosin dynamics and vesicle trafficking during cytokinesis. Because cytokinesis failures have been associated with premalignant disease and cancer, our studies suggest novel insight into molecular circuits involving the oncogene GOLPH3 in cytokinesis.

Organization and Evolution of Drosophila Terminin: Similarities and Differences between Drosophila and Human Telomeres.

Drosophila lacks telomerase and fly telomeres are elongated by occasional transposition of three specialized retroelements. Drosophila telomeres do not terminate with GC-rich repeats and are assembled independently of the sequence of chromosome ends. Recent work has shown that Drosophila telomeres are capped by the terminin complex, which includes the fast-evolving proteins HOAP, HipHop, Moi, and Ver. These proteins, which are not conserved outside Drosophilidae and closely related Diptera, localize and function exclusively at telomeres, protecting them from fusion events. Other proteins required to prevent end-to-end fusion in flies include HP1, Eff/UbcD1, ATM, the components of the Mre11-Rad50-Nbs (MRN) complex, and the Woc transcription factor. These proteins do not share the terminin properties; they are evolutionarily conserved non-fast-evolving proteins that do not accumulate only at telomeres and do not serve telomere-specific functions. We propose that following telomerase loss, Drosophila rapidly evolved terminin to bind chromosome ends in a sequence-independent manner. This hypothesis suggests that terminin is the functional analog of the shelterin complex that protects human telomeres. The non-terminin proteins are instead likely to correspond to ancestral telomere-associated proteins that did not evolve as rapidly as terminin because of the functional constraints imposed by their involvement in diverse cellular processes. Thus, it appears that the main difference between Drosophila and human telomeres is in the protective complexes that specifically associate with the DNA termini. We believe that Drosophila telomeres offer excellent opportunities for investigations on human telomere biology. The identification of additional Drosophila genes encoding non-terminin proteins involved in telomere protection might lead to the discovery of novel components of human telomeres.

Terminin: a protein complex that mediates epigenetic maintenance of Drosophila telomeres.

In most organisms, telomeres are extended by telomerase and contain GC-rich repeats. Drosophila telomeres are elongated by occasional transposition of specialized retroelements rather than telomerase activity, and are assembled independently of the sequence of the DNA termini. Recent work has shown that Drosophila telomeres are capped by a complex, we call terminin, which includes HOAP, HipHop, Moi and Ver; these are fast-evolving proteins that prevent telomere fusion, directly interact with each other, and appear to localize and function only at telomeres. With the possible exception of Ver that contains an OB fold domain structurally similar to the Stn1 OB fold, none of the terminin proteins is evolutionarily conserved outside the Drosophila species. Human telomeres are protected by the shelterin complex, which comprises six proteins that bind chromosome ends in a sequence-dependent manner. Shelterin subunits are not fast-evolving proteins and are not conserved in flies, but localize and function only at telomeres like the terminin components. Based on these findings, we propose that concomitant with telomerase loss Drosophila rapidly evolved terminin to bind chromosome ends in a sequence-independent fashion, and that terminin is functionally analogous to shelterin.

Verrocchio, a Drosophila OB fold-containing protein, is a component of the terminin telomere-capping complex.

Drosophila telomeres are elongated by transposition of specialized retroelements rather than telomerase activity, and are assembled independently of the terminal DNA sequence. Drosophila telomeres are protected by terminin, a complex that includes the HOAP (Heterochromatin Protein 1/origin recognition complex-associated protein) and Moi (Modigliani) proteins and shares the properties of human shelterin. Here we show that Verrocchio (Ver), an oligonucleotide/oligosaccharide-binding (OB) fold-containing protein related to Rpa2/Stn1, interacts physically with HOAP and Moi, is enriched only at telomeres, and prevents telomere fusion. These results indicate that Ver is a new terminin component; we speculate that, concomitant with telomerase loss, Drosophila evolved terminin to bind chromosome ends independently of the DNA sequence.

Drosophila HP1c is regulated by an auto-regulatory feedback loop through its binding partner Woc.

HP1 is a major component of chromatin and regulates gene expression through its binding to methylated histone H3. Most eukaryotes express at least three isoforms of HP1 with similar domain architecture. However, despite the common specificity for methylated histone H3, the three HP1 isoforms bind to different regions of the genome. Most of the studies so far focused on the HP1a isoform and its role in transcriptional regulation. As HP1a requires additional factors to bind methylated chromatin in vitro, we wondered whether another isoform might also require additional targeting factors. Indeed, we found that HP1c interacts with the DNA binding factors Woc and Row and requires Woc to become targeted to chromatin in vivo. Moreover, we show that the interaction between HP1c and Woc constitutes a transcriptional feedback loop that operates to balance the concentration of HP1c within the cell. This regulation may prevent HP1c from binding to methylated heterochromatin.

The Drosophila modigliani (moi) gene encodes a HOAP-interacting protein required for telomere protection.

Several proteins have been identified that protect Drosophila telomeres from fusion events. They include UbcD1, HP1, HOAP, the components of the Mre11-Rad50-Nbs (MRN) complex, the ATM kinase, and the putative transcription factor Woc. Of these proteins, only HOAP has been shown to localize specifically at telomeres. Here we show that the modigliani gene encodes a protein (Moi) that is enriched only at telomeres, colocalizes and physically interacts with HOAP, and is required to prevent telomeric fusions. Moi is encoded by the bicistronic CG31241 locus. This locus produces a single transcript that contains 2 ORFs that specify different essential functions. One of these ORFs encodes the 20-kDa Moi protein. The other encodes a 60-kDa protein homologous to RNA methyltransferases that is not required for telomere protection (Drosophila Tat-like). Moi and HOAP share several properties with the components of shelterin, the protein complex that protects human telomeres. HOAP and Moi are not evolutionarily conserved unlike the other proteins implicated in Drosophila telomere protection. Similarly, none of the shelterin subunits is conserved in Drosophila, while most human nonshelterin proteins have Drosophila homologues. This suggests that the HOAP-Moi complex, we name "terminin," plays a specific role in the DNA sequence-independent assembly of Drosophila telomeres. We speculate that this complex is functionally analogous to shelterin, which binds chromosome ends in a sequence-dependent manner.

Autosomal mutations affecting Y chromosome loops in Drosophila melanogaster.

BACKGROUND: The Y chromosome of Drosophila melanogaster harbors several genes required for male fertility. The genes for these fertility factors are very large in size and contain conspicuous amounts of repetitive DNA and transposons. Three of these loci (ks-1, kl-3 and kl-5) have the ability to develop giant lampbrush-like loops in primary spermatocytes, a cytological manifestation of their active state in these cells. Y-loops bind a number of non-Y encoded proteins, but the mechanisms regulating their development and their specific functions are still to be elucidated. RESULTS: Here we report the results of a screen of 726 male sterile lines to identify novel autosomal genes controlling Y-loop function. We analyzed mutant testis preparations both in vivo and by immunofluorescence using antibodies directed against Y-loop-associated proteins. This screen enabled us to isolate 17 mutations at 15 loci whose wild-type function is required for proper Y-loop morphogenesis. Six of these loci are likely to specifically control loop development, while the others display pleiotropic effects on both loops and meiotic processes such as spermiogenesis, sperm development and maturation. We also determined the map position of the mutations affecting exclusively Y-loop morphology. CONCLUSION: Our cytological screening permitted us to identify novel genetic functions required for male spermatogenesis, some of which show pleiotropic effects. Analysis of these mutations also shows that loop development can be uncoupled from meiosis progression. These data represent a useful framework for the characterization of Y-loop development at a molecular level and for the study of the genetic control of heterochromatin.

SUMO-binding motifs mediate the Rad60-dependent response to replicative stress and self-association.

In fission yeast, the replication checkpoint is enforced by the kinase Cds1 (human Chk2), which regulates both cell cycle progression and DNA repair factors to ensure that the genome is faithfully duplicated prior to mitosis. Cds1 contains a forkhead-associated domain that mediates its interaction with phosphorylated residues in target proteins. One target of Cds1 is the essential nuclear protein Rad60, which contains the unique structural feature of tandem SUMO homology domains at its C terminus. Hypomorphic mutants of Rad60 cause profound defects in DNA repair and replication stress tolerance. To explore the physiological significance of the Cds1-Rad60 interaction, we have examined the phosphorylation of Rad60 by Cds1 in vitro and the in vivo phosphorylation of Rad60 in response to replication blocks. We find that the N terminus but not the SUMO-like domain of Rad60 is phosphorylated in both conditions. Three important Rad60 phosphorylation sites were identified: Thr(72), Ser(32), and Ser(34). Rad60 Thr(72) mediates the Cds1-Rad60 interaction and is required for the Cds1-dependent phosphorylation of Rad60 in response to replication arrest. Phosphorylation of Rad60 Ser(32) and Ser(34) in a putative SUMO-binding motif is critical for the survival of replication stress. In addition, mutation of Rad60 Ser(32) and Ser(34) to alanine is lethal in cells deleted for the RecQ DNA helicase Rqh1. Finally, we find that Rad60 self-associates via its C-terminal SUMO-like domain and putative SUMO-binding motifs.

The putative Drosophila transcription factor woc is required to prevent telomeric fusions.

Woc is a Drosophila zinc finger protein that shares homology with the human polypeptides ZNF261 and ZNF198 implicated in mental retardation and leukemia syndromes. We show that mutations in the woc gene cause frequent telomeric fusions in Drosophila brain cells. Woc localizes to all telomeres and most interbands of polytene chromosomes. In interbands, Woc precisely colocalizes with the initiating forms of RNA polymerase II (Pol II). To characterize the role of woc in telomere maintenance, we analyzed its relationships with Su(var)205, cav, atm, and rad50, four genes that prevent telomeric fusions; Su(var)205 and cav encode HP1 and HP1/ORC Associated Protein (HOAP), respectively. woc mutants displayed normal telomeric accumulations of both HP1 and HOAP, and mutations in cav, Su(var)205, atm, and rad50 did not affect Woc localization on polytene chromosome telomeres. Collectively, our results indicate that Woc is a transcription factor with a telomere-capping function independent of those of Su(var)205, cav, atm, and rad50.

The Drosophila HOAP protein is required for telomere capping.

HOAP (HP1/ORC-associated protein) has recently been isolated from Drosophila melanogaster embryos as part of a cytoplasmic complex that contains heterochromatin protein 1 (HP1) and the origin recognition complex subunit 2 (ORC2). Here, we show that caravaggio, a mutation in the HOAP-encoding gene, causes extensive telomere-telomere fusions in larval brain cells, indicating that HOAP is required for telomere capping. Our analyses indicate that HOAP is specifically enriched at mitotic chromosome telomeres, and strongly suggest that HP1 and HOAP form a telomere-capping complex that does not contain ORC2.