In vivo activities of heparan sulfate differentially modified by NDSTs during development.

Heparan sulfate proteoglycans (HSPGs) serve as co-receptors for growth factor signaling during development. It is well known that the level and patterns of sulfate groups of heparan sulfate (HS) chains, or HS fine structures, have a major impact on HSPG function. On the other hand, the physiological significance of other structural features of HS, including NS/NA domain organization, remains to be elucidated. A blueprint of the HS domain structures is mainly controlled by HS N-deacetylase/N-sulfotransferases (NDSTs). To analyze in vivo activities of differentially modified HS, we established two knock-in (KI) Drosophila strains with the insertion of mouse Ndst1 (mNdst1) or Ndst2 (mNdst2) in the locus of sulfateless (sfl), the only Drosophila NDST. In these KI lines, mNDSTs are expressed from the sfl locus, in the level and patterns identical to the endogenous sfl gene. Thus, phenotypes of Ndst1 KI and Ndst2KI animals reflect the ability of HS structures made by these enzymes to rescue sfl mutation. Remarkably, we found that mNdst1 completely rescued the loss of sfl. mNdst2 showed a limited rescue ability, despite a higher level of HS sulfation compared to HS in mNdst1 KI. Our study suggests that independent of sulfation levels, additional HS structural features controlled by NDSTs play key roles during tissue patterning.

Endogenous epitope tagging of a JAK/STAT ligand Unpaired1 in Drosophila.

Unpaired1 (Upd1) is a ligand of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway in Drosophila. In this study, using the CRISPR/Cas9 technique, we generate a transgenic fly strain in which a hemagglutinin (HA) epitope tag sequence is inserted into the endogenous locus of the upd1 gene. Anti-HA antibody staining confirms that the distribution of the epitope-tagged Upd1::HA in various tissues is consistent with upd1 expression patterns revealed by previous studies. This transgenic fly strain will be useful in studying the expression, localization, and association partners of Upd1, and thus will contribute to understanding how activation of the JAK/STAT pathway is regulated.

Drosophila MOV10 regulates the termination of midgut regeneration.

The molecular mechanisms by which stem cell proliferation is precisely controlled during the course of regeneration are poorly understood. Namely, how a damaged tissue senses when to terminate the regeneration process, inactivates stem cell mitotic activity, and organizes ECM integrity remain fundamental unanswered questions. The Drosophila midgut intestinal stem cell (ISC) offers an excellent model system to study the molecular basis for stem cell inactivation. Here, we show that a novel gene, CG6967 or dMOV10, is induced at the termination stage of midgut regeneration, and shows an inhibitory effect on ISC proliferation. dMOV10 encodes a putative component of the microRNA (miRNA) gene silencing complex (miRISC). Our data, along with previous studies on the mammalian MOV10, suggest that dMOV10 is not a core member of miRISC, but modulates miRISC activity as an additional component. Further analyses identified direct target mRNAs of dMOV10-containing miRISC, including Daughter against Dpp (Dad), a known inhibitor of BMP/TGF-ß signaling. We show that RNAi knockdown of Dad significantly impaired ISC division during regeneration. We also identified six miRNAs that are induced at the termination stage and their potential target transcripts. One of these miRNAs, mir-1, is required for proper termination of ISC division at the end of regeneration. We propose that miRNA-mediated gene regulation contributes to the precise control of Drosophila midgut regeneration.

KEOPS complex promotes homologous recombination via DNA resection.

KEOPS complex is one of the most conserved protein complexes in eukaryotes. It plays important roles in both telomere uncapping and tRNA N6-threonylcarbamoyladenosine (t6A) modification in budding yeast. But whether KEOPS complex plays any roles in DNA repair remains unknown. Here, we show that KEOPS complex plays positive roles in both DNA damage response and homologous recombination-mediated DNA repair independently of its t6A synthesis function. Additionally, KEOPS displays DNA binding activity in vitro, and is recruited to the chromatin at DNA breaks in vivo, suggesting a direct role of KEOPS in DSB repair. Mechanistically, KEOPS complex appears to promote DNA end resection through facilitating the association of Exo1 and Dna2 with DNA breaks. Interestingly, inactivation of both KEOPS and Mre11/Rad50/Xrs2 (MRX) complexes results in synergistic defect in DNA resection, revealing that KEOPS and MRX have some redundant functions in DNA resection. Thus we uncover a t6A-independent role of KEOPS complex in DNA resection, and propose that KEOPS might be a DSB sensor to assist cells in maintaining chromosome stability.

Yeast KEOPS complex regulates telomere length independently of its t6A modification function.

In Saccharomyces cerevisiae, the highly conserved Sua5 and KEOPS complex (including five subunits Kae1, Bud32, Cgi121, Pcc1 and Gon7) catalyze a universal tRNA modification, namely N6-threonylcarbamoyladenosine (t6A), and regulate telomere replication and recombination. However, whether telomere regulation function of Sua5 and KEOPS complex depends on the t6A modification activity remains unclear. Here we show that Sua5 and KEOPS regulate telomere length in the same genetic pathway. Interestingly, the telomere length regulation by KEOPS is independent of its t6A biosynthesis activity. Cytoplasmic overexpression of Qri7, a functional counterpart of KEOPS in mitochondria, restores cytosolic tRNA t6A modification and cell growth, but is not sufficient to rescue telomere length in the KEOPS mutant kae1Δ cells, indicating that a t6A modification-independent function is responsible for the telomere regulation. The results of our in vitro biochemical and in vivo genetic assays suggest that telomerase RNA TLC1 might not be modified by Sua5 and KEOPS. Moreover, deletion of KEOPS subunits results in a dramatic reduction of telomeric G-overhang, suggesting that KEOPS regulates telomere length by promoting G-overhang generation. These findings support a model in which KEOPS regulates telomere replication independently of its function on tRNA modification.

Tethering telomerase to telomeres increases genome instability and promotes chronological aging in yeast.

Chronological aging of the yeast Saccharomyces cerevisiae is attributed to multi-faceted traits especially those involving genome instability, and has been considered to be an aging model for post-mitotic cells in higher organisms. Telomeres are the physical ends of eukaryotic chromosomes, and are essential for genome integrity and stability. It remains elusive whether dysregulated telomerase activity affects chronological aging. We employed the CDC13-EST2 fusion gene, which tethers telomerase to telomeres, to examine the effect of constitutively active telomerase on chronological lifespan (CLS). The expression of Cdc13-Est2 fusion protein resulted in overlong telomeres (2 to 4 folds longer than normal telomeres), and long telomeres were stably maintained during long-term chronological aging. Accordingly, genome instability, manifested by accumulation of extra-chromosomal rDNA circle species, age-dependent CAN1 marker-gene mutation frequency and gross chromosomal rearrangement frequency, was significantly elevated. Importantly, inactivation of Sch9, a downstream kinase of the target of rapamycin complex 1 (TORC1), suppressed both the genome instability and accelerated chronological aging mediated by CDC13-EST2 expression. Interestingly, loss of the CDC13-EST2 fusion gene in the cells with overlong telomeres restored the regular CLS. Altogether, these data suggest that constitutively active telomerase is detrimental to the maintenance of genome stability, and promotes chronological aging in yeast.