Mars promotes dTACC dephosphorylation on mitotic spindles to ensure spindle stability

Somatic genetic rescue of a germline ribosome assembly defect

Indirect somatic genetic rescue (SGR) of a germline mutation is thought to be rare in inherited Mendelian disorders. Here, we establish that acquired mutations in the EIF6 gene are a frequent mechanism of SGR in Shwachman-Diamond syndrome (SDS), a leukemia predisposition disorder caused by a germline defect in ribosome assembly. Biallelic mutations in the SBDS or EFL1 genes in SDS impair release of the anti-association factor eIF6 from the 60S ribosomal subunit, a key step in the translational activation of ribosomes. Here, we identify diverse mosaic somatic genetic events (point mutations, interstitial deletion, reciprocal chromosomal translocation) in SDS hematopoietic cells that reduce eIF6 expression or disrupt its interaction with the 60S subunit, thereby conferring a selective advantage over non-modified cells. SDS-related somatic EIF6 missense mutations that reduce eIF6 dosage or eIF6 binding to the 60S subunit suppress the defects in ribosome assembly and protein synthesis across multiple SBDS-deficient species including yeast, Dictyostelium and Drosophila. Our data suggest that SGR is a universal phenomenon that may influence the clinical evolution of diverse Mendelian disorders and support eIF6 suppressor mimics as a therapeutic strategy in SDS.

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.

The microtubule-associated protein HURP recruits the centrosomal protein TACC3 to regulate K-fiber formation and support chromosome congression

Kinetochore fibers (K-fibers) are microtubule bundles attached to chromosomes. Efficient K-fiber formation is required for chromosome congression, crucial for faithful chromosome segregation in cells. However, the mechanisms underlying K-fiber formation before chromosome biorientation remain unclear. Depletion of hepatoma up-regulated protein (HURP), a RanGTP-dependent microtubule-associated protein localized on K-fibers, has been shown to result in low-efficiency K-fiber formation. Therefore, here we sought to identify critical interaction partners of HURP that may modulate this function. Using co-immunoprecipitation and bimolecular fluorescence complementation assays, we determined that HURP interacts directly with the centrosomal protein transforming acidic coiled coil-containing protein 3 (TACC3), a centrosomal protein, both in vivo and in vitro through the HURP^1-625 region. We found that HURP is important for TACC3 function during kinetochore microtubule assembly at the chromosome region in prometaphase. Moreover, HURP regulates stable lateral kinetochore attachment and chromosome congression in early mitosis by modulation of TACC3. These findings provide new insight into the coordinated regulation of K-fiber formation and chromosome congression in prometaphase by microtubule-associated proteins.

Protein Phosphatases Involved in Regulating Mitosis: Facts and Hypotheses

Almost all eukaryotic proteins are subject to post-translational modifications during mitosis and cell cycle, and in particular, reversible phosphorylation being a key event. The recent use of high-throughput experimental analyses has revealed that more than 70% of all eukaryotic proteins are regulated by phosphorylation; however, the mechanism of dephosphorylation, counteracting phosphorylation, is relatively unknown. Recent discoveries have shown that many of the protein phosphatases are involved in the temporal and spatial control of mitotic events, such as mitotic entry, mitotic spindle assembly, chromosome architecture changes and cohesion, and mitotic exit. This implies that certain phosphatases are tightly regulated for timely dephosphorylation of key mitotic phosphoproteins and are essential for control of various mitotic processes. This review describes the physiological and pathological roles of mitotic phosphatases, as well as the versatile role of various protein phosphatases in several mitotic events.

Identification of Multiple Loci Associated with Social Parasitism in Honeybees

In colonies of the honeybee Apis mellifera, the queen is usually the only reproductive female, which produces new females (queens and workers) by laying fertilized eggs. However, in one subspecies of A. mellifera, known as the Cape bee (A. m. capensis), worker bees reproduce asexually by thelytoky, an abnormal form of meiosis where two daughter nucleii fuse to form single diploid eggs, which develop into females without being fertilized. The Cape bee also exhibits a suite of phenotypes that facilitate social parasitism whereby workers lay such eggs in foreign colonies so their offspring can exploit their resources. The genetic basis of this switch to social parasitism in the Cape bee is unknown. To address this, we compared genome variation in a sample of Cape bees with other African populations. We find genetic divergence between these populations to be very low on average but identify several regions of the genome with extreme differentiation. The regions are strongly enriched for signals of selection in Cape bees, indicating that increased levels of positive selection have produced the unique set of derived phenotypic traits in this subspecies. Genetic variation within these regions allows unambiguous genetic identification of Cape bees and likely underlies the genetic basis of social parasitism. The candidate loci include genes involved in ecdysteroid signaling and juvenile hormone and dopamine biosynthesis, which may regulate worker ovary activation and others whose products localize at the centrosome and are implicated in chromosomal segregation during meiosis. Functional analysis of these loci will yield insights into the processes of reproduction and chemical signaling in both parasitic and non-parasitic populations and advance understanding of the process of normal and atypical meiosis.

Synergy between multiple microtubule-generating pathways confers robustness to centrosome-driven mitotic spindle formation

The mitotic spindle is defined by its organized, bipolar mass of microtubules, which drive chromosome alignment and segregation. Although different cells have been shown to use different molecular pathways to generate the microtubules required for spindle formation, how these pathways are coordinated within a single cell is poorly understood. We have tested the limits within which the Drosophila embryonic spindle forms, disrupting the inherent temporal control that overlays mitotic microtubule generation, interfering with the molecular mechanism that generates new microtubules from preexisting ones, and disrupting the spatial relationship between microtubule nucleation and the usually dominant centrosome. Our work uncovers the possible routes to spindle formation in embryos and establishes the central role of Augmin in all microtubule-generating pathways. It also demonstrates that the contributions of each pathway to spindle formation are integrated, highlighting the remarkable flexibility with which cells can respond to perturbations that limit their capacity to generate microtubules.

The Drosophila microtubule-associated protein mars stabilizes mitotic spindles by crosslinking microtubules through its N-terminal region

Correct segregation of genetic material relies on proper assembly and maintenance of the mitotic spindle. How the highly dynamic microtubules (MTs) are maintained in stable mitotic spindles is a key question to be answered. Motor and non-motor microtubule associated proteins (MAPs) have been reported to stabilize the dynamic spindle through crosslinking adjacent MTs. Mars, a novel MAP, is essential for the early development of Drosophila embryos. Previous studies showed that Mars is required for maintaining an intact mitotic spindle but did not provide a molecular mechanism for this function. Here we show that Mars is able to stabilize the mitotic spindle in vivo. Both in vivo and in vitro data reveal that the N-terminal region of Mars functions in the stabilization of the mitotic spindle by crosslinking adjacent MTs.

Specific Cooperation Between Imp-α2 and Imp-β/Ketel in Spindle Assembly During Drosophila Early Nuclear Divisions

The multifunctional factors Imp-α and Imp-β are involved in nuclear protein import, mitotic spindle dynamics, and nuclear membrane formation. Furthermore, each of the three members of the Imp-α family exerts distinct tasks during development. In Drosophila melanogaster, the imp-α2 gene is critical during oogenesis for ring canal assembly; specific mutations, which allow oogenesis to proceed normally, were found to block early embryonic mitosis. Here, we show that imp-α2 and imp-β genetically interact during early embryonic development, and we characterize the pattern of defects affecting mitosis in embryos laid by heterozygous imp-α2(D14) and imp-β(KetRE34) females. Embryonic development is arrested in these embryos but is unaffected in combinations between imp-β(KetRE34) and null mutations in imp-α1 or imp-α3. Furthermore, the imp-α2(D14)/imp-β(KetRE34) interaction could only be rescued by an imp-α2 transgene, albeit not imp-α1 or imp-α3, showing the exclusive imp-α2 function with imp-β. Use of transgenes carrying modifications in the major Imp-α2 domains showed the critical requirement of the nuclear localization signal binding (NLSB) site in this process. In the mutant embryos, we found metaphase-arrested mitoses made of enlarged spindles, suggesting an unrestrained activity of factors promoting spindle assembly. In accordance with this, we found that Imp-β(KetRE34) and Imp-β(KetD) bind a high level of RanGTP/GDP, and a deletion decreasing RanGTP level suppresses the imp-β(KetRE34) phenotype. These data suggest that a fine balance among Imp-α2, Imp-β, RanGTP, and the NLS cargos is critical for mitotic progression during early embryonic development.

RanGTP is required for meiotic spindle organization and the initiation of embryonic development in Drosophila

RanGTP is important for chromosome-dependent spindle assembly in Xenopus extracts. Here we report on experiments to determine the role of the Ran pathway on microtubule dynamics in Drosophila oocytes and embryos. Females expressing a dominant-negative form of Ran have fertility defects, suggesting that RanGTP is required for normal fertility. This is not, however, because of a defect in acentrosomal meiotic spindle assembly. Therefore, RanGTP does not appear to be essential or sufficient for the formation of the acentrosomal spindle. Instead, the most important function of the Ran pathway in spindle assembly appears to be in the tapering of microtubules at the spindle poles, which might be through regulation of proteins such as TACC and the HURP homolog, Mars. One consequence of this spindle organization defect is an increase in the nondisjunction of achiasmate chromosomes. However, the meiotic defects are not severe enough to cause the decreased fertility. Reductions in fertility occur because RanGTP has a role in microtubule assembly that is not directly nucleated by the chromosomes. This includes microtubules nucleated from the sperm aster, which are required for pronuclear fusion. We propose that following nuclear envelope breakdown, RanGTP is released from the nucleus and creates a cytoplasm that is activated for assembling microtubules, which is important for processes such as pronuclear fusion. Around the chromosomes, however, RanGTP might be redundant with other factors such as the chromosome passenger complex.

Mitotic phosphatases: from entry guards to exit guides

While the importance of protein kinases for the spatial and temporal control of mitotic events has long been recognized, mitotic phosphatases have only recently come into the limelight. It is now well established that protein phosphatases counteract mitotic kinases, so contributing to the generation of switch-like responses at mitotic stage transitions. In addition, the timely dephosphorylation of mitotic phosphoproteins by tightly regulated phosphatases is required for the assembly and stability of the mitotic spindle, the initiation of anaphase, and exit from mitosis. Mitotic phosphatases also emerge as effectors of the DNA damage and spindle assembly checkpoints. These new findings show that protein phosphatases regulate every step of mitosis and provide novel insights into the dynamic and versatile nature of mitotic phosphoregulation.

mars and tousled-like kinase act in parallel to ensure chromosome fidelity in Drosophila

Background

High levels of Hepatoma Up-Regulated Protein (HURP) and Tousled-Like Kinase (TLK) transcripts are found in hepatocellular carcinoma. HURP overexpression induces anchorage-independent growth of 293-T cells and enhances a rough-eye phenotype resulting from tlk overexpression in Drosophila. In addition, both HURP and Mars, a Drosophila HURP sequence homologue, promote polymerization of mitotic spindles. Thus, the genetic interaction of mars with tlk might be required for accurate chromosome segregation.

Methods

To reveal whether chromosome fidelity was decreased, the frequency of gynandromorphy, an individual with both male and female characteristics, and of non-disjunction were measured in the progeny from parents with reduced mars and/or tlk activities and analyzed by Student's t-test. To show that the genetic interaction between mars and tlk is epistatic or parallel, a cytological analysis of embryos with either reduced or increased activities of mars and/or tlk was used to reveal defects in mitotic-spindle morphology and chromosome segregation.

Results

A significant but small fraction of the progeny from parents with reduced mars activity showed gynandromorphy and non-disjunction. Results of cytological analysis revealed that the decrease in chromosome fidelity was a result of delayed polymerization of the mitotic spindle, which led to asynchronous chromosome segregation in embryos that had reduced mars activity. By removing one copy of tousled-like kinase (tlk) from flies with reduced mars activity, chromosome fidelity was further reduced. This was indicated by an increased in the non-disjunction rate and more severe asynchrony. However, the morphology of the mitotic spindles in the embryos at metaphase where both gene activities were reduced was similar to that in mars embryos. Furthermore, tlk overexpression did not affect the morphology of the mitotic spindles and the cellular localization of Mars protein.

Conclusion

Chromosome fidelity in progeny from parents with reduced mars and/or tlk activity was impaired. The results from cytological studies revealed that mars and tlk function in parallel and that a balance between mars activity and tlk activity is required for cells to progress through mitosis correctly, thus ensuring chromosome fidelity.

Mars, a Drosophila protein related to vertebrate HURP, is required for the attachment of centrosomes to the mitotic spindle during syncytial nuclear divisions

The formation of the mitotic spindle is controlled by the microtubule organizing activity of the centrosomes and by the effects of chromatin-associated Ran-GTP on the activities of spindle assembly factors. In this study we show that Mars, a Drosophila protein with sequence similarity to vertebrate hepatoma upregulated protein (HURP), is required for the attachment of the centrosome to the mitotic spindle. More than 80% of embryos derived from mars mutant females do not develop properly due to severe mitotic defects during the rapid nuclear divisions in early embryogenesis. Centrosomes frequently detach from spindles and from the nuclear envelope and nucleate astral microtubules in ectopic positions. Consistent with its function in spindle organization, Mars localizes to nuclei in interphase and associates with the mitotic spindle, in particular with the spindle poles, during mitosis. We propose that Mars is an important linker between the spindle and the centrosomes that is required for proper spindle organization during the rapid mitotic cycles in early embryogenesis.