Identification and characterization of mitotic mutations in Drosophila

Phenotypic characterization of diamond (dind), a Drosophila gene required for multiple aspects of cell division

Many genes are required for the assembly of the mitotic apparatus and for proper chromosome behavior during mitosis and meiosis. A fruitful approach to elucidate the mechanisms underlying cell division is the accurate phenotypic characterization of mutations in these genes. Here, we report the identification and characterization of diamond (dind), an essential Drosophila gene required both for mitosis of larval brain cells and for male meiosis. Larvae homozygous for any of the five EMS-induced mutations die in the third-instar stage and exhibit multiple mitotic defects. Mutant brain cells exhibit poorly condensed chromosomes and frequent chromosome breaks and rearrangements; they also show centriole fragmentation, disorganized mitotic spindles, defective chromosome segregation, endoreduplicated metaphases, and hyperploid and polyploid cells. Comparable phenotypes occur in mutant spermatogonia and spermatocytes. The dind gene encodes a non-conserved protein with no known functional motifs. Although the Dind protein exhibits a rather diffuse localization in both interphase and mitotic cells, fractionation experiments indicate that some Dind is tightly associated with the chromatin. Collectively, these results suggest that loss of Dind affects chromatin organization leading to defects in chromosome condensation and integrity, which in turn affect centriole stability and spindle assembly. However, our results do not exclude the possibility that Dind directly affects some behaviors of the spindle and centrosomes.

Diverse mitotic and interphase functions of condensins in Drosophila

The condensin complex has been implicated in the higher-order organization of mitotic chromosomes in a host of model eukaryotes from yeasts to flies and vertebrates. Although chromosomes paradoxically appear to condense in condensin mutants, chromatids are not properly resolved, resulting in chromosome segregation defects during anaphase. We have examined the role of different condensin complex components in interphase chromatin function by examining the effects of various condensin mutations on position-effect variegation in Drosophila melanogaster. Surprisingly, most mutations affecting condensin proteins were often found to result in strong enhancement of variegation in contrast to what might be expected for proteins believed to compact the genome. This suggests either that the role of condensin proteins in interphase differs from their expected role in mitosis or that the way we envision condensin's activity needs to be modified to accommodate alternative possibilities.

Invadolysin: a novel, conserved metalloprotease links mitotic structural rearrangements with cell migration

The cell cycle is widely known to be regulated by networks of phosphorylation and ubiquitin-directed proteolysis. Here, we describe IX-14/invadolysin, a novel metalloprotease present only in metazoa, whose activity appears to be essential for mitotic progression. Mitotic neuroblasts of Drosophila melanogaster IX-14 mutant larvae exhibit increased levels of nuclear envelope proteins, monopolar and asymmetric spindles, and chromosomes that appear hypercondensed in length with a surrounding halo of loosely condensed chromatin. Zymography reveals that a protease activity, present in wild-type larval brains, is missing from homozygous tissue, and we show that IX-14/invadolysin cleaves lamin in vitro. The IX-14/invadolysin protein is predominantly found in cytoplasmic structures resembling invadopodia in fly and human cells, but is dramatically relocalized to the leading edge of migrating cells. Strikingly, we find that the directed migration of germ cells is affected in Drosophila IX-14 mutant embryos. Thus, invadolysin identifies a new family of conserved metalloproteases whose activity appears to be essential for the coordination of mitotic progression, but which also plays an unexpected role in cell migration.

The HIV-Tat protein induces chromosome number aberrations by affecting mitosis

To analyze the effects of the HIV-Tat-tubulin interaction, we microinjected HIV-Tat purified protein into Drosophila syncytial embryos. Following the Tat injection, altered timing of the cortical nuclear cycles was observed; specifically, the period between the nuclear envelope breakdown and anaphase initiation was lengthened as was the period between anaphase initiation and the formation of the next nuclear envelope. These two periods correspond to kinetochore alignment at metaphase and to mitosis exit, respectively. We also demonstrated that these two delays are the consequence of damage specifically induced by Tat on kinetochore alignment and on the timing of sister chromatid segregation at anaphase. Furthermore, we show that the expression of Tat in Drosophila larvae brain cells produces a significant percentage of polyploid and aneuploid cells. The results reported here indicate that Tat impairs the mitotic process and that Tat-tubulin interaction appears to be responsible for the observed defects. The presence of polyploid and aneuploid cells is consistent with a delay or arrest in the M phase of a substantial fraction of the cells expressing Tat, suggesting that mitotic spindle checkpoints are overridden following Tat expression.

Flattening Drosophila cells for high-resolution light microscopic studies of mitosis in vitro

Here we briefly review techniques used to flatten cells that otherwise round in culture, so that their division can be more clearly analyzed in vitro by high resolution light microscopy. We then describe an agar overlay procedure for use with isolated Drosophila neuroblasts, which promotes their long-term viability while also allowing for correlative studies of the same cell in the living and fixed state. This same procedure can also be used to obtain high temporal and spatial resolution images of mitosis and cytokinesis in cultured Drosophila Schneider S2 cells, which are a popular model for RNAi studies.

The Drosophila kinesin-like protein KLP67A is essential for mitotic and male meiotic spindle assembly

We have performed a mutational analysis together with RNA interference to determine the role of the kinesin-like protein KLP67A in Drosophila cell division. During both mitosis and male meiosis, Klp67A mutations cause an increase in MT length and disrupt discrete aspects of spindle assembly, as well as cytokinesis. Mutant cells exhibit greatly enlarged metaphase spindle as a result of excessive MT polymerization. The analysis of both living and fixed cells also shows perturbations in centrosome separation, chromosome segregation, and central spindle assembly. These data demonstrate that the MT plus end-directed motor KLP67A is essential for spindle assembly during mitosis and male meiosis and suggest that the regulation of MT plus-end polymerization is a key determinant of spindle architecture throughout cell division.

Arp2/3-dependent pseudocleavage [correction of psuedocleavage] furrow assembly in syncytial Drosophila embryos

BACKGROUND

In syncytial blastoderm Drosophila embryos, actin caps assemble during telophase. As the cell cycle progresses through interphase, these small caps expand and fuse to form pseudocleavage furrows that are structurally related to the cleavage furrows that assemble during somatic cell division. The molecular mechanism driving cell cycle coordinated actin reorganization from the caps to the furrows is not understood.

RESULTS

We show that Drosophila embryos contain a typical Arp2/3 complex and that components of this complex localize to the margins of the expanding caps, to mature pseudocleavage furrows, and to somatic cell cleavage furrows during the postcellularization embryonic divisions. A mutation that disrupts the arpc1 subunit of Arp2/3 leads to spindle fusions that are characteristic of pseudocleavage furrow disruption. By contrast, this mutation does not significantly affect nuclear positioning during interphase, which is dependent on actin cap function. In vivo analysis of actin reorganization demonstrates that the arpc1 mutation does not prevent assembly of small actin caps but blocks cap expansion and furrow assembly as the cell cycle progresses through interphase. The scrambled gene is also required for cap expansion and furrow assembly, and Scrambled is required for Arp2/3 localization to the cap margins.

CONCLUSIONS

The Drosophila Arp2/3 complex and Scrambled protein are required for actin cap expansion and pseudocleavage furrow formation during the syncytial blastoderm divisions. We propose that Scrambled-dependent localization of Arp2/3 to the margins of the expanding caps triggers local actin polymerization that drives cap expansion and pseudocleavage furrow assembly.

Kinetochore dynein: its dynamics and role in the transport of the Rough deal checkpoint protein

We describe the dynamics of kinetochore dynein-dynactin in living Drosophila embryos and examine the effect of mutant dynein on the metaphase checkpoint. A functional conjugate of dynamitin with green fluorescent protein accumulates rapidly at prometaphase kinetochores, and subsequently migrates off kinetochores towards the poles during late prometaphase and metaphase. This behaviour is seen for several metaphase checkpoint proteins, including Rough deal (Rod). In neuroblasts, hypomorphic dynein mutants accumulate in metaphase and block the normal redistribution of Rod from kinetochores to microtubules. By transporting checkpoint proteins away from correctly attached kinetochores, dynein might contribute to shutting off the metaphase checkpoint, allowing anaphase to ensue.

Chromosome elasticity and mitotic polar ejection force measured in living Drosophila embryos by four-dimensional microscopy-based motion analysis

BACKGROUND

Mitosis involves the interaction of many different components, including chromatin, microtubules, and motor proteins. Dissecting the mechanics of mitosis requires methods of studying not just each component in isolation, but also the entire ensemble of components in its full complexity in genetically tractable model organisms.

RESULTS

We have developed a mathematical framework for analyzing motion in four-dimensional microscopy data sets that allows us to measure elasticity, viscosity, and forces by tracking the conformational movements of mitotic chromosomes. We have used this approach to measure, for the first time, the basic biophysical parameters of mitosis in wild-type Drosophila melanogaster embryos. We found that Drosophila embryo chromosomes are significantly less rigid than the much larger chromosomes of vertebrates. Anaphase kinetochore force and nucleoplasmic viscosity were comparable with previous estimates in other species. Motion analysis also allowed us to measure the magnitude of the polar ejection force exerted on chromosome arms during metaphase by individual microtubules. We find the magnitude of this force to be approximately 1 pN, a number consistent with force generation either by collision of growing microtubules with chromosomes or by single kinesin motors.

CONCLUSIONS

Motion analysis allows noninvasive mechanical measurements to be made in complex systems. This approach should allow the functional effects of Drosophila mitotic mutants on chromosome condensation, kinetochore forces, and the polar ejection force to be determined.

Centrosomes and the Scrambled protein coordinate microtubule-independent actin reorganization

In Drosophila syncytial blastoderm embryos, centrosomes specify the position of actin-based interphase caps and mitotic furrows. Mutations in the scrambled locus prevent assembly of mitotic furrows, but do not block actin cap formation. The scrambled gene encodes a protein that localizes to the mitotic furrows and centrosomes. Sced localization, actin reorganization from caps into mitotic furrows, and centrosome-coordinated assembly of actin caps are not blocked by microtubule disruption. Our results indicate that centrosomes may coordinate assembly of cortical actin caps through a microtubule-independent mechanism, and that Scrambled mediates a second microtubule-independent process that drives mitotic furrow assembly.

DNA-replication/DNA-damage-dependent centrosome inactivation in Drosophila embryos

During early embryogenesis of Drosophila melanogaster, mutations in the DNA-replication checkpoint lead to chromosome-segregation failures. Here we show that these segregation failures are associated with the assembly of an anastral microtubule spindle, a mitosis-specific loss of centrosome function, and dissociation of several components of the gamma-tubulin ring complex from a core centrosomal structure. The DNA-replication inhibitor aphidicolin and DNA-damaging agents trigger identical mitotic defects in wild-type embryos, indicating that centrosome inactivation is a checkpoint-independent and mitosis-specific response to damaged or incompletely replicated DNA. We propose that centrosome inactivation is part of a damage-control system that blocks chromosome segregation when replication/damage checkpoint control fails.