rasiRNAs, DNA damage, and embryonic axis specification.

Drosophila repeat-associated small interfering RNAs (rasiRNAs) have been implicated in retrotransposon and stellate locus silencing. However, mutations in the rasiRNA pathway genes armitage, spindle-E, and aubergine disrupt embryonic axis specification, triggering defects in microtubule organization and localization of osk and grk mRNAs during oogenesis. We show that mutations in mei-41 and mnk, which encode ATR and Chk2 kinases that function in DNA damage signal transduction, dramatically suppress the cytoskeletal and RNA localization defects associated with rasiRNA mutations. In contrast, stellate and retrotransposon silencing are not restored in mei-41 and mnk double mutants. We also find that armitage, aubergine, and spindle-E mutations lead to germ-line-specific accumulation of gamma-H2Av foci, which form at DNA double-strand breaks, and that mutations in armi lead to Chk2-dependent phosphorylation of Vasa, an RNA helicase required for axis specification. The Drosophila rasiRNA pathway thus appears to suppress DNA damage in the germ line, and mutations in this pathway block axis specification by activating an ATR/Chk2-dependent DNA damage response that disrupts microtubule polarization and RNA localization.

In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway.

Drosophila bicoid mRNA is synthesized in the nurse cells and transported to the oocyte where microtubules and Exuperantia protein mediate localization to the anterior pole. Fluorescent bicoid mRNA injected into the oocyte displays nonpolar microtubule-dependent transport to the closest cortical surface, and the oocyte microtubule cytoskeleton lacks clear axial asymmetry. Nonetheless, bicoid mRNA injected into the nurse cell cytoplasm, withdrawn, and injected into a second oocyte shows microtubule-dependent transport to the anterior cortex. Nurse cells require microtubules and Exuperantia to support anterior transport of bicoid mRNA, and microtubules are required for bicoid mRNA-Exuperantia particle coassembly. We propose that microtubule-dependent Exuperantia-bicoid mRNA complex formation in the nurse cell cytoplasm allows anterior-specific transport on a grossly nonpolar oocyte microtubule network.

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.

CPEB, maskin, and cyclin B1 mRNA at the mitotic apparatus: implications for local translational control of cell division.

In Xenopus development, the expression of several maternal mRNAs is regulated by cytoplasmic polyadenylation. CPEB and maskin, two factors that control polyadenylation-induced translation are present on the mitotic apparatus of animal pole blastomeres in embryos. Cyclin B1 protein and mRNA, whose translation is regulated by polyadenylation, are colocalized with CPEB and maskin. CPEB interacts with microtubules and is involved in the localization of cyclin B1 mRNA to the mitotic apparatus. Agents that disrupt polyadenylation-induced translation inhibit cell division and promote spindle and centrosome defects in injected embryos. Two of these agents inhibit the synthesis of cyclin B1 protein and one, which has little effect on this process, disrupts the localization of cyclin B1 mRNA and protein. These data suggest that CPEB-regulated mRNA translation is important for the integrity of the mitotic apparatus and for cell division.

Actin cytoskeleton: putting a CAP on actin polymerization.

Two recent studies have identified a Drosophila homolog of cyclase-associated protein (CAP) as a developmentally important negative regulator of actin polymerization that may also directly mediate signal transduction.

Perinuclear localization and insulin responsiveness of GLUT4 requires cytoskeletal integrity in 3T3-L1 adipocytes.

The GLUT4 glucose transporter resides mostly in perinuclear membranes in unstimulated 3T3-L1 adipocytes and is acutely translocated to the cell surface in response to insulin. Using a novel method to purify intracellular GLUT4-enriched membranes, we identified by mass spectrometry the intermediate filament protein vimentin and the microtubule protein alpha-tubulin as components of these membranes. Immunoelectron microscopy of the GLUT4-containing membranes also revealed their association with these cytoskeletal proteins. Disruption of intermediate filaments and microtubules in 3T3-L1 adipocytes by microinjection of a vimentin-derived peptide of the helix initiation 1A domain caused marked dispersion of perinuclear GLUT4 to peripheral regions of the cells. Inhibition of the microtubule-based motor dynein by brief cytoplasmic acidification of cultured adipocytes also dispersed perinuclear GLUT4 and inhibited insulin-stimulated GLUT4 translocation to the cell surface. Insulin sensitivity was restored as GLUT4 was again concentrated near the nucleus upon recovery of cells in physiological buffer. These data suggest that GLUT4 trafficking to perinuclear membranes of cultured adipocytes is directed by dynein and is required for optimal GLUT4 regulation by insulin.

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.

The Drosophila ATM homologue Mei-41 has an essential checkpoint function at the midblastula transition.

BACKGROUND: Drosophila embryogenesis is initiated by 13 rapid syncytial mitotic divisions that do not require zygotic gene activity. This maternally directed cleavage phase of development terminates at the midblastula transition (MBT), at which point the cell cycle slows dramatically, membranes surround the cortical nuclei to form a cellular blastoderm, and zygotic gene expression is first required. RESULTS: We show that embryos lacking Mei-41, a Drosophila homologue of the ATM tumor suppressor, proceed through unusually short syncytial mitoses, fail to terminate syncytial division following mitosis 13, and degenerate without forming cells. A similar cleavage-stage arrest is produced by mutations in grapes, which encodes a homologue of the Checkpoint-1 kinase. We present biochemical, cytological and genetic data indicating that Mei-41 and Grapes are components of a conserved DNA-replication/damage checkpoint pathway that triggers inhibitory phosphorylation of the Cdc2 kinase and mediates resistance to replication inhibitors and DNA-damaging agents. This pathway is nonessential during postembryonic development, but it is required to terminate the cleavage stage at the MBT. Cyclins are required for Cdc2 kinase activity, and mutations in cyclin A and cyclin B bypass the requirement for mei-41 at the MBT. These mutations do not restore wild-type syncytial cell-cycle timing or the embryonic replication checkpoint, however, suggesting that Mei-41-mediated inhibition of Cdc2 has an additional essential function at the MBT. CONCLUSIONS: The Drosophila DNA-replication/damage checkpoint pathway can be activated by externally triggered DNA damage or replication defects throughout the life cycle, and under laboratory conditions this inducible function is nonessential. During early embryogenesis, however, this pathway is activated by developmental cues and is required for the transition from maternal to zygotic control of development at the MBT.

In vivo analyses of cytoplasmic transport and cytoskeletal organization during Drosophila oogenesis: characterization of a multi-step anterior localization pathway.

Anterior patterning of the Drosophila embryo depends on localization of bicoid (bcd) mRNA to the anterior pole of the developing oocyte, and bcd mRNA localization requires both the exuperantia (exu) gene and an intact microtubule cytoskeleton. To gain insight into the mechanism of anterior patterning, we have used time lapse laser scanning confocal microscopy to analyze transport of particles containing a Green Fluorescent Protein-Exu fusion (GFP-Exu), and to directly image microtubule organization in vivo. Our observations indicate that microtubules are required for three forms of particle movement within the nurse cells, while transport through the ring canals linking the nurse cells and oocyte appears to be independent of both microtubules and actin filaments. As particles enter the oocyte, a final microtubule-dependent step directs movement to the oocyte cortex. However, our observations and previous studies suggest that the polarity of the oocyte microtubule network is not in itself sufficient to generate anterior asymmetry, and that additional factors are required to restrict morphogens to the anterior pole. Based on these observations, we propose a multi-step anterior localization pathway.

Oocyte differentiation: a motor makes a difference.

In a variety of developmental systems, asymmetric mitoses precede, and are essential for, cellular differentiation. Recent studies demonstrate a role for the motor protein cytoplasmic dynein in generating the mitotic asymmetries that lead to Drosophila oocyte differentiation.

DNA-replication checkpoint control at the Drosophila midblastula transition.

Embryogenesis is typically initiated by a series of rapid mitotic divisions that are under maternal genetic control. The switch to zygotic control of embryogenesis at the midblastula transition is accompanied by significant increases in cell-cycle length and gene transcription, and changes in embryo morphology. Here we show that mutations in the grapes (grp) checkpoint 1 kinase homologue in Drosophila block the morphological and biochemical changes that accompany the midblastula transition, lead to a continuation of the maternal cell-cycle programme, and disrupt DNA-replication checkpoint control of cell-cycle progression. The timing of the midblastula transition is controlled by the ratio of nuclei to cytoplasm (the nucleocytoplasmic ratio), suggesting that this developmental transition is triggered by titration of a maternal factor by the increasing mass of nuclear material that accumulates during the rapid embryonic mitoses. Our observations support a model for cell-cycle control at the midblastula transition in which titration of a maternal component of the DNA-replication machinery slows DNA synthesis and induces a checkpoint-dependent delay in cell-cycle progression. This delay may allow both completion of S phase and transcription of genes that initiate the switch to zygotic control of embryogenesis.

Anastral meiotic spindle morphogenesis: role of the non-claret disjunctional kinesin-like protein.

We have used time-lapse laser scanning confocal microscopy to directly examine microtubule reorganization during meiotic spindle assembly in living Drosophila oocytes. These studies indicate that the bipolarity of the meiosis I spindle is not the result of a duplication and separation of centrosomal microtubule organizing centers (MTOCs). Instead, microtubules first associate with a tight chromatin mass, and then bundle to form a bipolar spindle that lacks asters. Analysis of mutant oocytes indicates that the Non-Claret Disjunctional (NCD) kinesin-like protein is required for normal spindle assembly kinetics and stabilization of the spindle during metaphase arrest. Immunolocalization analyses demonstrate that NCD is associated with spindle microtubules, and that the centrosomal components gamma-tubulin, CP-190, and CP-60 are not concentrated at the meiotic spindle poles. Based on these observations, we propose that microtubule bundling by the NCD kinesin-like protein promotes assembly of a stable bipolar spindle in the absence of typical MTOCs.

Mutations that perturb poly(A)-dependent maternal mRNA activation block the initiation of development.

Translational recruitment of maternal mRNAs is an essential process in early metazoan development. To identify genes required for this regulatory pathway, we have examined a collection of Drosophila female-sterile mutants for defects in translation of maternal mRNAs. This strategy has revealed that maternal-effect mutations in the cortex and grauzone genes impair translational activation and cytoplasmic polyadenylation of bicoid and Toll mRNAs. Cortex embryos contain a bicoid mRNA indistinguishable in amount, localization, and structure from that in wild-type embryos. However, the bicoid mRNA in cortex embryos contains a shorter than normal polyadenosine (poly(A)) tail. Injection of polyadenylated bicoid mRNA into cortex embryos allows translation demonstrating that insufficient polyadenylation prevents endogenous bicoid mRNA translation. In contrast nanos mRNA, which is activated by a poly(A)-independent mechanism, is translated in cortex embryos, indicating that the block in maternal mRNA activation is specific to a class of mRNAs. Cortex embryos are fertilized, but arrest at the onset of embryogenesis. Characterization of grauzone mutations indicates that the phenotype of these embryos is similar to cortex. These results identify a fundamental pathway that serves a vital role in the initiation of development.

The cytoskeleton and morphogenesis of the early Drosophila embryo.

Drosophila embryogenesis begins with thirteen mitotic divisions that occur without cytokinesis. During these syncytial divisions, a series of stereotyped nuclear movements produce a syncytial blastoderm embryo that is characterized by a uniform monolayer of cortical nuclei. Inhibitor studies indicate that actin filaments and microtubules mediate the coordinated nuclear movements of the syncytial stages of embryogenesis. Recent genetic and cytological analyses provide new insight into the functions of specific microtubule and actin filament arrays in organizing the syncytial embryo, and these may lead to the identification of novel regulatory and structural components of the cytoskeleton.

Cytoskeletal functions during Drosophila oogenesis.

Organismal morphogenesis is driven by a complex series of developmentally coordinated changes in cell shape, size, and number. These changes in cell morphology are in turn dependent on alterations in basic cytoarchitecture. Elucidating the mechanisms of development thus requires an understanding of the cytoskeletal elements that organize the cytoplasm of differentiating cells. Drosophila oogenesis has emerged as a versatile system for the study of cytoskeletal function during development. A series of highly coordinated changes in cytoskeletal organization are required to produce a mature Drosophila oocyte, and these cytoskeletal transformations are amenable to a variety of experimental approaches. Genetic, molecular, and cytological studies have shed light on the specific functions of the cytoskeleton during oogenesis. The results of these studies are reviewed here, and their mechanistic implications are considered.

Premature microtubule-dependent cytoplasmic streaming in cappuccino and spire mutant oocytes.

Embryonic axis specification in Drosophila melanogaster is achieved through the asymmetric subcellular localization of morphogenetic molecules within the oocyte. The cappuccino and spire loci are required for both posterior and dorsoventral patterning. Time-lapse confocal microscopic analyses of living egg chambers demonstrated that these mutations induce microtubule reorganization and the premature initiation of microtubule-dependent ooplasmic streaming. As a result, microtubule organization is altered and bulk ooplasm rapidly streams during the developmental stages in which morphogens are normally localized. These changes in oocyte cytoarchitecture and dynamics appear to disrupt axial patterning of the embryo.