Inscuteable-dependent apical localization of the microtubule-binding protein Cornetto suggests a role in asymmetric cell division

A modular toolset of phiC31-based fluorescent protein tagging vectors for Drosophila

The Drosophila transgenic technology and fluorescent protein fusions are powerful tools to analyze protein expression patterns, subcellular localization and protein dynamics. Recently, the Drosophila transgenic technology has been improved by the highly efficient phiC31 site-specific integration system. Many new and improved fluorescent proteins with desirable advantages have been developed. However, the phiC31 system and the newly developed fluorescent proteins have not been systematically applied in Drosophila transgenic vectors. Here, we have constructed a modular toolset of C-terminal fluorescent protein fusion vectors based on phiC31 site-specific integration system for the generation of transgenic Drosophila lines. These cloning vectors contain a variety of fluorescent tags, including blue, cyan, green or red fluorescent proteins, photoactivatable or photoswitchable fluorescent proteins, fluorescent timers, photosensitizers and bimolecular fluorescence complementation tags. These vectors provide a range of transcriptional regulation options including UAST, UASP, UASC, LexAop, QUAS, Ubi, αTub67C and αTub84B promoters, and two screening marker options including white and vermilion gene. The vectors have been tested in vivo and can produce fluorescent chimeric proteins that are functional.

Proteomics approach to study the functions of Drosophila myosin VI through identification of multiple cargo-binding proteins

Myosin VI is a molecular motor implicated in many processes, and it likely associates with a variety of cargoes that specify its functions. Although it is critical to Drosophila development, little is known about its cellular roles. To reveal its involvement in specific pathways, we sought to identify the binding partners of Drosophila myosin VI. We used affinity chromatography and mass spectrometry to discover interacting proteins, which we tested for direct binding. Using this approach, we found that the microtubule-associated protein Cornetto bound myosin VI, and we demonstrated a role for both in secretion of the lipidated morphogen Hedgehog. We also identified a number of other binding proteins, and further characterization of their interactions with myosin VI will advance our understanding of the roles of these complexes in cellular and developmental processes. Thus, our method has provided us the means to gain valuable insight into the multifaceted roles of a motor protein in vivo.

Multiple centrosomes: together they stand, divided they fall

Cells with extra centrosomes rely entirely on centrosome clustering mechanisms to assemble a bipolar spindle and to divide in a bipolar fashion. To identify the pathways involved in suppression of multipolarity, Kwon, Godinho, and colleagues (pp. 2189-2203) have set up a genome-wide screen in Drosophila S2 cells. Surprisingly, they found that efficient clustering requires a large number of proteins associated with a variety of cellular functions.

Mechanisms to suppress multipolar divisions in cancer cells with extra centrosomes

Multiple centrosomes in tumor cells create the potential for multipolar divisions that can lead to aneuploidy and cell death. Nevertheless, many cancer cells successfully divide because of mechanisms that suppress multipolar mitoses. A genome-wide RNAi screen in Drosophila S2 cells and a secondary analysis in cancer cells defined mechanisms that suppress multipolar mitoses. In addition to proteins that organize microtubules at the spindle poles, we identified novel roles for the spindle assembly checkpoint, cortical actin cytoskeleton, and cell adhesion. Using live cell imaging and fibronectin micropatterns, we found that interphase cell shape and adhesion pattern can determine the success of the subsequent mitosis in cells with extra centrosomes. These findings may identify cancer-selective therapeutic targets: HSET, a normally nonessential kinesin motor, was essential for the viability of certain extra centrosome-containing cancer cells. Thus, morphological features of cancer cells can be linked to unique genetic requirements for survival.

Extrinsic cues orient the cell division axis in Drosophila embryonic neuroblasts

Cell polarity must be integrated with tissue polarity for proper development. The Drosophila embryonic central nervous system (CNS) is a highly polarized tissue; neuroblasts occupy the most apical layer of cells within the CNS, and lie just basal to the neural epithelium. Neuroblasts are the CNS progenitor cells and undergo multiple rounds of asymmetric cell division, ;budding off' smaller daughter cells (GMCs) from the side opposite the epithelium, thereby positioning neuronal/glial progeny towards the embryo interior. It is unknown whether this highly stereotypical orientation of neuroblast divisions is controlled by an intrinsic cue (e.g. cortical mark) or an extrinsic cue (e.g. cell-cell signal). Using live imaging and in vitro culture, we find that neuroblasts in contact with epithelial cells always ;bud off' GMCs in the same direction, opposite from the epithelia-neuroblast contact site, identical to what is observed in vivo. By contrast, isolated neuroblasts 'bud off' GMCs at random positions. Imaging of centrosome/spindle dynamics and cortical polarity shows that in neuroblasts contacting epithelial cells, centrosomes remained anchored and cortical polarity proteins localize at the same epithelia-neuroblast contact site over subsequent cell cycles. In isolated neuroblasts, centrosomes drifted between cell cycles and cortical polarity proteins showed a delay in polarization and random positioning. We conclude that embryonic neuroblasts require an extrinsic signal from the overlying epithelium to anchor the centrosome/centrosome pair at the site of epithelial-neuroblast contact and for proper temporal and spatial localization of cortical Par proteins. This ensures the proper coordination between neuroblast cell polarity and CNS tissue polarity.

Asymmetric distribution of EGFR receptor during mitosis generates diverse CNS progenitor cells

It has been debated whether asymmetric distribution of cell surface receptors during mitosis could generate asymmetric cell divisions by yielding daughters with different environmental responsiveness and, thus, different fates. We have found that in mouse embryonic forebrain ventricular and subventricular zones, the EGFR can distribute asymmetrically during mitosis in vivo and in vitro. This occurs during divisions yielding two Nestin+ progenitor cells, via an actin-dependent mechanism. The resulting sibling progenitor cells respond differently to EGFR ligand in terms of migration and proliferation. Moreover, they express different phenotypic markers: the EGFRhigh daughter usually has radial glial/astrocytic markers, while its EGFRlow sister lacks them, indicating fate divergence. Lineage trees of cultured cortical glioblasts reveal repeated EGFR asymmetric distribution, and asymmetric divisions underlie formation of oligodendrocytes and astrocytes in clones. These data suggest that asymmetric EGFR distribution contributes to forebrain development by creating progenitors with different proliferative, migratory, and differentiation responses to ligand.

inscuteable mRNA Localization Is Dynein-Dependent and Regulates Apicobasal Polarity and Spindle Length in Drosophila Neuroblasts

Drosophila neuroblasts undergo asymmetric divisions along the apicobasal axis to produce two daughter cells of unequal size and different developmental fate. Inscuteable (Insc) protein functions as part of an apically localized complex to coordinate orientation of the mitotic spindle and basal sorting of cell fate determinants. insc mRNA transcripts also localize apically in neuroblasts, yet the mechanism underpinning this process and its developmental significance are unknown. Here, we show that the Egalitarian (Egl)/Bicaudal-D (BicD)/dynein mRNA transport machinery mediates apical localization of insc mRNA transcripts in neuroblasts, and we provide evidence that insc localization is required for efficient apical targeting of Insc protein. egl and BicD mutant neuroblasts display defects in apicobasal polarity, which is consistent with apical Insc activity being reduced. Also, we observe shortened mitotic spindles at metaphase in egl, BicD, and insc mutant neuroblasts and demonstrate a previously unknown, dose-dependent requirement for Insc in augmenting metaphase spindle length. We conclude that localization of insc mRNA transcripts in neuroblasts confers maximal levels of apical Insc activity, which is required for accurate control of metaphase spindle length, division orientation, and asymmetric cell division.

Identification of critical domains and putative partners for the Caenorhabditis elegans spindle component LIN-5

Successful cell division requires proper assembly, placement and functioning of the spindle apparatus that segregates the chromosomes. The Caenorhabditis elegans gene lin-5 encodes a novel coiled-coil component of the spindle required for spindle positioning and chromosome segregation. To gain further insights into lin-5 function, we screened for dominant suppressors of the partial loss-of-function phenotype associated with the mutation lin-5(ev571ts ), and isolated 68 suppressing mutations. Eight out of the ten suppressors sequenced contained intragenic missense mutations immediately upstream of the lesion in lin-5(ev571ts ). These probably help to stabilize protein-protein interactions mediated by the coiled-coil domain. This domain was found to be required for binding to several putative LIN-5 interacting (LFI) proteins identified in yeast two-hybrid screens. Interestingly, interaction with the coiled-coil protein LFI-1 was specifically reduced by the lin-5(ev571ts ) mutation and restored by a representative intragenic suppressor mutation. Immunostaining experiments showed that LIN-5 and LFI-1 may co-localize around the kinetochore microtubules during metaphase, indicating potential interaction in vivo. The coiled-coil domain of LIN-5 was also found to mediate homodimerization, while the C-terminal region of LIN-5 was sufficient for interaction with GPR-1, a recently identified component of a LIN-5 spindle-regulatory complex. A single amino-acid substitution in the N-terminal region of LIN-5, encoded by the e1457 allele, abolished all LIN-5 interactions. Taken together, our results indicate that the spindle functions of LIN-5 depend on interactions with multiple protein partners, and that these interactions are mediated through several different domains of LIN-5.

Asymmetric localization of LGN but not AGS3, two homologs ofDrosophilapins, in dividing human neural progenitor cells

Human neural progenitor cells (hNPCs) can be recovered from postmortem human brains and used to study the molecular basis of neurogenesis. Human NPCs are being used to investigate the molecular basis of cell fate determination during stem cell divisions, based on comparison with the Drosophila model system. Drosophila neuroblasts and sensory organ precursors undergo well-defined asymmetric cell divisions (ACD), under the control of a genetically defined set of apical and basal determinants that are localized tightly and dynamically during division. We show by indirect immunofluorescence, confocal microscopy, and time-lapse video-microscopy that LGN and AGS3, two human homologs of the Drosophila ACD determinant Pins, have distinct patterns of localization in hNPCs. When cells are grown under conditions favoring proliferation, LGN is distributed asymmetrically in a cell cycle-dependent manner; it localizes to one side of the dividing cell and segregates into one of the daughter cells. When the cells are grown under conditions favoring differentiation, LGN accumulates in double foci similar to those containing the mitotic apparatus protein NuMA, and in a pattern shown previously for LGN and NuMA in differentiated cells. AGS3, a slightly more distant Pins homolog than LGN, does not show asymmetric localization in these cells. The progenitor cell marker nestin also localizes asymmetrically in colcemid-treated hNPCs and colocalizes with LGN. The results suggest that hNPCs undergo ACD and that similar molecular pathways may underlie these divisions in Drosophila and human cells.

Quantitative analysis of bristle number in Drosophila mutants identifies genes involved in neural development

BACKGROUND

The identification of the function of all genes that contribute to specific biological processes and complex traits is one of the major challenges in the postgenomic era. One approach is to employ forward genetic screens in genetically tractable model organisms. In Drosophila melanogaster, P element-mediated insertional mutagenesis is a versatile tool for the dissection of molecular pathways, and there is an ongoing effort to tag every gene with a P element insertion. However, the vast majority of P element insertion lines are viable and fertile as homozygotes and do not exhibit obvious phenotypic defects, perhaps because of the tendency for P elements to insert 5' of transcription units. Quantitative genetic analysis of subtle effects of P element mutations that have been induced in an isogenic background may be a highly efficient method for functional genome annotation.

RESULTS

Here, we have tested the efficacy of this strategy by assessing the extent to which screening for quantitative effects of P elements on sensory bristle number can identify genes affecting neural development. We find that such quantitative screens uncover an unusually large number of genes that are known to function in neural development, as well as genes with yet uncharacterized effects on neural development, and novel loci.

CONCLUSIONS

Our findings establish the use of quantitative trait analysis for functional genome annotation through forward genetics. Similar analyses of quantitative effects of P element insertions will facilitate our understanding of the genes affecting many other complex traits in Drosophila.