Bazooka recruits Inscuteable to orient asymmetric cell divisions in Drosophila neuroblasts

Asymmetric cell divisions can be generated by the segregation of determinants into one of the two daughter cells. In Drosophila, neuroblasts divide asymmetrically along the apical-basal axis shortly after their delamination from the neuroectodermal epithelium. Several proteins, including Numb and Miranda, segregate into the basal daughter cell and are needed for the determination of its correct cell fate. Both the apical-basal orientation of the mitotic spindle and the localization of Numb and Miranda to the basal cell cortex are directed by Inscuteable, a protein that localizes to the apical cell cortex before and during neuroblast mitosis. Here we show that the apical localizaton of Inscuteable requires Bazooka, a protein containing a PDZ domain that is essential for apical-basal polarity in epithelial cells. Bazooka localizes with Inscuteable in neuroblasts and binds to the Inscuteable localization domain in vitro and in vivo. In embryos lacking both maternal and zygotic bazooka function, Inscuteable no longer localizes asymmetrically in neuroblasts and is instead uniformly distributed in the cytoplasm. Mitotic spindles in neuroblasts are misoriented in these embryos, and the proteins Numb and Miranda fail to localize asymmetrically in metaphase. Our results suggest that direct binding to Bazooka mediates the asymmetric localization of Inscuteable and connects the asymmetric division of neuroblasts to the axis of epithelial apical-basal polarity.     

Fat cells reactivate quiescent neuroblasts via TOR and glial insulin relays in Drosophila

Many stem, progenitor and cancer cells undergo periods of mitotic quiescence from which they can be reactivated. The signals triggering entry into and exit from this reversible dormant state are not well understood. In the developing Drosophila central nervous system, multipotent self-renewing progenitors called neuroblasts undergo quiescence in a stereotypical spatiotemporal pattern. Entry into quiescence is regulated by Hox proteins and an internal neuroblast timer. Exit from quiescence (reactivation) is subject to a nutritional checkpoint requiring dietary amino acids. Organ co-cultures also implicate an unidentified signal from an adipose/hepatic-like tissue called the fat body. Here we provide in vivo evidence that Slimfast amino-acid sensing and Target of rapamycin (TOR) signalling activate a fat-body-derived signal (FDS) required for neuroblast reactivation. Downstream of this signal, Insulin-like receptor signalling and the Phosphatidylinositol 3-kinase (PI3K)/TOR network are required in neuroblasts for exit from quiescence. We demonstrate that nutritionally regulated glial cells provide the source of Insulin-like peptides (ILPs) relevant for timely neuroblast reactivation but not for overall larval growth. Conversely, ILPs secreted into the haemolymph by median neurosecretory cells systemically control organismal size but do not reactivate neuroblasts. Drosophila thus contains two segregated ILP pools, one regulating proliferation within the central nervous system and the other controlling tissue growth systemically. Our findings support a model in which amino acids trigger the cell cycle re-entry of neural progenitors via a fat-body-glia-neuroblasts relay. This mechanism indicates that dietary nutrients and remote organs, as well as local niches, are key regulators of transitions in stem-cell behaviour.     

The tumour-suppressor genes lgl and dlg regulate basal protein targeting in Drosophila neuroblasts

Drosophila neuroblasts are a model system for studying asymmetric cell division: they divide unequally to produce an apical neuroblast and a basal ganglion mother cell that differ in size, mitotic activity and developmental potential. During neuroblast mitosis, an apical protein complex orients the mitotic spindle and targets determinants of cell fate to the basal cortex, but the mechanism of each process is unknown. Here we show that the tumour-suppressor genes lethal giant larvae (lgl) and discs large (dlg) regulate basal protein targeting, but not apical complex formation or spindle orientation, in both embryonic and larval neuroblasts. Dlg protein is apically enriched and is required for maintaining cortical localization of Lgl protein. Basal protein targeting requires microfilament and myosin function, yet the lgl phenotype is strongly suppressed by reducing levels of myosin II. We conclude that Dlg and Lgl promote, and myosin II inhibits, actomyosin-dependent basal protein targeting in neuroblasts.     

Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres

Drosophila Crumbs (Crb) is required for apical-basal polarity and is an apical determinant in embryonic epithelia. Here, we describe properties of Crb that control the position and integrity of the photoreceptor adherens junction and photosensitive organ, or rhabdomere. In contrast to normal photoreceptor adherens junctions and rhabdomeres, which span the depth of the retina, adherens junctions and rhabdomeres of Crb-deficient photoreceptors initially accumulate at the top of the retina and fail to maintain their integrity as they stretch to the retinal floor. We show that Crb controls localization of the adherens junction through its intracellular domain containing a putative binding site for a protein 4.1 superfamily protein (FERM). Although loss of Crb or overexpression of the FERM binding domain causes mislocalization of adherens junctions, they do not result in a significant loss of photoreceptor polarity. Mutations in CRB1, a human homologue of crb, are associated with photoreceptor degeneration in retinitis pigmentosa 12 (RP12) and Leber congenital amaurosis (LCA). The intracellular domain of CRB1 behaves similarly to its Drosophila counterpart when overexpressed in the fly eye. Our studies may provide clues for mechanisms of photoreceptor degeneration in RP12 and LCA.     

The Rab8 GTPase regulates apical protein localization in intestinal cells

A number of proteins are known to be involved in apical/basolateral transport of proteins in polarized epithelial cells. The small GTP-binding protein Rab8 was thought to regulate basolateral transport in polarized kidney epithelial cells through the AP1B-complex-mediated pathway. However, the role of Rab8 (Rab8A) in cell polarity in vivo remains unknown. Here we show that Rab8 is responsible for the localization of apical proteins in intestinal epithelial cells. We found that apical peptidases and transporters localized to lysosomes in the small intestine of Rab8-deficient mice. Their mislocalization and degradation in lysosomes led to a marked reduction in the absorption rate of nutrients in the small intestine, and ultimately to death. Ultrastructurally, a shortening of apical microvilli, an increased number of enlarged lysosomes, and microvillus inclusions in the enterocytes were also observed. One microvillus inclusion disease patient who shows an identical phenotype to Rab8-deficient mice expresses a reduced amount of RAB8 (RAB8A; NM_005370). Our results demonstrate that Rab8 is necessary for the proper localization of apical proteins and the absorption and digestion of various nutrients in the small intestine.     

Telomere-telomere recombination provides an express pathway for telomere acquisition

DNA termini from Tetrahymena and Oxytricha, which bear C4A2 and C4A4 repeats respectively, can support telomere formation in Saccharomyces cerevisiae by serving as substrates for the addition of yeast telomeric C1-3A repeats. Previously, we showed that linear plasmids with 108 base pairs of C4A4 DNA (YLp108CA) efficiently acquired telomeres, whereas plasmids containing 28-64 base pairs of C4A4 DNA also promoted telomere formation, but with reduced efficiency. Although many of the C4A4 termini on these plasmids underwent recombination with a C4A2 terminus, the mechanism of telomere-telomere recombination was not established. We now report the sequence of the C4A4 ends from the linear plasmids. The results provide strong evidence for a novel recombination process involving a gene conversion event that requires little homology, occurs at or near the boundary of telomeric and nontelomeric DNA, and resembles the recombination process involved in bacteriophage T4 DNA replication.     

Rapid spread of an inherited incompatibility factor in California Drosophila

In Drosophila simulans in California, an inherited cytoplasmic incompatibility factor reduces egg hatch when infected males mate with uninfected females. The infection is spreading at a rate of more than 100 km per year; populations in which the infection was rare have become almost completely infected within three years. Analyses of the spread using estimates of selection in the field suggest dispersal distances far higher than those found by direct observation of flies. Hence, occasional long-distance dispersal, possibly coupled with local extinction and recolonization, may be important to the dynamics. Incompatibility factors that can readily spread through natural populations may be useful for population manipulation and important as a post-mating isolating mechanism.     

A human natural killer cell subset provides an innate source of IL-22 for mucosal immunity

Natural killer (NK) cells are classically viewed as lymphocytes that provide innate surveillance against virally infected cells and tumour cells through the release of cytolytic mediators and interferon (IFN)-gamma. In humans, blood CD56(dim) NK cells specialize in the lysis of cell targets. In the lymph nodes, CD56(bright) NK cells secrete IFN-gamma cooperating with dendritic cells and T cells in the generation of adaptive responses. Here we report the characterization of a human NK cell subset located in mucosa-associated lymphoid tissues, such as tonsils and Peyer's patches, which is hard-wired to secrete interleukin (IL)-22, IL-26 and leukaemia inhibitory factor. These NK cells, which we refer to as NK-22 cells, are triggered by acute exposure to IL-23. In vitro, NK-22-secreted cytokines stimulate epithelial cells to secrete IL-10, proliferate and express a variety of mitogenic and anti-apoptotic molecules. NK-22 cells are also found in mouse mucosa-associated lymphoid tissues and appear in the small intestine lamina propria during bacterial infection, suggesting that NK-22 cells provide an innate source of IL-22 that may help constrain inflammation and protect mucosal sites.     

Transcript localization of four opsin genes in the three visual organs of Drosophila; RH2 is ocellus specific

Drosophila and other Dipteran flies have three different kinds of visual organs; in the adult a pair of compound eyes and three dorsal ocelli; and in the larva a pair of internal photoreceptor organs. They develop in distinct ways, yet have certain features in common. All three organs use retinal-derived chromophores, coupled to distinct opsins, to provide a diversity of spectral sensitivities. Four opsin genes have been identified thus far in Drosophila; Rh1, Rh2, Rh3 and Rh4 (refs 6-11). We have used in situ hybridization to study the messenger RNAs expressed by these four opsin genes in all three visual organs. Rh1, Rh3 and Rh4 are already known to be expressed in different subsets of cells in the compound eye. We found that, in contrast, opsin Rh2 is the predominant opsin expressed in the ocelli. Opsin Rh1 is known to be expressed in the larval photoreceptor. We found that Rh3 and Rh4 are as well, but not Rh2. The ocellar-specific gene expression of Rh2 is of particular interest for its possible bearing on the function of the ocellus.     

无线传感器网络中一种改进的加权质心定位算法研究

在无线传感器网络的应用中节点的定位信息是非常重要的。本论文提出一种改进的加权质心算法,该算法是基于平面多边形定点凹凸性原理进行实现的,算法中选择自由传播模型和对数距离路径分布模型来计算无线电波传播过程。改进的质心算法能够提高定位的精度,仿真实验验证了该算法的平均定位误差,仿真实验结果表明,在相同的条件下,该算法的定位精度高于加权质心算法。     

Genome reshuffling of the copia element in an inbred line of Drosophila melanogaster

Mobile genetic elements are found in the genomes of many organisms, and because of their effects on genes and their ability to induce chromosomal rearrangements they are an important source of genetic variability. Transposition rates are usually found to be low, estimated at around 10(-3) per generation. Higher rates of transposition are observed, however, in crosses between certain strains of Drosophila melanogaster ('hybrid dysgenesis'), which can lead to a dramatic rearrangement of many mobile elements ('transposition bursts'). We have studied the chromosomal distribution of mdg-1 and copia mobile elements in 17 highly inbred lines of D. melanogaster, after 69 generations of sib-mating. Most lines show no changes, but one showed a complete reshuffling of the copia element. We conclude that the transpositions of the copia element in this line occurred rapidly in a few generations. This phenomenon, distinct from 'transposition bursts' in that only copia elements are involved, may account for the instability sometimes observed in inbred lines and may be important in creating genetic variability in highly homozygous populations.     

The genomic mutation rate for fitness in Drosophila.

The mutation rate per genome for local affecting fitness is crucial in theories of the evolution of sex and recombination and of outbreeding mechanisms. Mutational variation in fitness may also be important in the evolution of mate choice in animals. No information is available on the rate at which spontaneous mutations with small effects on fitness arise, although viability (probability of survival to adulthood) has been studied in Drosophila melanogaster. These experiments involved the accumulation of spontaneous mutations in the virtual absence of natural selection, in a set of independently maintained lines with a common origin. The rates of decline in mean and increase in variance among lines permit estimation of limits to the mean number of new mutations arising per generation (U) and the average homozygous effect of a new mutation of minor effect(s). For the second chromosome of D. melanogaster, the value of U is at least 0.17 (ref. 7), and (1-h)s is less than 0.02, where hs is the average decline in fitness of heterozygotes. As the second chromosome is about 40% of the genome, these data indicate a mutation rate per haploid genome of at least 0.42 for viability. Here we present similar data on the effects of homozygous spontaneous mutations on a measure of fitness in D. melanogaster.