Two distinct modes of guidance signalling during collective migration of border cells

Although directed migration is a feature of both individual cells and cell groups, guided migration has been studied most extensively for single cells in simple environments. Collective guidance of cell groups remains poorly understood, despite its relevance for development and metastasis. Neural crest cells and neuronal precursors migrate as loosely organized streams of individual cells, whereas cells of the fish lateral line, Drosophila tracheal tubes and border-cell clusters migrate as more coherent groups. Here we use Drosophila border cells to examine how collective guidance is performed. We report that border cells migrate in two phases using distinct mechanisms. Genetic analysis combined with live imaging shows that polarized cell behaviour is critical for the initial phase of migration, whereas dynamic collective behaviour dominates later. PDGF- and VEGF-related receptor and epidermal growth factor receptor act in both phases, but use different effector pathways in each. The myoblast city (Mbc, also known as DOCK180) and engulfment and cell motility (ELMO, also known as Ced-12) pathway is required for the early phase, in which guidance depends on subcellular localization of signalling within a leading cell. During the later phase, mitogen-activated protein kinase and phospholipase Cgamma are used redundantly, and we find that the cluster makes use of the difference in signal levels between cells to guide migration. Thus, information processing at the multicellular level is used to guide collective behaviour of a cell group.     

Cell-autonomous action of zebrafish spt-1 mutation in specific mesodermal precursors

In zebrafish, as in Xenopus, the well-orchestrated cell movements of gastrulation can be dissected into several components, including epiboly, involution, convergence and extension. Embryos homozygous for the recessive lethal mutation spt-1(b104) or 'spadetail' have a complex set of defects in the trunk of the embryo that may arise secondarily after loss of one of these movements, convergence, from those precursors that would normally have given rise to trunk somitic mesoderm. We have now tested this hypothesis by transplanting cells between wild-type and mutant embryos, to identify the cells that spt-1 affects directly. Our results show that the mutation autonomously affects only those mesodermal precursors located along the lateral margin of the early gastrula blastoderm. Other mesodermal cells and all ectodermal precursors seem not to require function of the wild-type gene. Our findings reveal an unexpectedly delicate genetic control of vertebrate gastrulation.     

Collinear activation of Hoxb genes during gastrulation is linked to mesoderm cell ingression

The vertebral column exhibits segmentation and regionalization along the antero-posterior axis. During embryogenesis, the rhythmic production of the precursors of the vertebrae, the somites, imposes a segmented aspect to the spine, whereas the spine's regional differentiation is controlled by Hox genes. Here we show that in the paraxial mesoderm, Hoxb genes are first activated in a temporal collinear fashion in precursors located in the epiblast lateral to the primitive streak. Our data suggest that collinear activation of Hoxb genes regulates the flux of cells from the epiblast to the streak and thus directly controls the establishment of the genes' characteristic nested expression domains in the somites. This suggests that establishment of the spatial co-linearity in the embryo is directly controlled by the Hox genes themselves.     

Mutant Drosophila embryos in which all cells adopt a neural fate

In the Drosophila embryo, early developmental decisions lead to all cells adopting one of several initial fates, such as those characteristic of the germ layers. The central nervous system is formed subsequently from the neurogenic region of the ectoderm, in which progenitor cells of the neuroblasts and ventral epidermis are intermingled. Two classes of genes govern the segregation of neuroblasts and peripheral sensory organs. The pro-neural class of genes, for example, the achaete-scute complex, participates in the initial decision to make each uniquely positioned neuroblast or sensory organ, but are initially expressed in groups of cells. The segregation of a neuroblast or sensory organ from an equivalent group of equipotential cells involves a mechanism of lateral inhibition whereby the future epidermal cells are prevented from engaging in the primary dominant neural fate. In the absence of this inhibitory signal, all cells of the group will become neural by default. The neurogenic class of genes is thought to mediate these cell interactions. Here we report that cells in embryos mutant for shaggy which are unable to adopt any of the early initial fates, instead develop neural characteristics.     

Selective activation of Lyt 2+ precursor T cells by ligation of the antigen receptor

Resting T lymphocytes may be activated either physiologically, by the specific recognition of antigen in association with molecules encoded by the major histocompatibility complex (MHC), or non-physiologically using mitogens such as concanavalin A (Con A). The former activation process is difficult to analyse because resting precursor T cells specific for a particular antigen-MHC combination can only be isolated in the presence of a large excess of bystander cells of irrelevant specificity; clonal populations of uniform specificity are not useful for studying the activation of naive T cells because there is no reason to believe that such cloned cells ever return to the state of resting precursors. Mitogens may activate a large fraction of resting T cells, but analysis is again complicated because the target molecule(s) of most mitogens is unknown and the relationship of this kind of activation to physiological induction by antigen plus MHC molecules remains unclear. By using a monoclonal antibody specific for the antigen receptors on approximately 25% of all T cells of both Lyt 2+ and Lyt 2- subsets, we have studied the induction of lymphokine responsiveness in resting normal T cells. This antibody, immobilized on Sepharose beads, is sufficient to activate Lyt 2+ T cells, but not Lyt 2- T cells, to clonal expansion in the presence of a mixture of lymphokines (10% rat spleen Con A supernatant). We report here that clonal growth of the T cells obeys single-hit kinetics in limiting-dilution microcultures, suggesting that a single cell type is limiting. We conclude that cytotoxic T-lymphocyte (Tc) precursors require only ligation of the antigen receptor before they become responsive to lymphokines, whereas helper T-lymphocyte (Th) precursors require additional signals.     

白蜡虫(Ericerus pela Chavannes)雌虫神经分泌系统的形态及组织学研究

应用Ewen’s改良多聚醛品红染色方法,对雌性白蜡虫(Ericerus pela Cha-vannes)成虫神经分泌系统的形态及组织学进行了研究.确定了在前脑中存在有两群属于“A”型神经分泌细胞.愈合腹神经节中亦有分散的“A”型神经分泌细胞.脑神经分泌细胞中的神经分泌粒通过其轴突传送到心侧体中释放.心侧体为神经血器官.虫体在进入成体阶段后到产卵期间,其脑中的神经分泌细胞在形态及染色性上都没有明显差异.     

RNA sequencing of pancreatic circulating tumour cells implicates WNT signalling in metastasis

Circulating tumour cells (CTCs) shed into blood from primary cancers include putative precursors that initiate distal metastases. Although these cells are extraordinarily rare, they may identify cellular pathways contributing to the blood-borne dissemination of cancer. Here, we adapted a microfluidic device for efficient capture of CTCs from an endogenous mouse pancreatic cancer model and subjected CTCs to single-molecule RNA sequencing, identifying Wnt2 as a candidate gene enriched in CTCs. Expression of WNT2 in pancreatic cancer cells suppresses anoikis, enhances anchorage-independent sphere formation, and increases metastatic propensity in vivo. This effect is correlated with fibronectin upregulation and suppressed by inhibition of MAP3K7 (also known as TAK1) kinase. In humans, formation of non-adherent tumour spheres by pancreatic cancer cells is associated with upregulation of multiple WNT genes, and pancreatic CTCs revealed enrichment for WNT signalling in 5 out of 11 cases. Thus, molecular analysis of CTCs may identify candidate therapeutic targets to prevent the distal spread of cancer.     

Social interactions among epithelial cells during tracheal branching morphogenesis

Many organs are composed of tubular networks that arise by branching morphogenesis in which cells bud from an epithelium and organize into a tube. Fibroblast growth factors (FGFs) and other signalling molecules have been shown to guide branch budding and outgrowth, but it is not known how epithelial cells coordinate their movements and morphogenesis. Here we use genetic mosaic analysis in Drosophila melanogaster to show that there are two functionally distinct classes of cells in budding tracheal branches: cells at the tip that respond directly to Branchless FGF and lead branch outgrowth, and trailing cells that receive a secondary signal to follow the lead cells and form a tube. These roles are not pre-specified; rather, there is competition between cells such that those with the highest FGF receptor activity take the lead positions, whereas those with less FGF receptor activity assume subsidiary positions and form the branch stalk. Competition appears to involve Notch-mediated lateral inhibition that prevents extra cells from assuming the lead. There may also be cooperation between budding cells, because in a mosaic epithelium, cells that cannot respond to the chemoattractant, or respond only poorly, allow other cells in the epithelium to move ahead of them.     

Acetylcholine inhibits identified interneurons in the cat lateral geniculate nucleus

The transmission of visual information from retina to cortex through the dorsal lateral geniculate nucleus (LGNd) is controlled by non-retinal inputs. Enhanced visually evoked responses in cat LGNd relay cells during periods of increased alertness have been attributed in large part to increased rate of acetylcholine (ACh) release by fibres ascending from the brainstem reticular formation. ACh can modulate geniculate visual responses in vivo, but comparatively little is known about the underlying ionic mechanisms of these cholinergic actions. Although direct excitation of LGNd relay neurons has been shown in vitro, the situation is complicated because cholinergic axons form numerous and complex synapses not only with relay cells, but also with inhibitory interneurons, and electrical activation of the brainstem cholinergic neurons reduces inhibitory postsynaptic potentials in the LGNd. We report here that morphologically characterized interneurons in the cat LGNd possess distinctive electrophysiological properties in comparison with those of relay cells and are inhibited by ACh through a muscarinic receptor-mediated increase in potassium conductance. Together the direct excitation of relay cells and inhibition of intrageniculate interneurons allow the ascending cholinergic system to exert a powerful facilitatory influence over the transfer of visual information to the cerebral cortex.     

Neurogenic phenotypes and altered Notch processing in Drosophila Presenilin mutants

Presenilin proteins have been implicated both in developmental signalling by the cell-surface protein Notch and in the pathogenesis of Alzheimer's disease. Loss of presenilin function leads to Notch/lin-12-like mutant phenotypes in Caenorhabditis elegans and to reduced Notch1 expression in the mouse paraxial mesoderm. In humans, presenilins that are associated with Alzheimer's disease stimulate overproduction of the neurotoxic 42-amino-acid beta-amyloid derivative (Abeta42) of the amyloid-precursor protein APP. Here we describe loss-of-function mutations in the Drosophila Presenilin gene that cause lethal Notch-like phenotypes such as maternal neurogenic effects during embryogenesis, loss of lateral inhibition within proneural cell clusters, and absence of wing margin formation. We show that presenilin is required for the normal proteolytic production of carboxy-terminal Notch fragments that are needed for receptor maturation and signalling, and that genetically it acts upstream of both the membrane-bound form and the activated nuclear form of Notch. Our findings provide evidence for the existence of distinct processing sites or modifications in the extracellular domain of Notch. They also link the role of presenilin in Notch signalling to its effect on amyloid production in Alzheimer's disease.     

Plant biomarkers in aerosols record isotopic discrimination of terrestrial photosynthesis

Carbon uptake by the oceans and by the terrestrial biosphere can be partitioned using changes in the (12)C/(13)C isotopic ratio (delta(13)C) of atmospheric carbon dioxide, because terrestrial photosynthesis strongly discriminates against (13)CO(2), whereas ocean uptake does not. This approach depends on accurate estimates of the carbon isotopic discrimination of terrestrial photosynthesis (Delta; ref. 5) at large regional scales, yet terrestrial ecosystem heterogeneity makes such estimates problematic. Here we show that ablated plant wax compounds in continental air masses can be used to estimate Delta over large spatial scales and at less than monthly temporal resolution. We measured plant waxes in continental air masses advected to Bermuda, which are mainly of North American origin, and used the wax isotopic composition to estimate Delta simply. Our estimates indicate a large (5 6 per thousand) seasonal variation in Delta of the temperate North American biosphere, with maximum discrimination occurring in late spring, coincident with the onset of production. We suggest that the observed seasonality arises from several factors, including seasonal shifts in the proportions of production by C(3) and C(4) plants, and environmentally controlled adjustments in the photosynthetic discrimination of C(3)-plant-dominated ecosystems.     

关于岩石侧压系数的认识误区

针对石油工程中岩石侧压系数的预测问题,研究了介质的水平应力形成机理与侧压系数的计算方法,分析了水、土与岩石3种介质水平应力的区别与侧压系数的工程意义,并探讨了岩石的侧压系数预测方法。研究认为由于介质的结构与流动性不同,水和土的水平应力都是自重产生的,这与岩石的水平应力产生机理完全不同,岩石的水平应力主要是构造作用产生的。由于构造作用的强弱差异大,岩石的水平应力变化范围较大,侧压系数在-∞~∞,因此岩石的侧压系数不能沿用土介质侧压系数的计算方法。同时,岩石的侧压系数是通过实测地应力得到的,其预测结果并没有实际意义。     

Endstopped neurons in the visual cortex as a substrate for calculating curvature

Neurons in the visual cortex typically respond selectively to the orientation, and velocity and direction of movement, of moving-bar stimuli. These responses are generally thought to provide information about the orientation and position of lines and edges in the visual field. Some cells are also endstopped, that is selective for bars of specific lengths. Hubel and Wiesel first observed that endstopped hypercomplex cells could respond to curved stimuli and suggested they might be involved in detection of curvature, but the exact relationship between endstopping and curvature has never been determined. We present here a mathematical model relating endstopping to curvature in which the difference in response of two simple cells gives rise to endstopping and varies in proportion to curvature. We also provide physiological evidence that endstopped cells in area 17 of the cat visual cortex are selective for curvature, whereas non-endstopped cells are not, and that some are selective for the sign of curvature. The prevailing view of edge and curve determination is that orientations are selected locally by the class of simple cortical cells and then integrated to form global curves. We have developed a computational theory of orientation selection which shows that measurements of orientation obtained by simple cells are not sufficient because there will be strong, incorrect responses from cells whose receptive fields (RFs) span distinct curves (Fig. 1). If estimates of curvature are available, however, these inappropriate responses can be eliminated. Curvature provides the key to structuring the network that underlies our theory and distinguishes it from previous lateral inhibition schemes.     

Functions of the colour-opponent and broad-band channels of the visual system

The colour-opponent and broad-band channels of the primate visual system originate in the retina and remain segregated through several neural stations in the visual system. Until now inferences about their function in vision have been based primarily on studies examining single-cell receptive field properties which have shown that the colour-opponent retinal ganglion cells have small receptive fields, produce sustained responses and receive spatially segregated inputs from different cone types; the broad-band cells have large receptive fields, respond transiently and receive cone inputs that are not spatially separated. We have now examined the visual capacities of rhesus monkeys before and after interrupting either of these channels with small lesions at the lateral geniculate nucleus. Here we report that the colour-opponent channel is essential for the processing of colour, texture, fine pattern and fine stereopsis, whereas the broad-band channel is crucial for the perception of fast flicker and motion. Little or no deficits were found in brightness and coarse-shape discrimination, low spatial frequency stereopsis and contrast sensitivity after the disruption of either of the channels.     

Chronic polyarthritis caused by mammalian DNA that escapes from degradation in macrophages

A large amount of chromosomal DNA is degraded during programmed cell death and definitive erythropoiesis. DNase II is an enzyme that digests the chromosomal DNA of apoptotic cells and nuclei expelled from erythroid precursor cells after macrophages have engulfed them. Here we show that DNase II-/-IFN-IR-/- mice and mice with an induced deletion of the DNase II gene develop a chronic polyarthritis resembling human rheumatoid arthritis. A set of cytokine genes was strongly activated in the affected joints of these mice, and their serum contained high levels of anti-cyclic citrullinated peptide antibody, rheumatoid factor and matrix metalloproteinase-3. Early in the pathogenesis, expression of the gene encoding tumour necrosis factor (TNF)-alpha was upregulated in the bone marrow, and administration of anti-TNF-alpha antibody prevented the development of arthritis. These results indicate that if macrophages cannot degrade mammalian DNA from erythroid precursors and apoptotic cells, they produce TNF-alpha, which activates synovial cells to produce various cytokines, leading to the development of chronic polyarthritis.     

Selective responses of visual cortical cells do not depend on shunting inhibition

Theoretical analyses of the electrical behaviour of the highly branched processes of nerve cells has focused attention on the possibility that single cells perform complex logical operations rather than simply summing their synaptic inputs. In particular, it has been suggested that the orientation and direction selectivity of cells in the visual cortex results from the action of a nonlinear 'shunting' inhibition that emulates an AND-NOT logical operation. The characteristic biophysical feature of this proposed inhibitory mechanism is that it evokes a large and relatively sustained increase in the conductance of the neuronal membrane while leaving the membrane potential unaffected. This shunting mechanism contrasts with linear 'summative' inhibition in which conductance changes are less prominent, and inhibition is achieved by hyperpolarization of the membrane potential. In a direct experimental test of the hypothesis that the selectivity of visual cortical neurons depends on shunting inhibition we found no evidence for the large conductance changes predicted by the theory.     

Dephosphorylated SRp38 acts as a splicing repressor in response to heat shock

The cellular response to stresses such as heat shock involves changes in gene expression. It is well known that the splicing of messenger RNA precursors is generally repressed on heat shock, but the factors responsible have not been identified. SRp38 is an SR protein splicing factor that functions as a general repressor of splicing. It is activated by dephosphorylation and required for splicing repression in M-phase cells. Here we show that SRp38 is also dephosphorylated on heat shock and that this dephosphorylation correlates with splicing inhibition. Notably, depletion of SRp38 from heat-shocked cell extracts derepresses splicing, and adding back dephosphorylated SRp38 specifically restores inhibition. We further show that dephosphorylated SRp38 interacts with a U1 small nuclear ribonucleoprotein particle (snRNP) protein, and that this interaction interferes with 5'-splice-site recognition by the U1 snRNP. Finally, SRp38-deficient DT40 cells show an altered cell-cycle profile consistent with a mitotic defect; they are also temperature sensitive and defective in recovery after heat shock. SRp38 thus plays a crucial role in cell survival under stress conditions by inhibiting the splicing machinery.     

Cell lineage analysis reveals multipotency of some avian neural crest cells

A major question in developmental biology is how precursor cells give rise to diverse sets of differentiated cell types. In most systems, it remains unclear whether the precursors can form many or all cell types (multipotent or totipotent), or only a single cell type (predetermined). The question of cell lineage is central to the neural crest because it gives rise to numerous and diverse derivatives including peripheral neurons, glial and Schwann cells, pigment cells, and cartilage. Although the sets of derivatives arising from different populations of neural crest cells have been well-documented, relatively little is known about the developmental potentials of individual neural crest cells. We have iontophoretically microinjected the vital dye, lysinated rhodamine dextran (LRD) into individual dorsal neural tube cells to mark unambiguously their descendants. Many of the resulting labelled clones consisted of multiple cell types, as judged by both their location and morphology. Cells as diverse as sensory neurons, presumptive pigment cells, ganglionic supportive cells, adrenomedullary cells and neural tube cells were found within individual clones. Our results indicate that at least some neural crest cells are multipotent before their departure from the neural tube.     

Sensitivity gains in chemosensing by lasing action in organic polymers

Societal needs for greater security require dramatic improvements in the sensitivity of chemical and biological sensors. To meet this challenge, increasing emphasis in analytical science has been directed towards materials and devices having highly nonlinear characteristics; semiconducting organic polymers (SOPs), with their facile excited state (exciton) transport, are prime examples of amplifying materials. SOPs have also been recognized as promising lasing materials, although the susceptibility of these materials to optical damage has thus far limited applications. Here we report that attenuated lasing in optically pumped SOP thin films displays a sensitivity to vapours of explosives more than 30 times higher than is observed from spontaneous emission. Critical to this achievement was the development of a transducing polymer with high thin-film quantum yield, a high optical damage threshold in ambient atmosphere and a record low lasing threshold. Trace vapours of the explosives 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) introduce non-radiative deactivation pathways that compete with stimulated emission. We demonstrate that the induced cessation of the lasing action, and associated sensitivity enhancement, is most pronounced when films are pumped at intensities near their lasing threshold. The combined gains from amplifying materials and lasing promise to deliver sensors that can detect explosives with unparalleled sensitivity.