Planar polarized actomyosin contractile flows control epithelial junction remodelling

Force generation by Myosin-II motors on actin filaments drives cell and tissue morphogenesis. In epithelia, contractile forces are resisted at apical junctions by adhesive forces dependent on E-cadherin, which also transmits tension. During Drosophila embryonic germband extension, tissue elongation is driven by cell intercalation, which requires an irreversible and planar polarized remodelling of epithelial cell junctions. We investigate how cell deformations emerge from the interplay between force generation and cortical force transmission during this remodelling in Drosophila melanogaster. The shrinkage of dorsal-ventral-oriented ('vertical') junctions during this process is known to require planar polarized junctional contractility by Myosin II (refs 4, 5, 7, 12). Here we show that this shrinkage is not produced by junctional Myosin II itself, but by the polarized flow of medial actomyosin pulses towards 'vertical' junctions. This anisotropic flow is oriented by the planar polarized distribution of E-cadherin complexes, in that medial Myosin II flows towards 'vertical' junctions, which have relatively less E-cadherin than transverse junctions. Our evidence suggests that the medial flow pattern reflects equilibrium properties of force transmission and coupling to E-cadherin by α-Catenin. Thus, epithelial morphogenesis is not properly reflected by Myosin II steady state distribution but by polarized contractile actomyosin flows that emerge from interactions between E-cadherin and actomyosin networks.     

Antero-posterior tissue polarity links mesoderm convergent extension to axial patterning

Remodelling its shape, or morphogenesis, is a fundamental property of living tissue. It underlies much of embryonic development and numerous pathologies. Convergent extension (CE) of the axial mesoderm of vertebrates is an intensively studied model for morphogenetic processes that rely on cell rearrangement. It involves the intercalation of polarized cells perpendicular to the antero-posterior (AP) axis, which narrows and lengthens the tissue. Several genes have been identified that regulate cell behaviour underlying CE in zebrafish and Xenopus. Many of these are homologues of genes that control epithelial planar cell polarity in Drosophila. However, elongation of axial mesoderm must be also coordinated with the pattern of AP tissue specification to generate a normal larval morphology. At present, the long-range control that orients CE with respect to embryonic axes is not understood. Here we show that the chordamesoderm of Xenopus possesses an intrinsic AP polarity that is necessary for CE, functions in parallel to Wnt/planar cell polarity signalling, and determines the direction of tissue elongation. The mechanism that establishes AP polarity involves graded activin-like signalling and directly links mesoderm AP patterning to CE.     

Differential positioning of adherens junctions is associated with initiation of epithelial folding

During tissue morphogenesis, simple epithelial sheets undergo folding to form complex structures. The prevailing model underlying epithelial folding involves cell shape changes driven by myosin-dependent apical constriction. Here we describe an alternative mechanism that requires differential positioning of adherens junctions controlled by modulation of epithelial apical-basal polarity. Using live embryo imaging, we show that before the initiation of dorsal transverse folds during Drosophila gastrulation, adherens junctions shift basally in the initiating cells, but maintain their original subapical positioning in the neighbouring cells. Junctional positioning in the dorsal epithelium depends on the polarity proteins Bazooka and Par-1. In particular, the basal shift that occurs in the initiating cells is associated with a progressive decrease in Par-1 levels. We show that uniform reduction of the activity of Bazooka or Par-1 results in uniform apical or lateral positioning of junctions and in each case dorsal fold initiation is abolished. In addition, an increase in the Bazooka/Par-1 ratio causes formation of ectopic dorsal folds. The basal shift of junctions not only alters the apical shape of the initiating cells, but also forces the lateral membrane of the adjacent cells to bend towards the initiating cells, thereby facilitating tissue deformation. Our data thus establish a direct link between modification of epithelial polarity and initiation of epithelial folding.     

Adherens junctions inhibit asymmetric division in the Drosophila epithelium

Asymmetric division is a fundamental mechanism for generating cellular diversity. In the central nervous system of Drosophila, neural progenitor cells called neuroblasts undergo asymmetric division along the apical-basal cellular axis. Neuroblasts originate from neuroepithelial cells, which are polarized along the apical-basal axis and divide symmetrically along the planar axis. The asymmetry of neuroblasts might arise from neuroblast-specific expression of the proteins required for asymmetric division. Alternatively, both neuroblasts and neuroepithelial cells could be capable of dividing asymmetrically, but in neuroepithelial cells other polarity cues might prevent asymmetric division. Here we show that by disrupting adherens junctions we can convert the symmetric epithelial division into asymmetric division. We further confirm that the adenomatous polyposis coli (APC) tumour suppressor protein is recruited to adherens junctions, and demonstrate that both APC and microtubule-associated EB1 homologues are required for the symmetric epithelial division along the planar axis. Our results indicate that neuroepithelial cells have all the necessary components to execute asymmetric division, but that this pathway is normally overridden by the planar polarity cue provided by adherens junctions.     

Planar cell polarity signalling controls cell division orientation during zebrafish gastrulation

Oriented cell division is an integral part of pattern development in processes ranging from asymmetric segregation of cell-fate determinants to the shaping of tissues. Despite proposals that it has an important function in tissue elongation, the mechanisms regulating division orientation have been little studied outside of the invertebrates Caenorhabditis elegans and Drosophila melanogaster. Here, we have analysed mitotic divisions during zebrafish gastrulation using in vivo confocal imaging and found that cells in dorsal tissues preferentially divide along the animal-vegetal axis of the embryo. Establishment of this animal-vegetal polarity requires the Wnt pathway components Silberblick/Wnt11, Dishevelled and Strabismus. Our findings demonstrate an important role for non-canonical Wnt signalling in oriented cell division during zebrafish gastrulation, and indicate that oriented cell division is a driving force for axis elongation. Furthermore, we propose that non-canonical Wnt signalling has a conserved role in vertebrate axis elongation, orienting both cell intercalation and mitotic division.     

Group A Streptococcus tissue invasion by CD44-mediated cell signalling.

Streptococcus pyogenes (also known as group A Streptococcus, GAS), the agent of streptococcal sore throat and invasive soft-tissue infections, attaches to human pharyngeal or skin epithelial cells through specific recognition of its hyaluronic acid capsular polysaccharide by the hyaluronic-acid-binding protein CD44 (refs 1, 2). Because ligation of CD44 by hyaluronic acid can induce epithelial cell movement on extracellular matrix, we investigated whether molecular mimicry by the GAS hyaluronic acid capsule might induce similar cellular responses. Here we show that CD44-dependent GAS binding to polarized monolayers of human keratinocytes induced marked cytoskeletal rearrangements manifested by membrane ruffling and disruption of intercellular junctions. Transduction of the signal induced by GAS binding to CD44 on the keratinocyte surface involved Rac1 and the cytoskeleton linker protein ezrin, as well as tyrosine phosphorylation of cellular proteins. Studies of bacterial translocation in two models of human skin indicated that cell signalling triggered by interaction of the GAS capsule with CD44 opened intercellular junctions and promoted tissue penetration by GAS through a paracellular route. These results support a model of host cytoskeleton manipulation and tissue invasion by an extracellular bacterial pathogen.     

Drosophila Stardust is a partner of Crumbs in the control of epithelial cell polarity.

The polarized architecture of epithelial cells depends on the highly stereotypic distribution of cellular junctions and other membrane-associated protein complexes. In epithelial cells of the Drosophila embryo, three distinct domains subdivide the lateral plasma membrane. The most apical one comprises the subapical complex (SAC). It is followed by the zonula adherens (ZA) and, further basally, by the septate junction. A core component of the SAC is the transmembrane protein Crumbs, the cytoplasmic domain of which recruits the PDZ-protein Discs Lost into the complex. Cells lacking crumbs or the functionally related gene stardust fail to organize a continuous ZA and to maintain cell polarity. Here we show that stardust provides an essential component of the SAC. Stardust proteins colocalize with Crumbs and bind to the carboxy-terminal amino acids of its cytoplasmic tail. We introduce two different Stardust proteins here: one MAGUK protein, characterized by a PDZ domain, an SH3 domain and a guanylate kinase domain; and a second isoform comprising only the guanylate kinase domain. The Stardust proteins represent versatile candidates as structural and possibly regulatory constituents of the SAC, a crucial element in the control of epithelial cell polarity.     

3-RRR平面并联机构的刚度特性分析

导出了3-RRR平面并联机构基于守恒协调转换刚度矩阵机构刚度结构位形部分的解析表达式和外力作用影响部分的不完全解析表达式.给出了求机构刚度映射的一般方法,求出了不同参数下3-RRR平面并联机构的刚度映射曲线,并对机构的刚度特性进行了分析和讨论.研究发现,3-RRR平面并联机构的刚度是机构构型的函数,并与驱动力和关节刚度成比例;动平台中心趋于工作空间边界刚度增大,在工作空间的边界附近形成一个刚度较强的环状区域,而在工作空间的内部形成一个刚度较弱的区域.     

Scatter factor is a fibroblast-derived modulator of epithelial cell mobility

Various factors are known to regulate cell growth and differentiation, but less is known of agents which affect movement and positioning, particularly in epithelial-mesenchymal interactions. Cultured human embryo fibroblasts release a protein with a relative molecular mass (Mr) of approximately 50,000 (50K) that affects epithelial cells by causing a disruption of junctions, an increase in local motility and a scattering of contiguous sheets of cells. To investigate specificity, a range of cells has been examined for the ability to produce the factor and for sensitivity to its action. Most freshly isolated normal epithelia and epithelia from cell lines of normal tissue, but not epithelia from tumour cell lines or fibroblasts, were sensitive to scatter factor. In contrast, production of the factor, as identified by activity and by chromatography, was restricted to embryonic fibroblasts and certain variants of 3T3 and BHK21 cells and their transformed derivatives. We conclude that the scatter factor is a paracrine effector of epithelial-mesenchymal interaction, which affects the intercellular connections and mobility of normal epithelial cells. The factor might be involved in epithelial migration, such as occurs in embryogenesis or wound healing.     

Control of organ shape by a secreted metalloprotease in the nematode Caenorhabditis elegans.

The molecular controls governing organ shape are poorly understood. In the nematode Caenorhabditis elegans, the gonad acquires a U-shape by the directed migration of a specialized 'leader' cell, which is located at the tip of the growing gonadal 'arm'. The gon-1 gene is essential for gonadal morphogenesis: in gon-1 mutants, no arm elongation occurs and somatic gonadal structures are severely malformed. Here we report that gon-1 encodes a secreted protein with a metalloprotease domain and multiple thrombospondin type-1-like repeats. This motif architecture is typical of a small family of genes that include bovine procollagen I N-protease (P1NP), which cleaves collagen, and murine ADAMTS-1, the expression of which correlates with tumour cell progression. We find that gon-1 is expressed in two sites, leader cells and muscle, and that expression in each site has a unique role in forming the gonad. We speculate that GON-1 controls morphogenesis by remodelling basement membranes and that regulation of its activity is crucial for achieving organ shape.     

Rac function and regulation during Drosophila development

Rac GTPases regulate the actin cytoskeleton to control changes in cell shape. To date, the analysis of Rac function during development has relied heavily on the use of dominant mutant isoforms. Here, we use loss-of-function mutations to show that the three Drosophila Rac genes, Rac1, Rac2 and Mtl, have overlapping functions in the control of epithelial morphogenesis, myoblast fusion, and axon growth and guidance. They are not required for the establishment of planar cell polarity, as had been suggested on the basis of studies using dominant mutant isoforms. The guanine nucleotide exchange factor, Trio, is essential for Rac function in axon growth and guidance, but not for epithelial morphogenesis or myoblast fusion. Different Rac activators thus act in different developmental processes. The specific cellular response to Rac activation may be determined more by the upstream activator than the specific Rac protein involved.     

A mirror-symmetric cell division that orchestrates neuroepithelial morphogenesis

The development of cell polarity is an essential prerequisite for tissue morphogenesis during embryogenesis, particularly in the development of epithelia. In addition, oriented cell division can have a powerful influence on tissue morphogenesis. Here we identify a novel mode of polarized cell division that generates pairs of neural progenitors with mirror-symmetric polarity in the developing zebrafish neural tube and has dramatic consequences for the organization of embryonic tissue. We show that during neural rod formation the polarity protein Pard3 is localized to the cleavage furrow of dividing progenitors, and then mirror-symmetrically inherited by the two daughter cells. This allows the daughter cells to integrate into opposite sides of the developing neural tube. Furthermore, these mirror-symmetric divisions have powerful morphogenetic influence: when forced to occur in ectopic locations during neurulation, they orchestrate the development of mirror-image pattern formation and the consequent generation of ectopic neural tubes.     

Generation of cell polarity in plants links endocytosis, auxin distribution and cell fate decisions

Dynamically polarized membrane proteins define different cell boundaries and have an important role in intercellular communication-a vital feature of multicellular development. Efflux carriers for the signalling molecule auxin from the PIN family are landmarks of cell polarity in plants and have a crucial involvement in auxin distribution-dependent development including embryo patterning, organogenesis and tropisms. Polar PIN localization determines the direction of intercellular auxin flow, yet the mechanisms generating PIN polarity remain unclear. Here we identify an endocytosis-dependent mechanism of PIN polarity generation and analyse its developmental implications. Real-time PIN tracking showed that after synthesis, PINs are initially delivered to the plasma membrane in a non-polar manner and their polarity is established by subsequent endocytic recycling. Interference with PIN endocytosis either by auxin or by manipulation of the Arabidopsis Rab5 GTPase pathway prevents PIN polarization. Failure of PIN polarization transiently alters asymmetric auxin distribution during embryogenesis and increases the local auxin response in apical embryo regions. This results in ectopic expression of auxin pathway-associated root-forming master regulators in embryonic leaves and promotes homeotic transformation of leaves to roots. Our results indicate a two-step mechanism for the generation of PIN polar localization and the essential role of endocytosis in this process. It also highlights the link between endocytosis-dependent polarity of individual cells and auxin distribution-dependent cell fate establishment for multicellular patterning.     

Live-cell delamination counterbalances epithelial growth to limit tissue overcrowding

The development and maintenance of an epithelium requires finely balanced rates of growth and cell death. However, the mechanical and biochemical mechanisms that ensure proper feedback control of tissue growth, which when deregulated contribute to tumorigenesis, are poorly understood. Here we use the fly notum as a model system to identify a novel process of crowding-induced cell delamination that balances growth to ensure the development of well-ordered cell packing. In crowded regions of the tissue, a proportion of cells undergo a serial loss of cell-cell junctions and a progressive loss of apical area, before being squeezed out by their neighbours. This path of delamination is recapitulated by a simple computational model of epithelial mechanics, in which stochastic cell loss relieves overcrowding as the system tends towards equilibrium. We show that this process of delamination is mechanistically distinct from apoptosis-mediated cell extrusion and precedes the first signs of cell death. Overall, this analysis reveals a simple mechanism that buffers epithelia against variations in growth. Because live-cell delamination constitutes a mechanistic link between epithelial hyperplasia and cell invasion, this is likely to have important implications for our understanding of the early stages of cancer development.     

A positive feedback mechanism governs the polarity and motion of motile cilia

Ciliated epithelia produce fluid flow in many organ systems, ranging from the respiratory tract where it clears mucus to the ventricles of the brain where it transports cerebrospinal fluid. Human diseases that disable ciliary flow, such as primary ciliary dyskinesia, can compromise organ function or the ability to resist pathogens, resulting in recurring respiratory infections, otitis, hydrocephaly and infertility. To create a ciliary flow, the cilia within each cell need to be polarized coordinately along the planar axis of the epithelium, but how polarity is established in any ciliated epithelia is not known. Here we analyse the developmental mechanisms that polarize cilia, using the ciliated cells in the developing Xenopus larval skin as a model system. We show that cilia acquire polarity through a sequence of events, beginning with a polar bias set by tissue patterning, followed by a refinement phase. Our results indicate that during refinement, fluid flow is both necessary and sufficient in determining cilia polarity. These findings reveal a novel mechanism in which tissue patterning coupled with fluid flow act in a positive feedback loop to direct the planar polarity of cilia.     

合成水滑石的插入化学

从开发新型微孔材料的观点出发，评述了合成水滑石的结构、合成方法及其在催化领域中的应用进展．     

Fibronectin requirement in branching morphogenesis

Many organs, including salivary glands, lung and kidney, are formed during embryonic development by epithelial branching. In branching morphogenesis, repetitive epithelial cleft and bud formation create the complex three-dimensional branching structures characteristic of many organs. Although the mechanisms are poorly understood, one might involve the site-specific accumulation of some regulatory protein. Here we show that the extracellular matrix protein fibronectin is essential for cleft formation during the initiation of epithelial branching. Fibronectin messenger RNA and fibrils appeared transiently and focally in forming cleft regions of submandibular salivary-gland epithelia, accompanied by an adjacent loss of cadherin localization. Decreasing the fibronectin concentration by using small interfering RNA and inhibition by anti-fibronectin or anti-integrin antibodies blocked cleft formation and branching. Exogenous fibronectin accelerated cleft formation and branching. Similar effects of fibronectin suppression and augmentation were observed in developing lung and kidney. Mechanistic studies revealed that fibrillar fibronectin can induce cell-matrix adhesions on cultured human salivary epithelial cells with a local loss of cadherins at cell-cell junctions. Thus, fibronectin expression is required for cleft formation in branching morphogenesis associated with the conversion of cell-cell adhesions to cell-matrix adhesions.