Robustness of the BMP morphogen gradient in Drosophila embryonic patterning

Developmental patterning relies on morphogen gradients, which generally involve feedback loops to buffer against perturbations caused by fluctuations in gene dosage and expression. Although many gene components involved in such feedback loops have been identified, how they work together to generate a robust pattern remains unclear. Here we study the network of extracellular proteins that patterns the dorsal region of the Drosophila embryo by establishing a graded activation of the bone morphogenic protein (BMP) pathway. We find that the BMP activation gradient itself is robust to changes in gene dosage. Computational search for networks that support robustness shows that transport of the BMP class ligands (Scw and Dpp) into the dorsal midline by the BMP inhibitor Sog is the key event in this patterning process. The mechanism underlying robustness relies on the ability to store an excess of signalling molecules in a restricted spatial domain where Sog is largely absent. It requires extensive diffusion of the BMP-Sog complexes, coupled with restricted diffusion of the free ligands. We show experimentally that Dpp is widely diffusible in the presence of Sog but tightly localized in its absence, thus validating a central prediction of our theoretical study.     

Type IV collagens regulate BMP signalling in Drosophila

Dorsal-ventral patterning in vertebrate and invertebrate embryos is mediated by a conserved system of secreted proteins that establishes a bone morphogenetic protein (BMP) gradient. Although the Drosophila embryonic Decapentaplegic (Dpp) gradient has served as a model to understand how morphogen gradients are established, no role for the extracellular matrix has been previously described. Here we show that type IV collagen extracellular matrix proteins bind Dpp and regulate its signalling in both the Drosophila embryo and ovary. We provide evidence that the interaction between Dpp and type IV collagen augments Dpp signalling in the embryo by promoting gradient formation, yet it restricts the signalling range in the ovary through sequestration of the Dpp ligand. Together, these results identify a critical function of type IV collagens in modulating Dpp in the extracellular space during Drosophila development. On the basis of our findings that human type IV collagen binds BMP4, we predict that this role of type IV collagens will be conserved.     

Spatial bistability of Dpp-receptor interactions during Drosophila dorsal-ventral patterning

In many developmental contexts, a locally produced morphogen specifies positional information by forming a concentration gradient over a field of cells. However, during embryonic dorsal-ventral patterning in Drosophila, two members of the bone morphogenetic protein (BMP) family, Decapentaplegic (Dpp) and Screw (Scw), are broadly transcribed but promote receptor-mediated signalling in a restricted subset of expressing cells. Here we use a novel immunostaining protocol to visualize receptor-bound BMPs and show that both proteins become localized to a sharp stripe of dorsal cells. We demonstrate that proper BMP localization involves two distinct processes. First, Dpp undergoes directed, long-range extracellular transport. Scw also undergoes long-range movement, but can do so independently of Dpp transport. Second, an intracellular positive feedback circuit promotes future ligand binding as a function of previous signalling strength. These data elicit a model in which extracellular Dpp transport initially creates a shallow gradient of BMP binding that is acted on by positive intracellular feedback to produce two stable states of BMP-receptor interactions, a spatial bistability in which BMP binding and signalling capabilities are high in dorsal-most cells and low in lateral cells.     

The molecular nature of the zebrafish tail organizer

Based on grafting experiments, Mangold and Spemann showed the dorsal blastopore lip of an amphibian gastrula to be able to induce a secondary body axis. The equivalent of this organizer region has been identified in different vertebrates including teleosts. However, whereas the graft can induce ectopic head and trunk, endogenous and ectopic axes fuse in the posterior part of the body, raising the question of whether a distinct organizer region is necessary for tail development. Here we reveal, by isochronic and heterochronic transplantation, the existence of a tail organizer deriving from the ventral margin of the zebrafish embryo, which is independent of the dorsal Spemann organizer. Loss-of-function experiments reveal that bone morphogenetic protein (BMP), Nodal and Wnt8 signalling pathways are required for tail development. Moreover, stimulation of naive cells by a combination of BMP, Nodal and Wnt8 mimics the tail-organizing activity of the ventral margin and induces surrounding tissues to become tail. In contrast to induction of the vertebrate head, known to result from the triple inhibition of BMP, Nodal and Wnt, here we show that induction of the tail results from the triple stimulation of BMP, Nodal and Wnt8 signalling pathways.     

BMP inhibition-driven regulation of six-3 underlies induction of newt lens regeneration

Lens regeneration in adult newts is a classic example of how cells can faithfully regenerate a complete organ through the process of transdifferentiation. After lens removal, the pigment epithelial cells of the dorsal, but not the ventral, iris dedifferentiate and then differentiate to form a new lens. Understanding how this process is regulated might provide clues about why lens regeneration does not occur in higher vertebrates. The genes six-3 and pax-6 are known to induce ectopic lenses during embryogenesis. Here we tested these genes, as well as members of the bone morphogenetic protein (BMP) pathway that regulate establishment of the dorsal-ventral axis in embryos, for their ability to induce lens regeneration. We show that the lens can be regenerated from the ventral iris when the BMP pathway is inhibited and when the iris is transfected with six-3 and treated with retinoic acid. In intact irises, six-3 is expressed at higher levels in the ventral than in the dorsal iris. During regeneration, however, only expression in the dorsal iris is significantly increased. Such an increase is seen in ventral irises only when they are induced to transdifferentiate by six-3 and retinoic acid or by BMP inhibitors. These data suggest that lens regeneration can be achieved in noncompetent adult tissues and that this regeneration occurs through a gene regulatory mechanism that is more complex than the dorsal expression of lens regeneration-specific genes.     

The evolutionarily conserved BMP-binding protein Twisted gastrulation promotes BMP signalling

Dorsal-ventral patterning in vertebrate and Drosophila embryos requires a conserved system of extracellular proteins to generate a positional information gradient. The components involved include bone morphogenetic proteins (BMP/Dpp), a BMP antagonist (Chordin/Short gastrulation; Chd/Sog) and a secreted metalloproteinase (Xolloid/Tolloid) that cleaves Chd/Sog. Here we describe Xenopus Twisted gastrulation (xTsg), another member of this signalling pathway. xTsg is expressed ventrally as part of the BMP-4 synexpression group and encodes a secreted BMP-binding protein that is a BMP signalling agonist. The data suggest a molecular mechanism by which xTsg dislodges latent BMPs bound to Chordin BMP-binding fragments generated by Xolloid cleavage, providing a permissive signal that allows high BMP signalling in the embryo. Drosophila Tsg also binds BMPs and is expressed dorsally, supporting the proposal that the dorsal-ventral axis was inverted in the course of animal evolution.     

Local inhibition and long-range enhancement of Dpp signal transduction by Sog

Extracellular gradients of signalling molecules can specify different thresholds of gene activity in development. A gradient of Decapentaplegic (Dpp) activity subdivides the dorsal ectoderm of the Drosophila embryo into amnioserosa and dorsal epidermis. The proteins Short gastrulation (Sog) and Tolloid (Tld) are required to shape this gradient. Sog has been proposed to form an inhibitory complex with either Dpp or the related ligand Screw, and is subsequently processed by the protease Tld. Paradoxically, Sog appears to be required for amnioserosa formation, which is specified by peak Dpp signalling activity. Here we show that the misexpression of sog using the even-skipped stripe-2 enhancer redistributes Dpp signalling in a mutant background in which dpp is expressed throughout the embryo. Dpp activity is diminished near the Sog stripe and peak Dpp signalling is detected far from this stripe. However, a tethered form of Sog suppresses local Dpp activity without augmenting Dpp activity at a distance, indicating that diffusion of Sog may be required for enhanced Dpp activity and consequent amnioserosa formation. The long-distance stimulation of Dpp activity by Sog requires Tld, whereas Sog-mediated inhibition of Dpp does not. The heterologous Dpp inhibitor Noggin inhibits Dpp signalling but fails to augment Dpp activity. These results suggest an unusual strategy for generating a gradient threshold of growth-factor activity, whereby Sog and its protease specify peak Dpp signalling far from a localized source of Sog.     

Determination of spatial domains of zygotic gene expression in the Drosophila embryo by the affinity of binding sites for the bicoid morphogen

The maternal gene bicoid is a key component of the system that determines the pattern of the anterior half of Drosophila embryos. The bicoid protein forms a concentration gradient in early embryos, and is known to bind DNA. Specific binding sites are now shown to confer expression in a region of the embryo that depends on their affinity for bicoid protein: sites of high affinity allow expression further down the bicoid protein gradient than sites of low affinity.     

Establishment of developmental precision and proportions in the early Drosophila embryo

During embryonic development, orderly patterns of gene expression eventually assign each cell in the embryo its particular fate. For the anteroposterior axis of the Drosophila embryo, the first step in this process depends on a spatial gradient of the maternal morphogen Bicoid (Bcd). Positional information of this gradient is transmitted to downstream gap genes, each occupying a well defined spatial domain. We determined the precision of the initial process by comparing expression domains in different embryos. Here we show that the Bcd gradient displays a high embryo-to-embryo variability, but that this noise in the positional information is strongly decreased ('filtered') at the level of hunchback (hb) gene expression. In contrast to the Bcd gradient, the hb expression pattern already includes the information about the scale of the embryo. We show that genes known to interact directly with Hb are not responsible for its spatial precision, but that the maternal gene staufen may be crucial.     

Isolation of the dorsal locus of Drosophila

The establishment of embryonic polarity is a crucial step in pattern formation and morphogenesis. In the fruitfly Drosophila melanogaster, embryonic polarity depends primarily on genes expressed in the female during oogenesis. Mutations in these 'maternal effect' genes can lead to major disruptions in normal pattern formation. Two classes of maternal genes essential for the establishment of polarity in the embryo have been identified. Lesions in one class, the 'bicaudal' genes, disrupt the anterior-posterior axis; lesions in the other class disrupt dorsal-ventral polarity, and in the most extreme cases embryos fail to form any ventral or lateral structures. Genetic studies suggest that the anterior-posterior and dorsal-ventral axes may be independent as the defects observed in mutants from each class seem to be restricted to one axis only. The dorsal (dl) locus is one of the maternal effect genes involved in the establishment of dorsal-ventral polarity. Homozygous dl females produce embryos exhibiting the mutant phenotype--complete lack of dorsal-ventral polarity in the strongest alleles--irrespective of the genotype of the father. Although dl is a maternal effect locus and must be expressed during oogenesis, the gene product, or a substance depending on the normal function of the dl gene, seems to be active early in embryogenesis, as the dl phenotype can be partially rescued by injection of cytoplasm from wild-type cleavage-stage embryos. Here we report the molecular cloning of the dorsal locus and a study of its expression.     

舞龙运动员的身体形态、机能、素质研究

通过文献资料、专家访谈、归纳分析等方法,分析了舞龙运动员的身体形态、机能、素质.研究结果表明:舞龙运动的竞技性发展对运动员的体型、体能等要求提高.身体形态方面要求运动员身体匀称、体型优美、肌肉发达等,生理机能方面要求心肺功能好、平衡机能强等,身体素质方面要求灵活性强、柔韧好、爆发力大、耐力好等.旨在为舞龙教练员今后在选材及科学训练中提供一些理论依据和实践性方法.     

Krüppel requirement for knirps enhancement reflects overlapping gap gene activities in the Drosophila embryo

Segmental pattern formation in Drosophila proceeds in a hierarchical manner whereby the embryo is stepwise divided into progressively finer regions until it reaches its final metameric form. Maternal genes initiate this process by imparting on the egg a distinct antero-posterior polarity and by directing from the two polar centres the activities of the zygotic genes. The anterior system is strictly dependent on the product of the maternal gene bicoid (bcd), without which all pattern elements in the anterior region of the embryo fail to develop. The posterior system seems to lack such a morphogen. Rather, the known posterior maternal determinants simply define the boundaries within which abdominal segmentation can occur, and the process that actively generates the abdominal body pattern may be entirely due to the interactions between the zygotic genes. The most likely candidates among the zygotic genes that could fulfil the role of initiating the posterior pattern-forming process are the gap genes, as they are the first segmentation genes to be expressed in the embryo. Here we describe the interactions between the gap genes Krüppel (Kr), knirps (kni) and tailless (tll). We show that kni expression is repressed by tll activity, whereas it is directly enhanced by Kr activity. Thus, Kr activity is present throughout the domain of kni expression and forms a long-range protein gradient, which in combination with kni activity is required for abdominal segmentation of the embryo.     

RBF神经网络与模糊理论相结合的磨矿分级智能控制方法

将RBF神经网络和模糊理论结合起来,提出了一种基于RBF神经网络和模糊理论实现智能控制的方法。该方法能够有效克服磨矿效率和旋流器入口压力等波动引起的扰动,使磨矿浓度和溢流粒度的波动减小,为浮选过程产品品位改善及产量提高创造了有利条件,在技术上实现了优化磨矿分级过程。该分析过程相对简单,网络学习训练时间少,学习精度高,估计值与分析值拟合非常好。仿真表明这类智能控制器可用于难以建立数学模型的控制系统。     

Twisted gastrulation can function as a BMP antagonist

Bone morphogenetic proteins (BMPs), including the fly homologue Decapentaplegic (DPP), are important regulators of early vertebrate and invertebrate dorsal-ventral development. An evolutionarily conserved BMP regulatory mechanism operates from fly to fish, frog and mouse to control the dorsal-ventral axis determination. Several secreted factors, including the BMP antagonist chordin/Short gastrulation (SOG), modulate the activity of BMPs. In Drosophila, Twisted gastrulation (TSG) is also involved in dorsal-ventral patterning, yet the mechanism of its function is unclear. Here we report the characterization of the vertebrate Tsg homologues. We show that Tsg can block BMP function in Xenopus embryonic explants and inhibits several ventral markers in whole-frog embryos. Tsg binds directly to BMPs and forms a ternary complex with chordin and BMPs. Coexpression of Tsg with chordin leads to a more efficient inhibition of the BMP activity in ectodermal explants. Unlike other known BMP antagonists, however, Tsg also reduces several anterior markers at late developmental stages. Our data suggest that Tsg can function as a BMP inhibitor in Xenopus; furthermore, Tsg may have additional functions during frog embryogenesis.     

Localized maternal orthodenticle patterns anterior and posterior in the long germ wasp Nasonia

The Bicoid (Bcd) gradient in Drosophila has long been a model for the action of a morphogen in establishing embryonic polarity. However, it is now clear that bcd is a unique feature of higher Diptera. An evolutionarily ancient gene, orthodenticle (otd), has a bcd-like role in the beetle Tribolium. Unlike the Bcd gradient, which arises by diffusion of protein from an anteriorly localized messenger RNA, the Tribolium Otd gradient forms by translational repression of otd mRNA by a posteriorly localized factor. These differences in gradient formation are correlated with differences in modes of embryonic patterning. Drosophila uses long germ embryogenesis, where the embryo derives from the entire anterior-posterior axis, and all segments are patterned at the blastoderm stage, before gastrulation. In contrast, Tribolium undergoes short germ embryogenesis: the embryo arises from cells in the posterior of the egg, and only anterior segments are patterned at the blastoderm stage, with the remaining segments arising after gastrulation from a growth zone. Here we describe the role of otd in the long germband embryo of the wasp Nasonia vitripennis. We show that Nasonia otd maternal mRNA is localized at both poles of the embryo, and resulting protein gradients pattern both poles. Thus, localized Nasonia otd has two major roles that allow long germ development. It activates anterior targets at the anterior of the egg in a manner reminiscent of the Bcd gradient, and it is required for pre-gastrulation expression of posterior gap genes.     

发散步长准则下的增量聚合梯度算法

针对目标函数是若干光滑函数之和的优化问题,提出采用发散步长准则的增量聚合梯度算法。与增量梯度算法一样,增量聚合梯度算法的每次迭代也只需要计算其中一个函数的梯度。目前关于增量聚合梯度算法的研究主要是采用常值步长的增量聚合梯度算法,这一算法要求目标函数二阶连续可微且强凸,且常值步长的选取依赖最优点的二阶导数;而发散步长准则不依赖目标函数。在目标函数的梯度有界且李普希兹连续假设条件下,证明了采用发散步长的增量聚合梯度算法的收敛性;最后,通过数值例子验证了算法的收敛性,并与采用相同步长准则的增量梯度算法进行比较;数值结果表明对于某些优化问题,增量聚合梯度算法比采用相同步长的增量梯度算法更有效。     

A molecular gradient in early Drosophila embryos and its role in specifying the body pattern

After fertilization, the protein products of the Drosophila homeobox gene caudal (cad) accumulate in a concentration gradient spanning the anteroposterior axis of the developing embryo. Mutations in the cad gene that reduce or eliminate the gradient cause abnormal zygotic expression of at least one segmentation gene (fushi tarazu) and alter the global body pattern.