A morphogen gradient model for pattern regulation. II. Time description of global morphogen formation and field compartmentalization

In a model for pattern regulation, use was made of local and global morphogens S and Sigma. Sigma is produced from the S-degradation and it is decomposed by first order kinetics while it diffuses along the field. We solve exactly the partial differential equation for the distribution of Sigma in one spatial dimension when its source S is monotonie (for simplicity, linear or generally a power function). Assuming that S and Sigma react reversibly with an allosteric protein P according to a sequential scheme, we derive the evolution in time of the field separation into compartments. At equilibrium the relative extent of each compartment is constant (for variable field size) and so pattern regulation is achieved.     

Hox genes and morphological identity: axial versus lateral patterning in the vertebrate mesoderm

The successful organization of the vertebrate body requires that local information in the embryo be translated into a functional, global pattern. Somite cells form the bulk of the musculoskeletal system. Heterotopic transplants of segmental plate along the axis from quail to chick were performed to test the correlation between autonomous morphological patterning and Hox gene expression in somite subpopulations. The data presented strengthen the correlation of Hox gene expression with axial specification and focus on the significance of Hox genes in specific derivatives of the somites. We have defined two anatomical compartments of the body based on the embryonic origin of the cells making up contributing structures: the dorsal compartment, formed from purely somitic cell populations; and the ventral compartment comprising cells from somites and lateral plate. The boundary between these anatomical compartments is termed the somitic frontier. Somitic tissue transplanted between axial levels retains both original Hox expression and morphological identity in the dorsal compartment. In contrast, migrating lateral somitic cells crossing the somitic frontier do not maintain donor Hox expression but apparently adopt the Hox expression of the lateral plate and participate in the morphology appropriate to the host level. Dorsal and ventral compartments, as defined here, have relevance for experimental manipulations that influence somite cell behavior. The correlation of Hox expression profiles and patterning behavior of cells in these two compartments supports the hypothesis of independent Hox codes in paraxial and lateral plate mesoderm.     

Multiple morphogens and rapid elongation promote segmental patterning during development

The vertebrate hindbrain is segmented into rhombomeres (r) initially defined by distinct domains of gene expression. Previous studies have shown that noise-induced gene regulation and cell sorting are critical for the sharpening of rhombomere boundaries, which start out rough in the forming neural plate (NP) and sharpen over time. However, the mechanisms controlling simultaneous formation of multiple rhombomeres and accuracy in their sizes are unclear. We have developed a stochastic multiscale cell-based model that explicitly incorporates dynamic morphogenetic changes (i.e. convergent-extension of the NP), multiple morphogens, and gene regulatory networks to investigate the formation of rhombomeres and their corresponding boundaries in the zebrafish hindbrain. During pattern initiation, the short-range signal, fibroblast growth factor (FGF), works together with the longer-range morphogen, retinoic acid (RA), to specify all of these boundaries and maintain accurately sized segments with sharp boundaries. At later stages of patterning, we show a nonlinear change in the shape of rhombomeres with rapid left-right narrowing of the NP followed by slower dynamics. Rapid initial convergence improves boundary sharpness and segment size by regulating cell sorting and cell fate both independently and coordinately. Overall, multiple morphogens and tissue dynamics synergize to regulate the sizes and boundaries of multiple segments during development.     

Compartments in the abdomen of Drosophila and the role of the engrailed locus

A clonal analysis has shown that the dorsal surface of the first abdominal segment of Drosophila melanogaster is subdivided into anterior and posterior compartments. Cells of the posterior compartment grow up to but not beyond the anterior-posterior compartment border within the first abdominal segment and the intersegmental border that defines the boundary between the first and second abdominal segments. Growing within these boundaries, a narrow band of tissue clonally isolated from the adjoining tissue is formed. When these posterior cells are deficient for the engrailed locus, however, neither the compartment nor the segment border is maintained. The implications, that compartmentalization is essential for segmentation, and that all insect segments are subdivided by anterior and posterior compartments, are discussed.     

器官的构造是如何形成的——以果蝇翅为例

在器官发育过程中,细胞是如何接收到指令,在特定的位置形成特定的细胞形貌,来组建一个正确的三维构造实现器官的功能,这是生物学中的最基本问题之一。在发育的早期,选择者基因通过赋予细胞以不同的亲和性把组织划分为若干个隔间区域。隔间边界细胞作为组织者通过分泌信号分子(器官成形素)来促进细胞的存活和增殖,控制细胞的分化和命运,以及确保正确的细胞形貌发生。器官成形素的空间时序性表达以及随后细胞对这些信号分子的反应是正确形成组织构造的关键环节。根据国际最新的研究进展,本文综述了构造形成的机制和主流假说,并以果蝇翅的发育为例,讨论了TGF-β家族器官成形素Dpp在翅发育中的作用机制。     

Notch signaling is not sufficient to define the affinity boundary between dorsal and ventral compartments

The developing limbs of Drosophila are subdivided into distinct cells populations known as compartments. Short-range interaction between cells in adjacent compartments induces expression of signaling molecules at the compartment boundaries. In addition to serving as the sources of long-range signals, compartment boundaries prevent mixing of the adjacent cell populations. One model for boundary formation proposes that affinity differences between compartments are defined autonomously as one aspect of compartment-specific cell identity. An alternative is that the affinity boundary depends on signaling between compartments. Here, we present evidence that the dorsal selector gene apterous plays a role in establishing the dorsoventral affinity boundary that is independent of Notch-mediated signaling between dorsal and ventral cells.     

Network topology: patterns and mechanisms in plant-herbivore and host-parasitoid food webs

1.?Biological communities are organized in complex interaction networks such as food webs, which topology appears to be non-random. Gradients, compartments, nested subsets and even combinations of these structures have been shown in bipartite networks. However, in most studies only one pattern is tested against randomness and mechanistic hypotheses are generally lacking. 2.?Here we examined the topology of regional, coexisting plant-herbivore and host-parasitoid food webs to discriminate between the mentioned network patterns. We also evaluated the role of species body size, local abundance, regional frequency and phylogeny as determinants of network topology. 3.?We found both food webs to be compartmented, with interaction range boundaries imposed by host phylogeny. Species degree within compartments was mostly related to their regional frequency and local abundance. Only one compartment showed an internal nested structure in the distribution of interactions between species, but species position within this compartment was unrelated to species size or abundance. 4.?These results suggest that compartmentalization may be more common than previously considered, and that network structure is a result of multiple, hierarchical, non-exclusive processes.     

Compartment boundaries: Sorting cells with tension

The subdivision of proliferating tissues into groups of non-intermingling sets of cells, termed compartments, is a common process of animal development. Signaling between adjacent compartments induces the local expression of morphogens that pattern the surrounding tissue. Sharp and straight boundaries between compartments stabilize the source of such morphogens during tissue growth and, thus, are of crucial importance for pattern formation. Signaling pathways required to maintain compartment boundaries have been identified, yet the physical mechanisms that maintain compartment boundaries remained elusive. Recent data now show that a local increase in actomyosin-based mechanical tension on cell bonds is vital for maintaining compartment boundaries in Drosophila.Key words: Drosophila, wing imaginal disc, compartment boundary, cell sorting, mechanical tensionCompartments were first identified in the wings and abdomen of insects by clonal analysis.^1,2 When single cells were genetically marked during early development, the descendant cells (‘clone’) grew up in the adult structure to a boundary line (the compartment boundary), and frequently ran along it, but never extended to the other side. These experiments revealed that, in Drosophila, the developing wing is subdivided during embryogenesis into anterior (A) and posterior (P) compartments (Fig. 1A) and later, during larval development, into dorsal (D) and ventral (V) compartments. Compartments were subsequently identified in different parts of the fly, including the leg, haltere, head and abdomen.^3–7 More recently, lineage tracing also revealed compartments in vertebrate embryos,^8–16 indicating that the formation of compartments is a common strategy during both insect and vertebrate development.Open in a separate windowFigure 1Increased cell bond tension at compartment boundaries in Drosophila. (A) The Drosophila wing imaginal disc is subdivided into anterior (A) and posterior (P) compartments. (B) Myosin II and F-actin (green lines) are enriched at the cell bonds between anterior cells and posterior cells compared to cell bonds elsewhere in the tissue. Mechanical tension (arrows) on cell bonds along the A/P boundary is increased. (C) Measurement of cell bond tension by laser ablation. Arrowheads depict the site of ablation. The two vertices at the ends of the ablated cell bond are displaced. (D and E) Sequential images of an E-cadherin-GFP-labelled cell bond within the anterior compartment (D) or at the A/P boundary (E) before and after laser ablation in wing imaginal discs. (F) Each parasegment of the Drosophila embryo is subdivided into anterior and posterior compartments. (G) Chromophore-assisted laser inactivation (CALI) to locally reduce Myosin II (green lines) in cells along the parasegment boundary (boxed area). As a consequence, dividing cells at the parasegment boundary intermingle.Meinhardt''s theoretical work on pattern formation proposed that boundaries between compartments act as reference lines for positional information during tissue development, and that they serve as sources of morphogen synthesis.^17,18 Indeed, many compartment boundaries, both in insects and vertebrates, have by now been shown experimentally to be associated with signaling centers that produce morphogens (reviewed in refs. ¹⁹ and ²⁰). The defined position and shape of signaling centers is important for the establishment of precise morphogen gradients and patterning.^21,22 In growing tissues, however, the position and shape of signaling centers is challenged by cell rearrangements that take place during cell division.^23,24 By inducing signaling centers along stable and straight compartment boundaries, precise morphogen gradients can be maintained in proliferating tissues.²⁵ Compartment boundaries therefore play vital roles during the patterning of proliferating tissues.How are straight and sharp compartment boundaries maintained despite cell re-arrangements caused by cell division? The maintenance of compartment boundaries often requires local signaling between cells from the two adjacent compartments. In the developing hindbrain, for example, signaling by Eph receptors and ephrins is required to maintain the boundaries between adjacent rhombomeres.^26,27 In the developing wing of the fly, signaling downstream of Hedgehog and Dpp is required to maintain the A/P boundary,^28–31 and Notch signaling is required to maintain the D/V boundary.^32,33 The physical mechanisms maintaining compartment boundaries, however, remained elusive for a long time. Two recent papers, by Landsberg et al. and Monier et al. now provide evidence that actomyosin-dependent tension on cell bonds is an important mechanism to maintain straight and sharp compartment boundaries.^34,35A longstanding hypothesis posed that the sorting of cells at compartment boundaries is due to differences in the affinities between cells of adjacent compartments.³⁶ Earlier theoretical work by Malcom Steinberg had indeed proposed that differences in the adhesiveness of cells lead to cell sorting.³⁷ Steinberg''s hypothesis was based on the important insight that cell sorting closely resembles the separation of immiscible liquids and that quantitative differences in cell properties suffice to explain cell sorting. Cadherins are a class of Ca²⁺-dependent cell adhesion molecules that can confer differential cell adhesion in vitro and in vivo.^38–40 Circumstantial evidence indicates that cadherins may play a role in maintaining compartment boundaries. In the telencephalon of mouse embryos, for example, the interface between cells expressing R-cadherin and cells expressing cadherin-6 coincide with the cortico-striatal compartment boundary.¹¹ Interestingly, cortical cells ectopically expressing cadherin-6 sort into the striatal compartment, and the reverse is observed for striatal cells engineered to express R-cadherin. In addition to cadherins, further cell adhesion proteins have been implicated in maintaining compartment boundaries. In the Drosophila wing imaginal disc, an epithelium that gives rise to the adult wing, the two leucine-rich-repeat domain proteins Capricious and Tartan are expressed specifically in cells of the dorsal compartment.⁴¹ Strikingly, forced expression of either of these proteins in the dorsal compartment can restore a normal straight and sharp D/V boundary in mutants for apterous, the selector gene required to establish this boundary.⁴¹More recent hypotheses to explain the sorting of cells in animal development are based on differential surface contraction⁴² or differential interfacial tension.⁴³ These hypotheses do not treat cells as liquid molecules, as Steinberg''s differential adhesion hypothesis does, but emphasize that cells can generate mechanical tension that allows them to contract the surface to neighboring cells. Minimizing cell surfaces at interfaces between different cell populations could contribute to cell sorting.Mechanical tension in cells can be generated by tensile elements located at the cellular cortex underlying the plasma membrane, including contractile actomyosin filaments (reviewed in ref. ⁴⁴). Irvine and colleagues made the important observation that, in Drosophila wing imaginal discs, Filamentous (F)-actin and the motor protein non-muscle Myosin II (Myosin II) were enriched at adherens junctions along the D/V boundary,^45,46 indicating a distinct mechanical property of bonds between cells along this compartment boundary. Moreover, these authors found that in mutants for zipper, which encodes myosin heavy chain, the D/V boundary was irregular,⁴⁶ showing a requirement for Myosin II in maintaining this boundary.Landsberg et al. show that F-actin and Myosin II were also enriched on cell bonds along the A/P boundary in Drosophila wing imaginal discs, and that also the A/P boundary was irregular in zipper mutants.³⁴ Moreover, they now provide direct evidence that mechanical tension at cell bonds along the A/P boundary is increased (Fig. 1B). Differences in mechanical tension on cell bonds have been proposed to result in differences in the shape of cells and the angles between bonds of cells.^24,47 Landsberg et al. demonstrate that the two rows of cells along the A/P boundary display a unique shape and that angles between cell bonds along the A/P boundary are widened, providing evidence that mechanical tension is elevated along these cell bonds.³⁴ Distinct shapes have also been previously reported for cells along compartment boundaries in Oncopeltus,⁴⁸ indicating that they are commonly associated with compartment boundaries.Ablation of cell bonds generates displacements of the corners (vertices) of the ablated bonds, providing direct evidence for tension on cell bonds.⁴⁹ Landsberg et al. ablated individual cell bonds in wing imaginal discs using an UV laser beam, and quantified the displacements of the two vertices of the ablated cell bonds (Fig. 1C–E). The relative initial velocities with which these vertices are separated in response to laser ablation is a relative measure of cell bond tension.⁵⁰ Ablation of cell bonds within the anterior compartment and the posterior compartment resulted in similar initial velocities.³⁴ However, when cell bonds along the A/P boundary were ablated, the initial velocity of vertex separation was approximately 2.5-fold higher.³⁴ Displacements of cell vertices after laser ablation were strongly reduced in the presence of Y-27632, a drug that specifically inhibits Rho-kinase,⁵¹ which is a major activator of Myosin II.⁵² These results suggest that actomyosin-based cell bond tension along the A/P boundary is increased 2.5-fold compared to the tension on cell bonds located elsewhere.Is a local increase in cell bond tension sufficient to maintain straight interfaces between proliferating groups of cells? To test this, Landsberg et al. simulated the growth of a tissue based on a vertex model.²⁴ In this model, the network of adherens junctions in a tissue is described by polygons characterized by the position of vertices. Stable configurations of this network are local minima of an energy function that describes the area elasticity of cells, cell bond tension, and the elasticity of cell perimeters. In these simulations, two adjacent cell populations, anterior and posterior compartments, separated by a straight and sharp interface, are introduced into this network. Tissue growth is simulated by randomly selecting a cell, increasing its area two-fold, and dividing the cell at a random angle. The energy in the whole network is then minimized and the procedure is repeated. Simulation of tissue growth renders the initially straight and sharp interface between the two compartments rough and irregular.³⁴ However, by increasing locally cell bond tension at the interface between the two simulated compartments, the interface remains straight.³⁴ These computer simulations provide evidence that a local increase in cell bond tension is sufficient to maintain straight boundaries between compartments in proliferating tissues.Monier et al. analyzed boundaries in the Drosophila embryo.³⁵ The embryonic epidermis is subdivided into parasegments, and cells from adjacent parasegments do not intermingle⁵³ (Fig. 1F). Similar to the D/V and A/P boundaries of larval wing imaginal discs, the authors found that the parasegment boundaries also display elevated levels of F-actin and Myosin II.³⁵ Injection of the Rho-kinase inhibitor Y-27632 into embryos, or expression of a dominant-negative form of zipper, resulted in cell sorting defects at the parasegment boundaries. Live imaging of embryos furthermore showed that mitotic cells locally deform the parasegment boundaries, but that the boundaries straighten out at the onset of cytokinesis. When Myosin II activity was locally reduced by chromophore-assisted laser inactivation (CALI), the parasegment boundaries failed to straighten out after cells had divided, and anterior and posterior cells partially intermingled³⁵ (Fig. 1G). These results demonstrate an important role for Myosin II in separating anterior and posterior cells at parasegment boundaries.Cell sorting is a general phenomenon of developing animals not restricted to compartment boundaries. A well-studied example is the sorting out of cells from the different germ layers during gastrulation. Interestingly, during zebrafish gastrulation, differential actomyosin-dependent cell-cortex tension has recently been implicated in the sorting out of cells from different germ layers.⁵⁴ A differential mechanical tension might, therefore, be a general mechanism to prevent the mixing of cells in developing animals.Does differential cell adhesion play a role in regulating mechanical tension? At least two contributions can be envisioned. First, cell bond tension depends on both contractile forces along cell bonds as well as the strength of adhesion between neighboring cells.^24,43 Elevating contractile forces can increase cell bond tension, whereas increasing adhesive contacts between cells can release tension. Differences in the adhesion between neighboring cells along compartment boundaries, compared to the remaining cells within the compartments, could therefore contribute to the maintenance of compartment boundaries. Second, differential expression of some cell adhesion molecules results in a local increase of F-actin and Myosin II. For example, interfaces between cells expressing the cell adhesion molecule Echinoid and cells lacking Echinoid display elevated levels of F-actin and Myosin II in Drosophila wing imaginal discs.⁵⁵ Therefore, it seems plausible that, at least in some cases, the increase of F-actin and Myosin II at compartment boundaries could be the consequence of the differential expression of adhesion molecules. In this model, differential cell adhesion would play an indirect role in maintaining compartment boundaries by resulting in local enrichment of F-actin and Myosin II, which in turn could lead to an elevated mechanical tension.The local enrichment of F-actin and Myosin II at distinct sites within cells, and a presumed modulation of tensile stresses, is not restricted to compartment boundaries, but appears to be common to diverse developmental processes. In gastrulating Drosophila embryos, for example, tissue elongation is driven by cell intercalation that depends on the enrichment of Myosin II on shrinking cell bonds.^56,57 Similarly, during mesoderm invagination of Drosophila embryos, F-actin and Myosin II accumulate in a central weblike structure at the apical side of cells resulting in apical cell constriction.⁵⁸ Recruitment of F-actin and Myosin II to this medial web can be induced by expression of an activated form of Wasp, a known regulator of actin polymerization, providing a mechanism for the local enrichment of actomyosin within cells.⁵⁹ In addition to biochemical mechanisms, mechanical signals have also been shown to help localize Myosin II to specific sites within cells. During germband elongation in the Drosophila embryo, for example, cell bonds that are under high tension have elevated levels of Myosin II, and the experimental application of mechanical force is sufficient to recruit Myosin II to the cell cortex.⁶⁰ Increased tension at cell bonds along compartment boundaries might, therefore, be also a consequence of both biochemical and mechanical mechanisms. It will be interesting to investigate the nature of these mechanisms, and how they are linked to the developmental signals that control the formation of compartment boundaries.     

Changes in muscle activity in response to different impact forces affect soft tissue compartment mechanical properties

Electromyographic (EMG) activity is associated with several tasks prior to landing in walking and running including positioning the leg, developing joint stiffness and possibly control of soft tissue compartment vibrations. The concept of muscle tuning suggests one reason for changes in muscle activity pattern in response to small changes in impact conditions, if the frequency content of the impact is close to the natural frequency of the soft tissue compartments, is to minimize the magnitude of soft tissue compartment vibrations. The mechanical properties of the soft tissue compartments depend in part on muscle activations and thus it was hypothesized that changes in the muscle activation pattern associated with different impact conditions would result in a change in the acceleration transmissibility to the soft tissue compartments. A pendulum apparatus was used to systematically administer impacts to the heel of shod male participants. Wall reaction forces, EMG of selected leg muscles, soft tissue compartment and shoe heel cup accelerations were quantified for two different impact conditions. The transmissibility of the impact acceleration to the soft tissue compartments was determined for each subject/soft tissue compartment/shoe combination. For this controlled impact situation it was shown that changes in the damping properties of the soft tissue compartments were related to changes in the EMG intensity and/or mean frequency of related muscles in response to a change in the impact interface conditions. These results provide support for the muscle tuning idea--that one reason for the changes in muscle activity in response to small changes in the impact conditions may be to minimize vibrations of the soft tissue compartments that are initiated at heel-strike.     

hedgehog and engrailed: pattern formation and polarity in the Drosophila abdomen

Like the Drosophila embryo, the abdomen of the adult consists of alternating anterior (A) and posterior (P) compartments. However the wing is made by only part of one A and part of one P compartment. The abdomen therefore offers an opportunity to compare two compartment borders (A/P is within the segment and P/A intervenes between two segments), and ask if they act differently in pattern formation. In the embryo, abdomen and wing P compartment cells express the selector gene engrailed and secrete Hedgehog protein whilst A compartment cells need the patched and smoothened genes in order to respond to Hedgehog. We made clones of cells with altered activities of the engrailed, patched and smoothened genes. Our results confirm (1) that the state of engrailed, whether 'off' or 'on', determines whether a cell is of A or P type and (2) that Hedgehog signalling, coming from the adjacent P compartments across both A/P and P/A boundaries, organises the pattern of all the A cells. We have uncovered four new aspects of compartments and engrailed in the abdomen. First, we show that engrailed acts in the A compartment: Hedgehog leaves the P cells and crosses the A/P boundary where it induces engrailed in a narrow band of A cells. engrailed causes these cells to form a special type of cuticle. No similar effect occurs when Hedgehog crosses the P/A border. Second, we look at the polarity changes induced by the clones, and build a working hypothesis that polarity is organised, in both compartments, by molecule(s) emanating from the A/P but not the P/A boundaries. Third, we show that both the A and P compartments are each divided into anterior and posterior subdomains. This additional stratification makes the A/P and the P/A boundaries fundamentally distinct from each other. Finally, we find that when engrailed is removed from P cells (of, say, segment A5) they transform not into A cells of the same segment, but into A cells of the same parasegment (segment A6).     

Cell segregation in the vertebrate hindbrain relies on actomyosin cables located at the interhombomeric boundaries

Segregating cells into compartments during embryonic development is essential for growth and pattern formation. Physical mechanisms shaping compartment boundaries were recently explored in Drosophila, where actomyosin‐based barriers were revealed to be important for keeping cells apart. In vertebrates, interhombomeric boundaries are straight interfaces, which often serve as signaling centers that pattern the surrounding tissue. Here, we demonstrate that in the hindbrain of zebrafish embryos cell sorting sharpens the molecular boundaries and, once borders are straight, actomyosin barriers are key to keeping rhombomeric cells segregated. Actomyosin cytoskeletal components are enriched at interhombomeric boundaries, forming cable‐like structures in the apical side of the neuroepithelial cells by the time morphological boundaries are visible. When myosin II function is inhibited, cable structures do not form, leading to rhombomeric cell mixing. Downregulation of EphA4a compromises actomyosin cables and cells with different rhombomeric identity intermingle, and the phenotype is rescued enhancing myosin II activity. Moreover, enrichment of actomyosin structures is obtained when EphA4 is ectopically expressed in even‐numbered rhombomeres. These findings suggest that mechanical barriers act downstream of EphA/ephrin signaling to segregate cells from different rhombomeres.     

JNK Signalling Controls Remodelling of the Segment Boundary through Cell Reprogramming during Drosophila Morphogenesis

Segments are fundamental units in animal development which are made of distinct cell lineages separated by boundaries. Although boundaries show limited plasticity during their formation for sharpening, cell lineages make compartments that become tightly restricted as development goes on. Here, we characterize a unique case of breaking of the segment boundary in late drosophila embryos. During dorsal closure, specific cells from anterior compartments cross the segment boundary and enter the adjacent posterior compartments. This cell mixing behaviour is driven by an anterior-to-posterior reprogramming mechanism involving de novo expression of the homeodomain protein Engrailed. Mixing is accompanied by stereotyped local cell intercalation, converting the segment boundary into a relaxation compartment important for tension-release during morphogenesis. This process of lineage switching and cell remodelling is controlled by JNK signalling. Our results reveal plasticity of segment boundaries during late morphogenesis and a role for JNK-dependent developmental reprogramming in this process.     

Novel tissue units of regional differentiation in the gut epithelium of Drosopbila,as revealed by P-element-mediated detection of enhancer

We analysed spatial patterns of expression of a lacZ reporter gene in the gut of Drosophila larvae that had been transformed with a P-element-lacZ vector to identify regional differences in gene expression. lacZ-positive epithelial cells formed distinct domains with discrete transverse and longitudinal boundaries along the gut tube. Boundaries were often found at sites at which morphological boundaries were not obvious. The gut epithelium was subdivided into 36 compartments by the boundaries. We refer to these novel compartments as tissue compartments. The lacZ-positive domain of each strain appeared as a single tissue compartment or as a combination of several tissue compartments. The tissue compartment is considered to be a unit of regional differentiation. The spatial organization of the tissue compartments may represent the floor plan, determined by genes that control the regional differentiation of this nonsegmental organ. Correspondence to: R. Murakami     

Analysis of the compartments of the wing of Drosophila melanogaster mosaic for a temperature-sensitive mutation that reduces mitotic rate

Gynandromorphs of Drosophila melanogaster were analysed in which the female tissue was normal but the male tissue was hemizygous for a temperature-sensitive mutation, l(1)ts1126, which reduces mitotic rate. In gynandromorphs grown at restrictive temperature, the slow-growing l(1)ts1126 tissue survives preferentially when it is segregated from the wild-type tissue, i.e., when it occupies an entire imaginal disc or an entire anterior or posterior compartment within a disc. Mosaic compartments composed of both male, l(1)ts1126, and female wild-type tissue are found less frequently at restrictive temperature than at permissive temperature and when present, are composed mainly of wild-type tissue with very small patches of l(1)ts1126. These very small patches are found almost exclusively along the borders defining compartments. The implications of these results to theories concerning the way in which the compartment boundaries may be maintained is considered. In gynandromorphs grown at restrictive temperature, the size of compartments composed entirely of l(1)ts1126 tissue is drastically reduced, relative to those composed of wild-type tissue. The observations support the hypothesis that the sizes of the anterior and posterior compartments are autonomously controlled.     

Differential Activity of Drosophila Hox Genes Induces Myosin Expression and Can Maintain Compartment Boundaries

Compartments are units of cell lineage that subdivide territories with different developmental potential. In Drosophila, the wing and haltere discs are subdivided into anterior and posterior (A/P) compartments, which require the activity of Hedgehog, and into dorsal and ventral (D/V) compartments, needing Notch signaling. There is enrichment in actomyosin proteins at the compartment boundaries, suggesting a role for these proteins in their maintenance. Compartments also develop in the mouse hindbrain rhombomeres, which are characterized by the expression of different Hox genes, a group of genes specifying different structures along their main axis of bilaterians. We show here that the Drosophila Hox gene Ultrabithorax can maintain the A/P and D/V compartment boundaries when Hedgehog or Notch signaling is compromised, and that the interaction of cells with and without Ultrabithorax expression induces high levels of non-muscle myosin II. In the absence of Ultrabithorax there is occasional mixing of cells from different segments. We also show a similar role in cell segregation for the Abdominal-B Hox gene. Our results suggest that the juxtaposition of cells with different Hox gene expression leads to their sorting out, probably through the accumulation of non-muscle myosin II at the boundary of the different cell territories. The increase in myosin expression seems to be a general mechanism used by Hox genes or signaling pathways to maintain the segregation of different groups of cells.     

Generation of nonidentical compartments in vesicular transport systems

How can organelles communicate by bidirectional vesicle transport and yet maintain different protein compositions? We show by mathematical modeling that a minimal system, in which the basic variables are cytosolic coats for vesicle budding and membrane-bound soluble N-ethyl-maleimide–sensitive factor attachment protein receptors (SNAREs) for vesicle fusion, is sufficient to generate stable, nonidentical compartments. A requirement for establishing and maintaining distinct compartments is that each coat preferentially packages certain SNAREs during vesicle budding. Vesicles fuse preferentially with the compartment that contains the highest concentration of cognate SNAREs, thus further increasing these SNAREs. The stable steady state is the result of a balance between this autocatalytic SNARE accumulation in a compartment and the distribution of SNAREs between compartments by vesicle budding. The resulting nonhomogeneous SNARE distribution generates coat-specific vesicle fluxes that determine the size of compartments. With nonidentical compartments established in this way, the localization and cellular transport of cargo proteins can be explained simply by their affinity for coats.     

C2C12 myocytes lack an insulin-responsive vesicular compartment despite dexamethasone-induced GLUT4 expression

Insulin regulates the uptake of glucose into skeletal muscle and adipocytes by redistributing the tissue-specific glucose transporter GLUT4 from intracellular vesicles to the cell surface. To date, GLUT4 is the only protein involved in insulin-regulated vesicular traffic that has this tissue distribution, thus raising the possibility that its expression alone may allow formation of an insulin-responsive vesicular compartment. We show here that treatment of differentiating C2C12 myoblasts with dexamethasone, acting via the glucocorticoid receptor, causes a >or=10-fold increase in GLUT4 expression but results in no significant change in insulin-stimulated glucose transport. Signaling from the insulin receptor to its target, Akt2, and expression of the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor, or SNARE, proteins syntaxin 4 and vesicle-associated membrane protein are normal in dexamethasone-treated C2C12 cells. However, these cells show no insulin-dependent trafficking of the insulin-responsive aminopeptidase or the transferrin receptor, respective markers for intracellular GLUT4-rich compartments and endosomes that are insulin responsive in mature muscle and adipose cells. Therefore, these data support the hypothesis that GLUT4 expression by itself is insufficient to establish an insulin-sensitive vesicular compartment.     

Compartmental modulation of abdominal Hox expression by engrailed and sloppy-paired patterns the fly ectoderm

In Drosophila, segmentation genes partition the early embryo into reiterative segments along the anterior-posterior axis, while Hox genes assign segments their identities. Each segment is also subdivided into distinct anterior (A) and posterior (P) compartments based on the expression of the engrailed (en) segmentation gene. Differences in Hox expression often correlate with compartmental boundaries, but the genetic basis for these differences is not well understood. In this study, we extend previous results to describe a genetic circuit that controls the differential expression of two Hox genes, Ultrabithorax (Ubx) and abdominal-A (abd-A), within the A and P compartments of the abdominal ectoderm. Consistent with earlier findings, we show that en is essential for high Abd-A levels and low Ubx levels in the P compartment, whereas sloppy-paired (slp) is required for high Ubx levels in the A compartment. Overall, these results demonstrate that the compartmental expression of Ubx and abd-A is established through a repressive regulatory network between en, slp, Ubx and abd-A. We also show that abd-A expression in the P compartment is important for the formation of abdominal-specific cell types, suggesting that en and slp modulation of Hox expression within the A and P compartments is essential for embryonic patterning.     

ATP-coupled transport of vesicular stomatitis virus G protein. Functional boundaries of secretory compartments

The oligosaccharide processing intermediates of the vesicular stomatitis virus strain ts045 G protein were used to identify ATP- and temperature-sensitive steps in the constitutive pathway of protein transfer to the cell surface. In addition to the initial ATP-sensitive step required for export from the endoplasmic reticulum (Balch, W. E., Elliott, M. M., and Keller, D. S. (1986) J. Biol. Chem. 261, 14681-14689), two distinct ATP-sensitive steps functionally dissect the Golgi into at least 3 compartments: a cis compartment containing the trimming enzyme mannosidase I, a medial compartment conferring resistance to endoglycosidase H, and a trans compartment containing terminal glycosyl transferases. A fourth ATP-sensitive step is required for export of G protein from the trans Golgi to the cell surface. A high threshold of cellular ATP (70% of the control) was required for maximal rates of transport between Golgi compartments. Transport between compartments is inhibited at 40% of the normal cellular ATP pool. Only a single temperature-sensitive step localized to the endoplasmic reticulum inhibited transport of ts045 G protein to the cell surface. The data suggest that ATP-sensitive steps punctuate transport of protein between compartmental boundaries of the secretory pathway.