The microtubule end-binding proteins EB1 and Patronin modulate the spatiotemporal dynamics of myosin and pattern pulsed apical constriction

Apical constriction powers amnioserosa contraction during Drosophila dorsal closure. The nucleation, movement and dispersal of apicomedial actomyosin complexes generates pulsed apical constrictions during early closure. Persistent apicomedial and circumapical actomyosin complexes drive unpulsed constrictions that follow. Here, we show that the microtubule end-binding proteins EB1 and Patronin pattern constriction dynamics and contraction kinetics by coordinating the balance of actomyosin forces in the apical plane. We find that microtubule growth from moving Patronin platforms governs the spatiotemporal dynamics of apicomedial myosin through the regulation of RhoGTPase signaling by transient EB1-RhoGEF2 interactions. We uncover the dynamic reorganization of a subset of short non-centrosomally nucleated apical microtubules that surround the coalescing apicomedial myosin complex, trail behind it as it moves and disperse as the complex dissolves. We demonstrate that apical microtubule reorganization is sensitive to Patronin levels. Microtubule depolymerization compromised apical myosin enrichment and altered constriction dynamics. Together, our findings uncover the importance of reorganization of an intact apical microtubule meshwork, by moving Patronin platforms and growing microtubule ends, in enabling the spatiotemporal modulation of actomyosin contractility and, through it, apical constriction.

Cytoskeletal tension and Bazooka tune interface geometry to ensure fusion fidelity and sheet integrity during dorsal closure

Epithelial fusion establishes continuity between the separated flanks of epithelial sheets. Despite its importance in creating resilient barriers, the mechanisms that ensure stable continuity and preserve morphological and molecular symmetry upon fusion remain unclear. Using the segmented embryonic epidermis whose flanks fuse during Drosophila dorsal closure, we demonstrate that epidermal flanks modulate cell numbers and geometry of their fusing fronts to achieve fusion fidelity. While fusing flanks become more matched for both parameters before fusion, differences persisting at fusion are corrected by modulating fusing front width within each segment to ensure alignment of segment boundaries. We show that fusing cell interfaces are remodelled from en-face contacts at fusion to an interlocking arrangement after fusion, and demonstrate that changes in interface length and geometry are dependent on the spatiotemporal regulation of cytoskeletal tension and Bazooka/Par3. Our work uncovers genetically constrained and mechanically triggered adaptive mechanisms contributing to fusion fidelity and epithelial continuity.

Cadherin switching during the formation and differentiation of the Drosophila mesoderm - implications for epithelial-to-mesenchymal transitions

Epithelial-to-mesenchymal transition (EMT) is typically accompanied by downregulation of epithelial (E-) cadherin, and is often additionally accompanied by upregulation of a mesenchymal or neuronal (N-) cadherin. Snail represses transcription of the E-cadherin gene both during normal development and during tumour spreading. The formation of the mesodermal germ layer in Drosophila, considered a paradigm of a developmental EMT, is associated with Snail-mediated repression of E-cadherin and the upregulation of N-cadherin. By using genetic manipulation to remove or overexpress the cadherins, we show here that the complementarity of cadherin expression is not necessary for the segregation or the dispersal of the mesodermal germ layer in Drosophila. However, we discover different effects of E- and N-cadherin on the differentiation of subsets of mesodermal derivatives, which depend on Wingless signalling from the ectoderm, indicating differing abilities of E- and N-cadherin to bind to and sequester the common junctional and signalling effector β-catenin. These results suggest that the downregulation of E-cadherin in the mesoderm might be required to facilitate optimal levels of Wingless signalling.

Local, cell-nonautonomous feedback regulation of myosin dynamics patterns transitions in cell behavior: a role for tension and geometry?

Signals, stresses, and myosin-dependent contractility influence cell geometry, tension, myosin dynamics, and pulsed constriction in the amnioserosa both cell-autonomously and -nonautonomously and feedback regulate cell behavior. Cell delamination is a locally patterned, nonautonomously regulated transition from pulsed to unpulsed constriction.

How robust patterns of tissue dynamics emerge from heterogeneities, stochasticities, and asynchronies in cell behavior is an outstanding question in morphogenesis. A clear understanding of this requires examining the influence of the behavior of single cells on tissue patterning. Here we develop single-cell manipulation strategies to uncover the origin of patterned cell behavior in the amnioserosa during Drosophila dorsal closure. We show that the formation and dissolution of contractile, medial actomyosin networks previously shown to underlie pulsed apical constrictions in the amnioserosa are apparently asynchronous in adjacent cells. We demonstrate for the first time that mechanical stresses and Rho1 GTPase control myosin dynamics qualitatively and quantitatively, in amplitude and direction, both cell autonomously and nonautonomously. We then demonstrate that interfering with myosin-dependent contractility in single cells also influences pulsed constrictions cell nonautonomously. Our results suggest that signals and stresses can feedback regulate the amplitude and spatial propagation of pulsed constrictions through their influence on tension and geometry. We establish the relevance of these findings to native closure by showing that cell delamination represents a locally patterned and collective transition from pulsed to unpulsed constriction that also relies on the nonautonomous feedback control of myosin dynamics.

Spatial, temporal and molecular hierarchies in the link between death, delamination and dorsal closure

Dead cells in most epithelia are eliminated by cell extrusion. Here, we explore whether cell delamination in the amnioserosa, a seemingly stochastic event that results in the extrusion of a small fraction of cells and known to provide a force for dorsal closure, is contingent upon the receipt of an apoptotic signal. Through the analysis of mutant combinations and the profiling of apoptotic signals in situ, we establish spatial, temporal and molecular hierarchies in the link between death and delamination. We show that although an apoptotic signal is necessary and sufficient to provide cell-autonomous instructions for delamination, its induction during natural delamination occurs downstream of mitochondrial fragmentation. We further show that apoptotic regulators can influence both delamination and dorsal closure cell non-autonomously, presumably by influencing tissue mechanics. The spatial heterogeneities in delamination frequency and mitochondrial morphology suggest that mechanical stresses may underlie the activation of the apoptotic cascade through their influence on mitochondrial dynamics. Our results document for the first time the temporal propagation of an apoptotic signal in the context of cell behaviours that accomplish morphogenesis during development. They highlight the importance of mitochondrial dynamics and tissue mechanics in its regulation. Together, they provide novel insights into how apoptotic signals can be deployed to pattern tissues.

Grainy head promotes expression of septate junction proteins and influences epithelial morphogenesis

Transcription factors of the Grainy head (Grh) family are required in epithelia to generate the impermeable apical layer that protects against the external environment. This function is conserved in vertebrates and invertebrates, despite the differing molecular composition of the protective barrier. Epithelial cells also have junctions that create a paracellular diffusion barrier (tight or septate junctions). To examine whether Grh has a role in regulating such characteristics, we used an epidermal layer in the Drosophila embryo that has no endogenous Grh and lacks septate junctions, the amnioserosa. Expression of Grh in the amnioserosa caused severe defects in dorsal closure, a process similar to wound closure, and induced robust expression of the septate junction proteins Coracle, Fasciclin 3 and Sinuous. Grh-binding sites are present within the genes encoding these proteins, consistent with them being direct targets. Removal of Grh from imaginal disc cells caused a reduction in Fasciclin 3 and Coracle levels, suggesting that Grh normally fine tunes their epithelial expression and hence contributes to barrier properties. The fact that ectopic Grh arrests dorsal closure also suggests that this dynamic process relies on epithelia having distinct adhesive properties conferred by differential deployment of Grh.

Analysis and reconstitution of the genetic cascade controlling early mesoderm morphogenesis in the Drosophila embryo

To understand how transcription factors direct developmental events, it is necessary to know their target or 'effector' genes whose products mediate the downstream cell biological events. Whereas loss of a single target may partially or fully recapitulate the phenotype of loss of the transcription factor, this does not mean that this target is the only direct mediator. For a complete understanding of the pathway it is necessary to identify the full set of targets that together are sufficient to carry out the programme initiated by the transcription factor, which has not yet been attempted for any pathway. In the case of the transcriptional activator Twist, which acts at the top of the mesodermal developmental cascade in Drosophila, two targets, Snail and Fog, are known to be necessary for the first morphogenetic event, the orderly invagination of the mesoderm. We use a system of reconstituting loss of Twist function by transgenes expressing Snail and Fog independently of Twist to analyse the sufficiency of these factors-a loss of function assay for additional gene functions to assess what further functions might be needed downstream of Twist. Confirming and extending previous studies, we show that Snail plays an essential role, allowing basic cell shape changes to take place. Fog and at least two other genes are needed to accelerate and coordinate shape changes. Furthermore, this study represents the first step in the systematic reconstruction of the morphogenetic programme downstream of Twist.

Tensin Stabilizes Integrin Adhesive Contacts in Drosophila

We report the functional characterization of the Drosophila ortholog of tensin, a protein implicated in linking integrins to the cytoskeleton and signaling pathways. A tensin null was generated and is viable with wing blisters, a phenotype characteristic of loss of integrin adhesion. In tensin mutants, mechanical abrasion is required during wing expansion to cause wing blisters, suggesting that tensin strengthens integrin adhesion. The localization of tensin requires integrins, talin, and integrin-linked kinase. The N-terminal domain and C-terminal PTB domain of tensin provide essential recruitment signals. The intervening SH2 domain is not localized on its own. We suggest a model where tensin is recruited to sites of integrin adhesion via its PTB and N-terminal domains, localizing the SH2 domain so that it can interact with phosphotyrosine-containing proteins, which stabilize the integrin link to the cytoskeleton.

Novel Functions for Integrins in Epithelial Morphogenesis

Dorsal closure during Drosophila embryogenesis provides a valuable model for epithelial morphogenesis and wound healing. Previous studies have focused on two cell populations, the dorsal epidermis and the extraembryonic amnioserosa. Here, we demonstrate that there is an additional player, the large yolk cell. We find that integrins are expressed in the amnioserosa and yolk cell membrane and that they are required for three processes: (1) assembly of an intervening extracellular matrix, (2) attachment between these two cell layers, and (3) contraction of the amnioserosa cells. We also provide evidence for integrin-extracellular matrix interactions occurring between the lateral surfaces of the amnioserosa cell and the leading edge epidermis that effectively mediate cell-cell adhesion. Thus, dorsal closure shares mechanistic similarities with vertebrate epithelial morphogenetic events, including epiboly, that also employ an underlying substrate.

Cell movements during gastrulation: come in and be induced

The conversion of an epithelial monolayer into a multilayered structure consisting of the three germ layers, ectoderm, mesoderm and endoderm, constitutes a conserved theme in the early development of animals. This is accomplished by morphogenetic movements that occur during gastrulation and serve not only to generate shape but also to ensure that cells receive the right signals at the right time. Recent evidence of the role of molecular interactions facilitated by cell movements in continuously defining the chick 'organizer' during gastrulation challenges the notion that it is a fixed cell population derived from an exclusive cell lineage.

The role of the paternal genome in the development of the mouse germ line

The mouse germ line originates at 6.5 days post coitum (dpc) in the proximal epiblast, apparently in response to signals from the primitive endoderm or the extraembryonic mesoderm [1,2]. Some studies have implied a significant role for imprinted genes in germ-line development [3,4]. These genes, whose expression is determined by their parental origin [5], serve complementary functions during mammalian development [6-9] and exert striking reciprocal phenotypic effects on androgenetic (AG: two paternal genomes) and parthenogenetic (GG/PG: two maternal genomes) cells [3,4,10]. This may include a fundamental effect on germ-cell development because PG but not AG cells can differentiate into viable gametes [3,4,11], suggesting that the maternal genome is obligatory for development of the mammalian germ line. Here we show unequivocally that AG cells can differentiate into germ cells, and that in chimeras with normal cells they produce functional sperm. These studies establish that the paternal and maternal genomes can individually provide both the signal and the response required for the specification of germ cells in mammals.

Genomic imprinting and the differential roles of parental genomes in brain development

Certain genes are expressed either from the maternal or the paternal genome as a result of genomic imprinting, a process that confers functional differences on parental genomes during mammalian development. In this study we focus on the cumulative effects of imprinted genes on brain development by examining the fate of androgenetic (Ag: duplicated paternal genome) and parthenogenetic/gynogenetic (Pg/Gg: duplicated maternal genome) cells in chimeric embryos. Striking cell autonomous differences in the phenotypic properties of the uniparental cells were observed. Ag cells contributed substantially to the hypothalamic structures and not the cortex. By contrast, Pg/Gg cells contributed substantially to the cortex, striatum and hippocampus but not to the hypothalamic structures. Furthermore growth of the brain was enhanced by Pg/Gg and retarded by Ag cells. We propose that genomic imprinting may be responsible for a change in strategy controlling brain development in mammals. In particular, genomic imprinting may have facilitated a rapid non-linear expansion of the brain, especially the cortex, during development over evolutionary time.

Age-specific sero-prevalence of amoebiasis and giardiasis in southern Indian infants and children

The age-specific sero-prevalence of amoebiasis and giardiasis was estimated in 91 pediatric diarrhoea and in 70 non-diarrhoeal cases from Southern India. Anti-amoeba/giardia IgG assays on 20 children with inflammatory bowel disease from the UK yielded base-line levels in a non-endemic symptomatic population. IgG, IgM, and IgA levels were estimated to E. histolytica and G. lamblia using an ELISA. Concomittant faecal examinations were done for the Indian children. There was a significant correlation between acquisition of sero-positivity and age. A rise in the IgG response to both organisms was evident between 38 and 47 and 13-24 months, respectively, in diarrhoeal and non-diarrhoeal cases. An appreciable IgM response occurred predominantly in diarrhoea cases and at a younger age (less than 24 months). IgA responses were low. Anti-protozoal IgG levels in the UK children were negligible. There was no relationship between faecal excretion and sero-positivity. The study shows an age-related antibody response to E. histolytica and G. lamblia.

Serum antibody response in children with Giardia lamblia infection and identification of an immunodominant 57-kilodalton antigen

Giardia lamblia antigens which react with sera from children with G. lamblia infection were investigated by sodium-dodecyl polyacrylamide gel electrophoresis and immunoblotting. Serum IgG, IgM and IgA response to the antigens were immunochemically characterized. Serum antibodies from all giardiasis patients, but none of the controls, was found to react with a 57-kilodalton antigen. The 57 kDa antigen elicited IgG and IgA but not IgM antibodies. The protein nature of the 57 kDa antigen was demonstrated by loss of antibody recognition after trypsin treatment of G. lamblia trophozoites. Subcellular fractionation of G. lamblia trophozoites followed by SDS-PAGE and immunoblotting showed that the 57 kDa antigen was probably not a component of the cytoskeleton.

Intestinal amoebiasis and giardiasis in southern Indian infants and children

The role of Entamoeba histolytica and Giardia lamblia as causative agents of paediatric diarrhoea was studied in a southern Indian population. Relationship between infant feeding practices, co-existing malnutrition and the occurrence of intestinal amoebiasis and giardiasis was also examined. The subjects were 361 paediatric patients with acute diarrhoea and 70 hospitalized control children without diarrhoea. Faecal samples from cases and controls were examined for the protozoal pathogens using faecal preservatives, permanent staining and formalin-ether concentration. Bacteriological studies were conducted on 244 of the 361 cases. A high prevalence of invasive amoebiasis was seen in the 0-6 month (12.5%) and 7-12 month (20.3%) age groups. Giardiasis was uncommon under 6 months (2.1%) but occurred in 8-10% of all other age groups. Invasive amoebiasis occurred mainly in children on weaning foods (45.9%) but also [corrected] in exclusively breast-fed children (13.5%). Giardiasis was not seen in exclusively breast-fed infants, but commonly occurred in older children on normal diets. There was no association between amoebiasis or giardiasis and malnutrition.