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H Skaer

Publications and source records attributed to H Skaer.

15 recordsLinked to original sources
PubMed2001

Myoblast diversification and ectodermal signaling in Drosophila.

The flight muscles of Drosophila derive from myoblasts found on the third instar disc. We demonstrate that these myoblasts already show distinctive properties and examine how this diversity is generated. In the late larva, Vestigial and low levels of Cut are expressed in myoblasts that will contribute to the indirect flight muscles. Other myoblasts, which express high levels of Cut but no Vestigial, are required for the formation of the direct flight muscles. Vestigial and Cut expression are stabilized by a mutually repressive feedback loop. Vestigial expression begins in the embryo in a subset of adult myoblasts, and Wingless signaling is required later to maintain this expression. Thus, myoblasts are divided into identifiable populations, consistent with their allocation to different muscles, and ectodermal signals act to maintain these differences.

Animals↗
PubMed2000

Coordinating cell fate and morphogenesis in Drosophila renal tubules.

Using the renal tubules of Drosophila as an example, we explore how cell specification leads to the morphogenetic movements that underlie the generation of tissue architecture. Taking two stages of development, we show first that the tubule cells are allocated by signalling between the endodermal and ectodermal compartments of the posterior gut. Activation of the Wnt pathway patterns the ectodermal anlage, resulting in the expression of tubule genes in a subset of cells and their eversion from the hindgut to form the tubule primordia. We argue that early gene expression directs these morphogenetic movements but not the complete programme of tubule differentiation. In the second example we show that the allocation of the mitogenic tip cell lineage in each tubule is required not only for the normal pattern of cell division but also for the stereotyped three-dimensional arrangement of the mature tubules. Analysis of mutants in which the tip cell lineage is misspecified reveals that both daughters of the tip cell progenitor are required for the tubules to navigate through the body cavity, so that the distal tips locate in their characteristic positions. We show that the regulator of Rac, Myoblast city is essential for this second morphogenetic process.

Animals↗
PubMed2000

Cell polarity: Nailing Crumbs to the scaffold.

Recent studies suggest that the cytoplasmic tail of the Crumbs protein establishes membrane territories in polarised epithelial cells by stabilising a PDZ-domain protein complex. The isolation of a human Crumbs homologue required for normal vision revives questions about the role of Crumbs in cell-cell interactions.

Alternative Splicing↗
PubMed2000

Multiple signalling pathways establish cell fate and cell number in Drosophila malpighian tubules.

A unique cell, the tip mother cell, arises in the primordium of each Drosophila Malpighian tubule by lateral inhibition within a cluster of achaete-expressing cells. This cell maintains achaete expression and divides to produce daughters of equivalent potential, of which only one, the tip cell, adopts the primary fate and continues to express achaete, while in the other, the sibling cell, achaete expression is lost (M. Hoch et al., 1994, Development 120, 3439-3450). In this paper we chart the mechanisms by which achaete expression is differentially maintained in the tip cell lineage to stabilise cell fate. First, wingless is required to maintain the expression of achaete in the tubule primordium so that wingless mutants lack tip cells. Conversely, increasing wingless expression results in the persistence of achaete expression in the cell cluster. Second, Notch signalling is restricted by the asymmetric segregation of Numb, as the tip mother cell divides, so that achaete expression is maintained only in the tip cell. In embryos mutant for Notch tip cells segregate at the expense of sibling cells, whereas in numb neither daughter cell adopts the tip cell fate resulting in tubules with two sibling cells. Conversely, when numb is overexpressed two tip cells segregate and tubules have no sibling cells. Analysis of cell proliferation in the developing tubules of embryos lacking Wingless after the critical period for tip cell allocation reveals an additional requirement for wingless for the promotion of cell division. In contrast, alteration in the expression of numb has no effect on the final tubule cell number.

Animals↗
PubMed1999

bloated tubules (blot) encodes a Drosophila member of the neurotransmitter transporter family required for organisation of the apical cytocortex.

We have identified a novel member of the vertebrate sodium- and chloride-dependent neurotransmitter symporter family from Drosophila melanogaster. This gene, named bloated tubules (blot), shows significant sequence similarity to a subgroup of vertebrate orphan transporters. blot transcripts are maternally supplied and during embryogenesis exhibit a complex and dynamic pattern in a subset of ectodermally derived epithelia, notably in the Malpighian tubules, and in the nervous system. Animals mutant for this gene are larval lethals, in which the Malpighian tubule cells are distended with an enlarged and disorganised apical surface. Embryos lacking the maternal component of blot expression die during early stages of development. They show an inability to form actin filaments in the apical cortex, resulting in impaired syncytial nuclear divisions, severe defects in the organisation of the cortical cytoskeleton, and a failure to cellularise. For the first time, a neurotransmitter transporter-like protein has been implicated in a function outside the nervous system. The isolation of blot thus provides the basis for an analysis of the relationship between the function of this putative transporter and epithelial morphogenesis.

Amino Acid Sequence↗
PubMed1999

Airway patterning: A paradigm for restricted signalling.

Intercellular signalling is limited by the range of cell responsiveness, often mediated by repressors. A recently identified repressor, Sprouty, inhibits MAP kinase signalling in flies, mice and humans and has a conserved function in patterning the airways of these divergent species.

Animals↗
PubMed1997

Morphogenesis: FGF branches out.

Signalling via tissue-specific receptors for fibroblast growth factor is important in tracheal branching and mesodermal patterning in Drosophila; both processes may involve a common primary response - detachment and directed migration of the target cells - that is conserved from worms to mammals.

Animals↗
PubMed1996

The Drosophila genes crumbs and stardust are involved in the biogenesis of adherens junctions.

Morphogenetic movements of epithelia during development underlie the normal elaboration of the final body plan. The tissue integrity critical for these movements is conferred by anchorage of the cytoskeleton by adherens junctions, initially spot and later belt-like, zonular structures, which encircle the apical side of the cell. Loss-of-function mutations in the Drosophila genes crumbs and stardust lead to the loss of cell polarity in most ectodermally derived epithelia, followed in some, such as the epidermis, by extensive apoptosis. Here we show that both mutants fail to establish proper zonulae adherentes in the epidermis. Our results suggest that the two genes are involved in different aspects of this process. Further, they are compatible with the hypothesis that crumbs delimits the apical border, where the zonula adherens usually forms and where Crumbs protein is normally most abundant. In contrast, stardust seems to be required at an earlier stage for the assembly of the spot adherence junctions. In both mutants, the defect observed at the ultrastructural level are preceded by a misdistribution of Armadillo and DE-cadherin, the homologues of beta-catenin and E-cadherin, respectively, which are two constituents of the vertebrate adherens junctions. Strikingly, expansion of the apical membrane domain in epidermal cells by overexpression of crumbs also abolishes the formation of adherens junctions and results in the disruption of tissue integrity, but without loss of membrane polarity. This result supports the view that membrane polarity is independent of the formation of adherens junctions in epidermal cells.

Animals↗
PubMed1994

Sequential fates in a single cell are established by the neurogenic cascade in the Malpighian tubules of Drosophila.

In each Malpighian tubule of Drosophila, one cell is singled out, the tip cell, whose function during embryogenesis is to promote cell division in its neighbours. We follow the segregation of this cell, explore the genetic interactions that underlie its specification and demonstrate that tip cell allocation closely resembles neurogenesis. The tip cell arises by division of a tip mother cell, which is selected from a cluster of equivalent cells in each tubule primordium. Each cluster is marked out by the expression of proneural genes and the selection of a single cell from each group involves lateral inhibition, mediated by the neurogenic genes. We confirm the mitogenic role of the tip cell during embryogenesis by mutational analysis and show that it subsequently adopts a second fate, differentiating neural characteristics. We demonstrate that both stages in the differentiation of this cell are established by the same sequence of genetic interactions, which have not previously been shown to occur outside the neurogenic ectoderm.

Animals↗
PubMed1993

The Drosophila EGF receptor homologue (DER) is required for Malpighian tubule development.

Defects in the locus Egfr, encoding the Drosophila EGF receptor homologue (DER), affect the development of the Malpighian tubules. They form as much shorter structures than in wild-type embryos, containing a reduced number of cells. The severity of this phenotype in seven alleles that we have analysed correlates with other embryonic defects caused by Egfr mutations. Interestingly the two pairs of tubules are affected with different severity, with a greater reduction in cell number in the posterior pair than in the anterior. Temperature shift experiments indicate a role for this receptor in the regulation of tubule cell division. We also suggest that an additional role for DER in the allocation of cells to the tubule primordia is possible.

Animals↗
PubMed1992

Cell proliferation and rearrangement in the development of the malpighian tubules of the hemipteran, Rhodnius prolixus.

The pattern of cell activities resulting in the generation of a simple tubular epithelium, the Malpighian tubules of Rhodnius prolixus, is examined during embryogenesis. The anlage of the tubules is shown to be of ectodermal origin. An evolving pattern of cycling cells in the primordia is revealed using the immunocytochemical localization of a substituted nucleotide, BUdR. A cell unique to each tubule is shown not to enter the cell cycle but to be required for the proliferation of the remaining cells in the tubules. Furthermore, the activity of this cell imposes on each tubule a clear proximo-distal axis.

Animals↗
PubMed1991

Secretion and movement of wingless protein in the epidermis of the Drosophila embryo.

The segment polarity gene wingless encodes a cysteine rich protein which is essential for pattern formation in Drosophila. Using polyclonal antibodies against the product of the wingless gene, we demonstrate that this protein is secreted in the embryo and that it is taken up by neighbouring cells. The protein can be found two or three cell diameters away from the cells in which it is synthesized. We discuss the possible mechanisms which are responsible for this spatial distribution and its regulation during embryogenesis.

Alleles↗