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A Bejsovec

Publications and source records attributed to A Bejsovec.

At least 19 recordsLinked to original sources
PubMed2006

Flying at the head of the pack: Wnt biology in Drosophila.

The fruitfly, Drosophila melanogaster, has been of central importance in analysing the mechanics of cellular processes. Classic forward genetic screens in the fly have identified many of the genes that define critical cell signaling pathways, for example. Our understanding of the Wnt pathway, in particular, has benefited from the many advantages that the fly offers as a model system. Here, I review the history of these discoveries and highlight the utility of the fly in dissecting the molecular workings of Wnt signal transduction.

Animals↗
PubMed2001

Drosophila APC2 and Armadillo participate in tethering mitotic spindles to cortical actin.

Proper positioning of mitotic spindles ensures equal allocation of chromosomes to daughter cells. This often involves interactions between spindle and astral microtubules and cortical actin. In yeast and Caenorhabditis elegans, some of the protein machinery that connects spindles and cortex has been identified but, in most animal cells, this process remains mysterious. Here, we report that the tumour suppressor homologue APC2 and its binding partner Armadillo both play roles in spindle anchoring during the syncytial mitoses of early Drosophila embryos. Armadillo, alpha-catenin and APC2 all localize to sites of cortical spindle attachment. APC2-Armadillo complexes often localize with interphase microtubules. Zeste-white 3 kinase, which can phosphorylate Armadillo and APC, is also crucial for spindle positioning and regulates the localization of APC2-Armadillo complexes. Together, these data suggest that APC2, Armadillo and alpha-catenin provide an important link between spindles and cortical actin, and that this link is regulated by Zeste-white 3 kinase.

Actins↗
PubMed2000

Non-equivalent roles of Drosophila Frizzled and Dfrizzled2 in embryonic wingless signal transduction.

The highly conserved Wnt family of growth factors is essential for generating embryonic pattern in many animal species [1]. In the fruit fly Drosophila, most Wnt-mediated patterning is performed by a single family member, Wingless (Wg), acting through its receptors Frizzled (Fz) and DFrizzled2 (Dfz2). In the ventral embryonic epidermis, Wg signaling generates two different cell-fate decisions: the production of diverse denticle types and the specification of naked cuticle separating the denticle belts. Mutant alleles of wg disrupt these cellular decisions separately [2], suggesting that some aspect of ligand-receptor affinity influences cell-fate decisions, or that different receptor complexes mediate the distinct cellular responses. Here, we report that overexpression of Dfz2, but not Fz, rescues the mutant phenotype of wgPE2, an allele that produces denticle diversity but no naked cuticle. Fz was able to substitute for Dfz2 only under conditions where the Wg ligand was present in excess. The wgPE2 mutant phenotype was also sensitive to the dosage of glycosaminoglycans, suggesting that the mutant ligand is excluded from the receptor complex when proteoglycans are present. We conclude that wild-type Wg signaling requires efficient interaction between ligand and the Dfz2-proteoglycan receptor complex to promote the naked cuticle cell fate.

Animals↗
PubMed1999

Wnt signalling shows its versatility.

Wnt signalling controls many different cell fate choices in a wide variety of animal species. Recent studies have revealed that regulatory interactions at several steps in the pathway can modify its outcome, helping to explain how the same pathway can, in different contexts, have very different characteristics and consequences.

Animals↗
PubMed1999

Drosophila APC2 is a cytoskeletally-associated protein that regulates wingless signaling in the embryonic epidermis.

The tumor suppressor adenomatous polyposis coli (APC) negatively regulates Wingless (Wg)/Wnt signal transduction by helping target the Wnt effector beta-catenin or its Drosophila homologue Armadillo (Arm) for destruction. In cultured mammalian cells, APC localizes to the cell cortex near the ends of microtubules. Drosophila APC (dAPC) negatively regulates Arm signaling, but only in a limited set of tissues. We describe a second fly APC, dAPC2, which binds Arm and is expressed in a broad spectrum of tissues. dAPC2's subcellular localization revealed colocalization with actin in many but not all cellular contexts, and also suggested a possible interaction with astral microtubules. For example, dAPC2 has a striking asymmetric distribution in neuroblasts, and dAPC2 colocalizes with assembling actin filaments at the base of developing larval denticles. We identified a dAPC2 mutation, revealing that dAPC2 is a negative regulator of Wg signaling in the embryonic epidermis. This allele acts genetically downstream of wg, and upstream of arm, dTCF, and, surprisingly, dishevelled. We discuss the implications of our results for Wg signaling, and suggest a role for dAPC2 as a mediator of Wg effects on the cytoskeleton. We also speculate on more general roles that APCs may play in cytoskeletal dynamics.

Actins↗
PubMed1999

Cellular mechanisms of wingless/Wnt signal transduction.

Wg/Wnt signaling regulates cell proliferation and differentiation in species as divergent as nematodes, flies, frogs, and humans. Many components of this highly conserved process have been characterized and work from a number of laboratories is beginning to elucidate the mechanism by which this class of secreted growth factor triggers cellular decisions. The Wg/Wnt ligand apparently binds to Frizzled family receptor molecules to initiate a signal transduction cascade involving the novel cytosolic protein Dishevelled and the serine/threonine kinase Zeste-white 3/GSK3. Antagonism of Zw3 activity leads to stabilization of Armadillo/beta-catenin, which provides a transactivation domain when complexed with the HMG box transcription factor dTCF/LEF-1 and thereby activates expression of Wg/Wnt-responsive genes. The Wg/Wnt ligands pass through the secretory pathway and associate with extracellular matrix components; recent work shows that sulfated glycosaminoglycans are essential for proper transduction of the signal. Mutant forms of Wg in Drosophila reveal separable aspects of Wg function and suggest that proper transport of the protein across cells is essential for cell fate specification. Complex interactions with the Notch and EGF/Ras signaling pathways also play a role in cell fate decisions during different phases of Drosophila development. These many facets of Wg/Wnt signaling have been elucidated through studies in a variety of species, each with powerful and unique experimental approaches. The remarkable conservation of this pathway suggests that Wg/Wnt signal transduction represents a fundamental mechanism for the generation of diverse cell fates in animal embryos.

Amino Acid Sequence↗
PubMed1999

Directionality of wingless protein transport influences epidermal patterning in the Drosophila embryo.

Active endocytotic processes are required for the normal distribution of Wingless (Wg) protein across the epidermal cells of each embryonic segment. To assess the functional consequences of this broad Wg distribution, we have devised a means of perturbing endocytosis in spatially restricted domains within the embryo. We have constructed a transgene expressing a dominant negative form of shibire (shi), the fly dynamin homologue. When this transgene is expressed using the GAL4-UAS system, we find that Wg protein distribution within the domain of transgene expression is limited and that Wg-dependent epidermal patterning events surrounding the domain of expression are disrupted in a directional fashion. Our results indicate that Wg transport in an anterior direction generates the normal expanse of naked cuticle within the segment and that movement of Wg in a posterior direction specifies diverse denticle cell fates in the anterior portion of the adjacent segment. Furthermore, we have discovered that interfering with posterior movement of Wg rescues the excessive naked cuticle specification observed in naked (nkd) mutant embryos. We propose that the nkd segment polarity phenotype results from unregulated posterior transport of Wg protein and therefore that wild-type Nkd function may contribute to the control of Wg movement within the epidermal cells of the segment.

Alleles↗
PubMed1998

Drosophila Tcf and Groucho interact to repress Wingless signalling activity.

Wingless/Wnt signalling directs cell-fate choices during embryonic development. Inappropriate reactivation of the pathway causes cancer. In Drosophila, signal transduction from Wingless stabilizes cytosolic Armadillo, which then forms a bipartite transcription factor with the HMG-box protein Drosophila Tcf (dTcf) and activates expression of Wingless-responsive genes. Here we report that in the absence of Armadillo, dTcf acts as a transcriptional repressor of Wingless-responsive genes, and we show that Groucho acts as a corepressor in this process. Reduction of dTcf activity partially suppresses wingless and armadillo mutant phenotypes, leading to derepression of Wingless-responsive genes. Furthermore, overexpression of wild-type dTcf enhances the phenotype of a weak wingless allele. Finally, mutations in the Drosophila groucho gene also suppress wingless and armadillo mutant phenotypes as Groucho physically interacts with dTcf and is required for its full repressor activity.

Animals↗
PubMed1998

Functional analysis of Wingless reveals a link between intercellular ligand transport and dorsal-cell-specific signaling.

The Drosophila segment polarity gene wingless (wg) is essential for cell fate decisions in the developing embryonic epidermis. Wg protein is produced in one row of cells near the posterior of every segment and is secreted and distributed throughout the segment to generate wild-type pattern elements. Ventrally, epidermal cells secrete a diverse array of anterior denticle types and a posterior expanse of naked cuticle; dorsally, a stereotyped pattern of fine hairs is secreted. We describe three new wg alleles that promote naked cuticle cell fate but show reduced denticle diversity and dorsal patterning. These mutations cause single amino acid substitutions in a cluster of residues that are highly conserved throughout the Wnt family. By manipulating expression of transgenic proteins, we demonstrate that all three mutant molecules retain the intrinsic capacity to signal ventrally but fail to be distributed across the segment. Thus, movement of Wg protein through the epidermal epithelium is essential for proper ventral denticle specification and this planar movement is distinct from the apical-basal transcytosis previously described in polarized epithelia. Furthermore, ectopic overexpression of the mutant proteins fails to rescue dorsal pattern elements. Thus we have identified a region of Wingless that is required for both the transcytotic process and signal transduction in dorsal cell populations, revealing an unexpected link between these two aspects of Wg function.

Alleles↗
PubMed1997

Armadillo coactivates transcription driven by the product of the Drosophila segment polarity gene dTCF.

The vertebrate transcription factors TCF (T cell factor) and LEF (lymphocyte enhancer binding factor) interact with beta-catenin and are hypothesized to mediate Wingless/Wnt signaling. We have cloned a maternally expressed Drosophila TCF family member, dTCF. dTCF binds a canonical TCF DNA motif and interacts with the beta-catenin homolog Armadillo. Previous studies have identified two regions in Armadillo required for Wingless signaling. One of these interacts with dTCF, while the other constitutes a transactivation domain. Mutations in dTCF and expression of a dominant-negative dTCF transgene cause a segment polarity phenotype and affect expression of the Wingless target genes engrailed and Ultrabithorax. Epistasis analysis positions dTCF downstream of armadillo. The Armadillo-dTCF complex mediates Wingless signaling as a bipartite transcription factor.

Amino Acid Sequence↗
PubMed1997

Negative regulation of Armadillo, a Wingless effector in Drosophila.

Drosophila Armadillo and its vertebrate homolog beta-catenin play essential roles both in the transduction of Wingless/Wnt cell-cell signals and in the function of cell-cell adherens junctions. Wingless and Wnts direct numerous cell fate choices during development. We generated a mutant protein, Armadillo(S10), with a 54 amino acid deletion in its N-terminal domain. This mutant is constitutively active in Wingless signaling; its activity is independent of both Wingless signal and endogenous wild-type Armadillo. Armadillo's role in signal transduction is normally negatively regulated by Zeste-white 3 kinase, which modulates Armadillo protein stability. Armadillo(S10) is more stable than wild-type Armadillo, suggesting that it is less rapidly targeted for degradation. We show that Armadillo(S10) has escaped from negative regulation by Zeste white-3 kinase, and thus accumulates outside junctions even in the absence of Wingless signal. Finally, we present data implicating kinases in addition to Zeste white-3 in Armadillo phosphorylation. We discuss two models for the negative regulation of Armadillo in normal development and discuss how escape from this regulation contributes to tumorigenesis.

Amino Acid Sequence↗
PubMed1997

Wingless signaling generates pattern through two distinct mechanisms.

wingless (wg) and its vertebrate homologues, the Wnt genes, play critical roles in the generation of embryonic pattern. In the developing Drosophila epidermis, wg is expressed in a single row of cells in each segment, but it influences cell identities in all rows of epidermal cells in the 10- to 12-cell-wide segment. Wg signaling promotes specification of two distinct aspects of the wild-type intrasegmental pattern: the diversity of denticle types present in the anterior denticle belt and the smooth or naked cuticle constituting the posterior surface of the segment. We have manipulated the expression of wild-type and mutant wg transgenes to explore the mechanism by which a single secreted signaling molecule can promote these distinctly different cell fates. We present evidence consistent with the idea that naked cuticle cell fate is specified by a cellular pathway distinct from the denticle diversity-generating pathway. Since these pathways are differentially activated by mutant Wg ligands, we propose that at least two discrete classes of receptor for Wg may exist, each transducing a different cellular response. We also find that broad Wg protein distribution across many cell diameters is required for the generation of denticle diversity, suggesting that intercellular transport of the Wg protein is an essential feature of pattern formation within the epidermal epithelium. Finally, we demonstrate that an 85 amino acid region not conserved in vertebrate Wnts is dispensable for Wg function and we discuss structural features of the Wingless protein required for its distinct biological activities.

Animals↗
PubMed1995

Signaling activities of the Drosophila wingless gene are separately mutable and appear to be transduced at the cell surface.

The Drosophila segment polarity gene wingless encodes an intercellular signaling molecule that transmits positional information during development of the embryonic epidermis. We have explored the mechanism of wg signal transduction by perturbing cellular processes genetically and by performing structure/function analysis of the Wg protein. We present evidence that Wingless protein may transduce signal at the cell surface and that Wg may bind to its cell surface receptor without necessarily activating it. We demonstrate that two specific signaling activities of the Wg molecule can be disrupted independently by mutation. Sequence analysis indicates that these different signaling activities are not promoted by discrete functional domains, but rather than the overall conformation of the molecule may control distinct signaling functions. We conclude that wg signaling may involve complex interactions between the Wg ligand and its cell surface receptor molecule(s) and that some of this complexity resides within the Wg ligand itself.

Amino Acid Sequence↗
PubMed1993

Segment polarity gene interactions modulate epidermal patterning in Drosophila embryos.

Each segment of a Drosophila larva shows a precisely organized pattern of cuticular structures, indicating diverse cellular identities in the underlying epidermis. Mutations in the segment polarity genes alter the cuticle pattern secreted by the epidermal cells; these mutant patterns provide clues about the role that each gene product plays in the development of wild-type epidermal pattern. We have analyzed embryos that are multiply mutant for five key patterning genes: wingless, patched, engrailed, naked and hedgehog. Our results indicate that wild-type activity of these five segment polarity genes can account for most of the ventral pattern elements and that their gene products interact extensively to specify the diverse cellular identities within the epidermis. Two pattern elements can be correlated with individual gene action: wingless is required for formation of naked cuticle and engrailed is required for formation of the first row of denticles in each abdominal denticle belt. The remaining cell types can be produced by different combinations of the five gene activities. wingless activity generates the diversity of cell types within the segment, but each specific cell identity depends on the activity of patched, engrailed, naked and hedgehog. These molecules modulate the distribution and interpretation of wingless signalling activity in the ventral epidermal cells and, in addition, each can contribute to pattern through a pathway independent of the wingless signalling pathway.

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↗
PubMed1991

Roles of wingless in patterning the larval epidermis of Drosophila.

The larval epidermis of Drosophila shows a stereotyped segmentally repeating pattern of cuticular structures. Mutants deficient for the wingless gene product show highly disrupted patterning of the larval cuticle. We have manipulated expression of the wg gene product to assess its role in this patterning process. We present evidence for four distinct phases of wg function in epidermal cells: (1) an early requirement in engrailed-expressing cells to establish and maintain stable expression of en, (2) a discrete period when wg and en gene products act in concert to generate positional values in the anterior portion of the ventral segment and all values of the dorsal and lateral epidermis, (3) a progressive function (dependent on prior interaction with the en-expressing cells) in conferring positional values to cells within the posterior portion of the segment, and (4) a late continuous requirement for maintaining some ventral positional values.

Animals↗
PubMed1990

Functions of the myosin ATP and actin binding sites are required for C. elegans thick filament assembly.

We have determined the positions and sequences of 31 dominant mutations affecting a C. elegans muscle myosin heavy chain gene. These mutations alter thick filament structure in heterozygotes by interfering with the ability of wild-type myosin to assemble into stable thick filaments. These assembly-disruptive mutations are missense alleles affecting the globular head of myosin. The most strongly dominant alleles alter highly conserved residues of the myosin ATP binding site, indicating that functions of the myosin ATPase are important for thick filament assembly. Other alleles alter the site at which myosin binds actin.

Actin Cytoskeleton↗