Biomedical subjects

M Bienz

Publications and source records attributed to M Bienz.

At least 55 records · Page 3Linked to original sources

PubMed1994

Homeotic genes and positional signalling in the Drosophila viscera.

Homeotic genes play a key regulatory role in the two innermost germ layers of Drosophila, the visceral mesoderm and the endoderm. Here, they control, and are controlled by, genes that encode extracellular proteins involved in signalling. Examples of these regulatory interactions are an inductive process between the two germ layers, and an indirect autoregulatory loop of a homeotic gene in the visceral mesoderm. Both these mechanisms define or redefine spatial domains of homeotic gene expression, by way of intercellular communication. The homeotic genes that are expressed in the viscera control morphogenesis and differentiation of the larval midgut.

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PubMed1994

Imaginal disc silencers from Ultrabithorax: evidence for Polycomb response elements.

Silencers from the Drosophila homeotic gene Ultrabithorax (Ubx) require hunchback (hb) and Polycomb (Pc) to suppress the activity of embryonic enhancers outside the Ubx domain. Embryonic silencing is initiated by hb protein which binds to the silencers to repress Ubx, thereby defining the Ubx domain. Here, we study silencing during subsequent development by examining expression patterns in imaginal discs conferred by individual Ubx fragments and pair-wise combinations thereof. We find that fragments which mediate silencing in anterior regions of imaginal discs contain embryonic silencers and hb target sites. One exception to this is a fragment called BXD which is not under hb control itself, but whose silencing activity depends on combination with fragments containing hb protein binding sites. Since silencing by BXD also requires Pc function, this suggests that BXD contains target sites for Pc or for Pc-like proteins. We propose that stable silencing of Ubx is achieved through cooperation between hb and Pc target sites.

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PubMed1993

Indirect autoregulation of a homeotic Drosophila gene mediated by extracellular signaling.

Commitments to developmental pathways are often made and maintained in groups of cells. Such commitments are conferred by the products of selector genes, many of which are homeobox genes. Homeobox genes can maintain their expression by directly autoregulating their own transcription. Here, we report a case where positive autoregulation of Ultrabithorax, a homeotic Drosophila gene, is at least partly indirect and mediated by the extracellular signal molecules that are products of the genes wingless and decapentaplegic. Indirect autoregulatory mechanisms may be used to ensure coordinate maintenance of selector gene activity in groups of cells.

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PubMed1993

Silencers in abdominal-B, a homeotic Drosophila gene.

Homeotic genes determine the developmental fates of cells. Restriction of their expression along the body axis is of prime importance for normal development. We searched for cis-regulatory sequences within Abdominal-B (Abd-B), a homeotic Drosophila gene, by testing genomic Abd-B fragments for their ability to confer beta-galactosidase (beta-gal) expression in transformed embryos. One of the Abd-B fragments, called IAB5, mediates a beta-gal pattern restricted along the body axis to the Abd-B expression domain. Alterations of the IAB5 pattern in gap mutants provide evidence that the protein products of the gap genes hunchback, Krüppel and knirps act as repressors through IAB5. The anterior Abd-B expression limit is apparently determined by Krüppel repression, whereas the knirps repressor may be responsible for the graded Abd-B expression within the Abd-B domain. IAB5 and two other fragments called MCP and FAB show region-specific silencing activity: they suppress at a distance beta-gal expression mediated by a linked heterologous enhancer. Silencing requires hunchback as well as Polycomb function and evidently provides maintenance of Abd-B expression limits throughout embryogenesis. We conclude that transcriptional repression is a key mechanism operating at multiple levels to control Abd-B expression. The striking similarities between the control of Abd-B and of Ultrabithorax, another homeotic Drosophila gene, may point to a universal principle underlying homeotic gene regulation.

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PubMed1993

Dissection of an indirect autoregulatory response of a homeotic Drosophila gene.

Homeotic genes often use autoregulation as a mechanism to maintain their expression. Autoregulation of Ultrabithorax (Ubx) in the visceral mesoderm is at least partly indirect and mediated by extracellular signalling from wingless (wg) and decapentaplegic (dpp). Ubx controls the localized expression of these two extracellular proteins. Here, we identify separate wg and dpp response elements within upstream sequences of Ubx. Our evidence suggests that there are two distinct response factors each of which, after signal-induced activation, mediates transcriptional activation through its cognate element, whereas each element is recognized by a repressor in the absence of the corresponding signal. We show that the response factors and other components for transmission of the wg and, probably, of the dpp signal are present throughout the midgut mesoderm. Thus, there may be ubiquitous repression, preventing Ubx autoregulation throughout the visceral mesoderm, which is relieved locally by wg and dpp signalling. Evidently, the two signals convey positional information, allowing visceral mesoderm cells to reassess their position at advanced stages of embryogenesis and to decide whether or not to maintain expression of a homeotic gene.

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PubMed1992

Segmental determination in Drosophila conferred by hunchback (hb), a repressor of the homeotic gene Ultrabithorax (Ubx).

The activity of homeotic genes in Drosophila cells determines segment-specific morphogenesis. Here, we provide evidence that the product of hunchback (hb), a segmentation gene, acts as a direct repressor or "silencer" of the homeotic gene Ultrabithorax (Ubx) and thus prevents ectopic activity of this gene: we show, by stable integration of reporter gene constructs, that hb protein binding sites are capable of repressing at a distance the activity of an embryonic Ubx enhancer outside the Ubx expression domain. This silencing activity is observed at advanced embryonic stages, at a time when the hb gene product is no longer detectable or required, and is dependent on the function of Polycomb (Pc). We propose a working hypothesis as to how hb protein in a "hit-and-run" fashion may effect stable and heritable silencing of the Ubx gene throughout advanced stages of development, thus mediating repression of this homeotic gene outside its realm of function.

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PubMed1992

Sharp anterior boundary of homeotic gene expression conferred by the fushi tarazu protein.

Parasegmental boundaries in the Drosophila embryo are delimited by the products of the fushi tarazu (ftz) and even-skipped (eve) genes. We show here that these act through particular key control regions of the homeotic gene Ultrabithorax (Ubx) to generate ftz- or eve-like stripe patterns of beta-galactosidase expression. Footprint analysis and tests in transformed embryos of constructs bearing mutated footprint regions suggest that ftz protein acts directly as a transcriptional activator of Ubx. Its activity outside the Ubx expression domain is suppressed by hunchback (hb), a repressor of Ubx. Some DNA binding sites for ftz protein are adjacent to, others overlap binding sites for hb protein, and we provide evidence that ftz protein competes with hb protein for DNA binding and/or for transcriptional activation. This competition mechanism results in a sharp anterior expression boundary. Direct activation of homeotic gene control regions by ftz (or eve) protein may be a regulatory step which is generally used to align expression of homeotic genes with parasegmental boundaries.

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PubMed1992

A cis-element mediating Ultrabithorax autoregulation in the central nervous system.

We dissected an upstream control region (a BXD fragment) from the homeotic gene Ultrabithorax (Ubx) of Drosophila which confers a Ubx-like expression pattern in the embryonic ectoderm. We found several distinct enhancer elements spread through the whole BXD fragment each of which is active in transformed embryos, mediating a different pattern of beta-galactosidase expression in the ventral nerve cord. The strongest of these patterns mimics Ubx expression within the Ubx domain. This pattern is strictly dependent on Ubx function. Thus, the BXD control region contains a Ubx response element, suggesting that positive autoregulation of Ubx may occur in the central nervous system of the developing embryo.

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PubMed1992

Molecular mechanisms of determination in Drosophila.

A number of Drosophila proteins have been identified that play key roles in the establishment of active or inactive states of selector gene expression. Interactions between these proteins and their target selector genes are beginning to be understood, shaping our molecular view as to how stable determination of cells is achieved.

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PubMed1992

Function of an Ultrabithorax minigene in imaginal cells.

An Ultrabithorax (Ubx) minigene constructed from three key Ubx control regions is capable of supporting development of Ubx null mutants throughout larval life and beyond to pharate flies, thereby rescuing the larval lethality due to the homeotic mutation. The cuticle of these flies shows that the minigene provides at least partial Ubx function in each of the four compartments whose morphogenetic pathways are determined by Ubx. We analyse beta-galactosidase patterns in imaginal discs conferred by each individual Ubx control region. From the comparison of these patterns with Ubx expression in Cbx mutants, we infer that long-range repressor elements in the chromosomal Ubx gene play an important role in the generation of Ubx expression patterns in imaginal discs. Expression and function of our Ubx minigenes indicate that Ubx control regions are capable of functioning properly out of context and detached from their normal chromosomal location within the homeotic gene complex.

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PubMed1992

Induction of labial expression in the Drosophila endoderm: response elements for dpp signalling and for autoregulation.

Extracellular signal proteins induce the homeotic gene labial (lab) to high levels of localised expression in the endoderm of Drosophila embryos. We aimed to identify cis-regulatory elements within the lab gene that respond to this induction by analysing the activity of stably integrated reporter gene constructs. Dissection of lab 5' flanking sequences reveals two types of response elements. One of these mediates lab dependent activity, providing evidence that lab induction in the endoderm is autoregulatory. The other element, to a large extent independent of lab function, responds to decapentaplegic (dpp), a signal molecule related to mammalian TGF-beta. Our evidence suggests that lab induction in the endoderm reflects coordinate action of two distinct factors one of which may be lab protein itself, and another whose localised activity or expression in the midgut depends on the dpp signal.

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PubMed1991

Long range repression conferring boundaries of Ultrabithorax expression in the Drosophila embryo.

In an attempt to reconstruct the embryonic expression pattern of the homeotic gene Ultrabithorax (Ubx) by stable integration of fusion constructs, we identified three key control regions called PBX, ABX and BXD. Each of these confers an expression pattern mimicking certain aspects of Ubx expression. The PBX and ABX patterns are limited to the Ubx domain with anterior boundaries at parasegments 6 and 5. In contrast, the BXD pattern extends from head to tail. PBX or ABX expression boundaries are imposed on the BXD pattern, if PBX or ABX is linked to BXD. These boundaries, although not the PBX and ABX expression limits themselves, are dependent on Polycomb function. We conclude that PBX and ABX are recognized by repressors which act across large distances to suppress BXD activity. Stable and heritable Ubx expression boundaries are thus mediated by this process of long range repression.

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PubMed1991

Functional dissection of Drosophila abdominal-B protein.

The Abdominal-B gene is unique among homeotic Drosophila genes as it encodes two proteins m and r which confer different functions. The m protein corresponds to an r protein with a large N-terminal extension; the two proteins contain the same homeodomain. We have used a transient co-transfection assay, based on HeLa cells, to analyse the intrinsic function of m and r protein in activating transcription. We find two strong transcriptional activation domains in the common part of the two proteins. The m-specific exon contains additional transcriptional activation potential. Despite this, the m protein is a weaker transcriptional activator than the r protein. Apparently, there are inhibitory sequences in the m-specific exon which, in the embryo, may have a role in masking r function in the intact m protein.

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PubMed1991

Target sequences for hunchback in a control region conferring Ultrabithorax expression boundaries.

Boundaries of Ultrabithorax expression are mediated by long-range repression acting through the PBX or ABX control region. We show here that either of these control regions confers an early band of beta-galactosidase expression which is restricted along the anteroposterior axis of the blastoderm embryo. This band is succeeded by a stripe pattern with very similar anteroposterior limits. Dissection of the PBX control region demonstrates that the two patterns are conferred by distinct cis-regulatory sequences contained within separate PBX subfragments. We find several binding sites for hunchback protein within both PBX subfragments. Zygotic hunchback function is required to prevent ectopic PBX expression. Moreover, the PBX pattern is completely suppressed in embryos containing uniformly distributed maternal hunchback protein. Our results strongly suggest that hunchback protein directly binds to the PBX control region and acts as a repressor to specify the boundary positions of the PBX pattern.

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PubMed1990

Induction across germ layers in Drosophila mediated by a genetic cascade.

We report an induction process occurring between two germ layers in the Drosophila embryo that involves a cascade of five interacting genes. Two of these, Ultrabithorax and abdominal-A, encode nuclear homeobox proteins; each of them is expressed in one of two adjacent parasegments in the visceral mesoderm and directs expression in its parasegment of a separate target gene, decapentaplegic in parasegment 7 and wingless in parasegment 8. The activity of both target genes is required for normal expression of another homeotic gene, labial, in cells of the adhering midgut epithelium. Their products are putative extracellular proteins, which presumably act as signals between the two germ layers. Positional instruction of this kind may be needed since the endoderm, unlike the mesoderm, appears unsegmented at first as it originates from two primordia near the embryonic poles, outside the realm of segmentation genes.

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PubMed1990

Expression of Abdominal-B homeoproteins in Drosophila embryos.

The Abdominal-B (Abd-B) gene determines development of the posteriormost segments in Drosophila. Genetic and molecular analysis suggested that it consists of two genetically separable functions that are conferred by two related homeoproteins termed m and r. We have raised an antiserum against Abd-B protein to describe the patterns of Abd-B protein expression during embryonic development. The pattern of r protein expression, as deduced by analysis of Abd-B mutants, is restricted to ps14 and 15 in all germ layers and observes a parasegmental boundary at its anterior margin of expression. In contrast, the pattern of m protein expression is unusual as its level in the ectoderm increases from ps10 to ps13 in parasegmental steps. Its anterior margin of expression is highly dynamic shifting anteriorly across more than 3 parasegments during midembryonic development. Evidently, the control mechanisms of m and r protein expression are considerably different. Moreover, an antibody-positive Abd-B mutant suggests that these differ, in the case of m protein expression, to some extent in individual germ layers.

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PubMed1989

Coordinate action of a proximal homeoprotein binding site and a distal sequence confers the Ultrabithorax expression pattern in the visceral mesoderm.

Spatially regulated expression of the homeotic gene Ultrabithorax (Ubx) in the visceral mesoderm can be mimicked in transformed Drosophila embryos by expression of a Ubx--beta-galactosidase fusion gene. Here we show that a proximal homeoprotien binding sequence downstream of the Ubx transcription start site, the B element, is required for this pattern. A distal upstream Ubx sequence, but not the B element, is sufficient to confer the pattern if linked to an hsp70 TATA box in a heterologous construct. The pattern in this case requires Ubx function, like endogenous Ubx expression in the visceral mesoderm, suggesting that the distal upstream sequence contains an important target sequence for autoregulation. We propose that the B element, in the context of the Ubx promoter, functions to mediate enhancer function of the distal sequence. Thus, the visceral mesoderm pattern requires coordinate action of a proximal and a distal regulatory element.

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PubMed1989

Homeotic gene expression in the visceral mesoderm of Drosophila embryos.

The visceral mesoderm adhering to the midgut constitutes an internal germ layer of the Drosophila embryo that stretches along most of the anteroposterior axis (parasegment 2-13). Most cells of the midgut visceral mesoderm express exclusively one of five homeotic genes. Three of these genes, Antennapedia, Ultrabithorax and abdominal-A are active in parasegmental domains characteristic for this germ layer as they are nonoverlapping and adjacent. The common boundaries between these domains depend on mutual regulatory interactions between the three genes. The same genes function to control gut morphogenesis. Two further homeotic genes Sex combs reduced and Abdominal-B are expressed at both ends of the midgut visceral mesoderm, although absence of their expression does not appear to affect gut morphogenesis. There are no regulatory interactions between these two and the other homeotic genes. As a rule, the anterior limit of each homeotic gene domain in the visceral mesoderm is shifted posteriorly by one parasegment compared to the ectoderm. The domains result from a set of regulatory processes that are distinct from the ones ruling in other germ layers.

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