Biomedical subjects

B Sanson

Publications and source records attributed to B Sanson.

9 recordsLinked to original sources

PubMed2001

Generating patterns from fields of cells. Examples from Drosophila segmentation.

In Drosophila, a cascade of maternal, gap, pair-rule and segment polarity genes subdivides the antero/posterior axis of the embryo into repeating segmental stripes. This review summarizes what happens next, i.e. how an intrasegmental pattern is generated and controls the differentiation of specific cell types in the epidermis. Within each segment, cells secreting the signalling molecules Wingless (the homologue of vertebrate Wnt-1) and Hedgehog are found in narrow stripes on both sides of the parasegmental boundary. The Wingless and Hedgehog organizing activities help to establish two more stripes per segment that localize ligands for the Epidermal Growth Factor and the Notch signalling pathways, respectively. These four signals then act at short range and in concert to control epidermal differentiation at the single cell level across the segment. This example from Drosophila provides a paradigm for how organizers generate precise patterns, and ultimately different cell types, in a naïve field of cells.

Animals↗

PubMed2000

Endoribonuclease RegB from bacteriophage T4 is necessary for the degradation of early but not middle or late mRNAs.

The RegB endoribonuclease from bacteriophage T4 cleaves early mRNAs specifically in the middle of the sequence GGAG. We show here that RegB is required for the degradation of bulk T4 early mRNA. In the absence of RegB, the chemical half-life of early transcripts is increased nearly fourfold, whereas their functional half-life is increased twofold. RegB also regulates the translation of several prereplicative genes. The synthesis of several early proteins is down-regulated, probably as a consequence of RegB cleavages in the Shine-Dalgarno sequence of these genes. The synthesis of several other proteins is up-regulated, suggesting that processing by RegB might improve translation by changing the conformation of a transcript. In contrast, RegB does not affect the average half-life of middle and late mRNA. An analysis of the susceptibility to RegB of many GGAG motifs carried by these mRNA species showed that most middle and all late GGAG-carrying mRNAs escape RegB processing in spite of the fact that the enzyme is acting at least until ten minutes post-infection. The sensitivity or resistance to RegB observed during phage infection could be reproduced in uninfected Escherichia coli cells and in vitro. This shows that the GGAG-carrying RNAs that are uncut during T4 infection are not substrates, whatever the period of the T4 cycle when the transcripts are made.

Bacteriophage T4↗

PubMed1999

Engrailed and hedgehog make the range of Wingless asymmetric in Drosophila embryos.

In many instances, remote signaling involves the transport of secreted molecules. Here, we examine the spread of Wingless within the embryonic epidermis of Drosophila. Using two assays for Wingless activity (specification of naked cuticle and repression of rhomboid transcription), we found that Wingless acts at a different range in the anterior and posterior directions. We show that this asymmetry follows in part from differential distribution of the Wingless protein. Transport or stability is reduced within engrailed-expressing cells, and farther posteriorward Wingless movement is blocked at the presumptive segment boundary and perhaps beyond. We demonstrate the role of hedgehog in the formation of this barrier.

Animals↗

PubMed1999

A screen for identifying genes interacting with armadillo, the Drosophila homolog of beta-catenin.

Drosophila Armadillo is a multifunctional protein implicated in both cell adhesion, as a catenin, and cell signaling, as part of the Wingless signal transduction pathway. We have generated viable fly stocks with alterations in the level of Armadillo available for signaling. Flies from one stock overexpress Armadillo and, as a result, have increased vein material and bristles in the wings. Flies from the other stock have reduced cytoplasmic Armadillo following overexpression of the intracellular domain of DE-cadherin. These flies display a wing-notching phenotype typical of wingless mutations. Both misexpression phenotypes can be dominantly modified by removing one copy of genes known to encode members of the wingless pathway. Here we describe the identification of further mutations that dominantly modify the Armadillo misexpression phenotypes. These mutations are in genes encoding three different functions: establishment and maintenance of adherens junctions, cell cycle control, and Egfr signaling.

Animals↗

PubMed1996

Uncoupling cadherin-based adhesion from wingless signalling in Drosophila.

The Wnt genes encode secreted glycoproteins used in intercellular communication at multiple steps during development. Signalling by Wingless, the Drosophila Wnt-1 homologue, requires the activity of Armadillo, the homologue of vertebrate beta-catenin, which is a component of the cadherin/catenin complex at adherens junctions. The genetic link between wingless and armadillo suggests that cell fate specification and cell-cell adhesion might be controlled concurrently. For instance, in one extreme view, Wingless could specify cell fate entirely by modulating cell adhesion. Alternatively, it might signal independently of adherens junctions. To distinguish between these alternatives, we have expressed two polypeptides that have opposite effects on cadherin-dependent adhesion: full-length Drosophila E-cadherin and a dominant-negative truncated form. We found that overexpression of either construct mimics wingless phenotypes, thereby uncoupling changes in adhesion from signalling effects. We demonstrate that both constructs titrate Armadillo from a 'signalling' pool which is functionally distinct from the junctional pool.

Animals↗

PubMed1996

Compartments, wingless and engrailed: patterning the ventral epidermis of Drosophila embryos.

Recent experiments on the wing disc of Drosophila have shown that cells at the interface between the anterior and posterior compartments drive pattern formation by becoming the source of a morphogen. Here we ask whether this model applies to the ventral embryonic epidermis. First, we show that interfaces between posterior (engrailed ON) and anterior (engrailed OFF) cells are required for pattern formation. Second, we provide evidence that Wingless could play the role of the morphogen, at least within part of the segmental pattern. We looked at the cuticular structures that develop after different levels of uniform Wingless activity are added back to unsegmented embryos (wingless- engrailed-). Because it is rich in landmarks, the T1 segment is a good region to analyse. There, we find that the cuticle formed depends on the amount of added Wingless activity. For example, a high concentration of Wingless gives the cuticle elements normally found near the top of the presumed gradient. Unsegmented embryos are much shorter than wild type. If Wingless activity is added in stripes, the embryos are longer than if it is added uniformly. We suggest that the Wingless gradient landscape affects the size of the embryo, so that steep slopes would allow cells to survive and divide, while an even distribution of morphogen would promote cell death. Supporting the hypothesis that Wingless acts as a morphogen, we find that these stripes affect, at a distance, the type of cuticle formed and the planar polarity of the cells.

Animals↗

PubMed1995

Post-transcriptional controls in bacteriophage T4: roles of the sequence-specific endoribonuclease RegB.

Gene regB of bacteriophage T4 encodes a sequence-specific endoribonuclease which introduces cuts in early phage messenger RNAs. In most cases, cutting takes place in the middle of the tetranucleotide GGAG. Efficient cleavages occur in the motifs located in intergenic regions, some of them being Shine-Dalgarno sequences. When located in a coding sequence, this tetranucleotide is poorly recognized or not at all. In this article, we have reviewed the properties of the RegB endoribonuclease, with emphasis on its possible roles in T4 development. We show that the nuclease RegB plays at least two roles: (i) it inactivates a sub-class of early mRNA by cleaving Shine-Dalgarno sequences, and (ii) it is necessary for the degradation of early mRNAs, but not of middle and late mRNAs. Accordingly, we found that middle and late mRNAs avoid processing by RegB, probably for different reasons. Most of the middle mRNAs (mRNAs initiated at MotA-dependent promoters) do not contain the motif GGAG in their intergenic regions, whereas about one-third of the late genes have this motif as Shine-Dalgarno sequence. It is not yet known whether the RNase is inactivated early in the phage cycle, or whether it remains active but cannot recognize late mRNAs as substrates.

Bacteriophage T4↗

PubMed1993

Dual role of the sequence-specific bacteriophage T4 endoribonuclease RegB. mRNA inactivation and mRNA destabilization.

Gene regB of bacteriophage T4 encodes a sequence-specific endoribonuclease that introduces cuts in early phage messenger RNAs. Cutting takes place specifically in the middle of the tetranucleotide GGAG, as soon as the first minute of infection. Out of the 20 processing sites so far identified, seven are in Shine-Dalgarno sequences. The others are localized in intercistronic regions or within coding sequences. In the latter case, cutting efficiency is much lower. regB-dependent cleavages can occur within AU-rich sequences downstream of processed GGAG motifs that are not in effective translation initiation sites. We looked for possible consequences of regB-dependent cuts on gene expression in two early regions of the T4 chromosome. In the comC alpha region, none of the three major RegB cleavage sites is in a Shine-Dalgarno sequence, and in the motA region the unique regB-dependent processing site is found within the Shine-Dalgarno sequence of the gene. We find that in the region of gene comC alpha, RegB decreases two- to threefold the chemical half-life of early transcripts, but does not change the functional half-life of mRNAs coding for protein ComC alpha. The amount of MotA protein synthesized by the wild-type is half that obtained in a regB mutant infection. We show that this is a direct consequence of mRNA processing by RegB at the Shine-Dalgarno sequence of motA. This regB-mediated translation inhibition is not accompanied by an important modification in motA mRNA chemical half-life. We show that rapid shut-off of MotA protein synthesis that occurs soon after infection results both from RegB processing within the translation initiation region of motA and from early transcription inhibition followed by regB-independent breakdown of the motA mRNA.

Bacterial Proteins↗

PubMed1992

Sequence and characterization of the bacteriophage T4 comC alpha gene product, a possible transcription antitermination factor.

We have sequenced a 1,340-bp region of the bacteriophage T4 DNA spanning the comC alpha gene, a gene which has been implicated in transcription antitermination. We show that comC alpha, identified unambiguously by sequencing several missense and nonsense mutations within the gene, codes for an acidic polypeptide of 141 residues, with a predicted molecular weight of 16,680. We have identified its product on one- and two-dimensional gel systems and found that it migrates abnormally as a protein with a molecular weight of 22,000. One of the missense mutations (comC alpha 803) is a glycine-to-arginine change, and the resulting protein exhibits a substantially faster electrophoretic mobility. The ComC alpha protein appears immediately after infection. Its rate of synthesis is maximum around 2 to 3 min postinfection (at 37 degrees C) and then starts to decrease slowly. Some residual biosynthesis is still detectable during the late period of phage development.

Amino Acid Sequence↗