The role of feedback in the formation of morphogen territories.

In this paper, we consider a mathematical model for the formation of spatial morphogen territories of two key morphogens: Wingless (Wg) and Decapentaplegic (DPP), involved in leg development of Drosophila. We define a gene regulatory network (GRN) that utilizes autoactivation and cros-sinhibition (modeled by Hill equations) to establish and maintain stable boundaries of gene expression. By computational analysis we find that in the presence of a general activator, neither autoactivation, nor cross-inhibition alone are sufficient to maintain stable sharp boundaries of morphogen production in the leg disc. The minimal requirements for a self-organizing system are a coupled system of two morphogens in which the autoactivation and cross-inhibition have Hill coefficients strictly greater than one. In addition, the GRN modeled here describes the regenerative responses to genetic manipulations of positional identity in the leg disc

The role of feedback in the formation of morphogen territories.

In this paper, we consider a mathematical model for the formation of spatial morphogen territories of two key morphogens: Wingless (Wg) and Decapentaplegic (DPP), involved in leg development of Drosophila. We define a gene regulatory network (GRN) that utilizes autoactivation and cros-sinhibition (modeled by Hill equations) to establish and maintain stable boundaries of gene expression. By computational analysis we find that in the presence of a general activator, neither autoactivation, nor cross-inhibition alone are sufficient to maintain stable sharp boundaries of morphogen production in the leg disc. The minimal requirements for a self-organizing system are a coupled system of two morphogens in which the autoactivation and cross-inhibition have Hill coefficients strictly greater than one. In addition, the GRN modeled here describes the regenerative responses to genetic manipulations of positional identity in the leg disc

Wingless directly represses DPP morphogen expression via an armadillo/TCF/Brinker complex.

BackgroundSpatially restricted morphogen expression drives many patterning and regeneration processes, but how is the pattern of morphogen expression established and maintained? Patterning of Drosophila leg imaginal discs requires expression of the DPP morphogen dorsally and the wingless (WG) morphogen ventrally. We have shown that these mutually exclusive patterns of expression are controlled by a self-organizing system of feedback loops that involve WG and DPP, but whether the feedback is direct or indirect is not known.Methods/findingsBy analyzing expression patterns of regulatory DNA driving reporter genes in different genetic backgrounds, we identify a key component of this system by showing that WG directly represses transcription of the dpp gene in the ventral leg disc. Repression of dpp requires a tri-partite complex of the WG mediators armadillo (ARM) and dTCF, and the co-repressor Brinker, (BRK), wherein ARM.dTCF and BRK bind to independent sites within the dpp locus.Conclusions/significanceMany examples of dTCF repression in the absence of WNT signaling have been described, but few examples of signal-driven repression requiring both ARM and dTCF binding have been reported. Thus, our findings represent a new mode of WG mediated repression and demonstrate that direct regulation between morphogen signaling pathways can contribute to a robust self-organizing system capable of dynamically maintaining territories of morphogen expression

A 2.8 kb fragment of the dpp enhancer is sufficient for activation and repression of <i>dpp</i> in the leg disc.

<p>A: Schematic representation of the <i>dpp</i> locus and the 6 enhancer fragments used in this study. The <i>dpp</i> transcription unit is centered around 86 kb (arrow). [Map coordinates (in kilobases) from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000142#pone.0000142-Blackman1" target="_blank">[19]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000142#pone.0000142-Bergstrom1" target="_blank">[52]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0000142#pone.0000142-St.Johnston1" target="_blank">[53]</a>. The leg disc enhancer is located between 20–30 kb downstream of the <i>dpp</i> coding region. Filled stars represent dTCF-binding sites confirmed by footprinting, open stars are predicted sites and pentagons are BRK binding sites. Arrowheads indicate fusion to the ß galactosidase reporter gene. APRD refers to the 4 relevant domains A (region required for Activation), P (proximal TCF sites), R (repressor domain), D (distal TCF sites). B–E: 3rd instar leg imaginal discs with dorsal up and anterior to the left. B: Normal <i>dpp</i> mRNA expression detected by <i>in situ</i> hybridization. Bracket indicates ventral region, where <i>dpp</i> is repressed. C: A 10 kb <i>dpp</i> enhancer fragment (BS3.0) drives expression of lacZ in a stripe that recapitulates normal <i>dpp</i> expression including ventral repression (bracket). D: Expression driven by the 2.8 kb APRD <i>dpp</i> enhancer fragment mimics <i>dpp</i> mRNA and BS3.0 expression. Again, note ventral repression (bracket). E: Ventral repression is lost (bracket) in the 2.3 kb APR- fragment which has a 500 bp region of APRD that contains the distal cluster of dTCF binding sites (D) deleted. F: An 800 bp fragment (AP--, BS3.1) containing the proximal cluster of dTCF sites (P) is not sufficient for ventral repression (bracket). G: The AP-D fragment does not show ventral repression (bracket). Sequences in the 1.4 kb between the proximal and distal dTCF sites do not contain dTCF sites but are required for ventral repression.</p

The <i>dpp</i> enhancer responds to WG signaling

<p>A–D: 3rd instar leg imaginal discs. Dorsal is up, anterior is to the left. Expression of the 2.8 kb APRD reporter fragment is monitored by β-galactosidase activity. A: In wild type leg discs (mesothoracic shown), APRD>LacZ expression is repressed in the ventral region (bracket). B: WG signaling is required for ventral repression. In a pair of everting prothoracic leg discs from a <i>wg^ts</i> larva, ventral repression of APRD>LacZ is lost after shifting to restrictive temperature (brackets). C: Expression of the APRD reporter is repressed ventrally in Hs>Gal4; UAS>DNdTCF animals reared at 18° (bracket). DNdTCF is a dominant negative form of dTCF that cannot bind ARM. These animals and their discs are small compared to their non DNdTCF bearing sibs even when maintained continuously at low temperature, presumably due to low level expression of Hs>Gal4. However, these control animals maintained at low temperature do survive as viable, mophologically intact adults. D: When heat shocked in late third instar, repression is lost within 2.5 hours (bracket). At least 6 animals of each genotype were examined and all legs exhibited the same responses.</p

Identification of familial and de novo microduplications of 22q11.21-q11.23 distal to the 22q11.21 microdeletion syndrome region

Deletions of the 22q11.2 region distal to the 22q11.21 microdeletion syndrome region have recently been described in individuals with mental retardation and congenital anomalies. Because these deletions are mediated by low-copy repeats (LCRs), located distal to the 22q11.21 DiGeorge/velocardiofacial microdeletion region, duplications are predicted to occur with a frequency equal to the deletion. However, few microduplications of this region have been reported. We report the identification of 18 individuals with microduplications of 22q11.21-q11.23. The duplication boundaries for all individuals are within LCRs distal to the DiGeorge/velocardiofacial microdeletion region. Clinical records for nine subjects reveal shared characteristics, but also several examples of contradicting clinical features (e.g. macrocephaly versus microcephaly and upslanting versus downslanting palpebral fissures). Of 12 cases for whom parental DNA samples were available for testing, one is de novo and 11 inherited the microduplication from a parent, three of whom reportedly have learning problems or developmental delay. The variable phenotypes and preponderance of familial cases obfuscate the clinical relevance of the molecular data and emphasize the need for careful parental assessments and clinical correlations