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Identification of Ind transcription activation and repression domains required for dorsoventral patterning of the CNS.

ABSTRACT: Specification of cell fates across the dorsoventral axis of the central nervous system in Drosophila involves the subdivision of the neuroectoderm into three domains that give rise to three columns of neural precursor cells called neuroblasts. Ventral nervous system defective (Vnd), intermediate neuroblasts defective (Ind) and muscle segment homeobox (Msh) are expressed in the three columns from ventral to dorsal, respectively. The products of these genes play multiple important roles in formation and specification of the embryonic nervous system. Ind, for example, is known to play roles in two important processes. First, Ind is essential for formation of neuroblasts conjunction with SoxB class transcription factors. Sox class transcription factors are known to specify neural stem cells in vertebrates. Second, Ind plays an important role in patterning the CNS in conjunction with, vnd and msh, which is also similar to how vertebrates pattern their neural tube. This work focuses two important aspects of Ind function. First, we used multiple approaches to identify and characterize specific domains within the protein that confer repressor or activator ability. Currently, little is known about the presence of activation or repression domains within Ind. Here, we show that transcriptional repression by Ind requires multiple conserved domains within the protein, and that Ind has a transcriptional activation domain. Specifically, we have identified a novel domain, the Pst domain, that has transcriptional repression ability and appears to act independent of interaction with the co-repressor Groucho. This domain is highly conserved among insect species, but is not found in vertebrate Gsh class homeodomain proteins. Second, we show that Ind can and does repress vnd expression, but does so in a stage specific manner. We conclude from this that the function of Ind in regulating vnd expression is one of refinement and maintenance of the dorsal border.

SUBMITTER: Von Ohlen TL

PROVIDER: S-EPMC2703006 | biostudies-literature | 2009 Jul

REPOSITORIES: biostudies-literature

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Identification of Ind transcription activation and repression domains required for dorsoventral patterning of the CNS.

Von Ohlen Tonia L TL Moses Cade C

Mechanisms of development 20090405 7

Specification of cell fates across the dorsoventral axis of the central nervous system in Drosophila involves the subdivision of the neuroectoderm into three domains that give rise to three columns of neural precursor cells called neuroblasts. Ventral nervous system defective (Vnd), intermediate neuroblasts defective (Ind) and muscle segment homeobox (Msh) are expressed in the three columns from ventral to dorsal, respectively. The products of these genes play multiple important roles in formati ...[more]

PMID: 19348939

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Project description:Many members of the animal kingdom display coat or skin color differences along their dorsoventral axis. To determine the mechanisms that control regional differences in pigmentation, we have studied how a classical mouse mutation, droopy ear (de(H)), affects dorsoventral skin characteristics, especially those under control of the Agouti gene. Mice carrying the Agouti allele black-and-tan (a(t)) normally have a sharp boundary between dorsal black hair and yellow ventral hair; the de(H) mutation raises the pigmentation boundary, producing an apparent dorsal-to-ventral transformation. We identify a 216 kb deletion in de(H) that removes all but the first exon of the Tbx15 gene, whose embryonic expression in developing mesenchyme correlates with pigmentary and skeletal malformations observed in de(H)/de(H) animals. Construction of a targeted allele of Tbx15 confirmed that the de(H) phenotype was caused by Tbx15 loss of function. Early embryonic expression of Tbx15 in dorsal mesenchyme is complementary to Agouti expression in ventral mesenchyme; in the absence of Tbx15, expression of Agouti in both embryos and postnatal animals is displaced dorsally. Transplantation experiments demonstrate that positional identity of the skin with regard to dorsoventral pigmentation differences is acquired by E12.5, which is shortly after early embryonic expression of Tbx15. Fate-mapping studies show that the dorsoventral pigmentation boundary is not in register with a previously identified dermal cell lineage boundary, but rather with the limb dorsoventral boundary. Embryonic expression of Tbx15 in dorsolateral mesenchyme provides an instructional cue required to establish the future positional identity of dorsal dermis. These findings represent a novel role for T-box gene action in embryonic development, identify a previously unappreciated aspect of dorsoventral patterning that is widely represented in furred mammals, and provide insight into the mechanisms that underlie region-specific differences in body morphology.

Project description:We have compared the dorsoventral development of hemichordates and chordates to deduce the organization of their common ancestor, and hence to identify the evolutionary modifications of the chordate body axis after the lineages split. In the hemichordate embryo, genes encoding bone morphogenetic proteins (Bmp) 2/4 and 5/8, as well as several genes for modulators of Bmp activity, are expressed in a thin stripe of ectoderm on one midline, historically called "dorsal." On the opposite midline, the genes encoding Chordin and Anti-dorsalizing morphogenetic protein (Admp) are expressed. Thus, we find a Bmp-Chordin developmental axis preceding and underlying the anatomical dorsoventral axis of hemichordates, adding to the evidence from Drosophila and chordates that this axis may be at least as ancient as the first bilateral animals. Numerous genes encoding transcription factors and signaling ligands are expressed in the three germ layers of hemichordate embryos in distinct dorsoventral domains, such as pox neuro, pituitary homeobox, distalless, and tbx2/3 on the Bmp side and netrin, mnx, mox, and single-minded on the Chordin-Admp side. When we expose the embryo to excess Bmp protein, or when we deplete endogenous Bmp by small interfering RNA injections, these expression domains expand or contract, reflecting their activation or repression by Bmp, and the embryos develop as dorsalized or ventralized limit forms. Dorsoventral patterning is independent of anterior/posterior patterning, as in Drosophila but not chordates. Unlike both chordates and Drosophila, neural gene expression in hemichordates is not repressed by high Bmp levels, consistent with their development of a diffuse rather than centralized nervous system. We suggest that the common ancestor of hemichordates and chordates did not use its Bmp-Chordin axis to segregate epidermal and neural ectoderm but to pattern many other dorsoventral aspects of the germ layers, including neural cell fates within a diffuse nervous system. Accordingly, centralization was added in the chordate line by neural-epidermal segregation, mediated by the pre-existing Bmp-Chordin axis. Finally, since hemichordates develop the mouth on the non-Bmp side, like arthropods but opposite to chordates, the mouth and Bmp-Chordin axis may have rearranged in the chordate line, one relative to the other.

Project description:Background:Bone morphogenetic proteins (BMPs) are of central importance for dorsal-ventral (DV) axis specification. They are core components of a signalling cascade that includes the BMP ligand decapentaplegic (DPP) and its antagonist short gastrulation (SOG) in Drosophila melanogaster. These components are very ancient, with orthologs involved in DV patterning in both protostomes and deuterostomes. Despite such strong conservation, recent comparative work in insects has revealed interesting differences in the way the patterning function of the DV system is achieved in different species. Results:In this paper, we characterise the expression patterns of the principal components of the BMP DV patterning system, as well as its signalling outputs and downstream targets, in the non-cyclorrhaphan moth midge Clogmia albipunctata (Diptera: Psychodidae). We previously reported ventral expression patterns of dpp in the pole regions of C. albipunctata blastoderm embryos. Strikingly, we also find ventral sog and posteriorly restricted tkv expression, as well as expanded polar activity of pMad. We use our results from gene knock-down by embryonic RNA interference to propose a mechanism of polar morphogen shuttling in C. albipunctata. We compare these results to available data from other species and discuss scenarios for the evolution of DV signalling in the holometabolan insects. Conclusions:A comparison of gene expression patterns across hemipteran and holometabolan insects reveals that expression of upstream signalling factors in the DV system is very variable, while signalling output is highly conserved. This has two major implications: first, as long as ligand shuttling and other upstream regulatory mechanisms lead to an appropriately localised activation of BMP signalling at the dorsal midline, it is of less importance exactly where the upstream components of the DV system are expressed. This, in turn, explains why the early-acting components of the DV patterning system in insects exhibit extensive amounts of developmental systems drift constrained by highly conserved downstream signalling output.

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Identification of Ind transcription activation and repression domains required for dorsoventral patterning of the CNS.

Publications

Identification of Ind transcription activation and repression domains required for dorsoventral patterning of the CNS.

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