The maternal NF-kappaB/dorsal gradient of Tribolium castaneum: dynamics of early dorsoventral patterning in a short-germ beetle

In the long-germ insect Drosophila melanogaster dorsoventral polarity is induced by localized Toll-receptor activation which leads to the formation of a nuclear gradient of the rel/ NF-kappaB protein Dorsal. Peak levels of nuclear Dorsal are found in a ventral stripe spanning the entire length of the blastoderm embryo allowing all segments and their dorsoventral subdivisions to be synchronously specified before gastrulation. We show that a nuclear Dorsal protein gradient of similar anteroposterior extension exists in the short-germ beetle, Tribolium castaneum, which forms most segments from a posterior growth zone after gastrulation. In contrast to Drosophila, (i) nuclear accumulation is first uniform and then becomes progressively restricted to a narrow ventral stripe, (ii) gradient refinement is accompanied by changes in the zygotic expression of the Tribolium Toll-receptor suggesting feedback regulation and, (iii) the gradient only transiently overlaps with the expression of a potential target, the Tribolium twist homolog, and does not repress Tribolium decapentaplegic. No nuclear Dorsal is seen in the cells of the growth zone of Tribolium embryos, indicating that here dorsoventral patterning occurs by a different mechanism. However, Dorsal is up-regulated and transiently forms a nuclear gradient in the serosa, a protective extraembryonic cell layer ultimately covering the whole embryo.     

sog and dpp exert opposing maternal functions to modify toll signaling and pattern the dorsoventral axis of the Drosophila embryo

The short gastrulation (sog) and decapentaplegic (dpp) genes function antagonistically in the early Drosophila zygote to pattern the dorsoventral (DV) axis of the embryo. This interplay between sog and dpp determines the extent of the neuroectoderm and subdivides the dorsal ectoderm into two territories. Here, we present evidence that sog and dpp also play opposing roles during oogenesis in patterning the DV axis of the embryo. We show that maternally produced Dpp increases levels of the I(kappa)B-related protein Cactus and reduces the magnitude of the nuclear concentration gradient of the NF(kappa)B-related Dorsal protein, and that Sog limits this effect. We present evidence suggesting that Dpp signaling increases Cactus levels by reducing a signal-independent component of Cactus degradation. Epistasis experiments reveal that sog and dpp act downstream of, or in parallel to, the Toll receptor to reduce translocation of Dorsal protein into the nucleus. These results broaden the role previously defined for sog and dpp in establishing the embryonic DV axis and reveal a novel form of crossregulation between the NF(kappa)B and TGF(beta) signaling pathways in pattern formation.     

HMG boxes of DSP1 protein interact with the rel homology domain of transcription factors

Formation of the dorsoventral axis in Drosophila melanogaster is mediated through control of the expression of several genes by the morphogen Dorsal. In the ventral part of the embryo Dorsal activates twist and represses zen amongst others. Recently, several proteins have been shown to assist Dorsal in the repression of zen, one of which is DSP1, a HMG box protein that was isolated as a putative co-repressor of Dorsal. In this report we used a DSP1 null mutant to ascertain in vivo the involvement of DSP1 in Dorsal-mediated repression of zen but not in the activation of twist. We show that Dorsal has the ability to interact with DSP1 in vitro as well as with rat HMG1. Using truncated versions of the proteins we located the domains of interaction as being the HMG boxes for DSP1 and HMG1 and the Rel domain for Dorsal. Finally, studies of the zen DNA binding properties of Dorsal and another related Rel protein (Gambif1 from Anopheles gambiae) revealed that their DNA binding affinities were increased in the presence of DSP1 and HMG1.     

The polarity of the dorsoventral axis in the Drosophila embryo is defined by an extracellular signal

Twelve maternal effect loci are required for the production of Drosophila embryos with a correct dorsoventral axis. Analysis of mosaic females indicates that the expression of the genes nudel, pipe, and windbeutel is required in the somatic tissue, presumably in the follicle cells that surround the oocyte. Thus, information coming from outside the egg cell influences dorsoventral pattern formation during embryogenesis. In transplantation experiments, the perivitelline fluid from the compartment surrounding the embryo can restore dorsoventral pattern to embryos from females mutant for nudel, pipe, or windbeutel. The positioning of the transplanted pervitelline fluid also determines the polarity of the restored dorsoventral axis. We propose that the polarizing activity, normally present at the ventral side of the egg, is a ligand for the Toll receptor. Presumably, local activation of the Toll protein by the ligand initiates the formation of the nuclear concentration gradient of the dorsal protein, thereby determining dorsoventral pattern.     

A gradient of nuclear localization of the dorsal protein determines dorsoventral pattern in the Drosophila embryo

The dorsoventral axis of the Drosophila embryo is determined by a morphogen gradient established by the action of 12 maternal-effect genes: the dorsal group genes and cactus. One of the dorsal group genes, dorsal (dl), encodes the putative morphogen. Although no overall asymmetry in the distribution of dorsal protein is observed, a gradient of nuclear concentration of dl protein is established during cleavage stages, with a maximum at the ventral side of the egg. At the dorsal side of the egg, the protein remains in the cytoplasm. Nuclear localization of the dl protein, and hence gradient formation, is blocked in dorsalizing alleles of all of the other dorsal group genes, while in ventralizing mutants nuclear localization extends to the dorsal side of the egg. A correlation between dl protein distribution and embryonic pattern in mutant embryos indicates that the nuclear concentration of the dl protein determines pattern along the dorsoventral axis.     

Spätzle regulates the shape of the Dorsal gradient in the Drosophila embryo

Dorsal-ventral polarity of the Drosophila embryo is established by a nuclear gradient of Dorsal protein, generated by successive gurken-Egfr and sp?tzle-Toll signaling. Overexpression of extracellular Sp?tzle dramatically reshapes the Dorsal gradient: the normal single peak is broadened and then refined to two distinct peaks of nuclear Dorsal, to produce two ventral furrows. This partial axis duplication, which mimics the ventralized phenotype caused by reduced gurken-Egfr signaling, arises from events in the perivitelline fluid of the embryo and occurs at the level of Sp?tzle processing or Toll activation. The production of two Dorsal peaks is addressed by a model that invokes action of a diffusible inhibitor, which is proposed to normally regulate the slope of the Dorsal gradient.