Terminal Patterning Research Articles

Developmental Evolution: Torso — a Story with Different Ends?

The Torso pathway patterns the ends of the Drosophila embryo. Now, it has been found to control axis elongation in the short germ insect Tribolium. This result raises the issue of the ancestral function of the Torso pathway and its evolution.

Tribolium mae expression suggests roles in terminal and midline patterning and in the specification of mesoderm

In Drosophila, the Mae protein ("modulator of the activity of Ets") regulates receptor tyrosine kinase (RTK)-dependent mitogen-activated protein kinase (MAPK) signaling. Mae has been shown to bind the Yan and Pointed-P2 transcription factors, thereby changing their ability to activate or repress target genes. In this work we show that the mae ortholog of the red flour beetle Tribolium castaneum (Tc'mae) is active at the posterior, but not the anterior pole of the blastoderm. Since MAPK signaling is known to be active at both poles, Tc'Mae could function to modulate terminal MAPK signaling to differentiate the developmental programs at the anterior and posterior poles of the Tribolium blastoderm embryo. Tc'mae is also expressed along the midline of the germband, similar as in Drosophila, where it is involved in the patterning of midline cells. Before gastrulation and in the growth zone, Tc'mae is active in mesoderm precursor cells. This suggests that in short germ embryos MAPK signaling may also be involved in the specification of mesoderm.

Capicua integrates input from two maternal systems in Drosophila terminal patterning

In Drosophila, the maternal terminal system specifies cell fates at the embryonic poles via the localised stimulation of the Torso receptor tyrosine kinase (RTK). Signalling by the Torso pathway relieves repression mediated by the Capicua and Groucho repressors, allowing the restricted expression of the zygotic terminal gap genes tailless and huckebein. Here we report a novel positive input into tailless and huckebein transcription by maternal posterior group genes, previously implicated in abdomen and pole cell formation. We show that absence of a subset of posterior group genes, or their overactivation, leads to the spatial reduction or expansion of the tailless and huckebein posterior expression domains, respectively. We demonstrate that the terminal and posterior systems converge, and that exclusion of Capicua from the termini of posterior group mutants is ineffective, accounting for reduced terminal gap gene expression in these embryos. We propose that the terminal and posterior systems function coordinately to alleviate transcriptional silencing by Capicua, and that the posterior system fine-tunes Torso RTK signalling output, ensuring precise spatial domains of tailless and huckebein expression.

An F1 genetic screen for maternal-effect mutations affecting embryonic pattern formation in Drosophila melanogaster.

Large-scale screens for female-sterile mutations have revealed genes required maternally for establishment of the body axes in the Drosophila embryo. Although it is likely that the majority of components involved in axis formation have been identified by this approach, certain genes have escaped detection. This may be due to (1) incomplete saturation of the screens for female-sterile mutations and (2) genes with essential functions in zygotic development that mutate to lethality, precluding their identification as female-sterile mutations. To overcome these limitations, we performed a genetic mosaic screen aimed at identifying new maternal genes required for early embryonic patterning, including zygotically required ones. Using the Flp-FRT technique and a visible germline clone marker, we developed a system that allows efficient screening for maternal-effect phenotypes after only one generation of breeding, rather than after the three generations required for classic female-sterile screens. We identified 232 mutants showing various defects in embryonic pattern or morphogenesis. The mutants were ordered into 10 different phenotypic classes. A total of 174 mutants were assigned to 86 complementation groups with two alleles on average. Mutations in 45 complementation groups represent most previously known maternal genes, while 41 complementation groups represent new loci, including several involved in dorsoventral, anterior-posterior, and terminal patterning.

Role of 14-3-3 proteins in early Xenopus development

14-3-3 proteins are intracellular dimeric phosphoserine/threonine-binding molecules that participate in signal transduction and checkpoint control pathways. 14-3-3 proteins are required for normal eye development, brain function, and terminal patterning in Drosophila melanogaster, but the role of 14-3-3 proteins in vertebrate development is undefined. In this work an unphosphorylated peptide inhibitor of 14-3-3, R18, was used to determine the role of 14-3-3 proteins in Xenopus embryonic development. Biochemical analysis demonstrated that R18 was specific and efficient at attenuating global 14-3-3 activities in Xenopus embryos. Microinjection experiments showed a requirement for 14-3-3 function in mesodermal specification. Inhibition of 14-3-3 resulted in embryos with axial patterning defects and reduced expression of mesodermal marker genes. These phenotypic defects were caused by impaired fibroblast growth factor signaling in R18-injected embryos. These results establish the importance of 14-3-3 proteins in vertebrate embryonic development.

Conserved and divergent aspects of terminal patterning in the beetle Tribolium castaneum.

To infer similarities and differences in terminal pattern formation in insects, we analyzed several of the key genes of this process in the beetle Tribolium castaneum. We cloned two genes of the terminal pattern cascade, namely tailless (tll) and forkhead (fkh), from Tribolium and studied their expression patterns. In addition, we analyzed the pattern of MAP kinase activation at blastoderm stage as a possible signature for torso-dependent signaling. Further, we analyzed the late expression of the previously cloned Tribolium caudal (Tc-cad) gene. Finally, we used the upstream region of Tc-tll to drive a reporter gene construct in Drosophila. We find that this construct is activated at the terminal regions in Drosophila, suggesting that the torso-dependent pathway is conserved between the species. We show that most of the expression patterns of the genes studied here are similar in Drosophila and Tribolium, suggesting conserved functions. There is, however, one exception, namely the early function of Tc-tll at the posterior pole. In Drosophila, the posterior tll expression is involved in the direct regulation of the target genes of the terminal pathway. In Tribolium, posterior Tc-tll expression occurs only for a short time and ceases before the target genes known from Drosophila are activated. Thus, we infer that Tc-tll does not function as a direct regulator of segmentation genes at the posterior end. It is more likely to be involved in the early specification of a group of "terminal" cells, which begin to differentiate only at a later stage of embryogenesis, when much of the abdominal segmentation process is complete. Thus, there appears to have been a major shift in tll function during the evolutionary transition from short germ to long germ embryogenesis.

Relief of gene repression by Torso RTK signaling: role of capicua in Drosophila terminal and dorsoventral patterning

Differentiation of the embryonic termini in Drosophila depends on signaling by the Tor RTK, which induces terminal gene expression by inactivating at the embryonic poles a uniformly distributed repressor activity that involves the Gro corepressor. Here, we identify a new gene, cic, that acts as a repressor of terminal genes regulated by the Tor pathway. cic also mediates repression along the dorsoventral axis, a process that requires the Dorsal morphogen and Gro, and which is also inhibited by Tor signaling at the termini. cic encodes an HMG-box transcription factor that interacts with Gro in vitro. We present evidence that Tor signaling regulates terminal patterning by inactivating Cic at the embryo poles. cic has been evolutionarily conserved, suggesting that Cic-like proteins may act as repressors regulated by RTK signaling in other organisms.

Huckebein repressor activity in Drosophila terminal patterning is mediated by Groucho.

The Groucho corepressor mediates negative transcriptional regulation in association with various DNA-binding proteins in diverse developmental contexts. We have previously implicated Groucho in Drosophila embryonic terminal patterning, showing that it is required to confine tailless and huckebein terminal gap gene expression to the pole regions of the embryo. Here we reveal an additional requirement for Groucho in this developmental process by establishing that Groucho mediates repressor activity of the Huckebein protein. Putative Huckebein target genes are derepressed in embryos lacking maternal groucho activity and biochemical experiments demonstrate that Huckebein physically interacts with Groucho. Using an in vivo repression assay, we identify a functional repressor domain in Huckebein that contains an FRPW tetrapeptide, similar to the WRPW Groucho-recruitment domain found in Hairy-related repressor proteins. Mutations in Huckebein's FRPW motif abolish Groucho binding and in vivo repression activity, indicating that binding of Groucho through the FRPW motif is required for the repressor function of Huckebein. Taken together with our earlier results, these findings show that Groucho-repression regulates sequential aspects of terminal patterning in Drosophila.

Torso signalling regulates terminal patterning in Drosophila by antagonising Groucho-mediated repression.

Patterning of the non-segmental termini of the Drosophila embryo depends on signalling via the Torso receptor tyrosine kinase (RTK). Activation of Torso at the poles of the embryo triggers restricted expression of the zygotic gap genes tailless (tll) and huckebein (hkb). In this paper, we show that the Groucho (Gro) corepressor acts in this process to confine terminal gap gene expression to the embryonic termini. Embryos lacking maternal gro activity display ectopic tll and hkb transcription; the former leads, in turn, to lack of abdominal expression of the Krüppel and knirps gap genes. We show that torso signalling permits terminal gap gene expression by antagonising Gro-mediated repression. Thus, the corepressor Gro is employed in diverse developmental contexts and, probably, by a variety of DNA-binding repressors.

Three maternal coordinate systems cooperate in the patterning of the Drosophila head

In contrast to the segmentation of the embryonic trunk region which has been extensively studied, relatively little is known about the development and segmentation of the Drosophila head. Proper development of the cephalic region requires the informational input of three of the four maternal coordinate systems. Head-specific gene expression is set up in response to a complex interaction between the maternally provided gene products and zygotically expressed genes. Several zygotic genes involved in head development have recently been characterized. A genetic analysis suggests that the segmentation of the head may use a mechanism different from the one acting in the trunk. The two genes of the sloppy paired locus (slp1 and slp2) are also expressed in the embryonic head. slp1 plays a predominant role in head formation while slp2 is largely dispensible. A detailed analysis of the slp head phenotype suggests that slp is important for the development of the mandibular segment as well as two adjacent pregnathal segments (antennal and ocular). Our analysis of regulatory interactions of slp with maternal and zygotic genes suggests that it behaves like a gap gene. Thus, phenotype and regulation of slp support the view that slp acts as a head-specific gap gene in addition to its function as a pair-rule and segment polarity gene in the trunk. We show that all three maternal systems active in the cephalic region are required for proper slp expression and that the different systems cooperate in the patterning of the head. The terminal and anterior patterning system appear to be closely linked. This cooperation is likely to involve a direct interaction between the bcd morphogen and the terminal system. Low levels of terminal system activity seem to potentiate bcd as an activator of slp, whereas high levels down-regulate bcd rendering it inactive. Our analysis suggests that dorsal, the morphogen of the dorsoventral system, and the head-specific gap gene empty spiracles act as repressor and corepressor in the regulation of slp. We discuss how positional information established independently along two axes can act in concert to control gene regulation in two dimensions.

BDNF in the development of the visual system of Xenopus

To explore the role of BDNF during Xenopus visual system development, the expression of BDNF and TrkB, as well as the effect of BDNF during retinal ganglion cell (RGC) development in culture, was examined. BDNF mRNA was found to be expressed in both the developing eye and tectum, peaking at stage 45 46 . The expression of BDNF coincided with RGC expression of full-length trkB transcripts, suggesting a functional role for BDNF. In culture, BDNF significantly increased the number of RGCs. The ability of BDNF to rescue differentiated RGCs that had projected to the tectum, the time course of the effect, and the lack of mitogenic response indicate that this neurotrophin promotes survival. The expression of BDNF and TrkB and the responsiveness of RGCs to BDNF coincide with retinal axon terminal arborization and patterning. Our results indicate that BDNF is a relevant neurotrophin for Xenopus RGC development and suggest that it plays a role during visual system patterning.

The relationship between retinal axon ingrowth, terminal morphology, and terminal patterning in the optic tectum of the frog.

AbstractThe mode of entry of retinal ganglion cell (RGC) axons and the detailed morphology of their arbors has been studied in the tectal lobes of the frog Ranapipiens. These tecta were subjected intact to HRP histochemistry and subsequently flal‐mounted for examination under a compound microscope. Subsets of RGC axons were labelled by inserting small pellets of solidified HRP into the optic chiasm region or into the tectal neuropil itself. Annular or semiannular patterns of retinal ganglion cell arbors were always observed after placement of HRP in the chiasm region. HRP pellets in the tectal neuropil labelled segments of annuli in caudal tectal regions. These tectal patterns corresponded to a similar annular distribution of HRP‐filled RGC bodies in the topographically appropriate regions of the contralateral retina.The morphologies and trajectories of RGC axons were found to vary systematically depending on the age of the animal and the region of the tectal lobe examined. In rostral tectum the tangential spread of large‐caliber RGC axon arbors appears to decrease with increasing animal age and in large adult frogs there is a general tendency for arbors in caudal tectum to be larger than arbors in rostral tectum. In addition, axons contributing to terminal annuli in the central region of young and old frog tecta travel in a strict rostrocaudal direction over the anterior regions of the lobe. A predominantly rostrocaudal trajectory is also established by axons terminating in lateral and medial regions of the tecta of young postmetamorphic frogs. In the lateral and medial regions of mature optic tecta, however, axons arborizing in peripheral terminal annuli turn rather abruptly off the medial and lateral optic tracts and establish radially oriented trajectories. The annular patterning of terminal arbors and the age‐ and position‐dependent differences in arbor morphology are discussed in the context of a developmental sequence where arbors from newly arrived RGC axons distribute themselves along the tectal borders.The question of whether such a high degree of fiber and terminal ordering actually dictates synaptic patterning in the tectal lobe has also been addressed in this study by using the stereotyped striped pattern found in the dually innervated tectal lobes of three‐eyed frogs (Constantine‐Paton and Law, 78;Law and Constantine‐Paton, '81). The presentresults indicate that eye‐specific segregation in the terminal zone is independent of bundling of axons in the optic tract. Stripes are also shown to be independent of the path taken by axons en route to the tectum and of the way in which axons enter the tectal plate. These observations and our earlier conclusions (Reh et al., '83) concerning order in the Rana optic nerve are discussed in terms of recent models of retino tectal development which postulate a role for passive “extra‐tectal” factors in the formation of this visual map.