The decapentaplegic core promoter region plays an integral role in the spatial control of transcription

Dpp signaling determines regional stem cell identity in the regenerating adult Drosophila gastrointestinal tract

The gastrointestinal tract is lined by a series of epithelia that share functional requirements but also have distinct, highly specialized roles. Distinct populations of somatic stem cells (SCs) regenerate these epithelia, yet the mechanisms that maintain regional identities of these SCs are not well understood. Here, we identify a role for the BMP-like Dpp signaling pathway in diversifying regenerative processes in the adult gastrointestinal tract of Drosophila. Dpp secreted from enterocytes at the boundary between the posterior midgut and the middle midgut (MM) sets up a morphogen gradient that selectively directs copper cell (CC) regeneration from gastric SCs in the MM and thus determines the size of the CC region. In vertebrates, deregulation of BMP signaling has been associated with Barrett's metaplasia, wherein the squamous esophageal epithelium is replaced by a columnar epithelium, suggesting that the maintenance of regional SC identities by BMP is conserved.

Analysis and recognition of the GAGA transcription factor binding sites in Drosophila genes

The transcription factor GAGA, encoded by the gene Trl, controls expression of many Drosophila melanogaster genes. We have compiled the presently largest sample (120 sites) of published nucleotide sequences with experimentally confirmed binding to GAGA protein. Analysis of the sample has demonstrated that despite an apparent structural diversity of the GAGA sites, they fall into four distinct groups, namely, (1) the sites containing two GAG trinucleotides with no more than one nucleotide substitution in each and separated by spacers with a length of 1 or 3 nucleotides (GAGnGAG and GAGnnnGAG); (2) the sites containing a single GAGAG motif; (3) (GA)(3-9) microsatellite repeats; and (4) the sites corresponding to three and more direct repeats of GAG trinucleotide homolog and its inverted repeats separated by spacers of various lengths. Using the software package SITECON, the methods were elaborated for recognizing the sites of GAGnGAG (method 1) and GAGnnnGAG (method 2) types in DNA sequences. Experimental verification confirmed the ability to interact with the GAGA factor for 72% of the sites predicted using method 1 and 94.5% of the sites predicted by method 2. Application of the experimentally verified methods to analyzing the localization of potential GAGA binding sites in the target genes of this transcription factor has demonstrated that the 5'-untranslated regions (5'UTRs) and first introns are enriched for these sites (two-threefold relative to the average occurrence frequency in the D. melanogaster genome) as compared with a moderate enrichment (not exceeding 1.5-fold) of promoter regions (-4000/+200 bp or -1000/+100 bp).

Binding site number variation and high-affinity binding consensus of Myb-SANT-like transcription factor Adf-1 in Drosophilidae

There is a growing interest in the evolution of transcription factor binding sites and corresponding functional change of transcriptional regulation. In this context, we have examined the structural changes of the ADF-1 binding sites at the Adh promoters of Drosophila funebris and D. virilis. We detected an expanded footprinted region in D. funebris that contains various adjacent binding sites with different binding affinities. ADF-1 was described to direct sequence-specific DNA binding to sites consisting of the multiple trinucleotide repeat . The ADF-1 recognition sites with high binding affinity differ from this trinucleotide repeat consensus sequence and a new consensus sequence is proposed for the high-affinity ADF-1 binding sites. In vitro transcription experiments with the D. funebris and D. virilis ADF-1 binding regions revealed that stronger ADF-1 binding to the expanded D. funebris ADF-1 binding region only moderately lead to increased transcriptional activity of the Adh gene. The potential of this regional expansion is discussed in the context of different ADF-1 cellular concentrations and maintenance of the ADF-1 stimulus. Altogether, evolutionary change of ADF-1 binding regions involves both, rearrangements of complex binding site cluster and also nucleotide substitutions within sites that lead to different binding affinities.

Developmental gene regulation in the era of genomics

Genetic experiments over the last few decades have identified many developmental control genes critical for pattern formation and cell fate specification during the development of multicellular organisms. A large fraction of these genes encode transcription factors and signaling molecules, show highly dynamic expression patterns during development, and are deeply evolutionarily conserved and deregulated in various human diseases such as cancer. Because of their importance in development, evolution, and disease, a fundamental question in biology is how these developmental control genes are regulated in such an extensive and precise fashion. Using genomics methods, it has become clear that developmental control genes are a distinct group of genes with special regulatory characteristics. However, a systematic analysis of these characteristics has not been presented. Here we review how developmental control genes were discovered, evaluate their genome-wide regulation and gene structure, discuss emerging evidence for their mode of regulation, and estimate their overall abundance in the genome. Understanding the global regulation of developmental control genes may provide a new perspective on development in the era genomics.

Mad is required for wingless signaling in wing development and segment patterning in Drosophila

A key question in developmental biology is how growth factor signals are integrated to generate pattern. In this study we investigated the integration of the Drosophila BMP and Wingless/GSK3 signaling pathways via phosphorylations of the transcription factor Mad. Wingless was found to regulate the phosphorylation of Mad by GSK3 in vivo. In epistatic experiments, the effects of Wingless on wing disc molecular markers (senseless, distalless and vestigial) were suppressed by depletion of Mad with RNAi. Wingless overexpression phenotypes, such as formation of ectopic wing margins, were induced by Mad GSK3 phosphorylation-resistant mutant protein. Unexpectedly, we found that Mad phosphorylation by GSK3 and MAPK occurred in segmental patterns. Mad depletion or overexpression produced Wingless-like embryonic segmentation phenotypes. In Xenopus embryos, segmental border formation was disrupted by Smad8 depletion. The results show that Mad is required for Wingless signaling and for the integration of gradients of positional information.

Analysis of the shortvein cis-regulatory region of thedecapentaplegic gene ofDrosophila melanogaster

In mammals, the Transforming Growth Factor-beta (TGF-beta) superfamily controls a variety of developmental processes. In Drosophila, by contrast, a single member of the superfamily, decapentaplegic (dpp) performs most TGF-beta developmental functions. The complexity of dpp functions is reflected in the complex cis-regulatory sequences that flank the gene. Dpp is divided into three regions: Hin, including the protein-coding exons; disk, including 3' cis-regulatory sequences; and shortvein (shv), including noncoding exons and 5' cis-regulatory sequences. We analyzed the cis-regulatory structure of the shortvein region using a nested series of rearrangement breakpoints and rescue constructs. We delimit the molecular regions responsible for three mutant phenotypes: larval lethality, wing venation defects, and head capsule defects. Multiple overlapping elements are responsible for larval lethality and wing venation defects. However, the area regulating head capsule formation is distinct, and resides 5' to these elements. We have demonstrated this by isolating and describing two novel dpp alleles, which affect only the adult head capsule.

Design and function of transcriptional switches in Drosophila

Extensive genetic and biochemical analysis of Drosophila melanogaster has made this system an important model for characterization of transcriptional regulatory elements and factors. Given the striking conservation of transcriptional controls in metazoans, general principles derived from studies of Drosophila are expected to continue to illuminate transcriptional regulation in other systems, including vertebrates. With improvement in technologies for genetic manipulation of insects, research in Drosophila will also aid the design of systems for controlled expression of genes in other hosts. This review focuses on recent advances from Drosophila in analysis of the functional components of transcriptional switches, including basal promoters, enhancers, boundary elements, and maintenance elements.

Analysis and function of transcriptional regulatory elements: insights from Drosophila

Analysis of gene expression is assuming an increasingly important role in elucidating the molecular basis of insect biology. Transcriptional regulation of gene expression is directed by a variety of cis-acting DNA elements that control spatial and temporal patterns of expression. This review summarizes current knowledge about properties of transcriptional regulatory elements, based largely on research in Drosophila melanogaster, and outlines ways that new technologies are providing tools to facilitate the study of transcriptional regulatory elements in other insects.

Drosophila OVO regulates ovarian tumor transcription by binding unusually near the transcription start site

Evolutionarily conserved ovo loci encode developmentally regulated, sequence-specific, DNA-binding, C(2)H(2)-zinc-finger proteins required in the germline and epidermal cells of flies and mice. The direct targets of OVO activity are not known. Genetic experiments suggest that ovo acts in the same regulatory network as ovarian tumor (otu), but the relative position of these genes in the pathway is controversial. Three OVO-binding sites exist in a compact regulatory region that controls germline expression of the otu gene. Interestingly, the strongest OVO-binding site is very near the otu transcription start, where basal transcriptional complexes must function. Loss-of-function, gain-of-function and promoter swapping constructs demonstrate that OVO binding near the transcription start site is required for OVO-dependent otu transcription in vivo. These data unambiguously identify otu as a direct OVO target gene and raise the tantalizing possibility that an OVO site, at the location normally occupied by basal components, functions as part of a specialized core promoter.

The homeodomain transcription factor Xvent-2 mediates autocatalytic regulation of BMP-4 expression in Xenopus embryos

Like other genes of the transforming growth factor-beta family, the BMP-4 gene is regulated by an autocatalytic loop. In Xenopus embryos this loop can be ectopically induced by injection of BMP-2 RNA. However, cycloheximide treatment subsequent to BMP-2 overexpression revealed that BMP signaling is not direct but requires additional factor(s). As putative mediator we have identified Xvent-2 which is activated by BMP-2/4 signaling and, in turn, activates BMP-4 transcription. Using promoter/reporter assays we have delineated Xvent-2 responsive elements within the BMP-4 gene. We further demonstrate that Xvent-2 which has recently been characterized as a transcriptional repressor can also act, context dependent, as an activator binding two copies of a 5'-CTAATT-3' motif in the second intron of the BMP-4 gene. Replacement of Xvent-2 target sites within the goosecoid (gsc) promoter by the BMP-4 enhancer converts Xvent-2 caused repression of gsc to strong activation. This switch is obviously due to adjacent nucleotides probably binding a transcriptional co-activator interacting with Xvent-2. A model is presented describing the mechanism of BMP-4 gene activation in Xenopus embryos at the early gastrula stage.

Mesenchymal expression of nuclear factor-kappaB inhibits epithelial growth and branching in the embryonic chick lung

It is becoming increasingly recognized that the ubiquitous, inducible transcription factor nuclear factor-kappaB (NF-kappaB) is involved in developmental processes. For example, NF-kappaB acts as a mediator of epithelial-mesenchymal interactions in the developing chick limb. We investigated the role of NF-kappaB in directing the branching morphogenesis of the developing chick lung, a process which relies on epithelial-mesenchymal communication. High level expression of relA was found in the mesenchyme surrounding the nonbranching structures of the lung but was not detected either in the mesenchyme surrounding the branching structures of the distal lung or in the developing lung epithelium. Specific inhibition of mesenchymal NF-kappaB in lung cultures resulted in increased epithelial budding. Conversely, expression of a trans-dominant activator of NF-kappaB in the lung mesenchyme repressed budding. Ectopic expression of RelA was sufficient to inhibit the ability of the distal mesenchyme to induce epithelial bud formation. Cellular proliferation in the mesenchyme was inhibited by hyperactivation of NF-kappaB in the mesenchyme of lung cultures. Interestingly, increased NF-kappaB activity in the mesenchyme also decreased the proliferation of the associated epithelium, while inhibition of NF-kappaB activity increased cellular proliferation in lung cultures. Expression patterns of several genes which are known to influence lung branching morphogenesis were altered in response to changes in mesenchymal NF-kappaB activity, including fgf10, bmp-4, and tgf-beta1. Thus NF-kappaB represents the first transcription factor reported to function within the lung mesenchyme to limit growth and branching of the adjacent epithelium.

The initiator element of the Drosophila beta2 tubulin gene core promoter contributes to gene expression in vivo but is not required for male germ-cell specific expression

The tissue-specific expression of the Drosophila beta 2 tubulin gene ( B2t ) is accomplished by the action of a 14-bp activator element (beta2UE1) in combination with certain regulatory elements of the TATA-less, Inr-containing B2t core promoter. We performed an in vivo analysis of the Inr element function in the B2t core promoter using a transgenic approach. Our experiments demonstrate that the Inr element acts as a functional cis -regulatory element in vivo and quantitatively regulates tissue-specific reporter expression in transgenic animals. However, our mutational analysis of the Inr element demonstrates no essential role of the Inr in mediating tissue specificity of the B2t promoter. In addition, a downstream element seems to affect promoter activity in combination with the Inr. In summary, our data show for the first time the functionality of the Inr element in an in vivo background situation in Drosophila.

Limb and skin abnormalities in mice lacking IKKalpha

The gene encoding inhibitor of kappa B (IkappaB) kinase alpha (IKKalpha; also called IKK1) was disrupted by gene targeting. IKKalpha-deficient mice died perinatally. In IKKalpha-deficient fetuses, limb outgrowth was severely impaired despite unaffected skeletal development. The epidermal cells in IKKalpha-deficient fetuses were highly proliferative with dysregulated epidermal differentiation. In the basal layer, degradation of IkappaB and nuclear localization of nuclear factor kappa B (NF-kappaB) were not observed. Thus, IKKalpha is essential for NF-kappaB activation in the limb and skin during embryogenesis. In contrast, there was no impairment of NF-kappaB activation induced by either interleukin-1 or tumor necrosis factor-alpha in IKKalpha-deficient embryonic fibroblasts and thymocytes, indicating that IKKalpha is not essential for cytokine-induced activation of NF-kappaB.

Transcriptional regulation of the Bmp2 gene. Retinoic acid induction in F9 embryonal carcinoma cells and Saccharomyces cerevisiae

Bmp2, a highly conserved member of the transforming growth factor-beta gene family, is crucial for normal development. Retinoic acid, combined with cAMP analogs, sharply induces the Bmp2 mRNA during the differentiation of F9 embryonal carcinoma cells into parietal endoderm. Retinoic acid (RA) also induces the Bmp2 gene in chick limb buds. Since normal Bmp2 expression may require an endogenous retinoid signal and aberrant Bmp2 expression may cause some aspects of RA-induced teratogenesis, we studied the mechanism underlying the induction of Bmp2. Measurements of the Bmp2 mRNA half-life and nuclear run-on assays indicated that RA stimulated the transcription rate of the Bmp2 gene. The results of ribonuclease protection and primer extension assays indicated that Bmp2 transcription started 2,127 nucleotides upstream of the translation start site in F9 cells. To identify genetic elements controlling this transcription rate increase, upstream and downstream genomic sequences flanking the Bmp2 gene were screened using chloramphenicol acetyltransferase reporter genes in F9 cells and beta-galactosidase reporter genes in Saccharomyces cerevisiae that were cotransformed with retinoic acid receptor and retinoid X receptor expression plasmids. RA-dependent transcriptional activation was detected between base pairs -2,373 and -2,316 relative to the translation start site. We also identified a required Sp1 binding site between -2,308 and -2,298. The data indicate that Bmp2 is directly regulated by retinoic acid-bound receptors and Sp1.

Experimental induction of BMP-4 expression leads to apoptosis in the paraxial and lateral plate mesoderm

In the avian embryo, epithelialization of the segmental plate and formation of an epithelial dermomyotome depend on signals from the neural tube and the ectoderm overlying the paraxial mesoderm. In this study, we report that ectoderm removal in combination with barrier insertion between the axial organs and the segmental plate leads to an induction of BMP-4 expression in the paraxial mesoderm. In the lateral plate, ectoderm removal alone leads to an increase of BMP-4 expression. Application of BMP-4 protein results in a lack of epithelialization of the paraxial mesoderm. In order to investigate whether the loss of epithelial structures after these manipulations can be attributed to a change in cell fate, a change in cell proliferation, or the induction of apoptosis, the paraxial mesoderm was tested for expression of Msx-2, BMP-2, BMP-4, and BMP-7. Moreover, BrdU and TUNEL staining were carried out. The inhibition of epithelialization after ectoderm removal alone and after segregation of the axial organs is accompanied neither by an increase in apoptosis nor by a reduction of the proliferation rate in the paraxial mesoderm. On the other hand, an ectopic BMP-4 expression in the paraxial mesoderm after ectoderm removal in combination with barrier insertion coincides with the occurrence of apoptotic cells and reduction of proliferation rate in this tissue. Increase of apoptosis and decrease in cell proliferation are observed in the paraxial and lateral plate mesoderm also after application of BMP-4 protein.

A core region of the mafK gene IN promoter directs neurone-specific transcription in vivo

BACKGROUND

MafK serves as a required subunit of erythroid transcription factor NF-E2 and also functions with various heterodimeric CNC family proteins. MafK expression begins in early mesoderm and is observed in mesenchymal and haematopoietic cells, as well as in neurones during mouse development. In mesodermal descendants, MafK mRNA begins with a distal first exon (called IM), whereas the mRNA in neurones begins with a proximal first exon (IN).

RESULTS

To elucidate the mechanisms that underlie the tissue-specific transcription of the mafK gene, and to gain insights into the functions of MafK during neural development, we analysed the activity of the mafK IN promoter. A detailed investigation of mafK expression in the embryonic spinal cord revealed that IN-initiated mRNA is expressed in the ventral side of the spinal cord. Transient transfection analysis of reporter plasmids bearing the IN promoter and upstream regions revealed that the 'core' region of this promoter (nt -67 to -9) is active and that its integrity is crucial for this activity. The core region was also capable of directing the tissue-specific transcription of a reporter gene in neural cells of the spinal cord in transgenic mice in vivo.

CONCLUSION

These results demonstrate that the specific expression of mafK in neural cells is determined, at least in part, by the core region of the IN promoter.

Structural and functional analysis of the BMP-4 promoter in early embryos of Xenopus laevis

The Xenopus laevis BMP-4 gene shows an evolutionary conserved structure containing two coding exons and a leader exon. The transcripts which are detected after zygotic activation of the gene in ventral mesoderm of late blastula stage embryos do either contain the leader exon or begin within the first intron. Luciferase reporter/promoter studies revealed multiple elements being required for the activation and for the spatial control of transcription. These elements are located within the upstream region and within the second intron and they interact with a most proximal located basal promoter being indispensable for transcriptional activation. The auto-activatory capacity of BMP-4 is mediated by several enhancer elements being responsive not only to BMP-4 but also to BMP-2 signaling. BMP-2 might thus function as a natural activator of the BMP-4 gene in the early embryo. Since reporter activity obtained with distinct BMP-2/4 responsive promoter deletion mutants is simultaneously inhibited by the dominant negative BMP receptor as well as by chordin, we suggest that down-regulation of the BMP-4 gene by chordin results from an interference with the auto-regulatory loop at the level of protein-protein interactions.

Inhibition of NF-kappaB activity results in disruption of the apical ectodermal ridge and aberrant limb morphogenesis

In Drosophila, the Dorsal protein establishes the embryonic dorso-ventral axis during development. Here we show that the vertebrate homologue of Dorsal, nuclear factor-kappa B (NF-kappaB), is vital for the formation of the proximo-distal organizer of the developing limb bud, the apical ectodermal ridge (AER). Transcription of the NF-kappaB proto-oncogene c-rel is regulated, in part, during morphogenesis of the limb bud by AER-derived signals such as fibroblast growth factors. Interruption of NF-kappaB activity using viral-mediated delivery of an inhibitor results in a highly dysmorphic AER, reduction in overall limb size, loss of distal elements and reversal in the direction of limb outgrowth. Furthermore, inhibition of NF-kappaB activity in limb mesenchyme leads to a reduction in expression of Sonic hedgehog and Twist but derepresses expression of the bone morphogenetic protein-4 gene. These results are the first evidence that vertebrate NF-kappaB proteins act to transmit growth factor signals between the ectoderm and the underlying mesenchyme during embryonic limb formation.

Regulation of Drosophila Adh promoter switching by an initiator-targeted repression mechanism

The stage-specific expression of the Drosophila alcohol dehydrogenase (Adh) gene is achieved through the alternate activation of two tandem promoters. The proximal promoter is active primarily during late embryonic development and early larval stages, while the distal promoter is active in late third instar larvae and adults. Here, we provide evidence that this Adh promoter switch is regulated by a zinc finger repressor protein (AEF-1) that is expressed predominantly in adult flies and targets the initiator region of the proximal promoter. We propose that AEF-1 plays a critical role in Adh promoter switching by blocking interactions between a component of the general transcription machinery and the initiator region of the proximal promoter.

The zebrafish BMP4 gene: sequence analysis and expression pattern during embryonic development

We have isolated zebrafish BMP4 gene from a zebrafish genomic DNA library. The size of the isolated BMP4 gene was approximately 14.9 kb. The isolated gene contained two exons which formed the complete coding region together with part of the 3'-noncoding region. The deduced BMP4 protein sequence contained 400 amino acids. Sequence comparison showed that it shared 73% amino acid sequence identity with that of human and mouse BMP4. An intron with a size of 8,963 bp was present between two coding exons. Danio retroposon A (DANA)-like retroposon was located in the intron. It contained four conserved boxes and was flanked by a pair of direct repeats of 9 nucleotide sequence (GTTTTAATA). During embryonic development of the zebrafish, a 3.8-kb BMP4 mRNA was detected from gastrula stage up to a month-old hatching larvae via Northern blot analysis. In addition, the use of reverse transcription polymerase chain reaction further demonstrated the presence of BMP4 mRNA in both the early developmental stages (i.e., cleavage and blastula) and in adult fish. Developmental expression of BMP4 protein was also analyzed. Trace amounts of an 18-kD protein were detected at pharyngula stage, while the production increased from hatching larvae to adult fish. In adult fish, the expression of BMP4 mRNA was observed in brain, heart, digestive tracts, testes, and jaw. The results suggest that the zebrafish BMP4 gene may play important roles during zebrafish development.

Twist-mediated activation of the NK-4 homeobox gene in the visceral mesoderm of Drosophila requires two distinct clusters of E-box regulatory elements

NK-4, also called msh2 and tinman, encodes a homeodomain transcription factor that is required for the development of the dorsal mesoderm and its derivatives in the Drosophila embryo. Genetic analyses indicate that NK-4 resides downstream of the mesodermal determinant twist, which encodes a basic helix-loop-helix-type transcription factor. However, the regulation of NK-4 by twist remains poorly understood. Using expression assays in cultured cells and transgenic flies, we show that two distinct clusters of E-box regulatory sequences, present upstream of the NK-4 gene, mediate NK-4 expression in the visceral mesoderm. These elements are conserved between the Drosophila melanogaster and Drosophila virilis NK-4 genes and serve as binding sites for Twist (E1 cluster) and NK-4 (E2 cluster) proteins. In cultured cells, Twist and NK-4 binding results in activation of NK-4 gene expression. In transgenic animals, the E1 and E2 clusters are functionally connected, and both elements are required for NK-4 activation in cells of the visceral mesoderm and also for NK-4 repression in cells of the somatic musculature. These results demonstrate that NK-4 is a direct transcriptional target for Twist and its own gene product in visceral mesodermal cells, supporting the idea that twist and NK-4 function in the subdivision of the mesoderm during Drosophila embryogenesis.

Drosophila Jun kinase regulates expression of decapentaplegic via the ETS-domain protein Aop and the AP-1 transcription factor DJun during dorsal closure

During Drosophila embryogenesis, ectodermal cells of the lateral epithelium stretch in a coordinated fashion to internalize the amnioserosa cells and close the embryo dorsally. This process, dorsal closure, requires two signaling pathways: the Drosophila Jun-amino-terminal kinase (DJNK) pathway and the Dpp pathway. We have identified mutations in DJun and show that DJNK controls dorsal closure by activating DJun and inactivating the ETS repressor Aop/Yan by phosphorylation. DJun and Aop regulate dpp expression in the most dorsal row of cells. Secreted Dpp then instructs more ventrally located cells to stretch. Our results provide a causal link between the DJNK and Dpp pathways during dorsal closure. Interestingly, in vertebrates, transforming growth factor-beta and c-Jun regulate collagenase gene expression during wound healing, a process that also involves the closing of an epithelial sheath.

Bone morphogenetic proteins in the nervous system

Bone morphogenetic proteins (BMPs) are a rapidly expanding subclass of the transforming growth factor superfamily. BMP ligands and receptor subunits are present throughout neural development within discrete regions of the embryonic brain and within neural crest-derived pre- and post-migratory zones. BMPs initially inhibit the formation of neuroectoderm during gastrulation while, within the neural tube, they act as gradient morphogens to promote the differentiation of dorsal cell types and intermediate cell types throughout co-operative signaling. In the peripheral nervous system, BMPs act as instructive signals for neuronal lineage commitment and promote graded stages of neuronal differentiation. By contrast, within the CNS, these same factors promote astroglial lineage elaboration from embryonic subventricular zone progenitor cells, with concurrent suppression of the neuronal or oligodendroglial lineages, or both. In addition, BMPs act on more lineage-restricted embryonic CNS progenitor cells to promote regional neuronal survival and cellular differentiation. Furthermore, these versatile cytokines induce selective apoptosis of discrete rhombencephalic neural crest-associated cellular populations. These observations suggest that the BMPs exhibit a broad range of cellular and context-specific effects during multiple stages of neural development.

A conserved signaling pathway: the Drosophila toll-dorsal pathway

The Toll-Dorsal pathway in Drosophila and the interleukin-1 receptor (IL-1R)-NF-kappa B pathway in mammals are homologous signal transduction pathways that mediate several different biological responses. In Drosophila, genetic analysis of dorsal-ventral patterning of the embryo has defined the series of genes that mediate the Toll-Dorsal pathway. Binding of extracellular ligand activates the transmembrane receptor Toll, which requires the novel protein Tube to activate the cytoplasmic serine/threonine kinase Pelle. Pelle activity controls the degradation of the Cactus protein, which is present in a cytoplasmic complex with the Dorsal protein. Once Cactus is degraded in response to signal, Dorsal is free to move into the nucleus where it regulates transcription of specific target genes. The Toll, tube, pelle, cactus, and dorsal genes also appear to be involved in Drosophila immune response. Because the IL-1R-NF-kappa B pathway plays a role in vertebrate innate immunity and because plant homologues of the Toll-Dorsal pathway are important in plant disease resistance, it is likely that this pathway arose before the divergence of plants and animals as a defense against pathogens.

Bone morphogenetic proteins: multifunctional regulators of vertebrate development

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