Conservation of dorsal-ventral patterning in arthropods and chordates

TGF beta-related pathways. Roles in Caenorhabditis elegans development

Genetic and molecular analysis in Caenorhabditis elegans has produced new insights into how TGF beta-related pathways transduce signals and the developmental processes in which they function. These pathways are essential regulators of dauer formation, body-size determination, male copulatory structures and axonal guidance. Here, we review the insights that have come from standard molecular genetic experiments and discuss how the recently completed genome sequence has contributed to our understanding of these pathways.

The role of brinker in mediating the graded response to Dpp in early Drosophila embryos

Brinker (Brk), a novel protein with features of a transcriptional repressor, regulates the graded response to Decapentaplegic (Dpp) in appendage primordia of Drosophila. Here, we show that in the embryo brk also has differential effects on Dpp target genes, depending on the level of Dpp activity required for their activation. Low-level target genes, like dpp itself, tolloid and early zerknullt, show strong ectopic expression in ventrolateral regions of brk mutant embryos; intermediate-level target genes like pannier show weak ectopic expression, while high-level target genes like u-shaped and rhomboid are not affected. Ectopic target gene activation in the absence of brk is independent of Dpp, Tkv and Medea, indicating that Dpp signaling normally antagonizes brk's repression of these target genes. brk is expressed like short gastrulation (sog) in ventrolateral regions of the embryo abutting the dpp domain. Here, both brk and sog antagonize the antineurogenic activity of Dpp so that only in brk sog double mutants is the neuroectoderm completely deleted.

TGF-beta family signal transduction in Drosophila development: from Mad to Smads

The transforming growth factor-beta (TGF-beta) superfamily encompasses a large group of soluble extracellular proteins that are potent regulators of development in both vertebrates and invertebrates. Drosophila TGF-beta family members include three proteins with homology to vertebrate bone morphogenetic proteins (BMPs): Decapentaplegic (Dpp), Screw, and Glass bottom boat-60A. Genetic studies of Dpp signaling led to the identification of Smad proteins as central mediators of signal transduction by TGF-beta family members. Work in mammalian tissue culture has elucidated a biochemical model for signal transduction, in which activation of receptor serine-threonine kinase activity leads to phosphorylation of specific Smad proteins and translocation of heteromeric Smad protein complexes to the nucleus. Once in the nucleus Smad proteins interact with other DNA binding proteins to regulate transcription of specific target genes. Dissection of Dpp-response elements from genes expressed during embryonic mesoderm patterning and midgut morphogenesis provides important insights into the contributions of Smad proteins and tissue-specific transcription factors to spatial regulation of gene expression. Genetic studies in Drosophila are now expanding to include multiple BMP ligands and receptors and have uncovered activities not explained by the current signal transduction model. Identification of more ligand sequences and demonstration of a functional Drosophila activin-like signal transduction pathway suggest that all TGF-beta signal transduction pathways are present in flies.

A gradient of BMP activity specifies dorsal-ventral fates in early Xenopus embryos

BMP-4 is an extracellular signalling molecule belonging to the TGF-beta superfamily that plays a central role in dorsoventral patterning in vertebrate gastrulae. We review the evidence indicating that BMP-4 acts as a morphogen, specifying dorsoventral positional values in a concentration-dependent manner. An activity gradient of BMP-4 is established not by simple diffusion from a localised source, but by diffusion of inhibitory binding proteins that act on a uniform level of BMP-4 protein. These in turn are regulated by the activity of tolloid-related metalloproteases, such as Xenopus xolloid and zebrafish tolloid.

Sea urchin TgBMP2/4 gene encoding a bone morphogenetic protein closely related to vertebrate BMP2 and BMP4 with maximal expression at the later stages of embryonic development

We have cloned a gene fragment (named TgBMP2/4) that encodes a protein homologous to vertebrate bone morphogenetic protein (BMP) 2 and BMP4 in the sea urchin Tripneustes gratilla. This peptide sequence contains 204 amino acids with 7 conserved cysteine residues at the C-terminus of the coding region and a cluster of basic amino acids that may serve as a signal for proteolytic cleavage. Sequence comparison and phylogenetic analyses reveal that TgBMP2/4 is closely related to vertebrate BMP2 and BMP4 as well as to amphioxus BMP2/4, with similarity levels ranging from 90% to 94% at the mature C-terminal domain. Northern blot analyses show that a 6.3-kb TgBMP2/4 mRNA appears first at the mesenchyme blastula stage and increases to a maximal level at the gastrula and pluteus stages. This expression pattern is different from that of a BMP2/4-related gene previously found in sea urchin.

brinker is a target of Dpp in Drosophila that negatively regulates Dpp-dependent genes

Growth and patterning of the Drosophila wing is controlled in part by the long-range organizing activities of the Decapentaplegic protein (Dpp). Dpp is synthesized by cells that line the anterior side of the anterior/posterior compartment border of the wing imaginal disc. From this source, Dpp is thought to generate a concentration gradient that patterns both anterior and posterior compartments. Among the gene targets that it regulates are optomotor blind (omb), spalt (sal), and daughters against dpp (dad). We report here the molecular cloning of brinker (brk), and show that brk expression is repressed by dpp. brk encodes, a protein that negatively regulates Dpp-dependent genes. Expression of brk in Xenopus embryos indicates that brk can also repress the targets of a vertebrate homologue of Dpp, bone morphogenetic protein 4 (BMP-4). The evolutionary conservation of Brk function underscores the importance of its negative role in proportioning Dpp activity.

Conservation of the expression and function of apterous orthologs in Drosophila and mammals

The Drosophila apterous (ap) gene encodes a protein of the LIM-homeodomain family. Many transcription factors of this class have been conserved during evolution; however, the functional significance of their structural conservation is generally not known. ap is best known for its fundamental role as a dorsal selector gene required for patterning and growth of the wing, but it also has other important functions required for neuronal fasciculation, fertility, and normal viability. We isolated mouse (mLhx2) and human (hLhx2) ap orthologs, and we used transgenic animals and rescue assays to investigate the conservation of the Ap protein during evolution. We found that the human protein LHX2 is able to regulate correctly ap target genes in the fly, causes the same phenotypes as Ap when ectopically produced, and most importantly rescues ap mutant phenotypes as efficiently as the fly protein. In addition, we found striking similarities in the expression patterns of the Drosophila and murine genes. Both mLhx2 and ap are expressed in the respective nerve cords, eyes, olfactory organs, brain, and limbs. These results demonstrate the conservation of Ap protein function across phyla and argue that aspects of its expression pattern have also been conserved from a common ancestor of insects and vertebrates.

Transducing the Dpp morphogen gradient in the wing of Drosophila: regulation of Dpp targets by brinker

Dpp, a TGFbeta, organizes pattern in the Drosophila wing by acting as a graded morphogen, activating different targets above distinct threshold concentrations. Like other TGFbetas, Dpp appears to induce transcription directly via activation of a SMAD, Mad. However, here we demonstrate that Dpp can also control gene expression indirectly by downregulating the expression of the brinker gene, which encodes a putative transcription factor that functions to repress Dpp targets. The medial-to-lateral Dpp gradient along the anterior-posterior axis is complemented by a lateral-to-medial gradient of Brinker, and the presence of these two opposing gradients may function to allow cells to detect small differences in Dpp concentration and respond by activating different target genes.

The Drosophila gene brinker reveals a novel mechanism of Dpp target gene regulation

decapentaplegic (dpp), a Drosophila member of the TGFbeta family of secreted molecules, functions as a long-range morphogen in patterning of the embryo and the adult appendages. Dpp signals via the SMAD proteins Mad and Medea. Here we show that in the absence of brinker (brk), Mad is not required for the activation of Dpp target genes that depend on low levels of Dpp. brk encodes a novel protein with features of a transcriptional repressor. brk itself is negatively regulated by Dpp. Dpp signaling might relieve brk's repression of low-level target genes either by transcriptional repression of brk or by antagonizing a repressor function of brk at the target gene promoters.

Regulation of BMP signaling by the BMP1/TLD-related metalloprotease, SpAN

We have used the Xenopus embryo as a test system for analyzing the activity of SpAN, a sea urchin metalloprotease in the astacin family containing BMP1 and tolloid. Embryos expressing SpAN initiated gastrulation on a time scale indistinguishable from controls, but invagination of the vegetal pole was subsequently delayed by several hours. At tailbud stages the most severely affected embryos were completely ventralized, lacking all dorsal structures. Molecular analysis of injected embryos, using probes for both dorsal (xgsc and xnot) and ventral (xhox3 and xwnt8) mesoderm, indicates that SpAN ventralizes dorsal mesoderm during gastrula stages. These results mirror those previously obtained with BMP4, suggesting that SpAN may enhance the activity of this ventralizing factor. Consistent with this suggestion, we have shown that SpAN blocks the dorsalizing activity of noggin and chordin, two inhibitory binding proteins for BMP4, but not that of a dominant-negative receptor for BMP4. In contrast, a dominant-negative SpAN, in which the metalloprotease domain has been deleted, dorsalizes ventral mesoderm, a phenotype that can be rescued by coexpressing either SpAN or XBMP1. This suggests that SpAN is mimicking a Xenopus metalloprotease responsible for regulating the activity of Xenopus BMPs during gastrulation. Moreover, our results raise the possibility that SpAN may function to facilitate BMP signaling in early sea urchin embryos.

The Drosophila activin receptor baboon signals through dSmad2 and controls cell proliferation but not patterning during larval development

The TGF-beta superfamily of growth and differentiation factors, including TGF-beta, Activins and bone morphogenetic proteins (BMPs) play critical roles in regulating the development of many organisms. These factors signal through a heteromeric complex of type I and II serine/threonine kinase receptors that phosphorylate members of the Smad family of transcription factors, thereby promoting their nuclear localization. Although components of TGF-beta/Activin signaling pathways are well defined in vertebrates, no such pathway has been clearly defined in invertebrates. In this study we describe the role of Baboon (Babo), a type I Activin receptor previously called Atr-I, in Drosophila development and characterize aspects of the Babo intracellular signal-transduction pathway. Genetic analysis of babo loss-of-function mutants and ectopic activation studies indicate that Babo signaling plays a role in regulating cell proliferation. In mammalian cells, activated Babo specifically stimulates Smad2-dependent pathways to induce TGF-beta/Activin-responsive promoters but not BMP-responsive elements. Furthermore, we identify a new Drosophila Smad, termed dSmad2, that is most closely related to vertebrate Smads 2 and 3. Activated Babo associates with dSmad2 but not Mad, phosphorylates the carboxy-terminal SSXS motif and induces heteromeric complex formation with Medea, the Drosophila Smad4 homolog. Our results define a novel Drosophila Activin/TGF-beta pathway that is analogous to its vertebrate counterpart and show that this pathway functions to promote cellular growth with minimal effects on patterning.

Dorsoventral patterning in the Drosophila central nervous system: the intermediate neuroblasts defective homeobox gene specifies intermediate column identity

One of the first steps in neurogenesis is the diversification of cells along the dorsoventral axis. In Drosophila the central nervous system develops from three longitudinal columns of cells: ventral cells that express the vnd/nk2 homeobox gene, intermediate cells, and dorsal cells that express the msh homeobox gene. Here we describe a new Drosophila homeobox gene, intermediate neuroblasts defective (ind), which is expressed specifically in the intermediate column cells. ind is essential for intermediate column development: Null mutants have a transformation of intermediate to dorsal column neuroectoderm fate, and only 10% of the intermediate column neuroblasts develop. The establishment of dorsoventral column identity involves negative regulation: Vnd represses ind in the ventral column, whereas ind represses msh in the intermediate column. Vertebrate genes closely related to vnd (Nkx2.1 and Nkx2.2), ind (Gsh1 and Gsh2), and msh (Msx1 and Msx3) are expressed in corresponding ventral, intermediate, and dorsal domains during vertebrate neurogenesis, raising the possibility that dorsoventral patterning within the central nervous system is evolutionarily conserved.

Conservation of the spiralian developmental program: cell lineage of the nemertean, Cerebratulus lacteus

Lineage tracers were injected into individual blastomeres in embryos of the indirect-developing nemertean Cerebratulus lacteus through the formation of the fourth quartet of micromeres. Subsequent development was followed to the formation of feeding pilidium larvae to establish their ultimate fates. Results showed that these blastomeres have unique fates, and their clones give rise to highly predictable regions of the larval body. As in other spiralians, four discrete cell quadrants can be identified. For the most part, their identities are homologous to the typical spiralian A, B, C, and D cell quadrants. In some respects their fates differ from the typical spiralian fate map; however, these can be understood in terms of simple modifications of the early cleavage program. Unlike most spiralians, the first quartet micromeres in the eight-celled embryo are larger than the corresponding vegetal macromeres, and generate most of the larval ectoderm. All four of these micromeres contribute to the apical organ and generate four bilaterally situated domains of ectoderm, where the progeny of the 1a and 1d micromeres lie to the left of the median plane while those of 1b and 1c lie to the right. Unlike the progeny of the first quartet, those of the second quartet are situated in left (2a), ventral (2b), right (2c), and dorsal (2d) positions. The third quartet micromeres generate clones situated in a bilaterally symmetrical fashion similar to those of the first quartet. The alternating axial relationships exhibited by successive micromere quartets are a characteristic of spiralian development. Unlike other spiralian larvae possessing a ciliary band, the pilidium larval ciliary band is formed by all blastomeres of the first and second micromere quartets, as well as 3c and 3d. Ectomesoderm is derived from two blastomeres (3a and 3b), which give rise to the extensive array of the larval muscle cells. C. lacteus also possesses a true mesentoblast (4d) which gives rise to a pair of mesodermal bandlets, and scattered mesenchymal cells. The dual origin of the mesoderm, as both ectomesoderm and endomesoderm, appears to be a condition present in all spiralians. The gut is formed by all the fourth quartet micromeres as well as the vegetal macromeres (4A, 4B, 4C, 4D). Despite differences in the determination of axial properties and some modifications in quadrant fates, nemerteans appear to be constructed on the typical spiralian developmental platform.

Disruption of primary mesenchyme cell patterning by misregulated ectodermal expression of SpMsx in sea urchin embryos

The patterning of the mesoderm of the sea urchin embryo is a classical paradigm of epithelial mesenchymal interactions in organogenesis, yet little is known of its molecular basis. Here we address the role of the homeobox gene, SpMsx, a member of the highly conserved Msx gene family, in this process. Msx genes have been shown to function in the dorsoventral patterning of the central nervous system in Drosophila and in a variety epithelial-mesenchymal interactions in vertebrates. We showed previously that the SpMsx gene is expressed during embryogenesis in a complex and dynamic pattern consistent with roles in the development of subpopulations of endoderm, mesoderm, and oral ectoderm. To perturb this pattern of expression and thus probe the function of SpMsx, we injected SpMsx mRNA into single-cell zygotes and monitored development morphologically and with a series of territory-specific molecular markers. RT-PCR analysis revealed that injected SpMsx transcripts persisted at least until the gastrula stage in amounts comparable to endogenous levels. Injected embryos exhibited deficiencies in the organization of primary and secondary mesenchyme cells within the blastocoelic cavity, as well as abnormalities in spicule number and shape. Defects in the endoderm were also common, including reduced or absent archenterons. Micromere transplantation experiments revealed that the defects in skeletogenic mesenchyme patterning were non-cell autonomous, consistent with findings that cell-cell interactions between ectoderm and the progenitors of the skeletogenic mesenchyme, the primary mesenchyme cells (PMCs), are important both for PMC guidance and spicule morphogenesis. Our data, taken together with observations in other organisms on the role of Msx genes in embryonic signaling processes, particularly involving the BMP pathway, suggest that SpMsx may be a part of the mechanism by which the ectoderm influences both the arrangement of primary mesenchyme cells within the blastocoel and the shapes of the skeletal rods.

The zebrafish organizer

Signals from the organizer play a crucial role in patterning the vertebrate embryo. Recent molecular analysis of zebrafish mutations has established an essential role for BMP2 and chordin in organizer function and has identified one-eyed pinhead as a novel EGF-like gene involved in prechordal plate and endoderm formation. In addition, embryological studies have suggested that the zebrafish organizer is induced by extraembryonic cues and have defined two novel organizing centers that pattern the nervous system along the anterior-posterior axes.

Smad6 functions as an intracellular antagonist of some TGF-beta family members during Xenopus embryogenesis

BACKGROUND

Bone morphogenetic proteins (BMPs) transmit signals via the intracellular protein Smad1, which is phosphorylated by ligand bound receptors, translocates to the nucleus, and functions to activate BMP target genes. Recently, a subclass of Smad proteins has been shown to inhibit, rather than transduce, BMP signalling, either by binding to the intracellular domain of BMP receptors, thereby preventing phosphorylation-mediated activation of Smad1, or by binding directly to Smad1, thereby inhibiting its ability to activate gene transcription.

RESULTS

We have identified a Xenopus Smad (Smad6) that is 52% identical to mammalian Smad6, an inhibitory Smad. The spatial pattern of expression of Smad6 changes dynamically during embryogenesis and is similar to that of BMP-4 at the tailbud stage. Overexpression of Smad6 in Xenopus embryos phenocopies the effect of blocking BMP-4 signalling, leading to dorsalization of mesoderm and neuralization of ectoderm. Xenopus Smad6 completely blocks the activity of exogenous BMP-4, and, unlike human Smad6, partially blocks the activity of activin, in a mesoderm induction assay. We also find that Smad6 protein accumulates at the membrane in some cells but is partially or completely restricted to nuclei of most overexpressing cells.

CONCLUSIONS

We have identified an inhibitory Xenopus Smad, Smad6, that functions as an intracellular antagonist of activin and BMP-4 signalling. Our finding that Smad6 protein is partially or completely restricted to nuclei of most overexpressing cells suggests that it may employ a novel or additional mechanism of action to antagonize TGF-beta family signalling other than that reported for other inhibitory Smads.

The Xcad-2 gene can provide a ventral signal independent of BMP-4

Patterning of the marginal zone in the Xenopus embryo has been attributed to interactions between dorsal genes expressed in the organizer and ventral-specific genes. In this antagonistic interplay of activities, BMP-4, a gene that is not expressed in the organizer, provides a strong ventralizing signal. The Xenopus caudal type homeobox gene, Xcad-2, which is expressed around the blastopore with a gap over the dorsal lip, was analyzed as part of the ventral signal. Xcad-2 was shown to efficiently repress during early gastrula stages the dorsal genes gsc, Xnot-2, Otx-2, XFKH1 and Xlim-1, while it positively regulates the ventral genes, Xvent-1 and Xvent-2, with Xpo exhibiting a strong positive response to Xcad-2 overexpression. Xcad-2 was also capable of inducing BMP-4 expression in the organizer region. Support for a ventralizing role for Xcad-2 was obtained from co-injection experiments with the dominant negative BMP receptor which was used to block BMP-4 signaling. Under lack-of-BMP-signaling conditions Xcad-2 could still regulate dorsal and ventral gene expression and restore normal development, suggesting that it can act downstream of BMP-4 signaling or independently of it. Xcad-2 could also inhibit secondary axis formation and dorsalization induced by the dominant negative BMP receptor. Xcad-2 was also shown to efficiently reverse the dorsalizing effects of LiCl. These results place Xcad-2 as part of the ventralizing gene program which acts during early gastrula stages and can execute its ventralizing function in the absence of BMP signaling.

Molecular insights into cardiac development

Recent discoveries have led to a greater appreciation of the diverse mechanisms that underlie cardiac morphogenesis. Genetic strategies (primarily gene targeting approaches in mice) have significantly broadened research in cardiovascular developmental biology by illuminating new pathways involved in heart development and by allowing the genetic evaluation of pathways that have previously been implicated in these events. Advances have also been made using biochemical and cell- and tissue-based approaches. This review summarizes the author's interpretation of current trends in the effort to understand the molecular basis of cardiac-development, with an emphasis on insights obtained from genetic models.

Xenopus Smad8 acts downstream of BMP-4 to modulate its activity during vertebrate embryonic patterning

Bone morphogenetic proteins (BMPs) participate in the development of nearly all organs and tissues. BMP signaling is mediated by specific Smad proteins, Smad1 and/or Smad5, which undergo serine phosphorylation in response to BMP-receptor activation and are then translocated to the nucleus where they modulate transcription of target genes. We have identified a distantly related member of the Xenopus Smad family, Smad8, which lacks the C-terminal SSXS phosphorylation motif present in other Smads, and which appears to function in the BMP signaling pathway. During embryonic development, the spatial pattern of expression of Smad8 mirrors that of BMP-4. We show that an intact BMP signaling pathway is required for its expression. Overexpression of Smad8 in Xenopus embryos phenocopies the effect of blocking BMP-4 signaling, leading to induction of a secondary axis on the ventral side of intact embryos and to direct neural induction in ectodermal explants. Furthermore, Smad8 can block BMP-4-mediated induction of ventral mesoderm-specific gene expression in ectodermal explants. Overexpression of Smad8 within dorsal cells, however, causes patterning defects that are distinct from those reported in BMP-4-deficient embryos, suggesting that Smad8 may interact with additional signaling pathways. Indeed, overexpression of Smad8 blocks expression of Xbra in whole animals, and partially blocks activin signaling in animal caps. In addition, Smad8 inhibits involution of mesodermal cells during gastrulation, a phenotype that is not observed following blockade of activin or BMPs in Xenopus. Together, these results are consistent with the hypothesis that Smad8 participates in a negative feedback loop in which BMP signaling induces the expression of Smad8, which then functions to negatively modulate the amplitude or duration of signaling downstream of BMPs and, possibly, downstream of other transforming growth factor-beta (TGF-beta) family ligands.

From genes to individuals: developmental genes and the generation of the phenotype

The success of the genetic approach to developmental biology has provided us with a suite of genes that are involved in the regulation of ontogenetic pathways. It is therefore time to ask whether and how such genes might be involved in the generation of adaptive phenotypes. Unfortunately, the current results do not provide a clear answer. Most of the genes that have been studied by developmental biologists affect early embryonic traits with significant effects on the whole organism. These genes are often highly conserved which allows us to do comparative studies even across phyla. However, whether the same genes are also involved in short-term ecological adaptations remains unclear. The suggestion that early acting ontogenetic genes may also affect late phenotypes comes from the genetic analysis of quantitative traits like bristle numbers in Drosophila. A rough mapping of the major loci affecting these traits shows that these loci might correspond to well known early acting genes. On the other hand, there are also many minor effect loci that are as yet uncharacterized. We suggest that these minor loci might correspond to a different class of genes. In comparative studies of randomly drawn cDNAs from Drosophila we find that there is a large group of genes that evolve fast and that are significantly under-represented in normal genetic screens. We speculate that these genes might provide a large, as yet poorly understood, reservoir of genes that might be involved in the evolution of quantitative traits and short-term adaptations.

Conserved Spätzle/Toll signaling in dorsoventral patterning of Xenopus embryos

The Spätzle/Toll signaling pathway controls ventral axis formation in Drosophila by generating a gradient of nuclear Dorsal protein. Dorsal controls the downstream regulators dpp and sog, whose patterning functions are conserved between insects and vertebrates. Although there is no experimental evidence that the upstream events are conserved as well, we set out to ask if a vertebrate embryo can respond to maternal components of the fly Dorsal pathway. Here we demonstrate a dorsalizing activity for the heterologous Easter, Spätzle and Toll proteins in UV-ventralized Xenopus embryos, which is inhibited by a co-injected dominant Cactus variant. We conclude that the Dorsal signaling pathway is a component of the conserved dorsoventral (d/v) patterning system in bilateria.

Chordin regulates primitive streak development and the stability of induced neural cells, but is not sufficient for neural induction in the chick embryo

We have investigated the role of Bone Morphogenetic Protein 4 (BMP-4) and a BMP antagonist, chordin, in primitive streak formation and neural induction in amniote embryos. We show that both BMP-4 and chordin are expressed before primitive streak formation, and that BMP-4 expression is downregulated as the streak starts to form. When BMP-4 is misexpressed in the posterior area pellucida, primitive streak formation is inhibited. Misexpression of BMP-4 also arrests further development of Hensen's node and axial structures. In contrast, misexpression of chordin in the anterior area pellucida generates an ectopic primitive streak that expresses mesoderm and organizer markers. We also provide evidence that chordin is not sufficient to induce neural tissue in the chick. Misexpression of chordin in regions outside the future neural plate does not induce the early neural markers L5, Sox-3 or Sox-2. Furthermore, neither BMP-4 nor BMP-7 interfere with neural induction when misexpressed in the presumptive neural plate before or after primitive streak formation. However, chordin can stabilise the expression of early neural markers in cells that have already received neural inducing signals. These results suggest that the regulation of BMP signalling by chordin plays a role in primitive streak formation and that chordin is not sufficient to induce neural tissue.

Memory suppressor genes: inhibitory constraints on the storage of long-term memory

Synaptic plasticity, the ability of neurons to alter the strength of their synaptic connections with activity and experience, is thought to play a critical role in memory storage. Molecular studies of gene expression during long-lasting synaptic plasticity related to memory storage initially focused on the identification of positive regulators. More recent work has revealed that the establishment of long-lasting synaptic plasticity and long-term memory also requires the removal of inhibitory constraints. By analogy to tumor suppressor genes, which restrain cell proliferation, we propose that these inhibitory constraints of memory storage, which restrain synapse growth, be termed memory suppressor genes.

Control of EGF receptor activation in Drosophila

Diverse biological and developmental functions are mediated by signalling through the epidermal growth factor (EGF) receptor. In flies, many different mechanisms are used to control and restrict EGF-R signalling, including ligand processing, ligand variety (an inhibitor as well as activators), transcription and perhaps subcellular localization of the receptor. Since the components of EGF-R signalling have been well conserved, understanding these different modes of receptor regulation in flies should lead to general insights into the strategies of receptor activation.

Daughters against dpp modulates dpp organizing activity in Drosophila wing development

The family of TGF-beta signalling molecules play inductive roles in various developmental contexts. One member of this family, Drosophila Decapentaplegic (Dpp) serves as a morphogen that patterns both the embryo and adult. We have now isolated a gene, Daughters against dpp (Dad), whose transcription is induced by Dpp. Dad shares weak homology with Drosophila Mad (Mothers against dpp), a protein required for transduction of Dpp signals. In contrast to Mad or the activated Dpp receptor, whose overexpression hyperactivates the Dpp signalling pathway, overexpression of Dad blocks Dpp activity. Expression of Dad together with either Mad or the activated receptor rescues phenotypic defects induced by each protein alone. Dad can also antagonize the activity of a vertebrate homologue of Dpp, bone morphogenetic protein, as evidenced by induction of dorsal or neural fate following overexpression in Xenopus embryos. We conclude that the pattern-organizing mechanism governed by Dpp involves a negative-feedback circuit in which Dpp induces expression of its own antagonist, Dad. This feedback loop appears to be conserved in vertebrate development.

Bmp-4 acts as a morphogen in dorsoventral mesoderm patterning in Xenopus

The marginal zone is a ring of tissue that gives rise to a characteristic dorsoventral pattern of mesoderm in amphibian embryos. Bmp-4 is thought to play an important role in specifying ventral mesodermal fate. Here we show (1) that different doses of Bmp-4 are sufficient to pattern four distinct mesodermal cell types and to pattern gene expression in the early gastrula marginal zone into three domains, (2) that there is a graded requirement for a Bmp signal in mesodermal patterning, and (3) that Bmp-4 has long-range activity which can become graded in the marginal zone by the antagonizing action of noggin. The results argue that Bmp-4 acts as a morphogen in dorsoventral patterning of mesoderm.

Fish are like flies are like frogs: conservation of dorsal-ventral patterning mechanisms

Genetic analysis of Drosophila has shown that a morphogenetic gradient of the Transforming Growth Factor-beta family member dpp patterns the embryonic dorsal-ventral axis. Molecular and embryological evidence from Xenopus has strongly suggested a similar role for Bmp-4, the dpp homolog, in patterning the dorsal-ventral axis of chordates. A recent report has now identified mutations in two genes, dino and swirl, that disrupt dorsal-ventral patterning in the zebrafish Danio rerio. Characterization of these mutations parallels findings from Drosophila, thus establishing a genetic framework for the analysis of dorsal-ventral patterning in a vertebrate.

Cell determination strategies in the Drosophila eye

Cells in the Drosophila eye are determined by inductive signalling. Here I describe a new model of eye development that explains how simple intercellular signals could specify the diverse cell types that constitute the ommatidium. This model arises from the recent observation that the Drosophila homologue of the EGF receptor (DER) is used reiteratively to trigger the differentiation of each of the cell types--successive rounds of DER activation recruit first the photoreceptors, then cone and finally pigment cells. It seems that a cell's identity is not determined by the specific signal that induces it, but is instead a function of the state of the cell when it receives the signal. DER signalling is activated by the ligand, Spitz, and inhibited by the secreted protein, Argos. Spitz is initially produced by the central cells in the ommatidium and diffuses over a small distance. Argos has a longer range, allowing it to block more distal cells from being activated by low levels of Spitz; I have termed this interplay between a short-range activator and a long-range inhibitor ‘remote inhibition’. Since inductive signalling is common in many organisms and its components have been conserved, it is possible that the logic of signalling may also be conserved.

The Xenopus dorsalizing factor noggin ventralizes Drosophila embryos by preventing DPP from activating its receptor

noggin is expressed in the Spemann organizer region of the Xenopus embryo and can promote dorsal cell fates within the mesoderm and neural development within the overlying ectoderm. Here, we show that noggin promotes ventral development in Drosophila, specifying ventral ectoderm and CNS in the absence of all endogenous ventral-specific zygotic gene expression. We utilize constitutively active forms of the dpp receptors to demonstrate that noggin blocks dpp signaling upstream of dpp receptor activation. These results suggest a mechanistic basis for the action of Spemann's organizer during Xenopus development and provide further support for the conservation of dorsal-ventral patterning mechanisms between arthropods and chordates.