Identification of maverick, a novel member of the TGF-beta superfamily in Drosophila

Drosophila Smad2 degradation occurs independently of linker phosphorylations

TGF-β signals are important for proliferation, differentiation, and cell fate determination during embryonic development and tissue homeostasis in adults. Drosophila Activin/TGF-β signals are transduced intracellularly when its transcription factor dSmad2 (also called Smad on X or Smox) is C-terminally phosphorylated by pathway receptors. Recently, it has been shown that receptor-activated dSmad2 undergoes bulk degradation, however, the mechanism of how this occurs is unknown. Here we investigated if two putative linker phosphorylation sites are involved in dSmad2 degradation. We demonstrate that degradation of activated-dSmad2 occurs independently of threonine phosphorylation at linker sites 252 and 277. We also show that dSmad2 degradation is not carried out by cellular proteasomes.

Ret and Substrate-Derived TGF-β Maverick Regulate Space-Filling Dendrite Growth in Drosophila Sensory Neurons

Dendrite morphogenesis is a highly regulated process that gives rise to stereotyped receptive fields, which are required for proper neuronal connectivity and function. Specific classes of neurons, including Drosophila class IV dendritic arborization (C4da) neurons, also feature complete space-filling growth of dendrites. In this system, we have identified the substrate-derived TGF-β ligand maverick (mav) as a developmental signal promoting space-filling growth through the neuronal Ret receptor. Both are necessary for radial spreading of C4da neuron dendrites, and Ret is required for neuronal uptake of Mav. Moreover, local changes in Mav levels result in directed dendritic growth toward regions with higher ligand availability. Our results suggest that Mav acts as a substrate-derived secreted signal promoting dendrite growth within not-yet-covered areas of the receptive field to ensure space-filling dendritic growth.

The Drosophila Ret gene functions in the stomatogastric nervous system with the Maverick TGFβ ligand and the Gfrl co-receptor

The RET receptor tyrosine kinase is crucial for the development of the enteric nervous system (ENS), acting as a receptor for Glial cell line-derived neurotrophic factor (GDNF) via GFR co-receptors. Drosophila has a well-conserved RET homolog (Ret) that has been proposed to function independently of the Gfr-like co-receptor (Gfrl). We find that Ret is required for development of the stomatogastric (enteric) nervous system in both embryos and larvae, and its loss results in feeding defects. Live imaging analysis suggests that peristaltic waves are initiated but not propagated in mutant midguts. Examination of axons innervating the midgut reveals increased branching but the area covered by the branches is decreased. This phenotype can be rescued by Ret expression. Additionally, Gfrl shares the same ENS and feeding defects, suggesting that Ret and Gfrl might function together via a common ligand. We identified the TGFβ family member Maverick (Mav) as a ligand for Gfrl and a Mav chromosomal deficiency displayed similar embryonic ENS defects. Our results suggest that the Ret and Gfrl families co-evolved before the separation of invertebrate and vertebrate lineages.

A Novel, Noncanonical BMP Pathway Modulates Synapse Maturation at the Drosophila Neuromuscular Junction

At the Drosophila NMJ, BMP signaling is critical for synapse growth and homeostasis. Signaling by the BMP7 homolog, Gbb, in motor neurons triggers a canonical pathway—which modulates transcription of BMP target genes, and a noncanonical pathway—which connects local BMP/BMP receptor complexes with the cytoskeleton. Here we describe a novel noncanonical BMP pathway characterized by the accumulation of the pathway effector, the phosphorylated Smad (pMad), at synaptic sites. Using genetic epistasis, histology, super resolution microscopy, and electrophysiology approaches we demonstrate that this novel pathway is genetically distinguishable from all other known BMP signaling cascades. This novel pathway does not require Gbb, but depends on presynaptic BMP receptors and specific postsynaptic glutamate receptor subtypes, the type-A receptors. Synaptic pMad is coordinated to BMP’s role in the transcriptional control of target genes by shared pathway components, but it has no role in the regulation of NMJ growth. Instead, selective disruption of presynaptic pMad accumulation reduces the postsynaptic levels of type-A receptors, revealing a positive feedback loop which appears to function to stabilize active type-A receptors at synaptic sites. Thus, BMP pathway may monitor synapse activity then function to adjust synapse growth and maturation during development.

Synaptic activity and synapse development are intimately linked, but our understanding of the coupling mechanisms remains limited. Anterograde and retrograde signals together with trans-synaptic complexes enable intercellular communications. How synapse activity status is monitored and relayed across the synaptic cleft remains poorly understood. The Drosophila NMJ is a very powerful genetic system to study synapse development. BMP signaling modulates NMJ growth via a canonical, Smad-dependent pathway, but also synapse stability, via a noncanonical, Smad-independent pathway. Here we describe a novel, noncanonical BMP pathway, which is genetically distinguishable from all other known BMP pathways. This pathway does not contribute to NMJ growth and instead influences synapse formation and maturation in an activity-dependent manner. Specifically, phosphorylated Smad (pMad in flies) accumulates at active zone in response to active postsynaptic type-A glutamate receptors, a specific receptor subtype. In turn, synaptic pMad functions to promote the recruitment of type-A receptors at synaptic sites. This positive feedback loop provides a molecular switch controlling which flavor of glutamate receptors will be stabilized at synaptic locations as a function of synapse status. Since BMP signaling also controls NMJ growth and stability, BMP pathway offers an exquisite means to monitor the status of synapse activity and coordinate NMJ growth with synapse maturation and stabilization.

The Maternal Maverick/GDF15-like TGF-β Ligand Panda Directs Dorsal-Ventral Axis Formation by Restricting Nodal Expression in the Sea Urchin Embryo

Specification of the dorsal-ventral axis in the highly regulative sea urchin embryo critically relies on the zygotic expression of nodal, but whether maternal factors provide the initial spatial cue to orient this axis is not known. Although redox gradients have been proposed to entrain the dorsal-ventral axis by acting upstream of nodal, manipulating the activity of redox gradients only has modest consequences, suggesting that other factors are responsible for orienting nodal expression and defining the dorsal-ventral axis. Here we uncover the function of Panda, a maternally provided transforming growth factor beta (TGF-β) ligand that requires the activin receptor-like kinases (Alk) Alk3/6 and Alk1/2 receptors to break the radial symmetry of the embryo and orient the dorsal-ventral axis by restricting nodal expression. We found that the double inhibition of the bone morphogenetic protein (BMP) type I receptors Alk3/6 and Alk1/2 causes a phenotype dramatically more severe than the BMP2/4 loss-of-function phenotype, leading to extreme ventralization of the embryo through massive ectopic expression of nodal, suggesting that an unidentified signal acting through BMP type I receptors cooperates with BMP2/4 to restrict nodal expression. We identified this ligand as the product of maternal Panda mRNA. Double inactivation of panda and bmp2/4 led to extreme ventralization, mimicking the phenotype caused by inactivation of the two BMP receptors. Inhibition of maternal panda mRNA translation disrupted the early spatial restriction of nodal, leading to persistent massive ectopic expression of nodal on the dorsal side despite the presence of Lefty. Phylogenetic analysis indicates that Panda is not a prototypical BMP ligand but a member of a subfamily of TGF-β distantly related to Inhibins, Lefty, and TGF-β that includes Maverick from Drosophila and GDF15 from vertebrates. Indeed, overexpression of Panda does not appear to directly or strongly activate phosphoSmad1/5/8 signaling, suggesting that although this TGF-β may require Alk1/2 and/or Alk3/6 to antagonize nodal expression, it may do so by sequestering a factor essential for Nodal signaling, by activating a non-Smad pathway downstream of the type I receptors, or by activating extremely low levels of pSmad1/5/8. We provide evidence that, although panda mRNA is broadly distributed in the early embryo, local expression of panda mRNA efficiently orients the dorsal-ventral axis and that Panda activity is required locally in the early embryo to specify this axis. Taken together, these findings demonstrate that maternal panda mRNA is both necessary and sufficient to orient the dorsal-ventral axis. These results therefore provide evidence that in the highly regulative sea urchin embryo, the activity of spatially restricted maternal factors regulates patterning along the dorsal-ventral axis.

Panda, a member of the TGF-beta family of signaling molecules, is encoded by maternal mRNA and helps to break radial symmetry and orient the dorsal-ventral axis of the developing sea urchin embryo.

A key event during development of bilaterians is specification of the anterior-posterior and dorsal-ventral axes of the embryo. In some species, such as the fly Drosophila, this process relies on the activity of maternal determinants localized into the egg during oogenesis. However, in other animals, such as mammals or echinoderms, which are renowned for the developmental plasticity of their embryos, there is presently no evidence for maternal determinants controlling axis formation, and how these embryonic axes emerge from radially symmetrical embryos remains unknown. In the sea urchin embryo, specification of the dorsal-ventral axis critically relies on the localized expression of the TGF-β ligand Nodal in the presumptive ventral territory, but what controls the spatially restricted expression of nodal is not known. We discovered that in the sea urchin embryo, the initial restriction of nodal expression is directed by another TGF-β ligand that is expressed maternally, which we named Panda. Panda is both necessary for the early spatial restriction of nodal and sufficient to orient the dorsal-ventral axis when misexpressed locally. Altogether, our findings suggest that Panda may act as a maternal signal that defines the orientation of the dorsal-ventral axis. Thus, an antagonism between Nodal and maternal Panda signaling drives dorsal-ventral axis formation in the sea urchin embryo.

Mechanisms of BMP-Receptor Interaction and Activation

Bone morphogenetic proteins (BMPs), together with the eponymous transforming growth factor (TGF) β and the Activins form the TGFβ superfamily of ligands. This protein family comprises more than 30 structurally highly related proteins, which determine formation, maintenance, and regeneration of tissues and organs. Their importance for the development of multicellular organisms is evident from their existence in all vertebrates as well as nonvertebrate animals. From their highly specific functions in vivo either a strict relation between a particular ligand and its cognate cellular receptor and/or a stringent regulation to define a distinct temperospatial expression pattern for the various ligands and receptor is expected. However, only a limited number of receptors are found to serve a large number of ligands thus implicating highly promiscuous ligand-receptor interactions instead. Since in tissues a multitude of ligands are often found, which signal via a highly overlapping set of receptors, this raises the question how such promiscuous interactions between different ligands and their receptors can generate concerted and highly specific cellular signals required during embryonic development and tissue homeostasis.

Whole transcriptome responses among females of the filariasis and arbovirus vector mosquito Culex pipiens implicate TGF-β signaling and chromatin modification as key drivers of diapause induction

Culex pipiens mosquitoes are important disease vectors inhabiting temperate zones, worldwide. The seasonal reduction in temperature and photoperiod accompanying late summer and early fall prompts female mosquitoes to enter diapause, a stage of developmental arrest and physiological conditioning that enhances survival during the winter months. To investigate the molecular mechanisms underlying diapause induction, we used custom whole transcriptome microarrays to identify differences in gene expression following exposure to nondiapause (long days, 25 °C) and diapause-inducing (short days, 18 °C) environmental conditions. Using a two-way ANOVA, we identified 1130 genes that were differentially expressed. We used the expression of these genes across three time points to construct a gene co-expression network comprising five modules. Genes in modules 1, 2, and 3 were largely up-regulated, while genes in modules 4 and 5 were down-regulated when compared to nondiapause conditions. Pathway enrichment analysis of the network modules revealed some potential regulatory mechanisms driving diapause induction. Module 1 was enriched for genes in the TGF-ß and Wnt signaling pathways; module 2 was enriched for genes involved in insect hormone biosynthesis, specifically, ecdysone synthesis; module 3 was enriched for genes involved in chromatin modification; and module 5 was enriched for genes in the circadian rhythm pathway. Our results suggest that TGF-β signaling and chromatin modification are key drivers for the integration of environmental signals into the diapause induction phase in C. pipiens mosquitoes.

Ancient and diverged TGF-β signaling components in Nasonia vitripennis

The transforming growth factor beta (TGF)-β signaling pathway and its modulators are involved in many aspects of cellular growth and differentiation in all metazoa. Although most of the core components of the pathway are highly conserved, many lineage-specific adaptations have been observed including changes regarding paralog number, presence and absence of modulators, and functional relevance for particular processes. In the parasitic jewel wasp Nasonia vitripennis, the bone morphogenetic proteins (BMPs), one of the major subgroups of the TGF-β superfamily, play a more fundamental role in dorsoventral (DV) patterning than in all other insects studied so far. However, Nasonia lacks the BMP antagonist Short gastrulation (Sog)/chordin, which is essential for polarizing the BMP gradient along the DV axis in most bilaterian animals. Here, we present a broad survey of TGF-β signaling in Nasonia with the aim to detect other lineage-specific peculiarities and to identify potential mechanisms, which explain how BMP-dependent DV pattering occurs in the early Nasonia embryo in the absence of Sog.

Transforming growth factor β/activin signaling functions as a sugar-sensing feedback loop to regulate digestive enzyme expression

Organisms need to assess their nutritional state and adapt their digestive capacity to the demands for various nutrients. Modulation of digestive enzyme production represents a rational step to regulate nutriment uptake. However, the role of digestion in nutrient homeostasis has been largely neglected. In this study, we analyzed the mechanism underlying glucose repression of digestive enzymes in the adult Drosophila midgut. We demonstrate that glucose represses the expression of many carbohydrases and lipases. Our data reveal that the consumption of nutritious sugars stimulates the secretion of the transforming growth factor β (TGF-β) ligand, Dawdle, from the fat body. Dawdle then acts via circulation to activate TGF-β/Activin signaling in the midgut, culminating in the repression of digestive enzymes that are highly expressed during starvation. Thus, our study not only identifies a mechanism that couples sugar sensing with digestive enzyme expression but points to an important role of TGF-β/Activin signaling in sugar metabolism.

Strategies for exploring TGF-β signaling in Drosophila

The TGF-β pathway is an evolutionarily conserved signal transduction module that mediates diverse biological processes in animals. In Drosophila, both the BMP and Activin branches are required for viability. Studies rooted in classical and molecular genetic approaches continue to uncover new developmental roles for TGF-β signaling. We present an overview of the secreted ligands, transmembrane receptors and cellular Smad transducer proteins that compose the core pathway in Drosophila. An assortment of tools have been developed to conduct tissue-specific loss- and gain-of-function experiments for these pathway components. We discuss the deployment of these reagents, with an emphasis on appropriate usage and limitations of the available tools. Throughout, we note reagents that are in need of further improvement or development, and signaling features requiring further study. A general theme is that comparison of phenotypes for ligands, receptors, and Smads can be used to map tissue interactions, and to separate canonical and non-canonical signaling activities. Core TGF-β signaling components are subject to multiple layers of regulation, and are coupled to context-specific inputs and outputs. In addition to fleshing out how TGF-β signaling serves the fruit fly, we anticipate that future studies will uncover new regulatory nodes and modes and will continue to advance paradigms for how TGF-β signaling regulates general developmental processes.

Retrograde bone morphogenetic protein signaling shapes a key circadian pacemaker circuit

The neuropeptide pigment-dispersing factor (PDF) synchronizes molecular oscillations within circadian pacemakers in the Drosophila brain. It is expressed in the small ventral lateral neurons (sLNvs) and large ventral lateral neurons, the former being indispensable for maintaining behavioral rhythmicity under free-running conditions. How PDF circuits develop the specific connectivity traits that endow such global behavioral control remains unknown. Here, we show that mature sLNv circuits require PDF signaling during early development, acting through its cognate receptor PDFR at postsynaptic targets. Yet, axonal defects by PDF knockdown are presynaptic and become apparent only after metamorphosis, highlighting a delayed response to a signal released early on. Presynaptic expression of constitutively active bone morphogenetic protein (BMP) receptors prevents pdfr mutants misrouting phenotype, while sLNv-restricted downregulation of BMP signaling components phenocopied pdf(01). Thus, we have uncovered a novel mechanism that provides an early "tagging" of synaptic targets that will guide circuit refinement later in development.

Hyperactive BMP signaling induced by ALK2(R206H) requires type II receptor function in a Drosophila model for classic fibrodysplasia ossificans progressiva

BACKGROUND

Fibrodysplasia Ossificans Progressiva (FOP) is an autosomal dominant disorder characterized by episodic deposition of heterotopic bone in place of soft connective tissue. All FOP-associated mutations map to the BMP type I receptor, ALK2, with the ALK2(R206H) mutant form found in the vast majority of patients. The mechanism(s) regulating the expressivity of hyperactive ALK2(R206H) signaling throughout a patient's life is not well understood.

RESULTS

In Drosophila, human ALK2(R206H) receptor induces hyperactive BMP signaling. As in vertebrates, elevated signaling associated with ALK2(R206H) in Drosophila is ligand-independent. We found that a key determinant for ALK2(R206H) hyperactivity is a functional type II receptor. Furthermore, our results indicate that like its Drosophila ortholog, Saxophone (Sax), wild-type ALK2 can antagonize, as well as promote, BMP signaling.

CONCLUSIONS

The dual function of ALK2 is of particular interest given the heterozygous nature of FOP, as the normal interplay between such disparate behaviors could be shifted by the presence of ALK2(R206H) receptors. Our studies provide a compelling example for Drosophila as a model organism to study the molecular underpinnings of complex human syndromes such as FOP.

Neuroendocrine regulation of Drosophila metamorphosis requires TGFbeta/Activin signaling

In insects, initiation of metamorphosis requires a surge in the production of the steroid hormone 20-hydroxyecdysone from the prothoracic gland, the primary endocrine organ of juvenile larvae. Here, we show that blocking TGFβ/Activin signaling, specifically in the Drosophila prothoracic gland, results in developmental arrest prior to metamorphosis. The terminal, giant third instar larval phenotype results from a failure to induce the large rise in ecdysteroid titer that triggers metamorphosis. We further demonstrate that activin signaling regulates competence of the prothoracic gland to receive PTTH and insulin signals, and that these two pathways act at the mRNA and post-transcriptional levels, respectively, to control ecdysone biosynthetic enzyme expression. This dual regulatory circuitry may provide a cross-check mechanism to ensure that both developmental and nutritional inputs are synchronized before initiating the final genetic program leading to reproductive adult development. As steroid hormone production in C. elegans and mammals is also influenced by TGFβ/Activin signaling, this family of secreted factors may play a general role in regulating developmental transitions across phyla.

Activin signaling functions upstream of Gbb to regulate synaptic growth at the Drosophila neuromuscular junction

Activins are members of the TGF-ss superfamily of secreted growth factors that control a diverse array of processes in vertebrates including endocrine function, cell proliferation, differentiation, immune response and wound repair. In Drosophila, the Activin ligand Dawdle (Daw) has been shown to regulate several aspects of neuronal development such as embryonic axonal pathfinding, neuroblast proliferation in the larval brain and growth cone motility in the visual system. Here we identify a novel role for Activin signaling in regulating synaptic growth at the larval neuromuscular junction (NMJ). Mutants for Daw, the Activin type I receptor Baboon (Babo), and the signal transducer dSmad2, display reduced NMJ size suggesting that Daw utilizes a canonical Activin signal-transduction pathway in this context. Additionally, loss of Daw/Babo activity affects microtubule stability, axonal transport and distribution of Futsch, the Drosophila microtubule associated protein 1B (MAP1B) homolog. We find that Babo signaling is required postsynaptically in the muscle, in contrast to the well-characterized retrograde BMP/Gbb signal that is required for synaptic growth and function in presynaptic cells. Finally, we show that the Daw/Babo pathway acts upstream of gbb, and is involved in maintenance of muscle gbb expression, suggesting that Activins regulate NMJ growth by modulating BMP activity through transcriptional regulation of ligand expression.

The Drosophila Activin-like ligand Dawdle signals preferentially through one isoform of the Type-I receptor Baboon

How TGF-beta-type ligands achieve signaling specificity during development is only partially understood. Here, we show that Dawdle, one of four Activin-type ligands in Drosophila, preferentially signals through Babo(c), one of three isoforms of the Activin Type-I receptor that are expressed during development. In cell culture, Dawdle signaling is active in the presence of the Type-II receptor Punt but not Wit, demonstrating that the Type-II receptor also contributes to the specificity of the signaling complex. During development, different larval tissues express unique combinations of these receptors, and ectopic expression of Babo(c) in a tissue where it is not normally expressed at high levels can make that tissue sensitive to Dawdle signaling. These results reveal a mechanism by which distinct cell types can discriminate between different Activin-type signals during development as a result of differential expression of Type-I receptor isoforms.

The regulation of TGFbeta signal transduction

Transforming growth factor beta (TGFbeta) pathways are implicated in metazoan development, adult homeostasis and disease. TGFbeta ligands signal via receptor serine/threonine kinases that phosphorylate, and activate, intracellular Smad effectors as well as other signaling proteins. Oligomeric Smad complexes associate with chromatin and regulate transcription, defining the biological response of a cell to TGFbeta family members. Signaling is modulated by negative-feedback regulation via inhibitory Smads. We review here the mechanisms of TGFbeta signal transduction in metazoans and emphasize events crucial for embryonic development.

TGF-beta1 genotype and phenotype in breast cancer and their associations with IGFs and patient survival

Transforming growth factor-beta (TGF-beta)-mediated signals play complicated roles in the development and progression of breast tumour. The purposes of this study were to analyse the genotype of TGF-beta1 at T29C and TGF-beta1 phenotype in breast tumours, and to evaluate their associations with IGFs and clinical characteristics of breast cancer. Fresh tumour samples were collected from 348 breast cancer patients. TGF-beta1 genotype and phenotype were analysed with TaqMan and ELISA, respectively. Members of the IGF family in tumour tissue were measured with ELISA. Cox proportional hazards regression analysis was performed to assess the association of TGF-beta1 and disease outcomes. Patients with the T/T (29%) genotype at T29C had the highest TGF-beta1, 707.9 pg mg(-1), followed by the T/C (49%), 657.8 pg mg(-1), and C/C (22%) genotypes, 640.8 pg mg(-1), (P=0.210, T/T vs C/C and C/T). TGF-beta1 concentrations were positively correlated with levels of oestrogen receptor, IGF-I, IGF-II and IGFBP-3. Survival analysis showed TGF-beta1 associated with disease progression, but the association differed by disease stage. For early-stage disease, patients with the T/T genotype or high TGF-beta1 had shorter overall survival compared to those without T/T or with low TGF-beta1; the hazard ratios (HR) were 3.54 (95% CI: 1.21-10.40) for genotype and 2.54 (95% CI: 1.10-5.89) for phenotype after adjusting for age, grade, histotype and receptor status. For late-stage disease, however, the association was different. The T/T genotype was associated with lower risk of disease recurrence (HR=0.13, 95% CI: 0.02-1.00), whereas no association was found between TGF-beta1 phenotype and survival outcomes. The study suggests a complex role of TGF-beta1 in breast cancer progression, which supports the finding of in vitro studies that TGF-beta1 has conflicting effects on tumour growth and metastasis.

TGFbeta signaling in Tribolium: vertebrate-like components in a beetle

The cytokines of the TGFbeta superfamily are highly conserved in evolution and elicit a diverse range of cellular responses in all metazoa. In Drosophila, the signaling pathways of the two TGFbeta subfamilies, Activins and Bone Morphogenetic Proteins (BMPs), have been well studied. To address the question of whether the findings from Drosophila are representative of insects in general, we analyzed the components of TGFbeta-signaling present in the genome of the beetle Tribolium castaneum. We were able to identify orthologs of the BMPs Decapentaplegic and Glass bottom boat, of the Activins Activinbeta and Dawdle, as well as orthologs of the less well-known ligands Myoglianin and Maverick, together with orthologs of all TGFbeta receptors and cytoplasmic signal transducers present in Drosophila. This indicates that the diversity of TGFbeta signaling components is generally well conserved between Drosophila and Tribolium. However, the genome of the beetle-and of the bee Apis mellifera-lacks an ortholog of the Drosophila BMP Screw but does contain a vertebrate-like BMP10 homolog which is not found in Drosophila. Concerning BMP inhibitors, Tribolium displays an even more vertebrate-like ensemble of components. We found two orthologs of the vertebrate DAN family, Dan and Gremlin, and show embryonic expression of a vertebrate-like BAMBI ortholog, all of which are absent in Drosophila. This suggests that Tribolium might have retained a more ancestral composition of TGFbeta signaling components and that TGFbeta signaling underwent considerable change in the Drosophila lineage. Tribolium is an excellent model to study the function of these ancestral signaling components in insects.

The BMP ligand Gbb gates the expression of synaptic homeostasis independent of synaptic growth control

Inhibition of postsynaptic glutamate receptors at the Drosophila NMJ initiates a compensatory increase in presynaptic release termed synaptic homeostasis. BMP signaling is necessary for normal synaptic growth and stability. It remains unknown whether BMPs have a specific role during synaptic homeostasis and, if so, whether BMP signaling functions as an instructive retrograde signal that directly modulates presynaptic transmitter release. Here, we demonstrate that the BMP receptor (Wit) and ligand (Gbb) are necessary for the rapid induction of synaptic homeostasis. We also provide evidence that both Wit and Gbb have functions during synaptic homeostasis that are separable from NMJ growth. However, further genetic experiments demonstrate that Gbb does not function as an instructive retrograde signal during synaptic homeostasis. Rather, our data indicate that Wit and Gbb function via the downstream transcription factor Mad and that Mad-mediated signaling is continuously required during development to confer competence of motoneurons to express synaptic homeostasis.

How to innervate a simple gut: familiar themes and unique aspects in the formation of the insect enteric nervous system

Like the vertebrate enteric nervous system (ENS), the insect ENS consists of interconnected ganglia and nerve plexuses that control gut motility. However, the insect ENS lies superficially on the gut musculature, and its component cells can be individually imaged and manipulated within cultured embryos. Enteric neurons and glial precursors arise via epithelial-to-mesenchymal transitions that resemble the generation of neural crest cells and sensory placodes in vertebrates; most cells then migrate extensive distances before differentiating. A balance of proneural and neurogenic genes regulates the morphogenetic programs that produce distinct structures within the insect ENS. In vivo studies have also begun to decipher the mechanisms by which enteric neurons integrate multiple guidance cues to select their pathways. Despite important differences between the ENS of vertebrates and invertebrates, common features in their programs of neurogenesis, migration, and differentiation suggest that these relatively simple preparations may provide insights into similar developmental processes in more complex systems.

Cg-TGF-beta, a TGF-beta/activin homologue in the Pacific Oyster Crassostrea gigas, is involved in immunity against Gram-negative microbial infection

Transforming growth factor-beta (TGF-beta) members represent a widespread protein superfamily in the animal kingdom, but few members have been characterised in lophotrochozoans, a major clade of invertebrates. Here, we report the identification of Crassostrea gigas-TGF-beta (Cg-TGF-beta), a homologue of vertebrate TGF-beta and activin, from the bivalve mollusc C. gigas. Phylogenetic analysis suggests an early ancestral origin of this subgroup of TGF-beta superfamily member. Investigation of the spatio-temporal expression of Cg-TGF-beta gene by real-time quantitative RT-PCR showed a ubiquitous pattern in all adult tissues. These findings imply that Cg-TGF-beta has multiple functions as described for its vertebrate counterparts. Moreover, Cg-TGF-beta was upregulated in haemocytes during infection by a Gram-negative bacterium, suggesting that it could act as a cytokine involved in immunity in molluscs.

The metalloprotease Tolloid-related and its TGF-β-like substrate Dawdle regulateDrosophilamotoneuron axon guidance

Proper axon pathfinding requires that growth cones execute appropriate turns and branching at particular choice points en route to their synaptic targets. Here we demonstrate that the Drosophila metalloprotease tolloid-related (tlr) is required for proper fasciculation/defasciculation of motor axons in the CNS and for normal guidance of many motor axons enroute to their muscle targets. Tlr belongs to a family of developmentally important proteases that process various extracellular matrix components, as well as several TGF-β inhibitory proteins and pro-peptides. We show that Tlr is a circulating enzyme that processes the pro-domains of three Drosophila TGF-β-type ligands, and, in the case of the Activin-like protein Dawdle (Daw), this processing enhances the signaling activity of the ligand in vitro and in vivo. Null mutants of daw, as well as mutations in its receptor babo and its downstream mediator Smad2, all exhibit axon guidance defects that are similar to but less severe than tlr. We suggest that by activating Daw and perhaps other TGF-β ligands, Tlr provides a permissive signal for axon guidance.

RTK and TGF-beta signaling pathways genes in the sea urchin genome

The Receptor Tyrosine kinase (RTK) and TGF-beta signaling pathways play essential roles during development in many organisms and regulate a plethora of cellular responses. From the genome sequence of Strongylocentrotus purpuratus, we have made an inventory of the genes encoding receptor tyrosine kinases and their ligands, and of the genes encoding cytokines of the TGF-beta superfamily and their downstream components. The sea urchin genome contains at least 20 genes coding for canonical receptor tyrosine kinases. Seventeen of the nineteen vertebrate RTK families are represented in the sea urchin. Fourteen of these RTK among which ALK, CCK4/PTK7, DDR, EGFR, EPH, LMR, MET/RON, MUSK, RET, ROR, ROS, RYK, TIE and TRK are present as single copy genes while pairs of related genes are present for VEGFR, FGFR and INSR. Similarly, nearly all the subfamilies of TGF-beta ligands identified in vertebrates are present in the sea urchin genome including the BMP, ADMP, GDF, Activin, Myostatin, Nodal and Lefty, as well as the TGF-beta sensu stricto that had not been characterized in invertebrates so far. Expression analysis indicates that the early expression of nodal, BMP2/4 and lefty is restricted to the oral ectoderm reflecting their role in providing positional information along the oral-aboral axis of the embryo. The coincidence between the emergence of TGF-beta-related factors such as Nodal and Lefty and the emergence of the deuterostome lineage strongly suggests that the ancestral function of Nodal could have been related to the secondary opening of the mouth which characterizes this clade, a hypothesis supported by functional data in the extant species. The sea urchin genome contains 6 genes encoding TGF-beta receptors and 4 genes encoding prototypical Smad proteins. Furthermore, most of the transcriptional activators and repressors shown to interact with Smads in vertebrates have orthologues in echinoderms. Finally, the sea urchin genome contains an almost complete repertoire of genes encoding extracellular modulators of BMP signaling including Chordin, Noggin, Sclerotin, SFRP, Gremlin, DAN and Twisted gastrulation. Taken together, these findings indicate that the sea urchin complement of genes of the RTK and TGF-beta signaling pathways is qualitatively very similar to the repertoire present in vertebrates, and that these genes are part of the common genetool kit for intercellular signaling of deuterostomes.

TGFbeta superfamily members in spermatogenesis: setting the stage for fertility in mouse and Drosophila

Male germ cell development involves a tightly controlled sequence of differentiation switches, from the time that this lineage is specified in the embryo to the moment of sperm release for transport from the testis. Recent research findings and technological advances have allowed key mediators of developmental switches to be identified, and several members of the TGFbeta superfamily of ligands have been implicated at distinct points of male gamete formation and spermatogenesis. This paper reviews the multiple points of control mediated by TGFbeta superfamily signalling molecules during differentiation of the mammalian male germ cell. Comparisons have been made with the Drosophila testis for which genetic analysis has yielded new information concerning the roles of TGFbeta signalling in early germ cell differentiation.

Synaptic strengthening mediated by bone morphogenetic protein-dependent retrograde signaling in the Drosophila CNS

Retrograde signaling is an essential component of synaptic development and physiology. Previous studies show that bone morphogenetic protein (BMP)-dependent retrograde signaling is required for the proper development of the neuromuscular junction (NMJ) in Drosophila. These studies, moreover, raised the significant possibility that the development of central motor circuitry might similarly be reliant on such signaling. To test this hypothesis, retrograde signaling between postsynaptic motoneurons and their presynaptic interneurons is examined. Postsynaptic expression of an adenylate cyclase encoded by rutabaga (rut), is sufficient to strengthen synaptic transmission at these identified central synapses. Results are presented to show that the underlying mechanism is dependent on BMP retrograde signaling. Thus, presynaptic expression of an activated TGF-beta receptor, thickvien (tkv), or postsynaptic expression of a TGF-beta ligand, glass-bottom boat (gbb), is sufficient to phenocopy strengthening of synaptic transmission. In the absence of gbb, endogenous synaptic transmission is significantly weakened and, moreover, postsynaptic overexpression of rut is unable to potentiate synaptic function. Potentiation of presynaptic neurotransmitter release, mediated by increased postsynaptic expression of gbb, is dependent on normal cholinergic activity, indicative that either the secretion of this retrograde signal, or its transduction, is activity dependent. Thus, in addition to the development of the NMJ and expression of myoactive FMRFamide-like peptides in specific central neurons, the results of the present study indicate that this retrograde signaling cascade also integrates the development and function of central motor circuitry that controls movement in Drosophila larvae.

Transforming growth factor-βs and related gene products in mosquito vectors of human malaria parasites: signaling architecture for immunological crosstalk

The participation of a divergent mosquito transforming growth factor-beta (TGF-beta) and mammalian TGF-beta1 in the Anopheles stephensi response to malaria parasite development [Infect. Genet. Evol. 1 (2001) 131-141; Infect. Immun. 71 (2003) 3000-3009] suggests that a network of Anopheles TGF-beta ligands and signaling pathways figure prominently in immune defense of this important vector group. To provide a basis for identifying the roles of these proteins in Anopheles innate immunity, we identified six predicted TGF-beta ligand-encoding genes in the Anopheles gambiae genome, including two expressed, diverged copies of 60A, the first evidence of ligand gene duplication outside of chordates. In addition to five predicted type I and II receptors, we identified three Smad genes in the A. gambiae genome that would be predicted to support both TGF-beta/Activin- and bone morphogenetic protein (BMP)-like signaling. All three Smad genes are expressed in an immunocompetent A. stephensi cell line and in the A. stephensi midgut epithelium, confirming that a conserved signaling architecture is in place to support signaling by divergent exogenous and endogenous TGF-beta superfamily proteins.

The BMP homolog Gbb provides a retrograde signal that regulates synaptic growth at the Drosophila neuromuscular junction

We show that the BMP ortholog Gbb can signal by a retrograde mechanism to regulate synapse growth of the Drosophila neuromuscular junction (NMJ). gbb mutants have a reduced NMJ synapse size, decreased neurotransmitter release, and aberrant presynaptic ultrastructure. These defects are similar to those we observe in mutants of BMP receptors and Smad transcription factors. However, whereas these BMP receptors and signaling components are required in the presynaptic motoneuron, Gbb expression is required in large part in postsynaptic muscles; gbb expression in muscle rescues key aspects of the gbb mutant phenotype. Consistent with this notion, we find that blocking retrograde axonal transport by overexpression of dominant-negative p150/Glued in neurons inhibits BMP signaling in motoneurons. These experiments reveal that a muscle-derived BMP retrograde signal participates in coordinating neuromuscular synapse development and growth.

TGF-beta signaling activates steroid hormone receptor expression during neuronal remodeling in the Drosophila brain

Metamorphosis of the Drosophila brain involves pruning of many larval-specific dendrites and axons followed by outgrowth of adult-specific processes. From a genetic mosaic screen, we recovered two independent mutations that block neuronal remodeling in the mushroom bodies (MBs). These phenotypically indistinguishable mutations affect Baboon function, a Drosophila TGF-beta/activin type I receptor, and dSmad2, its downstream transcriptional effector. We also show that Punt and Wit, two type II receptors, act redundantly in this process. In addition, knocking out dActivin around the mid-third instar stage interferes with remodeling. Binding of the insect steroid hormone ecdysone to distinct ecdysone receptor isoforms induces different metamorphic responses in various larval tissues. Interestingly, expression of the ecdysone receptor B1 isoform (EcR-B1) is reduced in activin pathway mutants, and restoring EcR-B1 expression significantly rescues remodeling defects. We conclude that the Drosophila Activin signaling pathway mediates neuronal remodeling in part by regulating EcR-B1 expression.

Regulation of BMP and Activin Signaling in Drosophila

Cytokines of the TGF-beta superfamily act through an evolutionarily conserved signaling pathway to elicit a diverse range of biological responses in vertebrates as well as invertebrates. Drosophila has proved to be a powerful system to unravel the profound complexities underlying the regulation of this superficially simple signaling system for two reasons--the availability of sophisticated genetic tools and the restricted number of core signaling components compared to vertebrates. A BMP signaling pathway in Drosophila that regulates growth, differentiation and morphogenesis of the embryo and the larva has been extensively characterized. This work has provided major insights into how gradients of secreted proteins can be established and maintained in vivo, allowing a single ligand to induce multiple cell fates rather than function as an on-off switch. More recently, an activin signaling pathway has also been delineated that is required for growth and neuronal function during development. This review provides an overview of TGF-beta signaling in Drosophila with emphasis on the extensive modulation of signaling activity both within and outside the cell, that enables ligands to trigger specific and context-dependent effects.