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

Bone morphogenetic protein signaling: the pathway and its regulation

In the mid-1960s, bone morphogenetic proteins (BMPs) were first identified in the extracts of bone to have the remarkable ability to induce heterotopic bone. When the Drosophila gene decapentaplegic (dpp) was first identified to share sequence similarity with mammalian BMP2/BMP4 in the late-1980s, it became clear that secreted BMP ligands can mediate processes other than bone formation. Following this discovery, collaborative efforts between Drosophila geneticists and mammalian biochemists made use of the strengths of their respective model systems to identify BMP signaling components and delineate the pathway. The ability to conduct genetic modifier screens in Drosophila with relative ease was critical in identifying the intracellular signal transducers for BMP signaling and the related transforming growth factor-beta/activin signaling pathway. Such screens also revealed a host of genes that encode other core signaling components and regulators of the pathway. In this review, we provide a historical account of this exciting time of gene discovery and discuss how the field has advanced over the past 30 years. We have learned that while the core BMP pathway is quite simple, composed of 3 components (ligand, receptor, and signal transducer), behind the versatility of this pathway lies multiple layers of regulation that ensures precise tissue-specific signaling output. We provide a sampling of these discoveries and highlight many questions that remain to be answered to fully understand the complexity of BMP signaling.

Primary cilia: The central role in the electromagnetic field induced bone healing

Primary cilia have emerged as the cellular “antenna” that can receive and transduce extracellular chemical/physical signals, thus playing an important role in regulating cellular activities. Although the electromagnetic field (EMF) is an effective treatment for bone fractures since 1978, however, the detailed mechanisms leading to such positive effects are still unclear. Primary cilia may play a central role in receiving EMF signals, translating physical signals into biochemical information, and initiating various signalingsignaling pathways to transduce signals into the nucleus. In this review, we elucidated the process of bone healing, the structure, and function of primary cilia, as well as the application and mechanism of EMF in treating fracture healing. To comprehensively understand the process of bone healing, we used bioinformatics to analyze the molecular change and associated the results with other studies. Moreover, this review summarizedsummarized some limitations in EMFs-related research and provides an outlook for ongoing studies. In conclusion, this review illustrated the primary cilia and related molecular mechanisms in the EMF-induced bone healing process, and it may shed light on future research.

DCAF7 regulates cell proliferation through IRS1-FOXO1 signaling

Cell proliferation is dependent on growth factors insulin and IGF1. We sought to identify interactors of IRS1, the most proximal mediator of insulin/IGF1 signaling, that regulate cell proliferation. Using proximity-dependent biotin identification (BioID), we detected 40 proteins displaying proximal interactions with IRS1, including DCAF7 and its interacting partners DYRK1A and DYRK1B. In HepG2 cells, DCAF7 knockdown attenuated cell proliferation by inducing cell cycle arrest at G2. DCAF7 expression was required for insulin-stimulated AKT phosphorylation, and its absence promoted nuclear localization of the transcription factor FOXO1. DCAF7 knockdown induced expression of FOXO1-target genes implicated in G2 cell cycle inhibition, correlating with G2 cell cycle arrest. In Drosophila melanogaster, wing-specific knockdown of DCAF7/wap caused smaller wing size and lower wing cell number; the latter recovered upon double knockdown of wap and dfoxo. We propose that DCAF7 regulates cell proliferation and cell cycle via IRS1-FOXO1 signaling, of relevance to whole organism growth.

Single-cell Senseless protein analysis reveals metastable states during the transition to a sensory organ fate

Cell fate decisions can be envisioned as bifurcating dynamical systems, and the decision that Drosophila cells make during sensory organ differentiation has been described as such. We extended these studies by focusing on the Senseless protein which orchestrates sensory cell fate transitions. Wing cells contain intermediate Senseless numbers before their fate transition, after which they express much greater numbers of Senseless molecules as they differentiate. However, the dynamics are inconsistent with it being a simple bistable system. Cells with intermediate Senseless are best modeled as residing in four discrete states, each with a distinct protein number and occupying a specific region of the tissue. Although the states are stable over time, the number of molecules in each state vary with time. The fold change in molecule number between adjacent states is invariant and robust to absolute protein number variation. Thus, cells transitioning to sensory fates exhibit metastability with relativistic properties.

Deltex modulates Dpp morphogen gradient formation and affects the Dpp signaling in Drosophila

Deltex (Dx) is a context-dependent regulator of Notch signaling and regulates Notch signaling in a non-canonical fashion by facilitating the endocytosis of its receptor. In an RNAi- based modifier screen of kinases and phosphatases Thickveins (Tkv), the receptor of Decapentaplegic (Dpp), was identified as one of the interactors of Dx. Dpp, a Drosophila TGF-β/Bone Morphogenetic Protein homolog acts as a morphogen to specify cell fate along the anterior-posterior axis of the wing. Tight regulation of Dpp signaling is thus indispensable for its proper functioning. Here we present Dx as a novel modulator of Dpp signaling. We show evidence for the very first time that dx genetically interacts with dpp and its pathway components. Immunocytochemical analysis shows that Dx co-localizes with Dpp and its receptor Tkv in the Drosophila third instar larval tissues. Further, Dx is also seen to modulate the expression of dpp and its target genes. Here, we attribute this modulation to the endocytosis and trafficking of Dpp through Dx. This study thus presents a whole new avenue of Dpp signaling regulation via the cytoplasmic protein Dx.

Damage-responsive neuro-glial clusters coordinate the recruitment of dormant neural stem cells in Drosophila

Recruitment of stem cells is crucial for tissue repair. Although stem cell niches can provide important signals, little is known about mechanisms that coordinate the engagement of disseminated stem cells across an injured tissue. In Drosophila, adult brain lesions trigger local recruitment of scattered dormant neural stem cells suggesting a mechanism for creating a transient stem cell activation zone. Here, we find that injury triggers a coordinated response in neuro-glial clusters that promotes the spread of a neuron-derived stem cell factor via glial secretion of the lipocalin-like transporter Swim. Strikingly, swim is induced in a Hif1-α-dependent manner in response to brain hypoxia. Mammalian Swim (Lcn7) is also upregulated in glia of the mouse hippocampus upon brain injury. Our results identify a central role of neuro-glial clusters in promoting neural stem cell activation at a distance, suggesting a conserved function of the HIF1-α/Swim/Wnt module in connecting injury-sensing and regenerative outcomes.

Mad dephosphorylation at the nuclear pore is essential for asymmetric stem cell division

Stem cells divide asymmetrically to generate a stem cell and a differentiating daughter cell. Yet, it remains poorly understood how a stem cell and a differentiating daughter cell can receive distinct levels of niche signal and thus acquire different cell fates (self-renewal versus differentiation), despite being adjacent to each other and thus seemingly exposed to similar levels of niche signaling. In the Drosophila ovary, germline stem cells (GSCs) are maintained by short range bone morphogenetic protein (BMP) signaling; the BMP ligands activate a receptor that phosphorylates the downstream molecule mothers against decapentaplegic (Mad). Phosphorylated Mad (pMad) accumulates in the GSC nucleus and activates the stem cell transcription program. Here, we demonstrate that pMad is highly concentrated in the nucleus of the GSC, while it quickly decreases in the nucleus of the differentiating daughter cell, the precystoblast (preCB), before the completion of cytokinesis. We show that a known Mad phosphatase, Dullard (Dd), is required for the asymmetric partitioning of pMad. Our mathematical modeling recapitulates the high sensitivity of the ratio of pMad levels to the Mad phosphatase activity and explains how the asymmetry arises in a shared cytoplasm. Together, these studies reveal a mechanism for breaking the symmetry of daughter cells during asymmetric stem cell division.

Temporal flexibility of gene regulatory network underlies a novel wing pattern in flies

Developmental genes can be coopted to generate evolutionary novelties by changing their spatial regulation. However, developmental genes seldom act independently, but rather work in a gene regulatory network (GRN). How is it possible to recruit a single gene from a whole GRN? What are the properties that allow parallel cooptions of the same genes during evolution? Here, we show that a novel engrailed gene expression underlies a novel wing color pattern in flies. We show that cooption is facilitated 1) because of GRN flexibility over development and 2) because every single gene of the GRN has its own functional time window. We suggest these two temporal properties could explain why the same gene can be independently recruited several times during evolution.

Organisms have evolved endless morphological, physiological, and behavioral novel traits during the course of evolution. Novel traits were proposed to evolve mainly by orchestration of preexisting genes. Over the past two decades, biologists have shown that cooption of gene regulatory networks (GRNs) indeed underlies numerous evolutionary novelties. However, very little is known about the actual GRN properties that allow such redeployment. Here we have investigated the generation and evolution of the complex wing pattern of the fly Samoaia leonensis. We show that the transcription factor Engrailed is recruited independently from the other players of the anterior–posterior specification network to generate a new wing pattern. We argue that partial cooption is made possible because 1) the anterior–posterior specification GRN is flexible over time in the developing wing and 2) this flexibility results from the fact that every single gene of the GRN possesses its own functional time window. We propose that the temporal flexibility of a GRN is a general prerequisite for its possible cooption during the course of evolution.

The Mediator CDK8-Cyclin C complex modulates Dpp signaling in Drosophila by stimulating Mad-dependent transcription

Dysregulation of CDK8 (Cyclin-Dependent Kinase 8) and its regulatory partner CycC (Cyclin C), two subunits of the conserved Mediator (MED) complex, have been linked to diverse human diseases such as cancer. Thus, it is essential to understand the regulatory network modulating the CDK8-CycC complex in both normal development and tumorigenesis. To identify upstream regulators or downstream effectors of CDK8, we performed a dominant modifier genetic screen in Drosophila based on the defects in vein patterning caused by specific depletion or overexpression of CDK8 or CycC in developing wing imaginal discs. We identified 26 genomic loci whose haploinsufficiency can modify these CDK8- or CycC-specific phenotypes. Further analysis of two overlapping deficiency lines and mutant alleles led us to identify genetic interactions between the CDK8-CycC pair and the components of the Decapentaplegic (Dpp, the Drosophila homolog of TGFβ, or Transforming Growth Factor-β) signaling pathway. We observed that CDK8-CycC positively regulates transcription activated by Mad (Mothers against dpp), the primary transcription factor downstream of the Dpp/TGFβ signaling pathway. CDK8 can directly interact with Mad in vitro through the linker region between the DNA-binding MH1 (Mad homology 1) domain and the carboxy terminal MH2 (Mad homology 2) transactivation domain. Besides CDK8 and CycC, further analyses of other subunits of the MED complex have revealed six additional subunits that are required for Mad-dependent transcription in the wing discs: Med12, Med13, Med15, Med23, Med24, and Med31. Furthermore, our analyses confirmed the positive roles of CDK9 and Yorkie in regulating Mad-dependent gene expression in vivo. These results suggest that CDK8 and CycC, together with a few other subunits of the MED complex, may coordinate with other transcription cofactors in regulating Mad-dependent transcription during wing development in Drosophila.

Ordered patterning of the sensory system is susceptible to stochastic features of gene expression

Sensory neuron numbers and positions are precisely organized to accurately map environmental signals in the brain. This precision emerges from biochemical processes within and between cells that are inherently stochastic. We investigated impact of stochastic gene expression on pattern formation, focusing on senseless (sens), a key determinant of sensory fate in Drosophila. Perturbing microRNA regulation or genomic location of sens produced distinct noise signatures. Noise was greatly enhanced when both sens alleles were present in homologous loci such that each allele was regulated in trans by the other allele. This led to disordered patterning. In contrast, loss of microRNA repression of sens increased protein abundance but not sensory pattern disorder. This suggests that gene expression stochasticity is a critical feature that must be constrained during development to allow rapid yet accurate cell fate resolution.

Bone Morphogenetic Protein Glass Bottom Boat (BMP5/6/7/8) and its receptor Wishful Thinking (BMPRII) are required for injury-induced allodynia in Drosophila

Background Chronic pain affects millions of people worldwide; however, its cellular and molecular mechanisms have not been completely elucidated. It is thought that chronic pain is triggered by nociceptive sensitization, which produces elevated nocifensive responses. A model has been developed in Drosophila melanogaster to investigate the underlying mechanisms of chronic pain using ultraviolet-induced tissue injury to trigger thermal allodynia, a nociceptive hypersensitivity to a normally innocuous stimulus. Larvae were assayed for their behavioral latencies to produce a distinct avoidance response under different thermal conditions. Previously, Decapentaplegic, a member of the Bone Morphogenetic Protein (BMP) family and orthologous to mammalian BMP2/4, was shown to be necessary for the induction of allodynia. Here, we further investigate the BMP pathway to identify other essential molecules necessary to activate the nociceptive sensitization pathway. Results Using the GAL4-UAS-RNAi system to induce a cell-specific knockdown of gene expression, we further explored BMP pathway components to identify other key players in the induction of nociceptive sensitization by comparing the responses of manipulated animals to those of controls. Here, we show that a second BMP, Glass Bottom Boat, and its receptor Wishful Thinking are both necessary for injury-induced thermal allodynia since the formation of sensitization was found to be severely attenuated when either of these components was suppressed. The effects on pain perception appear to be specific to the sensitization system, as the ability to respond to a normally noxious stimulus in the absence of injury was left intact, and no nociceptor morphological defects were observed. Conclusion These results provide further support of the hypothesis that the BMP pathway plays a crucial role in the development of nociceptive sensitization. Because of its strong conservation between invertebrates and mammals, the BMP pathway may be worthy of future investigation for the development of targeted treatments to alleviate chronic pain.

bantam microRNA is a negative regulator of the Drosophila decapentaplegic pathway

Decapentaplegic (Dpp), the Drosophila homolog of the vertebrate bone morphogenetic protein (BMP2/4), is crucial for patterning and growth in many developmental contexts. The Dpp pathway is regulated at many different levels to exquisitely control its activity. We show that bantam (ban), a microRNA, modulates Dpp signaling activity. Over expression of ban decreases phosphorylated Mothers against decapentaplegic (Mad) levels and negatively affects Dpp pathway transcriptional target genes, while null mutant clones of ban upregulate the pathway. We provide evidence that dpp upregulates ban in the wing imaginal disc, and attenuation of Dpp signaling results in a reduction of ban expression, showing that they function in a feedback loop. Furthermore, we show that this feedback loop is important for maintaining anterior-posterior compartment boundary stability in the wing disc through regulation of optomotor blind (omb), a known target of the pathway. Our results support a model that ban functions with dpp in a negative feedback loop.

Anchor negatively regulates BMP signalling to control Drosophila wing development

G protein-coupled receptors play particularly important roles in many organisms. The novel Drosophila gene anchor is an orthologue of vertebrate GPR155. However, the roles of anchor in molecular functions and biological processes, especially in wing development, remain unknown. Knockdown of anchor resulted in an increased wing size and additional and thickened veins. These abnormal wing phenotypes were similar to those observed in BMP signalling gain-of-function experiments. We observed that the BMP signalling indicator p-Mad was significantly increased in wing discs in which anchor RNAi was induced in larvae and accumulated abnormally in intervein regions in pupae. Furthermore, the expression of target genes of the BMP signalling pathway was examined using a lacZ reporter, and the results indicated that omb and sal were substantially increased in anchor-knockdown wing discs. An investigation of genetic interactions between Anchor and the BMP signalling pathway revealed that the thickened and ectopic vein tissues were rescued by knocking down BMP levels. These results suggested that Anchor functions to negatively regulate BMP signalling during wing development and vein formation.

TGF-β Family Signaling in Drosophila

The transforming growth factor β (TGF-β) family signaling pathway is conserved and ubiquitous in animals. In Drosophila, fewer representatives of each signaling component are present compared with vertebrates, simplifying mechanistic study of the pathway. Although there are fewer family members, the TGF-β family pathway still regulates multiple and diverse functions in Drosophila. In this review, we focus our attention on several of the classic and best-studied functions for TGF-β family signaling in regulating Drosophila developmental processes such as embryonic and imaginal disc patterning, but we also describe several recently discovered roles in regulating hormonal, physiological, neuronal, innate immunity, and tissue homeostatic processes.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Signaling Cross Talk between TGF-β/Smad and Other Signaling Pathways

Cytokines of the transforming growth factor β (TGF-β) family, including TGF-βs, bone morphogenic proteins (BMPs), activins, and Nodal, play crucial roles in embryonic development and adult tissue homeostasis by regulating cell proliferation, survival, and differentiation, as well as stem-cell self-renewal and lineage-specific differentiation. Smad proteins are critical downstream mediators of these signaling activities. In addition to regulating the transcription of direct target genes of TGF-β, BMP, activin, or Nodal, Smad proteins also participate in extensive cross talk with other signaling pathways, often in a cell-type- or developmental stage-specific manner. These combinatorial signals often produce context-, time-, and location-dependent biological outcomes that are critical for development. This review discusses recent progress in our understanding of the cross talk between Smad proteins and signaling pathways of Wnt, Notch, Hippo, Hedgehog (Hh), mitogen-activated protein (MAP), kinase, phosphoinositide 3-kinase (PI3K)-Akt, nuclear factor κB (NF-κB), and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

Drosophila Dullard functions as a Mad phosphatase to terminate BMP signaling

Bone morphogenetic proteins (BMPs) are growth factors that provide essential signals for normal embryonic development and adult tissue homeostasis. A key step in initiating BMP signaling is ligand induced phosphorylation of receptor Smads (R-Smads) by type I receptor kinases, while linker phosphorylation of R-Smads has been shown to cause BMP signal termination. Here we present data demonstrating that the phosphatase Dullard is involved in dephosphorylating the Drosophila R-Smad, Mad, and is integral in controlling BMP signal duration. We show that a hypomorphic Dullard allele or Dullard knockdown leads to increased Mad phosphorylation levels, while Dullard overexpression resulted in reduced Mad phosphorylations. Co-immunoprecipitation binding assays demonstrate phosphorylated Mad and Dullard physically interact, while mutation of Dullard’s phosphatase domain still allowed Mad-Dullard interactions but abolished its ability to regulate Mad phosphorylations. Finally, we demonstrate that linker and C-terminally phosphorylated Mad can be regulated by one of two terminating mechanisms, degradation by proteasomes or dephosphorylation by the phosphatase Dullard.

dFoxO promotes Wingless signaling in Drosophila

The Wnt/β-catenin signaling is an evolutionarily conserved pathway that regulates a wide range of physiological functions, including embryogenesis, organ maintenance, cell proliferation and cell fate decision. Dysregulation of Wnt/β-catenin signaling has been implicated in various cancers, but its role in cell death has not yet been fully elucidated. Here we show that activation of Wg signaling induces cell death in Drosophila eyes and wings, which depends on dFoxO, a transcription factor known to be involved in cell death. In addition, dFoxO is required for ectopic and endogenous Wg signaling to regulate wing patterning. Moreover, dFoxO is necessary for activated Wg signaling-induced target genes expression. Furthermore, Arm is reciprocally required for dFoxO-induced cell death. Finally, dFoxO physically interacts with Arm both in vitro and in vivo. Thus, we have characterized a previously unknown role of dFoxO in promoting Wg signaling, and that a dFoxO-Arm complex is likely involved in their mutual functions, e.g. cell death.

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 expression of the T-box selector gene midline in the leg imaginal disc is controlled by both transcriptional regulation and cell lineage

The Drosophila Tbx20 homologs midline and H15 act as selector genes for ventral fate in Drosophila legs. midline and H15 expression defines the ventral domain of the leg and the two genes are necessary and sufficient for the development of ventral fate. Ventral-specific expression of midline and H15 is activated by Wingless (Wg) and repressed by Decapentaplegic (Dpp). Here we identify VLE, a 5 kb enhancer that drives ventral specific expression in the leg disc that is very similar to midline expression. Subdivision of VLE identifies two regions that mediate both activation and repression and third region that only mediates repression. Loss- and gain-of-function genetic mosaic analysis shows that the activating and repressing regions respond to Wg and Dpp signaling respectively. All three repression regions depend on the activity of Mothers-against-decapentaplegic, a Drosophila r-Smad that mediates Dpp signaling, and respond to ectopic expression of the Dpp target genes optomoter-blind and Dorsocross 3. However, only one repression region is responsive to loss of schnurri, a co-repressor required for direct repression by Dpp-signaling. Thus, Dpp signaling restricts midline expression through both direct repression and through the activation of downstream repressors. We also find that midline and H15 expression are both subject to cross-repression and feedback inhibition. Finally, a lineage analysis indicates that ventral midline-expressing cells and dorsal omb-expressing cells do not mix during development. Together this data indicates that the ventral-specific expression of midline results from both transcriptional regulation and from a lack of cell-mixing between dorsal and ventral cells.

Efficient retina formation requires suppression of both Activin and BMP signaling pathways in pluripotent cells

Retina formation requires the correct spatiotemporal patterning of key regulatory factors. While it is known that repression of several signaling pathways lead to specification of retinal fates, addition of only Noggin, a known BMP antagonist, can convert pluripotent Xenopus laevis animal cap cells to functional retinal cells. The aim of this study is to determine the intracellular molecular events that occur during this conversion. Surprisingly, blocking BMP signaling alone failed to mimic Noggin treatment. Overexpressing Noggin in pluripotent cells resulted in a concentration-dependent suppression of both Smad1 and Smad2 phosphorylation, which act downstream of BMP and Activin signaling, respectively. This caused a decrease in downstream targets: endothelial marker, xk81, and mesodermal marker, xbra. We treated pluripotent cells with dominant-negative receptors or the chemical inhibitors, dorsomorphin and SB431542, which each target either the BMP or Activin signaling pathway. We determined the effect of these treatments on retina formation using the Animal Cap Transplant (ACT) assay; in which treated pluripotent cells were transplanted into the eye field of host embryos. We found that inhibition of Activin signaling, in the presence of BMP signaling inhibition, promotes efficient retinal specification in Xenopus tissue, mimicking the affect of adding Noggin alone. In whole embryos, we found that the eye field marker, rax, expanded when adding both dominant-negative Smad1 and Smad2, as did treating the cells with both dorsomorphin and SB431542. Future studies could translate these findings to a mammalian culture assay, in order to more efficiently produce retinal cells in culture.

Mad linker phosphorylations control the intensity and range of the BMP-activity gradient in developing Drosophila tissues

The BMP ligand Dpp, operates as a long range morphogen to control many important functions during Drosophila development from tissue patterning to growth. The BMP signal is transduced intracellularly via C-terminal phosphorylation of the BMP transcription factor Mad, which forms an activity gradient in developing embryonic tissues. Here we show that Cyclin dependent kinase 8 and Shaggy phosphorylate three Mad linker serines. We demonstrate that linker phosphorylations control the peak intensity and range of the BMP signal across rapidly developing embryonic tissues. Shaggy knockdown broadened the range of the BMP-activity gradient and increased high threshold target gene expression in the early embryo, while expression of a Mad linker mutant in the wing disc resulted in enhanced levels of C-terminally phosphorylated Mad, a 30% increase in wing tissue, and elevated BMP target genes. In conclusion, our results describe how Mad linker phosphorylations work to control the peak intensity and range of the BMP signal in rapidly developing Drosophila tissues.

Long-chain Acyl-CoA synthetase 4A regulates Smad activity and dorsoventral patterning in the zebrafish embryo

Long-chain polyunsaturated fatty acids (LC-PUFA) and their metabolites are critical players in cell biology and embryonic development. Here we show that long-chain acyl-CoA synthetase 4a (Acsl4a), an LC-PUFA activating enzyme, is essential for proper patterning of the zebrafish dorsoventral axis. Loss of Acsl4a results in dorsalized embryos due to attenuated bone morphogenetic protein (Bmp) signaling. We demonstrate that Acsl4a modulates the activity of Smad transcription factors, the downstream mediators of Bmp signaling. Acsl4a promotes the inhibition of p38 mitogen-activated protein kinase and the Akt-mediated inhibition of glycogen synthase kinase 3, critical inhibitors of Smad activity. Consequently, introduction of a constitutively active Akt can rescue the dorsalized phenotype of Acsl4a-deficient embryos. Our results reveal a critical role for Acsl4a in modulating Bmp-Smad activity and provide a potential avenue for LC-PUFAs to influence a variety of developmental processes.

Brain tumor regulates neuromuscular synapse growth and endocytosis in Drosophila by suppressing mad expression

The precise regulation of synaptic growth is critical for the proper formation and plasticity of functional neural circuits. Identification and characterization of factors that regulate synaptic growth and function have been under intensive investigation. Here we report that brain tumor (brat), which was identified as a translational repressor in multiple biological processes, plays a crucial role at Drosophila neuromuscular junction (NMJ) synapses. Immunohistochemical analysis demonstrated that brat mutants exhibited synaptic overgrowth characterized by excess satellite boutons at NMJ terminals, whereas electron microscopy revealed increased synaptic vesicle size but reduced density at active zones compared with wild-types. Spontaneous miniature excitatory junctional potential amplitudes were larger and evoked quantal content was lower at brat mutant NMJs. In agreement with the morphological and physiological phenotypes, loss of Brat resulted in reduced FM1-43 uptake at the NMJ terminals, indicating that brat regulates synaptic endocytosis. Genetic analysis revealed that the actions of Brat at synapses are mediated through mothers against decapentaplegic (Mad), the signal transduction effector of the bone morphogenetic protein (BMP) signaling pathway. Furthermore, biochemical analyses showed upregulated levels of Mad protein but normal mRNA levels in the larval brains of brat mutants, suggesting that Brat suppresses Mad translation. Consistently, knockdown of brat by RNA interference in Drosophila S2 cells also increased Mad protein level. These results together reveal an important and previously unidentified role for Brat in synaptic development and endocytosis mediated by suppression of BMP signaling.

New insights into extracellular and post-translational regulation of TGF-β family signalling pathways

Members of the transforming growth factor-β (TGF-β) family of secreted proteins are present in all multicellular animals. TGF-β proteins are versatile intercellular signalling molecules that orchestrate cell fate decisions during development and maintain homeostasis in adults. The Smad family of signal transducers implements TGF-β signals in responsive cells. Given the ability of TGF-β ligands to induce dramatic responses in target cells, numerous regulatory mechanisms exist to prevent unintended consequences. Here we review new reports of extracellular and post-translational regulation in Drosophila and vertebrates. Extracellular topics include the regulation of TGF-β signalling range and the coordination between tissue morphogenesis and TGF-β signalling. Post-translational topics include the regulation of TGF-β signal transduction by Gsk3-β phosphorylation of Smads and by cycles of Smad mono- and deubiquitylation. Extension of the ubiquitylation data to the Hippo pathway is also discussed.

Activin receptor inhibition by Smad2 regulates Drosophila wing disc patterning through BMP-response elements

Imaginal disc development in Drosophila requires coordinated cellular proliferation and tissue patterning. In our studies of TGFβ superfamily signaling components, we found that a protein null mutation of Smad2, the only Activin subfamily R-Smad in the fruit fly, produces overgrown wing discs that resemble gain of function for BMP subfamily signaling. The wing discs are expanded specifically along the anterior-posterior axis, with increased proliferation in lateral regions. The morphological defect is not observed in mutants for the TGFβ receptor baboon, and epistasis tests showed that baboon is epistatic to Smad2 for disc overgrowth. Rescue experiments indicate that Baboon binding, but not canonical transcription factor activity, of Smad2 is required for normal disc growth. Smad2 mutant discs generate a P-Mad stripe that is narrower and sharper than the normal gradient, and activation targets are correspondingly expressed in narrowed domains. Repression targets of P-Mad are profoundly mis-regulated, with brinker and pentagone reporter expression eliminated in Smad2 mutants. Loss of expression requires a silencer element previously shown to be controlled by BMP signaling. Epistasis experiments show that Baboon, Mad and Schnurri are required to mediate the ectopic silencer output in the absence of Smad2. Taken together, our results show that loss of Smad2 permits promiscuous Baboon activity, which represses genes subject to control by Mad-dependent silencer elements. The absence of Brinker and Pentagone in Smad2 mutants explains the compound wing disc phenotype. Our results highlight the physiological relevance of substrate inhibition of a kinase, and reveal a novel interplay between the Activin and BMP pathways.

Bridging Decapentaplegic and Wingless signaling in Drosophila wings through repression of naked cuticle by Brinker

Wnts and bone morphogenetic proteins (BMPs) are signaling elements that are crucial for a variety of events in animal development. In Drosophila, Wingless (Wg, a Wnt ligand) and Decapentaplegic (Dpp, a BMP homolog) are thought to function through distinct signal transduction pathways and independently direct the patterning of the wing. However, recent studies suggest that Mothers against Dpp (Mad), the key transducer of Dpp signaling, might serve as a node for the crosstalk between these two pathways, and both positive and negative roles of Mad in Wg signaling have been suggested. Here, we describe a novel molecular mechanism by which Dpp signaling suppresses Wg outputs. Brinker (Brk), a transcriptional repressor that is downregulated by Dpp, directly represses naked cuticle (nkd), which encodes a feedback inhibitor of Wg signaling, in vitro and in vivo. Through genetic studies, we demonstrate that Brk is required for Wg target gene expression in fly wing imaginal discs and that loss or gain of brk during wing development mimics loss or gain of Wg signaling, respectively. Finally, we show that Dpp positively regulates the expression of nkd and negatively regulates the Wg target gene Distal-less (Dll). These data support a model in which different signaling pathways interact via a negative-feedback mechanism. Such a mechanism might explain how organs coordinate inputs from multiple signaling cues.

An inwardly rectifying K+ channel is required for patterning

Mutations that disrupt function of the human inwardly rectifying potassium channel KIR2.1 are associated with the craniofacial and digital defects of Andersen-Tawil Syndrome, but the contribution of Kir channels to development is undefined. Deletion of mouse Kir2.1 also causes cleft palate and digital defects. These defects are strikingly similar to phenotypes that result from disrupted TGFβ/BMP signaling. We use Drosophila melanogaster to show that a Kir2.1 homolog, Irk2, affects development by disrupting BMP signaling. Phenotypes of irk2 deficient lines, a mutant irk2 allele, irk2 siRNA and expression of a dominant-negative Irk2 subunit (Irk2DN) all demonstrate that Irk2 function is necessary for development of the adult wing. Compromised Irk2 function causes wing-patterning defects similar to those found when signaling through a Drosophila BMP homolog, Decapentaplegic (Dpp), is disrupted. To determine whether Irk2 plays a role in the Dpp pathway, we generated flies in which both Irk2 and Dpp functions are reduced. Irk2DN phenotypes are enhanced by decreased Dpp signaling. In wild-type flies, Dpp signaling can be detected in stripes along the anterior/posterior boundary of the larval imaginal wing disc. Reducing function of Irk2 with siRNA, an irk2 deletion, or expression of Irk2DN reduces the Dpp signal in the wing disc. As Irk channels contribute to Dpp signaling in flies, a similar role for Kir2.1 in BMP signaling may explain the morphological defects of Andersen-Tawil Syndrome and the Kir2.1 knockout mouse.

Comment on "Dynamics of dpp signaling and proliferation control"

Wartlick et al. (Research Articles, 4 March 2011, p. 1154) reported that growth rates in the Drosophila wing disc correlate with increasing Dpp signaling levels, suggesting that the rate of Dpp increase determines the cell-cycle length. Contradicting their model, we found that cells in which the increase of Dpp signaling levels was genetically abrogated grew at rates comparable to those of wild-type cells.

The kinase Sgg modulates temporal development of macrochaetes in Drosophila by phosphorylation of Scute and Pannier

Evolution of novel structures is often made possible by changes in the timing or spatial expression of genes regulating development. Macrochaetes, large sensory bristles arranged into species-specific stereotypical patterns, are an evolutionary novelty of cyclorraphous flies and are associated with changes in both the temporal and spatial expression of the proneural genes achaete (ac) and scute (sc). Changes in spatial expression are associated with the evolution of cis-regulatory sequences, but it is not known how temporal regulation is achieved. One factor required for ac-sc expression, the expression of which coincides temporally with that of ac-sc in the notum, is Wingless (Wg; also known as Wnt). Wingless downregulates the activity of the serine/threonine kinase Shaggy (Sgg; also known as GSK-3). We demonstrate that Scute is phosphorylated by Sgg on a serine residue and that mutation of this residue results in a form of Sc with heightened proneural activity that can rescue the loss of bristles characteristic of wg mutants. We suggest that the phosphorylated form of Sc has reduced transcriptional activity such that sc is unable to autoregulate, an essential function for the segregation of bristle precursors. Sgg also phosphorylates Pannier, a transcriptional activator of ac-sc, the activity of which is similarly dampened when in the phosphorylated state. Furthermore, we show that Wg signalling does not act directly via a cis-regulatory element of the ac-sc genes. We suggest that temporal control of ac-sc activity in cyclorraphous flies is likely to be regulated by permissive factors and might therefore not be encoded at the level of ac-sc gene sequences.

Regulation of BMP activity and range in Drosophila wing development

Bone morphogenetic protein (BMP) signaling controls development and maintenance of many tissues. Genetic and quantitative approaches in Drosophila reveal that ligand isoforms show distinct function in wing development. Spatiotemporal control of BMP patterning depends on a network of extracellular proteins Pent, Ltl and Dally that regulate BMP signaling strength and morphogen range. BMP-mediated feedback regulation of Pent, Ltl, and Dally expression provides a system where cells actively respond to, and modify, the extracellular morphogen landscape to form a gradient that exhibits remarkable properties, including proportional scaling of BMP patterning with tissue size and the modulation of uniform tissue growth. This system provides valuable insights into mechanisms that mitigate the influence of variability to regulate cell-cell interactions and maintain organ function.

A heat shock protein and Wnt signaling crosstalk during axial patterning and stem cell proliferation

Both Wnt signaling and heat shock proteins play important roles in development and disease. As such, they have been widely, though separately, studied. Here we show a link between a heat shock protein and Wnt signaling in a member of the basal phylum, Cnidaria. A heat shock at late gastrulation in the clonal marine hydrozoan, Hydractinia, interferes with axis development, specifically inhibiting head development, while aboral structures remain unaffected. The heat treatment upregulated Hsc71, a constitutive Hsp70 related gene, followed by a transient upregulation, and long-term downregulation, of Wnt signaling components. Downregulating Hsc71 by RNAi in heat-shocked animals rescued these defects, resulting in normal head development. Transgenic animals, ectopically expressing Hsc71, had similar developmental abnormalities as heat-shocked animals in terms of both morphology and Wnt3 expression. We also found that Hsc71 is upregulated in response to ectopic Wnt activation, but only in the context of stem cell proliferation and not in head development. Hsc71's normal expression is consistent with a conserved role in mitosis and apoptosis inhibition. Our results demonstrate a hitherto unknown crosstalk between heat shock proteins and Wnt/β-catenin signaling. This link likely has important implications in understanding normal development, congenital defects and cancer biology.

Phosphorylation of Mad controls competition between wingless and BMP signaling

Bone morphogenetic proteins (BMPs) and Wnts are growth factors that provide essential patterning signals for cell proliferation and differentiation. Here, we describe a molecular mechanism by which the phosphorylation state of the Drosophila transcription factor Mad determines its ability to transduce either BMP or Wingless (Wg) signals. Previously, Mad was thought to function in gene transcription only when phosphorylated by BMP receptors. We found that the unphosphorylated form of Mad was required for canonical Wg signaling by interacting with the Pangolin-Armadillo transcriptional complex. Phosphorylation of the carboxyl terminus of Mad by BMP receptor directed Mad toward BMP signaling, thereby preventing Mad from functioning in the Wg pathway. The results show that Mad has distinct signal transduction roles in the BMP and Wnt pathways depending on its phosphorylation state.

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

Turing-Child field underlies spatial periodicity in Drosophila and planarians

The regular spatial periodicity manifested in Drosophila gene expression has been considered as a test case for the involvement of a Turing system in biology. It was expected--if such involvement exists--to find a spatially periodic protein distribution where the proteins are Turing morphogens. The failure to find such a periodic distribution of Turing proteins, and the experimental findings of the involvement of different combinations of regulatory proteins and different binding sites for the different stripes of a periodic gene expression, has resulted in the dismissal of the involvement of a Turing system in Drosophila periodicity and segmentation. But if one is willing to allow a Turing system in the level of post-translational modification of proteins instead of in the protein level, one can explain the regular spatial periodicity of gene expression. The source of the spatial periodicity of gene expression does not lie in the regulatory proteins, but in the spatially periodic post-translational modification of these broadly distributed upstream regulatory proteins. The post-translational modification provides the missing spatial information for the regular pattern of 14 stripes. We report that such a field with segmental spatial periodicity that can affect downstream proteins and modify them post-translationally and periodically has been observed. This is the Turing-Child (TC) field. We explain the recent observation in Drosophila of phosphorylated transcription factor distributed with segmental periodicity, the disappearance of the spatially periodic gene expression when the regulatory protein loses its normal ability to be phosphorylated, and the spatially periodic segmental groove formation. Just as the reduction of Turing wavelength causes the appearance of 14 stripes in Drosophila so it causes the appearance of bipolar 2-headed Planaria.

Drosophila Smad2 opposes Mad signaling during wing vein development

In the vertebrates, the BMP/Smad1 and TGF-β/Smad2 signaling pathways execute antagonistic functions in different contexts of development. The differentiation of specific structures results from the balance between these two pathways. For example, the gastrula organizer/node of the vertebrates requires a region of low Smad1 and high Smad2 signaling. In Drosophila, Mad regulates tissue determination and growth in the wing, but the function of dSmad2 in wing patterning is largely unknown. In this study, we used an RNAi loss-of-function approach to investigate dSmad2 signaling during wing development. RNAi-mediated knockdown of dSmad2 caused formation of extra vein tissue, with phenotypes similar to those seen in Dpp/Mad gain-of-function. Clonal analyses revealed that the normal function of dSmad2 is to inhibit the response of wing intervein cells to the extracellular Dpp morphogen gradient that specifies vein formation, as measured by expression of the activated phospho-Mad protein. The effect of dSmad2 depletion in promoting vein differentiation was dependent on Medea, the co-factor shared by Mad and dSmad2. Furthermore, double RNAi experiments showed that Mad is epistatic to dSmad2. In other words, depletion of Smad2 had no effect in Mad-deficient wings. Our results demonstrate a novel role for dSmad2 in opposing Mad-mediated vein formation in the wing. We propose that the main function of dActivin/dSmad2 in Drosophila wing development is to antagonize Dpp/Mad signaling. Possible molecular mechanisms for the opposition between dSmad2 and Mad signaling are discussed.

Spemann's organizer and the self-regulation of embryonic fields

Embryos and developing organs have the remarkable ability of self-regenerating after experimental manipulations. In the Xenopus blastula half-embryos can regenerate the missing part, producing identical twins. Studies on the molecular nature of Spemann's organizer have revealed that self-regulation results from the battle between two signaling centers under reciprocal transcriptional control. Long-range communication between the dorsal and ventral sides is mediated by the action of growth factor antagonists - such as the BMP antagonist Chordin - that regulate the flow of BMPs within the embryonic morphogenetic field. BMPs secreted by the dorsal Spemann organizer tissue are released by metalloproteinases of the Tolloid family, which cleave Chordin at a distance of where they were produced. The dorsal center secretes Chordin, Noggin, BMP2 and ADMP. The ventral center of the embryo secretes BMP4, BMP7, Sizzled, Crossveinless-2 and Tolloid-related. Crossveinless-2 binds Chordin/BMP complexes, facilitating their flow towards the ventral side, where BMPs are released by Tolloid allowing peak BMP signaling. Self-regulation occurs because transcription of ventral genes is induced by BMP while transcription of dorsal genes is repressed by BMP signals. This assures that for each action of Spemann's organizer there is a reaction in the ventral side of the embryo. Because both dorsal and ventral centers express proteins of similar biochemical activities, they can compensate for each other. A novel biochemical pathway of extracellular growth factor signaling regulation has emerged from these studies in Xenopus. This remarkable dorsal-ventral positional information network has been conserved in evolution and is ancestral to all bilateral animals.

Integration of BMP and Wnt signaling via vertebrate Smad1/5/8 and Drosophila Mad

BMPs pattern the dorsal-ventral axis of vertebrate embryos. Smad1/5/8 transduces the BMP signal, and receives phosphorylation inputs from both MAPK and GSK3. Phosphorylation of Smad1 by MAPK and GSK3 result in its polyubiquitination and transport to the centrosome where it is degraded by the proteasome. These linker phosphorylations inhibit BMP/Smad1signaling by shortening its duration. Wnt, which negatively regulates GSK3 activity, prolongs the BMP/Smad1 signal. Remarkably, linker-phosphorylated Smad1 has been shown to be inherited asymmetrically during cell division. Drosophila contains a single Smad1/5/8 homologue, Mad, and is stabilized by phosphorylation-resistant mutations at GSK3 sites, causing Wingless-like effects. We summarize here the significance of linker-phosphorylated Smad1/Mad in relation to signal intensity and duration, and how this integrates the Wnt and BMP pathways during cell differentiation.