A feed-forward loop coupling extracellular BMP transport and morphogenesis in Drosophila wing

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.

Engineered kinases as a tool for phosphorylation of selected targets in vivo

Reversible protein phosphorylation by kinases controls a plethora of processes essential for the proper development and homeostasis of multicellular organisms. One main obstacle in studying the role of a defined kinase–substrate interaction is that kinases form complex signaling networks and most often phosphorylate multiple substrates involved in various cellular processes. In recent years, several new approaches have been developed to control the activity of a given kinase. However, most of them fail to regulate a single protein target, likely hiding the effect of a unique kinase–substrate interaction by pleiotropic effects. To overcome this limitation, we have created protein binder-based engineered kinases that permit a direct, robust, and tissue-specific phosphorylation of fluorescent fusion proteins in vivo. We show the detailed characterization of two engineered kinases based on Rho-associated protein kinase (ROCK) and Src. Expression of synthetic kinases in the developing fly embryo resulted in phosphorylation of their respective GFP-fusion targets, providing for the first time a means to direct the phosphorylation to a chosen and tagged target in vivo. We presume that after careful optimization, the novel approach we describe here can be adapted to other kinases and targets in various eukaryotic genetic systems to regulate specific downstream effectors.

The developing wing crossvein of Drosophila melanogaster: a fascinating model for signaling and morphogenesis

The Drosophila wing has been used as a model for studying tissue growth, morphogenesis and pattern formation. The wing veins of Drosophila are composed of two distinct structures, longitudinal veins and crossveins. Although positional information of longitudinal veins is largely defined in the wing imaginal disc during the larval stage, crossvein primordial cells appear to be naive until the early pupal stage. Here, we first review how wing crossveins have been investigated in the past. Then, the developmental mechanisms underlying crossvein formation are summarized. This review focuses on how a conserved trafficking mechanism of BMP ligands is utilized for crossvein formation, and how various co-factors play roles in sustaining BMP signalling. Recent findings further reveal that crossvein development serves as an excellent model to address how BMP signal and dynamic cellular processes are coupled. This comprehensive review illustrates the uniqueness, scientific value and future perspectives of wing crossvein development as a model.

Mechano-chemical feedback mediated competition for BMP signalling leads to pattern formation

Developmental patterning is thought to be regulated by conserved signalling pathways. Initial patterns are often broad before refining to only those cells that commit to a particular fate. However, the mechanisms by which pattern refinement takes place remain to be addressed. Using the posterior crossvein (PCV) of the Drosophila pupal wing as a model, into which bone morphogenetic protein (BMP) ligand is extracellularly transported to instruct vein patterning, we investigate how pattern refinement is regulated. We found that BMP signalling induces apical enrichment of Myosin II in developing crossvein cells to regulate apical constriction. Live imaging of cellular behaviour indicates that changes in cell shape are dynamic and transient, only being maintained in those cells that retain vein fate competence after refinement. Disrupting cell shape changes throughout the PCV inhibits pattern refinement. In contrast, disrupting cell shape in only a subset of vein cells can result in a loss of BMP signalling. We propose that mechano-chemical feedback leads to competition for the developmental signal which plays a critical role in pattern refinement.

Regulation of spatial distribution of BMP ligands for pattern formation

Bone morphogenetic proteins (BMPs), members of the transforming growth factor-ß (TGF-ß) family, have been shown to contribute to embryogenesis and organogenesis during animal development. Relevant studies provide support for the following concepts: (a) BMP signals are evolutionarily highly conserved as a genetic toolkit; (b) spatiotemporal distributions of BMP signals are precisely controlled at the post-translational level; and (c) the BMP signaling network has been co-opted to adapt to diversified animal development. These concepts originated from the historical findings of the Spemann-Mangold organizer and the subsequent studies about how this organizer functions at the molecular level. In this Commentary, we focus on two topics. First, we review how the BMP morphogen gradient is formed to sustain larval wing imaginal disc and early embryo growth and patterning in Drosophila. Second, we discuss how BMP signal is tightly controlled in a context-dependent manner, and how the signal and tissue dynamics are coupled to facilitate complex tissue structure formation. Finally, we argue how these concepts might be developed in the future for further understanding the significance of BMP signaling in animal development.

Coupling between dynamic 3D tissue architecture and BMP morphogen signaling during Drosophila wing morphogenesis

Tissue morphogenesis is a dynamic process often accompanied by cell patterning and differentiation. Although how conserved growth factor signaling affects cell and tissue shapes has been actively studied, much less is known about how signaling and dynamic morphogenesis are mutually coordinated. Our study shows that BMP signaling and 3D morphogenesis of the Drosophila pupal wing are tightly coupled. These findings are highlighted by the fact that the directionality of BMP signal is changed from lateral planar during the inflation stage to interplanar after re-apposition of the dorsal and ventral wing epithelia. We suspect that the dynamic interplay between planar and interplanar signaling linked to tissue shape changes is likely to be used across species in many developing organs.

At the level of organ formation, tissue morphogenesis drives developmental processes in animals, often involving the rearrangement of two-dimensional (2D) structures into more complex three-dimensional (3D) tissues. These processes can be directed by growth factor signaling pathways. However, little is known about how such morphological changes affect the spatiotemporal distribution of growth factor signaling. Here, using the Drosophila pupal wing, we address how decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signaling and 3D wing morphogenesis are coordinated. Dpp, expressed in the longitudinal veins (LVs) of the pupal wing, initially diffuses laterally within both dorsal and ventral wing epithelia during the inflation stage to regulate cell proliferation. Dpp localization is then refined to the LVs within each epithelial plane, but with active interplanar signaling for vein patterning/differentiation, as the two epithelia appose. Our data further suggest that the 3D architecture of the wing epithelia and the spatial distribution of BMP signaling are tightly coupled, revealing that 3D morphogenesis is an emergent property of the interactions between extracellular signaling and tissue shape changes.

The extracellular protease AdamTS-B inhibits vein formation in the Drosophila wing

Vein patterning in the Drosophila wing provides a powerful tool to study regulation of various signaling pathways. Here we show that the ADAMTS extracellular protease AdamTS-B (CG4096) is expressed in the embryonic wing imaginal disc precursor cells and the wing imaginal disc, and functions to inhibit wing vein formation. Knock-down of AdamTS-B displayed posterior crossveins (PCVs) with either extra branches or deltas, or wider PCVs, and a wandering distal tip of the L5 longitudinal vein. Conversely, over-expression of AdamTS-B resulted in a complete absence of the PCV, an incomplete anterior crossvein, and missing distal end of the L5 longitudinal vein. We conclude that AdamTS-B inhibits wing vein formation through negative regulation of signaling pathways, possibly BMP as well as Egfr, displaying the complexity of roles for this family of extracellular proteases.

Quantitative Morphological Variation in the Developing Drosophila Wing

Quantitative genetic variation in morphology is pervasive in all species and is the basis for the evolution of differences among species. The measurement of morphological form in adults is now beginning to be combined with comparable measurements of form during development. Here we compare the shape of the developing wing to its adult form in a holometabolous insect, Drosophila melanogaster. We used protein expression patterns to measure shape in the developing precursors of the final adult wing. Three developmental stages were studied: late larval third instar, post-pupariation and in the adult fly. We studied wild-type animals in addition to mutants of two genes (shf and ds) that have known effects on adult wing shape and size. Despite experimental noise related to the difficulty of comparing developing structures, we found consistent differences in wing shape and size at each developmental stage between genotypes. Quantitative comparisons of variation arising at different developmental stages with the variation in the final structure enable us to determine when variation arises, and to generate hypotheses about the causes of that variation. In addition we provide linear rules allowing us to link wing morphology in the larva, with wing morphology in the pupa. Our approach provides a framework to analyze quantitative morphological variation in the developing fly wing. This framework should help to characterize the natural variation of the larval and pupal wing shape, and to measure the contribution of the processes occurring during these developmental stages to the natural variation in adult wing morphology.

Wing vein development in the sawfly Athalia rosae is regulated by spatial transcription of Dpp/BMP signaling components

Wing venation among insects serves as an excellent model to address how diversified patterns are produced. Previous studies suggest that evolutionarily conserved Decapentaplegic (Dpp)/Bone Morphogenetic Protein (BMP) signal plays a critical role in wing vein development in the dipteran Drosophila melanogaster and the hymenopteran sawfly Athalia rosae. In sawfly, dpp is ubiquitously expressed in the wing during prepupal stages, but Dpp/BMP signal is localized in the future vein cells. Since localized BMP signaling involves BMP binding protein Crossveinless (Cv), redistribution of BMP ligands appears to be crucial for sawfly wing vein formation. However, how ubiquitously expressed ligands lead to a localized signal remains to be addressed. Here, we found that BMP binding protein short gastrulation (Sog) is highly expressed in the intervein cells. Our data also reveal that BMP type I receptors thickveins (Tkv) and saxophone (Sax) are highly expressed in intervein cells and at lower levels in the vein progenitor cells. RNAi knockdown of Ar-tkv or Ar-sax indicates that both receptors are required for localized BMP signaling in the wing vein progenitor cells. Taken together, our data suggest that spatial transcription of core- and co-factors of the BMP pathway sustain localized BMP signaling during sawfly wing vein development.

The integrated stress response regulates BMP signalling through effects on translation

Background

Developmental pathways must be responsive to the environment. Phosphorylation of eIF2α enables a family of stress-sensing kinases to trigger the integrated stress response (ISR), which has pro-survival and developmental consequences. Bone morphogenetic proteins (BMPs) regulate multiple developmental processes in organisms from insects to mammals.

Results

Here we show in Drosophila that GCN2 antagonises BMP signalling through direct effects on translation and indirectly via the transcription factor crc (dATF4). Expression of a constitutively active GCN2 or loss of the eIF2α phosphatase dPPP1R15 impairs developmental BMP signalling in flies. In cells, inhibition of translation by GCN2 blocks downstream BMP signalling. Moreover, loss of d4E-BP, a target of crc, augments BMP signalling in vitro and rescues tissue development in vivo.

Conclusion

These results identify a novel mechanism by which the ISR modulates BMP signalling during development.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0503-x) contains supplementary material, which is available to authorized users.

Scribbled Optimizes BMP Signaling through Its Receptor Internalization to the Rab5 Endosome and Promote Robust Epithelial Morphogenesis

Epithelial cells are characterized by apical-basal polarity. Intrinsic factors underlying apical-basal polarity are crucial for tissue homeostasis and have often been identified to be tumor suppressors. Patterning and differentiation of epithelia are key processes of epithelial morphogenesis and are frequently regulated by highly conserved extrinsic factors. However, due to the complexity of morphogenesis, the mechanisms of precise interpretation of signal transduction as well as spatiotemporal control of extrinsic cues during dynamic morphogenesis remain poorly understood. Wing posterior crossvein (PCV) formation in Drosophila serves as a unique model to address how epithelial morphogenesis is regulated by secreted growth factors. Decapentaplegic (Dpp), a conserved bone morphogenetic protein (BMP)-type ligand, is directionally trafficked from longitudinal veins (LVs) into the PCV region for patterning and differentiation. Our data reveal that the basolateral determinant Scribbled (Scrib) is required for PCV formation through optimizing BMP signaling. Scrib regulates BMP-type I receptor Thickveins (Tkv) localization at the basolateral region of PCV cells and subsequently facilitates Tkv internalization to Rab5 endosomes, where Tkv is active. BMP signaling also up-regulates scrib transcription in the pupal wing to form a positive feedback loop. Our data reveal a unique mechanism in which intrinsic polarity genes and extrinsic cues are coupled to promote robust morphogenesis.

Epithelial morphogenesis is one of the key processes in animal development. Evolutionarily conserved growth factors frequently instruct patterning and differentiation in morphogenesis. However, little is known about how extracellular cues and epithelial morphogenesis are mutually coordinated in vivo. Wing posterior crossvein (PCV) development in Drosophila provides an excellent system for understanding how bone morphogenetic protein (BMP) signaling regulates patterning and differentiation of epithelia. We find that the apical-basal polarity gene Scribbled (Scrib) is required for PCV formation by optimizing BMP signaling in the PCV region as follows. First, Scrib regulates BMP type-I receptor Thickveins (Tkv) localization basally. Second, Scrib facilitates Tkv internalization to the Rab5 endosomes to optimize signal transduction after receptor-ligand binding. Third, BMP signaling up-regulates scrib transcription in the pupal wing to form a positive feedback loop. These results suggest that coupling between epithelial polarity genes and conserved growth factors play crucial roles in patterning and differentiation of epithelia.

Modulation of BMP signalling by integrins

The bone morphogenetic protein (BMP) pathway is a major conserved signalling pathway with diverse roles in development and homeostasis. Given that cells exist in three-dimensional environments, one important area is to understand how the BMP pathway operates within such complex cellular environments. The extracellular matrix contains information regarding tissue architecture and its mechanical properties that is transmitted to the cell via integrin receptors. In this review, I describe various examples of modulation of the BMP pathway by integrins. In the case of the Drosophila embryo and some cell line-based studies, integrins have been found to enhance BMP responses through different mechanisms, such as enhancement of BMP ligand–receptor binding and effects on Smad phosphorylation or stability. In these contexts, BMP-dependent activation of integrins is a common theme. However, I also discuss examples where integrins inhibit the BMP pathway, highlighting the context-dependent nature of integrin–BMP cross-talk.

lolal Is an Evolutionarily New Epigenetic Regulator of dpp Transcription during Dorsal-Ventral Axis Formation

Secreted ligands in the Dpp/BMP family drive dorsal–ventral (D/V) axis formation in all Bilaterian species. However, maternal factors regulating Dpp/BMP transcription in this process are largely unknown. We identified the BTB domain protein longitudinals lacking-like (lolal) as a modifier of decapentaplegic (dpp) mutations. We show that Lolal is evolutionarily related to the Trithorax group of chromatin regulators and that lolal interacts genetically with the epigenetic factor Trithorax-like during Dpp D/V signaling. Maternally driven Lolal^HA is found in oocytes and translocates to zygotic nuclei prior to the point at which dpp transcription begins. lolal maternal and zygotic mutant embryos display significant reductions in dpp, pMad, and zerknullt expression, but they are never absent. The data suggest that lolal is required to maintain dpp transcription during D/V patterning. Phylogenetic data revealed that lolal is an evolutionarily new gene present only in insects and crustaceans. We conclude that Lolal is the first maternal protein identified with a role in dpp D/V transcriptional maintenance, that Lolal and the epigenetic protein Trithorax-like are essential for Dpp D/V signaling and that the architecture of the Dpp D/V pathway evolved in the arthropod lineage after the separation from vertebrates via the incorporation of new genes such as lolal.

Integration of Orthogonal Signaling by the Notch and Dpp Pathways in Drosophila

The transcription factor Suppressor of Hairless and its coactivator, the Notch intracellular domain, are polyglutamine (pQ)-rich factors that target enhancer elements and interact with other locally bound pQ-rich factors. To understand the functional repertoire of such enhancers, we identify conserved regulatory belts with binding sites for the pQ-rich effectors of both Notch and BMP/Dpp signaling, and the pQ-deficient tissue selectors Apterous (Ap), Scalloped (Sd), and Vestigial (Vg). We find that the densest such binding site cluster in the genome is located in the BMP-inducible nab locus, a homolog of the vertebrate transcriptional cofactors NAB1/NAB2 We report three major findings. First, we find that this nab regulatory belt is a novel enhancer driving dorsal wing margin expression in regions of peak phosphorylated Mad in wing imaginal discs. Second, we show that Ap is developmentally required to license the nab dorsal wing margin enhancer (DWME) to read out Notch and Dpp signaling in the dorsal compartment. Third, we find that the nab DWME is embedded in a complex of intronic enhancers, including a wing quadrant enhancer, a proximal wing disc enhancer, and a larval brain enhancer. This enhancer complex coordinates global nab expression via both tissue-specific activation and interenhancer silencing. We suggest that DWME integration of BMP signaling maintains nab expression in proliferating margin descendants that have divided away from Notch-Delta boundary signaling. As such, uniform expression of genes like nab and vestigial in proliferating compartments would typically require both boundary and nonboundary lineage-specific enhancers.

The Gyc76C Receptor Guanylyl Cyclase and the Foraging cGMP-Dependent Kinase Regulate Extracellular Matrix Organization and BMP Signaling in the Developing Wing of Drosophila melanogaster

The developing crossveins of the wing of Drosophila melanogaster are specified by long-range BMP signaling and are especially sensitive to loss of extracellular modulators of BMP signaling such as the Chordin homolog Short gastrulation (Sog). However, the role of the extracellular matrix in BMP signaling and Sog activity in the crossveins has been poorly explored. Using a genetic mosaic screen for mutations that disrupt BMP signaling and posterior crossvein development, we identify Gyc76C, a member of the receptor guanylyl cyclase family that includes mammalian natriuretic peptide receptors. We show that Gyc76C and the soluble cGMP-dependent kinase Foraging, likely linked by cGMP, are necessary for normal refinement and maintenance of long-range BMP signaling in the posterior crossvein. This does not occur through cell-autonomous crosstalk between cGMP and BMP signal transduction, but likely through altered extracellular activity of Sog. We identify a novel pathway leading from Gyc76C to the organization of the wing extracellular matrix by matrix metalloproteinases, and show that both the extracellular matrix and BMP signaling effects are largely mediated by changes in the activity of matrix metalloproteinases. We discuss parallels and differences between this pathway and other examples of cGMP activity in both Drosophila melanogaster and mammalian cells and tissues.

Signaling between cells regulates many processes, including the choices cells make between different fates during development and regeneration, and misregulation of such signaling underlies many human pathologies. To understand how such signals control developmental decisions, it is necessary to elucidate both how cells regulate and respond to different levels of signaling, and how different types of signals combine and regulate each other. We have used genetic screening in the fruitfly Drosophila melanogaster to identify mutations that reduce or eliminate signals carried by Bone Morphogenetic Proteins (BMPs), and show that BMP signaling is sensitive Gyc76C, a peptide receptor that stimulates the production of cGMP in cells. We identify downstream intracellular effectors of this cGMP activity, but provide evidence that the effects on the BMP pathway are not mediated at the intracellular level, but rather through cGMP’s effects upon the extracellular matrix and matrix-remodeling proteinases, which in turn affects the activity of extracellular BMP-binding proteins. We discuss differences and parallels with other examples of cGMP activity in Drosophila melanogaster and mammals.

Making quantitative morphological variation from basic developmental processes: Where are we? The case of the Drosophila wing

One of the aims of evolutionary developmental biology is to discover the developmental origins of morphological variation. The discipline has mainly focused on qualitative morphological differences (e.g., presence or absence of a structure) between species. Studies addressing subtle, quantitative variation are less common. The Drosophila wing is a model for the study of development and evolution, making it suitable to investigate the developmental mechanisms underlying the subtle quantitative morphological variation observed in nature. Previous reviews have focused on the processes involved in wing differentiation, patterning and growth. Here, we investigate what is known about how the wing achieves its final shape, and what variation in development is capable of generating the variation in wing shape observed in nature. Three major developmental stages need to be considered: larval development, pupariation, and pupal development. The major cellular processes involved in the determination of tissue size and shape are cell proliferation, cell death, oriented cell division and oriented cell intercalation. We review how variation in temporal and spatial distribution of growth and transcription factors affects these cellular mechanisms, which in turn affects wing shape. We then discuss which aspects of the wing morphological variation are predictable on the basis of these mechanisms. Developmental Dynamics 244:1058-1073, 2015. © 2015 Wiley Periodicals, Inc.

Insights into the molecular mechanisms underlying diversified wing venation among insects

Insect wings are great resources for studying morphological diversities in nature as well as in fossil records. Among them, variation in wing venation is one of the most characteristic features of insect species. Venation is therefore, undeniably a key factor of species-specific functional traits of the wings; however, the mechanism underlying wing vein formation among insects largely remains unexplored. Our knowledge of the genetic basis of wing development is solely restricted to Drosophila melanogaster. A critical step in wing vein development in Drosophila is the activation of the decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signalling pathway during pupal stages. A key mechanism is the directional transport of Dpp from the longitudinal veins into the posterior crossvein by BMP-binding proteins, resulting in redistribution of Dpp that reflects wing vein patterns. Recent works on the sawfly Athalia rosae, of the order Hymenoptera, also suggested that the Dpp transport system is required to specify fore- and hindwing vein patterns. Given that Dpp redistribution via transport is likely to be a key mechanism for establishing wing vein patterns, this raises the interesting possibility that distinct wing vein patterns are generated, based on where Dpp is transported. Experimental evidence in Drosophila suggests that the direction of Dpp transport is regulated by prepatterned positional information. These observations lead to the postulation that Dpp generates diversified insect wing vein patterns through species-specific positional information of its directional transport. Extension of these observations in some winged insects will provide further insights into the mechanisms underlying diversified wing venation among insects.

Coordination of wing and whole-body development at developmental milestones ensures robustness against environmental and physiological perturbations

Development produces correctly patterned tissues under a wide range of conditions that alter the rate of development in the whole body. We propose two hypotheses through which tissue patterning could be coordinated with whole-body development to generate this robustness. Our first hypothesis states that tissue patterning is tightly coordinated with whole-body development over time. The second hypothesis is that tissue patterning aligns at developmental milestones. To distinguish between our two hypotheses, we developed a staging scheme for the wing imaginal discs of Drosophila larvae using the expression of canonical patterning genes, linking our scheme to three whole-body developmental events: moulting, larval wandering and pupariation. We used our scheme to explore how the progression of pattern changes when developmental time is altered either by changing temperature or by altering the timing of hormone synthesis that drives developmental progression. We found the expression pattern in the wing disc always aligned at moulting and pupariation, indicating that these key developmental events represent milestones. Between these milestones, the progression of pattern showed greater variability in response to changes in temperature and alterations in physiology. Furthermore, our data showed that discs from wandering larvae showed greater variability in patterning stage. Thus for wing disc patterning, wandering does not appear to be a developmental milestone. Our findings reveal that tissue patterning remains robust against environmental and physiological perturbations by aligning at developmental milestones. Furthermore, our work provides an important glimpse into how the development of individual tissues is coordinated with the body as a whole.

Studies of morphogens: keep calm and carry on

Morphogens are signaling factors that direct cell fate and tissue development at a distance from their source, and various modes of transport and interpretation have been suggested for morphogens. The recent EMBO Workshop on 'Morphogen gradients', which took place in Oxford, UK in June 2013, centered on the formation and interpretation of such morphogen gradients during development. This meeting allowed an exchange of views in light of recent results. Here, we provide a brief overview of the talks, organized in relation to several major themes of discussion at the meeting: (1) morphogen gradient formation; (2) morphogen gradient interpretation; (3) signaling networks and feedback in morphogenesis; (4) emergence of patterns; (5) scaling of patterns; (6) the control of growth; and (7) new techniques in the field.

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.