The Drosophila DPP signal is produced by cleavage of its proprotein at evolutionary diversified furin-recognition sites

The prodomain of bone morphogenetic protein 2 promotes dimerization and cleavage of BMP6 homodimers and BMP2/6 heterodimers

Bone morphogenetic protein 2 (BMP2) and BMP6 are key regulators of systemic iron homeostasis. All BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments, but nothing is known about how BMP2 or BMP6 homodimeric or heterodimeric precursor proteins are proteolytically activated. Here, we conducted in vitro cleavage assays, which revealed that BMP2 is sequentially cleaved by furin at two sites, initially at a site upstream of the mature ligand, and then at a site adjacent to the ligand domain, while BMP6 is cleaved at a single furin motif. Cleavage of both sites of BMP2 is required to generate fully active BMP2 homodimers when expressed in Xenopus embryos or liver endothelial cells, and fully active BMP2/6 heterodimers in Xenopus. We analyzed BMP activity in Xenopus embryos expressing chimeric proteins consisting of the BMP2 prodomain and BMP6 ligand domain, or vice versa. We show that the prodomain of BMP2 is necessary and sufficient to generate active BMP6 homodimers and BMP2/6 heterodimers, whereas the BMP6 prodomain cannot generate active BMP2 homodimers or BMP2/6 heterodimers. We examined BMP2 and BMP6 homodimeric and heterodimeric ligands generated from native and chimeric precursor proteins expressed in Xenopus embryos. Whereas native BMP6 is not cleaved when expressed alone, it is cleaved to generate BMP2/6 heterodimers when co-expressed with BMP2. Furthermore, BMP2-6 chimeras are cleaved to generate BMP6 homodimers. Our findings reveal an important role for the BMP2 prodomain in dimerization and proteolytic activation of BMP6.

The BMP2 prodomain promotes dimerization and cleavage of BMP6 homodimers and BMP2/6 heterodimers in vivo

Bone morphogenetic protein 2 (BMP2) and BMP6 are key regulators of systemic iron homeostasis. All BMPs are generated as inactive precursor proteins that dimerize and are cleaved to generate the bioactive ligand and inactive prodomain fragments, but nothing is known about how BMP2 or BMP6 homodimeric or heterodimeric precursor proteins are proteolytically activated. Here, we conducted in vitro cleavage assays, which revealed that BMP2 is sequentially cleaved by furin at two sites, initially at a site upstream of the mature ligand, and then at a site adjacent to the ligand domain, while BMP6 is cleaved at a single furin motif. Cleavage of both sites of BMP2 is required to generate fully active BMP2 homodimers when expressed in Xenopus embryos or liver endothelial cells, and fully active BMP2/6 heterodimers in Xenopus . We analyzed BMP activity in Xenopus embryos expressing chimeric proteins consisting of the BMP2 prodomain and BMP6 ligand domain, or vice versa. We show that the prodomain of BMP2 is necessary and sufficient to generate active BMP6 homodimers and BMP2/6 heterodimers, whereas the BMP6 prodomain cannot generate active BMP2 homodimers or BMP2/6 heterodimers. We examined BMP2 and BMP6 homodimeric and heterodimeric ligands generated from native and chimeric precursor proteins expressed in Xenopus embryos. Whereas native BMP6 is not cleaved when expressed alone, it is cleaved to generate BMP2/6 heterodimers when co-expressed with BMP2. Furthermore, BMP2-6 chimeras are cleaved to generate BMP6 homodimers. Our findings reveal an important role for the BMP2 prodomain in dimerization and proteolytic activation of BMP6.

Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Bilateria encompass the vast majority of the animal phyla. As the name states, they are bilaterally symmetric, that is with a morphologically clear main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and with a left side being roughly a mirror image of their right side. Bone morphogenetic protein (BMP) signalling has widely conserved functions in the formation and patterning of the second, dorso-ventral (DV) body axis, albeit to different extents in different bilaterian species. Whilst initial findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities amongst these evolutionarily very distant species, more recent analyses featuring other models revealed considerable diversity in the mechanisms underlying dorsoventral patterning. In fact, as phylogenetic sampling becomes broader, we find that this axis patterning system is so evolvable that even its core components can be deployed differently or lost in different model organisms. In this review, we will try to highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not belong to Bilateria. Future research combining functional analyses and modelling is bound to give us some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie.

Proprotein convertase cleavage of Ictalurid herpesvirus 1 spike-like protein ORF46 is modulated by N-glycosylation

Viral spike proteins undergo a special maturation process that enables host cell receptor recognition, membrane fusion, and viral entry, facilitating effective virus infection. Here, we investigated the protease cleavage features of ORF46, a spike-like protein in Ictalurid herpesvirus 1 (IcHV-1) sharing similarity with spikes of Nidovirales members. We noted that during cleavage, full-length ORF46 is cleaved into ∼55-kDa and ∼100-kDa subunits. Moreover, truncation or site-directed mutagenesis at the recognition sites of proprotein convertases (PCs) abolishes this spike cleavage, highlighting the crucial role of Arg506/Arg507 and Arg668/Arg671 for the cleavage modification. ORF46 cleavage was suppressed by specific N-glycosylation inhibitors or mutation of its specific N-glycosylation sites (N192, etc.), suggesting that glycoprotein ORF46 cleavage is modulated by N-glycosylation. Notably, PCs and N-glycosylation inhibitors exhibited potent antiviral effects in host cells. Our findings, therefore, suggested that PCs cleavage of ORF46, modulated by N-glycosylation, is a potent antiviral target for fish herpesviruses.

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.

aristaless1 has a dual role in appendage formation and wing color specification during butterfly development

BACKGROUND

Highly diverse butterfly wing patterns have emerged as a powerful system for understanding the genetic basis of phenotypic variation. While the genetic basis of this pattern variation is being clarified, the precise developmental pathways linking genotype to phenotype are not well understood. The gene aristaless, which plays a role in appendage patterning and extension, has been duplicated in Lepidoptera. One copy, aristaless1, has been shown to control a white/yellow color switch in the butterfly Heliconius cydno, suggesting a novel function associated with color patterning and pigmentation. Here we investigate the developmental basis of al1 in embryos, larvae, and pupae using new antibodies, CRISPR/Cas9, RNAi, qPCR assays of downstream targets, and pharmacological manipulation of an upstream activator.

RESULTS

We find that Al1 is expressed at the distal tips of developing embryonic appendages consistent with its ancestral role. In developing wings, we observe Al1 accumulation within developing scale cells of white H. cydno during early pupation while yellow scale cells exhibit little Al1 at this time point. Reduced Al1 expression is also associated with yellow scale development in al1 knockouts and knockdowns. We propose that Al1 expression in future white scales might be related to an observed downregulation of the enzyme Cinnabar and other genes that synthesize and transport the yellow pigment, 3-hydroxykynurenine (3-OHK). Finally, we provide evidence that Al1 activation is under the control of Wnt signaling.

CONCLUSIONS

We propose a model in which high levels of Al1 during early pupation, which are mediated by Wnt, are important for melanic pigmentation and specifying white portions of the wing while reduced levels of Al1 during early pupation promote upregulation of proteins needed to move and synthesize 3-OHK, promoting yellow pigmentation. In addition, we discuss how the ancestral role of aristaless in appendage extension may be relevant in understanding the cellular mechanism behind color patterning in the context of the heterochrony hypothesis.

Molecular Evolution of Transforming Growth Factor-β (TGF-β) Gene Family and the Functional Characterization of Lamprey TGF-β2

The transforming growth factor-βs (TGF-βs) are multifunctional cytokines capable of regulating a wide range of cellular behaviors and play a key role in maintaining the homeostasis of the immune system. The TGF-β subfamily, which is only present in deuterostomes, expands from a single gene in invertebrates to multiple members in jawed vertebrates. However, the evolutionary processes of the TGF-β subfamily in vertebrates still lack sufficient elucidation. In this study, the TGF-β homologs are identified at the genome-wide level in the reissner lamprey (Lethenteron reissneri), the sea lamprey (Petromyzon marinus), and the Japanese lamprey (Lampetra japonica), which are the extant representatives of jawless vertebrates with a history of more than 350 million years. The molecular evolutionary analyses reveal that the lamprey TGF-β subfamily contains two members representing ancestors of TGF-β2 and 3 in vertebrates, respectively, but TGF-β1 is absent. The transcriptional expression patterns show that the lamprey TGF-β2 may play a central regulatory role in the innate immune response of the lamprey since it exhibits a more rapid and significant upregulation of expression than TGF-β3 during lipopolysaccharide stimuli. The incorporation of BrdU assay reveals that the lamprey TGF-β2 recombinant protein exerts the bipolar regulation on the proliferation of the supraneural myeloid body cells (SMB cells) in the quiescent and LPS-activated state, while plays an inhibitory role in the proliferation of quiescent and activated leukocytes in lampreys. Furthermore, caspase-3/7 activity analysis indicates that the lamprey TGF-β2 protects SMB cells from apoptosis after serum deprivation, in contrast to promoting apoptosis of leukocytes. Our composite results offer valuable clues to the origin and evolution of the TGF-β subfamily and imply that TGF-βs are among the most ancestral immune regulators in vertebrates.

Zika virus NS3 protease induces bone morphogenetic protein-dependent brain calcification in human fetuses

The most frequent fetal birth defect associated with prenatal Zika virus (ZIKV) infection is brain calcification, which in turn may potentially affect neurological development in infants. Understanding the mechanism could inform the development of potential therapies against prenatal ZIKV brain calcification. In perivascular cells, bone morphogenetic protein (BMP) is an osteogenic factor that undergoes maturation to activate osteogenesis and calcification. Here, we show that ZIKV infection of cultivated primary human brain pericytes triggers BMP2 maturation, leading to osteogenic gene expression and calcification. We observed extensive calcification near ZIKV⁺ pericytes of fetal human brain specimens and in vertically transmitted ZIKV⁺ human signal transducer and activator of transcription 2-knockin mouse pup brains. ZIKV infection of primary pericytes stimulated BMP2 maturation, inducing osteogenic gene expression and calcification that were completely blocked by anti-BMP2/4 neutralizing antibody. Not only did ZIKV NS3 expression alone induce BMP2 maturation, osteogenic gene expression and calcification, but purified NS3 protease also effectively cleaved pro-BMP2 in vitro to generate biologically active mature BMP2. These findings highlight ZIKV-induced calcification where the NS3 protease subverts the BMP2-mediated osteogenic signalling pathway to trigger brain calcification.

Proteolytic Cleavages in the VEGF Family: Generating Diversity among Angiogenic VEGFs, Essential for the Activation of Lymphangiogenic VEGFs

Specific proteolytic cleavages turn on, modify, or turn off the activity of vascular endothelial growth factors (VEGFs). Proteolysis is most prominent among the lymph-angiogenic VEGF-C and VEGF-D, which are synthesized as precursors that need to undergo enzymatic removal of their C- and N-terminal propeptides before they can activate their receptors. At least five different proteases mediate the activating cleavage of VEGF-C: plasmin, ADAMTS3, prostate-specific antigen, cathepsin D, and thrombin. All of these proteases except for ADAMTS3 can also activate VEGF-D. Processing by different proteases results in distinct forms of the "mature" growth factors, which differ in affinity and receptor activation potential. The "default" VEGF-C-activating enzyme ADAMTS3 does not activate VEGF-D, and therefore, VEGF-C and VEGF-D do function in different contexts. VEGF-C itself is also regulated in different contexts by distinct proteases. During embryonic development, ADAMTS3 activates VEGF-C. The other activating proteases are likely important for non-developmental lymphangiogenesis during, e.g., tissue regeneration, inflammation, immune response, and pathological tumor-associated lymphangiogenesis. The better we understand these events at the molecular level, the greater our chances of developing successful therapies targeting VEGF-C and VEGF-D for diseases involving the lymphatics such as lymphedema or cancer.

T-box transcription factors Dorsocross and optomotor-blind control Drosophila leg patterning in a functionally redundant manner

The T-box genes are essential transcription factors during limb development. In Drosophila, Dorsocross (Doc) and optomotor-blind (omb), members of the Tbx2 and Tbx6 families, are best studied in the Drosophila wing development. Despite prominently expressed in leg discs, the specific function of these genes in leg growth is still not revealed. Here we demonstrated that Doc and omb regulated the morphogenesis of leg intermediate regions in a functionally redundant manner. Loss of Doc or omb individually did not result in any developmental defects of the legs, but loss of both genes induced significant defects in femur and proximal tibia of the adult legs. These genes located in the dorsal domain, where the Doc region expanded and cross-overlapped with the omb region corresponding to the presumptive leg intermediate region. We detected that the normal epithelial folds in the leg discs were disrupted along with dorsal repression of cell proliferation and activation of cell apoptosis when Doc and omb were both reduced. Furthermore, the dorsal expression of dachshund (dac), a canonical leg developmental gene specifying the leg intermediate region, was maintained by Doc and omb. Meanwhile, the Notch pathway was compromised in the dorsal domain when these genes were reduced, which might contribute to the joint defect of the adult leg intermediate regions. Our study provides cytological and genetic evidence for understanding the redundant function of Doc and omb in leg morphogenesis.

Identification of Genes Involved in the Differentiation of R7y and R7p Photoreceptor Cells in Drosophila

The R7 and R8 photoreceptor cells of the Drosophila compound eye mediate color vision. Throughout the majority of the eye, these cells occur in two principal types of ommatidia. Approximately 35% of ommatidia are of the pale type and express Rh3 in R7 cells and Rh5 in R8 cells. The remaining 65% are of the yellow type and express Rh4 in R7 cells and Rh6 in R8 cells. The specification of an R8 cell in a pale or yellow ommatidium depends on the fate of the adjacent R7 cell. However, pale and yellow R7 cells are specified by a stochastic process that requires the genes spineless, tango and klumpfuss. To identify additional genes involved in this process we performed genetic screens using a collection of 480 P{EP} transposon insertion strains. We identified genes in gain of function and loss of function screens that significantly altered the percentage of Rh3 expressing R7 cells (Rh3%) from wild-type. 36 strains resulted in altered Rh3% in the gain of function screen where the P{EP} insertion strains were crossed to a sevEP-GAL4 driver line. 53 strains resulted in altered Rh3% in the heterozygous loss of function screen. 4 strains showed effects that differed between the two screens, suggesting that the effect found in the gain of function screen was either larger than, or potentially masked by, the P{EP} insertion alone. Analyses of homozygotes validated many of the candidates identified. These results suggest that R7 cell fate specification is sensitive to perturbations in mRNA transcription, splicing and localization, growth inhibition, post-translational protein modification, cleavage and secretion, hedgehog signaling, ubiquitin protease activity, GTPase activation, actin and cytoskeletal regulation, and Ser/Thr kinase activity, among other diverse signaling and cell biological processes.

Drosophila jumu modulates apoptosis via a JNK-dependent pathway and is required for other processes in wing development

Previous studies in several model organisms have revealed that members of the Forkhead (Fkh) transcription factor family have multiple functions. Drosophila Jumeau (Jumu), a member of this family, participates in cardiogenesis, hematopoiesis and immune system homeostasis. Here, we show that loss of jumu function positively regulates or triggers apoptosis via a JNK-dependent pathway in wing development. jumu mutants showed reduced wing size and increased apoptosis. Moreover, we observed a loss of the anterior cross vein (ACV) phenotype that was similar to that observed in wings in which JNK signaling has been ectopically activated. The JNK signaling markers puckered (puc) and p-JNK were also significantly increased in the wing discs of jumu mutants. In addition, apoptosis induced by the loss of jumu was rescued by knocking down JNK, indicating a role for JNK in reducing jumu-induced apoptosis. Jumu could also control wing margin development via the positive regulation of cut expression, and the observed wing margin defect did not result from a loss of jumu-induced apoptosis. Further, jumu deficiency in the pupal wing could induce multiple wing hairs via a Rho1-mediated planar cell polarity pathway, but abnormal Rho1 expression was not why jumu loss induced apoptosis via a JNK-dependent pathway in wing discs.

A Neuron-Glial Trans-Signaling Cascade Mediates LRRK2-Induced Neurodegeneration

Pathogenic mutations in leucine-rich repeat kinase 2 (LRRK2) induce an age-dependent loss of dopaminergic (DA) neurons. We have identified Furin 1, a pro-protein convertase, as a translational target of LRRK2 in DA neurons. Transgenic knockdown of Furin1 or its substrate the bone morphogenic protein (BMP) ligand glass bottom boat (Gbb) protects against LRRK2-induced loss of DA neurons. LRRK2 enhances the accumulation of phosphorylated Mad (pMad) in the nuclei of glial cells in the vicinity of DA neurons but not in DA neurons. Consistently, exposure to paraquat enhances Furin 1 levels in DA neurons and induces BMP signaling in glia. In support of a neuron-glial signaling model, knocking down BMP pathway members only in glia, but not in neurons, can protect against paraquat toxicity. We propose that a neuron-glial BMP-signaling cascade is critical for mediating age-dependent neurodegeneration in two models of Parkinson's disease, thus opening avenues for future therapeutic interventions.

TGF-β Prodomain Alignments Reveal Unexpected Cysteine Conservation Consistent with Phylogenetic Predictions of Cross-Subfamily Heterodimerization

Evolutionary relationships between prodomains in the TGF-β family have gone unanalyzed due to a perceived lack of conservation. We developed a novel approach, identified these relationships, and suggest hypotheses for new regulatory mechanisms in TGF-β signaling. First, a quantitative analysis placed each family member from flies, mice, and nematodes into the Activin, BMP, or TGF-β subfamily. Second, we defined the prodomain and ligand via the consensus cleavage site. Third, we generated alignments and trees from the prodomain, ligand, and full-length sequences independently for each subfamily. Prodomain alignments revealed that six structural features of 17 are well conserved: three in the straitjacket and three in the arm. Alignments also revealed unexpected cysteine conservation in the "LTBP-Association region" upstream of the straitjacket and in β8 of the bowtie in 14 proteins from all three subfamilies. In prodomain trees, eight clusters across all three subfamilies were present that were not seen in the ligand or full-length trees, suggesting prodomain-mediated cross-subfamily heterodimerization. Consistency between cysteine conservation and prodomain clustering provides support for heterodimerization predictions. Overall, our analysis suggests that cross-subfamily interactions are more common than currently appreciated and our predictions generate numerous testable hypotheses about TGF-β function and evolution.

Muscle-derived Dpp regulates feeding initiation via endocrine modulation of brain dopamine biosynthesis

In animals, the brain regulates feeding behavior in response to local energy demands of peripheral tissues, which secrete orexigenic and anorexigenic hormones. Although skeletal muscle is a key peripheral tissue, it remains unknown whether muscle-secreted hormones regulate feeding. In Drosophila, we found that decapentaplegic (dpp), the homolog of human bone morphogenetic proteins BMP2 and BMP4, is a muscle-secreted factor (a myokine) that is induced by nutrient sensing and that circulates and signals to the brain. Muscle-restricted dpp RNAi promotes foraging and feeding initiation, whereas dpp overexpression reduces it. This regulation of feeding by muscle-derived Dpp stems from modulation of brain tyrosine hydroxylase (TH) expression and dopamine biosynthesis. Consistently, Dpp receptor signaling in dopaminergic neurons regulates TH expression and feeding initiation via the downstream transcriptional repressor Schnurri. Moreover, pharmacologic modulation of TH activity rescues the changes in feeding initiation due to modulation of dpp expression in muscle. These findings indicate that muscle-to-brain endocrine signaling mediated by the myokine Dpp regulates feeding behavior.

Ex vivo Drosophila Wing Imaginal Disc Culture and Furin Inhibitor Assay

Furin is an evolutionarily conserved proprotein convertase (PC) family enzyme with a broad range of substrates that are essential for developmental, homeostatic, and disease pathways. Classical genetic approaches and in vitro biochemical or cell biological assays identified that precursor forms of most growth factor family proteins are processed by Furin. To quantitatively assess the potential role of Furin in cleaving and modulating intercellular dispersion of a Drosophila signaling protein, we developed a simple assay by combining genetics, ex vivo organ culture, pharmacological treatment, and imaging analyses. The protocol herein describes how to ex vivo culture Drosophila wing imaginal discs expressing a fluorescently tagged Drosophila Fibroblast Growth Factor (FGF, Branchless/Bnl) over a long period of time in the presence of Furin inhibitors and monitor the cleavage and intercellular dispersion of the truncated Bnl parts using microscopy. Although the assay described here is for assessing the effect of Furin inhibition on Bnl cleavage in the Drosophila larval wing imaginal disc, the principle and methodology can easily be adopted for any other signals, tissue systems, or organisms. This strategy and protocol provide an assay for examining Furin activity on a specific substrate by directly visualizing the spatiotemporal distribution of its truncated parts in an ex vivo-cultured organ.

Drosophila FGF cleavage is required for efficient intracellular sorting and intercellular dispersal

Bnl controls tracheal development in Drosophila, but it is unclear how this fibroblast growth factor is prepared for tissue-specific dispersal. Sohr et al. find that Furin1 cleaves Bnl in the Golgi, which polarizes its sorting to the basal surface of the source cells and determines its range of cytoneme-mediated intercellular dispersion, signaling, and branching morphogenesis.

How morphogenetic signals are prepared for intercellular dispersal and signaling is fundamental to the understanding of tissue morphogenesis. We discovered an intracellular mechanism that prepares Drosophila melanogaster FGF Branchless (Bnl) for cytoneme-mediated intercellular dispersal during the development of the larval Air-Sac-Primordium (ASP). Wing-disc cells express Bnl as a proprotein that is cleaved by Furin1 in the Golgi. Truncated Bnl sorts asymmetrically to the basal surface, where it is received by cytonemes that extend from the recipient ASP cells. Uncleavable mutant Bnl has signaling activity but is mistargeted to the apical side, reducing its bioavailability. Since Bnl signaling levels feedback control cytoneme production in the ASP, the reduced availability of mutant Bnl on the source basal surface decreases ASP cytoneme numbers, leading to a reduced range of signal/signaling gradient and impaired ASP growth. Thus, enzymatic cleavage ensures polarized intracellular sorting and availability of Bnl to its signaling site, thereby determining its tissue-specific intercellular dispersal and signaling range.

Proteolytic Activation of Bmps: Analysis of Cleavage in Xenopus Oocytes and Embryos

Bone morphogenetic proteins (Bmps) are synthesized as inactive precursors that are cleaved to generate active ligands, along with prodomain fragments that can modulate growth factor activity. Here we provide three protocols that can be used to examine the process of proteolytic activation of Bmps. The first protocol describes how to generate radiolabeled Bmp precursor proteins in Xenopus oocytes and then analyze the time course of precursor cleavage by recombinant enzymes in vitro. The second protocol details how to analyze cleavage of radiolabeled precursor proteins in Xenopus oocytes over time using pulse-chase analysis and autoradiography. This protocol can also be used to analyze folding and cleavage of radiolabeled precursor proteins at steady state. Finally, the third protocol details methods for isolating Bmp cleavage products from the blastocoele of Xenopus embryos and then analyzing them on immunoblots.

Analysis of novel domain-specific mutations in the zebrafish ndr2/cyclops gene generated using CRISPR-Cas9 RNPs

Nodal-related protein (ndr2) is amember of the transforming growth factor type β superfamily of factors and is required for ventral midline patterning of the embryonic central nervous system in zebrafish. In humans, mutations in the gene encoding nodal cause holoprosencephaly and heterotaxy. Mutations in the ndr2 gene in the zebrafish (Danio rerio) lead to similar phenotypes, including loss of the medial floor plate, severe deficits in ventral forebrain development and cyclopia. Alleles of the ndr2 gene have been useful in studying patterning of ventral structures of the central nervous system. Fifteen different ndr2 alleles have been reported in zebrafish, of which eight were generated using chemical mutagenesis, four were radiation-induced and the remaining alleles were obtained via random insertion, gene targeting (TALEN) or unknown methods. Therefore, most mutation sites were random and could not be predicted a priori. Using the CRISPR-Cas9 system from Streptococcus pyogenes, we targeted distinct regions in all three exons of zebrafish ndr2 and observed cyclopia in the injected (G₀) embryos.We show that the use of sgRNA-Cas9 ribonucleoprotein (RNP) complexes can cause penetrant cyclopic phenotypes in injected (G₀) embryos. Targeted polymerase chain reaction amplicon analysis using Sanger sequencing showed that most of the alleles had small indels resulting in frameshifts. The sequence information correlates with the loss of ndr2 activity. In this study, we validate multiple CRISPR targets using an in vitro nuclease assay and in vivo analysis using embryos. We describe one specific mutant allele resulting in the loss of conserved terminal cysteine-coding sequences. This study is another demonstration of the utility of the CRISPR-Cas9 system in generating domain-specific mutations and provides further insights into the structure-function of the ndr2 gene.

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.

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.

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.

Regulation of neuroblast proliferation by surface glia in the Drosophila larval brain

Despite the importance of precisely regulating stem cell division, the molecular basis for this control is still elusive. Here, we show that surface glia in the developing Drosophila brain play essential roles in regulating the proliferation of neural stem cells, neuroblasts (NBs). We found that two classes of extracellular factors, Dally-like (Dlp), a heparan sulfate proteoglycan, and Glass bottom boat (Gbb), a BMP homologue, are required for proper NB proliferation. Interestingly, Dlp expressed in perineural glia (PG), the most outer layer of the surface glia, is responsible for NB proliferation. Consistent with this finding, functional ablation of PG using a dominant-negative form of dynamin showed that PG has an instructive role in regulating NB proliferation. Gbb acts not only as an autocrine proliferation factor in NBs but also as a paracrine survival signal in the PG. We propose that bidirectional communication between NBs and glia through TGF-β signaling influences mutual development of these two cell types. We also discuss the possibility that PG and NBs communicate via direct membrane contact or transcytotic transport of membrane components. Thus, our study shows that the surface glia acts not only as a simple structural insulator but also a dynamic regulator of brain development.

Diet regulates membrane extension and survival of niche escort cells for germline homeostasis via insulin signaling

Diet is an important regulator of stem cell homeostasis, however, the underlying mechanisms of this regulation are not fully known. Here, we report that insulin signaling mediates dietary maintenance of Drosophila ovarian germline stem cells (GSCs) by promoting the extension of niche escort cell (EC) membranes to wrap around GSCs. This wrapping may facilitate the delivery of BMP stemness factors from ECs in the niche to GSCs. In addition to the effects on GSCs, insulin signaling-mediated regulation of EC number and protrusions controls the division and growth of GSC progeny. The effects of insulin signaling on EC membrane extension are, at least in part, driven by enhanced translation of Failed axon connections (Fax) via Ribosomal protein S6 kinase. Fax is a membrane protein that may participate in Abl-regulated cytoskeletal dynamics and is known to be involved in axon bundle formation. Therefore, we conclude that dietary cues stimulate insulin signaling in the niche to regulate EC cellular structure, probably via Fax-dependent cytoskeleton remodeling. This mechanism enhances intercellular contact and facilitates homeostatic interactions between somatic and germline cells in response to diet.

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.

Parallel Activin and BMP signaling coordinates R7/R8 photoreceptor subtype pairing in the stochastic Drosophila retina

Drosophila color vision is achieved by comparing outputs from two types of color-sensitive photoreceptors, R7 and R8. Ommatidia (unit eyes) are classified into two subtypes, known as 'pale' or 'yellow', depending on Rhodopsin expression in R7 and R8. Subtype specification is controlled by a stochastic decision in R7 and instructed to the underlying R8. We find that the Activin receptor Baboon is required in R8 to receive non-redundant signaling from the three Activin ligands, activating the transcription factor dSmad2. Concomitantly, two BMP ligands activate their receptor, Thickveins, and the transcriptional effector, Mad. The Amon TGFβ processing factor appears to regulate components of the TGFβ pathway specifically in pale R7. Mad and dSmad2 cooperate to modulate the Hippo pathway kinase Warts and the growth regulator Melted; two opposing factors of a bi-stable loop regulating R8 Rhodopsin expression. Therefore, TGFβ and growth pathways interact in postmitotic cells to precisely coordinate cell-specific output.

Tissue-specific regulation of BMP signaling by Drosophila N-glycanase 1

Mutations in the human N-glycanase 1 (NGLY1) cause a rare, multisystem congenital disorder with global developmental delay. However, the mechanisms by which NGLY1 and its homologs regulate embryonic development are not known. Here we show that Drosophila Pngl encodes an N-glycanase and exhibits a high degree of functional conservation with human NGLY1. Loss of Pngl results in developmental midgut defects reminiscent of midgut-specific loss of BMP signaling. Pngl mutant larvae also exhibit a severe midgut clearance defect, which cannot be fully explained by impaired BMP signaling. Genetic experiments indicate that Pngl is primarily required in the mesoderm during Drosophila development. Loss of Pngl results in a severe decrease in the level of Dpp homodimers and abolishes BMP autoregulation in the visceral mesoderm mediated by Dpp and Tkv homodimers. Thus, our studies uncover a novel mechanism for the tissue-specific regulation of an evolutionarily conserved signaling pathway by an N-glycanase enzyme.

DNA carries the information needed to build and maintain an organism, and units of DNA known as genes contain coded instructions to build other molecules, including enzymes. Sometimes, genes can become faulty and develop mutations that can affect how an embryo develops and lead to diseases. For example, people with mutations in the gene that encodes an enzyme called N-glycanase 1 experience many problems with their nervous system, gut and other organs.

Normally, N-glycanase 1 helps the body remove specific sugar molecules from some proteins in the cells, and is also thought to be important during embryonic development. As an embryo develops, its cells undergo a series of transformations, which is regulated by different molecules and signaling pathways. For example, a pathway known as BMP signaling plays an important role in many tissues. Problems with this pathway can lead to many diseases throughout the body, including the gut, where it helps cells to develop.

Previous research has shown that fruit flies lacking the gene that codes for an equivalent N-glycanase enzyme (which is called Pngl in flies) cannot develop properly into adults. However, until now it was not known what type of cells need the N-glycanase enzyme in any organism, or if NGLY1 is essential for important signaling pathways like BMP signaling. Now, Galeone et al. have used genetically modified flies to test how losing Pngl affected their development.

The results first showed that engineering Pngl-deficient fruit flies to produce the human enzyme eliminated their problems; these flies developed and survived like normal flies. This confirmed that that the human and fly enzymes can perform equivalent roles. Galeone et al. then discovered that Pngl plays two distinct roles in a group of cells that surround the fruit fly’s gut tissue and give rise to the cells that eventually form the muscle layer in the gut. In the larvae, Pngl was required to empty the gut, which is a necessary step before the larvae can develop into an adult. Moreover, Pngl is needed for BMP signaling in the gut, and when flies had the enzyme removed, some parts of their gut could not from properly.

This study will provide a framework to improve our understanding of how BMP signaling is regulated in humans. A next step will be to test if some of the symptoms experienced by patients without a working copy of the gene for N-glycanase 1 are caused by a faulty BMP-signaling system in specific tissues. If this is the case, it could provide new opportunities to treat some of these symptoms.

Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense

Invading pathogens provoke robust innate immune responses in Dipteran insects, such as Drosophila melanogaster In a systemic bacterial infection, a humoral response is induced in the fat body. Gram-positive bacteria trigger the Toll signaling pathway, whereas gram-negative bacterial infections are signaled via the immune deficiency (IMD) pathway. We show here that the RNA interference-mediated silencing of Furin1-a member of the proprotein convertase enzyme family-specifically in the fat body, results in a reduction in the expression of antimicrobial peptides. This, in turn, compromises the survival of adult fruit flies in systemic infections that are caused by both gram-positive and -negative bacteria. Furin1 plays a nonredundant role in the regulation of immune responses, as silencing of Furin2, the other member of the enzyme family, had no effect on survival or the expression of antimicrobial peptides upon a systemic infection. Furin1 does not directly affect the Toll or IMD signaling pathways, but the reduced expression of Furin1 up-regulates stress response factors in the fat body. We also demonstrate that Furin1 is a negative regulator of the Janus kinase/signal transducer and activator of transcription signaling pathway, which is implicated in stress responses in the fly. In summary, our data identify Furin1 as a novel regulator of humoral immunity and cellular stress responses in Drosophila-Aittomäki, S., Valanne, S., Lehtinen, T., Matikainen, S., Nyman, T. A., Rämet, M., Pesu, M. Proprotein convertase Furin1 expression in the Drosophila fat body is essential for a normal antimicrobial peptide response and bacterial host defense.

Adaptive protein divergence of BMP ligands takes place under developmental and evolutionary constraints

The bone morphogenetic protein (BMP) signaling network, comprising evolutionary conserved BMP2/4/Decapentaplegic (Dpp) and Chordin/Short gastrulation (Sog), is widely utilized for dorsal-ventral (DV) patterning during animal development. A similar network is required for posterior crossvein (PCV) formation in the Drosophila pupal wing. Although both transcriptional and post-transcriptional regulation of co-factors in the network gives rise to tissue-specific and species-specific properties, their mechanisms are incompletely understood. In Drosophila, BMP5/6/7/8-type ligands, Screw (Scw) and Glass bottom boat (Gbb), form heterodimers with Dpp for DV patterning and PCV development, respectively. Sequence analysis indicates that the Scw ligand contains two N-glycosylation motifs: one being highly conserved between BMP2/4- and BMP5/6/7/8-type ligands, and the other being Scw ligand specific. Our data reveal that N-glycosylation of the Scw ligand boosts BMP signaling both in cell culture and in the embryo. In contrast, N-glycosylation modifications of Gbb or Scw ligands reduce the consistency of PCV development. These results suggest that tolerance for structural changes of BMP5/6/7/8-type ligands is dependent on developmental constraints. Furthermore, gain and loss of N-glycosylation motifs in conserved signaling molecules under evolutionary constraints appear to constitute flexible modules to adapt to developmental processes.

Dpp spreading is required for medial but not for lateral wing disc growth

Drosophila Decapentaplegic (Dpp) has served as a paradigm to study morphogen-dependent growth control. However, the role of a Dpp gradient in tissue growth remains highly controversial. Two fundamentally different models have been proposed: the 'temporal rule' model suggests that all cells of the wing imaginal disc divide upon a 50% increase in Dpp signalling, whereas the 'growth equalization model' suggests that Dpp is only essential for proliferation control of the central cells. Here, to discriminate between these two models, we generated and used morphotrap, a membrane-tethered anti-green fluorescent protein (GFP) nanobody, which enables immobilization of enhanced (e)GFP::Dpp on the cell surface, thereby abolishing Dpp gradient formation. We find that in the absence of Dpp spreading, wing disc patterning is lost; however, lateral cells still divide at normal rates. These data are consistent with the growth equalization model, but do not fit a global temporal rule model in the wing imaginal disc.

The prodomain of BMP4 is necessary and sufficient to generate stable BMP4/7 heterodimers with enhanced bioactivity in vivo

Bone morphogenetic proteins 4 and 7 (BMP4 and BMP7) are morphogens that signal as either homodimers or heterodimers to regulate embryonic development and adult homeostasis. BMP4/7 heterodimers exhibit markedly higher signaling activity than either homodimer, but the mechanism underlying the enhanced activity is unknown. BMPs are synthesized as inactive precursors that dimerize and are then cleaved to generate both the bioactive ligand and prodomain fragments, which lack signaling activity. Our study reveals a previously unknown requirement for the BMP4 prodomain in promoting heterodimer activity. We show that BMP4 and BMP7 precursor proteins preferentially or exclusively form heterodimers when coexpressed in vivo. In addition, we show that the BMP4 prodomain is both necessary and sufficient for generation of stable heterodimeric ligands with enhanced activity and can enable homodimers to signal in a context in which they normally lack activity. Our results suggest that intrinsic properties of the BMP4 prodomain contribute to the relative bioactivities of homodimers versus heterodimers in vivo. These findings have clinical implications for the use of BMPs as regenerative agents for the treatment of bone injury and disease.

Expression of the myostatin gene in the adductor muscle of the Pacific lion-paw scallop Nodipecten subnodosus in association with growth and environmental conditions

The cDNA sequence of the myostatin gene in the Pacific lion-paw Nodipecten subnodosus (Ns-mstn) was characterized, and the temporal expression during grow-out was analyzed for the first time in a scallop. Ns-mstn encodes a 459-amino-acid protein in which two propeptide proteolytic sites were identified, the previously recognized (RSKR) and a second one at position 266-269 aa (RRKR). The alternative furin cleavage site could be related with post-translational processing, or it could be a tissue-specific mechanism for signaling activity. The Ns-mstn transcript was located by in situ hybridization in sarcomeres and around the nucleus of muscle fibers. The temporal expression analysis by qPCR in the adductor muscle showed that Ns-mstn expression was significantly different (P < 0.05) between months during the grow-out period, increasing largely during the summer months when both biomass and muscle weight did not increase or even decreased; muscle fiber size and number were found to decrease significantly. Exogenous and endogenous factors such as high temperature and low food availability, as well as gametogenesis and reproduction, can be associated with the growth pattern and Ns-mstn expression changes. Our results indicate that MSTN is involved in adductor muscle growth regulation in N. subnodosus as it occurs in vertebrate skeletal muscle although Ns-mstn expression in non-muscle organs/tissues suggests additional functions.

Prodomain removal enables neto to stabilize glutamate receptors at the Drosophila neuromuscular junction

Stabilization of neurotransmitter receptors at postsynaptic specializations is a key step in the assembly of functional synapses. Drosophila Neto (Neuropillin and Tolloid-like protein) is an essential auxiliary subunit of ionotropic glutamate receptor (iGluR) complexes required for the iGluRs clustering at the neuromuscular junction (NMJ). Here we show that optimal levels of Neto are crucial for stabilization of iGluRs at synaptic sites and proper NMJ development. Genetic manipulations of Neto levels shifted iGluRs distribution to extrajunctional locations. Perturbations in Neto levels also produced small NMJs with reduced synaptic transmission, but only Neto-depleted NMJs showed diminished postsynaptic components. Drosophila Neto contains an inhibitory prodomain that is processed by Furin1-mediated limited proteolysis. neto null mutants rescued with a Neto variant that cannot be processed have severely impaired NMJs and reduced iGluRs synaptic clusters. Unprocessed Neto retains the ability to engage iGluRs in vivo and to form complexes with normal synaptic transmission. However, Neto prodomain must be removed to enable iGluRs synaptic stabilization and proper postsynaptic differentiation.

Synapse development is initiated by genetic programs, but is coordinated by neuronal activity, by communication between the pre- and postsynaptic compartments, and by cellular signals that integrate the status of the whole organisms and its developmental progression. The molecular mechanisms underlining these processes are poorly understood. In particular, how neurotransmitter receptors are recruited and stabilized at central synapses remain the subject of intense research. The Drosophila NMJ is a glutamatergic synapse similar in composition and physiology with mammalian central excitatory synapses. Like mammals, Drosophila utilizes auxiliary subunit(s) to modulate the formation and function of glutamatergic synapses. We have previously reported that Neto is an auxiliary protein essential for functional glutamate receptors and for organization of postsynaptic specializations. Here we report that synapse assembly and NMJ development are exquisitely sensitive to postsynaptic Neto levels. Furthermore, we show that Neto activity is controlled by Furin-type proteases, which regulate the processing and maturation of many developmentally important proteins, from growth factors and neuropeptides to extracellular matrix components. Such concerted control may serve to coordinate synapse assembly with synapse growth and developmental progression.

Morphogen transport: theoretical and experimental controversies

According to morphogen gradient theory, extracellular ligands produced from a localized source convey positional information to receiving cells by signaling in a concentration-dependent manner. How do morphogens create concentration gradients to establish positional information in developing tissues? Surprisingly, the answer to this central question remains largely unknown. During development, a relatively small number of morphogens are reiteratively deployed to ensure normal embryogenesis and organogenesis. Thus, the intracellular processing and extracellular transport of morphogens are tightly regulated in a tissue-specific manner. Over the past few decades, diverse experimental and theoretical approaches have led to numerous conflicting models for gradient formation. In this review, we summarize the experimental evidence for each model and discuss potential future directions for studies of morphogen gradients. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Simultaneous rather than ordered cleavage of two sites within the BMP4 prodomain leads to loss of ligand in mice

ProBMP4 is generated as a latent precursor that is sequentially cleaved at two sites within the prodomain to generate an active ligand. An initial cleavage occurs adjacent to the ligand domain, which generates a non-covalently associated prodomain/ligand complex that is subsequently dissociated by cleavage at an upstream site. An outstanding question is whether the two sites need to be cleaved sequentially and in the correct order to achieve proper control of BMP4 signaling during development. In the current studies, we demonstrate that mice carrying a knock-in point mutation that causes simultaneous rather than sequential cleavage of both prodomain sites show loss of BMP4 function and die during mid-embryogenesis. Levels of mature BMP4 are severely reduced in mutants, although levels of precursor and cleaved prodomain are unchanged compared with wild type. Our biochemical analysis supports a model in which the transient prodomain/ligand complex that forms during sequential cleavage plays an essential role in prodomain-mediated stabilization of the mature ligand until it can acquire protection from degradation by other means. By contrast, simultaneous cleavage causes premature release of the ligand from the prodomain, leading to destabilization of the ligand and loss of signaling in vivo.

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.

Cleavage of the Drosophila screw prodomain is critical for a dynamic BMP morphogen gradient in embryogenesis

Dorsoventral patterning of the Drosophila embryo is regulated by graded distribution of bone morphogenetic proteins (BMPs) composed of two ligands, decapentaplegic (Dpp) a BMP2/4 ortholog and screw (Scw) a BMP5/6/7/8 family member. scw(E1) encodes an unusual allele that was isolated as a dominant enhancer of partial loss-of-function mutations in dpp. However, the molecular mechanisms that underlie this genetic interaction remain to be addressed. Here we show that scw(E1) contains a mutation at the furin cleavage site within the prodomain that is crucial for ligand production. Furthermore, our data show that Scw(E1) preferentially forms heterodimers with Dpp rather than homotypic dimers, providing a possible explanation for the dominant negative phenotype of scw(E1) alleles. The unprocessed prodomain of Scw(E1) remains in a complex with the Dpp:Scw heterodimer, and thus could interfere with interaction of the ligand with the extracellular matrix, or the kinetics of processing/secretion of the ligand in vivo. These data reveal novel mechanisms by which post-translational regulation of Scw can modulate Dpp signaling activity.

Regulation of TGFβ and related signals by precursor processing

Secreted cytokines of the TGFβ family are found in all multicellular organisms and implicated in regulating fundamental cell behaviors such as proliferation, differentiation, migration and survival. Signal transduction involves complexes of specific type I and II receptor kinases that induce the nuclear translocation of Smad transcription factors to regulate target genes. Ligands of the BMP and Nodal subgroups act at a distance to specify distinct cell fates in a concentration-dependent manner. These signaling gradients are shaped by multiple factors, including proteases of the proprotein convertase (PC) family that hydrolyze one or several peptide bonds between an N-terminal prodomain and the C-terminal domain that forms the mature ligand. This review summarizes information on the proteolytic processing of TGFβ and related precursors, and its spatiotemporal regulation by PCs during development and various diseases, including cancer. Available evidence suggests that the unmasking of receptor binding epitopes of TGFβ is only one (and in some cases a non-essential) function of precursor processing. Future studies should consider the impact of proteolytic maturation on protein localization, trafficking and turnover in cells and in the extracellular space.

Fine-tuned shuttles for bone morphogenetic proteins

Bone morphogenetic proteins (BMPs) are potent secreted signaling factors that trigger phosphorylation of Smad transcriptional regulators through receptor complex binding at the cell-surface. Resulting changes in target gene expression impact critical cellular responses during development and tissue homeostasis. BMP activity is tightly regulated in time and space by secreted modulators that control BMP extracellular distribution and availability for receptor binding. Such extracellular regulation is key for BMPs to function as morphogens and/or in the formation of morphogen activity gradients. Here, we review shuttling systems utilized to control the distribution of BMP ligands in tissue of various geometries, developing under different temporal constraints. We discuss the biological advantages for employing specific strategies for BMP shuttling and roles of varied ligand forms.

Multistep molecular mechanism for bone morphogenetic protein extracellular transport in the Drosophila embryo

In the Drosophila embryo, formation of a bone morphogenetic protein (BMP) morphogen gradient requires transport of a heterodimer of the BMPs Decapentaplegic (Dpp) and Screw (Scw) in a protein shuttling complex. Although the core components of the shuttling complex--Short Gastrulation (Sog) and Twisted Gastrulation (Tsg)--have been identified, key aspects of this shuttling system remain mechanistically unresolved. Recently, we discovered that the extracellular matrix protein collagen IV is important for BMP gradient formation. Here, we formulate a molecular mechanism of BMP shuttling that is catalyzed by collagen IV. We show that Dpp is the only BMP ligand in Drosophila that binds collagen IV. A collagen IV binding-deficient Dpp mutant signals at longer range in vivo, indicating that collagen IV functions to immobilize free Dpp in the embryo. We also provide in vivo evidence that collagen IV functions as a scaffold to promote shuttling complex assembly in a multistep process. After binding of Dpp/Scw and Sog to collagen IV, protein interactions are remodeled, generating an intermediate complex in which Dpp/Scw-Sog is poised for release by Tsg through specific disruption of a collagen IV-Sog interaction. Because all components are evolutionarily conserved, we propose that regulation of BMP shuttling and immobilization through extracellular matrix interactions is widely used, both during development and in tissue homeostasis, to achieve a precise extracellular BMP distribution.

You're going to need a bigger (glass bottom) boat

Signaling molecules of the transforming growth factor (TGF)-β family are generated from proprotein precursors containing prodomain sequences that are typically removed to allow signaling by the mature ligands. A form of a TGF-β family ligand that remains covalently attached to its prodomain but retains signaling activity has been identified. Glass bottom boat (Gbb), a Drosophila homolog of the bone morphogenetic protein 5/6/7/8 subfamily, is active as a carboxyl-terminal fragment of the proprotein (Gbb15) that is generated by a conventional processing event common to TGF-β ligands. Unexpectedly, a larger form (Gbb38) produced by processing at a newly identified furin site in the prodomain is also secreted and active. Contrary to the present paradigm in which TGF-β ligands require dissociation of the entire prodomain for activity, Gbb38 is active in cell culture and in vivo without additional processing at conventional sites. The large form can restore the viability of gbb mutant animals but has distinct signaling properties compared with the conventional form. Production of multiple functional ligands from one proprotein is a potential mechanism to fine-tune TGF-β signaling outputs. Mutations in TGF-β family members have been linked to human diseases, several of which affect potential furin cleavage sites in prodomains. However, given the diversity of potential furin processing sites and prodomain functions, direct experimentation will be required to determine whether production of active jumbo ligands is a general feature of TGF-β superfamily members.

A large bioactive BMP ligand with distinct signaling properties is produced by alternative proconvertase processing

Dimers of conventional transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) ligands are composed of two 100- to 140-amino acid peptides that are produced through the proteolytic processing of a proprotein precursor by proconvertases, such as furin. We report the identification of an evolutionarily conserved furin processing site in the amino terminus (NS) of the Glass bottom boat (Gbb; the Drosophila ortholog of vertebrate BMP5, 6, and 7) proprotein that generates a 328-amino acid, active BMP ligand distinct from the conventional 130-amino acid ligand. Gbb38, the large ligand form of Gbb, exhibited greater signaling activity and a longer range than the shorter form Gbb15. The abundance of Gbb15 and Gbb38 varied among different tissues, raising the possibility that differential processing could account for tissue-specific behaviors of BMPs. In human populations, mutations that abolished the NS cleavage site in BMP4, BMP15, or anti-Müllerian hormone were associated with cleft lip with or without cleft palate (BMP4), premature ovarian failure (BMP15), and persistent Müllerian duct syndrome (anti-Müllerian hormone), suggesting the importance of NS processing during development. The identification of this large BMP ligand form and the functional differences between large and small ligands exemplifies the potential for differential proprotein processing to substantially affect BMP and TGF-β signaling output in different tissue and cellular contexts.

Different requirements for proteolytic processing of bone morphogenetic protein 5/6/7/8 ligands in Drosophila melanogaster

Bone morphogenetic proteins (BMPs) are synthesized as proproteins that undergo proteolytic processing by furin/subtilisin proprotein convertases to release the active ligand. Here we study processing of BMP5/6/7/8 proteins, including the Drosophila orthologs Glass Bottom Boat (Gbb) and Screw (Scw) and human BMP7. Gbb and Scw have three functional furin/subtilisin proprotein convertase cleavage sites; two between the prodomain and ligand domain, which we call the Main and Shadow sites, and one within the prodomain, which we call the Pro site. In Gbb each site can be cleaved independently, although efficient cleavage at the Shadow site requires cleavage at the Main site, and remarkably, none of the sites is essential for Gbb function. Rather, Gbb must be processed at either the Pro or Main site to produce a functional ligand. Like Gbb, the Pro and Main sites in Scw can be cleaved independently, but cleavage at the Shadow site is dependent on cleavage at the Main site. However, both Pro and Main sites are essential for Scw function. Thus, Gbb and Scw have different processing requirements. The BMP7 ligand rescues gbb mutants in Drosophila, but full-length BMP7 cannot, showing that functional differences in the prodomain limit the BMP7 activity in flies. Furthermore, unlike Gbb, cleavage-resistant BMP7, although non-functional in rescue assays, activates the downstream signaling cascade and thus retains some functionality. Our data show that cleavage requirements evolve rapidly, supporting the notion that changes in post-translational processing are used to create functional diversity between BMPs within and between species.

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.

Crimpy inhibits the BMP homolog Gbb in motoneurons to enable proper growth control at the Drosophila neuromuscular junction

The BMP pathway is essential for scaling of the presynaptic motoneuron arbor to the postsynaptic muscle cell at the Drosophila neuromuscular junction (NMJ). Genetic analyses indicate that the muscle is the BMP-sending cell and the motoneuron is the BMP-receiving cell. Nevertheless, it is unclear how this directionality is established as Glass bottom boat (Gbb), the known BMP ligand, is active in motoneurons. We demonstrate that crimpy (cmpy) limits neuronal Gbb activity to permit appropriate regulation of NMJ growth. cmpy was identified in a screen for motoneuron-expressed genes and encodes a single-pass transmembrane protein with sequence homology to vertebrate Cysteine-rich transmembrane BMP regulator 1 (Crim1). We generated a targeted deletion of the cmpy locus and find that loss-of-function mutants exhibit excessive NMJ growth. In accordance with its expression profile, tissue-specific rescue experiments indicate that cmpy functions neuronally. The overgrowth in cmpy mutants depends on the activity of the BMP type II receptor Wishful thinking, arguing that Cmpy acts in the BMP pathway upstream of receptor activation and raising the possibility that it inhibits Gbb activity in motoneurons. Indeed, the cmpy mutant phenotype is strongly suppressed by RNAi-mediated knockdown of Gbb in motoneurons. Furthermore, Cmpy physically interacts with the Gbb precursor protein, arguing that Cmpy binds Gbb prior to the secretion of mature ligand. These studies demonstrate that Cmpy restrains Gbb activity in motoneurons. We present a model whereby this inhibition permits the muscle-derived Gbb pool to predominate at the NMJ, thus establishing the retrograde directionality of the pro-growth BMP pathway.

Feedback regulation of Drosophila BMP signaling by the novel extracellular protein larval translucida

The cellular response to the Drosophila BMP 2/4-like ligand Decapentaplegic (DPP) serves as one of the best-studied models for understanding the long-range control of tissue growth and pattern formation during animal development. Nevertheless, fundamental questions remain unanswered regarding extracellular regulation of the ligand itself, as well as the nature of the downstream transcriptional response to BMP pathway activation. Here, we report the identification of larval translucida (ltl), a novel target of BMP activity in Drosophila. Both gain- and loss-of-function analyses implicate LTL, a leucine-rich repeat protein, in the regulation of wing growth and vein patterning. At the molecular level, we demonstrate that LTL is a secreted protein that antagonizes BMP-dependent MAD phosphorylation, indicating that it regulates DPP/BMP signaling at or above the level of ligand-receptor interactions. Furthermore, based on genetic interactions with the DPP-binding protein Crossveinless 2 and biochemical interactions with the glypican Dally-like, we propose that LTL acts in the extracellular space where it completes a novel auto-regulatory loop that modulates BMP activity.

Analysis of Thisbe and Pyramus functional domains reveals evidence for cleavage of Drosophila FGFs

BACKGROUND

As important regulators of developmental and adult processes in metazoans, Fibroblast Growth Factor (FGF) proteins are potent signaling molecules whose activities must be tightly regulated. FGFs are known to play diverse roles in many processes, including mesoderm induction, branching morphogenesis, organ formation, wound healing and malignant transformation; yet much more remains to be learned about the mechanisms of regulation used to control FGF activity.

RESULTS

In this work, we conducted an analysis of the functional domains of two Drosophila proteins, Thisbe (Ths) and Pyramus (Pyr), which share homology with the FGF8 subfamily of ligands in vertebrates. Ths and Pyr proteins are secreted from Drosophila Schneider cells (S2) as smaller N-terminal fragments presumably as a result of intracellular proteolytic cleavage. Cleaved forms of Ths and Pyr can be detected in embryonic extracts as well. The FGF-domain is contained within the secreted ligand portion, and this domain alone is capable of functioning in the embryo when ectopically expressed. Through targeted ectopic expression experiments in which we assay the ability of full-length, truncated, and chimeric proteins to support cell differentiation, we find evidence that (1) the C-terminal domain of Pyr is retained inside the cell and does not seem to be required for receptor activation and (2) the C-terminal domain of Ths is secreted and, while also not required for receptor activation, this domain does plays a role in limiting the activity of Ths when present.

CONCLUSIONS

We propose that differential protein processing may account for the previously observed inequalities in signaling capabilities between Ths and Pyr. While the regulatory mechanisms are likely complex, studies such as ours conducted in a tractable model system may be able to provide insights into how ligand processing regulates growth factor activity.