A Complete Domain Structure of Drosophila Tolloid Is Required for Cleavage of Short Gastrulation

Characterization of tolloid-mediated cleavage of the GDF8 procomplex

Growth differentiation factor 8 (GDF8), a.k.a. myostatin, is a member of the larger TGFβ superfamily of signaling ligands. GDF8 has been well characterized as a negative regulator of muscle mass. After synthesis, GDF8 is held latent by a noncovalent complex between the N-terminal prodomain and the signaling ligand. Activation of latent GDF8 requires proteolytic cleavage of the prodomain at residue D99 by a member of the tolloid family of metalloproteases. While tolloid proteases cleave multiple substrates, they lack a conserved consensus sequence. Here, we investigate the tolloid cleavage site of the GDF8 prodomain to determine what residues contribute to tolloid recognition and subsequent proteolysis. Using sequential alanine mutations, we identified several residues adjacent to the scissile bond, including Y94, that when mutated, abolish tolloid-mediated activation of latent GDF8. Using the astacin domain of Tll1 (Tolloid Like 1) we determined that prodomain mutants were more resistant to proteolysis. Purified latent complexes harboring the prodomain mutations, D92A and Y94A, impeded activation by tolloid but could be fully activated under acidic conditions. Finally, we show that co-expression of GDF8 WT with prodomain mutants that were tolloid resistant, suppressed GDF8 activity. Taken together our data demonstrate that residues towards the N-terminus of the scissile bond are important for tolloid-mediated activation of GDF8 and that the tolloid-resistant version of the GDF8 prodomain can function dominant negative to WT GDF8.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Fell Muir Lecture: Collagen fibril formation in vitro and in vivo

It is a great honour to be awarded the Fell Muir Prize for 2016 by the British Society of Matrix Biology. As recipient of the prize, I am taking the opportunity to write a minireview on collagen fibrillogenesis, which has been the focus of my research for 33 years. This is the process by which triple helical collagen molecules assemble into centimetre-long fibrils in the extracellular matrix of animals. The fibrils appeared a billion years ago at the dawn of multicellular animal life as the primary scaffold for tissue morphogenesis. The fibrils occur in exquisite three-dimensional architectures that match the physical demands of tissues, for example orthogonal lattices in cornea, basket weaves in skin and blood vessels, and parallel bundles in tendon, ligament and nerves. The question of how collagen fibrils are formed was posed at the end of the nineteenth century. Since then, we have learned about the structure of DNA and the peptide bond, understood how plants capture the sun's energy, cloned animals, discovered antibiotics and found ways of editing our genome in the pursuit of new cures for diseases. However, how cells generate tissues from collagen fibrils remains one of the big unsolved mysteries in biology. In this review, I will give a personal account of the topic and highlight some of the approaches that my research group are taking to find new insights.

Mammalian tolloid proteinases: role in growth factor signalling

Tolloid proteinases are essential for tissue patterning and extracellular matrix assembly. The members of the family differ in their substrate specificity and activity, despite sharing similar domain organization. The mechanisms underlying substrate specificity and activity are complex, with variation between family members, and depend on both multimerization and substrate interaction. In addition, enhancers, such as Twisted gastrulation (Tsg), promote cleavage of tolloid substrate, chordin, to regulate growth factor signalling. Although Tsg and mammalian tolloid (mTLD) are involved in chordin cleavage, no interaction has been detected between them, suggesting Tsg induces a change in chordin to increase susceptibility to cleavage. All members of the tolloid family bind the N terminus of latent TGFβ‐binding protein‐1, providing support for their role in TGFβ signalling.

Amnioserosa development and function in Drosophila embryogenesis: Critical mechanical roles for an extraembryonic tissue

Despite being a short-lived, extraembryonic tissue, the amnioserosa plays critical roles in the major morphogenetic events of Drosophila embryogenesis. These roles involve both cellular mechanics and biochemical signaling. Its best-known role is in dorsal closure-well studied by both developmental biologists and biophysicists-but the amnioserosa is also important during earlier developmental stages. Here, we provide an overview of amnioserosa specification and its role in several key developmental stages: germ band extension, germ band retraction, and dorsal closure. We also compare embryonic development in Drosophila and its relative Megaselia to highlight how the amnioserosa and its roles have evolved. Placed in context, the amnioserosa provides a fascinating example of how signaling, mechanics, and morphogen patterns govern cell-type specification and subsequent morphogenetic changes in cell shape, orientation, and movement. Developmental Dynamics 245:558-568, 2016. © 2016 Wiley Periodicals, Inc.

Synthetic enzyme-substrate tethering obviates the Tolloid-ECM interaction during Drosophila BMP gradient formation

Members of the Tolloid family of metalloproteinases liberate BMPs from inhibitory complexes to regulate BMP gradient formation during embryonic dorsal-ventral axis patterning. Here, we determine mechanistically how Tolloid activity is regulated by its non-catalytic CUB domains in the Drosophila embryo. We show that Tolloid, via its N-terminal CUB domains, interacts with Collagen IV, which enhances Tolloid activity towards its substrate Sog, and facilitates Tsg-dependent stimulation of cleavage. In contrast, the two most C-terminal Tld CUB domains mediate Sog interaction to facilitate its processing as, based on our structural data, Tolloid curvature positions bound Sog in proximity to the protease domain. Having ascribed functions to the Tolloid non-catalytic domains, we recapitulate embryonic BMP gradient formation in their absence, by artificially tethering the Tld protease domain to Sog. Our studies highlight how the bipartite function of Tolloid CUB domains, in substrate and ECM interactions, fine-tune protease activity to a particular developmental context.

DOI:http://dx.doi.org/10.7554/eLife.05508.001

The body of an animal is a highly organised structure of tissues and organs that contain cells with specialised roles. To achieve this level of organisation, it is important that the cells in the embryo know their location and receive the correct instructions on how to develop, when to divide or move. Many animals are roughly symmetrical about an imaginary line that runs from their head to their tail; a developing embryo can provide its cells with information about their position along this head-to-tail axis and the axis that runs from its front to its back.

Setting up the front-to-back axis in the embryo involves a family of proteins called the bone morphogenetic proteins (or BMPs). These proteins can bind to other proteins that act as signals to provide instructions to cells. However, many of the BMPs are unable to perform this job because they are trapped by inhibitory molecules that bind to them instead.

Enzymes belonging to the Tolloid family can break down these inhibitors to release the BMPs. Together, the inhibitors and Tolloid enzymes create a gradient of BMP activity across the embryo. The side of the embryo with the highest levels of active BMPs sets the position of the back of the body, while the opposite side—which has the lowest levels of active BMPs—becomes the front. However, it is not clear how Tolloid is controlled to create the BMP gradient.

Different parts of the Tolloid enzyme have different roles; one portion of the enzyme breaks down the inhibitory molecules, and there are also several so-called ‘non-catalytic domains’. Winstanley et al. used a combination of approaches to study how Tolloid is controlled in fruit fly embryos. The experiments show that two non-catalytic domains at one end of Tolloid help the enzyme to bind to the inhibitory molecules. At the other end of the Tolloid enzyme, another non-catalytic domain can bind to a structural protein called Collagen IV. This enhances the ability of the enzyme to break down the inhibitory molecules and release the BMPs.

These findings reveal how Tolloid's non-catalytic domains can fine-tune the activity of this enzyme to create the gradient of BMP activity that is needed to set the front-to-back direction in animal embryos. Future studies will focus on identifying other proteins that bind to the non-catalytic domains of Tolloid in order to further control its activity during development.

DOI:http://dx.doi.org/10.7554/eLife.05508.002

Identification of nickel response genes in abnormal early developments of sea urchin by differential display polymerase chain reaction

Bioassays and biomarkers have been previously developed to assess the effects of heavy metal contaminants on the early life stages of the sea urchin. In this study, malformation in the early developmental processes was observed in sea urchin (Strongylocentrotus intermedius) larvae exposed to 10 ppm Ni for over 30 h. The most critical stage at which the triggering of nickel effects takes place is thought to be the blastula stage, which occurs after fertilization in larval development. To investigate the molecular-level responses of sea urchin exposed to heavy metal stress and to explore the differentially expressed genes that are induced or repressed by nickel, differential display polymerase chain reaction (DD-PCR) was used with sea urchin mRNAs. The malformation-related genes expressed in the early life stages of the sea urchin were cloned from larvae exposed to 10 ppm of nickel for 15 h, and accessed via DD-PCR. Sequence analysis results revealed that each of the genes evidenced high homology with EGF2, PCSK9, serine/threonine protein kinase, apolipophorin precursor protein, and MGC80921 protein/transcript variant 2. This result may prove useful in the development of novel biomarkers for the assessment of heavy metal stresses on sea urchin embryos.

Structure and properties of the Ca(2+)-binding CUB domain, a widespread ligand-recognition unit involved in major biological functions

CUB domains are 110-residue protein motifs exhibiting a β-sandwich fold and mediating protein-protein interactions in various extracellular proteins. Recent X-ray structural and mutagenesis studies have led to the identification of a particular CUB domain subset, cbCUB (Ca(2+)-binding CUB domain). Unlike other CUB domains, these harbour a homologous Ca(2+)-binding site that underlies a conserved binding site mediating ionic interaction between two of the three conserved acidic Ca(2+) ligands and a basic (lysine or arginine) residue of a protein ligand, similar to the interactions mediated by the low-density lipoprotein receptor family. cbCUB-mediated protein-ligand interactions usually involve multipoint attachment through several cbCUBs, resulting in high-affinity binding through avidity, despite the low affinity of individual interactions. The aim of the present review is to summarize our current knowledge about the structure and functions of cbCUBs, which represent the majority of the known CUB repertoire and are involved in a variety of major biological functions, including immunity and development, as well as in various cancer types. Examples discussed in the present review include a wide range of soluble and membrane-associated human proteins, as well as some archaeal and invertebrate proteins. The fact that these otherwise unrelated proteins share a common Ca(2+)-dependent ligand-binding ability suggests a mechanism inherited from very primitive ancestors. The information provided in the present review should stimulate further investigations on the crucial interactions mediated by cbCUB-containing proteins.

Enzymatic regulation of pattern: BMP4 binds CUB domains of Tolloids and inhibits proteinase activity

In Xenopus embryos, a dorsal-ventral patterning gradient is generated by diffusing Chordin/bone morphogenetic protein (BMP) complexes cleaved by BMP1/Tolloid metalloproteinases in the ventral side. We developed a new BMP1/Tolloid assay using a fluorogenic Chordin peptide substrate and identified an unexpected negative feedback loop for BMP4, in which BMP4 inhibits Tolloid enzyme activity noncompetitively. BMP4 binds directly to the CUB (Complement 1r/s, Uegf [a sea urchin embryonic protein] and BMP1) domains of BMP1 and Drosophila Tolloid with high affinity. Binding to CUB domains inhibits BMP4 signaling. These findings provide a molecular explanation for a long-standing genetical puzzle in which antimorphic Drosophila tolloid mutant alleles displayed anti-BMP effects. The extensive Drosophila genetics available supports the relevance of the interaction described here at endogenous physiological levels. Many extracellular proteins contain CUB domains; the binding of CUB domains to BMP4 suggests a possible general function in binding transforming growth factor-beta (TGF-beta) superfamily members. Mathematical modeling indicates that feedback inhibition by BMP ligands acts on the ventral side, while on the dorsal side the main regulator of BMP1/Tolloid enzymatic activity is the binding to its substrate, Chordin.

Scube1 is expressed during facial development in the mouse

Scube1 encodes a secreted plasma membrane-associated protein characterized by a N-terminal signal peptide sequence, multiple EGF domains, a N-linked glycosylated spacer region and a C-terminal CUB region. Here, we describe expression of the mouse Scube1 gene during early craniofacial development. Transcripts were identified in the nervous system, within the ventral neural tube, telencephalon and trigeminal ganglion. In addition, strong regionally restricted expression was found in the facial processes, including the medial and lateral nasal processes, maxilla and mandible, and caudal pharyngeal arches. During tooth development, Scube1 localized to the dental papilla of both incisor and molar teeth. Together, these data suggest a potential role for Scube1 during early craniofacial development.

Molecular determinants of Xolloid action in vivo

Xld (Xolloid) is a member of the Tolloid family of metalloproteases found in embryos of the frog Xenopus laevis. It cleaves Chordin, an inhibitory binding protein for BMP2/4, releasing fragments with reduced affinity for these important ventralizing signals. As a consequence, increasing Xld activity ventralizes Xenopus embryos. We have used this phenotype as an assay to determine the requirement for the C-terminal, nonprotease component of Xld for in vivo activity. This part of the protein is composed of five complement C1r/C1s-sea urchin epidermal growth factor-BMP1 (CUB) and two epidermal growth factor domains, which are thought to be involved in protein-protein interactions and may confer substrate specificity. Our results show that the protease coupled to CUB1 and CUB2 is the minimum domain structure required to ventralize Xenopus embryos and to block the dorsal axis-inducing activity of Chordin. Xld-CUB1-CUB2 cleaves Chordin, and a protease-inactive version co-precipitates Chordin. Our results indicate that the first and second CUB domains bind Chordin and present it to the protease domain. Protease-inactive Xld blocks the cleavage of Chordin by wild-type Xld and dorsalizes injected Xenopus embryos. We find that protease-inactive Xld-CUB1-CUB2 does not share this activity and that all of the C-terminal domains are required to generate the dorsalized phenotype.

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

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

Insights into how CUB domains can exert specific functions while sharing a common fold: conserved and specific features of the CUB1 domain contribute to the molecular basis of procollagen C-proteinase enhancer-1 activity

Procollagen C-proteinase enhancers (PCPE-1 and -2) are extracellular glycoproteins that can stimulate the C-terminal processing of fibrillar procollagens by tolloid proteinases such as bone morphogenetic protein-1. They consist of two CUB domains (CUB1 and -2) that alone account for PCPE-enhancing activity and one C-terminal NTR domain. CUB domains are found in several extracellular and plasma membrane-associated proteins, many of which are proteases. We have modeled the structure of the CUB1 domain of PCPE-1 based on known three-dimensional structures of CUB-containing proteins. Sequence alignment shows conserved amino acids, notably two acidic residues (Asp-68 and Asp-109) involved in a putative surface-located calcium binding site, as well as a conserved tyrosine residue (Tyr-67). In addition, three residues (Glu-26, Thr-89, and Phe-90) are found only in PCPE CUB1 domains, in putative surface-exposed loops. Among the conserved residues, it was found that mutations of Asp-68 and Asp-109 to alanine almost completely abolished PCPE-1 stimulating activity, whereas mutation of Tyr-67 led to a smaller reduction of activity. Among residues specific to PCPEs, mutation of Glu-26 and Thr-89 had little effect, whereas mutation of Phe-90 dramatically decreased the activity. Changes in activity were paralleled by changes in binding of different PCPE-1 mutants to a mini-procollagen III substrate, as shown by surface plasmon resonance. We conclude that PCPE-stimulating activity requires a calcium binding motif in the CUB1 domain that is highly conserved among CUB-containing proteins but also that PCPEs contain specific sites that could become targets for the development of novel anti-fibrotic therapies.