hch-1, a gene required for normal hatching and normal migration of a neuroblast in C. elegans, encodes a protein related to TOLLOID and BMP-1.

Proteins of the tolloid/bone morphogenetic protein (BMP)-1 family play important roles in the differentiation of cell fates. Among those proteins are BMP-1, which plays a role in cartilage and bone formation in mammals, the TOLLOID protein, which is required for the establishment of the dorsoventral axis of Drosophila embryos and BP10/SpAN, which are thought to act in the morphogenesis of sea urchins. These proteins have some properties in common. First, they contain the astacin metalloprotease domain, the CUB domain and the epidermal growth factor-like domain. Second, they are expressed in embryos at stages expected for their role in cell differentiation. Third, at least BMP-1 and TOLLOID are thought to interact with proteins of the transforming growth factor-beta family. We report that the hch-1 gene of the nematode Caenorhabditis elegans encodes a tolloid/BMP-1 family protein. The protein has the characteristic domains common to the tolloid/ BMP-1 family. Like other members of the family, it is expressed in embryos. However, the phenotype of hch-1 mutants shows that it is required for normal hatching and normal migration of a post-embryonic neuroblast. Furthermore, in spite of its expression in embryogenesis, it is not required for the viability of embryos. These results show new functions of the tolloid/BMP-1 family proteins and give insight into their evolution.     

Crystal structure and functional analysis of Drosophila Wind,a protein-disulfide isomerase-related protein

In the developing Drosophila melanogaster embryo, dorsal-ventral patterning displays an absolute requirement for the product of the essential windbeutel gene, Wind. In homozygous windbeutel mutant flies, dorsal-ventral patterning fails to initiate because of the failure of the Golgi-resident proteoglycan-modifying protein, Pipe, to exit the endoplasmic reticulum, and this leads to the death of the embryo. Here, we describe the three-dimensional structure of Wind at 1.9-A resolution and identify a candidate surface for interaction with Pipe. This represents the first crystal structure of a eukaryotic protein-disulfide isomerase-related protein of the endoplasmic reticulum to be described. The dimeric protein is composed of an N-terminal thioredoxin domain and a C-terminal alpha-helical domain unique to protein-disulfide isomerase D proteins. Although Wind carries a CXXC motif that is partially surface accessible, this motif is redox inactive, and the cysteines are not required for the targeting of Pipe to the Golgi. However, both domains are required for targeting Pipe to the Golgi, and, although the mouse homologue ERp28 cannot replace the function of Wind, exchange of the Wind D-domain with that of ERp28 allows for efficient Golgi transport of Pipe.     

Isolation and characterization of a sea urchin hsp 70 gene segment

Three clones containing Paracentrotus lividus sea urchin DNA sequences which cross-hybridize to Drosophila heat shock protein (hsp) 70 gene were isolated. The sequence arrangements in the three cloned DNA inserts were compared by restriction and cross-hybridization analysis. The results showed that they contain four different genes related to one Drosophila hsp 70 gene. One of these genes was subcloned, and two of the isolated fragments were shown to hybridize to genomic DNA and to RNA from heat-treated sea urchin embryo.     

The Tolkin Gene Is a Tolloid/Bmp-1 Homologue That Is Essential for Drosophila Development

The Drosophila decapentaplegic (dpp) gene, a member of the tranforming growth factor β superfamily of growth factors, is critical for specification of the embryonic dorsal-ventral axis, for proper formation of the midgut, and for formation of Drosophila adult structures. The Drosophila tolloid gene has been shown to genetically interact with dpp. The genetic interaction between tolloid and dpp suggests a model in which the tolloid protein participates in a complex containing the DPP ligand, its protease serving to activate DPP, either directly or indirectly. We report here the identification and cloning of another Drosophila member of the tolloid/bone morphogenic protein (BMP) 1 family, tolkin, which is located 700 bp 5' to tolloid. Its overall structure is like tolloid, with an N-terminal metalloprotease domain, five complement subcomponents C1r/C1s, Uegf, and Bmp1 (CUB) repeats and two epidermal growth factor (EGF) repeats. Its expression pattern overlaps that of tolloid and dpp in early embryos and diverges in later stages. In larval tissues, both tolloid and tolkin are expressed uniformly in the imaginal disks. In the brain, both tolloid and tolkin are expressed in the outer proliferation center, whereas tolkin has another stripe of expression near the outer proliferation center. Analysis of lethal mutations in tolkin indicate it is vital during larval and pupal stages. Analysis of its mutant phenotypes and expression patterns suggests that its functions may be mostly independent of tolloid and dpp.     

A protein of the basal lamina of the sea urchin embryo

The purification, biochemical characterization and functional features of a novel extracellular matrix protein are described. This protein is a component of the basal lamina found in embryos from the sea urchin species Paracentrotus lividus and Hemicentrotus pulcherrimus . The protein has been named PI-200 K or Hp-200 K, respectively, because of the species from which it was isolated and its apparent molecular weight in SDS-PAGE under reducing conditions. It has been purified from unfertilized eggs where it is found packed within cytoplasmic granules, and has different binding affinities to type I collagen and heparin, as assessed by affinity chromatography columns. By indirect immunofluorescence experiments it was shown that, upon fertilization, the protein becomes extracellular, polarized at the basal surface of ectoderm cells, and on the surface of primary mesenchyme cells at the blastula and gastrula stages. The protein serves as an adhesive substrate, as shown by an in vitro binding assay where cells dissociated from blastula embryos were settled on 200K protein-coated substrates. To examine the involvement of the protein in morphogenesis of sea urchin embryo, early blastula embryos were microinjected with anti-200K Fab fragments and further development was followed. When control embryos reached the pluteus stage, microinjected embryos showed severe abnormalities in arms and skeleton elongation and patterning. On the basis of current results, it was proposed that 200K protein is involved in the regulation of sea urchin embryo skeletogenesis.     

Genetic Screens to Identify Elements of the Decapentaplegic Signaling Pathway in Drosophila

Pathways for regulation of signaling by transforming growth factor-β family members are poorly understood at present. The best genetically characterized member of this family is encoded by the Drosophila gene decapentaplegic (dpp), which is required for multiple events during fly development. We describe here the results of screens for genes required to maximize dpp signaling during embryonic dorsal-ventral patterning. Screens for genetic interactions in the zygote have identified an allele of tolloid, as well as two novel alleles of screw, a gene recently shown to encode another bone morphogenetic protein-like polypeptide. Both genes are required for patterning the dorsalmost tissues of the embryo. Screens for dpp interactions with maternally expressed genes have identified loss of function mutations in Mothers against dpp and Medea. These mutations are homozygous pupal lethal, engendering gut defects and severely reduced imaginal disks, reminiscent of dpp mutant phenotypes arising during other dpp-dependent developmental events. Genetic interaction phenotypes are consistent with reduction of dpp activity in the early embryo and in the imaginal disks. We propose that the novel screw mutations identified here titrate out some component(s) of the dpp signaling pathway. We propose that Mad and Medea encode rate-limiting components integral to dpp pathways throughout development.     

Evolution of the dorsal-ventral patterning network in the mosquito, Anopheles gambiae

The dorsal-ventral patterning of the Drosophila embryo is controlled by a well-defined gene regulation network. We wish to understand how changes in this network produce evolutionary diversity in insect gastrulation. The present study focuses on the dorsal ectoderm in two highly divergent dipterans, the fruitfly Drosophila melanogaster and the mosquito Anopheles gambiae. In D. melanogaster, the dorsal midline of the dorsal ectoderm forms a single extra-embryonic membrane, the amnioserosa. In A. gambiae, an expanded domain forms two distinct extra-embryonic tissues, the amnion and serosa. The analysis of approximately 20 different dorsal-ventral patterning genes suggests that the initial specification of the mesoderm and ventral neurogenic ectoderm is highly conserved in flies and mosquitoes. By contrast, there are numerous differences in the expression profiles of genes active in the dorsal ectoderm. Most notably, the subdivision of the extra-embryonic domain into separate amnion and serosa lineages in A. gambiae correlates with novel patterns of gene expression for several segmentation repressors. Moreover, the expanded amnion and serosa anlage correlates with a broader domain of Dpp signaling as compared with the D. melanogaster embryo. Evidence is presented that this expanded signaling is due to altered expression of the sog gene.     

A conserved role for the nodal signaling pathway in the establishment of dorso-ventral and left-right axes in deuterostomes

Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have analyzed the function of the Nodal signaling pathway during the embryogenesis of the sea urchin, a non-chordate organism. We found that Nodal signaling plays a central role in axis specification in the sea urchin, but surprisingly, its first main role appears to be in ectoderm patterning and not in specification of the endoderm and mesoderm germ layers as in vertebrates. Starting at the early blastula stage, sea urchin nodal is expressed in the presumptive oral ectoderm where it controls the formation of the oral-aboral axis. A second conserved role for nodal signaling during vertebrate evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment occurring only on the left side. We found that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We have shown that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. This result shows that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms and that this role for nodal is conserved among the deuterostomes. We will discuss the implications regarding the reference axes of the sea urchin and the ancestral function of the nodal gene in the last section of this review.     

The sea urchin kinome: a first look

This paper reports a preliminary in silico analysis of the sea urchin kinome. The predicted protein kinases in the sea urchin genome were identified, annotated and classified, according to both function and kinase domain taxonomy. The results show that the sea urchin kinome, consisting of 353 protein kinases, is closer to the Drosophila kinome (239) than the human kinome (518) with respect to total kinase number. However, the diversity of sea urchin kinases is surprisingly similar to humans, since the urchin kinome is missing only 4 of 186 human subfamilies, while Drosophila lacks 24. Thus, the sea urchin kinome combines the simplicity of a non-duplicated genome with the diversity of function and signaling previously considered to be vertebrate-specific. More than half of the sea urchin kinases are involved with signal transduction, and approximately 88% of the signaling kinases are expressed in the developing embryo. These results support the strength of this nonchordate deuterostome as a pivotal developmental and evolutionary model organism.     

Bone morphogenetic protein-1 (BMP-1). Identification of the minimal domain structure for procollagen C-proteinase activity

Bone morphogenetic protein-1 (BMP-1) is a shorter spliced variant of mammalian tolloid (mTld), both of which cleave the C-propeptides of type I procollagen during the synthesis of extracellular matrix collagen fibrils. The fact that BMP-1 and mTld both exhibit procollagen C-proteinase (PCP) activity and that BMP-1 is the smaller variant might indicate that BMP-1 comprises the minimal required sequences for PCP activity. BMP-1 comprises a metalloproteinase domain, three CUB domains, and an epidermal growth factor (EGF)-like domain, which is located between the second and third CUB (complement components C1r/C1s, the sea urchin protein Uegf, and BMP-1) domains. In this study we showed the following. 1) The CUB1 domain is required for secretion of the molecule. Domain swapping experiments, in which CUB1 and other CUB domains were interchanged, resulted in retention of the proteins by cells. Therefore, CUB1 and its location immediately adjacent to the metalloproteinase domain are essential for secretion of the protein. 2) Mutants lacking the EGF-like and CUB3 domains exhibited full C-proteinase activity. In contrast, mutants lacking the CUB2 domain were poor C-proteinases. 3) Further studies showed that Glu-483 on the beta4-beta5 loop of CUB2 is essential for C-proteinase activity of BMP-1. In conclusion, the study showed that the minimal domain structure for PCP activity is considerably shorter than expected and comprises the metalloproteinase domain and the CUB1 and CUB2 domains of BMP-1.     

Overexpression and mechanistic characterization of blastula protease 10, a metalloprotease involved in sea urchin embryogenesis and development

Blastula protease 10 (BP10) is a metalloenzyme involved in sea urchin embryogenesis, which has been assigned to the astacin family of zinc-dependent endopeptidases. It shows greatest homology with the mammalian tolloid-like genes and contains conserved structural motifs consistent with astacin, tolloid, and bone morphogenetic protein 1. Astacin, a crustacean digestive enzyme, has been proposed to carry out hydrolysis via a metal-centered mechanism that involves a metal-coordinated "tyrosine switch." It has not been determined if the more structurally complex members of this family involved in eukaryotic development share this mechanism. The recombinant BP10 has been overexpressed in Escherichia coli, its metalloenzyme nature has been confirmed, and its catalytic properties have been characterized through kinetic studies. BP10 shows significant hydrolysis toward gelatin both in its native zinc-containing form and copper derivative. The copper derivative of BP10 shows a remarkable 960% rate acceleration toward the hydrolysis of the synthetic substrate N-benzoyl-arginine-p-nitroanilide when compared with the zinc form. The enzyme also shows calcium-dependent activation. These are the first thorough mechanistic studies reported on BP10 as a representative of the more structurally complex members of astacin-type enzymes in deuterostomes, which can add supporting data to corroborate the metal-centered mechanism proposed for astacin and the role of the coordinated Tyr. We have demonstrated the first mechanistic study of a tolloid-related metalloenzyme involved in sea urchin embryogenesis.