Developmental expression of a cell-surface protein involved in calcium uptake and skeleton formation in sea urchin embryos

Evolution of nacre- and prisms-related shell matrix proteins in the pen shell, Atrina pectinata

The molluscan shell is a good model for understanding the mechanisms underlying biomineralization. It is composed of calcium carbonate crystals and many types of organic molecules, such as the matrix proteins, polysaccharides, and lipids. The pen shell Atrina pectinata (Pterioida, Pinnidae) has two shell microstructures: an outer prismatic layer and an inner nacreous layer. Similar microstructures are well known in pearl oysters (Pteriidae), such as Pinctada fucata, and many kinds of shell matrix proteins (SMPs) have been identified from their shells. However, the members of SMPs that consist of the nacreous and prismatic layers of Pinnidae bivalves remain unclear. In this study, we identified 114 SMPs in the nacreous and prismatic layers of A. pectinata, of which only seven were found in both microstructures. 54 of them were found to bind calcium carbonate. Comparative analysis of nine molluscan shell proteomes showed that 69 of 114 SMPs of A. pectinata were found to have sequential similarity with at least one or more SMPs of other molluscan species. For instance, nacrein, tyrosinase, Pif/BMSP-like, chitinase (CN), chitin-binding proteins, CD109, and Kunitz-type serine proteinase inhibitors are widely shared among bivalves and gastropods. Our results provide new insights for understanding the complex evolution of SMPs related to nacreous and prismatic layer formation in the pteriomorph bivalves.

Initial stages of calcium uptake and mineral deposition in sea urchin embryos

Sea urchin larvae have an endoskeleton consisting of two calcitic spicules. We reconstructed various stages of the formation pathway of calcium carbonate from calcium ions in sea water to mineral deposition and integration into the forming spicules. Monitoring calcium uptake with the fluorescent dye calcein shows that calcium ions first penetrate the embryo and later are deposited intracellularly. Surprisingly, calcium carbonate deposits are distributed widely all over the embryo, including in the primary mesenchyme cells and in the surface epithelial cells. Using cryo-SEM, we show that the intracellular calcium carbonate deposits are contained in vesicles of diameter 0.5-1.5 μm. Using the newly developed airSEM, which allows direct correlation between fluorescence and energy dispersive spectroscopy, we confirmed the presence of solid calcium carbonate in the vesicles. This mineral phase appears as aggregates of 20-30-nm nanospheres, consistent with amorphous calcium carbonate. The aggregates finally are introduced into the spicule compartment, where they integrate into the growing spicule.

Glycobiology of reproductive processes in marine animals: the state of the art

Glycobiology is the study of complex carbohydrates in biological systems and represents a developing field of science that has made huge advances in the last half century. In fact, it combines all branches of biomedical research, revealing the vast and diverse forms of carbohydrate structures that exist in nature. Advances in structure determination have enabled scientists to study the function of complex carbohydrates in more depth and to determine the role that they play in a wide range of biological processes. Glycobiology research in marine systems has primarily focused on reproduction, in particular for what concern the chemical communication between the gametes. The current status of marine glycobiology is primarily descriptive, devoted to characterizing marine glycoconjugates with potential biomedical and biotechnological applications. In this review, we describe the current status of the glycobiology in the reproductive processes from gametogenesis to fertilization and embryo development of marine animals.

Proteomic analysis of sea urchin (Strongylocentrotus purpuratus) spicule matrix

BACKGROUND

The sea urchin embryo has been an important model organism in developmental biology for more than a century. This is due to its relatively simple construction, translucent appearance, and the possibility to follow the fate of individual cells as development to the pluteus larva proceeds. Because the larvae contain tiny calcitic skeletal elements, the spicules, they are also important model organisms for biomineralization research. Similar to other biominerals the spicule contains an organic matrix, which is thought to play an important role in its formation. However, only few spicule matrix proteins were identified previously.

RESULTS

Using mass spectrometry-based methods we have identified 231 proteins in the matrix of the S. purpuratus spicule matrix. Approximately two thirds of the identified proteins are either known or predicted to be extracellular proteins or transmembrane proteins with large ectodomains. The ectodomains may have been solubilized by partial proteolysis and subsequently integrated into the growing spicule. The most abundant protein of the spicule matrix is SM50. SM50-related proteins, SM30-related proteins, MSP130 and related proteins, matrix metalloproteases and carbonic anhydrase are among the most abundant components.

CONCLUSIONS

The spicule matrix is a relatively complex mixture of proteins not only containing matrix-specific proteins with a function in matrix assembly or mineralization, but also: 1) proteins possibly important for the formation of the continuous membrane delineating the mineralization space; 2) proteins for secretory processes delivering proteinaceous or non-proteinaceous precursors; 3) or proteins reflecting signaling events at the cell/matrix interface. Comparison of the proteomes of different skeletal matrices allows prediction of proteins of general importance for mineralization in sea urchins, such as SM50, SM30-E, SM29 or MSP130. The comparisons also help point out putative tissue-specific proteins, such as tooth phosphodontin or specific spicule matrix metalloproteases of the MMP18/19 group. Furthermore, the direct sequence analysis of peptides by MS/MS validates many predicted genes and confirms the existence of the corresponding proteins.

The sea urchin (Strongylocentrotus purpuratus) test and spine proteomes

BACKGROUND

The organic matrix of biominerals plays an important role in biomineral formation and in determining biomineral properties. However, most components of biomineral matrices remain unknown at present. In sea urchin, which is an important model organism for developmental biology and biomineralization, only few matrix components have been identified and characterized at the protein level. The recent publication of the Strongylocentrotus purpuratus genome sequence rendered possible not only the identification of possible matrix proteins at the gene level, but also the direct identification of proteins contained in matrices of skeletal elements by in-depth, high-accuracy, proteomic analysis.

RESULTS

We identified 110 proteins as components of sea urchin test and spine organic matrix. Fourty of these proteins occurred in both compartments while others were unique to their respective compartment. More than 95% of the proteins were detected in sea urchin skeletal matrices for the first time. The most abundant protein in both matrices was the previously characterized spicule matrix protein SM50, but at least eight other members of this group, many of them only known as conceptual translation products previously, were identified by mass spectrometric sequence analysis of peptides derived from in vitro matrix degradation. The matrices also contained proteins implicated in biomineralization processes previously by inhibition studies using antibodies or specific enzyme inhibitors, such as matrix metalloproteases and members of the mesenchyme-specific MSP130 family. Other components were carbonic anhydrase, collagens, echinonectin, a alpha2-macroglobulin-like protein and several proteins containing scavenger receptor cysteine-rich domains. A few possible signal transduction pathway components, such as GTP-binding proteins, a semaphorin and a possible tyrosine kinase were also identified.

CONCLUSION

This report presents the most comprehensive list of sea urchin skeletal matrix proteins available at present. The complex mixture of proteins identified in matrices of the sea urchin skeleton may reflect many different aspects of the mineralization process. Because LC-MS/MS-based methods directly measures peptides our results validate many predicted genes and confirm the existence of the corresponding proteins. Considering the many newly identified matrix proteins, this proteomic study may serve as a road map for the further exploration of biomineralization processes in an important model organism.

Differential distribution of spicule matrix proteins in the sea urchin embryo skeleton

Spicule matrix proteins are the products of primary mesenchyme cells, and are present in calcite spicules of the sea urchin embryo. To study their possible roles in skeletal morphogenesis, monoclonal antibodies against SM50, SM30 and another spicule matrix protein (29 kDa) were obtained. The distribution of these proteins in the embryo skeleton was observed by immunofluorescent staining. In addition, their distribution inside the spicules was examined by a 'spicule blot' procedure, direct immunoblotting of proteins embedded in crystallized spicules. Our observations showed that SM50 and 29 kDa proteins were enriched both outside and inside the triradiate spicules of the gastrulae, and also existed in the corresponding portions of growing spicules in later embryos and micromere cultures. The straight extensions of the triradiate spicules and thickened portions of body rods in pluteus spicules were also rich in these proteins. The SM30 protein was only faintly detected along the surface of spicules. By examination using the spicule blot procedure, however, SM30 was clearly detectable inside the body rods and postoral rods. These results indicate that SM50 and 29 kDa proteins are concentrated in radially growing portions of the spicules (normal to the c-axis of calcite), while SM30 protein is in the longitudinally growing portions (parallel to the c-axis). Such differential distribution suggests the involvement of these proteins in calcite growth during the formation of three-dimensionally branched spicules.

Ultrastructural localization of proteins involved in sea urchin biomineralization

Three skeletal tissues of the adult echinoid Paracentrotus lividus (the pedicellaria primordium, the test, and the tooth) were immunolabeled with three sera raised against the total mineralization organic matrix and two specific matrix proteins (SM30 and SM50) from the embryo of the echinoid Strongylocentrotus purpuratus. Two conventional chemical fixation protocols and two high-pressure freezing/freeze-substitution protocols were tested. One conventional protocol is recommended for its good preservation of the ultrastructure, and one high-pressure freezing/freeze-substitution protocol is recommended for its good retention of antigenicity. Immunolabeling was obtained in the three adult tissues. It was confined to the active skeleton-forming cells and to the structured organic matrix. The results indicate that the matrix proteins follow the classical routes of secretory protein assembly and export and suggest that SM30 and SM50 are a part of the tridimensional network formed by the organic matrix before the onset of mineralization. They show that the genetic program of part of skeletogenesis is conserved among different calcification models and developmental stages.

A putative role for carbohydrates in sea urchin gastrulation

Many studies have examined the effects of lectins on embryonic development. Recently, it has been shown that lectins actually enter the blastocoel of sea urchin embryos without microinjection and bind to specific cell types. The present study was performed to examine the effects of lectins on sea urchin gastrulation. Strongylocentrotus purpuratus sea urchin embryos were incubated with several lectins at concentrations from 0.01 microgram/ml to 100 micrograms/ml at 15-28 h in the presence or absence of the preferential binding sugars. The most interesting findings were that the mannose specific lectins Lens culinaris agglutinin (LcH) which binds to secondary mesenchyme cells involved in archenteron anchoring and Pisum sativum (PSA) caused exogastrulation. Wheat germ agglutinin (WGA) which binds to primary mesenchyme cells involved in skeletogenesis caused defective skeletogenesis. Our findings suggest that D-mannose-like residues (LcH and PSA specific sugar) may function in archenteron development and anchoring, while N-acetyl-D-glucosamine-like groups (WGA specific sugar) may contribute to control of primary mesenchyme positioning and function. Specific carbohydrate-containing receptors may, therefore, be of importance in specific gastrulation events.

The sea urchin larva, a suitable model for biomineralisation studies in space (IML-2 ESA Biorack experiment '24-F urchin')

By the ESA Biorack 'F-24 urchin' experiment of the IML-2 mission, for the first time the biomineralisation process in developing sea urchin larvae could be studied under real microgravity conditions. The main objectives were to determine whether in microgravity the process of skeleton formation does occur correctly compared to normal gravity conditions and whether larvae with differentiated skeletons do 'de-mineralise'. These objectives have been essentially achieved. Postflight studies on the recovered 'sub-normal' skeletons focused on qualitative, statistical and quantitative aspects. Clear evidence is obtained that the basic biomineralisation process does actually occur normally in microgravity. No significant differences are observed between flight and ground samples. The sub-normal skeleton architectures indicate, however, that the process of positioning of the skeletogenic cells (determining primarily shape and size of the skeleton) is particularly sensitive to modifications of environmental factors, potentially including gravity. The anatomical heterogeneity of the recovered skeletons, interpreted as long term effect of an accidental thermal shock during artificial egg fertilisation (break of climatisation at LSSF), masks possible effects of microgravity. No pronounced demineralisation appears to occur in microgravity; the magnesium component of the skeleton seems yet less stable than the calcium. On the basis of these results, a continuation of biomineralisation studies in space, with the sea urchin larva as model system, appears well justified and desirable.

Primary mesenchyme cell migration in the sea urchin embryo: distribution of directional cues

The directional migration of the primary mesenchyme cells (PMCs) of the sea urchin embryo is a critical step in the process of gastrulation. Although interactions between the migrating cells and the blastocoel environment are necessary for guiding the PMCs to their subequatorial target site, the nature of these interactions and the localization of guidance cues involved in directing the cells are not yet known. Previous studies have suggested that PMC migration is the result of random exploration and selective trapping at the target site by a pattern of adhesiveness in the ectoderm or basal lamina. To better characterize the distribution of guidance cues in the blastocoel we used a combination of time-lapse microscopy, microsurgery, and fluorescence photoablation to study the behavior of the migrating cells. By using fluorescence time-lapse microscopy, and a two-dimensional random-walk analysis of cell trajectories, we demonstrated that fluorescently labeled PMCs injected near the animal pole move in a directed fashion over a relatively long distance to reach the target site. This suggests that guidance cues are distributed globally throughout the embryo and are not restricted to the immediate ring area. To further test this hypothesis we investigated the migratory behavior of PMCs that were prevented from interacting directly with the target site. First, we examined the behavior of PMCs injected into animal embryo fragments lacking the target site. We found that PMCs move to the vegetal-most area of such embryo fragments, regardless of their size. Second, we studied the effects of photoablating a stripe of ectoderm between PMCs injected at the animal pole region (APR) and the target site. PMCs were found to accumulate along the ablated stripe and were unable to cross it for up to 6 hr after ablation. We also examined the migratory behavior of endogenous PMCs in embryos treated with lithium, a vegetalizing agent which shifts the position of the PMC ring toward the animal pole. We found that PMCs accumulated along an ablated stripe of ectoderm positioned below the shifted target site, suggesting that endogenous PMCs follow a set of directional cues to the target site which may be similar to those used by PMCs injected into the APR. As a whole, these results suggest that migrating PMCs follow a set of directional cues that are widely distributed throughout the blastocoel and that may be arranged in a gradient.

Characterization of a homolog of human bone morphogenetic protein 1 in the embryo of the sea urchin, Strongylocentrotus purpuratus

A cDNA clone encoding a protein homologous to human bone morphogenetic protein 1 (huBMP1) was isolated from a sea urchin embryo cDNA library. This sea urchin gene, named suBMP, encodes a protein of M(r) of 72 × 10(3). The deduced amino acid sequence of suBMP shares 72% sequence similarity (55% identity) with that of huBMP1. Like huBMP1 it also contains an N-terminal metalloendoprotease domain that shares sequence similarity with the astacin protease from crayfish, a C-terminal domain that is similar to the repeat domain found in C1r or C1s serine proteases, and an EGF-like segment. Although suBMP mRNA was detectable at a low level in the unfertilized egg, maximal expression of mRNA was observed at hatched blastula stage, with only a modest decrease in level at later stages of development. In situ hybridization studies revealed that suBMP mRNA is found in both ectodermal and primary mesenchyme cells in hatched blastula-stage embryos. Maximal expression of suBMP was observed at mesenchyme blastula, just before the onset of primitive skeleton (spicule) formation. SuBMP was found by immunoelectronmicroscopy in all cell types in late gastrula stage embryos. The antibody gold particles appeared in small clusters in the cytoplasm, on the surface of the cells and within the blastocoel. This distribution of suBMP, coupled with the finding that it was associated with membranes but was released by sodium carbonate treatment, suggests that the protein is secreted, and subsequently associates with a cell surface component. Two models for the possible function of suBMP in spiculogenesis in the sea urchin embryo are discussed.

Mesodermal cell interactions in the sea urchin embryo: properties of skeletogenic secondary mesenchyme cells

An interaction between the two principal populations of mesodermal cells in the sea urchin embryo, primary and secondary mesenchyme cells (PMCs and SMCs, respectively), regulates SMC fates and the process of skeletogenesis. In the undisturbed embryo, skeletal elements are produced exclusively by PMCs. Certain SMCs also have the ability to express a skeletogenic phenotype; however, signals transmitted by the PMCs direct these cells into alternative developmental pathways. In this study, a combination of fluorescent cell-labeling methods, embryo microsurgery and cell-specific molecular markers have been used to study the lineage, numbers, normal fate(s) and developmental potential of the skeletogenic SMCs. Previous fate-mapping studies have shown that SMCs are derived from the veg2 layer of blastomeres of the 64-cell-stage embryo and from the small micromeres. By specifically labeling the small micromeres with 5-bromodeoxyuridine, we demonstrate that descendants of these cells do not participate in skeletogenesis in PMC-depleted larvae, even though they are the closest lineal relatives of PMCs. Skeletogenic SMCs are therefore derived exclusively from the veg2 blastomeres. Because the SMCs are a heterogeneous population of cells, we have sought to gain information concerning the normal fate(s) of skeletogenic SMCs by determining whether specific cell types are reduced or absent in PMC(−) larvae. Of the four known SMC derivatives: pigment cells, blastocoelar (basal) cells, muscle cells and coelomic pouch cells, only pigment cells show a major reduction (> 50%) in number following SMC skeletogenesis. We therefore propose that the PMC-derived signal regulates a developmental switch, directing SMCs to adopt a pigment cell phenotype instead of a default (skeletogenic) fate. Ablation of SMCs at the late gastrula stage does not result in the recruitment of any additional skeletogenic cells, demonstrating that, by this stage, the number of SMCs with skeletogenic potential is restricted to 60–70 cells. Previous studies showed that during their switch to a skeletogenic fate, SMCs alter their migratory behavior and cell surface properties. In this study, we demonstrate that during conversion, SMCs become insensitive to the PMC-derived signal, while at the same time they acquire PMC-specific signaling properties.

Differential expression of the msp130 gene among skeletal lineage cells in the sea urchin embryo: a three dimensional in situ hybridization analysis

In order to examine the ontogeny of tissue-specific expression of the msp130 gene during early embryogenesis of the sea urchin, we have developed a whole-mount, non-radioactive in situ hybridization protocol suitable for these embryos. This protocol is adapted from the existing technology of immunohistochemical localization of digoxygenin-labelled hybridization probes in tissue sections. Transcript distribution patterns in the whole embryo are seen in three dimensions, and at much higher resolution and sensitivity than can be achieved using radioactive probes and sectioned material. We have traced the ontogeny of expression of the skeleton-specific gene, msp130, during the development of Strongylocentrotus purpuratus. Transcripts are first detected at the blastula stage, in micromere-lineage cells just prior to ingression. Appearance of msp130 transcripts remains strictly limited to this lineage through the pluteus stage. Estimated from the relative intensity of staining of the PMCs of an embryo, the relative abundance of msp130 transcripts is uniform among the 32 cells of this lineage in secondary mesenchyme blastulae and in gastrulae, indicating that expression is homogeneous among these cells up to the early prism stage. However, the relative intensity of stain, and therefore abundance of transcripts, changes dramatically and in a consistent pattern among the PMCs of an embryo during prism and pluteus stages, suggesting that these cells switch from an autonomous mode of regulation of the msp130 gene, to an inductive mode. In the pluteus larva, the highest levels of expression occur in those cells associated with the rapidly growing tips of the spicular skeleton.

Secondary mesenchyme of the sea urchin embryo: ontogeny of blastocoelar cells

Secondary mesenchyme in sea urchin embryos is released into the blastocoel after primary mesenchyme, and although these cells have been recognized for some time, we lack knowledge about many fundamental aspects of their origin and fate. Here we documented the ontogeny of one of the principal, and least well-known, types of cells derived from secondary mesenchyme. The blastocoelar cells arise from mesenchyme released from the tip of the archenteron following the initial phase of gastrulation. The cells migrate with their cell bodies suspended in the blastocoel, rather than being apposed to the basal lamina like primary mesenchyme. The cells extend numerous fine filopodia to form a network of cytoplasmic processes around the gut, along the skeletal rods, and within the larval arms. Once the network is formed, the cells maintain their positions, although they actively translocate vesicles and cytoplasm along their filopodia. Cell counts indicate there is an initial recruitment of cells during gastrulation, followed by a more gradual increase in cell number after the larva begins to feed. Lineage studies in which 16-cell-stage macromeres were injected with horseradish peroxidase indicate that almost all of the macromere-derived mesenchyme forms pigment cells and blastocoelar cells. We propose that blastocoelar cells are a distinct subset of secondary mesenchyme that forms fibroblast-like cells in the blastocoel of sea urchin embryos.

Improved binding of acidic bone matrix proteins to cationized filters during solid phase assays

A number of commercially available matrix filter supports have been designed for the immobilization of proteins following either electrotransfer from sodium dodecyl sulfate (SDS) polyacrylamide gels or direct application during dot blotting assays. These matrices differ with respect to chemical composition, charge, pore size, and degree of hydrophobicity. It follows that the properties of the protein(s) of interest will greatly influence the degree to which they interact with and ultimately bind to various filters. Acidic bone proteins contain diverse post-translational modifications that influence their interactions with solid phase matrices such as those used in immunoblotting (Western or dot blotting) or ion binding (overlay) procedures. This communication describes the results of a study comparing binding of various mixtures of non-collagenous acidic bone matrix phosphoproteins as well as purified osteopontin and osteocalcin to various filters including nitrocellulose and cationized paper or nylon. Based on our findings, we recommend the use of cationized filters for solid phase assays requiring the binding of these acidic macromolecules to background supports.

Cell type specification during sea urchin development

Recent discoveries indicate that cell lineages and fates play a key role in the establishment of spatially restricted gene expression during sea urchin development. Unique sets of founder cells generate five territories of gene expression by means of an invariant pattern of complete cleavage. Cell lineage analysis demonstrates that the second embryonic axis, the oral-aboral axis, is specified with reference to the first cleavage plane. In the undisturbed embryo, clones that contribute to one territory or another begin to appear at the third cleavage, and founder cell segregation to all five territories is completed by the sixth cleavage. Founder cell segregation is a key feature of mechanisms that establish the spatially defined gene activity of sea urchin embryogenesis.

Identification of the rat bone 60K acidic glycoprotein as alpha 2HS-glycoprotein

Previous reports have described an Mr 60,000-64,000 glycoprotein present in guanidium chloride (GdmCl)/EDTA extracts of bovine and rat bone. We have purified this protein from the long bones of rats and have raised polyclonal antibodies to the purified protein. The 60K glycoprotein has amino acid and carbohydrate compositions that are similar to those reported for the 60-64K protein(s). Several lines of evidence indicate that the 60K bone glycoprotein is the rat homologue of human alpha 2HS-glycoprotein. First, immunochemical data demonstrated that the 60K bone glycoprotein was present in serum as well as in EDTA/GdmCl extracts of bone. Second, immunolocalization and metabolic labelling experiments showed that the 60K protein is synthesized in liver and not in bone cells, although it is sequestered in vascularized regions of bone matrix. Finally, the NH2-terminal sequence for the rat 60K bone glycoprotein was highly similar to that of the human alpha 2HS-glycoprotein A chain. A surprising finding was that small amounts of contaminating 60K/alpha 2HS-glycoprotein were found in several protein fractions purified by ion-exchange chromatography of bone EDTA/GdmCl extracts. Because this protein was found to be highly immunogenic, the presence of anti-60K antibodies in anti-sera prepared against purified bone proteins should be considered as a potential problem.

Inhibition of glycoprotein processing blocks assembly of spicules during development of the sea urchin embryo

Previous studies have implicated an 130-kD glycoprotein containing complex, N-linked oligosaccharide chain(s) in the process of spicule formation in sea urchin embryos. To ascertain whether the processing of high mannose oligosaccharides to complex oligosaccharides is necessary for spiculogenesis, intact embryos and cultures of spicule-forming primary mesenchyme cells were treated with glycoprotein processing inhibitors. In both the embryonic and cell culture systems 1-deoxymannojirimycin (1-MMN) and, to a lesser extent, 1-deoxynojirimycin (1-DNJ) inhibited spicule formation. These inhibitors did not affect gastrulation in whole embryos or filopodial network formation in cell cultures. Swainsonine (SWSN) and castanospermine (CSTP) had no effect in either system. Further analysis revealed the following: (a) 1-MMN entered the embryos and blocked glycoprotein processing in the 24-h period before spicule formation as assessed by a twofold increase in endoglycosidase H sensitivity among newly synthesized glycoproteins upon addition of 1-MMN; (b) 1-MMN did not affect general protein synthesis until after its effects on spicule formation were observed; (c) Immunoblot analysis with an antibody directed towards the polypeptide chain of the 130-kD protein (mAb A3) demonstrated that 1-MMN did not affect the level of the polypeptide that is known to be synthesized just before spicule formation; (d) 1-MMN and 1-DNJ almost completely abolished (greater than 95%) the appearance of mAb 1223 reactive complex oligosaccharide moiety associated with the 130-kD glycoprotein; CSTP and SWSN had much less of an effect on expression of this epitope. These results indicate that the conversion of high mannose oligosaccharides to complex oligosaccharides is required for spiculogenesis in sea urchin embryos and they suggest that the 130-kD protein is one of these essential complex glycoproteins.

The "lecithotrophic" sea urchin Heliocidaris erythrogramma lacks typical yolk platelets and yolk glycoproteins

The sea urchin Heliocidaris tuberculata undergoes typical development, forming an echinoid pluteus larva, whereas H. erythrogramma undergoes direct development via a highly modified, nonfeeding larva. Using a polyclonal antibody prepared against yolk glycoproteins from the typical developer Stronglyocentrotus purpuratus, we found that H. tuberculata contains cross-reactive proteins in abundance, but H. erythrogramma does not. In addition, we used immunoelectron microscopy to demonstrate that unfertilized eggs of H. tuberculata contain yolk platelets, but those of H. erythrogramma do not.

Loss of yolk platelets and yolk glycoproteins during larval development of the sea urchin embryo

The fate of the yolk platelets and their constituent yolk glycoproteins was studied in Strongylocentrotus purpuratus eggs and embryos cultured through the larval stage. Previous studies have shown that the yolk glycoproteins undergo limited proteolysis during early embryonic development. We present evidence that the yolk glycoproteins stored in the yolk platelets exist as large, disulfide-linked complexes that are maintained even after limited proteolysis have occurred. We provide additional evidence that acidification of the yolk platelet may activate a latent thiol protease in the yolk platelet that is capable of correctly processing the major yolk glycoprotein into the smaller yolk glycoproteins. Because we previously showed that these yolk glycoproteins are not catabolized during early embryonic development, it was of interest to study their fate during larval development. Using a specific polyclonal antibody to a yolk glycoprotein, we found that both yolk glycoproteins and the yolk platelets disappeared in feeding, Day 7, larval stage embryos, but that starvation did not significantly affect the levels of the yolk glycoproteins. We also found that the yolk glycoproteins reappeared in 30-day-old premetamorphosis larvae.

Ontogeny and characterization of mesenchyme antigens of the sea urchin embryo

A monoclonal antibody, Sp12, binds to cortical granules, the hyaline layer, and skeletogenic, chromogenic, and blastocoelar mesenchyme of sea urchin eggs and embryos. Adult urchins also express Sp12 antigens in the dermal layer of the test and spines. Antigen is expressed on the surface of primary mesenchyme cells after they have entered the blastocoel, and by two secondary mesenchyme derivatives--the blastocoelar cells after they have been released from the tip of the archenteron, and the pigment cells in prism stage embryos. Immunogold localizations show antigen on the surfaces of mesenchyme, within membrane bounded vesicles, and associated with the Golgi apparatus. Western blots of antigens immunoprecipitated from seven developmental stages reveal twelve antigens ranging in Mr from 35 k to 240 k. Most of these antigens appear, disappear or change Mr over the first five days of development. Characterizations of this complex array of antigens show that the epitope recognized by Sp12 is eliminated by proteolytic enzymes and endoglycosidase F, while immunoreactivity is only reduced by periodate oxidation. As well, calcium magnesium free seawater extracts a subset of antigens different from that retained by crude membrane preparations. It is proposed that the mesenchyme of sea urchin embryos produces a family of developmentally regulated cell surface and extracellular matrix glycoproteins which all exhibit a carbohydrate epitope recognized by Sp12.

A calcium-binding, asparagine-linked oligosaccharide is involved in skeleton formation in the sea urchin embryo

We have previously identified a 130-kD cell surface protein that is involved in calcium uptake and skeleton formation by gastrula stage embryos of the sea urchin Strongylocentrotus purpuratus (Carson et al., 1985. Cell. 41:639-648). A monoclonal antibody designated mAb 1223 specifically recognizes the 130-kD protein and inhibits Ca+2 uptake and growth of the CaCO3 spicules produced by embryonic primary mesenchyme cells cultured in vitro. In this report, we demonstrate that the epitope recognized by mAb 1223 is located on an anionic, asparagine-linked oligosaccharide chain on the 130-kD protein. Combined enzymatic and chemical treatments indicate that the 1223 oligosaccharide contains fucose and sialic acid that is likely to be O-acetylated. Moreover, we show that the oligosaccharide chain containing the 1223 epitope specifically binds divalent cations, including Ca+2. We propose that one function of this negatively charged oligosaccharide moiety on the surfaces of primary mesenchyme cells is to facilitate binding and sequestration of Ca+2 ions from the blastocoelic fluid before internalization and subsequent deposition into the growing CaCO3 skeleton.

Structure of a major yolk glycoprotein and its processing pathway by limited proteolysis are conserved in echinoids

To study the fate of the yolk glycoproteins found in eggs and embryos of the sea urchin, Strongylocentrotus purpuratus, a polyclonal antibody to a 90-kDa polymannose glycoprotein found in the embryo was prepared. Immunoblot analysis of total proteins over the course of development showed that this antibody recognized a family of glycoproteins. Concomitant with the disappearance of the major 160-kDa yolk glycoprotein of the egg during embryogenesis, glycoproteins with a lower molecular mass appeared. These glycoproteins (115, 108, 90, 83, and 68 kDa) were purified from S. purpuratus and analyzed by limited proteolysis and peptide mapping. This analysis revealed that these glycoproteins were cleavage products derived from the major yolk glycoprotein. The antibody to the 90-kDa glycoprotein in S. purpuratus embryos was used to identify a homologous set of yolk glycoproteins with similar molecular masses in the embryos of three other species in the class Echinoidea: Arbacia punctulata, Lytechinus pictus, and Dendraster excentricus. However, eggs from other echinoderm classes and from Xenopus laevis, Drosophila melanogaster, and the chicken did not contain any cross-reactive molecules. Cross-reactivity within the class Echinoidea was not due to a common carbohydrate epitope, because the antibody recognized the glycoproteins even after the N-linked carbohydrate side chains were enzymatically removed. The major yolk glycoprotein (160-170 kDa) from each of the three sea urchin species was purified and analyzed. Comparison of the physical and chemical properties of these glycoproteins revealed striking similarities in pI and in amino acid and monosaccharide composition. The results of peptide mapping also supported the conclusion that the 160- to 170-kDa glycoproteins from the four echinoids are structurally homologous glycoproteins containing N-linked polymannose chains. Immunolocalization by electron microscopy in S. purpuratus showed that the yolk glycoproteins remained within the yolk platelet throughout development, and that externalization of the 160-kDa glycoprotein or its cleavage products was not detectable.

Developmental distribution of a cell surface glycoprotein in the sea urchin Strongylocentrotus purpuratus

Recent studies from this laboratory have shown that an antigen recognized by a monoclonal antibody (MAb 1223) displays a bimodal distribution of expression in development of the embryo of Strongylocentrotus purpuratus. This molecule is specifically localized to the primary mesenchyme cells of the embryo, but is also found within the egg. In the current study, immunoelectron microscopy was used to determine the subcellular distribution of the antigen and to determine its fate during early stages of development of the embryo. In eggs, the epitope recognized by MAb 1223 was localized to the cortical vesicles. Immunoblot analysis of an isolated cell surface complex (CSC) that contained the cortical vesicles revealed the presence of a 130-kDa protein, as well as immunoreactive components of higher molecular weight. Upon fertilization, the antigen was exocytosed from the cortical vesicles and became associated with the hyaline layer, the fertilization envelope, and the plasma membrane. Subsequently, the epitope could be detected within small vesicles and yolk platelets. By 60 min postfertilization, the amount of epitope detected intracellularly or in the perivitelline compartment was greatly reduced. At later stages of development, when formation of the embryonic skeleton occurred, the 1223 antigen was principally localized to the Golgi complex and to the syncytial cell surface of the primary mesenchyme cells. Thus, the results of this study suggest that in S. purpuratus the 1223 antigen is stored and secreted from the cortical vesicles of the egg, degraded after fertilization, and then later expressed on the surface of the primary mesenchyme cells.

Growth of linear spicules in cultured primary mesenchymal cells of sea urchin embryos is bidirectional

The growth of spicules of the sea urchin embryo was studied in a simple in vitro system in which primary mesenchymal cells attach to the substrata, migrate and fuse via filopodia, and subsequently deposit CaCO3, which in most cases is in the form of linear rods. The use of autoradiographs following 45Ca2+ pulse-chase labeling revealed that in such a system linear spicules that formed had two focal points of growth. Elongation occurred by addition of approximately equivalent amounts of Ca2+ to both ends of each rod. Multiform spicules having variable numbers of elongation sites (tips) also showed a similar pattern of Ca2+ deposition. Thus, the growth of both linear and relatively complex skeletal forms is apparently accomplished by the same basic mechanism.