A putative role for carbohydrates in sea urchin gastrulation

Heterologous expression of newly identified galectin-8 from sea urchin embryos produces recombinant protein with lactose binding specificity and anti-adhesive activity

Galectin family members specifically bind beta-galactoside derivatives and are involved in different cellular events, including cell communication, signalling, apoptosis, and immune responses. Here, we report a tandem-repeat type galectin from the Paracentrotus lividus sea urchin embryo, referred to as Pl-GAL-8. The 933nt sequence encodes a protein of 34.73 kDa, containing the conserved HFNPRF and WGxExR motifs in the two highly similar carbohydrate-recognition domains (CRD). The three-dimensional protein structure model of the N-CRD confirms the high evolutionary conservation of carbohydrate binding sites. The temporal gene expression is regulated during development and transcripts localize at the tip of the archenteron at gastrula stage, in a subset of the secondary mesenchyme cells that differentiate into blastocoelar (immune) cells. Functional studies using a recombinant Pl-GAL-8 expressed in bacteria demonstrate its hemo-agglutinating activity on human red blood cells through the binding to lactose, as well as its ability in inhibiting the adhesion of human Hep-G2 cells to the substrate. The recent implications in autoimmune diseases and inflammatory disorders make Gal-8 an attractive candidate for therapeutic purposes. Our results offer a solid basis for addressing the use of the new Pl-GAL-8 in functional and applicative studies, respectively in the developmental and biomedical fields.

A role for polyglucans in a model sea urchin embryo cellular interaction

The enzymatic activities of commercially prepared glycosidases were verified by direct chemical assays using defined substrates and fixed and live sea urchin (Lytechinus pictus) embryos to determine if a model cellular interaction of interest to developmental biologists for over a century (interaction of archenteron tip and roof of the blastocoel) was mediated by glycans. Glycosidases (active and denatured) were incubated with microdissected archenterons and blastocoel roofs in a direct assay to learn if their enzymatic activities could prevent the normal adhesive interaction. Of the five glycosidases tested only β-amylase (an exoglycosidase) immediately inhibited the interaction at relatively low unit activity. α-Amylase (an endoglycosidase) had no measurable effect, while other glycosidases (α-glucosidase, β-glucosidase, β-galactosidase) only substantially inhibited adhesion after a 12-h incubation. We demonstrated that the five glycosidases were active (not inhibited) in the presence of embryo materials, and that cleaved sugars could be detected directly after incubation of some enzymes with the embryos. The biochemical purity of the enzymes was examined using gel electrophoresis under denaturing conditions, and the absence of contaminating proteases was confirmed using Azocoll™ substrate. As we cannot entirely rule out the presence of minor contaminating enzymatic activities, only inhibitions of adhesion after very short incubations with enzyme were considered significant and biologically relevant. Although glycans in indirect experiments have been implicated in mediating the interaction of the tip of the archenteron and roof of the blastocoel, to our knowledge, this is the first study that directly implicates polyglucans with terminal 1,4-linked glucose residues in this adhesive event.

Use of specific glycosidases to probe cellular interactions in the sea urchin embryo

We present an unusual and novel model for initial investigations of a putative role for specifically conformed glycans in cellular interactions. We have used alpha- and ss-amylase and alpha- and ss-glucosidase in dose-response experiments evaluating their effects on archenteron organization using the NIH designated sea urchin embryo model. In quantitative dose-response experiments, we show that defined activity levels of alpha-glucosidase and ss-amylase inhibited archenteron organization in living Lytechinus pictus gastrula embryos, whereas all concentrations of ss-glucosidase and alpha-amylase were without substantial effects on development. Product inhibition studies suggested that the enzymes were acting by their specific glycosidase activities and polyacrylamide gel electrophoresis suggested that there was no detectable protease contamination in the active enzyme samples. The results provide evidence for a role of glycans in sea urchin embryo cellular interactions with special reference to the possible structural conformation of these glycans based on the differential activities of the alpha- and ss-glycosidases.

Exogenous hyalin and sea urchin gastrulation. Part IV: a direct adhesion assay - progress in identifying hyalin's active sites

In Strongylocentrotus purpuratus the hyalins are a set of three to four rather large glycoproteins (hereafter referred to as 'hyalin'), which are the major constituents of the hyaline layer, the developing sea urchin embryo's extracellular matrix. Recent research from our laboratories has shown that hyalin is a cell adhesion molecule involved in sea urchin embryo-specific cellular interactions. Other laboratories have shown it to consist of 2-3% carbohydrate and a cloned, sequenced fragment demonstrated repeat domains (HYR) and non-repeat regions. Interest in this molecule has increased because HYR has been identified in organisms as diverse as bacteria, flies, worms, mice and humans, as well as sea urchins. Our laboratories have shown that hyalin appears to mediate a specific cellular interaction that has interested investigators for over a century, archenteron elongation/attachment to the blastocoel roof. We have shown this finding by localizing hyalin on the two components of the cellular interaction and by showing that hyalin and anti-hyalin antibody block the cellular interaction using a quantitative microplate assay. The microplate assay, however, has limitations because it does not directly assess hyalin's effects on the adhesion of the two components of the interaction. Here we have used an elegant direct assay that avoids the limitations, in which we microdissected the two components of the adhesive interaction and tested their re-adhesion to each other, thereby avoiding possible factors in the whole embryos that could confound or confuse results. Using both assays, we found that mild periodate treatment (6 h to 24 h in sodium acetate buffer with 0.2 M sodium periodate at 4 degrees C in the dark) of hyalin eliminates its ability to block the cellular interaction, suggesting that the carbohydrate component(s) may be involved in hyalin's specific adhesive function. This first step is important in identifying the molecular mechanisms of a well known cellular interaction in the NIH-designated sea urchin embryo model, a system that has led to the discovery of scores of physiological mechanisms, including those involved in human health and disease.

Hyalin is a cell adhesion molecule involved in mediating archenteron-blastocoel roof attachment

The US National Institutes of Health has designated the sea urchin embryo as a model organism because around 25 discoveries in this system have led to insights into the physiology of higher organisms, including humans. Hyalin is a large glycoprotein in the hyaline layer of sea urchin embryos that functions to maintain general adhesive relationships in the developing embryo. It consists of the hyalin repeat domain that has been identified in organisms as diverse as bacteria, worms, flies, mice, sea urchins and humans. Here we show, using a polyclonal antibody raised against the 11.6 S species of hyalin, that it localizes at the tip of the archenteron and on the roof of the blastocoel exactly where these two structures bond in an adhesive interaction that has been of interest for over a century. In addition, the antibody blocks the interaction between the archenteron tip and blastocoel roof. These results, in addition to other recent findings from this laboratory that will be discussed, suggest that hyalin is involved in mediating this cellular interaction. This is the first demonstration that suggests that hyalin functions as a cell adhesion molecule in many organisms, including humans.

Exogenous hyalin and sea urchin gastrulation, Part II: hyalin, an interspecies cell adhesion molecule

The 330 kDa fibrillar glycoprotein hyalin is a well known component of the sea urchin embryo extracellular hyaline layer. Only recently, the main component of hyalin, the hyalin repeat domain, has been identified in organisms as widely divergent as bacteria and humans using the GenBank database and therefore its possible function has garnered a great deal of interest. In the sea urchin, hyalin serves as an adhesive substrate in the developing embryo and we have recently shown that exogenously added purified hyalin from Strongylocentrotus purpuratus embryos blocks a model cellular interaction in these embryos, archenteron elongation/attachment to the blastocoel roof. It is important to demonstrate the generality of this result by observing if hyalin from one species of sea urchin blocks archenteron elongation/attachment in another species. Here we show in three repeated experiments, with 30 replicate samples for each condition, that the same concentration of S. purpuratus hyalin (57 microg/ml) that blocked the interaction in living S. purpuratus embryos blocked the same interaction in living Lytechinus pictus embryos. These results correspond with the known crossreactivity of antibody against S. purpuratus hyalin with L. pictus hyalin. We propose that hyalin-hyalin receptor binding may mediate this adhesive interaction. The use of a microplate assay that allows precise quantification of developmental effects should help facilitate identification of the function of hyalin in organisms as divergent as bacteria and humans.

Carbohydrate-based experimental therapeutics for cancer, HIV/AIDS and other diseases

This review, primarily for general readers, briefly presents experimental approaches to therapeutics of cancer, HIV/AIDS and various other diseases based on advances in glycobiology and glycochemistry. Experimental cancer and HIV/AIDS vaccines are being developed in attempts to overcome weak immunological responses to carbohydrate-rich surface antigens using carriers, adjuvants and novel carbohydrate antigen constructs. Current carbohydrate-based vaccines are used for typhus, pneumonia, meningitis; vaccines for anthrax, malaria and leishmaniasis are under development. The link between O-linked beta-N-acetylglucosamine glycosylation and protein phosphorylation in diseases including diabetes and Alzheimer's disease is also explored. Carbohydrate-associated drugs that are in current use or under development, such as heparan sulfate binders, lectins, acarbose, aminoglycosides, tamiflu and heparin, and technologies using carbohydrate and lectin microarrays that offer improved diagnostic and drug development possibilities, are described. Advances in carbohydrate synthesis, analysis and manipulation through the emerging fields of glycochemistry and glycobiology are providing new approaches to disease therapeutics.

Cyclodextrin, a probe for studying adhesive interactions

In this short communication, we introduce alpha-cyclodextrin as a new probe to study mechanisms of adhesive interactions. We show that this cyclic polysaccharide, that consisting of six glucosyl residues linked by alpha-1,4 bonds, was the only sugar of 22 tested that dramatically blocked a specific cellular interaction in the sea urchin embryo (p<0.001 compared with non-sugar controls). A total of 150-400 embryos were sampled for each concentration of each sugar tested. Mechanisms of cellular interactions have been studied for almost a century and they still remain poorly understood. Cyclodextrin is an exciting new tool that can be utilized for investigating these mechanisms.

A novel approach to study adhesion mechanisms by isolation of the interacting system

For decades most investigations into mechanisms of adhesive interactions have examined whole organisms or single cells. Results using whole organisms are often unclear because it may not be known if a probe used in an experiment is directly affecting the cellular interaction under study or if it is an indirect effect resulting from action on some other structure or pathway. Here we develop a novel approach to isolate the structural components of a cellular interaction by dissecting them out of the organism to study them in a pristine environment away from all confounding factors. We used the adhesion between the archenteron and blastocoel roof of the sea urchin gastrula stage embryo as a model that can be replicated in many other developmental and pathological systems. The isolated components of the cellular interaction and those in the whole organism possessed identical cell surface receptors and adhesive affinities.

Embryotoxic activity and differential binding of plant-derived carbohydrate-recognizing proteins towards the sea urchin embryo cells

The embryotoxic activity and differential binding of plant-derived carbohydrate-recognizing proteins on sea urchin (Lytechinus variegatus) embryo cells was investigated. IC50 doses for toxicity on larvae development varied from 0.6 up to 96.3 microg ml(-1) and these effects were largely reversed by previously heating the proteins. Changes in the glycoconjungate status of the cell surface were assessed by time-course binding of the proteins during embryogenesis according to their carbohydrate-binding specificity. Glucose/mannose binding-proteins bound embryo cells at the same stage of development, at a similar stage to the N-acetylglucosamine/N-acetylneuraminic acid binding-protein (WGA) and earlier than galactose specific ones. FITC-conjugates of these proteins confirmed the above results and revealed the presence of specific and differential receptors for them. Inhibition assays using inhibitory glycoproteins significantly diminished the labelled patterns of FITC-conjugates. In conclusion, the assayed proteins exhibited embryotoxicity and their binding requirements were useful for following changes in the pattern of cell surface glycoconjugates on embryo cells of sea urchin. This property could be useful in analyzing other cell types.

Gastrulation in the sea urchin embryo: a model system for analyzing the morphogenesis of a monolayered epithelium

Processes of gastrulation in the sea urchin embryo have been intensively studied to reveal the mechanisms involved in the invagination of a monolayered epithelium. It is widely accepted that the invagination proceeds in two steps (primary and secondary invagination) until the archenteron reaches the apical plate, and that the constituent cells of the resulting archenteron are exclusively derived from the veg2 tier of blastomeres formed at the 60-cell stage. However, recent studies have shown that the recruitment of the archenteron cells lasts as late as the late prism stage, and some descendants of veg1 blastomeres are also recruited into the archenteron. In this review, we first illustrate the current outline of sea urchin gastrulation. Second, several factors, such as cytoskeletons, cell contact and extracellular matrix, will be discussed in relation to the cellular and mechanical basis of gastrulation. Third, differences in the manner of gastrulation among sea urchin species will be described; in some species, the archenteron does not elongate stepwise but continuously. In those embryos, bottle cells are scarcely observed, and the archenteron cells are not rearranged during invagination unlike in typical sea urchins. Attention will be also paid to some other factors, such as the turgor pressure of blastocoele and the force generated by blastocoele wall. These factors, in spite of their significance, have been neglected in the analysis of sea urchin gastrulation. Lastly, we will discuss how behavior of pigment cells defines the manner of gastrulation, because pigment cells recently turned out to be the bottle cells that trigger the initial inward bending of the vegetal plate.

Carbohydrate involvement in cellular interactions in sea urchin gastrulation

The sea urchin embryo is a model for studying cellular interactions that occur in higher organisms because of its availability, transparency, and accessibility to molecular probes. In previous studies, we found that the mannose/glucose-binding lectin Lens culinaris agglutinin entered living sea urchin embryos, bound to specific cell types and caused exogastrulation, when the developing gut (archenteron) falls out of the embryo proper. We have proposed that the lectin bound to sugar-containing ligands, thus preventing attachment of the archenteron to the blastocoel roof, resulting in exogastrulation. Here, we have continued our study of cellular interactions in this model using Lytechinus pictus sea urchin embryos, and have found that inhibitors of glycoprotein/proteoglycan synthesis, tunicamycin and sodium selenate, and the specific glycosidases, beta-amylase, alpha-glucosidase, and alpha-mannosidase, all inhibit archenteron organization, elongation, and attachment to the blastocoel roof in viable swimming embryos. We also show that single cells obtained by disaggregation of 32-h-old sea urchin embryos bind to L. culinaris agglutinin- and concanavalin A-derivatized beads; the binding is blocked by alpha-methyl mannose, but not l-fucose. These cells also bind to beads derivatized with mannan. These results provide evidence for a role of carbohydrate-containing molecules in cellular interactions in sea urchin gastrulation. In a second set of experiments, we found that the supernatant obtained by disaggregation of 24-32-h-old L. pictus embryos in calcium- and magnesium-free sea water contains molecules that cause exogastrulation, archenteron disorganization, inhibition of archenteron elongation and inhibition of archenteron attachment to the blastocoel roof in viable swimming embryos. We propose that the supernatant contains ligands and/or receptors that mediate archenteron development and attachment to the blastocoel roof and are released when embryos are disaggregated into single cells. These studies may lead to a better understanding of the molecular basis of mechanisms that control cellular interactions during development.