Supramolecular structure of a new family of circular proteoglycans mediating cell adhesion in sponges

Adhesion of freshwater sponge cells mediated by carbohydrate-carbohydrate interactions requires low environmental calcium

Marine ancestors of freshwater sponges had to undergo a series of physiological adaptations to colonize harsh and heterogeneous limnic environments. Besides reduced salinity, river-lake systems also have calcium concentrations far lower than seawater. Cell adhesion in sponges is mediated by calcium-dependent multivalent self-interactions of sulfated polysaccharide components of membrane-bound proteoglycans named aggregation factors. Cells of marine sponges require seawater average calcium concentration (10 mM) to sustain adhesion promoted by aggregation factors. We demonstrate here that the freshwater sponge Spongilla alba can thrive in a calcium-poor aquatic environment and that their cells are able to aggregate and form primmorphs with calcium concentrations 40-fold lower than that required by marine sponges cells. We also find that their gemmules need calcium and other micronutrients to hatch and generate new sponges. The sulfated polysaccharide purified from S. alba has sulfate content and molecular size notably lower than those from marine sponges. Nuclear magnetic resonance analyses indicated that it is composed of a central backbone of non- and 2-sulfated α- and β-glucose units decorated with branches of α-glucose. Assessments with atomic force microscopy/single-molecule force spectroscopy show that S. alba glucan requires 10-fold less calcium than sulfated polysaccharides from marine sponges to self-interact efficiently. Such an ability to retain multicellular morphology with low environmental calcium must have been a crucial evolutionary step for freshwater sponges to successfully colonize inland waters.

Modelling the early evolution of extracellular matrix from modern Ctenophores and Sponges

Animals (metazoans) include some of the most complex living organisms on Earth, with regard to their multicellularity, numbers of differentiated cell types, and lifecycles. The metazoan extracellular matrix (ECM) is well-known to have major roles in the development of tissues during embryogenesis and in maintaining homoeostasis throughout life, yet insight into the ECM proteins which may have contributed to the transition from unicellular eukaryotes to multicellular animals remains sparse. Recent phylogenetic studies place either ctenophores or poriferans as the closest modern relatives of the earliest emerging metazoans. Here, we review the literature and representative genomic and transcriptomic databases for evidence of ECM and ECM-affiliated components known to be conserved in bilaterians, that are also present in ctenophores and/or poriferans. Whereas an extensive set of related proteins are identifiable in poriferans, there is a strikingly lack of conservation in ctenophores. From this perspective, much remains to be learnt about the composition of ctenophore mesoglea. The principal ECM-related proteins conserved between ctenophores, poriferans, and bilaterians include collagen IV, laminin-like proteins, thrombospondin superfamily members, integrins, membrane-associated proteoglycans, and tissue transglutaminase. These are candidates for a putative ancestral ECM that may have contributed to the emergence of the metazoans.

Origin and Evolution of the Sponge Aggregation Factor Gene Family

Although discriminating self from nonself is a cardinal animal trait, metazoan allorecognition genes do not appear to be homologous. Here, we characterize the Aggregation Factor (AF) gene family, which encodes putative allorecognition factors in the demosponge Amphimedon queenslandica, and trace its evolution across 24 sponge (Porifera) species. The AF locus in Amphimedon is comprised of a cluster of five similar genes that encode Calx-beta and Von Willebrand domains and a newly defined Wreath domain, and are highly polymorphic. Further AF variance appears to be generated through individualistic patterns of RNA editing. The AF gene family varies between poriferans, with protein sequences and domains diagnostic of the AF family being present in Amphimedon and other demosponges, but absent from other sponge classes. Within the demosponges, AFs vary widely with no two species having the same AF repertoire or domain organization. The evolution of AFs suggests that their diversification occurs via high allelism, and the continual and rapid gain, loss and shuffling of domains over evolutionary time. Given the marked differences in metazoan allorecognition genes, we propose the rapid evolution of AFs in sponges provides a model for understanding the extensive diversification of self-nonself recognition systems in the animal kingdom.

Carbohydrate-Carbohydrate Interactions Mediated by Sulfate Esters and Calcium Provide the Cell Adhesion Required for the Emergence of Early Metazoans

Early metazoans had to evolve the first cell adhesion mechanism addressed to maintain a distinctive multicellular morphology. As the oldest extant animals, sponges are good candidates for possessing remnants of the molecules responsible for this crucial evolutionary innovation. Cell adhesion in sponges is mediated by the calcium-dependent multivalent self-interactions of sulfated polysaccharides components of extracellular membrane-bound proteoglycans, namely aggregation factors. Here, we used atomic force microscopy to demonstrate that the aggregation factor of the sponge Desmapsamma anchorata has a circular supramolecular structure and that it thus belongs to the spongican family. Its sulfated polysaccharide units, which were characterized via nuclear magnetic resonance analysis, consist preponderantly of a central backbone composed of 3-α-Glc1 units partially sulfated at 2- and 4-positions and branches of Pyr(4,6)α-Gal1→3-α-Fuc2(SO3)1→3-α-Glc4(SO3)1→3-α-Glc→4-linked to the central α-Glc units. Single-molecule force measurements of self-binding forces of this sulfated polysaccharide and their chemically desulfated and carboxyl-reduced derivatives revealed that the sulfate epitopes and extracellular calcium are essential for providing the strength and stability necessary to sustain cell adhesion in sponges. We further discuss these findings within the framework of the role of molecular structures in the early evolution of metazoans.

Catch bond interaction between cell-surface sulfatase Sulf1 and glycosaminoglycans

In biological adhesion, the biophysical mechanism of specific biomolecular interaction can be divided in slip and catch bonds, respectively. Conceptually, slip bonds exhibit a reduced bond lifetime under increased external force and catch bonds, in contrast, exhibit an increased lifetime (for a certain force interval). Since 2003, a handful of biological systems have been identified to display catch bond properties. Upon investigating the specific interaction between the unique hydrophilic domain (HD) of the human cell-surface sulfatase Sulf1 against its physiological glycosaminoglycan (GAG) target heparan sulfate (HS) by single molecule force spectroscopy (SMFS), we found clear evidence of catch bond behavior in this system. The HD, ∼320 amino acids long with dominant positive charge, and its interaction with sulfated GAG-polymers were quantitatively investigated using atomic force microscopy (AFM) based force clamp spectroscopy (FCS) and dynamic force spectroscopy (DFS). In FCS experiments, we found that the catch bond character of HD against GAGs could be attributed to the GAG 6-O-sulfation site whereas only slip bond interaction can be observed in a GAG system where this site is explicitly lacking. We interpreted the binding data within the theoretical framework of a two state two path model, where two slip bonds are coupled forming a double-well interaction potential with an energy difference of ΔE ≈ 9 kBT and a compliance length of Δx ≈ 3.2 nm. Additional DFS experiments support this assumption and allow identification of these two coupled slip-bond states that behave consistently within the Kramers-Bell-Evans model of force-mediated dissociation.

Analysis of DNA interactions using single-molecule force spectroscopy

Protein-DNA interactions are involved in many biochemical pathways and determine the fate of the corresponding cell. Qualitative and quantitative investigations on these recognition and binding processes are of key importance for an improved understanding of biochemical processes and also for systems biology. This review article focusses on atomic force microscopy (AFM)-based single-molecule force spectroscopy and its application to the quantification of forces and binding mechanisms that lead to the formation of protein-DNA complexes. AFM and dynamic force spectroscopy are exciting tools that allow for quantitative analysis of biomolecular interactions. Besides an overview on the method and the most important immobilization approaches, the physical basics of the data evaluation is described. Recent applications of AFM-based force spectroscopy to investigate DNA intercalation, complexes involving DNA aptamers and peptide- and protein-DNA interactions are given.

Age-related nanostructural and nanomechanical changes of individual human cartilage aggrecan monomers and their glycosaminoglycan side chains

The nanostructure and nanomechanical properties of aggrecan monomers extracted and purified from human articular cartilage from donors of different ages (newborn, 29 and 38 year old) were directly visualized and quantified via atomic force microscopy (AFM)-based imaging and force spectroscopy. AFM imaging enabled direct comparison of full length monomers at different ages. The higher proportion of aggrecan fragments observed in adult versus newborn populations is consistent with the cumulative proteolysis of aggrecan known to occur in vivo. The decreased dimensions of adult full length aggrecan (including core protein and glycosaminoglycan (GAG) chain trace length, end-to-end distance and extension ratio) reflect altered aggrecan biosynthesis. The demonstrably shorter GAG chains observed in adult full length aggrecan monomers, compared to newborn monomers, also reflects markedly altered biosynthesis with age. Direct visualization of aggrecan subjected to chondroitinase and/or keratanase treatment revealed conformational properties of aggrecan monomers associated with chondroitin sulfate (CS) and keratan sulfate (KS) GAG chains. Furthermore, compressive stiffness of chemically end-attached layers of adult and newborn aggrecan was measured in various ionic strength aqueous solutions. Adult aggrecan was significantly weaker in compression than newborn aggrecan even at the same total GAG density and bath ionic strength, suggesting the importance of both electrostatic and non-electrostatic interactions in nanomechanical stiffness. These results provide molecular-level evidence of the effects of age on the conformational and nanomechanical properties of aggrecan, with direct implications for the effects of aggrecan nanostructure on the age-dependence of cartilage tissue biomechanical and osmotic properties.

Characterization of collagenous matrix assembly in a chondrocyte model system

Collagen is a major component of the newly synthesized pericellular microenvironment of chondrocytes. Collagen types II, IX, and XI are synthesized and assembled into higher ordered complexes by a mechanism in which type XI collagen plays a role in nucleation of new fibrils, and in limiting fibril diameter. This study utilizes a cell line derived from the Swarm rat chondrosarcoma that allows the accumulation and assembly of pericellular matrix. Immunofluorescence and atomic force microscopy were used to assess early intermediates of fibril formation. Results indicate that this cell line synthesizes and secretes chondrocyte-specific pericellular matrix molecules including types II, IX, and XI collagen and is suitable for the study of newly synthesized collagen matrix under the experimental conditions used. AFM data indicate that small fibrils or assemblies of microfibrils are detectable and may represent precursors of the approximately 20 nm thin fibrils reported in cartilage. Treatment with hyaluronidase indicates that the dimensions of the small fibrils may be dependent upon the presence of hyaluronan within the matrix. This study provides information on the composition and organization of the newly synthesized extracellular matrix that plays a role in establishing the material properties and performance of biological materials such as cartilage.

Assessing carbohydrate-carbohydrate interactions by NMR spectroscopy: the trisaccharide epitope from the marine sponge Microciona prolifera

WEAK RECOGNITION PROCESSES: Weak calcium-mediated carbohydrate-carbohydrate interactions have been detected by DOSY and TRNOESY NMR methods by employing a gold glyconanoparticle as a multivalent system. In addition, 3D models of trisaccharide-Ca(II)-trisaccharide complexes based on results from molecular dynamics simulations are proposed. Diffusion-ordered NMR spectroscopy (DOSY-NMR) and TR-NOESY-NMR experiments are used to detect ligand binding to macromolecular receptors. These techniques have been applied to detect weak carbohydrate-carbohydrate self-recognition in solution, making use of sugar-decorated gold nanoparticles as the "macromolecule" and the same carbohydrate as the ligand. Changes in the diffusion coefficient of the free carbohydrate in the presence of the glyconanoparticle (only with Ca(II) ions in the sample solution), as well as changes in the sign of the sugar NOE peaks--positive for the free sugar (in the presence or absence of Ca(II)) and negative for the sugar only in the simultaneous presence of the glyconanoparticle and Ca(II) ions--have been taken as proof of weak Ca(II)-mediated carbohydrate-carbohydrate interactions in solution. Although different methods such as SPR, TEM, and AFM have been used in the past to detect carbohydrate-carbohydrate interactions with the aid of gold nanoparticles and gold selfassembled monolayers, they are restricted to high-affinity ranges. The methods used in this study allow expansion of the number of techniques to tackle this relevant biological problem, also for approaching ligand-receptor interactions below the high-affinity range. Additionally, 3D models of trisaccharide-Ca(II)-trisaccharide complexes based on results from molecular dynamics simulations are proposed.

Adhesion forces in the self-recognition of oligosaccharide epitopes of the proteoglycan aggregation factor of the marine sponge Microciona prolifera

Cell aggregation in the marine sponge Microciona prolifera is mediated by a multimillion molecular-mass aggregation factor, termed MAF. Earlier investigations revealed that the cell aggregation activity of MAF depends on two functional domains: (i) a Ca(2+)-independent cell-binding domain and (ii) a Ca(2+)-dependent proteoglycan self-interaction domain. Structural analysis of involved carbohydrate fragments of the proteoglycan in the self-association established a sulfated disaccharide beta-D: -GlcpNAc3S-(1-->3)-alpha-L: -Fucp and a pyruvated trisaccharide beta-D: -Galp4,6(R)Pyr-(1-->4)-beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp. Recent UV, SPR, and TEM studies, using BSA conjugates and gold nanoparticles of the synthetic sulfated disaccharide, clearly demonstrated self-recognition on the disaccharide level in the presence of Ca(2+)-ions. To determine binding forces of the carbohydrate-carbohydrate interactions for both synthetic MAF oligosaccharides, atomic force microscopy (AFM) studies were carried out. It turned out that, in the presence of Ca(2+)-ions, the force required to separate the tip and sample coated with a self-assembling monolayer of thiol-spacer-containing beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp-(1-->O)(CH(2))(3)S(CH(2))(6)S- was found to be quantized in integer multiples of 30 +/- 6 pN. No binding was observed between the two monolayers in the absence of Ca(2+)-ions. Cd(2+)-ions could partially induce the self-interaction. In contrast, similar AFM experiments with thiol-spacer-containing beta-D: -Galp4,6(R)Pyr-(1-->4)-beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp-(1-->O)(CH(2))(3)S(CH(2))(6)S- did not show a binding in the presence of Ca(2+)-ions. Also TEM experiments of gold nanoparticles coated with the pyruvated trisaccharide could not make visible aggregation in the presence of Ca(2+)-ions. It is suggested that the self-interaction between the sulfated disaccharide fragments is stronger than that between the pyruvated trisaccharide.

The glycosylation and pharmacokinetics of CTLA4Ig produced in rice cells

Cytotoxic T-lymphocyte antigen 4-immunoglobulin (CTLA4Ig) has immunosuppressive activity and the ability to induce immune tolerance. There has been no report of its glycosylation ratio or of the role of its glycans. We investigated the terminal sialylation of rice cell-derived recombinant human CTLA4Ig (rrhCTLA4Ig) using lectins. The glycosylation ratios of rrhCTLA4Ig and Chinese hamster ovary (CHO) cell-derived recombinant human CTLA4Ig (crhCTLA4Ig) were evaluated by chemical deglycosylation. After intravenous (i.v.) or subcutaneous (s.c.) administration to rats, the pharmacokinetics of rrhCTLA4Ig and crhCTLA4Ig as well as of their deglycosylated forms were evaluated. rrhCTLA4Ig does not have terminal sialic acids and its glycosylation ratio was slightly lower than that of crhCTLA4Ig. Its terminal elimination half-life (T(1/2)) was shorter than that of crhCTLA4Ig following i.v. administration. However, the half-life was significantly prolonged and was similar with that of crhCTLA4Ig following s.c. administration. Moreover, the deglycosylated forms of both preparations were cleared from the circulation faster than the native forms. These results suggest that the presence of glycans on rrhCTLA4Ig and crhCTA4Ig are important for their in vivo stability. In addition, the glycan structure of rrhCTLA4Ig is more effective in maintaining in vivo stability after s.c. administration than after i.v. administration although the glycans on rrhCTLA4Ig lack terminal sialic acids, suggesting that its glycans have the potential for in vivo stability.

Analysis of carbohydrate-carbohydrate interactions using gold glyconanoparticles and oligosaccharide self-assembling monolayers

Carbohydrates are the most extended structures exposed at the surface of most cells. These carbohydrate chains, when arranged in polyvalent clusters, offer a rich supply of low-affinity binding sites, making them a reliable and flexible system to regulate cell adhesion and recognition. The very first model system for cell-cell recognition by means of carbohydrate-carbohydrate interactions in the animal kingdom came from a primitive invertebrate animal: the marine sponge. During the past 50 years, studies have shown that highly repetitive carbohydrate motives on extracellular proteoglycan supramolecular complexes of marine sponge cells are involved in the species-specific adhesion. In this chapter, some glyconanotechnology procedures are described for the detailed investigation of the role of a carbohydrate epitope in the marine sponge cell recognition. The various protocols are generally applicable in other areas of glycoscience.

Proteoglycan mechanics studied by single-molecule force spectroscopy of allotypic cell adhesion glycans

Early Metazoans had to evolve the first cell adhesion system addressed to maintaining stable interactions between cells constituting different individuals. As the oldest extant multicellular animals, sponges are good candidates to have remnants of the molecules responsible for that crucial innovation. Sponge cells associate in a species-specific process through multivalent calcium-dependent interactions of carbohydrate structures on an extracellular membrane-bound proteoglycan termed aggregation factor. Single-molecule force spectroscopy studies of the mechanics of aggregation factor self-binding indicate the existence of intermolecular carbohydrate adhesion domains. A 200-kDa aggregation factor glycan (g200) involved in cell adhesion exhibits interindividual differences in size and epitope content which suggest the existence of allelic variants. We have purified two of these g200 distinct forms from two individuals of the same sponge species. Comparison of allotypic versus isotypic g200 binding forces reveals significant differences. Surface plasmon resonance measurements show that g200 self-adhesion is much stronger than its binding to other unrelated glycans such as chondroitin sulfate. This adhesive specificity through multiple carbohydrate binding domains is a type of cooperative interaction that can contribute to explain some functions of modular proteoglycans in general. From our results it can be deduced that the binding strength/surface area between two aggregation factor molecules is comparable with that of focal contacts in vertebrate cells, indicating that strong carbohydrate-based cell adhesions evolved at the very start of Metazoan history.

Channel-forming membrane proteins as molecular sensors

Membrane channels are typically around or less than 1 nm in diameter and a description of the flow through them requires a molecular approach called nanofluidic. The ion current through channels is extremely sensitive to pore sizes. It is tempting to use the ion current to probe conformational changes of the channel or, for a fixed channel conformation, the current can be used to follow binding of molecules to the pore surfaces. Here we show the sensitivity of this method. It is possible to observe the passage of single isolated molecules through the channel and it is possible to discriminate between different passing molecules. Bioengineering allows us to modify channel surfaces and the affinity to different host molecules. Combining engineered proteins with the appropriated detection technique will allow a new type of molecular sensor.

Carbohydrate-carbohydrate interaction as a major force initiating cell-cell recognition

Sponges were the earliest multicellular organisms to evolve through the development of cell recognition and adhesion processes mediated by cell surface proteoglycans. Information on sponges has an extra added value because, as a group, they are the oldest Metazoans alive and contribute more to our understanding of life on earth than knowledge of other animal groups. Although the proteoglycans are emerging as key players in various physiological and pathophysiological cellular events, little is known about the carbohydrate moiety of the proteoglycan molecule. Until recently there was no evidence provided for the existence of specific and biologically significant carbohydrate-carbohydrate interaction. We show here that the interaction between single oligosaccharides of surface proteoglycans is relatively strong (in the 200-300 piconewtons range) and in the same range as other relevant biological interactions, like those between antibodies and antigens. This carbohydrate-carbohydrate recognition is highly species-specific and perfectly mimics specific cell-cell recognition. Both the strength and the species-specificity of the carbohydrate-carbohydrate interaction are guaranteed by polyvalency, by compositional and architectural differences between carbohydrates, and by the arrangement of the carbohydrate chain in a three-dimensional context. Ca(2+)-ions are essential and probably provide coordinating forces. Our findings confirm the existence and character of species-specific carbohydrate-carbohydrate recognition fundamental to cell recognition and adhesion events.

Comparison of molecular mechanisms mediating cell contact phenomena in model developmental systems: an exploration of universality

Are there universal molecular mechanisms associated with cell contact phenomena during metazoan ontogenesis? Comparison of adhesion systems in disparate model systems indicates the existence of unifying principles. Requirements for multicellularity are (a) the construction of three-dimensional structures involving a crucial balance between adhesiveness and motility; and (b) the establishment of integration at molecular, cellular, tissue, and organismal levels of organization. Mechanisms for (i) cell-cell and cell-substrate adhesion, (ii) cell movement, (iii) cell-cell communication, (iv) cellular responses, (v) regulation of these processes, and (vi) their integration with patterning, growth, and other developmental processes are all crucial to metazoan development, and must have been present for the emergence and radiation of Metazoa. The principal unifying themes of this review are the dynamics and regulation of cell contact phenomena. Our knowledge of the dynamic molecular mechanisms underlying cell contact phenomena remains fragmentary. Here we examine the molecular bases of cell contact phenomena using extant model developmental systems (representing a wide range of phyla) including the simplest i.e. sponges, and the eukaryotic protist Dictyostelium discoideum, the more complex Drosophila melanogaster, and vertebrate systems. We discuss cell contact phenomena in a broad developmental context. The molecular language of cell contact phenomena is complex; it involves a plethora of structurally and functionally diverse molecules, and diverse modes of intermolecular interactions mediated by protein and/or carbohydrate moieties. Reasons for this are presumably the necessity for a high degree of specificity of intermolecular interactions, the requirement for a multitude of different signals, and the apparent requirement for an increasingly large repertoire of cell contact molecules in more complex developmental systems, such as the developing vertebrate nervous system. However, comparison of molecular models for dynamic adhesion in sponges and in vertebrates indicates that, in spite of significant differences in the details of the way specific cell-cell adhesion is mediated, similar principles are involved in the mechanisms employed by members of disparate phyla. Universal requirements are likely to include (a) rapidly reversible intermolecular interactions; (b) low-affinity intermolecular interactions with fast on-off rates; (c) the compounding of multiple intermolecular interactions; (d) associated regulatory signalling systems. The apparent widespread employment of molecular mechanisms involving cadherin-like cell adhesion molecules suggests the fundamental importance of cadherin function during development, particularly in epithelial morphogenesis, cell sorting, and segregation of cells.

Carbohydrate–carbohydrate interactions in cell recognition

Obtaining a better understanding of the molecular basis of cell recognition remains an important challenge with regard to the social functioning of cells in multicellular systems. The wide structural diversity of carbohydrates allows many combinatorial possibilities for fine-tuning cell-cell and cell-matrix recognition in multicellular organisms. Direct carbohydrate-carbohydrate interaction would endow both the flexibility and the specificity of reversible contacts at the cell surface during the formation, maintenance and pathogenesis of tissues. The recent development of methods for the characterization of such interactions will help to expand our knowledge of the mechanisms that trigger early events in cell recognition.

Carbohydrate-carbohydrate interaction provides adhesion force and specificity for cellular recognition

The adhesion force and specificity in the first experimental evidence for cell-cell recognition in the animal kingdom were assigned to marine sponge cell surface proteoglycans. However, the question whether the specificity resided in a protein or carbohydrate moiety could not yet be resolved. Here, the strength and species specificity of cell-cell recognition could be assigned to a direct carbohydrate-carbohydrate interaction. Atomic force microscopy measurements revealed equally strong adhesion forces between glycan molecules (190-310 piconewtons) as between proteins in antibody-antigen interactions (244 piconewtons). Quantitative measurements of adhesion forces between glycans from identical species versus glycans from different species confirmed the species specificity of the interaction. Glycan-coated beads aggregated according to their species of origin, i.e., the same way as live sponge cells did. Live cells also demonstrated species selective binding to glycans coated on surfaces. These findings confirm for the first time the existence of relatively strong and species-specific recognition between surface glycans, a process that may have significant implications in cellular recognition.

Molecular recognition between glyconectins as an adhesion self-assembly pathway to multicellularity

The appearance of multicellular forms of life has been tightly coupled to the ability of an organism to retain its own anatomical integrity and to distinguish self from non-self. Large glycoconjugates, which make up the outermost cell surface layer of all Metazoans, are the primary candidates for the primordial adhesion and recognition functions in biological self-assembly systems. Atomic force microscopy experiments demonstrated that the binding strength between a single pair of Porifera cell surface glyconectin 1 glycoconjugates from Microciona prolifera can hold the weight of 1600 cells, proving their adhesion functions. Here, measurement of molecular self-recognition of glyconectins (GNs) purified from three Porifera species was used as an experimental model for primordial xenogeneic self/non-self discrimination. Physicochemical and biochemical characterization of the three glyconectins, their glycans, and peptides using gel electrophoresis, ultracentrifugation, NMR, mass spectrometry, glycosaminoglycan-degrading enzyme treatment, amino acid and carbohydrate analyses, and peptide mapping showed that GNs define a new family of proteoglycan-like molecules exhibiting species-specific structures with complex and repetitive acidic carbohydrate motives different from the classical proteoglycans and mucins. In functional self-assembly color-coded bead, cell, and blotting assays, glyconectins displayed species-specific recognition and adhesion. Affinity-purified monospecific polyclonal antibodies prepared against GN1, -2, and -3 glycans selectively inhibited cell adhesion of the respective sponge species. These results together with species-specific coaggregation of GN carbohydrate-coated beads with cells showed that GN glycans are functional in cell recognition and adhesion. The specificity of carbohydrate-mediated homophilic GN interactions in Porifera approaches the binding selectivity of the evolutionarily advanced immunoglobulin superfamily. Xenoselectivity of primordial glyconectin to glyconectin recognition may be a new paradigm in the self-assembly and non-self discrimination pathway of cellular adhesion leading to multicellularity.

Deglycosylation of glycoproteins with trifluoromethanesulphonic acid: elucidation of molecular structure and function

The alteration of proteins by post-translational modifications, including phosphorylation, sulphation, processing by proteolysis, lipid attachment and glycosylation, gives rise to a broad range of molecules that can have an identical underlying protein core. An understanding of glycosylation of proteins is important in clarifying the nature of the numerous variants observed and in determining the biological roles of these modifications. Deglycosylation with TFMS (trifluoromethanesulphonic acid) [Edge, Faltynek, Hof, Reichert, and Weber, (1981) Anal. Biochem. 118, 131-137] has been used extensively to remove carbohydrate from glycoproteins, while leaving the protein backbone intact. Glycosylated proteins from animals, plants, fungi and bacteria have been deglycosylated with TFMS, and the most extensively studied types of carbohydrate chains in mammals, the N-linked, O-linked and glycosaminoglycan chains, are all removed by this procedure. The method is based on the finding that linkages between sugars are sensitive to cleavage by TFMS, whereas the peptide bond is stable and is not broken, even with prolonged deglycosylation. The relative susceptibility of individual sugars in glycosidic linkage varies with the substituents at C-2 and the occurrence of amido and acetyl groups, but even the most stable sugars are removed under conditions that are sufficiently mild to prevent scission of peptide bonds. The post-translational modifications of proteins have been shown to be required for diverse biological functions, and selective procedures to remove these modifications play an important role in the elucidation of protein structure and function.

Individual cartilage aggrecan macromolecules and their constituent glycosaminoglycans visualized via atomic force microscopy

Atomic force microscopy was used in ambient conditions to directly image dense and sparse monolayers of bovine fetal epiphyseal and mature nasal cartilage aggrecan macromolecules adsorbed on mica substrates. Distinct resolution of the non-glycosylated N-terminal region from the glycosaminoglycan (GAG) brush of individual aggrecan monomers was achieved, as well as nanometer-scale resolution of individual GAG chain conformation and spacing. Fetal aggrecan core protein trace length (398+/-57 nm) and end-to-end length (257+/-87 nm) were both larger than that of mature aggrecan (352+/-88 and 226+/-81 nm, respectively). Similarly, fetal aggrecan GAG chain trace length (41+/-7 nm) and end-to-end (32+/-8 nm) length were both larger than that of mature aggrecan GAG (32+/-5 and 26+/-7 nm, respectively). GAG-GAG spacing along the core protein was significantly smaller in fetal compared to mature aggrecan (3.2+/-0.8 and 4.4+/-1.2nm, respectively). Together, these differences between the two aggrecan types were likely responsible for the greater persistence length of the fetal aggrecan (110 nm) compared to mature aggrecan (82 nm) calculated using the worm-like chain model. Measured dimensions and polymer statistical analyses were used in conjunction with the results of Western analyses, chromatographic, and carbohydrate electrophoresis measurements to better understand the dependence of aggrecan structure and properties on its constituent GAG chains.

Cell adhesion-related proteins as specific markers of sponge cell types involved in allogeneic recognition

Sponge immunocyte identification is of interest to comparative immunologists since characterizing these cells will allow investigations into the mechanisms of non-self recognition in the oldest animal phylum. Here, we report that polyclonal antibodies raised against the core protein of a proteoglycan involved in cell adhesion in the marine sponge Microciona prolifera are specific markers for archaeocytes, the totipotent sponge cells. Archaeocytes are mobilized upon allogeneic contact and they accumulate in the contact zone. A second type of cell, the gray cells, are specifically recognized by monoclonal antibodies raised against CD44, a hyaluronan receptor. Gray cells do also accumulate in the contact area. Specific staining of a third sponge cell type, the rhabdiferous cells, shows that these do not accumulate upon allografting. These specific cell markers allow tracking of archaeocytes and gray cells, and show that they play an active role in sponge allogeneic reactions.