Biochemical and immunological characterization of collagen molecules from echinothurioid sea urchin Asthenosoma ijimai

Diverse and Productive Source of Biopolymer Inspiration: Marine Collagens

Marine biodiversity is expressed through the huge variety of vertebrate and invertebrate species inhabiting intertidal to deep-sea environments. The extraordinary variety of "forms and functions" exhibited by marine animals suggests they are a promising source of bioactive molecules and provides potential inspiration for different biomimetic approaches. This diversity is familiar to biologists and has led to intensive investigation of metabolites, polysaccharides, and other compounds. However, marine collagens are less well-known. This review will provide detailed insight into the diversity of collagens present in marine species in terms of their genetics, structure, properties, and physiology. In the last part of the review the focus will be on the most common marine collagen sources and on the latest advances in the development of innovative materials exploiting, or inspired by, marine collagens.

Biomineralization of Schlumbergerella floresiana, a significant carbonate-producing benthic foraminifer

Most foraminifera that produce a shell are efficient biomineralizers. We analyzed the calcitic shell of the large tropical benthic foraminifer Schlumbergerella floresiana. We found a suite of macromolecules containing many charged and polar amino acids and glycine that are also abundant in biomineralization proteins of other phyla. As neither genomic nor transcriptomic data are available for foraminiferal biomineralization yet, de novo-generated sequences, obtained from organic matrices submitted to ms blast database search, led to the characterization of 156 peptides. Very few homologous proteins were matched in the proteomic database, implying that the peptides are derived from unknown proteins present in the foraminiferal organic matrices. The amino acid distribution of these peptides was queried against the uniprot database and the mollusk uniprot database for comparison. The mollusks compose a well-studied phylum that yield a large variety of biomineralization proteins. These results showed that proteins extracted from S. floresiana shells contained sequences enriched with glycine, alanine, and proline, making a set of residues that provided a signature unique to foraminifera. Three of the de novo peptides exhibited sequence similarities to peptides found in proteins such as pre-collagen-P and a group of P-type ATPases including a calcium-transporting ATPase. Surprisingly, the peptide that was most similar to the collagen-like protein was a glycine-rich peptide reported from the test and spine proteome of sea urchin. The molecules, identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry analyses, included acid-soluble N-glycoproteins with its sugar moieties represented by high-mannose-type glycans and carbohydrates. Describing the nature of the proteins, and associated molecules in the skeletal structure of living foraminifera, can elucidate the biomineralization mechanisms of these major carbonate producers in marine ecosystems. As fossil foraminifera provide important paleoenvironmental and paleoclimatic information, a better understanding of biomineralization in these organisms will have far-reaching impacts.

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.

Cell adhesion and communication: a lesson from echinoderm embryos for the exploitation of new therapeutic tools

In this chapter, we summarise fundamental findings concerning echinoderms as well as research interests on this phylum for biomedical and evolutionary studies. We discuss how current knowledge of echinoderm biology, in particular of the sea urchin system, can shed light on the understanding of important biological phenomena and in dissecting them at the molecular level. The general principles of sea urchin embryo development are summarised, mainly focusing on cell communication and interactions, with particular attention to the cell-extracellular matrix and cell-cell adhesion molecules and related proteins. Our purpose is not to review all the work done over the years in the field of cellular interaction in echinoderms. On the contrary, we will rather focus on a few arguments in an effort to re-examine some ideas and concepts, with the aim of promoting discussion in this rapidly growing field and opening new routes for research on innovative therapeutic tools.

Distinct maturations of N-propeptide domains in fibrillar procollagen molecules involved in the formation of heterotypic fibrils in adult sea urchin collagenous tissues

We have characterized the primary structure of a new sea urchin fibrillar collagen, the 5alpha chain, including nine repeats of the sea urchin fibrillar module in its N-propeptide. By Western blot and immunofluorescence analyses, we have shown that 5alpha is co-localized in adult collagenous ligaments with the 2alpha fibrillar collagen chain and fibrosurfin, two other extracellular matrix proteins possessing sea urchin fibrillar modules. At the ultrastructural level, the 5alpha N-propeptide is detected at the surface of fibrils, suggesting the retention of this domain in mature collagen molecules. Biochemical characterization of pepsinized collagen molecules extracted from the test tissue (the endoskeleton) together with a matrix-assisted laser desorption ionization time-of-flight analysis allowed us to determine that 5alpha is a quantitatively minor fibrillar collagen chain in comparison with the 1alpha and 2alpha chains. Moreover, 5alpha forms heterotrimeric molecules with two 1alpha chains. Hence, as in vertebrates, sea urchin collagen fibrils are made up of quantitatively major and minor fibrillar molecules undergoing distinct maturation of their N-propeptide regions and participating in the formation of heterotypic fibrils.

Characterization of fibrosurfin, an interfibrillar component of sea urchin catch connective tissues

The Sea URchin Fibrillar (SURF) domain is a four-cysteine module present in the amino-propeptide of the sea urchin 2alpha fibrillar collagen chain. Despite numerous international genome and expressed sequence tag projects, computer searches have so far failed to identify similar domains in other species. Here, we have characterized a new sea urchin protein of 2656 amino acids made up of a series of epidermal growth factor-like and SURF modules. From its striking similarity to the modular organization of fibropellins, we called this new protein fibrosurfin. This protein is acidic with a calculated pI of 4.12. Eleven of the 17 epidermal growth factor-like domains correspond to the consensus sequence of calcium-binding type. By Western blot and immunofluorescence analyses, this protein is not detectable during embryogenesis. In adult tissues, fibrosurfin is co-localized with the amino-propeptide of the 2alpha fibrillar collagen chain in several collagenous ligaments, i.e., test sutures, spine ligaments, peristomial membrane, and to a lesser extent, tube feet. Finally, immunogold labeling indicates that fibrosurfin is an interfibrillar component of collagenous tissues. Taken together, the data suggest that proteins possessing SURF modules are localized in the vicinity of mineralized tissues and could be responsible for the unique properties of sea urchin mutable collagenous tissues.

Sea urchin fibrillar collagen 2alpha chain participates in heterotrimeric molecules of (1alpha)(2)2alpha stoichiometry

In sea urchin, two fibrillar collagen chains (alpha1 and alpha2) have been characterized by molecular biology while two biochemically detected chains (alpha1 and alpha2) have been reported. Here, to determine the relationship between these results, Western-blotting and Edman degradation sequencing of the amino-termini of pepsinized sea urchin fibrillar collagen chains were performed. The data demonstrate that the 2alpha chain corresponds to the alpha2 chain and is involved in the formation of heterotrimeric molecules [(1alpha)(2)2alpha].

Growth Factor Modulation of Substrate-Specific Morphological Patterns in Aplysia Bag Cell Neurons

Components of the extracellular matrix (ECM) can act not only as passive substrates for neuronal attachment and outgrowth but also as active sites for signal transduction. Thus, specific ECM components may modulate effects of growth factors (GFs) that play an important role in structural changes in development and adult neuronal plasticity. In this study we examined the interaction of cultured Aplysia bag cell neurons (BCNs) with components of ECM and different GFs. Different ECM substrata induce a substrate-specific BCN morphology: BCNs grown on collagen or poly-l-lysine have larger soma diameter and more extensive neurite outgrowth than BCNs grown on laminin or fibronectin. BCNs also interact in a substrate-dependent way with GFs: BDNF treatment leads to a reduction of outgrowth on poly-l-lysine but an enhancement on fibronectin and laminin. CNTF reduces the soma diameter on collagen IV but enlarges it on laminin or fibronectin. In contrast, NGF induces a reduction of both soma diameter and outgrowth, on all substrata. Plating of BCNs in the presence of anti-β1-integrin reduces adhesion to fibronectin but does not change outgrowth. In contrast, RGD peptides block adhesion to laminin and poly-l-lysine and, additionally, reduce outgrowth on laminin. These data suggest that BCNs use different β1-integrin-dependent as well as RGD-dependent mechanisms for adhesion and outgrowth on different ECM substrata, providing possible sites of modulation by specific GFs.

Flexilin: a new extracellular matrix glycoprotein localized on collagen fibrils

We have immunopurified and characterized a new glycoprotein of the extracellular matrix, using a monoclonal antibody obtained after immunization with fibril-associated collagens extracted from bovine tendon. In polyacrylamide gels, the protein migrates at about 350 kDa molecular mass. The protein is insensitive to bacterial collagenase, and no disulfide-linked aggregates could be detected; sugars were stained with periodic acid-Schiff's reagent. Amino acid analysis and sequencing of tryptic peptides failed to detect any similarity with known proteins. By rotary shadowing experiments, the protein was observed as flexible, unbranched structures, approximately 150 nm long, with a small globule at one end. Investigation of the tissue distribution of the protein in fetal bovine tissues by immunofluorescence resulted in labeling in extracellular matrices with loosely packed collagen fibrils, such as the peritendineum, embryonic skin and kidney glomeruli; cornea, cartilage matrix and bone were not labeled. Ultrastructural immunolocalization in dermis and in mesangium of glomeruli showed that the protein always occurred in the vicinity of collagen fibrils. In view of its tissue distribution and molecular shape, we postulate that this protein is important in the properties of the extrafibrillar environment. By reference to its shape as observed by rotary shadowing, we propose the name 'flexilin' for this extracellular matrix glycoprotein.

Characterizations of sea urchin fibrillar collagen and its cDNA clone

Collagens were isolated from the adult test of the sea urchin species, Hemicentrotus pulcherrimus and Strongylocentrotus purpuratus, and their molecular properties were compared with those of Asthenosoma ijimai collagen. Collagens from H. pulcherrimus and S. purpuratus comprised two major alpha-chains (alpha 120 and alpha 90) and a minor chain (alpha 140), while collagen from A. ijimai contained four alpha-chains (alpha 1, alpha 2, alpha 3 and alpha 4). Based on their molecular and immunological properties, the alpha 90 chain of H. pulcherrimus and S. purpuratus, and the alpha 2 and alpha 4 chains of A. ijimai are grouped together, while the alpha 120 and alpha 140 chains of H. pulcherrimus and S. purpuratus, and the alpha 1 and alpha 3 chains of A. ijimai are classified into another group. It is likely that collagen molecules of sea urchins are heterotrimers composed of these two types of alpha-chains. A cDNA of collagen was cloned from the cDNA library prepared from mRNA of H. pulcherrimus test and denoted as Hpcol1. This clone contained sequences for uninterrupted triple helical domain (378 amino acids), carboxyl telopeptide (28 amino acids) and carboxyl propeptide (225 amino acids). This structure is characteristic for fibril-forming collagens and was shown to encode alpha 120 and alpha 140 chains of H. pulcherrimus collagen. Hpcol1-mRNA was expressed in embryos as early as the prism stage.

Molecular structure and functional morphology of echinoderm collagen fibrils

The collagenous tissues of echinoderms, which have the unique capacity to rapidly and reversibly alter their mechanical properties, resemble the collagenous tissues of other phyla in consisting of collagen fibrils in a nonfibrillar matrix. Knowledge of the composition and structure of their collagen fibrils and interfibrillar matrix is thus important for an understanding of the physiology of these tissues. In this report it is shown that the collagen molecules from the fibrils of the spine ligament of a sea-urchin and the deep dermis of a sea-cucumber are the same length as those from vertebrate fibrils and that they assemble into fibrils with the same repeat period and gap/overlap ratio as do those of vertebrate fibrils. The distributions of charged residues in echinoderm and vertebrate molecules are somewhat different, giving rise to segment-long-spacing crystallites and fibrils with different banding patterns. Compared to the vertebrate pattern, the banding pattern of echinoderm fibrils is characterized by greatly increased stain intensity in the c3 band and greatly reduced stain intensity in the a3 and b2 bands. The fibrils are spindle-shaped, possessing no constant-diameter region throughout their length. The shape of the fibrils is mechanically advantageous for their reinforcing role in a discontinuous fiber-composite material.

Native collagen fibrils from echinoderms are molecularly bipolar

Collagen fibrils are generally assumed to be cylinders with uniform diameters (except possibly at their ends) and to be composed of molecules all of which have the same polarity. These assumptions have been largely untested because of the extreme difficulty associated with isolating entire native fibrils. Intact collagen fibrils are readily extracted from certain echinoderms, however, and we have therefore analyzed the molecular structure of these fibrils. Our electron microscopic analyses show the above assumptions to be false: echinoderm fibrils, which previously have been shown to be symmetrically spindle shaped, are also molecularly bipolar. Their constituent molecules have their N-termini oriented toward the nearest fibril end, and they are antiparallel in the fibril center. The shape and molecular arrangement of these fibrils have implications for fibrillogenesis.

Characterization of heterotrimeric collagen molecules in a sea-pen (Cnidaria, Octocorallia)

The collagen of a primitive invertebrate, the sea-pen Veretillum Cnidaria, Octocorallia), was studied with respect to its molecular-chain composition. The soft extracellular tissues (mesoglea) were solubilized by limited pepsin proteolysis and the collagen was isolated by selective precipitation at 0.7 M NaCl under acidic conditions. The pepsinized molecules were 260 nm in length, as demonstrated by electron microscope studies of rotary-shadowed molecules and of the segment-long-spacing crystallites obtained by dialysis against ATP. SDS/PAGE of the extract produced two main bands susceptible to bacterial collagenase, designated as the alpha 1 and alpha 2 chain, which were differentiated clearly by their CNBr cleavage products and the higher glycosylation rate of the alpha 2 chain. The latter finding corresponds with the high hydroxylysine content of the alpha 2 chain. The alpha 1/alpha 2 chain ratio observed in SDS/PAGE and the fact that only one peak was obtained by concanavalin-A affinity chromatography of a non-denatured 0.7 M NaCl extract demonstrate the alpha 1 [alpha 2]2 molecular structure of this collagen. These results contrast with data on the structure of other coelenterates (i.e. [alpha]3 for sea anemone collagen molecules and alpha 1 alpha 2 alpha 3 for jellyfish collagen molecules). They are discussed in relation to the evolution of collagen.

Primary mesenchyme cells of the sea urchin embryo require an autonomously produced, nonfibrillar collagen for spiculogenesis

A collagen molecule in the sea urchin embryo was characterized by analysis of a 2.7-kb cDNA clone. This clone, Spcoll, was obtained by screening a gastrula stage Strongylocentrotus purpuratus cDNA library with a 237-bp genomic clone encoding a collagen-like sequence previously isolated by Venkatesan et al. (1986). DNA sequence analysis of the cDNA clone demonstrated the nonfibrillar nature of the encoded molecule--13 interruptions of the Gly-X-Y repeat motif were found in the 85-kDa open reading frame. The mRNA of approximately 9 kb accumulated specifically in mesenchyme cells of the embryo through development to the pluteus larva. Polyclonal antibodies generated against a Spcoll-beta-galactosidase fusion protein were utilized to identify and localize the native Spcoll. This collagen molecule of approximately 210 kDa was deposited into the blastocoel by the primary mesenchyme cells. When primary mesenchyme cells were cultured in vitro, Spcoll was secreted into the media and accumulated at sites of cell-substrate interaction. Addition of anti-Spcoll antibodies to primary mesenchyme cell cultures selectively inhibited spiculogenesis, whereas other antibodies had no inhibitory effect. Since collagen is not a component of the organic matrix of spicules (Benson et al., 1986), these results suggest that the autonomous production of Spcoll by differentiating mesenchyme cells in turn influences the point in differentiation at which these cell initiate biomineralization.