Role of a structural glycoprotein of corneal stroma in transplantation immunity

A tribute to Ladislas Robert

This Thematic Minireview Series of Matrix Biology focused on elastin, from structure to disease celebrates the memory of Ladislas Robert, a pioneer in Matrix Biology in France and Europe. Since his first publication on elastin and elastases in 1957, the huge development in matrix biology led to major findings on elastic fibers and their component proteins including elastin architecture, the role of fibrillins and microfibril-binding proteins on elastin assembly, the effects of sequence variants of human tropoelastin on its assembly, structure and functions, the role of elastin peptides in health and diseases, the identification of neuraminidase-1 as a member of the elastin receptor complex, and the fate of elastic fibers upon aging, which are reviewed in this series. Two other reviews, focused on the design and use of elastin-like recombinamers as biomaterials, and on the circadian rhythms in skin and other elastic tissues, complete this series.

[50 years of connective tissue research: from the French Connective Tissue Club to the French Society of Extracellular Matrix Biology]

The history of connective tissue research began in the late 18th century. However, it is only 50 years later that the concept of connective tissue was shaped. It took another fifty years before biochemical knowledge of extracellular matrix macromolecules began to emerge in the first half of the 20th century. In 1962, thanks to Ladislas and Barbara Robert, back from the US, the first society called "French Connective Tissue Club" was created in Paris. The first board was constituted of Albert Delaunay, Suzanne Bazin and Ladislas Robert. Very quickly, under the influence of these pioneers, national and international meetings were organized and, in 1967, a "Federation of the European Connective Tissue Clubs" was created at the initiative of Ladislas Robert (Paris) and John Scott (Manchester). It spread rapidly to the major European nations. In 1982 the transformation of "Clubs" in "Societies" occurred, a name more in line with the requirements of the time. In 2008, the "French Connective Tissue Society" became the "French Society of Extracellular Matrix Biology" ("Société Française de Biologie de la Matrice Extracellulaire", SFBMEc), to better highlight the importance of the extracellular matrix in the biology of living organisms. The SFBMEc's mission today is to promote and develop scientific exchanges between academic, industrial, and hospital laboratories involved in research on the extracellular matrix. SFBMEc organizes or subsidizes scientific meetings and awards scholarships to Ph.D. students or post-docs to participate in international conferences. It includes 200 to 250 members from different disciplines, developing strong interactions between scientists, clinicians and pathologists. It is present all around the French territory in many research laboratories. During these last 50 years, the extraordinary advances made possible by the development of new investigation techniques, in particular molecular biology, cell and tissue imaging, molecular modeling, etc., have permitted a considerable increase of the knowledge in the field of connective tissue.

[Eye connective tissues: cornea and vitreous body]

The authors, ophtalmologist (Y.P.) and basic scientists (J.L.-R and L.R.), collaborated on eye-research since 1962 on normal and pathological aspects of eye tissues, considered as specialized forms of connective tissues, and on specific aspects of the physiology and pathology of the eye. This date coincides with the foundation of the French Society of Connective Tissues, which celebrates the 50th anniversary of its creation. We shall present here some of our work on the ontogenetic and phylogenetic aspects of the cornea, on its structure, function and regulation in normal and pathological states, taken from a large number of publications of our laboratories. Our work on cornea started with the study of the morphogenesis of its lamellar structure, made of collagen fibers and proteoglycans. This led us to the isolation and characterization of structural (or matrix) glycoproteins, a new class of matrix components, present also in all other connective tissues, and to the study of their biosynthesis by keratocytes. Corneal wounds and regeneration were also studied, as well as some corneal pathologies such as keratoconus. The confrontation of quantitative morphological methods with biochemical procedures were to yield important results on the mechanisms of the maintenance of corneal structure and function. Another series of studies concerned the vitreous where we detected, besides previously characterized components, such as hyaluronan and collagens, fibronectin which plays an important role in the adhesion of hyaluronan to the collagen network. Its age-dependent modifications were also studied, with a special focus on the role of reactive oxygen species (ROS)-mediated degradation of hyaluronan, especially important for the aging of the vitreous.

Fifty years of structural glycoproteins

During decades preceding and following the last war, a favourite subject of biochemists was to study glycoproteins. One class of these substances, found in connective tissues were characterised as polysaccharides, most of them found to be linked to proteins, designated later as glycosaminoglycans and proteoglycans. Another family of glycoconjugates represented epithelial mucins as found in the gastro-intestinal and respiratory tracts and conduits. A third family of glycoconjugates is represented by circulating glycoproteins isolated from the blood plasma, mostly studied by medical biochemists in relation to pathological conditions comprising those increasing during the inflammatory reaction: acute phase glycoproteins. Their study suggested that they might be derived from connective tissues. Although inflammatory glycoproteins derive mostly from the liver, the possibility of connective tissue origin of glycoproteins remained open. Using cornea, an avascular tissue, we could show that connective tissues also synthesize glycoproteins. We proposed to designate them "structural glycoproteins" (SGP-s) to distinguish them from circulating, blood-born glycoproteins coming from the liver. They play locally "structural" roles in connective tissues where they are synthesized. Soon after fibronectin was identified and shown to mediate cell-matrix interactions. A large family of glycoproteins were then isolated from a variety of sources, cells, tissues others than liver, confirming our original hypothesis. The first experiments on these glycoproteins were published from 1961/1962 giving the opportunity to recapitulate this biochemical adventure 50 years later, together with the celebration of the foundation of the first connective tissue society in Europe, as described in the first article in this issue.

Effect of procyanidolic oligomers of Vitis vinifera on the biosynthesis and excretion of corneal glycosaminoglycans

Glycosaminoglycans (GAG) as hyaluronan and proteoglycans (PG) as dermatan sulfate (DSPG) and keratan sulfate (KSPG) play important role in the structure and physiological functions of cornea. Several corneal pathologies are associated with qualitative and quantitative alterations of GAG and/or PG biosynthesis. For these reasons we investigated the effect of procyanidolic oligomers (PCO) on the biosynthesis and excretion of GAG-s in bovine corneal explant cultures. This was accomplished by the determination of their chemical components, hexoses, hexosamines and uronic acids as well as the incorporation of (35)S-sulfate in the presence and without PCO in the culture medium. In presence of 1 mg/ml PCO total hexoses, hexosamines and uronic acids increased at 5 and 24 h of incubation, hexoses less than the two other components. Stimulation of biosynthesis concerned essentially the uronic acid containing GAG-s, DS and hyaluronan. (35)S-sulfate incorporation decreased in presence of PCO, showing a decreased sulfation of GAG-s. The most striking effect was however the strongly increased excretion of neosynthesized GAG-s in the culture medium. This effect could be beneficial in decreasing corneal hydration in inflammatory conditions, together with the previously demonstrated protection of corneal macromolecules against proteolytic degradation.

Lectin-binding sites in quick-frozen, freeze-substituted extracellular matrix of rabbit corneal stroma: a comparative electron microscopic study with different embedding procedures

The efficiency of Epon, Lowicryl K4M and LR White resins for electron microscopy was tested on the reactivity of carbohydrate residues to lectins in quick-frozen, freeze-substituted extracellular matrix of rabbit corneal stroma, without the use of any chemical fixation. Three lectins conjugated to gold particles were used in a post-embedding method: Wheat germ agglutinin, Concanavalin A and Ricinus communis agglutinin 1. Best gold-labelling was obtained with Lowicryl K4M for Wheat germ agglutinin, and with both Lowicryl K4M and Epon for Concanavalin A and Ricinus communis agglutinin 1. Tissues embedded in LR White gave less intense labelling for the three lectins used in comparison with the two other resins. This study demonstrates the crucial role of resins in the optimal preservation of biological activity of carbohydrate residues in cryotechnically-processed cornea for electron microscopy.

Isolation and preliminary characterization of a structural glycoprotein complex from bovine corneal stroma

After extraction of bovine corneal stroma with 1M CaC1(2) and subsequent digestion of the insoluble residue with purified bacterial collagenase, two crude structural glycoprotein (SPG) fractions were obtained; one which precipitated upon dialysis against water of the collagenase solubilized material and the other which was extracted by 8M urea from the collagenase insoluble material. Amino acid analyses of these crude SGP fractions indicated that they were primarily non-collagenous but that very small amounts of collagen-derived amino acids were present. Upon gel filtration of these SGP fractions on Sepharose 4B-CL, void volume fractions were isolated from each of the crude fractions which were enriched, relative to the original crude fractions, in the collagen-derived amino acids. Carbohydrate analysis indicated that the void volume fractions had the properties of glycoproteins rather than proteoglycans. Upon disulfide reduction and SDS-PAGE, each of these fractions was resolved into five major protein bands with molecular weights of 155,000, 137,000, 117,000 82,000 and 34,000. Only the three largest bands contained the collagen derived amino acids. These data are consistent with the presence within bovine corneal stroma of a large structural glycoprotein complex comprised of at least five protein components associated through disulfide bonds. Collagen apparently is associated with three of these, either through covalent crosslinkage, or as part of the primary structure.

The antigenicity of soluble porcine elastins: I. Measurement of antibody by a radioimmunoassay

Immunization of rabbits with either porcine tropoelastin, lung alpha-elastin, or aortic alpha-elastin resulted in the production of antibodies against the respective antigens. The assay of antibody activity with a radioimmunoassay indicated a high degree of cross reactivity between these three soluble elastin preparations. Through the use of competitive protein binding experiments it was possible to detect antigen-specific differences between tropoelastin and the two alpha-elastins. No antigenic differences were observed between lung or aortic alpha-elastin by any of the assay procedures uded in this investigation. Absorption of the various antisera with either insolubilized tropoelastin, lung elastin or aortic elastin allowed the preparation of non-crossreactive antibodies and provided further evidence for the antigenic differences between tropoelastin and the soluble alpha-elastins. The implications of the findings, as they relate to our current understanding of the molecular and structural arrangement of elastin and the elastin precursor, are discussed.