Chondroitin sulphate and keratan sulphate are almost isosteric

Structural collagen alterations in macular corneal dystrophy occur mainly in the posterior stroma

PURPOSE

Collagen fibrils in the corneal stroma in macular corneal dystrophy, on average, are more closely spaced than in the normal cornea. This study was conducted to investigate if this occurs uniformly across the stroma or is more prevalent at certain stromal depths.

METHODS

Microbeam synchrotron X-ray fiber diffraction patterns were obtained in 25 microm steps across the whole thickness of a thin strip of a macular corneal dystrophy cornea obtained at keratoplasty. Data were analyzed for mean collagen interfibrillar spacing at all positions. Serum was analyzed immunochemically to determine immunophenotype, and transmission electron microscopy was carried out to visualize stromal ultrastructure.

RESULTS

Keratan sulphate was not detectable in blood serum, classifying the disease as macular corneal dystrophy type I. Collagen interfibrillar spacing dropped linearly with stromal depth from the anterior to posterior cornea, measuring 5-10% less in the posterior 100 microm of the MCD stroma compared to the anterior 100 microm (p < 0.001). Isolated pockets of collagen fibrils with unusually large diameters were identified in the deep stroma.

CONCLUSIONS

Collagen fibril spacing is reduced and large-diameter collagen fibrils are seen in macular corneal dystrophy type I, with the deep stroma affected more. We speculate that the ultrastructural abnormalities are more prevalent in the posterior stroma because the structural influence of sulphated keratan sulphate glycosaminoglycans/proteoglycans is high in this region of the cornea.

Structural and biochemical aspects of keratan sulphate in the cornea

Keratan sulphate (KS) is the predominant glycosaminoglycan (GAG) in the cornea of the eye, where it exists in proteoglycan (PG) form. KS-PGs have long been thought to play a pivotal role in the establishment and maintenance of the array of regularly-spaced and uniformly- thin collagen fibrils which make up the corneal stroma. This characteristic arrangement of fibrils allows light to pass through the cornea. Indeed, perturbations to the synthesis of KS-PG core proteins in genetically altered mice lead to structural matrix alterations and corneal opacification. Similarly, mutations in enzymes responsible for the sulphation of KS-GAG chains are causative for the inherited human disease, macular corneal dystrophy, which is manifested clinically by progressive corneal cloudiness starting in young adulthood.

An immunological study of glycosaminoglycans in the connective tissue of bovine and cod skeletal muscle

The presence of sulfated glycosaminoglycans (GAGs) was demonstrated in the connective tissue of bovine and cod skeletal muscle by histochemical staining using Alcian blue added MgCl(2) (0.06 M and 0.4 M, respectively). For further identification of the sulfated GAGs, a panel of monoclonal antibodies, 1B5, 2B6, 3B3 and 5D4 was used that recognizes epitopes in chondroitin-0-sulfate (C0S), chondroitin-4-sulfate/dermatan sulfate (C4S/DS), chondroitin-6-sulfate (C6S) and keratan sulfate (KS), respectively. Light microscopy and Western blotting techniques showed that in bovine and cod muscle C0S and C6S were primarily localized pericellularly, whereas cod exhibited a more intermittent staining. C4S was expressed around the separate cells and also in the perimysium and myocommata. In contrast to bovine muscle, which hardly expressed highly sulfated KS, cod exhibited a very strong and consistent staining. Western blotting showed that C0S and C6S were mainly associated with proteoglycans (PGs) of high molecular sizes in both species. Contrary to bovine muscle, C4S in cod was associated with molecules of various sizes. Both cod and bovine muscle contained KSPGs of similar sizes as C4S. KSPGs of different sizes and buoyant densities, sensitive to keratanase I and II were found expressed in cod.

The relationship between proteoglycan synthesis in Swarm chondrocytes and pathways of cellular energy and UDP-sugar metabolism

The effect of anaerobic culture conditions and various metabolic inhibitors on 35S-proteoglycan synthesis, UDP-sugar pools, and the ATP pool were investigated in confluent, primary, day 1 cultures of Swarm chondrosarcoma chondrocytes. (i) Incubation under a nitrogen atmosphere for 6 h did not affect 35S-proteoglycan synthesis or the pool size for UDP-glucuronate, other UDP-sugars, or ATP. Incubation with 5 mM sodium azide brought about a 40% reduction of proteoglycan synthesis in the first 30 min but no further change over the subsequent 90 min. UDP-Glucuronate, other UDP-sugar pools, and the ATP level were not affected by azide treatment. The results indicate that proteoglycan synthesis and its energy requirements can be supported entirely by anaerobic metabolism in these cells. (ii) 35S-Proteoglycan synthesis, UDP-sugar production, and nucleotide triphosphate pools were inhibited in a concentration-dependent fashion with sodium iodoacetate. A > 70% reduction of the ATP pool after 30 min treatment suggests that glycolysis is a major target for iodoacetate. Lactate production was inhibited by 40% after 3 h treatment with 10(-4) M iodoacetate. (iii) Glutamine deprivation resulted in a 60% contraction in the UDP-N-acetylhexosamine pool and markedly inhibited 35S-proteoglycan and 3H-protein synthesis. At the same time the UDP-glucose pool expanded to 200%, but the UDP-glucuronate pool was unchanged. The sum of the UDP-N-acetylhexosamine and UDP-hexose pools remained constant. Restoration of glutamine to previously depleted cultures resulted in excessive expansion of the UDP-N-acetylhexosamine pool and excessive contraction of the UDP-hexose pool before both adjusted to normal levels. The UDP-xylose pool was very small. No increases were observed during inhibition of proteoglycan synthesis induced by glutamine deprivation. (iv) 6-Diazo-5-oxo-L-norleucine (DON), a glutamine analogue and amino transferase inhibitor, induced a further contraction of the UDP-N-acetylhexosamine pool and a further decrease in proteoglycan synthesis in glutamine-deprived cultures. Thus cultures use endogenous glutamine during exogenous glutamine deprivation. DON unaccountably stimulated expansion of the UDP-glucuronate pool by 180%, irrespective of whether glutamine was present or not.

On the polylactose nature of chondroitin and keratan sulphates

The recognition that keratan and chondroitin sulphates are based on the same polylactose backbone simplifies the study of heteroduplex formation between them, suggests insights into structure-function relationships in tissues, and permits new definitions of the linkage regions.

The chemical morphology of age-related changes in human intervertebral disc glycosaminoglycans from cervical, thoracic and lumbar nucleus pulposus and annulus fibrosus

Hyaluronan (HA), chondroitin and keratan sulphates (CS, KS), collagen and dry weights were measured in the annulus fibrosus and nucleus pulposus of human cervical, thoracic and lumbar intervertebral discs aged 36-79 y. Alcian blue-critical electrolyte concentration (CEC) staining of sections extended the results. The collagen, total polyanion, HA, CS and KS contents of the nucleus pulposus and annulus fibrosus were plotted for all 3 regions against age. Regional differences and age-related trends were found. For regional differences, the collagen content of the nucleus pulposus was highest in cervical discs and lowest in lumbar discs. In contrast, the total polyanion content of the nucleus pulposus was highest in lumbar discs and lowest in cervical discs. These differences were seen in fetal and adult discs. With respect to age-related trends, the collagen content of the annulus fibrosus was higher in adults and children than in neonates and infants. The collagen content of the nucleus pulposus increased with age in thoracic and lumbar discs, but it was consistently high in cervical discs. There was generally a downward trend of total polyanion and CS with increase in age. This was quite consistent for the annulus fibrosus in all regions and there were dramatic decreases in the lumbar nucleus pulposus in all adults compared with infants and children. These trends were least evident in the cervical nucleus pulposus where infant values were low. CS changes correlated with water content. HA and KS increased in all discs with increasing maturity. Oversulphated KS, absent from fetal discs, reached mature levels by 10 y. Many of the changes occurred before maturity. Glycosaminoglycan (GAG) levels correlated with increasing compressive loads. Higher collagen levels in the cervical nucleus pulposus correlated with greater ranges of torsional and shearing strains in cervical discs. High GAG levels in cervical annulus fibrosus probably facilitate lamellar movements during torsional and flexional movements by lubrication and increase of tissue compressibility. Increased KS/CS ratios before maturity correlated with decreased disc blood supply. Ambient O2 tensions may determine KS/CS balance, the former consuming little O2 during biosynthesis.

Structure and biological functions of keratan sulfate proteoglycans

The skeletal and corneal keratan sulfate proteoglycans show a different metabolic and structural heterogeneity. The domain structure of the carbohydrate chain has been shown to be different in various animal species. There are two major types of skeletal keratan sulfate proteoglycans with and without fucose. The protein cores of the corneal chicken keratan sulfate proteoglycan (lumican) and those of another small keratan sulfate proteoglycan (fibromodulin) have been sequenced. Keratan sulfate oligosaccharides belong to the members of an antigen family of the poly-N-acetyllactosamine series. Monoclonal antibodies and immunoassay procedures for keratan sulfate proteoglycans have been prepared. In osteoarthritis, no significant specific increase of keratan sulfate has been found. Keratan sulfate is a functional substitute for chondroitin sulfate in O2-deficient tissues.

The nomenclature of glycosaminoglycans and proteoglycans

1. The proposed terms and codes provide concise, quantitative information on the polymer backbone, state of oxidation, patterns of sulphation and epimerization, and proportions of monomeric units. Oversulphated domains and special units are easily recognized. 2. They are intermediate between general statements (e.g. 'chondroitin sulphate') and detailed primary structures. 3. They bring terminology into line with current analytical techniques, which depend largely on enzymes developed by S. Suzuki and co-workers. Current enzyme nomenclature (chondroitinase ABC, AC) can still be used. 4. They are not primarily intended to be spoken, although some codes are easily articulated. 5. They have capacity and flexibility to accommodate future developments.

Non-electrostatic factors govern the hydrodynamic properties of articular cartilage proteoglycan

The hydrodynamic frictional resistance to water flow exerted by articular cartilage proteoglycan is shown to be similar to that of proteoglycan isolated from Swarm rat chondrosarcoma, and independent of the state of aggregation of the proteoglycan. Frictional resistance is dependent, however, on the chain segments of the constituent chondroitin-sulphate and keratan-sulphate chains of the proteoglycan. Frictional resistance offered by chondroitin sulphate was independent of pH over the range 3.2-8.7. This confirms previous studies, associated with varying ionic strength and chemical modification of ionic groups of chondroitin sulphate, which showed that the frictional resistance offered by this molecule is independent of electrostatic factors. Water-structure-breaking and hydrogen-bond-breaking solvents were also without major effects on the flow resistance offered by chondroitin sulphate. An overall secondary structure of chondroitin sulphate was not evident, as it showed no significant difference to dextran in terms of its temperature dependence of relative viscosity. Local regions of rigid secondary structure, as manifested through inter-residue hydrogen bonding between sugar residues, is likely to control flow resistance as periodate-oxidized chondroitin sulphate and periodate-oxidized and reduced preparations showed a significant decrease in their frictional resistance to water.

Oxygen and the connective tissues

Avascular connective tissues (cartilage, discs, cornea) change with maturation and aging, particularly in large animals, where diffusion paths are longest. It is suggested that the changes in such tissues are responses to increasing difficulties in obtaining oxygen. Two almost identical structural polymers are made in these tissues: chondroitin sulphate, which requires large amounts of oxygen for biosynthesis and keratan sulphate, which requires relatively little. The observed balance of these polymers in the tissue is proposed to depend on the control of biosynthesis by the ambient oxygen tension, and/or selective breakdown.