The hydrating effect of lathyrogenic compounds on chick-embryo cartilage in vivo

Chemical and biomechanical characterization of hyperhomocysteinemic bone disease in an animal model

BACKGROUND

Classical homocystinuria is an autosomal recessive disorder caused by cystathionine beta-synthase (CBS) deficiency and characterized by distinctive alterations of bone growth and skeletal development. Skeletal changes include a reduction in bone density, making it a potentially attractive model for the study of idiopathic osteoporosis.

METHODS

To investigate this aspect of hyperhomocysteinemia, we supplemented developing chicks (n = 8) with 0.6% dl-homocysteine (hCySH) for the first 8 weeks of life in comparison to controls (n = 10), and studied biochemical, biomechanical and morphologic effects of this nutritional intervention.

RESULTS

hCySH-fed animals grew faster and had longer tibiae at the end of the study. Plasma levels of hCySH, methionine, cystathionine, and inorganic sulfate were higher, but calcium, phosphate, and other indices of osteoblast metabolism were not different. Radiographs of the lower limbs showed generalized osteopenia and accelerated epiphyseal ossification with distinct metaphyseal and suprametaphyseal lucencies similar to those found in human homocystinurics. Although biomechanical testing of the tibiae, including maximal load to failure and bone stiffness, indicated stronger bone, strength was proportional to the increased length and cortical thickness in the hCySH-supplemented group. Bone ash weights and IR-spectroscopy of cortical bone showed no difference in mineral content, but there were higher Ca2+/PO4(3-) and lower Ca2+/CO3(2-) molar ratios than in controls. Mineral crystallization was unchanged.

CONCLUSION

In this chick model, hyperhomocysteinemia causes greater radial and longitudinal bone growth, despite normal indices of bone formation. Although there is also evidence for an abnormal matrix and altered bone composition, our finding of normal biomechanical bone strength, once corrected for altered morphometry, suggests that any increase in the risk of long bone fracture in human hyperhomocysteinemic disease is small. We also conclude that the hCySH-supplemented chick is a promising model for study of the connective tissue abnormalities associated with homocystinuria and an important alternative model to the CBS knock-out mouse.

Loss of decorin from the surface zone of articular cartilage in a chick model of osteoarthritis

The objective of this study was to immunolocalize decorin and to assess changes as a result of pyridoxine (PN) deficiency in chick articular cartilage from femoral condyles. After maintenance on a normal diet for the first two weeks after hatching, 15 broiler chickens were deprived of this vitamin for 6 weeks. It was previously shown that the ankle joints of PN-deficient animals are swollen with effusions. They also present an abnormal gait, enlarged bony margins, and fissuring of the articular cartilages. Milder changes (no fissures) were also shown in the knee joints. Data from a previous study were suggestive that sulfated glycosaminoglycans are lost from the knee cartilage surface into synovial fluid. The current study was focused on the small proteoglycan, decorin, which coats the surface of collagen fibrils and may regulate their morphology. To examine decorin in normal and PN-deficient articular cartilage, a monoclonal antibody to an epitope on the protein core of decorin was used for immunohistochemical staining of tissue sections and for Western Blot analysis of cartilage extracts. Reduction of staining with the antibody was demonstrated in the tangential surface zone of PN-deficient cartilage, and Western Blot analysis showed reduced intensity of decorin bands compared to normal controls. These data suggest that a lack of decorin may play a role in the enlargement of collagen bundles in the tangential zone of PN-deficient articular cartilage as observed in a previous electron microscopic study.

Vitamin B6 deficiency experimentally-induced bone and joint disorder: microscopic, radiographic and biochemical evidence

In the present study the effect of pyridoxine deficiency on the ultrastructure and morphology of bone and its metabolism was examined in the rapidly growing chick. Pyridoxine-deficient animals had tibias of reduced dry weight and cortical thickness. Histomorphometry demonstrated a disproportionately high eroded surface, lower amount of osteoid tissue and reduced mineralized trabecular width. Anterior-posterior radiographs of the tibiotarsometatarsal joint showed reduced secondary ossification centres and coarse trabeculation. Decalcified metaphyseal cartilage showed irregular trabeculas and a markedly reduced amount of Fast-green counterstain matrix suggesting that there is less collagen present and in turn less availability for matrix to be laid down for later calcification. Plasma activity of the bone alkaline phosphatase isoenzyme (EC 3.1.3.1) was decreased. Plasma Ca and PO4 levels did not vary. The present bone study referring to a pseudo-lathyritic state in which collagen maturation is not completely achieved supports the hypothesis that pyridoxine is an essential nutrient for the connective tissue matrix.

The possible role of neutrophil proteinases in damage to articular cartilage

The proteolytic degradation of articular cartilage that is seen in the arthritides affects both of the major structural components of the tissue, proteoglycan and collagen. Neutrophil leucocytes are abundant in the synovial fluid of the inflamed joints, and we have considered whether the large quantities of neutral proteinases carried by these cells could contribute to the cartilage degradation. The two neutrophil serine proteinases have been isolated, and shown to break down both proteoglycan and collagen in articular cartilage. The enzymes attacked the non-helical terminal peptides of the collagen, eliminating the cross-links, thus destabilizing and solubilizing, the fibres. The soluble collagen then denatured spontaneously, and was further degraded. Although large quantities of the neutrophil proteinases are probably released in the synovial fluid each day, the inhibitory capacity of the fluid is seldom, if ever, saturated. Nevertheless, immunologically mediated release of the neutrophil enzymes in 'frustrated endocytosis' at the cartilage surface could give rise to the generalized damage that has been reported by others.

The degradation of articular collagen by neutrophil proteinases

The action of the serine proteinase (EC 3.4.21--)of human neutrophil leucocytes, elastase and cathepsin G, on cartilage and tendon was investigated. With cartilage, both enzymes first degraded the proteoglycan, then solubilized collagen by an attack on the terminal peptides, destroying the inter- and intramolecular cross-links. There was little degradation of the helical region of the type II collagen. Elastase also solubilized type I collagen from tendon, though this was less susceptible than cartilage collagen, and attacked the terminal peptides and perhaps the helical region of type I skin collagen in solution. Cathepsin G had little or no effect on type I collagen of skin or tendon. Since massive infiltration of joint tissues by neutrophil leucocytes is a prominent feature of inflammatory joint disease, it may well be that elastase and cathepsin G make a significant contribution to the tissue damage that occurs.