Collagen biochemistry of osteopetrotic bone: I. Quantitative changes in bone collagen cross-links in virus-induced avian osteopetrosis

Pyridoxine deficiency affects biomechanical properties of chick tibial bone

The mechanical integrity of bone is dependent on the bone matrix, which is believed to account for the plastic deformation of the tissue, and the mineral, which is believed to account for the elastic deformation. The validity of this model is shown in this study based on analysis of the bones of vitamin B6-deficient and vitamin B6-replete chick bones. In this model, when B6-deficient and control animals are compared, vitamin B6 deficiency has no effect on the mineral content or composition of cortical bone as measured by ash weight (63 +/- 6 vs. 58 +/- 3); mineral to matrix ratio of the FTIR spectra (4.2 +/- 0.6 vs. 4.5 +/- 0.2), line-broadening analyses of the X-ray diffraction 002 peak (beta 002 = 0.50 +/- 0.1 vs. 0.49 +/- 0.01), or other features of the infrared spectra. In contrast, collagen was significantly more extractable from vitamin B6-deficient chick bones (20 +/- 2% of total hydroxyproline extracted vs. 10 +/- 3% p < or = 0.001). The B6-deficient bones also contained an increased amount of the reducible cross-links DHLNL, dehydro-dihydroxylysinonorleucine, (1.03 +/- 0.07 vs. 0.84 +/- 0.13 p < or = 0.001); and a nonsignificant increase in HLNL, dehydro-hydroxylysinonorleucine, (0.51 +/- 0.03 vs. 0.43 +/- 0.03, p < or = 0.10). There were no significant changes in bone length, bone diameter, or area moment of inertia. In four-point bending, no significant changes in elastic modulus, stiffness, offset yield deflection, or fracture deflection were detected. However, fracture load in the B6-deficient animals was decreased from 203 +/- 35 MPa to 151 +/- 23 MPa, p < or = 0.01, and offset yield load was decreased from 165 +/- 9 MPa to 125 +/- 14 MPa, p < or = 0.05. Since earlier histomorphometric studies had demonstrated that the B6-deficient bones were osteopenic, these data suggest that although proper cortical bone mineralization occurred, the alterations of the collagen resulted in changes to bone mechanical performance.

Changes in the ratio of non-calcified collagen to calcified collagen in human vertebrae with advancing age

Bone loss associated with aging is associated primarily with a decline in bone formation. To try and further understand the nature of this process we have used a biochemical approach which relies on the fact that osteoid is susceptible to enzymatic degradation whereas calcified collagen is protected by the mineral phase against proteolytic digestion. Our findings show a statistically significant inverse relationship between osteoid and age (r = 0.70 female, r = 0.47 male). A closer relationship was observed when age was related to the ratio of osteoid to bone (r = 0.73 female, r = 0.56 male). In both cases, the observed linear decline begins at an early age and becomes marked with advancing age. Histologic observations illustrate these findings showing decreased osteoid and osteoblasts in the older vertebral specimens compared to the younger ones. Even though the mechanism for osteoid calcification seems to remain unimpaired, the decline of a calcifiable matrix in the presence of normal bone turnover could lead to bone loss.

Vitamin D deficiency causes a selective reduction in deposition of transforming growth factor beta in rat bone: possible mechanism for impaired osteoinduction

We demonstrated previously that implants of bone matrix prepared from vitamin D-deficient (-D) rats were less osteoinductive and contained less extractable mitogenic activity compared with control implants prepared from vitamin D-replete (+D) rats and proposed that bone from -D rats is deficient in one or more specific growth factors. To test this hypothesis, bones from rats that were fed either +D or -D diets and kept in the dark for 8 wk were extracted and assayed for insulin-like growth factors I and II (IGF-I and IGF-II) and transforming growth factor beta (TGF-beta), the three most abundant growth factors in rat bone, and osteocalcin. Serum calcium, 25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] were determined at sacrifice. In -D rats, there were significant reductions in serum calcium, 25-hydroxyvitamin D3, and 1,25(OH)2D3 and skeletal TGF-beta but no differences in extractable skeletal protein, IGF-I, IGF-II, or osteocalcin compared with +D rats. To determine whether 1,25(OH)2D3 increased TGF-beta production by bone cells, we treated mouse calvaria for 6 days and mouse osteoblasts for 2 days with 10 nM 1,25(OH)2D3. Production of TGF-beta was increased almost 100% by 1,25(OH)2D3. We conclude that vitamin D deficiency reduces deposition of TGF-beta in rat bone and that diminished skeletal TGF-beta could contribute to the previously observed decrease in osteoinduction in implants from -D rat bone. The findings support the possibility that vitamin D and bone-derived TGF-beta are required for normal repair of the skeleton.

Collagen cross-linking in bovine gingiva

Tissue samples from 5 different sites of 6 bovine mandibles were used to quantify collagen cross-links as moles cross-link per mole of collagen. Gingiva contained primarily 3 reducible cross-links, deH-DHLNL, deH-HHMD, and deH-HLNL, and relatively small amounts of non-reducible cross-links. The cross-link density varied from site to site within the same mandible. DeH-DHLNL showed the most significant increase from anterior to posterior; the content in the retromolar region was 3-fold greater than in the anterior lingual site. Histologically, the posterior samples, especially the retromolar gingiva, contained less mature (thinner) collagen bundles. These findings suggest that the maturity of bovine mandibular gingival collagen varies at different sites.

Simultaneous determination of the reducible and nonreducible cross-links of connective tissue. Analysis of mineralized and nonmineralized bone collagen

Secondary amine cross-links occur in collagen and elastin from a number of tissue sources. Quantification of these cross-links by amino acid analysis is complicated by the problem of separating cross-links, which are often minor components, from the more common amino acids and also because relatively large amounts of a cross-link are required to determine a color factor. A specific radioactive labeling method has been developed and used to quantify cross-links in bone collagen. Primary amines such as lysine and hydroxylysine are first guanidinated with 3,5-dimethylpyrazole-1-carboxamidine nitrate (DMPC). Secondary amines, which are unreactive with DMPC, are then quantitatively cyanoethylated with [14C]acrylonitrile. This procedure can be used to detect any secondary amine cross-link, with higher sensitivity than ninhydrin analysis, in peptide form as well as in acid hydrolysates. It is applied here in conjunction with [3H]NaBH4 reduction to simultaneously quantify Schiff base cross-links and amounts of in vivo reduction of Schiff bases in mineralized versus nonmineralized bovine bone.

Nonmineralized and mineralized bone collagen in bone of immobilized monkeys

Monkey bones from different time periods of immobilization and reambulation of the monkeys were assessed histologically. The bone was also assessed biochemically for nonmineralized collagen in bone. Results indicate that more nonmineralized bone is present in monkeys that have been immobilized, and upon reambulation, these values tend to be normalized to control bone.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Nonmineralized and mineralized compartments of bone: the role of pyridinoline in nonmineralized collagen

Collagen tryptic peptides obtained from the nonmineralized and mineralized compartments of diaphyseal bone have different distributions of intermolecular crosslinks. Pyridinoline, a collagen crosslink thought to be associated with chronologically older bone, was detected in peptides from nonmineralized collagen but not from mineralized collagen. Mineralization may prevent collagen maturation; conversely, pyridinoline in nonmineralized collagen may decrease the intermolecular distances among collagen chains in fibrils and preclude mineralization.

The influence of 1-hydroxyethane-1,1-diphosphonate and dichloromethanediphosphonate on lysine hydroxylation and cross-link formation in rat bone, cartilage and skin collagen

The effects in vivo of dichloromethanediphosphonate and 1-hydroxyethane 1,1-diphosphonate on collagen solubility, hydroxylation of lysine and proline and on the formation of collagen intermolecular cross-links were studied by using rat bone, cartilage and skin tissues. Dichloromethanediphosphonate decreased bone collagen solubility both in acetic acid and after pepsin treatment. Although none of the diphosphonates had any effect on the hydroxylation of proline, dichloromethane-diphosphonate, but not 1-hydroxyethane-1,1-diphosphonate, increased the number of hydroxylysine residues in the alpha-chains of bone, skin and cartilage collagen. The stimulatory effect was dose-dependent. The dichloromethanediphosphonate-mediated increase in hydroxylysine residues in bone and cartilage was manifested in an increase of dihydroxylysinonorleucine, the cross-link that is formed by the condensation of two hydroxylysine residues. The cross-link hydroxylysinonorleucine, a condensation product of hydroxylysine and lysine, on the other hand, was decreased. The total number of intermolecular cross-links was not changed by the diphosphonate.

Collagen and noncollagen protein synthesis in chick limb bud cells infected with a virus that causes osteoblastoma

Infection of cultured stage 24 (day 4.5) chick limb cells (spot cultures) with myeloblastosis-associated virus 2-(osteo) (MAV-2(0)) produces cell density-dependent alterations of collagen and noncollagen protein synthesis over the course of 8 days in culture. Collagen synthesis in infected, low density cultures (100 k cells/spot) was maximal on day 4 and increased 12 fold compared with maximal values on days 5-6 for uninfected counterparts. The patterns of collagen synthesis were similar in uninfected or MAV-2(0)-infected cultures seeded at 200 (medium density) or 400 k cells (high density) per spot with peaks on days 5-6. Noncollagen protein synthesis (NCP) in infected, low density cultures was maximal on day 4 and increased 15.7 fold compared with maximal values on days 5-6 for uninfected counterparts. The patterns for NCP synthesis were similar for uninfected and MAV-2(0)-infected cultures seeded at medium density (peak on day 5). However, in MAV-2(0)-infected , high density cultures, NCP synthesis was maximal on day 4 and increased 2.9 fold compared with maximal values on days 5-6 for uninfected counterparts. Although kinetics of cell division and DNA synthesis were similar in uninfected and MAV-2(0)-infected limb bud cells, the virus may have diverse affects on different cell populations at the three cell densities employed in these experiments.