Mineralized tissue protein profiles in the Australian form of bovine osteogenesis imperfecta

The bone cell biology of osteogenesis imperfecta

Osteogenesis Imperfecta (OI) has been defined as a heritable connective tissue disorder with variable severity of clinical expression. OI is a type I collagen based disease. Consequently, much research has focused on identifying specific mutations in the pro-alpha (I) genes. Our interest in OI lies in the metabolism of the non-collagenous proteins (NCPs) of the bone matrix. Although type I collagen is the most abundant protein in bone extracellular matrix, it is the NCPs which bind to, modify and have the potential to regulate that collagen matrix. Our approach has been to determine the levels of the NCPs for both OI and age-matched controls. Most recently, we have utilized an in vitro human osteoblast system to study normal and OI NCP metabolism (Fedarko et al. J. Bone Min. Res. 7, 921-930, 1992). It is our hypothesis that the altered stoichiometry of collagen and NCPs is, in part, responsible for the phenotypic variation of the disease.

Osteogenesis imperfecta in Holstein-Friesian calves

Eight calves with osteogenesis imperfecta were born in a Danish Holstein-Friesian herd during a two-year period. In total 92 calves were born (84 normal), and all were sired by a clinically normal Holstein-Friesian bull. The defect was probably due to a de novo dominant mutation present as a gonadal mosaicism in the bull. Affected calves were characterised by multiple fractures, congenital bone deformations, general joint laxity, dentinogenesis imperfecta, and light blue sclerae. The skin seemed normal. Electron microscopical studies revealed slightly decreased average diameter of cutaneous collagen fibrils, while the diameter of collagen fibrils in tendons and ligaments was severely reduced. Abnormalities of collagen type I from skin and compact bone were not detected by biochemical analyses.

Defective pro alpha 2(I) collagen synthesis in a recessive mutation in mice: a model of human osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable disorder of connective tissue associated with fractures, osteopenia, and short stature. OI results from mutations affecting the pro alpha 1 or pro alpha 2 gene of type I collagen. We describe a strain of mice with a nonlethal recessively inherited mutation (oim) that results in phenotypic and biochemical features that simulate moderate to severe human OI. The phenotype of homozygous oim mice includes skeletal fractures, limb deformities, generalized osteopenia, and small body size. Their femurs are smaller and demonstrate marked cortical thinning and fewer medullary trabeculae than those of wild-type mice. Breeding studies show the mutation is inherited in most crosses as a single recessive gene on chromosome 6, near the murine Cola-2 gene. Biochemical analysis of skin and bone, as well as isolated dermal fibroblast cultures, demonstrate that alpha 1(I) homotrimeric collagen accumulates in these tissues and is secreted by fibroblasts. Short labeling studies in fibroblasts demonstrate an absence of pro alpha 2(I) collagen chains. Nucleotide sequencing of the cDNA encoding the COOH-propeptide reveals a G deletion at pro alpha 2(I) nucleotide 3983; this results in an alteration of the sequence of the last 48 amino acids. The oim mouse will facilitate the study of type I collagen-related skeletal disease.

Extracellular matrix formation by osteoblasts from patients with osteogenesis imperfecta

Extracellular matrix proteins synthesized by bone cells isolated from 16 patients with different forms of osteogenesis imperfecta (OI) were analyzed in vitro. Specific components of the extracellular matrix by OI and age-matched cultures were investigated by steady-state radiolabeling followed by quantitation of label into specific proteins and comparison of OI cultures to those of age-matched controls. The in vitro proliferation of OI bone cells was found to be lower than that of control cells. In seven patients, abnormalities of the alpha 1(I) and/or alpha 2(I) chains of type I collagen were detected by gel electrophoresis. In two of these patients, the mutations in the COLIA1 and COLIA2 genes have been previously identified. Although the amount of total protein synthesized by the cells in culture was the same for OI bone cells and age-matched control cells, OI bone cells showed a significantly reduced synthesis of not only collagen but also other bone matrix glycoproteins. The synthesis of osteonectin (SPARC/BM40) and three proteoglycans [a large chondroitin sulfate proteoglycan, biglycan (PGI), and decorin (PGII)] was found to be decreased in OI cells. The reduction was most pronounced at the developmental age at which these macromolecules reach maximal levels during normal development.

Osteogenesis imperfecta: changes in noncollagenous proteins in bone

The noncollagenous proteins osteonectin, bone sialoprotein, osteocalcin, the small proteoglycan decorin (PG II), and alpha 2-HS glycoprotein (which is synthesized in the liver but highly concentrated in bone) were measured in extracts of cortical bone from 3 type I, 2 type II, 8 type III and 13 type IV patients with osteogenesis imperfecta (OI) and from 7 control subjects. Osteonectin was found to be reduced in the bone of all OI patients. The bone from severely affected type III OI patients contained the lowest levels of osteonectin. In contrast, bone sialoprotein was found to be elevated in the bones of OI patients. The highest levels were found in individuals classified as type IV patients. Osteocalcin and alpha 2-HS glycoprotein concentrations were increased in all OI patients. Decorin levels were not significantly altered in OI bones compared to controls. These changes in the concentrations of the noncollagenous proteins may contribute to the fragility of the OI bone by interfering with complete mineralization and/or normal tissue architecture.

Localization of endogenous osteocalcin in neonatal rat bone and its absence in articular cartilage: effect of warfarin treatment

Immunocytochemistry after cryoultramicrotomy was used to localize endogenous osteocalcin in bone (calvaria, femoral diaphysis) and epiphyseal femoral cartilage from 8-day-old rats treated (or mot) for 7 days with warfarin. Ultrathin frozen sections were incubated with goat antiserum against rat osteocalcin at high dilutions (2 x 10(-4) to 2 x 10(-6). In calvaria and femur of untreated rats, endogenous osteocalcin was observed in osteoblasts (cytoplasm and nucleus) and in the collagenous matrix. Osteocalcin appeared progressively in osteoblasts and bone matrix in the mineralization front, then increased in the regions of extended calcification. Osteocalcin was also detected in osteocytes but was not as abundant as in osteoblasts. In bone samples of warfarin-treated rats, endogenous osteocalcin was only detected in bone matrix but not in osteoblasts. Furthermore, osteocalcin was only observed if antiserum was not very dilute (2 x 10(-2). In cartilage (hypertrophied and degenerative zones), osteocalcin was not observed in matrix and chondrocytes. However, it was found in the vicinity of matrix vesicles at the initial loci of calcification. Osteocalcin was never detected in the cartilage of warfarin-treated rats. Our results provide ultrastructural immunocytological evidence for the localization of endogenous osteocalcin in osteoblasts, the presence of osteocalcin in bone matrix and a direct gradient between the presence of osteocalcin and the calcification process. Osteocalcin is absent from cartilage, except possibly close to calcifying matrix vesicles. Warfarin inhibits the formation of osteocalcin.

An evaluation of serum osteocalcin in Paget's disease of bone and its response to diphosphonate treatment

We found that serum bone gamma-carboxyglutamic acid-containing protein (BGP) (osteocalcin) had lower sensitivity and specificity for measurement of disease activity in Paget's disease of bone than other biochemical measures of disease activity. The administration of diphosphonates induced suppression of urinary hydroxyproline excretion and a subsequent decrease in alkaline phosphatase values, but no consistent change in BGP values. Serum BGP measurements have limited value as a screening test for Paget's disease or for monitoring treatment of the disorder.

Osteogenesis imperfecta

Major advances have occurred in the classification of OI and in the definition of underlying molecular defects. A clearer understanding of the pathogenesis of OI and of the relationships between the phenotypes and genotypes should emerge. The study of induced mutations in selected regions of the collagen genes with expression in cultured cells or transgenic mice should hasten this process. These advances will also provide a basis for studies into the large number of other genetically determined connective tissue disorders that are grouped together as the skeletal dysplasias. The results of recent studies in OI are providing a unique insight into many aspects of collagen and connective tissue biochemistry, physiology and pathology.

Osteonectin content in human osteogenesis imperfecta bone shows a range similar to that of two bovine models of OI

Samples of human osteogenesis imperfecta (OI) bone were analyzed for osteonectin content by SDS gel electrophoresis and immunodetection on Western blots. The OI bone osteonectin content varied from normal to severely depressed. Previously, we showed that two clinically identical but genetically unrelated bovine models of OI were differentiated biochemically by their bone osteonectin content: one OI model had normal bone osteonectin while the other was severely depressed in this parameter. The data in this pilot study suggest that further investigation of bone osteonectin content may prove useful in the clinical assessment of human OI cases.

Two bovine models of osteogenesis imperfecta exhibit decreased apatite crystal size

In recent years advances have been made in detailing the changes in both collagen and noncollagenous proteins caused by a variety of mutations leading to osteogenesis imperfecta. Much less, however, is known about the mineral phase in the affected bone. In this report, we measured the crystallinity of the apatite in bovine OI bone. Line broadening of the 002 reflection (which estimates changes in the long or c axis of the crystals) and of the 310 reflection (which estimates changes in the thickness of the crystals) both show large decreases (30 and 35% respectively). Transmission electron micrograph measurements indicate that these changes were most probably a result of smaller crystals. No decrease in the ash weight of the bone was observed.