Extracellular matrix formation by osteoblasts from patients with osteogenesis imperfecta

Altered Sox9 and FGF signaling gene expression in Aga2 OI mice negatively affects linear growth

Osteogenesis imperfecta (OI), or brittle bone disease, is a disorder characterized by bone fragility and increased fracture incidence. All forms of OI also feature short stature, implying an effect on endochondral ossification. Using the Aga2+/- mouse, which has a mutation in type I collagen, we show an affected growth plate primarily due to a shortened proliferative zone. We used single-cell RNA-Seq analysis of tibial and femoral growth plate tissues to understand transcriptional consequences on growth plate cell types. We show that perichondrial cells, which express abundant type I procollagen, and growth plate chondrocytes, which were found to express low amounts of type I procollagen, had ER stress and dysregulation of the same unfolded protein response pathway as previously demonstrated in osteoblasts. Aga2+/- proliferating chondrocytes showed increased FGF and MAPK signaling, findings consistent with accelerated differentiation. There was also increased Sox9 expression throughout the growth plate, which is expected to accelerate early chondrocyte differentiation but reduce late hypertrophic differentiation. These data reveal that mutant type I collagen expression in OI has an impact on the cartilage growth plate. These effects on endochondral ossification indicate that OI is a biologically complex phenotype going beyond its known impacts on bone to negatively affect linear growth.

Discoidin domain receptors; an ancient family of collagen receptors has major roles in bone development, regeneration and metabolism

The extracellular matrix (ECM) niche plays a critical role in determining cellular behavior during bone development including the differentiation and lineage allocation of skeletal progenitor cells to chondrocytes, osteoblasts, or marrow adipocytes. As the major ECM component in mineralized tissues, collagen has instructive as well as structural roles during bone development and is required for bone cell differentiation. Cells sense their extracellular environment using specific cell surface receptors. For many years, specific β1 integrins were considered the main collagen receptors in bone, but, more recently, the important role of a second, more primordial collagen receptor family, the discoidin domain receptors, has become apparent. This review will specifically focus on the roles of discoidin domain receptors in mineralized tissue development as well as related functions in abnormal bone formation, regeneration and metabolism.

Deciphering the Relevance of Bone ECM Signaling

Bone mineral density, a bone matrix parameter frequently used to predict fracture risk, is not the only one to affect bone fragility. Other factors, including the extracellular matrix (ECM) composition and microarchitecture, are of paramount relevance in this process. The bone ECM is a noncellular three-dimensional structure secreted by cells into the extracellular space, which comprises inorganic and organic compounds. The main inorganic components of the ECM are calcium-deficient apatite and trace elements, while the organic ECM consists of collagen type I and noncollagenous proteins. Bone ECM dynamically interacts with osteoblasts and osteoclasts to regulate the formation of new bone during regeneration. Thus, the composition and structure of inorganic and organic bone matrix may directly affect bone quality. Moreover, proteins that compose ECM, beyond their structural role have other crucial biological functions, thanks to their ability to bind multiple interacting partners like other ECM proteins, growth factors, signal receptors and adhesion molecules. Thus, ECM proteins provide a complex network of biochemical and physiological signals. Herein, we summarize different ECM factors that are essential to bone strength besides, discussing how these parameters are altered in pathological conditions related with bone fragility.

Calcification of Human Saphenous Vein Associated with Endothelial Dysfunction: A Pilot Histopathophysiological and Demographical Study

While the pathophysiology and clinical significance of arterial calcifications have been studied extensively, minimal focus has been placed on venous calcification deposition. In this study, we evaluated the association between calcium deposition in human saphenous vein (HSV), endothelial function, and patient demographic risk factors. Fifty-four HSV segments were collected at the time of coronary artery bypass graft (CABG) surgery. The presence or absence of calcium deposits was visualized using the Von Kossa staining method. Endothelial function was determined by measuring muscle tissue contraction with phenylephrine and relaxation with carbachol in a muscle bath. Additional segments of vein underwent histologic evaluation for preexisting intimal thickness and extracellular matrix (ECM) deposition. Patient demographics data were obtained through our institution’s electronic medical record, with patient consent. Calcium was present in 16 of 54 samples (29.6%). Veins with calcium deposits had significantly greater intimal-to-medial thickness ratios (p = 0.0058) and increased extracellular collagen deposition (p = 0.0077). Endothelial relaxation was significantly compromised in calcified veins vs. those without calcium (p = 0.0011). Significant patient risk factors included age (p = 0.001) and preoperative serum creatinine (p = 0.017). Calcified veins can be characterized as having endothelial dysfunction with increased basal intimal thickness and increased ECM deposition. Patient risk factors for calcium deposits in veins were similar to those for arteries, namely, advanced age and kidney disease. Further studies are needed to determine the effect of preexisting vein calcification on short- and long-term graft patency.

Osteoblast Malfunction Caused by Cell Stress Response to Procollagen Misfolding in α2(I)-G610C Mouse Model of Osteogenesis Imperfecta

Glycine (Gly) substitutions in collagen Gly-X-Y repeats disrupt folding of type I procollagen triple helix and cause severe bone fragility and malformations (osteogenesis imperfecta [OI]). However, these mutations do not elicit the expected endoplasmic reticulum (ER) stress response, in contrast to other protein-folding diseases. Thus, it has remained unclear whether cell stress and osteoblast malfunction contribute to the bone pathology caused by Gly substitutions. Here we used a mouse with a Gly610 to cysteine (Cys) substitution in the procollagen α2(I) chain to show that misfolded procollagen accumulation in the ER leads to an unusual form of cell stress, which is neither a conventional unfolded protein response (UPR) nor ER overload. Despite pronounced ER dilation, there is no upregulation of binding immunoglobulin protein (BIP) expected in the UPR and no activation of NF-κB signaling expected in the ER overload. Altered expression of ER chaperones αB crystalline and HSP47, phosphorylation of EIF2α, activation of autophagy, upregulation of general stress response protein CHOP, and osteoblast malfunction reveal some other adaptive response to the ER disruption. We show how this response alters differentiation and function of osteoblasts in culture and in vivo. We demonstrate that bone matrix deposition by cultured osteoblasts is rescued by activation of misfolded procollagen autophagy, suggesting a new therapeutic strategy for OI. © 2016 American Society for Bone and Mineral Research.

Makings of a brittle bone: Unexpected lessons from a low protein diet study of a mouse OI model

Glycine substitutions in type I collagen appear to cause osteogenesis imperfecta (OI) by disrupting folding of the triple helix, the structure of which requires Gly in every third position. It is less clear, however, whether the resulting bone malformations and fragility are caused by effects of intracellular accumulation of misfolded collagen on differentiation and function of osteoblasts, effects of secreted misfolded collagen on the function of bone matrix, or both. Here we describe a study originally conceived for testing how reducing intracellular accumulation of misfolded collagen would affect mice with a Gly610 to Cys substitution in the triple helical region of the α2(I) chain. To stimulate degradation of misfolded collagen by autophagy, we utilized a low protein diet. The diet had beneficial effects on osteoblast differentiation and bone matrix mineralization, but also affected bone modeling and suppressed overall animal growth. Our more important observations, however, were not related to the diet. They revealed how altered osteoblast function and deficient bone formation by each cell caused by the G610C mutation combined with increased osteoblastogenesis might make the bone more brittle, all of which are common OI features. In G610C mice, increased bone formation surface compensated for reduced mineral apposition rate, resulting in normal cortical area and thickness at the cost of altering cortical modeling process, retaining woven bone, and reducing the ability of bone to absorb energy through plastic deformation. Reduced collagen and increased mineral density in extracellular matrix of lamellar bone compounded the problem, further reducing bone toughness. The latter observations might have particularly important implications for understanding OI pathophysiology and designing more effective therapeutic interventions.

SPARC/osteonectin in mineralized tissue

Secreted protein acidic and rich in cysteine (SPARC/osteonectin/BM40) is one of the most abundant non-collagenous protein expressed in mineralized tissues. This review will focus on elucidating functional roles of SPARC in bone formation building upon results from non-mineralized cells and tissues, the phenotype of SPARC-null bones, and recent discoveries of human diseases with either dysregulated expression of SPARC or mutations in the gene encoding SPARC that give rise to bone pathologies. The capacity of SPARC to influence pathways involved in extracellular matrix assembly such as procollagen processing and collagen fibril formation as well as the capacity to influence osteoblast differentiation and osteoclast activity will be addressed. In addition, the potential for SPARC to regulate cross-linking of extracellular matrix proteins by members of the transglutaminase family of enzymes is explored. Elucidating defined biological functions of SPARC in terms of bone formation and turnover are critical. Further insight into specific cellular mechanisms involved in the formation and homeostasis of mineralized tissues will lead to a better understanding of disease progression.

The long bone deformity of osteogenesis imperfecta III: analysis of structural changes carried out with scanning electron microscopic morphometry

The wedges of the mid-diaphyseal osteotomies carried out to correct the femoral and/or tibial native deformity in type III osteogenesis imperfecta (OI III) were used to study the remodeling patterns and lamellar organization at the level of the major deformity. Histology and scanning electron microscopy (SEM) morphology showed abnormal cortical remodeling characterized by the failure to form a cylinder of compact bone with a regular marrow canal. Atypical, flattened, and large resorption lacunae with a wide resorption front on one side and systems of parallel lamellae on the opposite side were observed, resembling those formerly reported as drifting osteons. SEM morphometry documented a higher percentage of nonossified vascular/resorption area (44.3 %) in OI than in controls (13.6 %), a lower density of secondary osteons, and lower values for the parameters expressing the individual osteon size. The mean osteon total area, the mean central canal area, and the mean osteon bone area of two selected, randomized populations of secondary osteons were significantly higher (p < 0.001, p = 0.028, and p < 0.001, respectively) in control bones than in OI. The mean ossified matrix area was not significantly different, but the mean secondary osteon number and mean density were higher in controls (both p < 0.001). Osteon wedges were carried out to correct the native deformity of OI III and morphologic analysis suggested that the abnormal remodeling pattern (with "drifting osteons") may result from the altered load and tensile stresses on the deformed tubular bones.

In vitro analysis of bone phenotypes in Col1a1 and Jagged1 mutant mice using a standardized osteoblast cell culture system

The mouse is a valuable model organism for studying bone biology and for unravelling pathological processes in skeletal disorders. In vivo methods like X-ray analysis, DXA measurements, pQCT and μCT are available to investigate the bone phenotype of mutant mice. However, the descriptive nature of such methods does not provide insights into the cellular and molecular bases of the observed bone alterations. Thus, first-line investigations might be complemented by cell culture-based methods to characterize the pathological processes at the cellular level independent from systemic influences. By combining well-established assays, we designed a comprehensive test system to investigate the cellular and molecular phenotype of primary calvarial osteoblasts in mutant mice compared to wild-type controls as a first-line phenotyping method. The compilation of 9 different quantifiable assays allows assessment of general properties of cell growth and investigation of bone-specific parameters at the functional, protein and RNA level in a kinetic fashion throughout a 3-week culture period, thus maximizing the chance to discover and explain new phenotypes in mutant mice. By analyzing mutant mouse lines for Col1a1 and Jag1 (Delta-Notch pathway) that both showed clear alterations in several bone-related parameters we could demonstrate the usefulness of our cell culture system to discriminate between primary (Col1a1) and secondary effects (Jag1) in osteoblasts.

Thrombospondin-2 and SPARC/osteonectin are critical regulators of bone remodeling

Thrombospondin-2 (TSP2) and osteonectin/BM-40/SPARC are matricellular proteins that are highly expressed by bone cells. Mice deficient in either of these proteins show phenotypic alterations in the skeleton, and these phenotypes are most pronounced under conditions of altered bone remodeling. For example, TSP2-null mice have higher cortical bone volume and are resistant to bone loss associated with ovariectomy, whereas SPARC-null mice have decreased trabecular bone volume and fail to demonstrate an increase in bone mineral density in response to a bone-anabolic parathyroid hormone treatment regimen. In vitro, marrow stromal cell (MSC) osteoprogenitors from TSP2-null mice have increased proliferation but delayed formation of mineralized matrix. Similarly, in cultures of SPARC-null MSCs, osteoblastic differentiation and mineralized matrix formation are decreased. Overall, both TSP2 and SPARC positively influence osteoblastic differentiation. Intriguingly, both of these matricellular proteins appear to impact MSC fate through mechanisms that could involve the Notch signaling system. This review provides an overview of the role of TSP2 and SPARC in regulating bone structure, function, and remodeling, as determined by both in vitro and in vivo studies.

Accentuated osteoclastic response to parathyroid hormone undermines bone mass acquisition in osteonectin-null mice

Matricellular proteins play a unique role in the skeleton as regulators of bone remodeling, and the matricellular protein osteonectin (SPARC, BM-40) is the most abundant non-collagenous protein in bone. In the absence of osteonectin, mice develop progressive low turnover osteopenia, particularly affecting trabecular bone. Polymorphisms in a regulatory region of the osteonectin gene are associated with bone mass in a subset of idiopathic osteoporosis patients, and these polymorphisms likely regulate osteonectin expression. Thus it is important to determine how osteonectin gene dosage affects skeletal function. Moreover, intermittent administration of parathyroid hormone (PTH) (1-34) is the only anabolic therapy approved for the treatment of osteoporosis, and it is critical to understand how modulators of bone remodeling, such as osteonectin, affect skeletal response to anabolic agents. In this study, 10 week old female wild type, osteonectin-haploinsufficient, and osteonectin-null mice (C57Bl/6 genetic background) were given 80 microg/kg body weight/day PTH(1-34) for 4 weeks. Osteonectin gene dosage had a profound effect on bone microarchitecture. The connectivity density of trabecular bone in osteonectin-haploinsufficient mice was substantially decreased compared with that of wild type mice, suggesting compromised mechanical properties. Whereas mice of each genotype had a similar osteoblastic response to PTH treatment, the osteoclastic response was accentuated in osteonectin-haploinsufficient and osteonectin-null mice. Eroded surface and osteoclast number were significantly higher in PTH-treated osteonectin-null mice, as was endosteal area. In vitro studies confirmed that PTH induced the formation of more osteoclast-like cells in marrow from osteonectin-null mice compared with wild type. PTH treated osteonectin-null bone marrow cells expressed more RANKL mRNA compared with wild type. However, the ratio of RANKL:OPG mRNA was somewhat lower in PTH treated osteonectin-null cultures. Increased expression of RANKL in response to PTH could contribute to the accentuated osteoclastic response in osteonectin-/- mice, but other mechanisms are also likely to be involved. The molecular mechanisms by which PTH elicits bone anabolic vs. bone catabolic effects remain poorly understood. Our results imply that osteonectin levels may play a role in modulating the balance of bone formation and resorption in response to PTH.

Osteonectin/SPARC polymorphisms in Caucasian men with idiopathic osteoporosis

Animal models suggest a role for osteonectin/SPARC in determination of bone mass. We found haplotypes consisting of three single nucleotide polymorphisms (SNPs) in the 3' untranslated region (UTR) of the osteonectin gene are associated with bone density in Caucasian men with idiopathic osteoporosis.

INTRODUCTION

Osteonectin is a matricellular protein regulating matrix assembly, osteoblast differentiation, and survival. Animal studies indicate that osteonectin is essential for normal bone mass. The 3' UTR is a regulatory region controlling mRNA stability, trafficking, and translation, and we determined whether osteonectin 3' UTR haplotypes could be associated with bone mass and/or idiopathic osteoporosis.

METHODS

Single strand conformation polymorphism and allele-specific PCR analysis were used to assess alleles at osteonectin cDNA bases 1046, 1599, and 1970, using genomic DNA from middle-aged Caucasian men with idiopathic, low turnover osteoporosis (n = 56) and matched controls (n = 59). Bone density was measured by DXA at spine, hip and radius. Allele and haplotype frequencies were analyzed by Chi square analysis and Fisher's exact test.

RESULTS

Five common osteonectin 3' UTR haplotypes were identified. The frequency of one haplotype (1046C-1599C-1970T) was higher in controls compared with patients, and this haplotype was also associated with higher bone densities at multiple sites in patients. In contrast, a second haplotype (1046C-1599G-1970T) was associated with lower bone densities in patients at multiple sites.

CONCLUSIONS

Osteonectin regulates skeletal remodeling and bone mass in animals, and haplotypes in the 3' UTR of this gene are associated with bone density in Caucasian men with idiopathic osteoporosis.

Gene targeting of mutant COL1A2 alleles in mesenchymal stem cells from individuals with osteogenesis imperfecta

Mesenchymal stem cells (MSCs) are adult cells with the capacity to differentiate into multiple cell types, including bone, fat, cartilage, and muscle cells. In order to effectively utilize autologous MSCs in cell-based therapies, precise genetic manipulations are required to eliminate the effects of disease-causing mutations. We previously used adeno-associated virus (AAV) vectors to target and inactivate mutant COL1A1 genes in MSCs from individuals with the brittle bone disorder, osteogenesis imperfecta (OI). Here we have used AAV vectors to inactivate mutant COL1A2 genes in OI MSCs, thereby demonstrating that both type I collagen genes responsible for OI can be successfully targeted. We incorporated improved vector designs so as to minimize the consequences of random integration, facilitate the removal of potential antigens, and avoid unwanted exon skipping. MSCs targeted at mutant COL1A2 alleles produced normal type I procollagen and formed bone, thereby demonstrating their therapeutic potential.

Osteogenesis imperfecta:epidemiology and pathophysiology

Osteogenesis imperfecta (OI) is the most common of the inherited connective tissue disorders that primarily affect bone. However, it is a systemic disorder, as evidenced by the occurrence of ocular complications, dentinogenesis imperfecta, hearing loss, joint laxity, restrictive pulmonary disease, and short stature. The OI classification initially included four phenotypes (I-IV) involving COL1A1 and COL1A2 mutations. Three new phenotypes have been added, of which one, type VII, is the result of mutations of the cartilage-associated protein (CRTAP) gene. Investigation of recessive forms of OI particularly reported among South African blacks have revealed mutations involving both the CRTAP gene and the leucine proline-enriched proteoglycan 1 (LEPRE1) gene, each involved in collagen proline-3 hydroxylation. Issues related to the treatment of OI with bisphosphonates involve patient selection, evaluation of the results of treatment, and the duration of treatment. Also, questions exist regarding the difference in treatment response between children and adults with OI. Other treatment options, such as recombinant human parathyroid hormone (1-34), Rank ligand inhibitors, and stem cell technology, are being evaluated or are of future investigative interest.

Insights into material and structural basis of bone fragility from diseases associated with fractures: how determinants of the biomechanical properties of bone are compromised by disease

Minimal trauma fractures in bone diseases are the result of bone fragility. Rather than considering bone fragility as being the result of a reduced amount of bone, we recognize that bone fragility is the result of changes in the material and structural properties of bone. A better understanding of the contribution of each component of the material composition and structure and how these interact to maintain whole bone strength is obtained by the study of metabolic bone diseases. Disorders of collagen (osteogenesis imperfecta and Paget's disease of bone), mineral content, composition and distribution (fluorosis and osteomalacia); diseases of high remodeling (postmenopausal osteoporosis, hyperparathyroidism, and hyperthyroidism) and low remodeling (osteopetrosis, pycnodysostosis); and other diseases (idiopathic male osteoporosis, corticosteroid-induced osteoporosis) produce abnormalities in the material composition and structure that lead to bone fragility. Observations in patients and in animal models provide insights on the biomechanical consequences of these illnesses and the nature of the qualities of bone that determine its strength.

Zebrafish and medaka as models for bone research including implications regarding space-related issues

Teleost fish develop bones directly from mesenchymal condensations and from cartilage precursors. At the cellular level, the involved cell populations share many features with their mammalian counterparts. In addition, several genes are already described in fish showing high homology in amino acid sequence and expression with the corresponding genes of tetrapods that are involved in bone metabolism. Therefore, analysis of the underlying molecular mechanism in fish, in particular zebrafish and medaka, will increase the knowledge in teleosts. Furthermore, it will help to identify novel genes and regulatory pathways of bone homeostasis and skeletal disorders also in higher vertebrates, including disorders caused by altered gravity.

Constitutive expression of thrombospondin 1 in MC3T3-E1 osteoblastic cells inhibits mineralization

Thrombospondin 1 (TSP1) is a multifunctional extracellular glycoprotein present mainly in the fetal and adult skeleton. Although an inhibitory effect of TSP1 against pathological mineralization in cultured vascular pericytes has been shown, its involvement in physiological mineralization by osteoblasts is still unknown. To determine the role of TSP1 in biomineralization, mouse osteoblastic MC3T3-E1 cells were cultured in the presence of antisense phosphorothioate oligodeoxynucleotides complementary to the TSP1 sequence. The 18- and 24-mer antisense oligonucleotides caused concentration-dependent increases in the number of mineralized nodules, acid-soluble calcium deposition in the cell/matrix layer, and alkaline phosphatase activity within 9 days, without affecting cell proliferation. The corresponding sense or scrambled oligonucleotides did not affect these parameters. In the antisense oligonucleotide-treated MC3T3-E1 cells, thickened extracellular matrix, well-developed cell processes, increased intracellular organelles, and collagen fibril bundles were observed. On the other hand, the addition of TSP1 to the culture decreased the production of a mineralized matrix by MC3T3-E1 cells. Furthermore, MC3T3-E1 clones overexpressing mouse TSP1 were established and assayed for TSP1 protein and their capacity to mineralize. TSP1 dose-dependently inhibited mineralization by these cells both in vitro and in vivo. These results indicate that TSP1 functions as an inhibitory regulator of bone mineralization and matrix production by osteoblasts to sustain bone homeostasis.

Role of adult mesenchymal stem cells in bone tissue engineering applications: current status and future prospects

Mesenchymal stem cells (MSCs) have been demonstrated as an attractive cell source for tissue-engineering applications because of their ability to be easily isolated and expanded from adult bone marrow aspirates and their versatility for pluripotent differentiation into mesenchymal tissues. This review highlights advances and progress in bone reconstruction techniques for both the repair of site-specific bone defects and the attenuation of musculoskeletal disease symptoms associated with osteoporosis and osteogenesis imperfecta. Despite the enormous potential benefits of MSCs within these approaches, conventional tissue culture methods limit the clinical utility of these cells because of the gradual loss of both their proliferative and differentiation potential during ex vivo expansion. Novel strategies to overcome these limitations are discussed including cultivation in the presence of basic fibroblastic growth factor 2, induction of ectopotic telomerase expression, and ex vivo expansion on various collagenous biomaterials. In addition, this review also outlines mechanistic theories on the potential role of MSC-extracellular matrix interactions in mediating the retention of MSC proliferative and differentiation capacity after ex vivo expansion on collagenous biomaterials.

Dynamic expression of sparc precedes formation of skeletal elements in the Medaka (Oryzias latipes)

Sparc is a secreted calcium-binding glycoprotein that regulates mineralization of bone tissues in mammals. In other vertebrates, its function remains largely unclear. Here, we describe the isolation, genomic organization and expression of the sparc gene in the teleost Medaka (Oryzias latipes), an established vertebrate model for developmental studies. During earliest stages of Medaka embryogenesis, sparc is expressed in the sclerotome compartment of the somites that gives rise to precursor cells of the axial skeleton. Importantly, in this area its expression precedes that of twist-1, which is a crucial regulator of osteoblast formation. Dynamic expression is also found in the floor plate of the neural tube and the notochord. Both structures are passed by migrating skeletal precursors shortly before they differentiate and form the vertebrae. In general, sparc is expressed before the formation and mineralization of bone elements and expression of bone markers like collagen type 1a in the fins and axial skeleton of Medaka embryos. It is also expressed in several non-skeletal tissues of embryos and adult fish, suggesting possible other functions not related to bone mineralization. Taken together, the Medaka sparc gene represents an excellent marker for early sclerotome development. Its restricted and highly dynamic expression suggests a novel function during migration of sclerotome cells and their differentiation into early vertebrae. This marker thus allows the analysis of early skeletal development and formation of extracellular bone matrix in this vertebrate model.

In vitro comparison of equine cancellous bone graft donor sites and tibial periosteum as sources of viable osteoprogenitors

OBJECTIVE

To compare the osteogenic potential of cancellous bone of conventional graft sites with that of one nonconventional site (fourth coccygeal vertebra) and to investigate the tibial periosteum as a donor site with respect to osteogenic potential.

STUDY DESIGN

In vitro osteogenic cell culture system.

SAMPLE POPULATION

Eight adult horses.

METHODS

Cancellous bone or tibial periosteum was aseptically collected and cut into bone chips or periosteal strips of 1 to 2 mm(3) for primary explant cultures. After 2 weeks, primary tissue cultures that yielded a population of osteogenic cells were counted and subcultured at 1 x 10(5) cells/35-mm dish in osteogenic media. After 7 to 10 days, subcultures were stained with Von Kossa (VK) to assess mineralized bone nodule formation. VK-positive bone nodules were counted as osteoprogenitors and compared among 3 donor sites, which provided consistent primary osteogenic cells (tuber coxae, fourth coccygeal vertebra, periosteum) using ANOVA (P <.05).

RESULTS

Sternal and tibial bone yielded viable osteogenic cells from 25% and 50% of horses, respectively, whereas yields from tuber coxae, coccygeal vertebra, and periosteum were 75%, 100%, and 100%, respectively. Tuber coxae and periosteum had significantly greater numbers of osteoprogenitors compared with fourth coccygeal vertebra.

CONCLUSIONS

Among the conventional donor sites, tuber coxae most consistently yielded viable osteogenic cells with an acceptable percentage of osteoprogenitors. Sternal and tibial sites were unreliable in providing osteogenic cells. Two new donor sites, the fourth coccygeal vertebra and tibial periosteum, were tissues with good osteogenic potential.

CLINICAL RELEVANCE

When a source of transplantable viable osteoprogenitor cells is desired, use of the tuber coxae as a conventional donor site is warranted. Use of tibial periosteum or fourth coccygeal vertebra as reliable sources of transplantable osteoprogenitors should be considered.

Age-related changes in human bone proteoglycan structure. Impact of osteogenesis imperfecta

Proteoglycans (PGs) are a family of molecules that undergo extensive post-translational modifications that include addition of glycosaminoglycan (GAG) chains as well as N- and O-linked oligosaccharides to the protein core. PG composition and structure have been reported to alter with age. To test whether the post-translational modifications to PGs can serve as in vitro surrogate end point markers for chronological age, the extent of GAG modifications was determined for PGs derived from normal human bone cells of 14 donors (age range, fetal to 60 years). Isolated cells were steady state radiolabeled with (35)SO(4)(2-) and [(3)H]GlcN. For biglycan and decorin, iduronate content was linearly correlated with age (increased 1.5x between fetal and age 60 years). For the syndecan-like heparan sulfate PG, the N-sulfation of post-natal cells increased over 3.5-fold until reaching a plateau during the 4th decade of life. The amount of O-linked oligosaccharides was also found to decrease as a function of increasing normal donor age, whereas the specific activity of the metabolic precursor pool remained constant regardless of donor age. These age-related changes in post-translational modifications were then used to demonstrate that osteoblasts derived from patients with osteogenesis imperfecta did not exhibit facets of a pre-mature aging, but rather were arrested in a fetal-like phenotypic state. A growth matrix rich in thrombospondin altered PG metabolism in osteoblastic cells, resulting in the production and secretion of the fetal-like (rich in O-linked oligosaccharides) forms of decorin and biglycan. This effect was qualitatively different from the effect of transforming growth factor-beta, which predominantly altered GAGs rather than O-linked oligosaccharides. No other Arg-Gly-Asp protein (fibronectin, vitronectin, type I collagen, osteopontin, and bone sialoprotein) showed any detectable effect on PG metabolism in bone cells. These results indicate that a proper matrix stoichiometry is critical for metabolism of PGs.

Advances in osteogenesis imperfecta

Considerable progress has been made in many aspects of osteogenesis imperfecta. The international Sillence classification of osteogenesis imperfecta is being expanded to include a greater range of subgroups of patients. Attempts are being made to identify the genes causing forms of osteogenesis imperfecta and related syndromes that are not caused by mutations of the Type I collagen genes. In medium-term studies, bisphosphonate treatment has been shown to be the first method of treatment to improve the clinical course of the disease significantly. Somatic cell therapy, using allogeneic bone marrow and mesenchymal stromal cell transplantation, are in their early phases of development for use in humans with osteogenesis imperfecta. Somatic gene therapy, which aims to inactivate the mutation, is being evaluated in laboratory and animal studies.

Defects of type I procollagen metabolism correlated with decrease of prolidase activity in a case of lethal osteogenesis imperfecta

We have studied the structure and metabolism of type I procollagen in a case of perinatal lethal osteogenesis imperfecta (OI) type II. Cultured skin fibroblasts from the proband synthesized both normal and abnormal forms of type I procollagen. Some abnormal, overmodified molecules were secreted by OI cells, although less efficiently than normal molecules from control cells. The OI fibroblasts accumulated large amounts of abnormal proalpha1(I) and proalpha2(I) chains intracellularly. The extracellular collagenolytic activity was decreased compared to control cells. Furthermore, OI cells produced less type I procollagen and demonstrated lower capacity to synthesize DNA than control cells. We have found that in contrast to prolinase activity, the activity of prolidase (an enzyme essential for collagen synthesis and cell growth) is also significantly reduced in OI cells. No differences were found in the amount of the enzyme protein recovered from both the OI and control cells. However, we found that expressions of beta1 integrin and insulin-like growth factor-I receptor (receptors known to play an important role in up regulation of prolidase activity) were decreased in OI cells compared to control cells. The decrease in prolidase activity may provide an important mechanism of altered cell growth and collagen metabolism involved in producing the perinatal lethal form of the OI phenotype.

A role for decorin in the structural organization of periodontal ligament

Decorin is a small leucine-rich proteoglycan that interacts with several matrix molecules, including various types of collagen and growth factors, and suppresses the growth of neoplastic cells by an epidermal growth factor (EGF) receptor-mediated pathway. Decorin is abundantly expressed in the periodontal connective tissues during development and tissue maintenance. In periodontal disease, which is one of the most common diseases in the human kind, the level of decorin is decreased in the periodontal connective tissue. Abnormal expression of decorin may also associate with certain inherited disorders that involve increased susceptibility to severe periodontal disease in the early childhood. Therefore, we investigated the periodontal tissues of mice with targeted disruption of the decorin gene. Gross and microscopic analyses showed that decorin-deficient mice appeared to have normal tooth development and eruption, and there were no signs of periodontal disease. However, electron microscopic analysis revealed abnormal morphology and organization of the collagen fibrils in the periodontal ligament. The number of periodontal ligament fibroblasts in the decorin-deficient mice was also increased about two-fold as compared with the wild-type mice. In cell culture, ectopic overexpression of decorin in NIH 3T3 fibroblasts or decorin added exogenously to periodontal fibroblasts suppressed cell growth. However, blocking the EGF receptor tyrosine kinase activity did not prevent the decorin-elicited growth suppression in periodontal fibroblasts. Additionally, decorin did not induce a marked increase in the relative expression of p21 mRNA in periodontal fibroblasts. Therefore, decorin appeared to regulate growth of normal periodontal fibroblasts by a mechanism distinct from that reported for neoplastic cells. The findings demonstrate that decorin plays a role in the organization of collagen fibrils and regulates cell proliferation in the periodontal ligament.

Basic fibroblast growth factor destabilizes osteonectin mRNA in osteoblasts

Osteonectin (secreted protein acidic and rich in cysteine, 40-kDa basement membrane) is a glycoprotein abundantly expressed in bone and in other tissues undergoing active remodeling. Fibroblast growth factors (FGFs) are important in skeletal development and fracture repair, events associated with extracellular matrix remodeling. We used the murine osteoblastic cell line MC3T3 to determine whether basic FGF (bFGF) regulates osteonectin expression in bone. Northern blot analysis showed that bFGF decreased osteonectin transcripts in a dose- and time-dependent manner. This regulation was independent of the mitogenic effect of bFGF but was dependent on new protein synthesis. Immunoprecipitation of [35S]methionine-cysteine osteoblast-conditioned medium and cell layer proteins showed that bFGF decreased osteonectin synthesis. Nuclear runoff assays failed to reveal regulation of osteonectin gene transcription by bFGF. However, bFGF dramatically decreased the stability of osteonectin mRNA in transcriptionally arrested osteoblasts. This destabilization of osteonectin mRNA may be one means by which bFGF regulates extracellular matrix remodeling.

Biglycan gene promoter activity in osteosarcoma cells is regulated by cyclic AMP

The pericellular proteoglycan biglycan is among the major secretory products of osteoblasts and articular chondrocytes but the regulatory agents and signal transduction pathways that ultimately lead to alterations in biglycan gene expression are poorly defined. We report here on the transcriptional up-regulation of biglycan in MG-63 osteosarcoma cells by agents that increase intracellular cAMP levels. Transfection of these cells with biglycan promoter luciferase reporter fusion genes and subsequent treatment with forskolin or the cAMP analog 8-Bromo-cAMP resulted in an up to 3.8-fold stimulation of biglycan promoter activity. This effect could be prevented with the compound KT5720, a specific inhibitor of the cAMP-dependent protein kinase. Up-regulation of transcription is also reflected at the level of mRNA expression, since biglycan mRNA steady state levels in MG-63 cells increased approximately 2-fold after 24 hours of forskolin treatment. These data suggest that elevated levels of intracellular cAMP increase transcription from the biglycan promoter in bone cells and implicate for the first time the cAMP/protein kinase A signal transduction pathway in the regulation of biglycan gene expression.

Multiexon deletions in the type I collagen COL1A2 gene in osteogenesis imperfecta type IB. Molecules containing the shortened alpha2(I) chains show differential incorporation into the bone and skin extracellular matrix

Osteogenesis imperfecta (OI) type IB is a rare subset of the mildest form of OI, clinically characterized by moderate bone fragility, blue sclera, and dentinogenesis imperfecta. Cultured skin fibroblasts from two unrelated individuals (OI-197 and OI-165) with the typical features of OI type IB produced shortened alpha2(I) chains. Reverse transcription-polymerase chain reaction of the alpha2(I)-cDNA revealed deletions in the triple helical domain of 5 exons (exons 7-11) in OI-197, and 8 exons (exons 10-17) in OI-165. This exon skipping was caused by genomic deletions in one allele of COL1A2 with the breakpoints located in introns 6 and 11 in OI-197, and introns 9 and 17 in OI-165. The secretion and deposition of the mutant collagen into the matrix was measured in vitro in cultures of skin fibroblasts and bone osteoblasts, grown in the presence of ascorbic acid to induce collagen matrix formation and maturation, as well as in collagen extracts from skin and bone. The secretion of mutant collagen was impaired and long term cultures of fibroblasts showed that the mutant collagen was not incorporated into the mature collagenous matrix produced in vitro by skin fibroblasts from both patients. Likewise, the shortened alpha2(I) chain was not demonstrable in skin extracts. In contrast, bone extracts from OI-197 showed the presence of the mutant collagen. This incorporation of the abnormal collagen into the mature matrix was also demonstrated in long term cultures of the patient's osteoblasts. The deposition of the mutant collagen by bone osteoblasts but not by skin fibroblasts demonstrates a tissue specificity in the incorporation of mutant collagen into the matrix which may explain the primary involvement of bone and not skin in these patients.

Collagen peptides in osteogenesis imperfecta, idiopathic juvenile osteoporosis and Ehlers--Danlos syndrome

We evaluated the potential of the carboxy-terminal propeptide of type I procollagen (PICP), the carboxy-terminal telopeptide of collagen I (ICTP), and the amino-terminal propeptide of type III procollagen (PIIINP) to differentiate osteogenesis imperfecta (OI) from Ehlers-Danlos syndrome (EDS) and idiopathic juvenile osteoporosis (IJO) in paediatric patients. Markedly decreased serum concentrations of PICP were found in type I OI, while in IJO they were much less diminished, and in EDS they were near to normal. In type III and IV OI, the serum PICP level was lowered in prepubertal patients, whereas at puberty it was comparable to that in controls. Serum ICTP and PIIINP levels in patients with OI did not differ significantly from the levels in EDS and IJO. Measurements of serum PICP levels seem to be useful in discriminating OI from EDS and IJO in prepubertal children. In pubertal children, however, they lose their diagnostic power.

An ultrastructural and immunogold localization study of proteoglycans associated with the osteocytes of fetal bone in osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a rare, heterogeneous, inherited connective tissue disorder frequently caused by abnormalities of type I collagen. It is characterized by bone fragility, osteopenia, and progressive skeletal deformities. Electron microscopy of three OI type II fetal bone samples revealed numerous large osteocyte lacunae. In addition, there was a perilacunar osteoid-like band of collagen surrounding the osteocytes, which was unmineralized and morphologically unusual. Furthermore, large osteocyte lacunae contained fine particles and filamentous material similar to the expected ultrastructural appearance of proteoglycans. More detailed examination was carried out using histochemical and immunogold localization of proteoglycans at light and ultrastructural levels. These tests and the use of electron probe X-ray microanalysis confirmed that the material in the osteocyte lacunae was proteoglycan. In contrast, in the age- and site-matched normal fetal bone, all the osteocyte lacunae appeared negative for proteoglycan. Proteoglycans are regarded as inhibitors of calcification. Our observation of substantial amounts of proteoglycan in abnormally enlarged osteocytic lacunae of some OI fetal bone suggests association with the abnormal bone of this particular subtype of OI type II.

Long-term extracellular matrix metabolism by cultured human osteogenesis imperfecta osteoblasts

Osteopenia due to deficient extracellular matrix synthesis is a hallmark of osteogenesis imperfecta (OI), Previous studies carried out within 72 h of osteoblast subculture, at an early stage of matrix synthesis, indicated that for osteoblasts derived from human OI patients the total amounts of collagen, osteonectin, and three proteoglycans were significantly reduced, while total amounts of thrombospondin, fibronectin, and matrix hyaluronan were elevated compared with age-matched controls. The current study was undertaken to follow OI osteoblast matrix metabolism as that matrix is synthesized, deposited, and matured. Steady-state metabolic radiolabeling was used to follow the metabolism of collagen, hyaluronan, and total proteoglycan by OI and normal osteoblasts for up to 5 weeks. Trabecular osteoblasts from non-OI controls showed an increase in total and matrix-associated collagen synthesis during the first and second week, reaching steady-state levels by week 4. In contrast, cultured OI osteoblasts did not increase either the total (medium + matrix-associated) or matrix-associated collagen during the entire 5-week period. Proteoglycan synthesis exhibited a pattern similar to that for collagen. OI-derived proteoglycans differed from controls in that levels in OI cultures did not reflect the normal time-dependent increase in total proteoglycan and proteoglycan matrix deposition. OI osteoblast hyaluronan synthesis was increased when compared with age-matched controls during 4 weeks of culture. In contrast, the ratios of calcium to phosphorus solublized from control and the OI extracellular matrix were not statistically different. Thus, with respect to the synthesis of collagen, proteoglycans, and hyaluronan, OI osteoblasts fail to parallel controls in depositing and elaborating extracellular matrix during 35 days in culture.

Deficient expression of the small proteoglycan decorin in a case of severe/lethal osteogenesis imperfecta

In osteogenesis imperfecta (OI) the effects of mutations in type I collagen genes generally reflect their nature and localization. Unrelated individuals sharing identical mutations present, in general, similar clinical phenotypes. However, in some such cases the clinical phenotype differs. This variable clinical expression could be the result of abnormalities in other connective tissue proteins. Since decorin is a component of connective tissue, binds to type I collagen fibrils and plays a role in matrix assembly, we studied decorin production in skin fibroblasts from OI patients. Cultured fibroblasts from one patient with extremely severe osteogenesis imperfecta (classified as type II/III) who has an alpha 1(I)gly415ser mutation were found to secrete barely detectable amounts of decorin into culture medium. Western blotting using antibodies raised against decorin confirmed the reduction of the decorin core protein and Northern blot analysis showed decorin mRNA levels below the limit of detection. Cells from a patient, with a less severe phenotype, bearing a mutation in the same position of the triple helix (alpha 1(I)gly415) expressed decorin normally. The different clinical phenotypes could be due to the differing genetic backgrounds of the patients so it is tempting to conclude that in our most severely affected patient the absence of decorin aggravates the clinical phenotype.

Extracellular matrix stoichiometry in osteoblasts from patients with osteogenesis imperfecta

In previous work, we compared the steady-state levels of specific matrix components in human bone cells derived from patients with osteogenesis imperfecta (OI) to those of age-matched controls. A remarkable finding was the observation that there was a reduction not only in the total levels of collagen, but also in osteonectin and three proteoglycans (a large chondroitin sulfate proteoglycan, biglycan, and decorin). This pattern was observed in patients with and without detectable collagen defects. More recent analysis of extracellular matrix composition have yielded that, compared with age-matched controls, bone cells from OI patients produced higher steady-state levels of fibronectin and thrombospondin. The percentage of these two proteins incorporated into the cell layer pool was also higher in OI than in age-matched controls. In addition, the steady-state levels of hyaluronan and a heparan sulfate proteoglycan were analyzed in both OI and age-matched controls. Although the total (medium + cell layer) steady-state levels of hyaluronan were reduced by 1/3, the percentage of the hyaluronan in the cell layer pool of patients with OI increased between 100-250% of age-matched control. Thus the matrix elaborated by human OI bone cells is not only quantitatively different but also qualitatively distinct from that of age-matched controls. Not only have specific bone cell matrix components (collagen, osteonectin, the large chondroitin sulfate proteoglycan, biglycan, and decorin) been found to be present in reduced levels in OI bone cells, but some matrix components (thrombospondin, fibronectin, and hyaluronan) have also been found to be present in elevated levels in the matrix of OI cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The strength of a calcified tissue depends in part on the molecular structure and organization of its constituent mineral crystals in their organic matrix

High-voltage electron-microscopic tomographic (3D) studies of the ultrastructural interaction between mineral and organic matrix in a variety of calcified tissues reveal different crystal structural and organizational features in association with their respective organic matrices. In brittle or weak pathologic or ectopic calcifications, including examples of osteogenesis imperfecta, calciphylaxis, calcergy, and dermatomyositis, hydroxyapatite crystals occur in various sizes and shapes and are oriented and aligned with respect to collagen in a manner which is distinct from that found in normal calcified tissues. A model of collagen-mineral interaction is proposed which may account for the observed crystal structures and organization. The results indicate that the ultimate strength, support, and other mechanical properties provided by a calcified tissue are dependent in part upon the molecular structure and arrangement of its constituent mineral crystals within their organic matrix.

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.

Tomographic imaging of collagen-mineral interaction: implications for osteogenesis imperfecta

The novel method of high voltage electron microscopic tomography (3D) has been applied for the first time to examine ultrastructural features and spatial relations between collagen fibrils and mineral crystals in a mouse mutant (oim/oim) which replicates a moderate to severe form of osteogenesis imperfecta. The animal produces collagen consisting of the alpha1(I) homotrimer and has a brittle calcified skeleton. Three-dimensional image reconstructions of the Achilles tendons, which were found to mineralize in the mutant mice, revealed that their composite crystals were different in their structural appearance and spatial association with collagen compared to that determined in normal calcified tissues. These results indicate that the nature of the organic matrix of a mineralizing tissue critically influences the formation, structure, and location of the constituent mineral and, further, the data are interpreted as suggesting that the unusual structural and organizational interaction between mineral and collagen underlies the inherent brittleness and weakness of calcification in this model of osteogenesis imperfecta.

OIM and related animal models of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by fragile bones, skeletal deformity, and growth retardation. This heritable disorder of connective tissue is the result of mutations affecting the COL1A1 and COL1A2 genes of type I collagen. Progress in OI research has been limited because of dependence on human fibroblast and osteoblast specimens and the absence of a naturally occurring animal model for this genetic disorder. Recent technology in molecular biology has led to the development of transgenic models of OI based on site directed mutagenesis of type I collagen genes. OIM is a naturally occurring model which incorporates both the phenotypic and biochemical defects of moderate to severe osteogenesis imperfecta. This powerful tool permits the development of models based on different type I collagen mutations. The collagen type I mutation in OIM is a C propeptide deletion which impairs the production of normal pro-alpha2(I). Tissues in OIM contain only [pro-alpha1(I)]3 homotrimer. Thus, although several animal models are now available for research in osteogenesis imperfecta few are viable or fully mimic human disease disorders. OIM duplicates the phenotype and biochemistry of human disease and has a normal life span.

Effects of ascorbic acid on collagen matrix formation and osteoblast differentiation in murine MC3T3-E1 cells

Treatment of mouse MC3T3-E1 cells with ascorbic acid initiates the formation of a collagenous extracellular matrix and synthesis of several osteoblast-related proteins. We recently showed that ascorbic acid dramatically increases alkaline phosphatase and osteocalcin mRNAs and that this induction is blocked by inhibitors of collagen triple-helix formation (Franceschi and Iyer, J Bone Miner Res 7:235). In the present study, the relationship between collagen matrix formation and osteoblast-specific gene expression is explored in greater detail. Kinetic studies revealed that ascorbic acid increased proline hydroxylation in the intracellular procollagen pool within 1 h and stimulated the cleavage of type I collagen propeptides beginning at 2.5 h. Mature alpha 1(I) and alpha 2(I) collagen components were first detected at 10 h and continued to increase in both cell layer and culture medium for up to 72 h. Ascorbic acid also increased the rate of procollagen secretion from cell layers to culture medium. The secretion of another matrix protein, fibronectin, was only slightly affected. Alkaline phosphatase or its mRNA was first detected 2-3 days after ascorbic acid addition, but osteocalcin mRNA was not seen until day 6. Two inhibitors of collagen triple-helix formation, ethyl-3,4-dihydroxybenzoate and 3,4-dehydroproline, inhibited procollagen hydroxylation and alkaline phosphatase induction. 3,4-Dehydroproline also inhibited the induction of alkaline phosphatase and osteocalcin mRNAs. Surprisingly, induction was not blocked if cells were exposed to ascorbic acid before inhibitor addition. Alkaline phosphatase was also partially inhibited if cells were grown in the presence of purified bacterial collagenase. These results indicate that the induction of osteoblast markers by ascorbic acid does not require the continuous hydroxylation and processing of procollagens and suggest that a stable, possibly matrix-associated signal is generated at early times after ascorbic acid addition that allows subsequent induction of osteoblast-related genes.

In vitro expression of osteoblastic markers in cells isolated from normal fetal and postnatal human bone and from bone of patients with osteogenesis imperfecta

We studied the expression of osteoblastic markers in cultured cells isolated from the bone of 15 patients with different clinical forms of osteogenesis imperfecta (OI) and of seven fetal and postnatal controls. Cultured bone cells of ten OI patients produced abnormal collagen type I. Similar to controls, OI bone cells produced predominantly collagen type I with traces of collagen types III and V. The 1,25(OH)2 vitamin D3-stimulated synthesis of osteocalcin, a specific osteoblastic marker protein, was similar in OI bone cells and age-matched controls. Bone cells from fetal controls and from patients with the perinatal lethal OI type II produced less osteocalcin than bone cells from postnatal controls and surviving OI patients. OI bone cells responded to parathyroid hormone (PTH) by increased production of cAMP similar to controls. Bone cells from fetal controls and from OI type II donors showed a decreased response to PTH. Activity of the bone-liver-kidney isoenzyme alkaline phosphatase (AP) was detected in all control and OI bone cells. The expression of all osteoblastic markers was similar in bone cells producing abnormal collagen type I. These observations show that OI bone cells in vitro express a pattern of osteoblastic markers similar to age-matched control bone cells indicating that osteoblastic differentiation is not altered by the underlying defects of collagen type I metabolism in OI bone cells.

Effects of transforming growth factor beta on cells derived from bone and callus of patients with osteogenesis imperfecta

We studied the influence of transforming growth factor beta (TGF-beta) on cultured bone cells derived from two patients with osteogenesis imperfecta (OI) and from human controls. Additionally, cells from a hyperplastic callus that had developed spontaneously at the femur of the patient in Case 1 and cells from a normal fracture callus were included in the study. TGF-beta increased the synthesis of total protein and collagen of all cells without changing the pattern of interstitial collagens. Proliferation was stimulated by TGF-beta in the OI bone cells from Case 1, in cells from the central part of the hyperplastic callus, and in cells from the fracture callus. In Case 2, proliferation of bone cells was decreased by low concentrations of TGF-beta. Alkaline phosphatase (AP) activity was enhanced by TGF-beta in normal human bone cells, not affected in bone cells from the patient in Case 2 or in cells from the central part of the hyperplastic callus, and inhibited in bone cells and cells from the peripheral part of the hyperplastic callus of Case 1 and in cells from the fracture callus. We conclude that TGF-beta has common and specific effects on cultured human cells derived from different types of skeletal tissues. Simultaneous stimulation of collagen synthesis and AP activity by TGF-beta was restricted to normal human bone cells and might reflect their mature state of osteoblastic differentiation. Cells derived from bone of both patients with OI, from the hyperplastic callus, and from the fracture callus showed a different response pattern to TGF-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Collagen crosslinks and mineral crystallinity in bone of patients with osteogenesis imperfecta

In cortical bone samples from patients with osteogenesis imperfecta (OI), the concentrations of hydroxypyridinium cross-linking amino acids in collagen were measured by reversed-phase HPLC and the x-axis crystallinity of the apatite mineral phase was determined by x-ray diffraction. Bone samples from three patients with type I, nine patients with type III, and eight patients with type IV OI were analyzed and compared with human bone from nine controls. The concentration of the two chemical forms of the mature collagen crosslinking amino acids, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP), and the ratio HP/LP were found to be alike in bone collagen of OI patients and healthy controls. However, the c-axis crystallinity of the apatite was found to be reduced in the type III and IV OI patients compared with controls. Regression analysis of crosslink concentrations and c-axis crystallinity in OI bones did not show any correlation. Therefore, collagen molecules deposited in the extracellular matrix of OI bone appear to fulfill the structural requirements for the action of the enzyme lysyl oxidase, such that a normal concentration of intermolecular crosslinks is formed compared with healthy bone. Consequently, crosslink formation and apatite crystal growth seem to be regulated independently in OI bone.