Demonstration of an osteoblast defect in two cases of human malignant osteopetrosis. Correction of the phenotype after bone marrow transplant

CSF1R as a Therapeutic Target in Bone Diseases: Obvious but Not so Simple

PURPOSE OF REVIEW

The purpose of the review is to summarize the expression and function of CSF1R and its ligands in bone homeostasis and constraints on therapeutic targeting of this axis.

RECENT FINDINGS

Bone development and homeostasis depends upon interactions between mesenchymal cells and cells of the mononuclear phagocyte lineage (MPS), macrophages, and osteoclasts (OCL). The homeostatic interaction is mediated in part by the systemic and local production of growth factors, macrophage colony-stimulating factor (CSF1), and interleukin 34 (IL34) that interact with a receptor (CSF1R) expressed exclusively by MPS cells and their progenitors. Loss-of-function mutations in CSF1 or CSF1R lead to loss of OCL and macrophages and dysregulation of postnatal bone development. MPS cells continuously degrade CSF1R ligands via receptor-mediated endocytosis. As a consequence, any local or systemic increase or decrease in macrophage or OCL abundance is rapidly reversible. In principle, both CSF1R agonists and antagonists have potential in bone regenerative medicine but their evaluation in disease models and therapeutic application needs to carefully consider the intrinsic feedback control of MPS biology.

Successful hematopoietic stem cell transplantation for osteopetrosis using reduced intensity conditioning

BACKGROUND

Infantile malignant osteopetrosis (IMO) is an autosomal recessive condition characterized by defective osteoclast activity, with hematopoietic bone marrow transplant being the only available cure. Over the past several years, new conditioning regimes and donor options have emerged, thus extending the possibility of cure to a greater number of patients and improving the outcomes of bone marrow transplant. Here we detail the outcomes of bone marrow transplant in a cohort of 31 patients treated with a combination of fludarabine, treosulphan, thiotepa, and antithymocyte globulin.

PROCEDURES

Thirty-one patients with IMO who underwent hematopoietic stem cell transplantation with fludarabine, treosulphan, thiotepa, and antithymocyte globulin at our center from 2012 to 2017 are retrospectively reviewed in this study. Twenty-six patients were transplanted from 10/10 matched donors (13 from siblings, 11 from unrelated, and two from extended family donors), four from 9/10 matched unrelated donors, and one from a 9/10 matched family donor.

RESULTS

Overall survival was 100% with a median follow-up of 363 days (range 74-1891). There were 12 cases of acute graft versus host disease (GvHD) (38.7%), no cases of veno-occlusive disease, and eight cases of hypercalcemia (25.8%). Almost 80% of patients suffered viral reactivations with two cases of Epstein-Barr-virus-driven post-transplant lymphoproliferative disease. All cases of GvHD and viral reactivation were successfully treated.

CONCLUSIONS

We conclude that transplantation in children with IMO using fludarabine, treosulphan, thiotepa, and antithymocyte globulin is safe and effective and should be performed as early as possible following diagnosis, prior to the development of severe disease sequelae.

Peroxidase enzymes inhibit osteoclast differentiation and bone resorption

Myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are heme-containing enzymes, well known for their antimicrobial activity, are released in abundance by innate immune infiltrates at sites of inflammation and injury. We have discovered new and previously unrecognised roles for heme peroxidases in extracellular matrix biosynthesis, angiogenesis, and bone mineralisation, all of which play an essential role in skeletal integrity. In this study we used in vitro models of osteoclastogenesis to investigate the effects of heme peroxidase enzymes on osteoclast differentiation and bone resorbing activity, pertinent to skeletal development and remodelling. Receptor activator of nuclear factor kappa B-ligand (RANKL) stimulates the formation of tartate-resistant acid phosphatase (TRAP) positive multinucleated cells and increases bone resorption when cultured with human peripheral blood mononuclear cells (PBMCs) or the RAW264.7 murine monocytic cell line. When RANKL was added in combination with either MPO or EPO, a dose-dependent inhibition of osteoclast differentiation and bone resorption was observed. Notably, peroxidases had no effect on the bone resorbing activity of mature osteoclasts, suggesting that the inhibitory effect of the peroxidases was limited to osteoclast precursor cells. Mechanistically, we observed that osteoclast precursor cells readily internalize peroxidases, and inhibited the phosphorylation of JNK, p38 MAPK and ERK1/2, important signalling molecules central to osteoclastogenesis. Our findings suggest that peroxidase enzymes, like MPO and EPO, may play a fundamental role in inhibiting RANKL-induced osteoclast differentiation at inflammatory sites of bone fracture and injury. Therefore, peroxidase enzymes could be considered as potential therapeutic agents to treat osteolytic bone disease and aberrant bone resorption.

An overview of the regulation of bone remodelling at the cellular level

OBJECTIVES

To review the current literature on the regulation of bone remodelling at the cellular level.

DESIGN AND METHODS

The cellular activities of the cells in the basic multicellular unit (BMU) were evaluated.

RESULTS

Bone remodelling requires an intimate cross-talk between osteoclasts and osteoblasts and is tightly coordinated by regulatory proteins that interact through complex autocrine/paracrine mechanisms. Osteocytes, bone lining cells, osteomacs, and vascular endothelial cells also regulate bone remodelling in the BMU via cell signalling networks of ligand-receptor complexes. In addition, through secreted and membrane-bound factors in the bone microenvironment, T and B lymphocytes mediate bone homeostasis in osteoimmunology.

CONCLUSIONS

Osteoporosis and other bone diseases occur because multicellular communication within the BMU is disrupted. Understanding the cellular and molecular basis of bone remodelling and the discovery of novel paracrine or coupling factors, such as RANKL, sclerostin, EGFL6 and semaphorin 4D, will lay the foundation for drug development against bone diseases.

Bisphosphonates: effects on osteoblast

PURPOSE

Bisphosphonates are synthetic analogues of pyrophosphate usually used in treating bone disorders such as osteoporosis, Paget's disease, fibrous dysplasia, hypercalcemia of malignancy, and inflammation-related bone loss. Though therapeutic effects of bisphosphonates depend primarily on their inhibitory effect on osteoclasts, increasing attention is being given to other effector cells, such as osteoblasts. This review focuses on the presumed effect of bisphosphonates on osteoblasts.

METHODS

A review of the literature was conducted to evaluate the pharmacodynamic effects of bisphosphonates including inhibition of osteoclasts and apoptosis of osteocytes and osteoblasts as well as their potential stimulatory effects on the proliferation of osteoblasts.

RESULTS

Studies have demonstrated that bisphosphonates may stimulate proliferation of osteoblasts and inhibit apoptosis of osteocytes and osteoblasts.

CONCLUSION

Considering that osteoblasts may be involved in bone disorders, such as osteoporosis, osteopetrosis, osteogenesis imperfecta, and Paget's disease, and that bisphosphonates may stimulate proliferation of osteoblasts and inhibit apoptosis of osteocytes and osteoblasts, it is conceivable that a role for bisphosphonates exists in these diseases beyond merely the osteoclast influence.

Genetic diseases of bones and joints

Genetic factors play roles in many diseases. Often these factors are ill defined and unpredictable. Other diseases are caused by specific single gene mutations and are passed to offspring in Mendelian inheritance patterns. There are over 5000 documented Mendelian disorders; over 500 of these affect bones and joints. Some of these single gene disorders affect many tissues, and the skeletal system is one of many organ systems involved. The surgical pathologist must often diagnose these disorders. Important examples are neurofibromatosis, Gaucher's disease, and alkaptonuria. Other single gene disorders almost exclusively affect the skeleton. These disorders are the skeletal dysplasias and 372 have been documented. These disorders are classified using radiographic, clinical, and molecular data. The most common dysplasias are osteogenesis imperfecta, achondroplasia, and osteopetrosis. The surgical pathologist usually does not play a role in the diagnosis of skeletal dysplasias. However, histologic studies often elucidate the pathophysiologic basis of these diseases and proper collection of tissues is important for the evolving understanding of the molecular basis of these disorders.

Altered mineralization of human osteoarthritic osteoblasts is attributable to abnormal type I collagen production

OBJECTIVE

Bone tissue in osteoarthritis (OA) is composed of abundant undermineralized osteoid matrix. The aim of this study was to investigate the mechanisms responsible for this abnormal matrix, using in vitro OA subchondral osteoblasts.

METHODS

Primary normal and OA osteoblasts were prepared from tibial plateaus. Phenotype was determined by alkaline phosphatase activity, and osteocalcin, osteopontin, prostaglandin E2 (PGE2), and transforming growth factor beta1 (TGFbeta1) were assessed by enzyme-linked immunosorbent assay. Expression of COL1A1 and COL1A2 was determined by real-time polymerase chain reaction. The production of type I collagen was determined by the release of its C-terminal propeptide and Western blot analysis. In vitro mineralization was evaluated by alizarin red staining. Inhibition of TGFbeta1 expression was performed using a small interfering RNA technique.

RESULTS

Mineralization of OA osteoblasts was reduced compared with mineralization of normal osteoblasts, even in the presence of bone morphogenetic protein 2 (BMP-2). Alkaline phosphatase and osteocalcin levels were elevated in OA osteoblasts compared with normal osteoblasts, whereas osteopontin levels were similar. The COL1A1-to-COL1A2 messenger RNA ratio was 3-fold higher in OA osteoblasts compared with normal osteoblasts, and the production of collagen by OA osteoblasts was increased. Because TGFbeta1 inhibits BMP-2-dependent mineralization, and because TGFbeta1 levels are approximately 4-fold higher in OA osteoblasts than in normal osteoblasts, inhibiting TGFbeta1 levels in OA osteoblasts corrected the abnormal COL1A1-to-COL1A2 ratio and increased alizarin red staining.

CONCLUSION

Elevated TGFbeta1 levels in OA osteoblasts are responsible, in part, for the abnormal ratio of COL1A1 to COL1A2 and for the abnormal production of mature type I collagen. This abnormal COL1A1-to-COL1A2 ratio generates a matrix that blunts mineralization in OA osteoblasts.

Genetics, pathogenesis and complications of osteopetrosis

Human osteopetrosis is a rare genetic disorder caused by osteoclast failure, which ranges widely in severity. In the most severe forms, deficient bone resorption prevents enlargement of bone cavities, impairing development of bone marrow, leading to hematological failure. Closure of bone foramina causes cranial nerve compression with visual and hearing deterioration. Patients also present with osteosclerosis, short stature, malformations and brittle bones. This form is fatal in infancy, has an autosomal recessive inheritance and is cured with hematopoietic stem cell transplantation, with a rate of success <50% and unsatisfactory rescue of growth and visual deterioration. It relies on loss-of-function mutations of various genes, including the TCIRG1 gene, encoding for the a3 subunit of the H+ATPase and accounting for >50% of cases, the ClCN7 and the OSTM1 genes, which have closely related function and account for approximately 10% of cases, also presenting with neurodegeneration. Further genes are implicated in rare forms with various severities and association with other syndromes and, recently, the RANKL gene has been found to be mutated in a subset of patients lacking osteoclasts. Autosomal recessive osteopetrosis may also have intermediate severity, with a small number of cases due to loss-of-function mutations of the CAII or the PLEKHM1 genes. Dominant negative mutations of the ClCN7 gene cause the so-called Albers-Schönberg disease, which represents the most frequent and heterogeneous form of osteopetrosis, ranging from asymptomatic to intermediate/severe, thus suggesting additional genetic/environmental determinants affecting penetrance. Importantly, recent work has demonstrated that osteoblasts may also contribute to the pathogenesis of the disease, either because they are affected by intrinsic defects, or because their activity may be enhanced by deregulated osteoclasts abundantly present in most forms. Therapy is presently unsatisfactory and effort is necessary to unravel the gene defects yet unrecognized and identify new treatments to improve symptoms and save life.

The shunt from the cyclooxygenase to lipoxygenase pathway in human osteoarthritic subchondral osteoblasts is linked with a variable expression of the 5-lipoxygenase-activating protein

Osteoarthritis (OA) is characterized by articular cartilage degradation and hypertrophic bone changes with osteophyte formation and abnormal bone remodeling. Two groups of OA patients were identified via the production of variable and opposite levels of prostaglandin E₂ (PGE₂) or leukotriene B₄ (LTB₄) by subchondral osteoblasts, PGE₂ levels discriminating between low and high subgroups. We studied whether the expression of 5-lipoxygenase (5-LO) or 5-LO-activating protein (FLAP) is responsible for the shunt from prostaglandins to leukotrienes. FLAP mRNA levels varied in low and high OA groups compared with normal, whereas mRNA levels of 5-LO were similar in all osteoblasts. Selective inhibition of cyclooxygenase-2 (COX-2) with NS-398-stimulated FLAP expression in the high OA osteoblasts subgroup, whereas it was without effect in the low OA osteoblasts subgroup. The addition of PGE₂ to the low OA osteoblasts subgroup decreased FLAP expression but failed to affect it in the high OA osteoblasts subgroup. LTB₄ levels in OA osteoblasts were stimulated about twofold by 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) plus transforming growth factor-β (TGF-β), a situation corresponding to their effect on FLAP mRNA levels. Treatments with 1,25(OH)₂D₃ and TGF-β also modulated PGE₂ production. TGF-β stimulated PGE₂ production in both OA osteoblast groups, whereas 1,25(OH)₂D₃ alone had a limited effect but decreased the effect of TGF-β in the low OA osteoblasts subgroup. This modulation of PGE₂ production was mirrored by the synthesis of COX-2. IL-18 levels were only slightly increased in a subgroup of OA osteoblasts compared with normal; however, no relationship was observed overall between IL-18 and PGE₂ levels in normal and OA osteoblasts. These results suggest that the shunt from the production of PGE₂ to LTB₄ is through regulation of the expression of FLAP, not 5-LO, in OA osteoblasts. The expression of FLAP in OA osteoblasts is also modulated differently by 1,25(OH)₂D₃ and TGF-β depending on their endogenous low and high PGE₂ levels.

Abnormal insulin-like growth factor 1 signaling in human osteoarthritic subchondral bone osteoblasts

Insulin-like growth factor (IGF)-1 is a key factor in bone homeostasis and could be involved in bone tissue sclerosis as observed in osteoarthritis (OA). Here, we compare the key signaling pathways triggered in response to IGF-1 stimulation between normal and OA osteoblasts (Obs). Primary Obs were prepared from the subchondral bone of tibial plateaus of OA patients undergoing knee replacement or from normal individuals at autopsy. Phenotypic characterization of Obs was evaluated with alkaline phosphatase and osteocalcin release. The effect of IGF-1 on cell proliferation, alkaline phosphatase and collagen synthesis was evaluated in the presence or not of 50 ng/ml IGF-1, whereas signaling was studied with proteins separated by SDS-PAGE before western blot analysis. We also used immunoprecipitation followed by western blot analysis to detect interactions between key IGF-1 signaling elements. IGF-1 receptor (IGF-1R), Shc, Grb2, insulin receptor substrate (IRS)-1, and p42/44 mitogen-activated protein kinase (MAPK) levels were similar in normal and OA Obs in the presence or absence of IGF-1. After IGF-1 stimulation, the phosphorylation of IGF-1R in normal and OA Obs was similar; however, the phosphorylation of IRS-1 was reduced in OA Ob. In addition, the PI3K pathway was activated similarly in normal and OA Obs while that for p42/44 MAPK was higher in OA Obs compared to normal. p42/44 MAPK can be triggered via an IRS-1/Syp or Grb2/Shc interaction. Interestingly, Syp was poorly phosphorylated under basal conditions in normal Obs and was rapidly phosphorylated upon IGF-1 stimulation, yet Syp showed a poor interaction with IRS-1. In contrast, Syp was highly phosphorylated in OA Obs and its interaction with IRS-1 was very strong initially, yet rapidly dropped with IGF-1 treatments. The interaction of Grb2 with IRS-1 progressively increased in response to IGF-1 in OA Obs whereas this was absent in normal Ob. IGF-1 stimulation altered alkaline phosphatase in Ob, an effect reduced in the presence of PD98059, an inhibitor of p42/44 MAPK signaling, whereas neither IGF-1 nor PD98059 had any significant effect on collagen synthesis. In contrast, cell proliferation was higher in OA Obs compared to normal under basal conditions, and IGF-1 stimulated more cell proliferation in OA Obs than in normal Ob, an effect totally dependent on p42/44 MAPK activiy. The altered response of OA Obs to IGF-1 may be due to abnormal IGF-1 signaling in these cells. This is mostly linked with abnormal IRS-1/Syp and IRS-1/Grb2 interaction in these cells.

Modulation of insulin-like growth factor 1 levels in human osteoarthritic subchondral bone osteoblasts

Human osteoarthritis (OA) is characterized by cartilage loss, bone sclerosis, osteophyte formation and inflammation of the synovial membrane. We previously reported that OA osteoblasts (Ob) show abnormal phenotypic characteristics possibly responsible for bone sclerosis and that two subgroups of OA patients can be identified by low or high endogenous production of prostaglandin E2 (PGE2) by OA Ob. Here, we determined that the elevated PGE2 levels in the high OA subgroup were linked with enhanced cyclooxygenase-2 (COX-2) protein levels compared to normal and low OA Ob. A linear relationship was observed between endogenous PGE2 levels and insulin-like growth factor 1 (IGF-1) levels in OA Ob. As parathyroid hormone (PTH) and PGE2 are known stimulators of IGF-1 production in Ob, we next evaluated their effect in OA Ob. Both subgroups increased their IGF-1 production similarly in response to PGE2, while the high OA subgroup showed a blunted response to PTH compared to the low OA group. Conversely, only the high OA group showed a significant inhibition of IGF-1 production when PGE2 synthesis was reduced with Naproxen, a non-steroidal antiinflammatory drug (NSAID) that inhibits cyclooxygenases (COX). The PGE2-dependent stimulation of IGF-1 synthesis was due in part to the cAMP/protein kinase A pathway since both the direct inhibition of this pathway with H-89 and the inhibition of EP2 or EP4 receptors, linked to cAMP production, reduced IGF-1 synthesis. The production of the most abundant IGF-1 binding proteins (IGFBPs) in bone tissue, IGFBP-3, -4, and -5, was lower in OA compared to normal Ob independently of the OA group. Under basal condition, OA Ob expressed similar IGF-1 mRNA to normal Ob; however, PGE2 stimulated IGF-1 mRNA expression more in OA than normal Ob. These data suggest that increased IGF-1 levels correlate with elevated endogenous PGE2 levels in OA Ob and that higher IGF-1 levels in OA Ob could be important for bone sclerosis in OA.

Limited rescue of osteoclast-poor osteopetrosis after successful engraftment by cord blood from an unrelated donor

We report on a case of osteoclast-poor osteopetrosis who received a hematopoietic stem cell graft and, despite hematological engraftment, showed little signs of response in the skeletal defect. Clinical and laboratory studies supported the concept that the bone microenvironment remained abnormal, thus reducing the clinical response to transplantation.

INTRODUCTION

Osteopetrosis is a rare genetic disorder characterized by severely reduced bone resorption resulting from a defect in either osteoclast development (osteoclast-poor osteopetrosis) or activation (osteoclast-rich osteopetrosis). Patients with osteoclast-rich osteopetrosis can be rescued by allogenic hematopoietic stem cell transplantation; however, little information exists concerning the success of transplantation as a treatment for osteoclast-poor osteopetrosis. We report on a child with osteoclast-poor osteopetrosis whose diagnosis was delayed, consequently receiving a cord blood transplant from an unrelated donor at the age of 8 years. Engraftment was deemed successful by peripheral blood genotyping, although >3 years after transplantation there was little rescue of the skeletal defect and anemia, and extramedullary hematopoiesis persisted.

MATERIALS AND METHODS

Peripheral blood mononuclear cells from the osteopetrosis patient, before and after transplantation, were used to generate osteoclasts in vitro in the presence of macrophage colony-stimulating factor (M-CSF) and RANKL.

RESULTS

Before transplantation few, small mononuclear osteoclasts formed (F-actin ring-positive cells, co-localizing with vitronectin receptor [alphavbeta3 integrin] and TRACP) associated with occasional, small resorption lacunae. Low levels of collagen C-terminal telopeptide (CTx) fragments were released from these cultures as assessed by ELISA (CrossLaps; patient, 12.85 nM; control, 448.6 nM). In contrast, osteoclasts formed in cultures after transplantation formed to a similar degree to control cultures from healthy individuals: large numbers of osteoclasts containing numerous nuclei were present, and approximately 50% of the surface of bone slices was resorbed, associated with intermediate levels of collagen fragment release (116.48 nM). The culture data reflect the histopathology and radiological findings and also support previous studies showing that neither M-CSF nor RANKL rescues osteoclast-poor osteopetrosis.

CONCLUSIONS

This is the first case reported in which a successful hematopoietic engraftment failed to correct an osteopetrotic skeletal defect, and this finding may be credited to the age at which the child was transplanted.

Clinical, genetic, and cellular analysis of 49 osteopetrotic patients: implications for diagnosis and treatment

BACKGROUND

Osteopetrosis, a genetic disease characterised by osteoclast failure, is classified into three forms: infantile malignant autosomal recessive osteopetrosis (ARO), intermediate autosomal recessive osteopetrosis (IRO), and autosomal dominant osteopetrosis (ADO).

METHODS

We studied 49 patients, 21 with ARO, one with IRO, and 27 with type II ADO (ADO II).

RESULTS

Most ARO patients bore known or novel (one case) ATP6i (TCIRG1) gene mutations. Six ADO II patients had no mutations in ClCN7, the only so far recognised gene implicated, suggesting involvement of yet unknown genes. Identical ClCN7 mutations produced differing phenotypes with variable degrees of severity. In ADO II, serum tartrate resistant acid phosphatase was always elevated. Bone alkaline phosphatase (BALP) was generally low, but osteocalcin was high, suggesting perturbed osteoblast differentiation or function. In contrast, BALP was high in ARO patients. Elevated osteoclast surface/bone surface was noted in biopsies from most ARO patients. Cases with high osteoclasts also showed increased osteoblast surface/bone surface. ARO osteoclasts were morphologically normal, with unaltered formation rates, intracellular pH handling, and response to acidification. Their resorption activity was greatly reduced, but not abolished. In control osteoclasts, all resorption activity was abolished by combined inhibition of proton pumping and sodium/proton antiport.

CONCLUSIONS

These findings provide a rationale for novel therapies targeting pH handling mechanisms in osteoclasts and their microenvironment.

Subchondral and trabecular bone metabolism regulation in canine experimental knee osteoarthritis

OBJECTIVE

To determine trabecular and subchondral bone metabolic changes in experimental canine osteoarthritis (OA).

METHODS

OA was induced in 19 dogs by transection of the anterior cruciate ligament (ACL) of the right knee through a stab wound. Dogs were sacrificed at 8 (n=7) and 12 weeks (n=12) after surgery. Non-operated normal dogs (n=6) were used as controls. After sacrifice, samples were obtained from the weight-bearing area of medial tibial plateaus. Explants and cell cultures were prepared from subchondral and trabecular bone. Osteocalcin (Oc), cellular alkaline phosphatase (ALPase), urokinase plasminogen-activator (uPA), prostaglandin E2 (PGE2), metalloproteinase (MMP) and nitric oxide (NO) were measured using standard procedures.

RESULTS

ALPase production was significantly increased only at week 12 in subchondral and trabecular bone, while an increase in Oc was noted at week 8. uPA and MMP activity were increased significantly at week 12 in subchondral bone, while PGE2 levels were significantly higher in subchondral and trabecular bone at week 12 compared to normal. A decrease in NO production appeared late at week 12 in trabecular bone, whereas NO levels from subchondral bone were significantly increased compared to normal at week 8.

DISCUSSION

Intense bone remodeling takes place in both subchondral and trabecular bone in the knee following ACL transection. This process seems to occur around week 12, although Oc and NO appeared to be involved earlier at 8 weeks. These results suggest that not only subchondral but also trabecular bone metabolism is altered in this OA model.

Osteocalcin synthesis by human osteoblasts from normal and osteoarthritic bone after vitamin D3 stimulation

Alterations in osteoblast metabolism are involved in the pathogenesis of typical subchondral bone changes in osteoarthritis (OA). Osteocalcin is a specific bone protein, synthesised by the osteoblasts, which can be considered a marker of metabolic activity of these cells. In this study we correlated osteocalcin production from human osteoblasts isolated from healthy and osteoarthritic subjects to the degree of cartilage damage, before and after stimulation with 1,25(OH)2-vitamin D3, the active metabolite of vitamin D3. We isolated human osteoblasts from cancellous bone of healthy subjects and from subchondral bone of osteoarthritic subjects and considered the osteoblasts corresponding to different degrees of cartilage damage as different cell populations. We determined the osteocalcin production in normal and osteoarthritic osteoblasts from maximal and minimal cartilage damage areas both under basal conditions and after vitamin D3 stimulation. Compared to normal osteoblasts, under basal conditions osteocalcin production is significantly greater in osteoarthritic osteoblasts, corresponding both to maximal and minimal damage joint areas. No differences were observed between osteoblasts from maximal and minimal damage areas. The response of osteoblasts to vitamin D3 stimulation appeared to be proportional to the degree of joint damage, as the vitamin D3-induced increase in osteocalcin is proportionally greater in maximally damaged osteoblasts compared to minimally damaged ones. Thus, after vitamin D3 stimulation, a significant increase in osteocalcin production by maximally damaged osteoblasts compared to the minimally damaged ones was observed. This study confirms abnormal osteoarthritic osteoblast behaviour and indicates that osteoblasts from different areas of the same affected joint may be metabolically different, supporting the hypothesis that subchondral osteoblasts may play an essential role in the pathogenesis of OA.

Treatment with licofelone prevents abnormal subchondral bone cell metabolism in experimental dog osteoarthritis

OBJECTIVES

To determine if treatment with licofelone, a combined 5-lipoxygenase and cyclo-oxygenase inhibitor, in vivo in experimental dog osteoarthritis can modify bone cell metabolism in long term in vitro subchondral osteoblast cell cultures (Ob).

METHODS

Group 1 received sectioning of the anterior cruciate ligament (ACL) of the right knee with no active treatment (placebo group). Groups 2 and 3 received sectioning of the ACL of the right knee, and were given licofelone (2.5 or 5.0 mg/kg daily by mouth, respectively) for eight weeks beginning the day after surgery. Primary Ob were prepared from the subchondral bone plate. Levels of phenotypic markers (alkaline phosphatase activity, osteocalcin release), and urokinase plasminogen activator (uPA) and insulin-like growth factor-1 (IGF-I) levels, were evaluated in each group. Lastly, prostaglandin E(2) (PGE(2)) and leucotriene B(4) levels were evaluated.

RESULTS

No significant differences in alkaline phosphatase activity or osteocalcin release from Ob between the three groups, under either basal or 1,25(OH)(2)D(3) induction were seen. In contrast, treatment with licofelone reduced uPA and IGF-I levels in Ob. PGE(2) levels, which were still raised in the placebo group, were decreased sharply by licofelone. A relationship was found between licofelone treatment and either the reduction in the size of lesions on tibial plateaus or the levels of uPA, IGF-I, or PGE(2).

CONCLUSIONS

Licofelone treatment prevents and/or delays the abnormal metabolism of subchondral osteoblasts in this model. Licofelone reduced PGE(2) levels after long term Ob, suggesting that the reduction in uPA and IGF-I levels is linked, at least in part, to this reduction.

Osteoclast diseases

Osteoclasts are the only cells capable of resorbing mineralised bone, dentine and cartilage. Osteoclasts act in close concert with bone forming osteoblasts to model the skeleton during embryogenesis and to remodel it during later life. A number of inherited human conditions are known that are primarily caused by a defect in osteoclasts. Most of these are rare monogenic disorders, but others, such as the more common Paget's disease, are complex diseases, where genetic and environmental factors combine to result in the abnormal osteoclast phenotype. Where the genetic defect gives rise to ineffective osteoclasts, such as in osteopetrosis and pycnodysostosis, the result is the presence of too much bone. However, the phenotype in many osteoclast diseases is a combination of osteosclerosis with osteolytic lesions. In such conditions, the primary defect is hyperactivity of osteoclasts, compensated by a secondary increase in osteoblast activity. Rapid progress has been made in recent years in the identification of the causative genes and in the understanding of the biological role of the proteins encoded. This review discusses the known osteoclast diseases with particular emphasis on the genetic causes and the resulting osteoclast phenotype. These human diseases highlight the critical importance of specific proteins or signalling pathways in osteoclasts.

Osteopetrosis and Glanzmann's thrombasthenia in a child

Autosomal recessive osteopetrosis is a rare, fatal disease characterized by accumulation of excessive bone mass due to defective bone resorption. The pathogenesis of osteopetrosis is controversial. Osteoblast-osteoclast interaction defects, incorrect differentiation of osteoclasts, abnormal contact between osteoclast and extracellular matrix, and abolished signaling are included in this process. Recently, mutations in the gene of the vacuolar proton pump have been described in some cases of recessive osteopetrosis. Glanzmann's thrombasthenia (GT) is a rare hereditary qualitative platelet disorder characterized by a lifelong bleeding tendency due to quantitative and qualitative abnormalities of the platelet integrin alpha(IIb) beta3. Several mutations on either integrin alpha(IIb) [glycoprotein (GP) IIb] or integrin beta(3) (GP IIIa) were reported in GT. We report on a patient with autosomal recessive osteopetrosis concurrently diagnosed with variant type Glanzmann's thrombasthenia. To our knowledge, our patient was the first case reported in the literature in which osteopetrosis and Glanzmann's thrombasthenia were diagnosed together.

Genotype-phenotype relationship in human ATP6i-dependent autosomal recessive osteopetrosis

Autosomal-recessive osteopetrosis is a severe genetic disease caused by osteoclast failure. Approximately 50% of the patients harbor mutations of the ATP6i gene, encoding for the osteoclast-specific a3 subunit of V-ATPase. We found inactivating ATP6i mutations in four patients, and three of these were novel. Patients shared macrocephaly, growth retardation and optic nerve alteration, osteosclerotic and endobone patterns, and high alkaline phosphatase and parathyroid hormone levels. Bone biopsies revealed primary spongiosa lined with active osteoblasts and high numbers of tartrate-resistant acid phosphatase (TRAP)-positive, a3 subunit-negative, morphologically unremarkable osteoclasts, some of which located in shallow Howship lacunae. Scarce hematopoietic cells and abundant fibrous tissue containing TRAP-positive putative osteoclast precursors were noted. In vitro osteoclasts were a3-negative, morphologically normal, with prominent clear zones and actin rings, and TRAP activity more elevated than in control patients. Podosomes, alphaVbeta3 receptor, c-Src, and PYK2 were unremarkable. Consistent with the finding in the bone biopsies, these cells excavated pits faintly stained with toluidine blue, indicating inefficient bone resorption. Bone marrow transplantation was successful in all patients, and posttransplant osteoclasts showed rescue of a3 subunit immunoreactivity.

Study of the role of leukotriene B()4 in abnormal function of human subchondral osteoarthritis osteoblasts: effects of cyclooxygenase and/or 5-lipoxygenase inhibition

OBJECTIVE

To compare the effect of licofelone, NS-398 (an inhibitor of cyclooxygenase 2 [COX-2]), and BayX-1005 (an inhibitor of 5-lipoxygenase activating protein) on the production of leukotriene B(4) (LTB(4)) and prostaglandin E(2) (PGE(2)), and on cell biomarkers by human osteoarthritis (OA) subchondral osteoblasts.

METHODS

Primary in vitro osteoblasts were prepared from subchondral bone specimens obtained from OA patients and autopsy subjects. LTB(4) and PGE(2) levels were measured by enzyme-linked immunosorbent assay in conditioned media of osteoblasts incubated in the presence or absence of licofelone, NS-398, or BayX-1005. The effect of these drugs or of the addition of LTB(4) on alkaline phosphatase (AP) activity and osteocalcin release by OA and normal osteoblasts was determined. The presence of LTB(4) receptors in normal and OA osteoblasts was evaluated by Western blot analysis.

RESULTS

OA osteoblasts produced variable levels of PGE(2) and LTB(4) compared with normal osteoblasts. Licofelone, at the maximal dose used, inhibited production of PGE(2) and LTB(4) by OA osteoblasts by a mean +/- SEM of 61.2 +/- 6.4% and 67.0 +/- 7.6%, respectively. NS-398 reduced PGE(2) production by 75.8 +/- 5.3%. BayX-1005 inhibited LTB(4) production in OA osteoblasts by 38.7 +/- 14.5% and marginally affected PGE(2) levels (reduction of 14.8 +/- 5.3%). Licofelone dose-dependently stimulated 1,25-dihydroxyvitamin D-induced AP activity while inhibiting osteocalcin release. BayX-1005 partly reproduced these effects, but NS-398 failed to affect them. LTB(4) dose-dependently inhibited AP activity in OA osteoblasts, while its effect on osteocalcin depended on endogenous LTB(4) levels in these cells. In normal osteoblasts, LTB(4) dose-dependently stimulated osteocalcin, whereas it failed to influence AP. LTB(4) receptors BLT1 and BLT2 were present in normal and OA osteoblasts.

CONCLUSION

Licofelone inhibits the production of PGE(2) and LTB(4). Selective effects of licofelone on AP and osteocalcin occur via its role on LTB(4) production. Because LTB(4) can modify cell biomarkers in OA and normal osteoblasts, our results suggest licofelone could modify abnormal bone remodeling in OA.

Can altered production of interleukin-1beta, interleukin-6, transforming growth factor-beta and prostaglandin E(2) by isolated human subchondral osteoblasts identify two subgroups of osteoarthritic patients

OBJECTIVE

To determine the capacity of human subchondral osteoarthritic osteoblasts (Ob) to produce interleukin (IL)-1beta, IL-6, transforming growth factor-beta (TGF-beta) and prostaglandin E(2) (PGE(2)), and determine if a relationship exists between IL-1beta, TGF-beta, PGE(2) and IL-6 production.

METHODS

We measured the abundance of IL-1beta, IL-6, TGF-beta and PGE(2) using very sensitive ELISA in conditioned-media of human primary subchondral Ob from normal individuals and osteoarthritic patients. Selective inhibition of IL-6 or IL-6 receptor signaling was performed to determine its effect on PGE(2) production whereas the inhibiton of PGE(2) production was performed to determine its effect on IL-6 production. The expression of bone cell markers and urokinase plasminogen activator (uPA) activity was also determined.

RESULTS

Osteoarthritic Ob produced all these factors with greater variability than normal cells. Interestingly, the production of IL-6 and PGE(2) by osteoarthritic Ob separated patients into two subgroups, those whose Ob produced levels comparable to normal (low producers) and those whose Ob produced higher levels (high producers). In those cells classified as high osteoarthritic Ob, PGE(2) and IL-6 levels were increased two- to three-fold and five- to six-fold, respectively, compared with normal. In contrast, while using their IL-6 and PGE(2) production to separate osteoarthritic Ob into low and high producers, we found that IL-1beta levels were similar in normal and all osteoarthritic Ob. Using the same criteria, TGF-beta levels were increased in all osteoarthritic Ob compared with normal. Reducing PGE(2) synthesis by Indomethacin [a cyclo-oxygenase (COX) -1 and -2 inhibitor] reduced IL-6 levels in all osteoarthritic Ob, whereas Naproxen (a more selective COX-2 inhbitor) reduced PGE(2) and IL-6 levels only in the high osteoarthritic group. Conversely, PGE(2) addition to osteoarthritic Ob enhanced IL-6 production in both groups. Moreover, the addition of parathyroid hormone also stimulated IL-6 production to similar normal levels in both osteoarthritic groups. In contrast, using an antibody against IL-6 or IL-6 receptors did not reduce PGE(2) levels in either group. The evaluation of alkaline phosphatase activity, osteocalcin release, collagen type I and uPA activity in osteoarthritic Ob failed to show any differences between these cells regardless to which subgroup they were assigned.

CONCLUSIONS

These results indicate that IL-6 and PGE(2) production by subchondral Ob can discriminate two subgroups of osteoarthritic patients that cannot otherwise be separated by their expression of cell markers, and that endogenous PGE(2) levels influence IL-6 synthesis in osteoarthritic Ob.

Apparent cure of a newborn with malignant osteopetrosis using prednisone therapy

A newborn girl with hemorrhagic purpura, suspected neonatal sepsis, and pale and dry skin was lethargic with remarkable hepatosplenomegaly, convergent strabismus, severe anemia, and elevated alkaline phosphatase activity. Radiographs showed a generalized increase in bone density, small medullary cavities, sclerosis of the skull and vertebrae, transverse wavy stripes of sclerotic bone in the metaphyses, and bone-in-bone appearance in phalanges of hands and feet. On this basis, she was diagnosed with malignant infantile osteopetrosis. On the first day of life, the infant was given a blood transfusion and vitamin K (1 mg intravenously [iv]). Corticosteroid therapy was started with prednisone (2 mg/kg per day). She showed marked improvement of symptoms. On the 26th day and 42nd day of life, she received additional blood transfusions. On the 49th day, the patient was discharged and corticosteroid therapy was continued at a regimen of 5 mg/day. Subsequent blood sample analyses revealed normal values for age. At 1 year of life, a bone marrow sample showed normal white and red cell lineages. X-ray confirmed attenuation of the bone sclerosis; therefore, bone marrow transplantation (BMT) was not implemented. At the age of 1.5 years, prednisone therapy was discontinued gradually and withdrawn before the age of 2 years. Subsequent follow-up showed normalization of all radiological and hematologic parameters. At present, the patient is 3 years old and appears healthy with apparently complete regression of the disease.

Endogenous prostaglandin E2 and insulin-like growth factor 1 can modulate the levels of parathyroid hormone receptor in human osteoarthritic osteoblasts

Subchondral bone sclerosis may be important for the onset and/or progression of cartilage loss/damage in human osteoarthritis (OA). OA osteoblasts are resistant to parathyroid hormone (PTH) stimulation, which could explain bone sclerosis via the inhibition of PTH-dependent catabolism. Here, we investigated the molecular mechanism(s) responsible for reduced PTH-dependent cyclic adenosine monophosphate (cAMP) synthesis in OA subchondral osteoblasts. Although cholera toxin (CTX) increased basal cAMP formation in these cells, it failed to stimulate PTH-dependent cAMP synthesis, whereas pertussis toxin (PTX) did not inhibit basal cAMP, yet diminished PTH-dependent cAMP production. Binding of 125I-PTH indicated lower PTH receptor levels in OA than in normal osteoblasts (-50.5 +/- 9.5%). This could be attributed to either reduced expression of the PTH receptor (PTH-R) or altered recycling of existing pools of receptors. Reverse-transcription polymerase chain reaction (RT-PCR) analysis indicated decreased PTH-R messenger RNA (mRNA) levels in OA cells that were highly variable (ranging from -10% to -60%), a situation that reflects disease severity. Interestingly, OA osteoblasts produced more prostaglandin E2 (PGE2) than normal osteoblasts, and using naproxen, a cyclo-oxygenase inhibitor, increased PTH-dependent cAMP formation to a level similar to normal osteoblasts. Because heterologous desensitization can explain a decrease in PTH binding but cannot account for reduced PTH-R expression, we looked at the possible effect of insulin-like growth factor 1 (IGF-1) on this parameter. Blocking IGF-1 signaling with a neutralizing receptor antibody increased 125I-PTH binding in both normal and OA osteoblasts. Conversely, treatments with IGF-1 receptor (IGF-1R) antibody only slightly increased the levels of PTH-R mRNA whereas the addition of IGF-1 significantly reduced PTH-R mRNA levels (-24.1 +/- 7.1%), yet neither PGE2 nor naproxen modified PTH-R levels. These results suggest that both IGF-1 signaling and PGE2 formation repress PTH-dependent response in OA osteoblasts, a situation that can contribute to abnormal bone remodeling and bone sclerosis in OA.

Thiazide diuretics affect osteocalcin production in human osteoblasts at the transcription level without affecting vitamin D3 receptors

Besides their natriuretic and calciuretic effect, thiazide diuretics have been shown to decrease bone loss rate and improve bone mineral density. Clinical evidence suggests a specific role of thiazides on osteoblasts, because it reduces serum osteocalcin (OC), an osteoblast-specific protein, yet the mechanisms implicated are unknown. We therefore investigated the role of hydrochlorothiazide (HCTZ) on OC production by the human osteoblast-like cell line MG-63. HCTZ dose-dependently (1-100 microM) inhibited 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]-induced OC release by these cells (maximal effect, -40-50% and p < 0.005 by analysis of variance [ANOVA]) as measured by ELISA. This effect of HCTZ on OC release was caused by a direct effect on OC gene expression because Northern blot analysis revealed that OC messenger RNA (mRNA) levels were reduced in the presence of increasing doses of the diuretic (-47.2+/-4.0%; p < 0.0001 by paired ANOVA with 100 microM 13.6+/-0.49 pmol/mg protein/15 minutes; p < 0.05) in MG-63 cells. Reducing extracellular Ca2+ concentration with 0.5 mM EDTA or 0.5 mM ethylene glycol-bis(beta-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) only partly prevented the inhibitory effect of the diuretic on OC secretion (maximal effect, -22.5+/-6.9%), suggesting that thiazide-dependent Ca2+ influx is not sufficient to elicit the inhibition of OC secretion. Because OC production is strictly dependent on the presence of 1,25(OH)2D3 in human osteoblasts, we next evaluated the possible role of HCTZ on vitamin D3 receptors (VDR) at the mRNA and protein levels. Both Northern and Western blot analyses showed no effect of HCTZ (1-100 microM) on VDR levels. The presence of EGTA in the culture media reduced slightly the VDR mRNA levels under basal condition but this was not modified in the presence of increasing levels of HCTZ. The OC gene promoter also is under the control of transcription factors such as Yin Yang 1 (YY1) and cFOS. Western blot analysis revealed no changes in YY1 levels in response to HCTZ either in the presence or in the absence of 0.5 mM EGTA in the culture media. In contrast, HCTZ induced a dose-dependent increase in cFOS levels (p < 0.002 by ANOVA), a situation prevented by incubation with EGTA. These studies indicate that HCTZ inhibits OC mRNA expression independently of an effect on VDR, YY1, or extracellular Ca2+ levels but involves changes in cFOS levels. As OC retards bone formation/mineralization, the inhibition of OC production by HCTZ could explain its preventive role in bone loss rate.

In vitro pathological model of osteopenia to test orthopaedic biomaterials

The association of in vitro tests and in vivo bone implants, has significantly improved the characterization of biomaterials for orthopaedic devices before their clinical use. However, neither cell cultures nor most animals models used for these tests entirely reflect the clinical conditions in which biomaterials are implanted. Pathological animal models are considered to substantially improve our knowledge of osteointegration of biomaterials; for this reason researchers increasingly use aged, osteopenic or arthritic animals in their experimental tests. The development of "pathological cell cultures" would also be of great importance for the study of biomaterials. It would allow a complete material evaluation, beginning with a biocompatibility test to a more finalized and specific preclinical evaluation. The present study, looks at the possibility of using cell culture methodology for the improvement of in vitro biomaterials characterization in the case of osteopenia. Cultures derived from normal (NB-OST) rats were compared to those of osteopenic (OB-OST) rats, by testing the osteoblasts against common parameters of characterization. Moreover, the reaction of these cultures to two biological glasses of known in vivo behavior (both in normal and osteopenic bone) by means of parameters on biocompatibility and bone formation index, was evaluated. Our results showed that there was no evidence of differences between the NB-OST and OB-OST cultures. After 6 days of culturing, the bioglass that did not osteointegrate in osteopenic animals, did not induce cytotoxicity in NB-OST, but a significative reduction of viability and Osteocalcin level in OB-OST was observed. We think that these data should stimulate researchers to develop further tests in order to improve preliminary in vitro comprehension on biomaterials.

Study of the nonresorptive phenotype of osteoclast-like cells from patients with malignant osteopetrosis: a new approach to investigating pathogenesis

Osteopetrosis manifests as failure of osteoclastic bone resorption. The cause of the disease lies either in the hematopoietic lineage or in the bone marrow stromal microenvironment. It has not been possible to define the cell type involved in the various forms of the human disease because of the inability to form human osteoclasts in vitro. Using the recently described method for generating human osteoclasts from peripheral blood in coculture with rat osteoblastic UMR 106 cells, we demonstrate that a defect lies in the mature osteoclast-like cells in four cases of this disease. Control and osteopetrotic cocultures generated large numbers of osteoclast-like cells (calcitonin and vitronectin receptor positive, and F-actin ring-positive cells) with similar morphology. Bone resorption did not occur in three of the four osteopetrotic cultures. In case 1, in which bone resorption was identified, the area of resorption was negligible compared with the number of osteoclast-like cells in the culture and was detected only by scanning electron microscopy. In contrast, up to 20% of the bone surface in controls was resorbed. The normal and osteopetrotic osteoclast-like cells had a similar phenotype except that two of the osteopetrotic cases did not express CD44 and two expressed CD44 weakly, whereas CD44 was strongly expressed in the controls. This study shows that it is possible to reproduce in vitro the pathological features of human osteopetrosis, and the assay provides a means of acquiring a greater understanding of the pathogenesis of human osteopetrosis.

Abnormal regulation of urokinase plasminogen activator by insulin-like growth factor 1 in human osteoarthritic subchondral osteoblasts

OBJECTIVE

Subchondral bone sclerosis is a common feature of osteoarthritis (OA), but the mechanisms responsible for this condition remain unresolved. We investigated the role of insulin-like growth factor 1 (IGF-1) and urokinase plasminogen activator (uPA) in human osteoblasts from subchondral bone obtained from the tibial plateaus of OA patients and normal individuals.

METHODS

Primary in vitro osteoblasts were prepared from subchondral bone specimens obtained from OA patients at surgery and from normal individuals at autopsy. Levels of uPA and PA inhibitor 1 (PAI-1) levels were determined under basal conditions and after IGF-1 stimulation in conditioned media from osteoblasts by enzyme-linked immunosorbent assay. The activity of uPA was evaluated by specific substrate hydrolysis and zymography under basal conditions and after plasminogen stimulation, in the presence and absence of added IGF-1. Plasmin activity was also evaluated by specific substrate hydrolysis.

RESULTS

Levels of uPA released by OA osteoblasts were significantly higher than normal. Addition of IGF-1 to osteoblasts significantly reduced uPA protein levels only in OA patients (P < 0.05). In contrast, the addition of uPA to osteoblasts did not modify IGF-1 levels in either normal or OA osteoblasts. Basal uPA activity was higher in OA than in normal osteoblasts. Interestingly, IGF-1 enhanced basal uPA activity in OA specimens in a dose-dependent manner. Addition of plasminogen promoted uPA activity in both normal and OA osteoblasts via a positive feedback loop due to plasmin generation, since this activity was inhibited by both PAI-1 and alpha2-antiplasmin. Unexpectedly, incubation with IGF-1 inhibited this positive feedback of plasminogen-dependent uPA activity in OA osteoblasts, but not in normal osteoblasts, in a dose-dependent manner. Hence, normal osteoblasts were relatively insensitive to IGF-1, whereas the same treatment reduced both uPA levels and plasminogen-dependent uPA activity in OA osteoblasts while it increased basal uPA activity in OA osteoblasts. This could not be explained by PAI-1 protein levels, which were similar in normal and OA osteoblasts in the presence and absence of IGF-1. IGF-1 also reduced plasmin activity in OA osteoblasts while it did not modify this activity in normal osteoblasts.

CONCLUSION

These results suggest that in OA osteoblasts, the uPA/plasmin system functions normally, yet IGF-1 inhibits the positive feedback of plasmin on uPA activity. This inhibition may contribute to abnormal IGF-1- and uPA-dependent bone remodeling, ultimately leading to abnormal bone sclerosis in OA.

Transplantability and therapeutic effects of bone marrow-derived mesenchymal cells in children with osteogenesis imperfecta

In principle, transplantation of mesenchymal progenitor cells would attenuate or possibly correct genetic disorders of bone, cartilage and muscle, but clinical support for this concept is lacking. Here we describe the initial results of allogeneic bone marrow transplantation in three children with osteogenesis imperfecta, a genetic disorder in which osteoblasts produce defective type I collagen, leading to osteopenia, multiple fractures, severe bony deformities and considerably shortened stature. Three months after osteoblast engraftment (1.5-2.0% donor cells), representative specimens of trabecular bone showed histologic changes indicative of new dense bone formation. All patients had increases in total body bone mineral content ranging from 21 to 29 grams (median, 28), compared with predicted values of 0 to 4 grams (median, 0) for healthy children with similar changes in weight. These improvements were associated with increases in growth velocity and reduced frequencies of bone fracture. Thus, allogeneic bone marrow transplantation can lead to engraftment of functional mesenchymal progenitor cells, indicating the feasibility of this strategy in the treatment of osteogenesis imperfecta and perhaps other mesenchymal stem cell disorders as well.

Dissociation between bone resorption and bone formation in osteopenic transgenic mice

Bone mass is maintained constant in vertebrates through bone remodeling (BR). BR is characterized by osteoclastic resorption of preexisting bone followed by de novo bone formation by osteoblasts. This sequence of events and the fact that bone mass remains constant in physiological situation lead to the assumption that resorption and formation are regulated by each other during BR. Recent evidence shows that cells of the osteoblastic lineage are involved in osteoclast differentiation. However, the existence of a functional link between the two activities, formation and resorption, has never been shown in vivo. To define the role of bone formation in the control of bone resorption, we generated an inducible osteoblast ablation mouse model. These mice developed a reversible osteopenia. Functional analyses showed that in the absence of bone formation, bone resorption continued to occur normally, leading to an osteoporosis of controllable severity, whose appearance could be prevented by an antiresorptive agent. This study establishes that bone formation and/or bone mass do not control the extent of bone resorption in vivo.

The protein tyrosine kinase p60c-Src is not implicated in the pathogenesis of the human autosomal recessive form of osteopetrosis: a study of 13 children

Osteopetrosis has been described in mice generated by homozygous gene disruption of c-src gene encoding for the p60c-Src protein tyrosine kinase (Src-/- mice). The similarities of bone histologic findings in this murine model to those observed in some patients first seen with autosomal recessive osteopetrosis, "malignant" osteopetrosis, led us to investigate the potential role of p60c-Src in the pathogenesis of malignant osteopetrosis in 13 children. In 4 patients a c-src mutation was ruled out by an intragenic microsatellite segregation study. In the other 9 we analyzed p60c-Src expression and function, as well as c-src sequence. The expression was normal in all of the patients tested. In addition, the tyrosine phosphorylation and kinase activity of p60c-Src were also normal in all of the patients. Moreover, in these patients, sequences of the coding region of c-src were identical to the published sequence of the human c-src complementary DNA. These results exclude a role for c-src in the pathogenesis of human malignant osteopetrosis in the 13 patients analyzed.

Fourier transform infrared microspectroscopic analysis of bones of osteocalcin-deficient mice provides insight into the function of osteocalcin

Osteocalcin, the gamma-carboxyglutamic acid-containing protein, which in most species is the predominant noncollagenous protein of bone and dentin, has been postulated to play roles in bone formation and remodeling. Recently, genetic studies showed that osteocalcin acts as an inhibitor of osteoblast function. Based on von Kossa staining and measurement of mineral apposition rates in tetracycline-labeled bones, osteocalcin knockout animals were reported to have no detectable alterations in bone mineralization. To test the hypothesis that, in addition to regulating osteoblastic activity, osteocalcin is involved in regulating mineral properties, a more sensitive assay of mineralization, Fourier transform infrared microspectroscopy (FT-IRM) was used to study thin sections of femora of 4-week-, 6-month- (intact and ovariectomized), and 9-month-old wild-type and osteocalcin-knockout mice. FT-IRM spectra provided spatially resolved measures of relative mineral and carbonate contents, and parameters indicative of apatite crystal size and perfection. No differences were detected in the mineral properties of the 4-week-old knockout and wild-type mice indicating that the mineralization process was not altered at this time point. Six-month-old wild-type animals had higher mineral contents (mineral:matrix ratios) in cortical as compared with trabecular bones; mineral contents in knockout and wild-type bones were not different. At each age studied, carbonate:phosphate ratios tended to be greater in the wild-type as compared with knockout animals. Detailed analysis of the phosphate nu1,nu3 vibrations in the spectra from 6-month-old wild-type animals indicated that the crystals were larger/more perfect in the cortical as opposed to the trabecular bones. In contrast, in the knockout animals' bones at 6 months, there were no differences between trabecular and cortical bone in terms of carbonate content or crystallite size and perfection. Spectral parameters of the cortical and trabecular bone of the knockout animals resembled those in the wild-type trabecular bone and differed from wild-type cortical bone. In ovariectomized 6-month-old animals, the mineral content (mineral:matrix ratio) in the wild-type cortices increased from periosteum to endosteum, whereas, in the knockout animals' bones, the mineral:matrix ratio was constant. Ovariectomized knockout cortices had lower carbonate:phosphate ratios than wild-type, and crystallite size and perfection resembled that in wild-type trabeculae, and did not increase from periosteum to endosteum. These spatially resolved data provide evidence that osteocalcin is required to stimulate bone mineral maturation.

Osteoblast-like cells from human subchondral osteoarthritic bone demonstrate an altered phenotype in vitro: possible role in subchondral bone sclerosis

OBJECTIVE

Osteoarthritis (OA) is accompanied by subchondral bone sclerosis. The present study was undertaken to determine whether osteoblast-like cells in patients with OA show an abnormal phenotype that could contribute to this sclerosis.

METHODS

Explants and primary in vitro osteoblast-like cell cultures were prepared from subchondral bone specimens from OA patients or from bone removed at autopsy from individuals showing no signs of OA or metabolic bone disease. We measured the abundance and activity of urokinase plasminogen activator (uPA), and the levels of PA inhibitor (PAI-1) and insulin-like growth factor 1 (IGF-1) in conditioned media from both explants and osteoblast-like cells. The expression of osteoblast phenotypic biomarkers was also evaluated.

RESULTS

OA explants showed increased levels and activity of uPA, no changes in PAI-1 abundance, and increases in IGF-1 release, as compared with preparations from normal individuals. In vitro primary osteoblast-like cells showed results similar to the ex vivo findings for uPA, PAI-1, and IGF-1. Primary OA osteoblast-like cells also expressed higher alkaline phosphatase activity and osteocalcin release than normal cells, both under basal conditions and with 1,25(OH)2D3 (1,25-dihydroxyvitamin D) stimulation. Conversely, OA osteoblast-like cells showed blunted cAMP synthesis in response to human parathyroid hormone and prostaglandin E2 in contrast to the finding with normal osteoblast-like cells, a result that could not be attributed to altered adenylate cyclase activity.

CONCLUSION

Ex vivo and in vitro results indicate similar altered activities of OA osteoblasts as compared with normal cells. This suggests that an altered phenotype of subchondral osteoblasts may be a contributing factor in human OA.