Abnormalities of phosphoprotein gene expression in three osteopetrotic rat mutations: elevated mRNA transcripts, protein synthesis, and accumulation in bone of mutant animals

Signaling and transcriptional regulation in osteoblast commitment and differentiation

The major event that triggers osteogenesis is the transition of mesenchymal stem cells into bone forming, differentiating osteoblast cells. Osteoblast differentiation is the primary component of bone formation, exemplified by the synthesis, deposition and mineralization of extracellular matrix. Although not well understood, osteoblast differentiation from mesenchymal stem cells is a well-orchestrated process. Recent advances in molecular and genetic studies using gene targeting in mouse enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. Osteoblast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. We review Wnt signaling pathway and Runx2 regulation network, which are critical for osteoblast differentiation. Many other factors and signaling pathways have been implicated in regulation of osteoblast differentiation in a network manner, such as the factors Osterix, ATF4, and SATB2 and the TGF-beta, Hedgehog, FGF, ephrin, and sympathetic signaling pathways. This review summarizes the recent advances in the studies of signaling transduction pathways and transcriptional regulation of osteoblast cell lineage commitment and differentiation. The knowledge of osteoblast commitment and differentiation should be applied towards the development of new diagnostic and therapeutic alternatives for human bone diseases.

Decreased c-Src expression enhances osteoblast differentiation and bone formation

c-src deletion in mice leads to osteopetrosis as a result of reduced bone resorption due to an alteration of the osteoclast. We report that deletion/reduction of Src expression enhances osteoblast differentiation and bone formation, contributing to the increase in bone mass. Bone histomorphometry showed that bone formation was increased in Src null compared with wild-type mice. In vitro, alkaline phosphatase (ALP) activity and nodule mineralization were increased in primary calvarial cells and in SV40-immortalized osteoblasts from Src(-/-) relative to Src(+/+) mice. Src-antisense oligodeoxynucleotides (AS-src) reduced Src levels by approximately 60% and caused a similar increase in ALP activity and nodule mineralization in primary osteoblasts in vitro. Reduction in cell proliferation was observed in primary and immortalized Src(-/-) osteoblasts and in normal osteoblasts incubated with the AS-src. Semiquantitative reverse transcriptase-PCR revealed upregulation of ALP, Osf2/Cbfa1 transcription factor, PTH/PTHrP receptor, osteocalcin, and pro-alpha 2(I) collagen in Src-deficient osteoblasts. The expression of the bone matrix protein osteopontin remained unchanged. Based on these results, we conclude that the reduction of Src expression not only inhibits bone resorption, but also stimulates osteoblast differentiation and bone formation, suggesting that the osteogenic cells may contribute to the development of the osteopetrotic phenotype in Src-deficient mice.

Higher osteoclastic demineralization and highly mineralized cement lines with osteocalcin deposition in a mandibular cortical bone of autosomal dominant osteopetrosis type II: ultrastructural and undecalcified histological investigations

In this study we report on histological and ultrastructural investigations of the mandibular cortical bone in a case of autosomal dominant osteopetrosis type II complicated by mandibular osteomyelitis. Histologically, there was a marked increase in the number and size of osteoclasts on the inner bone surface. An undecalcified preparation showed a pair of deeply stained (highly demineralized) and stain-phobic (highly mineralized) layers on the bone surface just beneath the osteoclasts. The layers were incorporated into the bone matrix during the remodeling process as thickened cement lines. A contact microradiogram of the cortical bone revealed highly mineralized layers at the cement lines, which were closely correlated with immunohistochemical evidence of deposition of osteocalcin at the thickened cement lines. Ultrastructural examination showed that the osteoclasts had a typical clear zone, but they were deficient in ruffled border formation and had numerous lysosomal vacuoles containing dense substances. An electron-dense amorphous material layer was present on the bone surface just beneath the osteoclasts as well as at the cement lines. The layer was partly composed of a short fibrillar material, and it partially revealed the lamellar structure. Consequently, an osteoclastic malfunction might be primarily involved in the process of bone matrix resorption rather than demineralization, resulting in higher demineralization and abnormal material deposition on the bone surface and at the cement lines. Furthermore, evidence of active osteoclastic bone resorption with a brush border formation at the bone involved in the inflammatory lesion in this case suggests that the osteoclastic malfunction is influenced and recovered by a microenvironment such as inflammatory cytokines.

Insulin-like growth factor-I (IGF-I) and IGF-I receptor (IGF-IR) immunoreactivity in normal and osteopetrotic (toothless, tl/tl) rat tibia

Insulin-like growth factor-I (IGF-I) plays a major role in regulating cell growth. This study examined the immunohistochemical distribution of IGF-I and IGF-I receptor (IGF-IR) in tibias from normal and osteopetrotic (toothless, tl/tl) rats, following treatment with colony stimulating factor-1 (CSF-1). In normal rats, immunoreactivity for IGF-I and IGF-IR was detected in cells of the articular and epiphyseal cartilage, secondary ossification centres, zones of resting and proliferating chondrocytes and bone marrow. Bone marrow cells immunoreactive for IGF-I and IGF-IR were significantly reduced in the tl/tl rat (p < 0.001) compared with normal animals. Treatment of tl/tl rats with CSF-1 increased immunoreactivity for IGF-I and IGF-IR in bone marrow cells as well as the number of TRAP positive osteoclasts. This increase was the result of recruitment of a range of hematopoietic cell types, including eosinophils, polymorphs and a substantial number of monocyte-like cells demonstrating strong immunoreactivity to IGF-I/IGF-IR. The differences in relative immunoreactivity for IGF-I/IGF-IR by bone marrow cells in untreated and CSF-1-treated tl/tl rats indicate a CSF-1-dependent recruitment of cells bearing surface IGF-IRs which may be mediated by an increase in local or systemic IGF-I.

Abnormalities in bone cell function and endochondral ossification in the osteopetrotic toothless rat

The toothless (tl) rat is an osteopetrotic mutation whose excess skeletal mass is produced by a defect in its skeletal microenvironment. Its skeletal sclerosis fails to be cured by bone marrow transplantation but is largely reversed by exogenous administration of colony stimulating factor 1 which increases osteoclast neogenesis and resorptive activity. Recent studies have also indicated abnormalities in growth plate cartilage morphology and in osteoblast number and function in the tl rat. The present histomorphometric study examined static and kinetic parameters of bone cell and cartilage function in young (3-5-week-old) animals of tl stock for evidence of tissue level dysfunction. Mineralization of growth plate cartilage in mutants occurred only in the lateral regions of the growth plate, not in the central region, and longitudinal bone growth was significantly reduced (36%-61%) in mutants at the ages examined. Bone volume and trabecular thickness were greater in mutants despite significant reductions in their osteoblast populations and bone formation rates (two to threefold lower). Mutants also showed progressive age- and metaphyseal site-related decreases in osteoblast numbers which, compared to normal littermates, may relate to differences in osteoprogenitor cell pools, osteoblast lifespan, or resorption-derived skeletal growth factors locally available to support and maintain normal osteoblast phenotype. Osteoclast number per millimeter bone perimeter was reduced 96-fold in mutants and showed no age- or metaphyseal site-related changes. This study presents evidence in support of defects in chondrocyte and bone cell function in the tl rat and reveals the specific tissue locations where they occur.

Use of bone cell cultures to study skeletal pathology

We describe procedures for the isolation, culture, and analysis of neonatal osteoblasts from osteopetrotic (toothless (tl) and osteopetrosis [op]) rats and normal littermates. Normal osteoblasts produce and mineralize an extracellular matrix indistinguishable from that of well-characterized fetal rat osteoblasts in vitro. Mutant (tl and op) cultures show an early abnormal pattern of cell proliferation and a later premature, extensive mineralization which mimic the mutant phenotype in vivo. In cocultures with normal osteoclasts, mutant (tl) osteoblasts also show a greatly reduced ability to orchestrate bone resorption, as revealed by pit formation in bone slices, in response to physiologic mediators. These phenomena in vitro are consistent with the behavior of mutant osteoblasts and osteoclasts in vivo and suggest that more definitive microscopic analyses of osteoblasts from each mutation in vitro will provide insights on the roles of osteoblasts in the compromised bone resorption which characterizes the osteopetroses as well as their role in osteoclast ontogeny. This study shows that when their behavior is confirmed in vivo, bone cell cultures offer rigorous systems for understanding skeletal cell dysfunction in normal and pathological development.

Decreased growth hormone receptor expression in long bones from toothless (osteopetrotic) rats and restoration by treatment with colony-stimulating factor-1

Growth hormone (GH) is known to regulate growth and development of skeletal tissues. This study examined the distribution of growth hormone receptor (GHR) expression in tibias from normal and osteopetrotic tl/tl rats. For normal 2 week-old rats, GHR expression was detected immunocytochemically in cells of the articular and epiphyseal cartilage, primary and secondary ossification centres, zone of resting cartilage and bone marrow. Within the marrow, GHR immunopositive cells were concentrated in the central cone and largely excluded from the zone of immature progenitors at the periphery. For the marrow haemopoietic compartment, GHR expression was almost restricted to the nucleus in large mononuclear cells, adipocytes and megakaryocytes. A population of small lymphocytelike cells in the marrow periphery expressed GHR on the plasma membrane. GHR was not detected in mature erythroid cells, macrophages, granulocytes, or osteoclasts. The expression of GHR was significantly reduced in bone marrow cells of the tl/tl rat (p < 0.001) compared with normal animals. Injection of recombinant CSF-1 into tl/tl rats every 48 hours for 2 weeks from birth restored GHR-positive cells to the central core of the marrow space. The most striking change was the appearance of substantial numbers of mononuclear cells expressing abundant GHR on the cell surface. We infer that these cells are a novel subset of CSF-1 responsive cells involved in bone resorption. The differences in relative expression of GHR by bone marrow cells in untreated and CSF-1-treated tl/tl rats suggests a CSF-1-dependent recruitment of cells bearing surface GHRs.

Regulation of bone sialoprotein and osteopontin mRNA expression by dexamethasone and 1,25-dihydroxyvitamin D3 in rat bone organ cultures

Bone sialoprotein (BSP) and osteopontin (OPN) are prominent components of the extracellular matrix of mineralized connective tissues that have been implicated in the formation and remodelling of bone. Although these proteins have similar biochemical properties and are expressed by bone cells during bone formation it has been suggested that they have different functions and that their expression is regulated independently by hormones and cytokines. The precise role of these proteins has, however, yet to be firmly established. Since steroid hormones strongly influence the formation of bone we have analyzed the effects of glucocorticoids and 1,25 dihydroxyvitamin D3 (1,25-(OH)2D3) on the expression of BSP and OPN mRNAs in developing rat bone in vitro using in situ hybridization. In these studies it has been possible to identify the nature and spatial distribution of the cells that respond to these hormones by changes in sialoprotein expression. When cultured in the presence of the synthetic glucocorticoid, dexamethasone (dex), expression of BSP mRNA by hypertrophic cartilage cells in the tibiae and mandible was dramatically increased as were the number of responding cells indicating that glucocorticoids promote differentiation of hypertrophic cartilage cells as well as osteoblasts. Dexamethasone also stimulated a marked (> 5-fold) increase in OPN expression by osteoblasts and cells lining endosteal and periosteal bone surfaces. In contrast to dex, 1,25-(OH)2D3 suppressed BSP expression in osteoblastic cells whereas OPN expression was strongly (> 5-fold) stimulated in all three cultured bone tissues. Histological examination of the tissues showed that cell viability was retained over the culture period. However, in the presence of 1,25-(OH)2D3 considerable resorption of the tissue was evident, with cement and reversal lines being prominent. The increased expression of BSP and OPN by dex is consistent with the stimulation of bone formation by glucocorticoids, whereas the differential effects of 1,25-(OH)2D3 on BSP and OPN may reflect a stimulation of bone remodelling.

Regulation of osteopontin expression in osteoblasts

Osteopontin (OPN) is a prominent bone matrix protein that is synthesized by osteoblastic cells. To elucidate the function of OPN in bone we studied the regulated expression of the rat OPN protein during bone formation in vivo and in vitro. OPN mRNA is expressed by preosteoblastic cells early in bone formation, but the highest expression is observed in mature osteoblasts at sites of bone remodelling. A low-phosphorylated, 55-kDa form of OPN is produced by the preosteoblastic cells, whereas osteoblasts produce a highly phosphorylated, 44-kDa protein; the two forms of OPN corresponding to pp69 and pp62 in transformed rat cells. The synthesis of the 55-kDa OPN correlates with the formation of a 'cement' matrix that is synthesized prior to bone deposition, whereas the 44-kDa OPN synthesized by osteoblasts associates rapidly with hydroxyapatite, possibly regulating crystal growth, and may also provide a substratum for osteoclast attachment. Expression of OPN mRNA is upregulated by growth and differentiation factors (PDGF, EGF, TGF-beta and BMP-7/OP-1) and by mechanical stress, which promote bone formation, as well as by osteotropic hormones (retinoic acid and vitamin D3), which can promote bone resorption and remodelling. However, OPN mRNA is down-regulated by bisphosphonates, which abrogate bone resorption. Regulation of OPN expression is, therefore, consistent with a multiplicity of functions for OPN that involve specific structural motifs in both the synthesis and resorption of bone.

Aberrant gene expression in cultured mammalian bone cells demonstrates an osteoblast defect in osteopetrosis

Osteopetrosis is a skeletal condition in which a generalized radioopacity of bone is caused by reduced resorption of bone by osteoclasts. However, it has recently been shown that during skeletal development in several osteopetrotic rat mutations specific aberrations occur in gene expression reflecting the activity of the bone forming cells, osteoblasts, and the development of tissue organization. To evaluate their pathogenetic significance, progressive osteoblast differentiation was studied in vitro. Primary cultures of normal osteoblasts undergo a sequential expression of cell growth and tissue-related genes associated with development of skeletal tissue. We report that osteoblast cultures can be established from one of these mutants, toothless; that these cells in vitro exhibit similar aberrations in gene expression during cell proliferation and extracellular matrix formation and mineralization observed in vivo; and that an accelerated maturation sequence by mutant osteoblasts mimics the characteristic skeletal sclerosis of this disease. These data are the first direct evidence for an intrinsic osteoblast defect in osteopetrosis and establish an in vitro model for the study of heritable skeletal disorders.