Gene expression during skeletal development in three osteopetrotic rat mutations. Evidence for osteoblast abnormalities

Pleiotropic Impacts of Macrophage and Microglial Deficiency on Development in Rats with Targeted Mutation of the Csf1r Locus

We have produced Csf1r-deficient rats by homologous recombination in embryonic stem cells. Consistent with the role of Csf1r in macrophage differentiation, there was a loss of peripheral blood monocytes, microglia in the brain, epidermal Langerhans cells, splenic marginal zone macrophages, bone-associated macrophages and osteoclasts, and peritoneal macrophages. Macrophages of splenic red pulp, liver, lung, and gut were less affected. The pleiotropic impacts of the loss of macrophages on development of multiple organ systems in rats were distinct from those reported in mice. Csf1r^-/- rats survived well into adulthood with postnatal growth retardation, distinct skeletal and bone marrow abnormalities, infertility, and loss of visceral adipose tissue. Gene expression analysis in spleen revealed selective loss of transcripts associated with the marginal zone and, in brain regions, the loss of known and candidate novel microglia-associated transcripts. Despite the complete absence of microglia, there was little overt phenotype in brain, aside from reduced myelination and increased expression of dopamine receptor-associated transcripts in striatum. The results highlight the redundant and nonredundant functions of CSF1R signaling and of macrophages in development, organogenesis, and homeostasis.

Effects of microgravity on osteoclast bone resorption and osteoblast cytoskeletal organization and adhesion

Exposure to microgravity has been associated with several physiological changes in astronauts, including an osteoporosis-like loss in bone mass. Despite many in vivo and in vitro studies in both microgravity and simulated microgravity conditions, the mechanism for bone loss is still not clear. The lack of weight-bearing forces makes microgravity an ideal physical stimulus to assess bone cell responses. In this work, we conduct a unique investigation of the effects of microgravity on bone-producing osteoblasts and, in parallel, on bone-resorbing osteoclasts. An increase in total number of discrete resorption pits is observed in osteoclasts that experienced microgravity versus ground controls. We further show that osteoblasts exposed to 5 days of microgravity have shorter and wavier microtubules (MTs), smaller and fewer focal adhesions, and thinner cortical actin and stress fibers. Space-flown osteoblasts present extended cell shapes as well as significantly more disrupted and often fragmented or condensed nuclei. The absence of gravitational forces therefore causes both an increase in bone resorption by osteoclasts, and a decrease in osteoblast cellular integrity. The observed effects on both major bone cell types likely accelerate bone loss in microgravity environments, and additionally offer a potential explanation to the development of disuse osteoporosis on Earth.

Expression of secreted frizzled related protein 1, a Wnt antagonist, in brain, kidney, and skeleton is dispensable for normal embryonic development

Secreted frizzled related protein-1 (sFRP1), an antagonist of Wnt signaling, regulates cell proliferation, differentiation and apoptosis and negatively regulates bone formation. The spatial and temporal pattern of endogenous sFRP1 expression and loss-of-function were examined in the sFRP1-LacZ knock-in mouse (sFRP1-/-) during embryonic development and post-natal growth. beta-gal activity representing sFRP1 expression is robust in brain, skeleton, kidney, eye, spleen, abdomen, heart and somites in early embryos, but sFRP1 gene inactivation in these tissues did not compromise normal embryonic and post-natal development. Kidney histology revealed increased numbers of glomeruli in KO mice, observed after 5 years of breeding. In the skeleton, we show sFRP1 expression is found in relation to the mineralizing front of bone tissue during skeletal development from E15.5 to birth. Trabecular bone volume and bone mineral density in the sFRP1-/- mouse compared to WT was slightly increased during post-natal growth. Calvarial osteoblasts from newborn sFRP1-/- mice exhibited a 20% increase in cell proliferation and differentiation at the early stages of osteoblast maturation. sFRP1 expression was observed in osteoclasts, but this did not affect osteoclast number or activity. These findings have identified functions for sFRP1 in kidney and bone that are not redundant with other sFRPs. In summary, the absence of major organ abnormalities, the enhanced bone formation and a normal life span with no detection of spontaneous tumors suggests that targeting sFRP1 can be used as a therapeutic strategy for increasing bone mass in metabolic bone disorders or promoting fracture healing by modulating Wnt signaling.

Cloning and characterization of osteoactivin, a novel cDNA expressed in osteoblasts

Osteoblast development is a complex process involving the expression of specific growth factors and regulatory proteins that control cell proliferation, differentiation, and maturation. In this study, we used the rat mutation, osteopetrosis (op), to examine differences in skeletal gene expression between mutant op and normal littermates. Total RNA isolated from long bone and calvaria was used as a template for mRNA differential display. One of many cDNAs that were selectively expressed in either normal or mutant bone was cloned and sequenced and found to share some homology to the human nmb and Pmel 17 genes. This novel cDNA was named osteoactivin. Osteoactivin has an open reading frame of 1716 bp that encodes a protein of 572 amino acids with a predicted molecular weight of 63.8 kD. Protein sequence analysis revealed the presence of a signal peptide and a cleavage site at position 23. The protein also has thirteen predicted N-linked glycosylation sites and a potential RGD integrin recognition site at position 556. Northern blot analysis confirmed that osteoactivin was 3- to 4-fold overexpressed in op versus normal bone. RT-PCR analysis showed that osteoactivin is most highly expressed in bone compared with any of the other non-osseous tissues examined. In situ hybridization analysis of osteoactivin in normal bone revealed that it is primarily expressed in osteoblasts actively engaged in bone matrix production and mineralization. In primary rat osteoblast cultures, osteoactivin showed a temporal pattern of expression being expressed at highest levels during the later stages of matrix maturation and mineralization and correlated with the expression of alkaline phosphatase and osteocalcin. Our findings show that osteoactivin expression in bone is osteoblast-specific and suggest that it may play an important role in osteoblast differentiation and matrix mineralization. Furthermore, osteoactivin overexpression in op mutant bone may be secondary to the uncoupling of bone resorption and formation resulting in abnormalities in osteoblast gene expression and function.

Molecular characterization of celtix-1, a bromodomain protein interacting with the transcription factor interferon regulatory factor 2

Transcriptional control at the G1/S-phase transition of the cell cycle requires functional interactions of multimeric promoter regulatory complexes that contain DNA binding proteins, transcriptional cofactors, and/or chromatin modifying enzymes. Transcriptional regulation of the human histone H4/n gene (FO108) is mediated by Interferon Regulatory Factor-2 (IRF-2), as well as other histone gene promoter factors. To identify proteins that interact with cell-cycle regulatory factors, we performed yeast two-hybrid analysis with IRF-2 and identified a novel human protein termed Celtix-1 which binds to IRF-2. Celtix-1 contains several phylogenetically conserved domains, including a bromodomain, which is found in a number of transcriptional cofactors. Using a panel of IRF-2 deletion mutants in yeast two-hybrid assays, we established that Celtix-1 contacts the C-terminus of IRF-2. Celtix-1 directly interacts with IRF-2 based on binding studies with glutathione S-transferase (GST)/IRF-2 fusion proteins, and immunofluorescence studies suggest that Celtix-1 and IRF-2 associate in situ. Celtix-1 is distributed throughout the nucleus in a heterodisperse pattern. A subset of Celtix-1 colocalizes with the hyperacetylated forms of histones H3 and H4, as well as with the hyperphosphorylated, transcriptionally active form of RNA polymerase II. We conclude that the bromodomain protein Celtix-1 is a novel IRF-2 interacting protein that associates with transcriptionally active chromatin in situ.

Connective tissue growth factor: what's in a name?

Connective tissue growth factor (CTGF) is a member of the recently described CCN gene family which contains CTGF itself, cyr61, nov, elm1, Cop1, and WISP-3. CTGF is transcriptionally activated by several factors although its stimulation by transforming growth factor beta (TGF-beta) has attracted considerable attention. CTGF acts to promote fibroblast proliferation, migration, adhesion, and extracellular matrix formation, and its overproduction is proposed to play a major role in pathways that lead to fibrosis, especially those that are TGF-beta-dependent. This includes fibrosis of major organs, fibroproliferative diseases, and scarring. CTGF also appears to play a role in the extracellular matrix remodeling that occurs in normal physiological processes such as embryogenesis, implantation, and wound healing. However, recent advances have shown that CTGF is involved in diverse autocrine or paracrine actions in several other cell types such as vascular endothelial cells, epithelial cells, neuronal cells, vascular smooth muscle cells, and cells of supportive skeletal tissues. Moreover, in some circumstances CTGF has negative effects on cell growth in that it can be antimitotic and apoptotic. In light of these discoveries, CTGF has been implicated in a diverse variety of processes that include neovascularization, transdifferentiation, neuronal scarring, atherosclerosis, cartilage differentiation, and endochondral ossification. CTGF has thus emerged as a potential important effector molecule in both physiological and pathological processes and has provided a new target for therapeutic intervention in fibrotic diseases.

Cloning the full-length cDNA for rat connective tissue growth factor: implications for skeletal development

The mammalian osteopetroses represent a pathogenetically diverse group of skeletal disorders characterized by excess bone mass resulting from reduced osteoclastic bone resorption. Abnormalities involving osteoblast function and skeletal development have also been reported in many forms of the disease. In this study, we used the rat mutation, osteopetrosis (op), to examine differences in skeletal gene expression between op mutants and their normal littermates. RNA isolated from calvaria and long bones was used as a template for mRNA-differential display. Sequence information for one of the many cDNA that were selectively expressed in either normal or mutant bone suggested that it is the rat homologue of connective tissue growth factor (CTGF) previously cloned in the human, mouse, and other species. A consensus sequence was assembled from overlapping 5'-RACE clones and used to confirm the rat CTGF cDNA protein coding region. Northern blot analysis confirmed that this message was highly (8- to 10-fold) over-expressed in op versus normal bone; it was also upregulated in op kidney but none of the other tissues (brain, liver, spleen, thymus) examined. In primary rat osteoblast cultures, the CTGF message exhibits a temporal pattern of expression dependent on their state of differentiation. Furthermore, CTGF expression is regulated by prostaglandin E(2), a factor known to modulate osteoblast differentiation. Since members of the CTGF family regulate the expression of specific genes, such as collagen and fibronectin, we propose that CTGF may play a previously unreported role in normal skeletal modeling/remodeling. Its dramatic over-expression in the op mutant skeleton may be secondary to the uncoupling of bone resorption and bone formation resulting in dysregulation of osteoblast gene expression and function.

Mechanisms of osteoclast dysfunction in human osteopetrosis: abnormal osteoclastogenesis and lack of osteoclast-specific adhesion structures

Osteoclasts from a patient affected by osteopetrosis were examined in vivo and in vitro. Iliac crest biopsy revealed an osteosclerotic pattern, with prominent numbers of osteoclasts noted for hypernuclearity and incomplete adherence to the bone surface. A population comprising tartrate-resistant acid phosphatase (TRAP)-positive, multinucleated and mononuclear cells, and alkaline phosphatase-positive stromal fibroblasts was obtained in vitro from bone marrow. Mononuclear TRAP-positive precursors spontaneously fused in culture to form giant osteoclast-like cells. These cells expressed the osteoclast marker MMP-9 and calcitonin receptor, and lacked the macrophage marker, Fc receptor. Expression and distribution of c-src, c-fms, and CD68, and response to steroid hormones relevant to osteoclast differentiation and function were apparently normal, whereas cell retraction in response to calcitonin was impaired. TRAP-positive multinucleated cells did not form osteoclast-specific adhesion structures (clear zone, podosomes, or actin rings). Bone resorption rate was severely reduced in vitro. Focal adhesions and stress fibers were observed en lieu of podosomes and actin rings. Adhesion structures contained low levels of immunoreactive vitronectin receptor, most of this integrin being retained in cytoplasmic vesicles. These data provide the first characterization of abnormal differentiation and function of human osteopetrotic osteoclast-like cells.

Facial development and type III collagen RNA expression: concurrent repression in the osteopetrotic (Toothless,tl) rat and rescue after treatment with colony-stimulating factor-1

The toothless (osteopetrotic) mutation in the rat is characterized by retarded development of the anterior facial skeleton. Growth of the anterior face in rats occurs at the premaxillary-maxillary suture (PMMS). To identify potential mechanisms for stunted facial growth in this mutation we compared the temporospatial expression of collagen I (Col I) and collagen III (Col III) RNA around this suture in toothless (tl) rats and normal littermates by in situ hybridization of specific riboprobes in sagittal sections of the head. In normal rats, the suture is S shaped at birth and becomes highly convoluted by 10 days with cells in the center (fibroblasts and osteoblast progenitors) expressing Col III RNA and those at the periphery (osteoblasts) expressing no Col III RNA but high amounts of Col I RNA throughout the growth phase (the first 2 postnatal weeks). In the mutant PMMS, cells were reduced in number, less differentiated, and fewer osteoblasts were encountered. Expression of Col I RNA was at normal levels, but centrosutural cells expressed Col III RNA only after day 6 and then only weakly. A highly convoluted sutural shape was never achieved in mutants during the first 2 postnatal weeks. Treatment of tl rats with the cytokine CSF-1 improved facial growth and restored cellular diversity and Col III RNA expression in the PMMS to normal levels. Taken together, these data suggest that normal facial growth in rats is related to expression of Col III RNAby osteoblast precursors in the PMMS, that these cells are deficient in the tl mutation and are rescued following treatment with CSF-1.

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.

Normalization of periodontal tissues in osteopetrotic mib mutant rats, treated with CSF-1

The osteopetrotic mib mutation in rats causes defects in the skeletal bone tissue in young animals. These defects, i.e. slow bone remodelling, changes in both crystallinity and mineral content, are transient and undergo normalization, even without any treatment in 6-wk-old animals. Treatment with CSF-1 (colony stimulating factor-1) accelerates the normalization process in skeletal bones. The periodontal tissues around the apices of incisors show abnormalities caused by the slow remodelling process of the mandible bone tissue, the deficiency of osteoclasts and their abnormal morphology, as well as the disorganization of periodontal ligament fibres. In contrast to the skeletal tissues, these abnormalities would not undergo spontaneous normalization. Under treatment with colony stimulating factor 1 (CSF-1), the primitive bone trabeculae of mandible are resorbed and the normalization of the number of osteoclasts and their cytology occurs. The organization of the periodontal ligament fibres is partially restored, resembling the histological structure of the normal one.

Actin mRNA isoforms are differentially sorted in normal osteoblasts and sorting is altered in osteoblasts from a skeletal mutation in the rat

Actin isoform sorting has been shown to occur in a variety of cell types in culture. To this list we add osteoblasts, in which we show by in situ hybridization that beta-actin is distributed primarily in cell processes and on one side of the nucleus and gamma-actin has a perinuclear distribution. Osteoblasts from the skeletal mutation toothless (tl), evaluated under identical conditions, fail to sort these actin isoforms differentially and exhibit diffuse labeling as their major manifestation. Northern analyses of actin mRNAs showed no differences between normal and mutant cultures. Shortened osteoblast life span and an inability to direct osteoclast-mediated bone resorption have recently been demonstrated in tl mutants. The present results suggest that a failure of osteoblasts to sort actin mRNAs may be related to one or both of these pathological manifestations in this mutation and represent, to our knowledge, the first correlation of an actin mRNA-sorting abnormality with a mammalian disease.

Hypophosphatemia and the development of rickets in osteopetrotic (op/op) mice

Our previous work has shown that op/op mice hyperabsorb dietary calcium in the vitamin D-deficient state and shunt that calcium into bone. Under these conditions, the op/op mice are hypocalcemic. The purpose of this study was to examine calcium metabolism and bone mineralization in vitamin D-deficient op/op mice. First, the op/op mice and their normal littermates were placed on a vitamin D-deficient, low phosphorus diet to limit bone mineralization. Under these circumstances, op/op mice survived, even when calcium was also removed from the diet. If the diet contained phosphate, op/op mice died from hypocalcemic tetany when calcium was also removed from the diet. Furthermore, serum calcium levels became similar to wild type in the op/op mice administered the vitamin D-deficient, low phosphorus diet, and op/op mice were able to increase serum calcium in response to 1,25-dihydroxyvitamin D3. The op/op mice developed rickets when their serum phosphorus level was too low to support bone mineralization. The op/op mice became hypophosphatemic on regimens in which normal mice were able to maintain normal serum phosphorus levels. It appears that the op/op mouse simply requires a higher dietary calcium and phosphorus level to prevent rickets and hypocalcemic tetany since the bone is not available as a source of these minerals. However, the ability of the op/op mouse to mineralize bone at low serum calcium and phosphorus levels remains unexplained.

Ultrastructural evidence of abnormally short and maldistributed actin stress fibers in osteopetrotic (toothless) rat osteoblasts in situ after detergent perfusion

Osteopetrosis is a heterogenous group of metabolic bone diseases characterized by a generalized increase in skeletal mass, the product of reduced bone resorption and interceptions in the development and/or function of osteoclasts. In one such mutation in the rat, toothless (tl), osteoblasts are absent from older bone surfaces and there is evidence for aberrant osteoblast gene expression and function. Given the emerging appreciation of the role of osteoblasts in the differentiation and activation of osteoclasts, we have examined ultrastructural features of the cytoskeleton of normal and mutant osteoblasts after perfusion fixation with the non-ionic detergent Triton X-100. This procedure reduces the electron density of the cytoplasm, rendering visible the microfilamentous network in osteoblasts. In normal osteoblasts a prominent system of stress fibers (bundles of actin microfilaments) run parallel to the cell membrane adjacent to osteoid surfaces, stretching for 75% of that distance. However, only 50% of mutant (tl) osteoblasts had stress fibers and in these cells stress fibers were either significantly shorter (18% of normal) or distributed intracellularly rather than along the osteoid surface. In mutant osteoblasts without stress fibers, 20% showed ultrastructural signs of cell degeneration. Given the role of stress fibers in cellular attachment ot extracellular matrices, these observations suggest that the reduced number of osteoblasts in tl rats may be related to their inability to organize actin filaments and adhesion plaques for attachment to bone surfaces. We propose that a feature of osteopetrosis in the tl rat is a disruption of the mechanisms that regulate the synthesis, sorting, and/or assembly of actin.

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.

The heterogeneity of the osteopetroses reflects the diversity of cellular influences during skeletal development

Experimental studies of the mammalian osteopetroses, characterized by generalized skeletal sclerosis, have illuminated a variety of mechanisms by which bone resorption can be reduced. We review recent data implicating a diverse group of growth factors, proto-oncogenes, and immune regulators that can influence skeletal development and account for the heterogeneity of the osteopetroses. Furthermore, similar studies are likely to continue to provide for improved clinical management of both osteopetrotic children and the localized and generalized osteopenias.

Osteoblasts from the toothless (osteopetrotic) mutation in the rat are unable to direct bone resorption by normal osteoclasts in response to 1,25-dihydroxyvitamin D

Osteopetrosis describes a diversified group of metabolic bone disorders characterized by a generalized, skeletal sclerosis resulting from reduced osteoclast-mediated bone resorption. The toothless (tl) osteopetrotic mutation in the rat is characterized by few osteoclasts and the inability to be cured by transplants of hemopoietic stem cells. This implies that the defect(s) responsible for reduced osteoclast activity in tl rats is within the skeletal microenvironment (cells or matrices). Osteoblasts and their products are known to play a role in regulating bone resorption and abnormalities in the osteoblast population in tl rats have been reported. The purpose of this study was to determine whether osteoblasts isolated from tl mutant rats, when cultured with normal osteoclasts, could increase bone resorption (pit formation) in response to stimulation by 1,25 dihydroxyvitamin D (1,25(OH)2D). The addition of 1,25(OH)2D produced a highly significant response in normal osteoblast cocultures but no response in mutant cultures. A dose response study with 1,25(OH)2D (10(-6) to 10(-9)M) revealed that mutant osteoblasts are unable to increase osteoclast activity. These data indicate that the vitamin D receptor-signal transduction pathway in tl rats needs to be examined.

Administration of colony stimulating factor-1 to toothless osteopetrotic rats normalizes osteoblast, but not osteoclast, gene expression

The toothless (tl) osteopetrotic mutation in the rat is characterized by generalized skeletal sclerosis, a severe reduction in the numbers of osteoclasts, monocytes, and macrophages, and absence of tooth eruption. Studies examining gene expression in bone-derived cells of tl rats and their normal littermates have shown that genes related to osteoblast function are aberrantly expressed in tl rats compared to normal littemates. We have previously shown that exogenous administration of colony stimulating factor-1 (CSF-1) to tl rats results in a dramatic reduction of the skeletal sclerosis and significant increases in the number of osteoclasts. Thus, we examined the effects of CSF-1 on osteoblast and osteoclast gene expression in tl rats as demonstrated by Northern blot analysis. While osteoblast-related gene expression as reflected by mRNA levels of alkaline phosphatase, osteocalcin, osteopontin, and type I collagen was normalized, osteoclast-related gene expression, as reflected by mRNA levels of carbonic anhydrase II and tartrate-resistant adenosine triphosphatase, remained significantly lower in CSF-1-treated tl rats compared to untreated normal littermates. Since previous studies have not demonstrated the CSF-1 receptor on osteoblasts, these results suggest that osteoblast abnormalities in tl rats are an effect of the osteopetrotic condition rather than the cause of the disease.

Bone metabolism in the osteopetrotic rat mutation microphthalmia blanc

We have examined parameters of bone metabolism in a new mutation, microphthalmia blanc (mib), in the rat exhibiting a skeletal sclerosis at birth that improves with age. There were no significant differences in the rate of bone formation during the first postnatal month except a temporary reduction in mutants at 3 weeks that coincided with compromised nutrition at weaning. At birth the ruffled border in mutant osteoclasts was absent or poorly developed and mRNA analyses of mutant bone compared to normal bone showed significant reductions in the messages for the osteoclast-specific genes carbonic andydrase II and tartrate-resistant ATPase. These distinctive ultrastructural and molecular differences were not present 1 month later. These data show that the transient osteopetrosis in mib rats results from a perinatal reduction in ultrastructural and enzymatic features of active osteoclasts and is not complicated by elevations in bone formation. The molecular basis for both the production and resolution of these abnormalities deserves further study.

Temporal variation of c-Fos proto-oncogene expression during osteoblast differentiation and osteogenesis in developing rat bone

To delineate the implication of c-fos protooncogenic in the osteogenie process, we have investigated the temporal pattern of c-fos mRNA expression in fetal and neonatal rat bone during intramembranous and endochondral bone formation. Northern blot analysis of mRNA extracted from calvaria and femur showed that expression of c-fos, Histone H4, and osteocalcin mRNAs followed a temporal sequence during bone development. The levels of histone H4 mRNA, a marker of cell proliferation, were high at early stages of fetal development of calvaria and femur, and decreased until birth. In both the postnatal calvaria and femur, c-fos mRNA levels increased transiently at birth and preceded a rise in osteocalcin transcripts, a marker of the mature osteoblast phenotype. The immunohistochemical analysis showed that c-Fos protein was expressed in osteoprogenitor cells in the perichondrium and periosteum, and not in mature osteoblasts which expressed markers of differentiated osteoblasts such as type-I collagen, bone sialoprotein, and osteocalcin. Thus, the transient c-fos proto-oncogene expression during the postnatal life that precedes the osteocalcin expression may be involved in the transition from the precursor state to mature osteoblasts. These results suggest that c-fos proto-oncogene may play an important role in osteogenesis during rat postnatal life.

Models for the study of cementogenesis

Cementum is a mineralized tissue that acts to connect the periodontal ligament to the tooth root surface. Its composition is very much like bone, being comprised mainly of type I collagen, inorganic mineral and noncollagenous proteins, however the origin of the cells and factors necessary for cementum formation have yet to be elucidated. Our laboratory has focused on the role that adhesion molecules, and their cell surface receptors, play in the formation of cementum and tooth root. In order to study this, we used a mouse molar as a model system. This system enabled us to study the formation of four distinct mineralized tissues; bone, cementum, dentin and enamel at various stages of their development. For these studies, we initiated experiments to examine potential cementoblast progenitor cells, in vitro. As a first step, we show that dental papilla and dental follicle cells, n vitro, obtained from molar tissues at day 21 of development, induce mineralized nodules, in vitro. In addition, we obtained tissues from mice where defects in root development may exist and determined bone sialoprotein (BSP) protein expression, a mineralized tissue specific adhesion molecule, in such tissues. As discussed here, we found that osteopetrotic (op/op) mice have delayed and/or defective root development and BSP does not localize in the dental tissues, at day 33 of development. In addition, dentin formation was defective and odontoblasts appeared immature, based on morphological examination. In contrast, the day 33 control molars demonstrated positive staining for BSP localized to root cementum, with normal formation of dentin.

Understanding bone cell biology requires an integrated approach: reliable opportunities to study osteoclast biology in vivo

The relative simplicity of all in vitro methods to study bone cell biology will at best result in oversimplification of the development and functional capacity of the skeleton in vivo. We have shown this to be true for selected aspects of bone cell biology, but numerous other examples are available. One alternative is to undertake skeletal research in vivo. It is important that those in bone research be willing to move increasingly in this direction not only to understand the true complexities of skeletal versatility, but also to avoid repetition and perpetuation of erroneous or irrelevant conclusions which waste resources. Toward this end we have described two situations, osteopetrosis and tooth eruption, in which reproducible abrogations or local activations of bone resorption can be examined in vivo. The application of emerging molecular and morphological techniques that permit the subcellular dissection of metabolic pathways and their precise cellular localization, such as a combination of the variety of in situ hybridization technologies with PCR, antisense probes, and antibody blockase, will allow the investigator greater control of variables in vivo. We expect that these technologies, largely worked out in vitro, combined with highly selected, appropriate models, as we have ourlined here for osteoclast biology, will make research in vivo less intimidating and increase the frequency with which the real biology is studied directly.

Interdependence of skeletal sclerosis and elevated circulating levels of 1,25-dihydroxyvitamin D in osteopetrotic (op and tl) rats

Osteopetrosis describes a heterogeneous group of inherited, metabolic bone disorders characterized by reduced bone resorption which coexists with elevated circulating levels of 1,25-dihydroxyvitamin D [1,25(OH)2D]. To determine whether or not skeletal sclerosis and high concentrations of 1,25(OH)2D are interdependent, this study used two distinct, nonallelic osteopetrotic mutations in the rat, osteopetrosis (op) and toothless (tl). The op rat is a mutation in which skeletal sclerosis can be cured (mutant) or induced (normal) following the transfer of normal or mutant osteoclast progenitors, respectively. Although these procedures are ineffective in rats of tl stock, infusions of pharmacological doses of macrophage colony-stimulating factor (CSF-1) can stimulate bone resorption and eliminate most of the excess skeletal matrix in tl mutants. This study examined the effects of cure/induction in neonatal mutant/normal rats of op stock and CSF-1 infusions in mutant rats of tl stock on skeletal (bone resorption) and serum [1,25(OH)2D] parameters as a function of time after treatment. Osteopetrotic mutants transplanted (cured) with normal spleen cells demonstrated cellular changes in osteoclast phenotype within 2-3 days followed by histologic and radiographic evidence for increased bone resorption that culminated in a normal appearance of the skeleton by 4 weeks. The markedly elevated serum levels of 1,25(OH)2D observed in untreated mutants fell significantly in transplanted mutants by the end of the first week and were similar to those in normal littermates at 3 and 4 weeks. Normal littermates transplanted (induced) with mutant spleen cells showed a progressive increase in skeletal sclerosis paralleled by significant increases in circulating levels of 1,25(OH)2D.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Responsiveness of gene expression markers of osteoblastic and osteoclastic activity to calcitonin in the appendicular and axial skeleton of the rat in vivo

We have previously shown that calcitonin (CT), an inhibitor of bone resorption, increases vertebral, but not femoral bone density in the rat. To address the physiologic responses associated with these effects on bone mineral density (BMD), we assessed mRNA transcripts reflecting activities of osteoblasts (type I collagen, osteocalcin, osteopontin, and alkaline phosphatase), osteoclasts [tartrate-resistant acid phosphatase (TRAP)], and cell proliferation (histone H4) in the spine and femur of these rats. CT increased spine BMD while increasing type I collagen and decreasing TRAP and histone mRNAs. In the femur, where CT had no effect on BMD, it decreased type I collagen and histone H4 mRNA but did not affect TRAP. CT had no effect on the gene expression of osteocalcin, osteopontin, or alkaline phosphatase at either site. The results indicate that selective alterations in gene expression, as reflected by steady state mRNA levels, are consistent with the changes observed by BMD measurement, and can more clearly define the specific contribution from osteoblast and osteoclast activity. This study demonstrates a heterogeneity in response of the axial and appendicular skeleton to CT, reflected by alterations in gene expression that provide a basis for understanding the observed BMD responses to various pharmacologic interventions.

Transcriptionally active nuclei isolated from intact bone reflect modified levels of gene expression in skeletal development and pathology

Transcriptional regulation of gene expression in vivo in bone, associated with normal development or skeletal disorders, to date, has not been studied. We report the successful isolation of nuclei that are transcriptionally active from normal and osteopetrotic rat bone. Transcription rates of cell growth and bone-related genes (including histone H4, c-fos, c-jun, TGF beta 1, beta 2 macroglobulin, collagen, fibronectin, osteocalcin, osteopontin, and tartrate resistant acid phosphatase) change as a function of calvarial development from birth to 6 weeks and are selectively modified in osteopetrotic animals. Additionally, nuclei isolated from intact bone yield promoter binding factors. Bone nuclei, which transcribe faithfully and contain the normal complement of nuclear protein factors, offer a powerful approach for investigating in vivo gene regulation in skeletal development and pathology.

2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits differentiation of normal diploid rat osteoblasts in vitro

The influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent halogenated aromatic hydrocarbon, on the development of bone tissue-like organization in primary cultures of normal diploid calvarial-derived rat osteoblasts was examined. Initially, when placed in culture, these cells actively proliferate while expressing genes associated with biosynthesis of the bone extracellular matrix. Then, post-proliferatively, genes are expressed that render the osteoblast competent for extracellular matrix mineralization and maintenance of structural as well as functional properties of the mature bone-cell phenotype. Our results indicate that, in the presence of TCDD, proliferation of osteoblasts was not inhibited but post-confluent formation of multicellular nodules that develop bone tissue-like organization was dramatically suppressed. Consistent with TCDD-mediated abrogation of bone nodule formation, expression of alkaline phosphatase and osteocalcin was not upregulated post-proliferatively. These findings are discussed within the context of TCDD effects on estrogens and vitamin D-responsive developmental gene expression during osteoblast differentiation and, from a broader biological perspective, on steroid hormone control of differentiation.

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

Osteoclast abnormalities that characterize osteopetrosis, a disorder of bone resorption, may derive from aberrant signals from the osteoblast or the bone matrix. In the present studies, both synthesis and the bone matrix content of the major bone phosphoprotein component, osteopontin, were found to be elevated in three osteopetrotic rat mutations (ia, op, and tl). In whole bone, a twofold increase in the content of the characteristic amino acid O-phosphoserine for osteopontin occurred in op and tl mutant long bone, but a smaller (15%) and more variable increase was observed in ia mutant rat long bone. Extraction of the bone matrix components and partial purification by reverse phase chromatography showed a twofold increase in a phosphoprotein fraction relative to other noncollagenous components. Amino acid analysis and staining characteristics of SDS-PAGE fractionated proteins indicated this to be osteopontin. Organ cultures of calvarial bone from 4 day ia osteopetrotic mutant and normal rats in the presence of 3H-proline showed increased synthesis of this 60 kD protein, which was stimulated by vitamin D. Preparation of total cellular RNA from bone of 2- and 6-week-old mutants and normal rats supported increased synthesis of osteopontin as reflected by hybridization with osteopontin cDNA probe, showing significantly higher levels of mRNA transcripts in ia (3-5 fold), tl (1.4-2 fold), and op (6-25 fold) mutant bone compared to normal littermates. The changes in osteopontin mRNA levels in mutant bone were also examined in relation to other growth and phenotype-expressed genes. The findings of increased accumulation of osteopontin in osteopetrotic bone and increased synthesis by osteoblasts are interesting in light of the previously reported decrease in bone osteocalcin content (Endocrinology, 126:966, 1990), confirmed here by decreased osteocalcin mRNA transcripts. Such aberrations in the composition of skeletal extracellular matrix could be a reflection of or a contributing factor to the osteoclast abnormalities of some of these osteopetrotic disorders.

Estrogen modulates the mRNA levels for cancellous bone protein of ovariectomized rats

This study was undertaken to examine the effects of ovariectomy and 17 beta-estradiol (E2) on the gene expression of type 1 collagen, osteocalcin and the protooncogen, c-myc, in cancellous bone. Female Sprague-Dawley rats, aged 95 days, were divided into 4 groups. Group 1 was sham operated and Groups 2-4 were ovariectomized. Groups 3 and 4 received daily injections of 160 ng and 1600 ng E2/kg body weight, respectively. Groups 1 and 2 received the solvent vehicle. All animals were sacrificed after 14 days. The femurs were dissected out and cancellous bone scraped from the distal metaphysis. RNA was isolated from the cancellous bone, immobilized on filters or size-fractionated by agarose gel electrophoresis and adsorbed on filters which were then hybridized with specific cDNA probes. Ovariectomy resulted in a significant increase in the mRNAs of type 1 collagen, osteocalcin and c-myc. The increase was suppressed in animals that received 17 beta-estradiol injections. In addition, ovariectomy caused the expected decrease in cancellous bone in the proximal tibia and increased osteoclast and osteoblast numbers. The ovariectomy-induced changes were prevented by 17 beta-estradiol administration. These findings suggest that the lack of ovarian hormones shortly after ovariectomy up-regulates and estrogen administration down-regulates the expression of important cancellous bone matrix proteins as well as the protooncogen, c-myc.

The cell biology of bone

Bone remodelling and repair are accomplished by the co-ordinated activity of cells of the osteoclast and osteoblast lineages. Small changes in the balance between formation and resorption will, when magnified by repeated cycles, lead to significant reduction in bone mass and strength, ultimately resulting in fracture. This review focuses on the cellular features of bone remodelling and the known regulators of bone cell function. These include systemic and local factors, both soluble and contained within the complex extracellular matrix of bone.

Effect of gallium nitrate in vitro and in normal rats

Gallium nitrate (GN) is an inhibitor of bone resorption and thereby may result in a change in coupled bone formation. In the present investigation the effects of GN on bone formation were studied in the rat osteosarcoma (ROS) 17/2.8 cell line and normal diploid rat osteoblasts (ROB) in vitro and the femur of rats treated in vivo, measuring mRNA levels for two osteoblast parameters, type I collagen, a marker of matrix formation, and osteocalcin, a bone specific protein and also histone H4, a marker of cell proliferation. GN, at 50 microM for 3 h, increased type I collagen mRNA levels by 132% in ROS 17/2.8 cells and by 122% in proliferating ROB cells. Osteocalcin (OC) mRNA levels were decreased by 61% in ROS 17/2.8 cells and by 97% in differentiated ROB cells. These changes occurred in the absence of any effects on cell proliferation. Seventy-day-old female rats were then treated with GN, 0.5 mg/kg/day, for 3 weeks. As previously reported, GN decreased serum calcium levels, but had no effect on lumbar or femoral bone density. In contrast to the in vitro effects, GN had no effect on type I collagen steady-state mRNA levels in the femur; however, it decreased OC steady-state mRNA levels in the femur by 58%. These results suggest that GN has similar in vitro effects in transformed and normal osteoblasts, while the collagen-stimulatory effects observed in vitro cannot be extrapolated to in vivo models. The consistent inhibition of osteocalcin in vitro and in vivo suggests a more specific target for GN that may relate to its effects in inhibiting bone resorption in normal rats.

Glucocorticoids promote development of the osteoblast phenotype by selectively modulating expression of cell growth and differentiation associated genes

To understand the mechanisms by which glucocorticoids promote differentiation of fetal rat calvaria derived osteoblasts to produce bone-like mineralized nodules in vitro, a panel of osteoblast growth and differentiation related genes that characterize development of the osteoblast phenotype has been quantitated in glucocorticoid-treated cultures. We compared the mRNA levels of osteoblast expressed genes in control cultures of subcultivated cells where nodule formation is diminished, to cells continuously (35 days) exposed to 10(-7) M dexamethasone, a synthetic glucocorticoid, which promotes nodule formation to levels usually the extent observed in primary cultures. Tritiated thymidine labelling revealed a selective inhibition of internodule cell proliferation and promotion of proliferation and differentiation of cells forming bone nodules. Fibronectin, osteopontin, and c-fos expression were increased in the nodule forming period. Alkaline phosphatase and type I collagen expression were initially inhibited in proliferating cells, then increased after nodule formation to support further growth and mineralization of the nodule. Expression of osteocalcin was 1,000-fold elevated in glucocorticoid-differentiated cultures in relation to nodule formation. Collagenase gene expression was also greater than controls (fivefold) with the highest levels observed in mature cultures (day 35). At this time, a rise in collagen and TGF beta was also observed suggesting turnover of the matrix. Short term (48 h) effects of glucocorticoid on histone H4 (reflecting cell proliferation), alkaline phosphatase, osteopontin, and osteocalcin mRNA levels reveal both up or down regulation as a function of the developmental stage of the osteoblast phenotype. A comparison of transcriptional levels of these genes by nuclear run-on assays to mRNA levels indicates that glucocorticoids exert both transcriptional and post-transcriptional effects. Further, the presence of glucocorticoids enhances the vitamin D3 effect on gene expression. Those genes which are upregulated by 1,25(OH)2D3 are transcribed at an increased rate by dexamethasone, while those genes which are inhibited by vitamin D3 remain inhibited in the presence of dexamethasone and D3. We propose that the glucocorticoids promote changes in gene expression involved in cell-cell and cell-extracellular matrix signaling mechanisms that support the growth and differentiation of cells capable of osteoblast phenotype development and bone tissue-like organization, while inhibiting the growth of cells that cannot progress to the mature osteoblast phenotype in fetal rat calvarial cultures.

Tissue-specific expression of bone proteins in femora of growing rats

Total cellular RNA was extracted from bone cells of three different femoral compartments of 2-mo-old rats. The intact femora were first incubated with collagenase to obtain periosteal cells. The bisected periosteum-free diaphyses and metaphyses were then incubated with collagenase to obtain enriched populations of endosteal and cancellous bone cells, respectively. The total cellular RNA from these three tissues was separated by size using agarose gel electrophoresis, transferred to nylon filters, hybridized to 32P-labeled cDNA probes for glyceraldehyde-3-phosphate dehydrogenase (GAP), pre-pro-alpha (I) type I collagen (collagen), osteocalcin (BGP), and alkaline phosphatase (AP), and the cDNA/mRNA hybrids were visualized by radioautography. Bone matrix deposition was measured in each tissue compartment by tetracycline-based dynamic bone histomorphometry. The bone formation and apposition rates were greatest in the periosteum and least in metaphysis. Mean mRNA levels for collagen and BGP were positively correlated with mean bone formation and mineral apposition rates. Interestingly, mean AP mRNA levels were not correlated with indexes of bone formation. These results demonstrate that the steady-state mRNA levels for bone matrix proteins in femora show pronounced site specificity and correlate with the rates of bone matrix deposition.

The effects of colony-stimulating factor-1 on tooth eruption in the toothless (osteopetrotic) rat in relation to the critical periods for bone resorption during tooth eruption

The toothless (tl) rat is an osteopetrotic mutation characterized by a generalized skeletal sclerosis, reduced bone resorption, few osteoclasts and a total absence of erupted teeth. This mutation is not cured by bone marrow transplants from normal littermates. It is known that the skeletal defects in tl rats are greatly improved after treatment with colony-stimulating factor-1 (CSF-1). This investigation concerns the effects of CSF-1 on the development and eruption of the dentition of tl rats. Untreated tl rats had no erupted teeth by 56 days after birth, and the roots of incisors and molars were severely distorted by compression against bone. The apex of the mandibular incisor did not extend past the first molar and continued growth of its apical end produced odontoma-like masses consisting of distorted dentine and enamel matrices. In addition, few osteoclasts were seen on alveolar bone surfaces surrounding the developing teeth. Mutants given CSF-1 were characterized by delayed eruption of all molars and sometimes incisors. The incidence of incisor eruption was related inversely to the age at which CSF-1 treatment began. Molars of treated tl rats had well-developed roots similar to those in normal rats. Treated mutants had numerous osteoclasts in alveolar bone and well-developed haemopoietic marrow spaces in the mandible. Histochemical staining for both tartrate-resistant acid phosphatase and tartrate-resistant acid ATPase was reduced or negligible in osteoclasts of untreated tl rats, heavy in normal osteoclasts and of intermediate intensity in CSF-1-treated mutants.(ABSTRACT TRUNCATED AT 250 WORDS)

Downregulation of histone H4 gene transcription during postnatal development in transgenic mice and at the onset of differentiation in transgenically derived calvarial osteoblast cultures

In vivo regulation of cell cycle dependent human histone gene expression was examined in transgenic mice using a fusion construct containing 6.5 kB of a human H4 promoter linked to the chloramphenicol acetyltransferase (CAT) reporter gene. Transcriptional control of histone gene expression, as a function of proliferative activity, was determined. We established the relationship between DNA replication dependent H4 mRNA levels (Northern blot analysis) and H4 promoter activity (CAT assay) during postnatal development in a broad spectrum of tissues. In most tissues sampled in adult animals, the cellular representation of H4 gene transcripts declined in parallel with promoter activity. This result is consistent with transcriptional control of H4 gene expression at the cessation of proliferation. Interestingly, while H4 mRNA was detectable at very low levels post-proliferatively in brain, promoter activity persisted in adult brain, where most of the cells are terminally differentiated. This dissociation between histone gene promoter activity and histone mRNA accumulation points to the possibility of post-transcriptional regulation of histone gene expression in brain. Cultures of osteoblasts were prepared from calvaria of transgenic mice carrying the H4 promoter/CAT reporter construct. In contrast to the brain, in these bone-derived cells, we established by immunohistochemistry that the transition to the quiescent, differentiated state is associated with a transcriptionally mediated downregulation of histone gene expression at the single cell level.

Alveolar bone remodeling after tooth extraction in normal and osteopetrotic (ia) rats

One of the osteopetrotic mutations in the rat, incisors-absent (ia), exhibits generalized skeletal sclerosis and failure or delay of tooth eruption, characteristics of other osteopetrotic mutations. Osteopetrosis in ia rats is known to be due to a reduction in bone resorption, the result of the inability of ia osteoclasts to elaborate a ruffled border. During healing of extraction wounds, especially the initial period, osteoclastic resorption of alveolar bone is considered to be a significant feature, followed by new bone formation. We have studied extraction wound healing in osteopetrotic (ia) rats histologically in order to determine if their systemic reduction in bone resorption changes the sequence or rate of alveolar bone healing within and outside the socket after tooth extraction, In ia rats, the healing process was delayed in comparison to that of normal rats. Many osteoclasts were observed on the surface of alveolar bone, but there was little evidence of resorption. Bone formation in the socket following bone resorption was reduced and the newly formed trabeculae were irregular. In contrast, the quantity of resorption-independent (periosteal) new bone formation outside the socket was exaggerated compared to normal animals. These data indicate that the disturbance of new bone formation in the socket is probably related to the reduction in osteoclastic bone resorption.

Concepts of osteoblast growth and differentiation: basis for modulation of bone cell development and tissue formation

The combined application of molecular, biochemical, histochemical, and ultrastructural approaches has defined a temporal sequence of gene expression associated with development of the bone cell phenotype in primary osteoblast cultures. The peak levels of expressed genes reflect a developmental sequence of bone cell differentiation characterized by three principal periods: proliferation, extracellular matrix maturation and mineralization, and two restriction points to which the cells can progress but cannot pass without further signals. The regulation of cell growth and bone-specific gene expression has been examined during this developmental sequence and is discussed within the context of several unique concepts. These are (1) that oncogene expression in proliferating osteoblasts contributes to the suppression of genes expressed postproliferatively, (2) that hormone modulation of a gene is dependent upon the maturational state of the osteoblast, and (3) that chromatin structure and the presence of nucleosomes contribute to three-dimensional organization of gene promoters that support synergistic and/or antagonistic activities of physiologic mediators of bone cell growth and differentiation.

Congenitally osteosclerotic (os/os) rabbits are not cured by bone marrow transplantation from normal littermates

The osteopetrotic (os) rabbit is a lethal mutation of autosomal recessive inheritance characterized by hypocalcemia, hypophosphatemia, fibrosis of marrow spaces, and ultrastructural abnormalities in both osteoclasts and osteoblasts. Procedures involving the transplantation of cells from normal hemopoietic tissues, which are sources of osteoclast precursors, are known to cure osteopetrosis in several mutations including some children. We tested the ability of transplanted bone marrow and/or spleen from normal littermates to reverse the skeletal sclerosis in os rabbits. Treatment of 15 neonatal mutants consisted of immunosuppression by whole-body irradiation followed by transplantation of normal bone marrow and/or spleen cell suspensions. This treatment failed to prolong life span or to cure osteopetrosis judged radiographically and histologically for up to 3 weeks posttreatment, the longest time of survival. These data indicate that transplantation of stem cells from multiple hemopoietic tissues, procedures known to cure osteopetrosis in other mutations, is not effective in the os rabbit. These results support the hypothesis that the skeletal microenvironment is not capable of supporting the development and function of normal osteoclasts in this mutation.