The skeletal effects of colony-stimulating factor-1 in toothless (osteopetrotic) rats: persistent metaphyseal sclerosis and the failure to restore subepiphyseal osteoclasts

The influence of M-CSF on fracture healing in a mouse model

Macrophage colony-stimulating factor 1 (M-CSF) is known to play a critical role during fracture repair e.g. by recruiting stem cells to the fracture site and impacting hard callus formation by stimulating osteoclastogenesis. The aim of this experiment was to study the impact of systemic M-CSF application and its effect on bony healing in a mouse model of femoral osteotomy. Doing so, we studied 61 wild type (wt) mice (18-week-old female C57BL/6) which were divided into three groups: (1) femoral osteotomy, (2) femoral osteotomy + stabilization with external fixator and (3) femoral osteotomy + stabilization with external fixator + systemic M-CSF application. Further, 12 op/op mice underwent femoral osteotomy and served as proof of concept. After being sacrificed at 28 days bony bridging was evaluated ex vivo with µCT, histological and biomechanical testing. Systemic M-CSF application impacted osteoclasts numbers, which were almost as low as found in op/op mice. Regarding callus size, the application of M-CSF in wt mice resulted in significantly larger calluses compared to wt mice without systemic M-CSF treatment. We further observed an anabolic effect of M-CSF application resulting in increased trabecular thickness compared to wt animals without additional M-CSF application. Systemic M-CSF application did not alter biomechanical properties in WT mice. The impact of M-CSF application in a mouse model of femoral osteotomy was oppositional to what we were expecting. While M-CSF application had a distinct anabolic effect on callus size as well as trabecular thickness, this on bottom line did not improve biomechanical properties. We hypothesize that in addition to the well-recognized negative effects of M-CSF on osteoclast numbers this seems to further downstream cause a lack of feedback on osteoblasts. Ultimately, continuous M-CSF application in the absence of co-stimulatory signals (e.g. RANKL) might overstimulate the hematopoietic linage in favor of tissue macrophages instead of osteoclasts.

Marrow Adiposity and Hematopoiesis in Aging and Obesity: Exercise as an Intervention

PURPOSE OF REVIEW

Changes in the bone marrow microenvironment, which accompany aging and obesity, including increased marrow adiposity, can compromise hematopoiesis. Here, we review deleterious shifts in molecular, cellular, and tissue activity and consider the potential of exercise to slow degenerative changes associated with aging and obesity.

RECENT FINDINGS

While bone marrow hematopoietic stem cells (HSC) are increased in frequency and myeloid-biased with age, the effect of obesity on HSC proliferation and differentiation remains controversial. HSC from both aged and obese environment have reduced hematopoietic reconstitution capacity following bone marrow transplant. Increased marrow adiposity affects HSC function, causing upregulation of myelopoiesis and downregulation of lymphopoiesis. Exercise, in contrast, can reduce marrow adiposity and restore hematopoiesis. The impact of marrow adiposity on hematopoiesis is determined mainly through correlations. Mechanistic studies are needed to determine a causative relationship between marrow adiposity and declines in hematopoiesis, which could aid in developing treatments for conditions that arise from disruptions in the marrow microenvironment.

The cast of clasts: catabolism and vascular invasion during bone growth, repair, and disease by osteoclasts, chondroclasts, and septoclasts

Three named cell types degrade and remove skeletal tissues during growth, repair, or disease: osteoclasts, chondroclasts, and septoclasts. A fourth type, unnamed and less understood, removes nonmineralized cartilage during development of secondary ossification centers. "Osteoclasts," best known and studied, are polykaryons formed by fusion of monocyte precursors under the influence of colony stimulating factor 1 (CSF)-1 (M-CSF) and RANKL. They resorb bone during growth, remodeling, repair, and disease. "Chondroclasts," originally described as highly similar in cytological detail to osteoclasts, reside on and degrade mineralized cartilage. They may be identical to osteoclasts since to date there are no distinguishing markers for them. Because osteoclasts also consume cartilage cores along with bone during growth, the term "chondroclast" might best be reserved for cells attached only to cartilage. "Septoclasts" are less studied and appreciated. They are mononuclear perivascular cells rich in cathepsin B. They extend a cytoplasmic projection with a ruffled membrane and degrade the last transverse septum of hypertrophic cartilage in the growth plate, permitting capillaries to bud into it. To do this, antiangiogenic signals in cartilage must give way to vascular trophic factors, mainly vascular endothelial growth factor (VEGF). The final cell type excavates cartilage canals for vascular invasion of articular cartilage during development of secondary ossification centers. The "clasts" are considered in the context of fracture repair and diseases such as arthritis and tumor metastasis. Many observations support an essential role for hypertrophic chondrocytes in recruiting septoclasts and osteoclasts/chondroclasts by supplying VEGF and RANKL. The intimate relationship between blood vessels and skeletal turnover and repair is also examined.

Elevated levels of macrophage colony-stimulating factor in human fracture healing

Macrophage colony-stimulating factor (M-CSF) plays a unique role in bone remodeling. However, to our knowledge, no data on the role of M-CSF in fracture healing in humans have been published so far. This study addressed this issue. One hundred and thirteen patients with long-bone fractures were included in the study and divided into two groups, according to their course of fracture healing. The first group contained 103 patients with normal fracture healing. Ten patients with impaired fracture healing formed the second group of the study. Volunteers donated blood samples as control. Serum samples were collected over a period of 6 months, following a standardized time schedule. In addition, M-CSF levels were measured in fracture hematoma and serum of 11 patients with bone fractures. M-CSF concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Fracture hematoma contained significantly higher M-CSF concentrations compared to M-CSF concentrations in patient's serum. M-CSF levels in fracture hematoma and in patient's serum were both significantly higher than M-CSF concentrations measured in serum of healthy controls. Highly elevated M-CSF serum concentrations were found in patients with physiological fracture healing over the entire observation period. Significant differences in the M-CSF serum concentration between patients with normal fracture healing and patients with impaired fracture healing were not observed. This study indicates, for the first time, to our knowledge, a possible local and systemic involvement of M-CSF in humans during fracture healing.

The impact of colony-stimulating factor-1 on fracture healing: an experimental study

The role of colony stimulating factor-1 (CSF-1) in the regulation of osteoclasts and bone remodeling suggests that CSF-1 may also be involved in regulation of bone healing. The ability of CSF-1 to promote healing of bone defects was tested in a rabbit model. Twenty-four New Zeeland rabbits were included in the study. Animals were assigned to two groups: the control group (n = 12) was treated by plate fixation. The animals in the second group (n = 12) were also stabilized by conventional plating and received additionally CSF-1 for 2 weeks systemically. Histologic, histomorphometric, and radiologic examinations were performed to evaluate the healing process at 4, 8, and 12 weeks following surgery. Animals that were treated by CSF-1 produced a significantly higher amount of mineralized bone over the first 8 weeks after fracture compared to the control animals. Furthermore, a higher number of osteoclasts was found in CSF-1-treated animals within the first 8 weeks, compared to the controls. The present data emphasize for the first time the importance of CSF-1 in the bone healing. The use of CSF-1 in addition to conventional fixation might be a novel approach for the treatment of bone defects.

Cell-to-cell contact is critical for the survival of hematopoietic progenitor cells on osteoblasts

Osteoblasts constitute part of the stromal cell support system in marrow for hematopoiesis, however little is known as to how they interact with hematopoietic stem cells (HSCs). In vitro studies have demonstrated that the survival of HSCs in co-culture with osteoblasts requires intimate cell-to-cell contact. This suggests that the osteoblast-derived factor(s) that supports stem cell activities are produced in very small quantities, are rapidly turned over, may be membrane-anchored and/or require the engagement of cell-cell adhesion molecules that are yet to be determined. In the present report we found that the survival of hematopoietic progenitor cells on osteoblasts is dependent upon the engagement of VLA-4 (alpha4beta1) and VLA-5 (alpha5beta1) receptors using function blocking antibodies. Cell-to-cell contact is required to support progenitor activity, but can be replaced if receptor-ligand engagement of the VLA-4 and LFA-1 complexes is provided through the use of recombinant ligands (fibronectin, ICAM-1, VCAM-1). Moreover, once these receptors were engaged, conditioned medium derived from HSCs grown on osteoblast ligands supported significantly greater hematopoietic progenitors in vitro than did osteoblast-conditioned or HSC-conditioned medium alone. While the molecules present in the co-cultured medium remain to be identified, the data suggest that hematopoietic cells cooperate with osteoblasts to assemble the various marrow microenvironments by directing the synthesis of osteoblast-derived cytokines to improve HSC survival.

The osteopetrotic mutation toothless (tl) is a loss-of-function frameshift mutation in the rat Csf1 gene: Evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification

The toothless (tl) mutation in the rat is a naturally occurring, autosomal recessive mutation resulting in a profound deficiency of bone-resorbing osteoclasts and peritoneal macrophages. The failure to resorb bone produces severe, unrelenting osteopetrosis, with a highly sclerotic skeleton, lack of marrow spaces, failure of tooth eruption, and other pathologies. Injections of CSF-1 improve some, but not all, of these. In this report we have used polymorphism mapping, sequencing, and expression studies to identify the genetic lesion in the tl rat. We found a 10-base insertion near the beginning of the open reading of the Csf1 gene that yields a truncated, nonfunctional protein and an early stop codon, thus rendering the tl rat CSF-1(null). All mutants were homozygous for the mutation and all carriers were heterozygous. No CSF-1 transcripts were identified in rat mRNA that would avoid the mutation via alternative splicing. The biology and actions of CSF-1 have been elucidated by many studies that use another naturally occurring mutation, the op mouse, in which a single base insertion also disrupts the reading frame. The op mouse has milder osteoclastopenia and osteopetrosis than the tl rat and recovers spontaneously over the first few months of life. Thus, the tl rat provides a second model in which the functions of CSF-1 can be studied. Understanding the similarities and differences in the phenotypes of these two models will be important to advancing our knowledge of the many actions of CSF-1.

Mutation of macrophage colony stimulating factor (Csf1) causes osteopetrosis in the tl rat

Osteopetrosis results from a heterogeneous group of congenital bone diseases that display inadequate osteoclastic bone resorption. We recently mapped tl (toothless), a mutation that causes osteopetrosis in rats, to a genetic region predicted to include the rat Csf1 gene. In this study, we sequenced the coding sequence of the rat Csf1 gene to determine if a mutation in Csf1 could be responsible for the tl phenotype. Sequencing revealed a 10-base insertion in the coding sequence of mutant animals that produces a frameshift and generates a stop codon early in the mutant Csf1 coding sequence. The 41 amino acid polypeptide predicted to be produced from the Csf1 promoter would have only the first nine amino acids of the wild-type rat protein. These data suggest that osteopetrosis develops in tl/tl rats because they cannot produce functional mCsf, a growth factor required for osteoclast differentiation and activation.

Serum creatine kinase isoenzyme BB in mammalian osteopetrosis

In mammalian osteopetrosis the different mutations exemplify reduced bone resorption leading to net accumulation of bone. Recently, high blood levels of creatine kinase-BB have been reported in some human forms, suggesting it as a marker of osteopetrosis. In the current study serum creatine kinase-BB was evaluated in relation to known osteoclastic pathophysiology in two human types of autosomal dominant osteopetrosis at baseline and after stimulation with triiodothyronine and in four different rodent mutations. Creatine kinase-BB was increased markedly in Type 2 autosomal dominant osteopetrosis and in the incisors absent rat, both characterized by large numbers of giant osteoclasts, and did not change significantly after stimulation. Although creatine kinase-BB was unchanged in Type 1 autosomal dominant osteopetrosis at baseline and after stimulation, the rodent counterparts characterized by small osteoclasts, microphthalmic and osteopetrotic mice and toothless rats, had significantly decreased levels. Similar differences were observed in both types of autosomal dominant osteopetrosis compared with controls concerning tartrate resistant acid phosphatase. Creatine kinase-BB in mammalian osteopetrosis is related to osteoclastic number and size, where it probably reflects the differentiation and maturation of inactive bone resorbing cells. The isoenzyme does not seem to be a valuable screening marker for osteopetrosis.

Conditioned medium of estrogen-treated osteoblasts inhibits osteoclast maturation and function in vitro

The increase of bone resorption and reduction of bone mass in postmenopausal women can be prevented by treatment with estrogen. Although it is well established that estrogen treatment normalizes the increased bone turnover, the mechanism by which estrogen exerts its protective influence at the cellular and molecular level in bone remains elusive. It has been shown that osteoblasts are involved in osteoclast development and osteoclastic bone resorption. In this work we examine the effect of estrogen (E2) on osteoclast-mediated bone resorption via the medium conditioned by osteoblast cultures. The conditioned medium collected from osteoblast cultures without (CM) or with 0.1 nmol/L 17beta-estradiol (E-CM) was mixed in a 1:1 ratio with fresh osteoclast culture medium. Osteoclasts were isolated from the bone marrow of 3-day-old NMRI mice and cultured on bovine bone slices. The total number of multinucleated tartrate-resistant alkaline phosphatase (TRAP)-positive cells in cultures with CM and E-CM was similar to that of cells incubated in control medium. However, the number of osteoclasts containing more than three nuclei was significantly smaller in the cultures containing E-CM. The total area of resorption was only slightly decreased in cultures containing CM, but was markedly inhibited in cultures with E-CM. In osteoblast cultures, the production of interleukin (IL)-1 and IL-6, but not of TNF-alpha, was reduced by 0.1 nmol/L E2. Our data suggest that E2 treatment of osteoblasts decreases the production of factor(s) that induces osteoclast differentiation to multinucleated cells with a higher capacity for bone resorption.

The toothless osteopetrotic rat has a normal vitamin D-binding protein-macrophage activating factor (DBP-MAF) cascade and chondrodysplasia resistant to treatments with colony stimulating factor-1 (CSF-1) and/or DBP-MAF

The osteopetrotic rat mutation toothless (tl) is characterized by little or no bone resorption, few osteoclasts and macrophages, and chondrodysplasia at the growth plates. Short-term treatment of tl rats with colony-stimulating factor-1 (CSF-1) has been shown to increase the number of osteoclasts and macrophages, producing dramatic resolution of skeletal sclerosis at some, but not all, sites. Defects in production of vitamin D-binding protein-macrophage activating factor (DBP-MAF) have been identified in two other independent osteopetrotic mutations of the rat (op and ia), and two in the mouse (op and mi), in which macrophages and osteoclasts can be activated by the administration of exogenous DBP-MAF. The present studies were undertaken to examine the histology and residual growth defects in tl rats following longer CSF-1 treatments, to investigate the possibility that exogenous DBP-MAF might act synergistically with CSF-1 to improve the tl phenotype, and to assess the integrity of the endogenous DBP-MAF pathway in this mutation. CSF-1 treatment-with or without DBP-MAF-induced resorption of metaphyseal bone to the growth plate on the marrow side, improved slightly but did not normalize long bone growth, and caused no improvement in the abnormal histology of the growth plate. Injections of lysophosphatidylcholine (lyso-Pc) to prime macrophage activation via the DBP-MAF pathway raised superoxide production to similar levels in peritoneal macrophages from both normal and mutant animals, indicating no defect in the DBP-MAF pathway in tl rats. Interestingly, pretreatments with CSF-1 alone also increased superoxide production, although the mechanism for this remains unknown. In summary, we find that, unlike other osteopetrotic mutations investigated to date, the DBP-MAF pathway does not appear to be defective in the tl rat; that additional DBP-MAF does not augment the beneficial skeletal effects seen with CSF-1 alone; and that the growth plate chondrodystrophy seen in this mutation is unaffected by either molecule. Thus, the tl mutation intercepts the function of a gene required for both normal endochondral ossification and bone resorption, thereby uncoupling the coordination of skeletal metabolism required for normal long bone growth.

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.

Reciprocal gene expression of osteoclastogenesis inhibitory factor and osteoclast differentiation factor regulates osteoclast formation

Osteoblasts/stromal cells support the formation of osteoclast-like cells (OCL) from osteoclast progenitor cells via expressing a membrane-associated protein, osteoclast differentiation factor (ODF), in the presence of osteotropic factors, whereas the cells secrete a substantial amount of osteoclastogenesis inhibitory factor (OCIF) in the unstimulated state. There are both OCL formation-supporting and the nonsupporting cell lines in osteoblasts/stromal cell lineages. The mechanism that divides osteoblasts/stromal cell lines into the two types is not known. The present study reports that OCL formation-supporting cell line ST2 showed a greatly increased level of ODF mRNA, whereas their OCIF mRNA was drastically diminished in the presence of 1alpha, 25(OH)2-dihydroxyvitamin D3 or prostaglandin E2. In contrast, MC3T3-E1 cells lacking OCL formation-supporting ability did not show a decrease in OCIF mRNA in response to the factors, despite a similar increase in ODF mRNA as ST2 cells. However, inactivated MC3T3-E1 cells secreting nothing supported OCL formation in coculture with human promyelocytic cells, HL60. On the contrary, ST2 cells did not support OCL formation from HL60 cells when cocultured in medium conditioned by 1alpha, 25(OH)2 vitamin D3-treated MC3T3-E1. These findings indicate that reciprocal gene expression of ODF and OCIF in osteoblasts/stromal cells is essential for supporting OCL formation.

Auditory ossicle abnormalities and hearing loss in the toothless (osteopetrotic) mutation in the rat and their improvement after treatment with colony-stimulating factor-1

Osteopetrosis describes a group of skeletal metabolic diseases of heterogeneous etiology and varied severity that produces a generalized accumulation of skeletal mass, the result of reduced bone resorption. Inherited in a variety of species including humans, the most severe forms are lethal. Among common features are progressive blindness and deafness of controversial etiologies for which there are no universally effective treatments. We have studied the auditory responsiveness and auditory ossicle quantitative histomorphology and temporal bone vasculature in the toothless (tl) rat, a lethal osteopetrotic mutation with few osteoclasts, very low bone turnover, and limited angiogenesis in the axial skeleton. Compared with normal littermates, 3-week-old mutants showed significantly reduced auditory responsiveness, a hearing loss due to abnormalities in both form and tissue composition of the stapes, and little capillary sprouting in the vascular bed of the temporal bone. Treatment of mutants with colony-stimulating factor 1 (CSF-1), known to greatly reduce sclerosis in the axial skeleton, significantly improved hearing, stapedial form and tissue composition, and angiogenesis in the temporal bone. In normal rats, the stapes consisted of 89.3% bone, 9.1% mineralized cartilage, and 0.8% porosity. In osteopetrotic rats, the stapes consisted of 48.3% bone, 35.9% mineralized cartilage, and 15.9% porosity, while after CSF-1 treatment, the bone content increased to 55.2%, cartilage was decreased to 21.7%, and porosity increased to 23.0%, respectively. This is the first demonstration of an auditory abnormality in an osteopetrotic animal mutation and shows that the hearing loss in tl rats can be significantly improved 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.

The role of osteoblasts in the hematopoietic microenvironment

Hematopoietic stem cell differentiation occurs in direct proximity to osteoblasts within the bone marrow cavity. Despite this striking affiliation, surprisingly little is known about the precise cellular and molecular impact of osteoblasts on the bone marrow microenvironment. Recently, it has been proposed that human osteoblasts support the growth of primitive human hematopoietic cells in vitro and possibly in vivo. Evidence to support this hypothesis is reviewed as follows: the influence of osteoblasts on osteoclast development; the participation of osteoblasts in long-term bone marrow cultures; the production of positive hematopoietic regulatory molecules by osteoblasts; the production of cell-cycle inhibitory factors by osteoblasts, and cell-cell interactions between early hematopoietic cells and osteoblasts.

The effects of colony-stimulating factor-1 on the number and ultrastructure of osteoclasts in toothless (tl) rats and osteopetrotic (op) mice

The role of colony-stimulating factor-1 (CSF-1 or M-CSF) in osteoclast development is illustrated by observations that administration of exogenous CSF-1 increases osteoclast number and improves the skeletal sclerosis of two osteopetrotic mutations, toothless (tl) in the rat and osteopetrotic (op) in the mouse. We examined the effects of CSF-1 treatment on the number and ultrastructure of osteoclasts in the tibial metaphysis of normal and mutant animals of both stocks to understand the similarities and differences between these two mutations. Osteoclasts from normal animals of both stocks were abundant and possessed the ultrastructural features of active cells. These included apical areas in contact with mineralized surfaces with tightly apposed clear zones, extensive ruffled borders, and a vacuolated cytoplasm with numerous mitochondria. In toothless rats osteoclasts were difficult to locate and those present had poorly defined ruffled borders, fewer cytoplasmic vacuoles, and a basal membrane with both smooth and ruffled areas. Large lipid accumulations were often found near tl osteoclasts. Osteoclasts in op mice were difficult to find, but more numerous than in tl rats. Unlike tl osteoclasts, those of op mice possessed very well developed ruffled borders, small clear zones, and large electron-dense cytoplasmic inclusions. These cells also had unusual basal membranes with both smooth and ruffled regions. CSF-1 treatment increased the number of osteoclasts in both mutant stocks, normalizing the numbers in op mice, but not tl rats. CSF-1 injections caused dramatic changes in the morphology of tl osteoclasts, including increased incidence and size of ruffled borders and cytoplasmic vacuolization. The growth factor had little effect on ruffled borders or clear zones in op mice. Interestingly, mutant osteoclasts of both stocks exhibited a ruffled basal membrane in response to CSF-1 treatment. This increase in membrane ruffling may reflect the ability of CSF-1 to promote rapid formation of osteoclasts from mononuclear precursors in a more permissive microenvironment. Our data indicate that CSF-1 is not required for the development of at least some osteoclasts. The differences in response to CSF-1 treatment which we report lead us to speculate that additional factors may be involved in osteoclastogenesis.

Alteration of mineral crystallinity and collagen cross-linking of bones in osteopetrotic toothless (tl/tl) rats and their improvement after treatment with colony stimulating factor-1

A common feature of various types of mammalian osteopetroses is a marked increase in bone mass accompanied by spontaneous bone fractures. The toothless (tl/tl) rat osteopetrotic mutation is characterized by drastically reduced bone resorption due to a profound deficiency of osteoclasts and their precursors. An altered bone morphology has also been observed. The mutants cannot be cured by bone marrow transplantation, but skeletal defects are greatly reduced after treatment with colony stimulating factor 1 (CSF-1). The objectives of this study were to characterize mineral and collagen matrices in cancellous and compact bone isolated from long bones of 6-week-old normal littermates, tl/tl osteopetrotic mutants and mutants (tl/tl) treated with CSF-1. There were no differences in bone mineral content, but a significant decrease in the crystallinity of mineral evaluated by the method based on electron paramagnetic resonance spectrometry was observed in all bones of tl/tl mutants as compared to that of controls. Within the collagen matrix, slight decreases in the labile cross-links, but significant increases in the content of the stable cross-links, pyridinoline, and deoxypyridinoline, were observed in both cancellous and compact bone of osteopetrotic mutants. In tl/tl mutants treated with human recombinant CSF-1, the normalization of the crystallinity of bone mineral as well as collagen cross-links was found. Our results indicate that remodeling of bone matrix in tl/tl mutants is highly suppressed, but that after treatment with CSF-1, this activity recovers significantly. Taken together, these data provide further support for the hypothesis that CSF-1 is an essential factor for normal osteoclast differentiation and bone remodelling.

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.

Animal models of osteopetrosis: the impact of recent molecular developments on novel strategies for therapeutic intervention

Osteopetrosis comprises a group of rare metabolic diseases of skeletal development that are characterized by a generalized increase in skeletal mass resulting from reduced osteoclast-mediated bone resorption. Specific immune regulators and growth factors that influence osteoclast ontogeny and/or activation have been implicated in the pathogenesis of some of the naturally occurring mutations associated with osteopetrosis in animals. Most recently, loss-of-function experiments using transgenic mice with targeted disruptions of the c-src or c-fos proto-oncogenes have resulted in different osteoclast abnormalities that produce osteopetrosis. The information gained from these mutations in animals should continue to provide new understanding of the molecular defects associated with osteopetrosis, and to broader aspects of skeletal pathology; this should result in more effective therapeutic intervention in humans.

Heterogeneity of colony stimulating factor-1 gene expression in the skeleton of four osteopetrotic mutations in rats and mice

Congenital osteopetrosis in mammals is an inherited bone disease caused by aberrations in osteoclast development and/or function. Colony-stimulating factor-1 (CSF-1) promotes formation of osteoclasts and is produced by osteoblasts. Recently, two osteopetrotic mutations (op mouse and tl rat) have been shown to have reductions in CSF-1 activity, and CSF-1 injections improve the skeletal manifestations in each. Several different CSF-1 transcripts have been described in mouse and human soft tissues, and differential expression of CSF-1 transcripts has been documented. Thus, we compared gene expression for CSF-1 as reflected by mRNA levels in the bones of tl rats and op mice, and also two other osteopetrotic rat mutations (ia and op). In op mouse calvaria the 4.6 kb transcript was reduced while the 2.3 kb transcript was absent. However, no differences were detected in the levels of these transcripts in mutant and normal calvaria of tl stock. In contrast, CSF-1 transcript levels were elevated in op rat mutants and variable in ia mutants compared to normal littermates. Osteoblast cultures derived from neonatal animals of tl and op rat stock showed the same differences seen in calvarial bone in vivo. The mRNA expression of another growth factor, TGF-beta 1, paralleled that of CSF-1 in vivo and in vitro in the rat mutations. These data demonstrate the emerging molecular heterogeneity among osteopetrotic mutations and underscore the need to evaluate the contributions of these and other cytokines to osteoclast differentiation and function in each mutation.

Synthesis of membrane- and matrix-bound colony-stimulating factor-1 by cultured osteoblasts

Colony-stimulating factor-1 (CSF-1) is synthesized as a secreted or membrane-bound molecule. We investigated whether osteoblastic cells produce these forms of CSF-1. Glutaraldehyde-fixed cell layers supported proliferation of the macrophage cell line BAC1.2F5, suggesting the presence of membrane- or/and matrix-associated CSF-1. Furthermore, CSF-1 activity could be either extracted from the matrix or released from the cell membrane. A neutralizing antiserum against CSF-1 inhibited these activities. After labeling the cellular proteins with [35S] met/cys or [35S] SO4(2-), CSF-1 was immunoprecipitated and analyzed by SDS-PAGE. Under nonreducing conditions, bands with MW more than 200, 200, 100, and 50 kd were detected. These bands shifted to lower MW under reducing conditions. Treatment with chondroitin lyase ABC decreased the MW of the 200 kd monomer, proving the proteoglycan structure. Much smaller quantities of CSF-1 were found in the matrix extract than in the conditioned medium. Transforming growth factor beta (TGF-beta) increased both the synthesis of CSF-1 and its accumulation in the matrix. CSF-1 released with trypsin from the membrane fraction yielded on SDS-PAGE a band with MW of 60 and 30 kd under nonreducing and reducing conditions, respectively. Transcripts encoding both the secreted and the membrane-associated forms of the cytokine were detected in osteoblasts by reverse transcription polymerase chain reaction. These data indicate that osteoblastic cells produce the secreted forms, either remaining in the culture supernatant, or being associated to the matrix, and the membrane associated form of CSF-1.

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.

Detection of transcripts and binding sites for colony-stimulating factor-1 during bone development

Colony-stimulating factor-1 (CSF-1), originally characterized as the growth factor for the cells of the mononuclear phagocytic system, has been shown to be essential for osteoclast formation. The aim of the present study was twofold: (i) to investigate the expression of transcripts encoding CSF-1; and (ii) to detect binding sites for CSF-1 during bone development. As a model, metatarsal rudiments from embryonic mice of different ages were used, an in vivo system allowing one to follow osteoclast formation. In 16-day-old embryos, proliferating osteoclast precursors are located on the outer surface of the rudiments. They differentiate subsequently to post-mitotic precursors. At 18 days, the precursors fuse and the mature osteoclasts invade the mineralized cartilage of the rudiments to excavate the future bone marrow cavity. Within this study, in situ hybridization on sections of whole paws from 17-day-old embryos revealed CSF-1 transcripts to be present in cells lining the outside of the midregion of the metatarsals. One day later, cells containing CSF-1 mRNA were found within the mineralized cartilage. The levels of transcripts encoding CSF-1 were further increased in the bone rudiments of newborn animals. Binding sites for CSF-1 on cells in close proximity of the metatarsals were detected at embryonic age 17 days, but not before. At this stage, cells binding CSF-1 were located on the periosteum of the midregion of the metatarsal rudiment. At 18 days, cells expressing high levels of CSF-1 binding sites had invaded the mineralized cartilage.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of vitamin D binding protein-macrophage activating factor (DBP-MAF) infusion on bone resorption in two osteopetrotic mutations

Osteopetrosis is a heterogeneous group of bone diseases characterized by an excess accumulation of bone and a variety of immune defects. Osteopetrosis (op) and incisors absent (ia) are two nonallelic mutations in the rat which demonstrated these skeletal defects as a result of reduced bone resorption. Osteopetrotic (op) rats have severe sclerosis as a result of reduced numbers of osteoclasts which are structurally abnormal. The sclerosis in ia rats is not as severe as in op mutants; they have elevated numbers of osteoclasts, but they are also morphologically abnormal, lacking a ruffled border. Both of these mutations have defects in the inflammation-primed activation of macrophages. They demonstrate independent defects in the cascade involved in the conversion of vitamin D binding protein (DBP) to a potent macrophage activating factor (DBP-MAF). Because this factor may also play a role in the pathogenesis of osteoclastic dysfunction, the effects of ex vivo-generated DBP-MAF were evaluated on the skeletal system of these two mutations. Newborn ia and op rats and normal littermate controls were injected with DBP-MAF or vehicle once every 4 days from birth until 2 weeks of age, at which time bone samples were collected to evaluate a number of skeletal parameters. DBP-MAF treated op rats had an increased number of osteoclasts and the majority of them exhibited normal structure. There was also reduced bone volume in the treated op animals and an associated increased cellularity of the marrow spaces. The skeletal sclerosis was also corrected in the ia rats; the bone marrow cavity size was significantly enlarged and the majority of the osteoclasts appeared normal with extensive ruffled borders.

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.

Microvascular pattern in the metaphysis during bone growth

BACKGROUND

Little is known about the three-dimensional micromorphology of vessels in the growth zone of long bones, where significant vasculogenesis occurs. Therefore, we examined the microvascular pattern of the femoral metaphysis.

METHODS

Six-week-old normal rats of either sex were used. We cast the femurs of 14 rats with Mercox for scanning electron microscopy (SEM), and in 10 rats we prepared tissue sections of femurs for light (LM) and transmission electron microscopy (TEM).

RESULTS

In the LM, calcified cartilage was found to define cylindrical compartments beneath the last row of hypertrophied chondrocytes of the metaphyseal growth plate. These compartments ran in the bone's longitudinal axis and contained a single capillary profile. Endothelial cells of these capillaries often showed increased cytoplasmic volume and loose texture of nuclear chromatin. Cast metaphyses by SEM showed numerous parallel vascular loops with nodular protrusions 10-12 microns in diameter at their tips. The loops had ascending and descending limbs with a luminal diameter of 10-14 microns. Small projections 4-5 microns in diameter and delicate crests were sometimes found on the tip of the larger nodes. In a 100 x 100 microns area, there were 14-17 large nodes. By TEM, capillary sprouts were identified at the level beneath the last row of hypertrophied chondrocytes. These capillaries had voluminous endothelial cells rich in free ribosomes and rough endoplasmic reticulum. Endothelial cell nuclei were rounded and showed loose chromatin texture. Endothelial cells were connected by intermediate junctions and there was no basal lamina. Deeper into the metaphysis, arterioles and sinusoids were present.

CONCLUSIONS

We conclude that the metaphyseal plate of the growing rat offers an optimal model to study vasculogenesis. Capillary sprouts can be readily identified, measured, and counted because they are located within a plane bordering against avascular cartilage. In addition, by using microvascular corrosion casting in SEM not only capillary sprouting per se but also different stages of neovascularization, indicated by differently sized nodular projections at the tip of vascular loops, can be studied in the growing long bone.

Is the osteopetrotic (op/op mutant) mouse completely deficient in expression of macrophage colony-stimulating factor?

The op/op mouse has a mutation in the macrophage colony-stimulating (CSF-1) gene. The phenotype of gross deficiency in the macrophage and osteoclast lineages corrects significantly with age, suggesting that other factors can substitute for CSF-1. This review examines the evidence that the op/op mouse is completely CSF-1 deficient and considers the possibility that alternative splicing within the CSF-1 gene might bypass the mutation, yielding an incompletely penetrant phenotype.

CSF-1 treatment promotes angiogenesis in the metaphysis of osteopetrotic (toothless, tl) rats

It has recently been shown that following treatment with colony-stimulating factor-1 (CSF-1) the osteopetrotic condition in toothless (tl) rats greatly improves and growth is accelerated. We have examined the effects of such treatment on the microvasculature of the distal femoral chondro-osseous junction, a site where bone growth in length is coordinated with angiogenesis. Vascular casts and ultrastructural analyses of this region showed that, compared to untreated normal rats, untreated mutants showed little bone growth or angiogenesis. When mutants were treated with CSF-1 angiogenesis was markedly accelerated. These data show a remarkable effect of this growth factor on angiogenesis in this osteopetrotic mutation. Whether this effect of CSF-1 on angiogenesis is direct or indirect is not known and indicates that its effects on the normal microvasculature deserve further study.

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.

Neonatal reductions in osteoclast number and function account for the transient nature of osteopetrosis in the rat mutation microphthalmia blanc (mib)

We have examined the general and skeletal manifestations of osteopetrosis in a new, mild osteopetrotic mutation in the rat, microphalmia blanc (mib). Newborn mutant (mib) rats exhibit the typical skeletal deformities and sclerosis of osteopetrosis at birth, which are reduced significantly during the first postnatal month but don't disappear entirely up to 8 months later. Osteoclast numbers, staining for TRAP and TraATPase, and bone resorption are reduced in mutants during the first 2 postnatal weeks but improve by 1 month. In mutants, serum concentrations of calcium and phosphorus are normal, but 1,25(OH)2 D levels are higher at 1 week than those in normal littermates. Neonatally, mutants exhibit extramedullary hemopoiesis in the spleen. These results are interpreted to mean that the transient perinatal skeletal sclerosis in mib rats is caused by reduced production and function of osteoclasts in this period. The recent description of transient, perinatal osteopetrosis in a child suggests that analyses of the early differences between mild and severe animal mutations might distinguish those children with osteopetrosis who need treatment from those who do not.

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.

Role of colony-stimulating factor-1 in bone metabolism

Colony-stimulating factor-1 (CSF-1) is a cytokine required for proliferation, differentiation, activity, and survival of cells of the mononuclear phagocytic system. The growth factor is synthesized as a soluble, matrix, or membrane associated molecule. The specific functions of these forms are not clear. However, some data suggest a dependence of the development of various populations of tissue macrophages on the locally expressed and presented cytokine. Deficiency in CSF-1, as is the case in the murine mutant strain op/op, results in low numbers of macrophages and monocytes and, most striking, leads to osteopetrosis due to a virtual absence of osteoclasts. Using the op/op mutation as a model, CSF-1 was established as one of the growth factors for osteoclasts. The expression of CSF-1 receptors, encoded by the proto-oncogene c-fms, by osteoclast precursors and osteoclasts, suggested an effect of this cytokine not only during osteoclast formation but also on the mature cells. In fact, CSF-1 was shown to inhibit the resorbing activity, to stimulate migration, and to support survival of isolated osteoclasts in vitro. By these actions on cells of the osteoclast lineage, CSF-1 induces recruitment of new osteoclasts, leading to a net increase of bone resorption, and might govern the spatial distribution of resorption sites within the bone. During these processes, locally expressed and presented forms of the growth factor may play a crucial role, as will be discussed in this article.