Blocking substance P signaling reduces musculotendinous and dermal fibrosis and sensorimotor declines in a rat model of overuse injury

Purpose/Aim: Substance P-NK-1R signaling has been implicated in fibrotic tendinopathies and myositis. Blocking this signaling with a neurokinin 1 receptor antagonist (NK1RA) has been proposed as a therapeutic target for their treatment.Materials and Methods: Using a rodent model of overuse injury, we pharmacologically blocked Substance P using a specific NK1RA with the hopes of reducing forelimb tendon, muscle and dermal fibrogenic changes and associated pain-related behaviors. Young adult rats learned to pull at high force levels across a 5-week period, before performing a high repetition high force (HRHF) task for 3 weeks (2 h/day, 3 days/week). HRHF rats were untreated or treated in task weeks 2 and 3 with the NK1RA, i.p. Control rats received vehicle or NK1RA treatments.Results: Grip strength declined in untreated HRHF rats, and mechanical sensitivity and temperature aversion increased compared to controls; these changes were improved by NK1RA treatment (L-732,138). NK1RA treatment also reduced HRHF-induced thickening in flexor digitorum epitendons, and HRHF-induced increases of TGFbeta1, CCN2/CTGF, and collagen type 1 in flexor digitorum muscles. In the forepaw upper dermis, task-induced increases in collagen deposition were reduced by NK1RA treatment.Conclusions: Our findings indicate that Substance P plays a role in the development of fibrogenic responses and subsequent discomfort in forelimb tissues involved in performing a high demand repetitive forceful task.

Src is a major signaling component for CTGF induction by TGF-beta1 in osteoblasts

Connective tissue growth factor (CTGF/CCN2) is induced by transforming growth factor beta1 (TGF-beta1) where it acts as a downstream mediator of TGF-beta1 induced matrix production in osteoblasts. We have shown the requirement of Src, Erk, and Smad signaling for CTGF induction by TGF-beta1 in osteoblasts; however, the potential interaction among these signaling pathways remains undetermined. In this study we demonstrate that TGF-beta1 activates Src kinase in ROS17/2.8 cells and that treatment with the Src family kinase inhibitor PP2 prevents Src activation and CTGF induction by TGF-beta1. Additionally, inhibiting Src activation prevented Erk activation, Smads 2 and 3 activation and nuclear translocation by TGF-beta1, demonstrating that Src is an essential upstream signaling partner of both Erk and Smads in osteoblasts. MAPKs such as Erk can modulate the Smad pathway directly by mediating the phosphorylation of Smads or indirectly through activation/inactivation of required nuclear co-activators that mediate Smad DNA binding. When we treated cells with the Erk inhibitor, PD98059, it inhibited TGF-beta1-induced CTGF protein expression but had no effect on Src activation, Smad activation or Smad nuclear translocation. However PD98059 impaired transcriptional complex formation on the Smad binding element (SBE) of the CTGF promoter, demonstrating that Erk activation was required for SBE transactivation. These data demonstrate that Src is an essential upstream signaling transducer of Erk and Smad signaling with respect to TGF-beta1 in osteoblasts and that Smads and Erk function independently but are both essential for forming a transcriptionally active complex on the CTGF promoter in osteoblasts.

Molecular requirements for induction of CTGF expression by TGF-beta1 in primary osteoblasts

Connective tissue growth factor (CTGF/CCN2) is a cysteine rich, extracellular matrix protein that acts as an anabolic growth factor to regulate osteoblast differentiation and function. In osteoblasts, CTGF is induced by TGF-beta1 where it acts as a downstream mediator of TGF-beta1 induced matrix production. The molecular mechanisms that control CTGF induction by TGF-beta1 in osteoblasts are not known. To assess the role of individual Smads in mediating the induction of CTGF by TGF-beta1, we used specific Smad siRNAs to block Smad expression. These studies demonstrated that Smads 3 and 4, but not Smad 2, are required for TGF-beta1 induced CTGF promoter activity and expression in osteoblasts. Since the activation of MAPKs (Erk, Jnk and p38) by TGF-beta1 is cell type specific, we were interested in determining the role of individual MAPKs in TGF-beta1 induction of CTGF promoter activity and expression. Using dominant negative (DN) mutants for Erk, Jnk and p38, we demonstrated that the expression of DN-Erk caused a significant inhibition of TGF-beta1 induced CTGF promoter activity. In contrast, the expression of DN-p38 or DN-Jnk failed to inhibit activation of CTGF promoter activity. To confirm the vital role of Erk, we used the Erk inhibitor (PD98059) to block its activation, demonstrating that it prevented TGF-beta1 activation of the CTGF promoter and up-regulation of CTGF expression in osteoblasts. Since Src can also act as a downstream signaling effector for TGF-beta in some cell types, we determined its role in TGF-beta1 induction of CTGF in osteoblasts. Treatment of osteoblasts with a Src family kinase inhibitor, PP2, or the expression of two independent kinase-dead Src mutant constructs caused significant inhibition of TGF-beta1 induced CTGF promoter activity and expression. Additionally, blocking Src activation prevented Erk activation by TGF-beta1 demonstrating a role for Src as an upstream mediator of Erk in regulating CTGF expression in osteoblasts. To investigate the involvement of the TGF-beta1 response element (TRE) and the SMAD binding element (SBE) in CTGF induction, we cloned the rat CTGF proximal promoter (-787 to +1) containing the TRE and SBE motifs into a pGL3-Luciferase reporter construct. Using a combination of CTGF promoter deletion constructs and site-directed mutants, we demonstrated the unique requirement of both the TRE and SBE for CTGF induction by TGF-beta1 in osteoblasts. Electro-mobility shift assays using specific probes containing the TRE, SBE or both showed TGF-beta1 inducible complexes that can be ablated by mutation of the respective motif, confirming their requirement for TGF-beta1 induced CTGF promoter activity. In conclusion, these studies demonstrate that CTGF induction by TGF-beta1 in osteoblasts involves Smads 3 and 4, the Erk and Src signaling pathways, and requires both the TRE and SBE motifs in the CTGF proximal promoter.

Connective tissue growth factor (CTGF/CCN2) is a downstream mediator for TGF-beta1-induced extracellular matrix production in osteoblasts

Connective tissue growth factor (CTGF/CCN2) is a cysteine-rich, extracellular matrix (ECM) protein that acts as an anabolic growth factor to regulate osteoblast differentiation and function. Recent studies have identified CTGF as a downstream effector of transforming growth factor-beta1 (TGF-beta1) for certain functions in specific cell types. In this study, we examined the role of CTGF as a downstream mediator of TGF-beta1-induced ECM production and cell growth in osteoblasts. Using primary cultures, we demonstrated that TGF-beta1 is a potent inducer of CTGF expression in osteoblasts, and that this induction occurred at all stages of osteoblast differentiation from the proliferative through mineralization stages. TGF-beta1 treatment of osteoblasts increased the expression and synthesis of the ECM components, collagen and fibronectin. When CTGF-specific siRNA was used to prevent TGF-beta1 induction of CTGF expression, it also inhibited collagen and fibronectin production, thereby demonstrating the requirement of CTGF for their up-regulation. To examine the effects of TGF-beta1 on osteoblast cell growth, cultures were treated with TGF-beta1 during the proliferative stage. Cell number was significantly reduced and the cells exhibited a decrease in G1 cyclin expression, consistent with TGF-beta1-induced cell-cycle arrest. Cultures transfected with CTGF siRNA prior to TGF-beta1 treatment showed an even greater reduction in cell number, suggesting that TGF-beta1-induced growth arrest is independent of CTGF in osteoblasts. Collectively, these data demonstrate for the first time that CTGF is an essential downstream mediator for TGF-beta1-induced ECM production in osteoblasts, but these two growth factors function independently regarding their opposing effects on osteoblast proliferation.

Identification and Characterization of the Genes Encoding Human and Mouse Osteoactivin

Osteoactivin (OA) is more highly expressed in the bones of osteopetrotic mutant rats (op/op) than in those of their normal littermates and is the homologue of human nmb, a cDNA more highly expressed in melanoma-derived cell lines of low metastatic potential, and of mouse DC-HIL, which has been implicated in endothelial cell adhesion. The human OA gene is found on chromosome 7p15.1 and consists of 11 exons spanning 28.3 kb. Murine OA is encoded by a highly similar gene of 11 exons spanning 20.2 kb on mouse chromosome 6. Human OA uses the same transcriptional initiation site in both bone and kidney as was reported for melanoma cells. OA is expressed in primary human and mouse osteoblast cultures at all stages of differentiation, with increased levels observed concurrently with the expression of osteoblast phenotype markers. OA is also expressed in a wide variety of human and mouse tissues as determined by RT-PCR analysis. Immunohistochemical investigation of OA expression in late mouse embryonic development showed very high, cell-specific expression in the nervous system, basal layer of the skin, germinal cells of hair follicles, and in the forming nephrons of the kidney. Continuing investigation of the cell-specific expression of OA in bone as well as in other tissues will lead to a better understanding of its function in the development of these cell types.

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.

Evidence that the rat osteopetrotic mutation toothless (tl) is not in the TNFSF11 (TRANCE, RANKL, ODF, OPGL) gene

The toothless (tl) osteopetrotic mutation in the rat affects an osteoblast-derived factor that is required for normal osteoclast differentiation. Although the genetic locus remains unknown, the phenotypic impact of the tl mutation on multiple systems has been well characterized. Some of its actions are similar to tumornecrosis factor superfamily member 11(TNFSF11; also called TRANCE, RANKL, ODF and OPGL) null mice. TNFSF11 is a recently described member of the tumor necrosis factor superfamily which, when expressed by activated T cells, enhances the survival of antigen-presenting dendritic cells, and when expressed by osteoblasts, promotes the differentiation and activation of osteoclasts. The skeletal similarities between tl rats and TNFSF11(-/-) mice include 1) profound osteoclastopenia (TNFSF11-null mice, 0% and tl rats 0-1% of normal); 2) persistent, non-resolving osteopetrosis that results from 3) a defect not in the osteoclast lineage itself, but in an osteoblast-derived, osteoclastogenic signal; and 4) a severe chondrodysplasia of the growth plates of long bones not seen in other osteopetrotic mutations. The latter includes thickening of the growth plate with age, disorganization of chondrocyte columns, and disturbances of chondrocyte maturation. These striking similarities prompted us to undertake studies to rule in or out a TNFSF11 mutation in the tl rat. We looked for expression of TNFSF11 mRNA in tl long bones and found it to be over-expressed and of the correct size. We also tested TNFSF11 protein function in the tl rat. This was shown to be normal by flow cytometry experiments in which activated, spleen-derived T-cells from tl rats exhibited normal receptor binding competence, as measured by a recombinant receptor assay. We also found that tl rats develop histologically normal mesenteric and peripheral lymph nodes, which are absent from TNFSF11-null mice. Next, we found that injections of recombinant TNFSF11, which restores bone resorption in null mice, had no therapeutic effect in tl rats. Finally, gene mapping studies using co-segregation of polymorphic markers excluded the chromosomal region containing the TNFSF11 gene as harboring the mutation responsible for the tl phenotype. We conclude that, despite substantial phenotypic similarities to TNFSF11(-/-) mice, the tl rat mutation is not in the TNFSF11 locus, and that its identification must await the results of further studies.

Reduced bone resorption in toothless (osteopetrotic) rats--an abnormality of osteoblasts related to their inability to activate osteoclast activity in vitro

Osteopetrosis is a heterogeneous group of metabolic bone disorders characterized by reduced bone resorption. In the toothless (tl) osteopetrotic rat mutation there are few osteoclasts and mutants are not cured by bone marrow transplants. This suggests that the defect(s) in tl rats is within the skeletal microenvironment and not one of stem cell incompetence. Osteoblasts are known to play a role in bone resorption and abnormalities in these cells have been reported in tl rats. We explored the ability of osteoblasts from tl rats to activate resorption by normal osteoclasts when co-cultured in the presence of 1,25-dihydroxyvitamin D (1,25(OH)2D). Stimulation with 1,25(OH)2D produced a highly significant response in normal osteoblast co-cultures, but no response was observed in mutant cultures over a wide dose range. Ligand-binding studies demonstrated no abnormalities in vitamin D receptor (VDR) affinity, but mutant osteoblasts had reduced VDR numbers. Taken together with the demonstrated resistance of these mutants to the hypercalcemic effects of 1,25(OH)2D and parathyroid hormone in vivo, these data implicate osteoblasts in the pathogenesis of this mutation.

Endochondral bone formation in toothless (osteopetrotic) rats: failures of chondrocyte patterning and type X collagen expression

The pacemaker of endochondral bone growth is cell division and hypertrophy of chondrocytes. The developmental stages of chondrocytes, characterized by the expression of collagen types II and X, are arranged in arrays across the growth zone. Mutations in collagen II and X genes as well as the absence of their gene products lead to different, altered patterns of chondrocyte stages which remain aligned across the growth plate (GP). Here we analyze GP of rats bearing the mutation toothless (tl) which, apart from bone defects, develop a progressive, severe chondrodystrophy during postnatal weeks 3 to 6. Mutant GP exhibited disorganized, non-aligned chondrocytes and mineralized metaphyseal bone but without cartilage mineralization or cartilaginous extensions into the metaphysis. Expression of mRNA coding for collagen types II (Col II) and X (Col X) was examined in the tibial GP by in situ hybridization. Mutant rats at 2 weeks exhibited Col II RNA expression and some hypertrophied chondrocytes (HC) but no Col X RNA was detected. By 3rd week, HC had largely disappeared from the central part of the mutant GP and Col II RNA expression was present but weak and in 2 separate bands. Peripherally the GP contained HC but without Col X RNA expression. This abnormal pattern was exacerbated by the fourth week. Bone mineralized but cartilage in the GP did not. These data suggest that the tl mutation involves a regulatory function for chondrocyte maturation, including Col X RNA synthesis and mineralization, and that the GP abnormalities are related to the Col X deficiency. The differences in patterning in the tl rat GP compared to direct Col X mutations may be explained by compensatory effects.

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.

Influence of estrogen deficiency and replacement on T-cell populations in rat lymphoid tissues and organs

Estrogen deficiency following ovariectomy or menopause results in bone loss. Although evidence strongly suggests that the immune system is involved in the pathogenesis of estrogen-deficient osteoporosis, it is not clear what role, if any, the T-lymphocyte plays in this process. Therefore, we examined the distribution of T-cell subsets in lymphoid organs and tissues, under varying estrogenic states in the rat. Six-month-old female Sprague-Dawley rats, ovariectomized (Ovx) and sham-operated, were randomized 5 d post-surgery into six groups to receive the following treatments: (A) sham/placebo; (B) sham/low-dose E2; (C) sham/high-dose E2; (D) Ovx/placebo; (E) Ovx/low-dose E2; (F) Ovx/high-dose E2. Half of the treated rats (groups A-F) were sacrificed on d 14; the remainder on d 28. Following euthanasia, mononuclear cells were isolated from the thymus, peripheral blood, spleen, lymph node and bone marrow, and were labeled for flow cytometric analysis using mouse anti-rat monoclonal antibodies directed against CD5, CD4, and CD8 antigenic markers. In the thymus, ovariectomy caused a dramatic increase and E2 treatment caused a dose-dependent decrease in weight that was proportional to the number of thymocytes. In the bone marrow, ovariectomy caused a significant reduction in the percentage of all T-cell subsets examined and this effect persisted throughout the duration of the study. Estrogen replacement therapy at the low-dose reversed the effects of ovariectomy and high-dose E2 treatment caused an increase in T-cell subsets in both the sham and Ovx groups, an effect that was more pronounced at d 14 compared with d 28. Although the percentages of some T-cell subsets in the other lymphoid organs/tissues were altered by ovariectomy or E2 treatment at d 0 and 14, all these changes had normalized by d 28 except for CD5 and CD4 cells in peripheral blood. In summary, with the exception of T-lymphocytes in the bone marrow, the effects of varying estrogenic states on T-cells were variable and transient. The influence of estrogen status on bone marrow T-lymphocytes suggests that these cells may play a role in mediating the effects of estrogen on bone turnover and warrant additional studies focusing on the functional role of T-cells in the bone marrow compartment.

Skeletal resistance to 1,25-dihydroxyvitamin D3 in osteopetrotic rats

The osteopetrotic (op/op) rat mutation is a lethal mutation in which decreased osteoclast function (bone resorption) coexists with markedly elevated serum levels of 1 ,25-dihydroxyvitamin D3[1,25(OH)2D3]. Increased circulating levels of 1,25(OH)2D3 have been reported in other osteopetrotic animal mutations and in some osteopetrotic children. This study examined the effects of 1,25(OH)2D3 infusions on serum and skeletal parameters in normal and mutant rats of op stock. We also examined vitamin D receptor expression and binding in bone cells from op normal and mutant animals. Four-week-old normal and mutant rats were infused either with propylene glycol (used as controls) or with 12.5-125 ng of 1,25(OH)2D3/d using osmotic minipumps implanted subcutaneously for 1 wk. Sera were analyzed for calcium, phosphorus, and 1,25(OH)2D3 levels. Histomorphometric analyses of proximal tibiae from treated normal (50 ng/d) and op mutant (125 ng/d) rats and their vehicle-infused controls were performed. Normal animals infused with 1,25(OH)2D3 exhibited a dose-dependent increase in serum calcium levels. Histomorphometric analyses of metaphyseal bone within the primary spongiosae region showed that 1,25(OH)2D3 increased osteoclast number with a reduction in osteoblast surface associated with a decrease in growth plate cartilage thickness. However, similar analyses on secondary spongiosae showed a decrease in osteoclast number and surface associated with an anabolic response. Op mutants infused with 1,25(OH)2D3 did not exhibit any change in serum calcium levels or histomorphometric parameters related to growth plate cartilage and metaphyseal bone compared with mutant controls. Vitamin D mRNA and protein levels were increased twoto threefold in op mutants compared to age-matched normal rats. However, binding affinity of 1,25(OH)2D3 to its receptor was similar between op mutant and normal animals. High dose calcitriol therapy, under the conditions and period of treatment used in this study, failed to stimulate bone turnover in op rats, suggesting that they are resistant to the skeletal effects of 1,25(OH)2D3. The failure of osteoclast activation in response to 1,25(OH)2D3 treatment may be associated with osteoblast incompetence in this mutation.

Sutures, growth plates and the craniofacial base--experimental studies in the toothless (tl-osteopetrotic) rat

The craniofacial skeleton develops from a base in which coordinated growth at sutures and growth centres assures the development of normal form. In this report we describe features of retarded postnatal craniofacial development in the osteopetrotic mutation, toothless (tl), in the rat in which bone growth in both the nasal area and the cranial base is reduced, suggesting that the mutation affects bone formation in sutures and growth plates. We began a systematic search for potential mechanisms by analysing the expression in time and intensity of RNA coding for collagens type I (Col I) and type III (Col III) analysed by in situ hybridisation of cells in the premaxillary-maxillary suture (PMMS). In the centre of the PMMS of tl rats, cells expressing Col I and Col III appeared later than in normal littermates and exhibited lower signal. During osteoblast recruitment from the suture centre into the bone domains, Col III RNA expression is switched off. Osteoblasts expressing Col I in abundance, but no Col III, appeared in the flanking bone regions of tl rats later than in normal littermates. It is proposed that the tl mutation restricts the number of available osteoblast progenitor cells, and that the shortage of these cells affects bone growth in the PMMS and in the cranial base. Additional analyses are needed to test this hypothesis and to understand the developmental dynamics in the cranial base.

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.

Cellular and molecular strategies for studying the regulation of bone resorption using the toothless (osteopetrotic) mutation in the rat

The division of labor among cells of the skeleton is distinct and diverse and the regulation of these cells is interdependent. Osteoclasts are the cellular source of bone resorption and signals for their development and activation come, at least in part, from bone and other cells in the local environment. Studies of isolated cells have identified some factors in the developmental cascade of osteoclasts but there is little understanding of the sequence and local concentrations, not to mention other factors, needed for both the development of competent osteoclasts and for coordinated bone resorption. We review the skeletal biology of one osteopetrotic mutation in the rat, toothless, in which bone resorption is severely reduced because of a failure in the development and function of osteoclasts. Furthermore, we review the advantages and limitations of a relatively new method, differential display of mRNA (DD), that identifies differences in gene expression in two or more populations of cells. We present a strategy and preliminary data for the application of DD to this mutation. We propose that application of this method to these and other skeletal diseases, with the appropriate controls and confirmations, will provide data about pathogenetic pathways and has a high probability for identifying new regulators of skeletal development and turnover.

The role of the T-lymphocyte in estrogen deficiency osteopenia

Our laboratory has previously demonstrated that the T-lymphocyte is critical in the development of cyclosporin A-induced osteopenia in the rat model. A similar state of osteopenia is induced by estrogen depletion in the ovariectomized (OVX) rat, which is the animal model of postmenopausal bone loss. However, the role of the immune system, and particularly the T-lymphocyte, in estrogen deplete osteopenia has not been elucidated. We used the Rowett athymic nude rat as our model of T-lymphocyte deficiency. In this study, the experimental rats were divided into four groups as follows: (1) sham-operated Rowett heterozygous (rnu/+) euthymic rats (control group); (2) OVX Rowett heterozygous (rnu/+) euthymic rats; (3) sham-operated Rowett homozygous (rnu/rnu) athymic nude rats, which are T-lymphocyte deficient; and (4) ovariectomized Rowett homozygous (rnu/rnu) rats. Rats were weighed, and venous blood was taken in weeks 2, 4, and 6 for determination of serum osteocalcin. Serum 1,25-dihydroxyvitamin D (1,25(OH)2D) was determined on the day of sacrifice. Following sacrifice, histomorphometry was performed on double-labeled proximal tibial metaphyses. Flow cytometric analysis of splenic mononu-clear cell isolates stained for OX19-positive (CD5) T-lymphocytes was performed. T-lymphocyte analysis revealed significant reductions in both athymic nude groups, while OVX euthymic rats demonstrated a diminished number of T-cells relative to their sham-operated counterparts. Histomorphometric data indicated that both OVX groups exhibited a significant loss of trabecular volume, with associated increases in indices for bone formation and resorption, with resorption likely outstripping formation, resulting in osteopenia. Serum osteocalcin was significantly elevated in the ovariectomized euthymic group throughout the experimental period compared with the control group (p < 0.01); it was elevated in the ovariectomized athymic group on week 4 only (p < 0.01 vs. control). It appears that the T-lymphocyte may not be an essential component in the pathogenesis of estrogen deficiency osteopenia. The contribution of circulating T-lymphocytes as well as other T-lymphocyte-rich organs needs to be explored further.

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.

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.

Expression of the vitamin D receptor in the intestine and kidney of osteopetrotic rats

Osteopetrosis describes a heterogeneous group of metabolic bone disorders characterized by a generalized skeletal sclerosis. Because reduced bone resorption coexists with elevated plasma levels of 1,25-dihydroxyvitamin D [1,25-(OH)2D] in several osteopetrotic animals and children, skeletal resistance to this hormone has been proposed. In some mutations, such as the osteopetrotic (op) rat, the inability of 1,25-(OH)2D to elicit a skeletal response has been demonstrated. It is not known whether this resistance is localized to the skeleton or involves all target tissues. This study examined vitamin D receptor (VDR) status in the intestine and kidney from op rats and their normal littermates from 2-8 weeks of age. Quantitation of unoccupied VDR levels by Scatchard analysis demonstrated a delayed pattern of VDR expression in the intestine of op rats compared with their normal littermates; unoccupied VDR levels were up-regulated in op mutants from 5-8 weeks. Western analysis of 6-week-old mutant and normal intestinal, chromatin-associated protein revealed that total VDR levels were consistently and significantly elevated in all of the mutants examined. In op kidney, VDR numbers did not change as a function of age and were significantly down-regulated from 2-6 weeks of age compared with age-matched normal littermates. VDR affinity was similar in age-matched mutant and normal rats in both the intestine and kidney. In summary, these data suggest that skeletal resistance to 1,25-(OH)2D in op mutants is not the result of a generalized receptor defect resulting in reduced numbers or affinity. Furthermore, the up-regulation of intestinal VDR observed in older (5- to 8-week-old) mutants may reflect a compensatory mechanism to help establish and maintain normal serum calcium and phosphorus levels.

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.

Regulation of mineral homeostasis in osteopetrotic (op) rats

The osteopetrotic (op) rat is a lethal mutation characterized by severe skeletal sclerosis resulting from reduced bone resorption. Although the skeletal manifestations have been studied extensively, little is known about mineral homeostasis in this mutation. This paucity of data prompted us to undertake this study quantitating circulating levels of calcium, phosphorus, 1,25-dihydroxyvitamin D [1,25(OH)2D] and parathyroid hormone (PTH) in op mutants and normal rats between 2 and 8 wk of age. Calcium and phosphorus levels were significantly lower in op mutants at younger ages; both parameters normalized by 6 wk. Serum levels of 1,25(OH)2D were markedly elevated in op rats at all ages and showed no signs of normalization. Serum PTH levels were also elevated at most ages, with the greatest increase occurring when op mutants were severely hypocalcemic. These results demonstrate that, in op mutants, changes in circulating PTH and calcium levels were interdependent; however, levels of 1,25(OH)2D did not change despite normalization of serum calcium and phosphorus. The latter deserves further investigation and supports the hypothesis of a localized (skeletal) resistance to 1,25(OH)2D.

Comparison of characteristics of Q beta replicase-amplified assay with competitive PCR assay for Chlamydia trachomatis

In order to study infections due to Chlamydia trachomatis, we have compared semiquantitative PCR and Q beta replicase-amplified assays for detection of this organism. The PCR assay was directed against the C. trachomatis 16S rRNA gene. Quantitation was accomplished by adding known amounts of a plasmid containing a truncated segment of the 16S rRNA gene target to chlamydia-containing samples and then amplifying with a common primer set. The Q beta replicase assay consisted of reversible target capture of C. trachomatis 16S rRNA, which was followed by amplification of an RNA detector probe in the presence of the enzyme Q beta replicase. In a clinical matrix, the lower limit of detection of both the PCR and Q beta replicase assays was five elementary bodies. The Q beta replicase and PCR assays were quantitative over 10,000- and 1,000-fold ranges of organisms, respectively. Analysis of the effects of endocervical matrix on amplification was accomplished by examining 94 endocervical specimens by each technique. Both assays detected five of six culture-confirmed specimens as well as three culture-negative specimens. PCR inhibitors were detected in 13 specimens. The Q beta replicase assay, in contrast, showed no evidence of sample inhibition. The Q beta replicase and PCR assays should allow quantitative investigation of infections due to C. trachomatis. In addition, because it targets highly labile RNA, the Q beta replicase assay may facilitate investigations into the role of active persisting infection in culture-negative inflammatory conditions.

Novel, ultrasensitive, Q-beta replicase-amplified hybridization assay for detection of Chlamydia trachomatis

A sensitive, nonisotopic hybridization assay termed "dual capture" is described. The assay rapidly and specifically detects very low levels of target nucleic acids and organisms. The assay is based on the principles of sandwich hybridization, reversible target capture, and Q-Beta replicase amplification. The assay can be completed in less than 4 h, and in the described model format, it detects Chlamydia trachomatis rRNA or rDNA. Up to 96 samples can be analyzed simultaneously. The assay employs two types of probes: a test-specific capture probe, which mediates the cycling of the target probe complex on and off derivatized magnetic beads, and a replicatable RNA detector molecule containing a sequence complementary to and adjacent to the capture probe site on the target. Following reversible target capture, detection of the signal is accomplished by replication of the detector molecule by Q-Beta replicase in the presence of propidium iodide. A specific assay signal can be detected from as few as 1,000 molecules above the background. In a limited study of 94 urogenital samples the assay detected five of the six culture-positive samples and did not detect the C. trachomatis target in 85 of the 88 culture-negative samples.

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)

A defect in the inflammation-primed macrophage-activation cascade in osteopetrotic rats

Macrophages were activated by administration of lysophosphatidylcholine (lyso-Pc) or dodecylglycerol (DDG) to wild-type rats but not in osteopetrotic (op) mutant rats. In vitro treatment of wild-type rat peritoneal cells with lyso-Pc or DDG efficiently activated macrophages whereas treatment of op mutant rat peritoneal cells with lyso-Pc or DDG did not activate macrophages. The inflammation-primed macrophage activation cascade in rats requires participation of B lymphocytes and vitamin D binding protein (DBP). Lyso-Pc-inducible beta-galactosidase of wild-type rat B lymphocytes can convert DBP to the macrophage-activating factor (MAF), whereas B lymphocytes of the op mutant rats were shown to be deficient in lyso-Pc-inducible beta-galactosidase. DBP is conserved among mammalian species. Treatment of human DBP (Gc1 protein) with commercial glycosidases yields an extremely high titrated MAF as assayed on mouse and rat macrophages. Because the enzymatically generated MAF (GcMAF) bypasses the role of lymphocytes in macrophage activation, the op mutant rat macrophages were efficiently activated by administration of a small quantity (100 pg/rat) of GcMAF. Likewise, in vitro treatment of op rat peritoneal cells with as little as 40 pg GcMAF/ml activated macrophages.

A defect in the inflammation-primed macrophage-activation cascade in osteopetrotic rats

Macrophages were activated by administration of lysophosphatidylcholine (lyso-Pc) or dodecylglycerol (DDG) to wild-type rats but not in osteopetrotic (op) mutant rats. In vitro treatment of wild-type rat peritoneal cells with lyso-Pc or DDG efficiently activated macrophages whereas treatment of op mutant rat peritoneal cells with lyso-Pc or DDG did not activate macrophages. The inflammation-primed macrophage activation cascade in rats requires participation of B lymphocytes and vitamin D binding protein (DBP). Lyso-Pc-inducible beta-galactosidase of wild-type rat B lymphocytes can convert DBP to the macrophage-activating factor (MAF), whereas B lymphocytes of the op mutant rats were shown to be deficient in lyso-Pc-inducible beta-galactosidase. DBP is conserved among mammalian species. Treatment of human DBP (Gc1 protein) with commercial glycosidases yields an extremely high titrated MAF as assayed on mouse and rat macrophages. Because the enzymatically generated MAF (GcMAF) bypasses the role of lymphocytes in macrophage activation, the op mutant rat macrophages were efficiently activated by administration of a small quantity (100 pg/rat) of GcMAF. Likewise, in vitro treatment of op rat peritoneal cells with as little as 40 pg GcMAF/ml activated macrophages.

Experimental studies of osteopetrosis in laboratory animals

Osteopetrosis is a metabolic bone disease characterized by a systemic increase in skeletal mass. It results from a defect in the production or function of osteoclasts and is inherited in nine genetically distinct osteopetrotic animal mutations and man. Studies of these mutations have revealed that osteopetrosis is a complex, heterogeneous disorder in its expression, etiology, and response to treatment by bone marrow transplantation or by hormone/growth factor therapy. These animal mutations have been valuable tools for probing the pathogenesis and treatment of osteopetrosis, and information obtained from these studies has been used clinically for the treatment of humans with osteopetrosis. In addition, studies of these mutations have contributed significantly to understanding normal bone cell biology, including the origin of the osteoclast and the significance of colony-stimulating factor-1 in osteoclast development. The resistance of some of these mutations to cure by stem cell transplantation and hormone therapy, coupled with similar observations and experiences in the human condition, indicates that these animal mutations will continue to serve important roles in the development of alternative therapies to treat resistant forms of the disease. These studies are bound to improve the understanding of normal bone biology by providing additional insights into the regulation of osteoclasts by osteoblasts and their products or by other elements of the skeletal microenvironment.

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.

Coexistence of reduced function of natural killer cells and osteoclasts in two distinct osteopetrotic mutations in the rat

Recent evidence suggesting that immune cells and their products (cytokines) play an important role in the regulation of skeletal development and function, particularly of the osteoclast, implies that immune cell dysfunction may be involved in the pathogenesis of certain skeletal disorders. The mammalian osteopetroses are a pathogenetically heterogeneous group of skeletal disorders characterized by skeletal sclerosis resulting from reduced osteoclast-mediated bone resorption. Using a 51Cr-release microcytotoxicity assay we demonstrated that splenic natural killer (NK) cell activity was significantly reduced in two distinctly different osteopetrotic mutations in the rat, osteopetrosis (op) and toothless (tl). To determine whether this reduction in NK cell-mediated cytotoxicity is caused by decreased cell number and/or function in these osteopetrotic mutants, we quantitated NK cells by analyzing mononuclear cell suspensions labeled for two-color fluorescence with OX8 and OX19 monoclonal antibodies in a fluorescence-activated cell sorter. Flow cytometry of these double-labeled cells revealed that the percentage of NK cells (OX8+/OX19- subset) in op and tl spleens was not significantly different from that of normal spleens. These results suggest that NK cells in these osteopetrotic mutants are functionally defective. Thus aberrations in osteoclast and NK cell function coexist in these mutations, and their developmental relationships deserve further study.

Effects of 1,25 dihydroxyvitamin D on osteoclast number and cytochemistry in normal and osteopetrotic (os) rabbits

Osteoclast-mediated bone resorption is increased in response to 1,25 dihydroxyvitamin D (1,25[OH]2D or calcitriol). Osteopetrosis is a metabolic bone disease characterized by defective, osteoclast-mediated bone resorption, which co-exists with elevated serum 1,25-(OH)2D levels in some osteopetrotic children and animals. We examined the effects of high doses of calcitriol on osteoclast number and cytochemistry in both normal and osteopetrotic (os) rabbits. Calcitriol was continuously infused at doses of 0.5, 2.5, or 25 micrograms/kg/day via subcutaneously implanted osmotic minipumps for a period of 7 days. Following treatment, the proximal tibial metaphyses were processed for histomorphometric and cytochemical analyses. Sections were stained for tartrate-resistant acid phosphatase (TrAP) or acid ATPase (TraATPase). Osteoclasts were significantly reduced in untreated os rabbits compared with age-matched normal littermates between birth and 3 weeks of age (41-46% of normal). Whereas most normal osteoclasts (85%) stained heavily for TrAP or TraATPase, less than half of os osteoclasts were heavily stained for these acid hydrolases. Infusions of 1,25(OH)2D resulted in elevations of osteoclast numbers in both normal and os rabbits, but the number of osteoclasts remained significantly lower in mutants than in normal littermates at any given dose. Calcitriol infusions also resulted in a significant increase in the percentage of os osteoclasts staining heavily for TrAP and TraATPase. These results suggest that in response to 1,25(OH)2D normal osteoclasts increase their production of acid hydrolases before increasing cell numbers and that, in spite of high levels of endogenous calcitriol, os rabbits can respond to exogenous 1,25(OH)2D as evidenced by increased osteoclast number and cytochemical staining, even though these osteoclasts fail to resorb the excess skeletal matrix.(ABSTRACT TRUNCATED AT 250 WORDS)

Defective osteoclast differentiation and function in the osteopetrotic (os) rabbit

We tested the ability of normal osteoclast progenitors found in neonatal liver and bone marrow to develop into functional osteoclasts when co-cultured with metatarsals from newborn osteopetrotic rabbits; the latter inherit an osteoclast incompetence resistant to cure by bone marrow transplantation. This system, developed by Burger and colleagues, has been shown to produce normal, functional osteoclasts when used with normal metatarsals. Our study tested the competence of the mutant skeletal microenvironment for differentiation of normal osteoclasts. Mutant and normal metatarsals were cultured alone or with normal liver, spleen, or bone marrow for up to 14 days. All normal cultures possessed a marrow cavity and contained numerous osteoclasts with cytochemical characteristics (tartrate-resistant acid phosphatase) of active cells. Mutant metatarsals co-cultured with normal spleen, liver, or bone marrow failed to develop a marrow cavity (evidence in itself of reduced bone resorption) and had osteoclasts reduced in both numbers and cytochemically detectable activity. Similar metatarsal cultures of an osteopetrotic rat mutation (incisors--absent) curable by bone-marrow transplantation exhibited marrow cavity development in mutant metatarsals co-cultured with normal spleen. These data suggest that the skeletal environment of osteopetrotic rabbits contains an inhibitor or lacks a promoter of osteoclast differentiation and function.

Relationship of abnormalities in dental and skeletal development in the osteopetrotic (os) rabbit

Osteopetrosis is a metabolic bone disease characterized by reduced bone resorption of heterogenous cause. The rabbit mutation is lethal and exhibits ultrastructural aberrations in osteoclasts and osteoblasts together with hypocalcemia, hypophosphatemia and failure to be cured by bone marrow transplantation. We have studied dental abnormalities in mutants from birth to 3 wk using radiographic, cytologic and autoradiographic methods. Radiographs show hypoplasia of most teeth in mutants. The maxillary incisor is smaller and more curved and the mandibular incisor thin and straight compared to normal littermates. The first 3 molars in both arches are unerupted and of distorted shape while the last 2 are less affected. Microscopically areas of ankylosis of mutant incisors and the first 3 molars were commonly encountered even at birth. Osteoclasts were numerous. Autoradiograms of 3H-proline incorporation showed strong periosteal and weak endosteal labeling of bone in both mutants and normal littermates. Dentin labeling in mutant incisors and molars was not less than that in normal rabbits and sites of ankylosis in mutants exhibited labeling. These data indicate that osteopetrotic rabbits exhibit major aberrations in shape and eruption of incisors and most molars and that these effects are not due to lack of dentin formation. Early ankylosis, perhaps secondary to congenital reduction of bone resorption, appears to be the major cause of the dental abnormalities which are less severe in the youngest (posterior) teeth.

Osteoclast biology in the osteopetrotic (op) rat

Osteopetrosis is a metabolic bone disease characterized by reduced bone resorption. From experimental studies of various osteopetrotic mutations has emerged the hypothesis that each is unique with respect to mechanisms whereby osteoclast development and/or function are reduced. The osteopetrotic (op) mutation in the rat was discovered in Fatty/ORL stock over a decade ago. The paucity of data about osteoclast biology in this mutation prompted this study of cytological, cytochemical, and ultrastructural features of osteoclasts. In op rats, osteoclasts are significantly reduced in number, but are larger and more vacuolated than in normal littermates. Mutant osteoclasts can form ruffled borders and clear zones, but their ability to fragment and excavate bone surfaces is greatly impaired. Cytoplasmic vacuoles in op osteoclasts are randomly distributed and greatly enlarged, and they stain weakly for two cytochemical characteristics of osteoclasts, tartrate-resistant acid phosphatase and acid ATPase. These findings suggest that an abnormality in the lysosomal/vacuolar system, an important component of the resorptive mechanism, may be involved in the interception of osteoclast function in this mutation.

Treatment of congenital osteopetrosis in the rabbit with high-dose 1,25-dihydroxyvitamin D

Osteopetrosis is a congenital metabolic bone disease characterized by skeletal sclerosis resulting from defective osteoclast-mediated bone resorption. Osteopetrosis has been described in several animal species (mouse, rat, and rabbit) and in children. Bone marrow transplantation, originally shown to reverse the skeletal sclerosis in some animal mutations, has been effective in curing osteopetrosis in some children. Unfortunately, not all children with osteopetrosis are candidates for or respond to bone marrow transplantation. Recent studies have shown that several animal mutations and some children inheriting osteopetrosis have significantly elevated serum levels of 1,25-(OH)2D. Based on the possibility that there may be a resistance to 1,25-(OH)2D, high-dose calcitriol therapy has been used to treat some children and stimulated some parameters of resorption. In this study, we have examined the effects of high-dose calcitriol therapy on various serum and skeletal parameters in the osteopetrotic rabbit. Mutant rabbits and normal littermates were given continuous infusions of calcitriol via subcutaneously implanted osmotic minipumps for 2 weeks at a dose of 0.5, 2.5, or 25 micrograms/kg/per day. Untreated mutant rabbits are hypocalcemic and hypophosphatemic in the presence of elevated serum 1,25-(OH)2 levels in comparison with their normal littermates. Calcitriol infusions resulted in dose-dependent increases in circulating 1,25-(OH)2D levels in both normal and mutant rabbits. However, evaluation of other serum parameters and the skeletal response demonstrated significant differences between osteopetrotic and normal rabbits. At the highest dose, normal animals rapidly became hypercalcemic and osteoporotic, accompanied by weight loss and a failure to thrive; mutants remained hypocalcemic and osteopetrotic but did not exhibit the deleterious physical effects seen in treated normal littermates. Although the number of osteoclasts increased in both mutants and normals, osteoclast phenotype in the former remained abnormal. These data indicate that although very high levels of circulating 1,25-(OH)2D were achieved in osteopetrotic mutants, activation of osteoclast-mediated bone resorption with subsequent improvement of skeletal sclerosis was not observed.

Calcitonin inhibits accumulation of cyclic AMP in stimulated peritoneal macrophages from normal rats but not from osteopetrotic (incisors-absent) littermates

Macrophages and osteoclasts derive from related cell lines. In osteopetrotic mutants the function of osteoclasts is greatly reduced compared to that in normal animals or children and macrophage function is variably affected depending upon the mutation. To further explore macrophage function in osteopetrosis we examined the regulation of cyclic AMP production in macrophages from mutants and normal littermates of the osteopetrotic stock incisors-absent (ia) in the rat. Surface stimulation by latex particles of elicited peritoneal macrophages from normal or osteopetrotic (ia) mutant rats caused an identical increase in the accumulation of cyclic AMP. This effect was inhibited in normal animals by coincubation of macrophages with calcitonin (CT) but this inhibition was either absent or less marked in macrophages from mutant littermates. In contrast to human monocytes preincubation of rat macrophages with pertussis toxin did not relieve this inhibition. This implies that rat peritoneal macrophages respond to CT by a different mechanism. These results demonstrate altered macrophage function in osteopetrotic animals and may be functionally related to the reduced CT binding previously described in ia osteoclasts. Furthermore, the coexistence of reduced function of macrophages and osteoclasts in the ia mutation suggests that macrophages and osteoclasts share a common progenitor.

Bone resorption in osteosclerotic mice is reduced in vitro

Osteopetrosis in mammals results from a congenital reduction in bone resorption. Calvarial organ cultures were used to measure bone resorption in osteosclerotic (oc/oc) mice and their normal littermates. Measurements of cell-mediated resorption indicate that baseline isotope release by mutant calvariae was only 57% of that observed in normal littermates and isotope release by mutant bone in the presence of parathyroid hormone (PTH) was only 60% of that in normal controls. However, the response of oc calvariae to PTH was not different from normal bone when considered with respect to baseline resorption. These data indicate that bone resorption in oc mice is reduced in both its basal level and in response to PTH and suggest that oc mice are unable to establish normal baseline resorption which may in turn compromise their responsiveness to PTH.

Skeletal biology in the toothless (osteopetrotic) rat

The toothless (tl) rat is a nonlethal osteopetrotic mutation characterized by the presence of few osteoclasts and the failure to be cured by bone-marrow transplantation. We examined the skeletal biology of tl rats and normal littermates up to 6 weeks after birth. Osteoclasts in tl rats were small, reduced 25-fold in number, and had greatly reduced concentrations of acid hydrolases. Bone shape internally and externally reflected reduced bone resorption, and tl rats were hypophosphatemic and mildly hypocalcemic at 2 weeks. These data indicate that the basic defect in tl rats is one of differentiation of osteoclasts and, coupled with the observation that normal bone-marrow cells cannot develop into osteoclasts in the tl skeleton, suggest that the defect lies in the skeletal micro-environment.

Bone cell biology: the regulation of development, structure, and function in the skeleton

Bone cells compose a population of cells of heterogeneous origin but restricted function with respect to matrix formation, mineralization, and resorption. The local, mesenchymal origin of the cells which form the skeleton contrasts with their extraskeletal, hemopoietic relatives under which bone resorption takes place. However, the functions of these two diverse populations are remarkably related and interdependent. Bone cell regulation, presently in its infancy, is a complicated cascade involving a plethora of local and systemic factors, including some components of the skeletal matrices and other organ systems. Thus, any understanding of bone cell regulation is a key ingredient in understanding not only the development, maintenance, and repair of the skeleton but also the prevention and treatment of skeletal disorders.

Osteopetrosis in the toothless rat: failure of osteoclast differentiation and function

Toothless is an osteopetrotic mutation in the rat characterized by reduced bone resorption, few osteoclasts and failure to be cured by bone marrow transplants from normal littermates. The latter could be due to the production of bone that cannot recruit or activate normal osteoclasts or their precursors. We tested this hypothesis using a metatarsal organ culture system in which metatarsals are cultured with various tissues which act as sources of osteoclast precursors. Mutant metatarsals cultured alone or with mutant tissue had numerous large mononuclear cells, few osteoclasts, no marrow spaces and no evidence of bone resorption. Culture of mutant metatarsals with normal liver or spleen caused no changes in osteoclast number or bone resorption. Cultures of normal metatarsals had large osteoclasts, bone resorption and marrow spaces. Histochemical analyses demonstrated that strong tartrate-resistant acid phosphatase (TRAP) staining was present in all osteoclasts and in some mononuclear cells in normal metatarsals. Mutant metatarsals before or after culture contained no TRAP-positive osteoclasts and an occasional, weakly staining mononuclear cell only when co-cultured with normal spleen which contained TRAP-positive mononuclear cells. We interpret these data to mean that the mutant skeletal environment is unable to support the development and differentiation of normal osteoclasts. These observations are consistent with the failure of toothless rats to be cured by bone marrow transplants from normal littermates.

Ultrastructure of the giant cell infiltrate of subcutaneously implanted bone particles in rats and mice

The giant cells of soft tissues and those of mineralized tissues (osteoclasts) have distinctly different cell surface receptors and ultrastructural characteristics. Recently, the removal of dead bone particles in a subcutaneous environment has been described as a prototype of bone resorption, and a major issue is whether the giant cells that surround these ectopic bone implants and the processes involved in the disruption of bone surfaces are the same as those in the skeleton. We have compared the cytology and ultrastructure of giant cells recruited to subcutaneously implanted isogeneic bone particles with similar features of osteoclasts in metaphyseal bone of young normal rats and mice. Giant cells on surfaces of bone particles 2, 3, and 4 weeks after implantation were multinucleated, had a homogeneous, nonvacuolated cytoplasm, and had a bone surface interface unremarkable by light microscopy. In a few cells randomly distributed, small cytoplasmic vacuoles were present and large vacuoles were noted next to the bone surface at high magnification. By transmission electron microscopy, folded membrane configurations forming extensive interdigitations with adjacent cells were prominent features on most surfaces of giant cells. In instances where these interdigitations abutted bone surfaces, configuration resembling a ruffled border were noted, but these regions were always part of two different cells when examined at lower magnification or in serial sections. Breakdown of bone particles appeared to be by phagocytosis of small pieces and subsequent intracellular digestion in electron-dense cytoplasmic vacuoles. Osteoclasts from these same young animals were smaller with fewer nuclei, had cytoplasmic vacuoles concentrated next to bone surfaces, and had characteristic ruffled borders and clear zones. These results confirm those of others that native osteoclasts and multinucleated giant cells on dead bone particles are distinctly different with respect to both ultrastructure and mechanism of disruption of bone surfaces.

Processing of concanavalin A-receptor complexes by rat osteoclasts in vitro

The osteoclast is a large multinucleate cell that is widely accepted as the primary effector cell responsible for normal bone resorption. In a previous study, we demonstrated that concanavalin A (con A) has a dose-dependent biphasic effect on the bone-resorbing capacity of osteoclasts, using a 45Ca bone-organ culture system; bone resorption was stimulated at low concentrations and inhibited at high concentrations. The mitogenic property of con A in lymphocyte cultures is well documented; therefore con A has been used extensively to study the manner in which lymphocytes and other mononuclear cells process the cell-bound lectin. In this study, we have investigated the processing of con A-receptor complexes by osteoclasts in culture, using con A-FITC to evaluate the redistribution of cell-bound con A via epifluorescence microscopy and using con A-ferritin to determine whether the lectin receptor complexes are internalized. The osteoclasts were obtained from the long bones of newborn rats and allowed to attach to glass coverslips at 37 degrees C. Following attachment, the nonadherent cells were removed by rinsing. The adherent osteoclasts were preincubated in 50 micrograms/ml con A-FITC or con A-ferritin at 4 degrees C for 10 min, washed to remove unbound con A, and incubated at 37 degrees C for 15 or 30 min in the absence of con A. Positive controls were fixed immediately after preincubation at 4 degrees C; negative controls were preincubated in con A-FITC and alpha-methyl mannoside, the haptenic inhibitor of con A binding. The results demonstrate that redistribution and endocytosis of con A-receptor complexes occurs within 30 min. These findings confirm the hypothesis that cell-bound con A can alter the structure and activity of osteoclast membrane components in a manner similar to that observed in mononuclear cell cultures. The internalization of con A may be important in altering osteoclastic activity by mediating intracellular mechanisms involved in the bone-resorbing process.

Reactions of the UVRABC excision nuclease with DNA damaged by diamminedichloroplatinum(II)

Mutants of Escherichia coli, which are blocked in excision repair (uvrA6, uvrB5, or uvrC34) are exceptionally sensitive to the antitumor drug cis-Pt(II)(NH3)2Cl2 (cis-DDP) but not the trans isomer. Plasmid DNA, damaged by either the cis or trans compound and treated with the UVRABC excision nuclease was cut as shown by conversion of supercoiled DNA to relaxed forms. All three protein products of the uvrA, uvrB, and uvrC genes were required for incision. End-labeled fragments damaged with cis-DDP and reacted with the UVRABC nuclease were cut at the 8th phosphodiester bond 5' and at the 4th phosphodiester bond 3' to adjacent GG's. DNA treated with trans-DDP was not cut appreciably at adjacent GG's by the repair enzyme as subsequent analysis of reaction products after enzyme digestion gave a pattern similar to those obtained with control untreated fragments. The results indicate that the UVRABC nuclease may promote cell survival by the removal of adjacent GG's which are crosslinked by cis-Pt(II)(NH3)2Cl2.

The effects of lectins on the interaction between macrophages and bone in vitro. A morphological and functional study

Recent studies have demonstrated that the attachment of elicited rat macrophages to bone is mediated by specific saccharides located on the cell and/or bone surfaces. We have used a macrophage-bone culture system to study the effects of two lectins, concanavalin A (con A) and soybean agglutinin (SBA), on the morphology of macrophage attachment to a devitalized bone surface and subsequent functional activity. Macrophages were obtained from 3- to 4-week-old rats by peritoneal lavage and the adherent pool was used to prepare cell suspensions. Con A-treated, SBA-treated or control cell suspensions were aliquoted onto the endocranial surface of devitalized rat calvariae. The cells were allowed to attach for 1 h at 37 degrees C, after which, the bone samples were removed from culture and prepared for scanning electron microscopy (SEM). The morphology of con A-treated macrophages attached to bone was markedly different from that of control or SBA-treated cells. Con A altered the attachment and subsequent spreading of macrophages on bone as visualized by SEM. Furthermore, the number of con A-treated cells that attached to bone and the average surface area of cell membrane apposed to the matrix was significantly different from that of control or SBA-treated cells. A 45Ca bone-release assay was performed to evaluate the functional significance of the morphological findings. Lectin-treated or control cell suspensions were allowed to attach to the endocranial surface of 45Ca pre-labeled calvariae for 1 h. Following attachment, the samples were cultured for 72 h.(ABSTRACT TRUNCATED AT 250 WORDS)