αSMA Osteoprogenitor Cells Contribute to the Increase in Osteoblast Numbers in Response to Mechanical Loading.

Bone is a dynamic tissue that site-specifically adapts to the load that it experiences. In response to increasing load, the cortical bone area is increased, mainly through enhanced periosteal bone formation. This increase in area is associated with an increase in the number of bone-forming osteoblasts; however, the origin of the cells involved remains unclear. Alpha-smooth muscle actin (αSMA) is a marker of early osteoprogenitor cells in the periosteum, and we hypothesized that the new osteoblasts that are activated by loading could originate from αSMA-expressing cells. Therefore, we used an in vivo fate-mapping approach in an established axial loading model to investigate the role of αSMA-expressing cells in the load-induced increase in osteoblasts. Histomorphometric analysis was applied to measure the number of cells of different origin on the periosteal surface in the most load-responsive region of the mouse tibia. A single loading session failed to increase the number of periosteal αSMA-expressing cells and osteoblasts. However, in response to multiple episodes of loading, the caudal, but not the cranial, periosteal surface was lined with an increased number of osteoblasts originating from αSMA-expressing cells 5 days after the initial loading session. The proportion of osteoblasts derived from αSMA-labeled progenitors increased by 70% (p < 0.05), and the proportion of αSMA-labeled cells that had differentiated into osteoblasts was doubled. We conclude that αSMA-expressing osteoprogenitors can differentiate and contribute to the increase in periosteal osteoblasts induced by mechanical loading in a site-specific manner.

FGF2 Enhances Odontoblast Differentiation by αSMA+ Progenitors In Vivo.

The goal of this study was to examine the effects of early and limited exposure of perivascular cells expressing α (αSMA) to fibroblast growth factor 2 (FGF2) in vivo. We performed in vivo fate mapping by inducible Cre-loxP and experimental pulp injury in molars to induce reparative dentinogenesis. Our results demonstrate that early delivery of exogenous FGF2 to exposed pulp led to proliferative expansion of αSMA-tdTomato+ cells and their accelerated differentiation into odontoblasts. In vivo lineage-tracing experiments showed that the calcified bridge/reparative dentin in FGF2-treated pulps were lined with an increased number of Dspp+ odontoblasts and devoid of BSP+ osteoblasts. The increased number of odontoblasts derived from αSMA-tdTomato+ cells and the formation of reparative dentin devoid of osteoblasts provide in vivo evidence for the stimulatory effects of FGF signaling on odontoblast differentiation from early progenitors in dental pulp.

Mineral trioxide aggregate improves healing response of periodontal tissue to injury in mice.

BACKGROUND AND OBJECTIVE: Mineral trioxide aggregate (MTA) is a biomaterial used in endodontic procedures as it exerts beneficial effects on regenerative processes. In this study, we evaluate the effect of MTA on healing of periodontal ligament (PDL) and surrounding tissue, following injury, in a transgenic mouse model and on the differentiation of murine mesenchymal progenitor cells in vitro. MATERIAL AND METHODS: We used an inducible Cre-loxP in vivo fate mapping approach to examine the effects of MTA on the contributions of descendants of cells expressing the αSMA-CreERT2 transgene (SMA9+ ) to the PDL and alveolar bone after experimental injury to the root furcation on the maxillary first molars. Col2.3GFP was used as a marker to identify mature osteoblasts, cementoblasts and PDL fibroblasts. The effects of MTA were examined 2, 17 and 30 days after injury and compared histologically with sealing using an adhesive system. The effects of two dilutions of medium conditioned with MTA on proliferation and differentiation of mesenchymal progenitor cells derived from bone marrow (BMSC) and periodontal ligament (PDLC) in vitro were examined using the PrestoBlue viability assay, alkaline phosphatase and Von Kossa staining. The expression of markers of differentiation was assessed using real-time PCR. RESULTS: Histological analyses showed better repair in teeth restored with MTA, as shown by greater expansion of SMA9+ progenitor cells and Col2.3GFP+ osteoblasts compared with control teeth. We also observed a positive effect on differentiation of SMA9+ progenitors into osteoblasts and cementoblasts in the apical region distant from the site of injury. The in vitro data showed that MTA-conditioned medium reduced cell viability and osteogenic differentiation in both PDLC and BMSC, indicated by reduced von Kossa staining and lower expression of osteocalcin and bone sialoprotein. In addition, cultures grown in the presence of MTA had marked decreases in SMA9+ and Col2.3GFP+ areas as compared with osteogenic medium, confirming reduced osteogenesis. CONCLUSION: MTA promotes regeneration of injured PDL and alveolar bone, reflected as contribution of progenitors (SMA9+ cells) into osteoblasts (Col2.3GFP+ cells). In vitro, MTA-conditioned medium fails to promote osteogenic differentiation of both PDLC and BMSC.

FGF Signaling Prevents the Terminal Differentiation of Odontoblasts.

Members of the fibroblast growth factor (FGF) family play essential and important roles in primary and reparative dentinogenesis, with conflicting results regarding their effects on odontoblast differentiation. Our recent studies showed that the effects of FGF2 on cells in odontoblast lineage were stage-specific and depended on the stage of cell maturity. Continuous exposure of pulp cells to FGF2 inhibited odontoblast differentiation, whereas early and limited exposure of pulp cells to FGF2 resulted in marked increases in odontoblast differentiation. The purpose of this study was to evaluate the cellular and molecular mechanisms regulating the inhibitory effects of FGF2 on odontoblast differentiation. To do so, we examined the effects of the addition of FGF2 during the differentiation/mineralization phase of the in vitro growth of pulp cultures derived from a series of green fluorescent protein reporter transgenic mice that display stage-specific activation of transgenes during odontoblast differentiation. Our results showed that this treatment first stimulated the differentiation of remaining progenitors in pulp cultures into functional odontoblasts but prevented their differentiation into mature odontoblasts. In addition, this treatment inhibited expression of markers of osteogenesis. Furthermore, we demonstrated that the inhibitory effects of FGF2 on odontoblast differentiation were mediated through activation of FGFR/MEK/Erk1/2 signaling and downregulation of bone morphogenetic protein signaling, with negative and positive roles in the expression of Dmp1 and Dspp, respectively, during the advanced stage of odontoblast differentiation.

αSMA-Expressing Perivascular Cells Represent Dental Pulp Progenitors In Vivo.

The goal of this study was to examine the contribution of perivascular cells to odontoblasts during the development, growth, and repair of dentin using mouse molars as a model. We used an inducible, Cre-loxP in vivo fate-mapping approach to examine the contributions of the descendants of cells expressing the αSMA-CreERT2 transgene to the odontoblast lineage. In vivo lineage-tracing experiments in molars showed the contribution of αSMA-tdTomato+ cells to a small number of newly formed odontoblasts during primary dentinogenesis. Using an experimental pulp exposure model in molars to induce reparative dentinogenesis, we demonstrate the contribution of αSMA-tdTomato+ cells to cells secreting reparative dentin. Our results demonstrate that αSMA-tdTomato+ cells differentiated into Col2.3-GFP+ cells composed of both Dspp+ odontoblasts and Bsp+ osteoblasts. Our findings identify a population of mesenchymal progenitor cells capable of giving rise to a second generation of odontoblasts during reparative dentinogenesis. This population also makes a small contribution to odontoblasts during primary dentinogenesis.

Increased chemotaxis and activity of circulatory myeloid progenitor cells may contribute to enhanced osteoclastogenesis and bone loss in the C57BL/6 mouse model of collagen-induced arthritis.

Our study aimed to determine the functional activity of different osteoclast progenitor (OCP) subpopulations and signals important for their migration to bone lesions, causing local and systemic bone resorption during the course of collagen-induced arthritis in C57BL/6 mice. Arthritis was induced with chicken type II collagen (CII), and assessed by clinical scoring and detection of anti-CII antibodies. We observed decreased trabecular bone volume of axial and appendicular skeleton by histomorphometry and micro-computed tomography as well as decreased bone formation and increased bone resorption rate in arthritic mice in vivo. In the affected joints, bone loss was accompanied with severe osteitis and bone marrow hypercellularity, coinciding with the areas of active osteoclasts and bone erosions. Flow cytometry analysis showed increased frequency of putative OCP cells (CD3- B220- NK1.1- CD11b-/lo CD117+ CD115+ for bone marrow and CD3- B220- NK1.1- CD11b+ CD115+ Gr-1+ for peripheral haematopoietic tissues), which exhibited enhanced differentiation potential in vitro. Moreover, the total CD11b+ population was expanded in arthritic mice as well as CD11b+ F4/80+ macrophage, CD11b+ NK1.1+ natural killer cell and CD11b+ CD11c+ myeloid dendritic cell populations in both bone marrow and peripheral blood. In addition, arthritic mice had increased expression of tumour necrosis factor-α, interleukin-6, CC chemokine ligand-2 (Ccl2) and Ccl5, with increased migration and differentiation of circulatory OCPs in response to CCL2 and, particularly, CCL5 signals. Our study characterized the frequency and functional properties of OCPs under inflammatory conditions associated with arthritis, which may help to clarify crucial molecular signals provided by immune cells to mediate systemically enhanced osteoresorption.

Gene-expression analysis of cementoblasts and osteoblasts.

BACKGROUND AND OBJECTIVE: Cementum and bone are similar mineralized tissues, but cementum accumulates much more slowly than bone, does not have vasculature or innervation and does not undergo remodeling. Despite these differences, there are no well-established markers to distinguish cementoblasts from other mature mineralizing cells such as osteoblasts and odontoblasts. The purpose of this study was to assess differences in gene expression between cementoblasts and osteoblasts using gene profiling of cell populations isolated directly from osteocalcin-green fluorescent protein (OC-GFP) transgenic mice. MATERIAL AND METHODS: OC-GFP reporter mice were used as they show labeling of cementoblasts, osteoblasts and odontoblasts, but not of periodontal ligament fibroblasts, within the periodontium. We sorted cells digested from the molar root surface to isolate OC-GFP(+) cementoblasts. Osteoblasts were isolated from calvarial digests. Microarray analysis was performed, and selected results were confirmed by real-time PCR and immunostaining or in situ hybridization. RESULTS: Microarray analysis identified 95 genes that were expressed at least two-fold higher in cementoblasts than in osteoblasts. Our analysis indicated that the Wnt signaling pathway was differentially regulated, as were genes related to skeletal development. Real-time PCR confirmed that expression of the Wnt inhibitors Wnt inhibitory factor 1 (Wif1) and secreted frizzled-related protein 1 (Sfrp1) was elevated in cementoblasts compared with osteoblasts, and Wif1 expression was localized to the apical root region. In addition, the transcription factor BARX homeobox 1 (Barx1) was expressed at higher levels in cementoblasts, and immunohistochemistry indicated that BARX1 was expressed in apical cementoblasts and cementocytes, but not in osteoblasts or odontoblasts. CONCLUSION: The OC-GFP mouse provides a good model for selectively isolating cementoblasts, and allowed for identification of differentially expressed genes between cementoblasts and osteoblasts.

Enhanced Dentinogenesis of Pulp Progenitors by Early Exposure to FGF2.

Members of the fibroblast growth factor (FGF) family play essential and important roles in primary and reparative dentinogenesis. Although there appears to be a general agreement on the effects of FGF signaling on the proliferation of pulp cells, there are conflicting results regarding its effects on odontoblast differentiation. We recently examined the effects of continuous exposure of dental pulp cells to FGF2 and showed that the effects of FGF2 on differentiation of progenitor cells into odontoblasts were stage specific and dependent on the stage of cell maturity. The purpose of this study was to gain further insight into cellular and molecular mechanisms regulating the stimulatory effects of FGF2 on odontoblast differentiation. To do so, we examined the effects of early and limited exposure of pulp cells from a series of green fluorescent protein (GFP) reporter transgenic mice that display stage-specific activation of transgenes during odontoblast differentiation to FGF2. Our results showed that early and limited exposure of pulp cells to FGF2 did not have significant effects on the extent of mineralization but induced significant increases in the expression of Dmp1 and Dspp and the number of DMP1-GFP(+) and DSPP-Cerulean(+) odontoblasts. Our results also showed that the stimulatory effects of FGF2 on odontoblast differentiation were mediated through FGFR/MEK/Erk1/2 signaling, increases in Bmp2, and activation of the BMP/BMPR signaling pathway. These observations show that early and limited exposure of pulp cells to FGF2 alone promotes odontoblast differentiation and provides critical insight for applications of FGF2 in dentin regeneration.

In vivo identification of periodontal progenitor cells.

The periodontal ligament contains progenitor cells; however, their identity and differentiation potential in vivo remain poorly characterized. Previous results have suggested that periodontal tissue progenitors reside in perivascular areas. Therefore, we utilized a lineage-tracing approach to identify and track periodontal progenitor cells from the perivascular region in vivo. We used an alpha-smooth muscle actin (αSMA) promoter-driven and tamoxifen-inducible Cre system (αSMACreERT2) that, in combination with a reporter mouse line (Ai9), permanently labels a cell population, termed 'SMA9'. To trace the differentiation of SMA9-labeled cells into osteoblasts/cementoblasts, we utilized a Col2.3GFP transgene, while expression of Scleraxis-GFP was used to follow differentiation into periodontal ligament fibroblasts during normal tissue formation and remodeling following injury. In uninjured three-week-old SMA9 mice, tamoxifen labeled a small population of cells in the periodontal ligament that expanded over time, particularly in the apical region of the root. By 17 days and 7 weeks after labeling, some SMA9-labeled cells expressed markers indicating differentiation into mature lineages, including cementocytes. Following injury, SMA9 cells expanded, and differentiated into cementoblasts, osteoblasts, and periodontal ligament fibroblasts. SMA9-labeled cells represent a source of progenitors that can give rise to mature osteoblasts, cementoblasts, and fibroblasts within the periodontium.

Bone-specific overexpression of NPY modulates osteogenesis.

OBJECTIVES: Neuropeptide Y (NPY) is a peptide involved in the regulation of appetite and energy homeostasis. Genetic data indicates that NPY decreases bone formation via central and peripheral activities. NPY is produced by various cell types including osteocytes and osteoblasts and there is evidence suggesting that peripheral NPY is important for regulation of bone formation. We sought to investigate the role of bone-derived NPY in bone metabolism. METHODS: We generated a mouse where NPY was over-expressed specifically in mature osteoblasts and osteocytes (Col2.3NPY) and characterized the bone phenotype of these mice in vivo and in vitro. RESULTS: Trabecular and cortical bone volume was reduced in 3-month-old animals, however bone formation rate and osteoclast activity were not significantly changed. Calvarial osteoblast cultures from Col2.3NPY mice also showed reduced mineralization and expression of osteogenic marker genes. CONCLUSIONS: Our data suggest that osteoblast/osteocyte-derived NPY is capable of altering osteogenesis in vivo and in vitro and may represent an important source of NPY for regulation of bone formation. However, it is possible that other peripheral sources of NPY such as the sympathetic nervous system and vasculature also contribute to peripheral regulation of bone turnover.

Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes.

The therapeutic goal of increasing bone mass by co-treatment of parathyroid hormone (PTH) and an osteoclast inhibitor has been complicated by the undefined contribution of osteoclasts to the anabolic activity of PTH. To determine whether active osteoclasts are required at the time of PTH administration, we administered a low dose of the transient osteoclast inhibitor salmon calcitonin (sCT) to young rats receiving an anabolic PTH regimen. Co-administration of sCT significantly blunted the anabolic effect of PTH as measured by peripheral quantitative computer tomography (pQCT) and histomorphometry in the femur and tibia, respectively. To determine gene targets of sCT, we carried out quantitative real time PCR and microarray analysis of metaphyseal samples 1.5, 4 and 6.5h after administration of a single injection of PTH, sCT or PTH+sCT. Known targets of PTH action, IL-6, ephrinB2 and RANKL, were not modified by co-administration with sCT. Surprisingly, at all time points, we noted a significant upregulation of sclerostin mRNA by sCT treatment, as well as down-regulation of two other osteocyte gene products, MEPE and DMP1. Immunohistochemistry confirmed that sCT administration increased the percentage of osteocytes expressing sclerostin, suggesting a mechanism by which sCT reduced the anabolic effect of PTH. Neither mRNA for CT receptor (Calcr) nor labeled CT binding could be detected in sclerostin-enriched cells differentiated from primary calvarial osteoblasts. In contrast, osteocytes freshly isolated from calvariae expressed a high level of Calcr mRNA. Furthermore immunohistochemistry revealed co-localization of CT receptor (CTR) and sclerostin in some osteocytes in calvarial sections. Taken together these data indicate that co-treatment with sCT can blunt the anabolic effect of PTH and this may involve direct stimulation of sclerostin production by osteocytes. These data directly implicate calcitonin as a negative regulator of bone formation through a previously unsuspected mechanism.

Defining a visual marker of osteoprogenitor cells within the periodontium.

BACKGROUND AND OBJECTIVE: Cells with osteoprogenitor potential are present within periodontal tissues during development and in postnatal life. To identify an osteoprogenitor population, this study utilized a transgenic model in which an alpha-smooth muscle actin (alphaSMA) promoter directed green fluorescent protein (GFP) expression. MATERIAL AND METHODS: Observation of GFP expression was complemented with analysis of osteogenic differentiation by determining the expression of RNA of bone markers, by histochemical staining for alkaline phosphatase and by the detection of mineralized nodules using xylenol orange. Flow cytometry was utilized to determine the proliferative potential and cell-surface phenotype of cultured alphaSMA-positive cells. RESULTS: alphaSMA-GFP expression was detected within the dental follicle and in the apical region of the root (i.e. areas rich in vascularization) but not in mature bone. alphaSMA-GFP expression was observed during the early stages of primary cultures derived from the dental follicle and periodontal ligament and was diminished in areas undergoing mineralization. Intense alkaline phosphatase activity and the presence of mineralized nodules was observed 2 wk after osteogenic induction. Consequently, the expression of bone sialoprotein, osteocalcin and dentin matrix protein-1 was increased. Flow cytometry revealed that in vitro expansion enriched for an alphaSMA-GFP-positive population in which 55-65% of cells expressed the cell-surface markers Thy1(+) and Sca1(+). The alphaSMA-GFP-positive population exhibited high proliferative and osteogenic potentials when compared with an alphaSMA-GFP-negative population. CONCLUSION: Our data indicate that the alphaSMA promoter can be used to identify a population of osteoprogenitor cells residing within the dental follicle and periodontal ligament that can differentiate into mature osteoblasts.

In vivo and in vitro comparison of the effects of FGF-2 null and haplo-insufficiency on bone formation in mice.

We previously reported that deletion of the Fgf2 gene (Fgf2-/-) resulted in decreased bone mass in adult mice. This study examines the effect of haplo-insuffiency (Fgf2+/-) on bone loss in vertebrae from these mutant mice. Fgf2+/+ mice attained peak bone mass at 8-9 months of age. In contrast BMD was significantly reduced in vertebrae from adult (8-9) Fgf2+/- mice. Exogenous FGF-2 rescued reduced bone nodule formation in Fgf2+/- and Fgf2-/- cultures. Runx2 mRNA was reduced in cultures from Fgf2+/- and Fgf2-/- mice. FGF receptor2 mRNA and protein were markedly reduced in Fgf2+/- and Fgf2-/- mice. Decreased bone formation in Fgf2 mutant mice may correlate with impaired FGFR signaling, decreased Runx2 gene expression.

Alteration of newly induced endochondral bone formation in adult mice without tumour necrosis factor receptor 1.

Tumour necrosis factor (TNF)-alpha, a major proinflammatory cytokine, exerts its role on bone cells through two receptors (TNFR1 and TNFR2). TNFR1, but not TNFR2, is expressed by osteoblasts and its function in bone formation in vivo is not fully understood. We compared in vivo new bone formation in TNFR1-deficient (TNFR1(-/-)) mice and wild-type mice, using two models of bone formation: intramembranous ossification following tibial marrow ablation and endochondral ossification induced by bone morphogenetic protein (BMP)-2. Intramembranous osteogenesis in TNFR1(-/-) mice did not differ from the wild-type mice either in histomorphometric parameters or mRNA expression of bone-related markers and inflammatory cytokines. During endochondral osteogenesis, TNFR1(-/-) mice formed more cartilage (at post-implantation day 9), followed by more bone and bone marrow (at day 12). mRNAs for BMP-2, -4 and -7 were increased during the endochondral differentiation sequence in TNFR1(-/-) mice. The expression of receptor activator of NF-kappa B ligand (RANKL) and receptor activator of NF-kappa B (RANK), as assessed by quantitative reverse transcription polymerase chain reaction (RT-PCR), was also increased significantly during endochondral ossification in TNFR1(-/-) mice. In conclusion, signalling through the TNFR1 seems to be a negative regulator of new tissue formation during endochondral but not intramembranous osteogenesis in an adult organism. BMPs and RANKL and its receptor RANK may be involved in the change of local environment in the absence of TNFR1 signalling.

Dentin matrix protein 1 expression during osteoblastic differentiation, generation of an osteocyte GFP-transgene.

Our previous studies have demonstrated that promoter-green fluorescent protein (GFP) transgenes can be used to identify and isolate populations of cells at the preosteoblastic stage (pOBCol3.6GFP) and at the mature osteoblastic stage (pOBCol2.3GFP) in living primary bone cell cultures. This strategy forms the basis for appreciating the cellular heterogeneity of lineage and relating gene function to cell differentiation. A weakness of this approach was the lack of a selective marker for late osteoblasts and mature osteocytes in the mineralized matrix. In this study, we have examined the expression of DMP-1 mRNA in murine marrow stromal and calvarial osteoblast cultures, and in bone, and calvaria in vivo. Furthermore, we have generated transgenic mice utilizing a mouse DMP1 cis-regulatory system to drive GFP as a marker for living osteocytes. Transgene expression was directed to mineralized tissues and showed a high correlation with the expression of the endogenous gene. Osteocyte-restricted expression of GFP was observed in histological sections of femur and calvaria and in primary cell cultures. Generation of this transgenic model will facilitate studies of gene expression and biological functions in these terminally differentiated bone cells.

Stage specific inhibition of osteoblast lineage differentiation by FGF2 and noggin.

Fibroblast growth factor 2 (FGF2) and noggin are two unrelated ligands of two distinctly different signaling pathways that have a similar inhibitory effect on osteoblast differentiation. Because of their differences, we postulated that they probably acted at a different stage within the osteoprogenitor differentiation pathway. This study was performed on primary murine bone cell cultures under conditions where alkaline phosphatase (AP) and type I collagen expression (Col1a1) were observed by day 7 (preosteoblast stage), followed by bone syaloprotein (BSP) at day 11 (early osteoblast) and osteocalcin (OC) by day 15-18 (mature osteoblast stage). FGF2 completely inhibited expression of AP and the mRNA transcript for Col1a1, while noggin showed only a partial inhibition of these markers of preosteoblast differentiation. However, the markers of differentiated osteoblasts (BSP and OC) were completely inhibited in both the FGF2 and noggin treated cultures, suggesting that noggin acts at later point in the osteoprogenitor differentiation pathway than FGF2. To further verify that the inhibition was occurring at a different stage of osteoblasts development, primary cultures derived from transgenic mice harboring segments of the collagen promoter driving green fluorescent protein (GFP) that activate at different levels of osteoblast differentiation were analyzed. Consistent with the endogenous markers, pOBCol3.6GFP and pOBCOL2.3GFP transgene activity was completely inhibited by continuous addition of FGF2, while noggin showed partial inhibition of pOBCol3.6GFP and complete inhibition of the pOBCol2.3GFP transgene. Upon removal of either agent, endogenous and GFP markers of osteoblast differentiation reappeared although at a different temporal pattern. This work demonstrates that FGF2 and noggin can reversibly modulate osteoblast lineage differentiation at different maturational stages. These agents may be useful to enrich for and maintain a population of osteoprogenitor cells at a defined stage of differentiation.

Col1a1-GFP transgene expression in developing incisors.

Previous studies have shown that terminal differentiation of odontoblasts is accompanied by dramatic increases in type I collagen synthesis. Recently transgenic mice in which green fluorescent protein (GFP) expression is under the control of the rat 3.6 (pOBCol3.6GFPtpz) and 2.3 (pOBCol2.3GFPemd) Col1a1 promoter fragments were generated. Our analysis of these GFP-expressing transgenic mice shows that the 2.3-kb promoter fragment directs strong expression of GFP only to bones and teeth, whereas the 3.6-kb fragment of promoter directs strong expression of GFP in bone and tooth, as well as in other type I collagen producing tissues. Our observations of incisors in these transgenic mice show high levels of GFP expression in functional odontoblasts and in differentiated osteoblasts. These observations show that expression of GFP reporter genes closely follow the patterns of expression of alpha 1(I) collagen in various tissues including odontoblasts.

Osteoblastic response to the defective matrix in the osteogenesis imperfecta murine (oim) mouse.

This work examines the cellular pathophysiology associated with the weakened bone matrix found in a murine model of osteogenesis imperfecta murine (oim). Histomorphometric analysis of oim/oim bone showed significantly diminished bone mass, and the osteoblast and osteoclast histomorphometric parameters were increased in the oim/oim mice, compared with wild-type (+/+) mice. To assess osteoblast activity, a rat Col1a1 promoter linked to the chloramphenicol acetyltransferase reporter transgene was bred into the oim model. At 8 d and 1 month of age, no difference in transgene activity between oim and control mice was observed. However, at 3 months of age, chloramphenicol acetyl transferase activity was elevated in oim/oim;Tg/Tg, compared with +/+;Tg/Tg and oim/+;Tg/Tg. High levels of urinary pyridinoline crosslinks in the oim/oim;Tg/Tg mice were present at all ages, reflecting continuing high bone resorption. Our data portray a state of ineffective osteogenesis in which the mutant mouse never accumulates a normal quantity of bone matrix. However, it is only after the completion of the rapid growth phase that the high activity of the oim/oim osteoblast can compensate for the high rate of bone resorption. This relationship between bone formation and resorption may explain why the severity of osteogenesis imperfecta decreases after puberty is completed. The ability to quantify high bone turnover and advantages of using a transgene that reflects osteoblast lineage activity make this a useful model for studying interventions designed to improve the bone strength in osteogenesis imperfecta.