Conditional loss of CaMKK2 in Osterix-positive osteoprogenitors enhances osteoblast function in a sex-divergent manner

Ca2+/calmodulin-dependent protein kinase kinase 2 (CaMKK2) is a multi-functional, serine/threonine protein kinase with predominant roles in inflammation, systemic energy metabolism, and bone remodeling. We previously reported that global ablation of CaMKK2 or its systemic pharmacological inhibition led to bone mass accrual in mice by stimulating osteoblasts and inhibiting osteoclasts. However, a direct, cell-intrinsic role for the kinase in the osteoblast lineage has not been established. Here we report that conditional deletion of CaMKK2 from osteoprogenitors, using the Osterix 1 (Osx1) - GFP::Cre (tetracycline-off) mouse line, resulted in increased trabecular bone mass due to an acute stimulation of osteoblast function in male and female mice. The acute simulation of osteoblasts and bone formation following conditional ablation of osteoprogenitor-derived CaMKK2 was sustained only in female mice. Periosteal bone formation at the cortical bone was enhanced only in male conditional knockout mice without altering cortical bone mass or strength. Prolonged deletion of CaMKK2 in early osteoblasts was accompanied by a stimulation of osteoclasts in both sexes, indicating a coupling effect. Notably, alterations in trabecular and cortical bone mass were absent in the doxycycline-removed "Cre-only" Osx1-GFP::Cre mice. Thus, the increase in osteoblast function at the trabecular and cortical bone surfaces following the conditional deletion of CaMKK2 in osteoprogenitors is indicative of a direct but sex-divergent role for the kinase in osteoblasts.

The complement factor H-related protein-5 (CFHR5) exacerbates pathological bone formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory disease, characterized by excessive new bone formation. We previously reported that the complement factor H-related protein-5 (CFHR5), a member of the human factor H protein family, is significantly elevated in patients with AS compared to other rheumatic diseases. However, the pathophysiological mechanism underlying new bone formation by CFHR5 is not fully understood. In this study, we revealed that CFHR5 and proinflammatory cytokines (TNF, IL-6, IL-17A, and IL-23) were elevated in the AS group compared to the HC group. Correlation analysis revealed that CFHR5 levels were not significantly associated with proinflammatory cytokines, while CFHR5 levels in AS were only positively correlated with the high CRP group. Notably, treatment with soluble CFHR5 has no effect on clinical arthritis scores and thickness at hind paw in curdlan-injected SKG, but significantly increased the ectopic bone formation at the calcaneus and tibia bones of the ankle as revealed by micro-CT image and quantification. Basal CFHR5 expression was upregulated in AS-osteoprogenitors compared to control cells. Also, treatment with CFHR5 remarkedly induced bone mineralization status of AS-osteoprogenitors during osteogenic differentiation accompanied by MMP13 expression. We provide the first evidence demonstrating that CFHR5 can exacerbate the pathological bone formation of AS. Therapeutic modulation of CFHR5 could be promising for future treatment of AS. KEY MESSAGES: Serum level of CFHR5 is elevated and positively correlated with high CRP group of AS patients. Recombinant CFHR5 protein contributes to pathological bone formation in in vivo model of AS. CFHR5 is highly expressed in AS-osteoprogenitors compared to disease control. Recombinant CFHR5 protein increased bone mineralization accompanied by MMP13 in vitro model of AS.

Osteoprogenitor recruitment and differentiation during intracortical bone remodeling of adolescent humans

BACKGROUND

Recruitment and proliferation of osteoprogenitors during the reversal-resorption phase, and their differentiation into mature bone-forming osteoblasts is crucial for initiation of bone formation during bone remodeling. This study investigates the osteoprogenitors' gradual recruitment, proliferation, and differentiation into bone-forming osteoblasts within intracortical remodeling events of healthy adolescent humans.

METHODS

The study was conducted on cortical bone specimens from 11 adolescent human controls - patients undergoing surgery due to coxa valga. The osteoprogenitor recruitment route and differentiation into osteoblasts were backtracked using immunostainings and in situ hybridizations with osteoblastic markers (CD271/NGFR, osterix/SP7, COL3A1 and COL1A1). The osteoblastic cell populations were defined based on the pore surfaces, and their proliferation index (Ki67), density and number/circumference were estimated in multiplex-immunofluorescence (Ki67, TRAcP, CD34) stained sections.

RESULTS

During the reversal-resorption phase, osteoclasts are intermixed with (COL3A1+NFGR+) osteoblastic reversal cells, which are considered to be osteoprogenitors of (COL1A1+SP7+) bone-forming osteoblasts. Initiation of bone formation requires a critical density of these osteoprogenitors (43 ± 9 cells/mm), which is reached though proliferation (4.4 ± 0.5 % proliferative) and even more so through recruitment of osteoprogenitors, but challenged by the ongoing expansion of the canal circumference. These osteoprogenitors most likely originate from osteoblastic bone lining cells and mainly lumen osteoprogenitors, which expand their population though proliferation (4.6 ± 0.3 %) and vascular recruitment. These lumen osteoprogenitors resemble canopy cells above trabecular remodeling sites, and like canopy cells they extend above bone-forming osteoblasts where they may rejuvenate the osteoblast population during bone formation.

CONCLUSION

Initiation of bone formation during intracortical remodeling requires a critical density of osteoprogenitors on eroded surfaces, which is reached though proliferation and recruitment of local osteoprogenitors: bone lining cells and lumen osteoprogenitors.

Elevated BMPR2 expression amplifies osteoblast differentiation in ankylosing spondylitis

Objective

Bone morphogenetic protein receptor type 2 (BMPR2) has been associated with radiographic changes in ankylosing spondylitis (AS), but further characterization of the cellular signaling pathway in osteoprogenitor (OP) is not clearly understood. The aim of this study was to investigate the expression of BMPR2 and bone morphogenetic protein 2 (BMP2)-mediated responsibility in AS.

Methods

We collected 10 healthy control (HC) and 14 AS-OPs derived from facet joints. Subsequently, we then conducted RNA sequencing with two samples per group and selected BMP-related genes. Facet joint tissues and derived primary OPs were evaluated by validation of selected RNA sequencing data, immunohistochemistry, and comparison of osteogenic differentiation potential.

Results

Based on RNA-sequencing analysis, we found that BMPR2 expression is higher in AS-OPs compared to in HC-OPs. We also validated the increased BMPR2 expression in facet joint tissues with AS and its derived OPs in messenger RNA and protein levels. Additionally, primary AS-OPs showed much greater response to osteogenic differentiation induced by BMP2 and a higher capacity for smad1/5/8-induced RUNX2 expression compared to HCs.

Conclusion

The expression of BMPR2 was found to be significantly increased in facet joint tissues of patients with AS. These findings suggest that BMPR2 may play a role in the BMP2-mediated progression of AS.

AcanCreER lacks specificity to chondrocytes and targets periosteal progenitors in the fractured callus

Aggrecan (Acan) is a large proteoglycan molecule constituting the extracellular matrix of cartilage, secreted by chondrocytes. To specifically target the chondrocyte lineage, researchers have widely used the AcanCreER mouse model. Evaluation of specificity and efficiency of recombination, requires Cre animals to be crossed with reporter mice. In order to accurately interpret data from Cre models, it is imperative to consider A) the amount of recombination occurring in cells/tissues that are not intended for targeting (i.e., non-specific expression), B) the efficiency of Cre recombination, which can depend on dose and duration of tamoxifen treatment, and C) the activation of CreER without tamoxifen induction, known as "Cre leakage." Using a highly sensitive reporter mouse (Ai9, tdTomato), we performed a comprehensive analysis of the AcanCreER system. Surprisingly, we observed expression in cells within the periosteum. These cells expand at a stage when chondrocytes are not yet present within the forming callus tissue (Acan/Ai9+ cells). In pulse-chase experiments, we confirmed that fibroblastic Acan/Ai9+ cells within the periosteum can directly give rise to osteoblasts. Our results show that Acan/Ai9+ is not specific for the chondrocyte lineage in the fracture callus or with the tibial holes. The expression of AcanCreER in periosteal progenitor cells complicates the interpretation of studies evaluating the transition of chondrocytes to osteoblasts (termed transdifferentiation). Awareness of these issues and the limitations of the system will lead to better data interpretation.

Development and validation of a new method to isolate, expand, and differentiate circulating osteogenic precursor (COP) cells

Circulating osteogenic precursor (COP) cells are a population of progenitor cells in the peripheral blood with the capacity to form bone in vitro and in vivo. They have characteristics of the mesenchymal stem and progenitor pool found in the bone marrow; however, more recently, a population of COP cells has been identified with markers of the hematopoietic lineage such as CD45 and CD34. While this population has been associated with several bone pathologies, a lack of cell culture models and inconsistent characterization has limited mechanistic research into their behavior and physiology. In this study, we describe a method for the isolation of CD45+/CD34+/alkaline phosphatase (ALP) + COP cells via fluorescence-activated cell sorting, as well as their expansion and differentiation in culture. Hematopoietic COP cells are a discreet population within the monocyte fraction of the peripheral blood mononuclear cells, which form proliferative, fibroblastoid colonies in culture. Their expression of hematopoietic markers decreases with time in culture, but they express markers of osteogenesis and deposit calcium with differentiation. It is hoped that this will provide a standard for their isolation, for consistency in future research efforts, to allow for the translation of COP cells into clinical settings.

Isolation and establishment of skin-derived and mesenchymal cells from south American bat Artibeus planirostris (Chiroptera - Phyllostomidae)

Animal models represent a crucial tool for biological research, so the establishment of new cultures is fundamental for the discovery of new therapies and the understanding of mechanisms of cell development in the most diverse animals. Here, we report the successful establishment of two new primary cell cultures derived from a South American bat (Artibeus planirostris). The establishment of a new bat culture can help in the investigation of new zoonoses since bats have been proposed as carriers of these diseases. We evaluated the chromosomal stability of cells from different passages. Primary cultures were collected from ear tissues and bone marrow of A. planirostris. Cultures were expanded, and osteogenic and adipogenic inductions were conducted for 21 days. For osteogenic differentiation, the medium was supplemented with 0.1 μM dexamethasone, 3 mM β-glycerophosphate, and 10 μM L-ascorbic acid 2-phosphate. For adipogenic differentiation, the medium was supplemented with 5 μM rosiglitazone, 0.4 μM insulin, 0.1 mM indomethacin, and 0.1 μM dexamethasone. After the induction period, the cells were stained with Alizarin Red to assess osteogenic differentiation and Oil Red O to assess adipogenic differentiation. We observed the appearance of lipid droplets in adipocytes and the extracellular deposition of calcium matrix by osteocytes, indicating that bone marrow-derived cells and skin-derived cells of A. planirostris could successfully differentiate into these lineages. Also, the number of chromosomes remained stable for both primary cultures during passages 2, 4, 6, and 8.

Inhibition of the mitochondrial permeability transition improves bone fracture repair

Bone fracture is accompanied by trauma, mechanical stresses, and inflammation - conditions known to induce the mitochondrial permeability transition. This phenomenon occurs due to opening of the mitochondrial permeability transition pore (MPTP) promoted by cyclophilin D (CypD). MPTP opening leads to more inflammation, cell death and potentially to disruption of fracture repair. Here we performed a proof-of-concept study and tested a hypothesis that protecting mitochondria from MPTP opening via inhibition of CypD improves fracture repair. First, our in vitro experiments indicated pro-osteogenic and anti-inflammatory effects in osteoprogenitors upon CypD knock-out or pharmacological inhibition. Using a bone fracture model in mice, we observed that bone formation and biomechanical properties of repaired bones were significantly increased in CypD knock-out mice or wild type mice treated with a CypD inhibitor, NIM811, when compared to controls. These effects were evident in young male but not female mice, however in older (13 month-old) female mice bone formation was also increased during fracture repair. In contrast to global CypD knock-out, mesenchymal lineage-specific (Prx1-Cre driven) CypD deletion did not result in improved fracture repair. Our findings implicate MPTP in bone fracture and suggest systemic CypD inhibition as a modality to promote fracture repair.

Effects of dihydrotestosterone on osteoblast activity in curdlan-administered SKG mice and osteoprogenitor cells in patients with ankylosing spondylitis

Background

Ankylosing spondylitis (AS) is characteristically male-predominant, and progressive spinal ankylosis affects male patients more severely; however, the hormonal effects in males with AS are poorly understood.

Methods

In the present study, the regulatory effects of dutasteride, a 5-α reductase inhibitor that blocks the conversion of testosterone to dihydrotestosterone (DHT), were examined in curdlan-administered male SKG mice to determine spinal bone formation, bone metabolism-related markers, and interleukin (IL)-17A cytokine and T cell populations. In addition, the effects of DHT on primary osteoprogenitors from the facet joints of AS patients were assessed based on osteoblast-related parameters. DHT level was measured, and the correlation with modified Stoke Ankylosing Spondylitis Spinal Score (mSASSS) was analyzed in AS patients.

Results

In curdlan-administered SKG mice, dutasteride treatment resulted in an increased accumulation of hydroxyapatite in the spine which was positively correlated with serum IL-17A levels. In the analysis of bone metabolism-related molecules, a decrease in sclerostin levels was observed in the sera in the dutasteride group. Continuous exposure to DHT resulted in fewer calcium deposits in AS osteoprogenitors during osteoblast differentiation. DHT-treated AS osteoprogenitors showed decreased osteocalcin and increased DKK1 and SOST1 mRNA expression, supporting the results of the in vivo experiments. Treatment with dutasteride upregulated bone formation in the spine of curdlan-administered SKG mice and DHT treatment downregulated osteoblast differentiation in vitro.

Conclusions

Treatment with dutasteride affected the bone formation in the spine of curdlan-treated SKG mice, and DHT treatment attenuated osteoblast differentiation in vitro. Therefore, contrary to what could be expected if osteoblasts contributed to spinal ankylosis, DHT inhibition might increase rather than decrease the progression of spinal ankylosis despite the higher levels of DHT observed in many AS patients.

Nestin-GFP transgene labels skeletal progenitors in the periosteum

The periosteum is critical for bone repair and contains skeletal stem cells (SSCs), but these cells are still poorly characterized. In the bone marrow, cells expressing the Nes-GFP transgene have been described to be SSCs. Here, we investigated whether Nes-GFP expression also typifies SSCs in the periosteum. We show that in adult mice, Nes-GFP cells are present in the periosteum and localize closely to blood vessels, but periosteal Nes-GFP cells express SSC and progenitor markers differently compared to Nes-GFP cells in the bone marrow. Periosteal Nes-GFP cells show in vitro clonogenicity and tri-lineage differentiation potential and they can form bone in vivo. Shortly after fracture, they start to proliferate and they contribute to the osteoblast pool during the repair process. However, periosteal Nes-GFP cells are not slow dividing nor self-renewing in vivo. These results indicate that in adult mice, periosteal Nes-GFP expressing cells are skeletal progenitors rather than true SSCs, and they participate in the fracture healing process.

β-aminoisobutyric acid accelerates the proliferation and differentiation of MC3T3-E1 cells via moderate activation of ROS signaling

BACKGROUD

Osteoporosis is one of the bone-metabolic diseases associated with decreased bone renewal and bone mineral density. β-aminoisobutyric acid (BAIBA), a natural thymine catabolite, can reduce inflammation in skeletal muscle and alleviate hepatic endoplasmic reticulum stress. However, the roles of BAIBA in osteoblast proliferation and differentiation remain largely unknown.

METHODS

The cultured MC3T3-E1 cells received various treatments in this study, including BAIBA alone, H₂O₂ alone, BAIBA plus N-acetyl-l-cysteine and BAIBA plus apocynin. Cell proliferation was determined by CCK-8 assay and ³H-Thymidine incorporation. Cell differentiation was evaluated by determining mRNA level of differentiation makers and ALP, and ALP activity. Reactive oxygen species (ROS) were determined by DHE staining while superoxide anion level and NAD(P)H oxidase activity were determined by the lucigenin-derived chemiluminescence method. The content of hydrogen peroxide (H₂O₂) was detected using a commercial kit. The level of NOX1, NOX2 and NOX4 was determined by Western-blot or qRT-PCR.

RESULTS

We show that treatment of BAIBA stimulated the proliferation of MC3T3-E1 osteoprogenitor cells and enhanced the gene expression of osteoblast differentiation markers. Incubation of MC3T3-E1 cells with BAIBA evoked increases in NAD(P)H oxidase-derived reactive oxygen species (ROS). Scavenging of reactive oxygen species (N-acetyl-l-cysteine) or inhibition of NAD(P)H oxidase (apocynin) abolished the BAIBA-elicited proliferation and differentiation of MC3T3-E1 cells.

CONCLUSION

Our results provide the first evidence that BAIBA stimulates proliferation and differentiation of osteoprogenitor cells via activation of NAD(P)H oxidase/ROS signaling.

Decreased osteoprogenitor proliferation precedes attenuation of cancellous bone formation in ovariectomized rats treated with sclerostin antibody

Sclerostin antibody (Scl-Ab) stimulates bone formation, which with long-term treatment, attenuates over time. The cellular and molecular mechanisms responsible for the attenuation of bone formation are not well understood, but in aged ovariectomized (OVX) rats, the reduction in vertebral cancellous bone formation is preceded by a reduction in osteoprogenitor (OP) number and significant induction of signaling pathways known to suppress mitogenesis and cell cycle progression in the osteocyte (OCy) (Taylor et al., 2016). To determine if the reduction in OP number is associated with a decrease in proliferation, aged OVX rats were administered vehicle or Scl-Ab for 9 or 29 days and implanted with continuous-delivery 5-bromo-2′-deoxyuridine (BrdU) mini-osmotic pumps 5 days prior to necropsy. The total number of BrdU-labeled osteoblasts (OB) was quantified in vertebral cancellous bone to indirectly assess the effects of Scl-Ab treatment on OP proliferation at the time of activation of modeling-based bone formation at day 9 and at the time of maximal mineralizing surface, initial decrease in OP number, and transcriptional changes in the OCy at day 29. Compared with vehicle, Scl-Ab resulted in an increase in the total number of BrdU-positive OB (+260%) at day 9 that decreased with continued treatment (+50%) at day 29. These differences in proliferation occurred at time points when the increase in total OB number was significant and similar in magnitude. These findings suggest that reduced OP proliferation contributes to the decrease in OP numbers, an effect that would limit the OB pool and contribute to the attenuation of bone formation that occurs with long-term Scl-Ab treatment.

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Sclerostin antibody stimulates bone formation (BF) that attenuates over time.

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Osteoprogenitor (OP) proliferation increases early with treatment.

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Attenuation of BF is preceded by a decrease in OP proliferation.

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Decrease is coincident with molecular signaling consistent with cell cycle arrest.

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Decreased OP proliferation contributes to the attenuation of BF.

Loss of ephrinB1 in osteogenic progenitor cells impedes endochondral ossification and compromises bone strength integrity during skeletal development

The EphB receptor tyrosine kinase family and their ephrinB ligands have been implicated as mediators of skeletal development and bone homeostasis in humans, where mutations in ephrinB1 contribute to frontonasal dysplasia and coronal craniosynostosis. In mouse models, ephrinB1 has been shown to be a critical factor mediating osteoblast function. The present study examined the functional importance of ephrinB1 during endochondral ossification using the Cre recombination system with targeted deletion of ephrinB1 (EfnB1^fl/fl) in osteogenic progenitor cells, under the control of the osterix (Osx:Cre) promoter. The Osx:EfnB1^-/- mice displayed aberrant bone growth during embryonic and postnatal skeletal development up to 4weeks of age, when compared to the Osx:Cre controls. Furthermore, compared to the Osx:Cre control mice, the Osx:EfnB1^-/- mice exhibited significantly weaker and less rigid bones, with a reduction in trabecular/ cortical bone formation, reduced trabecular architecture and a reduction in the size of the growth plates at the distal end of the femora from newborn through to 4weeks of age. The aberrant bone formation correlated with increased numbers of tartrate resistant acid phosphatase positive osteoclasts and decreased numbers of bone lining osteoblasts in 4week old Osx:EfnB1^-/- mice, compared to Osx:Cre control mice. Taken together, these observations demonstrate the importance of ephrinB1 signalling between cells of the skeleton required for endochondral ossification.

Signaling pathways effecting crosstalk between cartilage and adjacent tissues: Seminars in cell and developmental biology: The biology and pathology of cartilage

Endochondral ossification, the mechanism responsible for the development of the long bones, is dependent on an extremely stringent coordination between the processes of chondrocyte maturation in the growth plate, vascular expansion in the surrounding tissues, and osteoblast differentiation and osteogenesis in the perichondrium and the developing bone center. The synchronization of these processes occurring in adjacent tissues is regulated through vigorous crosstalk between chondrocytes, endothelial cells and osteoblast lineage cells. Our knowledge about the molecular constituents of these bidirectional communications is undoubtedly incomplete, but certainly some signaling pathways effective in cartilage have been recognized to play key roles in steering vascularization and osteogenesis in the perichondrial tissues. These include hypoxia-driven signaling pathways, governed by the hypoxia-inducible factors (HIFs) and vascular endothelial growth factor (VEGF), which are absolutely essential for the survival and functioning of chondrocytes in the avascular growth plate, at least in part by regulating the oxygenation of developing cartilage through the stimulation of angiogenesis in the surrounding tissues. A second coordinating signal emanating from cartilage and regulating developmental processes in the adjacent perichondrium is Indian Hedgehog (IHH). IHH, produced by pre-hypertrophic and early hypertrophic chondrocytes in the growth plate, induces the differentiation of adjacent perichondrial progenitor cells into osteoblasts, thereby harmonizing the site and time of bone formation with the developmental progression of chondrogenesis. Both signaling pathways represent vital mediators of the tightly organized conversion of avascular cartilage into vascularized and mineralized bone during endochondral ossification.

Transient Canonical Wnt Stimulation Enriches Human Bone Marrow Mononuclear Cell Isolates for Osteoprogenitors

Activation of the canonical Wnt signaling pathway is an attractive anabolic therapeutic strategy for bone. Emerging data suggest that activation of the Wnt signaling pathway promotes bone mineral accrual in osteoporotic patients. The effect of Wnt stimulation in fracture healing is less clear as Wnt signaling has both stimulatory and inhibitory effects on osteogenesis. Here, we tested the hypothesis that transient Wnt stimulation promotes the expansion and osteogenesis of a Wnt‐responsive stem cell population present in human bone marrow. Bone marrow mononuclear cells (BMMNCs) were isolated from patients undergoing hip arthroplasty and exposed to Wnt3A protein. The effect of Wnt pathway stimulation was determined by measuring the frequency of stem cells within the BMMNC populations by fluorescence‐activated cell sorting and colony forming unit fibroblast (CFU‐F) assays, before determining their osteogenic capacity in in vitro differentiation experiments. We found that putative skeletal stem cells in BMMNC isolates exhibited elevated Wnt pathway activity compared with the population as whole. Wnt stimulation resulted in an increase in the frequency of skeletal stem cells marked by the STRO‐1^bright/Glycophorin A⁻ phenotype. Osteogenesis was elevated in stromal cell populations arising from BMMNCs transiently stimulated by Wnt3A protein, but sustained stimulation inhibited osteogenesis in a concentration‐dependent manner. These results demonstrate that Wnt stimulation could be used as a therapeutic approach by transient targeting of stem cell populations during early fracture healing, but that inappropriate stimulation may prevent osteogenesis. Stem Cells2016;34:418–430

Multicolor flow cytometry-based cellular phenotyping identifies osteoprogenitors and inflammatory cells in the osteoarthritic subchondral bone marrow compartment

PURPOSE

The cellular component of subchondral bone is thought to be responsible for aberrant bone remodeling in osteoarthritis (OA). Direct phenotypical analysis of the cellular compartment is critical to better understand the OA disease process. This study provides proof-of-principle for flow cytometry-based phenotyping of isolated subchondral trabecular bone (STB) marrow cells without prior use of cell culture techniques.

METHODS

Tibial plateaus were obtained from OA patients undergoing total knee arthroplasty. Subchondral bone chips were digested with collagenase IA and single cell suspensions were directly phenotyped using flow cytometry. Cells were analyzed for the expression of alkaline phosphatase (ALP) as osteoblast/osteoprogenitor marker and monocyte/macrophage markers (CD14, CD68, HLA-DR, CD115).

RESULTS

MTT staining revealed abundant viable cells in the bone marrow compartment of STB prior to digestion, which were efficiently released by collagenase. Within the CD45-negative cell fraction, approximately 20% of the cells were positive for the early osteoblast/osteoprogenitor marker ALP. Within the CD45+ hematopoietic cell fraction, the majority of cells were of monocytic origin (>80%) displaying strong surface expression of CD14. Discreet macrophage populations (CD14+/HLA-DR+/CD68+) and putative osteoclast progenitors (CD45+/HLA-DR-/CD115+) were unequivocally identified. Osteoblast, macrophage and osteoclast progenitor presence in the subchondral bone unit (SBU) was confirmed by (immuno)histochemical staining for osteocalcin, CD68 and tartrate-resistant acid phosphatase, respectively.

CONCLUSIONS

Flow cytometric analysis is a valuable methodology to study the cellular compartment of STB marrow. This method provides a proof of principle that the whole resident cell population can be directly phenotypically characterized without the prior use of cell culture techniques.

Epidermal growth factor receptor (EGFR) signaling promotes proliferation and survival in osteoprogenitors by increasing early growth response 2 (EGR2) expression

Maintaining bone architecture requires continuous generation of osteoblasts from osteoprogenitor pools. Our previous study of mice with epidermal growth factor receptor (EGFR) specifically inactivated in osteoblast lineage cells revealed that EGFR stimulates bone formation by expanding the population of mesenchymal progenitors. EGFR ligands are potent regulators for the osteoprogenitor pool, but the underlying mechanisms are largely unknown. Here we demonstrate that activation of EGFR increases the number of osteoprogenitors by promoting cell proliferation and suppressing either serum depletion-induced or TNFα-induced apoptosis mainly through the MAPK/ERK pathway. Mouse calvarial organ culture revealed that EGF elevated the number of proliferative cells and decreased the number of apoptotic cells, which led to increased osteoblasts. Microarray analysis of MC3T3 cells, an osteoprogenitor cell line, revealed that EGFR signaling stimulates the expression of MCL1, an antiapoptotic protein, and a family of EGR transcription factors (EGR1, -2, and -3). The up-regulation of MCL1 and EGR2 by EGF was further confirmed in osteoprogenitors close to the calvarial bone surface. Overexpression of NAB2, a co-repressor for EGRs, attenuated the EGF-induced increase in osteoprogenitor number. Interestingly, knocking down the expression of EGR2, but not EGR1 or -3, resulted in a similar effect. Using inhibitor, adenovirus overexpression, and siRNA approaches, we demonstrate that EGFR signaling activates the MAPK/ERK pathway to stimulate the expression of EGR2, which in turn leads to cell growth and MCL1-mediated cell survival. Taken together, our data clearly demonstrate that EGFR-induced EGR2 expression is critical for osteoprogenitor maintenance and new bone formation.