Dimorphic effects of Notch signaling in bone homeostasis. Nat Med. 2008;14:299-305. [PMID: 18297084 DOI: 10.1038/nm1712]

Understanding the role of Notch in osteosarcoma

The Notch pathway has been described as an oncogene in osteosarcoma, but the myriad functions of all the members of this complex signaling pathway, both in malignant cells and nonmalignant components of tumors, make it more difficult to define Notch as simply an oncogene or a tumor suppressor. The cell-autonomous behaviors caused by Notch pathway manipulation may vary between cell lines but can include changes in proliferation, migration, invasiveness, oxidative stress resistance, and expression of markers associated with stemness or tumor-initiating cells. Beyond these roles, Notch signaling also plays a vital role in regulating tumor angiogenesis and vasculogenesis, which are vital aspects of osteosarcoma growth and behavior in vivo. Further, osteosarcoma cells themselves express relatively low levels of Notch ligand, making it likely that nonmalignant cells, especially endothelial cells and pericytes, are the major source of Notch activation in osteosarcoma tumors in vivo and in patients. As a result, Notch pathway expression is not expected to be uniform across a tumor but likely to be highest in those areas immediately adjacent to blood vessels. Therapeutic targeting of the Notch pathway is likewise expected to be complicated. Most pharmacologic approaches thus far have focused on inhibition of gamma secretase, a protease of the presenilin complex. This enzyme, however, has numerous other target proteins that would be expected to affect osteosarcoma behavior, including CD44, the WNT/β-catenin pathway, and Her-4. In addition, Notch plays a vital role in tissue and organ homeostasis in numerous systems, and toxicities, especially GI intolerance, have limited the effectiveness of gamma secretase inhibitors. New approaches are in development, and the downstream targets of Notch pathway signaling also may turn out to be good targets for therapy. In summary, a full understanding of the complex functions of Notch in osteosarcoma is only now unfolding, and this deeper knowledge will help position the field to better utilize novel therapies as they are developed.

The role of Notch signaling in bone development and disease

During the last decade a considerable amount of data have been accumulated regarding the role of intracellular signaling pathways in the pathogenesis of human diseases. One of these, Notch signaling, well known for its significance in cellular development and tissue morphogenesis, has been increasingly recognized as a crucial participant in the pathogenetic mechanisms underlying certain skeletal disorders. A better understanding of the biology and regulation of this multifaceted pathway is considered an important step towards clarification of the pathogenesis of various skeletal diseases and the development of novel targets for therapeutic purposes.

Analysis of αSMA-labeled progenitor cell commitment identifies notch signaling as an important pathway in fracture healing

Fracture healing is a regenerative process that involves coordinated responses of many cell types, but characterization of the roles of specific cell populations in this process has been limited. We have identified alpha smooth muscle actin (αSMA) as a marker of a population of mesenchymal progenitor cells in the periosteum that contributes to osteochondral elements during fracture healing. Using a lineage tracing approach, we labeled αSMA-expressing cells, and characterized changes in the periosteal population during the early stages of fracture healing by histology, flow cytometry, and gene expression profiling. In response to fracture, the αSMA-labeled population expanded and began to differentiate toward the osteogenic and chondrogenic lineages. The frequency of mesenchymal progenitor cell markers such as Sca1 and PDGFRα increased after fracture. By 6 days after fracture, genes involved in matrix production and remodeling were elevated. In contrast, genes associated with muscle contraction and Notch signaling were downregulated after fracture. We confirmed that activating Notch signaling in αSMA-labeled cells inhibited differentiation into osteogenic and adipogenic lineages in vitro and ectopic bone formation in vivo. By characterizing changes in a selected αSMA-labeled progenitor cell population during fracture callus formation, we have shown that modulation of Notch signaling may determine osteogenic potential of αSMA-expressing progenitor cells during bone healing.

Notch signaling: targeting cancer stem cells and epithelial-to-mesenchymal transition

Notch signaling is an evolutionarily conserved pathway involved in cell fate control during development, stem cell self-renewal, and postnatal tissue differentiation. Roles for Notch in carcinogenesis, the biology of cancer stem cells, tumor angiogenesis, and epithelial-to-mesenchymal transition (EMT) have been reported. This review describes the role of Notch in the "stemness" program in cancer cells and in metastases, together with a brief update on the Notch inhibitors currently under investigation in oncology. These agents may be useful in targeting cancer stem cells and to reverse the EMT process.

Mutations in NOTCH2 in patients with Hajdu-Cheney syndrome

The Hajdu-Cheney syndrome is a very rare disease that affects several organ system, leading to severe osteoporosis and other abnormalities. We describe clinical and genetic findings of nine patients with this disease.

INTRODUCTION

The Hajdu-Cheney syndrome (HCS) is a rare autosomal dominant disorder characterized by severe osteoporosis, acroosteolysis of the distal phalanges, renal cysts, and other abnormalities. Recently, heterozygous mutations in NOTCH2 were identified as the cause of HCS.

METHODS

Nine patients with typical presentations of HCS took part in this study: five affected patients from two small families and four sporadic cases. Peripheral blood DNA was obtained and exome sequencing performed in one affected individual per family and in all four sporadic cases. Sanger sequencing confirmed mutations in all patients.

RESULTS

One of the identified mutations was introduced in a plasmid encoding NOTCH2. Wild-type and mutant NOTCH2 were transiently expressed in HEK293 cells to assess intracellular localization after ligand activation. Deleterious heterozygous mutations in the last NOTCH2 exon were identified in all patients; five of the six mutations were novel.

CONCLUSION

Consistent with previous reports, all mutations are predicted to result in a loss of the proline/glutamic acid/serine/threonine sequence, which harbors signals for degradation, therefore suggesting activating mutations. One of the six mutations furthermore predicted disruption of the second nuclear localization signal of NOTCH2, but the mutant revealed normal nuclear localization after transfection, which is consistent with the proposed gain-of-function mechanism as the cause of this autosomal dominant disease. Our findings confirm that heterozygous NOTCH2 mutations are the cause of HCS and expand the mutational spectrum of this disorder.

Notch signaling in osteocytes differentially regulates cancellous and cortical bone remodeling

Notch receptors play a role in skeletal development and homeostasis, and Notch activation in undifferentiated and mature osteoblasts causes osteopenia. In contrast, Notch activation in osteocytes increases bone mass, but the mechanisms involved and exact functions of Notch are not known. In this study, Notch1 and -2 were inactivated preferentially in osteocytes by mating Notch1/2 conditional mice, where Notch alleles are flanked by loxP sequences, with transgenics expressing Cre directed by the Dmp1 (dentin matrix protein 1) promoter. Notch1/2 conditional null male and female mice exhibited an increase in trabecular bone volume due to an increase in osteoblasts and decrease in osteoclasts. In male null mice, this was followed by an increase in osteoclast number and normalization of bone volume. To activate Notch preferentially in osteocytes, Dmp1-Cre transgenics were crossed with Rosa(Notch) mice, where a loxP-flanked STOP cassette is placed between the Rosa26 promoter and Notch1 intracellular domain sequences. Dmp1-Cre(+/-);Rosa(Notch) mice exhibited an increase in trabecular bone volume due to decreased bone resorption and an increase in cortical bone due to increased bone formation. Biomechanical and chemical properties were not affected. Osteoprotegerin mRNA was increased, sclerostin and dickkopf1 mRNA were decreased, and Wnt signaling was enhanced in Dmp1-Cre(+/-);Rosa(Notch) femurs. Botulinum toxin A-induced muscle paralysis caused pronounced osteopenia in control mice, but bone mass was preserved in mice harboring the Notch activation in osteocytes. In conclusion, Notch plays a unique role in osteocytes, up-regulates osteoprotegerin and Wnt signaling, and differentially regulates trabecular and cortical bone homeostasis.

Jagged1 immobilization to an osteoconductive polymer activates the Notch signaling pathway and induces osteogenesis

Treatment of nonunion fractures is a significant problem. Common therapeutics, including autologous bone grafts and bone morphogenetic proteins, show well-established limitations. Therefore, a need persists for the identification of novel clinical therapies to promote healing. The Notch signaling pathway regulates bone development. Clinically, loss-of-function mutations to the Notch ligand Jagged1 decrease bone mass and increase fracture risk. Jagged1 is also the most highly upregulated ligand during fracture repair, identifying it as a potential target to promote bone formation. Therefore, the objective of this study was to develop a clinically translatable construct comprised of Jagged1 and an osteoconductive scaffold, and characterize its activity in human mesenchymal stem cells (hMSC). We first evaluated the effects of Jagged1 directly immobilized to a novel poly(β-amino ester) relative to indirect coupling via antibody. Direct was more effective at activating hMSC Notch target gene expression and osteogenic activity. We then found that directly immobilized Jagged1 constructs induced osteoblast differentiation. This is the first study to demonstrate that Jagged1 delivery transiently activates Notch signaling and increases osteogenesis. A positive correlation was found between Jagged1-induced Notch and osteogenic expression. Collectively, these results indicate that Jagged1 coupled to an osteogenic biomaterial could promote bone tissue formation during fracture healing.

Systemic inhibition of canonical Notch signaling results in sustained callus inflammation and alters multiple phases of fracture healing

The Notch signaling pathway is an important regulator of embryological bone development, and many aspects of development are recapitulated during bone repair. We have previously reported that Notch signaling components are upregulated during bone fracture healing. However, the significance of the Notch pathway in bone regeneration has not been described. Therefore, the objective of this study was to determine the importance of Notch signaling in regulating bone fracture healing by using a temporally controlled inducible transgenic mouse model (Mx1-Cre;dnMAML^f/-) to impair RBPjκ-mediated canonical Notch signaling. The Mx1 promoter was synthetically activated resulting in temporally regulated systemic dnMAML expression just prior to creation of bilateral tibial fractures. This allowed for mice to undergo unaltered embryological and post-natal skeletal development. Results showed that systemic Notch inhibition prolonged expression of inflammatory cytokines and neutrophil cell inflammation, and reduced the proportion of cartilage formation within the callus at 10 days-post-fracture (dpf) Notch inhibition did not affect early bone formation at 10dpf, but significantly altered bone maturation and remodeling at 20dpf. Increased bone volume fraction in dnMAML fractures, which was due to a moderate decrease in callus size with no change in bone mass, coincided with increased trabecular thickness but decreased connectivity density, indicating that patterning of bone was altered. Notch inhibition decreased total osteogenic cell density, which was comprised of more osteocytes rather than osteoblasts. dnMAML also decreased osteoclast density, suggesting that osteoclast activity may also be important for altered fracture healing. It is likely that systemic Notch inhibition had both direct effects within cell types as well as indirect effects initiated by temporally upstream events in the fracture healing cascade. Surprisingly, Notch inhibition did not alter cell proliferation. In conclusion, our results demonstrate that the Notch signaling pathway is required for the proper temporal progression of events required for successful bone fracture healing.

HES1, a target of Notch signaling, is elevated in canine osteosarcoma, but reduced in the most aggressive tumors

BACKGROUND

Hairy and enhancer of split 1 (HES1), a basic helix-loop-helix transcriptional repressor, is a downstream target of Notch signaling. Notch signaling and HES1 expression have been linked to growth and survival in a variety of human cancer types and have been associated with increased metastasis and invasiveness in human osteosarcoma cell lines. Osteosarcoma (OSA) is an aggressive cancer demonstrating both high metastatic rate and chemotherapeutic resistance. The current study examined expression of Notch signaling mediators in primary canine OSA tumors and canine and human osteosarcoma cell lines to assess their role in OSA development and progression.

RESULTS

Reverse transcriptase - quantitative PCR (RT-qPCR) was utilized to quantify HES1, HEY1, NOTCH1 and NOTCH2 gene expression in matched tumor and normal metaphyseal bone samples taken from dogs treated for appendicular OSA at the Colorado State University Veterinary Teaching Hospital. Gene expression was also assessed in tumors from dogs with a disease free interval (DFI) of <100 days compared to those with a DFI > 300 days following treatment with surgical amputation followed by standard chemotherapy. Immunohistochemistry was performed to confirm expression of HES1. Data from RT-qPCR and immunohistochemical (IHC) experiments were analyzed using REST2009 software and survival analysis based on IHC expression employed the Kaplan-Meier method and log rank analysis. Unbiased clustered images were generated from gene array analysis data for Notch/HES1 associated genes. Gene array analysis of Notch/HES1 associated genes suggested alterations in the Notch signaling pathway may contribute to the development of canine OSA. HES1 mRNA expression was elevated in tumor samples relative to normal bone, but decreased in tumor samples from dogs with a DFI < 100 days relative to those with a DFI > 300 days. NOTCH2 and HEY1 mRNA expression was also elevated in tumors relative to normal bone, but was not differentially expressed between the DFI tumor groups. Survival analysis confirmed an association between decreased HES1 immunosignal and shorter DFI.

CONCLUSIONS

Our findings suggest that activation of Notch signaling occurs and may contribute to the development of canine OSA. However, association of low HES1 expression and shorter DFI suggests that mechanisms that do not alter HES1 expression may drive the most aggressive tumors.

Mesenchymal Stem Cells in Bone Regeneration

SIGNIFICANCE

Mesenchymal stem cells (MSCs) play a key role in fracture repair by differentiating to become bone-forming osteoblasts and cartilage-forming chondrocytes. Cartilage then serves as a template for additional bone formation through the process of endochondral ossification.

RECENT ADVANCES

Endogenous MSCs that contribute to healing are primarily derived from the periosteum, endosteum, and marrow cavity, but also may be contributed from the overlying muscle or through systemic circulation, depending on the type of injury. A variety of growth factor signaling pathways, including BMP, Wnt, and Notch signaling, influence MSC proliferation and differentiation. These MSCs can be therapeutically manipulated to promote differentiation. Furthermore, MSCs can be harvested, cultivated, and delivered to promote bone healing.

CRITICAL ISSUES

Pharmacologically manipulating the number and differentiation capacity of endogenous MSCs is one potential therapeutic approach to improve healing; however, ideal agents to influence signaling pathways need to be developed and additional therapeutics that activate endogenous MSCs are needed. Whether isolated and purified, MSCs participate directly in the healing process or serve a bystander effect and indirectly influence healing is not well defined.

FUTURE DIRECTIONS

Studies must focus on better understanding the regulation of endogenous MSCs durings fracture healing. This will reveal novel molecules and pathways to therapeutically target. Similarly, while animal models have demonstrated efficacy in the delivery of MSCs to promote healing, more research is needed to understand ideal donor cells, cultivation methods, and delivery before stem cell therapy approaches can be utilized to repair bone.

Hairy and Enhancer of Split-related with YRPW motif (HEY)2 regulates bone remodeling in mice

Notch induces Hairy and Enhancer of Split-related with YRPW motif (Hey)1, Hey2, and HeyL expression in osteoblasts, but the contributions of these genes to the skeletal effects of Notch are not fully understood. HEY1 misexpression has limited skeletal impact, female HeyL null mice display increased bone mass, and Hey2 inactivation is developmentally lethal. To inactivate Hey2 in immature or mature osteoblasts, Hey2(loxP/loxP) mice were crossed with transgenics expressing CRE under the control of the osterix (Osx-Cre) or osteocalcin (Oc-Cre) promoters to generate Osx-Cre(+/-);Hey2(Δ/Δ) or Oc-Cre(+/-);Hey2(Δ/Δ) mice. Trabecular bone volume increased in 3-month-old Osx-Cre(+/-);Hey2(Δ/Δ) and Oc-Cre(+/-);Hey2(Δ/Δ) male mice and in 1-month-old Oc-Cre(+/-);Hey2(Δ/Δ) female mice, although 3-month-old Oc-Cre(+/-);Hey2(Δ/Δ) females developed osteopenia. Alkaline phosphatase liver/bone/kidney (ALPL) expression and activity were suppressed in osteoblasts from Oc-Cre(+/-);Hey2(Δ/Δ) mice of both sexes. To overexpress HEY2 in osteoblasts, transgenic mice where a 3.6-kb fragment of the rat collagen type-I α1 promoter directs HEY2 expression were created. Three-month-old Hey2 transgenic males exhibited decreased osteoblast activity and increased bone resorption and developed osteopenia at 6 months of age. Hey2 transgenic females exhibited reduced osteoblast number and function, but no changes in bone resorption. HEY2 overexpression in osteoblasts from mice of both sexes inhibited ALPL expression and activity and suppressed osteocalcin transcripts in cells from male mice only. HEY2 overexpression in osteoblasts from male mice enhanced bone resorption by co-cultured splenocytes and induced interleukin-6, a molecule that promotes osteoclastogenesis. In conclusion, HEY2 decreases skeletal mass and regulates bone remodeling in male mice.

Notch Signaling is Associated with ALDH Activity and an Aggressive Metastatic Phenotype in Murine Osteosarcoma Cells

Osteosarcoma (OS) is the most common primary malignancy of bone, and pulmonary metastatic disease accounts for nearly all mortality. However, little is known about the biochemical signaling alterations that drive the progression of metastatic disease. Two murine OS cell populations, K7M2 and K12, are clonally related but differ significantly in their metastatic phenotypes and therefore represent excellent tools for studying metastatic OS molecular biology. K7M2 cells are highly metastatic, whereas K12 cells display limited metastatic potential. Here we report that the expression of Notch genes (Notch1, 2, 4) are up-regulated, including downstream targets Hes1 and Stat3, in the highly metastatic K7M2 cells compared to the less metastatic K12 cells, indicating that the Notch signaling pathway is more active in K7M2 cells. We have previously described that K7M2 cells exhibit higher levels of aldehyde dehydrogenase (ALDH) activity. Here we report that K7M2 cell ALDH activity is reduced with Notch inhibition, suggesting that ALDH activity may be regulated in part by the Notch pathway. Notch signaling is also associated with increased resistance to oxidative stress, migration, invasion, and VEGF expression in vitro. However, Notch inhibition did not significantly alter K7M2 cell proliferation. In conclusion, we provide evidence that Notch signaling is associated with ALDH activity and increased metastatic behavior in OS cells. Both Notch and ALDH are putative molecular targets for the treatment and prevention of OS metastasis.

Notch signaling in skeletal health and disease

Notch receptors are single-pass transmembrane proteins that determine cell fate. Upon Notch ligand interactions, proteolytic cleavages release the Notch intracellular domain, which translocates to the nucleus to regulate the transcription of target genes, including Hairy enhancer of split (Hes) and Hes related to YRPW motif (Hey). Notch is critical for skeletal development and activity of skeletal cells, and dysregulation of Notch signaling is associated with human diseases affecting the skeleton. Inherited or sporadic mutations in components of the Notch signaling pathway are associated with spondylocostal dysostosis, spondylothoracic dysostosis and recessive brachydactyly, diseases characterized by skeletal patterning defects. Inactivating mutations of the Notch ligand JAG1 or of NOTCH2 are associated with Alagille syndrome, and activating mutations in NOTCH2 are associated with Hajdu-Cheney syndrome (HCS). Individuals affected by HCS exhibit osteolysis in distal phalanges and osteoporosis. NOTCH is activated in selected tumors, such as osteosarcoma, and in breast cancer cells that form osteolytic bone metastases. In conclusion, Notch regulates skeletal development and bone remodeling, and gain- or loss-of-function mutations of Notch signaling result in important skeletal diseases.

Notch signaling and bone remodeling

Notch signaling plays context-dependent roles in the development and maintenance of many cell types and tissues in mammals. In the skeleton, both osteoblasts and osteoclasts require Notch signaling for proper differentiation and function, and the specific roles of Notch are dependent on the differentiation status of the cell. The recent discovery of activating NOTCH2 mutations as the cause of Hajdu-Cheney syndrome has highlighted the significance of Notch signaling in human bone physiology.

Mitogen-activated protein kinase pathways in osteoblasts

Mitogen-activated protein kinases (MAPKs) are ancient signal transducers well characterized as mediators of inflammation and neoplastic transformation. Recent work has expanded our understanding of their developmental functions, particularly in the regulation of bone mass via control of osteoblast differentiation. Here, we review the functions of MAPK pathways in osteoblasts, including a consideration of MAPK substrates. In particular, MAPKs function to regulate the key transcriptional mediators of osteoblast differentiation, with ERK and p38 MAPKs phosphorylating RUNX2, the master regulator of osteoblast differentiation. ERK also activates RSK2, which in turn phosphorylates ATF4, a transcriptional regulator of late-stage osteoblast synthetic functions. The MAP3Ks and MAP2Ks upstream of MAPKs have also been investigated, and significant differences have been found in the wiring of MAPK pathways in osteoblasts relative to other tissues. Thus, the investigation of MAPKs in osteoblasts has both revealed critical mechanisms for the maintenance of bone mass and added to our understanding of how the individual components of MAPK pathways function in concert in a complex in vivo system.

Androgen receptor agonism promotes an osteogenic gene program in preadipocytes

Androgens regulate body composition by interacting with the androgen receptor (AR) to control gene expression in a tissue-specific manner. To identify novel regulatory roles for AR in preadipocytes, we created a 3T3-L1 cell line stably expressing human AR. We found AR expression is required for androgen-mediated inhibition of 3T3-L1 adipogenesis. This inhibition is characterized by decreased lipid accumulation, reduced expression of adipogenic genes, and induction of genes associated with osteoblast differentiation. Collectively, our results suggest androgens promote an osteogenic gene program at the expense of adipocyte differentiation.

Hand in glove: brain and skull in development and dysmorphogenesis

The brain originates relatively early in development from differentiated ectoderm that forms a hollow tube and takes on an exceedingly complex shape with development. The skull is made up of individual bony elements that form from neural crest- and mesoderm-derived mesenchyme that unite to provide support and protection for soft tissues and spaces of the head. The meninges provide a protective and permeable membrane between brain and skull. Across evolutionary and developmental time, dynamic changes in brain and skull shape track one another so that their integration is evidenced in two structures that fit soundly regardless of changes in biomechanical and physiologic functions. Evidence for this tight correspondence is also seen in diseases of the craniofacial complex that are often classified as diseases of the skull (e.g., craniosynostosis) or diseases of the brain (e.g., holoprosencephaly) even when both tissues are affected. Our review suggests a model that links brain and skull morphogenesis through coordinated integration of signaling pathways (e.g., FGF, TGFβ, Wnt) via processes that are not currently understood, perhaps involving the meninges. Differences in the earliest signaling of biological structure establish divergent designs that will be enhanced during morphogenesis. Signaling systems that pattern the developing brain are also active in patterning required for growth and assembly of the skull and some members of these signaling families have been indicated as causal for craniofacial diseases. Because cells of early brain and skull are sensitive to similar signaling families, variation in the strength or timing of signals or shifts in patterning boundaries that affect one system (neural or skull) could also affect the other system and appropriate co-adjustments in development would be made. Interactions of these signaling systems and of the tissues that they pattern are fundamental to the consistent but labile functional and structural association of brain and skull conserved over evolutionary time obvious in the study of development and disease.

Identification of gender-specific candidate genes that influence bone microarchitecture in chromosome 1

Studies on the identification of the genetic basis for sexual dimorphism in peak bone mass are obviously important for providing novel therapeutic approaches to prevent or treat metabolic bone diseases. Our goal in this study was to identify the bone microstructure that could lead to differences in volumetric bone mineral density (vBMD) and new candidate genes that regulate the gender effect on bone. We used a congenic line of mice that carry the BMD1-4 locus from CAST/EiJ (CAST) mice in a C57BL/6J (B6) background and show greater vBMD in female, but not male, congenics compared to age- and gender-matched B6 mice. To assess the vBMD variations between the two lines of mice, we performed μCT measurements and found no difference in cortical bone volume by tissue volume (BV/TV) between congenics and B6 mice. However, trabecular BV/TV was significantly greater in female, but not male, congenics compared to corresponding B6 mice, which was due to increased trabecular thickness but not reduced trabecular separation, suggesting that bone formation, but not bone resorption, is responsible for the trabecular bone phenotype observed in the female, but not male, congenics. To identify the gender candidate genes, we determined the polymorphisms between B6 and CAST within the BMD1-4 locus and performed gene expression profiling. We identified EF-hand calcium binding domain (Efcab2), consortin, connexin sorting protein (Cnst), and presenilin 2 (Psen2) as potential candidate genes that regulate bone mass by influencing trabecular thickness in a gender-specific manner.

Notch signaling coordinates cellular proliferation with differentiation during zebrafish fin regeneration

Zebrafish can completely regenerate amputated fins via formation of a blastema, a proliferative mass of undifferentiated precursor cells. During regenerative growth, blastema proliferation must be tightly coordinated with cellular differentiation, but little is known about how this is achieved. Here, we show that Notch signaling is essential for maintenance of blastema cells in a proliferative undifferentiated state. We found that the Notch pathway is activated in response to fin amputation in the highly proliferative region of the blastema. Chemical interference with Notch signaling resulted in a complete block of regeneration. Notch signaling was not required for the earliest known cellular processes during blastema formation, i.e. dedifferentiation and migration of osteoblasts, but specifically interfered with proliferation of blastema cells. Interestingly, overactivation of the pathway via misexpression of the intracellular domain of the Notch receptor (NICD) likewise inhibited regenerative outgrowth. In NICD-overexpressing fins, overall blastemal cell proliferation was not enhanced, but expanded into proximal regions where cellular differentiation normally occurs. Similarly, blastemal and epidermal gene expression territories invaded proximal regions upon sustained Notch activation. Concomitantly, NICD overexpression suppressed differentiation of osteoblasts and caused an expansion of the undifferentiated blastema. Together, these data suggest that Notch signaling activity maintains blastemal cells in a proliferative state and thus coordinates proliferation with differentiation during regenerative growth.

Notch regulates blastema proliferation and prevents differentiation during adult zebrafish fin regeneration

Zebrafish have the capacity to regenerate several organs, including the heart and fins. Fin regeneration is epimorphic, involving the formation at the amputation plane of a mass of undifferentiated, proliferating mesenchymal progenitor-like cells, called blastema. This tissue provides all the cell types that form the fin, so that after damage or amputation the fin pattern and structure are fully restored. How blastema cells remain in this progenitor-like state is poorly understood. Here, we show that the Notch pathway plays an essential role during fin regeneration. Notch signalling is activated during blastema formation and remains active throughout the regeneration process. Chemical inhibition or morpholino-mediated knockdown of Notch signalling impairs fin regeneration via decreased proliferation accompanied by reduced expression of Notch target genes in the blastema. Conversely, overexpression of a constitutively active form of the Notch1 receptor (N1ICD) in the regenerating fin leads to increased proliferation and to the expansion of the blastema cell markers msxe and msxb, as well as increased expression of the proliferation regulator aldh1a2. This blastema expansion prevents regenerative fin outgrowth, as indicated by the reduction in differentiating osteoblasts and the inhibition of bone regeneration. We conclude that Notch signalling maintains blastema cells in a plastic, undifferentiated and proliferative state, an essential requirement for fin regeneration.

Notch gain of function inhibits chondrocyte differentiation via Rbpj-dependent suppression of Sox9

Notch signaling plays a critical role during development by directing the binary cell fate decision between progenitors and differentiated cells. Previous studies have shown sustained Notch activation in cartilage leads to chondrodysplasia. Genetic evidence indicates that Notch regulates limb bud mesenchymal stem cell differentiation into chondrocytes via an Rbpj-dependent Notch pathway. However, it is still unknown how Notch governs chondrogenesis in the axial skeleton where Notch serves a primary patterning function. We hypothesized that both Rbpj-dependent and Rbpj-independent Notch signaling mechanisms might be involved. Cartilage-specific Notch gain-of-function (GOF) mutant mice display chondrodysplasia accompanied by loss of Sox9 expression in vertebrae. To evaluate the contribution of an Rbpj-dependent Notch signaling to this phenotype, we deleted Rbpj on the Notch GOF background. These mice showed persistent spine abnormalities characterized by "butterfly" vertebrae suggesting that removal of Rbpj does not fully rescue the axial skeleton deformities caused by Notch GOF. However, Sox9 protein level was restored in Rbpj-deficient Notch GOF mice compared with Notch GOF mutants, demonstrating that regulation of Sox9 expression is canonical or Rbpj-dependent. To further understand the molecular basis of this regulation, we performed chromatin immunoprecipitation (ChIP) assays and detected the recruitment of the Rbpj/NICD transcription complex to Rbpj-binding sites upstream of the Sox9 promoter. The association of the Rbpj/NICD complex with the Sox9 promoter is associated with transcriptional repression of Sox9 in a cellular model of chondrocyte differentiation. Hence, Notch negatively regulates chondrocyte differentiation in the axial skeleton by suppressing Sox9 transcription, and Rbpj-independent Notch signaling mechanisms may also contribute to axial skeletogenesis.

Osteoblast lineage-specific effects of notch activation in the skeleton

Transgenic overexpression of the Notch1 intracellular domain inhibits osteoblast differentiation and causes osteopenia, and inactivation of Notch1 and Notch2 increases bone volume transiently and induces osteoblastic differentiation. However, the biology of Notch is cell-context-dependent, and consequences of Notch activation in cells of the osteoblastic lineage at various stages of differentiation and in osteocytes have not been defined. For this purpose, Rosa(Notch) mice, where a loxP-flanked STOP cassette placed between the Rosa26 promoter and the NICD coding sequence, were crossed with transgenics expressing the Cre recombinase under the control of the Osterix (Osx), Osteocalcin (Oc), Collagen 1a1 (Col2.3), or Dentin matrix protein1 (Dmp1) promoters. At 1 month, Osx-Cre;Rosa(Notch) and Oc-Cre;Rosa(Notch) mice exhibited osteopenia due to impaired bone formation. In contrast, Col2.3-Cre;Rosa(Notch) and Dmp1-Cre;Rosa(Notch) exhibited increased femoral trabecular bone volume due to a decrease in osteoclast number and eroded surface. In the four lines studied, cortical bone was either not present, was porous, or had the appearance of trabecular bone. Oc-Cre;Rosa(Notch) and Col2.3-Cre;Rosa(Notch) mice exhibited early lethality so that their adult phenotype was not established. At 3 months, Osx-Cre;Rosa(Notch) and Dmp1-Cre;Rosa(Notch) mice displayed increased bone volume, and increased osteoblasts although calcein-demeclocycline labels were diffuse and fragmented, indicating abnormal bone formation. In conclusion, Notch effects in the skeleton are cell-context-dependent. When expressed in immature osteoblasts, Notch arrests their differentiation, causing osteopenia, and when expressed in osteocytes, it causes an initial suppression of bone resorption and increased bone volume, a phenotype that evolves as the mice mature.

Notch suppresses nuclear factor of activated T cells (NFAT) transactivation and Nfatc1 expression in chondrocytes

Notch1 to Notch4 transmembrane receptors determine cell fate, and release of the Notch intracellular domain (NICD) in the cytoplasm induces gene expression. Notch regulates endochondral ossification, but it is not clear whether Notch interacts with signals controlling chondrocyte differentiation. Nuclear factor of activated T cells (Nfatc) transcription factors regulate chondrogenesis, and we asked whether Notch modifies Nfat signaling in chondrocytes. Notch was induced in teratocarcinoma ATDC5 chondrogenic cells infected with a retroviral vector, where the cytomegalovirus (CMV) promoter directs NICD expression. NICD suppressed chondrocyte differentiation and inhibited Nfat transactivation and Nfatc1 expression. Notch was activated in chondrocytes from Rosa(Notch) mice, where the Rosa26 promoter is upstream of a loxP-flanked STOP cassette and NICD. To excise the STOP cassette and express NICD, Rosa(Notch) chondrocytes were infected with an adenoviral vector where the CMV promoter directs Cre expression (Ad-CMV-Cre). Notch1 and Notch2 mediate the effects of Notch in skeletal cells, and to inhibit Notch signaling, chondrocytes from mice homozygous for Notch1 and Notch2 alleles targeted with loxP sites were infected with Ad-CMV-Cre. NICD suppressed chondrogenic nodules formation and expression of selected chondrocyte gene markers, induced Col10a1 and Mmp13, and suppressed Nfat transactivation and Nfatc1 expression, whereas inactivation of Notch1 and Notch2 did not affect chondrocyte differentiation. To investigate Nfatc1 function in chondrocytes, Nfatc1 was induced in Rosa(Notch) chondrocytes overexpressing NICD or controls. Nfatc1 suppressed chondrocyte differentiation and opposed Col10a1 induction by Notch. In conclusion, Notch suppresses Nfat transactivation in chondrocytes and Notch and Nfatc1 regulate chondrocyte differentiation.

Effect of radiation on the Notch signaling pathway in osteoblasts

Notch signaling has been shown to be important in osteoblast differentiation. Therapeutic radiation has been shown to alter the skeletal system, yet little information is available on the changes in Notch signaling in irradiated osteoblasts. The purpose of this study was to analyze the effect of radiation therapy with 2 and 4 Gy on Notch signaling in osteoblasts. In order to assess the radiation damage on osteoblast differentiation, total RNA and protein were collected three days after exposure to radiation. The effects of radiation on Notch signaling at the early and terminal stages of osteoblastic MC3T3-E1 cell differentiation was analyzed by qRT-PCR and western blot analysis. Our study applied a previously established method to induce MC3T3-E1 cell differentiation into osteoblasts and osteoblast precursors. Our results showed that the expression of Notch receptors (Notch1-4), ligands (Jagged1, Jagged2 and Delta1), target of Notch signaling (Hes1) and markers (ALP, M-CSF, RANKL and OPG) were altered following 2 and 4 Gy of irradiation. The present research did not indicate a strong relationship between Notch1 regulation and suppression of osteoblast differentiation. We found Hes1 may play a role in the radiation effect on osteoblast differentiation. Our results indicate that radiated osteoblast precursors and osteoblasts promoted osteoclast differentiation and proliferation.

Glycogen synthase kinase 3β promotes osteogenic differentiation of murine adipose-derived stromal cells

Although the role of glycogen synthase kinase 3β (GSK3β) in osteogenic differentiation of bone marrow-derived mesenchymal stromal cells (BMSCs) is well-characterized as a negative regulator of β-catenin, its effect on osteogenesis of adipose-derived stromal cells (ADSCs) is poorly understood. Here, we show that GSK3β positively regulates osteogenic differentiation of murine ADSCs. Gain-of-function studies showed that GSK3β promotes in vitro osteogenesis of ADSCs. Regulation of GSK3β activity in ADSCs, either by small interfering RNA (siRNA)-mediated GSK3β silencing or by pharmacological inhibitors, blunted osteogenesis and the expression of osteogenic markers. Importantly, we demonstrated that transgenic mice, engineered to overexpress the constitutively active GSK3β (GSK3β-S9A) mutant, exhibited a marked increase in osteogenesis, whereas expression of the catalytically inactive GSK3β (GSK3β-K85A) in mice inhibits osteogenic differentiation. Molecular analyses showed that the enhanced osteoblast differentiation induced by GSK3β was mediated by downregulation of β-catenin. Remarkably, β-catenin silencing enhances osteogenesis and osteoblast marker gene expression such as alkaline phosphatase (ALP) and osterix. Taken together, these findings demonstrate a novel role for GSK3β in the regulation of osteogenic differentiation in ADSCs.

MAML1 enhances the transcriptional activity of Runx2 and plays a role in bone development

Mastermind-like 1 (MAML1) is a transcriptional co-activator in the Notch signaling pathway. Recently, however, several reports revealed novel and unique roles for MAML1 that are independent of the Notch signaling pathway. We found that MAML1 enhances the transcriptional activity of runt-related transcription factor 2 (Runx2), a transcription factor essential for osteoblastic differentiation and chondrocyte proliferation and maturation. MAML1 significantly enhanced the Runx2-mediated transcription of the p6OSE2-Luc reporter, in which luciferase expression was controlled by six copies of the osteoblast specific element 2 (OSE2) from the Runx2-regulated osteocalcin gene promoter. Interestingly, a deletion mutant of MAML1 lacking the N-terminal Notch-binding domain also enhanced Runx2-mediated transcription. Moreover, inhibition of Notch signaling did not affect the action of MAML1 on Runx2, suggesting that the activation of Runx2 by MAML1 may be caused in a Notch-independent manner. Overexpression of MAML1 transiently enhanced the Runx2-mediated expression of alkaline phosphatase, an early marker of osteoblast differentiation, in the murine pluripotent mesenchymal cell line C3H10T1/2. MAML1(-/-) embryos at embryonic day 16.5 (E16.5) had shorter bone lengths than wild-type embryos. The area of primary spongiosa of the femoral diaphysis was narrowed. At E14.5, extended zone of collagen type II alpha 1 (Col2a1) and Sox9 expression, markers of chondrocyte differentiation, and decreased zone of collagen type X alpha 1 (Col10a1) expression, a marker of hypertrophic chondrocyte, were observed. These observations suggest that chondrocyte maturation was impaired in MAML1(-/-) mice. MAML1 enhances the transcriptional activity of Runx2 and plays a role in bone development.

Development of the endochondral skeleton

Much of the mammalian skeleton is composed of bones that originate from cartilage templates through endochondral ossification. Elucidating the mechanisms that control endochondral bone development is critical for understanding human skeletal diseases, injury response, and aging. Mouse genetic studies in the past 15 years have provided unprecedented insights about molecules regulating chondrocyte formation, chondrocyte maturation, and osteoblast differentiation, all key processes of endochondral bone development. These include the roles of the secreted proteins IHH, PTHrP, BMPs, WNTs, and FGFs, their receptors, and transcription factors such as SOX9, RUNX2, and OSX, in regulating chondrocyte and osteoblast biology. This review aims to integrate the known functions of extracellular signals and transcription factors that regulate development of the endochondral skeleton.

Disruption of the transcription factor RBP-J results in osteopenia attributable to attenuated osteoclast differentiation

The transcription factor recombination signal binding protein-Jκ (RBP-J) is the critical transcription factor downstream to all four mammalian Notch receptors. Although it has been reported that Notch signaling pathway is involved in bone remodeling, the importance of RBP-J in osteoclastogenesis has not been fully explored. To investigate the role of RBP-J in osteoclastogenesis, we conditionally deleted RBP-J systemically in bone marrow (BM) or specifically in macrophages. We found that disruption of RBP-J in BM resulted in an obvious decrease in trabecular bone mass associated with an increase in osteoclasts, leading to osteopenia. Disruption of RBP-J in macrophages phenocopied the phenotypes of RBP-J deletion in BM with respect to osteoclastogenesis, suggesting that the osteopenia in RBP-J deficient mice is essentially resulted from increased osteoclastogenesis. Furthermore, we found that RBP-J deletion in osteoclasts resulted in a dramatic increase in tartrate-resistant acid phosphatase expression. These findings demonstrate a negatively role of RBP-J in the differentiation of osteoclasts and suggest that Notch pathway may be a new therapeutic target for bone diseases related to increased osteoclastogenesis.

Nuclear factor of activated T-cells (NFAT)C2 inhibits Notch receptor signaling in osteoblasts

Notch receptors regulate osteoblastogenesis, and Notch activation induces cleavage and nuclear translocation of the Notch intracellular domain (NICD), which associates with Epstein-Barr virus latency C-promoter binding factor-1/suppressor of hairless/lag-1 (CSL) and induces transcription of Notch target genes, such as hairy enhancer of split-related with YRPW motif (Hey)1 and Hey2. Nuclear factors of activated T-cells (NFAT) are transcription factors that regulate osteoclastogenesis, but their function in osteoblasts is not clear. Notch inhibits NFATc1 transcription, but interactions between Notch and NFAT are understood poorly. To determine the regulation of NFAT expression by Notch, osteoblasts from Rosa(Notch) mice, where NICD is transcribed following excision of a loxP flanked STOP cassette, were used. Alternatively, wild-type C57BL/6 osteoblasts were exposed to the Notch ligand Delta-like (Dll)1 to induce Notch signaling or to bovine serum albumin as control. In Rosa(Notch) osteoblasts, Notch suppressed NFATc1 expression, increased Nfatc2 mRNA by post-transcriptional mechanisms, and had no effect on NFATc3 and NFATc4 transcripts. Induction of Nfatc2 transcripts by Notch was confirmed in C57BL/6 osteoblasts exposed to Dll1. To investigate NFATc2 function in osteoblasts, constitutively active NFATc2 was overexpressed in Rosa(Notch) osteoblasts. NFATc2 suppressed Notch transactivation and expression of Hey genes. Electrophoretic mobility shift assays revealed that NFATc2 and CSL bind to similar DNA sequences, and chromatin immunoprecipitation indicated that NFATc2 displaced CSL from the Hey2 promoter. The effects of NICD and NFATc2 in Rosa(Notch) osteoblasts were assessed, and both proteins inhibited osteoblast function. In conclusion, Notch stabilizes Nfatc2 transcripts, NFATc2 suppresses Notch signaling, and both proteins inhibit osteoblast function.

LRF-mediated Dll4 repression in erythroblasts is necessary for hematopoietic stem cell maintenance

Hematopoietic stem cells (HSCs) are the most primitive cells in the hematopoietic system and are under tight regulation for self-renewal and differentiation. Notch signals are essential for the emergence of definitive hematopoiesis in mouse embryos and are critical regulators of lymphoid lineage fate determination. However, it remains unclear how Notch regulates the balance between HSC self-renewal and differentiation in the adult bone marrow (BM). Here we report a novel mechanism that prevents HSCs from undergoing premature lymphoid differentiation in BM. Using a series of in vivo mouse models and functional HSC assays, we show that leukemia/lymphoma related factor (LRF) is necessary for HSC maintenance by functioning as an erythroid-specific repressor of Delta-like 4 (Dll4) expression. Lrf deletion in erythroblasts promoted up-regulation of Dll4 in erythroblasts, sensitizing HSCs to T-cell instructive signals in the BM. Our study reveals novel cross-talk between HSCs and erythroblasts, and sheds a new light on the regulatory mechanisms regulating the balance between HSC self-renewal and differentiation.

Hypoxia inhibits the differentiation of mesenchymal stem cells into osteoblasts by activation of Notch signaling

Postnatal bone marrow contains mesenchymal stem cells (MSCs) that are osteoblast precursors with great therapeutic potential. The oxygen tension in bone marrow is about 1-7% pO2 which is much lower than that of the external environment. The effect of these hypoxic conditions on MSC differentiation is not fully understood. In this study, we show that hypoxia inhibits osteogenic differentiation of MSCs, and that this effect is associated with increased levels of Notch1 and increased activity of Notch signaling. Knockdown of Notch1 in MSCs by shRNA markedly rescues the impaired osteogenic differentiation of MSCs. Furthermore, Notch1 physiologically binds to Runx2 and inhibits its transcriptional activity. Thus, hypoxia inhibits MSC differentiation into osteoblasts by activating the Notch pathway.

Genetics of osteoporosis from genome-wide association studies: advances and challenges

Osteoporosis is among the most common and costly diseases and is increasing in prevalence owing to the ageing of our global population. Clinically defined largely through bone mineral density, osteoporosis and osteoporotic fractures have reasonably high heritabilities, prompting much effort to identify the genetic determinants of this disease. Genome-wide association studies have recently provided rapid insights into the allelic architecture of this condition, identifying 62 genome-wide-significant loci. Here, we review how these new loci provide an opportunity to explore how the genetics of osteoporosis can elucidate its pathophysiology, provide drug targets and allow for prediction of future fracture risk.

Osteoblastogenesis regulation signals in bone remodeling

Bone remodeling is essential for adult bone homeostasis. The failure of this process often leads to the development of osteoporosis, a present major global health concern. The most important factor that affects normal bone remodeling is the tightly controlled and orchestrated regulation of osteoblasts and osteoclasts. The present review summarized the recent discoveries related to osteoblast regulation from several signals, including transforming growth factor-β, bone morphogenetic proteins, Wnt signal, Notch, Eph-Ephrin interaction, parathyroid hormone/parathyroid hormone-related peptide, and the leptin-serotonin-sympathetic nervous systemic pathway. The awareness of these mechanisms will facilitate further research that explores bone remodeling and osteoporosis. Future investigations on the endogenous regulation of osteoblastogenesis will increase the current knowledge required for the development of potential drug targets in the treatment of osteoporosis.

Notch and disease: a growing field

Signals through the Notch receptors are used throughout development to control cellular fate choices. Our intention here is to provide an overview of the involvement of Notch signaling in human disease, which, keeping pace with the known biology of the pathway, manifests itself in a pleiotropic fashion. A pathway with such broad action in normal development, a profound involvement in the biology of adult stem cells and intricate and complex controls governing its activity, poses numerous challenges. We provide an overview of Notch related pathologies identified thus far and emphasize aspects that have been modeled in experimental systems in order to understand the underlying pathobiology and, hopefully, help the definition of rational therapeutic avenues.

miR-34s inhibit osteoblast proliferation and differentiation in the mouse by targeting SATB2

miR-34b and -c inhibit osteoblast proliferation and differentiation by decreasing the levels of cell cycle proteins and of the nuclear matrix protein SATB2.

A screen of microRNAs preferentially expressed in osteoblasts identified members of the miR-34 family as regulators of osteoblast proliferation and/or differentiation. Osteoblast-specific gain- and loss-of-function experiments performed in vivo revealed that miR-34b and -c affected skeletogenesis during embryonic development, as well as bone mass accrual after birth, through two complementary cellular and molecular mechanisms. First, they inhibited osteoblast proliferation by suppressing Cyclin D1, CDK4, and CDK6 accumulation. Second, they inhibited terminal differentiation of osteoblasts, at least in part through the inhibition of SATB2, a nuclear matrix protein that is a critical determinant of osteoblast differentiation. Genetic evidence obtained in the mouse confirmed the importance of SATB2 regulation by miR-34b/c. These results are the first to identify a family of microRNAs involved in bone formation in vivo and to identify a specific genetic pathway by which these microRNAs regulate osteoblast differentiation.

Cartilage-specific RBPjκ-dependent and -independent Notch signals regulate cartilage and bone development

The Notch signaling pathway has emerged as an important regulator of endochondral bone formation. Although recent studies have examined the role of Notch in mesenchymal and chondro-osteo progenitor cell populations, there has yet to be a true examination of Notch signaling specifically within developing and committed chondrocytes, or a determination of whether cartilage and bone formation are regulated via RBPjκ-dependent or -independent Notch signaling mechanisms. To develop a complete understanding of Notch signaling during cartilage and bone development we generated and compared general Notch gain-of-function (Rosa-NICD(f/+)), RBPjκ-deficient (Rbpjκ(f/f)), and RBPjκ-deficient Notch gain-of-function (Rosa-NICD(f/+);Rbpjκ(f/f)) conditional mutant mice, where activation or deletion of floxed alleles were specifically targeted to mesenchymal progenitors (Prx1Cre) or committed chondrocytes (inducible Col2Cre(ERT2)). These data demonstrate, for the first time, that Notch regulation of chondrocyte maturation is solely mediated via the RBPjκ-dependent pathway, and that the perichodrium or osteogenic lineage probably influences chondrocyte terminal maturation and turnover of the cartilage matrix. Our study further identifies the cartilage-specific RBPjκ-independent pathway as crucial for the proper regulation of chondrocyte proliferation, survival and columnar chondrocyte organization. Unexpectedly, the RBPjκ-independent Notch pathway was also identified as an important long-range cell non-autonomous regulator of perichondral bone formation and an important cartilage-derived signal required for coordinating chondrocyte and osteoblast differentiation during endochondral bone development. Finally, cartilage-specific RBPjκ-independent Notch signaling likely regulates Ihh responsiveness during cartilage and bone development.

miRNA-34c regulates Notch signaling during bone development

During bone homeostasis, osteoblast and osteoclast differentiation is coupled and regulated by multiple signaling pathways and their downstream transcription factors. Here, we show that microRNA 34 (miR-34) is significantly induced by BMP2 during osteoblast differentiation. In vivo, osteoblast-specific gain of miR-34c in mice leads to an age-dependent osteoporosis due to the defective mineralization and proliferation of osteoblasts and increased osteoclastogenesis. In osteoblasts, miR-34c targets multiple components of the Notch signaling pathway, including Notch1, Notch2 and Jag1 in a direct manner, and influences osteoclast differentiation in a non-cell-autonomous fashion. Taken together, our results demonstrate that miR-34c is critical during osteoblastogenesis in part by regulating Notch signaling in bone homeostasis. Furthermore, miR-34c-mediated post-transcriptional regulation of Notch signaling in osteoblasts is one possible mechanism to modulate the proliferative effect of Notch in the committed osteoblast progenitors which may be important in the pathogenesis of osteosarcomas. Therefore, understanding the functional interaction of miR-34 and Notch signaling in normal bone development and in bone cancer could potentially lead to therapies modulating miR-34 signaling.

Integration of BMP, Wnt, and notch signaling pathways in osteoblast differentiation

Bone marrow-derived mesenchymal stem cells (MSCs) are multipotent progenitors that can commit to osteoblast, chondrocyte, adipocyte, and several other lineages. The proper utilization of stem cells for clinical applications requires an integrated understanding of multiple signal inputs that control maintenance of stemness, proliferation, commitment, and differentiation. Various signaling pathways have been implicated in the regulation of MSC differentiation; however, complexities of pathway interactions, as well as seemingly contradictory results in the literature, create an often confusing and disjointed knowledge base. Several recent publications explore the integration of signaling pathways such as BMP, Wnt, Notch, Hedgehog, and Fibroblast Growth Factors in MSC osteoblast differentiation. The transcription factor Cbfa1/Runx2 has been implicated in these pathways as a potential focal point for signaling integration. This review will outline the current understanding of these pathways and indicate where both spatiotemporal effects during differentiation and comparable experimental conditions need to be considered in order to clarify the outcome(s) of differing regulatory levels of these signaling pathways.

Runx2 promotes both osteoblastogenesis and novel osteoclastogenic signals in ST2 mesenchymal progenitor cells

We profiled the global gene expression of a bone marrow-derived mesenchymal pluripotent cell line in response to Runx2 expression. Besides osteoblast differentiation, Runx2 promoted the osteoclastogenesis of co-cultured splenocytes. This was attributable to the upregulation of many novel osteoclastogenic genes and the downregulation of anti-osteoclastogenic genes.

INTRODUCTION

In addition to being a master regulator for osteoblast differentiation, Runx2 controls osteoblast-driven osteoclastogenesis. Previous studies profiling gene expression during osteoblast differentiation had limited focus on Runx2 or paid little attention to its role in mediating osteoblast-driven osteoclastogenesis.

METHODS

ST2/Rx2(dox), a bone marrow-derived mesenchymal pluripotent cell line that expresses Runx2 in response to Doxycycline (Dox), was used to profile Runx2-induced gene expression changes. Runx2-induced osteoblast differentiation was assessed based on alkaline phosphatase staining and expression of classical marker genes. Osteoclastogenic potential was evaluated by TRAP staining of osteoclasts that differentiated from primary murine splenocytes co-cultured with the ST2/Rx2(dox) cells. The BeadChip™ platform (Illumina) was used to interrogate genome-wide expression changes in ST2/Rx2(dox) cultures after treatment with Dox or vehicle for 24 or 48 h. Expression of selected genes was also measured by RT-qPCR.

RESULTS

Dox-mediated Runx2 induction in ST2 cells stimulated their own differentiation along the osteoblast lineage and the differentiation of co-cultured splenocytes into osteoclasts. The latter was attributable to the stimulation of osteoclastogenic genes such as Sema7a, Ltc4s, Efnb1, Apcdd1, and Tnc as well as the inhibition of anti-osteoclastogenic genes such as Tnfrsf11b (OPG), Sema3a, Slco2b1, Ogn, Clec2d (Ocil), Il1rn, and Rspo2.

CONCLUSION

Direct control of osteoblast differentiation and concomitant indirect control of osteoclast differentiation, both through the activity of Runx2 in pre-osteoblasts, constitute a novel mechanism of coordination with a potential crucial role in coupling bone formation and resorption.

Physiological notch signaling maintains bone homeostasis via RBPjk and Hey upstream of NFATc1

Notch signaling between neighboring cells controls many cell fate decisions in metazoans both during embryogenesis and in postnatal life. Previously, we uncovered a critical role for physiological Notch signaling in suppressing osteoblast differentiation in vivo. However, the contribution of individual Notch receptors and the downstream signaling mechanism have not been elucidated. Here we report that removal of Notch2, but not Notch1, from the embryonic limb mesenchyme markedly increased trabecular bone mass in adolescent mice. Deletion of the transcription factor RBPjk, a mediator of all canonical Notch signaling, in the mesenchymal progenitors but not the more mature osteoblast-lineage cells, caused a dramatic high-bone-mass phenotype characterized by increased osteoblast numbers, diminished bone marrow mesenchymal progenitor pool, and rapid age-dependent bone loss. Moreover, mice deficient in Hey1 and HeyL, two target genes of Notch-RBPjk signaling, exhibited high bone mass. Interestingly, Hey1 bound to and suppressed the NFATc1 promoter, and RBPjk deletion increased NFATc1 expression in bone. Finally, pharmacological inhibition of NFAT alleviated the high-bone-mass phenotype caused by RBPjk deletion. Thus, Notch-RBPjk signaling functions in part through Hey1-mediated inhibition of NFATc1 to suppress osteoblastogenesis, contributing to bone homeostasis in vivo.

Osteoporosis is a disease caused by disruption of the balance between bone formation and resorption resulting in a net loss of bone mass. Although anti-resorptive agents are the current mainstay of osteoporosis therapy, novel strategies to promote bone formation are critically needed for more effective prevention and treatment of the disease. Notch signaling, an evolutionally conserved mechanism among multi-cellular organisms, was recently shown to control bone formation and therefore represents a potential target pathway for novel bone-promoting therapeutics. In this study we elucidate the intracellular signaling mechanism through which Notch controls bone formation, providing a molecular framework that may guide future drug development.

TNF-induced osteoclastogenesis and inflammatory bone resorption are inhibited by transcription factor RBP-J

Tumor necrosis factor (TNF) plays a key role in the pathogenesis of inflammatory bone resorption and associated morbidity in diseases such as rheumatoid arthritis and periodontitis. Mechanisms that regulate the direct osteoclastogenic properties of TNF to limit pathological bone resorption in inflammatory settings are mostly unknown. Here, we show that the transcription factor recombinant recognition sequence binding protein at the J(κ) site (RBP-J) strongly suppresses TNF-induced osteoclastogenesis and inflammatory bone resorption, but has minimal effects on physiological bone remodeling. Myeloid-specific deletion of RBP-J converted TNF into a potent osteoclastogenic factor that could function independently of receptor activator of NF-κB (RANK) signaling. In the absence of RBP-J, TNF effectively induced osteoclastogenesis and bone resorption in RANK-deficient mice. Activation of RBP-J selectively in osteoclast precursors suppressed inflammatory osteoclastogenesis and arthritic bone resorption. Mechanistically, RBP-J suppressed induction of the master regulator of osteoclastogenesis (nuclear factor of activated T cells, cytoplasmic 1) by attenuating c-Fos activation and suppressing induction of B lymphocyte-induced maturation protein-1, thereby preventing the down-regulation of transcriptional repressors such as IRF-8 that block osteoclast differentiation. Thus, RBP-J regulates the balance between activating and repressive signals that regulate osteoclastogenesis. These findings identify RBP-J as a key upstream negative regulator of osteoclastogenesis that restrains excessive bone resorption in inflammatory settings.

Notch regulation of bone development and remodeling and related skeletal disorders

Notch signaling mediates cell-to-cell interactions that are critical for embryonic development and tissue renewal. In the canonical signaling pathway, the Notch receptor is cleaved following ligand binding, resulting in the release and nuclear translocation of the Notch intracellular domain (NICD). NICD induces gene expression by forming a ternary complex with the DNA binding protein CBF1/Rbp-Jk, Suppressor of Hairless, Lag1, and Mastermind-Like (Maml). Hairy Enhancer of Split (Hes) and Hes related with YRPW motif (Hey) are classic Notch targets. Notch canonical signaling plays a central role in skeletal development and bone remodeling by suppressing the differentiation of skeletal cells. The skeletal phenotype of mice misexpressing Hes1 phenocopies partially the effects of Notch misexpression, suggesting that Hey proteins mediate most of the skeletal effects of Notch. Dysregulation of Notch signaling is associated with diseases affecting human skeletal development, such as Alagille syndrome, brachydactyly and spondylocostal dysostosis. Somatic mutations in Notch receptors and ligands are found in tumors of the skeletal system. Overexpression of NOTCH1 is associated with osteosarcoma, and overexpression of NOTCH3 or JAGGED1 in breast cancer cells favors the formation of osteolytic bone metastasis. Activating mutations in NOTCH2 cause Hajdu-Cheney syndrome, which is characterized by skeletal defects and fractures, and JAG1 polymorphisms, are associated with variations in bone mineral density. In conclusion, Notch is a regulator of skeletal development and bone remodeling, and abnormal Notch signaling is associated with developmental and postnatal skeletal disorders.

Notch signaling components are upregulated during both endochondral and intramembranous bone regeneration

Previous studies have demonstrated that Notch signaling regulates endochondral and intramembranous bone formation by controlling cell proliferation and differentiation. Notch signaling has also been shown to regulate healing in a variety of tissues. The objective of this study was to characterize and compare activation of the Notch signaling pathway during endochondral and intramembranous bone healing using tibial fracture and calvarial defect injury models, respectively. Bilateral tibial fractures or bilateral 1.5 mm diameter calvarial defects were created in mice, and tissues were harvested at 0, 5, 10, and 20 days post-fracture. Gene expression of Notch signaling components was upregulated during both tibial fracture and calvarial defect healing, with expression generally higher during tibial fracture healing. The most highly expressed ligand and receptor during healing, Jag1 and Notch2 (specifically the activated receptor, known as NICD2), were similarly localized in mesenchymal cells during both modes of healing, with expression decreasing during chondrogenesis, but remaining present in osteoblasts at all stages of maturity. Results suggest that in addition to embryological bone development, Notch signaling regulates both endochondral and intramembranous bone healing.