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

Tissue-Specific Cultured Human Pericytes: Perivascular Cells from Smooth Muscle Tissue Have Restricted Mesodermal Differentiation Ability

Microvascular pericytes (PCs) are considered the adult counterpart of the embryonic mesoangioblasts, which represent a multipotent cell population that resides in the dorsal aorta of the developing embryo. Although PCs have been isolated from several adult organs and tissues, it is still controversial whether PCs from different tissues exhibit distinct differentiation potentials. To address this point, we investigated the differentiation potentials of isogenic human cultured PCs isolated from skeletal (sk-hPCs) and smooth muscle tissues (sm-hPCs). We found that both sk-hPCs and sm-hPCs expressed known pericytic markers and did not express endothelial, hematopoietic, and myogenic markers. Both sk-hPCs and sm-hPCs were able to differentiate into smooth muscle cells. In contrast, sk-hPCs, but not sm-hPCs, differentiated in skeletal muscle cells and osteocytes. Given the reported ability of the Notch pathway to regulate skeletal muscle and osteogenic differentiation, sk-hPCs and sm-hPCs were treated with N-[N-(3,5- difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (DAPT), a known inhibitor of Notch signaling. DAPT treatment, as assessed by histological and molecular analysis, enhanced myogenic differentiation and abolished osteogenic potential of sk-hPCs. In contrast, DAPT treatment did not affect either myogenic or osteogenic differentiation of sm-hPCs. In summary, these results indicate that, despite being isolated from the same anatomical niche, cultured PCs from skeletal muscle and smooth muscle tissues display distinct differentiation abilities.

[Up-regulation of Notch1 inhibits proliferation and differentiation of osteoclast in vitro]

OBJECTIVE

This study aimed to explore the effect of the up-regulation of Notch1 on osteoclastogenesis induced to osteoclasts by receptor activator for nuclear factor-kappaB ligand (RANKL) and macrophage colony-stimulating factors (MCSF) in vitro.

METHODS

The bone marrow stem cells (BMSCs) of Rosa(-notch1) mice were cultured and induced to osteoclasts by RANKL and MCSF. The BMSCs were transfected with the Ad-Cre-green fluorescent protein (GFP) virus or Ad-GFP virus. Total RNA from cells was extracted, and the gene expression levels of Notch1, Notch2, Notch3, Notch4, Deltal, Delta3, Delta4, Jagged1, Hes1, and tartrate resistant acid phosphatase (TRAP) were detected at the defined stage by reverse transcription-polymerase chain reaction (RT-PCR). Osteoclast formation was analyzed by TRAP assay.

RESULTS

The number of TRAP-positive multinuclear cells of the experimental group significantly decreased compared with that of the control group. The mRNA expression levels of Notch1, Notch3, Jagged1, Delta3, and Hesl of the experimental group were significantly higher than those of the control group, whereas the TRAP mRNA expression of the experimental group was significantly lower than that of the control group (P<0.05).

CONCLUSION

Up-regulation of Notch1 inhibit osteoclastogenesis of BMSCs induced by RANKL and MCSF in vitro.

[Up-regulation of Notch1 inhibits proliferation and differentiation of osteoclast in vitro]

OBJECTIVE

This study aimed to explore the effect of the up-regulation of Notch1 on osteoclastogenesis induced to osteoclasts by receptor activator for nuclear factor-kappaB ligand (RANKL) and macrophage colony-stimulating factors (MCSF) in vitro.

METHODS

The bone marrow stem cells (BMSCs) of Rosa(-notch1) mice were cultured and induced to osteoclasts by RANKL and MCSF. The BMSCs were transfected with the Ad-Cre-green fluorescent protein (GFP) virus or Ad-GFP virus. Total RNA from cells was extracted, and the gene expression levels of Notch1, Notch2, Notch3, Notch4, Deltal, Delta3, Delta4, Jagged1, Hes1, and tartrate resistant acid phosphatase (TRAP) were detected at the defined stage by reverse transcription-polymerase chain reaction (RT-PCR). Osteoclast formation was analyzed by TRAP assay.

RESULTS

The number of TRAP-positive multinuclear cells of the experimental group significantly decreased compared with that of the control group. The mRNA expression levels of Notch1, Notch3, Jagged1, Delta3, and Hesl of the experimental group were significantly higher than those of the control group, whereas the TRAP mRNA expression of the experimental group was significantly lower than that of the control group (P<0.05).

CONCLUSION

Up-regulation of Notch1 inhibit osteoclastogenesis of BMSCs induced by RANKL and MCSF in vitro.

Anabolic actions of Notch on mature bone

Notch controls skeletogenesis, but its role in the remodeling of adult bone remains conflicting. In mature mice, the skeleton can become osteopenic or osteosclerotic depending on the time point at which Notch is activated or inactivated. Using adult EGFP reporter mice, we find that Notch expression is localized to osteocytes embedded within bone matrix. Conditional activation of Notch signaling in osteocytes triggers profound bone formation, mainly due to increased mineralization, which rescues both age-associated and ovariectomy-induced bone loss and promotes bone healing following osteotomy. In parallel, mice rendered haploinsufficient in γ-secretase presenilin-1 (Psen1), which inhibits downstream Notch activation, display almost-absent terminal osteoblast differentiation. Consistent with this finding, pharmacologic or genetic disruption of Notch or its ligand Jagged1 inhibits mineralization. We suggest that stimulation of Notch signaling in osteocytes initiates a profound, therapeutically relevant, anabolic response.

Key transcription factors in the differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multipotent cells that represent a promising source for regenerative medicine. MSCs are capable of osteogenic, chondrogenic, adipogenic and myogenic differentiation. Efficacy of differentiated MSCs to regenerate cells in the injured tissues requires the ability to maintain the differentiation toward the desired cell fate. Since MSCs represent an attractive source for autologous transplantation, cellular and molecular signaling pathways and micro-environmental changes have been studied in order to understand the role of cytokines, chemokines, and transcription factors on the differentiation of MSCs. The differentiation of MSC into a mesenchymal lineage is genetically manipulated and promoted by specific transcription factors associated with a particular cell lineage. Recent studies have explored the integration of transcription factors, including Runx2, Sox9, PPARγ, MyoD, GATA4, and GATA6 in the differentiation of MSCs. Therefore, the overexpression of a single transcription factor in MSCs may promote trans-differentiation into specific cell lineage, which can be used for treatment of some diseases. In this review, we critically discussed and evaluated the role of transcription factors and related signaling pathways that affect the differentiation of MSCs toward adipocytes, chondrocytes, osteocytes, skeletal muscle cells, cardiomyocytes, and smooth muscle cells.

Aberrant Notch Signaling in the Bone Marrow Microenvironment of Acute Lymphoid Leukemia Suppresses Osteoblast-Mediated Support of Hematopoietic Niche Function

More than half of T-cell acute lymphoblastic leukemia (T-ALL) patients harbor gain-of-function mutations in the intracellular domain of Notch1. Diffuse infiltration of the bone marrow commonly occurs in T-ALL and relapsed B-cell acute lymphoblastic leukemia patients, and is associated with worse prognosis. However, the mechanism of leukemia outgrowth in the marrow and the resulting biologic impact on hematopoiesis are poorly understood. Here, we investigated targetable cellular and molecular abnormalities in leukemia marrow stroma responsible for the suppression of normal hematopoiesis using a T-ALL mouse model and human T-ALL xenografts. We found that actively proliferating leukemia cells inhibited normal hematopoietic stem and progenitor cell (HSPC) proliferation and homing to the perivascular region. In addition, leukemia development was accompanied by the suppression of the endosteum-lining osteoblast population. We further demonstrated that aberrant Notch activation in the stroma plays an important role in negatively regulating the expression of CXLC12 on osteoblasts and their differentiation. Notch blockade reversed attenuated HSPC cycling, leukemia-associated abnormal blood lineage distribution, and thrombocytopenia as well as recovered osteoblast and HSPC abundance and improved the hematopoietic-supportive functions of osteoblasts. Finally, we confirmed that reduced osteoblast frequency and enhanced Notch signaling were also features of the marrow stroma of human ALL tissues. Collectively, our findings suggest that therapeutically targeting the leukemia-infiltrated hematopoietic niche may restore HSPC homeostasis and improve the outcome of ALL patients.

NOTCH signaling in skeletal progenitors is critical for fracture repair

Fracture nonunions develop in 10%-20% of patients with fractures, resulting in prolonged disability. Current data suggest that bone union during fracture repair is achieved via proliferation and differentiation of skeletal progenitors within periosteal and soft tissues surrounding bone, while bone marrow stromal/stem cells (BMSCs) and other skeletal progenitors may also contribute. The NOTCH signaling pathway is a critical maintenance factor for BMSCs during skeletal development, although the precise role for NOTCH and the requisite nature of BMSCs following fracture is unknown. Here, we evaluated whether NOTCH and/or BMSCs are required for fracture repair by performing nonstabilized and stabilized fractures on NOTCH-deficient mice with targeted deletion of RBPjk in skeletal progenitors, maturing osteoblasts, and committed chondrocytes. We determined that removal of NOTCH signaling in BMSCs and subsequent depletion of this population result in fracture nonunion, as the fracture repair process was normal in animals harboring either osteoblast- or chondrocyte-specific deletion of RBPjk. Together, this work provides a genetic model of a fracture nonunion and demonstrates the requirement for NOTCH and BMSCs in fracture repair, irrespective of fracture stability and vascularity.

Role of Notch signaling in regulating innate immunity and inflammation in health and disease

The Notch signaling pathway is conserved from Drosophila to mammals and is critically involved in developmental processes. In the immune system, it has been established that Notch signaling regulates multiple steps of T and B cell development in both central and peripheral lymphoid organs. Relative to the well documented role of Notch signaling in lymphocyte development, less is known about its role in regulating myeloid lineage development and function, especially in the context of acute and chronic inflammation. In this review article, we will describe the evidence accumulated during the recent years to support a key regulatory role of the Notch pathway in innate immune and inflammatory responses and discuss the potential implications of such regulation for pathogenesis and therapy of inflammatory disorders.

Analysis of new bone, cartilage, and fibrosis tissue in healing murine allografts using whole slide imaging and a new automated histomorphometric algorithm

Histomorphometric analysis of histologic sections of normal and diseased bone samples, such as healing allografts and fractures, is widely used in bone research. However, the utility of traditional semi-automated methods is limited because they are labor-intensive and can have high interobserver variability depending upon the parameters being assessed, and primary data cannot be re-analyzed automatically. Automated histomorphometry has long been recognized as a solution for these issues, and recently has become more feasible with the development of digital whole slide imaging and computerized image analysis systems that can interact with digital slides. Here, we describe the development and validation of an automated application (algorithm) using Visiopharm’s image analysis system to quantify newly formed bone, cartilage, and fibrous tissue in healing murine femoral allografts in high-quality digital images of H&E/alcian blue-stained decalcified histologic sections. To validate this algorithm, we compared the results obtained independently using OsteoMeasure^TM and Visiopharm image analysis systems. The intraclass correlation coefficient between Visiopharm and OsteoMeasure was very close to one for all tissue elements tested, indicating nearly perfect reproducibility across methods. This new algorithm represents an accurate and labor-efficient method to quantify bone, cartilage, and fibrous tissue in healing mouse allografts.

Canonical Notch activation in osteocytes causes osteopetrosis

Activation of Notch1 in cells of the osteoblastic lineage inhibits osteoblast differentiation/function and causes osteopenia, whereas its activation in osteocytes causes a distinct osteopetrotic phenotype. To explore mechanisms responsible, we established the contributions of canonical Notch signaling (Rbpjκ dependent) to osteocyte function. Transgenics expressing Cre recombinase under the control of the dentin matrix protein-1 (Dmp1) promoter were crossed with Rbpjκ conditional mice to generate Dmp1-Cre(+/-);Rbpjκ(Δ/Δ) mice. These mice did not have a skeletal phenotype, indicating that Rbpjκ is dispensable for osteocyte function. To study the Rbpjκ contribution to Notch activation, Rosa(Notch) mice, where a loxP-flanked STOP cassette is placed between the Rosa26 promoter and the NICD coding sequence, were crossed with Dmp1-Cre transgenic mice and studied in the context (Dmp1-Cre(+/-);Rosa(Notch);Rbpjκ(Δ/Δ)) or not (Dmp1-Cre(+/-);Rosa(Notch)) of Rbpjκ inactivation. Dmp1-Cre(+/-);Rosa(Notch) mice exhibited increased femoral trabecular bone volume and decreased osteoclasts and bone resorption. The phenotype was reversed in the context of the Rbpjκ inactivation, demonstrating that Notch canonical signaling was accountable for the phenotype. Notch activation downregulated Sost and Dkk1 and upregulated Axin2, Tnfrsf11b, and Tnfsf11 mRNA expression, and these effects were not observed in the context of the Rbpjκ inactivation. In conclusion, Notch activation in osteocytes suppresses bone resorption and increases bone volume by utilization of canonical signals that also result in the inhibition of Sost and Dkk1 and upregulation of Wnt signaling.

Hajdu Cheney Mouse Mutants Exhibit Osteopenia, Increased Osteoclastogenesis, and Bone Resorption

Notch receptors are determinants of cell fate and function and play a central role in skeletal development and bone remodeling. Hajdu Cheney syndrome, a disease characterized by osteoporosis and fractures, is associated with NOTCH2 mutations resulting in a truncated stable protein and gain-of-function. We created a mouse model reproducing the Hajdu Cheney syndrome by introducing a 6955C→T mutation in the Notch2 locus leading to a Q2319X change at the amino acid level. Notch2(Q2319X) heterozygous mutants were smaller and had shorter femurs than controls; and at 1 month of age they exhibited cancellous and cortical bone osteopenia. As the mice matured, cancellous bone volume was restored partially in male but not female mice, whereas cortical osteopenia persisted in both sexes. Cancellous bone histomorphometry revealed an increased number of osteoclasts and bone resorption, without a decrease in osteoblast number or bone formation. Osteoblast differentiation and function were not affected in Notch2(Q2319X) cells. The pre-osteoclast cell pool, osteoclast differentiation, and bone resorption in response to receptor activator of nuclear factor κB ligand in vitro were increased in Notch2(Q2319X) mutants. These effects were suppressed by the γ-secretase inhibitor LY450139. In conclusion, Notch2(Q2319X) mice exhibit cancellous and cortical bone osteopenia, enhanced osteoclastogenesis, and increased bone resorption.

Glucocorticoid-Induced Osteoporosis

Osteoporosis is among the most devastating side effects of glucocorticoid (GC) therapy for the management of inflammatory and auto-immune diseases. Evidence from both humans and mice indicate deleterious skeletal effects within weeks of pharmacological GC administration, both related and unrelated to a decrease in bone mineral density (BMD). Osteoclast numbers and bone resorption are also rapidly increased, and together with osteoblast inactivation and decreased bone formation, these changes lead the fastest loss in BMD during the initial disease phase. Bone resorption then decreases to sub-physiological levels, but persistent and severe inhibition of bone formation leads to further bone loss and progressively increased fracture risk, up to an order of magnitude higher than that observed in untreated individuals. Bone forming osteoblasts are thus considered the main culprits in GC-induced osteoporosis (GIO). Accordingly, we focus this review primarily on deleterious effects on osteoblasts: inhibition of cell replication and function and acceleration of apoptosis. Mediating these adverse effects, GCs target pivotal regulatory mechanisms that govern osteoblast growth, differentiation and survival. Specifically, GCs inhibit growth factor pathways, including Insulin Growth Factors, Growth Hormone, Hepatocyte Growth/Scatter Factor and IL6-type cytokines. They also inhibit downstream kinases, including PI3-kinase and the MAP kinase ERK, the latter attributable in part to direct transcriptional stimulation of MAP kinase phosphatase 1. Most importantly, however, GCs inhibit the Wnt signaling pathway, which plays a pivotal role in osteoblast replication, function and survival. They transcriptionally stimulate expression of Wnt inhibitors of both the Dkk and Sfrp families, and they induce reactive oxygen species (ROS), which result in loss of ß-catenin to ROS-activated FoxO transcription factors. Identification of dissociated GCs, which would suppress the immune system without causing osteoporosis, is proving more challenging than initially thought, and GIO is currently managed by co-treatment with bisphosphonates or PTH. These drugs, however, are not ideally suited for GIO. Future therapeutic approaches may aim at GC targets such as those mentioned above, or newly identified targets including the Notch pathway, the AP-1/Il11 axis and the osteoblast master regulator RUNX2.

The role of microRNAs in bone remodeling

Bone remodeling is balanced by bone formation and bone resorption as well as by alterations in the quantities and functions of seed cells, leading to either the maintenance or deterioration of bone status. The existing evidence indicates that microRNAs (miRNAs), known as a family of short non-coding RNAs, are the key post-transcriptional repressors of gene expression, and growing numbers of novel miRNAs have been verified to play vital roles in the regulation of osteogenesis, osteoclastogenesis, and adipogenesis, revealing how they interact with signaling molecules to control these processes. This review summarizes the current knowledge of the roles of miRNAs in regulating bone remodeling as well as novel applications for miRNAs in biomaterials for therapeutic purposes.

mTORC1 Prevents Preosteoblast Differentiation through the Notch Signaling Pathway

The mechanistic target of rapamycin (mTOR) integrates both intracellular and extracellular signals to regulate cell growth and metabolism. However, the role of mTOR signaling in osteoblast differentiation and bone formation is undefined, and the underlying mechanisms have not been elucidated. Here, we report that activation of mTOR complex 1 (mTORC1) is required for preosteoblast proliferation; however, inactivation of mTORC1 is essential for their differentiation and maturation. Inhibition of mTORC1 prevented preosteoblast proliferation, but enhanced their differentiation in vitro and in mice. Activation of mTORC1 by deletion of tuberous sclerosis 1 (Tsc1) in preosteoblasts produced immature woven bone in mice due to excess proliferation but impaired differentiation and maturation of the cells. The mTORC1-specific inhibitor, rapamycin, restored these in vitro and in vivo phenotypic changes. Mechanistically, mTORC1 prevented osteoblast maturation through activation of the STAT3/p63/Jagged/Notch pathway and downregulation of Runx2. Preosteoblasts with hyperactive mTORC1 reacquired the capacity to fully differentiate and maturate when subjected to inhibition of the Notch pathway. Together, these findings identified the role of mTORC1 in osteoblast formation and established that mTORC1 prevents preosteoblast differentiation and maturation through activation of the Notch pathway.

The coordinated activities of osteoblasts and osteoclasts in bone deposition and resorption form the internal structure of bone. Disruption of the balance between bone formation and resorption results in loss of bone mass and causes bone diseases such as osteoporosis. Current therapies for osteoporosis are limited to anti-resorptive agents, while bone diseases due to reduced osteoblast activity, such as senile osteoporosis, urgently require targeted treatment and novel strategies to promote bone formation. mTORC1 has emerged as a critical regulator of bone formation and is therefore a potential target in the development of novel bone-promoting therapeutics. Identifying the detailed function of mTORC1 in bone formation and clarifying the underlying mechanisms may uncover useful therapeutic targets. In this study, we reveal the role of mTORC1 in osteoblast formation. mTORC1 stimulated preosteoblast proliferation but prevented their differentiation and attenuated bone formation via activation of the Notch pathway. Pharmaceutical coordination of the pathways and agents in preosteoblasts may be beneficial in bone formation.

RECQL4 Regulates p53 Function In Vivo During Skeletogenesis

RECQ DNA helicases play critical roles in maintaining genomic stability, but their role in development has been less well studied. Rothmund-Thomson syndrome, RAPADILINO, and Baller-Gerold syndrome are rare genetic disorders caused by mutations in the RECQL4 gene. These patients have significant skeletal developmental abnormalities including radial ray, limb and craniofacial defects. To investigate the role of Recql4 in the developing skeletal system, we generated Recql4 conditional knockout mice targeting the skeletal lineage. Inactivation of Recql4 using the Prx1-Cre transgene led to limb abnormalities and craniosynostosis mimicking the major bone findings in human RECQL4 patients. These Prx1-Cre(+) ;Recql4(fl/fl) mice as well as Col2a1-Cre(+) ;Recql4(fl/fl) mice exhibited growth plate defects and an increased p53 response in affected tissues. Inactivation of Trp53 in these Recql4 mutants resulted in genetic rescue of the skeletal phenotypes, indicating an in vivo interaction between Recql4 and Trp53, and p53 activation as an underlying mechanism for the developmental bone abnormalities in RECQL4 disorders. Our findings show that RECQL4 is critical for skeletal development by modulating p53 activity in vivo.

Genetic regulation of bone metabolism in the chicken: similarities and differences to Mammalian systems

Birds have a unique bone physiology, due to the demands placed on them through egg production. In particular their medullary bone serves as a source of calcium for eggshell production during lay and undergoes continuous and rapid remodelling. We take advantage of the fact that bone traits have diverged massively during chicken domestication to map the genetic basis of bone metabolism in the chicken. We performed a quantitative trait locus (QTL) and expression QTL (eQTL) mapping study in an advanced intercross based on Red Junglefowl (the wild progenitor of the modern domestic chicken) and White Leghorn chickens. We measured femoral bone traits in 456 chickens by peripheral computerised tomography and femoral gene expression in a subset of 125 females from the cross with microarrays. This resulted in 25 loci for female bone traits, 26 loci for male bone traits and 6318 local eQTL loci. We then overlapped bone and gene expression loci, before checking for an association between gene expression and trait values to identify candidate quantitative trait genes for bone traits. A handful of our candidates have been previously associated with bone traits in mice, but our results also implicate unexpected and largely unknown genes in bone metabolism. In summary, by utilising the unique bone metabolism of an avian species, we have identified a number of candidate genes affecting bone allocation and metabolism. These findings can have ramifications not only for the understanding of bone metabolism genetics in general, but could also be used as a potential model for osteoporosis as well as revealing new aspects of vertebrate bone regulation or features that distinguish avian and mammalian bone.

In this work we seek to further the understanding of bone genetics by mapping bone traits and gene expression in the chicken. Bone in female birds is special due to egg production. In this study, we combine the genetic mapping of bone traits with bone gene expression to find candidate quantitative trait genes that explain the differences between wild and domestic chickens in terms of bone production. The concept of combining genetic mapping and gene expression mapping is not new, and has already been successful in isolating bone-related genes in mammals, however this is the first time it has been applied to an avian system with such unique bone modelling processes. We aim to reveal new molecular mechanisms of bone regulation, and many of the candidates we find are new, highlighting the potential this technique has to identify the potential differences between avian and mammalian bone biology.

Ubiquitin E3 Ligase LNX2 is Critical for Osteoclastogenesis In Vitro by Regulating M-CSF/RANKL Signaling and Notch2

The Notch signaling pathway plays a crucial role in skeletal development and homeostasis by regulating the proliferation and differentiation of osteoblasts and osteoclasts. However, the molecular mechanisms modulating the level and activity of Notch receptors in bone cells remain unknown. In this study, we uncovered that LNX2, an E3 ubiquitin ligase and Notch inhibitor Numb binding protein, was up-regulated during osteoclast differentiation. Knocking-down LNX2 expression in bone marrow macrophages by lentivirus-mediated short hairpin RNAs markedly inhibited osteoclast formation. Decreased LNX2 expression attenuated macrophage colony-stimulating factor (M-CSF)-induced ERK and AKT activation and RANKL-stimulated activation of NF-κB and JNK pathways; therefore, accelerated osteoclast apoptosis. Additionally, loss of LNX2 led to an increased accumulation of Numb, which promoted the degradation of Notch and caused a reduction of the expression of the Notch downstream target gene, Hes1. We conclude that LNX2 regulates M-CSF/RANKL and the Notch signaling pathways during osteoclastogenesis.

Losartan increases bone mass and accelerates chondrocyte hypertrophy in developing skeleton

Angiotensin receptor blockers (ARBs) are a group of anti-hypertensive drugs that are widely used to treat pediatric hypertension. Recent application of ARBs to treat diseases such as Marfan syndrome or Alport syndrome has shown positive outcomes in animal and human studies, suggesting a broader therapeutic potential for this class of drugs. Multiple studies have reported a benefit of ARBs on adult bone homeostasis; however, its effect on the growing skeleton in children is unknown. We investigated the effect of Losartan, an ARB, in regulating bone mass and cartilage during development in mice. Wild type mice were treated with Losartan from birth until 6 weeks of age, after which bones were collected for microCT and histomorphometric analyses. Losartan increased trabecular bone volume vs. tissue volume (a 98% increase) and cortical thickness (a 9% increase) in 6-weeks old wild type mice. The bone changes were attributed to decreased osteoclastogenesis as demonstrated by reduced osteoclast number per bone surface in vivo and suppressed osteoclast differentiation in vitro. At the molecular level, Angiotensin II-induced ERK1/2 phosphorylation in RAW cells was attenuated by Losartan. Similarly, RANKL-induced ERK1/2 phosphorylation was suppressed by Losartan, suggesting a convergence of RANKL and angiotensin signaling at the level of ERK1/2 regulation. To assess the effect of Losartan on cartilage development, we examined the cartilage phenotype of wild type mice treated with Losartan in utero from conception to 1 day of age. Growth plates of these mice showed an elongated hypertrophic chondrocyte zone and increased Col10a1 expression level, with minimal changes in chondrocyte proliferation. Altogether, inhibition of the angiotensin pathway by Losartan increases bone mass and accelerates chondrocyte hypertrophy in growth plate during skeletal development.

Bone marrow stroma-derived miRNAs as regulators, biomarkers and therapeutic targets of bone metastasis

MicroRNAs (miRNAs) are short, endogenous RNA molecules that have essential roles in regulating gene expression. They control numerous physiological and cellular processes, including normal bone organogenesis and homeostasis, by enhancing or inhibiting bone marrow cell growth, differentiation, functional activity and crosstalk of the multiple cell types within the bone. Hence, elucidating miRNA targets in bone marrow stromal cells has revealed novel regulations during bone development and maintenance. Moreover, recent studies have detailed the capacity for bone stromal miRNAs to influence bone metastasis from a number of primary carcinomas by interfering with bone homeostasis or by directly influencing metastatic tumor cells. Owing to the current lack of good diagnostic biomarkers of bone metastases, such changes in bone stromal miRNA expression in the presence of metastatic lesions may become useful biomarkers, and may even serve as therapeutic targets. In particular, cell-free and exosomal miRNAs shed from bone stromal cells into circulation may be developed into novel biomarkers that can be routinely measured in easily accessible samples. Taken together, these findings reveal the significant role of bone marrow stroma-derived miRNAs in the regulation of bone homeostasis and bone metastasis.

Osteoblast dysfunctions in bone diseases: from cellular and molecular mechanisms to therapeutic strategies

Several metabolic, genetic and oncogenic bone diseases are characterized by defective or excessive bone formation. These abnormalities are caused by dysfunctions in the commitment, differentiation or survival of cells of the osteoblast lineage. During the recent years, significant advances have been made in our understanding of the cellular and molecular mechanisms underlying the osteoblast dysfunctions in osteoporosis, skeletal dysplasias and primary bone tumors. This led to suggest novel therapeutic approaches to correct these abnormalities such as the modulation of WNT signaling, the pharmacological modulation of proteasome-mediated protein degradation, the induction of osteoprogenitor cell differentiation, the repression of cancer cell proliferation and the manipulation of epigenetic mechanisms. This article reviews our current understanding of the major cellular and molecular mechanisms inducing osteoblastic cell abnormalities in age-related bone loss, genetic skeletal dysplasias and primary bone tumors, and discusses emerging therapeutic strategies to counteract the osteoblast abnormalities in these disorders of bone formation.

Transient gamma-secretase inhibition accelerates and enhances fracture repair likely via Notch signaling modulation

Approximately 10% of skeletal fractures result in healing complications and non-union, while most fractures repair with appropriate stabilization and without pharmacologic intervention. It is the latter injuries that cannot be underestimated as the expenses associated with their treatment and subsequent lost productivity are predicted to increase to over $74 billion by 2015. During fracture repair, local mesenchymal stem/progenitor cells (MSCs) differentiate to form new cartilage and bone, reminiscent of events during skeletal development. We previously demonstrated that permanent loss of gamma-secretase activity and Notch signaling accelerates bone and cartilage formation from MSC progenitors during skeletal development, leading to pathologic acquisition of bone and depletion of bone marrow derived MSCs. Here, we investigated whether transient and systemic gamma-secretase and Notch inhibition is capable of accelerating and enhancing fracture repair by promoting controlled MSC differentiation near the fracture site. Our radiographic, microCT, histological, cell and molecular analyses reveal that single and intermittent gamma-secretase inhibitor (GSI) treatments significantly enhance cartilage and bone callus formation via the promotion of MSC differentiation, resulting in only a moderate reduction of local MSCs. Biomechanical testing further demonstrates that GSI treated fractures exhibit superior strength earlier in the healing process, with single dose GSI treated fractures exhibiting bone strength approaching that of un-fractured tibiae. These data further establish that transient inhibition of gamma-secretase activity and Notch signaling temporarily increases osteoclastogenesis and accelerates bone remodeling, which coupled with the effects on MSCs likely explains the accelerated and enhanced fracture repair. Therefore, we propose that the Notch pathway serves as an important therapeutic target during skeletal fracture repair.

Comparative transcriptome profiling in human bicuspid aortic valve disease using RNA sequencing

Intrinsic valvular degeneration and dysfunction is the most common complication of bicuspid aortic valve (BAV) disease. Phenotypically, it ranges from calcific aortic stenosis to redundant or prolapsing regurgitant leaflets. The underlying molecular mechanism underpinning phenotype heterogeneity of valvular degeneration in BAV is poorly understood. We used RNA sequencing (RNA-seq) to identify genes and pathways responsible for the development of valvular degeneration in BAV, compared with tricuspid aortic valve (TAV). Comparative transcriptome analysis was performed on total RNA of aortic valve tissues of patients with diseased BAV ( n = 5) and calcified TAV ( n = 3). RNA-seq findings were validated by RT-qPCR. A total of 59 and 177 genes were significantly up- and downregulated, respectively, in BAV compared with TAV. Hierarchical clustering indicated heterogeneity within the BAV group, separating those with heavy calcification (BAVc) from those with redundant leaflets and/or minimal calcification (BAVr). Interestingly, the gene expression profile of the BAVc group closely resembled the TAV, with shared up- and downregulation of inflammatory and NOTCH1 signaling pathways, respectively. Downregulation of matrix protease ADAMTS9 and protein aggrecan were observed in BAVr compared with TAV. Dysregulation of fetal gene programs were also present, with notable downregulation of SEMA6B and SEMA3F in BAVr and BAVc compared with TAV, respectively. Upregulation of TBX20 was observed exclusively in BAVr compared with BAVc. In conclusion, diverging molecular mechanisms underpin phenotype heterogeneity of valvular degeneration in BAV and data from the present study suggest that there may be shared mechanisms leading to calcification in BAV and TAV. Recognition of these pathways is fundamental to improve our understanding of the molecular basis of human BAV disease.

Multiple myeloma-related bone disease: state-of-art and next future treatments

SUMMARY 

Multiple myeloma (MM) is a plasma cell malignancy associated with the development of life-threatening and/or severe osteolytic lesions, which significantly worsen the quality of life of affected patients. MM-related bone disease (BD) is the result of an overwhelming osteoclastic activity, while osteoblast-mediated bone formation is inhibited. Bisphosphonates are still the mainstay of therapy for BD. However, these drugs are associated with mid long-term sequelae. In this work, we review the pathogenesis and currently available therapies of MM-related BD. We describe the most recent and promising findings that may translate in changing the clinical practice in the next future.

Signaling pathways regulating the specification and differentiation of the osteoblast lineage

Tissue engineering is an approach to the regeneration of tissues that uses a combination of cell sources, signaling factors and scaffolds. Among these three components, signaling factors for bone regeneration have not yet been established, and it is necessary to better understand osteoblast progenitors as a target cells. Several lines of evidence have revealed that, during bone formation, mesenchymal cells are specified and differentiate into osteoblasts through several stages of precursors. The osteoblast lineage is defined by the expression of stage-specific transcription factors. The specification and differentiation are organized by a variety of signaling pathways including hedgehog (Hh), Wnt, Notch, bone morphogenetic protein (BMP) and transforming growth factor-beta (TGFβ). In this review we integrate the known functions of these signaling pathways and discuss future tasks to gain a better understanding of the signaling network in osteogenesis for tissue engineering.

Osteocytes mediate the anabolic actions of canonical Wnt/β-catenin signaling in bone

Osteocytes, >90% of the cells in bone, lie embedded within the mineralized matrix and coordinate osteoclast and osteoblast activity on bone surfaces by mechanisms still unclear. Bone anabolic stimuli activate Wnt signaling, and human mutations of components along this pathway underscore its crucial role in bone accrual and maintenance. However, the cell responsible for orchestrating Wnt anabolic actions has remained elusive. We show herein that activation of canonical Wnt signaling exclusively in osteocytes [dominant active (da)βcat(Ot) mice] induces bone anabolism and triggers Notch signaling without affecting survival. These features contrast with those of mice expressing the same daß-catenin in osteoblasts, which exhibit decreased resorption and perinatal death from leukemia. daßcat(Ot) mice exhibit increased bone mineral density in the axial and appendicular skeleton, and marked increase in bone volume in cancellous/trabecular and cortical compartments compared with littermate controls. daßcat(Ot) mice display increased resorption and formation markers, high number of osteoclasts and osteoblasts in cancellous and cortical bone, increased bone matrix production, and markedly elevated periosteal bone formation rate. Wnt and Notch signaling target genes, osteoblast and osteocyte markers, and proosteoclastogenic and antiosteoclastogenic cytokines are elevated in bones of daßcat(Ot) mice. Further, the increase in RANKL depends on Sost/sclerostin. Thus, activation of osteocytic β-catenin signaling increases both osteoclasts and osteoblasts, leading to bone gain, and is sufficient to activate the Notch pathway. These findings demonstrate disparate outcomes of β-catenin activation in osteocytes versus osteoblasts and identify osteocytes as central target cells of the anabolic actions of canonical Wnt/β-catenin signaling in bone.

Disruption of cell-cell contact-mediated notch signaling via hydrogel encapsulation reduces mesenchymal stem cell chondrogenic potential: winner of the Society for Biomaterials Student Award in the Undergraduate Category, Charlotte, NC, April 15 to 18, 2015

Cell-cell contact-mediated Notch signaling is essential for mesenchymal stem cell (MSC) chondrogenesis during development. However, subsequent deactivation of Notch signaling is also required to allow for stem cell chondrogenic progression. Recent literature has shown that Notch signaling can also influence Wnt/β-catenin signaling, critical for MSC differentiation, through perturbations in cell-cell contacts. Traditionally, abundant cell-cell contacts, consistent with development, are emulated in vitro using pellet cultures for chondrogenesis. However, cells are often encapsulated within biomaterials-based scaffolds, such as hydrogels, to improve therapeutic cell localization in vivo. To explore the role of Notch and Wnt/β-catenin signaling in the context of hydrogel-encapsulated MSC chondrogenesis, we compared signaling and differentiation capacity of MSCs in both hydrogels and traditional pellet cultures. We demonstrate that encapsulation within poly(ethylene glycol) hydrogels reduces cell-cell contacts, and both Notch (7.5-fold) and Wnt/β-catenin (84.7-fold) pathway activation. Finally, we demonstrate that following establishment of cell-cell contacts and transient Notch signaling in pellet cultures, followed by Notch signaling deactivation, resulted in a 1.5-fold increase in MSC chondrogenesis. Taken together, these findings support that cellular condensation, and establishment of initial cell-cell contacts is critical for MSC chondrogenesis, and this process is inhibited by hydrogel encapsulation.

Control of hair cell development by molecular pathways involving Atoh1, Hes1 and Hes5

Atoh1, Hes1 and Hes5 are crucial for normal inner ear hair cell development. They regulate the expression of each other in a complex network, while they also interact with many other genes and pathways, such as Notch, FGF, SHH, WNT, BMP and RA. This paper summarized molecular pathways that involve Atoh1, Hes1, and Hes5. Some of the pathways and gene regulation mechanisms discussed here were studied in other tissues, yet they might inspire studies in inner ear hair cell development. Thereby, we presented a complex regulatory network involving these three genes, which might be crucial for proliferation and differentiation of inner ear hair cells.

Hajdu-Cheney syndrome: a review

Hajdu Cheney Syndrome (HCS), Orpha 955, is a rare disease characterized by acroosteolysis, severe osteoporosis, short stature, specific craniofacial features, wormian bones, neurological symptoms, cardiovascular defects and polycystic kidneys. HCS is rare and is inherited as autosomal dominant although many sporadic cases have been reported. HCS is associated with mutations in exon 34 of NOTCH2 upstream the PEST domain that lead to the creation of a truncated and stable NOTCH2 protein with enhanced NOTCH2 signaling activity. Although the number of cases with NOTCH2 mutations reported are limited, it would seem that the diagnosis of HCS can be established by sequence analysis of exon 34 of NOTCH2. Notch receptors are single-pass transmembrane proteins that determine cell fate, and play a critical role in skeletal development and homeostasis. Dysregulation of Notch signaling is associated with skeletal developmental disorders. There is limited information about the mechanisms of the bone loss and acroosteolysis in HCS making decisions regarding therapeutic intervention difficult. Bone antiresorptive and anabolic agents have been tried to treat the osteoporosis, but their benefit has not been established. In conclusion, Notch regulates skeletal development and bone remodeling, and gain-of-function mutations of NOTCH2 are associated with HCS.

NOTCH-Mediated Maintenance and Expansion of Human Bone Marrow Stromal/Stem Cells: A Technology Designed for Orthopedic Regenerative Medicine

Human bone marrow-derived stromal/stem cells (BMSCs) have great therapeutic potential for treating skeletal disease and facilitating skeletal repair, although maintaining their multipotency and expanding these cells ex vivo have proven difficult. Because most stem cell-based applications to skeletal regeneration and repair in the clinic would require large numbers of functional BMSCs, recent research has focused on methods for the appropriate selection, expansion, and maintenance of BMSC populations during long-term culture. We describe here a novel biological method that entails selection of human BMSCs based on NOTCH2 expression and activation of the NOTCH signaling pathway in cultured BMSCs via a tissue culture plate coated with recombinant human JAGGED1 (JAG1) ligand. We demonstrate that transient JAG1-mediated NOTCH signaling promotes human BMSC maintenance and expansion while increasing their skeletogenic differentiation capacity, both ex vivo and in vivo. This study is the first of its kind to describe a NOTCH-mediated methodology for the maintenance and expansion of human BMSCs and will serve as a platform for future clinical or translational studies aimed at skeletal regeneration and repair.

Notch signaling in the malignant bone marrow microenvironment: implications for a niche-based model of oncogenesis

Fueled by the growing interest in stem cell biology and the promise of regenerative medicine, study of the hematopoietic stem cell (HSC) microenvironment has provided critical insights into normal and malignant hematopoiesis. Notch receptor signaling in this microenvironment is a critical regulator of HSC fate and differentiation. Notch signaling also has the potential to modulate the growth of various malignant cell types, as evidenced by the growing list of hematologic cancers and other malignancies associated with either mutations in Notch genes or alterations in Notch signaling. In both health and disease, activation of Notch signaling predominantly exerts influence through stromal cell interactions with the tumor or stem cell microenvironments. Definitive evidence from transgenic mouse models has shown that alterations in stromal cell signaling from the bone marrow niche can induce malignant outgrowth of preleukemic clones and leukemia. Understanding how Notch receptor signals in the bone marrow microenvironment govern stem cell behavior will advance our understanding of cancer pathogenesis in hematologic malignancies and may have implications for treating metastatic solid tumors involving bone. These microenvironmental interactions are potential therapeutic targets for treating and preventing a variety of diseases, including bone marrow failure disorders, myelodysplastic syndromes, leukemia, and lymphoma.

Nemo-like kinase regulates postnatal skeletal homeostasis

Nemo-like kinase (Nlk) is related to the mitogen-activated protein (MAP) kinases and known to regulate signaling pathways involved in osteoblastogenesis. In vitro Nlk suppresses osteoblastogenesis, but the consequences of the Nlk inactivation in the skeleton in vivo are unknown. To study the function of Nlk, Nlk(loxP/loxP) mice, where the Nlk exon2 is flanked by lox(P) sequences, were mated with mice expressing the Cre recombinase under the control of the paired-related homeobox gene 1 (Prx1) enhancer (Prx1-Cre), the Osterix (Osx-Cre) or the osteocalcin/bone gamma carboxyglutamate protein (Bglap-Cre) promoter. Prx1-Cre;Nlk(Δ/Δ) mice did not exhibit a skeletal phenotype except for a modest increase in trabecular number and connectivity observed only in 3-month-old male mice. Osx-Cre;Nlk(Δ/Δ) male and female mice exhibited an increase in trabecular bone volume secondary to an increased trabecular number at 3 months of age. Bone histomorphometry revealed a decrease in osteoclast number and eroded surface in male mice, and decreased osteoblast number and function in female mice. Expression of osteoprotegerin mRNA was increased in calvarial extracts, explaining the decreased osteoclast and osteoblast number. The conditional deletion of Nlk in mature osteoblasts (Bglap-Cre;Nlk(Δ/Δ) ) resulted in no skeletal phenotype in 1- to 6-month-old male or female mice. In conclusion, when expressed in undifferentiated osteoblasts, Nlk is a negative regulator of skeletal homeostasis possibly by targeting signals that regulate osteoclastogenesis and bone resorption.

Notch activation as a driver of osteogenic sarcoma

Osteogenic sarcoma (OS) is a deadly skeletal malignancy whose cause is unknown. We report here a mouse model of OS based on conditional expression of the intracellular domain of Notch1 (NICD). Expression of the NICD in immature osteoblasts was sufficient to drive the formation of bone tumors, including OS, with complete penetrance. These tumors display features of human OS; namely, histopathology, cytogenetic complexity, and metastatic potential. We show that Notch activation combined with loss of p53 synergistically accelerates OS development in mice, although p53-driven OS is not Rbpj dependent, which demonstrates a dual dominance of the Notch oncogene and p53 mutation in the development of OS. Using this model, we also reveal the osteoblasts as the potential sources of OS.

Downregulation of Notch modulators, tetraspanin 5 and 10, inhibits osteoclastogenesis in vitro

Genetic studies in human and mice have pinpointed an essential role of Notch signaling in osteoblast and osteoclast differentiation during skeletal development and bone remodeling. However, the factors and pathways regulating Notch activation in bone cells remain largely unknown. In this in vitro study, we have provided evidence that two of the TspanC8 subfamily members of tetraspanins, Tspan-5 and Tspan-10, are up-regulated during osteoclast differentiation and knockdown of their expression by shRNAs dramatically inhibits osteoclastogenesis. Loss of Tspan-5 and Tspan-10 in osteoclast lineage cells results in attenuation of ADAM10 maturation and Notch activation. Therefore, these two tetraspanins play a critical role in osteoclast formation, at least in part, by modulating Notch signaling pathway.

Multiple myeloma mesenchymal stromal cells: Contribution to myeloma bone disease and therapeutics

Multiple myeloma is a hematological malignancy in which clonal plasma cells proliferate and accumulate within the bone marrow. The presence of osteolytic lesions due to increased osteoclast (OC) activity and suppressed osteoblast (OB) function is characteristic of the disease. The bone marrow mesenchymal stromal cells (MSCs) play a critical role in multiple myeloma pathophysiology, greatly promoting the growth, survival, drug resistance and migration of myeloma cells. Here, we specifically discuss on the relative contribution of MSCs to the pathophysiology of osteolytic lesions in light of the current knowledge of the biology of myeloma bone disease (MBD), together with the reported genomic, functional and gene expression differences between MSCs derived from myeloma patients (pMSCs) and their healthy counterparts (dMSCs). Being MSCs the progenitors of OBs, pMSCs primarily contribute to the pathogenesis of MBD because of their reduced osteogenic potential consequence of multiple OB inhibitory factors and direct interactions with myeloma cells in the bone marrow. Importantly, pMSCs also readily contribute to MBD by promoting OC formation and activity at various levels (i.e., increasing RANKL to OPG expression, augmenting secretion of activin A, uncoupling ephrinB2-EphB4 signaling, and through augmented production of Wnt5a), thus further contributing to OB/OC uncoupling in osteolytic lesions. In this review, we also look over main signaling pathways involved in the osteogenic differentiation of MSCs and/or OB activity, highlighting amenable therapeutic targets; in parallel, the reported activity of bone-anabolic agents (at preclinical or clinical stage) targeting those signaling pathways is commented.

MicroRNAs as regulators of bone homeostasis and bone metastasis

MicroRNAs (miRNAs) are short, endogenous RNAs that have essential roles in regulating gene expression through the disruption of target genes. The miRNA-induced suppression can occur through Argonaute-mediated cleavage of target mRNAs or by translational inhibition. System-wide studies have underscored the integral role that miRNAs play in regulating the expression of essential genes within bone marrow stromal cells. The miRNA expression has been shown to enhance or inhibit cell differentiation and activity, and elucidating miRNA targets within bone marrow cells has revealed novel regulations during normal bone development. Importantly, multiple studies have shown that miRNA misexpression mediates the progression of bone-related pathologies, including osteopetrosis and osteoporosis, as well as the development and progression of osteosarcoma. Furthermore, recent studies have detailed the capacity for miRNAs to influence bone metastasis from a number of primary carcinomas. Taken together, these findings reveal the significant clinical potential for miRNAs to regulate bone homeostasis, as well as to mediate bone-related pathologies.

Targeting Notch1 signaling pathway positively affects the sensitivity of osteosarcoma to cisplatin by regulating the expression and/or activity of Caspase family

Background

The introduction of cisplatin has improved the long-term survival rate in osteosarcoma patients. However, some patients are intrinsically resistant to cisplatin. This study reported that the activation of Notch1 is positively correlated with cisplatin sensitivity, evidenced by both clinical and in vitro data.

Results

In this study, a total 8 osteosarcoma specimens were enrolled and divided into two groups according to their cancer chemotherapeutic drugs sensitivity examination results. The relationship between Notch1 expression and cisplatin sensitivity of osteosarcoma patients was detected by immunohistochemistry and semi-quantitative analysis. Subsequently, two typical osteosarcoma cell lines, Saos-2 and MG63, were selected to study the changes of cisplatin sensitivity by up-regulating (NICD1 plasmid transfeciton) or decreasing (gamma-secretase complex inhibitor DAPT) the activation state of Notch1 signaling pathway. Our results showed a significant correlation between the expression of Notch1 and cisplatin sensitivity in patient specimens. In vitro, Saos-2 with higher expression of Notch1 had significantly better cisplatin sensitivity than MG63 whose Notch1 level was relatively lower. By targeting regulation in vitro, the cisplatin sensitivity of Saos-2 and MG63 had significantly increased after the activation of Notch1 signaling pathway, and vice versa. Further mechanism investigation revealed that activation/inhibition of Notch1 sensitized/desensitized cisplatin-induced apoptosis, which probably depended on the changes in the activity of Caspase family, including Caspase 3, Caspase 8 and Caspase 9 in these cells.

Conclusions

Our data clearly demonstrated that Notch1 is critical for cisplatin sensitivity in osteosarcoma. It can be used as a molecular marker and regulator for cisplatin sensitivity in osteosarcoma patients.

NOTCH inhibits osteoblast formation in inflammatory arthritis via noncanonical NF-κB

NOTCH-dependent signaling pathways are critical for normal bone remodeling; however, it is unclear if dysfunctional NOTCH activation contributes to inflammation-mediated bone loss, as observed in rheumatoid arthritis (RA) patients. We performed RNA sequencing and pathway analyses in mesenchymal stem cells (MSCs) isolated from transgenic TNF-expressing mice, a model of RA, to identify pathways responsible for decreased osteoblast differentiation. 53 pathways were dysregulated in MSCs from RA mice, among which expression of genes encoding NOTCH pathway members and members of the noncanonical NF-κB pathway were markedly elevated. Administration of NOTCH inhibitors to RA mice prevented bone loss and osteoblast inhibition, and CFU-fibroblasts from RA mice treated with NOTCH inhibitors formed more new bone in recipient mice with tibial defects. Overexpression of the noncanonical NF-κB subunit p52 and RELB in a murine pluripotent stem cell line increased NOTCH intracellular domain-dependent (NICD-dependent) activation of an RBPjκ reporter and levels of the transcription factor HES1. TNF promoted p52/RELB binding to NICD, which enhanced binding at the RBPjκ site within the Hes1 promoter. Furthermore, MSC-enriched cells from RA patients exhibited elevated levels of HES1, p52, and RELB. Together, these data indicate that persistent NOTCH activation in MSCs contributes to decreased osteoblast differentiation associated with RA and suggest that NOTCH inhibitors could prevent inflammation-mediated bone loss.

Jagged1 is essential for osteoblast development during maxillary ossification

Maxillary hypoplasia occurs due to insufficient maxillary intramembranous ossification, leading to poor dental occlusion, respiratory obstruction and cosmetic deformities. Conditional deletion of Jagged1 (Jag1) in cranial neural crest (CNC) cells using Wnt1-cre; Jagged1(f/f) (Jag1CKO) led to maxillary hypoplasia characterized by intrinsic differences in bone morphology and density using μCT evaluation. Jag1CKO maxillas revealed altered collagen deposition, delayed ossification, and reduced expression of early and late determinants of osteoblast development during maxillary ossification. In vitro bone cultures on Jag1CKO mouse embryonic maxillary mesenchymal (MEMM) cells demonstrated decreased mineralization that was also associated with diminished induction of osteoblast determinants. BMP receptor expression was dysregulated in the Jag1CKO MEMM cells suggesting that these cells were unable to respond to BMP-induced differentiation. JAG1-Fc rescued in vitro mineralization and osteoblast gene expression changes. These data suggest that JAG1 signaling in CNC-derived MEMM cells is required for osteoblast development and differentiation during maxillary ossification.

Notch1 and Notch2 expression in osteoblast precursors regulates femoral microarchitecture

Notch receptors regulate cell differentiation and function. Notch1 and Notch2 inactivation in osteoblasts and osteocytes increases cancellous bone volume, but the function of Notch signaling in osteoblast precursors is unknown. To inactivate Notch signaling in immature osteoblastic cells, mice homozygous for conditional Notch1 and Notch2 alleles (Notch1(loxP/loxP);Notch2(loxP/loxP)) were crossed with mice where the osterix (Osx) promoter, regulated by a Tet-Off cassette, governs Cre expression (Osx-Cre). Notch1(loxP/loxP);Notch2(loxP/loxP) control and Osx-Cre(+/-);Notch1(Δ/Δ);Notch2(Δ/Δ) experimental littermate cohorts were obtained. To prevent the effects of embryonic Osx-Cre expression, doxycycline was administered to pregnant dams, but not to newborns. Recombination of conditional alleles was documented in calvarial DNA extracts from 1month old mice. Notch1 and Notch2 inactivation did not affect femoral microarchitecture at 1month of age. Cancellous bone volume was higher and structure model index was lower in 3 and 6 month old Osx-Cre(+/-);Notch1(Δ/Δ);Notch2(Δ/Δ) mice than in control littermates and the effect was more pronounced in female mice. One month old Osx-Cre(+/-);Notch1(Δ/Δ);Notch2(Δ/Δ) male mice transiently exhibited an increase in osteoblast number and a modest suppression in bone resorption. Osx-Cre(+/-);Notch1(Δ/Δ);Notch2(Δ/Δ) female mice displayed a tendency toward increased bone formation at 3months of age, although bone remodeling was suppressed in 6month old Osx-Cre(+/-);Notch1(Δ/Δ);Notch2(Δ/Δ) female mice. Notch1 and Notch2 inactivation increased porosity and reduced thickness of cortical bone. These effects were modest and more evident in 3 and 6 month old female than in male mice of the same age. In conclusion, Notch1 and Notch2 expression in osteoblast precursors regulates cancellous bone volume and microarchitecture.

PKCδ is required for Jagged-1 induction of human mesenchymal stem cell osteogenic differentiation

JAG1, the gene for the Jagged-1 ligand (Jag1) in the Notch signaling pathway, is variably mutated in Alagille Syndrome (ALGS). ALGS patients have skeletal defects, and additionally JAG1 has been shown to be associated with low bone mass through genome-wide association studies. Plating human osteoblast precursors (human mesenchymal stem cells-hMSCs) on Jag1 is sufficient to induce osteoblast differentiation; however, exposure of mouse MSC (mMSC) to Jag1 actually inhibits osteoblastogenesis. Overexpression of the notch-2 intracellular domain (NICD2) is sufficient to mimic the effect of Jag1 on hMSC osteoblastogenesis, while blocking Notch signaling with a γ-secretase inhibitor or with dominant-negative mastermind inhibits Jag1-induced hMSC osteoblastogenesis. In pursuit of interacting signaling pathways, we discovered that treatment with a protein kinase C δ (PKCδ) inhibitor abrogates Jag1-induced hMSC osteoblastogenesis. Jag1 results in rapid PKCδ nuclear translocation and kinase activation. Furthermore, Jag1 stimulates the physical interaction of PKCδ with NICD. Collectively, these results suggest that Jag1 induces hMSC osteoblast differentiation through canonical Notch signaling and requires concomitant PKCδ signaling. This research also demonstrates potential deficiencies in using mouse models to study ALGS bone abnormalities.

Notch signaling in mesenchymal stem cells harvested from geriatric mice

OBJECTIVES

Morbidity associated with geriatric fractures may be attributed, in part, to compromised mesenchymal stem cell (MSC) function within the fracture callus. The Notch signaling pathway is important for the healing of nonskeletal tissues in an age-dependent manner, but the effect of Notch on age-dependent fracture healing and MSC dysfunction has not been substantiated. The objective of this study was to examine Notch signaling in MSCs obtained from young and geriatric mice.

METHODS

Marrow-derived MSCs were harvested from the femora of 5- and 25-month-old C57BL/6 mice. We assessed in vivo MSC number using CFU-F, proliferation using an Alamar Blue assay, osteoblast differentiation by Alizarin Red S staining, and adipogenic differentiation using Oil Red O staining. Notch receptor and ligand expression was assessed using quantitative PCR, and Notch signaling was assessed by evaluating Notch target gene expression (Hey and HES) under basal conditions and when cells were plated to Jagged-1 ligand.

RESULTS

MSC from geriatric mice exhibit reduced MSC number (CFU-F), proliferation, adipogenesis, and inconsistent osteogenesis. The highest expressed Notch receptor is Notch 2, and the highest expressed ligand is Jagged-1, but there were no differences in ligand and receptor gene expression between young and old MSCs. Interestingly, geriatric MSCs show decreased basal Notch signaling activity but are fully responsive to Jagged-1 stimulation.

CONCLUSIONS

These data suggest that therapeutic targeting of Notch signaling should be explored in clinical therapies to improve geriatric fracture healing.

Endothelial Notch activity promotes angiogenesis and osteogenesis in bone

Blood vessel growth in the skeletal system and osteogenesis seem to be coupled, suggesting the existence of molecular crosstalk between endothelial and osteoblastic cells. Understanding the nature of the mechanisms linking angiogenesis and bone formation should be of great relevance for improved fracture healing or prevention of bone mass loss. Here we show that vascular growth in bone involves a specialized, tissue-specific form of angiogenesis. Notch signalling promotes endothelial cell proliferation and vessel growth in postnatal long bone, which is the opposite of the well-established function of Notch and its ligand Dll4 in the endothelium of other organs and tumours. Endothelial-cell-specific and inducible genetic disruption of Notch signalling in mice not only impaired bone vessel morphology and growth, but also led to reduced osteogenesis, shortening of long bones, chondrocyte defects, loss of trabeculae and decreased bone mass. On the basis of a series of genetic experiments, we conclude that skeletal defects in these mutants involved defective angiocrine release of Noggin from endothelial cells, which is positively regulated by Notch. Administration of recombinant Noggin, a secreted antagonist of bone morphogenetic proteins, restored bone growth and mineralization, chondrocyte maturation, the formation of trabeculae and osteoprogenitor numbers in endothelial-cell-specific Notch pathway mutants. These findings establish a molecular framework coupling angiogenesis, angiocrine signals and osteogenesis, which may prove significant for the development of future therapeutic applications.

Crucial role of Notch signaling in osteogenic differentiation of periodontal ligament stem cells in osteoporotic rats

Estrogen deficiency-induced osteoporosis typically occurs in postmenopausal women and has been strongly associated with periodontal diseases. Periodontal ligament stem cells (PDLSCs) isolated from the periodontal ligament can differentiate into many types of specialized cells, including osteoblast-like cells that contribute to periodontal tissue repair. The Notch signaling pathway is highly conserved and associated with self-renewal potential and cell-fate determination. Recently, several studies have focused on the relationship between Notch signaling and osteogenic differentiation. However, the precise mechanisms underlying this relationship are largely unknown. We have successfully isolated PDLSCs from both ovariectomized (OVX) and sham-operated rats. Both the mRNA and protein levels of Notch1 and Jagged1 were upregulated when PDLSCs were cultured in osteogenic induction media. Mineralization assays showed decreased calcium deposits in OVX-PDLSCs treated with a γ-secretase inhibitor compared with control cells. Thus Notch signaling is important in maintaining the osteogenic differentiation of PDLSCs in osteoporotic rats, which help in the development of a potential therapeutic strategy for periodontal disease in postmenopausal women.

Aberrant ADAM10 expression correlates with osteosarcoma progression

Background

Osteosarcoma is the most common type of bone cancer and is notorious for its rapid progression. The Notch signaling pathway has recently been shown to be involved in osteosarcoma. As a major sheddase of Notch receptors, ADAM10 has been implicated in many types of cancers, but its role in osteosarcoma has not been investigated. Previous studies have shown that the expression of CD31 was significantly elevated in metastatic osteosarcoma; however, its expression in nonmetastatic groups is not known. In addition, the mysterious multinucleated giant cell in giant cell-rich osteosarcoma was previously regarded as an osteoclast-like cell, but its exact identity is unclear.

Method

Tissue chip samples from 40 cases of nonmetastatic osteosarcoma were stained for cytoplasmic ADAM10, activated Notch1 and CD31. Osteoclasts in tumor sections were also stained for tartrate-resistant acid phosphatase (TRAP).

Results

Immunofluorescence staining revealed that ADAM10 expression significantly increased with the progression of osteosarcoma as well as in osteoblastic osteosarcoma, whereas the expression of the Notch intracellular domain (NICD) and CD31 was not significantly altered between different pathological stages. In addition, multinucleated giant cells in giant cell-rich osteosarcoma were also found to coexpress CD31, ADAM10 and NICD, but were negative for TRAP staining.

Conclusions

Our results highlight the importance of ADAM10 in the progression of osteosarcoma and suggest that the protein might be a potential therapeutic target in osteosarcoma treatment. This study also demonstrates that the multinucleated giant cell is an angiogenic tumor cell, rather than an osteoclast, and involves ADAM10/Notch1 signaling activation.

RBP-Jκ-dependent Notch signaling is required for murine articular cartilage and joint maintenance

OBJECTIVE

Osteoarthritis (OA) is a degenerative disease resulting in severe joint cartilage destruction and disability. While the mechanisms underlying the development and progression of OA are poorly understood, gene mutations have been identified within cartilage-related signaling molecules, implicating impaired cell signaling in OA and joint disease. The Notch pathway has recently been identified as a crucial regulator of growth plate cartilage development, and components are expressed in joint tissue. This study was undertaken to investigate a novel role for Notch signaling in joint cartilage development, maintenance, and the pathogenesis of joint disease in a mouse model.

METHODS

We performed the first mouse gene study in which the core Notch signaling component, RBP-Jκ, was tissue specifically deleted within joints. The Prx1Cre transgene removed Rbpjk loxP-flanked alleles in mesenchymal joint precursor cells, while the Col2Cre(ERT2) transgene specifically deleted Rbpjk in postnatal chondrocytes. Murine articular chondrocyte cultures were also used to examine Notch regulation of gene expression.

RESULTS

Loss of Notch signaling in mesenchymal joint precursor cells did not affect embryonic joint development in mice, but rather, resulted in an early, progressive OA-like pathology. Additionally, partial loss of Notch signaling in murine postnatal cartilage resulted in progressive joint cartilage degeneration and an age-related OA-like pathology. Inhibition of Notch signaling altered the expression of the extracellular matrix (ECM)-related factors type II collagen (COL2A1), proteoglycan 4, COL10A1, matrix metalloproteinase 13, and ADAMTS.

CONCLUSION

Our findings indicate that the RBP-Jκ-dependent Notch pathway is a novel pathway involved in joint maintenance and articular cartilage homeostasis, a critical regulator of articular cartilage ECM-related molecules, and a potentially important therapeutic target for OA-like joint disease.

Vitamin D: direct effects of vitamin D metabolites on bone: lessons from genetically modified mice

The vitamin D endocrine system has clear beneficial effects on bone as demonstrated by prevention of rickets in children and by reducing the risk of osteomalacia or osteoporosis in adults or elderly subjects. Depending on the design of the study of genetically modified animals, however, 1,25(OH)2D and the vitamin D receptor (VDR) may have no effect, beneficial or even deleterious direct effects on bone. We present here a comprehensive model of the direct effects of vitamin D on bone. In case of sufficient calcium supply, vitamin D and its metabolites can improve the calcium balance and facilitate mineral deposition in bone matrix largely without direct effects on bone cells, although some beneficial effects may occur via mature osteoblasts, as demonstrated in mice with osteoblast-specific overexpression of VDR or 1α-hydroxylase. In case of calcium deficiency, however, 1,25(OH)2D enhances bone resorption, whereas simultaneously inhibiting bone mineralization, so as to defend serum calcium homeostasis at the expense of bone mass. This dual role probably provides a survival benefit for land vertebrates living in a calcium-poor environment.

Notch signaling in skeletal stem cells

The skeleton originates from stem cells residing in the sclerotome and neural crest that undergo proliferation, migration, and commitment. The development of the skeletal stem cells is influenced by many signaling pathways that govern cell fate determination, proliferation, differentiation, and apoptosis. This review will focus on Notch signaling functions in regulating the different cell types that form the skeletal system as well as the interplay between them to maintain homeostasis. Osteochondroprogenitors require Notch signaling to maintain multipotency and to prevent premature differentiation into osteoblasts. Subsequently, overactivation of Notch signaling suppresses osteoblast maturation. Moreover, Notch signaling in osteochondroprogenitors is required for chondrocyte proliferation and hypertrophy and suppresses terminal differentiation. Translational studies demonstrated a crucial role of Notch signaling in osteosarcoma and osteoarthritis, where concepts derived from developmental pathways are often recapitulated. This brings hope of taking advantage of the molecular mechanisms learned from development to approach the pathological processes underlying abnormal bone/cartilage metabolism or tumorigenesis. Pharmacological agents that target Notch receptors or ligands in a tissue-specific fashion would offer new opportunities for treating bone/cartilage diseases caused by dysregulation of Notch signaling.