NOTCH2 promotes osteoclast maturation and metabolism and modulates the transcriptome profile during osteoclastogenesis.

Notch signaling plays a key regulatory role in bone remodeling and NOTCH2 enhances osteoclastogenesis, an effect that is mostly mediated by its target gene Hes1. In the present study, we explored mechanisms responsible for the enhanced osteoclastogenesis in bone marrow-derived macrophages (BMM) from Notch2tm1.1Ecan, harboring a NOTCH2 gain-of-function mutation, and control mice. Notch2tm1.1Ecan mice are osteopenic and have enhanced osteoclastogenesis. Bulk RNA-Seq and gene set enrichment analysis of Notch2tm1.1Ecan BMMs cultured in the presence of macrophage colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand revealed enrichment of genes associated with enhanced cell metabolism, aerobic respiration, and mitochondrial function, all associated with osteoclastogenesis. These pathways were not enhanced in the context of a Hes1 inactivation. Analysis of single cell RNA-Seq data of pooled control and Notch2tm1.1Ecan BMMs treated with M-CSF or M-CSF and receptor activator of NF-κB ligand for 3 days identified 11 well-defined cellular clusters. Pseudotime trajectory analysis indicated a trajectory of clusters expressing genes associated with osteoclast progenitors, osteoclast precursors, and mature cells. There were an increased number of cells expressing gene markers associated with the osteoclast and with an unknown, albeit related, cluster in Notch2tm1.1Ecan than in control BMMs as well as enhanced expression of genes associated with osteoclast progenitors and precursors in Notch2tm1.1Ecan cells. In conclusion, BMM cultures display cellular heterogeneity, and NOTCH2 enhances osteoclastogenesis, increases mitochondrial and metabolic activity of osteoclasts, and affects cell cluster allocation in BMMs.

NOTCH2 sensitizes the chondrocyte to the inflammatory response of tumor necrosis factor α.

Notch regulates the immune and inflammatory response and has been associated with the pathogenesis of osteoarthritis in humans and preclinical models of the disease. Notch2tm1.1Ecan mice harbor a NOTCH2 gain-of-function and are sensitized to osteoarthritis, but the mechanisms have not been explored. We examined the effects of tumor necrosis factor α (TNFα) in chondrocytes from Notch2tm1.1Ecan mice and found that NOTCH2 enhanced the effect of TNFα on Il6 and Il1b expression. Similar results were obtained in cells from a conditional model of NOTCH2 gain-of-function, Notch22.1Ecan mice, and following the expression of the NOTCH2 intracellular domain in vitro. Recombination signal-binding protein for immunoglobulin Kappa J region partners with the NOTCH2 intracellular domain to activate transcription; in the absence of Notch signaling it inhibits transcription, and Rbpj inactivation in chondrocytes resulted in Il6 induction. Although TNFα induced IL6 to a greater extent in the context of NOTCH2 activation, there was a concomitant inhibition of Notch target genes Hes1, Hey1, Hey2, and Heyl. Electrophoretic mobility shift assay demonstrated displacement of recombination signal-binding protein for immunoglobulin Kappa J region from DNA binding sites by TNFα explaining the increased Il6 expression and the concomitant decrease in Notch target genes. NOTCH2 enhanced the effect of TNFα on NF-κB signaling, and RNA-Seq revealed increased expression of pathways associated with inflammation and the phagosome in NOTCH2 overexpressing cells in the absence and presence of TNFα. Collectively, NOTCH2 has important interactions with TNFα resulting in the enhanced expression of Il6 and inflammatory pathways in chondrocytes.

Hairy and enhancer of split 1 is a primary effector of NOTCH2 signaling and induces osteoclast differentiation and function.

Notch2tm1.1Ecan mice, which harbor a mutation replicating that found in Hajdu-Cheney syndrome, exhibit marked osteopenia because of increased osteoclast number and bone resorption. Hairy and enhancer of split 1 (HES1) is a Notch target gene and a transcriptional modulator that determines osteoclast cell fate decisions. Transcript levels of Hes1 increase in Notch2tm1.1Ecan bone marrow-derived macrophages (BMMs) as they mature into osteoclasts, suggesting a role in osteoclastogenesis. To determine whether HES1 is responsible for the phenotype of Notch2tm1.1Ecan mice and the skeletal manifestations of Hajdu-Cheney syndrome, Hes1 was inactivated in Ctsk-expressing cells from Notch2tm1.1Ecan mice. Ctsk encodes the protease cathepsin K, which is expressed preferentially by osteoclasts. We found that the osteopenia of Notch2tm1.1Ecan mice was ameliorated, and the enhanced osteoclastogenesis was reversed in the context of the Hes1 inactivation. Microcomputed tomography revealed that the downregulation of Hes1 in Ctsk-expressing cells led to increased bone volume/total volume in female mice. In addition, cultures of BMMs from CtskCre/WT;Hes1Δ/Δ mice displayed a decrease in osteoclast number and size and decreased bone-resorbing capacity. Moreover, activation of HES1 in Ctsk-expressing cells led to osteopenia and enhanced osteoclast number, size, and bone resorptive capacity in BMM cultures. Osteoclast phenotypes and RNA-Seq of cells in which HES1 was activated revealed that HES1 modulates cell-cell fusion and bone-resorbing capacity by supporting sealing zone formation. In conclusion, we demonstrate that HES1 is mechanistically relevant to the skeletal manifestation of Notch2tm1.1Ecan mice and is a novel determinant of osteoclast differentiation and function.

Activation of Notch3 in osteoblasts/osteocytes causes compartment-specific changes in bone remodeling.

Notch receptors maintain skeletal homeostasis. NOTCH1 and 2 have been studied for their effects on bone remodeling. Although NOTCH3 plays a significant role in vascular physiology, knowledge about its function in other cellular environments, including bone, is limited. The present study was conducted to establish the function of NOTCH3 in skeletal cells using models of Notch3 misexpression. Microcomputed tomography demonstrated that Notch3 null mice did not have appreciable bone phenotypes. To study the effects of the NOTCH3 activation in the osteoblast lineage, BGLAP-Cre or Dmp1-Cre transgenics were crossed with RosaNotch3 mice, where the NOTCH3 intracellular domain is expressed following the removal of a loxP-flanked STOP cassette. Microcomputed tomography demonstrated that BGLAP-Cre;RosaNotch3 and Dmp1-Cre;RosaNotch3 mice of both sexes exhibited an increase in trabecular bone and in connectivity, with a decrease in cortical bone and increased cortical porosity. Histological analysis revealed a decrease in osteoclast number and bone resorption in trabecular bone and an increase in osteoclast number and void or pore area in cortical bone of RosaNotch3 mice. Bone formation was either decreased or could not be determined in Cre;RosaNotch3 mice. NOTCH3 activation in osteoblasts inhibited Alpl (alkaline phosphatase) and Bglap (osteocalcin) and induced Tnfsf11 (RANKL) and Tnfrsf11b (osteoprotegerin) mRNA, possibly explaining the trabecular bone phenotype. However, NOTCH3 induced Tnfsf11 and suppressed Tnfrsf11b in osteocytes, possibly explaining the cortical porosity. In conclusion, basal NOTCH3 is dispensable for skeletal homeostasis, whereas activation of NOTCH3 in osteoblasts/osteocytes inhibits osteoclastogenesis and bone resorption in cancellous bone but increases intracortical remodeling and causes cortical porosity.

Antisense oligonucleotides targeting Notch2 ameliorate the osteopenic phenotype in a mouse model of Hajdu-Cheney syndrome.

Notch receptors play critical roles in cell-fate decisions and in the regulation of skeletal development and bone remodeling. Gain-of-function NOTCH2 mutations can cause Hajdu-Cheney syndrome, an untreatable disease characterized by osteoporosis and fractures, craniofacial developmental abnormalities, and acro-osteolysis. We have previously created a mouse model harboring a point 6955C→T mutation in the Notch2 locus upstream of the PEST domain, and we termed this model Notch2tm1.1Ecan Heterozygous Notch2tm1.1Ecan mutant mice exhibit severe cancellous and cortical bone osteopenia due to increased bone resorption. In this work, we demonstrate that the subcutaneous administration of Notch2 antisense oligonucleotides (ASO) down-regulates Notch2 and the Notch target genes Hes-related family basic helix-loop-helix transcription factor with YRPW motif 1 (Hey1), Hey2, and HeyL in skeletal tissue from Notch2tm1.1Ecan mice. Results of microcomputed tomography experiments indicated that the administration of Notch2 ASOs ameliorates the cancellous osteopenia of Notch2tm1.1Ecan mice, and bone histomorphometry analysis revealed decreased osteoclast numbers in Notch2 ASO-treated Notch2tm1.1Ecan mice. Notch2 ASOs decreased the induction of mRNA levels of TNF superfamily member 11 (Tnfsf11, encoding the osteoclastogenic protein RANKL) in cultured osteoblasts and osteocytes from Notch2tm1.1Ecan mice. Bone marrow-derived macrophage cultures from the Notch2tm1.1Ecan mice displayed enhanced osteoclastogenesis, which was suppressed by Notch2 ASOs. In conclusion, Notch2tm1.1Ecan mice exhibit cancellous bone osteopenia that can be ameliorated by systemic administration of Notch2 ASOs.

Protease-Activated Receptor 1 Deletion Causes Enhanced Osteoclastogenesis in Response to Inflammatory Signals through a Notch2-Dependent Mechanism.

We found that protease-activated receptor 1 (PAR1) was transiently induced in cultured osteoclast precursor cells. Therefore, we examined the bone phenotype and response to resorptive stimuli of PAR1-deficient (knockout [KO]) mice. Bones and bone marrow-derived cells from PAR1 KO and wild-type (WT) mice were assessed using microcomputed tomography, histomorphometry, in vitro cultures, and RT-PCR. Osteoclastic responses to TNF-α (TNF) challenge in calvaria were analyzed with and without a specific neutralizing Ab to the Notch2-negative regulatory region (N2-NRR Ab). In vivo under homeostatic conditions, there were minimal differences in bone mass or bone cells between PAR1 KO and WT mice. However, PAR1 KO myeloid cells demonstrated enhanced osteoclastogenesis in response to receptor activator of NF-κB ligand (RANKL) or the combination of RANKL and TNF. Strikingly, in vivo osteoclastogenic responses of PAR1 KO mice to TNF were markedly enhanced. We found that N2-NRR Ab reduced TNF-induced osteoclastogenesis in PAR1 KO mice to WT levels without affecting WT responses. Similarly, in vitro N2-NRR Ab reduced RANKL-induced osteoclastogenesis in PAR1 KO cells to WT levels without altering WT responses. We conclude that PAR1 functions to limit Notch2 signaling in responses to RANKL and TNF and moderates osteoclastogenic response to these cytokines. This effect appears, at least in part, to be cell autonomous because enhanced osteoclastogenesis was seen in highly purified PAR1 KO osteoclast precursor cells. It is likely that this pathway is involved in regulating the response of bone to diseases associated with inflammatory signals.

The Hajdu Cheney mutation sensitizes mice to the osteolytic actions of tumor necrosis factor α.

Hajdu Cheney syndrome (HCS) is characterized by craniofacial developmental abnormalities, acro-osteolysis, and osteoporosis and is associated with gain-of-NOTCH2 function mutations. A mouse model of HCS termed Notch2tm1.1Ecan harboring a mutation in exon 34 of Notch2 replicating the one found in HCS was used to determine whether the HCS mutation sensitizes the skeleton to the osteolytic effects of tumor necrosis factor α (TNFα). TNFα injected over the calvarial vault caused a greater increase in osteoclast number, osteoclast surface, and eroded surface in Notch2tm1.1Ecan mice compared with littermate WT controls. Accordingly, the effect of TNFα on osteoclastogenesis was greatly enhanced in cultures of bone marrow-derived macrophages (BMMs) from Notch2tm1.1Ecan mice when compared with the activity of TNFα in control cultures. TNFα induced the expression of Notch2 and Notch2 mutant mRNA by ∼2-fold, possibly amplifying the NOTCH2-dependent induction of osteoclastogenesis. The effect of TNFα on osteoclastogenesis in Notch2tm1.1Ecan mutants depended on NOTCH2 activation because it was reversed by anti-NOTCH2 negative regulatory region and anti-jagged 1 antibodies. The inactivation of Hes1 prevented the TNFα effect on osteoclastogenesis in the context of the Notch2tm1.1Ecan mutation. In addition, the induction of Il1b, but not of Tnfa and Il6, mRNA by TNFα was greater in Notch2tm1.1Ecan BMMs than in control cells, possibly contributing to the actions of TNFα and NOTCH2 on osteoclastogenesis. In conclusion, the HCS mutation enhances TNFα-induced osteoclastogenesis and the inflammatory bone-resorptive response possibly explaining the acro-osteolysis observed in affected individuals.

Nuclear factor of activated T cells 1 and 2 are required for vertebral homeostasis.

The present study defines the function of nuclear factor of activated T cells (NFAT)c1 and NFATc2 in osteoblast function in vivo and in vitro. Nfatc1loxP/loxP , Nfatc2loxP/loxP , and Nfatc1loxP/loxP ;Nfatc2loxP/loxP conditional mice were mated with BGLAP-Cre transgenics to inactivate Nfatc1 and Nfatc2 singly and in combination in osteoblasts. Microcomputed tomography demonstrated that male and female conditionally inactivated Nfatc1, Nfatc2 and dual Nfatc1;Nfatc2 mice had osteopenia at Lumbar 3 (L3) sites when compared to littermate controls. However, the Nfatc1 and Nfatc2 inactivation singly and in combination in Bglap-expressing osteoblasts did not result in an appreciable phenotype at femoral sites. Bone histomorphometry of L3 confirmed the osteopenic phenotype and demonstrated that Nfatc1;Nfatc2 inactivated male mice had a significant decrease in osteoblast number and in osteoblast surface and osteoid surface. The dual downregulation of Nfatc1 and Nfatc2 in bone marrow stromal cells caused a decrease in Alpl and Bglap expression, confirming a role of these transcription factors in osteoblast function. In conclusion, our studies reveal that NFATc1 and NFATc2 are necessary for optimal vertebral, but not femoral, bone homeostasis in vivo and osteoblast differentiation in vitro.

An antibody to Notch3 reverses the skeletal phenotype of lateral meningocele syndrome in male mice.

Lateral meningocele syndrome (LMS), a genetic disorder characterized by meningoceles and skeletal abnormalities, is associated with NOTCH3 mutations. We created a mouse model of LMS (Notch3tm1.1Ecan ) by introducing a tandem termination codon in the Notch3 locus upstream of the proline (P), glutamic acid (E), serine (S) and threonine (T) domain. Microcomputed tomography demonstrated that Notch3tm1.1Ecan mice exhibit osteopenia. The cancellous bone osteopenia was no longer observed after the intraperitoneal administration of antibodies directed to the negative regulatory region (NRR) of Notch3. The anti-Notch3 NRR antibody suppressed the expression of Hes1, Hey1, and Hey2 (Notch target genes), and decreased Tnfsf11 (receptor activator of NF Kappa B ligand) messenger RNA in Notch3tm1.1Ecan osteoblast (OB) cultures. Bone marrow-derived macrophages (BMMs) from Notch3tm1.1Ecan mutants exhibited enhanced osteoclastogenesis in culture, and this was increased in cocultures with Notch3tm1.1Ecan OB. Osteoclastogenesis was suppressed by anti-Notch3 NRR antibodies in Notch3tm1.1Ecan OB/BMM cocultures. In conclusion, the cancellous bone osteopenia of Notch3tm1.1Ecan mutants is reversed by anti-Notch3 NRR antibodies.

The lateral meningocele syndrome mutation causes marked osteopenia in mice.

Lateral meningocele syndrome (LMS) is a rare genetic disorder characterized by neurological complications and osteoporosis. LMS is associated with mutations in exon 33 of NOTCH3 leading to a truncated protein lacking sequences for NOTCH3 degradation and presumably causing NOTCH3 gain of function. To create a mouse model reproducing human LMS-associated mutations, we utilized CRISPR/Cas9 to introduce a tandem termination codon at bases 6691-6696 (ACCAAG→TAATGA) and verified this mutation (Notch3tm1.1Ecan ) by DNA sequencing of F1 mice. One-month-old male and female heterozygous Notch3tm1.1Ecan mice had cancellous and cortical bone osteopenia but exhibited no obvious neurological alterations, and histopathology of multiple organs revealed no abnormalities. Microcomputed tomography of these mutants revealed a 35-60% decrease in cancellous bone volume associated with a reduction in trabecular number and decreased connectivity. During maturation, cancellous and cortical bones were restored in female but not in male mice, which exhibited cancellous bone osteopenia at 4 months. Cancellous bone histomorphometry revealed increased osteoblast and osteocyte numbers and a modest increase in osteoclast surface and bone formation rate. Notch3tm1.1Ecan calvarial osteoblasts had increased proliferation and increased bone γ-carboxyglutamate protein (Bglap) and TNF superfamily member 11 (Tnfsf11) mRNA levels and lower Tnfrsf11b levels. Tnfsf11 mRNA was increased in osteocyte-rich femora from Notch3tm1.1Ecan mice. Cultures of bone marrow-derived macrophages from Notch3tm1.1Ecan mice revealed increased osteoclast formation, particularly in cocultures with osteoblasts from Notch3tm1.1Ecan mice. In conclusion, the Notch3tm1.1Ecan mutation causes osteopenia despite an increase in osteoblast proliferation and function and is associated with enhanced Tnfsf11 expression in osteoblasts and osteocytes.

The Hajdu Cheney Mutation Is a Determinant of B-Cell Allocation of the Splenic Marginal Zone.

The neurogenic locus notch homolog protein (Notch)-2 receptor is a determinant of B-cell allocation, and gain-of-NOTCH2-function mutations are associated with Hajdu-Cheney syndrome (HCS), a disease presenting with osteoporosis and acro-osteolysis. We generated a mouse model reproducing the HCS mutation (Notch2HCS), and heterozygous global mutant mice displayed gain-of-Notch2 function. In the mutant spleen, the characteristic perifollicular rim marking the marginal zone (MZ), which is the interface between the nonlymphoid red pulp and the lymphoid white pulp, merged with components of the white pulp. As a consequence, the MZ of Notch2HCS mice occupied most of the splenic structure. To explore the mechanisms involved, lymphocyte populations from the bone marrow and spleen were harvested from heterozygous Notch2HCS mice and sex-matched control littermates and analyzed by flow cytometry. Notch2HCS mice had an increase in CD21/35highCD23- splenic MZ B cells of approximately fivefold and a proportional decrease in splenic follicular B cells (CD21/35intCD23+) at 1, 2, and 12 months of age. Western blot analysis revealed that Notch2HCS mutant splenocytes had increased phospho-Akt and phospho-Jun N-terminal kinase, and gene expression analysis of splenic CD19+ B cells demonstrated induction of Hes1 and Hes5 in Notch2HCS mutants. Anti-Notch2 antibodies decreased MZ B cells in control and Notch2HCS mice. In conclusion, Notch2HCS mutant mice have increased mature B cells in the MZ of the spleen.

Induction of the Hajdu-Cheney Syndrome Mutation in CD19 B Cells in Mice Alters B-Cell Allocation but Not Skeletal Homeostasis.

Mice harboring Notch2 mutations replicating Hajdu-Cheney syndrome (Notch2tm1.1ECan) have osteopenia and exhibit an increase in splenic marginal zone B cells with a decrease in follicular B cells. Whether the altered B-cell allocation is responsible for the osteopenia of Notch2tm1.1ECan mutants is unknown. To determine the effect of NOTCH2 activation in B cells on splenic B-cell allocation and skeletal phenotype, a conditional-by-inversion (COIN) Hajdu-Cheney syndrome allele of Notch2 (Notch2[ΔPEST]COIN) was used. Cre recombination generates a permanent Notch2ΔPEST allele expressing a transcript for which sequences coding for the proline, glutamic acid, serine, and threonine-rich (PEST) domain are replaced by a stop codon. CD19-Cre drivers were backcrossed into Notch2[ΔPEST]COIN/[ΔPEST]COIN to generate CD19-specific Notch2ΔPEST/ΔPEST mutants and control Notch2[ΔPEST]COIN/[ΔPEST]COIN littermates. There was an increase in marginal zone B cells and a decrease in follicular B cells in the spleen of CD19Cre/WT;Notch2ΔPEST/ΔPEST mice, recapitulating the splenic phenotype of Notch2tm1.1ECan mice. The effect was reproduced when the NOTCH1 intracellular domain was induced in CD19-expressing cells (CD19Cre/WT;RosaNotch1/WT mice). However, neither CD19Cre/WT;Notch2ΔPEST/ΔPEST nor CD19Cre/WT;RosaNotch1/WT mice had a skeletal phenotype. Moreover, splenectomies in Notch2tm1.1ECan mice did not reverse their osteopenic phenotype. In conclusion, Notch2 activation in CD19-expressing cells determines B-cell allocation in the spleen but has no skeletal consequences.

Sustained Notch2 signaling in osteoblasts, but not in osteoclasts, is linked to osteopenia in a mouse model of Hajdu-Cheney syndrome.

Individuals with Hajdu-Cheney syndrome (HCS) present with osteoporosis, and HCS is associated with NOTCH2 mutations causing deletions of the proline-, glutamic acid-, serine-, and threonine-rich (PEST) domain that are predicted to enhance NOTCH2 stability and cause gain-of-function. Previously, we demonstrated that mice harboring Notch2 mutations analogous to those in HCS (Notch2HCS) are severely osteopenic because of enhanced bone resorption. We attributed this phenotype to osteoclastic sensitization to the receptor activator of nuclear factor-κB ligand and increased osteoblastic tumor necrosis factor superfamily member 11 (Tnfsf11) expression. Here, to determine the individual contributions of osteoclasts and osteoblasts to HCS osteopenia, we created a conditional-by-inversion (Notch2COIN ) model in which Cre recombination generates a Notch2ΔPEST allele expressing a Notch2 mutant lacking the PEST domain. Germ line Notch2COIN inversion phenocopied the Notch2HCS mutant, validating the model. To activate Notch2 in osteoclasts or osteoblasts, Notch2COIN mice were bred with mice expressing Cre from the Lyz2 or the BGLAP promoter, respectively. These crosses created experimental mice harboring a Notch2ΔPEST allele in Cre-expressing cells and control littermates expressing a wild-type Notch2 transcript. Notch2COIN inversion in Lyz2-expressing cells had no skeletal consequences and did not affect the capacity of bone marrow macrophages to form osteoclasts in vitro In contrast, Notch2COIN inversion in osteoblasts led to generalized osteopenia associated with enhanced bone resorption in the cancellous bone compartment and with suppressed endocortical mineral apposition rate. Accordingly, Notch2 activation in osteoblast-enriched cultures from Notch2COIN mice induced Tnfsf11 expression. In conclusion, introduction of the HCS mutation in osteoblasts, but not in osteoclasts, causes osteopenia.

Nuclear factor of activated T cells 2 is required for osteoclast differentiation and function in vitro but not in vivo.

Nuclear factor of activated T cells (NFAT) c2 is important for the immune response and it compensates for NFATc1 for its effects on osteoclastogenesis, but its role in this process is not established. To study the function of NFATc2 in the skeleton, Nfatc2loxP/loxP mice, where the Nfact2 exon 2 is flanked by loxP sequences, were created and mated with mice expressing the Cre recombinase under the control of the Lyz2 promoter. Bone marrow-derived macrophage (BMM) from Lyz2Cre/WT ;Nfatc2Δ/Δ mice cultured in the presence of macrophage-colony stimulating factor and receptor activator of NF-κB ligand exhibited a decrease in the number and size of osteoclasts and a smaller sealing zone when compared to BMMs from Nfatc2loxP/loxP littermate controls. Bone resorption was decreased in osteoclasts from Lyz2Cre/WT ;Nfatc2Δ/Δ mice. This demonstrates that NFATc2 is necessary for optimal osteoclast maturation and function in vitro. Male and female Lyz2Cre/WT ;Nfatc2Δ/Δ mice did not exhibit an obvious skeletal phenotype by microcomputed tomography (µCT) at either 1 or 4 months of age when compared to Nfatc2loxP/loxP sex-matched littermates. Bone histomorphometry confirmed the µCT results, and conditional 4-month-old Lyz2Cre/WT ;Nfatc2Δ/Δ mice did not exhibit changes in parameters of bone histomorphometry. In conclusion, NFATc2 is necessary for optimal osteoclastogenesis in vitro, but its downregulation in the myeloid lineage has no consequences in skeletal remodeling in vivo.

Glucocorticoids inhibit notch target gene expression in osteoblasts.

Glucocorticoids in excess suppress osteoblast function and cause osteoporosis. We demonstrated that cortisol induces the expression of selected Notch receptors in osteoblasts, revealing a potential mechanism for the skeletal effects of glucocorticoids. However, it remains to be determined whether increased expression of Notch receptors results into enhanced signaling. Following activation of Notch, its intracellular domain (NICD) binds to the DNA-associated protein recombination signal binding protein for immunoglobulin kappa-J region (RBPJ) and induces the expression of target genes such as Hey1, Hey2, and HeyL. To determine whether glucocorticoids modulate Notch signaling in the skeleton, 1 month old wild-type mice were administered prednisolone or placebo and sacrificed after 72 h, and gene expression was analyzed in femoral bone. Prednisolone induced Tsc22d3, a glucocorticoid target gene, and suppressed Hey1 and HeyL expression, which is indicative of inhibited Notch receptor activity or direct Hey downregulation. To determine the mechanisms of Hey suppression, wild-type osteoblast-enriched cells were seeded on the Notch cognate ligand Delta-like (DLL)1 or transfected with constructs expressing the NOTCH1 NICD fragment and exposed to either cortisol or vehicle. Cortisol opposed the induction of mRNA and heterogeneous nuclear RNA for Hey1, Hey2, and HeyL by DLL1, but had no effect on mRNA stability, indicating that glucocorticoids inhibit Hey expression by transcriptional mechanisms. Transactivation studies and electrophoretic mobility shift assays revealed that cortisol did not oppose RBPJ-mediated transcription or RBPJ/DNA interactions, respectively. In conclusion, glucocorticoids suppress expression of Hey1, Hey2, and HeyL in osteoblasts by RBPJ-independent transcriptional mechanisms.

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.

Effects of Sex and Notch Signaling on the Osteocyte Cell Pool.

Osteocytes play a fundamental role in mechanotransduction and skeletal remodeling. Sex is a determinant of skeletal structure, and female C57BL/6J mice have increased osteoblast number in cancellous bone when compared to male mice. Activation of Notch in the skeleton causes profound cell-context dependent changes in skeletal physiology. To determine the impact of sex and of Notch signaling on the osteocyte cell pool, we analyzed cancellous and cortical bone of 1-6-month-old C57BL/6J or 129SvJ/C57BL/6J mice and determined the osteocyte number/area. There was an age-dependent decline in osteocyte number in cancellous bone of male but not female mice, so that 6-month-old female mice had a greater number of osteocytes than male littermates. Although differences between male and female mice were modest, female mice had ∼10-15% greater number of osteocytes/area. RNA sequence analysis of osteocyte-rich preparations did not reveal differences between sexes in the expression of genes known to influence bone homeostasis. Neither the activation of Notch1 nor the concomitant inactivation of Notch1 and Notch2 in Osterix (Sp7) or Dentin matrix protein 1 (Dmp1) expressing cells had a pronounced and consistent effect on cancellous or cortical bone osteocyte number in either sex. Moreover, inactivation of Notch1 and Notch2 in Dmp1 expressing cells did not influence the bone loss in a muscle immobilization model of skeletal unloading. In conclusion, cancellous bone osteocytes decline with age in male mice, cortical osteocytes are influenced by sex in younger mice, but osteocyte cell density is not affected substantially by Notch signaling. J. Cell. Physiol. 232: 363-370, 2017. © 2016 Wiley Periodicals, Inc.

Hairy and Enhancer of Split-Related With YRPW Motif-Like (HeyL) Is Dispensable for Bone Remodeling in Mice.

Notch induces Hairy Enhancer of Split (Hes)1 and Hes-related with YRPW motif (Hey) Hey1, Hey2 and Hey-like (HeyL) expression in osteoblasts, but it is not known whether any of these target genes mediates the effect of Notch in the skeleton. We demonstrated that Notch1 activation in osteoblasts/osteocytes induces Hes1, Hey1, Hey2, and HeyL, but HeyL was induced to a greater extent than other target genes. To characterize HeyL null mice for their skeletal phenotype, microcomputed tomography (µCT) and histomorphometric analysis of HeyL null and sex-matched littermate controls was performed. µCT demonstrated modest cancellous bone osteopenia in 1 month old male mice and normal microarchitecture in 3 month old male HeyL null mice. Female HeyL null mice were not different from controls at either 1 or 3 months of age. Bone histomorphometry did not demonstrate differences between HeyL null mice of either sex and littermate controls. In conclusion, HeyL null mice do not exhibit an obvious skeletal phenotype demonstrating that HeyL is dispensable for skeletal homeostasis. J. Cell. Biochem. 118: 1819-1826, 2017. © 2016 Wiley Periodicals, Inc.

The Dmp1-SOST Transgene Interacts With and Downregulates the Dmp1-Cre Transgene and the Rosa(Notch) Allele.

Activation of Notch1 in osteocytes of Rosa(Notch) mice, where a loxP-flanked STOP cassette and the Nicd coding sequence were targeted to the reverse orientation splice acceptor (Rosa)26 locus, causes osteopetrosis associated with suppressed Sost expression and enhanced Wnt signaling. To determine whether Sost downregulation mediates the effects of Notch activation in osteocytes, Rosa(Notch) mice were crossed with transgenics expressing Cre recombinase or SOST under the control of the dentin matrix protein (Dmp)1 promoter. Dmp1-SOST transgenics displayed vertebral osteopenia and a modest femoral cancellous and cortical bone phenotype, whereas hemizygous Dmp1-Cre transgenics heterozygous for the Rosa(Notch) allele (Dmp1-Cre;Rosa(Notch)) exhibited osteopetrosis. The phenotype of Notch activation in osteocytes was prevented in Dmp1-Cre;Rosa(Notch) mice hemizygous for the Dmp1-SOST transgene. The effect was associated with downregulated Notch signaling and suppressed Dmp1 and Rosa26 expression. To test whether SOST regulates Notch expression in osteocytes, cortical bone cultures from Dmp1-Cre;Rosa(Notch) mice or from Rosa(Notch) control littermates were exposed to recombinant human SOST. The addition of SOST had only modest effects on Notch target gene mRNA levels and suppressed Dmp1, but not Cre or Rosa26, expression. These findings suggest that prevention of the Dmp1-Cre;Rosa(Notch) skeletal phenotype by Dmp1-SOST is not secondary to SOST expression but to interactions among the Dmp1-SOST and Dmp1-Cre transgenes and the Rosa26 locus. In conclusion, the Dmp1-SOST transgene suppresses the expression of the Dmp1-Cre transgene and of Rosa26.

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.