Effects of Sex and Notch Signaling on the Osteocyte Cell Pool

Induction of a NOTCH3 Lehman syndrome mutation in osteocytes causes osteopenia in male C57BL/6J mice

Lateral Meningocele or Lehman Syndrome (LMS) is associated with NOTCH3 mutations causing deletions of the PEST domain and a gain-of-NOTCH3 function. We demonstrated that Notch3em1Ecan mice harboring Notch3 mutations analogous to those found in LMS are osteopenic because of enhanced bone resorption. To determine the contribution of specific cell lineages to the phenotype, we created a conditional-by-inversion (Notch3COIN) model termed Notch3em2Ecan in which Cre recombination generates a Notch3INV allele expressing a NOTCH3 mutant lacking the PEST domain. Germ line Notch3COIN inversion caused osteopenia and phenocopied the Notch3em1Ecan mutant, validating the model. To induce the mutation in osteocytes, smooth muscle and endothelial cells, Notch3COIN mice were bred with mice expressing Cre from the Dmp1, Sm22a and Cdh5 promoters, respectively, creating experimental mice harboring Notch3INV alleles in Cre-expressing cells and control littermates harboring Notch3COIN alleles. Notch3COIN inversion in osteocytes led to femoral and vertebral cancellous bone osteopenia, whereas Notch3COIN inversion in mural Sm22a or endothelial Cdh5-expressing cells did not result in a skeletal phenotype. In conclusion, introduction of the LMS mutation in osteocytes but not in vascular cells causes osteopenia and phenocopies Notch3em1Ecan global mutant mice.

Botulinum Toxin A and Osteosarcopenia in Experimental Animals: A Scoping Review

We conducted a scoping review to investigate the effects of intramuscular injection of Botulinum Toxin A (BoNT-A) on bone morphology. We investigated if the muscle atrophy associated with Injection of BoNT-A had effects on the neighboring bone. We used the search terms: osteopenia, bone atrophy, Botulinum Toxin A, Micro-CT, mice or rat. The following databases were searched: Medline, Embase, PubMed and the Cochrane Library, between 1990 and 2020. After removal of duplicates, 228 abstracts were identified of which 49 studies satisfied our inclusion and exclusion criteria. The majority of studies (41/49) reported a quantitative reduction in at least one measure of bone architecture based on Micro-CT. The reduction in the ratio of bone volume to tissue volume varied from 11% to 81% (mean 43%) according to the experimental set up and study time points. While longer term studies showed muscle recovery, no study showed complete recovery of all bone properties at the termination of the study. In experimental animals, intramuscular injection of BoNT-A resulted in acute muscle atrophy and acute degradation of the neighboring bone segment. These findings may have implications for clinical protocols in the use of Botulinum Toxin in children with cerebral palsy, with restraint recommended in injection protocols and consideration for monitoring bone density. Clinical studies in children with cerebral palsy receiving injections of Botulinum are indicated.

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.

Modulation of Notch1 signaling regulates bone fracture healing

Fracture healing involves interactions of different cell types, driven by various growth factors, and signaling cascades. Periosteal mesenchymal progenitor cells give rise to the majority of osteoblasts and chondrocytes in a fracture callus. Notch signaling has emerged as an important regulator of skeletal cell proliferation and differentiation. We investigated the effects of Notch signaling during the fracture healing process. Increased Notch signaling in osteochondroprogenitor cells driven by overexpression of Notch1 intracellular domain (NICD1) (αSMACreERT2 mice crossed with Rosa-NICD1) during fracture resulted in less cartilage, more mineralized callus tissue, and stronger and stiffer bones after 3 weeks. Periosteal cells overexpressing NICD1 showed increased proliferation and migration in vitro. In vivo data confirmed that increased Notch1 signaling caused expansion of alpha-smooth muscle actin (αSMA)-positive cells and their progeny including αSMA-derived osteoblasts in the callus without affecting osteoclast numbers. In contrast, anti-NRR1 antibody treatment to inhibit Notch1 signaling resulted in increased callus cartilage area, reduced callus bone mass, and reduced biomechanical strength. Our study shows a positive effect of induced Notch1 signaling on the fracture healing process, suggesting that stimulating the Notch pathway could be beneficial for fracture repair.

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.

Contextual Regulation of Skeletal Physiology by Notch Signaling

PURPOSE OF REVIEW

This article reviews the past 2 years of research on Notch signaling as it relates to bone physiology, with the goal of reconciling seemingly discrepant findings and identifying fruitful areas of potential future research.

RECENT FINDINGS

Conditional animal models and high-throughput omics have contributed to a greater understanding of the context-dependent role of Notch signaling in bone. However, significant gaps remain in our understanding of how spatiotemporal context and epigenetic state dictate downstream Notch phenotypes. Biphasic activation of Notch signaling orchestrates progression of mesenchymal progenitor cells through the osteoblast lineage, but there is a limited understanding of ligand- and receptor-specific functions. Paracrine Notch signaling through non-osteoblastic cell types contributes additional layers of complexity, and we anticipate impactful future work related to the integration of these cell types and signaling mechanisms.

Postnatal Skeletal Deletion of Dickkopf-1 Increases Bone Formation and Bone Volume in Male and Female Mice, Despite Increased Sclerostin Expression

The Wnt antagonist Dickkopf-1 (Dkk1) is a negative regulator of osteoblast function and bone mass. However, because of the lack of appropriate models, many aspects of its role in the regulation of postnatal bone turnover and its cellular source have remained unknown. In this study, we deleted Dkk1 postnatally and in different cell types using various Cre-drivers (Rosa26-ERT2-Cre, Osx-cre, Dmp1-Cre) and assessed to which extent cells of the osteoblastic lineage contribute to the effects of Dkk1 on bone turnover and homeostasis. Female and male mice were examined at 12 weeks of age. Mice with a global or cell type-specific deletion of Dkk1 showed a two- to threefold higher bone volume compared with their Cre-negative littermates. The mineral apposition rate and the bone formation rate were increased two- to fourfold in all three mouse lines, despite a significant increase in systemic and skeletal levels of sclerostin. Dkk1 deletion further reduced the number of osteoclasts about twofold, which was accompanied by a strong decrease in the receptor activator of nuclear factor-κB ligand/osteoprotegerin mRNA ratio in femoral bone. Despite similar increases in bone mass, the deletion of Dkk1 in osterix-expressing cells reduced circulating Dkk1 significantly (males, -79%; females, -77%), whereas they were not changed in Dkk1^fl/fl ;Dmp1-Cre mice. However, both lines showed significantly reduced Dkk1 mRNA levels in bone. In summary, we show that lack of Dkk1 in cells of the osteoblastic lineage leads to high bone mass with increased bone formation, despite increased levels of sclerostin. Moreover, the majority of systemic Dkk1 appears to originate from osteoprogenitors but not from mature osteoblasts or osteocytes. Nevertheless, the amount of Dkk1 produced locally by more mature osteogenic cells is sufficient to modulate bone mass. Thus, this study highlights the importance of local Wnt signaling on postnatal bone homeostasis. © 2018 American Society for Bone and Mineral Research.

Bone Structural Characteristics and Response to Bisphosphonate Treatment in Children With Hajdu-Cheney Syndrome

CONTEXT

Hajdu-Cheney syndrome (HJCYS) is a rare, multisystem bone disease caused by heterozygous mutations in the NOTCH2 gene. Histomorphometric and bone ultrastructural analyses in children have not been reported and sparse evidence exists on response to bisphosphonate (BP) therapy.

OBJECTIVE

To investigate clinical and bone histomorphometric characteristics, bone matrix mineralization, and the response of bone geometry and density to BP therapy.

PATIENTS

Five children with HJCYS (three males) between 6.7 and 15.3 years of age.

INTERVENTIONS

Various BP regimens (pamidronate, zoledronic acid, and alendronate) were used for between 1 and 10 years.

MAIN OUTCOME MEASURES

Pretreatment transiliac bone biopsy specimens and peripheral quantitative computed tomography results were available in four and three subjects, respectively. Bone histomorphometry and quantitative backscattered electron imaging were performed in two patients. The response to BP was monitored using dual-energy X-ray absorptiometry and peripheral quantitative computed tomography.

RESULTS

Three patients had previously unreported NOTCH2 mutations. Histomorphometry demonstrated increased bone resorption and osteoclast numbers, increased heterogeneity of mineralization, and immature, woven bone. Trabecular bone formation was normal or elevated. Radius cortical thickness and density and lumbar spine bone mineral density were reduced at baseline and increased in response to BP therapy, which was not sustained after therapy discontinuation.

CONCLUSIONS

Increased bone resorption and low cortical thickness are consistent with the effect of activating NOTCH2 mutations, which stimulate osteoclastogenesis. The increase in lumbar spine bone density and radial cortical thickness and density by BP therapy provides evidence of beneficial treatment effects in children with HJCYS.

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.