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

Mechanosignaling in Osteoporosis: When Cells Feel the Force

Bone is a highly mechanosensitive tissue, where mechanical signaling plays a central role in maintaining skeletal homeostasis. Mechanotransduction regulates the balance between bone formation and resorption through coordinated interactions among bone cells. Key mechanosensing structures-including the extracellular/pericellular matrix (ECM/PCM), integrins, ion channels, connexins, and primary cilia, translate mechanical cues into biochemical signals that drive bone adaptation. Disruptions in mechanotransduction are increasingly recognized as an important factor in osteoporosis. Under pathological conditions, impaired mechanical signaling reduces bone formation and accelerates bone resorption, leading to skeletal fragility. Defects in mechanotransduction disrupt key pathways involved in bone metabolism, further exacerbating bone loss. Therefore, targeting mechanotransduction presents a promising pharmacological strategy for osteoporosis treatment. Recent advances have focused on developing drugs that enhance bone mechanosensitivity by modulating key mechanotransduction pathways, including integrins, ion channels, connexins, and Wnt signaling. A deeper understanding of mechanosignaling mechanisms may pave the way for novel therapeutic approaches aimed at restoring bone mass, mechanical integrity, and mechanosensitive bone adaptation.

The influence of microorganisms on bone homeostasis in apical periodontitis

OBJECTIVE

This review aims to provide an overview of the role of microorganisms in the onset and progression of periapical diseases, particularly regarding their effects on bone homeostasis.

DESIGN

The search for this narrative review was conducted in PubMed, Web of Science and Google Scholar using relevant keywords, including checking reference lists of journal articles by hand searching.

RESULTS

Microorganisms directly promote osteoclasts through pathways such as nuclear factor-κB (NF-κB) and extracellular regulated protein kinases (ERK), while inhibiting osteoblasts function by interfering with the wingless-related integration site (Wnt)/β-catenin pathway in the periapical area. Moreover, microorganisms indirectly regulate periapical bone homeostasis by inducing programmed cell death and modulating the immune microenvironment through the activation of innate immunity via pattern-recognition receptors (PRRs) and subsequent cascades of responses. Among these microorganisms, Enterococcus faecalis, Porphyromonas gingivalis and Fusobacterium nucleatum play significant roles.

CONCLUSION

Microorganisms regulate pathways such as NF-ĸB and Wnt/β-catenin, as well as programmed cell death and the immune microenvironment in the periapical area, thereby disrupting bone homeostasis.

CD_99 G1 neutrophils modulate osteogenic differentiation of mesenchymal stem cells in the pathological process of ankylosing spondylitis

OBJECTIVES

This study aimed to identify the types and heterogeneity of cells within the spinal enthesis and investigate the underlying mechanisms of osteogenesis.

METHODS

Single-cell RNA sequencing was used to identify cell populations and their gene signatures in the spinal enthesis of five patients with ankylosing spondylitis (AS) and three healthy individuals. The transcriptomes of 40 065 single cells were profiled and divided into 7 clusters: neutrophils, monocytic cells, granulomonocytic progenitor_erythroblasts, T cells, B cells, plasma cells and stromal cells. Real-time quantitative PCR, immunofluorescence, flow cytometry, osteogenesis induction, alizarin red staining, immunohistochemistry, short hairpin RNA and H&E staining were applied to validate the bioinformatics analysis.

RESULTS

Pseudo-time analysis showed two differentiation directions of stromal cells from the mesenchymal stem cell subpopulation MSC-C2 to two Cxcl12-abundant-reticular (CAR) cell subsets, Osteo-CAR and Adipo-CAR, within which three transcription factors, C-JUN, C-FOS and CAVIN1, were highly expressed in AS and regulated the osteogenesis of mesenchymal stem cells. A novel subcluster of early-stage neutrophils, CD99_G1, was elevated in AS. The proinflammatory characteristics of monocyte dendritic cell progenitor-recombinant adiponectin receptor 2 monocytic cells were explored. Interactions between Adipo-CAR cells, CD99_G1 neutrophils and other cell types were mapped by identifying ligand-receptor pairs, revealing the recruitment characteristics of CD99_G1 neutrophils by Adipo-CAR cells and the pathogenesis of osteogenesis induced in AS.

CONCLUSIONS

Our results revealed the dynamics of cell subpopulations, gene expression and intercellular interactions during AS pathogenesis. These findings provide new insights into the cellular and molecular mechanisms of osteogenesis and will benefit the development of novel therapeutic strategies.

Antisense oligonucleotides targeting a NOTCH3 mutation in male mice ameliorate the cortical osteopenia of lateral meningocele syndrome

Lateral Meningocele Syndrome (LMS) is a monogenic disorder associated with NOTCH3 pathogenic variants that result in the stabilization of NOTCH3 and a gain-of-function. A mouse model (Notch3em1Ecan) harboring a 6691-TAATGA mutation in the Notch3 locus that results in a functional outcome analogous to LMS exhibits cancellous and cortical bone osteopenia. We tested Notch3 antisense oligonucleotides (ASOs) specific to the Notch36691-TAATGA mutation for their effects on Notch3 downregulation and on the osteopenia of Notch3em1Ecan mice. Twenty-four mouse Notch3 mutant ASOs were designed and tested for toxic effects in vivo, and 12 safe ASOs were tested for their impact on the downregulation of Notch36691-TAATGA and Notch3 mRNA in osteoblast cultures from Notch3em1Ecan mice. Three ASOs downregulated Notch3 mutant transcripts specifically and were tested in vivo for their effects on the bone microarchitecture of Notch3em1Ecan mice. All three ASOs were well tolerated. One of these ASOs had more consistent effects in vivo and was studied in detail. The Notch3 mutant ASO downregulated Notch3 mutant transcripts in osteoblasts and bone marrow stromal cells and had no effect on other Notch receptors. The subcutaneous administration of Notch3 mutant ASO at 50 mg/Kg decreased Notch36691-TAATGA mRNA in bone without apparent toxicity; microcomputed tomography demonstrated that the ASO ameliorated the cortical osteopenia of Notch3em1Ecan mice but not the cancellous bone osteopenia. In conclusion, a Notch3 ASO that downregulates Notch3 mutant expression specifically ameliorates the cortical osteopenia in Notch3em1Ecan mice. ASOs may become useful strategies in the management of monogenic disorders affecting the skeleton.

Piezo1-mediated fluid shear stress promotes OPG and inhibits RANKL via NOTCH3 in MLO-Y4 osteocytes

Piezo1, a mechanosensitive ion channel, participates in a variety of biological processes in maintaining bone homeostasis. As the most abundant cells in bones of the mammals, osteocytes play an essential role in bone formation, remodeling, and bone mass maintenance. Here, by exposing MLO-Y4 osteocytes to the fluid shear stress (FSS) microenvironment, we explored the effect of Piezo1-mediated FSS on the expression of the molecules critical to the process of bone formation and resorption, Receptor Activator of Nuclear Factor-Kappa-B Ligand (RANKL) and Osteoprotegerin (OPG). It was found that 9 dyne/cm² loading for 30 minutes showed an upregulation trend on Piezo1 when MLO-Y4 osteocytes were exposed to an FSS microenvironment. FSS promotes the expression of OPG and inhibits the expression of RANKL. The blocker of Piezo1, GsMTx4, downregulates the effect of FSS on the expression of these two molecules. In addition, NOTCH3 was involved in this process. Thus, the results demonstrated that Piezo1-mediated FSS promotes the expression of OPG and inhibits the expression of RANKL via NOTCH3 in MLO-Y4 osteocytes.

Mechanisms of bone remodeling and therapeutic strategies in chronic apical periodontitis

Chronic periapical periodontitis (CAP) is a typical oral disease in which periodontal inflammation caused by an odontogenic infection eventually leads to bone loss. Uncontrolled infections often lead to extensive bone loss around the root tip, which ultimately leads to tooth loss. The main clinical issue in the treatment of periapical periodontitis is the repair of jawbone defects, and infection control is the first priority. However, the oral cavity is an open environment, and the distribution of microorganisms through the mouth in jawbone defects is inevitable. The subversion of host cell metabolism by oral microorganisms initiates disease. The presence of microorganisms stimulates a series of immune responses, which in turn stimulates bone healing. Given the above background, we intended to examine the paradoxes and connections between microorganisms and jaw defect repair in anticipation of new ideas for jaw defect repair. To this end, we reviewed the microbial factors, human signaling pathways, immune cells, and cytokines involved in the development of CAP, as well as concentrated growth factor (CGF) and stem cells in bone defect repair, with the aim of understanding the impact of microbial factors on host cell metabolism to inform the etiology and clinical management of CAP.

Use of antisense oligonucleotides to target Notch3 in skeletal cells

Notch receptors are determinants of cell fate and function, and play an important role in the regulation of bone development and skeletal remodeling. Lateral Meningocele Syndrome (LMS) is a monogenic disorder associated with NOTCH3 pathogenic variants that result in the stabilization of NOTCH3 and a gain-of-function. LMS presents with neurological developmental abnormalities and bone loss. We created a mouse model (Notch3em1Ecan) harboring a 6691TAATGA mutation in the Notch3 locus, and heterozygous Notch3em1Ecan mice exhibit cancellous and cortical bone osteopenia. In the present work, we explored whether Notch3 antisense oligonucleotides (ASO) downregulate Notch3 and have the potential to ameliorate the osteopenia of Notch3em1Ecan mice. Notch3 ASOs decreased the expression of Notch3 wild type and Notch36691-TAATGA mutant mRNA expressed by Notch3em1Ecan mice in osteoblast cultures without evidence of cellular toxicity. The effect was specific since ASOs did not downregulate Notch1, Notch2 or Notch4. The expression of Notch3 wild type and Notch36691-TAATGA mutant transcripts also was decreased in bone marrow stromal cells and osteocytes following exposure to Notch3 ASOs. In vivo, the subcutaneous administration of Notch3 ASOs at 25 to 50 mg/Kg decreased Notch3 mRNA in the liver, heart and bone. Microcomputed tomography demonstrated that the administration of Notch3 ASOs ameliorates the cortical osteopenia of Notch3em1Ecan mice, and ASOs decreased femoral cortical porosity and increased cortical thickness and bone volume. However, the administration of Notch3 ASOs did not ameliorate the cancellous bone osteopenia of Notchem1Ecan mice. In conclusion, Notch3 ASOs downregulate Notch3 expression in skeletal cells and their systemic administration ameliorates cortical osteopenia in Notch3em1Ecan mice; as such ASOs may become useful strategies in the management of skeletal diseases affected by Notch gain-of-function.