Runx2-mediated regulation of the zinc finger Osterix/Sp7 gene

Fam102a translocates Runx2 and Rbpjl to facilitate Osterix expression and bone formation

Bone remodeling maintains the robustness of the bone tissue by balancing bone resorption by osteoclasts and bone formation by osteoblasts. Although these cells together play a crucial role in bone remodeling, only a few reports are available on the common factors involved in the differentiation of the two types of cells. Here, we show family with sequence similarity 102 member A (Fam102a) as a bone-remodeling factor that positively regulates both osteoclast and osteoblast differentiation. Fam102a regulates osteoblast differentiation by controlling recombination signal binding protein for immunoglobulin κ J region-like (Rbpjl). The Fam102a-Rbpjl axis promotes the nuclear translocation of transcription factors and enhances the expression of Osterix, a transcription factor essential for osteoblast differentiation. The deletion of Fam102a or a functional mutation in Rbpjl leads to osteopenia accompanied by reduced osteoblastic bone formation. Thus, the Fam102a-Rbpjl axis plays an important role in osteoblasts and this finding provides insights into bone remodeling.

Bone development by Hedgehog and Wnt signaling, Runx2, and Sp7

Hedgehog and canonical Wnt signaling pathways and the transcription factors Runx2 and Sp7 are essential for osteoblast differentiation. Ihh is necessary for the commitment of perichondrial mesenchymal cells to Runx2+ osteoprogenitors and for the formation of the bone collar and primary spongiosa. Runx2 is needed for osteoblast differentiation during both endochondral and intramembranous ossification. It regulates the commitment of mesenchymal cells to osteoblast-lineage cells and their proliferation by inducing the expression of Hedgehog, Fgf, Wnt, Pthlh signaling pathway genes, and Dlx5. The Runx2-induced expression of Fgfr2 and Fgfr3 is important for the proliferation of osteoblast-lineage cells. Runx2 induces Sp7 expression and Runx2+ osteoprogenitors become Runx2+Sp7+ preosteoblasts. Runx2, Sp7, and canonical Wnt signaling induce the differentiation of preosteoblasts into osteoblasts. Canonical Wnt signaling, but not Sp7, enhances the proliferation of osteoblast-lineage cells. In mature osteoblasts, Runx2 plays an important role in the expression of major bone matrix protein genes, including Col1a1, Col1a2, Spp1, Ibsp, and Bglap/Bglap2. The canonical Wnt signaling pathway is also crucial for bone formation by mature osteoblasts. Sp7 is needed for osteocytes to acquire a sufficient number of processes and a reduction in these processes results in osteocyte apoptosis and cortical porosity.

Advances in the mechanisms of vascular calcification in chronic kidney disease

Vascular calcification (VC) is common in patients with advanced chronic kidney disease (CKD).A series of factors, such as calcium and phosphorus metabolism disorders, uremic toxin accumulation, inflammation and oxidative stress and cellular senescence, cause osteoblast-like differentiation of vascular smooth muscle cells, secretion of extracellular vesicles, and imbalance of calcium regulatory factors, which together promote the development of VC in CKD. Recent advances in epigenetics have provided better tools for the investigation of VC etiology and new approaches for finding more accurate biomarkers. These advances have not only deepened our understanding of the pathophysiological mechanisms of VC in CKD, but also provided valuable clues for the optimization of clinical predictors and the exploration of potential therapeutic targets. The aim of this article is to provide a comprehensive overview of the pathogenesis of CKD VC, especially the new advances made in recent years, including the various key factors mentioned above. Through the comprehensive analysis, we expect to provide a solid theoretical foundation and research direction for future studies targeting the specific mechanisms of CKD VC, the establishment of clinical predictive indicators and the development of potential therapeutic strategies.

Metabolically activated energetic materials mediate cellular anabolism for bone regeneration

The understanding of cellular energy metabolism activation by engineered scaffolds remains limited, posing challenges for therapeutic applications in tissue regeneration. This study presents biosynthesized poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3HB-co-4HB)] and its major degradation product, 3-hydroxybutyrate (3HB), as endogenous bioenergetic fuels that augment cellular anabolism, thereby facilitating the progression of human bone marrow-derived mesenchymal stem cells (hBMSCs) towards osteoblastogenesis. Our research demonstrated that 3HB markedly boosts in vitro ATP production, elevating mitochondrial membrane potential and capillary-like tube formation. Additionally, it raises citrate levels in the tricarboxylic acid (TCA) cycle, facilitating the synthesis of citrate-containing apatite during hBMSCs osteogenesis. Furthermore, 3HB administration significantly increased bone mass in rats with osteoporosis induced by ovariectomy. The findings also showed that P(3HB-co-4HB) scaffold substantially enhances long-term vascularized bone regeneration in rat cranial defect models. These findings reveal a previously unknown role of 3HB in promoting osteogenesis of hBMSCs and highlight the metabolic activation of P(3HB-co-4HB) scaffold for bone regeneration.

Hakai, a novel Runx2 interacting protein, augments osteoblast differentiation by rescuing Runx2 from Smurf2-mediated proteasome degradation

Runt-related transcription factor 2 (Runx2) is a key regulator of osteoblast differentiation and bone formation. In Runx2-deficient embryos, skeletal development ceases at the cartilage anlage stage. These embryos die of respiratory failure upon birth and display a complete absence of bone and cartilage mineralization. Here, we identified Hakai, a type of E3 ubiquitin ligase as a potential Runx2 interacting partner through affinity pulldown-based proteomic approach. Subsequently, we observed that similar to Runx2, Hakai was downregulated in osteopenic ovariectomized rats, suggesting its involvement in bone formation. Consistent with this observation, Hakai overexpression significantly enhanced osteoblast differentiation in mesenchyme-like C3H10T1/2 as well as primary rat calvaria osteoblast (RCO) cells in vitro. Conversely, overexpression of a catalytically inactive Hakai mutant (C109A) exhibited minimal to no effect, whereas Hakai depletion markedly reduced endogenous Runx2 levels and impaired osteogenic differentiation in both C3H10T1/2 and RCOs. Mechanistically, Hakai physically interacts with Runx2 and enhances its protein turnover by rescuing it from Smad ubiquitination regulatory factor 2 (Smurf2)-mediated proteasome degradation. Wild-type Hakai but not Hakai-C109A inhibited Smurf2 protein levels through proteasome-mediated degradation. These findings underscore Hakai's functional role in bone formation, primarily through its positive modulation of Runx2 protein turnover by protecting it from Smurf2-mediated ubiquitin-proteasomal degradation. Collectively, our results demonstrate Hakai as a promising novel therapeutic target for osteoporosis.

Roles of Sp7 in osteoblasts for the proliferation, differentiation, and osteocyte process formation

Background

Zinc finger-containing transcription factor Osterix/Specificity protein-7 (Sp7) is an essential transcription factor for osteoblast differentiation. However, its functions in differentiated osteoblasts remain unclear and the effects of osteoblast-specific Sp7 deletion on osteocytes have not been sufficiently studied.

Methods

Sp7 floxneo/floxneo mice, in which Sp7 expression was 30 % of that in wild-type mice because of disturbed splicing by neo gene insertion, and osteoblast-specific knockout (Sp7 fl/fl;Col1a1-Cre) mice using 2.3-kb Col1a1 enhanced green fluorescent protein (EGFP)-Cre were examined by micro-computed tomography (micro-CT), bone histomorphometry, serum markers, and histological analyses. The expression of osteoblast and osteocyte marker genes was examined by real-time reverse transcription (RT)-PCR analysis. Osteoblastogenesis, osteoclastogenesis, and regulation of the expression of collagen type I alpha 1 chain (Col1a1) were examined in primary osteoblasts.

Results

Femoral trabecular bone volume was higher in female Sp7 floxneo/floxneo and Sp7 fl/fl;Col1a1-Cre mice than in the respective controls, but not in males. Bromodeoxyuridine (BrdU)-positive osteoblastic cells were increased in male Sp7 fl/fl;Col1a1-Cre mice, and osteoblast number and the bone formation rate were increased in tibial trabecular bone in female Sp7 fl/fl;Col1a1-Cre mice, although osteoblast maturation was inhibited in female Sp7 fl/fl;Col1a1-Cre mice as shown by the increased expression of an immature osteoblast marker gene, secreted phosphoprotein 1 (Spp1), and reduced expression of a mature osteoblast marker gene, bone gamma-carboxyglutamate protein/bone gamma-carboxyglutamate protein 2 (Bglap/Bglap2). Furthermore, alkaline phosphatase activity was increased but mineralization was reduced in the culture of primary osteoblasts from Sp7 fl/fl;Col1a1-Cre mice. Therefore, the accumulated immature osteoblasts in Sp7 fl/fl;Col1a1-Cre mice was likely compensated for the inhibition of osteoblast maturation at different levels in males and females. Vertebral trabecular bone volume was lower in both male and female Sp7 fl/fl;Col1a1-Cre mice than in the controls and the osteoblast parameters and bone formation rate in females were lower in Sp7 fl/fl;Col1a1-Cre mice than in Sp7 fl/fl mice, suggesting differential regulatory mechanisms in long bones and vertebrae. The femoral cortical bone was thin and porous in Sp7 floxneo/floxneo and Sp7 fl/fl;Col1a1-Cre mice of both sexes, the number of canaliculi was reduced, and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL)-positive lacunae and the osteoclasts were increased, whereas the bone formation rate was similar in Sp7 fl/fl;Col1a1-Cre and Sp7 fl/fl mice. The serum levels of total procollagen type 1 N-terminal propeptide (P1NP), a marker for bone formation, were similar, while those of tartrate-resistant acid phosphatase 5b (TRAP5b), a marker for bone resorption, were higher in Sp7 fl/fl;Col1a1-Cre mice. Osteoblasts were less cuboidal, the expression of Col1a1 and Col1a1-EGFP-Cre was lower in Sp7 fl/fl;Col1a1-Cre mice, and overexpression of Sp7 induced Col1a1 expression.

Conclusions

Our studies indicated that Sp7 inhibits the proliferation of immature osteoblasts, induces osteoblast maturation and Col1a1 expression, and is required for osteocytes to acquire a sufficient number of processes for their survival, which prevents cortical porosity.

The translational potential of this article

This study clarified the roles of Sp7 in differentiated osteoblasts in proliferarion, maturation, Col1a1 expression, and osteocyte process formation, which are required for targeting SP7 in the development of therapies for osteoporosis.

Stem Cell and Regenerative Therapies for the Treatment of Osteoporotic Vertebral Compression Fractures

Osteoporotic vertebral compression fractures (OVCFs) significantly increase morbidity and mortality, presenting a formidable challenge in healthcare. Traditional interventions such as vertebroplasty and kyphoplasty, despite their widespread use, are limited in addressing the secondary effects of vertebral fractures in adjacent areas and do not facilitate bone regeneration. This review paper explores the emerging domain of regenerative therapies, spotlighting stem cell therapy's transformative potential in OVCF treatment. It thoroughly describes the therapeutic possibilities and mechanisms of action of mesenchymal stem cells against OVCFs, relying on recent clinical trials and preclinical studies for efficacy assessment. Our findings reveal that stem cell therapy, particularly in combination with scaffolding materials, holds substantial promise for bone regeneration, spinal stability improvement, and pain mitigation. This integration of stem cell-based methods with conventional treatments may herald a new era in OVCF management, potentially improving patient outcomes. This review advocates for accelerated research and collaborative efforts to translate laboratory breakthroughs into clinical practice, emphasizing the revolutionary impact of regenerative therapies on OVCF management. In summary, this paper positions stem cell therapy at the forefront of innovation for OVCF treatment, stressing the importance of ongoing research and cross-disciplinary collaboration to unlock its full clinical potential.

PAI-1 transfected-conditioned media promotes osteogenic differentiation of hBMSCs

Reconstruction of injured bone remains challenging in the clinic owing to the lack of suitable bone grafts. The utilization of PAI-1 transfected-conditioned media (P-CM) has demonstrated its ability to facilitate the differentiation process of mesenchymal stem cells (MSCs), potentially serving as a crucial mediator in tissue regeneration. This research endeavored to explore the therapeutic potential of P-CM concerning the differentiation of human bone marrow mesenchymal stem cells (hBMSCs). To assess new bone formation, a rat calvaria critical defect model was employed, while in vitro experiments involved the use of the alizarin Red-S mineral induction test. In the rat calvaria critical defect model, P-CM treatment resulted in significan new bone formation. In vitro, P-CM treated hBMSCs displayed robust osteogenesis compared to the control group, as demonstrated by the mineral induction test using alizarin Red-S. P-CM with hydroxyapatite/β-tricalcium phosphate/fibrin gel treatment significantly exhibited new bone formation, and the expression of osteogenic associated markers was enhanced in the P-CM-treated group. In conclusion, results demonstrate that P-CM treatment significantly enhanced the osteogenic differantiation efficiency and new bone formation, thus could be used as an ideal therapeutic biomolecule for constructing bone-specific implants, especially for orthopedic and dental applications.

Is the Cis-Element CACCC-Box a Master Regulatory Element during Cardiovascular Disease? A Bioinformatics Approach from the Perspective of the Krüppel-like Family of Transcription Factors

The CACCC-box motif emerges as a pivotal cis-regulatory element implicated in diverse developmental processes and diseases, particularly cardiovascular diseases (CVDs). This study centers on the intricate interplay between the CACCC-box and its binding proteins such as: the Krüppel-Like Family (KLF) of transcription factors as primary effectors in the context of CVDs. Our analysis was through a bioinformatics approach, which revealed significant transcriptional activity among KLF subgroup 2, exhibiting the highest number of interactions focusing on the established roles: pluripotency, cancer, and cardiovascular development and diseases. Our analysis reveals KLF's interactions with GATA4, MEF2C, NKX2.5 and other ~90 potential genes that participate in the regulation of the hypertrophic environment (or CVDs' Environment). Also, the GO analysis showed that genes containing the motif CACCC were enriched for multiple CVDs; in combination with STRING analysis, these results pointed to a link between KLFs and these diseases. The analysis further identifies other potential CACCC-box binding factors, such as SP family members, WT1, VEZF1, and -SALL4, which are implicated in cardiac contraction, remodeling, and inflammation processes.

Calcitonin gene-related peptide regulates periodontal tissue regeneration

Calcitonin gene-related peptide (CGRP), a neuropeptide composed of 37 amino acids secreted from the sensory nerve endings, reportedly possesses various physiological effects, such as vasodilation and neurotransmission. Recently, there have been increasing reports of the involvement of CGRP in bone metabolism; however, its specific role in the pathogenesis of periodontitis, particularly in the repair and healing processes, remains to be elucidated. Therefore, this study aimed to investigate dynamic expression patterns of CGRP during the destruction and regeneration processes of periodontal tissues in a mouse model of experimental periodontitis. We also explored the effects of CGRP on periodontal ligament cells, which can differentiate to hard tissue-forming cells (cementoblasts or osteoblasts). Our findings demonstrated that CGRP stimulation promotes the differentiation of periodontal ligament cells into hard tissue-forming cells. Experimental results using a ligature-induced periodontitis mouse model also suggested fluctuations in CGRP expression during periodontal tissue healing, underscoring the vital role of CGRP signaling in alveolar bone recovery. The study results highlight the important role of nerves in the periodontal ligament not only in sensory reception in the periphery, as previously known, but also in periodontal tissue homeostasis and tissue repair processes.

BRD4 binds to active cranial neural crest enhancers to regulate RUNX2 activity during osteoblast differentiation

Cornelia de Lange syndrome (CdLS) is a congenital disorder featuring facial dysmorphism, postnatal growth deficits, cognitive disability and upper limb abnormalities. CdLS is genetically heterogeneous, with cases arising from mutation of BRD4, a bromodomain protein that binds and reads acetylated histones. In this study, we have modeled CdLS facial pathology through mouse neural crest cell (NCC)-specific mutation of BRD4 to characterize cellular and molecular function in craniofacial development. Mice with BRD4 NCC loss of function died at birth with severe facial hypoplasia, cleft palate, mid-facial clefting and exencephaly. Following migration, BRD4 mutant NCCs initiated RUNX2 expression for differentiation to osteoblast lineages but failed to induce downstream RUNX2 targets required for lineage commitment. BRD4 bound to active enhancers to regulate expression of osteogenic transcription factors and extracellular matrix components integral for bone formation. RUNX2 physically interacts with a C-terminal domain in the long isoform of BRD4 and can co-occupy osteogenic enhancers. This BRD4 association is required for RUNX2 recruitment and appropriate osteoblast differentiation. We conclude that BRD4 controls facial bone development through osteoblast enhancer regulation of the RUNX2 transcriptional program.

Turtle skull development unveils a molecular basis for amniote cranial diversity

Amniote skulls display diverse architectural patterns including remarkable variations in the number of temporal arches surrounding the upper and lower temporal fenestrae. However, the cellular and molecular basis underlying this diversification remains elusive. Turtles are a useful model to understand skull diversity due to the presence of secondarily closed temporal fenestrae and different extents of temporal emarginations (marginal reduction of dermal bones). Here, we analyzed embryos of three turtle species with varying degrees of temporal emargination and identified shared widespread coexpression of upstream osteogenic genes Msx2 and Runx2 and species-specific expression of more downstream osteogenic genes Sp7 and Sparc in the head. Further analysis of representative amniote embryos revealed differential expression patterns of osteogenic genes in the temporal region, suggesting that the spatiotemporal regulation of Msx2, Runx2, and Sp7 distinguishes the temporal skull morphology among amniotes. Moreover, the presence of Msx2- and/or Runx2-positive temporal mesenchyme with osteogenic potential may have contributed to their extremely diverse cranial morphology in reptiles.

Early induction of Hes1 by bone morphogenetic protein 9 plays a regulatory role in osteoblastic differentiation of a mesenchymal stem cell line

Bone morphogenic protein 9 (BMP9) is one of the most potent inducers of osteogenic differentiation among the 14 BMP members, but its mechanism of action has not been fully demonstrated. Hes1 is a transcriptional regulator with basic helix-loop-helix (bHLH) domain and is a well-known Notch effector. In this study, we investigated the functional roles of early induction of Hes1 by BMP9 in a mouse mesenchymal stem cell line, ST2. Hes1 mRNA was transiently and periodically induced by BMP9 in ST2, which was inhibited by BMP signal inhibitors but not by Notch inhibitor. Interestingly, Hes1 knockdown in ST2 by siRNA increased the expression of osteogenic differentiation markers such as Sp7 and Ibsp and matrix mineralization in comparison with control siRNA transfected ST2. In contrast, forced expression of Hes1 by using the Tet-On system suppressed the expression of osteogenic markers and matrix mineralization by BMP9. We also found that the early induction of Hes1 by BMP9 suppressed the expression of Alk1, an essential receptor for BMP9. In conclusion, BMP9 rapidly induces the expression of Hes1 via the SMAD pathway in ST2 cells, which plays a negative regulatory role in osteogenic differentiation of mesenchymal stem cells induced by BMP9.

Lnc-PPP2R1B Mediates the Alternative Splicing of PPP2R1B by Interacting and Stabilizing HNRNPLL and Promotes Osteogenesis of MSCs

Osteogeinc differentiation from mesenchymal stem cells (MSCs) into osteoblasts is a key step for bone tissue engineering in regenerative medicine. The insight into regulatory mechanism of osteogenesis of MSCs facilitates achieving better recovery effect. Long non-coding RNAs are regarded as a family of important moderators in osteogenesis. In this study, we found a novel lncRNA, lnc-PPP2R1B was up-regulated during osteogenesis of MSCs by Illumina HiSeq transcritome sequencing. We demonstrated lnc-PPP2R1B overexpression promoted osteogenesis and knockdown of lnc-PPP2R1B inhibited osteogenesis of MSCs. Mechanically, it physically interacted with and up-regulated heterogeneous nuclear ribonucleoprotein L Like (HNRNPLL), which is a master regulator of activation-induced alternative splicing in T cells. We found lnc-PPP2R1B knockdown or HNRNPLL knockdown decreased transcript-201 of Protein Phosphatase 2A, Regulatory Subunit A, Beta Isoform (PPP2R1B) while increased transcript-203 of PPP2R1B, and did not affect transcript-202/204/206. PPP2R1B is a constant regulatory subunit of protein phosphatase 2 (PP2A), which activates Wnt/β-catenin pathway by removing phosphorylation and stabilization of β-catenin and translocation into nucleus. The transcript-201 retained exon 2 and 3, compared to transcript-203. And it was reported the exon 2 and 3 of PPP2R1B were one part of B subunit binding domain on A subunit in PP2A trimer, and therefore retaining exon 2 and 3 promised formation and enzyme function of PP2A. Finally, lnc-PPP2R1B promoted ectopic osteogenesis in vivo. Conclusively, lnc-PPP2R1B mediated alternative splicing of PPP2R1B through retaining exon 2 and 3 by interacting with HNRNPLL and then promoted osteogenesis, which may facilitate an in-depth understanding of function and mechanism of lncRNAs in osteogenesis. Lnc-PPP2R1B interacted with HNRNPLL, and regulated alternative splicing of PPP2R1B through retaining exon 2 and 3, which preserved enzyme function of PP2A and enhanced dephosphorylation and nuclear translocation of β-catenin, thereby promoting Runx2 and OSX expression and then osteogenesis. And it provided experimental data and potential target for promoting bone formation and bone regeneration.

Particulate beta-tricalcium phosphate and hydroxyapatite doped with silver promote in vitro osteoblast differentiation in MC3T3-E1 cells

BACKGROUND

Calcium phosphates including β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp) have been widely used for bone regeneration application because of their high osteoconductive activities. In addition, various kinds of inorganic ions enhance differentiation, proliferation, and mineralization of osteoblasts. However, information about the effects of silver-doped β-TCP [β-TCP (Ag)] and HAp [HAp (Ag)] particles on osteogenic differentiation is not available yet.

OBJECTIVE

We focused on the impact of β-TCP (Ag) and HAp (Ag) particles on the osteogenic differentiation of MC3T3-E1 osteoblast precursor cells.

METHODS

MC3T3-E1 osteoblast precursor cells were pre-treated by β-TCP (Ag) or HAp (Ag). And then the medium was changed to differentiation medium. Subsequently, osteoblast differentiation-related markers were determined.

RESULTS

We found that treatment with β-TCP (Ag) or HAp (Ag) particles increased alkaline phosphatase activity in MC3T3-E1 cells. Expression of osteoblast differentiation-related genes also increased after treatment with β-TCP (Ag) or HAp (Ag) particles, a response thought to be regulated by zinc finger-containing transcription factor osterix. The ratio of the receptor activator of nuclear factor kappa-B ligand (RANKL) to osteoprotegerin (OPG) was decreased by β-TCP (Ag) and HAp (Ag) particles.

CONCLUSION

Silver doping of β-TCP and HAp particles is effective for bone regeneration.

Salidroside alleviates dexamethasone-induced inhibition of bone formation via transforming growth factor-beta/Smad2/3 signaling pathway

Glucocorticoid-induced osteoporosis is the third epidemic osteoporosis following postmenopausal and senileosteoporosis. According to one study, salidroside made ovariectomized rats' bones strong. Salidroside's potential for treating glucocorticoid-induced osteoporosis remains unproven. This study aimed to investigate the protective effect and mechanism of salidroside on dexamethasone-induced osteogenic differentiation and bone formation in MC3T3-E1 cells and zebrafish. The study proved that salindroside had no harmful impact on MC3T3E1 cells. Salidroside significantly relieved dexamethasone-induced inhibition of ALP (alkaline phosphatase) activity and mineralization in MC3T3-E1 cells, and promoted osteogenic differentiation of cells. Salidroside increased the expression of osteopontin (OPN), runt-related transcription factor 2 (Runx2), osterix (Osx), transforming growth factor-beta (TGF-β) proteins and promoted the phosphorylation of Smad2/3 in MC3T3-E1 cells treated with dexamethasone. In addition, the effect of salidroside in relieving dexamethasone-induced inhibition of osteogenic differentiation in MC3T3-E1 cells can be blocked by TGF-β receptor type I/II inhibitor (LY2109761). At the same time, we found that salidroside significantly alleviated the inhibition of dexamethasone-induced bone formation in zebrafish and promoted the mineralization of zebrafish skulls. LY2109761 reversed the protective impact of salidroside on dexamethasone-mediated bone impairment in zebrafish. These findings suggested that salidroside alleviated dexamethasone-induced inhibition of osteogenic differentiation and bone formation via TGF-β/Smad2/3 signaling pathway.

Cranium growth, patterning and homeostasis

Craniofacial development requires precise spatiotemporal regulation of multiple signaling pathways that crosstalk to coordinate the growth and patterning of the skull with surrounding tissues. Recent insights into these signaling pathways and previously uncharacterized progenitor cell populations have refined our understanding of skull patterning, bone mineralization and tissue homeostasis. Here, we touch upon classical studies and recent advances with an emphasis on developmental and signaling mechanisms that regulate the osteoblast lineage for the calvaria, which forms the roof of the skull. We highlight studies that illustrate the roles of osteoprogenitor cells and cranial suture-derived stem cells for proper calvarial growth and homeostasis. We also discuss genes and signaling pathways that control suture patency and highlight how perturbing the molecular regulation of these pathways leads to craniosynostosis. Finally, we discuss the recently discovered tissue and signaling interactions that integrate skull and cerebrovascular development, and the potential implications for both cerebrospinal fluid hydrodynamics and brain waste clearance in craniosynostosis.

Runx2 and Nell-1 in dental follicle progenitor cells regulate bone remodeling and tooth eruption

Dental follicles are necessary for tooth eruption, surround the enamel organ and dental papilla, and regulate both the formation and resorption of alveolar bone. Dental follicle progenitor cells (DFPCs), which are stem cells found in dental follicles, differentiate into different kinds of cells that are necessary for tooth formation and eruption. Runt‐related transcription factor 2 (Runx2) is a transcription factor that is essential for osteoblasts and osteoclasts differentiation, as well as bone remodeling. Mutation of Runx2 causing cleidocranial dysplasia negatively affects osteogenesis and the osteoclastic ability of dental follicles, resulting in tooth eruption difficulties. Among a variety of cells and molecules, Nel-like molecule type 1 (Nell-1) plays an important role in neural crest-derived tissues and is strongly expressed in dental follicles. Nell-1 was originally identified in pathologically fused and fusing sutures of patients with unilateral coronal synostosis, and it plays indispensable roles in bone remodeling, including roles in osteoblast differentiation, bone formation and regeneration, craniofacial skeleton development, and the differentiation of many kinds of stem cells. Runx2 was proven to directly target the Nell-1 gene and regulate its expression. These studies suggested that Runx2/Nell-1 axis may play an important role in the process of tooth eruption by affecting DFPCs. Studies on short and long regulatory noncoding RNAs have revealed the complexity of RNA-mediated regulation of gene expression at the posttranscriptional level. This ceRNA network participates in the regulation of Runx2 and Nell-1 gene expression in a complex way. However, non-study indicated the potential connection between Runx2 and Nell-1, and further researches are still needed.

Sp7 Action in the Skeleton: Its Mode of Action, Functions, and Relevance to Skeletal Diseases

Osteoblast differentiation is a tightly regulated process in which key transcription factors (TFs) and their target genes constitute gene regulatory networks (GRNs) under the control of osteogenic signaling pathways. Among these TFs, Sp7 works as an osteoblast determinant critical for osteoblast differentiation. Following the identification of Sp7 and a large number of its functional studies, recent genome-scale analyses have made a major contribution to the identification of a "non-canonical" mode of Sp7 action as well as "canonical" ones. The analyses have not only confirmed known Sp7 targets but have also uncovered its additional targets and upstream factors. In addition, biochemical analyses have demonstrated that Sp7 actions are regulated by chemical modifications and protein-protein interaction with other transcriptional regulators. Sp7 is also involved in chondrocyte differentiation and osteocyte biology as well as postnatal bone metabolism. The critical role of SP7 in the skeleton is supported by its relevance to human skeletal diseases. This review aims to overview the Sp7 actions in skeletal development and maintenance, particularly focusing on recent advances in our understanding of how Sp7 functions in the skeleton under physiological and pathological conditions.

Effects of BMSC-Derived EVs on Bone Metabolism

Extracellular vesicles (EVs) are small membrane vesicles that can be secreted by most cells. EVs can be released into the extracellular environment through exocytosis, transporting endogenous cargo (proteins, lipids, RNAs, etc.) to target cells and thereby triggering the release of these biomolecules and participating in various physiological and pathological processes. Among them, EVs derived from bone marrow mesenchymal stem cells (BMSC-EVs) have similar therapeutic effects to BMSCs, including repairing damaged tissues, inhibiting macrophage polarization and promoting angiogenesis. In addition, BMSC-EVs, as efficient and feasible natural nanocarriers for drug delivery, have the advantages of low immunogenicity, no ethical controversy, good stability and easy storage, thus providing a promising therapeutic strategy for many diseases. In particular, BMSC-EVs show great potential in the treatment of bone metabolic diseases. This article reviews the mechanism of BMSC-EVs in bone formation and bone resorption, which provides new insights for future research on therapeutic strategies for bone metabolic diseases.

Selenium nanoparticles stimulate osteoblast differentiation via BMP-2/MAPKs/β-catenin pathway in diabetic osteoporosis

Aim: To evaluate whether selenium nanoparticles (SeNPs) can stimulate bone formation and inhibit the bone loss involved in hyperglycemia-induced osteoporosis. Methods: Rat osteoblastic UMR-106 cells were used for in vitro studies and female Sprague–Dawley rats were used for type 2 diabetes-associated osteoporosis in vivo study. Results: In vitro studies show that SeNPs promote osteoblast differentiation via modulating alkaline phosphatase (ALP) activity, and promoting calcium nodule formation and collagen content. The authors also provide evidence regarding the involvement of the BMP-2/MAPKs/β-catenin pathway in preventing diabetic osteoporosis. Further, in vivo and ex vivo studies suggested that SeNPs can preserve mechanical and microstructural properties of bone. Conclusion: To the best of our knowledge, this study provides the first evidence regarding the therapeutic benefits of SeNPs in preventing diabetes-associated bone fragility.

Plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects that prevents menopausal-related osteoporosis in mice

Osteoporosis is caused by enhanced bone resorption and relatively reduced bone formation. There is an unmet need to develop new agents with both antiresorptive and anabolic effects to treat osteoporosis, although drugs with either effect alone are available. A small molecular compound, plumbagin, was reported to inhibit receptor activator of nuclear factor kappa-B ligand-induced osteoclast (OC) differentiation by inhibiting IκBα phosphorylation-mediated canonical NF-κB activation. However, the key transcriptional factor RelA/p65 in canonical NF-κB pathway functions to promote OC precursor survival but not terminal OC differentiation. Here, we found that plumbagin inhibited the activity of NF-κB inducing kinase, the key molecule that controls noncanonical NF-κB signaling, in an ATP/ADP-based kinase assay. Consistent with this, plumbagin inhibited processing of NF-κB2 p100 to p52 in the progenitor cells of both OCs and osteoblasts (OBs). Interestingly, plumbagin not only inhibited OC but also stimulated OB differentiation in vitro. Importantly, plumbagin prevented trabecular bone loss in ovariectomized mice. This was associated with decreased OC surfaces on trabecular surface and increased parameters of OBs, including OB surface on trabecular surface, bone formation rate, and level of serum osteocalcin, compared to vehicle-treated mice. In summary, we conclude that plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects on bone and could represent a new class of agent for the prevention and treatment of osteoporosis.

Nanoplatforms for Promoting Osteogenesis in Ovariectomy-Induced Osteoporosis in the Experimental Model

Background:

Osteoporosis is a debilitating bone ailment characterized by the obvious loss of bone mass and bone microarchitecture impairment.

Objective:

This study aimed to illuminate the in vivo usefulness of nanotechnology as a treatment for osteoporosis via analyzing the effectiveness of nano-hydroxyapatite (nHa), nano-hydroxy- apatite/chitosan (nHa/C), and nano-hydroxyapatite/silver (nHa/S) in mitigation of osteoporosis in ovariectomized rats.

Method:

The characterization of the nHa, nHa/C, and nHa/S was carried out using TEM, SEM, FTIR, and Zeta potential measurements. This in vivo study included 48 adult female rats that were randomized into six groups (8 rats/group): (1) Sham-operated control, (2) osteoporotic, (3) nHa, (4) nHa/C, (5) nHa/S, and (6) Fosamax®. Serum osterix level was quantified using ELISA. Femur bone morphogenetic protein 2 and SMAD1 mRNA levels were evaluated by qPCR. The femur bones were scanned by DEXA for measurement of bone mineral density and bone mineral content. In ad-dition, a histopathological examination of femur bones was performed.

Results:

The present approach denoted that the treatment with nHa, nHa/C, or nHa/S yields a signif-icant rise in serum level of osterix and mRNA levels of bone morphogenetic protein 2 and SMAD1 as well as significant enhancements of bone tissue minerals.

Conclusion:

The findings affirmed the potency of nHa, nHa/C, and nHa/S as auspicious nanoplat-forms for repairing bone defects in the osteoporotic rat model. The positive effect of the inspected nanoformulations arose from bone formation indicators in serum and tissue, and additionally, the reinforcement of bone density and content, which were verified by the histopathological description of bone tissue sections.

Cellulose Acetate-g-Polycaprolactone Copolymerization Using Diisocyanate Intermediates and the Effect of Polymer Chain Length on Surface, Thermal, and Antibacterial Properties

The need for biodegradable and biocompatible polymers is growing quickly, particularly in the biomedical and environmental industries. Cellulose acetate, a natural polysaccharide, can be taken from plants and modified with polycaprolactone to improve its characteristics for a number of uses, including biomedical applications and food packaging. Cellulose acetate-g-polycaprolactone was prepared by a three-step reaction: First, polymerization of ε-caprolactone via ring-opening polymerization (ROP) reaction using 2-hydroxyethyl methacrylate (HEMA) and functionalization of polycaprolactone(PCL) by introducing NCO on the hydroxyl end of the HEMA-PCL using hexamethyl lenediisocyanate(HDI) were carried out. Then, the NCO–HEMA-PCL was grafted onto cellulose acetate (using the “grafting to” method). The polycaprolactone grafted cellulose acetate was confirmed by FTIR, the thermal characteristics of the copolymers were investigated by DSC and TGA, and the hydrophobicity was analyzed via water CA measurement. Introducing NCO-PCL to cellulose acetate increased the thermal stability. The contact angle of the unreacted PCL was higher than that of cellulose acetate-g-PCL, and it increased when the chain length increased. The CA-g-PCL50, CA-g-PCL100, and CA-g-PCL200 showed very high inhibition zones for all three bacteria tested (E. coli, S. aureus, and P. aeruginosa).

Inhibition of TET-mediated DNA demethylation suppresses osteoblast differentiation

DNA methylation is an epigenetic modification critical for the regulation of chromatin structure and gene expression during development and disease. The ten-eleven translocation (TET) enzyme family catalyzes the hydroxymethylation and subsequent demethylation of DNA by oxidizing 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Little is known about TET protein function due to a lack of pharmacological tools to manipulate DNA hydroxymethylation levels. In this study, we examined the role of TET-mediated DNA hydroxymethylation during BMP-induced C2C12 osteoblast differentiation using a novel cytosine-based selective TET enzyme inhibitor, Bobcat339 (BC339). Treatment of C2C12 cells with BC339 increased global 5mC and decreased global 5hmC without adversely affecting cell viability, proliferation, or apoptosis. Furthermore, BC339 treatment inhibited osteoblast marker gene expression and decreased alkaline phosphatase activity during differentiation. Methylated DNA immunoprecipitation and bisulfite sequencing showed that inhibition of TET with BC339 led to increased 5mC at specific CpG-rich regions at the promoter of Sp7, a key osteoblast transcription factor. Consistent with promoter 5mC marks being associated with transcriptional repression, luciferase activity of an Sp7-promoter-reporter construct was repressed by in vitro DNA methylation or BC339. Chromatin immunoprecipitation analysis confirmed that TET2 does indeed occupy the promoter region of Sp7. Accordingly, forced overexpression of SP7 rescued the inhibition of osteogenic differentiation by BC339. In conclusion, our data suggest that TET-mediated DNA demethylation of genomic regions, including the Sp7 promoter, plays a role in the initiation of osteoblast differentiation. Furthermore, BC339 is a novel pharmacological tool for the modulation of DNA methylation dynamics for research and therapeutic applications.

Super-Enhancer-Associated Long Non-Coding RNA LINC01485 Promotes Osteogenic Differentiation of Human Bone Marrow Mesenchymal Stem Cells by Regulating MiR-619-5p/RUNX2 Axis

OBJECTIVE

To investigate the mechanisms of super-enhancer-associated LINC01485/miR-619-5p/RUNX2 signaling axis involvement in osteogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs).

METHODS

Osteogenic differentiation of hBMSCs was induced in vitro. The expression levels of LINC01485 and miR-619-5p during osteogenesis were measured using quantitative real-time polymerase chain reaction (qRT-PCR). Osteogenic differentiation was examined by qRT-PCR, western blot, alkaline phosphatase (ALP) staining, ALP activity measurement, and Alizarin Red S (ARS) staining assays. Thereafter, the effects of LINC01485 and miR-619-5p on osteogenic differentiation of hBMSCs were evaluated by performing loss- and gain-of-function experiments. Subsequently, a fluorescence in situ hybridization (FISH) assay was employed to determine the cellular localization of LINC01485. Bioinformatics analysis, RNA antisense purification (RAP) assay, and dual-luciferase reporter assays were conducted to analyze the interactions of LINC01485, miR-619-5p, and RUNX2. Rescue experiments were performed to further delineate the role of the competitive endogenous RNA (ceRNA) signaling axis consisting of LINC01485/miR-619-5p/RUNX2 in osteogenic differentiation of hBMSCs.

RESULTS

The expression of LINC01485 was up-regulated during osteogenic differentiation of hBMSCs. The overexpression of LINC01485 promoted osteogenic differentiation of hBMSCs by up-regulating the expression of osteogenesis-related genes [e.g., runt-related transcription factor 2 (RUNX2), osterix (OSX), collagen type 1 alpha 1 (COL1A1), osteocalcin (OCN), and osteopontin (OPN)], and increasing the activity of ALP. ALP staining and ARS staining were also found to be increased upon overexpression of LINC01485. The opposing results were obtained upon LINC01485 interference in hBMSCs. miR-619-5p was found to inhibit osteogenic differentiation. FISH assay displayed that LINC01485 was mainly localized in the cytoplasm. RAP assay results showed that LINC01485 bound to miR-619-5p, and dual-luciferase reporter assay verified that LINC01485 bound to miR-619-5p, while miR-619-5p and RUNX2 bound to each other. Rescue experiments illustrated that LINC01485 could promote osteogenesis by increasing RUNX2 expression by sponging miR-619-5p.

CONCLUSION

LINC01485 could influence RUNX2 expression by acting as a ceRNA of miR-619-5p, thereby promoting osteogenic differentiation of hBMSCs. The LINC01485/miR-619-5p/RUNX2 axis might comprise a novel target in the bone tissue engineering field.

Zfhx4 regulates endochondral ossification as the transcriptional platform of Osterix in mice

Endochondral ossification is regulated by transcription factors that include SRY-box transcription factor 9, runt-related protein 2 (Runx2), and Osterix. However, the sequential and harmonious regulation of the multiple steps of endochondral ossification is unclear. This study identified zinc finger homeodomain 4 (Zfhx4) as a crucial transcriptional partner of Osterix. We found that Zfhx4 was highly expressed in cartilage and that Zfhx4 deficient mice had reduced expression of matrix metallopeptidase 13 and inhibited calcification of cartilage matrices. These phenotypes were very similar to impaired chondrogenesis in Osterix deficient mice. Coimmunoprecipitation and immunofluorescence indicated a physical interaction between Zfhx4 and Osterix. Notably, Zfhx4 and Osterix double mutant mice showed more severe phenotype than Zfhx4 deficient mice. Additionally, Zfhx4 interacted with Runx2 that functions upstream of Osterix. Our findings suggest that Zfhx4 coordinates the transcriptional network of Osterix and, consequently, endochondral ossification.

LncRNA, PLXDC2-OT promoted the osteogenesis potentials of MSCs by inhibiting the deacetylation function of RBM6/SIRT7 complex and OSX specific isoform

Bone regeneration and remodeling are complex physiological processes that are regulated by key transcription factors. Understanding the regulatory mechanism of key transcription factors on the osteogenic differentiation of mesenchymal stem cells (MSCs) is a key issue for successful bone regeneration and remodeling. In the present study, we investigated the regulatory mechanism of the histone deacetylase Sirtuin 7 (SIRT7) on the key transcription factor OSX and osteogenesis of MSCs. In this study, we found that SIRT7 knockdown increased ALP activity and in vitro mineralization and promoted the expression of the osteogenic differentiation markers DSPP, DMP1, BSP, OCN, and the key transcription factor OSX in MSCs. In addition, SIRT7 could associate with RNA binding motif protein 6 (RBM6) to form a protein complex. Moreover, RBM6 inhibited ALP activity, the expression of DSPP, DMP1, BSP, OCN, and OSX in MSCs, and the osteogenesis of MSCs in vivo. Then, the SIRT7/RBM6 protein complex was shown to downregulate the level of H3K18Ac in the OSX promoter by recruiting SIRT7 to the OSX promoter and inhibiting the expression of OSX isoforms 1 and 2. Furthermore, lncRNA PLXDC2-OT could associate with the SIRT7/RBM6 protein complex to diminish its binding and deacetylation function in the OSX promoter and its inhibitory function on OSX isoforms 1 and 2 and to promote the osteogenic potential of MSCs.

The Development of Molecular Biology of Osteoporosis

Osteoporosis is one of the major bone disorders that affects both women and men, and causes bone deterioration and bone strength. Bone remodeling maintains bone mass and mineral homeostasis through the balanced action of osteoblasts and osteoclasts, which are responsible for bone formation and bone resorption, respectively. The imbalance in bone remodeling is known to be the main cause of osteoporosis. The imbalance can be the result of the action of various molecules produced by one bone cell that acts on other bone cells and influence cell activity. The understanding of the effect of these molecules on bone can help identify new targets and therapeutics to prevent and treat bone disorders. In this article, we have focused on molecules that are produced by osteoblasts, osteocytes, and osteoclasts and their mechanism of action on these cells. We have also summarized the different pharmacological osteoporosis treatments that target different molecular aspects of these bone cells to minimize osteoporosis.

Psoralen accelerates osteogenic differentiation of human bone marrow mesenchymal stem cells by activating the TGF-β/Smad3 pathway

Psoralen, one of the active ingredients in Psoralea corylifolia, has been previously reported to regulate the osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). A previous study revealed that psoralen can regulate the expression levels of microRNA-488 and runt-related transcription factor 2 (Runx2) to promote the osteogenic differentiation of BMSCs. However, the underlying signalling pathway in this process remains to be fully elucidated. BMSCs have also been confirmed to play a key role in the occurrence and development of osteoporosis, and are expected to be potential seed cells in the treatment of osteoporosis. In order to explore the potential signalling pathways of psoralen acting on BMSCs, in the present study, human BMSCs (hBMSCs) were treated with different concentrations of psoralen (0.1, 1, 10 and 100 µmol/l) and the TGF-β receptor I (RI) inhibitor SB431542 (5 µmol/l) in vitro for 3, 7 or 14 days. Cell Counting Kit-8 and MTT assays were used to measure cell proliferation and cell viability of hBMSCs following psoralen administration. Alkaline phosphatase (ALP) activity and alizarin red S staining were used to assess the osteogenic differentiation ability of hBMSCs. Reverse transcription-quantitative PCR and western blotting were used to measure the expression of osteogenic differentiation-related genes [bone morphogenetic protein 4 (BMP4), osteopontin (OPN), Runx2 and Osterix] and proteins associated with the TGF-β/Smad3 pathway [TGF-β1, TGF-β RI, phosphorylated (p-)Smad and Smad3]. Psoralen was found to increase the proliferation and viability of hBMSCs. Although different concentrations of psoralen enhanced ALP activity and the calcified nodule content in hBMSCs, the enhancement effects were more potent at lower concentrations (0.1, 1 and 10 µmol/l). The expression of BMP4, OPN, Osterix, Runx2, TGF-β1, TGF-β RI and p-Smad3 was also promoted by psoralen at lower concentrations (0.1, 1 and 10 µmol/l). In addition, whilst SB431542 could inhibit calcium deposition and osteogenic differentiation-related gene expression in hBMSCs, psoralen effectively reversed the inhibitory effects of SB431542. In conclusion, psoralen accelerates the osteogenic differentiation of hBMSCs by activating the TGF-β/Smad3 pathway, which may be valuable for the future clinical treatment of osteoporosis.

MACF1 promotes osteoblast differentiation by sequestering repressors in cytoplasm

Osteoblast differentiation leading to bone formation requires a coordinated transcriptional program. Osteoblastic cells with low level of microtubule actin crosslinking factor 1 (MACF1) show reduced osteoblast differentiation ability, however, the comprehensive mechanism of MACF1's action remains unexplored. In the current study, we found that MACF1 knockdown suppressed osteoblast differentiation by altering the transcriptome dynamics. We further identified two MACF1-interacted proteins, cyclin-dependent kinase 12 (CDK12) and MYST/Esa1-associated factor 6 (MEAF6), and two MACF1-interacted transcription factors (TFs), transcription factor 12 (TCF12) and E2F transcription factor 6 (E2F6), which repress osteoblast differentiation by altering the expression of osteogenic TFs and genes. Moreover, we found that MACF1 regulated cytoplasmic-nuclear localization of itself, TCF12 and E2F6 in a concentration-dependent manner. MACF1 oppositely regulates the expression of TCF12 and transcription factor 7 (TCF7), two TFs that drive osteoblast differentiation to opposite directions. This study reveals that MACF1, a cytoskeletal protein, acts as a sponge for repressors of osteoblast differentiation to promote osteoblast differentiation and contributes to a novel mechanistic insight of osteoblast differentiation and transcription dynamics.

Single-Cell RNA-Sequencing Reveals the Breadth of Osteoblast Heterogeneity

The current paradigm of osteoblast fate is that the majority undergo apoptosis, while some further differentiate into osteocytes and others flatten and cover bone surfaces as bone lining cells. Osteoblasts have been described to exhibit heterogeneous expression of a variety of osteoblast markers at both transcriptional and protein levels. To explore further this heterogeneity and its biological significance, Venus‐positive (Venus⁺) cells expressing the fluorescent protein Venus under the control of the 2.3‐kb Col1a1 promoter were isolated from newborn mouse calvariae and subjected to single‐cell RNA sequencing. Functional annotation of the genes expressed in 272 Venus⁺ single cells indicated that Venus⁺ cells are osteoblasts that can be categorized into four clusters. Of these, three clusters (clusters 1 to 3) exhibited similarities in their expression of osteoblast markers, while one (cluster 4) was distinctly different. We identified a total of 1920 cluster‐specific genes and pseudotime ordering analyses based on established concepts and known markers showed that clusters 1 to 3 captured osteoblasts at different maturational stages. Analysis of gene co‐expression networks showed that genes involved in protein synthesis and protein trafficking between endoplasmic reticulum (ER) and Golgi are active in these clusters. However, the cells in these clusters were also defined by extensive heterogeneity of gene expression, independently of maturational stage. Cells of cluster 4 expressed Cd34 and Cxcl12 with relatively lower levels of osteoblast markers, suggesting that this cell type differs from actively bone‐forming osteoblasts and retain or reacquire progenitor properties. Based on expression and machine learning analyses of the transcriptomes of individual osteoblasts, we also identified genes that may be useful as new markers of osteoblast maturational stages. Taken together, our data show much more extensive heterogeneity of osteoblasts than previously documented, with gene profiles supporting diversity of osteoblast functional activities and developmental fates. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Early B-cell Factor1 (Ebf1) promotes early osteoblast differentiation but suppresses osteoblast function

Early B cell factor 1 (Ebf1) is a transcription factor that regulates B cell, neuronal cell and adipocyte differentiation. We and others have shown that Ebf1 is expressed in osteoblasts and that global deletion of Ebf1 results in increased bone formation in vivo. However, as Ebf1 is expressed in multiple tissues and cell types, it has remained unclear, which of the phenotypic changes in bone are derived from bone cells. The aim of this study was to determine the cell-autonomous and differentiation stage-specific roles of Ebf1 in osteoblasts. In vitro, haploinsufficient Ebf1^+/- calvarial cells showed impaired osteoblastic differentiation indicated by lower alkaline phosphatase (ALP) activity and reduced mRNA expression of osteoblastic genes, while overexpression of Ebf1 in wild type mouse calvarial cells led to enhanced osteoblast differentiation with increased expression of Osterix (Osx). We identified a putative Ebf1 binding site in the Osterix promoter by ChIP assay in MC3T3-E1 osteoblasts and showed that Ebf1 was able to activate Osx-luc reporter construct that included this Ebf1 binding site, suggesting that Ebf1 indeed regulates osteoblast differentiation by inducing Osterix expression. To reconcile our previous data and that of others with our novel findings, we hypothesized that Ebf1 could have a dual role in osteoblast differentiation promoting early but inhibiting late stages of differentiation and osteoblast function. To test this hypothesis in vivo, we generated conditional Ebf1 knockout mice, in which Ebf1 deletion was targeted to early or late osteoblasts by crossing Ebf1^fl/fl mice with Osx- or Osteocalcin (hOC)-Cre mouse lines, respectively. Deletion of Ebf1 in early Ebf1_Osx^-/- osteoblasts resulted in significantly increased bone volume and trabecular number at 12 weeks by μCT analysis, while Ebf1_hOC^-/- mice did not have a bone phenotype. To conclude, our data demonstrate that Ebf1 promotes early osteoblast differentiation by regulating Osterix expression. However, Ebf1 inhibits bone accrual in the Osterix expressing osteoblasts in vivo but it is redundant in the maintenance of mature osteoblast function.

Wrong place, wrong time: Runt-related transcription factor 2/SATB2 pathway in bone development and carcinogenesis

Upregulation or aberrant expression of genes such as special AT-rich sequence-binding protein 2 (SATB2) is necessary for normal cell differentiation and tissue development and is often associated with carcinogenesis and metastatic progression. SATB2 is a critical transcription factor for biological development of various specialized cell lineages, such as osteoblasts and neurons. The dysregulation of SATB2 expression has recently been associated with various types of cancer, while the mechanisms and pathways by which it mediates tumorigenesis are not well elucidated. Runt-related transcription factor 2 (RUNX2) is a master regulator for osteogenesis, and it shares common pathways with SATB2 to regulate bone development. Interestingly, these two transcription factors co-occur in several epithelial and mesenchymal cancers and are linked by multiple cancer-related proteins and microRNAs. This review examines the interactions between RUNX2 and SATB2 in a network necessary for normal bone development and the circumstances in which the expression of RUNX2 and SATB2 in the wrong place and time leads to carcinogenesis.

Chronic Intermittent Hypobaric Hypoxia Enhances Bone Fracture Healing

The effect of chronic intermittent hypobaric hypoxia (CIHH) on bone fracture healing is not elucidated. The present study aimed to investigate the role of CIHH on bone fracture healing and the mechanism. The Sprague-Dawley rats were randomly divided into the CIHH group and control group and monitored for 2, 4, or 8 weeks after femoral fracture surgery. Bone healing efficiency was significantly increased in the CIHH group as evidenced by higher high-density bone volume fractions, higher bone mineral density, higher maximum force, and higher stiffness. Histologically, the CIHH group exhibited superior bone formation, endochondral ossification, and angiogenic ability compared with the control group. The expression of HIF-1α and its downstream signaling proteins VEGF, SDF-1/CXCR4 axis were increased by the CIHH treatment. Moreover, the expression of RUNX2, osterix, and type I collagen in the callus tissues were also up-regulated in the CIHH group. In conclusion, our study demonstrated that CIHH treatment improves fracture healing, increases bone mineral density, and increases bone strength via the activation of HIF-1α and bone production-related genes.

Bone Marrow Mesenchymal Stem Cells-Derived Exosomal MicroRNA-150-3p Promotes Osteoblast Proliferation and Differentiation in Osteoporosis

At present, much more studies have focused on the role of microRNAs in osteoporosis, but the more specific role of microRNA-150-3p (miR-150-3p) in osteoporosis still needs full exploration. We aim at investigating the role of miR-150-3p in osteoporosis and at exploring the related mechanisms. Bone marrow mesenchymal stem cells (BMSCs) were cultured, from which exosomes were isolated. Osteoporosis models were established by ovariectomy and injected with transfected BMSCs exosomes. Bone formation markers in serum, histopathological changes and miR-150-3p, runt-related transcription factor 2 (Runx2) and Osterix expression, and osteoblast apoptosis in femoral tissues were detected. Osteoblasts were isolated and co-cultured with the transfected BMSCs-derived exosomes. Osteoblast proliferation, cell differentiation, and apoptosis, along with miR-150-3p, Runx2, and Osterix expression in osteoblasts were detected. In vivo experiment demonstrated that miR-150-3p, Runx2, and Osterix expression was decreased whereas bone formation markers were decreased in osteoporosis. BMSCs exosomes attenuated osteoporosis, which was further improved by upregulated miR-150-3p in exosomes whereas it was impaired by downregulated miR-150-3p in exosomes. In vitro experiments declared decreased miR-150-3p, Runx2, and Osterix expression; suppressed proliferation; and encouraged apoptosis in osteoblasts in osteoporosis. BMSCs exosomes promoted osteoblast proliferation and differentiation and inhibited apoptosis, which was strengthened by raised exosomal miR-150-3p whereas it was disrupted by inhibited exosomal miR-150-3p. Our study elucidates that exosomal miR-150-3p promotes osteoblast proliferation and differentiation in osteoporosis and provides a new clue for the treatment of patients with osteoporosis.

Neural Cell Adhesion Molecule Regulates Osteoblastic Differentiation Through Wnt/β-Catenin and PI3K-Akt Signaling Pathways in MC3T3-E1 Cells

Neural cell adhesion molecule (NCAM) is involved in cell multi-directional differentiation, but its role in osteoblast differentiation is still poorly understood. In the present study, we investigated whether and how NCAM regulates osteoblastic differentiation. We found that NCAM silencing inhibited osteoblast differentiation in pre-osteoblastic MC3T3-E1 cells. The function of NCAM was further confirmed in NCAM-deficient mesenchymal stem cells (MSCs), which also had a phenotype with reduced osteoblastic potential. Moreover, NCAM silencing induced decrease of Wnt/β-catenin and Akt activation. The Wnt inhibitor blocked osteoblast differentiation, and the Wnt activator recovered osteoblast differentiation in NCAM-silenced MC3T3-E1 cells. We lastly demonstrated that osteoblast differentiation of MC3T3-E1 cells was inhibited by the PI3K-Akt inhibitor. In conclusion, these results demonstrate that NCAM silencing inhibited osteoblastic differentiation through inactivation of Wnt/β-catenin and PI3K-Akt signaling pathways.

Phosphate as a Signaling Molecule

Phosphorus plays a vital role in diverse biological processes including intracellular signaling, membrane integrity, and skeletal biomineralization; therefore, the regulation of phosphorus homeostasis is essential to the well-being of the organism. Cells and whole organisms respond to changes in inorganic phosphorus (Pi) concentrations in their environment by adjusting Pi uptake and altering biochemical processes in cells (local effects) and distant organs (endocrine effects). Unicellular organisms, such as bacteria and yeast, express specific Pi-binding proteins on the plasma membrane that respond to changes in ambient Pi availability and transduce intracellular signals that regulate the expression of genes involved in cellular Pi uptake. Multicellular organisms, including humans, respond at a cellular level to adapt to changes in extracellular Pi concentrations and also have endocrine pathways which integrate signals from various organs (e.g., intestine, kidneys, parathyroid glands, bone) to regulate serum Pi concentrations and whole-body phosphorus balance. In mammals, alterations in the concentrations of extracellular Pi modulate type III sodium-phosphate cotransporter activity on the plasma membrane, and trigger changes in cellular function. In addition, elevated extracellular Pi induces activation of fibroblast growth factor receptor, Raf/mitogen-activated protein kinase/ERK kinase (MEK)/extracellular signal-regulated kinase (ERK) and Akt pathways, which modulate gene expression in various mammalian cell types. Excessive Pi exposure, especially in patients with chronic kidney disease, leads to endothelial dysfunction, accelerated vascular calcification, and impaired insulin secretion.

New physiological insights into the phenomena of deer antler: A unique model for skeletal tissue regeneration

Generally, mammals are unable to regenerate complex tissues and organs however the deer antler provides a rare anomaly to this rule. This osseous cranial appendage which is located on the frontal bone of male deer is capable of stem cell-based organogenesis, annual casting, and cyclic de novo regeneration. A series of recent studies have classified this form of regeneration as epimorphic stem cell based. Antler renewal is initiated by the activation of neural crest derived pedicle periosteal cells (PPCs) found residing within the pedicle periosteum (PP), these PPCs have the potential to differentiate into multiple lineages. Other antler stem cells (ASCs) are the reserve mesenchymal cells (RMCs) located in the antlers tip, which develop into cartilage tissue. Antlerogenic periosteal cells (APCs) found within the antlerogenic periosteum (AP) form the tissues of both the pedicle and first set of antlers. Antler stem cells (ASCs) further appear to progress through various stages of activation, this coordinated transition is considered imperative for stem cell-based mammalian regeneration. The latest developments have shown that the rapid elongation of the main beam and antler branches are a controlled form of tumour growth, regulated by the tumour suppressing genes TP73 and ADAMTS18. Both osteoclastogenesis, as well as osteogenic and chondrogenic differentiation are also involved. While there remains much to uncover this review both summarises and comprehensively evaluates our existing knowledge of tissue regeneration in the deer antler. This will assist in achieving the goal of in vitro organ regeneration in humans by furthering the field of modern regenerative medicine.

The Translational potential of this article

As a unique stem cell-based organ regeneration process in mammals, the deer antler represents a prime model system for investigating mechanisms of regeneration in mammalian tissues. Novel ASCs could provide cell-based therapies for regenerative medicine and bone remodelling for clinical application. A greater understanding of this process and a more in-depth defining of ASCs will potentiate improved clinical outcomes.

The Transcription Factor HAND1 Is Involved in Cortical Bone Mass through the Regulation of Collagen Expression

Temporal and/or spatial alteration of collagen family gene expression results in bone defects. However, how collagen expression controls bone size remains largely unknown. The basic helix-loop-helix transcription factor HAND1 is expressed in developing long bones and is involved in their morphogenesis. To understand the functional role of HAND1 and collagen in the postnatal development of long bones, we overexpressed Hand1 in the osteochondroprogenitors of model mice and found that the bone volumes of cortical bones decreased in Hand1^Tg/+;Twist2-Cre mice. Continuous Hand1 expression downregulated the gene expression of type I, V, and XI collagen in the diaphyses of long bones and was associated with decreased expression of Runx2 and Sp7/Osterix, encoding transcription factors involved in the transactivation of fibril-forming collagen genes. Members of the microRNA-196 family, which target the 3' untranslated regions of COL1A1 and COL1A2, were significantly upregulated in Hand1^Tg/+;Twist2-Cre mice. Mass spectrometry revealed that the expression ratios of alpha 1(XI), alpha 2(XI), and alpha 2(V) in the diaphysis increased during postnatal development in wild-type mice, which was delayed in Hand1^Tg/+;Twist2-Cre mice. Our results demonstrate that HAND1 regulates bone size and morphology through osteochondroprogenitors, at least partially by suppressing postnatal expression of collagen fibrils in the cortical bones.

Ascorbic Acid-PEI Carbon Dots with Osteogenic Effects as miR-2861 Carriers to Effectively Enhance Bone Regeneration

Nucleic acid transfer has shown significant potential in the treatment of bone damage because of its long lasting local effect and lower cost. Nonviral vectors, such as nanomaterials, with higher biocompatibility are increasedly applied in the study of bone defect repair. Carbon dots with various reactive groups on the surface not only provide a unique surface to carry therapeutic genes, but also some carbon dots have been reported to promote osteogenic differentiation. The bone regeneration effect of carbon dots in vivo, however, is rarely investigated. MiR-2861 has revealed osteogenic differentiation effects. In the current study, we created ascorbic acid-PEI carbon dots (CD), which were able to carry miR-2861, by the microwave-assisted pyrolysis method. Results demonstrated that CD had excellent fluorescence stability leading to good fluorescence imaging in vitro and in vivo. CD was efficiently internalized into bone marrow stromal cells (BMSCs) through the clathrin-mediated endocytosis pathway and distributed in the mitochondria, endoplasmic reticulum, lysosome, and nucleus. Results from alkaline phosphatase staining, alizarin red staining, and reverse transcription real-time PCR (RT-QPCR) showed that our CD indeed had osteogenic effects in vitro. Flow cytometry data indicated that CD could efficiently deliver miR-2861 into BMSCs in vitro, and CD carrying miR-2861 (CD@miR) had the strongest osteogenic effects. Analyses of hematology, serum biochemistry, and histology showed that CD and CD@miR did not have cytotoxicity and had higher biocompatibility in vivo. Most interestingly, CD and miR-2861 in the CD@miR could act synergistically to promote osteogenic differentiation in vitro and new bone regeneration in vivo remarkably. Our results clearly indicate that the osteogenic CD delivering osteogenic therapeutic gene, miR-2861, can obtain much stronger bone regeneration ability, suggesting that our CD has great potential in future clinical application.

RUNX2 co-operates with EGR1 to regulate osteogenic differentiation through Htra1 enhancers

Runt-related transcription factor 2 (Runx2) has been shown to regulate osteoblast differentiation by directly or indirectly regulating numerous osteoblast-related genes. However, our understanding of the transcriptional mechanisms of RUNX2 is mainly restricted to its transactivation, while the mechanism underlying its inhibitory effect during osteoblast differentiation remains largely unknown. Here, we incorporated the anti-RUNX2 chromatin immunoprecipitation (ChIP) sequencing in MC3T3-E1 cells and RNA-sequencing of parietal bone from Runx2 heterozygous mutant mice, to identify the putative genes negatively regulated by RUNX2. We identified HtrA serine peptidase 1 (Htra1) as a target gene and found ten candidate Htra1 enhancers potentially regulated by RUNX2, among which seven were verified by dual-luciferase assays. Furthermore, we investigated the motifs in the vicinity of RUNX2-binding sites and identified early growth response 1 (EGR1) as a potential partner transcription factor (TF) potentially regulating Htra1 expression, which was subsequently confirmed by Re-ChIP assays. RUNX2 and EGR1 co-repressed Htra1 and increased the expression levels of other osteoblast marker genes, such as osterix, osteocalcin, and osteoprotegerin at the messenger RNA and protein level. Moreover, Alizarin red staining combined with alkaline phosphatase (ALP) staining showed decreased calcified nodules and ALP activity in the siRUNX2+siEGR1 group compared with siRUNX2 group. Our findings revealed the detailed mechanism of the inhibitory function of RUNX2 towards its downstream genes, along with its partner TFs, to promote osteoblast differentiation.

Phytochemicals impact on osteogenic differentiation of mesenchymal stem cells

Medicinal plants have always been utilized for the prevention and treatment of the spread of different diseases all around the world. To name some traditional medicine that has been used over centuries, we can refer to phytochemicals such as naringin, icariin, genistein, and resveratrol gained from plants. Osteogenic differentiation and mineralization of stem cells can be the result of specific bioactive compounds from plants. One of the most appealing choices for therapy can be mesenchymal stem cells (MSCs) because it has a great capability of self-renewal and differentiation into three descendants, namely, endoderm, mesoderm, and ectoderm. Stem cell gives us the glad tidings of great advances in tissue regeneration and transplantation field for treatment of diseases. Using plant bioactive phytochemicals also holds tremendous promises in treating diseases such as osteoporosis. The purpose of the present review article thus is to investigate what are the roles and consequences of phytochemicals on osteogenic differentiation of MSCs.

The phytoestrogen glabrene prevents osteoporosis in ovariectomized rats through upregulation of the canonical Wnt/β-catenin signaling pathway

This study systematically investigated the effects of phytoestrogen glabrene on postmenopausal osteoporosis in an ovariectomy (OVX) rat model. Glabrene administration (25, 50, and 100 mg/kg) for 13 weeks can significantly slow down the body weight gain and slightly increase the uterus weight of OVX rats. The increased levels of U-Ca, U-P levels, urine DPD/creatinine, serum ALP, OCN, triglycerides, and total cholesterol induced by OVX were dramatically inhibited in rats, whereas no difference occurred for S-Ca and S-P in all groups. Furthermore, glabrene can enhance bone mineral density of the right femur, fourth-lumbar vertebra and tibia and improve biomechanical parameters, such as femoral neck loading force, three-point bending of the tibia, and vertebral compression in OVX rats. Moreover, glabrene greatly suppressed the expression of TRAP protein but increased OPG and BGP protein expression in tibia tissue of OVX rats. In addition, OVX-induced reduction of Lrp-5, β-catenin, Runx2, and Osx protein expression was all restored by glabrene treatment. The present study indicated that glabrene might be a potential alternative medicine for the prevention and treatment of postmenopausal osteoporosis via activation of the Wnt/β-catenin signaling pathway.

MicroRNAs are critical in regulating smooth muscle cell mineralization and apoptosis during vascular calcification

Vascular calcification refers to the pathological deposition of calcium and phosphate minerals into the vasculature. It is prevalent in atherosclerosis, ageing, type 2 diabetes mellitus and chronic kidney disease, thus, increasing morbidity and mortality from these conditions. Vascular calcification shares similar mechanisms with bone mineralization, with smooth muscle cells playing a critical role in both processes. In the last decade, a variety of microRNAs have been identified as key regulators for the differentiation, phenotypic switch, proliferation, apoptosis, cytokine production and matrix deposition in vascular smooth muscle cells during vascular calcification. Therefore, this review mainly discusses the roles of microRNAs in the pathophysiological mechanisms of vascular calcification in smooth muscle cells and describes several interventions against vascular calcification by regulating microRNAs. As the exact mechanisms of calcification remain not fully elucidated, having a better understanding of microRNA involvement in vascular calcification may give impetus to development of novel therapeutics for the control and treatment of vascular calcification.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

Evaluation of Osteogenic/Cementogenic Modulating Potential of PAI-1 Transfected Media for Stem Cells

AIM OF THE STUDY

In vitro evaluation of the effects of plasminogen activator inhibitor-1 (PAI-1) transfected-conditioned media (P-CM) on the differentiation of human periodontal ligament stem cells (hPDLSCs) and human periapical follicular stem cells (hPAFSCs).

MATERIALS AND METHODS

The hPDLSCs and hPAFSCs received from impacted third molars were treated with P-CM and viability, as well as differentiation of the cells were evaluated. Plasmids were constructed according to standard techniques, and all sequences were validated by proper enzyme digestion and sequencing. Chinese hamster ovarian (CHO) cells were transfected with pcDNA3.1-hPAI-1 plasmid to obtain P-CM, followed by western blotting and PAI-1-specific ELISA kit to evaluate the proteins of P-CM. The cell viability of hPDLSCs and hPAFSCs were analyzed using MTT assay after 48 h of incubation. Alizarin red S staining was performed to evaluate the differentiation of hPDLSCs and hPAFSCs. The reverse transcription-polymerase chain reaction was used to observe the expression levels of osteogenic/cementogenic marker genes. The human cytokine antibody array was applied for further analysis of cytokine expression in P-CM.

RESULTS

P-CM significantly promoted the differentiation of hPDLSCs and hPAFSCs and upregulated the expression of osteogenic/cementogenic marker genes in vitro. Furthermore, rhPAI-1 promoted mineralized nodules formation of hPDLSCs and hPAFSCs, and we identified that other proteins, RANTES and IL-6, were highly expressed in P-CM.

CONCLUSIONS

P-CM promoted the differentiation of hPDLSCs and hPAFSCs by upregulating the expression of RANTES and IL-6, and interaction between PAI-1 and RANTES/IL-6 signaling may be involved in P-CM-induced osteogenic/cementogenic differentiation.

The Implication of Spatial Statistics in Human Mesenchymal Stem Cell Response to Nanotubular Architectures

INTRODUCTION

In recent years there has been ample interest in nanoscale modifications of synthetic biomaterials to understand fundamental aspects of cell-surface interactions towards improved biological outcomes. In this study, we aimed at closing in on the effects of nanotubular TiO2 surfaces with variable nanotopography on the response on human mesenchymal stem cells (hMSCs). Although the influence of TiO2 nanotubes on the cellular response, and in particular on hMSC activity, has already been addressed in the past, previous studies overlooked critical morphological, structural and physical aspects that go beyond the simple nanotube diameter, such as spatial statistics.

METHODS

To bridge this gap, we implemented an extensive characterization of nanotubular surfaces generated by anodization of titanium with a focus on spatial structural variables including eccentricity, nearest neighbour distance (NND) and Voronoi entropy, and associated them to the hMSC response. In addition, we assessed the biological potential of a two-tiered honeycomb nanoarchitecture, which allowed the detection of combinatory effects that this hierarchical structure has on stem cells with respect to conventional nanotubular designs. We have combined experimental techniques, ranging from Scanning Electron (SEM) and Atomic Force (AFM) microscopy to Raman spectroscopy, with computational simulations to characterize and model nanotubular surfaces. We evaluated the cell response at 6 hrs, 1 and 2 days by fluorescence microscopy, as well as bone mineral deposition by Raman spectroscopy, demonstrating substrate-induced differential biological cueing at both the short- and long-term.

RESULTS

Our work demonstrates that the nanotube diameter is not sufficient to comprehensively characterize nanotubular surfaces and equally important parameters, such as eccentricity and wall thickness, ought to be included since they all contribute to the overall spatial disorder which, in turn, dictates the overall bioactive potential. We have also demonstrated that nanotubular surfaces affect the quality of bone mineral deposited by differentiated stem cells. Lastly, we closed in on the integrated effects exerted by the superimposition of two dissimilar nanotubular arrays in the honeycomb architecture.

DISCUSSION

This work delineates a novel approach for the characterization of TiO2 nanotubes which supports the incorporation of critical spatial structural aspects that have been overlooked in previous research. This is a crucial aspect to interpret cellular behaviour on nanotubular substrates. Consequently, we anticipate that this strategy will contribute to the unification of studies focused on the use of such powerful nanostructured surfaces not only for biomedical applications but also in other technology fields, such as catalysis.

Post-natal bone physiology

Post-natal bone development is characterized by substantial longitudinal bone growth and changes in skeletal size and shape. Bone is in a dynamic process of continuous remodeling which helps to regulate calcium homeostasis, repair micro-damage to bones from everyday stress, and to shape the skeleton during growth. Bone growth is regulated by systemic hormones and locally generated factors. Understanding their mechanisms of action enables us to obtain a better appreciation of the cellular and molecular basis of bone remodeling and could therefore be valuable in approaches to new therapies. This article will review molecular and cellular control of skeletal growth in the post-natal period, the physiology of each bone cell with their systemic and local regulators, as well as the physiology of bone remodeling.