Osteoblast programmed cell death (apoptosis): modulation by growth factors and cytokines

Mycobacterium tuberculosis specific protein Rv1509 modulates osteoblast and osteoclast differentiation via TLR2 signaling

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M.tb), is one of the most ancient diseases recorded. In cases of bone TB, it significantly disrupts bone homeostasis, though the precise mechanisms are poorly understood and effective treatment targets are scarce. Our study investigated the role of Rv1509 in the pathogenesis of bone TB. We found that Rv1509 enhances the differentiation of bone marrow macrophages (BMMs) into osteoclasts by activating the TLR2 pathway, which stimulates the production of IL-6 and TNF-α. This, in turn, indirectly inhibits osteoblast differentiation and mineralization. Additionally, Rv1509 directly impairs osteoblast function and enhances the secretion of RANKL via TLR2 signaling, creating a detrimental RANKL/OPG imbalance that promotes osteoclast differentiation and bone degradation. Notably, the injection of Rv1509 into mouse skulls led to extensive bone damage, highlighting its significant role as a virulence factor in the pathogenesis of bone TB.

Ionic release from bioactive SiO2-CaOCME/poly(tetrahydrofuran)/poly(caprolactone) hybrids drives human-bone marrow stromal cell osteogenic differentiation

This study demonstrates that dissolution products of inorganic/organic SiO2-CaOCME/PTHF/PCL-diCOOH hybrid (70S30CCME-CL) drive human bone marrow stromal cells (h-BMSCs) down an osteogenic pathway with the production of mineralised matrix. We investigated osteogenesis through combined analyses of mRNA dynamics for key markers and targeted staining of mineralised matrix. We demonstrate that h-BMSCs undergo accelerated differentiation in vitro in response to the 70S30CCME-CL ionic milieu, as compared to incubation with osteogenic media. Extracts from 70S30CCME-CL promote osteogenesis by inducing changes in cellular metabolic activity, promoting changes in cell morphology consistent with the osteogenic lineage, and by enhancing mineralisation of hydroxyapatite in the extracellular matrix. Additionally, our results show that 70S30CCME-CL hybrids prove sustained functional resilience by maintaining osteostimulatory effects despite cumulated dissolution cycles. In co-differentiation medium, 70S30CCME-CL ionic release can modulate signalling pathways associated with non-osteogenic functions, further supporting their potential for bone regeneration applications. Overall, our study provides compelling experimental evidence that the 70S30CCME-CL hybrid is a promising biomaterial for bone tissue regeneration applications.

Exploring the Osteoinductive Potential of Bacterial Pyomelanin Derived from Pseudomonas aeruginosa in a Human Osteoblast Model

Alkaptonuria (AKU) is a genetically determined disease associated with disorders of tyrosine metabolism. In AKU, the deposition of homogentisic acid polymers contributes to the pathological ossification of cartilage tissue. The controlled use of biomimetics similar to deposits observed in cartilage during AKU potentially may serve the development of new bone regeneration therapy based on the activation of osteoblasts. The proposed biomimetic is pyomelanin (PyoM), a polymeric biomacromolecule synthesized by Pseudomonas aeruginosa. This work presents comprehensive data on the osteoinductive, pro-regenerative, and antibacterial properties, as well as the cytocompatibility, of water-soluble (PyoMsol) or water-insoluble (PyoMinsol) PyoM. Both variants of PyoM support osteoinductive processes as well as the maturation of osteoblasts in cell cultures in vitro due to the upregulation of bone-formation markers, osteocalcin (OC), and alkaline phosphatase (ALP). Furthermore, the cytokines involved in these processes were elevated in cell cultures of osteoblasts exposed to PyoM: tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-10. The PyoM variants are cytocompatible in a wide concentration range and limit the doxorubicin-induced apoptosis of osteoblasts. This cytoprotective PyoM activity is correlated with an increased migration of osteoblasts. Moreover, PyoMsol and PyoMinsol exhibit antibacterial activity against staphylococci isolated from infected bones. The osteoinductive, pro-regenerative, and antiapoptotic effects achieved through PyoM stimulation prompt the development of new biocomposites modified with this bacterial biopolymer for medical use.

[Research progress on the regulatory cell death of osteoblasts in periodontitis]

Periodontitis is a chronic inflammatory disease characterized by progressive destruction of alveolar bone. The most critical mechanism underlying alveolar bone destruction is the imbalance of bone homeostasis, where osteoblast-mediated bone matrix synthesis plays an important role in regulating bone homeostasis. Regulated cell death is instrumental in both the inflammatory microenvironment and the regulation of bone homeostasis. Chronic inflammation, oxidative stress, and other factors can be directly involved in mitochondrial and death receptor-mediated signaling pathways, modulating B-cell lymphoma 2 family proteins and cysteine aspartic acid specific protease (caspase) activity, thereby affecting osteoblast apoptosis and alveolar bone homeostasis. Chronic inflammation and cellular damage induce osteoblast necroptosis via the RIPK1/RIPK3/MLKL signaling pathway, exacerbating the inflammatory response and accelerating alveolar bone destruction. Stimuli such as pathogenic microorganisms and cellular injury may also activate caspase-1-dependent or independent signaling pathways and gasdermin D family proteins, promoting osteoblast pyroptosis and releasing pro-inflammatory cytokines to mediate alveolar bone damage. Iron overload and lipid peroxidation in periodontitis can trigger ferroptosis in osteoblasts, impacting their survival and function, ultimately leading to bone homeostasis imbalance. This article focuses on the mechanism of periodontal disease affecting bone homeostasis through regulatory cell death, aiming to provide research evidence for the treatment of periodontitis and alveolar bone homeostasis imbalance.

Sex hormone-binding globulin promotes the osteogenic differentiation potential of equine adipose-derived stromal cells by activating the BMP signaling pathway

Background

Musculoskeletal injuries and chronic degenerative diseases pose significant challenges in equine health, impacting performance and overall well-being. Sex Hormone-Binding Globulin (SHBG) is a glycoprotein determining the bioavailability of sex hormones in the bloodstream, and exerting critical metabolic functions, thus impacting the homeostasis of many tissues including the bone.

Methods

In this study, we investigated the potential role of SHBG in promoting osteogenesis and its underlying mechanisms in a model of equine adipose-derived stromal cells (ASCs). An SHBG-knocked down model has been established using predesigned siRNA, and cells subjected to osteogenic induction medium in the presence of exogenous SHBG protein. Changes in differentiation events where then screened using various analytical methods.

Results

We demonstrated that SHBG treatment enhances the expression of key osteoconductive regulators in equine ASCs CD34+ cells, suggesting its therapeutic potential for bone regeneration. Specifically, SHBG increased the cellular expression of BMP2/4, osteocalcin (OCL), alkaline phosphatase (ALP), and osteopontin (OPN), crucial factors in early osteogenesis. Furthermore, SHBG treatment maintained adequate apoptosis and enhanced autophagy during osteogenic differentiation, contributing to bone formation and remodeling. SHBG further targeted mitochondrial dynamics, and promoted the reorganization of the mitochondrial network, as well as the expression of dynamics mediators including PINK, PARKIN and MFN1, suggesting its role in adapting cells to the osteogenic milieu, with implications for osteoblast maturation and differentiation.

Conclusion

Overall, our findings provide novel insights into SHBG's role in bone formation and suggest its potential therapeutic utility for bone regeneration in equine medicine.

Efferocytosis and Bone Dynamics

PURPOSE OF REVIEW

This review summarizes the recently published scientific evidence regarding the role of efferocytosis in bone dynamics and skeletal health.

RECENT FINDINGS

Several types of efferocytes have been identified within the skeleton, with macrophages being the most extensively studied. Efferocytosis is not merely a 'clean-up' process vital for maintaining skeletal homeostasis; it also plays a crucial role in promoting resolution pathways and orchestrating bone dynamics, such as osteoblast-osteoclast coupling during bone remodeling. Impaired efferocytosis has been associated with aging-related bone loss and various skeletal pathologies, including osteoporosis, osteoarthritis, rheumatoid arthritis, and metastatic bone diseases. Accordingly, emerging evidence suggests that targeting efferocytic mechanisms has the potential to alleviate these conditions. While efferocytosis remains underexplored in the skeleton, recent discoveries have shed light on its pivotal role in bone dynamics, with important implications for skeletal health and pathology. However, there are several knowledge gaps and persisting technical limitations that must be addressed to fully unveil the contributions of efferocytosis in bone.

Legumain is a paracrine regulator of osteoblast differentiation and mediates the inhibitory effect of TGF-β1 on osteoblast maturation

Transforming growth factor-beta 1 (TGF-β1) is a critical regulator of skeletal homeostasis and has diverse effects on osteoblastogenesis. To date, the mechanisms behind the intriguing inhibitory effect of TGF-β1 on osteoblast maturation are not fully understood. Here, we demonstrate a novel mechanism by which TGF-β1 modulates osteoblast maturation through the lysosomal protease legumain. We observed that addition of TGF-β1 to osteogenic cultures of human bone marrow derived mesenchymal stromal (stem) cells enhanced legumain activity and secretion, in-spite of decreased legumain mRNA expression, suggesting post-transcriptional regulation. We further showed that osteogenic cells internalize and activate prolegumain, associated with inhibited osteoblast maturation, revealing legumain as a paracrine regulator of osteoblast maturation. Interestingly, TGF-β1 treatment exacerbated legumain internalization and activity, and showed an additive effect on legumain-induced inhibition of osteoblast maturation. Importantly, pharmacological inhibition of legumain abolished the inhibitory effect of TGF-β1 on osteoblast maturation. Our findings reveal that TGF-β1 inhibits osteoblast maturation by stimulating secretion and activity of endogenous legumain, as well as enhancing internalization and activation of extracellular prolegumain. Therefore, our study provides a deeper understanding of the complex regulation of osteoblastogenesis and unveils a novel TGF-β1-legumain axis in regulation of osteoblast maturation, offering novel insights for possible therapeutic interventions related to bone diseases associated with aberrant TGF-β1 signaling.

Cell life-or-death events in osteoporosis: All roads lead to mitochondrial dynamics

Mitochondria exhibit heterogeneous shapes and networks within and among cell types and tissues, also in normal or osteoporotic bone tissues with complex cell types. This dynamic characteristic is determined by the high plasticity provided by mitochondrial dynamics and is stemmed from responding to the survival and functional requirements of various bone cells in a specific microenvironments. In contrast, mitochondrial dysfunction, induced by dysregulation of mitochondrial dynamics, may act as a trigger of cell death signals, including common apoptosis and other forms of programmed cell death (PCD). These PCD processes consisting of tightly structured cascade gene expression events, can further influence the bone remodeling by facilitating the death of various bone cells. Mitochondrial dynamics, therefore, drive the bone cells to stand at the crossroads of life and death by integrating external signals and altering metabolism, shape, and signal-response properties of mitochondria. This implies that targeting mitochondrial dynamics displays significant potential in treatment of osteoporosis. Considerable effort has been made in osteoporosis to emphasize the parallel roles of mitochondria in regulating energy metabolism, calcium signal transduction, oxidative stress, inflammation, and cell death. However, the emerging field of mitochondrial dynamics-related PCD is not well understood. Herein, to bridge the gap, we outline the latest knowledge on mitochondrial dynamics regulating bone cell life or death during normal bone remodeling and osteoporosis.

Exploring the Role of Hormones and Cytokines in Osteoporosis Development

The disease of osteoporosis is characterized by impaired bone structure and an increased risk of fractures. There is a significant impact of cytokines and hormones on bone homeostasis and the diagnosis of osteoporosis. As defined by the World Health Organization (WHO), osteoporosis is defined as having a bone mineral density (BMD) that is 2.5 standard deviations (SD) or more below the average for young and healthy women (T score < -2.5 SD). Cytokines and hormones, particularly in the remodeling of bone between osteoclasts and osteoblasts, control the differentiation and activation of bone cells through cytokine networks and signaling pathways like the nuclear factor kappa-B ligand (RANKL)/the receptor of RANKL (RANK)/osteoprotegerin (OPG) axis, while estrogen, parathyroid hormones, testosterone, and calcitonin influence bone density and play significant roles in the treatment of osteoporosis. This review aims to examine the roles of cytokines and hormones in the pathophysiology of osteoporosis, evaluating current diagnostic methods, and highlighting new technologies that could help for early detection and treatment of osteoporosis.

Induction of osteoblast apoptosis stimulates macrophage efferocytosis and paradoxical bone formation

Apoptosis is crucial for tissue homeostasis and organ development. In bone, apoptosis is recognized to be a main fate of osteoblasts, yet the relevance of this process remains underexplored. Using our murine model with inducible Caspase 9, the enzyme that initiates intrinsic apoptosis, we triggered apoptosis in a proportion of mature osteocalcin (OCN+) osteoblasts and investigated the impact on postnatal bone development. Osteoblast apoptosis stimulated efferocytosis by osteal macrophages. A five-week stimulation of OCN+ osteoblast apoptosis in 3-week-old male and female mice significantly enhanced vertebral bone formation while increasing osteoblast precursors. A similar treatment regimen to stimulate osterix+ cell apoptosis had no impact on bone volume or density. The vertebral bone accrual following stimulation of OCN+ osteoblast apoptosis did not translate in improved mechanical strength due to disruption of the lacunocanalicular network. The observed bone phenotype was not influenced by changes in osteoclasts but was associated with stimulation of macrophage efferocytosis and vasculature formation. Phenotyping of efferocytic macrophages revealed a unique transcriptomic signature and expression of factors including VEGFA. To examine whether macrophages participated in the osteoblast precursor increase following osteoblast apoptosis, macrophage depletion models were employed. Depletion of macrophages via clodronate-liposomes and the CD169-diphtheria toxin receptor mouse model resulted in marked reduction in leptin receptor+ and osterix+ osteoblast precursors. Collectively, this work demonstrates the significance of osteoblast turnover via apoptosis and efferocytosis in postnatal bone formation. Importantly, it exposes the potential of targeting this mechanism to promote bone anabolism in the clinical setting.

The therapeutic mechanism of Compound Lurong Jiangu Capsule for the treatment of cadmium-induced osteoporosis: network pharmacology and experimental verification

Background

Among bone diseases, osteoporosis-like skeleton, such as trabecular thinning, fracture and so on, is the main pathological change of cadmium-induced osteoporosis(Cd-OP), accompanied by brittle bone and increased fracture rate. However, the mechanism underlying cadmium-induced osteoporosis has remained elusive. Compound Lurong Jiangu Capsule (CLJC) is an experienced formula for the treatment of bone diseases, which has the effect of tonifying kidney and strengthening bones, promoting blood circulation and relieving pain.

Objective

Network pharmacology and molecular docking technology combined with experiments were used to investigate the potential mechanism of CLJC in treating Cd-OP.

Method

The active compounds and corresponding targets of each herb in CLJC were searched in the TCMSP and BATMAN-TCM databases. The DisGeNet, OMIM, and GeneCards databases searched for Cd-OP targets. The relationship between both of them was visualized by establishing an herb-compound-target network using Cytoscape 3.9.1 software. Gene ontology (GO), and Kyoto encyclopedia of genes and genomes (KEGG) pathway enrichment analyses were performed after determining the intersection of the targets from CLJC and Cd-OP. What's more, molecular docking was performed to validate the results. All of them were aim to obtain hud signaling pathways for further study. Finally, BAX, BCL-2, and CASPASE-3 were screened and selected for further experiments, which included bone imaging and reconstruction analysis (Micro-CT), hematoxylin-eosin Staining (HE), and western blot (WB).

Results

106 common targets from CLJC and Cd-OP targets were identified. KEGG pathway analysis suggested that multiple signaling pathways, such as the pathways in cancer, may play roles in treatment. Verification of the molecular docking was successful. Here we showed that Cd-OP displayed Tb.Th and Tb.N significantly reduced and even broke, irregular proliferation of bone cortex, uneven and loose trabecular bone arrangement, changed in apoptosis-related proteins, such as significant upregulation of CASPASE-3, BAX protein and significant downregulation of BCL-2 protein in vivo, while CLJC rescued these phenotypes.

Conclusion

This study revealed that CLJC can reduce the expression of apoptosis-related proteins, and multiple components and multiple targets inhibit Cd-OP through apoptosis signaling pathway.

HIF-1α activation impairs dendrites formation and elongation in osteocytogenesis

Osteocytes are terminally differentiated cells derived from osteoblasts and are deeply embedded within the bone matrix. They play a critical role in bone remodeling by generating a lacuno-canalicular network (LCN) and controlling the transport of nutrients. Due to the absence of blood vessels within the bone matrix, it is widely believed that osteocytes develop in a hypoxic environment. However, the mechanisms of osteocytogenesis and the role of oxygen sensing in this process remain unclear. Hypoxia-inducible factors (HIFs) are major transcriptional factors involved in oxygen sensing. Previous studies have shown that accumulation of HIFs in osteoblasts leads to abnormal bone remodeling, potentially linked with the alterations of the LCN network. Specifically, HIF-1α is hypothesized to play a more significant role in regulating bone remodeling compared to HIF-2α. Therefore, we investigated the functions of HIF-1α in dendrite formation and the establishment of the LCN network during osteocytogenesis. Immunostaining and scanning electron microscopy revealed that the E11 protein aggregates to form a ring structure that defines the site for dendrite initiation. This process is followed by activation of the ERM/RhoA pathway and recruitment of matrix metalloproteinase 14 (MMP14) to facilitate extracellular matrix degradation, enabling dendrite elongation. However, both hypoxic treatment and overexpression of HIF-1α impair ring formation, resulting in reduced ERM/RhoA activity and decreased matrix degradation capability. These findings suggest that abnormal HIF-1α activity in local areas could contribute to impaired LCN network formation and abnormal bone remodeling observed in bone diseases such as osteopenia and aging.

Aggregation of human osteoblasts unlocks self-reliant differentiation and constitutes a microenvironment for 3D-co-cultivation with other bone marrow cells

Skeletal bone function relies on both cells and cellular niches, which, when combined, provide guiding cues for the control of differentiation and remodeling processes. Here, we propose an in vitro 3D model based on human fetal osteoblasts, which eases the study of osteocyte commitment in vitro and thus provides a means to examine the influences of biomaterials, substances or cells on the regulation of these processes. Aggregates were formed from human fetal osteoblasts (hFOB1.19) and cultivated under proliferative, adipo- and osteoinductive conditions. When cultivated under osteoinductive conditions, the vitality of the aggregates was compromised, the expression levels of the mineralization-related gene DMP1 and the amount of calcification and matrix deposition were lower, and the growth of the spheroids stalled. However, within spheres under growth conditions without specific supplements, self-organization processes occur, which promote extracellular calcium deposition, and osteocyte-like cells develop. Long-term cultivated hFOB aggregates were free of necrotic areas. Moreover, hFOB aggregates cultivated under standard proliferative conditions supported the co-cultivation of human monocytes, microvascular endothelial cells and stromal cells. Overall, the model presented here comprises a self-organizing and easily accessible 3D osteoblast model for studying bone marrow formation and in vitro remodeling and thus provides a means to test druggable molecular pathways with the potential to promote life-long bone formation and remodeling.

Current perspectives on the multiple roles of osteoclasts: Mechanisms of osteoclast-osteoblast communication and potential clinical implications

Bone remodeling is a complex process involving the coordinated actions of osteoblasts and osteoclasts to maintain bone homeostasis. While the influence of osteoblasts on osteoclast differentiation is well established, the reciprocal regulation of osteoblasts by osteoclasts has long remained enigmatic. In the past few years, a fascinating new role for osteoclasts has been unveiled in promoting bone formation and facilitating osteoblast migration to the remodeling sites through a number of different mechanisms, including the release of factors from the bone matrix following bone resorption and direct cell-cell interactions. Additionally, considerable evidence has shown that osteoclasts can secrete coupling factors known as clastokines, emphasizing the crucial role of these cells in maintaining bone homeostasis. Due to their osteoprotective function, clastokines hold great promise as potential therapeutic targets for bone diseases. However, despite long-standing work to uncover new clastokines and their effect in vivo, more substantial efforts are still required to decipher the mechanisms and pathways behind their activity in order to translate them into therapies. This comprehensive review provides insights into our evolving understanding of the osteoclast function, highlights the significance of clastokines in bone remodeling, and explores their potential as treatments for bone diseases suggesting future directions for the field.

The bone ecosystem facilitates multiple myeloma relapse and the evolution of heterogeneous drug resistant disease

Multiple myeloma (MM) is an osteolytic malignancy that is incurable due to the emergence of treatment resistant disease. Defining how, when and where myeloma cell intrinsic and extrinsic bone microenvironmental mechanisms cause relapse is challenging with current biological approaches. Here, we report a biology-driven spatiotemporal hybrid agent-based model of the MM-bone microenvironment. Results indicate MM intrinsic mechanisms drive the evolution of treatment resistant disease but that the protective effects of bone microenvironment mediated drug resistance (EMDR) significantly enhances the probability and heterogeneity of resistant clones arising under treatment. Further, the model predicts that targeting of EMDR deepens therapy response by eliminating sensitive clones proximal to stroma and bone, a finding supported by in vivo studies. Altogether, our model allows for the study of MM clonal evolution over time in the bone microenvironment and will be beneficial for optimizing treatment efficacy so as to significantly delay disease relapse.

Role of MAPKs in TGF-β1-induced maturation and mineralization in human osteoblast-like cells

OBJECTIVES

Our study aimed to clarify the role of mitogen-activated protein kinases (MAPKs) in transforming growth factor (TGF)-β1-stimulated mineralization in the human osteoblast-like MG63 cells.

METHODS

The viability of MG63 cells under TGF-β1 stimulation was assessed by MTS assay. Western blotting determined TGF-β1-mediated activation of extracellular signal-related protein kinase (ERK), p38, and c-Jun amino-terminal kinase (JNK). Mineralization-related gene expression was examined by quantitative real-time PCR, and mineral deposition levels were evaluated by alizarin red S staining.

RESULTS

TGF-β1 had no effect on MG63 cell proliferation. Activation of p38 was observed at 3 h post TGF-β1 stimulation. Moreover, JNK phosphorylation was upregulated by TGF-β1 from 1 to 6 h post stimulation, but had no activation on ERK phosphorylation throughout the experimental period. Treatment with JNK inhibitor diminished the alizarin red S-stained area in a dose-dependent manner. Mineral deposition was unaffected by MEK inhibitor, whereas p38 inhibitor increased the red-stained area. Gene expression levels of ALP and BSP were significantly decreased under treatment with JNK inhibitor and p38 inhibitor. The MEK inhibitor had no effect on the TGF-β1-mediated upregulation of ALP and BSP. Although all three inhibitors suppressed expression of COL I, none were found to stimulate expression of OCN.

CONCLUSIONS

Human osteoblast-like MG63 cells maturation and mineralization are induced through JNK activation of MAPK signaling in response to TGF-β1.

A novel lncRNA GM15416 regulates osteoblast apoptosis and differentiation through the c-Fos/Fas axis and mitigates osteoporosis

Osteoporosis (OP) is a common systemic bone disorder, and the programmed cell death of osteoblasts is closely linked to the development of osteoporosis. Previous studies have shown that c-fos can cause osteoblast apoptosis. Furthermore, it has been demonstrated that long non-coding RNA (lncRNA) plays a pervasive role in regulating the biology of osteoblasts. Nevertheless, the precise role and mechanism of long non-coding RNA (lncRNA) in relation to c-Fos at the transcriptional level in osteoblast cell death remain uncertain. Compared with normal osteoblasts, serum deprivation resulted in significant upregulation of the transcription factor c-Fos and apoptosis-related Fas proteins in osteoblasts. In addition, the expression of lncRNA GM15416 related to c-Fos was significantly increased. The results showed that overexpression of c-Fos leads to an increase in downstream Fas protein, which subsequently leads to osteoblast apoptosis and hinders osteogenesis. On the contrary, a decrease in lncRNA GM15416 expression leads to a decrease in c-Fos/Fas expression, which hinders osteoblast apoptosis and promotes osteogenesis. Our results suggest that lncRNA GM15416 exerts inhibitory effects on osteoblast apoptosis and acts as a preventive factor against osteoporosis. As a result, GM15416 emerges as an important lncRNA associated with osteoporosis and holds potential as a future therapeutic target.

Impaired Efferocytosis Enables Apoptotic Osteoblasts to Escape Osteoimmune Surveillance During Aging

Macrophage efferocytosis of apoptotic osteoblasts (apoOBs) is a key osteoimmune process for bone homeostasis. However, apoOBs frequently accumulate in aged bone marrow, where they may mount proinflammatory responses and progressive bone loss. The reason why apoOBs are not cleared during aging remains unclear. In this study, it is demonstrated that aged apoOBs upregulate the immune checkpoint molecule CD47, which is controlled by SIRT6-regulated transcriptional pausing, to evade clearance by macrophages. Using osteoblast- and myeloid-specific gene knockout mice, SIRT6 is further revealed to be a critical modulator for apoOBs clearance via targeting CD47-SIRPα checkpoint. Moreover, apoOBs activate SIRT6-mediated chemotaxis to recruit macrophages by releasing apoptotic vesicles. Two targeting delivery strategies are developed to enhance SIRT6 activity, resulting in rejuvenated apoOBs clearance and delayed age-related bone loss. Collectively, the findings reveal a previously unknown linkage between immune surveillance and bone homeostasis and targeting the SIRT6-regulated mechanism can be a promising therapeutic strategy for age-related bone diseases.

Osteoprogenitor recruitment and differentiation during intracortical bone remodeling of adolescent humans

BACKGROUND

Recruitment and proliferation of osteoprogenitors during the reversal-resorption phase, and their differentiation into mature bone-forming osteoblasts is crucial for initiation of bone formation during bone remodeling. This study investigates the osteoprogenitors' gradual recruitment, proliferation, and differentiation into bone-forming osteoblasts within intracortical remodeling events of healthy adolescent humans.

METHODS

The study was conducted on cortical bone specimens from 11 adolescent human controls - patients undergoing surgery due to coxa valga. The osteoprogenitor recruitment route and differentiation into osteoblasts were backtracked using immunostainings and in situ hybridizations with osteoblastic markers (CD271/NGFR, osterix/SP7, COL3A1 and COL1A1). The osteoblastic cell populations were defined based on the pore surfaces, and their proliferation index (Ki67), density and number/circumference were estimated in multiplex-immunofluorescence (Ki67, TRAcP, CD34) stained sections.

RESULTS

During the reversal-resorption phase, osteoclasts are intermixed with (COL3A1+NFGR+) osteoblastic reversal cells, which are considered to be osteoprogenitors of (COL1A1+SP7+) bone-forming osteoblasts. Initiation of bone formation requires a critical density of these osteoprogenitors (43 ± 9 cells/mm), which is reached though proliferation (4.4 ± 0.5 % proliferative) and even more so through recruitment of osteoprogenitors, but challenged by the ongoing expansion of the canal circumference. These osteoprogenitors most likely originate from osteoblastic bone lining cells and mainly lumen osteoprogenitors, which expand their population though proliferation (4.6 ± 0.3 %) and vascular recruitment. These lumen osteoprogenitors resemble canopy cells above trabecular remodeling sites, and like canopy cells they extend above bone-forming osteoblasts where they may rejuvenate the osteoblast population during bone formation.

CONCLUSION

Initiation of bone formation during intracortical remodeling requires a critical density of osteoprogenitors on eroded surfaces, which is reached though proliferation and recruitment of local osteoprogenitors: bone lining cells and lumen osteoprogenitors.

Influences of Aged Bone Marrow Macrophages on Skeletal Health and Senescence

PURPOSE OF REVIEW

The purpose of this review is to discuss the role of macrophages in the regulation of skeletal health with age, particularly in regard to both established and unexplored mechanisms in driving inflammation and senescence.

RECENT FINDINGS

A multitude of research has uncovered mechanisms of intrinsic aging in macrophages, detrimental factors released by these immune cells, and crosstalk from senescent mesenchymal cell types, which altogether drive age-related bone loss. Furthermore, bone marrow macrophages were recently proposed to be responsible for the megakaryocytic shift during aging and overall maintenance of the hematopoietic niche. Studies on extra-skeletal macrophages have shed light on possible conserved mechanisms within bone and highlight the importance of these cells in systemic aging. Macrophages are a critically important cell type in maintaining skeletal homeostasis with age. New discoveries in this area are of utmost importance in fully understanding the pathogenesis of osteoporosis in aged individuals.

Assembling the Puzzle Pieces. Insights for in Vitro Bone Remodeling

As a highly dynamic organ, bone changes during throughout a person's life. This process is referred to as 'bone remodeling' and it involves two stages - a well-balanced osteoclastic bone resorption and an osteoblastic bone formation. Under normal physiological conditions bone remodeling is highly regulated that ensures tight coupling between bone formation and resorption, and its disruption results in a bone metabolic disorder, most commonly osteoporosis. Though osteoporosis is one of the most prevalent skeletal ailments that affect women and men aged over 40 of all races and ethnicities, currently there are few, if any safe and effective therapeutic interventions available. Developing state-of-the-art cellular systems for bone remodeling and osteoporosis can provide important insights into the cellular and molecular mechanisms involved in skeletal homeostasis and advise better therapies for patients. This review describes osteoblastogenesis and osteoclastogenesis as two vital processes for producing mature, active bone cells in the context of interactions between cells and the bone matrix. In addition, it considers current approaches in bone tissue engineering, pointing out cell sources, core factors and matrices used in scientific practice for modeling bone diseases and testing drugs. Finally, it focuses on the challenges that bone regenerative medicine is currently facing.

Megakaryocyte Secreted Factors Regulate Bone Marrow Niche Cells During Skeletal Homeostasis, Aging, and Disease

The bone marrow microenvironment contains a diverse array of cell types under extensive regulatory control and provides for a novel and complex mechanism for bone regulation. Megakaryocytes (MKs) are one such cell type that potentially acts as a master regulator of the bone marrow microenvironment due to its effects on hematopoiesis, osteoblastogenesis, and osteoclastogenesis. While several of these processes are induced/inhibited through MK secreted factors, others are primarily regulated by direct cell-cell contact. Notably, the regulatory effects that MKs exert on these different cell populations has been found to change with aging and disease states. Overall, MKs are a critical component of the bone marrow that should be considered when examining regulation of the skeletal microenvironment. An increased understanding of the role of MKs in these physiological processes may provide insight into novel therapies that can be used to target specific pathways important in hematopoietic and skeletal disorders.

Hyperglycemia exerts disruptive effects on the secretion of TGF-β1 and its matrix ligands, decorin and biglycan, by mesenchymal sub-populations and macrophages during bone repair

Introduction

Bone has a high capacity for repair, but for patients with uncontrolled type 2 diabetes mellitus (T2DM), the associated hyperglycemia can significantly delay osteogenic processes. These patients respond poorly to fracture repair and bone grafts, leading to lengthy care plans due to arising complications. Mesenchymal stromal cells (MSCs) and M2 macrophages are both major sources of transforming growth factor-β1 (TGF-β1), a recognized mediator for osteogenesis and whose bioavailability and activities are further regulated by matrix small leucine-rich proteoglycans (SLRPs), decorin and biglycan. The aim of this study was to investigate how in vivo and in vitro hyperglycemic (HGly) environments can influence the levels of TGF-β1, decorin, and biglycan during bone repair, with additional consideration for how long-term glucose exposure and cell aging can also influence this process.

Results

Following bone healing within a T2DM in vivo model, histological and immunolabeling analyses of bone tissue sections confirmed delayed healing, which was associated with significantly elevated TGF-β1 levels within the bone matrices of young diabetic rats, compared with their normoglycemic (Norm) and aged counterparts. Studies continued to assess in vitro the effects of normal (5.5 mM) and high (25 mM) glucose exposure on the osteogenic differentiation of compact bone-derived mesenchymal stromal cells (CB-MSCs) at population doubling (PD)15, characterized to contain populations of lineage-committed osteoblasts, and at PD150, where transit-amplifying cells predominate. Short-term glucose exposure increased TGF-β1 and decorin secretion by committed osteoblasts but had a lesser effect on transit-amplifying cells. In contrast, the long-term exposure of CB-MSCs to high glucose was associated with decreased TGF-β1 and increased decorin secretion. Similar assessments on macrophage populations indicated high glucose inhibited TGF-β1 secretion, preventing M2 formation.

Discussion

Collectively, these findings highlight how hyperglycemia associated with T2DM can perturb TGF-β1 and decorin secretion by MSCs and macrophages, thereby potentially influencing TGF-β1 bioavailability and signaling during bone repair.

Bioactive peptides for boosting stem cell culture platform: Methods and applications

Peptides, short protein fragments, can emulate the functions of their full-length native counterparts. Peptides are considered potent recombinant protein alternatives due to their specificity, high stability, low production cost, and ability to be easily tailored and immobilized. Stem cell proliferation and differentiation processes are orchestrated by an intricate interaction between numerous growth factors and proteins and their target receptors and ligands. Various growth factors, functional proteins, and cellular matrix-derived peptides efficiently enhance stem cell adhesion, proliferation, and directed differentiation. For that, peptides can be immobilized on a culture plate or conjugated to scaffolds, such as hydrogels or synthetic matrices. In this review, we assess the applications of a variety of peptides in stem cell adhesion, culture, organoid assembly, proliferation, and differentiation, describing the shortcomings of recombinant proteins and their full-length counterparts. Furthermore, we discuss the challenges of peptide applications in stem cell culture and materials design, as well as provide a brief outlook on future directions to advance peptide applications in boosting stem cell quality and scalability for clinical applications in tissue regeneration.

Research Progress of Macrophages in Bone Regeneration

Bone tissue regeneration plays an increasingly important role in contemporary clinical treatment. The reconstruction of bone defects remains a huge challenge for clinicians. Bone regeneration is regulated by the immune system, in which inflammation is an important regulating factor in bone formation and remodeling. As the main cells involved in inflammation, macrophages play a key role in osteogenesis by polarizing into different phenotypes during different stages of bone regeneration. Considering this, this review mainly summarizes the function of macrophage in bone regeneration based on mesenchymal stem cells (MSCs), osteoblasts, osteoclasts, and vascular cells. In conclusion, anti-inflammatory macrophages (M2) have a greater potentiality to promote bone regeneration than M0 and classically activated proinflammatory macrophages (M1). In the fracture and bone defect models, tissue engineering materials can induce the transition from M1 to M2, alter the bone microenvironment, and promote bone regeneration through interactions with bone-related cells and blood vessels. The review provides a further understanding of macrophage polarization behavior in the evolving field of bone immunology.

Caspase-9 driven murine model of selective cell apoptosis and efferocytosis

Apoptosis and efficient efferocytosis are integral to growth, development, and homeostasis. The heterogeneity of these mechanisms in different cells across distinct tissues renders it difficult to develop broadly applicable in vivo technologies. Here, we introduced a novel inducible caspase-9 (iCasp9) mouse model which allowed targeted cell apoptosis and further facilitated investigation of concomitant efferocytosis. We generated iCasp9+/+ mice with conditional expression of chemically inducible caspase-9 protein that is triggered in the presence of Cre recombinase. In vitro, bone marrow cells from iCasp9+/+ mice showed expression of the iCasp9 protein when transduced with Cre-expressing adenovirus. Treatment of these cells with the chemical dimerizer (AP20187/AP) resulted in iCasp9 processing and cleaved caspase-3 upregulation, indicating successful apoptosis induction. The in vivo functionality and versatility of this model was demonstrated by crossing iCasp9+/+ mice with CD19-Cre and Osteocalcin (OCN)-Cre mice to target CD19+ B cells or OCN+ bone-lining osteoblasts. Immunofluorescence and/or immunohistochemical staining in combination with histomorphometric analysis of EGFP, CD19/OCN, and cleaved caspase-3 expression demonstrated that a single dose of AP effectively induced apoptosis in CD19+ B cells or OCN+ osteoblasts. Examination of the known efferocytes in the target tissues showed that CD19+ cell apoptosis was associated with infiltration of dendritic cells into splenic B cell follicles. In the bone, where efferocytosis remains under-explored, the use of iCasp9 provided direct in vivo evidence that macrophages are important mediators of apoptotic osteoblast clearance. Collectively, this study presented the first mouse model of iCasp9 which achieved selective apoptosis, allowing examination of subsequent efferocytosis. Given its unique feature of being controlled by any Cre-expressing mouse lines, the potential applications of this model are extensive and will bring forth more insights into the diversity of mechanisms and cellular effects induced by apoptosis including the physiologically important efferocytic process that follows.

Conditional Loss of Nmp4 in Mesenchymal Stem Progenitor Cells Enhances PTH-Induced Bone Formation

Activation of bone anabolic pathways is a fruitful approach for treating severe osteoporosis, yet FDA-approved osteoanabolics, eg, parathyroid hormone (PTH), have limited efficacy. Improving their potency is a promising strategy for maximizing bone anabolic output. Nmp4 (Nuclear Matrix Protein 4) global knockout mice exhibit enhanced PTH-induced increases in trabecular bone but display no overt baseline skeletal phenotype. Nmp4 is expressed in all tissues; therefore, to determine which cell type is responsible for driving the beneficial effects of Nmp4 inhibition, we conditionally removed this gene from cells at distinct stages of osteogenic differentiation. Nmp4-floxed (Nmp4fl/fl ) mice were crossed with mice bearing one of three Cre drivers including (i) Prx1Cre+  to remove Nmp4 from mesenchymal stem/progenitor cells (MSPCs) in long bones; (ii) BglapCre+  targeting mature osteoblasts, and (iii) Dmp1Cre+  to disable Nmp4 in osteocytes. Virgin female Cre+  and Cre- mice (10 weeks of age) were sorted into cohorts by weight and genotype. Mice were administered daily injections of either human PTH 1-34 at 30 μg/kg or vehicle for 4 weeks or 7 weeks. Skeletal response was assessed using dual-energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and serum analysis for remodeling markers. Nmp4fl/fl ;Prx1Cre+  mice virtually phenocopied the global Nmp4-/- skeleton in the femur, ie, a mild baseline phenotype but significantly enhanced PTH-induced increase in femur trabecular bone volume/total volume (BV/TV) compared with their Nmp4fl/fl ;Prx1Cre- controls. This was not observed in the spine, where Prrx1 is not expressed. Heightened response to PTH was coincident with enhanced bone formation. Conditional loss of Nmp4 from the mature osteoblasts (Nmp4fl/fl ;BglapCre+ ) failed to increase BV/TV or enhance PTH response. However, conditional disabling of Nmp4 in osteocytes (Nmp4fl/fl ;Dmp1Cre+ ) increased BV/TV without boosting response to hormone under our experimental regimen. We conclude that Nmp4-/- Prx1-expressing MSPCs drive the improved response to PTH therapy and that this gene has stage-specific effects on osteoanabolism. © 2022 American Society for Bone and Mineral Research (ASBMR).

The epigenetic impact of suberohydroxamic acid and 5‑Aza‑2'‑deoxycytidine on DNMT3B expression in myeloma cell lines differing in IL‑6 expression

Gene inactivation of the cyclin-dependent kinase inhibitors p16^INK4a, p15^INK4b and p21^WAF is frequently mediated by promoter gene methylation, whereas histone deacetylases (HDACs) control gene expression through their ability to deacetylate proteins. The effect of suberohydroxamic acid (SBHA) and 5-Aza-2′-deoxycytidine (Decitabine) (DAC) treatments on the transcription of CDKN2A, CDKN2B and CDKN1A genes, and their effects on molecular biological behavior were examined in two myeloma cell lines, RPMI8226 and U266, which differ in p53-functionality and IL-6 expression. In both tested myeloma cell lines, a non-methylated state of the CDKN2B gene promoter region was detected with normal gene expression, and the same level of p15^INK4b protein was detected by immunocytochemical staining. Furthermore, in myeloma cells treated with SBHA and DAC alone, the expression of both p15^INK4b and p21^WAF was significantly upregulated in RPMI8226 cells (p53-functional, without IL-6 expression), whereas in the U266 cell line (p53 deleted, expressing IL-6) only p21^WAF expression was significantly increased. Moreover, the analysis revealed that treatment with DAC induced DNMT3B enhancement in U266 cells. In conclusion, in myeloma cells with IL-6 expression, significantly increased DNMT3B expression indicated the tumorigenic consequences of 5-Aza-2′deoxycytidine treatment, which requires careful use in diseases involving epigenetic dysregulation, such as multiple myeloma (MM).

Single-cell spatiotemporal analysis reveals cell fates and functions of transplanted mesenchymal stromal cells during bone repair

Mesenchymal stromal cells (MSCs) transplantation could enhance bone repair. However, the cell fate of transplanted MSCs, in terms of their local distribution and spatial associations with other types of cells were poorly understood. Here, we developed a single-cell 3D spatial correlation (sc3DSC) method to track transplanted MSCs based on deep tissue microscopy of fluorescent nanoparticles (fNPs) and immunofluorescence of key proteins. Locally delivered fNP-labeled MSCs enhanced tibial defect repair, increased the number of stem cells and vascular maturity in mice. fNP-MSCs persisted in the defect throughout repair. But only a small portion of transplanted cells underwent osteogenic differentiation (OSX+); a significant portion has maintained their expression of mesenchymal stem cell and skeletal stem cell markers (SCA-1 and PRRX1). Our results contribute to the optimization of MSC-based therapies. The sc3DSC method may be useful in studying cell-based therapies for the regeneration of other tissue types or disease models.

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Transplanted marrow stromal cells associated with vessels during bone defect repair

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Small proportion of transplanted cells differentiated into osteogenic cells

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A proportion of transplanted cells maintained expressions of stem cell markers

Closing cones create conical lamellae in secondary osteonal bone

Lamellae are sheets of mineralized collagen 1-20 µm thick, extending over hundreds of µm in bone tissue, occupying bone's structural hierarchy at a level above collagen fibres and osteocytes, and below osteons and trabeculae. Osteons are tubular arrangements of lamellae surrounding central neurovascular canals. Lamellae in osteons are usually described as concentric cylinders based on their annular appearance in transverse section. In this review, I provide a perspective on current understanding of the relationship between geometry of the bone formation front and the shape of lamellae produced at it, reaching the conclusion that the 'closing cone' bone formation front in secondary osteonal remodelling must necessarily result in cone-shaped lamellae in the mature secondary osteon. Secondary osteons replace primary osteons through a tunnelling process of bone turnover, meaning that conical lamellae may become more common in older and damaged bone which is at greatest risk of fracture. Visualization and measurement of three-dimensional lamellar shape over hundreds of microns is needed to provide data for accurate micromechanical simulations. Treating secondary osteonal lamellae as a 'stack of cones' rather than 'nested cylinders' may have important implications for our appreciation of bone's function as a load-bearing tissue and of its behaviour in fracture.

Skeletal Aging

Aging represents the single greatest risk factor for chronic diseases, including osteoporosis, a skeletal fragility syndrome that increases fracture risk. Optimizing bone strength throughout life reduces fracture risk. Factors critical for bone strength include nutrition, physical activity, and vitamin D status, whereas unhealthy lifestyles, illnesses, and certain medications (eg, glucocorticoids) are detrimental. Hormonal status is another important determinant of skeletal health, with sex steroid concentrations, particularly estrogen, having major effects on bone remodeling. Aging exacerbates bone loss in both sexes and results in imbalanced bone resorption relative to formation; it is associated with increased marrow adiposity, osteoblast/osteocyte apoptosis, and accumulation of senescent cells. The mechanisms underlying skeletal aging are as diverse as the factors that determine the strength (and thus fragility) of bone. This review updates our current understanding of the epidemiology, pathophysiology, and treatment of osteoporosis and provides an overview of the underlying hallmark mechanisms that drive skeletal aging.

The Hypothesis of the Prolonged Cell Cycle in Turner Syndrome

Turner syndrome (TS) is a chromosomal disorder that is caused by a missing or structurally abnormal second sex chromosome. Subjects with TS are at an increased risk of developing intrauterine growth retardation, low birth weight, short stature, congenital heart diseases, infertility, obesity, dyslipidemia, hypertension, insulin resistance, type 2 diabetes mellitus, metabolic syndrome, and cardiovascular diseases (stroke and myocardial infarction). The underlying pathogenetic mechanism of TS is unknown. The assumption that X chromosome-linked gene haploinsufficiency is associated with the TS phenotype is questioned since such genes have not been identified. Thus, other pathogenic mechanisms have been suggested to explain this phenotype. Morphogenesis encompasses a series of events that includes cell division, the production of migratory precursors and their progeny, differentiation, programmed cell death, and integration into organs and systems. The precise control of the growth and differentiation of cells is essential for normal development. The cell cycle frequency and the number of proliferating cells are essential in cell growth. 45,X cells have a failure to proliferate at a normal rate, leading to a decreased cell number in a given tissue during organogenesis. A convergence of data indicates an association between a prolonged cell cycle and the phenotypical features in Turner syndrome. This review aims to examine old and new findings concerning the relationship between a prolonged cell cycle and TS phenotype. These studies reveal a diversity of phenotypic features in TS that could be explained by reduced cell proliferation. The implications of this hypothesis for our understanding of the TS phenotype and its pathogenesis are discussed. It is not surprising that 45,X monosomy leads to cellular growth pathway dysregulation with profound deleterious effects on both embryonic and later stages of development. The prolonged cell cycle could represent the beginning of the pathogenesis of TS, leading to a series of phenotypic consequences in embryonic/fetal, neonatal, pediatric, adolescence, and adulthood life.

Advancing Treatment of Bone Metastases through Novel Translational Approaches Targeting the Bone Microenvironment

Bone metastases represent a lethal condition that frequently occurs in solid tumors such as prostate, breast, lung, and renal cell carcinomas, and increase the risk of skeletal-related events (SREs) including pain, pathologic fractures, and spinal cord compression. This unique metastatic niche consists of a multicellular complex that cancer cells co-opt to engender bone remodeling, immune suppression, and stromal-mediated therapeutic resistance. This review comprehensively discusses clinical challenges of bone metastases, novel preclinical models of the bone and bone marrow microenviroment, and crucial signaling pathways active in bone homeostasis and metastatic niche. These studies establish the context to summarize the current state of investigational agents targeting BM, and approaches to improve BM-targeting therapies. Finally, we discuss opportunities to advance research in bone and bone marrow microenvironments by increasing complexity of humanized preclinical models and fostering interdisciplinary collaborations to translational research in this challenging metastatic niche.

Role of the Interaction of Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors 1 and 2 in Bone-Related Cells

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine expressed by macrophages, monocytes, and T cells, and its expression is triggered by the immune system in response to pathogens and their products, such as endotoxins. TNF-α plays an important role in host defense by inducing inflammatory reactions such as phagocytes and cytocidal systems activation. TNF-α also plays an important role in bone metabolism and is associated with inflammatory bone diseases. TNF-α binds to two cell surface receptors, the 55kDa TNF receptor-1 (TNFR1) and the 75kDa TNF receptor-2 (TNFR2). Bone is in a constant state of turnover; it is continuously degraded and built via the process of bone remodeling, which results from the regulated balance between bone-resorbing osteoclasts, bone-forming osteoblasts, and the mechanosensory cell type osteocytes. Precise interactions between these cells maintain skeletal homeostasis. Studies have shown that TNF-α affects bone-related cells via TNFRs. Signaling through either receptor results in different outcomes in different cell types as well as in the same cell type. This review summarizes and discusses current research on the TNF-α and TNFR interaction and its role in bone-related cells.

Parathyroid hormone signaling in mature osteoblasts/osteocytes protects mice from age-related bone loss

Aging is accompanied by osteopenia, characterized by reduced bone formation and increased bone resorption. Osteocytes, the terminally differentiated osteoblasts, are regulators of bone homeostasis, and parathyroid hormone (PTH) receptor (PPR) signaling in mature osteoblasts/osteocytes is essential for PTH-driven anabolic and catabolic skeletal responses. However, the role of PPR signaling in those cells during aging has not been investigated. The aim of this study was to analyze the role of PTH signaling in mature osteoblasts/osteocytes during aging. Mice lacking PPR in osteocyte (Dmp1-PPRKO) display an age-dependent osteopenia characterized by a significant decrease in osteoblast activity and increase in osteoclast number and activity. At the molecular level, the absence of PPR signaling in mature osteoblasts/osteocytes is associated with an increase in serum sclerostin and a significant increase in osteocytes expressing 4-hydroxy-2-nonenals, a marker of oxidative stress. In Dmp1-PPRKO mice there was an age-dependent increase in p16Ink4a/Cdkn2a expression, whereas it was unchanged in controls. In vitro studies demonstrated that PTH protects osteocytes from oxidative stress-induced cell death. In summary, we reported that PPR signaling in osteocytes is important for protecting the skeleton from age-induced bone loss by restraining osteoclast's activity and protecting osteocytes from oxidative stresses.

Cytokine-scavenging nanodecoys reconstruct osteoclast/osteoblast balance toward the treatment of postmenopausal osteoporosis

Imbalance between osteoblasts and osteoclasts accounts for the incidence and deterioration of postmenopausal osteoporosis. Abnormally elevated RANKL and TNF-α levels after menopause promote osteoclast formation and inhibit osteoblast differentiation, respectively. Here, nanodecoys capable of scavenging RANKL and TNF-α were developed from preosteoclast (RAW 264.7 cell) membrane–coated poly(lactic-co-glycolic acid) (PLGA) nanoparticles, which inhibited osteoporosis and maintained bone integrity. The nanodecoys effectively escaped from macrophage capture and enabled prolonged blood circulation after systemic administration. The abundant RANK and TNF-α receptor (TNF-αR) on the cell membranes effectively neutralized RANKL and TNF-α to prevent osteoclastogenesis and promote osteoblastogenesis, respectively, thus reversing the progression of osteoporosis in the ovariectomized (OVX) mouse model. These biomimetic nanodecoys provide an effective strategy for reconstructing the osteoclast/osteoblast balance and hold great potentials for the clinical management of postmenopausal osteoporosis.

The molecular etiology and treatment of glucocorticoid-induced osteoporosis

Glucocorticoid-induced osteoporosis (GIOP) is the most common form of secondary osteoporosis, accounting for 20% of osteoporosis diagnoses. Using glucocorticoids for >6 months leads to osteoporosis in 50% of patients, resulting in an increased risk of fracture and death. Osteoblasts, osteocytes, and osteoclasts work together to maintain bone homeostasis. When bone formation and resorption are out of balance, abnormalities in bone structure or function may occur. Excess glucocorticoids disrupt the bone homeostasis by promoting osteoclast formation and prolonging osteoclasts' lifespan, leading to an increase in bone resorption. On the other hand, glucocorticoids inhibit osteoblasts' formation and facilitate apoptosis of osteoblasts and osteocytes, resulting in a reduction of bone formation. Several signaling pathways, signaling modulators, endocrines, and cytokines are involved in the molecular etiology of GIOP. Clinically, adults ≥40 years of age using glucocorticoids chronically with a high fracture risk are considered to have medical intervention. In addition to vitamin D and calcium tablet supplementations, the major therapeutic options approved for GIOP treatment include antiresorption drug bisphosphonates, parathyroid hormone N-terminal fragment teriparatide, and the monoclonal antibody denosumab. The selective estrogen receptor modulator can only be used under specific condition for postmenopausal women who have GIOP but fail to the regular GIOP treatment or have specific therapeutic contraindications. In this review, we focus on the molecular etiology of GIOP and the molecular pharmacology of the therapeutic drugs used for GIOP treatment.

Transcriptional networks controlling stromal cell differentiation

Stromal progenitors are found in many different tissues, where they play an important role in the maintenance of tissue homeostasis owing to their ability to differentiate into parenchymal cells. These progenitor cells are differentially pre-programmed by their tissue microenvironment but, when cultured and stimulated in vitro, these cells - commonly referred to as mesenchymal stromal cells (MSCs) - exhibit a marked plasticity to differentiate into many different cell lineages. Loss-of-function studies in vitro and in vivo have uncovered the involvement of specific signalling pathways and key transcriptional regulators that work in a sequential and coordinated fashion to activate lineage-selective gene programmes. Recent advances in omics and single-cell technologies have made it possible to obtain system-wide insights into the gene regulatory networks that drive lineage determination and cell differentiation. These insights have important implications for the understanding of cell differentiation, the contribution of stromal cells to human disease and for the development of cell-based therapeutic applications.

NMDA Receptor Hypofunction in the Aging-Associated Malfunction of Peripheral Tissue

Glutamatergic transmission through NMDA receptors (NMDARs) is important for the function of peripheral tissues. In the bone, NMDARs and its co-agonist, D-serine participate in all the phases of the remodeling. In the vasculature, NMDARs exerts a tonic vasodilation decreasing blood perfusion in the corpus cavernosum and the filtration rate in the renal glomerulus. NMDARs are relevant for the skin turnover regulating the proliferation and differentiation of keratinocytes and the formation of the cornified envelope (CE). The interference with NMDAR function in the skin leads to a slow turnover and repair. As occurs with the brain and cognitive functions, the manifestations of a hypofunction of NMDARs resembles those observed during aging. This raises the question if the deterioration of the glomerular vasculature, the bone remodeling and the skin turnover associated with age could be related with a hypofunction of NMDARs. Furthermore, the interference of D-serine and the effects of its supplementation on these tissues, suggest that a decrease of D-serine could account for this hypofunction pointing out D-serine as a potential therapeutic target to reduce or even prevent the detriment of the peripheral tissue associated with aging.

Too much death can kill you: inhibiting intrinsic apoptosis to treat disease

Apoptotic cell death is implicated in both physiological and pathological processes. Since many types of cancerous cells intrinsically evade apoptotic elimination, induction of apoptosis has become an attractive and often necessary cancer therapeutic approach. Conversely, some cells are extremely sensitive to apoptotic stimuli leading to neurodegenerative disease and immune pathologies. However, due to several challenges, pharmacological inhibition of apoptosis is still only a recently emerging strategy to combat pathological cell loss. Here, we describe several key steps in the intrinsic (mitochondrial) apoptosis pathway that represent potential targets for inhibitors in disease contexts. We also discuss the mechanisms of action, advantages and limitations of small-molecule and peptide-based inhibitors that have been developed to date. These inhibitors serve as important research tools to dissect apoptotic signalling and may foster new treatments to reduce unwanted cell loss.

Regulation of Osteoblast Differentiation by Cytokine Networks

Osteoblasts, which are bone-forming cells, play pivotal roles in bone modeling and remodeling. Osteoblast differentiation, also known as osteoblastogenesis, is orchestrated by transcription factors, such as runt-related transcription factor 1/2, osterix, activating transcription factor 4, special AT-rich sequence-binding protein 2 and activator protein-1. Osteoblastogenesis is regulated by a network of cytokines under physiological and pathophysiological conditions. Osteoblastogenic cytokines, such as interleukin-10 (IL-10), IL-11, IL-18, interferon-γ (IFN-γ), cardiotrophin-1 and oncostatin M, promote osteoblastogenesis, whereas anti-osteoblastogenic cytokines, such as tumor necrosis factor-α (TNF-α), TNF-β, IL-1α, IL-4, IL-7, IL-12, IL-13, IL-23, IFN-α, IFN-β, leukemia inhibitory factor, cardiotrophin-like cytokine, and ciliary neurotrophic factor, downregulate osteoblastogenesis. Although there are gaps in the body of knowledge regarding the interplay of cytokine networks in osteoblastogenesis, cytokines appear to be potential therapeutic targets in bone-related diseases. Thus, in this study, we review and discuss our osteoblast, osteoblast differentiation, osteoblastogenesis, cytokines, signaling pathway of cytokine networks in osteoblastogenesis.

Longitudinal Functional Study of Murine Aging: A Resource for Future Study Designs

Aging is characterized by systemic declines in tissue and organ functions. Interventions that slow these declines represent promising therapeutics to protect against age-related disease and improve the quality of life. In this study, several interventions associated with lifespan extension in invertebrates or improvement of age-related disease were tested in mouse models to determine if they were effective in slowing tissue aging in a broad spectrum of functional assays. Benzoxazole, which extends the lifespan of Caenorhabditis elegans, slowed age-related femoral bone loss in mice. Rates of change were established for clinically significant parameters in untreated mice, including kyphosis, blood glucose, body composition, activity, metabolic measures, and detailed parameters of skeletal aging in bone. These findings have implications for the study of preclinical physiological aging and therapies targeting aging. Finally, an online application was created that includes the calculated rates of change and that enables power and variance to be calculated for many clinically important metrics of aging with an emphasis on bone. This resource will help in future study designs employing novel interventions in aging mice. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Heterogeneity of murine periosteum progenitors involved in fracture healing

The periosteum is the major source of cells involved in fracture healing. We sought to characterize progenitor cells and their contribution to bone fracture healing. The periosteum is highly enriched with progenitor cells, including Sca1+ cells, fibroblast colony-forming units, and label-retaining cells compared to the endosteum and bone marrow. Using lineage tracing, we demonstrate that alpha smooth muscle actin (αSMA) identifies long-term, slow-cycling, self-renewing osteochondroprogenitors in the adult periosteum that are functionally important for bone formation during fracture healing. In addition, Col2.3CreER-labeled osteoblast cells contribute around 10% of osteoblasts but no chondrocytes in fracture calluses. Most periosteal osteochondroprogenitors following fracture can be targeted by αSMACreER. Previously identified skeletal stem cell populations were common in periosteum but contained high proportions of mature osteoblasts. We have demonstrated that the periosteum is highly enriched with skeletal progenitor cells, and there is heterogeneity in the populations of cells that contribute to mature lineages during periosteal fracture healing.

Time-lapsed imaging of nanocomposite scaffolds reveals increased bone formation in dynamic compression bioreactors

Progress in bone scaffold development relies on cost-intensive and hardly scalable animal studies. In contrast to in vivo, in vitro studies are often conducted in the absence of dynamic compression. Here, we present an in vitro dynamic compression bioreactor approach to monitor bone formation in scaffolds under cyclic loading. A biopolymer was processed into mechanically competent bone scaffolds that incorporate a high-volume content of ultrasonically treated hydroxyapatite or a mixture with barium titanate nanoparticles. After seeding with human bone marrow stromal cells, time-lapsed imaging of scaffolds in bioreactors revealed increased bone formation in hydroxyapatite scaffolds under cyclic loading. This stimulatory effect was even more pronounced in scaffolds containing a mixture of barium titanate and hydroxyapatite and corroborated by immunohistological staining. Therefore, by combining mechanical loading and time-lapsed imaging, this in vitro bioreactor strategy may potentially accelerate development of engineered bone scaffolds and reduce the use of animals for experimentation.

Schädli et al. present a bioreactor system that combines mechanical loading with longitudinal microCT imaging to assess bone mineralization in a poly(lactic-co-glycolic acid) (PLGA) scaffold reinforced with nanoparticles. This approach allows rapid and rigorous evaluation of engineered bone scaffolds performance in vitro and might reduce the use of animals for experimentation.

Bioglass/carbonate apatite/collagen composite scaffold dissolution products promote human osteoblast differentiation

OssiMend® Bioactive (Collagen Matrix Inc., NJ) is a three-component porous composite bone graft device of 45S5 Bioglass/carbonate apatite/collagen. Our in vitro studies showed that conditioned media of the dissolution products of OssiMend Bioactive stimulated primary human osteoblasts to form mineralized bone-like nodules in vitro in one week, in basal culture media (no osteogenic supplements). Osteoblast differentiation was followed by gene expression analysis and a mineralization assay. In contrast, the dissolution products from commercial OssiMend (Bioglass-free carbonate apatite/collagen scaffolds), or from 45S5 Bioglass particulate alone, did not induce the mineralization of the extracellular matrix, but did induce osteoblast differentiation to mature osteoblasts, evidenced by the strong upregulation of BGLAP and IBSP mRNA levels. The calcium ions and soluble silicon species released from 45S5 Bioglass particles and additional phosphorus release from OssiMend mediated the osteostimulatory effects. Medium conditioned with OssiMend Bioactive dissolution had a much higher concentration of phosphorus and silicon than media conditioned with OssiMend and 45S5 Bioglass alone. While OssiMend and OssiMend Bioactive led to calcium precipitation in cell culture media, OssiMend Bioactive produced a higher concentration of soluble silicon than 45S5 Bioglass and higher dissolution of phosphorus than OssiMend. These in vitro results suggest that adding 45S5 Bioglass to OssiMend produces a synergistic osteostimulation effect on primary human osteoblasts. In summary, dissolution products of a Bioglass/carbonate apatite/collagen composite scaffold (OssiMend® Bioactive) stimulate human osteoblast differentiation and mineralization of extracellular matrix in vitro without any osteogenic supplements. The mineralization was faster than for dissolution products of ordinary Bioglass.

The effect of TNF-α on osteoblasts in metal wear-induced periprosthetic bone loss

Aims

This study aimed to examine the effects of tumour necrosis factor-alpha (TNF-α) on osteoblasts in metal wear-induced bone loss.

Methods

TNF-α immunoexpression was examined in periprosthetic tissues of patients with failed metal-on-metal hip arthroplasties and also in myeloid MM6 cells after treatment with cobalt ions. Viability and function of human osteoblast-like SaOs-2 cells treated with recombinant TNF-α were studied by immunofluorescence, terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay, western blotting, and enzyme-linked immunosorbent assay (ELISA).

Results

Macrophages, lymphocytes, and endothelial cells displayed strong TNF-α immunoexpression in periprosthetic tissues containing metal wear debris. Colocalization of TNF-α with the macrophage marker CD68 and the pan-T cell marker CD3 confirmed TNF-α expression in these cells. Cobalt-treated MM6 cells secreted more TNF-α than control cells, reflecting the role of metal wear products in activating the TNF-α pathway in the myeloid cells. While TNF-α did not alter the immunoexpression of the TNF-receptor 1 (TNF-R1) in SaOs-2 cells, it increased the release of the soluble TNF-receptor 1 (sTNF-R1). There was also evidence for TNF-α-induced apoptosis. TNF-α further elicited the expression of the endoplasmic reticulum stress markers inositol-requiring enzyme (IRE)-1α, binding-immunoglobulin protein (BiP), and endoplasmic oxidoreductin1 (Ero1)-Lα. In addition, TNF-α decreased pro-collagen I α 1 secretion without diminishing its synthesis. TNF-α also induced an inflammatory response in SaOs-2 cells, as evidenced by the release of reactive oxygen and nitrogen species and the proinflammatory cytokine vascular endothelial growth factor.

Conclusion

The results suggest a novel osteoblastic mechanism, which could be mediated by TNF-α and may be involved in metal wear debris-induced periprosthetic bone loss.

Cite this article: Bone Joint Res 2020;9(11):827–839.

Potential Role of Lycopene in the Prevention of Postmenopausal Bone Loss: Evidence from Molecular to Clinical Studies

Osteoporosis is a metabolic bone disease characterized by reduced bone mineral density, which affects the quality of life of the aging population. Furthermore, disruption of bone microarchitecture and the alteration of non-collagenous protein in bones lead to higher fracture risk. This is most common in postmenopausal women. Certain medications are being used for the treatment of osteoporosis; however, these may be accompanied by undesirable side effects. Phytochemicals from fruits and vegetables are a source of micronutrients for the maintenance of bone health. Among them, lycopene has recently been shown to have a potential protective effect against bone loss. Lycopene is a lipid-soluble carotenoid that exists in both all-trans and cis-configurations in nature. Tomato and tomato products are rich sources of lycopene. Several human epidemiological studies, supplemented by in vivo and in vitro studies, have shown decreased bone loss following the consumption of lycopene/tomato. However, there are still limited studies that have evaluated the effect of lycopene on the prevention of bone loss in postmenopausal women. Therefore, the aim of this review is to summarize the relevant literature on the potential impact of lycopene on postmenopausal bone loss with molecular and clinical evidence, including an overview of bone biology and the pathophysiology of osteoporosis.

Estrogens decrease osteoclast number by attenuating mitochondria oxidative phosphorylation and ATP production in early osteoclast precursors

Loss of estrogens at menopause is a major cause of osteoporosis and increased fracture risk. Estrogens protect against bone loss by decreasing osteoclast number through direct actions on cells of the myeloid lineage. Here, we investigated the molecular mechanism of this effect. We report that 17β-estradiol (E₂) decreased osteoclast number by promoting the apoptosis of early osteoclast progenitors, but not mature osteoclasts. This effect was abrogated in cells lacking Bak/Bax-two pro-apoptotic members of the Bcl-2 family of proteins required for mitochondrial apoptotic death. FasL has been previously implicated in the pro-apoptotic actions of E₂. However, we show herein that FasL-deficient mice lose bone mass following ovariectomy indistinguishably from FasL-intact controls, indicating that FasL is not a major contributor to the anti-osteoclastogenic actions of estrogens. Instead, using microarray analysis we have elucidated that ERα-mediated estrogen signaling in osteoclast progenitors decreases "oxidative phosphorylation" and the expression of mitochondria complex I genes. Additionally, E₂ decreased the activity of complex I and oxygen consumption rate. Similar to E₂, the complex I inhibitor Rotenone decreased osteoclastogenesis by promoting osteoclast progenitor apoptosis via Bak/Bax. These findings demonstrate that estrogens decrease osteoclast number by attenuating respiration, and thereby, promoting mitochondrial apoptotic death of early osteoclast progenitors.

BMP9 is a potential therapeutic agent for use in oral and maxillofacial bone tissue engineering

Oral and maxillofacial surgery is often challenging due to defective bone healing owing to the microbial environment of the oral cavity, the additional involvement of teeth and esthetic concerns. Insufficient bone volume as a consequence of aging and some oral and maxillofacial surgical procedures, such as tumor resection of the jaw, may further impact facial esthetics and cause the failure of certain procedures, such as oral and maxillofacial implantation. Bone morphogenetic protein (BMP) 9 (BMP9) is one of the most effective BMPs to induce the osteogenic differentiation of different stem cells. A large cross-talk network that includes the BMP9, Wnt/β, Hedgehog, EGF, TGF-β and Notch signaling pathways finely regulates osteogenesis induced by BMP9. Epigenetic control during BMP9-induced osteogenesis is mainly dependent on histone deacetylases (HDACs), microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), which adds another layer of complexity. As a result, all these factors work together to orchestrate the molecular and cellular events underlying BMP9-related tissue engineering. In this review, we summarize our current understanding of the SMAD-dependent and SMAD-independent BMP9 pathways, with a particular focus on cross-talk and cross-regulation between BMP9 and other major signaling pathways in BMP9-induced osteogenesis. Furthermore, recently discovered epigenetic regulation of BMP9 pathways and the molecular and cellular basis of the application of BMP9 in tissue engineering in current oral and maxillofacial surgery and other orthopedic-related clinical settings are also discussed.

Multifactorial effects of hyperglycaemia, hyperinsulinemia and inflammation on bone remodelling in type 2 diabetes mellitus

Bones undergo continuous cycles of bone remodelling that rely on the balance between bone formation and resorption. This balance allows the bone to adapt to changes in mechanical loads and repair microdamages. However, this balance is susceptible to upset in various conditions, leading to impaired bone remodelling and abnormal bones. This is usually indicated by abnormal bone mineral density (BMD), an indicator of bone strength. Despite this, patients with type 2 diabetes mellitus (T2DM) exhibit normal to high BMD, yet still suffer from an increased risk of fractures. The activity of the bone cells is also altered as indicated by the reduced levels of bone turnover markers in T2DM observed in the circulation. The underlying mechanisms behind these skeletal outcomes in patients with T2DM remain unclear. This review summarises recent findings regarding inflammatory cytokine factors associated with T2DM to understand the mechanisms involved and considers potential therapeutic interventions.