Mechanical loading stimulates BMP7, but not BMP2, production by osteocytes

Changes in load distribution after unilateral condylar fracture: A finite element model study

OBJECTIVE

Premature dental contact on the fractured side and a contralateral open bite are signs of a unilaterally fractured condyle of the temporomandibular joint (TMJ). The lateral pterygoid muscle pulls the condyle inwards, causing angulation of the fractured part and shortening of the ramus. This imbalance after fracture might change the load in both TMJs and consequently induce remodeling. The present study aimed to calculate this change in load. It is hypothesized to decrease on the fractured side and increase on the non-fractured side.

DESIGN

For these calculations, a finite element model (FEM) was used. In the FEM, shortening of the ramus varied from 2 mm to 16 mm; angulation, from 6.25° to 50°.

RESULTS

After fracture, load on the non-fractured side increased, but only at maximal mouth opening (MMO). Simultaneously, load on the fractured side decreased, at both timepoints, i.e., MMO and closed mouth. When comparing all simulations at those time points, i.e., from 2 mm and 6.25° to 16 mm and 50°, the load in the fractured condyle declines steadily. However, for both timepoints, a threshold stands out around 6 mm shortening and 18.75° angulation: visualization of the fractured condyle showed, apart from load on the condylar head, a second point of load more medial in the TMJ which was most evident in the 6 mm - 18.75° simulation.

CONCLUSIONS

These findings could implicate that the balance between both TMJs is more difficult to restore after a fracture with more than 6 mm shortening and more than 18.75° angulation.

Contact stress distribution after unilateral condylar fracture with angulation of the fractured part: A finite element model study

After a fracture of the condyle, the head of the condyle is often pulled inwards, which causes the fractured part to angulate medially. This change can cause a disbalance in the masticatory system. The disbalance could lead to contact stress differences within the temporomandibular joints (TMJs) which might induce remodelling within the TMJ to restore the balance. The contact stress in the fractured condyle during open and closing movements is expected to decrease, while the contact stress in the non-fractured condyle will increase. In a clinical situation this is hard to investigate. Therefore, a finite element model (FEM) was used. In the FEM a fractured right condyle with an angulation was induced, which was placed at different degrees, varying from 5° to 50° in steps of 5°. This study shows only minor differences in amount of contact stress between the fractured and the non-fractured condyle. The amount of contact stress in the condyles does not increase with a higher degree of angulation. However, with larger angulations, the contact stress within the fractured condyle is more centralized. Clinically, this more centralized area could be associated with complaints, such as pain. In conclusion, due to the more centralized contact stress in the fractured condyle, one would expect some minor remodelling on the fractured side with more angulation.

The Role of Sclerostin in Rheumatic Diseases: A Review

Systemic connective tissue disorders constitute a heterogenous group of autoimmune diseases with the potential to affect a range of organs. Rheumatoid arthritis (RA) is a chronic, progressive, autoimmune inflammatory disease affecting the joints. Systemic lupus erythematosus (SLE) may manifest with multiple system involvement as a result of inflammatory response to autoantibodies. Spondyloarthropathies (SpAs) such as ankylosing spondylitis (AS) or psoriatic arthritis (PsA) are diseases characterised by the inflammation of spinal joints, paraspinal tissues, peripheral joints and enthesitis as well as inflammatory changes in many other systems and organs. Physiologically, sclerostin helps to maintain balance in bone tissue metabolism through the Wnt/β-catenin pathway, which represents a major intracellular signalling pathway. This review article aims to present the current knowledge on the role of sclerostin in the Wnt/β-catenin pathway and its correlation with clinical data from RA, SLE, AS and PsA patients.

Load distribution after unilateral condylar fracture with shortening of the ramus: a finite element model study

OBJECTIVES

After a fracture of the condyle, the fractured ramus is often shortened, which causes premature dental contact on the fractured side and a contralateral open bite. The imbalance could change the load in the temporomandibular joints (TMJs). This change could lead to remodelling of the TMJs to compensate for the imbalance in the masticatory system. The load in the non-fractured condyle is expected to increase, and the load in the fractured condyle to decrease.

MATERIALS AND METHODS

These changes cannot be measured in a clinical situation. Therefore a finite element model (FEM) of the masticatory system was used. In the FEM a fractured right condyle with shortening of the ramus was induced, which varied from 2 to 16 mm.

RESULTS

Results show that, with a larger shortening of the ramus, the load in the fractured condyle decreases and the load in the non-fractured condyle increases. In the fractured condyle during closed mouth a major descent in load, hence a cut-off point, was visible between a shortening of 6 mm and 8 mm.

CONCLUSIONS

In conclusion, the change of load could be associated with remodelling on both condyles due to shortening of the ramus.

CLINICAL RELEVANCE

The cut-off point implies that shortening over 6 mm could present more difficulty for the body to compensate.

Large-scale osteocyte lacunar morphological analysis of transiliac bone in normal and osteoporotic premenopausal women

Bone's ability to adapt is governed by the network of embedded osteocytes, which inhabit individual pores called lacunae. The morphology of these lacunae and their resident osteocytes are known to change with age and diseases such as postmenopausal osteoporosis. However, it is unclear whether alterations in lacunar morphology are present in younger populations with osteoporosis. To investigate this, we implemented a previously validated methodology to image and quantify the three-dimensional morphometries of lacunae on a large scale with ultra-high-resolution micro-computed tomography (microCT) in transiliac bone biopsies from three groups of premenopausal women: control n = 39; idiopathic osteoporosis (IOP) n = 45; idiopathic low BMD (ILBMD) n = 19. Lacunar morphometric parameters were measured in both trabecular and cortical bone such as lacunar density (Lc.N/BV), lacunar volume (Lc.V), and lacunar sphericity (Lc.Sr). These were then compared against each other and also with previously measured tissue morphometries such as bone volume density (BV/TV), trabecular separation (Tb.Sp), trabecular number (Tb.N), and others. We detected no differences in lacunar morphology between the IOP, ILBMD and healthy premenopausal women. In contrast, we did find significant differences between lacunar morphologies including Lc.N/BV, Lc. V, and Lc. Sr in cortical and trabecular regions within all three groups (p < 0.001), which was consistent with our previous findings on a subgroup of the healthy group. Furthermore, we discovered strong correlations between Lc. Sr from trabecular regions with the measured BV/TV (R = -0.90, p < 0.05). The findings and comprehensive lacunar dataset we present here will be a crucial foundation for future investigations of the relationship between osteocyte lacunar morphology and disease.

Glucocorticoid counteracts cellular mechanoresponses by LINC01569-dependent glucocorticoid receptor-mediated mRNA decay

Mechanical stimuli on cells and mechanotransduction are essential in many biological and pathological processes. Glucocorticoid is an important hormone, roles, and mechanisms of which in cellular mechanotransduction remain unknown. Here, we report that glucocorticoid counteracted cellular mechanoresponses dependently on a novel long noncoding RNA (lncRNA), LINC01569 Further, LINC01569 mediated glucocorticoid effects on mechanotransduction by destabilizing messenger RNA (mRNA) of mechanosensors including early growth response protein 1 (EGR1), Cbp/P300-interacting transactivator 2 (CITED2), and bone morphogenic protein 7 (BMP7) in glucocorticoid receptor-mediated mRNA decay (GMD) manner. Mechanistically, LINC01569 directly bound to the GMD factor Y-box-binding protein 1 (YBX1). Then, the LINC01569-YBX1 complex was guided to the mRNAs of EGR1, CITED2, and BMP7 through specific LINC01569-mRNA interaction, thereby contributing to the successful assembly of GMD complex and triggering GMD. Our results uncovered roles of glucocorticoid in cellular mechanotransduction and novel lncRNA-dependent GMD machinery and provided potential strategy for early intervention in mechanical disorder-associated diseases.

Bone's Response to Mechanical Loading in Aging and Osteoporosis: Molecular Mechanisms

Mechanotransduction is pivotal in the maintenance of homeostasis in different tissues and involves multiple cell signaling pathways. In bone, mechanical stimuli regulate the balance between bone formation and resorption; osteocytes play a central role in this regulation. Dysfunctions in mechanotransduction signaling or in osteocytes response lead to an imbalance in bone homeostasis. This alteration is very relevant in some conditions such as osteoporosis and aging. Both are characterized by increased bone weakness due to different causes, for example, the increase of osteocyte apoptosis that cause an alteration of fluid space, or the alteration of molecular pathways. There are intertwined yet very different mechanisms involved among the cell-intrinsic effects of aging on bone, the cell-intrinsic and tissue-level effects of estrogen/androgen withdrawal on bone, and the effects of reduced mechanical loading on bone, which are all involved to some degree in how aged bone fails to respond properly to stress/strain compared to younger bone. This review aims at clarifying how the cellular and molecular pathways regulated and induced in bone by mechanical stimulation are altered with aging and in osteoporosis, to highlight new possible targets for antiresorptive or anabolic bone therapeutic approaches.

Bone turnover biomarkers, disease activity, and MRI changes of sacroiliac joints in patients with spondyloarthritis

The lack of valid biomarkers in patients with spondyloarthritis (SpA) requires searching for additional options to increase sacroiliac joint (SIJ) evaluation effectiveness. We assessed the serum levels of bone turnover markers and their relationships with active and chronic changes in SIJs using magnetic resonance imaging (MRI), indices, and laboratory parameters of disease activity in SpA patients. 102 patients with SpA and 15 healthy subjects were included. Testing of serum levels of transforming growth factor-beta (TGF-β1), Wnt3, sclerostin, and Dickkopf-1 (Dkk-1) was conducted. Active inflammatory lesions in SIJs were evaluated using Spondyloarthritis Research Consortium of Canada (SPARCC) MRI SIJ score, and chronic changes using the Danish scoring method. Bath Ankylosing Spondylitis Disease Activity Index, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), Ankylosing Spondylitis Disease Activity Scores with CRP, and ESR were used to assess disease activity. Serum levels of Dkk-1, TGF-β1, and sclerostin were significantly lower in SpA patients compared to healthy controls. The serum levels of Dkk-1 positively correlated with CRP. Dkk-1 had a significant negative correlation with Danish score. The sclerostin serum level had a weak negative correlation with the active inflammatory MRI SIJ lesions. There were positive correlations between TGF-β1 and sclerostin with Dkk-1, and negative correlation between Wnt3 and sclerostin. Dkk-1 positively correlated with CRP and negatively with chronic SIJ changes by Danish score. Sclerostin negatively correlated with the active SIJ lesions by SPARCC. This suggests that Dkk-1 and sclerostin are the most promising candidates to reveal inflammation and bone turnover in patients with SpA.

Bone, a Secondary Growth Site of Breast and Prostate Carcinomas: Role of Osteocytes

Bone is the primarily preferred site for breast and prostate cancer to metastasize. Bone metastases are responsible for most deaths related to breast and prostate cancer. The bone's particular microenvironment makes it conducive for the growth of cancer cells. Studies on bone metastasis have focused on the interaction between cancer cells and the bone microenvironment. Osteocytes, the most common cell type of bone tissue, have received little attention in bone metastasis, although they are master signal sensors, integrators, and skeleton transducers. They play an important role in regulating bone mass by acting on both osteoblasts and osteoclasts, through the release of proteins such as sclerostin, Dickkopf-1 (DKK-1), and fibroblast growth factor 23 (FGF23). Osteocytes have been extensively re-evaluated, in light of their multiple functions: with different experimental approaches, it has been shown that, indeed, osteocytes are actively involved in the colonization of bone tissue by cancer cells. The present review focuses on recent research on the role that osteocytes play in bone metastasis of breast and prostate cancers. Moreover, the studies here summarized open up perspectives for new therapeutic approaches focused on modulating the activity of osteocytes to improve the condition of the bone metastatic patients. A better understanding of the complex interactions between cancer cells and bone-resident cells is indispensable for identifying potential therapeutic targets to stop tumor progression and prevent bone metastases.

Osteocyte-Mediated Translation of Mechanical Stimuli to Cellular Signaling and Its Role in Bone and Non-bone-Related Clinical Complications

PURPOSE OF REVIEW

Osteocytes comprise > 95% of the cellular component in bone tissue and produce a wide range of cytokines and cellular signaling molecules in response to mechanical stimuli. In this review, we aimed to summarize the molecular mechanisms involved in the osteocyte-mediated translation of mechanical stimuli to cellular signaling, and discuss their role in skeletal (bone) diseases and extra-skeletal (non-bone) clinical complications.

RECENT FINDINGS

Two decades before, osteocytes were assumed as a dormant cells buried in bone matrix. In recent years, emerging evidences have shown that osteocytes are pivotal not only for bone homeostasis but also for vital organ functions such as muscle, kidney, and heart. Osteocyte mechanotransduction regulates osteoblast and osteoclast function and maintains bone homeostasis. Mechanical stimuli modulate the release of osteocyte-derived cytokines, signaling molecules, and extracellular cellular vesicles that regulate not only the surrounding bone cell function and bone homeostasis but also the distant organ function in a paracrine and endocrine fashion. Mechanical loading and unloading modulate the osteocytic release of NO, PGE₂, and ATPs that regulates multiple cellular signaling such as Wnt/β-catenin, RANKL/OPG, BMPs, PTH, IGF1, VEGF, sclerostin, and others. Therefore, the in-depth study of the molecular mechanism of osteocyte mechanotransduction could unravel therapeutic targets for various bone and non-bone-related clinical complications such as osteoporosis, sarcopenia, and cancer metastasis to bone.

BMP7-overexpressing bone marrow-derived mesenchymal stem cells (BMSCs) are more effective than wild-type BMSCs in healing fractures

Bone fractures are a worldwide public health concern. Previous studies have demonstrated that bone morphogenetic protein-7 (BMP7) gene transfer or mesenchymal stem cells (MSCs) transplantation may be a promising novel therapeutic approach. Therefore, the aim of the present study was to observe the effect of bone BMP7 transfer to MSCs on fracture healing. Bone marrow-derived MSCs (BMSCs) from New Zealand white rabbits were isolated and identified using flow cytometry. A recombinant BMP7 overexpressing adenovirus vector (Adv) was constructed and transfected into BMSCs. The expression of BMP7 was detected by reverse transcription-polymerase chain reaction, immunofluorescence and western blotting. The present study additionally investigated the effect of BMP7 on the differentiation capacity of BMSCs. Finally, tissue-engineered bone was created with support material to verify the effect of BMP7-BMSCs on fracture healing. The results demonstrated that the expression of BMP7 was increased at the mRNA and protein levels in BMSCs following transfection with BMP7 overexpressing Adv. The results additionally demonstrated that the expression of BMP7 enhanced the differentiation capacity of bone marrow mesenchymal stem cells and had a promotional effect on fracture healing. Overall, these data suggest that Adv-BMP7 is useful for introducing foreign genes into BMSCs and will be a powerful gene therapy tool for bone regeneration and other tissue engineering applications in the future.

Strontium and strontium ranelate: Historical review of some of their functions

The review covers historical and last decade's scientific literature on the biological and clinical role of strontium (Sr) and strontium ranelate (Sr RAN). It enrols the description of the main effects of Sr on supportive tissue, its proven and possible morphopathogenetical mechanisms and the interaction with the bone, and especially focuses on the Sr ability to inhibit osteoclasts and affect the programmed cell death. The main experimental and clinical experience regarding the Sr RAN influence in the treatment of osteoporosis and the search for correct doses is also highlighted. The review gives insight into the role of Sr/Sr RAN on stem cells, apoptosis, animal and clinical research.

Silk fibroin as biomaterial for bone tissue engineering

Silk fibroin (SF) is a fibrous protein which is produced mainly by silkworms and spiders. Its unique mechanical properties, tunable biodegradation rate and the ability to support the differentiation of mesenchymal stem cells along the osteogenic lineage, have made SF a favorable scaffold material for bone tissue engineering. SF can be processed into various scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified, which provides an impressive toolbox and allows SF scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing SF, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

STATEMENT OF SIGNIFICANCE

Silk fibroin is a natural biomaterial with remarkable biomedical and mechanical properties which make it favorable for a broad range of bone tissue engineering applications. It can be processed into different scaffold forms, combined synergistically with other biomaterials to form composites and chemically modified which provides a unique toolbox and allows silk fibroin scaffolds to be tailored to specific applications. This review discusses and summarizes recent advancements in processing silk fibroin, focusing on different fabrication and functionalization methods and their application to grow bone tissue in vitro and in vivo. Potential areas for future research, current challenges, uncertainties and gaps in knowledge are highlighted.

Wnt/β-catenin signaling plays a key role in the development of spondyloarthritis

Spondyloarthritis (SpA) is a group of diseases consisting of psoriatic arthritis (PsA), reactive arthritis, arthritis related to inflammatory bowel disease (a subgroup of juvenile idiopathic arthritis), and ankylosing spondylitis (the prototype of SpA). Axial bone formation and the combination of concurrent erosion and new bone formation are specific characteristics of SpA disease. The use of antiproinflammatory cytokines, such as inhibitors of tumor necrosis factor α (TNF-α), appears to be the greatest advance in the treatment of SpA over the past 20 years. However, TNF-α blockers do not halt new bone formation. Recent clinical observations and animal studies demonstrate that Wnt signaling proteins and natural Wnt inhibitors, such as DKK1 and sclerostin, are likely to play important roles in the process of ankylosis in SpA, and could potentially serve as therapeutic targets for the treatment of SpA.

Review of the cellular and biological principles of distraction osteogenesis: An in vivo bioreactor tissue engineering model

Distraction osteogenesis (DO) is a widely used technique in plastic and orthopaedic surgery. During the process, mechanical force is applied to fractured bone to enhance the regenerative processes and induce new bone formation. Although there is an abundance of literature on the clinical process of DO, there is a distinct lack of focus on the underlying biological principles governing this process. DO follows the basic premises of tissue engineering. The mechanical stress stimulates mesenchymal stem cell differentiation down an osteoblastic lineage on a matrix background. The aim of this review is to give an overview of the current knowledge of the molecular mechanism governing this process.

Genetic risk factors for the development of osteonecrosis in children under age 10 treated for acute lymphoblastic leukemia

Osteonecrosis is a dose-limiting toxicity in the treatment of pediatric acute lymphoblastic leukemia (ALL). Prior studies on the genetics of osteonecrosis have focused on patients ≥10 years of age, leaving the genetic risk factors for the larger group of children <10 years incompletely understood. Here, we perform the first evaluation of genetic risk factors for osteonecrosis in children <10 years. The discovery cohort comprised 82 cases of osteonecrosis and 287 controls treated on Children's Oncology Group (COG) standard-risk ALL protocol AALL0331 (NCT00103285, https://clinicaltrials.gov/ct2/show/NCT00103285), with results tested for replication in 817 children <10 years treated on COG protocol AALL0232 (NCT00075725, https://clinicaltrials.gov/ct2/show/NCT00075725). The top replicated single nucleotide polymorphisms (SNPs) were near bone morphogenic protein 7 [BMP7: rs75161997, P = 5.34 × 10(-8) (odds ratio [OR] 15.0) and P = .0498 (OR 8.44) in the discovery and replication cohorts, respectively] and PROX1-antisense RNA1 (PROX1-AS1: rs1891059, P = 2.28 × 10(-7) [OR 6.48] and P = .0077 [OR 3.78] for the discovery and replication cohorts, respectively). The top replicated nonsynonymous SNP, rs34144324, was in a glutamate receptor gene (GRID2, P = 8.65 × 10(-6) [OR 3.46] and P = .0136 [OR 10.8] in the discovery and replication cohorts, respectively). In a meta-analysis, the BMP7 and PROX1-AS1 variants (rs75161997 and rs1891059, respectively) met the significance threshold of <5 × 10(-8). Top replicated SNPs were enriched in enhancers active in mesenchymal stem cells, and analysis of annotated genes demonstrated enrichment in glutamate receptor and adipogenesis pathways. These data may provide new insights into the pathophysiology of osteonecrosis.

Bone morphogenetic protein signaling in bone homeostasis

Bone morphogenetic proteins (BMPs) are cytokines belonging to the transforming growth factor-β (TGF-β) superfamily. They play multiple functions during development and tissue homeostasis, including regulation of the bone homeostasis. The BMP signaling pathway consists in a well-orchestrated manner of ligands, membrane receptors, co-receptors and intracellular mediators, that regulate the expression of genes controlling the normal functioning of the bone tissues. Interestingly, BMP signaling perturbation is associated to a variety of low and high bone mass diseases, including osteoporosis, bone fracture disorders and heterotopic ossification. Consistent with these findings, in vitro and in vivo studies have shown that BMPs have potent effects on the activity of cells regulating bone function, suggesting that manipulation of the BMP signaling pathway may be employed as a therapeutic approach to treat bone diseases. Here we review the recent advances on BMP signaling and bone homeostasis, and how this knowledge may be used towards improved diagnosis and development of novel treatment modalities. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Mechanical loading reduces inflammation-induced human osteocyte-to-osteoclast communication

Multiple factors contribute to bone loss in inflammatory diseases such as rheumatoid arthritis (RA), but circulating inflammatory factors and immobilization play a crucial role. Mechanical loading prevents bone loss in the general population, but the effects of mechanical loading in patients with RA are less clear. Therefore, we aimed to investigate whether mechanical stimuli reverse the stimulatory effect of RA serum on osteocyte-to-osteoclast communication. Human primary osteocytes were pretreated with 10 % RA serum or healthy control serum for 7 days, followed by 1 h ± mechanical loading by pulsating fluid flow (PFF). Nitric oxide (NO) and prostaglandin E2 were measured in the medium. Receptor activator of nuclear factor-kappaB ligand (RANKL), osteoprotegerin (OPG), interleukin-6 (IL-6), cyclooxygenase-2 (COX2), matrix-extracellular phosphoglycoprotein (MEPE), cysteine-rich protein 61 (CYR61), and SOST gene expression was quantified by qPCR. Osteoclast precursors were cultured with PFF-conditioned medium (PFF-CM) or static-conditioned medium (stat-CM), and osteoclast formation was assessed. RA serum alone did not affect IL-6, CYR61, COX2, MEPE, or SOST gene expression in osteocytes. However, RA serum enhanced the RANKL/OPG expression ratio by 3.4-fold, while PFF nullified this effect. PFF enhanced NO production to the same extent in control serum (2.6-3.5-fold) and RA serum-pretreated (2.7-3.6-fold) osteocytes. Stat-CM from RA serum-pretreated osteocytes enhanced osteoclastogenesis compared with stat-CM from control serum-pretreated osteocytes, while PFF nullified this effect. In conclusion, RA serum, containing inflammatory factors, did not alter the intrinsic capacity of osteocytes to sense mechanical stimuli, but upregulated osteocyte-to-osteoclast communication. Mechanical loading nullified this upregulation, suggesting that mechanical stimuli could contribute to the prevention of osteoporosis in inflammatory disease.

Biocompatibility of Polypyrrole with Human Primary Osteoblasts and the Effect of Dopants

Polypyrrole (PPy) is a conducting polymer that enables controlled drug release upon electrical stimulation. We characterized the biocompatibility of PPy with human primary osteoblasts, and the effect of dopants. We investigated the biocompatibility of PPy comprising various dopants, i.e. p-toluene sulfonate (PPy-pTS), chondroitin sulfate (PPy-CS), or dodecylbenzenesulfonate (PPy-DBS), with human primary osteoblasts. PPy-DBS showed the roughest appearance of all surfaces tested, and its wettability was similar to the gold-coated control. The average number of attached cells was 45% higher on PPy-DBS than on PPy-CS or PPy-pTS, although gene expression of the proliferation marker Ki-67 was similar in osteoblasts on all surfaces tested. Osteoblasts seeded on PPy-DBS or gold showed similar vinculin attachment points, vinculin area per cell area, actin filament structure, and Feret's diameter, while cells seeded on PPY-CS or PPY-pTS showed disturbed focal adhesions and were enlarged with disorganized actin filaments. Osteoblasts grown on PPy-DBS or gold showed enhanced alkaline phosphatase activity and osteocalcin gene expression, but reduced osteopontin gene expression compared to cells grown on PPy-pTS and PPy-CS. In conclusion, PPy doped with DBS showed excellent biocompatibility, which resulted in maintaining focal adhesions, cell morphology, cell number, alkaline phosphatase activity, and osteocalcin gene expression. Taken together, conducting polymers doped with DBS are well tolerated by osteoblasts. Our results could provide a basis for the development of novel orthopedic or dental implants with controlled release of antibiotics and pharmaceutics that fight infections or focally enhance bone formation in a tightly controlled manner.

Dynamics and cellular localization of Bmp2, Bmp4, and Noggin transcription in the postnatal mouse skeleton

Transcription of BMPs and their antagonists in precise spatiotemporal patterns is essential for proper skeletal development, maturation, maintenance, and repair. Nevertheless, transcriptional activity of these molecules in skeletal tissues beyond embryogenesis has not been well characterized. In this study, we used several transgenic reporter mouse lines to define the transcriptional activity of two potent BMP ligands, Bmp2 and Bmp4, and their antagonist, Noggin, in the postnatal skeleton. At 3 to 4 weeks of age, Bmp4 and Noggin reporter activity was readily apparent in most cells of the osteogenic or chondrogenic lineages, respectively, whereas Bmp2 reporter activity was strongest in terminally differentiated cells of both lineages. By 5 to 6 months, activity of the reporters had generally abated; however, the Noggin and Bmp2 reporters remained remarkably active in articular chondrocytes and persisted there indefinitely. We further found that endogenous Bmp2, Bmp4, and Noggin transcript levels in postnatal bone and cartilage mirrored the activity of their respective reporters in these tissues. Finally, we found that the activity of the Bmp2, Bmp4, and Noggin reporters in bone and cartilage at 3 to 4 weeks could be recapitulated in both osteogenic and chondrogenic culture models. These results reveal that Bmp2, Bmp4, and Noggin transcription persists to varying degrees in skeletal tissues postnatally, with each gene exhibiting its own cell type-specific pattern of activity. Illuminating these patterns and their dynamics will guide future studies aimed at elucidating both the causes and consequences of aberrant BMP signaling in the postnatal skeleton.

Nitric oxide is involved in the down-regulation of SOST expression induced by mechanical loading

Mechanical stimulation reduces sclerostin expression in rodents. However, few data are available about the effect of physical stimuli in human systems. Recently we observed that the demethylating agent AzadC induces SOST expression in bone cells. This allowed us in this study to explore the effect of mechanical loading on SOST expression by subjecting AzadC-treated human bone cells to pulsating fluid flow (PFF). PFF significantly decreased the AzadC-induced expression of SOST. This effect persisted for at least 24 h, and in fact SOST expression was lower at 24 h after PFF treatment than at 1 h after PFF treatment (PFF/static ratio 0.47 ± 0.04 vs. 0.63 ± 0.03 respectively, p = 0.03). The PFF-induced decrease in SOST expression was not due to a change in the methylation profile of the SOST promoter. However, PFF stimulated nitric oxide (NO) synthesis, which appeared essential for the PFF effect on SOST expression. In fact, the NO synthase inhibitor 1400 W prevented the effect of PFF on SOST expression. Moreover, the NO-donor SNAP decreased SOST mRNA in bone organ cultures. The conditioned medium (CM) of cells subjected to PFF induced a 38 ± 4 % decrease in SOST expression (p = 0.03) in static cultures and diminished the transcriptional activity of reporter vectors with the cloned SOST promoter (Static-CM: 1.47 ± 0.10 vs.

PFF-CM

0.78 ± 0.09, p = 0.02). This is consistent with a PFF-induced secretion of factors that modulate SOST. Our results suggest that NO and other soluble factors are involved in the inhibition of SOST expression by PFF.

Vitamin D endocrine system and osteocytes

The physiological role of the osteocyte, the most numerous of the three bone cell types, was significantly underestimated until recently. It is now known that they not only coordinate bone remodeling but also have an endocrine function as part of the regulatory network for calcium and phosphate homeostasis. Vitamin D and osteocytes interact in numerous ways to accomplish these activities. The major source of active vitamin D (1,25(OH)2D3) is the kidney but there is evidence that osteocytes can produce it as well. Renal 1,25(OH)2D3 regulates osteocyte production of fibroblast growth factor 23 (FGF23), a powerful phosphaturic factor with far-reaching physiological effects. The function of 1,25(OH)2D3 produced by osteocytes themselves is poorly understood and is an area of active research. Osteocytes affect local bone remodeling by producing regulatory factors for osteoblasts and osteoclasts in response to mechanical loading and to endocrine signals such as serum 1,25(OH)2D3. In addition, 1,25(OH)2D3 may inhibit mineralization in osteocyte lacunae. Whether 1,25(OH)2D3 has a role in osteocytic perilacunar remodeling is currently unknown. This short review presents the current state of our knowledge about the physiologically and clinically significant roles of vitamin D signaling in osteocytes.

Osteocyte differentiation is regulated by extracellular matrix stiffness and intercellular separation

Osteocytes are terminally differentiated bone cells, derived from osteoblasts, which are vital for the regulation of bone formation and resorption. ECM stiffness and cell seeding density have been shown to regulate osteoblast differentiation, but the precise cues that initiate osteoblast-osteocyte differentiation are not yet understood. In this study, we cultured MC3T3-E1 cells on (A) substrates of different chemical compositions and stiffnesses, as well as, (B) substrates of identical chemical composition but different stiffnesses. The effect of cell separation was investigated by seeding cells at different densities on each substrate. Cells were evaluated for morphology, alkaline phosphatase (ALP), matrix mineralisation, osteoblast specific genes (Type 1 collagen, Osteoblast specific factor (OSF-2)), and osteocyte specific proteins (dentin matrix protein 1 (DMP-1), sclerostin (Sost)). We found that osteocyte differentiation (confirmed by dendritic morphology, mineralisation, reduced ALP, Col type 1 and OSF-2 and increased DMP-1 and Sost expression) was significantly increased on soft collagen based substrates, at low seeding densities compared to cells on stiffer substrates or those plated at high seeding density. We propose that the physical nature of the ECM and the necessity for cells to establish a communication network contribute substantially to a concerted shift toward an osteocyte-like phenotype by osteoblasts in vitro.

Role of BMPs in the regulation of sclerostin as revealed by an epigenetic modifier of human bone cells

Sclerostin, encoded by the SOST gene, is specifically expressed by osteocytes. However osteoblasts bear a heavily methylated SOST promoter and therefore do not express SOST. Thus, studying the regulation of human SOST is challenged by the absence of human osteocytic cell lines. Herein, we explore the feasibility of using the induction of SOST expression in osteoblasts by a demethylating agent to study the mechanisms underlying SOST transcription, and specifically, the influence of bone morphogenetic proteins (BMPs). Microarray analysis and quantitative PCR showed that AzadC up-regulated the expression of several BMPs, including BMP-2, BMP-4 and BMP-6, as well as several BMP downstream targets. Recombinant BMP-2 increased the transcriptional activity of the SOST promoter cloned into a reporter vector. Likewise, exposing cells transfected with the vector to AzadC also resulted in increased transcription. On the other hand, inhibition of the canonical BMP signaling blunted the effect of AzadC on SOST. These results show that the AzadC-induced demethylation of the SOST promoter in human osteoblastic cells may be a valuable tool to study the regulation of SOST expression. As a proof of concept, it allowed us to demonstrate that BMPs stimulate SOST expression by a mechanism involving BMPR1A receptors and downstream Smad-dependent pathways.

Strontium ranelate affects signaling from mechanically-stimulated osteocytes towards osteoclasts and osteoblasts

Strontium Ranelate (SrRan) is used to decrease the risk of bone fractures. Any factor that alters the release of paracrine signals by osteocytes in response to mechanical stimuli potentially affects bone mass and structure, and thus fracture resistance. We hypothesized that SrRan affects paracrine signaling from mechanically-stimulated osteocytes towards osteoclast-precursors and osteoblasts. MLO-Y4 osteocytes were cultured for 24h with SrRan (0.1-3mM) and either or not mechanically stimulated by pulsating fluid flow (PFF; 0.7 ± 0.3 Pa, 5 Hz) for 60 min. Nitric oxide (NO) and prostaglandin E(2) (PGE(2)) release, and expression of mechanoresponsive genes were quantified. Conditioned medium (CM) from osteocytes was added to mouse bone marrow cells for 7 days to assess osteoclastogenesis, or MC3T3-E1 osteoblasts for 4-16 days to measure osteogenic gene expression. SrRan (3mM) enhanced NO and PGE(2) release to the same extent in static osteocytes (NO: 1.6-fold; PGE(2): 2.8-fold) and PFF-stimulated osteocytes (NO: 1.3-fold; PGE(2): 2.6-fold). CM from PFF-treated osteocytes without SrRan enhanced Ki67 expression but reduced Runx2 and Ocn expression in osteoblasts. This effect on gene expression was not observed with CM obtained from osteocytes treated with the combination of PFF and 3mM SrRan. CM from PFF-treated osteocytes inhibited osteoclastogenesis by 1.9-fold. The combination of PFF and 3mM SrRan reduced osteocyte-stimulated osteoclastogenesis even more strongly (4.3-fold). In conclusion, SrRan affects paracrine signaling between mechanically-stimulated MLO-Y4 osteocytes and both osteoblasts and osteoclast precursors. The positive effects of SrRan on bone fracture resistance may thus be partly explained by altered paracrine signaling by osteocytes.

Variation in osteocytes morphology vs bone type in turtle shell and their exceptional preservation from the Jurassic to the present

Here we describe variations in osteocytes derived from each of the three bone layers that comprise the turtle shell. We examine osteocytes in bone from four extant turtle species to form a morphological 'baseline', and then compare these with morphologies of osteocytes preserved in Cenozoic and Mesozoic fossils. Two different morphotypes of osteocytes are recognized: flattened-oblate osteocytes (FO osteocytes), which are particularly abundant in the internal cortex and lamellae of secondary osteons in cancellous bone, and stellate osteocytes (SO osteocytes), principally present in the interstitial lamellae between secondary osteons and external cortex. We show that the morphology of osteocytes in each of the three bone layers is conserved through ontogeny. We also demonstrate that these morphological variations are phylogenetically independent, as well as independent of the bone origin (intramembranous or endochondral). Preservation of microstructures consistent with osteocytes in the morphology in Cenozoic and Mesozoic fossil turtle bones appears to be common, and occurs in diverse diagenetic environments including marine, freshwater, and terrestrial deposits. These data have potential to illuminate aspects of turtle biology and evolution previously unapproachable, such as estimates of genome size of extinct species, differences in metabolic rates among different bones from a single individual, and potential function of osteocytes as capsules for preservation of ancient biomolecules.

Control of osteogenesis by the canonical Wnt and BMP pathways in vivo: cooperation and antagonism between the canonical Wnt and BMP pathways as cells differentiate from osteochondroprogenitors to osteoblasts and osteocytes

Although many regulators of skeletogenesis have been functionally characterized, one current challenge is to integrate this information into regulatory networks. Here, we discuss how the canonical Wnt and Smad-dependent BMP pathways interact together and play antagonistic or cooperative roles at different steps of osteogenesis, in the context of the developing vertebrate embryo. Early on, BMP signaling specifies multipotent mesenchymal cells into osteochondroprogenitors. In turn, the function of Wnt signaling is to drive these osteochondroprogenitors towards an osteoblastic fate. Subsequently, both pathways promote osteoblast differentiation, albeit with notable mechanistic differences. In osteocytes, the ultimate stage of osteogenic differentiation, the Wnt and BMP pathways exert opposite effects on the control of bone resorption by osteoclasts. We describe how the dynamic molecular wiring of the canonical Wnt and Smad-dependent BMP signaling into the skeletal cell genetic programme is critical for the generation of bone-specific cell types during development.

The nuclear localization of MGF receptor in osteoblasts under mechanical stimulation

Mechano-growth factor (MGF) has emerged as an important mechanosensitive player in bone repair, but understanding of MGF function is hampered by the fact that MGF receptor and the underlying pathways remain unknown. In this study, fluorescein isothiocyanate (FITC)-labeled MGF-Ct24E (FITC-MGF) was used to determine the subcellular localization of MGF receptor in osteoblasts. After the primary osteoblasts were exposed to stretch with the strain at 10 %, and/or loaded with 50 ng/ml exogenous MGF-Ct24E, cells were incubated with the different concentrations of FITC-MGF (0.01, 0.1, and 1 mg/ml) followed by flow cytometry and laser scanning confocal microscope analysis. Our results showed that the fluorescence intensity and cell population internalizing FITC-MGF increased with the concentration of FITC-MGF. And all the cells were labeled with fluorescence at 1 mg/ml. Notably, FITC-MGF had nuclear localization when osteoblasts were exposed to stretch and/or 50 ng/ml MGF-Ct24E added, compared to the evident cytoplasmic localization in the static culture group. The nuclear localization of FITC-MGF in response to mechanical loading was found to associate with high expression of proliferating cell nuclear antigen, suggesting MGF and its receptor could serve as potential messengers that replay information in nuclei to control cell proliferation.