A subset of osteoblasts expressing high endogenous levels of PPARgamma switches fate to adipocytes in the rat calvaria cell culture model

POT1a deficiency in mesenchymal niches perturbs B-lymphopoiesis

Protection of telomeres 1a (POT1a) is a telomere binding protein. A decrease of POT1a is related to myeloid-skewed haematopoiesis with ageing, suggesting that protection of telomeres is essential to sustain multi-potency. Since mesenchymal stem cells (MSCs) are a constituent of the hematopoietic niche in bone marrow, their dysfunction is associated with haematopoietic failure. However, the importance of telomere protection in MSCs has yet to be elucidated. Here, we show that genetic deletion of POT1a in MSCs leads to intracellular accumulation of fatty acids and excessive ROS and DNA damage, resulting in impaired osteogenic-differentiation. Furthermore, MSC-specific POT1a deficient mice exhibited skeletal retardation due to reduction of IL-7 producing bone lining osteoblasts. Single-cell gene expression profiling of bone marrow from POT1a deficient mice revealed that B-lymphopoiesis was selectively impaired. These results demonstrate that bone marrow microenvironments composed of POT1a deficient MSCs fail to support B-lymphopoiesis, which may underpin age-related myeloid-bias in haematopoiesis.

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

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

Dexamethasone Induces Changes in Osteogenic Differentiation of Human Mesenchymal Stromal Cells via SOX9 and PPARG, but Not RUNX2

Despite the huge body of research on osteogenic differentiation and bone tissue engineering, the translation potential of in vitro results still does not match the effort employed. One reason might be that the protocols used for in vitro research have inherent pitfalls. The synthetic glucocorticoid dexamethasone is commonly used in protocols for trilineage differentiation of human bone marrow mesenchymal stromal cells (hBMSCs). However, in the case of osteogenic commitment, dexamethasone has the main pitfall of inhibiting terminal osteoblast differentiation, and its pro-adipogenic effect is well known. In this work, we aimed to clarify the role of dexamethasone in the osteogenesis of hBMSCs, with a particular focus on off-target differentiation. The results showed that dexamethasone does induce osteogenic differentiation by inhibiting SOX9 expression, but not directly through RUNX2 upregulation as it is commonly thought. Rather, PPARG is concomitantly and strongly upregulated, leading to the formation of adipocyte-like cells within osteogenic cultures. Limiting the exposure to dexamethasone to the first week of differentiation did not affect the mineralization potential. Gene expression levels of RUNX2, SOX9, and PPARG were simulated using approximate Bayesian computation based on a simplified theoretical model, which was able to reproduce the observed experimental trends but with a different range of responses, indicating that other factors should be integrated to fully understand how dexamethasone influences cell fate. In summary, this work provides evidence that current in vitro differentiation protocols based on dexamethasone do not represent a good model, and further research is warranted in this field.

Runx2 silencing promotes adipogenesis via down-regulation of DLK1 in chondrogenic differentiating MSCs

BACKGROUND

For cartilage regeneration, stem cells are a promising cell source; however, even the advances made in the differentiation of stem cells into precursor-differentiated cartilage cells have not been successful with respect to reprograming these cells to achieve complete differentiation and fully functioning cells until now. Previous findings suggest that Runx2 plays a major role in chondrocyte differentiation and maturation. Although targeting Runx2 has enhanced some chondrocyte properties, the adipogenic lineage shift has eventually occurred in these cells. The present study mainly aimed to reveal the mechanism of this adipogenesis.

METHODS

To create inducible artificial shRNA-miR expressing vectors, the designed short hairpin RNAs (shRNAs) were inserted into the pri-mir-30 backbone, cloned into lentiviral pLVET-Tet-on, and transducted into mesenchymal stem cells (MSCs). Runx2 gene was silenced in MSCs either for 1 week or 4 weeks and cultured in the chondrogenic medium. At days 7, 14 and 28, cells were harvested, and chondrogenesis, adipogenesis and hypertrophic states were examined using histochemical staining and a real-time polymerase chain reaction assay.

RESULTS

The results showed that the designed shRNA-miR effectively targeted Runx2 in mRNA and protein levels. Chondrogenic markers were up-regulated in constantly silenced Runx2 group; however, adipogenic markers and fat droplets appeared gradually. DLK1 gene was also significantly down-regulated in this group, and overexpression of DLK1 abrogated adipogenesis in the Runx2 targeted group.

CONCLUSIONS

Based on these results, it can be concluded that DLK1 is responsible for the lineage shift in Runx2 targeted chondrogenic differentiating MSCs.

Alternating Differentiation and Dedifferentiation between Mature Osteoblasts and Osteocytes

Osteocytes are terminally differentiated osteoblasts embedded in the bone matrix. Evidence indicates that cells in the mesenchymal lineage possess plasticity. However, whether or not osteocytes have the capacity to dedifferentiate back into osteoblasts is unclear. This study aimed to clarify the dedifferentiation potential of osteocytes. Mouse calvarial osteoblasts were isolated and maintained in normal two-dimensional (2D) or collagen gel three-dimensional (3D) cultures. In 2D cultures, osteoblasts exhibited a typical fibroblast-like shape with high Alpl and minimal Sost, Fgf23, and Dmp1 expression and osteoblasts formed mineralised nodules. When these osteoblasts were transferred into 3D cultures, they showed a stellate shape with diminished cytoplasm and numerous long processes and expression of Alpl decreased while Sost, Fgf23, and Dmp1 were significantly increased. These cells were in cell cycle arrest and showed suppressed mineralisation, indicating that they were osteocytes. When these osteocytes were recovered from 3D cultures and cultured two-dimensionally again, they regained adequate cytoplasm and lost the long processes, resulting in a fibroblast-like shape. These cells showed high Alpl and low Sost, Fgf23, and Dmp1 expression with a high mineralisation capability, indicating that they were osteoblasts. This report shows that osteocytes possess the capacity to dedifferentiate back into mature osteoblasts without gene manipulation.

The Late Osteoblast/Preosteocyte Cell Line MLO-A5 Displays Mesenchymal Lineage Plasticity In Vitro and In Vivo

The process of osteoblast switching to alternative mesenchymal phenotypes is incompletely understood. In this study, we tested the ability of the osteoblast/preosteocyte osteogenic cell line, MLO-A5, to also differentiate into either adipocytes or chondrocytes. MLO-A5 cells expressed a subset of skeletal stem cell markers, including Sca-1, CD44, CD73, CD146, and CD166. Confluent cultures of cells underwent differentiation within 3 days upon the addition of osteogenic medium. The same cultures were capable of undergoing adipogenic and chondrogenic differentiation under lineage-appropriate culture conditions, evidenced by lineage-specific gene expression analysis by real-time reverse-transcription-PCR, and by Oil Red O and alcian blue (pH 2.5) staining, respectively. Subcutaneous implantation of MLO-A5 cells in a gel foam into NOD SCID mice resulted in a woven bone-like structure containing embedded osteocytes and regions of cartilage-like tissue, which stained positive with both alcian blue (pH 2.5) and safranin O. Together, our findings show that MLO-A5 cells, despite being a strongly osteogenic cell line, exhibit characteristics of skeletal stem cells and display mesenchymal lineage plasticity in vitro and in vivo. These unique characteristics suggest that this cell line is a useful model with which to study aging and disease-related changes to the mesenchymal lineage composition of bone.

How is mechanobiology involved in mesenchymal stem cell differentiation toward the osteoblastic or adipogenic fate?

Mechanobiology plays a major role in transducing physical cues from the dynamic cellular environment into biochemical modifications that promote cell-specific differentiation paths. Mesenchymal stem cells in the bone marrow or in other mesenchymal tissues will differentiate according to the expression of transcription factors (TFs) that govern their lineage commitment. The favoring of either osteogenic or adipogenic differentiation relies on TF expression as well as mechanical properties of the cells' niche that are translated into the activation of certain signaling pathways. Physical factors can induce significant shifts in bipotential lineage commitment between osteogenesis and adipogenesis. The stiffness of the extracellular matrix (ECM) surrounding a cell, varying greatly from rigid environments close to the bone surface to softer regions in the bone marrow, can influence the path of differentiation. Additionally, mechanical loading through exercise appears to favor osteogenesis whereas disuse conditions seem to promote adipogenesis.

Adipocytes spectrum - From homeostasia to obesity and its associated pathology

Firstly identified by anatomists, the fat tissue is nowadays an area of intense research due to increased global prevalence of obesity and its associated diseases. Histologically, there are four types of fat tissue cells which are currently recognized (white, brown, beige, and perivascular adipocytes). Therefore, in this study we are reviewing the most recent data regarding the origin, structure, and molecular mechanisms involved in the development of adipocytes. White adipocytes can store triglycerides as a consequence of lipogenesis, under the regulation of growth hormone or leptin and adiponectin, and release fatty acids resulted from lipolysis, under the regulation of the sympathetic nervous system, glucocorticoids, TNF-α, insulin, and natriuretic peptides. Brown adipocytes possess a mitochondrial transmembrane protein thermogenin or UCP1 which allows heat generation. Recently, thermogenic, UCP positive adipocytes have been identified in the subcutaneous white adipose tissue and have been named beige adipocytes. The nature of these cells is still controversial, as current theories are suggesting their origin either by transdifferentiation of white adipocytes, or by differentiation from an own precursor cell. Perivascular adipocytes surround most of the arteries, exhibiting a supportive role and being involved in the maintenance of intravascular temperature. Thoracic perivascular adipocytes resemble brown adipocytes, while abdominal ones are more similar to white adipocytes and, consequently, are involved in obesity-induced inflammatory reactions. The factors involved in the regulation of adipose stem cells differentiation may represent potential pathways to inhibit or to divert adipogenesis. Several molecules, such as pro-adipogenic factors (FGF21, BMP7, BMP8b, and Cox-2), cell surface proteins or receptors (Asc-1, PAT2, P2RX5), and hypothalamic receptors (MC4R) have been identified as the most promising targets for the development of future therapies. Further investigations are necessary to complete the knowledge about adipose tissue and the development of a new generation of therapeutic tools based on molecular targets.

Osteoblast versus Adipocyte: Bone Marrow Microenvironment-Guided Epigenetic Control

The commitment and differentiation of bone marrow mesenchymal stem cells (MSCs) is tightly controlled by the local environment ensuring lineage differentiation balance and bone homeostasis. However, pathological conditions linked with osteoporosis have changed the bone marrow microenvironment, shifting MSCs’ fate to favor adipocytes over osteoblasts, and consequently leading to decreased bone mass with marrow fat accumulation. Multiple questions related to the underlying mechanisms remain to be answered. As recent findings have confirmed the fundamental role of the epigenetic mechanism in connecting environmental signals with gene expression and stem cell differentiation, a regulatory network in the bone marrow microenvironment, epigenetic modulation, gene expression, and MSC differentiation begins to emerge. This review discusses how pathological environmental factors affect MSCs’ fate by epigenetic modulating lineage-specific genes. We conclude that manipulating local environments and/or the epigenetic regulatory machinery that target the adipocyte differentiation pathway might be a therapeutic implication of bone loss diseases such as osteoporosis.

The Roles of Long Non-Protein-Coding RNAs in Osteo-Adipogenic Lineage Commitment

Osteoblasts and adipocytes share a common mesenchymal progenitor in the bone marrow. This implies that a reciprocal relationship exists between osteogenic and adipogenic differentiation. Further, cells of osteoblast lineage transdifferentiate into adipocytes under some circumstances. Dysregulation of osteo-adipogenic fate-determination leads to bone diseases such as osteoporosis, accompanied by an increase in bone marrow adipose tissue. Thus, the fine-tuning of osteo-adipogenesis is necessary for bone homeostasis. Osteo-adipogenic progression is governed by a complex crosstalk of extrinsic signals, transcription factors, and epigenetic factors. Long non-protein-coding RNAs (lncRNAs) act in part as epigenetic regulators in a broad range of biological activities, such as chromatin organization, transcriptional regulation, post-translational modifications, and histone modification. In this review, we highlight the roles of epigenetic regulators, particularly lncRNAs, in the osteo-adipogenic lineage commitment of bone marrow mesenchymal stem cells and the adipogenic transdifferentiation of osteoblasts.

Bone marrow sFRP5 level is negatively associated with bone formation markers

Secreted frizzled-related protein 5 (sFRP5) level in bone marrow environment is inversely correlated with bone formation markers, suggesting that it decreases bone mass by inhibiting bone formation. Besides, it functions in a local fashion when regulating bone metabolism. sFRP5 may be a target when developing anti-osteoporotic agents.

INTRODUCTION

The purpose of the study is to investigate the relationship between bone marrow sFRP5 level and bone turnover state.

METHODS

Eighty-three total knee arthroplasty patients were enrolled in this study. Data were collected prospectively and reviewed retrospectively. Lumbar spine and femoral neck bone mineral density (BMD), marrow adipose tissue (MAT) sFRP5 messenger RNA (mRNA) expression level, sFRP5 concentrations in marrow fluid and serum, concentrations of bone formation and resorption markers were measured for each participant.

RESULTS

Marrow fluid sFRP5 concentration was positively correlated with both MAT sFRP5 expression (p = 0.040) and serum sFRP5 concentration (p = 0.043). Significantly positive correlation existed between MAT sFRP5 expression level and BMD (p < 0.05). Marrow fluid sFRP5 concentration had a moderate but not significant positive association with BMD. MAT sFRP5 was negatively related to serum bone formation markers including N-terminal propeptide of type 1 procollagen (P1NP) (p = 0.011), osteocalcin (OC), and alkaline phosphatase (ALP). Marrow fluid and serum sFRP5 concentrations also had mild negative correlations with bone formation markers but reached no significance. There was no significant correlation between bone resorption marker β-crosslaps (β-CTX) and sFRP5. The mRNA expression level of MAT sFRP5 was positively related with those of MAT leptin, peroxisome proliferator-activated receptor-γ (PPARγ), and adiponectin, and its correlation with leptin was statistically significant (p = 0.026).

CONCLUSIONS

Bone marrow sFRP5 level is closely correlated with BMD and bone formation markers. sFRP5 may be a potential negative regulator of bone mass by inhibiting bone formation. It may exert its effects on bone metabolism in a paracrine, rather than endocrine manner.

Effects of collagen membrane application and cortical bone perforation on de novo bone formation in periosteal distraction: an experimental study in a rabbit calvaria

OBJECTIVES

The aim of the present study was to assess the impact of collagen membrane application and cortical bone perforations in periosteal distraction osteogenesis.

STUDY DESIGN

A total of 32 New Zealand rabbits were randomized into four experimental groups, considering two treatment modalities. Calvarial bone was perforated or left intact (P+/-). In half the animals, the distraction mesh was covered with a collagen membrane (M+/-). All animals were subjected to a 7-day latency period and a 10-day distraction period. The samples were harvested after 4-week and 8-week consolidation periods and analyzed histologically and by means of micro-computed tomography.

RESULTS

Primary, woven bone observed at the 4-week consolidation period was gradually replaced by lamellar bone at the 8-week consolidation period. Significant increase in bone volume was found in all groups (P < .001) and in bone mineral density in groups I (P-/M-; P < .001), III (P+/M-; P < .001), and IV (P+/M+; P = .013). Group III (P+/M-) showed significantly more new bone at the 8-week consolidation period compared with the other three groups (P = .001), with no differences observed in bone mineral density between groups at a given time-point.

CONCLUSIONS

In the present model, cortical bone perforations have more impact on the osteogenic process compared with the application of a collagen membrane.

The Role of Bone Marrow Microenvironment in Governing the Balance between Osteoblastogenesis and Adipogenesis

In the adult bone marrow, osteoblasts and adipocytes share a common precursor called mesenchymal stem cells (MSCs). The plasticity between the two lineages has been confirmed over the past decades, and has important implications in the etiology of bone diseases such as osteoporosis, which involves an imbalance between osteoblasts and adipocytes. The commitment and differentiation of bone marrow (BM) MSCs is tightly controlled by the local environment that maintains a balance between osteoblast lineage and adipocyte. However, pathological conditions linked to osteoporosis can change the BM microenvironment and shift the MSC fate to favor adipocytes over osteoblasts, and consequently decrease bone mass with marrow fat accumulation. This review discusses the changes that occur in the BM microenvironment under pathological conditions, and how these changes affect MSC fate. We suggest that manipulating local environments could have therapeutic implications to avoid bone loss in diseases like osteoporosis.

Intrinsic Sex-Linked Variations in Osteogenic and Adipogenic Differentiation Potential of Bone Marrow Multipotent Stromal Cells

Bone formation and aging are sexually dimorphic. Yet, definition of the intrinsic molecular differences between male and female multipotent mesenchymal stromal cells (MSCs) in bone is lacking. This study assessed sex-linked differences in MSC differentiation in 3-, 6-, and 9-month-old C57BL/6J mice. Analysis of tibiae showed that female mice had lower bone volume fraction and higher adipocyte content in the bone marrow compared to age-matched males. While both males and females lost bone mass in early aging, the rate of loss was higher in males. Similar expression of bone- and adipocyte-related genes was seen in males and females at 3 and 9 months, while at 6 months, females exhibited a twofold greater expression of these genes. Under osteogenic culture conditions, bone marrow MSCs from female 3- and 6-month-old mice expressed similar levels of bone-related genes, but significantly greater levels of adipocyte-related genes, than male MSCs. Female MSCs also responded to rosiglitazone-induced suppression of osteogenesis at a 5-fold lower (10 nM) concentration than male MSCs. Female MSCs grown in estrogen-stripped medium showed similar responses to rosiglitazone as MSCs grown in serum containing estrogen. MSCs from female mice that had undergone ovariectomy before sexual maturity also were sensitive to rosiglitazone-induced effects on osteogenesis. These results suggest that female MSCs are more sensitive to modulation of differentiation by PPARγ and that these differences are intrinsic to the sex of the animal from which the MSCs came. These results also may explain the sensitivity of women to the deleterious effects of rosiglitazone on bone.

Relationship between Sloan-Kettering virus expression and mammalian follicular development

Sloan-Kettering virus gene, a product of a cellular proto-oncogene c-Ski is a unique nuclear pro-oncoprotein and belongs to the Ski/Sno proto-oncogene family. The aim of the present study was to locate Ski protein in rat ovaries in order to find insights into the possible involvement of Ski in follicular development. First, expression of c-Ski mRNA in the ovaries of adult female rats was confirmed by RT-PCR. Then, ovaries obtained on the day of estrus were subjected to immunohistochemical analysis for Ski and proliferating cell nuclear antigen (PCNA) in combination with terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). RT-PCR and in situ hybridization revealed that c-Ski mRNA was expressed in the ovaries of the adult rat on the day of estrous and localized mainly in the granulose cells. Ski was expressed in granulosa cells that were positive for TUNEL, but negative for PCNA, regardless of the shape and size of follicles. Expression of Ski in TUNEL-positive granulosa cells, but not in PCNA-positive granulosa cells, was also verified in rats having atretic follicles with double staining. These results indicate that Ski is profoundly expressed in the granulosa cells of atretic follicles, but not in growing follicles. Based on the present findings, Ski may play a role in the apoptosis of granulosa cells during follicular atresia.

Loss of Gsα early in the osteoblast lineage favors adipogenic differentiation of mesenchymal progenitors and committed osteoblast precursors

In humans, aging and glucocorticoid treatment are associated with reduced bone mass and increased marrow adiposity, suggesting that the differentiation of osteoblasts and adipocytes may be coordinately regulated. Within the bone marrow, both osteoblasts and adipocytes are derived from mesenchymal progenitor cells, but the mechanisms guiding the commitment of mesenchymal progenitors into osteoblast versus adipocyte lineages are not fully defined. The heterotrimeric G protein subunit Gs α activates protein kinase A signaling downstream of several G protein-coupled receptors including the parathyroid hormone receptor, and plays a crucial role in regulating bone mass. Here, we show that targeted ablation of Gs α in early osteoblast precursors, but not in differentiated osteocytes, results in a dramatic increase in bone marrow adipocytes. Mutant mice have reduced numbers of mesenchymal progenitors overall, with an increase in the proportion of progenitors committed to the adipocyte lineage. Furthermore, cells committed to the osteoblast lineage retain adipogenic potential both in vitro and in vivo. These findings have clinical implications for developing therapeutic approaches to direct the commitment of mesenchymal progenitors into the osteoblast lineage.

Lentiviral delivery of PPARγ shRNA alters the balance of osteogenesis and adipogenesis, improving bone microarchitecture

INTRODUCTION

Skeletal aging is associated not only with alterations in osteoblast (OB) and osteoclast (OC) number and activity within the basic metabolic unit, but also with increased marrow adiposity. Peroxisome proliferator-activated receptor gamma (PPARγ) is commonly considered the master transcriptional regulator of adipogenesis, however, it has known roles in osteoblast and osteoclast function as well. Here, we designed a lentiviral delivery system for PPARγ shRNA, and examined its effects in vitro on bone marrow stromal cells (BMSC) and in a mouse intramedullary injection model.

METHODS

PPARγ shRNA was delivered by a replication-deficient lentiviral vector, after in vitro testing to confirm purity, concentration, and efficacy for Pparg transcript reduction. Next, control green fluorescent protein lentivirus or PPARγ shRNA expressing lentivirus were delivered by intramedullary injection into the femoral bone marrow of male SCID mice. Analyses included daily monitoring of animal health, and postmortem analysis at 4 weeks. Postmortem analyses included high resolution microcomputed tomography (microCT) reconstructions and analysis, routine histology and histomorphometric analysis, quantitative real time polymerase chain reaction analysis of Pparg transcript levels, and immunohistochemical analysis for markers of adipocytes (PPARγ, fatty acid binding protein 4 [FABP4]), osteoblasts (alkaline phosphatase [ALP], osteocalcin [OCN]), and osteoclasts (tartrate-resistant acid phosphatase [TRAP], Cathepsin K).

RESULTS

In vitro, PPARγ shRNA delivery significantly reduced Pparg expression in mouse BMSC, accompanied by a significant reduction in lipid droplet accumulation. In vivo, a near total reduction in mature marrow adipocytes was observed at 4 weeks postinjection. This was accompanied by significant reductions in adipocyte-specific markers. Parameters of trabecular bone were significantly increased by both microCT and histomorphometric analysis. By immunohistochemical staining and semi-quantification, a significant increase in OCN+osteoblasts and decrease in TRAP+multinucleated osteoclasts was observed with PPARγ shRNA treatment.

DISCUSSION

These findings suggest that acute loss of PPARγ in the bone marrow compartment has a significant role beyond anti-adipose effects. Specifically, we found pro-osteoblastogenic, anti-osteoclastic effects after PPARγ shRNA treatment, resulting in improved trabecular bone architecture. Future studies will examine the isolated and direct effects of PPARγ shRNA on OB and OC cell types, and it may help determine whether PPARγ antagonists are potential therapeutic agents for osteoporotic bone loss.

Identification of novel regulators of osteoblast matrix mineralization by time series transcriptional profiling

Bone mineralization is a carefully orchestrated process, regulated by a number of promoters and inhibitors that function to ensure effective hydroxyapatite formation. Here we sought to identify new regulators of this process through a time series microarray analysis of mineralising primary osteoblast cultures over a 27 day culture period. To our knowledge this is the first microarray study investigating murine calvarial osteoblasts cultured under conditions that permit both physiological extracellular matrix mineralization through the formation of discrete nodules and the terminal differentiation of osteoblasts into osteocytes. RT-qPCR was used to validate and expand the microarray findings. We demonstrate the significant up-regulation of >6,000 genes during the osteoblast mineralization process, the highest-ranked differentially expressed genes of which were those dominated by members of the PPAR-γ signalling pathway, namely Adipoq, Cd36 and Fabp4. Furthermore, we show that the inhibition of this signalling pathway promotes matrix mineralisation in these primary osteoblast cultures. We also identify Cilp, Phex, Trb3, Sox11, and Psat1 as novel regulators of matrix mineralization. Further studies examining the precise function of the identified genes and their interactions will advance our understanding of the mechanisms underpinning biomineralization.

Distal-less homeobox 5 inhibits adipogenic differentiation through the down-regulation of peroxisome proliferator-activated receptor γ expression

Distal-less homeobox 5 (Dlx5) is a positive regulator of osteoblast differentiation that contains a homeobox domain. Because there are possible reciprocal relationships between osteogenic and adipogenic differentiation of bone marrow mesenchymal stem cells (MSCs), we examined the regulatory role of Dlx5 in adipogenic differentiation in this study. Adipogenic stimuli suppressed the expression levels of Dlx5 mRNA in mouse bone marrow stromal cells. Over-expression of Dlx5 inhibited adipogenic differentiation in human bone marrow MSCs and 3T3-L1 preadipocytic cells whereas knockdown of Dlx5 enhanced adipogenic differentiation in 3T3-L1 cells. Over-expression of Dlx5 suppressed the expression of adipogenic marker genes, including CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ). Dlx5-mediated suppression of adipogenic differentiation was overcome by over-expression of PPARγ but not by that of cAMP response element binding protein (CREB) or C/EBPα. Dlx5 decreased the transcriptional activity of CREB and C/EBPα in a dose-dependent manner. Dlx5 directly bound to CREB and C/EBPα and prevented them from binding to and subsequently transactivating the PPARγ promoter. These results suggest that Dlx5 plays an important regulatory role in fate determination of bone marrow MSCs toward the osteoblast lineage through the inhibition of adipocyte differentiation as well as the direct stimulation of osteoblast differentiation.

Vitamin C deficiency accelerates bone loss inducing an increase in PPAR-γ expression in SMP30 knockout mice

Senescence marker protein (SMP) 30 knockout (KO) mice display symptoms of scurvy, including spontaneous bone fractures, and this was considered to be induced by a failure of collagen synthesis owing to vitamin C deficiency. However, low bone mineral density is also known to be associated with spontaneous bone fracture. Therefore, we investigated the effects of vitamin C deficiency on the balance between osteoblasts and osteoclasts in SMP30 KO mice as evidenced by histopathology. All mice were fed a vitamin C-free diet, and only one group (KV) mice were given water containing 1.5 g/l of vitamin C, whereas wild-type (WT) and KO mice were given normal drinking tap water without vitamin C for 16 weeks. After 16 weeks, all femur samples were removed for histopathological examination. The femurs of KO mice showed significantly reduced bone area and decreased number of osteoblasts compared with those of WT mice and KV mice. KO mice also exhibited the lowest level of alkaline phosphatase (ALP) expression in their femurs. However, KO mice showed the most elevated expression of the receptor activator of nuclear factor kappa-B ligand (RANKL). Moreover, KO mice had the strongest peroxisome proliferator-activated receptor (PPAR)-γ expression level in their osteoblasts and the highest number of TUNEL-positive bone marrow cells. Therefore, we concluded that vitamin C deficiency plays an important role in spontaneous bone fracture by inhibiting osteoblast differentiation and promoting transition of osteoblasts to adipocytes, and this could in turn be related to the increased PPAR-γ expression.

Myeloid Elf-1-like factor stimulates adipogenic differentiation through the induction of peroxisome proliferator-activated receptor γ expression in bone marrow

Myeloid Elf-1 like factor (MEF) is one of the Ets transcription factors known to regulate cell proliferation and differentiation. A previous report has shown that osteoblast-specific MEF transgenic mice (Col1a1-MEF TG mice) have low bone mass but high bone marrow adiposity. In the present study, we explored a previously unappreciated mechanism whereby MEF promotes adipogenesis in bone marrow. An adipogenic colony-forming unit assay showed that bone marrow cells derived from Col1a1-MEF TG mice had a higher adipogenic differentiation potential compared to those from wild-type. The levels of adipogenic marker genes expression in 3T3L1 cells were higher when co-cultured with Col1a1-MEF TG bone marrow cells than with wild-type cells. MC3T3-E1 preosteoblasts transfected with MEF secreted higher levels of 15-deoxy-delta (12, 14)-prostaglandin J(2), a potent endogenous ligand of peroxisome proliferator-activated receptor γ (PPARγ), under adipogenic conditions. MEF overexpression increased the adipogenic marker genes expression including PPARγ and lipid droplet accumulation in MC3T3-E1 preosteoblasts and 3T3L1 preadipocytes. Endogenous MEF expression levels increased as adipocyte differentiation proceeded. Knockdown of MEF by siRNA suppressed expression levels of adipogenic marker genes including PPARγ. MEF directly bound to the MEF binding element on the mouse PPARγ promoter, transactivating promoter activity. Immunohistochemical staining of tibia sections demonstrated that bone lining cells and bone marrow cells express higher levels of PPARγ protein in Col1a1-MEF TG mice than in wild-type mice. These results suggest that MEF transactivates PPARγ expression, which, in turn, enhances adipogenic differentiation. Furthermore, MEF overexpressing osteoblasts secrete higher levels of adipogenic factors, creating a marrow microenvironment that favors adipogenesis.

Down-regulation of type I Runx2 mediated by dexamethasone is required for 3T3-L1 adipogenesis

Runx2, a runt-related transcriptional factor family member, is involved in the regulation of osteoblast differentiation. Interestingly, it is abundant in growth-arrested 3T3-L1 preadipocytes and was dramatically down-regulated during adipocyte differentiation. Knockdown of Runx2 expression promoted 3T3-L1 adipocyte differentiation, whereas overexpression inhibited adipocyte differentiation and promoted the trans-differentiation of 3T3-L1 preadipocytes to bone cells. Runx2 was down-regulated specifically by dexamethasone (DEX). Only type I Runx2 was expressed in 3T3-L1 preadipocytes. Using luciferase assay and chromatin immunoprecipitation-quantitative PCR analysis, it was found that DEX repressed this type of Runx2 at the transcriptional level through direct binding of the glucocorticoid receptor (GR) to a GR-binding element in the Runx2 P2 promoter. Further studies indicated that GR recruited histone deacetylase 1 to the Runx2 P2 promoter which then mediated the deacetylation of histone H4 and down-regulated Runx2 expression. Runx2 might play its repressive role through the induction of p27 expression, which blocked 3T3-L1 adipocyte differentiation by inhibiting mitotic clonal expansion. Taken together, we identified Runx2 as a new downstream target of DEX and explored a new pathway between DEX, Runx2, and p27 which contributed to the mechanism of the 3T3-L1 adipocyte differentiation.

The EP4-ERK-dependent pathway stimulates osteo-adipogenic progenitor proliferation resulting in increased adipogenesis in fetal rat calvaria cell cultures

We previously reported that fetal rat calvaria (RC) cells are osteo-adipogenic bipotential and that PGE(2) receptors EP2 and EP4 are involved in bone nodule formation via both common and distinct MAPK pathways in RC cell cultures. Because PGE(2) participates in multiple biological processes including adipogenesis, it is of interest to determine the additional role(s) of PGE(2) in RC cells. PGE(2) increased the number of adipocyte colonies when RC cells were treated during proliferation but not other development stages. Of four EP agonists tested, the EP4 agonist ONO-AE1-437 (EP4A) was the most effective in promoting adipogenesis. Concomitantly, EP4A increased the number of cells with BrdU labeling and gene expression of CCAAT/enhancer binding protein (C/EBP)δ and c-fos but not peroxisome proliferator-activated receptor γ2 and C/EBPα. Amongst MAPK inhibitors, U0126, an inhibitor of MEK1/2, abrogated the EP4A-dependent effects. Our results suggest that the PGE(2)-EP4-ERK pathway increases the number of osteo-adipogenic bipotential progenitor cells, with a resultant increase in adipogenesis in RC cell cultures.

Bone vs. fat: embryonic origin of progenitors determines response to androgen in adipocytes and osteoblasts

Although androgen is considered an anabolic hormone, the consequences of androgen receptor (AR) overexpression in skeletally-targeted AR-transgenic lines highlight the detrimental effect of enhanced androgen sensitivity on cortical bone quality. A compartment-specific anabolic response is observed only in male and not in female AR3.6-transgenic (tg) mice, with increased periosteal bone formation and calvarial thickening. To identify anabolic signaling cascades that have the potential to increase bone formation, qPCR array analysis was employed to define expression differences between AR3.6-tg and wild-type (WT) periosteal tissue. Notably, categories that were significantly different between the two genotypes included axonal guidance, CNS development and negative regulation of Wnt signaling with a node centered on stem cell pathways. Further, fine mapping of AR3.6-tg calvaria revealed that anabolic thickening in vivo is not uniform across the calvaria, occurring only in frontal and in not parietal bones. Multipotent fraction 1 progenitor populations from both genotypes were cultured separately as frontal bone neural crest stem-like cells (fNCSC) and parietal bone mesenchymal stem-like cells (pMSC). Both osteoblastic and adipogenic differentiation in these progenitor populations was influenced by embryonic lineage and by genotype. Adipogenesis was enhanced in WT fNCSC compared to pMSC, but transgenic cultures showed strong suppression of lipid accumulation only in fNCSC cells. Osteoblastogenesis was significantly increased in transgenic fNCSC cultures compared to WT, with elevated alkaline phosphatase (ALP) activity and induction of mineralization and nodule formation assessed by alizarin red and von Kossa staining. Osteocalcin (OC) and ALP mRNA levels were also increased in fNCSC cultures from AR3.6-tg vs. WT, but in pMSC cultures ALP mRNA levels, mineralization and nodule formation were decreased in AR3.6-tg cells. Expression differences identified by array in long bone periosteal tissue from AR3.6-tg vs. WT were recapitulated in the fNCSC samples while pMSC profiles reflected cortical expression. These observations reveal the opposing effects of androgen signaling on lineage commitment and osteoblast differentiation that is enhanced in cells derived from a neural crest origin but inhibited in cells derived from a mesodermal origin, consistent with in vivo compartment-specific responses to androgen. Combined, these results highlight the complex action of androgen in the body that is dependent on the embryonic lineage and developmental origin of the cell. Further, these data these data suggest that the periosteum surrounding long bone is derived from neural crest.

Ectonucleotide pyrophosphatase/phosphodiesterase-1 (ENPP1) protein regulates osteoblast differentiation

ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase-1) is an established regulator of tissue mineralization. Previous studies demonstrated that ENPP1 is expressed in differentiated osteoblasts and that ENPP1 influences matrix mineralization by increasing extracellular levels of inorganic pyrophosphate. ENPP1 is also expressed in osteoblastic precursor cells when stimulated with FGF2, but the role of ENPP1 in preosteoblastic and other precursor cells is unknown. Here we investigate the function of ENPP1 in preosteoblasts. We find that ENPP1 expression is critical for osteoblastic differentiation and that this effect is not mediated by changes in extracellular concentration levels of phosphate or pyrophosphate or ENPP1 catalytic activity. MC3T3E1(C4) preosteoblastic cells, in which ENPP1 expression was suppressed by ENPP1-specific shRNA, and calvarial cells isolated from Enpp1 knock-out mice show defective osteoblastic differentiation upon stimulation with ascorbate, as indicated by a lack of cellular morphological change, a lack of osteoblast marker gene expression, and an inability to mineralize matrix. Additionally, MC3T3E1(C4) cells, in which wild type or catalytic inactive ENPP1 expression was increased, exhibited an increased tendency to differentiate, as evidenced by increased osteoblast marker gene expression and increased mineralization. Notably, treatment of cells with inorganic phosphate or pyrophosphate inhibited, as opposed to enhanced, expression of multiple genes that are expressed in association with osteoblast differentiation, matrix deposition, and mineralization. Our results indicate that ENPP1 plays multiple and distinct roles in the development of mineralized tissues and that the influence of ENPP1 on osteoblast differentiation and gene expression may include a mechanism that is independent of its catalytic activity.