Maf promotes osteoblast differentiation in mice by mediating the age-related switch in mesenchymal cell differentiation

c-MAF deletion in adult C57BL/6J mice induces cataract formation and abnormal differentiation of lens fiber cells

The transcription factor c-MAF is a member of the large MAF family, members of which possess transactivation and bZIP domains. c-MAF plays an important role in lens formation, T-lymphocyte differentiation, hypertrophic chondrocyte differentiation, and kidney development in mouse embryos. However, because homozygous deletion of c-Maf in C57BL/6J mice causes embryonic lethality, the functions of c-MAF in adult mice remain largely uninvestigated. To address this issue, we generated c-Maf floxed (c-Maf^fl/fl) C57BL/6J mice and established tamoxifen-inducible c-Maf knockout mice (c-Maf^fl/fl; CAG-Cre-ER^TM mice, c-Maf^ΔTAM). After tamoxifen injection, adult c-Maf^ΔTAM mice showed successful deletion of c-Maf protein and developed severe cataracts; cataracts are also seen in human patients who have mutations in the c-MAF DNA binding domain. Furthermore, adult c-Maf^ΔTAM mice exhibited abnormal lens structure and impaired differentiation of lens fiber cells. In summary, we established c-Maf^fl/fl and c-Maf^ΔTAM C57BL/6J mice, which can be useful animal models for the investigation of c-MAF function in various developmental stages and can also be used as a disease model for cataracts.

Identification of a key gene module associated with glucocorticoid- induced derangement in bone mineral density in patients with asthma

Derangement in bone mineral density (BMD) caused by glucocorticoid is well-known. The present study aimed to find key biological pathways associated with low BMD after glucocorticoid treatment in asthmatics using gene expression profiles of peripheral blood cells. We utilized immortalized B cells (IBCs) from 32 childhood asthmatics after multiple oral glucocorticoid bursts and peripheral blood mononuclear cells (PBMCs) from 17 adult asthmatics after a long-term use of oral glucocorticoid. We searched co-expressed gene modules significantly related with the BMD Z score in childhood asthmatics and tested if these gene modules were preserved and significantly associated with the BMD Z score in adult asthmatics as well. We identified a gene module composed of 199 genes significantly associated with low BMD in both childhood and adult asthmatics. The structure of this module was preserved across gene expression profiles. We found that the cellular metabolic pathway was significantly enriched in this module. Among 18 hub genes in this module, we postulated that 2 genes, CREBBP and EP300, contributed to low BMD following a literature review. A novel biologic pathway identified in this study highlighted a gene module and several genes as playing possible roles in the pathogenesis of glucocorticoid- induced derangement in BMD.

Biological Effects of Anti-RANKL Antibody and Zoledronic Acid on Growth and Tooth Eruption in Growing Mice

The anti-bone resorptive drugs denosumab, an anti-human-RANKL antibody, and zoledronic acid (ZOL), a nitrogen-containing bisphosphonate, have recently been applied for treatment of pediatric patients with bone diseases, though details regarding their effects in growing children have yet to be fully elucidated. In the present study, we administered these anti-resorptive drugs to mice from the age of 1 week and continued once-weekly injections for a total of 7 times. Mice that received the anti-RANKL antibody displayed normal growth and tooth eruption, though osteopetrotic bone volume gain in long and alveolar bones was noted, while there were nearly no osteoclasts and a normal of number osteoblasts observed. In contrast, ZOL significantly delayed body growth, tooth root formation, and tooth eruption, with increased osteoclast and decreased osteoblast numbers. These findings suggest regulation of tooth eruption via osteoblast differentiation by some types of anti-resorptive drugs.

Phosphoproteomic profiling reveals a defined genetic program for osteoblastic lineage commitment of human bone marrow-derived stromal stem cells

Bone marrow-derived mesenchymal stem cells (MSCs) differentiate into osteoblasts upon stimulation by signals present in their niche. Because the global signaling cascades involved in the early phases of MSCs osteoblast (OB) differentiation are not well-defined, we used quantitative mass spectrometry to delineate changes in human MSCs proteome and phosphoproteome during the first 24 h of their OB lineage commitment. The temporal profiles of 6252 proteins and 15,059 phosphorylation sites suggested at least two distinct signaling waves: one peaking within 30 to 60 min after stimulation and a second upsurge after 24 h. In addition to providing a comprehensive view of the proteome and phosphoproteome dynamics during early MSCs differentiation, our analyses identified a key role of serine/threonine protein kinase D1 (PRKD1) in OB commitment. At the onset of OB differentiation, PRKD1 initiates activation of the pro-osteogenic transcription factor RUNX2 by triggering phosphorylation and nuclear exclusion of the histone deacetylase HDAC7.

Bone loss caused by dopaminergic degeneration and levodopa treatment in Parkinson's disease model mice

Accumulating evidence have shown the association of Parkinson’s disease (PD) with osteoporosis. Bone loss in PD patients, considered to be multifactorial and a result of motor disfunction, is a hallmark symptom that causes immobility and decreased muscle strength, as well as malnutrition and medication. However, no known experimental evidence has been presented showing deleterious effects of anti-PD drugs on bone or involvement of dopaminergic degeneration in bone metabolism. Here, we show that osteoporosis associated with PD is caused by dopaminergic degeneration itself, with no deficit of motor activity, as well as treatment with levodopa, the current gold-standard medication for affected patients. Our findings show that neurotoxin-induced dopaminergic degeneration resulted in bone loss due to accelerated osteoclastogenesis and suppressed bone formation, which was associated with elevated prolactin. On the other hand, using an experimental model of postmenopausal osteoporosis, dopaminergic degeneration did not result in exacerbation of bone loss due to estrogen deficiency, but rather reduction of bone loss. Thus, this study provides evidence for the regulation of bone metabolism by the dopaminergic system through both gonadal steroid hormone-dependent and -independent functions, leading to possible early detection of osteoporosis development in individuals with PD.

Singleton-Merten Syndrome-like Skeletal Abnormalities in Mice with Constitutively Activated MDA5

Singleton-Merten syndrome (SMS) is a type I interferonopathy characterized by dental dysplasia, aortic calcification, skeletal abnormalities, glaucoma, and psoriasis. A missense mutation in IFIH1 encoding a cytoplasmic viral RNA sensor MDA5 has recently been identified in the SMS patients as well as in patients with a monogenic form of lupus. We previously reported that Ifih1^gs/+ mice express a constitutively active MDA5 and spontaneously develop lupus-like nephritis. In this study, we demonstrate that the Ifih1^gs/+ mice also exhibit SMS-like bone abnormalities, including decreased bone mineral density and thin cortical bone. Histological analysis revealed a low number of osteoclasts, low bone formation rate, and abnormal development of growth plate cartilages in Ifih1^gs/+ mice. These abnormalities were not observed in Ifih1^gs/+ ・Mavs^-/- and Ifih1^gs/+ ・Ifnar1^-/- mice, indicating the critical role of type I IFNs induced by MDA5/MAVS-dependent signaling in the bone pathogenesis of Ifih1^gs/+ mice, affecting bone turnover. Taken together, our findings suggest the inhibition of type I IFN signaling as a possible effective therapeutic strategy for bone disorders in SMS patients.

Nicotinamide mononucleotide promotes osteogenesis and reduces adipogenesis by regulating mesenchymal stromal cells via the SIRT1 pathway in aged bone marrow

Mesenchymal stromal cells (MSCs) can differentiate to various cell types including osteoblasts, chondrocytes, and adipocytes. This cellular flexibility contributes to widespread clinical use of MSCs in tissue repair. However, challenges remain in efficient cellular expansion of MSCs for stem cell therapy. Current MSC culture methods have resulted in reduced self-renewal of MSCs and compromised therapeutic outcomes. This study identifies that nicotinamide mononucleotide (NMN), a key natural NAD⁺ intermediate, effectively encourages MSC expansion in vitro and in vivo. The in vitro expanded MSCs had heightened osteogenesis, but reduced adipogenesis. Furthermore, NMN supplementation stimulated osteogenesis of endogenous MSCs, and protected bone from aging and irradiation induced damage in mice. Mechanistically, we found that NMN treatment upregulated SIRT1. Genetically overexpressing SIRT1 in MSCs by using Prx1 cre; ColA1^{flox-stop-flox-SIRT1} mice promoted osteogenesis and reduced adipogenesis in aged mice. Overall, our data demonstrate that NMN promoted MSC self-renewal with strengthened osteogenesis and reduced adipogenesis via upregulating SIRT1 in aged mice.

Autoregulation of Osteocyte Sema3A Orchestrates Estrogen Action and Counteracts Bone Aging

Osteocyte survival is key to bone homeostasis and is perturbed in menopause and aging. However, it remains unknown how osteocyte-mediated maintenance of the skeleton is regulated by the osteoprotective factor semaphorin 3A (Sema3A), a secreted protein that is known to reduce bone resorption and enhance bone formation. Here, we show that estrogen induces osteocyte expression of Sema3A, which acts on its receptor on osteocytes to promote their survival and maintain bone homeostasis. Postnatal global and conditional deletion of Sema3a in osteoblastic cells resulted in a severe osteoporotic phenotype marked by fewer osteocytes. This phenotype was recapitulated by osteocyte-specific deficiency of either Sema3A or its receptor component neuropilin-1 (Nrp1). A stimulator of soluble guanylate cyclase-cGMP signaling mimicked Sema3A action and ameliorated bone loss after ovariectomy. We further show that serum levels of SEMA3A decreased with age or after menopause in humans. Thus, we provide a mechanistic insight into the estrogen action and a promising therapeutic approach to protect against bone-related aging.

Exosomes of pasteurized milk: potential pathogens of Western diseases

Milk consumption is a hallmark of western diet. According to common believes, milk consumption has beneficial effects for human health. Pasteurization of cow's milk protects thermolabile vitamins and other organic compounds including bioactive and bioavailable exosomes and extracellular vesicles in the range of 40-120 nm, which are pivotal mediators of cell communication via systemic transfer of specific micro-ribonucleic acids, mRNAs and regulatory proteins such as transforming growth factor-β. There is compelling evidence that human and bovine milk exosomes play a crucial role for adequate metabolic and immunological programming of the newborn infant at the beginning of extrauterine life. Milk exosomes assist in executing an anabolic, growth-promoting and immunological program confined to the postnatal period in all mammals. However, epidemiological and translational evidence presented in this review indicates that continuous exposure of humans to exosomes of pasteurized milk may confer a substantial risk for the development of chronic diseases of civilization including obesity, type 2 diabetes mellitus, osteoporosis, common cancers (prostate, breast, liver, B-cells) as well as Parkinson's disease. Exosomes of pasteurized milk may represent new pathogens that should not reach the human food chain.

Osteogenesis and aging: lessons from mesenchymal stem cells

Aging is a high risk factor for the development of osteoporosis, a multifactorial age-related progressive disease characterized by reduced bone mass and increased risk of fractures. At the cellular level, the mesenchymal stem cell pool in the bone marrow niche shows a biased differentiation into adipogenesis at the cost of osteogenesis. This differentiation shift leads to decreased bone formation, contributing to the etiology of osteoporosis. This review will focus on the most recent/relevant molecular findings driving this functional impairment of mesenchymal stem cells in the aging process.

The Spectrum of Fundamental Basic Science Discoveries Contributing to Organismal Aging

Aging research has undergone unprecedented advances at an accelerating rate in recent years, leading to excitement in the field as well as opportunities for imagination and innovation. Novel insights indicate that, rather than resulting from a preprogrammed series of events, the aging process is predominantly driven by fundamental non-adaptive mechanisms that are interconnected, linked, and overlap. To varying degrees, these mechanisms also manifest with aging in bone where they cause skeletal fragility. Because these mechanisms of aging can be manipulated, it might be possible to slow, delay, or alleviate multiple age-related diseases and their complications by targeting conserved genetic signaling pathways, controlled functional networks, and basic biochemical processes. Indeed, findings in various mammalian species suggest that targeting fundamental aging mechanisms (eg, via either loss-of-function or gain-of-function mutations or administration of pharmacological therapies) can extend healthspan; ie, the healthy period of life free of chronic diseases. In this review, we summarize the evidence supporting the role of the spectrum of fundamental basic science discoveries contributing to organismal aging, with emphasis on mammalian studies and in particular aging mechanisms in bone that drive skeletal fragility. These mechanisms or aging hallmarks include: genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Because these mechanisms are linked, interventions that ameliorate one hallmark can in theory ameliorate others. In the field of bone and mineral research, current challenges include defining the relative contributions of each aging hallmark to the natural skeletal aging process, better understanding the complex interconnections among the hallmarks, and identifying the most effective therapeutic strategies to safely target multiple hallmarks. Based on their interconnections, it may be feasible to simultaneously interfere with several fundamental aging mechanisms to alleviate a wide spectrum of age-related chronic diseases, including osteoporosis. © 2018 American Society for Bone and Mineral Research.

Clonal Analysis Delineates Transcriptional Programs of Osteogenic and Adipogenic Lineages of Adult Mouse Skeletal Progenitors

Bone, cartilage, and marrow adipocytes are generated by skeletal progenitors, but the relationships between lineages and mechanisms controlling their differentiation are poorly understood. We established mouse clonal skeletal progenitors with distinct differentiation properties and analyzed their transcriptome. Unipotent osteogenic and adipogenic cells expressed specific transcriptional programs, whereas bipotent clones combined expression of those genes and did not show a unique signature. We tested potential regulators of lineage commitment and found that in the presence of interferon-γ (IFNγ) adipogenic clones can be induced to osteogenesis and that their adipogenic capacity is inhibited. Analysis of IFNγ-regulated genes showed that lineage signatures and fate commitment of skeletal progenitors were controlled by EGR1 and EGR2. Knockdown experiments revealed that EGR1 is a positive regulator of the adipogenic transcriptional program and differentiation capacity, whereas EGR2 inhibits the osteogenic program and potency. Therefore, our work revealed transcriptional signatures of osteogenic and adipogenic lineages and mechanism triggering cell fate.

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Bone marrow osteo- and adipogenic progenitors have specific transcriptional profiles

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Bipotent progenitors combine expression of osteogenic and adipogenic programs

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IFNγ inhibits adipogenesis and induces osteogenesis via downregulation of Egr1/Egr2

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Egr1 maintains adipogenic and Egr2 suppresses osteogenic lineage commitment

Hyperactivation of Nrf2 leads to hypoplasia of bone in vivo

Keap1 is a negative regulator of Nrf2, a master transcription factor that regulates cytoprotection against oxidative and electrophilic stresses. Although several studies have suggested that the Keap1-Nrf2 system contributes to bone formation besides the maintenance of redox homeostasis, how Nrf2 hyperactivation by Keap1 deficiency affects the bone formation remains to be explored, as the Keap1-null mice are juvenile lethal. To overcome this problem, we used viable Keap1-deficient mice that we have generated by deleting the esophageal Nrf2 in Keap1-null mice (NEKO mice). We found that the NEKO mice exhibit small body size and low bone density. Although nephrogenic diabetes insipidus has been observed in both the NEKO mice and renal-specific Keap1-deficient mice, the skeletal phenotypes are not recapitulated in the renal-specific Keap1-deficient mice, suggesting that the skeletal phenotype by Nrf2 hyperactivation is not related to the renal phenotype. Experiments with primary culture cells derived from Keap1-null mice showed that differentiation of both osteoclasts and osteoblasts was attenuated, showing that impaired differentiation of osteoblasts rather than osteoclasts is responsible for bone hypoplasia caused by Nrf2 hyperactivation. Thus, we propose that the appropriate control of Nrf2 activity by Keap1 is essential for maintaining bone homeostasis.

Basal p53 expression is indispensable for mesenchymal stem cell integrity

Marrow-resident mesenchymal stem cells (MSCs) serve as a functional component of the perivascular niche that regulates hematopoiesis. They also represent the main source of bone formed in adult bone marrow, and their bifurcation to osteoblast and adipocyte lineages plays a key role in skeletal homeostasis and aging. Although the tumor suppressor p53 also functions in bone organogenesis, homeostasis, and neoplasia, its role in MSCs remains poorly described. Herein, we examined the normal physiological role of p53 in primary MSCs cultured under physiologic oxygen levels. Using knockout mice and gene silencing we show that p53 inactivation downregulates expression of TWIST2, which normally restrains cellular differentiation to maintain wild-type MSCs in a multipotent state, depletes mitochondrial reactive oxygen species (ROS) levels, and suppresses ROS generation and PPARG gene and protein induction in response to adipogenic stimuli. Mechanistically, this loss of adipogenic potential skews MSCs toward an osteogenic fate, which is further potentiated by TWIST2 downregulation, resulting in highly augmented osteogenic differentiation. We also show that p53^−/− MSCs are defective in supporting hematopoiesis as measured in standard colony assays because of decreased secretion of various cytokines including CXCL12 and CSF1. Lastly, we show that transient exposure of wild-type MSCs to 21% oxygen upregulates p53 protein expression, resulting in increased mitochondrial ROS production and enhanced adipogenic differentiation at the expense of osteogenesis, and that treatment of cells with FGF2 mitigates these effects by inducing TWIST2. Together, these findings indicate that basal p53 levels are necessary to maintain MSC bi-potency, and oxygen-induced increases in p53 expression modulate cell fate and survival decisions. Because of the critical function of basal p53 in MSCs, our findings question the use of p53 null cell lines as MSC surrogates, and also implicate dysfunctional MSC responses in the pathophysiology of p53-related skeletal disorders.

MiR-351 negatively regulates osteoblast differentiation of MSCs induced by (+)-cholesten-3-one through targeting VDR

Our previous reports indicated that (+)-cholesten-3-one induces osteogenic differentiation of bone marrow mesenchymal stem cells (MSCs) by activating vitamin D receptor (VDR). However, whether and how miRNAs modulate osteogenic differentiation induced by (+)-cholesten-3-one have not been explored. In this study, miRNA array profiling and further validation by quantitative real-time PCR revealed that miR-351 was downregulated during (+)-cholesten-3-one-induced osteogenic differentiation of MSCs. Overexpression of miR-351 by miR-351 precursor transfection markedly inhibited the expression of osteoblast-specific genes, such as alkaline phosphatase (ALP), collagen type II, osteopontin (OPN), and runt-related transcription factor 2 (RUNX2), which consequently decreased a number of calcium mineralized nodules. Inhibition of miR-351 function by anti-miR-351 promoted expression of osteoblast-specific genes. Our results suggest that miR-351 is a negative regulator of osteoblast differentiation of MSCs induced by (+)-cholesten-3-one. Target prediction analysis tools and experimental validation by luciferase 3'UTR reporter assay identified VDR as a direct target of miR-351. miR-351 inhibited the expression of the VDR, which played a critical role in the control of osteogenic differentiation of MSCs. Importantly, overexpression of VDR significantly abolished the inhibitory effect of miR-351 on (+)-cholesten-3-one induced osteogenic differentiation. Taken together, our results demonstrate that miR-351 negatively regulates osteoblast differentiation of MSCs induced by (+)-cholesten-3-one through targeting VDR. These findings provid evidence that miR-351 can bea possible therapeutic target for bone repair and regeneration.

Oxidative stress induces imbalance of adipogenic/osteoblastic lineage commitment in mesenchymal stem cells through decreasing SIRT1 functions

With rapidly ageing populations worldwide, the incidence of osteoporosis has reached epidemic proportions. Reactive oxygen species (ROS), a by‐product of oxidative stress and ageing, has been thought to induce osteoporosis by inhibiting osteogenic differentiation of mesenchymal stem cells (MSCs). However, specific mechanisms of how ROS results in alterations on MSC differentiation capacity have been inconsistently reported. We found that H₂O₂, an ROS, simultaneously induced MSC lineage commitment towards adipogenesis and away from osteogenesis at the functional as well as transcriptional level. In addition, H₂O₂ decreased the activities of SIRT1, a histone deacetylase and longevity gene. By silencing and reconstituting SIRT1 in MSCs, we demonstrated that H₂O₂ exerted its disparate effects on adipogenic/osteoblastic lineage commitment mainly through modulating SIRT1 expression levels. Treatment with resveratrol, a SIRT1 agonist, can also reverse this ROS‐induced adipogenesis/osteogenesis lineage imbalance. Moreover, SIRT1 regulation of RUNX2 transcriptional activity was mediated through deacetylation of the ROS‐sensitive transcription factor FOXO3a. Taken together, our data implicate SIRT1 as playing a vital role in ROS‐directed lineage commitment of MSCs by modulating two lineages simultaneously. Our findings on the critical role of SIRT1 in ROS/age‐related perturbations of MSC differentiation capacity highlight this molecule as a target for maintenance of MSC stemness as well as a potential anabolic target in osteoporosis.

Effects of deficiency of Kelch-like ECH-associated protein 1 on skeletal organization: a mechanism for diminished nuclear factor of activated T cells cytoplasmic 1 during osteoclastogenesis

Kelch-like ECH-associated protein 1 (Keap1) binds to nuclear factor E2 p45-related factor 2 (Nrf2), a transcription factor for antioxidant enzymes, to suppress Nrf2 activation. The role of oxidative stress in many diseases supports the possibility that processes that are associated with Nrf2 activation might offer therapeutic potential. Nrf2 deficiency induces osteoclastogenesis, which is responsible for bone loss, by activating receptor activator of NF-κB ligand (RANKL)-mediated signaling; however, the effects of Keap1 deficiency remain unclear. By using Keap1-deficient newborn mice, we observed that talus and calcaneus bone formation was partially retarded and that osteoclast number was reduced in vivo without severe gross abnormalities. In addition, Keap1-deficient macrophages were unable to differentiate into osteoclasts in vitrovia attenuation of RANKL-mediated signaling and expression of nuclear factor of activated T cells cytoplasmic 1 (NFATc1), a key transcription factor that is involved in osteoclastogenesis. Furthermore, Keap1 deficiency up-regulated the expression of Mafb, a negative regulator of NFATc1. RANKL-induced mitochondrial gene expression is required for down-regulation of IFN regulatory factor 8 (IRF-8), a negative transcriptional regulator of NFATc1. Our results indicate that Keap1 deficiency down-regulated peroxisome proliferator-activated receptor-γ coactivator 1β and mitochondrial gene expression and up-regulated Irf8 expression. These results suggest that the Keap1/Nrf2 axis plays a critical role in NFATc1 expression and osteoclastogenic progression.-Sakai, E., Morita, M., Ohuchi, M., Kido, M. A., Fukuma, Y., Nishishita, K., Okamoto, K., Itoh, K., Yamamoto, M., Tsukuba, T. Effects of deficiency of Kelch-like ECH-associated protein 1 on skeletal organization: a mechanism for diminished nuclear factor of activated T cells cytoplasmic 1 during osteoclastogenesis.

Biological effects of anti-RANKL antibody administration in pregnant mice and their newborns

Denosumab, a fully human monoclonal antibody that neutralizes receptor activator of nuclear factor-κB ligand (RANKL) and blocks osteoclast differentiation, has received approval in Japan for use as an anti-resorptive drug for osteoporosis and skeletal-related events (SREs) in patients with solid cancer. Denosumab is contraindicated during pregnancy, though the effects of blocking RANKL activity on pregnant mothers and their newborns are unclear. We used mice to investigate the effects of an anti-RANKL antibody on maternal and newborn health. Mothers injected with the anti-RANKL antibody had increased bone mass as compared with the controls, while osteoclast number and the level of tartrate-resistant acid phosphatase (TRAP) in serum were increased at the end of pregnancy. Newborn mice exposed to the antibody in utero were normally born, but showed increased bone mass and died within 48 h after birth. None of the newborns were found to have milk in their stomachs, suggesting that they died due to a maternal defect in lactation. Consistent with this, anti-RANKL antibody-injected mothers displayed impaired mammary gland development. However, fostering by healthy surrogate mothers rescued only 33% of the antibody-exposed newborns, suggesting that neonatal mortality was due, at least in part, to an intrinsic defect in the newborns. Our findings show that anti-RANKL antibody administration during pregnancy results in not only an undesirable increase in bone mass, but also has harmful effects on newborn survival.

Exposure to tetrabromobisphenol A induces cellular dysfunction in osteoblastic MC3T3-E1 cells

This study was undertaken to investigate the possible involvement of oxidative stress in tetrabromobisphenol A (TBBPA)-induced toxicity in osteoblastic MC3T3-E1 cells. To examine the potential effect of TBBPA on cultured osteoblastic cells, we measured cell viability, apoptosis, reactive oxygen species (ROS), mitochondrial superoxide, and mitochondrial parameters including adenosine triphosphate (ATP) level, cardiolipin content, cytochrome c release, cyclophilin levels, and differentiation markers in osteoblastic MC3T3-E1 cells. TBBPA exposure for 48 h caused the apoptosis and cytotoxicity of MC3T3-E1 cells. TBBPA also induced ROS and mitochondrial superoxide production in a concentration-dependent manner. These results suggest that TBBPA induces osteoblast apoptosis and ROS production, resulting in bone diseases. Moreover, TBBPA induced cardiolipin peroxidation, cytochrome c release, and decreased ATP levels which induced apoptosis or necrosis. TBBPA decreased the differentiation markers, collagen synthesis, alkaline phosphatase activity, and calcium deposition in cells. Additionally, TBBPA decreased cyclophilin A and B releases. Taken together, these data support the notion that TBBPA inhibits osteoblast function and has detrimental effects on osteoblasts through a mechanism involving oxidative stress and mitochondrial dysfunction.

The role of the progressive ankylosis protein (ANK) in adipogenic/osteogenic fate decision of precursor cells

The progressive ankylosis protein (ANK) is a transmembrane protein that transports intracellular pyrophosphate (PPi) to the extracellular milieu. In this study we show increased fatty degeneration of the bone marrow of adult ank/ank mice, which lack a functional ANK protein. In addition, isolated bone marrow stromal cells (BMSCs) isolated from ank/ank mice showed a decreased proliferation rate and osteogenic differentiation potential, and an increased adipogenic differentiation potential compared to BMSCs isolated from wild type (WT) littermates. Wnt signaling pathway PCR array analysis revealed that Wnt ligands, Wnt receptors and Wnt signaling proteins that stimulate osteoblast differentiation were expressed at markedly lower levels in ank/ank BMSCs than in WT BMSCs. Lack of ANK function also resulted in impaired bone fracture healing, as indicated by a smaller callus formed and delayed bone formation in the callus site. Whereas 5weeks after fracture, the fractured bone in WT mice was further remodeled and restored to original shape, the fractured bone in ank/ank mice was not fully restored and remodeled to original shape. In conclusion, our study provides evidence that ANK plays a critical role in the adipogenic/osteogenic fate decision of adult mesenchymal precursor cells. ANK functions in precursor cells are required for osteogenic differentiation of these cells during adult bone homeostasis and repair, whereas lack of ANK functions favors adipogenic differentiation.

Perfluorooctanoic acid induces mitochondrial dysfunction in MC3T3-E1 osteoblast cells

Perfluorooctanoic acid (PFOA), a stable organic perfluorinated compound, is an emerging persistent organic pollutant, found widely in human and wildlife populations. Recent evidence suggests that exposure to environmental toxicants can be associated with higher risks of osteoporosis and fractures. We studied the cellular toxicology of PFOA in MC3T3-E1osteoblast cells. To examine the effect of PFOA, we measured cell viability, reactive oxygen species (ROS), mitochondrial superoxide, and mitochondrial parameters including adenosine triphosphate (ATP) level, mitochondrial membrane potential (MMP), cardiolipin content, and cytochrome c release in MC3T3-E1 cells. Incubating MC3T3-E1 cells in different concentrations of PFOA for 48 h resulted in a concentration-dependent decrease in cell viability and significant inductions of ROS and mitochondrial superoxide. Moreover, PFOA induced MMP collapse, cardiolipin peroxidation, cytochrome c release, and decreased ATP levels, which in turn induced apoptosis or necrosis. When osteoblast differentiation markers were assessed, PFOA treatment caused a significant reduction in alkaline phosphatase activity, collagen synthesis, and mineralization in the cells. In summary, we found an ROS- and mitochondria-mediated pathway for the induction of cell damage by PFOA in MC3T3-E1 cells. Together, our results indicate that mitochondrial toxicity could be a plausible mechanism for the toxic effects of PFOA on osteoblast function.

Development of an efficient screening system to identify novel bone metabolism-related genes using the exchangeable gene trap mutagenesis mouse models

Despite numerous genetic studies on bone metabolism, understanding of the specific mechanisms is lacking. We developed an efficient screening system to identify novel genes involved in bone metabolism using mutant mouse strains registered with the Exchangeable Gene Trap Clones (EGTC) database. From 1278 trap clones in the EGTC database, 52 candidate lines were selected in the first screening, determined based on "EST profile", "X-gal", "Related article", and "Novel gene". For the second screening, bone morphometric analysis, biomechanical strength analysis, bone X-gal staining, etc. were performed on candidate lines. Forty-two male trap lines (80.8%) showed abnormalities with either bone morphometric analysis or biomechanical strength analysis. In the screening process, X-gal staining was significantly efficient (P = 0.0057). As examples, Lbr and Nedd4 trap lines selected using the screening system showed significant bone decrease and fragility, suggesting a relationship with osteoblast differentiation. This screening system using EGTC mouse lines is extremely efficient for identifying novel genes involved in bone metabolism. The gene trap lines identified as abnormal using this screening approach are highly likely to trap important genes for bone metabolism. These selected trap mice will be valuable for use as novel bio-resources in bone research.

Acute transcriptional up-regulation specific to osteoblasts/osteoclasts in medaka fish immediately after exposure to microgravity

Bone loss is a serious problem in spaceflight; however, the initial action of microgravity has not been identified. To examine this action, we performed live-imaging of animals during a space mission followed by transcriptome analysis using medaka transgenic lines expressing osteoblast and osteoclast-specific promoter-driven GFP and DsRed. In live-imaging for osteoblasts, the intensity of osterix- or osteocalcin-DsRed fluorescence in pharyngeal bones was significantly enhanced 1 day after launch; and this enhancement continued for 8 or 5 days. In osteoclasts, the signals of TRAP-GFP and MMP9-DsRed were highly increased at days 4 and 6 after launch in flight. HiSeq from pharyngeal bones of juvenile fish at day 2 after launch showed up-regulation of 2 osteoblast- and 3 osteoclast- related genes. Gene ontology analysis for the whole-body showed that transcription of genes in the category “nucleus” was significantly enhanced; particularly, transcription-regulators were more up-regulated at day 2 than at day 6. Lastly, we identified 5 genes, c-fos, jun-B-like, pai-1, ddit4 and tsc22d3, which were up-regulated commonly in the whole-body at days 2 and 6, and in the pharyngeal bone at day 2. Our results suggested that exposure to microgravity immediately induced dynamic alteration of gene expression levels in osteoblasts and osteoclasts.

GDF11 Inhibits Bone Formation by Activating Smad2/3 in Bone Marrow Mesenchymal Stem Cells

Growth differentiation factor 11 (GDF11) is a member of the transforming growth factor-β superfamily. Recent studies confirmed that GDF11 plays an important role in regulating the regeneration of brain, skeletal muscle, and heart during aging; however, its role in bone metabolism remains unclear. Thus, the aim of this study was to determine the effects of GDF11 on bone metabolism, including bone formation and bone resorption, both in vitro and in vivo. Our results showed that GDF11 inhibited osteoblastic differentiation of bone marrow mesenchymal stem cells in vitro. Mechanistically, GDF11 repressed Runx2 expression by inducing SMAD2/3 phosphorylation during osteoblast differentiation. Moreover, intraperitoneal injection of GDF11 inhibited bone formation and accelerated age-related bone loss in mice. Our results also showed that GDF11 had no effect on osteoclast differentiation or bone resorption both in vitro and in vivo. These results provide a further rationale for the therapeutic targeting of GDF11 for the treatment of age-related osteoporosis.

c-Maf regulates pluripotency genes, proliferation/self-renewal, and lineage commitment in ROS-mediated senescence of human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are therapeutically relevant multilineage and immunomodulatory progenitors. Ex vivo expansion of these rare cells is necessary for clinical application and can result in detrimental senescent effects, with mechanisms still largely unknown. We found that vigorous ex vivo expansion of human adipose tissue-derived MSCs (hAMSCs) results in proliferative decline, cell cycle arrest, and altered differentiation capacity. This senescent phenotype was associated with reactive oxygen species (ROS) accumulation, and with increased expression of G1 cell -cycle inhibitors— p15^INK4b and p16^INK4a — but decreased expression of pluripotency genes—Oct-4, Sox-2, Nanog, and c-Myc—as well as c-Maf a co-factor of MSC lineage-specific transcription factor and sensitive to oxidative stress. These global changes in the transcriptional and functional programs of proliferation, differentiation, and self-renewal were all mediated by ROS-induced suppression of c-Maf, as evidenced by binding of c-Maf to promoter regions of multiple relevant genes in hAMSCs which could be reduced by exogenous ROS. Our findings implicate the strong effects of ROS on multiple stem cell functions with a central role for c-Maf in stem cell senescence.

Loss of Rictor with aging in osteoblasts promotes age-related bone loss

Osteoblast dysfunction is a major cause of age-related bone loss, but the mechanisms underlying changes in osteoblast function with aging are poorly understood. This study demonstrates that osteoblasts in aged mice exhibit markedly impaired adhesion to the bone formation surface and reduced mineralization in vivo and in vitro. Rictor, a specific component of the mechanistic target of rapamycin complex 2 (mTORC2) that controls cytoskeletal organization and cell survival, is downregulated with aging in osteoblasts. Mechanistically, we found that an increased level of reactive oxygen species with aging stimulates the expression of miR-218, which directly targets Rictor and reduces osteoblast bone surface adhesion and survival, resulting in a decreased number of functional osteoblasts and accelerated bone loss in aged mice. Our findings reveal a novel functional pathway important for age-related bone loss and support for miR-218 and Rictor as potential targets for therapeutic intervention for age-related osteoporosis treatment.

Wedelolactone enhances osteoblastogenesis by regulating Wnt/β-catenin signaling pathway but suppresses osteoclastogenesis by NF-κB/c-fos/NFATc1 pathway

Bone homeostasis is maintained by formation and destruction of bone, which are two processes tightly coupled and controlled. Targeting both stimulation on bone formation and suppression on bone resorption becomes a promising strategy for treating osteoporosis. In this study, we examined the effect of wedelolactone, a natural product from Ecliptae herba, on osteoblastogenesis as well as osteoclastogenesis. In mouse bone marrow mesenchymal stem cells (BMSC), wedelolactone stimulated osteoblast differentiation and bone mineralization. At the molecular level, wedelolactone directly inhibited GSK3β activity and enhanced the phosphorylation of GSK3β, thereafter stimulated the nuclear translocation of β-catenin and runx2. The expression of osteoblastogenesis-related marker gene including osteorix, osteocalcin and runx2 increased. At the same concentration range, wedelolactone inhibited RANKL-induced preosteoclastic RAW264.7 actin-ring formation and bone resorption pits. Further, wedelolactone blocked NF-kB/p65 phosphorylation and abrogated the NFATc1 nuclear translocation. As a result, osteoclastogenesis-related marker gene expression decreased, including c-src, c-fos, and cathepsin K. In ovariectomized mice, administration of wedelolactone prevented ovariectomy-induced bone loss by enhancing osteoblast activity and inhibiting osteoclast activity. Together, these data demonstrated that wedelolactone facilitated osteoblastogenesis through Wnt/GSK3β/β-catenin signaling pathway and suppressed RANKL-induced osteoclastogenesis through NF-κB/c-fos/NFATc1 pathway. These results suggested that wedelolacone could be a novel dual functional therapeutic agent for osteoporosis.

Long Non-coding RNA H19 Inhibits Adipocyte Differentiation of Bone Marrow Mesenchymal Stem Cells through Epigenetic Modulation of Histone Deacetylases

Bone marrow mesenchymal stem cells (BMSCs) exhibit an increased propensity toward adipocyte differentiation accompanied by a reduction in osteogenesis in osteoporotic bone marrow. However, limited knowledge is available concerning the role of long non-coding RNAs (lncRNAs) in the differentiation of BMSCs into adipocytes. In this study, we demonstrated that lncRNA H19 and microRNA-675 (miR-675) derived from H19 were significantly downregulated in BMSCs that were differentiating into adipocytes. Overexpression of H19 and miR-675 inhibited adipogenesis, while knockdown of their endogenous expression accelerated adipogenic differentiation. Mechanistically, we found that miR-675 targeted the 3' untranslated regions of the histone deacetylase (HDAC) 4-6 transcripts and resulted in deregulation of HDACs 4-6, essential molecules in adipogenesis. In turn, trichostatin A, an HDAC inhibitor, significantly reduced CCCTC-binding factor (CTCF) occupancy in the imprinting control region upstream of the H19 gene locus and subsequently downregulated the expression of H19. These results show that the CTCF/H19/miR-675/HDAC regulatory pathway plays an important role in the commitment of BMSCs into adipocytes.

Cell Fate and Differentiation of Bone Marrow Mesenchymal Stem Cells

Osteoblasts and bone marrow adipocytes originate from bone marrow mesenchymal stem cells (BMMSCs) and there appears to be a reciprocal relationship between adipogenesis and osteoblastogenesis. Alterations in the balance between adipogenesis and osteoblastogenesis in BMMSCs wherein adipogenesis is increased relative to osteoblastogenesis are associated with decreased bone quality and quantity. Several proteins have been reported to regulate this reciprocal relationship but the exact nature of the signals regulating the balance between osteoblast and adipocyte formation within the bone marrow space remains to be determined. In this review, we focus on the role of Transducin-Like Enhancer of Split 3 (TLE3), which was recently reported to regulate the balance between osteoblast and adipocyte formation from BMMSCs. We also discuss evidence implicating canonical Wnt signalling, which plays important roles in both adipogenesis and osteoblastogenesis, in regulating TLE3 expression. Currently, there is demand for new effective therapies that target the stimulation of osteoblast differentiation to enhance bone formation. We speculate that reducing TLE3 expression or activity in BMMSCs could be a useful approach towards increasing osteoblast numbers and reducing adipogenesis in the bone marrow environment.

Intercellular Communication between Keratinocytes and Fibroblasts Induces Local Osteoclast Differentiation: a Mechanism Underlying Cholesteatoma-Induced Bone Destruction

Bone homeostasis is maintained by a balance in activity between bone-resorbing osteoclasts and bone-forming osteoblasts. Shifting the balance toward bone resorption causes osteolytic bone diseases such as rheumatoid arthritis and periodontitis. Osteoclast differentiation is regulated by receptor activator of nuclear factor κB ligand (RANKL), which, under some pathological conditions, is produced by T and B lymphocytes and synoviocytes. However, the mechanism underlying bone destruction in other diseases is little understood. Bone destruction caused by cholesteatoma, an epidermal cyst in the middle ear resulting from hyperproliferation of keratinizing squamous epithelium, can lead to lethal complications. In this study, we succeeded in generating a model for cholesteatoma, epidermal cyst-like tissue, which has the potential for inducing osteoclastogenesis in mice. Furthermore, an in vitro coculture system composed of keratinocytes, fibroblasts, and osteoclast precursors was used to demonstrate that keratinocytes stimulate osteoclast differentiation through the induction of RANKL in fibroblasts. Thus, this study demonstrates that intercellular communication between keratinocytes and fibroblasts is involved in the differentiation and function of osteoclasts, which may provide the molecular basis of a new therapeutic strategy for cholesteatoma-induced bone destruction.

Epidermal differentiation gene regulatory networks controlled by MAF and MAFB

Numerous regulatory factors in epidermal differentiation and their role in regulating different cell states have been identified in recent years. However, the genetic interactions between these regulators over the dynamic course of differentiation have not been studied. In this Extra-View article, we review recent work by Lopez-Pajares et al. that explores a new regulatory network in epidermal differentiation. They analyze the changing transcriptome throughout epidermal regeneration to identify 3 separate gene sets enriched in the progenitor, early and late differentiation states. Using expression module mapping, MAF along with MAFB, are identified as transcription factors essential for epidermal differentiation. Through double knock-down of MAF:MAFB using siRNA and CRISPR/Cas9-mediated knockout, epidermal differentiation was shown to be impaired both in-vitro and in-vivo, confirming MAF:MAFB's role to activate genes that drive differentiation. Lopez-Pajares and collaborators integrated 42 published regulator gene sets and the MAF:MAFB gene set into the dynamic differentiation gene expression landscape and found that lncRNAs TINCR and ANCR act as upstream regulators of MAF:MAFB. Furthermore, ChIP-seq analysis of MAF:MAFB identified key transcription factor genes linked to epidermal differentiation as downstream effectors. Combined, these findings illustrate a dynamically regulated network with MAF:MAFB as a crucial link for progenitor gene repression and differentiation gene activation.

Mediator MED23 cooperates with RUNX2 to drive osteoblast differentiation and bone development

How lineage specifiers are regulated during development is an outstanding question, and the molecular regulation of osteogenic factor RUNX2 remains to be fully understood. Here we report that the Mediator subunit MED23 cooperates with RUNX2 to regulate osteoblast differentiation and bone development. Med23 deletion in mesenchymal stem cells or osteoblast precursors results in multiple bone defects similar to those observed in Runx2(+/-) mice. In vitro, Med23-deficient progenitor cells are refractory to osteoblast differentiation, and Med23 deficiency reduces Runx2-target gene activity without changing Runx2 expression. Mechanistically, MED23 binds to RUNX2 and modulates its transcriptional activity. Moreover, Med23 deficiency in osteoprogenitor cells exacerbates the skeletal abnormalities observed in Runx2(+/-) mice. Collectively, our results establish a genetic and physical interaction between RUNX2 and MED23, suggesting that MED23 constitutes a molecular node in the regulatory network of anabolic bone formation and related diseases.

Surface Engineering of Nanostructured Titanium Implants with Bioactive Ions

Surface nanofeatures and bioactive ion chemical modification are centrally important in current titanium (Ti) oral implants for enhancing osseointegration. However, it is unclear whether the addition of bioactive ions definitively enhances the osteogenic capacity of a nanostructured Ti implant. We systematically investigated the osteogenesis process of human multipotent adipose stem cells triggered by bioactive ions in the nanostructured Ti implant surface. Here, we report that bioactive ion surface modification (calcium [Ca] or strontium [Sr]) and resultant ion release significantly increase osteogenic activity of the nanofeatured Ti surface. We for the first time demonstrate that ion modification actively induces focal adhesion development and expression of critical adhesion–related genes (vinculin, talin, and RHOA) of human multipotent adipose stem cells, resulting in enhanced osteogenic differentiation on the nanofeatured Ti surface. It is also suggested that fibronectin adsorption may have only a weak effect on early cellular events of mesenchymal stem cells (MSCs) at least in the case of the nanostructured Ti implant surface incorporating Sr. Moreover, results indicate that Sr overrides the effect of Ca and other important surface factors (i.e., surface area and wettability) in the osteogenesis function of various MSCs (derived from human adipose, bone marrow, and murine bone marrow). In addition, surface engineering of nanostructured Ti implants using Sr ions is expected to exert additional beneficial effects on implant bone healing through the proper balancing of the allocation of MSCs between adipogenesis and osteogenesis. This work provides insight into the future surface design of Ti dental implants using surface bioactive ion chemistry and nanotopography.

p38α MAPK Regulates Lineage Commitment and OPG Synthesis of Bone Marrow Stromal Cells to Prevent Bone Loss under Physiological and Pathological Conditions

Bone marrow-derived mesenchymal stromal cells (BM-MSCs) are capable of differentiating into osteoblasts, chondrocytes, and adipocytes. Skewed differentiation of BM-MSCs contributes to the pathogenesis of osteoporosis. Yet how BM-MSC lineage commitment is regulated remains unclear. We show that ablation of p38α in Prx1+ BM-MSCs produced osteoporotic phenotypes, growth plate defects, and increased bone marrow fat, secondary to biased BM-MSC differentiation from osteoblast/chondrocyte to adipocyte and increased osteoclastogenesis and bone resorption. p38α regulates BM-MSC osteogenic commitment through TAK1-NF-κB signaling and osteoclastogenesis through osteoprotegerin (OPG) production by BM-MSCs. Estrogen activates p38α to maintain OPG expression in BM-MSCs to preserve the bone. Ablation of p38α in BM-MSCs positive for Dermo1, a later BM-MSC marker, only affected osteogenic differentiation. Thus, p38α mitogen-activated protein kinase (MAPK) in Prx1+ BM-MSCs acts to preserve the bone by promoting osteogenic lineage commitment and sustaining OPG production. This study thus unravels previously unidentified roles for p38α MAPK in skeletal development and bone remodeling.

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p38α deletion in Prx1+ BM-MSCs led to osteoporosis and cartilage anomaly

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p38α controls proliferation and tri-lineage differentiation of Prx1+ BM-MSCs

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p38α regulates osteoclastogenesis through OPG production by BM-MSCs

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The BM-MSC p38-OPG axis participates in estrogen deficiency-induced osteoporosis

Plasmid DNA-coding p62 as a bone effective anti-inflammatory/anabolic agent

We recently reported that a DNA plasmid coding p62-SQSTM1 acts as an effective anti tumor vaccine against both transplantable mouse tumors and canine spontaneous mammary neoplasms. Here we report the unexpected finding that intramuscular delivery of p62 DNA exerts a powerful anti-osteoporotic activity in a mouse model of inflammatory bone loss (i.e, ovariectomy) by combining bone-sparing and osteo-synthetic effects. Notably, the suppression of osteoporosis by p62DNA was associated with a sharp down-regulation of master inflammatory cytokines, and up-regulation of endogenous p62 protein by bone-marrow stromal cells. The present data provide a solid rational to apply p62 DNA vaccine as a safe, new therapeutic for treatment of inflammatory related bone loss diseases.

Male Osteoporosis in the Elderly

Osteoporosis is now recognized as an important public health problem in elderly men as fragility fractures are complicated by increased morbidity, mortality, and social costs. This review comprises an overview of recent findings in pathophysiology, diagnosis, and treatment of male osteoporosis, with particular regard to the old population.

Transmembrane protein 64 reciprocally regulates osteoblast and adipocyte differentiation by modulating Wnt/β-catenin signaling

Age-related osteoporosis is associated with a reciprocal decrease in bone formation and an increase in adiposity in the bone marrow niche. We previously reported Transmembrane protein 64 (Tmem64) to be an important regulator of osteoclast function; however, its precise role in osteoblasts has not yet been established. Here, we showed that ablation of the Tmem64 gene in mice resulted in markedly increased osteoblast and reduced adipocyte differentiation from bone marrow-derived stromal cells (BMSCs). Conversely, Tmem64 overexpression inhibited osteogenesis and accelerated adipogenesis. Furthermore, BMSCs isolated from Tmem64 knockout mice formed a greater number of colony-forming unit-osteoblasts and a lower number of colony-forming unit-adipocytes than the wild type controls. Mechanistically, the expression level of β-catenin, the key Wnt signaling molecule, increased significantly, and its nuclear translocation was enhanced in Tmem64-deficient cells. Introduction of Tmem64 significantly suppressed β-catenin-mediated transcriptional activity in an in vitro co-transfection experiment as well as during an in vivo experiment involving BAT-Gal reporter mice. These results demonstrate that Tmem64 plays an important role in the regulation of mesenchymal lineage allocation by modulating Wnt/β-catenin signaling.

Suppression of EZH2 Prevents the Shift of Osteoporotic MSC Fate to Adipocyte and Enhances Bone Formation During Osteoporosis

During osteoporosis, the shift of mesenchymal stem cell (MSC) lineage commitment to adipocyte leads to the imbalance between bone mass and fat, which increases the risk of fracture. The Enhancer of Zeste homology 2 (EZH2), which methylates histone H3 on lysine 27 (H3K27me3), controls MSC cell lineage commitment. However, whether EZH2 is related to osteoporosis remains elusive. In our study, we found EZH2 expression was significantly increased in osteoporotic MSCs. EZH2 directly increased H3K27me3 levels on promoters of Wnt1, Wnt6, and Wnt10a to silence Wnt gene transcription. The inhibition of Wnt/β-catenin signaling shifted MSC cell lineage commitment to adipocyte. Knockdown of EZH2 by lentivirus-expressing shRNA rescued the abnormal fate of osteoporotic MSC. By employing the H3K27me3 inhibitor DZNep, we effectively derepressed Wnt signaling and improved osteogenic differentiation of osteoporotic MSCs in vitro. Furthermore, in vivo administration of DZNep successfully increased bone formation and repressed excessive bone marrow fat formation in osteoporotic mice. Noteworthy, DZNep treatment persistently enhanced osteogenic differentiation of endogenous MSCs. In conclusion, our study demonstrated that redundant EZH2 shifted MSC cell lineage commitment to adipocyte, which contributed to the development of osteoporosis. We also provided EZH2 as a novel therapeutic target for improving bone formation during osteoporosis.

Mesenchymal stem cell aging: Mechanisms and influences on skeletal and non-skeletal tissues

The aging population and the incidence of aging-related diseases such as osteoporosis are on the rise. Aging at the tissue and organ levels usually involves tissue stem cells. Human and animal model studies indicate that aging affects two aspects of mesenchymal stem cell (MSC): a decrease in the bone marrow MSC pool and biased differentiation into adipocyte at the cost of osteoblast, which underlie the etiology of osteoporosis. Aging of MSC cells is also detrimental to some non-skeletal tissues, in particular the hematopoietic system, where MSCs serve as a niche component. In addition, aging compromises the therapeutic potentials of MSC cells, including cells isolated from aged individuals or cells cultured for many passages. Here we discuss the recent progress on our understanding of MSC aging, with a focus on the effects of MSC aging on bone remodeling and hematopoiesis and the mechanisms of MSC aging.

A LncRNA-MAF:MAFB transcription factor network regulates epidermal differentiation

Progenitor differentiation requires remodeling of genomic expression; however, in many tissues, such as epidermis, the spectrum of remodeled genes and the transcription factors (TFs) that control them are not fully defined. We performed kinetic transcriptome analysis during regeneration of differentiated epidermis and identified gene sets enriched in progenitors (594 genes), in early (159 genes), and in late differentiation (387 genes). Module mapping of 1,046 TFs identified MAF and MAFB as necessary and sufficient for progenitor differentiation. MAF:MAFB regulated 393 genes altered in this setting. Integrative analysis identified ANCR and TINCR lncRNAs as essential upstream MAF:MAFB regulators. ChIP-seq analysis demonstrated MAF:MAFB binding to known epidermal differentiation TF genes whose expression they controlled, including GRHL3, ZNF750, KLF4, and PRDM1. Each of these TFs rescued expression of specific MAF:MAFB target gene subsets in the setting of MAF:MAFB loss, indicating they act downstream of MAF:MAFB. A lncRNA-TF network is thus essential for epidermal differentiation.

S-nitrosoglutathione reductase-dependent PPARγ denitrosylation participates in MSC-derived adipogenesis and osteogenesis

Bone marrow-derived mesenchymal stem cells (MSCs) are a common precursor of both adipocytes and osteoblasts. While it is appreciated that PPARγ regulates the balance between adipogenesis and osteogenesis, the roles of additional regulators of this process remain controversial. Here, we show that MSCs isolated from mice lacking S-nitrosoglutathione reductase, a denitrosylase that regulates protein S-nitrosylation, exhibited decreased adipogenesis and increased osteoblastogenesis compared with WT MSCs. Consistent with this cellular phenotype, S-nitrosoglutathione reductase-deficient mice were smaller, with reduced fat mass and increased bone formation that was accompanied by elevated bone resorption. WT and S-nitrosoglutathione reductase-deficient MSCs exhibited equivalent PPARγ expression; however, S-nitrosylation of PPARγ was elevated in S-nitrosoglutathione reductase-deficient MSCs, diminishing binding to its downstream target fatty acid-binding protein 4 (FABP4). We further identified Cys 139 of PPARγ as an S-nitrosylation site and demonstrated that S-nitrosylation of PPARγ inhibits its transcriptional activity, suggesting a feedback regulation of PPARγ transcriptional activity by NO-mediated S-nitrosylation. Together, these results reveal that S-nitrosoglutathione reductase-dependent modification of PPARγ alters the balance between adipocyte and osteoblast differentiation and provides checkpoint regulation of the lineage bifurcation of these 2 lineages. Moreover, these findings provide pathophysiological and therapeutic insights regarding MSC participation in adipogenesis and osteogenesis.

DNA methyltransferase 3a regulates osteoclast differentiation by coupling to an S-adenosylmethionine-producing metabolic pathway

Metabolic reprogramming occurs in response to the cellular environment to mediate differentiation, but the fundamental mechanisms linking metabolic processes to differentiation programs remain to be elucidated. During osteoclast differentiation, a shift toward more oxidative metabolic processes occurs. In this study we identified the de novo DNA methyltransferase 3a (Dnmt3a) as a transcription factor that couples these metabolic changes to osteoclast differentiation. We also found that receptor activator of nuclear factor-κB ligand (RANKL), an essential cytokine for osteoclastogenesis, induces this metabolic shift towards oxidative metabolism, which is accompanied by an increase in S-adenosylmethionine (SAM) production. We found that SAM-mediated DNA methylation by Dnmt3a regulates osteoclastogenesis via epigenetic repression of anti-osteoclastogenic genes. The importance of Dnmt3a in bone homeostasis was underscored by the observations that Dnmt3a-deficient osteoclast precursor cells do not differentiate efficiently into osteoclasts and that mice with an osteoclast-specific deficiency in Dnmt3a have elevated bone mass due to a smaller number of osteoclasts. Furthermore, inhibition of DNA methylation by theaflavin-3,3'-digallate abrogated bone loss in models of osteoporosis. Thus, this study reveals the role of epigenetic processes in the regulation of cellular metabolism and differentiation, which may provide the molecular basis for a new therapeutic strategy for a variety of bone disorders.

Transcriptional factors, Mafs and their biological roles

The Maf family of transcription factors is characterized by a typical bZip structure; these transcription factors act as important regulators of the development and differentiation of many organs and tissues, including the kidney. The Maf family consists of two subgroups that are characterized according to their structure: large Maf transcription factors and small Maf transcription factors. The large Maf subgroup consists of four proteins, designated as MAFA, MAFB, c-MAF and neural retina-specific leucine zipper. In particular, MAFA is a distinct molecule that has been attracting the attention of researchers because it acts as a strong transactivator of insulin, suggesting that Maf transcription factors are likely to be involved in systemic energy homeostasis. In this review, we focused on the regulation of glucose/energy balance by Maf transcription factors in various organs.

The Role of MKP-1 in the Anti-Proliferative Effects of Glucocorticoids in Primary Rat Pre-Osteoblasts

Glucocorticoid (GC)-induced osteoporosis has been attributed to a GC-induced suppression of pre-osteoblast proliferation. Our previous work identified a critical role for mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) in mediating the anti-proliferative effects of GCs in immortalized pre-osteoblasts, but we subsequently found that MKP-1 null mice were not protected against the pathological effects of GCs on bone. In order to reconcile this discrepancy, we have assessed the effects of GCs on proliferation, activation of the MAPK ERK1/2 and MKP-1 expression in primary adipose-derived stromal cells (ADSCs) and ADSC-derived pre-osteoblasts (ADSC-OBs). ADSCs were isolated by means of collagenase digestion from adipose tissue biopsies harvested from adult male Wistar rats. ADSC-OBs were prepared by treating ADSCs with osteoblast differentiation media for 7 days. The effects of increasing concentrations of the GC dexamethasone on basal and mitogen-stimulated cell proliferation were quantified by tritiated thymidine incorporation. ERK1/2 activity was measured by Western blotting, while MKP-1 expression was quantified on both RNA and protein levels, using semi-quantitative real-time PCR and Western blotting, respectively. GCs were strongly anti-proliferative in both naïve ADSCs and ADSC-OBs, but had very little effect on mitogen-induced ERK1/2 activation and did not upregulate MKP-1 protein expression. These findings suggest that the anti-proliferative effects of GCs in primary ADSCs and ADSC-OBs in vitro do not require the inhibition of ERK1/2 activation by MKP-1, which is consistent with our in vivo findings in MKP-1 null mice.

Runx2-I isoform contributes to fetal bone formation even in the absence of specific N-terminal amino acids

The Runt-related transcription factor 2 (Runx2) gene encodes the transcription factor Runx2, which is the master regulator of osteoblast development; insufficiency of this protein causes disorders of bone development such as cleidocranial dysplasia. Runx2 has two isoforms, Runx2-II and Runx2-I, and production of each isoform is controlled by a unique promoter: a distal promoter (P1) and a proximal promoter (P2), respectively. Although several studies have focused on differences and similarities between the two Runx2 isoforms, their individual roles in bone formation have not yet been determined conclusively, partly because a Runx2-I-targeted mouse model is not available. In this study, we established a novel Runx2-manipulated mouse model in which the first ATG of Runx2-I was replaced with TGA (a stop codon), and a neomycin-resistant gene (neo) cassette was inserted at the first intron of Runx2-I. Homozygous Runx2-Ineo/neo mice showed severely reduced expression of Runx2-I, whereas Runx2-II expression was largely retained. Runx2-Ineo/neo mice showed neonatal lethality, and in these mice, intramembranous ossification was more severely defective than endochondral ossification, presumably because of the greater involvement of Runx2-I, compared with that of Runx2-II in intramembranous ossification. Interestingly, the depletion of neo rescued the above-described phenotypes, indicating that the isoform-specific N-terminal region of Runx2-I is not functionally essential for bone development. Taken together, our results provide a novel clue leading to a better understanding of the roles of Runx2 isoforms in osteoblast development.