Calcineurin/NFAT signaling in osteoblasts regulates bone mass

Segmental bone defects: from cellular and molecular pathways to the development of novel biological treatments

Several conditions in clinical orthopaedic practice can lead to the development of a diaphyseal segmental bone defect, which cannot heal without intervention. Segmental bone defects have been traditionally treated with bone grafting and/or distraction osteogenesis, methods that have many advantages, but also major drawbacks, such as limited availability, risk of disease transmission and prolonged treatment. In order to overcome such limitations, biological treatments have been developed based on specific pathways of bone physiology and healing. Bone tissue engineering is a dynamic field of research, combining osteogenic cells, osteoinductive factors, such as bone morphogenetic proteins, and scaffolds with osteoconductive and osteoinductive attributes, to produce constructs that could be used as bone graft substitutes for the treatment of segmental bone defects. Scaffolds are usually made of ceramic or polymeric biomaterials, or combinations of both in composite materials. The purpose of the present review is to discuss in detail the molecular and cellular basis for the development of bone tissue engineering constructs.

NFAT, immunity and cancer: a transcription factor comes of age

Nuclear factor of activated T cells (NFAT) was first identified more than two decades ago as a major stimulation-responsive DNA-binding factor and transcriptional regulator in T cells. It is now clear that NFAT proteins have important functions in other cells of the immune system and regulate numerous developmental programmes in vertebrates. Dysregulation of these programmes can lead to malignant growth and cancer. This Review focuses on recent advances in our understanding of the transcriptional functions of NFAT proteins in the immune system and provides new insights into their potential roles in cancer development.

Progressive ankylosis protein (ANK) in osteoblasts and osteoclasts controls bone formation and bone remodeling

The progressive ankylosis gene (ank) encodes a transmembrane protein that transports intracellular inorganic pyrophosphate (PP(i)) to the extracellular milieu. ank/ank mice, which express a truncated nonfunctional ANK, showed a markedly reduced bone mass, bone-formation rate, and number of tartrate-resistant acid phosphatase-positive (TRAP(+)) multinucleated osteoclasts. ANK function deficiency suppressed osteoblastic differentiation of ank/ank bone marrow stromal cells, as indicated by the decrease in the expression of bone marker genes, including osterix, reduced alkaline phosphatase activity, and mineralization. Runx2 gene expression levels were not altered. Conversely, overexpression of ANK in the preosteoblastic cell line MC3T3-E1 resulted in increased expression of bone marker genes, including osterix. Whereas runx2 expression was not altered in ANK-overexpressing MC3T3-E1 cells, runx2 transcriptional activity was increased. Extracellular PP(i) or P(i) stimulated osteoblastogenic differentiation of MC3T3-E1 cells or partially rescued delayed osteoblastogenic differentiation of ank/ank bone marrow stromal cells. A loss of PP(i) transport function ANK mutation also stimulated osteoblastogenic differentiation of MC3T3-E1 cells. Furthermore, ANK function deficiency suppressed the formation of multinucleated osteoclasts from ank/ank bone marrow cells cultured in the presence of macrophage colony-stimulating factor and receptor activator of nuclear factor-kappaB ligand. In conclusion, ANK is a positive regulator of osteoblastic and osteoclastic differentiation events toward a mature osteoblastic and osteoclastic phenotype.

Cherubism gene Sh3bp2 is important for optimal bone formation, osteoblast differentiation, and function

INTRODUCTION

Cherubism is a human genetic disorder that causes bilateral symmetrical enlargement of the maxilla and the mandible in children. It is caused by mutations in SH3BP2. The exact pathogenesis of the disorder is an area of active research. Sh3bp2 knock-in mice were developed by introducing a Pro416Arg mutation (Pro418Arg in humans) in the mouse genome. The osteoclast phenotype of this mouse model was recently described.

METHODS

We examined the bone phenotype of the cherubism mouse model, the role of Sh3bp2 during bone formation, osteoblast differentiation, and osteoblast function.

RESULTS

We observed delays in early postnatal development of homozygous Sh3bp2(KI/KI) mice, which exhibited increased growth plate thickness and significantly decreased trabecular bone thickness and bone mineral density. Histomorphometric and microcomputed tomography analyses showed bone loss in the cranial and appendicular skeletons. Sh3bp2(KI/KI) mice also exhibited a significant decrease in osteoid formation that indicated a defect in osteoblast function. Calvarial osteoblast cell cultures had decreased alkaline phosphatase expression and mineralization, suggesting reduced differentiation potential. Gene expression of osteoblast differentiation markers such as collagen type I, alkaline phosphatase, and osteocalcin were decreased in osteoblast cultures from Sh3bp2(KI/KI) mice.

CONCLUSIONS

These data suggest that Sh3bp2 regulates bone homeostasis through not only osteoclast-specific effects, but also through effects on osteoblast differentiation and function.

Activation of NFAT signaling in podocytes causes glomerulosclerosis

Mutant forms of TRPC6 can activate NFAT-dependent transcription in vitro via calcium influx and activation of calcineurin. The same TRPC6 mutants can cause FSGS, but whether this involves an NFAT-dependent mechanism is unknown. Here, we generated mice that allow conditional induction of NFATc1. Mice with NFAT activation in nascent podocytes in utero developed proteinuria and glomerulosclerosis postnatally, resembling FSGS. NFAT activation in adult mice also caused progressive proteinuria and FSGS. Ultrastructural studies revealed podocyte foot process effacement and deposition of extracellular matrix. NFAT activation did not initially affect expression of podocin, synaptopodin, and nephrin but reduced their expression as glomerular injury progressed. In contrast, we observed upregulation of Wnt6 and Fzd9 in the mutant glomeruli before the onset of significant proteinuria, suggesting a potential role for Wnt signaling in the pathogenesis of NFAT-induced podocyte injury and FSGS. These results provide in vivo evidence for the involvement of NFAT signaling in podocytes, proteinuria, and glomerulosclerosis. Furthermore, this study suggests that NFAT activation may be a key intermediate step in the pathogenesis of mutant TRPC6-mediated FSGS and that suppression of NFAT activity may contribute to the antiproteinuric effects of calcineurin inhibitors.

Essential role of nuclear factor of activated T cells (NFAT)-mediated Wnt signaling in osteoblast differentiation induced by strontium ranelate

The antiosteoporotic treatment strontium ranelate (SrRan) was shown to increase bone mass and strength by dissociating bone resorption and bone formation. To identify the molecular mechanisms of action of SrRan on osteoblasts, we investigated its effects on calcineurin-NFAT (nuclear factor of activated T cells) signaling, an important calcium sensitive pathway controlling bone formation. Using murine MC3T3-E1 and primary murine osteoblasts, we demonstrate that SrRan induces NFATc1 nuclear translocation, as shown by immunocytochemical and Western blot analyses. Molecular analysis showed that SrRan increased NFATc1 transactivation in osteoblasts, an effect that was fully abrogated by the calcineurin inhibitors cyclosporin A or FK506, confirming that SrRan activates NFATc1 signaling in osteoblasts. This has functional implications because calcineurin inhibitors blunted the enhanced osteoblast replication and expression of the osteoblast phenotypic markers Runx2, alkaline phosphatase, and type I collagen induced by SrRan. We further found that SrRan increased the expression of Wnt3a and Wnt5a as well as beta-catenin transcriptional activity in osteoblasts, and these effects were abolished by calcineurin inhibitors. The Wnt inhibitors sFRP1 and DKK1 abolished SrRan-induced osteoblast gene expression. Furthermore, blunting the Wnt5a receptor Ryk or RhoA that acts downstream of Ryk abrogated cell proliferation and osteoblast gene expression induced by SrRan. These results indicate that activation of NFATc1 and downstream canonical and non-canonical Wnt signaling pathways mediate SrRan-induced osteoblastic cell replication and differentiation, which provides novel insights into the mechanisms of action of this antiosteoporotic agent in osteoblastogenesis.

Connective tissue growth factor (CTGF) transactivates nuclear factor of activated T-cells (NFAT) in cells of the osteoblastic lineage

Connective tissue growth factor (CTGF), a member of the Cyr 61, CTGF, Nov (CCN) family of proteins, regulates multiple cellular functions. Overexpression of CTGF in vivo causes osteopenia, but in vitro CTGF can induce osteoblastogenesis. To investigate mechanisms involved in the effects of CTGF on osteoblastic cell differentiation, we examined whether CTGF modifies the activity of nuclear factor of activated T-cells (NFATc) 1, a transcription factor that cooperates with osterix in the formation of new bone. CTGF increased the transactivation of a transiently transfected reporter construct, where 9 NFAT binding sites direct the expression of luciferase (9xNFAT-Luc) and the activity of the Regulators of calcineurin 1 exon 4 (Rcan1.4) promoter, an NFAT target gene. We postulated that CTGF could modify the phosphorylation of NFAT by regulating glycogen synthase kinase 3beta (GSK3beta). CTGF increased the mRNA levels of Protein kinase cyclic guanosine monophosphate (cGMP) dependent type II (Prkg2), the gene encoding for cGMP dependent protein kinase II (cGKII) which phosphorylates GSK3beta. Accordingly, CTGF induced GSK3beta phosphorylation and decreased the active pool of GSK3beta, a kinase that phosphorylates NFAT and leads to its nuclear export. As a consequence, CTGF favored the nuclear localization of NFATc1. Downregulation of PRKG2 by RNA interference reversed the effect of CTGF on the transactivation of the 9xNFAT reporter construct and the Rcan 1.4 promoter, confirming the role of cGKII in the activation of NFAT by CTGF. In conclusion, CTGF enhances NFAT signaling through the induction of cGKII and the phosphorylation of GSK3beta.

Parathyroid hormone induction of cyclooxygenase-2 in murine osteoblasts: role of the calcium-calcineurin-NFAT pathway

Murine MC3T3-E1 and MC-4 cells were stably transfected with -371/+70 bp of the murine cyclooxygenase-2 (COX-2) promoter fused to a luciferase reporter (Pluc371) or with Pluc371 carrying site-directed mutations. Mutations were made in (1) the cAMP response element (CRE) at -57/-52 bp, (2) the activating protein-1 (AP-1)-binding site at -69/-63 bp, (3) the nuclear factor of activated T-cells (NFAT)-binding site at -77/-73 bp, and (4) both the AP-1 and NFAT sites, which comprise a composite consensus sequence for NFAT/AP-1. Single mutation of CRE, AP-1, or NFAT sites decreased parathyroid hormone (PTH)-stimulated COX-2 promoter activity 40% to 60%, whereas joint mutation of NFAT and AP-1 abrogated the induction. On electrophoretic mobility shift analysis, PTH stimulated binding of phosphorylated CREB to an oligonucleotide spanning the CRE and binding of NFATc1, c-Fos, and c-Jun to an oligonucleotide spanning the NFAT/AP-1 composite site. Mutation of the NFAT site was less effective than mutation of the AP-1 site in competing binding to the composite element, suggesting that cooperative interactions of NFATc1 and AP-1 are more dependent on NFAT than on AP-1. Both PTH and forskolin, an activator of adenylyl cyclase, stimulated NFATc1 nuclear translocation. PTH- and forskolin-stimulated COX-2 promoter activity was inhibited 56% to 80% by calcium chelation or calcineurin inhibitors and 60% to 98% by protein kinase A (PKA) inhibitors. These results indicate an important role for the calcium-calcineurin-NFAT signaling pathway in the PTH induction of COX-2 and suggest that cross-talk between the cAMP/PKA pathway and the calcium-calcineurin-NFAT pathway may play a role in other functions of PTH in osteoblasts.

Calcineurin plays an important role in the shell formation of pearl oyster (Pinctada fucata)

Calcineurin (CN) is a multifunctional protein involved in many important physiological processes in mammalians, but the function of CN in mollusks is still largely unknown. In the present study, through the shell regeneration system, the changes of enzymatic activity of CN were determined in the process of shell regeneration in pearl oyster Pinctada fucata. CN was activated immediately and continuously in the shell regeneration process. The speed of shell regeneration was measured and the ultrastructure of inner shell surface was observed by scanning electron microscopy after inhibiting CN by intramuscular injection of immunosuppresant cyclosporine A (CsA). The results showed that the speed of shell regeneration was delayed and the morphology of calcite and aragonite in the inner shell surface became abnormal when CN was inhibited by CsA. Meanwhile, RT-PCR analysis revealed that the expression of P. fucata BMP-2 in mantle tissue decreased with CsA injection. In vitro secretion level of proteoglycans (PGs) in primary cultures of mantle cells was also decreased when mantle cells were exposed to CsA. Taken together, our results, for the first time, show that CN is involved in the shell formation through regulating the expression of Pf-BMP-2 in mantle tissue, which controls the secretion of PGs/GAGs of the mantle epithelial cells.

Blimp1-mediated repression of negative regulators is required for osteoclast differentiation

Regulation of irreversible cell lineage commitment depends on a delicate balance between positive and negative regulators, which comprise a sophisticated network of transcription factors. Receptor activator of NF-kappaB ligand (RANKL) stimulates the differentiation of bone-resorbing osteoclasts through the induction of nuclear factor of activated T cells c1 (NFATc1), the essential transcription factor for osteoclastogenesis. Osteoclast-specific robust induction of NFATc1 is achieved through an autoamplification mechanism, in which NFATc1 is constantly activated by calcium signaling while the negative regulators of NFATc1 are suppressed. However, it has been unclear how such negative regulators are repressed during osteoclastogenesis. Here we show that B lymphocyte-induced maturation protein-1 (Blimp1; encoded by Prdm1), which is induced by RANKL through NFATc1 during osteoclastogenesis, functions as a transcriptional repressor of anti-osteoclastogenic genes such as Irf8 and Mafb. Overexpression of Blimp1 leads to an increase in osteoclast formation, and Prdm1-deficient osteoclast precursor cells do not undergo osteoclast differentiation efficiently. The importance of Blimp1 in bone homeostasis is underscored by the observation that mice with an osteoclast-specific deficiency in the Prdm1 gene exhibit a high bone mass phenotype caused by a decreased number of osteoclasts. Thus, NFATc1 choreographs the determination of cell fate in the osteoclast lineage by inducing the repression of negative regulators as well as through its effect on positive regulators.

Corosolic acid stimulates osteoblast differentiation by activating transcription factors and MAP kinases

Recently, the use of anabolic agents to enhance bone mass has been a source of interest. Previous work by us suggested that corosolic acid (2alpha-hydroxyursolic acid), an active component of banaba leaves (Lagerstroemia speciosa L.), potentially stimulates the differentiation of mouse osteoblasts. Therefore, the present study investigated whether corosolic acid stimulates osteoblast differentiation, and its possible mechanisms of action. At low concentrations (up to 5 microm), corosolic acid significantly stimulated osteoblast differentiation and mineralization without cytotoxicity. Corosolic acid induced NF-kappaB and MAP kinase activity at an early stage of osteoblast differentiation and increased the activity of the transcription factor AP-1 during late-stage osteoblast differentiation. These results suggest that the anabolic effects of corosolic acid upon osteoblast differentiation could result from its activation of transcription factors and MAP kinases.

Transcriptional regulation of bone and joint remodeling by NFAT

Osteoporosis and arthritis are highly prevalent diseases and a significant cause of morbidity and mortality worldwide. These diseases result from aberrant tissue remodeling leading to weak, fracture-prone bones or painful, dysfunctional joints. The nuclear factor of activated T cells (NFAT) transcription factor family controls diverse biologic processes in vertebrates. Here, we review the scientific evidence that links NFAT-regulated gene transcription to bone and joint pathology. A particular emphasis is placed on the role of NFATs in bone resorption and formation by osteoclasts and osteoblasts, respectively. In addition, emerging data that connect NFATs with cartilage biology, angiogenesis, nociception, and neurogenic inflammation are explored. The goal of this article is to highlight the importance of tissue remodeling in musculoskeletal disease and situate NFAT-driven cellular responses within this context to inspire future research endeavors.

Nuclear factor of activated T cells mediates fluid shear stress- and tensile strain-induced Cox2 in human and murine bone cells

Mechanical loading such as interstitial fluid shear stress and tensile strain stimulates bone cells, which respond by changing bone mass and structure to maintain optimal skeletal architecture. Bone cells also adapt to bone implants and altered mechanical loading. Osseous integration between host bone and implants is a prerequisite for the stability of implants. Fluctuating fluid pressure and interfacial strains occur between bone cells and implants due to mechanical loading during walking and other daily activities. In this study, we examined the signaling mechanism by which mechanical stimulation activates a novel transcription factor in human and mouse bone cells. Nuclear factor of activated T cells (NFAT) is one of the transcription factors that act downstream of the Ca(++)/calcineurin (Ca(++)/Cn) network: a well-known pathway of inflammation. In this study, we hypothesized that NFAT2 is activated in response to mechanical stimulation and mediates Cox2 expression. Fluid shear stress and tensile strain results in nuclear translocation of NFAT in cells of the osteoblastic lineage. A peptide inhibitor of the Cn/NFAT axis was found to block the mechanical stimulation-mediated Cox2 induction. Further, chromatin immunoprecipitation assay shows direct interaction between NFAT2 and the human Cox2 promoter region. Additionally, CnAbeta knockout calvarial bone cells were found to be less sensitive than control bone cells to mechanical stimulation. Our study provides new evidence for a novel role for NFAT in bone mechanotransduction in the context of cytokine gene induction in bone cells.

High-density association study of 383 candidate genes for volumetric BMD at the femoral neck and lumbar spine among older men

Genetics is a well-established but poorly understood determinant of BMD. Whereas some genetic variants may influence BMD throughout the body, others may be skeletal site specific. We initially screened for associations between 4608 tagging and potentially functional single nucleotide polymorphisms (SNPs) in 383 candidate genes and femoral neck and lumbar spine volumetric BMD (vBMD) measured from QCT scans among 862 community-dwelling white men >or=65 yr of age in the Osteoporotic Fractures in Men Study (MrOS). The most promising SNP associations (p < 0.01) were validated by genotyping an additional 1156 white men from MrOS. This analysis identified 8 SNPs in 6 genes (APC, DMP1, FGFR2, FLT1, HOXA, and PTN) that were associated with femoral neck vBMD and 13 SNPs in 7 genes (APC, BMPR1B, FOXC2, HOXA, IGFBP2, NFATC1, and SOST) that were associated with lumbar spine vBMD in both genotyping samples (p < 0.05). Although most associations were specific to one skeletal site, SNPs in the APC and HOXA gene regions were associated with both femoral neck and lumbar spine BMD. This analysis identifies several novel and robust genetic associations for volumetric BMD, and these findings in combination with other data suggest the presence of genetic loci for volumetric BMD that are at least to some extent skeletal-site specific.

Negative feedback Inhibition of NFATc1 by DYRK1A regulates bone homeostasis

DYRK1A is a serine/threonine kinase that has been linked to mental retardation associated with Down syndrome. In the present report, we describe a previously unknown role for DYRK1A in bone homeostasis. The protein expression of DYRK1A increased during osteoclast differentiation. In vitro studies in osteoclasts revealed that DYRK1A inhibited osteoclastogenesis. Whereas DYRK1A phosphorylated and inhibited the osteoclastogenic transcription factor NFATc1, forced expression of NFATc1 induced DYRK1A expression, suggesting a negative feedback loop. Transgenic mice overexpressing DYRK1A by the extent of the increased gene dosage in Down syndrome exhibited significantly reduced bone mass despite the decreased osteoclastogenesis, which is reminiscent of osteoporotic bone phenotype in Down syndrome patients. In these mice, attenuated osteoblast differentiation and function in the presence of extra DYRK1A overrode the effect of impaired osteoclastogenesis. However, impeded osteoclastogenesis in DYRK1A transgenic mice was proven to be beneficial in protecting bone loss induced by inflammation or estrogen deficiency. These results provide novel insight into the role for DYRK1A in bone homeostasis as well as in bone destructive diseases, in which modulation of DYRK1A might be used as a strategy to treat unregulated bone resorption.

Ca2+-NFATc1 signaling is an essential axis of osteoclast differentiation

Osteoclasts are unique, multinucleated giant cells that decalcify and degrade the bone matrix. They originate from hematopoietic cells and their differentiation is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor-kappaB (NF-kappaB) ligand (RANKL), as well as macrophage-colony stimulating factor (M-CSF). Recent studies have unveiled the precise molecular mechanism underlying osteoclastogenesis. In particular, the discovery of nuclear factor of activated T cells c1 (NFATc1), the master regulator of osteoclastogenesis, has proven to be a breakthrough in this field. NFATc1 is activated by Ca2+ signaling induced by the activation of the immunoglobulin-like receptor signaling associated with immunoreceptor tyrosine-based activation motif (ITAM)-harboring adapters. The long-lasting Ca2+ oscillation, which is evident during osteoclastogenesis, may ensure the robust induction of NFATc1 through an autoamplification mechanism. Thus, intracellular Ca2+ is a critical attribute of osteoclastogenic signaling. In addition, osteoclasts are exposed to a very high extracellular Ca2+ concentration ([Ca2+]o) in the bone microenvironment and respond to the change in [Ca2+]o by increasing the intracellular Ca2+, which regulates diverse cellular functions. Investigation of the molecular mechanisms underlying the regulation of intracellular Ca2+ dynamics may open up new directions for therapeutic strategies in bone disease.

Mechanical loading regulates NFATc1 and beta-catenin signaling through a GSK3beta control node

Mechanical stimulation can prevent adipogenic and improve osteogenic lineage allocation of mesenchymal stem cells (MSC), an effect associated with the preservation of beta-catenin levels. We asked whether mechanical up-regulation of beta-catenin was critical to reduction in adipogenesis as well as other mechanical events inducing alternate MSC lineage selection. In MSC cultured under strong adipogenic conditions, mechanical load (3600 cycles/day, 2% strain) inactivated GSK3beta in a Wnt-independent fashion. Small interfering RNA targeting GSK3beta prevented both strain-induced induction of beta-catenin and an increase in COX2, a factor associated with increased osteoprogenitor phenotype. Small interfering RNA knockdown of beta-catenin blocked mechanical reduction of peroxisome proliferator-activated receptor gamma and adiponectin, implicating beta-catenin in strain inhibition of adipogenesis. In contrast, the effect of both mechanical and pharmacologic inhibition of GSK3beta on the putative beta-catenin target, COX2, was unaffected by beta-catenin knockdown. GSK3beta inhibition caused accumulation of nuclear NFATc1; mechanical strain increased nuclear NFATc1, independent of beta-catenin. NFATc1 knockdown prevented mechanical stimulation of COX2, implicating NFATc1 signaling. Finally, inhibition of GSK3beta caused association of RNA polymerase II with the COX2 gene, suggesting transcription initiation. These results demonstrate that mechanical inhibition of GSK3beta induces activation of both beta-catenin and NFATc1 signaling, limiting adipogenesis via the former and promoting osteoblastic differentiation via NFATc1/COX2. Our novel findings suggest that mechanical loading regulates mesenchymal stem cell differentiation through inhibition of GSK3beta, which in turn regulates multiple downstream effectors.

Concerted stimuli regulating osteo-chondral differentiation from stem cells: phenotype acquisition regulated by microRNAs

Bone and cartilage are being generated de novo through concerted actions of a plethora of signals. These act on stem cells (SCs) recruited for lineage-specific differentiation, with cellular phenotypes representing various functions throughout their life span. The signals are rendered by hormones and growth factors (GFs) and mechanical forces ensuring proper modelling and remodelling of bone and cartilage, due to indigenous and programmed metabolism in SCs, osteoblasts, chondrocytes, as well as osteoclasts and other cell types (eg T helper cells).This review focuses on the concerted action of such signals, as well as the regulatory and/or stabilizing control circuits rendered by a class of small RNAs, designated microRNAs. The impact on cell functions evoked by transcription factors (TFs) via various signalling molecules, also encompassing mechanical stimulation, will be discussed featuring microRNAs as important members of an integrative system. The present approach to cell differentiation in vitro may vastly influence cell engineering for in vivo tissue repair.

Stimulatory effect of undecylenic acid on mouse osteoblast differentiation

Natural compounds with bone-forming (or anabolic) activity have been recently focused on in bone research. The present study investigated the effect of undecylenic acid (UA) on osteoblast differentiation in mouse osteoblastic MC3T3-E1 subclone 4 cells and primary mouse calvarial cells. Low concentrations of UA (up to 5 microM) exhibited no cytotoxicity and significantly increased the expression and activity of alkaline phosphatase (early differentiation marker of osteoblast) and calcium deposition with the induction of expression of the osteocalcin gene in both cells. Interestingly, at low concentration of UA, the induction of NF-kappaB p65 translocation into nucleus and the up-regulation of AP-1 and NFATc1 transcript levels were also observed, suggesting that the stimulatory effect of UA on osteoblast differentiation could be mediated through the activation of transcription factors. Additionally, although the patterns of UA-induced activation of MAP kinases (JNK and p38) were not completely consistent with the increase of both ALP activity and calcium deposition by UA, MAP kinases might be partially involved in the biological function of UA during the early and late stages of osteoblast differentiation.

NFATc1 mediates HDAC-dependent transcriptional repression of osteocalcin expression during osteoblast differentiation

We previously reported that the in vivo and in vitro suppression of Nuclear Factor of Activated T Cells (NFAT) signaling increases osteoblast differentiation and bone formation. To investigate the mechanism by which NFATc1 regulates osteoblast differentiation, we established an osteoblast cell line that overexpresses a constitutively active NFATc1 (ca-NFATc1). The activation of NFATc1 significantly inhibits osteoblast differentiation and function, demonstrated by inhibition of alkaline phosphatase activity and mineralization as well as a decrease in gene expression of early and late markers of osteoblast differentiation such as osterix and osteocalcin, respectively. By focusing on the specific role of NFATc1 during late differentiation, we discovered that the inhibition of osteocalcin gene expression by NFATc1 was associated with a repression of the osteocalcin promoter activity, and a decrease in TCF/LEF transactivation. Also, overexpression of NFATc1 completely blocked the decrease in total histone deacetylase (HDAC) activity during osteoblast differentiation and prevented the hyperacetylation of histones H3 and H4. Mechanistically, we show by Chromatin Immunoprecipitation (ChIP) assay that the overexpression of NFATc1 sustains the binding of HDAC3 on the proximal region of the osteocalcin promoter, resulting in complete hypoacetylation of histones H3 and H4 when compared to GFP-expressing osteoblasts. In contrast, the inhibition of NFATc1 nuclear translocation either by cyclosporin or by using primary mouse osteoblasts with deleted calcineurin b1 prevents HDAC3 from associating with the proximal regulatory site of the osteocalcin promoter. These preliminary results suggest that NFATc1 acts as a transcriptional co-repressor of osteocalcin promoter, possibly in an HDAC-dependent manner.

Mechanoregulation of proliferation

The proliferation of all nontransformed adherent cells is dependent upon the development of mechanical tension within the cell; however, little is known about the mechanisms by which signals regulated by mechanical tension are integrated with those regulated by growth factors. We show here that Skp2, a component of a ubiquitin ligase complex that mediates the degradation of several proteins that inhibit proliferation, is upregulated when increased mechanical tension develops in intact smooth muscle and that its upregulation is critical for the smooth muscle proliferative response to increased mechanical tension. Notably, whereas growth factors regulate Skp2 at the level of protein stability, we found that mechanical tension regulates Skp2 at the transcriptional level. Importantly, we demonstrate that the calcium-regulated transcription factor NFATc1 is a critical mediator of the effect of increased mechanical tension on Skp2 transcription. These findings identify Skp2 as a node at which signals from mechanical tension and growth factors are integrated to regulate proliferation, and they define calcium-NFAT-Skp2 signaling as a critical pathway in the mechanoregulation of proliferation.

Osteoimmunology: crosstalk between the immune and bone systems

INTRODUCTION

The interaction between the immune and skeletal systems has long been acknowledged, but investigation into rheumatoid arthritis (RA) as well as the various bone phenotypes found in immunocompromised gene-deficient mice has highlighted the importance of the dynamic interplay between the two systems. This has led to the recent emergence and subsequent rapid evolution of the field of osteoimmunology. BONE DESTRUCTION WITH ARTHRITIS AS A RANKL DISEASE: In the bone destruction associated with RA, IL-17-producing helper T cells (T(H)17) play a major role by inducing receptor activator of nuclear factor-kappaB ligand (RANKL). RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity.

NEW PLAYERS IN OSTEOIMMUNOLOGY

In addition to cellular interactions via cytokines, the immune and skeletal systems share various molecules, including transcription factors, signaling molecules, and membrane receptors.

CONCLUSION

The scope of osteoimmunology has grown to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases of both the immune and skeletal systems.

Endocardial cells are a distinct endothelial lineage derived from Flk1+ multipotent cardiovascular progenitors

Identification of multipotent cardiac progenitors has provided important insights into the mechanisms of myocardial lineage specification, yet has done little to clarify the origin of the endocardium. Despite its essential role in heart development, characterization of the endocardial lineage has been limited by the lack of specific markers of this early vascular subpopulation. To distinguish endocardium from other vasculature, we generated an NFATc1-nuc-LacZ BAC transgenic mouse line capable of labeling this specific endothelial subpopulation at the earliest stages of cardiac development. To further characterize endocardiogenesis, embryonic stem cells (ESCs) derived from NFATc1-nuc-LacZ blastocysts were utilized to demonstrate that endocardial differentiation in vitro recapitulates the close temporal-spatial relationship observed between myocardium and endocardium seen in vivo. Endocardium is specified as a cardiac cell lineage, independent from other vascular populations, responding to BMP and Wnt signals that enhance cardiomyocyte differentiation. Furthermore, a population of Flk1+ cardiovascular progenitors, distinct from hemangioblast precursors, represents a mesodermal precursor of the endocardial endothelium, as well as other cardiovascular lineages. Taken together, these studies emphasize that the endocardium is a unique cardiac lineage and provides further evidence that endocardium and myocardium are derived from a common precursor.

Misexpression of CCAAT/enhancer binding protein beta causes osteopenia

CCAAT/enhancer binding proteins (C/EBPs) are expressed by osteoblasts and adipocytes during differentiation. C/EBP beta is critical for adipogenesis; however, its role in osteoblastogenesis is unclear, and its function in the postnatal skeleton is not known. To study C/EBP beta in osteoblasts in vivo, we created transgenic mice expressing full length C/EBP beta under the control of a 3.8 kb fragment of the human osteocalcin promoter. Two transgenic lines were established in a friend leukemia virus strain B genetic background, and compared with wild type littermate controls. Both C/EBP beta transgenic lines exhibited osteopenia, with a 30% decrease in bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. Bone marrow stromal cells from C/EBP beta transgenics showed reduced mineralization, and reduced alkaline phosphatase mRNA levels. Calvarial osteoblasts from C/EBP beta transgenics displayed reduced alkaline phosphatase activity. To determine the consequences of the Cebpb deletion in vivo, the phenotype of Cebpb null mice was compared with that of wild type controls of identical genetic composition. Cebpb null mice exhibited reduced weight, body fat, and bone mineral density, and decreased bone volume, due to a decrease in trabecular number. The number of osteoblasts and osteoclasts per bone perimeter was not changed. C/EBP beta downregulation by RNA interference in calvarial osteoblasts had no effect on osteoblast differentiation/function. The phenotype of the Cebpb inactivation may be secondary to systemic indirect effects, and to direct effects of C/EBP beta in osteoblasts. In conclusion, C/EBP beta plays a role in mesenchymal cell differentiation and its misexpression in vivo causes osteopenia.

Requirement for balanced Ca/NFAT signaling in hematopoietic and embryonic development

NFAT transcription factors are highly phosphorylated proteins residing in the cytoplasm of resting cells. Upon dephosphorylation by the phosphatase calcineurin, NFAT proteins translocate to the nucleus, where they orchestrate developmental and activation programs in diverse cell types. NFAT is rephosphorylated and inactivated through the concerted action of at least 3 different kinases: CK1, GSK-3, and DYRK. The major docking sites for calcineurin and CK1 are strongly conserved throughout vertebrate evolution, and conversion of either the calcineurin docking site to a high-affinity version or the CK1 docking site to a low-affinity version results in generation of hyperactivable NFAT proteins that are still fully responsive to stimulation. In this study, we generated transgenic mice expressing hyperactivable versions of NFAT1 from the ROSA26 locus. We show that hyperactivable NFAT increases the expression of NFAT-dependent cytokines by differentiated T cells as expected, but exerts unexpected signal-dependent effects during T cell differentiation in the thymus, and is progressively deleterious for the development of B cells from hematopoietic stem cells. Moreover, progressively hyperactivable versions of NFAT1 are increasingly deleterious for embryonic development, particularly when normal embryos are also present in utero. Forced expression of hyperactivable NFAT1 in the developing embryo leads to mosaic expression in many tissues, and the hyperactivable proteins are barely tolerated in organs such as brain, and cardiac and skeletal muscle. Our results highlight the need for balanced Ca/NFAT signaling in hematopoietic stem cells and progenitor cells of the developing embryo, and emphasize the evolutionary importance of kinase and phosphatase docking sites in preventing inappropriate activation of NFAT.

Mechanotransduction in osteoblast regulation and bone disease

Osteoblasts are key components of the bone multicellular unit and have a seminal role in bone remodeling, which is an essential function for the maintenance of the structural integrity and metabolic capacity of the skeleton. The coordinated function of skeletal cells is regulated by several hormones, growth factors and mechanical cues that act via interconnected signaling networks, resulting in the activation of specific transcription factors and, in turn, their target genes. Bone cells are responsive to mechanical stimuli and this is of pivotal importance in developing biomechanical strategies for the treatment of osteodegenerative diseases. Here, we review the molecular pathways and players activated by mechanical stimulation during osteoblastic growth, differentiation and activity in health, and consider the role of mechanostimulatory approaches in treating various bone pathophysiologies.

Leveraging existing biological knowledge in the identification of candidate genes for facial dysmorphology

Background

In response to the frequently overwhelming output of high-throughput microarray experiments, we propose a methodology to facilitate interpretation of biological data in the context of existing knowledge. Through the probabilistic integration of explicit and implicit data sources a functional interaction network can be constructed. Each edge connecting two proteins is weighted by a confidence value capturing the strength and reliability of support for that interaction given the combined data sources. The resulting network is examined in conjunction with expression data to identify groups of genes with significant temporal or tissue specific patterns. In contrast to unstructured gene lists, these networks often represent coherent functional groupings.

Results

By linking from shared functional categorizations to primary biological resources we apply this method to craniofacial microarray data, generating biologically testable hypotheses and identifying candidate genes for craniofacial development.

Conclusion

The novel methodology presented here illustrates how the effective integration of pre-existing biological knowledge and high-throughput experimental data drives biological discovery and hypothesis generation.

From streaming-potentials to shear stress: 25 years of bone cell mechanotransduction

Mechanical loads are vital regulators of skeletal mass and architecture as evidenced by the increase in bone formation following the addition of exogenous loads and loss of bone mass following their removal. While our understanding of the molecular mechanisms by which bone cells perceive changes in their mechanical environment has increased rapidly in recent years, much remains to be learned. Here, we outline the effects of interstitial fluid flow, a potent biophysical signal induced by the deformation of skeletal tissue in response to applied loads, on bone cell behavior. We focus on the molecular mechanisms by which bone cells are hypothesized to perceive interstitial fluid flow, the cell signaling cascades activated by fluid flow, and the use of this signal in tissue engineering protocols.

NFATc1 in inflammatory and musculoskeletal conditions

The nuclear factor of activated T-cells (NFAT) family of transcription factors specify developmental pathways and cell fate in vertebrates. NFATc1, in particular, is crucial to multiple seemingly unrelated biologic processes, including heart valve formation, T-cell activation, osteoclast development, and the mitigation of hair follicle stem cell proliferation. Here, we review how our recently generated NFATc1 conditional knockout mouse has contributed to our understanding of this transcription factor in inflammatory and musculoskeletal conditions and their treatment.

Tie2Cre-mediated inactivation of plexinD1 results in congenital heart, vascular and skeletal defects

PlexinD1 is a membrane-bound receptor that mediates signals derived from class 3 secreted semaphorins. Although semaphorin signaling in axon guidance in the nervous system has been extensively studied, functions outside the nervous system including important roles in vascular patterning have also been demonstrated. Inactivation of plexinD1 leads to neo-natal lethality, structural defects of the cardiac outflow tract, peripheral vascular abnormalities, and axial skeletal morphogenesis defects. PlexinD1 is expressed by vascular endothelial cells, but additional domains of expression have also been demonstrated including in lymphocytes, osteoblasts, neural crest and the central nervous system. Hence, the cell-type specific functions of plexinD1 have remained unclear. Here, we describe the results of tissue-specific gene inactivation of plexinD1 in Tie2 expressing precursors, which recapitulates the null phenotype with respect to congenital heart, vascular, and skeletal abnormalities resulting in neonatal lethality. Interestingly, these mutants also have myocardial defects not previously reported. In addition, we demonstrate functions for plexinD1 in post-natal retinal vasculogenesis and adult angiogenesis through the use of inducible cre-mediated deletion. These results demonstrate an important role for PlexinD1 in embryonic and adult vasculature.

Baicalein stimulates osteoblast differentiation via coordinating activation of MAP kinases and transcription factors

The identification of anabolic agents that directly stimulate bone formation has recently attracted greater interest. Here, baicalein was identified as a natural compound that stimulates the differentiation of mouse osteoblastic MC3T3-E1 subclone 4 cells. Baicalein induced the activation of NF-kappaB in the initiation stage of osteoblast differentiation, and it activated the MAP kinase/NF-kappaB signaling pathway and induced the expression of osteoblast differentiation markers in the early stage. In the late stage, baicalein stimulated the calcium deposition with the activation of MAP kinases and AP-1 family members such as Fra-1 and Fra-2. Another transcription factor, NFATc1, was slightly induced by baicalein in the late stage. Thus, baicalein could stimulate the osteoblast differentiation via the activation of complexly coordinated signaling pathways that include MAP kinases and transcription factors such as NF-kappaB, AP-1, and NFATc1.

DYRK3 dual-specificity kinase attenuates erythropoiesis during anemia

During anemia erythropoiesis is bolstered by several factors including KIT ligand, oncostatin-M, glucocorticoids, and erythropoietin. Less is understood concerning factors that limit this process. Experiments performed using dual-specificity tyrosine-regulated kinase-3 (DYRK3) knock-out and transgenic mice reveal that erythropoiesis is attenuated selectively during anemia. DYRK3 is restricted to erythroid progenitor cells and testes. DYRK3-/- mice exhibited essentially normal hematological profiles at steady state and reproduced normally. In response to hemolytic anemia, however, reticulocyte production increased severalfold due to DYRK3 deficiency. During 5-fluorouracil-induced anemia, both reticulocyte and red cell formation in DYRK3-/- mice were elevated. In short term transplant experiments, DYRK3-/- progenitors also supported enhanced erythroblast formation, and erythropoietic advantages due to DYRK3-deficiency also were observed in 5-fluorouracil-treated mice expressing a compromised erythropoietin receptor EPOR-HM allele. As analyzed ex vivo, DYRK3-/- erythroblasts exhibited enhanced CD71posTer119pos cell formation and 3HdT incorporation. Transgenic pA2gata1-DYRK3 mice, in contrast, produced fewer reticulocytes during hemolytic anemia, and pA2gata1-DYRK3 progenitors were compromised in late pro-erythroblast formation ex vivo. Finally, as studied in erythroid K562 cells, DYRK3 proved to effectively inhibit NFAT (nuclear factor of activated T cells) transcriptional response pathways and to co-immunoprecipitate with NFATc3. Findings indicate that DYRK3 attenuates (and possibly apportions) red cell production selectively during anemia.

NFATc1 in mice represses osteoprotegerin during osteoclastogenesis and dissociates systemic osteopenia from inflammation in cherubism

Osteoporosis results from an imbalance in skeletal remodeling that favors bone resorption over bone formation. Bone matrix is degraded by osteoclasts, which differentiate from myeloid precursors in response to the cytokine RANKL. To gain insight into the transcriptional regulation of bone resorption during growth and disease, we generated a conditional knockout of the transcription factor nuclear factor of activated T cells c1 (Nfatc1). Deletion of Nfatc1 in young mice resulted in osteopetrosis and inhibition of osteoclastogenesis in vivo and in vitro. Transcriptional profiling revealed NFATc1 as a master regulator of the osteoclast transcriptome, promoting the expression of numerous genes needed for bone resorption. In addition, NFATc1 directly repressed osteoclast progenitor expression of osteoprotegerin, a decoy receptor for RANKL previously thought to be an osteoblast-derived inhibitor of bone resorption. "Cherubism mice", which carry a gain-of-function mutation in SH3-domain binding protein 2 (Sh3bp2), develop osteoporosis and widespread inflammation dependent on the proinflammatory cytokine, TNF-alpha. Interestingly, deletion of Nfatc1 protected cherubism mice from systemic bone loss but did not inhibit inflammation. Taken together, our study demonstrates that NFATc1 is required for remodeling of the growing and adult skeleton and suggests that NFATc1 may be an effective therapeutic target for osteoporosis associated with inflammatory states.

Unique ERalpha cistromes control cell type-specific gene regulation

Estrogens play an important role in normal physiology and in a variety of pathological states involving diverse tissues including breast and bone. The mechanism by which estrogens exert cell type- and disease-specific effects, however, remains to be explained. We have compared the gene expression profile of the MCF7 breast cancer cell line with that of the osteoblast-like cell line U2OS-ERalpha by expression microarrays. We find that fewer than 10% of the 17beta-estradiol (E2)-regulated genes are common to both cell types. We have validated this in primary calvarial osteoblasts. To dissect the mechanism underlying the cell type-specific E2 regulation of gene expression in MCF7 and U2OS-ERalpha cells, we compared the ERalpha binding sites on DNA in the two cell types by performing chromatin immunoprecipitation (ChIP) on genomic tiling arrays (ChIP-on-chip). Consistent with the distinct patterns of E2-regulated gene expression in these two cell lines, we find that the vast majority of ERalpha binding sites are also cell type specific and correlate both in position and number with cell type-specific gene regulation. Interestingly, although the forkhead factor FoxA1 plays a critical role in defining the ERalpha cistrome in MCF7 cells, it is not expressed in U2OS-ERalpha cells, and forkhead motifs are not enriched in the ERalpha cistrome in these cells. Finally, the ERalpha cistromes are correlated with cell type-specific epigenetic histone modifications. These results support a model for the cell type-specific action of E2 being driven primarily through specific ERalpha occupancy of epigenetically marked cis-regulatory regions of target genes.

Osteosarcoma development and stem cell differentiation

Osteosarcoma is the most common nonhematologic malignancy of bone in children and adults. The peak incidence occurs in the second decade of life, with a smaller peak after age 50. Osteosarcoma typically arises around the growth plate of long bones. Most osteosarcoma tumors are of high grade and tend to develop pulmonary metastases. Despite clinical improvements, patients with metastatic or recurrent diseases have a poor prognosis. Here, we reviewed the current understanding of human osteosarcoma, with an emphasis on potential links between defective osteogenic differentiation and bone tumorigenesis. Existing data indicate osteosarcoma tumors display a broad range of genetic and molecular alterations, including the gains, losses, or arrangements of chromosomal regions, inactivation of tumor suppressor genes, and the deregulation of major signaling pathways. However, except for p53 and/or RB mutations, most alterations are not constantly detected in the majority of osteosarcoma tumors. With a rapid expansion of our knowledge about stem cell biology, emerging evidence suggests osteosarcoma should be regarded as a differentiation disease caused by genetic and epigenetic changes that interrupt osteoblast differentiation from mesenchymal stem cells. Understanding the molecular pathogenesis of human osteosarcoma could ultimately lead to the development of diagnostic and prognostic markers, as well as targeted therapeutics for osteosarcoma patients.

Skeletal overexpression of connective tissue growth factor impairs bone formation and causes osteopenia

Connective tissue growth factor (CTGF), a member of the CCN family of proteins, is expressed in skeletal cells, and the ctgf null mutation leads to neonatal lethality due to defects in skeletal development. To define the function of CTGF in the postnatal skeleton, we created transgenic mice overexpressing CTGF under the control of the human osteocalcin promoter. CTGF transgenic female and male mice exhibited a significant decrease in bone mineral density, compared with wild-type littermate controls. Bone histomorphometry revealed that CTGF overexpression caused decreased trabecular bone volume due to impaired osteoblastic activity because mineral apposition and bone formation rates were decreased. Osteoblast and osteoclast number and bone resorption were not altered. Calvarial osteoblasts and stromal cells from CTGF transgenics displayed decreased alkaline phosphatase and osteocalcin mRNA levels and reduced bone morphogenetic protein (BMP) signaling mothers against decapentaplegic, Wnt/beta-catenin, and IGF-I/Akt signaling. In conclusion, CTGF overexpression in vivo causes osteopenia, secondary to decreased bone formation, possibly by antagonizing BMP, Wnt, and IGF-I signaling and activity.

SH3BP2 is an activator of NFAT activity and osteoclastogenesis

Heterozygous activating mutations in exon 9 of SH3BP2 have been found in most patients with cherubism, an unusual genetic syndrome characterized by excessive remodeling of the mandible and maxilla due to spontaneous and excessive osteoclastic bone resorption. Osteoclasts differentiate after binding of sRANKL to RANK induces a number of downstream signaling effects, including activation of the calcineurin/NFAT (nuclear factor of activated T cells) pathway. Here, we have investigated the functional significance of SH3BP2 protein on osteoclastogenesis in the presence of sRANKL. Our results indicate that SH3BP2 both increases nuclear NFATc1 in sRANKL treated RAW 264.7 preosteoclast cells and enhances expression of tartrate resistant acid phosphatase (TRAP), a specific marker of osteoclast differentiation. Moreover, overexpression of SH3BP2 in RAW 264.7 cells potentiates sRANKL-stimulated phosphorylation of PLCgamma1 and 2, thus providing a mechanistic pathway for the rapid translocation of NFATc1 into the nucleus and increased osteoclastogenesis in cherubism.

Synaptotagmin VII regulates bone remodeling by modulating osteoclast and osteoblast secretion

Maintenance of bone mass and integrity requires a tight balance between resorption by osteoclasts and formation by osteoblasts. Exocytosis of functional proteins is a prerequisite for the activity of both cells. In the present study, we show that synaptotagmin VII, a calcium sensor protein that regulates exocytosis, is associated with lysosomes in osteoclasts and bone matrix protein-containing vesicles in osteoblasts. Absence of synaptotagmin VII inhibits cathepsin K secretion and formation of the ruffled border in osteoclasts and bone matrix protein deposition in osteoblasts, without affecting the differentiation of either cell. Reflecting these in vitro findings, synaptotagmin VII-deficient mice are osteopenic due to impaired bone resorption and formation. Therefore, synaptotagmin VII plays an important role in bone remodeling and homeostasis by modulating secretory pathways functionally important in osteoclasts and osteoblasts.

Notch signaling in osteoblasts

Bone remodeling is the result of the coordinated activity of osteoblasts, which form new matrix, and osteoclasts, which resorb bone. Notch proteins are single-pass transmembrane receptors that determine cell fate. Recent gain-of-function and loss-of-function experiments reveal a suppressive effect of Notch in osteoblast and osteoclast differentiation in development and in the postnatal bone, which establishes a role for Notch signaling in bone remodeling.

Osteoclast-osteoblast communication

Cells in osteoclast and osteoblast lineages communicate with each other through cell-cell contact, diffusible paracrine factors and cell-bone matrix interaction. Osteoclast-osteoblast communication occurs in a basic multicellular unit (BMU) at the initiation, transition and termination phases of bone remodeling. At the initiation phase, hematopoietic precursors are recruited to the BMU. These precursors express cell surface receptors including c-Fms, RANK and costimulatory molecules, such as osteoclast-associated receptor (OSCAR), and differentiate into osteoclasts following cell-cell contact with osteoblasts, which express ligands. Subsequently, the transition from bone resorption to formation is mediated by osteoclast-derived 'coupling factors', which direct the differentiation and activation of osteoblasts in resorbed lacunae to refill it with new bone. Bidirectional signaling generated by interaction between ephrinB2 on osteoclasts and EphB4 on osteoblast precursors facilitates the transition. Such interaction is likely to occur between osteoclasts and lining cells in the bone remodeling compartment (BRC). At the termination phase, bone remodeling is completed by osteoblastic bone formation and mineralization of bone matrix. Here, we describe molecular communication between osteoclasts and osteoblasts at distinct phases of bone remodeling.

Molecular genetic studies of gene identification for osteoporosis

This review comprehensively summarizes the most important and representative molecular genetics studies of gene identification for osteoporosis published up to the end of September 2007. It is intended to constitute a sequential update of our previously published reviews covering the available data up to the end of 2004. Evidence from candidate gene-association studies, genome-wide linkage and association studies, as well as functional genomic studies (including gene-expression microarray and proteomics) on osteogenesis and osteoporosis, are reviewed separately. Studies of transgenic and knockout mice models relevant to osteoporosis are summarized. The major results of all studies are tabulated for comparison and ease of reference. Comments are made on the most notable findings and representative studies for their potential influence and implications on our present understanding of genetics of osteoporosis. The format adopted by this review should be ideal for accommodating future new advances and studies.

Transcription factors controlling osteoblastogenesis

The recent development of molecular biology and mouse genetics and the analysis of the skeletal phenotype induced by genetic mutations in humans led to a better understanding of the role of transcription factors that govern bone formation. This review summarizes the role of transcription factors in osteoblastogenesis and provides an integrated perspective on how the activities of multiple classes of factors are coordinated for the complex process of developing the osteoblast phenotype. The roles of Runx2, the principal transcriptional regulator of osteoblast differentiation, Osterix, beta-Catenin and ATF which act downstream of Runx2, and other transcription factors that contribute to the control of osteoblastogenesis including the AP1, C/EBPs, PPARgamma and homeodomain, helix-loop-helix proteins are discussed. This review also updates the regulation of transcription factor expression by signaling factors and hormones that control osteoblastogenesis.

NFATc1 balances quiescence and proliferation of skin stem cells

Quiescent adult stem cells reside in specialized niches where they become activated to proliferate and differentiate during tissue homeostasis and injury. How stem cell quiescence is governed is poorly understood. We report here that NFATc1 is preferentially expressed by hair follicle stem cells in their niche, where its expression is activated by BMP signaling upstream and it acts downstream to transcriptionally repress CDK4 and maintain stem cell quiescence. As stem cells become activated during hair growth, NFATc1 is downregulated, relieving CDK4 repression and activating proliferation. When calcineurin/NFATc1 signaling is suppressed, pharmacologically or via complete or conditional NFATc1 gene ablation, stem cells are activated prematurely, resulting in precocious follicular growth. Our findings may explain why patients receiving cyclosporine A for immunosuppressive therapy display excessive hair growth, and unveil a functional role for calcium-NFATc1-CDK4 circuitry in governing stem cell quiescence.