Fibroblast growth factor receptor 1 signaling in the osteo-chondrogenic cell lineage regulates sequential steps of osteoblast maturation

Wnt co-receptors Lrp5 and Lrp6 differentially mediate Wnt3a signaling in osteoblasts

Wnt3a is a major regulator of bone metabolism however, very few of its target genes are known in bone. Wnt3a preferentially signals through transmembrane receptors Frizzled and co-receptors Lrp5/6 to activate the canonical signaling pathway. Previous studies have shown that the canonical Wnt co-receptors Lrp5 and Lrp6 also play an essential role in normal postnatal bone homeostasis, yet, very little is known about specific contributions by these co-receptors in Wnt3a-dependent signaling. We used high-throughput sequencing technology to identify target genes regulated by Wnt3a in osteoblasts and to elucidate the role of Lrp5 and Lrp6 in mediating Wnt3a signaling. Our study identified 782 genes regulated by Wnt3a in primary calvarial osteoblasts. Wnt3a up-regulated the expression of several key regulators of osteoblast proliferation/ early stages of differentiation while inhibiting genes expressed in later stages of osteoblastogenesis. We also found that Lrp6 is the key mediator of Wnt3a signaling in osteoblasts and Lrp5 played a less significant role in mediating Wnt3a signaling.

Defects in chondrocyte maturation and secondary ossification in mouse knee joint epiphyses due to Snorc deficiency

OBJECTIVE

The role of Snorc, a novel cartilage specific transmembrane proteoglycan, was studied during skeletal development using two Snorc knockout mouse models. Hypothesizing that Snorc, like the other transmembrane proteoglycans, may be a coreceptor, we also studied its interaction with growth factors.

METHODS

Skeletal development was studied in wild type (WT) and Snorc knockout mice during postnatal development by whole mount staining, X-ray imaging, histomorphometry, immunohistochemistry and qRT-PCR. Snorc promoter activity was studied by applying the LacZ reporter expressed by the targeting construct. Slot blot binding and cell proliferation assays were used to study the interaction of Snorc with several growth factors.

RESULTS

Snorc expression was localized in the knee epiphyses especially to the prehypertrophic chondrocytes delineating the cartilage canals and secondary ossification center (SOC). Snorc was demonstrated to have a glycosaminoglycan independent affinity to FGF2 and it inhibited FGF2 dependent cell growth of C3H101/2 cells. In Snorc deficient mice, SOCs in knee epiphyses were smaller, and growth plate (GP) maturation was disturbed, but total bone length was not affected. Central proliferative and hypertrophic zones were enlarged with higher extracellular matrix (ECM) volume and rounded chondrocyte morphology at postnatal days P10 and P22. Increased levels of Ihh and Col10a1, and reduced Mmp13 mRNA expression were observed at P10.

CONCLUSIONS

These findings suggest a role of Snorc in regulation of chondrocyte maturation and postnatal endochondral ossification. The interaction identified between recombinant Snorc core protein and FGF2 suggest functions related to FGF signaling.

Pulmonary fibrosis requires cell-autonomous mesenchymal fibroblast growth factor (FGF) signaling

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive pulmonary scarring, decline in lung function, and often results in death within 3-5 five years after diagnosis. Fibroblast growth factor (FGF) signaling has been implicated in the pathogenesis of IPF; however, the mechanism through which FGF signaling contributes to pulmonary fibrosis remains unclear. We hypothesized that FGF receptor (FGFR) signaling in fibroblasts is required for the fibrotic response to bleomycin. To test this, mice with mesenchyme-specific tamoxifen-inducible inactivation of FGF receptors 1, 2, and 3 (Col1α2-CreER; TCKO mice) were lineage labeled and administered intratracheal bleomycin. Lungs were collected for histologic analysis, whole lung RNA and protein, and dissociated for flow cytometry and FACS. Bleomycin-treated Col1α2-CreER; TCKO mice have decreased pulmonary fibrosis, collagen production, and fewer α-smooth muscle actin-positive (αSMA+) myofibroblasts compared with controls. Freshly isolated Col1α2-CreER; TCKO mesenchymal cells from bleomycin-treated mice have decreased collagen expression compared with wild type mesenchymal cells. Furthermore, lineage labeled FGFR-deficient fibroblasts have decreased enrichment in fibrotic areas and decreased proliferation. These data identify a cell autonomous requirement for mesenchymal FGFR signaling in the development of pulmonary fibrosis, and for the enrichment of the Col1α2-CreER-positive (Col1α2+) mesenchymal lineage in fibrotic tissue following bleomycin exposure. We conclude that mesenchymal FGF signaling is required for the development of pulmonary fibrosis, and that therapeutic strategies aimed directly at mesenchymal FGF signaling could be beneficial in the treatment of IPF.

Non-myogenic Contribution to Muscle Development and Homeostasis: The Role of Connective Tissues

Skeletal muscles belong to the musculoskeletal system, which is composed of bone, tendon, ligament and irregular connective tissue, and closely associated with motor nerves and blood vessels. The intrinsic molecular signals regulating myogenesis have been extensively investigated. However, muscle development, homeostasis and regeneration require interactions with surrounding tissues and the cellular and molecular aspects of this dialogue have not been completely elucidated. During development and adult life, myogenic cells are closely associated with the different types of connective tissue. Connective tissues are defined as specialized (bone and cartilage), dense regular (tendon and ligament) and dense irregular connective tissue. The role of connective tissue in muscle morphogenesis has been investigated, thanks to the identification of transcription factors that characterize the different types of connective tissues. Here, we review the development of the various connective tissues in the context of the musculoskeletal system and highlight their important role in delivering information necessary for correct muscle morphogenesis, from the early step of myoblast differentiation to the late stage of muscle maturation. Interactions between muscle and connective tissue are also critical in the adult during muscle regeneration, as impairment of the regenerative potential after injury or in neuromuscular diseases results in the progressive replacement of the muscle mass by fibrotic tissue. We conclude that bi-directional communication between muscle and connective tissue is critical for a correct assembly of the musculoskeletal system during development as well as to maintain its homeostasis in the adult.

Physiological Bone Remodeling: Systemic Regulation and Growth Factor Involvement

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

Contribution of FGFR1 Variants to Craniofacial Variations in East Asians

FGFR1 plays an important role in the development of the nervous system as well as the regulation of the skeletal development and bone homeostasis. Mutations in FGFR1 genes affect skull development, specifically suture and synchondrosis, resulting in craniosynostosis and facial abnormalities. We examined subjects with normal skull morphology for genetic polymorphisms that might be associated with normal craniofacial variations. Genomic DNA was obtained from 216 Japanese and 227 Korean subjects. Four FGFR1 SNPs, namely, rs881301, rs6996321, rs4647905, and rs13317, were genotyped. These SNPs were tested for association with craniofacial measurements obtained from lateral and posteroanterior cephalometries, in which principle component analysis was performed to compress the data of the craniofacial measurements. We observed that SNPs rs13317 and rs6996321 were correlated with the overall head size and midfacial development, indicating that FGFR1 SNPs played crucial roles in the normal variation of human craniofacial morphology. Subjects with the derived alleles of SNPs rs13317 and rs6996321 had a small face and a facial pattern associated with a retruded midface and relatively wide-set eyes. These facial features were similar to but were milder than those of individuals with Pfeiffer syndrome, which is caused by a dysfunctional mutation in FGFR1.

Characterization and comparison of osteoblasts derived from mouse embryonic stem cells and induced pluripotent stem cells

New developments in stem cell biology offer alternatives for the reconstruction of critical-sized bone defects. One of these developments is the use of induced pluripotent stem (iPS) cells. These stem cells are similar to embryonic stem (ES) cells, but can be generated from adult somatic cells and therefore do not raise ethical concerns. Proper characterization of iPS-derived osteoblasts is important for future development of safe clinical applications of these cells. For this reason, we differentiated mouse ES and iPS cells toward osteoblasts using osteogenic medium and compared their functionality. Immunocytochemical analysis showed significant expression of bone markers (osteocalcin and collagen type I) in osteoblasts differentiated from ES and iPS cells on days 7 and 30. An in vitro mineralization assay confirmed the functionality of osteogenically differentiated ES and iPS cells. Gene expression arrays focusing on osteogenic differentiation were performed in order to compare the gene expression pattern in both differentiated and undifferentiated ES cells and iPS cells. We observed a significant upregulation of osteogenesis-related genes such as Runx2, osteopontin, collagen type I, Tnfsf11, Csf1, and alkaline phosphatase upon osteogenic differentiation of the ES and iPS cells. We further validated the expression of key osteogenic genes Runx2, osteopontin, osteocalcin, collagen type I, and osterix in both differentiated and undifferentiated ES and iPS cells by means of quantified real-time polymerase chain reaction. We conclude that ES and iPS cells are similar in their osteogenic differentiation capacities, as well as in their gene expression patterns.

The Signaling Pathways Involved in Chondrocyte Differentiation and Hypertrophic Differentiation

Chondrocytes communicate with each other mainly via diffusible signals rather than direct cell-to-cell contact. The chondrogenic differentiation of mesenchymal stem cells (MSCs) is well regulated by the interactions of varieties of growth factors, cytokines, and signaling molecules. A number of critical signaling molecules have been identified to regulate the differentiation of chondrocyte from mesenchymal progenitor cells to their terminal maturation of hypertrophic chondrocytes, including bone morphogenetic proteins (BMPs), SRY-related high-mobility group-box gene 9 (Sox9), parathyroid hormone-related peptide (PTHrP), Indian hedgehog (Ihh), fibroblast growth factor receptor 3 (FGFR3), and β-catenin. Except for these molecules, other factors such as adenosine, O₂ tension, and reactive oxygen species (ROS) also have a vital role in cartilage formation and chondrocyte maturation. Here, we outlined the complex transcriptional network and the function of key factors in this network that determine and regulate the genetic program of chondrogenesis and chondrocyte differentiation.

The Morphogenesis of Cranial Sutures in Zebrafish

Using morphological, histological, and TEM analyses of the cranium, we provide a detailed description of bone and suture growth in zebrafish. Based on expression patterns and localization, we identified osteoblasts at different degrees of maturation. Our data confirm that, unlike in humans, zebrafish cranial sutures maintain lifelong patency to sustain skull growth. The cranial vault develops in a coordinated manner resulting in a structure that protects the brain. The zebrafish cranial roof parallels that of higher vertebrates and contains five major bones: one pair of frontal bones, one pair of parietal bones, and the supraoccipital bone. Parietal and frontal bones are formed by intramembranous ossification within a layer of mesenchyme positioned between the dermal mesenchyme and meninges surrounding the brain. The supraoccipital bone has an endochondral origin. Cranial bones are separated by connective tissue with a distinctive architecture of osteogenic cells and collagen fibrils. Here we show RNA in situ hybridization for col1a1a, col2a1a, col10a1, bglap/osteocalcin, fgfr1a, fgfr1b, fgfr2, fgfr3, foxq1, twist2, twist3, runx2a, runx2b, sp7/osterix, and spp1/ osteopontin, indicating that the expression of genes involved in suture development in mammals is preserved in zebrafish. We also present methods for examining the cranium and its sutures, which permit the study of the mechanisms involved in suture patency as well as their pathological obliteration. The model we develop has implications for the study of human disorders, including craniosynostosis, which affects 1 in 2,500 live births.

Ablation of Perlecan Domain 1 Heparan Sulfate Reduces Progressive Cartilage Degradation, Synovitis, and Osteophyte Size in a Preclinical Model of Posttraumatic Osteoarthritis

OBJECTIVE

To investigate the role of the heparan sulfate (HS) proteoglycan perlecan (HSPG-2) in regulating fibroblast growth factor (FGF) activity, bone and joint growth, and the onset and progression of posttraumatic osteoarthritis (OA) in a mouse gene-knockout model.

METHODS

Maturational changes were evaluated histologically in the knees of 3-, 6-, and 12-week-old wild-type (WT) mice and Hspg2(Δ3-/Δ3-) mice (Hspg2 lacking domain 1 HS, generated by ablation of exon 3 of perlecan). Cartilage damage, subchondral bone sclerosis, osteophytosis, and synovial inflammation were scored at 4 and 8 weeks after surgical induction of OA in WT and Hspg2(Δ3-/Δ3-) mice. Changes in cartilage expression of FGF-2, FGF-18, HSPG-2, FGF receptor 1 (FGFR-1), and FGFR-3 were examined immunohistochemically. Femoral head cartilage from both mouse genotypes was cultured in the presence or absence of interleukin-1α (IL-1α), FGF-2, and FGF-18, and the content and release of glycosaminoglycan (GAG) and expression of messenger RNA (mRNA) for key matrix molecules, enzymes, and inhibitors were quantified.

RESULTS

No effect of perlecan HS ablation on growth plate or joint development was detected. After induction of OA, Hspg2(Δ3-/Δ3-) mice had significantly reduced cartilage erosion, osteophytosis, and synovitis. OA-induced loss of chondrocyte expression of FGF-2, FGF-18, and HSPG-2 occurred in both genotypes. Expression of FGFR-1 after OA induction was maintained in WT mice, while FGFR-3 loss after OA induction was significantly reduced in Hspg2(Δ3-/Δ3-) mice. There were no genotypic differences in GAG content or release between unstimulated control cartilage and IL-1α-stimulated cartilage. However, IL-1α-induced cartilage expression of Mmp3 mRNA was significantly reduced in Hspg2(Δ3-/Δ3-) mice. Cartilage GAG release in either the presence or absence of IL-1α was unaltered by FGF-2 in both genotypes. In cartilage cultures with FGF-18, IL-1α-stimulated GAG loss was significantly reduced only in Hspg2(Δ3-/Δ3-) mice, and this was associated with maintained expression of Fgfr3 mRNA and reduced expression of Mmp2/Mmp3 mRNA.

CONCLUSION

Perlecan HS has significant roles in directing the development of posttraumatic OA, potentially via the alteration of FGF/HS/FGFR signaling. These data suggest that the chondroprotection conferred by perlecan HS ablation could be attributed, at least in part, to the preservation of FGFR-3 and increased FGF signaling.

FGF signaling in the osteoprogenitor lineage non-autonomously regulates postnatal chondrocyte proliferation and skeletal growth

Fibroblast growth factor (FGF) signaling is important for skeletal development; however, cell-specific functions, redundancy and feedback mechanisms regulating bone growth are poorly understood. FGF receptors 1 and 2 (Fgfr1 and Fgfr2) are both expressed in the osteoprogenitor lineage. Double conditional knockout mice, in which both receptors were inactivated using an osteoprogenitor-specific Cre driver, appeared normal at birth; however, these mice showed severe postnatal growth defects that include an ∼50% reduction in body weight and bone mass, and impaired longitudinal bone growth. Histological analysis showed reduced cortical and trabecular bone, suggesting cell-autonomous functions of FGF signaling during postnatal bone formation. Surprisingly, the double conditional knockout mice also showed growth plate defects and an arrest in chondrocyte proliferation. We provide genetic evidence of a non-cell-autonomous feedback pathway regulating Fgf9, Fgf18 and Pthlh expression, which led to increased expression and signaling of Fgfr3 in growth plate chondrocytes and suppression of chondrocyte proliferation. These observations show that FGF signaling in the osteoprogenitor lineage is obligately coupled to chondrocyte proliferation and the regulation of longitudinal bone growth.

Hedgehog inhibits β-catenin activity in synovial joint development and osteoarthritis

Both the WNT/β-catenin and hedgehog signaling pathways are important in the regulation of limb development, chondrocyte differentiation, and degeneration of articular cartilage in osteoarthritis (OA). It is not clear how these signaling pathways interact in interzone cell differentiation and synovial joint morphogenesis. Here, we determined that constitutive activation of hedgehog signaling specifically within interzone cells induces joint morphological changes by selectively inhibiting β-catenin-induced Fgf18 expression. Stabilization of β-catenin or treatment with FGF18 rescued hedgehog-induced phenotypes. Hedgehog signaling induced expression of a dominant negative isoform of TCF7L2 (dnTCF7L2) in interzone progeny, which may account for the selective regulation of β-catenin target genes observed. Knockdown of TCF7L2 isoforms in mouse chondrocytes rescued hedgehog signaling-induced Fgf18 downregulation, while overexpression of the human dnTCF7L2 orthologue (dnTCF4) in human chondrocytes promoted the expression of catabolic enzymes associated with OA. Similarly, expression of dnTCF4 in human chondrocytes positively correlated with the aggrecanase ADAMTS4. Consistent with our developmental findings, activation of β-catenin also attenuated hedgehog-induced or surgically induced articular cartilage degeneration in mouse models of OA. Thus, our results demonstrate that hedgehog inhibits selective β-catenin target gene expression to direct interzone progeny fates and articular cartilage development and disease. Moreover, agents that increase β-catenin activity have the potential to therapeutically attenuate articular cartilage degeneration as part of OA.

A combined series of Fgf9 and Fgf18 mutant alleles identifies unique and redundant roles in skeletal development

Fibroblast growth factor (FGF) signaling is a critical regulator of skeletal development. Fgf9 and Fgf18 are the only FGF ligands with identified functions in embryonic bone growth. Mice lacking Fgf9 or Fgf18 have distinct skeletal phenotypes; however, the extent of overlapping or redundant functions for these ligands and the stage-specific contributions of FGF signaling to chondrogenesis and osteogenesis are not known. To identify separate versus shared roles for FGF9 and FGF18, we generated a combined series of Fgf9 and Fgf18 null alleles. Analysis of embryos lacking alleles of Fgf9 and Fgf18 shows that both encoded ligands function redundantly to control all stages of skeletogenesis; however, they have variable potencies along the proximodistal limb axis, suggesting gradients of activity during formation of the appendicular skeleton. Congenital absence of both Fgf9 and Fgf18 results in a striking osteochondrodysplasia and revealed functions for FGF signaling in early proximal limb chondrogenesis. Additional defects were also noted in craniofacial bones, vertebrae, and ribs. Loss of alleles of Fgf9 and Fgf18 also affect the expression of genes encoding other key intrinsic skeletal regulators, including IHH, PTHLH (PTHrP), and RUNX2, revealing potential direct, indirect, and compensatory mechanisms to coordinate chondrogenesis and osteogenesis.

Evidence-Based Theory for Integrated Genome Regulation of Ontogeny--An Unprecedented Role of Nuclear FGFR1 Signaling

Genetic experiments have positioned the fgfr1 gene at the top of the gene hierarchy that governs gastrulation, as well as the subsequent development of the major body axes, nervous system, muscles, and bones, by affecting downstream genes that control the cell cycle, pluripotency, and differentiation, as well as microRNAs. Studies show that this regulation is executed by a single protein, the nuclear isoform of FGFR1 (nFGFR1), which integrates signals from development‐initiating factors, such as retinoic acid (RA), and operates at the interface of genomic and epigenomic information. nFGFR1 cooperates with a multitude of transcriptional factors (TFs), and targets thousands of genes encoding for mRNAs, as well as miRNAs in top ontogenic networks. nFGFR1 binds to the promoters of ancient proto‐oncogenes and tumor suppressor genes, in addition to binding to metazoan morphogens that delineate body axes, and construct the nervous system, as well as mesodermal and endodermal tissues. The discovery of pan‐ontogenic gene programming by integrative nuclear FGFR1 signaling (INFS) impacts our understanding of ontogeny, as well as developmental pathologies, and holds new promise for reconstructive medicine, and cancer therapy. J. Cell. Physiol. 231: 1199–1218, 2016. © 2016 The Authors. Journal of Cellular Physiology published by Wiley Periodicals, Inc.

The cartilage extracellular matrix as a transient developmental scaffold for growth plate maturation

The cartilage growth plate is a specialized developmental tissue containing characteristic zonal arrangements of chondrocytes. The proliferative and differentiative states of chondrocytes are tightly regulated at all stages including the initial limb bud and rudiment cartilage stages of development, the establishment of the primary and secondary ossification centers, development of the growth plates and laying down of bone. A multitude of spatio-temporal signals, including transcription factors, growth factors, morphogens and hormones, control chondrocyte maturation and terminal chondrocyte differentiation/hypertrophy, cell death/differentiation, calcification and vascular invasion of the growth plate and bone formation during morphogenetic transition of the growth plate. This involves hierarchical, integrated signaling from growth and factors, transcription factors, mechanosensory cues and proteases in the extracellular matrix to regulate these developmental processes to facilitate progressive changes in the growth plate culminating in bone formation and endochondral ossification. This review provides an overview of selected components which have particularly important roles in growth plate biology including collagens, proteoglycans, glycosaminoglycans, growth factors, proteases and enzymes.

Conditional Deletion of Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Gene Reveals Its Essential Role in Chondrocyte Function and Endochondral Bone Formation

The hypoxic growth plate cartilage requires hypoxia-inducible factor (HIF)-mediated pathways to maintain chondrocyte survival and differentiation. HIF proteins are tightly regulated by prolyl hydroxylase domain-containing protein 2 (Phd2)-mediated proteosomal degradation. We conditionally disrupted the Phd2 gene in chondrocytes by crossing Phd2 floxed mice with type 2 collagen-α1-Cre transgenic mice and found massive increases (>50%) in the trabecular bone mass of long bones and lumbar vertebra of the Phd2 conditional knockout (cKO) mice caused by significant increases in trabecular number and thickness and reductions in trabecular separation. Cortical thickness and tissue mineral density at the femoral middiaphysis of the cKO mice were also significantly increased. Dynamic histomorphometric analyses revealed increased longitudinal length and osteoid surface per bone surface in the primary spongiosa of the cKO mice, suggesting elevated conversion rate from hypertrophic chondrocytes to mineralized bone matrix as well as increased bone formation in the primary spongiosa. In the secondary spongiosa, bone formation measured by mineralizing surface per bone surface and mineral apposition rate were not changed, but resorption was slightly reduced. Increases in the mRNA levels of SRY (sex determining region Y)-box 9, osterix (Osx), type 2 collagen, aggrecan, alkaline phosphatase, bone sialoprotein, vascular endothelial growth factor, erythropoietin, and glycolytic enzymes in the growth plate of cKO mice were detected by quantitative RT-PCR. Immunohistochemistry revealed an increased HIF-1α protein level in the hypertrophic chondrocytes of cKO mice. Infection of chondrocytes isolated from Phd2 floxed mice with adenoviral Cre resulted in similar gene expression patterns as observed in the cKO growth plate chondrocytes. Our findings indicate that Phd2 suppresses endochondral bone formation, in part, via HIF-dependent mechanisms in mice.

Effects of FGFR Signaling on Cell Proliferation and Differentiation of Apert Dental Cells

The Apert syndrome is a rare congenital disorder most often arising from S252W or P253R mutations in fibroblast growth factor receptor (FGFR2). Numerous studies have focused on the regulatory role of Apert FGFR2 signaling in bone formation, whereas its functional role in tooth development is largely unknown. To investigate the role of FGFR signaling in cell proliferation and odontogenic differentiation of human dental cells in vitro, we isolated dental pulp and enamel organ epithelia (EOE) tissues from an Apert patient carrying the S252W FGFR2 mutation. Apert primary pulp and EOE cells were established and shown to exhibit normal morphology and express alkaline phosphatase under differentiation conditions. Similar to control cells, Apert dental pulp and EOE cells expressed all FGFRs, with highest levels of FGFR1 followed by FGFR2 and low levels of FGFR3 and FGFR4. However, Apert cells had increased cell growth compared with control cells. Distinct from previous findings in osteoblast cells, gain-of-function S252W FGFR2 mutation did not upregulate the expression of epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFRα), but elevated extracellular signal-regulated kinase (ERK) signaling in cells after EGF stimulation. Unexpectedly, there was little effect of the S252W mutation on odontogenic gene expression in dental pulp and EOE cells. However, after inhibition of total FGFR signaling or ERK signaling, the expression of odontogenic genes was upregulated in both dental cell types, indicating the negative effect of whole FGFR signaling on odontogenic differentiation. This study provides novel insights on FGFR signaling and a common Apert FGFR2 mutation in the regulation of odontogenic differentiation of dental mesenchymal and epithelial cells.

Mesenchymal fibroblast growth factor receptor signaling regulates palatal shelf elevation during secondary palate formation

BACKGROUND

Palatal shelf elevation is an essential morphogenetic process during secondary palate closure and failure or delay of palatal shelf elevation is a common cause of cleft palate, one of the most common birth defects in humans. Here, we studied the role of mesenchymal fibroblast growth factor receptor (FGFR) signaling during palate development by conditional inactivation of Fgfrs using a mesenchyme-specific Dermo1-Cre driver.

RESULTS

We showed that Fgfr1 is expressed throughout the palatal mesenchyme and Fgfr2 is expressed in the medial aspect of the posterior palatal mesenchyme overlapping with Fgfr1. Mesenchyme-specific disruption of Fgfr1 and Fgfr2 affected palatal shelf elevation and resulted in cleft palate. We further showed that both Fgfr1 and Fgfr2 are expressed in mesenchymal tissues of the mandibular process but display distinct expression patterns. Loss of mesenchymal FGFR signaling reduced mandibular ossification and lower jaw growth resulting in abnormal tongue insertion in the oral-nasal cavity.

CONCLUSIONS

We propose a model to explain how redundant Fgfr1 and Fgfr2 expression in the palatal and mandibular mesenchyme regulates shelf medial wall protrusion and growth of the mandible to coordinate the craniofacial tissue movements that are required for palatal shelf elevation.

Wntless spatially regulates bone development through β-catenin-dependent and independent mechanisms

BACKGROUND

Canonical and noncanonical Wnt signaling pathways both play pivotal roles in bone development. Wntless/GPR177 is a chaperone protein that is required for secretion of all Wnt ligands. We previously showed that deletion of Wntless within mature osteoblasts severely impaired postnatal bone homeostasis.

RESULTS

In this study, we systemically evaluated how deletion of Wntless in different stages of osteochondral differentiation affected embryonic bone development, by crossing Wntless (Wls)-flox/flox mice with strains expressing cre recombinase behind the following promoters: Osteocalcin, Collagen 2a1, or Dermo1. Ex vivo µCT and whole-mount skeletal staining were performed to examine skeletal mineralization. Histology and immunohistochemistry were used to evaluate cellular differentiation and alterations in Wnt signaling. In this work, we found that Wntless regulated chondrogenesis and osteogenesis through both canonical and noncanonical Wnt signaling.

CONCLUSIONS

These findings provide more insight into the requirements of different Wnt-secretion cell types critical for skeletal development.

Mesenchymal progenitor cells for the osteogenic lineage

Mesenchymal progenitors of the osteogenic lineage provide the flexibility for bone to grow, maintain its function and homeostasis. Traditionally, colony-forming-unit fibroblasts (CFU-Fs) have been regarded as surrogates for mesenchymal progenitors; however, this definition cannot address the function of these progenitors in their native setting. Transgenic murine models including lineage-tracing technologies based on the cre-lox system have proven to be useful in delineating mesenchymal progenitors in their native environment. Although heterogeneity of cell populations of interest marked by a promoter-based approach complicates overall interpretation, an emerging complexity of mesenchymal progenitors has been revealed. Current literatures suggest two distinct types of bone progenitor cells; growth-associated mesenchymal progenitors contribute to explosive growth of bone in early life, whereas bone marrow mesenchymal progenitors contribute to the much slower remodeling process and response to injury that occurs mainly in adulthood. More detailed relationships of these progenitors need to be studied through further experimentation.

Fibroblast growth factor signaling in skeletal development and disease

Fibroblast growth factor (FGF) signaling pathways are essential regulators of vertebrate skeletal development. FGF signaling regulates development of the limb bud and formation of the mesenchymal condensation and has key roles in regulating chondrogenesis, osteogenesis, and bone and mineral homeostasis. This review updates our review on FGFs in skeletal development published in Genes & Development in 2002, examines progress made on understanding the functions of the FGF signaling pathway during critical stages of skeletogenesis, and explores the mechanisms by which mutations in FGF signaling molecules cause skeletal malformations in humans. Links between FGF signaling pathways and other interacting pathways that are critical for skeletal development and could be exploited to treat genetic diseases and repair bone are also explored.

The PTH-Vitamin D-FGF23 axis

Fibroblast growth factor 23 (FGF23) has emerged as an important regulator of phosphate and vitamin D homeostasis. It is important to understand how FGF23 interacts with vitamin D and parathyroid hormone (PTH) in a FGF23-Vitamin D-PTH axis to regulate mineral homeostasis. In this review, we discuss the genomic structure, and transcriptional, translational, and posttranslational regulation of FGF23. We describe its interaction with PTH and vitamin D, disorders of altered FGF23 states, and emerging therapies for diseases of FGF23 based upon these findings. This discussion helps redefine the role of PTH and vitamin D in relation to a complex bone-kidney-parathyroid loop, and points to areas within this complicated field in need of further clarification and research.

The Fibroblast Growth Factor signaling pathway

The signaling component of the mammalian Fibroblast Growth Factor (FGF) family is comprised of eighteen secreted proteins that interact with four signaling tyrosine kinase FGF receptors (FGFRs). Interaction of FGF ligands with their signaling receptors is regulated by protein or proteoglycan cofactors and by extracellular binding proteins. Activated FGFRs phosphorylate specific tyrosine residues that mediate interaction with cytosolic adaptor proteins and the RAS-MAPK, PI3K-AKT, PLCγ, and STAT intracellular signaling pathways. Four structurally related intracellular non-signaling FGFs interact with and regulate the family of voltage gated sodium channels. Members of the FGF family function in the earliest stages of embryonic development and during organogenesis to maintain progenitor cells and mediate their growth, differentiation, survival, and patterning. FGFs also have roles in adult tissues where they mediate metabolic functions, tissue repair, and regeneration, often by reactivating developmental signaling pathways. Consistent with the presence of FGFs in almost all tissues and organs, aberrant activity of the pathway is associated with developmental defects that disrupt organogenesis, impair the response to injury, and result in metabolic disorders, and cancer. For further resources related to this article, please visit the WIREs website.

ERK1 and ERK2 regulate chondrocyte terminal differentiation during endochondral bone formation

Chondrocytes in the epiphyseal cartilage undergo terminal differentiation prior to their removal through apoptosis. To examine the role of ERK1 and ERK2 in chondrocyte terminal differentiation, we generated Osterix (Osx)-Cre; ERK1(-/-) ; ERK2(flox/flox) mice (conditional knockout Osx [cKOosx]), in which ERK1 and ERK2 were deleted in hypertrophic chondrocytes. These cKOosx mice were grossly normal in size at birth, but by 3 weeks of age exhibited shorter long bones. Histological analysis in these mice revealed that the zone of hypertrophic chondrocytes in the growth plate was markedly expanded. In situ hybridization and quantitative real-time PCR analyses demonstrated that Matrix metalloproteinase-13 (Mmp13) and Osteopontin expression was significantly decreased, indicating impaired chondrocyte terminal differentiation. Moreover, Egr1 and Egr2, transcription factors whose expression is restricted to the last layers of hypertrophic chondrocytes in wild-type mice, were also strongly downregulated in these cKOosx mice. In transient transfection experiments in the RCS rat chondrosarcoma cell line, the expression of Egr1, Egr2, or a constitutively active mutant of MEK1 increased the activity of an Osteopontin promoter, whereas the MEK1-induced activation of the Osteopontin promoter was inhibited by the coexpression of Nab2, an Egr1 and Egr2 co-repressor. These results suggest that MEK1-ERK signaling activates the Osteopontin promoter in part through Egr1 and Egr2. Finally, our histological analysis of cKOosx mice demonstrated enchondroma-like lesions in the bone marrow that are reminiscent of human metachondromatosis, a skeletal disorder caused by mutations in PTPN11. Our observations suggest that the development of enchondromas in metachondromatosis may be caused by reduced extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK MAPK) signaling.

A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

Decades of work have identified the signaling pathways that regulate the differentiation of chondrocytes during bone formation, from their initial induction from mesenchymal progenitor cells to their terminal maturation into hypertrophic chondrocytes. Here, we review how multiple signaling molecules, mechanical signals and morphological cell features are integrated to activate a set of key transcription factors that determine and regulate the genetic program that induces chondrogenesis and chondrocyte differentiation. Moreover, we describe recent findings regarding the roles of several signaling pathways in modulating the proliferation and maturation of chondrocytes in the growth plate, which is the 'engine' of bone elongation.

Membrane and integrative nuclear fibroblastic growth factor receptor (FGFR) regulation of FGF-23

Fibroblastic growth factor receptor 1 (FGFR1) signaling pathways are implicated in the regulation of FGF-23 gene transcription, but the molecular pathways remain poorly defined. We used low molecular weight (LMW, 18 kDa) FGF-2 and high molecular weight (HMW) FGF-2 isoforms, which, respectively, activate cell surface FGF receptors and intranuclear FGFR1, to determine the roles of membrane FGFRs and integrative nuclear FGFR1 signaling (INFS) in the regulation of FGF-23 gene transcription in osteoblasts. We found that LMW-FGF-2 induced NFAT and Ets1 binding to conserved cis-elements in the proximal FGF-23 promoter and stimulated FGF-23 promoter activity through PLCγ/calcineurin/NFAT and MAPK pathways in SaOS-2 and MC3T3-E1 osteoblasts. In contrast, HMW-FGF-2 stimulated FGF-23 promoter activity in osteoblasts through a cAMP-dependent binding of FGFR1 and cAMP-response element-binding protein (CREB) to a conserved cAMP response element (CRE) contiguous with the NFAT binding site in the FGF-23 promoter. Mutagenesis of the NFAT and CRE binding sites, respectively, inhibited the effects of LMW-FGF-2 and HMW-FGF-23 to stimulate FGF-23 promoter activity. FGF-2 activation of both membrane FGFRs and INFS-dependent FGFR1 pathways may provide a means to integrate systemic and local regulation of FGF-23 transcription under diverse physiological and pathological conditions.

FGFR1 signaling in hypertrophic chondrocytes is attenuated by the Ras-GAP neurofibromin during endochondral bone formation

Aberrant fibroblast growth factor receptor 3 (FGFR3) signaling disrupts chondrocyte proliferation and growth plate size and architecture, leading to various chondrodysplasias or bone overgrowth. These observations suggest that the duration, intensity and cellular context of FGFR signaling during growth plate chondrocyte maturation require tight, regulated control for proper bone elongation. However, the machinery fine-tuning FGFR signaling in chondrocytes is incompletely defined. We report here that neurofibromin, a Ras-GAP encoded by Nf1, has an overlapping expression pattern with FGFR1 and FGFR3 in prehypertrophic chondrocytes, and with FGFR1 in hypertrophic chondrocytes during endochondral ossification. Based on previous evidence that neurofibromin inhibits Ras-ERK signaling in chondrocytes and phenotypic analogies between mice with constitutive FGFR1 activation and Nf1 deficiency in Col2a1-positive chondrocytes, we asked whether neurofibromin is required to control FGFR1-Ras-ERK signaling in maturing chondrocytes in vivo. Genetic Nf1 ablation in Fgfr1-deficient chondrocytes reactivated Ras-ERK1/2 signaling in hypertrophic chondrocytes and reversed the expansion of the hypertrophic zone observed in mice lacking Fgfr1 in Col2a1-positive chondrocytes. Histomorphometric and gene expression analyses suggested that neurofibromin, by inhibiting Rankl expression, attenuates pro-osteoclastogenic FGFR1 signaling in hypertrophic chondrocytes. We also provide evidence suggesting that neurofibromin in prehypertrophic chondrocytes, downstream of FGFRs and via an indirect mechanism, is required for normal extension and organization of proliferative columns. Collectively, this study indicates that FGFR signaling provides an important input into the Ras-Raf-MEK-ERK1/2 signaling axis in chondrocytes, and that this input is differentially regulated during chondrocyte maturation by a complex intracellular machinery, of which neurofibromin is a critical component.

Bringing new life to damaged bone: the importance of angiogenesis in bone repair and regeneration

Bone has the unique capacity to heal without the formation of a fibrous scar, likely because several of the cellular and molecular processes governing bone healing recapitulate the events during skeletal development. A critical component in bone healing is the timely appearance of blood vessels in the fracture callus. Angiogenesis, the formation of new blood vessels from pre-existing ones, is stimulated after fracture by the local production of numerous angiogenic growth factors. The fracture vasculature not only supplies oxygen and nutrients, but also stem cells able to differentiate into osteoblasts and in a later phase also the ions necessary for mineralization. This review provides a concise report of the regulation of angiogenesis by bone cells, its importance during bone healing and its possible therapeutic applications in bone tissue engineering. This article is part of a Special Issue entitled "Stem Cells and Bone".

Secreted klotho protein attenuates osteogenic differentiation of human bone marrow mesenchymal stem cells in vitro via inactivation of the FGFR1/ERK signaling pathway

Increasing evidence indicates that the osteogenic differentiation of mesenchymal stem cells (MSCs) is related to bone formation, heterotopic ossification, and even vascular calcification. Therefore, it is essential to understand the microenvironment that regulates these processes. The Klotho gene plays an important role in tissue mineralization, and its secreted protein functions as a hormone. We investigated the effects of secreted Klotho protein on the osteogenesis of human bone marrow MSC (hBMSCs). To this end, the cells received osteogenic medium with or without Klotho protein. The results showed that osteoblast-specific gene expression and mineral deposition were decreased when MSCs were incubated with Klotho. Klotho reduced the expression of fibroblast growth factor receptor 1 (FGFR1) and phosphorylated extracellular signal-regulated kinase 1/2. However, both MEK and FGFR1 inhibitors delayed bone mineral formation more than Klotho. These data suggest that secreted Klotho protein attenuates the osteogenic differentiation of hBMSCs in vitro through FGFR1/ERK signaling.

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

A subset of chondrogenic cells provides early mesenchymal progenitors in growing bones

The hallmark of endochondral bone development is the presence of cartilaginous templates, in which osteoblasts and stromal cells are generated to form mineralized matrix and support bone marrow hematopoiesis. However, the ultimate source of these mesenchymal cells and the relationship between bone progenitors in fetal life and those in later life are unknown. Fate-mapping studies revealed that cells expressing cre-recombinases driven by the collagen II (Col2) promoter/enhancer and their descendants contributed to, in addition to chondrocytes, early perichondrial precursors prior to Runx2 expression and, subsequently, to a majority of osteoblasts, Cxcl12 (chemokine (C-X-C motif) ligand 12)-abundant stromal cells and bone marrow stromal/mesenchymal progenitor cells in postnatal life. Lineage-tracing experiments using a tamoxifen-inducible creER system further revealed that early postnatal cells marked by Col2-creER, as well as Sox9-creER and aggrecan (Acan)-creER, progressively contributed to multiple mesenchymal lineages and continued to provide descendants for over a year. These cells are distinct from adult mesenchymal progenitors and thus provide opportunities for regulating the explosive growth that occurs uniquely in growing mammals.

Deletion of Fgfr1 in osteoblasts enhances mobilization of EPCs into peripheral blood in a mouse endotoxemia model

Endothelial progenitor cells (EPCs) contribute to neovascularization and vascular repair, and may exert a beneficial effect on the clinical outcome of sepsis. Osteoblasts act as a component of "niche" in bone marrow, which provides a nest for stem/progenitor cells and are involved in the formation and maintenance of stem/progenitor cells. Fibroblast growth factor receptor 1 (FGFR1) can regulate osteoblast activity and influence bone mass. So we explored the role of FGFR1 in EPC mobilization. Male mice with osteoblast-specific knockout of Fgfr1 (Fgfr1(fl/fl);OC-Cre) and its wild-type littermates (Fgfr1(fl/fl) ) were used in this study. Mice intraperitoneally injected with lipopolysaccharide (LPS) were used to measure the number of circulating EPCs in peripheral blood and serum stromal cell-derived factor 1α (SDF-1α). The circulating EPC number and the serum level of SDF-1α were significantly higher in Fgfr1(fl/fl);OC-Cre mice than those in Fgfr1(fl/fl) mice after LPS injection. In cell culture system, SDF-1α level was also significantly higher in Fgfr1(fl/fl);OC-Cre osteoblasts compared with that in Fgfr1(fl/fl) osteoblasts after LPS treatment. TRAP staining showed that there was no significant difference between the osteoclast activity of septic Fgfr1(fl/fl) and Fgfr1(fl/fl);OC-Cre mice. This study suggests that targeted deletion of Fgfr1 in osteoblasts enhances mobilization of EPCs into peripheral blood through up-regulating SDF-1α secretion from osteoblasts.

Prostate cancer cell-stromal cell crosstalk via FGFR1 mediates antitumor activity of dovitinib in bone metastases

Bone is the most common site of prostate cancer (PCa) progression to a therapy-resistant, lethal phenotype. We found that blockade of fibroblast growth factor receptors (FGFRs) with the receptor tyrosine kinase inhibitor dovitinib has clinical activity in a subset of men with castration-resistant PCa and bone metastases. Our integrated analyses suggest that FGF signaling mediates a positive feedback loop between PCa cells and bone cells and that blockade of FGFR1 in osteoblasts partially mediates the antitumor activity of dovitinib by improving bone quality and by blocking PCa cell-bone cell interaction. These findings account for clinical observations such as reductions in lesion size and intensity on bone scans, lymph node size, and tumor-specific symptoms without proportional declines in serum prostate-specific antigen concentration. Our findings suggest that targeting FGFR has therapeutic activity in advanced PCa and provide direction for the development of therapies with FGFR inhibitors.

Signaling pathways involved in osteogenesis and their application for bone regenerative medicine

Bone regeneration is a well organized but complex physiological process, in which different cell types and their activated signaling pathways are involved. In bone regeneration and remodeling processes, mesenchymal stem cells (MSCs) have a crucial role, and their differentiation during these processes is regulated by specific signaling molecules (growth factors/cytokines and hormones) and their activated intracellular networks. Especially the utilization of the molecular machinery seems crucial to consider prior to developing bone implants, bone-substitute materials, and cell-based constructs for bone regeneration. The aim of this review is to provide an overview of the signaling mechanisms involved in bone regeneration and remodeling and the osteogenic potential of MSCs to become a key cellular resource for such regeneration and remodeling processes. Additionally, an overview of possibilities to beneficially exploit cell signaling processes to optimize bone regeneration is provided.

Osteocyte-specific deletion of Fgfr1 suppresses FGF23

Increases in fibroblastic growth factor 23 (FGF23 or Fgf23) production by osteocytes result in hypophosphatemia and rickets in the Hyp mouse homologue of X-linked hypophosphatemia (XLH). Fibroblastic growth factor (FGF) signaling has been implicated in the pathogenesis of Hyp. Here, we conditionally deleted FGF receptor 1 (FGFR1 or Fgfr1) in osteocytes of Hyp mice to investigate the role of autocrine/paracrine FGFR signaling in regulating FGF23 production by osteocytes. Crossing dentin matrix protein 1 (Dmp1)-Cre;Fgfr1^null/+ mice with female Hyp;Fgfr1^flox/flox mice created Hyp and Fgfr1 (Fgfr1^Dmp1-cKO)-null mice (Hyp;Fgfr1^Dmp1-cKO) with a 70% decrease in bone Fgfr1 transcripts. Fgfr1^Dmp1-cKO-null mice exhibited a 50% reduction in FGF23 expression in bone and 3-fold reduction in serum FGF23 concentrations, as well as reductions in sclerostin (Sost), phosphate regulating endopeptidase on X chromosome (PHEX or Phex), matrix extracellular phosphoglycoprotein (Mepe), and Dmp1 transcripts, but had no demonstrable alterations in phosphate or vitamin D homeostasis or skeletal morphology. Hyp mice had hypophosphatemia, reductions in 1,25(OH)₂D levels, rickets/osteomalacia and elevated FGF2 expression in bone. Compared to Hyp mice, compound Hyp;Fgfr1^Dmp1-cKO-null mice had significant improvement in rickets and osteomalacia in association with a decrease in serum FGF23 (3607 to 1099 pg/ml), an increase in serum phosphate (6.0 mg/dl to 9.3 mg/dl) and 1,25(OH)₂D (121±23 to 192±34 pg/ml) levels, but only a 30% reduction in bone FGF23 mRNA expression. FGF23 promoter activity in osteoblasts was stimulated by FGFR1 activation and inhibited by overexpression of a dominant negative FGFR1(TK−), PLCγ and MAPK inhibitors. FGF2 also stimulated the translation of an FGF23 cDNA transfected into osteoblasts via a FGFR1 and PI3K/Akt-dependent mechanism. Thus, activation of autocrine/paracrine FGF pathways is involved in the pathogenesis of Hyp through FGFR1-dependent regulation of FGF23 by both transcriptional and post-transcriptional mechanisms. This may serve to link local bone metabolism with systemic phosphate and vitamin D homeostasis.

Recent research on the growth plate: Recent insights into the regulation of the growth plate

For most bones, elongation is driven primarily by chondrogenesis at the growth plates. This process results from chondrocyte proliferation, hypertrophy, and extracellular matrix secretion, and it is carefully orchestrated by complex networks of local paracrine factors and modulated by endocrine factors. We review here recent advances in the understanding of growth plate physiology. These advances include new approaches to study expression patterns of large numbers of genes in the growth plate, using microdissection followed by microarray. This approach has been combined with genome-wide association studies to provide insights into the regulation of the human growth plate. We also review recent studies elucidating the roles of bone morphogenetic proteins, fibroblast growth factors, C-type natriuretic peptide, and suppressor of cytokine signaling in the local regulation of growth plate chondrogenesis and longitudinal bone growth.

SHP2 regulates chondrocyte terminal differentiation, growth plate architecture and skeletal cell fates

Loss of PTPN11/SHP2 in mice or in human metachondromatosis (MC) patients causes benign cartilage tumors on the bone surface (exostoses) and within bones (enchondromas). To elucidate the mechanisms underlying cartilage tumor formation, we investigated the role of SHP2 in the specification, maturation and organization of chondrocytes. Firstly, we studied chondrocyte maturation by performing RNA-seq on primary chondrocyte pellet cultures. We found that SHP2 depletion, or inhibition of the ERK1/2 pathway, delays the terminal differentiation of chondrocytes from the early-hypertrophic to the late-hypertrophic stage. Secondly, we studied chondrocyte maturation and organization in mice with a mosaic postnatal inactivation of Ptpn11 in chondrocytes. We found that the vertebral growth plates of these mice have expanded domains of early-hypertrophic chondrocytes that have not yet terminally differentiated, and their enchondroma-like lesions arise from chondrocytes displaced from the growth plate due to a disruption in the organization of maturation and ossification zones. Furthermore, we observed that lesions from human MC patients also display disorganized chondrocyte maturation zones. Next, we found that inactivation of Ptpn11 in Fsp1-Cre-expressing fibroblasts induces exostosis-like outgrowths, suggesting that loss of SHP2 in cells on the bone surface and at bone-ligament attachment sites induces ectopic chondrogenesis. Finally, we performed lineage tracing to show that exostoses and enchondromas in mice likely contain mixtures of wild-type and SHP2-deficient chondrocytes. Together, these data indicate that in patients with MC, who are heterozygous for inherited PTPN11 loss-of-function mutations, second-hit mutations in PTPN11 can induce enchondromas by disrupting the organization and delaying the terminal differentiation of growth plate chondrocytes, and can induce exostoses by causing ectopic chondrogenesis of cells on the bone surface. Furthermore, the data are consistent with paracrine signaling from SHP2-deficient cells causing SHP2-sufficient cells to be incorporated into the lesions.

Patients with the inherited disorder, metachondromatosis (MC), develop multiple benign cartilage tumors during childhood. MC patients carry heterozygous loss-of-function mutations in the PTPN11 gene, and their cartilage tumors likely arise when the second PTPN11 allele is lost due to a somatic mutation. PTPN11 encodes a phosphatase called SHP2 that is involved in a variety of signaling pathways. Here, we use mouse models and cell culture assays to investigate the mechanisms by which loss of SHP2 promotes cartilage tumor formation. We show that cartilage tumors that form inside bones (enchondromas) likely arise due to disorganized growth and delayed terminal differentiation of growth plate chondrocytes, while cartilage tumors that form on the bone surface (exostoses) can arise due to ectopic chondrogenesis of fibroblast-like cells that surround bones. We also suggest that paracrine signals from SHP2-deficient cells cause neighboring SHP2-sufficient cells to contribute to exostoses and enchondromas. Finally, we provide in vitro data that the ERK1/2 pathway is regulated by SHP2 and promotes chondrocyte terminal differentiation. Together, our data provide insight into the mechanisms underlying cartilage tumor formation and implicate SHP2 as a key regulator of chondrocyte specification, organization and maturation.

Adipose-derived stem cells: a review of signaling networks governing cell fate and regenerative potential in the context of craniofacial and long bone skeletal repair

Improvements in medical care, nutrition and social care are resulting in a commendable change in world population demographics with an ever increasing skew towards an aging population. As the proportion of the world's population that is considered elderly increases, so does the incidence of osteodegenerative disease and the resultant burden on healthcare. The increasing demand coupled with the limitations of contemporary approaches, have provided the impetus to develop novel tissue regeneration therapies. The use of stem cells, with their potential for self-renewal and differentiation, is one potential solution. Adipose-derived stem cells (ASCs), which are relatively easy to harvest and readily available have emerged as an ideal candidate. In this review, we explore the potential for ASCs to provide tangible therapies for craniofacial and long bone skeletal defects, outline key signaling pathways that direct these cells and describe how the developmental signaling program may provide clues on how to guide these cells in vivo. This review also provides an overview of the importance of establishing an osteogenic microniche using appropriately customized scaffolds and delineates some of the key challenges that still need to be overcome for adult stem cell skeletal regenerative therapy to become a clinical reality.

Role of FGF/FGFR signaling in skeletal development and homeostasis: learning from mouse models

Fibroblast growth factor (FGF)/fibroblast growth factor receptor (FGFR) signaling plays essential roles in bone development and diseases. Missense mutations in FGFs and FGFRs in humans can cause various congenital bone diseases, including chondrodysplasia syndromes, craniosynostosis syndromes and syndromes with dysregulated phosphate metabolism. FGF/FGFR signaling is also an important pathway involved in the maintenance of adult bone homeostasis. Multiple kinds of mouse models, mimicking human skeleton diseases caused by missense mutations in FGFs and FGFRs, have been established by knock-in/out and transgenic technologies. These genetically modified mice provide good models for studying the role of FGF/FGFR signaling in skeleton development and homeostasis. In this review, we summarize the mouse models of FGF signaling-related skeleton diseases and recent progresses regarding the molecular mechanisms, underlying the role of FGFs/FGFRs in the regulation of bone development and homeostasis. This review also provides a perspective view on future works to explore the roles of FGF signaling in skeletal development and homeostasis.

Inactivation of Vhl in osteochondral progenitor cells causes high bone mass phenotype and protects against age-related bone loss in adult mice

Previous studies have shown that disruption of von Hippel-Lindau gene (Vhl) coincides with activation of hypoxia-inducible factor α (HIFα) signaling in bone cells and plays an important role in bone development, homeostasis, and regeneration. It is known that activation of HIF1α signaling in mature osteoblasts is central to the coupling between angiogenesis and bone formation. However, the precise mechanisms responsible for the coupling between skeletal angiogenesis and osteogenesis during bone remodeling are only partially elucidated. To evaluate the role of Vhl in bone homeostasis and the coupling between vascular physiology and bone, we generated mice lacking Vhl in osteochondral progenitor cells (referred to as Vhl cKO mice) at postnatal and adult stages in a tamoxifen-inducible manner and changes in skeletal morphology were assessed by micro-computed tomography (µCT), histology, and bone histomorphometry. We found that mice with inactivation of Vhl in osteochondral progenitor cells at the postnatal stage largely phenocopied that of mice lacking Vhl in mature osteoblasts, developing striking and progressive accumulation of cancellous bone with increased microvascular density and bone formation. These were accompanied with a significant increase in osteoblast proliferation, upregulation of differentiation marker Runx2 and osteocalcin, and elevated expression of vascular endothelial growth factor (VEGF) and phosphorylation of Smad1/5/8. In addition, we found that Vhl deletion in osteochondral progenitor cells in adult bone protects mice from aging-induced bone loss. Our data suggest that the VHL-mediated signaling in osteochondral progenitor cells plays a critical role in bone remodeling at postnatal/adult stages through coupling osteogenesis and angiogenesis. © 2014 American Society for Bone and Mineral Research.

FGFR3 induces degradation of BMP type I receptor to regulate skeletal development

Fibroblast growth factors (FGFs) and their receptors (FGFRs) play significant roles in vertebrate organogenesis and morphogenesis. FGFR3 is a negative regulator of chondrogenesis and multiple mutations with constitutive activity of FGFR3 result in achondroplasia, one of the most common dwarfisms in humans, but the molecular mechanism remains elusive. In this study, we found that chondrocyte-specific deletion of BMP type I receptor a (Bmpr1a) rescued the bone overgrowth phenotype observed in Fgfr3 deficient mice by reducing chondrocyte differentiation. Consistently, using in vitro chondrogenic differentiation assay system, we demonstrated that FGFR3 inhibited BMPR1a-mediated chondrogenic differentiation. Furthermore, we showed that FGFR3 hyper-activation resulted in impaired BMP signaling in chondrocytes of mouse growth plates. We also found that FGFR3 inhibited BMP-2- or constitutively activated BMPR1-induced phosphorylation of Smads through a mechanism independent of its tyrosine kinase activity. We found that FGFR3 facilitates BMPR1a to degradation through Smurf1-mediated ubiquitination pathway. We demonstrated that down-regulation of BMP signaling by BMPR1 inhibitor dorsomorphin led to the retardation of chondrogenic differentiation, which mimics the effect of FGF-2 on chondrocytes and BMP-2 treatment partially rescued the retarded growth of cultured bone rudiments from thanatophoric dysplasia type II mice. Our findings reveal that FGFR3 promotes the degradation of BMPR1a, which plays an important role in the pathogenesis of FGFR3-related skeletal dysplasia.

A Ser252Trp mutation in fibroblast growth factor receptor 2 (FGFR2) mimicking human Apert syndrome reveals an essential role for FGF signaling in the regulation of endochondral bone formation

A S252W mutation of fibroblast growth factor receptor 2 (FGFR2), which is responsible for nearly two-thirds of Apert syndrome (AS) cases, causes retarded development of the skeleton and skull malformation resulting from premature fusion of the craniofacial sutures. We utilized a Fgfr2^+/S252W mouse (a knock-in mouse model mimicking human AS) to demonstrate decreased bone mass due to reduced trabecular bone volume, reduced bone mineral density, and shortened growth plates in the long bones. In vitro bone mesenchymal stem cells (BMSCs) culture studies revealed that the mutant mice showed reduced BMSC proliferation, a reduction in chondrogenic differentiation, and reduced mineralization. Our results suggest that these phenomena are caused by up-regulation of p38 and Erk1/2 phosphorylation. Treatment of cultured mutant bone rudiments with SB203580 or PD98059 resulted in partial rescue of the bone growth retardation. The p38 signaling pathway especially was found to be responsible for the retarded long bone development. Our data indicate that the S252W mutation in FGFR2 directly affects endochondral ossification, resulting in growth retardation of the long bone. We also show that the p38 and Erk1/2 signaling pathways partially mediate the effects of the S252W mutation of FGFR2 on long bone development.

Deficiency of Sef is associated with increased postnatal cortical bone mass by regulating Runx2 activity

Sef (similar expression to fgf genes) is a feedback inhibitor of fibroblast growth factor (FGF) signaling and functions in part by binding to FGF receptors and inhibiting their activation. Genetic studies in mice and humans indicate an important role for fibroblast growth factor signaling in bone growth and homeostasis. We, therefore, investigated whether Sef had a function role in skeletal acquisition and remodeling. Sef expression is increased during osteoblast differentiation in vitro, and LacZ staining of Sef+/- mice showed high expression of Sef in the periosteum and chondro-osseous junction of neonatal and adult mice. Mice with a global deletion of Sef showed increased cortical bone thickness, bone volume, and increased periosteal perimeter by micro-computed tomography (micro-CT). Histomorphometric analysis of cortical bone revealed a significant increase in osteoblast number. Interestingly, Sef-/- mice showed very little difference in trabecular bone by micro-CT and histomorphometry compared with wild-type mice. Bone marrow cells from Sef-/- mice grown in osteogenic medium showed increased proliferation and increased osteoblast differentiation compared with wild-type bone marrow cells. Bone marrow cells from Sef-/- mice showed enhanced FGF2-induced activation of the ERK pathway, whereas bone marrow cells from Sef transgenic mice showed decreased FGF2-induced signaling. FGF2-induced acetylation and stability of Runx2 was enhanced in Sef-/- bone marrow cells, whereas overexpression of Sef inhibited Runx2-responsive luciferase reporter activity. Bone marrow from Sef-/- mice showed enhanced hematopoietic lineage-dependent and osteoblast-dependent osteoclastogenesis and increased bone resorptive activity relative to wild-type controls in in vitro assays, whereas overexpression of Sef inhibited osteoclast differentiation. Taken together, these studies indicate that Sef has specific roles in osteoblast and osteoclast lineages and that its absence results in increased osteoblast and osteoclast activity with a net increase in cortical bone mass.

FGF2 stimulates osteogenic differentiation through ERK induced TAZ expression

TAZ (transcriptional coactivator with PDZ-binding motif) is a transcriptional modulator that regulates mesenchymal stem cell differentiation. It stimulates osteogenic differentiation while inhibiting adipocyte differentiation. FGFs (fibroblast growth factors) stimulate several signaling proteins to regulate their target genes, which are involved in cell proliferation, differentiation, and cell survival. Within this family, FGF2 stimulates osteoblast differentiation though a mechanism that is largely unknown. In this report, we show that TAZ mediates FGF2 signaling in osteogenesis. We observed that FGF2 increases TAZ expression by stimulating its mRNA expression. Depletion of TAZ using small hairpin RNA blocked FGF2-mediated osteogenic differentiation. FGF2 induced TAZ expression was stimulated by ERK (extracellular signal-regulated kinase) activation and the inhibition of ERK blocked TAZ expression. FGF2 increased nuclear localization of TAZ and, thus, facilitated the interaction of TAZ and Runx2, activating Runx2-mediated gene transcription. Taken together, these results suggest that TAZ is an important mediator of FGF2 signaling in osteoblast differentiation.

Comparative analysis of a teleost skeleton transcriptome provides insight into its regulation

An articulated endoskeleton that is calcified is a unifying innovation of the vertebrates, however the molecular basis of the structural divergence between terrestrial and aquatic vertebrates, such as teleost fish, has not been determined. In the present study long-read next generation sequencing (NGS, Roche 454 platform) was used to characterize acellular perichondral bone (vertebrae) and chondroid bone (gill arch) in the gilthead sea bream (Sparus auratus). A total of 15.97 and 14.53Mb were produced, respectively from vertebrae and gill arch cDNA libraries and yielded 32,374 and 28,371 contigs (consensus sequences) respectively. 10,455 contigs from vertebrae and 10,625 contigs from gill arches were annotated with gene ontology terms. Comparative analysis of the global transcriptome revealed 4249 unique transcripts in vertebrae, 4201 unique transcripts in the gill arches and 3700 common transcripts. Several core gene networks were conserved between the gilthead sea bream and mammalian skeleton. Transcripts for putative endocrine factors were identified in acellular gilthead sea bream bone suggesting that in common with mammalian bone it can act as an endocrine tissue. The acellular bone of the vertebra, in contrast to current opinion based on histological analysis, was responsive to a short fast and significant (p<0.05) down-regulation of several transcripts identified by NGS, osteonectin, osteocalcin, cathepsin K and IGFI occurred. In gill arches fasting caused a significant (p<0.05) down-regulation of osteocalcin and up-regulation of MMP9.