Coordination of chondrogenesis and osteogenesis by hypertrophic chondrocytes in endochondral bone development

Cellular Communication in Bone Homeostasis and the Related Anti-osteoporotic Drug Development

Background：Intercellular crosstalk among osteoblast, osteoclast, osteocyte and chondrocyte is involved in the precise control of bone homeostasis. Disruption of this cellular and molecular signaling would lead to metabolic bone diseases such as osteoporosis. Currently a number of anti-osteoporosis interventions are restricted by side effects, complications and long-term intolerance. This review aims to summarize the bone cellular communication involved in bone remodeling and its usage to develop new drugs for osteoporosis. Methods：We searched PubMed for publications from 1 January 1980 to 1 January 2018 to identify relevant and latest literatures, evaluation and prospect of osteoporosis medication were summarized. Detailed search terms were 'osteoporosis', 'osteocyte', 'osteoblast', 'osteoclast', 'bone remodeling', 'chondrocyte', 'osteoporosis treatment', 'osteoporosis therapy', 'bisphosphonates', 'denosumab', 'Selective Estrogen Receptor Modulator (SERM)', 'PTH', 'romosozumab', 'dkk-1 antagonist', 'strontium ranelate'. Results：A total of 170 papers were included in the review. About 80 papers described bone cell interactions involved in bone remodeling. The remaining papers were focused on the novel advanced and new horizons in osteoporosis therapies. Conclusion：There exists a complex signal network among bone cells involved in bone remodeling. The disorder of cell-cell communications may be the underlying mechanism of osteoporosis. Current anti-osteoporosis therapies are effective but accompanied by certain drawbacks simultaneously. Restoring the abnormal signal network and individualized therapy are critical for ideal drug development.

A New Look at Causal Factors of Idiopathic Scoliosis: Altered Expression of Genes Controlling Chondroitin Sulfate Sulfation and Corresponding Changes in Protein Synthesis in Vertebral Body Growth Plates

Background: In a previous report, we demonstrated the presence of cells with a neural/glial phenotype on the concave side of the vertebral body growth plate in Idiopathic Scoliosis (IS) and proposed this phenotype alteration as the main etiological factor of IS. In the present study, we utilized the same specimens of vertebral body growth plates removed during surgery for Grade III-IV IS to analyse gene expression. We suggested that phenotype changes observed on the concave side of the vertebral body growth plate can be associated with altered expression of particular genes, which in turn compromise mechanical properties of the concave side. Methods: We used a Real-Time SYBR Green PCR assay to investigate gene expression in vertebral body growth plates removed during surgery for Grade III-IV IS; cartilage tissues from human fetal spine were used as a surrogate control. Special attention was given to genes responsible for growth regulation, chondrocyte differentiation, matrix synthesis, sulfation and transmembrane transport of sulfates. We performed morphological, histochemical, biochemical, and ultrastructural analysis of vertebral body growth plates. Results: Expression of genes that control chondroitin sulfate sulfation and corresponding protein synthesis was significantly lower in scoliotic specimens compared to controls. Biochemical analysis showed 1) a decrease in diffused proteoglycans in the total pool of proteoglycans; 2) a reduced level of their sulfation; 3) a reduction in the amount of chondroitin sulfate coinciding with raising the amount of keratan sulfate; and 4) reduced levels of sulfation on the concave side of the scoliotic deformity. Conclusion: The results suggested that altered expression of genes that control chondroitin sulfate sulfation and corresponding changes in protein synthesis on the concave side of vertebral body growth plates could be causal agents of the scoliotic deformity.

Effects of Thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3) and their Metabolites on Osteoblast Differentiation

Studies involving human genetic mutations and mutant mouse models have provided irrevocable evidence for a key role for thyroid hormones (THs) in the regulation of skeletal growth. While T3 binds to TH receptors with higher affinity than T4, T4 occupied TH receptors have also been reported in the nucleus under euthyroid conditions raising the possibility that T4 bound nuclear receptors may be biologically relevant in thyroid syndromes with elevated free T4 and reduced T3 levels. We, therefore, evaluated the direct effects of T4, T3, and their metabolites (rT3 and T2) in stimulating osteoblast differentiation using MC3T3-E1 preosteoblasts which do not produce detectable levels of deiodinases. Under serum-free conditions, a 24-h treatment of MC3T3-E1 cells with THs and their metabolites caused a dose-dependent increase in the expression of osteoblast differentiation markers, osterix, and osteocalcin. Circulating concentrations of T3 (~1 ng/ml) and T4 (~30 ng/ml) showed similar potency in stimulating osteoblast differentiation marker expression, while rT3 and T2 were less potent than T3 and T4. Moreover, T3 and T4 treatments elevated the IGF-1 mRNA level suggesting the involvement of IGF-1 signaling in the TH regulation of osteoblast differentiation. We conclude that an elevated T4 level in the absence of T3 may exert stimulatory effects on osteoblast differentiation. The establishment of cell-specific effects of T4 on osteoblasts may provide a strategy to generate T4 mimics that exert skeletal specific effects without the confounding T3 effects on other tissues.

RE1-Silencing Transcription Factor (Rest) is a Novel Regulator of Osteoblast Differentiation

RE1-silencing transcription factor (Rest) has been identified as a master negative regulator of neuronal differentiation. Nothing is known about Rest function in bone cells. In this study, we examined the Rest expression levels and role during osteoblast differentiation. We found that Rest is abundantly expressed in bone marrow stromal cells, calvarial osteoblasts, and MC3T3-E1 osteoblasts. Treatment of primary osteoblasts with ascorbic acid (AA) down regulated Rest mRNA expression at an early stage, but not in later stages of differentiation. Consistent with treatment of primary cultures, AA treatment of MC3T3-E1 cells significantly reduced Rest protein expression at day 3 and at day 8 after initiation of osteoblast differentiation. Treatment of bone marrow stromal cells with BMP-2 and dexamethasone, but not IGF-I for 3 days greatly decreased Rest mRNA expression. To test the function of Rest during osteoblast differentiation, Rest expression was knocked down in MC3T3-E1 cell subclones segregated on the basis of ALP activity (differentiation status) using lentivirus expressing shRNA against Rest. An 80% knockdown of Rest expression decreased Osterix (Osx) expression by 52-57% and as a result, increased both basal and AA induced ALP expression and activity in the subclone that expresses low basal level of ALP (undifferentiated). By contrast, a 98% knockdown of Rest expression in cells that express high basal levels of ALP (differentiated cells) caused a significant reduction in Osx expression, basal and AA induced ALP expression and activity. These data suggest that Rest regulates early osteoblast differentiation via modulating Rest expression that is independent of Osx expression.

Endochondral ossification model system: designed cell fate of human epiphyseal chondrocytes during long-term implantation

The aim of this study is to establish a recapitulation system of human endochondral ossification as a paradigm of developmental engineering. Chondrocytes were isolated from the epiphyseal cartilage of the supernumerary digits of infants with polydactyly. In vivo studies showed that implanted chondrocytes exhibited cartilaginous regeneration over a short period of time and subsequent endochondral ossification with a marrow cavity. Tracing studies revealed that cells of donor origin at the periphery of the cartilage migrated into the center of the cartilage and transformed into osteoblasts, adipocytes, and endothelial cells. Bone marrow was formed through anastomosis with the recipient endothelial system at 13 weeks, and from the migration of recipient hematopoietic cells at 50 weeks. This study provides a human endochondral ossification model system with transdifferentiation of the donor cells at the periphery of the cartilage. J. Cell. Physiol. 230: 1376-1388, 2015. © 2015 Wiley Periodicals, Inc., A Wiley Company.

Comparative proteomic analysis of hypertrophic chondrocytes in osteoarthritis

Background

Osteoarthritis (OA) is a multi-factorial disease leading progressively to loss of articular cartilage and subsequently to loss of joint function. While hypertrophy of chondrocytes is a physiological process implicated in the longitudinal growth of long bones, hypertrophy-like alterations in chondrocytes play a major role in OA. We performed a quantitative proteomic analysis in osteoarthritic and normal chondrocytes followed by functional analyses to investigate proteome changes and molecular pathways involved in OA pathogenesis.

Methods

Chondrocytes were isolated from articular cartilage of ten patients with primary OA undergoing knee replacement surgery and six normal donors undergoing fracture repair surgery without history of joint disease and no OA clinical manifestations. We analyzed the proteome of chondrocytes using high resolution mass spectrometry and quantified it by label-free quantification and western blot analysis. We also used WebGestalt, a web-based enrichment tool for the functional annotation and pathway analysis of the differentially synthesized proteins, using the Wikipathways database. ClueGO, a Cytoscape plug-in, is also used to compare groups of proteins and to visualize the functionally organized Gene Ontology (GO) terms and pathways in the form of dynamical network structures.

Results

The proteomic analysis led to the identification of a total of ~2400 proteins. 269 of them showed differential synthesis levels between the two groups. Using functional annotation, we found that proteins belonging to pathways associated with regulation of the actin cytoskeleton, EGF/EGFR, TGF-β, MAPK signaling, integrin-mediated cell adhesion, and lipid metabolism were significantly enriched in the OA samples (p ≤10⁻⁵). We also observed that the proteins GSTP1, PLS3, MYOF, HSD17B12, PRDX2, APCS, PLA2G2A SERPINH1/HSP47 and MVP, show distinct synthesis levels, characteristic for OA or control chondrocytes.

Conclusion

In this study we compared the quantitative changes in proteins synthesized in osteoarthritic compared to normal chondrocytes. We identified several pathways and proteins to be associated with OA chondrocytes. This study provides evidence for further testing on the molecular mechanism of the disease and also propose proteins as candidate markers of OA chondrocyte phenotype.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-015-9085-6) contains supplementary material, which is available to authorized users.

Functional analyses reveal the essential role of SOX6 and RUNX2 in the communication of chondrocyte and osteoblast

SUMMARY

This study provides novel evidence that sex determining region Y (SRY)-box (SOX6) and runt-related transcription factor 2 (RUNX2) play essential roles in the communication of chondrocyte and osteoblast. Our findings open a new avenue to the limited understanding of the coordination effect between chondrogenesis and osteogenesis.

INTRODUCTION

Sox6 and Runx2 are two new susceptibility genes for osteoporosis identified by genome-wide association studies, but the functions of these genes in osteogenesis remain unclear. Both genes are essential transcription factors in chondrogenesis, which reminds us that SOX6 and RUNX2 might be involved in the coordination of chondrogenesis and osteogenesis. Therefore, this study aimed to investigate the functions of SOX6 and RUNX2 in the coupling regulation of chondrogenesis and osteogenesis.

METHODS

We established a chondrogenic differentiation model of ATDC5 cell and profiled the expression of SOX6 and RUNX2 during chondroblast differentiation. We co-cultured osteoblast cells with ATDC5 cells in different differentiation stages and examined the proliferation, cell cycle progression, apoptosis, and differentiation of osteoblast cells. SOX6 or RUNX2 was knocked down using specific siRNA and the effect was examined.

RESULTS

During chondrogenic differentiation, SOX6 and RUNX2 expressed sequentially in the proliferating and hypertrophic stages. Proliferative ATDC5 cells stimulated the multiplication of osteoblasts and promoted more osteoblasts to enter S-phase. Hypertrophic ATDC5 cells enhanced the differentiation of osteoblasts. Yet, the apoptosis of osteoblasts was neither affected by proliferating nor hypertrophic ATDC5 cells. Knockdown of SOX6 in proliferating ATDC5 cells significantly repressed the stimulation of osteoblast multiplication, whereas depletion of RUNX2 in hypertrophic ATDC5 cells retarded the expression of osteoblastic markers.

CONCLUSIONS

Our study first reveals that SOX6 and RUNX2 play important roles in the chondrogenesis-osteogenesis coordination. This finding enriches the limited understanding about this coordination and unravels the novel function of SOX6 and RUNX2 in the endochondral ossification.

Drynaria fortunei J. Sm. improves the bone mass of ovariectomized rats through osteocalcin-involved endochondral ossification

AIM OF THIS STUDY

Our previous study showed that Drynaria fortunei J. Sm. (Kunze), a traditional Chinese medical herb, can promote osteoblast differentiation and maturation. This study was further aimed to confirm the traditional effects of Kunze on the bone mass of ovariectomized rats.

MATERIALS AND METHODS

Female Wistar rats were given an ovariectomy and then administered the water extract of Kunze (WEK). Systemic and tissue toxicities of WEK were assessed. A biomechanical test, bone mineral contents, and bone histomorphometry were analyzed to determine the effects of the WEK on the bone mass. Levels of osteocalcin (OCN) in bone tissues were determined by immunohistochemistry and immunoblotting. The effects of naringin, one of the bioactive compounds of the WEK, on the bone mass were evaluated.

RESULTS

A bilateral ovariectomy in rats caused a time-dependent decrease in levels of serum 17β-estradiol. Exposure of ovariectomized rats to the WEK at 0.5 and 1g/kg body weight/day for 1, 2, 3, and 6 months did not induce systemic or tissue toxicities. Biomechanical testing and a bone mineral content analysis showed that the ovariectomy decreased the bone torsion force and bone ash in time-dependent manners. In comparison, after exposure to the WEK, the ovariectomy-induced reductions in the bone torsion force and bone ash were significantly alleviated. In parallel, results of a bone histomorphometric assay further revealed that the ovariectomy caused significant diminution in the production of prehypertrophic chondrocytes and trabecular bone but enhanced hypertrophic chondrocyte numbers in the growth plate. However, exposure to the WEK lowered ovariectomy-induced changes in these cellular events. As to the mechanism, the WEK increased OCN biosynthesis in bone tissues of ovariectomized rats. Administration of naringin to ovariectomized rats caused significant amelioration of the bone strength, bone mineral contents, and trabecular bone amounts.

CONCLUSIONS

This study shows that the WEK can translationally promote the bone mass in ovariectomized rats through stimulating OCN-involved endochondral ossification.

Bone tissue remodeling and development: focus on matrix metalloproteinase functions

Bone-forming cells originate from distinct embryological layers, mesoderm (axial and appendicular bones) and ectoderm (precursor of neural crest cells, which mainly form facial bones). These cells will develop bones by two principal mechanisms: intramembranous and endochondral ossification. In both cases, condensation of multipotent mesenchymal cells occurs, at the site of the future bone, which differentiate into bone and cartilage-forming cells. During long bone development, an initial cartilaginous template is formed and replaced by bone in a coordinated and refined program involving chondrocyte proliferation and maturation, vascular invasion, recruitment of adult stem cells and intense remodeling of cartilage and bone matrix. Matrix metalloproteinases (MMPs) are the most important enzymes for cleaving structural components of the extracellular matrix (ECM), as well as other non-ECM molecules in the ECM space, pericellular perimeter and intracellularly. Thus, the bioactive molecules generated act on several biological events, such as development, tissue remodeling and homeostasis. Since the discovery of collagenase in bone cells, more than half of the MMP members have been detected in bone tissues under both physiological and pathological conditions. Pivotal functions of MMPs during development and bone regeneration have been revealed by knockout mouse models, such as chondrocyte proliferation and differentiation, osteoclast recruitment and function, bone modeling, coupling of bone resorption and formation (bone remodeling), osteoblast recruitment and survival, angiogenesis, osteocyte viability and function (biomechanical properties); as such alterations in MMP function may alter bone quality. In this review, we look at the principal properties of MMPs and their inhibitors (TIMPs and RECK), provide an up-date on their known functions in bone development and remodeling and discuss their potential application to Bone Bioengineering.

Haploinsufficiency of osterix in chondrocytes impairs skeletal growth in mice

Osterix (Osx) is essential for both intramembranous or endochondral bone formation. Osteoblast-specific ablation of Osx using Col1α1-Cre resulted in osteopenia, because of impaired osteoblast differentiation in adult mice. Since Osx is also known to be expressed in chondrocytes, we evaluated the role of Osx expressed in chondrocytes by examining the skeletal phenotype of mice with conditional disruption of Osx in Col2α1-expressing chondrocytes. Surprisingly, Cre-positive mice that were homozygous for Osx floxed alleles died after birth. Alcian blue and alizarin red staining revealed that the lengths of skeleton, femur, and vertebrae were reduced by 21, 26, and 14% (P < 0.01), respectively, in the knockout (KO) compared with wild-type mice. To determine if haploid insufficiency of Osx in chondrocytes influenced postnatal skeletal growth, we compared skeletal phenotype of floxed heterozygous mice that were Cre-positive or Cre-negative. Body length was reduced by 8% (P < 0.001), and areal BMD of total body, femur, and tibia was reduced by 5, 7, and 8% (P < 0.05), respectively, in mice with conditional disruption of one allele of Osx in chondrocytes. Micro-CT showed reduced cortical volumetric bone mineral density and trabecular bone volume to total volume in the femurs of Osx(flox/+);col2α1-Cre mice. Histological analysis revealed that the impairment of longitudinal growth was associated with disrupted growth plates in the Osx(flox/+);col2α1-Cre mice. Primary chondrocytes isolated from KO embryos showed reduced expression of chondral ossification markers but elevated expression of chondrogenesis markers. Our findings indicate that Osx expressed in chondrocytes regulates bone growth in part by regulating chondrocyte hypertrophy.

Expression of glycosaminoglycan epitopes during zebrafish skeletogenesis

BACKGROUND

The zebrafish is an important developmental model. Surprisingly, there are few studies that describe the glycosaminoglycan composition of its extracellular matrix during skeletogenesis. Glycosaminoglycans on proteoglycans contribute to the material properties of musculo skeletal connective tissues, and are important in regulating signalling events during morphogenesis. Sulfation motifs within the chain structure of glycosaminoglycans on cell-associated and extracellular matrix proteoglycans allow them to bind and regulate the sequestration/presentation of bioactive signalling molecules important in musculo-skeletal development.

RESULTS

We describe the spatio-temporal expression of different glycosaminoglycan moieties during zebrafish skeletogenesis with antibodies recognising (1) native sulfation motifs within chondroitin and keratan sulfate chains, and (2) enzyme-generated neoepitope sequences within the chain structure of chondroitin sulfate (i.e., 0-, 4-, and 6-sulfated isoforms) and heparan sulfate glycosaminoglycans. We show that all the glycosaminoglycan moieties investigated are expressed within the developing skeletal tissues of larval zebrafish. However, subtle changes in their patterns of spatio-temporal expression over the period examined suggest that their expression is tightly and dynamically controlled during development.

CONCLUSIONS

The subtle differences observed in the domains of expression between different glycosaminoglycan moieties suggest differences in their functional roles during establishment of the primitive analogues of the skeleton.

The transcription factor activating enhancer-binding protein epsilon (AP-2ε) regulates the core promoter of type II collagen (COL2A1)

The transcription factor activating enhancer-binding protein epsilon (AP-2ε) was recently shown to be expressed during late chondrocyte differentiation, especially in hypertrophic chondrocytes. In this study, we were able to reveal that the promoter of the type II collagen (COL2A1) gene, encoding the extracellular matrix protein type II collagen, is specifically regulated by AP-2ε. Expression of COL2A1 is downregulated at the transition of chondroblasts into hypertrophic chondrocytes and our data provide evidence that AP-2ε is involved in this process. In reporter gene assays, overexpression of AP-2ε in cartilaginous cell lines resulted in a significant reduction in COL2A1 core promoter activity of ~ 45%. Furthermore, we found that this process is dose-dependent and highly specific for the epsilon isoform. Computational analysis offered only a single putative AP-2-binding motif within the chosen promoter fragment but site-directed mutagenesis revealed this motif to be regulatory inactive. After expanding our screening to motifs containing minor differences from the classical AP-2 consensus sequence (5'-GCCN3 GGC-3'), we determined the sequence 5'-GCCCAGGC-3' ranging from position -128 to -135 bp as an important regulatory site and responsible for COL2A1 downregulation through AP-2ε. Interaction of AP-2ε with the COL2A1 promoter at this site was confirmed by chromatin immunoprecipitation and electromobility shift assay. Further, our experiments suggest that at least one additional factor is involved in this process. This is the first study to prove regulation of COL2A1 by AP-2ε highlighting the role of the transcription factor as a modulator of cartilage development.

Role of the TGF-β/BMP-7/Smad pathways in renal diseases

TGF-β (transforming growth factor-β) and BMP-7 (bone morphogenetic protein-7), two key members in the TGF-β superfamily, play important but diverse roles in CKDs (chronic kidney diseases). Both TGF-β and BMP-7 share similar downstream Smad signalling pathways, but counter-regulate each other to maintain the balance of their biological activities. During renal injury in CKDs, this balance is significantly altered because TGF-β signalling is up-regulated by inducing TGF-β1 and activating Smad3, whereas BMP-7 and its downstream Smad1/5/8 are down-regulated. In the context of renal fibrosis, Smad3 is pathogenic, whereas Smad2 and Smad7 are renoprotective. However, this counter-balancing mechanism is also altered because TGF-β1 induces Smurf2, a ubiquitin E3-ligase, to target Smad7 as well as Smad2 for degradation. Thus overexpression of renal Smad7 restores the balance of TGF-β/Smad signalling and has therapeutic effect on CKDs. Recent studies also found that Smad3 mediated renal fibrosis by up-regulating miR-21 (where miR represents microRNA) and miR-192, but down-regulating miR-29 and miR-200 families. Therefore restoring miR-29/miR-200 or suppressing miR-21/miR-192 is able to treat progressive renal fibrosis. Furthermore, activation of TGF-β/Smad signalling inhibits renal BMP-7 expression and BMP/Smad signalling. On the other hand, overexpression of renal BMP-7 is capable of inhibiting TGF-β/Smad3 signalling and protects the kidney from TGF-β-mediated renal injury. This counter-regulation not only expands our understanding of the causes of renal injury, but also suggests the therapeutic potential by targeting TGF-β/Smad signalling or restoring BMP-7 in CKDs. Taken together, the current understanding of the distinct roles and mechanisms of TGF-β and BMP-7 in CKDs implies that targeting the TGF-β/Smad pathway or restoring BMP-7 signalling may represent novel and effective therapies for CKDs.

Leader genes in osteogenesis: a theoretical study

Little is still known about the molecular mechanisms involved in the process of osteogenesis. In this paper, the leader genes approach, a new bioinformatics method which has already been experimentally validated, is adopted in order to identify the genes involved in human osteogenesis. Interactions among genes are then calculated and genes are ranked according to their relative importance in this process. In total, 167 genes were identified as being involved in osteogenesis. Genes were divided into 4 groups, according to their main function in the osteogenic processes: skeletal development; cell adhesion and proliferation; ossification; and calcium ion binding. Seven genes were consistently identified as leader genes (i.e. the genes with the greatest importance in osteogenesis), while 14 were found to have slightly less importance (class B genes). It was interesting to notice that the larger part of leader and class B genes belonged to the cell adhesion and proliferation or to the ossification sub-groups. This finding suggested that these two particular sub-processes could play a more important role in osteogenesis. Moreover, among the 7 leader genes, it is interesting to notice that RUNX2, BMP2, SPARC, PTH play a direct role in bone formation, while the 3 other leader genes (VEGF, IL6, FGF2) seem to be more connected with an angiogenetic process. Twenty-nine genes have no known interactions (orphan genes). From these results, it may be possible to plan an ad hoc experimentation, for instance by microarray analyses, focused on leader, class B and orphan genes, with the aim to shed new light on the molecular mechanisms underlying osteogenesis.

Development of an efficient, non-viral transfection method for studying gene function and bone growth in human primary cranial suture mesenchymal cells reveals that the cells respond to BMP2 and BMP3

Background

Achieving efficient introduction of plasmid DNA into primary cultures of mammalian cells is a common problem in biomedical research. Human primary cranial suture cells are derived from the connective mesenchymal tissue between the bone forming regions at the edges of the calvarial plates of the skull. Typically they are referred to as suture mesenchymal cells and are a heterogeneous population responsible for driving the rapid skull growth that occurs in utero and postnatally. To better understand the molecular mechanisms involved in skull growth, and in abnormal growth conditions, such as craniosynostosis, caused by premature bony fusion, it is essential to be able to easily introduce genes into primary bone forming cells to study their function.

Results

A comparison of several lipid-based techniques with two electroporation-based techniques demonstrated that the electroporation method known as nucleofection produced the best transfection efficiency. The parameters of nucleofection, including cell number, amount of DNA and nucleofection program, were optimized for transfection efficiency and cell survival. Two different genes and two promoter reporter vectors were used to validate the nucleofection method and the responses of human primary suture mesenchymal cells by fluorescence microscopy, RT-PCR and the dual luciferase assay. Quantification of bone morphogenetic protein (BMP) signalling using luciferase reporters demonstrated robust responses of the cells to both osteogenic BMP2 and to the anti-osteogenic BMP3.

Conclusions

A nucleofection protocol has been developed that provides a simple and efficient, non-viral alternative method for in vitro studies of gene and protein function in human skull growth. Human primary suture mesenchymal cells exhibit robust responses to BMP2 and BMP3, and thus nucleofection can be a valuable method for studying the potential competing action of these two bone growth factors in a model system of cranial bone growth.

An activating Fgfr3 mutation affects trabecular bone formation via a paracrine mechanism during growth

The fibroblast growth factor receptor 3 (FGFR3) plays a critical role in the regulation of endochondral ossification. Fgfr3 gain-of-function mutations cause achondroplasia, the most common form of dwarfism, and a spectrum of chondrodysplasias. Despite a significant number of studies on the role of FGFR3 in cartilage, to date, none has investigated the influence of Fgfr3-mediated effects of the growth plate on bone formation. We studied three mouse models, each expressing Fgfr3 mutation either ubiquitously (CMV-Fgfr3(Y367C/+)), in chondrocytes (Col II-Fgfr3(Y367C/+)) or in mature osteoblasts (Col I-Fgfr3(Y367C/+)). Interestingly, we demonstrated that dwarfism with a significant defect in bone formation during growth was only observed in mouse models expressing mutant Fgfr3 in the cartilage. We observed a dramatic reduction in cartilage matrix mineralization and a strong defect of primary spongiosa. Anomalies of primary spongiosa were associated with an increase in osteoclast recruitment and a defect of osteoblasts at the mineralization front. A significant decrease in bone volume, trabecular thickness and number was also observed in the trabecular bone. Interestingly, no anomalies in proliferation and differentiation of primary osteoblasts from CMV-Fgfr3(Y367C/+) mice were observed. Based on these data, we excluded a potential function of Fgfr3 directly on osteoblasts at 3 weeks of age and we obtained evidence that the disorganization of the growth plate is responsible for the anomalies of the trabecular bone during bone formation. Herein, we propose that impaired FGFR3 signaling pathways may affect trabecular bone formation via a paracrine mechanism during growth. These results redefine our understanding of endochondral ossification in FGFR3-related chondrodysplasias.

Biocompatibility and resorption of a radiopaque premixed calcium phosphate cement

Calcium phosphate cements (CPC) are used as bone void filler in various orthopedic indications; however, there are some major drawbacks regarding mixing, transfer, and injection of traditional CPC. By using glycerol as mixing liquid, a premixed calcium phosphate cement (pCPC), some of these difficulties can be overcome. In the treatment of vertebral fractures the handling characteristics need to be excellent including a high radio-opacity for optimal control during injection. The aim of this study is to evaluate a radiopaque pCPC regarding its resorption behavior and biocompatibility in vivo. pCPC and a water-based CPC were injected into a Ø 4-mm drilled femur defect in rabbits. The rabbits were sacrificed after 2 and 12 weeks. Cross sections of the defects were evaluated using histology, electron microscopy, and immunohistochemical analysis. Signs of inflammation were evaluated both locally and systemically. The results showed a higher bone formation in the pCPC compared to the water-based CPC after 2 weeks by expression of RUNX-2. After 12 weeks most of the cement had been resorbed in both groups. Both materials were considered to have a high biocompatibility since no marked immunological response was induced and extensive bone ingrowth was observed. The conclusion from the study was that pCPC with ZrO(2) radiopacifier is a promising alternative regarding bone replacement material and may be suggested for treatment of, for example, vertebral fractures based on its high biocompatibility, fast bone ingrowth, and good handling properties.

Activin-like kinase 3 is important for kidney regeneration and reversal of fibrosis

Molecules associated with the transforming growth factor β (TGF-β) superfamily, such as bone morphogenic proteins (BMPs) and TGF-β, are key regulators of inflammation, apoptosis and cellular transitions. Here we show that the BMP receptor activin-like kinase 3 (Alk3) is elevated early in diseased kidneys after injury. We also found that its deletion in the tubular epithelium leads to enhanced TGF-β1-Smad family member 3 (Smad3) signaling, epithelial damage and fibrosis, suggesting a protective role for Alk3-mediated signaling in the kidney. A structure-function analysis of the BMP-Alk3-BMP receptor, type 2 (BMPR2) ligand-receptor complex, along with synthetic organic chemistry, led us to construct a library of small peptide agonists of BMP signaling that function through the Alk3 receptor. One such peptide agonist, THR-123, suppressed inflammation, apoptosis and the epithelial-to-mesenchymal transition program and reversed established fibrosis in five mouse models of acute and chronic renal injury. THR-123 acts specifically through Alk3 signaling, as mice with a targeted deletion for Alk3 in their tubular epithelium did not respond to therapy with THR-123. Combining THR-123 and the angiotensin-converting enzyme inhibitor captopril had an additive therapeutic benefit in controlling renal fibrosis. Our studies show that BMP signaling agonists constitute a new line of therapeutic agents with potential utility in the clinic to induce regeneration, repair and reverse established fibrosis.