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

Soluble Klotho causes hypomineralization in Klotho-deficient mice

The type I transmembrane protein αKlotho (Klotho) serves as a coreceptor for the phosphaturic hormone fibroblast growth factor 23 (FGF23) in kidney, while a truncated form of Klotho (soluble Klotho, sKL) is thought to exhibit multiple activities, including acting as a hormone, but whose mode(s) of action in different organ systems remains to be fully elucidated. FGF23 is expressed primarily in osteoblasts/osteocytes and aberrantly high levels in the circulation acting via signaling through an FGF receptor (FGFR)-Klotho coreceptor complex cause renal phosphate wasting and osteomalacia. We assessed the effects of exogenously added sKL on osteoblasts and bone using Klotho-deficient (kl/kl) mice and cell and organ cultures. sKL induced FGF23 signaling in bone and exacerbated the hypomineralization without exacerbating the hyperphosphatemia, hypercalcemia and hypervitaminosis D in kl/kl mice. The same effects were seen in rodent bone models in vitro, in which we also detected formation of a sKL complex with FGF23-FGFR and decreased Phex (gene responsible for X-linked hypophosphatemic rickets (XLH)/osteomalacia) expression. Further, sKL-FGF23-dependent hypomineralization in vitro was rescued by soluble PHEX. These data suggest that exogenously added sKL directly participates in FGF23 signaling in bone and that PHEX is a downstream effector of the sKL-FGF23-FGFR axis in bone.

AKT signaling displays multifaceted functions in neural crest development

AKT signaling is an essential intracellular pathway controlling cell homeostasis, cell proliferation and survival, as well as cell migration and differentiation in adults. Alterations impacting the AKT pathway are involved in many pathological conditions in human disease. Similarly, during development, multiple transmembrane molecules, such as FGF receptors, PDGF receptors or integrins, activate AKT to control embryonic cell proliferation, migration, differentiation, and also cell fate decisions. While many studies in mouse embryos have clearly implicated AKT signaling in the differentiation of several neural crest derivatives, information on AKT functions during the earliest steps of neural crest development had remained relatively scarce until recently. However, recent studies on known and novel regulators of AKT signaling demonstrate that this pathway plays critical roles throughout the development of neural crest progenitors. Non-mammalian models such as fish and frog embryos have been instrumental to our understanding of AKT functions in neural crest development, both in neural crest progenitors and in the neighboring tissues. This review combines current knowledge acquired from all these different vertebrate animal models to describe the various roles of AKT signaling related to neural crest development in vivo. We first describe the importance of AKT signaling in patterning the tissues involved in neural crest induction, namely the dorsal mesoderm and the ectoderm. We then focus on AKT signaling functions in neural crest migration and differentiation.

Fibroblast Growth Factor 2 and Its Receptors in Bone Biology and Disease

The fibroblast growth factor (FGF) regulatory axis is phylogenetically ancient, evolving into a large mammalian/human gene family of 22 ligands that bind to four receptor tyrosine kinases for a complex physiologic system controlling cell growth, differentiation, and metabolism. The tissue targets for the primary FGF function are mainly in cartilage and in bone for morphogenesis, mineralization, and metabolism. A multitude of complexities in the FGF ligand-receptor signaling pathways have made translation into therapies for FGF-related bone disorders such as osteomalacia, osteoarthritis, and osteoporosis difficult but not impossible.

Loss of Fgfr1 in chondrocytes inhibits osteoarthritis by promoting autophagic activity in temporomandibular joint

Temporomandibular joint osteoarthritis (TMJ OA) is a common degenerative disease with few effective disease-modifying treatments in the clinic. Fibroblast growth factor (FGF) signaling is implicated in articular cartilage homeostasis, but the functional roles of FGFR1 in TMJ OA remain largely unknown. In this study, we report that deletion of Fgfr1 in TMJ chondrocytes delayed TMJ OA progression in the age-associated spontaneous OA model and the abnormal dental occlusion OA model. Immunohistochemical staining revealed that Fgfr1 deficiency decreased the expressions of MMP13 (matrix metalloproteinase-13), ADAMTS5 (a disintegrin and metalloproteinase with thrombospondin motifs 5), and COL10A1 but increased aggrecan expression level in two TMJ OA models. Furthermore, our data show that inactivation of FGFR1 signaling may promote autophagic activity in TMJ. FGFR1 inhibitor decreased the expressions of Mmp13, Adamts5, and Runx2 in IL-1β-stimulated condylar chondrocytes, whereas autophagy inhibitors abrogated the protective effects of the FGFR1 inhibitor. Thus, our study indicates inactivated FGFR1 signaling ameliorates TMJ OA progression partially by promoting autophagic activity. Manipulation of this signaling may be a potential therapeutic approach to modify TMJ OA.

Growth factors FGF8 and FGF2 and their receptor FGFR1, transcriptional factors Msx-1 and MSX-2, and apoptotic factors p19 and RIP5 participate in the early human limb development

The expression pattern of fibroblast growth factors FGF8 and FGF2 and their receptor FGFR1, transcription factors MSX-1 and MSX-2, as well as cell proliferation (Ki-67) and cell death associated caspase-3, p19 and RIP5 factors were analyzed in histological sections of eight 4th-9th-weeks developing human limbs by immunohistochemistry and semi-thin sectioning. Increasing expression of all analyzed factors (except FGF8) characterized both the multilayered human apical ectodermal ridge (AER), sub-ridge mesenchyme (progress zone) and chondrocytes in developing human limbs. While cytoplasmic co-expression of MSX-1 and MSX-2 was observed in both limb epithelium and mesenchyme, p19 displayed strong cytoplasmic expression in non-proliferating cells. Nuclear expression of Ki-67 proliferating cells, and partly of MSX-1 and MSX-2 was detected in the whole limb primordium. Strong expression of factors p19 and RIP5, both in the AER and mesenchyme of human developing limbs indicates their possible involvement in control of cell senescence and cell death. In contrast to animal studies, expression of FGFR1 in the surface ectoderm and p19 in the whole limb primordium might reflect interspecies differences in limb morphology. Expression of FGF2 and downstream RIP5 gene, and transcription factors Msx-1 and MSX-2 did not show human-specific changes in expression pattern. Based on their spatio-temporal expression during human limb development, our study indicates role of FGFs and Msx genes in stimulation of cell proliferation, limb outgrowth, digit elongation and separation, and additionally MSX-2 in control of vasculogenesis. The cascade of orchestrated gene expressions, including the analyzed developmental factors, jointly contribute to the complex human limb development.

Progress of Regenerative Therapy in Orthopedics

PURPOSE OF REVIEW

To conduct a thorough appraisal of recent and inventive advances in the field of bone tissue engineering using biomaterials, cell-based research, along with the incorporation of biomimetic properties using surface modification of scaffolds.

RECENT FINDINGS

This paper will provide an overview on different biomaterials and emerging techniques involved in the fabrication of scaffolds, brief description of signaling pathways involved in osteogenesis, and the effect of surface modification on the fate of progenitor cells. The current strategies used for regenerative medicine like cell therapy, gene transfer, and tissue engineering have opened numerous therapeutic avenues for the treatment of various disabling orthopedic disorders. Precise strategy utilized for the reconstruction, restoration, or repair of the bone-related tissues exploits cells, biomaterials, morphogenetic signals, and appropriate mechanical environment to provide the basic constituents required for creating new tissue. Combining all the above strategies in clinical trials would pave the way for successful "bench to bedside" transformation in bone healing.

The Long Pentraxin 3 Plays a Role in Bone Turnover and Repair

Pentraxin 3 (PTX3) is an inflammatory mediator acting as a fluid-phase pattern recognition molecule and playing an essential role in innate immunity and matrix remodeling. Inflammatory mediators also contribute to skeletal homeostasis, operating at multiple levels in physiological and pathological conditions. This study was designed to investigate the role of PTX3 in physiological skeletal remodeling and bone healing. Micro-computed tomography (μCT) and bone histomorphometry of distal femur showed that PTX3 gene-targeted female and male mice (ptx3^−/−) had lower trabecular bone volume than their wild-type (ptx3^+/+) littermates (BV/TV by μCT: 3.50 ± 1.31 vs 6.09 ± 1.17 for females, p < 0.0001; BV/TV 9.06 ± 1.89 vs 10.47 ± 1.97 for males, p = 0.0435). In addition, μCT revealed lower trabecular bone volume in second lumbar vertebra of ptx3^−/− mice. PTX3 was increasingly expressed during osteoblast maturation in vitro and was able to reverse the negative effect of fibroblast growth factor 2 (FGF2) on osteoblast differentiation. This effect was specific for the N-terminal domain of PTX3 that contains the FGF2-binding site. By using the closed transversal tibial fracture model, we found that ptx3^−/− female mice formed significantly less mineralized callus during the anabolic phase following fracture injury compared to ptx3^+/+ mice (BV/TV 17.05 ± 4.59 vs 20.47 ± 3.32, p = 0.0195). Non-hematopoietic periosteal cells highly upregulated PTX3 expression during the initial phase of fracture healing, particularly CD51⁺ and αSma⁺ osteoprogenitor subsets, and callus tissue exhibited concomitant expression of PTX3 and FGF2 around the fracture site. Thus, PTX3 supports maintenance of the bone mass possibly by inhibiting FGF2 and its negative impact on bone formation. Moreover, PTX3 enables timely occurring sequence of callus mineralization after bone fracture injury. These results indicate that PTX3 plays an important role in bone homeostasis and in proper matrix mineralization during fracture repair, a reflection of the function of this molecule in tissue homeostasis and repair.

Glycans modify mesenchymal stem cell differentiation to impact on the function of resulting osteoblasts

Glycans are inherently heterogeneous, yet glycosylation is essential in eukaryotes, and glycans show characteristic cell type-dependent distributions. By using an immortalized human mesenchymal stromal cell (MSC) line model, we show that both N- and O-glycan processing in the Golgi functionally modulates early steps of osteogenic differentiation. We found that inhibiting O-glycan processing in the Golgi prior to the start of osteogenesis inhibited the mineralization capacity of the formed osteoblasts 3 weeks later. In contrast, inhibition of N-glycan processing in MSCs altered differentiation to enhance the mineralization capacity of the osteoblasts. The effect of N-glycans on MSC differentiation was mediated by the phosphoinositide-3-kinase (PI3K)/Akt pathway owing to reduced Akt phosphorylation. Interestingly, by inhibiting PI3K during the first 2 days of osteogenesis, we were able to phenocopy the effect of inhibiting N-glycan processing. Thus, glycan processing provides another layer of regulation that can modulate the functional outcome of differentiation. Glycan processing can thereby offer a novel set of targets for many therapeutically attractive processes.

Summary: Both N- and O-glycan processing modulate MSC differentiation early during osteogenesis to influence mineral formation. Inhibition of N-glycan processing increases mineralization.

Paracrine Fibroblast Growth Factor Initiates Oncogenic Synergy with Epithelial FGFR/Src Transformation in Prostate Tumor Progression

Cross talk of stromal-epithelial cells plays an essential role in both normal development and tumor initiation and progression. Fibroblast growth factor (FGF)-FGF receptor (FGFR)-Src kinase axis is one of the major signal transduction pathways to mediate this cross talk. Numerous genomic studies have demonstrated that expression levels of FGFR/Src are deregulated in a variety of cancers including prostate cancer; however, the role that paracrine FGF (from stromal cells) plays in dysregulated expression of epithelial FGFRs/Src and tumor progression in vivo is not well evaluated. In this study, we demonstrate that ectopic expression of wild-type FGFR1/2 or Src kinase in epithelial cells was not sufficient to initiate prostate tumorigenesis under a normal stromal microenvironment in vivo. However, paracrine FGF10 synergized with ectopic expression of epithelial FGFR1 or FGFR2 to induce epithelial-mesenchymal transition. Additionally, paracrine FGF10 sensitized FGFR2-transformed epithelial cells to initiate prostate tumorigenesis. Next, paracrine FGF10 also synergized with overexpression of epithelial Src kinase to high-grade tumors. But loss of the myristoylation site in Src kinase inhibited paracrine FGF10-induced prostate tumorigenesis. Loss of myristoylation alters Src levels in the cell membrane and inhibited FGF-mediated signaling including inhibition of the phosphotyrosine pattern and FAK phosphorylation. Our study demonstrates the potential tumor progression by simultaneous deregulation of proteins in the FGF/FGFRs/Src signal axis and provides a therapeutic strategy of targeting myristoylation of Src kinase to interfere with the tumorigenic process.

Effects of fin fold mesenchyme ablation on fin development in zebrafish

The evolution of the tetrapod limb involved an expansion and elaboration of the endoskeletal elements, while the fish fin rays were lost. Loss of fin-specific genes, and regulatory changes in key appendicular patterning genes have been identified as mechanisms of limb evolution, however their contributions to cellular organization and tissue differences between fins and limbs remains poorly understood. During early larval fin development, hoxa13a/hoxd13a-expressing fin fold mesenchyme migrate through the median and pectoral fin along actinotrichia fibrils, non-calcified skeletal elements crucial for supporting the fin fold. Fin fold mesenchyme migration defects have previously been proposed as a mechanism of fin dermal bone loss during tetrapod evolution as it has been shown they contribute directly to the fin ray osteoblast population. Using the nitroreductase/metronidazole system, we genetically ablated a subset of hoxa13a/hoxd13a-expressing fin fold mesenchyme to assess its contributions to fin development. Following the ablation of fin fold mesenchyme in larvae, the actinotrichia are unable to remain rigid and the median and pectoral fin folds collapse, resulting in a reduced fin fold size. The remaining cells following ablation are unable to migrate and show decreased actinodin1 mesenchymal reporter activity. Actinodin proteins are crucial structural component of the actinotrichia. Additionally, we show a decrease in hoxa13a, hoxd13a, fgf10a and altered shha, and ptch2 expression during larval fin development. A continuous treatment of metronidazole leads to fin ray defects at 30dpf. Fewer rays are present compared to stage-matched control larvae, and these rays are shorter and less defined. These results suggest the targeted hoxa13a/hoxd13a-expressing mesenchyme contribute to their own successful migration through their contributions to actinotrichia. Furthermore, due to their fate as fin ray osteoblasts, we propose their initial ablation, and subsequent disorganization produces truncated fin dermal bone elements during late larval stages.

Multiple faces of fibroblast growth factor-23

PURPOSE OF REVIEW

This review examines the role of fibroblast growth factor-23 (FGF-23) in mineral metabolism, innate immunity and adverse cardiovascular outcomes.

RECENT FINDINGS

FGF-23, produced by osteocytes in bone, activates FGFR/α-Klotho (α-Kl) complexes in the kidney. The resulting bone-kidney axis coordinates renal phosphate reabsorption with bone mineralization, and creates a counter-regulatory feedback loop to prevent vitamin D toxicity. FGF-23 acts to counter-regulate the effects of vitamin D on innate immunity and cardiovascular responses. FGF-23 is ectopically expressed along with α-Kl in activated macrophages, creating a proinflammatory paracrine signaling pathway that counters the antiinflammatory actions of vitamin D. FGF-23 also inhibits angiotensin-converting enzyme 2 expression and increases sodium reabsorption in the kidney, leading to hypertension and left ventricular hypertrophy. Finally, FGF-23 is purported to cause adverse cardiac and impair neutrophil responses through activation of FGFRs in the absence of α-Kl. Although secreted forms of α-Kl have FGF-23 independent effects, the possibility of α-Kl independent effects of FGF-23 is controversial and requires additional experimental validation.

SUMMARY

FGF-23 participates in a bone-kidney axis regulating mineral homeostasis, proinflammatory paracrine macrophage signaling pathways, and in a bone-cardio-renal axis regulating hemodynamics that counteract the effects of vitamin D.

Combinatorial Analysis of Growth Factors Reveals the Contribution of Bone Morphogenetic Proteins to Chondrogenic Differentiation of Human Periosteal Cells

Successful application of cell-based strategies in cartilage and bone tissue engineering has been hampered by the lack of robust protocols to efficiently differentiate mesenchymal stem cells into the chondrogenic lineage. The development of chemically defined culture media supplemented with growth factors (GFs) has been proposed as a way to overcome this limitation. In this work, we applied a fractional design of experiment (DoE) strategy to screen the effect of multiple GFs (BMP2, BMP6, GDF5, TGF-β1, and FGF2) on chondrogenic differentiation of human periosteum-derived mesenchymal stem cells (hPDCs) in vitro. In a micromass culture (μMass) system, BMP2 had a positive effect on glycosaminoglycan deposition at day 7 (p < 0.001), which in combination with BMP6 synergistically enhanced cartilage-like tissue formation that displayed in vitro mineralization capacity at day 14 (p < 0.001). Gene expression of μMasses cultured for 7 days with a medium formulation supplemented with 100 ng/mL of BMP2 and BMP6 and a low concentration of GDF5, TGF-β1, and FGF2 showed increased expression of Sox9 (1.7-fold) and the matrix molecules aggrecan (7-fold increase) and COL2A1 (40-fold increase) compared to nonstimulated control μMasses. The DoE analysis indicated that in GF combinations, BMP2 was the strongest effector for chondrogenic differentiation of hPDCs. When transplanted ectopically in nude mice, the in vitro-differentiated μMasses showed maintenance of the cartilaginous phenotype after 4 weeks in vivo. This study indicates the power of using the DoE approach for the creation of new medium formulations for skeletal tissue engineering approaches.

Fibroblast Growth Factor Receptor 2 (FGFR2) Mutation Related Syndromic Craniosynostosis

Craniosynostosis results from the premature fusion of cranial sutures, with an incidence of 1 in 2,100-2,500 live births. The majority of cases are non-syndromic and involve single suture fusion, whereas syndromic cases often involve complex multiple suture fusion. The fibroblast growth factor receptor 2 (FGFR2) gene is perhaps the most extensively studied gene that is mutated in various craniosynostotic syndromes including Crouzon, Apert, Pfeiffer, Antley-Bixler, Beare-Stevenson cutis gyrata, Jackson-Weiss, Bent Bone Dysplasia, and Seathre-Chotzen-like syndromes. The majority of these mutations are missense mutations that result in constitutive activation of the receptor and downstream molecular pathways. Treatment involves a multidisciplinary approach with ultimate surgical fixation of the cranial deformity to prevent further sequelae. Understanding the molecular mechanisms has allowed for the investigation of different therapeutic agents that can potentially be used to prevent the disorders. Further research efforts are need to better understand screening and effective methods of early intervention and prevention. Herein, the authors provide a comprehensive update on FGFR2-related syndromic craniosynostosis.

Inducible Activation of FGFR2 in Adult Mice Promotes Bone Formation After Bone Marrow Ablation

Apert syndrome is one of the most severe craniosynostoses, resulting from gain-of-function mutations in fibroblast growth factor receptor 2 (FGFR2). Previous studies have shown that gain-of-function mutations of FGFR2 (S252W or P253R) cause skull malformation of human Apert syndrome by affecting both chondrogenesis and osteogenesis, underscoring the key role of FGFR2 in bone development. However, the effects of FGFR2 on bone formation at the adult stage have not been fully investigated. To investigate the role of FGFR2 in bone formation, we generated mice with tamoxifen-inducible expression of mutant FGFR2 (P253R) at the adult stage. Mechanical bone marrow ablation (BMX) was performed in both wild-type and Fgfr2 mutant (MT) mice. Changes in newly formed trabecular bone were assessed by micro-computed tomography and bone histomorphometry. We found that MT mice exhibited increased trabecular bone formation and decreased bone resorption after BMX accompanied with a remarkable increase in bone marrow stromal cell recruitment and proliferation, osteoblast proliferation and differentiation, and enhanced Wnt/β-catenin activity. Furthermore, pharmacologically inhibiting Wnt/β-catenin signaling can partially reverse the increased trabecular bone formation and decreased bone resorption in MT mice after BMX. Our data demonstrate that gain-of-function mutation in FGFR2 exerts a Wnt/β-catenin-dependent anabolic effect on trabecular bone by promoting bone formation and inhibiting bone resorption at the adult stage. © 2017 American Society for Bone and Mineral Research.

Breast cancer cell-derived fibroblast growth factors enhance osteoclast activity and contribute to the formation of metastatic lesions

Fibroblast growth factors (FGFs) and their receptors (FGFRs) have been implicated in promoting breast cancer growth and progression. While the autocrine effects of FGFR activation in tumor cells have been extensively studied, little is known about the effects of tumor cell-derived FGFs on cells in the microenvironment. Because FGF signaling has been implicated in the regulation of bone formation and osteoclast differentiation, we hypothesized that tumor cell-derived FGFs are capable of modulating osteoclast function and contributing to growth of metastatic lesions in the bone. Initial studies examining FGFR expression during osteoclast differentiation revealed increased expression of FGFR1 in osteoclasts during differentiation. Therefore, studies were performed to determine whether tumor cell-derived FGFs are capable of promoting osteoclast differentiation and activity. Using both non-transformed and transformed cell lines, we demonstrate that breast cancer cells express a number of FGF ligands that are known to activate FGFR1. Furthermore our results demonstrate that inhibition of FGFR activity using the clinically relevant inhibitor BGJ398 leads to reduced osteoclast differentiation and activity in vitro. Treatment of mice injected with tumor cells into the femurs with BGJ398 leads to reduced osteoclast activity and bone destruction. Together, these studies demonstrate that tumor cell-derived FGFs enhance osteoclast function and contribute to the formation of metastatic lesions in breast cancer.

Local delivery of recombinant human FGF7 enhances bone formation in rat mandible defects

Fibroblast growth factor 7 (FGF7) plays an important role in regulating the proliferation, migration, and differentiation of cells. However, the role of FGF7 in bone formation is not yet fully understood. We examined the effect of FGF7 on bone formation using a rat model of mandible defects. Rats underwent mandible defect surgery and then either scaffold treatment alone (control group) or FGF7-impregnated scaffold treatment (FGF7 group). Micro-CT and histological analyses revealed that the FGF7 group exhibited greater bone formation than did the control group 10 weeks after surgery. With the exception of total porosity (%), all bone parameters had higher values in the FGF7 group than in the control group at each follow-up after surgery. The FGF7 group showed greater expression of osteogenic markers, such as runt-related transcription factor 2, osterix, osteocalcin, bone morphogenetic protein 2, osteopontin, and type I collagen in newly formed bone than did the control group. The delivery of FGF7 also increased the messenger RNA expression of stromal-cell-derived factor 1 (SDF-1) and CXCR4 in newly formed bone in the FGF7 group compared with the control group. Further, addition of exogenous FGF7 induced migration of rat bone marrow stromal cells and increased the expression of SDF-1 and CXCR4 in the cells. Furthermore, the addition of FGF7 augmented mineralization in the cells with increased expression of osteogenic markers, and this augmentation was significantly suppressed by an inhibitor specific for c-Jun N-terminal kinase (SP600125) or extracellular-signal-regulated kinase (PD98059). Collectively, these results suggest that local delivery of FGF7 increases bone formation in a mandible defect with enhanced osteogenesis and chemoattraction.

Definition of a Critical Size Osteochondral Knee Defect and its Negative Effect on the Surrounding Articular Cartilage in the Rat

BACKGROUND

Joint trauma is predisposing to the incidence of osteoarthritis (OA) of the knee. There is a limited knowledge on the impact of posttraumatic osteochondral defects on the whole joint. This study was designed to define a critical size osteochondral defect in the knee of rats and to investigate a possible association between osteochondral defects and degeneration of the surrounding joint surface.

METHODS

Cylindrical osteochondral defects of different sizes were created in the knee joint of rats. The natural course of these lesions was studied by macroscopic observation, histology, and immunohistochemistry. Gene expression of the articular cartilage surrounding the defects in vivo and of articular chondrocytes cultured in vitro in IL1β and fibroblast growth factor 2 (FGF2) supplemented media was evaluated by quantitative polymerase chain reaction (qPCR).

RESULTS

In defects of 0.9 mm diameter, spontaneous joint surface healing was observed but also upward advancing of the subchondral bone plate at 16 weeks. Larger 1.4 mm diameter defects were critical size, not resulting in successful healing at any time point. Importantly, the articular cartilage surrounding the defects expressed FGF2 and IL1β, but not ACAN and Col2. Chondrocytes cultured in IL1β and FGF2 supplemented media lost the natural fibroblast growth factor receptors - FGFr1/FGFr3 balance and showed decreased viability.

CONCLUSIONS

A critical size osteochondral defect was defined as 1.4 mm in diameter in rat. Subchondral bone plate advancement occured rapidly. The articular cartilage surrounding osteochondral defects showed catabolic activity with expression of IL1β, FGF2 and a disturbed FGFr1/FGFr3 balance, potentially initiating a process of early osteoarthritic disease.

Expression of RMRP RNA is regulated in chondrocyte hypertrophy and determines chondrogenic differentiation

Mutations in the RMRP-gene, encoding the lncRNA component of the RNase MRP complex, are the origin of cartilage-hair hypoplasia. Cartilage-hair hypoplasia is associated with severe dwarfism caused by impaired skeletal development. However, it is not clear why mutations in RMRP RNA lead to skeletal dysplasia. Since chondrogenic differentiation of the growth plate is required for development of long bones, we hypothesized that RMRP RNA plays a pivotal role in chondrogenic differentiation. Expression of Rmrp RNA and RNase MRP protein subunits was detected in the murine growth plate and during the course of chondrogenic differentiation of ATDC5 cultures, where Rmrp RNA expression was found to be correlated with chondrocyte hypertrophy. Genetic interference with Rmrp RNA expression in ATDC5 cultures caused a deregulation of chondrogenic differentiation, with a prominent impact on hypertrophy and changes in pre-rRNA processing and rRNA levels. Promoter reporter studies showed that Rmrp RNA expression responds to chondrogenic morphogens. Chondrogenic trans-differentiation of cartilage-hair hypoplasia fibroblasts was impaired with a pronounced impact on hypertrophic differentiation. Together, our data show that RMRP RNA expression is regulated during different stages of chondrogenic differentiation and indicate that RMRP RNA may play a pivotal role in chondrocyte hypertrophy, with potential consequences for CHH pathobiology.

Large numbers of explanatory variables, a semi-descriptive analysis

Data with a relatively small number of study individuals and a very large number of potential explanatory features arise particularly, but by no means only, in genomics. A powerful method of analysis, the lasso [Tibshirani R (1996) J Roy Stat Soc B 58:267-288], takes account of an assumed sparsity of effects, that is, that most of the features are nugatory. Standard criteria for model fitting, such as the method of least squares, are modified by imposing a penalty for each explanatory variable used. There results a single model, leaving open the possibility that other sparse choices of explanatory features fit virtually equally well. The method suggested in this paper aims to specify simple models that are essentially equally effective, leaving detailed interpretation to the specifics of the particular study. The method hinges on the ability to make initially a very large number of separate analyses, allowing each explanatory feature to be assessed in combination with many other such features. Further stages allow the assessment of more complex patterns such as nonlinear and interactive dependences. The method has formal similarities to so-called partially balanced incomplete block designs introduced 80 years ago [Yates F (1936) J Agric Sci 26:424-455] for the study of large-scale plant breeding trials. The emphasis in this paper is strongly on exploratory analysis; the more formal statistical properties obtained under idealized assumptions will be reported separately.

FGFR3 deficient mice have accelerated fracture repair

Bone fracture healing is processed through multiple biological stages that partly recapitulates the skeletal development process. FGFR3 is a negative regulator of chondrogenesis during embryonic stage and plays an important role in both chondrogenesis and osteogenesis. We have investigated the role of FGFR3 in fracture healing using unstabilized fracture model and found that gain-of-function mutation of FGFR3 inhibits the initiation of chondrogenesis during cartilage callus formation. Here, we created closed, stabilized proximal tibia fractures with an intramedullary pin in Fgfr3^-/-mice and their littermate wild-type mice. Fracture healing was evaluated by radiography, micro-CT, histology, and real-time polymerase chain reaction (RT-PCR) analysis. The fractured Fgfr3^-/- mice had increased formation of cartilaginous callus, more fracture callus, and more rapid endochondral ossification in fracture sites with up-regulated expressions of chondrogenesis related gene. The fractures of Fgfr3^-/- mice healed faster with accelerated fracture callus mineralization and up-regulated expression of osteoblastogenic genes. The healing of fractures in Fgfr3^-/- mice was accelerated in the stage of formation of cartilage and endochondral ossification. Downregulation of FGFR3 activity can be considered as a potential bio-therapeutic strategy for fracture treatment.

Identification and in silico characterization of p.G380R substitution in FGFR3, associated with achondroplasia in a non-consanguineous Pakistani family

Background

The dimerization efficiency of FGFR3 transmembrane domain plays a critical role in the formation of a normal skeleton through the negative regulation of bone development. Recently, gain-of-function mutations in the transmembrane domain of FGFR3 has been described associated with an aberrant negative regulation, leading to the development of achondroplasia-group disorders, including achondroplasia (ACH), hypochondroplasia (HCH) and thanatophoric dysplasia (TD). Here, we describe a non-consanguineous Pakistani family with achondroplasia to explain hereditary basis of the disease.

Methods

PCR-based linkage analysis using microsatellite markers was employed to localize the disease gene. Gene specific intronic primers were used to amplify the genomic DNA from all affected as well as phenotypically healthy individuals. Amplified PCR products were then subjected to Sanger sequencing and RFLP analysis to identify a potentially pathogenic mutation. The impact of identified mutation on FGFR3 protein’s structure and stability was highlighted through different bioinformatics tools.

Results

Genetic screening of the family revealed a previously reported heterozygous c.1138 G > A (p.G380R) mutation in the coding exon 8 of FGFR3 gene. Identified genetic variation was confirmed in all affected individuals while healthy individuals and controls were found genotypically normal. The results were further validated by RFLP analysis as c.1138 G > A substitution generates a unique recognition site for SfcI endonuclease. Following SfcI digestion, the electrophoretic pattern of three bands/DNA fragments for each patient is indicative of heterozygous status of the disease allele. In silico studies of the mutant FGFR3 protein predicted to adversely affect the stability of FGFR3 protein.

Conclusions

Mutation in the transmembrane domain may adversely affect the dimerization efficiency and overall stability of the FGFR3, leading to a constitutively active protein. As a result, an uncontrolled intracellular signaling or negative bone growth regulation leads to achondroplasia. Our findings support the fact that p.G380R is a common mutation among diverse population of the world and like other countries, can be used as a molecular diagnosis marker for achondroplasia in Pakistan.

Electronic supplementary material

The online version of this article (doi:10.1186/s13000-017-0642-3) contains supplementary material, which is available to authorized users.

Effects of klotho deletion from bone during chronic kidney disease

Klotho is a type I transmembrane protein that acts as a permissive co-receptor for FGF23 and helps to maintain proper mineral metabolism. Mice carrying a loss-of-function mutation in either the Klotho or Fgf23 gene develop many similar phenotypes including osteoporosis. Based on these observations it was hypothesized that the bone phenotypes in Klotho- and Fgf23-null mice may be mediated through a common signaling pathway. Recent improvements in antibody specificity have shown that osteoblasts and osteocytes, which produce FGF23, also express low amount of membrane Klotho. But, the role of Klotho in bone is still largely unclear. In this review we summarize the literature and show that Klotho has an FGF23 dependent and independent effect in bone.

-Glial and stem cell expression of murine Fibroblast Growth Factor Receptor 1 in the embryonic and perinatal nervous system

BACKGROUND

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are involved in the development and function of multiple organs and organ systems, including the central nervous system (CNS). FGF signaling via FGFR1, one of the three FGFRs expressed in the CNS, stimulates proliferation of stem cells during prenatal and postnatal neurogenesis and participates in regulating cell-type ratios in many developing regions of the brain. Anomalies in FGFR1 signaling have been implicated in certain neuropsychiatric disorders. Fgfr1 expression has been shown, via in situ hybridization, to vary spatially and temporally throughout embryonic and postnatal development of the brain. However, in situ hybridization lacks sufficient resolution to identify which cell-types directly participate in FGF signaling. Furthermore, because antibodies raised against FGFR1 commonly cross-react with other members of the FGFR family, immunocytochemistry is not alone sufficient to accurately document Fgfr1 expression. Here, we elucidate the identity of Fgfr1 expressing cells in both the embryonic and perinatal mouse brain.

METHODS

To do this, we utilized a tgFGFR1-EGFPGP338Gsat BAC line (tgFgfr1-EGFP+) obtained from the GENSAT project. The tgFgfr1-EGFP+ line expresses EGFP under the control of a Fgfr1 promoter, thereby causing cells endogenously expressing Fgfr1 to also present a positive GFP signal. Through simple immunostaining using GFP antibodies and cell-type specific antibodies, we were able to accurately determine the cell-type of Fgfr1 expressing cells.

RESULTS

This technique revealed Fgfr1 expression in proliferative zones containing BLBP+ radial glial stem cells, such as the cortical and hippocampal ventricular zones, and cerebellar anlage of E14.5 mice, in addition to DCX+ neuroblasts. Furthermore, our data reveal Fgfr1 expression in proliferative zones containing BLBP+ cells of the anterior midline, hippocampus, cortex, hypothalamus, and cerebellum of P0.5 mice, in addition to the early-formed GFAP+ astrocytes of the anterior midline.

DISCUSSION

Understanding when during development and where Fgfr1 is expressed is critical to improving our understanding of its function during neurodevelopment as well as in the mature CNS. This information may one day provide an avenue of discovery towards understanding the involvement of aberrant FGF signaling in neuropsychiatric disorders.

Low-level laser therapy (780 nm) combined with collagen sponge scaffold promotes repair of rat cranial critical-size defects and increases TGF-β, FGF-2, OPG/RANK and osteocalcin expression

The aim of this study was to evaluate the effect of collagen sponge scaffold (CSS) implantation associated with low-level laser therapy (LLLT) on repairing bone defects. A single 5-mm cranial defect was surgically created in forty Wistar rats, which then received one of the following four interventions (n = 10 per group): no treatment (G0); bone defect implanted with collagen sponge scaffold (CSS) alone (G1); defect treated with low-level laser therapy (LLLT) (wavelength 780 nm; total energy density 120 J/cm² ; power 50 mW) alone (G2); and CSS associated with LLLT treatment (G3). After surgery, animals in each group were euthanized at 21 days and 30 days (n = 5 per euthanasia time group). Bone formation was monitored by X-ray imaging analysis. Biopsies were collected and processed for histological analysis and immunohistochemical evaluation of transforming growth factor-beta (TGF-β), fibroblast growth factor-2 (FGF-2), osteoprotegerin (OPG) and receptor activator of nuclear factor ƙ (RANK). Osteocalcin (OCN) was detected by immunofluorescence analysis. Compared to the G0 group, defects in the 30-day G3 group exhibited increased bone formation, both by increase in radiopaque areas (P < 0.01) and by histomorphometric analysis (P < 0.001). The histopathological analysis showed a decreased number of inflammatory cells (P < 0.001). The combined CCS + LLLT (G3) treatment also resulted in the most intense immunostaining for OPG, RANK, FGF-2 and TGF-β, and the most intense and diffuse OCN immunofluorescent labelling at 30 days postsurgery (G3 vs. G0 group, P < 0.05). Therefore, the use of CCS associated with LLLT could offer a synergistic advantage in improving the healing of bone fractures.

2-Aminopyrimidine Derivatives as New Selective Fibroblast Growth Factor Receptor 4 (FGFR4) Inhibitors

A series of 2-aminopyrimidine derivatives were designed and synthesized as highly selective FGFR4 inhibitors. One of the most promising compounds 2n tightly bound FGFR4 with a K_d value of 3.3 nM and potently inhibited its enzymatic activity with an IC₅₀ value of 2.6 nM, but completely spared FGFR1/2/3. The compound selectively suppressed proliferation of breast cancer cells harboring dysregulated FGFR4 signaling with an IC₅₀ value of 0.38 μM. Furthermore, 2n exhibited extraordinary target specificity in a Kinome-wide screen against 468 kinases, with S(35) and S(10) selectivity scores of 0.01 and 0.007 at 1.0 μM, respectively.

Cutaneous skeletal hypophosphatemia syndrome (CSHS) is a multilineage somatic mosaic RASopathy

BACKGROUND

We recently demonstrated multilineage somatic mosaicism in cutaneous skeletal hypophosphatemia syndrome (CSHS), which features epidermal or melanocytic nevi, elevated fibroblast growth factor (FGF)-23, and hypophosphatemia, finding identical RAS mutations in affected skin and bone.

OBJECTIVE

We sought to: (1) provide an updated overview of CSHS; (2) review its pathobiology; (3) present a new patient with CSHS; and (4) discuss treatment modalities.

METHODS

We searched PubMed for "nevus AND rickets," and "nevus AND hypophosphatemia," identifying cases of nevi with hypophosphatemic rickets or elevated serum FGF-23. For our additional patient with CSHS, we performed histopathologic and radiographic surveys of skin and skeletal lesions, respectively. Sequencing was performed for HRAS, KRAS, and NRAS to determine causative mutations.

RESULTS

Our new case harbored somatic activating HRAS p.G13 R mutation in affected tissue, consistent with previous findings. Although the mechanism of FGF-23 dysregulation is unknown in CSHS, interaction between FGF and MAPK pathways may provide insight into pathobiology. Anti-FGF-23 antibody KRN-23 may be useful in managing CSHS.

LIMITATIONS

Multilineage RAS mutation in CSHS was recently identified; further studies on mechanism are unavailable.

CONCLUSION

Patients with nevi in association with skeletal disease should be evaluated for serum phosphate and FGF-23. Further studies investigating the role of RAS in FGF-23 regulation are needed.

Pin1, the Master Orchestrator of Bone Cell Differentiation

Pin1 is an enzyme that specifically recognizes the peptide bond between phosphorylated serine or threonine (pS/pT-P) and proline. This recognition causes a conformational change of its substrate, which further regulates downstream signaling. Pin1^-/- mice show developmental bone defects and reduced mineralization. Pin1 targets RUNX2 (Runt-Related Transcription Factor 2), SMAD1/5, and β-catenin in the FGF, BMP, and WNT pathways, respectively. Pin1 has multiple roles in the crosstalk between different anabolic bone signaling pathways. For example, it controls different aspects of osteoblastogenesis and increases the transcriptional activity of Runx2, both directly and indirectly. Pin1 also influences osteoclastogenesis at different stages by targeting PU.1 (Purine-rich nucleic acid binding protein 1), C-FOS, and DC-STAMP. The phenotype of Pin1^-/- mice has led to the recent identification of multiple roles of Pin1 in different molecular pathways in bone cells. These roles suggest that Pin1 can be utilized as an efficient drug target in congenital and acquired bone diseases. J. Cell. Physiol. 232: 2339-2347, 2017. © 2016 Wiley Periodicals, Inc.

Regulation of Bone Metabolism

Bone is formed through the processes of endochondral and intramembranous ossification. In endochondral ossification primary mesenchymal cells differentiate to chondrocytes and then are progressively substituted by bone, while in intramembranous ossification mesenchymal stem cells (MSCs) differentiate directly into osteoblasts to form bone. The steps of osteogenic proliferation, differentiation, and bone homeostasis are controlled by various markers and signaling pathways. Bone needs to be remodeled to maintain integrity with osteoblasts, which are bone-forming cells, and osteoclasts, which are bone-degrading cells.In this review we considered the major factors and signaling pathways in bone formation; these include fibroblast growth factors (FGFs), bone morphogenetic proteins (BMPs), wingless-type (Wnt) genes, runt-related transcription factor 2 (RUNX2) and osteoblast-specific transcription factor (osterix or OSX).

Klotho expression in long bones regulates FGF23 production during renal failure

Circulating levels of bone-derived fibroblast growth factor 23 (FGF23) increase early during acute and chronic kidney disease and are associated with adverse outcomes. Membrane-bound Klotho acts as a permissive coreceptor for FGF23, and its expression was recently found in osteoblasts/osteocytes. We hypothesized that Klotho in bone cells is part of an autocrine feedback loop that regulates FGF23 expression during renal failure. Thus, we induced renal failure in mice with targeted deletion of Klotho in long bones. Uremic wild-type (KL^fl/fl ) and knockout (Prx1-Cre;KL^fl/fl ) mice both responded with reduced body weight, kidney atrophy, hyperphosphatemia, and increased bone turnover. Importantly, long bones of Prx1-Cre;KL^fl/fl mice but not their axial skeleton failed to increase FGF23 expression as observed in uremic KL^fl/fl mice. Consequently, Prx1-Cre;KL^fl/fl mice had significantly lower serum FGF23 and parathyroid hormone levels, and higher renal 1-α-hydroxylase expression, serum 1,25-dihydroxyvitamin D, and calcium levels than KL^fl/fl mice. These results were confirmed in two independent models of renal failure, adenine diet induced and 5/6 nephrectomy. Moreover, FGF23-treated bone cells required Klotho to increase FGF23 mRNA and ERK phosphorylation. In summary, our novel findings show that Klotho in bone is crucial for inducing FGF23 production upon renal failure. We propose the presence of an autocrine feedback loop in which Klotho senses the need for FGF23.-Kaludjerovic, J., Komaba, H., Sato, T., Erben, R. G., Baron, R., Olauson, H., Larsson, T. E., Lanske, B. Klotho expression in long bones regulates FGF23 production during renal failure.

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.

Genetic Syndromes Affecting Kidney Development

Renal anomalies are common birth defects that may manifest as a wide spectrum of anomalies from hydronephrosis (dilation of the renal pelvis and calyces) to renal aplasia (complete absence of the kidney(s)). Aneuploidies and mosaicisms are the most common syndromes associated with CAKUT. Syndromes with single gene and renal developmental defects are less common but have facilitated insight into the mechanism of renal and other organ development. Analysis of underlying genetic mutations with transgenic and mutant mice has also led to advances in our understanding of mechanisms of renal development.

A New Method to Study Heterodimerization of Membrane Proteins and Its Application to Fibroblast Growth Factor Receptors

The activity of receptor tyrosine kinases (RTKs) is controlled through their lateral association in the plasma membrane. RTKs are believed to form both homodimers and heterodimers, and the different dimers are believed to play unique roles in cell signaling. However, RTK heterodimers remain poorly characterized, as compared with homodimers, because of limitations in current experimental methods. Here, we develop a FRET-based methodology to assess the thermodynamics of hetero-interactions in the plasma membrane. To demonstrate the utility of the methodology, we use it to study the hetero-interactions between three fibroblast growth factor receptors-FGFR1, FGFR2, and FGFR3-in the absence of ligand. Our results show that all possible FGFR heterodimers form, suggesting that the biological roles of FGFR heterodimers may be as significant as the homodimer roles. We further investigate the effect of two pathogenic point mutations in FGFR3 (A391E and G380R) on heterodimerization. We show that each of these mutations stabilize most of the heterodimers, with the largest effects observed for FGFR3 wild-type/mutant heterodimers. We thus demonstrate that the methodology presented here can yield new knowledge about RTK interactions and can further our understanding of signal transduction across the plasma membrane.

Chondrocyte FGFR3 Regulates Bone Mass by Inhibiting Osteogenesis

Chondrogenesis can regulate bone formation. Fibroblast growth factor receptor 3, highly expressed in chondrocytes, is a negative regulator of bone growth. To investigate whether chondrocyte FGFR3 regulates osteogenesis, thereby contributing to postnatal bone formation and bone remodeling, mice with conditional knock-out of Fgfr3 in chondrocytes (mutant (MUT)) were generated. MUT mice displayed overgrowth of bone with lengthened growth plates. Bone mass of MUT mice was significantly increased at both 1 month and 4 months of age. Histological analysis showed that osteoblast number and bone formation were remarkably enhanced after deletion of Fgfr3 in chondrocytes. Chondrocyte-osteoblast co-culture assay further revealed that Fgfr3 deficiency in chondrocytes promoted differentiation and mineralization of osteoblasts by up-regulating the expressions of Ihh, Bmp2, Bmp4, Bmp7, Wnt4, and Tgf-β1, as well as down-regulating Nog expression. In addition, osteoclastogenesis was also impaired in MUT mice with decreased number of osteoclasts lining trabecular bone, which may be related to the reduced ratio of Rankl to Opg in Fgfr3-deficient chondrocytes. This study reveals that chondrocyte FGFR3 is involved in the regulation of bone formation and bone remodeling by a paracrine mechanism.

The Basic Science of Bone Marrow Aspirate Concentrate in Chondral Injuries

There has been great interest in bone marrow aspirate concentrate (BMAC) as a cost effective method in delivering mesenchymal stem cells (MSCs) to aid in the repair and regeneration of cartilage defects. Alongside MSCs, BMAC contains a range of growth factors and cytokines to support cell growth following injury. However, there is paucity of information relating to the basic science underlying BMAC and its exact biological role in supporting the growth and regeneration of chondrocytes. The focus of this review is the basic science underlying BMAC in relation to chondral damage and regeneration.

HDAC6 deficiency or inhibition blocks FGFR3 accumulation and improves bone growth in a model of achondroplasia

Mutations that cause increased and/or inappropriate activation of FGFR3 are responsible for a collection of short-limbed chondrodysplasias. These mutations can alter receptor trafficking and enhance receptor stability, leading to increased receptor accumulation and activity. Here, we show that wildtype and mutant activated forms of FGFR3 increase expression of the cytoplasmic deacetylase HDAC6 (Histone Deacetylase 6) and that FGFR3 accumulation is compromised in cells lacking HDAC6 or following treatment of fibroblasts or chondrocytes with small molecule inhibitors of HDAC6. The reduced accumulation of FGFR3 was linked to increased FGFR3 degradation that occurred through a lysosome-dependent mechanism. Using a mouse model of Thanatophoric Dysplasia Type II (TDII) we show that both HDAC6 deletion and treatment with the small molecule HDAC6 inhibitor tubacin reduced FGFR3 accumulation in the growth plate and improved endochondral bone growth. Defective endochondral growth in TDII is associated with reduced proliferation and poor hypertrophic differentiation and the improved bone growth was associated with increased chondrocyte proliferation and expansion of the differentiation compartment within the growth plate. These findings further define the mechanisms that control FGFR3 accumulation and contribute to skeletal pathology caused by mutations in FGFR3.

Trim37-deficient mice recapitulate several features of the multi-organ disorder Mulibrey nanism

Mulibrey nanism (MUL) is a rare autosomal recessive multi-organ disorder characterized by severe prenatal-onset growth failure, infertility, cardiopathy, risk for tumors, fatty liver, and type 2 diabetes. MUL is caused by loss-of-function mutations in TRIM37, which encodes an E3 ubiquitin ligase belonging to the tripartite motif (TRIM) protein family and having both peroxisomal and nuclear localization. We describe a congenic Trim37 knock-out mouse (Trim37^−/−) model for MUL. Trim37^−/− mice were viable and had normal weight development until approximately 12 months of age, after which they started to manifest increasing problems in wellbeing and weight loss. Assessment of skeletal parameters with computer tomography revealed significantly smaller skull size, but no difference in the lengths of long bones in Trim37^−/− mice as compared with wild-type. Both male and female Trim37^−/− mice were infertile, the gonads showing germ cell aplasia, hilus and Leydig cell hyperplasia and accumulation of lipids in and around Leydig cells. Male Trim37^−/− mice had elevated levels of follicle-stimulating and luteinizing hormones, but maintained normal levels of testosterone. Six-month-old Trim37^−/− mice had elevated fasting blood glucose and low fasting serum insulin levels. At 1.5 years Trim37^−/− mice showed non-compaction cardiomyopathy, hepatomegaly, fatty liver and various tumors. The amount and morphology of liver peroxisomes seemed normal in Trim37^−/− mice. The most consistently seen phenotypes in Trim37^−/− mice were infertility and the associated hormonal findings, whereas there was more variability in the other phenotypes observed. Trim37^−/− mice recapitulate several features of the human MUL disease and thus provide a good model to study disease pathogenesis related to TRIM37 deficiency, including infertility, non-alcoholic fatty liver disease, cardiomyopathy and tumorigenesis.

Summary: A congenic Trim37-deficient mouse model recapitulates several features of the human disorder Mulibrey nanism, and thus provides a good model to study disease pathogenesis related to TRIM37 deficiency.

Functional characteristics of mesenchymal stem cells derived from the adipose tissue of a patient with achondroplasia

Mesenchymal stem cells (MSCs) can be isolated from various tissues including bone marrow, adipose tissue, skin dermis, and umbilical Wharton's jelly as well as injured tissues. MSCs possess the capacity for self-renewal and the potential for differentiation into adipogenic, osteogenic, and chondrogenic lineages. However, the characteristics of MSCs in injured tissues, such as achondroplasia (ACH), are not well known. In this study, we isolated MSCs from human subcutaneous adipose (ACH-SAMSCs) tissue and circumjacent human adipose tissue of the cartilage (ACH-CAMSCs) from a patient with ACH. We then analyzed the characterization of ACH-SAMSCs and ACH-CAMSCs, compared with normal human dermis-derived MSCs (hDMSCs). In flow cytometry analysis, the isolated ACH-MSCs expressed low levels of CD73, CD90, and CD105, compared with hDMSCs. Moreover, both ACH- SAMSCs and ACH-CAMSCs had constitutionally overactive fibroblast growth factor receptor 3 (FGFR3) and exhibited significantly reduced osteogenic differentiation, compared to enhanced adipogenic differentiation. The activity of extracellular signal-regulated kinases 1/2 (ERK1/2) and p38 mitogen-activated protein kinases (p38 MAPK) was increased in ACH-MSCs. In addition, the efficacy of osteogenic differentiation was slightly restored in osteogenic differentiation medium with MAPKs inhibitors. These results suggest that they play essential roles in MSC differentiation toward adipogenesis in ACH pathology. In conclusion, the identification of the characteristics of ACH-MSCs and the favoring of adipogenic differentiation via the FGFR3/MAPK axis might help to elucidate the pathogenic mechanisms relevant to other skeletal diseases and could provide targets for therapeutic interventions.

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.

MicroRNA variants as genetic determinants of bone mass

Single nucleotide polymorphisms (SNPs) are the most abundant genetic variants that contribute to the heritability of bone mass. MicroRNAs (miRNAs, miRs) are key post-transcriptional regulators that modulate the differentiation and function of skeletal cells by targeting multiple genes in the same or distinct signaling pathways. SNPs in miRNA genes and miRNA binding sites can alter miRNA abundance and mRNA targeting. This review describes the potential impact of miRNA-related SNPs on skeletal phenotype. Although many associations between SNPs and bone mass have been described, this review is limited to gene variants for which a function has been experimentally validated. SNPs in miRNA genes (miR-SNPs) that impair miRNA processing and alter the abundance of mature miRNA are discussed for miR-146a, miR-125a, miR-196a, miR-149 and miR-27a. SNPs in miRNA targeting sites (miR-TS-SNPs) that alter miRNA binding are described for the bone remodeling genes bone morphogenetic protein receptor 1 (Bmpr1), fibroblast growth factor 2 (Fgf2), osteonectin (Sparc) and histone deacetylase 5 (Hdac5). The review highlights two aspects of miRNA-associated SNPs: the mechanism for altering miRNA mediated gene regulation and the potential of miR-associated SNPs to alter osteoblast, osteoclast or chondrocyte differentiation and function. Given the polygenic nature of skeletal diseases like osteoporosis and osteoarthritis, validating the function of additional miRNA-associated SNPs has the potential to enhance our understanding of the genetic determinants of bone mass and predisposition to selected skeletal diseases.

Articular cartilage tissue engineering: the role of signaling molecules

Effective early disease modifying options for osteoarthritis remain lacking. Tissue engineering approach to generate cartilage in vitro has emerged as a promising option for articular cartilage repair and regeneration. Signaling molecules and matrix modifying agents, derived from knowledge of cartilage development and homeostasis, have been used as biochemical stimuli toward cartilage tissue engineering and have led to improvements in the functionality of engineered cartilage. Clinical translation of neocartilage faces challenges, such as phenotypic instability of the engineered cartilage, poor integration, inflammation, and catabolic factors in the arthritic environment; these can all contribute to failure of implanted neocartilage. A comprehensive understanding of signaling molecules involved in osteoarthritis pathogenesis and their actions on engineered cartilage will be crucial. Thus, while it is important to continue deriving inspiration from cartilage development and homeostasis, it has become increasingly necessary to incorporate knowledge from osteoarthritis pathogenesis into cartilage tissue engineering.

Smad signal pathway regulates angiogenesis via endothelial cell in an adipose-derived stromal cell/endothelial cell co-culture, 3D gel model

Co-implantation of adipose-derived stromal cells (ASCs) and endothelial cells (ECs) can markedly expedite the formation of functional microvascular beds and provides possible methods for cell-based revascularization therapies to treat various diseases. Furthermore, we investigated the role of TGFβ/Smad signaling pathway for angiogenesis in a three-dimensional (3D) collagen gel model established in vitro with co-culture between ASCs and ECs. We found that angiogenesis was attenuated in the co-culture gels after inhibition of ALK5/Smad2/3 with SB431542. Genes coding for VEGF-A, VEGF-B, VE-ca, FGF-1, PDGF, BMP-4, and BMP-7 were significantly reduced in both mono-cultured and co-cultured ECs. Furthermore, the decrease in co-cultured ECs was prominent relative to mono-cultured ECs. Taken together, these findings suggest that in the co-culture between ASCs and ECs, TGFβ/Smad signal pathway regulates angiogenesis via ECs; moreover, the findings that the co-cultured ECs were regulated more significantly than mono-cultured ECs suggest that suppression of Smad signal pathway may regulate the paracrine secretion of ASCs to further modulate angiogenesis of ECs.

FGFR3/fibroblast growth factor receptor 3 inhibits autophagy through decreasing the ATG12-ATG5 conjugate, leading to the delay of cartilage development in achondroplasia

FGFR3 (fibroblast growth factor receptor 3) is a negative regulator of endochondral ossification. Gain-of-function mutations in FGFR3 are responsible for achondroplasia, the most common genetic form of dwarfism in humans. Autophagy, an evolutionarily conserved catabolic process, maintains chondrocyte viability in the growth plate under stress conditions, such as hypoxia and nutritional deficiencies. However, the role of autophagy and its underlying molecular mechanisms in achondroplasia remain elusive. In this study, we found activated FGFR3 signaling inhibited autophagic activity in chondrocytes, both in vivo and in vitro. By employing an embryonic bone culture system, we demonstrated that treatment with autophagy inhibitor 3-MA or chloroquine led to cartilage growth retardation, which mimics the effect of activated-FGFR3 signaling on chondrogenesis. Furthermore, we found that FGFR3 interacted with ATG12-ATG5 conjugate by binding to ATG5. More intriguingly, FGFR3 signaling was found to decrease the protein level of ATG12-ATG5 conjugate. Consistently, using in vitro chondrogenic differentiation assay system, we showed that the ATG12-ATG5 conjugate was essential for the viability and differentiation of chondrocytes. Transient transfection of ATG5 partially rescued FGFR3-mediated inhibition on chondrocyte viability and differentiation. Our findings reveal that FGFR3 inhibits the autophagic activity by decreasing the ATG12-ATG5 conjugate level, which may play an essential role in the pathogenesis of achondroplasia.

Biomimetic supramolecular designs for the controlled release of growth factors in bone regeneration

The extracellular matrix (ECM) of tissues is an assembly of insoluble macromolecules that specifically interact with soluble bioactive molecules and regulate their distribution and availability to cells. Recapitulating this ability has been an important target in controlled growth factor delivery strategies for tissue regeneration and requires the design of multifunctional carriers. This review describes the integration of supramolecular interactions on the design of delivery strategies that encompass self-assembling and engineered affinity components to construct advanced biomimetic carriers for growth factor delivery. Several glycan- and peptide-based self-assemblies reported in the literature are highlighted and commented upon. These examples demonstrate how molecular design and chemistry are successfully employed to create versatile multifunctional molecules which self-assemble/disassemble in a precisely predicted manner, thus controlling compartmentalization, transport and delivery. Finally, we discuss whether recent advances in the design and preparation of supramolecular delivery systems have been sufficient to drive real translation towards a clinical impact.

Craniosynostosis

Craniosynostosis is the premature fusion of one or more of the cranial sutures. About 8% of the patients have familial or syndromic forms of synostosis, and in the remainder it occurs as a spontaneous isolated defect. Familial craniosynostosis syndromes are typically transmitted as an autosomal dominant trait resulting in disruption of the fibroblast growth factor receptor pathway. Familiarity with the characteristic head shapes resulting from craniosynostosis allows bedside diagnosis and differentiation from positional plagiocephaly. Because of the risks associated with untreated craniosynostosis, surgical treatment is usually undertaken soon after diagnosis. Current surgical methods include open calvarial reconstruction, minimally invasive strip craniectomy with use of postoperative molding helmet, minimally invasive strip craniectomy with spring implantation, and cranial distraction. Early referral to a pediatric craniofacial center allows all treatment options to be explored.

Cell cycle control, DNA damage repair, and apoptosis-related pathways control pre-ameloblasts differentiation during tooth development

Background

Ameloblast differentiation is the most critical stepwise process in amelogenesis, and it is controlled by precise molecular events. To better understand the mechanism controlling pre-ameloblasts (PABs) differentiation into secretory ameloblasts (SABs), a more precise identification of molecules and signaling networks will elucidate the mechanisms governing enamel formation and lay a foundation for enamel regeneration.

Results

We analyzed transcriptional profiles of human PABs and SABs. From a total of 28,869 analyzed transcripts, we identified 923 differentially expressed genes (DEGs) with p < 0.05 and Fold-change > 2. Among the DEGs, 647 genes showed elevated expression in PABs compared to SABs. Notably, 38 DEGs displayed greater than eight-fold changes. Comparative analysis revealed that highly expressed genes in PABs were involved in cell cycle control, DNA damage repair and apoptosis, while highly expressed genes in SABs were related to cell adhesion and extracellular matrix. Moreover, coexpression network analysis uncovered two highly conserved sub-networks contributing to differentiation, containing transcription regulators (RUNX2, ETV1 and ETV5), solute carrier family members (SLC15A1 and SLC7A11), enamel matrix protein (MMP20), and a polymodal excitatory ion channel (TRPA1).

Conclusions

By combining comparative analysis and coexpression networks, this study provides novel biomarkers and research targets for ameloblast differentiation and the potential for their application in enamel regeneration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1783-y) contains supplementary material, which is available to authorized users.

MicroRNA 224 Regulates Ion Transporter Expression in Ameloblasts To Coordinate Enamel Mineralization

Enamel mineralization is accompanied by the release of protons into the extracellular matrix, which is buffered to regulate the pH value in the local microenvironment. The present study aimed to investigate the role of microRNA 224 (miR-224) as a regulator of SLC4A4 and CFTR, encoding the key buffering ion transporters, in modulating enamel mineralization. miR-224 was significantly downregulated as ameloblasts differentiated, in parallel with upregulation of SLC4A4 and CFTR. Overexpression of miR-224 downregulated SLC4A4 and CFTR expression in cultured human epithelial cells. A microRNA luciferase assay confirmed the specific binding of miR-224 to the 3' untranslated regions (UTRs) of SLC4A4 and CFTR mRNAs, thereby inhibiting protein translation. miR-224 agomir injection in mouse neonatal incisors resulted in normal enamel length and thickness, but with disturbed organization of the prism structure and deficient crystal growth. Moreover, the enamel Ca/P ratio and microhardness were markedly reduced after miR-224 agomir administration. These results demonstrate that miR-224 plays a pivotal role in fine tuning enamel mineralization by modulating SLC4A4 and CFTR to maintain pH homeostasis and support enamel mineralization.