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

Analysis of the causal relationship between gut microbiota and bone remodeling growth factor from the gene association

BACKGROUND

A growing body of evidence indicates a close association between the gut microbiota (GM) and the bone remodeling (BR) process, raising suspicions that the GM may actively participate in BR by modulating the levels of growth factors. However, the precise causal relationship between them remains unclear. Due to many confounding factors, many microorganisms related to BR growth factors have not been identified. We aimed to elucidate the causal relationship between the GM and BR growth factors.

METHODS

We evaluated the genome-wide association study (GWAS) summary statistics for GM and five common growth factors associated with BR: namely, bone morphogenetic proteins (BMP), transforming growth factors(TGF), insulin growth factors (IGFs), epidermal growth factors (EGFs), and fibroblast growth factors (FGF). The causal relationship between the GM and BR growth factors was studied by double-sample Mendelian randomized analysis. We used five Mendelian randomization (MR) methods, including inverse variance-weighted (IVW), MR-Egger, simple mode, weighted median, and weighted model methods.

RESULTS

Through MR analysis, a total of 56 bacterial genera were co-identified as associated with BMP, TGF, IGF, EGF, and FGF. Among them, eight genera were found to have a causal relationship with multiple growth factors: Marvinbryantia was causally associated with BMP-6 (P = 0.018, OR = 1.355) and TGF-β2 (P = 0.002, OR = 1.475); Lachnoclostridium, BMP-7 (P = 0.021, OR = 0.73) and IGF-1 (P = 0.046, OR = 0.804); Terrisporobacter, TGF-β (P = 0.02, OR = 1.726) and FGF-23 levels (P = 0.016, OR = 1.76); Ruminiclostridium5, TGF-β levels (P = 0.024, OR = 0.525) and FGFR-2 (P = 0.003, OR = 0.681); Erysipelatoclostridium, TGF-β2 (P = 0.001, OR = 0.739) and EGF and its receptor (EGFR) (P = 0.012, OR = 0.795); Eubacterium_brachy_group, FGFR-2 (P = 0.045, OR = 1.153) and EGF (P = 0.013, OR = 0.7); Prevotella9 with EGFR (P = 0.022, OR = 0.818) and FGFR-2 (P = 0.011, OR = 1.233) and Faecalibacterium with FGF-23 (P = 0.02, OR = 2.053) and IGF-1 (P = 0.005, OR = 0.843).

CONCLUSION

We confirmed the causal relationship between the GM and growth factors related to BR, which provides a new perspective for the study of BR, through targeted regulation of specific bacteria to prevent and treat diseases and growth factor-mediated BR disorders.

Nutrient and Hormonal Effects on Long Bone Growth in Healthy and Obese Children: A Literature Review

Longitudinal bone growth is mediated through several mechanisms including macro- and micronutrients, and endocrine and paracrine hormones. These mechanisms can be affected by childhood obesity as excess adiposity may affect signaling pathways, place undue stress on the body, and affect normal physiology. This review describes the physiology of the epiphyseal growth plate, its regulation under healthy weight and obesity parameters, and bone pathology following obesity. A literature review was performed utilizing PubMed, PMC, NIH, and the Cochrane Database of Systematic Reviews pertinent to hormonal and nutritional effects on bone development, child obesity, and pathologic bone development related to weight. The review indicates a complex network of nutrients, hormones, and multi-system interactions mediates long bone growth. As growth of long bones occurs during childhood and the pubertal growth spurt, pediatric bones require adequate levels of minerals, vitamins, amino acids, and a base caloric supply for energy. Recommendations should focus on a nutrient-dense dietary approach rather than restrictive caloric diets to maintain optimal health. In conclusion, childhood obesity has profound multifaceted effects on the developing musculoskeletal system, ultimately causing poor nutritional status during development. Weight loss, under medical supervision, with proper nutritional guidelines, can help counteract the ill effects of childhood obesity.

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Fgf9 regulates bone marrow mesenchymal stem cell fate and bone-fat balance in osteoporosis by PI3K/AKT/Hippo and MEK/ERK signaling

Bone-fat balance is crucial to maintain bone homeostasis. As common progenitor cells of osteoblasts and adipocytes, bone marrow mesenchymal stem cells (BMSCs) are delicately balanced for their differentiation commitment. However, the exact mechanisms governing BMSC cell fate are unclear. In this study, we discovered that fibroblast growth factor 9 (Fgf9), a cytokine expressed in the bone marrow niche, controlled bone-fat balance by influencing the cell fate of BMSCs. Histomorphology and cytodifferentiation analysis showed that Fgf9 loss-of-function mutation (S99N) notably inhibited bone marrow adipose tissue (BMAT) formation and alleviated ovariectomy-induced bone loss and BMAT accumulation in adult mice. Furthermore, in vitro and in vivo investigations demonstrated that Fgf9 altered the differentiation potential of BMSCs, shifting from osteogenesis to adipogenesis at the early stages of cell commitment. Transcriptomic and gene expression analyses demonstrated that FGF9 upregulated the expression of adipogenic genes while downregulating osteogenic gene expression at both mRNA and protein levels. Mechanistic studies revealed that FGF9, through FGFR1, promoted adipogenic gene expression via PI3K/AKT/Hippo pathways and inhibited osteogenic gene expression via MAPK/ERK pathway. This study underscores the crucial role of Fgf9 as a cytokine regulating the bone-fat balance in adult bone, suggesting that FGF9 is a potentially therapeutic target in the treatment of osteoporosis.

Therapeutic avenues in bone repair: Harnessing an anabolic osteopeptide, PEPITEM, to boost bone growth and prevent bone loss

The existing suite of therapies for bone diseases largely act to prevent further bone loss but fail to stimulate healthy bone formation and repair. We describe an endogenous osteopeptide (PEPITEM) with anabolic osteogenic activity, regulating bone remodeling in health and disease. PEPITEM acts directly on osteoblasts through NCAM-1 signaling to promote their maturation and formation of new bone, leading to enhanced trabecular bone growth and strength. Simultaneously, PEPITEM stimulates an inhibitory paracrine loop: promoting osteoblast release of the decoy receptor osteoprotegerin, which sequesters RANKL, thereby limiting osteoclast activity and bone resorption. In disease models, PEPITEM therapy halts osteoporosis-induced bone loss and arthritis-induced bone damage in mice and stimulates new bone formation in osteoblasts derived from patient samples. Thus, PEPITEM offers an alternative therapeutic option in the management of diseases with excessive bone loss, promoting an endogenous anabolic pathway to induce bone remodeling and redress the imbalance in bone turnover.

Construction of a Lentiviral Vector for Fgfr2 Overexpression and its Impact on the Biological Behavior of Cranial Suture Mesenchymal Stem Cells

OBJECTIVE

Suture mesenchymal stem cells (SuSCs), possessing self-renewal and multilineage differentiation abilities, play a crucial role in cranial bone growth. However, the impact of the disease-causing fibroblast growth factor receptor 2 (FGFR2) mutation on SuSCs in Crouzon syndrome has not been explored. This study aims to employ a lentivirus to overexpress Fgfr2 and investigate its role in the pathogenesis of Crouzon syndrome.

METHODS

Starting with the prevalent FGFR2 mutation site in patients with Crouzon syndrome, a lentiviral vector carrying the Fgfr2.C361Y mutation was developed and transfected into SuSCs, with a determined multiplicity of infection values. The experimental group, SuSCs+Fgfr2.C361Y, was compared with the empty vector and normal SuSC groups. Cell proliferation, cycle, apoptosis, and osteogenic functionality were assessed using CCK-8 assays, flow cytometry, ALP activity assays, and real-time quantitative polymerase chain reaction.

RESULTS

The lentiviral vector effectively infected SuSCs, leading to heightened Fgfr2 expression, with optimal multiplicity of infection values of 80. The experimental group demonstrated decreased proliferation activity and a higher apoptosis rate compared with controls (P < 0.05). After osteogenic induction, the experimental group showed significantly higher ALP activity than controls (P < 0.05). Real-time quantitative polymerase chain reaction indicated lower mRNA expression levels of Gli1, Axin2, Pcna, Cdk2, and Bcl-2 in the experimental group than controls, whereas Bax, Runx2, and Bmp-2 showed higher expression (P < 0.05).

CONCLUSION

This study constructed a lentivirus vector to upregulate Fgfr2 expression in SuSCs, suppressing stem cell stemness by inhibiting proliferation, promoting apoptosis, and accelerating premature osteogenic differentiation, resulting in premature suture closure. These findings establish the groundwork for further understanding the pathogenesis of Crouzon syndrome.

Advances in the role and mechanism of fibroblasts in fracture healing

With the development of social population ageing, bone fracture has become a global public health problem due to its high morbidity, disability and mortality. Fracture healing is a complex phenomenon involving the coordinated participation of immigration, differentiation and proliferation of inflammatory cells, angioblasts, fibroblasts, chondroblasts and osteoblasts which synthesize and release bioactive substances of extracellular matrix components, Mortality caused by age-related bone fractures or osteoporosis is steadily increasing worldwide as the population ages. Fibroblasts play an important role in the process of fracture healing. However, it is not clear how the growth factors and extracellular matrix stiffness of the bone-regeneration microenvironment affects the function of osteoblasts and fibroblasts in healing process. Therefore, this article focuses on the role of fibroblasts in the process of fracture healing and mechanisms of research progress.

Cellular senescence in skeletal disease: mechanisms and treatment

The musculoskeletal system supports the movement of the entire body and provides blood production while acting as an endocrine organ. With aging, the balance of bone homeostasis is disrupted, leading to bone loss and degenerative diseases, such as osteoporosis, osteoarthritis, and intervertebral disc degeneration. Skeletal diseases have a profound impact on the motor and cognitive abilities of the elderly, thus creating a major challenge for both global health and the economy. Cellular senescence is caused by various genotoxic stressors and results in permanent cell cycle arrest, which is considered to be the underlying mechanism of aging. During aging, senescent cells (SnCs) tend to aggregate in the bone and trigger chronic inflammation by releasing senescence-associated secretory phenotypic factors. Multiple signalling pathways are involved in regulating cellular senescence in bone and bone marrow microenvironments. Targeted SnCs alleviate age-related degenerative diseases. However, the association between senescence and age-related diseases remains unclear. This review summarises the fundamental role of senescence in age-related skeletal diseases, highlights the signalling pathways that mediate senescence, and discusses potential therapeutic strategies for targeting SnCs.

LncRNA00638 promotes the osteogenic differentiation of periodontal mesenchymal stem cells from periodontitis patients under static mechanical strain

BACKGROUND

The osteogenic differentiation capacity of periodontal mesenchymal stem cells (PDLSCs) can be influenced by different levels of static mechanical strain (SMS) in an inflammatory microenvironment. Long non-coding RNAs (lncRNAs) are involved in various physiological processes. However, the mechanisms by which lncRNAs regulate the osteogenic differentiation of PDLSCs remain unclear.

METHODS

We investigated the responses of PDLSCs obtained from periodontitis patients and healthy people to 8% and 12%SMS. Gene microarray and bioinformatics analyses were implemented and identified lncRNA00638 as a target gene for the osteogenesis of PDLSCs from periodontitis patients under SMS. Competing endogenous RNA (ceRNA) network analysis was applied and predicted interactions among lncRNA00638, miRNA-424-5p, and fibroblast growth factor receptor 1 (FGFR1). Gene expression levels were regulated by lentiviral vectors. Cell Counting Kit-8 assays, alkaline phosphatase assays, and Alizarin Red S staining were used to examine the osteogenic potential. RT-qPCR and Western blot were performed to detect the expression levels of related genes and proteins.

RESULTS

We found that 8% and 12% SMS exerted distinct effects on HPDLSCs and PPDLSCs, with 12% SMS having the most significant effect. By microarray analysis, we detected differentially expressed lncRNAs/mRNAs between 12% SMS strained and static PPDLSCs, among which lncRNA00638 was detected as a positive target gene to promote the osteogenic differentiation of PPDLSCs under SMS loading. Mechanistically, lncRNA00638 may act as a ceRNA for miR-424-5p to compete with FGFR1. In this process, lncRNA00638 and miR-424-5p suppress each other and form a network to regulate FGFR1.

CONCLUSIONS

Our findings demonstrate that the lncRNA00638/miRNA-424-5p/FGFR1 regulatory network is actively involved in the regulation of PDLSC osteogenic differentiation from periodontitis patients under SMS loading, which may provide evidence for optimizing orthodontic treatments in patients with periodontitis.

Developmental impairments of craniofacial bone and cartilage in transgenic mice expressing FGF10

Mutations in a common extracellular domain of fibroblast growth factor receptor (FGFR)-2 isoforms (type IIIb and IIIc) cause craniosynostosis syndrome and chondrodysplasia syndrome. FGF10, a major ligand for FGFR2-IIIb and FGFR1-IIIb, is a key participant in the epithelial-mesenchymal interactions required for morphogenetic events. FGF10 also regulates preadipocyte differentiation and early chondrogenesis in vitro, suggesting that FGF10-FGFR signaling may be involved in craniofacial skeletogenesis in vivo. To test this hypothesis, we used a tet-on doxycycline-inducible transgenic mouse model (FGF10 Tg) to overexpress Fgf10 from embryonic day 12.5. Fgf10 expression was 73.3-fold higher in FGF10 Tg than in wild-type mice. FGF10 Tg mice exhibited craniofacial anomalies, such as a short rostrum and mandible, an underdeveloped (cleft) palate, and no tympanic ring. Opposite effects on chondrogenesis in different anatomical regions were seen, e.g., hyperplasia in the nasal septum and hypoplasia in the mandibular condyle. We found an alternative splicing variant of Fgfr2-IIIb with a predicted translation product lacking the transmembrane domain, and suggesting a soluble form of FGFR2-IIIb (sFGFR2-IIIb), differentially expressed in some of the craniofacial bones and cartilages. Thus, excessive FGF10 may perturb signal transduction of the FGF-FGFR, leading to craniofacial skeletal abnormalities in FGF10 Tg mice.

Fgfr1 deficiency in osteocytes leads to increased bone mass by enhancing Wnt/β-catenin signaling

Osteoporosis (OP) is the most common skeletal disease in middle-aged and elderly people. A comprehensive understanding of the pathogenesis of osteoporosis is important. Fibroblast growth factor receptor 1 (FGFR1) is an important molecule for skeletal development and bone remodeling. Osteocytes are the most numerous cells in bone and play critical roles in bone homeostasis, however the effect of FGFR1 on osteocytes is still unclear. To clarify the direct effects of FGFR1 on osteocytes, we conditionally deleted Fgfr1 in osteocytes with Dentin matrix protein 1 (Dmp1)-Cre. We found that mice lacking Fgfr1 in osteocytes (Fgfr1^f/f;Dmp-cre, MUT) showed increased trabecular bone mass at 2 and 6 months of age, which resulted from enhanced bone formation and decreased bone resorption. Furthermore, the cortical bone was thicker in WT mice than that in MUT mice at 2 and 6 months of age. Histological analysis showed that MUT mice had a decreased number of osteocytes but an increased number of osteocyte dendrites. We further found that mice lacking Fgfr1 in osteocytes showed enhanced activation of β-catenin signaling. The expression of sclerostin, an inhibitor of Wnt/β-catenin signaling, was obviously decreased in MUT mice. Furthermore, we found that FGFR1 can inhibit the expression of β-catenin and decrease the activity of β-catenin signaling. In brief, our study showed that FGFR1 in osteocytes can regulate bone mass by regulating Wnt/β-catenin signaling, providing genetic evidence that FGFR1 plays essential roles in osteocytes during bone remodeling and suggesting that FGFR1 is a potential therapeutic target for the prevention of bone loss.

Targeting fibroblast growth factor receptors causes severe craniofacial malformations in zebrafish larvae

Background and Objective

A key pathway controlling skeletal development is fibroblast growth factor (FGF) and FGF receptor (FGFR) signaling. Major regulatory functions of FGF signaling are chondrogenesis, endochondral and intramembranous bone development. In this study we focus on fgfr2, as mutations in this gene are found in patients with craniofacial malformations. The high degree of conservation between FGF signaling of human and zebrafish (Danio rerio) tempted us to investigate effects of the mutated fgfr2 ^sa10729 allele in zebrafish on cartilage and bone formation.

Methods

We stained cartilage and bone in 5 days post fertilization (dpf) zebrafish larvae and compared mutants with wildtypes. We also determined the expression of genes related to these processes. We further investigated whether pharmacological blocking of all FGFRs with the inhibitor BGJ398, during 0-12 and 24-36 h post fertilization (hpf), affected craniofacial structure development at 5 dpf.

Results

We found only subtle differences in craniofacial morphology between wildtypes and mutants, likely because of receptor redundancy. After exposure to BGJ398, we found dose-dependent cartilage and bone malformations, with more severe defects in fish exposed during 0-12 hpf. These results suggest impairment of cranial neural crest cell survival and/or differentiation by FGFR inhibition. Compensatory reactions by upregulation of fgfr1a, fgfr1b, fgfr4, sp7 and dlx2a were found in the 0-12 hpf group, while in the 24-36 hpf group only upregulation of fgf3 was found together with downregulation of fgfr1a and fgfr2.

Conclusions

Pharmacological targeting of FGFR1-4 kinase signaling causes severe craniofacial malformations, whereas abrogation of FGFR2 kinase signaling alone does not induce craniofacial skeletal abnormalities. These findings enhance our understanding of the role of FGFRs in the etiology of craniofacial malformations.

Tissue-Wide Expression of Genes Related to Vitamin D Metabolism and FGF23 Signaling following Variable Phosphorus Intake in Pigs

Calcium (Ca) and phosphorus (P) homeostasis is maintained by several regulators, including vitamin D and fibroblast growth factor 23 (FGF23), and their tissue-specific activation and signaling cascades. In this study, the tissue-wide expression of key genes linked to vitamin D metabolism (CYP2R1, CYP27A1, CYP27B1, CYP24A1, GC, VDR) and FGF23 signaling (FGF23, FGFR1-4, KL) were investigated in pigs fed conventional (trial 1) and divergent P diets (trial 2). The tissue set comprised kidney, liver, bone, lung, aorta, and gastrointestinal tract sections. Expression patterns revealed that non-renal tissues and cells (NRTC) express genes to form active vitamin D [1,25(OH)₂D₃] according to site-specific requirements. A low P diet resulted in higher serum calcitriol and increased CYP24A1 expression in the small intestine, indicating local suppression of vitamin D signaling. A high P diet prompted increased mRNA abundances of CYP27B1 for local vitamin D synthesis, specifically in bone. For FGF23 signaling, analyses revealed ubiquitous expression of FGFR1-4, whereas KL was expressed in a tissue-specific manner. Dietary P supply did not affect skeletal FGF23; however, FGFR4 and KL showed increased expression in bone at high P supply, suggesting regulation to balance mineralization. Specific NRTC responses influence vitamin D metabolism and P homeostasis, which should be considered for a thrifty but healthy P supply.

FGFR2 accommodates osteogenic cell fate determination in human mesenchymal stem cells

The multilineage differentiation potential of human mesenchymal stem cells (hMSCs) underpins their clinical utility for tissue regeneration. Control of such cell-fate decisions is tightly regulated by different growth factors/cytokines and their cognate receptors. Fibroblast growth factors (FGFs) are among such factors critical for osteogenesis. However, how FGF receptors (FGFRs) help to orchestrate osteogenic progression remains to be fully elucidated. Here, we studied the protein levels of FGFRs during osteogenesis in human adult bone marrow-derived MSCs and discovered a positive correlation between FGFR2 expression and alkaline phosphatase (ALP) activity, an early marker of osteogenesis. Through RNA interference studies, we confirmed the role of FGFR2 in promoting the osteogenic differentiation of hMSCs. Knockdown of FGFR2 resulted in downregulation of pro-osteogenic genes and upregulation of pro-adipogenic genes and adipogenic commitment. Moreover, under osteogenic induction, FGFR2 knockdown resulted in upregulation of Enhancer of Zeste Homolog 2 (EZH2), an epigenetic enzyme that regulates MSC lineage commitment and suppresses osteogenesis. Lastly, we show that serial-passaged hMSCs have reduced FGFR2 expression and impaired osteogenic potential. Our study suggests that FGFR2 is critical for mediating osteogenic fate by regulating the balance of osteo-adipogenic lineage commitment. Therefore, examining FGFR2 levels during serial-passaging of hMSCs may prove useful for monitoring their multipotency.

Abnormal eruption of teeth in relation to FGFR1 heterozygote mutation: a rare case of osteoglophonic dysplasia with 4-year follow-up

Background

We report a case and its 4-year follow-up of Osteoglophonic dysplasia (OD), a rare disease that disturbs both skeletal and dental development, which is usually caused by heterozygous FGFR1 mutations.

Case presentation

This article presents a case where a 6-year-old male patient suffered dysregulation of tooth eruption and was diagnosed with osteogenic dysplasia from a fibroblast growth factor receptor 1 (FGFR1) heterozygote mutation. However, the number of teeth is within the normal range, and their roots are well developed. Several interventions were implemented with varying degrees of results. The details of the 4-year follow-up showed that the signs of OD were more pronounced, including dwarfism, frontal bossing, delayed skeletal maturation, anteverted nares, micrognathia, and prominent ears, but the patient’s impacted teeth and edentulous jaws remained unchanged.

Conclusions

FGFR1 heterozygote mutation and OD present significant difficulty for teeth eruption and subsequent intervention. Further measures ought to be taken in recognizing various symptoms presented by the patient. This case supports the significance of careful inquiry, comprehensive physical examination and correct diagnosis as indispensable steps for clinical practice in patients with unerupted teeth. Additionally, the detailed case and its 4-year follow-up length may provide new insights into osteogenic dysplasia and patients with impacted teeth while encouraging further exploration in treatment methods.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-022-02069-6.

Excessive osteoclast activation by osteoblast paracrine factor RANKL is a major cause of the abnormal long bone phenotype in Apert syndrome model mice

The fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling pathway plays important roles in the development and growth of the skeleton. Apert syndrome caused by gain‐of‐function mutations of FGFR2 results in aberrant phenotypes of the skull, midface, and limbs. Although short limbs are representative features in patients with Apert syndrome, the causative mechanism for this limb defect has not been elucidated. Here we quantitatively confirmed decreases in the bone length, bone mineral density, and bone thickness in the Apert syndrome model of gene knock‐in Fgfr2^S252W/+ (EIIA‐Fgfr2^S252W/+) mice. Interestingly, despite these bone defects, histological analysis showed that the endochondral ossification process in the mutant mice was similar to that in wild‐type mice. Tartrate‐resistant acid phosphatase staining revealed that trabecular bone loss in mutant mice was associated with excessive osteoclast activity despite accelerated osteogenic differentiation. We investigated the osteoblast–osteoclast interaction and found that the increase in osteoclast activity was due to an increase in the Rankl level of osteoblasts in mutant mice and not enhanced osteoclastogenesis driven by the activation of FGFR2 signaling in bone marrow‐derived macrophages. Consistently, Col1a1‐Fgfr2^S252W/+ mice, which had osteoblast‐specific expression of Fgfr2 S252W, showed significant bone loss with a reduction of the bone length and excessive activity of osteoclasts was observed in the mutant mice. Taken together, the present study demonstrates that the imbalance in osteoblast and osteoclast coupling by abnormally increased Rankl expression in Fgfr2^S252W/+ mutant osteoblasts is a major causative mechanism for bone loss and short long bones in Fgfr2^S252W/+ mice.

Modern approaches on stem cells and scaffolding technology for osteogenic differentiation and regeneration

The process of bone repair has always been a natural mystery. Although bones do repair themselves, supplemental treatment is required for the initiation of the self-regeneration process. Predominantly, surgical procedures are employed for bone regeneration. Recently, cell-based therapy for bone regeneration has proven to be more effective than traditional methods, as it eliminates the immune risk and painful surgeries. In clinical trials, various stem cells, especially mesenchymal stem cells, have shown to be more efficient for the treatment of several bone-related diseases, such as non-union fracture, osteogenesis imperfecta, osteosarcoma, and osteoporosis. Furthermore, the stem cells grown in a suitable three-dimensional scaffold support were found to be more efficient for osteogenesis. It has been shown that the three-dimensional bioscaffolds support and simulate an in vivo environment, which helps in differentiation of stem cells into bone cells. Bone regeneration in patients with bone disorders can be improved through modification of stem cells with several osteogenic factors or using stem cells as carriers for osteogenic factors. In this review, we focused on the various types of stem cells and scaffolds that are being used for bone regeneration. In addition, the molecular mechanisms of various transcription factors, signaling pathways that support bone regeneration and the senescence of the stem cells, which limits bone regeneration, have been discussed.

Expression and related mechanisms of miR-330-3p and S100B in an animal model of cartilage injury

Objective

To investigate the roles of and relationship between microRNA (miR)-330-3p and S100 calcium-binding protein B (S100B) in an animal model of cartilage injury.

Methods

This study included 30 New Zealand male rabbits randomly divided into three groups: an intervention group, a model group and a sham surgery control group. Modelling was performed in the intervention and model groups, but in the sham surgery group, only the skin was cut. After modelling, the intervention and model groups were injected with the miR-330-3p overexpression vector GV268-miR-330-3p or the control GV268-N-ODN vector, respectively, twice a week for 7 weeks.

Results

Levels of interleukin-1β and tumour necrosis factor-α in the synovial fluid were significantly higher in the model group than in the intervention and control groups. The level of miR-330-3p in the cartilage tissue was significantly higher in the control group than in the model group but it was significantly lower compared with the intervention group. Levels of S100B, fibroblast growth factor receptor 1 and fibroblast growth factor-2 in the cartilage tissue of rabbits in the model group were significantly higher compared with the control and intervention groups.

Conclusion

These findings demonstrate that the upregulation of miR-330-3p can inhibit the expression of S100B.

The hypertrophic chondrocyte: To be or not to be

Hypertrophic chondrocytes are the master regulators of endochondral ossification; however, their ultimate cell fates cells remain largely elusive due to their transient nature. Historically, hypertrophic chondrocytes have been considered as the terminal state of growth plate chondrocytes, which are destined to meet their inevitable demise at the primary spongiosa. Chondrocyte hypertrophy is accompanied by increased organelle synthesis and rapid intracellular water uptake, which serve as the major drivers of longitudinal bone growth. This process is delicately regulated by major signaling pathways and their target genes, including growth hormone (GH), insulin growth factor-1 (IGF-1), indian hedgehog (Ihh), parathyroid hormone-related protein (PTHrP), bone morphogenetic proteins (BMPs), sex determining region Y-box 9 (Sox9), runt-related transcription factors (Runx) and fibroblast growth factor receptors (FGFRs). Hypertrophic chondrocytes orchestrate endochondral ossification by regulating osteogenic-angiogenic and osteogenic-osteoclastic coupling through the production of vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and matrix metallopeptidases-9/13 (MMP-9/13). Hypertrophic chondrocytes also indirectly regulate resorption of the cartilaginous extracellular matrix, by controlling formation of a special subtype of osteoclasts termed "chondroclasts". Notably, hypertrophic chondrocytes may possess innate potential for plasticity, reentering the cell cycle and differentiating into osteoblasts and other types of mesenchymal cells in the marrow space. We may be able to harness this unique plasticity for therapeutic purposes, for a variety of skeletal abnormalities and injuries. In this review, we discuss the morphological and molecular properties of hypertrophic chondrocytes, which carry out important functions during skeletal growth and regeneration.

Circular RNA Circ_0005564 promotes osteogenic differentiation of bone marrow mesenchymal cells in osteoporosis

Circular RNA (CircRNA) plays a potential role in bone formation. We aimed to study the circRNAs expression profiles and their functions in osteogenic differentiation of human bone marrow stromal cells (BMSCs). Firstly, we established osteogenic differentiation of BMSCs displaying increased mRNA expression of osteogenic differentiation marker (RUNX2, OPN, and OCN), increased ALP activity and protein expression, and increased mineralized nodules formation, as well as morphological alteration. Then, we employed high-throughput sequencing to analyze circRNA expression and found that 3440 and 3893 circRNAs in non-induced and induced groups, respectively. We further validated the 10 differentially expressed circRNAs with the most significant difference between induced and non-induced groups. Among these ten circRNAs, five of them with more than one miRNA binding site were used to construct a ceRNA network exhibiting 81 miRNAs and 182 target mRNAs. Furthermore, among these five circRNAs, we found only circ_0005564 significantly reduced the mRNA expression of RUNX2, OPN, and OCN. The circularity of circ_0005564 was verified. Our results showed that knockdown of circ_0005564 inhibited osteoblast differentiation in BMSCs. Taken together, our study demonstrates that circ_0005564 is a potential positive regulator of osteogenic differentiation of BMSCs.

Effects of nanocrystalline hydroxyapatite concentration and skeletal site on bone and cartilage formation in rats

Most fractures heal by a combination of endochondral and intramembranous ossification dependent upon strain and vascularity at the fracture site. Many biomaterials-based bone regeneration strategies rely on the use of calcium phosphates such as nano-crystalline hydroxyapatite (nHA) to create bone-like scaffolds. In this study, nHA was dispersed in reactive polymers to form composite scaffolds that were evaluated both in vitro and in vivo. Matrix assays, immunofluorescent staining, and Western blots demonstrated that nHA influenced mineralization and subsequent osteogenesis in a dose-dependent manner in vitro. Furthermore, nHA dispersed in polymeric composites promoted osteogenesis by a similar mechanism as particulated nHA. Scaffolds were implanted into a 2-mm defect in the femoral diaphysis or metaphysis of Sprague-Dawley rats to evaluate new bone formation at 4 and 8 weeks. Two formulations were tested: a poly(thioketal urethane) scaffold without nHA (PTKUR) and a PTKUR scaffold augmented with 22 wt% nHA (22nHA). The scaffolds supported new bone formation in both anatomic sites. In the metaphysis, augmentation of scaffolds with nHA promoted an intramembranous healing response. Within the diaphysis, nHA inhibited endochondral ossification. Immunohistochemistry was performed on cryo-sections of the bone/scaffold interface in which CD146, CD31, Endomucin, CD68, and Myeloperoxidase were evaluated. No significant differences in the infiltrating cell populations were observed. These findings suggest that nHA dispersed in polymeric composites induces osteogenic differentiation of adherent endogenous cells, which has skeletal site-specific effects on fracture healing. STATEMENT OF SIGNIFICANCE: Understanding the mechanism by which synthetic scaffolds promote new bone formation in preclinical models is crucial for bone regeneration applications in the clinic where complex fracture cases are seen. In this study, we found that dispersion of nHA in polymeric scaffolds promoted in vitro osteogenesis in a dose-dependent manner through activation of the PiT1 receptor and subsequent downstream Erk1/2 signaling. While augmentation of polymeric scaffolds with nHA enhanced intramembranous ossification in metaphyseal defects, it inhibited endochondral ossification in diaphyseal defects. Thus, our findings provide new insights into designing synthetic bone grafts that complement the skeletal site-specific fracture healing response.

Novel FGFR1 Variants Are Associated with Congenital Scoliosis

FGFR1 encodes a transmembrane cytokine receptor, which is involved in the early development of the human embryo and plays an important role in gastrulation, organ specification and patterning of various tissues. Pathogenic FGFR1 variants have been associated with Kallmann syndrome and hypogonadotropic hypogonadism. In our congenital scoliosis (CS) patient series of 424 sporadic CS patients under the framework of the Deciphering disorders Involving Scoliosis and COmorbidities (DISCO) study, we identified four unrelated patients harboring FGFR1 variants, including one frameshift and three missense variants. These variants were predicted to be deleterious by in silico prediction and conservation analysis. Signaling activities and expression levels of the mutated protein were evaluated in vitro and compared to that of the wild type (WT) FGFR1. As a result, the overall protein expressions of c.2334dupC, c.2339T>C and c.1261A>G were reduced to 43.9%, 63.4% and 77.4%, respectively. By the reporter gene assay, we observed significantly reduced activity for c.2334dupC, c.2339T>C and c.1261A>G, indicating the diminished FGFR1 signaling pathway. In conclusion, FGFR1 variants identified in our patients led to only mild disruption to protein function, caused milder skeletal and cardiac phenotypes than those reported previously.

Deletion of Fibroblast growth factor 9 globally and in skeletal muscle results in enlarged tuberosities at sites of deltoid tendon attachments

BACKGROUND

The growth of most bony tuberosities, like the deltoid tuberosity (DT), rely on the transmission of muscle forces at the tendon-bone attachment during skeletal growth. Tuberosities distribute muscle forces and provide mechanical leverage at attachment sites for joint stability and mobility. The genetic factors that regulate tuberosity growth remain largely unknown. In mouse embryos with global deletion of fibroblast growth factor 9 (Fgf9), the DT size is notably enlarged. In this study, we explored the tissue-specific regulation of DT size using both global and targeted deletion of Fgf9.

RESULTS

We showed that cell hypertrophy and mineralization dynamics of the DT, as well as transcriptional signatures from skeletal muscle but not bone, were influenced by the global loss of Fgf9. Loss of Fgf9 during embryonic growth led to increased chondrocyte hypertrophy and reduced cell proliferation at the DT attachment site. This endured hypertrophy and limited proliferation may explain the abnormal mineralization patterns and locally dysregulated expression of markers of endochondral development in Fgf9^null attachments. We then showed that targeted deletion of Fgf9 in skeletal muscle leads to postnatal enlargement of the DT.

CONCLUSION

Taken together, we discovered that Fgf9 may play an influential role in muscle-bone cross-talk during embryonic and postnatal development.

Signaling Pathways in Bone Development and Their Related Skeletal Dysplasia

Bone development is a tightly regulated process. Several integrated signaling pathways including HH, PTHrP, WNT, NOTCH, TGF-β, BMP, FGF and the transcription factors SOX9, RUNX2 and OSX are essential for proper skeletal development. Misregulation of these signaling pathways can cause a large spectrum of congenital conditions categorized as skeletal dysplasia. Since the signaling pathways involved in skeletal dysplasia interact at multiple levels and have a different role depending on the time of action (early or late in chondrogenesis and osteoblastogenesis), it is still difficult to precisely explain the physiopathological mechanisms of skeletal disorders. However, in recent years, significant progress has been made in elucidating the mechanisms of these signaling pathways and genotype–phenotype correlations have helped to elucidate their role in skeletogenesis. Here, we review the principal signaling pathways involved in bone development and their associated skeletal dysplasia.

The Potential of FGF-2 in Craniofacial Bone Tissue Engineering: A Review

Bone is a hard-vascularized tissue, which renews itself continuously to adapt to the mechanical and metabolic demands of the body. The craniofacial area is prone to trauma and pathologies that often result in large bone damage, these leading to both aesthetic and functional complications for patients. The "gold standard" for treating these large defects is autologous bone grafting, which has some drawbacks including the requirement for a second surgical site with quantity of bone limitations, pain and other surgical complications. Indeed, tissue engineering combining a biomaterial with the appropriate cells and molecules of interest would allow a new therapeutic approach to treat large bone defects while avoiding complications associated with a second surgical site. This review first outlines the current knowledge of bone remodeling and the different signaling pathways involved seeking to improve our understanding of the roles of each to be able to stimulate or inhibit them. Secondly, it highlights the interesting characteristics of one growth factor in particular, FGF-2, and its role in bone homeostasis, before then analyzing its potential usefulness in craniofacial bone tissue engineering because of its proliferative, pro-angiogenic and pro-osteogenic effects depending on its spatial-temporal use, dose and mode of administration.

Fgf9 Negatively Regulates Bone Mass by Inhibiting Osteogenesis and Promoting Osteoclastogenesis Via MAPK and PI3K/AKT Signaling

Fibroblast growth factor 9 (Fgf9) is a well-known factor that regulates bone development; however, its function in bone homeostasis is still unknown. Previously, we identified a point mutation in the FGF9 gene (p.Ser99Asn, S99N) and generated an isogeneic knock-in mouse model, which revealed that this loss-of-function mutation impaired early joint formation and was responsible for human multiple synostosis syndrome 3 (SYNS3). Moreover, newborn and adult S99N mutant mice exhibited significantly increased bone mass, suggesting that Fgf9 also participated in bone homeostasis. Histomorphology, tomography, and serological analysis of homozygous newborns and heterozygous adults showed that the Fgf9^S99N mutation immensely increased bone mass and bone formation in perinatal and adult bones and decreased osteoclastogenesis in adult bone. An in vitro differentiation assay further revealed that the S99N mutation enhanced bone formation by promoting osteogenesis and mineralization of bone marrow mesenchymal stem cells (BMSCs) and attenuating osteoclastogenesis of bone marrow monocytes (BMMs). Considering the loss-of-function effect of the S99N mutation, we hypothesized that Fgf9 itself inhibits osteogenesis and promotes osteoclastogenesis. An in vitro differentiation assay revealed that Fgf9 prominently inhibited BMSC osteogenic differentiation and mineralization and showed for the first time that Fgf9 promoted osteoclastogenesis by enhancing preosteoclast aggregation and cell-cell fusion. Furthermore, specific inhibitors and in vitro differentiation assays were used and showed that Fgf9 inhibited BMSC osteogenesis mainly via the MEK/ERK pathway and partially via the PI3K/AKT pathway. Fgf9 also promoted osteoclastogenesis as a potential costimulatory factor with macrophage colony-stimating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by coactivating the MAPK and PI3K/AKT signaling pathways. Taken together, our study demonstrated that Fgf9 is a negative regulator of bone homeostasis by regulating osteogenesis and osteoclastogenesis and provides a potential therapeutic target for bone degenerative diseases. © 2020 American Society for Bone and Mineral Research (ASBMR).

Cadmium induced skeletal underdevelopment, liver cell apoptosis and hepatic energy metabolism disorder in Bufo gargarizans larvae by disrupting thyroid hormone signaling

Cadmium (Cd) is hazardous to human health and it is also highly detrimental to amphibian life. In this study, Bufo gargarizans larvae were exposed to environmentally relevant Cd concentrations of 5, 100 and 200 μg L^-1 from Gosner stage (Gs) 26 to Gs 42 of metamorphic climax about 6 weeks. The results showed thyroid structural injuries and thyroid signaling disruption were induced by high Cd exposure (100 and 200 μg L^-1). Moreover, tadpole skeleton including whole body, vertebrata, forelimb and hindlimb was developmentally delayed by high Cd exposure through downregulating the mRNA expressions of genes involved with skeletal ossification and growth pathway. Moreover, liver histopathological injuries were caused by high Cd exposure featured by hepatocytes malformation, nuclear degeneration and increasing melanomacrophage centers. Meanwhile, liver apoptosis rate showed on the rise in a dose-dependent way and Cd stimulated liver apoptosis by upregulating mRNA expressions of genes related to extrinsic and intrinsic apoptosis pathways. Furthermore, high Cd caused hepatic glucometabolism disorder by decreasing the genetic expressions associated with glycolysis and mitochondrial oxidative phosphorylation. In addition, liver lipid metabolism was disrupted by high Cd exposure through downregulating mRNA levels of genes related to fatty oxidation and upregulating mRNA levels of genes related to fatty acid synthesis. We suggested that Cd did great harm to tadpole health by disturbing thyroid function, skeletal growth, liver cell apoptosis signaling and hepatic energy metabolism pathway.

FGF2: a key regulator augmenting tendon-to-bone healing and cartilage repair

Ligament/tendon and cartilage injuries are clinically common diseases that perplex most clinicians. Because of the lack of blood vessels and nerves, their self-repairing abilities are rather poor. Therefore, surgeries are necessary and also widely used to treat ligament/tendon or cartilage injuries. However, after surgery, there are still many problems that affect healing. In recent years, it has been found that exogenous FGF2 plays an important role in the repair of ligament/tendon and cartilage injuries and exerts a synergistic effect with endogenous FGF2. Therefore, FGF2 can be used as a new type of biomolecule to accelerate tendon-to-bone healing and cartilage repair after injury.

Mechanisms of bone development and repair

Bone development occurs through a series of synchronous events that result in the formation of the body scaffold. The repair potential of bone and its surrounding microenvironment - including inflammatory, endothelial and Schwann cells - persists throughout adulthood, enabling restoration of tissue to its homeostatic functional state. The isolation of a single skeletal stem cell population through cell surface markers and the development of single-cell technologies are enabling precise elucidation of cellular activity and fate during bone repair by providing key insights into the mechanisms that maintain and regenerate bone during homeostasis and repair. Increased understanding of bone development, as well as normal and aberrant bone repair, has important therapeutic implications for the treatment of bone disease and ageing-related degeneration.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

Fibroblast growth factor signalling in osteoarthritis and cartilage repair

Regulated fibroblast growth factor (FGF) signalling is a prerequisite for the correct development and homeostasis of articular cartilage, as evidenced by the fact that aberrant FGF signalling contributes to the maldevelopment of joints and to the onset and progression of osteoarthritis. Of the four FGF receptors (FGFRs 1-4), FGFR1 and FGFR3 are strongly implicated in osteoarthritis, and FGFR1 antagonists, as well as agonists of FGFR3, have shown therapeutic efficacy in mouse models of spontaneous and surgically induced osteoarthritis. FGF18, a high affinity ligand for FGFR3, is the only FGF-based drug currently in clinical trials for osteoarthritis. This Review covers the latest advances in our understanding of the molecular mechanisms that regulate FGF signalling during normal joint development and in the pathogenesis of osteoarthritis. Strategies for FGF signalling-based treatment of osteoarthritis and for cartilage repair in animal models and clinical trials are also introduced. An improved understanding of FGF signalling from a structural biology perspective, and of its roles in skeletal development and diseases, could unlock new avenues for discovery of modulators of FGF signalling that can slow or stop the progression of osteoarthritis.

FGFR2 Is Required for AEC2 Homeostasis and Survival after Bleomycin-induced Lung Injury

Alveolar epithelial cell (AEC) injury is central to the pathogenesis of pulmonary fibrosis. Epithelial FGF (fibroblast growth factor) signaling is essential for recovery from hyperoxia- and influenza-induced lung injury, and treatment with FGFs is protective in experimental lung injury. The cell types involved in the protective effect of FGFs are not known. We hypothesized that FGF signaling in type II AECs (AEC2s) is critical in bleomycin-induced lung injury and fibrosis. To test this hypothesis, we generated mice with tamoxifen-inducible deletion of FGFR1-3 (fibroblast growth factor receptors 1, 2, and 3) in surfactant protein C-positive (SPC⁺) AEC2s (SPC triple conditional knockout [SPC-TCKO]). In the absence of injury, SPC-TCKO mice had fewer AEC2s, decreased Sftpc (surfactant protein C gene) expression, increased alveolar diameter, and increased collagen deposition. After intratracheal bleomycin administration, SPC-TCKO mice had increased mortality, lung edema, and BAL total protein, and flow cytometry and immunofluorescence revealed a loss of AEC2s. To reduce mortality of SPC-TCKO mice to less than 50%, a 25-fold dose reduction of bleomycin was required. Surviving bleomycin-injured SPC-TCKO mice had increased collagen deposition, fibrosis, and ACTA2 expression and decreased epithelial gene expression. Inducible inactivation of individual Fgfr2 or Fgfr3 revealed that Fgfr2, but not Fgfr3, was responsible for the increased mortality and lung injury after bleomycin administration. In conclusion, AEC2-specific FGFR2 is critical for survival in response to bleomycin-induced lung injury. These data also suggest that a population of SPC⁺ AEC2s require FGFR2 signaling for maintenance in the adult lung. Preventing epithelial FGFR inhibition and/or activating FGFRs in alveolar epithelium may therefore represent a novel approach to treating lung injury and reducing fibrosis.

Fibroblast growth factors signaling in bone metastasis

Many solid tumors metastasize to bone, but only prostate cancer has bone as a single, dominant metastatic site. Recently, the FGF axis has been implicated in cancer progression in some tumors and mounting evidence indicate that it mediates prostate cancer bone metastases. The FGF axis has an important role in bone biology and mediates cell-to-cell communication. Therefore, we discuss here basic concepts of bone biology, FGF signaling axis, and FGF axis function in adult bone, to integrate these concepts in our current understanding of the role of FGF axis in bone metastases.

Glucocorticoids Decrease Longitudinal Bone Growth in Pediatric Kidney Transplant Recipients by Stimulating the FGF23/FGFR3 Signaling Pathway

Renal transplantation (RTx) is an effective therapy to improve clinical outcomes in pediatric patients with terminal chronic kidney disease. However, chronic immunosuppression with glucocorticoids (GCs) reduces bone growth and BMD. The mechanisms causing GC-induced growth impairment have not been fully clarified. Fibroblast growth factor 23 (FGF23) is a peptide hormone that regulates phosphate homeostasis and bone growth. In pathological conditions, FGF23 excess or abnormal FGF receptors (FGFR) activity leads to bone growth impairment. Experimental data indicate that FGF23 expression is induced by chronic GC exposure. Therefore, we hypothesize that GCs impair bone growth by increasing FGF23 expression, which has direct effects on bone growth plate. In a post hoc analysis of a multicentric randomized clinical trial of prepubertal RTx children treated with early GC withdrawal or chronic GC treatment, we observed that GC withdrawal was associated with improvement in longitudinal growth and BMD, and lower plasma FGF23 levels as compared with a chronic GC group. In prepubertal rats, GC-induced bone growth retardation correlated with increased plasma FGF23 and bone FGF23 expression. Additionally, GC treatment decreased FGFR1 expression whereas it increased FGFR3 expression in mouse tibia explants. The GC-induced bone growth impairment in tibiae explants was prevented by blockade of FGF23 receptors using either a pan-FGFR antagonist (PD173074), a C-terminal FGF23 peptide (FGF23180-205) which blocks the binding of FGF23 to the FGFR-Klotho complex or a specific FGFR3 antagonist (P3). Finally, local administration of PD173074 into the tibia growth plate ameliorated cartilage growth impairment in GC-treated rats. These results show that GC treatment partially reduces longitudinal bone growth via upregulation of FGF23 and FGFR3 expression, thus suggesting that the FGF23/Klotho/FGFR3 axis at the growth plate could be a potential therapeutic target for the management of GC-induced growth impairment in children.

Osteocyte Death and Bone Overgrowth in Mice Lacking Fibroblast Growth Factor Receptors 1 and 2 in Mature Osteoblasts and Osteocytes

Fibroblast growth factor (FGF) signaling pathways have well-established roles in skeletal development, with essential functions in both chondrogenesis and osteogenesis. In mice, previous conditional knockout studies suggested distinct roles for FGF receptor 1 (FGFR1) signaling at different stages of osteogenesis and a role for FGFR2 in osteoblast maturation. However, the potential for redundancy among FGFRs and the mechanisms and consequences of stage-specific osteoblast lineage regulation were not addressed. Here, we conditionally inactivate Fgfr1 and Fgfr2 in mature osteoblasts with an Osteocalcin (OC)-Cre or Dentin matrix protein 1 (Dmp1)-CreER driver. We find that young mice lacking both receptors or only FGFR1 are phenotypically normal. However, between 6 and 12 weeks of age, OC-Cre Fgfr1/Fgfr2 double- and Fgfr1 single-conditional knockout mice develop a high bone mass phenotype with increased periosteal apposition, increased and disorganized endocortical bone with increased porosity, and biomechanical properties that reflect increased bone mass but impaired material properties. Histopathological and gene expression analyses show that this phenotype is preceded by a striking loss of osteocytes and accompanied by activation of the Wnt/β-catenin signaling pathway. These data identify a role for FGFR1 signaling in mature osteoblasts/osteocytes that is directly or indirectly required for osteocyte survival and regulation of bone mass during postnatal bone growth. © 2019 American Society for Bone and Mineral Research.

A Tri-fusion Reporter Mouse Reveals Tissue-Specific FGF1B Promoter Activity in vivo

Transgenic mice harboring imaging reporters take full advantage of imaging technologies in studies using living mice. Here, we established a tri-fusion multimodal reporter gene containing fragments from firefly luciferase, enhanced green fluorescent protein, and herpes simplex virus type 1 thymidine kinase and generated tri-fusion reporter Tg mice. Fibroblast growth factor type 1 (FGF1), a multifunctional mitogen to a wide range of tissues, regulates proliferation of neural stem cells of the brain, where FGF1 expression is initiated through activation of the FGF1B (F1B) promoter. The reporter mouse under the control of the human F1B promoter enables visualization in vivo where F1B activity is elevated, including tissues not only in the brain but also in the nasopharynx, skull, spine, and testes, particularly in Leydig cells. Treating Tg mice with the alkylating agent busulfan, which is known to eradicate Leydig cells and disrupt spermatogenesis in mice, eliminated the reporter signals. Restoring Leydig cells recovered reporter expression, indicating that the reporter can be used as a surrogate marker for Leydig cells. The F1B tri-fusion reporter mouse model can be utilized in longitudinal monitoring of the health status of the male reproductive system, such as in studies exploring the toxicity of chemicals to spermatogenesis.

Review of the Pathways Involved in the Osteogenic Differentiation of Adipose-Derived Stem Cells

Grafts and prosthetic materials used for the repair of bone defects are often accompanied by comorbidity and rejection. Therefore, there is an immense need for novel approaches to combating the issues surrounding such defects. Because of their accessibility, substantial proportion, and osteogenic differentiation potential, adipose-derived stem cells (ASCs) make for an ideal source of bone tissue in regenerative medicine. However, efficient induction of ASCs toward an osteoblastic lineage in vivo is met with challenges, and many signaling pathways must come together to secure osteoblastogenesis. Among them are bone morphogenic protein, wingless-related integration site protein, Notch, Hedgehog, fibroblast growth factor, vascular endothelial growth factor, and extracellular regulated-signal kinase. The goal of this literature review is to conglomerate the present research on these pathways to formulate a better understanding of how ASCs are most effectively transformed into bone in the context of tissue engineering.

Nuclear Fibroblast Growth Factor Receptor Signaling in Skeletal Development and Disease

PURPOSE OF REVIEW

Fibroblast growth factor receptor (FGFR) signaling regulates proliferation and differentiation during development and homeostasis. While membrane-bound FGFRs play a central role in these processes, the function of nuclear FGFRs is also critical. Here, we highlight mechanisms for nuclear FGFR translocation and the effects of nuclear FGFRs on skeletal development and disease.

RECENT FINDINGS

Full-length FGFRs, internalized by endocytosis, enter the nucleus through β-importin-dependent mechanisms that recognize the nuclear localization signal within FGFs. Alternatively, soluble FGFR intracellular fragments undergo nuclear translocation following their proteolytic release from the membrane. FGFRs enter the nucleus during the cellular transition between proliferation and differentiation. Once nuclear, FGFRs interact with chromatin remodelers to alter the epigenetic state and transcription of their target genes. Dysregulation of nuclear FGFR is linked to the etiology of congenital skeletal disorders and neoplastic transformation. Revealing the activities of nuclear FGFR will advance our understanding of 20 congenital skeletal disorders caused by FGFR mutations, as well as FGFR-related cancers.

Molecular signaling in bone cells: Regulation of cell differentiation and survival

The achievement of proper bone mass and architecture, and their maintenance throughout life requires the concerted actions of osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells. The differentiation and activity of osteoblasts and osteoclasts are regulated by molecules produced by matrix-embedded osteocytes, as well as by cross talk between osteoblasts and osteoclasts through secreted factors. In addition, it is likely that direct contact between osteoblast and osteoclast precursors, and the contact of these cells with osteocytes and cells in the bone marrow, also modulates bone cell differentiation and function. With the advancement of molecular and genetic tools, our comprehension of the intracellular signals activated in bone cells has evolved significantly, from early suggestions that osteoblasts and osteoclasts have common precursors and that osteocytes are inert cells in the bone matrix, to the very sophisticated understanding of a network of receptors, ligands, intracellular kinases/phosphatases, transcription factors, and cell-specific genes that are known today. These advances have allowed the design and FDA-approval of new therapies to preserve and increase bone mass and strength in a wide variety of pathological conditions, improving bone health from early childhood to the elderly. We have summarized here the current knowledge on selected intracellular signal pathways activated in osteoblasts, osteocytes, and osteoclasts.

Fibroblast growth factors in skeletal development

Fibroblast growth factors (FGFs) and their receptors (FGFRs) are expressed throughout all stages of skeletal development. In the limb bud and in cranial mesenchyme, FGF signaling is important for formation of mesenchymal condensations that give rise to bone. Once skeletal elements are initiated and patterned, FGFs regulate both endochondral and intramembranous ossification programs. In this chapter, we review functions of the FGF signaling pathway during these critical stages of skeletogenesis, and explore skeletal malformations in humans that are caused by mutations in FGF signaling molecules.

Transcriptome analyses reveal molecular mechanisms that regulate endochondral ossification in amphibian Bufo gargarizans during metamorphosis

BACKGROUND

A developmental transition from aquatic to terrestrial existence is one of the most important events in the evolution of terrestrial vertebrates. Amphibian metamorphosis is a classic model to study this transition. The development of the vertebrate skeleton can reflect its evolutionary history. Endochondral ossification serves a vital role in skeletal development. Thus, we sought to unravel molecular mechanisms that regulate endochondral ossification during Bufo gargarizans metamorphosis.

METHODS

The alizarin red-alcian blue double staining method was used to visualize the skeletal development of B. gargarizans during metamorphosis. RNA sequencing (RNA-seq) was used to explore the transcriptome of B. gargarizans in four key developmental stages during metamorphosis. Real-time quantitative PCR (RT-qPCR) was used to validate the expression patterns of endochondral ossification related genes.

RESULTS

Endochondral ossification increased gradually in skeletal system of B. gargarizans during metamorphosis. A total of 137,264 unigenes were assembled and 44,035 unigenes were annotated. 10,352 differentially expressed genes (DEGs) were further extracted among four key developmental stages. In addition, 28 endochondral ossification related genes were found by searching for DEG libraries in B. gargarizans. Of the 28 genes, 10 genes were validated using RT-qPCR.

CONCLUSIONS

The exquisite coordination of the 28 genes is essential for regulation of endochondral ossification during B. gargarizans metamorphosis.

GENERAL SIGNIFICANCE

The present study will not only provide an invaluable genomic resource and background for further research of endochondral ossification in amphibians but will also aid in enhancing our understanding of the evolution of terrestrial vertebrates.

Monoclonal antibody targeting of fibroblast growth factor receptor 1c causes cardiac valvulopathy in rats

Fibroblast Growth Factors (FGFs) and their receptors (FGFRs) have been proposed as potential drug targets for the treatment of obesity. The aim of this study was to assess the potential toxicity in rats of three anti-FGFR1c mAbs with differential binding activity prior to clinical development. Groups of male rats received weekly injections of either one of two FGFR1c-specific mAbs or an FGFR1c/FGFR4-specific mAb at 10 mg/kg for up to 4 weeks. All three mAbs caused significant reductions in food intake and weight loss leading to some animals being euthanized early for welfare reasons. In all three groups given these mAbs, microscopic changes were seen in the bones and heart valves. In the bones of the femoro-tibial joint, thickening of the diaphyseal cortex of long bones, due to deposition of well organized new lamellar bone, indicated that an osteogenic effect was observed. In the heart, valvulopathy described as an endocardial myxomatous change affecting the mitral, pulmonary, tricuspid and aortic valves was observed in all mAb-treated animals. The presence of FGFR1 mRNA expression in the heart valves was confirmed using in situ hybridization. Targeting the FGF-FGFR1c pathway with anti-FGFR1c mAbs leads to drug induced valvulopathy in rats. In effect, this precluded the development of these mAbs as potential anti-obesity drugs. The valvulopathy observed was similar to that described for fenfluramine and dexafenfluramine. The pathogenesis of the drug-induced valvulopathy is considered FGFR1c-mediated, based on the specificity of the mAbs and FGFR1 mRNA expression in the heart valves.

Association between the FGFR1 rs13317 single nucleotide polymorphism and orbitale-nasion depth based on cephalometric images

The fibroblast growth factor receptor 1 (FGFR1) gene plays an important role in craniofacial morphogenesis. In our previous study, an association between FGFR1 single nucleotide polymorphisms (SNPs) and craniofacial morphology was demonstrated in Japanese and Korean subjects. The present study aimed to evaluate the relationship between a common FGFR1 SNP (rs13317) with craniofacial morphology, increasing the number of measurements and examining Egyptian subjects (n = 191) in addition to the Japanese (n = 211) and Korean (n = 226) subjects. Genotyping for rs13317 was performed using the TaqMan assay, and its associations with 81 craniofacial measurements derived from lateral and posteroanterior cephalograms were analyzed by multiple regression analysis controlling sex and facial size. The results from each of the populations were then statistically combined. In the Egyptian subjects, rs13317 was significantly associated with the nasion-orbitale depth (P = 0.00040), and a suggestive association was also observed in the Japanese (P = 0.037) and Korean subjects (P = 0.045). The combined analysis revealed that only the nasion-orbitale depth showed a significant association (P = 0.000062) and that several measurements showed a suggestive association. Our results strongly indicate that rs13317 is associated with a smaller depth between the nasion and orbitale, representing a relative protrusion of the cheekbones and retrusion of the nasal root. A similar characteristic is also observed in individuals with Pfeiffer syndrome, which is caused by a dysfunctional FGFR1 mutation.

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.

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.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Effects of strontium ranelate treatment on osteoblasts cultivated onto scaffolds of trabeculae bovine bone

Blocks of Bovine bone have shown promising results as implantable scaffolds to promote bone regeneration. Strontium ranelate (SrR) is both an antiresorptive and an anabolic drug that has been indicated for oral administration to treat osteoporosis. Few studies, however, have investigated the local effects of SrR and its use in association with biomaterials thus far. In this work, we investigated SrR effects in cultures of primary osteoblasts (PO, from Wistar rats calvaria) and immortalized osteoblasts (IO, from MC3T3-E1 cell line) cultivated as a monolayer or in association with scaffolds of bovine bone in mineralized (MBB) and demineralized (DBB) forms. The optimum dose to induce SrR effects on cell viability was established as 0.1 mM. Our results suggested that the local administration of SrR is biocompatible and non-cytotoxic. In addition, SrR appeared to accelerate primary osteoblast cell differentiation by enhancing alkaline phosphatase activity, the expression of osteogenic differentiation markers, the synthesis of the organic matrix, and a decrease of Ca²⁺ ions in mineralized nodules. DBB was found to be a better scaffold material to promote PO and IO cell proliferation. Exposing the proteins of the demineralized bone matrix might improve scaffold osteoconductive properties. Our results indicated the importance of further investigation of the administration of SrR at sites of bone repair. The association of SrR and bone grafts suggests the possibility of using SrR as a co-adjuvant for bone tissue bioengineering and in bone regeneration therapies.