[Note: This is the 3rd guest post by the coworker who has been contributing in writing posts for the website. The previous they wrote about was on statin HERE and bone morphogenic proteins BMPs HERE,  Thanks Nicki.]

FGFs

Fibroblast growth factors (FGFs) and their receptors (FGFRs) negatively regulate longitudinal bone growth. Activating FGFR3 mutations impair growth, causing human skeletal dysplasias, whereas inactivating mutations stimulate growth. Systemic administration of FGF-2 to mice stimulates bone growth at low doses but inhibits growth at high doses. In organ culture, FGF-2 inhibits growth by decreasing growth plate chondrocyte proliferation, hypertrophy and cartilage matrix synthesis. Local FGF-2 infusion accelerates ossification of growth plate cartilage. Thus, FGFs may regulate both growth plate chondrogenesis and ossification.

Fibroblast growth factor (FGF) signaling is essential for endochondral bone formation. Most previous work in this area has focused on embryonic chondrogenesis. To explore the role of FGF signaling in the postnatal growth plate, we quantitated expression of FGFs and FGF receptors (FGFRs) and examined both their spatial and temporal regulation.

Toward this aim, rat proximal tibial growth plates and surrounding tissues were microdissected, and specific mRNAs were quantitated by real-time RT-PCR. To assess the FGF system without bias, we first screened for expression of all known FGFs and major FGFR isoforms. Perichondrium expressed FGFs 1, 2, 6, 7, 9, and 18 and, at lower levels, FGFs 21 and 22. Growth plate expressed FGFs 2, 7, 18, and 22. Perichondrial expression was generally greater than growth plate expression, supporting the concept that perichondrial FGFs regulate growth plate chondrogenesis. Nevertheless, FGFs synthesized by growth plate chondrocytes may be physiologically important because of their proximity to target receptors. In growth plate, we found expression of FGFRs 1, 2, and 3, primarily, but not exclusively, the c isoforms. FGFRs 1 and 3, thought to negatively regulate chondrogenesis, were expressed at greater levels and at later stages of chondrocyte differentiation, with FGFR1 upregulated in the hypertrophic zone and FGFR3 upregulated in both proliferative and hypertrophic zones. In contrast, FGFRs 2 and 4, putative positive regulators, were expressed at earlier stages of differentiation, with FGFR2 upregulated in the resting zone and FGFR4 in the resting and proliferative zones. FGFRL1, a presumed decoy receptor, was expressed in the resting zone.

With increasing age and decreasing growth velocity, FGFR2 and 4 expression was downregulated in proliferative zone. Perichondrial FGF1, FGF7, FGF18, and FGF22 were upregulated.

In summary, we have analyzed the expression of all known FGFs and FGFRs in the postnatal growth plate using a method that is quantitative and highly sensitive. This approach identified ligands and receptors not previously known to be expressed in growth plate and revealed a complex pattern of spatial regulation of FGFs and FGFRs in the different zones of the growth plate. We also found temporal changes in FGF and FGFR expression which may contribute to growth plate senescence and thus help determine the size of the adult skeleton.

The family of FGFs constitutes at least 22 members that interact with at least four receptors (FGFR) and are major regulators of embryonic bone development.Both FGF1 and -2 as well as FGFR1, -2, and -3 are expressed in chondrocytes.In humans, activating mutations in the FGFR3 cause achondroplasia,the most common type of human dwarfism (97% of mutations have a Gly to Arg mutation in codon 380).Other forms of chondrodysplasia due to mutations in the FGFR3 gene include hypochondroplasia, a milder form of dwarfism and two severe types, SADDAN (severe achondroplasia with developmental delay and acanthosis nigricans), and thanatophoric dysplasia.Conversely, mice with an inactivating mutation in the FGFR3 gene demonstrate increased longitudinal growth. In addition, overexpression of FGF2 slows longitudinal growth.Only very recently, mice lacking FGF18 have been generated. These mice demonstrated a phenotype similar to that observed in mice lacking FGFR3, including expanded proliferating and hypertrophic zones, increased proliferation, differentiation, and Ihh signaling.In addition, FGF18 deficiency leads to delayed ossification and decreased expression of osteogenic markers, not seen in the FGFR3 knockout phenotype, which prompted the authors to suggest that FGF18 coordinates chondrogenesis and osteogenesis through FGFR3 and -2, respectively. In addition, FGF18 appeared to act as a physiological ligand for FGFR3 in the growth plate. These studies indicate that FGFR signaling reduces growth by inhibiting proliferation and differentiation.

Mancilla et al.  studied the effects of FGF2 on chondrocyte differentiation in a metatarsal organ culture system and found three growth-inhibiting mechanisms for FGF2: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and at high concentrations, decreased synthesis of cartilage matrix. Recently, a mouse model for thanatophoric dysplasia characterized by severe dwarfism was used to study the relationship between FGF signaling and the Ihh/PTHrP feedback loop. In these newborn mice with an activated FGFR3, Ihh and PTHrP mRNA expression were both down-regulated. In the same study, embryonic metatarsals from wild-type mice were cultured in the presence of FGF2, and similar results were found. Interestingly, FGF inhibited chondrocyte proliferation by down-regulating Ihh expression. Moreover, FGF and PTHrP signals independently inhibited chondrocyte differentiation. It was concluded that FGFR3 and PTHrP/Ihh signals act through two integrated parallel pathways that mediate both overlapping and distinct functions during longitudinal bone growth. In a recent study by Minina et al. , using a limb culture system, it was found that FGF and BMP signaling are antagonistic in the regulation of chondrocyte proliferation and in Ihh expression and the process of hypertrophic differentiation. The balance between the two adjusts the pace of the differentiation process to the proliferation rate.

Abstract

In vivo, fibroblast growth factor-2 (FGF-2) inhibits longitudinal bone growth. Similarly, activating FGF receptor 3 mutations impair growth in achondroplasia and thanatophoric dysplasia. To investigate the underlying mechanisms, we chose a fetal rat metatarsal organ culture system that would maintain growth plate histological architecture. Addition of FGF-2 to the serum-free medium inhibited longitudinal growth. We next assessed each major component of longitudinal growth: proliferation, cellular hypertrophy, and cartilage matrix synthesis. Surprisingly, FGF-2 stimulated proliferation, as assessed by [3H]thymidine incorporation. However, autoradiographic studies demonstrated that this increased proliferation occurred only in the perichondrium, whereas decreased labeling was seen in the proliferative and epiphyseal chondrocytes. FGF-2 also caused a marked decrease in the number of hypertrophic chondrocytes. To assess cartilage matrix synthesis, we measured 35SO4 incorporation into newly synthesized glycosaminoglycans. Low concentrations (10 ng/ml) of FGF-2 stimulated cartilage matrix production, but high concentrations (1000 ng/ml) inhibited matrix production. We conclude that FGF-2 inhibits longitudinal bone growth by three mechanisms: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and, at high concentrations, decreased cartilage matrix production. These effects may explain the impaired growth seen in patients with achondroplasia and related skeletal dysplasias.

Abstract

FGF21

Fibroblast Growth Factor 21 (FGF21) modulates glucose and lipid metabolism during fasting. In addition, previous evidence indicates that increased expression of FGF21 during chronic food restriction is associated with reduced bone growth and Growth Hormone (GH) insensitivity. In light of the inhibitory effects on growth plate chondrogenesis mediated by other FGFs, we hypothesized that FGF21 causes growth inhibition by acting directly at the long bones′ growth plate. We first demonstrated the expression of FGF21, FGFR1 and FGFR3 (two receptors known to be activated by FGF21), and β-klotho (a co-receptor required for the FGF21-mediated receptor binding and activation) in fetal and 3-week old mouse growth plate chondrocytes. We then cultured mouse growth plate chondrocytes in the presence of graded concentrations of rhFGF21 (0.01-10 μg/ml). Higher concentrations of FGF21 (5 and 10 μg/ml) inhibited chondrocyte thymidine incorporation and collagen X mRNA expression. 10 ng/ml GH stimulated chondrocyte thymidine incorporation and collagen X mRNA expression, with both effects being prevented by the addition in the culture medium of FGF21 in a concentration-dependent manner. In addition, FGF21 reduced GH binding in cultured chondrocytes. In cells transfected with FGFR1 siRNA or ERK 1 siRNA, the antagonistic effects of FGF21 on GH action were all prevented, supporting a specific effect of this growth factor in chondrocytes. Our findings suggest that increased expression of FGF21 during food restriction causes growth attenuation by antagonizing the GH stimulatory effects on chondrogenesis directly at the growth plate. In addition, high concentrations of FGF21 may directly suppress growth plate chondrocyte proliferation and differentiation.

Abstract

Endochondral ossification is a major mode of bone formation that occurs as chondrocytes undergo proliferation, hypertrophy, cell death, and osteoblastic replacement. We have identified a role for fibroblast growth factor receptor 3 (FGFR-3) in this process by disrupting the murine Fgfr-3 gene to produce severe and progressive bone dysplasia with enhanced and prolonged endochondral bone growth. This growth is accompanied by expansion of proliferating and hypertrophic chondrocytes within the cartilaginous growth plate. Thus, FGFR-3 appears to regulate endochondral ossification by an essentially negative mechanism, limiting rather than promoting osteogenesis. In light of these mouse results, certain human disorders, such as achondroplasia, can be interpreted as gain-of-function mutations that activate the fundamentally negative growth control exerted by the FGFR-3 kinase.

Conclusion: It appears that the Fibroblast Growth Factors, The FGF 1,2,3,4 all regulate the endochondral ossification process. There are a few studies that showed low rates of the FGF seems to increase longitudinal growth of the growth plates on the perineum outer layer but at high levels, all of the FGFs seems to inhibit the bone lengthening and slow down the growth process. As stated above

“”We conclude that FGF-2 inhibits longitudinal bone growth by three mechanisms: decreased growth plate chondrocyte proliferation, decreased cellular hypertrophy, and, at high concentrations, decreased cartilage matrix production””

The mechanism that is guessed to inhibit it is by down-regulating Ihh expression. The FGF21 does the same thing.