FGF signaling regulates mesenchymal differentiation and skeletal patterning along the limb bud proximodistal axis.
"Fibroblast growth factors (FGFs) are signals from the apical ectodermal ridge (AER) that are essential for limb pattern formation along the proximodistal (PD) axis. We conditionally inactivated fgf receptor 2 (Fgfr2) in the mouse AER to terminate all AER functions; for comparison, we inactivated both Fgfr1 and Fgfr2 in limb mesenchyme to block mesenchymal AER-FGF signaling. imb skeletal phenotypes resulting from loss of AER-FGF signals cannot simply be a consequence of excessive mesenchymal cell death but also must be a consequence of reduced mesenchymal proliferation and a failure of mesenchymal differentiation, which occur following loss of both Fgf4 and Fgf8. chondrogenic primordia formation, marked by initial Sox9 expression in limb mesenchyme, is an essential component of the PD patterning process and that a key role for AER-FGF signaling is to facilitate SOX9 function and to ensure progressive establishment of chondrogenic primordia along the PD axis."
"Of the four FGFs expressed in the AER, Fgf4 and Fgf8 are necessary for limb skeletal formation and when both genes are disrupted prior to limb bud initiation, the entire limb skeleton fails to form"
"when Fgf4 and Fgf8 are disrupted after limb bud initiation, transient AER-FGF signaling allows formation of a severely hypoplastic skeleton"
"stylopod agenesis after loss of Fgf8 is not due to reduced mesenchymal cell numbers, but rather to the failure of Sox9-expressing cells to undergo further chondrogenic differentiation."
"In the absence of both Gli3{up in LSJL} and Plzf, Sox9 is not expressed in early limb mesenchyme, which, as we suggest, results in failure of proximal chondrogenic primordia formation and subsequent failure of proximal skeletal element formation."
Antagonistic effects of FGF4 on BMP induction of apoptosis and chondrogenesis in the chick limb bud.
"We have implanted beads impregnated with [BMPs and FGFs] into chick limb buds between stages 20 and 26. Embryos were sacrificed at the time the bone chondrocyte condensations first appear (stages 27-28). Implantation of beads containing BMPs at the earlier stages (20-22) caused apoptosis to occur, in the most severe cases leading to complete limb degeneration. Application of FGF4, either in the same, or in a different bead, prevented the BMP-induced apoptosis. apoptosis observed on removal of the AER prior to stage 23 of development could be brought about by BMPs. The action of epithelial FGF in preventing BMP-mediated apoptosis in the mesenchyme would define a novel aspect of epithelial-mesenchymal interactions. Implanting the BMP4 beads into the core of the limb bud a day later (stages 25-26) caused intense chondrogenesis rather than apoptosis. FGF4 could again nullify this effect and by itself caused a reduction in bone size. This is the reverse of the functional relationship these growth factors have in mouse tooth specification (where it is BMP4 that inhibits the FGF8 function), and suggests that the balance between the effects of FGFs and BMPs could control the size of the chondrocyte precursor cell pool. In this way members of these two growth factor families could control the size of appendages when they are initially formed."
"Three FGF receptors that affect bone growth have reasonably distinct expression patterns within developing bone, with a major part of the expression of Fgfr-1 being in the periostium, that of Fgfr-2 in the hypertrophic cartilage, and that of Fgfr-3 in the resting cartilage, there is considerable overlap of expression in the chondrocyte condensations of the early limb bud"
"Dach1 was expressed in the distal mesenchyme of the early embryonic mouse limb bud and subsequently became restricted to the tips of digital cartilages. Dach1 protein was localized to postmitotic, prehypertrophic, and early hypertrophic chondrocytes during the initiation of ossification centers, but Dach1 was not expressed in growth plates that exhibited extensive ossification. Dach1 colocalized with Runx2/Cbfa1 in chondrocytes but not in the forming bone collar or primary spongiosa. Dach1 also colocalized with cyclin-dependent kinase inhibitors p27 (Kip1) and p57 (Kip2) in chondrocytes of the growth plate and in the epiphysis before the formation of the secondary ossification center. Because fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), and hedgehog molecules (Hh) regulate skeletal patterning of the limb bud and chondrocyte maturation in developing endochondral bones, we investigated the regulation of Dach1 by these growth and differentiation factors. Expression of Dach1 in 11 days postcoitus mouse limb buds in organ culture was up-regulated by implanting beads soaked in FGF1, 2, 8, or 9 but not FGF10. BMP4-soaked beads down-regulated Dach1 expression, whereas Shh and bovine serum albumin had no effect. FGF4 or 8 could substitute for the apical ectodermal ridge in maintaining Dach1 expression in the limb buds. FGFR2 and FGFR3 [overlapped] with Dach1 expression during skeletal patterning and chondrocyte maturation. Dach1 is a target gene of FGF signaling during limb skeletal development, and Dach1 may function as an intermediary in the FGF signaling pathway regulating cell proliferation or differentiation."
"Dach2, which overlaps in expression pattern with Dach1, may functionally compensate for the loss of Dach1 during embryonic morphogenesis"
"[Dach1] was expressed in mesenchymal cells before chondrocyte differentiation but was excluded from proliferating chondrocytes. It was re-expressed in postmitotic, prehypertrophic, and early hypertrophic chondrocytes, but not in the terminal stages of hypertrophy. By implanting growth factor–soaked beads into early limb buds in organ culture, we found that Dach1 expression was regulated by FGF signaling, consistent with its coexpression with FGF receptors throughout limb development."
"Dach1 and Mtsh1 may participate in the same developmental or cellular pathways during morphogenesis of the limb and other structures"<-Mtsh1 is upregulated by FGF8 and downregulated by BMP4.
"[We took] mesenchymal stromal cells (MSCs) from the chorion of human full term placenta from 15 donors. Chorionic MSCs revealed homologous fibroblast-like morphology and expressed CD73, CD29, CD105, and CD90. The hematopoietic stem cell markers including HLA DR, CD11b, CD34, CD79a, and CD45 were not expressed. The growth kinetics of their serial passage was steady at the later passages (passage 10). The multilineage capability of chorionic MSCs was demonstrated by successful adipogenic, osteogenic and chondrogenic differentiation and associated gene expression. Chorionic MSCs expressed genes associated with undifferentiated cells (NANOG, OCT4, REX1) and cardiogenic or neurogenic markers such as SOX2, FGF4, NES, MAP2, and NF. TERT was negative in all the samples. chorionic MSCs undifferentiated stem cells [are] less likely to be transformed into cancer cells. A low HLA DR expression suggests that chorionic MSCs may serve as a great source of stem cells for transplantation because of their immune-privileged status and their immunosuppressive effect. chorionic MSCs are pluripotent stem cells that are probably less differentiated than BM-MSCs, and they have considerable potential for use in cell-based therapies."
"SOX2 regulates FGF4 expression"
Synergistic effect of fibroblast growth factor-4 in ectopic bone formation induced by bone morphogenetic protein-2.
"After subcutaneously implanting recombinant human BMP-2 (rhBMP-2) in rats, we examined the expression of FGF-4 and FGF receptors (FGFRs) mRNAs and the effect of exogenous recombinant human FGF-4 (rhFGF-4) on bone formation. Three days after implantation, the pellets containing rhBMP-2 were surrounded by fibroblastic mesenchymal cells; on day 7, cartilage tissue appeared; on day 10, hypertrophic chondrocytes and a small amount of mineralized tissue were observed; and, on day 14, the amount of mineralized tissue increased. FGF-4 expression appeared at early stages (days 3 and 7) and its expression decreased at later stages (days 10, 14, and 21), whereas FGFRs were expressed continuously. on days 3 and 7, FGF-4, and FGFR subtypes 1 and 2 (FGFR-1 and FGFR-2) were expressed in mesenchymal cells and chondrocytes, and in the area of alkaline phosphatase (ALP) expression. On day 10, FGF-4 was not detected, whereas the expression of FGFR-1 and FGFR-2 was detectable in the area of alkaline phosphatase (ALP) expression. Injection of rhFGF-4 on days 2, 3, and 4 enhanced the mineralized tissue formation induced by rhBMP-2; however, neither rhFGF-4 treatment on days 6, 7, and 8 nor rhFGF-4 treatment on days 9, 10, and 11 influenced the amount of rhBMP-2-induced mineralization. Our results indicate that FGF-4 and FGFR signals play important roles during rhBMP-2-induced bone formation. The combination of rhBMP-2 and rhFGF-4 would be useful for bone augmentation."
"At day 10, although FGF-4 was undetectable, FGFR-1 and FGFR-2 were expressed"
"FGF-4 expression during rhBMP-2-induced ectopic bone formation strongly suggests that the gene implicated in embryonic tissue development is activated during the BMP-2-induced ectopic bone formation process in adult animals"
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