Two FGF receptor genes are differentially expressed in epithelial and mesenchymal tissues during limb formation and organogenesis in the mouse

Therapeutic potential of growth factors in pulmonary emphysematous condition

Pulmonary emphysema is a major manifestation of chronic obstructive pulmonary disease (COPD), which is characterized by progressive destruction of alveolar parenchyma with persistent inflammation of the small airways. Such destruction in the distal respiratory tract is irreversible and irreparable. All-trans-retinoic acid was suggested as a novel therapy for regeneration of lost alveoli in emphysema. However, profound discrepancies were evident between studies. At present, no effective therapeutic options are available that allow for the regeneration of lost alveoli in emphysematous human lungs. Recently, some reports on rodent's models have suggested the beneficial effects of various growth factors toward alveolar maintenance and repair processes.

Epigenetic and epigenomic mechanisms shape sarcoma and other mesenchymal tumor pathogenesis

Sarcomas comprise a large number of rare, histogenetically heterogeneous, mesenchymal tumors. Cancers such as Ewing's sarcoma, liposarcoma, rhabdomyosarcoma and synovial sarcoma can be generated by the transduction of mesenchymal stem cell progenitors with sarcoma-pathognomonic oncogenic fusions, a neoplastic transformation process accompanied by profound locus-specific and pangenomic epigenetic alterations. The epigenetic activities of histone-modifying and chromatin-remodeling enzymes such as SUV39H1/KMT1A, EZH2/KMT6A and BMI1 are central to epigenetic-regulated transformation, a property we coin oncoepigenic. Sarcoma-specific oncoepigenic aberrations modulate critical signaling pathways that control cell growth and differentiation including several miRNAs, Wnt, PI3K/AKT, Sav-RASSF1-Hpo and regulators of the G1 and G2/M checkpoints of the cell cycle. Herein an overview of the current knowledge of this rapidly evolving field that will undoubtedly uncover additional oncoepigenic mechanisms and yield druggable targets in the near future is discussed.

Upregulation of fibroblast growth factor receptor 2 and 3 in the late stages of fetal lung development in the nitrofen rat model

PURPOSE

Nitrofen model of congenital diaphragmatic hernia (CDH) has been widely used to investigate the pathogenesis of pulmonary hypoplasia (PH). Fibroblast growth factor (FGF) signaling pathway plays a fundamental role in fetal lung development. FGF7 and FGF10, which are critical for lung morphogenesis, have been reported to be downregulated in nitrofen-induced PH. FGF signaling is mediated by a family of four single transmembrane receptors, FGFR1-4. FGFR2 and FGFR3 have been shown to be expressed predominantly in the late stages of developing lungs. In addition, the upregulation of FGFR2 gene expression has been associated with severe defects in lung development and resulted in arrested alveologenesis similar to PH seen in the nitrofen model. Furthermore, FGFR3(-/-)FGFR4(-/-) double mutants showed thinner mesenchyme and larger air spaces. We designed this study to test the hypothesis that FGFR gene expression is upregulated in the late stages of lung development in the nitrofen CDH model.

METHODS

Pregnant rats were exposed to either olive oil or nitrofen on day 9 of gestation (D9). Cesarean section was performed and fetuses were harvested on D18 and D21. Fetal lungs were divided into three groups: control, nitrofen without CDH [CDH(-)], and nitrofen with CDH [CDH(+)] (n = 24 at each time-point). Pulmonary gene expression levels of FGFR1-4 were analyzed by real-time RT-PCR. Immunohistochemistry was also performed to evaluate protein expression/distribution at each time-point.

RESULTS

The relative messenger RNA expression levels of pulmonary FGFR2 and FGFR3 on D21 were significantly increased in CDH(-) (6.38 ± 1.93 and 7.84 ± 2.86, respectively) and CDH(+) (7.09 ± 2.50 and 7.25 ± 3.43, respectively) compared to controls (P < 0.05 and P < 0.01, respectively), whereas no significant alteration was observed on D18. There were no differences in FGFR1 and FGFR4 expression at both time-points. Increased immunoreactivity of FGFR2 and FGFR3, mainly in the distal epithelium and mesenchyme, was observed in the nitrofen-induced hypoplastic lungs on D21 compared to controls.

CONCLUSION

Upregulation of FGFR2 and FGFR3 pulmonary gene expression in the late stages of fetal lung development may disrupt FGFR-mediated alveologenesis resulting in PH in the CDH model.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) are expressed throughout the developing kidney. Several early studies have shown that exogenous fibroblast growth factors (Fgfs) affect growth and maturation of the metanephric mesenchyme (MM) and ureteric bud (UB). Transgenic mice that over-express a dominant negative receptor isoform develop renal aplasia/severe dysplasia, confirming the importance of Fgfrs in renal development. Furthermore, global deletion of Fgf7, Fgf10, and Fgfr2IIIb (isoform that binds Fgf7 and Fgf10) in mice leads to small kidneys with fewer collecting ducts and nephrons. Deletion of Fgfrl1, a receptor lacking intracellular signaling domains, causes severe renal dysgenesis. Conditional targeting of Fgf8 from the MM interrupts nephron formation. Deletion of Fgfr2 from the UB results in severe ureteric branching and stromal mesenchymal defects, although loss of Frs2α (major signaling adapter for Fgfrs) in the UB causes only mild renal hypoplasia. Deletion of both Fgfr1 and Fgfr2 in the MM results in renal aplasia with defects in MM formation and initial UB elongation and branching. Loss of Fgfr2 in the MM leads to many renal and urinary tract anomalies as well as vesicoureteral reflux. Thus, Fgfr signaling is critical for patterning of virtually all renal lineages at early and later stages of development.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) consist of four signaling family members and one nonsignaling "decoy" receptor, Fgfr-like 1 (Fgfrl1), all of which are expressed in the developing kidney. Several studies have shown that exogenous fibroblast growth factors (Fgfs) affect growth and maturation of the metanephric mesenchyme (MM) and ureteric bud (UB) in cultured tissues. Transgenic and conditional knockout approaches in whole animals have shown that Fgfr1 and Fgfr2 (predominantly the IIIc isoform) in kidney mesenchyme are critical for early MM and UB formation. Conditional deletion of the ligand, Fgf8, in nephron precursors or global deletion of Fgfrl1 interrupts nephron formation. Fgfr2 (likely the IIIb isoform signaling downstream of Fgf7 and Fgf10) is critical for ureteric morphogenesis. Moreover, Fgfr2 appears to act independently of Frs2α (the major signaling adapter for Fgfrs) in regulating UB branching. Loss of Fgfr2 in the MM leads to many kidney and urinary tract anomalies, including vesicoureteral reflux. Thus Fgfr signaling is critical for patterning of virtually all renal lineages at early and later stages of development.

Identification and characterization of an inhibitory fibroblast growth factor receptor 2 (FGFR2) molecule, up-regulated in an Apert Syndrome mouse model

AS (Apert syndrome) is a congenital disease composed of skeletal, visceral and neural abnormalities, caused by dominant-acting mutations in FGFR2 [FGF (fibroblast growth factor) receptor 2]. Multiple FGFR2 splice variants are generated through alternative splicing, including PTC (premature termination codon)-containing transcripts that are normally eliminated via the NMD (nonsense-mediated decay) pathway. We have discovered that a soluble truncated FGFR2 molecule encoded by a PTC-containing transcript is up-regulated and persists in tissues of an AS mouse model. We have termed this IIIa–TM as it arises from aberrant splicing of FGFR2 exon 7 (IIIa) into exon 10 [TM (transmembrane domain)]. IIIa–TM is glycosylated and can modulate the binding of FGF1 to FGFR2 molecules in BIAcore-binding assays. We also show that IIIa–TM can negatively regulate FGF signalling in vitro and in vivo. AS phenotypes are thought to result from gain-of-FGFR2 signalling, but our findings suggest that IIIa–TM can contribute to these through a loss-of-FGFR2 function mechanism. Moreover, our findings raise the interesting possibility that FGFR2 signalling may be a regulator of the NMD pathway.

FGF signaling pathway in the developing chick lung: expression and inhibition studies

Background

Fibroblast growth factors (FGF) are essential key players during embryonic development. Through their specific cognate receptors (FGFR) they activate intracellular cascades, finely regulated by modulators such as Sprouty. Several FGF ligands (FGF1, 2, 7, 9, 10 and 18) signaling through the four known FGFRs, have been implicated in lung morphogenesis. Although much is known about mammalian lung, so far, the avian model has not been explored for lung studies.

Methodology/Principal Findings

In this study we provide the first description of fgf10, fgfr1-4 and spry2 expression patterns in early stages of chick lung development by in situ hybridization and observe that they are expressed similarly to their mammalian counterparts. Furthermore, aiming to determine a role for FGF signaling in chick lung development, in vitro FGFR inhibition studies were performed. Lung explants treated with an FGF receptor antagonist (SU5402) presented an impairment of secondary branch formation after 48 h of culture; moreover, abnormal lung growth with a cystic appearance of secondary bronchi and reduction of the mesenchymal tissue was observed. Branching and morphometric analysis of lung explants confirmed that FGFR inhibition impaired branching morphogenesis and induced a significant reduction of the mesenchyme.

Conclusions/Significance

This work demonstrates that FGFRs are essential for the epithelial-mesenchymal interactions that determine epithelial branching and mesenchymal growth and validate the avian embryo as a good model for pulmonary studies, namely to explore the FGF pathway as a therapeutic target.

Fibroblast growth factor receptors in in vitro and in vivo chondrogenesis: relating tissue engineering using adult mesenchymal stem cells to embryonic development

Adult mesenchymal stem cells (MSCs) are considered promising candidate cells for therapeutic cartilage and bone regeneration. Because tissue regeneration and embryonic development may involve similar pathways, understanding common pathways may lead to advances in regenerative medicine. In embryonic limb development, fibroblast growth factor receptors (FGFRs) play a role in chondrogenic differentiation. The aim of this study was to investigate and compare FGFR expression in in vivo embryonic limb development and in vitro chondrogenesis of MSCs. Our study showed that in in vitro chondrogenesis of MSCs three sequential stages can be found, as in embryonic limb development. A mesenchymal condensation (indicated by N-cadherin) is followed by chondrogenic differentiation (indicated by collagen II), and hypertrophy (indicated by collagen X). FGFR1-3 are expressed in a stage-dependent pattern during in vitro differentiation and in vivo embryonic limb development. In both models FGFR2 is clearly expressed by cells in the condensation phase. No FGFR expression was observed in differentiating and mature hyaline chondrocytes, whereas hypertrophic chondrocytes stained strongly for all FGFRs. To evaluate whether stage-specific modulation of chondrogenic differentiation in MSCs is possible with different subtypes of FGF, FGF2 and FGF9 were added to the chondrogenic medium during different stages in the culture process (early or late). FGF2 and FGF9 differentially affected the amount of cartilage formed by MSCs depending on the stage in which they were added. These results will help us understand the role of FGF signaling in chondrogenesis and find new tools to monitor and control chondrogenic differentiation.

Twist1 activity thresholds define multiple functions in limb development

The basic helix-loop-helix transcription factor Twist1 is essential for normal limb development. Twist1(-/-) embryos die at midgestation. However, studies on early limb buds found that Twist1(-/-) mutant limb mesenchyme has an impaired response to FGF signaling from the apical ectodermal ridge, which disrupts the feedback loop between the mesenchyme and AER, and reduces and shifts anteriorly Shh expression in the zone of polarizing activity. We have combined Twist1 null, hypomorph and conditional alleles to generate a Twist1 allelic series that survives to birth. As Twist1 activity is reduced, limb skeletal defects progress from preaxial polydactyly to girdle reduction combined with hypoplasia, aplasia or mirror symmetry of all limb segments. With reduced Twist1 activity there is striking and progressive upregulation of ectopic Shh expression in the anterior of the limb, combined with an anterior shift in the posterior Shh domain, which is expressed at normal intensity, and loss of the posterior AER. Consequently limb outgrowth is initially impaired, before an ectopic anterior Shh domain expands the AER, promoting additional growth and repatterning. Reducing the dosage of FGF targets of the Etv gene family, which are known repressors of Shh expression in anterior limb mesenchyme, strongly enhances the anterior skeletal phenotype. Conversely this and other phenotypes are suppressed by reducing the dosage of the Twist1 antagonist Hand2. Our data support a model whereby multiple Twist1 activity thresholds contribute to early limb bud patterning, and suggest how particular combinations of skeletal defects result from differing amounts of Twist1 activity.

Heparan sulfate in lung morphogenesis: The elephant in the room

Heparan sulfate (HS) is a structurally complex polysaccharide located on the cell surface and in the extracellular matrix, where it participates in numerous biological processes through interactions with a vast number of regulatory proteins such as growth factors and morphogens. HS is crucial for lung development; disruption of HS synthesis in flies and mice results in a major aberration of airway branching, and in mice, it results in neonatal death as a consequence of malformed lungs and respiratory distress. Epithelial-mesenchymal interactions governing lung morphogenesis are directed by various diffusible proteins, many of which bind to, and are regulated by HS, including fibroblast growth factors, sonic hedgehog, and bone morphogenetic proteins. The majority of research into the molecular mechanisms underlying defective lung morphogenesis and pulmonary pathologies, such as bronchopulmonary dysplasia and pulmonary hypoplasia associated with congenital diaphragmatic hernia (CDH), has focused on abnormal protein expression. The potential contribution of HS to abnormalities of lung development has yet to be explored to any significant extent, which is somewhat surprising given the abnormal lung phenotype exhibited by mutant mice synthesizing abnormal HS. This review summarizes our current understanding of the role of HS and HS-binding proteins in lung morphogenesis and will present in vitro and in vivo evidence for the fundamental importance of HS in airway development. Finally, we will discuss the future possibility of HS-based therapeutics for ameliorating insufficient lung growth associated with lung diseases such as CDH.

Are purified or expanded cord blood-derived CD133+ cells better at improving cardiac function?

Endothelial progenitor cells (EPCs), which express the CD133 marker, can differentiate into mature endothelial cells (ECs) and create new blood vessels. Normal angiogenesis is unable to repair the injured tissues that result from myocardial infarction (MI). Patients who have high cardiovascular risks have fewer EPCs and their EPCs exhibit greater in vitro senescence. Human umbilical cord blood (HUCB)-derived EPCs could be an alternative to rescue impaired stem cell function in the sick and elderly. The aim of this study was to purify HUCB-derived CD133(+) cells, expand them in vitro and evaluate the efficacy of the purified and expanded cells in treating MI in rats. CD133(+) cells were selected for using CD133-coupled magnetic microbeads. Purified cells stained positive for EPC markers. The cells were expanded and differentiated in media supplemented with fetal calf serum and basic fibroblast growth factor, insulin-like growth factor-I and vascular endothelial growth factor (VEGF). Differentiation was confirmed by lack of staining for EPC markers. These expanded cells exhibited increased expression of mature EC markers and formed tubule-like structures in vitro. Only the expanded cells expressed VEGF mRNA. Cells were expanded up to 70-fold during 60 days of culture, and they retained their functional activity. Finally, we evaluated the therapeutic potential of purified and expanded CD133(+) cells in treating MI by intramyocardially injecting them into a rat model of MI. Rats were divided into three groups: A (purified CD133(+) cells-injected); B (expanded CD133(+) cells-injected) and C (saline buffer-injected). We observed a significant improvement in left ventricular ejection fraction for groups A and B. In summary, CD133(+) cells can be purified from HUCB, expanded in vitro without loosing their biological activity, and both purified and expanded cells show promising results for use in cellular cardiomyoplasty. However, further pre-clinical testing should be performed to determine whether expanded CD133(+) cells have any clinical advantages over purified CD133(+) cells.

Genetic control of bone formation

In the past few years, our molecular understanding of bone formation has continued to increase. This review aims to present a comprehensive view of the current state of knowledge in the field. Thus, it will cover our current knowledge of chondrogenesis and osteoblastogenesis. It will also cover the most salient aspects of osteoblast function.

Conditional gene inactivation reveals roles for Fgf10 and Fgfr2 in establishing a normal pattern of epithelial branching in the mouse lung

Fibroblast growth factor 10 (FGF10) signaling through FGF receptor 2 (FGFR2) is required for lung initiation. While studies indicate that Fgf10 and Fgfr2 are also important at later stages of lung development, their roles in early branching events remain unclear. We addressed this question through conditional inactivation of both genes in mouse subsequent to lung initiation. Inactivation of Fgf10 in lung mesenchyme resulted in smaller lobes with a reduced number of branches. Inactivation of Fgfr2 in lung epithelium resulted in disruption of lobes and small epithelial outgrowths that arose arbitrarily along the main bronchi. In both mutants, there was an increase in cell death. Also, the expression patterns of key signaling molecules implicated in branching morphogenesis were altered and a proximal lung marker was expanded distally. Our results indicate that both Fgf10 and Fgfr2 are required for a normal branching program and for proper proximal-distal patterning of the lung.

Fibroblast growth factor 10 plays a causative role in the tracheal cartilage defects in a mouse model of Apert syndrome

Patients with Apert syndrome (AS) display a wide range of congenital malformations including tracheal stenosis, which is a disease characterized by a uniform cartilaginous sleeve in place of a normally ribbed cartilagenous trachea. We have studied the cellular and molecular basis of this phenotype in a mouse model of AS (Fgfr2c(+/Delta) mice), which shows ectopic expression of Fgfr2b in mesenchymal tissues. Here we report that tracheal stenosis is associated with increased proliferation of mesenchymal cells, where the expression of Fgf10 and its upstream regulators Tbx4 and Tbx5 are abnormally elevated. We show that Fgf10 has a critical inductive role in tracheal stenosis, as genetic knockdown of Fgf10 in Fgfr2c(+/Delta) mice rescues this phenotype. These novel findings demonstrate a regulatory role for Fgf10 in tracheal development and shed more light on the underlying cause of tracheal defects in AS.

Convergent signalling through Fgfr2 regulates divergent craniofacial morphogenesis

Fibroblast growth factor receptor 2 (Fgfr2) has two splice variants IIIb and IIIc, which are unique in function and localization. Signalling through Fgfr2IIIb controls epithelial-mesenchymal interactions, which regulate morphogenesis during the development of several organs including the palate and tooth. In this study, we confirm that molar tooth development in Fgfr2IIIb(-/-) mice is arrested early in development and that the molar teeth of Fgf10(-/-) mice develop through all the normal stages of morphogenesis. We show that the molar phenotype of Fgfr2IIIb(-/-) mice is, in part, owing to reduced cell proliferation in both epithelial and mesenchymal compartments. We also show that the developing molar teeth of Fgf10(-/-) mice exhibit reduced cell proliferation. However, this reduction is not sufficient to arrest molar development. Recent evidence has indicated that Fgfr2IIIb/Fgf10 signalling is active in the calvaria in some pathological situations as heterozygous deletion of Fgfr2 exon IIIc in mice leads to ectopic expression of Fgfr2IIIb in the calvarial bones and causes craniosynostosis. Here, we investigate the mRNA expression of Fgfr2IIIb and Fgfr2IIIc as well as their ligands Fgf3, -7 and -10 in the developing murine tooth, palate and calvaria. We show that Fgf7 is expressed in the calvarial mesenchyme adjacent to the developing frontal bone and Fgf10 is expressed by osteoprogenitors in the developing frontal bone condensation. Taken together, we highlight the overlapping roles of Fgfr2IIIb/Fgf10 signalling in controlling epithelial-mesenchymal interactions during normal palate and tooth morphogenesis and how elevated signalling through Fgfr2IIIb/Fgf10 solely within the mesenchyme can result in abnormal calvarial morphogenesis.

Evidence that Fgf10 contributes to the skeletal and visceral defects of an Apert syndrome mouse model

Apert syndrome (AS) is a severe congenital disease caused by mutations in fibroblast growth factor receptor-2 (FGFR2), and characterised by craniofacial, limb, visceral, and neural abnormalities. AS-type FGFR2 molecules exert a gain-of-function effect in a ligand-dependent manner, but the causative FGFs and their relative contribution to each of the abnormalities observed in AS remains unknown. We have generated mice that harbour an AS mutation but are deficient in or heterozygous for Fgf10. The genetic knockdown of Fgf10 can rescue the skeletal as well as some of the visceral defects observed in this AS model, and restore a near normal level of FgfR2 signaling involving an apparent switch between ERK(p44/p42) and p38 phosphorylation. Surprisingly, it can also yield de novo cleft palate and blind colon in a subset of the compound mutants. These findings strongly suggest that Fgf10 contributes to AS-like pathologies and highlight a complexity of Fgf10 function in different tissues.

Genetic variants in FGFR2 and FGFR4 genes and skin cancer risk in the Nurses' Health Study

Background

The human fibroblast growth factor (FGF) and its receptor (FGFR) play an important role in tumorigenesis. Deregulation of the FGFR2 gene has been identified in a number of cancer sites. Overexpression of the FGFR4 protein has been linked to cutaneous melanoma progression. Previous studies reported associations between genetic variants in the FGFR2 and FGFR4 genes and development of various cancers.

Methods

We evaluated the associations of four genetic variants in the FGFR2 gene highly related to breast cancer risk and the three common tag-SNPs in the FGFR4 gene with skin cancer risk in a nested case-control study of Caucasians within the Nurses' Health Study (NHS) among 218 melanoma cases, 285 squamous cell carcinoma (SCC) cases, 300 basal cell carcinoma (BCC) cases, and 870 controls.

Results

We found no evidence for associations between these seven genetic variants and the risks of melanoma and nonmelanocytic skin cancer.

Conclusion

Given the power of this study, we did not detect any contribution of genetic variants in the FGFR2 or FGFR4 genes to inherited predisposition to skin cancer among Caucasian women.

Fibroblast growth factor 9 signaling inhibits airway smooth muscle differentiation in mouse lung

In mammalian lungs, airway smooth muscle cells (airway SMCs) are present in the proximal lung adjacent to bronchi and bronchioles, but are absent in the distal lung adjacent to terminal sacs that expand during gas exchange. Evidence suggests that this distribution is essential for the formation of a functional respiratory tree, but the underlying genetic mechanism has not been elucidated. In this study, we test the hypothesis that fibroblast growth factor 9 (Fgf9) signaling is essential to restrict SMC differentiation to the proximal lung. We show that loss of Fgf9 or conditional inactivation of Fgf receptors (Fgfr) 1 and 2 in mouse lung mesenchyme results in ectopic SMCs. Our data support a model where FGF9 maintains a SMC progenitor population by suppressing differentiation and promoting growth. This model also represents our findings on the genetic relationship between FGF9 and sonic hedgehog (SHH) in the establishment of airway SMC pattern.

Genetic mosaic analysis reveals FGF receptor 2 function in terminal end buds during mammary gland branching morphogenesis

FGF signaling is associated with breast cancer and is required for mammary placode formation in the mouse. In this study, we employed a genetic mosaic analysis based on Cre-mediated recombination to investigate FGF receptor 2 (Fgfr2) function in the postnatal mammary gland. Mosaic inactivation of Fgfr2 by the MMTV-Cre transgene enabled us to compare the behavior of Fgfr2 null and Fgfr2 heterozygous cells in the same gland. Fgfr2 null cells were at a competitive disadvantage to their Fgfr2 heterozygous neighbors in the highly proliferative terminal end buds (TEBs) at the invasion front, owing to a negative effect of loss of Fgfr2 function on cell proliferation. However, Fgfr2 null cells were tolerated in mature ducts. In these genetic mosaic mammary glands, the epithelial network is apparently built by TEBs that over time are composed of a progressively larger proportion of Fgfr2-positive cells. However, subsequently, most cells lose Fgfr2 function, presumably due to additional rounds of Cre-mediated recombination. Using an independent strategy to create mosaic mammary glands, which employed an adenovirus-Cre that acts only once, we confirmed that Fgfr2 null cells were out-competed by neighboring Fgfr2 heterozygous cells. Together, our data demonstrate that Fgfr2 functions in the proliferating and invading TEBs, but it is not required in the mature ducts of the pubertal mammary gland.

Biological impact of the fibroblast growth factor family on articular cartilage and intervertebral disc homeostasis

Two members of the fibroblast growth factor (FGF) family, basic FGF (bFGF) and FGF-18, have been implicated in the regulation of articular and intervertebral disc (IVD) cartilage homeostasis. Studies on bFGF from a variety of species have yielded contradictory results with regards to its precise role in cartilage matrix synthesis and degradation. In contrast, FGF-18 is a well-known anabolic growth factor involved in chondrogenesis and articular cartilage repair. In this review, we examined the biological actions of bFGF and FGF-18 in articular and IVD cartilage, the specific cell surface receptors bound by each factor, and the unique signaling cascades and molecular pathways utilized to exert their biological effects. Evidence suggests that bFGF selectively activates FGF receptor 1 (FGFR1) to exert degradative effects in both human articular chondrocytes and IVD tissue via upregulation of matrix-degrading enzyme activity, inhibition of matrix production, and increased cell proliferation resulting in clustering of cells seen in arthritic states. FGF-18, on the other hand, most likely exerts anabolic effects in human articular chondrocytes by activating FGFR3, increasing matrix formation and cell differentiation while inhibiting cell proliferation, leading to dispersed cells surrounded by abundant matrix. The results from in vitro and in vivo studies suggest the potential usefulness of bFGF and FGFR1 antagonists, as well as FGF-18 and FGFR3 agonists, as potential therapies to prevent cartilage degeneration and/or promote cartilage regeneration and repair in the future.

Cystic adenomatoid malformations are induced by localized FGF10 overexpression in fetal rat lung

Fibroblast growth factor-10 (FGF10) is a mesenchymal growth factor, involved in epithelial and mesenchymal interactions during lung branching morphogenesis. In the present work, FGF10 overexpression was transiently induced in a temporally and spatially restricted manner, during the pseudoglandular or canalicular stages of rat lung development, by trans-uterine ultrasound-guided intraparenchymal microinjections of adenoviral vector encoding the rfgf10 transgene. The morphologic and histologic classification of the resulting malformations were dependent upon developmental stage and location. Overexpression of FGF10 restricted to the proximal tracheobronchial tree during the pseudoglandular phase resulted in large cysts lined by tall columnar epithelium composed primarily of Clara cells with a paucity of Type II pneumocytes, resembling bronchiolar type epithelium. In contrast, FGF10 overexpression in the distal lung parenchyma during the canalicular phase resulted in small cysts lined by cuboidal epithelial cells composed of primarily Type II pneumocytes resembling acinar epithelial differentiation. The cystic malformations induced by FGF10 overexpression appear to closely recapitulate the morphology and histology of the spectrum of human congenital cystic adenomatoid malformation (CCAM). These findings support a role for FGF10 in the induction of human CCAM and provide further mechanistic insight into the role of FGF10 in normal and abnormal lung development.

Activator-inhibitor dynamics of vertebrate limb pattern formation

The development of the vertebrate limb depends on an interplay of cellular differentiation, pattern formation, and tissue morphogenesis on multiple spatial and temporal scales. While numerous gene products have been described that participate in, and influence, the generation of the limb skeletal pattern, an understanding of the most salient feature of the developing limb--its quasiperiodic arrangement of bones, requires additional organizational principles. We review several such principles, drawing on concepts of physics and chemical dynamics along with molecular genetics and cell biology. First, a "core mechanism" for precartilage mesenchymal condensation is described, based on positive autoregulation of the morphogen transforming growth factor (TGF)-beta, induction of the extracellular matrix (ECM) protein fibronectin, and focal accumulation of cells via haptotaxis. This core mechanism is shown to be part of a local autoactivation-lateral inhibition (LALI) system that ensures that the condensations will be regularly spaced. Next, a "bare-bones" model for limb development is described in which the LALI-core mechanism is placed in a growing geometric framework with predifferentiated "apical," differentiating "active," and irreversibly differentiated "frozen" zones defined by distance from an apical source of a fibroblast growth factor (FGF)-type morphogen. This model is shown to account for classic features of the developing limb, including the proximodistal (PD) emergence over time of increasing numbers of bones. We review earlier and recent work suggesting that the inhibitory component of the LALI system for condensation may not be a diffusible morphogen, and propose an alternative mechanism for lateral inhibition, based on synchronization of oscillations of a Hes mediator of the Notch signaling pathway. Finally, we discuss how viewing development as an interplay between molecular-genetic and dynamic physical processes can provide new insight into the origin of congenital anomalies.

Multiscale models for vertebrate limb development

Dynamical systems in which geometrically extended model cells produce and interact with diffusible (morphogen) and nondiffusible (extracellular matrix) chemical fields have proved very useful as models for developmental processes. The embryonic vertebrate limb is an apt system for such mathematical and computational modeling since it has been the subject of hundreds of experimental studies, and its normal and variant morphologies and spatiotemporal organization of expressed genes are well known. Because of its stereotypical proximodistally generated increase in the number of parallel skeletal elements, the limb lends itself to being modeled by Turing-type systems which are capable of producing periodic, or quasiperiodic, arrangements of spot- and stripe-like elements. This chapter describes several such models, including, (i) a system of partial differential equations in which changing cell density enters into the dynamics explicitly, (ii) a model for morphogen dynamics alone, derived from the latter system in the "morphostatic limit" where cell movement relaxes on a much slower time-scale than cell differentiation, (iii) a discrete stochastic model for the simplified pattern formation that occurs when limb cells are placed in planar culture, and (iv) several hybrid models in which continuum morphogen systems interact with cells represented as energy-minimizing mesoscopic entities. Progress in devising computational methods for handling 3D, multiscale, multimodel simulations of organogenesis is discussed, as well as for simulating reaction-diffusion dynamics in domains of irregular shape.

Advances in the molecular pathogenesis of craniofacial conditions

The impact that the understanding of fibroblast growth factor receptor (FGFR) biology and its relevance to the pathogenesis of the craniosynostoses has made cannot be underestimated. As the genetic and molecular pathology of other conditions become increasingly understood, there is much hope that robust and relevant animal models of these conditions may be generated. From these models-and in conjunction with laboratory studies in vitro-comes a real hope of improved therapeutic strategies. The future lies in increased cooperation between clinicians working in high-volume centers and basic scientists. This article decribes the results of a decade of research in which the molecular pathology of the craniosynostoses was unravelled. The understanding of the importance of FGFR mutations to the genetic etiology of craniosynostosis opened up novel studies in developmental biology in various tissues. Such studies describe the functional effects of FGFR mutations. Investigations of FGFR expression in human craniofacial development have related functional molecular studies to human craniosynostosis syndromes, which provides a link between the gene mutation and the affected child.

FGFR1 over-expression in primary rhabdomyosarcoma tumors is associated with hypomethylation of a 5' CpG island and abnormal expression of the AKT1, NOG, and BMP4 genes

Rhabdomyosarcoma (RMS), the most common pediatric soft tissue sarcoma likely results from abnormal proliferation and differentiation during skeletal myogenesis. Multiple genetic alterations are associated with the three RMS histopathological subtypes, embryonal, alveolar, and pleomorphic adult variant. Recently, we reported the novel amplification of the FGFR1 gene in a RMS tumor. The involvement of FGFR1 in RMS was now further studied in primary tumors and RMS cell lines by mutation screening, quantitative RNA expression, and methylation analyses. No mutation was found by DHPLC and sequencing of the entire FGFR1 coding sequence and exon-intron boundaries. However, FGFR1 over-expression was detected in all primary RMS tumors and cell lines tested. A hypomethylation of a CpG island upstream to FGFR1 exon 1 was identified in the primary RMS tumors, using sodium bisulfite modification method, suggesting a molecular mechanism to FGFR1 over-expression. Expression analysis of additional genes, AKT1, NOG and its antagonist BMP4, which interact downstream to FGFR1, demonstrated expression differences between primary RMS tumors and normal skeletal muscles. Our data suggest an important role for FGFR1 and FGFR1-downstream genes in RMS tumorigenesis and a possible association with the deregulation of proliferation and differentiation of skeletal myoblasts in RMS.

Changes in gene-expression during development of the murine molar tooth germ

In a matter of a few days the murine tooth germ develops into a complex, mineralized, structure. Murine 30K microarrays were used to examine gene expression in the mandibular first molar tooth germs isolated at 15.5dpc and at 2DPN. Microarray results were validated using real-time RT-PCR. The results suggested that only 25 genes (3 without known functions) exhibited significantly higher expression at 15.5dpc compared to 2DPN. In contrast, almost 1400 genes exhibited significantly (P<0.015) higher expression at 2DPN compared to 15.5dpc, about half of which were genes with unknown functions. More than 50 of the 783 known genes exhibited higher than 10-fold increase in expression at 2DPN, amongst these were genes coding for enamel matrix proteins which were expressed several 100-fold higher at 2DPN. GO and KEGG analysis showed highly significant associations between families of the 783 known genes and cellular functions relating to energy metabolism, protein metabolism, regulation of cell division, cell growth and apoptosis. The use of bioinformatics analysis therefore yielded a functional profile in agreement with known differences in tissue morphology and cellular composition between these two stages. Such data is therefore useful in directing attention towards genes, or cellular activities, which likely are worthy of further studies as regards their involvement in odontogenesis.

Aberrant fibroblast growth factor receptor signaling in bladder and other cancers

Fibroblast growth factors (FGFs) are potent mitogens, morphogens, and inducers of angiogenesis, and FGF signaling governs the genesis of diverse tissues and organs from the earliest stages. With such fundamental embryonic and homeostatic roles, it follows that aberrant FGF signaling underlies a variety of diseases. Pathological modifications to FGF expression are known to cause salivary gland aplasia and autosomal dominant hypophosphatemic rickets, while mutations in FGF receptors (FGFRs) result in a range of skeletal dysplasias. Anomalous FGF signaling is also associated with cancer development and progression. Examples include the overexpression of FGF2 and FGF6 in prostate cancer, and FGF8 overexpression in breast and prostate cancers. Alterations in FGF signaling regulators also impact tumorigenesis, which is exemplified by the down-regulation of Sprouty 1, a negative regulator of FGF signaling, in prostate cancer. In addition, several FGFRs are mutated in human cancers (including FGFR2 in gastric cancer and FGFR3 in bladder cancer). We recently identified intriguing alterations in the FGF pathway in a novel model of bladder carcinoma that consists of a parental cell line (TSU-Pr1/T24) and two sublines with increasing metastatic potential (TSU-Pr1-B1 and TSU-Pr1-B2), which were derived successively through in vivo cycling. It was found that the increasingly metastatic sublines (TSU-Pr1-B1 and TSU-Pr1-B2) had undergone a mesenchymal to epithelial transition. FGFR2IIIc expression, which is normally expressed in mesenchymal cells, was increased in the epithelial-like TSU-Pr1-B1 and TSU-Pr1-B2 sublines and FGFR2 knock-down was associated with the reversion of cells from an epithelial to a mesenchymal phenotype. These observations suggest that modified FGF pathway signaling should be considered when studying other cancer types.

Identification of Novel Wilms' Tumor Suppressor Gene Target Genes Implicated in Kidney Development

The Wilms' tumor suppressor gene (WT1) encodes a zinc finger transcription factor that is vital during development of several organs including metanephric kidneys. Despite the critical regulatory role of WT1, the pathways and mechanisms by which WT1 orchestrates development remain elusive. To identify WT1 target genes, we performed a genome-wide expression profiling analysis in cells expressing inducible WT1. We identified a number of direct WT1 target genes, including the epidermal growth factor (EGF)-family ligands epiregulin and HB-EGF, the chemokine CX3CL1, and the transcription factors SLUG and JUNB. The target genes were validated using quantitative reverse transcriptase-polymerase chain reaction, small interfering RNA knockdowns, chromatin immunoprecipitation, and luciferase reporter analyses. Immunohistochemistry of fetal kidneys confirmed that a number of the WT1 target genes had overlapping expression patterns with the highly restricted spatiotemporal expression of WT1. Finally, using an in vitro embryonic kidney culture assay, we found that the addition of recombinant epiregulin, amphiregulin, CX3CL1, and interleukin-11 significantly enhanced ureteric bud branching morphogenesis. Our genome-wide screen implicates WT1 in the transcriptional regulation of the EGF-family of growth factors as well as the CX3CL1 chemokine during nephrogenesis.

FGF9 regulates early hypertrophic chondrocyte differentiation and skeletal vascularization in the developing stylopod

Gain-of-function mutations in fibroblast growth factor (FGF) receptors result in chondrodysplasia and craniosynostosis syndromes, highlighting the critical role for FGF signaling in skeletal development. Although the FGFRs involved in skeletal development have been well characterized, only a single FGF ligand, FGF18, has been identified that regulates skeletal development during embryogenesis. Here we identify Fgf9 as a second FGF ligand that is critical for skeletal development. We show that Fgf9 is expressed in the proximity of developing skeletal elements and that Fgf9-deficient mice exhibit rhizomelia (a disproportionate shortening of proximal skeletal elements), which is a prominent feature of patients with FGFR3-induced chondrodysplasia syndromes. Although Fgf9 is expressed in the apical ectodermal ridge in the limb bud, we demonstrate that the Fgf9-/- limb phenotype results from loss of FGF9 functions after formation of the mesenchymal condensation. In developing stylopod elements, FGF9 promotes chondrocyte hypertrophy at early stages and regulates vascularization of the growth plate and osteogenesis at later stages of skeletal development.

Overexpression of SR proteins and splice variants modulates chondrogenesis

Fibronectin alternative exon EIIIA is largely included in undifferentiated mesenchymal cells of the developing limb bud, whereas the exon is excluded in differentiated chondrocytes. Inclusion of exon EIIIA in chondrocytic cells is increased by overexpression of SRp40, and, to a lesser extent, SRp75, but not SRp55. RT-PCR analysis using real-time PCR revealed that the levels of the mRNAs for these three proteins did not vary significantly in chick chondrocytes versus mesenchymal cells of the developing limb bud. However, a variant spliced form of SRp40, termed, SRp40LF, is detected preferentially in chondrocytes and in chondrifying mesenchymal cells. Forced overexpression of SRp40 or SRp75, but not SRp55, enhanced chondrogenic differentiation of chick limb mesenchymal cells in a high-density micromass assay. Overexpression of SRp40LF, which produces a truncated form of SRp40, also was strongly pro-chondrogenic. In a HeLa cell-based assay, SRp40LF fails to substitute for SRp40 in mediating an increase in exon EIIIA inclusion, suggesting that the latter event is not essential for the pro-chondrogenic effect. These results demonstrate the ability of these highly conserved splicing factors to modulate chondrogenesis and are consistent with earlier results that implicated exon EIIIA-containing isoforms of fibronectin in formation of chondrogenic condensations.

Fibroblast growth factor promotes the development of deep cerebellar nuclear neurons in dissociated mouse cerebellar cultures

Neurons of the deep cerebellar nuclei and excitatory cerebellar interneurons arise from the rhombic lip of the cerebellar anlage. In contrast, Purkinje cells and inhibitory interneurons arise in the neuroepithelium of the fourth ventricle. During development, the projection neurons of the cerebellar nuclei are born first (embryo age (E)9-E12 in mouse) followed closely by the Purkinje cells (E10-E13). Cerebellar interneurons arise later and differentiate postnatally. We have examined the development of cerebellar nuclear neurons in primary cultures. Embryonic cerebella from E15 to E18 pups were cultured 21 days in vitro. Three distinct classes of large neurons were identified: those expressing calbindin, typical of Purkinje cells; those expressing neurogranin (Golgi cells); and a third class expressing parvalbumin but not calbindin, consistent with the morphology of large projection neurons of the cerebellar nuclei. These neurons also express Tbr1, a specific antigenic marker of cerebellar nuclear neurons. Birthdating by using BrdU incorporation shows that the putative DCN neurons are not born in vitro. To confirm their identity the E18 cerebellum was dissected into cerebellar nuclear-containing (ventral) and -lacking (dorsal) halves, which were then dissociated and cultured separately. Only the ventral cultures produce putative cerebellar nuclear neurons. In contrast to E15-E18 cultures, dissociated E13-E14 cerebella in vitro do not yield putative cerebellar nuclear neurons. However, E14 cultures do produce them when fibroblast growth factors are added to the medium. We conclude that FGF signaling is required for the maturation of cerebellar nuclear neurons.

Role of fibroblast growth factor receptor signaling in kidney development

Fibroblast growth factor receptors (Fgfrs) are expressed in the ureteric bud and metanephric mesenchyme of the developing kidney. Furthermore, in vitro and in vivo studies have shown that exogenous fibroblast growth factors (Fgfs) increase growth and maturation of the metanephric mesenchyme and ureteric bud. Deletion of fgf7, fgf10, and fgfr2IIIb (the receptor isoform that binds Fgf7 and Fgf10) in mice lead to smaller kidneys with fewer collecting ducts and nephrons. Overexpression of a dominant negative receptor isoform in transgenic mice has revealed more striking defects including renal aplasia or severe dysplasia. Moreover, deletion of many fgf ligands and receptors in mice results in early embryonic lethality, making it difficult to determine their roles in kidney development. Recently, conditional targeting approaches revealed that deletion of fgf8 from the metanephric mesenchyme interrupts nephron formation. Furthermore, deletion of fgfr2 from the ureteric bud resulted in both ureteric bud branching and stromal mesenchymal patterning defects. Deletion of both fgfr1 and fgfr2 in the metanephric mesenchyme resulted in renal aplasia, characterized by defects in metanephric mesenchyme formation and initial ureteric bud elongation and branching. Thus, Fgfr signaling is critical for growth and patterning of all renal lineages at early and later stages of kidney development.

Fibroblast growth factors 1, 2, 17, and 19 are the predominant FGF ligands expressed in human fetal growth plate cartilage

Fibroblast growth factors (FGF) regulate bone growth, but their expression in human cartilage is unclear. Here, we determined the expression of entire FGF family in human fetal growth plate cartilage. Using reverse transcriptase PCR, the transcripts for FGF1, 2, 5, 8-14, 16-19, and 21 were found. However, only FGF1, 2, 17, and 19 were detectable at the protein level. By immunohistochemistry, FGF17 and 19 were uniformly expressed within the growth plate. In contrast, FGF1 was found only in proliferating and hypertrophic chondrocytes whereas FGF2 localized predominantly to the resting and proliferating cartilage. In addition, only the 18 kD isoform of FGF2 was found in resting chondrocytes while proliferating chondrocytes also synthesized 22 kD and 24 kD FGF2, similar to in vitro cultivated chondrocytes. In cell growth experiments, FGF1, 2, and 17 but not FGF19 inhibited the proliferation of FGFR3-expressing rat chondrosarcoma chondrocytes (RCS) with relative potency FGF2 >> FGF1 = FGF17. We conclude that FGF1, 2, 17, and 19 are the predominant FGF ligands present in developing human cartilage that are, with the exception of FGF19, experimentally capable of inhibiting chondrocyte proliferation.

Advances in understanding etiology of achondroplasia and review of management

PURPOSE OF REVIEW

A summary of management and current research in achondroplasia (OMIM 100800). The most common nonlethal skeletal dysplasia, achondroplasia presents a distinct clinical picture evident at birth. Substantial information is available concerning the natural history of this dwarfing disorder. Diagnosis is made by clinical findings and radiographic features. Characteristic features include short limbs, a relatively large head with frontal bossing and midface hypoplasia, trident hands, muscular hypotonia, and thoracolumbar kyphosis. Children commonly have recurrent ear infections, delayed motor milestones, and eventually develop bowed legs and lumbar lordosis. People with achondroplasia are generally of normal intelligence.

RECENT FINDINGS

The genetic cause of achondroplasia was discovered in 1994. Subsequent research efforts are designed to better characterize the underlying possible biochemical mechanisms responsible for the clinical findings of achondroplasia as well as to develop possible new therapies and/or improve intervention.

SUMMARY

Establishing a diagnosis of achondroplasia allows families and clinicians to provide anticipatory care for affected children. Although the primary features of achondroplasia affect the skeleton, a multidisciplinary approach to care for children with achondroplasia helps families and clinicians understand the clinical findings and the natural history of achondroplasia in order to improve the outcome for each patient.

Sustained thymopoiesis and improvement in functional immunity induced by exogenous KGF administration in murine models of aging

Age-related thymopoietic insufficiency has been proposed to be related to either defects in lymphohematopoietic progenitors or the thymic microenvironment. In this study, we examined whether keratinocyte growth factor (KGF), an epithelial cell-specific growth factor, could increase thymopoietic capacity in aged mice by restoration of the function of thymic epithelial cells (TECs). The thymic cellularity in KGF-treated aged mice increased about 4-fold compared to placebo-treated mice, resulting in an equivalent thymic cellularity to young mice. Enhanced thymopoiesis was maintained for about 2 months after a single course of KGF, and sustained improvement was achieved by administration of monthly courses of KGF. With the enhanced thymopoiesis after KGF treatment, the number of naive CD4 T cells in the periphery and T-cell-dependent antibody production improved in aged mice. KGF induced increased numbers of TECs and intrathymic interleukin-7 (IL-7) production and reorganization of cortical and medullary architecture. Furthermore, KGF enhanced thymopoiesis and normalized TEC organization in klotho (kl/kl) mice, a model of premature degeneration and aging, which displays thymopoietic defects. The result suggests that TEC damage is pathophysiologically important in thymic aging, and KGF therapy may be clinically useful in improving thymopoiesis and immune function in the elderly.

Progressive mRNA decay establishes an mkp3 expression gradient in the chick limb bud

The apical ectodermal ridge (AER) controls limb outgrowth and patterning, such that its removal causes changes in mesodermal gene expression, cell death and limb truncation. Fibroblast growth factor (FGF) family members are expressed in the AER and can rescue limb bud outgrowth after AER removal. Cells localized underneath the AER are maintained in an undifferentiated state by the FGFs produced by the AER. MAPK phosphatase 3 (mkp3) is a downstream effector of FGF8 signalling during limb bud development and is expressed in the distal limb mesenchyme. The present work evidences a gradient of mkp3 transcripts along the chick limb bud, in a distal to proximal direction. mkp3 transcription occurs only in the most distal limb bud cells and its mRNA gradient throughout the limb results from progressive mRNA decay. We show that FGF8-soaked beads induce ectopic mkp3 expression, indicating that AER-derived FGF8 protein may activate mkp3 in the distal mesenchyme.

Fibroblast growth factor 10 is required for survival and proliferation but not differentiation of intestinal epithelial progenitor cells during murine colon development

Epithelial-mesenchymal interactions that govern the development of the colon from the primitive gastrointestinal tract are still unclear. In this study, we determine the temporal-spatial expression pattern of Fibroblast growth factor 10 (Fgf10), a key developmental gene, in the colon at different developmental stages. We found that Fgf10 is expressed in the mesenchyme of the distal colon, while its main receptor Fgfr2-IIIb is expressed throughout the entire intestinal epithelium. We demonstrate that Fgf10 inactivation leads to decreased proliferation and increased cell apoptosis in the colonic epithelium at E10.5, therefore resulting in distal colonic atresia. Using newly described Fgf10 hypomorphic mice, we show that high levels of FGF10 are dispensable for the differentiation of the colonic epithelium. Our work unravels for the first time the pivotal role of FGF10 in the survival and proliferation of the colonic epithelium, biological activities which are essential for colonic crypt formation.

FGF-10 and its receptor exhibit bidirectional paracrine targeting to urothelial and smooth muscle cells in the lower urinary tract

Control of the regenerative properties of urothelial tissue would greatly aid the clinician in the management of urinary tract disease and disorders. Fibroblast growth factor 10 (FGF-10) is a mitogen which is particularly promising as a protein therapy for urothelial injury. The spatial synthesis, transport, targeting, and mechanistic pathway of FGF-10 and its receptor were studied in a human urothelial cell culture model and in fixed sections of lower urinary tract tissue. Synthesis of FGF-10 was restricted to mesenchymal fibroblasts, and secreted FGF-10 exhibited paracrine transport to two proximal sites, transitional epithelium and smooth muscle cell bundles, both of which were also the exclusive sites of FGF-10 receptor synthesis. The addition of recombinant FGF-10 to quiescent urothelial cells in vitro was sufficient to stimulate DNA synthesis. This stimulation was through a pathway independent of the epidermal growth factor receptor pathway. Deconvolution, light and transmission electron microscopic studies captured FGF-10 and its receptor in association with the urothelial cell surface, in cytoplasm, and within nuclei, observations that describe the mechanism that transduces the mitogenic signal in these tissues. Localization of the FGF-10 receptor to the superficial urothelial layer is clinically significant because intravesical administration of FGF-10 may provide the clinician a means to control the turnover of transitional epithelium in bladder disorders such as interstitial cystitis.

Identification of FGF-dependent genes in the Drosophila tracheal system

The embryonic development of the tracheal system of the fruit fly Drosophila provides a paradigm for genetic studies of branching morphogenesis. Efforts of many laboratories have identified Branchless (Bnl, a fibroblast growth factor homologue) and Breathless (Btl, the receptor homologue) as crucial factors at many stages of tracheal system development. The downstream targets of the Bnl/Btl signalling cascade, however, remain mostly unknown. Misexpression of the bnl gene results in specific tracheal phenotypes that lead to larval death. We characterised the transcriptional profiles of targeted over-expression of bnl in the embryonic trachea and of loss-of-function bnl(P1) mutant embryos. Gene expression data was mapped to high-throughput in situ hybridisation based ImaGO-annotation. Thus, we identified and confirmed by quantitative PCR 13 Bnl-dependent genes that are expressed in cells within and outside of the tracheal system.

Levels of mesenchymal FGFR2 signaling modulate smooth muscle progenitor cell commitment in the lung

Fibroblast growth factor (FGF) signaling has been shown to regulate lung epithelial development but its influence on mesenchymal differentiation has been poorly investigated. To study the role of mesenchymal FGF signaling in the differentiation of the mesenchyme and its impact on epithelial morphogenesis, we took advantage of Fgfr2c(+/Delta) mice, which due to a splicing switch express Fgfr2b in mesenchymal tissues and manifest Apert syndrome-like phenotypes. Using a set of in vivo and in vitro studies, we show that an autocrine FGF10-FGFR2b signaling loop is established in the mutant lung mesenchyme, which has several consequences. It prevents the entry of the smooth muscle progenitors into the smooth muscle cell (SMC) lineage and results in reduced fibronectin and elastin deposition. Levels of Fgf10 expression are raised within the mutant mesenchyme itself. Epithelial branching as well as epithelial levels of FGF and canonical Wnt signaling is dramatically reduced. These defects result in arrested development of terminal airways and an "emphysema like" phenotype in postnatal lungs. Our work unravels part of the complex interactions that govern normal lung development and may be pertinent to understanding the basis of respiratory defects in Apert syndrome.

FGF10 is required for cell proliferation and gland formation in the stomach epithelium of the chicken embryo

The development of digestive organs in vertebrates involves active epithelial-mesenchymal interactions. In the chicken proventriculus (glandular stomach), the morphogenesis and cytodifferentiation of the epithelium are controlled by the inductive signaling factors that are secreted from the underlying mesenchyme. Previous studies have shown that Fgf10 is expressed in the developing chicken proventricular mesenchyme, whereas its receptors are present in the epithelium. In our present study, we show that FGF10 is an early mesenchymal signal that is critically associated with the developmental processes in the proventricular epithelium. Furthermore, virus-mediated Fgf10 overexpression in ovo results in a hypermorphic epithelial structure and an increase in epithelial cell number. In contrast, the overexpression of a secreted FGFR2b (sFGFR2b), an FGF10 antagonist, blocks cell proliferation and gland formation in the proventricular epithelium in ovo. This downregulation of proliferative activity was subsequently found to retard gland formation and also to delay differentiation of the epithelium. These results demonstrate that FGF10 signaling, mediated by FGFR1b and/or FGFR2b, is required for proliferation and gland formation in the epithelium in the developing chick embryo.

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

Mutations in fibroblast growth factor receptors (Fgfrs) 1-3 cause skeletal disease syndromes in humans. Although these Fgfrs are expressed at various stages of chondrocyte and osteoblast development, their function in specific skeletal cell types is poorly understood. Using conditional inactivation of Fgfr1 in osteo-chondrocyte progenitor cells and in differentiated osteoblasts, we provide evidence that FGFR1 signaling is important for different stages of osteoblast maturation. Examination of osteogenic markers showed that inactivation of FGFR1 in osteo-chondro-progenitor cells delayed osteoblast differentiation, but that inactivation of FGFR1 in differentiated osteoblasts accelerated differentiation. In vitro osteoblast cultures recapitulated the in vivo effect of FGFR1 on stage-specific osteoblast maturation. In immature osteoblasts, FGFR1 deficiency increased proliferation and delayed differentiation and matrix mineralization, whereas in differentiated osteoblasts, FGFR1 deficiency enhanced mineralization. Furthermore, FGFR1 deficiency in differentiated osteoblasts resulted in increased expression of Fgfr3, a molecule that regulates the activity of differentiated osteoblasts. Mice lacking Fgfr1, either in progenitor cells or in differentiated osteoblasts, showed increased bone mass as adults. These data demonstrate that signaling through FGFR1 in osteoblasts is necessary to maintain the balance between bone formation and remodeling through a direct effect on osteoblast maturation.

Regulated expression of FLRT genes implies a functional role in the regulation of FGF signalling during mouse development

Within the mammalian genome, there are many multimember gene families that encode membrane proteins with extracellular leucine rich repeats which are thought to act as cell adhesion or signalling molecules. We previously showed that the members of the NLRR gene family are expressed in a developmentally restricted manner in the mouse with NLRR-1 being expressed in the developing myotome. The FLRT gene family shows a similar genomic layout and predicted protein secondary structure to the NLRRs so we analysed expression of the three FLRT genes during mouse development. FLRTs are glycosylated membrane proteins expressed at the cell surface which localise in a homophilic manner to cell-cell contacts expressing the focal adhesion marker vinculin. Each member of the FLRT family has a distinct, highly regulated expression pattern, as was seen for the NLRR family. FLRT3 has a provocative expression pattern during somite development being expressed in regions of the somite where muscle precursor cells migrate from the dermomyotome and move into the myotome, and later in myotomal precursors destined to migrate towards their final destination, for example, those that form the ventral body wall. FLRT3 is also expressed at the midbrain/hindbrain boundary and in the apical ectodermal ridge, regions where FGF signalling is known to be important, suggesting that the role for FLRT3 in FGF signalling identified in Xenopus is conserved in mammals. FLRT1 is expressed at brain compartmental boundaries and FLRT2 is expressed in a subset of the sclerotome, adjacent to the region that forms the syndetome, suggesting that interaction with FGF signalling may be a general property of FLRT proteins. We confirmed this by showing that all FLRTs can interact with FGFR1 and FLRTs can be induced by the activation of FGF signalling by FGF-2. We conclude that FLRT proteins act as regulators of FGF signalling, being induced by the signal and then able to interact with the signalling receptor, in many tissues during mouse embryogenesis. This process may, in part, be dependent on homophilic intercellular interactions between FLRT molecules.

The role of hormones, growth factors and their receptors in pituitary tumorigenesis

Numerous factors have been shown to govern adenohypophysial cell proliferation. Human and animal models have documented that the hypothalamic trophic hormone growth hormone-releasing hormone stimulates cell proliferation, and prolonged stimulation leads to tumor formation. Similarly, lack of dopaminergic inhibition of lactotrophs and lack of feedback suppression by adrenal, gonadal or thyroid hormones are implicated, perhaps through hypothalamic stimulatory mechanisms, in pituitary adenoma formation superimposed on hyperplasia. However, most pituitary tumors are not associated with underlying hyperplasia. Overexpression of growth factors and their receptors, such as EGF, TGFalpha, EGF-R and VEGF has been identified in pituitary adenomas, and reduction of follistatin expression has been implicated in gonadotroph adenomas. Aberrant expression of members of the FGF family, an FGF antisense gene and FGF receptors have all been described in pituitary adenomas. The clonal composition of pituitary adenomas attests to the molecular basis of pituitary tumorigenesis, however, the evidence suggests that these various hypophysiotropic hormones and growth factors likely play a role as promoters of tumor cell growth in genetically transformed cells.