Comparative analysis of the tissue distribution of three fibroblast growth factor receptor mRNAs during amphibian morphogenesis

An essential role for FGF receptor signaling in lens development

Since the days of Hans Spemann, the ocular lens has served as one of the most important developmental systems for elucidating fundamental processes of induction and differentiation. More recently, studies in the lens have contributed significantly to our understanding of cell cycle regulation and apoptosis. Over 20 years of accumulated evidence using several different vertebrate species has suggested that fibroblast growth factors (FGFs) and/or fibroblast growth factor receptors (FGFRs) play a key role in lens development. FGFR signaling has been implicated in lens induction, lens cell proliferation and survival, lens fiber differentiation and lens regeneration. Here we will review and discuss historical and recent evidence suggesting that (FGFR) signaling plays a vital and universal role in multiple aspects of lens development.

Possible involvement of a fibroblast growth factor 9 (FGF9)-FGF receptor-3-mediated pathway in adult pig retinal ganglion cell survival in vitro

The expression and potential roles of fibroblast growth factors (FGF) and their cognate FGF receptors (FGFR) in adult mammalian retinal ganglion cells (RGC) are poorly known. We show that FGFR-3 and FGFR-4 are especially pronounced on RGC and amacrine cell bodies in adult pig inner retinae both in vivo and in vitro. Western blotting revealed distinct profiles for each receptor. Expression of each FGFR and effects of the preferred ligand for FGFR-3, FGF9, upon RGC survival and neurite outgrowth were examined in primary retinal cell cultures: whereas there was no stimulation of neuritogenesis, RGC survival was promoted in a dose-dependent manner (ED(50) approximately 500 pg/ml, mean maximal increase of 60%) and could be completely blocked by addition of FGF9 neutralising antibody. Experiments with three additional FGF (FGF1, FGF2, and FGF4) showed no stimulation of RGC survival above control levels. Taken together, these data suggest that the ligand-receptor couple FGF9-FGFR-3 may function to promote survival of adult mammalian RGC, and their application might be beneficial in retinal degenerative diseases such as glaucoma.

The roles of Hedgehogs and Fibroblast Growth Factors in eye development and retinal cell rescue

Knowledge of normal eye development is crucial for the development of retinal rescue strategies. I shall focus on two signalling pathways that affect retinal development. Fibroblast growth factors function in retinal cell proliferation, retinal ganglion cell axon guidance and target recognition, craniofacial patterning and lens induction. Hedgehog proteins are required for progression of the neurogenic wave, cell proliferation, photoreceptor differentiation, retinal ganglion cell axon growth and craniofacial patterning. These signalling pathways have pleiotropic effects, can interact and have the potential to be used therapeutically. The zebrafish model organism may be well suited to studying how signalling pathways interact.

All for one and one for all: condensations and the initiation of skeletal development

Condensation is the pivotal stage in the development of skeletal and other mesenchymal tissues. It occurs when a previously dispersed population of cells gathers together to differentiate into a single cell/tissue type such as cartilage, bone, muscle, tendon, kidney, and lung and is the earliest stage during organ formation when tissue-specific genes are upregulated. We present a synopsis of our current understanding of how condensations are initiated and grown, how their boundaries and sizes are set, how condensation ceases, and how overt differentiation begins. Extracellular matrix molecules, cell surface receptors and cell adhesion molecules, such as fibronectin, tenascin, syndecan, and N-CAM, initiate condensation formation and set condensation boundaries. Hox genes (Hoxd-11-13) and other transcription factors (CFKH-1, MFH-1, osf-2), modulate the proliferation of cells within condensations. Cell adhesion is ensured indirectly through Hox genes (Hoxa-2, Hoxd-13), and directly via cell adhesion molecules (N-CAM and N-cadherin). Subsequent growth of condensations is regulated by BMPs, which activate Pax-2, Hoxa-2 and Hoxd-11 among other genes. Growth of a condensation ceases when Noggin inhibits BMP signalling, setting the stage for transition to the next stage of skeletal development, namely overt cell differentiation. BioEssays 22:138-147, 2000.

Expression of Msx genes in regenerating and developing limbs of axolotl

Msx genes, homeobox-containing genes, have been isolated as homologues of the Drosophila msh gene and are thought to play important roles in the development of chick or mouse limb buds. We isolated two Msx genes, Msx1 and Msx2, from regenerating blastemas of axolotl limbs and examined their expression patterns using Northern blot and whole mount in situ hybridization during regeneration and development. Northern blot analysis revealed that the expression level of both Msx genes increased during limb regeneration. The Msx2 expression level increased in the blastema at the early bud stage, and Msx1 expression level increased at the late bud stage. Whole mount in situ hybridization revealed that Msx2 was expressed in the distal mesenchyme and Msx1 in the entire mesenchyme of the blastema at the late bud stage. In the developing limb bud, Msx1 was expressed in the entire mesenchyme, while Msx2 was expressed in the distal and peripheral mesenchyme. The expression patterns of Msx genes in the blastemas and limb buds of the axolotl were different from those reported for chick or mouse limb buds. These expression patterns of axolotl Msx genes are discussed in relation to the blastema or limb bud morphology and their possible roles in limb patterning.

Fibroblast and epidermal growth factor receptor expression in Xenopus oocytes displays distinct calcium oscillatory patterns

Electrophysiological study performed with the voltage clamp technique was used to examine the intracellular calcium pathway activated by tyrosine kinase receptor members. Three FGF receptors from Pleurodeles PR1, PR3, PR4, homologs to human receptors, and the human EGF receptor were expressed in Xenopus oocytes. Under FGF1, FGF2 and FGF4 stimulation, PR1 and PR3 display a one phase inward chloride calcium dependent current superimposed by sustained oscillations, whereas PR4 did not show any oscillations. These currents were dependent on intracellular calcium mobilisation, as the responses were reduced by caffeine (10 mM). Solely PR4 responses were affected by an extracellular calcium depleted solution suggesting the involvement of concomitant extracellular and intracellular calcium intervention in the calcium chloride current, whereas PR1 and PR3 did not. Under EGF stimulation, the EGF receptor elicits a two component inward current composed of an undelayed rapid transient dependent on intracellular calcium store recruitment followed by a second slower current dependent on calcium influx. The specific pattern and amplitude of the calcium oscillations induced by the combinatorial action of growth factors on their receptors could be relevant in numerous calcium dependent cell functions.

Regulation of lens regeneration by fibroblast growth factor receptor 1

Lens regeneration in vivo is restricted to some urodeles only. After removal of the lens, this remarkable event is initiated from the dorsal iris. The pigmented epithelial cells from the dorsal iris dedifferentiate and subsequently transdifferentiate to form the regenerating lens. This property of the dorsal iris implies specific regulation along the dorsal-ventral axis. To date, no known genes are known to be specifically expressed in the dedifferentiating cells and to be involved in lens regeneration. In this paper, we show that FGFR-1 expression and function is correlated with the process of lens regeneration from the dorsal iris. Following lentectomy, FGFR-1 protein is specifically present in the dedifferentiating pigment epithelial cells in the dorsal iris, but is absent from the ventral iris. Subsequently, FGFR-1 protein is present throughout the process of lens regeneration and fiber differentiation. Furthermore, we show that an FGFR-1-specific inhibitor is able to inhibit the process of transdifferentiation and lens regeneration. In this sense, FGFR-1 can be regarded as the first known lens regeneration-associated factor.

Spatio-temporal expression of FGFR 1, 2 and 3 genes during human embryo-fetal ossification

Mutations in FGFR 1-3 genes account for various human craniosynostosis syndromes, while dwarfism syndromes have been ascribed exclusively to FGFR 3 mutations. However, the exact role of FGFR 1-3 genes in human skeletal development is not understood. Here we describe the expression pattern of FGFR 1-3 genes during human embryonic and fetal endochondral and membranous ossification. In the limb bud, FGFR 1 and FGFR 2 are initially expressed in the mesenchyme and in epidermal cells, respectively, but FGFR 3 is undetectable. At later stages, FGFR 2 appears as the first marker of prechondrogenic condensations. In the growing long bones, FGFR 1 and FGFR 2 transcripts are restricted to the perichondrium and periosteum, while FGFR 3 is mainly expressed in mature chondrocytes of the cartilage growth plate. Marked FGFR 2 expression is also observed in the periarticular cartilage. Finally, membranous ossification of the skull vault is characterized by co-expression of the FGFR 1-3 genes in preosteoblasts and osteoblasts. In summary, the simultaneous expression of FGFR 1-3 genes in cranial sutures might explain their involvement in craniosynostosis syndromes, whereas the specific expression of FGFR 3 in chondrocytes does correlate with the involvement of FGFR 3 mutations in inherited defective growth of human long bones.

Conservation of fibroblast growth factor function in lens regeneration

In urodele amphibians, lens induction during development and regeneration occurs through different pathways. During development, the lens is induced from the mutual interaction of the ectoderm and the optic vesicle, whereas after lentectomy the lens is regenerated through the transdifferentiation of the iris-pigmented epithelial cells. Given the known role of fibroblast growth factors (FGFs) during lens development, we examined whether or not the expression and the effects of exogenous FGF during urodele lens regeneration were conserved. In this paper, we describe expression of FGF-1 and its receptors, FGFR-2 (KGFR and bek variants) and FGFR-3, in newts during lens regeneration. Expression of these genes was readily observed in the dedifferentiating pigmented epithelial cells, and the levels of expression were high in the lens epithelium and the differentiating fibers and lower in the retina. These patterns of expression implied involvement of FGFs in lens regeneration. To further elucidate this function, we examined the effects of exogenous FGF-1 and FGF-4 during lens regeneration. FGF-1 or FGF-4 treatment in lentectomized eyes resulted in the induction of abnormalities reminiscent to the ones induced during lens development in transgenic mice. Effects included transformation of epithelial cells to fiber cells, double lens regeneration, and lenses with abnormal polarity. These results establish that FGF molecules are key factors in fiber differentiation, polarity, and morphogenesis of the lens during regeneration even though the regenerating lens is induced by a different mechanism than in lens development. In this sense, FGF function in lens regeneration and development should be regarded as conserved. Such conservation should help elucidate the mechanisms of lens regeneration in urodeles and its absence in higher vertebrates.

Ca2+ oscillations induced by fibroblast growth factor 2 in Xenopus oocytes expressing fibroblast growth factor receptors

Double electrode voltage clamp technique was used to follow precisely the calcium signalling pathway activated by FGF receptors from a normal and a carcinogenous cell environment. Functional FGF receptors were expressed in Xenopus oocytes following either the injection of PFR1 cRNA from Pleurodeles, an homologue of the human FGFR1 mRNA, or breast cancer MCF7 cells total mRNA. Cytosolic calcium oscillations were monitored through the endogenous Ca(2+)-dependent Cl- channel activity from both RNA injected systems, under FGF2 treatment. The Ca(2+)-dependent Cl- channel was demonstrated using the Cl- channel blocker SITS (250 microM) and by the determination of the reversal potential of the Cl- ions close to -20 mV. The FGF2-evoked Ca(2+)-dependent Cl- current was abolished by external application of genistein (10 microM, tyrosine kinase inhibitor), neomycin (10 mM, phosphatidylinositol turnover inhibitor), caffeine (10 mM, inhibitor of Ins(1,4,5)P3-mediated release of intracellular calcium), and injection of BAPTA (50 microM, calcium chelator) or heparin (2 micrograms/ml, inhibitor of the binding of Ins(1,4,5)P3). The recorded current was independent of extracellular Ca2+ but involved tyrosine kinase phosphorylation and intracellular Ins(1,4,5)P3 sensitive stores. External application of heparin enhanced the oscillatory Ca2+ rise, suggesting a role for the heparan sulfates in the regulatory mechanism of the FGF receptors. The similarities in the Ca(2+)-dependent Cl- current obtained in PFR1 and total MCF7 FGF receptors expressing oocytes are discussed.

Acidic and basic fibroblast growth factor messenger RNA and protein show increased expression in adult compared to developing normal and dystrophic rat retina

To further elucidate the possible roles of fibroblast growth factors (FGFs) in retinal pathophysiology, messenger RNA levels of acidic and basic FGF (aFGF and bFGF, respectively) were measured throughout embryonic and postnatal development until adulthood in normal and dystrophic (Royal College of Surgeons, RCS) rat retinas using sensitive reverse transcription-coupled polymerase chain reaction (PCR) techniques. In normal rats, both aFGF and bFGF transcript levels remained steadily low throughout embryogenesis and up until 7 d of postnatal age. By 13 d bFGF mRNA had increased 30-fold, and by adulthood (4 mo) levels were 150 times greater than in newborn retina. Dystrophic RCS retinas followed the same basic pattern, except that bFGF expression levels were increased relative to normal rats: By 4 d postnatal RCS retinas contained three times more bFGF mRNA than normal, by 7 d they contained six times more, and by 10 d they contained eight times more. In contrast, aFGF mRNA levels rose only threefold between embryonic and adult stages, and did not show any differences between normal and RCS rats. In parallel, staining of lightly fixed frozen sections of young (< 20 d) normal rat retina with antibodies to bFGF revealed only faint labeling of neural cells, whereas adult retinal sections were labeled strongly, especially within the photoreceptor layer. Twenty-day RCS rat retina showed detectable bFGF-like immunoreactivity. Hence, these data indicate that major aFGF and bFGF expression occurs only late in retinal maturation, suggesting these factors act principally as survival factors, especially for photoreceptors. In addition, the increased expression in a degenerative mutant strain may indicate the early onset of general cellular stress.

Fibroblast growth factor receptors and regeneration of the eye lens

If the eye lens of the adult newt, Notophthalmus viridescens, is removed, a new lens will regenerate and only from the dorsal, not the ventral, iris. The source, pigmented epithelial cells, would normally no longer divide, but upon lentectomy they do re-enter the cell cycle and form lens. The cause for this capability is unknown, but the mitogenic Fibroblast Growth Factors and their receptors may be involved. We have demonstrated that FGF receptors are present and operative in lens regeneration, since receptor-directed mitotoxins inhibit regeneration; heterogeneity and differential density in FGF-binding and receptor localization in iris sectors is also present. We propose that the spatial distribution of FGF receptors, especially the amphibian homolog of FGFR-3, is important in initiation of regeneration of eye lens.

A truncated FGF receptor blocks neural induction by endogenous Xenopus inducers

We have examined the role of fibroblast growth factor (FGF) signalling in neural induction. The approach takes advantage of the fact that both noggin and the dominant negative mutant activin receptor (delta1XAR1) directly induce neural tissues in the absence of dorsal mesoderm. A truncated FGF receptor (XFD) is co-expressed with noggin or delta1XAR1 in both whole embryos and isolated animal caps. We demonstrate that inhibition of FGF signalling prevents neural induction by both factors. Furthermore, neural induction by organizers (the dorsal lip of blastopore and Hensen's node) is also blocked by inhibiting FGF signalling in ectoderm. It has been proposed that the specification of anterior neuroectoderm, including the cement gland, occurs in a sequential manner as gastrulation proceeds. We show that the specification of the most anterior neuroectoderm by noggin may occur before gastrulation and does not require FGF signalling, since both the cement gland marker XCG-1 and the anterior neural marker Otx-2 are normally expressed in ectodermal explants co-injected with noggin and XFD RNAs, but the cement gland cells are poorly differentiated. In contrast, the expression of both genes induced by CSKA.noggin, which is expressed after the mid-blastula transition, is strongly inhibited by the presence of XFD. Therefore the noggin-mediated neural induction that takes place at gastrula stages is abolished in the absence of FGF signalling. Since inhibition of FGF signalling blocks the neuralizing effect of different neural inducers that function through independent mechanisms, we propose that FGF receptor-related-signalling is required for the response to inducing signals of ectodermal cells from gastrula.

Changes in the expression of fibroblast growth factor receptors mark distinct stages of chondrogenesis in vitro and during chick limb skeletal patterning

Members of the fibroblast growth factor (FGF) family of growth factors are key regulators of limb skeletal patterning and growth. Abnormal expression of FGFs or mutations in their receptors (fgfrs) result in skeletal disorders. Here we show that changes in the expression of fgfrs are intrinsic properties of differentiating cartilage. In mesenchymal micromass cultures differentiating into cartilage, as in ovo, fgfr 1 mRNA was found predominantly in undifferentiated, proliferating mesenchyme, fgfr 2 in precartilage cell aggregates, and fgfr 3 in differentiating cartilage nodules. Thus, our data suggest that switches in the expression of fgfr 1, 2, and 3 mRNAs are associated with phases of cartilage patterning both in vitro and in ovo, and mark distinct stages in the development of the limb skeleton.