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8/10/2019 Functions and Regulations of Fibroblast Growth Factor Signaling During
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Functions and regulations of fibroblast growth factor signaling during
embryonic development
Bernard Thisse, Christine Thisse *
Institut de Genetique et de Biologie Moleculaire et Cellulaire, UMR 7104, CNRS/INSERM/ULP, 1 rue Laurent Fries, BP 10142, CU de Strasbourg,
67404 ILLKIRCH cedex, France
Received for publication 16 June 2005, revised 29 July 2005, accepted 5 September 2005
Available online 10 October 2005
Abstract
Fibroblast growth factors (FGF) are secreted molecules which function through the activation of specific tyrosine kinases receptors, the FGF
receptors that transduce the signal by activating different pathways including the Ras/MAP kinase and the phospholipase-C gamma pathways.
FGFs are involved in the regulation of many developmental processes including patterning, morphogenesis, differentiation, cell proliferation or
migration. Such a diverse set of activities requires a tight control of the transduction signal which is achieved through the induction of different
feedback inhibitors such as the Sproutys, Sef and MAP kinase phosphatase 3 which are responsible for the attenuation of FGF signals, limiting
FGF activities in time and space.
D 2005 Elsevier Inc. All rights reserved.
Keywords: FGF; FGF receptors; Ras/MAP kinase; HSPG; Sprouty; Sef; MKP3
Introduction
Fibroblast growth factors represent a large family of secreted
molecules. Upon binding to their cognate receptors, FGFs
activate signal transduction pathways which are required for
multiple developmental processes. The wide-ranging biological
roles of FGFs, the variety of signaling pathways activated and
the complex and dynamic expressions of FGF ligands and
receptors imply that FGF signaling must be tightly regulated.
The scope of the present review is to provide readers with
updated information about FGF signaling. After a short
introduction on FGFs evolutionary history and structural
characteristics and on FGF receptors (FGFRs) particularities,various functions of the FGF signaling will be presented,
focusing on their implication in development. The last part of
the review will describe the attenuation of FGF signaling mainly
dependent on the Ras/MAP kinase signaling pathway, through
the action of different factors, such as the Sproutys, Sef or MAP
kinase phosphatase 3. Recent publication of various studies has
led to controversial results and we will present the raising
debate on how FGF can be regulated and on how multiple
feedback regulators of the FGF pathway act at different levels ofthe signal transduction cascades to attenuate FGF signaling.
Key structural properties of FGFs and FGF Receptors
FGFs represent an extended family of polypeptide growth
factors found through evolution, ranging from nematode to
human whereas they have not been identified in unicellular
organisms (for review,Itoh and Ornitz, 2004). Ininvertebrates,
only three Drosophila genes (branchless,pyramus and thisbe)
have been found and two (egl-17 and let-756) in Caenorhab-
ditis elegans. In contrast, in vertebrates, a large number ofFGF
genes has been identified: 10 FGFs in zebrafish (FGF2 4, 6,8,10 ,17a,17b,18,24), 6 inXenopus(FGF2 4,810), 13 in
chicken (FGF1 4, 810, 12, 13, 16, 1820), 22 genes in
mouse (FGF1 18, 2023) and human (FGF1 14, 1623).
FGFgenes are scattered throughout the genome, indicating
that the FGF gene family was generated both by gene and
chromosomal duplication and translocation during evolution
(for reviewOrnitz and Itoh, 2001).
All FGFs share an internal core of similarity as well as a
high affinity for heparin. FGFs act through binding and
activation of FGFRs which are tyrosine kinase receptors that
contain three immunoglobulin-like domains and a heparin-
0012-1606/$ - see front matterD
2005 Elsevier Inc. All rights reserved.doi:10.1016/j.ydbio.2005.09.011
* Corresponding author.
E-mail address: [email protected] (C. Thisse).
Developmental Biology 287 (2005) 390 402
www.elsevier.com/locate/ydbio
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binding sequence (Lee et al., 1989; Johnson et al., 1990,Fig.
1). FGFR forms are expressed in two possible ways: by the
expression of splice variants of a given FGFR gene or by the
expression of different FGFR genes (Johnson and Williams,
1993). Alternative splicing specifies the sequence of the
carboxy-terminal half of the Ig domain III, resulting in either
IIIb or IIIc isoforms (Miki et al., 1992; Chellaiah et al., 1994).
This alternative splicing event is regulated in a tissue-specific
manner and dramatically affects ligand receptor binding
specificity (seePowers et al., 2000).
Fig. 1. FGF receptors and FGF signal transduction. FGFRs are modular proteins comprising 3 immunoglobulin domains (IgI, IgII and IgIII). IgI and IgII are
separated by an acidic box (AD). IgII contains a heparin binding domain (HBD). The IgIII domain is followed by a unique transmembrane (TM), a juxtamembrane
(JM) and a kinase domain (KD) interrupted by an interkinase domain (IKD). FGF ligands linked to heparin sulfate proteoglycan (HSPG) bind to IgII and IgIII of
FGFR. This results in the dimerization and the subsequent transactivation by phosphorylation of specific tyrosine residues. The main two transduction pathways
involve the phospholipase C-g(PLCg) and the Ras/MAP kinase. The SH2 domain of the PLCginteracts with the phosphorylated Y766 of the activated receptor. The
activated PLCg hydrolyzes the phosphatidyl-inositol-4,5-diphosphate (PIP2) to inositol-1,4,5-triphophate (IP3) and the diacylglycerol (DAG). IP3 releases Ca 2+
while DAG activates the protein kinase C-y (PKCy). Activated PKCy activates Raf by phosphorylating its S338 and stimulates the downstream pathway in a Ras
independent manner. The main pathway involves the interaction of the docking protein FRS2awith the amino-acid residues 407 433 (Xu et al., 1998). This protein
is activated by phosphorylation on multiple tyrosine residues and subsequently interacts and activates Grb2 linked to Sos, a nucleotide exchange factor involved in
the activation of Ras. Activated Ras then activates Raf which stimulates MEK which in turn phosphorylates the MAP kinase ERK. This last activated component
translocates to the nucleus and phosphorylates specific transcription factors of the Ets family which in turn activate expression of specific FGF target genes. P:phosphorylation.
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Role of Heparin Sulfate Proteoglycans in FGF ligand
binding
In addition to high affinity for their receptors, members of
the FGF family have strong affinity for heparin (Burgess and
Maciag, 1989). FGF FGFR interaction and signaling are
further regulated by the spatial and temporal expression ofendogenous heparan sulfate proteoglycan (HSPG). HSPGs are
cell-surface and extracellular matrix macromolecules that
comprise a core protein to which heparan sulfate (HS)
glycosaminoglycan (GAG) chains are attached. HSPGs play
crucial roles in regulating key developmental pathways such as
the FGF signaling (for review, Lin, 2004). The genetic
evidences of such a role for HSPGs come from the isolation
of mutants, both in invertebrates and vertebrates that show
defective FGF signaling (Lin et al., 1999; Nybakken and
Perrimon, 2002). Biochemical (Nakato and Kimata, 2002) and
cristallographic studies (Schlessinger et al., 2000; Pellegrini et
al., 2000) have revealed the importance of 60 sulfation of HSfor FGF signaling which appears to be required for FGF1
FGFR2 and FGF2FGFR1 interactions. HSPGs were found to
directly interact with FGFs and their receptors in a ternary
complex on the cell surface (Ornitz, 2000; Pellegrini, 2001).
One possibility could be that Heparin/HSPGs within the FGF
signaling complex facilitate the FGFFGFR interaction. The
second possibility could be that Heparin/HSPGs enhance the
stability of a FGF FGFR complex. Examination of the
spatiotemporal changes in HS to promote complexes formation
with FGFRs and FGFs led to the finding that these changes
depend on glycosyltransferases and sugar sulfotransferases.
Furthermore, for each FGF FGFR pair examined, unique
developmental HS binding patterns were identified that
correlate with different HS binding requirements, as well as
variations in FGF signaling. These results suggested that each
FGFFGFR combination seeks distinct HS domains that are
spatially and temporally regulated during development. These
domains are unique to HS and distinguish it from heparin,
which is highly sulfated and lacks any domain structure,
explaining the ability of these FGFFGFR pairs to interact
equally with heparin in vitro. Importantly, the HS activity
necessary for ternary complex assembly does not represent the
additive binding requirements of individual FGFs or FGFRs;
rather, it represents requirements dictated by the synergistic
interaction of the FGFs, HS and FGFRs. Finally, given that HSspecifically mediates each FGFFGFR interaction examined,
these results suggest that developmental changes in HS may
also specifically modulate the signaling of other families of
morphogens and growth factors (Allen and Rapraeger, 2003).
In addition, these interactions limit FGF diffusion and release
into interstitial spaces, regulating FGF activity in a given tissue
(Moscatelli, 1987; Flauennhaft et al., 1990).
FGF transduction pathway
FGFR transmits extracellular signals to various cytoplasmic
signal transduction pathways through tyrosine phosphorylation
(Fig. 1). FGFRs exist as inactivated monomers, which are
activated when two FGF molecules connected by a heparan
sulfate proteoglycan bind to the extracellular IgII and IgIII
domains of the receptor leading to its homodimerization
(Schlessinger et al., 2000). This dimerization brings together
the intracellular domain of the receptor leading to transauto-
phosphorylation of several critical tyrosine residues. Activation
of the receptor allows proteins containing Src homology (SH2)or phosphotyrosine binding (PTB) domains to bind to sequence
recognition motifs in FGFR1, resulting in phosphorylation and
activation of these proteins (Pawson, 1995; Forman-Kay and
Pawson, 1999; Dhalluin et al., 2000). For example, the
Phospholipase C-g (PLCg) was identified to be associated
with FGFR following ligand-dependent activation. This asso-
ciation is due to binding between the SH2 domain of PLCgand
Tyr 766 of FGFR1. Activated PLCg can hydrolyze phospha-
tidylinositol-4,5-diphosphate (PIP2) to inositol-1, 4, 5-triphos-
phate (IP3) and diacylglycerol (DAG). IP3 induces Ca2+
release from intracellular stores, whereas DAG is a protein
kinase C activator. The activated protein kinase C-y (PKCy) isable to phosphorylate and to stimulate Raf therefore leading to
activate the MAP kinase pathway in a Ras-independent manner
(Ueda et al., 1996).
The main signaling activated through the stimulation of
FGFRs is the Ras/MAP kinase pathway. Links between
elements of this pathway and the activated FGFR involve the
interaction of the juxtamembrane domain of FGFR with the
FGFR substrate 2a (FRS2a) which is a membrane-anchored
docking protein. FRS2 lacks an SH2 domain, but contains a
phosphotyrosine binding domain (Kouhara et al., 1997), which
was shown to mediate phosphotyrosine-independent interac-
tion with amino-acid residues 407433 in FGFR1. Activation
of FGFR1 leads to tyrosine phosphorylation of FRS2 at several
sites. This allows the binding of Grb2, a small adaptor
molecule, which exists in complex with the nucleotide
exchange factor Sos and is involved in the activation of the
GTP-binding protein Ras. In this complex, Sos catalyzes the
exchange of GDP for GTP on Ras, promoting Ras activation as
well as downstream elements of the pathway, comprising Raf,
MEK and MAP kinases (ERK1, 2), the last member of which
finally enters the nucleus and phosphorylates target transcrip-
tion factors (Sternberg and Alberola-Ila, 1998). Among these
are the Ets domain containing factors ERM and PEA3
(Wasylyk et al., 1998; Sharrocks, 2001). They belong to a
family of transcription factors, which share a winged-helixloophelix domain with which they bind to DNA as
monomers. They are able to form heterocomplexes with
transcription factors and are activated most prominently by
the MAP kinase(Wasylyk et al., 1998).
A study performed during the course of somite development
revealed the contribution of these Ets-domain containing
proteins activated by FGFs. The FGF signal secreted from
the myotome induces formation of a scleraxis (Scx)-expressing
tendon progenitor population in the sclerotome. Brent and
Tabin (2004) showed that transcriptional activation by Pea3
and Erm in response to FGF signaling is both necessary and
sufficient for Scx expression in the somite. They proposed that
the domain of the somitic tendon progenitors is regulated both
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by the restricted expression of Pea3 and Erm, and by the
precise spatial relationship between these Ets transcription
factors and the FGF signal originating in the myotome.
Implication of FGF signaling during development
FGFs were described to be implicated in multiple roles suchas patterning, morphogenesis, differentiation, regulation of cell
proliferation or migration (for review see Goldfarb, 1996;
Powers et al., 2000; Coumoul and Deng, 2003). In the next
paragraphs, we will illustrate major functions of FGF signaling
during development.
Early patterning and dorso-ventral axis formation
Induction and patterning of mesoderm is one of the earliest
events in which FGF signaling was revealed to be essential.
The first mesodermal inducer identified was basic FGF (Slack
et al., 1987), for which its role is evolutionarily conserved.The effect of FGF on mesoderm formation was initially
studied in Xenopus in which not only FGF was described to
have a mesoderm inducing capacity (Slack et al., 1987;
Kimelman and Kirschner, 1987; Kimelman et al., 1988) but
components of the FGF signaling pathway such as FGFRs,
HSPGs, FRS2, Grb2 and the Ras downstream cascade, when
inhibited, were shown to block mesoderm formation and to
induce both gastrulation and posterior defects (Whitman and
Melton, 1992; MacNicol et al., 1993; Umbhauer et al., 1995;
LaBonne, 1995; Gotoh, 1995).
Involvement of FGFs in the establishment of the dorso-
ventral (D/V) axis has been extensively studied using the
zebrafish model. Genetic analysis revealed that BMP2b/BMP7
heterodimers (Schmid et al., 2000)act as morphogens secreted
ventrally and are responsible for the specification of ventral cell
fates. The expression of BMPgenes, initially activated in the
whole blastula becomes progressively restricted ventrally. This
ventral restriction coincides with the spreading of FGF activity
from the dorsal side of the embryo, suggesting an implication
of FGF signaling in the dorsal downregulation of BMP gene
expression. Consistently, general activation of the FGF/Ras/
MAP kinase signaling pathway inhibits BMP gene expression
in the whole blastula. Conversely, inhibition of FGF signaling
causes BMPgene expression to enlarge dorsally, leading to an
expansion of ventral cell fates. Altogether, this shows that FGFsignaling is essential to delimit the expression domain ofBMPs
in blastula stage embryos. Therefore, FGFs act upstream of the
ventral morphogens and function as initial signals for the
establishment of the D/V patterning (Furthauer et al., 2004).
FGF signaling was also identified to be implicated in
ventro-posterior mesodermal tissue maintenance. Consistently,
blocking FGF signaling by dominant-negative FGFR results in
deletions of tail and trunk structures (Amaya et al., 1991;
Griffin et al., 1995). As well, targeted disruption of mouse
FGFR1 leads to the disorganization of axial mesoderm and to a
defective development of the posterior structures. FGFs control
the specification and maintenance of mesoderm by regulation
of T box genes (Griffin et al., 1995; Smith et al., 1991; Strong
et al., 2000; Sun et al., 1999; Ciruna and Rossant, 2001; Griffin
et al., 1998; Zhao et al., 2003). One FGF mesodermal target is
brachyury, required for posteriormesoderm and axis formation
in mouse, zebrafish andXenopus(Smith et al., 1991; Herrmann
et al., 1990; Halpern et al., 1993; Conlon et al., 1996). In
Xenopus, Brachyury induces eFGF, which is required for
myogenic celllineage (Stanley et al., 2001; Fisher et al., 2002)and vice versa (Isaacs et al., 1994; Schulte-Merker and Smith,
1995).
Role of FGFs on cell movements
Evidence has been provided that FGF signaling plays a
direct role in cell movements during convergence extension
(Nutt et al., 2001). In the mouse, FGF signaling may affect
gastrulation movements via the transcription factor snail
(Ciruna and Rossant, 2001). FGFR1 mutant embryos die at
late gastrulation, showing defects in cell migration, cell fate
specification and patterning. FGFR1 mutant cells may fail tomigrate properly because they maintain high levels of E-
cadherin resulting from a snail downregulation. An additional
study performed in the mouse revealed that, in the absence of
both FGF8 and FGF4, epiblast cells move into the streak and
undergo an epithelial-to-mesenchymal transition; however,
most cells then fail to move away from the streak. As a
consequence, no embryonic mesoderm- or endoderm-derived
tissues develop demonstrating that signaling via FGF8 and/or
FGF4 is required for cell migration away from the primitive
streak(Sun et al., 1999).
Moreover, there is an increasing evidence for a direct
chemotactic response of migrating cells in response to FGF
signals. As such, FGF2 and FGF8 are potent chemoattractants
in migration of mesencephalic neural crest cells (Kubota and
Itoh, 2000) and FGF2 and FGF4 attract mesenchymal cells
during limb bud development in the mouse(Webb et al., 1997;
Li and Muneoka, 1999). Also, FGF10 exerts a potent
chemoattractive effect to direct lung distal epithelial buds to
their destination(Park et al., 1998). Furthermore, FGFs can act
as chemotactic signals to coordinate cell movements during
gastrulation. In the chick, cells can be guided by environmental
signals and beads coated with FGFs alter their trajectories. The
observed movement patterns of anterior streak cells can be
explained by an FGF8-mediated chemorepulsion of cells away
from the streak followed by chemoattraction towards an FGF4signal produced by the forming notochord(Yang et al., 2002).
Neural induction and FGF signaling
Several studies on neural induction have led to the ?default
modelX which proposes that ectodermal cells are fated to
become neural by default. In this model, cells are normally
inhibited from a neural fate by BMPs that are expressed in the
ectoderm, from which they must be released for neural
induction to occur (for reviews, Hemmati-Brivanlou and
Melton, 1997; Weinstein and Hemmati-Brivanlou, 1999).
Additional data have suggested that FGF signaling plays a
crucial role for an early step of neural induction in the chick. In
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this specie, induction of neural tissue precedes the formation of
the Hensens node. FGFs are sufficient to induce preneural
markers and inhibition of FGF signaling blocks neural
induction by Hensens node (Streit et al., 2000; Wilson et al.,
2000; Alvarez et al., 1998; Storey et al., 1998). FGF signals
repress BMP expression which leads to induction of neural
tissue. On the contrary, when FGFs are inhibited, BMPexpression is maintained and neural fate is blocked (Wilson
et al., 2000). FGFs may block BMP signaling during neural
induction of the chick by inducing Churchill which is required
for the expression of Smad-interacting protein 1 (Sip1) in the
neural plate. Sip1 binds and represses Smad1/5 blocking
therefore BMP signaling (Sheng et al., 2003).
In Xenopus, the role of FGF signaling in neural induction
has also been intensively studied. However, its implication in
neural induction has led to conflicting results. Several
investigators suggested that FGF signaling is required directly
for neural induction (Launay et al., 1996; Hongo et al., 1999;
Streit et al., 2000; Bertrand et al., 2003). In most cases,however, experiments were performed in a background where
the BMP signaling was partially attenuated (Kengaku and
Okamoto, 1995; Lamb and Harland, 1995; Ribisi et al., 2000).
In addition, several studies using dominant negative FGFR1
have led to different results showing that neural tissue forms
even when FGF signaling is inhibited (Kroll and Amaya, 1996;
Ribisi et al., 2000; Barnett et al., 1998; McGrew et al., 1997;
Holowacz and Sokol, 1999). It appears that FGF signaling
required for anterior neural development may be mediated
through FGFRs other than FGFR1, such as through FGFR4a
(Hongo et al., 1999).
In contrast, other studies report a requirement for FGFs in
the antero-posterior patterning rather than the initial induction
of the neural plate(Holowacz and Sokol, 1999). FGFs are able
to change anterior neural fate to more posterior neural cell
types (Umbhauer et al., 2000; Lamb and Harland, 1995; Ribisi
et al., 2000; Holowacz and Sokol, 1999; Cox and Hemmati-
Brivanlou, 1995). This may thus involve interactions with the
Wnt signaling pathway (McGrew et al., 1997; Kazanskaya et
al., 2000; Domingos et al., 2001; Kudoh et al., 2002).
Finally, a controversial study provides evidence that BMP
inhibition is required in the chick, but only as a relatively late
step. BMP inhibition would not be sufficient to cause
competent cells to acquire neural fates either in chick or in
Xenopus. In the frog, FGF synergizes with BMP inhibition toinduce neural markers. In chick, inhibition of BMP signaling
with Wnt antagonists and/or FGF is not sufficient for neural
induction. Neural induction would not occur by default but
would rather involve a succession of signaling events with
some players which would remain to be identified(Linkerand
Stern, 2004).
Midbrain hindbrain patterning
Early patterning of the vertebrate presumptive midbrain and
rhombomere 1 (rh1) is regulated by a local organizer located at
the mid/hindbrain junction(Liu and Joyner, 2001), a territory
expressing FGF8. This factor has organizer properties and is
required for midbrain and cerebellum development(Meyers et
al., 1998, Reifers et al., 1998). FGF17 and FGF18 are also
expressed in the mid/hindbrain junction in broader domains
than FGF8, including posterior midbrain(Maruoka et al., 1998;
Xu et al., 1998). Loss-of-function of FGF17 in the mouse
results in truncation of the posterior midbrain and reduced
proliferation of the anterior cerebellum. Removal of one copyof fgf8 in a fgf17 mutant background leads to exaggerated
cerebellum phenotype(Xu et al., 2000). To complicate matters,
fgf8 is differentially spliced to generate FGF8a and FGF8b
isoforms. FGF8a misexpression causes expansion of the
midbrain and FGF8b transforms the midbrain into cerebellum.
The initial effect of FGF8b is to reduce growth of the midbrain.
Therefore, FGF8a and FGF8b have distinct activities, both on
growth and regulation of gene expression (reviewed in Liu and
Joyner, 2001; Sato et al., 2001).
Interactions of FGF8a and FGF8b with FGF17 and FGF18
have been studied. It appears that FGF8b after being induced in
the presumptive rh1 territory induces FGF18 in surroundingtissue, further extending the gradient of FGF RNA expression.
FGF8b maintains two negative feedback loops by inducing the
expression of the negative feedback FGF inhibitors Spry1 and
Spry2 and repressing FGFR2 and FGFR3 (Liu et al., 2003).
FGF17, FGF18 proteins and possibly FGF8a as well as a low
level of FGF8b then regulate proliferation of the midbrain and
cerebellum(Liu et al., 2003; Chi et al., 2003).
Limb induction and morphogenesis
The role of FGFs has been established in induction,
initiation and maintenance of limb development (reviewed in
Martin, 1998). Limb bud initiation is triggered by FGF8
inducing the expression of FGF10. FGF10 then in turn induces
FGF8 in ectodermal cells resulting in the formation of the
apical ectodermal ridge (AER). FGFs from the AER maintain
cell proliferation in the progress zone (a population of
undifferentiated mesenchymal cells located near the distal tip
of the limb bud, adjacent to the AER). FGF2 (Fallon et al.,
1994), FGF4 (Laufer et al., 1994)and FGF8(Crossley etal.,
1996) induce sonic hedgehog (shh) expression in the zone of
polarizing activity (ZPA). Outgrowth and patterning of the limb
result from the combined effects of FGF and Shh and
regulation of various genes, including members of the Hox
family (reviewed inMartin, 1998).A first model proposed for explaining the patterning of the
limb development along the proximaldistal (PD) axis is the
progress zone model(Summerbell et al., 1973) which explains
that as the limb grows, cells are pushed out of the progress zone
and become fixed in the positional value that they acquired.
Cells that leave the proximal zone early will adopt a proximal
fate and cells that leave the progress zone late will form distal
elements (Sauders, reviewed in Niswander, 2003). The
conclusion from this model is that the PD fate is specified
progressively. Recent studies have provided an alternative
model on how the AER may influence the PD pattern. This
second model proposes that all PD fates are already specified
within the early limb bud rather than progressively (Dudley et
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al., 2002). In the absence of FGF8 in mice, the proximal
element of the limb is reduced or even lost. However, the distal
elements are formed quite normally because other FGFs are
activated (Lewandoski et al., 2000; Moon and Capecchi, 2000).
These results of loss of proximal but not of distal elements
cannot be interpreted within the progress zone model. In
addition, new experimental data have proposed that FGFsfunction is first to influence the initial size of the limb (Sun et
al., 2002). In FGF8 mutant mice, early limb bud is smaller.
This phenotype is detected right after FGF8 expression is
detectable, excluding the possibility that FGF8 alters cell
proliferation or cell death. Instead, FGF8 would (by affecting
morphogenetic movements or cell adhesion) influence the
number of cells in the limb bud by preventing their death. In
mutant limbs, death of proximal but not distal mesenchyme can
be observed. The PD precartilage condensations are formed but
smaller. Sun et al. (2002)proposed that the AER generates an
adequate number of mesenchymal cells for correctly sized
condensations to form. When the AER is disrupted, the numberof skeletal elements is decreased resulting in smaller or in
absence of condensations arguing that the FGFs function is to
control mesenchymal cell number but not the PD patterning.
Bone formation
The importance of FGF signaling in skeletal development
was first revealed with the discovery that a point mutation in
the transmembrane domain of FGFR3 is the etiology of
achondroplasia, the most common genetic form of dwarfism
in human (Rousseau et al., 1994; Shiang et al., 1994). A major
role of FGFs and FGFRs has then been scored by finding
missense mutations resulting in more than 15 human disorders,
ranging from skeletal dysplasias, craniosysnostosis syndromes
or short stature (for reviewCoumoul and Deng, 2003). In the
bone, FGF2 and FGF9 transcripts are found in mesenchymal
cells and osteoblats (Gonzalez et al., 1996; Kim et al., 1998).
FGF18 is expressed in mesenchymal cells, in differentiating
osteoblasts during calvarial bone development and in the
perichondrium of long bones (Ohbayashi et al., 2002). The
action of FGFs is dependent on the spatiotemporal pattern of
expression of FGFRs like FGFR1 and FGFR2 which are
located in mesenchymal cells during condensation of mesen-
chyme prior to deposition of bone matrix at early stages of long
bone development and in the cranial sutures (Orr-Urtreger etal., 1991; Delezoide et al., 1998). Later, they are found in
preosteoblasts and osteoblasts together with FGFR3 (Molteni
et al., 1999). Overexpression of FGF2 in mouse induces
abnormal bone formation (Coffin et al., 1995)whereas its loss-
of-function leads to inhibition of bone formation (Monteroet
al., 2000). Mice lacking FGF18 display ossification and cranial
sutures closure delay (Liu et al., 2002; Ohbayashi et al., 2002).
Altogether, FGF signaling appears to regulate cell proliferation
and differentiation positively in membranous and long bone
osteogenesis. FGF2 was shown also to act in vivo on osteoblast
precursor cells replication to increase osteoblast number. FGF
signaling also controls genes involved in osteoblast apoptosis.
High FGF signaling may reduce apoptosis of immature
osteoblasts and thereby enhance the osteoblast population,
whereas continuous signaling may promote apoptosis in more
mature osteoblasts, therefore limit the early increase in the
osteoblasts pool. FGFs interact with other growth factors
signaling pathways to regulate osteoblast function. For
example, FGF2 and FGFR2 inhibit the expression of the
BMP antagonist Noggin in the cranial sutures, resulting inincreased BMP4 activity and suture fusion (Warren et al.,
2003). Therefore, FGF signaling can control cranial suture
fusion indirectly through BMP signaling. FGFs regulate also
genes involved in matrix degradation(Kawaguchi et al., 1995)
and induce the expression of tissue inhibitors of metallopro-
teinases (Delany and Canalis, 1998; Varghese et al., 1995)
indicating that FGFs (mainly FGF2) may modulate bone matrix
proteolysis by regulating collagenase activity. Overall, activa-
tion of FGF signaling isable to regulate genes involved at all
steps of osteogenesis (Fig. 2). This array of genes may
participate in the regulation of cell progression from osteopro-
genitor cells to the end of the osteoblast life (for review, Marie,2003). Further studies have led to the identification of signaling
pathways that are involved in FGF signaling in osteoblasts,
mainly MAP kinase signal transduction cascade(Mansukhani
et al., 2000; Debiais et al., 2001).
Attenuation of FGF signaling
The multiple range of biological effects of FGFs and the
variety of signaling pathways activated by this family of
ligands imply that FGF signaling must be tightly regulated
regarding timing, duration and spread of the signal. This can be
achieved by both positive and negative feedback loops. FGF
signaling can be regulated at many levels in the extracelular
space and within the cell. A growing number of proteins have
been identified that specifically regulate the activity of FGF
signaling pathways through FGFRs. Many of these regulatory
factors belong to the FGF synexpression group (Furthauer et
al., 2002; Niehrs and Meinhardt, 2002; Tsang et al., 2002). Not
only are these factors coexpressed with FGFs but they are
themselves regulated by FGF signaling and inhibit FGF
signaling by establishing a negative feedback loop.
The Sprouty proteins
The first feedback regulator of the FGF pathway isolated isSprouty (Spry). Spry was originally identified as an antagonist
of FGF signaling that patterns apical branching in Drosophila
tracheae (Hacohen et al., 1998). In spry mutants, ectopic
airway branches are induced due to excess of FGF signaling.
Spry was then described to be a general inhibitor of receptor
tyrosine kinase (RTK) signaling (for review,Kim and Bar-Sagi,
2004). Spry proteins are evolutionarily conserved with four
members (Spry1, 2, 3, 4) identified in mammals (Dikic and
Giordano, 2003). These proteins share a conserved carboxy-
terminal cystein-rich domain, necessary for their specific
localization and function. Their amino-terminal part is diver-
gent except that they possess an invariant tyrosine phospho-
rylation site. It is thought that this sequence divergence dictates
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various functions by mediating different protein protein
interactions. Gain- and/or loss-of-functions studies in the
mouse, Xenopus and zebrafish (Minowada et al., 1999;
Mailleux et al., 2001; Nutt et al., 2001; Furthauer et al.,
2001, 2004) have demonstrated that Sprys antagonize FGF
signaling.Spryexpression is induced by the FGF signaling and
has various biological consequences, including regulation of
cell proliferation and differentiation, endocytic sorting of
activated RTKs, control of cytoplasmic calcium concentration,
migratory behavior of cells, retro-control of D/V patterning and
lung and bone development. Spry genes contribute also to
reciprocal epithelial mesenchymal and stromal signaling
controlling ureteric branching, which involves the coordination
of Fgf/Wnt11/Gdnf pathways during mammalian kidney
development(Chi et al., 2004).
Study of Sprys function has led to intense investigation and
controversy(Fig. 3). Works from many groups have identified
Sprys as negative regulators of the MAP kinase ERK.
However, contradictory reports have identified Spry inhibitionupstream of Ras(Casci et al., 1999; Hanafusa et al., 2002) or at
the level of Raf(Reich et al., 1999; Sasaki et al., 2003; Yusoff
et al., 2002). Moreover, Spry family members have varying
interacting partners, such as c-Cbl, Grb2, Raf1 and FRS2 (for
review, Dikic and Giordano, 2003).
The mechanisms by which Sprys interfere with ERK
activation have been extensively studied on EGF signaling
pathway and have come from the finding that Spry proteins are
regulated by tyrosine phosphorylation (Tyr55 in spry2, Tyr53
in Spry1/4) in response to stimuli. Phosphorylated Tyr55 serves
as a docking site for the SH2 domain of c-Cbl (Egan et al.,
2002; Wong et al., 2002; Rubin et al., 2003; Hall et al., 2003 ).
Usually, c-Cbl binds to phosphorylated tyrosines on the
activated EGFR, which in turn results in its ubiquination,
endocytosis and degradation through theproteasomal/lysosom-
al pathways (Dikic and Giordano, 2003). As Spry proteins
phosphorylated on Tyr 55 can also interact with c-Cbl, they are
in direct competition with activated EGFRs for binding to this
protein. As a result, the EGFR is not ubiquinated, internalized
or degraded in the presence of Spry, leading to prolonged cell-
surface exposure of the receptor and sustained signaling
activity. Therefore, in this context, Sprys function as positive
regulators of the EGF signaling resulting in an upregulation of
the MAP kinase ERK activity. A similar degradation of Spry2
by c-Cbl may also be true for FGFR signaling (Hall et al.,
2003).
However, most of the studies performed on FGF signaling
pathway show that Sprys associate with Grb2. For example,
mouse Spry2 and Xspry1 can interact with the SH2 domain of
Grb2 after FGFR-induced phosphorylation of Tyr55(Hanafusa
et al., 2002). As a result, the interaction of Grb2 with FRS2 is
blocked and FGF-induced signal transduction is repressed. Inaddition, Spry2 and 4 have been shown to bind directly to Raf
through a Raf-binding motif in their C-terminal domain and
suppress phosphorylation of Raf on Ser338 by the Raf kinase
PKCy (Sasaki et al., 2003).
Altogether, Sprys function through different pathways,
which determine the overall effect on RTK signaling. First,
binding of c-Cbl to tyrosine-phosphorylated Spry results in the
stabilization of EGFR and in sustained ERK activity. Second,
binding of Grb2 to tyrosine-phosphorylated Spry can interrupt
the FGFR/Ras/Raf/MAPK pathway upstream of Ras. Finally,
binding of the C-terminal domain of Spry by its Raf-binding
motif inhibits Raf phosphorylation by PLCg-activated PKCy.
However, the criteria to decide between Spry different
Fig. 2. FGF signaling regulates osteogenesis. During intramembranous bone development, bone formation is characterized by the proliferation of mesenchymal cells
followed by their commitment to become osteoprogenitor cells. They differentiate into preosteoblasts then into bone matrix-forming mature osteoblasts. FGFs are
required for each step of bone formation through FGFRs to control various genes involved in osteoblast commitment, differentiation and apoptosis. ALP: alkaline
phosphatase, BSP: bone sialoprotein, COL I: collagen type I, OC: osteocalcin, OP: osteospondin, ON: osteonectin (adapted from P. J.Marie, 2003).
B. Thisse, C. Thisse / Developmental Biology 287 (2005) 390 402396
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Fig. 3. Models for the attenuation of FGF signaling by Sprys. (I) Spry acts downstream of the FGFR and upstream of Ras by binding to the adaptor protein Grb2 at the le
the activation of Ras and the subsequent stimulation of the downstream elements of the pathway. (II) Spry also acts downstream of Ras by directly binding Raf. This is ach
located in its C-terminal part and this prevents both activation by Ras and phosphorylation of Ser338 by the PKC y. This explains how spry can inhibit the transduct
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functions are not yet known and will have to be defined in the
near future.
Other FGF synexpression group members
In addition to sprys, several other genes involved in the
regulation of the FGF pathway have been isolated through thecourse of large-scale in situ hybridization screens (Gawantka et
al., 1998; Thisse et al., 2001; Kudoh et al., 2001) designed to
identify genes showing expression pattern similar or over-
lapping with FGFs. Various genes were isolated such as
XFLRT3 (Bottcher et al., 2004), Sef (Furthauer et al., 2002;
Tsang et al., 2002), the MAP kinase phosphatase 3 (MKP3,
also called Pyst1, Eblaghie et al., 2003; Tsang et al., 2004),
PEA3 (for polyoma virus enhancer activator 3) and ERM
(Munchberg et al., 1999). These last two factors are the
downstream players of the FGF signaling pathway while the
others, also induced by FGF stimulation are involved in the
attenuation of the signal (for review, Tsang and Dawid, 2004).
XFLRT3
This gene encodes a transmembrane protein characterized
by a cluster of leucine-rich repeats and one FNIII domain
within the extracellular region. Its expression is induced after
activation of FGF signaling and downregulated after inhibition
of this pathway. XFLRT3 signaling results in phosphorylation
of the MAP kinase ERK and is blocked by the MAP kinase
phosphatase 1. In gain- and loss-of-function experiments
performed in Xenopus, FLRT3 was shown to phenocopy the
Fig. 4. FGF signaling and its regulation. The stimulation of FGFR by FGF ligands results in the activation of specific target genes. Among them are several feedback
inhibitors which are involved in the attenuation of FGF signaling. Spry acts at the level of Grb2 and/or at the level of Raf. Sef and XFLRT3 are both located at the
membrane and interact with FGFR. Sef functions as a negative regulator while XFLRT3 enhances the FGF activity resulting in the phosphorylation of the MAP
kinase ERK. Sef was shown also to affect the phosphorylation of the MAP kinase, either directly at the level of ERK or by preventing the phosphorylation of ERK byMEK. Finally, MKP3 negatively regulates the FGF signaling pathway by dephosphorylation of activated MAP kinase.
B. Thisse, C. Thisse / Developmental Biology 287 (2005) 390 402398
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FGF signaling (Bottcher et al., 2004). Surprisingly, in zebra-
fish, although FLRT3 is expressed in well known centers of
FGF signaling, neither gain nor loss of FGF signaling appears
to directly affect FLRT3 expression. Inactivating FLRT3
through injection of antisense morpholinos impairs conver-
gence-extension movements that direct axial elongation but
fails to mimick phenotypes observed after either loss or gainfunction of FGF activity. These observations suggest that, in
zebrafish, FLRT3 may be involved in directing cell movements
but does not appear to be related to the FGF pathway
(Furthauer M., Thisse, B. and Thisse, C., unpublished data).
Sef
The Sef protein is conserved across zebrafish, mouse and
human but not in invertebrates. Homology searches with Sef
revealed that it encodes a putative transmembrane protein, with
weak similarities to the Interleukin-17 receptor and a putative
tyrosine phosphorylation site into the intracellular region(Furthauer et al., 2002; Tsang et al., 2002). In zebrafish, loss-
or gain-of-function studies revealed Sef to function as an
antagonist of FGF signaling and to interfere with FGF signal
transduction by acting dowstream or at the level of MEK and
upstream (or at the level) of the MAP kinase (Furthauer et al.,
2002). Further studies performed using human Sef (hSef)
showed that it binds to activated forms of MEK, inhibits the
dissociation of the MEK ERK complex and blocks nuclear
translocation of activated ERK. Consequently, hSef inhibits
phosphorylation and activation of the nuclear ERK substrate
Elk-1, while it does not affect phosphorylation of the
cytoplasmic ERK substrate RSK2. Downregulation of endog-
enous hSef by hSef siRNA enhances the stimulus-induced
ERK nuclear translocation and the activity of Elk-1. These
results favor the hypothesis that hSef acts as a spatial regulator
for ERK signaling by targeting ERK to the cytoplasm (Toriiet
al., 2004). Other reports showed that the intracellular domain of
Sef is required for its inhibitory function and its interaction
with FGFR1 and FGFR2 (Tsang et al., 2002; Kovalenko et al.,
2003, Xiong et al., 2003). Ectopic expression of Sef blocks the
phosphorylation of FRS2. The conclusion of these studies was
that Sef acts at the level of FGFRs blocking the Ras and PI3K
pathways downstream. To complicate the further understanding
of Sef function, alternative spliced isoforms of Sef were found
in human (Preger et al., 2004). Sef-b which lacks the signalpeptide is localized in the cytoplasm and can suppress FGF
signaling downstream of MEK. Because FGFs deliver multiple
biological responses, it is possible that like Spry, Sef can inhibit
the FGF pathway at various points depending on the context, in
order to allow fine-tuning of signal regulation.
MAP kinase phosphatase 3 (MKP3)
Two distinct domains are characteristic of MKP3 proteins,
the N-terminal region, which contains a high-affinity ERK/
MAPK binding domain and the C-terminal domain, which
constitute a dual specificity phosphatase (Stewart et al., 1999;
Zhang et al., 2003; Zhao and Zhang, 2001). MKP3 negatively
regulates the MAP kinase cascade through dephosphorylation
of activated MAPK proteins. Upon binding of phosphorylated
MAPKs to the MAPK-binding domain in MKP3, a conforma-
tional activation of the C-terminal phosphatase domain is
achieved, leading to the inactivation of MAPKs (Camps et al.,
2000; Fjeld et al., 2000; Zhao and Zhang, 2001). In chicken,
ectopic expression of MKP3 results in the disruption of limboutgrowth, a phenotype characteristic of FGF signaling
blockage (Kawakami et al., 2003; Eblaghie et al., 2003). In
zebrafish, MKP3 limits the extent of FGF activity of the
gastrula embryo(Tsang et al., 2004).
Concluding remarks
A substantial amount of important work has been done to
establish the role of FGF signaling during development showing
that FGFs regulate cell fate and specification of germ layers.
FGF downstream elements have been characterized and it
becomes evident that a growing number of these factors arethemselves tightly regulated by FGFs. It is clear from various
studies that multiple feedback inhibitors exist and that they act at
different levels of the signal transduction cascades to attenuate
FGF signal(Fig.4). Regulation can occur at different levels of
the signal transduction pathways including the membrane down
to the level of phosphorylation of the MAP kinase. Some of the
feedback inhibitors are involved in more than one process (Sef,
Spry) and negatively regulate other RTK signaling pathways.
The next challenge will be to integrate the intricate
interactions between these different regulators within the
FGF pathway and between different RTK pathways. In the
future, it will be important to assess the pathways employed by
FGFs in different cells and define the determinants for
signaling specificity. This can include feedback regulation,
threshold-type responses and posttranslational modifications.
This will be complicated by the fact that these modulators
combine a wide range of activity. The complete understanding
of these mechanisms will have diverse implications in biology
as well as in therapeutic approaches.
Acknowledgment
We thank the ?FGF communityXfor help and apologize for
not having being able to cite as many papers as we would have
wished because of limited space.
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