Department of Molecular Genetics, Weizmann Institute of Science, Rehovot 76100, Israel
e-mail: benny.shilo{at}weizmann.ac.il
SUMMARY
The epidermal growth factor receptor (EGFR) signaling cascade represents one of the cardinal pathways that transmits information between cells during development in a broad range of multicellular organisms. Most of the elements that constitute the core EGFR signaling module, as well as a variety of negative and positive modulators, have been identified. Although this molecular pathway is utilized multiple times during development, the spatial and temporal features of its signaling can be modified to fit a particular developmental setting. Recent work has unraveled the various mechanisms by which the EGFR pathway can be modulated.
Introduction
A common paradigm of the cardinal signaling pathways that direct the
development of all multicellular organisms is the repeated use of the same
signaling cascades at numerous developmental decisions. This strategy raises
several problems. First, how can the same pathway dictate a multitude of
different cell fates? The prevalent solution seems to lie in the combinatorial
context of the promoters that are activated in each setting. For example,
different combinations of signaling pathways and tissue-specific enhancers
allow epidermal growth factor (EGF) receptor signaling to activate distinct
promoters in different tissues (Flores et
al., 2000; Halfon et al.,
2000
; Xu et al.,
2000
).
A second problem posed by the use of the same signaling pathway in different tissue settings is how to modulate signaling parameters according to the unique requirements of each tissue. For example, for each signaling scenario, the duration of signaling could be extended or restricted, the pathway may be activated only once or multiple times, and the range of activation could be limited or widespread. Because each of the signaling pathways relies on a `hard-wired' cascade of signaling modules, how can these different signaling features be achieved?
This review focuses on the EGF receptor signaling pathway in
Drosophila, and highlights the common aspects and differences between
the Drosophila pathway and the pathway in C. elegans and in
vertebrates. [For a recent review of the diverse functions of the
Drosophila EGFR pathway during development see Shilo
(Shilo, 2003).] Although the
receptor is activated by secreted ligands, the pathway predominantly mediates
short-range signaling, i.e. activation is restricted either to the cells
producing the signal or to cells positioned 1-2 cell diameters adjacent to the
signal source. A variety of regulatory modes have evolved to maintain this
restricted signaling range. Here, I discuss the mechanistic features that
underlie how the EGFR pathway can be activated in different modes: as a single
burst, as reiterative activation cycles within the same tissue, or by relay to
another tissue. Central to these modes is the distinction between the cell(s)
that provides the signal and the cells that are activated. Different
strategies for achieving this distinction are discussed. The ability to
compartmentalize the responses to EGFR activation within the receiving cells
and its implications are also addressed, as is the inter-relationship between
the EGFR and Notch pathways in Drosophila and C.
elegans.
The EGFR pathway in Drosophila: the basic hardware
In Drosophila, EGFR is the sole receptor of the pathway. The
cascade downstream of the receptor is the canonical RAS/RAF/MEK/MAPK pathway.
In most instances, the cascade downstream of the receptor appears to be
unbranched. As such, induction of gene expression by EGFR, mostly through the
Pointed ETS transcriptional activator, represents the universal output of the
pathway (Gabay et al., 1996;
O'Neill et al., 1994
). Only
two outputs that are not transcriptional have been described so far. In the
developing eye and in embryonic midline glial cells, EGFR antagonizes
apoptosis by MAPK-induced phosphorylation and inactivation of the
pro-apoptotic protein HID (Bergmann et al.,
1998
; Bergmann et al.,
2002
). In the migrating follicle border cells of the egg chamber,
EGFR mediates guided cell migration in response to attraction by the ligand
Gurken produced in the egg (Duchek and
Rorth, 2001
).
A key way in which the EGFR pathway is regulated is through the generation
of activate ligands of the pathway. There are four EGFR ligands in
Drosophila: Spitz, Keren, Gurken and Vein
(Table 1). Vein is produced as
a secreted protein that does not require processing for its activity
(Schnepp et al., 1996). The
other three ligands are produced as inactive membrane-bound precursors. Only
upon cleavage and the release of the extracellular EGF-containing domain is
the active ligand generated (Schweitzer et
al., 1995
). Spitz represents the cardinal ligand that is used in
the numerous developmental contexts in which EGFR operates.
|
Rhomboids and EGFR activation
The characterization of the key molecules that process Spitz has provided a
deeper understanding of how EGFR activation is spatially and temporally
controlled. Like spitz, Star is also broadly expressed in most
developmental settings, although in some cases its expression domain is
confined (Heberlein and Rubin,
1991). Conversely, the expression of rhomboid is
extremely dynamic (Bier et al.,
1990
), and precedes the appearance of EGFR-induced MAPK activation
(dpERK) (Gabay et al., 1997
).
Ectopic rhomboid expression leads to EGFR activation in a wide range
of tissues (Golembo et al.,
1996a
; Sturtevant et al.,
1993
), indicating that Rhomboid is the limiting factor and all
other components are ubiquitous. Thus, the complex array of enhancers that
regulate the rhomboid gene contains the `blueprint' for the dynamic
pattern of EGFR activation throughout Drosophila development. An
example of this can be seen in denticle-belt specification, where the
expression of rhomboid in defined cell rows determines the position
of the future denticle belts in each abdominal segment. Expression of
rhomboid is induced in two rows by Hedgehog, and in another row by
Serrate, triggering Notch signaling. Conversely, Wingless restricts the domain
of rhomboid expression (either directly or indirectly)
(Alexandre et al., 1999
).
Rhomboid is the founding member of a conserved gene family
(Wasserman and Freeman, 1998),
the function of some of its members being an intramembrane protease
(Urban et al., 2001
). The
highest degree of conservation among this family lies within the transmembrane
domains, which contain the catalytic site. Active intramembrane proteases
belonging to this family have been identified in species from bacteria to
humans (Koonin et al., 2003
).
In Drosophila, seven members of the family have been identified
(Wasserman et al., 2000
). Only
three have so far been shown to be involved in EGFR signaling. Rhomboid 1 is
the cardinal player in this context. Rhomboid 2/BRHO/STET is expressed in the
germline and was suggested to be required for Gurken processing during
oogenesis (Guichard et al.,
2000
). An analysis of stet mutants has shown that STET is
required in germline cells at the early stages of both oogenesis and
spermatogenesis, and that, in its absence, somatic cells fail to enwrap the
germline cells and to provide them with a microenvironment for their
differentiation (Schulz et al.,
2002
). Rhomboid 3/Roughoid is expressed in the eye, where it is
partially redundant to Rhomboid 1
(Wasserman et al., 2000
). In
addition, it is required in the embryo to facilitate the repulsion from the
midline of the tracheal ganglionic branch
(Gallio et al., 2004
), and it
might also cooperate with Rhomboid 1 to promote the viability of smooth
cuticle-producing cells in the ventral epidermis
(Urban et al., 2004
).
|
In cell culture, all three Rhomboid proteins, as well as Rhomboid 4, can
cleave the three EGF ligands (Ghiglione et
al., 2002; Urban et al.,
2002
). In accordance with their different expression patterns,
only rhomboid 1 mutants are lethal. It will be interesting to explore
whether there are functional differences between them, beyond their patterns
of expression. As will be discussed below, the intracellular sites of action
of Rhomboid proteins may be distinct, giving rise to interesting twists in the
regulation of EGFR signaling.
EGFR ligand processing in vertebrates
In vertebrates, four EGF receptors participate in signaling. The first,
EGFR, is activated by a set of seven ligands and undergoes homodimerization.
The remaining receptors (ERBB2, ERBB3 and ERBB4) are usually activated as
heterodimers by four ligands termed neuregulins. All ligands are produced as
precursors with a single transmembrane domain
(Table 1) (reviewed by
Falls, 2003;
Harris et al., 2003
). The
more-prevalent heterodimeric pair, ERBB2/ERBB3, is complementary, as the
former does not bind ligands, while the latter has an inactive kinase domain.
Phosphorylation of tyrosines on ERBB3 by ERBB2 following dimerization leads to
signaling (reviewed by Citri et al.,
2003
).
TGF is an EGFR ligand that has been studied in detail. It contains a
signal peptide, EGF domain, transmembrane and cytoplasmic domains. In contrast
to previous reports, recent work demonstrates that although the precursor form
of TGF
can mediate interaction between cells by virtue of its binding
to EGFR, it does not lead to activation of the receptor
(Borrell-Pages et al., 2003
).
Thus, cleavage is essential to produce a potent ligand. The cleavage
machinery, however, appears to be different in vertebrates. The accumulating
evidence suggests that membrane metalloproteases of the ADAM family, which are
active on the cell surface, cleave the ligand immediately above the
transmembrane domain. Especially revealing has been the observation that mice
in which the gene encoding TACE/ADAM17 metalloprotease has been inactivated
exhibit a TGF
mutant phenotype
(Blobel, 2005
;
Peschon et al., 1998
).
Fibroblasts from these knockout mice are defective in the shedding of several
EGFR ligands (Merlos-Suarez et al.,
2001
; Peschon et al.,
1998
; Sunnarborg et al.,
2002
). Other ADAM proteins have also been implicated in EGFR
ligand processing (Fischer et al.,
2004
). Because the ectodomain shedding machinery is located at the
cell surface, trafficking of the ligand precursors is essential. Two
PDZ-domain proteins that interact with the extreme C terminus of TGF
are required for its trafficking to the membrane
(Fernandez-Larrea et al.,
1999
).
ADAM metalloproteases have a fairly broad substrate specificity that
depends upon the domain(s) that mediates their association with the substrate,
rather than on a defined consensus sequence for cleavage. For example,
TACE/ADAM17 cleaves not only TGF, but also tumor necrosis factor (TNF),
the TNF receptor and L-selectin. It remains to be determined whether
particular metalloproteases have a preference for distinct EGFR ligands
(Harris et al., 2003
). In
addition, the ability to modulate the activity of metalloproteases has a
decisive regulatory effect on EGFR ligand cleavage. Activated G-protein
coupled receptors (GPCRs) stimulate the activity of ADAM proteins, leading to
the processing of the EGFR ligand heparin-binding EGF (HB-EGF), and thus to
EGFR activation (Prenzel et al.,
1999
; Wetzker and Bohmer,
2003
). Osmotic and oxidative stress may also induce EGFR
activation by stimulating metalloprotease-mediated cleavage of EGFR ligands
(Fischer et al., 2004
).
Taken together, the emerging strategies for controlling the activation of
EGFR/ERBB signaling in vertebrates are based on the restricted expression of
ligands and on the particular combination of ligands that are expressed by a
given cell type. ADAM metalloprotease-induced cleavage of the plasma membrane
generates the potent ligands. The repertoire of ADAM proteins, as well as the
stimulation of ADAM activity by external signals, may modulate the efficiency
of ligand cleavage. However, as neither the ligand trafficking machinery
(Urena et al., 1999), nor the
ADAM proteins are dedicated just to EGFR/ERBB activation, the ligand
processing system in vertebrates is less stringently regulated. In cases where
spatially restricted activation of EGFR is required, such as in the definition
of the feather inter-bud territory in chick, the restricted expression of EGF
has been observed (Atit et al.,
2003
).
Positive- and negative-feedback responses
The transcriptional induction of modulators of the pathway by EGFR
signaling plays a major role in shaping responses to this pathway. This
section discusses such modulators and their modes of action, whereas the
subsequent section addresses their roles in shaping the spatial and temporal
features of activation. In the Drosophila embryonic ventral ectoderm
and follicle cells, the original activation of the EGFR pathway is amplified
by inducing the expression of the ligand Vein
(Golembo et al., 1999;
Wasserman and Freeman, 1998
).
It is interesting to note that because Vein represents a ligand that is weaker
than Spitz [as measured by its capacity to activate EGFR in cell culture
(Schnepp et al., 1998
) or to
induce the appearance of phosphorylated MAPK (dpERK) and target genes in
embryos (Golembo et al.,
1999
)] this feedback response may allow the spread of lower levels
of activation to more distant cells. A second positive-feedback response in
Drosophila and C. elegans, as discussed below, is the
induction of rhomboid expression, which occurs only in restricted
instances, as it relies not only on EGFR activation, but also on the
convergence of additional signaling pathways
(Dobens et al., 2000
;
Dutt et al., 2004
;
Peri and Roth, 2000
;
Sapir et al., 1998
;
Wasserman and Freeman,
1998
).
Implicit in the short range of action of the EGFR pathway in
Drosophila, is the presence of multiple negative regulators of the
pathway. One class includes constitutively expressed elements such as CBL, an
E3 ligase that recognizes the activated, endocytosed EGFR by virtue of its P-Y
motifs, and induces its ubiquitination and degradation. CBL may also enhance
the endocytosis of EGFR, following ligand binding. Although CBL is broadly
expressed, it only modulates EGFR signaling in the follicle cells, which
receive the Gurken signal from the oocyte. In cbl-mutant cells, EGFR
is hyperactivated, leading to the repression of genes such as pipe
(Pai et al., 2000). Another
constitutive repressor is YAN (AOP - FlyBase), an ETS-domain transcriptional
repressor that blocks the DNA-binding site of Pointed. Following the
activation of MAPK, the phosphorylation of YAN leads to its nuclear export and
degradation (Rebay and Rubin,
1995
; Tootle et al.,
2003
). Finally, a recent report has suggested that a class II
Phosphoinositide 3-kinase may be constitutively recruited to activated EGFR to
attenuate signaling (MacDougall et al.,
2004
).
The other known negative regulators of EGFR signaling in Drosophila, including Argos, Kekkon and Sprouty, are transcriptionally induced by the pathway. Their role is to inhibit signaling in cells that are more distant from the source (Fig. 1). The inducibility of negative regulators endows the EGFR signaling pathway with a short-range signaling mode, irrespective of the nature of the tissue in which signaling takes place.
Argos is a secreted protein that has an EGF domain
(Freeman et al., 1992). It is
induced only in the cells where pronounced EGFR activation takes place
(Golembo et al., 1996b
). Based
on the atypical EGF domain of Argos and on its capacity to bind EGFR, it was
initially assumed to function as a competitive inhibitor of ligand/receptor
binding (Jin et al., 2000
).
Recent detailed biochemical studies have demonstrated that Argos associates
predominantly with Spitz, to form nonfunctional heterodimers
(Klein et al., 2004
). The
mechanism is similar to the known inhibitors of the bone morphogenetic protein
(BMP) pathway, such as Chordin/SOG, Noggin and Follistatin, which sequester
the BMP ligands (De Robertis and Kuroda,
2004
). This mode of action has two appealing features. First, it
provides an effective way of restricting the range of Spitz action. One can
actually consider Argos as a ligand `sieve', which allows only a small number
of Spitz molecules to reach and activate more distant cells. Recent
computational analysis has demonstrated that the short-range activity of Argos
is sufficient to restrict the range of Spitz diffusion and buffer fluctuations
in the levels of Spitz or EGFR (Reeves et
al., 2005
). Second, because Argos binds the ligand and not the
receptor, it is tempting to consider the possibility that Argos may
specifically sequester some ligands but not others. For instance, in the
ventral ectoderm, the source of Spitz is restricted to a single row of cells,
the midline glial cells. In the adjacent ectodermal cells, which receive the
highest level of signal, both argos and vein are induced
(Gabay et al., 1996
;
Golembo et al., 1999
). Argos
may restrict the amount of Spitz that reaches more distant cells, while
allowing Vein to diffuse readily and elicit a lower level of EGFR activation.
Argos-like genes have not been identified in vertebrates to date.
|
Sprouty proteins have a conserved carboxy-terminal cysteine-rich domain
that is necessary for their specific localization and function. The amino
terminus of Sprouty is divergent among organisms, except for a conserved
tyrosine residue. Sprouty exerts its inhibitory effect on receptor tyrosine
kinase (RTK) signaling by intercepting essential elements of the RAS/MAPK
cascade through diverse mechanisms (Kim
and Bar-Sagi, 2004). It is interesting to note that, in addition
to the transcriptional induction of sprouty by EGFR activation, the
protein must also undergo tyrosine phosphorylation at a distinct site, in
order to carry out its repressive activity
(Hanafusa et al., 2002
;
Rubin et al., 2003
). The
double switch for Sprouty activation may ensure that it functions only in
regions where pronounced activation by RTKs takes place. In parallel, the
tyrosine phosphatase SHP2, which is a positive element in signaling by RTKs,
has been shown to attenuate the repression of Sprouty by dephosphorylating the
crucial residue (Hanafusa et al.,
2004
).
Recently, Sprouty family proteins have been shown to not only attenuate the
response to RTK signaling, but also to mediate a developmental switch
(Sivak et al., 2005). In
Xenopus tropicalis, two members each of the sprouty and the
related spred genes were identified. This study showed that their
spatial patterns of expression during FGF-induced mesoderm differentiation are
similar. However, sprouty RNA levels are high during the gastrula
stage, whereas spred RNA levels are high in the subsequent neurula
stage. Sprouty selectively inhibits mesoderm spreading, whereas SPRED inhibits
mesoderm specification. The different activities of the two inhibitors stem
from their capacity to selectively attenuate different branches of FGF-induced
signaling. Sprouty inhibits FGF-receptor induced PLC
activation,
whereas SPRED selectively attenuates MAPK signaling.
Another interesting tier of negative regulation to consider may operate at
the level of the cells sending the signal, to restrict the amount of ligand
that is released. When the cleaved form of Spitz was expressed in a variety of
tissues, it was retained in the ER and was not secreted to activate
neighboring cells. This retention requires the activity of phospholipase
C (termed Small wing, Sl). In sl mutants, hyperactivation of
EGFR is observed due to the excessive release of the ligand, specifically in
the eye (Schlesinger et al.,
2004
; Thackeray et al.,
1998
). It is possible that, in this tissue, some of the cleaved
ligand is normally generated in the ER, rather than in a more advanced
secretory compartment. Both Rhomboid 1 and Rhomboid 3 are required for Spitz
processing in the eye (Wasserman et al.,
2000
). In contrast to Rhomboid 1, Rhomboid 3 may generate the
cleaved ligand already in the ER. For example, when Rhomboid 3 was expressed
with the Spitz precursor in cultured cells, the cleaved ligand was observed to
accumulate within the cells, indicating that Spitz could be cleaved in the
absence of Spitz trafficking by Star
(Urban et al., 2002
). The
biological impact of cleaving and retaining Spitz in the ER for eye
development is not known. The retention of cleaved Spitz may simply prevent
the secretion of ligand generated in the ER. However, if the ER cleavage mode
in the eye is predominant, the retention of cleaved Spitz may affect the
overall level of ligand that is eventually secreted by these cells.
Different modes of EGFR signaling
This section discusses how the common EGFR signaling cassette can be
adjusted to generate different modes of signaling that are suited to each
tissue. In many cases, the signaling event is executed once. In other
instances, the capacity to process the ligand is relayed to adjacent cells of
the same tissue, or to a neighboring tissue, leading to reiterated EGFR
signaling. Finally, there are cases where low level, trophic EGFR signaling is
necessary to maintain cell viability, for example in the eye disc
(Baonza et al., 2001).
Single burst
Most cases of EGFR signaling fall into this category. In the typical case,
the restricted expression of Rhomboid in a distinct cell(s) provides a source
of active ligand that will activate EGFR in the same cells or in adjacent
cells. Cells undergoing the highest level of activation induce expression of
Argos to modulate the response. Kekkon, and in some instances Sprouty, are
also induced, usually at a broader range
(Fig. 1,
Fig. 2A). Patterning of the
embryonic ventral ectoderm, as described above, is an example of a single
burst situation. The cSPI signal emanates from a single row of midline glial
cells, and the induction of Argos expression in the cells receiving the
highest levels of EGFR activation ensures the spatially restricted induction
of EGFR target genes.
Multiple activation cycles
Multiple EGFR activation cycles within the same tissue
This scenario involves the most complex setting in terms of signaling, as
it is based on discrete and successive bursts of EGFR activation within the
same tissue. How the range of each burst is controlled, how the final
cumulative outcome of all bursts is restricted, and how discrete bursts,
rather than a continuous signal, can be obtained are central issues. Below, I
outline several mechanistic solutions to these challenges.
In the Drosophila embryonic ectoderm, oenocytes (secretory cells
of epidermal origin) are formed in the dorsal-most cluster of sensory organ
precursors. A single cell expressing Rhomboid provides a signal to induce and
recruit up to six oenocytes. Time-lapse movies show that this induction takes
place in two bursts, and that three cells are recruited in each round
(Brodu et al., 2004). A
recruitment burst has a typical duration of
1 hour. The generation of
these signaling bursts does not stem from the discontinuous production of EGFR
ligand, as the enhanced, continuous production of ligand from the original
source cell maintained the cyclic EGFR activation pattern, while giving rise
to extra bursts. Remarkably, it is the capacity of the activated cells that
are immediately adjacent to the source to express the secreted inhibitor
Argos, and the topology of these cells, that provides the basis for generating
discrete activation bursts. Once these cells delaminate, the ligand can
effectively activate the next group of cells
(Fig. 2B).
|
Distinct cell types are induced in the eye, depending upon the combination
of transcription factors within the cells being activated, and on the
signaling pathways that converge on EGFR signaling in these cells
(Flores et al., 2000;
Xu et al., 2000
). Thus, every
round of activation must be discrete in space and time. In other words, the
new signal should emanate only from the newly differentiated cells, rather
than be provided continuously from the primary cells. The prevailing notion is
that the reiterative EGFR activation cycles in the eye stem from expanding the
expression of Rhomboid 1 and Rhomboid 3 following EGFR signaling
(Baonza et al., 2001
)
(Fig. 2C).
What mechanisms keep the range of Spitz signaling tightly restricted in the
eye? One mechanism might involve the induction of negative regulators such as
Argos and Sprouty, which are indeed necessary for correct ommatidial
differentiation (Casci et al.,
1999; Freeman et al.,
1992
; Kramer et al.,
1999
). It is also possible that the original cells that generated
the signal stop producing the cleaved ligand, after the new cohort of cells
begin to express rhomboid.
Relaying the signal source to another tissue
In some developmental contexts, the response to the signal in the receiving
cells needs strengthening. One way to achieve this is to endow the receiving
cells with the capacity to process their own ligand, by inducing the
expression of rhomboid in response to EGFR signaling.
An example of this strategy can be found in the differentiation of follicle
cells in the ovary, where the Gurken signal, emanating from the oocyte,
activates EGFR in the follicle cells. Gurken first activates EGFR in follicle
cells at the posterior part of the egg-chamber. Subsequently, following the
migration of the oocyte nucleus, Gurken activates the dorsoanterior cells
where rhomboid becomes a target gene of the pathway
(Sapir et al., 1998;
Wasserman and Freeman, 1998
).
This induction of Rhomboid by EGFR signaling also requires activation of the
BMP pathway, which synergizes with EGFR activation only when the follicle
cells have completed their migration over the oocyte nucleus
(Dobens et al., 2000
;
Peri and Roth, 2000
). Once
Rhomboid is expressed, it facilitates the processing of Spitz expressed by the
follicle cells (Fig. 2D). This
type of signaling relay can fulfill several functions. First, it might amplify
the signal, by expanding the ligand source. Second, it can perpetuate the
signal after the original source has faded.
Another example of signal relay has been recently identified during
EGFR-controlled vulval development in C. elegans
(Dutt et al., 2004).
Interestingly, the original expression of the EGF-like ligand LIN-3 in the
inducing anchor cell of the developing gonad does not require the Rhomboid
homolog ROM-1 for its cleavage nor for the induction of the proximal vulval
precursor cells. ROM-1 is required, however, in the vulval precursor cells,
which receive the primary LIN-3 signal, to increase the range of EGFR
signaling to more distal cells. An intriguing model to explain this invokes
the metalloprotease-based cleavage of a short splice variant of LIN-3 in the
anchor cell, which induces primary vulval cell fates in the adjacent cells,
and is followed by the relaying of the signal to secondary cells in a
ROM-1-dependent fashion.
Compartmentalization within signal-receiving cells
The previous sections have highlighted the importance of intracellular localization and trafficking for the correct processing of EGFR ligands in Drosophila. In this section, examples of the asymmetric segregation of EGFR within the cells that receive the signal will be discussed.
In the ventricular and sub-ventricular zones of the mouse embryonic
forebrain, EGFR is distributed asymmetrically between daughter cells during
mitosis, by an actin-dependent mechanism
(Sun et al., 2005). The
resulting progenitor cells respond differentially to EGFR activation in terms
of migration, proliferation and marker expression: the cells with high EGFR
levels give rise to astrocytes, whereas cells with low EGFR levels generate
oligodendrocytes. This asymmetric segregation of a signaling receptor, which
was previously described for Notch, provides a mechanism for generating
further diversity within developing neurons, by altering their sensitivity to
the same external cues according to the levels of a receptor that they
display.
The intracellular segregation of the EGFR ligand Neuregulin and the EGF
receptors ERBB2 and ERBB4 is crucial in human airway epithelia. Neuregulin is
present exclusively on the apical membranes of this epithelium, whereas the
receptors are restricted to the basolateral surface. When the epithelium is
intact, the receptors are not activated and the proliferation rate of the
tissue is low. Upon disruption of epithelial integrity, the receptors
encounter the ligand at the wound edges and cell proliferation ensues, leading
to the restoration of epithelial integrity
(Vermeer et al., 2003).
Conversely, the same basolateral localization of an EGF receptor is used in
other biological settings to enhance the signal, when the ligand and receptor
are present on adjacent surfaces of interacting cells. In C. elegans,
the LET-23 EGFR localizes to the basolateral membranes of polarized vulval
epithelial cells (Kaech et al.,
1998). The anchor cell secretes LIN-3 into the basal extracellular
space that abuts the vulval precursor cells. The juxtaposition of the receptor
thus sensitizes the receiving cells to the signal. Mutations in lin-2,
lin-7 and lin-10 compromise LET-23 localization and lead to
reduced signaling. The encoded proteins contain PDZ domains and form a protein
complex that binds the LET-23 cytoplasmic tail
(Kaech et al., 1998
). The PDZ
proteins and the interactions between them are conserved in vertebrates and
are necessary for the localization of ERBB receptors at the basolateral
epithelial surface of polarized MDCK cells
(Shelly et al., 2003
).
Targeting the receptors to the basolateral domain is achieved by the
N-terminal part of human LIN7, which binds the kinase domain of the receptors.
Once targeted to the basolateral surface, the human LIN7 PDZ domain stabilizes
ERBB2 at this position.
Inter-relationship between EGFR and Notch signaling
In most developmental settings, signaling from EGFR is integrated with
signaling from other pathways. The most detailed studies have addressed the
interaction between EGFR and Notch signaling. In some cases, these
interactions reinforce signaling. For example, induction of the Pax2
gene in the Drosophila cone cell requires the simultaneous binding of
Pointed (triggered by EGFR) and SU(H) (triggered by Notch)
(Flores et al., 2000)
(Fig. 3A). In parallel to the
activation of EGFR in the future cone cell, Spitz also induces the expression
of Delta in the photoreceptor cell, providing the Notch signal to the cone
cell (Tsuda et al., 2002
)
(Fig. 3B). This mode of
activation represents a `feed-forward' loop, which is used in multiple
transcriptional settings (Milo et al.,
2002
). The activation of EGFR signaling at two junctions to
produce the final output buffers the system against transient fluctuations in
signaling to ensure that only sustained EGFR activation will lead to a
response.
In C. elegans vulval development, mutual repression between the
EGFR and Notch pathways contributes to the generation of distinct cell types.
EGFR activation in the primary vulval-precursor cells induces the expression
of Delta-like ligands (DSL) and repressors of Notch signaling. In parallel,
activation of Notch in the secondary cells restricts the expression of DSLs,
and induces repressors of EGFR signaling
(Yoo et al., 2004)
(Fig. 3C). In addition to
Notch, another mechanism has recently been shown to be involved in restricting
EGFR activation. The DEP-1 receptor tyrosine phosphatase binds to and
dephosphorylates activated EGFR. Expression of DEP-1 is repressed by EGFR
activation in the primary cells, and is induced by a Notch-independent
mechanism in the secondary cells (Berset et
al., 2005
). The use of parallel mechanisms that restrict EGFR
activation in the secondary cells converts the graded activation by the ligand
LIN-3 into a binary EGFR-activation response.
|
In contrast to the autocrine activation of EGFR in the wing veins,
activation of EGFR is paracrine in many biological settings. In most, if not
all of these cases, the cells that produce the signal are refractive to it.
From a developmental viewpoint, this refractivity may allow the sending and
responding cell populations to maintain distinct identities. Several
mechanisms underlying the refractivity of the sending cells to EGFR activation
have been identified, the most prevalent of which is the suppression of the
transcriptional response. For example, the R8 photoreceptor cells in
Drosophila are the first cells to differentiate in the eye. Their
differentiation does not require EGFR activation, and they provide the initial
source of ligand for the recruitment of additional cells. It was shown that in
the R8 cells, expression of the transcription factor Senseless prevents the
nuclear transduction of EGFR activation, by blocking the transcriptional
responses to Pointed (Frankfort and
Mardon, 2004). The transmembrane protein Echinoid, which contains
L1 repeats, associates with EGFR and is phosphorylated by it. Echinoid
attenuates EGFR signaling either by promoting receptor endocytosis, or by
recruiting phosphatases to the receptor complex. In the absence of Echinoid,
extra R8 cells are induced (Rawlins et
al., 2003
; Spencer and Cagan,
2003
).
In other cases, the mechanistic basis for refractivity is not known. For
example, in the C1 sensory organ precursor cell in Drosophila, which
provides the ligand for oenocyte differentiation, no activation of MAPK is
observed, as monitored by dpERK antibodies, whereas prominent activation is
detected in the adjacent cells (Brodu et
al., 2004). In this case, the importance of preventing EGFR
activation in the ligand-producing cell may stem from the necessity to
maintain it as a stable signaling source that will not produce Argos.
Concluding remarks
This review has explored the ability to apply subtle twists to the highly conserved EGFR pathway, to generate a wide and varied array of signaling modes that are adapted to the particular constraints of each tissue. Given the central role of EGFR signaling in development, it is conceivable that evolutionary selection for variation in these `subtle twists' could change the morphology of tissues. For example, in any tissue where the rhomboid promoter is induced by EGFR activation, the outcome is dramatic because of the reiterative activation of the EGFR pathway. Restricting the intracellular distribution of the receptor or the ligands in polarized cells is also a strategy to modulate the signaling level.
Although the confined range of activation is a hallmark of EGFR signaling, little is known about the conversion of graded EGFR activation into sharp transcriptional response borders. Do these thresholds rely on multiple binding sites for the MAPK-activated transcriptional activators on the promoters of target genes, or are other mechanisms involved? As outlined above, cooperation with pathways such as Notch can facilitate sharp EGFR-response borders.
It is interesting to consider the evolution of distinct ligand-cleavage modes. Clearly, the EGFR ligands share a common ancestral molecule. In some organisms, one mode of ligand cleavage was replaced by another. Whether this change was driven by selection pressures that make one strategy advantageous in a particular setting, or whether it was a random event is a question that is likely to remain open. Interestingly, both strategies of cleavage appear to co-exist in C. elegans. Future studies should reveal whether the Rhomboid family of proteins are involved in the processing of EGFR ligands in vertebrates, or whether the two modes of cleavage are completely distinct.
ACKNOWLEDGMENTS
I would like to thank Eyal Schejter for critical reading of the manuscript, and members of my laboratory for stimulating discussions. B.S. is supported by a grant from the Israel Science Foundation and the Moross Cancer Institute, and is an incumbent of the Hilda and Cecil Lewis chair in Molecular Genetics.
REFERENCES
Alexandre, C., Lecourtois, M. and Vincent, J.
(1999). Wingless and Hedgehog pattern Drosophila
denticle belts by regulating the production of short-range signals.
Development 126,5689
-5698.
Alvarado, D., Rice, A. H. and Duffy, J. B.
(2004). Knockouts of Kekkon1 define sequence elements essential
for Drosophila epidermal growth factor receptor inhibition.
Genetics 166,201
-211.
Atit, R., Conlon, R. A. and Niswander, L. (2003). EGF signaling patterns the feather array by promoting the interbud fate. Dev. Cell 4, 231-240.[CrossRef][Medline]
Baker, N. E. and Yu, S. Y. (2001). The EGF receptor defines domains of cell cycle progression and survival to regulate cell number in the developing Drosophila eye. Cell 104,699 -708.[Medline]
Baonza, A., Casci, T. and Freeman, M. (2001). A primary role for the epidermal growth factor receptor in ommatidial spacing in the Drosophila eye. Curr. Biol. 11,396 -404.[CrossRef][Medline]
Bergmann, A., Agapite, J., McCall, K. and Steller, H. (1998). The Drosophila gene hid is a direct molecular target of Ras-dependent survival signaling. Cell 95,331 -341.[CrossRef][Medline]
Bergmann, A., Tugentman, M., Shilo, B. Z. and Steller, H. (2002). Regulation of cell number by MAPK-dependent control of apoptosis: a mechanism for trophic survival signaling. Dev. Cell 2,159 -170.[CrossRef][Medline]
Berset, T. A., Hoier, E. F. and Hajnal, A.
(2005). The C. elegans homolog of the mammalian tumor suppressor
Dep-1/Scc1 inhibits EGFR signaling to regulate binary cell fate decisions.
Genes Dev. 19,1328
-1340.
Bier, E., Jan, L. Y. and Jan, Y. N. (1990). rhomboid, a gene required for dorsoventral axis establishment and peripheral nervous system development in Drosophila melanogaster. Genes Dev. 4,190 -203.[Abstract]
Blobel, C. P. (2005). ADAMs: key components in EGFR signalling and development. Nat. Rev. Mol. Cell Biol. 6,32 -43.[CrossRef][Medline]
Borrell-Pages, M., Rojo, F., Albanell, J., Baselga, J. and
Arribas, J. (2003). TACE is required for the activation of
the EGFR by TGF-alpha in tumors. EMBO J.
22,1114
-1124.
Brodu, V., Elstob, P. R. and Gould, A. P. (2004). EGF receptor signaling regulates pulses of cell delamination from the Drosophila ectoderm. Dev. Cell 7, 885-895.[CrossRef][Medline]
Casci, T., Vinos, J. and Freeman, M. (1999). Sprouty, an intracellular inhibitor of Ras signaling. Cell 96,655 -665.[CrossRef][Medline]
Citri, A., Skaria, K. B. and Yarden, Y. (2003). The deaf and the dumb: the biology of ErbB-2 and ErbB-3. Exp. Cell Res. 284,54 -65.[CrossRef][Medline]
de Celis, J. F., Bray, S. and Garcia-Bellido, A.
(1997). Notch signalling regulates veinlet expression and
establishes boundaries between veins and interveins in the Drosophila wing.
Development 124,1919
-1928.
De Robertis, E. M. and Kuroda, H. (2004). Dorsal-ventral patterning and neural induction in Xenopus embryos. Annu. Rev. Cell Dev. Biol. 20,285 -308.[CrossRef][Medline]
Dobens, L. L., Peterson, J. S., Treisman, J. and Raftery, L.
A. (2000). Drosophila bunched integrates opposing DPP and EGF
signals to set the operculum boundary. Development
127,745
-754.
Duchek, P. and Rorth, P. (2001). Guidance of
cell migration by EGF receptor signaling during Drosophila oogenesis.
Science 291,131
-133.
Dutt, A., Canevascini, S., Froehli-Hoier, E. and Hajnal, A. (2004). EGF signal propagation during C. elegans vulval development mediated by ROM-1 rhomboid. PLoS Biol. 2, e334.[CrossRef][Medline]
Falls, D. L. (2003). Neuregulins: functions, forms, and signaling strategies. Exp. Cell Res. 284, 14-30.[CrossRef][Medline]
Fernandez-Larrea, J., Merlos-Suarez, A., Urena, J. M., Baselga, J. and Arribas, J. (1999). A role for a PDZ protein in the early secretory pathway for the targeting of proTGF-alpha to the cell surface. Mol. Cell 3,423 -433.[CrossRef][Medline]
Fischer, O. M., Hart, S., Gschwind, A., Prenzel, N. and Ullrich,
A. (2004). Oxidative and osmotic stress signaling in tumor
cells is mediated by ADAM proteases and heparin-binding epidermal growth
factor. Mol. Cell. Biol.
24,5172
-5183.
Flores, G. V., Duan, H., Yan, H., Nagaraj, R., Fu, W., Zou, Y., Noll, M. and Banerjee, U. (2000). Combinatorial signaling in the specification of unique cell fates. Cell 103, 75-85.[CrossRef][Medline]
Fortini, M. E., Rebay, I., Caron, L. A. and Artavanis-Tsakonas, S. (1993). An activated Notch receptor blocks cell-fate commitment in the developing Drosophila eye. Nature 365,555 -557.[CrossRef][Medline]
Frankfort, B. J. and Mardon, G. (2004).
Senseless represses nuclear transduction of Egfr pathway activation.
Development 131,563
-570.
Freeman, M. (1996). Reiterative use of the EGF receptor triggers differentiation of all cell types in the Drosophila eye. Cell 87,651 -660.[CrossRef][Medline]
Freeman, M. (1997). Cell determination
strategies in the Drosophila eye. Development
124,261
-270.
Freeman, M., Klambt, C., Goodman, C. S. and Rubin, G. M. (1992). The argos gene encodes a diffusible factor that regulates cell fate decisions in the Drosophila eye. Cell 69,963 -975.[CrossRef][Medline]
Gabay, L., Scholz, H., Golembo, M., Klaes, A., Shilo, B. Z. and
Klambt, C. (1996). EGF receptor signaling induces pointed P1
transcription and inactivates Yan protein in the Drosophila embryonic ventral
ectoderm. Development
122,3355
-3362.
Gabay, L., Seger, R. and Shilo, B. Z. (1997).
In situ activation pattern of Drosophila EGF receptor pathway during
development. Science
277,1103
-1106.
Gallio, M., Englund, C., Kylsten, P. and Samakovlis, C.
(2004). Rhomboid 3 orchestrates Slit-independent repulsion of
tracheal branches at the CNS midline. Development
131,3605
-3614.
Ghiglione, C., Carraway, K. L., 3rd, Amundadottir, L. T., Boswell, R. E., Perrimon, N. and Duffy, J. B. (1999). The transmembrane molecule kekkon 1 acts in a feedback loop to negatively regulate the activity of the Drosophila EGF receptor during oogenesis. Cell 96,847 -856.[CrossRef][Medline]
Ghiglione, C., Bach, E. A., Paraiso, Y., Carraway, K. L., 3rd,
Noselli, S. and Perrimon, N. (2002). Mechanism of activation
of the Drosophila EGF Receptor by the TGFalpha ligand Gurken during oogenesis.
Development 129,175
-186.
Ghiglione, C., Amundadottir, L., Andresdottir, M., Bilder, D.,
Diamonti, J. A., Noselli, S., Perrimon, N. and Carraway, I. K.
(2003). Mechanism of inhibition of the Drosophila and mammalian
EGF receptors by the transmembrane protein Kekkon 1.
Development 130,4483
-4493.
Golembo, M., Raz, E. and Shilo, B. Z. (1996a).
The Drosophila embryonic midline is the site of Spitz processing, and induces
activation of the EGF receptor in the ventral ectoderm.
Development 122,3363
-3370.
Golembo, M., Schweitzer, R., Freeman, M. and Shilo, B. Z.
(1996b). Argos transcription is induced by the Drosophila EGF
receptor pathway to form an inhibitory feedback loop.
Development 122,223
-230.
Golembo, M., Yarnitzky, T., Volk, T. and Shilo, B. Z.
(1999). Vein expression is induced by the EGF receptor pathway to
provide a positive feedback loop in patterning the Drosophila embryonic
ventral ectoderm. Genes Dev.
13,158
-162.
Guichard, A., Roark, M., Ronshaugen, M. and Bier, E. (2000). brother of rhomboid, a rhomboid-related gene expressed during early Drosophila oogenesis, promotes EGF-R/MAPK signaling. Dev. Biol. 226,255 -266.[CrossRef][Medline]
Gur, G., Rubin, C., Katz, M., Amit, I., Citri, A., Nilsson, J.,
Amariglio, N., Henriksson, R., Rechavi, G., Hedman, H. et al.
(2004). LRIG1 restricts growth factor signaling by enhancing
receptor ubiquitylation and degradation. EMBO J.
23,3270
-3281.
Halfon, M. S., Carmena, A., Gisselbrecht, S., Sackerson, C. M., Jimenez, F., Baylies, M. K. and Michelson, A. M. (2000). Ras pathway specificity is determined by the integration of multiple signal-activated and tissue-restricted transcription factors. Cell 103,63 -74.[CrossRef][Medline]
Hanafusa, H., Torii, S., Yasunaga, T. and Nishida, E. (2002). Sprouty1 and Sprouty2 provide a control mechanism for the Ras/MAPK signalling pathway. Nat. Cell. Biol. 4, 850-858.[CrossRef][Medline]
Hanafusa, H., Torii, S., Yasunaga, T., Matsumoto, K. and
Nishida, E. (2004). Shp2, an SH2-containing protein-tyrosine
phosphatase, positively regulates receptor tyrosine kinase signaling by
dephosphorylating and inactivating the inhibitor Sprouty. J. Biol.
Chem. 279,22992
-22995.
Harris, R. C., Chung, E. and Coffey, R. J. (2003). EGF receptor ligands. Exp. Cell Res. 284,2 -13.[CrossRef][Medline]
Hasson, P., Egoz, N., Winkler, C., Volohonsky, G., Jia, S., Dinur, T., Volk, T., Courey, A. J. and Paroush, Z. (2005). EGFR signaling attenuates Groucho-dependent repression to antagonize Notch transcriptional output. Nat. Genet. 37,101 -105.[Medline]
Heberlein, U. and Rubin, G. M. (1991). Star is required in a subset of photoreceptor cells in the developing Drosophila retina and displays dosage sensitive interactions with rough. Dev. Biol. 144,353 -361.[CrossRef][Medline]
Jimenez, G., Guichet, A., Ephrussi, A. and Casanova, J.
(2000). Relief of gene repression by torso RTK signaling: role of
capicua in Drosophila terminal and dorsoventral patterning. Genes
Dev. 14,224
-231.
Jin, M. H., Sawamoto, K., Ito, M. and Okano, H.
(2000). The interaction between the Drosophila secreted protein
argos and the epidermal growth factor receptor inhibits dimerization of the
receptor and binding of secreted spitz to the receptor. Mol. Cell.
Biol. 20,2098
-2107.
Kaech, S. M., Whitfield, C. W. and Kim, S. K. (1998). The LIN-2/LIN-7/LIN-10 complex mediates basolateral membrane localization of the C. elegans EGF receptor LET-23 in vulval epithelial cells. Cell 94,761 -771.[CrossRef][Medline]
Kim, H. J. and Bar-Sagi, D. (2004). Modulation of signalling by Sprouty: a developing story. Nat. Rev. Mol. Cell Biol. 5,441 -450.[CrossRef][Medline]
Klein, D. E., Nappi, V. M., Reeves, G. T., Shvartsman, S. Y. and Lemmon, M. A. (2004). Argos inhibits epidermal growth factor receptor signalling by ligand sequestration. Nature 430,1040 -1044.[CrossRef][Medline]
Kolodkin, A. L., Pickup, A. T., Lin, D. M., Goodman, C. S. and
Banerjee, U. (1994). Characterization of Star and its
interactions with sevenless and EGF receptor during photoreceptor cell
development in Drosophila. Development
120,1731
-1745.
Koonin, E. V., Makarova, K. S., Rogozin, I. B., Davidovic, L., Letellier, M. C. and Pellegrini, L. (2003). The rhomboids: a nearly ubiquitous family of intramembrane serine proteases that probably evolved by multiple ancient horizontal gene transfers. Genome Biol. 4,R19 .[CrossRef][Medline]
Kramer, S., Okabe, M., Hacohen, N., Krasnow, M. A. and Hiromi,
Y. (1999). Sprouty: a common antagonist of FGF and EGF
signaling pathways in Drosophila. Development
126,2515
-2525.
Laederich, M. B., Funes-Duran, M., Yen, L., Ingalla, E., Wu, X.,
Carraway, K. L., 3rd and Sweeney, C. (2004). The leucine-rich
repeat protein LRIG1 is a negative regulator of ErbB family receptor tyrosine
kinases. J. Biol. Chem.
279,47050
-47056.
Lee, J. R., Urban, S., Garvey, C. F. and Freeman, M. (2001). Regulated intracellular ligand transport and proteolysis control EGF signal activation in Drosophila. Cell 107,161 -171.[CrossRef][Medline]
MacDougall, L. K., Gagou, M. E., Leevers, S. J., Hafen, E. and
Waterfield, M. D. (2004). Targeted expression of the class II
phosphoinositide 3-kinase in Drosophila melanogaster reveals lipid
kinase-dependent effects on patterning and interactions with receptor
signaling pathways. Mol. Cell. Biol.
24,796
-808.
Mayer, U. and Nusslein-Volhard, C. (1988). A group of genes required for pattern formation in the ventral ectoderm of the Drosophila embryo. Genes Dev. 2,1496 -1511.[Abstract]
Merlos-Suarez, A., Ruiz-Paz, S., Baselga, J. and Arribas, J.
(2001). Metalloprotease-dependent protransforming growth
factor-alpha ectodomain shedding in the absence of tumor necrosis
factor-alpha-converting enzyme. J. Biol. Chem.
276,48510
-48517.
Milo, R., Shen-Orr, S., Itzkovitz, S., Kashtan, N., Chklovskii,
D. and Alon, U. (2002). Network motifs: simple building
blocks of complex networks. Science
298,824
-827.
Neuman-Silberberg, F. S. and Schupbach, T. (1993). The Drosophila dorsoventral patterning gene gurken produces a dorsally localized RNA and encodes a TGF alpha-like protein. Cell 75,165 -174.[CrossRef][Medline]
O'Neill, E. M., Rebay, I., Tjian, R. and Rubin, G. M. (1994). The activities of two Ets-related transcription factors required for Drosophila eye development are modulated by the Ras/MAPK pathway. Cell 78,137 -147.[CrossRef][Medline]
Pai, L. M., Barcelo, G. and Schupbach, T. (2000). D-cbl, a negative regulator of the Egfr pathway, is required for dorsoventral patterning in Drosophila oogenesis. Cell 103,51 -61.[CrossRef][Medline]
Peri, F. and Roth, S. (2000). Combined
activities of Gurken and decapentaplegic specify dorsal chorion structures of
the Drosophila egg. Development
127,841
-850.
Peschon, J. J., Slack, J. L., Reddy, P., Stocking, K. L.,
Sunnarborg, S. W., Lee, D. C., Russell, W. E., Castner, B. J., Johnson, R. S.,
Fitzner, J. N. et al. (1998). An essential role for
ectodomain shedding in mammalian development. Science
282,1281
-1284.
Prenzel, N., Zwick, E., Daub, H., Leserer, M., Abraham, R., Wallasch, C. and Ullrich, A. (1999). EGF receptor transactivation by G-protein-coupled receptors requires metalloproteinase cleavage of proHB-EGF. Nature 402,884 -888.[CrossRef][Medline]
Rawlins, E. L., White, N. M. and Jarman, A. P.
(2003). Echinoid limits R8 photoreceptor specification by
inhibiting inappropriate EGF receptor signalling within R8 equivalence groups.
Development 130,3715
-3724.
Rebay, I. and Rubin, G. M. (1995). Yan functions as a general inhibitor of differentiation and is negatively regulated by activation of the Ras1/MAPK pathway. Cell 81,857 -866.[CrossRef][Medline]
Reeves, G. T., Kalifa, R., Klein, D. E., Lemmon, M. A. and Shvartsman, S. Y. (2005). Computational analysis of EGFR inhibition by Argos. Dev. Biol. doi:10.1016/j.ydbio.2005.05.013 .[CrossRef]
Roch, F., Jimenez, G. and Casanova, J. (2002). EGFR signalling inhibits Capicua-dependent repression during specification of Drosophila wing veins. Development 129,993 -1002.[Medline]
Rubin, C., Litvak, V., Medvedovsky, H., Zwang, Y., Lev, S. and Yarden, Y. (2003). Sprouty fine-tunes EGF signaling through interlinked positive and negative feedback loops. Curr. Biol. 13,297 -307.[CrossRef][Medline]
Rutledge, B. J., Zhang, K., Bier, E., Jan, Y. N. and Perrimon, N. (1992). The Drosophila spitz gene encodes a putative EGF-like growth factor involved in dorsal-ventral axis formation and neurogenesis. Genes Dev. 6,1503 -1517.[Abstract]
Sapir, A., Schweitzer, R. and Shilo, B. Z.
(1998). Sequential activation of the EGF receptor pathway during
Drosophila oogenesis establishes the dorsoventral axis.
Development 125,191
-200.
Schlesinger, A., Kiger, A., Perrimon, N. and Shilo, B. Z. (2004). Small wing PLCgamma is required for ER retention of cleaved Spitz during eye development in Drosophila. Dev. Cell 7,535 -545.[CrossRef][Medline]
Schnepp, B., Grumbling, G., Donaldson, T. and Simcox, A. (1996). Vein is a novel component in the Drosophila epidermal growth factor receptor pathway with similarity to the neuregulins. Genes Dev. 10,2302 -2313.[Abstract]
Schnepp, B., Donaldson, T., Grumbling, G., Ostrowski, S.,
Schweitzer, R., Shilo, B. Z. and Simcox, A. (1998). EGF
domain swap converts a drosophila EGF receptor activator into an inhibitor.
Genes Dev. 12,908
-913.
Schulz, C., Wood, C. G., Jones, D. L., Tazuke, S. I. and Fuller,
M. T. (2002). Signaling from germ cells mediated by the
rhomboid homolog stet organizes encapsulation by somatic support cells.
Development 129,4523
-4534.
Schweitzer, R., Shaharabany, M., Seger, R. and Shilo, B. Z. (1995). Secreted Spitz triggers the DER signaling pathway and is a limiting component in embryonic ventral ectoderm determination. Genes Dev. 9,1518 -1529.[Abstract]
Shelly, M., Mosesson, Y., Citri, A., Lavi, S., Zwang, Y., Melamed-Book, N., Aroeti, B. and Yarden, Y. (2003). Polar expression of ErbB-2/HER2 in epithelia. Bimodal regulation by Lin-7. Dev. Cell 5,475 -486.[CrossRef][Medline]
Shilo, B. Z. (2003). Signaling by the Drosophila epidermal growth factor receptor pathway during development. Exp. Cell Res. 284,140 -149.[CrossRef][Medline]
Simcox, A. A., Grumbling, G., Schnepp, B., Bennington-Mathias, C., Hersperger, E. and Shearn, A. (1996). Molecular, phenotypic, and expression analysis of vein, a gene required for growth of the Drosophila wing disc. Dev. Biol. 177,475 -489.[CrossRef][Medline]
Sivak, J. M., Petersen, L. F. and Amaya, E. (2005). FGF signal interpretation is directed by Sprouty and Spred proteins during mesoderm formation. Dev. Cell 8, 689-701.[CrossRef][Medline]
Sotillos, S. and De Celis, J. F. (2005). Interactions between the Notch, EGFR, and decapentaplegic signaling pathways regulate vein differentiation during Drosophila pupal wing development. Dev. Dyn. 232,738 -752.[CrossRef][Medline]
Spencer, S. A. and Cagan, R. L. (2003).
Echinoid is essential for regulation of Egfr signaling and R8 formation during
Drosophila eye development. Development
130,3725
-3733.
Strutt, H. and Strutt, D. (2003). EGF signaling and ommatidial rotation in the Drosophila eye. Curr. Biol. 13,1451 -1457.[CrossRef][Medline]
Sturtevant, M. A., Roark, M. and Bier, E. (1993). The Drosophila rhomboid gene mediates the localized formation of wing veins and interacts genetically with components of the EGF-R signaling pathway. Genes Dev. 7, 961-973.[Abstract]
Sun, Y., Goderie, S. K. and Temple, S. (2005). Asymmetric distribution of EGFR receptor during mitosis generates diverse CNS progenitor cells. Neuron 45,873 -886.[CrossRef][Medline]
Sunnarborg, S. W., Hinkle, C. L., Stevenson, M., Russell, W. E.,
Raska, C. S., Peschon, J. J., Castner, B. J., Gerhart, M. J., Paxton, R. J.,
Black, R. A. et al. (2002). Tumor necrosis factor-alpha
converting enzyme (TACE) regulates epidermal growth factor receptor ligand
availability. J. Biol. Chem.
277,12838
-12845.
Thackeray, J. R., Gaines, P. C., Ebert, P. and Carlson, J.
R. (1998). small wing encodes a phospholipase C-(gamma) that
acts as a negative regulator of R7 development in Drosophila.
Development 125,5033
-5042.
Tootle, T. L., Lee, P. S. and Rebay, I. (2003).
CRM1-mediated nuclear export and regulated activity of the Receptor Tyrosine
Kinase antagonist YAN require specific interactions with MAE.
Development 130,845
-857.
Tsruya, R., Schlesinger, A., Reich, A., Gabay, L., Sapir, A. and
Shilo, B. Z. (2002). Intracellular trafficking by Star
regulates cleavage of the Drosophila EGF receptor ligand Spitz.
Genes Dev. 16,222
-234.
Tsuda, L., Nagaraj, R., Zipursky, S. L. and Banerjee, U. (2002). An EGFR/Ebi/Sno pathway promotes delta expression by inactivating Su(H)/SMRTER repression during inductive notch signaling. Cell 110,625 -637.[CrossRef][Medline]
Urban, S., Lee, J. R. and Freeman, M. (2001). Drosophila rhomboid-1 defines a family of putative intramembrane serine proteases. Cell 107,173 -182.[CrossRef][Medline]
Urban, S., Lee, J. R. and Freeman, M. (2002). A
family of Rhomboid intramembrane proteases activates all Drosophila
membrane-tethered EGF ligands. EMBO J.
21,4277
-4286.
Urban, S., Brown, G. and Freeman, M. (2004).
EGF receptor signalling protects smooth-cuticle cells from apoptosis during
Drosophila ventral epidermis development. Development
131,1835
-1845.
Urena, J. M., Merlos-Suarez, A., Baselga, J. and Arribas, J.
(1999). The cytoplasmic carboxy-terminal amino acid determines
the subcellular localization of proTGF-(alpha) and membrane type matrix
metalloprotease (MT1-MMP). J. Cell Sci.
112,773
-784.
Vermeer, P. D., Einwalter, L. A., Moninger, T. O., Rokhlina, T., Kern, J. A., Zabner, J. and Welsh, M. J. (2003). Segregation of receptor and ligand regulates activation of epithelial growth factor receptor. Nature 422,322 -326.[CrossRef][Medline]
Wasserman, J. D. and Freeman, M. (1998). An autoregulatory cascade of EGF receptor signaling patterns the Drosophila egg. Cell 95,355 -364.[CrossRef][Medline]
Wasserman, J. D., Urban, S. and Freeman, M.
(2000). A family of rhomboid-like genes: Drosophila rhomboid-1
and roughoid/rhomboid-3 cooperate to activate EGF receptor signaling.
Genes Dev. 14,1651
-1663.
Wetzker, R. and Bohmer, F. D. (2003). Transactivation joins multiple tracks to the ERK/MAPK cascade. Nat. Rev. Mol. Cell Biol. 4,651 -657.[CrossRef][Medline]
Xu, C., Kauffmann, R. C., Zhang, J., Kladny, S. and Carthew, R. W. (2000). Overlapping activators and repressors delimit transcriptional response to receptor tyrosine kinase signals in the Drosophila eye. Cell 103,87 -97.[CrossRef][Medline]
Yoo, A. S., Bais, C. and Greenwald, I. (2004).
Crosstalk between the EGFR and LIN-12/Notch pathways in C. elegans vulval
development. Science
303,663
-666.
zur Lage, P. and Jarman, A. P. (1999).
Antagonism of EGFR and notch signalling in the reiterative recruitment of
Drosophila adult chordotonal sense organ precursors.
Development 126,3149
-3157.