Zoologisches Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
* Authors for correspondence (e-mail: hafen{at}zool.unizh.ch and nairz{at}zool.unizh.ch)
Accepted 29 April 2003
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SUMMARY |
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Key words: Drosophila, DMKP3, CL100, Eye dual-specificity phosphatase, Signal transduction
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INTRODUCTION |
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A switch mechanism requires the possibility to also counteract the
stimulatory activity of the dual-specificity MAPK kinases. This is achieved by
phosphatases capable of dephosphorylating either the threonine residue or the
tyrosine residue [serine/threonine phosphatases (STPs) or protein tyrosine
phosphatases (PTPs)], or both [dual-specificity phosphatases (DSPs)]
(Camps et al., 2000;
Keyse, 2000
). As DSPs exhibit
a high specificity towards MAP kinases and within those to a subset of the
family, they have also been designated MKPs (for MAP kinase phosphatases).
DSPs are comprised of an N-terminal CH2 domain (for Cdc25 homology) implicated
in substrate binding, which also contains a basic docking site that directly
binds to the negatively charged common docking (CD) domain of MAPKs
(Theodosiou and Ashworth,
2002
). Upon MAPK binding the phosphatases undergo a conformational
transition that stimulates the activity of the C-terminal catalytic domain
(Camps et al., 1998
). The
prevalence of this interaction is illustrated by a dominant ERK mutation
termed Sevenmaker, which affects the charge of the CD domain such
that the physical interaction of ERK with its DSP is greatly impaired. Thereby
the phosphatase activity is compromised and ERK kept in an activated state
(Bott et al., 1994
;
Brunner et al., 1994
;
Chu et al., 1996
). Flies
carrying the dominant Sevenmaker mutation are viable, but display
multiple phenotypes characteristic of an overactive RAS pathway, for example
rough eyes because of the recruitment of extra photoreceptor cells. Numerous
other studies have established the Drosophila eye as an excellent
model to genetically dissect ERK signaling
(Dickson and Hafen, 1994
;
Freeman, 1998
).
The Drosophila compound eye is composed of approximately 800
ommatidia, each built up of an equivalent of 19 cells, eight of which are
neuronal photoreceptor cells. Photoreceptors contain specialized microvillar
stacks of membrane termed `rhabdomeres'. The rhabdomere of the R7
photoreceptor neuron is situated in the center of the ommatidial unit on top
of that of the R8 cell. The rhabdomeres of the remaining six outer
photoreceptors are arranged such that ommatidia appear in two different chiral
forms. Chirality is conveyed by the R3 and R4 cells, which adopt an
asymmetrical position within the ommatidium
(Fig. 7A)
(Wolff and Ready, 1993).
|
In contrast to the R8 cell, the remaining photoreceptors are dependent on
high and/or sustained Ras pathway activity
(Halfar et al., 2001).
Overactivation of ERK by constitutively active RAS or receptor tyrosine
kinases results in severe differentiation defects
(Bishop and Corces, 1988
;
Lesokhin et al., 1999
;
Lowy and Willumsen, 1993
).
This phenotype is mimicked by loss-of-function mutations in negative
regulators of the RAS signaling pathway, like Gap1 or the ETS transcriptional
inhibitor Yan. Surprisingly, apart from PTP-ER, mutations in genes
coding for ERK phosphatases have not been identified based on a similar
phenotype. It is thus possible that various phosphatases perform redundant
functions on ERK. Redundancy could explain why mutants of the mouse DSP MKP1
and the C. elegans lip-1 are fully viable
(Dorfman et al., 1996
;
Berset et al., 2001
). Likewise,
HE-PTP knockout mice devoid of the ERK tyrosine phosphatase are phenotypically
normal and the corresponding Drosophila PTP-ER mutants only exhibit
slight defects (Gronda et al.,
2001
; Karim and Rubin,
1999
).
Here we show that mammalian dual specificity phosphatases MKP3 and MKP4 and its Drosophila homolog DMKP3 (MKP3 FlyBase) selectively inhibit ERK in vivo. Analysis of Dmkp3 loss of function mutations reveals that DMKP3 performs redundant and non-redundant functions on ERK together with the tyrosine-phosphatase PTP-ER. Our results further suggest that RAS signaling is not only required within the photoreceptors to properly differentiate, but also performs a function in surrounding cells to shape the developing ommatidium. Together, we provide evidence that ERK is negatively regulated by an interplay of different phosphatases in a cell-context-dependent manner.
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MATERIALS AND METHODS |
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Germline transformation
MKP3, MKP4, MKP5, M3/6 and CL100 encoding cDNAs were generously provided by
M. Muda and S. Arkinstall (Serono Pharmaceuticals), and Dmkp3
full-length cDNA SD06439 was obtained from Research Genetics. The cDNAs were
either subcloned into the Drosophila transformation vectors
pUAST (Brand and Perrimon,
1993) or into the sevE/hsp70P vector pDN448 (kindly
provided by D. Nellen). Drosophila germline transformation of the
y w1118 stock was performed as previously described
(Basler et al., 1991
). Several
independent transformant lines were established per construct.
Generation of Dmkp3 mutations by transposase-mediated
P-element mobilization and reversion mutagenesis
EP3142 was mobilized in an isogenized and phenotypically wild-type
stock by 2-3 transposase
(Robertson et al., 1988
) and
mosaic males were crossed to sev-GAL4 virgins. Individuals of the F1
progeny with rough eyes potentially bearing a reoriented EP-element were
analyzed by PCR for exhibiting an inverted EP-element using a P3' and a
Dmkp3-specific primer. Six of 131 independent positives had an
EP-insertion closer to the Start-codon, four of which had the new EP
integrated into the 5' UTR of Dmkp3. The insertion-sites
upstream to the ATG are as follows: 483 bp
(Dmkp34), 132 bp (Dmkp31),
128 bp (Dmkp32) and 11 bp
(Dmkp33). All tested lines except for
Dmkp33, which has undergone more complex changes (data not
shown), still harbor the original EP at 1023 bp. To generate point
mutations in Dmkp3, we treated Dmkp311 males with
20 or 25 mM EMS according to Lewis and Bacher
(Lewis and Bacher, 1968
) and
crossed them to sev-GAL4 or GMR-GAL4 females. Among 7500 F1
flies six lines transmitted and exhibited a mutation in the ORF. Mutations
were first identified by DHPLC as described
(Nairz et al., 2002
) and then
confirmed by DNA sequencing. The nucleotide changes are (compare with
Fig. 3): Dmkp36: GCC
GTC; Dmkp37: CCC
CTC; Dmkp38: CAC
TAC;
Dmkp39: ACA
ATA; Dmkp310: GGA
AGA; Dmkp35: AGG
AAG.
As the Dmkp35 mutation affects a splice-acceptor site
(underlined in the triplet), the expected frameshift by one base was
corroborated by analysis of Dmkp35 cDNA.
|
RNA in situ hybridization
In situ hybridization on eye imaginal discs was performed essentially as
described (Lehmann and Tautz,
1994; O'Neill and Bier,
1994
). DIG-labeled ssRNA was in vitro transcribed from
Dmkp3 cDNA subcloned into the pCRII-TOPO vector (Invitrogen) by T7
and SP6 polymerase (Roche). Samples were stained for 1 hour.
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RESULTS |
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We first established an assay to test for substrate preferences of DSPs in
transgenic flies. Based on the assumption that overexpression of a specific
inhibitor should phenocopy the loss-of-function phenotype of the target, the
assay was calibrated with DSPs, whose substrate preferences have been
extensively characterized. As a readout, developmental processes known to
depend on JNK or ERK activity, respectively, were selected. Misexpression of
the JNK and p38 dual specificity phosphatases M3/6 (from mouse), human MKP5
and the Drosophila JNK phosphatase Puckered in Drosophila
embryos cause a dorsal-open phenotype akin to the JNK/basket
loss-of-function phenotype (Fig.
1F,I and data not shown)
(Riesgo-Escovar et al., 1996).
However, the same phosphatases do not influence ERK-dependent developmental
programs like photoreceptor and wing vein differentiation
(Fig. 1D,E,G,H)
(Schweitzer and Shilo, 1997
).
Conversely, the ERK phosphatase MKP3 (from rat) does not disrupt
embryogenesis, but interferes with eye and wing development when overexpressed
in Drosophila (Fig.
1J-L). Three conclusions can be drawn from these experiments:
First, the classification of DSPs according to substrate specificity partially
derived from in vitro data is qualitatively recapitulated in vivo. Second, the
substrate specificity is evolutionarily conserved. Third, even high levels of
an ERK-or JNK-specific phosphatase do not affect the other MAPK pathway,
suggesting that these phosphatases have a very high degree of substrate
specificity.
|
The putative Drosophila dual specificity phosphatase DMKP has no
phosphatase activity on an artificial substrate, but JNK and ERK phosphatase
activity in vitro (Lee et al.,
2000). DMKP overexpression neither interferes with photoreceptor
development nor causes a dorsal-open phenotype, indicating that it might have
other substrates than JNK and ERK or that it is a weak phosphatase (data not
shown).
Misexpression of Dmkp3 in wing and eye imaginal discs partially inhibits vein and photoreceptor formation, but overexpression in the embryonic epidermis does not affect dorsal closure (Fig. 1P-R), classifying DMKP3 as an ERK-specific phosphatase.
Dmkp3 interacts genetically with components of the Ras pathway
To further corroborate that DMKP3 acts as a negative regulator of ERK
signaling, its position in the RAS/ERK pathway was determined by genetic
epistasis experiments. The sev-Dmkp3 overexpression phenotype is
dominantly enhanced by a Ras or a ERK
(rl10A) mutation (Fig.
2D,G and data not shown), indicating that RAS and ERK levels are
limiting when DMKP3 is overexpressed. Conversely, high DMKP3 levels are
sufficient to curb overactivation of the RAS/ERK pathway caused by activating
mutations in genes coding for different pathway components. Expression of
sevS11, RasV12 and
raftorY9 transgenes in the eye cause ERK activation and
formation of extra R7 photoreceptors, but co-overexpression of Dmkp3
almost completely suppresses the eye phenotypes
(Fig. 2E,H and data not shown).
Finally, the interaction of Dmkp3 with the ERK
gain-of-function allele Sevenmaker (rlSEM) was
tested. In the wing, rlSEM causes extra veins, which are
not eliminated by co-overexpression of Dmkp3; i.e.
rlSEM is epistatic to Dmkp3
(Fig. 2A-C). In contrast, the
rlSEM rough-eye phenotype caused by the formation of
additional R7 cells is almost completely suppressed by high Dmkp3
levels (Fig. 2F,I). It is
possible that the cell context-dependent sensitivity of RlSem to
Dmkp3 overexpression is because of different expression levels. This
hypothesis is supported by data from Chu et al.
(Chu et al., 1996) who showed
that the mammalian Sevenmaker homologue ERK2D319N is inactivated by
DSPs in COS-7 cells at high, but not in NIH3T3 cells at lower expression. The
genetic interactions are consistent with a function of DMKP3 between Raf and
RlSEM and strongly support the interpretation that ERK is the main
target of DMKP3 (Kim et al.,
2002
).
|
DMKP3 and PTP-ER perform redundant as well as non-redundant functions
on ERK
The prototypical mutation activating RAS signaling at the level of ERK is
the gain-of-function rlSem allele.
rlSem/+ flies are viable, but display multiple phenotypes
characteristic of a pathway overactivation. These include female sterility,
additional wing veins and rough eyes because of the recruitment of multiple R7
photoreceptor cells (Brunner et al.,
1994) (Fig. 2B,F).
UAS-rlSEM flies possess some additional wing veins even in
the absence of a GAL4 driver, thus suggesting a subtle activation of
the pathway (Fig. 4B).
Dmkp3 mutants are viable and fertile and exhibit a mild, but
significant increase in wing vein material reminiscent of
UAS-rlSEM (Fig.
4A,E). In addition, they are slightly rough-eyed. A requirement
for DMKP3 during eye development is consistent with its expression in
third-instar eye imaginal discs posterior to the morphogenetic furrow where
photoreceptor differentiation occurs (Fig.
3C-E). Dmkp3 expression is not under transcriptional
control of the RAS pathway, because the expression pattern is virtually
unchanged in a sev-rasV12 background (data not shown).
|
|
The loss of ommatidial asymmetry has been associated with an altered Notch
and Delta activity in the initially equivalent R3 and R4 precursor cells. High
Delta levels in both precursors would lead to R3/R3-type ommatidia and high
Notch activity to facets containing two R4s
(Cooper and Bray, 1999;
Fanto and Mlodzik, 1999
;
Tomlinson and Struhl, 1999
).
If Dmkp3 participated in the Notch-Delta interaction directly (for
example as a Notch target) (Berset et al.,
2001
), one would expect to see only ommatidia of either type.
However, ommatidia exhibiting the R3/R3 and the R4/R4 shape are detected at
approximately the same frequency (Fig.
5A4,A5). Moreover, in contrast to its C. elegans homolog
lip-1 (Berset et al.,
2001
), Dmkp3 does not appear to be a transcriptional
target of activated Notch (data not shown).
In order to confirm the interpretation of ommatidial shapes with a
molecular marker, R4-differentiation was followed in
Dmkp3- eye imaginal discs. In contrast to the wild type,
occasionally two or no cells in Dmkp3- ommatidial clusters
express the R4 marker E(spl)m0.5
(Cooper and Bray 1999
)
(Fig. 5C,D), thus corroborating
that both R3 and R4 are affected by the absence of Dmkp3. Likewise,
using a R7-specific lacZ line (P. Maier and E.H., unpublished) in a
Dmkp3- background, additional R7 cells are detectable
(data not shown). Preclusters devoid of R4 staining very probably give rise to
symmetrical R3/R3-type ommatidia (with or without an extra photoreceptor) or
to ommatidia missing an outer photoreceptor. Preclusters containing two
R4-positive cells will differentiate to R4/R4 ommatidia or to ommatidia
containing an extra photoreceptor (Fig.
5A).
The weak rough eye phenotype associated with loss of PTP-ER function is caused by the occasional recruitment of one or more extra R7 cells, which may be accompanied by a loss of an outer photoreceptor cell. R3 and R4 cells are unaffected (Fig. 5E). The unequal Dmkp3 and PTP-ER loss-of-function phenotypes suggest that the two phosphatases perform non-overlapping functions during photoreceptor differentiation. In agreement with this hypothesis, eyes of the double mutant pupae or eyes containing Dmkp3- clones in a PTP-ER- background feature ommatidia characteristic of either single mutant (Fig. 5F and data not shown). These results demonstrate that during eye development, DMKP3 and PTP-ER exert non-redundant functions in the specification of photoreceptor cells.
DMKP3 is required both in R3/R4 photoreceptors and outside of the
ommatidial precluster
Unlike Delta and Notch, which are required specifically in R3 and R4,
respectively, DMKP3 influences both R3 and R4 differentiation. DMKP3 function
could thus reside in R3 and R4 cells. An alternative, but not exclusive,
possibility is that DMKP3 is needed in cells, which will not differentiate as
photoreceptors, but interfere with R3/R4 development. According to the model
that DMKP3 is required autonomously in R3 and R4 photoreceptors, a
Dmkp3- shape would always be associated with a
Dmkp3- genotype in those cells. If, however, DMKP3 were
not required in R3 and R4, but exhibited a non-autonomous effect, one would
expect to find mosaic ommatidia featuring a Dmkp3-
phenotype, but a Dmkp3+ genotype in R3 and/or R4
cells.
The predictions were tested in Dmkp35J4-Dmkp3+ mosaic ommatidia with Dmkp3- shapes. In 156 eye sections 83 mosaic ommatidia with a Dmkp3- morphology were found. Ten of them were not analyzed, because their symmetry and their location at the equator did not allow their chirality to be determined. In the unambiguous 73 mosaic ommatidia photoreceptors R6, R7, R1, R5 and R2 had a relatively high likelihood of being Dmkp3+ decreasing in the listed order. Five ommatidia were Dmkp3+ either for the R3 or the R4, one was wild-type for R3, R4 and the extra cell (Fig. 6A,B). In 12 of 37 mosaics exhibiting an additional inner or outer photoreceptor the extra cell was Dmkp3+ (Fig. 6B). Of the Dmkp3+ extra cells, which according to their location are most probably misdifferentiated mystery cells, two were outer photoreceptors and ten were R7 cells.
|
These data and the detection of Dmkp3+ cells in mutant ommatidia formally demonstrates that DMKP3 function is not absolutely essential in any of the photoreceptors and also not required in the mystery cell. However, there is a strong autonomous Dmkp3 component in R3 and R4 cells. We thus conclude that R3 and R4 differentiation is dependent both on DMKP3 activity within the precursor cells and on DMKP3 function in cells surrounding the ommatidial precluster.
Ommatidial preclusters contain one or two mystery cells, whereas only one
cell could be followed by the clonal assay. Formally, we cannot dismiss the
model assigning autonomy to the mystery cell. However, it is highly unlikely.
Tomlinson and Struhl (Tomlinson and
Struhl, 1999) found that even the most unrelated cell pair of an
ommatidial precluster, R3 and R4, has an 0.375 chance of being derived from
the same clone. Assuming a similar probability for both mystery cells, the
chance of undetected cells being Dmkp3- in all 12 cases is
3.55x10-3 [(1-0.375)12].
Our data predict that Dmkp3- clones should manifest non-autonomous effects. The examination of Dmkp35 and Dmkp32 clones in eye imaginal discs indeed revealed occasional R4/R4 ommatidia outside of the clonal boundaries (Fig. 6D-H and data not shown). Only ommatidia directly bordering the clone were affected, indicating that DMKP3 action is short-range. Similarly, ommatidia exhibiting a Dmkp3- shape can be found in wild-type tissue close to Dmkp35J4 clones in adult eye sections (Fig. 6C). These genotypically wild-type ommatidia further stress our notion that the requirement for DMKP3 in R3 and R4 cells is not absolute.
Cells outside of the ommatidial precluster are competent to respond
to a Notch-Delta interaction
Although DMKP3 is very unlikely to have a function in the mystery cell(s),
Dmkp3- ommatidia often contain a mystery cell having
differentiated as a photoreceptor (Fig.
5A). Conceivably, R3, R4 and some surrounding cells determine the
fate of the mystery cells in a DMKP3-dependent manner. For example, they could
be required for their timely withdrawal from the preclusters. If this process
were badly timed, the lagging cells would disrupt the Notch-Delta interactions
between R3 and R4 precursors and thereby result in their own
misdifferentiation. This model makes the prediction that in situations in
which the mystery cell remains between R3 and R4 precursors their interaction
should be inhibited, but their competence to interact should be
unaffected.
The constitutively active sevenless allele
sevS11 has been shown to be sufficient to reprogram
mystery and cone cells to a R7 fate (Basler
et al., 1991). In a sevS11 background the
wrongly differentiating cells may frequently separate the R3/R4 precursors,
which then may be free for Notch-Delta-mediated interactions with other cells.
We tested this possibility by following the R4 marker in
sevS11 eye imaginal discs and found that cells outside the
ommatidial preclusters can adopt a R4 fate
(Fig. 6I-K). This result can
also explain why sevS11 ommatidia may not only contain
extra R7, but occasionally also extra outer photoreceptor cells
(Fig. 6L).
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DISCUSSION |
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Although some of the phosphatases tested possess anti-p38 activity in
vitro, we could not compare their overexpression phenotypes with those of
p38 and p38ß null mutants because these null phenotypes are not
known. Moreover, the p38 kinase Licorne and the p38 kinase kinase DMEKK1
loss-of-function phenotypes are very divergent
(Inoue et al., 2001
;
Suzanne et al., 1999
). We
therefore cannot rule out that some aspects of the phenotypes described, such
as the extreme dorsal hole or the wing-duplication effect caused by CL100
overexpression, may also be because of an effect on p38.
In our assays CL100 also caused very rough eyes and strong loss of wing
veins suggesting that it is a potent phosphatase for both JNK and ERK. The
wing-duplication phenotype may be the result of the downregulation of EGFR
signaling because expression of a dominant negative form of RAF or RAS or
reduction of EGFR and CNK dosis leads to a similar phenotype
(Baonza et al., 2000). Because
we have, however, never observed wing duplications by expression of DMKP3,
CL100 is either more potent or has substrates other than ERK. Indeed, a
wing-duplication phenotype is also observed in twins mutants encoding
a regulatory subunit of the STP PP2A
(Uemura et al., 1993
).
DMKP3 is a negative regulator of the RAS/MAPK pathway
Several lines of evidence indicate that DMKP3 is an ERK-specific
phosphatase and that it cooperates with PTP-ER. (1) DMKP3 dephosphorylates ERK
but not JNK in vitro (Kim et al.,
2002). (2) Overexpression of DMKP3 produces phenotypes resembling
those of ERK but not JNK loss-of-function mutations. (3) Epistasis experiments
using Dmkp3 gain-of-function and loss-of-function alleles indicate
that DMKP3 acts in the RAS/ERK pathway in the eye and the wing. (4) The
synthetic lethality of PTP-ER-; Dmkp3- double
mutants is rescued by reducing ERK levels by half.
This interaction is reminiscent of the yeast DSP Yvh1 and the tyrosine
phosphatase Ptp2, which have little effect when mutated alone, but double
mutants are sporulation defective (Park et
al., 1996). As there are five additional MKPs in the
Drosophila genome (Morrison et
al., 2000
), negative regulation of ERK by a combinatorial network
of those phosphatases will probably reveal high redundancy as well.
DMKP3 functions in R3 and R4 and in surrounding non-neuronal cells
during ommatidial differentiation
In Dmkp3 mutant eyes, both R3 and R4 cells are misspecified in a
small fraction of ommatidia. DMKP3 has an autonomous and a non-autonomous role
in specifying R3 and R4. The autonomous DMKP3 function derives from the high,
albeit not complete correlation of a Dmkp3- phenotype and
a Dmkp3- genotype in the R3 and R4 cells. Because R3 and
R4 are the most distantly related cells in the precluster, the high incidence
of both R3 and R4 being mutant indicates a strong requirement for DMKP3
function in these cells. The evidence for a non-autonomous function of DMKP3
comes from phenotypically mutant ommatidia in which at least one cell of the
R3/R4 pair is wild-type and from phenotypically mutant and genotypically
wild-type ommatidia close to Dmkp3- clones.
Non-autonomous effects on outer photoreceptors were also observed for
groucho, argos, fat facets, liquid facets, sidekick and
atrophin clones (Cadavid et al.,
2000; Fanto et al.,
2003
; Fischer-Vize et al.,
1992a
; Fischer-Vize et al.,
1992b
; Freeman et al.,
1992
; Nguyen et al.,
1997
; Fanto et al.,
2003
). The results have been interpreted to indicate that
surrounding cells participate in photoreceptor differentiation. The data
presented here provide the first direct evidence that levels of RAS/ERK
activity in cells surrounding the growing ommatidial cluster can influence
ommatidial patterning. They may also explain why a Ras1
gain-of-function allele dominantly enhances the fat facets
(faf) loss-of-function phenotype, although faf function
resides outside the photoreceptors (Huang
and Fischer-Vize, 1996
; Li et
al., 1997
).
A model to account for the Dmkp3- ommatidial shape
From our results we infer that the misdifferentiation of
Dmkp3- ommatidia correlates with the behavior of the
mystery cell (Fig. 7B). The
mystery cell must leave the precluster to permit a physical interaction of R3
and R4 precursor cells to engage in a Notch-Delta-mediated specification of
the R3 and R4 fate. In the absence of DMKP3 in R3 and R4 precursors and in the
surrounding cell pool the mystery cell has a chance of being locked between R3
and R4, thus preventing the correct specification of its fate and that of the
R3 and R4 precursors. The presence of misspecified R3/R4 cells without an
intervening extra photoreceptor cells suggests that the mystery cell left the
cluster too late and thus interfered with R3/R4 development.
How could cells surrounding the mystery cell be involved in eliciting its
exit from the precluster? Conceivably, changes in cell adhesion, which may be
regulated by an ERK signal, play a major role in expunging the mystery cells
from the cluster. Upon recruitment of cells into the cluster, cell-cell
contacts between photoreceptor cells are tightened. The mystery cells cannot
adhere to the differentiating cells in the cluster and are expelled like melon
seeds. As DMKP3 is not required in the mystery cells, it is probable that it
is not the absolute value of cell-adhesive properties, but the relative amount
compared with its neighbors that influences their behavior. This model implies
that mutations altering cell-adhesive properties should lead to
Dmkp3--like ommatidia. Indeed, loss of sidekick
and atrophin, coding for adhesion molecules, result in a very similar
phenotype by affecting cells outside the cluster
(Fanto et al., 2003;
Nguyen et al., 1997
).
Furthermore, EGFR signaling and particularly ERK activity may not only
influence cell fate, but also directly or indirectly influence cell adhesion.
EGFR to ERK signaling has been shown to affect the adhesive properties of
mammalian cells (Xie et al.,
1998
), and recent evidence in Drosophila also points to a
role of EGFR in cell adhesion (Dumstrei et
al., 2002
). High ERK activity has also been found in migrating
cells, although activated ERK per se is insufficient to influence migration
(Duchek and Rorth, 2001
). The
possibility to modulate RAS pathway activity in Drosophila almost at
will may establish the developing eye as an interesting system in which the
connection between RAS signaling and cell adhesion within an epithelium can be
further analyzed.
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ACKNOWLEDGMENTS |
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