1 Department of Biology, Mudd Hall, Johns Hopkins University, 3400 N. Charles
Street, Baltimore, MD 21218, USA
2 Integrated Imaging Center, Department of Biology, Mudd Hall, Johns Hopkins
University, 3400 N. Charles Street, Baltimore, MD 21218, USA
* Author for correspondence (e-mail: vandoren{at}jhu.edu)
Accepted 29 May 2003
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SUMMARY |
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Key words: Drosophila, E-cadherin, fear of intimacy, Gonad coalescence, Organogenesis, Germline-soma interaction
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INTRODUCTION |
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In Drosophila, germ cells are formed at the posterior pole during the
syncytial blastoderm stage of embryogenesis. Gastrulation brings the germ
cells to the interior of the embryo, after which they actively migrate through
the midgut epithelium and into the mesoderm (reviewed by
Starz-Gaiano and Lehmann,
2001). They then make contact with specialized mesodermal cells
with which they form the gonad
(Sonnenblick, 1950
), known as
somatic gonadal precursors (SGPs) (Boyle et
al., 1997
). SGPs are specified in bilateral clusters within
parasegments (PS) 10, 11 and 12 (Brookman
et al., 1992
), and can be identified by their expression of the
nuclear proteins Eyes Absent (EYA) (Boyle
et al., 1997
) and ZFH-1
(Broihier et al., 1998
). They
arise within the eve domain of the dorsolateral mesoderm, where
groups of cells are selected to become either SGPs or fat body. The repressive
effects of serpent and the positive regulatory effects of
abdA limit the SGPs to PS10, 11 and 12
(Riechmann et al., 1998
;
Moore et al., 1998a
;
Hayes et al., 2001
). During
gonad formation, the three clusters of SGPs come together to form a band of
cells on each side of the embryo as the germ cells end their migration, and
the two cell types associate along PS10-12. Germ cells and SGPs then undergo
gonad coalescence to form a rounded structure in PS10.
In addition to SGPs that are specified in PS10, 11 and 12, there is an
additional cluster of somatic mesoderm cells, called msSGPs (for male-specific
SGPs), that arises in PS13 (DeFalco et
al., 2003). msSGPs can be distinguished from the SGPs by
co-expression of EYA and the nuclear protein Sox100B
(DeFalco et al., 2003
). As the
germ cells and SGPs coalesce to form the gonad, the msSGPs move anteriorly and
join the posterior of the gonad specifically in males
(DeFalco et al., 2003
).
Gonad coalescence involves the concerted movements of germ cells and SGPs
as they transition from a broad association of cells into a condensed and
organized gonad. Coalescence is complete by the start of embryonic stage 15,
with the gonad assuming a compact, spherical shape. Mutations in
iab4, a cis-regulatory region of abdA, or
eya specifically block gonad coalescence. In these mutants, initial
specification of the SGPs is normal, and the germ cells successfully migrate
to them. However, instead of coming together into a rounded organ, the SGPs
arrest in PS10-PS11 and the germ cells scatter throughout the embryo
(Cumberledge et al., 1992;
Boyle and DiNardo, 1995
;
Boyle et al., 1997
). In both
iab4 and eya mutants, this phenotype is attributed to a
failure in late SGP differentiation.
abdA and eya are examples of genes that specify SGP
identity. The downstream genes that coordinate the morphogenic movements of
gonad formation are still unknown. One candidate is the Drosophila
homolog of E-cadherin, a transmembrane cell adhesion molecule that plays a
major role in tissue morphogenesis (reviewed by
Tepass, 1999). E-cadherin
typically acts in homophilic cell adhesion, binding to E-cadherin molecules on
opposing cells and connecting to the cytoskeleton via its partner proteins
- and ß-catenin (reviewed by
Yap et al., 1997
).
Drosophila E-cadherin is encoded by the shotgun
(shg) locus (Tepass et al.,
1996
; Uemura et al.,
1996
). Mutations in shg cause defects in gonad
coalescence (Van Doren et al.,
2003
), but the role that E-cadherin plays in embryonic gonad
morphogenesis in Drosophila has not yet been analyzed. Another gene
thought to control gonad morphogenesis downstream of gonad cell identity is
fear of intimacy (foi). FOI is a member of a novel,
conserved family of transmembrane proteins of unknown function. foi
mutants exhibit defects in both gonad coalescence and tracheal branch fusion,
as is also observed in shg mutants
(Tanaka-Matakatsu et al.,
1996
; Van Doren et al.,
2003
). The similarities of the shg and foi
mutant phenotypes suggest that they may cooperate in the same or related
pathways to control morphogenic events.
We present work that furthers our understanding of both the cellular and molecular events that control gonad formation. Through a detailed analysis of gonad coalescence, we have found that germ cells and SGPs interact intimately from the moment they associate, with SGPs undergoing dramatic changes in cellular morphology as they individually ensheath each germ cell in the gonad. Furthermore, we show that Drosophila E-cadherin is upregulated in the gonad at the time of gonad coalescence, and its function is crucial for several aspects of gonad morphogenesis, including the ensheathment of germ cells by SGPs. E-cadherin expression is dependent on eya, providing a molecular link between cell identity and the morphogenic movements of gonad coalescence. Finally, we show that FOI is required for proper E-cadherin protein expression in the gonad, suggesting that improper regulation of E-cadherin is the basis for the foi mutant gonad phenotype.
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MATERIALS AND METHODS |
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Tissue fixation for anti-DCAD2 and anti-DCAD1 was as described
(Rothwell and Sullivan, 2000)
with the following modifications. Embryos were dechorionated in 50% bleach,
washed with 1x PBS + 0.1% Triton (PBTx), fixed in 4% formaldehyde in
1.75 ml PCM (100 mM PIPES pH 6.9, 1 mM CaCl2, 2 mM
MgSO4) and 8 ml heptane for 20 minutes at room temperature and
transferred to 3MM Whatman paper to allow the heptane to evaporate. Embryos
were then transferred to double-sided tape and covered with PBTx. Vitelline
membranes were removed by hand under a dissecting scope with a 25-gauge needle
and transferred in PBTx to a 2 ml screw cap vial. All subsequent incubations
were done on an upright shaker in BBTx (1x PBS, 1% BSA, 0.3% Triton).
Embryo fixation for all other antibodies and the immunolabeling protocol were
as described (Moore et al.,
1998b
). For most genotypes, embryos were staged by gut development
according to Campos-Ortega and Hartenstein
(Campos-Ortega and Hartenstein,
1985
). Because shg mutants display defects in gut
formation, shg embryos were aged for 11.5 hours at 25°C after egg
laying (AEL) to yield collections of embryos stage 15 and older. Embryos were
mounted in 70% glycerol containing 2.5% DABCO (Sigma) and viewed on a Leica NT
or Zeiss 510 Meta confocal microscope. Occasionally two Z-sections through a
stage 13 gonad were stacked together in order to display the gonad in one
image. Image brightness and contrast were adjusted with Adobe Photoshop 6.0.
Germ cell ensheathment was quantitated using single confocal sections through
embryos expressing UAS-mCD8-GFP and the mesoderm-specific
twist-Gal4. Percentage values, in 25% increments, were assigned to
each germ cell to represent the amount of its surface surrounded by GFP
signal.
Electron microscopy
Four-hour embryo collections from the mating of fafl females and
Fm7, Kruppel-Gal4, UAS-GFP/Y males were aged for 20 hours at
18°C. Female offspring were distinguished by the presence of the
Kruppel-GFP X chromosome. Embryos were dechorionated in 50% bleach and sorted
under a fluorescence dissecting scope. Male and female embryos were fixed
separately in 10 ml of heptane (previously saturated with 25% glutaraldehyde
and 2% acrolein contained in a 100 mM cacodylate buffer, pH 7.4) for 15
minutes at room temperature. The heptane was removed, and the embryos were
transferred to double-sided tape and covered with 3% formaldehyde, 2%
glutaraldehyde, 0.5% DMSO in 100 mM cacodylate buffer, pH 7.4. After 15
minutes, the embryos were hand-devitellinized with a 25-gauge needle and fixed
for an additional 2 hours at room temperature. Embryos were post-fixed in 1%
OsO4 containing 0.1% potassium ferrocyanide, 100 mM cacodylate and
5 mM CaCl2, pH 6.8 for 30 minutes at room temperature, then washed
in H2O four times over 10 minutes. They were transferred to 1%
thiocarbohydrazide contained in H2O for 5 minutes, washed four
times in H2O over 10 minutes, and transferred to 1%
OsO4/1% potassium ferrocyanide in cacodylate buffer, pH 6.8, for 5
minutes at room temperature. Embryos were then placed into Kellenberger's
uranyl acetate overnight at room temperature, dehydrated through a graded
series of ethanol and subsequently embedded in Spurr resin. Sagittal or
transverse sections were cut on a Leica UCT ultramicrotome, placed onto
monohole formvar/carbon coated grids, stained in 2% uranyl acetate and lead
citrate, and observed on a Philips EM 420 or 410 TEM. Images were recorded
using Kodak SO-163 electron image film or a Megaview III digital camera. The
cells in Fig. 2 were traced and
colored using plasma membranes as a guide with Adobe Photoshop 6.0.
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RESULTS |
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The images shown in Fig.
1C,D are male embryos, but we have also observed germ cell
ensheathment in female gonads (data not shown). This indicates that germ cell
ensheathment is not a sex-specific phenomenon, though there may be a small
difference in the extent to which male and female germ cells are ensheathed
[average % ensheathment/germ cell (n): female st15=70% (96); male
st15=86% (95)]. Interestingly, some germ cells begin to undergo cell division
at stage 15 (Sonnenblick,
1950; Asaoka-Taguchi et al.,
1999
; Deshpande et al.,
1999
) (A.B.J. and M.V.D., unpublished), yet the extent of germ
cell ensheathment does not decrease in older embryos. This indicates that
ensheathment remains an active process as SGPs are able to establish contact
with the newly formed surface between daughter germ cells. Finally, although
germ cell ensheathment has already occurred as the gonad forms, it is not
required for the compaction of the SGPs into PS10. Compaction occurs normally
in gonads that completely lack germ cells, such as in embryos derived from
mothers with weak mutations in oskar
(Brookman et al., 1992
)
(Fig. 1F).
To extend our analysis of gonad coalescence, we analyzed stage 14 wild-type
embryonic gonads by transmission electron microscopy (TEM). Electron
micrographs of the coalesced gonad confirm that somatic cells wrap around and
between germ cells (Fig. 2).
SGPs display long processes and a variety of shapes as they extend in
different directions to contact germ cells
(Fig. 2A). Germ cells, however,
are always very rounded in shape and lack processes or extensions. There is
also a high degree of soma-soma contact within the gonad and SGPs often
overlap each other as they surround germ cells, with a cellular process from
one SGP juxtaposed with a process from a neighboring SGP
(Fig. 2B). Previous work
suggested that there are two populations of SGPs: the interstitial cells that
associate with the germ cells inside the gonad and a separate group of cells
surrounding the perimeter of the gonad
(Poulson, 1950). However, we
have not observed a distinct population of somatic cells surrounding the
gonad, and we find that many of the SGPs contribute to both the interior and
the exterior of the gonad (e.g. purple cell in
Fig. 2A).
E-cadherin is expressed in both the germ cells and the SGPs
Our analysis of gonad coalescence reveals that specific and extensive
cell-cell contacts are made within the gonad as it forms. To determine how
these contacts are mediated and maintained, we turned our attention to the
cell adhesion molecule E-cadherin. Previous work had reported that
shg (Drosophila E-cadherin) mRNA is expressed within the
gonad (Tepass et al., 1996),
and we had observed a defect in gonad coalescence in shg mutants
(Van Doren et. al., 2003
). To
further investigate the role of E-cadherin in gonad coalescence, we first
examined its expression within the gonad using antibodies specific for
Drosophila E-cadherin (
DCAD2,
DCAD1)
(Oda et al., 1994
). Both
antibodies give similar staining patterns, and
DCAD2 immunoreactivity
is severely reduced in embryos homozygous for a deletion of shg (data
not shown), indicating that these antibodies reflect the localization of
E-cadherin protein within the gonad.
At stage 12, SGPs exist as three distinct clusters of cells (Fig. 3A). E-cadherin within the SGPs is indistinguishable from the background levels throughout the mesoderm at this time, in contrast to nearby tracheal tissue (Fig. 3A). By stage 13, when germ cells and SGPs are associated across PS10-12, E-cadherin is now clearly observed around germ cells (closed arrowhead, Fig. 3B) in the same regions where we observe ensheathment of germ cells by SGPs. E-cadherin is also observed between SGPs (open arrowhead, Fig. 3B). The pattern of E-cadherin remains similar as the gonad coalesces (stage 14, Fig. 3C). Additionally, as msSGPs approach and join the posterior of the male gonad, they too display clear E-cadherin staining (large arrow, Fig. 3C,F), similar to what is observed in the main body of the gonad.
At later stages, E-cadherin expression becomes highly concentrated at the
anterior end of the gonad (stage 17, Fig.
3D), which may be the result of increased protein expression or a
high density of somatic cells. This pattern appears to be male-specific, and
is likely to reflect E-cadherin expression in the developing proximal testis
structure known as the hub, since these cells co-express escargot
(A.B.J. and M.V.D., unpublished) and both escargot and E-cadherin
mark the hub in the adult testis (Kiger et
al., 2000; Tazuke et al.,
2002
).
In addition to E-cadherin, other components of classical cadherin complexes
are present in the gonad. Double labeling experiments with antibodies against
E-cadherin and Armadillo (ARM), the Drosophila homolog of
ß-catenin, reveal that localization of the two proteins overlaps almost
completely within the gonad (Fig.
3E). An antibody specific for -catenin also presents a
staining pattern similar to that of
DCAD2 in the gonad (data not
shown). We also observed small ring-like structures labeled with the DCAD2
antibody interspersed throughout the gonad at all stages of development (small
arrow in Fig. 3B,C). However,
these ring structures do not stain with
ARM antibodies and their
DCAD2 immunoreactivity is unchanged in embryos homozygous for a
deletion that removes shg/E-cadherin (in contrast to other
Drosophila E-cadherin staining in the gonad, which is greatly
reduced). Thus, either these structures represent highly stable complexes of
maternal E-cadherin, or they represent artifactual staining of the
DCAD2 antibody.
As E-cadherin is localized to sites of germ cell-soma contact, it may act
as a homophilic cell adhesion molecule to promote this interaction. In this
model, E-cadherin should be present on the plasma membranes of both germ cells
and SGPs. To test this, we expressed mCD8-GFP in either germ cells or SGPs and
looked for colocalization with E-cadherin. We find that DCAD2
colocalizes with
GFP at the cell surface of germ cells expressing
mCD8-GFP (Fig. 3F).
DCAD2 also labels migrating germ cells before they reach the site of
the gonad (Fig. 3G), providing
additional evidence that E-cadherin is present in the germ cells. When
mCD8-GFP is expressed in SGPs (Fig.
3H), we also see colocalization with E-cadherin. Furthermore,
E-cadherin staining is observed in gonads that lack germ cells
(Fig. 3I), confirming that it
is expressed in SGPs. Thus, E-cadherin appears to be on the cell surface of
both germ cells and SGPs, which is consistent with a role for this protein in
promoting cell-cell adhesion between these cell types. Interestingly,
E-cadherin does not appear to be localized to regions where germ cells are
contacting other germ cells (black arrowhead,
Fig. 3F) instead of SGPs. Thus,
it appears to be preferentially engaged in cell contacts between germ cells
and SGPs.
To investigate whether E-cadherin expression in the somatic gonad is dependent upon genes that specify SGP identity, we examined its localization in eya mutant embryos. Immunostaining in eya mutants reveals a lack of somatic E-cadherin in the gonad region, although germ cell expression is still clearly present (Fig. 3J). Antibodies against the SGP marker ZFH-1 show a decrease of ZFH-1-positive cells in the gonad region as expected. Some of the remaining ZFH-1 positive cells associate with germ cells, suggesting they are residual SGPs. Although a few of these remaining cells express some E-cadherin (not shown), most show a lack of E-cadherin expression (Fig. 3J). Thus, the SGP identity gene eya is required for proper E-cadherin expression in SGPs.
E-cadherin is required for gonad compaction and germ cell
ensheathment
We examined gonad formation in shg/E-cadherin mutants to determine
what aspects of gonad formation require E-cadherin. Interestingly, we find
that shg is required for both gonad compaction and germ cell
ensheathment. In shg mutants, gonad compaction is sometimes
initiated, but often does not proceed to completion
(Fig. 4A,B). The three clusters
of SGPs from PS10-12 are able to associate correctly with one another, and
with germ cells, to form a cohesive group. However, these cells often remain
loosely associated and spread over more than one parasegment, rather than
compacting tightly in PS10. In the most severe cases, compaction from PS10-12
to PS10 appears completely blocked (Fig.
4A). In weaker examples, compaction is initiated but not
completed, resulting in partially compacted and misshapen gonads
(Fig. 4B). Phenotypes are
stronger with the shgG317 allele (22% severe, 41% weak,
n=49) than with shg1H (3% severe, 50% weak,
n=36), consistent with previous observations on the relative
strengths of these alleles (Tepass et al.,
1996). shg/E-cadherin also has a strong maternal
contribution, which cannot be removed because it is required for oogenesis
(González-Reyes and St Johnston,
1998
; Godt and Tepass,
1998
). It is likely that the loss of both maternal and zygotic
E-cadherin would result in an increased penetrance of the more severe
phenotype.
|
To analyze the role of E-cadherin in germ cell ensheathment, we expressed mCD8-GFP in the mesoderm of shg mutants. Germ cell ensheathment is clearly defective in these embryos (Fig. 4E). shg mutant gonads exhibit a dramatic reduction in the extent to which germ cells are surrounded by SGP-derived mCD8-GFP, and we often saw gonads where germ cells showed little or no mCD8-GFP extending around them (Fig. 4E, compare with Fig. 1D). Again, the germ cell ensheathment defect was stronger in shgG317 than in shg1H. Interestingly, the severity of the germ cell ensheathment defect did not correlate with the severity of the gonad compaction defect.
Finally, we have observed that shg mutants also display defects in germ cell migration. Although part of this defect is likely to be to be due to a zygotic requirement for E-cadherin in tissues through which the germ cells move, our data indicate that there is also a requirement for E-cadherin in the germ cells themselves for proper migration to the gonad. First, we clearly see E-cadherin expression within the germ cells while they are migrating (Fig. 3H). Second, shgG317 exhibits a dominant, maternal effect on germ cell migration (Table 1). Offspring from heterozygous shgG317/+ females have a clear germ cell migration defect (Fig. 4F), independent of the zygotic genotype. Offspring from the reciprocal cross using shgG317/+ males show no germ cell migration defect. As the offspring from heterozygous females are viable, the dominant maternal effect is unlikely to be causing a global disruption of the embryonic tissues through which the germ cells are moving. Instead, it is likely to reflect a role for maternal E-cadherin within the germ cells themselves for proper migration.
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Interestingly, foi does not affect E-cadherin staining in the
msSGPs (Fig. 6C). As they join
the main body of the gonad, msSGPs in foi mutants
(Fig. 6C, arrow) display
E-cadherin immunoreactivity similar to that seen in msSGPs of wild-type
embryos (Fig. 3C,F, large
arrow). Double-staining for Sox100B and E-cadherin confirm the expression of
E-cadherin within msSGPs (data not shown). msSGPs represent an aspect of gonad
development that differs in its requirement for shg/E-cadherin versus
foi: these cells fail to associate correctly with the gonad in
shg mutants (Fig. 4F), but they behave normally in foi mutants
(DeFalco et al., 2003). This
suggests that the basis for the foi mutant phenotype may be due to
the effects of foi on E-cadherin.
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DISCUSSION |
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E-cadherin and gonad morphogenesis
Our detailed analysis of gonad coalescence has shown that it can be
subdivided into two processes: gonad compaction and germ cell ensheathment. In
gonad compaction, SGPs and germ cells physically condense together to create a
rounded organ. Germ cell ensheathment is characterized by the dramatic shape
changes of SGPs that produce thin cellular extensions that surround the germ
cells. Germ cells lack cellular extensions during gonad compaction, and need
not be present for compaction to occur. This suggests that SGPs provide the
`driving force' behind the movements of compaction and germ cells play a more
passive role.
Several pieces of data indicate that gonad compaction and germ cell ensheathment are distinct, separable events. Germ cell ensheathment is already apparent at stage 13, prior to the onset of compaction. In addition, compaction proceeds normally in agametic embryos, despite a lack of germ cell ensheathment. Furthermore, in mutants that affect gonad coalescence (shg, foi), we have observed examples of gonads with no ensheathment but a high degree of compaction, and also gonads with good ensheathment but little compaction (data not shown). Thus, gonad compaction and germ cell ensheathment are independent processes that together contribute to the proper architecture of the coalesced embryonic gonad. Both of these processes require the adhesion molecule E-cadherin.
How might Drosophila E-cadherin be functioning to promote gonad
morphogenesis? Differential cell adhesion mediated by E-cadherin has been
shown to govern cell sorting in vitro
(Steinberg and Takeichi, 1994)
and in at least one in vivo situation
(Godt and Tepass, 1998
;
González-Reyes and St Johnston,
1998
). It is possible to explain our observations of gonad
morphogenesis with a similar model of differential cell adhesion. In this
model, gonad compaction results from an increased affinity of SGPs for one
another relative to the surrounding mesoderm. Compaction would occur as SGPs
maximize their contacts with one another and minimize their contacts with the
surrounding mesoderm, hence forming a sphere. Contacts between SGPs and germ
cells might also play a role in compaction, but SGP-SGP affinity would be
sufficient to allow this process to occur in the absence of germ cells.
Consistent with this hypothesis, E-cadherin expression becomes more apparent
in SGPs relative to the surrounding mesoderm at the time that compaction is
initiated (Fig. 3). This is
likely to reflect an increase in E-cadherin expression or stability, but could
also conceivably result from a change in subcellular localization.
Upregulation of E-cadherin in the SGPs may contribute to an increase in
SGP-SGP adhesion during gonad compaction.
The process of germ cell ensheathment may also be controlled by differential cell adhesion, but between SGPs and germ cells. Ensheathment would occur as a result of SGPs maximizing their contacts with germ cells. This model requires that SGPs and germ cells have a higher affinity for each other than for their own cell type. A prediction of this model is that ensheathment would be blocked if germ cell-germ cell adhesion were increased, which is exactly what we observe (Fig. 5).
What role might E-cadherin, traditionally a homophilic cell adhesion
molecule, play in mediating the heterotypic interactions between SGPs and germ
cells during ensheathment? One possibility is the presence of additional
heterophilic adhesion molecules that promote specific adhesion between these
cell types. A candidate member of such a heterophilic adhesion system is
Neurotactin, which is present on SGPs and has been shown to promote
heterotypic cell adhesion (Barthalay et
al., 1990). In this case, E-cadherin could provide additional
`glue' that is required for ensheathment once the heterotypic specificity
between SGPs and germ cells is established. Alternatively, E-cadherin might
somehow be biased to act in a heterophilic manner. E-cadherin could interact
with a heterophilic binding partner (e.g.
Cepek et al., 1994
), or could
be biased by a modification or co-factor to bind preferentially to E-cadherin
molecules on heterotypic cells (e.g. a modified form of E-cadherin might
interact only with an unmodified form).
Gonad coalescence may represent an elegant example of organogenesis based on differential cell adhesion. A hierarchy of cell affinity (SGP-germ cell>SGP-SGP>SGP-surrounding mesoderm) can account for much of the observed gonad organization. This model requires cell movement for proper execution of cell sorting. Although the morphology of the germ cells suggests that they may not be highly motile at this time, further work is needed to determine the extent to which SGP versus germ cell movement contributes to this process. In addition, other mechanisms, such as cytoskeletal-derived contractile and protrusive forces, may also be important for compaction and ensheathment. The contribution of these different factors to the overall architecture of the gonad can now be further tested using our more detailed understanding of gonad coalescence.
E-cadherin and fear of intimacy
Embryos with mutations in the fear of intimacy gene share several
gonad defects with shg mutant embryos, including defects in gonad
compaction and germ cell ensheathment. Both genes are also required for
tracheal branch fusion (Tanaka-Matakatsu
et al., 1996; Van Doren et
al., 2003
), suggesting that Drosophila E-cadherin and FOI
may work together to promote all of these processes. Consistent with this, we
show that E-cadherin protein levels are severely reduced within the gonads of
foi mutants. E-cadherin expression is reduced in SGPs, which display
defective behaviors in foi mutants, but not msSGPs, which appear to
behave normally (Fig. 5). Thus,
gonad defects in foi mutants correlate strongly with the cells in
which E-cadherin expression is most affected, suggesting that this may be the
cause of the foi mutant phenotype.
There are several possible models for how FOI, a cell surface, multipass
transmembrane protein, might be affecting the levels of E-cadherin protein.
First, FOI could act as a receptor or channel that signals the beginning of
coalescence. Upregulation of E-cadherin in the SGPs could require such a
signal. Or, FOI might act to localize E-cadherin complexes to sites of germ
cell-soma and soma-soma contact within the gonad. As such, FOI could act
during the export of E-cadherin to the cell surface, or to localize E-cadherin
to specific sites of cell-cell contact. Alternatively, FOI might affect
E-cadherin levels by affecting its function as a cell adhesion molecule. It
has been suggested that the stability of E-cadherin is tightly linked to its
function in adhesion complexes, with reduced E-cadherin function leading to a
faster turnover of the protein (Tepass et
al., 1996). FOI might modulate E-cadherin function by acting as a
co-factor itself on the cell surface, or by acting as a transporter to alter
the concentration of a small molecule modulator of E-cadherin adhesion, such
as Ca2+.
Germline-soma interactions in gonad development
Germ cell ensheathment in the Drosophila embryonic gonad is an
example of a recurring theme in germ cell development; germ cells require
close contact with specialized somatic cells for their proper differentiation.
Germ cell-soma interaction has been shown to be essential for many phases of
germ cell development in diverse species. The proper sexual identity of the
germline is controlled by the soma in both the mouse and the fly
(Steinmann-Zwicky et al.,
1989; Adams and McLaren,
2002
). In addition, germ cells often exist as stem cells in the
adult gonad, dividing to produce one daughter that enters gametogenesis while
the other retains stem cell identity. Interaction between germline stem cells
and their somatic niche is essential for regulating cell division and stem
cell maintenance (reviewed by Spradling et
al., 2001
). Finally, during gametogenesis, differentiating germ
cells remain in close association with somatic cells that regulate their
development into sperm or egg.
Adhesive contacts and cell-cell junctions are crucial for soma-germline
signaling. Some somatic signals require specific cellular junctions, such as
gap junctions (Tazuke et al.,
2002; Kidder and Mhawi,
2002
). Even secreted signals, such as those governing the
regulation of germline stem cell maintanence, require the proper adhesion and
orientation between germline and soma
(Song et al., 2002
).
E-cadherin has been shown to play a crucial role in several examples of germ
cell-soma interaction, including in the stem cell niche and developing egg
chamber in Drosophila (Song et
al., 2002
; Godt and Tepass,
1998
; González-Reyes
and St Johnston, 1998
;
Niewiadomska et al., 1999
;
Geisbrecht and Montell,
2002
).
Regulation of germ cell development by the soma may begin as soon as the
gonad forms. There is evidence that the soma regulates sex determination and
the cell cycle in the mouse germline (Adams
and McLaren, 2002) and the pattern of germ cell gene expression in
Drosophila (Mukai et al.,
1999
) at very early stages. Thus, regulation by the soma is
crucial for every stage of germ cell development. We hypothesize that the
E-cadherin-dependent germ cell ensheathment we have observed in embryonic
gonads creates a nascent niche that allows the SGPs to regulate germ cell
development and the transition to germline stem cells.
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ACKNOWLEDGMENTS |
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REFERENCES |
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