Centre de Biologie du Développement, CNRS UMR 5547, 118 route de Narbonne, 31062 Toulouse, France
* Author for correspondence (e-mail: haenlin{at}cict.fr)
Accepted 9 August 2005
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SUMMARY |
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Key words: RUNX, GCM, Hematopoiesis, Cell fate choice, Drosophila
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Introduction |
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Despite the evolutionary distance between Drosophila and
vertebrates, many of the molecular pathways governing hematopoiesis have been
conserved (Evans et al., 2003).
In particular, transcription factors of the GATA, FOG and RUNX families, which
regulate several steps of hematopoiesis in vertebrates, also control
Drosophila hematopoiesis (Fossett
et al., 2001
; Lebestky et al.,
2000
; Rehorn et al.,
1996
). In the embryo, all prohemocytes express the GATA
transcription factor Serpent (SRP), which is required for blood cell precursor
specification and maintenance (Rehorn et
al., 1996
). From this pool of prohemocytes, two populations of
hematopoietic cells will emerge, plasmatocytes and crystal cells
(Lebestky et al., 2000
). SRP
participates in their differentiation and remains expressed in the
differentiated blood cells (Fossett et
al., 2003
; Waltzer et al.,
2002
). Furthermore, key lineage-specific factors are required for
the differentiation into these two cell types. On the one hand, the RUNX
transcription factor Lozenge (LZ) forms a functional complex with SRP to
induce crystal cell formation (Waltzer et
al., 2003
), and in a lz mutant no crystal cells appear
(Lebestky et al., 2000
). On
the other hand, the two related transcription factors Glial cells missing
(GCM) and GCM2 (also known as Glide and Glide2) are jointly required for
plasmatocyte differentiation (Alfonso and
Jones, 2002
; Bernardoni et al.,
1997
; Kammerer and Giangrande,
2001
). In the absence of both gcm and gcm2
(gcm/gcm2), plasmatocytes do not differentiate normally and
their number is strongly reduced, whereas crystal cell formation appears
unaffected (Alfonso and Jones,
2002
). In addition, enforced expression of GCM in crystal cells
converts them into plasmatocytes (Lebestky
et al., 2000
). By contrast, ectopic expression of LZ in
plasmatocytes induces crystal cell marker expression but does not repress
plasmatocyte cell fate (Waltzer et al.,
2003
).
It is proposed that, for their differentiation, crystal cells must express
lz but not gcm/gcm2, whereas plasmatocytes have to
express gcm/gcm2 but not lz
(Lebestky et al., 2000).
Interestingly, gcm/gcm2 expression is detected from stage 5
in the hematopoietic primordium (Alfonso
and Jones, 2002
; Bernardoni et
al., 1997
), whereas the onset of lz expression is only
detected later (at stage 10) (Lebestky et
al., 2000
). These observations raise several questions: (1) do the
crystal cell and the plasmatocytes precursors emerge from the same pool of
prohemocytes; (2) when do the two populations segregate; and (3) what is the
relationship between lz and gcm/gcm2 during blood
cell lineage choice?
To address these questions, we have undertaken an analysis of the mechanism
of segregation of the two embryonic blood cell lineages. We find that
gcm is expressed early on in all prohemocytes but is rapidly
downregulated in the anterior-most cells of the hematopoietic primordium,
which initiate lz expression by stage 7. Our results suggest that the
coordinated repression of gcm and activation of lz in the
anterior row of prohemocytes is a key step in the regulation of blood cell
lineage choice. In the absence of both gcm and gcm2, we
observe a striking increase in the number of crystal cells, indicating that
gcm/gcm2 actually regulates crystal cell development. We
further show that gcm/gcm2 inhibits crystal cell formation
in two steps: first by regulating the number of cells that initiate
lz expression, and second by interfering with lz maintenance
in these cells. Furthermore, contrary to what has been reported during larval
hematopoiesis (Duvic et al.,
2002; Lebestky et al.,
2003
), we demonstrate that Notch signalling is neither sufficient
nor required for crystal cell formation. Finally, our results indicate that
prohemocytes are bipotent progenitors, and that the interplay between
gcm/gmc2 and lz expression dictates the cell fate
choice.
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Materials and methods |
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Unless specified, crosses and embryo collections were performed at 25°C. To induce transient expression of GCM, hs-gal4; uas-gcm embryos were collected at 18°C for 3 hours, aged at 18°C for 6, 9 or 12 hours, heat shocked twice at 37°C for 20 minutes, and aged at 18°C for 16, 13 or 10 hours, respectively, before being processed for analysis.
Plasmids and transgenesis
The 1.5 kb upstream regulatory region of lz (nucleotides 234118 to
235562 on the genomic scaffold AE003446) was cloned into pCasper-hs43-lacZ to
generate pLZ-lacZ. The corresponding P{lz-lacZ} transgenic lines were
generated by standard P-element-mediated transformation into
w1118 flies.
In situ hybridization and antibody staining
The in situ hybridization technique and probes used have been previously
described by Waltzer et al. (Waltzer et
al., 2003).
For double fluorescent staining, the following antibodies were used: rabbit
anti-Serpent antibody (1/500) (Reuter,
1994), rabbit anti-ß-GAL antibody (1/500; Cappel
Pharmaceutical), goat anti-rabbit Alexa Fluor 488 (1/400; Molecular Probes),
sheep anti-DIG antibody (1/500; Roche), donkey anti-sheep antibody Alexa Fluor
488 (1/400; Molecular Probes) and/or anti-fluorescein AP (1/1000; Roche),
revealed with Fast Red substrate (Vector).
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Results |
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gcm is initially expressed in all prohemocytes but is rapidly downregulated in crystal cell precursors
GCM and GCM2 are co-expressed transcription factors that act redundantly to
induce plasmatocyte differentiation
(Alfonso and Jones, 2002) and
that have been suggested to be plasmatocyte specific
(Alfonso and Jones, 2002
;
Lebestky et al., 2000
). We
investigated whether gcm is expressed only in a sub-population of
prohemocytes that will give rise to plasmatocytes or in all prohemocytes,
including the prospective crystal cells. As shown
Fig. 1, at stage 5 (2 hours and
10 minutes to 2 hours and 50 minutes after egg laying; AEL), gcm and
srp transcripts co-localise in all of the cells that constitute the
hematopoietic primordium (Fig.
1A). gcm2 is also expressed in the hematopoietic anlage
from stage 5, but at a much lower level than gcm
(Alfonso and Jones, 2002
),
therefore precluding the precise analysis of its expression domain by
fluorescent in situ hybridization. Interestingly, at stage 6 (2 hours and 50
minutes to 3 hours AEL), gcm transcripts are no longer detected in
the anterior-most row of srp-expressing cells
(Fig. 1B). We extended this
observation by comparing the localisation of the gcm transcripts with
that of the SRP protein (Fig.
1C). Thus, although gcm is initially expressed in all
prohemocytes, its expression is subsequently downregulated in a sub-population
of prohemocytes.
To confirm that gcm is also expressed in the crystal cell
precursors, we analysed the expression of the P{lacZ} insertion in
gcm (gcmrA87), previously shown to recapitulate
gcm expression (Bernardoni et al.,
1997). Taking advantage of the long half-life of ß-gal, we
observed that almost all of the crystal cells (DoxA3-positive cells)
co-expressed ß-gal by stage 11 (Fig.
1K). However, ß-gal staining diminished progressively in the
crystal cell population at later stages
(Fig. 1L,M). For comparison,
SRP (Lebestky et al., 2000
) or
srp-gal4/uas-lacZ expression is maintained in crystal cells
throughout embryogenesis (Fig.
1N). Hence, although gcm is initially expressed in all
prohemocytes, it is rapidly repressed in the crystal cell precursors. Thus,
the transcriptional downregulation of gcm in the presumptive crystal
cell progenitors (and its maintenance in the remaining prohemocytes) is the
earliest known manifestation of a blood cell-lineage choice.
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Two fates for lozenge-expressing cells: crystal cells and plasmatocytes
Crystal cell precursors differentiate into two bilaterally symmetrical
groups of cells that remain localised around the proventriculus
(Lebestky et al., 2000). In
the course of our experiments with the uas-gal4 amplification system,
we noticed that some lz-gal4/uas-lacZ-positive cells were
localized outside the crystal cell clusters
(Fig. 1H,I) and did not express
DoxA3 (Fig. 3E),
suggesting that not all the cells that initiate lz expression
differentiate into crystal cells. This enticed us to investigate the fate of
all of the cells that initiate lz expression (so-called
lz+ progenitors). As shown in
Table 1, by stage 14, 60% of
the marked cells are clustered along the proventriculus and co-express the
crystal cell marker DoxA3. The remaining 40% are scattered throughout
the embryo, lack the expression of crystal cell-specific markers and
morphologically resemble macrophages (Fig.
1J). Therefore, only a fraction of the lz+
progenitors differentiates into crystal cells, while the rest become
plasmatocytes. These data, together with the observation that gcm is
initially expressed in all prohemocytes and is required for plasmatocyte
differentiation (Alfonso and Jones,
2002
; Kammerer and Giangrande,
2001
), led us to re-assess the role of gcm/gcm2 during
blood cell lineage choice.
|
|
To ensure that the above phenotypes were due to the lack of gcm
and/or gcm2 but not to another gene deleted by the deficiencies, we
also examined the phenotypes of gcm or gcm2 single mutants.
Lack of gcm resulted in a marked increase in the number of crystal
cells (Fig. 2L,
Table 1), whereas lack of
gcm2 alone did not significantly affect their development
(Fig. 2J). The phenotypes were
not enhanced when a gcm or gcm2 mutation was combined with
Df(2L)200 (Fig. 2K,M).
Nonetheless, the strongest phenotype was observed when both gcm and
gcm2 were deleted (Fig.
2D, Table 1). These
results indicate that gcm is primarily responsible for inhibiting
crystal cell development, and that its lack of function can be partially
compensated for by gcm2. To verify that gcm2 is also able to
inhibit crystal cell fate, we overexpressed gcm2 in crystal cell
precursors. Similar to what was shown for GCM
(Lebestky et al., 2000)
(Fig. 2N), lz-gal4-driven expression of GCM2 inhibited crystal cell formation
(Fig. 2O). Thus, both
gcm and gcm2 are capable of inhibiting crystal cell
formation.
Comparison between the expression patterns of the crystal cell-specific
marker DoxA3 and the panhemocyte marker peroxidasin
(Pxn) (Nelson et al.,
1994) indicated that most hemocytes do not acquire the crystal
cell fate in gcm/gcm2 mutant embryos (Fig.
2D, compared with
2R). Thus, in a
Df(2L)200 mutant embryo, it was shown previously that there are
approximately 250 PXN-labeled cells
(Alfonso and Jones, 2002
),
whereas we observed about 90 crystal cells, and 40% of them were mislocated
(Table 1). Because some
gcm activity is maternally contributed
(Bernardoni et al., 1997
), we
wondered whether this might explain why only some prohemocytes adopt a crystal
cell fate or some crystal cells migrate. However, even in embryos derived from
Df(2L)200 germline clones, ectopic crystal cells were still present
(Fig. 2P) and most hemocytes
did not differentiate as crystal cells
(Fig. 2S). Thus, although
gcm and gcm2 inhibit crystal cell development, their absence
is not sufficient to cause a complete switch in the fate of the hematopoietic
precursors from plasmatocytes to crystal cells.
|
We next analysed the fate of the lz+ progenitors. In stage 14 embryos, we observed a large increase in the number of lz-gal4/uas-lacZ-expressing cells in the absence of gcm/gcm2 (Table 1, Fig. 3D). Double labelling indicated that almost all of the lz-gal4/uas-lacZ-expressing cells, even those scattered throughout the embryo, co-expressed the crystal cell marker DoxA3 (Table 1, Fig. 3F). This is in marked contrast with the wild-type situation, where only the lz-gal4/uas-lacZ cells located around the proventriculus express DoxA3 (Table 1, Fig. 3E). Thus, in the absence of gcm/gcm2, all the lz+ progenitors, both those that remain near the proventriculus and those that migrate to distant positions, differentiate into crystal cells.
To ensure that the increase in crystal cells was directly related to a rise
in the number of lz+ progenitors and not to a modification
of the proliferation program, we determined the number of crystal cells formed
in the absence of cell proliferation. Accordingly, we introduced the
string (Drosophila cdc25) mutation, which blocks all
postblastodermal cell divisions (Edgar and
O'Farrell, 1990), and monitored the number of crystal cells formed
in wild-type or in Df(2L)200 mutant embryos. Even in a
string mutant context, lack of gcm/gcm2 induced a
twofold increase in the number of crystal cells
(Fig. 3, compare G with H).
In summary, the absence of gcm/gcm2 results in an increase in the number of lz+ progenitors and allows all of them to differentiate into crystal cells. These data suggest that gcm and gcm2 inhibit crystal cell differentiation by a two-step process. First, they limit the induction of lz expression to a subset of prohemocytes, thereby regulating the number of crystal cell precursors. Second, they inhibit the acquisition of the crystal cell fate in 40% of the lz+ progenitors, which take on a plasmatocyte fate instead.
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All of the cells initiating lz expression differentiate into crystal cells in the absence of gcm/gcm2, whereas 40% of them do not in a wild-type situation. To address the possibility, that gcm also inhibits the maintenance of lz, we induced gcm overexpression at different times during embryogenesis using a hs-gal4 driver (see Materials and methods for details). Interestingly, transient ectopic expression of GCM around stage 9, 10 or 11 still repressed crystal cell formation (Fig. 4L-N). Therefore gcm probably also inhibits the maintenance of lz expression.
As lz expression must be maintained for crystal cell
differentiation (Lebestky et al.,
2000), we wondered whether lz expression might be
auto-activated. Reminiscent of its capacity to induce ectopic crystal cell
markers (Waltzer et al.,
2003
), twi-gal4-driven LZ expression activated
lz-lacZ in the srp-expressing domains
(Fig. 4I). Indeed, SRP and LZ
form a functional complex to induce crystal cell development
(Waltzer et al., 2003
).
Therefore we ascertained whether they also cooperate to regulate lz
expression. Whereas we observed restricted ectopic activation of
lz-lacZ by SRP or LZ alone (Fig.
4H,I), lz-lacZ was strongly activated throughout the
mesoderm when SRP and LZ were co-expressed
(Fig. 4J). Thus SRP and LZ
cooperate to maintain lz expression via a positive-feedback loop.
If gcm (and gcm2) antagonizes crystal cell development only by repressing lz expression, we surmised that we might rescue crystal cell formation by uncoupling lz expression from gcm regulation. Accordingly, we co-expressed LZ and GCM under the control of the srp-gal4 driver and monitored crystal cell marker expression. As shown in Fig. 4, LZ induced DoxA3 expression to a similar extent in the presence or in the absence of overexpressed GCM, indicating that GCM does not inhibit LZ function (compare Fig 4Q with 4R). All together, these data demonstrate that gcm and gcm2 repress crystal cell formation by inhibiting both the induction and the maintenance of lz transcription.
Notch signalling is not required for crystal cell formation in the embryo
During larval hematopoiesis, Notch signalling in the lymph gland is
required and is sufficient to induce crystal cell formation
(Duvic et al., 2002;
Lebestky et al., 2003
).
Furthermore, the number of crystal cells is decreased in a Notch
mutant embryo, suggesting that Notch signalling might also be required for
embryonic crystal cell formation (Lebestky
et al., 2003
). Since the remaining crystal cells in a
Notch mutant might reflect Notch maternal contribution
(Lebestky et al., 2003
), we
generated Notch mutant germ line clones. We still observed crystal
cells, both in embryos derived from Notch germline clones
(12.8±2.6 crystal cells; n=21) and in Notch zygotic
mutants (15.9±3.5 crystal cells; n=19;
Fig. 5B,C). Therefore, although
Notch participates in embryonic crystal cell development, it is clearly not
required. To test whether ectopic Notch signalling activates crystal cell
formation in the embryo, we expressed an activated form of Notch in all of the
prohemocytes using the srp-gal4 driver or in plasmatocytes using the
pg33 driver. In neither case did we observe more crystal cells
(Fig. 5D,E). Therefore,
contrary to the previous suggestion
(Lebestky et al., 2003
), we
show that Notch signalling is neither sufficient nor required to induce
crystal cell formation in the embryo.
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Discussion |
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gcm/gcm2 play a pivotal role in the plasmatocyte versus crystal cell developmental decision during embryonic hematopoiesis
It was shown previously that gcm and gcm2 are required
for the proper differentiation of plasmatocytes, and GCM and GCM2 were claimed
to be plasmatocyte-specific lineage transcription factors that are not
involved in crystal cell development
(Alfonso and Jones, 2002;
Lebestky et al., 2000
). By
contrast, our results clearly demonstrate that gcm and gcm2
inhibit crystal cell formation. Furthermore, we detected the expression of
gcm in all of the prohemocytes. including the prospective crystal
cell precursors, at stage 5, a result confirmed by tracing gcm-lacZ
expression into early differentiating crystal cells. Thus, gcm and
gcm2 participate in blood cell fate segregation by regulating not
only plasmatocyte development but also that of crystal cells.
gcm and gcm2 have been most intensively studied during
neurogenesis, where they are required to promote glial cell development at the
expense of neuronal cell fate (Van De Bor
and Giangrande, 2002). We show here that they also regulate a
binary cell fate choice during hematopoiesis. However, although their
expression is restricted to glial precursors during neurogenesis
(Alfonso and Jones, 2002
;
Kammerer and Giangrande, 2001
;
Vincent et al., 1996
), they
are initially expressed in all prohemocytes irrespective of their subsequent
fate. Furthermore, in the absence of gcm/gcm2, whereas
almost all presumptive glial cells are transformed into neurons
(Alfonso and Jones, 2002
;
Kammerer and Giangrande, 2001
;
Vincent et al., 1996
), only a
small proportion of the presumptive plasmatocytes adopts a crystal cell fate.
Therefore, the function and mechanism of action of gcm/gcm2
in regulating cell fate choice during neurogenesis and hematopoiesis are
different.
gcm and gcm2 interfere with crystal cell development at different levels
We have deduced that gcm and gcm2 control crystal cell
formation by a two-step process. First, gcm/gcm2 determines
the number of crystal cell precursors by restricting the initiation of
lz expression in the prohemocyte population
(Fig. 7). In the absence of
gcm/gcm2, we observed more lz+ progenitors,
correlating with a greater number of differentiated crystal cells at later
stages. Our data indicate that gcm is expressed early in the entire
hematopoietic primordium but is rapidly downregulated in the prospective
lz expression domain. Maintaining GCM or GCM2 expression in the
lz+ progenitors inhibited crystal cell differentiation.
Thus, repressing gcm/gcm2 expression in the anterior
population of prohemocytes is most probably a prerequisite for the emergence
of crystal cells.
Second, gcm and gcm2 regulate the proportion of
lz+ progenitors that ultimately differentiate in crystal
cells: whereas 40% of these cells differentiate into plasmatocytes in
wild-type embryos, all of them become crystal cells in the absence of
gcm/gcm2. Lebestky et al. already noted that some
lz-expressing cells differentiate into plasmatocytes and suggested
that this might be due to the de novo activation of gcm expression in
these cells (Lebestky et al.,
2000). Our results extend their observations and demonstrate that
gcm participates in this process, although it is not re-expressed in
the lz+ cells. Our data further suggest that the residual
gcm activity present in the lz+ progenitors may
contribute to the relative plasticity in the fate of these progenitors,
allowing some of them to differentiate into plasmatocytes. In summary, we
provide compelling evidence that gcm and gcm2 play a key
role in regulating cell fate choice in prohemocytes and
lz+ progenitors.
|
It has been shown that lz function is continuously required to
promote crystal cell development (Lebestky
et al., 2000). Here, we have identified an enhancer of lz
that is transactivated by the SRP/LZ complex. This observation suggests that,
once initiated, lz expression can be maintained by a positive
autoregulatory feedback loop, thereby providing a simple mechanism to
stabilise crystal cell lineage commitment. This enhancer contains several
RUNX-binding sites and we are currently investigating the role of these sites
in lz autoregulation. Interestingly, the three mammalian homologues
of the RUNX factor LZ contain several conserved RUNX-binding sites in their
promoters (Otto et al., 2003
).
Furthermore, RUNX2 maintains its own expression through an auto-activation
loop in differentiated osteoblasts (Ducy et
al., 1999
), whereas RUNX3 inhibits RUNX1 expression in B
lymphocytes (Spender et al.,
2005
). Thus, auto- or cross-regulation might be a common feature
of the RUNX family. In addition, we showed that GCM/GCM2 repress
lz expression. However, no consensus GCM-binding sites are present in
the lz crystal cell-specific enhancer. Interestingly, it was recently
shown that zebrafish gcmb is expressed in macrophages
(Hanaoka et al., 2004
). Yet,
the putative functions of the two gcm homologues and their possible
interplays with RUNX factors have not been investigated during vertebrate
hematopoiesis.
Triggering blood cell fate choice
Because gcm is expressed early in the entire hematopoietic anlage,
it is tempting to speculate that prohemocytes are primed to differentiate into
plasmatocytes (i.e. macrophages). Thus, it appears likely that
Drosophila blood cells progenitors are not `naïve'. Similarly,
mammalian stem and progenitor blood cells express low levels of
lineage-affiliated genes and it has been suggested that they are primed for
differentiation (Graf, 2002).
Furthermore, from an evolutionary perspective, macrophages are certainly the
oldest and most pervasive blood cell type
(Lichanska and Hume, 2000
),
and it is remarkable that another hematopoietic cell type may have evolved
from this lineage through the use of a conserved RUNX transcription
factor.
Acquisition of crystal cell fate involves both the repression of the
primary fate (i.e. repression of gcm) and the activation of
lz. Our data show that these two steps are coordinated in space and
time. Nonetheless, the induction of lz is not the mere consequence of
relieving gcm/gcm2-mediated repression of lz but
requires an active and localised process. How gcm transcription is
repressed and lz activated in the anterior row of prohemocytes is
currently unknown. In the lymph gland, Notch/Serrate signalling is necessary
and sufficient to induce crystal cell formation by activating lz
expression (Duvic et al., 2002;
Lebestky et al., 2003
).
However our results demonstrate that, contrary to the situation in larvae,
Notch is not required for crystal cell formation in the embryo. In this
respect, it is interesting to note that neither gcm nor gcm2
is expressed in the lymph gland (B.A., unpublished). Hence, the process that
segregates crystal cells from plasmatocytes relies on different mechanisms in
the embryo and in the larval lymph gland. Similarly, in vertebrates, primitive
and definitive hematopoiesis may also depend on partially distinct programs
(Shepard and Zon, 2000
). For
instance, in mouse, the transcription factor PU.1 plays an essential role in
the emergence of definitive macrophages but does not seem to be required for
the formation of primitive macrophages in the yolk sac
(Lichanska et al., 1999
).
The coincident repression of gcm and activation of lz
between stages 6 and 7 in a row of prohemocytes is remarkable, as it suggests
that the head mesoderm is delicately patterned at this early stage of
development. The hematopoietic primordium is located in the posterior head
region, whose patterning involves several genes including buttonhead,
empty spiracles, orthodenticle and collier
(Crozatier et al., 1999).
However, mutations of these genes do not specifically suppress crystal cell or
plasmatocyte development (L.B., unpublished). Further work will thus be
required to understand the coordination permitting the silencing of
gcm and the activation lz that triggers the choice of one
fate at the expense of the other.
Resolving blood cell fate choice
It was shown that gcm can induce the differentiation of all of the
prohemocytes into plasmatocytes (Lebestky
et al., 2000). The data presented here demonstrate that, in the
absence of gcm/gcm2, lz can transform all of the hemocytes
to crystal cells. Thus, Drosophila prohemocytes are bipotent
progenitors. However, the incapacity of lz to repress gcm
(and thereby plasmatocyte fate) implies that the resolution of cell fate
choice does not rely on reciprocal antagonism between two `lineage-specific'
transcription factors like between GATA1 and PU.1 during myeloid/erythroid
cell fate choice in vertebrates (Galloway
et al., 2005
; Graf,
2002
; Rhodes et al.,
2005
). Instead, we propose that Drosophila embryonic
blood cell fate segregation is a process that can be divided into two
consecutive phases (Fig. 7). A
local cue triggers the process by downregulating gcm and activating
lz in the anterior population of prohemocytes, whereas gcm
expression is maintained in the remaining cells, which differentiate into
plasmatocytes. Then, in the lz+ progenitors, the relative
levels of LZ and GCM will dictate lineage choice. If the ratio of LZ to GCM is
high enough to overcome GCM-mediated repression of lz expression, LZ
can elicit its autoregulatory activation loop and the progenitor will
differentiate into a crystal cell. If not, GCM inhibits lz
autoactivation and the progenitor differentiates into a plasmatocyte. Such a
mechanism of segregation could provide some plasticity, because the size of a
population may be regulated at different times by physiological cues
influencing either the initiation event or the feed-back loop required for its
development.
In conclusion, our data shed light on the transition in vivo from bipotent hematopoietic progenitors to lineage-restricted precursors. Interestingly, the embryonic Drosophila cell fate choice occurs though an original mechanism distinct from that observed during larval hematopoiesis. Moreover, this process does not seem to involve reciprocal negative regulation between two `lineage-specific' transcription factors. Hence, the mechanisms leading to the resolution of hematopoietic lineages in vivo appears to be more complex and diverse than expected.
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ACKNOWLEDGMENTS |
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