Department of Cell Biology, Emory University School of Medicine, Atlanta, GA 30322, USA
* Author for correspondence (e-mail: Kbhat{at}cellbio.emory.edu)
Accepted 1 December 2003
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SUMMARY |
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Key words: Stem cell, Asymmetric division, Mitimere, Nubbin, Cyclin E, Drosophila
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Introduction |
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In the Drosophila CNS, neuroblasts undergo a self-renewing stem
cell type of asymmetric division, whereas their progeny, ganglion mother cells
(GMCs), undergo a non-self-renewing terminal asymmetric division
(Bate, 1976;
Thomas et al., 1984
;
Bhat and Schedl, 1994
;
Buescher et al., 1998
;
Wai et al., 1999
;
Lear et al., 1999
). Recently,
several genes, such as inscuteable (insc), bazooka,
miranda, numb (nb) and Notch (N), have been
shown to be required for the asymmetric division of neural precursor cells
(Buescher et al., 1998
;
Wai et al., 1999
;
Lear et al., 1999
;
Lu et al., 1999
;
Mehta and Bhat, 2001
;
Yedvobnick et al., 2004
). The
asymmetric divisions mediated by these proteins appear to be tied to the
asymmetric segregation of some of these proteins to one of the two daughter
cells during division. For example, during the division of GMC-1 of the
RP2/sib lineage, a most intensely studied CNS lineage (reviewed by
Bhat, 1999
), Insc localizes to
the apical end of GMC-1, whereas Nb segregates to the basal end. The cell that
inherits Nb is specified as RP2 because of the ability of Nb to block
Notch-signaling from specifying sib fate, whereas the cell that inherits Insc
is specified as sib by Notch. Thus, in insc mutants, both the
daughters of GMC-1 adopt an RP2 fate, whereas in nb mutants they
assume a sib fate (Buescher et al.,
1998
; Wai et al.,
1999
). Although there is a good understanding of the terminal
asymmetric division, not much is known about how a stem cell undergoes
self-renewing asymmetric division. Similarly, it is not known what prevents
GMCs from undergoing a stem cell type of asymmetric division. A previous study
implicated Prospero (Pros) in inhibiting the ability of GMCs to divide more
than once by preventing continued expression of cell-cycle genes
(Li and Vaessin, 2000
).
However, this previous study did not examine any GMCs for multiple rounds of
division. Pros is expressed in GMC-1 of the RP2/sib lineage and, in
pros loss-of-function mutant embryos, this GMC-1 identity is not
specified in nearly 95% of the hemisegments (this study). In
5% of the
remaining hemisegments, where GMC-1 identity is normally specified, the GMC-1s
divide only once to generate RP2 and sib cells, as in wild type (this study).
Moreover, in an hypomorphic allele of pros, GMC-1s are specified
properly in
50% of the hemisegments; however, these GMC-1s do not undergo
additional rounds of cell division either. These results suggest that this
lineage (and other GMC lineages) is sensitive for the loss of Pros activity in
terms of additional cell division.
It has been shown that the two POU proteins, Nubbin (Nub; also known as
Pdm1) and Mitimere (Miti; also known as Pdm2), are required for the
specification of identity of GMC-1 of the RP2/sib lineage in the
Drosophila nerve cord (Bhat and
Schedl, 1994; Bhat et al.,
1995
; Yeo et al.,
1995
). By contrast, a brief ectopic expression of these proteins
at high levels prior to GMC-1 division results in a symmetrical division of
GMC-1 to generate two GMC-1s, each of which subsequently divide to generate an
RP2 and a sib (Yang et al.,
1993
; Bhat et al.,
1995
). Consistent with these results is the finding that although
the levels of Nub and Miti are very high in a newly formed GMC-1, their levels
drop significantly prior to GMC-1 division
(Bhat and Schedl, 1994
;
Bhat et al., 1995
). These
results suggest that a downregulation of these two POU proteins is necessary
for the GMC-1 to exit the cell cycle and to undergo a terminal asymmetric
division.
Given these results, we investigated what would happen if miti or
nub genes were overexpressed in GMC-1 at high levels for a prolonged
period of time. We show that such an induction in GMC-1 causes it to
self-renew several times. However, each of these self-renewing divisions
generates a cell that becomes an RP2 or a sib. The self-renewing asymmetric
divisions in these embryos are due to a failure in the downregulation of
Cyclin E (CycE) in late GMC-1 and its unequal distribution between two
daughter cells. An overexpression of CycE in GMC-1 also causes GMC-1 to
undergo a similar type of self-renewing asymmetric division. Moreover, loss of
function of archipelago (ago), which downregulates CycE via
the degradation of the protein (Moberg et
al., 2001), causes a late GMC-1 to accumulate high levels of CycE
and results in its unequal distribution between two daughter cells. This
causes self-renewing asymmetric division of GMC-1. Finally, overexpression of
CycE also causes self-renewing asymmetric division in GMC1-1a of the aCC/pCC
lineage, indicating that downregulation of CycE is essential for other GMCs to
terminally divide into two distinct cells. These results show that when one of
the daughter cell of a GMC acquires high levels of CycE, it behaves as a GMC
with the ability to divide again, while the other differentiates into a
neuron. These results provide insight into how cells can undergo a stem cell
type of asymmetric division and maintain their pluripotency.
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Materials and methods |
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Overexpression experiments
miti, nub or CycE transgenic embryos were collected on
apple-juice agar plates for 15 minutes and appropriately aged embryos (see
text) were heat shocked at 37°C for 20, 25 or 90 minutes. The development
of these embryos post-induction was monitored in halocarbon oil and embryos
were fixed when they reached appropriate stages. To determine the localization
of Insc, 90-minute heat-shocked embryos were either fixed immediately after
the induction or allowed to recover for 20 minutes before fixation. Both
wild-type (heat shocked and non-heat shocked) and non-heat-shocked transgenic
embryos were used as controls. To specifically induce nub in GMC-1,
we crossed UAS-nub and UAS-CycE transgenic flies to
ftz-GAL4 flies, and the embryos were collected and stained with
appropriate antibodies.
Immunohistochemistry and whole-mount RNA in situ hybridization
Embryos were fixed and stained with various antibodies as described
previously (Bhat et al., 1995).
The various primary antibodies used, and their dilutions, were as follows:
anti-Eve (rabbit, 1:2000; or mouse, 1:5), anti-Zfh1 (mouse, 1:400), anti-Vnd
(mouse, 1:1000), Mab22C10 (mouse, 1:4), anti-Spectrin (mouse, 1:50), anti-Insc
(rabbit, 1:500), anti-Ftz (rabbit, 1:200) and anti-ßGal (rabbit, 1:3000
or mouse, 1:400). For CycE staining, the primary antibody incubation was
performed at room temperature. Whole-mount RNA in situ hybridization was
performed using standard procedures (Bhat
and Schedl, 1994
).
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Results |
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Even-skipped (Eve) staining of late stage 12 (10 hour)
mitiP embryos revealed hemisegments with five to seven
cells in a closely associated cluster in
63% of the hemisegments (total
number of hemisegments examined, n=510) in the location of RP2 and
sib cells, as opposed to an RP2 and a sib in wild type (compare
Fig. 1C and
1A). When late stage 14 to
early stage 15 (
13 hour) mitiP embryos were examined
for Eve, duplication of RP2 was observed in
45% of the hemisegments
(n=470), instead of a single RP2 as seen in wild type (compare
Fig. 1D and
1B). The multi-cell clusters
also generate three RP2s (Fig.
1E,F; 5% of the hemisegments, n=470) or a single RP2 (13%
of the hemisegments). These additional cells were indeed RP2s is indicated by
the expression of other RP2-specific genes, such as zfh1
(Fig. 1H), and their axon
morphology (Fig. 1J). The
multi-cell clusters that we observe during stage 12 appear be a collection of
GMC-1s (a maximum of two GMC-1s), RP2s and sibs. It seems likely from the
above results that there is a preference for the generation of sibs rather
than RP2s. Similar results were also observed following overexpression of
nub. Thus, our analysis indicates that mitiP and
nubP embryos can generate as many as three RP2s and two to
four sibs (see Fig. 1E).
Identical heat-shock experiments of wild-type embryos did not result in any of
these phenotypes. Furthermore, no phenotypes were observed in the
GMC1-1a
aCC/pCC lineage, a Miti or Nub-negative lineage. Therefore, we
conclude that the above phenotypes are not heat-induced artifacts.
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We further explored this in another way. Previous results have shown that
Ago downregulates CycE during exit from the cell cycle
(Moberg et al., 2001). Thus,
in ago mutants there is a lack of degradation of CycE protein and,
therefore, cells undergo additional rounds of mitosis. We examined the
GMC-1
RP2/sib lineage in embryos mutant for ago. Several
findings were of interest. First, in ago mutants the GMC-1 was
adopting a self-renewing asymmetric division pattern similar to
gain-of-function CycE or Miti/Nub. Thus, in hemisegments with 3-cell
phenotypes, two sibs and one RP2 (Fig.
7A), and two RP2s and one sib
(Fig. 7B,C) were observed.
Second, the late GMC-1 in the mutant had a higher level of CycE than in wild
type (Fig. 7D,E). Third, an
unequal distribution of CycE between the two daughters of a dividing GMC-1 was
observed (Fig. 7F,G; note the
continuity of Eve between the two daughter cells). As the transcription of
CycE (as judged by RNA in situ hybridization) ceases in late GMC-1
(data not shown) and Ago regulates CycE levels at the protein level, the
higher levels of CycE in one of the two daughters appears to be due to an
unequal distribution of the protein. Fourth, the level of CycE is nearly
undetectable among cells in a 3-cell cluster
(Fig. 7H,I) indicating that by
the time the self-renewed GMC-1 divides again, the level of CycE is
downregulated. Finally, we want to point out that the penetrance of the GMC-1
phenotype was low in ago mutants. The strongest allele,
ago3, had a penetrance of 5% (n=770),
ago1 had a penetrance of 2% (n=1100) and
ago3/df had a penetrance of 7% (n=770); the
upregulation of CycE in late GMC-1 was observed in 7% of the hemisegments in
ago3 embryos. Similarly, the percentage of hemisegments
with elevated levels of CycE in GMC-1, or the asymmetric segregation of CycE
between GMC-1 daughter cells, was 7% (n=336) in the strongest allele.
This correlation indicates that the upregulation of CycE and the observed
additional division of GMC-1 in ago mutants is not an artifact. The
low penetrance can be due to one of several possibilities, such as the
hypomorphic nature of the alleles, maternal deposition of ago
message, or the existence of genetic redundancy for ago during
embryonic development. Nonetheless, our results show that upregulation of CycE
in late GMC-1 results in this GMC adopting a self-renewing asymmetric division
pattern.
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We also find that the effect of upregulation of Miti on Insc localization is not restricted to GMC-1. This is indicated by the fact that Insc distribution is also non-asymmetric in several other GMCs (see Fig. 8L). However, overexpression of Miti (or Nub) in GMC1-1a, which gives rise to aCC/pCC neurons and is a Miti/Nub-negative GMC, has no effect on Insc localization (Fig. 8N), or on the division pattern of the GMC1-1a. This indicates that GMCs that do not normally express Miti are insensitive to its ectopic expression.
The division pattern of GMC-1RP2/sib lineage is not affected in embryos mutant for Prospero
A previous study indicated that Prospero (Pros) acts as a repressor of
cell-cycle genes and functions to restrict the number of divisions a GMC can
undergo to one (Li and Vaessin,
2000). As Pros is present in GMC-1 of the RP2/sib lineage, we
sought to determine (1) whether this GMC-1 undergoes multiple divisions, and
(2) whether such divisions are self-renewing asymmetric divisions. If the
answer to the above questions is in the affirmative, we can then start
examining whether the levels of Pros is affected in GMC-1 of the embryos
overexpressing miti/nub genes. Therefore, we examined pros
mutant embryos for the expression of Eve. In these pros mutant
embryos, we observed an Eve-positive RP2/sib lineage in
5% of the
hemisegments (n=336; we occasionally found pros mutant
embryos that had a fully penetrant missing Eve-positive RP2/sib, U or CQ
lineages). However, the division pattern of GMC-1 in those hemisegments was
normal and no additional RP2 or sib cells were observed in pros
mutant embryos (Fig. 9C,D,F,H).
U and CQ lineages are also formed in
20% and
15% of the of the
hemisegments, respectively (n=336), in pros mutant embryos,
although there was often a reduction in the number of these neurons. In wild
type, the number of Us is four, of which two are Eve and Zfh1 positive
(Fig. 9G). In the pros
mutant embryo where this lineage is formed, we observed usually one Eveand
Zfh1-positive U, and two Eve-positive Us
(Fig. 9H). Similarly, the
number of CQs observed in the mutant embryo is usually one
(Fig. 9J), as opposed to three
in wild type (Fig. 9I; only two
are visible in this focal plane). These results show that no additional
neurons are generated in these lineages in pros mutant embryos. If
Pros had a role in restricting the number of divisions the GMCs can undergo in
these lineages, these GMCs should have divided more than once in the
pros mutant embryo. As we did not observe such divisions, Pros is
unlikely to play a role in the self-renewing asymmetric division pathway
described here for GMC-1 and GMC1-1a.
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Discussion |
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The GMC-1 has the potential to undergo a self-renewing asymmetric division
The strongest evidence that a GMC-1 undergoes a self-renewing type of
asymmetric division in embryos overexpressing miti/nub or
CycE, and in embryos mutant for ago, comes from the presence
of hemisegments with two sibs and one RP2. There are two ways the second sib
cell can be generated: (1) a self-renewed GMC-1 generates another sib when it
divides; and (2) some other cell is transformed into a sib. The following set
of evidence indicates the former scenario. First, the second sib cell always
appears later in development, i.e. at 8.5 hours of age (as opposed to in
wild type where the GMC-1 terminally divides by
7.5 hours of age into an
RP2 and a sib). Second, the dynamics of Eve expression itself in the sib:
expression of eve is switched off in a sib during the asymmetric
division of GMC-1 and there is no de novo synthesis of Eve thereafter. If a
postmitotic cell from an Eve-negative lineage transforms into a sib, it would
be negative for Eve and would not be detected. The development of the other
Eve-positive neuronal lineages is normal in these embryos, thus it is unlikely
that a cell from those Eve-positive lineages is transformed into a sib. Third,
the Eve and Spectrin staining of UAS-nub; ftz-GAL4 embryos provides
more direct evidence for the self-renewal of GMC-1. In
8. 5-hour-old
UAS-nub; ftz GAL4 embryos, we can observe the larger GMC-1 (this
Eve-positive cell is Zfh1 negative, indicating that it is indeed a GMC-1)
undergoing asymmetric cytokinesis for the second time. From the heat-shock
induction experiments of nub or miti mutant embryos, it can
be argued that higher levels of these proteins in the parental NB4-2 cause
later born GMCs to adopt a GMC-1 fate. However, the GMC-1 self-renewing
phenotype observed following targeted expression of nub using the
ftz-GAL driver makes this scenario unlikely. Fourth, the results
obtained with the mitiP; insc and
mitiP; nb double mutant embryos, and the
mis-localization of Insc in GMC-1 of these embryos, are also consistent with
this conclusion.
The level, timing and duration of presence of Miti or Nub proteins determine the kinetics of GMC-1 self-renewal
Our results indicate that the level, timing and duration of presence of
Miti or Nub proteins determine the dynamics of the GMC-1 division pattern. For
example, the asymmetric divisions (which generate the 3-cell phenotypes) and
the symmetric divisions (which generate the 4-cell phenotype) were observed
when the transgenes were induced for 20-25 minutes. However, the multiple
cell-phenotype was observed only when the transgenes were induced for 90
minutes. Once the induction was stopped and the levels returned to normal, the
two GMC-1s appeared to exit from the cell cycle to generate postmitotic cells.
Similarly, when the transgene was induced with ftz-GAL4, only the
3-cell phenotypes, and not the 4-cell or multi-cell phenotypes were observed.
Thus, the following picture emerges from these results. Although high levels
of Miti and Nub proteins are required for the specification of GMC-1 identity,
their level must be downregulated in order for the GMC-1 to divide
asymmetrically into postmitotic RP2 and sib. Maintaining a high level of these
proteins in GMC-1 commits that cell to adopt a self-renewing stem cell type of
division pattern. The results described here also show that Miti and Nub
prevent GMC-1 from exiting the cell cycle by upregulation of CycE (see
below).
The self-renewal of GMC-1 in embryos expressing high levels of Miti or Nub is due to elevated levels of Cyclin E
Our results clearly show that upregulation of CycE in late GMC-1 is the
cause for the adoption of a self-renewing asymmetric division pattern. In
other words, presence of high levels of CycE in late GMC-1 and its unequal
distribution to one of the two daughter cells prevents this cell from exiting
the cell cycle. As this daughter cell still maintains the GMC-1 identity and
has sufficient CycE to divide again, a further asymmetric division(s) is
ensured. The cell that has lower amounts of CycE becomes committed to a
differentiation pathway (RP2 or sib).
What lines of evidence support this conclusion? First, in contrast with wild type, there is a significant amount of CycE present in a late GMC-1 in embryos overexpressing miti or nub. This CycE preferentially segregates to one of the two daughters of that GMC-1, usually the larger cell. When miti or nub genes are overexpressed only briefly, the level of CycE is downregulated after just one additional round of division, whereas with prolonged induction, the level is maintained at high levels in one or two cells of the multi-cell cluster for a prolonged duration of time.
Second, upregulation of CycE in a late GMC-1 is also observed in embryos
mutant for ago, which is known to regulate CycE levels
(Moberg et al., 2001). In
ago mutants, the two daughter cells of such a GMC-1 have unequal CycE
levels accompanied by a self-renewing asymmetric division phenotype. The CycE
is always downregulated after one additional GMC-1 division, which is
consistent with the finding that the self-renewal occurs only once in these
embryos. As penetrance in ago mutants is partial, and CycE is
downregulated in this lineage after just one additional division, there must
be additional factors that mediate the downregulation of CycE in this
lineage.
Third, embryos expressing high levels of CycE from a CycE transgene exhibit the same GMC-1 phenotypes as embryos expressing high levels of Miti or Nub. Thus, these results indicate that upregulation of CycE alone is sufficient for the GMC-1 to adopt a self-renewing type of division pattern. Finally, we find that mitiP phenotypes are dependent on CycE (data not shown). That is, we did not observe multi-cell clusters in mitiP; CycE double mutant embryos.
In wild type, the downregulation of CycE in GMCs appears to occur through switching off CycE transcription and degradation of the protein by factors such as Ago. At what level does Miti or Nub regulate CycE? As these POU genes are thought to be transcriptional activators, they can regulate transcription of CycE either directly or indirectly. However, this does not seem to be the case as expressing high levels of miti did not have a discernible effect on the levels of CycE mRNA in GMC-1, as assessed by whole-mount RNA in situ hybridization (data not shown). In addition, the putative promoter/enhancer region of CycE gene does not contain any consensus POU protein-binding sites. Therefore, it seems likely that Miti and Nub regulate factors that are involved in the degradation of CycE in late GMC-1.
The question arises as to how only one cell has a high level of CycE. There
are several ways this can happen. There might be an asymmetric degradation of
CycE. This scenario seems unlikely as there is only one of two daughter cells
with high levels of CycE in ago mutants. Given that Ago downregulates
CycE via a protein degradation mechanism
(Moberg et al., 2001), if
there was an asymmetric degradation, in those hemisegments where the levels of
CycE was elevated in GMC-1, we would initially expect both the daughter cells
to have high CycE levels. However, this was not the case. An asymmetric
transcription of the CycE gene also seems unlikely as the
transcription of CycE ceases prior to GMC-1 division, as judged by
whole-mount RNA in situ hybridization (data not shown). The most likely
possibility is that CycE is unequally distributed between the two daughter
cells of GMC-1. The unequal distribution of CycE could be a passive process
due to the size difference between daughter cells, especially in the
GMC-1
RP2/sib lineage. Moreover, we did not observe a cytoplasmic
crescent of CycE during mitosis. By contrast, it could also be an active
process. For instance, the size difference between an aCC and a pCC (or
between a GMC1-1a and an aCC) is very small, and the fact that GMC1-1a
undergoes a self-renewing asymmetric division suggests that the segregation of
CycE may not be entirely a passive process.
CycE can induce self-renewing asymmetric divisions in other GMCs
Finally, our results indicate that while a GMC that does not normally
express Miti or Nub is insensitive to its ectopic expression (e.g. GMC1-1a of
NB1-1; this GMC produces an aCC/pCC pair of neurons), a brief induction of
CycE in the same GMC causes it to undergo self-renewing asymmetric
division. Therefore, CycE can confer a stem cell type of division potential to
more than one GMC. Another important conclusion one can draw from this result
is that the segregation of CycE may be an active process. In the case of
GMC1RP2/sib lineage, the cytokinesis of GMC-1 is asymmetric, and the
size difference between an RP2 and a sib is significant. Thus, CycE can be
asymmetrically segregated because of this size difference. However, the size
difference between an aCC and a pCC (or between a GMC1-1a and an aCC) is very
small, and the fact that GMC1-1a undergoes a self-renewing asymmetric division
suggests that the segregation of CycE may not be entirely a passive process.
It is possible that the difference between the levels of CycE needed to retain
a cell within the cell cycle and the levels that do not support maintaining
the cell within the cell cycle is quite small. Thus, even a minor change in
the amount that a cell receives during division might be sufficient to make a
difference. Thus, the segregation of CycE can still be a passive process.
Nonetheless, these results reveal how a cell can adopt a self-renewing
asymmetric division potential through CycE.
A previous study implicated Pros in inhibiting the ability of GMCs to
divide more than once by preventing continued expression of cell-cycle genes
(Li and Vaessin, 2000). The
caveat of this study, however, is that none of the GMC lineage was examined
using cell-specific markers to determine whether GMCs continue to divide in
embryos mutant for pros. The conclusion that Pros inhibits GMC
division was mainly based on the presence of additional BrdU-positive cells in
late stage (post 15-hours-old) pros mutant embryos. Pros is expressed
in GMC-1 of the RP2/sib lineage and, in null alleles, this GMC-1 identity is
not specified (Doe et al.,
1991
) We found that in pros17, a
loss-of-function allele,
5% of the hemisegments had an RP2/sib lineage
specified. In these hemisegments, the GMC-1 divides only once to generate an
RP2 and a sib cell as in wild type (Fig.
9). Moreover, we also observed specification of U and CQ lineages
in
20% and
13% of the hemisegments, respectively, and no additional
cell division appeared to occur in these lineages. A previous study found that
the aCC/pCC neurons (from GMC1-1a) have an abnormal axon morphology, but it
did not find any additional neurons in this lineage
(Doe et al., 1991
). Similarly,
NB6-4 of the thoracic segment produced the normal number of progeny in
pros mutant embryos (Akiyama-Oda
et al., 2000
). These results suggest that Pros does not regulate
cell division in RP2/sib, U and CQ lineages, and possibly not in many other
neuronal lineages, and therefore it is unlikely to function in the
miti/nub pathway.
Relationship between elevated levels of Miti/Nub and localization of Insc
How is the specification of identity of one of the two progeny, either as
an RP2 or as a sib, from a self-renewing asymmetric division of GMC-1
regulated? (Specification of the other progeny as GMC-1 is by high levels of
CycE.) Our results indicate that specification of an RP2 versus a sib identity
to this differentiating cell is through a combination of low levels of CycE
and localization of Insc. This is indicated by the finding that overexpression
of Miti and Nub causes localization of Insc to be non-asymmetric.
Non-asymmetric Insc also causes non-asymmetric localization of Nb. The cell
that has lower levels of CycE and also has Nb becomes an RP2. Whenever the
cell with lower levels of CycE fails to inherit Nb (the effect of
overexpression of Miti or Nub on the localization of Insc is partially
penetrant) that cell will become a sib. That the generation of an RP2 during
the asymmetric division of GMC-1 is tied to Nb is also indicated by the
analysis of mitiP; nb embryos. Although the
self-renewal of GMC-1 in mitiP embryos is
nb-independent, the commitment of a progeny to become a sib is
nb-dependent. Thus, in 13-hour-old
mitiP; nb embryos, we observed multiple cells
adopting a sib fate. An often overlooked fact is that in insc mutants
the GMC-1 division is normal in
30% of the hemisegments (n=280)
despite having no insc. Similarly, the penetrance of the symmetrical
division of GMC-1 in pins (where Insc localization is affected as in
mitiP embryos) is also partial, indicating the presence of
additional (partially redundant) pathways for Insc that mediate asymmetric
fate specification. These very same additional pathways must also influence
the choice between a sib and an RP2 when the GMC-1 in
mitiP embryos undergoes a self-renewing type of asymmetric
division.
CycE and Ago are part of a mechanism that converts a normal cell into a
cancer cell. In ago mutants, CycE protein is not degraded and a
number of cancer cell lines carry a mutation in ago
(Moberg et al., 2001). Our
results showing that these genes are also involved in a stem cell type of
division suggests a commonality between stem cells and cancer cells. Our
results also provide a molecular mechanism of how self-renewing asymmetric
divisions are possible.
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ACKNOWLEDGMENTS |
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Footnotes |
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REFERENCES |
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