Cyclin E
Cyclin E is an essential regulator of S phase entry. Transcriptional regulation of Drosophila Cyclin E plays
an important role in the control of the G1 to S phase
transition during development. The first
comprehensive analysis of the transcriptional regulation
of a G1 phase cell cycle regulatory gene during
embryogenesis is presented here. Analysis of deficiencies, a genomic
transformant and reporter gene constructs all reveal
that Cyclin E transcription is controlled by a large and
complex cis-regulatory region containing tissue- and stage-specific
components. Separate regulatory elements for
transcription in epidermal cells during cell cycles 14-16,
central nervous system cells and peripheral nervous system
cells were found. An additional cis-regulatory element
drives transcription in thoracic epidermal cells that
undergo a 17th cell cycle when other epidermal cells have
arrested in G1 phase prior to terminal differentiation. The
complexity of Cyclin E transcriptional regulation argues
against a model in which Cyclin E transcription is regulated
simply and solely by G1 to S phase transcription regulators
such as RB, E2F and DP. Rather, this study demonstrates
that tissue-specific transcriptional regulatory mechanisms
are important components of the control of cyclin E
transcription and thus of cell proliferation in metazoans (Jones, 2000).
This analysis has revealed the presence of multiple
independent tissue-specific regulatory elements acting to
control DmcycE transcription during embryogenesis. These
include: (1) at least two different elements required for
expression in different cells of the peripheral nervous system;
(2) an element required for expression in the epidermal cells
and central nervous system, and (3) an element required for
expression in patches of thoracic epidermal cells that undergo
a 17th G1 phase-regulated cycle, in the PNS of the labial and
maxillary segments and in the posterior spiracle primordia.
These elements were identified by the presence or absence of
tissue-specific expression in a genomic transformant, in
animals carrying small regulatory region deletions generated
by P-element excisions and in genomic fragment-reporter gene
constructs. All these regions are detected upstream of the zygotic transcript, within the large region coding for three 5' exons and large introns of the maternal transcript. The results obtained with each of these different
approaches are consistent with the others in terms of the
location of the different elements. Members of one notable group of
elements that were not fully defined in this study are those
responsible for Cyclin E expression in endoreplicating tissues.
Only one element was identified that is responsible for
driving Cyclin E transcription in the central midgut.
Presumably elements responsible for regulating the remaining
patterns of DmcycE in embryonic endoreplicating tissues lie
further 5' or 3' of the genomic regions examined in this study.
The failure to define more than the one endoreplicative
regulatory sequence is surprising. It is possible that other
regulatory sequences lie outside the region studied, but it is also
possible that identification of these domains may be hampered
by the negative autoregulatory nature of Cyclin E expression in
these endoreplication domains. If the elements carrying the endoreplication
regulatory sequences also carry the autorepression sequences,
expression may be inhibited by the endogenous Cyclin E. It
should also be noted that additional complexity is likely to exist
in the regulation of Cyclin E transcription. For example,
Cyclin E was expressed in a subset of the epidermal thoracic
patch cells in Cyclin E 19L deficient embryos, indicating that
these patches may themselves be complex domains of Cyclin E
transcriptional regulation (Jones, 2000).
Although these analyses have identified separable tissue-specific
cis-acting elements in the Cyclin E regulatory region,
the factors that operate on these
elements to drive Cyclin E transcription remain to be elucidated. Some clues to the
trans-acting factors that could be regulating Cyclin E
transcription come from studies of cell cycle control
mechanisms in mammalian cells, where cyclin E
transcriptional regulation also plays an important role in
control of the G1 to S phase transition. A well-established model of
G1 phase transcriptional regulation in mammalian cells
postulates a cascade of events initiated by extracellular growth
factor signaling that leads to activation of the Cyclin D/Cdk4
complex, which in turn phosphorylates the tumor suppressor
Retinoblastoma (Rb), disassociating it from the S phase-specific
transcription factors E2F and DP and allowing
transcription of S phase-specific genes, such as cyclin E (Jones, 2000).
In Drosophila, where
G1 phase-regulatory events can be studied within a developmental context,
E2F and DP are required for embryonic Cyclin E
transcription in endoreplicating tissues, but are dispensable for
Cyclin E transcription in the CNS divisions that lack G1 and
G2 phases. It is not known whether E2F and DP
are required for Cyclin E expression in the early embryonic
PNS and epidermal divisions, because maternal products of
these genes may mask any requirement. Nonetheless, the fact
that these transcription factors are dispensable for Cyclin E
transcription in the CNS cells argues against the universality
of the mammalian model of cyclin E regulation by E2F. In
addition, the mammalian model predicts that E2F-dependent
transcriptional regulation of Cyclin E in endoreplicating tissues
should be mediated through an E2F/DP responsive element. In
the experiments described here, this model
is shown to be an oversimplification of Cyclin E regulation in Drosophila,
as only one element in the 16 kb analysed was found to drive
Cyclin E expression in a subset of endoreplicating tissues. Thus
a minimum of two regulatory elements are required for driving
Cyclin E expression in endoreplicating tissues. The fact that
expression of Cyclin E in these tissues has been shown to be
dependent on the E2F/DP complex appears at first sight to be
paradoxical. The paradox would be resolved if expression
requires activation by both E2F/DP and tissue-specific
activators. An absolute requirement for a developmental signal
cannot however exist, as ectopic expression of E2F and DP
together induces Cyclin E expression in all G1-arrested
epidermal cells. It remains possible that
the high levels of E2F and DP expressed following heat shock
induction of the respective transgenes override the tissue-specific
regulatory component (Jones, 2000).
Downregulation of Cyclin E transcription during cycle 16 is
essential for cycle 17 G1 arrest in epidermal cells prior to
differentiation. Significantly, the
regulatory element that drives constitutive Cyclin E expression
in the epidermal cells during cycles 14-16 shows
transcriptional downregulation, characteristic of wild-type
Cyclin E expression. If this downregulation requires active
repression of Cyclin E transcription, then the regulatory
sequences necessary for this repression must also be located in
the 1.0 kb regulatory element defined in this study. Alternatively, the
downregulation could be a consequence of the loss of
activation of Cyclin E transcription in the epidermis.
Cyclin E is known to be ectopically expressed in a subset of normally G1-arrested
epidermal cells in embryos that are deficient for RBF
(the Drosophila Rb homolog). However, Cyclin E transcription
is initially downregulated normally and a G1 cycle 17 arrest is
established in the absence of RBF. These data suggest that
Cyclin E is actively repressed in G1-arrested epidermal cells
and that RBF is required to maintain this repression. Although
dE2F or dDP do not result in ectopic Cyclin E transcription in
epidermal cells, a RBF/E2F/DP complex may still
mediate this repression if maternal E2F and DP are not
depleted in the respective mutant embryos at this stage. A
transcriptional repression mechanism of Cyclin E at this stage
may also be mediated by the second Drosophila E2F, E2F2,
which has been shown to act as a transcriptional repressor of
S phase genes in tissue culture cells.
Alternatively, E2F and/or DP may be needed to activate ectopic
Cyclin E expression in the absence of RBF. The initial
mechanism acting to downregulate Cyclin E transcription
remains to be determined. Further dissection of this regulatory
element may identify separate activation and repression
sequences (Jones, 2000).
The regulators of DmcycE transcription in other
developmental contexts may be more difficult to identify, since
many candidate genes may be involved. In some cases it is
possible to suggest the involvement of particular regulatory
genes. For example, the discrete expression of DmcycE in the
epidermal thoracic patches, which undergo a G1-regulated 17th
cell cycle, is much stronger in the first thoracic segment and is
absent in the abdominal segments. The products of the
homeotic genes of the Bithorax and Antennapedia Complexes,
which are key components of anterior/posterior patterning
are therefore candidate upstream regulators of Cyclin E
transcription in this tissue (Jones, 2000).
Understanding of cell cycle regulation has primarily
derived from the single cell yeast, from cultured cells and from
oocytes or very early embryos in which the complex patterning
of embryogenesis has not yet begun. In contrast, cell cycles in
the embryos of a multicellular organism respond to a variety
of developmental cues that give different tissue types and
different cell cycle kinetics. The significance of the relationship
between embryo patterning and cell cycle control is evident
from the pioneering work that has shown that regulation of the cycles that occur during gastrulation and germ band extension in Drosophila embryos is
mediated by the transcriptional regulation of the string
mitotic activator gene. This transcriptional regulation is
mediated by the patterning genes active during these stages.
The stg mitotic inducer gene exhibits a remarkably complex
transcriptional regulatory region that responds to a variety of
patterning genes to control cell cycle progression during early
embryogenesis. The
analysis of Cyclin E regulation offers an
interesting parallel with stg in that both genes exhibit
unexpected complexity in transcriptional regulation. It is
striking that both Cyclin E and stg, the first cell cycle regulators
to have their transcriptional regulation examined in a
developmental context, contain tissue-specific regulatory
elements. This indicates that transcriptional regulation is
mediated differently in distinct developmental contexts, such
as during the complex events of gastrulation when cycles 14-16 occur or during the 17th cycle in cells of the epidermal
thoracic patches (Jones, 2000).
Why is DmcycE transcriptional regulation so complex? One
clue comes from the observation that the G1 regulators
cyclin D, cyclin E, Rb and E2F/DP are highly conserved
between insects and mammals, but have no specific orthologs
in yeast. Perhaps these genes represent a
specialized mechanism that evolved to deal with G1 phase
regulation in multicellular organisms. Complex transcriptional
regulation of cell cycle genes such as Cyclin E may be part of
such a mechanism, which permitted the universal eukaryotic
cell cycle regulatory genes to be brought under the influence
of the more recently evolved transcriptional regulatory
mechanisms operating in different tissues and at different
developmental stages (Jones, 2000).
cyclin E expression at G1-S requires E2F, activation of E2F without Cyclin E is not sufficient for the induction of S phase. Early in G1, ectopic expression of Cyclin E alone can bypass E2F and induce S phase. Cyclin E is the downstream gene that couples E2F activity to G1 control. However it appears that S phase has a requirement for Cyclin E that is independent of transcription. However, not all embryonic cycles are similarly coupled to E2F activation. The rapidly proliferating CNS cells that exhibit no obvious G1 express Cyclin E constitutively and independently of E2F. Instead, Cyclin E expression activates E2F in the CNS. Thus, this tissue-specific E2F-independent transcription of Cyclin E reverses the hierarchical relationship between Cyclin E and E2F. Both hierarchies activate expression of the full complement of replication functions controlled by E2F with this caveat: whereas inactivation of E2F can produce a G1 when Cyclin E is downstream of E2F, an E2F-independent source of Cyclin E eliminates G1 (Duronio, 1995b).
The E2F transcription factor, a heterodimer of E2F and DP subunits, is capable of driving the G1-S
transition of the cell cycle. However, mice in which the E2F-1 gene has been disrupted develop
tumors, suggesting a negative role for E2F in controlling cell proliferation in some tissues. The
consequences of disrupting the DP genes have not been reported. A screen was carried out for mutations that
disrupt G1-S transcription late in Drosophila embryogenesis and five mutations in the dDP
gene were identified. Sequencing of dDP reveals the presence of
several important motifs, including the DNA-binding region, the DEF box that is predicted to be
required for DP/E2F heterodimerization, and three other highly homologous regions named
DP-conserved box 1 (DCB1), DCB2, and negatively charged box (NCB). Although mutations in dDP or dE2F nearly eliminate E2F-dependent G1-S transcription, S-phase
still occurs. Cyclin E has been shown to be essential for S-phase in late embryogenesis, but in dDP and
dE2F mutants the peaks of G1-S transcription of cyclin E are missing. Thus, greatly reduced levels of
cyclin E transcript suffice for DNA replication until late in development. Both dDP and dE2F are
necessary for viability, and mutations in the genes cause lethality at the late larval/pupal stage. The
mutant phenotypes reveal that both genes promote progression of the cell cycle (Royzman, 1997).
The striking observation from
the Drosophila dDP and dE2F mutants is that although cyclic transcription of cyclin E, PCNA, and
ribonucleotide reductase 2 (RNR2) is not detectable, S phase still occurs. Although the possibility that cyclic
transcription of these genes occurs at a low level driven by maternal pools of dDP and dE2F cannot be excluded, the
bursts of transcription that normally precede S phase are not essential for the G1-S transition. In these
mutants the cell cycle may be driven by basal levels of transcripts and post-transcriptional regulation.
The maternal pools of components of the replication machinery can persist until late in development, as
evidenced by the fact that mutations in PCNA and MCM2 cause late larval lethality (Royzman, 1997 and references).
E2F is a heterogenous transcription factor and its role in cell
cycle control results from the integrated activities of many different
E2F family members. Unlike mammalian cells, which have a large number of
E2F-related genes, the Drosophila genome encodes just two E2F
genes, de2f1 and E2F transcription factor 2 (de2f2). de2f1
and de2f2 provide different elements of E2F regulation, and
they have opposing functions during Drosophila< development.
dE2F1 and dE2F2 both heterodimerize with dDP and bind to the promoters
of E2F-regulated genes in vivo. dE2F1 is a potent activator of
transcription, and the loss of de2f1 results in the reduced
expression of E2F-regulated genes. In contrast, dE2F2 represses the
transcription of E2F reporters and the loss of de2f2 function
results in increased and expanded patterns of gene expression. The loss
of de2f1 function has previously been reported to compromise
cell proliferation. de2f1 mutant embryos have reduced
expression of E2F-regulated genes, low levels of DNA synthesis, and
hatch to give slow-growing larvae. These defects are due
in large part to the unchecked activity of dE2F2, since they can be
suppressed by mutation of de2f2. Examination of eye discs from
de2f1;de2f2 double-mutant animals reveals that relatively normal patterns of DNA synthesis can occur in the absence of
both E2F proteins. This study shows how repressor and activator E2Fs
are used to pattern transcription and how the net effect of E2F on cell
proliferation results from the interplay between two types of E2F
complexes that have antagonistic functions (Frolov, 2001).
The relatively normal patterns of cell proliferation in
de2f1;de2f2 mutants are, at first glance, difficult
to reconcile with the idea that E2F is a critical regulator of gene
expression and cell proliferation. The expression
of de2f1-regulated genes was examined in de2f2 and de2f1;
de2f2-mutant animals. Third instar eye discs were chosen for
this analysis to avoid the possible contribution of maternally supplied
products. Because Cyclin E is one of the best-known targets of E2F and
is rate limiting for S-phase entry the pattern of Cyclin E transcription was examined in the de2f2;de2f1 double-mutant animals. The normal pattern of Cyclin E expression is not altered in de2f2 mutant discs and a similar pattern is also evident in de2f2;de2f1 double
mutants. However in the absence of both dE2F1 and dE2F2, the variations
in Cyclin E expression are reduced and the pattern of
expression is less distinct. Northern analysis shows that
the steady-state level of Cyclin E transcripts is not
decreased in the absence of E2F proteins. These results are
consistent with evidence that de2f1 contributes to the pattern
of Cyclin E expression but is not required for Cyclin E transcription. The finding that E2F target genes are expressed in de2f2;de2f1 double mutants may explain, at least in part, why normal cell proliferation is possible in the absence of E2F proteins (Frolov, 2001).
Neurons and glia are often derived from common multipotent stem cells. In Drosophila, neural identity appears to be the default fate of these precursors. Stem cells that generate either neurons or glia transiently express neural stem
cell-specific markers. Further development as glia requires the activation of glial-specific regulators. However, this
must be accompanied by simultaneous repression of the alternate neural fate. The Drosophila transcriptional repressor Tramtrack is a key repressor of neuronal fates. It is expressed at high levels in all mature
glia of the embryonic central nervous system. Analysis of the temporal profile of Tramtrack expression in glia shows that it follows that of existing
glial markers. When expressed ectopically before neural stem cell formation, Tramtrack represses the neural stem cell-specific genes asense and
deadpan. Surprisingly, Tramtrack protein levels oscillate in a cell cycle-dependent manner in proliferating glia, with expression dropping before
replication, but re-initiating after S phase. Overexpression of Tramtrack blocks glial development by inhibiting S-phase and repressing expression of
the S-phase cyclin, cyclin E. Conversely, in tramtrack mutant embryos, glia are disrupted and undergo additional rounds of replication. It is proposed that
Tramtrack ensures stable mature glial identity by both repressing neuroblast-specific genes and controlling glial cell proliferation (Badenhorst, 2001).
The absence of Ttk69 from replicating glia implies that ectopic expression of Ttk69 may block normal glial development by inhibiting cell cycle progression. Whether ectopic expression of Ttk69 blocks entry into S-phase was examined. BrdU incorporation is inhibited by ectopic expression of Ttk69 using the Kr-Gal4 driver. In Ttk69-expressing segments of a Stage 10 embryo, the normal BrdU incorporation in the ventral neuroectoderm is inhibited. In the embryonic nervous system, entry into S phase is driven by bursts of transcription of S-phase cyclins -- specifically cyclin E. Heat-shock induced overexpression of Ttk69 blocks zygotic transcription of cyclin E. At earlier stages, maternally deposited cyclin E transcripts are unaffected by Ttk69 overexpression, indicating that Ttk69 affects transcript synthesis rather than stability, consistent with its role as a transcription repressor. Ectopic expression of Ttk69 was induced by a 1 hour heat-shock, after which embryos were processed immediately for in situ hybridization. The rapidity of repression of cyclin E and the presence of multiple Ttk69 consensus recognition sites in the cyclin E promoters both suggest that repression is direct (Badenhorst, 2001).
It is clear from the current study that, at least in some glial populations, Ttk69 has the ability to regulate proliferation. In the Drosophila glioblasts Ttk69 is expressed at low levels soon after glial specification, blocking neural genes. This expression is not constant, though, but oscillates during the cell cycle. Significantly, Ttk69 can not be detected when glia enter S phase and commence DNA replication. Like neuroblasts, glioblasts appear to delaminate in G2 of the cell cycle. The timing of BrdU incorporation indicates that immediately after mitosis they enter S phase and undergo DNA replication. Although Ttk69 is expressed in glia in G2, Ttk69 is not detected in glia that incorporate BrdU. Since Ttk69 can repress cyclin E expression, the absence of Ttk69 allows replication to occur. Once replication is complete, Ttk69 is expressed again. The BrdU incorporation experiments indicate that the LGB undergoes three synchronous cell divisions to produce eight longitudinal glia. This agrees well with a recent estimate of between 7-9 longitudinal glia obtained by DiI labeling of the longitudinal glioblast. After the third mitosis, longitudinal glia do not undergo replication but instead express higher levels of Ttk. By inhibiting cyclin E, high levels of Ttk69 would keep glia in G1 of the cell cycle. Similarly, differentiation of oligodendrocytes and Müller glia is accompanied by high levels of the cyclin-dependent kinase inhibitor p27, blocking re-entry into the cell cycle (Badenhorst, 2001).
Cell proliferation in the developing renal tubules of Drosophila is strikingly patterned, occurring in two phases to
generate a consistent number of tubule cells. The later phase of cell division is promoted by EGF receptor signaling
from a specialized subset of tubule cells, the tip cells, which express the protease Rhomboid and are thus able to
secrete the EGF ligand, Spitz. The response to EGF signaling, and in consequence cell division, is patterned by the specification of a second cell type in the tubules. These cells are primed to respond to EGF signaling
by the transcription of two pathway effectors, PointedP2, which is phosphorylated on pathway activation, and Seven up. While expression of
pointedP2 is induced by Wingless signaling, seven up is initiated in a subset of the PointedP2 cells through the activity of the proneural genes. To understand how the proneural and neurogenic genes pattern the response to EGFR activation, the expression of genes involved in transduction of the pathway was analyzed. The orphan nuclear-receptor svp functions downstream of the EGF receptor to promote cell divisions in the tubules. In the absence of Svp function, cycE and stg transcription is abolished, with a consequent reduction in EGFR-driven cell divisions. These late divisions in the tubules of stage 12 wild-type embryos were followed and it was found that BrdU incorporation (and hence, cell division) is confined within the svp-lacZ domain. These results define the svp domain of expression as including those cells which will divide in response to Egfr activation. However, the expression of svp-lacZ is initiated in a group of cells surrounding the tip mother cell, before the birth of the TC. This early onset of svp expression occurs before the late divisions start (cycle 17 onwards), when neither Svp function nor Egfr activation is required for cell proliferation. The pattern of gene expression observed suggests that the Svp-positive cells surrounding the tip mother cell derive from the proneural cluster (Sudarsan, 2002).
So far, relatively few mechanisms have been shown to be capable of regulating both cell proliferation and cell death in a coordinated manner. In a screen for Drosophila mutations that result in tissue overgrowth, salvador (sav), a gene that promotes both cell cycle exit and cell death was identified. Elevated Cyclin E and DIAP1 levels are found in mutant cells, resulting in delayed cell cycle exit and impaired apoptosis. Salvador contains two WW domains and binds to the Warts protein kinase. The human ortholog of salvador (hWW45) is mutated in several cancer cell lines. Thus, salvador restricts cell numbers in vivo by functioning as a dual regulator of cell proliferation and apoptosis (Tapon, 2002).
Elevated levels of Cyclin E protein are found in the basal nuclei of sav clones posterior to the MF. These are the nuclei of the undifferentiated cells that continue to proliferate in sav clones. Such discs were examined for levels of cyclin E RNA. When sav clones are generated using eyFLP, a large proportion of cells in third instar discs are mutant, and these discs contain large patches of mutant tissue. In wild-type discs, cyclin E RNA is expressed in a narrow stripe immediately posterior to the morphogenetic furrow. In discs containing sav clones, the stripe of expression is broader and more intense, indicating that cyclin E RNA levels are elevated in these discs. Thus, the increased level of Cyclin E protein is likely to result, at least in part, from an increase in cyclin E RNA levels (Tapon, 2002).
Although mutations that activate the Hedgehog (Hh) signaling pathway have been linked to several types of cancer, the molecular and cellular basis of Hh's ability to induce tumor formation is not well understood. A mutation in patched (ptc), an inhibitor of Hh signaling, was identified in a genetic screen for regulators of the Retinoblastoma (Rb) pathway in Drosophila. Hh signaling promotes transcription of Cyclin E and Cyclin D, two inhibitors of Rb, and principal regulators of the cell cycle during development in Drosophila. Upregulation of Cyclin E expression, accomplished through binding of Cubitus interruptus (Ci) to the Cyclin E promoter, mediates the ability of Hh to induce DNA replication. Upregulation of Cyclin D expression by Hh mediates the distinct ability of Hh to promote cellular growth. The discovery of a direct connection between Hh signaling and principal cell-cycle regulators provides insight into the mechanism by which deregulated Hh signaling promotes tumor formation (Duman-Scheel, 2002).
During eye development in Drosophila, initiation of neural differentiation, marked by an indentation referred to as the morphogenetic furrow, begins at the posterior end of the disc and passes anteriorly. Cells within the furrow arrest in G1 phase before differentiating. Cells located just posterior to the furrow exit G1 arrest and enter a synchronous S phase referred to as the second mitotic wave. Overexpression of the Drosophila Retinoblastoma-family gene (Rbf), an inhibitor of the S phase promoting transcription factor E2F, produces a 'rough' adult eye phenotype, characterized by loss of bristles and fusion of ommatidia. This phenotype results from delay of S phase progression in cells of the second mitotic wave as a consequence of inhibited E2F target gene expression. Loss of one copy of the ptc gene suppresses this rough eye phenotype and restores E2F target gene expression. The observed genetic interaction between Rbf and ptc suggests that the Hh signaling pathway might regulate the cell cycle during eye development (Duman-Scheel, 2002).
Hh is secreted from differentiating neurons located just posterior to cells entering S phase in the second mitotic wave. This expression pattern is consistent with the idea that reception of the Hh signal might be required for S phase entry in the second mitotic wave. To test this hypothesis, the effect of blocking Hh signaling during eye development was assessed. Cells with a mutated smoothened (smo) gene cannot respond to the Hh signal and fail to enter S phase in the second mitotic wave. Conversely, when Ci, the transcription factor that mediates Hh signaling, is overexpressed in the furrow, cells normally arrested in G1 enter S phase. Ectopic expression of Ci can also promote S phase in G1-arrested cells located in the wing margin and in the brain. Thus, Ci can induce S phase in a variety of tissues (Duman-Scheel, 2002).
Cyclin E is a principal regulator of S phase during Drosophila development. Although Cyclin E is an inhibitor of Rb, it also has additional Rb/E2F-independent cell-cycle roles. The initiation of high levels of Cyclin E expression in cells of the second mitotic wave that is located just anterior to neurons secreting Hh protein, is consistent with the idea that Hh signaling may regulate Cyclin E expression in the eye. Indeed, loss of smo results in reduction of Cyclin E levels in cells entering the second mitotic wave. Conversely, when Ci is overexpressed in furrow cells, high levels of Cyclin E transcript and protein can be detected within these cells. In the eye, the ability of Ci to induce high levels of Cyclin E expression is limited to the furrow region, where it is capable of inducing S phase. Overexpression of Ci is also capable of inducing high levels of Cyclin E transcript and protein expression in the wing. Although Cyclin E is an E2F target gene, the ability of Ci to promote Cyclin E expression in the presence of RBF-280 indicates that Hh can induce Cyclin E expression independently of E2F. Therefore, in addition to promoting expression of Cyclin D and activating E2F, Hh signaling also promotes expression of Cyclin E independently of E2F. The ability of the Cyclin E-dependent kinase inhibitor Dacapo (Dap) to inhibit the ability of Ci to induce S phase in the furrow and wing margin indicates that Cyclin E is the principal mediator of the ability of Hh to promote S phase (Duman-Scheel, 2002).
To investigate the mechanism by which Hh signaling induces Cyclin E transcription, the Cyclin E promoter was examined. Several sequences with homology to the consensus Ci-binding site were identified within the 5' regulatory region of Cyclin E. Ci was found to bind to three Ci-binding sites (A, B and C in gel shift competition experiments). To examine whether this interaction occurs in vivo, chromatin immunoprecipitation (ChIP) experiments were carried out. These assays demonstrate that Ci-binding sites A-C are occupied by Ci protein in vivo. Several lines of evidence indicate that the observed ability of Ci to bind to the Cyclin E promoter is important for regulation of Cyclin E expression in the developing eye. First, normal upregulation of Cyclin E expression in the second mitotic wave is disrupted in Cyclin EJP mutant flies, which bear a P element inserted adjacent to Ci-binding sites A-C. Also, flies carrying the 16.4 Cyclin E lacZ13 reporter, which contains all three Ci-binding sites, show upregulation of ß-galactosidase (ß-gal) expression in the second mitotic wave; this pattern resembles the endogenous pattern of Cyclin E expression. By contrast, upregulated ß-gal levels in the second mitotic wave are not observed in flies carrying reporter 13.2 Cyclin E lacZ13, which lacks Ci-binding sites A and B. Furthermore, overexpression of Ci in the furrow can drive ectopic ß-gal expression from reporter 16.4, but not from reporter 13.2. These experiments suggest that the presence of sites A and B is required for normal Hh-mediated upregulation of Cyclin E expression in the second mitotic wave. Taken together, these results provide strong evidence that Hh signaling promotes S phase through direct induction of Cyclin E expression by the transcription factor Ci (Duman-Scheel, 2002).
The investigation demonstrates that Hh signaling has a distinct ability to promote cellular growth, which is mediated by Cyclin D. In addition, Hh signaling can induce proliferation during development by promoting expression of Cyclin D and Cyclin E. This study reveals a direct connection between Hh signaling and induction of Cyclin E expression, which is accomplished through binding of Ci to the Cyclin E promoter. Upregulation of murine cyclin D1, D2 and E in response to Hh signaling has been observed. It is therefore likely that the mechanism for Cyclin E induction by Hh described here is conserved in mammals. Furthermore, because both overexpression of Ptc-1 or mutation of cyclin D1 produces a small mouse phenotype, it is likely that the ability of Hh to promote cellular growth through upregulation of D-type cyclins is also conserved in mice. Thus, constitutive Hh signaling (which promotes deregulated expression of G1/S cyclins that have been associated with diverse forms of human cancer) would promote both cell proliferation and growth in tumors. In contrast, during development, cell growth and proliferation must be carefully regulated and coordinated with the processes of cell patterning and differentiation. These same processes are also regulated by Hh signaling. This delicate balance is probably maintained by tight control of the temporal and spatial expression patterns of Hh targets and the molecules that regulate them (Duman-Scheel, 2002).
Cancer is a multistep process involving cooperation between oncogenic or tumor suppressor mutations and interactions between the tumor and surrounding normal tissue. This study is the first description of cooperative tumorigenesis in Drosophila, and uses a system that mimics the development of tumors in mammals. The MARCM system was used to generate mutant clones of the apical-basal cell polarity tumor suppressor gene, scribbled, in the context of normal tissue. scribbled mutant clones in the eye disc exhibit ectopic expression of cyclin E and ectopic cell cycles, but do not overgrow due to increased cell death mediated by the JNK pathway and the surrounding wild-type tissue. In contrast, when oncogenic Ras or Notch is expressed within the scribbled mutant clones, cell death is prevented and neoplastic tumors develop. This demonstrates that, in Drosophila, activated alleles of Ras and Notch can act as cooperating oncogenes in the development of epithelial tumors, and highlights the importance of epithelial polarity regulators in restraining oncogenes and preventing tumor formation (Brumby, 2003).
A clonal approach, more closely resembling the clonal nature of mammalian cancer, was used to analyze the effects of removing Scrib function on tumor formation. This analysis indicates that Drosophila scrib- tumors: (1) lose tissue architecture, including apical-basal cell polarity; (2) fail to differentiate properly; (3) exert non-cell-autonomous effects upon the surrounding wild-type tissue; (4) upregulate cyclin E and undergo excessive cell proliferation; (5) are restrained from overgrowing by the surrounding wild-type tissue via a JNK-dependent apoptotic response, and (6) show strong cooperation with oncogenic alleles of Ras and Notch to produce large amorphous tumors. These conclusions are summarized in a model for tumor development in Drosophila. It is suggested that the role of epithelial cell polarity regulators in restraining oncogenes is likely to be of general significance in mammalian tumorigenesis (Brumby, 2003).
The model suggests that the wild-type larval eye disc is a monolayered columnar epithelium, in which cell proliferation is tightly regulated. Cell architecture is maintained by the formation of adherens junctions, the apical localization of Scribbled, and adhesion to the basement membrane. Mutation of scrib results in loss of apical-basal polarity, leading to multilayering and rounding up of cells. scrib- tissue also shows impaired differentiation, and ectopic cyclin E expression (by an unknown mechanism) leads to ectopic cell proliferation. Unrestrained overgrowth and tumor formation of scrib- cells is held in check by compensatory JNK-mediated apoptosis, dependent upon the presence of surrounding wild-type cells. Secondary mutations are required to avoid this apoptotic fate. If JNK activity is blocked within scrib- cells, by expressing a dominant-negative form of JNK, apoptosis is prevented, resulting in tissue overgrowth and lethality. Even more aggressive overgrowth results from the addition of activating oncogenic alleles of Ras or Notch. In addition to promoting cell survival, these oncogenes must also promote tumor cell proliferation; however, it is proposed that other downstream effectors of these oncogenes are likely also to be important, since it was not possible to mimic the cooperative overgrowth effects of RasACT or NACT on scrib- tissue by simply blocking apoptosis and enhancing cell proliferation (Brumby, 2003).
scrib- clones ectopically express cyclin E and undergo ectopic S phases and mitoses. Since cyclin E is rate limiting for cell cycle progression in the developing eye, it is likely that upregulation of cyclin E in scrib- clones is critical for the ectopic cell proliferation. Indeed, alleles of scrib and lgl were originally isolated as dominant suppressors of a hypomorphic cyclin E allele, DmcycEJP, suggesting that these cell polarity genes normally play a critical role in limiting cyclin E expression. Currently being investigated is which signaling pathways are altered in scrib, dlg or lgl mutants that could be responsible for cyclin E upregulation. A recent study in human lung epithelial cells shows that disrupting cell polarity allows mixing of the heregulin-alpha ligand and the erbB2-4 receptor, which are normally physically separated, resulting in activation of the pathway and cell proliferation. Further studies are required to determine whether the ectopic expression of cyclin E observed in the absence of Scrib is simply a consequence of the tissue disorganization induced by disrupting cell polarity, or if Scrib has a direct role in limiting cell proliferation independent of cell polarity. Interestingly, the rounding up of cells in the absence of Scrib appears to be predominantly a cell-autonomous effect, yet clearly non-cell-autonomous defects are also apparent, including the upregulation of cyclin E. This would suggest that altered cell-cell interactions between wild-type and mutant cells can also alter signaling pathways within wild-type cells, and that the loss of apical-basal polarity and collapse of the columnar epithelium is not intrinsically responsible for the deregulated expression of cyclin E. A deeper understanding of the relationship between epithelial cell polarization and cell proliferation is clearly important for understanding the development of cancer, since a loss of cell polarity often accompanies tumor progression and metastasis (Brumby, 2003).
In Drosophila, activated Ras exerts its oncogenic effects through Raf and the MAPK pathway. Downstream targets of MAPK in the eye disc promote differentiation, cell survival and cell proliferation. This work also demonstrates that Ras can increase cyclin E protein levels in the eye disc. In combination with scrib-, the differentiation output of RasACT signaling appears to be attenuated, and the proliferative and anti-apoptotic responses prevail (Brumby, 2003).
This study has described a novel multi-hit model of tumorigenesis in Drosophila. Furthermore, although it has been suspected that disruptions to cell polarity could potentiate tumor progression and metastasis, this work demonstrates for the first time how the oncogenic effects of activated Ras and Notch are unleashed in the absence of epithelial polarity regulators. It is predicted that in mammals also, defects in apical-basal polarity could cooperate with oncogenes during neoplastic development. This approach in Drosophila can now be used to screen for novel oncogenes that, when specifically overexpressed in scrib- clones, are capable of inducing cooperative tumorigenesis, and can also be extended to identify cooperative interactions between other tumor suppressors and oncogenes within a whole animal context (Brumby, 2003).
E2F transcription factors can activate or actively repress transcription of their target genes. The role of active repression during normal development has not been analyzed in detail. dE2F1su89 is a novel allele of Drosophila E2f that disrupts E2f's association with RBF Drosophila retinoblastoma protein (Rb) homolog but retains its transcription activation function. Interestingly, the dE2F1su89 mutant, which has E2f activation by dE2F1su89 and active repression by E2f2, is viable and fertile with no gross developmental defects. In contrast, complete removal of active repression in de2f2;dE2F1su89 mutants results in severe developmental defects in macrochaetae and salivary glands, tissues with extensive endocycles, but not in tissues derived from mitotic cycles. The endoreplication defect results from a failure to downregulate the level of cyclin E during the gap phase of the endocycling cells. Importantly, reducing the gene dosage of cyclin E partially suppresses all the phenotypes associated with the endoreplication defect. These observations point to an important role for E2f-Rb complexes in the downregulation of cyclin E during the gap phase of endocycling cells in Drosophila development (Weng, 2003).
A novel allele of E2f1, dE2F1su89, was identified from a genetic screen for suppressors of the Rbf overexpression phenotype. Sequence analysis revealed that dE2F1su89 contains a single base pair mutation in the conserved Rb-binding domain that converts the conserved amino acid leucine at position 786 to glutamine. To test whether this mutation disrupts the interaction between Rbf and E2f, a yeast two-hybrid interaction assay was performed. E2F1su89 is unable to bind to Rbf. To demonstrate further the effect of this mutation with endogenous proteins, a co-immunoprecipitation experiment was carried out. While both E2f and Dp co-immunoprecipitate with Rb from wild-type embryo extracts, no E2f co-immunoprecipitates with Rb from the dE2F1su89 embryo extracts, even though similar levels of E2f protein are present in the two extracts. These results indicate that the endogenous dE2F1su89 and Rb proteins do not form a complex. Interestingly, Dp still co-immunoprecipitates with Rb from the dE2F1su89 embryo extracts, indicating that Dp can still form a complex with Rb in the dE2F1su89 mutant background, probably through the other Drosophila E2F protein, E2f2 (Weng, 2003).
The decreased number of endocycles could be due to a lengthening of the S phase or a lengthening of the gap phase. Lengthening of the S phase would lead to an increased number of cells that are in the S phase, while lengthening of the gap phase would decrease the number of cells that are in S phase at any given time. A decreased number of S phase nuclei was observed in e2f2;dE2F1su89 salivary glands compared with that in wild-type salivary glands. Thus the gap phase of the endocycles in the e2f2;dE2F1su89 mutants is significantly lengthened. e2f2;dE2F1su89 but not wild-type salivary gland cells accumulate high levels of cyclin E in some gap phase cells (cells that are not incorporating BrdU). Since downregulation of cyclin E levels is required for continuous endoreplication, the failure to downregulate cyclin E levels properly in these gap phase cells probably inhibits endoreplication and leads to severe defects in tissues that require extensive endoreplication during development. The observation that decreasing the gene dosage of cyclin E partially suppresses the e2f2;dE2F1su89 phenotypes such as salivary gland endoreplication defects, macrochaetae defects and lethality provides strong support for the idea that the failure to downregulate cyclin E levels in these gap phase cells is a cause for the observed defects in e2f2;dE2F1su89 endocycle tissues (Weng, 2003).
Although previous results established that cyclin E oscillation is critical for continuous endoreplication, it is not clear how cyclin E oscillation in endocycle cells is achieved. No cyclin E oscillation defect is observed in salivary gland cells in the dE2F1su89 mutants, suggesting that active repression by the E2f2-Rb complexes is sufficient to downregulate the level of cyclin E during the gap phase, even in the presence of the unregulated dE2F1su89. In contrast, removal of the dE2F2-Rb complexes in the dE2F1su89 background results in extensive defects in endocycle tissues and defective cyclin E downregulation in the gap phase of endocycling cells. These results argue strongly that the E2f-Rb complexes are required for the normal downregulation of cyclin E in the gap phase of endocycling cells. These results, in conjunction with the observation that E2F activity is required for cyclin E expression and S phase progression of endocycle cells, suggests a model in which E2f activation is required for S phase of the endocycles and active repression by E2f-Rb complexes is required during gap phase. It is interesting to note that even in the complete absence of Rb-E2f active repression, there are still significant levels of endoreplication, suggesting that the oscillation of cyclin E activity, although defective, can still occur to some extent in e2f2;dE2F1su89 mutants. It is possible that additional mechanisms such as protein degradation or binding to inhibitor proteins such as Dacapo can also contribute to the downregulation of cyclin E activity (Weng, 2003).
Tissue growth during animal development is tightly controlled so that the organism can develop harmoniously. The salvador gene, which encodes a scaffold protein, restricts cell number by coordinating cell-cycle exit and apoptosis during Drosophila development. Hippo (Hpo), the Drosophila ortholog of the mammalian MST1 and MST2 serine/threonine kinases, is a partner of Sav. Hippo was described in five publications that appeared simutaneously: Pantalacci (2003) identified Hippo in a yeast two-hybrid screen in a search for Salvador interacting proteins, Udan (2003) identifed and positionally cloned hippo in a mutagenesis screen for genes that regulate tissue growth, and Harvey (2003), Jia (2003) and Wu (2003) identified hippo in screens for genes that restrict growth and cell number. Loss of hpo function leads to sav-like phenotypes, whereas gain of hpo function results in the opposite phenotype. Whereas Sav and Hpo normally restrict cellular quantities of the Drosophila inhibitor of apoptosis protein DIAP1 (Thread), overexpression of Hpo destabilizes DIAP1 in cell culture. DIAP1 is phosphorylated in a Hpo-dependent manner in S2 cells and that Hpo can phosphorylate DIAP1 in vitro. Thus, Hpo may promote
apoptosis by reducing cellular amounts of DIAP1. In addition, Sav is an unstable protein that is stabilized by Hpo. It is
proposed that Hpo and Sav function together to restrict tissue growth in vivo (Pantalacci, 2003; Harvey, 2003; Jia, 2003; Udan, 2003 and Wu, 2003).
Mutations are described in hippo, which encodes a protein kinase most related to mammalian Mst1 and Mst2. Like wts and sav, hpo mutations result in increased tissue growth and impaired apoptosis characterized by elevated levels of the cell cycle regulator Cyclin E and apoptosis inhibitor DIAP1. Three alleles of hippo are lethal either
when homozygous or in trans to another allele. Eyes containing
hpo mutant clones and wild-type clones have an overrepresentation of mutant tissue when compared
to eyes containing clones of the wild-type parental chromosome suggesting
that the mutant tissue may have a relative growth advantage. Mutant
ommatidial facets are slightly larger than wild-type facets and
sometimes contain extra interommatidial bristles. When
homozygous clones of hpo were generated in other imaginal
discs using hsFLP, outgrowths of tissue were observed and portions of wings containing large hpo clones were larger
than the corresponding portion of a wild-type wing, indicating a
role for hpo in regulating organ size in tissues other than
the eye. Retinal sections of adult eyes containing hpo
clones reveal that mutant ommatidia appear to have the normal
complement and arrangement of photoreceptor cells. However,
hpo mutant ommatidia appear to have significantly more
tissue between adjacent ommatidia. Cell outlines are visualized more
readily in the pupal retina. In contrast to the single
layer of interommatidial cells observed in wild-type retinas, mutant
ommatidia have several additional interommatidial cells.
These phenotypic abnormalities are very similar to those observed in
sav or wts mutant clones (Harvey, 2003).
sav or wts mutant cells in the eye imaginal disc fail to exit
from the cell cycle at the appropriate time. The additional rounds of
cell division generate an excess of interommatidial cells. Elevated
levels of cyclin E protein detected in mutant cells may underlie the
delayed cell cycle exit (Harvey, 2003).
In a disc mosaic for
the wild-type parent chromosome, BrdU-incorporation was evident in
the anterior portion of the disc and in a narrow stripe, the second
mitotic wave (SMW), but not in the morphogenetic furrow (MF) or
posterior to the SMW where cells arrest in the G1 phase of the cell
cycle. In
hpo mosaic eye discs, the pattern of S phases was normal in
the anterior portion of the disc and in the SMW but in mutant
portions of the disc, BrdU-incorporating nuclei were observed
posterior to the SMW and also in the MF. Thus, hpo cells continue to
cycle when surrounding wild-type cells are arrested in
G1, indicating that hpo function is essential for timely
cell cycle exit. In discs containing clones of the wild-type parental
chromosome, mitoses, visualized with anti-phospho histone H3, were
observed in the anterior portion of the eye disc and in several rows
of developing ommatidia immediately posterior to the SMW. In discs
mosaic for hpo however, extra mitoses were seen in mutant
clones many ommatidial rows posterior to the SMW, indicating
that at least a subset of hpo mutant cells continue to
divide when wild-type cells are mitotically quiescent. These
abnormalities are similar to, though less severe than, those found in
sav and wts imaginal discs (Harvey, 2003).
Elevated levels of cyclin E were found in
hpo mutant clones immediately anterior to the MF, in the
SMW, and posterior to the SMW. Cyclin E expression appeared
normal in hpo clones in the most anterior portions of the
third instar larval eye-antennal disc. In contrast, cyclins A, B, and
D were expressed at normal levels throughout the disc. In wild-type discs cyclin E RNA is expressed in a narrow stripe corresponding to the SMW. In discs mosaic for hpo, the level of cyclin E RNA was elevated and the expression domain of cyclin E was broader.
Additionally, an increase in cyclin E mRNA was detected by
semiquantitative RT-PCR performed on eye imaginal discs composed
almost entirely of hpo mutant tissue.
This indicates that, at least in part, hpo regulates
cyclin E at the level of transcription or RNA stability but
additional posttranscriptional regulation is also possible. In
experiments where hpo function was reduced in S2 cells by
RNAi, the levels of cyclin E protein were increased without an
obvious change in RNA levels as assessed by Northern blotting. Thus,
hpo may be capable of regulating cyclin E levels at both
transcriptional and posttranscriptional levels (Harvey, 2003).
The cycling
properties of hpo mutant cells were analyzed by generating
clones of mutant cells in third instar larval wing discs. Larvae were
heat shocked 48 hr after egg deposition (AED) and wing discs were
dissected 120 hr AED. Following dissociation with trypsin and
staining with Hoechst, cells were subjected to flow cytometry. Cell
size, as gauged by forward scatter, and DNA content were measured and
found to be almost indistinguishable between wild-type and
hpo mutant cells (Harvey, 2003).
The number of cells in hpo mutant
clones is consistently larger than in wild-type sister clones. The median
population doubling time in hpo clones was 13.1 hr, which
is significantly shorter than that of the GFP-bearing tester
chromosome, which was 14.7 hr. By
comparison, clones derived from the parent FRT42D chromosome had a
population doubling time that was not significantly different from
the same tester chromosome. It is unlikely that the increased cell
numbers in hpo clones can be explained by a block in
apoptosis, since overexpression of the caspase inhibitor p35 in wing
disc cells at this stage of development does not appreciably alter
the population doubling time. Thus, cells appear to divide faster in
hpo clones. Since cell size is essentially unchanged, this
would imply that hpo cells have an increased rate of growth
(mass accumulation) and a commensurate increase in the rate of cell
division (Harvey, 2003).
In wild-type pupal retinas, excess interommatidial
cells are eliminated in a wave of apoptosis during the midpupal
stage. There is a defect in apoptosis
in sav and wts mutant tissue. As a result, the additional cells
generated by excess cell division in sav and wts
tissue are not eliminated and account for the increased number of
interommatidial cells (Harvey, 2003).
Little is known about how patterns of cell proliferation and arrest are generated during development, a time when tight regulation of the cell cycle is necessary. In this study, the mechanism by which the developmental signaling molecule Wingless generates G1 arrest in the presumptive Drosophila wing margin is examined in detail. Wg signaling promotes activity of the Drosophila retinoblastoma family (Rbf) protein, which is required for G1 arrest in the presumptive wing margin. Wg promotes Rbf function by repressing expression of the G1-S regulator Drosophila myc (dmyc). Ectopic expression of dMyc induces expression of Cyclin E, Cyclin D, and Cdk4, which can inhibit Rbf and promote G1-S progression. Thus, G1 arrest in the presumptive wing margin depends on the presence of Rbf, which is maintained by the ability of Wg signaling to repress dmyc expression in these cells. In addition to advancing the understanding of how patterned cell-cycle arrest is generated by the Wg signaling molecule during development, this study indicates that components of the Rbf/E2f pathway are targets of dMyc in Drosophila. Although Rbf/E2f pathway components mediate the ability of dMyc to promote G1 progression, dMyc appears to regulate growth independently of the RBF/E2f pathway (Duman-Scheel, 2004).
The results indicate why exclusion of dMyc from the ZNC is necessary for Rbf activity. Overexpression of dMyc leads to high levels of Cyclin E, Cyclin D, and Cdk4 transcripts. dMyc also regulates Cyclin E posttranscriptionally in Drosophila. G1-S Cyclins/Cdks function to phosphorylate and inhibit Rbf, suggesting that dMyc blocks Rbf activity through activation of G1-S Cyclins/Cdks. Thus, inhibition of dMyc by Wg helps to ensure that G1-S Cyclins/Cdks do not activate S phase. This idea is supported by the results that indicate that only a combination of both Dap and constitutively active Rbf (that cannot be regulated by Cyclins/Cdks) can restore G1 arrest when Wg signaling is blocked or when dMyc is expressed. These data suggest that either Cyclin D or Cyclin E activity can mediate the ability of dMyc to promote S phase in the ZNC. Coexpressing Dap alone with dMyc, which would block only Cyclin E/Cdk2 activity, does not restore G1 arrest. Furthermore, overexpression of dMyc in a cdk4 mutant background still results in ectopic S phases, suggesting that Cyclin E/Cdk2 also are sufficient to mediate dMyc's ability to promote G1 progression. Thus, either Cyclin D/Cdk4 or Cyclin E/Cdk2 is sufficient to mediate the ability of dMyc to promote G1 progression. The ability of Wg to inhibit dMyc expression is thus critical for RBF activation and G1 arrest in the ZNC. Still, it is possible that Wg promotes G1 arrest through other mechanisms that have not yet been uncovered. The observation that overexpression of a dominant-negative form of dTCF (dTCFDeltaN) with C96>Gal4 can promote S phase, even in a dmyc mutant background, supports this idea (Duman-Scheel, 2004).
It is likely that dMyc/dMax directly up-regulate transcription of Cyclin D and cdk4 in Drosophila. Myc/Max heterodimers regulate transcription by binding to various consensus sequences, such as the E box. Previous studies indicated that cMyc induces Cyclin D2 expression in mice by binding to two consensus E boxes in the Cyclin D2 promoter. cdk4 also was identified as a transcriptional target of c-Myc. Furthermore, it has been suggested that cdk4 is a transcriptional target of dMyc and Cyclin D is a transcriptional target of dMax. Although future studies should analyze the Drosophila Cyclin D and Cdk 4 regulatory regions in more detail, these results suggest that the observed ability of dMyc to induce Cyclin D and Cdk4 expression in the ZNC most likely occurs through transcriptional regulation of these proteins by dMyc/dMax. In contrast, Cyclin E was not identified as a target of dMyc or dMax. It is more likely that the ability of dMyc to induce growth in the wing indirectly leads to increased Cyclin E transcript levels (Duman-Scheel, 2004).
In the Drosophila CNS, neuroblasts undergo self-renewing
asymmetric divisions, whereas their progeny, ganglion mother cells (GMCs),
divide asymmetrically to generate terminal postmitotic neurons. It is not
known whether GMCs have the potential to undergo self-renewing asymmetric
divisions. It is also not known how precursor cells undergo self-renewing
asymmetric divisions. Maintaining high levels of Mitimere
or Nubbin, two POU proteins, in a GMC causes it to undergo self-renewing
asymmetric divisions. These asymmetric divisions are due to upregulation of Cyclin E in late GMC and its unequal distribution between two daughter cells. GMCs in an embryo overexpressing Cyclin E, or in an embryo mutant for archipelago, also undergo self-renewing asymmetric divisions.
Although the GMC self-renewal is independent of inscuteable and
numb, the fate of the differentiating daughter is
inscuteable and numb-dependent. These results reveal that
regulation of Cyclin E levels, and asymmetric distribution of Cyclin E and
other determinants, confer self-renewing asymmetric division potential to
precursor cells, and thus define a pathway that regulates such divisions.
These results add to understanding of maintenance and loss of
pluripotential stem cell identity (Bhat, 2004).
Maintenance of a self-renewing fate can be viewed as a state where
activities of certain genes maintain that state. Once the activity of such
genes is switched off, the cells become committed to a differentiation pathway. The results reported in this study indeed support this type of mechanism. That POU genes might be a class
of genes that maintain a self-renewing capacity is indicated by the fact that the Oct4 POU gene (Pou5f1 -- Mouse Genome Informatics), which is expressed in pluripotent stem cells of the mouse early embryo, is turned off when these cells begin to differentiate
(Rosner, 1990). Similarly, SCIP is expressed in the progenitors of oligodendrocytes, but it is downregulated when these cells are induced to differentiate (Collarini, 1992). The current results provide direct evidence that these genes can induce a cell that is
committed to a differentiation pathway to acquire a self-renewing capability in a lineage specific manner. Moreover, studies undertaken in the past several years using the Drosophila nervous system as a paradigm have revealed how asymmetry can be generated during cell division to produce two distinct postmitotic cells. However, there is very little information on how an asymmetric self-renewing division pattern is determined. In this paper, results are presented that provide insight into this particular process. (Bhat, 2004).
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: (1) 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); (2) 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. (3) 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, the larger GMC-1 (this Eve-positive cell is Zfh1 negative, indicating that it is indeed a GMC-1) can be observed 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. (4) The results obtained with the mitiP; insc and mitiP; nb double mutant embryos (P referring to prolonged expression), and the mis-localization of Insc in GMC-1 of these embryos, are also consistent with this conclusion. (Bhat, 2004).
These 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 (Bhat, 2004).
The 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. Since 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) (Bhat, 2004).
What lines of evidence support this conclusion? (1) 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 (Bhat, 2004).
(2) Upregulation of CycE in a late GMC-1 is also observed in embryos mutant for ago, which is known to regulate CycE levels. 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. Since 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 (Bhat, 2004).
(3) 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, mitiP phenotypes are found to be dependent on CycE. That is, no multi-cell
clusters were observed in mitiP; CycE double mutant embryos (Bhat, 2004).
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? Since
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 since expressing high levels of miti does not have a discernible effect on the levels of CycE mRNA in GMC-1, as assessed by whole-mount RNA in situ hybridization. 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 (Bhat, 2004).
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 since 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, if
there was an asymmetric degradation, in those hemisegments where the levels of CycE was elevated in GMC-1, it would initially be expected that both the daughter cells
would have high CycE levels. However, this was not the case. An asymmetric
transcription of the CycE gene also seems unlikely since the
transcription of CycE ceases prior to GMC-1 division, as judged by
whole-mount RNA in situ hybridization. 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, no cytoplasmic
crescent of CycE was observed 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 (Bhat, 2004).
Finally, the 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
GMC1-->RP2/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 are 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. (Bhat, 2004).
Pros has been implicated in inhibiting the ability of GMCs to
divide more than once by preventing continued expression of cell-cycle genes. 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. 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. Moreover, specification of U and CQ lineages was observed 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. Similarly,
NB6-4 of the thoracic segment produced the normal number of progeny in
pros mutant embryos. 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 (Bhat, 2004).
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.) The 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 Numb. The cell that has lower levels of CycE and also has Numb becomes an RP2. Whenever the cell with lower levels of CycE fails to inherit Numb (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 Numb is also indicated by the analysis of mitiP; numb embryos. Although the self-renewal of GMC-1 in mitiP embryos is
numb-independent, the commitment of a progeny to become a sib is
numb-dependent. Thus, in ~13-hour-old
mitiP; numb embryos, multiple cells
are observed adopting a sib fate. An often overlooked fact is that in insc mutants the GMC-1 division is normal in ~30% of the hemisegments 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 (Bhat, 2004).
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. The current
results showing that these genes are also involved in a stem cell type of
division suggest a commonality between stem cells and cancer cells. These
results also provide a molecular mechanism of how self-renewing asymmetric
divisions are possible (Bhat, 2004).
During central nervous system development, glial cells need to be in the
correct number and location, at the correct time, to enable axon guidance and
neuropile formation. Repair of the injured or diseased central nervous system
will require the manipulation of glial precursors, so that the number of glial
cells is adjusted to that of neurons, enabling axonal tracts to be rebuilt,
remyelinated and functional. To a large extent, the molecular mechanisms controlling
glial precursor proliferative potential are unknown. This study shows that glial
proliferation is regulated by interactions with axons and that the
Drosophila gene prospero is required to maintain the mitotic
potential of glia. During growth cone guidance, Prospero positively regulates
cycE promoting cell proliferation. Neuronal Vein activates the MAPKinase
signalling pathway in the glia with highest Prospero levels, coupling axon
extension with glial proliferation. Later on, Prospero maintains glial
precursors in an undifferentiated state by activating Notch and antagonising the
p27/p21 homologue Dacapo. This enables prospero-expressing cells alone to
divide further upon elimination of neurons and to adjust glial number to axons
during development (Griffiths, 2004).
The longitudinal glia (LG) of the Drosophila CNS share some features with
vertebrate oligodendrocyte precursors. Like oligodendrocytes, LG are also
produced in excess and the excess cells are eliminated through apoptosis. The
survival of both oligodendrocytes and at least some of the LG
depends on contact with axons and on Neuregulin/Vein.
There is also suggestive evidence that LG proliferation may be under
non-autonomous control. The LG originate from the segmentally repeated
longitudinal glioblasts (LGBs). DiI labelling of the LGB produces a clone of
variable number of progeny cells, resulting in between 7 and 10 progeny cells.
There is apoptosis in up to three cells in this lineage
in normal embryos, meaning that the resulting progeny of
the LG lineage if they were all to survive may be around 12 cells. This suggests
that the mitotic profile of the LGB lineage is not simply symmetrical and/or
perhaps LG precursor divisions are under non-autonomous control (Griffiths, 2004).
This study analyzed the mechanisms that regulate proliferation of the LG as they
interact with pioneer axons. Proliferation of the LG is shown to be regulated
by neurons and prospero (pros) is shown to play a key role in linking glial
proliferation and axon guidance. Early on, Pros enables glial proliferation in
response to pioneer neurons. Once the axonal bundles are formed, Pros maintains
glial precursors in an arrested, immature state, enabling pros-expressing
cells alone to divide further upon elimination of neurons (Griffiths, 2004).
This study has found novel roles of Pros in promoting cell proliferation and
preventing cell cycle exit. The glia reach the extending
growth cones in clusters of four cells, when cell division halts for some time.
Normally, two of these LG then divide, resulting in a total of six, which then
divide again, but since some LG die the real final number ranges between 8 and
11 cells. In pros mutants, the glia contact the pioneer neurons in
clusters of eight cells rather than the normal four, suggesting that LG divided
faster than normally in the presence of maternal CycE, skipping a G1 phase. The
division of four LG into six is also missed, thus changing the mitotic pattern
from its normal 4-6-12 to 4-8 (Griffiths, 2004).
Loss of
pros function causes a reduction in LG proliferation, which is manifested
in three ways. (1) In pros mutants, the first division of the two
anterior LG with highest Pros levels is missed, because there is no dpERK. (2)
LG do not divide at the normal times during axon guidance and fasiculation is not produced in
pros mutants, because of the absence of CycE.
Thus, although LG divided earlier in pros mutants, these divisions are
uncoupled from axon guidance. Thus, Pros changes the mitotic profile in the LG
from a simple symmetric pattern to a pattern in which the LG respond to incoming
axons. (3) In the absence of Pros, LG do not have the potential to
overproliferate when neurons are ablated (Griffiths, 2004).
Pros protein is
present in all dividing LG and in LG that retain mitotic potential. During
growth cone guidance and axonal fasciculation, Pros promotes LG proliferation of
the two LG that are able to respond to Vein and activate the MAP kinase pathway.
Vein induces LG cell division as well as cell survival of the two EGFR-positive
LG. Knock-down of Vein function with targeted RNAi exclusively directed to the the MP2 neurons is
sufficient to cause LG apoptosis. Loss of Vein function in genetic null embryos
reduces mitosis, also when apoptosis is blocked. Thus, the EGFR/MAPKinase
signalling pathway controls both cell survival and cell proliferation in these
two LG. The EGFR also controls both cell survival and cell proliferation in the
retina, in response to the ligand Spitz.
Later on, when the axonal fascicles are formed, Pros maintains the mitotic
potential in the LG by preventing them from exiting the cell cycle. In fact,
only Pros-positive LG can enter S phase upon ectopic expression of cycE.
In this way, at the end of embryogenesis, the LG are divided into Pros-positive
G1-arrested LG and Pros-negative LG, which have exited the cell cycle and are in
G0. Pros maintains the LG in the G1-arrested undifferentiated, immature
precursor state by positively regulating Notch and by antagonising Dacapo (Griffiths, 2004).
These findings on the roles of Pros in the LG during axon guidance
differ from Pros' neuroblast functions. In neuroblast lineages, Pros protein is
located in a crescent and it is distributed asymmetrically to the daughter cell
upon the division of the neuroblast. In the ganglion mother cell, Pros is
internalised into the nucleus, where it determines cell fate and it restricts
cell division. However, the progeny of
the LGlioblast [from the time in which they contact the pioneer axons (four-cell
stage)] divide apparently symmetrically, although asynchronously. During these
divisions, Pros is present in the nuclei of all dividing LG, and not in
crescents. Upon cell division, Pros is segregated symmetrically to the two
daughter cells and it is downregulated after cell division, at the time that the posterior LG migrate with the axons. Finally, during axon guidance, pros mutations
cause a reduction in LG proliferation rather than an excess, meaning that
pros is necessary for cell division to proceed (Griffiths, 2004).
Pros and its vertebrate homologue Prox1 can inhibit cell proliferation and promote cell cycle exit. In fact,
both in pros and Prox1 mutants, cell proliferation and the
expression of cyclin increase, and both Prox1 and Pros can promote
p27/dap expression. In the LG, Pros promotes cell
proliferation and it prevents cell cycle exit by antagonising Dap. Therefore,
Pros controls cell cycle genes in different ways in different cellular contexts.
Moreover, temporal regulation is crucial and Pros can both promote and antagonise dap
expression at different time points. Upon ectopic pros expression
the LG divide less and do
not express cycE. However, in the LG this may not be due to the promotion
of cell cycle exit but to the earlier halt of precursors in cell cycle
arrest (Griffiths, 2004).
These findings also contrast with the roles of Pros in
mixed neuro-glioblast lineages, where Pros is segregated asymmetrically to the
daughter cell that will become a glial cell. The LG is a glial-only lineage. In the
LG, Pros may control the fate of the two LG with higher Pros levels, which
signal through MAPKinase/dpERK. The results show that during axon guidance Pros
plays a primary role in the maintenance of the proliferative and undetermined
state (Griffiths, 2004).
The current findings on the non-autonomous regulation of
glial proliferation contrast with previous work that envisioned a
cell-autonomous proliferation profile determined by lineage identity.
Accordingly, the LGB would divide in a straightforward symmetrical fashion, into
2−4−8 cells. This conclusion
was based on the finding that BrdU is incorporated in four Repo-positive cells.
The data show that the incorporation of BrdU into four LG represents a narrow
time window in the LG lineage, and not the final division. In fact,
mitosis is detected in up to five LG at the same stage (Griffiths, 2004).
The finding of a
different LG profile has important implications. It means that the final number
of LG is not fixed at eight cells, but variable between 8 and 11, depending on
how many LG die. A final fixed number of eight LG could be achieved faster
through simple symmetrical divisions without considerable influence on final
glial cell mass. In fact, in Pros mutants a final number of eight cells is
achieved at an earlier time point, and these eight cells stretch out to occupy
the whole length of the segmental neuropile. However, the sequential increase
and adjustment in LG number deploys a restricted number of LG at sequential
steps in axonal patterning. This enables glia to be in the correct number at
discrete time points to enable axon guidance and fasciculation (Griffiths, 2004).
The first event in growth cone guidance occurs at the four-cell
stage, when LG stop dividing for some time and wait for the pioneer growth cones
to extend. At this time, the LG are in the first G1 phase in the lineage. The G1
phase is a characteristic time in which cells respond to growth factors to
signal through ERK, and
in the retina axons approach selectively precursors that are in G1.
As the growth cones approach, the
two anterior LG (with higher Pros levels) of the four-cell clusters divide in
response to Vein. Vein is produced by the MP2 pioneer neurons, which require LG
for axon guidance. By regulating both cell survival
and cell proliferation, Vein ensures that LG are present in the correct number
to enable growth cone guidance. Pros regulates the zygotic expression of CycE in
LG, thus introducing the first G1-S transition, and the fate of the EGFR
signalling cells. In this way, Pros modulates the timing of the response of glia
to a neuronal signal to divide. Subsequently, the LG continue to divide at times
in which axons undergo fasciculation and defasciculation. In this way, LG are
deployed in restricted numbers to enable sorting out of axons through time (Griffiths, 2004).
Stem cells have the remarkable ability to give rise to both self-renewing and differentiating daughter cells. Drosophila neural stem cells segregate cell-fate determinants from the self-renewing cell to the differentiating daughter at each division. This study shows that one such determinant, the homeodomain transcription factor Prospero, regulates the choice between stem cell self-renewal and differentiation. The in vivo targets of Prospero have been identified throughout the entire genome. Prospero represses genes required for self-renewal, such as stem cell fate genes and cell-cycle genes. Surprisingly, Prospero is also required to activate genes for terminal differentiation. In the absence of Prospero, differentiating daughters revert to a stem cell-like fate: they express markers of self-renewal, exhibit increased proliferation, and fail to differentiate. These results define a blueprint for the transition from stem cell self-renewal to terminal differentiation (Choksi, 2006).
To identify sites within the Drosophila genome to which Prospero binds, use was made of an in vivo binding-site profiling technique, DamID. DamID is an established method of determining the binding sites of DNA- or chromatin-associated proteins. Target sites identified by DamID have been shown to match targets identified by chromatin immunoprecipitation (ChIP). DamID enables binding sites to be tagged in vivo and later identified on DNA microarrays. In brief, the DNA or chromatin-binding protein of interest is fused to an Escherichia coli adenine methyltransferase (Dam), and the fusion protein is expressed in vivo. The DNA-binding protein targets the fusion protein to its native binding sites, and the Dam methylates local adenine residues in the sequence GATC. The sequences near the protein-DNA interaction site are thereby marked with a unique methylation tag, over approximately 2–5 kilobase pairs (kb) from the binding site. The tagged sequences can be isolated after digestion with a methylation-sensitive restriction enzyme, such as DpnI (Choksi, 2006).
Dam was fused to the N terminus of Prospero, and transgenic flies were generated. The fusion protein is expressed from the uninduced minimal Hsp70 promoter of the UAS vector, pUAST, as high levels of expression of Dam can result in extensive nonspecific methylation and cell death. As a control for nonspecific Dam activity, animals expressing Dam alone were generated. To assess the sites to which Prospero binds during neurogenesis, genomic DNA was extracted from stage 10–11 embryos, approximately 4–7 hr after egg laying (AEL), expressing either the Dam-Prospero fusion protein or the Dam protein alone. The DNA was digested with DpnI and amplified by PCR. DNA from Dam-Prospero embryos was labeled with Cy3, and control DNA with Cy5. The samples were then cohybridized to genomic microarrays. Microarrays were designed that tile the entire euchromatic Drosophila genome. A 60 base oligonucleotide was printed for approximately every 300 bp of genomic DNA, resulting in roughly 375,000 probes on a single array (Choksi, 2006).
Log-transformed ratios from four biological replicates (two standard dye configurations plus two swapped dye configurations) were normalized and averaged. Regions of the genome with a greater than 1.4-fold log ratio (corresponding to approximately a 2.6-fold enrichment) of Dam-Prospero to the control over a minimum of four adjacent genomic probes were identified as in vivo Prospero binding sites. Using these parameters, a total of 1,602 in vivo Prospero binding sites were identified in the Drosophila genome. This work demonstrates that it is possible to map in vivo binding sites across the whole genome of a multicellular organism (Choksi, 2006).
Prospero is known to regulate the differentiation of photoreceptors in the adult eye, and recently sites have been characterized to which Prospero can bind upstream of two Rhodopsin genes, Rh5 and Rh6. A variant of the Prospero consensus sequence is found four times upstream of Rh5 and four times upstream of Rh6. Prospero was shown to bind this sequence in vitro, by band shift assay, and also by a 1-hybrid interaction assay in yeast. In addition, deletion analysis demonstrated that the consensus sequence is required for the Pros-DNA interaction both in vivo and in vitro. It was found that 67% of in vivo binding sites contain at least one Prospero binding motif. Combining in vivo binding-site data with searches for the Prospero consensus sequence reveals 1,066 distinct sites within the Drosophila genome to which Prospero binds during embryogenesis (Choksi, 2006).
A total of 730 genes have one or more of the 1,066 Prospero binding sites located within 1 kb of their transcription unit. Statistical analyses to determine GO annotation enrichment on the members of the gene list that had some associated annotation (519) was performed by using a web-based set of tools, GOToolbox. Using Biological Process (GO: 0008150) as the broadest classification, a list was generated of overrepresented classes of genes (Choksi, 2006).
The three most significant classes of genes enriched in the list of putative Prospero targets are Cell Fate Commitment, Nervous System Development, and Regulation of Transcription. Utilizing GO annotation, it was found that nearly 41% of all annotated neuroblast fate genes (11 of 27) are located near Prospero binding sites and that approximately 9% of known cell-cycle genes are near Prospero binding sites. These include the neuroblast genes achaete (ac), scute (sc), asense (ase), aPKC, and mira and the cell-cycle regulators stg and CycE. In addition, it was found that the Drosophila homolog of the mammalian B lymphoma Mo-MLV insertion region 1 (Bmi-1) gene, Posterior sex combs, is located near a Prospero binding site. Bmi-1 is a transcription factor that has been shown to regulate the self-renewal of vertebrate hematopoetic stem cells. It is concluded that Prospero is likely to regulate neuroblast identity and self-renewal genes as well as cell-cycle genes directly, repressing their expression in the GMC (Choksi, 2006).
Prospero enters the nucleus of GMCs, and its expression is maintained in glial cells but not in neurons . Therefore the list of targets was searched for genes annotated as glial development genes. Prospero binds near 45% of genes involved in gliogenesis. Among the glial genes, it was found that the master regulator of glial development, glial cells missing (gcm), and gilgamesh (gish), a gene involved in glial cell migration, are both near Prospero binding sites and are likely directly activated by Prospero in glia (Choksi, 2006).
In summary, Prospero binds near, and is likely to regulate directly, genes required for the self-renewing neural stem cell fate such as cell-cycle genes. It was also found that Prospero binds near most of the temporal cascade genes: hb, Kruppel (Kr), nubbin (nub/pdm1), and grainyhead (grh) and to genes required for glial cell fate. The in vivo binding-site mapping experiments are supportive of a role for Prospero in regulating the fate of Drosophila neural precursors by directly controlling their mitotic potential and capacity to self-renew (Choksi, 2006).
The Drosophila ventral nerve cord develops in layers, in a manner analogous to the mammalian cortex. The deepest (most dorsal) layer of the VNC comprises the mature neurons, while the superficial layer (most ventral) is made up of the mitotically active, self-renewing neuroblasts. Neuroblast cell-fate genes and cell-cycle genes are normally expressed only in the most ventral cells, while Prospero is found in the nucleus of the more dorsally lying GMCs. If in GMCs, Prospero normally acts to repress neuroblast cell-fate genes and cell-cycle genes, then in a prospero mutant, expression of those genes should expand dorsally. Conversely, ectopically expressed Prospero should repress gene expression in the neuroblast layer.
The neuroblast genes mira, ase, and insc and the cell cycle genes CycE and stg show little or no expression in differentiated cells of wild-type stage 14 nerve cords. Expression of these neuroblast-specific genes was examined in the differentiated cells layer of prospero embryos and it was found that they are derepressed throughout the nerve cord of mutant embryos. mira, ase, insc, CycE, and stg are all ectopically expressed deep into the normally differentiated cell layer of the VNC. To check whether Prospero is sufficient to repress these genes, Prospero was expressed with the sca-GAL4 driver, forcing Prospero into the nucleus of neuroblasts. Prospero expression is sufficient to repress mira, ase, insc, CycE, and stg in the undifferentiated cell layer of the VNC. These data, combined with the Prospero binding-site data, demonstrate that Prospero is both necessary and sufficient to directly repress neuroblast genes and cell-cycle genes in differentiated cells. This direct repression of gene expression is one mechanism by which Prospero initiates the differentiation of neural stem cells (Choksi, 2006).
Having shown that Prospero directly represses genes required for neural stem cell fate, it was asked whether Prospero also directly activates GMC-specific genes. Alternatively, Prospero might regulate a second tier of transcription factors, which are themselves responsible for the GMC fate. Of the few previously characterized GMC genes, it was found that Prospero binds to eve and fushi-tarazu (ftz). In the list of targets several more GMC genes were expected to be found, but not genes involved in neuronal differentiation, since Prospero is not expressed in neurons. Surprisingly, however, it was foudn 18.8% of neuronal differentiation genes are located near Prospero binding sites (Choksi, 2006).
To determine Prospero's role in regulating these neuronal differentiation genes, in situ hybridization was carried out on prospero mutant embryos. Prospero was found to be necessary for the expression of a subset of differentiation genes, such as the adhesion molecules FasciclinI (FasI) and FasciclinII (FasII), which have roles in axon guidance and/or fasciculation. Netrin-B, a secreted protein that guides axon outgrowth, and Encore, a negative regulator of mitosis, also both require Prospero for proper expression. Therefore, in addition to directly repressing genes required for neural stem cell self-renewal, Prospero binds and activates genes that direct differentiation. These data suggest that Prospero is a binary switch between the neural stem cell fate and the terminally differentiated neuronal fate (Choksi, 2006).
To test to what extent Prospero regulates the genes to which it binds, genome-wide expression profiling was carried out on wild-type and prospero mutant embryos. While the DamID approach identifies Prospero targets in all tissues of the embryo, in this instance genes regulated by Prospero were assayed in the developing central nervous system. Small groups of neural stem cells and their progeny (on the order of 100 cells) were isolated from the ventral nerve cords of living late stage 12 embryos with a glass capillary. The cells were expelled into lysis buffer, and cDNA libraries generated by reverse transcription and PCR amplification. cDNA libraries prepared from neural cells from six wild-type and six prospero null mutant embryos were hybridized to full genome oligonucleotide microarrays, together with a common reference sample. Wild-type and prospero mutant cells were compared indirectly through the common reference (Choksi, 2006).
In the group of Prospero target genes that contain a Prospero consensus sequence within 1 kb of the transcription unit, 91 show reproducible differences in gene expression in prospero mutants. Seventy-nine percent of these genes (72) exhibit at least a 2-fold change in levels of expression. Many of the known genes involved in neuroblast fate determination and cell-cycle regulation (e.g., asense, deadpan, miranda, inscuteable, CyclinE, and string) show increased levels in a prospero mutant background, consistent with their being repressed by Prospero. Genes to which Prospero binds, but which do not contain an obvious consensus sequence, are also regulated by Prospero: CyclinA and Bazooka show elevated mRNA levels in the absence of Prospero, as does Staufen, which encodes a dsRNA binding protein that binds to both Miranda and to prospero mRNA (Choksi, 2006).
Expression of genes required for neuronal differentiation is decreased in the prospero mutant cells, consistent with Prospero being required for their transcription. These include zfh1 and Lim1, which specify neuronal subtypes, and FasI and FasII, which regulate axon fasciculation and path finding (Choksi, 2006).
The stem cell-like division of neuroblasts generates two daughters: a self-renewing neuroblast and a differentiating GMC. Prospero represses stem cell self-renewal genes and activates differentiation genes in the newly born GMC. In the absence of prospero, therefore, neuroblasts should give rise to two self-renewing neuroblast-like cells (Choksi, 2006).
The division pattern of individual neuroblasts was studied in prospero mutant embryos by labeling with the lipophilic dye, DiI. Individual cells were labeled at stage 6, and the embryos allowed to develop until stage 17. S1 or S2 neuroblasts were examined, as determined by their time of delamination. Wild-type neuroblasts generate between 2 and 32 cells, producing an average of 16.2 cells. Most of the clones exhibit extensive axonal outgrowth. In contrast, prospero mutant neuroblasts generate between 8 and 51 cells, producing an average of 31.8 cells. Moreover, prospero mutant neural clones exhibit few if any projections, and the cells are smaller in size. Thus, prospero mutant neuroblasts produce much larger clones of cells with no axonal projections, suggesting that neural cells in prospero mutants undergo extra divisions and fail to differentiate (Choksi, 2006).
Recently it was shown, in the larval brain, that clones of cells lacking Prospero or Brat undergo extensive cell division to generate undifferentiated tumors. Given that Prospero is nuclear in the GMC but not in neuroblasts, the expanded neuroblast clones in prospero mutant embryos might arise from the overproliferation of GMCs: the GMCs lacking Prospero may divide like neuroblasts in a self-renewing manner. It is also possible, however, that neuroblasts divide more frequently in prospero mutant embryos, giving rise to supernumerary GMCs that each divide only once. To distinguish between these two possibilities, the division pattern of individual GMCs was followed in prospero mutant embryos (Choksi, 2006).
S1 or S2 neuroblasts were labeled with DiI as before. After the first cell division of each neuroblast, the neuroblast was mechanically ablated, leaving its first-born GMC. All further labeled progeny derive, therefore, from the GMC. Embryos were allowed to develop until stage 17, at which time the number of cells generated by a single GMC was determined (Choksi, 2006).
To determine whether mutant GMCs are transformed to a stem cell-like state, stage 14 embryos were stained for the three neuroblast markers: Miranda (Mira), Worniu (Wor), and Deadpan (Dpn). In wild-type embryos at stage 14, the most dorsal layer of cells in the VNC consists mostly of differentiated neurons. As a result, few or none of the cells in this layer express markers of self-renewal. Mira-, Wor-, and Dpn- expressing cells are found on the midline only or in lateral neuroblasts of the differentiated cell layer of wild-type nerve cords. In contrast, a majority of cells in the differentiated cell layer of stage 14 prospero mutant embryos express all three markers: Mira is found cortically localized in most cells of the dorsal layer of prospero nerve cords; Wor is nuclear in most cells of mutant VNCs; Dpn is ectopically expressed throughout the nerve cord of prospero mutants (Choksi, 2006).
Expression of neuroblast markers in the ventral-most layer of the nerve cord (the neuroblast layer), to exclude the possibility that a general disorganization of cells within the VNC contributes to the increased number of Mira-, Wor-, and Dpn-positive cells in the dorsal layer. The number of neuroblasts in a prospero mutant embryo is normal in stage 14 embryos, as assayed by Wor, Dpn, and Mira expression. Thus, the increased expression of neuroblast markers in prospero mutants is the result of an increase in the total number of cells expressing these markers in the differentiated cell layer. It is concluded that prospero mutant neuroblasts divide to give two stem cell-like daughters. GMCs, which would normally terminate cell division and differentiate, are transformed into self-renewing neural stem cells that generate undifferentiated clones or tumors (Choksi, 2006).
Therefore, Prospero directly represses the transcription of many neuroblast genes and binds near most of the temporal cascade genes: hb, Kruppel (Kr), nubbin (nub/pdm1), and grainyhead (grh), which regulate the timing of cell-fate specification in neuroblast progeny. Prospero maintains hb expression in the GMC, and it has been suggested that this is through regulation of another gene, seven-up (svp). Prospero not only regulates svp expression directly but also maintains hb expression directly. In addition, Prospero maintains Kr expression and is likely to act in a similar fashion on other genes of the temporal cascade. Intriguingly, Prospero regulates several of the genes that direct asymmetric neuroblast division (baz, mira, insc, aPKC). aPKC has recently been shown to be involved in maintaining the self-renewing state of neuroblasts (Choksi, 2006).
Prospero initiates the expression of genes necessary for differentiation. This is particularly surprising since prospero is transcribed only in neuroblasts, not in GMCs or neurons. Prospero mRNA and protein are then segregated to the GMC. Prospero binds near eve and ftz, which have been shown to be downstream of Prospero, as well as to genes required for terminal neuronal differentiation, including the neural-cell-adhesion molecules FasI and FasII. Prospero protein is present in GMCs and not neurons, suggesting that Prospero initiates activation of neuronal genes in the GMC. The GMC may be a transition state between the neural stem cell and the differentiated neuron, providing a window during which Prospero functions to repress stem cell-specific genes and activate genes required for differentiation. There may be few, or no, genes exclusively expressed in GMCs (Choksi, 2006).
Prospero acts in a context-dependent manner, functioning alternately to repress or activate transcription. This implies that there are cofactors and/or chromatin remodeling factors that modulate Prospero's activity. In support of this, although Prospero is necessary and sufficient to repress neuroblast genes, forcing Prospero into the nuclei of neuroblasts is not sufficient to activate all of the differentiation genes to which it binds (Choksi, 2006).
Neuroblasts decrease in size with each division throughout embryogenesis. By the end of embryogenesis, they are similar in size to neurons. A subset of these embryonic neuroblasts becomes quiescent and is reactivated during larval life: they enlarge and resume stem cell divisions to generate the adult nervous system. Neuroblasts in prospero mutant embryos divide to produce two self-renewing daughters but still divide asymmetrically with respect to size, producing a large apical neuroblast and a smaller basal neuroblast-like cell. The daughter may be too small to undergo more than three additional rounds of division during embryogenesis. prospero mutant cells eventually stop dividing, and a small number occasionally differentiate. This suggests that there is an inherent size limitation on cell division. The segregation of Brat, or an additional cell fate determinant, to the daughter cell may also limit the potential of the prospero mutant cells to keep dividing (Choksi, 2006).
The Prox family of atypical homeodomain transcription factors has been implicated in initiating the differentiation of progenitor cells in contexts as varied as the vertebrate retina, forebrain, and lymphatic system. Prospero/Prox generally regulates the transition from a multipotent, mitotically active precursor to a differentiated, postmitotic cell. In most contexts, Prox1 acts in a similar fashion to Drosophila Prospero: to stop division and initiate differentiation (Choksi, 2006).
It is proposed that Prospero/Prox is a master regulator of the differentiation of progenitor cells. Many of the vertebrate homologs of the Drosophila Prospero targets identified in this study may also be targets of Prox1 in other developmental contexts. Prospero directly regulates several genes required for cell-cycle progression, and it is possible that Prox1 will regulate a similar set of cell-cycle genes during, for example, vertebrate retinal development. In addition, numerous Prospero target genes have been identified whose orthologs may be involved in the Prox-dependent differentiation of retina, lens, and forebrain precursors (Choksi, 2006).
Coordination between cell proliferation and cell death is essential to maintain homeostasis in multicellular organisms. In Drosophila, these two processes are regulated by a pathway involving the Ste20-like kinase Hippo (Hpo) and the NDR family kinase Warts (Wts; also called Lats). Hpo phosphorylates and activates Wts, which in turn, through unknown mechanisms, negatively regulates the transcription of cell-cycle and cell-death regulators such as cycE and diap1. Yorkie (Yki), the Drosophila ortholog of the mammalian transcriptional coactivator yes-associated protein (YAP), has been identified as a missing link between Wts and transcriptional regulation. Yki is required for normal tissue growth and diap1 transcription and is phosphorylated and inactivated by Wts. Overexpression of yki phenocopies loss-of-function mutations of hpo or wts, including elevated transcription of cycE and diap1, increased proliferation, defective apoptosis, and tissue overgrowth. Thus, Yki is a critical target of the Wts/Lats protein kinase and a potential oncogene (Huang, 2005).
Activation of yki leads to increased transcription of diap1 and CycE. The increased cell proliferation and decreased apoptosis resulting from yki overexpression are strikingly similar to those caused by loss of hpo, sav, or wts, suggesting that Yki functions in the Hpo pathway. To further explore this possibility, the transcription of cell-death inhibitor diap1 and cell-cycle regulator cycE, known targets of the Hpo pathway were examined. Elevated DIAP1 protein is detected in yki-overexpressing clones in the eye discs. This regulation is largely mediated at the level of diap1 transcription since the expression of thj5c8, a P[lacZ] enhancer trap reporter inserted into the diap1 locus, is similarly elevated in yki-overexpressing clones in a cell-autonomous manner. A cycE-lacZ reporter containing 16.4 kb of the 5′ regulatory sequence of cycE is also increased in yki-overexpressing clones, especially those close to the MF, although the effect is less profound than that observed with the diap1 reporter. Thus, like loss of hpo, sav, or wts, overexpression of yki results in increased transcription of diap1 and cycE. It is worth noting that previous analyses of hpo mutant clones also revealed a 'tighter' regulation of diap1: while diap1 transcription is elevated in all hpo mutant cells irrespective of their relative position to the MF, cycE transcription is only elevated in hpo mutant cells close to the MF (Wu, 2003). These observations suggest that diap1 might represent a more direct transcriptional target of the Hpo pathway (Huang, 2005).
Appropriate cell number and organ size in a multicellular organism are determined by coordinated cell growth, proliferation, and apoptosis. Disruption of these processes can cause cancer. Recent studies have identified the Large tumor suppressor (Lats)/Warts (Wts) protein kinase as a key component of a pathway that controls the coordination between cell proliferation and apoptosis. Growth inhibitory functions are described for a Mob superfamily protein, termed Mats (Mob as tumor suppressor), in Drosophila. Loss of Mats function results in increased cell proliferation, defective apoptosis, and induction of tissue overgrowth. Mats and Wts function in a common pathway. Mats physically associates with Wts to stimulate the catalytic activity of the Wts kinase. A human Mats ortholog (Mats1) can rescue the lethality associated with loss of Mats function in Drosophila. Since Mats1 is mutated in human tumors, Mats-mediated growth inhibition and tumor suppression is likely conserved in humans (Lai, 2005).
Cyclin E, a key regulator for the G1-S transition, is normally expressed in the second mitotic wave (SMW) of larval eye discs. In mats mosaic eye discs, Cyclin E levels are elevated in mutant clones located in the morphogenetic furrow (MF) and SMW regions. Moderate upregulation of Cyclin A and Cyclin B expression is also observed. Thus, an important mechanism for mats to control cell proliferation is to negatively regulate expression of key cell cycle regulators such as Cyclins. Interestingly, Cyclin E expression in mutant cells immediately anterior to the MF is much less elevated than that immediately posterior to the MF, suggesting that mats may use a different mechanism to restrict cell proliferation in cells anterior to the MF. The cell proliferation defects observed in mats mutants are similar to those caused by sav, wts, and hpo mutations (Lai, 2005).
A coordinate program of transcription of S-phase genes (DNA polymerase alpha, Proliferating cell nuclear antigen and the two ribonucleotide reductase subunits) can be induced by Cyclin E (the G1 cyclin). In Drosophila embryos, this program drives an intricate spatial and temporal pattern of gene expression that perfectly parallels the embryonic program of S-phase control. This dynamic pattern of expression is not disrupted by a string mutation that blocks the cell cycle. Thus, the transcriptional program is not a secondary consequence of cell cycle progression. It is likely that developmental signals control this transcriptional program and that its activation either directly or indirectly drives transition from G1 to S phase in the stereotyped embryonic pattern (Durano, 1995a).
The onset of cyclin expression in endoreduplicating tissues is not dependent on Cyclin E function, as Cyclin E expression is observed in CycE mutants. However, instead of being terminated rapidly, the expression is maintained in CycE mutants. Thus functional Cyclin E protein is required directly or indirectly to shut off its own expression. Consistent with this notion, it is found that overexpression of Cyclin E down-regulates the endogenous CycE transcription (Sauer, 1995).
To test if the cell cycle transcription of double parked (dup) is dependent on
E2F, embryos homozygous for a null allele of the dE2F1 subunit, dE2F91 were collected and hybridized with
dup riboprobes. The levels of dup transcript are
decreased in the endoreplicating gut and appear to be slightly
decreased in the CNS. A similar effect on dup
transcript is seen in embryos that are homozygous mutant for the
other subunit of the E2F transcription factor, of the genotype
dDPa2. Thus, dup is a downstream
target of the E2F transcription factor. Interestingly, yeast cdt1
transcription is also cell-cycle regulated. Expression of cdt1
is controlled by the G1-S transcription factor Cdc10 that, like E2F,
regulates transcription of many genes required for S phase (Hofmann, 1994). This suggests that cell cycle control of dup may
be conserved, and Dup may prove to be an important downstream target of
E2F in mammalian cells (Whittaker, 2000).
Cyclin E is required to regulate positively the transcription of S
phase genes in the nervous system and to downregulate these transcripts
in endo cycling cells.
The cyclinEl(2)305 and cyclin
EPZ5 mutations and a cyclin E deficiency,
Df(2L)TE35D1, have similar effects on double parked
transcripts. In these embryos, dup is not downregulated properly in the endoreplicating gut such that dup transcripts persist at higher levels than wild type in the anterior midgut, central midgut, and posterior midgut in later embryonic stages. In cyclin E mutant embryos dup
transcripts are reduced in the CNS, although not to as great an extent
as other S phase genes. Thus, dup expression also is regulated
by cyclin E (Whittaker, 2000).
The Cip/Kip family of cyclin-dependent kinase inhibitors (CKIs) has been implicated in mediating cell cycle arrest prior to terminal
differentiation. In many instances, increased expression of CKIs immediately precedes mitotic arrest. However, the mechanism that activates
CKI expression in cells that are about to stop dividing has remained elusive. This issue was addressed by investigating the
expression pattern of dacapo, a Cip/Kip CKI in Drosophila. The accumulation of Dacapo RNA and protein requires Cyclin
E and ectopic expression of Cyclin E can induce dacapo expression. The oscillation of the Cyclin E and Dacapo
proteins is tightly coupled during ovarian endocycles. These results argue for a mechanism where Cyclin E/Cdk activity induces Dacapo
expression but only within certain windows that are permissive for dacapo expression (de Nooij, 2000).
In a number of different tissues, expression of Dap occurs
precisely during the last mitotic division that cells undergo
before they terminally differentiate. For instance, in the
embryonic epidermis, a rapid accumulation of DAP mRNA
and Dap protein is detected only following S-phase 16,
just before these cells arrest in the G1-phase of cycle 17. Exit from the
cell cycle also requires alterations in the levels of Cyclins
and the activity of Cyclin-Cdk complexes. Is the
induction of dap directly regulated by the developmental
cues that dictate cell cycle exit? Or alternatively, is the
induction of DAP RNA and protein a response to an alteration in the activity of one of the other cell cycle regulators (de Nooij, 2000)?
To help distinguish between these possibilities, whether alterations in the levels of one of the
known cell cycle regulators would affect the normal pattern
of Dap expression was examined in different embryonic tissues. Embryos that were mutant for the G1-S regulators, cyclin E, E2F1 and DP and for the regulators that
mediate the G2-M transition, string (stg), cyclin A, rca1,
cyclin B and cdc2 were examined. With the notable exception of cyclin E mutants, no obvious abnormalities in the Dap expression
pattern could be observed in any of these mutants. Analysis of cyclin E (cycE) mutant embryos, however, shows significant differences in the
expression pattern of Dap. Dap protein levels are severely
reduced in the cells of the PNS and are almost absent in the
cells of the CNS of embryos homozygous or hemizygous for
the cycE null allele cycEAr95. The level of DAP
RNA is also reduced in the cells of the PNS and the CNS. This suggests that Cyclin E may regulate dap expression at least in part at the transcriptional level. Immunostaining of cycE mutant embryos with an anti-Elav antibody does not show significant abnormalities in the cellularity of the PNS and CNS,
suggesting that the loss of dap expression is not a secondary
consequence of a failure to form these tissues (de Nooij, 2000).
Interestingly, no reduction in Dap expression levels is
observed in the epidermis of cycE mutant embryos. However, cycE mutants have been shown to
complete all 16 epidermal divisions normally, possibly
due to the activity of residual maternal supplies of Cyclin
E. It is therefore likely that the residual supply of Cyclin E may also be sufficient to trigger Dap
expression in the epidermis. Alternatively, the apparently
normal expression of Dap in the cycE mutant epidermal
cells may reflect a difference between the mechanisms
that regulate Dap expression in the epidermis and in the
nervous system (de Nooij, 2000).
These results also show that normal Dap expression is not
contingent upon progression through the cell cycle.
Embryos mutant for string, which encodes a Cdc25-like
phosphatase, arrest in the G2 phase of cycle 14. Despite
the absence of S-phase progression and cell division in
string mutants, the onset of Dap expression occurs at the
appropriate developmental stage in both the epidermis and
the cells of the nervous system. These observations also show that Dap expression is not normally contingent upon cell cycle progression or on reaching cycle 16 in
the epidermis (de Nooij, 2000).
The reduced level of dap RNA in cycE mutant embryos
seems to indicate a role for Cyclin E in the transcriptional
regulation of dap. A potential mechanism for the transcriptional regulation of dap by Cyclin E could involve the E2F
transcription factor. It has been demonstrated previously
that E2F can mediate the activation of a 'G1-S transcriptional program' that, in the CNS, is dependent upon cyclin E
function. E2F can also activate stg expression during G2. However, while E2F1 and DP mutant embryos
clearly show a reduction in the RNA levels of the E2F
responsive genes PCNA and RNR2, Dap expression appears to be normal
in both E2F and DP mutant embryos (de Nooij, 2000).
Cyclin E is also expressed normally in the CNS in E2F
and DP mutant embryos. Thus, a redundant function for E2F in regulating dap expression cannot be ruled out, these experiments do not provide any evidence for E2F as a regulator of dap expression. Taken together, these results show that when Cyclin E activity is abolished in the nervous system, Dap expression is no longer observed thus indicating a requirement for Cyclin E activity in regulating Dap expression. This activity of Cyclin E does not appear to involve the activity of the E2F transcription factor. Moreover, Dap expression in either the epidermis or the nervous system is not contingent upon cell cycle progression (de Nooij, 2000).
In both eye and wing imaginal discs, ectopic Cyclin E can induce ectopic
expression of Dap. To determine whether the induction of Dap is due to an
increase in DAP RNA levels, RNA levels of
DAP were examined in whole-mount preparations by in situ hybridizations in
eye and wing discs in which Cyclin E was ectopically
expressed. In contrast to the dramatic increase in Dap
protein, relatively modest increases in the levels of DAP
RNA are observed in both the eye and wing discs. Nonetheless, these results are consistent with the notion that Cyclin E regulates DAP RNA levels, and furthermore, suggest that dap transcription may be responsive to
Cyclin E levels (de Nooij, 2000).
To test whether dap regulatory elements are responsive
to Cyclin E levels, several lines of transgenic flies were generated that contain a 2.7 kb fragment of the dap promoter region linked to a beta-galactosidase reporter. Immunohistochemical analysis of wing discs from flies
that contain the pCasdap2.7kb-lacZ transgene either
alone, or in the presence of the enGAL4 driver, shows no
significant lacZ staining in the posterior compartment of
the wing disc. In the presence of both the enGAL4 driver and a UAScycE transgene, increased beta-galactosidase activity is clearly detected in the region where Cyclin E is expressed. Thus Cyclin E can activate the expression of a reporter gene under the control of dap regulatory elements (de Nooij, 2000).
Since Cyclin E appears to be a requirement for the
expression of the Dap protein, Dap expression was examined
in the context of oscillating levels of Cyclin E in the nurse
cell nuclei in the ovary. Oogenesis normally starts with four
mitotic germ cell divisions which generate a 16-cell cyst.
Surrounded by somatically-derived follicle cells, each cyst
forms an individual egg chamber that ultimately gives rise
to a mature egg. One of the 16-germ cell nuclei arrests in the prophase of
meiosis I, becomes transcriptionally silent and is specified
as the oocyte nucleus. The remaining 15 cells, the nurse cells, proceed
through a series of endocycles. In wildtype ovaries, Dap expression is first detected in the germ cell nuclei in the germarium at a time when the four mitotic cyst cell divisions are about to be completed (region 2A of oogenesis). In the maturing egg chambers Dap is detected in the endocycling nurse cell nuclei at levels which, within an individual egg chamber, vary from very high to undetectable. This is likely to reflect an oscillation in the level of Dap protein during the endocycles, as has been postulated for Cyclin E. In contrast to the nurse cell nuclei, high levels of Dap are observed in the oocyte nucleus throughout oogenesis (de Nooij, 2000).
Consistent with the immunostaining, DAP RNA can be
observed in the germarium early in oogenesis, and subsequently, high levels of DAP RNA are found i