Notch
Notch and neurogenesis: The PNS and bristle formation (part 2/2) Genetically, Hairless (H) acts as an antagonist of most neurogenic genes and may insulate neural
precursor cells from inhibition. H function is required for commitment to the bristle sensory organ
precursor (SOP) cell fate and for daughter cell fates. Using Notch gain-of-function alleles and
conditional expression of an activated Notch transgene, enhanced signaling in these cases produces
H-like loss-of-function phenotypes by suppressing bristle SOP cell specification or by causing an
H-like transformation of sensillum daughter cell fates. Furthermore, adults carrying Notch gain of
function and H alleles exhibit synergistic enhancement of mutant phenotypes. Over-expression of
an H+ transgene product suppresses virtually all phenotypes generated by Notch gain-of-function
genotypes. Phenotypes resulting from over-expression of the H+ transgene blocked by the
Notch gain-of-function products, indicating a balance between Notch and H activity. The results
suggest that H insulates SOP cells from inhibition and indicate that H activity is suppressed by
Notch signaling (Lyman, 1995).
To learn about the acquisition of neural fate by ectodermal cells, a very early sign of neural commitment in Drosophila has been analyzed, namely the specific accumulation of achaete-scute complex (AS-C) proneural proteins in the cell that becomes a sensory organ mother cell (SMC). An AS-C enhancer has been analyzed that directs expression specifically in SMCs. To delimit the sequences responsible for expression in SMCs, subfragments of a 3.7-kb fragment immediately upstream of scute were assayed for their ability to drive lacZ expression in wing discs. The necessary sequences are within a 356-bp fragment. This fragment specifically directed expression in SMCs. It also promotes expression in SMCs of other imaginal discs and of the embryonic PNS, but not in neuroectoderm neuroblasts. The SMC enhancer is shown to promote macrochaetae formation. Interspecific sequence comparisons and site-directed mutagenesis show the presence of several conserved motifs necessary for enhancer action, some of them binding sites for proneural proteins. The conserved sequences contain three E boxes: these are putative binding sites for bHLH proteins of the Achaete, Scute, and Daughterless type. The most proximal of the three is adjacent to an N box, a site that can be recognized by the E(spl)-C bHLH proteins. In addition, there are three copies of a motif reminiscent of a consensus binding site for the NF-kappaB family of transcription factors (named alpha1, alpha2, and alpha3), and three copies of a T-rich motif (termed beta1, beta2 and beta3) that does not fit with known protein-binding sequences. In spite of considerable effort, the NF-kappaB family member binding to the alpha motifs has not been identified (Culi, 1998).
In order to promote transcription, the SMC enhancer requires, besides proneural proteins, either additional activating factors or the removal of inhibitors. Activating factors might interact with the alpha and beta boxes necessary for efficient enhancer action. To identify the minimum number of different motifs sufficient to constitute an SMC-specific enhancer, the enhancer activity of a synthetic oligonucleotide containing two E1 boxes and one alpha2 box were examined. It promotes beta-galactosidase accumulation only in SMCs, although a weak one. A four tandem repeat of the same oligonucleotide drives much stronger lacZ activity and this also occurs exclusively in SMCs. In contrast, a four tandem repeat with E1 boxes, but without alpha2 boxes, drives strong expression in many cells of proneural clusters. A four tandem repeat of alpha2 boxes without E1 boxes fails to drive expression. Hence, both E and alpha boxes are sufficient, in the context of the minienhancer, to constitute an SMC-specific enhancer (Culi, 1998).
Promoters of other genes share similar enhancer motifs. The asense sequences that direct expression in SMCs contain several E boxes necessary for optimal expression in SMCs. The corresponding DNA from D. virilis was sequenced and compared with that of D. melanogaster. Similar to the sc SMC-enhancer, the stretches of D. virilis conserved DNA contain E boxes, one N box, two alpha boxes, and one beta box, supporting the relevance of these boxes for SMC enhancer function. Moreover, the neurogenic gene Bearded, which is expressed in proneural clusters and SMCs, contains in its regulatory region one E box, necessary for its expression, and one motif identical to the alpha2 box. An evolutionarily conserved alpha box is also found within the regulatory region of rough, a homeobox gene important for restricting photoreceptor R8 specification (Culi, 1998).
Notch signaling prevents more than one of a proneural cluster from becoming SMCs. When the N pathway is not operative, as for instance in Su(H) larvae or in larvae harboring a Nts allele raised at a nonpermissive temperature, Ac and Sc proteins accumulate in many cells of proneural clusters at levels higher than in the wild type. The extra accumulation of Ac and Sc might be mediated by the cluster-specific enhancers, by the SMC enhancer (which under insufficient N signaling may promote expression in many cells of the proneural cluster as they become SMCs), or by both. To distinguish among these alternatives, an examination was carried out of the activity promoted by each type of enhancer, in both wild-type and in Nts discs. N inactivation allows the SMC enhancer to drive expression in many cells of proneural clusters. Expression can occur in contiguous cells, indicating the failure of lateral inhibition. In contrast, N inactivation does not modify the activity of the enhancer that drives expression in the vein L3 and TSM (twin sensilla of the wing margin) proneural clusters, although the accumulation of Sc in these clusters is increased. Hence, the SMC enhancer is responsible for most of the increased levels of proneural protein that occur in proneural clusters under insufficient N function (Culi, 1998).
N signaling, triggered by Ac-Sc in the emitter cell, promotes in the receptor cell the accumulation of E(spl)-C proteins, the main effectors of this signal. E(spl)-C proteins are detectable in proneural cluster cells, except for the SMCs. This correlates with the SMCs being the cells that signal maximally and inhibit their neighbors from acquiring the neural fate, while the SMCs themselves are not inhibited. Ectopic accumulation of E(spl)-C protein prevents SMCs from emerging, as detected by a neuralized enhancer trap line and the consequent absence of SOs in the adult fly. Overexpression of UAS-E(spl)-m8 or UAS-E(spl)-m7 transgenes driven by da-GAL4 or the C-253 GAL4 lines block the activity of the SMC enhancer and the development of the corresponding SOs. In contrast, either of these overexpressions allowed normal accumulation of Ac and Sc in proneural clusters despite the high levels of ectopic E(spl)-m8 mRNA, which are severalfold higher than those in the wild type. However, overexpression with presumably stronger GAL4 drivers does interfere with ac-sc expression in proneural clusters. Taken together these results indicate that the function of the SMC enhancer is more sensitive to E(spl)-C inhibition than are the proneural cluster enhancers, and suggest that the SMC enhancer is the main target of lateral inhibition mediated by the N pathway (Culi, 1998).
Does E(spl)-m8 bind to the SMC enhancer, given the inhibition of SMC enhancer function by E(spl)-C? E(spl)-m8 binds to the N box and, unexpectedly, also protects a broad region of the enhancer (nucleotides 142-182), which does not contain sequences that fit the E(spl)-C consensus binding site. Binding to an enhancer with a mutated N box is weaker, and binding to an enhancer without the N box and the second E(spl)-m8-binding site is undetectable. Remarkably, the removal of one or both binding sites does not modify the SMC specificity of the enhancer, as might be expected if these binding sites mediated the repression of enhancer function in response to N signaling. E(spl)-m8 is unable to bind to the synthetic SMC-specific minienhancer. These results were extended to other E(spl)-C proteins by verifying that [similar to E(spl)-m8] E(spl)-m5 binds to an oligonucleotide with the E1-N sequence, but not to oligonucleotides containing only E2 or E3 boxes. Thus, it is concluded that the E(spl)-C proteins restrict enhancer function to SMCs by a mechanism that does not require direct interaction with enhancer DNA. Thus the Enhancer of split bHLH proteins block the proneural gene self-stimulatory loop, possibly by antagonizing the action on the enhancer of the NF-kappaB-like factors or the proneural proteins. These data suggest a mechanism for SMC committment (Culi, 1998).
Asymmetric cell division is a widespread mechanism in developing tissues that leads to the generation of cell diversity. For the most part the basis of asymmetric cell division has been analyzed in neuroblasts in the process by which neuroblast division yields another neuroblast and a secondary precursor cell: the ganglion mother cell (GMC). In the embryonic central nervous system of Drosophila melanogaster, GMCs divide and produce postmitotic neurons that take on different cell fates. The current study analyses the process of binary fate decision of two pairs of sibling neurons that occurs during cell division in GMCs. This process is accomplished through the intrinsic fate determinant, Numb. GMCs have apical-basal polarity; Numb localization and the orientation of division are coordinated to segregate Numb to only one sibling cell. The correct positioning of Numb and the proper orientation of division require Inscuteable (Insc). Loss of insc results in the generation of equivalent sibling cells. These results provide evidence that sibling neuron fate decision is nonstochastic and normally depends on the presence of Numb in one of the two siblings. Moreover, the data suggest that the fate of some sibling neurons may be regulated by signals that do not require lateral interaction between the sibling cells (Buescher, 1998).
The focus for the analysis of the roles of insc, numb, and components of the N-signaling pathway in fate specification, was on the only two pairs of GMC-derived neurons for which sibling relationships have been established: the RP2/RP2sib and the aCC/pCC neurons. These neurons are derived from two GMCs that can be identified unambigously by their specific expression of the nuclear protein Even-skipped (Eve). GMC1-1a divides into the aCC/pCC neurons that have approximately equal size and continue to express Eve. However, at later stages of development, aCC is distinguished from pCC by the expression of Zfh-1 and 22C10 (a membrane associated antigen). aCC is a motoneuron and forms an ipsilateral projection that pioneers the intersegmental nerve. GMC4-2a divides to form the sibling neurons RP2/RP2sib that are morphologically distinguishable. In 88% of the hemisegments, the newborn siblings show a significant difference in the size of their nuclei and cell bodies. This asymmetry appears to be initiated during cell division. In GA1019 mutant embryos, in which GMC4-2a fails to complete cytokinesis, cells are formed that contain one large and one small nucleus. This strongly suggests that the difference in size is generated early, prior to the completion of cytokinesis. The larger cell always adopts the RP2 fate, which is characterized by the expression of Eve, Zfh-1, and 22C10. RP2 forms an antero-ipsilateral projection. The smaller sibling always adopts the RP2sib fate, which is characterized by a further decrease in cell and nuclear size and the loss of Eve immunreactivity. Zfh-1 and 22C10 expression have not been shown in RP2sib. These observations suggest that the cell and nuclear size difference may serve as an early physical marker that will allow one to differentiate between the two progeny of GMC4-2a, irrespective of the molecular markers they express later (Buescher, 1998).
Mutations in mastermind (mam),sanpodo, and Notch equalize aspects of sibling cell fate but retain the difference in cell and nuclear size of sibling neurons. In mam mutant embryos, both progeny of GMC4-2a can adopt the RP2 fate with respect to Eve, Zfh-1, and 22C10 expression. However, despite this apparent change from the RP2sib to the RP2 cell fate, the unequal sizes of the GMC4-2a daughter cells remain; that is, their sizes are unaffected. mam is required for the correct fate specification of RP2sib and pCC but not for that of RP2 and aCC. The requirement for mam suggests that N signaling may be involved in the resolution of distinct sibling neuron cell fate. Mutations in mam and N result in similar defects and support the notion that N signaling is required for the resolution of sibling neuron fate. In inscuteable mutant embryos, GMC1-1a and GMC4-2a are correctly formed and express normal levels of Eve (and in the case of GMC4-2a, also Pdm-1). However, GMC1-1a divides to form two sibling neurons that both adopt the aCC fate (94%) with respect to marker gene expression. Similarly, GMC4-2a division results in two sibling cells, both of which adopt the RP2 fate (96%) with respect to expression of Eve, Zfh-1, and 22C10, as well as axon morphology. This strongly suggests that in wild-type embryos, the divisions of GMC1-1a and GMC4-2a are asymmetric in an insc-dependent manner and produce sibling cells that are intrinsically different; loss of insc function leads to the generation of sibling neurons with equivalent cellular identities. Moreover, in contrast to mam, sanpodo, and Notch mutant embryos, the duplicated RP2s seen in insc mutants are equal with respect to their cell and nuclear size. These observations are consistent with the idea that the size difference seen in wild-type embryos is generated by an insc-dependent process during the GMC cell division and occurs prior to the events mediated by mam, spdo, and N that presumably act at the level of the postmitotic sibling cells. No size asymmetry between the sibling neurons should be generated in an insc background regardless of whether the other functions (e.g., spdo) are present or not (Buescher, 1998).
polychaetoid is required for cell fate specification in the eye. In pyd mutants a slight roughness of the eye is detected. This roughness is due to the presence of mispositioned and duplicated mechanosensory bristles and occasional enlarged, irregularly shaped facets. In addition, the rows of ommatidia are misaligned. In the midpupal stage eyes of pyd mutants, a small fraction of ommatidia are observed to have extra cone cells and/or primary pigment cells. Some ommatidia have too few cone cells. Ommatidia with extra cone cells also have extra photoreceptors. Reduction of Delta or Notch function in pyd mutants strongly enhances both the pyd extra bristle and rough eye phenotypes (Chen, 1996).
The Notch receptor is the central element in a cell signaling mechanism controlling a broad spectrum of
cell fate choices. Genetic modifier screens in Drosophila and subsequent molecular studies have
identified several Notch pathway components, but the biochemical nature of signaling is still elusive.
The results are described of a genetic modifier screen of the bristle phenotype of a gain-of-function
Notch allele, Abruptex16. Abruptex mutations interfere with lateral inhibition/specification events that
control the segregation of epidermal and sensory organ precursor lineages, thus inhibiting bristle
formation. Mutations that reduce Notch signaling suppress this phenotype. This screen of
approximately 50,000 flies led to the identification of a small number of dominant suppressors in seven
complementation groups. These include Notch, mastermind, Delta,
and Hairless, known components in the pathway, as well as two novel mutations: A122 and M285. A122, appears to interact with Notch only during bristle development. M285, displays extensive genetic interactions with the Notch pathway elements and appears, in general, capable of suppressing Notch gain-of-function phenotypes while enhancing Notch loss-of-function phenotypes, suggesting that it plays an important role in Notch signaling. The profile of the genetic interactions
documented with M285 is quite similar to that of mutations in other known components of the Notch
pathway. Three kismet alleles were isolated as weak suppressors of the Ax16 bristle phenotype. Interestingly,
mutations in kismet have been isolated independently as enhancers of the eye phenotype associated
with the expression of constitutively activated forms of the Notch receptor. kismet, which may encode a structural component of chromatin, does not display broad genetic interactions with Notch. It has therefore been suspected that the identification of these alleles through the eye screen may reflect its effect on the expression of the transgene by perturbing normal chromatin function rather than significant
interactions with Notch signaling. The fact that
such alleles were isolated in the bristle screen may be indicative of a link between Notch signaling and kismet function; however, further analysis is necessary before such a relationship can be established (Go, 1998b).
An early step in the development of the large mesothoracic
bristles (macrochaetae) of Drosophila is the expression of
the proneural genes of the achaete-scute complex (AS-C)
in small groups of cells (proneural clusters) of the wing
imaginal disc. This is followed by a much increased
accumulation of AS-C proneural proteins in the cell that
will give rise to the sensory organ, the SMC (sensory organ
mother cell). This accumulation is driven by cis-regulatory
sequences, SMC-specific enhancers, that permit self-stimulation
of the achaete, scute and asense proneural
genes. Negative interactions among the cells of the cluster,
triggered by the proneural proteins and mediated by the
Notch receptor (lateral inhibition), block this accumulation
in most cluster cells, thereby limiting the number of SMCs.
In
addition, proneural proteins trigger positive interactions among cells of the cluster
that are mediated by the Epidermal growth factor receptor
(Egfr) and the Ras/Raf pathway. These interactions,
which are termed 'lateral co-operation', are essential
for macrochaetae SMC emergence. Activation of the
Efgr/Ras pathway appears to promote proneural gene
self-stimulation mediated by the SMC-specific enhancers.
Excess Egfr signaling can overrule lateral inhibition and
allow adjacent cells to become SMCs and sensory organs.
Thus, the Egfr and Notch pathways act antagonistically
in notum macrochaetae determination (Culí, 2001).
Egfr-mediated lateral cooperation should tend to activate the
SMC-specific enhancers in many cells of the proneural clusters. This, however, is prevented by N signaling, which is
activated by Ac and Sc in the cells of the cluster. This signaling, by means of the bHLH proteins of the E(spl)-C, blocks the ac-sc-ase self-stimulatory loop promoted by the SMC-specific enhancers. However, within a proneural cluster the cells of the proneural field accumulate greater amounts of Ac-Sc proteins. As it has been
hypothesized that cells that signal the most are the least
inhibited by their neighbors, eventually, a cell of the proneural
field will be released from the inhibitory loop and its levels of
E(spl)-C bHLH protein will become minimal. This cell will turn on the ac-sc-ase self-stimulation and become an SMC. The SMC signals maximally to its
neighbors and prevents them from following the same fate
(lateral inhibition). These results add to this scenario the requirement for Egfr-mediated signaling for one cell of the proneural field to turn
on the ac-sc-ase self-stimulatory loops and become an SMC. According to this model, Ac-Sc activate both the N-and Egfr-mediated signaling pathways, with their SMC-suppressing
and SMC-promoting abilities, respectively, and
both signaling systems appear to act on the same SMC-specific
enhancers. Since an excess signaling by the N or the
Egfr pathway will either prevent SMC determination or
promote emergence of ectopic SMCs, the respective levels of
signaling should balance each other so that only one SMC is
determined at a time from each proneural cluster. How is this
balance accomplished? This is at present unclear. The large
enhancement of rho/ve mRNA in proneural clusters under
conditions of insufficient N signaling suggests that this
pathway may prevent the Rho/Ve-promoted activation of
Egfr from rising to excessively high levels. In contrast, the
insensitivity of the levels of E(spl)-m8 protein to the
overexpression of UAS-aos in proneural clusters suggests that
the Egfr pathway does not affect N signaling. Antagonistic
interactions between the N and the Egfr pathways are found
in other developing systems, as in the wing preveins and in the reiterative recruitment, from a long-lived atonal proneural cluster, of the precursors of the 70-80 scolopidia of the femoral chordotonal organs. In this later case, Egfr signaling promotes
commitment of neural precursors and the Dl-N interaction
prevents too many cells from being committed (Culí, 2001).
The requirement for WASp function in N-dependent cell fate decisions prompted a search for genetic interactions between WASp and N pathway elements, making use of the WASp adult bristle-loss phenotype. Although the N pathway is involved in a wide variety of cell fate decisions during fly development, use of conditional mutant alleles has been successful in demonstrating that loss-of-function mutations in N itself and in its ligands results in PNS neuronal preponderance and associated phenotypes, in both embryos and adults, including the pIIa-to-pIIb and sheath-to-neuron transformations suggested for WASp. A WASp;N double mutant was constructed using the temperature-sensitive Nts1 allele. At 25°C, Nts1 flies display a wild-type morphology, including a normal array of neurosensory bristles. Introducing this very mild N hypomorphic genotype into a WASp mutant background results in a strong enhancement of the WASp bristle-loss phenotype. Double mutant flies lack practically all bristles on regions of the cuticle such as the thorax, which is only partially affected by the WASp mutation alone (Ben-Yaacov, 2000).
In contrast to the enhancement achieved by reducing N function, significant suppression of the WASp bristle-loss phenotype can be observed when activity of the N pathway is even moderately elevated. The neurosensory bristle pattern of WASp mutant flies, which also lack one copy of the established N antagonist Hairless (H), is close to wild-type in appearance. These flies eclose normally. Similarly, a significant, if somewhat less dramatic rescue of the WASp phenotype is obtained using a gain-of-function allele of the N receptor itself. A transgenic construct (Nint.hs), in which the constitutively active, intracellular portion of N is expressed under the control of a heat-shock promoter, was introduced into a WASp mutant background. Mild (29°C) heat treatment of such flies, which has no noticeable effect on Nint.hs flies, when combined with a WASp mutation leads to significant restoration of the bristle pattern, particularly in abdominal segments. Sensitive genetic interactions can thus be demonstrated between WASp and elements of the N pathway, raising the possibility of a common functional framework (Ben-Yaacov, 2000).
Notch and neurogenesis: Chordotonal organs The selection of Drosophila sense organ
precursors (SOPs) for sensory bristles is a progressive
process: each neural equivalence group is transiently
defined by the expression of proneural genes (proneural
cluster), and neural fate is refined to single cells by Notch-Delta
lateral inhibitory signalling between the cells. Unlike
sensory bristles, SOPs of chordotonal (stretch receptor)
sense organs are tightly clustered. It has been shown that for
one large adult chordotonal SOP array (the adult femoral chordotonal sense organ), clustering results
from the progressive accumulation of a large number of
SOPs from a persistent proneural cluster. This is achieved
by a novel interplay of inductive epidermal growth factor-
receptor (EGFR) and competitive Notch signals. EGFR
acts in opposition to Notch signaling in two ways: it
promotes continuous SOP recruitment despite lateral
inhibition, and it attenuates the effect of lateral inhibition
on the proneural cluster equivalence group, thus
maintaining the persistent proneural cluster. SOP
recruitment is reiterative because the inductive signal
comes from previously recruited SOPs (zur Lage, 1999).
The adult femoral chordotonal sense organ arises from a group of some 70-80 SOPs. A developmental
analysis of Ato expression has revealed that these SOPs accumulate
over an extended period of time in the dorsal region of each leg
imaginal disc during the third larval instar and early pupa. The continued expression of Ato implies a sustained
requirement for proneural function throughout the process of
SOP accumulation. Unusually, Ato is persistently expressed in
a group of ectodermal cells identified as the proneural cluster (PNC). From this PNC, cells are funnelled inward into a cavity
formed by the folding of the disc. This invagination later
becomes visible as a distinctive 2-cell wide intrusion, which is
referred to as the 'stalk'. Cells at the deepest end of the stalk
undergo shape changes to form an amorphous inner SOP mass.
Invaginating cells are characterised by upregulation of Ato
expression, a characteristic of SOP commitment. Surprisingly, SOP markers (Ase protein and the A101
enhancer trap line) are not expressed in all
the stalk SOPs. Instead, these markers are only apparent in older
cells, particularly at the time when they become part of the inner
mass (which is therefore referred to as mature SOPs).
Despite this, entry into the stalk seems to mark SOP
commitment, since both the stalk and the mature SOPs are
absent in discs from ato mutant larvae. This
apparent intermediate stage may not have a counterpart in
external sense organ precursor formation, although there is
some evidence for multiple steps between the uncommitted cell
and the SOP (the so-called pre-sensory
mother cell state).
Initially, Ato remains activated in all invaginated SOPs. This
extended period of proneural gene expression is unusual since
AS-C proneural expression is typically switched off in SOPs
shortly after commitment. Later, at
approximately 6 hours before puparium formation (BPF), Ato expression is switched off
synchronously in the mature SOPs, although expression
remains in the stalk SOPs and the PNC. At this
point there is very little overlap between Ato and Ase or A101 (zur Lage, 1999).
The process of chordotonal SOP formation described above is
at odds in several respects with the well-known paradigm of
SOP selection for sensory bristles. In the latter, the solitary
SOP expresses Delta, which triggers expression in the PNC of
genes of the E(spl)-C, thereby
preventing further SOP commitment and forcing loss of AS-C
expression and neural competence. In the case of the femoral
chordotonal organ, newly committed cells from the PNC are
in contact with previously committed SOPs in the stalk, but are
apparently not receiving (or not responding to) lateral
inhibition signals from these to prevent their commitment.
Likewise, the presence of committed SOPs does not switch off
ato expression in the PNC. Nevertheless, components of the N-Dl
pathway are expressed in patterns consistent with lateral
inhibition. The newly formed SOPs express Dl, suggesting that
they send inhibitory signals, while the PNC expresses
mgamma, a member of the E(spl)-C, suggesting that these
cells are responding to the Notch-Delta signal.
Indeed, mgamma is coexpressed with ato in the PNC throughout the
development of the SOP cluster. Chordotonal SOP formation is
shown to be sensitive to N inhibitory signaling.
Strong activation of N signaling or its effectors can
inhibit chordotonal SOP formation.
Thus, N signaling has an important role to play: it acts to limit
the process of SOP selection from the PNC. Some mechanism,
however, must prevent N signaling from completely inhibiting
multiple SOP formation (zur Lage, 1999).
The progressive accumulation of chordotonal SOPs suggests
that a recruitment mechanism could explain the clustering of
SOPs. The Drosophila Egfr signaling pathway is involved
in a number of recruitment processes in development, and a role for Egfr signaling has been demonstrated in the
induction of embryonic chordotonal precursors (zur Lage,
1997). Although there appear to be significant differences in
the process of SOP formation in imaginal discs, as compared with
the embryo, it was asked whether Egfr signaling is also
involved in forming the femoral chordotonal cluster. To address
this question, the pathway was conditionally disrupted by
expressing a dominant negative form of Egfr protein. Expression of UAS-Egfr
DN results in a dramatic loss of chordotonal SOPs in
late third instar imaginal leg discs (as judged by Ase protein
expression or the A101 enhancer trap line). This demonstrates that Egfr signaling is
required for the process of femoral chordotonal SOP
formation. In contrast, the appearance of bristle SOPs is
unaffected, arguing against the possibility of a nonspecific
effect on SOPs in general (zur Lage, 1999).
To determine whether Egfr signaling controls SOP
number, expression of components of the Egfr
pathway that determine the level of signaling was forced, thus resulting
in hyperactivation of the pathway. pointed (pnt) is an effector gene that encodes a transcription
factor and is activated in cells responding to
Egfr signaling. Both rho and pnt
are expressed during chordotonal SOP formation. Indeed, forced expression of rho or pnt increases chordotonal SOP formation. Egfr could promote SOP formation by stimulating the
commitment of PNC cells or by stimulating proliferation of
SOPs. Both functions would be consistent with known Egfr
roles, but
the current investigations favour the former. Analysis of Ato
expression in leg discs in which rho has been misexpressed
reveals a large invagination of cells and a smaller PNC. Shrinking of the PNC was confirmed by the reduced
extent of mgamma expression. These observations are
consistent with an increased rate of SOP commitment upon
Egfr hyperactivation. Moreover, this effect is reminiscent of
the effect of N loss of function on Ato expression, suggesting
that Egfr signaling supplies the mechanism that interferes
with lateral inhibition of SOP commitment (zur Lage, 1999).
The opposing forces of Notch and Egfr signaling are thought to be played out through direct Notch and Egfr signaling between the epidermal proneural cells, which bear Notch, and the SOP, which sends inhibitory signals through the Delta ligand, and stimulatory signals through the Spitz ligand.
Reiterative recruitment alone cannot entirely explain the
accumulation of SOPs. Such an accumulation also relies on the persistence of the
competent pool of PNC cells from which SOPs can be
recruited. For AS-C PNCs, this does not occur, because the
mutual inhibition required for continued competence is
unstable and resolves quickly to a state of lateral inhibition
once the SOP emerges from the PNC.
This results in rapid shutdown of AS-C expression and hence
competence within the PNC. It is possible
that the members of E(spl)-C that are expressed in the PNC
(notably mgamma and mdelta) are less aggressive inhibitors of proneural
gene expression than the E(spl)-C members expressed in AS-C
PNCs (m5 and m8). The results obtained in the femoral SOP suggest, however, that
Egfr has a role to play in maintaining the PNC by partially
attenuating lateral inhibition on a PNC-wide scale. Thus, the
PNC is not completely shut off by inhibition from SOPs, but
instead kept in check, allowing continued mutual inhibition and
maintenance of competence but not allowing general SOP
commitment. Since neither rho nor dp-ERK are detected in the
PNC as a whole, this function of Egfr could be indirect and
achieved through partial attenuation of Dl signaling from the
stalk SOPs themselves. The trans- or auto-activation of EGFR
signaling between the stalk SOPs (as suggested by the co-expression
of dp-ERK and rho) might be an indicator
of this function. It is also possible, however, that Egfr
signaling is direct and that the dp-ERK antibody is not
sensitive enough to detect expression in the PNC cells (zur Lage, 1999).
Notch and sense-organ differentiation
in the Drosophila antenna The Drosophila antenna has a diversity of chemosensory organs within a single epidermal field. Three broad categories of sense-organs are known to be specified at the level of progenitor choice. However, little is known about how cell
fates within single sense-organs are specified. Selection of individual primary olfactory progenitors is followed by organization of groups
of secondary progenitors, which divide in a specific order to form a differentiated sensillum. The combinatorial expression of Prospero, Elav,
and Seven-up shows that there are three secondary progenitor fates. The lineages of these cells have been established by clonal analysis
and marker distribution following mitosis. High Notch signaling and the exclusion of these markers identifies PIIa; this cell gives rise to
the shaft and socket. The sheath/neuron lineage progenitor PIIb, expresses all three markers; upon division, Prospero asymmetrically
segregates to the sheath cell. In the coeloconica, PIIb undergoes an additional division to produce glia. PIIc is present in multiinnervated
sense-organs and divides to form neurons. An understanding of the lineage and development of olfactory sense-organs provides a handle
for the analysis of how olfactory neurons acquire distinct terminal fates (Sen, 2003).
In sibling cells, a bias in N signaling occurs because of
an asymmetric distribution of the membrane-associated protein
Numb (Nb), which binds the intracellular region of N
antagonizing its function. N signaling
can also be compromised by ectopic expression of a dominant
negative (DN-N) construct of the N receptor which
interferes with ligand-dependent signaling. The
sca-Gal4P309 insertion strain was used to drive UAS-mediated Nb or
DN-N activity in secondary progenitors. Expression of sca-Gal4P309-driven GFP is visualized in the proneural domains, the primary and secondary
progenitors, but is not detected in the majority of
sensory clusters after division (Sen, 2003).
Animals of sca-Gal4P309/UAS-Nb show a strong
reduction in external structures on the adult antenna concomitant with an increase in glial cell numbers. The defect was also observed although significantly
weaker in sca-Gal4P309/UAS-DN-N. In both genotypes, there appears to be an
increase in neuronal number since the fascicles appeared
thicker than in the controls. It is proposed that
these phenotypes are caused by a transformation of PIIa to
PIIb; in the coeloconic lineages, this would result in an
increase of glial cells (Sen, 2003).
To test this hypothesis, pros-Gal4 was used to drive expression
of DN-N in PIIb/PIIIb and PIIc but not PIIa. The numbers of external cells (sensillar shafts) from
all three sense-organ types was counted and was found to be comparable to
normal controls. This is consistent with the findings
that tormogen and trichogen cells arise from the PIIa,
which is not being manipulated in this genotype. There was
also no change in glial cell number, even though N activity
is being lowered in the PIIb progenitors. While it
is possible that ectopically expressed DN-N is not sufficient
to compromise N signaling, the favored explanation is that N
is not required in PIIb/PIIIb. This would imply a PIIa-to-PIIb switch, which in the coeloconic lineages, results in
excess of glial cells (in addition to neurons) (Sen, 2003).
If indeed N signaling plays a role in the binary choice
between secondary progenitor types, then gain of N activity
would be expected to increase the external cells (socket and
shaft) that arise from the PIIa lineage. The truncated cytoplasmic
domain of the N receptor (N-intra) is constitutively
active, and its misexpression creates a dominant gain-of-function
condition. Expression of Nintra early during sense organ
development interfers with olfactory progenitor
choice and subsequent recruitment of secondary progenitors. These effects of N could be avoided by exploiting the thermosensitive nature of the Gal4/UAS system (Sen, 2003).
sca-Gal4P309; UAS-Nintra animals were reared at 16°C
until 10 h APF and then shifted to 28°C to activate N
specifically in secondary progenitors. The adult antennae show
a variety of defects affecting the external structures of the
sensory units. There were cases of multiple
sockets and sensilla with two shafts arising from
a socket. While, in principle, such phenotypes
could be explained by a role of N in binary choice between
shaft and socket cells, the appearance of sensilla with four
sockets or two shafts with a single socket can only
be explained by invoking conversion of PIIb/PIIc to PIIa (Sen, 2003).
Notch and gliogenesis By using gain-of-function mutations it has been proposed
that vertebrate Notch promotes the glial fate. In
vivo glial cells are produced at the expense of neurons
in the peripheral nervous system of flies lacking Notch and
that constitutively activated Notch produces the opposite
phenotype. Notch acts as a genetic switch between neuronal
and glial fates by negatively regulating
glial cells missing, the gene required in the glial precursor
to induce gliogenesis. Moreover, Notch represses
neurogenesis or gliogenesis, depending on the sensory
organ type. Numb, which is asymmetrically localized in the
multipotent cell that activates the glial cell fate, inducing
glial cells at the expense of neurons. Thus, a cell-autonomous
mechanism inhibits Notch signaling (Van De Bor, 2001).
Strikingly, N seems to act in opposite directions
in fly and some vertebrate peripheral glial cells. Indeed,
gain-of-function N mutations promote differentiation of
Müller, radial and Schwann glial cells. Two possible
explanations can account for these results: (1) the genetic
switch between neuronal and glial fates has different
requirements in flies and vertebrates; (2) the role of N
depends on the subtype of glial cell. The analysis of other
classes of fly glial cells will help elucidate this point.
Preliminary analyses on the embryonic CNS
suggest that the response of central glial cells to N depends
on the subtype. The observation that
oligodendrocyte differentiation, like fly peripheral glial cells,
is repressed by N, also argues in favor of the second
hypothesis (Van De Bor, 2001).
One of the most striking results is that repression of the N
pathway throughout the development of the sensory organ
(obtained by N loss-of-function mutations or by Numb
overexpression) leads to sensory organs composed of six glial
cells. The competence to adopt
the glial fate is restricted to some cells of the sensory organ
lineage; the strongest phenotype observed upon overexpression
of gcm throughout the lineage is the differentiation of a
sensory organ composed of five cells, three of which are Repo-positive. Thus, gcm is not sufficient
to induce a IIa into IIb transformation. This indicates that the
pathway mediated by N and Numb affects the competence of
sensory organ cells to adopt the glial fate. In molecular terms,
this implies the control of expression of gcm regulators,
positive co-factors and/or repressors (Van De Bor, 2001).
In the Drosophila CNS glial cells
are known to be generated from glioblasts, which produce
exclusively glia or neuroglioblasts that bifurcate to produce
both neuronal and glial sublineages. The
genesis of a subset of glial cells, the subperineurial glia
(SPGs), involves a new mechanism and requires Notch. SPGs share direct sibling
relationships with neurons and are the products of
asymmetric divisions. This mechanism of specifying glial
cell fates within the CNS is novel and provides further
insight into regulatory interactions leading to glial cell
fate determination. Furthermore, Notch
signaling positively regulates glial cells missing
expression in the context of SPG development (Udolph, 2001).
In order to better understand how a complete lineage of a
specific NGB with all its progeny, including its glial cells,
might be created, NB1-1 was chosen for a detailed analysis. NB1-
1 has been extensively used for cell fate specification studies
and a sound basis of information about this NB lineage is
available. NB1-1 is a NB that develops differential lineages
in the thoracic versus the abdominal segments. Focus was placed on the abdominal NB1-1A because only
these abdominal NB1-1 lineages contain glia. In addition to the
aCC/pCC sibling neurons, which are the progeny of the first
GMC produced from this lineage, NB1-1A generates 2 to 3
glial cells and 4 to 5 clustered interneurons (cN), yielding a
total of 9 to 10 cells. The three glial cells belong to the group
of subperineurial glia (SPG) that lie at the periphery of the
nerve cord and enwrap the entire ventral nervous system. Two of the glia, the A- and B-SPGs, can be found in dorsal positions, with a third
glia, the LV-SPG, located at ventral positions of the nerve cord.
All SPGs, including the A- and B-SPG and LV-SPG of NB1-1A, are specifically labelled by two enhancer trap lines, M84
and P101 (Udolph, 2001 and references therein).
The expression patterns of the enhancer-trap lines M84 and
P101 are indistinguishable in abdominal segments: both are expressed in the SPGs including the A- and B-SPG and LV-SPG cells produced from NB1-1A.
To investigate the relationship between the three glial cells
derived from NB1-1A, M84/P101 (a stock double
homozygous for both the M84 and P101 insertions), embryos were stained with anti-ß-gal. In stage 12/13
embryos, a single M84/P101+ cell appears in dorsal positions
of each hemi-neuromere. Embryos double labelled
with anti-Even-skipped (anti-Eve) indicates that this cell is
located posterior to the Eve+ cluster of NB1-1 (aCC/pCC)
and NB7-1 progeny (CQ-neurons). Slightly
later, a second M84/P101+ cell appears anterior to the first cell. According to the positioning within the developing
nerve cord, the posterior cell represents the A-SPG, whereas
the anterior cell is the B-SPG. A third glial cell, representing
the LV-SPG, can be detected in ventral
positions only after the A- and B-SPG are already present,
suggesting that this cell is the last born glia within the lineage (Udolph, 2001).
As a first step toward elucidating the origin of the glial cells
of the NB1-1A lineage, the effects of loss of
function mutants in several genes, Notch, mastermind (mam)
and numb, which are known to affect the resolution of distinct
sibling cell fates, were tested for their effect on the development of A-, B- and
LV-SPGs. Embryos hemizygous/homozygous for a conditional Notch allele, Nts1, and also carrying one copy each of M84 and P101 (Nts1/M84/P101) were subjected to the non-permissive temperature of 29°>C after 6 hours of development. This regime allows Notch to function during the singling out of NBs and removes Notch during the crucial period when it is required for sibling cell fate resolution. Double staining with
anti-Eve and anti-ß-gal was performed. As expected,
in most hemisegments, Nts1/M84/P101 embryos duplicate the RP2 neuron at the expense of its sibling cell. Moreover, in
96% of the hemisegments, M84/P101+ cells could not
be found in typical dorsal or ventral positions. It is concluded that Notch function is required for the specification of the M84/P101 positive A-, B- and LV-SPGs. In wild-type embryos, M84/P101 is expressed in about eight SPGs per hemisegment, including the A- and B-SPGs and the LV-SPG (Udolph, 2001).
Removing Notch function results in the near complete
abolishment of all M84/P101 expression, indicating a more
general function for Notch in SPG specification.
These findings prompted an investigation whether Notch is
required in the specification of CNS glia in general. Experiments were performed in which glia-specific markers, either
enhancer trap lines (gcm-lacZrA87 and pnt-lacZ) or an antibody (anti-Repo) were used. gcm-lacZrA87 is an
insertion into the gcm gene and expresses ß-gal in the pattern
of gcm. pointed (pnt) is specifically expressed in glial cells and has been reported to act downstream of gcm. Nts1embryos were shifted to the non-permissive temperature after 6 hours of
development, and subsequently stained with anti-Eve and anti-ß-gal at late stage 16; taking into account the missing SPGs, both enhancer traps were expressed in a pattern reminiscent of wild-type embryos. However, an increase in the
number of ß-gal-positive cells was observed, which could be
in part due to a mild neurogenic phenotype caused by Nts1.
Furthermore, it was found that the glial-specific protein Repo
was widely expressed in the CNS of N55e11 (an amorphic N
allele) embryos. Thus, general glial specific markers
like gcm, pnt and repo are expressed in embryos that lack or
have strongly reduced Notch function, indicating that although
Notch is required for the formation of SPGs, there is not a
global requirement for Notch in the specification of all CNS glia (Udolph, 2001).
Another neurogenic gene, mastermind,
which has been linked to the Notch signaling pathway by its
genetic interactions with Notch and its strikingly similar
phenotype in early and late neurogenesis, was also tested. mam acts downstream of Notch during sibling cell fate
specification in the embryonic nervous system. The hypomorphic mam345 allele used in this
study shows only a mild hypertrophy of the nervous system but
clearly has an effect on sibling cell fate specification. A severe reduction (94%) of P101+ cells was observed in mam345;P101 embryos
similar to that seen with Nts1/M84/P101 embryos.
These data suggest that both genes are strictly required for the
specification of SPGs, most likely in a linear pathway.
However, it is unclear how Notch acts in the specification of
the SPGs. The possibility is considered that SPG glial cells could
arise from a series of asymmetric cell divisions, with Notch
being required to specify the glial daughters of these divisions (Udolph, 2001).
Based on its function as a negative regulator of Notch
signaling, the expected numb phenotype is opposite that of Notch
in terms of sibling cell fate transformation. The P101
expression pattern was tested in the background of a strong numb
mutation. In contrast to Notch and mam, additional
P101+ cells were found in the vicinity of the aCC/pCC position. In most
of the examined hemi-neuromeres, up to four ß-gal-positive cells were detected in dorsal positions close to aCC/pCC. This is indicative of a duplication of the A- and B-SPGs. Additional
P101+ cells with glial morphology were found in lateral and ventral
positions of the nerve cord, presumably duplications of other
SPGs. These findings are consistent with an
asymmetric cell division model for the genesis of the SPGs (Udolph, 2001).
Clonal analysis demonstrates that the loss of Notch
function results in the loss of SPGs and a concomitant gain of
neuronal cells within the NB1-1A lineage, consistent with the
notion that a sibling cell fate relationship exists between cluster
neurons and glial cells in this lineage and that Notch is required
for the asymmetric divisions that generate these postulated
neuron/glia sibling pairs. The data support a hypothesis that glial cells and cluster neuron s share a sibling relationship within the NB1-1A lineage. Taken together, the data favour a model in which a series of three GMCs produced from
NB1-1A can each divide asymmetrically to produce a neuron
and a glia, with Notch signaling required to specify the glia
fate, and Numb and the absence of Notch signaling required
for the neuronal fate (Udolph, 2001).
It is known that gcm acts as a master regulator in gliogenesis because it has been described as a binary switch between glia and neurons. As expected, gcm appears to be required for the formation of SPGs. P101 marker gene expression is abolished in embryos homozygous for gcm. The similarity of gcm and Notch phenotypes suggests the possibility that both genes share a common pathway required for SPG specification. It was reasoned that if Notch acts downstream of gcm, then overexpression of activated N in a gcm minus genetic background should result in additional SPGs; however, if Notch acts upstream of gcm, then in a gcm minus background the overexpression of activated N should not result in the production of additional P101+ cells. In a genetic background lacking gcm function, activated N expression is unable to induce SPG development as indicated by the loss of marker gene expression. These findings are consistent with the notion that Notch functions upstream of gcm in the context of SPG development (Udolph, 2001).
Two types of neuroectodermally derived glial progenitors in
the embryonic nervous system of Drosophila have been
described. Glioblasts (GB) generate only glial progeny, and
NGBs produce both neurons and glial cells within the same
lineage. A mechanism by which both neurons and glia
can arise within the lineage (NGB6-4T) of a thoracic NGB has
recently been described. NGB6-4T represents a thoracic-specific
NGB in which glial and neuronal sublineages bifurcate
from each other during the first division of the parental NGB. Only one
of the two daughters expresses Gcm, a master regulator of glial
cell fate that is involved in regulating the expression of other
glia-specific target genes. In the NGB6-4T lineage, the
asymmetric distribution of Gcm results in a cell that is
specified as a GB within a neuroglioblast lineage. The GB will
exclusively generate the glial components, whereas its sibling
will exclusively give rise to the neuronal components of the
lineage (Udolph, 2001).
This study reports a novel mechanism by which glia
can be generated during CNS development. The data suggest
that NB1-1A gives rise to a set of three glial cells through a
series of three GMC asymmetric divisions. Several lines of
evidence support the notion that within a NGB lineage
GMCs can produce both glial and neuronal cells. (1) Immunohistochemical analyses indicate that the A-, B- and LV-SPG
arise at different times in development, and their non-simultaneous
birth, in conjunction with the fact that their
formation can be differentially affected by inactivation of
Notch, suggests that they do not derive from a common
precursor; (2) mutations in genes involved
in specifying alternative sibling cell fates affect SPG
development in a fashion that suggests these cells are
siblings with non-glial components of the lineage; (3)
transplantation experiments indicate that the 3 SPGs share
sibling relationships with the neuronal components of the
NB1-1A lineage, the cluster neurons (cN). Finally, analysis
of two cell FLP-clones demonstrate that SPG glia and neurons
share a direct sibling relationship (Udolph, 2001).
This proposed mechanism for the genesis of SPGs
is fundamentally different from the ones described for GBs,
e.g. anterior glioblast, and for NGBs,
e.g. NGB6-4T and NGB5-6A. The first division of NB1-1A
produces a neurogenic GMC that gives rise to a pair of sibling
neuron s (aCC/pCC), and at the level of the first division of
the parental NB no GB sublineage is bifurcated. In addition,
the NB1-1A derived glial cells share a direct sibling
relationship with neurons and does not involve the generation
of a lineage internal GB at all. The asymmetric origin of glia
described here provides a novel mechanistic framework of glial
origin that might be used as a model system to gain further
insights into the regulatory networks involved in glial cell fate
specification in the CNS. It is concluded that the mechanisms
leading to glial cell fate specification are complex and that
multiple developmental mechanisms lead to glial cell fate
specification. These mechanisms will be likely to require
different molecular machinery that involve distinct sets of
genes or genetic hierarchies; for example, Notch being
required specifically for the SPGs but apparently not for other
types of glia. However, most of the glial cells (except midline
glia) strictly require the gcm gene. Thus, there seems to be a
common molecular basis that is context and cell specifically
regulated during development.
This study provides the first evidence that Notch is a crucial
component in the specification of a subclass of CNS glial
cells, the subperineurial glia (SPG). As revealed by the
transplantation experiments, Notch is not only required for the
expression of the M84/P101 marker, but loss of Notch function
leads to the loss of the SPG cell fate per se. In the case of NB1-1A, the data indicate that the loss of glia is accompanied by
the conversion of these cells into neurons. In contrast, Notch
gain of function results in the overproduction of SPG-like cells
at the expense of the cluster neurons. Hence, Notch signaling
is sufficient for the specification of these cells. A second gene,
gcm, is also crucially required for the specification of the
SPGs; in a gcm mutant background M84/P101 expression is
completely absent. If an activated form of the Notch protein is
ectopically expressed in the embryonic CNS of animals lacking
gcm function, SPGs do not form; in addition, M84/P101
expression cannot be detected in the CNS of these embryos.
This indicates that gcm function is strictly required for Notch-mediated
SPG specification, suggesting that Notch functions
upstream of gcm. It is interesting to note that Notch also
appears to play an instructive role in gliogenesis for
mammalian neural crest stem cells in culture (Udolph, 2001).
These results, in conjunction with what is already known about
the origin of the aCC and pCC neurons, as well as the terminal
lineage of NB1-1A, allow the following model to be proposed for the
NB1-1A lineage: in total, NB1-1A gives rise to five pairs of
sibling cells. The first pair comprises the aCC/pCC neurons
resulting from GMC1-1a. In addition, three later born GMCs
each give rise to one cluster neuron and one SPG-glia. A fifth
GMC gives birth to two additional cluster neurons. The order
of appearance and the position of the MP84/P101-labelled cells
would suggest that, of the three glia-producing GMCs, the
earliest born GMC gives rise to the A-SPG; the latest born
GMC gives rise to the LV-SPG; and the GMC born in the
middle gives rise to the B-SPG. The time of
birth of the fifth GMC in this lineage, which is postulated
to produce two cluster neurons, cannot be determined. Clearly, refinement and proof
for this proposal will require experiments that provide direct
temporal information (Udolph, 2001).
Apoptosis is prevalent during development of the central nervous system, yet very little is known about the signals that specify an apoptotic cell fate. The role of Numb/Notch signaling in the development of the serotonin lineage of Drosophila has been studied; it is necessary for regulating apoptosis. When Numb inhibits Notch signaling, cells undergo neuronal differentiation, whereas cells that maintain Notch signaling initiate apoptosis. The apoptosis inhibitor p35 can counteract Notch-mediated apoptosis and rescue cells within the serotonin lineage that normally undergo apoptosis. Furthermore, tumor-like overproliferation of cells is observed in the CNS when Notch signaling is reduced. These data suggest that the distribution of Numb during terminal mitotic divisions of the CNS can distinguish between a neuronal cell fate and programmed cell death (Lundell, 2003).
The segmented Drosophila nerve cord develops from stereotyped
division of 30 neuroblasts (NB) in each hemisegment. A pair
of serotonergic neurons in each hemisegment arises from NB7-3. The divisions of the NB7-3 lineage have recently been
determined using a combination of molecular markers and clonal analysis. NB7-3
produces three GMCs. GMC-1 produces two neurons: GW, a motoneuron, and EW1,
the more medial serotonergic neuron. GMC-2 produces EW2, the more lateral
serotonergic neuron. GMC-3 produces EW3, a neuron that synthesizes the
neuropeptide corazonin. The GW neuron projects an axon ipsilateral and
posteriorly, and the three EW interneurons all project axons anterior to the
posterior commissure (Lundell, 2003 and references therein).
The results of this study demonstrate that the intercellular Notch
signaling pathway can be modulated during terminal divisions of the CNS to
direct a choice between neuronal development and programmed cell death.
The division of GMC-1 produces two distinct neuronal cell fates: the EW1
interneuron and the GW motoneuron. In this division, genetic alteration in the
expression of Notch leads to switching between these two cell fates.
A loss of Notch activity in spdo mutants leads to two
Ddc/Hb-expressing EW1 cells and the overexpression of Notch leads to two Zfh-1
expressing GW cells. Therefore, Notch signaling must be inactivated during
development of the EW1 neuron. Numb appears to have a minor role in this
inactivation. In a numb1 mutant, 7% of the hemisegments do
not develop an EW1 neuron, and a similar number of numb1
hemisegments show two Zfh-1-expressing GW cells. This transformation from an
EW1 cell fate to a GW cell fate is what one would expect if Numb were
inhibiting Notch. However, most EW1 neurons develop normally in a
numb1 mutant and do not convert to the GW cell fate.
Therefore, inactivation of Notch signaling in EW1 is mostly independent of
Numb function. One possible explanation is that EW1 has a factor that is
redundant for Numb function, which can inhibit Notch signaling and is capable
of masking the effect of a numb1 mutation in most
hemisegments. The unique expression of Hb in GMC-1 progeny could be
responsible for establishing this redundancy. However, if a redundant
Numb-like factor does exist, it is insufficient to protect EW1 during
expression of the UAS-NotchACT transgene (Lundell, 2003).
During the divisions of GMC-2 and GMC-3, genetic alterations in the
expression of Notch lead to a switching between a neuronal cell fate and
apoptosis. A reduction of Notch signaling with either
spdoG104 or UAS-Numb embryos produces ectopic
NB7-3 cells that express Zfh-2. Conversely, the overexpression of Notch in
either UAS-NotchACT or numb1 embryos
led to an increase in TUNEL labeling of GMC-2 and GMC-3 progeny. Additionally,
inhibiting apoptosis with UAS-p35 or reducing Notch activity with
spdoG104 can rescue the numb1
phenotype. It is hypothesized that during the divisions of GMC-2 and GMC-3, Numb
partitions asymmetrically into EW2 and EW3 where it inactivates Notch
signaling and leads to neuronal development. The mitotic sisters of EW2 and
EW3 do not receive Numb, maintain Notch signaling and undergo apoptosis. The
difficulty in detecting wild-type hemisegments that have more than four
immunoreactive Eg cells, suggests that any other cells produced during
divisions of the NB7-3 lineage quickly undergo apoptosis (Lundell, 2003).
Ectopic Eg cells in the NB7-3 lineage can be induced at stage 15 by
H99, UAS-Numb, spdoG104 and UAS-p35. However, the ability of these alleles to produce ectopic Ddc and corazonin-containing
neurons at later stages is variable. No significant
ectopic Ddc or corazonin-containing cells were detected in either H99 or
UAS-Numb CNS. For UAS-Numb it was shown that the ectopic Eg
cells detected at stage 15 can undergo apoptosis. spdoG104
mutants produce only ectopic Ddc cells, but the reduction in the number of
corazonin-containing cells in general suggests that either GMC-3 does not
consistently form in these mutants or that GMC-3 progeny may convert from a
corazonin-containing cell fate to a serotonergic cell fate. UAS-p35
mutants produce both ectopic Ddc and corazonin-containing cells at low
frequency, but the allele is much more efficient at rescuing the EW neurons in
numb1 and UAS-Notch mutants. Therefore, apoptosis
is harder to reverse in cells that normally undergo apoptosis, than in the
cells genetically induced to undergo apoptosis. The ability of these various
alleles to produce ectopic Ddc- and corazonin-containing cells could be
influenced by mutant effects they cause outside the NB7-3 lineage or may
reflect different roles they have in the apoptotic pathway. The mechanism by
which Notch induces apoptosis in the NB7-3 lineage remains to be determined,
but the apoptotic genes reaper, grim and hid may be involved
because all three of these genes are deleted in the H99 allele (Lundell, 2003).
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Notch:
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| Protein Interactions | Post-transcriptional regulation of Notch mRNA
| Developmental Biology
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