islet/tailup
IslH, a 7 kb fragment located 6 to 13 kb upstream of the putative transcriptional start site, can direct expression in a pattern similar to that of the ISL protein. Since two or three of the 20 cells per hemisegment that normally express islet cells do not express protein directed by the IslH fragment, additional islet regulatory elements may lie outside of the genomic region of the IslH fragment (Thor, 1997).
Axon pathfinding and target choice are governed by cell type-specific responses to external cues. In the
Drosophila embryo, motorneurons with targets in the dorsal muscle field express the homeobox gene even-skipped and
this expression is necessary and sufficient to direct motor axons into the dorsal muscle field. Motorneurons projecting to ventral targets express the LIM homeobox gene islet, which is sufficient to direct axons to the
ventral muscle field (Thor, 1997). Thus, even-skipped complements the function of islet, and together these two genes constitute a
bimodal switch regulating axonal growth and directing motor axons to ventral or dorsal regions of the muscle field (Landgraf, 1999).
The LIM homeobox gene islet is sufficient to
direct motor axons via the ventral branch of the ISN (ISNb/d) into the ventral muscle field (Thor, 1997). The
implications of the findings with respect to Eve are that together eve and islet might constitute a bimodal switch that
directs motor axon growth either to ventral (islet) or dorsal (eve) regions of the muscle field. One
prediction of such an interpretation would be that the expression patterns of these two genes in
motorneurons are mutually exclusive. In the wild type, this is the case. Moreover, while the expression pattern of Eve remains unchanged when Islet is either
absent or ectopically expressed, it is found that ectopic Eve
expression throughout the CNS suppresses Islet expression in most motorneurons. In the wild type,
Islet is expressed medially in the dorsal RP1, 3, and 4 neurons and one ventral VUM
motorneuron, and laterally, in approximately four to five motorneurons. In stage 16 elav-GAL4; UAS-eve embryos, the Islet expression
pattern is markedly reduced: medially, Islet expression is consistently lost from the VUM
and from two of the three RP motorneurons; laterally, Islet expression is lost from a further four to six cells. However, the Islet expression pattern does not expand when Eve function is removed (Landgraf, 1999).
How do transcriptional regulators such as Eve and Islet direct patterns of axonal growth? The
phenotype observed in ectopic Eve embryos (fusion of the main nerve trunks and
failure of secondary nerve branching) is similar to, though more severe than, phenotypes produced in
embryos where general interaxonal adhesion is increased either by overexpression of the homophilic
CAM Fas II or by removal of its antagonist Beaten path (Beat). In such embryos, the two main nerve trunks (SN and ISN)
form, but secondary nerves fail to branch off. A test was carried out to see if ectopic Eve increases interaxonal
adhesion by downregulating the antiadhesive neural CAM antagonist Beat. No
significant changes were seen in the overall pattern or relative levels of BEAT mRNA expression in ectopic Eve embryos. The expression patterns of the
major neural CAMs Fas II, Fas III, and Connectin were examined in ectopic Eve or eve mutant
embryos, but no changes in their expression patterns were detected. To test if Eve might regulate the expression of another (as yet unidentified) neural CAM, it was
reasoned that beat might antagonize interaxonal adhesion mediated by such a CAM, just as beat
antagonizes adhesion mediated by Fas II and Connectin. When Eve and beat are ectopically co-expressed, the ectopic Eve phenotype of
excessive axonal fasciculation is partially rescued.
Thus, Eve directs motor axons to the dorsal region of the muscle field by suppressing expression of
the ventrally directing islet gene and by promoting adhesion to the ISN (Landgraf, 1999).
There is an interesting correlation between the expression of islet homologs in vertebrate and
invertebrate motorneurons. However, while all vertebrate motorneurons express islet-1 and/or islet-2, only a subset of motorneurons express islet in Drosophila
(Thor, 1997). Another subset expresses eve, and there may well be
further subsets expressing other genes that direct axons to different parts of the muscle field. For instance,
the dorsolateral muscles DO3-5 [11, 19, and 20] and DT1 [18] are innervated by at least four
intersegmental motorneurons that express neither eve nor islet. Thus, there may be a third gene that defines the dorsolateral sector of the
muscle field as the target area of these motorneurons. Interestingly though, the axonal
projections of the DO3-5 [11, 19, and 20] and DT1 [18] motorneurons are frequently affected by loss of
Eve function. This suggests that these motorneurons rely on the axons of the Eve-expressing cells for
pathfinding. In addition, motorneurons whose axons project through the SN express neither eve nor
islet, and their growth patterns are likely to be regulated by other genes.
Interestingly, the gene vab-7 in the nematode Caenorhabditis elegans (a homolog of the Drosophila eve gene)
is also expressed in a set of motorneurons that go to dorsal targets, and it is required for their correct
pathfinding (B. Esmaeili and J. Ahringer, personal communication to Landgraf, 1999). Thus, it appears that the function of
eve in directing patterns of motorneuron growth is an ancient one (Landgraf, 1999).
The dorsal ectoderm of the Drosophila embryo is
subdivided into different cell types by an activity gradient
of two TGFbeta signaling molecules, Decapentaplegic
and Screw. Patterning responses to this gradient
depend on a secreted inhibitor, Short gastrulation
and a newly identified transcriptional repressor, Brinker, which are expressed in neurogenic regions that abut the dorsal ectoderm. The expression of a
number of Dpp target genes has been examined in transgenic embryos that
contain ectopic stripes of Dpp, Sog and Brk expression.
These studies suggest that the Dpp/Scw activity gradient
directly specifies at least three distinct thresholds of gene
expression in the dorsal ectoderm of gastrulating embryos.
Brk was found to repress two target genes, tailup/islet and
pannier, that exhibit different limits of expression within
the dorsal ectoderm. These results suggest that the Sog
inhibitor and Brk repressor work in concert to establish
sharp dorsolateral limits of gene expression. Evidence is provided that the activation of Dpp/Scw target
genes depends on the Drosophila homolog of the CBP
histone acetyltransferase (Ashe, 2000).
All of the aforementioned genes are virtually silent in the
dorsal ectoderm of dpp-/dpp- embryos, while
changes in dpp+ gene dose cause altered patterns of expression. For example, increasing the number of dpp+ copies
from two to three to four results
in a sequential expansion of the hnt expression pattern, whereas
expression is lost in dpp/+ heterozygotes. In
contrast, ush is expressed in dpp/+ heterozygotes, although
there is a marked narrowing in the expression pattern as
compared with wild-type embryos.
Three copies of dpp+ cause an expansion of the ush pattern. Similarly, the tup expression pattern is narrower in
dpp/+ heterozygotes and expanded in embryos with three
copies of dpp.
Further evidence that hnt and ush are early targets of the Dpp
signaling pathway was obtained by analyzing transgenic
embryos that contain the dpp-coding sequence attached to the
eve stripe 2 enhancer. These embryos exhibit both
the endogenous dpp pattern in the dorsal ectoderm as well as an ectopic stripe of expression (Ashe, 2000).
Additional Dpp/Scw target genes were examined for
repression by the stripe2-brk transgene. Those showing altered
patterns of expression include tup, rho, hnt and Race. The normal tup expression pattern
encompasses both the presumptive amnioserosa and dorsal
regions of the dorsal epidermis. In transgenic
embryos, there is a gap in the pattern in regions where the
stripe2-brk fusion gene is expressed. These results
suggest that Brk represses tup, even though it appears to
respond to a different threshold of Dpp/Scw signaling than pnr.
Additional experiments were done to determine whether Brk
directly represses tup expression, or works indirectly by
inhibiting Dpp signaling (Ashe, 2000).
To examine the relative contributions of the Sog inhibitor and
the Brk repressor in establishing different thresholds of
Dpp/Scw signaling activity, target genes were analyzed in
gastrulation defective (gd) mutants that express either a
stripe2-sog or stripe2-brk transgene. Mutant embryos collected from gd-/gd - females lack a Dl nuclear gradient and therefore lack ventral tissues such as the mesoderm and neurogenic ectoderm. All tissues along the
dorsoventral axis form derivatives of the dorsal ectoderm,
mainly dorsal epidermis. Hereafter, such embryos are referred to as gd-. These mutants lack
endogenous sog and brk products, so that the stripe2 transgenes
represent the only source of expression. Although the stripe2-sog transgene inhibits Dpp signaling, it does not cause
activation of brk.
The pnr and tup expression patterns are derepressed in gd- mutants, and exhibit uniform staining in both dorsal and
ventral regions. These expanded patterns correlate
with the expanded expression of dpp, which is normally
repressed in ventral and lateral regions by the Dl gradient. As seen in wild-type embryos, the stripe2-brk transgene represses the anterior portion of the pnr expression pattern. In contrast, the
stripe2-sog transgene has virtually no effect on the pattern. These observations suggest that Brk is the key
determinant in establishing the lateral limits of the pnr pattern
at the boundary between the dorsal ectoderm and neurogenic
ectoderm. The failure of stripe2-sog to inhibit pnr expression
might be due to redundancy in the action of the Dpp and Scw
ligands. Perhaps either Scw alone or just one copy of dpp+ is
sufficient to activate pnr. This would be consistent with the
observation that the initial pnr expression pattern is essentially
normal in dpp-/dpp- and scw-/scw- mutant embryos (Ashe, 2000).
The limits of the tup expression pattern seem to depend on
both Sog and Brk. The introduction of the stripe2-brk transgene leads to a clear gap in the tup expression pattern, although there is a narrow stripe of repression in gd- mutants lacking the transgene. The stripe2-sog
transgene causes a more extensive gap in the tup pattern. The stripe2-brk transgene was also found to repress Race,
hnt and rho in this assay (Ashe, 2000).
In principle, the gap in the tup pattern caused by the stripe2-brk
transgene could be indirect, and caused by
the repression of dpp. Previous studies have shown that the
early dpp expression pattern expands into the ventral ectoderm
in brk- mutant embryos. To
investigate this possibility, tup expression was monitored in
brk- embryos, and in wild-type embryos carrying both the
stripe2-brk and stripe2-dpp transgenes.
The tup expression pattern exhibits a transient expansion in
brk- mutant embryos. However, this expansion is
only seen in early embryos, prior to the completion of
cellularization. By the onset of gastrulation, the pattern is
essentially normal. The stripe2-brk
transgene creates an early gap in the normal dpp expression
pattern in wild-type embryos. This observation raises
the possibility that the repression of tup and rho is indirectly mediated by the inhibition of Dpp signaling. To test this, the tup pattern was examined in embryos
carrying both the stripe2-brk and stripe2-dpp transgenes. As expected, the stripe2-dpp transgene alone causes a
local expansion of the tup pattern in the vicinity of the stripe
2 pattern. However, the simultaneous expression of
both stripe2-dpp and stripe2-brk leads to a clear gap in the tup
expression pattern. Thus, it would appear that Brk
can repress tup even in regions containing high levels of Dpp
signaling. Similar assays suggest that Race, hnt and rho are not
directly repressed by Brk (Ashe, 2000).
A summary is presented of Dpp signaling thresholds in the embryo. The Dpp/Scw activity
gradient presumably leads to a broad nuclear gradient of Mad and
Medea across the dorsal ectoderm of early embryos. It is conceivable
that the early lateral stripes of brk expression lead to the formation of
an opposing Brk repressor gradient through the limited diffusion of
the protein in the precellular embryo. Peak
levels of Dpp and Scw activity lead to the activation of Race and hnt
at the dorsal midline. The tup and ush patterns represent another
threshold of gene activity. The similar patterns might involve
different mechanisms of Dpp signaling since tup is repressed by Brk,
whereas ush is not. Finally, the broad pnr pattern
represents another threshold of gene activity. It is not inhibited by
Sog but is repressed by Brk. It is possible that tup and pnr are
differentially repressed by a Brk gradient. Low levels of Brk might
be sufficient to direct the lateral limits of the tup pattern, whereas
high levels may be required to repress pnr (Ashe, 2000).
The pattern of the external sensory organs (SO) in Drosophila depends on the
activity of the basic helix-loop-helix (bHLH) transcriptional activators
Achaete/Scute (Ac/Sc) that are expressed in clusters of cells (proneural
clusters) and provide the cells with the potential to develop a neural fate. In
the mesothorax, the GATA1 transcription factor Pannier (Pnr), together with its
cofactor Chip, activates ac/sc genes directly through binding to the
dorsocentral enhancer (DC) of ac/sc. The LIM-homeodomain (LIM-HD)
transcription factor Islet (Isl) was identified by genetic screening and its role
in the thoracic prepatterning was investigated. isl loss-of-function mutations
result in expanded Ac expression in DC and scutellar (SC) proneural clusters and
formation of ectopic sensory organs. Overexpression of Isl decreases proneural
expression and suppresses bristle development. Moreover, Isl is coexpressed with
Pnr in the posterior region of the mesothorax. In the DC proneural cluster, Isl
antagonizes Pnr activity both by dimerization with the DNA-binding domain of Pnr
and via competitive inhibition of the Chip-bHLH interaction. It is proposed that
sensory organ prepatterning relies on the antagonistic activity of individual
Chip-binding factors. The differential affinities of these binding-factors and
their precise stoichiometry are crucial in specifying prepatterns within the
different proneural clusters (Biryukova, 2005).
During Drosophila development, the
expression of transcription factors divides the dorsal thorax into three
domains -- one median and two lateral domains. The lateral domains are specified by
the homeobox-containing proteins of the iroquois-complex (iro),
whereas the GATA
factor Pnr is required to establish the median domain.
Within the mesothorax, Pnr together with
U-shaped (Ush) and Chip plays a key role in dorsal closure. This report
presents evidence that Isl is an essential regulator of the dorso-median
patterning of the thorax. isl− clones generated
adjacent to the thoracic midline, induce a strong cleft, suggesting that Isl is
required for proper dorsal closure during metamorphosis. Ectopic expression of Pnr leads to wing-to-thorax transformations, consistent with its role as medio-dorsal patterning factor. Ectopic
Isl expression does not exhibit this phenotype, excluding the LIM-HD factor from
a direct function as a prothoracic selector. Pnr is also known to activate
wingless (wg) in dorsal thorax. isl
loss-of-function has no significant effect on wg expression. However,
overexpressed Isl strongly reduces the size of the wg thoracic stripe.
This result is consistent with a repressive activity of Isl on Pnr (Biryukova, 2005).
Iro proteins and Pnr are direct activators of the proneural genes in their
respective domains. Pnr binds directly to the DC enhancer of ac/sc,
providing therefore region-specific control of the proneural prepatterning.
Flies with reduced or lack of Pnr function fail to activate ac/sc and to
develop DC and SC sensory organs. The proneural activity of Pnr is antagonized
by Ush, the vertebrate homologue of the FOG (friend of GATA). Ush is expressed
only in the dorsal-most cells of the medial region. As a consequence, the
segregation of the sensory organ precursors occurs along two stripes at the
border of the medial domain of Pnr expression, where Ush is absent or
insufficient to repress Pnr (Biryukova, 2005).
Several lines of evidence indicate that Isl
interferes with the proneural activity of Pnr as a repressor. (1) isl
loss-of-function mutants show an opposite phenotype with regard to Pnr or Chip
loss-of-function mutants: an excess of DC and SC sensory organs. (2) A genetic
synergism exists between PnrD and
isl− alleles. This genetic interaction is less
sensitive than that between PnrD and
ush−, implying an alternative route for Isl to
modulate the Pnr proneural activity. (3) Isl is coexpressed with Pnr within
the posterior mesothorax. (4) Isl modulates the activity of a DC:ac-lacZ
reporter. Loss-of-function isl mutants expand the DC:ac-lacZ expression
as in ush− or PnrD
constitutive mutants, whereas overexpressed Isl reduces the DC:ac-lacZ
expression (Biryukova, 2005).
In the DC region, the regulation of Pnr concentration is critical
for the proper position and shape of the DC proneural cluster. Isl expression
overlaps with the dorsal-most domain of Pnr and DC proneural activity coincides
with the posterior border of Isl expression. Therefore, it proposed that both Isl
and Ush restrict Pnr activity in the mesothorax. Interestingly, the regulation
of the concentration of the mammalian Pnr ortholog, GATA-1, is similarly
critical for proper erythroid, megakaryocytic, eosinophilic and mast cell
lineages (Biryukova, 2005).
Ush behaves as either an activator or a repressor of Pnr,
depending on developmental context. No evidence was found for a direct Isl-Ush
interaction by GST pull down assay: Ush, Pnr and Isl could be co-immunoprecipitated
from transient transfected S2 cells.
Both Ush and Isl may behave as positive cofactors of Pnr for nonneural
activities, such as cardiac development, embryonic dorsal closure and
metamorphosis. Several reports emphasize the role of the Pnr homolog, GATA-1 and
Isl1 in human blood disorders.
It seems likely that GATA:Islet interactions represent a conserved
mechanism to specify different cell fates in humans and other organisms (Biryukova, 2005).
Isl proteins are known as positive regulators of transcription in vertebrates.
In flies, Isl mediates repression of Pnr-driven proneural activity via binding to the
DNA-binding domain of Pnr. Interestingly, these interactions are less specific
than for the Pnr-Ush interaction, where the amino-terminal zinc finger of
Pnr is specifically involved (Biryukova, 2005).
Genetic
analyses of mutants reveal that the DC and the SC proneural clusters show
differential sensitivities during neurogenesis. Ush mutants display
ectopic DC bristles and a few additional SC bristles. This phenotype is similar
to PnrD constitutive mutants, in which Pnr-Ush
interactions are greatly reduced. In contrast, isl mutants show the
opposite phenotype, with a large excess of SC bristles and a few additional DC
bristles. The ChipE mutant exhibits antagonistic
phenotypes: lack of DC bristles, reflecting Pnr loss-of-function and an excess of SC
bristles, reflecting Isl loss-of-function. The differential topography of DC and
SC enhancer binding sites presumably underlies differential
transcription-complex binding affinities (Biryukova, 2005).
Chip is the ortholog of Ldb factors that are
ubiquitous multiadaptor proteins in vertebrates. Each Ldb-dependent
developmental event is specified by modification of the transcriptional complex
and is dependent on the stoichiometry of the region-specific Ldb partners. During normal
development of the thorax, different partners of Chip (i.e., Isl, Ap and Pnr) are
expressed in the same region. The
ChipE mutant is highly sensitive to the dosage of these
factors. In ChipE flies, removing one copy of either
Pnr or Isl causes pupal lethality associated with extreme morphogenetic
phenotypes. Removing one copy of Ap, however, rescues the Pnr-dependent
phenotypes of ChipE flies. Taken together, these
results indicate selective competition between the different partners of Chip,
suggesting that hierarchical protein interactions depending on differential
affinities and the strict stoichiometry of Chip and its partners, are critical
to establish proper transcriptional codes within different proneural
fields (Biryukova, 2005).
isl mutants were isolated in genetic screens for dominant
enhancers of the ChipE phenotype. This study demonstrates that
the LIM-HD transcription factor Isl can bind to the LID of Chip. The binding of the
LID domain of Chip with LIM domains has been conserved throughout evolution as has Chip binding
with bHLHs proteins. LID contains two subdomains: a small N-terminal hydrophobic β
patch (VMVV) followed by a large α helix. ChipE
mutation has a single substitution that changes an Arg to Trp (R504W) in the
middle of the α helix. This residue is highly conserved among species and
mediates high-affinity contact with the LIM domains. Interestingly, the R504W substitution in
Chip abolishes, or strongly reduces, both interactions with the bHLHs and also
interactions with Isl. This result implies that bHLHs and Isl recognize the same
site within the LID domain of Chip. The data argue that competition between
bHLHs and Isl for the LID domain of Chip may be critical for modulating the
activity of transcription complexes during development. In vertebrates, the NLI
homolog of Chip mediates direct coupling of the proneural bHLH factors Ngn2,
NeuroM and the LIM-HD transcription factors (Isl1 and Lhx3). This interaction
leads to transcriptional synergism and the synchronization of motor neuron
subtype specification with neurogenesis in the embryonic spinal cord of chicken.
This work demonstrates that Isl is able to interfere with proneural
activity of Chip-Pnr-bHLH transcription complex and therefore, Isl is thought to be able to antagonize proneural specification (Biryukova, 2005).
Interestingly, the
ChipE mutation has little or no effect on interactions
with other LIM-containing factors, such as Ap and dLMO, suggesting that
different factors have different affinities with the Chip LID domain. Therefore,
the ChipE mutation changes the hierarchy of the
affinities among the different partners of Chip in the mesothorax (Biryukova, 2005).
A transcription-complex 'cassette' model is proposed for the specification of
region-specific patterns of specialized cell types. In this model, the presence
of one of a number of alternative binding factors modifies the specificity of a
core transcription complex. This model makes the prediction that, while the core
components of the transcription complex will be strongly conserved in evolution,
the specificity cassette components will vary significantly between species
showing divergent morphogenetic patterns. Comparison of these variable
components in related species should provide insights into the fundamental
mechanisms of encoding the pattern of differentiated cell types within
morphogenetic fields (Biryukova, 2005).
Dorsal vessel morphogenesis in Drosophila melanogaster serves as a superb system with which to study the cellular and genetic bases of heart tube formation. A cardioblast-expressed Toll-GFP transgene was used to screen for additional genes involved in heart development and tailup was identified as a locus essential for normal dorsal vessel formation. tailup, related to vertebrate islet1, encodes a LIM homeodomain transcription factor expressed in all cardioblasts and pericardial cells of the heart tube as well as in associated lymph gland hematopoietic organs and alary muscles that attach the dorsal vessel to the epidermis. A transcriptional enhancer regulating expression in these four cell types was identified and used as a tailup-GFP transgene with additional markers to characterize dorsal vessel defects resulting from gene mutations. Two reproducible phenotypes were observed in mutant embryos: hypoplastic heart tubes with misaligned cardioblasts and the absence of most lymph gland and pericardial cells. Conversely, a significant expansion of the lymph glands and abnormal morphology of the heart were observed when tailup was overexpressed in the mesoderm. Tailup was shown to bind to two DNA recognition sequences in the dorsal vessel enhancer of the Hand basic helix-loop-helix transcription factor gene, with one site proven to be essential for the lymph gland, pericardial cell, and Svp/Doc cardioblast expression of Hand. Together, these results establish Tailup as being a critical new transcription factor in dorsal vessel morphogenesis and lymph gland formation and place this regulator directly upstream of Hand in these developmental processes (Tao, 2007).
Thus, Tup is a newly discovered player in the regulatory network controlling dorsal vessel morphogenesis and hematopoietic organ formation. Tup is expressed in all cardioblast and pericardial cells of the heart tube, prohemocytes of the lymph glands, and alary muscles needed to secure the dorsal vessel to the epidermis. Phenotypic studies demonstrate a requirement for tup function in three of these cells types. tup mutant embryos exhibit a hypoplastic dorsal vessel, with a variable number of cardioblasts that fail to organize into a heart tube structure. It appears that correct numbers of cardioblasts are not specified in mutant embryos, since gaps were observed in the bilateral cardioblast rows early in the process of dorsal vessel formation. Missing cardioblasts included cells of both the Tin- and Svp/Doc-positive subclasses. The late cardioblast misalignment phenotype is likely due to the dorsal closure and germ band retraction defects known to occur in tup embryos (Tao, 2007).
While the degree of cardioblast hypoplasia is variable in mutant embryos, the severe reduction in prohemocytes of the lymph glands and pericardial cells surrounding the contractile tube is fully penetrant. The Collier (Col) protein serves as an excellent marker for lymph gland primordia and the posterior signaling centers of lymph glands associated with the mature dorsal vessel. Since Col expression is normal in tup mutants, Tup function is not required for the early specification of lymph gland primordia within the dorsal mesoderm. However, the severe reduction of several mature lymph gland markers such as tup-GFP, Hand-GFP, Srp, and Odd suggests that either prohemocytes are present within lymph glands with Tup activity essential for expression of all four of these indicator genes or the cells are absent due to defects in prohemocyte proliferation and/or programmed cell death. The latter is an attractive possibility since Hand knockout embryos show ectopic apoptosis among lymph gland progenitor cells (Tao, 2007).
A function for the Hand basic helix-loop-helix transcription factor has been reported for cardioblast, pericardial, and lymph gland cells. This is the same set of dorsal vessel and hematopoietic cells that require Tup function. Through analysis of the Hand cardiac and hematopoietic enhancer, Tup was demonstrated to be a direct transcriptional regulator of Hand in these cell types. Specifically, mutation of the single Tup-2 element in the Hand cardiac and hematopoietic regulatory module resulted in a dramatic loss or reduction of Hand enhancer activity in prohemocytes, pericardial cells, and the Svp/Doc cardioblast subtype. These findings invoke two possibilities. (1) tup phenotypes may be due to the lack of Hand expression and function in cardioblasts, pericardial cells, and lymph gland progenitors. However, Tup function is likely to be even more critical for cardiogenic and hematopoietic events; forced expression of tup results in the production of excess prohemocytes, while the ectopic expression of Hand does not. Thus, Tup can be considered to be a seminal upstream regulator of genetic and cellular events controlling lymph gland formation. (2) Tin and GATA factors have been shown to regulate the Hand cardiac and hematopoietic enhancer. Thus, it is possible the Hand cardiac and hematopoietic transcription occurs due to combinatorial control, specifically via Tup and Doc cofunction in Svp/Doc-expressing cardioblasts and Tup and Srp coactivity in lymph gland progenitors. Ample evidence exists for the function of multiple interacting transcription factors in the regulation of heart and blood cell gene expression in Drosophila. To summarize regulatory aspects of its function, the data showing that Tup is a direct transcriptional activator of Hand expression in lymph glands, pericardial cells, and Svp/Doc-positive cardioblasts through the HCH enhancer module are compelling. Likewise, Tup serves as either a direct or indirect regulator of srp expression in lymph gland cells and odd expression in lymph gland and pericardial cells (Tao, 2007).
See the embryonic expression pattern of islet/tail up at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site.
islet transcripts are first detectable at embryonic stage 10 in precursors of the heart and aorta (the dorsal vessel), the pharynx (gut ectoderm), and the amnioserosa. By stage 16, this expression is restricted to the heart and aorta, plus the alary and pharyngeal muscles, similar to islet-1 expression in the developing vertebrate pharynx, heart and aorta (Korzh, 1993). Expression in the Drosophila CNS commences at the beginning of germ band retraction (stage 12) in subsets of cells in the ventral cord and brain. ISL transcripts are not present in neuroblasts; based on the dorsal location of the expressing cells within the ventral nerve cord, isl expression appears to be confined to neuronal progeny. Slightly later in development, a second wave of newly expressing islet cells occurs in the ventral cord, such that by stage 13, there are 20 islet-expressing cells per hemisegment (Thor, 1997).
islet expression appears to be restricted to postmitotic neurons. isl expressing cells constitute a subset of motor neurons and interneurons. All of the islet motor neurons innervate muscles located ventrally in the body wall. The majority of the islet motor neurons exit the ventral cord in the segmental nerve branches b and d (SNb and d). Based on their medial position within the ventral cord and their pattern of terminal arborization over ventral muscles 6, 7, and 13, three of the islet motor neurons can be identified as RP1, RP3 and RP4. The remaining SN islet motor neurons lie in more lateral positions within the ventral cord, projecting in SNd to innervate ventral muscles 15, 16 and 17 or in SNb along with the RPs to muscles 6, 7, 12, 14 and 30 (Thor, 1997).
Two additional islet-expressing motor neurons exit the ventral cord in the transverse nerve (TN). The TN is prefigured by two exit glia that lie at the dorsal midline of the ventral cord and extend long processes out to the periphery. The two islet TN motor neurons leave the ventral cord at the dorsal midline and project ipsilaterally along the exit glia. One is a motor neuron innervating ventral muscle 25 (TMN25), while the other (TMNp) contacts the ventral process of the lateral bipolar dendrite neuron (LBD) which in turn innervates the alary muscles attached to the heat and aorta. It is probable that the TMNp directly synapses onto the peripheral LBD neuron, analogous to vertebrate sympathetic preganglionic motor neurons that synapse onto postganglionic neurons lying outside the spinal cord (Thor, 1997).
islet interneurons belong to several different classes based on their morphology. Class I and II interneurons project either ipsi- or contralaterally and extend axons within the connectives, forming two discrete fascicles within the longitudinal connectives. A third class is composed of local interneurons that project across the midline and terminate contralaterally within the same segment. apterous is expressed in a small subset of ipsilaterally projecting interneurons that form a single fascicle in the connective, similar to the Class I and II islet interneurons (Lundgren, 1995). AP and ISL are expressed in nonoverlapping sets of neurons. In addition, both apterous and islet expressing neuronal subsets also project axons along different pathways (Thor, 1997).
Proper information processing in neural circuits requires establishment of specific connections between pre- and postsynaptic neurons. Targeting specificity of neurons is instructed by cell-surface receptors on the growth cones of axons and dendrites, which confer responses to external guidance cues. Expression of cell-surface receptors is in turn regulated by neuron-intrinsic transcriptional programs. In the Drosophila olfactory system, each projection neuron (PN) achieves precise dendritic targeting to one of 50 glomeruli in the antennal lobe. PN dendritic targeting is specified by lineage and birth order, and their initial targeting occurs prior to contact with axons of their presynaptic partners, olfactory receptor neurons. A search was performed for transcription factors (TFs) that control PN-intrinsic mechanisms of dendritic targeting. Two POU-domain TFs, acj6 and drifter have been identified as essential players. After testing 13 additional candidates, four TFs were identified, (LIM-homeodomain TFs islet and lim1, the homeodomain TF cut, and the zinc-finger TF squeeze) and the LIM cofactor Chip, that are required for PN dendritic targeting. These results begin to provide insights into the global strategy of how an ensemble of TFs regulates wiring specificity of a large number of neurons constituting a neural circuit (Komiyama, 2007).
For technical simplicity, larval born GH146-Gal4-positive PNs, originating from three neuroblast lineages, anterodorsal (adPNs), lateral (lPNs), and ventral (vPNs), were studied. Out of ~25 classes defined by their glomerular targets, focus was placed on 17 classes whose target glomeruli are reliably recognized across different animals. The MARCM technique allows visualization and genetic manipulation of PNs in neuroblast and single-cell clones in otherwise heterozygous animals, so PN-intrinsic programs can be studied for dendritic targeting. GH146 is expressed only in postmitotic PNs (Komiyama, 2007).
acj6 and drifter have been identified as lineage-specific regulators of PN dendritic targeting. To identify additional transcription factors (TFs) that regulate dendritic targeting of different PN classes, candidates were tested that have been shown to regulate neuronal subtype specification and targeting specificity and have available loss-of-function mutants. The following was tested; (1) the expression of candidate genes in PNs at 18 hr after puparium formation (APF) when PN dendrites are in the process of completing their initial targeting, and/or (2) their requirement in PNs by examining dendritic targeting in homozygous mutant MARCM clones (Komiyama, 2007).
In addition to the eight genes described below, five other TFs were examined that were not pursued because of the lack of expression in GH146-PNs at 18 hr APF (aristaless and pdm-1) or the lack of targeting defects in homozygous mutant PNs (abrupt [abk02807], kruppel [Kr1], and Dichaete [Dichaete87]) (Komiyama, 2007).
LIM-HD factors and PN targeting: LIM-homeodomain (LIM-HD) TFs are involved in multiple events during neuronal development. Most functions of LIM-HD factors require the LIM domain-binding cofactor, which is represented in Drosophila by ubiquitously expressed Chip. Chip antibody revealed ubiquitous expression of Chip in cells around the antennal lobe (AL) including all GH146-PNs at 18 hr APF (Komiyama, 2007).
The requirement of Chip in PN dendritic targeting was tested. Wild-type adPNs, lPNs, and vPNs target stereotyped sets of glomeruli. PNs homozygous for a Chip null allele (Chipe5.5) failed to target most of the correct glomeruli and occupied inappropriate glomeruli. Most adPN and lPN clones (12/13) also mistargeted a fraction of dendrites to the structure ventral to the AL, the suboesophaegeal ganglion (SOG). Thus, Chip is required for targeting specificity of most, if not all, PN classes studied here, and Chip-interacting proteins including LIM-HD factors likely play important roles in PN dendritic targeting (Komiyama, 2007).
Five LIM-HD factors have been characterized in Drosophila: apterous, arrowhead, islet, lim1, and lim3. apterous, arrowhead, or lim3 were not pursued because they are not expressed in GH146-PNs at 18 hr APF (apterous) or they do not have targeting defects in PNs homozygous for null alleles (lim337Bd6 and awh16) (Komiyama, 2007).
Islet antibody detected Islet expression in ~50% adPNs and most lPNs but not in vPNs at 18 hr APF and adult. isl−/− adPNs failed to target many (but not all) of the normal target glomeruli, including VA1lm, VA3, and VM7. In addition, DA1, a lPN target, was often specifically mistargeted. Defects of isl−/− lPNs were very similar to Chip−/− lPN defects. A fraction of dendrites often mistargeted to the SOG. Within the AL, dendrites were diffusely spread, although DA1 and DL3 were always correctly innervated. Targeting of isl−/− vPNs was normal, consistent with their lack of Islet expression (Komiyama, 2007).
Lim1 antibody revealed Lim1 expression in most or all vPNs, but not in adPNs or lPNs in adults. The expression pattern appears similar at 18 hr APF, although vPNs are difficult to identify unambiguously at early stages. lim1−/− adPNs showed no defects, consistent with the lack of Lim1 expression. lim1−/− lPNs rarely showed a cell number decrease, but in clones in which the cell number was normal, lim1−/− lPNs targeted correct glomeruli. In contrast, lim1−/− vPNs showed a specific targeting defect. Wild-type vPNs innervate DA1 and VA1lm densely because of the single vPNs that specifically innervate these glomeruli, in addition to the diffuse innervation all over the AL contributed by the pan-AL vPN. In lim1−/− vPNs, DA1 innervation was greatly reduced and sometimes undetectable. Therefore, lim1 is required for dendritic targeting by a single vPN class, vDA1, despite its general expression in vPNs. lim1 might be redundant with other factors in non-DA1 vPNs. It was note that phenotypes of islet and lim1 combined are only a subset of the Chip phenotype. Additional Chip phenotype may be explained by non-Lim-HD molecules interacting with Chip (Komiyama, 2007).
\
islet mutants are embryonic lethal and have defects in CNS development as well as in the organization of heart, aorta and alary muscles. Neuron survival and axon extention is normal in islet mutants. Thus the islet gene, like apterous is not necessary for survival of the expressing neurons. However, islet mutant interneurons and motor neurons exhibit striking pathfinding defects. The Class I and II interneurons fail to form their distinct fascicles in the longitudinal connective. Similarly, axons of the Class III local interneurons often appear defasciculated and highly disorganized. In islet mutants, SNb motoneurons project to the ventral muscle area but show a range of defects in target selection. The most common phenotype is a failure to innervate the cleft between muscle 12 and 13. This is often coupled with SNb motor axons leaving the muscle field altogether and joining the transverse nerve, and a frequent lack of innervation of the muscle 6/7 and 6/13 clefts (Thor, 1997).
In almost half the mutant flies, the TMN25 and TMNp neurons fail to enter the TN and instead project inappropriately within the CNS, sending axons either across the midline or into adjacent segements. In many segments, they appear to be stalled along their path. However, the exit glia that pioneer the TN and act as a substrate for the TMN neurons are presentnd appear morphologically normal in mutants, suggesting that the pathfinding defects of the TMNS are due to a failure to recognize the exit glia. The peripheral lateral bipolar dendrite neuron (LBD) neurons, which also normally project in the TN, extend abnormal ventral processes into the ventral muscle area, often joining the SNb. Since no islet expression is detected in the LBD of wild-type embryos, this phenotype is nonautonomous and likely the result of either a failure to synapse within the islet-expressing TMNp neuron or a lack of innervation of the ventral muscles by the SNb motor neurons (Thor, 1997). A similar invasion of the ventral muscle region by projects from the TN has been observed in situations of weak SNb innervation (Chang, 1996 and Kopczynski, 1996).
dHb9 (FlyBase designation: Extra-extra [Exex]), the Drosophila homolog of vertebrate Hb9, encodes a factor central to motorneuron (MN) development. Exex regulates neuronal fate by restricting expression of Lim3 and Even-skipped The ISNb MN phenotypes of Exex exhibit similarity to those of Lim3 and Islet. Lim3 and Islet are two LIM-HD proteins that are required for the development of ISNb-projecting axons (Thor, 1997; Thor, 1999). As noted, ISNb-MNs express Exex and require Exex function for their differentiation, suggesting that Exex might interact with Lim3 and Islet to regulate neuronal fate. To investigate this, the relative expression patterns and genetic interactions between these genes were examined. To this end, Lim3- and Islet-specific antibodies were generated because prior expression analyses of Lim3 and Islet used gene-specific reporter constructs (Thor, 1997; Thor, 1999) and such reporter constructs often identify only a subset of a gene's expression profile (Broihier, 2002).
It was found that Lim3 is expressed in about 40 neurons per hemisegment -- this is many more neurons than previously identified by reporter gene expression. Of particular interest, Lim3 is expressed in all Exex-positive neurons as well as in several lateral Exex-negative neurons, including the Eve-positive EL interneurons. Therefore, like Exex, Lim3 is expressed in MNs projecting in the primary and secondary branches of both the SN and ISN. Since previous work has demonstrated that Lim3 is expressed in the TN nerve (Thor, 1999), it is concluded that Lim3 is expressed in all motor axon branches. These results suggest that all ventrally and laterally projecting MNs may express Lim3 (Broihier, 2002).
Despite the near identity of the Exex and Lim3 expression patterns, Exex and Lim3 do not activate each other's expression in these cells. Exex expression initiates normally in lim3 mutants and Lim3 expression in Exex-expressing cells also initiates normally in exex mutants. These data demonstrate that Exex and Lim3 are activated independently of one another in coexpressing cells and suggest that they act in parallel to specify neuronal identity. In addition, the striking similarity of the Exex and Lim3 expression patterns suggests coregulation of Lim3 and Exex by a largely overlapping set of transcriptional regulators (Broihier, 2002).
More limited overlap is found in the expression patterns of Exex and Islet. Islet is expressed in roughly 30 neurons per hemisegment, the majority of which are located laterally in the CNS. Exex and Islet are coexpressed in three discrete neuronal populations: the medial ISNb-projecting RP MNs, a pair of mediolateral interneurons corresponding to the serotonergic interneurons of the CNS, and a compact cluster of six lateral neurons. As observed for Exex and Lim3, Exex and Islet do not regulate each other's expression -- Islet expression is normal in exex mutant embryos and Exex expression is normal in isl mutant embryos. These results indicate that exex and isl do not fall into a simple linear hierarchy and suggest they act in parallel to specify neuronal fate (Broihier, 2002).
To investigate whether exex and Islet act in parallel, isl;exex double mutants were constructed and axonal organization was analyzed in these embryos. isl or exex single mutant embryos exhibit no overt defects in the overall architecture of the CNS. In contrast, isl;exex double mutant embryos exhibit clear defects in the organization of the axonal scaffold. For example, the anterior and posterior commissures are thinner than in wild-type and frequently only one commissure forms per segment. In addition, the longitudinal connectives are thinner than in wild-type and often veer toward or away from the midline (Broihier, 2002).
The defects in axonal organization in isl;exex double mutants have suggested these embryos might exhibit pronounced defects in motor axon projections. Whereas the axonal phenotypes of both single mutants are confined to the ISNb nerve branch, double mutant embryos display widespread defects. In isl;exex double mutants, the organization of motor axons into five nerve branches usually occurs, though axonal outgrowth is substantially delayed relative to wild-type. In addition, the penetrance of ISNb phenotypes in isl;exex double mutant embryos is dramatically higher than in exex single mutants. In 96% of hemisegments, the ISNb either bypasses the ventral muscle domain and extends along the ISN, or stalls shortly after it defasciculates from the ISN. Furthermore, defects are observed in the main ISN branch. In 32% of hemisegments, ISN axons defasciculate inappropriately, giving the ISN a 'frayed' appearance. At lower frequency (5%), the ISNs from adjacent hemisegments fuse. The ISN phenotypes are consistent with the presence of Exex-positive axons in the ISN and demonstrate that like ISNb, the ISN is sensitive to exex levels. Since it is unclear whether Isl is expressed in ISN-projecting neurons, the ISN phenotype in isl;exex embryos may result from loss of isl and exex activity either in common or distinct neuronal populations. In conclusion, the widespread axonal phenotypes in isl;exex double mutant embryos indicate that isl and exex act in parallel to regulate neuronal differentiation. Furthermore, the fact that the isl;exex double mutant reveals a role for exex in regulating ISN-projecting axons suggests that exex may genetically interact with other factors to control the outgrowth of additional motor axon branches (Broihier, 2002).
Expression analyses indicate that Exex and Lim3 are expressed widely in ventrally and laterally projecting MNs. In contrast, Eve has been shown to be expressed in dorsally projecting MNs, suggesting that Exex/Lim3 and Eve might label nonoverlapping MN populations. This is, in fact, what is observed since Exex and Eve label mutually exclusive neuronal subsets. Lim3 and Eve also identify nonoverlapping sets of MNs, since they are only coexpressed in the EL interneurons. Together with other expression analyses, these data show that Exex/Lim3 are expressed in the majority of Eve-negative MNs and demonstrate that Exex/Lim3 and Eve identify distinct MN classes (Broihier, 2002).
The LIM-HD gene tailup has been categorised as a prepattern gene that antagonises the formation of sensory bristles on the notum of Drosophila by downregulating the expression of the proneural achaete-scute genes. tup has an earlier function in the development of the imaginal wing disc; namely, the specification of the notum territory. Absence of tup function causes cells of this anlage to upregulate different wing-hinge genes and to lose expression of some notum genes. Consistently, these cells differentiate hinge structures or modified notum cuticle. The LIM-HD co-factors Chip and Sequence-specific single-stranded DNA-binding protein (Ssdp) are also necessary for notum specification. This suggests that Tup acts in this process in a complex with Chip and Ssdp. Overexpression of tup, together with araucan, a `pronotum' gene of the iroquois complex (Iro-C), synergistically reinforces the weak capacity of either gene, when overexpressed singly, to induce ectopic notum-like development. Whereas the Iro-C genes are activated in the notum anlage by EGFR signalling, tup is positively regulated by Dpp signalling. These data support a model in which the EGFR and Dpp signalling pathways, with their respective downstream Iro-C and tup genes, converge and cooperate to commit cells to the notum developmental fate (de Navascues, 2007).
Tup has been categorised as a prepattern factor that controls the
expression of the proneural achaete-scute genes in the third instar
wing disc. This study shows that tup functions earlier in the
development of the dorsal mesothorax. Loss of tup causes a range of
phenotypes, which taken together indicate interference with the assignment of
cells to form notum. Thus, depending on the time of induction of the clones
and their location multiple effects are observed; the formation of notum-like cuticle with
altered cell-cell adhesion properties, the generation of ectopic wing-hinge
structures including tegulae, sclerites or sensilla typical of the proximal
wing, or even the loss of the entire heminotum. Consistent with these adult
phenotypes, in third instar wing discs tup mutant cells can
upregulate genes typically expressed at high levels in the wing-hinge
territory of the disc, such as zfh2, msh, sal and the lacZ
insertion line l(2)09261. Concomitantly, notum-expressed genes such
as eyg, ush and pnr are generally repressed, although in
some cases tup cells may abnormally express notum and hinge genes
together. These data indicate that notum tup cells undergo
transformation towards either an altered notum fate or a hinge fate. Moreover,
the activation of hinge markers in wild-type cells surrounding some
tup clones might reflect the presence of ectopic notum/hinge borders,
which are known to promote non-autonomous effects (de Navascues, 2007).
Unequivocal notum-to-hinge transformations are consistently observed in
clones induced during the first larval instar. In later-induced clones, this
phenotype becomes less manifest and the modified notum cuticle phenotype
becomes prevalent. Accordingly, the upregulation of hinge marker genes and the
converse downregulation of notum genes in the notum territory are most
consistently observed in first instar-induced clones. This suggests that the
requirement for the 'pronotum' function of tup progressively
decreases as development advances. Lesions associated with tup clones
can appear anywhere within the notum, although each particular phenotype shows
a degree of topographic specificity. Interestingly, the activation of hinge
genes and the repression of notum genes are best shown in early-induced clones
located in the presumptive medial notum. Probably, these clones, which are
normally large, do not yield adult structures, since the expected large regions of
mutant cuticle have not been recovered. The clones might give rise to flies
lacking part or most of a heminotum. The dynamic expression pattern of
tup fits well with the spatial distribution of these phenotypes and
the early requirement for tup function for the development of the
notum. Indeed, tup is expressed very early in the wing disc, when it
has less than 100 cells, and the expression occurs within the region that will
form the notum. It is concluded that, similar to other LIM-HD factors such as Ap
and the vertebrate Tup homologue Isl1, Tup is required for the proper specification of not only cell types, but also developing territories (de Navascues, 2007).
Tup is known to bind the co-factor Chip.
Since, in dorsal compartment specification, Chip functions in a
2Ap-2Chip-2Sspd hexamer, it was asked whether a similar 2Tup-2Chip-2Sspd complex
might mediate Tup function in notum specification. The results support this
interpretation. The loss of either Chip or Ssdp upregulates hinge genes
(zfh2, msh), represses a notum marker (eyg), and induces
cuticular defects similar to those associated with tup clones.
Moreover, an excess of Chip would be expected to titrate Tup and/or Ssdp in
incomplete complexes and mimic the loss-of-function phenotype of
notum-to-hinge transformation, as was experimentally observed (de Navascues, 2007).
By contrast, during the later process of sensory organ formation, Tup
appears to act by sequestering both Chip and Pnr, thus preventing activation
of the proneural genes achaete-scute.
This negative function of Tup does not seem relevant for notum specification,
where both Tup and Chip work as positive effectors. Moreover, the Tup
homeodomain is dispensable for titrating Chip and Pnr,
but this is not the case for its 'pronotum' function.
Interestingly, a missense mutation within the LIM-interacting interacting
domain of Chip (ChipE) severely reduces its ability to
interact with Tup and suppresses the negative regulation by Tup of bristle
formation. However, homozygous ChipE flies have no
defects in notum specification. This suggests that a residual interaction between
ChipE and Tup might persist, as additionally suggested by the
suppression of the extra bristles present in ChipE
individuals by UAS-tup overexpression.
A weak interaction between Tup and Chip, which might only permit the formation
of low levels of hexameric complex, might still allow proper notum
specification. This suggestion agrees with the fact that
tupd03613, a strong hypomorphic allele (as substantiated
by its embryonic lethality over the null tupex4, allows proper notum formation in homozygosis (de Navascues, 2007).
Similarly to tup, Iro-C also has a 'pronotum' function. However,
their roles are not entirely equivalent. Anywhere within the notum territory,
loss of Iro-C during first or second instar induces a clear switch to hinge
fate. By contrast, loss of tup causes an assortment of
different combinations of derepressed hinge genes and repressed notum genes.
Moreover, many tup clones induced during the second larval instar,
and even some induced in the first, can develop recognisable notum cuticle.
Thus, it is proposed that tup reinforces/stabilises the commitment of
cells to develop as notum, a commitment imposed mainly by Iro-C. This
reinforcement or stabilisation might be most necessary in the proximal part of
the disc, where expression of ara/caup ceases after the second
instar, but that of tup persists. This might account for the
derepression of hinge genes being most manifest in this region. Depending on
the location and time of Tup deprival, its loss may be inconsequential or lead
to a partial or even a complete loss of notum commitment. Such diversity of
consequences led to an exploration of whether tup might act on target genes
by affecting chromatin remodelling. However, no genetic interactions have been
found with Polycomb (Pc, Scr+Pcl+esc) or trithorax
(trx, osa, brm, Trl, lawc) group genes (de Navascues, 2007).
In contrast to the absolute requirement for Iro-C for notum specification,
overexpression of UAS-ara can impose a notum fate only on the wing
anlage, and only when provided early in the development of the disc. An extra
notum with mirror-image disposition versus the extant notum is generated at
the expense of the wing, a phenotype identical to that resulting from early
deprivation of Wg function. Since UAS-ara overexpression can interfere with
wg expression, Wg deprival probably explains the formation
of the extra notum. Thus, by itself, overexpression of UASara
probably lacks a genuine potential for imposing the notum fate. Similar notum
duplications arise upon early and strong overexpression of UAS-tup
(MD638, dpp-Gal4 and ptc-Gal4 drivers) and, again, they
probably result from inhibition of Wg activity.
Consistent with this interpretation, weaker and later expression of either
UAS-tup or UAS-ara (C765 driver) has little or no capacity to promote notum fate. However, when coexpressed, these transgenes are effective in imposing the notum fate and this should not be attributed to Wg depletion. Indeed, the transformation consists of an expansion of the notum tissue, rather than a notum
duplication. Moreover, as detected by the onset of the ectopic
expression of notum markers (eyg, DC-lacZ), the transformation occurs
in late third instar discs (J.deN., unpublished) that have a nearly wild-type
morphology and a distinguishable wing pouch. This indicates that
these markers are activated in territories previously specified as wing, hinge
or pleura, and subsequently forced to acquire notum identity. Moreover,
overexpression of the Wg pathway antagonists UAS-Axin or
UAS-dTCFDN (dTCF or pan with the same driver failed to transform wing towards notum. Finally, the activation of eyg and the formation of notum tissue in the sternopleurite, a derivative of the leg disc, also attest
to the capacity of tup plus ara to commit cells to develop
as notum (de Navascues, 2007).
It is well established that signalling by the EGFR pathway is essential for
notum development. Its inhibition prevents activation of Iro-C and the growth
of the notum territory. By contrast, Dpp negatively regulates Iro-C and restricts
its domain of expression at both its distal and proximal borders. The
data indicate a novel function of Dpp in notum development; namely, the
activation or maintenance of tup expression in second and third
instar discs. In the notum region of the early disc, Dpp signalling occurs at
low levels, but the results suggest that these are sufficient for
activating tup. Expression of tup is largely independent on
EGFR signalling. Thus, EGFR and Dpp signalling seem to cooperate in specifying
notum identity to the cells of the proximal part of the disc by activating
their respective 'pronotum' downstream genes, Iro-C and tup (de Navascues, 2007).
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Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
islet/tailup:
Biological Overview
| Evolutionary Homologs
| Regulation
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| Effects of Mutation
date revised: 25 August 2007
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