apterous
See the embryonic expression pattern of ap at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site
apterous is first expressed at 6 hours in a segmentally repeated subset of mesodermal cells in postions where ventral and mediolateral muscle precursors arise. The expression later disappears. apterous plays a role in embryonic muscle patterning, and mutations lead to a loss of a specific set of muscles. Misexpression of apterous causes development of ectopic muscles (Bourgouin, 1992). Many neural cells express apterous, including cells of the brain. In the ventral nerve cord (CNS), Apterous is detected at 10 hours when neurons begin extending axons. In neuroblasts apterous expression is reduced to only three cells per segment. The growth cones of the developing interneurons, derived from the apterous expressing neuroblasts, fasiculate preferentially to one other. In addition to the interneurons, apterous is expressed in the PNS in four lateral cells of each thoracic hemisegment. The resulting axons eventually fasiculate the with anterior/posterior bundle (Lundgren, 1995).
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).
The expression patterns of the murine genes Lhx2 and Msx1 and their Drosophila orthologs apterous and muscle-segment homeobox are described and compared. Lhx2 and Msx1 show complementary patterns of expression in most tissues, including the neural and cranial epithelium, pituitary gland, olfactory organs, and neural tube; in contrast, Lhx2 and Msx1 are coexpressed in the developing limbs. Strikingly, the spatial relationship between ap and msh expression in Drosophila is very reminiscent of the expression of their murine orthologs. ap and msh show complementary expression in the leg and antennal imaginal discs, and brain and ventral ganglion of the central nervous system (CNS), but both are coexpressed in the wing imaginal disc. These observations suggest conservation in the regulation of these genes between Drosophila and mice (Lu, 2000).
Lhx2 and Msx1 are found to be coexpressed in the progress zone of the developing mouse limbs. However, Lhx2 is excluded from the tip of the limb bud corresponding to the apical ectodermal ridge
(AER), whereas Msx1 is expressed in this area. The Drosophila
genes msh and ap also exhibit overlapping expression
patterns in the wing imaginal disc, particularly within the
dorsal compartment. msh is also expressed
in the anterior mesopleura where ap is not expressed. Similar expression profiles are observed in haltere discs (Lu, 2000).
The spatial relationship between Lhx2 and Msx1 expression was examined in other embryonic areas. In contrast to the limbs, Lhx2 and Msx1 have reciprocal expression patterns in other regions, including
mutually exclusive domains within the same tissue and
juxtaposed domains in adjacent tissues. For instance,
Msx1 and Lhx2 are both expressed in the olfactory epithelium, however Msx1 is restricted to the anterior region,
while Lhx2 is in the posterior region that precisely fills
the Msx1-negative area. Sagittal views
reveal that Msx1 is expressed in the medial and lateral
nasal processes, whereas Lhx2 seems to label the epithelium of the vomeronasal organ. In addition, Msx1 and Lhx2 are both expressed in the dorsal neural tube, however, Msx1 is prominent at the roof plate,
whereas Lhx2 is directly lateral to the Msx1-positive zone
in the marginal layer of the dorsal commissures (Lu, 2000).
The second type of reciprocal expression is that seen in adjacent tissue layers. For instance, Msx1 is expressed in the developing anterior pituitary, called Rathke's pouch, while Lhx2 is expressed in the base of the diencephalon and its infundibular evagination, which will form the posterior pituitary. In addition, Msx1 is expressed in the cranial epithelium, whereas Lhx2 is expressed in the underlying neural epithelium. Indeed, Lhx2 is absent in the roof between the telencephalic hemispheres, which is a region strongly labeled by Msx1 (Lu, 2000).
ap and msh reciprocal expression has been found in other Drosophila tissues. ap is expressed in a ring-like domain corresponding to the presumptive fourth tarsal
segment of the leg discs, whereas msh is expressed in two
arc-like domains that flank the ap territory. In
the eye-antennal disc, msh is broadly expressed in the eye
portion of the disc and in several ring-like domains within
the antenna region, the stronger of which is found in the
second antennal segment. In contrast, ap is specifically
expressed in the center of the antenna disc, where it shares
discrete areas of overlapping and complementary expression with msh. It was also found that ap and msh have mutually exclusive domains within the brain and ventral ganglion. Thus the data provide previously unreported
expression profiles for msh in Drosophila larval stages,
and uncover a conserved spatial relationship between the expression of msh/ap and Msx1/Lhx2 genes during evolution (Lu, 2000).
Analysis was made of the expression and function of Apterous during embryonic brain development of Drosophila. Expression of Ap in the embryonic brain begins at early stage 12 and is subsequently found in approximately 200 protocerebral neurons and in 4 deutocerebral neurons. Brain glia do not express Ap. Most of the Ap-expressing neurons are interneurons and project their axons across the midline to the contralateral hemisphere; members of a smaller subset project their axons into the ventral nerve cord. A few Ap-expressing neurons project to the ring gland, suggesting that these neurons are neurosecretory cells. In ap loss-of-function mutants, some of the protocerebral and deutocerebral interneurons that express Ap in the wild type show axon pathfinding errors and fasciculation defects in the brain, notably in the fascicles of the brain commissure. In contrast, the interneurons that project to the ring gland do not appear to be affected in ap mutants. Thus, in brain development, Ap is required for correct axon guidance and fasciculation of interneurons, and Ap-expressing cells may also be involved in the brain neuroendocrine system (Herzig, 2001).
Expression of Ap is first seen in the embryonic protocerebrum in approximately 80 cells at early stage 12. Expression of Ap in the embryonic brain is restricted to the protocerebrum until stage 13. At stage 14, approximately 130 Ap-expressing cells are found in the anterior part of the protocerebrum. Additionally, two pairs of cells in the deutocerebrum begin to express Ap; these cells are initially located at the outer surface of the developing deutocerebrum, proximal to the developing antennal nerve. Subsequently, during the process of head involution and germband retraction, these deutocerebral Ap-expressing cells move inward and by the end of embryogenesis are located on the inner surface of the developing deutocerebrum, in close association with the developing frontal connective. Their number remains constant until the end of embryogenesis, whereas the number of Ap-expressing cells in the protocerebrum increases to approximately 200 in stage-17 embryos. In the tritocerebrum, no Ap expression is seen during embryogenesis. Thus, in contrast to the metameric expression of Ap in each of the neuromeres of the VNC, Ap expression in the anterior brain is restricted to the protocerebral and deutocerebral neuromeres (Herzig, 2001).
In order to visualize the neuronal processes of Ap-expressing cells and to further document their localization in the embryonic brain, the GAL4-UAS system consisting of an apGAL4 driver and a UAS-tau-lacZ reporter was used. Double labeling of apGAL4/UAS-tau-lacZ embryos with anti-AP and anti-TAU-ß-GAL antibodies has revealed nearly perfect coexpression of Ap and tau-ß-gal in the embryonic brain. In the protocerebrum only a few neurons ectopically express apGAL4-driven tau-ß-gal and in the deutocerebrum there is no ectopic expression at all. This indicates that within the CNS, apGAL4-driven reporter gene expression represents the endogenous Ap expression pattern (Herzig, 2001).
An analysis of the projection patterns of the Ap neurons in apGAL4/UAS-tau-lacZ embryos indicates that most of the Ap-expressing neurons in the brain are interneurons; with the exception of putative neurosecretory processes, none of the labeled axons extends out of the CNS. The majority of the labeled axons are seen within the protocerebral hemineuromeres or projecting across the primary brain commissure at the level of the protocerebrum. The labeled axons in the primary brain commissure run in several distinct fascicles including one large fascicle and at least four smaller fascicles. Descending Ap axons are also seen projecting from some of the Ap-expressing protocerebral neurons through the deutocerebrum and tritocerebrum into longitudinal pathways of the VNC connectives. In the VNC connectives, these protocerebral axons project in close association with labeled axons from Ap-expressing VNC interneurons. The two pairs of Ap-expressing neurons in the deutocerebrum send processes towards the primary brain commissure; however, these do not cross the midline of the embryonic brain. In the developing VNC, as in the brain of the embryo, most of the Ap neurons are also interneurons. However, in their axonal projection patterns, the Ap-expressing VNC interneurons differ from those in the brain in that they project ipsilaterally within the longitudinal connectives and fasciculate tightly with their ipsilateral segmental homologs without ever crossing the midline (Herzig, 2001).
Double labeling of apGAL4/UAS-tau-lacZ embryos with anti-EN and anti-TAU-ß-GAL antibodies demonstrates that the Ap-expressing protocerebral interneurons are located anterior to the protocerebral En-expressing cells; it also demonstrates that the deutocerebral Ap-expressing cells are located at the same anteroposterior level as the deutocerebral En-expressing cells. The localization of Ap-expressing neurons in the brain is characterized further by double-labeling experiments of apGAL4/UAS-tau-lacZ embryos with anti-Empty spiracles (Ems) and anti-TAU-ß-GAL antibodies. The head gap gene ems is expressed in the anterior deutocerebrum and the anterior tritocerebrum. Deutocerebral Ap expression is seen at a level anterior to the tritocerebral expression domain of Ems and posterior to the deutocerebral Ems expression, thus assigning these pairs of Ap-expressing cells to the posterior deutocerebrum (Herzig, 2001).
To characterize sub-populations of Ap-expressing cells in the proto- or deuto-cerebrum further, double-labeling experiments of apGAL4/UAS-tau-lacZ embryos were carried out with anti-Eyeless (Ey) and anti-TAU-ß-GAL antibodies. Recent studies have shown that the pax gene eyeless is expressed in specific subsets of each embryonic brain neuromere as well as in the embryonic mushroom body primordia. Only a limited set of Ap-expressing cells overlap with Ey expression in the medial and lateral parts of the anteriormost protocerebrum. In particular, the position of a cluster of cells coexpressing Ap and Ey coincides with the position of the developing mushroom bodies, suggesting that Ap is expressed in the progeny of the embryonic mushroom body neuroblasts (Herzig, 2001). In addition to the numerous Ap-expressing interneurons, a small set of Ap-expressing cells projects axons out of the protocerebrum through the nervi corporis cardiaci into the developing ring gland. The ring gland is a neurosecretory structure that consists of the ventral corpora cardiaca, the medial thoracic glands and the dorsal corpus allatum. By combining fasciclin II immunostaining, which labels both the developing ring gland and the developing nervi corporis cardiaci, with apGAL4/UAS-tau-lacZ labeling, a small fascicle of axons deriving from Ap-expressing protocerebral neurons can be seen to project from each half of the protocerebrum along the nervi corporis cardiaci towards the developing ring gland. This axon fascicle reaches the ventral part of the developing ring gland by the end of embryogenesis. The neuronal innvervation of the neurosecretory ring gland by Ap-expressing neurons suggests that these neurons are in fact protocerebral neuroendocrine cells. Corresponding with this notion, the ap gene is also expressed in the dFMRFa-positive SP2 interneuron of the developing protocerebrum. Similarly, studies on the role of Ap in the VNC indicate that a subset of the Ap-expressing cells there are also neuroendocrine. In the VNC, Ap contributes to the initiation and maintenance of expression of the FMRF neuropeptide gene in the neuroendocrine Tv neurons, which project to the neurohemal organs (Herzig, 2001).
apterous expression is not detected in early second instar wing discs, but is activated in its apparently mature pattern in early-mid second instar discs. The early expression of apterous in a dorsally restricted pattern indicates that a D/V boundary exists in early second instar discs, even before the developing disc field contains 200 cells (Williams, 1993).
Mechanisms composing Drosophila's clock are conserved within the animal kingdom. To learn how such clocks influence behavioral and physiological rhythms, the complement of circadian transcripts in adult Drosophila heads was determined. High-density oligonucleotide arrays were used to collect data in the form of three 12-point time course experiments spanning a total of 6 days. Analyses of 24 hr Fourier components of the expression patterns revealed significant oscillations for ~400 transcripts. Based on secondary filters and experimental verifications, a subset of 158 genes showed particularly robust cycling and many oscillatory phases. Circadian expression is associated with genes involved in diverse biological processes, including learning and memory/synapse function, vision, olfaction, locomotion, detoxification, and areas of metabolism. Data collected from three different clock mutants (per0, tim01, and ClkJrk), are consistent with both known and novel regulatory mechanisms controlling circadian transcription (Claridge-Chang, 2001).
A genome-wide expression analysis was performed aimed at identifying all transcripts from the fruit fly head that exhibit circadian oscillations in their expression. By taking time points every 4 hr, a data set was obtained that has a high enough sampling rate to reliably extract 24 hr Fourier components. Time course experiments spanning a day of entrainment followed by a day of free-running were performed to take advantage of both the self-sustaining property of circadian patterns and the improved amplitude and synchrony of circadian patterns found during entrainment. 36 RNA isolates from wild-type adult fruit fly heads, representing three 2 day time courses, were analyzed on high-density oligonucleotide arrays. Each array contained 14,010 probe sets (each composed of 14 pairs of oligonucleotide features) including ~13,600 genes annotated from complete sequence determination of the Drosophila genome. To identify different regulatory patterns underlying circadian transcript oscillations, four-point time course data was colleced from three strains of mutant flies with defects in clock genes (per0, tim01, and ClkJrk) during a single day of entrainment. Because all previously known clock-controlled genes cease to oscillate in these mutants but exhibit changes in their average absolute expression levels, the analysis of the mutant data was focused on changes in absolute expression levels rather than on evaluations of periodicity (Claridge-Chang, 2001).
To organize the 158 statistically significant circadian transcripts in a way that was informed by the data, hierarchical clustering was performed. Both the log ratio wild-type data (normalized per experiment) and the log ratios for each of the three clock mutants (normalized to the entire data set) were included to achieve clusters that have both a more or less uniform phase and a uniform pattern of responses to defects in the circadian clock. One of the most interesting clusters generated by this organization is the per cluster. This cluster contains genes that have an expression peak around ZT16 and a tendency to be reduced in expression in the ClkJrk mutant. Strikingly, all genes previously known to show this pattern of oscillation (per, tim, vri) are found in this cluster (Claridge-Chang, 2001).
All genes of the apterous (ap) cluster are defined by both the oscillatory phase of their expression pattern (average phase ZT17) and by a distinct expression profile in the three clock mutants. Although the 6 hr sampling interval in the mutant data makes it difficult to reliably detect oscillations, it seems that the majority of the genes in this cluster shows some degree of periodicity in the three mutant light-dark regime (LD) time courses. Although it cannot be ruled out that there are circadian oscillations independent from the known clock genes, the hypothesis that there may be a light-driven response underlying the observed mutant expression pattern is favored. The genes in this group may, therefore, be regulated not only by the circadian clock, but also by a direct light-dependent mechanism. It should be mentioned that evidence of gene expression patterns that are purely light-driven in wild-type flies was sought, but little indication was found of such regulation. Instead, genes with both a strong light-driven oscillation and a weak circadian component were encountered. apterous (ap) encodes a LIM-homeobox transcription factor, which is known to act both in neural development and in neuropeptide expression. The ap cluster includes the genes for the transcription factor moira, the synaptic regulator syndapin, two septins (Sep1 and CG9699), and two ATP binding cassette (ABC) transporters (CG6162, CG9990). In terms of chromosomal organization, CG6166, the gene adjacent to CG6162 on chromosome 3R is homologous to CG9990 and coregulated with CG6162 and CG9990 (Claridge-Chang, 2001).
The developing wing disc of Drosophila is divided into
distinct lineage-restricted compartments along both the
anterior/posterior (A/P) and dorsal/ventral (D/V) axes. At
compartment boundaries, morphogenic signals pattern the
disc epithelium and direct appropriate outgrowth and
differentiation of adult wing structures. However, the mechanisms
by which affinity boundaries are established and
maintained are not completely understood.
Compartment-specific adhesive differences and inter-compartment
signaling have both been implicated in this
process. The selector gene apterous is expressed in
dorsal cells of the wing disc and is essential for D/V
compartmentalization, wing margin formation, wing
outgrowth and dorsal-specific wing structures. To better
understand the mechanisms of Ap function and
compartment formation, aspects of the ap
mutant phenotype have been rescued with genes known to be downstream of
Ap. Fringe, a secreted protein involved
in modulation of Notch signaling, is sufficient to rescue
D/V compartmentalization, margin formation and wing
outgrowth when appropriately expressed in an ap mutant
background. When Fng and alphaPS1 (Multiple edematous wings, a dorsally expressed
integrin subunit) are co-expressed, a nearly normal-looking
wing is generated. However, these wings are entirely of
ventral identity. These results demonstrate that a
number of wing development features, including D/V
compartmentalization and wing vein formation, can
occur independently of dorsal identity and that inter-compartmental
signaling, refined by Fng, plays the crucial
role in maintaining the D/V affinity boundary. In addition,
it is clear that key functions of the ap selector gene are mediated by only a small number of downstream effectors (OíKeefe, 2001).
These results suggest that intercompartmental signaling is
sufficient to maintain the D/V affinity boundary. In the
absence of dorsal identity, compartmental
defects associated with ap mutant wing discs can be rescued with the
molecule Fng. This argues that signaling between
compartments mediated by Fng and Notch, and not the
autonomous acquisition of compartment-specific affinity as
an aspect of cell identity, plays the crucial role in D/V
compartmentalization. Consistent with this are previous
findings that both fng and Notch mutant clones generated in
the dorsal compartment do not respect the D/V boundary,
despite the fact that they likely retain dorsal identity.
While the ap alleles used in this study are not molecularly-defined
nulls, these allelic combinations clearly reduce Ap function sufficiently
to eliminate dorsal identity. Based on both sensory bristle and
wing vein morphologies, the Fng and Fng+alphaPS1-rescued
wings consist entirely of ventral cell types. The possibility cannot be excluded that these ap allelic combinations might maintain small degrees of dorsal-specific affinity (independent of dorsal identity): the mutant
phenotypes indicate that any adhesive differences are clearly
not sufficient to maintain D/V compartmentalization (OíKeefe, 2001).
Prospective wing vein cells are identifiable in late third instar
wing discs by molecular markers such as rhomboid. Wing disc eversion results in apposition of dorsal and ventral vein components, and interplanar signaling
between the dorsal and ventral wing surfaces has been shown
to play a crucial role in wing vein differentiation. Clonal
analysis has demonstrated that mutations that disrupt or alter
vein formation, frequently have non-autonomous effects on the
opposite surface, and that these effects are particularly
dramatic when the genetic clone lies on the dorsal surface. These results suggest a dorsal-specific signal that induces differentiation of ventral veins. However,
when forced to differentiate without interplanar signaling, vein
structures are capable of forming on both surfaces, although
these veins are defective in terms of refinement and their
pattern of corrugation. In the Fng+alphaPS1-rescued wing there is no dorsal identity
and, therefore, no dorsal-specific signal directing ventral vein
differentiation. Despite this abnormality, vein components on
both surfaces differentiate appropriately based on their A/P and
proximal/distal position in the wing; these vein components have an entirely ventral
identity. This demonstrates that wing vein refinement,
alignment and pattern of corrugation can occur independently
of dorsal cell types. Although interplanar signaling is certainly
essential for proper wing vein differentiation, it is clear that a
dorsal-to-ventral signal is not required, and that ventral cell types autonomously contain all the information necessary for wing vein development (OíKeefe, 2001).
An emerging view of selector gene function is that these genes
may regulate large numbers of effector genes involved in
particular morphogenetic processes. For example, in the
differentiation of Drosophila haltere from wing, the
transcription factor Ultrabithorax regulates genes at many
levels of the wing patterning genetic cascade. So too, the selector homeoproteins Even-skipped
and Fushi tarazu (Ftz) have been shown to regulate either
directly or indirectly most genes during embryogenesis. However, fusion of the VP16 activation domain to Ftz has suggested that Ftz binds to and regulates
only a small number of target genes. The question is therefore unanswered as to whether the number
of genes regulated by selectors is large or small.
In the absence of normal ap selector gene function, the
expression of only two downstream effectors is sufficient to
rescue wing structures to a remarkable degree. This result
suggests that the compartment-specific selector gene ap
regulates only a small number of target genes during wing
development. It will be interesting to determine whether
selector genes with broader scopes of activities function in a
similar manner. Selectors that control the formation of entire
structures (such as eyeless) or entire body regions (such as the
Hox genes) presumably sit at the top of larger genetic
hierarchies than ap, and may control larger sets of target genes
to fulfill their developmental roles (OíKeefe, 2001).
Finally, although ap regulates only a small number of
downstream effectors to generate the overall morphology of the
wing, it may indeed regulate many genes to confer dorsal
identity. It is tempting to speculate, however, that Ap may
regulate only one additional gene, Dorsal wing (Tiong, 1995), in order to
specify dorsal cell fate in the wing. Loss-of-function mutations
in the Drosophila Dorsal wing locus result in dorsal-to-ventral
transformations in the wing blade, and ventral misexpression
of Dorsal wing produces ectopic dorsal structures (Tiong, 1995). While the gene corresponding to this phenotype has
yet to be characterized, Dorsal wing likely forms a crucial
component of Ap-dependent wing developmental processes (OíKeefe, 2001).
apterous, expressed in the ring gland (Botas, J. personal communication, 1996), has been implicated in the juvenile hormone system because mutations in apterous lead to hormone deficiency, defective histolysis of the larval fat body, arrested vitellogenesis, sterility, and aberrant sexual behavior, all of which are dependent on juvenile hormone (Shtorch, 1995).
Chip may encode an enhancer-facilitator, acting to facilitate the activity of distal enhancers. The mechanisms allowing remote enhancers to regulate promoters several kilobase pairs away are
unknown but are blocked by the Drosophila suppressor of Hairy-wing protein [su(Hw)] that binds to
gypsy retrovirus insertions between enhancers and promoters. su(Hw) bound to a gypsy insertion in the
cut gene also appears to act interchromosomally to antagonize enhancer-promoter interactions on the
homologous chromosome when activity of the Chip gene is reduced. Chip is needed for the wing margin enhancer of cut. The Chip mutation dominantly enhances the mutant phenotypes displayed by partially suppressed gypsy insertions in both cut and Ultrabithorax and is a homozygous larval lethal, indicating that Chip regulates multiple genes. Chip is normally required for wing margin enhancer function of cut because Chip mutations also enhance the cut wing phenotype of a cut mutation and heterozygotes for Chip display cut wing phenotypes when either scalloped or mastermind (mam) are also heterozygous mutant. Both Sc and Mam are known to regulate the cut distal enhancer, but in contrast to sd and mam mutants, Chip mutants display stronger genetic interactions with gypsy insertions than with wing margin enhancer deletions. Thus, in a heterozygous Chip mutant, a heterozygous gypsy insertion in cut displays a cut wing phenotype, whereas a heterozygous enhancer deletion does not. Dependence on the nature of the heterozygous lesion in the regulatory region strongly suggests that Chip directly regulates cut. More strikingly, it indicates that in a Chip heterozygote, a gypsy insertion is more deleterious to enhancer function than deletion of the enhancer. The simplest explanation is that su(Hw) bound to gypsy in one cut allele acts in a transvection-like manner (interchromosomally) to block the wing enhancer in the wild-type cut allele on a second chromosome. This implicates Chip in
enhancer-promoter communication (Morcillo, 1997 and references).
Chip was cloned and found to encode a homolog of the recently
discovered mouse Nli/Ldb1/Clim-2 and Xenopus Xldb1 proteins, which bind nuclear LIM domain proteins.
Chip protein interacts with the LIM domains in the Apterous homeodomain protein, and Chip interacts
genetically with apterous, showing that these interactions are important for Apterous function in vivo.
Importantly, Chip also appears to have broad functions beyond interactions with LIM domain proteins.
Chip is a ubiquitous chromosomal factor required for normal expression of diverse genes at many
stages of development. It is suggested that Chip cooperates with different LIM domain proteins and other
factors to structurally support remote enhancer-promoter interactions (Morcillo, 1997).
LIM domains are found in a variety of proteins, including cytoplasmic and nuclear LIM-only proteins,
LIM-homeodomain (LIM-HD) transcription factors and LIM-kinases. Although the ability of LIM domains
to interact with other proteins has been clearly established in vitro and in cultured cells, their in vivo
function is unknown. Drosophila was used to test the roles of the LIM domains of the LIM-HD family
member Apterous (Ap) in wing and nervous system development. Within the
embryonic ventral nerve cord (VNC), ap is expressed by three
of the approximately 200 neurons in each abdominal
hemisegment. Using promoter fusions
to the axon-targeting tau-lacZ reporter, it has been shown that the ap-expressing neurons are
interneurons that extend axons ipsilaterally and anteriorly along
a single pathway within each longitudinal connective. Upon reaching the adjacent anterior segment the
Ap neurons tightly fasciculate with their homologs, forming
a discrete axon bundle running the length of the VNC.
In ap P44 mutant embryos, the ap neurons fail to
recognize their appropriate pathway and instead
wander within the connective, failing to fasciculate
with one another (O'Keefe, 1998).
Using a rescuing assay of the ap mutant
phenotype, the LIM domains were found to be essential for Ap function. Expression of
LIM domains alone can act in a dominant-negative fashion to disrupt Ap function. The Ap LIM domains can
be replaced by those of another family member to generate normal wing structure, but LIM domains are not
interchangeable during axon pathfinding of the Ap neurons. ap GAL4 /UAS-ap-mediated phenotypic
rescue, in which ap promoter is used to express the UAS-ap transgene in wings and CNS, was used to ask whether the Ap LIM domains are required for
function. To generate ApdeltaLIM, the
LIM domains were specifically delimited, and the rest of the protein left intact.
This Ap derivative is unable to rescue any element of the ap
phenotype when expressed using ap GAL4 in ap mutant cells,
although ApdeltaLIM protein is present at high levels and is
properly localized to the nucleus as assayed with the anti-Ap
antibody. In ap GAL4 /ap P44 ; UAS-ap deltaLIM/+
adults, the wings remain ribbon-like outgrowths, devoid of any
identifiable structures. Using the UAS-tau-lacZ
transgene, the ap neurons were found to remain defasciculated,
indistinguishable from those of ap GAL4 /ap P44 mutant
individuals. Therefore, the LIM domains are
essential for ap function. The homeodomain
is also shown to be required for ap function.
The ApdeltaHD protein, lacking the homeodomain, acts in a dominant-negative fashion to disrupt
residual ap function. The ability to act as a dominant-negative
inhibitor of Ap function in the wing is not restricted to
the Ap LIM domains.
In contrast to the wing, neither ApdeltaHD nor IsletdeltaHD has any
dominant effects on the development of the ap neurons within
the CNS. This suggests that for these two developmental
processes, generation of wing structures and axon pathfinding,
there are differences in the protein interactions involving the
Ap LIM domains. A test was performed to determine whether the LIM domains of
a different LIM-HD family member might be interchangeable
with those of Ap. Drosophila Lim3 (S. Thomas, S. G. E. Andersson, A.
Tomlinson and J. B. Thomas, unpublished),
which is normally expressed in subsets of post-mitotic neurons,
none of which co-express Ap, was chosen. Conservation of LIM
domain sequence within the LIM-HD family ranges from 25%
to 86% aa identity. Ap and Lim3 are relatively divergent,
sharing only 37% identity within the LIM domains.
Expression of Lim3 in Ap cells results in lethality during
larval or early pupal stages. Lim3 can
partially rescue the ap wing phenotype.
Lim3 also partially rescues the ap neuronal pathfinding defects. The axons are more highly fasciculated than those
in ap mutants, but 74% of the segments still display clear
pathfinding errors. Thus, although Lim3 promotes some degree
of axon fasciculation and the formation of a rudimentary wing
margin in ap mutants, it is not interchangeable with Ap (O'Keefe, 1998).
To determine whether LIM domains are interchangeable
between Lim3 and Ap, a fusion was created between the N-terminal
half of Lim3, including the LIM domains, to the C-terminal
half of Ap, containing the homeodomain. This
Lim3:Ap chimera rescues the ap wing phenotype to the same
extent as full-length Ap. Thus, interchanging the LIM domains has no effect on
formation of the Ap-Chip complex in the generation of wing
structure. During pathfinding of the Ap neurons, the Ap LIM
domains are involved in additional protein interactions independent
of Chip. These interactions are specific to the Ap LIM domains and
cannot be mediated by the Lim3 LIM domains. Taken together, this
data suggests that LIM domains mediate different types of protein
interactions in different developmental processes and that LIM domains can participate in
conferring specificity of target gene selection (O'Keefe, 1998).
The unique expression of Lim1 in a subset of motoneurons and interneurons led to an examination of whether this expression
overlaps with other LIM homeodomain members. Characterization of the expression of a group of vertebrate LIM
homeodomain genes (Isl-1, Isl-2, Lim-1 and Lim-3) along
the chick spinal column has demonstrated that these genes overlap with one another in very distinct patterns (Tsuchida, 1994). Their spatial overlap demarcates regions of
motorneuron subclasses, suggesting that the LIM homeodomain genes confer an identity to pools of motorneurons
by their combinatorial expression. More recently, a combinatorial code for motorneuron pathway selection has been
demonstrated for isl and lim3 in Drosophila. In order to evaluate the expression of
Lim1 with respect to its LIM homeodomain relatives, a series of double labeling experiments were carried out using late stage embryos. These
embryos carried enhancers from either islet, lim3 or apterous that recapitulated their expression using a tau-LacZ or tau-c-myc fusion construct as a reporter. By double
staining for enhancer expression and the Lim1 protein, no overlap of expression was observed between Lim1 and Isl,
Ap or Lim3. All neurons that stain positively
for Lim1 in the nuclei lack enhancer expression within
their cell bodies. Similar to what is observed in vertebrates,
the LIM homeodomain genes that were analyzed in Drosophila are expressed in distinct subclasses of neurons within
the ventral nerve cord. The expression of Lim1 is confined
to a subset of motorneurons and interneurons that are lacking the other LIM homeodomain genes tested. To assess the
possibility that the absence of Lim1 in cells expressing
other LIM homeodomain proteins was due to repression
by these family members, the expression of
Lim1 was analyzed in ap and lim3 mutant embryos. In embryos with a
null mutation in ap, the expression of Lim1 remains
unchanged. Likewise, analysis of the Lim3-expressing RP
neurons in lim3 mutants shows no upregulation of Lim1. These results indicate that the exclusion of
Lim1 from cells expressing other LIM homeodomain
proteins is not a result of repression by these LIM homeodomain family members. In addition to the exclusive expression of Lim1, Ap and
Isl do not overlap, while Lim3
fails to overlap with Ap, but is found in a subset of Isl
positive cells. Thus, as in vertebrates,
the expression of these genes in the Drosophila nerve
cord may provide instructional cues for proper pathfinding
and target identity in the embryo (Lilly, 1999 and references therein).
LIM-homeodomain transcription factors are expressed in
subsets of neurons and are required for correct axon
guidance and neurotransmitter identity. The LIM-homeodomain
family member Apterous requires the
LIM-binding protein Chip to execute patterned outgrowth
of the Drosophila wing. To determine whether Chip is a
general cofactor for diverse LIM-homeodomain functions
in vivo, its role in the embryonic nervous
system was studied. Loss-of-function Chip mutations cause defects in
neurotransmitter production that mimic apterous and islet
mutants. Chip is also required cell-autonomously by
Apterous-expressing neurons for proper axon guidance,
and requires both a homodimerization domain and a LIM
interaction domain to function appropriately. Using a
Chip/Apterous chimeric molecule lacking domains
normally required for their interaction,
the complex was reconstituted and the axon guidance defects of
apterous mutants, of Chip mutants and of embryos doubly
mutant for both apterous and Chip were rescued. These results indicate
that Chip participates in a range of developmental
programs controlled by LIM-homeodomain proteins and
that a tetrameric complex comprising two Apterous
molecules bridged by a Chip homodimer is the functional
unit through which Apterous acts during neuronal
differentiation (van Meyel, 2000).
Chip is expressed in most, if not all, embryonic and larval
tissues. In wild-type embryos,
strong, nuclear Chip expression is found throughout the developing
VNC with no apparent subclasses of neurons excluded. A substantial fraction of embryonic Chip is contributed
maternally during oogenesis, and this maternally derived
expression is required for early embryonic segmentation. To estimate the relative contribution of
zygotic and maternally derived Chip to the embryonic VNC, homozygous embryos mutant for a Chip null
allele were examined. Derived from an intercross of
heterozygous parents, mutants are expected to retain half
the maternal and not any zygotic Chip expression. Little reduction of staining in mutant embryos is observed, relative to
Chip/+ heterozygotes. Thus it appears a
substantial fraction of Chip in the VNC is provided maternally.
Co-labelling embryos with anti-Ap and anti-Chip antibodies
reveals that Chip expression overlaps with all the Ap neurons
of the developing VNC (van Meyel, 2000).
If Chip were required for Ap function, elimination of Chip
might be expected to result in an ap-like phenotype. The
requirement of maternally supplied Chip in segmentation precluded an examination of the effects
of eliminating both maternal and zygotic Chip on neuronal
development. Thus, neurotransmitter expression
and axon guidance were examined in Chip mutants in which half of the
maternal and all of the zygotic Chip expression were absent.
In each thoracic hemisegment of the VNC, ap is expressed
in a lateral cluster of four neurons, one of which is the Tv
neuroendocrine cell that expresses the neurotransmitter
dFMRFa. In wild-type embryos, there
are a total of six Tv cells, one in each thoracic hemisegment.
In ap mutants, the Tv neurons are present, but half of all Tv
neurons stochastically fail to express dFMRFa. This regulation of dFMRFa by ap is transcriptional,
since expression of a fusion transgene comprising a 446 bp Tv
neuron-specific enhancer of the dFMRFa gene driving beta-galactosidase
(Tv-lacZ) is similarly reduced in ap
mutants. Ap binds in vitro to each of three sequences within
the enhancer, and mutagenesis of these sites has confirmed that
these sequences are important for Tv-lacZ expression in vivo (van Meyel, 2000).
To determine whether reduction of Chip results in an ap-like
reduction in transcriptional activation of dFMRFa, expression
of the Tv-lacZ reporter transgene was assayed in wild-type,
ap and Chip mutant embryos. Both ap and
Chip mutant embryos show decreased Tv-lacZ activity in
Tv neurons relative to wild-type controls,
implicating Chip in the establishment of this Ap-regulated
aspect of neuronal differentiation. The reduction of Tv-lacZ
activity is less severe in Chip null mutants than ap null
mutants, probably because of the maternally supplied Chip
remaining in Chip mutants. In embryos homozygous for an
antimorphic Chip mutation, Tv-lacZ
expression is reduced further than Chip null mutants but not
to the level of ap mutants (van Meyel, 2000).
Like Ap, the LIM HD protein Isl also regulates
neurotransmitter identity of embryonic neurons. There are
three dopaminergic cells per segment of the VNC, one
unpaired midline cell and a pair of dorsal lateral cells, all of which express Isl protein and thus represent a
subset of the isl interneurons. isl mutants show loss of
expression of tyrosine hydroxylase (TH), a rate-limiting
enzyme in the synthesis of dopamine. To test the role of Chip in the expression of
TH, late-stage wild-type and Chip mutant embryos were stained
with anti-TH antibodies. Homozygous Chip mutant
embryos retain TH expression in the ventral unpaired midline
cells, but few of the dorsal lateral cells express TH, and in those
that do, TH levels are significantly reduced relative to wild-type. In embryos homozygous for a Chip
antimorph, TH expression is greatly diminished in both the
ventral midline and dorsal lateral dopaminergic neurons. While it is clear that the paired dorsal TH cells are more
sensitive to the reduction in Chip dosage than the unpaired
ventral cells, the effects of the antimorphic Chip allele
suggest that TH production in the latter cells is also dependent
on Chip. From these results, together with the above results on
the expression of FMRFamide, it is concluded that Chip is
required for both Ap- and Islet-regulated neurotransmitter
production in the CNS (van Meyel, 2000).
The Notch pathway plays a crucial and universal role
in the assignation of cell fates during development. In
Drosophila, Notch is a transmembrane protein that acts as
a receptor of two ligands, Serrate and Delta. The current
model of Notch signal transduction proposes that Notch is
activated upon binding its ligands and that this leads to the
cleavage and release of its intracellular domain (also called
Nintra). Nintra translocates to the nucleus where it forms
a dimeric transcription activator with the Su(H) protein. In
contrast with this activation model, experiments with the
vertebrate homolog of Su(H), CBF1, suggest that, in
vertebrates, Nintra converts CBF1 from a repressor into an
activator. The role of Su(H) in Notch
signaling during the development of the wing of
Drosophila has been assessed. The results show that, during this process,
Su(H) can activate the expression of some Notch target
genes and that it can do so without the activation of the
Notch pathway or the presence of Nintra. In contrast, the
activation of other Notch target genes requires both Su(H)
and Nintra, and, in the absence of Nintra, Su(H) acts as a
repressor. The Hairless protein interacts
with Notch signaling during wing development and
inhibits the activity of Su(H). These results suggest that, in
Drosophila, the activation of Su(H) by Notch involves the
release of Su(H) from an inhibitory complex, which
contains the Hairless protein. After its release Su(H) can
activate gene expression in the absence of Nintra (Klein, 2000).
An examination was performed to see whether the degree of endogenous
Su(H) activation that results from the removal of H
is sufficient to elicit a biological effect. To assay this,
it was asked whether or not removal of H activity can
induce Su(H)-dependent development of the pouch in
wing discs in which Notch signaling is absent, such
as apterous and Presenilin mutant wing discs. Loss
of H function rescues the loss of wing development
of ap mutants: whereas ap mutants
have no wing pouch, ap;H double
mutants have large wing pouches with no margin
structures. The enlarged pouch of the
double mutant discs expresses spalt (sal) and the two
vg reporters, vgQE and vgBE, all of which are
expressed specifically in the wing pouch in a Notch/Su(H)-dependent manner and are not expressed in ap
mutants. In contrast, no wg expression
is induced in these double mutant discs,
suggesting that the observed rescue is likely to be due
to the activation of Su(H) in the double mutants. This
is strongly supported by the fact that Su(H);H double
mutants exhibit a small wing rudiment identical to
that of Su(H) mutants. Expression of UAS-vg by dpp-Gal4 in ap
mutant discs can recover the pouch-specific
expression domain of sal, suggesting that the activation of vg
expression by Su(H) is responsible for the recovered
sal expression in the ap;H double mutant wing discs.
Similar to overexpression of UAS-Su(H) in ap mutant
wing discs, the pouch in ap;H mutant discs develops
near the residual wg expression in the remaining
hinge. As
expected from the analysis of the wing discs, the
pharate adult ap;H double mutants have large wing
pouches, which are devoid of any margin like
structure such as innervated bristles (Klein, 2000).
The effects on wing development of removing H
in Psn mutants were examined. As in the case
of ap, loss of function of H effects a strong rescue
of the wing pouch in the Psn;H mutant discs in
comparison to the Psn mutant discs. However,
in this case, the morphology of the discs is more like wild type and, in contrast to ap;H mutant discs, the pouch
develops at its normal place. Closer
monitoring of double mutant discs reveals some expression of
wg and the vgBE along the DV boundary. This
suggests that, in contrast to the situation of ap mutants, in Psn
mutants, there is some activation of Notch and it seems that the
lack of H activity can enhance this residual signaling of Notch
at the DV boundary. This is remarkable considering that the
wing phenotype caused by the loss of Psn is stronger than that
caused by loss of Su(H) function.
Taken together, these results provide further evidence for a
positive transcriptional activity of Su(H). They further show
that H is an antagonist of Su(H) during early wing
development and that it suppresses the activity of Su(H) in the
absence of Notch signaling. The results also suggest that the
inactivation of H is sufficient to activate Su(H) and that the
activity of Notch is required to inactivate H during normal
development (Klein, 2000).
Compartment formation is a developmental process that requires the existence of barriers against intermixing between cell groups. In the Drosophila wing disc, the dorso-ventral (D/V) compartment boundary is defined by the expression of the apterous selector gene in the dorsal compartment. Ap activity is under control of dLMO (Beadex) which destabilizes the formation of the Ap-Chip complex. D/V boundary formation in the wing disc also depends on early expression of vestigial. These data suggest that vg is already required for wing cell proliferation before D/V compartmentalization. In addition, over-expression of vg can, to some extent, rescue the effect of the absence of ap on D/V boundary formation. Early Vg product regulates Ap activity by inducing dLMO and thus indirectly regulating ap target genes such as fringe and the PSalpha1 and PSalpha2 integrins. It is concluded that normal cell proliferation is necessary for ap expression at the level of the D/V boundary. This would be mediated by vg, which interacts in a dose-dependent way with ap (Delanoue, 2002).
During development of multicellular organisms, cells are often eliminated by apoptosis if they fail to receive appropriate signals from their surroundings. Short-range cell interactions support cell survival in the Drosophila wing imaginal disc. Evidence is presented showing that cells incorrectly specified for their position undergo apoptosis because they fail to express specific proteins that are found on surrounding cells, including the LRR transmembrane proteins Capricious and Tartan. Interestingly, only the extracellular domains of Capricious and Tartan are required, suggesting that a bidirectional process of cell communication is involved in triggering apoptosis. Evidence showing that activation of the Notch signal transduction pathway is involved in triggering apoptosis of cells misspecified for their dorsal-ventral position (Milán, 2002).
To determine whether apoptosis might be a general response of cells unable to engage in normal interactions with their neighbors, the effects caused by producing cells with inappropriate dorsal-ventral compartment identity were examined. Clones of cells expressing Apterous (Ap) were produced to examine the survival of D cells in the V compartment. Fewer than 20% of surviving Ap-expressing clones were of V compartment origin. Half of these had sorted out into the D compartment and so were in contact with other Ap-expressing cells. The remaining ~10% of clones were recovered in the V compartment. Ventral Apterous-expressing clones were round in shape and induced Wg expression at their borders. Expression of the Apterous inhibitor dLMO was used to produce cells with V identity in the D compartment. Only 30% of dLMO-expressing clones were of D compartment origin. Most of these had sorted out into the V compartment. Fewer than 5% of dLMO-expressing clones were recovered in the D compartment. These were round in shape and induced Wg expression at their borders. These observations suggested that dLMO-expressing clones are preferentially lost from the D compartment if they are unable to make contact with V cells. Likewise, Ap-expressing clones are preferentially lost from the V compartment if they are unable to make contact with D cells. Loss of the inappropriately specified cells was suppressed by coexpression of p35. Under these conditions 48% of dLMO and p35-expressing clones were of D origin, and 51% of clones expressing Ap and p35 were of V origin. This indicates that inappropriately positioned cells are lost by apoptosis. Apoptosis of these cells occurs when clones were induced in second instar. Clones induced during third instar survive equally in both compartments. Caps and Tartan are expressed in D cells under Ap control in second instar wing discs. Ectopic expression of Caps or Tartan cause clones to sort out toward the D compartment, suggesting that these proteins may confer a preferential affinity for D compartment cells. To test whether loss of Caps or Tartan expression contributes to the poor survival of dorsal dLMO-expressing clones, the recovery was measured of clones coexpressing dLMO with Caps or with Tartan. When coexpressed with Caps, 58% of dLMO-expressing clones were of dorsal origin and were recovered in the D compartment, compared to 30% when dLMO was expressed alone. Coexpression with Tartan yielded 54% dorsal dLMO-expressing clones. Expression of CapsDeltaC and TrnDeltaC is able to support survival of dLMO-expressing clones in the D compartment almost as effectively as the full-length proteins (Milán, 2002).
The Drosophila limb primordia are subdivided into compartments -- cell populations that do not mix during development. The wing is subdivided into dorsal (D) and ventral (V) compartments by the activity of the selector gene apterous in D cells. Apterous causes segregation of D and V cell populations by at least two distinct mechanisms. The LRR transmembrane proteins Capricious and Tartan are transiently expressed in D cells and contribute to initial segregation of D and V cells. Signaling between D and V cells mediated by Notch and Fringe contributes to the maintenance of the DV affinity boundary. Given that Notch is activated symmetrically, in D and V cells adjacent to the boundary, its role in boundary formation remains somewhat unclear. The roles of Apterous and Fringe activities in DV boundary formation have been re-examined and evidence is presented that Fringe cannot, by itself, generate an affinity difference between D and V cells. Although not sufficient, Fringe is required via Notch activation for expression of an Apterous-dependent affinity difference. It is proposed that Apterous controls expression of surface proteins that confer an affinity difference in conjunction with activated Notch. Thus, Apterous is viewed as instructive and Notch activity as essential, but permissive (Milán, 2003).
The LRR transmembrane proteins Capricious and Tartan contribute to DV
boundary formation, but their role is transient.
Maintenance of the boundary requires an additional mechanism. Notch activity
has been implicated in this process, but its role has
been questioned. Models for maintenance of the DV boundary must take into
account the fact that Notch is activated symmetrically in cells on either side
of the DV boundary. Therefore, an Ap-dependent process must be invoked to
confer a DV difference. One proposal is that Fringe mediates the required
Ap-dependent activity by acting in a Notch-independent manner, in addition to
its role in Notch signaling.
According to this view, confrontation of Fringe-expressing and non-expressing
cells should induce a cell affinity difference. Increasing
or decreasing Fringe activity has some effect, but does not produce affinity
differences comparable with those produced by manipulating Apterous activity.
Furthermore, the effects of restoring Fringe in D cells
that lack Apterous activity can be reproduced independently by blocking Notch
activation using Necd. Thus, it is unlikely that Fringe has a
Notch-independent role in DV cell interactions (Milán, 2003).
A second, very different, model proposes that Notch activation confers a
boundary-specific affinity state and that this is modulated into D and V
states by Apterous expression.
According to this model, there should be an affinity difference between
boundary cells and internal cells within a compartment but not between D and V
cells in the absence of Notch activity. This model proposes that Notch
activity is sufficient to produce an affinity difference and hence smooth
clone borders. However, clones of cells expressing the
activated Notch receptor do not exhibit this property. This model is also
difficult to reconcile with the observation that the borders of
fringe mutant clones in the D compartment are highly irregular. It is also
incompatible with the finding that restoring Notch activity in the absence of
Apterous function is not sufficient to generate a smooth DV boundary and
prevent mixing of D and V cells (Milán, 2003).
The results reported here support the view that Notch activity is needed
for cell affinity differences between D and V cells, but indicate that Notch
activation is not sufficient to cause these differences. A new model is proposed that
differs in one crucial respect from the model discussed above. The role
of Notch activation is considered to be permissive rather than instructive, and it is suggested that
Apterous controls expression of surface proteins in D and V cells. It is envisaged
that Notch activity is an essential co-factor in allowing cells to convert
this into an affinity state. In molecular terms, one possibility is that D and
V surface proteins form complexes with activated Notch (N*). In
this scenario D+N* and V+N* are the active components, D
and V are needed and instructive but have no activity alone. Interestingly, it
has been observed that loss of Notch activation only in one compartment does
not alter the DV affinity boundary. Thus,
production of either the dorsal (D+N*) or the ventral
(V+N*) boundary-specific cell state is sufficient to induce an
affinity difference with cells of the opposite compartment. Another plausible
molecular scenario is that Notch activity might control the subcellular
localization of the predicted D and V proteins (Milán, 2003).
These examples are presented to illustrate how Notch activity can be seen as a
permissive co-factor rather than as an instructive principle defining cell
affinity. Many other molecular explanations are possible. This model provides
a satisfactory explanation for how Notch can be required, but not sufficient
for boundary maintenance. The essential difference between the permissive and
instructive models for Notch function lies in the observation that Notch
activation leads to an affinity difference only in the context of
juxtaposition of cells with opposite DV identity. Notch activation per se does
not induce a robust affinity boundary, whereas clones expressing dLMO and
Necd do so only when Notch is not blocked in the cells outside
the clone. Comparable
results have been obtained with clones expressing Apterous and Necd (Milán, 2003).
Are the transmembrane proteins Serrate and Delta the D and V proteins,
respectively? Early in development, Serrate is expressed in D cells and Delta
in V cells. Late in development, both genes are regulated by Wg and are expressed in cells
adjacent to the Wg-expressing cells at the DV boundary. Given that the
Serrate- and Delta-expressing cells are offset from the DV boundary, it is
considered unlikely that they confer the D* and V*
activities. However, the possibility that they might
contribute to the establishment of the DV affinity boundary in collaboration
with Caps and Tartan cannot be excluded (Milán, 2003).
The interface between D and V cells behaves as an affinity boundary and as
a signaling center where Notch activation is required for the growth of the
wing disc. Clones of cells can be induced to sort into the opposite
compartment by manipulating Apterous or Fringe activities. A distinction can be made between crossing and pushing the DV boundary as
possible mechanisms. Cells with altered Apterous activity also have altered
Fringe activity. It is suggested that these clones can cross the boundary and mix
freely with cells in the opposite compartment because they change both their
affinity state and signaling properties. Clones in which only Fringe activity
is altered adopt signaling properties of the opposite compartment and displace
the signaling center relative to the endogenous compartment boundary. In
wild-type discs, symmetric activation of Notch and its targets leads to
symmetric growth of D and V compartments. If growth is symmetric with respect
to the displaced signaling center, the clone could be pushed into the opposite
compartment by growth of the surrounding tissue (Milán, 2003).
At first glance, differential growth might explain how cells could be
pushed to the interface between compartments. Can the model presented in the
preceding section explain why some dorsal fringe mutant clones become
able to mix with cells of the opposite compartment? Notch is not activated in V cells adjacent to fringe mutant clones
abutting the boundary. The model presented here suggests that these cells would become V instead of V+N*; hence, there would not be a sustained affinity difference
between fringe mutant D cell and the adjacent V cells. This may
explain why fringe mutant D cells can sometimes mix with V cells when
they are pushed into the V compartment. A similar case can be made to explain
how V cells expressing Fringe can be pushed into the D compartment and mix
with D cells. In both situations, it is noted that these clones form smooth
borders with the cells of the compartment of origin, suggesting symmetric
growth induced by Notch may contribute to the smoothness of the affinity
boundary. This type of 'pushing' mechanism provides a useful explanation for
the behavior of clones of cells that contact the DV boundary. It is noted that the
behavior of cells expressing Apterous and Fringe was not the same when the
entire P compartment was involved. P cells of ventral origin expressing
Apterous were able to sort into the dorsal posterior quadrant, but cells
expressing Fringe were not. It is suggested that this reflects an underlying
difference between cells that have acquired a fully dorsal affinity state from
those in which only the signaling properties have been altered. Fringe
activity clearly plays an important role in the maintaining the segregation of
D and V cells, but it is not the sole mediator of Apterous activity in this
process (Milán, 2003).
apterous:
Biological Overview
| Evolutionary Homologs
| Regulation
| Protein Interactions
| Developmental Biology
| References
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.