Ecdysone receptor
The steroid hormone ecdysone directs Drosophila metamorphosis via three heterodimeric receptors that differ according to which of three Ecdysone receptor isoforms encoded by the EcR gene (EcR-A, EcR-B1, or EcR-B2) is activated by the orphan nuclear receptor USP. Two classes of EcR mutations have been identified and molecularly mapped: those specific to EcR-B1, which uncouple metamorphosis, and embryonic-lethal mutations, which map to common sequences encoding the DNA-
and ligand-binding domains. The two mutations that are specific to the EcR-B1 isoform both generate stop codons that lie within the EcR-B1-specific exon 2 and are predicted to produce EcR-B1 proteins truncated after only 49 or 52 amino acids. Because neither mutation interferes with the synthesis or structure of EcR-A or EcR-B2, they provide a means for determining EcR-B1-specific functions. The EcR early mutants form a largely normal cuticle, which bears the proper number of ventral denticle belts. However, a number of more subtle defects are characteristic of these mutants, including reductions in denticle size and number, and occasional mouthpart aberrations. It should be noted that a maternal contribution of EcR-A mRNA and protein has been observed; hence, the phenotype of a full null EcR is apt to be more severe (Bender, 1997).
The EcR nonpupariating and prepupal mutant classes affect the early stages of metamorphosis, which commences with pupariation and the formation of the pupal case. Pupariation is preceded by a cessation of wandering and includes shortening of the larva, eversion of anterior spiracles, attachment to a solid surface, and hardening of the cuticle. The two EcR-B1 nonpupariating class mutants initiate wandering but fail to evert their spiracles, or to shorten and attach themselves to a solid surface, or to harden their cuticles. Despite failure of these early events, these mutants easily slip free of the larval cuticle when dissected, indicating that apolysis of the larval cuticle has been completed, and hence, that coordination of the early events of metamorphosis is disrupted. EcR-B1 is the predominant isoform in both imaginal and larval cells of the larval midgut; indeed, the imaginal cells do not express detectable EcR-A. Large polyploid cells of the larval epithelial layer line the lumen of the midgut. Interspersed with these larval cells are the midgut imaginal islands, small groups of diploid cells that will give rise to the adult gut. Within 2 hours after puparium formation, the cells of the midgut imaginal islands begin to proliferate. Ultimately the islands join to form a complete larval tube.
Neither cell type follows its normal developmental pathway at metamorphosis in EcR-B1 mutants. The imaginal cells of the midgut islands begin to proliferate but fail to form a tube surrounding the larval epithelial cells, and the larval epithelial cells fail to become condensed into a compact mass. The abdominal histoblasts, which give rise to the adult abdomen, express EcR-B1 but no detectable EcR-A. Counts of histoblast numbers in EcR-B1 mutants reveal that histoblast proliferation is initiated in these animals, but no replacement of larval cells is observed. Histoblast proliferation is limited to one or two cell doublings in these mutants. The BR-C, E74 and E75 early genes are submaximally or negligibly induced in the EcR-B1 mutant, relative to wild type. Transgenic expression of EcR-B1 in EcR-B1 mutant glands results in full repair of that loss. Expression of EcR-B2 isoform only partially restores the loss of EcR-B1, while expression of EcR-A isoform fails to restore the loss of EcR-B1 (Bender, 1997).
The expression of two Ecdysone receptor isoforms early in metamorphosis correlates with the ongoing developmental responses, and not with the developmental origin of the neuron cells. Thus, EcR-B1 is expressed predominantly in neurons undergoing regressive responses whereas EcR-A is predominant in neurons showing maturational responses. Although correlative in nature, these observations are consistent with the hypothesis that EcR-A and EcR-B1 trigger specific cellular responses during CNS metamorphosis. The recent isolation of EcR-B1 specific mutations has allowed tests of EcR-B1 functions in directing specific responses to ecdysteroids (Schubiger, 1998).
In contrast to mutations in EcR common exons, which inactivate all EcR isoforms and lead to embryonic lethality, mutations in an EcR-B1 specific exon result in a delayed death, which allows for larval viability, but lethality at the onset of metamorphosis. Loss of both EcR-B isoforms leads to an earlier death, predominantly at the first and second larval molt. The predominant time of death of EcR-B mutants is during the 1st and 2nd larval stages. The EcR-B lethal period can be contrasted with the strong embryonic lethality of EcR common exon mutations that inactivate all three EcR isoforms. The EcR-B lethal period also differs from that seen for an EcR-B1 stop codon mutant, since the EcR-B1 mutant shows little reduction in 2nd larval stage viability and has substantially greater numbers of surviving 3rd instar larvae than any of the EcR-B mutants. Thus the EcR-B lethal phase differs from that of both EcR null and EcR-B1 specific mutants. The EcR-B animals that arrest during larval development show defects in the process of larval molting. Animals arrested prior to the 2nd larval stage carry both 1st and 2nd larval mouth hooks, while those arrested prior to the 3rd larval stage carry both 2nd and 3rd larval mouth hooks (Schubiger, 1998).
A small fraction of the EcR-B mutants survive to the third larval stage. These animals feed and later come to the surface of the food, but do not wander up the sides of the vial, indicating that normal wandering behavior is affected. They then become sluggish and eventually immobile. Wild-type larvae empty their gut at the end of the wandering period, shortly before the animals contract and form the barrel-shaped puparium. The mutant animals empty their gut but do not contract or form a tanned puparium. In addition, the gas bubble that normally forms at about 3 hours after pupariation is not observed in EcR-B mutants. Nevertheless, some tissues in these arrested larvae continue to show developmental changes: the larval cuticle apolyzes and a large space between the cuticle and the epidermis appears at the anterior and posterior ends, although the tracheal trunks remain attached to the larval cuticle. Later, the mouth hooks are expelled, a process that normally occurs at 12 hours after pupariation, at the time of head eversion. It is evident that a new cuticle, presumably the pupal cuticle, has been secreted. Elongation of the imaginal discs in mutant animals begins normally up to a stage comparable to about 4 hours after pupariation. At this time the peripodial epithelium contracts in wild-type animals to allow the discs to evert to the exterior. In the mutants this contraction apparently does not occur and the uneverted discs continue to elongate, but within vesicles inside of the body. The appendage can then break through the peripodial membrane and elongate inside the body. In normal development the beginning of muscle histolysis is recognizable at 3 hours after pupariation by a loss of birefringence in the set of larval abdominal muscles that are histolyzed during the prepupal stage. In the mutants even at the time of mouth hook detachment the larval muscles are present and have retained their birefringence. In the gut, the larval gastric caecae (which normally histolyze during the prepupal stage) have shortened but are still present at the time of the developmental arrest (Schubiger, 1998).
At the onset of metamorphosis larval neurons lose their synaptic targets and begin to prune back their larval dendritic and axonal arbors. This first step in the remodeling process is preceded by high levels of EcR-B1 expression. Later during pupal development EcR-A is the predominant isoform expressed in these neurons; it is during this stage that adult-specific processes and synapses are made. From the correlation of the EcR expression patterns with the ongoing developmental changes, a functional role is thought to exist for the two EcR isoforms. For example, process regression is thought to be under the control of EcR-B1, whereas maturation is thought to be induced by EcR-A (Schubiger, 1998).
The null mutations for the EcR-B isoforms allow a test for a functional role of EcR isoform types. The investigation focussed on a small set of larval neurons that contain a peptide that cross-reacts with an antibody against the molluscan small cardio-active peptide B (SCP). These neurons are a subset of the FMRFamide expressing neurons. The SCP antibody strongly labels the neurons and reveals a characteristic projection pattern that can easily be followed as these larval neurons initiate remodeling. EcR-B1 is expressed by SCP labeled neurons in control white puparia, and these neurons, as well as many surrounding larval neurons, express EcR-B1 at the time of pupariation. The EcR-B mutants show no detectable EcR-B1 immunoreactivity, whereas the neurons of controls and mutants at the same stage both express EcR-A. The SCP labeling pattern in the ventral nerve cord of control 0 hour prepupa is distinguished by a prominent pair of ventral neurons in each thoracic neuromere (the Tv neurons) and several other neurons in the subesophageal ganglion. The neurites from these cells form a characteristic projection pattern that is particularly dense in the area of the subesophageal ganglion, forming a posteriorly directed branch. Shortly after pupariation the dendritic arbors begin to regress. By 5-6 hours after pupariation the SCP label is clearly reduced, and at the time of head eversion few dendrites remain (Schubiger, 1998).
The loss of labeled dendrites is thought to be caused by the regression of the dendritic arbors, rather than the loss of the neuropeptide from the finer processes. To test this interpretation the bovine microtubule-associated protein tau was expressed in a subset of the SCP-IR neurons. Using the GAL4-UAS system, GAL4 constructs were made with upstream sequences of the FMRFamide gene to drive UAS-tau in the Tv cells. Examination of the Tau pattern at pupariation and late prepupal stages reveals a clear reduction of processes in the older animals. This indicates that the reduced SCP labeling pattern seen as animals metamorphose reflects process regression and not loss of neuropeptide from neuronal processes. In the EcR-B mutants the SCP pattern, at a stage closely corresponding to pupariation, is indistinguishable from the control, with a dense mesh of dendrites in the anterior region of the ventral CNS. In the mutants that develop to the stage of mouth hook detachment, a stage comparable to head eversion, the SCP pattern remains larval, despite the clear advancement of CNS development. Process regression does not occur and the dense dendritic arbors typical of younger stages are still present. The lack of pruning in older stage animals is consistent, and typical for all alleles tested, either in combination with an EcR null mutation or as homozygotes. These results indicate that the EcR-B isoforms have an essential role in the pruning back process during neuronal remodeling. Since both EcR-B isoforms are eliminated in these mutants the pruning back cannot be ascribed to one or the other EcR-B isoform. Consequently, the SCP pattern was examined in a EcR-B1 specific mutant. Most of these mutant larvae reach the third instar and have a phenotype similar to the late stage arrested animals of other EcR-B mutants. At the time of mouth hook detachment clear pruning back has taken place. In all CNS preparations extensive process regression is seen, though the degree is variable, with patterns similar to controls between 6 hours after pupariation and head eversion. It is thought that blockage of neuronal process regression in EcR-B but not EcR-B1 mutants indicates functional redundancy of the EcR-B1 and EcR-B2 isoforms during dendritic pruning of SCP-IR neurons. Such apparent functional redundancy has been seen frequently in RAR knockout experiments in the mouse, with milder defects than expected if generated by loss of a single isoform (Schubiger, 1998).
Neuronal process remodeling occurs widely in the construction of both invertebrate and vertebrate nervous systems. During Drosophila metamorphosis, gamma
neurons of the mushroom bodies (MBs), the center for olfactory learning in insects, undergo pruning of larval-specific dendrites and axons followed by outgrowth of
adult-specific processes. To elucidate the underlying molecular mechanisms, a genetic mosaic screen was conducted and one ultraspiracle (usp) allele
defective in larval process pruning was discovered. Consistent with the notion that Usp forms a heterodimer with the Ecdysone receptor (EcR), it was found that the EcR-B1 isoform is
specifically expressed in the MBgamma neurons, and is required for the pruning of larval processes. Surprisingly, most identified primary EcR/Usp targets are
dispensable for MB neuronal remodeling. This study demonstrates cell-autonomous roles for EcR/Usp in controlling neuronal remodeling, potentially through novel
downstream targets (Lee, 2000).
The recently established MARCM (for mosaic analysis with a repressible cell marker) genetic mosaic system has allowed the study of functions of genes in various neural developmental processes in the Drosophila brain. The MARCM system allows unique labeling of homozygous mutant cells in a mosaic tissue, which is important for phenotypic analysis of individual mutant neurons in the complex brain. Because typical neurogenesis involves the generation of ganglion mother cells (GMCs) from neuroblasts followed by the formation of two postmitotic neurons from each GMC, the MARCM system can be used to mark the entire axonal and dendritic projections of single neurons if mitotic recombination occurs during GMC division (Lee, 2000 and references therein).
MARCM-based analysis has elucidated the cellular basis for the development of the mushroom bodies (MBs), including neuronal reorganization of the MB during metamorphosis. The MBs are prominent neuropils of the central brain that are essential for several forms of learning and memory. Each MB is composed of approximately 2,500 neurons, which are derived from four neuroblasts that undergo hundreds of asymmetric divisions through embryonic, larval, and pupal stages. Unlike most larval-born neurons, which are arrested as immature neurons until the pupal stage, MB neurons elaborate axonal and dendritic projections shortly after mitosis. In the larval brain, every MB neuron extends a single process from which dendrites branch out into the calyx. The axon extends further and then bifurcates into two major branches, one projecting medially and the other projecting dorsally. Interestingly, MB neurons generated prior to the mid-third instar stage, named gamma neurons, prune the medial and dorsal branches during early metamorphosis and subsequently project axons only into the medial gamma lobe of the adult MB. In contrast, the alpha'/beta' MB neurons that are born after the mid-third instar stage retain their larval projections during metamorphosis (Lee, 2000 and references therein).
Because the MARCM system further allows one to generate clones homozygous for any mutation of interest only in the uniquely labeled gamma neurons, the MB gamma neuron was used as a genetic model system to investigate the molecular mechanisms of neuronal remodeling. Both a forward genetic screen and a candidate gene approach have indicated that Usp, is essential for MB gamma neuron remodeling. The EcR-B1 isoform is specifically expressed in the MB neurons destined for remodeling, and it mediates the axonal pruning of MB gamma neurons independent of the surrounding cells. The individual functions of several ecdysone primary response genes, including Broad-Complex (BR-C), E74, and E75 were examined, and none of them were found to be essential for the EcR/USP-mediated MB remodeling. This study demonstrates cell-autonomous roles for EcR/USP in controlling MB neuronal remodeling, potentially through novel downstream targets (Lee, 2000).
Drosophila imaginal discs are specified and patterned during embryonic and
larval development, resulting in each cell acquiring a specific fate in the
adult fly. Morphogenesis and differentiation of imaginal tissues, however, does
not occur until metamorphosis, when pulses of the steroid hormone ecdysone
direct these complex morphogenetic responses. The ecdysone regulatory pathway controls wing morphogenesis and integrin expression during Drosophila metamorphosis. Mutations in the EcR ecdysone receptor gene and crooked legs (crol),
an ecdysone-inducible gene that encodes a family of zinc finger proteins, cause
similar defects in wing morphogenesis and cell adhesion: this indicates a role for
ecdysone in these morphogenetic responses. In some homo- and hetero-allelic crol combinations a few adult escapers can be recovered. All of these escapers also display wing defects. Fifty-seven percent of the adult escapers have held out wings with a partial (blister) or complete (balloon) separation of the
dorsal and ventral wing surfaces, while the remaining 43% have either malformed or completely unfolded wings. The blisters in crol mutant wings are generally large and do not appear to have sharp boundaries. These phenotypes indicate a role for crol in cell adhesion and wing morphogenesis, raising the possibility
that ecdysone signaling might play a role in regulating these processes. To test this hypothesis, the role of the ecdysone receptor in wing development was analyzed using the hypomorphic EcRk06210 allele. The wings of EcRk06210 homozygous mutants display cell adhesion and morphogenetic defects similar to those seen in crol mutants. At 18°C, 24% of the eclosed adults display venation defects, 32% have malformed wings, and 13% display wing blisters. The blisters are usually small and centrally located, although blistering of the entire wing (a balloon wing) can occasionally be seen. The most frequent venation defects include a small, extra vein that originates from the third longitudinal vein; an additional anterior crossvein, and a 'delta' thickening at the intersection between the posterior crossvein and the fourth longitudinal vein. crol and EcR mutations are shown to interact with mutations in genes encoding the integrin subunits. The frequency of blisters in if mutants is enhanced three- to four-fold by crol1 and crol2 and approximately sevenfold by crol3. In contrast, no interaction was observed between mysnj42 and crol1 or crol2, whereas the frequency of blisters and balloons in mysnj42 mutants is increased three- to four-fold by crol3, and the frequency of balloon wings increased approximately sevenfold (D'Avino, 2000).
alpha-Integrin transcription is regulated by ecdysone in
cultured larval organs and some changes in the temporal patterns of integrin
expression correlate with the ecdysone titer profile during metamorphosis.
Transcription of alpha- and beta-integrin subunits is also altered in crol and
EcR mutants, indicating that integrin expression is dependent upon crol and EcR function. The expression patterns of alphaPS1 and
alphaPS2 in culture appear almost identical. After ~1 h of culture in the presence of ecdysone, alphaPS1 and alphaPS2 transcript levels decrease rapidly, becoming very low by 8 h after hormone addition. The alphaPS3 (scab) gene consists of two transcription units [Long-alphaPS3 (L-alphaPS3) and Short-alphaPS3 (S-alphaPS3)] that initiate from different start sites. Interestingly, L-alphaPS3 mRNA accumulates rapidly in response to ecdysone, peaking by 6-8 h after hormone addition, while S-alphaPS3 transcription appears unaffected by the hormone. Similar to S-alphaPS3, betaPS mRNA levels remain uniform throughout the time course. Thus, only alpha-integrin subunits are regulated by ecdysone in cultured larval organs, and they are either induced or repressed in response to the hormone (D'Avino, 2000).
These findings suggest that altered integrin gene expression
in crol and EcR mutants lead to the defects observed
in wing morphogenesis and cell adhesion. However, integrins
also function in a wide range of other biological
pathways during development, including tissue morphogenesis,
cytoskeletal reorganization, memory, and gene
expression. These widespread functions
raise the possibility that ecdysone-regulated integrin expression
may control multiple events during metamorphosis. For example, the if V2 semilethal allele displays a
misshapen leg phenotype that resembles the defective legs
seen in crol mutants, indicating that alphaPS2 functions may be
recruited by the ecdysone pathway to regulate leg morphogenesis. Furthermore, since alphaPS3 has been proposed to mediate synaptic
rearrangements, its ecdysone-induced
expression in late third instar larvae may contribute
to the extensive neuronal remodeling that occurs in the
central nervous system during metamorphosis. Further studies of the tissue-specific functions of integrins during metamorphosis will provide a better
understanding of how these critical cell surface receptors
exert their multiple effects during development (D'Avino, 2000).
Oogenesis in Drosophila is regulated by the steroid hormone ecdysone and the sesquiterpenoid juvenile hormone. Response to ecdysone is mediated by a heteromeric receptor composed of the EcR and USP proteins. A temperature-sensitive EcR mutation, EcRA483T, has been identified from a previously isolated collection of EcR mutations. EcRA483T is predicted to affect all EcR protein products (EcR-A, EcR-B1, and EcR-B2) since it maps to a common exon encoding the ligand-binding domain. In wild-type females, both EcR-A and EcR-B1 are expressed in nurse cells and follicle cells throughout oogenesis. EcR mutant females raised at permissive temperature and then shifted to restrictive temperature exhibit severe reductions in fecundity. Oogenesis in EcR mutant females is defective, and the spectrum of oogenic defects includes the presence of abnormal egg chambers and loss of vitellogenic egg stages (Carney, 2000).
These observations are consistent with the notion that the defective and degenerating egg chambers observed in EcR mutant females may result from action of EcR and USP heterodimers in ovarian germ cells and/or somatic cells. Recent analysis of expression and function of the ecdysone-dependent E75 early gene during oogenesis has led to the proposal of an ecdysone-dependent checkpoint in midoogenesis. In this study, removal of EcR through creation of EcR germline clones appears to result in egg chamber arrest at stages 6-7, suggesting a requirement for EcR in the germline for progression through oogenesis. Thus, the reproductive defects in EcR mutant females described here are likely to result in part from loss of EcR function in germline cells (Carney, 2000).
Vitellogenesis begins with yolk protein synthesis in the fat body and ovarian follicle cells: ecdysone signaling has been implicated in the control of fat body yolk protein synthesis. It is presumed that active ecdysone receptors are present in the fat body. Because of the detection of ecdysone binding activity in this tissue, the presence of EcR protein in the adult fat body has not been examined directly. In experiments using a yolk protein polyclonal antiserum, small but reproducible decreases are seen in yolk protein production in EcR mutant females compared to controls between 12 and 30 hr following eclosion. It is speculated that the small reductions in yolk protein accumulation seen in EcR mutant females are due to a requirement for EcR in the fat body for yolk protein synthesis. If this is the case, methods that allow a more complete inactivation of EcR functions would be predicted to show more dramatic effects on yolk protein production (Carney, 2000).
The three Drosophila EcR isoforms differ only at their N termini; thus, they share the conserved ligand-binding domain transcriptional activation function (AF2) and only differ in the unconserved A/B region, which contains a second, isoform-specific, activation function (AF1). A dominant-negative mutant EcR (EcR-DN) has been developed: it was expressed in flies with the GAL4/UAS system, and it was used to block ecdysone signaling in eight tissues or groups of tissues. Localized EcR-DN arrests ecdysone-dependent development in the target cells and often (because of a molting checkpoint) arrests development globally. Simultaneously expressing individual wild-type EcR isoforms in the same target tissues suppresses the EcR-DN phenotype and identifies the rescuing isoform as sufficient to support the development of the target. Every isoform, and even an N-terminal truncated EcR that lacks any AF1, supports development in the fat body, eye discs, salivary glands, EH-secreting neurosecretory cells and in the dpp expression domain, implying that AF1 is dispensable in these tissues. By contrast, only EcR-A is able to support development in the margins of the wing discs, and only EcR-B2 can do so in the larval epidermis and the border cells of the developing egg chamber. In light of these results, the simplest explanations for the widespread spatial and temporal variations in EcR isoform titers appear untenable (Cherbas, 2003).
In comparison with most other transcriptional regulators, the nuclear
receptors are remarkably complex. This study is concerned with one aspect of that
complexity: the alternative pathways by which a nuclear receptor can activate
transcription. A typical nuclear receptor contains two activation surfaces: a
strongly conserved activation function (AF2) in its ligand-binding domain (LBD
or E region) and a
second activation function (AF1) within its unconserved N-terminal A/B region.
A great deal is known about the role of AF2 in seeding coactivator complexes,
and rather less about AF1. Nonetheless, it is clear from assays in different
settings that the two AFs play separable, though often complementary, roles.
Thus, depending upon the promoter being tested, its chromatin structure and
the host cell, full activation may require either AF or both (Cherbas, 2003 and references therein).
An attractive inference is that these differing AF requirements reflect the
diversity of the rate-limiting steps that can be involved in the assembly of
functional transcription complexes. If so, it is interesting to consider how
these requirements are organized globally. One plausible hypothesis is that AF
requirements are simply promoter specific. According to this idea, the set of
hormone-responsive promoters in any given cell type may approximate a random
sample of AF requirements. A corollary is that, if AF1-requiring promoters are
prevalent, every cell type should require AF1, and if the receptor exists in
multiple isoforms with different AF1s, every cell should require multiple
isoforms (Cherbas, 2003).
A very different hypothesis is suggested by observations on receptors with
alternative, isoform-specific AF1s and particularly by the behavior of such
receptors in developmental systems. Many nuclear receptors exist as
functionally distinct isoforms differing only in their N-terminal domains
(e.g. RARs alpha, ß and gamma, RXRs
alpha, ß and gamma, TRß, GR, PR, ERalpha and
EcR). In addition, receptor isotypes, generated from distinct genes, most often differ
in their N-terminal domains (e.g. TR and
ER). Where a receptor exists in several isoforms or isotypes,
these are generally distributed in a strongly tissue-specific manner. Differences in isoform titers can be quite striking, and
for some nuclear receptors there is a strong and unmistakable correlation with
developmental fate. Given that alternative AF1s have distinct transcriptional
effects and that they can activate distinct sets of target genes, it is
reasonable to suppose that the availability of particular AF1s (in specific
receptor isoforms or isotypes) may regulate tissue-specific gene expression.
Taken to the extreme, this hypothesis would imply that promoters that are
induced in all tissues require only AF2, while promoters whose hormone
response is tissue-specific require AF1, and in any given cell type the
AF1-specific promoters share a requirement for a specific receptor variant (Cherbas, 2003 and references therein).
These alternatives led to the following question: where isoform (hence
AF1) titers vary in a tissue-specific way, are they strongly correlated with
tissue-specific requirements for the corresponding AF1s? This question has
been addressed using mutant animals. For example, the phenotypes of
RXRalpha-null mice and mice homozygous for a deletion of the AF1 region of
RXRalpha, were compared. Animals that lacked AF1 exhibited localized defects,
suggesting specific AF1 requirements, though most RXRalpha functions
occurred normally. In a parallel approach, the roles of individual isoforms
have been assessed by using isoform-specific mutations. In
all these instances, the use of mutant organisms creates daunting interpretive
challenges, for it is difficult to decipher the direct, local consequences of
receptor failure in the face of numerous organism-wide defects (Cherbas, 2003 and references therein). This study adopts a different approach, arresting receptor function in targeted
developmental domains and then testing the abilities of particular receptor
isoforms to rescue development when expressed in those domains (Cherbas, 2003).
In Drosophila, there are three EcR isoforms that
differ only in their N-terminal regions, the three isoforms being derived from
a single structural gene by both alternative promoter usage and alternative
splicing. The potencies of the three isoforms have been tested in an
EcR-deficient Drosophila cell line, in yeast and in mammalian cells. These
studies confirm that the individual isoforms (and their isolated N-terminal
regions tested as fusions) differ markedly in their abilities to activate
particular test promoters, and that each A/B region contains an AF1 that can
activate transcription in an appropriate experimental setting. For example,
AF1s from isoforms B1 and B2, but not from A, are active when tested in
Drosophila cells with an artificial promoter derived from the
Drosophila Eip71CD gene. It is important to note here that these
specific isoform requirements are absolute; that is, increasing the titer of
an inactive isoform does not increase its ability to activate transcription (Cherbas, 2003 and references therein).
Little is known about the distribution of isoform EcR-B2, but it is clear
that isoforms EcR-A and EcR-B1 have very different tissue distributions, and
their relative titers in different tissues are well correlated with the fates
of those tissues during metamorphosis.
For example, immunohistochemistry shows that in third-instar larvae, B1
predominates in larval tissues that will die during metamorphosis, while A
predominates in the imaginal discs (Cherbas, 2003 and references therein).
Genetic studies suggest that the isoforms have overlapping but distinct
functions during fly development. Of mutations that eliminate all isoforms, only
isoform B1, or isoforms B1 and B2 have distinct lethal phases, leading to
death at hatching, at pupariation and during larval life, respectively. It is
known that ectopic expression of any single EcR isoform can partially rescue
development in EcR- animals, with the extent of rescue
depending on the isoform (Cherbas, 2003 and references therein).
The GAL4 driver system and
dominant-negative mutant EcRs were used in this study to arrest ecdysone receptor function in selected
developmental domains. Then, using the same driver system to express
individual EcR isoforms, it was asked which isoforms are sufficient to
restore and sustain development. Serendipitously, these experiments have
revealed a molting checkpoint -- a global block in development induced by
local lesions. The developmental arrest associated with that checkpoint is
noteworthy in its own right; in the present context it has proven
exceptionally useful for these experiments (Cherbas, 2003).
Dominant negative Ecdysone receptor proteins (EcR-DNs) EcR-F645A and EcR-W650A were constructed expecting that, like similar helix
12 mutant TRß proteins, they would lack the transcriptional activation function AF2
and that they might disrupt signaling by competing with wild-type EcR. Both
predictions were confirmed by assays in cultured cells, and the experiments presented in this study suggest that EcR-F645A
expression can block ecdysone signaling in any Drosophila cell. It is
suspected that in vivo EcR-F645A dimerizes with USP and binds to ecdysone
response elements, forming unliganded complexes that repress transcription
normally, but cannot be converted into activating complexes by ligand
(ecdysone). Although still unproven, this scheme conforms to observations
in cultured cells and in vitro and is consistent with the vertebrate
precedents. This mechanism would account for the interference of
EcR-F645A with ecdysone-mediated transcriptional activation. It seems
plausible that the mutant EcR also interferes with ecdysone-mediated
transcriptional repression, but these experiments did not address this directly (Cherbas, 2003).
The mutant EcRs retain the wild-type B1 N terminus, which has AF1 activity
in the wild-type molecule. If AF1 in the
mutant EcR mediates transcriptional activation, it would complicate the
interpretation of the results. However, this is not the case, for
several reasons: (1) co-repressors block AF1 activity; (2) when tested in
EcR-deficient cultured cells, under conditions where AF1 is functional in
wild-type EcR-B1, EcR-F645A completely fails to activate transcription in
response to ecdysone, and (3) in flies, F645A and W650A proteins based on EcR-B2,
EcR-A or EcR-C produce results that are qualitatively similar to those described in this study (Cherbas, 2003).
Despite residual uncertainties about the molecular details, it is clear
that the EcR-DNs block development in individual tissues much as one would
expect if ecdysone action had been arrested. Perhaps most
telling is the abrogation of all hormone-induced puffing changes in the
salivary gland polytene chromosomes. All the phenotypes observed are
associated with a reduction in the level of functional EcR/USP as they are
strongly enhanced in a background of lowered wild-type EcR titer and strongly
suppressed when extra wild-type EcR is supplied. Analysis in cultured cells shows that these mutant EcRs interfere
with wild-type EcR function only when they are present in substantial excess;
it is inferred that this is the case in the targeted fly tissues (Cherbas, 2003).
An unanticipated phenotypic consequence of targeted EcR-DN expression is
the global block that is referred to as a 'molting checkpoint'. When EcR-DNs are expressed in some tissues, they cause both local
phenotypes and a global effect: metamorphosis stops at the time of the next
ecdysone-dependent event. This is most clearly illustrated by EcR-DN
expression in the Serrate domain (referring to the use of the Serrate promoter to positionally regulate ectopic expression); in this case, expression of the EcR-DN is
restricted to the margins of the wing and leg discs, but development is
blocked in the entire animal. The term 'molting checkpoint' is used to describe
the global block. A similar phenomenon occurs in all other cases where EcR-DN
is expressed in epidermal cells. The expression patterns of the Eip, Dpp and
GMR drivers are more complex, opening the possibility that the global block
results from localized malfunction of crucial tissues such as neurosecretory
cells or tracheae; nonetheless, the similarity of the lethal phenotypes makes
it attractive to hypothesize that the molting checkpoint is a general
consequence of EcR-DN expression in epidermal cells (Cherbas, 2003).
A few basic properties of the molting check-point are important here. (1) Checkpoint arrest is enhanced when EcR levels are reduced and suppressed by
rescue constructs; thus, arrest behaves as a downstream consequence of aborted
receptor function. (2) The global block to development is invoked by stimuli other than
interference with ecdysone signaling. For example, expression of the cell
death gene reaper in the GMR domain can produce a similar response. If the molting checkpoint is triggered by some general
aspect of defective cells the checkpoint is probably closely related to
another phenomenon that has been known for many years, i.e. physical injuries,
regeneration or genetic disc-ablation delay in molting. (3) Imaginal discs from arrested animals can be induced to develop by hormone
treatment in vitro; it is inferred that metamorphosis stops because the ecdysone
titer is insufficient (Cherbas, 2003).
The temporal progression of the cell cycle must be coordinated and this
coordination is achieved by a series of checkpoints responding to aberrant
events. Similarly, the complex multicellular events of molting and
metamorphosis must be coordinated, and it is suggested that the style of this
coordination is similar: progress is delayed by a checkpoint invoked by
aberrant development. The checkpoint engendered great interest because it
revealed itself as a global effect in experiments designed to test purely
local phenotypes. It seems likely that the same phenomenon occurs in
experiments where targeted expression is not involved; i.e. mutations with
diverse, local effects may, because they invoke the checkpoint, reveal
themselves by the common phenotype of late larval or pupal lethality (Cherbas, 2003).
The checkpoint is important here because it provides a simple quantitative
measure of EcR-DN phenotypes. The frequency of adult eclosion was used as
a quantitative indicator of the localized effects of EcR-DN action in several
domains. It is the only available assay for the Serrate domain, where the
checkpoint acts so efficiently that no escapers were observed under any
conditions, and it is a convenient assay in the dpp domain. Its
validity is clear in the GMR domain, where the morphological defects
in escapers are proportional to the frequency of the block to pupation over a
wide range of temperatures and responder insertions. Lethality
caused by EcR-DN expression in the Lsp domain arises from a different
mechanism; the animals die late in adult development rather than simply
failing to molt (Cherbas, 2003).
For those drivers that yield viable, morphologically defective adults,
targeted expression of EcR-DNs provides a new way to identify
ecdysone-dependent developmental steps. Although none the EcR-DN-induced morphological phenotypes were investigated in detail, the results do
support several significant inferences (Cherbas, 2003).
In the GMR, dpp, Lsp2 and EH domains any EcR isoform will
support metamorphosis, and, for at least the first three, so will EcR-C. This
is even more impressive because two of the domains (GMR and
dpp) include diverse cell types. In these four domains,
ecdysone-induced transcriptional changes may be mediated by EcR-AF2, by
release of EcR/USP-mediated inhibition, or by USP. There is structural
evidence to suggest that USP may not be capable of activation and it
does not contribute to activation in a cell culture model system, but the first two possibilities are entirely
plausible and cannot be distinguished by the experiments described here (Cherbas, 2003).
The contrary result was also observed. In the Ser domain, only
EcR-A gives full rescue, and in the Eip and slbo domains
only EcR-B2 is effective. Although these effects may be exaggerated by
differences in the levels of expression of the responders used, the
clear-cut nature of the differences suggest that they are real. If the
interpretation of these results is correct, then in each of these domains at least one crucial promoter requires an isoform-specific EcR AF1 (Cherbas, 2003).
Salivary gland puffing might be expected to reveal gene-specific isoform
requirements with individual isoforms giving different uncoordinated
responses. Instead, the rescued puffing response, at least
during its pre-puparial stages, can be characterize by a single parameter -- its rate. Each of the
three isoforms supports the normal, coordinate response. It is thought to be simplest
to suppose that the AF1s play no role and that the puffing response is simply
a sensitive reporter of the expression levels for the three rescuing
transgenes. By contrast, earlier studies using heat shock pulses of single
isoforms in an EcR-null background, controlled approximately for protein
levels, observed the normal pattern rescued by B1>B2>>>A.
Plainly, it will require a more sophisticated experiment to determine with
confidence whether the isoforms differ in their ability to support the puffing
pathway (Cherbas, 2003).
At pupariation, the rescued puffing responses become uncoordinated. This
probably reflects the superimposition of the still-in-progress early response
with new gene activities induced by the declining ecdysone titer at
pupariation (Cherbas, 2003).
The results reported here must be put into the context of previous work on
the tissue-specific properties of the EcR isoforms. Using different approaches
to local rescue, others have demonstrated that isoforms B1 and B2 (but not A)
can support the remodeling of mushroom-body gamma neurons and of the
SCP-staining neurons . When EcR mutations are examined at the level of
the whole organism, the effects tend to be widespread. Thus, common region EcR
nulls are early embryonic lethals, and EcR-B1 nulls are nonpupariating lethals
with defects in the leg discs, the imaginal cells of the midgut islands, the
larval gut and the histoblasts. Animals null for both EcR-B1 and -B2 are early larval
lethals. The widespread nature of the defects in these mutant
animals makes it impossible to judge the localized requirements for EcR
isoforms (Cherbas, 2003).
In addition to testing the capacity of individual isoforms to support
development in diverse tissues, these experiments contribute to the catalog of
isoform distribution in those tissues. The distribution of isoforms is complex
both in time and space. Previous experiments used immunostaining to examine the
relative levels of isoforms A and B1 in late third-instar tissues and in
the CNS. The
measurements of the effects of isoform-specific mutations on EcR-DN phenotypes
contribute an estimate of the contribution of individual endogenous isoforms
in the particular times and places in which each driver is expressed. These
data imply major roles for isoform B2 in the larval fat body and epidermis. In
addition, only isoform B2 can rescue the EcR-DN effects in the slbo
domain, suggesting that it may also be the major isoform in the follicle
cells (Cherbas, 2003).
Two alternative models have been proposed for the distribution
of AF1-specific promoters. According to one, individual domains should exhibit
specific isoform requirements that can be predicted by their isoform contents.
The results lend no support to this idea. It is not known why isoform titers
vary, but they do not appear to be good predictors of isoform requirements.
Instead, the results
are consistent with the following picture: A small minority of promoters
require specific isoforms. As each responding tissue may contain several (or
many) critical promoters, specific AF1 requirements are not limited to a small
minority of tissues. Still, it is remarkable that tissues lacking even one
such critical promoter are not rare. In those tissues (about half of
the sample) AF1 is dispensable, and ecdysone effects are mediated by
AF2 or by relief of repression. In some cells, at least one critical promoter
does require a specific AF1. When many tissues lack a specific isoform,
developmental defects are likely and their phenotypes depend on both the
missing isoform and the intervention of the molting checkpoint (Cherbas, 2003).
The steroid hormone 20-hydroxyecdysone induces metamorphosis in insects. The receptor for the hormone is the ecdysone receptor, a heterodimer of two nuclear receptors, EcR and USP. In Drosophila the EcR gene encodes 3 isoforms (EcR-A, EcR-B1 and EcR-B2) that vary in their N-terminal region but not in their DNA binding and ligand binding domains. The stage and tissue specific distribution of the isoforms during metamorphosis suggests distinct functions for the different isoforms. By over-expressing the three isoforms in animals, results supporting this hypothesis were obtained. Tests were performed for the ability of the different isoforms to rescue the lack of dendritic pruning that is characteristic of mutants lacking both EcR-B1 and EcR-B2. By expressing the different isoforms specifically in the affected neurons, it was found that both EcR-B isoforms are able to rescue the neuronal defect cell autonomously, but EcR-A is less effective. The effect of over-expressing the isoforms was examined in a wild-type background. A sensitive period was determined when high levels of either EcR-B isoform are lethal, indicating that the low levels of EcR-B at this time are crucial to ensure normal development. Over-expressing EcR-A in contrast has no detrimental effect. However, high levels of EcR-A expressed in the posterior compartment suppress puparial tanning, and result in down-regulation of some of the tested target genes in the posterior compartment of the wing disc. EcR-B1 or EcR-B2 over-expression had little or no effect (Schubiger, 2003).
Loss of function of both EcR-B isoforms inhibits dendritic pruning by the larval Tv-neurons, a set of thoracic FMRFamide expressing neurons. However, since all cells in these animals are mutant, it was not known if the failure to prune dendrites was a cell autonomous defect. Consequently, it was asked if the dendritic pruning defect could be rescued by expressing the different EcR isoforms specifically in these neurons. Using the FG5-Gal4 driver containing regulatory sequences of the FMRFamide gene wild-type EcR isoforms as well as the GFP-labeled membrane marker mCD8 were expressed in EcR-B mutant animals. The cell-autonomous rescue of pruning in the Tv neurons shows that given the correct set of receptors in these neurons, ecdysone can induce a pruning response even in a mutant environment in which overall nervous system development is arrested. Both the pruning of these neurosecretory cells and the pruning of the mushroom body neurons require the presence of an EcR-B isoform. The pruning of the Tv-cells, however, is only inhibited when both EcR-B1 and EcR-B2 are absent. This differs from the mushroom body neurons that fail to undergo pruning when only EcR-B1 is missing. Nevertheless similarity across different neuronal types suggests that a general set of rules may be employed early in metamorphosis to remove larval specializations (Schubiger, 2003).
Regressive events that refine exuberant or inaccurate connections are
critical in neuronal development. Multi-photon, time-lapse imaging was used to
examine how dendrites of Drosophila dendritic arborizing (da) sensory neurons
are eliminated during early metamorphosis, and how intrinsic and extrinsic
cellular mechanisms control this deconstruction. Removal of the larval dendritic
arbor involves two mechanisms: local degeneration and branch retraction. In
local degeneration, major branch severing events entail focal disruption of the
microtubule cytoskeleton, followed by thinning of the disrupted region, severing
and fragmentation. Retraction was observed at distal tips of branches and in
proximal stumps after severing events. The pruning program of da neuron
dendrites is steroid induced; cell-autonomous dominant-negative inhibition of
steroid action blocks local degeneration, although retraction events still
occur. The data suggest that steroid-induced changes in the epidermis may
contribute to dendritic retraction. Finally, it was found that phagocytic blood cells
not only engulf neuronal debris but also attack and sever intact branches that
show signs of destabilization (Williams, 2005).
The abdominal sensory system of the adult fly is a mosaic consisting of
postembryonic neurons derived from the imaginal histoblast nests and genital
disc, and a subset of larval sensory neurons of embryonic origin. At
the onset of metamorphosis, larval sensory neurons either die or survive and
remodel. Among the dorsal neurons of abdominal segments 2-5, it was found that dorsal dendritic arborizing
ddaA, ddaF and ddaB die whilst ddaC, ddaD and ddaE survive (Williams, 2005).
Programmed cell death is a near ubiquitous phenomenon in the vertebrate
nervous system and essential for proper morphogenesis.
Surprisingly, most studies have focused on the cell body and paid little
attention to the fate of neuritic processes, even though the axons of most
projection neurons have reached their targets before the onset of cell death.
Time-lapse imaging was used to observe the dendrites of dying neurons in
intact animals. The distal branches of the ddaF develop swellings at 2 h APF
that become distinct beads, which accumulate tubulin. Between 3 and 4 h APF
branches undergo severing at multiple sites where they have thinned. This
beading propagates through the arbor in a proximal to distal direction with
proximal changes occurring 20-30 minutes ahead of the distal ones. This same
progression is seen in ddaB but with a lag of 1 hour. A similar degeneration
is found in the peripheral arbors of Rohon-Beard neurons, which also thin and
fragment when dying (Williams, 2005).
The destruction of peripheral processes has been most widely studied
following acute local trauma, where the axon branch distal to the lesion
undergoes a stereotyped degeneration by fragmentation, termed Wallerian
degeneration (Waller, 1850). There
are similarities between the degeneration of dendrites in the current study and the
axon pathology seen in Wallerian degeneration. (1) In vitro models of
Wallerian degeneration reveal a proximal to distal spread of beading in the
separated distal neurites. (2) The microtubule cytoskeleton is disrupted
following transection and the beaded morphology and redistribution of tubulin
seen in the da neuron dendrites resembles that seen in neurons treated with
microtubule depolymerizing drugs (Williams, 2005).
The expression of EcRDN in neurons that are fated
to die blocks their death and prevents the local degeneration seen between 3
and 6 h APF. This prompted an inquiry as to whether the degeneration
normally observed is due to cell death directly or is the result of a
hormone-induced microtubule disassembly program running in parallel with cell
death. When cell death was blocked by expressing p35, the dendrites showed no
signs of degeneration by 5 h APF, yet by 18 h APF their arbor was largely removed.
Thus, with p35, the
dendrites of these doomed cells prune at the same time and in the same way as
ddaD and ddaE, suggesting that under normal conditions, death
directly causes the dendrite degeneration observed in ddaA, ddaF and ddaB at
3-6 h APF. Similar observations made after killing da neurons by laser
ablation support this proposition (Williams, 2005).
Pruning of the dendrites of ddaD and ddaE starts between 6 and 10 h APF and
is largely complete by 24 h APF. Time-lapse movies reveal that dendrites undergo
deconstruction by two different cellular mechanisms: local degeneration and
branch retraction (Williams, 2005).
During local degeneration, branches are detached from the main arbor and
undergo fragmentation. Axons and dendrites of mushroom body
neurons undergo local degeneration: between 4 and 6 h APF the processes
undergo blebbing, and by 8 h APF most of the dendritic arbor has been removed.
By 18 h APF the axons have fragmented, and during that time there is no
evidence to suggest that gamma neurons undergo retraction. Similarly, larval
projection neurons in the olfactory lobe undergo local degeneration (Williams, 2005).
A key step in local degeneration appears to be severing, which it was possible
to visualize with time-lapse movies. Severing happens in one of two ways,
depending on position within the arbor. In proximal regions, severing is
preceded by thinning.
Once a branch has thinned, it severs and the stump retracts, while the
separated arbor undergoes fragmentation. It was found that beads adjacent to
thinning regions contain abundant Futsch and tub::GFP, whereas these are
lacking in the thinned regions, suggesting that the bulk of the microtubule
cytoskeleton is lost in these focal regions. This supports observations
that myc-marked tubulin disappears from pruning gamma
neurons before the axon is lost. Similarly, it has been shown that
just prior to shedding 'axosomes', the proximal region of a retracting
axon becomes devoid of organized microtubules. When neurons express
EcRDN, the caliber of the proximal branches does not change,
suggesting that a redistribution of the microtubule cytoskeleton fails and
proximal severing is subsequently blocked (Williams, 2005).
The second type of severing occurs at more distal sites within the arbor.
Here there is no
thinning and only occasional beading; thus it is possible that different
mechanisms are responsible for severing at distal sites. Nevertheless, distal
severing is also suppressed in neurons that express EcRDN,
suggesting that a destabilization is also important here (Williams, 2005).
After severing, both proximal and distally detached branches undergo
fragmentation. The branches thin while beads form along their length and break at multiple
sites, generating GFP blebs that are removed by phagocytes.
Although most of the events observed in branch fragmentation during pruning
appear similar to those seen in the arbors of dying da neurons, an exception
is the appearance of filopodia. When the
microtubule cytoskeleton and the membrane were simultaneously imaged in pruning neurons, it was often found that
filopodia are coincident with the areas of microtubule disorganization. Lateral filopodia
have been found when colcemid was applied to chick neurons in culture.
Filopodia do not appear on the arbors of the doomed cells since the cells are
undergoing programmed cell death. However, when cell death is blocked by
expression of p35 these cells later show abundant filopodia while their dendrites
are removed. This
suggests a fundamental difference between the dendrite removal observed during
the pruning of ddaD and ddaE, and that seen in cells that undergo programmed
cell death (Williams, 2005).
The primary branches of ddaD and ddaE can be retracted. These retraction
events occurred in 6 of 17 neurons imaged and the branches of such neurons often
possessed many filopodia. When retraction events happen, local degeneration
may still occur in secondary branches and on other primary branches nearby. It
is somewhat problematic to apportion what percentage of pruning is due to
retraction and what is due to severing and fragmentation, since they both appear
to happen simultaneously, e.g. where a branch is in the process of distal
retraction and suddenly is severed at a more proximal site (Williams, 2005).
The dendrites of neurons expressing EcRDN form bulb-like
structures on their distal tips and produce few if any filopodia. Local
degeneration is blocked in these neurons, but they do eventually remove their
dendrites by retraction. A number of scenarios could explain this retraction
phenotype. It is possible that EcRDN does not block all ecdysone
signaling and so the cell intrinsic ecdysone pruning program has not been
entirely eliminated. It is also possible, but unlikely, that the
EcRDN results in a non-specific 'neomorphic' since the specificity of
EcRDN has been demonstrated. The
other possibility is that EcRDN completely blocks ecdysone
signaling and that another parallel intrinsic signaling pathway plays a role
in dendrite retraction. It is most likely that EcRDN completely
blocks ecdysone signaling and that the phenotype that is seen reveals extrinsic
factors that are important for dendrite pruning under wild-type conditions.
The neuron-specific expression of EcRDN means that ecdysone
signaling is disrupted only in the neuron and that the local environment can
undergo its normal hormone induced changes (Williams, 2005).
A dorsal region of the epidermis is the target for the dendrites of ddaD
and ddaE. Two potential interactions could be taking place between the target
and pruning arbor. The epidermis could be sending an instructive signal
causing the arbor to prune or there could be a passive loss of epidermal
contacts as the larval epidermis is replaced. To explore the latter
possibility, movies were made at the time when ddaD and ddaE are pruning. It has
been widely held that the larval epidermal cells are removed only by the
migrating front of adult epidermal cells derived from the histoblast nest.
Surprisingly it was found that during early metamorphosis larval cells are
removed at sites distant from the histoblast migration front. The epidermal cells
are removed by phagocytic blood cells and the cells that are removed do not
show obvious signs of apoptosis before being contacted by the phagocyte. This
suggests that the larval cells are killed by a phagocytosis-induced cell death
as described in C. elegans and
Drosophila. Importantly, when a larval cell is removed, other cells
move to compensate for its loss. Thus the epidermis becomes a dynamic
substrate during prepupal and early pupal development, this movement may
result in shearing events in distal dendritic branches and could explain the
phenomenon of distal severing without thinning. Likewise, the disruption of
neuron-epidermal contacts may contribute to the retraction of primary branches
seen in an extreme form when neurons express the EcRDN (Williams, 2005).
Drosophila Set2 encodes a developmentally essential histone H3 lysine 36 (K36) methyltransferase. Larvae subjected to RNA interference-mediated (RNAi) suppression of Set2 lack Set2 expression and H3-K36 methylation, indicating that Set2 is the sole enzyme responsible for this modification in Drosophila. Set2 RNAi blocks puparium formation and adult development, and causes partial (blister) separation of the dorsal and ventral wing epithelia, defects suggesting a failure of the ecdysone-controlled genetic program. A transheterozygous EcR null mutation/Set2 RNAi combination produces a complete (balloon) separation of the wing surfaces, revealing a genetic interaction between the Ecdysone receptor (EcR) and Set2. Immunoprecipitation studies demonstrated that Set2 associates with the hyperphosphorylated form of RNA polymerase II (RNAPII) (Stabell, 2007).
The expression level of Set2 during development was determined by RT-PCR. Set2 is expressed throughout the life cycle of Drosophila. Whereas the level of expression is rather low in 0-6 h embryos it increases to an apparently steady level after that time-point, with a noticeable peak in late third instar larvae. It is interesting to note that the major expression of Set2 in adults is in the gonads (Stabell, 2007).
To investigate the function of Set2, RNA interference (RNAi) was used to lower Set2 mRNA and protein levels. Fly stocks were generated carrying the responder transposon UAS-dSet2.IR, in which a Gal4-dependent promoter expresses the inverted repeat of Set2 to yield a double-stranded RNA molecule. The pUdsGFP vector used for this purpose has an independent UAS-GFP marker so that a tissue exposed to RNAi will simultaneously show GFP expression. Different driver stocks, each expressing Gal4 in a temporally and spatially distinct developmental domain, were selected, and the stocks were crossed to yield hybrid progeny with targeted UAS-dSet2.IR expression (Stabell, 2007).
Initially, a salivary gland-specific driver, Sgs3-Gal4 was used. This allowed immunostaining to be used against Set2 and H3-K36Me, and at the same time visualization of nuclei with DAPI staining and GFP fluorescence. Immunostaining of wild type salivary glands with anti-Set2 and anti-H3K36Me, respectively, showed that both Set2 and dimethylated of H3-K36 are present in the cells. The effect of RNAi knockdown of Set2 by the Sgs3-Gal4 driver was analyzed by immunostaining. Salivary glands from UAS-dSet2/Sgs3-Gal4 larvae were dissected at 3 ± 1 h before puparium formation and immunostained with anti-Set2, anti-dimethyl H3-K4, anti-dimethyl H3-K27, anti-dimethyl H3-K36, anti-H3-9Me, and anti-acetyl H3-K9. Immunostaining with anti-Set2 reveals the absence of Set2 protein in salivary gland cells where UAS-dSet2.IR is expressed by the Sgs3-Gal4 driver. During this work two nuclei in one salivary gland lobe (out of 10 pairs) were found that did not express GFP but were clearly detectable by DAPI staining. Immunostaining with anti-H3-K36Me reveals the absence of H3-K36 dimethylation in salivary gland cells where UAS-dSet2.IR is expressed. Most importantly, however, UAS-dSet2.IR is not induced in the two salivary gland cells not expressing GFP, and here the H3-K36 dimethylation is clearly observable. Furthermore, fat body cells, where the Sgs3-Gal4 driver is not expressed, also show H3-K36 dimethylation in all nuclei. The reason to why the two cells have lost the ability to express the GFP-tagged UAS-dSet2.IR construct may be either loss of UAS or Gal4 sequences or the whole construct(s) prior to the onset of polyploidization. Methylation and acetylation of H3-K9, respectively, are unaffected by the absence of Set2. This also is true for H3-K4 and H3-K27, thus showing the specificity of Set2 RNAi. It is concluded that the Set2 gene encodes a HKMT responsible for all detectable H3-K36 dimethylation in Drosophila (Stabell, 2007).
Set2 interacts genetically with the ecdysone receptor: To test whether a knock down of Set2 affects the fly, the ubiquitously expressed da-Gal4 driver was crossed with UAS-dSet2.IR flies. All progeny are da-Gal4/UAS-dSet2.IR and develop normally until the end of the third larval instar. However, these RNAi larvae do not form their puparium and crawling larvae are found after 10-12 days. The larvae finally stop moving, and in a few cases melanin-less 'pseudo-prepupae' are formed, which maintain the elongated larval form and fail to evert the anterior spiracles. These observations may indicate a defect in ecdysone responses at puparium formation, similar to those reported for mutants in the ecdysone pathways, and thus implicates Set2 in this regulatory cascade (Stabell, 2007).
The defects in puparium formation seen in Set2 RNAi animals could result from either a decrease in the ecdysone titer or a decrease in the ability of the ecdysone signal to be transduced. To distinguish between these possibilities the effects were examined of feeding ecdysone to Set2 RNAi larvae. This method has been shown to effectively rescue phenotypes associated with ecdysone-deficient mutations. Mid- and late-third instar larvae were transferred to food either with or without 20-hydroxyecdysone (20E) for 6-8 h and scored on a 12 h basis. Feeding 20E to Set2 RNAi larvae did not rescue them to puparium formation. Therefore, it is concluded that ecdysone is not limiting in the da-Gal4/UAS-dSet2.IR animals and that Set2 functions downstream of ecdysone biosynthesis and release (Stabell, 2007).
The development of a flat bilayered wing from an imaginal disc monolayer involves four steps that occur twice during metamorphosis. The timing of these two rounds of wing morphogenesis correlates with the two main ecdysone peaks during metamorphosis; namely during mid- and late prepupal stages, and from 24 to 72 h after puparium formation, and it has been shown that ecdysone controls wing morphogenesis and cell adhesion by regulating integrin expression during metamorphosis. The effects of EcR mutations on integrin transcription and the essential role for integrins during wing morphogenesis are both principally restricted to the pupal stages. To investigate if Set2 might be involved in wing morphogenesis, Set2 RNAi was triggered in the wing discs by the ap-Gal4 driver. Induction of Set2 RNAi by the ap-Gal4 driver generates a blister in the proximal (towards thorax) part of the wings. The transparent blisters are initially large balloons filled with air and hemolymph that deflate after a while, leaving a scar that may cause the wing to bend up. This phenotype shows 100% penetrance (Stabell, 2007).
Ecdysone exerts its effects on development through a heterodimer of two nuclear receptors, encoded by EcR and usp. The ecdysone/EcR/USP complex then directly activates cascades of gene expression. To assess if Set2 and EcR genetically interacts in a common pathway controlling wing morphogenesis, whether the EcR null mutation EcRM554fs could dominantly affect the blistered wing phenotype of ap-Gal4/+;UAS-dSet2.IR/+ flies described above was tested. Therefore, the stock y w;EcRM554fs/CyO;UAS-dSet2.IR was synthesized and crossed reciprocally to y w;ap-Gal4/CyO flies. EcRM554fs dramatically enhances the blistered wing phenotype in all EcRM554fs/ap-Gal4;UAS-dSet2.IR/+ flies. The wing layers were completely separated (ballooned) and often one or both wings were held in a Dichaete-like fashion. The balloon soon becomes blackened and remains inflated for 1-2 days. Again, all flies of the genotype ap-Gal4/CyO;UAS-dSet2.IR/+ have the blistered wing phenotype. It should also be mentioned that heterozygous EcRM554fs/CyO;UAS-dSet2.IR/+ flies have normal wings. Taken together, these results suggest that Set2 and EcR may function in a common ecdysone signaling pathway during wing morphogenesis, and that this interaction may be dose sensitive (Stabell, 2007).
These results prompted an analysis by RT-PCR of the ecdysone receptor genes DHR3, DHR4 and reaper (rpr) in dSet2.IR animals around pupariation. While an apparently normal expression pattern for DHR3, DHR4 and rpr was observed at the same time points in control salivary glands, these genes show a clear decline in mRNA levels in Set2 RNAi salivary glands. Both DHR3 and DHR4 execute essential functions during development, including at the onset of metamorphosis. The parallel functions of these two nuclear receptors suggest that they act together to direct the switch from late-larval to prepupal genetic programmes. Although these results were obtained from Set2 RNAi in salivary glands it is believed that these genes, and probably several others, also are affected in da-Gal4/UAS-dSet2.IR larvae, thus explaining the failure of these larvae to pupariate. The fact that none of the studied genes is fully down regulated also suggests that the non-pupariating phenotype may be ascribed to a collective down regulation of essential genes involved in the larval-to-pupal transition. It is also interesting to note the decreased level of rpr mRNA, which is significant at +2 h. Although rpr is not involved in metamorphosis other than in programmed cell death, its down regulation supports the notion that Set2 is linked to the ecdysone regulatory hierarchy (Stabell, 2007).
Recent reports have shown that Set2 from various organisms binds to the hyperphosphorylated CTD of RNAPII, implying that K36 methylation plays an important role in the transcription elongation process. The presence of both the WW and SRI domains suggested that Set2 may associate with RNAPII also in Drosophila. To address this issue, extracts prepared from control or Set2 RNAi embryos were immunoprecipitated with antibodies directed against Ser5-phosphorylated CTD, followed by immunoblotting with antibodies directed against Set2 and Ser5-phosphorylated CTD form of RNAPII. Immunoprecipitation of Ser5-phosphorylated CTD resulted in strong immunoreactivity of both phosphorylated CTD and Set2 in control embryos whereas no Set2 is detected in extracts from RNAi embryos. This result was corroborated by showing co-localization of Set2 and elongating RNAPII on salivary gland chromosomes. While these results demonstrate that Set2 is associated with the elongating form of RNAPII in Drosophila, the precise role of this association is currently unclear. However, the fact that a loss of Set2/K36 methylation results in mutant phenotypes associated with defects in the ecdysone response indicates that Set2/K36 methylation plays an important role in the ecdysone regulatory hierarchy (Stabell, 2007).
Ecdysone receptor:
Biological Overview
| Evolutionary homologs
| Bombyx and Manduca prothoracicotropic hormone
| Regulation
| Targets of Activity
| Protein interactions
| Developmental Biology
| References
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