Gene name - ftz transcription factor 1
Synonyms - alphaftz-f1 - betaftz-f1
Cytological map position - 75C1--75D8
Function - transcription factor
Symbol - ftz-f1
Genetic map position - 3-
Classification - orphan nuclear receptor - zinc finger motif
Cellular location - nuclear
|Recent literature||Borsos, B. N., Pankotai, T., Kovacs, D., Popescu, C., Pahi, Z. and Boros, I. M. (2015). Acetylations of Ftz-F1 and histone H4K5 are required for the fine-tuning of ecdysone biosynthesis during Drosophila metamorphosis. Dev Biol 404(1):80-7. PubMed ID: 25959239
The molting during Drosophila development is tightly regulated by the ecdysone hormone. Several steps of the ecdysone biosynthesis have been already identified but the regulation of the entire process has not been clarified yet. Studies have shown that the dATAC histone acetyltransferase complex is necessary for the steroid hormone biosynthesis process. To reveal possible mechanisms controlled by dATAC assumptions were made that either dATAC may influence directly the transcription of Halloween genes involved in steroid hormone biosynthesis or it may exert an indirect effect on it by acetylating the Ftz-F1 transcription factor which regulates the transcription of steroid converting genes. This study shows that the lack of dATAC complex results in increased mRNA level and decreased protein level of Ftz-F1. In this context, decreased mRNA and increased protein levels of Ftz-F1 were detected upon treatment of Drosophila S2 cells with histone deacetylase inhibitor trichostatin A. Ftz-F1, the transcriptional activator of Halloween genes, is acetylated in S2 cells. In addition, ecdysone biosynthetic Halloween genes were found to be transcribed in S2 cells and their expression can be influenced by deacetylase inhibitors. Furthermore, H4K5 acetylation was detected at the regulatory regions of disembodied and shade Halloween genes, while H3K9 acetylation is absent on these genes. Based on these findings it is concluded that the dATAC HAT complex might play a dual regulatory role in Drosophila steroid hormone biosynthesis through the acetylation of Ftz-F1 protein and the regulation of the H4K5 acetylation at the promoters of Halloween genes.
|Kulshammer, E., Mundorf, J., Kilinc, M., Frommolt, P., Wagle, P. and Uhlirova, M. (2015). Interplay among Drosophila transcription factors Ets21c, Fos and Ftz-F1 drives JNK-mediated tumor malignancy. Dis Model Mech 8: 1279-1293. PubMed ID: 26398940
This study defines TF network that triggers an abnormal gene expression program promoting malignancy of clonal tumors, generated in Drosophila imaginal disc epithelium by gain of oncogenic Ras (RasV12) and loss of the tumor suppressor Scribble (scrib1). Malignant transformation of the rasV12scrib1 tumors requires TFs of distinct families, namely the bZIP protein Fos, the ETS-domain factor Ets21c and the nuclear receptor Ftz-F1, all acting downstream of Jun-N-terminal kinase (JNK). Depleting any of the three TFs improves viability of tumor-bearing larvae, and this positive effect can be enhanced further by their combined removal. Although both Fos and Ftz-F1 synergistically contribute to rasV12scrib1 tumor invasiveness, only Fos is required for JNK-induced differentiation defects and Matrix metalloprotease (MMP1) upregulation. In contrast, the Fos-dimerizing partner Jun is dispensable for JNK to exert its effects in rasV12scrib1 tumors. Interestingly, Ets21c and Ftz-F1 are transcriptionally induced in these tumors in a JNK- and Fos-dependent manner, thereby demonstrating a hierarchy within the tripartite TF network, with Fos acting as the most upstream JNK effector. Of the three TFs, only Ets21c can efficiently substitute for loss of polarity and cooperate with Ras(V12) in inducing malignant clones that, like rasV12scrib1 tumors, invade other tissues and overexpress MMP1 and the Drosophila insulin-like peptide 8 (Dilp8). While rasV12ets21c tumors require JNK for invasiveness, the JNK activity is dispensable for their growth. In conclusion, this study delineates both unique and overlapping functions of distinct TFs that cooperatively promote aberrant expression of target genes, leading to malignant tumor phenotypes.
|Akagi, K., Sarhan, M., Sultan, A. R., Nishida, H., Koie, A., Nakayama, T. and Ueda, H. (2016). A biological timer in the fat body comprised of Blimp-1, betaFTZ-F1 and Shade regulates pupation timing in Drosophila melanogaster. Development [Epub ahead of print]. PubMed ID: 27226323
During the development of multicellular organisms, many events occur with precise timing. In Drosophila, pupation occurs about 12 hours after puparium formation, and its timing is believed to be determined by the release of a steroid hormone, ecdysone (E), from the prothoracic gland. This study demonstrates that the ecdysone-20-monooxygenase, Shade, determines the pupation timing by converting E to 20-hydroxyecdysone (20E) in the fat body, which is the organ that senses nutritional status. The timing of shade expression is determined by its transcriptional activator βFTZ-F1. The βFTZ-F1 gene is activated after a decline in the expression of its transcriptional repressor Blimp-1, which is temporally expressed around puparium formation in response to a high titer of 20E. The expression level and stability of Blimp-1 is critical for the precise timing of pupation. Thus, it is proposed that Blimp-1 molecules function as sands in an hourglass for this precise developmental timer system. Furthermore, the data suggest a biological advantage results from both the use of a transcriptional repressor for the time determination, and association of developmental timing with nutritional status of the organism.
|Field, A., Xiang, J., Anderson, W. R., Graham, P. and Pick, L. (2016). Activation of Ftz-F1-responsive genes through Ftz/Ftz-F1 dependent enhancers. PLoS One 11: e0163128. PubMed ID: 27723822
The orphan nuclear receptor Ftz-F1 is expressed in all somatic nuclei in Drosophila embryos, but mutations result in a pair-rule phenotype. This was explained by the interaction of Ftz-F1 with the homeodomain protein Ftz that is expressed in stripes in the primordia of segments missing in either ftz-f1 or ftz mutants. Ftz-F1 and Ftz were shown to physically interact and coordinately activate the expression of ftz itself and engrailed by synergistic binding to composite Ftz-F1/Ftz binding sites. However, attempts to identify additional target genes on the basis of Ftz-F1/ Ftz binding alone has met with only limited success. To discern rules for Ftz-F1 target site selection in vivo and to identify additional target genes, a microarray analysis was performed comparing wildtype and ftz-f1 mutant embryos. Ftz-F1-responsive genes most highly regulated included engrailed and nine additional genes expressed in patterns dependent on both ftz and ftz-f1. Candidate enhancers for these genes were identified by combining BDTNP Ftz ChIP-chip data with a computational search for Ftz-F1 binding sites. Of eight enhancer reporter genes tested in transgenic embryos, six generated expression patterns similar to the corresponding endogenous gene and expression was lost in ftz mutants. These studies identified a new set of Ftz-F1 targets, all of which are co-regulated by Ftz. Comparative analysis of enhancers containing Ftz/Ftz-F1 binding sites that were or were not bona fide targets in vivo suggested that GAF negatively regulates enhancers that contain Ftz/Ftz-F1 binding sites but are not actually utilized. These targets include other regulatory factors as well as genes involved directly in morphogenesis, providing insight into how pair-rule genes establish the body pattern.
FTZ-F1 was first identified as a protein that binds to the zebra element, a 740 base pair DNA sequence upstream of the fushi tarazu (ftz) transcriptional start site. The zebra element is responsible for regulating fushi tarazu expression in seven stripes of alternating segment (pair rule) periodicity in the early embryo. Mutational alteration in the FTZ-F1 binding site results in a lack of expression of ftz in stripe 1, weaker, but detectable expression in stripes 2, 3 and 6, and normal staining in stripes 4, 5 and 7.
FTZ-F1 is found in both early and late forms, corresponding to an early protein found during the first few hours of embryonic development and a late protein, migrating on gels at a faster rate, appearing between 16 and 19 hours of development. DNA binding specificity cannot be distinguished between early and late forms. Besides binding to several sites in the zebra element, FTZ-F1 binds to additional sites within the ftz gene (Ueda, 1989)
FTZ-F1 also influences molting. Chromosomal puffing in the late prepupal salivary gland polytene chromosomes allows for an assessment of the order of gene activation. Puffing is evidence of a 'loosening' of the chromatin holding a gene together, and is thought to accompany gene activation. FTZ-F1 is directly involved in the regulation of the gene activation hierarchy in salivary gland chromosomes. Antibodies directed against FTZ-F1 protein detect staining of 166 loci in the late prepupal salivary gland polytene chromosomes, suggesting that FTZ-F1 regulates transcription of many genes active in polytene chromosomes. It is presumed that FTZ-F1 functions to regulate expression of the gene puffs to which it binds. 51 of these loci represent ecdysone-regulated puffs. Of 33 puffs that show increased activity after the peak of the 75CD puff (responsible for FTZ-F1 synthesis), 17 show reproducable staining for FTZ-F1. These include two prominent late prepupal puffs (74EF and 75B) encoding respectively ets-related and steroid receptor superfamily DNA binding proteins. These late prepupal puffs occur in the latter part of the early phase of the puffing hierarchy. Both 74EF and 75B are induced directly by the late larval and prepupal pulses of ecdysone. These results suggest that FTZ-F1 contributes to a significant fraction of the genes in the late prepupal phase of the molting regulatory hierarchy. Of interest is the observation that FTZ-F1 binds to the 75CD puff itself, raising the possibility of an autoregulatory interaction. Among the 25 puffs that show decreased activity in late prepupae, 7 are bound by FTZ-F1. It is therefore possible that FTZ-F1 may also participate in the repression of these puff loci (Lavorgna, 1993).
Where does FTZ-F1 fit into the hierarchy of regulatory genes expressed during metamorphosis? It has been suggested that its action may serve as a bridge between early and late gene expression during the process of metamorphosis. Early genes include Ecdysone receptor, the master regulator whose dimerization partner is Ultraspiracle. A third early protein, coded for by E75A, is another nuclear receptor superfamily member. These proteins are induced during the third instar larval period beginning during the fourth day of fly development. These early genes both repress their own expression and induce a large set of late genes. The induction of two late genes, E78A and DHR3, is delayed, relative to that of the early genes, apparently owing to an additional requirement for early ecdysone-induced protein synthesis. FTZ-F1, expressed during the fifth day of fly development, is repressed by both itself and ecdysone, thus restricting its expression to the brief interval of low ecdysone titer in midprepupae. FTZ-F1 appears to provide the competence for the "early genes" E54A, E75A, Broad Complex and E93 (an early gene expressed later in prepupal development at 5.5 to 6 days). These latter genes regulate the expression of late genes, expressed in late prepupal development and the pupal phase, beginning 5.5 days after fertilization (Woodard, 1994 and Thummel, 1995). Thus FTZ-F1 acts as a bridge between expression of the earliest genes involved in metamorphosis (Ecdysone receptor and E75) and the late genes.
Many of the 21 members of the nuclear receptor superfamily in Drosophila are transcriptionally regulated by the steroid hormone ecdysone and play a role during the onset of metamorphosis, including the EcR/USP ecdysone receptor heterodimer. The temporal patterns of expression for all detectable nuclear receptor transcripts were examined throughout major ecdysone-regulated developmental transitions in the life cycle: embryogenesis, a larval molt, puparium formation, and the prepupal-pupal transition. An unexpected close temporal relationship was found between DHR3, E75B, and betaFTZ-F1 expression after each major ecdysone pulse examined, reflecting the known cross-regulatory interactions of these genes in prepupae and suggesting that they act together at other stages in the life cycle. In addition, E75A, E78B, and DHR4 are expressed in a reproducible manner with DHR3, E75B, and betaFTZ-F1, suggesting that they intersect with this regulatory cascade. Finally, known ecdysone-inducible primary-response transcripts are coordinately induced at times when the ecdysteroid titer is low, implying the existence of novel, as yet uncharacterized, temporal signals in Drosophila (Sullivan, 2003).
Total RNA was isolated from two independent collections of embryos staged at 2-h intervals throughout the 24 h of Drosophila embryonic development. Five Northern blots were prepared using equal amounts of RNA from each time point. These blots were sequentially hybridized, stripped, and rehybridized with radioactive probes derived from each of the 21 nuclear receptor genes encoded by the Drosophila genome. This approach allowed the generation of time courses of nuclear receptor gene expression that could be directly compared between family members. The transcripts detected are consistent with reported sizes. Transcripts from eight nuclear receptor genes were not detectable during embryonic development: E75C, E78, CG16801, DHR38, DHR83, dsf, eg, and svp (Sullivan, 2003).
Transcripts from nine nuclear receptor genes can be detected at the earliest time point (0-2 h): usp, EcR-A, FTZ-F1, DHR39, DHR78, DHR96, dERR, dHNF-4, and tll. This expression is consistent with the known maternal contribution of usp, EcR, and FTZ-F1. The observation that transcripts from DHR39, DHR78, DHR96, dERR, and dHNF-4 are undetectable by the next time point examined (2-4 h) suggests that these mRNAs are maternally loaded and rapidly degraded. EcR-B and usp transcripts are induced in early embryos, up-regulated at 6-8 h after egg laying (AEL), and maintain expression through the end of embryogenesis, with down-regulation of EcR-B in late embryos. EcR-A, in contrast, is expressed for a relatively brief temporal window, at 8-14 h AEL (Sullivan, 2003).
Six nuclear receptor genes are expressed in brief intervals during midembryonic stages. DHR39 and E75A are initially induced at 4-6 and 6-8 h AEL, respectively, and peak at 8-12 h AEL. This is followed by induction of DHR3, DHR4, and E75B at 8-12 h AEL, followed by ßFTZ-F1 expression at 12-18 h AEL. DHR39 appears to exhibit an expression pattern reciprocal to that of ßFTZ-F1, with lowest levels of mRNA at 14-16 h AEL and reinduction at 16-18 h as ßFTZ-F1 is repressed. This is followed by a second peak of E75A transcription at 18-22 h AEL (Sullivan, 2003).
A second group of nuclear receptors, DHR78, DHR96, dHNF-4, and dERR, is more broadly expressed at low levels throughout embryogenesis. DHR78 accumulates above its constant low level of expression between 8 and 14 h AEL. dERR exhibits an apparent mRNA isoform switch between 14 and 18 h AEL. dHNF-4 regulation also appears complex, with two size classes of mRNA induced at approximately 8-10 h AEL. While the 4.6-kb dHNF-4 mRNA is expressed throughout embryogenesis, the 3.3-kb mRNA is down-regulated at 14-16 h AEL. This timing is consistent with observations that dHNF-4 is expressed primarily in the embryonic midgut, fat body, and Malpighian tubules. Finally, nuclear receptors known to exert essential functions in patterning the early embryo, tll, kni, and knrl, are expressed predominantly during early stages (Sullivan, 2003).
Two genes that are not members of the nuclear receptor superfamily, BR-C and E74, were also examined in this study, as transcriptional markers for ecdysone pulses during development. Unexpectedly, both of these genes are induced late in embryogenesis, several hours after the rise in ecdysone titer at 6 h AEL. An approximately 7-kb BR-C transcript is induced at 10-12 h AEL and is present through the end of embryogenesis while E74B is induced at 14-16 h AEL and repressed as E74A is expressed from 16-20 h AEL. This BR-C expression pattern is consistent with the identification of the BR-C Z3 isoform in specific neurons of the embryonic CNS (Sullivan, 2003).
First-instar larvae were synchronized as they molted to the second instar, aged and harvested at 4-h intervals throughout second-instar larval development. Two Northern blots were prepared using equal amounts of total RNA isolated from a single collection of animals. Each blot was sequentially hybridized, stripped, and rehybridized to detect nuclear receptor transcription. The following transcripts were not detectable during the second instar: E75C, dERR, CG16801, DHR83, dsf, eg, svp, tll, kn, and knrl (Sullivan, 2003).
EcR-B expression is induced in mid-second-instar larvae, but does not reach maximum levels until 68-72 h AEL, just before the molt. In contrast, usp is expressed throughout the instar. A sequential pattern of nuclear receptor expression is observed that resembles the pattern seen in midembryogenesis. DHR39 and E75A are expressed in the early second instar. This is followed by induction of E75B, E78B, DHR3, and DHR4, followed by expression of ßFTZ-F1 at the end of the instar. DHR39 again shows a pattern that is approximately reciprocal with ßFTZ-F1, with highest levels during the first half of the instar. Similarly, DHR78, DHR96, and dHNF-4 exhibit broad expression patterns throughout second-instar larval development. E74A, E75A, and DHR38 are coordinately up-regulated with EcR-B at the end of the instar, between 64-72 h AEL. Finally, an approximately 9-kb BR-C transcript is detected throughout the second-larval instar (Sullivan, 2003).
Nuclear receptor gene expression was also examined throughout the third larval instar and into the early stages of metamorphosis, encompassing the ecdysone-triggered larval-to-prepupal and prepupal-to-pupal transitions. Third-instar larvae were staged relative to the molt from the second instar and harvested at 4-h intervals throughout the 48 h of the instar. Prepupae were synchronized relative to puparium formation (±15 min) and harvested at 2-h intervals up to 16 h after puparium formation (APF). Total RNA was isolated from whole animals and analyzed by Northern blot hybridization. Five blots were prepared from two independent collections of animals. These blots were sequentially hybridized, stripped, and rehybridized to detect nuclear receptor gene expression. The following transcripts were not detectable during third-instar larval or prepupal stages: CG16801, DHR83, dsf, eg, svp, tll, kn, and knrl (Sullivan, 2003).
Most nuclear receptor genes show little or no detectable expression in early and mid-third-instar larvae, a time when the ecdysone titer is low. Similar to the pattern seen in second-instar larvae, usp is expressed at relatively low levels throughout the instar and up-regulated at puparium formation, while EcR-B is induced at approximately 100 h AEL and rapidly down-regulated at puparium formation. This is followed by a sequential pattern of nuclear receptor expression similar to that seen at earlier stages. DHR39, E75A, and E78B are induced at 116-120 h AEL, in concert with the late larval ecdysone pulse, followed by maximum accumulation of E75B, DHR3, and DHR4 at 0-4 h APF. ßFTZ-F1 is expressed from 6-10 h APF, with a pattern that is approximately reciprocal to that of DHR39. EcR-A is expressed in parallel with E75B, DHR3, and DHR4 in midprepupae, similar to their coordinate expression during embryogenesis (Sullivan, 2003).
DHR78, DHR96, and dHNF-4 continue to exhibit broad expression profiles throughout third-instar larval and prepupal development. An E75 isoform not detected in embryos or second-instar larvae, E75C, is also detectable at low levels throughout most of the third instar and up-regulated in correlation with the late-larval and prepupal pulses of ecdysone. DHR38 is detectable at very low levels in early third-instar larvae, in synchrony with the early induction of E74B and BR-C. E74B is repressed, E74A is induced, and BR-C transcripts are up-regulated in late third-instar larvae, in synchrony with the late-larval ecdysone pulse. The prepupal pulse of ecdysone occurs at 10-12 h APF, marking the prepupal-to-pupal transition. EcR-A, E75A, E78B, DHR4, dERR, E75C, dHNF-4, and E74A are all induced at 10-12 h APF, in apparent response to this hormone pulse. These results are consistent with a microarray analysis of gene expression at the onset of metamorphosis where the temporal profiles of about half of these genes have been reported (Sullivan, 2003).
Most nuclear receptors can be divided into one of four classes based on this study: (1) those that are expressed exclusively during early embryogenesis (kni, knrl, tll); (2) those that are expressed throughout development (usp, DHR78, DHR96, dHNF-4); (3) those that are expressed in a reproducible temporal cascade at each stage tested (E75A, E75B, DHR3, DHR4, FTZ-F1, DHR39), and (4) those that are undetectable in these assays (CG16801, DHR83, dsf, eg, svp) (Sullivan, 2003).
Three nuclear receptor genes appear to be expressed exclusively during early embryogenesis: kni, knrl, and tll. This restricted pattern of expression fits well with the functional characterization of these genes, which have been shown to act as key determinants of embryonic body pattern. Eight genes (usp, EcR, FTZ-F1, DHR39, DHR78, DHR96, dERR, and dHNF-4) were identified that appear to have maternally deposited transcripts and thus possible embryonic functions. Indeed, maternal functions have been defined for usp, EcR, and alphaFTZ-F1 (Sullivan, 2003).
Four nuclear receptor genes are broadly expressed through all stages examined: usp, DHR78, DHR96, and dHNF-4. dHNF-4 mRNA is first detectable at 6-10 h AEL, as the ecdysone titer begins to rise. In addition, peaks of dHNF-4 expression are seen at 0, 12, and 16 h APF, in synchrony with the E74 and E75C early ecdysone-inducible genes. These observations raise the interesting possibility that this orphan nuclear receptor is regulated by ecdysone (Sullivan, 2003).
DHR38 transcripts are difficult to detect in these assays. This is consistent with studies which used RT-PCR or riboprobes for this purpose. Nonetheless, DHR38 mRNA can be detected during third-instar larval development, consistent with the widespread expression reported in earlier studies. DHR38 expression peaks at late pupal stages, consistent with its essential role in adult cuticle formation (Sullivan, 2003).
dERR and E75C display related temporal profiles of expression that do not fit with other nuclear receptor genes described in this study. Both of these genes are specifically transcribed during prepupal development, with increases in expression at 0 and 10-12 h APF. dERR, but not E75C, is also expressed during embryogenesis, with an initial induction at approximately 6 h AEL. These increases occur in synchrony with ecdysone pulses, suggesting that these orphan nuclear receptor genes are hormone inducible, although in a stage-specific manner. Further studies of dERR regulation, as well as a genetic analysis of this locus, are currently in progress (Sullivan, 2003).
Interactions between the DHR3 and E75B orphan nuclear receptors contribute to appropriate ßFTZ-F1 regulation during the onset of metamorphosis. DHR3 is both necessary and sufficient to induce ßFTZ-F1 and appears to exert this effect directly, through two response elements in the ßFTZ-F1 promoter. E75B can heterodimerize with DHR3 and is sufficient to block the ability of DHR3 to induce ßFTZ-F1. These three factors thus define a cross-regulatory network that contributes to the timing of ßFTZ-F1 expression in midprepupae. ßFTZ-F1, in turn, acts as a competence factor that directs the appropriate genetic and biological responses to the prepupal pulse of ecdysone. The patterns of DHR3, E75B, and ßFTZ-F1 expression observed at the onset of metamorphosis are consistent with these regulatory interactions as well as the expression patterns reported in earlier studies (Sullivan, 2003).
Unexpectedly, the tight linkage of DHR3, E75B, and ßFTZ-F1 expression seen at the onset of metamorphosis is recapitulated at earlier stages, after each of the major ecdysone pulses examined, in midembryogenesis and second-instar larval development. This observation suggests that the regulatory interactions between these receptors is not restricted to metamorphosis, but rather may recur in response to each ecdysone pulse during development. It is possible that this regulatory cascade contributes to cuticle deposition, which is dependent on ecdysone signaling in embryos, larvae, and prepupae. In support of this proposal, DHR3 and ßFTZ-F1 mutants exhibit defects in larval molting, suggesting that they act together to regulate this early ecdysone response (Sullivan, 2003).
Three other orphan nuclear receptor genes, E75A, DHR4, and DHR39, are expressed in concert with DHR3, E75B, and ßFTZ-F1, after the embryonic, second-instar, and third-instar ecdysone pulses. A peak of E75A expression marks the start of each genetic cascade, correlating with the rising ecdysone titer in 6- to 8-h embryos, the first half of the second instar, and in late third-instar larvae. This is followed by DHR3, E75B, and DHR4 expression which, in turn, is followed by a burst of ßFTZ-F1 expression. E78B is expressed in synchrony with DHR4 in late second and third-instar larvae, but not in embryos. These patterns of expression raise the interesting possibility that E75A, DHR4, and E78B may intersect with the cross-regulatory network defined for DHR3, E75B, and ßFTZ-F1. E75B and E78B are related to the Rev-erb vertebrate orphan nuclear receptor and are both missing their DNA binding domain. E75B and E78B null mutants are viable and fertile, suggesting that they exert redundant regulatory functions. E75A mutants die during larval stages, with no known direct regulatory targets. DHR4 mutants have not yet been described, although recent work indicates that this gene exerts essential roles in genetic and biological responses to the late larval ecdysone pulse. Further functional studies of these nuclear receptor genes should provide insight into their possible contribution to the regulatory circuit defined by DHR3, E75B, and ßFTZ-F1 (Sullivan, 2003).
Interestingly, DHR39 displays a reproducible pattern of expression that is inversely related to that of ßFTZ-F1, defining possible repressive interactions. DHR39 and ßFTZ-F1 have a similar DNA binding domain (63% identity) and bind to identical response elements, suggesting that they may exert cross-regulatory interactions. Moreover, DHR39 can repress transcription through the same response element that is activated by ßFTZ-F1. It would be interesting to determine whether the reciprocal patterns of DHR39 and ßFTZ-F1 expression during development is of functional significance (Sullivan, 2003).
The transcription of BR-C, EcR, E74, and E75 has been extensively characterized during the onset of metamorphosis, due to their rapid and direct regulation by the steroid hormone ecdysone at this stage in development. Surprisingly, however, their expression appears to be disconnected from the high-titer ecdysteroid pulses during embryonic and second-instar larval stages. As expected, EcR is induced early in embryonic development, in coincidence with the rising ecdysone titer at 4-10 h AEL, with EcR-B transcripts appearing first followed by EcR-A. BR-C mRNA, however, is not seen until 10-12 h AEL and E74B mRNA is induced even later, at 14-16 h AEL, when the ecdysteroid titer has returned to a basal level. Both EcR-B and E74B are repressed from 16-20 h AEL as E74A and E75A are induced, a switch that has been linked to the high-titer ecdysone pulse in late third-instar larvae; however, this response occurs during late embryogenesis when the ecdysteroid titer is low. A similar observation has been made for E75A expression in the Manduca dorsal abdominal epidermis, where a brief burst of E75A mRNA is detected immediately before pupal ecdysis, after the ecdysteroid titer has returned to basal levels (Sullivan, 2003).
It thus seems likely that the second instar ecdysone pulse occurs during the first half of the instar. This profile is consistent with the early induction of E75A. EcR-B and E74A, however, are not induced until the second half of the second instar, with a peak at the end of the instar. BR-C mRNA levels remain steady throughout the second instar. Finally, EcR-B, E74B, and BR-C are induced in early to mid-third-instar larvae, a time when one or more low-titer ecdysone pulses may occur. It is curious that E74B is poorly expressed relative to E74A during embryonic and second-instar larval stages, disconnecting its expression from that of EcR. This pattern is not seen in studies that focused on the onset of metamorphosis. Taken together, the temporal profiles of early gene expression (EcR, BR-C, E74, E75A) during late embryonic and late second-instar larval stages appear to be unlinked to the known ecdysteroid pulses at these stages. This could indicate that these promoters are activated in a hormone-independent manner at these stages in the life cycle. Alternatively, these ecdysone primary-response genes may be induced by a novel temporal signal that remains to be identified (Sullivan, 2003).
Several lines of evidence indicate that 20-hydroxyecdysone is not the only temporal signal in Drosophila. A major metabolite of this hormone, 3-dehydro-20-hydroxyecdysone, was shown to be as effective as 20-hydroxyecdysone in inducing target gene transcription in the hornworm, Manduca sexta. Similarly, 3-dehydro-20-hydroxyecdysone is more efficacious than 20-hydroxyecdysone in inducing Fbp-1 transcription in the Drosophila larval fat body. A high-titer pulse of alpha-ecdysone, the precursor to 20-hydroxyecdysone, can drive the extensive proliferation of neuroblasts during early pupal development in Manduca. This is the first evidence that alpha-ecdysone is responsible for a specific response in insects. It is unlikely, however, that this signal is transduced through the EcR/USP heterodimer, which shows only very low transcriptional activity in response to this ligand. Rather, recent evidence indicates that alpha-ecdysone may activate DHR38 through a novel mechanism that does not involve direct hormone binding (Sullivan, 2003).
Studies of ecdysteroid-regulated gene expression in Drosophila have also provided evidence for hormone signaling pathways that may act independently of 20-hydroxyecdysone. Several studies have identified a large-scale switch in gene expression midway through the third larval instar, an event that has been referred to as the mid-third-instar transition. It is not clear whether this response is triggered by a low-titer ecdysteroid pulse, another hormonal signal, or in a hormone-independent manner. Similarly, the let-7 and miR-125 micro-RNAs are induced at the onset of metamorphosis in Drosophila in tight temporal correlation with the E74A early mRNA, but not in apparent response to 20-hydroxyecdysone. These studies indicate that 20-hydroxyecdysone cannot act as the sole temporal regulator during the Drosophila life cycle (Sullivan, 2003).
FTZ-F1 is a member of the nuclear hormone receptor superfamily. The conserved regions include the DNA binding domain that bears two potential Cys2-Cys2 zinc finger motifs, and the ligand binding domain of the nuclear receptor superfamily. The putative DNA binding domain is well conserved across the nuclear receptor superfamily, showing identity with all 20 invariant amino acids. Nonetheless, FTZ-R1 is somewhat distinct from the two major classes of nuclear receptors represented by those that bind either the glucocorticoid or the estrogen-thyroid hormone response elements (Lavorgna, 1991).
The C-terminal half of the FTZ-F1 protein shows sequence similarity to the ligand binding domain of hormone receptors. Sequence alignment identifies two core regions (region II and III) that are conserved across the nuclear receptor superfamily.. The FTZ-F1 sequence in these regions is most similar to the human COUP transcription factor 9 (also called erb-A related 3 or EAR3), and also to the Drosophila neural determination protein Sevenup. The sequence conservation in the ligand binding domain suggests that FTZ-F1 is a receptor for a hormonal ligand in Drosophila, but the nature of this ligand has yet to be determined (Lavorgna, 1991).
The late FTZ-F1 isoform (FTZ-F1ß) differs from the early isoform (FTZ-F1alpha) by replacement of the N-terminal 401 amino acids with a 173 amino acid segment. Both 5.6 and 4.8 kb mRNAs code for an identical late protein form. The late protein corresponds to the protein generated during the midprepupal stage that plays a role in ecdysone-induced gene expression (Lavorgna, 1993).
date revised: 19 Dec 96
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.