REGULATION (part 1/2)


The zinc finger transcription factors Spalt and Spalt-related have been implicated in multiple developmental processes. In the wing they are regulated by the secreted protein Decapentaplegic and participate in the positioning of the wing veins. The function of Spalt has also been analyzed during tracheal development and embryonic segmentation. The isolation and characterization of novel spalt/spalt-related alleles is reported. These genes cannot substitute for one another in the developmental processes studied. The mutants present embryonic or pupal lethality, with phenotypes consistent with the loss of spalt function. A detailed functional analysis of the DNA regions implicated in the regulation of these genes is presented. This regulation is complex, integrating the information from both negative and positive regulators, and it is modular, with discrete fragments of DNA directing expression to discrete regions in embryonic and larval tissues (Barrio, 1999).

spalt and salr are separated by approximately 70 kb of DNA and are transcribed from opposite strands. The two genes are oriented so that their directions of transcriptions converge. Some of the general enhancers of both sal and salr (AZA, AZP, AS, BE, ZO and AM), possibly analogous to a locus control region, exist 5' of the salr coding region (outside of the region between the two genes). Extensive transgenic analysis has identified modular regulatory domains not only for the imaginal discs, but also for other embryonic and larval structures, in some cases down to the level of specific cell clusters. The construct AZA, 5' of salr, directs expression specifically in the tracheae. Similarly, a region located 5' of sal directs sal expression in this organ. The construct BO, located between the two genes directs expression to all the oenocyte cells where sal and salr are normally expressed, while another construct, BI, also located between the two genes, directs expression only in a subset of these cells, Some constructs such as AZA, EME and BI show beta-gal expression in the PNS in subsets of cells where endogenous Sal expression has been found. Other constructs show marker expression in the epidermis, partially overlapping with the endogenous expression of sal: ZO and EMA in certain regions and BE, SE, and LA in stripes of cells (Barrio, 1999).

The regulation in the CNS appears to be more complex. Thus, several constructs (ZA, AK, and BAT derived from DNA between the two genes) show both endogenous and ectopic expression in specific cells in the CNS, while others (AZO, AM, EME, LA and BI) show expression only in cells where sal and salr are normally not expressed. This 'ectopic' expression is likely to be a consequence of the separation between putative transcriptional activator and repressor elements, a combination of which would be needed to restrict the expression of sal and salr to their normal domains (Barrio, 1999).

In the same 50-kb interval, including both the region between the two genes and the region 5' of salr, discrete regions have been identified that direct expression in imaginal discs and in the larval CNS. In the eye disc, expression in photoreceptors and cone cells is observed in the constructs BO and LA, indicating that the corresponding regulatory region is in the common interval between these two overlapping constructs. The expression of Sal and Salr is highest in the early photoreceptors, just anterior to the morphogenetic furrow, and declines thereafter as expression in the cone cells appears. Another small region located in the first large intron of salr, seems to direct expression in the cone cells (Barrio, 1999).

In some constructs (LA, SE ABO ABI and AL, all located between the two genes) beta-gal expression is detected in specific domains of the wing and haltere imaginal discs. Interestingly, the constructs that direct expression in the wing disc present the same pattern of expression in the haltere, which correlates with the fact that both types of disc follow the same developmental program. In the case of the constructs that are expressed in the wing blade, expression in the corresponding region (pouch) of the haltere disc is observed. Because sal and salr are normally not expressed in the haltere pouch, it is possible that this ectopic expression is a consequence of the lack of specific repressor sites that normally prevent expression in the haltere; a good candidate to mediate this repression is Ubx. In the larval CNS only a few constructs (ZO, EME, and BAT, all derived from the intergenic interval) are expressed in the same cells that express sal and salr endogenously; additionally, these constructs also result in ectopic expression in different regions of the CNS. Some lines (AS, BE, AL, ABO, and IRU) direct expression in different parts of the ring gland, like the corpus cardiacum (AS, ectopic with respect to sal); the ecdysteroid-producing cells (prothoracic gland; AZ, AL and IRU, endogenous with respect to sal), or the corpus allatum (ABO, ectopic with respect to sal). Taken together, these results suggest that Sal and Salr proteins have regained similar functionality although their genes cannot substitute for one another. The modular organization of cis-regulatory regions facilitates the diversification of these duplicated genes, and either new or nonrestrictive domains of expression may be acquired by rearrangement of such regions (Barrio, 1999).

Collaboration between Smads and a Hox protein in target gene repression

Hox proteins control the differentiation of serially iterated structures in arthropods and chordates by differentially regulating many target genes. It is yet unclear to what extent Hox target gene selection is dependent upon other regulatory factors and how these interactions might affect target gene activation or repression. Two Smad proteins, effectors of the Drosophila Dpp/TGF-ß pathway, that are genetically required for the activation of the spalt (sal) gene in the wing, collaborate with the Hox protein Ultrabithorax (Ubx) to directly repress sal in the haltere. The repression of sal is integrated by a cis-regulatory element (CRE) through a remarkably conserved set of Smad binding sites flanked by Ubx binding sites. If the Ubx binding sites are relocated at a distance from the Smad binding sites, the proteins no longer collaborate to repress gene expression. These results support an emerging view of Hox proteins acting in collaboration with a much more diverse set of transcription factors than has generally been appreciated (Walsh, 2007).

The activation of sal in the wing and its repression in the haltere are regulated by a 1.1 kb CRE, sal1.1 (Galant, 2002). Previous studies have shown that sal1.1 is directly repressed by Ubx in the haltere (Galant, 2002). In order to test whether Mad/Med binds to and directly represses the activity of the sal1.1 CRE in the haltere, candidate Mad/Med binding sites were sought in the sal1.1 CRE. One candidate Mad/Med binding site, M1 (5'-AGACGGGCAC-3'), was identified that lies between Ubx binding sites 5 and 6 in sal1.1, using binding site prediction and electrophoretic mobility shift assays (EMSAs). The sequence of M1 deviates somewhat from published Mad/Med silencer consensus binding sites (5'-AGAC-5 bp-GNCGYC-3') (Gao, 2005; Pyrowolakis, 2004), and Mad and Med bound with >10-fold and >25-fold lower affinities, respectively, to the M1 site than to the bam (Gao, 2005) and brk (Pyrowolakis, 2004) silencer elements (Walsh, 2007).

In order to test whether Mad/Med bound specifically to the M1 site, a series of point mutations were introduced within the M1 site, and their effect on protein binding was examined in vitro. Of four point mutations to the M1 site, the single mutation at position 808 reduced the binding of a Med fusion protein (GST-MedMH1) to M1 as compared with the wild-type sequence. The remaining three point mutations did not affect the affinity of GST-MedMH1 for the probe. These results suggest that Med might contact the sequence 5'-AGAC-3' in sal1.1. By contrast, the four individual point mutations each decreased, but did not abolish, binding of a Mad fusion protein (GST-MadN) in vitro, with the point mutation at bp 814 having the strongest effect. The weaker effect of the individual point mutations in M1 on Mad binding affinity in vitro is likely to be due to the affinity of MadN for both 5'-AGAC-3' Smad sites and GC-rich sequence. Combining these four mutations (sal798-824 kM1) had the greatest effect on GST-MadN binding to the probe. This analysis of individual point mutations indicates a putative orientation for a Mad/Med compound-binding site in the sal1.1 CRE (Walsh, 2007).

Most importantly, in transgenic flies, each point mutation of M1 introduced into an otherwise wild-type sal1.1 reporter construct caused derepression of the reporter gene lacZ in the haltere imaginal disc. The strength of derepression correlates with the decreased affinity of Mad for its binding site with the pm814 mutation, the strongest point mutation in vitro, showing the strongest level of derepression in vivo. Full derepression was observed when all four point mutations were combined into a sal1.1 reporter construct. No effect of mutations in M1 were observed on sal1.1-driven reporter gene expression in the wing as compared with the wild-type sal1.1 element or with endogenous sal expression, indicating that this site is not required for gene activation in the wing or haltere disc. Together, the biochemical, reporter gene and genetic evidence indicate that Mad/Med/Shn are directly required for sal repression in the haltere imaginal disc (Walsh, 2007).

This study demonstrates that Mad/Med and Ubx bind to adjacent sites in the sal1.1 CRE and that each protein is required for the direct repression of sal expression in the haltere. Furthermore, the sequence and spacing of Ubx and Smad binding sites are highly conserved and their proximity is required for target gene repression in the haltere. Because no evidence was found that these proteins interact directly, it is suggested this is an example of 'collaboration' or target gene co-regulation without direct cooperative interaction. These results have general implications for understanding how Hox proteins regulate diverse sets of target genes in animal development (Walsh, 2007).

The direct role for Smads in the repression of sal in the haltere is surprising in the light of previous genetic and molecular studies that had indicated that the Dpp pathway and Mad/Med were involved in sal activation in the wing. No direct evidence was found that this is the case and the fact that sal is activated in Mad and Med clones in the haltere indicates that sal is activated independently of Mad/Med in the flight appendages. The requirement for Mad/Med/Shn in shaping the pattern of sal expression in the wing appears to be indirect -- the protein complex represses the expression of brk, a repressor of sal, in cells in the central region of the developing wing and thereby permits sal expression (Walsh, 2007).

The Mad-Med-Shn complex is also active within cells in the central region of the haltere as a consequence of Dpp signaling. However, whereas sal is expressed and the sal1.1CRE is active in the wing, sal and the sal1.1 CRE are repressed in the haltere. These observations raise the question of how the Mad-Med-Shn complex selectively represses sal in the haltere but not in the wing disc? The results suggest that there are two key determinants in the selective repression of sal in the haltere. The first is collaboration with Ubx, which is expressed in the haltere and not in the wing disc. The second key determinant might be the affinity of Mad/Med binding to the sal CRE (Walsh, 2007).

The different responses of the brk and sal genes to Mad/Med/Shn suggests how the different affinities of proteins for binding sites might determine how available transcriptional regulatory inputs are integrated by CREs. Mad/Med binding to the brk CRE is of high affinity (Pyrowolakis, 2004) and apparently sufficient to impart repression, whereas that to the sal CRE is of much lower affinity and insufficient to impart repression in the wing. In the haltere, although Mad-Med-Shn or Ubx binding are alone insufficient, they act together either via simultaneous or sequential occupancy of their binding sites to repress sal (Walsh, 2007).

The requirement for two or more regulators to act together to control gene expression, i.e. combinatorial regulation, is fundamental to the generation of the great diversity of gene expression patterns by a finite set of transcription factors. Several previous studies have revealed the dual requirement for Hox and Smad functions for the activation of a target gene. Studies have suggested a general combinatorial mechanism for gene activation in which apparently separate transcriptional inputs act synergistically in gene activation and, in at least one case, the Hox response element and Dpp response element are separable. In this study, however, a requirement was observed for strict evolutionary conservation of the close topology of Hox and Smad binding sites in the sal CRE. It is suggested that collaboration is a distinct mode of combinatorial regulation in which two or more regulatory proteins must bind to nearby sites, but not necessarily to each other (Walsh, 2007).

The integration of Hox and Smad inputs could work through a number of possible mechanisms in the absence of direct physical interaction. One appealing possibility that might explain the requirement for the close proximity of binding sites is that Ubx and Mad-Med-Shn might interact with, and could therefore cooperatively recruit, the same co-repressor(s) for the repression of sal. Alternatively, if Mad-Med-Shn and Ubx bind sequentially to sal1.1, they might recruit different co-repressors and thereby orchestrate the assembly of a co-repressor complex. A third possibility is that because the Ubx and Mad/Med sites are embedded within a larger block of conserved regulatory DNA sequence in the sal1.1 CRE, the binding of other interacting transcription factors might also be involved in the repression of sal by Ubx and Mad-Med-Shn (Walsh, 2007).

These and recent results raise the question of whether collaboration is a general feature of target gene selection by Hox proteins. It is suggested that collaboration might be a widespread requirement for Hox function in vivo. This proposal is prompted by three observations: (1) Hox proteins alone have low DNA-binding specificity; (2) some, and perhaps all, Hox proteins might act as both repressors and activators; (3) Hox proteins regulate a great diversity of target genes that are also regulated by other transcription factors. In order to be such versatile regulators, it would be too great a constraint to require that Hox proteins always interact cooperatively with the diverse repertoire of transcription factors with which they act. Indeed, it may be argued that too much weight has been ascribed to the cooperative binding of Hox proteins and co-factors to DNA (Walsh, 2007).

Previously, much attention has focused on Exd and Hth, which interact with Hox proteins and bind cooperatively to DNA, thereby increasing Hox DNA-binding selectivity. However, it was only recently shown that the binding of these complexes alone was not sufficient to regulate target gene expression. Rather, Hox-Exd-Hth collaborate with and require the segmentation proteins Slp and En to repress the target gene Dll. This study has shown that the Exd- and Hth-independent target gene repression of sal requires collaboration between Ubx and Mad-Med-Shn. Although still a tiny sample of target genes, cases of transcription factors of various structural types acting as collaborators with Hox proteins are now available. The picture of Hox proteins relying on dedicated interacting co-factors such as Exd and Hth is expanding to a larger pool of collaborating transcription factors that modulate target gene selection (Walsh, 2007).

Indeed, collaboration might be the key to another unresolved mystery of the Hox proteins - the regulation of Hox protein activity. Some Hox proteins appear to act in both gene activation and repression; this is certainly the case for Ubx. This versatility would appear to be crucial to their role as sculptors of major features of body patterns, but how does the same transcription factor act positively in some contexts but negatively in others? There is evidence to suggest that the identity of the collaborating proteins and/or CRE sequences determines the 'sign' of Hox action (Walsh, 2007).

For instance, there is no evidence that the mere binding of Hox-Exd-Hth to a site determines the sign of Hox activity. These co-factors are involved in both Hox target gene activation (e.g., dpp in the midgut) and target repression (e.g.,Dll in the embryonic abdomen). But, in the latter case, En and Slp, two proteins that each harbor motifs for interaction with the co-repressor Groucho, are required collaborators for Dll repression. The roles of En and Slp in this instance might not be so much a matter of facilitating Hox target selection, but rather in regulating the sign of the output of the collaboration (Walsh, 2007).

Similar to the Hox proteins, the Smads can either activate or repress target genes. Furthermore, it has been demonstrated that the topology of Smad binding sites on DNA appears to be critical for determining whether a target gene is activated or repressed. In Drosophila, the topology of Mad and Med binding sites is critical for the recruitment of the co-repressor Shn. The recruitment of Shn was shown here to be necessary for sal repression. These two examples suggest that the positive or negative regulatory activity of a Hox protein depends on the context of surrounding binding sites and how they influence the activity of collaborating factors (Walsh, 2007).

The dependence of Hox proteins upon co-factors and collaborators indicates that, at the molecular level, Hox proteins are not 'master' regulatory proteins that dictate how target genes behave. Rather, they exert their great influence by virtue of their simple binding specificity, broad domains of expression and versatile, collaborative properties (Walsh, 2007).

Transcriptional Regulation (part 1/2)

Antennapedia serves as a strong repressor of sal (Wagner-Bernholz, 1991). Ectopic expression of Antp in the antennal disc represses sal. Recessive mutants of Antp have the opposite effect; they cause differentiation of antennal structures in the second leg disc.

The region-specific homeotic gene spalt is involved in the specification of terminal versus trunk structures during early Drosophila embryogenesis. Later in development spalt activity participates in specific processes during organogenesis and larval imaginal disc development. The multiple functions of spalt are reflected in distinct spatio-temporal expression patterns throughout development. spalt cis-regulatory sequences for region-specific and organ-specific expression are clustered. Their organization may provide the structural basis for the diversification of expression pattern within the spalt/spalt related/spalt adjacent gene complex. The transacting factor requirement has been examined for the blastodermal spalt expression domains. They are under the genetic control of maternal and gap gene products and these products are able to bind to corresponding spalt cis-acting sequences in vitro. The results suggest that the transacting factors, as defined by genetic studies, functionally interact with the spalt regulatory region. In addition, evidence is provided that a zygotic gene product of the terminal system, Tailless, cooperates with the maternal gene product Caudal and thereby activates gene expression in the terminal region of the embryo (Kuhnlein, 1997).

Genetic evidence is presented showing that lines, a Drosophila segment polarity gene that has yet to be cloned, is required for the function of the Abdominal-B protein. In lines mutant embryos Abdominal-B protein expression is normal but is incapable of promoting its normal function: formation of the posterior spiracles and specification of an eighth abdominal denticle belt. The tail and A8 segment of lin embryos are highly abnormal. The A8 denticle belt is replaced by naked cuticle that occasionally forms a few denticles less pigmented than the normal ventral denticles. This abnormal A8 cuticle does not resemble the cuticle of any region of the wild-type or of the lin mutant embryo. The absence of anal pads and the abnormal hindgut suggests abnormal development of abdominal segment 11, however, other aspects of the tail development are normal, such as the formation of an anal tuft. In lin embryos the sensory organs are formed at roughly correct positions but have an abnormal shape (Castelli-Gair, 1998).

The Abd-B gene directs the formation of the posterior spiracles by controlling downstream target genes. The defects associated with lines mutation arise because in lines mutant embryos the Abdominal-B protein cannot activate its direct target empty spiracles (ems) or other downstream genes, such as cut(ct) and spalt(sal), while it can still function as a repressor of Ultrabithorax and abdominal-A. ems is one gene required for the formation of posterior spiracles. ems expression in the posterior spiracles is regulated by Abd-B. In lin embryos the transcription of ems is not activated in the posterior spiracles, showing that lin is required for Abd-B to activate its direct downstream target. The other putative Abd-B downstream targets (cut and spalt are also required for the normal development of the posterior spiracles. The activation of ct and sal in the anlage of the posterior spiracles requires Abd-B function but their activation remains independent of one another and of ems, suggesting that all three genes are independently controlled by Abd-B. In lin mutants neither ct nor sal are activated in the anlage of the posterior spiracles. These results show that in lin mutant embryos, Abd-B is incapable of activating some of its targets. The requirement of lines for Abd-B function is not a specific property of the A8 segment. In wild-type embryos, ectopic Abd-B expression using the GAL4 targeting system results in the formation of ectopic posterior spiracles in segments anterior to A8. In contrast, ectopic Abd-B expression in lin mutants does not form ectopic posterior spiracles showing that no matter where the Abd-B protein is expressed in the embryo it requires lines to be fully functional (Castelli-Gair, 1998).

The effect of lin on Abd-B can be explained at the molecular level if lin is required for protein posttranscriptional modification or as a transcriptional cofactor of Abd-B. There is some evidence that the Abd-B protein is posttranslationally modified. If Lin were mediating this process, it would imply that such posttranscriptional modification is functional in vivo. Alternatively if Lines is a transcriptional cofactor of Abd-B, Lines would be interacting with Abd-B in a similar way to that proposed for Extradenticle with Ubx and Abd-A, or Ftz-F1 with Ftz. It is interesting that Exd does not have any effect on Abd-B protein binding or function, and that lin is specific for Abd-B but not for the other Hox genes tested. This suggests that different HOX proteins use different cofactors that contribute to the DNA binding specificity of the HOX proteins (Castelli-Gair, 1998).

The development of the posterior spiracles of Drosophila may serve as a model to link patterning genes and morphogenesis. A genetic cascade of transcription factors downstream of the Hox gene Abdominal-B subdivides the primordia of the posterior spiracles into two cell populations that develop using two different morphogenetic mechanisms. The inner cells that give rise to the spiracular chamber invaginate by elongating into 'bottle-shaped' cells. The surrounding cells give rise to a protruding stigmatophore by changing their relative positions in a process similar to convergent extension. In the larvae the spiracular chamber forms a very refractile filter, the filzkorper. The opening of the spiracular chamber, the stigma, is surrounded by four sensory organs; the spiracular hairs. Clones labeling the spiracular hairs show that each one is formed by four cells related by lineage, two neural and two support cells, the typical structure of a type I external sensory organ. When the larva is buried in the semi-liquid medium on which it feeds, the stigmatophore periscopes out of the medium allowing the larva to continue breathing. The genetic cascades regulating spiracular chamber, stigmatophore, and trachea morphogenesis are different but coordinated to form a functional tracheal system. In the posterior spiracle, this coordination involves the control of the initiation of cell invagination that starts in the cells closer to the trachea primordium and spreads posteriorly. As a result, the opening of the tracheal system shifts back from the spiracular branch of the trachea into the posterior spiracle cells (Hu, 1999).

Downstream of Abd-B the cascade can be subdivided into various levels. The activation of six genes -- cut, empty spiracles (ems), nubbin (nub), klumpfuss (klu), and spalt (sal) -- does not require expression any of the other genes studied, suggesting that these six genes are at the top of the cascade under Abd-B regulation. The cut, ems, nub, and klu genes are expressed in the spiracular chamber in overlapping patterns. The sal gene is not expressed in the spiracular chamber but in the cells that surround it and will form the stigmatophore. The exclusion of sal from the spiracular chamber is partly due to repression by cut, because in cut mutants sal is expressed at low levels in the internal part of the spiracle. Downstream of these putative Abd-B targets other genes are activated. These include the transcription factors grainyhead (grh), trachealess (trh) and engrailed (en) (Hu, 1999).

The spiracle phenotypes in mutants for the early Abd-B downstream genes have been analyzed. In sal mutants the stigmatophore does not form, resulting in embryos with a normal spiraclular chamber that does not protrude. Conversely, mutations in ems and cut affect the spiracular chamber but not the stigmatophore. Mutations for ems result in a spiracular chamber that lacks a filzkorper and is not connected to the trachea. In cut mutants the filzkorper is almost completely missing, but the trachea is still connected to the spiracular chamber and the spiracular hairs are also missing. In trh mutants, where the tracheal pits do not form and there is no tracheal network, the spiracular chamber cells still invaginate, forming a filzkorper. However, this filzkorper is shorter than that of the wild type probably due to a secondary requirement of trh, which is also expressed in the spiracular chamber cells. These results show that the spiracular chamber, the stigmatophore, and the trachea develop independently of one another. No phenotypes for either klu or nub could be detected, indicting that although these genes are expressed in the spiracle, they are either redundant or their function is not required for spiracle morphogenesis (Hu, 1999).

To follow the movements of the spiracular chamber cells as they invaginate, constructs were examined that were made with particular enhancers of the cut, ems, and grh genes, each of which drive expression of beta-gal in a subset of cells that express the cut gene at stage 11. These enhancers do not drive the whole spiracular expression of their genes, but are good tools for studying cell specification and the morphogenetic movements of the posterior spiracle cells. The expression of cut in the posterior spiracle is controlled by at least three different enhancers, two of which have been used in this study. From stage 13, the ct-A4.2 enhancer marks the precursors of the four spiracular hairs. The grh-D4 enhancer of the grh gene is expressed in a single group of cells in this area. The expression of ems in the spiracle is driven by at least by one enhancer: ems-1.2. From stage 11 this enhancer marks a group of cells abutting the tracheal pit. Double stainings of the cut-D2.3, ems-1.2, and grh-D4 lacZ constructs show that they are expressed in non-overlapping subsets of cells. The correlation of the expression of these three constructs allows the fate mapping of the spiracular chamber primordium when it is a two dimensional sheet of cells. The different spatial expression of these enhancers at stage 11 shows that the two-dimensional sheet of cells is already patterned and that the cells invaginate to precise positions during development (Hu, 1999 and references therein).

The connection of the posterior spiracle to the trachea is a regulated event. In mutants for the Drosophila FGF and FGF-receptor homologs branchless and breathless the tracheal pits do invaginate, but since they do not migrate toward one another, they do not form a continuous network. In contrast, in btl mutants, the posterior spiracle connects normally to the A8 spiracular branch of the trachea. In mutants for Abd-B the stigma of A8 does not slide posteriorly, but stays in the same position as in anterior abdominal segments, where the spiracular branch attaches to the outside epidermis. The contribution of the ems gene to coordination of morphogenetic movements has been examined. The spiracle-trachea connection occurs in cut and sal mutants but not in ems mutants. In ems mutants, invagination of the spiracle cells adjacent to the trachea does not occur, but more posterior cells of the spiracle invaginate normally. The elongation does not occur simultaneously in all cells, but starts in the more anterior ones and, in general, the invaginating cells keep contact with the external surface of the embryo. This results in the cells that have invaginated earlier being deeper in the spiracular chamber and more elongated. The defective invagination in ems mutants results in a spiracle without a lumen and with the tracheal opening located outside it. The results show that cell elongation and formation of a lumen are two independently controlled processes. The spiracles provide a good model for the study of cellular and molecular mechanisms controlling cell shape and cell rearrangements, two mechanisms which are used during the morphogenesis of a variety of organisms (Hu, 1999).

Cell migration during embryonic tracheal system development in Drosophila requires Dpp and Egf signaling to generate the archetypal branching pattern. Two genes encoding the transcription factors Knirps and Knirps related are shown to possess multiple and redundant functions during tracheal development. knirps/knirps related activity is necessary to mediate Dpp signaling that is required for tracheal cell migration and formation of the dorsal and ventral branches. The expression of kni and Knrl appears during stage 10 in the tracheal placodes. During primary branch formation, expression of kni and Knrl decreases and restricts to the dorsal- and ventral-most cells as well as visceral branch cells. kni and Knrl expression persists in the cells of dorsal, visceral, lateral trunk and ganglionic branches. Thus, kni and Knrl are expressed in the same spatio-temporal patterns, suggesting that kni and knrl may also share redundant functions during tracheal development (Chen, 1998).

In dorsal tracheal cells knirps/knirps related activity represses the transcription factor Spalt; this repression is essential for secondary and terminal branch formation. However, in cells of the dorsal trunk, spalt expression is required for normal anteroposterior cell migration and morphogenesis. spalt expression is maintained by the Egf receptor pathway and, hence, some of the opposing activities of the Egf and Dpp signaling pathways are mediated by spalt and knirps/knirps related. Furthermore, evidence is provided that the border between cells acquiring dorsal branch and dorsal trunk identity is established by the direct interaction of Knirps with a spalt cis-regulatory element (Chen, 1998).

It has been proposed that the Dpp and Egf signaling generates three different cell fates in the developing placode. This signaling confers the capacity of cells to migrate in distinct directions. kni/knrl activity have been shown to be necessary to mediate Dpp signaling for dorsal and ventral cell migration. In addition, repression of the Egfr signaling target sal by kni/knrl establishes a border between the dorsally and anteroposteriorly migrating dorsal branch and dorsal trunk cells, respectively. However, the repression of sal is not necessary for normal dorsoventral tracheal cell migration but rather for morphogenetic processes that occur independent of cell migration. Thus, tracheal cells that express sal and kni/knrl still adopt a dorsoventral migration behavior. Ectopic expression of kni/knrl in dorsal trunk cells has two consequences: (1) it represses Sal, which results in the lack of anteroposterior migration of dorsal trunk cells, and (2) ectopic kni/knrl leads primordial dorsal trunk cells to adopt a dorsoventral migration behavior. Thus, the observation that ectopic Dpp causes altered tracheal cell migration and lack of dorsal trunk formation is consistent with the proposal that these processes are mediated in part via kni/knrl. However, in contrast to ectopic Dpp, which inhibits visceral branch formation, ectopic kni/knrl tracheal expression does not affect anterior outgrowth of visceral branches. This observation is not unexpected since kni/knrl is expressed in visceral branch cells and is necessary for normal visceral branch morphogenesis. Thus, kni/knrl act within the genetic circuitry of visceral branch cell fate determination in a different way from the way these genes act during dorsal branch development. No mediation of dorsoventral cell migration is involved. kni/knrl may be part of a patterning system for visceral branch development within the Egfr signaling domain, whereas sal activity is necessary for dorsal trunk development (Chen, 1998 and references).

Pattern formation along the anterior-posterior (A/P) axis of the developing Drosophila wing depends on Decapentaplegic (Dpp), a member of the conserved transforming growth factor beta (TGF beta) family of secreted proteins. Dpp is expressed in a stripe along the A/P compartment boundary of the wing imaginal disc and forms a long-range concentration gradient with morphogen-like properties that generate distinct cell fates along the A/P axis. Dpp expression and Dpp signaling have been monitored in endocytosis-mutant wing imaginal discs that develop severe pattern defects specifically along the A/P wing axis. The results show that the size of the Dpp expression domain is expanded in endocytosis-mutant wing discs. However, this expansion does not result in a concommittant expansion of the functional range of Dpp activity but rather, results in its reduction, as indicated by the reduced expression domain of the Dpp target gene spalt. The data suggest that clathrin-mediated endocytosis, a cellular process necessary for membrane recycling and vesicular trafficking, participates in Dpp action during wing development. Genetic interaction studies suggest a link between the Dpp receptors and clathrin. Impaired endocytosis does not interfere with the reception of the Dpp signal or the intracellular processing of the mediation of the signal in the responder cells, but rather affects the secretion and/or the distribution of Dpp in the developing wing cells (Gonzalez-Gaitan, 1999).

The Drosophila tracheal tree consists of a tubular network of epithelial branches that constitutes the respiratory system. Groups of tracheal cells migrate towards stereotyped directions while they acquire specific tracheal fates. This work shows that the wingless/WNT signaling pathway is needed within the tracheal cells for the formation of the dorsal trunk (DT) and for fusion of the branches. These functions are achieved through the regulation of target genes, such as spalt in the dorsal trunk and escargot in the fusion cells. The pathway also aids tracheal invagination and helps guide the ganglionic branch. Moreover the wingless/WNT pathway displays antagonistic interactions with the Dpp pathway, which regulates branching along the dorsoventral axis. Remarkably, the wingless gene itself, acting through its canonical pathway, seems not to be absolutely required for all these tracheal functions. However, the artificial overexpression of wingless in tracheal cells mimics the overexpression of a constitutively activated Armadillo protein. The results suggest that another gene product, possibly a WNT, could help to trigger the wingless cascade in the developing tracheae (Llimargas, 2000).

The results indicate that the Wg pathway acts in DT formation by regulating sal expression. Although this regulation could be direct, it is also possible that the Wg pathway acts by regulating the number of cells that will later express sal under a Wg-independent control. Two pieces of evidence show that normally the Wg pathway is active only in the DT during primary branching. (1) The constitutive activation of Arm cause integration of the visceral branches (VBs) into DT. Such transformation would not be expected if the pathway were normally active in the VBs. (2) In the absence of tkv, the pathway must be constitutively activated to observe sal expression in every tracheal cell. If the pathway were normally active in all tracheal cells, the mere absence of tkv should be sufficient to allow sal expression. Moreover, the pathway seems to be required in the DT cells themselves. The expression of dominant negative Pangolin in all tracheal cells impairs DT formation, indicating that there is an autonomous requirement within the tracheal cells. When the expression of dominant negative Pangolin in the tracheae is maintained only in the DT cells, similar DT defects are observed (Llimargas, 2000).

The Dpp pathway is required to form the dorsal and ventral branches while the Wg pathway is required for DT formation. Accordingly, only the VBs and transverse connectives (TCs), which are independent of these two pathways, form in their absence. The Wg and the Dpp pathways are known to act antagonistically in several structures, such as the wing or the leg disc. Similarly, the Wg and the Dpp pathways have opposing activities during primary branching. Only when the Dpp pathway is not active is the Wg pathway able to confer DT identity to the tracheal cells. This antagonism is likely to be mediated through the negative regulation of sal by kni. sal is positively regulated by the Wg pathway and kni is activated by the Dpp pathway. However, when the Wg signal is constitutive in a wild type background, the VBs (which express kni) seem to adopt a DT identity. Remarkably, in the VBs, the Dpp pathway is neither active nor does it control kni expression. This indicates that kni expression is not sufficient to prevent sal regulation by the Wg pathway and that other targets of Dpp might also antagonize the WG/WNT pathway (Llimargas, 2000).

Sequencing of the Drosophila genome has revealed that there are 'silent' homologs of many important gene family members that were not detected by classic genetic approaches. Why have so many homologs been conserved during evolution? Perhaps each one has a different but important function in every system. Perhaps each one works independently in a different part of the body. Or, perhaps some are redundant. This study takes one well known gene family and analyzes how the individual members contribute to the making of one system, the tracheae. There are seven DWnt genes in the Drosophila genome, including wingless. The wg gene helps to pattern the developing trachea but is not responsible for all Wnt functions there. Each one of the seven DWnts was tested in several ways and evidence was found that wg and DWnt2 can function in the developing trachea: when both genes are removed together, the phenotype is identical or very similar to that observed when the Wnt pathway is shut down. DWnt2 is expressed near the tracheal cells in the embryo in a pattern different from wg's, but is also transduced through the canonical Wnt pathway. The seven DWnt genes vary in their effectiveness in specific tissues, such as the tracheae; moreover, the epidermis and the tracheae respond to DWnt2 and Wg differently. It is suggested that the main advantage of retaining a number of similar genes is that it allows more subtle forms of control and more flexibility during evolution (Llimargas, 2001).

The spalt (sal) gene (coding for a transcription factor) is expressed in the dorsal ectoderm, including some tracheal cells, during stage 10 and persists later in those tracheal cells that form the DT. sal is absolutely required for DT formation and is thus a good marker for DT cell identity. The most dorsal cells that express sal also coexpress DWnt2. The pattern of wg expression differs strikingly from that of DWnt2, although both gene products are made near the tracheal cells. In arm mutants, Sal is not expressed in tracheal cells and no DT is formed, suggesting that sal expression in tracheal cells depends on activation of the Wnt pathway. Thus, sal can be induced in the tracheal cells wherever either Wg or DWnt2 proteins are received (Llimargas, 2001).

Removal of Wg and DWnt2 proteins (in wg-DWnt2- embryos) eliminates detectable expression of sal in the presumptive tracheal cells of the DT, whereas in wg- embryos, very low levels of sal still can be detected in some embryos. The early expression of sal in the dorsal ectoderm still is observed in both wg- and wg-DWnt2- embryos. In wg-DWnt2- embryos, late kni expression in tracheal cells is normal, as is the case in arm mutants. In addition, dpp expression also is normal -- Dpp has been shown to inhibit sal expression by activating kni in tracheal cells. Thus, the lack of sal must be caused by the absence of direct or indirect stimulation by the DWnt pathway and not due to repression by the Dpp pathway (Llimargas, 2001).

The tubular epithelium of the Drosophila tracheal system forms a network with a stereotyped pattern consisting of cells and branches with distinct identity. The tracheal primordium undergoes primary branching induced by the FGF homolog Branchless; it differentiates cells with specialized functions such as fusion cells, which perform target recognition and adhesion during branch fusion, and extends branches toward specific targets. Specification of a unique identity for each primary branch is essential for directed migration, because a defect in either the Egfr or the Dpp pathway leads to a loss of branch identity and the misguidance of tracheal cell migration. The role of Wingless signaling in the specification of cell and branch identity in the tracheal system has been investigated. Wingless and its intracellular signal transducer, Armadillo, have multiple functions, including specifying the dorsal trunk through activation of Spalt expression and inducing differentiation of fusion cells in all fusion branches. Moreover, Wingless signaling regulates Notch signaling by stimulating Delta expression at the tip of primary branches. These activities of Wingless signaling together specify the shape of the dorsal trunk and other fusion branches (Chihara, 2000).

Wg signaling controls the formation of DT by regulating at least three target genes (sal, esg and Delta) in distinct ways. Sal is expressed in all DT cells and is required for directed migration along the anterior and posterior directions. Most of the cells in the Egfr domain can respond to Wg signaling by expressing Sal, and the expression of Sal is not affected by excess Delta. It is proposed that Sal expression is regulated by Wg signaling but not by Notch signaling, and that it serves as a major mediator of Wg signaling in determining DT identity. Regulation of Esg is more complex. Although Esg expression is stimulated by Wg signaling, it is normally limited to a single cell on each branch due to repression by Notch. Wg signaling activates Esg expression independently of Delta. It is proposed that Wg signaling bifurcates after activation of Arm, activating Esg on the one hand, and Delta on the other. Elevated Delta activates Notch in nearby cells, leading to repression of Esg in the stalk of tracheal branches. These combinatorial effects limit Esg expression to the tip of fusion branches. Stimulation of both positive and negative regulation of Esg by a single inductive signal comprises a self-limiting process of cell-fate determination and accounts for the assignment of single fusion cells that mark the end of the tracheal tubule. In combination with the specification of thick tubules through regulation of Sal, Wg signaling determines the shape of the tracheal tubule (Chihara, 2000).

The elbow B (elB) gene encodes a conserved nuclear protein with a single zinc finger. Expression of ElB is restricted to a specific subset of tracheal cells, namely the dorsal branch and the lateral trunks. Stalled or aberrant migration of these branches is observed in elB mutant embryos. Conversely, ElB misexpression in the trachea gives rise to absence of the visceral branch and an increase in the number of cells forming the dorsal branch. These results imply that the restricted expression of ElB contributes to the specification of distinct branch fates, as reflected in their stereotypic pattern of migration. Since elB loss-of-function tracheal phenotypes are reminiscent of defects in Dpp signaling, the relationship between ElB and the Dpp pathway was examined. By using pMad antibodies that detect the activation pattern of the Dpp pathway, it has been shown that Dpp signaling in the trachea is not impaired in elB mutants. In addition, expression of the Dpp target gene kni is unaltered. The opposite is true as well, because expression of elB is independent of Dpp signaling. ElB thus defines a parallel input, which determines the identity of the lateral trunk and dorsal branch cells. No ocelli (Noc) is the Drosophila protein most similar to ElB. Mutations in noc give rise to a similar tracheal phenotype. Noc is capable of associating with ElB, suggesting that they can function as a heterodimer. ElB also associates with the Groucho protein, indicating that the complex has the capacity to repress transcription of target genes. Indeed, in elB or noc mutants, expanded expression of tracheal branch-specific genes is observed (Dorfman, 2002).

In ElB misexpression embryos, several cells normally assigned to the dorsal trunk appear to be migrating into the dorsal branch, thus increasing the cell number in that branch. This suggests that there may also be a defect in specifying the dorsal trunk fate. Therefore, a dorsal trunk marker, Spalt (Sal), was followed in embryos misexpressing ElB. Sal is a transcription factor that is specifically expressed in the dorsal trunk cells and determines their identity. Ectopic ElB expression abolishes all Sal expression in the trachea. Surprisingly, this does not lead to an absence of the dorsal trunk, which is typical of sal mutant embryos, presumably because the btl-Gal4 driver induces the accumulation of ElB and abolishment of Sal expression only after execution of the normal Sal function in the dorsal trunk (Dorfman, 2002).

Spalt and sensory organ precursors

The nuclear proteins Spalt and Spalt-related belong to a conserved family of transcriptional regulators characterized by the presence of double zinc-finger domains. In the wing, they are regulated by the secreted protein Decapentaplegic and participate in the positioning of the wing veins. Regulatory regions in the spalt/spalt-related gene complex have been identifed that direct expression in the wing disc. The regulatory sequences are organized in independent modules, each of them responsible for expression in particular domains of the wing imaginal disc. In the thorax, spalt and spalt-related are expressed in a restricted domain that includes most proneural clusters of the developing sensory organs in the notum, and are regulated by the signaling molecules Wingless, Decapentaplegic and Hedgehog. spalt/spalt-related are found to participate in the development of sensory organs in the thorax, mainly in the positioning of specific proneural clusters. Later, the expression of at least spalt is eliminated from the sensory organ precursor cells and this is a requisite for the differentiation of these cells. It is postulated that spalt and spalt-related belong to a category of transcriptional regulators that subdivide the thorax into expression domains (prepattern) required for the localized activation of proneural genes (de Celis, 1999).

The genes sal and salr are expressed in identical patterns in the wing disc. The territory of expression includes a central stripe in the wing blade, anterior and posterior pleural regions and a subset of the proximal hinge and thorax. The region necessary to drive both sal and salr expression in the thorax and some domains of the wing is approximately 60 kb in size, and is located between the breakpoints of two translocations, FCK-25 and FCK-68. This DNA was cloned in fragments of 0.3 to 10 kb in front of the reporter lacZ gene to uncover the relevant regulatory elements in the wing disc. Several fragments direct expression of beta-gal in places where sal and salr are present in the thorax (AS, ABO, ABI and LA); hinge (EME, AL, LA); pleura (BE, EME, AK, BI), and wing blade (BE, EME, BO, LA, BI, AK). In most cases, the expression of beta-gal occurs both in subdomains where sal and salr are normally expressed and in specific ectopic domains. For each construct, beta-gal expression patterns are identical in (at least) three independent transformant lines, demonstrating that the complexity of each pattern is generated by the driving DNA, and is not due to insertion position effects. For instance, the constructs ABO and ABI, which overlap by approximately 2.2 kb, reproduce the endogenous expression of sal/salr in the posterior compartment of the thorax; they also express beta-gal in the anterior pleura, a place where these genes are not normally transcribed. Similarly, the construct LA is expressed in a pattern that includes part of the domain of endogenous sal/salr expression in the thorax and also in an anterior ectopic domain in the wing blade. Several constructs located 5' to the salr gene or in the large intron upstream of its coding region (BE, EME) direct generalized expression of beta-gal in the wing blade, in a pattern that includes the stripe of normal sal/salr expression but also adjacent ectopic anterior and posterior regions. The observation that consistent expression of beta-gal is driven by several constructs in places where sal and salr are not expressed suggests that the regulation of these genes involves the interplay of both activating and repressing regulatory sequences. The combined analysis of these beta-gal constructs, in a total of 92 independent transgenic lines, allows a broad localization of multiple DNA sequences responsible for sal and salr expression in the wing disc. It also suggests that the expression of sal/salr in the different parts of the wing disc is regulated in an independent manner. Previous work has shown that, in the wing blade, these two genes are regulated by Dpp, and the present molecular analysis predicts that the expression in thorax, hinge or pleura is achieved by a different set of factors (de Celis, 1999).

The sal and salr genes are expressed in only part of the thorax in three domains that have been defined with reference to en, wg and ci: the thoracic posterior compartment marked by En, an adjacent stripe anterior to the anteroposterior compartment boundary corresponding to the stripe of maximal accumulation of Ci, and a zone between the stripe of wg expression and the hinge. A fourth domain in the central thorax, from where only microchaetae develop, does not express sal/salr. To explore the regulatory mechanisms that localize sal/salr expression with respect to the anteroposterior compartment boundary and wg, experiments were performed in which genes that function in developmental signaling were expressed ectopically using the Gal4 system. A series of experiments led to the conclusion that, in the thorax, dual hh signaling is required to induce sal/salr expression: signaling through dpp and signaling that is dpp independent. Thus, expression of hh in clones within the central thorax (presumably accompanied by induction of dpp) leads to ectopic expression of sal/salr; interestingly, this ectopic expression is observed both in hh-expressing cells and in adjacent cells. In contrast, ectopic expression of dpp does not result in activation of sal transcription in the thorax or in the hinge, but it does so in the wing blade. The transcription factor Cubitus interruptus (Ci), a key mediator of Hedgehog signaling, was also experimentally mis-expressed in clones of cells. Ci is only able to activate sal ectopically in the wing blade, a place where ectopic expression of Ci results in novel expression of dpp, but not in the central thorax or wing hinge. In any other tissue studied to date, Hh signaling depends on Ci; since hh positively regulates sal in the thorax, the failure of ectopic Ci to activate sal expression there may be ascribed to the presence of countermanding repressors. However, even though dpp is not sufficient to induce sal/salr in the thorax, it is required. Thus, mitotic clones of Pka (corresponding to constitutive activation of hh signaling show cell autonomous expression of sal; in contrast, Pka;dpp double mutant clones do not express sal, indicating that, close to the Dpp source, hh and dpp signaling must cooperate to activate sal expression in the thorax. In agreement with this, the expression of sal can be reduced in tkv mutant cells, which have reduced levels of a Dpp receptor. The requirement of dpp function for induction of sal differs in different parts of the thorax. In the central thorax, where sal is normally not expressed, tkv clones have no effect. In region 2, where dpp is normally expressed, tkv clones result in reduced expression of sal. In other regions of the thorax, expression of sal is unaffected by the reduction of tkv (de Celis, 1999).

A prominent stripe of wg expression is seen in the thorax, within the sal non-expressing region 4 and close to the border of the sal-expressing region 3. This observation raises the possibility that wg may act as a repressor of sal expression in the thorax, and possibly in other regions of the wing disc. Indeed, in thoracic region 3, sal expression is repressed in and around clones of cells that overexpress wg. Furthermore, wg overexpression in the hinge region results in a reduction of sal expression, whereas a reduction of wg expression in the hinge of imaginal discs as a result of the regulatory mutation spade flag (wgspd) results in a consistent increase in sal expression. Reduction of wg expression in the thorax in a heteroallelic wingless combination results in the expansion of Sal expression. However, Sal is not expressed in all region 4, indicating that a repressor other than wg is responsible for the exclusion of Sal in this region. It is likely that both sal activation by hh/dpp and its repression by wg are mediated by the regulatory regions that have been identified as responsible for direct sal/salr expression in the thorax. The repressive action of wg is mediated by sequences contained within a fragment called the LA fragment. In these experiments, the inhibitory action of wg can be counteracted by sequences of the endogenous regulatory region located outside the LA fragment. Consistent with a requirement for both hh and dpp signaling to activate sal/salr in the thorax, the expression of beta-gal in the line LA is not modified when dpp is expressed ectopically (de Celis, 1999).

The domain of sal/salr expression in the thorax was mapped with more precision with respect to the emerging Sensory Organ Precursor cells (SOP), which can be identified in the disc using the reporter line neuralized-lacZ (neu-lacZ). The sensory organs included in the sal/salr domain are most of the lateral macrochaetae (ANP, PNP, ASA, PSA, APA, PPA) and also the ASC, PSC and PDC macrochaetae; the PS and ADC macrochaetae arise outside this domain. The names of the macrochaetae are: ADC and PDC, anterior and posterior dorsocentral; ASC and PSC, anterior and posterior scutellars; ASA and PSA, anterior and posterior supralar; ANP and PNP, anterior and posterior notopleural; APA and PPA, anterior and posterior postalar; PS, presutural. Other sensory organs in that region are tr1 and tr2 (sensillum tricoideum 1 and 2). sal and salr are not expressed in the central domain of the thorax, the region from which most of the microchaetae will develop during pupal development. This was confirmed by demonstrating that extramacrochaetae (emc), a negative regulator of the ac and sc genes that marks the microchaetae territory, is expressed in a domain nearly complementary to sal/salr. Sal is expressed in the domain that encompasses most of the macrochaetae proneural cell clusters. Interestingly, when specific cells of the proneural clusters are 'singled out' to form the SOP, sal expression is eliminated from these cells and their descendants. This is a requisite for SOP development, because when sal expression is experimentally maintained in SOP cells they do not differentiate. The localization of most macrochaetae SOPs within the territory of sal/salr expression in the thorax, and the dynamics of sal expression associated with proneural clusters (Sal+) and SOPs (Sal -), raises the possibility of a function for the sal/salr genes in sensory organ patterning. The requirement of sal and salr was studied by inducing clones of cells homozygous for a deficiency (Df(2L)32FP5), that includes both genes. Mutant clones in the thorax, marked with forked, were viable and of normal size, indicating that sal and salr are not required for the viability of thoracic cells. Loss of sal/salr causes the absence of two macrochaetae, the ANP and PNP, and also the anterior displacement of the PSC and PDC, which differentiate abnormally close to their anterior counterparts; the remaining seven macrochaetae are unaffected. A similar, albeit weaker, phenotype is observed in some allelic combinations in which sal and salr expression are reduced. Thus, combinations involving any salr allele over the deficiency of the complex causes the absence of the ANP macrochaetae (de Celis, 1999).

A sal/salr heteroallelic combination results in the reduction of sal/salr expression. In this genetic background, the ANP proneural cluster does not show ac expression and the ANP macrochaetae is not formed; these observations suggest that sal/salr contributes to the normal expression of ac, which is necessary for macrochaetae formation. However, most proneural clusters develop normally despite reduction of sal/salr function and many macrochaetae included in the expression domain of sal/salr differentiate normally in the total absence of these genes. Evidently, additional factors, besides sal and salr, participate in the transcriptional activation of ac/sc. These factors most likely correspond to previously characterized regulators of ac/sc, such as the genes of the iroquois complex (iro), which have similar, albeit weaker, and partially complementary effects compared to sal/salr on macrochaetae formation when they are expressed ectopically. As expected from the expression pattern of sal and salr, the differentiation of the microchaetae is not affected either in heteroallelic combinations or in homozygous deficiency clones. The influence of sal and salr on macrochaetae pattern formation has also been studied in experiments in which either of these genes was ectopically expressed in the thorax using the Gal4 system. In the case of salr, two different UAS lines, named UAS-salr1 and UAS-salr2, which produce low and high levels of Salr expression, respectively, were used in combination with any Gal4 driver. When sal or salr are expressed at low levels in all thoracic cells several extra macrochaetae differentiate in ectopic positions, indicating that sal and salr have the capability to promote SOP development. In addition, stronger ectopic expression of salr causes the absence or size reduction of some macrochaetae. These contradictory effects on the pattern of macrochaetae when sal/salr are ectopically expressed are associated with defective localization of proneural clusters in the imaginal disc. Thus, generalized low expression of salr causes a weak expansion of the APA proneural cluster, whereas higher levels of generalized salr expression result in a broader rearrangement of proneural clusters, including a larger expansion of the DC and NP (which now appear fused), and the apparent loss of the PA proneural cluster. These results indicate that the restriction and levels of sal/salr expression participate in the positioning of proneural clusters in the thorax. Other observations indicate that the elimination of sal expression from the SOPs observed in normal development is functionally significant, being a requisite for later SOP differentiation. Thus dynamic expression of sal, first sal upregulation and subsequently sal repression, are required for proper SOP development (de Celis, 1999).

Transforming growth factor ß signaling mediated by Decapentaplegic and Screw is known to be involved in defining the border of the ventral neurogenic region in the fruitfly. A second phase of Decapentaplegic signaling occurs in a broad dorsal ectodermal region. The dorsolateral peripheral nervous system forms within the region where this second phase of signaling occurs. Decapentaplegic activity is required for development of many of the dorsal and lateral peripheral nervous system neurons. Double mutant analysis of the Decapentaplegic signaling mediator Schnurri and the inhibitor Brinker indicates that formation of these neurons requires Decapentaplegic signaling, and their absence in the mutant is mediated by a counteracting repression by Brinker. Interestingly, the ventral peripheral neurons that form outside the Decapentaplegic signaling domain depend on Brinker to develop. The role of Decapentaplegic signaling on dorsal and lateral peripheral neurons is strikingly similar to the known role of Transforming growth factor ß signaling in specifying dorsal cell fates of the lateral (later dorsal) nervous system in chordates (Halocythia, zebrafish, Xenopus, chicken and mouse). It points to an evolutionarily conserved mechanism specifying dorsal cell fates in the nervous system of both protostomes and deuterostomes (Rusten, 2002).

The embryonic abdominal (A) PNS of Drosophila consists of three bilateral clusters of neurons (ventral, lateral and dorsal) per segment, which can be most especially appreciated in the serially homologous segments A1-A7. In order to investigate whether the second phase of Dpp signaling is necessary for patterning the PNS, mutant alleles for a gene involved in the Dpp signaling pathway, schnurri (shn), were examined. This gene encodes a zinc-finger transcription factor that is necessary for the transcription of some Dpp target genes and binds directly to the main Dpp mediator Mothers against Dpp (Mad). Unlike the zygotic mutants of dpp, scw, tolloid (tld) or mad, shn mutants have no effect on the initial dpp/scw governed dorsoventral patterning of the blastoderm. They express normally the early Dpp target genes, such as pannier (pnr, stage 7), dpp itself in the dorsal ectoderm (stage 9) and Krüppel (Kr) (which is a marker for the amnioserosa), showing that the dorsal ectoderm is correctly specified. By contrast, several Dpp target genes that are expressed following the second phase of Dpp signaling are affected in shn zygotic mutants: at stage 11, the expression of genes responsive to Dpp signaling, such as dad, pnr, spalt or dpp itself is reduced or lost. Thus, any failures in PNS formation, which are observed in shn mutant embryos, must originate from the second rather than the first phase of Dpp signaling and are likely to be mediated by Shn. PNS malformations were sought in strong shn zygotic mutant embryos using the ubiquitous PNS neuronal marker 22C10. Homozygous shn1 and shnk00401 fail to undergo dorsal closure and show severe defects of PNS development. A strong reduction in number of neurons is observed, especially in the dorsal and lateral PNS clusters, although it is difficult to determine exactly which neurons are affected because of the dorsal closure failure. Therefore, another allele, shnk04412, which does undergo dorsal closure, was also examined. In these embryos, position and identity of PNS neurons could be more clearly assigned. In homozygosity, as well as in transheterozygosity over shn1, this mutant shows a reduction in the number of dorsal and lateral neurons, similar to the other mutants analyzed. These results are consistent with a role for Shn-mediated Dpp signaling in the formation of the dorsal and lateral PNS (Rusten, 2002).

EGF receptor signaling regulates pulses of cell delamination from the Drosophila ectoderm: Activation of Spalt

Many different intercellular signaling pathways are known but, for most, it is unclear whether they can generate oscillating cell behaviors. Time-lapse analysis of Drosophila embryogenesis has been used to show that oenocytes delaminate from the ectoderm in discrete bursts of three. This pulsatile process has a 1 hour period, occurs without cell division, and requires a localized EGF receptor (EGFR) response. High-threshold EGFR targets are sequentially activated in rings of three cells, prefiguring the temporal pattern of delamination. Surprisingly, widespread misexpression of the relevant activating ligand, Spitz, is compatible with robust delamination pulses. A single chordotonal organ precursor (called C1) and its progeny provide the source of secreted Spi relevant for oenocyte induction. Although Spitz ligand becomes limiting after only two pulses, artificially prolonging its secretion generates up to six additional cycles, revealing a rhythmic underlying mechanism. These findings illustrate how intercellular signaling and cell movements can generate multiple cycles of a cell behavior, despite individual cells experiencing only one cycle of receptor activation (Brodu, 2004).

The induction of larval oenocytes in Drosophila has been used as a simple model system for investigating the developmental regulation of EGFR signaling. Oenocytes are induced from the dorsal ectoderm of abdominal segments by a fixed and highly restricted source of Spi. This triggers a local EGFR response within a ring of overlying dorsal ectodermal cells, termed a whorl, leading to the upregulation of numerous oenocyte-specification genes and subsequent cell delamination. The simple cell geometry of the oenocyte whorl, together with time-lapse microscopy, was used to explore the timing of Spi secretion, EGFR-target activation, early cell induction, and later cell delamination. These studies reveal that oenocytes delaminate in bursts of three and identify the cell-counting mechanism as an EGFR-dependent pulse generator converting the time window of Spi secretion into final oenocyte number. This represents the first example of a rhythmic cell behavior other than the cell cycle to be reported in the Drosophila embryo (Brodu, 2004).
Rather than delaminating from the ectoderm in a continuous stream, oenocyte precursors segregate in discrete well-separated bursts of three cells. Genetic backgrounds affecting the pattern of cell segregation but not early fate specification were used to show how these pulses are regulated by EGFR signaling. The signaling parameters regulating the time of onset, time of cessation, and in particular, the cyclical nature of cell delamination of oenocytes are discussed (Brodu, 2004).

Using a panel of markers for double- and single-ring stages, it was possible to place gene expression 'snapshots' in temporal order with the cell movements recorded in movies. Three generic EGFR targets (activated Rolled/ERK, Yan, and argos) and three oenocyte-specific EGFR targets (Sal, svplacZ, and svplacZΔ18) were analyzed. In wild-type embryos, the high-threshold EGFR outputs of argos and svplacZ expression, detectable Rolled activation, and strong Yan downregulation are all inner ring specific, whereas lower-threshold outputs such as Sal upregulation and svplacZΔ18 expression are present in both precursor rings. Delamination itself also appears to be a high-threshold EGFR response and is thus confined to the inner ring (Brodu, 2004).

Extradenticle and homothorax control adult muscle fiber identity in Drosophila

This study had identified a key role for the homeodomain proteins Extradenticle (Exd) and Homothorax (Hth) in the specification of muscle fiber fate in Drosophila. exd and hth are expressed in the fibrillar indirect flight muscles but not in tubular jump muscles, and manipulating exd or hth expression converts one muscle type into the other. In the flight muscles, exd and hth are genetically upstream of another muscle identity gene, salm, and are direct transcriptional regulators of the signature flight muscle structural gene, Actin88F. Exd and Hth also impact muscle identity in other somatic muscles of the body by cooperating with Hox factors. Because mammalian orthologs of exd and hth also contribute to muscle gene regulation, these studies suggest that an evolutionarily conserved genetic pathway determines muscle fiber differentiation (Bryantsev, 2012).

These results demonstrate a dramatic effect upon muscle fiber identity of the two factors Exd and Hth. The fact that muscle fiber type can be profoundly influenced by the activity of the two genes defines a central mechanism for the control of fiber identity and begins to expose the entire fiber specification pathway (Bryantsev, 2012).

A recent study identified salm as a controller of transition from tubular leg muscle to the fibrillar fiber type. Tubular leg muscles could be transformed into the fibrillar type by ectopic expression of salm. This study has expand these observations to show that the mechanism of Salm action is less straightforward: salm is expressed in the tubular jump muscle, suggesting that its pro-fibrillar action may require cooperation with additional factors. The current data suggest that Salm cofactors could be Exd and Hth: their absence in the jump muscle prevents this muscle from acquiring a fibrillar fiber phenotype despite its expression of salm; also, ectopic expression of salm in leg muscles promotes fibrillar fate, perhaps because the leg muscles also express exd and hth (Bryantsev, 2012).

It is also noted that, in the flight muscles, Exd and Hth maintain their localization in the absence of Salm. Moreover, despite the sustained accumulation of Exd and Hth, loss of Salm nevertheless results in transformation of the flight muscles toward a tubular fate. This indicates that Exd and Hth have at least some requirement for Salm to promote fibrillar muscle fate, and it will be interesting in the future to identify the respective roles of these factors directly interacting with other fiber-specific enhancers (Bryantsev, 2012).

This study also provides a direct mechanistic link between the determinants of fibrillar fate, exd/hth, and the actin gene characteristic of the flight muscles, Act88F. Whether fibrillar muscle genes are direct targets of Exd/Hth or Salm, or both, is yet to be determined; nevertheless, the identification of fiber-specific enhancers will provide new mechanistic insight into this process (Bryantsev, 2012).

Since diverse fiber types are characteristic of many vertebrate muscles, these findings may relate directly to vertebrate myogenesis. In zebrafish, slow muscle fate is promoted by the activities of PBX and MEIS, which are the vertebrate orthologs of Exd and Hth, respectively (Maves, 2007). In mice, PBX and MEIS are cofactors for myogenic determination genes, where they facilitate transcription factor binding to nonconsensus target sites, and this effect might function to fine tune muscle fiber fate (Heidt, 2007). Thus, diverse lines of evidence suggest a robust and conserved mechanism for fiber type specification, acting through PBX1/Exd and MEIS/Hth (Bryantsev, 2012).

Spalt in imaginal discs

Continued Transcriptional Regulation part 2/2

spalt: Biological Overview | Evolutionary Homologs | Targets of Activity | Developmental Biology | Effects of Mutation | References

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