argos

Transcriptional Regulation

argos expression in the ventral ectoderm is induced by the EGF-R pathway: argos is not expressed in EGF-R mutant embryos, while it is ectopically expressed in the entire ventral ectoderm following ubiquitous activation of the EGF-R pathway. argos expression appears to be triggered directly by the EGF-R pathway, since induction can also be observed in cell culture, following activation of EGF-R by its ligand, Spitz (Golembo, 1996).

EGF receptor signaling is required in neural recruitment during formation of Drosophila chordotonal sense organ clusters. A total of five neural precursors express atonal in abdominal segments, and this number is too few to explain the formation of the eight scolopidia in each abdominal segment. The remaining precursors require Egf-R signaling for their selection. Signaling by the founder precursors is initiated by atonal activating (directly or indirectly) rhomboid expression in the founder cells. It should be noted that in some developmental processes, rhomboid appears to function in the signal-receiving cells, such as in the patterning of ovarian follicle cells. It is not believed that this is the case in chordotonal-precursor formation, because rho is expressed in precursors that do not require rhomboid function (C1-C5 are formed even in rhomboid mutants). Signaling by these founder precursors, presumably through the EGF receptor ligand Spitz, then provokes a response in the surrounding ectodermal cells, as shown by the activation of expression of the Egf-R target genes pointed and argos. The signal and response then leads to recruitment of some of the ectodermal cells to the chordotonal precursor cell fate. Egf-R hyperactivation by misexpression of rhomboid results in excessive chordotonal precursor recruitment. Argos functions in a feedback mechanism to prevent the excess recruitment of additional ectodermal cells. The increase in the number of scolopidia caused by Egf-R hyperactivation is confined to an enlargement of existing cluster sizes: no new chordotonal clusters are formed. A two step mechanism is postulated for the formation of clusters of chordotonal precursors. In the first step, precursors C1-C5 are selected as founder precursors by the conventional route of proneural gene expression and lateral inhibition. In a distinct second phase, these precursors then signal to adjacent ectodermal cells via the Egf-r pathway, inducing some of them to become chordotonal precursors (secondary or recruited precursors). This two-step process is strongly reminiscent of the way atonal acts in neurogenesis in compound eyes. Here, atonal expression is initially refined by lateral inhibition, until atonal is expressed in only the founding R8 precursor, which then recruits R1-R7 in a mechanism that does not require the activation of atonal in these cells (zur Lage, 1997).

During oogenesis, Egfr activation is required for the establishment of the dorsal/ventral axis of the egg and the embryo. To examine how ectopic Egfr activation affects cell fate determination, an activated version of the protein was constructed. Expression of this activated form (lambda top) in the follicle cells of the ovary induces dorsal cell fates in both the follicular epithelium and the embryo. Different levels of expression result in different dorsal follicle cell fates. Among the anterior follicle cells, a minimum of three cell fates can be distinguished by their contribution to the final eggshell morphology. The most dorsal cells produce the midline/operculum region; dorsolateral cells secrete the respiratory appendages, and the ventral cells contribute to the main body. The three populations of follicle cells in the anterior of the developing egg chamber express different combinations of downstream genes. The dorsal midline cells express argos, kekkon1, rhomboid and pointed. The dorsolateral cells express rho and kek1. The ventral follicle cells are distinguished by the expression of CF2 (Queenan, 1997).

Even in cases where all the follicle cells covering the oocyte expresse lambda top, dorsal cell fates are expanded in the anterior, but not the posterior, of the egg. The expression of genes known to respond to Egfr activation (aos, kek 1 and rho) are also expanded in the presence of the lambda top construct. When lambda top is expressed in all the follicle cells covering the oocyte, kek 1 and argos expression are induced in follicle cells all along the anterior/posterior axis of the egg chamber. In contrast, rho RNA expression is only activated in the anterior of the egg chamber. These data indicate that the response to Egfr signaling is regulated by an anterior/posterior prepattern in the follicle cells. Expression of lambda top in the entire follicular epithelium results in an embryo dorsalized along the entire anterior/posterior axis. Expression of lambda top in anterior or posterior subpopulations of follicle cells results in regionally autonomous dorsalization of the embryos. This result indicates that subpopulations of follicle cells along the anterior/posterior axis can respond to Top/Egfr activation independently of one another (Queenan, 1997).

In addition to essential myogenic functions, mutant Mef2 adult females are weakly fertile and produce defective eggs. Mef2 is expressed in nurse and follicle cells of the wild-type egg chamber. The Mef2 oogenic phenotype has been analyzed and it has been shown that the gene is required for the normal patterning and differentiation of the centripetally migrating follicle cells (CMFCs) that are crucial for development of the anterior chorionic structures. Mef2 alleles exhibit a genetic interaction with a dominant-negative allele of thick veins (tkv), which encodes a type I receptor of the Decapentaplegic-signaling pathway. TKV mRNA is overexpressed in Mef2 mutant egg chambers, and, conversely, forced expression of Mef2 represses tkv expression. These results indicate roles for Mef2 in the regulation of tkv gene expression and Decapentaplegic signal transduction that are essential for proper determination and/or differentiation of the anterior follicle cells. Mef2 is also expressed in both nurse and follicle cells. No defects have been observed in the germ line, either the number of germ cells or the location of the oocyte within the egg chamber. Therefore, a possible requirement for Mef2 in germ-line cells remains to be elucidated (Mantrova, 1999).

At stage 10B, tkv is expressed in the ventral half of the CMFC in addition to two short stripes in the dorsal region of the oocyte-associated follicular epithelium. This expression pattern appears to be complementary to that of the Egfr blocker argos, which forms a T-shaped pattern along the dorsal CMFCs and dorsal midline. argos expression is induced by the highest level of Egfr signaling; Egfr in turn, reduces the signaling strength by blocking the interaction between the receptor and its ligands. Thus, the initial graded distribution of Egfr signaling, extending laterally from the anterodorsal midline of the O-FCs, is transformed into two ridges of the Egfr-signaling level just lateral to the dorsal midline. These two ridges define the two lines of O-FCs that ultimately produce the two dorsal appendages. Interestingly, argos expression is diminished in the Mef2 mutant, consistent with the observed mutant egg chambers possessing broad and fused appendages. Although the notion is favored that argos expression is modulated by Mef2 through the action of Tkv, it cannot be ruled out that Mef2 may directly control the transcription of argos (Mantrova, 1999).

In addition to regulating the expression pattern of argos, Mef2 may play a more general role in modulating the Egfr-signaling level. This is suggested by the presence of Mef2 mutant egg chambers with reduced and fused dorsal appendages, a phenotype typical of hypomorphic Egfr-signaling pathway mutants. Indeed, reduced expression of Egfr-signaling components such as rhomboid has been observed in Mef2 mutants. More detailed and expansive studies are needed to elucidate the possible interaction between the Dpp- and Egfr-signaling pathways with Mef2 as a potential mediator (Mantrova, 1999).

Terminal development is disrupted in the dead ringer maternal and zygotic mutant embryos. Both head and tail defects are invariably observed in dri maternal and zygotic mutant embryos. One of the most consistent and striking phenotypes is severe disruption of the cephalo-pharyngeal skeleton. Germline and zygotic dri mutant embryos still have a recognizable dorsal bridge, dorsal and ventral arms and mouth hooks, but the H-piece and lateralgraten are missing or severely malformed. In addition, the atypical anterior position of pharyngeal muscles, visualized using anti-muscle myosin immunostaining, indicates that head involution does not proceed properly (Shandala, 1999).

The appearance of these defects prompted an examination of genes that play a role in the formation of terminal structures. Expression of the terminal gene tailless and the genes buttonhead, empty spiracles, orthodenticle and argos was examined. Of these genes, disruption to only argos (aos) and buttonhead (btd) expression was observed. In wild-type embryos, aos is initially expressed at stage 5 in two terminal domains and a domain that flanks the position of the cephalic furrow. In embryos lacking dri maternal and zygotic product, expression of aos in the terminal domains is almost completely eliminated while expression in the region of the cephalic furrow is maintained, both before and after division into two stripes at the time of cephalic furrow formation. Zygotic aos mutant embryos exhibit head defects that are similar to those observed in maternal and zygotic dri mutant embryos, indicating that the dri mutant head defects are likely to be the result of loss of anterior aos expression in the dri mutant embryos. Analysis of btd expression reveals a regulatory relationship that accounts for another consistent dri mutant phenotype, the appearance of ectopic cephalic furrows. btd expression is found to be partially derepressed in the trunk of dri germline and zygotic mutant embryos. The cephalic furrow arises where expression of the head specific gap gene btd overlaps the first stripe of expression of the primary pair rule gene eve. The repetitive appearance of ectopic cephalic furrows is therefore likely to be the result of the coincident ectopic trunk expression of btd with the more posterior eve stripes. The ectopic furrows do not progress, most probably due to the incomplete derepression of btd in this region (Shandala, 1999).

The Drosophila EGF receptor (Egfr) is required for the specification of diverse cell fates throughout development. How the activation of Egfr controls the development of vein and intervein cells in the Drosophila wing has been examined. Two distinct events are involved in the determination and differentiation of wing vein cells: (1) the establishment of a positive feedback amplification loop, which drives Egfr signaling in larval stages (at this time, rhomboid, in combination with vein, initiates and amplifies the activity of Egfr in vein cells); (2) the late downregulation of Egfr activity [at this point, the inactivation of MAPK in vein cells is necessary for the maintenance of the expression of decapentaplegic (dpp) and becomes essential for vein differentiation. Subseqently, Egfr becomes activated in intervein territories. During the time that dpp is expressed in vein territiories, MAPK activity builds up in intervein territories, probably due to the presence of Vn, a weak Egfr activator. As a consequence, aos expression relocates to intervein territories. Together, these temporal and spatial changes in the activity of Egfr constitute an autoregulatory network that controls the definition of vein and intervein cell types (Martin-Blanco, 1999).

The reiterated use of Egfr is a common effector of differentiation. In the Drosophila eye, Egfr is required for the determination of all cell types. In this system, cell fate depends on the developmental stage at which the receptor is activated. By interfering with Egfr signaling activity, the specification of veins respond to the activation of receptor tyrosine kinase (RTK) signaling during larval stages, but continued activation of RTK signaling results in a failure of vein cells to differentiate. One explanation for these opposite effects could be that early activation of RTK signaling would specify vein cells, while late RTK signaling would implement intervein cell fates. Several observations provide support for this model. In pupae, MAPK is repressed in veins and activated in intervein cells. This activation of MAPK (and the expression of downstream genes, such as argos) responds to Ras signaling activity, and appears to be involved in the suppression of vein cell fates. Indeed, after ectopical activation of D-Raf during the pupal period, promoting intervein cell fates, the MAPK activity remains stimulated all over the wing blade (Martin-Blanco, 1999).

It seems that Egfr is the only receptor tyrosine kinase at work in the wing, able to activate Ras and Raf. While Egfr is ubiquitously expressed during larval imaginal disc development, EGFR mRNA levels are downregulated in the pupal period in presumptive vein cells. This downregulation of Egfr could be involved in the supression of MAPK activity in vein territories. Furthermore, when a dominant negative-Egfr (DN-Egfr) molecule is overexpressed, titrating the endogenous Egfr, in pupae, extra vein tissue is induced. MAPK dephosphorylation in veins could also be induced by other mechanisms; for instance, the early expression of the inhibitor ligand Argos in veins up to 24 hours APF could cooperate in the inactivation of MAPK in these territories (Martin-Blanco, 1999).

What is the function of this change of expression? The first effect of this developmental switch is a modification in the expression of downstream targets. As a consequence of the reduction in MAPK activity from vein cells, aos is eliminated from veins between 24 and 30 hours APF. Conversely, it is upregulated in intervein territories. This scenario is reminiscent of the induction of Egfr ligands in the ventral ectoderm. Here, the primary signal, Spitz induces a relay mechanism by triggering the expression of Vn (and Aos) in adjacent cells. Aos reduces the overall level of Egfr signaling, whereas Vn provides a lower level of activation, capable of inducing only the lateral cell fates. In the larval wing, high levels of Egfr signaling are achieved in veins through a positive feedback loop. Here, Egfr activity promotes the expression of Aos. It is suggested that Aos diffusion from veins could prevent adjacent cells from responding to the vein inductive signals and producing high levels of Egfr activity ('remote inhibition'). Consistently, aos mutant flies display small deltas and extra veins clustered around vein territories. On the contrary, Aos overexpression in larval stages induces the suppression of veins. It is also proposed that, in pupae, while Egfr activity (and Aos) in veins are lost, Vn and Aos expression in intervein cells will reach a competitive balance leading to the activation of Egfr and MAPK, and intervein cell specification (Martin-Blanco, 1999 and references therein).

The tissue-specific regulation of Vn signaling was investigated by examining vn transcriptional control and Vn target gene activation in the embryo and the wing. The results show a complex temporal and spatial regulation of vn transcription involving multiple signaling pathways and tissue-specific activation of Vn target genes. In the embryo, vn is a target of Spi/Egfr signaling mediated by the ETS transcription factor PointedP1 (PntP1). This establishes a positive feedback loop in addition to the negative feedback loop involving Aos. The simultaneous production of Vn provides a mechanism for dampening Aos inhibition and thus fine-tunes signaling. In the larval wing pouch, vn is not a target of Spi/Egfr signaling but is expressed along the anterior-posterior boundary in response to Hedgehog (Hh) signaling. Repression by Wingless (Wg) signaling further refines the vn expression pattern by causing a discontinuity at the dorsal-ventral boundary. The potential for vn to activate Egfr target genes correlates with its roles in development: vn has a minor role in embryogenesis and does not induce Egfr target genes such as aos and pntP1 in the embryo. Conversely, vn has a major role in wing development and Vn/Egfr signaling is a potent inducer of Egfr target genes in the wing disk. Spi also has the potential to induce Egfr target genes in the wing disk. However, the ligands appear to evoke specific responses that result in different patterns of target gene expression. Other factors modulate the potential of Vn so that induction of Vn/Egfr target genes in the wing pouch is cell specific (Wessells, 1999).

Differences between Vn and Spi are apparent in the patterns of target gene induction resultant from the ectopic expression of Vn and soluble Spitz (sSpi) in the wing pouch. Effects have been noted for three Egfr target genes: aos, pntP1, and kekkon-1 (kek1). In each case, a different response to the ligands is seen. Both ligands induce ectopic aos expression but Vn does so in a broader domain than sSpi, however, neither induces aos in the L3/L4 intervein region. In the embryo, the Egfr target gene pntP1 mediates aos induction by sSpi. Likewise in the wing, ectopic sSpi induces pntP1 expression in cells that also expressed aos. However, following ectopic Vn no detectable change in pntP1 expression is seen using in situ hybridization and only very weak induction of pntP1-lacZ is seen in a domain that does not correspond fully with ectopic aos expression. This suggests either that another transcription factor mediates the induction of aos in response to Vn or that PntP1 is capable of inducing aos, even when changes in its own expression level are too low to be detected by current methods (Wessells, 1999).

Localized activation of the Ras/Raf pathway by epidermal growth factor receptor (Egfr) signalling specifies the formation of veins in the Drosophila wing. However, little is known about how the Egfr signal regulates transcriptional responses during the vein/intervein cell fate decision. Evidence is provided that Egfr signaling induces expression of vein-specific genes by inhibiting the Capicua (Cic) HMG-box repressor, a known regulator of embryonic body patterning. Lack of Cic function causes ectopic expression of Egfr targets such as argos, ventral veinless and decapentaplegic and leads to formation of extra vein tissue. In vein cells, Egfr signaling downregulates Cic protein levels in the nucleus and relieves repression of vein-specific genes, whereas intervein cells maintain high levels of Cic throughout larval and pupal development, repressing the expression of vein-specific genes and allowing intervein differentiation. However, regulation of some Egfr targets such as rhomboid appears not to be under direct control of Cic, suggesting that Egfr signaling branches out in the nucleus and controls different targets via distinct mediator factors. These results support the idea that localized inactivation of transcriptional repressors such as Cic is a rather general mechanism for regulation of target gene expression by the Ras/Raf pathway (Roch, 2002).

There are two key aspects of Cic function as a developmental regulator: its ability to repress specific target genes in defined territories, and its inhibition by the Ras/Raf pathway to allow expression of those targets in complementary positions. In the blastoderm embryo, Cic is required for development of trunk body regions and represses genes mediating differentiation of terminal structures. Torso RTK activation at each pole of the embryo alleviates Cic-dependent repression and initiates the terminal gene expression program. A similar model is proposed for cic function during specification of vein versus intervein fate in the wing. Loss of cic function in the wing causes formation of ectopic vein tissue, implying that Cic mediates intervein specification by restricting vein formation to appropriate regions. In intervein territories, Cic behaves as a repressor of vein-specific genes such as argos and vvl but does not seem to affect directly the expression of blistered, which is required for the specification of intervein fates. Finally, Egfr signaling leads to downregulation of Cic protein levels in vein nuclei, thus relieving Cic-mediated repression and promoting vein development (Roch, 2002).

Convergent intercellular signals must be precisely integrated in order to elicit specific biological responses. During specification of muscle and cardiac progenitors from clusters of equivalent cells in the Drosophila embryonic mesoderm, the Ras/MAPK pathway -- activated by both epidermal and fibroblast growth factor receptors -- functions as an inductive cellular determination signal, while lateral inhibition mediated by Notch antagonizes this activity. A critical balance between these signals must be achieved to enable one cell of an equivalence group to segregate as a progenitor while its neighbors assume a nonprogenitor identity. Whether these opposing signals directly interact with each other has been investigated, and how they are integrated by the responding cells to specify their unique fates was been examined. Two distinct modes of lateral inhibition, one Notch based and a second based on the epidermal growth factor receptor antagonist Argos, are described that have complementary and reinforcing functions. Argos/Ras and Notch do not function independently; rather, several modes of cross-talk between these pathways have been uncovered. Ras induces Notch, its ligand Delta, and Argos. Delta and Argos then synergize to nonautonomously block a positive autoregulatory feedback loop that amplifies a fate-inducing Ras signal. This feedback loop is characterized by Ras-mediated upregulation of proximal components of both the epidermal and fibroblast growth factor receptor pathways. In turn, Notch activation in nonprogenitors induces its own expression and simultaneously suppresses both Delta and Argos levels, thereby reinforcing a unidirectional inhibitory response. These reciprocal interactions combine to generate the signal thresholds that are essential for proper specification of progenitors and nonprogenitors from groups of initially equivalent cells (Carmena, 2002).

This study involves the origin of two progenitors from a single cell cluster. The two progenitors are characterized by expression of the segmentation gene eve and are specified in a distinct temporal order in the Drosophila embryonic mesoderm. Progenitor 2 (P2) develops first; it originates from the preC2 cluster which develops into the C2 cluster and subsequently gives rise to a single P2 cell. P2 divides asymmetrically to give rise to two founder cells, one specific for a pair of persistently Eve-positive heart-associated or pericardial cells (EPCs) in every hemisegment and a second of previously undetermined identity. This second founder coexpresses Eve along with the gap gene Runt, with Eve levels rapidly fading but Runt persisting as development proceeds. By the time that Eve is evident in the EPCs, Runt labels a single somatic muscle, dorsal oblique muscle 2 (DO2). Runt is also detected in the muscle DO2 precursor during germband retraction (Carmena, 2002).

The second Eve progenitor, P15, which also has its origin in the preC2 cluster (which gives rise to a C15 cluster) forms later than P2 and divides asymmetrically to yield the founders of dorsal acute muscle 1 (DA1) and another cell whose fate cannot be followed since a specific, stably expressed marker for it is unavailable (Carmena, 2002).

Although the net effect of Ras and N signaling in the present system is the result of their antagonistic relationship, several forms of cooperative cross-talk also occur. For example, Ras activation induces the expression of Dl. Since the Ras signal is amplified by a positive feedback loop, this has the effect of biasing Dl expression to the emerging progenitor, thereby generating a polarized, nonautonomous inhibitory signal that acts on adjacent cells of the cluster. Aos is also a target of Ras activation, and Aos acts synergistically with the neurogenic pathway to block inductive RTK signaling. Thus, through its effects on the two inhibitory ligands, Dl and Aos, Ras cooperates with N to ensure that only one cell segregates as a progenitor from each equivalence group (Carmena, 2002).

Further cooperation is evident in the N-mediated down-regulation of Dl and Aos in prospective nonprogenitors, a combination of negative feedback and cross-talk that effectively prevents neighboring cells from sending an inhibitory signal to the emerging progenitor. Yan is yet another Ras-dependent component that reinforces the effect of N: when MAPK is suppressed in cells in which N is active, Yan is a functional repressor that blocks progenitor fate. Other examples of cooperation between Ras and N signaling include mammalian cell tumorigenesis and photoreceptor specification in the Drosophila eye (Carmena, 2002).

These results also revealed an effect of Aos on Htl- but not Egfr-dependent C2 cluster development. Although this could be interpreted as indicating a role for Aos in the inhibition of the Htl Fgfr, the idea that Aos is actually blocking basal and/or spontaneous levels of Egfr activation in C2 cells is favored. This interpretation is supported by the finding that a dominant negative form of Egfr suppressed the effect of aos loss-of-function not only in Egfr-dependent C15, but also in C2, which does not require Egfr for its specification. In this cluster, the requisite Aos expression is dependent on Htl activity (Carmena, 2002).

The stereotyped pattern of Drosophila wing veins is determined by the action of two morphogens, Hedgehog (Hh) and Decapentaplegic (Dpp), which act sequentially to organize growth and patterning along the anterior-posterior axis of the wing primordium. An important unresolved question is how positional information established by these morphogen gradients is translated into localized development of morphological structures such as wing veins in precise locations. In the current study, the mechanism has been examined by which two broadly expressed Dpp signaling target genes, optomotor-blind (omb) and brinker (brk), collaborate to initiate formation of the fifth longitudinal (L5) wing vein. omb is broadly expressed at the center of the wing disc in a pattern complementary to that of brk, which is expressed in the lateral regions of the disc and represses omb expression. A border between omb and brk expression domains is necessary and sufficient for inducing L5 development in the posterior regions. Mosaic analysis indicates that brk-expressing cells produce a short-range signal that can induce vein formation in adjacent omb-expressing cells. This induction of the L5 primordium is mediated by abrupt, which is expressed in a narrow stripe of cells along the brk/omb border and plays a key role in organizing gene expression in the L5 primordium. Similarly, in the anterior region of the wing, brk helps define the position of the L2 vein in combination with another Dpp target gene, spalt. The similar mechanisms responsible for the induction of L5 and L2 development reveal how boundaries set by dosage-sensitive responses to a long-range morphogen specify distinct vein fates at precise locations (Cook, 2004).

Extension of a previous analysis of ab in initiating L5 development has shown that ab functions early in L5 specification. Activation of all known vein genes, including rho, Dl, the caupolican and araucan genes of the Iroquois Complex (IroC), and argos, and repression of the intervein genes bs (also known as DSRF) and net, is lost in cells corresponding to the L5 primordium in ab¹ mutant wing discs. A determination was also made whether it is critical that ab expression is confined to a narrow stripe for regulating expression of vein or intervein genes. ab was ubiquitously misexpressed in the wing disc using the MS1096-GAL4 driver; such global activation of ab suppresses expression of vein genes, such as rho and Dl. This ab misexpression also caused vein-specific downregulation of the intervein gene bs, in the wing disc, but did not repress expression of other genes, including hh, ptc and dpp. This phenotype may result from unregulated production of a lateral inhibitory signal normally produced by vein cells to suppress vein development in adjacent intervein cells (Cook, 2004).

Lozenge directly activates argos and klumpfuss to regulate programmed cell death

Reducing the activity of the Drosophila Runx protein Lozenge (Lz) during pupal development causes a decrease in cell death in the eye. Lz-binding sites were identified in introns of argos (aos) and klumpfuss (klu); these genes were shown to be directly activated targets of Lz. Loss of either aos or klu reduces cell death, suggesting that Lz promotes apoptosis at least in part by regulating aos and klu. These results provide novel insights into the control of programmed cell death (PCD) by Lz during Drosophila eye development (Wildonger, 2005).

These findings, together with what is known about aos and klu, support the following model: Lz induces aos expression in cone cells, wherefrom Aos diffuses to antagonize EGFR activity in the surrounding 2° and 3° cells. The expression pattern of aos⁹²³-lacZ indicates that Lz also regulates aos expression in 2° and 3° cells, suggesting that these cells may also send antisurvival signals. The data further suggest that within the 2° and 3° cells, Lz activates klu, which antagonizes EGFR signaling downstream of the receptor. Lz also activates klu expression in cone and 1° cells, but it is unclear what function klu has in these cells. Although two phases of PCD during retinal development have been proposed, these experiments support a role for Lz in promoting only the EGFR-dependent phase. An alternative possibility is that the decrease in cell death in lz mutant retinas is due to an increase in 2° and 3° cell differentiation stimulated by an increase in EGFR signaling. However, given the large body of evidence demonstrating that lz normally functions to promote differentiation, a model in which lz acts to suppress differentiation is not favored (Wildonger, 2005).

The mammalian homolog of Lz, Runx1 (also known as AML1), is also a transcriptional regulator. In humans, translocations that affect Runx1 are associated with acute myelogenous leukemia (AML), which is characterized by the proliferation of undifferentiated hematopoietic cells. Effects on cell cycle regulators have been implicated in contributing to this overproliferation, but it is likely that PCD also plays a role . Changes in the amount of the apoptotic regulator Wilms Tumor 1 (WT1) are often found in AML patients. lz promotes cell death in the Drosophila eye in part by activating the expression of klu, the Drosophila homolog of WT1. It is suggested that these findings may be relevant to how Runx1 chimeras lead to the development of AML in humans. Furthermore, they suggest that WT1 may be a direct target of Runx1 (Wildonger, 2005).

Protein Interactions

Argos protein can inhibit the activation of EGF-R by Spitz (Schweitzer,1994). Presumably, it does this by binding competitively to the EGF receptor, but it is unclear whether Argos binding sends an active off signal, or whether it just fails to signal through the receptor. Alternatively, argos could send an off signal throught another receptor, or it could form non-functional heterodimers with Spitz (Golembo, 1996).

The activation of the Drosophila EGF receptor (DER) by its natural ligand Spitz is inhibited by Argos. Argos and Spitz both have an EGF-like domain, which in the case of Argos differs from that of Spitz and other EGF receptor agonists in that it has an extended B-loop of 20 amino acids instead of 10 amino acids. This B-loop contains an unusual cluster of charged residues. To investigate whether B-loop sequences are an important determinant for receptor activation and play a causal role in the antagonistic activity of Argos, three human (h)EGF mutants were constructed in which amino acids derived from the Argos B-loop were introduced. In one mutant (E3A4E/B10), the replacement of four amino acids in the B-loop of hEGF (123, E24, D27, and K28) by the corresponding Argos residues neither alters the binding affinity of the growth factor for the hEGF receptor nor does it change its ability to induce a mitogenic response. However, the insertion of 2 additional Argos residues (E3A4E/B12) or extension of the B-loop by 10 amino acids (E3A4E/B20) results in a significant loss of binding affinity. In spite of this, both E3A4E/B12 and E3A4E/B20 appear to be strong agonists for the hEGF receptor with similar dose-response curves for mitogenic activity and MAPK activation as wild-type hEGF. These data show that several nonconservative substitutions in the hEGF B-loop are tolerated without affecting receptor binding or activation. They show that receptor binding and receptor signaling efficiency can be uncoupled, which is a prerequisite for the development of receptor antagonists (van de Poll, 1997).

Members of the epidermal growth factor receptor (EGFR) or ErbB/HER family and their activating ligands are essential regulators of diverse developmental processes. Inappropriate activation of these receptors is a key feature of many human cancers, and its reversal is an important clinical goal. A natural secreted antagonist of EGFR signalling, called Argos, was identified in Drosophila. Argos functions by directly binding (and sequestering) growth factor ligands that activate EGFR. This study describes the 1.6-Å resolution crystal structure of Argos bound to Spitz, an EGFR ligand. Contrary to expectations, Argos contains no EGF-like domain. Instead, a trio of closely related domains (resembling a three-finger toxin fold) form a clamp-like structure around the bound EGF ligand. Although structurally unrelated to the receptor, Argos mimics EGFR by using a bipartite binding surface to entrap EGF. The individual Argos domains share unexpected structural similarities with the extracellular ligand-binding regions of transforming growth factor-β family receptors. The three-domain clamp of Argos also resembles the urokinase-type plasminogen activator (uPA) receptor, which uses a similar mechanism to engulf the EGF-like module of uPA. These results indicate that undiscovered mammalian counterparts of Argos may exist among other poorly characterized structural homologues. In addition, the structures presented in this study define requirements for the design of artificial EGF-sequestering proteins that would be valuable anti-cancer therapeutics (Klein, 2008).

Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.